Abstract
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.
