Specification
Title of invention : Steel plate and plated steel plate
Technical field
[0001]
The present invention relates to steel sheets and plated steel sheets. More specifically, the present invention relates to steel sheets and plated steel sheets having high strength and excellent elongation and bending workability, which are suitable as materials for use in applications such as automobiles, home appliances, mechanical structures, and construction.
This application claims priority based on Japanese Patent Application No. 2019-229403 filed in Japan on December 19, 2019, the contents of which are incorporated herein.
Background technology
[0002]
In recent years, from the perspective of global environmental protection, we have been working to reduce carbon dioxide emissions in many fields. Automobile manufacturers are actively developing technologies to reduce the weight of automobile bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of the car body because the emphasis is placed on improving crash resistance to ensure passenger safety. Therefore, in order to achieve both weight reduction of the vehicle body and collision resistance, thinning of members using high-strength steel sheets is being studied. For this reason, steel sheets having both high strength and excellent formability are strongly desired. Specifically, steel sheets used for inner plate members, structural members, suspension members, etc. of automobiles are often required to have high strength, elongation, and bending workability because bending is frequently used.
[0003]
As a steel sheet with excellent elongation, a dual phase steel sheet (hereinafter referred to as DP steel) composed of a composite structure of a soft ferrite phase and a hard martensite phase is known (for example, Patent Document 1). Although the DP steel sheet is excellent in elongation, voids may occur at the interface between the ferrite phase and the martensite phase, which are significantly different in hardness, and cracks may occur, so that the bending workability may be inferior in some cases.
[0004]
Patent Document 2 discloses a steel structure obtained by setting the cooling rate in the temperature range from the solidification of the slab to 1300 ° C. to 10 to 300 ° C./min and winding at 500 ° C. or higher and 700 ° C. or lower after finish rolling. has been proposed a high-strength hot-rolled steel sheet having a ferrite single phase and a tensile strength of 1180 MPa or more, and Patent Document 2 discloses that the high-strength hot-rolled steel sheet improves bending workability. However, the hot-rolled steel sheet described in Patent Document 2 is reheated without cooling the slab to below 900 ° C. where the ferrite phase begins to form, and is subjected to hot rolling, so the segregation formed during solidification is There was a problem that the bending workability was not stabilized because it was not sufficiently reduced.
[0005]
In Patent Document 3, by completing hot rolling within 5 hours after continuous casting, Ti exceeding the solubility is dissolved in γ, and fine TiC is formed along with ferrite transformation during coiling at 550 ° C. or higher and 700 ° C. or lower. A method for producing a steel sheet having a ferrite area fraction of 80% or more and a tensile strength of 980 MPa or more and the steel sheet have been proposed. However, in Patent Document 3 as well, continuous casting to the completion of hot finish rolling are performed in the austenite region in order to suppress the precipitation of coarse TiC, so bending workability may deteriorate due to Mn segregation.
prior art documents
patent literature
[0006]
Patent Document 1: Japanese Patent Laid-Open No. 6-128688
Patent Document 2: Japanese Patent Application Laid-Open No. 2014-194053
Patent Document 3: Japanese Patent Application Laid-Open No. 2014-208876
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0007]
The present invention has been made in view of the above-mentioned problems, and an object thereof is to provide a steel sheet and a plated steel sheet having high strength and excellent elongation and bending workability.
Means to solve problems
[0008]
By optimizing the chemical composition and manufacturing conditions of the steel sheet, the present inventors controlled the metallographic structure and Mn segregation of the steel sheet, thereby achieving high strength and excellent elongation and bending workability. was found to be able to manufacture
[0009]
The present invention was made based on the above findings, and the gist thereof is as follows.
[0010]
[1] A steel sheet according to an aspect of the present invention has a chemical composition, in mass%,
C: 0.05-0.20%,
Si: 0.005 to 2.00%,
Mn: 0.50 to 4.00%,
P: 0.100% or less,
S: 0.0100% or less,
sol. Al: 0.001 to 1.00%,
Ti: 0.15 to 0.40%,
N: 0.0010 to 0.0100%,
Nb: 0 to 0.100%,
　V: 0 to 1.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 2.00%,
B: 0 to 0.0020%,
Ca: 0 to 0.0100%,
　Mg: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0 to 0.0200%
and the balance consists of Fe and impurities,
The metal structure at a depth of 1/4 of the sheet thickness from the surface contains, in area fraction, 90% or more ferrite and less than 3% retained austenite, and the average grain size excluding the retained austenite is 10.0 μm or less. and the average aspect ratio of the crystal grains excluding the retained austenite is 0.3 or more, and the standard deviation of the Mn concentration is 0.60% by mass or less,
　The tensile strength is 980 MPa or more.
