TWI507536B - A hot-pressed steel sheet member, a method for manufacturing the same, and a steel sheet for hot pressing - Google Patents

Description

Hot-pressed steel sheet member, method for producing the same, and steel sheet for hot pressing Technical field

The present invention relates to a hot-pressed steel sheet member used for a mechanical structural part or the like, a method for producing the same, and a steel sheet for hot pressing.

Background technique

In order to reduce the weight of automobiles, we are striving to increase the strength of steel used in the car body and strive to reduce the weight of steel. In steel sheets widely used in automobiles, in general, as the strength is increased, press formability is lowered, and parts having complicated shapes are not easily manufactured. For example, as the ductility is lowered, the portion having a high degree of workability is broken, or the rebound is excessively large, resulting in deterioration in dimensional accuracy. Therefore, it is difficult to press a high-strength steel sheet, in particular, by press-forming a steel sheet having a tensile strength of 980 MPa or more. If the press forming is not performed by press forming, the high-strength steel sheet can be easily processed, but it is only applicable to the parts having the same cross section in the longitudinal direction.

Patent Documents 1 to 4 describe a method for obtaining high formability in a high-strength steel sheet by hot pressing. A high-strength hot-pressed steel sheet member can be obtained by forming a high-strength steel sheet with high precision by hot pressing.

On the other hand, hot-pressed steel sheet members also seek to improve ductility. However, the steel structure of the steel sheet obtained by the methods described in Patent Documents 1 to 4 is substantially Ma Tian loose iron single phase, it is not easy to improve ductility.

Further, in Patent Documents 5 to 7, the hot-pressed steel sheet member for the purpose of improving the ductility is described. However, the conventional hot-pressed steel sheet member is not easily combined with strength and ductility.

Patent Document 8 also describes a hot-pressed steel sheet member for the purpose of improving ductility. However, the manufacture of the hot-pressed steel sheet member requires complicated control, and there are other problems such as a decrease in productivity and an increase in manufacturing cost.

Prior technical literature Patent literature

Patent Document 1: British Patent Gazette No. 1490535

Patent Document 2: Japanese Patent Laid-Open No. Hei 10-96031

Patent Document 3: Japanese Patent Laid-Open Publication No. 2009-197253

Patent Document 4: Japanese Patent Laid-Open Publication No. 2009-35793

Patent Document 5: Japanese Patent Laid-Open Publication No. 2010-65292

Patent Document 6: Japanese Patent Laid-Open Publication No. 2010-65293

Patent Document 7: Japanese Patent Laid-Open Publication No. 2010-521584

Patent Document 8: Japanese Patent Laid-Open Publication No. 2010-131672

Summary of invention

An object of the present invention is to provide a hot-pressed steel sheet member which is excellent in strength and ductility without performing complicated control, a method for producing the same, and a steel sheet for hot pressing.

The inventors of the present application have repeatedly made efforts to review the above-mentioned problems, and have found that a chemical composition having a predetermined amount of C and Mn and more containing Ti is treated by hot pressing or the like under appropriate conditions, and has a predetermined The steel sheet for hot pressing of the steel structure can obtain a hot-pressed steel sheet member in which the steel structure includes a multiphase structure of the ferrite iron and the granulated iron without performing the complicated control as described in Patent Document 8. The inventors of the present application have further found that the hot-pressed steel sheet member has a high tensile strength of 980 MPa or more and has excellent ductility. Further, the inventors of the present application have thought of the aspects of the invention shown below.

(1) A hot-pressed steel sheet member having the chemical composition shown below: C: 0.10% to 0.24%, Si: 0.001% to 2.0%, Mn: 1.2% to 2.3%, sol. Al: by mass% 0.001% to 1.0%, Ti: 0.060% to 0.20%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Nb: 0% to 0.20%, V: 0% to 0.20%, Cr: 0%~1.0%, Mo: 0%~0.15%, Cu: 0%~1.0%, Ni: 0%~1.0% Ca: 0%~0.01%, Mg: 0%~0.01%, REM: 0%~0.01%, Zr: 0%~0.01%, B: 0%~0.005%, Bi:0 %~0.01%, the remaining part: Fe and impurities; in area%, with ferrite iron: 10%~70%, Ma Tian loose iron: 30%~90%, fat grain iron and 麻田散铁 total area ratio: 90%~100% steel structure; more than 90% of the total Ti in steel will precipitate, and the tensile strength is above 980MPa.

(2) The hot-pressed steel sheet member according to the above aspect, wherein the chemical composition is one or more selected from the group consisting of Nb: 0.003% to 0.20%, in terms of % by mass, V: 0.003% to 0.20%, Cr: 0.005% to 1.0%, Mo: 0.005% to 0.15%, Cu: 0.005% to 1.0%, and Ni: 0.005% to 1.0%.

(3) The hot-pressed steel sheet member according to (1) or (2), wherein the chemical composition is one by mass selected from the group consisting of the following Or two or more types: Ca: 0.0003% to 0.01%, Mg: 0.0003% to 0.01%, REM: 0.0003% to 0.01%, and Zr: 0.0003% to 0.01%.

(4) The hot-pressed steel sheet member according to any one of (1) to (3) wherein the chemical composition contains B: 0.0003% to 0.005% by mass%.

(5) The hot-pressed steel sheet member according to any one of (1) to (4) wherein the chemical composition contains, by mass%, Bi: 0.0003% to 0.01%.

(6) A steel sheet for hot pressing having the chemical composition shown below: C: 0.10% to 0.24%, Si: 0.001% to 2.0%, Mn: 1.2% to 2.3%, sol. Al: by mass% 0.001% to 1.0%, Ti: 0.060% to 0.20%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Nb: 0% to 0.20%, V: 0% to 0.20%, Cr: 0%~1.0%, Mo: 0%~0.15%, Cu: 0%~1.0%, Ni: 0%~1.0%, Ca: 0%~0.01%, Mg: 0%~0.01%, REM: 0%~0.01%, Zr: 0%~0.01%, B: 0%~0.005%, Bi: 0%~0.01%, the remainder: Fe and impurities; and more than 70% of the total Ti in the steel will precipitate.

