Study On Thermal/Mechanical Behavior And Microstructure Evolution Of Thick Continuous Casting Slab During High Temperature Quenching

Microalloyed steel has excellent properties and a wide range of application,which is the main product of domestic and foreign steel companies.The hot charging and heating rolling process of the continuous casting slab frequently heating up and sending cracks,which is a common problem in the industry.Based on the microalloyed steel thick slab continuous casting and cooling process,this paper is based on the micro-alloyed steel thick slab continuous casting and cooling process to carry out continuous casting slab online high temperature full continuous quenching key theory and process research,which has guiding significance for the high-efficiency,defect-free and green production of microalloyed steel.This thesis first studied the key temperature control parameters of microalloyed steel under different controlled cooling conditions,and measured the heat transfer coefficients of the inner and outer arc supercooling conditions of the casting slab based on the actual continuous casting slab laboratory.On this basis,combined with the actual thick slab continuous casting process of a steel plant,the three-dimensional temperature field and stress field finite element calculation model of the typical microalloy steel continuous casting and quenching process were constructed,and the continuous casting slab in the second The solidification and heat transfer law in the cold casting stream and the super-strong full-continuous quenching process of the thick slab at the end of the continuous caster obtain the following conclusions:(1)Through thermodynamic and kinetic analysis of the precipitation of Nb carbonitride in the continuous casting slab,it is calculated that the carbonitride grain boundary,uniform and dislocation nucleation mechanism nose points are respectively 920℃,840℃ and 780℃.The analysis of the precipitation state of the second phase particles of the continuous casting slab by transmission electron microscope shows that the quenching speed is controlled above 4℃/s to effectively control the chain precipitation of carbonitride.Combinng the Gleeble experimental analysis with the metallurgical microscope,the phase transition temperature of the slab structure at different cooling speeds is obtained and the structure of the quenching at different starting temperatures is analyzed.It is concluded that the quenching starting temperature can be effectively controlled by adjusting the quenching starting temperature above 800℃.which has a fundamental effect on the amount of ferrite grain boundary precipitation.(2)The empirical formula between the amount of water and the heat transfer coefficient is obtained by the inverse heat conduction method through the field measurement of the temperature of the experimental slab during the quenching experiment and the finite element simulation.Due to the water retention phenomenon in the inner arc of the cast slab during the quenching process,the empirical formulas for the water volume and heat transfer coefficient of the inner and outer arc cast slabs are solved separately,and the ratio of the inner and outer arc water volume can be solved by the empirical formula.(3)The empirical formula between the water volume and heat transfer coefficient in the secondary cooling and the mold summarized by the predecessors can be used to obtain the temperature change and force deformation of the slab in the traditional continuous casting process,and the traditional process is used as the premise to analyze Quenching process.When the casting speed is 0.9m/min,the quenching distance is 2m,and the inner and outer arc water flow densities are 3.36L/m2·s and 10.93L/m2·s,the inner and outer arc temperature and deformation of the slab are basically the same.The temperature of the center of the wide face and the narrow face and the corners are 407℃,354℃ and 308℃ respectively,and the quenching depth can reach 10mm or more;through the quantitative analysis of the quenching process,it can be seen that the casting slab after quenching with every increase of the drawing speed by 0.1m/min The temperature of the center of the wide face and the center of the narrow face and the corners are increased by 12℃,16℃,and 17℃ respectively,and the shrinkage of the casting slab decreases by 0.125mm every time the drawing speed is increased by 0.1m/min.(4)The quality of the quenching process continuous casting slab was tested by transmission electron microscope and metallographic microscope.The quenching process precipitates were dispersedly distributed,the grain size was significantly reduced,and the grain size was uniform,which effectively improved the structure and performance of the continuous casting slab.