| 摘要 |
PURPOSE:To obtain an advantageous partially hardened steel plate having a soft part and a hardened part on the steel plate by heating the desired place on the surface of a steel plate with a laser beam to produce the steel plate having different local hardness. CONSTITUTION:The desired place on the surface of a recrystallized soft steel plate is rapidly heated with a laser beam. In this case, the place is heated to a temp. higher than an A1 transformation point (about 720 deg.C) at which an austenitic layer emerges, and quenched to 500 deg.C to obtain a hard transformed structure of martensite, fine pearlite, etc. The hardened part is thus produced. And the quenching is carried out by blowing air, gaseous N2, water, etc. against the heated place. In addition, CO2, N2, Ar, H2, etc. which are weakly oxidative are used as the gas for laser, and also preferably used for cooling. When the partially hardened steel plate is used as the floor of an automobile, formability is ensured by the soft part, and the rigidity in use can be remarkably enhanced by the hardened part. Consequently, further gage-down can be made possible. |
