WO2020189530A1 - Steel sheet - Google Patents

Abstract

A steel sheet having the following chemical composition in mass%: C: greater than 0.10% and less than 0.55%, Si: at least 0.001% and less than 3.50%, Mn: greater than 4.00% and less than 9.00%, sil.Al: at least 0.001% and less than 3.00%, P: not more than 0.100%, S: not more than 0.010%, N: less than 0.050%, O: less than 0.020%, Cr: at least 0% and less than 2.00%, Mo: 0 to 2.00%, W: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Ti: 0 to 0.300%, Nb: 0 to 0.300%, V: 0 to 0.300%, B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Zr: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, Bi: 0 to 0.050%, and balance: Fe and impurities. In the L-cross section, at a position one-fourth of the thickness from the surface, the steel sheet has a metallographic composition, as an area ratio, of tempered martensite: 25 to 90%, ferrite: not more than 5%, retained austenite: 10 to 50%, and bainite: not more than 5%, wherein the area ratio of the total area of retained austenite crystal grains satisfying an area of at least 1 μm² and a grain circularity of at least 0.1, to the total area of the retained austenite, is less than 50%. The steel sheet satisfies C_Mnγ/C_Mnα ≥ 1.2.

Description

Steel plate

The present invention relates to steel sheets.

In recent years, efforts have been actively made to reduce the weight of the vehicle body by applying high-strength steel plates for the purpose of improving the fuel efficiency and collision safety of automobiles. However, in general, the higher the strength of the steel sheet, the lower the elongation that affects the formability and the toughness that affects the collision characteristics. Therefore, in the development of high-strength steel sheets, it is an important issue to increase the strength without lowering the elongation and toughness.

So-called TRIP steels that utilize the transformation-induced plasticity of retained austenite (residual γ) have been proposed so far in order to improve elongation (for example, Patent Document 1).

Residual austenite is obtained by concentrating and stabilizing C in austenite. For example, by incorporating a carbide precipitation inhibitoring element such as Si and Al into the steel sheet, it is possible to concentrate C in austenite during the bainite transformation that occurs in the steel sheet during the manufacturing stage of the steel sheet. In this technique, if the C content contained in the steel sheet is large, the austenite can be further stabilized and the amount of retained austenite can be increased. As a result, a steel sheet having excellent strength and elongation can be obtained.

Further, as a steel sheet having a larger amount of retained austenite than the above-mentioned TRIP steel and a ductility exceeding the above-mentioned TRIP steel, a steel to which Mn of 3.5% or more is added (Patent Document 2) and Mn of more than 4.0% are added. Steel (Non-Patent Document 1) has been proposed. Since the steel contains a large amount of Mn, the weight reduction effect on the members used is also remarkable.

Japanese Unexamined Patent Publication No. 5-59429 Japanese Unexamined Patent Publication No. 2013-76162

When a steel sheet is used as a structural member, welding is often performed on the steel sheet, but if the C content in the steel sheet is high, the weldability deteriorates, so that the use as a structural member is restricted. Therefore, it is desired to improve both the formability and the strength of the steel sheet without increasing the C content.

Further, the steel disclosed in Patent Document 2 and Non-Patent Document 1 requires a long-time heating process such as box annealing, and improvement in productivity is desired. However, the material design in a short-time heating process such as continuous annealing suitable for manufacturing high-strength steel sheets for automobile members has not been sufficiently studied, and the requirement for enhancing the elongation characteristics in that case has not been clarified.

Furthermore, in order to improve the collision characteristics, it is effective to increase the amount of impact energy absorption by significantly deforming the collision member, especially the front side member, but buckling due to the localization of the deformation. In order to prevent this, it was necessary to achieve both a high work hardening rate and good impact characteristics.

An object of the present invention is to solve the above problems and to provide a steel sheet having high strength, excellent uniform elongation characteristics and impact energy absorption ability.

The gist of the steel sheet of the present invention is the following steel sheet.

(1) The chemical composition of the steel sheet is mass%.
C: More than 0.10% and less than 0.55%,
Si: 0.001% or more and less than 3.50%,
Mn: More than 4.00% and less than 9.00%,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0% or more and less than 2.00%,
Mo: 0-2.00%,
W: 0 to 2.00%,
Cu: 0-2.00%,
Ni: 0 to 2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Zr: 0 to 0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%,
Remaining: Fe and impurities,
In the cross section parallel to the rolling direction and the plate thickness direction of the steel plate, the metal structure at a depth of 1/4 of the plate thickness from the surface is, in% area.
Tempering martensite: 25-90%,
Ferrite: 5% or less,
Residual austenite: 10-50%, and bainite: 5% or less,
Residual austenite crystal grains having an area of 1 μm ² or more and a grain circularity of 0.1 or more at a depth of 1/4 of the plate thickness from the surface of the cross section parallel to the rolling direction and the plate thickness direction of the steel sheet. The ratio of the total area of is less than 50% of the total area of the retained austenite.
Satisfy the following equation (i),
Steel plate.
C _Mnγ / C _Mnα ≧ 1.2 ・ ・ ・ (i)
However, the meanings of the symbols in the above equation (i) are as follows.
C _Mnγ : Average Mn concentration (mass%) in retained austenite
C _Mnα : Average Mn concentration (% by mass) in ferrite and tempered martensite

(2) The chemical composition is mass%.
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01-2.00%,
W: 0.01-2.00%,
Cu: 0.01 to 2.00%, and Ni: 0.01 to 2.00%
Contains one or more selected from
The steel sheet according to (1) above.

(3) The chemical composition is mass%.
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%, and V: 0.005 to 0.300%
Contains one or more selected from
The steel sheet according to (1) or (2) above.

(4) The chemical composition is mass%.
B: 0.0001 to 0.010%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 0.010%,
Zr: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%
Contains one or more selected from
The steel sheet according to any one of (1) to (3) above.

(5) The chemical composition is mass%.
Sb: 0.0005 to 0.050%,
Sn: 0.0005 to 0.050%, and Bi: 0.0005 to 0.050%
Contains one or more selected from
The steel sheet according to any one of (1) to (4) above.

