2020 Volume 106 Issue 7 Pages 438-447
Effect of heat-affected zone (HAZ) softening on tensile strength (TS) and total elongation (TEl) of spot welded 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 stress (TS*) of 1450 MPa and the fracture elongation (TEl*) of 7.0% was superior to that of hot stamping steel (the HS1 steel), and it was found that the TS* and the TEl* for the TM steel possessed 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 HAZ softening and was able to express a high maximum stress (TS*) of 1450 MPa for the TM steel of the spot welded specimen.