JPWO2020017606A1 - Steel plate - Google Patents

Abstract

It has a tensile strength of 1320 MPa or more and a product of tensile strength of 26000 MPa% or more and total elongation, which is useful for structural members, particularly automobile skeletal members, reinforcing members, and collision-resistant members, and has little variation in elongation characteristics. A steel sheet having a high Mn content is provided. % By mass, C: more than 0.18% and less than 0.45%, Si: 0.001% or more and less than 3.50%, Mn: more than 4.00% and less than 9.00%, and 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%, and optionally containing elements, the balance is iron and impurities, the tensile strength is 1320 MPa or more, the product of tensile strength and total elongation is 26000 MPa% or more, the total elongation A steel sheet having a standard deviation of 2.5 or less.

Description

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

In order to achieve both weight reduction and safety of automobile bodies and parts, the strength of steel sheets, which are these materials, has been increased. Generally, when the strength of a steel sheet is increased, the elongation is reduced and the formability of the steel sheet is impaired. Therefore, in order to use a high-strength steel sheet as a member for automobiles, it is necessary to enhance both the contradictory properties of strength and formability.

In order to improve the tensile strength and the elongation, so-called TRIP steel utilizing the transformation-induced plasticity of retained austenite has been proposed so far (for example, Patent Document 1).

Retained austenite is obtained by concentrating C in austenite so that austenite does not transform into another structure even at room temperature. As a technique for stabilizing austenite, it has been proposed to incorporate a carbide precipitation inhibiting element such as Si and Al into the steel sheet so as to concentrate C in the austenite during the bainite transformation that occurs in the steel sheet at the production stage of the steel sheet. There is. In this technique, if the C content contained in the steel sheet is large, austenite is further stabilized and the amount of retained austenite can be increased, and as a result, a steel sheet excellent in both strength and elongation can be manufactured. However, when a steel sheet is used as a structural member, welding is often performed on the steel sheet, but if the C content in the steel sheet is large, it becomes difficult to ensure sufficient weldability, and it is decided to use it as a structural member. There are restrictions. Therefore, it is desired to improve both strength and elongation of the steel sheet with a smaller C content.

As a steel sheet having a C content lower than that of the TRIP steel, a retained austenite content higher than that of the TRIP steel, and strength and ductility exceeding the TRIP steel, a steel containing more than 4.0% Mn added is proposed. (For example, Non-Patent Document 1).

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

Patent Document 2 discloses a steel sheet that is excellent in tensile strength and elongation by optimizing the manufacturing conditions of steel to which 2 to 6% of Mn is added.

JP2012-237054A JP-A-7-188834

Furukawa Kei, Matsumura O, Heat Treatment, Japan, Japan Heat Treatment Association, 1997, Vol. 37, No. 4, p. 204

In order to use a high-strength steel sheet as a member for automobiles, it is desired to secure the contradictory properties of strength and formability without lowering the weldability. Specifically, it is desired to have excellent elongation characteristics 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, for example, in the steel containing more than 4.0% Mn as disclosed in the above-mentioned Patent Document 1 and Patent Document 2 and Non-Patent Document 1, it is considered that the grain boundary and the heterophase interface become weak, The variation in elongation property in the tensile strength test becomes large. 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 characteristics. Such variation in elongation characteristics is remarkable when the tensile strength exceeds 1320 MPa.

Therefore, there is a demand for a steel sheet having a high tensile strength of 1320 MPa or more and a product of tensile strength and total elongation of 26000 MPa% or more, which has excellent elongation characteristics and high strength, and has a high content of Mn content with a small variation in the elongation characteristics. ..

In order to secure the characteristics that the tensile strength is 1320 MPa or more and the product of the tensile strength and the total elongation is 26000 MPa% or more in the steel sheet with a high Mn content, and further to reduce the variation in the elongation characteristics, the present inventors have The P content of the steel sheet with a high Mn content 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 no such steel sheet has ever been produced. We have found that it is effective to control by hot rolling conditions and annealing conditions.

