TWI826257B - Steel plate and manufacturing method - Google Patents

Abstract

To provide high-strength steel plates with excellent ammonia SCC resistance and low-temperature toughness for use in storage tanks for storing liquefied gas in energy carriers. The steel plate has a given composition, and the hardness characteristics of the steel plate are that at a depth of 0.5 mm from the surface of the steel plate, the average hardness is HV0.1 or less, the hardness deviation is 30 HV0.1 or less, and the hardness in the thickness direction of the steel plate is The maximum value is between 1.0mm and 1/4 of the plate thickness from the surface of the steel plate. The deviation of the hardness in the thickness direction is less than 70HV1. The metal structure of the steel plate is at a depth of 0.5mm from the surface of the steel plate. The volume ratio of the toughened iron structure is more than 90%.

Description

Steel plate and manufacturing method thereof

The present invention relates to a high-strength steel plate with excellent toughness and corrosion resistance. In particular, it relates to a high-strength steel plate that is suitable for structural members such as tanks used at low temperatures and in liquid ammonia environments. It has excellent low-temperature toughness and liquid ammonia stress corrosion cracking resistance. High-strength low-temperature steel plate and its manufacturing method.

With the increase in energy demand in recent years, the use of energy carriers to transport liquefied gas is becoming popular. For the efficient use of energy transport ships, LPG may be transported together with liquid ammonia in the tank. Recently, utilization of this liquid ammonia as a hydrogen carrier and liquid ammonia fuel has been promoted, and therefore, tanks for transporting and storing liquid ammonia are required to be enlarged.

Here, it is known that stress corrosion cracking (hereinafter referred to as ammonia SCC (Stress Corrosion Cracking)) caused by liquid ammonia occurs in carbon steel piping, storage tanks, tankers, pipelines, etc. that handle liquid ammonia. Therefore, for steel materials used in liquid ammonia environments, steel materials with low susceptibility to ammonia SCC should be used, or engineering measures should be taken to suppress ammonia SCC.

For example, it is known that the occurrence of ammonia SCC is related to the strength of the material. When using carbon steel, the yield strength (YS) of ammonia is controlled to 440 MPa or less to avoid stress corrosion cracking caused by ammonia. On the other hand, due to the recent increase in the size of tanks and the reduction of steel usage, there are increasing requirements for high-strength steel plates.

And because liquefied gases such as LPG and liquid ammonia are transported and stored at low temperatures, the steel plates used in tanks for storing these liquefied gases require excellent low-temperature toughness.

As mentioned above, patent documents 1 and 2 disclose technologies that can satisfy the low-temperature toughness and predetermined strength range required for a liquefied gas storage tank. The technology described in these documents achieves high low-temperature toughness by subjecting hot-rolled and cooled thick steel plates to several heat treatments, or by subjecting hot-rolled and water-cooled thick steel plates to several heat treatments. and established strength properties. [Prior technical literature] [Patent Document]

Patent Document 1: Japanese Patent Application Publication No. 10-140235 Patent Document 2: Japanese Patent Application Publication No. 10-168516

[Problem to be solved by the invention]

However, the methods described in the above-mentioned Patent Documents 1 and 2 require a plurality of heat treatments, and there is an economic problem in that costs related to heat treatment equipment and energy increase.

In order to solve the above problems, the object of the present invention is to provide a high-strength steel plate with excellent ammonia SCC resistance and low-temperature toughness and a manufacturing method thereof. The steel plate is used in storage tanks for containing liquefied gas in energy carriers, etc. . [Technical means to solve problems]

In order to achieve the above purpose, the inventors of the present case used a TMCP program (Thermo Mechanical Control Process) and an on-line induction heating device to conduct research on various factors related to the low-temperature toughness and strength characteristics of the steel plate. Study painstakingly. As a result, it was found that by making the steel plate contain a predetermined amount of elements such as C, Si, Mn, and Al, the volume ratio of the bainite structure at a position 0.5 mm from the surface of the steel plate becomes more than 90%. The metal structure is controlled in such a way that at a depth of 0.5mm from the surface of the steel plate, the average hardness is 230HV0.1 or less and the variation in hardness is 30HV0.1 or less, and the maximum value of the hardness in the thickness direction is By setting the hardness deviation in the thickness direction from 1.0mm to 1/4 of the plate thickness to 70HV1 or less from the surface of the steel plate, SCC resistance in a liquid ammonia environment can be effectively obtained, and multiple costly cycles can be omitted. heat treatment.

That is, the present invention was developed based on the above-mentioned knowledge, and the gist of the present invention is as follows. 1. A steel plate whose composition, in mass %, contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.010~0.060%, N: 0.0010% or more 0.0100% below, P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and the remainder is Fe and inevitable impurities. The hardness characteristics of this steel plate are that at a depth of 0.5mm from the surface of the steel plate, the average hardness is 230HV0.1 or less, the deviation of the hardness is 30HV0.1 or less, and the maximum value of the hardness in the thickness direction is at a depth of 0.5mm from the surface of the steel plate. From the position 1.0mm from the surface to 1/4 of the plate thickness, the deviation of the hardness in the plate thickness direction is 70HV1 or less. The metal structure of the steel plate has a volume fraction of the toughened iron structure at a depth of 0.5 mm from the surface of the steel plate of 90% or more.

