US-20260125784-A1 - HOT-ROLLED STEEL SHEET

Abstract

This hot-rolled steel sheet has a desired chemical composition, in a microstructure at a position of ¼ from a surface in a sheet thickness direction, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×10 5 to 8.0×10 5 , a standard deviation of Mn concentrations is 0.60 mass % or less, at the surface, an area ratio of a region where a Ni concentration is 0.2 mass % or more is 10.0% or more, an area ratio of a region where an O concentration is 3.0 mass % or more is 3.0% to 50.0%, and a maximum value of sphere equivalent diameters of oxides is 5.00 μm or less, and a tensile strength is 980 MPa or more.

Inventors

• Mutsumi YOSHITAKE
• Hiroshi Shuto
• Eisaku Sakurada
• Jun Ando
• Toshiki Sugiyama

Assignees

• NIPPON STEEL CORPORATION

Dates

Publication Date

20260507

Application Date

20231012

Priority Date

20221012

Claims (2)

1. 1 . A hot-rolled steel sheet comprising, in terms of mass %, as a chemical composition: C: 0.050% to 0.250%; Si: 0.05% to 3.00%; Mn: 1.00% to 4.00%; Ni: 0.02% to 2.00%; sol. Al: 0.001% to 2.000%; P: 0.100% or less; S: 0.0300% or less; N: 0.1000% or less; 0: 0.0100% or less; Ti: 0% to 0.500%; Nb: 0% to 0.500%; V: 0% to 0.500%; Cu: 0% to 2.00%; Cr: 0% to 20.00%; Mo: 0% to 1.00%; B: 0% to 0.0100%; Ca: 0% to 0.0200%; Mg: 0% to 0.0200%; REM: 0% to 0.1000%; Bi: 0% to 0.0200%; As: 0% to 0.100%; Zr: 0% to 1.00%; Co: 0% to 1.00%; Zn: 0% to 1.00%; W: 0% to 1.00%; Sn: 0% to 0.05%; a remainder comprising Fe and impurities, and the following formulas (A) and (B) are satisfied, wherein, a microstructure at a position of ¼ from a surface in a sheet thickness direction having, in terms of area %, residual austenite at less than 3.0%, ferrite at 15.0% or more and less than 60.0%, and pearlite at less than 5.0%, an Entropy value indicated by the following formula (1) is 10.7 or more, an Inverse difference normalized value indicated by the following formula (2) is 1.020 or more, and a Cluster Shade value indicated by the following formula (3) is −8.0×10 5 to 8.0×10 5 , which are obtained by analyzing SEM images of the microstructure with a gray level co-occurrence matrices method, a standard deviation of Mn concentrations is 0.60 mass % or less, at the surface, an area ratio of a region where a Ni concentration is 0.2 mass % or more is 10.0% or more, an area ratio of a region where an O concentration is 3.0 mass % or more is 3.0% to 50.0%, a maximum value of sphere equivalent diameters of oxides is 5.00 μm or less, and a tensile strength is 980 MPa or more, 0.06 % ≤ Ti + Nb + V ≤ 0.5 % , ( A ) Zr + Co + Zn + W ≤ 1. % , ( B ) here, each element symbol in the formulas (A) and (B) indicates the content of the element in terms of mass %, and 0% is substituted when the element is not contained, P(i,j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, L in the following formula (2) is possible Quantization levels of grayscale of the SEM images, i and j in the following formulas (2) and (3) are natural numbers from 1 to the L, p x and p y in the following formula (3) are indicated by the in the following formulas (4) and (5), [ Formula ⁢ 1 ] Entropy = - ∑ i ∑ j P ⁡ ( i , j ) ⁢ log ⁡ ( P ⁡ ( i , j ) ) ( 1 ) [ Formula ⁢ 2 ] Inverse ⁢ difference ⁢ normalized = ∑ i ∑ j P ⁡ ( i , j ) 1 + ❘ "\[LeftBracketingBar]" i + j ❘ "\[RightBracketingBar]" L ( 2 ) [ Formula ⁢ 3 ] Cluster ⁢ Shade = ∑ i ∑ j ( i + j - μ x - μ y ) 3 ⁢ P ⁡ ( i , j ) ( 3 ) [ Formula ⁢ 4 ] μ x = ∑ i ∑ j i ⁡ ( P ⁡ ( i , j ) ) ( 4 ) [ Formula ⁢ 5 ] μ y = ∑ i ∑ j j ⁡ ( P ⁢ ( i , j ) ) . _ ( 5 )
2. 2 . The hot-rolled steel sheet according to claim 1 , wherein the chemical composition comprises, in terms of mass %, one or more of: Ti: 0.001% to 0.500%; Nb: 0.001% to 0.500%; V: 0.001% to 0.500%; Cu: 0.01% to 2.00%; Cr: 0.01% to 2.00%; Mo: 0.01% to 1.00%; B: 0.0001% to 0.0100%; Ca: 0.0005% to 0.0200%; Mg: 0.0005% to 0.0200%; REM: 0.0005% to 0.1000%; Bi: 0.0005% to 0.0200%; As: 0.0010% to 0.100%; Zr: 0.010% to 1.00%; Co: 0.010% to 1.00%; Zn: 0.01% to 1.00%; W: 0.01% to 1.00%; and or Sn: 0.01% to 0.05%.