The steel sheet of the present disclosure was made based on the above findings, and the summary thereof is as follows.
(1) In mass%,
C: more than 0.18% and less than 0.45%,
Si: 0.001% or more and less than 3.50%,
Mn: more 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% to 2.00%,
W: 0.00% to 2.00%,
Cu: 0.00% to 2.00%,
Ni: 0.00% to 2.00%,
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,
The balance is iron and impurities,
The tensile strength is 1320 MPa or more and the product of the tensile strength and the total elongation is 26000 MPa% or more,
A steel sheet having a standard deviation of total elongation of 2.5 or less.
(2) In 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, 1 type or 2 types or more, The steel plate as described in said (1) characterized by the above-mentioned. ..
(3) In mass%,
Ti: 0.005% or more and 0.300% or less,
Nb: 0.005% or more and 0.300% or less, and V: 0.005% or more and 0.300% or less, or one or more kinds of the above (1) or (2). Steel plate described in.
(4) In 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, or one or more types thereof, and the above (1) to (3). The steel plate according to any one of 1.
(5) In 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, one kind or two or more kinds are contained, and the above (1) to (4) are included. The steel plate according to any one of 1.
(6) The above-mentioned (1) to (5), wherein the metal structure at the 1/4 position of the thickness from the surface in the L cross section of the steel sheet contains the retained austenite in an area ratio of 15% or more and 75% or less. ) The steel plate according to any one of.
(7) The steel sheet according to any one of (1) to (6) above, which has a hot-dip galvanized layer on the surface of the steel sheet.
(8) The steel sheet according to any one of (1) to (6) above, which has a galvannealed layer on the surface of the steel sheet.

According to the present disclosure, a structural member, particularly a skeletal member or reinforcing member of an automobile, and a collision resistant member, which has a tensile strength of 1320 MPa or more and a product of tensile strength of 26000 MPa% or more and total elongation, and It is possible to provide a steel sheet with a high content of Mn in which the variation in elongation characteristics is small.

As a result of intensive studies, the present inventors have found that the P content of a steel sheet having a high Mn content is 0.023 mass% or less and the B content is 0.0003 mass% or more and 0.010 mass% or less. In order to manufacture a steel sheet having controlled tensile strength of 1320 MPa or more and a product of tensile strength of 26000 MPa% or more and total elongation and having small variation in elongation characteristics, hot rolling conditions that are not existent in the prior art are controlled. It was found that it is effective to control with the annealing conditions.

That is, the variation of the total elongation that can occur in the steel sheet with a high Mn concentration is not the variation of the total elongation due to the solidification segregation of P by conventional casting and the formation of the band structure of the hot-rolled steel sheet, or the width during hot rolling. It is not the variation of the total elongation caused by the variation of the manufacturing method such as the uneven cooling in the direction, but the brittleness due to the segregation of P and the precipitation of B in the grain boundary between the fresh martensite (hard phase) and the retained austenite and the surrounding structure. Analysis, in other words, it became clear from the analysis that it is the essential variation of the steel sheet due to the grain boundary embrittlement. The present invention is directed to a steel sheet that suppresses this grain boundary embrittlement. Further, the present application proposes a manufacturing method including a heat treatment method for suppressing the grain boundary embrittlement.

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

1. Chemical Composition The reasons for defining the chemical composition of the steel sheet of the present disclosure as described above will be described. In the following description, “%” representing the content of each element means mass% unless otherwise specified.

