CN114729427A - Steel sheet and plated steel sheet - Google Patents

Abstract

The steel sheet of the present invention has a predetermined chemical composition, and has a microstructure at a position 1/4 depth from the surface of the sheet thickness containing 90% or more of ferrite and less than 3% of retained austenite in terms of area fraction, an average crystal grain size excluding the retained austenite of 10.0 [ mu ] m or less, an average aspect ratio of crystal grains excluding the retained austenite of 0.3 or more, a standard deviation of Mn concentration of 0.60 mass% or less, and a tensile strength of 980MPa or more.

Description

Steel sheet and plated steel sheet

Technical Field

The present invention relates to a steel sheet and a plated steel sheet. More specifically, the present invention relates to a steel sheet and a plated steel sheet having high strength and excellent elongation and bending workability, which are suitable as a material for use in automobiles, home appliances, machine structures, buildings, and the like.

The present application claims priority based on Japanese application No. 2019-229403 filed 12/19/2019, the contents of which are incorporated herein by reference.

Background

In recent years, reduction of carbon dioxide emissions has been dealt with in many fields from the viewpoint of global environmental conservation. In automobile factories, technology development for reducing the weight of a vehicle body for the purpose of low fuel consumption is actively being performed. However, in order to ensure passenger safety, emphasis is placed on improvement of collision resistance, and therefore, it is not easy to reduce the weight of the vehicle body. In order to achieve both weight reduction of the vehicle body and collision resistance, thinning of the member using a high-strength steel sheet has been studied. Therefore, a steel sheet having both high strength and excellent formability is strongly desired. Specifically, steel sheets used for inner panel members, structural members, running members, and the like of automobiles are often subjected to bending, and therefore are often required to have high strength, elongation, and bending workability.

As a steel sheet capable of obtaining excellent elongation, a Dual Phase (hereinafter, DP) steel sheet having a composite structure of a soft ferrite Phase and a hard martensite Phase is known (for example, patent document 1). DP steel sheets are excellent in elongation, and on the other hand, have a possibility of generating cracks due to voids from the interface between ferrite phase and martensite phase having significantly different hardness, and thus have a possibility of poor bending workability.

Patent document 2 proposes a high-strength hot-rolled steel sheet having a steel structure formed of a ferrite single phase and a tensile strength of 1180MPa or more, which is obtained by setting a cooling rate in a temperature range from solidification of a slab to 1300 ℃ to 10 to 300 ℃/min, and winding the steel sheet at 500 to 700 ℃ after finish rolling, and patent document 2 discloses that the high-strength hot-rolled steel sheet improves bendability. However, the hot-rolled steel sheet described in patent document 2 is subjected to reheating and hot rolling without cooling the slab to a temperature lower than 900 ℃ at which the ferrite phase starts to be generated, and therefore has the following problems: segregation formed during solidification may not be sufficiently reduced, and bending workability may be unstable.

Patent document 3 proposes a method for producing a steel sheet having a ferrite area fraction of 80% or more and a tensile strength of 980MPa or more, in which Ti exceeding solubility is dissolved in γ by completing hot rolling within 5 hours after continuous casting, and fine TiC is precipitated together with ferrite transformation in coiling at 550 to 700 ℃, and the steel sheet. However, in patent document 3, in order to suppress the precipitation of coarse TiC, since the precipitation is performed in the austenite region from the continuous casting until the completion of the finish hot rolling, Mn segregation may cause a reduction in bending workability.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 6-128688

Patent document 2: japanese patent laid-open publication No. 2014-194053

Patent document 3: japanese patent laid-open publication No. 2014-208876

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made in view of the above 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 for solving the problems

The present inventors have recognized that: by controlling the microstructure and Mn segregation of the steel sheet through optimization of the chemical composition and production conditions of the steel sheet, it is possible to produce a steel sheet and a plated steel sheet having high strength and excellent elongation and bending workability.

The present invention has been made based on the above-described knowledge, and the gist thereof is as follows.

[1] The chemical composition of the steel sheet according to one embodiment of the present invention contains, in mass%:

C：0.05～0.20％、

Si：0.005～2.00％、

Mn：0.50～4.00％、

p: less than 0.100 percent,

S: less than 0.0100%,

sol.Al：0.001～1.00％、

Ti：0.15～0.40％、

N：0.0010～0.0100％、

Nb：0～0.100％、

V：0～1.00％、

Mo：0～1.00％、

Cu：0～1.00％、

Ni：0～1.00％、

Cr：0～2.00％、

B：0～0.0020％、

Ca：0～0.0100％、

Mg：0～0.0100％、

REM：0～0.0100％、

Bi：0～0.0200％，

The remainder comprising Fe and impurities,

the microstructure at a position 1/4 deep from the surface of the sheet thickness contains 90% or more of ferrite and less than 3% of retained austenite in terms of area fraction, the average crystal grain diameter excluding the retained austenite is 10.0 [ mu ] m or less, the average aspect ratio of crystal grains excluding the retained austenite is 0.3 or more, the standard deviation of Mn concentration is 0.60 mass% or less,

the tensile strength is 980MPa or more.

[2] The steel sheet according to [1], wherein the chemical composition may contain 1 or 2 or more elements selected from the following elements in mass%:

Nb：0.001～0.100％、

V：0.005～1.00％、

Mo：0.001～1.00％、

Cu：0.02～1.00％、

Ni：0.02～1.00％、

Cr：0.02～2.00％、

B：0.0001～0.0020％、

Ca：0.0002～0.0100％、

Mg：0.0002～0.0100％、

REM: 0.0002 to 0.0100%, and

Bi：0.0001～0.0200％。

[3] a plated steel sheet according to another aspect of the present invention is the plated steel sheet according to any one of [1] and [2], wherein the plated layer is formed on a surface of the steel sheet.

