Abstract
The medium Mn steels are gaining increasing attention due to their excellent combination of mechanical properties and material cost. A cold-rolled 0.1C5Mn medium Mn steel with a ferrite matrix plus metastable austenite duplex microstructure was resistance spot-welded with various welding currents and times. The nugget size rose with the increase of heat input, but when the welding current exceeded the critical value, the tensile-shear load increased slowly and became unstable due to metal expulsion. The fusion zone exhibited a lath martensite microstructure, and the heat-affected zone was composed of a ferrite/martensite matrix with retained austenite. The volume fraction of retained austenite decreased gradually from the base metal to the fusion zone, while the microhardness presented a reverse varying trend. Interfacial failure occurred along the interface of the steel sheets with lower loading capacity. Sufficient heat input along with serious expulsion brought about high stress concentration around the weld nugget, and the joint failed in partial interfacial mode. Pull-out failure was absent in this study.
Highlights
Current trends in automobiles have mainly aimed at improving safety, weight reduction and enhanced fuel economy [1]
The nugget size mainly depended on the heat input during
The following conclusions can be drawn from the present study: (1) The nugget size increased linearly with the welding current from 10 to 15 kA (20 cyc, 4 kN); the tensile-shear load increased as well with the current but became unstable when the current exceeded 12 kA due to metal expulsion
Summary
Current trends in automobiles have mainly aimed at improving safety, weight reduction and enhanced fuel economy [1]. Advanced high-strength steels (AHSSs) offer an opportunity to meet the increasing requirements of the car body. AHSSs, exhibit desirable tensile properties with strength higher than 700 MPa, as well as remarkable elongation exceeding 50% [2]. Medium Mn steels, as third-generation AHSSs, are becoming attractive, with a good balance of material cost and mechanical properties. Research about medium Mn steel was first reported by Miller in 1972, in which 0.11C5.7Mn steel was annealed at 600 ◦ C for 16 h; the ultimate tensile strength (UTS) reached 878.4 MPa with a total elongation of 34% [4,5]. Studies relevant to medium Mn steel are mainly focused on (0.1–0.2)C(3–10)Mn (wt %) steels with a certain amount of austenite [6,7,8]. The final microstructure is usually controlled to be ferrite + austenite dual phases by inter-critical annealing between the start
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