High Carbon Steel Billets Research Articles

Study on the synergistic effect of electromagnetic stirring and mechanical reduction on the improvement of the internal homogeneity of high-carbon steel billet

The synergistic effect of electromagnetic stirring (EMS) and mechanical reduction on the improvement of the internal homogeneity has been studied during the continuous casting of high-carbon steel billet. Plant trials were conducted with different parameters both for mold electromagnetic stirring (M-EMS) and final electromagnetic stirring (F-EMS), at the same casting condition and with a unified mechanical reduction process. As the stirring intensity of EMS increases, both the initiation and termination of columnar to equiaxed transition occur at an earlier timing. The results also indicate that, EMS could result in a decline trend for the compactness of the casting billet to a certain extent, especially in the pure equiaxed crystal zone at the core part of the billet. Under the proper intensity setting of M-EMS and F-EMS, the crack susceptibility of the high-carbon steel billet was markedly decreased by enlarging the mixed crystal zone, which is not arrayed with distinct directionality while featured with a relatively high compactness. The effect of EMS on the solidification behavior, and therefore on the crack susceptibility of the high-carbon steel billet was further explained with the numerical simulation. Consequently, a superior inner quality of high-carbon steel billet was successfully achieved with the effective synergy of EMS and mechanical reduction.

Phase field simulation on the synergistic variation of solidification structure morphology and segregation for continuous casting process of high carbon steel billets

The Fe-0.82 wt.% C binary alloy was taken as an example to simulate the solidification process of high carbon steel continuous casting billet by phase field method. Synergistic changes of solidification structure morphology and solute element segregation at different solidification stages were studied. The thickness of solute diffusion layer at solidification interface decreased with increasing cooling rate and undercooling. Increasing the degree of undercooling and decreasing the cooling rate was beneficial to increase the degree of complexity. For the segregation of C element, in the early stage of solidification, the greater the degree of undercooling and the cooling rate, the smaller the segregation ratio; and in the late stage of solidification, the degree of undercooling had little effect on segregation ratio, and the greater the cooling rate, the greater the segregation ratio. Therefore, high undercooling and cooling rate should be maintained in the early stage of solidification.

On the Macrosegregation of Continuous Casting of High Carbon Steel Billet with Strand Reduction Process

A mathematical model of the macrosegregation of continuous cast high carbon steel billet was developed based upon a representative volume element, considering the flow of enriched liquid, solidification rate, and solidification shrinkage as well. It was found that a lower casting velocity, higher cooling intensity, and shorter solidification interval positively contributed to the inhibition of macrosegregation in a continuously cast billet when a mechanical reduction process was not applied. A numerical expression for the relative flow velocity of liquid was further proposed incorporating such aspects as casting velocity, densities of different phases, and the variation of cross section areas as well. The analysis based on this numerical expression indicated that the overall effect of the reduction process on the macrosegregation of billets depended not only on the reduction zone but also on the reduction amount and its distribution for the active reduction rolls. The test results of further practical plant trials demonstrated a reasonable agreement with the predictions obtained from the proposed numerical model, indicating the reliability of this analysis model to be employed for the continuous casting of high carbon steel billet with strand reduction process.

Investigation on the effect of reduction process on internal quality of high carbon steel billet and its evolution in as-rolled wire rod

The effect of reduction process, especially the reduction zone on the internal quality of continuous casting high carbon steel billets was studied by shifting active reduction rolls while at same casting condition, as well as identical reduction amount and gradient. In the present study, the heredity of the main internal defects emerged in billets with different reduction zones was also investigated after subsequent rolling process in terms of severity, regional distribution, and microstructure formation as well. For the investigated high carbon steel grade, the evolution of the internal quality in relation to the macrosegregation, internal crack and porosity illustrates the effectiveness and importance of applying mechanical reduction for the homogeneity improvement from the cast billet to the as-rolled wire rod product. Meanwhile, it is also suggested by the investigation the necessity of fine control for the implement of mechanical reduction process, or it may not have sufficient positive impact on homogeneity and compactness of billet and as-rolled product, but some other undesirable defects may even be induced by mechanical reduction.

