JP7485240B1 - Steel plates, components and their manufacturing methods - Google Patents

Abstract

The present invention provides a steel sheet, a member, and a manufacturing method thereof, which have a tensile strength of 980 MPa or more, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the sheet width direction, and have a specific range of component composition and steel structure, in which the total area ratio of quenched martensite and retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more is 20% or less, and the area ratio of C-enriched regions (SC≧0.5) having a C concentration of 0.5 mass% or more relative to the entire structure is 15% or less.

Description

The present invention relates to a steel plate, a member, and a manufacturing method thereof. More specifically, the present invention relates to a steel plate, a member, and a manufacturing method thereof, which has a tensile strength (TS) of 980 MPa or more and has excellent formability and material stability. The steel plate of the present invention is suitable as a material for automotive frame members.

In recent years, from the viewpoint of global environmental conservation, the strengthening of regulations on _CO2 emissions from automobiles has been promoted within the international framework. The most effective way to improve the fuel efficiency of automobiles is to reduce the weight of the automobile body by thinning the steel plates used in the automobile frame members. For this reason, the amount of high-strength steel plates used is increasing in order to contribute to low fuel consumption of automobiles. However, as the strength of steel plates increases, cracks are more likely to occur during press forming due to decreased ductility and decreased stretch flange formability. For this reason, steel plates with better ductility and stretch flange formability than conventional steel plates are desired.

In addition, from the perspective of carbon neutrality, it is necessary to use up all the material (improved yield), and steel plates are also required to have excellent material stability in the plate width direction. However, as the strength of steel plates increases, variations in formability such as ductility and hole expandability in the plate width direction become apparent, making them more susceptible to cracking during press forming, which makes it necessary to set restrictions on the blanking position, posing the issue of reduced yield.

In addition, as strength increases, the yield ratio (YR) (YR = yield strength YS/tensile strength TS) generally increases, which poses the problem of increased springback after forming.

As a technology to improve the formability of high-strength steel sheets, TRIP steel sheets with residual austenite dispersed in the structure have been developed. For example, Patent Document 1 discloses that austempering (carbon distribution associated with bainite transformation) is performed by annealing steel containing 0.04-0.12% C, 0.8-2.5% Si, and 0.5-2.0% Mn, and then holding it at 300-500°C for 10-900 seconds to generate 2-10% residual γ, resulting in a steel sheet with high ductility of TS×El≧21000MPa・% and high stretch flange formability of 70% or more.

In addition, Patent Document 2 discloses a technology for improving the ductility of steel sheets by using the principle of so-called Q&P; Quenching & Partitioning (quenching and partitioning of carbon from martensite to austenite), in which the sheet is cooled once during the cooling process to a temperature range between the martensite transformation start temperature (Ms point) and the martensite transformation completion temperature (Mf point), and then reheated and held to stabilize the retained austenite. Specifically, the technology discloses a steel sheet having a predetermined chemical composition, which is held at a first soaking temperature of 750°C or higher, cooled to a cooling stop temperature in the temperature range of 150 to 350°C, and then reheated to a temperature range of 350 to 500°C, thereby securing a retained austenite volume fraction of 5 to 15%, achieving both a TS of 980 MPa or higher and ductility with an elongation of 17% or higher, and having excellent hole expansion properties with a hole expansion ratio of 50% or higher, and a method for manufacturing the same.

On the other hand, DP steel (Dual Phase steel) has been developed as a steel sheet with a low yield ratio that is effective in reducing springback. A typical DP steel is a multi-phase steel with martensite dispersed in the ferrite structure, which is the main phase, and has a high TS, a low yield ratio, and excellent ductility. However, DP steel has the disadvantage of being inferior in stretch flange formability because cracks are easily generated due to stress concentration at the interface between ferrite and martensite. For example, Patent Document 3 and Patent Document 4 are technologies for improving the stretch flange formability of DP steel.

Patent Document 3 discloses a technology for suppressing deterioration of stretch flange formability by controlling the volume fraction of ferrite to 50% or more and the volume fraction of martensite to 3-30% relative to the total structure, while keeping the average grain size of ferrite to 10 μm or less and the average grain size of martensite to 5 μm or less.

Furthermore, Patent Document 4 discloses that the volume fraction of ferrite in the total structure is controlled to 5-30%, the volume fraction of martensite is controlled to 50-95%, and the ferrite is controlled to have an average grain size of 3 μm or less in equivalent circle diameter, and the martensite is controlled to have an average grain size of 6 μm or less in equivalent circle diameter, thereby improving ductility and stretch flange formability.

Patent No. 5515623 Patent No. 5821911 Japanese Patent No. 3936440 JP 2008-297609 A

Although the above-mentioned Patent Document 1 and Patent Document 4 disclose a method for manufacturing a steel sheet having excellent ductility and stretch flange formability, it is difficult to increase the strength to, for example, 780 MPa or more because it is necessary to form a large amount of ferrite of the soft phase. In addition, Patent Document 2 has excellent ductility and stretch flange formability, but since the YR is 0.8 or more, the dimensional accuracy may be impaired by springback during press forming. In addition, Patent Document 3 discloses a method for manufacturing a DP steel sheet having low YR and excellent stretch flange formability, but since it is a DP structure, the ductility is not necessarily sufficient. In addition, neither of the patent documents discloses a technology for suppressing the variation in ductility and stretch flange formability in the sheet width direction. Therefore, there is a demand for the development of a high-strength steel sheet having excellent ductility, excellent stretch flange, and excellent material stability in the sheet width direction.
In view of the above circumstances, the present invention aims to provide a steel plate, a member, and a manufacturing method thereof, which have a tensile strength (TS) of 980 MPa or more, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the plate width direction.

Here, the tensile strength refers to the tensile strength (TS) obtained in accordance with JIS Z2241 (2011).
Excellent press formability means that the yield ratio YR obtained in accordance with JIS Z2241 (2011) is 0.8 or less.
Excellent ductility means that the total elongation EL obtained in accordance with JIS Z2241 (2011) satisfies either (A) or (B) below.
(A) TS: 980 MPa or more and less than 1180 MPa, EL: 14.0% or more,
(B) When TS is 1,180 MPa or more, EL is 12.0% or more. Excellent stretch flange formability means that the hole expanding ratio λ (%) (= {(dd ₀ )/d ₀ }×100) obtained by a hole expanding test in accordance with the JFST1001 standard is 40% or more.
The term "excellent material stability in the sheet width direction" means that the sheet width of the region A in which the following formulas (1) and (2) are continuously satisfied with respect to EL and λ at the measurement position X in the sheet width direction is 80% or more of the total sheet width.
−10≦100×[(EL(%) at measurement position X in region A−EL(%) at center position of sheet width)/EL(%) at center position of sheet width]≦10 (1)
−10≦100×[(λ(%) at measurement position X in region A−λ(%) at center position of sheet width)/λ(%) at center position of sheet width]≦10 (2)
(In formulas (1) and (2), the measurement positions X are 23 positions in total among the 24 divided positions of the width W of the steel plate (23 contact positions of adjacent widths when the width W is divided into 24 equal widths). That is, the measurement positions X are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24, 9W/24, 10W/24, 11W/24, 12W/24, 13W/24, 14W/24, 15W/24, 16W/24, 17W/24, 18W/24, 19W/24, 20W/24, 21W/24, 22W/24, and 23W/24, totaling 23 positions.)
Here, for example, when the measurement position X where the formula (1) and the formula (2) are continuously satisfied is 2W/24 to 20W/24, the sheet width of the region A where the formula (1) and the formula (2) are continuously satisfied is 100×(20−2+1)/23=83% of the total sheet width.

