Abstract
The contraction of peritectic steels in the initial solidification has an important influence on the formation of surface defects of continuously cast slabs. In order to understand the contraction behavior of the initial solidification of steels in the mold, the solidification process and surface roughness in a commercial hypo-peritectic and several non-peritectic steels were investigated using Confocal Scanning Laser Microscope (CSLM). The massive transformation of delta-Fe (δ) to austenite (γ) was documented in the hypo-peritectic steel, which caused surface wrinkles and greatly increases the surface roughness of samples in the experiments. Surface roughness (Ra(δ→γ)) was calculated to evaluate the contraction level of the hypo-peritectic steel due to δ–γ transformation. The result shows that the surface roughness method can facilitate the estimation of the contraction level of peritectic transformation over a wide range of cooling rates.
Highlights
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]
Measurement of the stress and strain during solidification is a direct way to analyze the relationship between phase transformation shrinkage and the cooling conditions, for example, by use of high temperature tensile test apparatus to measure the strength during solidification [31] and submerged split-chill tensile (SSCT) testing for measurement of tensile forces [32]
Confocal Scanning Laser Microscope (CSLM) equipment was used to study the variation of the surface roughness of steels subjected to large cooling rates at the initial solidification stage
Summary
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]. Measurement of the stress and strain during solidification is a direct way to analyze the relationship between phase transformation shrinkage and the cooling conditions, for example, by use of high temperature tensile test apparatus to measure the strength during solidification [31] and submerged split-chill tensile (SSCT) testing for measurement of tensile forces [32] The former needs to accurately control the direction of dendrite growth with near equilibrium conditions of low cooling rate, whereas ensuring the uniform shell growth around the mold is required for SSCT tests. Peritectic transformation causes uneven deformation of the shell during the initial solidification, wrinkles are generated on the solidified surface on the microscopic scale [21,22,23] These wrinkles are caused by contraction strain and the relationship between stress strain and wrinkles (surface roughness) has been studied in several materials, including steels [33], brass sheets [34], and aluminum alloys [35]. The surface roughness method is put forward to provide a better insight into the shrinkage of hypo-peritectic steels during the initial solidification stage
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