TW202006154A - Steel plate - Google Patents

Abstract

Provided is a steel plate which is useful as a structural member, and specifically, as a vehicle skeleton member, reinforcing member, or collision resistance member, exhibits a tensile strength of 1,320 MPa or higher, exhibits a product of tensile strength and total elongation which is 26,000 MPa% or higher, exhibits little variation in elongation properties, and contains a high Mn concentration. The steel plate is characterized: by containing, in mass%, C in an amount greater than 0.18% and less than 0.45%, Si in an amount no less than 0.001% and less than 3.50%, Mn in an amount greater than 4.00% and less than 9.00%, sol. Al in an amount no less than 0.001% and less than 1.00%, B in an amount no less than 0.0003% and no greater than 0.010%, P in an amount no greater than 0.023%, S in an amount no greater than 0.010%, N in an amount less than 0.050%, O in an amount less than 0.020%, and optionally selected elements with the remainder constituting iron and impurities; and in that the tensile strength thereof is 1,320 MPa or higher, the product of tensile strength and total elongation thereof is 26,000 MPa% or higher, and the standard deviation in the total elongation thereof is no greater than 2.5.

Description

Steel plate

The present disclosure relates to an ultra-high-strength steel sheet having excellent formability, and more specifically to a steel sheet having a high Mn concentration and having excellent elongation characteristics and high strength.

Background of the invention In order to achieve both light weight and safety of automobile bodies and parts, the steel plates belonging to these blanks are continuing to develop toward high strength. In general, if the steel sheet is increased in strength, the elongation will decrease and the formability of the steel sheet will be impaired. Therefore, in order to use a high-strength steel sheet as a member for automobiles, it is necessary to increase both strength and formability which are opposite characteristics.

In order to improve the tensile strength and elongation, a steel that utilizes the metamorphic induced plasticity of the residual austenitic iron, so-called TRIP steel (for example: Patent Document 1) has been proposed.

The residual Vostian iron system is made by concentrating C in Vostian iron, thereby making Vostian iron not metamorphose into other organizations even at room temperature. As a technology for stabilizing Vostian iron, the following has been proposed: the steel sheet contains elements such as Si and Al that can suppress the precipitation of carbides, and during the manufacturing stage of the steel sheet, the steel is made to be C during the period when the steel sheet is toughened and the iron is deformed. It is concentrated in the Vostian iron. In this technique, the greater the C content in the steel sheet, the more stable Vostian iron can be, and the amount of residual Vostian iron can be increased. As a result, a steel sheet excellent in both strength and elongation can be produced. However, when a steel sheet is used as a structural member, it is often welded to the steel sheet. However, if the C content in the steel sheet is large, it is difficult to sufficiently ensure the weldability, and there is a limit in using it as a structural member. Therefore, it is desirable to increase both the strength and elongation of the steel sheet with a small amount of C content.

As a steel plate having less C content than the above TRIP steel, more residual iron in the field than the above TRIP steel, and greater strength and ductility than the above TRIP steel, a steel added with more than 4.0% of Mn has been proposed (for example: Non-Patent Document 1) .

Patent Document 1 discloses a steel sheet in which 3.5% or more of Mn is added, and by controlling the ferrite iron to 30% to 80%, its tensile strength and elongation are excellent.

Patent Document 2 discloses a steel sheet that has excellent tensile strength and extensibility by optimizing the conditions for manufacturing steel added with 2 to 6% of Mn.

Prior technical literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 2012-237054 Patent Document 2: Japanese Patent Laid-Open No. 7-1888834

Non-patent literature Non-Patent Document 1: Furukawa Kei, Matsumura Ryo, Heat Treatment, Japan, Japan Heat Treatment Association, 2009, Volume 37, Number 4, p.204

Summary of the invention Problems to be solved by invention In order to use a high-strength steel sheet as a member for automobiles, it is desirable to increase the strength and formability of the opposite characteristics before reducing the weldability. Specifically, it is desirable to have excellent tensile properties and high strength with a tensile strength of 1320 MPa or more, and a product of tensile strength and total elongation of 26000 MPa% or more.

However, it can be considered that in steels containing Mn greater than 4.0% as disclosed in Patent Document 1 and Patent Document 2 and Non-Patent Document 1, for example, the grain boundary and the heterogeneous interface become brittle and extend in the tensile strength test The variation in characteristics becomes larger. Therefore, even if the tensile strength and the product of the tensile strength and the total elongation satisfy the above characteristics, there is still room for improvement in the substantial tensile properties. As mentioned above, the variation of the elongation characteristics is obvious when the tensile strength exceeds 1320 MPa.

Therefore, a steel sheet with a high Mn concentration is desired, which has excellent tensile properties and high strength with a tensile strength of 1320 MPa or more, and a product of tensile strength and total elongation of 26000 MPa% or more, and has small variations in the tensile properties.

