The transformation behaviour and hot strength of 3CR12 during the continuous casting process

Abstract

Extensive research has been done over the years and has contributed quite a lot to the development of 3CR12 stainless steel. Nevertheless, there is still much to be understood about the behaviour of this steel during its production. One of the problems that are occasionally encountered is the side bulging effect i.e. the unconstrained narrow faces of the strand plastically bulge due to ferrostatic pressure from the liquid core of the strand at high temperatures. In general, this problem is prevalent in ferritic stainless steels as they exhibit a weaker hot strength than austenitic stainless steels. Coupled with side bulging, there is also strand width variation at high temperatures i.e. when the steel is in the ferrite-austenite dual phase region. Both of these dimensional changes to the slab profile create some processing problems in subsequent hot rolling operations when unacceptable width variations are encountered. This work was, therefore, motivated by the requirement to quantify the role that the metallurgical behaviour of this steel plays with regard to the above width variation problem. The research work involved studies of the <font face="symbol">d</font>-ferrite to austenite phase change during continuous cooling (simulating cooling during continuous casting) and the establishment of CCT diagrams, the influence of chemical composition on the austenite start temperature and the hot ductility and hot strength visà-vis the side bulging effect. The casting conditions in the mould are crucial to ensure that the solidification shell is thick enough to withstand the ferrostatic pressure exerted on the unconstrained narrow sides of the strand as it exits from the mould. Therefore, part of the solution lies in the study of the optimisation of the cooling rate, mould flux properties, casting speed, mould taper angle and the chemical composition of this steel, among other factors. Additions of austenite formers, within the specification range of 3CR12, should be favourable for a stronger solidification shell since austenite exhibits superior hot strength to <font face="symbol">d</font>-ferrite. The good hot ductility observed in 3CR12 may also provide a leeway for increasing the secondary cooling rate in order to form a thicker solidification shell soon after emerging from the mould. This may reduce the side bulging effect. This can be achieved without a risk of transverse cracking if the cooling is adjusted carefully. As long as the <font face="symbol">d</font>-ferrite to austenite phase ratio keeps fluctuating due to variations in (i) the chemical composition within the specification range and (ii) the cooling rate in the dual phase region from cast to cast, the strand width variation effect will persist. This is simply because of the effect these have on the ferrite to austenite phase ratios through the differences between the lattice structures of these two phases. Ferrite being less dense than austenite, occupies more volume than austenite, thereby affecting the slab width. The remedy to this problem is to control within stricter chemical composition limits in order to reduce or completely avert this width variation effect.