CN1860247A - High-strength thin steel sheet with excellent hole expandability and ductility - Google Patents

Abstract

A high-strength thin steel sheet excellent in hole expansibility and ductility, characterized in that: contains, in mass%, C: 0.01% -0.20%, Si: 1.5% or less, Al: 1.5% or less, Mn: 0.5% -3.5%, P: 0.2% or less, S: 0.0005% -0.009%, N: 0.009% or less, Mg: 0.0006% -0.01%, O: 0.005% or less; and Ti: 0.01% -0.20%, Nb: 0.01 to 0.10%, the balance being iron and unavoidable impurities, and the steel structure being mainly composed of ferrite, bainite and martensite, and the steel structure satisfying the following formula.

[Mg％]≥([O％]/16×0.8)×24 (1)；

[S％]≤([Mg％]/24－[O％]/16×0.8+0.00012)×32 (2);

[S％]≤0.0075/[Mn％] (3)。

Description

High-strength thin steel sheet with excellent hole expandability and ductility

technical field

The present invention relates to a sheet thickness of about 6.0 mm or less, which is mainly used as a stamped steel sheet for automobiles, and has a tensile strength of 590 N/ ^mm2 or more, and furthermore, 980 N/ ^mm2 or more, and is excellent in hole expandability and ductility High-strength thin steel plate and its manufacturing method.

Background technique

In recent years, as measures to reduce automobile fuel consumption, the demand for weight reduction of the vehicle body and cost reduction by integral molding of parts has intensified, and the development of hot-rolled high-strength steel sheets with excellent press formability has been progressing. Conventionally, a dual-phase steel sheet composed of ferrite and martensite structures is known as a hot-rolled steel sheet for processing.

The dual-phase steel plate is composed of a composite structure of a soft ferrite phase and a hard martensitic phase. Pores and cracks occur at the interface of the two phases with significantly different hardness, so there is a problem of poor hole expandability. Not suitable for applications requiring high hole expandability, such as parts.

In response to this, in JP-A-4-88125 and JP-A-3-180426, a method for producing a hot-rolled steel sheet with a structure mainly composed of bainite and excellent hole expandability has been proposed. Poor, so applicable parts are restricted.

As a technology that takes into account both hole expandability and ductility, ferrite was proposed in JP-A No. 6-293910, JP-A No. 2002-180188, JP-A No. 2002-180189, and JP-A No. 2002-180190. +Steel plate with mixed structure of bainite, but in the background of further weight reduction of automobiles and the complexity of parts, etc., higher hole expandability is required, and high formability and high strength that cannot be fully satisfied by the above-mentioned technologies are required change.

In addition, the inventors of the present invention found in JP-A-2001-342543 and JP-A-2002-20838 that the punching crack is an important point as means for improving hole expandability without deterioration of elongation. State, and found that through the miniaturization of (Ti, Nb)N, the punching cross-section can generate fine and uniform pores, which can alleviate the stress concentration during the hole expansion process and improve the hole expandability.

Furthermore, the use of Mg-based oxides has been proposed as a means for miniaturizing the above-mentioned (Ti,Nb)N. However, in this invention, only oxides are controlled, and the degree of freedom in oxygen control is small, and the total amount of limited free oxygen available after deoxidation is also small, and it is difficult to obtain a predetermined dispersion state, and it is difficult to obtain sufficient effects.

Contents of the invention

The present invention was made in order to solve the above-mentioned conventional problems, and to provide a high-strength thin steel sheet having a tensile strength of 590 N/mm ² or more, further 980 N/mm ² or more, and having both excellent hole expandability and ductility .

The inventors of the present invention have repeatedly conducted various experiments and experiments on the miniaturization of (Ti, Nb)N in order to alleviate the stress concentration during the hole expansion process and improve the hole expandability by forming fine and uniform pores in the cross section of the punched hole. seminar.

As a result, it was found that, conventionally, sulfides were considered to cause deterioration of hole expandability, but Mg-based sulfides precipitated at high temperature can act as nuclei for (Ti, Nb)N precipitates, and Mg-based sulfides precipitated at low temperature Competitive precipitation with (Ti, Nb)N suppresses the growth of (Ti, Nb)N, and as a result, Mg-based sulfide contributes to the improvement of pore expandability due to the miniaturization of TiN.

Moreover, it was found that in order to avoid the precipitation of Mn-based sulfides and obtain the above-mentioned effect through the precipitation of Mg-based sulfides, the addition of O, Mg, Mn, and S is required to meet certain conditions. Uniform miniaturization of finer (Ti, Nb)N can be easily realized compared with the material. And, based on this finding, the following inventions have been accomplished.

(1) A high-strength thin steel plate excellent in hole expandability and ductility, characterized in that: in mass %, it contains

C: 0.01% to 0.20%,

Si: 1.5% or less,

Al: 1.5% or less,

Mn: 0.5% to 3.5%,

P: 0.2% or less,

S: 0.0005% to 0.009%,

N: 0.009% or less,

Mg: 0.0006% to 0.01%,

O: 0.005% or less; and

Ti: 0.01% to 0.20%,

Nb: one or two of 0.01% to 0.10%, the remainder is composed of iron and unavoidable impurities, and Mn%, Mg%, S%, and O% satisfy the formulas (1) to (3), while The steel structure is mainly composed of one, two or more of ferrite, bainite, and martensite.

[Mg%]≥([O%]/16×0.8)×24 (1)

[S%]≤([Mg%]/24-[O%]/16×0.8+0.00012)×32 (2)

[S%]≤0.0075/[Mn%] (3)

(2) The high-strength steel sheet excellent in hole expandability and ductility according to the above (1), characterized in that in the composite precipitates of MgO, MgS and (Nb, Ti)N, 0.05 μm to 3.0 μm The precipitates contained 5.0×10 ² to 1.0×10 ⁷ per 1 square millimeter.

(3) The high-strength steel sheet excellent in hole expandability and ductility according to the above (1), wherein Al% and Si% also satisfy the formula (4) in terms of mass%.

[Si%]+2.2×[Al%]≥0.35 (4)

(4) The high-strength steel sheet excellent in hole expandability and ductility according to the above (2), wherein Al% and Si% also satisfy the formula (4) in mass%.

[Si%]+2.2×[Al%]≥0.35 (4)

(5) The high-strength steel sheet excellent in hole expandability and ductility according to any one of the above (1) to (4), characterized in that, in mass %, Ti%, C%, Mn%, and Nb% also satisfies formulas (5) to (7), and the steel structure is mainly composed of bainite, and the strength exceeds 980N/mm ² .

0.9≤48/12×[C%]/[Ti%]＜1.7 (5)

50227×[C%]-4479×[Mn%]＞-9860 (6)

811×[C%]+135×[Mn%]+602×[Ti%]+794×[Nb%]＞465 (7)

(6) The high-strength steel sheet excellent in hole expandability and ductility according to any one of the above (1) to (4), characterized in that, in mass %, C%, Si%, Al%, and The Mn% also satisfies formula (8), and the steel structure is mainly composed of ferrite and martensite, and the strength exceeds 590N/mm ² .

-100≤-300[C%]+105[Si%]-95[Mn%]+233[Al%] (8)

(7) The high-strength steel sheet excellent in hole expandability and ductility according to the above (6), characterized in that in the crystal grains of the steel structure, the ratio of the short diameter (ds) to the long diameter (dl) is Crystal grains having (ds/dl) of 0.1 or more were present in 80% or more.

(8) The high-strength steel sheet excellent in hole expandability and ductility according to the above (7), characterized in that, among the ferrite grains in the above-mentioned steel structure, grains having a grain size of 2 μm or more 80% or more are present.

(9) The high-strength steel sheet excellent in hole expandability and ductility according to any one of the above (1) to (4), characterized in that, in mass %, C%, Si%, Mn%, and Al% also satisfies formula (8), and the steel structure is mainly composed of ferrite and bainite, and the strength exceeds 590N/mm ² .

-100≤-300[C%]+105[Si%]-95[Mn%]+233[Al%] (8)

(10) The high-strength steel sheet excellent in hole expandability and ductility according to the above (9), wherein the ratio of the short diameter (ds) to the long diameter (dl) in the crystal grains of the steel structure is Crystal grains having (ds/dl) of 0.1 or more were present in 80% or more.

(11) The high-strength steel sheet excellent in hole expandability and ductility according to the above (10), wherein, among the ferrite grains in the above-mentioned steel structure, grains having a grain size of 2 μm or more 80% or more are present.