Means to solve the problem In the steel sheet containing a high Mn concentration, in order to ensure the characteristics of the tensile strength of 1320 MPa or more and the product of the tensile strength and the total elongation of 26000 MPa% or more, and to reduce the variation of the tensile properties, the inventors obtained the following Knowledge insight: The effective system is to control the P content of the steel plate with high Mn concentration to 0.023 mass% or less, and the B content to 0.0003 mass% or more and 0.010 mass% or less, and to use novel hot rolling conditions and annealing conditions. Control the steel plate as described above.

The steel plate of the present disclosure was created based on the above knowledge and knowledge, and its gist is as follows. (1) A steel plate characterized by: It contains in mass %: C: greater than 0.18% and less than 0.45%, Si: 0.001% or more and less than 3.50%, Mn: greater than 4.00% and less than 9.00%, sol.Al: 0.001% or more and less than 1.00%, B: 0.0003% or more and 0.010% or less, P: 0.023% or less, S: 0.010% or less, N: less than 0.050%, O: less than 0.020%, Cr: 0.00% or more and less than 2.00%, Mo: 0.00% or more and 2.00% or less, W: 0.00% or more and 2.00% or less, Cu: 0.00% or more and 2.00% or less, Ni: 0.00% or more and 2.00% or less, Ti: 0.000% or more and 0.300% or less, Nb: 0.000% or more and 0.300% or less, V: 0.000% or more and 0.300% or less, Ca: 0.000% or more and 0.010% or less, Mg: 0.000% or more and 0.010% or less, Zr: 0.000% or more and 0.010% or less, REM: 0.000% or more and 0.010% or less, Sb: 0.000% or more and 0.050% or less, Sn: 0.000% or more and 0.050% or less and Bi: 0.000% or more and 0.050% or less, and The rest is iron and impurities; The tensile strength of the steel plate is 1320 MPa or more, and the product of tensile strength and total elongation is 26000 MPa% or more; and The standard deviation of the total elongation is 2.5 or less. (2) The steel sheet as described in (1) above, which contains one or more of the following elements: In terms of mass %, Cr: 0.01% or more and less than 2.00%, Mo: 0.01% or more and 2.00% or less, W: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less and Ni: 0.01% or more and 2.00% or less. (3) The steel sheet as described in (1) or (2) above, which contains one or more of the following elements: In terms of mass %, Ti: 0.005% or more and 0.300% or less, Nb: above 0.005% and below 0.300% and V: 0.005% or more and 0.300% or less. (4) The steel sheet according to any one of (1) to (3) above, which contains one or more of the following elements: In terms of mass %, Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, Zr: 0.0001% or more and 0.0100% or less and REM: 0.0001% or more and 0.0100% or less. (5) The steel sheet according to any one of (1) to (4) above, which contains one or more of the following elements: In terms of mass %, Sb: 0.0005% or more and 0.0500% or less, Sn: 0.0005% or more and 0.0500% or less and Bi: 0.0005% or more and 0.0500% or less. (6) The steel sheet according to any one of the above (1) to (5), wherein the metal structure of the 1/4 position of the thickness from the surface of the L section of the steel sheet is 15% or more and 75% by area ratio % Of the remaining Vostian iron. (7) The steel sheet according to any one of (1) to (6) above, wherein the surface of the steel sheet has a hot-dip galvanized layer. (8) The steel sheet according to any one of (1) to (6) above, wherein the surface of the steel sheet has an alloyed galvanized layer.

Invention effect According to the present disclosure, it is possible to provide a steel plate with a high Mn concentration that is very useful for structural members, especially automobile frame members and reinforcing members, or even collision-resistant members, the steel sheet having a tensile strength of 1320 MPa or more and 26000 MPa% or more The product of the tensile strength and the total elongation, and the variation of the elongation characteristics is small.

Forms for carrying out the invention The inventors conducted intensive research, and as a result, obtained the following knowledge and insights: the P content of the steel plate with a high Mn concentration is controlled to 0.023 mass% or less, and the B content is controlled to 0.0003 mass% or more and 0.010 mass% or less, and Steel plates with a tensile strength of 1320 MPa or more and a product of a tensile strength of 26000 MPa% or more and total elongation can be efficiently manufactured with small variations in elongation characteristics under the control of novel hot rolling conditions and annealing conditions.

That is, it is clear from the analysis: the variation of the total elongation that occurs in the steel plate with a high Mn concentration is not due to the solidification segregation of P caused by the conventional casting and the accompanying formation of the band structure of the hot-rolled steel plate The resulting variation in the total elongation is not the variation in the total elongation due to uneven manufacturing methods such as uneven cooling in the width direction during hot rolling, but is due to the loss of iron (hard phase) and residual fertile The embrittlement caused by the segregation of grain boundaries and the precipitation of B in the field iron and its surrounding structures, in other words, the steel plate is essentially uneven due to the embrittlement of the grain boundaries. The present invention is directed to a steel sheet whose grain boundary embrittlement has been suppressed. In addition, a manufacturing method is also proposed in the present case, which includes a heat treatment method to suppress the embrittlement of the grain boundaries.