(6) A hot-dip galvanized layer is provided on the surface of the steel sheet.
The steel sheet according to any one of (1) to (5) above.

(7) An alloyed hot-dip galvanized layer is provided on the surface of the steel sheet.
The steel sheet according to any one of (1) to (5) above.

(8) The Charpy impact value at 0 ° C. is 20 J / cm ² or more.
The steel sheet according to any one of (1) to (7) above.

(9) The yield ratio of the steel sheet is more than 0.40 and less than 0.80.
The steel sheet according to any one of (1) to (8) above.

According to the present invention, it is possible to provide a steel sheet having high strength, excellent uniform elongation characteristics and impact energy absorption ability.

Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: More than 0.10% and less than 0.55% C is an extremely important element for increasing the strength of martensite and tempered martensite and ensuring retained austenite. In order to obtain a sufficient amount of austenite, a C content of more than 0.10% is required. On the other hand, if C is excessively contained, the toughness and weldability of the steel sheet are impaired. Therefore, the C content is more than 0.10% and less than 0.55%. The C content is preferably 0.12% or more, more preferably 0.15% or more, and even more preferably 0.20% or more. The C content is preferably 0.40% or less, more preferably 0.35% or less.

Si: 0.001% or more and less than 3.50% Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving workability. In addition, Si also has an action of improving the uniform elongation property of the steel sheet by suppressing the precipitation of cementite and promoting the residual of austenite. On the other hand, if Si is excessively contained, the plating property and chemical conversion treatment property of the steel sheet are impaired. Therefore, the Si content is set to 0.001% or more and less than 3.50%. The Si content is preferably 0.005% or more, and more preferably 0.010% or more. The Si content is preferably 3.00% or less, more preferably 2.50% or less.

Mn: More than 4.00% and less than 9.00% Mn is an element that stabilizes austenite and enhances hardenability. Further, in the steel sheet of the present invention, Mn is concentrated in austenite to further stabilize austenite. More than 4.00% Mn is required to stabilize austenite at room temperature. On the other hand, if the steel sheet contains excess Mn, the toughness is impaired. Therefore, the Mn content is set to more than 4.00% and less than 9.00%. The Mn content is preferably 4.50% or more, more preferably 4.80% or more. The Mn content is preferably 8.50% or less, more preferably 8.00% or less.

sol. Al: 0.001% or more and less than 3.00% Al is a deoxidizing agent, and sol. It is necessary to contain 0.001% or more as Al. In addition, Al also has an action of improving material stability because it widens the temperature range of the two-phase region at the time of annealing. The higher the Al content, the greater the effect, but if the Al content is excessive, it becomes difficult to maintain the surface texture, paintability, and weldability. Therefore, sol. The Al content is 0.001% or more and less than 3.00%. sol. The Al content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.020% or more. In addition, sol. The Al content is preferably 2.00% or less, more preferably 1.00% or less. In addition, "sol.Al" referred to in this specification means "acid-soluble Al".

P: 0.100% or less P is an impurity, and if the steel sheet contains an excess of P, the toughness and weldability are impaired. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.020% or less. The P content may be 0.001% or more, but since the steel sheet according to the present invention does not require P, it is preferable to reduce it as much as possible.

S: 0.010% or less S is an impurity, and if the steel sheet contains an excess of S, MnS stretched by hot rolling is generated, and formability such as bendability and hole expansion property is lowered. Therefore, the S content is 0.010% or less. The S content is preferably 0.007% or less, more preferably 0.003% or less. The S content may be 0.001% or more, but since the steel sheet according to the present invention does not require S, it is preferable to reduce it as much as possible.

N: Less than 0.050% N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness decreases. Therefore, the N content is set to less than 0.050%. The N content is preferably 0.010% or less, more preferably 0.006% or less. The N content may be 0.002% or more, but since the steel sheet according to the present invention does not require N, it is preferable to reduce it as much as possible.

O: Less than 0.020% O is an impurity, and if the steel sheet contains 0.020% or more of O, the uniform elongation property deteriorates. Therefore, the O content is set to less than 0.020%. The O content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.003% or less. The O content may be 0.001% or more, but since the steel sheet according to the present invention does not require O, it is preferable to reduce it as much as possible.

In addition to the above elements, the steel sheet of the present invention further contains the following amounts of Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and It may contain one or more selected from Bi.

Cr: 0% or more and less than 2.00% Mo: 0 to 2.00%
W: 0 to 2.00%
Cu: 0 to 2.00%
Ni: 0 to 2.00%
Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, surface scratches are likely to occur during hot rolling, and the strength of the hot rolled steel sheet becomes too high, which may reduce the cold rollability. Therefore, the Cr content is less than 2.00%, the Mo content is 2.00% or less, the W content is 2.00% or less, the Cu content is 2.00% or less, and the Ni content is 2.00%. It is as follows. In order to more reliably obtain the above-mentioned effects of these elements, it is preferable to contain at least 0.01% or more of the above-mentioned elements.

Ti: 0 to 0.300%
Nb: 0 to 0.300%
V: 0 to 0.300%
Since Ti, Nb, and V are elements that produce fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of the steel sheet. Therefore, one or more selected from Ti, Nb, and V may be contained. However, if these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased, and the cold rollability may be lowered. Therefore, the Ti content is 0.300% or less, the Nb content is 0.300% or less, and the V content is 0.300% or less. In order to obtain the above-mentioned effects of these elements more reliably, it is preferable to contain at least one of the above-mentioned elements in an amount of 0.005% or more.

B: 0 to 0.010%
Ca: 0 to 0.010%
Mg: 0 to 0.010%
Zr: 0 to 0.010%
REM: 0 to 0.010%
B, Ca, Mg, Zr, and REM (rare earth metals) improve the local ductility and hole expandability of steel sheets. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, the workability of the steel sheet may be deteriorated. Therefore, the B content is 0.010% or less, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the Zr content is 0.010% or less, and the REM content is 0.010%. It is as follows. Further, it is preferable that the total content of one or more elements selected from B, Ca, Mg, Zr, and REM is 0.030% or less. In order to more reliably obtain the above effects of these elements, it is preferable to contain at least one of the above elements in an amount of 0.0001% or more, and more preferably 0.0010% or more.

