Abstract: A steel for forming a high strength steel sheet contains, in mass %, C: at most 0.04%, Si: at most 0.4%, Mn: 0.5-3.0%, P: at most 0.15%, S: at most 0.03%, Al: at most 0.50%, N: at most 0.01%, and Mo: 0.01-1.0%. Steel sheet formed from the steel is suitable for use as automotive panels.

Abstract: A galvannealed steel sheet suited for use in automobiles, and a manufacturing method thereof. The steel sheet has tensile strength of 340 MPa or higher and paint bake hardenability of 10 MPa or higher, and excellent resistance to powdering when press formed, and excellent resistance to chipping in cold regions. The base metal of the galvannealed steel sheet consists essentially of C: 0.004-0.008%, Si: 2.5.times.P (%)-0.20%, Mn: 0.10-0.40%, P: 0.017-0.045%, sol. Al: 0.003-0.08%, Ti: 0.002-0.015%, Nb: 0.010-0.030 %, wherein Ti (%)+Nb (%): 0.012-0.035%. A steel sheet of the chemical composition described above is hot-dip galvanized, heated up to a Fe--Zn alloying temperature at a heating velocity of 20.degree. C./sec or higher, and upon completion of the Fe--Zn alloying process, cooled down from the Fe--Zn alloying temperature at a cooling velocity of 10.degree. C./sec or higher.