The Formation And Control Of Carbon Segregation Of Gear Steel In The Bloom Casting Process

The homogeneity of carbon in the cross section of caster is very important for gear steel wich effects the deformation of gear during hot treatment. During the production of20CrMnTi gear steel in Xiangtan Steel, the rolled bar had box segregation wich produced from cast bloom. So we focus on the problem in the paper, do researching on the carbon segregation of caster and its effect on the rolled bar. Finally propsing the solving means by adjusting the casting operations.By investigating the producing of gear steel,we found that the carbon distribution of cast bloom、rolled and gear were asymmetry.The carbon content is high in the center of gear and the situation were the same in the cast bloom and rolled bar.There was box segregation in the center of rolled bar which was corresponding with the CET of cast bloom,carbon segregation was related with etched macrostructure. By numerical simulation, we did research in the relations between cast operations and CET. The results showed that the area of exquized zone become smaller while increasing the over heat of steel. In order to control the carbon segregation,the over heat can be maintained a little higher.By analyzing the etched-macrostructure of cast blooms and corresponding rolled bars,we found that the macrostructure of rolled bars was related with cast blooms’.The light color zone in the center of rolled bars was responding with the exquized zone of cast blooms.We drilling the cast blooms to get samples for carbon content analysis basing on the their macrostructure and found that the carbon content riching in the columnar to exquized transition area(CET). The phenomenon will lead to carbon inhomogeneity in the cross section, and it’s effect will extend to the rolled bar and so the finished quality of products.So we changed the operations of cast bloom and expected to improve the average of carbon distribution especially in the1/2and3/4radius.The changes of casting operation including increasing the water flow rate(from0.27L/kg to0.35L/kg)and decreasing the strength of mold magnetic stirring(from400A to200A and0).Comparing the before and after the operation changes, we found that the rate of exquiaxed zone decreasing with the reducing of stirring strength and the carbon segregation pattern of cast blooms were different weather the magnetic stirring was on or off. What was more, The box segregation of rolled bar disappeared when the magnetic stirring was off, and the distribution of carbon content and hardness of rolled bar was the most homogeneous,so the its quality was improved. In the same time by examing metallographical structures we found the rolled bars made from cast blooms without magnetic stirring the structures was the most homogenous and the rank of band structure was the lowest.Above all,in order to control the carbon segregation of rolled bar we need to adjust the operations of of casting.Carbon is riching in CET and forming high carbon segregation in the center of cast bloom and causing the box segregation of rolled bar.Through the changing to openraton of casting,including maganetic stirring、water ratio、over heat and cast speed,we reducing the area of exquixed of cast bloom,then improving the box segregation of rolled bar.