JPH0657337A - Production of high strength galvannealed steel sheet excellent in formability - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for producing a high-strength galvannealed steel sheet having excellent formability. [Structure] C ≦ 0.005% by mass ratio, Si ≦ 2.0
%, Mn ≦ 3.0%, P ≦ 0.10% , S ≦ 0.01
%, A1: 0.01 to 0.10%, Ti: 0.008 to
0.03%, Nb: 0.008 to 0.03%, Cu:
0.8-2.0%, Ni: 0.4-1.0%, B: 0.
0001 to 0.0030%, if necessary Cr: 0.2
Steel containing either or both of 2.0 to 2.0% and 0.0001 to 0.0030% of Ca: the heating temperature, the rolling ratio in rough rolling, the finish rolling finish temperature, and the winding temperature. Hot-rolled by a limited method, the hot-rolled steel strip thus obtained is pickled and cold-rolled by a conventional method, and then, when passing through a continuous hot-dip galvanizing line, each of heating temperature and oxidation balance during heating Limit the conditions. Thereby, a high-strength galvannealed steel sheet having excellent elongation at break and r-value characteristics can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is molded and used as a typical example of automobiles and has a tensile strength of super 440 to super 59.
The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet which has 0 N / mm ² and is excellent in deep drawing property and overhanging property, and is also excellent in secondary workability.

[0002]

2. Description of the Related Art With the automobile industry as a typical example, the demand from users for high strength steel sheets having high formability is steadily increasing in the future due to the pursuit of amenities and global environmental problems. That is, it is the promotion of weight reduction by increasing the number of members having a more complicated shape and thinning the steel plate than ever before. Moreover, this is based on the addition of higher rust prevention than before,
In addition, economics are a prerequisite. Therefore, there is a need for a technique for producing a high-strength and rust-proof steel sheet having high formability at low cost, and naturally, various techniques have been proposed for this.

For example, as typified by JP-A-61-157625 and JP-A-61-217529, metastable austenite at room temperature is left in a steel sheet and transformation is induced by deformation of the austenite. There is a technique in which the fracture elongation property in uniaxial tension is significantly improved by plasticity (TRIP) as compared with the conventional high strength steel plate. This gives high tensile strength, for example 590 N / mm ² grade and 780 N / mm ² grade,
Moreover, because of its high n-value and extremely high overhanging property, it has been spotlighted recently. However, in the case of a steel sheet containing this retained austenite, r
Value (Average Rankford value: Rankfo in the rolling direction
rd value is rL, rolling right-angle direction is rC, rolling 45
R = (rL + rC + 2r)
D) / 4. ) Is at most about 1.0, and the problem is that there is a difficulty in deep drawability.

On the other hand, as a technique for producing a high strength steel sheet having a high deep drawability, that is, a high r value, for example, Japanese Patent Laid-Open No.
9-74232, JP-A-63-47338, and the like. These are all IF (Inte
rstial Free) steel is used as a base steel, and a solid solution strengthening element such as P or Mn is used as a strengthening method to increase the r value and the elongation at break, but when economically and industrially produced, the tensile strength is A maximum of 470 N / mm ² can be achieved, which is an unsuitable technique for further strengthening. On the other hand, Cu has been cited as an element that has been drawing attention in recent years.
-4429 and JP-A-3-33774. However, the former has a drawback that the elongation at break is low because it is a technique using the precipitation of Cu, and the latter is C
The solid solution strengthening of u is used, but the tensile strength is 440.
It is difficult to make it higher than N / mm ² .

[0005]

[Problems to be Solved by the Invention] As described above,
It is at this point that the present invention intends to solve the problem, because there is no existing manufacturing technique for an alloyed hot-dip galvanized steel sheet having a tensile strength of over 440 N / mm ² and a high r value and elongation at break.