[4] The plated steel sheet according to item [3], wherein the plating layer may be a hot-dip galvanized layer.

[5] The plated steel sheet according to item [4], wherein the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

Effects of the invention

According to the above aspect of the present invention, a steel sheet and a plated steel sheet having high strength and excellent elongation and bending workability can be provided. When the steel sheet or plated steel sheet of the present invention is used as a material for parts such as an inner panel member, a structural member, and a chassis member of an automobile, the steel sheet or plated steel sheet can be easily processed into a part shape, and the industrial contribution is extremely remarkable.

Detailed Description

The steel sheet and the plated steel sheet of the present embodiment will be described in detail below. First, the chemical composition of the steel sheet of the present embodiment will be described. However, the present invention is not limited to the configurations disclosed in the embodiments, and various modifications can be made without departing from the scope of the present invention.

The numerical limitation ranges described below include the lower limit and the upper limit. For values expressed as "below" or "above," the value is not included in the range of values. In the following description, "%" relating to the chemical composition of steel is "mass%".

< chemical composition of Steel >

(C：0.05～0.20％)

C combines with Ti and the like to form carbide, thereby improving the tensile strength of the steel. When the C content is less than 0.05%, it becomes difficult to obtain a tensile strength of 980MPa or more. Therefore, the C content is set to 0.05% or more. The C content is preferably set to 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. Further, weldability significantly deteriorates. Therefore, the C content is set to 0.20% or less. The C content is preferably 0.15% or less or 0.14% or less, more preferably 0.13% or less.

(Si：0.005～2.00％)

Si has an effect of improving the tensile strength of steel by improving solid solution strengthening and hardenability. In addition, Si also has an effect of suppressing precipitation of cementite. If the Si content is less than 0.005%, the above-described effect is hardly exhibited. Therefore, the Si content is set to 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 in the hot rolling step. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less or 1.50% or less, more preferably 1.30% or less.

(Mn：0.50～4.00％)

Mn has an effect of improving the tensile strength of steel by improving solid solution strengthening and hardenability. If the Mn content is less than 0.50%, ferrite transformation is excessively promoted, and carbides such as Ti are roughly precipitated together with ferrite transformation at high temperatures, making it difficult to obtain the tensile strength of the steel sheet of 980MPa or more. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 0.70% or more or 0.80% or more, and more preferably 1.00% or more. On the other hand, if the Mn content exceeds 4.00%, Mn segregation occurs at a high concentration, so that the standard deviation of the Mn concentration becomes large, and the bending workability is deteriorated. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less, and still more preferably 3.30% or less or 3.00% or less.

(Ti：0.15～0.40％)

Ti combines with C to form carbide, and the tensile strength of the steel sheet is improved by fine precipitation. In addition, Ti has an effect of suppressing coarsening of austenite grains by Ti nitrides to refine the metal structure. When the Ti content is less than 0.15%, it becomes difficult to obtain a tensile strength of 980MPa or more. Therefore, the Ti content is set to 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, if Ti is excessively contained, coarse nitrides and carbides are generated, and the elongation and bending workability are lowered. Therefore, the Ti content is set to 0.40% or less. The Ti content is preferably 0.38% or less, 0.35% or less, or 0.30% or less.

(sol.Al：0.001～1.00％)

Al has the following effects: in the steel making stage, steel is cleaned by deoxidation (generation of defects such as pores in steel is suppressed), and ferrite transformation is promoted. If the al content is less than 0.001%, the above-described effect becomes difficult to be exhibited. Therefore, the sol.al content is set to 0.001% or more. The al content is preferably 0.01% or more, more preferably 0.02% or more, or 0.03% or more. On the other hand, even if the sol.al content is set to exceed 1.00%, the effects due to the above-described actions are saturated, and an increase in refining cost is caused. Therefore, the sol.al content is set to 1.00% or less. The al content is preferably 0.80% or less, more preferably 0.60% or less or 0.10% or less. Al means acid-soluble Al.

(N：0.0010～0.0100％)

N has the following effects: ti nitrides are formed to suppress the coarsening of austenite during slab reheating and hot rolling and to refine the metal structure. When the N content is less than 0.0010%, the above-mentioned effects become difficult to be exerted. Therefore, the N content is set to 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, and the stretch flangeability of the steel sheet deteriorates. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0060% or less, 0.0050% or less, or 0.0045% or less.

(P: 0.100% or less)

P is an element contained in steel as an impurity, and has an effect of reducing the bending workability of the steel sheet. 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 still more preferably 0.020% or less. The content of P is preferably lower from the viewpoint of ensuring bending workability, because P is mixed as an impurity from the raw material, and the lower limit thereof does not need to be particularly limited. However, if the P content is excessively reduced, the manufacturing cost increases. From the viewpoint of production cost, the P content is preferably 0.001% or more or 0.003% or more, and more preferably 0.005% or more.

(S: 0.0100% or less)

S is an element contained as an impurity, and has an effect of reducing the bending workability of the steel sheet. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, more preferably 0.0060% or less, and still more preferably 0.0030% or less. S is mixed as an impurity in the raw material, and the lower limit thereof is not particularly limited, and the content of S is preferably lower from the viewpoint of securing the bending workability. However, if the S content is excessively reduced, the manufacturing cost increases. 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.

The remainder of the chemical composition of the steel sheet of the present embodiment contains Fe and impurities. In the present embodiment, the impurities are those which are mixed from ores and scraps as raw materials, manufacturing environments, and the like, and are allowed within a range where they do not adversely affect the steel sheet of the present embodiment.

