Study On Corner Structure Control Of SCM435 Cold Heading Steel Bloom During Continuous Casting Process

SCM435 cold heading steel is widely used in the manufacture of bolts,nuts and other fasteners in automobile,ship,engineering machinery,aerospace and many other fields because of its good mechanical properties.The base metal is usually produced by bloom continuous casting.However,in the actual production process,this kind of steel bloom often has corner transverse crack defects,which seriously affects the quality of rolled products and causes great economic losses to the steel enterprises.The frequent transverse crack in the corner of billet is a universal problem that restricts the high quality and high efficiency production of SCM435 cold heading steel.In this paper,the production process of SCM435 cold heading bloom continuous casting in a domestic steel plant is taken as the research object.The causes of transverse cracks in the corner of the billet are analyzed by means of metallographic examination and the static CCT curve of the steel is measured.Based on the production practice of SCM435 cold heading steel bloom,a three-dimensional solidification heat transfer model of billet was established to comprehensively explore the evolution rule of billet temperature field in the production process.On this basis,combined with the high temperature structure transformation mechanism of steel,a controlled cooling process for the corner structure control of SCM435 cold heading steel was proposed.The main conclusions are as follows:(1)Under the traditional casting process,the austenite grains in the corner of the inner and outer arc side of the slab are coarse and mixed obviously,the minimum diameter of the austenite grains is 1.5 mm,the maximum is about 8mm,the average grain size is about 3mm,and the grains are abnormally coarse;in addition,the pre eutectoid ferrite film with an average thickness of about 25 ?m is formed on the grain boundary of the austenite grains,which results in the low plasticity of the corner of the slab The defect of transverse crack at the corner of slab is caused.(2)The static CCT curve of SCM435 cold heading steel was measured,and the Ac3 temperature of the steel was determined to be 804?.When the cooling rate is 0.25?/s,the temperature of transformation from austenite to ferrite is about 780?,while when the cooling rate is increased,the temperature of transformation decreases gradually.When the cooling rate is greater than 5?/s,the temperature of Ar3 is about 460?.(3)Under the traditional process,the surface temperature distribution of the slab is uneven during the continuous casting process.The center temperature of the wide and narrow surface is about 1130?,and the temperature of the corner affected by two-dimensional heat transfer is about 1020?;the temperature of the corner at the beginning and the end of straightening is about 915? and 850? respectively;the temperature at the end of the secondary cooling is about 10? and the temperature at the end of straightening is about 15? for each 0.05 m/min increase of the casting speed The temperature at the end of secondary cooling is about 14? and that at the end of straightening is about 4? for every 0.05 L/kg of specific water.(4)Combined with the high temperature structure transformation mechanism of steel,a new secondary cooling zone controlled cooling process is proposed,which makes the corner temperature of slab drop below 460? quickly after leaving the mold,and then the temperature return to about 910? to meet the conditions of high temperature secondary phase transformation of corner structure;the solidification shrinkage of corner of slab increases gradually along the thickness direction,and the shrinkage at the end of strong cooling is about 3 mm,and then the temperature return process reduces the corner shrinkage The maximum displacement of the corner at the end of straightening along the thickness direction is 3.97 mm.Under the new technology,the equivalent stress of the corner of the slab is significantly increased,and under the new technology and the old technology,the equivalent stress of the new and the old technology is 67.0 MPa and 195.0 MPa respectively.When the secondary cooling zone is produced,the equivalent stress tends to be the same.(5)Through field verification,the average size of austenite grain in the corner of the new process is 800 ?m,which is 70%smaller than that of the original process;at the same time,the pre eutectoid ferrite film is completely eliminated;the average thickness of fine grain layer in the corner of the billet is about 17 mm,which significantly improves the plasticity of the corner structure of the billet,and effectively controls the occurrence of transverse crack in the corner of SCM435 cold heading bloom.