Abstract
In order to develop a new carburizing steel material that realizes an intermediate heat treatment-free process in parts manufacturing, the cold forgeability of the as-rolled steel and suppression of abnormal grain growth of austenite were studied. It was shown that adjustment of addition amount of Si, Mn and Cr, suppression of dynamic strain aging during cold forging, and an increase of ferrite fraction by controlled rolling contribute to the reduction of load of cold forging. However, Nb precipitation control by fully utilizing mill manufacturing processes was also necessary for suppression of abnormal grain growth of austenite. The as-rolled developed steel showed lower load of cold forging than spheroidized SCM420. The reduction of load of cold forging in the developed steel is due to the synergistic effects of the optimized balance of Si, Mn and Cr, suppression of dynamic strain aging by fixing N, and the increase in the ferrite fraction by low-temperature controlled rolling. Rotating bending fatigue strength is an indicator of resistance to breakage of gears. The rotating bending fatigue strength of the developed steel was excellent in comparison with that of SCM420. From this, the developed steel can prevent breakage of gears. According to the Si mapping by EPMA in the same figure, at the surface of samples after carburizing, grain boundary oxidation was suppressed in the developed steel in comparison with SCM420. A new steel for carburizing was developed by integrating these technologies, making it possible to eliminate annealing before cold forging and normalizing before carburizing simultaneously.