The steel sheet of the present embodiment may contain the following optional elements in place of part of Fe. Since the steel sheet of 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%.

(Nb：0～0.100％)

Nb is an optional element. Nb has the following effects: the tensile strength of the steel sheet is improved by precipitation strengthening of NbC while the grain size of ferrite is reduced to a finer size while the grain size of the steel sheet is suppressed from coarsening. In order to obtain these effects, the Nb content is preferably set to 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the above-described effects are saturated, and there is a possibility that an increase in rolling load at the time of finish rolling is caused. Therefore, when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.070% or less or 0.060% or less, and more preferably 0.030% or less.

(V：0～1.00％)

V is an optional element. V has the following effects: the steel is dissolved in the steel to increase the tensile strength of the steel sheet, and is precipitated as carbide, nitride, carbonitride, or the like in the steel, and is also precipitation-strengthened to increase the tensile strength of the steel sheet. In order to obtain these effects, the V content is preferably set to 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 be coarsened, which may cause a reduction 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, and still more preferably 0.60% or less or 0.30% or less.

(Mo：0～1.00％)

Mo is an optional element. Mo has the effect of improving the hardenability of steel and also forming carbides or carbonitrides to increase the strength of the steel sheet. In order to obtain these effects, the Mo content is preferably set to 0.001% or more. The 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 be improved. Therefore, when Mo is contained, the content of Mo is set to 1.00% or less. The Mo content is more preferably 0.80% or less, and still more preferably 0.60% or less or 0.30% or less.

(Cu：0～1.00％)

Cu is an optional element. Cu has an effect of improving toughness of steel and an effect of increasing tensile strength. In order to obtain these effects, the Cu content is preferably set to 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 excessively contained, weldability of the steel sheet may be reduced. 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, and still more preferably 0.30% or less or 0.10% or less.

(Ni：0～1.00％)

Ni is an optional element. Ni has an effect of improving toughness of steel and an effect of increasing tensile strength. In order to obtain these effects, the Ni content is preferably set to 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 excessively contained, the alloy cost increases, and there is a possibility that the toughness of the weld heat affected zone of the steel sheet deteriorates. 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, and still more preferably 0.30% or less or 0.10% or less.

(Cr：0～2.00％)

Cr is an optional element. Cr has the effect of improving the hardenability of steel and also forming carbides or carbonitrides to increase the strength of the steel sheet. In order to obtain this effect, the Cr content is preferably set to 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 conversion treatability 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.

(B：0～0.0020％)

B is an optional element. B has the effect of improving the tensile strength of the steel sheet by grain boundary strengthening or solid solution strengthening. In order to obtain this effect, the B content is preferably set to 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 B is contained in excess of 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, and still more preferably 0.0013% or less or 0.0010% or less.

(Ca：0～0.0100％)

Ca is an optional element. Ca has an effect of dispersing a large amount of fine oxides in molten steel to refine the microstructure of a steel sheet. Further, Ca has the following effects: s in molten steel is fixed as spherical CaS, thereby suppressing the generation of tensile inclusions such as MnS and improving the stretch-flangeability of a steel sheet. In order to obtain these effects, the Ca content is preferably set to 0.0002% or more. The Ca content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Ca content exceeds 0.0100%, the amount of CaO in the steel increases, and there is a possibility that the toughness of the steel sheet is adversely affected. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is more preferably 0.0050% or less, and still more preferably 0.0030% or less or 0.0020% or less.

(Mg：0～0.0100％)

Mg is an optional element. Mg has the following effects as with Ca: oxides and sulfides are formed in the molten steel, the formation of coarse MnS is suppressed, a large amount of fine oxides are dispersed, and the metal structure of the steel sheet is refined. In order to obtain these effects, the Mg content is preferably set to 0.0002% or more. The Mg content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Mg content exceeds 0.0100%, oxides in the steel increase, and there is a possibility that the toughness of the steel sheet is adversely affected. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less. The Mg content is more preferably 0.0050% or less, and still more preferably 0.0030% or less or 0.0025% or less.

(REM：0～0.0100％)

REM is an optional element. REM also has the following effects as Ca: oxides and sulfides are formed in the molten steel, the formation of coarse MnS is suppressed, a large amount of fine oxides are dispersed, and the metal structure of the steel sheet is refined. In the case of obtaining these effects, the REM content is preferably set to 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, and the toughness of the steel sheet may be adversely affected. Therefore, when REM is contained, the REM content is preferably set to 0.0100% or less. The REM content is more preferably 0.0050% or less, and still more preferably 0.0030% or less or 0.0020% or less.

Here, REM (rare earth) refers to 17 elements in total including Sc, Y, and lanthanoid. In the present embodiment, the content of REM refers to the total content of these elements.

(Bi：0～0.0200％)

Bi is an optional element. Bi has an effect of refining the solidification structure to improve the formability of the steel sheet. In order to obtain this effect, the Bi content is preferably set to 0.0001% or more. The Bi content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, if the Bi content exceeds 0.0200%, the above effects are saturated and the alloy cost increases. Therefore, when Bi is contained, the Bi content is set to 0.0200% or less. More preferably 0.0100% or less, still more preferably 0.0070% or less, or 0.0030% or less.