The REM referred to in the present specification refers to a total of 17 elements of Sc, Y, and lanthanoid, and the REM content refers to the total content of these elements. REM is also generally supplied as mischmetal, which is an alloy of a plurality of types of REM. Therefore, one or more individual elements may be added so that the REM content is within the above range. For example, when added in the form of mischmetal, the REM content is within the above range. It may be contained so as to become.

Sb: 0 to 0.050%
Sn: 0 to 0.050%
Bi: 0 to 0.050%
Sb, Sn, and Bi suppress that easily oxidizing elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Therefore, one or more selected from these elements may be contained. However, even if it is contained in an excessive amount, the above effect is saturated. Therefore, the Sb content is 0.050% or less, the Sn content is 0.050% or less, and the Bi content is 0.050% or less. In order to obtain the above-mentioned effects of these elements more reliably, it is preferable to contain at least one of the above-mentioned elements in an amount of 0.0005% or more, and more preferably 0.0010% or more.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. The "impurities" are unavoidably mixed from the steel raw material or scrap and / or from the steelmaking process, and examples thereof include elements that are allowed as long as they do not impair the characteristics of the steel sheet according to the present invention.

(B) Metal structure The metal structure of the steel sheet according to the present invention will be described. In the following description, "%" for the area ratio means "area%".

In a cross section (also referred to as “L cross section”) parallel to the rolling direction and the thickness direction of the steel sheet according to the present invention and passing through the central axis of the steel sheet, the metallographic structure at a depth of 1/4 of the plate thickness from the surface is determined. Contains 25-90% tempered martensite, 5% or less ferrite, 10-50% retained austenite, and 5% or less baynite. The fraction of each structure varies depending on the annealing conditions and affects the strength, uniform elongation characteristics, toughness, and yield ratio of the steel sheet. The reasons for the limitation of each organization will be explained in detail.

Tempering martensite: 25-90%
Tempered martensite is a structure that enhances the strength of steel sheets, improves uniform elongation properties and toughness, and provides an appropriate yield ratio. When the area ratio of tempered martensite is less than 25% or more than 90%, it becomes difficult to obtain sufficient strength, uniform elongation, toughness, and yield ratio. Therefore, the area ratio of tempered martensite is 25 to 90%.

The area ratio of tempered martensite is preferably 30% or more, more preferably 35% or more, and even more preferably 50% or more. From the viewpoint of hydrogen embrittlement, the area ratio of tempered martensite is preferably 80% or less, more preferably 75% or less, and even more preferably 70% or less.

Ferrite: 5% or less When the area ratio of ferrite in the metal structure is increased, the uniform elongation property and toughness are significantly deteriorated. In addition, when the ferrite area ratio exceeds 50%, the yield ratio tends to be too large. Therefore, the area ratio of ferrite is set to 5% or less. The area ratio of ferrite is preferably 3% or less, and more preferably 0%.

Residual austenite: 10-50%
Residual austenite is a structure that enhances the ductility of steel sheets, especially the uniform elongation characteristics of steel sheets, by transformation-induced plasticity. In addition, retained austenite can be transformed into the martensite phase by overhanging, drawing, stretching flange processing, or bending processing accompanied by tensile deformation, which contributes not only to various workability of the steel sheet but also to improvement of the strength of the steel sheet. To do. Therefore, the higher the area ratio of retained austenite, the more preferable.

However, in the steel sheet having the above-mentioned chemical composition, the area ratio of retained austenite is limited to 50%. If Mn of more than 9.00% is contained, the area ratio of retained austenite can be more than 50%, but in this case, the uniform elongation property and castability of the steel sheet are impaired.

Therefore, the area ratio of retained austenite is 10 to 50%. The area ratio of retained austenite is preferably 14% or more, more preferably 18% or more, and even more preferably 20% or more. In particular, when the area ratio of retained austenite is 18% or more, the product “TS × uEL” of tensile strength and uniform elongation becomes 15000 MPa ·% or more, and the uniform elongation characteristic can be maintained even at higher strength.

Bainite: 5% or less In the steel sheet according to the present invention, when bainite is present in the metal structure, MA (Martensite-Austenite constituent), which is a hard structure, is inherent in the bainite. The presence of MA reduces uniform elongation properties and toughness. In order not to deteriorate the uniform elongation property and toughness of the steel sheet, the area ratio of bainite is 5% or less, preferably 0%.

It is desirable that the residual structure other than tempered martensite, ferrite, retained austenite, and bainite in the metal structure of the steel plate according to the present invention is fresh martensite (that is, untempered martensite). Bainite may also include tempered bainite, but is not distinguished herein. Further, pearlite is unlikely to be contained, and is substantially 0%.

Fresh martensite has a hard structure and is effective in ensuring the strength of steel sheets. However, the lower the area ratio of fresh martensite, the higher the bendability of the steel sheet. Therefore, the area ratio of fresh martensite is preferably more than 0%, more preferably 1% or more, and even more preferably 3% or more. The area ratio of fresh martensite is preferably 55% or less, more preferably 45% or less, and even more preferably 20% or less.

Further, in the metal structure at a depth of 1/4 of the plate thickness from the surface of the L cross section of the steel sheet according to the present invention, retained austenite crystal grains having an area of 1 μm ² or more and a grain circularity of 0.1 or more. The total area of is less than 50% of the total area of retained austenite.

The area ratio of the retained austenite structure having a crystal grain area of 1 μm ² or more and the grain circularity of the crystal grains of 0.1 or more to the entire structure of the retained austenite is less than 50%. It is possible to obtain an excellent steel plate. When the retained austenite having a large crystal grain area and a large grain circularity occupies 50% or more of the entire structure of the retained austenite, the uniform elongation property and toughness of the steel sheet deteriorate.

Retained austenite with a crystal grain area of less than 1 μm ² , that is, a small crystal grain size, tends to uniformly concentrate Mn in a short time during annealing in the ferrite-austenite two-phase region, and has high stability. The transformation is delayed. Therefore, a steel sheet having excellent uniform elongation characteristics and toughness can be obtained.

