Kobe Steel, Ltd.

Kabushiki Kaisha Kobe Seiko Sho (kobe Steel, Ltd.)

Kabushiki Kaisha Kobe Seiko Sho

Shinshu Tlo Co., Ltd.

Hiroshi Akamizu

Shushi Ikeda

Yoichi Mukai

Koichi Makii

Koichi Sugimoto

Shunichi Hashimoto

Takahiro Kashima

Koh-ichi Sugimoto

Tatsuya Asai

Kenji Saito

Jiro Iwaya

Muneaki Ikeda

Yosuke Shindo

Fumio Yuse

Junichiro Kinugasa

Shinji Kozuma

Masahiro Nomura

Kouji Kasuya

Akihiko Nagasaka

Akinobu Kanda

Ryo Kikuchi

A dual phase steel sheet with good bake-hardening properties is provided. The steel sheet is characterized in containing (in terms of percent by mass) C: no less than 0.06% and less than 0.25%; Si+Al: 0.5 to 3%; Mn: 0.5 to 3%; P: no more than 0.15%; and S: no more than 0.02%; and also meeting the following condition (in terms of space factor) that retained austenite is at least 3%, bainite is at least 30%, and ferrite is no more than 50%, and further characterized in differing in stress larger than 50 MPa before and after application of 2% pre-strain and ensuing heat treatment for paint baking at 170&#xb0; C. for 20 minutes. The steel sheet has well-balanced strength and workability, exhibits good bake-hardening properties at the time of paint baking, and offers good resistance to natural aging.

Dual phase steel sheet with good bake-hardening properties

A high-strength cold-rolled steel sheet excellent in uniform elongation, including in percent by mass:

High-strength cold-rolled steel sheet excellent in uniform elongation and method for manufacturing same

A high-strength cold rolled steel sheet contains: 0.10 to 0.28% of C, 1.0 to 2.0% of Si, 1.0 to 3.0% of Mn, and 0.03 to 0.10% of Nb in terms of % by mass, Al is controlled to 0.5 or less, P is controlled to 0.15% or less, and S is controlled to 0.02% or less, and residual austenite accounts for 5 to 20%, bainitic ferrite accounts for 50% or more, and polygonal ferrite accounts for 30% or less (containing 0%), of the entire structure, and a mean number of residual austenite blocks is 20 or more as determined when the random area (15 &#x3bc;m&#xd7;15 &#x3bc;m) is observed by EBSP (electron back scatter diffraction pattern).

High-strength cold rolled steel sheet having excellent formability, and plated steel sheet

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 forged part is disclosed which comprises a base phase structure, comprising 30% or more of ferrite in terms of a space factor, and a second phase structure, comprising bainite and/or martensite, and retained austenite having an average gain diameter of 5 &#x3bc;m or less and a content represented by 50X[C]&#x3c;[V]&#x3c;150x[C], wherein [V] represents a space factor of the retained austenite (&#x3b3;R) and [C] represents the mass % of C in the forged part. Furthermore, a high-strength forged part is disclosed which comprises a base phase structure, comprising 50% or more of tempered bainite or tempered martensite in terms of a space factor, and a second phase structure, comprising martensite and 3% to 30% retained austenite in terms of a space factor, wherein the portion of the retained austenite and martensite having an aspect ratio of 2 or less is 25% or less in terms of a space factor.

Method for producing high-strength forged parts having high reduction of area

A high-strength cold-rolled steel sheet exhibiting an excellent strength-workability balance, including in percent by mass:

High strength cold rolled steel sheet and plated steel sheet excellent in the balance of strength and workability

A press forming method which can prevent a stretch-flange crack while using conventional press forming equipment at a time of press forming a formed member with flange includes steps of constructing a blank for press forming with a base body blank portion corresponding to flat base body of the formed member, and a concave flange blank portion formed by bending on a concave outer peripheral edge of the base body blank portion. The concave blank portion is constituted by a convex flange blank portion in which a stretch-flange deformation is generated, and an adjacent flange blank portion which is adjacent thereto. An outer peripheral edge of the convex flange blank portion is formed such that one end exists between two points on a profile line corresponding to an outer peripheral edge of the adjacent flange blank portion, the other end exists between two other points, and the one end and the other end are smoothly connected between the straight lines.

Press forming method of formed member with flange

To provide a high strength and low yield ratio cold rolled steel sheet having high elongation property and high flange drawing property, or a plated steel sheet made by plating the same. The high strength and low yield ratio cold rolled steel sheet or the plated steel sheet made by plating the same has such a constitution as 0.10 to 0.25% of C, 1.0 to 2.0% of Si and 1.5 to 3.0% of Mn, are contained in terms of weight percentage, while other elements are controlled such as Al within 0.2%, P within 0.15% and S within 0.02%, with residual austenite occupying at least 5%, bainitic ferrite occupying at least 60% (preferably 80% or more), and polygonal ferrite within 20% (containing 0%), so that a tensile strength is 980 MPa or higher, while an elongation (El in %), a flange drawing property (&#x3bb;in %), a tensile strength (TS in MPa) and a yield strength (YP in MPa) satisfy the following inequality (1): [(El &#xd7;&#x3bb;&#xd7;TS)/YP]&#x2267;645.

