12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Hot-dip galvanized (GI) and galvanneal (GA)-coated third generation steels have been developed and commercialized at minimum tensile strength levels at both 780 and 980 MPa. These coated steels feature a combination of superior formability which, with targeted steel compositions and unique processing, have been shown to result in more customer-friendly properties and excellent residual ductility after automotive assembly. The steel microstructure is developed by the application of a novel heat treatment that results in improved uniform and total elongation; strength, n-value, local formability, transverse uniformity, and reduced anisotropy. In addition to providing improved properties, the heat treatment provides other benefits. These include decreased alloy costs, improved production yields, and decreased silicon content. The low silicon hot-dipped steel compositions provide five areas of benefits. First, lower silicon improves coat-ability, which has been demonstrated in advanced laboratory simulations and commercial production; second, it allows galvanneal (GA) alloying at more moderate GA peak metal temperatures (PMT) and thus, less degradation of retained austenite, even with thicker 50 g/m2 class GA coatings; third, it reduces silicon-affected welding carbon equivalent, which helps to minimize liquid metal embrittlement (LME); fourth, the avoidance of high silicon helps to minimize the need for hot charging of thick slabs prior to hot rolling; fifth, steels which avoid high silicon typically have improved hot band de-scaling pickling and coated surface quality. Cooperative development efforts with automotive manufacturers have revealed properties that favorably combine to produce cold-stamped components that can potentially replace hot-stamped components of higher strength ratings. Significant yield strength increase after forming, combined with potent bake hardening results in a final yield strength that compares favorably with press hardened steels that can experience softening during automotive paint baking treatment. The result of these synergistic responses to vehicle assembly practices is higher residual ductility, and crash energy absorption with reduced fractures. The development of these steels benefited from advanced simulation equipment including a new direct-fired furnace (DFF) simulator as well as two hot-dip coating and annealing process simulators (an upgraded Surtec Research A-3 and a newly installed A-6). In addition to laboratory optimization of heat treatments and coat-ability, the steelmaking pilot operations and simulation equipment also heavily influenced capital expenditures at the manufacturing operations. Design of experiments optimized the process parameters prior to commercial trials.