Method of directly softening rolled machine structural steels

Details Method of directly softening rolled machine structural steels Method of directly softening rolled machine structural steels

C - Chemistry, Metallurgy

21

D

148/31

C21D 8/00 (2006.01) C21D 1/32 (2006.01) C21D 8/06 (2006.01) C22C 38/32 (2006.01) C22C 38/54 (2006.01)

Patent

CA 1290657

ABSTRACT A method of directly softening a rolled machine structural steel is provided. This method is characterized by that it comprises the steps of: hot rolling a steel conisting essentially of 0.2 - 0.65% C, less than 0.1% Si, 0.2 - 0.5% Mn, 0.0003 - 0.01% B, more than 0.5 - 1.7% Cr, 0.01 - 0.1% Al, all the percentage being on a weight basis, and the balance being Fe and incidental impurities, and may contain one or more other alloying elements selected from either one of or both of the groups (A) and (B), the group (A) consisting of not more than 1% Ni, 0.1 - 0.5% Mo and not more than 1% Cu, and the other group (B) consisting of 0.002 - 0.05% Ti, 0.005 - 0.05% Nb and 0.005 - 0.2% V, then subjecting said rolled steel to either one of the two softening treatments 1) or 2), treatment 1) com- prises slowly cooling the steel in a temperature range until transformation to pearlite is completed at a cooling rate of less than 15°C/min, and the treatment 2) comprises, isothermally holding said steel in a temperature range of 680 to 730 °C until the transformation to pearlite is completed and then to natural cooling, so that the steel can display a tensile strength less than a value expressed by a formula, 24 + 67 x Ceq (kg/mm2), specified by the carbon equivalent Ceq (kg/mm2) of the subject steel.

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