WO2024057669A1 - Steel sheet, member, and method for manufacturing the foregoing - Google Patents

Abstract

Provided are: a steel sheet and a member that have tensile strength of at least 780 MPa, excellent press-moldability, ductility, and expansion flange moldability, and excellent material stability in a sheet width direction; and a method of manufacturing the steel sheet and the member.　The present invention has a component composition and a steel structure with prescribed ranges. The combined surface modulus of quenched martensite and residual austenite with an aspect ratio of 3 or less and a circle-equivalent diameter of at least 2.0 μm, with respect to the total surface modulus of quenched martensite and residual austenite, is 20% or less. The surface modulus of a carbon-concentrated region where the carbon concentration is at least 0.5% by mass is 20% or less with respect to the entire structure.

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 780 MPa or more and excellent formability and material stability, and methods for manufacturing them. The present invention relates to steel plates and members used in various applications such as automobiles, and methods of manufacturing them.

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 making the steel plates used in automobile frame parts thinner, and the amount of high-strength steel plates used is increasing in order to contribute to the fuel efficiency of automobiles. . Generally, as the strength of steel sheets increases, cracks are more likely to occur during press forming due to a decrease in ductility and stretch flange formability. Therefore, a steel plate with improved ductility and stretch-flange formability compared to conventional steel sheets is desired. In addition, as the strength increases, the yield ratio: YR (YR = yield strength YS / tensile strength TS) increases, which increases springback after molding, resulting in a decrease in dimensional accuracy after press molding. Therefore, a steel plate with low YR is desired from the viewpoint of press formability.

On the other hand, from the perspective of carbon neutrality, the amount of CO ₂ emissions generated in all processes from material production to product production is also an important indicator. Therefore, if the amount of scrap generated when manufacturing a product increases, the total amount of CO ₂ emitted will increase, and there is a concern that the environmental burden will increase. For this reason, it is required to use up the material in the manufacturing process (improving yield), 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 in the sheet width direction and hole expandability become apparent. As a result of this, cracks are likely to occur during press molding, making it necessary to limit the blanking position in order to ensure quality, resulting in a lower yield.

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.

DP steel (Dual Phase steel) has been developed as a steel plate with low YR and excellent ductility that is effective in reducing springback. General DP steel is a multi-phase steel in which martensite is dispersed in a ferrite structure which is the main phase. 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. Techniques for improving the stretch flange formability of DP steel include, for example, Patent Document 2 and Patent Document 3.

In Patent Document 2, 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 the average grain size is fine ferrite with an equivalent circle diameter of 3 μm or less. It is disclosed that ductility and stretch flange formability can be improved by controlling the grain size to martensite having a circular equivalent diameter of 6 μm or less.

In addition, 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 grain size of martensite is controlled to be 10 μm or less. A technique has been disclosed in which deterioration in stretch flange formability is suppressed by controlling the crystal grain size to 5 μm or less.