In order to solve the above problems, the present inventors have conducted extensive research into various factors affecting press formability, ductility, stretch flange formability, and material stability of various thin steel sheets having a tensile strength of 980 MPa or more, from the viewpoints of the component composition and microstructure of the steel sheet, and manufacturing conditions. As a result, the inventors have found that the composition of the steel sheet is, in mass%, C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. It has been found that a high-strength cold-rolled steel sheet having excellent press formability, ductility, and stretch flange formability and excellent material stability in the sheet width direction (small material variation) can be obtained by using a steel structure containing Al: 0.005 to 0.50%, N: less than 0.015%, an area ratio of polygonal ferrite of 10% to 57%, a total area ratio of upper bainite, tempered martensite, and lower bainite of 40% to 80%, an area ratio of retained austenite (residual γ) of 3% to 15%, an area ratio of quenched martensite of 12% or less ( _{including 0} %), an aspect ratio of 3 or less and a total area ratio of quenched martensite and retained γ having a circle equivalent diameter of 1.6 μm or more of 20% or less, and an area ratio of a C-enriched region (S C≧0.5 ) having a C concentration of 0.5 mass% or more to the entire structure of 15% or less.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
[1] In mass%,
C: 0.05 to 0.20%,
Si: 0.40 to 1.50%,
Mn: 1.9 to 3.5%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.005 to 0.50%,
N: less than 0.015%;
The balance being iron and unavoidable impurities;
Area ratio of polygonal ferrite: 10% or more and 57% or less,
Total area ratio of upper bainite, tempered martensite and lower bainite: 40% or more and 80% or less,
Volume fraction of retained austenite: 3% or more and 15% or less,
Area ratio of quenched martensite: 12% or less (including 0%),
Further, the composition has a structure consisting of a remaining structure,
The total area ratio of the quenched martensite and the retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more is 20% or less,
A steel plate in which an area ratio of a C-enriched region (S _C≧0.5 ) having a C concentration of 0.5 mass% or more relative to the entire structure is 15% or less.
[2] The component composition further includes, in mass%,
Ti: 0.1% or less,
B: 0.01% or less,
The steel sheet according to [1], containing one or two selected from the following:
[3] The component composition further includes, in mass%,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1% or less,
Mo: 0.5% or less,
V: 0.5% or less,
Nb: 0.1% or less,
The steel sheet according to [1] or [2], containing one or more selected from the following:
[4] The component composition further includes, in mass%,
Mg: 0.0050% or less,
Ca: 0.0050% or less,
Sn: 0.1% or less,
Sb: 0.1% or less,
REM: 0.0050% or less,
The steel sheet according to any one of [1] to [3], containing one or more selected from the following:
[5] The steel sheet according to any one of [1] to [4], having a zinc-plated layer on a surface thereof.
[6] A member made using the steel plate according to any one of [1] to [5].
[7] A method for producing a steel sheet, comprising: subjecting a steel slab having a component composition according to any one of [1] to [4] to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet;
The annealing is
A holding step of heating the cold-rolled steel sheet to an annealing temperature of 750 to 880 ° C. and holding the annealing temperature for 10 to 500 seconds;
A first cooling step of cooling the temperature range from the annealing temperature to a first cooling stop temperature of 350 to 550 ° C. at a first average cooling rate of 2 to 50 ° C./s to the first cooling stop temperature;
A second cooling step of retaining the steel sheet at a retention temperature of 350 to 550° C. for 10 s to 60 s and then cooling the steel sheet to a second cooling stop temperature of 100 to 300° C. at a second average cooling rate of 3 to 50° C./s;
A reheating step of heating from the second cooling stop temperature to a reheating temperature of second cooling stop temperature + 50 ° C. or more and 340 ° C. or less at an average heating rate of 2.0 ° C./s or more;
and a third cooling step of cooling the steel sheet at a temperature range from the reheating temperature to 50°C at a third average cooling rate of 0.05 to 1.0°C/s while retaining the steel sheet for 100 s or more after the reheating step.
[8] The method for producing a steel sheet according to [7], wherein in the second cooling step, when the steel sheet is retained at a retention temperature of 350 to 550°C for 10 s to 60 s, a hot-dip galvanizing treatment or a hot-dip galvannealing treatment is performed on the surface of the steel sheet.
[9] The method for producing a steel sheet according to [7], further comprising electrolytic galvanizing treatment on the surface of the steel sheet after the annealing.
[10] A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of [1] to [5] to at least one of forming and joining to form a component.

According to the present invention, a steel sheet can be obtained which has a high strength, tensile strength TS of 980 MPa or more, excellent press formability, ductility and stretch flange formability, and excellent material stability in the sheet width direction.
The steel sheet of the present invention can be applied to, for example, automobile structural members having complex shapes, thereby achieving weight reduction of automobile bodies and reducing the environmental load by improving the yield during production.

The present invention will be specifically described below. Note that the present invention is not limited to the following embodiments.
The steel plate of the present invention contains, by mass%, C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.005 to 0.50%, N: less than 0.015%, with the balance being iron and unavoidable impurities. The steel plate of the present invention has a composition comprising: C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.005 to 0.50%, N: less than 0.015%, with the balance being iron and unavoidable impurities; an area ratio of polygonal ferrite: 10% to 57%, a total area ratio of upper bainite, tempered martensite, and lower bainite: 40% to 80%, and a residual The steel has a volume fraction of austenite: 3% or more and 15% or less, an area fraction of quenched martensite: 12% or less (including 0%), and a steel structure consisting of a remaining structure, in which the total area fraction of the quenched martensite and retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more is 20% or less relative to the total area fraction of the quenched martensite and retained austenite, and the area fraction of C-enriched regions (S _C≧0.5 ) having a C concentration of 0.5 mass% or more relative to the entire structure is 15% or less.
Hereinafter, the steel sheet of the present invention will be described in the order of the chemical composition and the steel structure. First, the reasons for limiting the chemical composition of the present invention will be described. In the following description, all percentages indicating the steel composition are mass% unless otherwise specified.

<C: 0.05 to 0.20%>
C is contained from the viewpoint of securing a predetermined strength through transformation strengthening and from the viewpoint of securing a predetermined amount of retained austenite (retained γ) to improve ductility. If the C content is less than 0.05%, these effects cannot be sufficiently secured.
On the other hand, if the C content exceeds 0.20%, the martensitic transformation start temperature (Ms point) decreases. As a result, in the third cooling step in which cooling is performed at a third average cooling rate of 0.05 to 1.0 ° C./s in the temperature range from the reheating temperature to 50 ° C., martensitic transformation and subsequent tempering of the martensite are not sufficiently performed. As a result, the formation of quenched martensite and a C-enriched region (S _{C ≧ 0.5} ) of 0.5 mass% or more is promoted, and the stretch flange formability and material stability in the sheet width direction are reduced.
Therefore, the C content is set to 0.05% or more and 0.20% or less, preferably 0.08% or more, and more preferably 0.18% or less.

<Si: 0.40 to 1.50%>
Silicon is included from the viewpoints of strengthening ferrite to increase strength, and of suppressing the formation of carbides in martensite and bainite to ensure a predetermined amount of retained γ and improve ductility. If the silicon content is less than 0.40%, these effects cannot be sufficiently ensured.
On the other hand, if the Si content exceeds 1.50%, carbon partitioning to untransformed austenite is excessively promoted, and the formation of a C-enriched region (S _C≧0.5 ) of 0.5 mass% or more is promoted, and the stretch flangeability and material stability in the sheet width direction are reduced. Therefore, the Si content is set to 0.40% or more and 1.50% or less. The Si content is preferably 0.60% or more. The Si content is preferably 1.20% or less.

<Mn: 1.9 to 3.5%>
Mn is added from the viewpoints of improving the hardenability of the steel sheet, suppressing excessive transformation of ferrite, promoting high strength through transformation strengthening, and further improving ductility by promoting the formation of retained austenite which, like Si, suppresses the formation of carbides in bainite and contributes to ductility. To obtain these effects, the Mn content needs to be 1.9% or more.
On the other hand, if the Mn content exceeds 3.5%, the bainite transformation is delayed, a predetermined amount of retained austenite cannot be secured, and ductility is reduced.
Furthermore, if the Mn content exceeds 3.5%, it becomes difficult to suppress the formation of coarse hardened martensite, and the stretch flange formability also deteriorates.
Therefore, the Mn content is set to 1.9% or more and 3.5% or less. The Mn content is preferably 2.1% or more. The Mn content is preferably 3.3% or less, and more preferably 3.0% or less.