(7) The steel sheet for hot press according to the above aspect, wherein the chemical composition is one by mass or two or more selected from the group consisting of Nb: 0.003% to 0.20%, in terms of % by mass, V: 0.003% to 0.20%, Cr: 0.005% to 1.0%, Mo: 0.005% to 0.15%, Cu: 0.005% to 1.0%, and Ni: 0.005% to 1.0%.

(8) The steel sheet for hot press according to the above aspect, wherein the chemical composition is one or more selected from the group consisting of: Ca: 0.0003% by mass%. ~0.01%, Mg: 0.0003% to 0.01%, REM: 0.0003% to 0.01%, and Zr: 0.0003% to 0.01%.

(9) The steel sheet for hot press according to any one of (6), wherein the chemical composition contains B: 0.0003% to 0.005% by mass%.

(10) The hot-pressed steel sheet according to any one of (6), wherein the chemical composition contains, by mass%, Bi: 0.0003% to 0.01%.

(11) A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing according to any one of (6) to (10) in a temperature range of from Ac ₃ to Ac ₃ + 100 ° C for 1 minute a step of ~10 minutes, and a step of performing hot pressing after the heating; the step of performing hot pressing has the steps of: performing the first cooling in a temperature range of 600 ° C to 750 ° C, and 150 ° C to 600 ° C a second cooling step in the temperature range; and an average cooling rate of 3 ° C / sec to 200 ° C / sec in the first cooling, and precipitation of ferrite iron in a temperature range of 600 ° C to 750 ° C, and In the second cooling, the average cooling rate is 10 ° C / sec to 500 ° C / sec.

According to the present invention, high tensile strength and excellent ductility can be obtained without complicated control.

Fig. 1 is a view showing a photograph of a metal structure of a hot-pressed steel sheet member of an embodiment.

Form for implementing the invention

Hereinafter, embodiments of the present invention will be described. Embodiment of the invention The state system relates to a hot-pressed steel sheet member having a tensile strength of 980 MPa or more.

First, the hot-pressed steel sheet member (hereinafter referred to as "steel sheet member") according to the embodiment of the present invention and the chemical composition of the hot-pressed steel sheet used for the production thereof will be described. In the following description, the content unit "%" of each element contained in the steel sheet member or the hot-pressed steel sheet is "% by mass" unless otherwise specified.

The steel composition of the present embodiment and the chemical composition of the hot-pressed steel sheet used for the production thereof are C: 0.10% to 0.24%, Si: 0.001% to 2.0%, and Mn: 1.2% to 2.3%, by mass%. sol.Al: 0.001% to 1.0%, Ti: 0.060% to 0.20%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Nb: 0% to 0.20%, V: 0% to 0.20 %, Cr: 0% to 1.0%, Mo: 0% to 0.15%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0%~0.01%, Zr: 0%~0.01%, B: 0%~0.005%, Bi: 0%~0.01%, and the remainder: Fe and impurities. The impurities may be exemplified by those contained in raw materials such as ore or scrap, which are included in the manufacturing steps.

(C: 0.10%~0.24%)

The C system is a very important element for improving the hardenability of the steel sheet for hot pressing and mainly determining the strength of the steel sheet member. When the C content of the steel sheet member is less than 0.10%, it is difficult to ensure the tensile strength of 980 MPa or more. Therefore, the C content is made 0.10% or more. When the C content of the hot-pressed steel sheet is more than 0.24%, the steel structure of the steel sheet member is a single phase of the granulated iron, and the deterioration of ductility is remarkable. Therefore, the C content is made 0.24% or less. From the viewpoint of weldability, the C content of the steel sheet member is preferably 0.21% or less, more preferably 0.18% or less.

(Si: 0.001%~2.0%)

The Si system is an element that effectively enhances the strength and ductility of the steel sheet member. When the Si content is less than 0.001%, the above effects are not easily obtained. Therefore, the Si content is made 0.001% or more. When the Si content is more than 2.0%, saturation is achieved by the effect of the above action, which is economically disadvantageous, and the wettability of the plating is markedly lowered, and the plating is unplated frequently. Therefore, the Si content is made 2.0% or less. From the viewpoint of further improving the ductility, the Si content is preferably 0.05% or more. From the viewpoint of improving the weldability, the Si content is preferably 0.2% or more. The Si content is preferably 0.6% or less from the viewpoint that the temperature at which the single phase of the Worstian iron is lower at the time of hot pressing. If the temperature is lower, the effect of shortening the heating time, improving the productivity, reducing the manufacturing cost, and suppressing the damage of the heating furnace can be obtained.

(Mn: 1.2% to 2.3%)

Mn is an element which is very effective in improving the hardenability of the steel sheet for hot pressing and ensuring the strength of the steel sheet member. When the Mn content is less than 1.2%, the above effects are not easily obtained. Therefore, the Mn content is made 1.2% or more. When the Mn content is more than 2.3%, the steel structure of the steel sheet member is a single phase of the granulated iron, and the deterioration of ductility is remarkable. Therefore, the Mn content is made 2.3% or less. The Mn content is preferably 1.4% or more from the viewpoint that the temperature at which the single phase of the Worth iron is hot at a low temperature (for example, 860 ° C or lower). The Mn content is preferably 2.2% or less, and preferably 2.1% or less, from the viewpoint of suppressing the steel structure of the steel sheet member to be a remarkable band shape and obtaining good bendability.

