Jfe Steel Corporation

Hidekazu Minami

Takeshi Yokota

Shinjiro Kaneko

Kazuhiro Seto

Yuji Tanaka

Takashi Kobayashi

Yoshimasa Funakawa

Tatsuya Nakagaito

Hiroshi Hasegawa

Yoshiyasu Kawasaki

Shotaro Terashima

Junya Tobata

Seigo Tsuchihashi

Shoji Tanaka

Takeshi Nishiyama

Yuki Toji

Reiko Sugihara

Katsuya Hoshino

Shunsuke Yamamoto

Takako Yamashita

Lingling Yang

Yusuke Wada

Yuki Takeda

Shinsuke Komine

Naoki Takayama

Kazunori Tahara

Kazuma Mori

Fusae Shiimori

Kana Sasaki

Michitoshi Saeki

A steel sheet having a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, high LME resistance, and good weld fatigue properties. The steel sheet has a specific chemical composition and a specific steel microstructure. Crystal grains containing an oxide of Si and/or Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in the range of 3 to 10 μm, the lowest Si concentration Land the lowest Mn concentration Lin the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration Tand a Mn concentration Tat a quarter thickness position of the steel sheet satisfy a specified formula.

Steel sheet, member, and method for producing them

A high strength steel sheet with a chemical composition containing C, Si, Mn, P, S, Al, N, Nb, Ti, and B, where [% Si]/[% Mn] is 0.10 or more and 0.60 or less, and free Ti content determined from Formula (1), below, is 0.001 mass % or more, with the balance being Fe and inevitable impurity. In a steel sheet surface layer, a total amount of Nb solute and 100 nm or smaller precipitate is 0.002 mass % or more.

High strength steel sheet, high strength coated or plated steel sheet, methods of producing same, and member

Provided are a high-strength hot-dip galvanized steel sheet that has a tensile strength of 1180 MPa or more and excellent ductility, stretch flangeability, bendability and LME resistance and can manufacture parts with high dimensional accuracy, and its manufacturing method. The high-strength steel sheet with a tensile strength of 1180 MPa or more has a chemical composition containing C, Si, Mn, P, S, Al and N, with [% Si], [% Mn], [% P], [% Mo] and [% Cr] satisfying a predetermined relationship and the balance being Fe and inevitable impurities, and a steel microstructure including ferrite, tempered martensite and bainite, and retained austenite, where the diffusible hydrogen amount in the steel sheet is 0.60 mass ppm or less, a thickness of softened surface layer is 5 μm or more and 150 μm or less, and a frequency of coincidence boundary in the steel sheet surface layer after a high-temperature tensile test is 0.45 or less.

High-strength steel sheet and method for manufacturing same

There is provided a high-strength steel sheet in which content amounts of a specific steel composition are limited and manufacturing conditions are optimized and that has a steel structure that contains ferrite and a harder phase and in which ferrite accounts for 30% to 70% in terms of area ratio, the harder phase contains bainitic ferrite, tempered martensite, martensite (including 0%), and retained austenite (including 0%), the maximum grain diameter of ferrite dis 2.5 μm or less, and the area ratio of ferrite Aand the average grain diameter of ferrite dsatisfy A[%]/d[βm]≥15.

High-strength steel sheet and method for manufacturing the same

A high-strength steel sheet comprises: a chemical composition containing C, Si, Mn, P, S, Al, N, Mo, Cr, Ca, and Sb with a balance consisting of Fe and inevitable impurities, wherein [% Si], [% Mn], [% P], [% Mo], and [% Cr] satisfy a predetermined relationship; a steel microstructure that contains ferrite, hard phase, and retained austenite and in which a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less; and a tensile strength of 980 MPa or more.

High-strength steel sheet and method for producing same

A high-strength hot-dip galvanized steel sheet includes a hot-dip galvanized layer on a surface of the steel sheet and has a specific component composition and a steel microstructure containing, on an area percentage basis, 90% to 100% of martensite and carbide-containing bainite in total and 0% to 10% of retained austenite, and containing prior austenite grains having an aspect ratio of 2.0 or less, in a region extending from 300 μm to 400 μm from the surface layer, in which the ratio of the average amount of C at 5 μm from the surface layer to the average amount of C at 70 μm from the surface layer is 0.2 to 0.8, and the ratio of the standard deviation of the amount of C to the average amount of C in a region extending from 300 μm to 400 μm from the surface layer is 0.40 or less.

High-strength hot-dip galvanized steel sheet and method for producing the same

A method of producing a hot-rolled steel sheet and a method for producing a cold-rolled full hard steel sheet are provided. The methods comprising heating a steel slab having a composition containing, in terms of mass %, C: 0.010% or more and 0.150% or less, Si: 0.20% or less, Mn: 1.00% or less, P: 0.100% or less, S: 0.0500% or less, Al: 0.001% or more and 0.100% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities, in which 0.002%≤[% P]+[% S]≤0.070% ([% M] denotes a content (mass %) of M element in steel) is satisfied.

Method for producing hot-rolled steel sheet and method for producing cold-rolled full hard steel sheet

A steel sheet is provided, having a tensile strength of 590 MPa or more, a particular composition and a steel structure that contains, in terms of area fraction, particular amounts of ferrite and martensite, in which the ferrite average crystal grain size is 20 μm or less, the martensite average size is 15 μm or less, the ratio of the average crystal grain size of the ferrite to the average size of the martensite (ferrite average crystal grain size/martensite average size) is 0.5 to 10.0, the ratio of the hardness of the martensite to the hardness of the ferrite (martensite hardness/ferrite hardness) is 1.0 or more and 5.0 or less, and, in the texture of the ferrite, the inverse intensity ratio of γ-fiber to the α-fiber is 0.8 or more and 7.0 or less.

