Hot stamping is a process by which advanced high strength steel is formed into complex shapes more efficiently than cold stamping. The process involves heating of the steel blanks until they are malleable, followed by formation and then rapid cooling in specially designed dies, creating in the process a transformed and hardened material.
Because of this ability to combine strength and complexity, press hardened parts accomplish in one relatively lightweight piece what would typically require thicker, heavier parts welded together in more than 1 process under cold stamping. The process is such that blanks are cut from a foil of steel. The patch-welded blanks are made to pass through the input feeder rollers with the help of pick and place robots into the furnace.
In the furnace, the blanks are heated up to a temperature of 940 degree Celsius. On passing through the furnace, the blanks come out red hot in a malleable state. This facilitates the shaping of blanks when made to pass through a die, beyond the realm of cold stamping. A blank with a hardness of 500 MPa, after hot stamping, returns with 1500 MPa hardness. The imported raw material (coated steel) used here includes boron, which gives it the hardness over materials used in conventional cold stamping processes.
In the die, cold water through a channel in the punch ensures punching in a controlled manner which in turn contributes to the hardness. At the output, the BIW parts manufactured include door impact beams, A-pillars, bottom and front rails and B-pillars among other parts that constitute the under-body and side-body parts. Loire Gestamp, a company from the Group specialising in the design and manufacture of press lines, has installed in the new plant a hot stamping line with a 1,200 tonne press, incorporating automatic connections for changing tools, thermal imagers and a monitoring system. At the last stage, unlike cold stamp parts, further processes like trimming, coldpiercing is possible only through lasers making it a crucial addition to the hot stamping lines. A rotating type fibre laser, 6-axis laser cutting machine is used for these.
In the quality checking process each fixture is checked for zero disparity before declaring the part as approved. It follows the first of acceptance, middle of acceptance and last of acceptance from an entire batch of product like in any standard manufacturing process. The hot stamped parts are later merged with cold stamped parts to make an assembly for supply to the customer.