Hereinafter, an example of the embodiment of the steel sheet of the present disclosure will be described.

1. Chemical composition The reason why the chemical composition of the steel sheet of the present disclosure is specified in the above manner will be explained. In the following description, the symbol "%" indicating the content of each element refers to mass% unless otherwise specified.

(C: greater than 0.18% and less than 0.45%) C is an element that is extremely important for improving the tensile strength of steel. In order to obtain a tensile strength above 1320 MPa, a C content greater than 0.18% is required. On the other hand, if too much C is contained, the weldability of the steel sheet is impaired, so the upper limit of the C content is set to less than 0.45%. From the viewpoint of improving tensile strength and total elongation, the lower limit of the C content is preferably 0.20% or more and more preferably 0.24% or more. If the lower limit of the C content is 0.20% or more, and the annealing time is set to 15000s or more at a predetermined annealing temperature in the later-described annealing, then the residual Vostian iron will be in terms of area ratio (area %) 15% or more, and the product of tensile strength and total elongation will be 28000MPa% or more. By setting the lower limit of the C content to 0.24% or more, controlling the B content to be described later to 0.0005% or more and 0.0040% or less, and setting the annealing time to 15000s or more at a predetermined annealing temperature in the later-described annealing An ultra-high-strength steel sheet that maintains the product of tensile strength and total elongation above 28,000 MPa% and whose tensile strength is above 1470 MPa is obtained. The upper limit of the C content is preferably 0.40% or less and more preferably 0.35% or less. By making the upper limit of the C content within the above range, the weldability of the steel sheet can be further improved.

(Si: 0.001% or more and less than 3.50%) Si suppresses the precipitation and coarsening of Xueming iron and carbon, and also has the effect of making it easy to control the Vostian iron generated during annealing. In order to obtain the above effect, it is necessary to have a Si content of 0.001% or more. The lower limit of Si content should be above 0.01%, and more preferably above 0.40%. Moreover, if the Si content becomes more than 1.00%, the strength-ductility balance can be maintained and the fatigue characteristics can be improved, so the lower limit of the Si content is more preferably 1.00% or more. On the other hand, if too much Si is contained, the plating properties and chemical conversion treatability of the steel sheet will be impaired, so the upper limit of the Si content is set to less than 3.50%. In addition, the upper limit of the Si content is preferably 3.00% or less and more preferably 2.50% or less.

(Mn: greater than 4.00% and less than 9.00%) Mn is an element that stabilizes Vostian iron and increases tensile strength and total elongation. In addition, in the disclosed steel sheet system, Mn is distributed to the Vostian iron to make the Vostian iron more stable. In order to stabilize the Vostian iron at room temperature, it must have more than 4.00% Mn. On the other hand, if the steel sheet contains too much Mn, the ductility is impaired, so the upper limit of the Mn content is set to less than 9.00%. The lower limit of Mn content should be more than 4.30%, more preferably 4.80% and more preferably 5.50%. The upper limit of Mn content should be below 8.00%, and more preferably below 7.50%. By setting the lower limit value and the upper limit value of the Mn content to the above-mentioned range, the Vostian iron can be more stabilized.

(sol.Al: 0.001% or more and less than 1.00%) Al is a deoxidizer and must contain 0.001% or more. In addition, Al expands the two-phase temperature range during annealing, so it also has the effect of improving the stability of the material. The greater the Al content, the greater the effect. However, if too much Al is contained, the surface properties, paintability, weldability, etc. will be deteriorated. Therefore, the upper limit of sol.Al is set to less than 1.00%. The lower limit of the sol.Al content should be above 0.005%, more preferably above 0.010% and even more preferably above 0.020%. The upper limit of sol.Al content should be 0.80% or less, and more preferably 0.60% or less. By setting the lower limit and upper limit of the sol.Al content to the above range, the balance between the deoxidizing effect and the effect of improving the stability of the material and the surface properties, paintability and weldability can be made better. In addition, in this specification, "sol.Al" means "acid soluble Al".

(B: 0.0003% or more and 0.010% or less) In order to exhibit excellent elongation characteristics in a steel sheet having a tensile strength of 1320 MPa or more, and to reduce the variation in elongation characteristics, the B series is an extremely important element. In order to obtain the product of the tensile strength of 26000 MPa% or more and the total elongation, and to suppress the variation of the total elongation, a B content of 0.0003% or more is required. On the other hand, if an excessive amount of B is contained, the toughness is impaired, so the upper limit of the B content is set to 0.010% or less. The lower limit of B content should be above 0.0005% and more preferably 0.0008%, and the upper limit of B content should be below 0.0040% and more preferably below 0.0030%.

(P: 0.023% or less) P is an impurity. If the steel sheet contains too much P, not only will the toughness and weldability be impaired, but also the effect of suppressing grain boundary embrittlement by adding B will be hindered, resulting in a significant decrease in elongation. Therefore, the upper limit of the P content is set to 0.023% or less. The upper limit of P content should be 0.020% or less, 0.015% or less and 0.012% or less. The steel plate of this embodiment does not necessarily have P, so it may not contain P substantially, and the lower limit of P content is 0.000%. The lower limit of P content can also be greater than 0.000% or more than 0.001%, and the smaller the P content, the better.