(12) A method of manufacturing a high-strength thin steel sheet excellent in hole expandability and ductility, characterized in that the steel composed of any one of the above-mentioned (1) to (4) components is formed in the Ar ₃ phase The rolling is completed at the final rolling temperature of the change point or above, followed by cooling at a cooling rate of 20°C/sec or above, and coiling at a temperature of less than 300°C. The steel structure is ferrite and martensite. A high-strength thin steel plate with a main structure and a strength exceeding 590N/ ^mm2 .

(13) A method of manufacturing a high-strength thin steel sheet excellent in hole expandability and ductility, characterized in that the steel composed of any one of the above-mentioned (1) to (4) components is formed in the Ar ₃ phase End rolling at the finish rolling temperature above the change point, then cool to 650°C to 750°C at a cooling rate of 20°C/sec or above, then perform air cooling at this temperature for 15 seconds or less, and then carry out again Cooling and coiling at a temperature of less than 300°C produces a high-strength thin steel sheet with a steel structure mainly composed of ferrite and martensite and a strength exceeding 590N/ ^mm2 .

(14) A method of manufacturing a high-strength thin steel sheet excellent in hole expandability and ductility, characterized in that the steel composed of any one of the above-mentioned (1) to (4) components is formed in the Ar ₃ phase The rolling is completed at the final rolling temperature of the change point or above, followed by cooling at a cooling rate of 20°C/sec or above, and coiling at a temperature of 300°C to 600°C. The steel structure is based on ferrite and shellfish. A high-strength thin steel plate with tenite as the main structure and a strength exceeding 590N/ ^mm2 .

(15) A method of manufacturing a high-strength thin steel sheet excellent in hole expandability and ductility, characterized in that the steel composed of any one of the above-mentioned (1) to (4) components is formed in the Ar ₃ phase End rolling at the finish rolling temperature above the change point, then cool to 650°C to 750°C at a cooling rate of 20°C/sec or above, then perform air cooling at this temperature for 15 seconds or less, and then carry out again Cool and coil at a temperature of 300°C to 600°C to produce a high-strength thin steel plate with a steel structure mainly composed of ferrite and bainite and a strength exceeding 590N/ ^mm2 .

Description of drawings

Fig. 1 is a graph showing the relationship between tensile strength and elongation.

Fig. 2 is a graph showing the relationship between tensile strength and hole expansion ratio.

Fig. 3 is a graph showing the relationship between tensile strength and elongation.

Fig. 4 is a graph showing the relationship between tensile strength and hole expansion ratio.

Fig. 5 is a graph showing the relationship between elongation and ds/dl.

Fig. 6 is a graph showing the relationship between elongation and the ratio of ferrite grains of 2 µm or more.

Fig. 7 is a graph showing the relationship between tensile strength and elongation.

Fig. 8 is a graph showing the relationship between tensile strength and hole expansion ratio.

Fig. 9 is a graph showing the relationship between elongation and ds/dl.

Fig. 10 is a graph showing the relationship between elongation and the ratio of ferrite grains of 2 µm or more.

Detailed ways

For the improvement of hole expandability, the present invention focuses on the end surface properties of punched holes, and adjusts the addition amount of O, Mg, Mn, and S under specified conditions, so that Mg-based oxides and sulfides are uniformly and finely precipitated, and punched holes are suppressed. When the coarse cracks occur, the end surface properties are uniformized, thereby improving the hole expandability.

Hereinafter, the constituent elements of the present invention will be described in detail.

First, the reasons for limiting the composition of the high-strength thin steel sheet of the present invention (steel sheet of the present invention) will be described. In addition, % means mass %.

C: is an element that affects the workability of steel, and when the content is large, the workability deteriorates. In particular, if it exceeds 0.20%, carbides (pearlite, cementite) harmful to hole expandability will be formed, so it is made 0.20% or less. However, when particularly high hole expandability is required, it is preferably set at 0.1% or less. Furthermore, from the viewpoint of ensuring the required strength, 0.01% or more is necessary.

Si: is an element effective in suppressing the generation of harmful carbides, increasing the ferrite ratio, and increasing the elongation, and is also effective in securing the strength of the material by solid solution strengthening. Therefore, it is preferable to add Si, but when the addition amount increases, not only the chemical conversion treatability but also the spot weldability will decrease, so the upper limit is made 1.5%.

Al: Like Si, it is an effective element for suppressing the formation of harmful carbides, increasing the ferrite ratio, and improving elongation. In particular, it is an essential element to balance ductility and chemical conversion treatability.

In addition, Al has been an element necessary for deoxidation in the past, and it is usually added in an amount of about 0.01 to 0.07%. As a result of repeated studies, the inventors have found that adding a large amount of Al in a low-Si system can also reduce the ductility. Next, improve chemical conversion processability.

However, when the amount of addition increases, not only the effect of improving the ductility is saturated, but also the chemical conversion treatability is lowered, and the spot weldability is also deteriorated, so the upper limit is made 1.5%. Especially under severe conditions of chemical conversion treatment, the upper limit is preferably set at 1.0%.

Mn: It is an element necessary to ensure the strength, and it needs to be added at least 0.50%. Furthermore, in order to secure hardenability and obtain stable strength, it is preferable to add more than 2.0%. However, when added in a large amount, micro-segregation or macro-segregation is likely to occur, and these segregations deteriorate the hole expandability, so the upper limit was made 3.5%.

P: It is an element which improves the strength of a steel plate, and is an element which improves corrosion resistance by simultaneous addition with copper. However, when the content is large, weldability, workability, and toughness will deteriorate. Therefore, the content is set at 0.2% or less. In particular, when the corrosion resistance is not a problem, it is preferable to set the content at 0.03% or less with emphasis on workability.

S: It is one of the most important additive elements in the present invention. S combines with Mg to form sulfide, which becomes the nucleus of (Ti, Nb)N, and by inhibiting the growth of (Ti, Nb)N, it is beneficial to the miniaturization of (Ti, Nb)N, and the hole expandability is greatly improved. improve.

In order to obtain this effect, addition of 0.0005% or more is necessary, preferably addition of 0.001% or more. However, excessive addition forms Mn-based sulfides and conversely deteriorates the hole expandability, so the upper limit is made 0.009%.

N: It is beneficial to generate (Ti, Nb)N, so it is better to be less in order to ensure workability. If it exceeds 0.009%, coarse TiN is formed and the workability deteriorates, so the amount of N is made 0.009% or less.

Mg: is one of the most important additive elements in the present invention. Mg combines with oxygen to form oxides, and combines with S to form sulfides. The generated Mg-based oxides and Mg-based sulfides are in a uniformly dispersed state in which precipitates are smaller in size than conventional steel without Mg addition.

These finely dispersed precipitates in the steel are beneficial to the finely dispersed (Ti, Nb)N and are effective in improving the hole expandability.

However, when the amount added is less than 0.0006%, the effect is not sufficient, and the addition of 0.0006% or more is necessary. In order to sufficiently obtain its effect, addition of 0.0015% or more is preferable.

On the other hand, the addition of more than 0.01% not only saturates the improvement effect but also degrades the purity of the steel and deteriorates the hole expandability and ductility, so the upper limit is made 0.01%.

O: is one of the most important additive elements in the present invention. Combining with Mg to form oxides is beneficial to the improvement of hole expandability. However, excessive addition degrades the purity of steel and causes deterioration of ductility, so the upper limit is made 0.005%.

Ti and Nb: are one of the most important additive elements in the present invention. Ti and Nb are elements that form carbides and are effective in increasing the strength, contribute to uniform hardness and improve hole expandability. In addition, Ti and Nb are considered to form fine and uniform nitrides with Mg-based oxides and Mg-based sulfides as nuclei, and this nitride can suppress coarse cracks by forming fine pores and suppressing stress concentration during punching. As a result, the hole expandability is dramatically improved.

In order to effectively exhibit the above effects, both Nb and Ti need to be added by at least 0.01% or more.

However, if the addition amount is too large, precipitation strengthening will lead to deterioration of ductility, so Ti is set to 0.20% and Nb is set to 0.10% as the upper limit. These elements have effects whether they are added individually or in combination.

Furthermore, in the steel sheet of the present invention, one, two or more of the following elements may be added.

Ca, Zr, REM: Controls the shape of sulfide-based inclusions and is effective for improving hole expandability. In order to obtain this effect, it is necessary to add at least 0.0005% or more of 1 type, 2 types or more. On the other hand, when added in a large amount, the purity of the steel will be deteriorated to the contrary, and the hole expandability and ductility will be impaired. Therefore, the upper limit is set at 0.01%.

Cu: is an element that improves corrosion resistance by compound addition with P. In order to obtain this effect, it is preferable to add 0.04% or more. However, adding a large amount increases hardenability and impairs ductility, so the upper limit is made 0.4%.

