WO2021187238A1

WO2021187238A1 - Steel sheet

Info

Publication number: WO2021187238A1
Application number: PCT/JP2021/009279
Authority: WO
Inventors: 吉田　充; 俊介谷口; 林　宏太郎; 洋志首藤
Original assignee: 日本製鉄株式会社
Priority date: 2020-03-19
Filing date: 2021-03-09
Publication date: 2021-09-23
Also published as: MX2022011351A; CN115298342B; CN115298342A; EP4123046A4; JP7277860B2; KR20220138402A; US20230140358A1; EP4123046A1; EP4123046B1; JPWO2021187238A1

Abstract

This steel sheet has a predetermined chemical composition, Ex. C determined by the formula Ex. C = (%C) – 12[(%Ti^*)/48 + (%V)/51 + (%Nb)/93 + (%Mo)/96 + (%W)/184] is 0.020% or less, the metallographic structure in a position at a depth of 1/4 of the sheet thickness from the surface thereof includes, in terms of area fraction, 60% or more of ferrite, 0-5% MA, and a total of 0-5% of pearlite and cementite, the remainder comprising bainite, and, in the metallographic structure, the average crystal grain size is 10.0 µm or less, the average aspect ratio of crystal grains is 0.30 or greater, the standard deviation of the Mn concentration is 0.60% by mass or less, Ti-based carbides having the Baker-Nutting orientation relationship in the ferrite are deposited in a semi-matched state, and the tensile strength of the steel sheet is 980 MPa or greater.

Description

Steel plate

The present invention relates to a steel sheet.
The present application claims priority based on Japanese Patent Application No. 2020-049120 filed in Japan on March 19, 2020, the contents of which are incorporated herein by reference.

In recent years, from the perspective of protecting the global environment, we have been working to reduce carbon dioxide emissions in many fields. Automakers are also actively developing technologies for reducing the weight of vehicle bodies with the aim of reducing fuel consumption. By reducing the weight of the steel material used, such as by reducing the thickness of the steel plate, the weight of the vehicle body can be easily reduced. However, in the case of automobiles, since the emphasis is also placed on improving collision resistance in order to ensure occupant safety, it is not possible to adopt weight reduction of the vehicle body by easily reducing the weight of steel used, and it is not easy to reduce the weight of the vehicle body. .. Therefore, in order to achieve both weight reduction of the vehicle body and collision resistance, it is being studied to thin the member by using a high-strength steel plate. On the other hand, a steel sheet applied to an automobile part is formed into a part shape, but when the strength of the steel sheet increases, the formability usually deteriorates. Therefore, it is strongly desired that a steel sheet applied to an automobile part has both high strength and excellent moldability. Specifically, steel sheets used for inner plate members, structural members, suspension members, etc. of automobiles are often subjected to stretch flange processing (hole expansion processing) and bending processing, so that they have high strength and are stretched and stretched flanges. It is required to have excellent properties and bendability.

For example, as shown in Patent Document 1, as a steel sheet capable of obtaining 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. .. However, while the DP steel sheet is excellent in elongation, voids may be generated from the interface between the ferrite phase and the martensite phase, which have significantly different hardness, and cracks may occur, so that the elongation flangeability and bending workability may be inferior. rice field.

Further, in Patent Document 2, it is obtained by setting the cooling rate in the temperature range from solidification of the slab to 1300 ° C. to 10 to 300 ° C./min and winding it at 500 ° C. or higher and 700 ° C. or lower after finish rolling. A high-strength hot-rolled steel sheet having a steel structure composed of a ferrite single phase and a tensile strength of 1180 MPa or more has been proposed. Patent Document 2 discloses that this high-strength hot-rolled steel sheet is excellent in bending workability. However, the high-strength hot-rolled steel sheet described in Patent Document 2 is manufactured by reheating the slab to less than 900 ° C. at which the ferrite phase begins to form and subjecting it to hot rolling. Therefore, there is a problem that the segregation formed at the time of solidification is not sufficiently reduced and the bending workability may not be stable. Further, in Patent Document 2, stretch flangeability is not considered.

Patent Document 3 states that Ti exceeding the solubility is solid-solved in γ by completing hot rolling within 5 hours after continuous casting, and fine TiC is formed along with ferrite transformation during winding 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 by precipitating the above, and a high-strength hot-rolled steel sheet obtained by the manufacturing method have been proposed. However, also in Patent Document 3, in order to suppress the precipitation of coarse TiC, since continuous casting to completion of hot finish rolling are performed in the austenite region, bending workability may be deteriorated due to Mn segregation. Further, in Patent Document 3, as in Patent Document 2, the stretch flangeability is not considered.

Japanese Patent Application Laid-Open No. 6-128688 Japanese Patent Application Laid-Open No. 2014-194053 Japanese Patent Application Laid-Open No. 2014-208876

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a steel sheet having high strength and excellent elongation, stretch flangeability, and bending workability. Here, the steel sheet of the present invention also includes a steel sheet having a coating such as a plating layer on the surface.

The present inventors examined a steel sheet having high strength, elongation, stretch flangeability and bending workability. As a result, by optimizing the chemical composition and manufacturing conditions, the metallographic structure of the steel sheet and the Mn segregation are controlled, and by controlling the precipitation form of Ti-based carbides, the strength is high and the elongation and elongation flanges are stretched. It was found that a steel sheet having excellent properties and bendability can be produced.

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

[1] The steel sheet according to one aspect of the present invention has a chemical composition of% by mass, C: 0.050 to 0.250%, Si: 0.005 to 2.000%, Mn: 0.10 to 3 .00%, P: 0.100% or less, S: 0.0100% or less, sol. Al: 0.001 to 1.00%, Ti: 0.150 to 0.400%, N: 0.0010 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 1.000% , Mo: 0 to 1.000%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Cr: 0 to 2.00%, W: 0 to 1.000%, B: 0 to It contains 0.0020%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Bi: 0 to 0.0200%, and the balance is Fe and impurities. Ex. C is 0.020% or less, and the metal structure at a depth of 1/4 of the plate thickness from the surface is 60% or more for ferrite, 0 to 5% for MA, and total of pearlite and cementite in area division. In the metal structure, the average crystal grain size is 10.0 μm or less, the average aspect ratio of the crystal grains is 0.30 or more, and the standard deviation of the Mn concentration is Ti-based carbides having a Baker-Nutting orientation relationship in the ferrite, which is 0.60% by mass or less, are precipitated in a semi-matched state, and the tensile strength is 980 MPa or more.
Ex. C = (% C) -12 {(% Ti ^* ) / 48+ (% V) / 51+ (% Nb) / 93+ (% Mo) / 96+ (% W) / 184} Equation (1) Here, the above (1) ^{"% Ti *} " in Eq. 1) is calculated from Eq. (2) below.
% Ti ^* =% Ti-48 × {(% N) / 14+ (% S) / 32} (2) Equation% C,% V,% Nb,% in the above equation (1) and the above equation (2) Mo,% W,% Ti,% N, and% S are the contents of C, V, Nb, Mo, W, Ti, N, and S in mass% in the steel sheet.
[2] The steel sheet according to [1] has a chemical composition of Nb: 0.001 to 0.100%, V: 0.005 to 1.000%, Mo: 0.001 to 1 in mass%. .000%, Cu: 0.02 to 1.00%, Ni: 0.02 to 1.00%, Cr: 0.02 to 2.00%, W: 0.02 to 1.000%, 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 It may contain one or more selected from the group consisting of 0.0200%.
[3] The steel sheet according to [1] or [2] may have a plating layer formed on its surface. [4] In the steel sheet according to [3], the plating layer may be a hot-dip galvanized layer.
[5] In the steel sheet according to [4], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.

