Research And Application For Raising Temperature Of Billet Under DROF Process

Rebar has been widely used in the construction industry.It is easy to be produced,has the large proportion in steel output,and the domestic production capacity exceeded the market demand in recent years.The rebar producers face fierce competition,and they are urgently to find the new technologies to reduce the cost by energy conservation and emission reduction.Under this background,a new technology for raising billet temperature was developed under the DROF(Direct Rolling with Free-Heating for bar and rod)process,and successfully applied on site.The main research works are as follows:(1)Based on the research on numerical simulation technology to multi-physical field coupling,an overall scheme was proposed for simulating the process of raising billet temperature,and a simulation software platform was established,which can be utilized to simulate the temperature field and solidification behavior in the process of billet continuous casting,and the simulation results can be used to determine the process parameters of raising billet temperature.It has been used to simulate the casting process instead of the trial,so as to shorten the development cycle,decrease development costs and reduce the number of experiments.(2)Based on the fundamental theory of heat transfer and solidification,the prediction model of the solidified shell thickness of the billet was established in order to eliminate the risk of breakout at the outlet of the mold due to raise the billet temperature.This model can not only be used to predict the solidified shell thickness in longitudinal direction,but also to predict the shell thickness in transverse section,and then check whether the cast billet can meet the requirement of critical shell thickness so as to avoid the occurrence of breakout accident.Through the calculation,we know that at the outlet of the mold,the shell thickness at the hot spot of the cast billet is generally less than that of the middle line of the section by 0.9?1.6mm.The results from trials on site verified that the model can accurately predict the breakout position of the billet during casting.(3)In order to meet the requirements of DROF process,a technical route was proposed to raise the billet temperature through comprehensive optimizing casting speed and cooling water distribution in the secondary zone.Mathematical models for raising billet temperature are developed,including the calculation model of casting billet temperature field,the temperature dropping curve on the billet surface,optimization model for distributing the secondary cooling water etc.The various factors have been taken into account in the models,and they are simple,easy to use and verify.The models have been used in the 150-170mm square billet production lines in three steel factories respectively.The billet temperature is raised for 70?133? at the outlet of the secondary cooling zone.(4)A prediction model is established in order to predict the location of the final solidification point in the casting process on real-time,so as to prevent the occurrence of the cutting breakout accident after raising billet temperature.The model was verified by the pin-shooting experiment through measuring the position of the steel nail melting zone,semi melting zone and non-melting zone.The results show that the deviation between the calculated value and the measured value of the shell thickness of the billet is less than 4.7%.The model has been applied to three production lines to raise billet temperature,and cutting breakout accident is never occurring.(5)The simulation of the phase transformation of the HRB400 rebar is conducted by the finite element method,with the help of the dynamic CCT curves.The simulation results were verified by metallographic experiments.Comparing with the THR(Traditional Hot Rolling)process,the pearlite content increased by about 4%?8%,and the ferrite content decreased by about 4%?7%in the DROF process.Moreover,the mechanical properties of rebar were compared through experiments,and it was found that the average yield strength and the tensile strength increased by 17 and 5MPa in the DROF comparing with the THR process respectively.The experimental results show that rebar produced by DROF can meet all needs of the standards and the user requirements.