(sol.Al (acid-soluble Al): 0.001% to 1.0%)

The Al system has an element that deoxidizes steel to restore the effect of the steel material. Al also has the effect of increasing the yield of carbonitride-forming elements such as Ti. sol.Al contains When the amount is less than 0.001%, the aforementioned effects are not easily obtained. Therefore, the sol. Al content is made 0.001% or more. In order to obtain the above effects more reliably, the sol. Al content is preferably 0.015% or more. When the sol.Al content is more than 1.0%, the decrease in weldability becomes remarkable, and oxide-based inclusions increase, and deterioration of surface texture becomes remarkable. Therefore, the sol. Al content is set to 1.0% or less. In order to obtain a better surface structure, the sol. Al content is preferably 0.080% or less.

(Ti: 0.060%~0.20%)

Ti is an element that promotes the deformation of ferrite and iron during hot pressing. By promoting the deformation of the ferrite and iron, the ductility of the steel sheet member is remarkably improved. Further, Ti is finely precipitated as a carbide, a nitride, or a carbonitride, and the steel structure of the steel sheet member is refined. When the Ti content is less than 0.060%, the ferrite iron deformation state is not sufficiently promoted, and the steel structure of the steel sheet member is liable to become a single phase of the granulated iron, and sufficient ductility is not obtained. Therefore, the Ti content is set to 0.060% or more. From the viewpoint of further increasing the ductility, the Ti content is preferably 0.075% or more. When the Ti content is more than 0.20%, coarse carbonitrides are formed during casting and hot rolling delay for obtaining a hot-pressed steel sheet, and deterioration of toughness is remarkable. Therefore, the Ti content is set to 0.20% or less. From the viewpoint of ensuring excellent toughness, the Ti content is preferably 0.18% or less, more preferably 0.15% or less.

(P: 0.05% or less)

P is not an essential element, for example, it is contained as an impurity in steel. From the viewpoint of weldability, the lower the P content, the better. In particular, when the P content is more than 0.05%, the weldability will be remarkably lowered. Therefore, the P content is made 0.05% or less. In order to ensure better weldability, the P content is preferably 0.018% or less. On the other hand, P has the effect of increasing the strength of steel by solid solution strengthening. use. In order to obtain this effect, it may contain 0.003% or more of P.

(S: 0.01% or less)

S is not an essential element, for example, it is contained as an impurity in steel. From the viewpoint of weldability, the lower the S content, the better. In particular, when the S content is more than 0.01%, the weldability will be remarkably lowered. Therefore, the S content is made 0.01% or less. In order to ensure better weldability, the S content is preferably 0.003% or less, more preferably 0.0015% or less.

(N: 0.01% or less)

N is not an essential element, for example, it is contained as an impurity in steel. From the viewpoint of weldability, the lower the N content, the better. In particular, when the N content is more than 0.01%, the weldability will be remarkably lowered. Therefore, the N content is made 0.01% or less. In order to ensure better weldability, the N content is preferably 0.006% or less.

Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM, Zr, B, and Bi are not essential elements, and may be limited to a steel plate member and a hot-pressed steel sheet, and may contain a predetermined amount as appropriate. element.

(Nb: 0%~0.20%, V: 0%~0.20%, Cr: 0%~1.0%, Mo: 0%~0.15%, Cu: 0%~1.0%, Ni: 0%~1.0%)

Nb, V, Cr, Mo, Cu, and Ni are all elements which improve the hardenability of the steel sheet for hot pressing, and can effectively ensure the strength of the steel sheet member. Therefore, one or two or more selected from the group consisting of these elements may be contained. However, when the content of either Nb or V is more than 0.20%, it is difficult to obtain hot rolling and cold rolling of the steel sheet for hot pressing, and the steel structure of the steel sheet member becomes a single phase of the granulated iron, and the ductility is deteriorated. The system is significant. Therefore, will Both the Nb content and the V content were set to 0.20% or less. When the content of Cr is more than 1.0%, it is difficult to ensure stable strength. Therefore, the Cr content is made 1.0% or less. When the content of Mo is more than 0.15%, the steel structure of the steel sheet member becomes a single phase of the granulated iron, and the deterioration of ductility becomes remarkable. Therefore, the Mo content is made 0.15% or less. When the content of any of Cu and Ni is 1.0%, it is economically disadvantageous to achieve saturation by the effect of the above-described action, and it is difficult to obtain hot rolling and cold rolling of the steel sheet for hot pressing. Therefore, both the Cu content and the Ni content are set to 1.0% or less. In order to ensure the stability of the strength of the steel sheet member, the Nb content and the V content are preferably 0.003% or more, and the Cr content, the Mo content, the Cu content, and the Ni content are preferably 0.005% or more. In other words, it satisfies "Nb: 0.003% to 0.20%", "V: 0.003% to 0.20%", "Cr: 0.005% to 1.0%", "Mo: 0.005% to 0.15%", and "Cu: 0.005%~ At least one of 1.0%" and "Ni: 0.005% to 1.0%" is preferred.

(Ca: 0%~0.01%, Mg: 0%~0.01%, REM: 0%~0.01%, Zr: 0%~0.01%)

Ca, Mg, REM, and Zr are elements which contribute to the control of inclusions, and in particular, contribute to the fine dispersion of inclusions and have an effect of improving toughness. Therefore, one or two or more selected from the group consisting of these elements may be contained. However, when any of the contents is more than 0.01%, there is a case where the surface texture is remarkably deteriorated. Therefore, the Ca content, the Mg content, the REM content, and the Zr content are all set to 0.01% or less. In order to improve the toughness, the Ca content, the Mg content, the REM content, and the Zr content are preferably 0.0003% or more. In other words, to satisfy "Ca: 0.0003% to 0.01%", "Mg: 0.0003% to 0.01%", "REM: 0.0003% to 0.01%", and "Zr: At least one of 0.0003% to 0.01%" is preferred.

REM (rare earth metal) refers to a total of 17 elements such as Sc, Y, and yttrium, and "REM content" means the total content of these 17 elements. The industry is added in the form of, for example, a rare earth metal alloy.

