Sumitomo Metal Industries, Inc.

Nippon Steel Sumitomo Metal Corporation

Toyota Jidosha Kabushiki Kaisha

Toyoda Iron Works Co., Ltd.

Tamotsu Toki

Kaori Kawano

Masayuki Wakita

Norio Imai

Kazuhito Imai

Toshirou Tomida

Nobusato Kojima

Masaru Takahashi

Masahiro Nakata

Masanori Yasuyama

Hitomi Nishibata

Atsushi Tomizawa

Naoaki Shimada

Hiroshi Takebayashi

Mitsuru Yoshida

Kazuya Ishii

Toyomitsu Nakamura

Yukihiro Yoshikawa

Masashi Yamamoto

Kunihiro Fukui

Masanobu Ichikawa

Akira Obayashi

Toshio Nakamori

Shuntaro Sudo

Toshinobu Nishibata

Hiroo Maeda

Mutsuo Sagara

Hirohito Masumoto

A heat treated galvannealed steel material having excellent post-painting corrosion resistance and a high strength which is suitable for use as an automotive part and a method for its manufacture are provided. A galvannealed steel material having a galvannealed coating on at least one side thereof is heat treated by heating at least a portion thereof to a temperature range in which hardening is possible. The coating remaining on the surface of at least a part of the portion which underwent heat treatment has a coating weight of at least 20 g/mand at most 80 g/mper side and an Fe content of at least 15% and at most 35%, an &#x3b7; phase is present in the coating, and the centerline average roughness Ra of the surface of the coating is at most 1.5 &#x3bc;m.

Heat treated galvannealed steel material and a method for its manufacture

A method of manufacturing a bent metal member comprises supporting the outer surface of a tubular metal material having a Zn-Fe alloy coating layer at two locations spaced in the axial direction of the metal material. The metal material is fed in its axial direction and heated at a position between the two locations to a temperature range of at least the Acpoint at a heating speed such that the rate of temperature increase is at least 3.0&#xd7;10&#xb0; C. per second, held so that the time for which the surface of the metal material is at a temperature of 8.0&#xd7;10&#xb0; C. or higher is at most 2 seconds, and then rapid cooled. A bending moment is imparted to the healed portion in two or three dimensions to manufacture a bent metal member having on its surface a Zn-based layer which contains an &#x3b7;, phase.

Bent metal member and a method for its manufacture

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof are provided. The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and said average crystal grain diameter D (&#x3bc;m) satisfy the relation defined by the formula (3) given below:1.2&#x2266;&#x2266;7&#x2003;&#x2003;formula (1)&#x2266;2.7+5000/(5+350&#xb7;C+40&#xb7;Mn)&#x2003;&#x2003;formula (2)&#x2266;0.1&#x2003;&#x2003;formula (3)wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.

Method of producing cold-rolled steel sheet

A cold-rolled steel sheet of carbon steel or low-alloy steel comprising C: 0.01 to 0.25% by mass with a ferrite main phase is has an average ferrite crystal grain diameter D (&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfying formulas (5,6) and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and D (&#x3bc;m) satisfying formula (3):1.2&#x2266;D&#x2266;9.3&#x2003;&#x2003;(5)&#x2266;5.0&#x2212;2.0&#xb7;Cr+5000/(5+350&#xb7;C+40&#xb7;Mn)2&#x2003;&#x2003;(6)&#x2266;0.1&#x2003;&#x2003;(3)and, at said &#xbc; inch depth, the area percentage of ferrite crystal grains the crystal grain diameter d (&#x3bc;m) of which satisfying formula (4) is at least 80%:/3&#x2266;&#x2266;3&#x2003;&#x2003;(4)wherein C, Cr and Mn represent mass % of the respective elements in the steel.

Cold-rolled steel sheet

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances, machine structures, and of constructing buildings, and a manufacturing method thereof is provided. The steel sheet is a hot-rolled steel sheet of a carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D(&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by formulas (1) and (2) given below and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and said average crystal grain diameter D(&#x3bc;m) satisfy the relation defined by the formula (3) given below:1.2&#x2266;D&#x2266;7&#x2003;&#x2003;formula (1)&#x2266;2.7+5000/(5+350&#xb7;C+40&#xb7;Mn)&#x2003;&#x2003;formula (2)&#x2266;0.1&#x2003;&#x2003;formula (3)wherein C and Mn represent the content (in % by mass) of the respective elements in the steel.

