The High Temperature Mechanical Properties And Constitutive Relations Of Continuous Casting Steel

Continuous casting is a mainstay of the modern steelmaking, and quality of continuous cast slab has important influence on the final product quality of steel. The high temperature thermo-mechanical behavior is the essential reason for the occurrence of cracking during continuous casting. Thus, knowledge of the high temperature thermo-mechanical behavior of carbon steel is required for achieving quality cast products. In this study, the high temperature deformation behaviors of UHSS and Micro-alloyed Carbon Steel are investigated with reheated type and solidified type thermal histories through the tensile tests using Gleeble-1500D system. The high temperature deformation behavior of UHSS and Micro-alloyed Carbon Steel, i.e., the characteristic temperatures, hot ductility curves and stress-strain curves, are obtained both in the solid state and in the mushy zone. Then, the influence of thermal history, strain rate and cooling rate on the high temperature mechanical property is analyzed. Finally, the constitutive relations both in solid state and in mushy zone are constructed according to the tensile test results.The characteristic temperatures such as zero strength temperature (ZST) and zero ductility temperature (ZDT) are measured under the cooling rates of 5℃/s and 15℃/s, respectively. The ZDT decreases as cooling rate increases, and the effect of cooling rate on the ZST can be nearly neglected. The corresponding solid fractions of ZDT and ZST of UHSS are 0.99 and 0.75, and that of Micro-alloyed Carbon Steel are around 0.99 and 0.70, respectively. According to the hot ductility curve, the temperature range is separated three parts, i.e., melting temperature～1200℃, 1200～900℃and 900～750℃. The temperature ranges of melting temperature～1200℃and 900～750℃are brittle temperature ranges, and the reduction of area in the temperature range of 1200～900℃are all above 60%.The tensile tests results with reheated type and solidified type thermal histories indicate that:the tensile strength of the two steel grades decreases with increasing test temperature and decreasing strain rate, and the tensile strength decreases rapidly when the temperature is heated into the mushy zone. The tensile strength decreases linearly with increasing temperature at the identical strain rate in the mushy zone. The tensile strength increases as the strain rate increases, and the tensile strength increases with the increasing strain rate generally in the mushy zone. The tensile strength with reheated type thermal history is higher than that with solidified type, and the effect of thermal history on the tensile strength can be contributed to the different activation energy for deformation. The tensile strength with solidified type thermal history increases as the cooling rate increases.The obtained stress-strain relations of y phase and 8 phase under different thermal histories of UHSS and Micro-alloyed Carbon Steel are analyzed with a simple viscoplastic constitutive equation involving variables of temperature, stress, strain rate and activation energy for deformation. The relation between solid fraction and temperature during solidification is calculated by a microsegregation model in mushy zone. Finally, the constitutive relations in mushy zone are constructed, and critical strain and critical stress for internal crack is obtained. The calculated flow curves are in good agreement with the experimentally measured ones, so the constitutive relation can be well recommended to describe the high temperature mechanical behavior of UHSS and Micro-alloyed Carbon Steel both in the solid state and in the mushy zone.