(B: 0%~0.005%)

The B system has an element that enhances the toughness of the steel sheet. Therefore, it is also possible to contain B. However, when the B content is more than 0.005%, the steel structure of the steel sheet member becomes a single phase of the granulated iron, and the deterioration of ductility is remarkable. Moreover, the hot workability is deteriorated, and it is useful to obtain the hot rolling of the hot-pressing steel sheet. Therefore, the B content is made 0.005% or less. In order to improve the toughness, the B content is preferably 0.0003% or more. In other words, the B content is preferably 0.0003% to 0.005%.

(Bi: 0%~0.01%)

The Bi system has an element which makes the steel structure uniform and improves the ductility. Therefore, Bi may also be contained. However, when the Bi content is more than 0.01%, hot workability is deteriorated, and it is difficult to obtain hot rolling of the steel sheet for hot pressing. Therefore, the Bi content is made 0.01% or less. In order to improve ductility, the Bi content is preferably 0.0003% or more. In other words, the Bi content is preferably 0.0003% to 0.01%.

Next, the steel structure of the steel sheet member of the present embodiment and the precipitates in the steel sheet member will be described. The steel plate member has a total area ratio of 90% to 100% of the total area ratio of the ferrite iron: 10% to 70%, the granulated iron: 30% to 90%, the ferrite iron and the granulated iron. Further, 90% or more of the total Ti in the steel is precipitated. Further, the numerical value relating to the steel structure, for example, may be a value related to the steel structure where the depth of the surface of the steel sheet member is 1/4 of the thickness of the steel sheet member (hereinafter, referred to as "1/4 depth position") representative The average value of the entire thickness direction of the steel sheet member. For example, when the thickness of the steel sheet member is 2.0 mm, it can be represented by a numerical value at a depth of 0.50 mm from the surface. This is because the steel structure at the 1/4 depth position indicates the average steel structure in the thickness direction of the steel sheet member.

(area ratio of fertilized iron: 10%~70%)

The ferrite iron which is precipitated into a mesh shape contributes to the improvement of the ductility of the steel sheet member. When the area ratio of the ferrite iron is less than 10%, the ferrite iron will not easily form a network and will not be sufficiently ductile. Therefore, the area ratio of the ferrite iron is set to 10% or more. When the area ratio of the ferrite iron is more than 70%, the area ratio of the granulated iron in the field is inevitably less than 30%, and it is not easy to ensure the tensile strength of 980 MPa or more. Therefore, the area ratio of the ferrite iron is set to 70% or less.

(Area area of Ma Tian loose iron: 30%~90%)

Ma Tian loose iron is important for the high strength of steel plate members. When the area ratio of the granulated iron is less than 30%, it is not easy to ensure the tensile strength of the steel sheet member of 980 MPa or more. Therefore, the area ratio of the granulated iron is set to 30% or more. When the area ratio of the granulated iron is more than 90%, the area ratio of the ferrite iron is inevitably less than 10%, and the full ductility is not obtained. Therefore, the area ratio of the granulated iron is set to 90% or less.

(Total area ratio of fertiliser iron and 麻田散铁: 90%~100%)

The steel structure of the hot-pressed steel sheet member according to the present embodiment is composed of ferrite iron and 麻田散铁, that is, the total area ratio of the ferrite iron and the granulated iron is preferably 100%. However, depending on the manufacturing conditions, it may also contain phases or structures other than ferrite iron and 麻田散铁, for example, selected from toughened iron, residual Worth iron, swarovski carbon iron, and Boron iron. One or two of the groups on. At this time, if the area ratio of the phase or the structure other than the ferrite iron and the granulated iron is more than 10%, the characteristics of the target may not be obtained due to the influence of the phase or the structure. Therefore, the area ratio of the phase or the structure other than the ferrite iron and the granulated iron is set to 10% or less. In other words, the total area ratio of the ferrite iron and the granulated iron is set to 90% or more.

The method of measuring the area ratio of each phase of the above steel structure can be carried out by a method well known to those skilled in the art to which the invention pertains. The area ratios are obtained, for example, from the average value of the value measured in the cross section perpendicular to the rolling direction and the value measured in the cross section perpendicular to the sheet width direction (the direction perpendicular to the rolling direction). In other words, for example, the average of the area ratios measured in the two cross sections.

(The ratio of precipitated Ti: 90% or more)

The precipitate of Ti helps to ensure the stable tensile strength of the steel sheet member. As described above, although the steel sheet member contains 0.060% to 0.20% of Ti, the ratio of Ti precipitated therein is less than 90%, and the above-described effects are hard to be obtained. Therefore, in the steel sheet member, the ratio of precipitation of total Ti in the steel is set to 90% or more. The precipitate of Ti is contained in the steel sheet member as, for example, a carbide, a nitride or a carbonitride. The amount of Ti precipitated in the steel sheet member can be specified by analysis of inductively coupled plasma (ICP) using the residue obtained by electrolytic extraction of the steel sheet member.

Such a steel sheet member can be produced by processing a predetermined steel sheet for hot pressing under predetermined conditions.

Here, the steel sheet for hot press used for producing the steel sheet member of the present embodiment will be described. In the steel sheet for hot pressing, the total Ti in the steel is precipitated. More than 70%.

The steel structure of the steel sheet for hot pressing is not particularly limited. This is because, as will be described later, the hot-pressed steel sheet is heated to a temperature of Ac ₃ or more at the time of hot pressing.

(The ratio of precipitated Ti: 70% or more)

When the proportion of precipitation in the total Ti contained in the hot-pressed steel sheet is less than 70%, the ferrite-grain metamorphism is less likely to occur during hot pressing, and the steel sheet member having the desired steel structure is not easily obtained. Therefore, in the steel sheet for hot press, the ratio of precipitation in the total Ti in the steel is 70% or more.

