Kobe Steel, Ltd.

Kabushiki Kaisha Kobe Seiko Sho

Naoki Mizuta

Tatsuya Asai

Toshio Murakami

Elijah Kakiuchi

Hideo Hata

Takayuki Yamano

Jiro Iwaya

Katsura Kajihara

Takayuki Kimura

Hiroyuki Omori

Tetsuji Hoshika

Noriyuki Jimbo

Takeshi Kojima

Provided is a method for manufacturing a steel member through hot press forming using a steel sheet that contains a comparatively large amount of Si. The steel sheet includes, in mass ratio, C: 0.15% to 0.35%, Si: 1.0% to 3.0%, Mn: 1.0% to 3.0%, Al: more than 0% up to 0.10%, Ti: ([N]&#xd7;48/14)% to 0.10% (where [N] denotes the amount of N in the steel sheet), B: 5 ppm to 50 ppm, P: more than 0% to less than 0.015%, S: more than 0% up to 0.010%, and N: more than 0% up to 0.010%, the balance being iron and unavoidable impurities. The average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 &#x3bc;m in a sheet thickness direction is 0.70 mass % or higher.

Steel plate for hot forming and manufacturing method of hot press formed steel member

In a method for obtaining a hot-press-formed steel member, a steel sheet containing-mass % of C,-mass % of Si,-mass % of Si and Al in total, and-mass % of Mn is heated at a heating temperature of not less than the Actransformation point. The steel sheet is hot-press formed for one or more times. The starting temperature of the hot pressing is not more than the heating temperature but not less than the Ms point. The average cooling rate from (Ms point&#x2212;)&#xb0; C. to&#xb0; C. is&#xb0; C./s or less. The hot-press-formed steel member has high strength, high tensile elongation, high bendability, excellent deformation characteristics at the time of collision crush, and excellent delayed fracture resistance.

Method of manufacturing hot-press-formed steel member

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

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

A press forming method forming an extended part by extension in a later forming period after deep drawing, the deep drawing process carried out with working at 100-250&#xb0; C., and the extension forming process carried out colder at less than 50&#xb0; C., whereby the extended part is formed by extension in a cup shaped low parts formed by deep drawing. Thus, press forming products containing formed elements can be deep drawing formed and extension formed at a high forming rate of 10 mm/sec or greater, which can assure high productivity. By making a steel plate temperature 100-350&#xb0; C. during press forming and by making the forming rate in the later forming period where extension forming is carried out slower than the forming rate in an earlier forming period where extension forming is not carried out, cracking of the extended part can be prevented and press forming limitations can be improved.

Press forming method for steel plate

Provided is a method for manufacturing a hot press formed steel member that has high strengths, an excellent balance between strength and ductility, and good deformation properties upon crush on collision (crashworthiness). The manufacturing method is highly efficient and allows a high degree of freedom with respect to the shape to be formed. The method manufactures a steel member by heating a steel sheet having a specific chemical composition and subjecting the steel sheet to at least one time of hot press forming. In the method, the heating temperature is equal to or higher than the Actransformation temperature, and a starting temperature of the hot press forming is in the range from the heating temperature to martensite start (Ms) temperature. Cooling from [(Ms temperature)&#x2212;150&#xb0; C.] down to 80&#xb0; C. is performed so that a tempering parameter (&#x3bb;) specified by Expression (1) is in the range from 7100 to 8030.

Manufacturing method for hot press formed steel member

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.

Growing community of inventors

Kakogawa, Japan

Naoki Mizuta