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

A new type of martensitic steel by utilizing the concept of Quenching and Partitioning (Q&P) has recently been established known also as “Low-End cycle”. A too high partitioning treatment temperature at 465°C will not provide the best metallurgical results of the Q&P-treatment which are possible to be achieved without the hot-dip coating at the last step of the metallurgical heat treatment chain. Recently a North American car maker has modified its Dual Phase steel specifications by replacing partly hard martensite with bainite preferably free of coarse carbides. The target is to balance formability by mainly improving hole expansion properties. Since 1984 it has been known from Japan that the best forming properties of the advanced high strength steels will be achieved with Tri-phase steels (ferrite- bainite-martensite). They have also proposed that the heat treatment cycle of Roll Quench/CAL having the direct cooling over 30°C/.sec from about 860 C to the end-point at 260°C and austempering at 260°C about 180 sec will provide superior mechanical properties for GIGA-steels with a very high hole expansion ratio, twice higher than Q&P980-GI presented at GDIS 2019. The purpose of this paper is to present a brief review of the work done to develop the new technology for simultaneous rapid cooling and hot-dip zinc coating to produce high formable GIGA-GI-steels with the similar heat treatment cycle to RQ/CAL and even with a higher cooling capability. This new technology will produce GI- coated AHSS/UHSS from lean/low-alloyed steel chemistries extremely cost-effectively and with improved coatability. The key design target has been that there is no bainite in steel strip before the zinc pot because the strip flatness problems during zinc off wiping has been found to be caused by transformation of bainite from austenite before zinc coating. Additionally the step time of strip at 450°C is less than five seconds i.e. bainite even with the low alloyed steels is formed after the zinc pot as Lower Bainite instead of Upper Bainite with coarse carbides. This paper will also briefly describe the production of GI-coated cold-formable UHSS-products with tensile strength over 1500 MPa from spot weldable steel chemistries (Si/C ~ 7 and Mn-eq <2.9%).