The non-isothermal crystallization characteristics of the electroslag remelting (ESR)-type TiO2-bearing slag were studied. The effect of crystallization characteristics of the slag on the in-mold performance was verified by ESR pilot trials operating in a static mode. The results showed that increasing TiO2 content decreased the crystallization temperature and liquidus temperature of the slag and suppressed the crystallization tendency of the slag. There is no change in the types of the crystalline phases in the slag with different TiO2 contents during cooling, i.e., 11CaO·7Al2O3·CaF2, CaTiO3, and CaF2. The sequence of crystal precipitation during cooling was 11CaO·7Al2O3·CaF2 to CaTiO3, and then CaF2 with increasing TiO2 content from 4.2 to 12.6 mass pct. With adding 16.8 mass pct TiO2 in the slag, the precipitation of 11CaO·7Al2O3·CaF2 and CaTiO3 took place simultaneously, followed by CaF2. With the increase in the TiO2 content from 4.2 to 16.8 mass pct in the slag, the dominant crystalline phase changed from 11CaO·7Al2O3·CaF2 to CaTiO3. The morphology of 11CaO·7Al2O3·CaF2 and CaTiO3 changed from faceted and bonelike to elliptically faceted and blocky, respectively. The morphology of CaF2 were spherical in all cases, irrespective of TiO2 contents in the slag. The decrease in the crystallization tendency of the slag with increasing TiO2 contents was attributed to the decrease in the activity of CaO in the slag. The increasing of TiO2 content in the slag are favorable for providing thin slag skin and stable heat flux across the slag skin, which improved the surface quality of the as-cast ESR ingot as demonstrated by ESR pilot trials operating in a static mode.

Crystallization Characteristics and In-Mold Performance of Electroslag Remelting-Type TiO 2 -Bearing Slag

This thesis has been made within the European Industrial Doctorate (EID) project called Mathematics and Materials Science for Steel Production and Manufacturing, abbreviated as MIMESIS, which has five partners: EFD Induction in Norway; SSAB, Outokumpu, and the University of Oulu in Finland; and Weierstrass Institute for Applied Analysis and Stochastics (WIAS) in Germany. The main aim of this work was to develop a steel composition and processing route suitable for making a slurry transportation pipeline with the aid of induction hardening, and to characterize the phase transformations and microstructures involved in the various stages of the processing route. A novel steel chemistry was designed based on metallurgical principles assisted by computational thermodynamics and kinetics. The designed composition is a medium-carbon, lowalloy steel microalloyed with niobium, in wt.% 0.40 C, 0.20 Si, 0.25 Mn, 0.50 Mo, 0.90 Cr, and 0.012 Nb. This was subsequently cast, thermomechanically rolled on a laboratory rolling mill to two bainitic microstructures, and finally subjected to the thermal cycles predicted to be encountered with the internal induction hardening of a typical pipe geometry. The phase transformations and microstructures found at various stages of the simulated production process have been characterized and algorithms developed to enable the optimization of microstructure and hardness through the pipe wall thickness.

Design, thermomechanical processing and induction hardening of a new medium-carbon steel microalloyed with niobium

High-temperature laser scanning confocal microscopy (HT-LSCM) employs the possibility of direct austenite grain growth observations. To ensure the results obtained are interpreted correctly, several influencing factors on the investigation outcome have to be taken into account. The present paper gives an overview of the basic experimental setup for in-situ grain growth observations and critically assesses the requirements concerning grain size measurement, materials and operational details. The extensive studies presented in this work indicate that the grain growth as seen on the sample surface is representative for the bulk material and allows the determination of an average grain size value and a full grain size distribution for every desired time step. The investigated material significantly influences the experimental outcome, which is why origin and thermal history of the sample always have to be taken into account for an interpretation of the results. Concerning the details of operation, a careful temperature referencing was proven to be a prerequisite to meet the desired temperatures within the sample. Temperature differences between set temperature and sample surface were shown to be ±30 °C following a non-linear behavior in relation to the absolute temperature. Oxidation of the sample surface can be prevented by Ar purging; however, evaporation of Mn was demonstrated to occur under standard experimental conditions. While the Mn loss did not impact the grain growth observations in this study, it is an important finding that should attract interest when using the HT-LSCM for the evaluation of other microstructural changes. Finally, some selected in-situ grain growth results are presented that demonstrate the unique potential of the HT-LSCM in determining the effect of specific alloying elements (Mo, Mn and Ni) on the grain growth kinetics as well as the impact of AlN precipitates. The achieved results feature a strong basis for grain growth modelling and the critical validation of simulation results, emphasizing the HT-LSCM as an efficient and reliable tool for various applications within steel research.

