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Title:
HIGH STRENGTH STEEL FOR MACHINE STRUCTURAL USE, EXCELLENT IN DELAYED FRACTURE CHARACTERISTIC, AND ITS PRODUCTION
Document Type and Number:
Japanese Patent JP3494799
Kind Code:
B2
Abstract:

PROBLEM TO BE SOLVED: To obtain the steel which is composed of martensitic structure and in which the length-to-width ratio of old austenite grains in the region between the surface layer and a position at a specific depth from surface layer is regulated to a specific value or above by regulating the temp. at the time of thread rolling of a bolt steel of specific composition and also regulating the cooling method after thread rolling.
SOLUTION: At the time of applying thread rolling to a steel having a composition consisting of, by weight, 0.15-0.50% C, 0.05-2.0% Si, 0.2-2.0% Mn, 0.005-0.1% Al, and the balance Fe, thread rolling is performed at least at 700-900°C and then water cooling is carried out without delay, by which martensitic strucutre is formed. Successively, the steel is heated to ≥250°C at ≥10°C/sec heating rate and tempered. By this method, the steel for machine structural use, in which the length-to-width ratio (aspect ratio) of old austenite grains in the region between the surface layer and a position at a depth of ≥300μm from the surface layer is regulated to ≥1.2 and which has ≥1300MPa tensile strength and excellent delayed fracture resistance, can be obtained.


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Inventors:
Shingo Yamazaki
Toshizo Tarui
Application Number:
JP7690396A
Publication Date:
February 09, 2004
Filing Date:
March 29, 1996
Export Citation:
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Assignee:
Nippon Steel Corporation
International Classes:
C21D6/00; C21D8/00; C21D9/00; C22C38/00; C22C38/06; C22C38/58; (IPC1-7): C22C38/00; C21D8/00; C21D9/00; C22C38/06; C22C38/58
Domestic Patent References:
JP7300652A
JP2236223A
JP6156268A
Attorney, Agent or Firm:
Tachio Chanogi (1 outside)



 
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