High strength and low yield ratio cold rolled steel sheet and method of manufacturing the same

Disclosed is a dual-phase steel sheet having low yield ratio, excellent in the balance for strength-elongation and for strength-stretch flange formability, and also excellent in bake hardening property containing (on the mass % basis).

Dual-phase steel sheet excellent in stretch flange formability and production method thereof

The present invention provide a TRIP-type composite structure steel plate of the TPF steel type in which elongation and stretch flange formability at room temperature are improved by controlling the morphology of the second-phase structure. In a composite structure sheet steel comprising 0.02 to 0.12% C, 0.5 to 2.0% Si+Al and 1.0 to 2.0% Mn, with the remainder being Fe and unavoidable impurities, and comprising 80% or more polygonal ferrite (steel structure space factor) and 1 to 7% retained austenite, with the remainder being bainite and/or martensite, wherein the elongation and stretch flange formability of the composite sheet steel are improved by reducing the number of bulky, massive second phases with an aspect ratio of 1:3 or less and a mean grain size of 0.5 &#x3bc;m or more in the second phase of this composite structure, which comprises retained austenite and martensite.

Composite structure sheet steel with excellent elongation and stretch flange formability

A high-strength forged part is disclosed which comprises a base phase structure, comprising 30% or more of ferrite in terms of a space factor, and a second phase structure, comprising bainite and/or martensite, and retained austenite having an average grain diameter of 5 &#x3bc;m or less and a content represented by 50&#xd7;[C]&#x3c;[V]&#x3c;150&#xd7;[C], wherein [V] represents a space factor of the retained austenite (&#x3b3;R) and [C] represents the mass % of C in the forged part. Furthermore, a high-strength forged part is disclosed which comprises a base phase structure, comprising 50% or more of tempered bainite or tempered martensite in terms of a space factor, and a second phase structure, comprising martensite and 3% to 30% retained austenite in terms of a space factor, wherein the portion of the retained austenite and martensite having an aspect ratio of 2 or less is 25% or less in terms of a space factor.

High-strength forged parts having high reduction of area and method for producing same

A high strength steel sheet having (2-1) a base phase structure, the base phase structure being tempered martensite or tempered bainite and accounting for 50% or more in terms of a space factor relative to the whole structure, or the base phase structure comprising tempered martensite or tempered bainite which accounts for 15% or more in terms of a space factor relative to the whole structure and further comprising ferrite, the tempered martensite or the tempered bainite having a hardness which satisfies the relation of Vickers hardness (Hv)&#x2267;500[C]+30[Si]+3[Mn]+50 where [ ] represents the content (mass %) of each element, and (2-2) a second phase structure comprising retained austenite which accounts for 3 to 30% in terms of a space factor relative to the whole structure and optionally further comprising bainite and/or martensite, the retained austenite having a C concentration (C&#x3b3;R) of 0.8% or more.

High strength steel sheet having excellent formability and method for production thereof

A high-strength steel sheet comprises carbon: 0.06 to 0.25 mass %, Si: 0.5 to 3.5 mass % and Mn: 0.7 to 4 mass %. Its mother structure is ferrite, its second phase structure comprises martensite and the residual austenite and the second phase structure measured by image analysis has an area fraction of 25% or less based on the total structure. The steel sheet satisfies the following requirements (1) to (3): (1) the volume fraction (Vt&#x3b3;hd R) of the residual austenite is 5% or more; (2) the ratio (SF&#x3b3;/Vt&#x3b3;) of the area fraction (SF&#x3b3;) of the residual austenite within ferrite to Vt&#x3b3;is 0.65 or more; and (3) the ratio [&#x3b1;2/(&#x3b1;1+&#x3b3;)] of the space factor (&#x3b1;2) of martensite to the second phase structure (&#x3b1;1+&#x3b3;) is 0.25 to 0.60. The steel sheet has excellent balance between strength and local elongation, and a low yield ratio.

High-strength steel sheet having excellent workability and production process therefor

A hot-dip galvanized steel sheet which is produced from a cold-rolled steel sheet, as a base steel sheet, consisting essentially of C: 0.010-0.06 wt %, Si: no more than 0.5 wt %, Mn: no less than 0.5 wt % and less than 2.0 wt %, P: no more than 0.20 wt %, S: no more than 0.01 wt %, Al: 0.005-0.10 wt %, N: no more than 0.005 wt %, Cr: no more than 1.0 wt %, Mn&plus;1.3Cr: 1.9-2.3 wt %, Fe: remainder, and having a structure composed of ferrite and a second phase containing martensite, said second phase in the structure accounting for no more than 20% in terms of area and martensite in the second phase accounting for no less than 50%, and which has a zinc-plated layer formed on the surface thereof by hot-dip galvanizing or hot-dip galvannealing. A process for production of said hot-dip galvanized steel sheet. This steel sheet has a composite structure containing martensite and yet it has a low strength (no higher than 500 MPa) and also has good strength-ductility balance.

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Kobe, Japan

Hiroshi Akamizu