(C: more than 0.18% and less than 0.45%)
C is an extremely important element for increasing the tensile strength of steel. In order to obtain a tensile strength of 1320 MPa or more, a C content exceeding 0.18% is required. On the other hand, if C is contained excessively, the weldability of the steel sheet is impaired, so the upper limit of C content was made less than 0.45%. From the viewpoint of increasing the tensile strength and the total elongation, the lower limit of the C content is preferably 0.20% or more, more preferably 0.24% or more. By setting the lower limit of the C content to 0.20% or more and the annealing time to be 15000 s or more at a predetermined annealing temperature in the annealing described later, the area ratio (area%) makes the retained austenite 15% or more. The product of tensile strength and total elongation is 28,000 MPa% or more. The lower limit of the C content is set to 0.24% or more and the B content to be described later is controlled to 0.0005% to 0.0040%, and in the annealing to be described later, the annealing time is set at a predetermined annealing temperature. By setting the tensile strength to 15,000 s or more, it becomes possible to obtain an ultrahigh-strength steel sheet having a tensile strength of 1470 MPa or more while maintaining the product of tensile strength and total elongation at 28000 MPa% or more. The upper limit of the C content is preferably 0.40% or less, more preferably 0.35% or less, and by setting the upper limit of the C content to the above range, the weldability of the steel sheet can be further improved. it can.

(Si: 0.001% or more and less than 3.50%)
Si also suppresses the precipitation and coarsening of cementite, and also has the effect of making it easier to control the austenite formed during annealing. To obtain the above effect, a Si content of 0.001% or more is required. The lower limit of the Si content is preferably 0.01% or more, more preferably 0.40% or more. Further, when the Si content is 1.00% or more, the fatigue property can be improved while maintaining the strength-ductility balance, so the lower limit of the Si content is more preferably 1.00% or more. .. On the other hand, if Si is contained excessively, the plating property and chemical conversion treatment property of the steel sheet are impaired, so the upper limit of the Si content was made less than 3.50%. Furthermore, the upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.

(Mn: more than 4.00% and less than 9.00%)
Mn is an element that stabilizes austenite and enhances tensile strength and total elongation. Further, in the steel sheet of the present disclosure, Mn is distributed in austenite to further stabilize austenite. More than 4.00% Mn is required to stabilize austenite at room temperature. On the other hand, if the steel sheet contains excessive Mn, the ductility is impaired, so the upper limit of the Mn content was made less than 9.00%. The lower limit of the Mn content is preferably 4.30% or more, more preferably 4.80% or more, still more preferably 5.50% or more. The upper limit of the Mn content is preferably 8.00% or less, more preferably 7.50% or less. By setting the lower limit value and the upper limit value of the Mn content within the above ranges, austenite can be further stabilized.

(Sol.Al: 0.001% or more and less than 1.00%)
Al is a deoxidizer, and it is necessary to contain 0.001% or more. In addition, Al widens the two-phase temperature range during annealing, and thus has the effect of enhancing the material stability. The larger the Al content, the greater the effect. However, if Al is excessively contained, surface properties, paintability, weldability, etc. are deteriorated. The upper limit of Al was set to less than 1.00%. sol. The lower limit of the Al content is preferably 0.005% or more, more preferably 0.010% or more, still more preferably 0.020% or more. sol. The upper limit of the Al content is preferably 0.80% or less, more preferably 0.60% or less. sol. By setting the lower limit and the upper limit of the Al content within the above ranges, the balance between the deoxidizing effect and the material stability improving effect and the surface properties, paintability, and weldability becomes better. As used herein, "sol.Al" means "acid-soluble Al".

(B: 0.0003% or more and 0.010% or less)
B is an extremely important element for developing excellent elongation characteristics and reducing variations in the elongation characteristics in a steel sheet having a tensile strength of 1320 MPa or more. In order to obtain the product of the tensile strength and the total elongation of 26000 MPa% or more and suppress the variation in the total elongation, the B content of 0.0003% or more is required. On the other hand, if B is contained excessively, the toughness is impaired, so the upper limit of the B content was made 0.010% or less. The lower limit of the B content is preferably 0.0005% or more, more preferably 0.0008% or more, and the upper limit of the B content is preferably 0.0040% or less, more preferably 0.0030%. It is as follows.

(P: 0.023% or less)
P is an impurity, and if the steel sheet contains P in an excessive amount, not only the toughness and weldability are impaired, but also the grain boundary embrittlement suppressing effect due to the addition of B is impaired, and elongation is significantly reduced. Therefore, the upper limit of the P content is 0.023% or less. The upper limit of the P content is preferably 0.020% or less, more preferably 0.015% or less, still more preferably 0.012% or less. Since the steel sheet according to the present embodiment does not require P, it does not need to contain P substantially, and the lower limit of the P content is 0.000%. The lower limit of the P content may be more than 0.000% or 0.001% or more, but the smaller the P content is, the more preferable.