(S: 0.010% or less) S is an impure substance. If the steel sheet contains too much S, elongation of MnS is generated by hot rolling, which leads to deterioration of formability such as bendability and hole expandability. Therefore, the upper limit of the S content is set to 0.010% or less. The upper limit of S content should be 0.007% or less, and more preferably 0.003% or less. The steel plate of this embodiment does not necessarily have S, so it may not contain S substantially, and the lower limit of S content is 0.000%. The lower limit of the S content can also be set to more than 0.000% or set to 0.001% or more, and the smaller the S content, the better.

(N: less than 0.050%) N is an impurity. If the steel plate contains more than 0.050% of N, the toughness will be impaired. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of N content should be 0.010% or less, and more preferably 0.006% or less. The steel plate of this embodiment does not necessarily have to be N, so it may be substantially free of N, and the lower limit of the N content is 0.000%. The lower limit of N content can also be set to more than 0.000% or set to 0.001% or more, and the smaller the N content, the better.

(O: less than 0.020%) O is an impurity. If the steel sheet contains 0.020% or more of O, ductility will deteriorate. Therefore, the upper limit of the O content is set to less than 0.020%. The upper limit of the O content should be 0.010% or less, more preferably 0.005% or less, and more preferably 0.003% or less. The steel plate of this embodiment does not necessarily have O, so it may be substantially free of O, and the lower limit of the O content is 0.000%. The lower limit of the O content can also be set to more than 0.000% or set to 0.001% or more, and the smaller the O content, the better.

The so-called "impurity" refers to components that are mixed from raw materials such as ore and scrap or various factors in the manufacturing process when manufacturing steel in the industry, and refers to what is allowed within the range that does not adversely affect the present invention.

The steel plate of this embodiment may further contain one or more elements selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi. However, the steel plate of this embodiment does not necessarily have Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn, and Bi, and therefore may not contain Cr, Mo , W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content can also be 0%. In this specification, REM refers to a total of 17 elements of Sc, Y, and lanthanides, and REM content refers to its content when REM is one type, and refers to their total content when it is two or more types. In addition, REM is generally supplied in the form of multiple types of REM alloys, that is, rare earth metal alloys. Therefore, one or more individual elements may be added so that the REM content is within the above range, or may be added, for example, in the form of a rare earth metal alloy, and the REM content is included in the above range.

(Cr: 0.00% or more and less than 2.00%) (Mo: 0.00% or more and 2.00% or less) (W: 0.00% or more and 2.00% or less) (Cu: 0.00% or more and 2.00% or less) (Ni: 0.00% or more and 2.00% or less) Since Cr, Mo, W, Cu, and Ni are not essential elements of the steel plate of this embodiment, they may not be included, and their respective contents are 0.00% or more. However, since Cr, Mo, W, Cu, and Ni-based elements enhance the strength of the steel sheet, they may also be included. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may also contain 0.01% or more of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. However, if the steel sheet contains excessive amounts of these elements, surface scratches during hot rolling are likely to occur, and the strength of the hot rolled steel sheet becomes too high and the cold rolling ductility decreases. Therefore, the content upper limit of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to 2.00% or less.

(Ti: 0.000% or more and 0.300% or less) (Nb: 0.000% or more and 0.300% or less) (V: 0.000% or more and 0.300% or less) Ti, Nb, and V are not essential elements of the steel sheet of this embodiment, so they may not be contained, and their respective contents are 0.000% or more. However, Ti, Nb, and V systems generate fine carbide, nitride, or carbonitride elements, so they can effectively increase the strength of the steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V. In order to obtain the effect of improving the strength of the steel sheet, the lower limit of the content of one or more elements selected from the group consisting of Ti, Nb, and V is preferably 0.005% or more. On the other hand, if too much of these elements are contained, the strength of the hot-rolled steel sheet may increase excessively and the cold-rolling ductility may decrease. Therefore, the content upper limit of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less.

(Ca: 0.000% or more and 0.010% or less) (Mg: 0.000% or more and 0.010% or less) (Zr: 0.000% or more and 0.010% or less) (REM: 0.000% or more and 0.010% or less)

Ca, Mg, Zr, and REM (rare earth metals) are not essential elements of the steel sheet of the present disclosure, so they may not be contained, and their respective contents are 0.000% or more. However, Ca, Mg, Zr and REM will improve the local ductility and hole expansion of the steel sheet. In order to obtain this effect, the lower limit of the content of one or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.0001% or more, and more preferably 0.001% or more . However, excessive amounts of these elements will deteriorate the workability of the steel plate, so the upper limit of the content of these elements should be set to 0.010% or less, and is selected from the group consisting of Ca, Mg, Zr and REM Or the total content of two or more elements should be set to 0.030% or less.