In retained austenite, if the area of crystal grains is 1 μm ² or more, that is, even if retained austenite having a large crystal grain size has a grain roundness of less than 0.1, most of the crystal grains are between martensites or between tempered martensites. Since it exists in, the transformation is delayed to the high distortion region side due to space constraints from the surroundings. Therefore, a steel sheet having excellent uniform elongation characteristics and toughness can be obtained.

Further, in the steel material of the present invention, the Mn concentration in the metal structure at a depth of 1/4 of the plate thickness from the surface of the L cross section satisfies the following formula (i).
C _Mnγ / C _Mnα ≧ 1.2 ・ ・ ・ (i)
However, the meanings of the symbols in the above equation (i) are as follows.
C _Mnγ : Average Mn concentration (mass%) in retained austenite
C _Mnα : Average Mn concentration (% by mass) in ferrite and tempered martensite

By heat-treating in a temperature range where the austenite phase fraction is 20 to 50%, Mn can be sufficiently concentrated in the portion where the austenite was. As a result, stable retained austenite can be obtained even after short-time annealing, and excellent uniform elongation characteristics, high strength, excellent toughness, and an appropriate yield ratio can be obtained.

When the lvalue of Eq. (I), which is the Mn concentration ratio between retained austenite and ferrite and tempered martensite, is 1.2 or more, Mn distribution is sufficient and retained austenite can be obtained by annealing for a short time. it can. Therefore, the lvalue in equation (i) is set to 1.2 or more. The lvalue of equation (i) is preferably 1.4 or more. Further, in order to suppress excessive stabilization of austenite and suppress a decrease in the effect of improving uniform elongation characteristics, the lvalue of Eq. (I) is preferably less than 2.0.

The area ratio of the metal structure, the area and grain circularity of the retained austenite crystal grains, and the calculation method of C _Mnγ and C _{Mn α} will be described below.

<Measurement method of area ratio of retained austenite>
The area ratio of retained austenite is measured by X-ray diffraction. First, a test piece having a width of 25 mm (length in the rolling direction), a length of 25 mm (length in the direction perpendicular to rolling), and a thickness in the plate thickness direction of the annealed sample as it is from the center of the main surface of the steel sheet. Cut out. Then, the test piece is chemically polished to reduce the plate thickness by 1/4 to obtain a test piece having a chemically polished surface. On the surface of the test piece, X-ray diffraction analysis with a measurement range of 2θ of 45 to 105 degrees is performed three times using a Co tube.

For the fcc phase, the integrated intensities of the peaks (111), (200), and (220) are obtained, and for the bcc phase, the integrated intensities of the peaks (110), (200), and (211) are obtained. The volume fraction of retained austenite is obtained by analyzing the integrated intensities and averaging the results of three X-ray diffraction analyzes, and the value is used as the area ratio of retained austenite.

<Measurement method of area ratio of tempered martensite, ferrite, bainite, and fresh martensite>
The area ratios of tempered martensite, ferrite, bainite, and fresh martensite are calculated from microstructure observation with a scanning electron microscope (SEM). After mirror polishing the L cross section of the steel sheet, a microstructure is revealed with 3% nital (3% nitric acid-ethanol solution). Then, at a magnification of 5000 times by SEM, a range of 0.1 mm in length (length in the plate thickness direction) × 0.3 mm in width (length in the rolling direction) at a depth position of 1/4 of the plate thickness from the surface of the steel plate. The microstructure can be observed and the area ratio of each tissue can be measured.

For tempered martensite, the area ratio is calculated by judging that among the white structures recognized by SEM observation, those whose substructure is confirmed in the crystal grains are tempered martensite. The area ratio is calculated by discriminating ferrite as a gray base structure. Bainite is a collection of lath-shaped crystal grains when observed by SEM, and is determined as a structure in which carbides extend in the same direction in the lath, and the area ratio is calculated.

Fresh martensite is recognized as a white tissue in the same way as retained austenite when observed by SEM. Therefore, it is difficult to distinguish between retained austenite and fresh martensite by SEM observation, but from the total area ratio of retained austenite and fresh martensite obtained by SEM observation, the retained austenite measured by X-ray diffraction method The area ratio of fresh martensite is calculated by subtracting the area ratio.

<Measurement method of residual austenite crystal grain area and grain roundness>
The grain circularity and crystal grain area are determined by performing backscattered electron diffraction (EBSP) analysis using the standard functions (Map and Grain Shape Circularity) of OIM Analysis 7 manufactured by TSL. Can be measured. Grain shape circularity is calculated by the following formula.
Grain roundness = 4πA / P ²
However, the meanings of the symbols in the above formula are as follows.
A: Area of crystal grains P: Peripheral length of crystal grains

<Measurement method of C _Mnγ and C _Mnα >
C _Mnγ / C _Mnα can be measured by EBSP, SEM, and electron probe microanalyzer (EPMA). EBSP and SEM can be used to identify retained austenite, ferrite, and tempered martensite, and EPMA can be _used to measure C _Mnγ and C _Mnα to calculate C _Mnγ / C _Mnα .

(C) Mechanical Properties Next, the mechanical properties of the steel sheet according to the present invention will be described.

When a steel sheet is used as a material for automobiles, the tensile strength (TS) of the steel sheet according to the present invention is preferably 780 MPa or more, preferably 980 MPa, in order to reduce the plate thickness by increasing the strength and contribute to weight reduction. It is more preferably 1180 MPa or more, and further preferably 1180 MPa or more. Further, in order to use the steel sheet according to the present invention for press forming, it is desirable that the uniform elongation (uEL) is also excellent. The TS × uEL of the steel sheet according to the present invention is preferably 12000 MPa ·% or more, and more preferably 15,000 MPa ·% or more.

The steel sheet according to the present invention also has excellent toughness. The steel sheet according to the present invention preferably has an impact value of 20 J / cm ² or more in the Charpy test at 0 ° C.