Investigation and Minimization of Slag Spot Surface Defects in Continuous Casting of High Carbon Steel Billets through Statistical Evaluation

Slag spot surface defects often appear during continuous casting of high carbon steel billets due to the solidification characteristics of molten steel in the mold. To target the problem of surface slag spot defects that occur frequently during the continuous casting of high-carbon steel strands, we analyzed the influence of molten steel superheat, accumulated service time and the water inlet temperature of the mold, the size of the submerged entry nozzle and the physical and chemical properties of the mold powder on the slag spot defects. The production practice shows that by adjusting the superheat of molten steel to 30–35 °C, the water inlet temperature of the mold is stable at 33–35 °C. To adjust the internal and external diameter of the immersion nozzle to 30–70 mm, the viscosity and melting temperature of the mold powder were adjusted from 0.45–0.55 Pa·s, 1100–1140 °C to 0.15–0.25 Pa·s, 1020–1060 °C. The final billet surface quality was improved significantly, the billet surface was smooth, the oscillation marks were relatively smooth and regular and the slag trench ratio was reduced from the original maximum of 40–50% to less than 1%.

Morphology characteristics of solidification structure in high-carbon steel billet based on fractal theory

Solidification structure, as solid skeleton, determines directly the properties of materials, and segregation defects (i.e., uneven distribution of alloy elements) are also affected by the morphology of solidification structure. The high-carbon steel is one of the typical high-end steels, and its solidification morphology is always more complicated and segregation defects are more serious. In this study, solidification morphology of high-carbon steel with 0.82 wt% carbon content in continuously cast was investigated based on fractal theory in order to evaluate solidification characteristics of the liquid steel and control the segregation defects. It was demonstrated that the fractal dimension calculated via the box count method is effective to characterize quantitatively the complexity of the overall morphology of solidification structure instead of only local features. Further, a positive linear correlation (fitting coefficients R2 > 0.95) between fractal dimension and solidification structure ratio (Rstr) was found, i.e., the fractal dimension increases with increasing Rstr. The phenomenon of dendrite morphology complexity, which increases with increasing Rstr, is called the increasingly complicated dendrite overall morphology. It can be attributed to the increasing constitutional undercooling. In addition, the entropy in solidification system calculated by fractal dimension increases gradually in direction from the outer to middle zones of the billet. This indicates that the chaos of the solidifying interface increases gradually, which contributes to the understanding the solidification process. This study proposed a new parameter that depicts the overall solidification morphology from self-similar perspective, thus providing guidance for us to control the solidification morphology and segregation defects delicately.

Continuous Casting of High Carbon Steel: How Does Hard Cooling Influence Solidification, Micro - and Macro Segregation?

One technology that is often employed in continuous casting of high-carbon steel billets to minimize centre - (or macro) segregation is hard secondary cooling. Investigations unanimously show, that hard cooling significantly reduces macro-segregation, but a mechanism for the reduced segregation is rarely given. In this paper the solidification of high carbon tire cord grade C80D cast as a 150x150 mm billet is calculated using the proprietary SMS Group solidification simulation package CHILL using steel properties calculated with the Thermo-Calc Software package and TCFE steels database. The obtained cooling rates in the billet for hard and soft secondary cooling are used to run solidification simulations considering solute redistribution using the diffusion module DICTRA. It is shown that for cooling rates achieved in continuous casting the steel solidifies far away from equilibrium. The solidification profile and solidus temperature lie in between the Scheil solidification model and the para-equilibrium Scheil model with carbon defined as a fast diffusing element. The calculated cooling rates and temperature gradients are used to simulate the solidification microstructure 20 mm from the billet surface using the phase field approach and the MICRESS® software package linked to Thermo-Calc through the TQ interface. This model clearly shows, that the most probable mechanism by which hard cooling reduces segregation is trapping of solutes between the intricately branched dendrite microstructure that results from the steep temperature gradients achieved when applying hard secondary cooling.