[2] In the steel sheet according to [1], the chemical composition is, in mass%,
Nb: 0.001 to 0.100%,
V: 0.005 to 1.00%,
Mo: 0.001 to 1.00%,
Cu: 0.02 to 1.00%,
Ni: 0.02 to 1.00%,
Cr: 0.02 to 2.00%,
B: 0.0001 to 0.0020%,
Ca: 0.0002 to 0.0100%,
　Mg: 0.0002 to 0.0100%,
REM: 0.0002 to 0.0100%, and
Bi: 0.0001 to 0.0200%
It may contain one or more selected from the group consisting of.
[0011]
[3] A plated steel sheet according to another aspect of the present invention has a plated layer formed on the surface of the steel sheet according to [1] or [2].
[4] In the plated steel sheet described in [3], the plated layer may be a hot-dip galvanized layer.
[5] In the plated steel sheet according to [4], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.
Effect of the invention
[0012]
According to the aspect of the present invention, it is possible to provide a steel sheet and a plated steel sheet having high strength and excellent elongation and bending workability. If the steel sheet or plated steel sheet according to the present invention is used as a material for parts such as inner plate members, structural members, and suspension parts of automobiles, it can be easily processed into a shape of parts, and the contribution to industry is extremely remarkable. be.
MODE FOR CARRYING OUT THE INVENTION
[0013]
The steel sheet and plated steel sheet according to this embodiment will be described in detail below. First, the chemical composition of the steel sheet according to this embodiment will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention.
The numerical limits described below include the lower and upper limits. Any numerical value indicated as "less than" or "greater than" excludes that value from the numerical range. In the following description, all percentages relating to the chemical composition of steel are percentages by mass.
[0014]

(C: 0.05-0.20%)
C increases the tensile strength of steel by combining with Ti and the like to form carbides. If the C content is less than 0.05%, it becomes difficult to obtain a tensile strength of 980 MPa or more. Therefore, the C content should be 0.05% or more. The C content is preferably 0.07% or more, 0.08% or more, or 0.10% or more. On the other hand, if the C content exceeds 0.20%, coarse carbides are formed and the bending workability of the steel sheet is lowered. In addition, weldability is remarkably deteriorated. Therefore, the C content should be 0.20% or less. The C content is preferably 0.15% or less or 0.14% or less, more preferably 0.13% or less.
[0015]
(Si: 0.005-2.00%)
Si has the effect of increasing the tensile strength of steel by increasing solid solution strengthening and hardenability. Si also has the effect of suppressing precipitation of cementite. If the Si content is less than 0.005%, it becomes difficult to exhibit the above effects. Therefore, the Si content should be 0.005% or more. The Si content is preferably 0.01% or more, 0.03% or more, or 0.10% or more. On the other hand, if the Si content exceeds 2.00%, the surface properties of the steel sheet are significantly deteriorated due to surface oxidation during the hot rolling process. Therefore, the Si content should be 2.00% or less. The Si content is preferably 1.60% or less or 1.50% or less, more preferably 1.30% or less.
[0016]
(Mn: 0.50-4.00%)
　Mn has the effect of increasing the tensile strength of steel by increasing solid-solution strengthening and hardenability. If the Mn content is less than 0.50%, ferrite transformation is excessively accelerated, and carbides such as Ti are coarsely precipitated together with ferrite transformation at high temperatures, making it difficult to obtain a steel sheet with a tensile strength of 980 MPa or more. Therefore, the Mn content should be 0.50% or more. The Mn content is preferably 0.70% or more or 0.80% or more, more preferably 1.00% or more. On the other hand, if the Mn content exceeds 4.00%, high-concentration Mn segregation occurs and the standard deviation of the Mn concentration increases, resulting in deterioration of bendability. Therefore, the Mn content should be 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less, even more preferably 3.30% or less or 3.00% or less.