Ni: is an element that suppresses hot cracking when Cu is added. In order to obtain this effect, it is preferable to add 0.02% or more. However, adding a large amount increases hardenability like Cu and impairs ductility, so the upper limit is made 0.3%.

Mo: is an effective element for suppressing the formation of cementite and improving hole expandability. In order to obtain this effect, it is necessary to add 0.02% or more. However, Mo is also an element that improves hardenability, and excessive addition reduces ductility, so the upper limit is made 0.5%.

V: An element that forms carbides and contributes to securing strength. In order to obtain this effect, it is necessary to add 0.02% or more. However, adding a large amount reduces the elongation and the cost of addition is high, so the upper limit is made 0.1%.

Cr: Like V, it is an element that forms carbides and contributes to securing strength. In order to obtain this effect, it is necessary to add 0.02% or more. However, Cr is also an element that improves hardenability, and adding a large amount reduces elongation, so the upper limit is made 1.0%.

B: It is an effective element for strengthening grain boundaries and improving secondary processing cracks that are a problem of ultra-high-strength steel. In order to obtain this effect, it is necessary to add 0.0003% or more. However, B is also an element that improves hardenability, and adding a large amount reduces ductility, so the upper limit is made 0.001%.

The inventors of the present invention conducted intensive studies to solve the above-mentioned problems. As a result, they found that by adjusting the amounts of O, Mg, Mn, and S added under predetermined conditions, it was possible to use Mg-based oxides and Mg-based sulfides to make (Nb , Ti) N finely dispersed.

That is, by sufficiently precipitating Mg-based oxides; and in the case of suppressing the precipitation of Mn-based sulfides, controlling the precipitation temperature of Mg-based sulfides to precipitate Mg-based sulfides, thereby being able to utilize the above-mentioned role as a nucleus, and inhibition of growth. Therefore, the following three relational expressions are derived. It will be explained below.

In the present invention, since Mg-based sulfides are used in addition to Mg-based oxides, Mg needs to be added in an amount of O or more. However, O and other elements such as Al also form oxides. As a result of intensive research by the present inventors, the effective O combined with Mg is 80% of the analyzed amount, and Mg is added in this amount or more. Sufficient sulfide that works for improvement of hole expandability is necessary. Therefore, the amount of Mg added needs to satisfy the formula (1).

On the other hand, S is an essential element for the formation of Mg-based sulfides, but when the amount of S is large, S becomes Mn-based sulfides. If the amount of precipitation of this Mn-based sulfide is small, it will exist in a composite state with Mg-based sulfide, and will not affect the deterioration of the hole expandability. It affects the properties of Mg-based sulfides and degrades hole expandability. Therefore, the amount of S needs to satisfy the formula (2) with respect to the amount of Mg and effective O.

In addition, under the condition that both the amount of Mn and the amount of S are large, Mn-based sulfides are precipitated at high temperature, the formation of Mg-based sulfides is suppressed, and the hole expandability cannot be sufficiently improved. Therefore, the amount of Mn and the amount of S need to satisfy the formula (3).

[Mg%]≥([O%]/16×0.8)×24 (1)

[S%]≤([Mg%]/24-[O%]/16×0.8+0.00012)×32 (2)

[S%]≤0.0075/[Mn%] (3)

In order to alleviate the stress concentration during the hole expansion process and improve the hole expandability by forming fine and uniform pores in the cross section of the punched hole, it is important to make the (Nb,Ti)N uniform and fine. When the size of (Nb,Ti)N is small, it cannot be the origin of fine and uniform pores. On the other hand, when the size is too large, it becomes the origin of coarse cracks.

In addition, it is considered that when the number of precipitates is small, the number of fine pores generated during punching is insufficient, and the effect of suppressing the occurrence of coarse cracks cannot be obtained.

As a result of intensive studies, the present inventors have found that composite precipitation of MgO and MgS can be utilized as a method for uniformly and finely depositing (Nb,Ti)N. Although the reason for this has not been confirmed, it was found that in the composite utilization of oxide plus sulfide, the size and density of the composite precipitates that exert the effect are different among the composite precipitates of MgO, MgS, and (Nb, Ti)N. Therefore, it is necessary to contain 5.0×10 ² to 1.0×10 ⁷ precipitates of 0.05 μm to 3.0 μm per 1 square mm. In this case, even if Al ₂ O ₃ or SiO ₂ is contained in the composite oxide, the effect will not be impaired, and a small amount of MnS will not impair the effect.

In addition, the dispersion state of the complex precipitates specified in the present invention can be quantitatively measured, for example, by the following method. Make an extracted replica sample from any part of the parent steel plate, observe it with a transmission electron microscope (TEM) at a magnification of 5000 to 20000 times, at least in an area of 5000 ^μm2 or more, preferably in an area of 50000 ^μm2 or more, The number of target composite inclusions is measured and converted into the number per unit area.

In this case, the identification of oxides and (Nb, Ti)N is carried out by compositional analysis by energy dispersive X-ray spectroscopy (EDS) attached to TEM, and crystal structure analysis by electron diffraction photographs obtained by TEM. . Where it is cumbersome to perform such identification for all complex inclusions to be measured, the following procedure can be followed briefly.

First, measure the number of objects with different shapes and sizes according to the above method, and among them, identify 10 or more of all the precipitates with different shapes and sizes according to the above method, and calculate the oxide and (Nb,Ti)N ratio. This ratio is then multiplied by the previously determined number of inclusions.

When the carbides in the steel interfere with the above-mentioned TEM observation, the carbides aggregated and coarsened or dissolved by heat treatment can facilitate the observation of the target composite inclusions.

Si and Al: are very important elements for controlling the structure to ensure ductility. However, Si may have surface irregularities called Si scale during the hot rolling process, which not only impairs the appearance of the product, but may also form a chemical conversion film when chemical conversion treatment and coating are applied after stamping. Poor and poor adhesion of the coating.

Therefore, a large amount of Si cannot be added in some cases for some applications that require strict chemical conversion treatability. At this time, in order to achieve both ductility and chemical conversion treatability, Al can be used instead of Si. However, when both Si and Al are added in large amounts, the ratio of the ferrite phase increases and the target strength cannot be obtained.

Therefore, in order to ensure sufficient strength and ensure ductility, the amount of Si and the amount of Al need to satisfy the formula (4). However, especially when elongation becomes a problem, it is preferable to set it to 0.9 or more.

[Si%]+2.2×[Al%]≥0.35 (4)

Next, the structure of the steel sheet of the present invention will be described.

The present invention is a technique for improving the cross-sectional properties during punching. Therefore, any phase including ferrite, bainite, and martensite in the steel structure can exhibit the desired effect.

However, since the steel structure affects the mechanical properties, the structure should be controlled according to the required mechanical properties.

(1) Steel plate mainly composed of bainite (steel plate B of the present invention)

In order to secure a strength exceeding 980 MPa, it is necessary to use structure strengthening as a strengthening mechanism, and in order to improve workability, especially hole expandability, the structure needs to be set to a structure mainly composed of bainite.

In this case, when the second phase is ferrite, the ductility is improved, so it is preferable to contain ferrite as the second phase. In addition, the steel plate B of the present invention does not hinder the effect of the present invention even if austenite remains in the structure, but coarse cementite and pearlite reduce the effect of Mg-based precipitates on improving the properties of the end surface, so it is not ideal.

Steel with a strength exceeding 980N/mm ² deteriorates in ductility and hole expandability as the strength increases. As a result of intensive research by the present inventors in order to solve the above-mentioned problems, it has been found that Mg-based precipitates can improve the hole expandability by improving the properties of the punched end surface, and as a means of ensuring both strength and ductility. In the structure, it is effective to specify the range of the component amounts of C, Mn, Ti, and Nb.

That is, the following three relational expressions were derived by making the most of the precipitation strengthening of TiC and clarifying the influence of the structure strengthening by Mn and C on the material. Hereinafter, description will be given.

When the addition amount of C is smaller than that of Ti, the elongation is deteriorated due to the increase of solid-solution Ti, so it is set to 0.9≦48/12×C/Ti. On the other hand, when the amount of C added is too high compared with Ti, TiC will precipitate during the heating of hot rolling, and not only the effect of increasing the strength cannot be obtained, but also the hole expandability will be accompanied by the increase of the amount of C in the second phase. deterioration.

Since this also leads to a decrease in the effect of the Mg-based precipitates on improving the properties of the end faces, the upper limit of 48/12×C/Ti is set to 1.7.

That is, the amount of Ti and the amount of C need to satisfy the formula (5).

0.9≤48/12×C/Ti＜1.7 (5)

Especially when emphasis is placed on hole expandability, it is preferable to set it to 1.0≦48/12×C/Ti<1.3.

