Improvement of Formability in Cold Rolling of Hot Band for Over 980 MPa Grade High Tensile Strength Steel Sheet

To produce high tensile strength steel sheets in cold rolling with good thickness accuracy, it is important to supply material hot bands without harmful fluctuations in their deformation resistance. For the purpose, it is effective to set a high coiling temperature in the hot rolling stage. On the other hand, it is well known that the surfaces of steel sheets easily form oxide at a high temperature in the case of steel grade which contains Si and Mn in its chemical composition. To avoid the formation of oxide, it is necessary to set a low coiling temperature in the hot rolling stage. By the conventional hot rolling conditions, it is difficult to satisfy conditions both to get uniform formability in cold rolling and to prevent the surface of hot strips from forming internal oxidation zone. This report describes the improvement on the mechanical property without harmful internal oxidation zone of hot strips by the proper heat histories in manufacturing the hot rolled 0.17C-1.3Si-2.0Mn high tensile steel sheet.