| The stainless steel coated carbon steel composite material has both the characteristics of corrosion resistance,heat resistance,wear resistance and aesthetic of stainless steel and the advantages of high strength,toughness and low cost of carbon steel.It is widely used in chemical industry,ship,petroleum and other fields.The stainless steel coated carbon steel billet is the raw material for forming stainless steel coated carbon steel composite material with short process and low cost.The continuous casting technology of liquid-solid composite is an advanced technology for efficient preparation of stainless steel coated carbon steel billet.However,the traditional continuous casting technology of liquid-solid composite is only suitable for the preparation of stainless steel coated carbon steel thin strip billet,and the carbon steel is prone to microstructure coarsening,oxidation and corrosion.The carbon steel near the cladding interface is easy to form a ferrite decarburization layer,and the carburized layer of the stainless steel is easy to form martensite and precipitate carbides,which greatly affects the improvement of the interfacial bonding strength of the subsequent formed stainless steel coated carbon steel composite material.In view of this,a new technology and equipment for the horizontal continuous casting process of bi-directional solidification liquid-solid composite for stainless steel coated carbon steel billet was developed.Based on the equipments and methods of metallographic microscope,scanning electron microscope,electron probe microanalysis,transmission electron microscope,universal testing machine and the finite element simulation,the basic theory of the technology of the horizontal continuous casting process of bi-directional solidification liquid-solid composite for stainless steel coated carbon steel billet was systematically studied.The mechanism of cladding layer and cladding interface formation and the stainless steel coated carbon steel billet strengthening was revealed.High quality and high performance stainless steel coated carbon steel billet was prepared by using 304 stainless steel and Q235 carbon steel.The main results are as follows:The influence of liquid-solid composite process on the temperature and bonding quality of the cladding interface of the stainless steel/carbon steel billet was clarified,and the mechanism of cladding interface formation and strengthening was revealed.The stainless steel melt contacted with the carbon steel to form a temperature gradient perpendicular to the cladding interface.With the increase of time,the temperature difference between stainless steel and carbon steel at the cladding interface gradually decreased,and the temperature of the cladding interface gradually decreased after reaching the maximum.With the increase of the temperature of the stainless steel melt or the carbon steel,the maximum temperature of the cladding interface gradually increased,the cladding interface transited from non-composite to a flat interface,and finally the corrosion occurred to form a wavy interface.The results showed that when the maximum temperature of the cladding interface was 1416℃,which was below the carbon steel solid phase line temperature,the surface of carbon steel formed cold shut,and the cladding interface cannot be combined.When the maximum temperature of the composite interface was 1510℃,which exceeded the carbon steel liquid phase line temperature,the solid fraction on the surface of carbon steel was 0.295,uneven corrosion occurred and the depth exceeded 5 cm,forming a wavy interface.When the maximum temperature of the cladding interface is 1490℃,it was between the solid/liquid phase line of carbon steel.The solid phase rate on the surface of carbon steel was 0.486,and the width of the micro-melting zone with free mixing of elements was 2.5 μm,and the solid phase rate increased towards the carbon steel.The Cr and C elements were mixed and diffused in the micro-melting zone,and the gradient distribution carbides coherent with the matrix were precipitated.The cladding interface realizes metallurgical bonding.The contribution of carbide precipitation strengthening was about 190 MPa,which significantly improved interfacial bonding strength.A novel technology and equipment for the horizontal continuous casting process of bi-directional solidification liquid-solid composite for stainless steel coated carbon steel billet were developed,and the influence of process parameters on the temperature field and solidification behavior of continuous casting process of liquid-solid composite was clarified.The idea of combining the principle of inversion solidification with the technology of continuous casting is proposed.The water-cooled mold for cooling and controlling the temperature of composite billet were designed,and a temperature-controlled inversion solidification device for controlling the temperature of stainless steel melt during the continuous casting process of liquid-solid composite was designed.The equipment for the horizontal continuous casting process of bi-directional solidification liquid-solid composite for stainless steel coated carbon steel billet was developed was developed.Based on the main structure of the equipment,a 3D finite element model was established and verified by temperature measurement experiments.The maximum error was 1.61%.Based on the results of finite element simulation and the horizontal continuous casting process of liquid-solid composite,it was found that the cladding layer underwent inversion solidification,remelting,secondary inversion solidification and forward solidification.Among them,the initial inversion solidification layer formed during