According to the above aspect of the present invention, it is possible to provide a steel sheet having high strength and excellent elongation, stretch flangeability and bending workability. The steel plate of the present invention is suitable as a material used for applications such as automobiles, home appliances, mechanical structures, and constructions, and in particular, as a material for parts such as inner plate members, structural members, and suspension members of automobiles. If used, it not only contributes to weight reduction of the vehicle body and improvement of collision resistance characteristics, but is also easy to process into a part shape.

Hereinafter, the steel plate according to the embodiment of the present invention (the steel plate according to the present embodiment) will be described in detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.

First, the chemical composition of the steel sheet according to the present embodiment will be described.
In the numerical limitation range displayed with "~" described below, the values at both ends are included in the range as the lower limit value and the upper limit value. However, the numerical value indicated as "less than" or "greater than" is not included in the numerical range. In the following description, all%s related to the chemical composition of the steel sheet are mass%.

<Chemical composition of steel sheet>
(C: 0.050 to 0.250%)
C is an element that enhances the tensile strength of steel by combining with Ti and the like to generate carbides. If the C content is less than 0.050%, it becomes difficult to obtain a tensile strength of 980 MPa or more. Therefore, the C content is set to 0.050% or more. It is preferably 0.070% or more.
On the other hand, if the C content exceeds 0.250%, there is a concern that the weldability may decrease. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less, more preferably 0.200% or less, and even more preferably 0.180% or less.

(Si: 0.005 to 2.000%)
Si is an element that has the effect of increasing the tensile strength of steel by strengthening the solid solution and increasing the hardenability. Si is an element that also has an effect of suppressing the precipitation of cementite. If the Si content is less than 0.005%, it becomes difficult to exert the above action. Therefore, the Si content is set to 0.005% or more. The Si content is preferably 0.010% or more.
On the other hand, when the Si content exceeds 2.000%, the surface properties of the steel sheet are significantly deteriorated due to surface oxidation in the hot rolling process. Therefore, the Si content is set to 2.000% or less. The Si content is preferably 1.500% or less, more preferably 1.300% or less.

(Mn: 0.10 to 3.00%)
Mn is an element that has the effect of increasing the tensile strength of steel by strengthening the solid solution and increasing the hardenability. If the Mn content is less than 0.10%, the ferrite transformation is excessively promoted, and at high temperatures, Ti-based carbides are coarsely precipitated together with the ferrite transformation. In this case, it becomes difficult to obtain the tensile strength of the steel sheet of 980 MPa or more. Therefore, the Mn content is set to 0.10% or more. The Mn content is preferably 0.30% or more, more preferably 0.50% or more.
On the other hand, if the Mn content exceeds 3.00%, the ferrite transformation and the bainite transformation are delayed, and the desired ferrite surface integral cannot be obtained. In this case, the elongation is lowered, or the stretch flangeability and the bending workability are lowered due to the generation of MA. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.50% or less, more preferably 2.00% or less, and even more preferably 1.50% or less.

(Sol.Al: 0.001 to 1.00%)
Al is an element having an action of purifying steel by deoxidation in the steelmaking stage. sol. If the Al content is less than 0.001%, it becomes difficult to exert the above action. Therefore, sol. The Al content is 0.001% or more. sol. The Al content is preferably 0.01% or more, more preferably 0.02% or more, still more preferably 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. The Al content is 1.00% or less. sol. The Al content is preferably 0.80% or less, more preferably 0.60% or less. sol. Al means acid-soluble Al.

(Ti: 0.150 to 0.400%)
Ti is an element that combines with C to form Ti-based carbides and contributes to the improvement of the tensile strength of the steel sheet. Further, Ti is an element having an action of forming a Ti nitride to suppress coarsening of austenite during slab reheating and hot rolling to refine the metal structure. If the Ti content is less than 0.150%, it becomes difficult to obtain a tensile strength of 980 MPa or more due to insufficient precipitation strengthening amount. Therefore, the Ti content is set to 0.150% or more. The Ti content is preferably 0.170% or more, more preferably 0.190% or more, and even more preferably 0.210% or more.
On the other hand, when the Ti content becomes excessive, coarse Ti-based carbides remain in the austenite in an unsolid solution, so that elongation and bending workability are lowered, and Ti having a Baker-Nutting orientation relationship that contributes to strength. The carbides are reduced and the strength is reduced. Therefore, the Ti content is set to 0.400% or less. The Ti content is preferably 0.380% or less, more preferably 0.350% or less.

(N: 0.0010 to 0.0100%)
N is an element having an action of suppressing coarsening of austenite during slab reheating and hot rolling and refining the metal structure by forming Ti nitride. If the N content is less than 0.0010%, it becomes difficult to exert the above action. 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.
On the other hand, when the N content exceeds 0.0100%, coarse Ti nitride is formed and the stretch flangeability of the steel sheet deteriorates. Therefore, the N content is 0.0100% or less. The N content is preferably 0.0060% or less, more preferably 0.0050% or less.

(P: 0.100% or less)
P is an element contained in steel as an impurity, and has an action of lowering the stretch flangeability and 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 even more preferably 0.020% or less. Although P is mixed as an impurity from the raw material, it is not necessary to limit the lower limit thereof, and it is preferable that the P content is lower from the viewpoint of ensuring bending workability. 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, more preferably 0.005% or more.

(S: 0.0100% or less)
S is an element contained as an impurity and has an action of lowering the stretch flangeability and 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 even more preferably 0.0030% or less. Although S is mixed as an impurity from the raw material, it is not necessary to limit the lower limit thereof, and it is preferable that the S content is lower from the viewpoint of ensuring 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 even more preferably 0.0010% or more.

