WO2024057670A1

WO2024057670A1 - Steel sheet, member, and methods for producing same

Info

Publication number: WO2024057670A1
Application number: PCT/JP2023/024255
Authority: WO
Inventors: 三周知場; 洋一郎松井; 琴未野口; 英之木村
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-09-15
Filing date: 2023-06-29
Publication date: 2024-03-21
Also published as: JPWO2024057670A1; JP7485240B1

Abstract

Provided are: a steel sheet that has a tensile strength of 980 MPa or more, excellent press moldability, ductility, and stretch-flangeability, and excellent material stability in the sheet width direction; a member; and methods for producing the same.　The steel sheet has a component composition and a steel structure of specific ranges. The total area ratio of hardened martensite and retained austenite having an aspect ratio of 3 or less and an equivalent circle diameter of 1.6 μm or more to the total area ratio of hardened martensite and retained austenite is 20% or less, and the area ratio of C-enriched regions (S_C≥0.5) in which the C concentration is 0.5 mass% or more to the total structure is 15% or less.

Description

Steel plates, members and their manufacturing methods

The present invention relates to steel plates, members, and methods of manufacturing them. More specifically, the present invention relates to steel plates and members having a tensile strength (TS) of 980 MPa or more and excellent formability and material stability, and methods for manufacturing them. The steel plate of the present invention is suitable as a material for automobile frame members.

In recent years, from the perspective of preserving the global environment, stricter regulations on _CO2 emissions from automobiles have been promoted within an international framework. The most effective way to improve the fuel efficiency of automobiles is to reduce the weight of automobile bodies by thinning the steel plates used in automobile frame members. For this reason, the amount of high-strength steel sheets used is increasing in order to contribute to lower fuel consumption of automobiles. However, as the strength of steel sheets increases, cracks are more likely to occur during press forming due to decreased ductility and stretch flange formability. Therefore, a steel plate with superior ductility and stretch-flange formability compared to conventional steel sheets is desired.

Additionally, from the perspective of carbon neutrality, material usage up (improved yield) is required, and steel sheets are also required to have excellent material stability in the width direction. However, as the strength of steel sheets increases, variations in formability such as ductility and hole expandability in the sheet width direction become apparent, and cracks are more likely to occur during press forming, so it is necessary to set limits on the blanking position. There is a problem of decreased yield.

Additionally, as the strength increases, the yield ratio: YR (YR=yield strength YS/tensile strength TS) generally increases, so there is a problem that springback after molding increases.

As a technology to improve the formability of high-strength steel sheets, TRIP steel sheets have been developed in which retained austenite is dispersed in the structure. For example, in Patent Document 1, steel containing C: 0.04 to 0.12%, Si: 0.8 to 2.5%, and Mn: 0.5 to 2.0% is annealed at 300 to 500°C. Austempering (carbon distribution accompanying bainite transformation) held for 10 to 900 seconds generates 2 to 10% residual γ, resulting in high ductility of TS×El≧21000MPa・% and high stretch flange formability of 70% or more. It is disclosed that a steel plate having the following properties can be obtained.

Furthermore, in Patent Document 2, during the cooling process, the temperature is once cooled to a temperature range between the martensitic transformation start temperature (Ms point) and the martensitic transformation completion temperature (Mf point), and then, the residual austenite is stabilized by reheating and holding. A technique has been disclosed for increasing the ductility of a steel sheet by utilizing the principle of so-called Q&P; Quenching & Partitioning (quenching and distribution of carbon from martensite to austenite). Specifically, a cold rolled steel sheet having a predetermined chemical composition is held at a first soaking temperature of 750°C or higher, then cooled to a cooling stop temperature in a temperature range of 150 to 350°C, and then heated to a temperature range of 350 to 500°C. By reheating to 100%, a retained austenite volume fraction of 5% to 15% is achieved, which achieves both TS of 980 MPa or more and ductility of 17% or more, and an excellent hole expansion rate of 50% or more. The present invention discloses a steel plate having good hole expandability and a method for manufacturing the same.

On the other hand, DP steel (Dual Phase steel) has been developed as a steel plate that has a low yield ratio that is effective in reducing springback. General DP steel is a multi-phase steel in which martensite is dispersed in the ferrite structure as the main phase, and has a high TS, low yield ratio, and excellent ductility. However, DP steel has the disadvantage of poor stretch flange formability because cracks are likely to occur due to stress concentration at the interface between ferrite and martensite. Examples of techniques for improving the stretch flange formability of DP steel include Patent Document 3 and Patent Document 4.

In Patent Document 3, the space factor of ferrite is controlled to be 50% or more and the space factor of martensite is controlled to 3 to 30% with respect to the entire structure, and the average crystal grain size of ferrite is controlled to be 10 μm or less, and the average crystal grain of martensite is controlled to be 10 μm or less. A technique is disclosed in which deterioration of stretch flange formability is suppressed by setting the diameter to 5 μm or less.

Furthermore, in Patent Document 4, the space factor of ferrite is controlled to 5 to 30% and the space factor of martensite to the entire structure is controlled to 50 to 95%, and fine ferrite particles with an average grain size of 3 μm or less in equivalent circle diameter are formed. It is disclosed that ductility and stretch flange formability can be improved by controlling the average grain size to martensite having a circular equivalent diameter of 6 μm or less.

Patent No. 5515623 Patent No. 5821911 Patent No. 3936440 Japanese Patent Application Publication No. 2008-297609

Although Patent Document 1 and Patent Document 4 mentioned above disclose a method for manufacturing a steel plate with excellent ductility and stretch-flange formability, it is necessary to form a large amount of soft phase ferrite, so for example, a high temperature of 780 MPa or more is required. Strengthening is difficult. Further, although Patent Document 2 has excellent ductility and stretch flange formability, since the steel plate has a YR of 0.8 or more, dimensional accuracy may be impaired due to springback during press forming. Further, Patent Document 3 discloses a method for manufacturing a DP steel sheet that has low YR and excellent stretch flange formability, but since it has a DP structure, ductility is not necessarily sufficient. Further, none of the patent documents discloses a technique for suppressing variations in ductility and stretch flange formability in the sheet width direction. Therefore, there is a need to develop a high-strength steel plate that has excellent ductility, excellent stretch flanges, and excellent material stability in the width direction.
In view of the above circumstances, the present invention provides a steel plate and member having a tensile strength (TS) of 980 MPa or more, excellent press formability, ductility and stretch flange formability, and excellent material stability in the width direction. The purpose of this invention is to provide methods for producing the same.

