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US-12624434-B2 - Hot-rolled steel sheet

US12624434B2US 12624434 B2US12624434 B2US 12624434B2US-12624434-B2

Abstract

A hot-rolled steel sheet has a predetermined chemical composition, a microstructure at a location of ¼ of a sheet thickness and at a location of 100 μm from a surface comprising, in area %, one or more of martensite and tempered martensite: 95% or more in total, and ferrite, bainite and pearlite: 5% or less in total, and an average dislocation density at the location of 100 μm from the surface is 1.2 times or more of an average dislocation density at the location of ¼ of the sheet thickness.

Inventors

  • Daisuke Ito
  • Akifumi SAKAKIBARA
  • Teruki Hayashida
  • Tatsuo Yokoi
  • Kazuhisa Kusumi

Assignees

  • NIPPON STEEL CORPORATION

Dates

Publication Date
20260512
Application Date
20221222
Priority Date
20220107

Claims (7)

  1. 1 . A hot-rolled steel sheet comprising, as a chemical composition, in % by mass, C: 0.050 to 0.150%, Si: 0.01 to 1.00%, Mn: 1.00 to 2.50%, P: 0.020% or less, S: 0.005% or less, N: 0.0050% or less, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, B: 0.0005 to 0.0050%, Nb: 0 to 0.100%, Cr: 0 to 1.00%, V: 0 to 0.30%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, and Ca: 0 to 0.0050%, wherein the remainder comprising Fe and impurities, Vc represented by the following Expressions (1) to (3) is 10 to 40, a microstructure at a location of ¼ of a sheet thickness and at a location of 100 μm from a surface comprises, in area %, one or more of martensite and tempered martensite: 95% or more in total, and ferrite, bainite and pearlite: 5% or less in total, an average dislocation density at the location of 100 μm from the surface is 1.2 times or more of an average dislocation density at the location of ¼ of the sheet thickness: when an effective amount of B≥0.0005% by mass is satisfied, Vc = 10 2.94 - 0.75 × ( 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr ) when an effective amount of B<0.0005% by mass is satisfied, Vc = 10 3.69 - 0.75 × ( 2.7 × C + 0.4 × Si + Mn + 0.45 × Ni + 0.8 × Cr ) , ( 1 ) an ⁢ effective ⁢ amount ⁢ of ⁢ B = 10.81 × ( B / 10.81 - solid ⁢ solution ⁢ N ⁢ amount / 14.01 ) , ( 2 ) and solid ⁢ solution ⁢ N ⁢ amount = 14.01 × ( N / 14.01 - Ti / 47.88 ) , ( 3 ) here, each element symbol in the above Expression (1) is the content of the element in % by mass, and when the element is not contained, 0 is substituted, B in the above Expression (2) is a B content in % by mass, when the effective amount of B is a negative value, the effective amount of B is set to 0, N and Ti in the above Expression (3) are their contents in % by mass, and when the solid solution N amount is a negative value, the solid solution N amount is 0.
  2. 2 . The hot-rolled steel sheet according to claim 1 , wherein the microstructure at the location of ¼ of the sheet thickness has, an average aspect ratio of prior austenite grains of 1.0 to 4.0.
  3. 3 . The hot-rolled steel sheet according to claim 1 , wherein the microstructure at the location of 100 μm from the surface has, a standard deviation of Vickers hardness of 15 Hv or less.
  4. 4 . The hot-rolled steel sheet according to claim 1 comprising, as the chemical composition, in % by mass, one or more of, Nb: 0.005 to 0.100%, Cr: 0.005 to 1.00%, V: 0.005 to 0.30%, Cu: 0.005 to 0.30%, Ni: 0.005 to 0.30%, and Ca: 0.0010 to 0.0050%.
  5. 5 . The hot-rolled steel sheet according to claim 4 , wherein the microstructure at the location of ¼ of the sheet thickness has, an average aspect ratio of prior austenite grains of 1.0 to 4.0.
  6. 6 . The hot-rolled steel sheet according to claim 1 , wherein the microstructure at the location of ¼ of the sheet thickness has, an average grain size of prior austenite grains of 5 to 40 μm.
  7. 7 . The hot-rolled steel sheet according to claim 6 , wherein the microstructure at the location of 100 μm from the surface has, a standard deviation of Vickers hardness of 15 Hv or less.

