Local laser softening of cold forming steels

The use of as-rolled cold forming steels opens new potential for lightweight construction in the automotive industry. Cold-rolled grades with a tensile strength up to 1100 MPa are on the market. However, the high strength limits the cold forming performance. A local softening in areas of high degree of deformation can be a solution to this conflict. In this work, basic studies on softening of a sheet metal were conducted. A fiber-coupled diode laser with a maximum laser power of 12 kW and a rectangular laser spot were used for laser heat treatment of the steel grade ZE 1100. It is shown that a local softening can be achieved even with short term heating. The softening is due to partly or full recrystallization of the microstructure. The tensile strength and the yield point can be reduced by about half, and the elongation at break can be increased by a factor of seven.