Next, the metal structure of the steel sheet will be explained. The steel sheet of the present embodiment has a microstructure at a depth of 1/4 from the surface of the sheet thickness that contains 90% or more of ferrite and less than 3% of retained austenite in terms of area fraction, has an average crystal grain size of 10.0 μm or less excluding the retained austenite, has an average aspect ratio of crystal grains excluding the retained austenite of 0.3 or more, and has a standard deviation of Mn concentration of 0.60 mass% or less. The reason why the metal structure at the depth position 1/4 where the distance from the surface of the steel sheet is the sheet thickness is defined is that the metal structure at this position is a typical metal structure of the steel sheet.

Cementite, pearlite, bainite, and martensite are also allowable as the metal structure other than ferrite and retained austenite.

(surface area fraction of ferrite: 90% or more)

The ferrite phase is required for obtaining good elongation and bending workability. If the area fraction of ferrite is less than 90%, cracks may be generated early at the phase boundary with a hard phase other than ferrite (cementite, pearlite, bainite, martensite, retained austenite, etc.), or the hard phase may be broken early, thereby decreasing the elongation and bending workability. Therefore, the area fraction of ferrite is set to 90% or more. The area fraction of ferrite is preferably 95% or more, 98% or more, and may be 100% (i.e., a single phase of ferrite).

(surface area fraction of retained austenite: less than 3%)

The retained austenite in the hard phase other than ferrite is transformed into very hard martensite by working, and the bending workability of the steel sheet is significantly deteriorated. Therefore, the area fraction of the retained austenite is set to be less than 3%. The area fraction of retained austenite is preferably 2% or less, more preferably 1% or less, and may be 0%.

(average crystal grain size excluding retained austenite: 10.0 μm or less)

Since the bending workability is deteriorated if the average crystal grain size other than the retained austenite is large (i.e., the crystal grains are coarse), the average crystal grain size other than the retained austenite is set to 10.0 μm or less. The average crystal grain size other than the 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, since the smaller the average crystal grain size excluding the retained austenite is, the more preferable it is. However, in the ordinary hot rolling, it is technically difficult to make fine grains such that the average crystal grain size other than the retained austenite is less than 1.0 μm, and therefore the average crystal grain size other than the retained austenite may be set to 1.0 μm or more, 2.0 μm or more, or 4.0 μm or more.

In the present embodiment, the "average crystal grain size" (excluding the residual austenite) "means an average crystal grain size obtained by defining, as crystal grains, a region surrounded by grain boundaries having a crystal orientation difference of 15 ° or more and having an equivalent circle diameter of 0.3 μm or more in the structure having a crystal structure bcc, that is, ferrite, bainite, martensite, and pearlite, and the crystal grain size of the residual austenite is not included in the average crystal grain size.

(average aspect ratio of grains other than the retained austenite: 0.3 or more)

In the present embodiment, the average aspect ratio of the crystal grains other than the retained austenite is 0.3 or more. The aspect ratio is a value obtained by dividing the length of the minor axis of the crystal grain by the length of the major axis, and is 0 to 1.0. The smaller the average aspect ratio of the grains other than the retained austenite, the flatter the grains, and the closer to 1.0, the equiaxed grains. When the average aspect ratio of the grains other than the retained austenite is less than 0.3, the number of flat grains increases, the anisotropy of the material increases, and the bending workability decreases. Therefore, the average aspect ratio of the crystal grains other than the retained austenite is set to 0.3 or more. The average aspect ratio of the crystal grains other than the retained austenite may be 0.4 or more, 0.5 or more, or 0.55 or more. Since the anisotropy becomes smaller and the workability is more excellent as the crystal grains are closer to equiaxed, the average aspect ratio of the crystal grains other than the retained austenite is better as being closer to 1.0. On the other hand, the average aspect ratio of the crystal grains other than the retained austenite may be 0.9 or less, 0.8 or less, or 0.6 or less.

In the present embodiment, the average crystal grain size excluding the retained austenite, the average aspect ratio of the crystal grains excluding the retained austenite, and the area fraction of the metal structure are determined as follows: the microstructure of the steel sheet cross section parallel to the rolling direction and the thickness direction at a position 1/4 depth from the steel sheet surface to the thickness was determined by Scanning Electron Microscope (SEM) observation and EBSD (Electron Back Scattering Diffraction) analysis using an EBSD analysis apparatus composed of a thermal field emission type scanning Electron microscope and an EBSD detector. The crystal orientation information was acquired at 0.2 μm intervals so as to distinguish fcc and bcc in a region of 200 μm in the rolling direction and 100 μm in the plate thickness direction centered at the 1/4 depth position where the distance surface of the steel plate was the plate thickness and the center position in the plate width direction. Crystal grain boundaries having a crystal orientation difference of 15 ° or more were identified using software (OIM Analysis (registered trademark) manufactured by AMETEK) attached to the EBSD analyzer. The average crystal particle diameter of bcc is determined by: the crystal grain size is determined by a method using the following formula (1), wherein a region surrounded by crystal grain boundaries having a crystal orientation difference of 15 ° or more and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain. In the following formula (1), D represents an average crystal grain size other than the retained austenite, N represents the number of crystal grains contained in an evaluation region of the average crystal grain size other than the retained austenite, Ai represents an area of the i-th crystal grain (i ═ 1, 2, … …, and N), and di represents an equivalent circle diameter of the i-th crystal grain.

Crystal grain boundaries having a crystal orientation difference of 15 ° or more are mainly ferrite grain boundaries, martensite and bainite lath block (block) boundaries. Under the conditions of JIS G0552: 2013, it is possible to calculate the grain size of ferrite grains having a difference in crystal orientation of less than 15 °, and further, lath blocks of martensite and bainite are not calculated. Therefore, the average crystal grain size other than the retained austenite in the present embodiment is a value obtained by EBSD analysis as described above. At the same time, the length of the major axis and the length of the minor axis of each crystal grain can be determined, and therefore, by using this method, the average aspect ratio of crystal grains other than the retained austenite can be determined.