Steel sheet, coated steel sheet, method for producing hot-rolled steel sheet, method for producing cold-rolled full hard steel sheet, method for producing heat-treated steel sheet, method for producing steel sheet, and method for producing coated steel sheet

A high-strength steel sheet having a tensile strength of 1,180 MPa or more, and specified chemical composition. The steel sheet includes a steel structure in which an area fraction of martensite having a carbon concentration of more than 0.7×[% C] and less than 1.5×[% C] is 55% or more, an area fraction of tempered martensite having a carbon concentration of 0.7×[% C] or less is 5% or more and 40% or less, a ratio of a carbon concentration in retained austenite to a volume fraction of retained austenite is 0.05 or more and 0.40 or less, and the martensite and the tempered martensite each have an average grain size of 5.3 μm or less, where [% C] represents the content, by mass %, of compositional element C in steel.

A high-strength cold-rolled steel sheet having a high yield ratio and excellent stretch flangeability and a method for manufacturing the steel sheet. The high-strength cold-rolled steel sheet has a chemical composition including, by mass %, C: 0.10 to 0.30%, Si: 0.50 to 2.00%, Mn: 2.5 to 4.0%, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, and B: 0.0003 to 0.0030%, with the balance being Fe and incidental impurities. N and Ti satisfy a specified formula, and the total area fraction of martensite and bainite is 95% or more. The number density of bainite grains having an area of 3 μmor more and a carbon concentration of less than 0.7C is 1200 grains/mmor less.

High-strength cold-rolled steel sheet and method for manufacturing the same

A production method for a high-strength steel sheet having a tensile strength TS of 780 MPa or more is provided. The production method comprises: heating a steel slab having a predetermined chemical composition; hotrolling the steel slab; coiling the hot-rolled sheet; subjecting the hot-rolled sheet to pickling treatment; holding the hot-rolled sheet in a pre-determined temperature range for predetermined time; cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; subjecting the cold-rolled sheet to first annealing treatment; cooling the cold-rolled sheet at a pre-determined average cooling rate; cooling the cold-rolled sheet to room temperature; reheating the clod-rolled sheet to perform second annealing treatment; cooling the cold-rolled sheet at a first average cooling rate; cooling the cold-rolled sheet at a second average cooling rate; reheating the cold-rolled sheet to a predetermined reheating temperature range; and holding the cold-rolled sheet in the reheating temperature range.

Production method for high-strength steel sheet

A high-strength steel sheet has a specific composition and a microstructure. In the microstructure, the area fraction of elongated ferrite phase grains having an aspect ratio of 3 or more is 1% or less, the average crystal grain size of martensite included in a region extending 50 &#x3bc;m from a surface of the steel sheet is 20 &#x3bc;m or less, the content of oxide particles having a minor axis length of 0.8 &#x3bc;m or less in the region extending 50 &#x3bc;m from the surface of the steel sheet is 1.0&#xd7;10particles/mor more, and the content of coarse oxide particles having a minor axis length of more than 1 &#x3bc;m in the region extending 50 &#x3bc;m from the surface of the steel sheet is 1.0&#xd7;10particles/mor less. The content of hydrogen trapped in the steel sheet is 0.05 ppm by mass or more.

High-strength steel sheet and method for producing the same

There is provided a high-strength steel sheet and a method for producing the same. The high-strength steel sheet has a specified chemical composition and a steel microstructure including, by area fraction, 75.0% or more tempered martensite, 1.0% or more and 20.0% or less fresh martensite, and 5.0% or more and 20.0% or less retained austenite. A hardness ratio of the fresh martensite to the tempered martensite is 1.5 or more and 3.0 or less, the ratio of the maximum KAM value in the tempered martensite in the vicinity of the heterophase interface between the tempered martensite and the fresh martensite to the average KAM value in the tempered martensite is 1.5 or more and 30.0 or less, and the average of ratios of grain sizes of prior austenite grains in the rolling direction to those in the thickness direction is 2.0 or less.

There is provided a high-strength steel sheet and a method for producing the same. The steel sheet has a specified chemical composition and a microstructure including, in terms of area percentage, 20.0% or more and 60.0% or less ferrite, 40.0% or more and 80.0% or less of a hard phase composed of bainitic ferrite, tempered martensite, fresh martensite, and retained austenite, 35.0% or more and 55.0% or less bainitic ferrite with respect to the entire hard phase, 20.0% or more and 40.0% or less tempered martensite with respect to the entire hard phase, 3.0% or more and 15.0% or less fresh martensite with respect to the entire hard phase, and 5.0% or more and 20.0% or less retained austenite with respect to the entire hard phase.

Provided is a high-strength steel sheet having a tensile strength (TS) of 750 MPa or more and excellent in ductility and stretch flangeability, in which the steel sheet has a predetermined chemical composition and a microstructure containing, in area ratio, ferrite: 50% to 90%, quenched martensite: 1% to 8%, tempered martensite: 3% to 40%, and retained austenite: 6% to 15%, the quenched martensite has an average grain size of 2.5 &#x3bc;m or less, the quenched martensite has an average circularity index of 0.50 or more, the circularity index being defined as 4 &#x3c0;M/D, where D is a perimeter of the quenched martensite and M is an area of the quenched martensite, and the steel sheet has a ratio of an area ratio of the quenched martensite fto a total area ratio of the quenched martensite and the tempered martensite f, f/f, of 50% or less.

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Tokyo, Japan

Hidekazu Minami