Fundamentals Of Scrap Melting In The Steelmaking Process

The utilization of scrap resources is one of the important measures to achieve green steel.There is still considerable development in the utilization of scrap due to the abundant and large output scrap resources in our country.The efficiency of scrap melting is the key to improving its industrial application value.It is necessary to analyze the scrap melting and optimize melting parameters.Here,the experimental research on the scrap melting and the simulation of scrap melting behavior by using Fluent software are carried out.The melting mechanism,heat transfer,and mass transfer phenomenon are analyzed.An evaluation system for the influencing factors of scrap melting is established which quantitatively analyzes the influence trend of various factors on the scrap melting.Finally,the mathematical model of the scrap control is established to calculate the amount and type of scrap under different smelting conditions.It provides theoretical guidance for the optimization of the addition mode of scrap.The main conclusions are as follows.(1)According to the experimental and simulated results,the scrap melting was subdivided into four stages of "①formation of the solidified layer ②rapid melting of the solidified layer③carburization④carburization+normal melting ".In addition,the carburization is more significant at lower bath temperature,larger scrap size,and smaller scrap specific surface area.The simulated results show that the carburization stage accounts for 33.8%of the melting time at 1573 K.It accounts for 20%of the melting time for the scrap of Φ15 mm.It accounts for 70%of the melting time for the scrap of S 比=52.7 m2/t.(2)Increasing the temperature of the molten pool and accelerating the stirring of the molten pool can promote the melting of scrap steel.Under the natural convection,the average melting rate increases from 0.0127 mm/s to 0.0671 mm/s when the bath temperature increases from 1573 K to 1723 K.Under forced convection,it is 0.061 mm/s when the roating speed is 423 r/min.It increases by 3.8 times compares with the condition of natural convection.The average melting rate increases by 0.4 times when the carbon content of scrap increases from 0.168wt%(0.0127 mm/s)to 0.49wt%(0.01774 mm/s).It decreases by 0.34 times when the diameter increases from 10 mm(0.0127 mm/s)to 15 mm(0.00833 mm/s).It increases by 4 times when the S比 increases from 52.7 m2/t to 121.2 m2/t.(3)The heat transfer coefficient is positively related to the bath temperature.It increases from to kw/(m2·K)to 22 kw/(m2·K)when the bath temperature increases from 1538 K to 1693 K.The mass transfer coefficients at different factors are obtained and the effect of various factors on them is "melt poll stirring>bath temperature>the conditions of scrap".The mass transfer coefficients.Furthermore,the mass transfer dimensionless correlations are calculated which is Sh=0.144(GrSc)0.325(1×108<GrSc<5×108)at natural convection and JD=0.133(Re)-0.356(1000<Re<4000)at forced convection.(4)The results obtained from EPMA and metallographic microscope show that the microstructure from the scrap edge to its inside is primary carbide→acicular martensite→original structure(dislocation martensite).The corresponding carbon concentration gradient is 4wt%→1-1.5wt%→0.2wt%.The change of interface carbon content is consistent with that of metallographic structure.The depth of the carbon diffusion layer increases with the increase of bath temperature which is 200μm and 220μm at 1573 K and 1623 K,respectively.A silicon-rich layer with a certain thickness is found where carbon content decreases rapidly.(5)The masmatical model based on correlation analysis and multiple regression analyzes the effect of various factors on scrap melting from a qualitative and quantitative perspective,respecticely.It is shown that bath temperature and melt poll stirring have the greatest influence on the scrap melting,followed by the specific surface area and carbon content of scrap.The regression equations of the average melting rate,the average mass melting speed,and the mass transfer coefficient are obtained.A theoretical evaluation model for the scrap melting is established.The obtained explicit functions between melting rate and influencing factors can be used to predict the scrap melting under actual smelting conditions.(6)The mathematical model of scrap control added is established from the heat balance and scrap melting time.The amount and type of scrap added undere different smelting conditions are calculated.The ideas on the reasonable matching of scrap added is proposed(maximum total amount of scrap obtained from the heat balance→amount of different types of scrap added within the specified melting time→maximum ratio of each type of scrap).The mathematical model provides technical ideas for the optimization of scrap addition mode.