Nippon Steel Corporation

Mutsumi Sakakibara

Hiroshi Shuto

Tatsuo Yokoi

Jun Ando

Teruki Hayashida

Kunio Hayashi

Akihiro Yamamoto

Kaoru Kawasaki

Natsuko Sugiura

Koutarou Hayashi

Akifumi Sakakibara

Kazumasa Tsutsui

Shinsuke Kai

Tetsu Asato

Yamamoto Akihiro

This hot-rolled steel sheet has a predetermined chemical composition, and in a case where the thickness is denoted by t, the metallographic structure at a t/4 position from the surface contains one or both of tempered martensite and lower bainite at a volume percentage of 90% or more, the tensile strength is 980 MPa or more, and the average Ni concentration on the surface is 7.0% or more.

Hot-rolled steel sheet and method for manufacturing same

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, by area %, ferrite is less than 15.0%, residual austenite is less than 3.0%, L/L, which is a ratio of a length Lof a grain boundary having a crystal orientation difference of 52° to a length Lof a grain boundary having a crystal orientation difference of 7° about a <110> direction is 0.10 to 0.18, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

Hot-rolled steel sheet

This hot rolled steel sheet is a hot rolled steel sheet including a base steel sheet and a scale formed on a surface of the base steel sheet, in which the base steel sheet has a predetermined chemical composition, the scale has a layer structure composed of wustite, magnetite, and hematite in order from the base steel sheet side or a layer structure composed of the wustite and the magnetite in order from the base steel sheet side, and, when a thickness of the scale is represented by s, a thickness of the hematite is represented by h, and a thickness of the magnetite is represented by m, the s, the h, and the m satisfy formula (1) and formula (2).()/<0.20  Formula (1)/4  Formula (2)

Hot rolled steel sheet

This hot-rolled steel sheet has a predetermined chemical composition, in which in a case where the thickness is denoted by t, a metallographic structure at a t/4 position from the surface includes, by area fraction, 77.0% to 97.0% of bainite or tempered martensite, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, in the metallographic structure, the average grain size excluding the residual austenite is 7.0 &#x3bc;m or less, the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0&#xd7;10carbides/mmor more, a tensile strength is 980 MPa or more, and an average Ni concentration on the surface is 7.0% or more.

A hot press-formed part according to an aspect of the present invention contains a predetermined chemical composition; in which a microstructure in a thickness &#xbc; portion includes, by unit vol %, tempered martensite: 20% to 90%, bainite: 5% to 75%, and residual austenite: 5% to 25%, and ferrite is limited to 10% or less; and a pole density of an orientation {211}&#x3c;011&#x3e; in the thickness &#xbc; portion is 3.0 or higher.

Hot press-formed part

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Tokyo, Japan

Mutsumi Sakakibara