Next, a method of manufacturing the steel sheet member according to the present embodiment, that is, a method of processing the steel sheet for hot pressing will be described. In the treatment of the hot-pressed steel sheet, the hot-pressed steel sheet is heated in a temperature range of Ac ₃ to Ac ₃ + 100 ° C for 1 minute to 10 minutes, and hot-pressed after the heating. In the hot pressing, the first cooling is performed in a temperature range of 600 ° C to 750 ° C, and the second cooling is performed in a temperature range of 150 ° C to 600 ° C. In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C. In the second cooling, the average cooling rate is set to 10 ° C / sec to 500 ° C / sec.

(Heating temperature of steel plate for hot pressing: Ac ₃ points ~ Ac ₃ points + 100 ° C temperature range)

The steel sheet for hot pressing, that is, the heating of the steel sheet for hot pressing is carried out in a temperature range of Ac ₃ points or more and Ac ₃ points + 100 ° C or lower. The Ac ₃ point is a temperature (unit: ° C) which is a single phase of Vostian iron as defined by the following experimental formula (i).

Ac ₃ = 910-203 × (C ^0.5 ) - 15.2 × Ni + 44.7 × Si + 104 × V + 31.5 × Mo-30 × Mn-11 × Cr - 20 × Cu + 700 × P + 400 × Al + 50 × Ti...(i)

Here, the element symbol in the above formula represents each of the chemical compositions of the steel sheet. The content of the element (unit: mass%).

When the heating temperature is less than Ac ₃ point, the steel structure of the steel sheet member tends to be uneven, and the tensile strength of the steel sheet member is unstable and the ductility is deteriorated. Therefore, the heating temperature is set to Ac ₃ or more. When the heating temperature is higher than Ac ₃ point + 100 ° C, the stability of the Worthfield iron grain boundary becomes too high, and it will not easily promote the fermented iron iron metamorphosis. As a result, the steel structure of the steel sheet member becomes a single phase of the granulated iron, and the deterioration of ductility is remarkable. Further, when the Ti content is less than 0.08%, the precipitate of Ti is easily dissolved. Therefore, the heating temperature is set to Ac ₃ point + 100 ° C or less. Further, from the viewpoint of suppressing damage to the heating furnace and improving productivity, the heating temperature is preferably 860 ° C or lower. By appropriately adjusting the composition of the hot-pressed steel sheet, the Worthite iron single phase can be formed at a temperature of 860 ° C or lower.

(Heating time of hot pressing steel plate: 1 minute to 10 minutes)

When the heating time is less than 1 minute, the single-phase structure of the Worthite iron tends to become uneven, and it is difficult to ensure stable strength. Therefore, the heating time is set to 1 minute or longer. When the heating time is longer than 10 minutes, the ferrite-iron deformation state is less likely to occur after cooling, and the steel structure of the steel sheet member becomes a single phase of the granulated iron, and the deterioration of ductility becomes remarkable. Also, productivity will drop significantly. Therefore, the heating time is set to 10 minutes or less.

Here, the heating time refers to the time from when the steel sheet temperature reaches the Ac ₃ point to when the heating is ended. Specifically, when the heating is completed, when the furnace is heated, the steel sheet is taken out from the heating furnace, and when the heating or the induction heating is performed, the current is turned on.

The average heating rate in the temperature range of Ac ₃ points or more and Ac ₃ points + 100 ° C or lower is preferably 0.2 ° C / sec or more and 100 ° C / sec or less. By setting the average heating rate to 0.2 ° C / sec or more, higher productivity can be ensured. Further, by setting the average heating rate to 100 ° C /sec or less, it is easy to control the heating temperature when heating in a normal furnace. In particular, when high-frequency heating or electric heating is performed, even if the average heating rate is more than 100 ° C / sec, the heating temperature is easily controlled, so the average heating rate may be more than 100 ° C / sec. The average heating rate in the temperature range of 700 ° C or higher and the Ac ₃ point or lower is preferably 1 ° C / sec or more and 10 ° C / sec or less. When the average heating rate in the temperature range is within this range, the steel structure of the steel sheet member will be more uniform, and the ductility can be further improved.

(Starting to precipitate the temperature of ferrite and iron: 600 ° C ~ 750 ° C)

The temperature at which the ferrite iron begins to precipitate during hot pressing will have an effect on the properties of the ferrite. When the ferrite iron is precipitated at more than 750 ° C, the ferrite iron will coarsen and the toughness deteriorates. When the ferrite iron is precipitated at less than 600 ° C, the difference in the density of the ferrite in the ferrite is high, and the ductility is deteriorated. Therefore, in the first cooling, the ferrite iron is precipitated in a temperature range of 600 ° C to 750 ° C.

(Average cooling rate of the first cooling: 3 ° C / sec ~ 200 ° C / sec)

The temperature at which the ferrite iron is precipitated, that is, the precipitation starting temperature of the ferrite iron, can be controlled by adjusting the average cooling rate of the hot pressing. For example, it is preferred to carry out the first cooling under the conditions determined by analyzing the thermal expansion curve. However, even if the precipitation starting temperature of the ferrite iron is in the range of 600 ° C to 750 ° C, and the average cooling rate of the first cooling is less than 3 ° C / sec, the ferrite iron is excessively deformed, and the steel sheet member is not easily The area ratio of the scattered iron is set to 30% or more, and there is a case where the tensile strength of 980 MPa or more is not obtained. Also, it is difficult to control the average cooling rate to be less than 3 ° C / sec only by air cooling or forced air cooling. Therefore, the average cooling rate of the first cooling is set to 3 ° C / sec. on. The average cooling rate is preferably 6 ° C /sec or more. Further, even if the precipitation starting temperature of the ferrite iron is in the range of 600 ° C to 750 ° C, and the average cooling rate of the first cooling is more than 200 ° C / sec, it is difficult to make the area ratio of the ferrite iron of the steel sheet member 10% or more. Failure to get good ductility. Therefore, the average cooling rate of the first cooling is set to 200 ° C / sec or less. The average cooling rate is preferably 60 ° C / sec or less.

