Kobe Steel, Ltd.

Kabushiki Kaisha Kobe Seiko Sho

Elijah Kakiuchi

Toshio Murakami

Tatsuya Asai

Naoki Mizuta

Hideo Hata

Katsura Kajihara

Yuichi Futamura

Kosuke Shibata

Shigeo Otani

Tadao Murata

A high-strength steel sheet has a chemical composition including 0.05% to 0.3% of C, 1% to 3% of Si, 0.5% to 3% of Mn, 0% to 0.1% of P, 0.001% to 0.1% of Al, and 0.002% to 0.03% of N, in mass percent; further includes iron and impurities; and has a structure including 50% to 90% of bainitic ferrite, 5% to 20% of retained austenite (&#x3b3;), a total of 10% to 50% of martensite and the retained austenite, and 0% to 40% of polygonal ferrite, in area percent based on the entire structure. The retained austenite has a carbon content (C&#x3b3;) of 0.5% to 1.2% by mass, an average equivalent circle diameter of 0.2 to 2 &#x3bc;m, and an average aspect ratio (maximum diameter/minimum diameter) of less than 3.0. The high-strength steel sheet excels both in elongation and deep drawability while having a strength of 980 MPa or more.

Highly formable high-strength steel sheet, warm working method, and warm-worked automobile part

A high strength cold-rolled steel sheet has a component composition containing specific amounts of C, Si, Mn, P, S, N and Al, respectively with a remainder being iron and inevitable impurities. The steel sheet contains 95% or more of martensite in terms of area ratio, and contains 5% or less (inclusive of 0%) of residual austenite and ferrite in terms of a total area ratio. An average size of a carbide is 60 nm or less in terms of an equivalent circle diameter, and a number density of the carbide having the equivalent circle diameter of 25 nm or more is 5.0&#xd7;10pieces or less per mm. The steel sheet has a yield strength of 1,180 MPa or more and a tensile strength of 1,470 MPa or more.

High-strength cold-rolled steel sheet having excellent bendability

This high-strength steel sheet contains, in mass %, 0.05 to 0.3% of C, 1 to 3% of Si, 0.5 to 3% of Mn, up to 0.1% (inclusive of 0%) of P, up to 0.01% (inclusive of 0%) of S, 0.001 to 0.1% of Al and 0.002 to 0.03% of N with the balance consisting of iron and unavoidable impurities, and has a microstructure which comprises, in area fraction relative to the microstructure, 40 to 85% of bainitic ferrite, 5 to 20% of retained austenite (&#x3b3;), 10 to 50% (in total) of martensite and &#x3b3;, and 5 to 40% of ferrite. The retained austenite (&#x3b3;) has a C concentration of 0.5 to 1.0 mass %, while the quantity of &#x3b3;present in the ferrite grains is 1% or more (in area fraction) relative to the microstructure.

High-strength steel sheet with excellent warm formability and process for manufacturing same

A hot-dip galvanized steel sheet contains specific amounts of C, Si, Mn, P, S, Ti, Al, and N with the remainder being iron and unavoidable impurities. Bainitic ferrite, martensite, retained &#x3b3;, and ferrite (&#x3b1;) are present each in a specific area ratio. The remainder &#x3b3; has a specific C concentration. Sub-grains in the recrystallized &#x3b1; and un-recrystallized &#x3b1; have a specific grain diameter. The surface area ratio of &#x3b1; and worked &#x3b1; having a grain diameter of 5 &#x3bc;m or more is 5% or less. The average particle diameter of TiC particles inside &#x3b1; grains is 10 nm or less.

High-strength hot-dip galvanized steel sheet having excellent yield strength and formability

This high-strength steel plate has a component composition including, by mass %, C: 0.02-0.3%, Si: 1-3%, Mn: 1.8-3%, P: 0.1% or less, S: 0.01% or less, Al: 0.001-0.1%, N: 0.002-0.03%, the rest consisting of iron and impurities. Said steel plate has a microstructure including, in terms of area ratio relative to the entire microstructure, each of the following phases: bainitic ferrite: 50-85%; retained &#x3b3;: 3% or greater; martensite+the aforementioned retained &#x3b3;: 10-45%; and ferrite: 5-40%. The C concentration (C&#x3b3;R) in the aforementioned retained austenite is 0.3-1.2 mass %, part or all of the N in the aforementioned component composition is solid solution N, and the amount of said solid solution N is 30-100 ppm.

High-strength steel sheet having excellent room-temperature formability and warm formability, and warm forming method thereof

This high-strength steel sheet has a component composition containing, in mass %, 0.02 to 0.3% C, 1 to 3% Si, 1.8 to 3% Mn, 0.1% or less P, 0.01% or less S, 0.001 to 0.1% Al, and 0.002 to 0.03% N, the remainder being iron and impurities. The high-strength steel sheet has a structure containing, in terms of area ratio relative to the entire structure, each of the following phases: 50 to 85% bainitic ferrite; 3% or more retained austenite (&#x3b3;); 10 to 45% martensite and the aforementioned retained austenite (&#x3b3;); and 5 to 40% ferrite. The ratio between the Mn concentration (Mn) in the retained austenite (&#x3b3;) and the average Mn concentration (Mn) in the entire structure is 1.2 or more (Mn/ Mn) based on the Mn concentration distribution obtained by means of EPMA line analysis. As a consequence, the high-strength steel sheet exhibits strength of 980 MPa or more and exerts excellent deep drawability.

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Kobe, Japan

Elijah Kakiuchi