(Sb: 0.000% or more and 0.050% or less) (Sn: 0.000% or more and 0.050% or less) (Bi: 0.000% or more and 0.050% or less) Sb, Sn, and Bi are not essential elements of the steel sheet of the present disclosure, so they may not be contained, and their respective contents are 0.000% or more. However, Sb, Sn, and Bi suppress the diffusion of easily oxidizable elements such as Mn, Si, and/or Al in the steel sheet to the surface of the steel sheet to form oxides, thereby improving the surface properties and plating properties of the steel sheet. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the content of each of these elements is greater than 0.050%, the effect will be saturated, so the upper limit of the content of these elements is set to 0.050% or less.

2. Mechanical properties Next, the mechanical characteristics of the steel sheet of this embodiment will be described.

The tensile strength of the steel plate of this embodiment is 1320 MPa or more, and preferably 1470 MPa or more. This is to reduce the thickness of the plate by increasing the strength when using a steel plate as a blank for structural materials such as automobiles, thereby contributing to weight reduction. The upper limit of the tensile strength of the steel plate is not particularly limited, and may be, for example, 1600 MPa or less. In addition, in order to subject the steel sheet of the present embodiment to plastic working such as press forming, and to provide the member after plastic working to the collision prevention member, it is preferable that the total elongation is excellent. At this time, the product of tensile strength and total elongation is 26,000 MPa% or more, and preferably 28,000 MPa% or more, more preferably 30,000 MPa% or more. The upper limit of the product of tensile strength and total elongation is not particularly limited, and may be, for example, 45000 MPa% or less. In addition, in order to increase the yield of plastically processed components and improve productivity during processing, the standard deviation of the total elongation is 2.5 or less. As disclosed above, the disclosed steel sheet has high strength, good elongation characteristics, and excellent formability, so it is most suitable for automotive parts such as pillars and front side members.

3. Metal structure Next, the metal structure of the steel plate of this embodiment will be described.

(The area ratio of residual austenitic iron in the metal structure of the 1/4t part of the steel plate: 15% or more and 75% or less) In the L section of the steel plate of this embodiment, the metal structure at the 1/4th position (also referred to as the 1/4t portion) of the thickness from the surface is preferably 15% to 75% of the residual austenitic iron in terms of area ratio . The area ratio of the residual Vostian iron will vary with the annealing conditions, and will affect the strength, elongation characteristics and other materials. The L cross section refers to a plane cut parallel to the thickness direction and the rolling direction and passing through the central axis of the rolling direction of the steel sheet.

The area ratio of residual Vostian iron was measured using the X-ray diffraction method. In the observation by the scanning electron microscope, it is difficult to distinguish between the residual Vostian iron and the Xintianmatian scattered iron, so the area ratio of the residual Vostian iron and Xinshengmatian scattered iron is determined by the following method. After mirror-polishing the L section of the steel plate, it is corroded with 3% nitrate etchant (3% nitric acid-ethanol solution), and then the microstructure of the 1/4 position of the thickness from the surface of the steel plate is observed using a scanning electron microscope to determine The total area ratio of residual Vossian iron and Shinsei Matian scattered iron. Next, the area ratio of the residual Vostian iron measured by the X-ray diffraction method was subtracted from the total area ratio of the residual Vostian iron and the fresh Ma Tian scattered iron to calculate the area ratio of the fresh Ma Tian scattered iron.

Residual Vostian iron is a structure that improves the ductility of steel plates by deforming induced plasticity, especially the uniform elongation characteristics of steel plates. Residual Vostian iron will be transformed into Matian loose iron due to stretching, stretching, flange extension or bending with accompanying tensile deformation, so it will also help to increase the strength of the steel plate. In order to obtain these effects, the lower limit of the area ratio of the residual austenite should be set to 15% or more, and more preferably 20% or more. If the area ratio of residual Vostian iron is above 15%, the product of tensile strength and total elongation will be above 28000MPa%. What's more, if the area ratio of the residual Vostian iron is more than 20%, the product of tensile strength and elongation will become more than 30,000 MPa%, and it will be able to maintain more excellent elongation characteristics even at higher strength. .

The higher the area ratio of residual Vossian iron, the better. However, in the steel plate having the above-mentioned chemical composition, the upper limit of the content of the residual austenitic iron is 75% in terms of area ratio. Although containing more than 9.0% of Mn, the residual austenitic iron can be greater than 75% in terms of area ratio, but in this case, the ductility and castability of the steel sheet will be impaired. In addition, considering the hydrogen embrittlement point of view, the area ratio of the residual austenitic iron is preferably 60% or less, more preferably 50% or less, and more preferably 45% or less.

The metal structure of the steel plate of the present embodiment may include tempered hemp iron, ferrite iron, toughened iron, and fresh hemp iron in addition to the residual austenitic iron. Newborn Ma Tian scattered iron is a hard structure, so the smaller the content of New Ma Tian scattered iron, the higher the flexibility and toughness of the steel plate. Therefore, from the standpoint of ensuring flexibility and toughness, the upper limit of the content of loose iron in Shinma Mata is preferably 30% or less in terms of area ratio, and more preferably 20% or less.