The rest of the chemical composition of the steel sheet according to this embodiment consists of Fe and impurities. In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., and are allowed as long as they do not adversely affect the steel sheet according to the present embodiment.
The steel sheet according to this embodiment may contain the following optional elements instead of a part of Fe. Since the steel sheet according to the present embodiment can solve the problem without containing an arbitrary element, the lower limit of the content of the arbitrary element is 0%.

(Nb: 0 to 0.100%)
Nb is an arbitrary element. Nb is an element that has the effect of increasing the tensile strength of the steel sheet by suppressing the coarsening of the crystal grain size of the steel sheet, by refining the ferrite grain size, and by precipitating and strengthening as NbC. In order to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, still more preferably 0.010% or more.
On the other hand, if the Nb content exceeds 0.100%, the above effects are saturated and there is a concern that the rolling load during finish rolling will increase. Therefore, when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.060% or less, more preferably 0.030% or less.

(V: 0 to 1.000%)
V is an arbitrary element. V is an element that dissolves 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 improving the tensile strength of the steel sheet by precipitation strengthening. be. In order to obtain these effects, the V content is preferably 0.005% or more. The V content is more preferably 0.010% or more, still more preferably 0.050% or more.
On the other hand, if the V content exceeds 1.000%, the carbides tend to become coarse and the bending workability may decrease. Therefore, when V is contained, the V content is set to 1.000% or less. The V content is preferably 0.800% or less, more preferably 0.600% or less.

(Mo: 0 to 1.000%)
Mo is an optional element. Mo is an element that has the effect of enhancing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel sheet. In order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more, further preferably 0.010% or more, and even more preferably 0.050% or more.
On the other hand, if the Mo content exceeds 1.000%, the crack sensitivity of a steel material such as a slab may increase. Therefore, when Mo is contained, the Mo content is 1.000% or less. The Mo content is more preferably 0.800% or less, still more preferably 0.600% or less.

(Cu: 0 to 1.00%)
Cu is an optional element. Cu is an element having the effect of improving the toughness of steel and the effect of increasing the tensile strength. In order to obtain these effects, the Cu content is preferably 0.02% or more.
On the other hand, if Cu is excessively contained, the weldability of the steel sheet may decrease. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less, more preferably 0.30% or less.

(Ni: 0 to 1.00%)
Ni is an optional element. Ni is an element that has the effect of improving the toughness of steel and the effect of increasing tensile strength. In order to obtain these effects, the Ni content is preferably 0.02% or more.
On the other hand, if Ni is excessively contained, 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 preferably 0.50% or less, more preferably 0.30% or less.

(Cr: 0 to 2.00%)
Cr is an arbitrary element. Cr is an element that has the effect of increasing the tensile strength by increasing the hardenability of steel. In order to obtain this effect, the Cr content is preferably 0.02% or more. The Cr content is more preferably 0.05% or more, still more preferably 0.10% or more.
On the other hand, if the Cr content becomes excessive, the chemical conversion processability deteriorates. Therefore, when Cr is contained, the Cr content is set to 2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00% or less, still more preferably 0.50% or less.

(W: 0 to 1.000%)
W is an arbitrary element. W is an element having the effect of forming carbides and carbonitrides and increasing the tensile strength. In order to obtain this effect, the W content is preferably 0.020% or more.
On the other hand, even if W is contained in a certain amount or more, the effect of the above action is saturated and the alloy cost increases. Therefore, when W is contained, the W content is set to 1.000% or less. The W content is preferably 0.800% or less.

(B: 0 to 0.0020%)
B is an arbitrary element. B is an element having an effect of increasing the tensile strength of the steel sheet by strengthening the grain boundaries and solid solution. In order to obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more.
On the other hand, if B is contained in excess of 0.0020%, not only the above effect is saturated but also 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.

(Ca: 0 to 0.0100%)
Ca is an optional element. Ca is an element that has the effect of dispersing a large number of fine oxides in molten steel and making the metal structure of the steel sheet finer. Further, Ca is an element having an effect of improving the stretch flangeability of the steel sheet by fixing S in the molten steel as a spherical CaS and suppressing the formation of stretching inclusions such as MnS. In order to obtain these effects, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, still more preferably 0.0010% or more.
On the other hand, if the Ca content exceeds 0.0100%, the amount of CaO in the steel may increase and the toughness of the steel sheet may deteriorate. Therefore, when Ca is contained, the Ca content is 0.0100% or less. The Ca content is preferably 0.0050% or less, more preferably 0.0030% or less.

(Mg: 0 to 0.0100%)
Mg is an optional element. Like Ca, Mg forms oxides and sulfides in molten steel to suppress the formation of coarse MnS, and also has the effect of dispersing a large number of fine oxides and refining the metal structure of the steel sheet. Is. In order to obtain these effects, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0005% or more, still more preferably 0.0010% or more.
On the other hand, if the Mg content exceeds 0.0100%, the oxide in the steel may increase and the toughness of the steel sheet may deteriorate. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less. The Mg content is preferably 0.0050% or less, more preferably 0.0030% or less.

(REM: 0 to 0.0100%)
REM is an optional element. Similar to Ca, REM also has the effect of forming oxides and sulfides in molten steel to suppress the formation of coarse MnS, dispersing a large number of fine oxides, and refining the metal structure of the steel sheet. It is an element. When these effects are obtained, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.0005% or more, still more preferably 0.0010% or more.
On the other hand, if the REM content exceeds 0.0100%, oxides in the steel may increase and the toughness of the steel sheet may deteriorate. Therefore, when REM is contained, the REM content is 0.0100% or less. The REM content is preferably 0.0050% or less, more preferably 0.0030% or less.
Here, REM (rare earth) refers to a total of 17 elements composed of Sc, Y and lanthanoids. In this embodiment, the REM content refers to the total content of these elements.

(Bi: 0 to 0.0200%)
Bi is an arbitrary element. Bi is an element having the effect of refining the solidified structure and improving the formability of the steel sheet. In order to obtain this effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0005% or more.
On the other hand, when 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 0.0200% or less. It is preferably 0.0100% or less, and more preferably 0.0070% or less.