Since the formation of ferrite is suppressed with an increase in the amount of Mn added, the ratio of the second phase increases and the securing of strength becomes easier, but this leads to a decrease in elongation. On the other hand, C hardens the second phase and deteriorates hole expandability, but improves elongation.

Therefore, in order to secure the required elongation at a tensile strength exceeding 980 N/mm ² , the amount of C and the amount of Mn need to satisfy the formula (6).

50227×C-4479×Mn＞-9860 (6)

In order to ensure workability, it is necessary to satisfy the above two formulas. If it is a steel plate at the level of 780N/ ^mm2 , it is relatively easy to ensure the strength while satisfying the above two formulas. However, in order to ensure the strength exceeding 980N/ ^mm2 , C which deteriorates the hole expandability and elongation is deteriorated. Mn is a last resort.

In order to secure a strength exceeding 980 N/mm ² , it is necessary to adjust the composition within a range that satisfies both the above two formulas and formula (7).

811×C+135×Mn+602×Ti+794×Nb＞465 (7)

Next, the manufacturing method will be described.

In order not to inhibit the formation of ferrite and to improve the hole expandability, it is necessary to set the finishing temperature at the Ar ₃ transformation point or higher. However, if the temperature is too high, the strength and ductility will decrease due to the coarsening of the structure, so it is preferable to set it at 950° C. or lower.

In order to suppress the formation of carbides harmful to the hole expandability and obtain a high hole expansion ratio, it is necessary to set the cooling rate to 20°C/s or more.

When the coiling temperature is lower than 300°C, martensite is formed and hole expandability deteriorates, so it is set to 300°C or higher.

In addition, although bainite formed at low temperature is not as serious as martensite, if it exists as the second phase, the hole expandability will deteriorate. Therefore, it is preferable to perform winding at 350° C. or above.

When the coiling temperature exceeds 600°C, pearlite and cementite harmful to the hole expandability are formed, so the coiling temperature is set to 600°C or lower.

Air cooling in continuous cooling is effective in increasing the occupancy of the ferrite phase and improving ductility. However, depending on the air cooling temperature and air cooling time, pearlite will be formed, and on the contrary, not only the ductility will be reduced, but also the hole expandability will be significantly reduced.

If the air cooling temperature is lower than 650°C, pearlite, which is harmful to the pore expandability, is formed early, so the air cooling temperature is set to 650°C or higher.

On the other hand, when the air cooling temperature exceeds 750°C, not only will the formation of ferrite be delayed, it will be difficult to obtain the effect of air cooling, and pearlite will be easily formed in the subsequent cooling, so the air cooling temperature is set at 750°C or the following.

Air cooling for more than 15 seconds not only saturates the increase of ferrite, but also imposes a burden on the subsequent cooling speed and winding temperature control. Therefore, the air cooling time is set to 15 seconds or less.

(2) Steel plate mainly composed of ferrite and martensite (steel plate FM of the present invention)

The end surface control technology is a technology related to the improvement of the hole expandability of the steel plate. Therefore, in order to ensure both the ductility and the hole expandability at a high value, it is necessary to ensure the elongation by the steel structure. Therefore, it is necessary to design the steel structure as a structure mainly composed of ferrite and martensite.

At this time, when ferrite exists at 50% or more, especially high ductility can be ensured, so it is preferable to set the ratio of ferrite at 50% or more. In addition, the steel plate FM of the present invention does not hinder the effect of the present invention even if austenite remains in the structure, but coarse cementite and pearlite reduce the effect of Mg-based precipitates on improving the properties of the end surface, so it is not ideal.

In hot rolling, a desired structure must be formed in a short period of time after finish rolling, and the composition of components has a strong influence on the formation of a desired structure. When the steel structure is mainly composed of ferrite and martensite, it is important to secure a ferrite ratio in order to improve ductility.

In order to secure a ferrite ratio effective for improving ductility, the respective amounts of C, Si, Mn, and Al need to satisfy the following formula (8). When the value of the formula (8) is less than -100, a sufficient amount of ferrite cannot be obtained, and the ratio of the second phase increases, so the ductility deteriorates.

-100≤-300[C%]+105[Si%]-95[Mn%]+233[Al%] (8)

The inventors of the present invention have improved the ductility of steel whose steel structure is mainly ferrite and martensite without reducing the effect of improving the hole expandability brought about by the Mg-based precipitates improving the properties of the punched end face. The means were intensively studied. As a result, it was found that controlling the shape of ferrite and the grain size of ferrite plays an effective role as a means for improving ductility. It will be explained below.

The shape of ferrite grains is one of the important indicators for improving the ductility of the steel sheet FM of the present invention. Generally, in a high-alloy composition system, there are many ferrite grains extending in the rolling direction. As a result of intensive studies, the present inventors have found that the stretched crystal grains lead to deterioration of ductility. Furthermore, it was found that, as an indicator, it is effective to reduce the probability of existence of crystal grains whose ratio (ds/dl) of the short axis (ds) to the long axis (dl) is less than 0.1.

In order to sufficiently obtain the ductility-enhancing effect by controlling ferrite crystal grains, it is necessary that crystal grains having a (ds/dl) ratio of 0.1 or more exist in 80% or more of ferrite crystals.

The grain size of ferrite is one of the important indicators for improving ductility in the present invention. Usually, crystal grains become finer with increasing strength. As a result of intensive research, the present inventors have found that for the same strength, ferrite with sufficiently grown crystal grains contributes to the improvement of ductility.

Also, in order to sufficiently obtain the improvement in ductility, it is necessary that 80% or more of ferrite grains have a grain size of 2 μm or more.

Next, the manufacturing method will be described.

In order to prevent the formation of ferrite and improve the hole expandability, it is necessary to set the finishing temperature at the Ar ₃ transformation point or higher. However, if the temperature is too high, the strength and ductility will decrease due to the coarsening of the structure, so it is preferable to set it at 950° C. or lower. In order to suppress the formation of carbides harmful to the hole expandability and obtain a high hole expansion ratio, a cooling rate of 20°C/s or more is necessary.

When the coiling temperature is 300°C or higher, martensite cannot be formed, the strength is lowered, and a predetermined strength cannot be ensured, so it is set to less than 300°C. In order to secure sufficient strength, and thereby sufficiently obtain improvement in elongation, it is preferable to set the winding temperature to 200° C. or lower.

Air cooling in continuous cooling is effective in increasing the occupancy of the ferrite phase and improving ductility. However, depending on the air cooling temperature and air cooling time, pearlite will be formed, and on the contrary, not only the ductility will be reduced, but also the hole expandability will be significantly reduced.

If the air cooling temperature is lower than 650°C, pearlite, which is detrimental to the cell expandability, will be formed early, so the air cooling temperature is set to 650°C or higher.

On the other hand, when the air cooling temperature exceeds 750°C, not only will the formation of ferrite be delayed, it will be difficult to obtain the air cooling effect, and pearlite will be easily formed in the subsequent cooling, so the air cooling temperature is set at 750°C or the following.

Air cooling for more than 15 seconds not only saturates the increase of ferrite, but also imposes a burden on the subsequent cooling speed and winding temperature control. Therefore, the air cooling time is set to 15 seconds or less.

(3) A steel plate mainly composed of ferrite and bainite (steel plate FB of the present invention)

The face control technology is a technology related to the improvement of hole expandability, so the hole expandability is strongly affected by the ductility and hole expandability (basic characteristics) of the base material. Especially for running parts, etc., there is a strong demand for hole expandability. As a basic characteristic, it is necessary to aim for a steel plate with a good balance between ductility and hole expandability, and to further improve hole expandability through end surface control technology.

Therefore, the steel structure needs to be designed as a structure with ferrite and bainite as the main body. At this time, when ferrite exists at 50% or more, especially high ductility can be ensured, so it is preferable to set the ferrite ratio at 50% or more.

In addition, the steel plate FB of the present invention does not hinder the effect of the present invention even if the austenite phase remains in the structure, but the coarse cementite and pearlite reduce the effect of the Mg-based precipitates on the end surface properties, so it is not ideal.

In hot rolling, a desired structure must be formed in a short period of time after finish rolling, and the composition of components has a strong influence on the formation of a desired structure. When the steel structure is mainly composed of ferrite and bainite, it is important to secure a ferrite ratio in order to improve ductility.

In order to secure a ferrite ratio effective for improving ductility, the respective amounts of C, Si, Mn, and Al need to satisfy the following formula (8). When the value of the formula (8) is less than -100, a sufficient amount of ferrite cannot be obtained, and the ratio of the second phase increases, so the ductility deteriorates.