2. Steel plate as described in 1 above, The aforementioned component composition, in mass %, further contains Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, and Mo: 0.01 ~0.50% and W: one or more of 0.01~1.00%.

3. Steel plates as described in the above 1 or 2, The aforementioned component composition is calculated in mass %, and further contains V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, and Ca: 0.0005 ～0.0200%, Mg: 0.0005～0.0200% and REM: 0.0005～0.0200%.

4. A method of manufacturing a steel plate, which involves hot rolling a steel material having the following chemical composition with a rolling end temperature equal to or above the Ar ₃ transformation point, and then performing acceleration from a cooling start temperature above the Ar ₃ transformation point Cool and then reheat. The aforementioned component composition, in terms of mass %, contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.010~0.060%, and N: 0.0010 % or more and 0.0100% or less, P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and the remainder is Fe and unavoidable impurities. After the aforementioned accelerated cooling, the cooling stop temperature is set to 200 to 600°C. range, and the cooling rate at a position 1/4 of the thickness of the steel plate is set to 20 to 120°C/s. The aforementioned reheating is to set the reaching temperature at a position of 1/4 of the thickness of the steel plate to 500°C or less, and This is performed until the temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680°C.

5. The manufacturing method of steel plate as described in the above 4, The composition of the aforementioned steel material, in terms of mass %, further contains Cu: 0.01 to 0.50%, Ni: 0.01 to 2.00%, Cr: 0.01 to 1.00%, Sn: 0.01 to 0.50%, Sb: 0.01 to 0.50%, At least one of Mo: 0.01 to 0.50% and W: 0.01 to 1.00%.

6. The manufacturing method of steel plate as described in the above 4 or 5, The composition of the aforementioned steel material, in terms of mass %, further contains V: 0.01 to 1.00%, Ti: 0.005 to 0.100%, Co: 0.01 to 1.00%, Nb: 0.005 to 0.100%, B: 0.0001 to 0.0100%, At least one of Ca: 0.0005～0.0200%, Mg: 0.0005～0.0200%, and REM: 0.0005～0.0200%. [Effects of the invention]

According to the present invention, it is possible to provide a high-strength steel plate that is excellent in low-temperature toughness, that is, impact resistance at low temperatures and ammonia SCC resistance, and is suitable for structural members such as tanks used in low-temperature and liquid ammonia environments. .

Embodiments of the present invention will be described below. In addition, the following "%" indicates the content of components (elements), unless otherwise stated, it refers to "mass %".

(1)About the ingredient composition Next, the chemical composition (chemical composition) of the steel plate will be explained.

C: 0.010～0.200% In order to improve the strength of the steel plate produced by cooling according to the present invention, C is the most effective element. In order to obtain this effect, the C content is specified to be 0.010% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at lower cost, the C content is preferably 0.013% or more. On the other hand, if the C content exceeds 0.200%, the toughness and weldability of the steel plate will deteriorate. Therefore, the C content is specified to be 0.200% or less. Furthermore, from the viewpoint of toughness and weldability, the C content is preferably 0.170% or less.

Si: 0.01～0.50% Si is added for deoxidation. In order to obtain this effect, the Si content is set to 0.01% or more. Furthermore, it is preferably 0.03% or more. On the other hand, if the Si content exceeds 0.50%, the toughness and weldability of the steel plate will deteriorate. Therefore, the Si content is set to 0.50% or less. Furthermore, from the viewpoint of toughness and weldability, the Si content is preferably 0.40% or less.

Mn: 0.50～2.50% Mn has the function of increasing the hardenability of steel, and is one of the important elements that must be added in order to meet the high strength as in the present invention. In order to obtain this effect, the Mn content is set to 0.50% or more. Furthermore, from the viewpoint of reducing the content of other alloy elements and manufacturing at lower cost, the Mn content is preferably 0.70% or more. On the other hand, if the Mn content exceeds 2.50%, the toughness and weldability of the steel plate will be reduced, and the alloy cost will become too high. Therefore, the Mn content is specified to be 2.50% or less. Furthermore, from the viewpoint of further suppressing decreases in toughness and weldability, the Mn content is preferably 2.30% or less.

Al: 0.010～0.060% Al functions as a deoxidizer. In order to obtain this effect, the Al content is set to 0.010% or more. On the other hand, if the Al content exceeds 0.060%, oxide inclusions will increase, resulting in a decrease in cleanliness and toughness. Therefore, the Al content is specified to be 0.060% or less. Furthermore, from the viewpoint of further preventing deterioration in toughness, the Al content is preferably 0.050% or less.

