A METHOD FOR PRODUCING HIGH STRENGTH STEELS HAVING EXCELLENT FORMABILITY AND HIGH IMPACT ENERGY ABSORPTION PROPERTIES

摘要

The object of the present invention is to provide high-strength steel sheets exhibiting high impact energy absorption properties, as steel sheets to be used for shaping and working into such parts to front side members which absorb impact energy upon collision, as well as a method for their production. The high-strength steel sheets of the invention which exhibit high impact energy absorption properties are press formable high-strength steel sheets with high flow stress during dynamic deformation characterized in that the microstructure of the steel sheets in their final form is a composite microstructure of a mixture of ferrite and/or bainite, either of which is the dominant phase, and a third phase including retained austenite at a volume fraction between 3% and 50%, wherein the difference between the static tensile strength Ãs when deformed in a strain rate range of 5 x 10 -4 ~ 5 x 10 -3 (1/s) after pre-deformation at an equivalent strain of greater than 0% and less than or equal to 10%, and the dynamic tensile strength Ãd when deformed at a strain rate of 5 x 10 7 ~ 5 x 10 3 (1/sec) after the pre-deformation, i.e. Ãd - Ãs, is at least 60 MPa, and the work hardening coefficient between 5% and 10% of a strain is at least 0.130.