Kobe Steel, Ltd.

Tomokazu Masuda

Toshio Murakami

Katsura Kajihara

Takehiro Tsuchida

Masaaki Miura

Muneaki Ikeda

Mutsuhisa Nagahama

Masaki Shimamoto

Hideo Hata

Nao Yoshihara

Koichi Sakamoto

Hiroshi Yaguchi

Shogo Murakami

Koichi Akazawa

Motohiro Horiguchi

Hiroshi Oura

Shinsuke Masuda

Akito Suzuki

Takayuki Naito

The present invention provides a steel material, such as a high-strength spring, that has excellent fatigue properties, and, more specifically, a steel material, such as the high-strength spring, that can improve the fatigue properties in a high-strength region more easily, without increasing an alloy cost. The steel material includes, in percent by mass, C: 0.5 to 1.0%, Si: 1.5 to 2.50%, Mn: 0.5 to 1.50%, P: more than 0% to 0.020% or less, S: more than 0% to 0.020% or less, Cr: more than 0% to 0.2% or less, Al: more than 0% to 0.010% or less, N: more than 0% to 0.0070% or less, and O: more than 0% to 0.0040% or less, and the balance consisting of iron and inevitable impurities, wherein Cr and Si contents satisfy a formula of Cr&#xd7;Si&#x2264;0.20, a ratio of tempered martensite in a steel microstructure is 80% or more by area, and a number density of particles of Cr-containing carbide or carbonitride having a circle-equivalent diameter of 50 nm or more in the steel microstructure is 0.10 particles/&#x3bc;mor less.

High-strength steel material having excellent fatigue properties

In a steel sheet having a specific chemical composition and having a microstructure including ferrite that is a soft first phase by 20-50% in terms of the area ratio, the remainder being tempered martensite and/or tempered bainite that is a hard second phase, the microstructure of steel of a surface layer section of the steel sheet from the surface to the depth of 100 &#x3bc;m and a center section of t/4-3t/4 (t is the sheet thickness) is controlled.

High strength cold-rolled steel sheet and manufacturing method therefor

In a high strength cold-rolled steel plate having a specific chemical composition, a soft first phase (ferrite) has an area ratio of 20-50%, the remainder being a hard second phase (tempered martensite and/or tempered bainite), among all the ferrite grains, ferrite grains that have an average grain diameter of 10-25 &#x3bc;m account for a total area ratio of 80% or more, the number of the cementite grains that have an equivalent circle diameter of 0.3 &#x3bc;m or more is more than 0.15 piece and 1.0 piece or less per 1 &#x3bc;mof ferrite, and the tensile strength is 980 MPa or more.

High strength cold-rolled steel sheet exhibiting little variation in strength and ductility, and manufacturing method for same

A high-strength cold-rolled steel sheet has a chemical composition including C of 0.05% to 0.30%, Si of greater than 0% to 3.0%, Mn of 0.1% to 5.0%, P of greater than 0% to 0.1%, S of greater than 0% to 0.02%, Al of 0.01% to 1.0%, and N of greater than 0% to 0.01%, in mass percent, with the remainder including iron and inevitable impurities. The steel sheet has a microstructure containing ferrite as a soft primary phase in an area percentage of 20% to 50% with the remainder including tempered martensite and/or tempered bainite as a hard secondary phase. The ferrite grains are adapted to contain cementite particles having an appropriate size in an appropriate number density.

High-strength cold-rolled steel sheet having small variations in strength and ductility and manufacturing method for the same

Disclosed is a steel for machine structural use including 0.05-0.8% of C, 0.03-2% of Si, 0.2-1.8% of Mn, 0.1-0.5% of Al, 0.0005-0.008% of B, and 0.002-0.015% of N, and including 0.03% of P or less (excluding 0%), 0.03% of S or less (excluding 0%), and 0.002% of O or less (excluding 0%), with the remainder comprising iron and unavoidable impurities. The ratio of BN/AlN precipitated in the steel is 0.020-0.2. Also disclosed is a case hardened steel component in which the ratio of BN/AlN deposited on the carburized or carbonitrided component surface is 0.01 or less and a manufacturing method for same. The steel for machine structural use exhibits excellent machinability in continuous cutting at high speeds using cemented carbide tools, and in interrupted cutting at low speeds using high-speed steel tools, as well as excellent impact performance, even after being subjected to a heat treatment such as quenching and tempering. Furthermore, the case hardened steel component exhibits excellent fatigue resistance, and particularly excellent pitting resistance.

Steel for machine structural use, manufacturing method for same, case hardened steel component, and manufacturing method for same

Provided is a steel for machine structural use which has excellent machinability (particularly, with respect to tool life) for both intermittent cutting with a high-speed steel tool and continuous cutting with a cemented carbide tool while maintaining strength properties required of the steel for machine structural use. Specifically, the steel for machine structural use contains C: 0.05-0.9 mass %, Si: 0.03-2 mass %, Mn: 0.2-1.8 mass %, P: 0.03 mass % or less, S: 0.03 mass % or less, Al: 0.1-0.5 mass %, N: 0.002-0.017 mass %, and O: 0.003 mass % or less, and contains one or more selected from a group consisting of Ti: 0.05 mass % or less (excluding 0 mass %) and B: 0.008 mass % or less (excluding 0 mass %), with the remainder being iron and unavoidable impurities, and satisfies all of the following inequalities (1)-(3) below: (1): [N]&#x2212;0.3[Ti]&#x2212;1.4[B]&#x3c;(0.0004/[Al])&#x2212;0.002; (2): [Ti]&#x2212;[N]/0.3&#x3c;0.005; and (3): [B]&#x2212;([N]&#x2212;0.3[Ti])/1.4&#x3c;0.003 when [Ti]&#x2212;[N]/0.3&#x3c;0 and [B]&#x3c;0.003 when [Ti]&#x2212;[N]/0.3&#x2267;0.

Steel for machine structural use

The present invention provides a steel for machine and structural use which is capable of maintaining mechanical characteristics such as strength by reducing a S content as well as of exhibiting excellent machinability (particularly tool life) in intermittent cutting (such as hobbing) with the high speed tool, and a method useful for producing the steel for machine and structural use. The steel for machine and structural use according to the invention secures 0.002% or more of solute N in the steel and has a chemical composition which is appropriately adjusted and satisfies a relationship of the following expression (1): (0.1&#xd7;[Cr]+[Al])/[O]&#x2267;150 . . . (1), in which [Cr], [Al], and [0] represent a Cr content (mass %), an Al content (mass %), and an O content (mass %), respectively.

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Kobe, Japan

Tomokazu Masuda