N：0.0010～0.0100% N helps to refine the structure and improve the toughness of the steel plate. In order to obtain this effect, the N content is set to 0.0010% or more. Preferably it is 0.0020% or more. On the other hand, if the N content exceeds 0.0100%, the toughness will decrease. Therefore, the N content is set to 0.0100% or less. Furthermore, from the viewpoint of further suppressing decreases in toughness and weldability, the N content is preferably 0.0080% or less. And when Ti is present, N can combine with Ti and precipitate in the form of TiN.

P: 0.020% or less P segregates at the grain boundaries, causing adverse effects such as reduced toughness and weldability. Therefore, the P content should be reduced as much as possible, as long as it is below 0.020%, it is acceptable. The lower limit of the P content is not particularly limited and may be 0%. However, P is an element that may remain in steel industrially and may exceed 0%. And because excessive reduction will lead to an increase in refining costs, from a cost perspective, the P content is preferably 0.0005% or more.

S: 0.0100% or less S exists in steel in the form of sulfide-based inclusions such as MnS, which becomes the starting point of damage and causes adverse effects such as a decrease in the toughness of the steel plate. Therefore, the S content should be reduced as much as possible, as long as it is below 0.0100%, it is acceptable. The lower limit of the S content is not particularly limited and may be 0%. However, S is an element that may remain in steel industrially and may exceed 0%. In addition, excessive reduction will lead to an increase in refining costs. From a cost perspective, the S content is preferably 0.0005% or more.

O: 0.0100% or less O forms oxides and becomes the starting point of damage, causing adverse effects such as lowering the toughness of the steel plate, so it is limited to less than 0.0100%. The O content is preferably 0.0050% or less, more preferably 0.0030% or less. The lower limit of the O content is not particularly limited and may be 0%. However, O is an element that may remain in steel industrially and may exceed 0%. In addition, excessive reduction will lead to an increase in refining costs. From a cost perspective, the O content is preferably 0.0010% or more.

In the composition of the steel sheet of the present invention, the remainder other than the above-mentioned components is Fe and inevitable impurities. However, the above-mentioned component composition may contain the elements described below as necessary.

Selected from Cu: 0.01～0.50%, Ni: 0.01～2.00%, Cr: 0.01～1.00%, Sn: 0.01～0.50%, Sb: 0.01～0.50%, Mo: 0.01～0.50%, and W: 0.01～1.00 1 or more of % Cu, Ni, Cr, Sn, Sb, Mo and W are elements that improve strength and ammonia SCC resistance, and one or more of these elements may be contained. In order to obtain this effect, when Cu is contained, the Cu content is preferably adjusted to 0.01% or more; when Ni is contained, the Ni content is preferably adjusted to 0.01% or more; and when Cr is contained, the Ni content is preferably adjusted to 0.01% or more. Adjust the Cr content to 0.01% or more; when it contains Sn, it is better to adjust the Sn content to 0.01% or more; when it contains Sb, it is better to adjust the Sb content to 0.01% or more; when it contains Mo , it is preferable to adjust the Mo content to 0.01% or more; and when W is contained, it is preferable to adjust the W content to 0.01% or more. On the other hand, if the Ni content is too high, the weldability will deteriorate and the alloy cost will increase. In addition, if the content of Cu, Cr, Sn, Sb, Mo and W is too high, the weldability and toughness will be deteriorated, which is also disadvantageous from the perspective of alloy cost. Therefore, it is preferable to adjust the Cu content to 0.50% or less, the Ni content to 2.00% or less, the Cr content to 1.00% or less, the Sn content to 0.50% or less, and the Sb content to 0.50% or less, The Mo content is adjusted to 0.50% or less, and the W content is adjusted to 1.00% or less. More preferably, the Cu content is adjusted to 0.40% or less, the Ni content is adjusted to 1.50% or less, the Cr content is adjusted to 0.80% or less, the Sn content is adjusted to 0.40% or less, the Sb content is adjusted to 0.40% or less, and The Mo content is adjusted to 0.40% or less, and the W content is adjusted to 0.80% or less.

V: 0.01～1.00% V has the effect of improving the strength of the steel plate and can be added arbitrarily. In order to obtain this effect, when adding V, it is preferable to set the V content to 0.01% or more. On the other hand, if the V content exceeds 1.00%, the weldability will deteriorate and the alloy cost will increase. Therefore, when adding V, it is preferable to set the V content to 1.00% or less. More preferably, the lower limit of the V content is set to 0.05% and the upper limit is set to 0.50%.

Ti: 0.005～0.100% Ti has a strong tendency to form nitrides and has the function of fixing N and reducing solid solution N. It can be added arbitrarily. In addition, Ti can improve the toughness of the base material and the welded part. In order to obtain these effects, when adding Ti, it is preferable to set the Ti content to 0.005% or more. More preferably, it is 0.007% or more. On the other hand, if the Ti content exceeds 0.100%, the toughness will be reduced. Therefore, when adding Ti, it is preferable to set the Ti content to 0.100% or less. Furthermore, it is more preferable to set the Ti content to 0.090% or less.