[0017]
(Ti: 0.15-0.40%)
Ti combines with C to form carbides and increases the tensile strength of the steel sheet through fine precipitation. In addition, Ti has the effect of suppressing the coarsening of austenite grains by Ti nitrides and refining the metal structure. If the Ti content is less than 0.15%, it becomes difficult to obtain a tensile strength of 980 MPa or more. Therefore, the Ti content should be 0.15% or more. The Ti content is preferably 0.17% or more, more preferably 0.19% or more, and most preferably 0.21% or more. On the other hand, when Ti is contained excessively, coarse nitrides and carbides are formed, which deteriorates elongation and bending workability. Therefore, the Ti content should be 0.40% or less. The Ti content is preferably 0.38% or less, 0.35% or less, or 0.30% or less.
[0018]
(sol. Al: 0.001 to 1.00%)
Al has the effect of deoxidizing the steel during the steelmaking stage to purify the steel (suppressing the occurrence of defects such as blowholes in the steel) and promoting ferrite transformation. sol. If the Al content is less than 0.001%, it becomes difficult to exhibit the above effects. Therefore, sol. Al content shall be 0.001% or more. sol. The Al content is preferably 0.01% or more, more preferably 0.02% or more or 0.03% or more. On the other hand, sol. Even if the Al content exceeds 1.00%, the effect of the above action is saturated and the refining cost increases. Therefore, sol. Al content is 1.00% or less. sol. The Al content is preferably 0.80% or less, more preferably 0.60% or less or 0.10% or less. In addition, sol. Al means acid-soluble Al.
[0019]
(N: 0.0010-0.0100%)
N has the effect of forming Ti nitrides, suppressing coarsening of austenite during slab reheating and hot rolling, and refining the metal structure. If the N content is less than 0.0010%, it becomes difficult to exhibit the above effects. Therefore, the N content should be 0.0010% or more. The N content is preferably 0.0015% or more, more preferably 0.0020% or more or 0.0030% or more. On the other hand, if the N content exceeds 0.0100%, coarse Ti nitrides are formed, degrading the stretch flangeability of the steel sheet. Therefore, the N content should be 0.0100% or less. The N content is preferably 0.0060% or less, 0.0050% or less, or 0.0045% or less.
[0020]
(P: 0.100% or less)
　P is contained in steel as an impurityIt is an element that reduces the bending workability of steel sheets. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.060% or less, more preferably 0.040% or less, and even more preferably 0.020% or less. Although P is mixed as an impurity from raw materials, there is no particular need to limit its lower limit, and from the viewpoint of ensuring bending workability, the lower the P content is, the better. However, excessively reducing the P content increases the manufacturing cost. From the viewpoint of production cost, the P content is preferably 0.001% or more or 0.003% or more, more preferably 0.005% or more.
[0021]
(S: 0.0100% or less)
S is an element contained as an impurity and has the effect of reducing the bending workability of the steel sheet. Therefore, the S content should be 0.0100% or less. The S content is preferably 0.0080% or less, more preferably 0.0060% or less, and even more preferably 0.0030% or less. Although S is mixed in as an impurity from the raw material, there is no particular need to limit its lower limit, and from the viewpoint of ensuring bending workability, the lower the S content, the better. However, excessively reducing the S content increases the manufacturing cost. From the viewpoint of production cost, the S content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.0010% or more.
[0022]
The remainder of the chemical composition of the steel sheet according to this embodiment consists of Fe and impurities. In the present embodiment, the term "impurities" refers to ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment or the like, and are permitted within a range that does not adversely affect the steel sheet according to the present embodiment.
The steel sheet according to the present embodiment may contain the following arbitrary elements instead of part of Fe. Since the steel sheet according to the present embodiment can solve the problem even if the optional element is not contained, the lower limit of the content when the optional element is not contained is 0%.
[0023]
(Nb: 0-0.100%)
　Nb is an arbitrary element. Nb has the effect of suppressing coarsening of the grain size of the steel sheet, refining the grain size of ferrite, and increasing the tensile strength of the steel sheet by strengthening the precipitation of NbC. To obtain these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more. On the other hand, when the Nb content exceeds 0.100%, the above effects are saturated and the rolling load during finish rolling may increase. Therefore, when Nb is contained, the Nb content should be 0.100% or less. The Nb content is preferably 0.070% or less or 0.060% or less, more preferably 0.030% or less.