Description

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet. Priority is claimed on Japanese Patent Application No. 2022-1416, filed Jan. 7, 2022, the content of which is incorporated herein by reference. BACKGROUND ART In recent years, the strength of steel sheets has been increasing to ensure the collision safety of automobiles and reduce the environmental load. In order to increase the strength of steel sheets, making microstructures have a martensite single phase is effective. Steel sheets in which microstructures have a martensite single phase have poor fatigue limit compared to composite structure steel sheets such as dual phase (DP) steel sheets and transformation induced plasticity (TRIP) steel sheets. For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet which has a martensite phase or tempered martensite phase as a main phase, has a structure in which a volume ratio of the main phase with respect to the entire structure is 90% or more, an average grain size of prior austenite grains is 20 μm or less in a cross section parallel to a rolling direction and 15 μm or less in a cross section perpendicular to the rolling direction, and an aspect ratio of the prior austenite grains in the cross section parallel to the rolling direction is 18 or less, and has excellent low temperature toughness. Patent Document 2 discloses a high-strength hot-rolled steel sheet in which a steel structure is composed of at least one of a martensite phase and a tempered martensite phase, which has a main phase in which an area ratio with respect to the entire steel structure is 95% or more, which contains cementite with an average grain size of 0.5 μm or less in a lath of the martensite phase and/or the tempered martensite phase, and in which the content of cementite is, in % by mass, 0.01 to 0.08%. CITATION LIST Patent Document Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2016-211073Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2018-188675 SUMMARY OF INVENTION Technical Problem The present inventors have found that the steel sheets described in Patent Documents 1 and 2 do not provide sufficient bendability and fatigue limit. An object of the present invention which has been made in view of the above circumstances is to provide a hot-rolled steel sheet having high strength and fatigue limit, and excellent bendability. Means for Solving the Problem The present inventors obtained the following findings as results of creative research and conceived the present invention. It was found that a hot-rolled steel sheet having high strength and fatigue limit, and excellent bendability can be obtained by setting martensite and tempered martensite are a main phase in a microstructure at a location of ¼ of a sheet thickness and at a location of 100 μm from a surface, and setting an average dislocation density at the location of 100 μm from the surface to 1.2 times or more of an average dislocation density at the location of ¼ of the sheet thickness. Also, the present inventors have found that strictly controlling chemical composition, finish rolling conditions and cooling conditions after finish rolling are particularly effective in order to obtaining the hot-rolled steel sheet. The gist of the present invention made on the basis of the above findings is as follows. (1) A hot-rolled steel sheet according to an aspect of the present invention comprising, as a chemical composition, in % by mass,C: 0.050 to 0.150%,Si: 0.01 to 1.00%,Mn: 1.00 to 2.50%,P: 0.020% or less,S: 0.005% or less,N: 0.0050% or less,Al: 0.001 to 0.100%,Ti: 0.001 to 0.100%,B: 0.0005 to 0.0050%,Nb: 0 to 0.100%,Cr: 0 to 1.00%,V: 0 to 0.30%,Cu: 0 to 0.30%,Ni: 0 to 0.30%, andCa: 0 to 0.0050%,wherein the remainder comprising Fe and impurities,Vc represented by the following Expressions (1) to (3) is 10 to 40,a microstructure at a location of ¼ of a sheet thickness and at a location of 100 μm from a surface comprises, in area %,one or more of martensite and tempered martensite: 95% or more in total, andferrite, bainite and pearlite: 5% or less in total,an average dislocation density at the location of 100 μm from the surface is 1.2 times or more of an average dislocation density at the location of ¼ of the sheet thickness:when an effective amount of B≥0.0005% by mass is satisfied, Vc=102.94-0.75×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr)when an effective amount of B<0.0005% by mass is satisfied, Vc=103.69-0.75×(2.7×C+0.4×Si+Mn+0.45×Ni+0.8×Cr),(1)an effective amount of B=10.81×(B/10.81−solid solution N amount/14.01) (2), and solid⁢ solution⁢ N⁢ amount=14.01×(N/14.01-Ti/47.88),(3)here, each element symbol in the above Expression (1) is the content of the element in % by mass, and when the element is not contained, 0 is substituted,B in the above Expression (2) is a B content in % by mass, when the effective amount of B is a negative value, the effective amount of B is set t