| 摘要 |
PURPOSE:To obtain a steel for a drill collar having superior machinability, quenching crack resistance and high workability and capable of improving the yield by adding a specified amount of C and providing a proper Ms point and proper hardenability. CONSTITUTION:This steel for a drill collar having superior quenching crack resistance contains 0.20-0.35% C and has >=310 deg.C Ms point (martensitic transformation start temp.) and hardenability D1 represented by a formula D1>=1.53d [where D1 is the critical diameter (in) calculated by a numerical formula prescribed by ASTM and d is the diameter (in) of a steel bar]. The steel makes a stage for cutting off a quenching-cracked part unnecessary and has high toughness and fatigue strength as well as said characteristics.
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