The steel sheet according to the present invention has an appropriate yield ratio. The yield ratio YR is the ratio of the yield stress (YS) to the tensile strength (TS) and is an index indicated by YS / TS. When YR is less than 0.80, a high work hardening rate can be obtained, and large energy absorption due to large deformation becomes possible. Further, when the YR is more than 0.40, the amount of impact energy absorbed at the initial stage of deformation can be sufficiently obtained. Therefore, the yield ratio YR of the steel sheet according to the present invention is preferably more than 0.40 and less than 0.80.

(D) Manufacturing Method Next, a manufacturing method for a steel sheet according to the present invention will be described. The steel sheet according to the embodiment of the present invention can be obtained by a manufacturing method including, for example, the following casting step, hot rolling step, cold rolling step, primary annealing step and secondary annealing step. Further, if necessary, a plating step may be further included.

<Casting process>
The steel sheet according to the present invention is prepared by melting steel having the above-mentioned chemical composition by a conventional method and casting it to prepare a steel material (hereinafter, also referred to as “slab”). As long as the steel sheet according to the present invention has the above-mentioned chemical composition, the molten steel may be melted by a normal blast furnace method, and the raw material scraps a large amount of scrap like the steel produced by the electric furnace method. It may include. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

<Hot rolling process>
Hot rolling can be performed using a normal continuous hot rolling line. Hot rolling is preferably carried out in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen.

Slab heating temperature: 1100 to 1300 ° C
The slab to be subjected to the hot rolling step is preferably heated before the hot rolling. By setting the temperature of the slab to be subjected to hot rolling to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the slab to be subjected to hot rolling to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. Therefore, the temperature of the slab to be subjected to hot rolling is preferably 1100 to 1300 ° C. In the present specification, the temperature means the surface temperature of the central portion of the main surface of the slab, the hot-rolled steel sheet, or the cold-rolled steel sheet.

The holding time in the slab heating temperature range is not particularly limited, but in order to improve the bendability, it is preferably 30 min or more, and more preferably 1 h or more. Further, in order to suppress excessive scale loss, it is preferably 10 hours or less, and more preferably 5 hours or less. When direct rolling or direct rolling is performed, the slab may be subjected to hot rolling as it is without being heat-treated.

Finish rolling start temperature: 750-1000 ° C
The finish rolling start temperature is preferably 750 to 1000 ° C. By setting the finish rolling start temperature to 750 ° C. or higher, the deformation resistance during rolling can be reduced. On the other hand, by setting the finish rolling start temperature to 1000 ° C. or lower, deterioration of the surface texture of the steel sheet due to intergranular oxidation can be suppressed.

Winding temperature: Less than 300 ° C After finish rolling, cool and wind at less than 300 ° C. As a result, a tempered martensite phase having an area ratio of 25% or more can be secured. When wound at 300 ° C. or higher, the hot-rolled sheet structure cannot be made into a full martensite structure, and Mn distribution and austenite reverse transformation are efficient in each of the heat treatment process of the hot-rolled steel sheet and the annealing process of the cold-rolled steel sheet. It becomes difficult to wake up.

Heat treatment of hot-rolled steel sheet:
The obtained hot-rolled steel sheet is heat-treated for 60 minutes or more in a temperature range where the austenite phase fraction is 20 to 50%. Mn is distributed to austenite by performing heat treatment in the temperature range where the austenite phase fraction is 20 to 50% in the temperature range of the two-phase region of Ac _{1 and} less than Ac _{3 of the} steel plate to obtain austenite. It stabilizes and contributes to excellent uniform elongation properties, high strength, excellent toughness, and an appropriate yield ratio. On the other hand, if the heat treatment is performed at a temperature where the austenite phase fraction is less than 20% or more than 50%, it becomes difficult to stabilize the austenite phase.

Also, when the heat treatment is performed in less than 60 minutes, it becomes difficult to stabilize the austenite phase. By performing heat treatment for 60 minutes or more at a temperature at which the austenite phase fraction is 20 to 50%, the metal structure at the position 1/4 of the thickness from the surface in the L cross section of the annealed steel sheet has an area ratio of 10% or more. Retained austenite can be included.

The temperature range in which the area ratio of austenite is 20 to 50% is determined by heating at a heating rate of 0.5 ° C / s from room temperature in an offline preliminary experiment, depending on the composition of the steel sheet, and from the volume change during heating, austenite It can be obtained by measuring the phase fraction. The holding time of the heat treatment is preferably 2 hours or more, more preferably 3 hours or more. From the viewpoint of productivity, the heat treatment holding time is preferably 10 hours or less, more preferably 8 hours or less.

Heat treatment is performed in a temperature range where the austenite phase fraction is 20 to 50%, and then cooling is performed. As a result, the Mn distribution state obtained by the heat treatment can be maintained.

<Cold rolling process>
The hot-rolled steel sheet after the heat treatment is pickled by a conventional method and then cold-rolled at a reduction rate of 30 to 70% to obtain a cold-rolled steel sheet. If the reduction ratio of cold rolling is less than 30%, the structure of the steel sheet after annealing cannot be refined, the reverse transformation of austenite is delayed, and retained austenite with a sufficient area ratio cannot be obtained. Further, from the viewpoint of suppressing breakage during cold rolling, the rolling reduction ratio of cold rolling is set to 70% or less.

It is preferable to perform light rolling of about 0% to 5% before or after cold rolling before or after pickling to correct the shape because it is advantageous in terms of ensuring flatness. In addition, light rolling before pickling improves pickling properties, promotes removal of surface-concentrating elements, and has the effect of improving chemical conversion treatment properties and plating treatment properties.

<Primary annealing process>
The cold-rolled steel sheet obtained through the cold rolling step is heated and held in a temperature range of more than 750 ° C. for 10 seconds or more to perform the first annealing. This annealing is referred to as "primary annealing" in the present invention. By primary annealing, the formation of ferrite in the final structure can be reduced to 5% or less in area ratio. This makes it possible to stably secure good uniform elongation characteristics and toughness. If the primary annealing temperature is lower than 750 ° C., ferrite formation in the final structure becomes excessive, and if the temperature is lowered, recrystallization may not proceed sufficiently.

Annealing may be performed in either an annealing furnace or a continuous annealing line, but it is preferable that both the primary annealing and the secondary annealing described later are performed using a continuous annealing line. Productivity can be improved by using a continuous annealing line. Annealing is preferably carried out in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen.