<P: 0.02% or less>
P is an element that strengthens steel, but if its content is high, it deteriorates spot weldability. Therefore, the P content is set to 0.02% or less, and preferably 0.01% or less. Although P may not be contained, it is very costly to reduce the P content to less than 0.001%, so the P content is preferably 0.001% or more. The P content is more preferably 0.002% or more, and further preferably 0.005% or more.

<S: 0.01% or less>
S has the effect of improving scale peeling during hot rolling and the effect of suppressing nitriding during annealing, but is an element that has a negative effect on spot weldability, bendability, and hole expandability. In order to reduce these negative effects, the S content is at least 0.01% or less, and preferably 0.0020% or less.
Although S may not be contained, it is very costly to reduce the S content to less than 0.0001%, so from the viewpoint of manufacturing costs, the S content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0015% or more.

<sol. Al: 0.005 to 0.50%
Al is contained for the purpose of deoxidation or obtaining residual γ. In order to perform stable deoxidation, the sol. Al content is set to 0.005% or more. It is preferable that the content is 0.1% or more.
On the other hand, if the sol. Al content exceeds 0.50%, the amount of coarse Al-based inclusions increases significantly, and the stretch flangeability decreases. Let us assume that.

<N: less than 0.015%>
N is an element that forms nitrides such as BN, AlN, and TiN in steel and reduces the stretch flangeability, so its content must be limited. Therefore, the N content is set to less than 0.015%.
Although N may not be contained, it is very costly to reduce the N content to less than 0.0001%. Therefore, the N content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The N content is more preferably 0.0005% or more, and further preferably 0.0015% or more.

The composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, with the balance being iron (Fe) and unavoidable impurities. Note that the composition of the steel sheet in the present invention preferably has a composition with the balance being Fe and unavoidable impurities.
The composition of the steel sheet of the present invention may contain, in addition to the above-mentioned components, one or more optional elements selected from the following (A) to (C):
(A) one or two selected from Ti: 0.1% or less, B: 0.01% or less,
(B) one or more selected from Cu: 1% or less, Ni: 1% or less, Cr: 1% or less, Mo: 0.5% or less, V: 0.5% or less, and Nb: 0.1% or less;
(C) One or more selected from Mg: 0.0050% or less, Ca: 0.0050% or less, Sn: 0.1% or less, Sb: 0.1% or less, and REM: 0.0050% or less

<Ti: 0.1% or less>
Ti has the effect of fixing N in steel as TiN, improving hot ductility, and improving the hardenability of B. It also has the effect of refining the structure by precipitation of TiC. In order to obtain these effects, it is desirable to set the Ti content to 0.002% or more. From the viewpoint of sufficiently fixing N, it is further preferable to set the Ti content to 0.008% or more. The Ti content is more preferably 0.010% or more.
On the other hand, if the Ti content exceeds 0.1%, it increases the rolling load and decreases the ductility due to an increase in the amount of precipitation strengthening, so if Ti is contained, the Ti content is set to 0.1% or less, preferably 0.05% or less, and more preferably 0.03% or less.

<B: 0.01% or less>
B is an element that improves the hardenability of steel and has the advantage of easily forming a predetermined area ratio of tempered martensite and/or bainite. Therefore, the B content is preferably 0.0005% or more. The B content is more preferably 0.0010% or more.
On the other hand, if the B content exceeds 0.01%, not only does the effect saturate, but it also causes a significant decrease in hot ductility and causes surface defects. Therefore, when B is contained, the B content is set to 0.01% or less. Preferably, the B content is 0.005% or less, and more preferably, 0.003% or less.

<Cu: 1% or less>
Cu improves corrosion resistance in the environment in which the automobile is used. In addition, the corrosion products of Cu cover the surface of the steel sheet, which has the effect of suppressing hydrogen penetration into the steel sheet. Cu is an element that is mixed in when scrap is used as a raw material, and by allowing the inclusion of Cu, recycled materials can be used as raw materials, thereby reducing manufacturing costs. From this viewpoint, it is preferable to contain Cu at 0.005% or more, and from the viewpoint of improving delayed fracture resistance, it is more preferable to contain Cu at 0.05% or more. More preferably, it is 0.10% or more. However, if the Cu content is too high, it will cause surface defects, so if Cu is contained, the Cu content is 1% or less.

<Ni: 1% or less>
Ni, like Cu, is an element that improves corrosion resistance. Ni also has the effect of suppressing the occurrence of surface defects that tend to occur when Cu is contained. For this reason, it is desirable to contain Ni at 0.01% or more. The Ni content is more preferably 0.04% or more, and even more preferably 0.06% or more.
However, if the Ni content is too high, the scale generation in the heating furnace becomes non-uniform, which may cause surface defects. It also leads to increased costs. For this reason, when Ni is contained, the Ni content is set to 1% or less. Preferably, the Ni content is 0.5% or less, and more preferably, 0.3% or less.

<Cr: 1% or less>
Cr can be added to improve the hardenability of the steel and to suppress the formation of carbides in martensite and upper/lower bainite. To obtain such effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, and further preferably 0.06% or more.
However, if an excessive amount of Cr is contained, the pitting corrosion resistance is deteriorated, so if Cr is contained, the Cr content is set to 1% or less.

<Mo: 0.5% or less>
Mo can be added due to its effect of improving the hardenability of steel and its effect of suppressing the formation of carbides in martensite and upper/lower bainite. To obtain such effects, the Mo content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, and even more preferably 0.2% or more.
However, Mo significantly deteriorates the chemical conversion treatability of the cold-rolled steel sheet, so if Mo is contained, the Mo content is set to 0.5% or less.

<V: 0.5% or less>
V can be contained due to the effects of improving the hardenability of steel, suppressing the formation of carbides in martensite and upper/lower bainite, refining the structure, and precipitating carbides to improve delayed fracture resistance. In order to obtain these effects, the V content is preferably 0.003% or more. The V content is more preferably 0.05% or more, and even more preferably 0.015% or more. Even more preferably, the V content is 0.02% or more, and even more preferably 0.05% or more. The V content is preferably 0.15% or more, and more preferably 0.25% or more.
However, since a large amount of V significantly deteriorates castability, when V is contained, the V content is set to 0.5% or less, preferably 0.4% or less, and more preferably 0.3% or less.

<Nb: 0.1% or less>
Nb can be added because of its effects of refining the steel structure to increase strength, promoting bainite transformation through grain refinement, improving bendability, and improving delayed fracture resistance. In order to obtain these effects, the Nb content is preferably 0.002% or more. The Nb content is more preferably 0.004% or more, and further preferably 0.010% or more.
However, if a large amount of Nb is contained, precipitation strengthening becomes too strong, and ductility decreases. In addition, it causes an increase in rolling load and deterioration of castability. For this reason, when Nb is contained, the Nb content is set to 0.1% or less. Preferably, the Nb content is 0.07% or less, and more preferably, 0.05% or less.

<Mg: 0.0050% or less>
Mg fixes O as MgO and contributes to improving formability such as bendability. Therefore, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more, and further preferably 0.0006% or more.
On the other hand, if a large amount of Mg is added, the surface quality and bendability are deteriorated, so when Mg is contained, the Mg content is set to 0.0050% or less, and preferably, the Mg content is set to 0.0030% or less.

<Ca: 0.0050% or less>
Ca fixes S as CaS and contributes to improving bendability and delayed fracture resistance. Therefore, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, and further preferably 0.0010% or more. On the other hand, when Ca is contained, the Ca content is 0.0050% or less, since adding a large amount of Ca deteriorates the surface quality and bendability. Preferably, the Ca content is 0.0040% or less.

<Sn: 0.1% or less>
Sn suppresses oxidation and nitridation of the surface layer of the steel sheet, and suppresses the resulting reduction in the content of C and B in the surface layer. This effect suppresses the generation of ferrite in the surface layer of the steel sheet, increases the strength, and improves the fatigue resistance. From this viewpoint, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.004% or more, and even more preferably 0.006% or more. More preferably, the Sn content is 0.01% or more, and even more preferably 0.05% or more.
On the other hand, if the Sn content exceeds 0.1%, castability deteriorates. In addition, Sn segregates at prior γ grain boundaries, deteriorating delayed fracture resistance. Therefore, when Sn is contained, the Sn content is set to 0.1% or less.

