Research On Key Technology Of High Speed Continuous Casting Of Ordinary Carbon Steel Billet

With the theme of high casting speed,the efficient billet continuous casting technology realizes the reasonable matching of the production rhythm of continuous casting and large converter,greatly improves the production efficiency,reduces the strand number of caster,significantly reduces the construction investment and production cost,and also increases the utilization of sensible heat of casting billet,which plays the purpose of energy saving and emission reduction,and is favored by all walks of life.At present,the casting speed of 150mm×150mm billets of ordinary carbon steel is generally between 2.8 and 3.5m/min,and there is a big gap compared to the advanced level at abroad.The difficulty lies in the fact that with the increase of casting speed,the chances of steel leakage,deflection and bulging are greatly increased.The realization of high casting speed is a comprehensive technology improvement that integrates technology,equipment,production operation and automatic control.In this paper,the self-developed mold taper design software was used to simulate and calculate that when the contraction of single shell at the outlet of 150mm×150mm copper tube reached 0.773mm,the maximum expansion displacement at the center of the hot surface of the copper tube was 0.228mm,and the maximum air gap was 0.817mm.Thus,the final ideal taper curve was obtained by modifying the hyperbolic taper of the copper tube.The upper part of the copper tube of the quincunx mold was convex with four sides.The middle and lower part of the copper tube was rectangular,the hyperbolic taper design was maintained,and the separation distance of the corner increased linearly,so as to realize the reasonable design of the copper tube inner cavity structure of the high speed casting.It was used to solve the problem of uneven contact between billet shell and copper tube.The test method and procedure of temperature measurement of mold copper tube were introduced in detail.The temperature at the center of wide surface and 1/2 thickness of the mold copper tube in B steel plant was measured by the selfdeveloped online collection system of copper tube temperature.The results were basically consistent with the simulation results of ANSYS three-dimensional finite element model,and the error rate was less than 6%.The measured results of the production process showed that the temperature field of the copper tube of the mold was unstable,and the temperature of each point was fluctuated aperiodically.The temperature in the meniscus area fluctuated greatly,while the temperature in the lower part of the copper tube fluctuated little.The temperature distribution on the cross section of the same height in the mold was not uniform:The smaller the oscillation of the vibration device was,the more uniform the temperature of the four sides of the copper tube became.The faster the casting speed was,the greater the temperature uneven tendency of four sides of the copper tube was.The lower the carbon content was,the higher the temperature of copper tube became.Taking the temperature test results of each surface 250mm from the meniscus as an example,the temperature difference of each surface of copper tube fluctuated in the range of 60-70℃ during the production of XGL steel,while the temperature difference of HRB400 and HPB300 fluctuated in the range of 20-30℃.When using 3.5m/min casting speed to produce HPB300 steel,the temperature of quincunx tube was higher than that of traditional copper tube,which proved that its heat transfer efficiency was higher.Based on the large amount of copper tube temperature data obtained from the temperature measurement experiment,the second type of interface heat transfer boundary conditions which were more accurate at high speed casting were obtained by iterative solution of the nonlinear inverse model.The three-dimensional finite element model which really conformed to the actual field was established by ANSYS to simulate the heat transfer of the copper tube in the mold at high casting speed of the billet:with the increase of casting speed,the temperature of the copper tube increased significantly.Choosing the proper cavity size of copper tube could promote the surface temperature distribution of billet shell along the section circumference to be more uniform.The cooling water flow had the least effect on heat flux.The maximum temperature of copper tube was 363.8℃ when casting speed was 4.0m/min for traditional mold with 150mm × 150mm section,and the temperature of corner was 108.5 ℃ lower than that of middle.The water-free seam crystallizer copper pipe technology was used to solve the problem of uneven cooling in traditional water joints.The copper tube of the high-efficiency mold with quincunx groove type could pass steel up to 15000t,a more uniform temperature distribution on the hot surface,a temperature difference between the corner and the middle was only 4.8℃,and the highest temperature on the hot surface of the copper tube can be reduced to 260.5℃,which reduced the deformation of the copper tube,increased the average heat flow by 7.8%,which was more favorable for the billet to achieve high casting speed.The calculation results of high speed casting secondary cooling heat transfer model were consistent with the infrared temperature test data of the billet.Based on this,the high pressure water secondary cooling process and equipment were developed:the pressure of the water supply main pipe was about 1.8MPa,and the specific water volume was 1.6-1.8L/kg.The determination criteria of metallurgical length and total length of secondary cooling zone of high casting speed billet were explained.The guiding parameters of length division of cooling zone,nozzle arrangement and selection,typical efficient secondary cooling system and so on were put forward,which laid a scientific foundation for secondary cooling design of high speed casting.Finally,the bulging capacity of the billet was predicted accurately by using the three-dimensional creep finite element bulging calculation system.Combined with the production statistics data,the design criteria of foot roll was put forward,and the densely guide roll segment of the billet caser with high casting speed was developed.If the casting speed exceeded 5.2m/min,it was necessary to add an independent densely guide roll segment to effectively control the bulging deformation of the casting billet.The key technology of high casting speed continuous casting of billet had been successfully applied in engineering practice:In the production of 150mm× 150mm billet HRB400,the industrial stable production speed reached 5.0m/min,and the production of 160mm×160mm billet HRB400 set a new industry record of 6.02m/min.The size and quality of billet produced by high speed caster could fully meet the requirements of subsequent rolling.