Co: 0.01～1.00% Co has the effect of improving the strength of the steel plate and can be added arbitrarily. In order to obtain this effect, when adding Co, it is preferable to set the Co content to 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the weldability will deteriorate and the alloy cost will increase. Therefore, when adding Co, it is preferable to set the Co content to 1.00% or less. More preferably, the lower limit of the Co content is 0.05% and the upper limit is 0.50%.

Nb: 0.005～0.100% Nb can precipitate in the form of carbonitride and reduce the particle size of former austenite, which has the effect of improving toughness. In order to obtain this effect, when adding Nb, it is preferable to set the Nb content to 0.005% or more. Furthermore, it is more preferably 0.007% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of NbC will precipitate, resulting in reduced toughness. Therefore, when adding Nb, it is preferable to set the Nb content to 0.100% or less. Furthermore, it is more preferably 0.060% or less.

B: 0.0001～0.0100% B has the effect of significantly improving the hardenability even when added in a trace amount. That is, the strength of the steel plate can be improved. In order to obtain this effect, when adding B, it is preferable to set the B content to 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, the weldability will decrease. Therefore, when adding B, it is preferable to set the B content to 0.0100% or less. More preferably, the lower limit of the B content is set to 0.0010% and the upper limit is set to 0.0030%.

Ca: 0.0005～0.0200% Ca combines with S and has the effect of suppressing the formation of MnS and the like extending long in the rolling direction. That is, by adding Ca, the morphology of the sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of the welded portion and the like. In order to obtain this effect, when adding Ca, it is preferable to set the Ca content to 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, the cleanliness of the steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when adding Ca, it is best to set the Ca content to 0.0200% or less. More preferably, the lower limit of the Ca content is 0.0020% and the upper limit is 0.0100%.

Mg: 0.0005～0.0200% Like Ca, Mg combines with S and has the effect of suppressing the formation of MnS and the like extending long in the rolling direction. That is, by adding Mg, the morphology of the sulfide-based inclusions can be controlled to be spherical, thereby improving the toughness of welded parts and the like. In order to obtain this effect, when Mg is added, it is preferable to set the Mg content to 0.0005% or more. On the other hand, if the Mg content exceeds 0.0200%, the cleanliness of the steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when Mg is added, it is preferable to set the Mg content to 0.0200% or less. More preferably, the lower limit of the Mg content is set to 0.0020% and the upper limit is set to 0.0100%.

REM: 0.0005～0.0200% Like Ca and Mg, REM (rare earth metal) combines with S and has the effect of suppressing the formation of MnS and the like extending long in the rolling direction. That is, by adding REM, the morphology of the sulfide-based inclusions can be controlled into a spherical shape, thereby improving the toughness of the welded portion and the like. In order to obtain this effect, when REM is added, the REM content is preferably 0.0005% or more. On the other hand, if the REM content exceeds 0.0200%, the cleanliness of the steel will decrease. Reduced clarity leads to reduced toughness. Therefore, when REM is added, the REM content is preferably 0.0200% or less. More preferably, the lower limit of the REM content is set to 0.0020% and the upper limit is set to 0.0100%.

(2) About hardness characteristics and metal structure The steel plate of the present invention, in addition to having the above-mentioned component composition, also has the following hardness characteristics, that is, the average hardness at a depth of 0.5 mm from the surface of the steel plate (also referred to as the 0.5 mm position in the present invention) is 230HV0.1 Below, the deviation of the hardness at the 0.5mm position is 30HV0.1 or less, and the maximum value of the hardness in the plate thickness direction is between 1.0mm and 1/4 of the plate thickness from the surface of the steel plate. The deviation of the hardness in the plate thickness direction It is below 70HV1. Furthermore, the steel plate of the present invention has a metal structure in which the volume ratio of the toughened iron structure (hereinafter also referred to as toughened iron) at the 0.5 mm position is 90% or more. The reason why the hardness characteristics and metal structure of the steel plate are limited as mentioned above will be explained below.

[At the 0.5mm position, the average hardness is 230HV0.1 or less, and the hardness variation is 30HV0.1 or less] At the 0.5mm position, the average hardness is made to be 230HV0.1 or less, and the variation in hardness is made to be 30HV0.1 or less. If there is a high hardness area on the extreme surface of the steel plate, specifically at a position 0.5 mm from the surface of the steel plate, it will promote stress corrosion cracking in a liquid ammonia environment. In addition, when there is a local area of high hardness, when stress is applied to the steel plate, stress concentration will occur and promote stress corrosion cracking. Therefore, in the steel plate of the present invention, excellent ammonia SCC resistance can be ensured by adjusting the hardness characteristics at the 0.5 mm position so that the average hardness becomes 230HV0.1 or less and the hardness variation becomes 30HV0.1 or less. The lower limit of the average hardness at the 0.5mm position is not particularly limited, but is preferably about 130HV0.1. The lower limit of the hardness deviation at the 0.5mm position can be 0HV0.1, which is about 10HV0.1 in industry. Here, the above-mentioned average hardness can be calculated by measuring the Vickers hardness at a position of 0.5 mm at a plurality of locations (for example, 100 points). The deviation of hardness refers to the standard deviation of Vickers hardness measured in order to find the average hardness.