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

However, although Patent Document 1 and Patent Document 2 disclose a method for producing a steel plate with excellent ductility and stretch-flange formability, it is necessary to form a large amount of soft phase ferrite, so that the tensile strength is 780 MPa, for example. It is difficult to further increase the strength. In addition, Patent Document 3 discloses a method for manufacturing a DP steel sheet that has low YR and excellent ductility and stretch-flange formability, but since it has a DP structure, the area ratio of martensitic structure is As ductility increases, ductility decreases, and fractures were observed at bending ridges where ductility is required for forming difficult-to-form parts such as center pillars, making it clear that ductility is not always 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, from the perspective of press formability, it is necessary to develop a high-strength steel plate that has low YR, excellent ductility and stretch-flange formability, and from the perspective of improving material yield, it has excellent material stability in the width direction. It has been demanded.
In view of the above circumstances, the present invention provides a steel plate and member having a tensile strength (TS) of 780 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 in accordance with JIS Z2241 (2011) satisfies any of the following (A) to (C).
(A) TS: 780 MPa or more and less than 980 MPa, EL: 16.0% or more,
(B) TS: 980 MPa or more and less than 1180 MPa, EL: 14.0% or more,
(C) 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 30% 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 780 MPa or more. 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.Al: 1.00% or less, N: less than 0.015%, the area ratio of polygonal ferrite is 10% or more and 80% or less, upper bainite, tempered martensite, and lower part. The total area ratio of bainite is 10% or more and 70% 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 15% or less (including 0%). Then, with respect to the total area of quenched martensite and residual γ, the total area ratio (space factor) of quenched martensite and residual γ with an aspect ratio of 3 or less and an equivalent circle diameter of 2.0 μm or more is 20 % or less, and by creating a steel structure in which the area ratio of the C-enriched region (SC _≧0.5 ) with respect to the entire structure is 20% or less with respect to the entire structure, excellent press formability is achieved. It has been found that a high-strength cold-rolled steel sheet having good ductility and stretch-flange formability and excellent material stability in the sheet width direction (with small material variations) can be obtained.

The present invention was 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: 1.00% or less,
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 80% or less,
Total area ratio of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less,
Volume fraction of retained austenite: 3% or more and 15% or less,
Area ratio of quenched martensite: 15% or less (including 0%),
Furthermore, it has a steel structure consisting of a residual structure,
The total area ratio of hardened martensite and retained austenite having an aspect ratio of 3 or less and an equivalent circle diameter of 2.0 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 20% 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 the following.
[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 150 to 360 °C, after retaining at a retention temperature of 350 to 550 °C for 10 seconds to 60 seconds;
A method for producing a steel sheet, comprising: a third cooling step in which cooling is performed in a temperature range from the second cooling stop temperature to 50° C. at a third average cooling rate: 0.05 to 1.0° C./s.
[8] The steel sheet according to [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 the steel sheet is retained 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 producing 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 780 MPa or more, has excellent press formability, ductility, and stretch flange formability, and has excellent material stability in the width direction.
When the steel sheet of the present invention is applied to a frame member of an automobile body, a difficult-to-form member with a complicated shape can be manufactured by cold pressing, and thus it can greatly contribute to reducing the weight of the automobile body. Furthermore, since the variation in material quality in the sheet width direction is small, the blanking position of the steel sheet is not restricted, and the material yield can be significantly improved.

The present invention will be explained in detail below. Note that the present invention is not limited to the following embodiments.
The steel plate of the present invention has C: 0.05 to 0.20%, Si: 0.40 to 1.50%, Mn: 1.9 to 3.5%, and P: 0.02% or less in mass %. , S: 0.01% or less, sol. The component composition contains Al: 1.00% or less, N: less than 0.015%, the balance is iron and inevitable impurities, and the area ratio of polygonal ferrite is 10% or more and 80% or less, and upper bainite. The total area ratio of tempered martensite and lower bainite is 10% or more and 70% or less, the volume ratio of retained austenite is 3% or more and 15% or less, and the area ratio of hardened martensite is 15% or less (0%). ), and further has a steel structure consisting of a residual structure, and has an aspect ratio of 3 or less with respect to the total area ratio of hardened martensite and retained austenite, and an equivalent circle diameter of 2.0 μm or more. The total area ratio of certain quenched martensite and retained austenite is 20% or less, and the area ratio of the C-enriched region (S _{C ≥ 0.5} ) where the C concentration is 0.5 mass% or more with respect to the entire structure is 20%. % or less.
The steel plate of the present invention will be described below in the order of its component composition and steel structure. First, the reason for limiting the component composition of the present invention will be explained. In the following description, all percentages indicating the components of steel are percentages by 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 the temperature range from the second cooling stop temperature to 50°C is cooled at a third average cooling rate: 0.05 to 1.0°C/s, martensite transformation and subsequent martensite transformation occur. Tempering is not performed sufficiently. Therefore, the formation of quenched martensite and a C enriched region 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: 0.40-1.50%>
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, promoting high strength through transformation strengthening, and, like Si, suppresses the formation of carbides in bainite and promotes the formation of retained austenite, which contributes to ductility. Contains from the viewpoint of improving. 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 significantly 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 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.

