Makoto Imanaka

Development of high-formability high-strength galvannealed steel sheet of tensile strength 440 MPa grade with excellent formability

Abstract Molybdenum and carbon were chosen as strengthening alloy elements for the production of high strength steel sheet with high press formability and excellent galvannealing properties. The strengthening effect of molybdenum addition was 6 MPa/0.1%, while that of carbon addition was 8.5 MPa/10ppm. The decrease in formability, especially elongation due to molybdenum (0.2%/0.1%) and the r value due to carbon (almost 0/l0ppm), was small. Steel containing carbon of 50–100ppm exhibited the highest formability with coiling temperature at 600°C. By applying these results, the high strength galvannealed sheet steel of TS 440 MPa grade with elongation of 35% and an r value of 1.5 was newly developed. This steel also exhibited good galvannealing properties.