Here, the tensile strength refers to tensile strength (TS) obtained in accordance with JIS Z2241 (2011).
"Excellent press formability" means that the yield ratio YR obtained according to JIS Z2241 (2011) is 0.8 or less.
"Excellent ductility" means that the total elongation EL obtained according to 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: 1180 MPa or more, EL: 12.0% or more Excellent stretch flange formability means hole expansion rate λ (%) (={(d -d ₀ )/d ₀ }×100) is 40% or more.
Excellent material stability in the sheet width direction means that the sheet width in region A continuously satisfies the following equations (1) and (2) regarding EL and λ at measurement position X in the sheet width direction. Refers to 80% or more of the width.
-10≦100×[(EL(%) at measurement position
-10≦100×[(λ(%) of measurement position
(In formulas (1) and (2), the measurement position In other words, the positions in the board width direction 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, 23W/24, a total of 23 locations, are defined as measurement positions X.)
Here, for example, if the measurement position X that continuously satisfies equations (1) and (2) is 2W/24 to 20W/24, then The plate width is 100×(20-2+1)/23=83% of the total plate width.

In order to solve the above-mentioned problems, the present inventors investigated various factors affecting press formability, ductility, stretch-flange formability, and material stability for various thin steel sheets having a tensile strength of 980 MPa or more, and investigated the chemical composition of the steel sheets. We conducted extensive studies from the viewpoints of microstructure, manufacturing conditions, and microstructure. As a result, 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. Contains Al: 0.005 to 0.50%, N: less than 0.015%, the area ratio of polygonal ferrite is 10% to 57%, and the total area of upper bainite, tempered martensite, and lower bainite The aspect ratio is 40% or more and 80% or less, the area ratio of retained austenite (residual γ) is 3% or more and 15% or less, and the area ratio of quenched martensite is 12% or less (including 0%). A C-enriched region (S _C≧0 By making the steel structure have an area ratio of 15% or less to the entire structure of _.5 ), it has excellent press formability, ductility and stretch flange formability, and also has excellent material stability in the sheet width direction ( It was discovered that high-strength cold-rolled steel sheets with small material variations can be obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In mass%,
C: 0.05-0.20%,
Si: 0.40 to 1.50%,
Mn: 1.9 to 3.5%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.005-0.50%,
N: Contains less than 0.015%,
A component composition in which the remainder consists of 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%),
Furthermore, it has a structure consisting of residual tissue,
The total area ratio of hardened martensite and retained austenite having an aspect ratio of 3 or less and an equivalent circle diameter of 1.6 μm or more is 20% or less with respect to the total area ratio of hardened martensite and retained austenite,
A steel plate in which the area ratio of C-enriched regions (SC _≧0.5 ) in which the C concentration is 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 plate according to [1], containing one or two selected from among the above.
[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 plate 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 plate according to any one of [1] to [3], containing one or more selected from among the above.
[5] The steel sheet according to any one of [1] to [4], which has a galvanized layer on the surface.
[6] A member using the steel plate according to any one of [1] to [5].
[7] After hot rolling, pickling and cold rolling a steel slab having the composition according to any one of [1] to [4], the obtained cold rolled steel plate is A method of manufacturing a steel plate that undergoes annealing,
The annealing is
A holding step of heating the cold rolled steel plate to an annealing temperature of 750 to 880°C and holding at the annealing temperature for 10 to 500 seconds;
A first cooling step of cooling the temperature range from the annealing temperature to the first cooling stop temperature of 350 to 550 ° C. to the first cooling stop temperature at a first average cooling rate of 2 to 50 ° C./s;
A second cooling step of cooling at a second average cooling rate: 3 to 50 °C/s to a second cooling stop temperature of 100 to 300 °C, after retaining at a retention temperature of 350 to 550 °C for 10 seconds to 60 seconds;
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 of 2.0 ° C / s or more,
After the reheating step, a third cooling step of cooling 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 seconds or more. , a method for manufacturing steel plates.
[8] The steel plate according to [7], wherein in the second cooling step, the steel plate surface is subjected to hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment when staying at a residence temperature of 350 to 550 ° C. for 10 seconds or more and 60 seconds or less. manufacturing method.
[9] The method for manufacturing a steel sheet according to [7], wherein after the annealing, the surface of the steel sheet is electrogalvanized.
[10] A method for manufacturing a member, comprising the step of subjecting the steel plate according to any one of [1] to [5] to at least one of forming and bonding to produce a member.

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

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: 0.05-0.20%>
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 (residual γ) to improve ductility. If the C content is less than 0.05%, these effects cannot be sufficiently ensured.
On the other hand, when the C content exceeds 0.20%, the martensitic transformation start temperature (Ms point) decreases. As a result, in the third cooling process in which cooling is performed in the temperature range from the reheating temperature to 50°C at a third average cooling rate: 0.05 to 1.0°C/s, martensitic transformation and subsequent tempering of martensite occur. is not done enough. As a result, the formation of quenched martensite and C-enriched regions of 0.5 mass% or more ( _SC≧0.5 ) is promoted, and stretch flange formability and material stability in the plate width direction are reduced.
Therefore, the C content is set to 0.05% or more and 0.20% or less. The C content is preferably 0.08% or more. Further, the C content is preferably 0.18% or less.

<Si: 0.40 to 1.50%>
Si is contained from the viewpoint of strengthening the ferrite and increasing its strength, and from the viewpoint of suppressing the formation of carbides in martensite and bainite to ensure a predetermined amount of residual γ and improving ductility. If the Si content is less than 0.40%, these effects cannot be sufficiently ensured.
On the other hand, when the Si content exceeds 1.50%, carbon distribution to untransformed austenite is excessively promoted, and the formation of a C-enriched region of 0.5 mass% or more ( _SC≧0.5 ) is promoted. , stretch flange formability and material stability in the plate width direction decrease. 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. Further, the Si content is preferably 1.20% or less.

<Mn: 1.9 to 3.5%>
Mn improves the hardenability of steel sheets, suppresses excessive transformation of ferrite, and promotes high strength through transformation strengthening, and similarly to Si, suppresses the formation of carbides in bainite and contributes to ductility. It is included from the viewpoint of further promoting the formation of retained austenite and further improving ductility. In order to obtain these effects, the Mn content needs to be 1.9% or more.
On the other hand, when the Mn content exceeds 3.5%, bainite transformation is delayed, a predetermined amount of retained austenite cannot be secured, and ductility decreases.
Moreover, when the Mn content exceeds 3.5%, it becomes difficult to suppress the formation of coarse quenched martensite, and 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. Further, the Mn content is preferably 3.3% or less, more preferably 3.0% or less.