Method of producing hot-rolled steel sheet

A steel sheet excellent in mechanical strength, workability and thermal stability and suited for use as a raw material in such fields of manufacturing automobiles, household electric appliances and machine structures and of constructing buildings, and a manufacturing method thereof are provided. The steel sheet is a hot-rolled steel sheet of carbon steel or low-alloy steel, the main phase of which is ferrite, and is characterized in that the average ferrite crystal grain diameter D (&#x3bc;m) at the depth of &#xbc; of the sheet thickness from the steel sheet surface satisfies the relations respectively defined by the formulas (1) and (2) given below and the increase rate X (&#x3bc;m/min) in average ferrite crystal grain diameter at 700&#xb0; C. at the depth of &#xbc; of the sheet thickness from the steel sheet surface and said average crystal diameter D (&#x3bc;m) satisfy the relation defined by the formula (3) given below:1.2&#x2266;D&#x2266;7&#x2003;&#x2003;formula (1)&#x2266;2.7+5000/(5+350&#xb7;C+40&#xb7;Mn)&#x2003;&#x2003;formula (2)&#x2266;0.1&#x2003;&#x2003;formula (3)wherein C and Mn represent the contents (in % by mass) of the respective elements in the steel.

Hot-rolled steel sheet and cold-rolled steel sheet and manufacturing method thereof

Hot press forming of a steel material is made possible by coating the surface of the steel material with a plated layer of zinc or a zinc alloy and forming thereon a barrier layer which prevents the vaporization of zinc even when heated to 700-1000&#xb0; C. Corrosion resistance can also be guaranteed without requiring post treatment, and hot press forming of high tensile steel sheet and stainless steel sheet is made possible. The barrier layer is formed by forming an upper plated layer, by surface oxidation, by contact with an oxidizing agent, by contact with Zn and an oxidizing agent, by anodic electrolysis, by cathodic electrolysis, or by coating with a ZnO sol.

Hot press forming method

A hot press-formed article having improved corrosion resistance and coating adhesion when coated is obtained by hot press forming of a zinc-based plated steel material. It has a zinc-based plating layer comprising an iron-zinc solid solution phase and atop it a zinc oxide layer. The average thickness of the zinc oxide layer which is the outermost layer is at most 2 &#x3bc;m. A hot press-formed article having a good appearance and excellent corrosion resistance, coating adhesion, post-coating corrosion resistance, and weldability has a layer () consisting essentially of an iron-zinc solid solution phase atop a base material steel sheet (). It also has a zinc oxide layer () with an average thickness of at most 5 &#x3bc;m as an uppermost layer, but it does not have a substantial amount of an iron-zinc intermetallic compound phase. The total amount of Al contained in the iron-zinc solid solution layer () and the zinc oxide layer () is at most 0.5 g/m, and/or the total amount of Al oxides contained in these layers is at most 5 mg/m.

Hot press-formed article

A batchwise hot dip plating method is performed by dipping a metallic material in a molten metal plating bath, following fluxing by dipping the metallic material in a bath of a fused salt flux (e.g., a mixture of cryolite and one or more alkali metal chlorides and optionally aluminum fluoride) having a melting temperature at least 5.degree. C. higher than the temperature of the molten metal plating bath, which also serves to preheat the metallic material. In the case of hot dip plating with an Al--Zn alloy, particularly a Zn/55% Al/0.5-2% Si alloy, a bare spot-free plated coating having good appearance can be formed by a reduced duration of dipping in the plating bath without post-plating treatment to remove flux residues. The use of a plating tank having a cross section of a round shape such as a semicircular shape or an oblong semielliptic shape, rather than a rectangular box shape, brings about a significantly extended service life of the plating tank.

Hot dip plating method and apparatus

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Nishinomiya, Japan

Tamotsu Toki