When a steel sheet for hot press having 70% or more of the total Ti in the Ti-ratio steel having the above chemical composition is used, the average cooling rate in the temperature range of 600 ° C or more and 750 ° C or less is 3 ° C / sec or more. At 200 ° C / sec or less, the ferrite iron can be precipitated in a temperature range of 600 ° C or more and 750 ° C or less.

(The average cooling rate of the second cooling: 10 ° C / sec ~ 500 ° C / sec)

It is important that the diffusion type metamorphism is less likely to occur in cooling in a temperature range of 150 ° C or more and 600 ° C or less. When the average cooling rate in the temperature range is less than 10 ° C / sec, the transition-type metamorphic toughened iron metamorphic state is likely to occur, and it is difficult to set the area ratio of the granulated iron of the steel sheet member to 30% or more, and it is difficult to ensure 980 MPa or more. Tensile strength is difficult. Therefore, the average cooling rate of the second cooling is set to 10 ° C /sec or more. From the viewpoint of more surely ensuring that the area ratio of the granulated iron is high, the average cooling rate is preferably 15 ° C /sec or more. It is difficult to make the average cooling rate of the second cooling more than 500 ° C / sec in a normal apparatus. Therefore, the average cooling rate in this temperature range is set to 500 ° C / sec or less. From the viewpoint of achieving more stable cooling, the average cooling rate is preferably 200 ° C / sec or less.

Between such a first cooling and a second cooling, a steel structure in which fine ferrite iron as shown in Fig. 1 is distributed in a mesh shape is obtained. Such a steel structure can be effective Improve ductility.

Further, in the second cooling, after the temperature reaches 600 ° C, the heat generated by the phase change state is likely to be extremely large. Therefore, when cooling in a temperature range of less than 600 ° C in the same manner as cooling in a temperature range of 600 ° C or higher, a sufficient average cooling rate cannot be ensured. Therefore, it is preferable to carry out the second cooling from 600 ° C to 150 ° C more strongly with the first cooling of 600 ° C. For example, it is preferable to use the following method.

In general, a steel mold used for molding a heated steel sheet is previously set to a normal temperature or a temperature of about 10 ° C, and the steel sheet is brought into contact with the mold to be cooled by hot pressing. Therefore, the average cooling rate can be controlled by, for example, a change in heat capacity as the size of the mold changes. The average cooling rate can also be controlled by changing the material of the mold to a dissimilar metal (such as Cu). The average cooling rate can also be controlled by using a water-cooled mold and changing the amount of cooling water flowing to the mold. A plurality of grooves are formed in advance in the mold, and the average cooling rate can be controlled by causing water to flow through the grooves during hot pressing. The hot press is lifted during hot pressing to allow water to flow through it, and the average cooling rate can also be controlled. The average cooling rate can also be controlled by adjusting the mold gap and changing the contact area between the mold and the steel sheet.

The method of increasing the cooling rate in the temperature range of 600 ° C or lower can be exemplified by the following three examples.

(a) Immediately after reaching 600 ° C, the steel sheet was moved to a mold having a different heat capacity or a mold at room temperature.

(b) Using water to cool the mold, the water flow in the mold is increased immediately after reaching 600 °C.

(c) After reaching 600 ° C, water was flowed between the mold and the steel sheet. In this method, the cooling rate can also be increased by increasing the amount of water corresponding to the temperature.

The form of hot pressing of the present embodiment is not particularly limited. Examples of the form of the forming include bending processing, drawing forming, stretch forming, hole expanding forming, and flange forming. The form of the forming can be appropriately selected depending on the type of the steel sheet member to be used. Representative examples of the steel sheet member include a door protection lever for a reinforcing member for an automobile and a bumper reinforcement beam. Further, if the steel sheet can be cooled immediately before or after the forming, thermoforming is not limited to hot pressing. For example, roll forming can also be performed as hot forming.

The steel sheet member of the present embodiment can be produced by the above-described predetermined hot-press steel sheet which has been subjected to such a series of treatments, that is, a steel sheet for hot pressing containing an appropriate C, Mn, and Ti contents. In other words, a hot-pressed steel sheet member having a desired steel structure and a tensile strength of 980 MPa and having excellent strength and ductility can be obtained without complicated control.

For example, the ductility can be evaluated by the full elongation (EL) of the tensile test. In the present embodiment, the total elongation of the tensile test is preferably 10% or more. The full elongation is preferably 14% or more.

Bead blasting can also be carried out after hot pressing and cooling. The scale can be removed by bead blasting. Since the bead blasting treatment also has the effect of introducing a compressive stress on the surface of the steel sheet member, it is possible to suppress the delayed fracture and improve the fatigue strength.

Further, in the method for producing a steel sheet member, the hot-pressed steel sheet is heated to a temperature range of Ac ₃ point or more and Ac ₃ point + 100 ° C or lower, and the Worthite iron is deformed and then molded. Therefore, the mechanical properties of the steel sheet for hot pressing at room temperature before heating are not critical. Therefore, for the steel sheet for hot pressing, for example, a hot-rolled steel sheet, a cold-rolled steel sheet, a plated steel sheet or the like can be used. The cold-rolled steel sheet can be exemplified by a full hard material and an annealed material. Examples of the plated steel sheet include an aluminum-based plated steel sheet and a zinc-based plated steel sheet. The manufacturing methods of these are not particularly limited.

