Kabushiki Kaisha Kobe Seiko Sho

Yamakawa Industrial Co.

Takuo Hosoda

Kazutoshi Shimogori

Toshio Fukuzuka

Ichiro Kokubo

Yasushi Torii

Hiroyoshi Yamakawa

Kohjiro Kitahata

Tsunahiro Yamakawa

Satoru Nito

Masakatsu Hata

Steel containing, on a weight percent basis, 0.01 to 0.15% of carbon, 0.05 to 0.50% of silicon, 0.20 to 1.0% of manganese, 0.01 to 0.1% of aluminum, 0.3 to 2.0% of copper, 0.1 to 2.0% of nickel, 0.015 to 0.1% of niobium and 0.0005 to 0.0050% of calcium, the balance of its composition being iron and unavoidable impurities, is heated to a temperature of 1100.degree. C. to 1250.degree. C., and hot rolled. The hot-rolled steel is coiled at a temperature of 350.degree. C. to 500.degree. C. to prepare a hot-rolled steel sheet having a tensile strength not exceeding 65 kgf/mm.sup.2. The sheet is cold worked until a working strain of at least 15% is set up. The cold-worked product is heated at a temperature of 400.degree. C. to 550.degree. C. for 0.5 to three hours to yield a part having a tensile strength of at least 80 kgf/mm.sup.2 for an automobile, particularly its automatic transmission system.

Process for manufacturing high-strength parts of an automobile

Disclosed herein is a rolled steel having excellent resistance to sulfide stress corrosion cracking. It comprises 0.20-0.40 wt. % C, .ltoreq.0.50 wt. % Si, .ltoreq.1.90 wt. % Mn, .ltoreq.0.06 wt. % Al, 0.0030-0.0090 wt. % N, 0.005-0.10 wt. % Nb, 0.005-0.050 wt. % Ti, and the balance iron and inevitable impurities. The rolled steel contains at least 4.times.10.sup.8 ultrafine carbonitride particles or 0.1 .mu.m or smaller per mm.sup.3 as a result that a bloom or slab having the above components is soaked, subjected at 1200.degree.-950.degree. C. to rough rolling under conditions of a total reduction percentage of 75% or lower, cooled at a cooling rate of 1.5.degree. C./sec. or higher after completion of the rough rolling and until the initiation of finish rolling, and then completed with its final rolling above the A.sub.1 transformation temperature. A process for producing the above rolled steel is also disclosed.

Method of producing rolled steel having excellent resistance to sulfide

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Akashi, Japan

Takuo Hosoda