Research On The Bimetallic Composite Roll Produced By The Electroslag Cladding Method

In recent years,with the coming out of advanced rolling mills and high-efficiency rolling technologies,the rolling production line has grown toward large-scale,high-speed and automation which makes the working conditions of rolls more stringent.As the roll quality has a direct impact on the production efficiency of the rolling mill,the surface quality of the rolled material and the rolling cost,research on the material and production process of the roll has become a common concerned issue of the roll and metallurgical industry both in China and abroad.In addition,the traditional single alloy roll can't meet the double requirements of its wear resistance and toughness in the rolling process at the same time,but for the bimetallic composite roller,due to its roll core and working layer can use different materials,it can better solve the contradiction between the wear resistance and toughness of a single alloy roll and greatly reduce the production cost of the roll at the same time.Therefore,the research,manufacture and use of high-quality,low-cost bimetallic composite roll will surely become a new direction for modern rolling.In this paper,based on the advantages of electroslag remelting technology,the bimetallic composite roll is selected as the research object,the process characteristics of composite roll production process under different conductive circuit schemes,influence of different process factors on the temperature field of the composite system,experimental exploration of composite roll produced by the electroslag remelting method,composite characteristics of different materials,bonding mechanism and the bonding quality of the bimetallic interface have been all studied.Firstly,based on the electromagnetic equations,flow equations and heat transfer equations,a 2D quasi-steady state mathematical model of producing bimetallic composite roll by the electroslag cladding method was developed.A numerical simulation has been done for the conventional conductive circuit with transformer ? short network?consumable electrode?liquid slag pool?roll core(electroslag ingot)?bottom tank?transformer(referred to as electrode?roll core)by using the Fluent software and the user define function(UDF)and user defined scalar(UDS)equations.The results illustrate that the loop current accumulates in the slag pool area between electrode and roll core and forms the highest temperature in this area under this conductive circuit scheme,which will cause an excessive melting of the roll core surface,as a result,it is harmful for obtaining a good bimetallic bonding and a uniform composition,microstructure and properties of the working layer.In the following experimental test of the composite roll by electroslag cladding method and a physical modeling test with a low melting point and transparent solution system,it has proved that the excessive melting phenomenon of the roll core surface always occurs.In summary,under the traditional electrode?roll core conductive circuit scheme,the roll core acts as a pole of the conductive circuit and it is difficult to avoid the surface melting phenomenon which is harmful for obtaining the ideal composite roll working layer and bimetallic interface performance.In view of the disadvantages of the traditional conductive circuit scheme with electrode?roll core,releasing the roll core from the conductive circuit effectively and controlling its surface temperature flexibly is the key to producing a high quality composite roll.For this purpose,an advanced current supplying mold technology and the 2D quasi-steady state mathematical model established in the previous part have been used to develop a new conductive circuit scheme with transformer?short network?consumable electrode? liquid slag pool ? current supplying mold? transformer(referred to as electrode?current supplying mold).The numerical simulation results illustrate that the loop current accumulates in the slag pool area between electrode and current supplying mold and forms the highest temperature in this area under this new conductive circuit scheme with the use of current supplying mold.Due to the increasing distance of the high temperature zone in the slag pool,it makes the surface temperature of the roll core has a more flexible adjustability.Among the various process factors,the electrical parameters,diameter of the roll core and depth of the slag pool in the conductive section have the biggest effects on the temperature distribution of the composite roll system,the effect of the distance between electrode and roll core surface is secondary and the least affected factor is the depth of electrode inserted in slag pool.An ideal surface temperature of roll core can be obtained through the reasonable matching of the process factors which is beneficial to achieving a good bimetallic bonding between working layer and roll core.Based on the above simulation study of the new conductive circuit scheme,an experimental test for producing GCr 15/45 bimetallic composite roll by electroslag cladding method has been carried out by using of the crucible electroslag furnace,slag flowing tank,electroslag remelting withdrawing furnaces,current supplying mold and metal level sensor.After a long term experimental exploration and experience summarization,a GCr 15/45 bimetallic composite roll casting billet with the diameter of 340 mm and compound height of 320 mm was finally produced.The cross section taken in the upper part of the composite section shows that the bimetallic interface is highly concentric and the thickness of composite layer is very uniform.In addition,there are no slag inclusions,pores,shrinkage holes and other defects at the bimetallic interface,it has a good metallurgical bonding.Through the longitudinal cross section of the composite billet,a tendency of the slag inclusions thickness gradually thins from the lower part to the upper part and eventually disappears at the bimetallic interface has been obtained.This is due to the fact that the temperature of composite roll system is gradually increasing and eventually reaching the steady state during the electroslag cladding process.As the roll core surface is heated to a different temperature,the bonding state of the bimetallic interface is also different too.Based on the equilibrium phase diagram calculation of the GCr15 and 45 steel by using the Thermo-Calc software,the single phase diffusion model was selected and the elements diffusion behavior at the bimetallic interface was calculated through the DICTRA software.The temperature variation with time function at the bimetallic interface was given through the comprehensive consideration of the numerical simulation results and the actual casting speed in the experimental test.In the present study,by comparing the line scanning results of the Cr element at the bimetallic interface with the calculation results of the Cr element diffusion at the same position,the interfacial bonding mechanism of bimetallic composite roll casting billet produced by the electroslag cladding method is strongly revealed to be the combined effects of the fusion and elements diffusion.The roll core was heated by the high temperature liquid slag pool and the working layer metal pool during the electroslag cladding process and its temperature rise was obvious.With the end of bimetallic electroslag cladding process and the extraction process of composite billet,a cooling process took place for the roll core.During the high temperature austenitizing process,the higher heating temperature and longer holding time lead to the formation of coarse austenite grains and some ferrite Widmanstatten structure at the surface area of roll core.However,it is exciting that these deficiencies can be eliminated and transformed into a uniform and fine microstructure though an appropriate heat treatment.Tensile strength and shear strength of the casting bimetallic interface specimens of GCr15/45 composite roll are 661 MPa and 282 MPa,respectively,and the tensile and shear fractures always occurs at the single material side rather than at the bimetallic interface.It fully proves that the interfacial bonding quality of the bimetallic composite roll produced by the electroslag cladding method is good enough.High speed steel roll has been widely used in the rolling industry due to its high hardness,good wear resistance and good red hardness.Based on the above new conductive circuit scheme,an experimental test of high speed steel/ductile cast iron bimetallic composite roll produced by electroslag cladding method has been carried out in this work.Due to the melting temperature characteristics of high speed steel and ductile cast iron,a low melting point slag system has been developed based on the FactSage software calculation and melting temperature measurement instrument.A high speed steel/ductile cast iron composite roll with a composite height of 264 mm was successfully produced with the previous equipment and experiences,then,a systematic study for the graphite morphology,alloying elements transition and microstructure changes at the working layer,roll core and bimetallic interface was given.The results show that the graphite morphology and matrix structure of the ductile cast iron have changed obviously.At the same time,due to the migration and diffusion of alloying elements at the bimetallic interface,a large number of carbides with different composition,morphology,content and distribution characteristics are formed.As a result,hardness of the bimetallic interface increases and the brittle fracture occurs easily in the tensile and impact tests.In spite of this,tensile strength of the casting specimens taken from the high speed steel/ductile cast iron bimetallic interface is 452 MPa,which satisfies the tensile strength requirement of the cast iron roll according to the Chinese national standard of cast iron rolls(GB/T 1504-2008).