The Influence of Composition on the Hot Cracking Susceptibility during Casting of Microalloyed Steels Processed to Simulate Thin Slab Casting Conditions

Abstract

One of the problems associated with the thin slab casting process route is the formation of surface defects such as transverse cracks in the continuously cast slab. The objective of this work was to determine the likelihood of developing transverse surface cracks for various microalloyed steel types under thin slab casting conditions. A technique was developed to simulate the cooling conditions experienced during the casting of thin slabs 50mm in thickness in small laboratory ingots. To simulate the slab straightening procedure, the cast ingots were deformed in a three point bend mode using strain rate conditions expected during the straightening of thin slabs. To generate significant cracking, surface strains greater than those expected during continuous casting had to be applied. The tests were conducted over a range of temperatures, and the degree of surface cracking assessed. Five steel types were examined based on 0.06%C, 1.5%Mn and with additions of V, Nb and N. Small cracks, up to 2mm long, were observed at austenite grain boundaries for all the steels across a wide temperature range. However, the Nb, V-N, and V-Nb steels exhibited cracks which were longer than this in some parts of the temperature range. The Nb steel exhibited the longest cracks (17mm at 820°C) over the widest temperature range (725-875°C) while the V-N and V-Nb steels exhibited cracks which were up to 15mm and 12mm long, respectively, in the temperature range 775-825°C. The most severe cracking was associated with the austenite-ferrite transformation, together with detrimental precipitate distributions. It was concluded that transverse cracking would be most likely to occur in the colder regions of the slab such as the slab corner, or in areas with a high level of water impaction.