Experimental Research Of Cold Rolled Strip Under Liquid Nitrogen Cooling And Lubricating Condition

Cooling and lubricating in cold rolling process is an important technological link, and it is the key technology of cold strip rolling process. In traditional cold rolling process, water/oil emulsions are often used in rolling process. But taking into account of existing problems of the traditional emulsion rolling, this paper puts forward a method of using liquid nitrogen to replace the water/oil emulsion for cooling and lubricating technology. It is possible to solve problems in traditional cold rolling process, for example, emulsion pollution, circulation system maintenance difficulties and the control of the strip surface quality and so on. The research aimed at the problems of liquid nitrogen cooling for cold rolling lubrication theory and design of the new cooling system. Through the research, we can make clear the cooling nature of liquid nitrogen in the cold rolling process, and it’s hopeful to set up the new cold rolling mill cooling system. In addition, it provides new ideas for a new generation of rolling cooling and lubrication technology development. The main work and results are as follows:(1) In this paper, through theoretical analysis and calculation of liquid nitrogen injection process, based on the theory of liquid nitrogen stable film boiling, puts forward the theory of heat transfer coefficient calculation model, and analyses the influence of a single variable liquid nitrogen injection coefficient of heat transfer. In the basis of the theoretical analysis, the influence of liquid nitrogen spray cooling roll experiment has been made,The various experimental parameters is given the influence of the heat exchange coefficient. Finally, each parameter is calculated by mathematical method and the model of the heat transfer coefficient formula of liquid nitrogen application of cold rolling mill roll temperature is given.(2) Liquid nitrogen applied in cold rolling, the strong cooling ability will have influences on the performance of the strip materials and roll, so low temperature characteristics of roll and strip material has been carried on the experimental research, respectively on the low temperature indentation hardness test, tensile and impact test. The ductile to brittle transition temperature of the strip is-20℃, with the cooling temperature drop down, the strip material and roll strength and hardness are increased.(3) Liquid nitrogen could reduce the strip surface residual oil and residual iron contents, improve after cold rolling strip steel surface cleaness, Compared with traditional lubricating way, cold-rolled strip steel under liquid nitrogen cooling and lubrication obtained lower temperature of strip and roll than those of traditional way; Using Hill rolling force calculated friction coefficient in rolling process, draw a liquid nitrogen cold rolling process, the friction coefficient determines the liquid nitrogen cooling lubrication conditions under boundary lubrication condition.(4)Liquid nitrogen application in flat rolling process has carried on the experimental study, results show that the liquid nitrogen in large and small elongation leveling skin pass rolling process showed a good elongation and rolling force control stability, application of liquid nitrogen also increased the roll genetic reproduction rate of strip steel, obtained the better surface roughness,at the same time prolonged the period of roll.