-100≤-300[C%]+105[Si%]-95[Mn%]+233[Al%] (8)

The inventors of the present invention have proposed a method for improving ductility without reducing the effect of improving hole expandability brought about by Mg-based precipitates improving the properties of the punched end face in steel whose steel structure is mainly ferrite and bainite. Intensive research was carried out. As a result, it was found that controlling the shape of ferrite and the grain size of ferrite plays an effective role as a means for improving ductility. It will be explained below.

The shape of ferrite is one of the important indicators for improving ductility in the present invention. Generally, in a high-alloy composition system, there are many ferrite grains extending in the rolling direction. As a result of intensive studies, the present inventors have found that the stretched crystal grains lead to deterioration of ductility. Furthermore, it was found that, as an indicator, it is effective to reduce the probability of existence of crystal grains whose ratio (ds/dl) of the short axis (ds) to the long axis (dl) is less than 0.1.

In order to sufficiently obtain the ductility-enhancing effect by controlling ferrite grains, it is necessary that, among ferrite grains, grains having a (ds/dl) ratio of 0.1 or more exist in 80% or more.

The grain size of ferrite is one of the important indicators for improving ductility in the present invention. Usually, crystal grains become finer with increasing strength. As a result of intensive research, the present inventors have found that for the same strength, ferrite with sufficiently grown crystal grains contributes to the improvement of ductility.

Also, in order to sufficiently contribute to the improvement of ductility, it is necessary that 80% or more of ferrite grains have a grain size of 2 μm or more.

Next, the manufacturing method will be described.

In order to prevent the formation of ferrite and improve the hole expandability, it is necessary to set the finishing temperature at the Ar ₃ transformation point or higher. However, if the temperature is too high, the strength and ductility will decrease due to the coarsening of the structure, so it is preferable to set it at 950° C. or lower.

In order to suppress the formation of carbides that are harmful to the hole expandability and obtain a high hole expansion ratio, it is necessary to have a cooling rate of 20° C./s or more.

When the coiling temperature is lower than 300°C, martensite is formed and hole expandability deteriorates, so it is set to 300°C or higher.

In addition, although bainite formed at low temperature is not as serious as martensite, if it exists as the second phase, the hole expandability will deteriorate, so it is preferable to coil at 350°C or higher.

When the coiling temperature exceeds 600°C, pearlite and cementite harmful to the hole expandability are formed, so the coiling temperature is set to 600°C or lower.

Air cooling in continuous cooling is effective in increasing the occupancy of the ferrite phase and improving ductility. However, depending on the air cooling temperature and air cooling time, pearlite will be formed, and on the contrary, not only the ductility will be reduced, but also the hole expandability will be significantly reduced.

If the air cooling temperature is lower than 650°C, pearlite, which is detrimental to the cell expandability, will be formed early, so the air cooling temperature is set to 650°C or higher.

On the other hand, when the air cooling temperature exceeds 750°C, not only will the formation of ferrite be delayed, it will be difficult to obtain the air cooling effect, and pearlite will be easily formed in the subsequent cooling, so the air cooling temperature is set at 750°C or the following.

Air cooling for more than 15 seconds not only saturates the increase of ferrite, but also imposes a burden on the subsequent cooling speed and winding temperature control. Therefore, the air cooling time is set to 15 seconds or less.

Next, the present invention will be described based on examples.

[Example 1]

Example of steel F according to the invention.

Steels having the composition and property values shown in Table 1 and Table 2 were melted, and continuous casting was performed according to a usual method to produce slabs. Symbols A to Z are steels according to the composition of the present invention, the amount of C added in steel a, the amount of Mn added in steel b, the amount of O added in steel c, the amount of S added in steel e, and the amount of S added in steel f. The amount of Mg added is outside the scope of the present invention.

Moreover, formula (5) of steel a, formula (3) and formula (6) of steel b, formula (1) and formula (2) of steel c, formula (4) of steel d, formula (2) of steel e ) and formula (3), formula (1) of f steel, formula (7) of g steel, outside the scope of the present invention. Also, the number of precipitates in steel f is outside the scope of the present invention.

These steels are heated in a heating furnace at a temperature of 1200° C. or higher, and hot-rolled steel sheets having a thickness of 2.6 to 3.2 mm are produced by hot rolling. The hot rolling conditions are shown in Table 3 and Table 4.

In Table 3 and Table 4, the cooling rates of A4 and J2, the air cooling start temperatures of B3 and F3, and the winding temperatures of E3, G3, and Q4 are outside the scope of the present invention.

The hot-rolled steel sheets thus obtained were subjected to a JIS No. 5 sheet tensile test and a hole-expanding test. Regarding hole expandability (λ), a 60° conical punch is used to extrude and expand a punching hole with a diameter of 10mm. The hole diameter (d) and the initial hole diameter (d0: 10mm) when the crack penetrates the thickness of the plate are expressed by λ=( d-d0)/d0×100 for evaluation.

Table 2 shows TS, E1 and λ of each test piece. Figure 1 shows the relationship between strength and elongation, and Figure 2 shows the relationship between strength and hole expansion (ratio). Compared with the comparison steel, the steel of the present invention is superior in elongation, hole expansion (ratio), or both. On the other hand, g1 steel did not obtain the targeted strength.

Thus, according to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet that ensures a predetermined strength of 980 N/mm ² and is excellent in hole expansion rate and ductility.

Table 1 steel C Si mn P S N Mg Al Nb Ti Ca o exam preparation quality% ABCDEFGHIJKLMNOPQRSTUVWXYZ 0.0620.0600.0550.0500.0600.0650.0500.0300.0800.0800.0500.0500.0600.0500.0400.0500.0500.0550.0550.0700.0700.0700.0500.0600.0600.0600 1.231.301.401.000.030.501.301.300.500.501.400.601.201.301.201.101.100.100.500.900.951.301.301.200.900.90 2.42.52.82.22.22.22.42.52.03.02.72.02.22.52.52.62.62.62.62.22.22.22.42.32.32.3 0.0040.0070.0060.0060.0060.0060.0080.0060.0100.0030.0200.0120.0150.0120.0110.0060.0090.0060.0090.0080.0080.0700.0070.00170.0617 0.00100.00200.00250.00100.00280.00280.00250.00200.00350.00180.00250.00300300.00250.00250.00200200200.00300300250.003003003003003003 0.0050.0030.0030.0040.0040.0040.0040.0030.0040.0020.0030.0030.0020.0030.0020.0040.0050.0020.0020.0020.0020.0020.0030.0030.0202 0.00230.00400.00300.00400.00300300.00440.00400.00170.00350.00800.00500.00100.00300300.00290220.00350.00300.00200320.003535 0.0350.0400.0500.0300.1800.2000.0360.0330.0321.3000.0340.0300.0050.8000.0300.0300.0370.4500.2000.0350.0350.0400.0340.0800.0300 0.0440.0350.0140.0350.0440.0440.0400.0500.0550.0350.0300.0900.0300.0350.0000.0370.0300.0300.0350.0400.0700.0350.0400.0300.030 0.1790.1700.1500.1700.1800.1800.1500.1300.1900.1950.1300.1900.1900.1300.1700.1240.1400.1400.1400.1700.1700.1550.1550.1700.1700.150 ---------0.003-0.002--0.0020.002-0.002-0.0020.0020.002-0.002-- 0.00140.00150.00150.00150.00100.00110.00150.00150.00150.00150.0070.00400.00120.00140.00150.00150.00250.00150.0015015015.0015.00150 Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel invention steel abcdefg 0.2100.0500.0600.0500.0550.0700.070 1.301.001.000.201.100.900.90 2.23.62.22.52.52.21.4 0.1200.0200.0200.0100.0100.0100.010 0.00300.00250.00300.00280.01000.00150.0020 0.0020.0020.0020.0020.0020.0020.002 0.00310.00400.00300.00290.00400.00030.0040 0.0050.0300.0350.0300.0200.0250.030 0.0300.0300.0350.0300.0200.0250.030 0.0800.1700.1700.1500.1500.1700.170 0.002-0.0020.0020.0020.0020.002 0.00150.00150.00600.00150.00150.00150.0007 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