[0006]

As a means for solving the above problems, as a result of intensive investigations by the present inventors, the main points are (1) C ≦ 0.005% by mass, Si ≦ 2. 0%, Mn ≦ 3.0%, P ≦ 0.07%, S ≦ 0.01%, Al: 0.01 to 0.10% Ti: 0.008 to 0.03% Nb: 0.008 to 0.03% Cu: 0.8 to 2.0% Ni: 0.4 to 1.0%, B: 0.0001 to 0.0030%, and hot-rolled steel from the balance Fe and inevitable impurities. In this case, the heating temperature is 1200 ° C. or lower, rolling in one pass of 35% or more is performed at least once in rough rolling, and the finish rolling temperature is set to Ar ₃ transformation point or higher and 350 ° C.
Wind at the following temperature. The hot-rolled steel strip thus obtained is pickled, cold-rolled by a conventional method, and then heated in a continuous hot-dip galvanizing line to 800 ° C. or higher and Ac ₃ transformation point or less, and the steel sheet surface before being dipped in a hot-dip zinc pot To 30 nm, the oxidation balance during heating is adjusted so that the Si concentration is 1.5 mg / m ² or less, and the alloying treatment is performed after immersion in the molten zinc pot. A method for manufacturing a high-strength galvanized steel sheet. (2) In the method described in (1), C is further contained in the steel.
r: 0.2 to 2.0% and Ca: 0.0001 to 0.00
A method for producing a high-strength galvannealed steel sheet with excellent formability, characterized by containing either or both of 30%. It was clarified that there are two points.

[0007]

Next, the reason for limiting the numerical values of the present invention will be described in detail. C: C can be further reduced by adding Ti and N
By using the IF steel due to the scavenging effect of the carbide forming element such as b, it is possible to obtain a high r value and a breaking elongation property. For that purpose, it is necessary to reduce it to 0.005% or less. The lower limit is not particularly specified, but the lower limit of C that can be achieved at present is about 0.0005%. Even if it is less than this, the present invention can exert the effect.
Preferably, the content is in the range of 0.0010 to 0.0025%. Si: Si is an element necessary for increasing the tensile strength in the present invention. Even if a higher Si content is contained, the content within the scope of the claims does not pose a problem because the balance of oxidation in the continuous hot-dip galvanizing line described later is adjusted. The upper limit is set to 2.0% because the cost is unnecessarily increased even if the content exceeds the upper limit. A preferred content range is 0.4 to 1.5%, although it varies depending on the tensile strength class.

Mn: Mn is also an element necessary for increasing the tensile strength in the present invention. The content exceeding 3.0% substantially exceeds the melting limit in the steelmaking process, so this was made the upper limit. 0.5 to 1.5% is a preferable content range. P: P is also a necessary element in the present invention because it is an element that can inexpensively increase the tensile strength. P
In order to slow down the alloying rate in the alloying process in the continuous hot-dip galvanizing line, the upper limit was made 0.07%. S: If S is high in content, it will unnecessarily form sulfides in the steel. Therefore, it is an unnecessary element in the present invention, and it is preferable to thoroughly reduce it. Therefore, the upper limit was set to 0.01%. Preferably 0.005
% Or less. Al: Al is necessary as a deoxidizing agent in the melting stage in steelmaking, and 0.01 to 0.10% was set in that range.

Ti: Ti is necessary as a fixed element of N / or N and S in steel. In order to exert the effect, it is necessary to contain at least 0.008% or more. On the other hand, if a large amount of Ti is contained, the powdering property of the plating layer is deteriorated and the r value is lowered due to the formation of a compound with P, which is unsuitable. Therefore, the upper limit was made 0.03%. Preferably, the content is 0.01 to 0.02%. Nb: Nb is necessary as a fixing element of C in steel.
For that purpose, the content of 0.008% or more is required. The upper limit is 0. to prevent an unnecessarily high cost increase.
It was set to 03%. Cu: Cu is used as a solid solution strengthening element in the present invention. This is because the elongation at break deteriorates if it is precipitated. In order to exert the effect, it is necessary to contain at least 0.8% or more. The upper limit is 2.0
Set to%. This is for the following reason.

