Study On Thermo-mechanical Behavior In Secondary Cooling Process Of Slab Continuous Casting

During the continuous casting of the slab, the heat transfer during the solidification process and the stress evolution process of the slab will directly have great influence on the quality of the slab. Therefore, studying the heat transfer and the stress distortion under the non-uniform cooling condition in the secondary cooling zone will make a significance in setting reasonable technological parameters of the continuous casting and improving the quality of the slab. Based on the continuous casting slab of 1600mm×220mm made from a steel plant in china, a Three-dimensional thermal-mechanical coupled model for slab continuous casting has been established by numerical simulation method under the practical continuous casting process and the spray cooling condition of Q235B. In order to enrich the heat transfer of solidification and stress distortion theory in the secondary cooling zone of continuous casting process and to provide theoretical foundation and guidance for the production, this study has analyzed the thermal behavior of temperature field of the solidified shell,the growth morphology of the shell and liquid core as well as the variation of two phase zone under the non-uniform cooling of the slab, the solidification shrinkage and stress evolution of the solidified shell have also been considered. The main conclusions are summarized as follows:(1)Non-Uniform solidification is a typical phenomenon during the continuous process of the slab. The slab will completely solidify at the 1/4 position of the width direction at the earliest, while at the 1/8 position at the latest, and the corresponding liquid cores below the meniscus are 20.42m and 20.82 respectively. When the 1/4 position at the width direction of the slab completely solidified, the thicknesses of the liquid cores between the center of the width and the 1/8 position of the width are 5.1m and 11.9m respectively.(2)The shrinkage distortion and the stress distribution of the solidified shell within the casting process changed dramatically during the continuous casing process of the slab, and both of them showed a dynamic variation along a width direction of the slab. In the upper part of the strand(the distance below the meniscus is less than 16m), there will be a remarkable shrinkage at the corner of the shell along the thickness direction, and the average shrinkage reached 0.23mm/m. While in the middle and lower part of the stream(the distance below the meniscus is greater than 16m), the most remarkable shrinkage will occur at 200m from the narrow face, and the average shrinkage dropped down to 0.212mm/m together with the transfer of the maximum of the shrinkage along the thickness direction from the corner to the central part in the width of the slab and a greater stress variation at the corner of the slab.(3) The variation of the casting speed will have different impacts on the solidification heat transfer and the stress transformation behavior of the shell in the stream. The overall temperature of the slab will rise up with the increasing of the casting speed, leading to the slower growth rate of the solidified shell and the longer length of the two phase zone. When the casting speed increased from 0.9m/min to l.lm/min, the two phase zone lengths of the 1/8, 1/4 and the central part of the width increased to 1.89m,1.8m and 1.85m respectively. Meanwhile, the thermal strain and thermal stress of the solidified shell decreased with the increasing of the casting speed. When the casing speed increased from 0.9m/min to 1.1m/min, the heat shrinkage at the corner and the central width of the solidification end of the slab will decrease by 0.66mm and 1.3mm respectively, and the corresponding thermal stress will decrease from 87.6MPa to 67MPa and from 28Mpa to 6.7MPa respectively.(4) Soft reduction will play a significant influence on the mechanical behavior of the slab. The overall strain will keep rising with a stepped tendency under the application of the soft press, and the maximum of the strain in the slab surface reached 0.043 at the end of the soft press. There will be a dramatic fluctuation of the equivalent stress at the corner of the slab, and the value kept increasing with the application of the soft press. When the soft press came to end, the equivalent stress at the corner of the slab is about 40MPa, and there will be a little fluctuation of stress in the central part and little rising trend of the stress in the soft reduction process.