(S: 0.010% or less)
S is an impurity, and when the steel sheet contains S in excess, MnS stretched by hot rolling is generated, resulting in deterioration of formability such as bendability and hole expandability. Therefore, the upper limit of the S content is 0.010% or less. The upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to this embodiment does not require S, it does not need to contain S substantially, and the lower limit of the S content is 0.000%. The lower limit of the S content may be more than 0.000% or 0.001% or more, but the smaller the S content, the more preferable.

(N: less than 0.050%)
N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is impaired. Therefore, the upper limit of the N content is less than 0.050%. The upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to the present embodiment does not require N, it does not have to contain N substantially, and the lower limit of the N content is 0.000%. The lower limit of the N content may be more than 0.000% or 0.001% or more, but the smaller the N content is, the more preferable.

(O: less than 0.020%)
O is an impurity, and if the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is less than 0.020%. The upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and further preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require O, it does not need to contain O substantially, and the lower limit of the O content is 0.000%. The lower limit of the O content may be more than 0.000% or 0.001% or more, but the smaller the O content is, the more preferable.

"Impurities" are components that are mixed by ores, raw materials such as scrap, and various factors of the manufacturing process when industrially manufacturing steel products, and are permitted within a range that does not adversely affect the present invention. Means

The steel sheet of the present embodiment is further one or two selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi. The above may be contained. However, the steel sheet according to the present embodiment does not require Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, so Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi may not be contained, that is, the lower limit of the content may be 0%. The REM referred to in the present specification refers to a total of 17 elements of Sc, Y, and a lanthanoid, and the REM content is the content of one REM, and the total content of two or more REMs. Refers to. REM is also generally supplied as a misch metal which is an alloy of a plurality of types of REM. Therefore, one kind or two or more kinds of individual elements may be added so as to be contained so that the REM content falls within the above range. For example, the individual elements may be added in the form of misch metal to have the REM content above. You may contain so that it may become the range of.

(Cr: 0.00% or more and less than 2.00%)
(Mo: 0.00% to 2.00%)
(W: 0.00% to 2.00%)
(Cu: 0.00% to 2.00%)
(Ni: 0.00% to 2.00%)
Cr, Mo, W, Cu, and Ni are not essential elements in the steel sheet according to the present embodiment, and thus may not be contained, and the content of each is 0.00% or more. However, Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet, and thus may be contained. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. .. However, if the steel sheet contains these elements in an excessive amount, surface scratches during hot rolling are likely to occur, and further, the strength of the hot rolled steel sheet may become too high, and the cold rolling property may deteriorate. Therefore, the upper limit of the content 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 for the steel sheet according to the present embodiment, and thus may not be contained, and the content of each is 0.000% or more. However, since Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, they are effective in improving 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 each 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 these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased and the cold rolling property may be deteriorated. Therefore, the upper limit of the content 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 metal) are not essential elements in the steel sheet of the present disclosure, and thus may not be contained, and the content of each is 0.000% or more. However, Ca, Mg, Zr, and REM improve the local ductility and hole expandability 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 Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably It is made 0.001% or more. However, since excessive amounts of these elements deteriorate the workability of the steel sheet, the upper limit of the content of each of these elements is set to 0.010% or less, and 1 selected from the group consisting of Ca, Mg, Zr, and REM. The total content of one kind or two or more kinds of elements is preferably 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 in the steel sheet of the present disclosure, and thus may not be contained, and the content of each is 0.000% or more. However, Sb, Sn, and Bi prevent Mn, Si, and/or Al and other easily oxidizable elements in the steel sheet from diffusing on the surface of the steel sheet to form oxides, and improve the surface properties and plating properties of the steel sheet. Increase. 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, more preferably 0.0010. % Or more. On the other hand, if the content of each of these elements exceeds 0.050%, the effect is saturated, so the upper limit of the content of each of these elements was made 0.050% or less.