<Sb: 0.1% or less>
Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, and suppresses the resulting reduction in the content of C and B in the surface layer. This effect suppresses the generation of ferrite in the surface layer of the steel sheet, increases the strength, and improves the fatigue resistance. From this viewpoint, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.004% or more, and even more preferably 0.006% or more. More preferably, the Sb content is 0.01% or more, and even more preferably 0.05% or more.
On the other hand, if the Sb content exceeds 0.1%, castability is deteriorated and the Sb segregates at the prior γ grain boundaries, deteriorating the delayed fracture resistance. Therefore, if Sb is contained, the Sb content is set to 0.1% or less.

<REM: 0.0050% or less>
REM is an element that suppresses the adverse effect of sulfides on stretch flangeability by making the shape of sulfides spheroidal, thereby improving stretch flangeability. In order to obtain these effects, the REM content is preferably 0.0005% or more. The REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more.
On the other hand, if the REM content exceeds 0.0050%, the effect of improving the stretch flangeability becomes saturated, so when REM is contained, the REM content is set to 0.0050% or less.

In the present invention, REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanide elements from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. The REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.

When the optional components are contained in amounts less than the lower limit, the optional elements contained in amounts less than the lower limit do not impair the effects of the present invention. Therefore, when the optional elements are contained in amounts less than the lower limit, the optional elements are considered to be contained as unavoidable impurities.

Next, we will explain the mechanical properties of the steel sheet (cold-rolled steel sheet with excellent material stability) that is the subject of this invention.

The steel plate of the present invention has a tensile strength (TS) of 980 MPa or more. There is no particular upper limit to the tensile strength, but from the viewpoint of compatibility with other properties, it is preferable that the tensile strength is 1,300 MPa or less.

In the steel sheet of the present invention, the total elongation EL is ensured to be 14.0% or more when TS is 980 MPa or more, and 12.0% or more when TS is 1180 MPa or more, thereby significantly improving the stability of press forming.
By ensuring that the hole expansion ratio λ is 40% or more, cracks during press forming can be suppressed, making it possible to apply the material to complex formed parts that are difficult to form. For this reason, λ is set to 40% or more.

In the steel sheet of the present invention, the sheet width of a region A which continuously satisfies the following formulas (1) and (2) with respect to EL and λ at a measurement position X in the sheet width direction is 80% or more of the total sheet width.
−10≦100×[(EL(%) at measurement position X in region A−EL(%) at center position of sheet width)/EL(%) at center position of sheet width]≦10 (1)
−10≦100×[(λ(%) at measurement position X in region A−λ(%) at center position of sheet width)/λ(%) at center position of sheet width]≦10 (2)
In formulas (1) and (2), the measurement positions X are 23 positions in total among the 24 division positions of the width W of the steel plate (23 contact positions of adjacent widths when the width W is divided into 24 equal widths). That is, the measurement positions X are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24, 9W/24, 10W/24, 11W/24, 12W/24, 13W/24, 14W/24, 15W/24, 16W/24, 17W/24, 18W/24, 19W/24, 20W/24, 21W/24, 22W/24, and 23W/24 in total, 23 positions in the width direction.
Here, for example, when the measurement position X where the formula (1) and the formula (2) are continuously satisfied is 2W/24 to 20W/24, the sheet width of the region A where the formula (1) and the formula (2) are continuously satisfied is 100×(20−2+1)/23=83% of the total sheet width.
In the steel plate of the present invention, the above-mentioned region A has a length in the plate width direction that is 80% or more of the total plate width.
That is, in the steel sheet of the present invention, the region in which the deviation of EL in the sheet width direction is 10% or less from the measured value at the sheet width center position and the deviation of λ in the sheet width direction is 10% or less from the measured value at the sheet width center position is set to be 80% or more of the entire sheet width region. The range of the non-steady portion is allowed to be up to 20% in total at both ends in the width direction.
The very end of the steel plate is not used to ensure quality because it comes into contact with other structures during transportation and work. Therefore, the usable effective plate width does not reach 100%. For this reason, it is preferable to make the effective plate width less than 100%.
By making the region where the deviation of EL in the sheet width direction is 10% or less of the measured value at the sheet width center position and the deviation of λ is 10% or less occupy 80% or more of the entire sheet width, the yield can be significantly improved, so in the present invention, the region where the deviation of EL in the sheet width direction is 10% or less of the measured value at the sheet width center position and the deviation of λ is 10% or less occupies 80% or more of the entire sheet width, preferably 85% or more.

<YR≦0.8, TS≧980MPa, EL≧14.0%>
<YR≦0.8, TS≧1180MPa, EL≧12.0%>
Tensile properties are evaluated by taking JIS No. 5 tensile test pieces from the center of the sheet width, and conducting tensile tests (in accordance with JIS Z2241 (2011)) with N=3. Each evaluation is based on the average value of the three points. A steel plate with a tensile strength of 980 MPa or more is considered to be a high-strength steel plate. A steel plate with a yield ratio YR of 0.8 or less is considered to have excellent press formability. A steel plate with a total elongation EL of 14.0% or more for TS: 980 MPa or more, and 12.0% or more for TS: 1180 MPa or more is considered to have excellent ductility.

<λ≧40%>
The evaluation of stretch flange formability is carried out by taking a test piece from the center position of the plate width, and performing a hole expansion test in accordance with the provisions of the Japan Iron and Steel Federation standard JFST1001 with N = 3. That is, after punching a sample of 100 mm x 100 mm square size using a punching tool with a punch diameter of 10 mm and clearance: 13%, a conical punch with an apex angle of 60 degrees is used to expand the hole until a crack penetrating the plate thickness occurs, so that the burrs generated during the formation of the punched hole are on the outside. In this case, d ₀ : initial hole diameter (mm), d: hole diameter at the time of crack occurrence (mm), the hole expansion ratio λ (%) = {(d - d ₀ ) / d ₀ } × 100 is obtained, and the average value of the three points obtained by carrying out the test is evaluated as λ. Steel having a λ of 40% or more is judged to have excellent hole expansion and stretch flangeability.

<Material stability evaluation in the plate width direction>
To evaluate the material stability in the width direction, 23 points (including the width center) of the evaluation material are sampled from both width directions at intervals of 100 mm or less from the width center position (the aforementioned 12W/24 position), and EL and λ are obtained at each position (measurement position X).The material stability in the width direction is evaluated by determining the ratio of the difference between the measurement value at the width center position and each position to the measurement value at the width center position.
Using EL and λ at the center position of the plate width as the standard, a group of consecutive measurements in which the difference in EL and λ is 10% or less is defined as a region in which the difference in EL and λ is 10% or less, and steel in which this region accounts for 80% or more of the total plate width is determined to have excellent material stability.
The width of the steel plate in the present invention is preferably 600 mm or more. The width of the steel plate in the present invention is preferably 1700 mm or less.

Next, the steel structure of the steel plate of the present invention will be explained.

<Area ratio of polygonal ferrite: 10% or more and 57% or less>
From the viewpoint of ensuring a low YR and high ductility, the area ratio of polygonal ferrite is set to 10% or more, and in order to obtain even higher ductility, it is preferably set to 20% or more.
On the other hand, if the polygonal ferrite exceeds 57%, a predetermined strength cannot be obtained, so the area ratio of polygonal ferrite is set to 57% or less, preferably 55% or less, and more preferably 50% or less.

<Total area ratio of upper bainite, tempered martensite and lower bainite: 40% or more and 80% or less>
In order to obtain a desired strength, the total area ratio of upper bainite, tempered martensite and lower bainite is set to 40% or more, and in order to obtain even higher strength, it is preferably set to 45% or more.
However, if the total area ratio of upper bainite, tempered martensite and lower bainite exceeds 80%, ductility decreases due to excessively high strength, so the area ratio is set to 80% or less, more preferably 75% or less.

