NEW ULTRA- LOW CARBON HIGH STRENGTH STEELS WITH IMPROVED BAKE HARDENABILITY FOR ENHANCED STRETCH FORMABILITY AND DENT RESISTANCE

摘要

The threat to future legislation regarding fuel consumption and emission has forced the automotive industry to develop lighter, more fuel-efficient vehicles.The two main objectives of this industry are the reduction in car weight and improvement in safety. To achieve this, most of the panels that make up the car bodies prefer high strength materials coupled with better formability. The performances of conventional high strength steels are not sufficient. It has been found out that bake hardenable steel sheets are an excellent solution to the above problem. These steels offer good formability (low yield strength for good shape fixability) during press operations and also higher final yield strength during paint baking operations for good dent resistance.Paint bake hardening is essentially the strain aging increment found after forming and aging for 20 minutes at 180°C. It is commonly assumed in the steel industry that the carbon and nitrogen in solution in the ferrite at the time of paint bake hardening is responsible for the bake hardening strengthening increment observed. However, numerous studies have shown that there is little correlation between the interstitial solute level and the measured paint bake hardening responseIt was tried to develop ultra low carbon steel that can provide a consistent BH increment of 100MPa, while retaining good formability. It is to maximize the paint bake hardening increment in ultra low carbon, high strength steel sheets in a consistent and reproducible manner and hence to obtain cost savings in steel plants from improved product yieldThe focus was to develop criteria for bake hardening based on the fundamental understanding of the interaction between the strain sources- temper rolling and pre- strain, interstitial character and content, and the bake hardening time-temperature response. Characterization of the grain boundary distribution, and hence establishing a correlation between the grain boundary hardening, segregation of interstitial solutes and the grain boundary mis-orientation have been established. This will tend to improve our understanding of the nature of the ferrite or final microstructure on the BH response. The processing condition dependence of the segregation of carbon in low carbon bake hardenable steel, taking account of the variation of the segregation on the individual grain boundaries and to elucidate the relation between the grain boundary segregation and the crystallographic orientation of grain boundaries is also accomplished.The successful completion of this program will result in large energy savings for steel plants and in the transportation sector.