<P: 0.02% or less>
P is an element that strengthens steel, but if its content is large, it deteriorates spot weldability. Therefore, the P content is 0.02% or less, preferably 0.01% or less. Note that although it is not necessary to contain P, it is preferable that the P content is 0.001% or more because reducing it to less than 0.001% requires a great deal of cost. The P content is more preferably 0.002% or more, and still more preferably 0.005% or more.

<S: 0.01% or less>
S has the effect of improving scale peelability during hot rolling and suppressing nitridation during annealing, but is an element that has an adverse effect on spot weldability, bendability, and hole expandability. In order to reduce these adverse effects, the S content is at least 0.01% or less, preferably 0.0020% or less.
Although it is not necessary to contain S, reducing the S content to less than 0.0001% requires a great deal of cost, so the S content is preferably 0.0001% or more from the viewpoint of manufacturing costs. The S content is more preferably 0.0005% or more, and still more preferably 0.0015% or more.

<sol. Al: 0.005-0.50%>
Al is contained for the purpose of deoxidizing or obtaining residual γ. In order to stably deoxidize, sol. Al content shall be 0.005% or more. sol. It is preferable that the Al content is 0.01% or more.
On the other hand, sol. If the Al content exceeds 0.50%, a large amount of Al-based coarse inclusions will increase, and stretch flange formability will deteriorate. For this reason, sol. Al content shall be 0.50% or less.

<N: less than 0.015%>
N is an element that forms nitrides such as BN, AlN, and TiN in steel, and reduces stretch flange formability, so it is necessary to limit its content. Therefore, the N content should be less than 0.015%.
Note that although it is not necessary to contain N, reducing the N content to less than 0.0001% requires a great deal of cost, so 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 even more preferably 0.0015% or more.

The component composition of the steel sheet in the present invention contains the above-mentioned component elements as basic components, and the remainder includes iron (Fe) and inevitable impurities. In addition, it is preferable that the component composition of the steel plate in the present invention has a component composition in which the balance consists of Fe and unavoidable impurities.
In addition to the above-mentioned components, the composition of the steel sheet of the present invention can appropriately contain one or more optional elements selected from the following (A) to (C).
(A) One or two types selected from Ti: 0.1% or less, B: 0.01% or less,
(B) 1 selected from 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 species or two or more species,
(C) One type 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, or 2 or more types

<Ti: 0.1% or less>
Ti fixes N in steel as TiN, and has the effect of improving hot ductility and the effect of B on improving hardenability. Further, the precipitation of TiC has the effect of making the structure finer. In order to obtain these effects, it is desirable that the Ti content be 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 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%, the rolling load will increase and the ductility will decrease due to an increase in the amount of precipitation strengthening, so if Ti is contained, the Ti content should be 0.1% or less. Preferably, the Ti content is 0.05% or less, 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 producing tempered martensite and/or bainite with a predetermined area ratio. Therefore, it is preferable that the B content is 0.0005% or more. Further, the B content is more preferably 0.0010% or more.
On the other hand, when the B content exceeds 0.01%, the effect not only becomes saturated, but 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, more preferably 0.003% or less.

<Cu: 1% or less>
Cu improves corrosion resistance in the automotive environment. Further, the corrosion products of Cu coat the surface of the steel sheet, which has the effect of suppressing hydrogen intrusion into the steel sheet. Cu is an element that is mixed in when scrap is used as a raw material, and by allowing Cu to be mixed in, recycled materials can be used as raw materials and manufacturing costs can be reduced. From this viewpoint, it is preferable to contain Cu in an amount of 0.005% or more, and from the viewpoint of improving delayed fracture resistance, it is more desirable to contain Cu in an amount of 0.05% or more. More preferably, it is 0.10% or more. However, if the Cu content becomes too large, surface defects will occur, so when Cu is contained, the Cu content is set to 1% or less.

<Ni: 1% or less>
Like Cu, Ni is also an element that has the effect of improving corrosion resistance. Further, Ni has the effect of suppressing the occurrence of surface defects that are likely to occur when Cu is included. For this reason, it is desirable to contain Ni in an amount of 0.01% or more. The Ni content is more preferably 0.04% or more, still more preferably 0.06% or more.
However, if the Ni content becomes too large, scale formation within the heating furnace will become uneven, which may even cause surface defects to occur. Moreover, it also causes an increase in costs. Therefore, when Ni is contained, the Ni content is set to 1% or less. Preferably, the Ni content is 0.5% or less, more preferably 0.3% or less.

<Cr: 1% or less>
Cr can be contained because of its effect of improving the hardenability of steel and suppressing the formation of carbides in martensite and upper/lower bainite. In order to obtain such effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.03% or more, and still more preferably 0.06% or more.
However, if Cr is contained excessively, the pitting corrosion resistance will deteriorate, so when Cr is contained, the Cr content is set to 1% or less.

<Mo: 0.5% or less>
Mo can be contained because it has the effect of improving the hardenability of steel and suppressing the formation of carbides in martensite and upper/lower bainite. In order to obtain such an effect, the Mo content is preferably 0.01% or more. The content is more preferably 0.03% or more, and even more preferably 0.06% or more. More preferably, the Mo content is 0.1% or more, even more preferably 0.2% or more.
However, since Mo significantly deteriorates the chemical conversion treatment property of cold rolled steel sheets, when Mo is contained, the Mo content is set to 0.5% or less.

<V: 0.5% or less>
V is included because it has the effect 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. be able to. 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 still more preferably 0.015% or more. Even more preferably, the V content is 0.02% or more, even more preferably 0.05% or more. The V content is preferably 0.15% or more, more preferably 0.25% or more.
However, if a large amount of V is contained, the castability will be significantly deteriorated, so when V is contained, the V content should be 0.5% or less. Preferably, the V content is 0.4% or less, more preferably 0.3% or less.

<Nb: 0.1% or less>
Nb can be contained because it has the effect of refining the steel structure and increasing its 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 still more preferably 0.010% or more.
However, if a large amount of Nb is contained, precipitation strengthening becomes too strong and ductility decreases. Moreover, this results in an increase in rolling load and deterioration in castability. Therefore, when Nb is contained, the Nb content is set to 0.1% or less. Preferably, the Nb content is 0.07% or less, 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 still more preferably 0.0006% or more.
On the other hand, if a large amount of Mg is added, the surface quality and bendability will deteriorate, so when Mg is included, the Mg content should be 0.0050% or less. Preferably, the Mg content is 0.0030% or less.