<Volume fraction of retained austenite (retained γ): 3% or more and 15% or less>
In order to ensure a desired ductility, it is effective to set the volume fraction of the retained austenite to 3% or more. Therefore, the volume fraction of the retained austenite is set to 3% or more, and preferably 5% or more.
On the other hand, if the retained austenite exceeds 15%, the stretch flange formability is deteriorated, so the retained austenite is set to 15% or less, more preferably 13% or less.

<Quenched martensite: 12% or less (including 0%)>
Since the hard quenched martensite structure reduces λ, its area ratio must be suppressed. In order to obtain a desired λ, the area ratio of quenched martensite is set to 12% or less. In order to obtain λ more stably, the area ratio of quenched martensite is preferably 10% or less.

<Remainder structure>
The steel structure is made up of the remaining structure other than the above. The area ratio of the remaining structure is preferably 5% or less. The remaining structure may be carbide or pearlite. These structures may be determined by SEM observation as described later.

<Total area ratio of quenched martensite and retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more relative to the total area ratio of quenched martensite and retained austenite: 20% or less>
The retained austenite becomes a hard martensite structure due to the TRIP effect during press forming, tensile processing, etc. Therefore, in the present invention, the quenched martensite and the retained austenite are controlled together from the viewpoint of stretch flangeability. If the quenched martensite or the retained austenite with a circle equivalent diameter of 1.6 μm or more is formed, voids are formed at the stress concentration part at the interface with other structures, and the desired stretch flangeability cannot be obtained.
Furthermore, when the aspect ratio of quenched martensite or retained austenite is 3 or less, stress concentration is likely to occur at the interface with other structures, promoting void formation and deteriorating stretch flange formability. Therefore, the total area ratio of quenched martensite and retained austenite with an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more is set to 20% or less, preferably 18% or less, relative to the total area ratio of quenched martensite and retained austenite.
The total area ratio of quenched martensite and retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 2.0 μm or more relative to the total area ratio of quenched martensite and retained austenite is not particularly limited, but since it is difficult to control it to 0% from the viewpoint of operability, it is preferably 2% or more, and more preferably 4% or more.
The above-mentioned equivalent circle diameter is preferably 20.0 μm or less.

<Area ratio of C-enriched regions (SC _≧0.5 ) in which the C concentration relative to the entire structure is 0.5 mass% or more: 15% or less>
The hardness of quenched martensite is determined by the amount of C dissolved in quenched martensite. When the hardness difference between quenched martensite and other structures increases, the formation of voids is promoted at the interface of the stress concentration portion. Structures containing a large amount of dissolved C are quenched martensite and retained austenite. Retained austenite is a structure that contributes to high ductility, and the C concentration is 0.5 mass% or more. However, if the area ratio of structures with a C concentration of 0.5 mass% or more is 15% or less for all constituent structures, it is possible to ensure stretch flange formability while improving ductility, and the material variation in the sheet width direction can be reduced, making it possible to manufacture steel sheets with excellent material stability. For this reason, the volume fraction of the C-enriched region (S _C≧0.5 ) with a C concentration of 0.5 mass% or more is set to 15% or less.
It is preferably 12% or less, and more preferably 10% or less.
Also, it is preferably 6% or more, and more preferably 8% or more.

Next, we will explain how to measure steel structure.

The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were measured by cutting out a cross section of the sheet thickness parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. Ten fields of view were observed at 5,000x magnification using an SEM at a quarter-thickness position, and the photographed structure was quantified by image analysis.
The polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest in the SEM.
Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM. If it is difficult to distinguish between upper bainite and polygonal ferrite, the area of ferrite with an aspect ratio of ≦2.0 is classified as polygonal ferrite, and the area of ferrite with an aspect ratio of >2.0 is classified as upper bainite, and the area ratio is calculated. Here, the aspect ratio is calculated by determining the major axis length a at which the particle length is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to the major axis length a, and a/b is the aspect ratio.
Tempered martensite and lower bainite are regions that are accompanied by a lath-shaped substructure and carbide precipitation inside when viewed under an SEM.
Hardened martensite (fresh martensite) is a blocky region that appears white under an SEM with no internal substructure visible.
The remaining structure is a carbide and/or pearlite structure, which can be confirmed by a white contrast under SEM. However, it is possible to distinguish between the carbide and pearlite structures because the carbide structure has a particle size of 1 μm or less and the pearlite structure has a lamellar (layer) shape.

The above-mentioned quantitative evaluation of the structure and the measurement of the aspect ratio and circle equivalent diameter of the quenched martensite and the retained austenite can be performed using image analysis software, for example, Image J (Fiji).
A cross section of the plate thickness parallel to the rolling direction is cut out, mirror-polished, and then etched with 1 vol% nital. At the 1/4 thickness position, 10 fields of view are observed at 5000x magnification with an SEM, and each structure is identified and quantitatively evaluated using the Trainable Weka segmentation method, which can identify regions using machine learning in Image J (Fiji). The aspect ratio and circle equivalent diameter of the quenched martensite and retained austenite can also be measured using a particle analysis program, which is a function of Image J, and only the quenched martensite and retained austenite identified as described above are extracted and measured.

The volume fraction of retained austenite is determined by chemically polishing the surface layer at 1/4 thickness and then performing X-ray diffraction. A Co-Kα source is used for the incident X-rays, and the volume fraction of retained austenite is calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Here, since the retained austenite is randomly distributed, the volume fraction of retained austenite determined by X-ray diffraction can be taken as the area fraction of retained austenite.

The area ratio S _C≧0.5 of the C-enriched region where the C concentration is 0.5 mass% or more is measured at a 1/4 position of the sheet thickness of the sheet thickness cross section parallel to the rolling direction using a JXA-8500F field emission electron probe microanalyzer (FE-EPMA). The measurement is performed by mapping analysis of the C concentration distribution with an acceleration voltage of 6 kV, an irradiation current of 7×10 ⁻⁸ A, and a minimum beam diameter, and the area ratio where the C concentration is 0.5 mass% or more is calculated.
However, to eliminate the effects of contamination, the background amount is subtracted so that the average value of C obtained in the analysis is equal to the carbon amount in the base material. In other words, if the average value of the measured carbon amount is greater than the carbon amount in the base material, the increase is considered to be contamination, and the true amount of C at each position is determined by subtracting the increase from the analysis value at each position.

Next, a method for producing a steel sheet according to the present invention will be described.
The method for producing a steel sheet of the present invention is a method for producing a steel sheet in which a steel slab having a component composition is subjected to hot rolling, pickling and cold rolling, and then the obtained cold-rolled steel sheet is annealed. The annealing includes a holding step of heating the cold-rolled steel sheet to an annealing temperature of 750 to 880°C and holding the annealing temperature for 10 to 500 seconds, a first cooling step of cooling the cold-rolled steel sheet to the first cooling stop temperature of 350 to 550°C at a first average cooling rate of 2 to 50°C/s in a temperature range from the annealing temperature to a first cooling stop temperature of 350 to 550°C, and a second cooling step of cooling the cold-rolled steel sheet to the first cooling stop temperature at a first average cooling rate of 2 to 50°C/s. The method includes a second cooling step of retaining the material at a retention temperature of 0°C for 10 s or more and 60 s or less, and then cooling to a second cooling stop temperature of 100 to 300°C at a second average cooling rate: 3 to 50°C/s; a reheating step of heating from the second cooling stop temperature to a reheating temperature of the second cooling stop temperature + 50°C or more and 340°C or less at an average heating rate: 2.0°C/s or more; and a third cooling step of cooling in a temperature range from the reheating temperature to 50°C at a third average cooling rate: 0.05 to 1.0°C/s while retaining the material for 100 s or more after the reheating step.

