Research And Application Of Temperature Controlling During Low-Temperature Coiling And U Cooling Strategy Based On Ultra-Fast Cooling

High-precision coil temperature control to meet the requirement of microstructure and mechanical properties is the main contents during cooling control after rolling. Based on the transformation project of cooling system after rolling which aims to enhance product quality of LYsteel.Hunan province, a further research of low-temperature control under ultra-fast cooling (UFC) was accomplished. Meanwhile, the U cooling strategy was also developed to improve or solve the non-uniformity of the mechanical property along the rolling depiction, which was caused by the uneven heat dissipation. Simultaneously, a satisfactory effect was got during the practice application. And the main contents and achievement of the research are as follows:(1) The heat conduction equation was established to analyze the temperature distribution along thickness during the Low-Temperature Coiling based on UFC. By dispersing the completely implicit infinite difference scheme of unconditional convergence, and inverted heat conduction method was used to calculate the equal average heat transfer coefficient. The result indicated that equal average heat transfer coefficient of UFC nozzles is about8000W/(m2-K). Under the condition, a further research was made to analyze the temperature distribution along thickness direction of different steel grade and thickness. The research demonstrates that the maximum ultra-fast cooling temperature (UFCT) difference between the surface and center is35℃with the thickness of12mm, meanwhile the difference is enhanced with the thickness increasing. And the temperature rising time is prolonging with the thickness increasing, thus, the temperature difference is also further enhanced.(2) The temperature field of the laminar cooling after ultra-fast cooling during the low-temperature coiling was analyzed systematically. The result indicated that the maximum instantaneous temperature deviation between the strip center and surface is up to35℃, and the deviation can be shortened according to the temperature rising during the succeeding air cooling. For the same thickness specifications of steel strip, the temperature deviation is decreasing during laminar main cooling process. For strip thickness of12mm, the deviation between the center and surface can reduce to10℃or even below according to the temperature rising.(3) According to the problems in practical production of Low-Temperature Coiling, with the precondition to meet the demands of mechanical property, reduce the influence on the property caused by the instantaneous temperature deviation, the front main cooling was adopted and the fine cooling zone nozzles was reduced to make sure the measured coil temperature meet the actual value. To deal with the problem of low temperature region after strip hot head, a special treatment to meet different target cooling temperature was made to solve the problem. Thus, the length and temperature control precision was improved significantly. For different steel grade and strip steel thickness with target CT lower than500℃, the field practice application indicates that the hit rates of coil temperature with the deviation is40℃is more than80%.(4) U Cooling control Strategy is researched and developed to solve the problem of uneven mechanical properties along the rolling direction which mainly caused by non-uniform heat dissipation after coiling. And U Cooling coefficient can be set flexibly to offset the uneven mechanical properties reasonably. The field practice shows that control temperature can be realized within18℃of the target value, coupled with high precision length control to meet the requirement of actual production. Meanwhile, the temperature field was analyzed under U cooling strategy, which provides references for the cooling coefficient setting.(5) According to achievements above mentioned in field practice, a satisfied result was achieved. And the practical production indicated that the precision of coiling temperature of low alloy structural steel and low-cost pipeline steel X70under Low-Temperature Coiling is higher than other cooling control model. The microstructure and mechanical properties of product is stable, and bring significant economic benefit for LYsteel.