With the current hot rolling ability, it is difficult to lower the heating temperature to 1050 ° C. or less. 1050 ° C
At the above hot rolling heating temperatures, so-called Cu hegging occurs, which is caused by Cu being concentrated at the grain boundaries on the iron surface. Ni is added in order to avoid this, but it is necessary to increase the Ni content as the Cu content increases.
Since Ni is an extremely expensive element, the inclusion of a large amount of Cu leads to an unnecessarily high material cost. In order to avoid this, the upper limit of the Cu content is set to 2.0%. Preferably, the content is 1.0 to 1.5%. Of course, if the temperature equivalent to the current hot rolling heating can be stabilized at 1050 ° C. or lower by technological innovation, it becomes unnecessary to add Ni. Therefore, the Cu content may be changed according to the tensile strength desired to be obtained as the final product, and it is not necessary to set the upper limit.

Here, Cu is used as a solid solution strengthening element, and it has been explained that it is not preferable because it deteriorates the elongation at break when it is precipitated. However, the alloying process after dipping hot-dip zinc in a continuous hot-dip galvanizing line was explained. In the above, even if Cu which is inevitably precipitated is contained, the present invention sufficiently exerts its effect. The present inventors have repeatedly conducted studies on this point as well. Regarding the precipitation of Cu, in some of the studied ones, only about several% or less of the total Cu content was observed, and the high elongation at break property due to the solid solution of Cu was maintained.

Ni: Ni is contained for the reasons described above. For that purpose, the content of at least 0.4 or more is necessary, and the content up to 1.0% is sufficient. B: B is an element that must be contained in order to prevent secondary processing brittleness in the final product. Therefore, it is necessary to contain at least 0.0001% or more. Also, 0.0
When the content exceeds 030%, the effect is saturated,
In addition, the r value is lowered, so this is set as the upper limit. A preferred content range is 0.0005 to 0.0020.
%. Further, if necessary, Cr is added in an amount of 0.2-2.
One or both of Ca may be contained in the range of 0.0001 to 0.0030% in the range of 0%.

The former, when obtaining the desired strength,
It is used when YP is further lowered and elongation at break is desired to be higher. If the content is 0.2% or less, the effect is not exhibited, and the addition of more than 2.0% is in the limit of melting in the steelmaking process in the case of the present invention based on the ultra-low carbon steel. Was set as the upper limit. The latter is for the treatment of sulfides when it becomes necessary to reduce the Ti content to the lower limit. 0.0
If the content is 001% or less, the effect is not exhibited, and if the content is 0.0030% or more, not only the effect is saturated but also a Ca-based compound is unnecessarily generated, which is unsuitable. A preferred content range is 0.0010 to 0.0025%.

Next, the reasons for limiting the manufacturing method will be described. Upon hot rolling, the heating temperature needs to be 1200 ° C. or lower. This is a condition that Ni is added to prevent Cu hegging, but it is also necessary to fulfill an auxiliary role from the hot rolling heating temperature. The lower limit does not have to be specified in particular, but the heating temperature may be set as a temperature required to secure the temperature until the end of finish rolling. Also, N of C and N
The low temperature heating is preferable also from the viewpoint of scavenging by b and Ti. A preferable heating temperature is 1150 ° C. or lower.

In the rough rolling, rolling of 35% or more per pass is performed at least once. This is defined as one of the reasons for limitation necessary for the present invention based on the results obtained by the present inventors through repeated test experiments. That is, recrystallization is performed in the heating stage in the continuous hot-dip galvanizing line after cold rolling,
It was found that this recrystallization becomes faster due to the limitation of the conditions in rough rolling. This is a very effective technique for the purpose of exerting high workability as in the present invention. The reason why such an effect is obtained is currently under study, but it is presumed that the difference in the morphology of minute carbides or nitrides that cannot be observed even with a transmission electron microscope is ultimately the cause. There is. After that, the finish rolling temperature is set to an Ar ₃ transformation point or higher and winding is performed at a temperature of 350 ° C. or lower. The former is limited to avoid unnecessary work ferrite, and the latter is defined to prevent unnecessary precipitation of Cu in the hot rolling stage.

