A Study about the Influence of Carbon Content in the Steel on the Casting Behavior

Abstract

In the casting process of steels with a C‐content ranging from 0.09 to 0.53 mass%, austenite is formed as secondary crystal phase by peritectic reaction between crystal of δ ferrite and residual melt. For unalloyed or micro‐alloyed steels the C‐content or C‐equivalent influences the casting behavior of steel in the mould, such as strand shell growth, crack formation, heat transfer, temperature fluctuation in the copper plate, mould level fluctuation and oscillation marks formation. The negative casting behavior like the uneven strand shell growth, the deep oscillation mark formation, the high mould level fluctuation, the crack formation on the strand surface were found mostly for steel with C‐content or Cp between 0.10–0.13 mass%. The strand shell structure (strand shell growth, mushy zone, δ + γ phase transformation) and shrinkage of the strand shell were simulated depending on the C‐content by means of mathematical simulation. On the basis of the simulation results and of the measured high temperature strength of steel the dependence of stiffness and the irregularity of the shrinkage of strand shell on the C‐content was investigated. It was found that the stiffness and irregularity of the shrinkage of the strand shell reach the maximum value at a C‐content of about 0.12 mass%.

Initial Solidification and Its Related Heat Transfer Phenomena in the Continuous Casting Mold

“…Mold cooling is a self‐limiting process essentially. As the shell grows thicker, its thermal resistance increases, causing the total thermal resistance to increase and the rate of heat removal decrease . When the shell formation in the mold reaches a limiting thickness that is dependent on the total heat removal in the mold, it would influence the further growth of the shell.…”

Analysis of Crack Susceptibility of Peritectic Steels Based on Surface Roughness

“…It is well known that during continuous casting, and especially high‐speed casting, surface longitudinal cracks form more easily in slabs of peritectic steel than in other steels . The occurrence of cracks is related to the initial ferrite shell undergoing a peritectic phase transition: liquid ( L ) + ferrite ( δ ) → austenite ( γ ).…”

“…A volume contraction of about 6% accompanies the progression of the incoherent δ / γ interface during peritectic transformation ( δ → γ ) due to the diverse crystal structure, and there is a 4% mismatch between the thermal contraction coefficient of δ and γ . These changes lead to a distinctly wrinkled shell, and the mismatch in the contraction coefficients and the uneven shell lead to the accumulation of stress and longitudinal cracking . For the continuous casting process, it is very important to distinguish between the cracking tendencies of specific steels because longitudinal cracking can be reduced by selecting a lower casting speed and special mold powders for a steel that is susceptible to cracking …”

A Novel Approach for Evaluating the Contraction of Hypo-Peritectic Steels during Initial Solidification by Surface Roughness

“…It is well known that hypo-peritectic steels are more sensitive to deep oscillations, surface shape variations, cracks, and even breakouts than low or high-carbon counterparts during continuous casting, especially at high casting speeds [ 1 , 2 , 3 , 4 , 5 ]. The surface quality problems are strongly related to the complicated peritectic transition process, which leads to nonuniform volume contraction of hypo-peritectic steels during the initial solidifying stage in the mold [ 4 , 5 , 6 , 7 , 8 ]. However, because peritectic steels have excellent mechanical properties, such as Advanced High Strength Steel and High Strength Low Alloy [ 9 , 10 ], and low manufacturing costs, they are still produced preferentially, even though surface defects are often encountered.…”

“…The subsequent growth of γ into the δ and L phases, respectively, is classed as the peritectic transformations. Generally, severe volume shrinkage and defect susceptibility occur when casting steels with carbon content between 0.10%C and 0.18%C, because the peritectic transformation of δ to γ coincides with the final solidification and ends in the solid during the early solidification stage, which can enhance the thermal contraction [ 7 , 8 , 9 , 13 ]. Therefore, the degree of contraction and defect susceptibility of steels are always evaluated according to the relationship between the carbon content and the position of the peritectic point.…”