Spot-Welded Tensile Properties in Automobile Ultrahigh-Strength TRIP-Aided Martensitic Steel Sheet

摘要

Effect of heat-affected zone (HAZ) softening on tensile strength (75) and total elongation (TEI) of spot-welded automobile ultrahigh strength TRIP-aided martensitic (TM) steel sheet was investigated for automobile applications. Tensile test was performed on an Instron type tensile testing machine at a crosshead speed of 3 mm/min (strain rate of 8.3×10~(-4) s~(-1)), using spot-welded specimen.The results are as follows.(1) The spot-welded specimen at the current value (/) of 6.5 kA for the TM steel with the maximum tensile stress (TS*) of 1 450 MPa and the fracture elongation (TEI*) of 7.0% was superior to that of hot stampted (HS1) steel sheet and it was found that the TS* and the TEI* for the TM steel possessed same level as those of base metal specimen for the HS1 steel with the tensile strength (TS) of 1 469 MPa and the total elongation (TEI) of 7.7%.(2) The TRIP effect for the TM steel with an excellent strength-ductility balance (TS×TEI) of 14.4 GPa% (i.e. the tensile strength (TS) of 1532 MPa and the total elongation (TEI) of 9.4%) suppressed the HAZ softening and was able to express a high maximum stress (TS*) of 1450 MPa of the spot-welded specimen for the TM steel.