The hot-rolled steel strip thus obtained is pickled, cold-rolled by a conventional method, and then passed through a continuous hot-dip galvanizing line. When passing this continuous hot-dip galvanizing line,
It is necessary to heat to 0 ° C or higher. This is due to the high r value and n
It is a condition necessary to obtain a value (that is, high deep drawability and overhanging property). To obtain this value, it is necessary to cause not only recrystallization but also normal grain growth to some extent as necessary. It is a condition. The upper limit differs depending on the actual component system and the intended strength class, but it is inappropriate because when heating up to the entire austenite region, phases other than polygonal ferrite will appear during cooling, so the Ac ₃ transformation The point was set as the upper limit of heating. Further, when heating here, the Si concentration amount up to 30 nm from the surface of the steel sheet before being immersed in the molten zinc pot is 1.5 mg / m ²
It is necessary to adjust the oxidation balance during heating so as to be as follows. This is a condition necessary for plating steel containing Si or P, which is difficult to plate by a conventional method, and is used in the present invention. Si concentration is 1.5 mg / m
^If it exceeds ² , non-plating may occur, or the alloying speed may be slowed, so that a problem such as an increase in cost may occur due to a slow stripping speed.

[0017]

【Example】

Example 1 The test steels shown in Table 1 were melted in a secondary refining equipment such as a converter and a vacuum degassing equipment, and continuously cast into a slab. In this table, steels A, B, C, D, E, F, G, H and I are within the scope of the present invention, and others are under the underlined components shown in the table which are outside the scope of the present invention. Is. All of these steels other than steel O were hot rolled under the conditions of hot rolling heating temperature = 1120 ° C., finish rolling end temperature = 920 ° C., and coiling temperature = 50 to 150 ° C. to obtain a coil having a plate thickness of 4.5 mm. The rolling rate of the third pass in the rough rolling was set to 37%. This hot rolled coil was pickled by a conventional method and then cold rolled to 0.8 mm.
In all of the cases, the continuous hot dip galvanizing line was passed under the maximum heating temperature condition which was not higher than the Ac ₃ transformation point. The oxidation balance during heating was adjusted so that the Si concentration amount at 30 nm from the surface of the steel sheet before being immersed in the molten zinc pot was 1.3 mg / m ² or less. Note that the measurement of the Si concentration amount up to 30 nm from the surface of the steel sheet before being immersed in the molten zinc pot was repeatedly studied by the present inventors, and the surface after the annealing test in the laboratory by the same heat cycle as the actual line was measured. The method obtained from analysis was adopted. This was subjected to ordinary temper rolling at 0.8 to 1.0% to obtain a product. The amount of plating adhered at the product stage was 40 to 50 g / m ² .

The mechanical test values and plating characteristics of the product thus obtained were evaluated. The tensile test is JIS Z2201.
Using the described No. 5 test piece, the test was carried out according to the method described in Z2241, and the yield point strength YP, tensile strength TS, and break elongation E
1 was measured. Further, the deep drawing formability and the overhang formability were evaluated by the r value and the n value in the tensile test. The r value was tested at a strain not exceeding the maximum load. Further, the true stress σ and the true strain ε are calculated from the relationship between the load and the elongation after the tensile test, and dσ / d
When ε−σ = 0, ε was taken as the n value. Furthermore, as the plating characteristics, the iron content of the Zn-Fe alloy layer, the appearance observation, and the powdering property were evaluated. Analyze the iron content,
For the powdering property, the front and back were evaluated by the tape peeling width on the punch side after the V bending of 60 degrees and the bending back. From the conventional results, when the plate thickness is 0.8 mm, the peeling width is 5 mm or less, and the powdering property is at a level at which there is no problem.