4. Manufacturing method Next, the manufacturing method of the steel plate of this embodiment will be described.

The steel plate of this embodiment is a method of smelting steel with the above chemical composition by a conventional method and casting it. After making a steel blank or steel ingot, it is heated and hot rolled, and then the obtained hot rolled steel plate is pickled and then cooled It is manufactured by rolling and annealing.

Hot rolling can be carried out in a general continuous hot rolling line. As long as the conditions described below are satisfied, the annealing may be performed by either an annealing furnace or a continuous annealing line. Annealing should be carried out under a reducing gas environment, or under a reducing gas environment such as nitrogen 98% and hydrogen 2%. By performing heat treatment in a reducing gas environment, the adhesion of scale to the surface of the steel plate can be suppressed. Furthermore, the cold-rolled steel sheet may be flat-rolled.

In order to obtain the mechanical properties of the steel sheet of the present disclosure, the hot rolling conditions and the annealing conditions are performed within the ranges shown below.

As long as the steel plate of the present embodiment has the above-mentioned chemical composition, the molten steel may be smelted by a general blast furnace method, or it may be a material such as steel made by an electric furnace method that contains a large amount of scrap in its raw materials. The steel embryo can be produced by a general continuous casting process or a thin flat steel embryo.

The above-mentioned steel blank or steel block is heated and hot rolled. The temperature of the steel material for hot rolling should be set to 1100°C or higher and 1300°C or lower. By setting the temperature of the steel material for hot rolling to be above 1100°C, the deformation resistance of the hot rolling delay can be made smaller. On the other hand, by setting the temperature of the steel material supplied for hot rolling to 1300° C. or lower, it is possible to suppress a decrease in productivity due to an increase in scale loss. In this specification, the temperature is measured at the center of the surface of the steel plate.

The time to maintain the temperature range of 1100°C or more and 1300°C or less before hot rolling is not particularly limited, but in order to improve the bendability, it is preferably 30 minutes or more and more preferably 1 hour or more. In addition, in order to suppress excessive scale loss, the time to maintain the temperature range of 1100°C or more and 1300°C or less is preferably 10 hours or less and more preferably 5 hours or less. In addition, the direct rolling or direct rolling delay can also be directly supplied to the hot rolling without heating treatment.

The finishing rolling start temperature is preferably set to 700°C or more and 1000°C or less. By setting the finishing temperature of finishing rolling to 700° C. or higher, the deformation resistance during rolling delay can be reduced. On the other hand, by setting the finishing rolling start temperature to 1000° C. or lower, the deterioration of the surface properties of the steel sheet due to grain boundary oxidation can be suppressed.

The hot-rolled steel sheet prepared by finishing rolling is cooled, coiled, and coiled. In order to suppress the grain boundary segregation of the brittle element in the steel sheet after annealing, it is desirable to cool the hot-rolled steel sheet after finishing rolling to 550°C or less at 10°C/sec or more. The coiling temperature after cooling is preferably set to 550°C or lower. By setting the coiling temperature below 550°C, internal oxidation can be suppressed, and subsequent pickling becomes easy. The coiling temperature is preferably below 530°C, more preferably below 500°C. The lower limit of the coiling temperature is not specified, and may be room temperature, for example.

After the coiled hot-rolled steel sheet is substantially cooled to room temperature, the hot-rolled steel sheet is tempered in a temperature range of 300°C to 530°C before cold rolling. By setting the tempering temperature of the hot-rolled steel sheet to 300° C. or higher, the cold-rolled delayed fracture can be more suppressed. On the other hand, by setting the tempering temperature of the hot-rolled steel sheet to 530° C. or lower, the variation in elongation characteristics can be reduced. The lower limit of the tempering temperature of the hot-rolled steel plate is preferably 350°C. The upper limit of the tempering temperature of hot-rolled steel plate should be below 500℃.

After the hot-rolled steel sheet is pickled in a conventional manner, cold rolling is performed to make a cold-rolled steel sheet.

Before cold rolling and before or after pickling, light rolling of more than 0% to about 5% is used to correct the shape, which is advantageous from the viewpoint of ensuring flatness, which is suitable. In addition, by performing mild rolling before pickling, the pickling property can be improved, the removal of surface thickening elements can be promoted, and the effect of improving the chemical conversion treatment property and the plating treatment property can be improved.

From the viewpoint of miniaturizing the structure of the steel sheet after annealing, the reduction ratio of cold rolling is preferably set to 20% or more. From the viewpoint of suppressing breakage during cold rolling, the reduction ratio of cold rolling is preferably set to 70% or less.