2. Mechanical Properties Next, the mechanical properties of the steel sheet according to the present embodiment will be described.

The tensile strength of the steel sheet according to the present embodiment is 1320 MPa or more, preferably 1470 MPa or more. This is because when a steel sheet is used as a material for a structural material for automobiles or the like, the strength is increased to reduce the sheet thickness and contribute to the weight reduction. The upper limit of the tensile strength of the steel sheet is not particularly specified, but may be 1600 MPa or less, for example. Further, in order to subject the steel sheet according to the present embodiment to plastic working such as press forming, and further to subject the plastically worked member to the anti-collision member, it is desirable that the total elongation is excellent. In that case, the product of tensile strength and total elongation is 26000 MPa% or more, preferably 28000 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, but may be, for example, 45,000 MPa% or less. Further, the standard deviation of the total elongation is 2.5 or less in order to increase the yield of the plastically worked member and improve the productivity during working. As described above, the steel sheet of the present disclosure has high strength, good elongation characteristics, and excellent formability, and therefore is most suitable for automobile parts such as pillars and front side members.

3. Metallographic Structure Next, the metallic structure of the steel sheet according to the present embodiment will be described.

(Area ratio of retained austenite in the metal structure of 1/4 t part of the steel plate: 15% or more and 75% or less)
In the L cross section of the steel sheet according to the present embodiment, the metal structure at the 1/4 position (also referred to as 1/4 t part) of the thickness from the surface preferably contains retained austenite in an area ratio of 15% or more and 75% or less. The area ratio of retained austenite changes depending on the annealing conditions and affects the material such as strength and elongation characteristics. The L cross section means a plane cut so as to pass through the central axis of the steel sheet in the rolling direction in parallel with the sheet thickness direction and the rolling direction.

The area ratio of retained austenite is measured using an X-ray diffraction method. In observation with a scanning electron microscope, it is difficult to distinguish between retained austenite and fresh martensite. Therefore, the area ratio of retained austenite and fresh martensite is measured by the following method. After mirror-polishing the L-section of the steel sheet, it was corroded with 3% Nital (3% nitric acid-ethanol solution), and the microstructure at 1/4 of the thickness was observed from the surface of the steel sheet with a scanning electron microscope, and remained. The total area ratio of austenite and fresh martensite is measured. Next, the area ratio of the residual austenite and the fresh martensite is subtracted from the total area ratio of the residual austenite and the fresh martensite to calculate the area ratio of the fresh martensite.

Retained austenite is a structure that enhances the ductility of a steel sheet, especially the uniform elongation property of the steel sheet, by transformation-induced plasticity. Retained austenite can transform into martensite by bulging accompanied by tensile deformation, drawing, stretch flange, or bending, and thus contributes to improvement in strength of the steel sheet. In order to obtain these effects, the lower limit of the area ratio of retained austenite is preferably 15% or more, more preferably 20% or more. When the area ratio of retained austenite is 15% or more, the product of tensile strength and total elongation is 28,000 MPa% or more. Further, when the area ratio of retained austenite is 20% or more, the product of tensile strength and elongation is 30,000 MPa% or more, and more excellent elongation properties can be maintained even at higher strength.

The higher the area ratio of retained austenite, the more preferable. However, in the steel sheet having the above-described chemical composition, the area ratio of 75% is the upper limit of the retained austenite content. If the Mn content exceeds 9.0%, the retained austenite can be made to exceed 75% in area ratio, but in this case, the ductility and castability of the steel sheet are impaired. Further, the area ratio of the retained austenite is preferably 60% or less, more preferably 50% or less, and further preferably 45% or less, from the viewpoint of hydrogen embrittlement.

The metal structure of the steel sheet according to the present embodiment may include tempered martensite, ferrite, bainite, and fresh martensite, in addition to retained austenite. Since fresh martensite has a hard structure, the smaller the content of fresh martensite, the higher the bendability and toughness of the steel sheet. Therefore, the upper limit of the content of fresh martensite is preferably 30% or less, and more preferably 20% or less in terms of area ratio from the viewpoint of ensuring bendability and toughness.