<Ca: 0.0050% or less>
Ca fixes S as CaS and contributes to improving bendability and delayed fracture resistance. For this reason, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. On the other hand, if a large amount of Ca is added, the surface quality and bendability will be deteriorated, so when Ca is contained, the Ca content should be 0.0050% or less. 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 thereby suppresses a reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing its strength and improving its fatigue resistance. From this point of view, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.004% or more, and still more preferably 0.006% or more. More preferably, the Sn content is 0.01% or more, even more preferably 0.05% or more.
On the other hand, when the Sn content exceeds 0.1%, castability deteriorates. Furthermore, Sn is segregated at the prior γ grain boundaries, deteriorating the delayed fracture resistance. Therefore, when Sn is contained, the Sn content is 0.1% or less.

<Sb: 0.1% or less>
Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, and thereby suppresses a reduction in the content of C and B in the surface layer. This effect suppresses the formation of ferrite in the surface layer of the steel sheet, increasing its strength and improving its fatigue resistance. From this point of view, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.004% or more, and still more preferably 0.006% or more. More preferably, the Sb content is 0.01% or more, even more preferably 0.05% or more.
On the other hand, when the Sb content exceeds 0.1%, castability deteriorates, and Sb segregates at prior γ grain boundaries, degrading delayed fracture resistance. Therefore, when 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 effects of sulfide on stretch flange formability and improves stretch flange formability by making the shape of sulfide spheroidal. 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 still more preferably 0.0020% or more.
On the other hand, if the REM content exceeds 0.0050%, the effect of improving stretch flange formability will be saturated, so when REM is contained, the REM content should be 0.0050% or less.

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

When the above-mentioned 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 above-mentioned arbitrary element is included in an amount less than the lower limit value, the above-mentioned arbitrary element is included as an unavoidable impurity.

Next, the mechanical properties of the steel sheet (cold-rolled steel sheet with excellent material stability) targeted by the present invention will be explained.

The steel plate of the present invention has a tensile strength (TS) of 980 MPa or more. Although the upper limit of the tensile strength is not particularly limited, from the viewpoint of coexistence with other properties, the tensile strength is preferably 1300 MPa or less.

In the steel plate of the present invention, the stability of press forming is significantly improved by ensuring the total elongation EL is 14.0% or more at TS: 980 MPa or more, and 12.0% or more at TS: 1180 MPa or more.
By ensuring a hole expansion rate λ of 40% or more, cracking during press molding can be suppressed, so that it can be applied to complex molded parts that are difficult to form. Therefore, λ is set to 40% or more.

In the steel plate of the present invention, regarding EL and λ at the measurement position That's all.
-10≦100×[(EL(%) at measurement position
-10≦100×[(λ(%) of measurement position
In equations (1) and (2), the measurement position contact points for each width). That is, the positions in the board width direction 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, 23W/24 A total of 23 locations are defined as measurement positions X.
Here, for example, if the measurement position X that continuously satisfies equations (1) and (2) is 2W/24 to 20W/24, then The plate width is 100×(20-2+1)/23=83% of the total plate width.
In the steel sheet of the present invention, the region A has a length in the sheet width direction of 80% or more of the total sheet width.
That is, in the steel sheet of the present invention, the deviation of EL in the sheet width direction is 10% or less with respect to the measured value at the sheet width center position, and the deviation of λ in the sheet width direction is less than 10% with respect to the measured value at the sheet width center position. The area where the thickness is 10% or less shall be 80% or more of the entire board width area. The range of the unsteady portion is allowed to be up to 20% in total at both ends in the width direction.
Because the end of the steel plate comes into contact with other structures during transportation and work processes, the end is not used to ensure quality. Therefore, the usable effective plate width does not reach 100%. Therefore, the effective plate width is preferably less than 100%.
By setting the area where the deviation of EL in the sheet width direction is 10% or less of the measured value at the center of the sheet width and the deviation of λ is 10% or less to 80% or more of the entire sheet width, yields are significantly improved. Therefore, in the present invention, the area where the deviation of EL in the board width direction is 10% or less of the measured value at the center of the board width, and the deviation of λ is 10% or less is 80% or more of the entire board width region. . Preferably it is 85% or more.

<YR≦0.8, TS≧980MPa, EL≧14.0%>
<YR≦0.8, TS≧1180MPa, EL≧12.0%>
For evaluation of tensile properties, a JIS No. 5 tensile test piece is taken from the center position of the plate width, and a tensile test (based on JIS Z2241 (2011)) is performed at N=3. Each evaluation is performed based on the average value of the three points. A steel plate having a tensile strength of 980 MPa or more is defined as a high-strength steel plate. A steel plate having a yield ratio YR of 0.8 or less is a steel plate having excellent press formability. A steel plate with excellent ductility has a total elongation EL of 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more.

<λ≧40%>
For evaluation of stretch flange formability, a test piece is taken from the center of the plate width, and a hole expansion test is conducted at N=3 in accordance with the Japan Iron and Steel Federation standard JFST1001. That is, after punching a sample with a square size of 100 mm x 100 mm using a punching tool with a punch diameter of 10 mm and a clearance of 13%, a conical punch with a 60 degree apex angle was used to remove the burrs generated when forming the punched hole on the outside. The hole is enlarged until a crack that penetrates through the plate thickness occurs. In this case, d ₀ is the initial hole diameter (mm), d is the hole diameter at the time of crack occurrence (mm), and the hole expansion rate λ (%) = {(d - d ₀ )/d ₀ }×100, The average value of the three points obtained during the test is evaluated as λ. Steel having a λ of 40% or more is judged to have excellent hole expandability and stretch flangeability.

<Evaluation of material stability in the board width direction>
To evaluate the material stability in the board width direction, 23 points were evaluated from both board width directions at intervals of 100 mm or less from the board width center position (the 12W/24 position mentioned above) (23 points include the board width center position). .) and determine EL and λ at each position (measurement position X). Then, the material stability in the board width direction is evaluated by determining the ratio of the difference between the measured value at the board width center position and each position to the measured value at the board width center position.
Using EL and λ at the center of the board width as a reference, consecutive measurement groups where the difference in EL and λ is 10% or less are defined as areas where the difference in EL and λ is 10% or less, and this area is defined for the entire board width. Steel having a ratio of 80% or more is judged to have excellent material stability.
In addition, the plate width of the steel plate in the present invention is preferably 600 mm or more. Moreover, the plate 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 low YR and high ductility, the area ratio of polygonal ferrite is 10% or more, and in order to obtain higher ductility, it is preferably 20% or more.
On the other hand, if the polygonal ferrite exceeds 57%, the desired strength cannot be obtained, so the area ratio of the polygonal ferrite is 57% or less, preferably 55% or less. More preferably it is 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 the 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 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, it is 75% or less.