<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.

<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: 1.00% or less>
Al is contained for the purpose of deoxidizing or obtaining residual γ. sol. Although the lower limit of Al is not particularly specified, in order to perform deoxidation stably, sol. The Al content is preferably 0.005% or more. sol. The Al content is more preferably 0.010% or more, and still more preferably 0.020% or more.
On the other hand, sol. If the Al content exceeds 1.00%, a large amount of Al-based coarse inclusions will increase, and stretch flange formability will deteriorate. For this reason, sol. Al content shall be 1.00% 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%. The N content is preferably 0.010% or less, more preferably 0.005% or less.
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 0.0010% or more, and still more preferably 0.0020% 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 such a 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 preferable to contain Cu in an amount of 0.05% or more. The Cu content is more preferably 0.10% or more. More preferably, the Cu content is 0.25% or more, even more preferably 0.50% 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, and even 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 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 Mo content is preferably 0.01% or more. The Mo content is more preferably 0.03% or more, and still 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 0.005% or more, and still more preferably 0.010% or more. Even more preferably, the V content is 0.020% or more, even more preferably 0.050% 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 0.3% or less, more preferably 0.2% 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 0.05% or less, more preferably 0.03% 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 0.0040% 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 0.008% or more, even more preferably 0.010% or more. The Sn content is preferably 0.030% or more, more preferably 0.060% 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 0.008% or more, even more preferably 0.010% or more. The Sb content is preferably 0.025% or more, more preferably 0.050% 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.
REM is not particularly limited, but preferably contains Sc, Y, Ce, and La.

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 780 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 sheet of the present invention, the total elongation EL is 16.0% or more when TS: 780 MPa or more, 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more is secured. Stability is greatly improved.
Since cracking during press molding can be suppressed by ensuring a hole expansion rate λ of 30% or more, λ is set to 30% 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, the deviation of EL in the board width direction is 10% or less with respect to the measured value at the board width center position, and the deviation of λ in the board width direction is 10% or less with respect to the measured value at the board width center position. The area should 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 set to be 80% or more of the entire board width region. . Preferably it is 85% or more.

<YR≦0.8, TS≧780MPa, EL≧16.0%>
<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 780 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 16.0% or more when TS: 780 MPa or more, 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more.

<λ≧30%>
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 30% or more is judged to have excellent hole expandability and stretch flangeability. Preferably it is 40% or more.

<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 80% 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 80%, the desired strength cannot be obtained, so the area ratio of the polygonal ferrite should be 80% or less, preferably 75% or less, and more preferably 70%.

<Total area ratio of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less>
In order to obtain desired strength, the total area ratio of upper bainite, tempered martensite, and lower bainite is set to 10% or more, and in order to obtain higher strength, it is preferably set to 15% or more.
However, if the total area ratio of upper bainite, tempered martensite, and lower bainite exceeds 70%, ductility decreases due to excessively high strength, so the area ratio is set to 70% or less. More preferably it is 65% or less, still more preferably 60% or less.

<Volume fraction of retained austenite (retained γ): 3% or more and 15% or less>
When the volume fraction of retained austenite is less than 3%, desired ductility cannot be ensured.
From the viewpoint of ductility, 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.

