Development Of Process And Control Technology For Direct Rolling With Free Heating For Bar And Rod Mill

The iron and steel industry in our country is facing serious challenges of excess production capacity,energy,resources,environment and other problems.How to effectively save energy and reduce cost has become an urgent problem to be solved.Under this background,this paper has carried on the thorough research on Direct Rolling with Free Heating for Bar and Rod Mill(DROF).In this process,the billet will direct rolling without coming into the furnace.This process can completely saving the fuel consumption of the furnace,and reducing the production costs by a large margin.Based on a reform project for a bar and rod production line in domestic,the research is focus on how to successfully implement the process of DROF,and the influence of this process on the load of mills and the performance of product.The main research contents and results are as follows:(1)In order to substantially increase the temperature of billet under safe conditions,the temperature closed-loop control system was developed.A method for second cooling water dynamic adjustment was promoted.The fuzzy control algorithm was used to optimize the intelligent control of the second cooling water distribution,which makes the billet temperature at cutting point higher than the conventional process for more than 100?.This provides a guarantee for DROF.(2)The cutting and transporting rhythm control system was developed.And the principle was put forward to guarantee the high rate of direct rolling.The control strategies were given depending on whether the casting speed can be adjusted continuously.And the knowledge base and rule set for cutting and transporting rhythm control was established to ensure transporting the billets to rough mill in shortest time,and have no negative impact on the rate of the fixed length and the product rate.(3)The effect of DROF on the rolling load of each mill was studied.The rolling experiments show that this process will increase the rolling load of each stand,especially the rough mills.To avoid rolling load overrun of several stands,the rolling force and energy parameters calculation model and optimizing algorithm of load margin for rough mills were studied.For the feature of bar and rod rough rolling,that flat roller and vertical roller could roll alternately,the iterative calculations were done alternately along the height and width of work piece.And the dichotomy was used during the iterative calculations in order to avoid iteration non-convergence.It provides a theoretical basis for the load distribution of bar and rod rough mills.(4)The effect of DROF on the micro structure and properties of the products was studied.Compared with the conventional rolling process,DROF can refine the grain size and improve the mechanical properties of the products.But due to the temperature difference between the head and tail of workpiece,the head grain refinement is more obvious,it's yield strength increased 10-30MPa,and the yield strength of the head is higher than the tail for 10?20MPa.(5)In order to ensure the longitudinal uniformity for the mechanical properties of product,the feasibility of using the existing cooling device to eliminate the temperature difference between the head and tail was analyzed,and the concrete strategy was promoted to dynamic adjusting the cooling water.The temperature field of the cooling process was simulated using ANSYS software,and the cooling process parameters were optimized according to the calculation results.In order to eliminate the temperature difference with linear distribution in the length direction of the workpiece,the relationship between required flow density and length of workpiece was parabolic.To reduce the temperature difference between the center and surface of workpiece,the length of the cooling device should be extended as far as possible under the conditions of the site.(6)In DROF process,the rebar produced by water cooling tends to rust during storage and transportation.In this paper,a method was proposed to improve the anti-rust property of hot rolled rebar,which uses oil-water emulsion cooling instead of water cooling after hot rolling.The thickness,morphology and structure of the oxide scale obtained using two cooling processes(water cooling,oil-water emulsion cooling)were investigated.The results of OM,SEM and EPMA analysis indicated that these two scales contained three layers:an outer Fe3O4 layer,an intermediate FeO layer with island-shaped pro-eutectoid Fe3O4,an inner eutectoid Fe3O4 layer.For the water cooled rebar,all three layers of oxide scale were relatively thin.Moreover,the scale had plenty of defects such as porosity,crack and so on.However,for the oil-water emulsion cooled rebar,all three layers of oxide scale were relatively thick and compact,which plays an important role in protecting the rebar from atmospheric rust.(7)This process has been successfully applied to a bar and rod production line in domestic.Based on the measured data,the practical application effect of DROF was evaluated.The results show that this process has obvious economic benefit and good social benefit.It plays a positive role in promoting the technical reconstruction and upgrading of bar and rod production line in China.