Development of Selective Laser-hardening of Steel Sheet

Abstract

Selective laser-hardening of steel sheet has been developed. By this method, only selected region of steel sheet can be hardened. So, forming process can be followed by hardening process. It is very effective, because steel sheet can be formed while it is ductile and after that the only region required to be hardened can be heat treated. It is necessary to develop two core technologies ; (1) uniformly heating of selected region of steel sheet and (2) realizing the optimized heat cycle for laser-hardening. As for (3), the special integration mirror for shaping a uniform laser beam of 25 mm × 25 mm square has been developed based on the ray-tracing method. Moreover, the combined optical system, which enlarges the uniformly heated area 9 times as large as the case of a uniform heat source, has been designed based on the heat conduction model for laser static heating. As for (2), the optimum heat cycle including soaking for hardening has been determined. And, in order to realize this heat cycle, a control system consisted of a non-contact temperature detector, a PID (proportional integral differential) controller and a laser power regulator, has been developed. Finally, by this selective laser-hardening method using the special integration mirror and the heat cycle control system, hardness of test piece of steel sheet has been increased from 130 to 200 in Vickers hardness.