Abnormal transient phenomena in the continuous casting process: Part 2

Abstract

This paper is the second of a series of two describing abnormal transient phenomena observed during online monitoring of a billet continuous casting machine. Special attention is paid in Part 2 to in mould solidification. A mould heat flux drift phenomenon (HFD) has been detected, but only for mould powder basicities larger than 0·8. The HFD is related to a decrease of the heat flux in the lower part of the mould and an increase in both the billet-mould friction force and mould thermocouple variability. Results of tests changing the mould powder grade during casting have provided help in explaining the HFD. The probable reason for the HFD is crystallisation of the glassy slag layer. The heat flux ratio parameter (HFR), defined as the ratio between the heat flux in the lower part of the mould and the heat flux in the upper part of the mould, has proved to be a good tool for judging the casting performance of a mould powder.     

Behaviour of inclusions in RH vacuum degasser

Abstract

Vacuum degassing processes are now an integral part of secondary steelmaking operations and besides the more obvious benefits of steel chemistry control to precise specifications, they also reduce the number of non-metallic inclusions. Care has to be taken, however, that the change in steel composition through loss of C, O, Mn, etc. does not result in a deleterious change in the composition of the remaining inclusions. Attempts in the past to determine the effect of vacuum on inclusion compositions have been through the use of thermodynamic models, following the inclusion engineering approach. The calculated inclusion compositions do not, however, compare well with the inclusions as analysed in samples taken from the liquid steel after the degassing operation. Clearly, it is important to take into account time dependent effects during degassing and this has been achieved by the development of a combined fluid flow-thermodynamic model. Using the computational fluid dynamics model, CFX, to establish the temperature, flow and species contours in a two-dimensional steel ladle under the influence of natural convection, the results are transferred as start conditions in a three-dimensional RH degasser model. A body force is then applied to simulate the argon bubbles that are injected into the up-leg of the degasser and changes in flow, temperature and species concentrations are calculated. Allowing for additions made during the process, the composition of the top slag and any local inclusions within the steel is predicted. The influence on top slag and inclusion chemistry of any glaze on the snorkels of the degasser is also taken into account.     

Kinetic model of desulphurisation by powder injection and blowing in RH refining of molten steel

Abstract

The desulphurisation by powder injection and blowing in the RH refining of molten steel and its mechanism have been considered and analysed. Based on the two-resistance mass transfer theory and the mass balance of sulphur in the system, a kinetic model for the process has been developed. The related parameters of the model, including the mass transfer coefficients and the effective amount of powder in the molten steel being treated for desulphurisation, have been reasonably determined. Modelling and predictions of the process of injecting and blowing the lime based powder flux under assumed operating modes with the different initial contents of sulphur and amounts of powder injected and blown in a RH degasser of 300 t capacity have been carried out using the model. The relevant circulation rate of the liquid steel and the powder injection and blowing rate were taken to be 100 t min^-1 and 150 kg min^-1, respectively. The initial contents of sulphur in the liquid steel to be treated and the amounts of powder injection and blowing were respectively assumed to be 0·007, 0·006, 0·005, 0·004, 0·003, 0·002 wt-%and 10, 8, 6, 5, 4, 3 kg/t steel. The total treatment time for desulphurisation under each mode was set at 24 min, equivalent to eight circulation cycles of the liquid steel to be treated. The results indicated that the predictions made by this model are in good agreement with data from industrial experiments and practice. By injecting and blowing the lime based powder flux with the composition of 85 wt-% lime (CaO) + 15 wt-%fluorspar (CaF₂ ) of 3–5 kg/t steel, it is possible to decrease the sulphur content in the molten steel to an ultralow level below (5–10) × 10^-4 wt-%from (60–80) × 10^-4 wt-%. The total treatment time needed will be 12–20 min. Intensifying the powder injection and blowing operation and increasing the circulation rate of the liquid steel may effectively increase the rate of the process in RH refining. The model may be expected to offer some useful information and a reliable basis for determining the reasonable process parameters and help in optimising the technology of desulphurisation by powder injection and blowing in the RH refining of molten steel.     

Slag entraining vortexing funnel formation during ladle teeming: similarity criteria and scale-up relationships

Abstract

There are several motivations for minimising slag entrainment during the teeming of steelmaking ladles. Cleaner steel, improved yield, and higher productivity are all at stake. As one of several identifiable contributors to slag entrainment, vortexing has received considerable attention in the past decade and a half. What is commonly referred to as 'vortexing' in fact comprises two distinct phenomena, namely, vortexing funnels and non-vortexing funnels, each controlled by entirely different sets of variables. Dimensionless correlations describing the two phenomena were determined, and validated, using separate sets of dimensional analyses and appropriately designed scale model experiments. The importance of these findings to the teeming of steel is discussed. Performance results of a patented 'vortex buster' device, developed on the basis of the understanding gained from these studies, and validated in water models as well as in a 12 ton tundish, are also presented.     

