Jfe Steel Corporation

Shinsuke Komine

Kentaro Sato

Tomohiro Sakaidani

Tatsuya Nakagaito

Takayuki Futatsuka

Yoshiyasu Kawasaki

Yuji Tanaka

Masaki Tada

Yuki Toji

Hidekazu Minami

Yusuke Kimata

Yoshie Obata

Seigo Tsuchihashi

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 12% or more and less than 30%; a total area fraction of tempered martensite and bainite is 55% or more and 85% 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 20% 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.

High-strength cold-rolled steel sheet, high-strength coated or plated steel sheet, method of producing high-strength cold-rolled steel sheet, and method of producing high-strength coated or plated steel sheet

A galvanized steel sheet including a steel sheet having a chemical composition with an equivalent carbon content Ceq of 0.60% or more and less than 0.85%, and a specified steel microstructure; and a galvanized layer on a surface of the steel sheet. The retained austenite has a solute C content of 0.6% or more by mass, and retained austenite grains with an aspect ratio of less than 2.0 constitute 50% or more of all retained austenite grains. In 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction, an L cross section in a 0- to 50-μm region from a surface of the steel sheet on a compression side has a number density of voids of 1500/mmor less, and the galvanized steel sheet has a tensile strength of 980 MPa or more.

Galvanized steel sheet, member, and method for producing them

A strain state of a bending outside surface of a test specimen is photographed by use of a camera without falling out of focus during a bending test. A bending test facility for a metal sheet material for an automobile body includes a supporting member that supports two supported portions of a flat test specimen formed of the metal sheet material for an automobile body, a punch for performing a bending test in which an area between the two supported portions of the test specimen is pressed from an opposite side to the supporting member so that the test specimen is bent to be deformed, and a camera for photographing, from the side of the supporting member, a bending outside surface of the test specimen during the bending test using the punch, with relative positions of the punch and the camera during the bending test being fixed.

Bending test facility and bending test method for metal sheet material for automobile body and bending performance evaluation method using the bending test facility

A collision performance evaluation test with few variations in test results with high accuracy in which a complicated phenomenon that actually occurs can be reproduced in a simple manner by considering the history of deformation in both the press forming and a collision. A collision performance evaluation test method for a metal sheet material for an automobile body is characterized in that a press working apparatus first forms a flat test specimen made of a metal sheet material to be tested into a V shape by primary bending, a bending test apparatus then deforms, by secondary bending, the test specimen formed by the primary bending in a direction intersecting with the primary bending direction, and a bending load and a bending stroke for the test specimen during the secondary bending deformation are recorded and evaluated.

Method and facility for collision performance evaluation test for metal sheet material for automobile body

A galvanized steel sheet includes: a steel sheet having a chemical composition satisfying an equivalent carbon content Ceq of 0.35% or more and less than 0.60%, and a specified steel microstructure; and a galvanized layer on a surface of the steel sheet. The retained austenite has a solute C content of 0.6% or more by mass, and retained austenite grains with an aspect ratio of less than 2.0 constitute 50% or more of all retained austenite grains. In 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction, an L cross section in a 0 to 50 μm region from a surface of the steel sheet on a compression side has a number density of voids of 1000/mmor less, and the galvanized steel sheet has a tensile strength of 590 MPa or more.

A steel sheet including a chemical composition satisfying an equivalent carbon content of 0.60% or more and less than 0.85%, and a steel microstructure with an area fraction of ferrite: less than 40%, tempered martensite and bainite: 40% or more in total, retained austenite: 3% to 15%, and ferrite, tempered martensite, bainite, and retained austenite: 93% or more in total. A 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction causes a change of 0.40 or more in (a grain size in a thickness direction)/(a grain size in a direction perpendicular to the thickness) of the tempered martensite in an L cross section in a 0- to 50-μm region from a surface of the steel sheet on a compression side. The steel sheet has a tensile strength of 980 MPa or more.

Steel sheet, member, and method for producing them

The high-strength hot-dip galvanized steel sheet, which includes a hot-dip galvanized coating layer on a surface of the steel sheet, has a component composition containing, in mass %, C: 0.07% to 0.20%, Si: 0.1% to 2.0%, Mn: 2.0% to 3.5%, P: 0.05% or less, S: 0.05% or less, and sol. Al: 0.005% to 0.1%, with the balance being Fe and incidental impurities; and has a steel microstructure containing, in area fraction, 60% or less of ferrite, 40% or more of tempered martensite, and 10% or less of fresh martensite and having a void number density of 1,500/mmor less in a bent portion in the VDA bending test.

High-strength hot-dip galvanized steel sheet and manufacturing method therefor

A high-strength cold-rolled steel sheet or high-strength coated steel sheet that has a tensile strength (TS) of 780 MPa or more and has high ductility, stretch-flangeability, and in-plane stability of stretch-flangeability and methods for producing the same. The high-strength cold-rolled steel sheet has a specified chemical composition and a microstructure comprising, by area fraction, in a range of 50% to 80% of ferrite, 8% or less of martensite with an average grain size of 2.5 &#x3bc;m or less, in a range of 6% to 15% of retained austenite, and in a range of 3% to 40% of tempered martensite. A ratio f/fbeing 50% or less, where fdenotes the area fraction of martensite and fdenotes the total area fraction of martensite and tempered martensite, and a standard deviation of the grain size of martensite at certain portions being 0.7 &#x3bc;m or less.

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Tokyo, Japan

Shinsuke Komine