(Ex.C: 0.020% or less)
C is precipitated as a Ti-based carbide and contributes to increasing the strength of the steel sheet. However, if C is contained in an amount exceeding the amount precipitated as a Ti-based carbide, this excess C produces pearlite, cementite, MA, etc., and as a result, the stretch flangeability and bendability are lowered. ..
Ex. Ex. C corresponds to the C content exceeding the amount precipitated as a Ti-based carbide. In the steel sheet according to this embodiment, this Ex. C is 0.020% or less. It is preferably 0.018% or less, and more preferably 0.015% or less. The lower limit is not particularly limited.
Ex. C = (% C) -12 {(% Ti ^* ) / 48+ (% V) / 51+ (% Nb) / 93+ (% Mo) / 96+ (% W) / 184} Equation (1) Here, (1) ) "% Ti ^* " in the formula is calculated from the following formula (2).
% Ti ^* =% Ti-48 × {(% N) / 14+ (% S) / 32} (2) Equations (1),% C,% V,% Nb,% Mo in equations (2), % W,% Ti,% N, and% S are the contents of C, V, Nb, Mo, W, Ti, N, and S in mass%, respectively, in the steel sheet.

Next, the metal structure of the steel sheet will be described. 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 60% or more of ferrite, 0 to 5% of MA, and 0 to 5% of pearlite and cementite in total, and the balance. Consists of bainite. Further, in the metal structure, the average crystal grain size is 10.0 μm or less, the average aspect ratio of the crystal grains is 0.30 or more, and the standard deviation of the Mn concentration is 0.60 mass% or less. In addition, Ti-based carbides having a Baker-Nutting orientation relationship in ferrite are precipitated in a semi-matched state.
Here, the reason for defining the metal structure at the position of 1/4 depth of the plate thickness in the plate thickness direction from the surface of the steel plate (the position of t / 4 from the surface when the plate thickness is t) is at this position. This is because the metal structure is a typical metal structure of a steel sheet.

(Surface integral of ferrite: 60% or more)
(MA area fraction: 0-5%)
(Total surface integral of pearlite and cementite: 0-5%)
(Remaining: Bainite organization)
Ferrite is required to obtain good elongation. If the surface integral is less than 60%, the elongation will decrease. Therefore, the surface integral of ferrite is set to 60% or more. The surface integral of ferrite is preferably 70% or more, more preferably 80% or more, and may be 100% (that is, ferrite single phase).
The metallographic structure may contain a small amount of MA in addition to ferrite, but it is acceptable if the surface integral is 5% or less. It is preferably 4% or less, more preferably 3% or less, and most preferably 2% or less. In addition, pearlite and cementite may precipitate, but it is permissible if the total surface integral is 5% or less. It is preferably 4% or less, more preferably 3% or less, and most preferably 2% or less. If the surface integral of MA is more than 5%, the bending workability and the hole expanding property are lowered. Alternatively, if the surface integral of pearlite and cementite is more than 5%, the hole-spreading property is lowered.
In the metallographic structure, the rest other than the above consists of bainite. Bainite has a small difference in hardness from ferrite precipitated and strengthened with Ti-based carbides. Therefore, as compared with MA (Martensite-Austenite Constituents), pearlite and cementite, the effect of reducing the hole-spreading property is small. Therefore, the rest of the tissue is bainite.

(Average crystal grain size: 10.0 μm or less)
If the average crystal grain size is large, the bending workability is lowered. Therefore, in the metal structure, the average crystal grain size is set to 10.0 μm or less. It is preferably 8.0 μm or less. The smaller the average crystal grain size, the more preferable, so the lower limit is not particularly limited. However, in ordinary hot rolling, it is technically difficult to make the particles finer so that the average crystal grain size is less than 1.0 μm. Therefore, the average crystal grain size may be 1.0 μm or more.
In the present embodiment, the "average crystal grain size" means that the crystal structure is bcc, that is, ferrite, bainite, martensite, and pearlite are surrounded by grain boundaries having a crystal orientation difference of 15 ° or more, and the diameter corresponding to a circle is 0. It means the average value of the crystal grain size in which the region of 3 μm or more is defined as the crystal grain, and the crystal grain size of retained austenite is not included in the average crystal grain size.

(Average aspect ratio of crystal grains: 0.30 or more)
In this embodiment, the average aspect ratio of the bcc crystal grains is 0.30 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 takes a value from 0 to 1.00. The smaller the average aspect ratio of the crystal grains, the flatter the crystal grains, and the closer to 1.00, the equiaxed grains. When the average aspect ratio of the crystal grains is less than 0.30, there are many flat crystal grains, the anisotropy of the material becomes large, and the stretch flangeability and the bending workability deteriorate. Therefore, the average aspect ratio of the crystal grains excluding retained austenite is set to 0.30 or more. The closer the crystal grains are to the equiaxed axis, the smaller the anisotropy and the better the processability. Therefore, the average aspect ratio of the crystal grains excluding retained austenite is better as it is closer to 1.00.

In the present embodiment, the average crystal grain size, the average aspect ratio of the crystal grains, and the area division of the metal structure are 1/4 depth from the surface of the steel plate to the plate thickness of the steel plate cross section parallel to the rolling direction and the plate thickness direction. Electron backscatter diffraction (Electron Backscatter Diffraction) and EBSD (Electron Backscattering Diffraction) observation and EBSD (Electron Backscatter Diffraction) Method) Obtained by analysis. Crystal orientation information is obtained by distinguishing fcc and bcc at intervals of 0.2 μm in a region of 200 μm in the rolling direction and 100 μm in the plate thickness direction centered on a 1/4 depth position of the plate thickness from the surface of the steel plate. Using the software attached to the EBSD analyzer (“OIMAnaisys®” manufactured by AMETEK, Inc.), the crystal grain boundaries having a crystal orientation difference of 15 ° or more are specified. The average crystal grain size of bcc is surrounded by crystal grain boundaries having a crystal orientation difference of 15 ° or more, and a region having a diameter equivalent to a circle of 0.3 μm or more is defined as a crystal grain, and the area average diameter is obtained.

The grain boundaries having a crystal orientation difference of 15 ° or more are mainly ferrite grain boundaries, martensite, and bainite block boundaries. In the method for measuring the ferrite grain size according to JIS G 0552: 2013, the grain size may be calculated even for ferrite grains with a crystal orientation difference of less than 15 °, and martensite and bainite blocks are not calculated. .. Therefore, as the average crystal grain size in this embodiment, the value obtained by EBSD analysis as described above is adopted. In the EBSD analysis, the length of the major axis and the length of the minor axis of each crystal grain are also obtained at the same time. Therefore, by adopting this method, the average aspect ratio of the bcc crystal grains can also be obtained.