[The maximum value of hardness in the plate thickness direction is between 1.0mm and 1/4 of the plate thickness from the surface of the steel plate] If the maximum value of the hardness of the steel plate is located at a certain distance from the surface, most of the hardness of the steel plate can be maintained while only the hardness of the surface layer is reduced. That is, it is possible to maintain the strength of the steel plate and ensure excellent ammonia-resistant SCC characteristics. Specifically, if the maximum value is located at a position smaller than 1.0 mm from the surface of the steel plate, the hardness at the 0.5 mm position cannot be sufficiently reduced. On the other hand, if the maximum value is located at a position exceeding 1/4 of the plate thickness from the surface of the steel plate, the steel plate itself cannot ensure sufficient strength. Therefore, in the steel plate of the present invention, the maximum value of hardness (Vickers hardness (HV1)) in the plate thickness direction is defined to be located between 1.0 mm and 1/4 of the plate thickness from the surface of the steel plate.

[The deviation of hardness in the plate thickness direction is 70HV1 or less] When the variation in hardness in the plate thickness direction is large, not only the uniform elongation of the steel plate decreases, but also the residual stress caused by the internal stress introduced during accelerated cooling increases, so there is a possibility that the ammonia-resistant SCC characteristics will deteriorate. Therefore, in the present invention, the variation in hardness in the plate thickness direction is specified to be 70 HV1 or less. Here, the Vickers hardness (HV1) is measured at a pitch of 0.5 mm along the plate thickness direction, and the above-mentioned deviation is calculated by finding the difference between the maximum value and the minimum value.

[The volume ratio of toughened iron at the 0.5mm position is more than 90%] In order to satisfy the strength characteristics and ammonia SCC resistance, the structure at the 0.5mm position must have a volume fraction of 90% or more of toughened iron. In the surface layer, if hard phases such as Asada iron structure and island-like Asada iron (MA) structure are formed, the hardness of the surface layer will increase and the variation in hardness within the steel plate will increase, resulting in poor material uniformity. That is, if the volume ratio of toughened iron is less than 90%, the volume of other structures, that is, fat grain iron, island-shaped Asada loose iron structure, Asada loose iron structure, pulverized iron structure, and Worthfield iron structure The fraction will increase and sufficient strength and/or ammonia SCC resistance cannot be obtained.

Here, the toughened iron system includes a structure called bainitic ferrite or granular ferrite that undergoes transformation during or after accelerated cooling that contributes to transformation strengthening. , and other structures after being tempered.

The remaining part of the structure accounting for less than 10% of the volume can also include hemp field loose iron structure in addition to the fat grain iron structure, Plein iron structure and Worthfield iron structure. Although the fraction of each structure in the remaining structure is not particularly limited, the remaining structure is preferably a Pleiron structure. In addition, the volume fraction of various metal structures can be measured according to the method described in the examples below.

(3)About manufacturing conditions In the manufacturing method of the present invention, a steel material having the same component composition as that of the steel plate is heated and hot-rolled, followed by accelerated cooling and then reheating. The following explains the reasons for limiting the manufacturing conditions of steel plates. First of all, the manufacturing conditions of steel are not particularly limited. For example, it is preferable to melt molten steel having the above-mentioned composition by a known melting method such as a converter, and to form a predetermined size by a known casting method such as a continuous casting method. Steel materials such as slabs. The ingot-block rolling method can also be used to make steel products such as flat blanks of a predetermined size.

The steel material thus obtained is directly hot-rolled without being cooled, or is heated again and then hot-rolled. This hot rolling is performed by setting the rolling end temperature to be equal to or higher than the Ar ₃ transformation point, followed by accelerated cooling from a cooling start temperature above the Ar ₃ transformation point under predetermined conditions, and then reheating under predetermined conditions. .

There is no particular limit to the heating temperature of steel. If the heating temperature is too low, the deformation resistance will increase and the load on the hot rolling mill will increase, which may make hot rolling difficult. On the other hand, if the temperature exceeds 1300° C., oxidation will become severe and the oxidation loss will increase, thereby possibly lowering the yield. For this reason, the heating temperature is preferably 950°C to 1300°C.

