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US-12624412-B2 - Steel sheet and manufacturing method thereof

US12624412B2US 12624412 B2US12624412 B2US 12624412B2US-12624412-B2

Abstract

This steel sheet has predetermined chemical composition and microstructure, in a crystal orientation distribution function of a texture at a sheet thickness ¼ position, when A/B that is a ratio of a maximum value A of pole densities at Φ=20° to 60° and φ 1 =30° to 90° in a cross section of φ 2 =45° to a maximum value B of pole densities at Φ=120° to 60° and φ 1 =30° to 90° in the cross section of φ 2 =45° is 1.50 or less, a total of the maximum value A and the maximum value B is 6.00 or less, and a tensile strength is 1030 MPa or more.

Inventors

  • Takashi YASUTOMI
  • Eisaku Sakurada

Assignees

  • NIPPON STEEL CORPORATION

Dates

Publication Date
20260512
Application Date
20211119
Priority Date
20210226

Claims (6)

  1. 1 . A steel sheet comprising, as a chemical composition, by mass %: C: 0.030% to 0.180%; Si: 0.030% to 1.400%; Mn: 1.60% to 3.00%; Al: 0.010% to 0.700%; P: 0.0800% or less; S: 0.0100% or less; N: 0.0050% or less; Ti: 0.020% to 0.180%; Nb: 0.010% to 0.050%; Mo: 0% to 0.600%; V: 0% to 0.300%; a total of Ti, Nb, Mo, and V: 0.100% to 1.130%; B: 0% to 0.0030%; Cr: 0% to 0.500%; and a remainder of Fe and impurities, in which a microstructure includes, in terms of area ratio, bainite: 80.0% or more, a total of fresh martensite and tempered martensite: 20.0% or less, and a total of pearlite, ferrite, and austenite: 20.0% or less, in a crystal orientation distribution function of a texture at a sheet thickness ¼ position, A/B that is a ratio of a maximum value A of pole densities at Φ=20° to 60° and φ 1 =30° to 90° in a cross section of φ 2 =45° to a maximum value B of pole densities at Φ=120° to 60° and φ 1 =30° to 90° in the cross section of φ 2 =45° is 1.50 or less, a total of the maximum value A and the maximum value B is 6.00 or less, and a tensile strength is 1030 MPa or more.
  2. 2 . The steel sheet according to claim 1 , wherein a proportion of the area ratio of the tempered martensite in the total of the area ratios of the fresh martensite and the tempered martensite is 80.0% or more.
  3. 3 . The steel sheet according to claim 1 , comprising, as the chemical composition, by mass %, one or more of, as the chemical composition: Mo: 0.001% to 0.600%, V: 0.010% to 0.300%, B: 0.0001% to 0.0030%, and Cr: 0.001% to 0.500%.
  4. 4 . A manufacturing method of the steel sheet according to claim 1 , comprising: a step of holding a slab having the chemical composition according to claim 1 in a temperature range of 1200° C. or higher for 30 minutes or longer; a step of applying a strain of 3% to 15% in a width direction to the slab after the holding; a step of performing finish rolling on the slab to which the strain has been applied so that a final rolling reduction is 24% to 60% and a finishing temperature is in a temperature range of 960° C. to 1060° C.; and a step of cooling the steel sheet after the finish rolling so that an average cooling rate in a temperature range of 900° C. to 650° C. becomes 30° C./second or faster and coiling the steel sheet in a temperature range of 400° C. to 580° C.
  5. 5 . The manufacturing method of the steel sheet according to claim 4 , further comprising: a step of holding the steel sheet after the coiling in a temperature range of 600° C. to 750° C. for 60 to 3010 seconds.
  6. 6 . The steel sheet according to claim 2 , comprising, as the chemical composition, by mass %, one or more of, as the chemical composition: Mo: 0.001% to 0.600%, V: 0.010% to 0.300%, B: 0.0001% to 0.0030%, and Cr: 0.001% to 0.500%.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a steel sheet and a manufacturing method thereof. Priority is claimed on Japanese Patent Application No. 2021-030349, filed Feb. 26, 2021, the content of which is incorporated herein by reference. BACKGROUND ART In recent years, weight reduction of automobiles and machine components has been underway. Designing an optimum shape as the component shape ensures stiffness and thereby makes it possible to reduce the weights of automobiles and machine components. Furthermore, in blank-formed components such as a press-formed component, the weights can be reduced by reducing the sheet thicknesses of component materials. However, in the case of attempting to ensure the strength properties of components such as static fracture strength and yield strength while reducing the sheet thicknesses, it becomes necessary to use high-strength materials. In particular, for automobile suspension components such as lower control arms, trailing arms, and knuckles, studies have begun about the application of higher than 780 MPa-grade steel sheets. These automobile suspension components are manufactured by performing burring, stretch flanging, bending forming, or the like on steel sheets. Therefore, steel sheets that are applied to these automobile suspension components are required to have excellent formability, particularly, excellent hole expansibility. For example, Patent Document 1 discloses a hot-rolled steel sheet in which, in a hot rolling step, the finishing temperature and the rolling reduction are set within predetermined ranges, thereby controlling the grain sizes and aspect ratios of prior austenite and reducing anisotropy. Patent Document 2 discloses a cold-rolled steel sheet in which, in a hot rolling step, the rolling reduction and the average strain rate are set within appropriate ranges in a predetermined finishing temperature range, thereby improving the toughness. In order to further reduce the weights of automobiles, machine components, and the like, it is also expected to apply steel sheets having a sheet thickness premised on a cold-rolled steel sheet to automobile suspension components. The techniques described in Patent Document 1 and Patent Document 2 are effective in the manufacturing of automobile suspension components to which a high strength steel sheet is applied. In particular, these techniques are important findings for obtaining an effect relating to the formability and impact properties of suspension components of automobiles having a complicated shape. However, automobile suspension components always receive cyclic loads attributed to weight-induced vibration, turning, obduction, and the like. Therefore, durability suitable for components is an important property. As described above, suspension components of automobiles are subjected to various formings. In a flat portion near the inside of an R portion that has been bent or bent and bent back, there are many places where the contact with a die is weak. Such a flat portion near the inside of the R portion has surface properties in which relatively sharp concaved parts are periodically formed due to the development of unevenness on the surface layer by forming and contact with a die at a weak load (hereinafter, a change in such surface properties will be referred to as forming damage). In a component including a portion damaged by forming (forming-damaged portion), stress and strain are likely to concentrate, and the component strength decreases. Therefore, for steel sheets that are formed and applied to automobile suspension components, it is required that the occurrence of forming damage can be suppressed. PRIOR ART DOCUMENT Patent Document [Patent Document 1] Japanese Patent No. 5068688[Patent Document 2] Japanese Patent No. 3858146 DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In view of the above-described circumstances, an object of the present invention is to provide a steel sheet having a high strength and excellent hole expansibility and being capable of suppressing the occurrence of forming damage and a manufacturing method thereof. Means for Solving the Problem As a result of original studies, the present inventors found that the occurrence of forming damage correlates with the texture of the surface layer of a steel sheet. The present inventors found that, in the texture of the surface layer of a steel sheet, in a case where the pole density is high and the symmetry is low, forming damage is likely to occur. Particularly, in a steel sheet having a tensile strength of 1030 MPa or more for which precipitation hardening has been used, since recrystallization is unlikely to occur during finish rolling, the pole density is high and the symmetry is low in the texture. The present inventors found that, in the texture of the surface layer of a steel sheet, the occurrence of forming damage can be suppressed by preferably controlling the ratio and total of pol