<Area ratio of quenched martensite: 15% 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 quenched martensite is set to 15% or less. In order to obtain λ more stably, the area ratio of hardened martensite is preferably 12% or less, more 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 2.0 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 2.0 μm or more is formed, voids are formed at stress concentration areas at the interface with other structures, which may deteriorate stretch flange formability.
Furthermore, when the aspect ratio of hardened martensite or retained austenite is 3 or less, stress concentration tends to occur at the interface with other structures, which may promote void formation and deteriorate stretch flange formability. Therefore, 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 shall be 20% or less of the total area ratio of hardened martensite and retained austenite. . This total area ratio is preferably 18% or less, more preferably 16% 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: 20% 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 20% 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 area ratio (occupation ratio) of the C-enriched region (SC _≧0.5 ) where the C concentration is 0.5 mass% or more is 20% or less. This area ratio is preferably 15% or less, more preferably 12% or less. Moreover, this area ratio is preferably 3% or more, more preferably 5% 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 carbide 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 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 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 the retained austenite is randomly distributed, the volume fraction of the retained austenite determined by X-ray diffraction can be taken as the area fraction of the 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 involves hot rolling, pickling and cold rolling a steel slab having the above-mentioned composition, and then annealing the obtained cold rolled steel plate. This is a manufacturing method, and the annealing includes a holding step of heating the cold rolled steel sheet to an annealing temperature of 750 to 880°C and holding at the annealing temperature for 10 to 500 seconds, and a holding step of heating the cold rolled steel plate to an annealing temperature of 350 to 550°C from the annealing temperature. A first cooling step in which the temperature range up to the first cooling stop temperature is set at a first average cooling rate of 2 to 50°C/s to the above first cooling stop temperature, and a residence temperature of 350 to 550°C for 10 seconds or more for 60 seconds. A second cooling step in which cooling is performed at a second average cooling rate of 3 to 50°C/s to a second cooling stop temperature of 150 to 360°C, and and a third cooling step in which the temperature range is cooled at a third average cooling rate: 0.05 to 1.0° C./s.

<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, recrystallization does not occur sufficiently, and the processed structure during cold rolling remains, reducing formability. Furthermore, when the annealing temperature (soaking temperature) is lower than 750°C, polygonal ferrite becomes excessive and C and Mn concentrate in the reverse transformed austenite. It is not possible to obtain sufficient bainite and retained austenite, and the desired strength and ductility cannot be secured. 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 increase, resulting in less than the specified amount. Upper bainite, tempered martensite, and lower bainite cannot be obtained, and not only the desired strength cannot be obtained, but also sufficient residual austenite cannot be obtained, and 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 residual austenite can be obtained, and desired ductility can be obtained. Bainite transformation has an incubation period and must be kept at the relevant temperature for a certain period of time. If the residence temperature range including the residence start temperature (=first cooling stop temperature) and residence end temperature is outside the range of 350 to 550°C, and/or the residence time (hereinafter also referred to as residence time) is less than 10 seconds. Therefore, the desired amount of bainite cannot be obtained, the formation of retained austenite is suppressed, and 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 the amount of remaining lumpy structure, making it impossible to obtain the desired λ. Therefore, the residence time is set to 10 seconds or more and 60 seconds or less.

[Second cooling step (2): Cooling to second cooling stop temperature of 150 to 360°C at second average cooling rate: 3 to 50°C/s]
After the above-mentioned residence, it is necessary to cool quickly so that carbon concentration in untransformed austenite does not progress too much. If the average cooling rate in the temperature range from the above residence end temperature to the cooling stop temperature of 150°C or more and 360°C or less is less than 3°C/s, carbon will concentrate into lumpy untransformed γ, and the quenched marten during final cooling will The amount of sites increases and λ decreases. Therefore, from the viewpoint of improving λ, the second average cooling rate in the temperature range from the residence end temperature to the second cooling stop temperature of 150° C. or more and 360° C. or less is set to 3° C./s or more. The second average cooling rate is preferably 5°C/s or more, more preferably 8°C/s or more. If the cooling rate in this temperature range becomes too high, the plate shape will deteriorate, so the cooling rate in this temperature range (second average cooling rate) is set to 50° C./s or less. Preferably it is 40°C/s or less. When the second cooling stop temperature exceeds 360° C., the area ratio of tempered martensite or lower bainite does not reach the predetermined area ratio, the area ratio of hardened martensite after annealing increases, and stretch flange formability deteriorates. Therefore, the second cooling stop temperature is set to 360° C. or lower.
On the other hand, if the second cooling stop temperature is less than 150°C, the effect of tempering martensite will not be sufficiently obtained during cooling to 50°C in the next step, and not only will the amount of hardened martensite increase, but also the C concentration will increase. The area (S _{C ≧0.5} ) becomes excessive and deteriorates stretch flange formability. Further, the volume fraction of retained austenite does not fall within the desired range, and sufficient ductility cannot be obtained.
Therefore, the second cooling stop temperature is set to 150°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".