Influence of the Speed of High-Carbon Steel Billet in the Patenting Unit on Its Final Structure and Mechanical Properties

Prestressed ferroconcrete structures are widely used at present. As a result, compressive stress is created in the concrete and tensile stress in the reinforcing rope. The stressed reinforcing rope is better able to withstand the external loads that it experiences throughout the life of the construction. Consequently, larger loads may be applied or, with unchanged load, the size of the construction may be decreased, with accompanying savings of concrete and steel. Today, it is important to develop a manufacturing technology for nanostructured reinforcing rope that may be used in prestressed concrete-steel constructions. This technology is based on patenting, in which the steel acquires the structure of a fine ferrite–carbide mixture characterized by high strength and improved deformability. In the present work, the influence of increased billet speed in the patenting unit on the final structure and mechanical properties of steel 80, 70, and 50 is investigated, with a view to increasing the productivity in patenting, without loss of strength or plasticity of the steel, in the production of blanks for nanostructured reinforcing rope that may be used in prestressed concrete-steel constructions. To determine the heat-treatment time and temperature, the Gleeble 3500 system is used to plot diagrams of the isothermal decomposition of undercooled austenite. In qualitative and quantitative analysis of the microstructure, the interlamellar spacing of the ferrite–carbide mixture is determined for different billet speeds in the patenting system. The mechanical properties are studied in tensile tests. It is found that, for all billet speeds, the interlamellar spacing of the ferrite–carbide mixture is practically the same and is optimal for subsequent drawing: 0.1–0.2 μm. Thanks to the fine structure of the ferrite–carbide mixture formed in patenting, the strength of the billet is increased. Hence, in subsequent drawing, the billet may withstand greater compression without fracture. In the production of patented billet for nanostructured reinforcing rope, its speed in the patenting unit may be increased to 5 m/min. Consequently, the productivity may be increased without loss of strength and plasticity of the billet.

Mathematical modelling of morphological transition and spot segregation in continuously-cast high carbon steel billets

In this work, the columnar-to-equiaxed transition (CET) and spot segregation phenomena in continuously-cast (CC) high carbon steel billets are investigated. Casting process is simulated for a 125 mm × 125 mm billet using a macroscopic thermal model. An empirical correlation, depicting the effect of melt temperature on heterogeneous nuclei density, is incorporated in the CET model to account for a more realistic variation of CET with melt superheat. A parametric study is also performed to see the influence of casting speed on CET and explore its theoretical limit for plant operation. The model is sufficiently general and can be utilised for different caster designs and processing conditions. Additionally, an attempt is made to estimate the degree of solute segregation in small spots along the billet centreline using a simple microsegregation model. The model predictions are compared with experimental observations on CC high-carbon steel billets and the results are encouraging.

Mathematical modelling of morphological transition and spot segregation in continuously-cast high carbon steel billets

In this work, the columnar-to-equiaxed transition (CET) and spot segregation phenomena in continuously-cast (CC) high carbon steel billets are investigated. Casting process is simulated for a 125 mm × 125 mm billet using a macroscopic thermal model. An empirical correlation, depicting the effect of melt temperature on heterogeneous nuclei density, is incorporated in the CET model to account for a more realistic variation of CET with melt superheat. A parametric study is also performed to see the influence of casting speed on CET and explore its theoretical limit for plant operation. The model is sufficiently general and can be utilised for different caster designs and processing conditions. Additionally, an attempt is made to estimate the degree of solute segregation in small spots along the billet centreline using a simple microsegregation model. The model predictions are compared with experimental observations on CC high-carbon steel billets and the results are encouraging.

Prevention of Scum Formation and Entrapment in High Carbon Steel Billets

Gradual build-up of scum over the liquid steel surface in the mould gives rise to entrapped exogenous inclusions and slag patch in the cast billets. The problem is more commonly observed during the open stream casting of continuous casting of high carbon (C > 0.6 mass%) steel billets. This problem has been quite commonly observed at Tata Steel during its billet casting. Present work aimed at eliminating the problem of scum formation during open stream billet casting of high carbon steel billets. This work involved experiments in the laboratory as well as in the plant. In addition, based on liquid steel composition thermodynamic calculations were carried out for predicting the possible oxide inclusions in Mn-Si deoxidised steels. Water model experiments were carried out in a full-scale billet caster physical model for investigating the influence of tundish outlet nozzle alignment on the entrapment of scum as slag patch over the billet surface. Based on these, cause of scum formation and its exact mechanism of entrapment in cast billets were established. Finally, countermeasures were recommended to the plant. Adequacy of the proposed countermeasures was established through a series of trials in the plant before implementation. Improvement obtained from the implementation over a period of time has been reported in the paper.

