Simulation Research On Temperature Controlling During Hot Delivery Process Of HSLA Steel Slab

Hot charging rolling technology has been widely used in the compact steel plantbecause of its great significance for saving energy and smoothing production process.However, high strength low alloyed slab with the increasingly widespread used, tendsto form more surface defects when the hot charging rolling process is applied. Thecracking problem has become the technical bottleneck for the large steel enterprises torealize process optimization and energy saving.This paper is based on the hot charge rolling technology of the second caster in thesteel plant of Chongqing Iron and Steel Company Limited. In order to realize the hotcharge rolling process of high strength low alloyed steel slab, the heat transferconditions of the direct roller hot charging, low-temperature hot charging and surfacequenching hot charging have been discussed. The heat transfer mathematical models ofthree different hot charging process mentioned above have been established. Thethermal physical parameters and coefficient of heat transfer have been considered, andthe finite element method was used to build the heat transfer models. Particular, thetemperature variation during the surface quenching hot charging process was analyzedas a key problem. The heat transfer behavior between cooling medium and hightemperature slab during the quenching process was discussed through experimentalstudy. The integrated heat transfer coefficient was regarded as a nonlinear function ofwater flow density and slab surface temperature, and was used in the surfacequenching hot charging model.Through the temperature simulation of the different charging processes, the resultshows that the charging temperature of the slab surface is between the two-regiontemperature with the direct roller hot charging procedure. So the direct roller hotcharging can’t be applied in the high strength low alloyed steel slab since theappearance of hot cracks. However for the low-temperature hot charging processwhich keeps the charging temperature below two-region temperature though stackcooling, this method can reduce the occurrence of hot cracks, but it takes a long timeand causes a large heat loss, the energy saving effect is greated reduced. The slabwhich experienced the surface quenching hot charging without offline, takes the sametime as direct roller charging procedure. After quenching process, the temperature ofthe slab surface drops below two-region temperature but the center remains high temperature. As the slab conveying to the reheating furnace, the surface temperature isallowed to recover above600℃from the residual heat in the centre of the slab. Theenergy-saving effect is still evident, and the total energy-saving is95%-75%of directroller hot charging.According to the different cooling rate during the quenching process, differentmicrostructure such as pearlite and ferrite, bainite, martensite, or their mixed structureswill be achieved. The pearlite and lower bainite are proved to be the effectivequenched microstructure according to the CCT diagram and the studies of the slabcracking conditions at reheating furnace with different quenching rates. At the sametime, combining with the specific conditions of quenching equipment at the steel plant,the cooling rate of the high strength low alloyed steel was determined.Combining the quenching requirements such as begin temperature, quenching rate,thickness of quenching layer and energy saving effect, the existing quenchingequipment has been optimized. The quenching parameters of the high strength lowalloyed steel slab were determined through numerical simulation. Considering theslower slab forward speed will reduce the productivity on the one hand, and largerwater flow rate will increase the thermal stress of the slab and lead to cracks on theother hand. Therefore, taking the median of the range mentioned above is mostappropriate. At the end of this paper, a surface quenching process was conducted insteel plant, the temperature variation and organization changes during the quenchingprocess was discussed. The result shows that, the surface quenching hot charging forhigh strength low alloyed slab has a bright future in the development and utilization.