| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for manufacturing alloy steel having high dimensional accuracy and impact resistance. SOLUTION: The method for manufacturing the alloy steel comprises: (1) a heating step of heating the steel alloy until the center of the alloy steel reaches a quenching initiation temperature Ta; (2) the first cooling step of cooling the alloy steel until the center of the alloy steel reaches a temperature T<SB>1F</SB>at such a cooling rate (C) as to make the center of the alloy steel cooled at the average cooling rate CR<SB>1</SB>which does not make ferrite and pearlite precipitate in the first cooling step; (3) the second cooling step of cooling the alloy steel until the center of the alloy steel reaches a temperature T<SB>2F</SB>which is higher than a martensite-transformation-starting temperature and a bainite-transformation-starting temperature in such a condition as a coefficient H<SB>2</SB>of heat transfer in the second cooling step is higher than a coefficient H<SB>1</SB>of heat transfer in the first cooling step; and (4) the third cooling step of cooling the alloy steel until the center of the alloy steel reaches a temperature T<SB>3F</SB>in such a condition as a coefficient H<SB>3</SB>of heat transfer in the third cooling step is higher than the coefficient H<SB>2</SB>of heat transfer. COPYRIGHT: (C)2008,JPO&INPIT
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