The steel sheet member of the present embodiment can also be produced by hot pressing along with preforming. For example, in a range in which the above-described conditions of heating and cooling are satisfied, a steel sheet for hot pressing of a predetermined shape is preliminarily formed into a mold of the same type, and a pressing pressure is applied to a mold of the same type, and hot pressing is performed by rapid cooling. Steel plate member. In this case, the type of the hot-pressed steel sheet and the steel structure thereof are not particularly limited, but it is preferable to use a soft and ductile steel sheet as much as possible in order to easily form the steel sheet. For example, the tensile strength is preferably 700 MPa or less. In order to obtain a soft steel sheet, the coiling temperature after hot rolling of the hot-rolled steel sheet is preferably 450 ° C or higher, and is preferably 700 ° C or less in order to reduce the scale loss. In the case of obtaining a soft steel sheet in the cold-rolled steel sheet, it is preferred to perform annealing, and the annealing temperature is preferably set to a temperature of Ac ₁ or higher and 900 ° C or lower. Further, the average cooling rate after annealing to room temperature is preferably equal to or lower than the critical cooling rate.

In addition, the above-described embodiments are merely examples for embodying the present invention, and the technical scope of the present invention cannot be limited thereto. In other words, it can be implemented in various forms without departing from the technical idea or main features of the invention.

Example

Next, an experiment conducted by the inventors of the present application will be described. In this experiment, 30 kinds of steel materials having a chemical composition shown in Table 1 were first used, and 30 kinds of materials to be tested having a thickness of 1.2 mm shown in Table 2 were produced. In addition, the remaining steel The remainder is Fe and impurities.

In the production of each material to be tested, hot rolling and cold rolling of the flat steel embryos which were melted in the laboratory were performed. The material No. 1 to be tested was produced by subjecting a cold-rolled steel sheet obtained by cold rolling to Al plating having a plating adhesion amount of 120 g/m ² per one surface. The material to be tested No. 2 was produced by hot-rolling steel sheets obtained by cold rolling, and subjected to hot-dip galvanizing with a plating adhesion amount of 60 g/m ² per one surface, followed by alloying treatment. In the alloying treatment, the Fe content in the hot-dip galvanized film was set to 15% by mass. Al plating and hot-dip galvanizing were performed using a plating simulator, the annealing temperature of the plating simulator was 820 ° C, and the average cooling rate from 820 ° C to 500 ° C was 5 ° C / sec.

After each test material was produced, a steel sheet having a thickness of 1.2 mm, a width of 100 mm, and a length of 200 mm was cut out from each of the materials to be tested, and heat treatment (heating and cooling) was carried out under the conditions shown in Table 2. In this heat treatment, a thermocouple was attached to a steel sheet, and the average cooling rate of the first cooling and the average cooling rate of the second cooling were measured. Moreover, the precipitation start temperature of the ferrite iron was determined from the analysis result of the change in the expansion ratio in the cooling.

After the heat treatment, the steel sheets were individually subjected to a tensile test and an observation of the steel structure. In the tensile test, the tensile strength (TS) and the total elongation (EL) were measured. Tensile strength and full elongation were measured using JIS No. 5 tensile test pieces taken from each steel sheet. The area ratio of the ferrite iron and the area ratio of the granulated iron were determined by observing the steel structure. The area ratio is an average value of the values calculated by analyzing the image of the electron microscope observation image by the cross section perpendicular to the rolling direction and the cross section perpendicular to the sheet width direction (the direction perpendicular to the rolling direction). The area of the field of view of the electron microscope observation was 8 mm ² . The results of these are shown in Table 3. In addition, although the steel sheet to be subjected to the tensile test and the steel structure is not subjected to hot pressing, the mechanical properties of the steel sheet are reflected in the mechanical structure of the hot-pressed steel sheet member which is produced by the same heat history as the heat treatment of the present experiment at the time of molding. nature. In other words, with or without the hot pressing of the forming, the mechanical properties thereafter are substantially the same as long as the thermal history is substantially the same.

As shown in Table 3, the materials to be tested No. 1, No. 4, No. 6, No. 8, No. 11, No. 15, No. 16, No. 18, No. 20, No. 22, No. .24, No. 26, No. 27, and No. 29 are examples of the present invention, and exhibit excellent tensile strength and ductility.

On the other hand, since the manufacturing conditions of the materials No. 2, No. 3, and No. 30 are outside the scope of the present invention, and the steel structure after the heat treatment is outside the scope of the present invention, sufficient tensile strength cannot be obtained. strength. The chemical composition of the steel materials of the materials No. 5, No. 14, No. 17, No. 19, No. 21, No. 23, and No. 28 is outside the scope of the present invention, and the steel structure after the heat treatment is also Outside of the scope of the present invention, sufficient ductility cannot be obtained. Since the chemical composition of the steel material of the material No. 7 to be tested was outside the scope of the present invention, sufficient ductility could not be obtained. Since the manufacturing conditions of the materials No. 9, No. 10, and No. 12 were outside the scope of the present invention, and the steel structure after the heat treatment was outside the scope of the present invention, sufficient ductility could not be obtained. The chemical composition of the steel material of the material No. 25 to be tested is outside the scope of the present invention, and the steel structure after the heat treatment is also outside the scope of the present invention, so that sufficient tensile strength cannot be obtained.

Industrial availability

The present invention can be applied to, for example, a manufacturing industry and a utilization industry of automobile body structure parts and the like which are excellent in tensile strength and ductility. The present invention can also be applied to the manufacturing industry and utilization industries of other mechanical structural parts.