<Hot rolling>
Methods for hot rolling a steel slab include a method of heating the slab and then rolling it, a method of directly rolling the slab after continuous casting without heating it, and a method of rolling the slab after continuous casting by subjecting it to a short-term heat treatment. Hot rolling may be carried out according to a conventional method, for example, the slab heating temperature may be 1100 to 1300 ° C, the soaking temperature may be 20 to 300 min, the finish rolling temperature may be Ar ₃ transformation point to Ar ₃ transformation point + 200 ° C, and the coiling temperature may be 400 to 720 ° C. From the viewpoint of suppressing plate thickness fluctuation and stably ensuring high strength, the coiling temperature is preferably 430 to 530 ° C.
The _Ar3 transformation point can be calculated from the components of the steel sheet and the following empirical formula (A).
Ar ₃ = 910 - 203 x [C] + 44.7 x [Mn] - 30 x [Si] + 700 x [P] + 400 x [sol. Al] - 20 x [B] + 31.5 x [Mo] + 104 x [V] + 400 x [Ti] ... (A)
In formula (A), [element] means the content (mass%) of each element. (An element that is not contained is represented as 0 (zero) mass%).

<Pickling>
The pickling may be carried out in a conventional manner.

<Cold rolling>
Cold rolling may be performed according to a conventional method, and the cumulative rolling reduction may be set to 30 to 85%. From the viewpoint of stably securing high strength and reducing anisotropy, the rolling reduction is preferably set to 35 to 85%. When the rolling load is high, softening annealing treatment can be performed at 450 to 730°C in a CAL (continuous annealing line) or BAF (box annealing furnace).

<Annealing>
A cold-rolled steel sheet manufactured according to a conventional method is annealed under the following conditions. The annealing equipment is not particularly limited, but from the viewpoints of productivity and ensuring the desired heating rate and cooling rate, it is preferable to carry out the annealing in a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL).

[Holding step: heating to an annealing temperature in the annealing temperature range of 750 to 880° C. and holding at the annealing temperature for 10 to 500 seconds]
If the annealing temperature (soaking temperature) is below 750°C, the amount of C and Mn concentrated in the reverse transformed austenite increases due to the excessive amount of polygonal ferrite. As a result, at least one of upper bainite, tempered martensite, lower bainite, and retained austenite cannot be obtained sufficiently, and the hardness of quenched martensite increases, making it impossible to ensure the desired strength, ductility, and stretch flange formability. Also, if the annealing temperature (soaking temperature) is below 750°C, recrystallization does not occur sufficiently, and the processed structure during cold rolling may remain, thereby reducing formability. For this reason, the annealing temperature (soaking temperature) is set to 750°C or higher.
On the other hand, if the annealing temperature (soaking temperature) exceeds 880°C, the austenite single phase temperature is obtained, the desired polygonal ferrite cannot be obtained, the YR increases, and the ductility decreases. Therefore, the annealing temperature (soaking temperature) is set to 880°C or less. The annealing temperature (soaking temperature) is preferably 850°C or less, and more preferably 830°C or less.

Furthermore, if the time for holding at the annealing temperature (soaking time) is less than 10 seconds, austenite is not sufficiently formed at the annealing temperature (soaking temperature), resulting in an excess of polygonal ferrite, and the specified amounts of upper bainite, tempered martensite, and lower bainite are not obtained. As a result, not only is the desired strength not obtained, but also sufficient retained austenite cannot be obtained, and the desired ductility is not secured.
On the other hand, if the time for holding at the annealing temperature (soaking time) exceeds 500 seconds, the structure will become significantly coarse, making it impossible to ensure the desired strength.
Therefore, the time for holding at the annealing temperature (soaking time) is 10 to 500 seconds. The time for holding at the annealing temperature (soaking time) is preferably 80 seconds or more, and more preferably 100 seconds or more. The time for holding at the annealing temperature (soaking time) is preferably 400 seconds or less, and more preferably 300 seconds or less.

[First cooling step: cooling to the first cooling stop temperature at a first average cooling rate of 2 to 50 ° C./s in the temperature range from the annealing temperature to the first cooling stop temperature of 350 to 550 ° C.]
After holding at a soaking temperature of 750°C to 880°C (after the above holding step), the steel sheet is cooled at a first average cooling rate of 2 to 50°C/s in the temperature range from the annealing temperature to a first cooling stop temperature of 350 to 550°C. If the first average cooling rate is less than 2°C/s, the operability decreases, so the first average cooling rate is set to 2°C/s or more. The first average cooling rate is preferably 5°C/s or more.
On the other hand, if the first average cooling rate is too high, the sheet shape deteriorates, so the first average cooling rate is set to 50° C./s or less. The first average cooling rate is preferably 40° C./s or less, and more preferably less than 30° C./s.
Here, the first average cooling rate is "(annealing temperature (°C) - first cooling stop temperature (°C)) / cooling time (seconds) from the annealing temperature to the first cooling stop temperature".

[Second cooling step (1): retention at a retention temperature of 350 to 550 ° C. for 10 s to 60 s]
At a temperature range (dwell temperature) below the first cooling stop temperature and between 350° C. and 550° C., upper bainite is formed, a predetermined amount of retained austenite is obtained, and the desired ductility is obtained. Bainite transformation has an incubation period, and the steel must be held for a certain period of time in a holding temperature range that includes the holding start temperature and holding end temperature.
If the residence temperature range is less than 350°C or more than 550°C, bainite transformation is suppressed, and as a result, the formation of retained austenite is suppressed, and the desired ductility cannot be obtained. Furthermore, if the residence temperature range is less than 350°C, martensitic transformation occurs, which unnecessarily increases the YR, and press formability may decrease. For this reason, the residence temperature range is set to 350 to 550°C.
Furthermore, if the residence time is less than 10 seconds, the desired amount of bainite is not obtained, and the formation of retained austenite is suppressed, so that the desired ductility is not obtained.
On the other hand, if the residence time exceeds 60 s, the concentration of C from bainite to blocky untransformed γ progresses, leading to an increase in coarse quenched martensite with a high C concentration, and the desired stretch flange formability and material stability in the sheet width direction cannot be obtained. Therefore, the residence time is set to 10 s or more and 60 s or less.

[Second cooling step (2): Cooling in the temperature range of 100 to 300 ° C. up to the second cooling stop temperature at a second average cooling rate of 3 to 50 ° C./s]
After the above-mentioned retention, it is necessary to rapidly cool the sheet so as to prevent a decrease in the amount of tempered martensite, the formation of coarse quenched martensite or retained austenite, and an excessive carbon concentration in the retained austenite, which are caused by excessive bainite transformation, from deteriorating the strength, stretch flangeability, and material stability in the sheet width direction. For this reason, the average cooling rate (second average cooling rate) in the temperature range from the retention end temperature to the cooling stop temperature of 100°C or more and 300°C or less is set to 3°C/s or more. The second average cooling rate is preferably 5°C/s.
On the other hand, if the second average cooling rate exceeds 50° C./s, the sheet shape deteriorates, so the second average cooling rate is set to 50° C./s or less.
If the second cooling stop temperature exceeds 300° C., the desired tempered martensite cannot be obtained, and as a result, coarse quenched martensite increases, making it impossible to obtain the desired stretch flange formability. For this reason, the second cooling stop temperature is set to 300° C. or less. The second cooling stop temperature is preferably 290° C. or less.
On the other hand, if the second cooling stop temperature is less than 100° C., the martensitic transformation proceeds excessively, and it is not possible to obtain a predetermined volume fraction of retained austenite, and it is not possible to obtain the desired ductility. For this reason, the cooling stop temperature is set to 100° C. or higher.
Here, the second average cooling rate is "retention end temperature (°C)-second cooling stop temperature (°C)/cooling time (seconds) from the retention end temperature to the second cooling stop temperature".

[Reheating step: heating from the second cooling stop temperature to a reheating temperature of 50° C. or more and 340° C. or less from the second cooling stop temperature at an average heating rate of 2.0° C./s or more]
The tempering effect of martensite is enhanced at higher temperatures, so by tempering at the second cooling stop temperature +50°C or higher, rather than at the second cooling stop temperature, carbon concentration is promoted, and the formation of retained austenite is promoted, resulting in improved stretch flange formability and improved ductility.
On the other hand, if the reheating temperature exceeds 340°C, carbide precipitation is promoted, carbon concentration is suppressed, and the desired retained austenite cannot be obtained, making it impossible to obtain the desired ductility. For this reason, the reheating temperature is set to be equal to or higher than the cooling stop temperature +50°C and equal to or lower than 340°C.
Moreover, if the average heating rate is less than 2.0° C./s, carbide precipitation is promoted rather than carbon distribution, and the desired retained austenite cannot be obtained. Therefore, the average heating rate is set to 2.0° C./s or more in the temperature range from the cooling stop temperature to 340° C. or less.
Here, the average heating rate is "reheating temperature (°C)-second cooling stop temperature (°C)/heating time from the second cooling stop temperature to the reheating temperature (seconds)".