Table 2 steel Right side of formula 1 The right side of formula 2 The right side of formula 3 left side of formula 4 middle of formula 5 Formula 6 on the left left side of formula 7 Number of precipitates/mm ² Ar ₃ ℃ exam preparation ABCDEFGHIJKLMNOPQRSTUVWXYZ 0.00170.00180.00140.00180.00120.00120.00130.00180100180.00180.0080.00480.00140170.00120.00180180300.0018018018018018.00 0.00470.00680.00680680.00620.00620.00790.00680480.00610.00610.013410.00410.00530.00620.00530.00450.00540.0068057061 0.00310.00300.00270.00340.00340.00340.00300.00380.00250.00280380.00300300.00290.00290.00290.00340.00330.00330.0033333333 1.311.391.511.070.430.941.381.370.573.361.470.671.213.061.271.171.181.090.940.981.031.391.371.380.900.97 1.391.411.471.181.331.441.330.921.681.641.541.051.261.540.941.611.431.571.571.651.651.811.291.411.601.41 -7815-8184-9779-7342-6840-6589-8238-9691-4940-9419-9582-6447-6840-8686-9188-9134-9134-8883-8883-6338-63386338-8238-7288-8288-72 522516524468489493487480493615507496484484472496500504508488512490485473481 2.1E+034.3E+033.7E+033.8E+033.9E+033.9E+035.1E+034.3E+033.1E+033.7E+034.0E+039.4E+034.5E+031.7E+033.2E+033.6E+033.5E +033.4E+032.5E+034.3E+033.8E+033.5E+034.5E+032.8E+034.0E+034.3E+03 743743729759728738755758744679741762761749751736737707718747748771754755747747 Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel invention steel abcdefg 0.00180.00180.00720.00180.00180.00180.0008 0.00560.0068-0.00180.00530.00680.00180.0081 0.00340.00210.00340.00300.00300.00340.0054 1.311.071.080.271.170.970.97 10.501.181.411.331.471.651.65 694-13613-6840-8686-8435-6338-2755 539653476492488476372 3.9E+034.5E+031.5E+033.6E+038.3E+033.0E+024.7F+03 712673757719741747798 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

* Among them, Ar ₃ =896-509(C%)+26.9(Si%)-63.5(Mn%)+229(P%)

table 3 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Elongation% Hole expansion ratio% Remark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teel Invention Steel Invention Steel Comparison Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel

Table 4 (continued from Table 3) steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Elongation% Hole expansion ratio% Remark O1O2P1P2Q1Q2Q3Q4R1R2S1S2T1T2U1U2V1V2V3W1W2W3X1X2Y1Y2Z1Z2a1b1c1d1e1f1g1 920910890900900890910920920920930910900910890890890900890920930910900930890910910910850900920900900910910 707070707015040407040100707040704070704070704070707070707070707070707070 670690680700670660--680-660720680-680-660660-700660-690-680690670680680680680670680680680 533445--3-523-4-34-33-3-43333434343 500480480500500480480200500500500480480500480480520400550500580480500480480400500400480480500480480520500 99999110221032102610161028993102010321028101810121034103610481003993103010181058102010121002997992100599510671178100110009108194 1414131313141314141414141616161616171714151415161616151675166141719 8787595964646940606660605960586056566169627465686161656610514568433944 Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention steel Invention steel Invention steel comparison steel comparison steel comparison steel comparison steel comparison steel comparison steel

[Example 2]

Example of steel FM according to the invention.

Steels having the component compositions and property values shown in Table 5 and Table 6 were melted, and continuous casting was performed according to a usual method to produce slabs. Symbols A to Z are steels according to the composition of the present invention, the amount of C added in steel a, the amount of Mn added in steel b, the amount of O added in steel c, the amount of S added in steel e, and the amount of S added in steel f. The amount of Mg added is outside the scope of the present invention.

Moreover, formula (3) and formula (8) of b steel, formula (1) and formula (2) of c steel, formula (4) of d steel, formula (2) and formula (3) of e steel, f Formula (1) for steel is outside the scope of the present invention. Also, the number of precipitates in steel f and steel g is outside the scope of the present invention.

These steels are heated in a heating furnace at a temperature of 1200° C. or higher, and hot-rolled steel sheets having a thickness of 2.6 to 3.2 mm are produced by hot rolling. The hot rolling conditions are shown in Table 7 and Table 8.

In Table 7 and Table 8, the cooling rates of A4 and J2, the air cooling start temperatures of B3 and F3, and the winding temperatures of E3, G3, and Q4 were outside the scope of the present invention.

The hot-rolled steel sheets thus obtained were subjected to a JIS No. 5 sheet tensile test and a hole-expanding test. Regarding hole expandability (λ), a 60° conical punch is used to extrude and expand a punching hole with a diameter of 10mm. The hole diameter (d) and the initial hole diameter (d0: 10mm) when the crack penetrates the thickness of the plate are expressed by λ=( d-d0)/d0×100 for evaluation.

TS, E1, and λ of each sample piece are shown in Table 7 and Table 8. Fig. 3 shows the relationship between strength and elongation, and Fig. 4 shows the relationship between strength and hole expansion ratio (ratio). It can be seen that the steel of the present invention is superior in elongation, hole expansion rate (ratio), or both properties compared with the comparative steel.

In addition, Table 9 and FIG. 5 show the relationship between the ratio (ds/dl) of the short diameter (ds) and the long diameter (dl) exceeding 0.1 and the elongation. It can be seen that when the ratio is 80% or more, high elongation can be stably obtained.

Also, Table 10 and FIG. 6 show the relationship between the ratio of ferrite grains of 2 μm or more and the elongation among the ferrite grains. It can be seen that when the ratio is 80% or more, high elongation can be stably obtained.

Thus, according to the present invention, a high-strength hot-rolled steel sheet excellent in hole expansion rate and ductility can be obtained.

table 5 steel C Si mn P S N Mg al Nb Ti Ca o exam preparation quality% ABCDEFGHIJKLMNOPQRSTUVWXYZ 0.0600.0550.0600.0600.0600.0650.0600.0600.0700.1700.0600.0650.0600.0600.0700.1300.0600.0800.0500.0600.0350.0400.0600.0600.0600.065 0.880.870.800.850.030.501.600.901.001.001.300.501.201.401.200.921.000.100.500.900.951.001.001.200.900.90 1.21.21.21.11.21.21.51.41.33.32.00.71.41.51.41.61.61.61.61.41.41.51.20.81.21.9 0.0180.0110.0150.0050.0060.0060.0110.0070.0100.0300.0200.0120.0150.0120.0110.0060.0150.0110.0150.0150.0120.0700.0080.00170.0617 0.00300.00230.00400200.00280.00280.00150370.00440.00180.00850.00300.00300350.00350.003003003003003003003003003003.00300.0030.00303300.00.00.00300300.00.00.00300.0030030003300003 that 0.0030.0030.0030.0040.0040.0040.0030.0030.0040.0020.0030.0020.0020.0030.0020.0040.0050.0010.0020.0020.0020.0020.0030.0030.0202 0.00300.00400.00200.00400.00230.00230.00300350.00170.00350.00800.00500.00100.00230.00170.00220.00350.00300.00200320.003535 0.0400.0280.0050.0020.1800.2000.0420.0320.0321.3000.0340.0300.0050.8000.0300.0300.0370.4500.2000.0350.0350.0400.0340.0800.0300 0.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0000.0200.0100.0000.0000.0000.0000.0000.0000.0000.0000 0.0250.0200.0200.0250.0250.0250.0200.0200.0300.0250.0250.0350.1900.0200.0200.0000.0100.0250.0250.0200.0250.0200.0200.0200.050.0250 --------------0.0020.002-0.002---0.002-0.002-- 0.00150.00070.00.00150.00100.00100.00150.00150.00150.00150.00150.00400.00120.00140100150.00150.00250.00150.0015015015.00150 Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel invention steel abcdefg 0.2100.0600.0600.0550.0560.0600.060 0.800.801.000.200.800.800.90 1.43.61.21.11.11.21.2 0.1200.0200.0200.0200.0200.0200.020 0.00300.00250.00300.00400.01000.00150.0040 0.0020.0020.0020.0020.0020.0020.002 0.00310.00400.00300.00290.00400.00030.0010 0.0050.0300.0350.0300.0300.0300.030 0.0000.0000.0000.0000.0000.0000.000 0.0200.0200.0200.0200.0200.0200.020 0.002----0.0020.002 0.00150.00150.00600.00150.00150.00150.0007 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

Table 6 steel Right side of formula 1 The right side of formula 2 The right side of formula 3 left side of formula 4 middle of formula 8 Number of precipitates/mm ² Ar ₃ ℃ exam preparation ABCDEFGHIJKLMNOPQRSTUVWXYZ 0.00180.00080.00.00180.00120.00120.00180.00180.00100180.00180.00480.00140170170.00120.00180.00300.00180180180180.0018 0.00540.00810.00680.00530.00530.00540.00610.00610.00610.0013410.00410.00530.00470.00530.004450.00540.00680410.006161 0.00610.00610.00630.00680.00610.00610.00500.00540.00230.00380.070.00540540.00470.00470.00540.00630.00940630.00399999 0.970.930.810.850.430.941.690.971.073.861.370.571.213.161.270.991.081.090.940.981.031.091.071.380.900.97 -33-35-47-33-89-3617-49-3243-64-27-24173-21-87-56-61-68-48-36-40-1951-39-96 3.8E+034.8E+033.3E+034.3E+033.2E+033.2E+033.0E+034.6E+033.5E+033.7E+034.3E+031.2E+044.5E+031.7E+033.4E+033.4E+033.0E +034.2E+033.0E+034.3E+033.8E+033.8E+034.5E+032.8E+034.0E+034.3E+03 815816814819790800815802807633778835812810806754794759786804817823818851815773 Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel invention steel abcdefg 0.00180.00180.00720.00180.00180.00180.0008 0.00560.0068-0.00180.00530.00680.00180.0041 0.00540.00210.00630.00680.00680.00630.0063 0.810.871.080.270.870.870.97 -111-269-19-93-30-41-31 3.9E+034.5E+031.5E+034.2E+038.3E+032.0E+022.5E+02 749663821808824815818 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