Effect of Dendritic Morphology and Central Segregation of Billet Castings on the Microstructure and Mechanical Property of Hot-Rolled Wire Rods

Microscopic dendritic morphology and central macrosegregation in continuously cast high carbon steel billets with a cross section 150 × 150 mm2 are investigated to elucidate their effect on the microstructure and mechanical property of the following hot-rolled wire rods, whose diameters are 10 and 6.5 mm, respectively. Experimental observations and numerical simulations for the steel billets under different casting conditions are carried out. It is found that the secondary dendrite arm spacing (SDAS) in the billets increases with the increase of superheat temperature, while decreases with the increasing specific water flow. Smaller SDAS in the billet center leads to lower segregation index of carbon (rC) and manganese (rMn) due to the low permeability in the final solidification stage. The measured rMn and rC in billets are in an approximately linear relationship. In addition, for the same rolling and cooling process, the severer carbon segregation in the center of the billets is, the lower the sorbite rate of the rods and the worse stability in the mechanical property of the rods. The tensile strength of wire rods can be improved by adjusting the casting parameters to decrease the SDAS and reduce the central carbon segregation of the billets.

Heat Transfer and Central Segregation of Continuously Cast High Carbon Steel Billet

A numerical model of heat transfer was developed to investigate the heat transfer of continuously cast billet with the aid of surface temperature tests by ThermaCAM™ researcher and nail shooting experiments. The effects of secondary cooling practice and casting speed on the solidification process and central segregation of carbon were investigated as well with the actual central segregation tests. The results show that the surface center and billet center temperatures exhibit a different pattern during solidification, and the solidified shell thickness is presented as an “S” type. With the increase of secondary cooling intensity and the decrease of casting speed, the end points of the solidus line and the liquidus line move forward, and the central segregation level of carbon decreases. The optimal casting condition is suggested for continuously cast high carbon billet with F-EMS (final electromagnetic stirring).

Control of Macrosegregation Behavior by Applying Final Electromagnetic Stirring for Continuously Cast High Carbon Steel Billet

Solidification behavior of continuously cast high carbon steel billets was investigated with an objective of producing high quality billets by determining the optimum final electromagnetic stirring (F-EMS) parameters. Characteristics of centerline segregation were analyzed for lots of billet samples collected from the plant through obtaining the carbon concentrations of drill chips, which were correlated with the operating parameters of the caster and stirrers, but a problem occurred that segregation control results of trial billets with the same casting and stirring parameters often have drastic fluctuations. An attempt was made to find out the induced reasons of this problem by measuring the electromagnetic torque, analyzing the secondary dendrite arm spacing (SDAS) and the corresponding cooling rate of the typical specimens, and observing the longitudinal profile of etched billet samples. Then a simple dynamic secondary cooling model was developed based on the solidified shell thickness control mode, by which the maximum carbon segregation index was reduced effectively, and thus the segregation fluctuation problem was basically solved. Finally, the most favourable stirring parameters were determined as the casting speed of 1.65 m/min, the liquid core thickness of 40 mm, stirring current of 360 A and frequency of 12 Hz.

Influence of electromagnetic stirring on transport phenomena in round billet continuous casting mould and macrostructure of high carbon steel billet

The characteristics of magnetic field, flow field, temperature profile and inclusion trajectories in a round billet continuous casting mould with electromagnetic stirring (M-EMS) were numerically simulated. An industrial plant trial was conducted to verify the magnetic field characteristics and investigate the influence of M-EMS on the solidification macrostructure of high carbon steel. The results indicate that the predicted magnetic field was in good agreement with the measured data, and the velocity patterns, temperature, inclusion trajectory distributions and macrostructure of steel are significantly modified using M-EMS during casting.

