Volume 36, Issue 4 - Special Issue: Cutting Edge of Computer Simulation of Solidification, Casting and Refining, Issue Editor: Keiji Nakajima and Qiang Wang

Intragranular ferrite (IGF), which nucleates from specific inclusion surfaces in low alloy steels, is the desired microstructure to improve mechanical properties of steel such as the toughness. This microstructure is especially important in the coarse grain heat affected zone (CGHAZ) of weldments. The latest review paper focusing on the role of non-metallic inclusions in the IGF formation in steels has been reported by Sarma et al. in 2009 (ISIJ int., 49(2009), 1063–1074). In recent years, large amount of papers have been presented to investigate different issues of this topic. This paper mainly highlights the frontiers of experimental and theoretical investigations on the effects of inclusion characteristics, such as the composition, size distribution and number density, on the IGF formation in low carbon low-alloyed steels, undertaken by the group of Applied Process Metallurgy, KTH Royal Institute of Technology. Related results reported in previous studies are also introduced. Also, plausible future work regarding various items of IGF formation is mentioned in each section. This work aims to give a better control of improving the steel quality during casting and in the heat affected zone (HAZ) of weldment, according to the concept of oxide metallurgy.

Mathematical models for dynamic heat radiation and convection boundary in directional solidification processes are established to simulate the temperature fields. Cellular automaton (CA) method and Kurz-Giovanola-Trivedi (KGT) growth model are used to describe nucleation and growth. Primary dendritic arm spacing (PDAS) and secondary dendritic arm spacing (SDAS) are calculated by the Ma-Sham (MS) and Furer-Wunderlin (FW) models respectively. The mushy zone shape is investigated based on the temperature fields, for both high-rate solidification (HRS) and liquid metal cooling (LMC) processes. The evolution of the microstructure and crystallographic orientation are analyzed by simulation and electron back-scattered diffraction (EBSD) technique, respectively. Comparison of the simulation results from PDAS and SDAS with experimental results reveals a good agreement with each other. The results show that LMC process can provide both dendritic refinement and superior performance for castings due to the increased cooling rate and thermal gradient.

The effects of rotational electromagnetic stirring (R-EMS) on dendrite growth characteristics and segregation control of bearing steel billet were investigated in continuous casting. The results show that applying R-EMS can promote columnar-equiaxed transition, increase the region of the equiaxed grain from 5 % to 45 %, decrease the secondary dendrite arm spacing (SDAS), and reduce the segregation of both carbon and sulfur. Meanwhile, the fragments of dendrite arms induced by R-EMS are observed. The length of the dendrite fragmentation is approximately 1.5 mm, 7–10 times the SDAS. Some fragments can partially remelt to become effective nuclei, and some fragments survive the solidification process.

The surface quality of slabs is closely related with the initial solidification at very first seconds of molten steel near meniscus in mold during continuous casting. The solidification, structure, and free deformation for given steels have been investigated in droplet experiments by aid of Laser Scanning Confocal Microscope. It is observed that the appearances of solidified shells for high carbon steels and some hyper-peritectic steels with high carbon content show lamellar, while that for other steels show spherical. Convex is formed along the chilling direction for most steels, besides some occasions that concave is formed for high carbon steel at times. The deformation degree decreases gradually in order of hypo-peritectic steel, ultra-low carbon steel, hyper-peritectic steel, low carbon steel, and high carbon steel, which is consistent with the solidification shrinkage in macroscope during continuous casting. Additionally, the microstructure of solidified shell of hypo-peritectic steel is bainite, while that of hyper-peritectic steel is martensite.

To investigate the formation of one kind of typical inter-dendritic crack around triple point region in continuous casting(CC) slab during the operation of soft reduction, fully coupled 3D thermo-mechanical finite element models was developed, also plant trials were carried out in a domestic continuous casting machine. Three possible types of soft reduction amount distribution (SRAD) in the soft reduction region were analyzed. The relationship between the typical inter-dendritic cracks and soft reduction conditions is presented and demonstrated in production practice. Considering the critical strain of internal crack formation, a critical tolerance for the soft reduction amount distribution and related casing parameters have been proposed for better contribution of soft reduction to the internal quality of slabs. The typical inter-dendritic crack around the triple point region had been eliminated effectively through the application of proposed suggestions for continuous casting of X70 pipeline steel in industrial practice.

High magnetic field (HMF) and solidification processes were changed during the solidification of both Cu-28 mass %Ag and Cu-72 mass %Ag alloys. The results indicated that the eutectic morphology in Cu-Ag alloys was affected by HMF, composition and solidification parameters. The lamellar spacing of Cu-28 mass %Ag alloy solidified by furnace-cooling was refined by the application of HMF owing to the decreased diffusion coefficient in mushy zone. The lamellar spacing in both Cu-28 mass %Ag sample held at the eutectic temperature and Cu-72 mass %Ag sample by slow controlling cooling was increased by HMF, which might be attributed to the dominated thermolectromagnetic convection. The lamellar spacing in Cu-72 mass %Ag alloys was increased compared with that of Cu-28 mass %Ag alloys because of the decreased growth rates. In Cu-28 mass %Ag alloy, however, fluid transverse velocity gradient was dominate rather than the growth rate and the imposition of HMF had reverse influences on the lamellar spacing. The lamellar-rod transition of Cu phase was promoted by HMF because of the increased Cu volume fraction in eutectic component. These results shed light on the dependence of eutectic morphology in Cu-Ag alloys on composition, external high magnetic field and solidification parameters.

