Kawasaki Steel has developed TS 590 MPa grade galvannealed sheet steels to reduce automobile's weight and improve anti-collision property of automotive bodies. Although the steels were heat-treated in the heat cycle of a continuous galvanizing line, in which quenching is difficult, favorable martensite phase was able to be obtained by Mo addition of 0.15 percent in conventional dual phase steels. Furthermore, the developed steels showed total elongation of 30 percent, apparent decrease in yield ratio and good spot weldability. Excellent surface quality of the coated steels was assured by good wettability by molten zinc during galvanizing, because no surface segregation of Mo occurred on the steel surface during recrystallization annealing. Powdering resistance of the galvannealed coatings was sufficient for automobile exterior panels. Low P content in steel, which did not retard galvannealing reaction, provided high productivity in continuous galvanizing lines.
展开▼
