Segregation Of Billet Castings And Its Heredity Effect On The Hot-rolled Products

With the rapid development of domestic steel industry,the market demand for quality steels has become increasingly important in recent years.The ambition of steel enterprises is to expand the steel varieties and improve the product qualities.The continuous casting(CC)procedure is the key component in the entire steel manufacturing process.Thus,it is very important to improve the quality of continuous casting billets.However,during the quick development of the technique of CC,the segregation issue in low alloy steel,high carbon steel,micro-alloyed steel is still a restrictive question to limit the whole product qualities.Segregation in billet is one of the main causes of the deterioration of steel uniformity and the reduction of the steel performance.Therefore,two typical steels including high carbon 70 steel and oil pipe steel were chosen to be the research objects to discuss the heredity effect of macro segregation or semi-macrosegregation in billets on the mechanical property or banded structure in hot-rolled products with the methods of mill trial,theoretical research and experiments.Finally,the ways to reduce the segregation of the billets and improve the steel properties from the very beginning of CC were suggested in this research.High carbon 70 steel is widely used in the sector plate spring and circular coil spring which can withstand vibration in the manufacturing of automobile,tractor or trains.However,serious quality problems frequently occur because of the inhomogeneity and instability of the hot rolling products originated from billet castings.For example,during the downstream drawing operation of the wire rods,the breakage problem often occurs due to the unstable mechanical properties from the uneven internal micro structures.The 70 steel billets and their hot-rolled wire rods were chosen to be the research objects.The chemical composition of billets and rods,mechanical property tests,observation on micro structure of rods were measured to discuss the effect of the central segregation of billets on the mechanical property of hot-rolled wire rods.The segregation index of C(1.02?1.26)is bigger than that of Mn(0.98?1.06).They changes in the same trend.The casting parameters are closely related to the dendrite morphology of the billets.The secondary dendrite arm spacing(SDAS)increases with the increasing superheat temperature of the molten steel,while decreases with the specific water flow.During the solidification progress,there is always surface temperature increasing from one cooling zone to the next,but the temperature rebound phenomena disappears at the location of 20mm below the surface.The local temperature rebound delays with the increasing distance to the shell surface.For the same billet castings,the sorbite rate of their smaller diameter hot-rolled rods is obviously higher than that of the bigger ones.The sorbite rate decreases with the increasing central carbon segregation of the billets,while the quantity of cementite network increases.For the same billet castings,their smaller diameter hot-rolled rods show better tensile strength than the bigger ones.The stability of the mechanical property of the rods is decreases with the increment of the central segregation index.With the continuous increase of the energy requirement,the development of oil pipe steels has been an important issue in the field of advanced steel materials.Hydrogen induced crack(HIC)is regarded as the main reason for the corrosion damage of tubes under hydrogenating environment.It is widely accepted the banded structure,resulted from the spot segregation,which is the critical factor influencing this HIC behavior.In this research,with the employment of theoretical calculation,physical experiment and numerical simulation,the mechanism on the formation of spot segregation in oil pipe steel will be revealed.The heredity of the morphology,dimension,location,metallographic structure and solute segregation ratio from spot segregation to banded structure during hot work process will be illustrated,and the critical parameters for the prevention on the occurrence of banded structure in oil pipe steel will be put forward.The ratio of the equiaxed crystal zone area increases with the increase of the M-EMS intensity.Moreover,the ratio of the total segregation spots area becomes larger with the increase of E.R.After hot deformation,the segregation spots are plastically deformed to form segregated bands in steel pipes.Through the thermo-mechanical analysis conducted by the Thermo-Calc software,there are three precipitated phases called KSI-CARBIDE,M23C6 and MC_SHP.During the precipitated phases,KSI-CARBIDE,M23C6 and MoC are related to Mo.The severe segregation of Mo,Mn and Cr in the segregated bands is in accordance with that in the spot segregation.It is necessary to control the degree of the spot segregation in as-cast products to suppress the formation of the segregated bands in oil pipe steels.It is strongly expected that the achievement will be the theoretical basis for solving the common problem of oil pipe steel.