Strengthening mechanism in hot-rolled high strength sheet steel with increase in tensile strength induced by bake hardening

A new type of bake-hardenable high strength hot-rolled sheet steel was developed, which shows remarkable increase in tensile strength (TS) as well as yield strength after strain age treatment such as paint baking to autobody. The new sheet steel possesses excellent crashworthiness and high fatigue strength, and also shows good formability equal to that of conventional high strength sheet steel. Furthermore, it shows good anti-aging property at room temperature. The mutually contradictory requirements of a high bake hardening capacity in TS and anti-aging property at room temperature are satisfied by optimizing the solute N content in the hot rolling process. The increased TS observed after paint baking is due to large work hardening relating with high dislocation density. In the case of developed steel, dislocations generated during prestrain are strongly locked by nitrogen after paint baking. Therefore, multiplication of dislocation encouraged at these locked dislocations to progress plastic deformation, resulting in high dislocation density. The external force required for movement of a dislocation in a dislocation group increases as the dislocation density becomes larger, leading to increase in TS. As the result of TEM investigation, nitrogen forms clusters and/or precipitates on locked dislocations after paint baking.