4. Manufacturing Method Next, a method of manufacturing the steel sheet according to the present embodiment will be described.

The steel sheet according to the present embodiment, the steel having the above-described chemical composition is melted by a conventional method, cast to produce a slab or a steel ingot, which is heated and hot rolled, the hot rolled steel sheet obtained Is pickled, cold-rolled, and annealed.

The hot rolling may be performed on a normal continuous hot rolling line. The annealing may be performed in either an annealing furnace or a continuous annealing line as long as the conditions described below are satisfied. Annealing is preferably carried out in a reducing atmosphere, for example in a reducing atmosphere of 98% nitrogen and 2% hydrogen. Heat treatment in a reducing atmosphere can prevent scale from adhering to the surface of the steel sheet. Further, skin pass rolling may be performed on the steel sheet after cold rolling.

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

As long as the steel sheet according to the present embodiment has the above-described chemical composition, the molten steel may be one produced by an ordinary blast furnace method, and like the steel produced by the electric furnace method, the raw material contains a large amount of scrap. May be included in. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

The above slab or steel ingot is heated and hot rolled. The temperature of the steel material subjected to hot rolling is preferably 1100°C or higher and 1300°C or lower. By setting the temperature of the steel material subjected to hot rolling to 1100° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300° C. or lower, it is possible to suppress the decrease in yield due to the increase in scale loss. In the present specification, the temperature is the temperature measured at the central position on the surface of the steel sheet.

The time of holding in the temperature range of 1100° C. or higher and 1300° C. or lower before hot rolling is not particularly specified, but in order to improve bendability, it is preferably 30 minutes or longer, and more preferably 1 hour or longer. preferable. Further, in order to suppress excessive scale loss, the time of holding in 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, when performing direct-feed rolling or direct rolling, it may be directly subjected to hot rolling without heat treatment.

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

The hot-rolled steel sheet obtained by finish rolling is cooled, wound, and formed into a coil. In order to suppress the grain boundary segregation of the embrittlement element in the annealed steel sheet, it is preferable to cool the hot-rolled steel sheet subjected to finish rolling at 10° C./sec or more and 550° C. or less. The winding temperature after cooling is preferably 550°C or lower. By setting the coiling temperature to 550° C. or lower, internal oxidation is suppressed and subsequent pickling becomes easy. The winding temperature is more preferably 530°C or lower, and further preferably 500°C or lower. The lower limit of the winding temperature is not particularly specified, but may be room temperature, for example.

After the rolled hot-rolled steel sheet is substantially cooled to room temperature, the hot-rolled steel sheet is tempered in a temperature range of 300° C. or higher and 530° C. or lower before cold rolling. By setting the tempering temperature of the hot-rolled steel sheet to 300° C. or higher, breakage during cold rolling can be further suppressed. On the other hand, by setting the tempering temperature of the hot-rolled steel sheet to 530° C. or lower, it is possible to reduce the variation in elongation characteristics. The lower limit of the tempering temperature of the hot rolled steel sheet is more preferably 350°C. The upper limit of the tempering temperature of the hot rolled steel sheet is preferably 500°C or lower.

The hot-rolled steel sheet is subjected to pickling by a conventional method and then cold-rolled to obtain a cold-rolled steel sheet.

It is preferable to perform a light rolling of more than 0% to about 5% before the cold rolling and before or after the pickling to correct the shape, which is advantageous in terms of ensuring flatness. Further, by performing light rolling before pickling, pickling properties are improved, removal of surface thickening elements is promoted, and chemical conversion treatment and plating treatment are improved.

From the viewpoint of refining the structure of the steel sheet after annealing, it is preferable that the reduction ratio of cold rolling is 20% or more. From the viewpoint of suppressing breakage during cold rolling, the reduction ratio of cold rolling is preferably 70% or less.