The cold-rolled steel sheet prepared through the above hot rolling step and cold rolling step is heated, and maintained in a temperature range of 600° C. or higher and lower than the Ac ₃ point for 10,000 seconds or longer to perform annealing. By setting the annealing temperature to 600° C. or higher and lower than the Ac ₃ point, the tensile strength and elongation characteristics can be improved. If the annealing time is less than 10,000 seconds, not only does the elongation decrease, but also the elongation characteristics vary. Although not being bound by theory, it can be considered that if the annealing time is less than 10,000 seconds, brittle elements such as P or S are likely to segregate at the grain boundary, and the density of precipitates at the grain boundary and the heterogeneous interface becomes larger, resulting in reduced elongation and elongation The variation in characteristics becomes larger. Therefore, the annealing time should be at least 10,000 seconds. In addition, by heating for more than 15000 seconds, the residual austenitic iron will become 15% or more, the elongation characteristics will be significantly improved, and it becomes easy to obtain the product of tensile strength and elongation of 28000 MPa% or more. The above effect will become apparent when the Mn content is 5.50% or more, and in terms of area ratio, the residual austenitic iron will become 20% or more, and the product of tensile strength and elongation will become 30000 MPa% or more. On the other hand, even if the annealing time exceeds 100,000 seconds, the effect is still saturated, so the annealing time should preferably be less than 100,000 seconds. The so-called out-of-phase interface refers to the interface of different structures such as Ma Tian scattered iron and residual Vostian iron. Here, for a variety of cold-rolled steel sheets containing C: 0.05% to 0.5%, Si: 0% to 3.5%, Mn: 0 to 9.0%, and Al: 0 to 2.0%, at a heating rate of 0.5 to 50°C/sec Measure and study Ac ₃ points, the results can be obtained as the Ac ₃ points: Ac ₃ =910-200√C+44Si-25Mn+44Al The Ac ₃ points can be calculated using this formula. The symbol of the element in the above formula is substituted into the content (mass %) of the corresponding element.

When the steel sheet is not to be plated, the cooling after the above annealing may be carried out directly until it reaches room temperature. In addition, when the steel plate is plated, it is performed as follows.

When the hot-dip galvanized steel sheet is manufactured by performing hot-dip galvanizing on the surface of the steel sheet, the steel sheet after the above annealing is reheated and cooled to a temperature range of 430 to 500°C, and then the steel sheet is immersed in a molten zinc plating bath to perform Hot-dip galvanizing. The conditions of the plating bath may be set within a general range. After the plating process, it can be cooled to room temperature.

When alloying hot-dip galvanizing is performed on the surface of the steel sheet to produce alloyed hot-dip galvanizing steel sheet, the steel sheet is melted at a temperature of 450 to 620°C before the steel sheet is subjected to hot-dip galvanizing treatment and before the steel sheet is cooled to room temperature Galvanized alloying treatment. The alloying treatment conditions may be set within a general range.

By manufacturing the steel sheet as described above, the steel sheet of this embodiment can be produced. Examples

The steel plate of the present disclosure will be described more specifically with reference to examples. However, the following example is an example of the disclosed steel plate, and the disclosed steel plate is not limited to the following examples.

1. Manufacturing evaluation steel plate Steel with the chemical composition shown in Table 1 was smelted in a converter, and a steel blank with a thickness of 245 mm was produced by continuous casting.

[Table 1]

The obtained steel blank was hot rolled under the conditions shown in Table 2 to produce a hot rolled steel plate with a thickness of 2.6 mm. Next, the obtained hot-rolled steel sheet was pickled and cold-rolled at the cold-rolling rate shown in Table 2 to produce a cold-rolled steel sheet.

[Table 2]

The obtained cold-rolled steel sheet was subjected to heat treatment under the conditions shown in Table 3 to produce an annealed cold-rolled steel sheet. The heat treatment of the cold-rolled steel sheet is carried out in a reducing gas environment of 98% nitrogen and 2% hydrogen.

[table 3]

For some examples of annealed cold-rolled steel sheets, after final annealing, the cooling after heating to 600°C was stopped at 460°C, and the cold-rolled steel sheets were immersed in a molten zinc plating bath at 460°C for 2 seconds. Hot-dip galvanizing. The conditions of the plating bath are the same as before. When the alloying treatment described below is not performed, after maintaining at 460°C, it is cooled to room temperature at an average cooling rate of 10°C/sec.

For some examples of annealed cold-rolled steel sheets, after performing hot-dip galvanizing treatment, alloying treatment is continuously performed without cooling to room temperature. It was heated to 520°C and maintained at 520°C for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 10°C/sec.

The annealed cold-rolled steel sheet prepared as described above was temper rolled at an elongation of 0.1%, and various steel sheets for evaluation were prepared.

2. Evaluation method For the annealed cold-rolled steel sheets prepared in the examples, the area ratio, tensile strength, and total elongation of the residual Vostian iron and Shinsei hemp iron were evaluated. The evaluation methods are as follows.

