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Effect of hydrogen on spot-welded tensile properties in ultrahigh-strength TRIP-aided martensitic steel (TM steel) sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 1 mm/min (strain rate of 2.8 × 10 − 4 /s), using base metal and spot-welded specimens with or without hydrogen charging.The results are as follows.(1) The difference between the tensile strength (TS) of 1 532 MPa for base metal specimen without hydrogen charging and the maximum stress (TS-H) of 1 126 MPa for the base metal specimen with hydrogen charging (ΔTS-H = TS − TS-H) in the TM steel was smaller than that of hot stampted steel (HS1 steel) and superior to that of HS1 steel. On the other hand, the TS-H of 725 MPa for the base metal specimen with hydrogen charging was halved in comparison with the TS of 1 438 MPa for base metal specimen without hydrogen charging in the HS1 steel. It is considered that this was because the retained austenite suppressed the strength reduction due to the hydrogen embrittlement of the TM steel.(2) The amount of hydrogen decreased in the order of the HS1 steel, the TM steel, and the tempered martensitic steel (HS7 steel), and the HS1 steel was the highest. This is thought to be due to the high dislocation density of the HS1 steel.(3) The difference between the maximum stress (TS-W) of the spot-welded specimen without hydrogen charging and the maximum stress (TS-WH) of the spot-welded specimen with hydrogen charging (ΔTS-WH = TS-W − TS-WH) in the TM steel and that of the HS1 steel were similar. It was considered that this is partly due to the effect of the stress concentration on heat affected zone (HAZ) softening of the hardness distribution of the spot-weld.

Effect of hydrogen on spot welded tensile properties in ultrahigh strength TRIP-aided martensitic (TM) steel sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 1 mm/min (strain rate of 2.8×10 −4 /s), using base metal and spot welded specimens with or without hydrogen charging. The results are as follows.(1) The difference between the tensile strength (TS) of 1532 MPa for base metal specimen without hydrogen charging and the maximum stress (TS-H) of 1126 MPa for the base metal specimen with hydrogen charging (ΔTS-H=TS−TS-H) in TM steel was smaller than that of hot stamped steel (HS1 steel) and superior to that of HS1 steel. On the other hand, the TS-H of 725 MPa for the base metal specimen with hydrogen charging was halved in comparison with the TS of 1438 MPa for base metal specimen without hydrogen charging in the HS1 steel. It is considered that this was because retained austenite suppressed the strength reduction due to hydrogen embrittlement of TM steel.(2) The amount of hydrogen decreased in the order of HS1 steel, TM steel, and tempered martensitic steel (HS7 steel), and HS1 steel was the highest. This is thought to be due to the high dislocation density of HS1 steel. (3) The difference between the maximum stress (TS-W) of spot welded specimen without hydrogen charging and the maximum stress (TS-WH) of spot welded specimen with hydrogen charging (ΔTS-WH=TS-W−TS-WH) in TM steel and that of HS1 steel were similar. It was considered that this is partly due to effect of stress concentration on heat affected zone (HAZ) softening of hardness distribution of spot weld.

Effect of heat-affected zone (HAZ) softening on tensile strength (TS) and total elongation (TEl) of spotwelded 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 1 450 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 1 469 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 1 532 MPa and the total elongation (TEl) of 9.4%) suppressed the HAZ softening and was able to express a high maximum stress (TS *) of 1 450 MPa of the spot-welded specimen for the TM steel.