[0019]

[Table 1]

[0020]

[Table 2]

In Table 2, No. The alphabets correspond to those in Table 1. As shown in Table 2, all of the methods according to the present invention showed excellent r value and n value, and the surface treatment characteristics were also excellent. The J1 material, which is a comparative method, had a poor r value due to the high C content in the steel. K1 material is
Since the amount of S was high, the n value was not good. L1 material has a large amount of Nb, and N1 material has a small amount of Ti.
I recognized the phenomenon that the temperature dropped. On the other hand, the M1 material had a high Nb content, and thus the El characteristic was deteriorated. O1 material is T
Since the amount of i was high, the r value characteristic was deteriorated and the powdering property was deteriorated.

Example 2 As shown in Table 3 of the H steels in Table 1, the influence of the change of the rolling ratio in the rough rolling on the tensile properties of the products was investigated. The manufacturing method other than the conditions shown in Table 2 was the same as H1 in Table 2.
As shown in Table 3, defining the rolling ratio in rough rolling is effective for obtaining a high quality material.

[0023]

[Table 3]

Example 3 Steel A in Table 1 was hot rolled under the conditions of hot rolling heating temperature = 1120 ° C., finish rolling finishing temperature = 900 ° C. and winding temperature = 50 ° C. to obtain a coil having a plate thickness of 4.5 mm. . In the rough rolling, the rolling temperature was 1070 ° C. in the third pass, and the rolling rate in this pass was 37%. This hot rolled coil was pickled by a conventional method, then cold rolled to 0.8 mm, and passed through a continuous hot dip galvanizing line under the conditions of the maximum heating temperature shown in Table 4. In addition, S at 30 nm from the surface of the steel sheet before being immersed in the molten zinc pot
The oxidation balance during heating was adjusted so that the concentration of i was 1.3 mg / m ² or less. Normal temper rolling is applied to this.
8 to 1.0% was applied to obtain a product. The amount of plating adhered at the product stage was 40 to 50 g / m ² . The tensile test value of the product thus obtained was evaluated. The method according to the present invention ensures high moldability, but the comparative method does not exhibit the effect due to insufficient recrystallization and grain growth.

[0025]

[Table 4]

Example 4 Steel E shown in Table 1 was used, and hot rolling and cold rolling were performed in the same manner as in Example 1
When a steel strip rolled under the same conditions as in (1) above was passed through a continuous hot-dip galvanizing line, the amount of Si concentration was adjusted to 30 nm from the steel plate surface as shown in Table 5. In addition,
The maximum heating temperature was 850 ° C. The plating appearance of the product thus obtained was evaluated. When the Si concentration is high, non-plating occurs and the product value does not appear.

Example 5 Steel H and Steel O in Table 1 were used to evaluate the secondary workability.
Steel O was made into a product by the same method as H1 material shown in Table 2. The secondary workability was indicated by the ductility-brittleness transition temperature (Tcr).
This is the transition temperature of the presence or absence of cracks when the edge of a cup formed with a drawing ratio of 2.0 was subjected to 10% widening forming by changing the temperature. The results are shown in Table 6. The O1 material has a high (bad) ductility-brittleness transition temperature, which may cause a practical problem.

[0028]

According to the present invention, an alloyed hot-dip galvanized high-strength steel sheet having excellent formability and excellent secondary workability can be industrially and economically manufactured, and is suitable for the recent environmental problems. Contributing to the industrial sector, especially the automobile industry, which is having a difficult time, is enormous.