The surface integral of ferrite is measured by the following method. Here, a region surrounded by crystal grain boundaries having a crystal orientation difference of 5 ° or more and having a diameter equivalent to a circle of 0.3 μm or more is defined as a crystal grain. In the crystal grains, the surface integral of the crystal grains whose value (GAM value) obtained by the Grain Average Composition analysis equipped in the OIMA analysis is 0.6 ° or less is calculated. By such a method, the surface integral of ferrite is obtained. The reason why the boundary with a crystal orientation difference of 5 ° or more is defined as a grain boundary when determining the surface integral of ferrite is that different metal structures generated by variants close to the same old austenite grain may not be distinguishable. be.
The surface integrals of pearlite and cementite are obtained by SEM observation of the metallographic structure exposed by nital corrosion. The surface integral of MA is obtained by observing the structure exposed by the repera corrosion with an optical microscope. The surface integral may be obtained by image analysis or by a point calculation method. For example, pearlite and cementite are observed in a region at a depth of 1/4 of the plate thickness from the surface of the steel sheet at a magnification of 1000 times for 3 or more fields of view (100 μm × 100 μm / field of view), and are obtained by a point calculation method with a lattice spacing of 5 μm. You can. The surface integral of MA is a point calculation method in which two or more visual fields (200 μm × 200 μm / visual field) are observed at a magnification of 500 times in a region at a depth of 1/4 of the plate thickness from the surface of the steel plate, and the grid spacing is 5 μm. You can ask for it.

(Standard deviation of Mn concentration: 0.60% by mass or less)
The standard deviation of the Mn concentration at a depth of 1/4 of the thickness of the steel sheet according to the present embodiment is 0.60% by mass or less. As a result, the local variation in tensile strength due to Mn segregation is reduced, and good bending workability can be stably obtained. The smaller the standard deviation value of the Mn concentration is, the more desirable it is, but due to the restrictions of the manufacturing process, the practical lower limit is 0.10% by mass.

The standard deviation of the Mn concentration is 1/4 depth from the surface of the steel sheet after the sample is sampled so that the cross section parallel to the rolling direction and the thickness direction of the steel sheet is the observation surface and the observation surface is mirror-polished. It is obtained by measuring the rolling position with an electron probe microanalyzer (EPMA). The measurement conditions are that the acceleration voltage is 15 kV, the magnification is 5000 times, and the distribution image in the range of 20 μm in the rolling direction of the sample and 20 μm in the plate thickness direction of the sample is measured. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration at 40,000 or more points is measured. Next, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentration obtained from all the measurement points.

(Ti carbide)
In the steel sheet according to the present embodiment, Ti-containing carbides (Ti-based carbides) are precipitated in the ferrite. Ti is an element having a high driving force for precipitation of carbides in ferrite, and it becomes easy to control the precipitation state of carbides by controlling the content and heat treatment. In addition, Ti-based carbides also have a high precipitation strengthening ability. Here, the Ti-based carbide refers to a carbide having a NaCl-type crystal structure containing Ti. If the carbide contains Ti, even if a small amount of other carbide-forming alloying elements are contained, the above-mentioned driving force is not significantly reduced, so that an effect can be obtained. Within the range of chemical composition defined in this embodiment, the Ti-based carbide may contain other carbide-forming alloying elements such as Mo, Nb, V, Cr and W. Further, in the Ti-based carbide, even if the carbonitride in which a part of the carbon is replaced with nitrogen does not change the precipitation state, the effect can be obtained.

(Ti-based carbides in ferrite precipitate in a semi-matched state)
When the ratio of Ti-based carbides whose interface with ferrite is a semi-matching interface to Ti-based carbides precipitated in ferrite with a Baker-Nutting orientation relationship is 50% or more, the stretch flangeability of the steel sheet is It becomes stable and good. The state in which "Ti-based carbides are precipitated in a semi-matched state" in the present embodiment refers to such a case. When the Ti-based carbide is not semi-matched precipitation, the hole-spreading property is lowered.
Whether or not the Ti-based carbide having the directional relationship of Baker-Nutting is in the semi-matched state is determined as follows. That is, in a thin film sample for a transmission electron microscope prepared from a depth of 1/4 of the plate thickness from the surface, the ring detector is detected by scanning transmission electron microscopy (magnification: 910,000 to 5,100,000 times). An annular dark-field scanning transmission electron microscope image in which the angle is set between 60 mrad and more and 200 mrad or less is photographed by incident an electron beam from the [001] direction of ferrite. Baker-Nutting orientation of plate-shaped particles with the ferrite (100) plane of the matrix as the crystal habit plane and plate-shaped particles with the (010) plane of the ferrite as the crystal habit plane. As the Ti-based carbide having a relationship, the crystal habit plane of the (100) plane of the particles having a plate-like shape with the (100) plane of ferrite as the crystal habit plane, or the (010) plane of ferrite as the crystal habit plane. If the number of crystal planes of the {010} plane of ferrite sandwiching the crystal habit plane of the (010) plane of the plate-shaped particles and the {01-1} plane of the Ti-based carbide are the same, it is considered to be in a matched state. If the number of crystal planes does not match, it is judged to be in a semi-matched state. When 20 or more Ti-based carbides are observed and 50% or more are in a semi-matched state, the Ti-based carbides having a Baker-Nutting orientation relationship of the steel material from which the observed thin film sample for a transmission electron microscope is collected are in a semi-matched state. Judge that.

Regarding the size of Ti-based carbides, in general, the larger the carbides, the smaller the number density tends to be. In the present invention, the thickness of the Ti-based carbide may be 1 nm or more and 5 nm or less from the viewpoint of ensuring the number density of the Ti-based carbides precipitated in the ferrite having a Baker-Nutting orientation relationship.

The thickness of Ti-based carbide is measured by the following method.
A thin film sample for a transmission electron microscope is prepared from a depth position of 1/4 in the plate thickness direction from the surface of the steel plate, and observed with a scanning transmission electron microscope (hereinafter, also referred to as "STEM"). In the Ti-based carbides having plate surfaces formed on the (100) plane and (010) plane of the ferrite observed in the STEM image taken by injecting an electron beam in the [001] direction of the ferrite, the ferrite [100] and [ Of the sizes of Ti-based carbides measured along the 010] direction, the length of the small side is defined as the thickness. In addition, when evaluating the thickness of Ti-based carbides, the interatomic distances of 10 unit lattices in the [100] direction and [010] direction of ferrite at locations where precipitates are not seen in the image, respectively. The scale is calibrated so that is 2.866 nm.

<Mechanical characteristics>
(Tensile strength: 980 MPa or more)
The steel sheet according to the present embodiment has high strength and excellent elongation, stretch flangeability and bending workability by controlling the metallographic structure, the precipitation form of Ti-based carbides and the Mn segregation. However, if the tensile strength of the steel sheet is small, the effects of reducing the weight of the vehicle body and improving the rigidity are small. Therefore, the tensile strength (TS) of the steel sheet according to this embodiment is set to 980 MPa or more. It is preferably 1080 MPa or more. The upper limit is not particularly specified, but as the tensile strength increases, press molding becomes difficult. Therefore, the tensile strength may be 1800 MPa or less.
In the steel sheet according to the present embodiment, in terms of formability, the target is TS × El, which is an index of the balance between strength and elongation, to be 14000 MPa ·% or more, and the index of the balance between strength and elongation and flangeability is used. The purpose is that TS × λ is 50,000 MPa ·% or more. TS × El is more preferably 15,000 MPa ·% or more. TS × λ is more preferably 55,000 MPa ·% or more, further preferably 60,000 MPa ·% or more, and even more preferably 65,000 MPa ·% or more.