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

<Quenched martensite: 12% or less (including 0%)>
Since the hard quenched martensitic structure lowers λ, it is necessary to suppress its area ratio. In order to obtain the desired λ, the area ratio of hardened martensite is set to 12% or less. In order to obtain λ more stably, the area ratio of hardened martensite is preferably 10% or less.

<Remaining organization>
Regarding the steel structure, other than the above, it consists of the remainder structure. The area ratio of the remaining tissue is preferably 5% or less. The remaining structure may be carbide or pearlite. These tissues may be determined by SEM observation as described later.

<Total area ratio of hardened martensite and retained austenite with an aspect ratio of 3 or less and an equivalent circle diameter of 1.6 μm or more relative to the total area ratio of hardened martensite and retained austenite: 20% or less>
The retained austenite becomes a hard martensitic structure due to the TRIP effect during press molding, tensile processing, etc. Therefore, in the present invention, from the viewpoint of stretch flangeability, quenched martensite and retained austenite are controlled together. When hardened martensite or retained austenite with a circular equivalent diameter of 1.6 μm or more is formed, voids are formed at stress concentration areas at the interface with other structures, making it impossible to obtain the desired stretch flange formability.
Furthermore, when the aspect ratio of quenched martensite or retained austenite is 3 or less, stress concentration tends to occur at the interface with other structures, which promotes void formation and deteriorates stretch flange formability. Therefore, with respect to the total area ratio of hardened martensite and retained austenite, the total area ratio of hardened martensite and retained austenite with an aspect ratio of 3 or less and an equivalent circle diameter of 1.6 μm or more is set to 20% or less. Preferably it is 18% or less.
There is no lower limit for the total area ratio of hardened martensite and retained austenite with an aspect ratio of 3 or less and an equivalent circle diameter of 2.0 μm or more, but there is no lower limit for the total area ratio of hardened martensite and retained austenite, but operability Since it is difficult to control the content to 0% from the viewpoint of the above, it is preferably set to 2% or more, 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 ) where the C concentration is 0.5 mass% or more relative to the whole tissue: 15% or less>
The hardness of quenched martensite is determined by the amount of C dissolved in the quenched martensite. When the difference in hardness between the hardened martensite and other structures increases, the formation of voids at the interface of the stress concentration area is promoted. The structures in which a large amount of solid solute C exists 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, but the area ratio of the structure with a C concentration of 0.5 mass% or more is 15% or less of all constituent structures. If so, stretch flange formability can be ensured while improving ductility, and variations in material quality in the sheet width direction can also be reduced, making it possible to manufacture a steel sheet with excellent material stability. Therefore, the space factor of the C-enriched region (SC _≧0.5 ) where the C concentration is 0.5 mass% or more is 15% or less.
Preferably it is 12% or less, more preferably 10% or less.
Moreover, it is preferably 6% or more, more preferably 8% or more.

Next, the method for measuring the steel structure will be explained.

To measure the area ratio of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite), cut out a cross section parallel to the rolling direction, mirror polish it, and then use 1 vol% nital. Corroded, 10 fields of view were observed at 1/4 thickness using SEM at 5000x magnification, and the photographed tissue photographs were quantified by image analysis.
Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. This is the area that appears blackest in the SEM.
Upper bainite is a ferritic structure with the formation of carbides or retained austenite 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 ≦2.0 is classified as polygonal ferrite, and the area with an aspect ratio >2.0 is classified as upper bainite, and the area ratio is calculated. do. Here, the aspect ratio is determined by determining the major axis length a where the particle length is the longest, and setting the particle length that crosses the particle longest in the direction perpendicular to it to be the minor axis length b, and a/b is the aspect ratio. Take the ratio.
The tempered martensite and lower bainite are regions with a lath-like substructure and carbide precipitation in the SEM.
Quenched martensite (fresh martensite) is a massive region that appears white with no underlying structure visible in the SEM.
The remaining structure is a carbide and/or pearlite structure, which can be confirmed by white contrast in SEM, but the carbide is a structure with a particle size of 1 μm or less, and the pearlite is a lamellar (layer) structure. It is possible to distinguish it from the fact that it has a similar structure.

The quantitative evaluation of the structure described above and the measurement of the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be performed using image analysis software such as Image J (Fiji).
A cross-section of the plate parallel to the rolling direction was cut out, polished to a mirror surface, corroded with 1 vol% nital, and observed at 1/4 thickness position with an SEM at 5000x magnification for 10 fields of view, and machine learning using Image J (Fiji) was performed. Each tissue can be identified and quantitatively evaluated using the Trainable Weka segmentation method that allows area identification. In addition, the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be measured using a particle analysis program that is also a function of Image J, and only the quenched martensite and retained austenite identified as above can be extracted and measured. do.

The volume fraction of retained austenite is determined by chemically polishing a 1/4 thickness position from the surface layer and using X-ray diffraction. A Co-Kα ray source is used for incident X-rays, and the volume of retained austenite is determined from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. Calculate the rate. Here, since 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 of the C-enriched region where the C concentration is 0.5 mass% _or more is measured using a JEOL field emission electron A line microanalyzer (FE-EPMA) JXA-8500F is used. Then, the C concentration distribution is measured by mapping analysis using an accelerating voltage of 6 kV, an irradiation current of 7×10 ⁻⁸ A, and a beam diameter of the minimum, and an area ratio at which the C concentration is 0.5 mass% or more is calculated.
However, in order to eliminate the influence of contamination, background components are subtracted so that the average value of C obtained in the analysis is equal to the carbon content of the base material. In other words, if the average value of the measured carbon content is greater than the carbon content of the base material, the increased amount is considered to be contamination, and the true value at each location is calculated by uniformly subtracting that increased amount from the analysis value at each location. Let the amount of C be .