Primary annealing temperature: Over 750 ° C. By setting the primary annealing temperature to over 750 ° C., the distribution of ferrite in the steel sheet after annealing can be made uniform, and uniform elongation characteristics and strength can be improved. The primary annealing temperature is preferably Ac ₃ points or more. Recrystallization can be significantly promoted by setting the primary annealing temperature to Ac ₃ or higher.

Here, the _three Ac points are calculated by the following method. C: More than 0.10% and less than 0.55%, Si: 0.001% or more and less than 3.50%, Mn: More than 4.00% and less than 9.00%, and Al: 0.001% or more and 3.00 As a result of measuring and examining ₃ points of Ac at a heating rate of 0.5 to 50 ° C./s for a plurality of types of cold-rolled steel sheets containing less than%, the following formula is obtained, and ₃ points of Ac are calculated using this formula. be able to.
Ac ₃ = 910-200√C + 44Si-25Mn + 44Al
However, the element symbol in the above formula represents the content (mass%) of each element contained in the steel.

On the other hand, the upper limit of the primary annealing temperature is preferably 950 ° C. or lower. By setting the annealing temperature to 950 ° C. or lower, damage to the annealing furnace can be suppressed and productivity can be improved.

Primary annealing time: 10 s or more In order to increase the martensite structure after cooling the primary annealing and make it a martensite-based structure, it is once held in a temperature range of over 750 ° C for the purpose of making it an austenite-based structure, but the annealing time. Is 10 s or more. If the annealing time is less than 10 s, the effect of the primary annealing may not be sufficiently obtained, and uniform elongation and toughness may decrease. From the viewpoint of productivity, the annealing time is preferably 300 s or less.

Average temperature rise: 5 to 30 ° C / s
The average heating rate from the heating start temperature (room temperature) to the annealing temperature in the primary annealing is preferably 5 to 30 ° C./s. By setting the heating rate in the primary annealing within this range, the area ratio of ferrite in the metallographic structure can be further reduced.

Final cooling temperature: less than 100 ° C. In the cooling after the primary annealing, the temperature is cooled from the primary annealing temperature to less than 100 ° C. The rasmartensite structure can be increased by lowering the final cooling temperature to less than 100 ° C. From the viewpoint of ensuring safety during transportation of the steel sheet, the final cooling temperature is preferably room temperature (50 ° C. or lower).

Further, it is preferable to cool at an average cooling rate of 2 to 2000 ° C./s from the primary annealing temperature to a temperature range of 500 ° C. or lower. By setting the average cooling rate after annealing to 2 ° C./s or higher, grain boundary segregation can be suppressed and bendability can be improved. On the other hand, by setting the average cooling rate to 2000 ° C./s or less, the temperature distribution of the steel sheet after the cooling is stopped becomes uniform, so that the flatness of the steel sheet can be further improved.

Preferably, the cooling stop temperature in cooling at an average cooling rate of 2 to 2000 ° C./s is 100 ° C. or higher. By setting the cooling stop temperature to 100 ° C. or higher, strain generation due to martensitic transformation can be suppressed, and the flatness of the steel sheet can be improved.

Further, after cooling the temperature range from the primary annealing temperature to 500 ° C. or lower at an average cooling rate of 2 to 2000 ° C./s, the temperature range is preferably maintained in the temperature range of 100 to 500 ° C. for 10 to 1000 s. By setting the holding time in the temperature range of 100 to 500 ° C. to 10 s or more, C distribution to austenite proceeds sufficiently and austenite can be increased in the structure before the final heat treatment, and as a result, after the final heat treatment. It is possible to suppress the formation of austenite in the structure of the tissue and further reduce the fluctuation of the strength characteristics. On the other hand, even if the holding time exceeds 1000 s, the effect of the above action is saturated and the productivity is only lowered. Therefore, the holding time in the temperature range of 100 to 500 ° C. is preferably 1000 s or less. It is preferably 300 s or less.

By setting the holding temperature to 100 ° C. or higher, the efficiency of the continuous annealing line can be improved. On the other hand, by setting the holding temperature to 500 ° C. or lower, grain boundary segregation can be suppressed and bendability can be improved.

<Secondary annealing process>
After performing the above primary annealing and cooling to room temperature, the mixture is heated to a temperature range of 600 ° C. or higher and less than ₃ points of Ac at an average heating rate of 1 to 40 ° C./s, and held at that heating temperature for 5 seconds or longer. Perform the second annealing. This annealing is referred to as "secondary annealing" in the present invention.

Secondary annealing temperature: 600 ° C. or higher and less than ₃ points Ac By setting the secondary annealing temperature to 600 ° C. or higher and less than ₃ points, the area ratio of ferrite can be reduced and the uniform elongation characteristics and toughness can be improved. When the secondary annealing temperature is set to Ac ₃ or higher, it becomes difficult to secure retained austenite in the subsequent cooling process. Further, since the Mn content is high and the martensitic transformation temperature is low, it is difficult to secure sufficient tempered martensite when the secondary annealing temperature is set to Ac ₃ or higher.

Secondary annealing time: From the viewpoint of dissolving cementite for 5 s or more and stably ensuring good toughness, the annealing time for holding in a temperature range of 600 ° C. or more and less than ₃ points of Ac is set to 5 s or more. From the viewpoint of productivity, the secondary annealing time is preferably 300 s or less.

Average heating rate: 1-40 ° C / s
The average rate of temperature rise in the secondary annealing is 1 ° C./s or higher, preferably 2 ° C./s or higher, and more preferably 3 ° C./s or higher. By raising the temperature at such an average heating rate, the area ratio of the ferrite phase can be reduced to 5% or less.

The average rate of temperature rise in the secondary annealing is less than 40 ° C./s, preferably less than 20 ° C./s, more preferably less than 10 ° C./s. By raising the temperature at such an average heating rate, the formation of coarse massive austenite, that is, retained austenite having an area of 1 μm ² or more and a grain circularity of 0.1 or more is suppressed, and the entire retained austenite is suppressed. The area ratio of coarse massive austenite to area can be less than 50%. If the rate of temperature rise is too fast, the driving force for austenite formation will increase, and austenite will be generated from the former austenite grain boundaries instead of martensite truss, resulting in an increase in coarse austenite.