The area fraction of ferrite was measured by the following method. Here, a region surrounded by crystal grain boundaries having a crystal orientation difference of 5 ° or more and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain. The area fraction of crystal grains having a value (GAM value) of 0.6 DEG or less, which is obtained by Grain Average misanalysis provided in OIM Analysis, in the crystal grains was calculated. By this method, the area fraction of ferrite is obtained. The reason why the boundary where the difference in crystal orientation is 5 ° or more is defined as the crystal grain boundary when the area fraction of ferrite is obtained is because: sometimes, different metallic structures formed from similar variants (variants) cannot be distinguished from the same prior austenite grain.

The area fraction of retained austenite is obtained by calculating the area fraction of the microstructure determined as fcc by EBSD analysis.

(standard deviation of Mn concentration: 0.60% by mass or less)

The standard deviation of the Mn concentration in the steel sheet of the present embodiment at a position 1/4 depth from the surface of the sheet thickness is 0.60 mass% or less. This reduces local variation in tensile strength due to Mn segregation, and can stably obtain good bending workability. The standard deviation of the Mn concentration may be 0.58 mass% or less, 0.55 mass% or less, or 0.52 mass% or less. The smaller the standard deviation of the Mn concentration, the more preferable the value, but the substantial lower limit is 0.10 mass% due to the restrictions of the production process. The standard deviation of the Mn concentration may be 0.12 mass% or more, 0.15 mass% or more, or 0.20 mass% or more.

The standard deviation of the Mn concentration was obtained by: the L-section of the steel sheet was mirror-polished, and the 1/4 depth position from the surface of the steel sheet, which was the sheet thickness, was measured by an Electron Probe Microanalyzer (EPMA). The measurement conditions were such that the acceleration voltage was set to 15kV, the magnification was set to 5000 times, and distribution images in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction were measured. More specifically, the measurement interval was set to 0.1 μm, and the Mn concentration at 40000 or higher was measured. Next, the standard deviation of the Mn concentration was obtained by calculating the standard deviation based on the Mn concentrations obtained from all the measurement points.

< mechanical Properties >

(tensile Strength: 980MPa or more)

The steel sheet of the present embodiment has high strength and excellent elongation and bending workability by controlling the microstructure and Mn segregation. However, if the tensile strength of the steel sheet is low, the effects of reducing the weight of the vehicle body, improving the rigidity, and the like are small. Therefore, the Tensile Strength (TS) of the steel sheet of the present embodiment is set to 980MPa or more. The tensile strength is preferably 1080MPa or more, 1130MPa or more, or 1180MPa or more. The upper limit is not particularly limited, but the tensile strength may be 1800MPa or less because press molding becomes difficult as the tensile strength becomes higher.

(balance of elongation and tensile Strength)

The steel sheet of the present embodiment has high strength and excellent elongation. Therefore, the steel sheet of the present embodiment is excellent in the balance between the elongation and the tensile strength, and TS × El as an index of the balance is preferably 15000MPa ·% or more, more preferably 16000MPa ·% or 17000MPa ·% or more.

Tensile strength and elongation of the steel sheet were measured according to JIS Z2241: the test specimen No. 5 specified in 2011 was evaluated for tensile strength and total elongation at break (El).

< production method >

The reason for limiting the production conditions of the steel sheet of the present embodiment will be described.

The present inventors confirmed that: the steel sheet of the present embodiment is obtained by a manufacturing method including the following heating step, hot rolling step, cooling step, and winding step.

[ heating Process ]

First, a slab or billet having the above-described chemical composition is heated. The slab to be subjected to hot rolling is preferably a slab obtained by continuous casting or casting and cogging, but may be one in which hot working or cold working is added thereto.

(residence time in the temperature range of 700 to 850 ℃ C. during heating: 900 seconds or more)

When a slab or billet to be subjected to hot rolling is heated, the slab or billet stays in a temperature range of 700 to 850 ℃ for 900 seconds or more. In the austenite transformation which occurs in the temperature range of 700 to 850 ℃, Mn is distributed between ferrite and austenite, and by extending this transformation time, Mn can diffuse in the ferrite region. This eliminates Mn microsegregation that is unevenly present in the slab, and significantly reduces the standard deviation of Mn concentration.

(heating temperature: 1280 ℃ C. or higher and SRT (. degree. C.) or higher)

The heating temperature of the slab or billet to be subjected to hot rolling is set to 1280 ℃ or higher and to a temperature SRT (c) represented by the following formula (2) or higher. When the heating temperature is less than 1280 ℃, the decrease in the standard deviation of the Mn concentration due to Mn diffusion during heating may become insufficient. When the heating temperature is lower than SRT (c), the solubility of Ti carbonitride becomes insufficient, and in either case, the tensile strength and bending workability of the steel sheet are reduced. Therefore, the temperature of the slab or billet to be subjected to hot rolling is set to 1280 ℃ or higher and SRT (c) or higher. Here, "the temperature of the slab or the billet is 1280 ℃ or more and SRT (c) or more" means that the temperature of the slab or the billet is higher than the higher temperature of 1280 ℃ and SRT (c).

On the other hand, when the heating temperature exceeds 1400 ℃, thick scale may be formed to reduce the yield or cause significant damage to the heating furnace, and therefore 1400 ℃ or lower is preferable.

SRT(℃)＝1630+90×ln([C]×[Ti]) (2)

Wherein [ element symbol ] in the above formula (2) represents the content of each element in mass%.

