Liquid Zn-assisted embrittlement during resistance spot welding of Zn-coated high strength steels induces risks of surface cracking. This paper presents a state-of-the-art review on the surface crack evolution due to liquid metal embrittlement during spot welding. Several aspects of cracking have been discussed such as macroscopic and microscopic features of liquid Zn-assisted cracks, prevalent crack locations in spot welds, sensitivity to welding conditions, and susceptibility of different types of steels and Zn-based coatings. Subsequently, the influence of governing factors like temperature, stress, microstructure, and nature of coating are presented by correlating thermo-mechanical studies with actual spot welding investigations on Zn-coated steels. Finally, the possible mechanisms of embrittlement are discussed and suitable methods to suppress surface crack formation during spot welding are suggested. This is the winning review of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.

Due to their high strength, formability and affordable cost, quenched and partitioned (Q&P) steels have shown the potential to reduce the mass of vehicles, thereby decreasing fuel consumption during service. Furthermore, because a lower mass of steel is used in each vehicle, energy consumption associated with the steelmaking process is also reduced. Q&P steels utilize the deformation-induced martensitic transformation (DIMT) of metastable retained austenite to enhance ductility and strain hardening. Accordingly, improvement of mechanical performance is contingent on the ability to precisely control the chemical and mechanical stability of austenite. Considering the multitude of factors that influence austenite stability, optimizing microstructures to delay necking or fracture is challenging, particularly as temperature and strain rate increase. Tensile tests of an intercritically annealed C-Mn-Si Q&P steel were performed over a range of strain rates (10 À4 to 10 À1 s À1 ) to evaluate effects on the DIMT and sheet tensile properties. As strain rates increased from 10 À4 to 10 À1 s À1 , the uniform elongation decreased from approximately 19 to 14 pct. This reduction in uniform elongation is associated with a decrease in the strain hardening exponent near the onset of strain localization. Based on experimental data from this study and review of previous research, it is postulated that the strengthening contribution of DIMT is controlled by competing effects of: (i) a decreasing chemical driving force for DIMT caused by deformation-induced heat accumulation at higher strain rates and (ii) an increasing number of martensite nucleation sites. This suggests that tailoring austenite stability for specific deformation conditions could enable further optimization of formability and vehicle crash behavior.

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Delamination and cracking related to segregations are mostly observed at the centerline of hot-rolled products. The delamination is related to heavy concentration of manganese sulfide inclusions originating from centerline segregation in slabs. In multiple cases, similar delamination or cracking is observed at locations away from the mid thickness plane of hot-rolled products during forming operations at customer end. This leads to rejection of materials. Metallographic investigation reveals segregation line with plenty of manganese sulfide stringers at the off-center location similar to observations in cases of centerline defects. Centerline segregation is a well-understood phenomenon, but the presence of off-center segregation line in hot-rolled products has not been systematically studied before. Several literatures report intercolumnar cracks in slabs to be filled with segregations. Also, they occur away from the centerline of continuously cast slabs. A laboratory-based experimental study was conducted using a slab suspected by S-printing to contain intercolumnar cracks. The aim of this study was to evaluate the effect of intercolumnar cracks on internal quality of rolled products. This paper reports the finding of this study that established a strong correlation between intercolumnar cracks in slabs and off-center cracking in hot-rolled products.

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