Recent studies have showed that the microstructure and mechanical properties of A356 alloy can be significantly improved when ultrasonic cavitation and solidification processing is used. This is because during the fabrication of A356 castings, ultrasonic cavitation processing plays an important role in degassing and refining the as-cast microstructure. In the present study, A356 alloy and Al 2 O 3 /SiC nanoparticles are used as the matrix alloy and the reinforcements, respectively. Nanoparticles are injected into the molten alloy and dispersed by ultrasonic cavitation. Ultrasonic cavitation was also applied during solidification of these nanocomposites. The microstructure and nanoparticle distribution of the cast samples have been investigated in detail. The current experimental results indicated that ultrasonic cavitation during solidification will greatly improve the microstructure of the samples. Al 2 O 3 and SiC nanoparticle reinforced nanocomposites have different nanoparticle distributions in the matrix.

Based on the calculation results of the two-dimensional unsteady heat transfer model for continuous slab casting with soft reduction process, a coupled thermo-elastic-plastic mathematical model has been developed to study the effects of the reduction amount and solid phase fraction in slab centerline on the deformation behavior of solidification shell in this paper. The calculated results show that the displacement along thickness direction of slab changes little in width center region of slab when the reduction amount keeps constant, then it increases and a peak of displacement appears in 0.075 m whose distance is far from the slab narrow face, finally, it decreases severely near the narrow face of slab. When the solid phase fraction is the same, the displacement of solidification shell toward to the width and thickness direction of slab increases with the increasing of reduction amount. When the solid phase fraction is 0.3, the reduction amount in reduction subzone is 0.15 mm and 0.25 mm, respectively, the effective reduction efficiency decreases from 0.463 to 0.436. When the total reduction amount is 5 mm, the solid phase fraction varies from 0.3 to 0.7, the accumulated displacement of solidification front increases from 0.238 mm to 0.805 mm, the reduction efficiency decreases from 0.46 to 0.32.

A dummy blade of Ni-based single crystal (SX) superalloy was modelled to investigate the stray grain (SG) defect by both experiment and simulation. Three construction manners of mould cluster (CMMC) with 0°, 45° and 90° assembling angle were considered. The experimental results reveal a strong dependence of SG sensitivity upon CMMC. Profuse SGs took place in the case of 0°, but almost no one shown in those of 45° and 90°. The FEM simulation results indicate that SG occurrence is not only determined by critical nucleation undercooling, but also the geometrical characteristic, shape of liquidus isotherm and other three important effects (shadow effect, channel effect and dimension effect). The tendency of SG formation increases with the increase of number of these effects. By means of the combination effect consisting of the above three effects, SG occurrence can be qualitatively predicted and process window is more accurately modified.

The electroslag remelting (ESR) process has been effectively applied to produce high grade special steels and super alloys based on the controllable solidification and chemical refining process. Due to the difficulties of precise measurements in a high temperature environment and the excessive expenses, mathematical models have been more and more attractive in terms of investigating the transport phenomena in ESR process. In this paper, the numerical models for different ESR processes made by our lab in last decade have been introduced. The first topic deals with traditional ESR process predicting the relationship between operating parameters and metallurgical parameters of interest. The second topic is concerning the new ESR technology process including ESR with current-conductive mould (CCM), ESR hollow ingot technology, electroslag casting with liquid metal(ESC LM), and so on. Finally, the numerical simulation of solidification microstructure with multi-scale model is presented, which reveals the formation mechanism of microstructure.

The present work develops a multi-region dynamic coupling model for fluid flow, heat transfer and arc–melt interaction in tungsten inert gas (TIG) welding using the dynamic mesh technique. The arc–weld pool unified model is developed on basis of magnetohydrodynamic (MHD) equations and the interface is tracked using the dynamic mesh method. The numerical model for arc is firstly validated by comparing the calculated temperature profiles and essential results with the former experimental data. For weld pool convection solution, the drag, Marangoni, buoyancy and electromagnetic forces are separately validated, and then taken into account. Moreover, the model considering interface deformation is adopted in a stationary TIG welding process with SUS304 stainless steel and the effect of interface deformation is investigated. The depression of weld pool center and the lifting of pool periphery are both predicted. The results show that the weld pool shape calculated with considering the interface deformation is more accurate.

For further lowering inclusions and improving the quality of steel, a new electromagnetic steel-teeming technology based on electromagnetic induction heating was proposed. To assess the proposed technology, an experimental platform that imitates the actual production condition of steelmakers was established. High temperature experiments were performed to investigate the melting length of Fe-C alloy under different power and frequency conditions. The heating effect was analyzed, and the method of magnetic shielding to reduce the power loss of power supply was put forward. The results show that when the power is 40 kW and frequency is 25 kHz, the melting length of the Fe-C alloy is 89.2 mm in 120 s, which meets the requirements of steel teeming. In addition, when magnetic shielding material is installed under the induction coil, the power loss is reduced by about 64 %, effectively improving the heating effect of power supply.