Effect of heat-affected zone (HAZ) softening on tensile strength (TS ) and total elongation (TEl ) 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 (I ) of 6.5 kA for the TM steel with the maximum tensile stress (TS ~(*)) of 1450 MPa and the fracture elongation (TEl ~(*)) of 7.0% was superior to that of hot stampted (HS1) steel sheet and it was found that the TS ~(*) and the TEl ~(*) for the TM steel possessed same level as those of base metal specimen for the HS1 steel with the tensile strength (TS ) of 1469 MPa and the total elongation (TEl ) of 7.7%.(2) The TRIP effect for the TM steel with an excellent strength-ductility balance (TS ×TEl ) of 14.4 GPa% (i.e. the tensile strength (TS ) of 1532 MPa and the total elongation (TEl ) 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.
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