[Third cooling step: cooling in the temperature range from the reheating temperature to 50° C. at a third average cooling rate of 0.05 to 1.0° C./s while retaining for 100 s or more]
If the third average cooling rate in the temperature range from the reheating temperature to 50°C exceeds 1.0°C/s, the tempering effect of martensite is not sufficiently obtained, and the C-enriched region ( _SC≧0.5 ) of 0.05 mass% or more increases, deteriorating the stretch flange formability and material stability in the sheet width direction. For this reason, the cooling rate in the temperature range from the reheating temperature to 50°C is set to 1.0°C/s or less.
In addition, by setting the cooling rate to 1.0°C/s or less in the temperature range from the reheating temperature to 50°C, the temperature variation in the sheet width direction can also be reduced, and the material stability in the sheet width direction can be further improved.
On the other hand, if the cooling rate (third average cooling rate) in the temperature range from the reheating temperature to 50° C. is slow, the treatment time becomes long and operability deteriorates. For this reason, the cooling rate (third average cooling rate) in the temperature range from the cooling stop temperature to 50° C. is set to 0.05° C./s or more.
Here, the third average cooling rate is "reheating temperature (° C.)−50° C./cooling time (seconds) from reheating temperature (° C.) to 50° C.".

The surface of the steel sheet may be subjected to a galvanizing treatment to obtain a steel sheet having a galvanized layer on the surface. The type of plating treatment is not particularly limited, and may be either hot-dip galvanizing or electrogalvanizing. Furthermore, as an alloying hot-dip galvanizing treatment, a plating treatment for alloying may be performed after hot-dip galvanizing.
Hot-dip galvanization is used for automotive steel sheets, etc. When hot-dip galvanization is performed, the steel sheet may be immersed in a hot-dip galvanizing bath after the holding step and the first cooling step in the above-mentioned annealing in a continuous annealing furnace at the upstream stage of a continuous hot-dip galvanizing line to form a hot-dip galvanized layer on the surface of the steel sheet, and further, may be subjected to an alloying treatment thereafter to produce an alloyed hot-dip galvanized steel sheet.
Specifically, in the above-mentioned second cooling step, when the steel sheet is retained at a retention temperature of 350 to 550° C. for 10 s to 60 s, the steel sheet surface can be subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. Moreover, the above-mentioned soaking and cooling steps and the plating step may each be performed in a separate line.
Alternatively, electrolytic galvanization can be carried out after annealing, i.e., after the third cooling step.

The thickness of the steel plate of the present invention thus obtained is preferably 0.5 mm or more. Also, the thickness of the steel plate of the present invention is preferably 2.0 mm or less.
The sheet width is preferably 600 mm or more. The sheet width of the steel sheet of the present invention is preferably 1700 mm or less.

Next, we will describe the components of the present invention and their manufacturing method.

The member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining. The manufacturing method of the member of the present invention includes a step of subjecting the steel plate of the present invention to at least one of forming and joining to obtain the member.

The steel plate of the present invention has a tensile strength of 980 MPa or more, is excellent in press formability, ductility, and stretch flange formability, and has excellent material stability in the plate width direction. Therefore, the member obtained using the steel plate of the present invention also has high strength, is excellent in press formability, ductility, and stretch flange formability, and has excellent material stability in the plate width direction. In addition, the use of the member of the present invention makes it possible to reduce weight. Therefore, the member of the present invention can be suitably used, for example, for vehicle body frame parts. The member of the present invention also includes a welded joint.

For the forming process, general processing methods such as press working can be used without restrictions. For the joining process, general welding methods such as spot welding and arc welding, riveting, crimping, etc. can be used without restrictions.

A slab having the composition shown in Table 1 produced by continuous casting was heated to 1200°C, soaked for 200 min, finished at 900°C, and coiled at 550°C. The slab was then subjected to a hot rolling process, followed by cold rolling at a rolling ratio of 50% to produce a cold-rolled steel sheet having a thickness of 1.4 mm. The cold-rolled steel sheet was then treated under the annealing conditions shown in Table 2 to produce the steel sheet of the present invention and the steel sheet of the comparative example.
The width of all the obtained steel plates was 1500 mm.

Some of the steel sheets (cold-rolled steel sheets: CR) were subjected to hot-dip galvanization treatment while being held at a holding temperature of 350 to 550°C for 10 to 60 s to produce hot-dip galvanized steel sheets (GI). Here, the steel sheets were immersed in a galvanizing bath at 440 to 550°C to produce hot-dip galvanized steel sheets, and then the coating weight was adjusted by gas wiping or the like. For hot-dip galvanization, a galvanizing bath containing 0.10% to 0.22% Al was used. Furthermore, some of the hot-dip galvanized steel sheets were subjected to alloying treatment after the hot-dip galvanizing treatment to produce alloyed hot-dip galvanized steel sheets (GA). Here, the alloying treatment was performed in a temperature range of 460 to 550°C. Some of the steel sheets (cold-rolled steel sheets: CR) were subjected to electroplating to produce electrogalvanized steel sheets (EG).

The steel structure was measured by the following method. The measurement results are shown in Table 3.
The area ratios of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and quenched martensite (fresh martensite) were measured by cutting out a cross section of the plate thickness parallel to the rolling direction, mirror-polishing it, and then etching it with 1 vol% nital. Ten fields of view were observed at a magnification of 5,000 times using an SEM at a position corresponding to 1/4 of the thickness, and the photographed structure was quantified by image analysis.
The polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. It is the area that appears the blackest in the SEM.
Upper bainite is a ferrite structure with the formation of carbides or retained austenite inside that appear white under SEM. When it is difficult to distinguish between upper bainite and polygonal ferrite, the area of ferrite with an aspect ratio of ≦2.0 was classified as polygonal ferrite, and the area with an aspect ratio of >2.0 was classified as upper bainite, and the area ratio was calculated. Here, the aspect ratio is calculated by determining the major axis length a at which the particle length is the longest, and the minor axis length b at the particle length when it crosses the particle the longest in the direction perpendicular to the major axis length a, and a/b is the aspect ratio.
Tempered martensite and lower bainite are regions that are accompanied by a lath-shaped substructure and carbide precipitation inside when viewed under an SEM.
Hardened martensite (fresh martensite) is a blocky region that appears white under an SEM with no internal substructure visible.
The remaining structure is a carbide and/or pearlite structure, which can be confirmed by a white contrast under SEM. Carbide is a structure with a particle size of 1 μm or less, and pearlite is a lamellar (layer) structure, so it can be distinguished.

The quantitative evaluation of the above-mentioned structure and the measurement of the aspect ratio and circle equivalent diameter of the quenched martensite and the retained austenite were performed using Image J (Fiji) as image analysis software.
A cross section of the plate thickness parallel to the rolling direction was cut out, mirror-polished, and then etched with 1 vol% nital. At the 1/4 thickness position, 10 fields of view were observed at 5000 times magnification with an SEM, and each structure was identified and quantitatively evaluated using the Trainable Weka segmentation method, which can identify regions using machine learning in Image J (Fiji). The aspect ratio and circle equivalent diameter of the quenched martensite and retained austenite can also be measured by a particle analysis program, which is a function of Image J, and only the quenched martensite and retained austenite identified as described above were extracted and measured.

