Study On Direct Rolling Process And Strengthening Mechanism Of Billet

In 2020,China's total steel production reaches 1.325 billion tons,and the total amount of bar products is about 500 million tons,accounting for about 38% of the entire total steel production.Therefore,new technologies for energy saving and emission reduction in bar enterprises are very important for the green and low-carbon development of the whole steel industry.The billet direct rolling process,a production process in which the continuous casting billet is cut off without any heating or short time edge make-up heat and then transported to the rolling mill for rolling,is a typical process interface technology.This technology significantly reduces the energy consumption of the rolling process and effectively reduces CO2 emissions,which is very important for China to achieve the goal of reaching the carbon peak by 2030.The distinctive feature of this technology is that the production line eliminates the heating furnace and makes full use of the sensible heat of the billet continuous casting process,which significantly reduces the energy consumption of the rolling process in the bar production process.However,there are three problems in the billet direct rolling process that need to be solved,which limit the application of this technology,including: the mismatch between the casting and rolling interfaces affects the direct rolling rate and yield of the continuous casting billet;the temperature difference between the head and tail of the continuous casting billet affects the stability of the product properties;and the direct rolling process lacks the precipitation and redissolution of the microalloy carbon and nitride in the heating furnace production process,and the strengthening effect is not obvious.In this context,this paper mainly researches the casting-rolling interface technology of billet direct rolling process to provide theoretical basis and technical support for the promotion and application of this technology,and the main research contents and conclusions are as follows.(1)Temperature field variation law and penetration rolling deformation mechanism of square billet direct rolling processUsing the finite element method,the influence of process parameters such as drawing speed,secondary cooling ratio water and casting superheat on the temperature andmetallurgical length of the billet out of the continuous casting billet is analyzed.Under the premise of ensuring that the solidification end does not exceed the position of the cut-off point,increasing the drawing speed is one of the most effective measures to raise the temperature of the cast billet.When the drawing speed is increased from 2m/min to 2.8m/min,the surface temperature of 150 square cast billet can be increased by 85?;the temperature difference between the heart surface of the cast billet before rolling in the direct rolling process is 160?,and after the same rough rolling pass deformation,the equivalent change in the heart of the cast billet in the direct rolling process of the square billet is increased by 4-6% than that in the heating furnace process,which is more favorable to the deformation penetration of the heart of the cast billet and improve the quality problems such as deviation of the heart of the cast billet.(2)Research on the queuing theory and articulation matching relationship of the casting-rolling interface of the billet direct rolling processThe best articulation of the billet direct rolling process casting-rolling interface requires the maximization of the billet direct rolling process yield and the maximization of the direct rolling rate.To maximize the yield of the direct rolling process,the throughput of the continuous casting process needs to meet the second flow rate of the rolling process,i.e.,the oversteel volume of the two processes is equal;to maximize the direct rolling rate of the direct rolling process,the waiting time of a single billet during the articulation of the casting-rolling interface needs to be less than the limit waiting time of the billet.The mathematical model of continuous billet queuing at the cast-roll interface of the square billet direct rolling process is constructed using the queuing theory approach to analyze the effects of different billet types,sizing,drawing speed and flow number on the average waiting time in the billet queuing system.For a common multi-stream continuous casting machine to a rolling line in China,the optimal control range of pulling speed and flow number under different conditions is optimized and calculated,providing a set of process parameters for the production selection of the direct rolling process.(3)Product quality stability control of billet direct rolling process and continuous casting billet temperature equalization processA process for reducing the temperature difference between the head and tail of the billet during the production of the billet direct rolling process was designed using the method of selective insulation in the continuous casting section.The temperature difference between the head and tail of the cast billet was reduced from 80? to 45? after the cut-off of the 6m,150 square continuous cast billet,resulting in a 40% narrowing of the fluctuation range of the mechanical properties of the bars in the same furnace from 100 MPa to 60 MPa,and a 50% narrowing of the fluctuation range of the mechanical properties of the bars in the length direction of the individual cast billet from 60 MPa to 30 MPa.(4)Study on the strengthening mechanism under the conditions of billet direct rolling processIn this paper,we studied the Nb(C,N)precipitation pattern under the direct rolling process.By using the characteristics of the direct rolling process and the strong water cooling process between the finishing rolls,we effectively reduced the final rolling temperature of the bars,which significantly increased the strain in the core of the billet,promoted the grain refinement and Nb(C,N)precipitation strengthening in the core,and improved the mechanical properties of the bars containing Nb.Through the above direct rolling production control rolling and cooling process optimization,the Nb-containing steel bars with full GB/T1499.2-2018 national standard requirements were produced,and the Nb alloying elements were controlled within 0.015%,reducing the addition of micro-alloy elements.