(Hot rolling) [Rolling completion temperature: Ar ₃ transformation point or above] In the present invention, after the steel material is heated to the above temperature, hot rolling is started, and the hot rolling is completed at or above the Ar ₃ transformation point. If the end temperature of rolling is lower than the Ar ₃ transformation point, fat particles of iron will be generated, which will worsen the uniformity of the material on the surface of the steel plate, increase the variation in hardness, and lead to poor ammonia SCC resistance. In addition, the generated fat iron particles will be affected by processing and deteriorate the toughness. Furthermore, the load on the hot rolling mill increases. Therefore, the rolling completion temperature of hot rolling in the present invention is set to be equal to or higher than the Ar ₃ transformation point. Preferably, the rolling completion temperature of hot rolling is a temperature above the Ar ₃ transformation point + 10°C. On the other hand, if the rolling end temperature exceeds 950°C, the structure may become coarsened and the toughness may deteriorate. Therefore, the rolling end temperature is preferably 950°C or lower. Here, the transformation point (°C) of Ar ₃ can be calculated according to the following formula. Ar ₃ (℃)=910-310×C-80×Mn-20×Cu-15×Cr-55×Ni-80×Mo where each element represents the content (mass %) of the element in the steel.

(Accelerated cooling) [Cooling start temperature: Ar ₃ transformation point or above] Next, the hot-rolled steel sheet is accelerated cooling from a cooling start temperature above the Ar ₃ transformation point. If the cooling start temperature is lower than the Ar ₃ transformation point, fat iron will be overproduced and the cooling rate will increase, causing it to coexist with the Asada loose iron structure or toughened iron with a large strength difference. As a result, the strength will be insufficient and the toughness will deteriorate. , thereby worsening ammonia SCC resistance. Therefore, the cooling start temperature is set to be above the Ar ₃ transformation point.

[Cooling rate at 1/4 of the thickness of the steel plate: 20~120℃/s] Accelerated cooling in which the cooling rate at 1/4 of the thickness of the steel plate is set to 20°C/s or more is an indispensable procedure in order to obtain a steel plate with high strength and high toughness. By cooling at a high cooling rate, it is possible to Obtain the strength-increasing effect based on metamorphosis enhancement. Therefore, in order to obtain this effect, the cooling rate at a position of 1/4 of the plate thickness of the steel plate during accelerated cooling according to the present invention is set to 20° C./s or more. On the other hand, if the cooling rate exceeds 120°C/s, the volume fraction of Asada loose iron becomes too high, resulting in a decrease in toughness. Therefore, the cooling rate at a position 1/4 of the thickness of the above-mentioned steel plate is specified to be 120°C/s or less. The cooling rate can be increased by an active cooling operation such as water cooling, and the cooling rate can be controlled by suitably performing the cooling operation intermittently (setting a period during which the cooling operation is stopped). In addition, it is difficult to directly measure the temperature at 1/4 of the thickness of the above-mentioned steel plate using physical methods. However, based on the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling, for example, using a data processing computer to perform differential calculations, the temperature distribution within the plate thickness section can be obtained in real time, especially for plate thickness 1 The temperature at the /4 position.

[Cooling stop temperature: 200～600℃] In the present invention, after the hot rolling is completed, predetermined accelerated cooling is performed to an arbitrarily set cooling stop temperature in the range of 200 to 600°C, whereby the fat grain iron and toughened iron are brought into a predetermined state in the center portion of the plate thickness. The volume ratio can improve the strength and toughness well. Here, if the cooling stop temperature is lower than 200° C., the volume fraction of the island-like Asada loose iron structure becomes too much, resulting in a decrease in toughness. On the other hand, if the cooling stop temperature exceeds 600°C, fat iron and plenum iron structures will be excessively generated, resulting in insufficient strength and poor toughness. Therefore, the cooling stop temperature is set in the range of 200 to 600°C. In addition, the cooling stop temperature in the present invention is the temperature at a position of 1/4 of the thickness of the steel plate.

(reheat) [Achieved temperature at 0.5 mm from the surface is 400 to 680°C] In the present invention, reheating must be performed after the aforementioned accelerated cooling. When a thick steel plate is cooled at an accelerated rate, the cooling rate of the surface portion of the steel plate becomes faster, and the surface portion of the steel plate is cooled to a lower temperature than the inside of the steel plate. Therefore, hard structures such as Asada loose iron are likely to form on the surface of the steel plate, which may lead to deterioration in ammonia SCC resistance. Therefore, in the present invention, the surface layer portion of the steel plate is reheated after accelerated cooling. This is because the hardness of the surface layer can be reduced. Preferably, reheating is performed immediately after accelerated cooling. Here, if the reheating temperature at a position 0.5 mm from the surface is less than 400°C, the hardness will not be sufficiently reduced. On the other hand, if it exceeds 680°C, the strength of the entire steel plate will be reduced, making it difficult to obtain the predetermined strength. . Therefore, the temperature reached at a position 0.5 mm from the surface during reheating after accelerated cooling is specified to be in the range of 400 to 680°C.

[The temperature reached at 1/4 of the thickness of the steel plate is 500°C or less] Also, when the temperature reached at 1/4 of the plate thickness of the steel plate during reheating exceeds 500°C, the strength decreases and the toughness deteriorates. Therefore, the temperature reached at 1/4 of the thickness of the steel plate during reheating is specified to be 500°C or less.