Claims (26)

A hot-pressed steel sheet member characterized by having the chemical composition shown below: C: 0.10% to 0.24%, Si: 0.001% to 2.0%, Mn: 1.2% to 2.3%, sol. Al: by mass% 0.001% to 1.0%, Ti: 0.060% to 0.20%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Nb: 0% to 0.20%, V: 0% to 0.20%, Cr: 0%~1.0%, Mo: 0%~0.15%, Cu: 0%~1.0%, Ni: 0%~1.0%, Ca: 0%~0.01%, Mg: 0%~0.01%, REM: 0% ~0.01%, Zr: 0%~0.01%, B: 0%~0.005%, Bi: 0%~0.01%, the remaining part: Fe and impurities; in area%, with ferrite iron: 10%~70%, Ma Tian loose iron: 30%~90%, fat iron and 麻田散铁Area ratio: 90%~100% steel structure; more than 90% of the total Ti in the steel will be precipitated, and the tensile strength is 980 MPa or more. The hot-pressed steel sheet member according to claim 1, wherein the chemical composition is one or more selected from the group consisting of Nb: 0.003% to 0.20%, and V: 0.003% by mass%. ~0.20%, Cr: 0.005% to 1.0%, Mo: 0.005% to 0.15%, Cu: 0.005% to 1.0%, and Ni: 0.005% to 1.0%. The hot-pressed steel sheet member according to claim 1 or 2, wherein the chemical composition is one by mass or more selected from the group consisting of: Ca: 0.0003% to 0.01%, Mg: by mass%: 0.0003%~0.01%, REM: 0.0003%~0.01%, and Zr: 0.0003%~0.01%. The hot-pressed steel sheet member according to claim 1 or 2, wherein the aforementioned chemical composition contains B: 0.0003% to 0.005% by mass%. The hot-pressed steel sheet member according to claim 3, wherein the chemical composition comprises B: 0.0003% to 0.005% by mass%. The hot-pressed steel sheet member according to claim 1 or 2, wherein the aforementioned chemical composition contains, by mass%, Bi: 0.0003% to 0.01%. The hot-pressed steel sheet member according to claim 3, wherein the chemical composition contains Bi: 0.0003% to 0.01% by mass%. The hot-pressed steel sheet member according to claim 4, wherein the chemical composition contains Bi: 0.0003% to 0.01% by mass%. The hot-pressed steel sheet member according to claim 5, wherein the chemical composition contains Bi: 0.0003% to 0.01% by mass%. A steel sheet for hot pressing, characterized by having the chemical composition shown below: C: 0.10% to 0.24%, Si: 0.001% to 2.0%, Mn: 1.2% to 2.3%, sol. Al: by mass% 0.001% to 1.0%, Ti: 0.060% to 0.20%, P: 0.05% or less, S: 0.01% or less, N: 0.01% or less, Nb: 0% to 0.20%, V: 0% to 0.20%, Cr: 0%~1.0%, Mo: 0% to 0.15%, Cu: 0% to 1.0%, Ni: 0% to 1.0%, Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 0.01%, Zr: 0%~0.01%, B: 0%~0.005%, Bi: 0%~0.01%, the remainder: Fe and impurities; and more than 70% of the total Ti in the steel will precipitate. The hot-pressing steel sheet according to claim 10, wherein the chemical composition is one by mass or more selected from the group consisting of Nb: 0.003% to 0.20%, and V: 0.003% by mass%. ~0.20%, Cr: 0.005% to 1.0%, Mo: 0.005% to 0.15%, Cu: 0.005% to 1.0%, and Ni: 0.005% to 1.0%. The hot-pressing steel sheet according to claim 10, wherein the chemical composition is one by mass or more selected from the group consisting of: Ca: 0.0003% to 0.01%, Mg: 0.0003%~0.01%, REM: 0.0003% to 0.01%, and Zr: 0.0003% to 0.01%. The hot-pressed steel sheet according to claim 10 or 11, wherein the chemical composition is B: 0.0003% to 0.005% by mass%. The steel sheet for hot pressing according to claim 12, wherein the chemical composition contains B: 0.0003% to 0.005% by mass%. The steel sheet for hot pressing according to claim 10 or 11, wherein the chemical composition contains Bi: 0.0003% to 0.01% by mass%. The hot-pressing steel sheet according to claim 12, wherein the chemical composition contains, by mass%, Bi: 0.0003% to 0.01%. The steel sheet for hot pressing according to claim 13, wherein the chemical composition contains Bi: 0.0003% to 0.01% by mass%. The steel sheet for hot pressing according to claim 14, wherein the chemical composition contains Bi: 0.0003% to 0.01% by mass%. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing according to claim 10 or 11 in a temperature range of Ac ₃ to Ac ₃ + 100 ° C for 1 minute to 10 minutes And the step of performing hot pressing after the heating; the step of performing hot pressing has the steps of: performing the first cooling in a temperature range of 600 ° C to 750 ° C, and performing the temperature range of 150 ° C to 600 ° C a second cooling step; in the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in a temperature range of 600 ° C to 750 ° C, and the second cooling is performed. In the middle, the average cooling rate is from 10 ° C / sec to 500 ° C / sec. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 12 in a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C, and in the second cooling, 10 ° C / sec ~ 500 ° C / sec as the average cooling rate. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 13 in a temperature range of from Ac ₃ to Ac ₃ + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C, and in the second cooling, 10 ° C / sec ~ 500 ° C / sec as the average cooling rate. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 14 in a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C, and in the second cooling, 10 ° C / sec ~ 500 ° C / sec as the average cooling rate. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 15 in a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C; and in the second cooling, 10 ° C / sec ~ 500 ° C / sec as the average cooling rate. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 16 in a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C, and in the second cooling, 10 ° C / sec ~ 500 ° C / sec as the average cooling rate. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 17 in a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C, and 10 ° C in the second cooling. / sec ~ 500 ° C / sec as the average cooling rate. A method for producing a hot-pressed steel sheet member, comprising: heating the steel sheet for hot pressing of claim 18 in a temperature range of Ac ₃ points to Ac ₃ points + 100 ° C for 1 minute to 10 minutes, and a step of performing hot pressing after heating; the step of performing hot pressing has a step of performing first cooling in a temperature range of 600 ° C to 750 ° C, and performing a second cooling in a temperature range of 150 ° C to 600 ° C In the first cooling, the average cooling rate is 3 ° C / sec to 200 ° C / sec, and the ferrite iron is precipitated in the temperature range of 600 ° C to 750 ° C, and in the second cooling, 10 ° C / sec ~ 500 ° C / sec as the average cooling rate.