Experience in Preventing Coil Break Formation in Hot Rolled Coil, PT Krakatau Steel

摘要

Steel demand for automotive application in Indonesia increases every year. Along with this, requirement of better surface quality become higher. Improving surface quality of steel requires elimination of all surface defects. One common type of defect which often occurs is coil break. Coil break is associated with yield point elongation (YPE) and spike yield phenomenon caused by interstitial atom, primarily nitrogen in steel solid solution. This work focused on how to avoid coil break formation in hot rolled steel products, especially low carbon steels with typical carbon and manganese content of 0.06% and 0.3% respectively. Process parameters studied were temperature and residence time of reheating furnace, finishing temperature, cooling temperature, and cooling rate of laminar cooling section.The investigation indicates that a higher reheating furnace temperature triggers formation of severe coil break, conform to recent studies. Increasing cooling rate of laminar cooling lead to softer coil breaks. These results are closely linked to the decreasing of free nitrogen interstitial atoms in steel solid solution and decreasing of ability of homogenously distributed nitrogen interstitial atoms to inhibit dislocation movements.