Development of process for thermomechanical treatment of ultrahigh strength steel prepared by electroslag refining

Abstract

Thermomechanical treatment (TMT) is the simultaneous use of work hardening, and grain refinement along with solid solution and precipitation strengthening. In this investigation, four alloys, with a base composition of 0·28%C, 1·0%Mn, 4·2%Cr, 1·0%Mo, 0·34%V, were prepared by electroslag refining (ESR) and by addition of small amounts of Ti and Nb and by increasing Cr and V to 4·8 and 0·48% respectively. In two of the alloys a yield strength in excess of 1550 MPa was obtained in the as cast quenched and tempered condition. Attempts were made to further increase the yield strength by thermomechanical treatment. The process parameters for thermomechanical treatment were optimised by adopting procedures such as calculation of stability of precipitates, hot compression test, determination of cooling rates in different coolants, and modelling of TTT and CCT diagrams. The process involved prerolling of the ESR ingot to a bar at 1200°C, followed by hot rolling in two passes starting from 950°C and finishing at 850°C with equal deformation of 25% in each pass to convert the bar into plates. These were immediately cooled in one of the cooling media: air, polymer–water solution (1 : 1·5) and oil. Yield strength in excess of 1750 MPa was obtained in oil cooled specimens of the alloy with titanium addition and that where Cr and V were increased. The niobium added specimen gave strengths, similar to that obtained for the base alloy, in spite of the fact that the as cast alloy had shown very high strengths, presumably because of the high soaking temperatures and grain growth. Air cooling gave the lowest strengths and oil cooling the highest.     

Numerical modelling of gas stirred ladles

Abstract

A finite element analysis of the flow in a gas stirred vessel is presented. Turbulence is modelled using the two equation k–L predictor/ε corrector scheme algorithm; two alternative studies are compared, with and without flotation in k and ε transport equations. The biphasic zone is considered as an homogeneous fluid with a reduced density – the quasi-single phase approach. This reduced density is estimated taking into account the slip velocity between the rising bubbles and the liquid according to correlations from the literature. The numerical results are compared with experimental water model data and then used to predict the flow in two industrial liquid steel ladles with twin eccentric Ar injectors.     

Microstructure in adiabatic shear bands in a pearlitic ultrahigh carbon steel

Abstract

Adiabatic shear bands, obtained in compression deformation at a strain rate of 4000 s^?1, in a pearlitic 1·3%C steel, were investigated. Shear bands initiated at 55% compression deformation with the width of the band equal to 14 μm. Nano-indentor hardness of the shear band was 11·5 GPa in contrast to the initial matrix hardness of 3·5 GPa. The high strength of the shear band is attributed to its creation from two sequential events. First, large strain deformation, at a high strain rate, accompanied by adiabatic heating, led to phase transformation to austenite. Second, retransformation upon rapid cooling occurred by a divorced eutectoid transformation (DET). The result is a predicted microstructure consisting of nano size carbide particles within a matrix of fine ferrite grains. It is proposed that the DET occurs in iron–carbon steels during high rate deformation in ball milling, ball drop tests and in commercial wire drawing.     

Ultrahigh strength hot rolled microalloyed steel: microstructure and properties

Abstract

A low carbon steel alloyed with Ni, Mn, Mo, Cu and microalloyed with Nb and Ti was prepared. Continuous cooling transformation behaviour of the steel was evaluated. Formation of polygonal or Widmanstätten ferrite is suppressed at high temperature and the 'C' curve is shifted to an extreme right. At lower temperatures a flat top 'C' curve with a mixed structure of bainite and martensite was obtained and the transformation temperatures do not vary much with a wide range of cooling rates. The steel was thermomechanically processed at different finishing temperatures and ultrahigh strength values were obtained as a result of austenite grain refinement, highly dislocated fine lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride at all finish rolling temperatures. The stable and large TiN/TiCN particles formed during casting have impaired the impact toughness values at ambient and at ?40°C temperatures.     

Degradation mechanisms of magnesia-chromite refractories in vacumm-oxygen decarburisation ladles during production of stainless steel

Abstract

Magnesia–chromite bricks are used as refractories for the refining of stainless steel in vacuum–oxygen decarburisation (VOD) ladles. Refractory wear is not uniform. In the present work, worn bricks from different zones in the ladle have been analysed, and a set of interdependent degradation mechanisms is proposed. Refractory wear as a function of position in the ladle is discussed. Slag infiltration and MgO dissolution from the refractory were observed in all samples, whereas FeO_x decomposition was seen at two levels in the high wear samples. First, partial decomposition of primary chromite crystals ( (Mg) [Fe³⁺, Cr, Al]₂ O₄ ) occurred at the hot face of the brick. Three layers were distinguished in the reacted chromite crystals and a reaction mechanism is presented. Second, a decrease in the FeO_x content of the magnesia phase occurred at the hot face of the brick. The negative effect of the presence of FeO_x in magnesia–chromite refractories is discussed and the influence of the ferrostatic pressure is demonstrated. Finally, the consequences of the following phenomena are discussed: increase in brick porosity, slag infiltration, corrosion, erosion of the partially liquid bonded refractory system, and spalling and cracking.