[0024]
(V: 0-1.00%)
　V is an arbitrary element. V forms a solid solution in steel to increase the tensile strength of the steel sheet, and also precipitates in the steel as carbides, nitrides, carbonitrides, etc., and has the effect of increasing the tensile strength of the steel sheet by precipitation strengthening. To obtain these effects, the V content is preferably 0.005% or more. The V content is more preferably 0.01% or more or 0.05% or more. On the other hand, if the V content exceeds 1.00%, the carbide tends to coarsen, which may lead to deterioration in bending workability. Therefore, when V is contained, the V content is set to 1.00% or less. The V content is more preferably 0.80% or less, more preferably 0.60% or less or 0.30% or less.
[0025]
(Mo: 0-1.00%)
Mo is an arbitrary element. Mo has the effect of increasing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel sheet. To obtain these effects, the Mo content is preferably 0.001% or more. Mo content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Mo content exceeds 1.00%, the cracking sensitivity of the slab may increase. Therefore, when Mo is contained, the content of Mo should be 1.00% or less. Mo content is more preferably 0.80% or less, more preferably 0.60% or less or 0.30% or less.
[0026]
(Cu: 0-1.00%)
　Cu is an arbitrary element. Cu has the effect of improving the toughness of steel and the effect of increasing tensile strength. To obtain these effects, the Cu content is preferably 0.02% or more. The Cu content is more preferably 0.04% or more or 0.08% or more. On the other hand, if Cu is contained excessively, the weldability of the steel sheet may deteriorate. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is more preferably 0.50% or less, even more preferably 0.30% or less or 0.10% or less.
[0027]
(Ni: 0-1.00%)
Ni is an arbitrary element. Ni has the effect of improving the toughness of steel and the effect of increasing tensile strength. To obtain these effects, the Ni content is preferably 0.02% or more. The Ni content is more preferably 0.10% or more or 0.15% or more. On the other hand, if Ni is contained excessively, the alloy cost increases, and the toughness of the weld heat affected zone of the steel sheet may deteriorate. Therefore, when Ni is contained, the Ni content is set to 1.00% or less. The Ni content is more preferably 0.50% or less, even more preferably 0.30% or less or 0.10% or less.
[0028]
(Cr: 0-2.00%)
　Cr is an arbitrary element. Cr has the effect of increasing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel sheet. To obtain this effect, the Cr content is preferably 0.02% or more. The Cr content is more preferably 0.05% or more or 0.10% or more. On the other hand, if Cr is contained excessively, the chemical convertibility deteriorates. Therefore, when Cr is contained, the Cr content is set to 2.00% or less. The Cr content is more preferably 1.50% or less, still more preferably 1.00% or less, and particularly preferably 0.50% or less.
[0029]
(B: 0-0.0020%)
B is an arbitrary element. B has the effect of increasing the tensile strength of the steel sheet by intergranular strengthening and solid-solution strengthening. To obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more or 0.0005% or more. On the other hand, even if the content of B exceeds 0.0020%, the above effect is saturated and the alloy cost increases. Therefore, when B is contained, the B content is set to 0.0020% or less. The B content is more preferably 0.0015% or less, still more preferably 0.0013% or less or 0.0010% or less.
[0030]
(Ca: 0-0.0100%)
　Ca is an arbitrary element. Ca has the effect of dispersing a large number of fine oxides in molten steel and refining the metal structure of the steel sheet. In addition, Ca has the effect of improving the stretch flangeability of the steel sheet by fixing S in the molten steel as spherical CaS and suppressing the formation of elongated inclusions such as MnS. To obtain these effects, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, when the Ca content exceeds 0.0100%, the amount of CaO in the steel increases, which may adversely affect the toughness of the steel sheet. Therefore, when Ca is contained, the Ca content shall be 0.0100% or less. The Ca content is more preferably 0.0050% or less, still more preferably 0.0030% or less or 0.0020% or less.
[0031]
(Mg: 0-0.0100%)
　Mg is an arbitrary element. Like Ca, Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and has the effect of refining the metal structure of the steel sheet. To obtain these effects, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, when the Mg content exceeds 0.0100%, oxides in the steel increase and adversely affect the toughness of the steel sheet. Therefore, when Mg is contained, the Mg content shall be 0.0100% or less. The Mg content is more preferably 0.0050% or less, still more preferably 0.0030% or less or 0.0025% or less.
