TiN/NbC Compound Particle Formation and High Temperature Mechanical Properties during Thin Slab Direct Rolling of HSLA steel

Abstract

Thin slab cast and direct rolling (TSDR) technologies are the most advanced steel processing routes. The initial TSDR process was developed for the commercial mild steel grades production. Due to the improvement of the technology and the industrial experience, a rapid expansion of the productrange has been achieved and higher strength steel grades are now becoming an important part of overall TSDR steel production. The present doctoral thesis focusses on the formation of precipitates during the TSDR processingof high strength low alloy (HSLA) steel. Compound two-phase particles were found in thin slab direct rolled Ti-added Nb HSLA steel after the rough rolling stage of an in-line strip processing line.The compound two-phase particles were composed of a cuboid Ti-rich (TixNb1-x)N (0.73≥x>0.60) core and a Nb-rich cap-shaped epitaxial deposit of (TixNb1-x)C (0.35≥x>0.06) or NbC formed on the {100}-type faces of the cuboid (TixNb1-x)N (0.73≥x>0.60) core. At the interface between the cuboid core and the cap-shaped deposit, the Ti/(Nb+Ti) atomic ratio was found to increase gradually from a low value of Ti/(Nb+Ti)≈0, on the cap-side of the particles, to a high value of Ti/(Nb+Ti)≈0.7, on the cuboid core side of theprecipitate. The fact that compound TiN /NbC two-phase particles were present in the matrix indicates that NbC has a greater thermodynamic stability, i.e. a lower solubility, in the presence of TiN precipitates. A kinetic precipitation model was used to comprare the conventional HSM and TSDR process and evaluate two possible mechanisms for the formation of the compound particles: a low cap/cuboid interfacial energy and a highmatrix/cuboid interfacial dislocation density. A kinetic precipitation model was also proposed to explain why the dislocation density around TiN was higher during rough rolling. The model is based on the formation of geometrically necessary dislocation around TiN precipitates. A high TiN volume fraction leads to a high density of geometrically necessary dislocationand a high density of heterogeneous nucleation sites for NbC. Consequently, the solute Nb content was lowered and the strain accumulation due to the suppression of the interpass recrystallization was limited. This results in the high softening ratio observed in multi-hit compression test, the low hardness observed in vickers micro-hardness and the low electrical resistivity. The high temperature mechanical tests using the torsion equipment were done to measure the TnRex, mean flow stress(MFS), critical strain and stress, peak strain and stress, activation energy for deformation and the coefficient of n.The activation energy and the coefficient of n are dependent on the material. The effect of temperature and strain rate on the high temperature mechanical property were also explained in detail.