The tensile strength and elongation of the steel sheet are evaluated by the tensile strength and the total elongation at break (El) using the No. 5 test piece specified in JIS Z 2241: 2011. The stretch flangeability of the steel sheet is evaluated by the hole expansion ratio (λ) specified in JIS Z 2256: 2010.

<Manufacturing method>
The reason for limiting the manufacturing conditions of the steel sheet according to the present embodiment will be described.
The present inventors have confirmed that the steel sheet according to the present embodiment can be obtained by a manufacturing method including the following heating step, hot rolling step, cooling step and winding step.

[Heating process]
First, a slab or steel piece having the above-mentioned chemical composition is heated. The slab or steel piece may be obtained by continuous casting or casting / slab rolling, but may be obtained by adding hot working or cold working to them.

(Dwelling time in the temperature range of 700 to 850 ° C during heating: 900 seconds or more)
When heating a slab or a piece of steel to be subjected to hot rolling, it is allowed to stay in a temperature range of 700 to 850 ° C. for 900 seconds or more. In the austenite transformation that occurs in the temperature range of 700 to 850 ° C., Mn is partitioned between ferrite and austenite. Therefore, Mn can be diffused in the ferrite region by lengthening the residence time and lengthening the transformation time. As a result, the Mn microsegregation unevenly distributed in the slab is eliminated, and the standard deviation of the Mn concentration becomes remarkably small.

(Heating temperature: 1280 ° C or higher and SRT (° C) or higher)
The heating temperature of the slab or steel piece to be subjected to hot rolling shall be 1280 ° C. or higher and the temperature SRT (° C.) or higher represented by the following equation (3). If the heating temperature is less than 1280 ° C., the reduction of the standard deviation of the Mn concentration due to the diffusion of Mn during heating may be insufficient. Further, if it is less than SRT (° C.), the solution of Ti carbonitride becomes insufficient, and in either case, the tensile strength and bending workability of the steel sheet are lowered. Therefore, the temperature of the slab or steel piece to be subjected to hot rolling is 1280 ° C. or higher and SRT (° C.) or higher. Here, "the temperature of the slab or steel piece is 1280 ° C. or higher and SRT (° C.) or higher" means that the temperature of the slab or steel piece is higher than the higher temperature of 1280 ° C. and SRT (° C.). , Or the higher temperature of 1280 ° C and SRT (° C), which means that the temperature of the slab or piece of steel is the same.
On the other hand, if the heating temperature exceeds 1400 ° C., a thick scale may be generated to reduce the yield or significantly damage the heating furnace. Therefore, the heating temperature is preferably 1400 ° C. or lower.
SRT (° C.) = 1630 + 90 × ln ([C] × [Ti])… (3)
However, the [element symbol] in the above equation (3) indicates the content of each element in mass%.

[Hot rolling process]
In the hot rolling step, the slab or steel piece after the heating step is subjected to multi-pass hot rolling using a plurality of rolling stands to obtain a hot-rolled steel sheet. The hot rolling process is divided into rough rolling and finish rolling performed after rough rolling.
Multi-pass hot rolling can be performed using a lever 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 stages.

(Time from the start of rough rolling to the completion of finish rolling: 600 seconds or less)
Since the precipitation of Ti-based carbides is promoted by rolling and starts to precipitate, if the time until the finish rolling is completed is too long, a large amount of coarse Ti-based carbides are precipitated in the austenite. In this case, the fine Ti-based carbides precipitated in the ferrite after finish rolling, which contribute to increasing the strength, are reduced, 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 to the completion of finish rolling is set to 600 seconds or less. It is preferably within 500 seconds, more preferably within 400 seconds, and most preferably within 320 seconds.
Normally, in the hot rolling process, the rolling reduction and rolling temperature are controlled according to the specifications of the rolling mill, the thickness and width of the coil to be manufactured, and the desired material, but from the start of rough rolling to the end of finish rolling. There is no overall control over time. The present inventors have newly found that the time from the start of rough rolling to the completion of finish rolling affects the precipitation state of Ti-based carbides.

(Total reduction rate in the temperature range of 850 to 1100 ° C: 90% or more)
By performing hot rolling in which the total rolling reduction in the temperature range of 850 to 1100 ° C. is 90% or more, the recrystallized austenite is mainly miniaturized and the strain energy is accumulated in the unrecrystallized austenite. Be promoted. As a result, the recrystallization of austenite is promoted and the atomic diffusion of Mn is promoted, and the standard deviation of the Mn concentration becomes small. Therefore, in hot rolling, the total reduction rate (cumulative reduction rate) in the temperature range of 850 to 1100 ° C. is set to 90% or more.
The total reduction rate in the temperature range of 850 to 1100 ° C. is when the inlet plate thickness before the first pass in rolling in this temperature range is t0 and the outlet plate thickness after the final pass in rolling in this temperature range is t1. , (T0-t1) / t0 × 100 (%).

(Finish rolling completion temperature FT (° C): TR (° C) or higher and 1080 ° C or lower)
When the FT (° C.) is less than TR (° C.) represented by the following equation (4), remarkably flat austenite is formed before cooling after finish rolling, and the metal structure elongated in the rolling direction is formed in the final product steel sheet. As a result, the average aspect ratio of the crystal grains having a bcc structure excluding retained austenite becomes smaller and the plastic anisotropy becomes larger. In this case, the elongation, stretch flangeability and / or bendability of the steel sheet is reduced. Therefore, the FT (° C.) is set to TR (° C.) or higher.
On the other hand, when the FT (° C.) exceeds 1080 ° C., the structure becomes coarse and the bendability of the steel sheet deteriorates. Therefore, the FT (° C.) is 1080 ° C. or lower. The FT (° C.) is preferably 1060 ° C. or lower.
The temperature during finish rolling refers to the surface temperature of the steel material and can be measured with a radiation thermometer or the like.
TR (° C.) = 805 + 385 x [Ti] + 584 x [Nb] (4)
However, the [element symbol] in the above equation (4) indicates the content of each element in mass%, and if it is not contained, 0 is substituted.