* Among them, Ar ₃ =896-509(C%)+26.9(Si%)-63.5(Mn%)+229(P%)

Table 7 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Elongation% Hole expansion ratio% Remark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teel Invention Steel Invention Steel Comparison Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel

Table 8 (continued from Table 7) steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Elongation% Hole expansion ratio% Remark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teel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel a1b1c1d1e1f1g1 850900920900900910910 70707070707070 680680680670680680680 3434343 100100100100100100100 683815604523493608516 20183125342933 40514092455050 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

Table 9 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² ds/dl≥0.1 ratio Elongation% Hole expansion ratio% Remark A1A5A6A7A8A9A10 920920920920920920920 70707070808080 680780760740720700660 4444444 100100100100100100100 608609610605605606611 91% 40% 70% 82% 88% 90% 92% 33242532333333 80808081818180 Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel

Table 10 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Ferrite grain ratio of 2μm or more Elongation% Hole expansion ratio% Remark B1B5B6B7B8B9B10 920860880880920960960 70707070708080 670670670730730670730 5444566 100100100100100100100 603603601600603605605 88506883909394 32252632333333 81818181818181 Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel

[Example 3]

Example of the steel plate FB of the present invention.

Steels having the component compositions and property values shown in Table 11 and Table 12 were melted, and continuous casting was performed according to a usual method to produce slabs. Symbols A to Z are steels according to the composition of the present invention, the amount of C added in steel a, the amount of Mn added in steel b, the amount of O added in steel c, the amount of S added in steel e, and the amount of S added in steel f. The amount of Mg added is outside the scope of the present invention.

Furthermore, formula (3) and formula (8) of steel b, formula (1) and formula (2) of steel c, formula (4) and formula (8) of steel d, formula (2) and formula of e steel (3), the formula (1) of steel f is outside the scope of the present invention. Also, the number of precipitates in steel f and steel g is outside the scope of the present invention.

These steels are heated in a heating furnace at a temperature of 1200° C. or higher, and hot-rolled steel sheets having a thickness of 2.6 to 3.2 mm are produced by hot rolling. The hot rolling conditions are shown in Table 13 and Table 14.

In Table 13 and Table 14, the cooling rates of A4 and J2, the air cooling start temperatures of B3 and F3, and the winding temperatures of E3, G3, and Q4 were outside the scope of the present invention.

The hot-rolled steel sheets thus obtained were subjected to a JIS No. 5 sheet tensile test and a hole-expanding test. Regarding hole expandability (λ), a 60° conical punch is used to extrude and expand a punching hole with a diameter of 10mm. The hole diameter (d) and the initial hole diameter (d0: 10mm) when the crack penetrates the thickness of the plate are expressed by λ=( d-d0)/d0×100 for evaluation.

TS, E1 and λ of each test piece are shown in Table 13 and Table 14. Fig. 7 shows the relationship between strength and elongation, and Fig. 8 shows the relationship between strength and hole expansion rate. It can be seen that the steel of the present invention is superior in elongation, hole expansion rate (ratio), or both properties compared with the comparative steel.

In addition, Table 15 and FIG. 9 show the relationship between the ratio (ds/dl) of the short diameter (ds) and the long diameter (dl) exceeding 0.1 and the elongation. It can be seen that when the ratio is 80% or more, high elongation can be stably obtained. In addition, Table 16 and FIG. 10 show the relationship between the proportion of grains having a diameter of 2 μm or more in the ferrite grains and the elongation. It can be seen that when the ratio is 80% or more, high elongation can be stably obtained.

Thus, according to the present invention, a high-strength steel sheet excellent in hole expansion rate and ductility can be obtained.

Table 11 steel C Si mn P S N Mg Al Nb Ti Ca o exam preparation quality% ABCDEFGHIJKLMNOPQRSTUVWXYZ 0.0390.0300.0320.0400.0390.0390.0400.0350.0300.1700.0500.0300.0600.0500.0400.1300.0300.0390.0300.0300.0350.0400.0350.0400.0030 0.921.001.000.900.030.500.950.901.000.501.300.601.201.401.200.921.000.100.500.700.951.001.001.200.900.90 1.21.31.21.41.21.22.02.01.33.32.00.71.41.51.41.61.61.61.61.21.41.50.80.81.21.9 0.0060.0090.0150.0050.0060.0060.0080.0070.0100.0300.0200.0120.0150.0120.0110.0060.0090.0060.0090.0080.0080.0700.0080.00170.0617 0.00280.00320.00400.00200280.00280.00190.00370.00440.00180.00850.00300.00300350.00350.003003003003003003003003003003.00300.0030.0030303.00.00.00300.00.00.00.00.00.00.00.00.0003 if 0.0040.0050.0030.0040.0040.0040.0020.0030.0040.0020.0030.0030.0020.0030.0020.0040.0050.0020.0020.0020.0020.0020.0030.0030.0202 0.00230.00170.00200.00400.00230.00230.00440.00350.00170.00350.00800.00500.00100230.00170170.00220.00350.002002002320.003535 0.0300.0370.0050.0020.1800.2000.0360.0330.0321.3000.0340.0300.0050.8000.0300.0300.0370.4500.2000.0350.0350.0400.0340.0800.0300 0.0370.0220.0280.0420.0370.0370.0360.0320.0280.0350.0300.0350.0300.0350.0000.0370.0200.0300.0350.0150.0300.0350.0150.0300.030 0.1240.1520.1500.1400.1240.1240.0810.0830.1600.1000.0500.0900.1900.0900.1700.1240.1400.1200.1200.0600.1300.1200.0800.1000.1000.1000 ---------0.003-0.002--0.0020.002-0.002-0.0020.0020.002-0.002-- 0.00140.00100.00150.00150.00100.00100110.00150.00150.00150.00150.00400.0070.00120.00140.00150.00150.00250.0015015015015.0015 Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel invention steel abcdefg 0.2100.0400.0300.0400.0400.0350.035 1.301.001.000.201.100.900.90 1.43.61.51.41.41.41.4 0.1200.0200.0200.0100.0100.0100.010 0.00300.00250.00300.00400.01000.00150.0040 0.0020.0020.0020.0020.0020.0020.002 0.00310.00400.00300.00290.00400.00030.0010 0.0050.0300.0350.0300.0300.0300.030 0.0150.0150.0350.0300.0200.0250.030 0.0800.0600.1400.1500.1500.1200.140 0.002-0.0020.0020.0020.0020.002 0.00150.00150.00600.00150.00150.00150.0007 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

Table 12 steel Right side of formula 1 The right side of formula 2 The right side of formula 3 left side of formula 4 middle of formula 8 Number of precipitates/mm ² Ar ₃ ℃ exam preparation ABCDEFGHIJKLMNOPQRSTUVWXYZ 0.00170.00120.00180.00180.00120.00120.00130.00180.00180180.00180.00480.00140170170120.00180180300300.0018018018.00 0.00470.00450.00680.00530.00530.00790.00610.00480.00610.001340.00410.00530.00470.00530.004450.00540.00680410.0057061 0.00610.00580.00560.00610.00610.00610.00380.00580.00230.00380.070.00540.00540470.00470.00470.00540.00940940930399999 0.991.081.010.900.430.941.030.971.073.361.370.671.213.161.270.991.081.090.940.781.031.001.071.380.900.97 -24-19-17-45-83-29-94-98-20-9-61-6-24176-12-87-47-48-62-41-36-402657-29-87 3.0E+032.8E+033.3E+034.3E+033.2E+033.2E+034.8E+034.6E+033.5E+033.7E+034.3E+031.2E+044.5E+031.7E+033.4E+033.4E+033.0E +034.2E+033.0E+034.3E+033.8E+033.8E+034.5E+032.8E+034.0E+034.3E+03 825827834815801813776777827620783855812815821754808779795825816823856861832788 Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel Invented steel invention steel abcdefg 0.00180.00180.00720.00180.00180.00180.0008 0.00560.0068-0.00180.00530.00680.00180.0041 0.00540.00210.00500.00540.00540.00540.0054 1.311.071.080.271.170.970.97 -58-242-38-117-23-42-42 3.9E+034.5E+031.5E+034.2E+038.3E+034.5E+022.5E+02 762678817794818816816 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