The cold-rolled steel sheet obtained through the hot rolling step and the cold rolling step is heated and annealed at a temperature range of 600° C. or higher and lower than Ac ₃ point for 10000 seconds or more. By setting the annealing temperature to 600° C. or higher and lower than the Ac ₃ point, the tensile strength can be increased and the elongation characteristics can be improved. If the annealing time is less than 10,000 seconds, not only the elongation decreases but also the variation in the elongation characteristics becomes apparent. Without being bound by theory, if the annealing time is less than 10,000 seconds, embrittlement elements such as P and S are likely to segregate at the grain boundaries, and further, the density of precipitates at the grain boundaries and the heterophasic interface increases. It is considered that the decrease in elongation and the variation in elongation characteristics increase. Therefore, the annealing time is set to 10,000 seconds or more. Further, by heating for 15000 seconds or more, the residual austenite becomes 15% or more, the elongation property is remarkably improved, and the product of tensile strength and elongation of 28000 MPa% or more is easily obtained. Such an effect becomes remarkable when the Mn content is 5.50% or more, the retained austenite has an area ratio of 20% or more, and the product of tensile strength and elongation is 30,000 MPa% or more. On the other hand, even if the annealing time exceeds 100,000 seconds, the effect is saturated, so the annealing time is preferably 100,000 seconds or less. The heterophase interface is an interface of different structures such as martensite and retained austenite. Here, a plurality of types of cold rolling containing C: 0.05% to 0.5%, Si: 0% to 3.5%, Mn: 0 to 9.0%, Al: 0 to 2.0%. As a result of measuring and examining Ac ₃ points at a heating rate of 0.5 to 50° C./sec for the steel sheet, the following formula is used as the Ac ₃ points:
Ac ₃ =910-200√C+44Si-25Mn+44Al
Is obtained, and the Ac ₃ point is calculated using this formula. The content (mass %) of the corresponding element is substituted for the element symbol in the above formula.

If the steel sheet is not plated, the cooling after the annealing may be performed as it is to room temperature. Moreover, when plating a steel plate, it is performed as follows.

When the surface of a steel sheet is subjected to hot dip galvanizing to produce a hot-dip galvanized steel sheet, the annealed steel sheet is heated again and cooled to a temperature range of 430 to 500° C., and then the hot-dip galvanizing bath is applied to the steel sheet. It is dipped in hot dip galvanizing process. The conditions of the plating bath may be within the usual range. After the plating treatment, it may be cooled to room temperature.

When the surface of the steel sheet is subjected to galvannealing to produce a galvannealed steel sheet, the steel sheet is subjected to hot dip galvanizing treatment and then cooled to room temperature at 450 to 620° C. The galvanizing alloying treatment is performed at a temperature. The alloying treatment conditions may be within the usual range.

The steel plate according to the present embodiment can be obtained by manufacturing the steel plate as described above.

The steel sheet of the present disclosure will be described more specifically with reference to examples. However, the following examples are examples of the steel sheet of the present disclosure, and the steel sheet of the present disclosure is not limited to the aspects of the following examples.

1. Production of Evaluation Steel Sheets Steels having the chemical components shown in Table 1 were melted in a converter and continuously cast to obtain 245 mm thick slabs.

The obtained slab was hot-rolled under the conditions shown in Table 2 to make a hot-rolled steel sheet having 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.

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 was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.

For some examples of cold-rolled annealed steel sheets, after the final annealing, further cooling after heating to 600° C. was stopped at 460° C., and the cold-rolled steel sheets were placed in a 460° C. hot dip galvanizing bath. It was dipped for 2 seconds to perform hot dip galvanizing treatment. The conditions of the plating bath are the same as conventional ones. When the alloying treatment described later was not performed, the temperature was maintained at 460° C. and then cooled to room temperature at an average cooling rate of 10° C./sec.

For some of the examples of the annealed cold rolled steel sheet, after the hot dip galvanizing treatment, the alloying treatment was performed without cooling to room temperature. The alloy was heated to 520° C., held at 520° C. for 5 seconds for alloying treatment, and then cooled to room temperature at an average cooling rate of 10° C./second.