Next, a method for manufacturing a steel plate according to the present invention will be explained.
The method for producing a steel plate of the present invention includes hot rolling, pickling, and cold rolling a steel slab having a chemical composition, and then annealing the obtained cold rolled steel plate. The annealing includes a holding step in which the cold rolled steel sheet is heated to an annealing temperature of 750 to 880°C and held at the annealing temperature for 10 to 500 seconds, and a holding step at 350 to 550°C from the annealing temperature. A first cooling step in which the temperature range up to the cooling stop temperature is cooled to the first cooling stop temperature with a first average cooling rate of 2 to 50°C/s, and a residence temperature of 350 to 550°C for 10 seconds to 60 seconds. After that, a second cooling step of cooling to a second cooling stop temperature of 100 to 300 ° C. at a second average cooling rate: 3 to 50 ° C / s, and a second cooling step of cooling from the second cooling stop temperature to the second cooling stop temperature: 2.0 A reheating step of heating at ℃/s or more to a reheating temperature of the second cooling stop temperature + 50℃ or more and 340℃ or less, and after the above reheating step, a third average cooling in the temperature range from the above reheating temperature to 50℃ It includes a third cooling step of cooling while retaining at a speed of 0.05 to 1.0° C./s for 100 seconds or more.

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

<Acid washing>
Pickling may be carried out according to a conventional method.

<Cold rolling>
Cold rolling may be carried out according to a conventional method, and the cumulative rolling ratio may be 30 to 85%. From the viewpoint of stably securing high strength and reducing anisotropy, the rolling ratio is preferably 35 to 85%. Note that when the rolling load is high, it is possible to perform softening annealing treatment at 450 to 730° C. in a CAL (continuous annealing line) or BAF (box annealing furnace).

<Annealing>
A cold rolled steel plate (cold rolled steel plate) manufactured according to a conventional method is annealed under the following conditions. Although the annealing equipment is not particularly limited, it is preferable to use a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL) from the viewpoint of productivity and ensuring desired heating and cooling rates.

[Holding step: heated to an annealing temperature in the annealing temperature range of 750 to 880°C and held at the annealing temperature for 10 to 500 seconds]
When the annealing temperature (soaking temperature) is lower than 750° C., the amount of polygonal ferrite becomes excessive, which increases the amount of C and Mn concentrated in the reverse transformed austenite. As a result, at least one of upper bainite, tempered martensite, lower bainite, and retained austenite cannot be obtained sufficiently, and the hardness of hardened martensite increases, resulting in desired strength, ductility, and stretch-flange formability. cannot be secured. Furthermore, when the annealing temperature (soaking temperature) is lower than 750°C, recrystallization does not occur sufficiently, and the processed structure during cold rolling remains, which may reduce formability. For this reason, the annealing temperature (soaking temperature) is set to 750°C or higher.
On the other hand, when the annealing temperature (soaking temperature) exceeds 880°C, the temperature becomes an austenite single phase temperature, the desired polygonal ferrite cannot be obtained, and the YR increases and the ductility decreases. Therefore, the annealing temperature (soaking temperature) is set to 880° C. or lower. The annealing temperature (soaking temperature) is preferably 850°C or lower, more preferably 830°C or lower.

In addition, if the time for holding at the above annealing temperature (soaking time) is less than 10 seconds, austenite will not be formed sufficiently at the above annealing temperature (soaking temperature), and polygonal ferrite will become excessive, resulting in a specified amount of Since upper bainite, tempered martensite, and lower bainite cannot be obtained, not only the desired strength cannot be obtained, but also sufficient residual austenite cannot be obtained, and the desired ductility cannot be secured.
On the other hand, if the time for holding at the above annealing temperature (soaking time) exceeds 500 seconds, the structure will significantly coarsen, making it impossible to secure the desired strength.
Therefore, the time for holding at the above annealing temperature (soaking time) is set to 10 to 500 seconds. The time for holding at the annealing temperature (soaking time) is preferably 80 seconds or more, more preferably 100 seconds or more. Further, the time for holding at the annealing temperature (soaking time) is preferably 400 seconds or less, more preferably 300 seconds or less.

[First cooling step: cooling the temperature range from the annealing temperature to the first cooling stop temperature of 350 to 550°C to the first cooling stop temperature at a first average cooling rate of 2 to 50°C/s]
After holding at a soaking temperature of 750°C to 880°C (after the above holding step), the temperature range from the above annealing temperature to the first cooling stop temperature of 350 to 550°C is set at a first average cooling rate of 2 to 50°C/s. Cool it down. If the cooling rate is less than 2°C/s, operability will deteriorate, 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 becomes too high, the plate shape will deteriorate, so it is set to 50° C./s or less. The first average cooling rate is preferably 40°C/s or less, 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 for 10 seconds or more and 60 seconds or less at a residence temperature of 350 to 550°C]
In a temperature range (retention temperature) below the first cooling stop temperature and from 350° C. to 550° C., upper bainite is formed, a predetermined retained austenite can be obtained, and desired ductility can be obtained. Bainite transformation has an incubation period, and must be allowed to stay in a residence temperature range that includes a residence start temperature and a residence end temperature for a certain period of time.
When the residence temperature range is less than 350°C or more than 550°C, bainite transformation is suppressed, resulting in suppressed formation of retained austenite, and desired ductility cannot be obtained. Furthermore, if the residence temperature range is less than 350° C., martensitic transformation occurs, which may unnecessarily increase YR and reduce press formability. Therefore, the residence temperature range is 350 to 550°C.
Furthermore, if the residence time is less than 10 seconds, the desired amount of bainite cannot be obtained, and as a result of suppressing the formation of retained austenite, the desired ductility cannot be obtained.
On the other hand, if the residence time exceeds 60 seconds, the concentration of C from bainite to lumpy untransformed γ progresses, leading to an increase in coarse quenched martensite with a high C concentration, resulting in the desired stretch flange formability and plate width direction. material stability cannot be obtained. Therefore, the residence time is set to 10 seconds or more and 60 seconds or less.

[Second cooling step (2): Cooling the temperature range up to the second cooling stop temperature of 100 to 300°C at a second average cooling rate: 3 to 50°C/s]
After the above residence, excessive bainite transformation causes a decrease in the amount of tempered martensite, the formation of coarse hardened martensite or retained austenite, and excessive carbon enrichment in the retained austenite, resulting in improved strength and stretch flange formability. In addition, it is necessary to cool the material quickly so that the stability of the material in the width direction of the board does not deteriorate. For this reason, the average cooling rate (second average cooling rate) in the temperature range from the residence 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 plate shape deteriorates, so the second average cooling rate is set to 50°C/s or less.
When the second cooling stop temperature exceeds 300° C., a predetermined tempered martensite cannot be obtained, and as a result, coarse quenched martensite increases, and desired stretch flange formability cannot be obtained. Therefore, the second cooling stop temperature is set to 300°C or less. The second cooling stop temperature is preferably 290°C or lower.
On the other hand, when the second cooling stop temperature is less than 100° C., martensitic transformation progresses excessively, making it impossible to obtain retained austenite at a predetermined volume fraction, and thus making it impossible to obtain desired ductility. Therefore, 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 residence end temperature to the second cooling stop temperature".