[ Hot Rolling Process ]

The method for manufacturing a steel sheet according to the present embodiment includes a hot rolling step of performing hot rolling on a slab or a billet after a heating step in a plurality of passes using a plurality of rolling stands to produce a hot-rolled steel sheet. The hot rolling process is divided into rough rolling and finish rolling performed subsequent to the rough rolling.

The multi-pass hot rolling may be performed by using a reversing mill or a tandem mill, but from the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the final several passes.

(time from rough rolling completion to finish rolling completion: 600 seconds or less)

Precipitation of carbonitrides of Ti or the like is promoted by rough rolling and precipitation is started, but if the time until completion of finish rolling is too long, a large amount of coarse carbonitrides are precipitated, while fine carbonitrides precipitated after finish rolling contributing to high strength are reduced, and the tensile strength of the steel sheet is remarkably reduced and the bending workability is lowered. Therefore, the time from the start of rough rolling (i.e., after the end of the heating step) to the completion of finish rolling is set to be within 600 seconds. The time from the start of rough rolling to the completion of finish rolling is preferably within 500 seconds, more preferably within 400 seconds.

(total reduction of pressure in 850 to 1100 ℃ C. temperature region: 90% or more)

By hot rolling with the total reduction ratio in the temperature range of 850 to 1100 ℃ set to 90% or more, it is possible to promote the accumulation of strain energy into unrecrystallized austenite, promote the recrystallization of austenite, promote the atomic diffusion of Mn, and reduce the standard deviation of the Mn concentration while mainly realizing the refinement of recrystallized austenite. Therefore, the total reduction ratio in the temperature range of 850 to 1100 ℃ is set to 90% or more.

The "total reduction ratio in a temperature range of 850 to 1100 ℃ means: when the inlet plate thickness before the first pass in the rolling in this temperature range is set to t0 and the outlet plate thickness after the final pass in the rolling in this temperature range is set to t1, (t0-t1)/t0 × 100 (%) is used.

(finishing temperature FT (. degree. C.): TR (. degree. C.) to 1080 ℃ C.)

When FT (c) is lower than TR (c) represented by the following formula (3), austenite that is significantly flat is formed before cooling after finish rolling, and the steel sheet of the final product has a metal structure elongated in the rolling direction, and the average aspect ratio of crystal grains other than the retained austenite is small, and the plastic anisotropy is large, and the elongation and bending workability of the steel sheet are reduced. Therefore, FT (. degree. C.) is set to TR (. degree. C.) or higher.

On the other hand, if FT (. degree. C.) exceeds 1080 ℃, austenite grains refined by hot rolling become coarse, and the bending workability of the steel sheet is lowered. Therefore, FT (. degree. C.) was set to 1080 ℃ or lower. FT (. degree. C.) is preferably 1060 ℃ or lower.

The temperature in the finish rolling refers to the surface temperature of the steel material, and can be measured by a radiation thermometer or the like.

TR(℃)＝805+385×[Ti]+584×[Nb] (3)

Wherein [ element symbol ] in the above formula (3) represents the content of each element in mass%, and 0 is substituted when not contained.

[ Cooling Process ]

The method for producing a steel sheet according to the present embodiment includes, as a step subsequent to the hot rolling step, a cooling step of cooling the hot-rolled steel sheet with water to a temperature range of 500 to 700 ℃ at an average cooling rate of 30 ℃/sec or more (water cooling). In the method for manufacturing a steel sheet according to the present embodiment, the cooling step is started within 3.0 seconds after the end of the hot rolling step.

(time from completion of finish rolling to initiation of Water-Cooling: within 3.0 seconds)

When the time from completion of finish rolling (i.e., after completion of hot rolling) to start of water cooling exceeds 3.0 seconds, the tensile strength and bending workability are lowered due to growth of austenite grains of fine grain size and coarse precipitation of carbonitride of Ti and the like. Therefore, in the method for manufacturing a steel sheet according to the present embodiment, water cooling is started within 3.0 seconds after completion of finish rolling. Preferably, the water cooling is started within 2.0 seconds, more preferably within 1.5 seconds after the finish rolling is completed.

(average Cooling Rate: 30 ℃/sec or more)

The average cooling rate is the following value: the value is obtained by dividing the amount of temperature drop from the start of water cooling (when the steel sheet is introduced into the cooling facility) to the end of water cooling (when the steel sheet is discharged from the cooling facility) immediately before coiling after completion of hot rolling by the time required from the start to the end of water cooling. When the average cooling rate is less than 30 ℃/sec, ferrite transformation occurs in a high temperature region, coarse carbonitride of Ti and the like is precipitated in ferrite grains, and the tensile strength is remarkably reduced. In addition, some or all of the crystal grains may be coarse, and the bending workability may be lowered. Therefore, the average cooling rate is set to 30 ℃/sec or more. The average cooling rate is preferably 40 ℃/sec or more, and more preferably 50 ℃/sec or more. The upper limit of the average cooling rate is not particularly limited, but is preferably 300 ℃/sec or less from the viewpoint of equipment cost.

In the cooling step, the hot-rolled steel sheet is cooled to a temperature range of 500 to 700 ℃ in view of the relationship with the winding temperature in the winding step described later.

[ coiling Process ]

The method for manufacturing a steel sheet according to the present embodiment includes a winding step of winding the hot-rolled steel sheet after the cooling step in a temperature range of 500 to 700 ℃.

(coiling temperature: 500 ℃ C. to 700 ℃ C.)