As a means of reheating after accelerated cooling, induction heating is preferably used. In particular, in order to concentrate heating on the surface layer of the steel plate, it is preferable to use high-frequency induction heating. After reheating, cooling can be performed appropriately. There is no particular limitation on the cooling after reheating. However, in the case of a thick steel plate with a thickness exceeding about 40 mm, the cooling rate becomes slow and there is a concern that the toughness deteriorates due to the agglomeration and coarsening of carbides. In this case, cooling by water cooling or mist can be performed after the reheating process.

By manufacturing the steel material having the above composition according to the above manufacturing conditions, a steel plate having the composition, hardness characteristics and metal structure according to the present invention can be obtained. The steel plate obtained in this way has excellent strength characteristics and toughness, and is a steel plate with excellent ammonia SCC resistance. Here, excellent strength characteristics refer to yield strength YS (when there is a yield point, it is the yield point YP, when there is no yield point, it is the 0.2% guaranteed stress σ0.2): 450MPa or more, tensile strength (TS): 570MPa or more, and uniform Elongation (uEl): more than 10%. Excellent toughness means vTrs is -30°C or less according to JIS Z 2241. Furthermore, a steel plate having these characteristics is the steel plate excellent in ammonia SCC resistance of the present invention.

In addition, according to the manufacturing method of the present invention, ordinary methods can be used for items not described in this specification. Example

The steel with the composition shown in Table 1 (steel types A to AI, the remainder is Fe and inevitable impurities) is made into a flat blank by the continuous casting method, and is hot-rolled and rolled in sequence according to the conditions shown in Table 2. Accelerate cooling and reheating to obtain thick steel plates (No. 1 to 50) with a thickness of 30 mm. For the obtained steel plate, the structure fraction of the metal structure at a position 0.5 mm from the surface of the steel plate was measured, the hardness characteristics were evaluated, the strength characteristics and toughness were evaluated, and the ammonia SCC resistance was evaluated. Each test method is as follows. These results are also listed in Table 2.

[The structure fraction of the metal structure at a position 0.5 mm from the surface of the steel plate] Samples were taken from each steel plate so that the 0.5 mm position became the observation surface. Next, the sample was mirror-polished and further etched with nital, and then a scanning electron microscope (SEM) was used to photograph an area of 10 mm × 10 mm at a magnification of 500 to 3000 times. Furthermore, the captured image is analyzed using an image analysis device to determine the area fraction of the microstructure (the tissue fraction of the metal structure). When the anisotropy of the microstructure is small, the area fraction is equivalent to the volume fraction, so in the present invention, the area fraction is regarded as the volume fraction.

In this embodiment, the determination when determining the fraction of the metal structure of the sample is performed as follows. That is, in the above-mentioned photographed image, the polygonal fat iron is determined to be fat iron, and the fat iron that grows into an elongated lath-shaped shape and contains an equivalent circle diameter of 0.05 μm or more The structure of the carbide is identified as toughened iron (B in Table 2).

[Hardness characteristics] For each steel plate, the Vickers hardness (HV0.1) of 100 points was measured at a position of 0.5 mm in accordance with JIS Z 2244, and the average value was calculated. The standard deviation of the 100-point Vickers hardness is also calculated as the deviation of the hardness at the 0.5 mm position. Here, the reason why HV0.1 is used instead of HV10, which is usually used for hardness measurement of steel plates, is that by measuring using HV0.1, the indentation becomes smaller and hardness information closer to the surface can be obtained. Microstructure is more sensitive to hardness information. The Vickers hardness (HV1) was also measured in the plate thickness direction, and the position of the maximum value of the hardness in the plate thickness direction (distance from the surface) was measured. Furthermore, the difference between the maximum value and the minimum value of the measured Vickers hardness (HV1) was calculated and used as the variation in hardness in the plate thickness direction.

[Strength characteristics] From the total thickness of each steel plate, a test piece No. 1B of JIS Z 2201 was taken so that the direction perpendicular to the rolling direction became the long side direction of the test piece, and a tensile test was performed according to the method described in JIS Z 2241, and the yield strength YS was measured. (When there is a yield point, it is the yield point YP, when there is no yield point, it is 0.2% guaranteed stress σ0.2), tensile strength (TS) and uniform elongation (uEl). Furthermore, a steel plate having excellent strength characteristics was evaluated as having a yield strength of 450 MPa or more, a tensile strength of 570 MPa or more, and a uniform elongation of 10% or more.

[Toughness] A portion of 1 mm was cut away from the surface side of each steel plate, and a V-notch test piece of JIS Z 2202 was taken so that the rolling direction was the long side direction of the test piece, and a Charpy impact test was performed in accordance with the procedures of JIS Z 2242. test), and measured vTrs (brittle transition temperature). Furthermore, steel sheets having vTrs of -30°C or less were evaluated as having excellent toughness.