* Among them, Ar ₃ =896-509(C%)+26.9(Si%)-63.5(Mn%)+229(P%)

Table 13 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Elongation% Hole expansion ratio% Remark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teel Invention Steel Invention Steel Comparison Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel

Table 14 (continued from Table 13) steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Elongation% Hole expansion ratio% Remark O1O2P1P2Q1Q2Q3Q4R1R2S1S2T1T2U1U2V1V2V3W1W2W3X1X2Y1Y2Z1Z2 920910890900900890910920920920930910900910890890890900890920930910900930890910910910 7070707070150404070401007070407040707040707040707070707070 670690680700670660--680-660720680-680-660660-700660-690-680690670680 533445--3-523-4-34-33-3-4333 500480480500500480480200500500500480480500480480520400550500580480500480480400500400 830820873883817807819769738750787777608630809821818798845820860822812802821811801791 24232121232422232524252330282322232321232222232223222323 1031101061031071081196011812811112413814011111811012211711099122112119111120112126 Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel Invention Steel a1b1c1d1e1f1g1 850900920900900910910 70707070707070 680680680670680680680 3434343 480480500480480520500 795859850782749788812 15122115242221 6010550115707875 Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel Compare Steel

Table 15 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time 5 Winding temperature ℃ Tensile strength N/mm ² ds/dl≥0.1 ratio Elongation% Hole expansion ratio% Remark A1A5A6A7A8A9A10 920920920920920920920 70707070808080 680780760740720700660 4444444 490490480500500490490 801801796806806801801 91% 30% 60% 82% 88% 90% 92% 24151623242424 112112113112112112112 Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel

Table 16 steel Finishing temperature ℃ Cooling rate ℃/s Air cooling start temperature ℃ Air cooling time s Winding temperature ℃ Tensile strength N/mm ² Ferrite grain ratio of 2μm or more Elongation% Hole expansion ratio% Remark B1B5B6B7B8B9B10 920860860880920960960 70707070708080 670670700730730670730 5444566 490490500490500500490 820820825820825825820 85% 60% 70% 83% 90% 93% 94% 23151623232324 110110109110109109110 Invention Steel Comparison Steel Invention Steel Invention Steel Invention Steel Invention Steel

According to the present invention, for high-strength steel sheets having a strength level of 590 N/mm ² or more, further 980 N/mm ² or more, it is possible to provide a high-strength thin steel sheet having an elongation-ductility balance that has never been achieved before. Therefore, the present invention is extremely useful for industries using high-strength steel sheets as base materials.

Claims (15)

1. A high-strength thin steel plate with excellent hole expandability and ductility, characterized in that: by mass %, it contains C: 0.01% to 0.20%, Si: 1.5% or less, Al: 1.5% or less, Mn: 0.5% to 3.5%, P: 0.2% or less, S: 0.0005% to 0.009%, N: 0.009% or less, Mg: 0.0006% to 0.01%, O: 0.005% or less; and Ti: 0.01% to 0.20%, Nb: one or two of 0.01% to 0.10%, the remainder is composed of iron and unavoidable impurities, and Mn%, Mg%, S%, and O% satisfy the formulas (1) to (3), while The steel structure is mainly composed of one, two or more of ferrite, bainite, and martensite. [Mg%]≥([O%]/16×0.8)×24 (1) [S%]≤([Mg%]/24-[O%]/16×0.8+0.00012)×32 (2) [S%]≤0.0075/[Mn%] (3) 2. The high-strength thin steel sheet excellent in hole expandability and ductility according to claim 1, characterized in that, among the composite precipitates of MgO, MgS and (Nb, Ti)N, the precipitates of 0.05 μm to 3.0 μm The substance contains 5.0×10 ² to 1.0×10 ⁷ per 1 square millimeter. 3. The high-strength thin steel sheet excellent in hole expandability and ductility according to claim 1, wherein Al% and Si% also satisfy formula (4) in terms of mass%. [Si%]+2.2×[Al%]≥0.35 (4) 4. The high-strength thin steel sheet excellent in hole expandability and ductility according to claim 2, wherein Al% and Si% also satisfy the formula (4) in terms of mass%. [Si%]+2.2×[Al%]≥0.35 (4) 5. The high-strength thin steel sheet excellent in hole expandability and ductility according to any one of claims 1 to 4, characterized in that, in terms of mass %, Ti%, C%, Mn% and Nb% also satisfy Formulas (5) to (7), at the same time, the steel structure is mainly composed of bainite, and the strength exceeds 980N/mm ² . 0.9≤48/12×[C%]/[Ti%]＜1.7 (5) 50227×[C%]-4479×[Mn%]＞-9860 (6) 811×[C%]+135×[Mn%]+602×[Ti%]+794×[Nb%]＞465 (7) 6. The high-strength thin steel sheet excellent in hole expandability and ductility according to any one of claims 1 to 4, characterized in that, in terms of mass%, C%, Si%, Al% and Mn% also satisfy Formula (8), at the same time, the steel structure is mainly composed of ferrite and martensite, and the strength exceeds 590N/mm ² . -100≤-300[C%]+105[Si%]-95[Mn%]+233[Al%] (8) 7. The high-strength thin steel sheet with excellent hole expandability and ductility according to claim 6, characterized in that, in the crystal grains of the steel structure, the ratio of the short diameter (ds) to the long diameter (dl) ( Crystal grains having ds/dl) of 0.1 or more were present in 80% or more. 8. The high-strength thin steel sheet excellent in hole expandability and ductility according to claim 7, characterized in that, among ferrite grains in the steel structure, grains having a grain size of 2 μm or more exist 80% or more. 9. The high-strength thin steel sheet with excellent hole expandability and ductility according to any one of claims 1 to 4, characterized in that, in terms of mass%, C%, Si%, Mn% and Al% also satisfy Formula (8), at the same time, the steel structure is mainly composed of ferrite and bainite, and the strength exceeds 590N/mm ² . -100≤-300[C%]+105[Si%]-95[Mn%]+233[Al%] (8) 10. The high-strength thin steel sheet with excellent hole expandability and ductility according to claim 9, characterized in that, in the grains of the steel structure, the ratio of the short diameter (ds) to the long diameter (dl) ( Crystal grains having ds/dl) of 0.1 or more were present in 80% or more. 11. The high-strength thin steel sheet excellent in hole expandability and ductility according to claim 10, characterized in that, among ferrite grains in the steel structure, grains having a grain size of 2 μm or more exist 80% or more. 12. A method for manufacturing a high-strength thin steel plate with excellent hole expandability and ductility, characterized in that: the steel composed of any one of claims 1 to 4 has an Ar ₃ transformation point or above End rolling at the final rolling temperature, then cooling at a cooling rate of 20°C/s or above, and coiling at a temperature lower than 300°C, the steel structure is mainly composed of ferrite and martensite , and high-strength thin steel plates with a strength exceeding 590N/mm ² . 13. A method for manufacturing a high-strength thin steel plate with excellent hole expandability and ductility, characterized in that: the steel composed of any one of claims 1 to 4 has an Ar ₃ transformation point or above End rolling at the finish rolling temperature, then cool to 650-750°C at a cooling rate of 20°C/sec or above, then perform air cooling at this temperature for 15 seconds or less, and then cool again. Coiling is carried out at a temperature of 300°C to produce a high-strength thin steel plate with a steel structure mainly composed of ferrite and martensite and a strength exceeding 590N/ ^mm2 . 14. A method of manufacturing a high-strength thin steel plate with excellent hole expandability and ductility, characterized in that: the steel composed of any one of claims 1 to 4 has an Ar ₃ transformation point or above End rolling at the final rolling temperature, then cooling at a cooling rate of 20°C/s or above, and coiling at a temperature of 300°C to 600°C. The steel structure is mainly composed of ferrite and bainite A high-strength thin steel plate with a high-strength structure and a strength exceeding 590N/mm ² . 15. A method for manufacturing a high-strength thin steel plate with excellent hole expandability and ductility, characterized in that: the steel composed of any one of claims 1 to 4 has an Ar ₃ transformation point or above End rolling at the finish rolling temperature, then cool to 650°C to 750°C at a cooling rate of 20°C/s or above, then perform air cooling at this temperature for 15 seconds or less, and then cool again, at 300°C Coiling is carried out at a temperature of ℃ to 600 ℃ to produce a high-strength thin steel plate with a steel structure mainly composed of ferrite and bainite and a strength exceeding 590N/mm ² .

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