The hot-rolled steel sheet is cooled to 700 ℃ or lower in the cooling step, and then wound at 500 to 700 ℃. When the coiling temperature is less than 500 ℃, ferrite transformation is insufficient, it becomes difficult to set the area fraction of ferrite to 90% or more in the microstructure, and precipitation of fine carbonitrides of Ti and the like in ferrite grains becomes insufficient, so that it becomes difficult to obtain a desired tensile strength, and the elongation also decreases. On the other hand, when the coiling temperature exceeds 700 ℃, carbonitride of Ti or the like grows roughly, and it becomes difficult to obtain a desired tensile strength.

In the present embodiment, the plated steel sheet may be produced by plating the surface of the steel sheet after the winding step. In the case of performing plating, there is no problem as long as plating is performed so as to satisfy the conditions of the method for manufacturing a steel sheet according to the present embodiment. The plating may be either electroplating or hot dip plating, and the plating species is not particularly limited, but is generally zinc-based plating including zinc plating and zinc alloy plating. Examples of the plated steel sheet include an electrogalvanized steel sheet, an electrogalvanized-nickel alloy steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, a hot-dip galvanized-aluminum alloy steel sheet, and the like. The plating adhesion amount may be a general amount. Before plating, Ni or the like may be attached to the surface as pre-plating.

In the production of the steel sheet of the present embodiment, a known temper rolling may be appropriately performed for the purpose of shape correction.

The thickness of the steel sheet of the present embodiment is not particularly limited, but when the thickness is too large, the metal structure formed in the surface layer and the inside of the steel sheet is significantly different, and therefore, it is preferably 6.0mm or less. On the other hand, if the thickness is too small, it becomes difficult to pass the steel sheet during hot rolling, and therefore, the thickness of the steel sheet is preferably 1.0mm or more in general. More preferably, the thickness of the steel sheet is 1.2mm or more.

Examples

Next, the effects of one embodiment of the present invention will be described more specifically by examples, but the conditions in the examples are one example of conditions adopted for confirming the feasibility and the effects of the present invention, and the present invention is not limited to this one example of conditions. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steel materials having a thickness of 250mm and having chemical compositions shown in tables 1-1 and 1-2 were hot-rolled under the conditions shown in Table 2 to obtain hot-rolled steel sheets having a thickness of 2.5 to 3.5 mm. A part of the obtained hot-rolled steel sheet was subjected to hot-dip galvanizing treatment at an annealing temperature of 700 ℃, and further to alloying treatment for evaluation of material quality. In tables 1-1 and 1-2, the contents of elements not intentionally added are set as blank columns. In addition, values outside the scope of the invention in tables 1-1 and 1-2 and values that are not preferable in table 2 are underlined.

[ tables 1-1]

Steel	C	Si	Mn	P	S	sol.AI	Ti	N	Nb	V	Mo	Cu
													A	0.101	0.051	1.85	0.011	0.0011	0.05	0.315	0.0028
B	0.085	0.042	1.87	0.007	0.0015	0.06	0.212	0.0035
													C	0.120	1.561	1.99	0.005	0.0019	0.08	0.190	0.0045
D	0.135	0.053	1.56	0.011	0.0009	0.09	0.365	0.0039
													E	0.112	0.064	1.32	0.009	0.0026	0.05	0.296	0.0032	0.065
F	0.109	0.111	1.85	0.010	0.0017	0.07	0.246	0.0043		0.102
													G	0.103	0.042	1.45	0.009	0.0012	0.05	0.280	0.0032			0.100
H	0.085	0.063	1.25	0.014	0.0002	0.06	0.312	0.0034				0.056
													I	0.105	0.025	1.23	0.010	0.0027	0.09	0.312	0.0037
J	0.081	0.079	1.82	0.016	0.0020	0.03	0.302	0.0044
													K	0.084	0.368	1.65	0.010	0.0027	0.04	0.285	0.0037
L	0.075	0.032	1.98	0.016	0.0018	0.06	0.279	0.0029
													M	0.086	0.025	1.65	0.010	0.0024	0.04	0.285	0.0030
N	0.073	0.039	1.31	0.010	0.0019	0.08	0.291	0.0038
													0	0.074	0.025	2.89	0.012	0.0015	0.05	0.295	0.0031
P	0.072	0.102	0.68	0.008	0.0021	0.03	0.286	0.0034
													Q	0.041	0.209	1.79	0.013	0.0017	0.07	0.172	0.0030
R	0.072	0.498	1.50	0.010	0.0006	0.08	0.125	0.0041
													S	0.110	0.046	2.15	0.007	0.0026	0.07	0.425	0.0031
T	0.221	0.517	1.86	0.012	0.0027	0.05	0.252	0.0039
													U	0.083	0.498	4.10	0.016	0.0030	0.06	0.212	0.0034
V	0.120	0.415	1.95	0.010	0.0015	0.35	0.165	0.0025	0.012	0.315

[ tables 1-2]

[ Table 2]

The area fraction of the microstructure at a position 1/4 depth from the surface of the steel sheet was determined, the average crystal grain size excluding the retained austenite, the average aspect ratio of the crystal grains excluding the retained austenite, and the standard deviation of the Mn concentration were determined.

The area fraction of the microstructure at 1/4 depth from the surface of the steel sheet as the sheet thickness, the average crystal grain size excluding the retained austenite, and the average aspect ratio of the crystal grains excluding the retained austenite were determined as follows: the distance from the steel sheet surface to the steel sheet cross section parallel to the rolling direction and the thickness direction was 1/4 depth positions of the sheet thickness and the metal structure at the center position in the sheet width direction was determined by Scanning Electron Microscope (SEM) observation and EBSD (Electron Back Scattering Diffraction) analysis using an EBSD analysis apparatus composed of a thermal field emission type scanning Electron microscope and an EBSD detector.