[Cooling process]
In the method for manufacturing a steel sheet according to the present embodiment, as a next step of the hot rolling step, the hot-rolled steel sheet is cooled with water at an average cooling rate of 45 ° C./sec or more to a temperature range of 650 to 800 ° C. Has a cooling process. Further, in the method for manufacturing a steel sheet according to the present embodiment, the cooling step is started within 3.0 seconds after the completion of the hot rolling step (after the completion of finish rolling).

(Time from the completion of finish rolling to the start of water cooling: within 3.0 seconds)
If the time from the completion of finish rolling to the start of water cooling exceeds 3.0 seconds, the tensile strength and bending workability deteriorate due to the growth of finely divided austenite crystal grains and the coarse precipitation of carbonitrides such as Ti. Therefore, in the method for manufacturing a steel sheet according to the present embodiment, water cooling is started within 3.0 seconds after the completion of finish rolling. It is preferably within 2.0 seconds, more preferably within 1.5 seconds.

(Average cooling rate from the start of water cooling after the completion of finish rolling to the water cooling stop temperature of 650 to 800 ° C: 45 ° C / sec or more)
When the average cooling rate to the water cooling stop temperature between 650 and 800 ° C. is less than 45 ° C./sec, coarse Ti-based carbides are precipitated in untransformed austenite or in transformed ferrite grains to obtain the desired strength. It becomes difficult. Therefore, the average cooling rate is 45 ° C./sec or higher. It is preferably 50 ° C./sec or higher, more preferably 55 ° C./sec or higher. The upper limit is not particularly limited, but is preferably 300 ° C./sec or less from the viewpoint of equipment cost. The average cooling rate is a value obtained by dividing the amount of temperature drop from the start of water cooling to the stop of water cooling by the required time after the completion of hot rolling.

(Dwelling time in the temperature range of 650 to 800 ° C: 5 to 50 seconds)
The steel sheet is cooled to 650 to 800 ° C. at an average cooling rate of 45 ° C./sec or higher, and then retained in the temperature range. If the residence time at 650 to 800 ° C. is short, it becomes difficult to obtain the desired ferrite surface integral, so the residence time needs to be 5 seconds or more. The residence time is preferably 7 seconds or more. On the other hand, if the residence time is long, pearlite is generated and the hole expanding property is lowered. Therefore, the residence time is set to 50 seconds or less in this temperature range. The residence time is preferably 40 seconds or less.
Further, while staying at 650 to 800 ° C., the ferrite transformation progresses and Ti-based carbides having a semi-matched interface are precipitated in the ferrite to obtain a steel sheet having excellent tensile strength and hole expandability. When Ti-based carbides are precipitated at a temperature higher than 800 ° C., they are coarsely precipitated and a desired number density cannot be obtained, making it difficult to obtain a desired tensile strength. On the other hand, when the Ti-based carbide is precipitated at a temperature lower than 650 ° C., the Ti-based carbide having a matching interface is precipitated and the hole expanding property is deteriorated.

(Average cooling rate in the temperature range of 550 to 650 ° C: 45 ° C / sec or more)
After the above retention, the steel sheet is cooled to a temperature of 550 ° C. or lower (winding temperature) so that the average cooling rate in the temperature range of 550 to 650 ° C. is 45 ° C./sec or more. If the average cooling rate is less than 45 ° C./sec, Ti-based carbides having a matching interface are precipitated during cooling, and the hole-spreading property is deteriorated. The upper limit of the average cooling rate is not particularly limited, but is preferably 300 ° C./sec or less from the viewpoint of equipment cost.

[Winding process]
(Taking temperature: 350 ° C or more and less than 550 ° C)
After the cooling step, the steel sheet is wound at 350 ° C. or higher and lower than 550 ° C. If the winding temperature is less than 350 ° C., untransformed austenite is transformed into martensite, and the hole-expanding property and bending workability are deteriorated. On the other hand, when the winding temperature is 550 ° C. or higher, Ti-based carbide having a matching interface is generated after winding, and the hole expanding property is lowered. The winding temperature is preferably 400 ° C. or higher and lower than 500 ° C.

In the present embodiment, a plated steel sheet having a plating layer may be obtained by plating the surface of the steel sheet after the winding step. Even in the case of plating, there is no problem as long as the plating is performed after satisfying the conditions of the steel sheet manufacturing method according to the present embodiment. The plating may be either electroplating or hot-dip plating, and the type of plating 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 electrozinc-nickel alloy plated steel sheet, a hot dip galvanized steel sheet, an alloyed hot dip galvanized steel sheet, and a hot dip galvanized steel sheet. The amount of plating adhered may be a general amount. Before plating, Ni or the like may be applied to the surface as pre-plating.
When producing the steel sheet according to the present embodiment, known temper rolling may be appropriately performed for the purpose of shape correction.

The plate thickness of the steel sheet according to the present embodiment is not particularly limited, but if the plate thickness is too thick, the metallographic structure generated between the surface layer of the steel sheet and the inside is significantly different, so 8.0 mm or less is preferable. More preferably, it is 6.0 mm or less. On the other hand, if the plate thickness is too thin, it becomes difficult to pass the plate during hot rolling, so 1.0 mm or more is generally preferable. More preferably, it is 1.2 mm or more.

Next, the effect of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one-condition example. The present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

A steel material having a chemical composition shown in Tables 1A and 1B (unit mass%, the balance is Fe and impurities) and having a plate thickness of 250 mm is hot-rolled under the conditions shown in Tables 2A and 2B to obtain a plate thickness of 2. A hot-rolled steel sheet of 5 to 3.5 mm was used. A part of the obtained hot-dip steel sheet was subjected to hot-dip galvanizing treatment at a quenching temperature of 700 ° C. and further alloying treatment to obtain a hot-dip galvanized steel sheet (GI) or an alloyed hot-dip galvanized steel sheet (GA).

With respect to the obtained steel sheet (hot-rolled steel sheet, plated steel sheet), the metal structure was observed at a position at a depth of 1/4 of the plate thickness from the surface of the steel sheet, and the area fraction of each structure and the average of the crystal grains having a bcc structure were observed. The crystal grain size, average aspect ratio, and standard deviation of Mn concentration were determined.