[0032]
(REM: 0-0.0100%)
REM is an arbitrary element. Like Ca, REM also forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and has the effect of refining the metal structure of the steel sheet. To obtain these effects, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the REM content exceeds 0.0100%, oxides in the steel increase, which may adversely affect the toughness of the steel sheet. Therefore, when REM is contained, the REM content is preferably 0.0100% or less. The REM content is more preferably 0.0050% or less, even more preferably 0.0030% or less or 0.0020% or less.
Here, REM (rare earth elements) refers to a total of 17 elements consisting of Sc, Y and lanthanides. In this embodiment, the content of REM refers to the total content of these elements.
[0033]
(Bi: 0-0.0200%)
Bi is an arbitrary element. Bi has the effect of refining the solidification structure and improving the formability of the steel sheet. To obtain this effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, when the Bi content exceeds 0.0200%, the above effect is saturated and the alloy cost increases. Therefore, when Bi is contained, the Bi content should be 0.0200% or less. It is more preferably 0.0100% or less, and even more preferably 0.0070% or less or 0.0030% or less.
[0034]
Next, the metal structure of the steel plate will be explained. In the steel sheet according to the present embodiment, the metal structure at a depth of 1/4 of the plate thickness from the surface contains 90% or more ferrite and less than 3% retained austenite in area fraction, and the average crystal excluding retained austenite. The grain size is 10.0 μm or less, the average aspect ratio of crystal grains excluding retained austenite is 0.3 or more, and the standard deviation of Mn concentration is 0.60% by mass or less. Here, the reason why the metallographic structure at the depth position of 1/4 of the plate thickness from the surface of the steel plate is specified is that the metallographic structure at this position is a representative metallographic structure of the steel plate.
In addition to ferrite and retained austenite, cementite, pearlite, bainite, and martensite are allowed as metal structures.
[0035]
(Area fraction of ferrite: 90% or more)
A ferrite phase is necessary to obtain good elongation and bending workability. If the area fraction of ferrite is less than 90%, cracks may occur early from the phase interface with hard phases other than ferrite (cementite, pearlite, bainite, martensite, retained austenite, etc.), and the hard phases may fracture early. elongation and bending workability are lowered. Therefore, the area fraction of ferrite is set to 90% or more. The area fraction of ferrite is preferably 95% or more or 98% or more, and may be 100% (ie single phase of ferrite).
[0036]
(Area fraction of retained austenite: less than 3%)
Among the hard phases other than ferrite, retained austenite significantly deteriorates the bending workability of steel sheets by transforming into extremely hard martensite during working. Therefore, the area fraction of retained austenite is set to less than 3%. The area fraction of retained austenite is preferably 2% or less, more preferably 1% or less, and may be 0%.
[0037]
(remainingAverage crystal grain size excluding stenite: 10.0 μm or less)
If the average crystal grain size excluding retained austenite is large (that is, the crystal grains are coarse), bending workability decreases, so the average crystal grain size excluding retained austenite is set to 10.0 μm or less. The average grain size excluding retained austenite is preferably 9.0 μm or less, 8.5 μm or less, or 8.0 μm or less. The lower limit is not particularly limited because the smaller the average crystal grain size excluding retained austenite, the better. However, in normal hot rolling, it is technically difficult to refine grains such that the average grain size excluding retained austenite is less than 1.0 μm, so the average grain size excluding retained austenite is 1.0 μm or more. , 2.0 μm or more, or 4.0 μm or more.
In the present embodiment, the “average grain size (excluding retained austenite)” refers to those having a crystal structure of bcc, that is, surrounded by grain boundaries with a crystal orientation difference of 15 ° or more in ferrite, bainite, martensite and pearlite. , and an average grain size defined as a region having an equivalent circle diameter of 0.3 μm or more as a crystal grain, and the grain size of retained austenite is not included in the average grain size.