The thus-obtained annealed cold-rolled steel sheet was temper-rolled at an elongation of 0.1% to prepare various evaluation steel sheets.

2. Evaluation Method The area ratio of retained austenite and fresh martensite, tensile strength, and total elongation of the annealed cold-rolled steel sheets obtained in each example were evaluated. The method of each evaluation is as follows.

(Metal structure)
The area ratio of retained austenite and fresh martensite was calculated from the structure observation by a scanning electron microscope and X-ray diffraction measurement. The L-section obtained by cutting the steel sheet in parallel with the rolling direction was mirror-polished, and then the microstructure was revealed with 3% Nital, and the microstructure at the 1/4 position from the surface was observed with a scanning electron microscope at a magnification of 5000 times. Was observed, and the total area ratio of the retained austenite and fresh martensite was calculated by image analysis (Photoshop (registered trademark)) in the range of 0.1 mm×0.3 mm. Further, a test piece having a width of 25 mm and a length of 25 mm was cut out from the obtained steel plate, and the test piece was subjected to chemical polishing to reduce the plate thickness by 1/4, and the surface of the test piece after chemical polishing was cut. , X-ray diffraction analysis using a Co tube was performed three times, the obtained profiles were analyzed, and the average of each was calculated to calculate the area ratio of retained austenite. The area ratio of the fresh martensite was calculated by subtracting the area ratio of the retained austenite calculated by X-ray diffraction measurement from the total area ratio of the retained austenite and fresh martensite calculated by scanning electron microscope observation.

(mechanical nature)
Five JIS No. 5 tensile test pieces were taken from one steel plate of 250 mm (direction perpendicular to the rolling direction) x 200 mm (direction parallel to the rolling direction) from the direction perpendicular to the rolling direction of the steel plate, and the tensile strength (TS) was obtained from each test piece. And total elongation (EL) were measured. The tensile test was carried out by using a JIS No. 5 tensile test piece by the method specified in JIS Z2241:2011. The measurement of the total elongation was performed by using a JIS No. 5 test piece by the method specified in JIS Z2241:2011. The tensile strength and the product of the tensile strength and the total elongation (TS×EL) were the average values of 5 times of the tensile test, and the standard deviation of the total elongation was calculated from the 5 times.

3. Evaluation results Table 4 shows the results of the above evaluations. Steel sheets having a tensile strength of 1320 MPa or more and a product of tensile strength of 26000 MPa% or more and total elongation and a standard deviation of elongation characteristics of 2.5 or less were evaluated as examples.

Claims (8)

In mass %,
C: more than 0.18% and less than 0.45%,
Si: 0.001% or more and less than 3.50%,
Mn: more 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% to 2.00%,
W: 0.00% to 2.00%,
Cu: 0.00% to 2.00%,
Ni: 0.00% to 2.00%,
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, with the balance being iron and impurities,
The tensile strength is 1320 MPa or more and the product of the tensile strength and the total elongation is 26000 MPa% or more,
A steel sheet having a standard deviation of total elongation of 2.5 or less. In 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 1 type or 2 types or more, The steel plate of Claim 1 characterized by the above-mentioned. In mass %,
Ti: 0.005% or more and 0.300% or less,
Nb: 0.005% or more and 0.300% or less, and V: 0.005% or more and 0.300% or less 1 type or 2 types or more, It is characterized by the above-mentioned. Steel plate. In 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 1 type or 2 types or more, It is characterized by the above-mentioned. The steel sheet according to one item. In 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 1 type or 2 types or more, It is characterized by containing any one of Claims 1-4. The steel sheet according to one item. The metal structure at a 1/4 position of the thickness from the surface in the L section of the steel sheet contains, in area ratio, 15% or more and 75% or less of retained austenite. Steel plate described in. The hot-dip galvanized layer is provided on the surface of the steel sheet, and the steel sheet according to any one of claims 1 to 6. The steel sheet according to any one of claims 1 to 6, which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.