the rapid inversion solidification stage can avoid the direct contact between high temperature stainless steel melt and carbon steel,which led to the coarsening or even excessive corrosion of carbon steel.In the remelting stage,the defects such as oxides and pores in the initial inversion solidification layer were removed.The cladding interface and stainless steel cladding layer of the billet were formed in the secondary inversion solidification and forward solidification stages.According to the heat transfer behavior of the continuous casting process of liquid-solid composite and the heat transfer characteristics of the cladding layer at different stages,a physical model of the formation of the cladding layer was established.The key factors affecting the quality of the cladding interface and the cladding layer were the maximum temperature of the carbon steel surface(Tmax),the remelting rate(vremelt)and the cooling rate(vcooling)of the cladding.With the increase of the temperature of stainless steel melt and inversion solidification device exit,The Tmax,vremelt and vcooling were significantly improved.When Tmax≤1427℃,vremelt≤0.41 mm·s-1,the initial cladding layer cannot be completely remelted before the formation of the secondary inversion solidification layer,and the defects such as oxides and pores in the cladding interface and cladding layer cannot be completely removed.When Tmax≥1509℃,the carbon steel fused.When the Tmax was between 1427℃ and 1509℃,the initial cladding layer could be completely re-melting to achieve the preparation of the stainless steel coated carbon steel billet.The formation mechanism and control method of surface defects and element segregation of stainless steel cladding during the horizontal continuous casting process of liquid-solid composite were clarified,and the composite mechanism of interface was revealed.When the length of the mushy zone exceeded 89.5 mm,the porosity on the surface of the billet increased,leading to a large uncoated area,which significantly reduced the quality of the billet.When the length of the mushy zone was less than 89.5 mm,only a few tiny pores appeared on the surface of the billet and the depth was less than 0.5 mm.When the length of the mushy zone was too long,the feeding of the stainless steel melt became insufficient,led to the formation of defects such as shrinkage cavities and porosity on the surface of the billet.With the increase of the temperature of the stainless steel melt and the inversion solidification device exit and reduce the continuous casting speed,the length of the mushy zone reduced and the feeding capacity of the stainless steel melt enhanced,led to the surface quality of the billet improved.With the increase of continuous casting speed and the decrease of the temperature of the stainless steel melt,the segregation of Cr element in the cladding layer decreased.When the continuous casting speed was 5 mm·s-1,the temperature of the stainless steel melt was 1550℃,and the thickness ratio of cladding and base was 4:15:4,the Cr segregation ratio near the interface was reduced to 1.25.After the surface of carbon steel underwent rapid inversion solidification and re-melting of cladding layer,the overall temperature of the carbon steel increased uniformly,and a micro-melting zone was formed where the gradient of the liquid phase volume fraction was small,and the width was large as the surface of the carbon steel contacted with the stainless steel melt.The C and Cr atoms were first uniformly mixed in the micromelting zone,and then the interface elements diffuse during the cooling and solidification process,and finally form uniformly distributed carbides.The formation of carbides at the cladding interface significantly reduced the difference of C activity between the cladding interface and the near-interface stainless steel.The decreasing of the chemical potential gradient of C atoms inhibited the diffusion of C atoms into stainless steel,eliminatee the decarburization layer at the nearinterface carbon steel,and also inhibited the precipitation of carbides in the carburizing layer.The variation law of mechanical properties of stainless steel coated carbon steel billet was clarified,and the mechanism of stainless steel coated carbon steel billet strengthening was revealed.With the decrease of continuous casting speed and the increase of the temperature of the inversion solidification exit,the tensile strength of the cladding interface area and the shear strength of the billet increased.The cladding interface of the billet formed the granular M23C6 carbides coherent with the matrix.It was also eliminated the decarburization layer at the near-interface carbon steel and the martensite and carbides at the stainless steel,as well as oxides and pores in the cladding interface and cladding layer,which significantly enhanced the interfacial bonding strength.In addition,the improvement of the interfacial bonding strength and the decrease of the dendrite spacing of the stainless steel cladding layer improved the tensile strength of the billet.When the continuous casting speed was 5 mm·s-1,the temperature of the stainless steel melt was 1550℃,the temperature of the inversion solidification device exit was 1450℃,and the thickness ratio of cladding and base was 4:15:4,the tensile strength of the stainless steel cladding layer,the carbon steel and the interface area of the stainless steel coated carbon steel billet and the shear strength of the billet could be as high as 555,509,577 MPa and 473 MPa,respectively.The tensile strength of the interface area was higher than that of the stainless steel cladding and the carbon steel. |
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