[0038]
(Average aspect ratio of crystal grains excluding retained austenite: 0.3 or more)
In the present embodiment, the average aspect ratio of crystal grains excluding retained austenite is 0.3 or more. The aspect ratio is a value obtained by dividing the length of the minor axis of a crystal grain by the length of the major axis, and takes a value of 0 to 1.0. The smaller the average aspect ratio of the crystal grains excluding retained austenite, the flatter the crystal grains, and the closer to 1.0, the equiaxed grains. If the average aspect ratio of the crystal grains excluding retained austenite is less than 0.3, there are many flat crystal grains, the anisotropy of the material increases, and the bending workability deteriorates. Therefore, the average aspect ratio of crystal grains excluding retained austenite is set to 0.3 or more. The average aspect ratio of grains excluding retained austenite may be 0.4 or more, 0.5 or more, or 0.55 or more. The closer the crystal grains are to equiaxed, the smaller the anisotropy and the better the workability. Therefore, the closer the average aspect ratio of the crystal grains, excluding retained austenite, to 1.0, the better. On the other hand, the average aspect ratio of grains other than retained austenite may be 0.9 or less, 0.8 or less, or 0.6 or less.
[0039]
In the present embodiment, the average grain size excluding retained austenite, the average aspect ratio of crystal grains excluding retained austenite, and the area fraction of the metal structure are the surface of the steel sheet in the cross section of the steel sheet parallel to the rolling direction and the thickness direction. A scanning electron microscope (SEM) observation and EBSD (Electron Back Scattering Diffraction (electron beam backscattering diffraction method) analysis. A region of 200 μm in the rolling direction and 100 μm in the thickness direction centered on the 1/4 depth position of the plate thickness from the surface of the steel plate and the center position in the plate width direction is separated by fcc and bcc at intervals of 0.2 μm. get information. Using software attached to the EBSD analysis device (“OIM Analysis (registered trademark)” manufactured by AMETEK), crystal grain boundaries with a crystal orientation difference of 15° or more are identified. The average crystal grain size of bcc is defined as a crystal grain as a region surrounded by crystal grain boundaries with a crystal orientation difference of 15 ° or more and having an equivalent circle diameter of 0.3 μm or more. Calculated by However, in the following (1) formula, D is the average grain size excluding retained austenite, N is the number of grains included in the evaluation region of the average grain size excluding retained austenite, Ai is the i-th (i = 1, 2, . . . , N), and di indicates the equivalent circle diameter of the i-th crystal grain.

The scope of the claims
[Claim 1]
The chemical composition, in mass%,
C: 0.05-0.20%,
Si: 0.005 to 2.00%,
Mn: 0.50 to 4.00%,
P: 0.100% or less,
S: 0.0100% or less,
sol. Al: 0.001 to 1.00%,
Ti: 0.15 to 0.40%,
N: 0.0010 to 0.0100%,
Nb: 0 to 0.100%,
　V: 0 to 1.00%,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 2.00%,
B: 0 to 0.0020%,
Ca: 0 to 0.0100%,
　Mg: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0 to 0.0200%
and the balance consists of Fe and impurities,
The metal structure at a depth of 1/4 of the sheet thickness from the surface contains, in area fraction, 90% or more ferrite and less than 3% retained austenite, and the average grain size excluding the retained austenite is 10.0 μm or less. and the average aspect ratio of the crystal grains excluding the retained austenite is 0.3 or more, and the standard deviation of the Mn concentration is 0.60% by mass or less,
　Tensile strength is 980 MPa or more
A steel plate characterized by:
[Claim 2]
The chemical composition, in % by mass,
Nb: 0.001 to 0.100%,
V: 0.005 to 1.00%,
Mo: 0.001 to 1.00%,
Cu: 0.02 to 1.00%,
Ni: 0.02 to 1.00%,
Cr: 0.02 to 2.00%,
B: 0.0001 to 0.0020%,
Ca: 0.0002 to 0.0100%,
　Mg: 0.0002 to 0.0100%,
REM: 0.0002 to 0.0100%, and
Bi: 0.0001 to 0.0200%
The steel sheet according to claim 1, comprising one or more selected from the group consisting of:
[Claim 3]
A plated steel sheet, wherein a plating layer is formed on the surface of the steel sheet according to claim 1 or 2.
[Claim 4]
The plated steel sheet according to claim 3, wherein the plated layer is a hot-dip galvanized layer.
[Claim 5]
The plated steel sheet according to claim 4, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.