The crystal orientation information was acquired at 0.2 μm intervals so as to distinguish fcc and bcc in a region of 200 μm in the rolling direction and 100 μm in the thickness direction centered at the 1/4 depth position where the distance from the surface of the steel sheet was the thickness and the center position in the width direction of the steel sheet. Crystal grain boundaries having a crystal orientation difference of 15 ° or more were identified using software (OIM Analysis (registered trademark) manufactured by AMETEK) attached to the EBSD analyzer. The average crystal particle diameter of bcc is determined by: the crystal grain size is determined by a method using the following formula (4) in which a region surrounded by crystal grain boundaries having a crystal orientation difference of 15 ° or more and having an equivalent circle diameter of 0.3 μm or more is defined as a crystal grain.

In the following formula (4), D represents an average crystal grain size other than the retained austenite, N represents the number of crystal grains contained in an evaluation region of the average crystal grain size other than the retained austenite, Ai represents an area of the i-th crystal grain (i ═ 1, 2, … …, and N), and di represents an equivalent circle diameter of the i-th crystal grain.

The area fraction of ferrite was measured by the following method. A region surrounded by crystal grain boundaries having a crystal orientation difference of 5 DEG or more and having an equivalent circle diameter of 0.3 [ mu ] m or more is defined as a crystal grain. The area fraction of crystal grains having a value (GAM value) of 0.6 DEG or less, which is obtained by Grain Average misanalysis provided in OIM Analysis, in the crystal grains was calculated. By this method, the area fraction of ferrite is obtained.

The area fraction of retained austenite is obtained by calculating the area fraction of the microstructure determined as fcc by EBSD analysis.

The standard deviation of the Mn concentration was obtained by: the L-section was mirror-polished so that the widthwise center position of the steel sheet became the measurement position, and then the widthwise center position of the steel sheet was measured by an Electron Probe Microanalyzer (EPMA) at a distance of 1/4 depths from the surface of the steel sheet. The measurement conditions were such that the acceleration voltage was set to 15kV, the magnification was set to 5000 times, and distribution images in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction were measured. More specifically, the measurement interval was set to 0.1 μm, and the Mn concentration at 40000 or more was measured. Next, the standard deviation of the Mn concentration was obtained by calculating the standard deviation based on the Mn concentrations obtained from all the measurement points.

In order to evaluate the mechanical properties of the obtained steel sheet, the steel sheet was subjected to the following tests in accordance with JIS Z2241: 2011 the tensile strength TS (MPa) and the total elongation at break El (%). The bending workability was evaluated by a 90 ° V bending test in which the bending radius was set to 2 times the sheet thickness.

Table 3 shows the results of the tests of the metal structure, texture, and mechanical properties. In table 3, values outside the scope of the invention are underlined. In table 3, GI in the column of "plating" represents a hot-dip galvanized layer, and GA represents an alloyed hot-dip galvanized layer.

The tensile strength was determined to be high when 980MPa or more was used.

The elongation is determined as a pass when the product of the tensile strength and the elongation at break (TS × El) (MPa ·%) is 15000MPa ·% or more, and the elongation is excellent as a high strength. The bending workability was tested 3 times, and all test pieces were rated as good (OK) for those who did not crack during the bending test, and rated as good (NG) for those who cracked 1 or more.

[ Table 3]

As shown in table 3, in the invention examples having the requirements of the present invention, TS × El and bending workability were all acceptable. On the other hand, in the comparative example which does not have at least one or more of the requirements of the present invention, at least one of TS, TS × El, and bending workability is not satisfactory.

Claims (5)

1. A steel sheet characterized by having a chemical composition containing, in mass%:

C：0.05～0.20％、

Si：0.005～2.00％、

Mn：0.50～4.00％、

p: less than 0.100 percent,

S: less than 0.0100%,

sol.Al：0.001～1.00％、

Ti：0.15～0.40％、

N：0.0010～0.0100％、

Nb：0～0.100％、

V：0～1.00％、

Mo：0～1.00％、

Cu：0～1.00％、

Ni：0～1.00％、

Cr：0～2.00％、

B：0～0.0020％、

Ca：0～0.0100％、

Mg：0～0.0100％、

REM：0～0.0100％、

Bi：0～0.0200％，

The remainder comprising Fe and impurities,

a microstructure at a depth of 1/4 from the surface of the sheet thickness contains 90% or more of ferrite and less than 3% of retained austenite in terms of area fraction, an average crystal grain diameter excluding the retained austenite is 10.0 [ mu ] m or less, an average aspect ratio of crystal grains excluding the retained austenite is 0.3 or more, and a standard deviation of Mn concentration is 0.60 mass% or less,

the tensile strength is 980MPa or more.

2. The steel sheet according to claim 1, wherein the chemical composition contains 1 or 2 or more elements selected from the group consisting of:

Nb：0.001～0.100％、

V：0.005～1.00％、

Mo：0.001～1.00％、

Cu：0.02～1.00％、

Ni：0.02～1.00％、

Cr：0.02～2.00％、

B：0.0001～0.0020％、

Ca：0.0002～0.0100％、

Mg：0.0002～0.0100％、

REM: 0.0002 to 0.0100%, and

Bi：0.0001～0.0200％。

3. a plated steel sheet, characterized in that a plating layer is formed on the surface of the steel sheet according to claim 1 or 2.

4. The plated steel sheet according to claim 3, wherein the plating layer is a hot-dip galvanized layer.

5. Plated steel sheet according to claim 4, characterized in that the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.