Average cooling rate: 5 ° C./s or higher After secondary annealing, the steel sheet is cooled to 100 ° C. or lower at an average cooling rate of 5 ° C./s or higher. If the average cooling rate is less than 5 ° C./s, soft bainite is excessively generated, and it may be difficult to secure high strength (tensile strength of 980 MPa or more) in the heat-treated steel material. Preferably, the average cooling rate is set to 500 ° C./s or less from the viewpoint of suppressing shrinkage of the steel sheet.

<Plating process>
When plating a steel sheet, it is manufactured as follows.

When hot-dip galvanized steel sheet is manufactured by hot-dip galvanizing the surface of the steel sheet, it is cooled to a temperature range of 430 to 500 ° C. at an average cooling rate of 5 ° C./s or more after secondary annealing, and then cold-rolled. The steel sheet is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing. The conditions of the plating bath may be within the normal range. After the plating treatment, the product is cooled to a temperature range of 100 ° C. or lower at an average cooling rate of 5 ° C./s or higher.

When the surface of a steel sheet is hot-dip galvanized to produce an alloyed hot-dip galvanized steel sheet, the temperature is 450 to 620 ° C. after the steel sheet is hot-dip galvanized and before the steel sheet is cooled to room temperature. The hot dip galvanizing process is alloyed at temperature. The alloying treatment conditions may be within the usual range. After the alloying treatment, the mixture is cooled to a temperature range of 100 ° C. or lower at an average cooling rate of 5 ° C./s or higher.

Skin pass rolling may be performed on the annealed steel sheet or the plated steel sheet. When skin pass rolling is performed, the rolling reduction of the skin pass rolling is preferably more than 0% and less than 5.0%. When hot-dip galvanizing or alloying hot-dip galvanizing is applied to the surface of the steel sheet, skin pass rolling is performed on the plated steel sheet.

By manufacturing the steel sheet as described above, the steel sheet according to the present invention can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

1. 1. Production of Steel Sheet for Evaluation The steel having the chemical components shown in Table 1 was melted in a converter and continuously cast to obtain a slab having a thickness of 245 mm.

The obtained steel material (slab) was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of about 2.4 mm. The obtained hot-rolled steel sheet was subjected to heat treatment, pickling, and cold rolling at the cold rolling ratio shown in Table 2 at a temperature and holding time at which the austenite phase fraction was shown in Table 2, and the thickness was 1.4 mm. Cold-rolled steel sheet was obtained. The hot rolling and heat treatment of the hot-rolled steel sheet were carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen.

The obtained cold-rolled steel sheet was subjected to primary annealing and secondary annealing under the conditions shown in Table 3 to prepare an annealed cold-rolled steel sheet. The two annealings on the cold-rolled steel sheet were carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen. The average heating rate from the heating start temperature (room temperature) to the annealing temperature in the primary annealing was 15 ° C./s. In the secondary annealing, the temperature was cooled from the annealing temperature to 100 ° C. or lower at an average cooling rate of 50 ° C./s.

For some examples of annealed cold-rolled steel sheets, cooling after secondary annealing was stopped at 460 ° C., and the cold-rolled steel sheets were immersed in a hot-dip galvanizing bath at 460 ° C. for 2 seconds to perform hot-dip galvanizing treatment. The conditions of the plating bath were the same as those of the conventional one. When the alloying treatment described later was not performed, the mixture was cooled to room temperature at an average cooling rate of 10 ° C./s after holding at 460 ° C.

For some examples of annealed cold-rolled steel sheets, after hot-dip galvanizing treatment, they were subsequently alloyed without being cooled to room temperature. It was heated to 520 ° C. and held at 520 ° C. for 5 s for alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./s.

The annealed cold-rolled steel sheet thus obtained was temper-rolled at an elongation rate of 0.1% to prepare various evaluation steel sheets.

2. 2. Evaluation method The areas of tempered martensite, ferrite, retained austenite, bainite, and fresh martensite were subjected to microstructure observation, tensile test, uniform elongation test, and toughness test on the annealed cold-rolled steel sheets obtained in each example. Rate, grain roundness and area of retained austenite grains, C _Mnγ / C _Mnα , and tensile strength, uniform elongation properties, toughness, and yield ratio were evaluated. The method of each evaluation is as follows.

<Area ratio of metal structure>
The area ratios of tempered martensite, ferrite, retained austenite, bainite, and fresh martensite were calculated from microstructure observation by SEM and X-ray diffraction measurement. The L cross section of the steel plate was mirror-polished, then the microstructure was revealed by 3% nital, and the microstructure was observed at a magnification of 5000 times by SEM at a position 1/4 from the surface, and 0.1 mm × 0. Image analysis (Photoshop®) over a range of 3 mm calculated the area ratios of tempered martensite, ferrite, and bainite, as well as the total area ratio of retained austenite and fresh martensite.

Further, a test piece having a width of 25 mm and a length of 25 mm was cut out from the obtained steel sheet, and the test piece was chemically polished to reduce the thickness by 1/4, and the surface of the test piece after the chemical polishing was subjected to chemical polishing. , X-ray diffraction analysis using Co tubes was performed three times, the obtained profiles were analyzed, and the area ratio of retained austenite was calculated by averaging each of them. The area ratio of fresh martensite was calculated by subtracting the area ratio of retained austenite from the total area ratio of retained austenite and fresh martensite obtained by SEM observation.

<Granularity and area of retained austenite crystal grains>
The grain circularity and area of the crystal grains are measured by performing backscattered electron diffraction (EBSP: Electron Back Scattering pattern) analysis using the standard functions (Map and Grain Shape Circularity) of OIM Analysis 7 manufactured by TSL. did.

The EBSP data measurement conditions are as follows. An SEM equipped with an OIM (Orientation Imaging Microscopy) detector at a position 1/4 of the thickness from the surface of the L cross section of the steel sheet, observes a region of 50 μm × 50 μm at a magnification of 500 times, and EBSP at a measurement interval of 0.1 μm. The data was measured. The EBSP data was measured for the five regions by the above method, and the average value was calculated.

