US-12624408-B2 - High-strength cold-rolled steel sheet, high-strength coated or plated steel sheet, method of producing high-strength cold-rolled steel sheet, method of producing high-strength coated or plated steel sheet, and automotive part

US12624408B2US 12624408 B2US12624408 B2US 12624408B2US-12624408-B2

Abstract

A high-strength cold-rolled steel sheet comprises: a chemical composition that contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1; and a steel microstructure in which: an area fraction of ferrite is 30% or more and 60% or less; a total area fraction of tempered martensite and bainite is 35% or more and 65% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 40% or less; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.

Inventors

Yuji Tanaka
Masaki Tada
Yuki Toji
Shinsuke KOMINE
Yoshie OBATA
Yusuke Kimata

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260512
Application Date: 20210805
Priority Date: 20201013

Claims (2)

1 . A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more as measured in accordance with JIS Z 2201, comprising: a chemical composition that consists of, in mass %, C: 0.06% or more and 0.15% or less, Si: 0.10% or more and 1.8% or less, Mn: 2.00% or more and 3.50% or less, P: 0.050% or less, S: 0.0050% or less, N: 0.0060% or less, Al: 0.010% or more and 1.0% or less, Ti: 0.005% or more and 0.075% or less, Nb: 0.005% or more and 0.075% or less, and B: 0.0002% or more and 0.0040% or less, and optionally at least one element selected from the group consisting of: V: 0.200% or less, Cr: 0.20% or less, Mo: 0.20% or less, Cu: 0.30% or less, Ni: 0.30% or less, Sb: 0.100% or less, Sn: 0.100% or less, Ca: 0.0050% or less, Mg: 0.0050% or less, REM: 0.0050% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.0200% or less, and Co: 0.100% or less, with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1, where [mol % N] and [mol % Ti] are respectively a content of N and a content of Ti in steel in mol %; and a steel microstructure in which: an area fraction of ferrite is 30% or more and 60% or less; a total area fraction of tempered martensite and bainite is 35% or more and 65% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 40% or less, where [% Mn] is a content of Mn in the steel in mass %; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.
2 . A high-strength coated or plated steel sheet comprising: the high-strength cold-rolled steel sheet according to claim 1 ; and a coated or plated layer on at least one side of the high-strength cold-rolled steel sheet.

Description

TECHNICAL FIELD The present disclosure relates to a high-strength cold-rolled steel sheet, a high-strength coated or plated steel sheet, a method of producing a high-strength cold-rolled steel sheet, a method of producing a high-strength coated or plated steel sheet, and an automotive part. BACKGROUND High-strength steel sheets are needed in order to achieve both crashworthiness and high fuel efficiency by weight reduction of automobiles. Moreover, automobile steel sheets having excellent ductility, stretch flangeability, and bendability are needed in order to improve formability by press working. JP 2015-193897 A (PTL 1) discloses a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and excellent ductility and bendability. JP 5464302 B2 (PTL 2) discloses a high-strength steel sheet with excellent balance between ductility and stretch flangeability, and a method of producing the same. CITATION LIST Patent Literature PTL 1: JP 2015-193897 APTL 2: JP 5464302 B2 SUMMARY Technical Problem However, stretch flangeability is not taken into account in PTL 1, and bendability is not taken into account in PTL 2. Thus, there is no steel sheet that satisfies all of strength, ductility, stretch flangeability, and bendability. It could therefore be helpful to provide a high-strength cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more and excellent ductility, stretch flangeability, and bendability, and a method of producing the same. Herein, “high strength” means that the tensile strength TS measured in accordance with JIS Z 2201 is 980 MPa or more. Moreover, “excellent elongation” means that the elongation El measured in accordance with JIS Z 2201 is 12% or more. Moreover, “excellent stretch flangeability” means that the hole expansion ratio (λ) measured in accordance with JIS Z 2256, which is an index of stretch flangeability, is 40% or more. Moreover, “excellent bendability” means that the VDA bending angle measured in accordance with the German Association of the Automotive Industry standard VDA 328-100 is 90° or more. Solution to Problem Upon careful examination, we discovered the following: (1) When a steel sheet containing Mn is annealed in the ferrite-austenite dual phase region, distribution of element (Mn distribution) occurs in which the Mn concentration in ferrite phase decreases whereas the Mn concentration in austenite phase increases.(2) When the steel sheet in which the Mn distribution has occurred is cooled at an appropriate cooling rate, austenite newly transforms into ferrite around ferrite low in Mn concentration as nuclei. Since the Mn concentration of the ferrite newly formed as a result of the transformation during cooling maintains the Mn concentration of the austenite before the transformation, the ferrite formed is high in Mn concentration.(3) The ferrite high in Mn concentration is harder than the ferrite low in Mn concentration. This hard ferrite is sandwiched between the soft ferrite low in Mn concentration and the hard bainite or tempered martensite, and thus has the effect of reducing the difference in hardness between soft phase (ferrite high in Mn concentration) and hard phase (bainite or tempered martensite). Consequently, the stretch flangeability of the high-strength cold-rolled steel sheet is improved.(4) Finely dispersing ferrite low in Mn concentration is effective in bendability improvement. The present disclosure is based on these discoveries. We thus provide the following. [1] A high-strength cold-rolled steel sheet comprising: a chemical composition that contains (consists of), in mass %, C: 0.06% or more and 0.15% or less, Si: 0.10% or more and 1.8% or less, Mn: 2.00% or more and 3.50% or less, P: 0.050% or less, S: 0.0050% or less, N: 0.0060% or less, Al: 0.010% or more and 1.0% or less, Ti: 0.005% or more and 0.075% or less, Nb: 0.005% or more and 0.075% or less, and B: 0.0002% or more and 0.0040% or less with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1, where [mol % N] and [mol % Ti] are respectively a content of N and a content of Ti in steel in mol %; and a steel microstructure in which: an area fraction of ferrite is 30% or more and 60% or less; a total area fraction of tempered martensite and bainite is 35% or more and 65% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 40% or less, where [% Mn] is a content of Mn in the steel in mass %; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.[2] The high-strength cold-rolled steel sheet according to [1], wherein the chemical composition further contain