[Ammonia SCC resistance] Ammonia SCC resistance is evaluated by performing a four-point bending test in a test solution and by a constant-potential anodic electrolysis promotion test in order to promote corrosion. Specifically, it is implemented according to the following procedures. A 5 mm thick × 15 mm × 115 mm test piece was taken from the surface of the steel plate, ultrasonic degreased in acetone for 5 minutes, and a stress equal to the yield strength was applied to each steel plate by four-point bending. The four-point bending test piece was placed in a test cell, filled with a solution of 12.5 g of ammonium carbamate and 1 L of liquid ammonia, and then controlled to +2.0 Vvs with a potentiostat. .Pt flows through the test piece and is immersed at room temperature (25°C). After 168 hours of immersion, the ammonia SCC resistance was judged to be "good" when no cracks were visible, and the ammonia SCC resistance was judged to be "poor" when cracks occurred.

As can be seen from Table 1 and Table 2, all of the invention examples have a yield strength YS of 450 MPa or more, a tensile strength TS of 570 MPa or more, a uniform elongation uEl of 10% or more, and vTrs is -30°C or less, thereby achieving low-temperature toughness. and steel plates with excellent ammonia SCC resistance.

On the other hand, although the component composition of Nos. 31 to 39 is within the scope of the present invention, the manufacturing method is not within the scope of the present invention, and the desired metal structure and/or hardness characteristics cannot be obtained. As a result, the yield strength YS, the tensile strength TS, the toughness at low temperatures, or the ammonia SCC resistance deteriorate.

Also in Nos. 40 to 50, the steel composition does not fall within the scope of the present invention, so the yield strength YS, tensile strength TS, toughness at low temperatures, or ammonia SCC resistance deteriorate. In the present invention, the chemical composition of steel can be regarded as the chemical composition of the steel plate.

Claims (6)

A steel plate whose composition, in mass %, contains C: 0.010~0.200%, Si: 0.01~0.50%, Mn: 0.50~2.50%, Al: 0.010~0.060%, N: 0.0010% or more and 0.0100% or less, P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and the remainder is Fe and inevitable impurities. The hardness characteristics of this steel plate are that at a depth of 0.5mm from the surface of the steel plate, the average hardness is 230HV0.1 or less, the deviation of the hardness is 30HV0.1 or less, and the maximum value of the hardness in the thickness direction is at a depth of 0.5mm from the surface of the steel plate. From the position 1.0mm from the surface to 1/4 of the plate thickness, the deviation of the hardness in the plate thickness direction is 70HV1 or less. The metal structure of the steel plate has a volume fraction of the toughened iron structure at a depth of 0.5 mm from the surface of the steel plate of 90% or more. The steel plate as described in claim 1, wherein, The aforementioned ingredients are calculated in mass %, It further contains Cu: 0.01～0.50%, Ni: 0.01～2.00%, Cr: 0.01～1.00%, Sn: 0.01～0.50%, Sb: 0.01～0.50%, Mo: 0.01～0.50% and W: 0.01～ 1.00% or more. The steel plate as described in claim 1 or 2, wherein, The aforementioned ingredients are calculated in mass %, It further contains V: 0.01~1.00%, Ti: 0.005~0.100%, Co: 0.01~1.00%, Nb: 0.005~0.100%, B: 0.0001~0.0100%, Ca: 0.0005~0.0200%, Mg: 0.0005~ 0.0200% and REM: one or more of 0.0005~0.0200%. A method of manufacturing a steel plate by performing hot rolling with a rolling end temperature equal to or above the Ar ₃ transformation point of a steel material having the following chemical composition, and then performing accelerated cooling from a cooling start temperature above the Ar ₃ transformation point, Next, reheating is performed, and the aforementioned component composition, in terms of mass %, contains C: 0.010 to 0.200%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, Al: 0.010 to 0.060%, and N: 0.0010% or more 0.0100% or less, P: 0.020% or less, S: 0.0100% or less, O: 0.0100% or less, and the remainder is Fe and unavoidable impurities. In the aforementioned accelerated cooling, the cooling stop temperature is set in the range of 200 to 600°C. Furthermore, the cooling rate at the 1/4 thickness position of the steel plate is set to 20 to 120°C/s. The aforementioned reheating is performed by setting the reaching temperature at the 1/4 thickness position of the steel plate to 500°C or less and proceeding to The temperature reached at a depth of 0.5 mm from the surface of the steel plate is in the range of 400 to 680°C. The manufacturing method of steel plate as described in claim 4, wherein, The composition of the aforementioned steel materials, in mass %, It further contains Cu: 0.01～0.50%, Ni: 0.01～2.00%, Cr: 0.01～1.00%, Sn: 0.01～0.50%, Sb: 0.01～0.50%, Mo: 0.01～0.50% and W: 0.01～ 1.00% or more. The manufacturing method of steel plate as described in claim 4 or 5, wherein, The composition of the aforementioned steel materials, in mass %, It further contains V: 0.01~1.00%, Ti: 0.005~0.100%, Co: 0.01~1.00%, Nb: 0.005~0.100%, B: 0.0001~0.0100%, Ca: 0.0005~0.0200%, Mg: 0.0005~ 0.0200% and REM: 0.0005~0.0200% or above.