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a hot-rolled steel sheet. Priority is claimed on Japanese Patent Application No. 2022-163955, filed on Oct. 12, 2022, the content of which is incorporated herein by reference. BACKGROUND ART In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Vehicle manufacturers are also actively developing techniques for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on improvement in collision resistance to secure the safety of the occupants. In order to achieve both vehicle body weight reduction and collision resistance, an investigation has been conducted to make a member thin by using a high-strength steel sheet. Therefore, there is a strong demand for a steel sheet having both high strength and excellent formability. In order to meet this demand, several techniques have been conventionally proposed. Since there are various working methods for vehicle members, the required formability differs depending on members to which the working methods are applied. Among these, sheet thickness reduction at critical fracture and ductility is placed as important indices for formability. The sheet thickness reduction at critical fracture is a value calculated from a sheet thickness of a tensile test piece before fracture and a minimum value of a sheet thickness of the tensile test piece after fracture. When the sheet thickness reduction at critical fracture is low, it is not preferable since early fracture become to be easily occurred when tensile strain is applied during press forming. Vehicle members are formed by press forming, and the press-formed blank sheet is often manufactured by highly productive shearing working. A blank sheet manufactured by shearing working needs to be excellent in terms of the end surface accuracy after shearing working. For example, when a secondary sheared surface consisting of a sheared surface, a fractured surface, and a sheared surface is occurred in the appearance of the end surface after shearing working (sheared end surface), the accuracy of the sheared end surface significantly deteriorates. The higher the strength of hot-rolled steel sheet, the more likely crack occurs from the bent inner during bending (hereinafter referred to as bent inner cracking). The mechanism of bent inner cracking is presumed to be as follows. A compressive stress is generated in the bent inner during bending. At first, the entire bent inner is uniformly deformed as the bending proceeds, but as the amount of deformation increases, the deformation cannot be borne by the uniform deformation alone, and strain is concentrated locally, causing the deformation to proceed (occurrence of a shear deformation band). By further growing the shear deformation band, cracking is occurred along the shear band from the surface of the bent inner and grows. The reason why the bent inner cracking are likely to occur as the strength is increased is presumably because the decrease of work hardenability associated with the increase in strength makes it difficult for uniform deformation to proceed, making it easier for deformation to become uneven, leading to the formation of the shear deformation band early in processing (or under loose processing conditions). There are various working method for vehicle members as described above, since a steel sheet is often subjected to bending, it is not preferable to occur bent inner cracking during bending. In addition, there is a case where a chemical conversion film is formed on a surface of a steel sheet for the purpose of such as improving corrosion resistance. When a Si content in a steel sheet is increased in order to increase the strength, oxides containing Si are likely to be generated and remain on a surface layer of the steel sheet, which may deteriorate a chemical convertibility of the steel sheet and prevent a sufficient formation of the chemical conversion film. For example, Patent Document 1 discloses a hot-rolled steel sheet that can be used as a raw material for cold-rolled steel sheet with excellent surface properties after press forming, in which the degrees of Mn segregation and P segregation in the center part of sheet thickness are controlled. PRIOR ART DOCUMENT Patent Document Patent Document 1: WO2020/044445 Non-Patent Document Non-Patent Document 1: J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596Non-Patent Document 2: D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546Non-Patent Document 3: K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485 SUMMARY OF INVENTION Technical Problem However, in Patent Document 1, a she