The area fraction of the metal structure at a depth of 1/4 of the plate thickness from the surface of the steel plate, the average crystal grain size and average aspect ratio of the crystal grains having a bcc structure are determined by the cross-section of the steel plate parallel to the rolling direction and the plate thickness direction. The metallographic structure at a depth of 1/4 of the plate thickness from the surface of the steel plate can be observed with a scanning electron microscope (SEM) using an EBSD analyzer composed of a thermal electric field radiation scanning electron microscope and an EBSD detector. It was determined by EBSD (Electron Backscattering Diffraction) analysis.
At that time, crystal orientation information is obtained by distinguishing fcc and bcc in a region of 200 μm in the rolling direction centered on the 1/4 depth position of the sheet thickness from the surface of the steel sheet and 100 μm in the plate thickness direction at 0.2 μm intervals. rice field. The crystal grain boundaries having a crystal orientation difference of 15 ° or more were identified using the software attached to the EBSD analyzer (“OIMAnalesis (registered trademark)” manufactured by AMETEK, Inc.). The average crystal grain size of bcc is surrounded by crystal grain boundaries having a crystal orientation difference of 15 ° or more, and a region having a circle-equivalent diameter determined to be bcc and having a diameter of 0.3 μm or more is defined as a crystal grain, and the area average diameter is defined as the crystal grain. I asked.

The surface integral of ferrite was measured by the following method.
A region surrounded by a grain boundary having a crystal orientation difference of 5 ° or more and having a diameter equivalent to a circle determined to be bcc and having a diameter of 0.3 μm or more was defined as a crystal grain. The surface integral of the crystal grains having a value (GAM value) of 0.6 ° or less obtained by the Grain Average Composition analysis equipped in the OIMA analysis was calculated in the crystal grains.

For the area fraction of pearlite and cementite, the metallographic structure exposed by nital corrosion in the region at a depth of 1/4 of the plate thickness from the surface of the steel plate was observed in 3 fields at a magnification of 1000 times using SEM. It was obtained by a point calculation method with a lattice spacing of 5 μm. As for the area fraction of MA, the structure exposed by the repera corrosion in the region at a depth of 1/4 of the plate thickness from the surface of the steel plate was observed in two fields at a magnification of 500 times using an optical microscope. It was obtained by a point calculation method with a lattice spacing of 5 μm.
Although not shown in the table, the rest of the metallographic structure was bainite.

The standard deviation of the Mn concentration is determined by mirror-polishing the cross section of the steel sheet parallel to the rolling direction and the sheet thickness direction, and then measuring the 1/4 depth position of the sheet thickness from the surface of the steel sheet with an electron probe microanalyzer (EPMA). Obtained. As the measurement conditions, the acceleration voltage was 15 kV, the magnification was 5000 times, and the distribution image in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction was measured. More specifically, the measurement interval was set to 0.1 μm, and the Mn concentration was measured at 40,000 or more places. Next, the standard deviation of the Mn concentration was obtained by calculating the standard deviation based on the Mn concentration obtained from all the measurement points.

In order to evaluate the mechanical properties of the obtained steel sheet, the tensile strength TS (MPa) and the total elongation at break El (%) were measured in accordance with JIS Z 2241: 2011. In addition, the hole expansion ratio (λ) was measured according to JIS Z 2256: 2010.
The bending workability was evaluated by a 90 ° V bending test in which the bending radius was twice the plate thickness.
Tables 3A and 3B show the test results of metallographic structure and mechanical properties.

The tensile strength was considered to be high when it was 980 MPa or more.
The elongation was considered to be excellent when the product of the tensile strength and the total elongation at break (TS × El) was 14,000 MPa ·% or more. Further, when TS × λ is 50,000 MPa ·% or more, the stretch flangeability is considered to be excellent. Bending workability was tested three times, and all test pieces that did not crack during the bending test were considered to have excellent bending workability (OK), and those that had one or more cracks were bent. The sex was not sufficient (NG).

As shown in Tables 3A and 3B, the invention examples satisfying the requirements of the present invention were excellent in all of TS, TS × El and bending workability. On the other hand, in the comparative example which does not satisfy at least one of the requirements of the present invention, at least one of TS, TS × El and bending workability was inferior.

According to the present invention, it is possible to provide a steel sheet having high strength and excellent elongation, stretch flangeability and bending workability. The steel plate of the present invention is suitable as a material used for applications such as automobiles, home appliances, mechanical structures, and constructions, and in particular, as a material for parts such as inner plate members, structural members, and suspension members of automobiles. If used, it not only contributes to weight reduction of the vehicle body and improvement of collision resistance characteristics, but is also easy to process into a part shape. Therefore, the steel sheet of the present invention has an extremely remarkable industrial contribution.

Claims

The chemical composition is mass%,
C: 0.050 to 0.250%,
Si: 0.005 to 2.000%,
Mn: 0.10 to 3.00%,
P: 0.100% or less,
S: 0.0100% or less,
sol. Al: 0.001 to 1.00%,
Ti: 0.150 to 0.400%,
N: 0.0010-0.0100%,
Nb: 0 to 0.100%,
V: 0 to 1.000%,
Mo: 0 to 1.000%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 2.00%,
W: 0 to 1.000%,
B: 0 to 0.0020%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0-0.0200%,
Containing, the balance consists of Fe and impurities,
Ex. Ex. C is 0.020% or less,
The metallographic structure at a depth of 1/4 of the plate thickness from the surface contains 60% or more of ferrite, 0 to 5% of MA, 0 to 5% of pearlite and cementite in total, and the balance is bainite. Consists of
In the metal structure
The average crystal grain size is 10.0 μm or less,
The average aspect ratio of the crystal grains is 0.30 or more,
The standard deviation of the Mn concentration is 0.60% by mass or less,
Ti-based carbides having a Baker-Nutting orientation relationship in the ferrite are precipitated in a semi-matched state.
The tensile strength is 980 MPa or more.
A steel plate characterized by that.
Ex. C = (% C) -12 {(% Ti * ) / 48+ (% V) / 51+ (% Nb) / 93+ (% Mo) / 96+ (% W) / 184} Equation (1) Here, the above (1) "% Ti * " in Eq. 1) is calculated from Eq. (2) below.
% Ti * =% Ti-48 × {(% N) / 14+ (% S) / 32} (2) Equation% C,% V,% Nb,% in the above equation (1) and the above equation (2) Mo,% W,% Ti,% N, and% S are the contents of C, V, Nb, Mo, W, Ti, N, and S in mass% in the steel sheet.
When the chemical composition is mass%,
Nb: 0.001 to 0.100%,
V: 0.005 to 1.000%,
Mo: 0.001 to 1.000%,
Cu: 0.02 to 1.00%,
Ni: 0.02 to 1.00%,
Cr: 0.02-2.00%,
W: 0.02 to 1.000%,
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: 0.0001-0.0200%
The steel sheet according to claim 1, wherein the steel sheet contains one kind or two or more kinds selected from the group consisting of.
The steel sheet according to claim 1 or 2, wherein a plating layer is formed on the surface of the steel sheet.
The steel sheet according to claim 3, wherein the plating layer is a hot-dip galvanized layer.
The steel sheet according to claim 4, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.