[Reheating step: 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 of 2.0°C/s or more]
The tempering effect of martensite is accelerated at higher temperatures. Therefore, by performing tempering at the second cooling stop temperature +50°C or higher, rather than performing tempering at the second cooling stop temperature, carbon concentration is promoted, the formation of retained austenite is promoted, and stretch flange formability is improved. At the same time, ductility can be improved.
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 a cooling stop temperature +50°C or more and 340°C or less.
Moreover, when the average heating rate is less than 2.0° C./s, carbide precipitation is promoted more than carbon distribution, and as a result, the desired retained austenite cannot be obtained. For this reason, the temperature range from the cooling stop temperature to 340°C or less is set to an average heating rate of 2.0°C/s or more.
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 seconds 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 martensite tempering effect will not be sufficiently obtained, and the C enriched region of 0.05 mass% or more ( S _{C ≧ 0.5} ) increases, which deteriorates stretch flange formability and material stability in the plate 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, temperature variations in the sheet width direction are also reduced, further improving material stability in the sheet width direction. You can also do it.
On the other hand, if the cooling rate (third average cooling rate) in the temperature range from the reheating temperature to 50° C. becomes slow, the processing time becomes long and operability deteriorates. Therefore, 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".

Alternatively, the surface of the steel sheet may be galvanized 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. Further, as the alloying hot-dip galvanizing treatment, a plating treatment in which alloying is performed after hot-dip galvanizing may be performed.
Hot-dip galvanizing is used for automobile steel sheets and the like. When applying hot-dip galvanizing, the steel sheet is immersed in a hot-dip galvanizing bath in a continuous annealing furnace at the front stage of the continuous hot-dip galvanizing line after the above-mentioned annealing holding step and first cooling step to form a hot-dip galvanized layer on the surface of the steel sheet. It is sufficient to form an alloyed galvanized steel sheet by subsequently performing an alloying treatment.
Specifically, in the second cooling step described above, when the steel sheet is retained at a residence temperature of 350 to 550° C. for 10 seconds or more and 60 seconds or less, hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment can be performed on the surface of the steel sheet. Further, the soaking and cooling steps and the plating step described above may be performed in separate lines.
Further, electrogalvanizing can be performed after annealing, that is, after the third cooling step.

The thickness of the steel plate of the present invention obtained as described above is preferably 0.5 mm or more. Further, the thickness of the steel plate of the present invention is preferably 2.0 mm or less.
Further, the plate width is preferably 600 mm or more. Moreover, it is preferable that the plate width of the steel plate of the present invention is 1700 mm or less.

Next, the member of the present invention and its manufacturing method will be explained.

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

The steel sheet of the present invention has 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. Therefore, members obtained using the steel sheet of the present invention also have high strength, excellent press formability, ductility, and stretch flange formability, and excellent material stability in the sheet width direction. Furthermore, by using the member of the present invention, it is possible to reduce the weight. Therefore, the member of the present invention can be suitably used for, for example, vehicle body frame parts. The members of the invention also include welded joints.

For the molding process, general processing methods such as press working can be used without restriction. In addition, as the joining process, general welding such as spot welding and arc welding, rivet joining, caulking joining, etc. can be used without limitation.

A slab manufactured by continuous casting having the composition shown in Table 1 was heated to 1200°C, and the soaking time was 200 min. Table 2 shows a cold-rolled steel sheet with a thickness of 1.4 mm manufactured by cold rolling at a rolling ratio of 50% after a hot rolling process with a finish rolling temperature of 900°C and a coiling temperature of 550°C. A steel plate of the present invention and a steel plate of a comparative example were manufactured by processing under the annealing conditions shown in .
The width of all the obtained steel plates was 1500 mm.

Note that some steel sheets (cold-rolled steel sheets: CR) are subjected to hot-dip galvanizing treatment when retained at a residence temperature of 350 to 550° C. for 10 seconds or more and 60 seconds or less, resulting in hot-dip galvanized steel sheets (GI). Here, a steel plate was immersed in a galvanizing bath at a temperature of 440° C. or higher and 550° C. or lower to perform hot-dip galvanizing treatment, and then the amount of plating deposited was adjusted by gas wiping or the like. For the hot-dip galvanizing, a galvanizing bath having an Al content of 0.10% or more and 0.22% or less was used. Further, some of the hot-dip galvanized steel sheets were subjected to alloying treatment after the hot-dip galvanizing treatment to obtain alloyed hot-dip galvanized steel sheets (GA). Here, alloying treatment was performed in a temperature range of 460° C. or higher and 550° C. or lower. Further, some of the steel plates (cold rolled steel plates: CR) were electroplated to form electrogalvanized steel plates (EG).

The steel structure was measured using the following method. The measurement results are shown in Table 3.
To measure the area ratio of polygonal ferrite, upper bainite, tempered martensite, lower bainite, and hardened martensite (fresh martensite), cut out a cross section parallel to the rolling direction, mirror polish it, and then use 1 vol% nital. Corroded, 10 fields of view were observed at 1/4 thickness using SEM at 5000x magnification, and the photographed tissue photographs were quantified by image analysis.
Polygonal ferrite is a relatively equiaxed ferrite with almost no carbides inside. This is the area that appears blackest in the SEM.
Upper bainite is a ferritic structure with the formation of carbides or retained austenite 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 ≦2.0 is classified as polygonal ferrite, and the area with an aspect ratio >2.0 is classified as upper bainite, and the area ratio is calculated. did. Here, the aspect ratio is determined by determining the major axis length a where the particle length is the longest, and setting the particle length that crosses the particle longest in the direction perpendicular to it to be the minor axis length b, and a/b is the aspect ratio. With ratio.
The tempered martensite and lower bainite are regions with a lath-like substructure and carbide precipitation in the SEM.
Quenched martensite (fresh martensite) is a massive region that appears white with no underlying structure visible in the SEM.
The residual structure is a carbide and/or pearlite structure, and is a structure that can be confirmed by white contrast in SEM. Carbide has a structure with a particle size of 1 μm or less, and pearlite has a lamellar structure, so they can be distinguished from each other.