<C _Mnγ / C _Mnα >
C _Mnγ / C _Mnα was measured by EBSP, SEM, and EPMA. Using EBSP and SEM, a region of 50 μm × 50 μm was observed at a magnification of 500, EBSP data was measured at a measurement interval of 0.1 μm, and retained austenite, ferrite, and tempered martensite were identified for the five regions. Next, for the identified austenite, ferrite and tempered martensite, point analysis by EPMA measurement was performed at 5 points and 5 regions, respectively, and the measured values were averaged to calculate C _Mnγ and C _Mnα , and C _Mnγ / C. _Mnα was determined.

<Tensile test method>
Take JIS No. 5 tensile test pieces from the direction perpendicular to the rolling direction of the steel sheet, measure the tensile strength (TS), uniform elongation (uEL), and yield stress (YS), and determine the TS × uEL and yield ratio (YR). Calculated. The tensile test was carried out using a JIS No. 5 tensile test piece by the method specified in JIS Z 2241: 2011. The uniform elongation test was carried out by the method specified in JIS Z 2241: 2011 using a JIS No. 5 test piece having a parallel portion length of 60 mm and a reference point distance of 50 mm as a reference for measuring strain. Uniform elongation is the elongation (strain measured between gauge points) obtained before reaching the maximum test strength (TS).

<Toughness test method>
Each steel material after the heat treatment was ground on the front and back surfaces so as to have a thickness of 1.2 mm for lamination to prepare a V-notch test piece. After stacking four of the test pieces and screwing them together, they were subjected to a Charpy impact test according to JIS Z 2242: 2005. The toughness was good when the impact value at 0 ° C. was 20 J / cm ² or more, and poor when it was less than 20 J / cm ² .

3. 3. Evaluation Results Table 4 shows the results of the above evaluation. An example in which TS × uEL of 12000 MPa ·% or more, yield ratio of more than 0.40 and less than 0.80, and good toughness were obtained, a steel sheet having high impact energy absorption capacity, excellent uniform elongation characteristics, and high strength. Evaluated as. In Table 4, it was decided that the impact energy absorption capacity was excellent when the yield ratio was more than 0.40 and less than 0.80 and the toughness was good.

As described above, the steel sheet according to the present invention has high strength, good uniform elongation characteristics, excellent moldability, and high impact energy absorption capacity (excellent YR and toughness). Ideal for structural parts of automobiles such as side members.

Claims (9)

1. The chemical composition of the steel sheet is mass%,
   C: More than 0.10% and less than 0.55%,
   Si: 0.001% or more and less than 3.50%,
   Mn: More than 4.00% and less than 9.00%,
   sol. Al: 0.001% or more and less than 3.00%,
   P: 0.100% or less,
   S: 0.010% or less,
   N: Less than 0.050%,
   O: Less than 0.020%,
   Cr: 0% or more and less than 2.00%,
   Mo: 0-2.00%,
   W: 0 to 2.00%,
   Cu: 0-2.00%,
   Ni: 0 to 2.00%,
   Ti: 0 to 0.300%,
   Nb: 0 to 0.300%,
   V: 0 to 0.300%,
   B: 0 to 0.010%,
   Ca: 0 to 0.010%,
   Mg: 0 to 0.010%,
   Zr: 0 to 0.010%,
   REM: 0-0.010%,
   Sb: 0 to 0.050%,
   Sn: 0 to 0.050%,
   Bi: 0 to 0.050%,
   Remaining: Fe and impurities,
   In the cross section parallel to the rolling direction and the plate thickness direction of the steel plate, the metal structure at a depth of 1/4 of the plate thickness from the surface is, in% area.
   Tempering martensite: 25-90%,
   Ferrite: 5% or less,
   Residual austenite: 10-50%, and bainite: 5% or less,
   Residual austenite crystal grains having an area of 1 μm ² or more and a grain circularity of 0.1 or more at a depth of 1/4 of the plate thickness from the surface of the cross section parallel to the rolling direction and the plate thickness direction of the steel sheet. The ratio of the total area of is less than 50% of the total area of the retained austenite.
   Satisfy the following equation (i),
   Steel plate.
   C _Mnγ / C _Mnα ≧ 1.2 ・ ・ ・ (i)
   However, the meanings of the symbols in the above equation (i) are as follows.
   C _Mnγ : Average Mn concentration (mass%) in retained austenite
   C _Mnα : Average Mn concentration (% by mass) in ferrite and tempered martensite
2. When the chemical composition is mass%,
   Cr: 0.01% or more and less than 2.00%,
   Mo: 0.01-2.00%,
   W: 0.01-2.00%,
   Cu: 0.01 to 2.00%, and Ni: 0.01 to 2.00%
   Contains one or more selected from
   The steel plate according to claim 1.
3. When the chemical composition is mass%,
   Ti: 0.005 to 0.300%,
   Nb: 0.005 to 0.300%, and V: 0.005 to 0.300%
   Contains one or more selected from
   The steel sheet according to claim 1 or 2.
4. When the chemical composition is mass%,
   B: 0.0001 to 0.010%,
   Ca: 0.0001 to 0.010%,
   Mg: 0.0001 to 0.010%,
   Zr: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%
   Contains one or more selected from
   The steel plate according to any one of claims 1 to 3.
5. When the chemical composition is mass%,
   Sb: 0.0005 to 0.050%,
   Sn: 0.0005 to 0.050%, and Bi: 0.0005 to 0.050%
   Contains one or more selected from
   The steel plate according to any one of claims 1 to 4.
6. A hot-dip galvanized layer is provided on the surface of the steel sheet.
   The steel plate according to any one of claims 1 to 5.
7. Having an alloyed hot-dip galvanized layer on the surface of the steel sheet.
   The steel plate according to any one of claims 1 to 5.
8. The Charpy impact value at 0 ° C. is 20 J / cm ² or more.
   The steel plate according to any one of claims 1 to 7.
9. The yield ratio of the steel sheet is more than 0.40 and less than 0.80.
   The steel plate according to any one of claims 1 to 8.