[Third cooling step: Cooling in the temperature range from the second cooling stop temperature to 50°C at a third average cooling rate: 0.05 to 1.0°C/s]
If the cooling rate in the temperature range from the second cooling stop temperature to 50°C exceeds 1.0°C/s, a sufficient martensite tempering effect will not be obtained, and the desired amount of retained austenite will not be obtained.
Furthermore, an increase in the C enriched region of 0.05 mass% or more (SC _≧0.5 ) deteriorates stretch flange formability and material stability in the plate width direction.
Therefore, the cooling rate in the temperature range from the second cooling stop temperature to 50°C (third average cooling rate) is set to 1.0°C/s or less. By setting the cooling rate (third average cooling rate) in the temperature range from the second cooling stop temperature to 50°C to 1.0°C/s or less, the temperature variation in the sheet width direction is also reduced, and the temperature variation in the sheet width direction is reduced. It is also possible to further improve material stability.
On the other hand, if the cooling rate in the temperature range from the cooling stop temperature to 50° C. becomes slow, the processing time becomes long and the 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.
The third average cooling rate is preferably 0.08°C/s or more, more preferably 0.10°C/s or more. Further, the third average cooling rate is preferably 0.80°C/s or less, more preferably 0.60°C/s or less.
Here, the third average cooling rate is "second cooling stop temperature (°C) - 50°C/cooling time (seconds) from the second cooling stop 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. Further, 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 bonding to produce a member.

The steel sheet of the present invention has a tensile strength of 780 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 500° 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 carbide 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. It was compared.
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 .

A JIS No. 5 tensile test piece and a hole expansion test piece were taken from the obtained steel plate, and a tensile test (based on JIS Z2241 (2011)) was conducted at N=3. Each evaluation was performed based on the average value of three points. Steel plates with a tensile strength of 780 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 EL of 16.0% or more when TS: 780 MPa or more, 14.0% or more when TS: 980 MPa or more, and 12.0% or more when TS: 1180 MPa or more was 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, The average value of the three points was evaluated as λ. Steels having a λ of 30% 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 the EL and λ at each position (measurement position X) were determined. Then, the material stability in the board width direction was 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 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 (10)

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: 1.00% or less,
   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 80% or less,
   Total area ratio of upper bainite, tempered martensite, and lower bainite: 10% or more and 70% or less,
   Volume fraction of retained austenite: 3% or more and 15% or less,
   Area ratio of quenched martensite: 15% 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 2.0 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 20% 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 claim 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 claim 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 claims 1 to 3, containing one or more selected from the following.
5. The steel sheet according to any one of claims 1 to 4, having a galvanized layer on the surface.
6. A member using the steel plate according to any one of claims 1 to 5.
7. 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 150 to 360 °C, after retaining at a retention temperature of 350 to 550 °C for 10 seconds to 60 seconds;
   A method for manufacturing a steel sheet, comprising a third cooling step of cooling in a temperature range from the second cooling stop temperature to 50° C. at a third average cooling rate: 0.05 to 1.0° C./s.
8. The method for manufacturing a steel sheet according to claim 7, 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 surface of the steel sheet is subjected to a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.
9. 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.
10. 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.