The quantitative evaluation of the structure described above and the measurement of the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite were performed using Image J (Fiji) as image analysis software.
A cross-section of the plate parallel to the rolling direction was cut out, polished to a mirror surface, corroded with 1 vol% nital, and observed at 1/4 thickness position with an SEM at 5000x magnification for 10 fields of view, and machine learning using Image J (Fiji) was performed. Each tissue was identified and quantitatively evaluated using the Trainable Weka segmentation method that allows region identification. In addition, the aspect ratio and equivalent circle diameter of quenched martensite and retained austenite can be measured using a particle analysis program that is also a function of Image J, and only the quenched martensite and retained austenite identified as above can be extracted and measured. did.

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

The area ratio of the C-enriched region where the C concentration is 0.5 mass% _or more is measured using a JEOL field emission electron A line microanalyzer (FE-EPMA) JXA-8500F was used. Then, the C concentration distribution was measured by mapping analysis using an accelerating voltage of 6 kV, an irradiation current of 7×10 ⁻⁸ A, and a minimum beam diameter, and an area ratio at which the C concentration was 0.5 mass% or more was calculated.
However, in order to eliminate the influence of contamination, background components were subtracted so that the average value of C obtained in the analysis was equal to the carbon content of the base material. In other words, if the average value of the measured carbon content is greater than the carbon content of the base material, the increased amount is considered to be contamination, and the true value at each location is calculated by uniformly subtracting that increased amount from the analysis value at each location. The amount of C was set to .

For evaluation of tensile properties, a JIS No. 5 tensile test piece was taken from the center position of the plate width, and a tensile test (based on JIS Z2241 (2011)) was performed at N=3. Each evaluation was performed based on the average value of 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. A total elongation of 14.0% or more at TS: 980 MPa or more, and a total elongation of 12.0% or more at TS: 1180 MPa or more were judged to be excellent in ductility.

In addition, for evaluation of stretch flange formability, a test piece was taken from the center position of the plate width, and a hole expansion test was conducted at N=3 in accordance with the Japan Iron and Steel Federation standard JFST1001. That is, after punching a sample with a square size of 100 mm x 100 mm using a punching tool with a punch diameter of 10 mm and a clearance of 13%, a conical punch with a 60 degree apex angle was used to remove the burrs generated when forming the punched hole on the outside. The hole was enlarged until a crack that penetrated through the plate thickness occurred. In this case, d ₀ is the initial hole diameter (mm), d is the hole diameter at the time of crack occurrence (mm), and the hole expansion rate λ (%) = {(d-d ₀ )/d ₀ }×100 is calculated. The average value of the three points was evaluated as λ. Steels having a λ of 40% or more were judged to have excellent hole expandability and stretch flangeability.

Regarding the material stability evaluation in the board width direction, 23 points were evaluated from both board width directions at intervals of 100 mm or less from the board width center position (12W/24 position (W: board width)). (including the width center position), and determine EL and λ at each position (measurement position X). Then, the material stability in the board width direction was evaluated by determining the ratio of the difference between the measured values at the board width center position and each position relative to the measured value at the center position.
Using EL and λ at the center of the board width as a reference, consecutive measurement groups where the difference in EL and λ is 10% or less are defined as areas where the difference in EL and λ is 10% or less, and this area is defined for the entire board width. Steel having a ratio of 80% or more was judged to have excellent material stability.
A case where the plate width of region A satisfying the following formulas (1) and (2) is 80% or more of the total plate width was judged to have excellent material stability in the plate width direction.
-10≦100×[(EL(%) at measurement position
-10≦100×[(λ(%) of measurement position
(In formulas (1) and (2), the measurement positions /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, 23W/24, a total of 23 locations as measurement position X.)

The measurement results are shown in Table 3.

The inventive examples shown in Tables 2 and 3 were excellent in strength, press formability, ductility, stretch flange formability, and material stability in the sheet width direction, whereas the comparative examples were inferior in any of the following. .

In addition, members obtained by forming, joining, and forming and joining the steel sheets of the invention examples have a high quality. It has high strength, excellent press formability, ductility, stretch flange formability, and material stability in the sheet width direction. It was found that it has excellent stretch flange formability and material stability in the width direction of the plate.

Claims

In mass%,
C: 0.05-0.20%,
Si: 0.40 to 1.50%,
Mn: 1.9 to 3.5%,
P: 0.02% or less,
S: 0.01% or less,
sol. Al: 0.005-0.50%,
N: Contains less than 0.015%,
A component composition in which the remainder consists of 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%),
Furthermore, a steel structure consisting of a residual structure,
has
The total area ratio of hardened martensite and retained austenite having an aspect ratio of 3 or less and an equivalent circle diameter of 1.6 μm or more is 20% or less with respect to the total area ratio of hardened martensite and retained austenite,
A steel plate in which the area ratio of C-enriched regions (SC ≧0.5 ) in which the C concentration is 0.5 mass% or more relative to the entire structure is 15% or less.
The component composition further includes, in mass%,
Ti: 0.1% or less,
B: 0.01% or less,
The steel plate according to claim 1, containing one or two selected from among the above.
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 plate according to claim 1 or 2, containing one or more selected from the following.
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 plate according to any one of claims 1 to 3, containing one or more selected from the following.
The steel sheet according to any one of claims 1 to 4, having a galvanized layer on the surface.
A member using the steel plate according to any one of claims 1 to 5.
After subjecting a steel slab having the composition according to any one of claims 1 to 4 to hot rolling, pickling and cold rolling, the obtained cold rolled steel plate is subjected to annealing. It is a manufacturing method,
The annealing is
A holding step of heating the cold rolled steel plate to an annealing temperature of 750 to 880°C and holding at the annealing temperature for 10 to 500 seconds;
A first cooling step of cooling the temperature range from the annealing temperature to the first cooling stop temperature of 350 to 550 ° C. to the first cooling stop temperature at a first average cooling rate of 2 to 50 ° C./s;
A second cooling step of cooling at a second average cooling rate: 3 to 50 °C/s to a second cooling stop temperature of 100 to 300 °C, after retaining at a retention temperature of 350 to 550 °C for 10 seconds to 60 seconds;
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 of 2.0 ° C / s or more,
After the reheating step, a third cooling step of cooling 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 seconds or more. , a method for manufacturing steel plates.
The method for producing a steel sheet according to claim 7, wherein in the second cooling step, the steel sheet surface is subjected to hot-dip galvanizing treatment or alloying hot-dip galvanizing treatment when retaining at a residence temperature of 350 to 550 ° C. for 10 seconds or more and 60 seconds or less. .
The method for manufacturing a steel sheet according to claim 7, wherein the surface of the steel sheet is subjected to electrogalvanizing treatment after the annealing.
A method for producing a member, the method comprising the step of subjecting the steel plate according to any one of claims 1 to 5 to at least one of forming and joining to produce a member.