IMPROVING STRENGTH-DUCTILITY BALANCE OF HIGH STRENGTH DUAL-PHASE STEELS BY ADDITION OF VANADIUM

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  For galvanized or galvannealed steels to be commercially successful,they must exhibit several attributes:(i)easy and inexpensive processing in the hot mill,cold mill and on the coating line,(ii)high strength with good formability and spot weldability,and(iii)good corrosion resistance,especially after cold forming.For good corrosion resistance,the coating must have sufficient coverage,be of uniform thickness,and most importantly,the coating must survive the cold stamping or forming operation.The purpose of this paper is to present research aiming at improving the steel substrate,such that high strength can be obtained while maintaining good global formability(tensile ductility),local formability(sheared-edge ductility),and good spot weldability.It is well-known that the strength of DP steels is controlled by several factors,including the amount of martensite found in the final microstructure.Recent research has revealed that the amount of austenite formed during intercritical annealing can be strongly influenced by the annealing temperature and the pre-annealing conditions of the hot band(coiling temperature)and cold band(%cold reduction).Current experiments have explored the combination of pre-annealing conditions and four annealing practices to help define the best practice to optimize the strength-formability balance in these higher strength DP steels.The steels used in these experiments contained(i)low carbon content for good spot weldability,(ii)the hardenability additions Mo and Cr for strength,and(iii)V for grain refinement,precipitation hardening and temper resistance.When processed correctly,these steels exhibited UTS levels up to 1000MPa,total elongation to 25%,reduction in area to 45%,and Hole Expansion Ratios to 50%.The results of this program will be presented and discussed.
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