Potential and limitations of direct austenite grain growth measurement by means of HT-LSCM

The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the susceptibility to the transverse crack can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate two types of double phase-transformation technologies. The distribution and morphology of ferrites under different cooling conditions were analyzed, and the effects of ferrite distribution and morphology on the double phase-transformation technologies were explored to obtain the suitable double phase-change technology for the continuous casting process. The results indicate that, under the thermal cycle TH0 [the specimens were cooled down to 913 K (640 °C) at a cooling rate of 5.0 K/s (5.0 °C/s)], the width of prior austenite grain boundaries was thick, and the dislocation density at grain boundaries was high. It had strong inhibition effect on crack propagation; under the thermal cycle TH1 [the specimens were cooled down to 1073 K (800 °C) at a cooling rate of 5.0 K/s (5.0 °C/s) and then to 913 K (640 °C) at a cooling rate of 1.0 K/s (1.0 °C/s)], the width of prior austenite grain boundary was thin, and the dislocation density at grain boundaries was low. It was beneficial to crack propagation. After the first phase change, the developed film-like ferrite along the austenite grain boundaries improved the nucleation conditions of new austenitic grains and removed the inhibition effect of the prior austenite grain boundaries on the austenite grain size.

Study of Ferrite During Refinement of Prior Austenite Grains in Microalloyed Steel Continuous Casting

Abstract The formation of coarse prior austenite grain is a key factor to promote transverse crack, and the transverse crack susceptibility can be reduced by refining the austenite grain size. In the present study, the high-temperature confocal laser scanning microscope (CLSM) was used to simulate and study the effect of two γ→α phase transformation conditions on the refinement of the prior austenite grains. Under the condition of the uniform distribution of the second phase precipitation, the effect of the distribution of the proeutectoid ferrite at different cooling rates and refinement of prior austenite grain were investigated. The results indicate that, at a cooling rate of 5.0°C s–1, the austenite grain size undergoing TH1 thermal cycle was 31% smaller than the austenite grain undergoing TH0 thermal cycle. Under TH0 cooling system, the proeutectoid ferrite was uniformly distributed in the austenite matrix; under TH1 cooling, the proeutectoid ferrite precipitated and mainly concentrated along the austenite grain boundaries to form developed-film-like ferrite, which is favorable to break the prior austenite grain boundaries. After the first phase transformation, the film-like ferrite improved the nucleation conditions of new austenite grains, thus more new austenite grains splitted the prior austenite grains, ultimately refining the prior austenite grains.

Effect of γ→α Phase Transformation on Refining Austenite Grains of Microalloyed Steel in Continuous Casting by Simulation

The austenite grain growth behaviour of microalloyed steel during continuous casting was simulated by confocal laser scanning microscope, and the effect of the cooling rate during the solidification, T γ (the starting temperature for austenite grain growth) and steel composition on the behaviour of austenite grain growth were analysed. Next, the model for predicting the austenite grain size, which is put forward by the Bernhard model, was revised. The results show that by the lab simulation of continuous casting initial solidification process at the cooling rate between 1·5 and 2·5°C s−1, the austenite grain size lying within the size range on the slab surface was obtained; T γ was calculated using equivalent carbon content CP and showed the exponential function relation to the latter; the revision factor K, describing the effect of microalloyed elements on austenite grain sizes, showed exponential function relation with PTi (precipitation amount of titanium); both T γ and K were used to revise th...

Study of austenite grain size of microalloyed steel by simulating initial solidification during continuous casting

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Study of austenite grain size of microalloyed steel by simulating initial solidification during continuous casting