Fatigue failures in low pressure die cast AlSi10Mg cylinder heads

Abstract

Exposed to both high and low cycle fatigue, the modern automotive cylinder head is certainly a demanding application for aluminium alloys. AlSi10Mg cylinder heads were investigated to identify the material issues which impact their performance. Fatigue cracks occurred at or very near the water jacket surface, shown to experience the highest residual stresses after quenching. The majority of these cracks were linked with either extensive porosity, insufficient level of modification or with Fe based intermetallic platelets. The predominance of such features near the water jacket surface is accounted for by the relatively slower solidification near the water jacket surface. Fatigue cracks were almost completely avoided by keeping the density index below 2 to control porosity, by improving the solidification rate to achieve SDAS smaller than 35 micrometres, by limiting the Fe level to a maximum of 0˙12 wt-%, by using a higher quality sand for the manufacture of water jacket cores and, finally, by adopting polymer water quenching practice during heat treatment.     

Determination of dynamic fracture toughness of heat affected zone of 9Cr–1Mo steel from instrumented drop weight tests

Abstract

The dynamic fracture toughness (K_1d) of the heat affected zone (HAZ) of 9Cr–1Mo steel at and below the nil ductility transition temperature has been estimated from instrumented drop weight test results. The presence of a significant microstructural gradient in the HAZ, comprising coarse, fine, and intercritical regions with sharp toughness differences, is reflected in the presence of distinctive load peaks in the load–time traces; the K_1d estimates for the coarse, fine, and intercritical regions are 53, 85, and 128 MPa m^1/2 respectively. The results from the 9Cr–1Mo steel were compared with those for AISI grade 403 martensitic stainless steel. The lack of distinctive multiple load peaks in the load–time traces for the latter is attributed to the absence of a steep toughness gradient, owing to a more or less uniform martensitic microstructure in the HAZ.     

Study on cascaded whole-leaf spring oscillation mechanism for mould in continuous casting

Abstract

A design method of a cascaded whole-leaf spring mechanism is proposed, which is a new oscillation guidance device for the mould in continuous casting. Then its prototype designed in this paper is produced in the lab, of which kinematics and dynamics simulations are carried out based on the rigid–flexible coupling virtual model. Simulation curves of the displacement and velocity of the mould are almost consistent with the ideal ones, which verifies the model built in this paper is rational. Furthermore, natural frequencies and mode shapes of the mechanism are calculated by dynamics simulation, and forces applied on leaf springs and revolute joints are analysed and effects of the basic parameters on these forces are also studied, which establish the basis for further studies and next application of this mechanism.     

Role of grain refining in hot cracking and macrosegregation in direct chill cast AA 7075 billets

Abstract

Direct chill (DC) casting experimental results are presented for an Al–Zn–Mg–Cu alloy with and without grain refining by attempting the inoculant addition either in the furnace or in the launder at two different casting speeds. Despite considerable structural refinement macro segregation remains unchanged with grain refining at a lower casting speed. Hot cracking, on the other hand, is totally prevented at this casting speed. Depending on the grain refining practice, either hot tears do not appear at all or prior cracks existing in the non-grain refined billet are completely healed. At a higher casting speed, the severity of segregation increases. The hot cracking tendency, however, has shown a distinct difference with grain refining method, with furnace-refined alloy resulting in a crack-free billet. But the billet grain refined in the launder exhibited hot cracking. Overall, these experimental results confirm the beneficiary effect of grain refining on hot cracking.     

Modelling of reoxidation inclusion formation in steel sand casting

Abstract

A model is developed that predicts the growth and motion of oxide inclusions during pouring, as well as their final locations on the surface of steel sand castings. Inclusions originate on the melt free surface, and their subsequent growth is controlled by oxygen transfer from the atmosphere. Inclusion motion is modelled in a Lagrangian sense, taking into account drag and buoyancy forces. The inclusion model is implemented in a general-purpose casting simulation code. The model is validated by comparing the simulation results to measurements made on production steel sand castings. Good overall agreement is obtained. In addition, parametric studies are performed to investigate the sensitivity of the predictions to various model parameters.     

Influence of material swelling on surface roughness in diamond turning of single crystals

Abstract

The effect of material swelling on the surface roughness in ultraprecision diamond turning has been investigated. Experimental results from the power spectrum analysis indicate that the profile of the tool marks is distorted by the effect of swelling of the materials being cut. A good correlation exists between the surface roughness and the amount of swelling that has occurred in the machined layer. Radically different surface roughness profiles were obtained when machining aluminium and copper single crystals with the same cutting plane and tool shape. The difference in the machining behaviour could not be accounted for by elastic recovery alone but could be explained by considering the plastic deformation induced in the machined layer.