(Metal Organization) The area ratios of the residual Vostian iron and the fresh Matian scattered iron are calculated based on the structure observation using a scanning electron microscope and X-ray diffraction measurement. For the L section obtained by cutting the steel plate parallel to the rolling direction, mirror polishing is performed, and then the microstructure is exposed with 3% nitrate etchant, and then observed with a scanning electron microscope at a magnification of 5000 times from the surface 1 The microstructure of the /4 position is calculated by image analysis (Photoshop (registered trademark)) for the area of 0.1 mm × 0.3 mm, and the total area ratio of the remaining Vostian iron and Shinseng Matian powder is calculated. In addition, a test piece having a width of 25 mm and a length of 25 mm was cut from the obtained steel plate, and the test piece was chemically polished to reduce the thickness of 1/4 of the thickness of the plate, and the surface of the chemically polished test piece was applied three times with a Co tube X-ray diffraction analysis of the ball, and analysis of the obtained curve, respectively averaged and calculated the area ratio of the residual Vostian iron. The area ratio of the residual Vostian iron calculated by observation with a scanning electron microscope and the total area ratio of the residual Vostian iron calculated by X-ray diffraction measurement are subtracted from the total area ratio of the residual Vostian iron calculated by X-ray diffraction measurement. The area ratio of iron.

(Mechanical properties) Take five JIS No. 5 tensile test pieces from a steel plate of 250 mm (direction perpendicular to the rolling direction) × 200 mm (direction parallel to the rolling direction) from a direction perpendicular to the rolling direction of the steel plate. The tensile strength (TS) and total elongation (EL) of each test piece were measured. The tensile test was performed using the JIS No. 5 tensile test piece, and the method was specified in JIS Z2241:2011. The measurement of the total elongation is carried out using a JIS No. 5 test piece in accordance with the method specified in JIS Z2241:2011. The tensile strength and the product of the tensile strength and the total elongation (TS×EL) are set to the average of five tensile tests, and the standard deviation of the total elongation is calculated based on the five times.

3. Evaluation results The above evaluation results are shown in Table 4. A steel plate having the following characteristics was evaluated as an example: the tensile strength of 1320 MPa or more and the product of the tensile strength of 26000 MPa% or more and the total elongation, and the standard deviation of the elongation characteristics was 2.5 or less.

[Table 4]

Claims (8)

A steel plate characterized by: It contains in mass %: C: greater than 0.18% and less than 0.45%, Si: 0.001% or more and less than 3.50%, Mn: greater than 4.00% and less than 9.00%, sol.Al: 0.001% or more and less than 1.00%, B: 0.0003% or more and 0.010% or less, P: 0.023% or less, S: 0.010% or less, N: less than 0.050%, O: less than 0.020%, Cr: 0.00% or more and less than 2.00%, Mo: 0.00% or more and 2.00% or less, W: 0.00% or more and 2.00% or less, Cu: 0.00% or more and 2.00% or less, Ni: 0.00% or more and 2.00% or less, Ti: 0.000% or more and 0.300% or less, Nb: 0.000% or more and 0.300% or less, V: 0.000% or more and 0.300% or less, Ca: 0.000% or more and 0.010% or less, Mg: 0.000% or more and 0.010% or less, Zr: 0.000% or more and 0.010% or less, REM: 0.000% or more and 0.010% or less, Sb: 0.000% or more and 0.050% or less, Sn: 0.000% or more and 0.050% or less and Bi: 0.000% or more and 0.050% or less, and The rest is iron and impurities; The tensile strength of the steel plate is 1320 MPa or more, and the product of tensile strength and total elongation is 26000 MPa% or more; and The standard deviation of the total elongation is 2.5 or less. If the steel plate of claim 1, it contains one or more of the following elements: In terms of mass %, Cr: 0.01% or more and less than 2.00%, Mo: 0.01% or more and 2.00% or less, W: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less and Ni: 0.01% or more and 2.00% or less. If the steel plate of claim 1 or 2, it contains one or more of the following elements: In terms of mass %, Ti: 0.005% or more and 0.300% or less, Nb: above 0.005% and below 0.300% and V: 0.005% or more and 0.300% or less. The steel sheet according to any one of claims 1 to 3 contains one or more of the following elements: In terms of mass %, Ca: 0.0001% or more and 0.0100% or less, Mg: 0.0001% or more and 0.0100% or less, Zr: 0.0001% or more and 0.0100% or less and REM: 0.0001% or more and 0.0100% or less. The steel sheet according to any one of claims 1 to 4 contains one or more of the following elements: In terms of mass %, Sb: 0.0005% or more and 0.0500% or less, Sn: 0.0005% or more and 0.0500% or less and Bi: 0.0005% or more and 0.0500% or less. The steel sheet according to any one of claims 1 to 5, wherein the metal structure of the 1/4 position of the thickness from the surface of the L section of the foregoing steel sheet contains 15% or more and 75% or less residual Vos in terms of area ratio Tian Tie. As in the steel sheet according to any one of claims 1 to 6, the surface of the foregoing steel sheet has a hot-dip galvanized layer. As in the steel sheet according to any one of claims 1 to 6, the surface of the foregoing steel sheet has an alloyed galvanized layer.