The volume fraction of retained austenite was determined by chemical polishing at a position 1/4 the thickness from the surface and then X-ray diffraction. A Co-Kα source was used for the incident X-rays, and the volume fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

The area ratio S _C≧0.5 of the C-enriched region where the C concentration is 0.5 mass% or more was measured at a 1/4 position of the sheet thickness in the sheet thickness cross section parallel to the rolling direction using a JXA-8500F field emission electron probe microanalyzer (FE-EPMA). The C concentration distribution was measured by mapping analysis with an acceleration voltage of 6 kV, an irradiation current of 7×10 ⁻⁸ A, and a minimum beam diameter, and the area ratio where the C concentration is 0.5 mass% or more was calculated.
However, in order to eliminate the effects of contamination, the background was subtracted so that the average value of C obtained in the analysis was equal to the carbon amount in the base material. In other words, if the average value of the measured carbon amount was greater than the carbon amount in the base material, the increase was considered to be contamination, and the true C amount at each position was determined by subtracting the increase uniformly from the analysis value at each position.

To evaluate the tensile properties, JIS No. 5 tensile test pieces were taken from the center position of the plate width, and tensile tests (in accordance with JIS Z2241 (2011)) were performed with N=3. Each evaluation was based on the average value of the three points. Steel plates with a tensile strength of 980 MPa or more were judged to have excellent strength. Steel plates with a yield ratio YR of 0.8 or less were judged to have excellent press formability. Total elongation of 14.0% or more for TS: 980 MPa or more and 12.0% or more for TS: 1180 MPa or more were judged to have excellent ductility.

In addition, the evaluation of stretch flange formability was performed by taking a test piece from the center position of the plate width, and performing a hole expansion test in accordance with the provisions of the Japan Iron and Steel Federation standard JFST1001 with N = 3. That is, after punching a sample of 100 mm x 100 mm square size using a punching tool with a punch diameter of 10 mm and a clearance of 13%, a conical punch with an apex angle of 60 degrees was used to expand the hole until a crack penetrating the plate thickness occurred, with the burrs generated during the formation of the punched hole being on the outside. In this case, d ₀ : initial hole diameter (mm), d : hole diameter at the time of crack occurrence (mm), the hole expansion ratio λ (%) = {(d - d ₀ ) / d ₀ } × 100 was obtained, and the average value of the three points performed was evaluated as λ. Steels having a λ of 40% or more were judged to have excellent hole expansion properties and excellent stretch flangeability.

For the evaluation of the material stability in the sheet width direction, 23 points (including the sheet width center position) of the evaluation material were sampled from both sheet width directions at intervals of 100 mm or less from the sheet width center position (position 12W/24 (W: sheet width)), and EL and λ were obtained at each position (measurement position X). The material stability in the sheet width direction was evaluated by determining the ratio of the difference between the measurement value at the sheet width center position and each position to the measurement value at the center position.
Using EL and λ at the center position of the plate width as the standard, a group of consecutive measurements in which the difference in EL and λ was 10% or less was defined as the region in which the difference in EL and λ was 10% or less, and steels in which this region accounted for 80% or more of the total plate width were determined to have excellent material stability.
When the sheet width of region A satisfying the following formulas (1) and (2) is 80% or more of the total sheet width, the sheet was determined to have excellent material stability in the sheet width direction.
−10≦100×[(EL(%) at measurement position X in region A−EL(%) at center position of sheet width)/EL(%) at center position of sheet width]≦10 (1)
−10≦100×[(λ(%) at measurement position X in region A−λ(%) at center position of sheet width)/λ(%) at center position of sheet width]≦10 (2)
(In formulas (1) and (2), the measurement positions X are 23 positions in total, which are 24 divided positions of the width W of the steel plate. That is, the measurement positions X are W/24, 2W/24, 3W/24, 4W/24, 5W/24, 6W/24, 7W/24, 8W/24, 9W/24, 10W/24, 11W/24, 12W/24, 13W/24, 14W/24, 15W/24, 16W/24, 17W/24, 18W/24, 19W/24, 20W/24, 21W/24, 22W/24, and 23W/24, which are 23 positions in total, as positions in the plate width direction.)

The measurement results are shown in Table 3.

The examples of the present invention shown in Tables 2 and 3 were excellent in strength, press formability, ductility, stretch flange formability, and material stability in the plate width direction, whereas the comparative examples were inferior in all of these aspects.

In addition, using the steel plate of the present invention, it was found that the components obtained by molding, the components obtained by joining, and the components obtained by further molding and joining have high strength and excellent press formability, ductility, stretch flange formability, and material stability in the width direction of the plate, similar to the steel plate of the present invention, because the steel plate of the present invention has high strength and is excellent in press formability, ductility, stretch flange formability, and material stability in the width direction of the plate.

Claims (9)

In mass percent,
C: 0.05 to 0.20%,
Si: 0.40 to 1.50%,
Mn: 1.9 to 3.5%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.005 to 0.50%,
N: less than 0.015%;
The balance being iron and unavoidable impurities;
Area ratio of polygonal ferrite: 10% or more and 57% or less,
Total area ratio of upper bainite, tempered martensite and lower bainite: 40% or more and 80% or less,
Volume fraction of retained austenite: 3% or more and 15% or less,
Area ratio of quenched martensite: 12% or less (including 0%),
a steel structure having a remaining structure with an area ratio of 5% or less ;
having
The total area ratio of the quenched martensite and the retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more is 20% or less,
A steel plate in which an area ratio of a C-enriched region (S _C≧0.5 ) having a C concentration of 0.5 mass% or more relative to the entire structure is 15% or less. The steel sheet according to claim 1, wherein the component composition contains one or more selected from the following (A) to (C):
(A) in mass %,
Ti: 0.1% or less,
B: 0.01% or less,
One or two selected from the following:
(B) in mass %,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1% or less,
Mo: 0.5% or less,
V: 0.5% or less,
Nb: 0.1% or less,
One or more selected from the following:
(C) in mass %,
Mg: 0.0050% or less,
Ca: 0.0050% or less,
Sn: 0.1% or less,
Sb: 0.1% or less,
REM: 0.0050% or less,
Select one or more of the following: The steel sheet according to claim 1, having a zinc-plated layer on its surface. The steel sheet according to claim 2, having a zinc-plated layer on its surface. A member made using the steel plate according to any one of claims 1 to 4. A method for producing a steel sheet, comprising the steps of: subjecting a steel slab having the composition according to claim 1 or 2 to hot rolling, pickling and cold rolling; and then annealing the resulting cold-rolled steel sheet,
The annealing is
A holding step of heating the cold-rolled steel sheet to an annealing temperature of 750 to 880 ° C. and holding the annealing temperature for 10 to 500 seconds;
A first cooling step of cooling the temperature range from the annealing temperature to a first cooling stop temperature of 350 to 550 ° C. at a first average cooling rate of 2 to 50 ° C./s to the first cooling stop temperature;
A second cooling step in which the steel sheet is held at a residence temperature of 350 to 550° C. for 10 s to 60 s and then cooled to a second cooling stop temperature of 100 to 300° C. at a second average cooling rate of 3 to 50° C./s;
A reheating step of heating from the second cooling stop temperature to a reheating temperature of second cooling stop temperature + 50 ° C. or more and 340 ° C. or less at an average heating rate of 2.0 ° C./s or more;
and a third cooling step of cooling the steel sheet in a temperature range from the reheating temperature to 50° C. at a third average cooling rate of 0.05 to 1.0° C./s for 100 s or more after the reheating step , the steel sheet having a steel structure including a residual structure having an area ratio of polygonal ferrite of 10% to 57%, a total area ratio of upper bainite, tempered martensite, and lower bainite of 40% to 80%, a volume ratio of retained austenite of 3% to 15%, an area ratio of quenched martensite of 12% or less (including 0%), and an area ratio of 5% or less, the total area ratio of quenched martensite and retained austenite having an aspect ratio of 3 or less and a circle equivalent diameter of 1.6 μm or more, being 20% or less, and the area ratio of C-enriched regions (S _C≧0.5 ) having a C concentration of 0.5 mass% or more relative to the entire structure being 15% or less . The method for manufacturing a steel sheet according to claim 6, wherein in the second cooling step, when the steel sheet is held at a holding temperature of 350 to 550°C for 10 to 60 seconds, the steel sheet surface is subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment. The method for manufacturing a steel sheet according to claim 6, further comprising electrolytic galvanizing the surface of the steel sheet after the annealing. A method for manufacturing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining processes to produce a component.