A new method to produce dual-phase steel

Abstract

Abstract In the current research, a new dual-phase (DP) steel produced by a novel technique consists of a combination of friction stir welding (FSW) and austenitization treatment using two different low carbon steels (interstitial free (IF) and St52). The microstructures of the IF layers in the DP steels produced by three different traverse speeds consisted of different regions including fine, coarse, and elongated ferrite grains. The IF steel and St52 layers of DP steels showed higher average hardness (about 150 HV and 240 HV, respectively) than initial IF steel and St52. By increment of the traverse speed, the hardness of the St52 layer in DP steels increased due to the lower grain growth of the St52 layer during FSW via higher traverse speed, and therefore the formation of finer martensite during austenitization. The yield and ultimate tensile strength of DP steel were 508.8 MPa and 682 MPa, respectively, which were much higher than the initial samples and other fabricated dual-phase steels. The work hardening rate – true strain curve of the sample made by 40 mm/min traverse speed showed discontinuous yielding with yield point phenomenon, while the other dual-phase steels exhibited continuous yielding. The fracture surfaces proved that the combination of FSW and austenitization is strong enough to accommodate the plastic deformation without delamination along the interfaces during the tensile test. The rupture mode of produced DP steels was a ductile fracture. However, by the increment of the traverse speed, both depth and number of dimples on the St52 layer decreased. Finally, the results indicated that using FSW and austenitization, the excellent strength-ductility balance achieved compared with the conventional trend in steels, which was mainly due to the formation of a proper laminated structure.