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[Procedure amendment]

[Submission date] January 11, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

[0006]

As a means for solving the above problems, as a result of intensive investigations by the present inventors, the main points are (1) C ≦ 0.005% by mass and Si ≦ 2. 0%, Mn ≦ 3.0%, P ≦ 0.10%, S ≦ 0.01%, A1: 0.01 to 0.10% Ti: 0.008 to 0.03% Nb: 0.008 to 0.03% Cu: 0.8 to 2.0% Ni: 0.4 to 1.0%, B: 0.0001 to 0.0030%, and hot-rolled steel from the balance Fe and inevitable impurities. In this case, the heating temperature is 1200 ° C. or lower, rolling in one pass of 35% or more is performed at least once in rough rolling, and the finish rolling temperature is set to Ar ₃ transformation point or higher and 350 ° C.
Wind at the following temperature. The hot-rolled steel strip thus obtained is pickled, cold-rolled by a conventional method, and then heated on a continuous hot-dip galvanizing line to 800 ° C. or higher and Ac ₃ transformation point or lower, and the steel sheet surface before being dipped in a hot-dip zinc pot To 30 nm, the oxidation balance during heating is adjusted so that the Si concentration is 1.5 mg / m ² or less, and alloying treatment is performed after immersion in a molten zinc pot. A method for manufacturing a high-strength galvanized steel sheet. (2) In the method described in (1), C is further contained in the steel.
r: 0.2 to 2.0% and Ca: 0.0001 to 0.00
A method for producing a high-strength galvannealed steel sheet with excellent formability, characterized by containing either or both of 30%. It was clarified that there are two points.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

Mn: Mn is also an element necessary for increasing the tensile strength in the present invention. The content exceeding 3.0% substantially exceeds the melting limit in the steelmaking process, so this was made the upper limit. 0.5 to 1.5% is a preferable content range. P: P is also a necessary element in the present invention because it is an element that can inexpensively increase the tensile strength. P
In order to slow down the alloying rate in the alloying process in the continuous hot-dip galvanizing line, the upper limit was made 0.10%.
Preferably, the content range is 0.03 to 0.07%. S: If S is high in content, it will unnecessarily form sulfides in the steel. Therefore, it is an unnecessary element in the present invention, and it is preferable to thoroughly reduce it. Therefore, the upper limit was set to 0.01%. Preferably 0.005
% Or less. A1: A1 is necessary as a deoxidizing agent in the melting stage in steel making, and 0.01 to 0.10% was set in that range.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location C22C 38/58 C23C 2/06 2/28 (72) Inventor Kunihiko Komiya 1 Kimitsu, Kimitsu-shi, Chiba Kimitsu Works, Nippon Steel Co., Ltd.

Claims (2)

[Claims] 1. A mass ratio of C ≦ 0.005%, Si ≦ 2.0%, Mn ≦ 3.0%, P ≦ 0.07%, S ≦ 0.01%, Al: 0.01 to 0. 10% Ti: 0.008 to 0.03% Nb: 0.008 to 0.03% Cu: 0.8 to 2.0% Ni: 0.4 to 1.0% B: 0.0001 to 0.0030% is contained, and when hot rolling the steel from the balance Fe and unavoidable impurities, the heating temperature is 1200 ° C. or less, and the rolling at least 1 pass 35% or more is performed at least once in the rough rolling to obtain the finish rolling temperature. Is above the Ar ₃ transformation point and 350 ° C
Wind at the following temperature. The hot-rolled steel strip thus obtained is pickled, cold-rolled by a conventional method, and then heated in a continuous hot-dip galvanizing line to 800 ° C. or higher and Ac ₃ transformation point or less, and the steel sheet surface before being dipped in a hot-dip zinc pot To 30 nm, the oxidation balance during heating is adjusted so that the Si concentration is 1.5 mg / m ² or less, and alloying treatment is performed after immersion in the molten zinc pot. A method for manufacturing a high-strength galvanized steel sheet. 2. The method according to claim 1, further comprising Cr: 0.2-2.0% and Ca: 0.0001- in the steel.
A method for producing a high-strength galvannealed steel sheet with excellent formability, characterized by containing either or both of 0.0030%.