Kobe Steel, Ltd.

Shinji Kozuma

Yoichi Mukai

Masaaki Miura

Yoshinobu Omiya

Masahiro Nomura

Hiroshi Akamizu

Muneaki Ikeda

Koichi Sugimoto

Fumio Yuse

Junichiro Kinugasa

Kouji Kasuya

Ikuro Hashimoto

A cold rolled steel sheet satisfying on the basis of percent by mass the chemical composition of 0.06-0.6% C, 0.1-2% Si, 0.01-3% Al, 1-4% Si+Al, 1-6% Mn, Si/Mn&#x2266;0.40, in which there exists 10 or more pieces/100 &#x3bc;mof Mn&#x2014;Si composite oxide having Mn&#x2014;Si atom ratio (Mn/Si) of 0.5 or over and major axis of from 0.01 &#x3bc;m to 5 &#x3bc;m and also having a covering ratio of 10% or below at which the surface of the steel sheet is covered with oxide containing Si as the main component.

High-strength cold-rolled steel sheet excellent in coating adhesion, workability and hydrogen embrittlement resistance, and steel component for automobile

The invention provides a high strength cold rolled steel sheet having excellent chemical conversion treatment property stably even Mo is added aiming high strengthening. The surface property of the cold rolled steel sheet satisfies that the characteristic of 10 &#x3bc;m or more of the maximum depth (Ry) of the unevenness and 30 &#x3bc;m or less of the average spacing (Sm) of the unevenness, and that either one or more preferably both of, the characteristic of the load length ratio (tp40) of the unevenness of the surface is 20% or less, and the characteristic of the difference of the load length ratios (tp60) and (tp40) is 60% or more, is satisfied, and the crack of 3 &#x3bc;m or less width and 5 &#x3bc;m or more depth does not exist on the surface.

High-strength cold rolled steel sheet excelling in chemical treatability

There is provided a high-strength hot rolled steel sheet excellent in phosphatability, wherein a maximum depth (Ry) of pits and bumps, existing on a surface thereof, is not less than 10 &#x3bc;m, and an average interval (Sm) of the pits and the bumps is not more than 30 &#x3bc;m, meeting either a requirement for a load length ratio (tp40) of the pits and the bumps on the surface at not more than 20%, or a requirement for a difference between a load length ratio (tp60) and the load length ratio (tp40), at not less than 60%, or both thereof. The high-strength hot rolled steel sheet is capable of exhibiting stable and excellent phosphatability even if Mo highly effective for reinforcement in strength is added thereto in expectation of a higher strength.

High-strength hot-rolled steel sheet excellent in chemical treatability

The invention relates to an ultrahigh-strength thin steel sheet excellent in the hydrogen embrittlement resistance, the steel sheet including, by weight %, 0.10 to 0.60% of C, 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% or less of P, 0.02% or less of S, 1.5% or less of Al, 0.003 to 2.0% of Cr, and a balance including iron and inevitable impurities; in which grains of residual austenite have an average axis ratio (major axis/minor axis) of 5 or more, the grains of the residual austenite have an average minor axis length of 1 &#x3bc;m or less, and the grains of the residual austenite have a nearest-neighbor distance between the grains of 1 &#x3bc;m or less.

Ultrahigh-strength thin steel sheet

A high-strength cold-rolled steel sheet is disclosed that comprises a steel including C: 0.05 to 0.13 mass %, Si: 0.5 to 2.5 mass %, and Mn: 0.5 to 3.5 mass %, as well as Mo: 0.05 to 0.6 mass % and/or Cr: 0.05 to 1.0 mass %. The steel sheet is of composite structure of a ferrite+a second phase wherein the second phase has an area rate of 30 to 70% and is combined approximately in a shape of a network; a circle-equivalent average ferrite grain size is not more than 10 &#x3bc;m; and a circle-equivalent diameter of ferrite grain aggregate that exists continuously in an area surrounded by the second phase is not more than 3 times of the average ferrite grain size. The steel sheet has a high-strength and satisfies a balance of elongation and stretch flange formability (ratio of hole expansion) at a higher level.

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Kakogawa, Japan

Shinji Kozuma