Control of Equiaxed Crystal Ratio of High Carbon Steel Billets by Circular Seam Cooling Nozzle

A circular seam cooling nozzle and its online control system have been developed to reduce the center segregation in high carbon steel billets by decreasing the superheat of the molten steel and improving the equiaxed crystal ratio based on the numerical results. An industrial experiment has been carried out on a 150 mm × 150 mm caster to investigate the effect of the circular seam cooling nozzle on the superheat removal of the molten steel. The results show that the circular seam cooling nozzle can be used to control the casting temperature in a closed loop control system. The online control system can be effectively adapted to the variation of operating parameters. The casting lasts about 4 h and about 400 t steel is successfully produced in a continuous operation. The removal of about 14°C superheat and the improvement of approximate 10% equiaxed crystal ratio can be achieved by the newly developed circular seam cooling nozzle.

Reducing the melt heating in the mold to enhance rail-steel quality

The macrostructure of continuous-cast high-carbon steel (in particular, rail steel) billet may be improved by introducing macrocooling additives to prevent overheating of the melt in the casting-machine mold. The liquidus temperature of the macrocooling additives is 5–20°C lower than for the cast steel. Macrocooling strip is alloyed with boron, which ensures additional modification of the rail steel.

Effect of electromagnetic stirring in mold on the macroscopic quality of high carbon steel billet

An industrial plant trial for optimizing the process parameters in a round billet continuous casting mold with electromagnetic stirring (M-EMS) was performed, in which the influences of stirring parameters with M-EMS on the solidification macrostructure of high carbon steel were investigated. The results show that the billet quality is not well controlled under the condition of working current and frequency with EMS, in which the subsurface crack of grade 1.0–2.0 ups to 38.09%, the central pipe of grade 1.0–1.5 reaches to 14.28%, and the central porosity of grade 1.5 is 14.29%. The parameters of current 260 A and frequency 8 Hz as the final optimum scheme has a remarkable effect for improving the macroscopic quality of billet, in which the subsurface crack, central pipe and skin blowhole are all disappeared, and the central porosity and carbon segregation are also well improved.

Quantification of the Solidification Microstructure in Continuously-Cast High-Carbon Steel Billets

In this work, an attempt has been made to investigate the relationship between the cast microstructure and solidification variables in industrial scale, continuously-cast (CC) high-carbon steel billets. Toward these, theoretical and experimental studies are undertaken to predict the evolution of dendrite arm spacing (DAS) in the columnar zone of CC billets. Several billet samples collected from the continuous casting shop of Tata Steel are used to characterize the solidification microstructure, and interdendritic arm spacings (both primary and secondary) are measured. Macrostructural examination of the billet samples indicates predominantly columnar structure in all billets. Dendrite arm spacings vary over a wide range indicating nonuniform secondary cooling. A mathematical model is also developed to describe the relationship between dendrite structures and solidification parameters. The model considers the effect of change of volume on solidification and provides a quantitative estimation of variation of DAS as a function of distance from the product surface. Results predicted by the mathematical model are compared with the experimental measurements and good agreement can be observed in this regard, thereby establishing the authenticity of the proposed formulation.

Morphology and Segregation in Continuously Cast High Carbon Steel Billets

Morphology and centerline macrosegregation in continuously cast high carbon steel billet samples were investigated in order to establish the casting behavior of a six strand curved mold billet caster without electro-magnetic stirring. Several billet samples were collected from the continuous casting shop of Tata Steel, India and experimental observations were correlated with the operating parameters of the caster. Macrostructural examination revealed predominantly columnar structure associated with high degree of segregation and porosities in all billets cast above 21 °C of tundish superheat. Centerline porosity was practically absent in the billet cast below 21°C. These billets showed prominent V-segregation and less prominent centerline segregation. Transition from U-segregation to V-segregation was observed around 21°C superheat. An attempt has been made to study the effectiveness of secondary cooling by measurements of the secondary dendrite arm spacing at various locations in billet samples. Finally, degree of segregation of constituent solute elements were correlated among themselves and applicability of one of the simple segregation models to the centerline macrosegregation has been tested.