| Metal clad plates are widely used in many engineering fields due to their excellent comprehensive properties,and hot rolling bonded stainless/carbon steel clad plate is one of the representative products,and is also one of new products in development.In order to further improve the composite quality,optimize and improve the rolling preparation technology,it is necessary and meaningful to further study and understand the rolling composite process of stainless steel and carbon steel and its impact on the mechanical properties of laminated products.In this dissertation,the composition,microstructure and mechanical properties of hot rolling bonded stainless/carbon steel clad plate are studied by experimental analysis and numerical simulation.The thick layered structure of clad plate is proposed,and the interface layer is defined.The thermal stability,mechanical behavior of the interface layer are revealed,and the mechanical model of the interface layer is established.Based on the new mechanical model of the interface layer,the deformation behavior of the thin rolling and stamping process of the clad plate are studied.The main research work and results are as follows.(1)The chemical composition,microstructure,mechanical properties,and the structure distribution characteristics of the hot rolling bonded stainless/carbon steel clad plate are characterized by experiments.The experimental results show that the clad plate has obvious delamination characteristics in the thickness direction,and based on this,the delamination structure of the clad plate in the thickness direction is proposed,which is composed of stainless steel,carburization layer of stainless steel,multi-element alloy layer,decarburization layer of carbon steel and carbon steel.The carburized layer of stainless steel,multi-element alloy layer and decarburized layer of carbon steel are the new material layers formed by the diffusion of elements between the clad and the substrate,which are different from the clad and the substrate.The distribution of microhardness of interface layer and the tensile stress-strain curves of each layer of clad plate are obtained.The experimental results show that the monotonous change of main element content along the thickness of the multi-element alloy layer determines that its microhardness distribution also has obvious gradient change characteristics;The increase of carbon content in the carburization layer of stainless steel changes the grain boundary morphology,improves the material strength but decreases the material ductility,and the microstructure of the carburization layer of stainless steel has little change in its thickness direction;The decrease of carbon content in the decarburization layer of carbon steel makes the pearlite inside disappear,decreases the material strength but improves the material ductility,and the microstructure of the decarburization layer of carbon steel has little change in its thickness direction.(2)The interface layer of the clad plate is characterized by experiments,and the growth law of the multi-element alloy layer and the distribution law of element content and mechanical properties are revealed.The research results show that the variation law of the thickness direction of the main elements in the multi-element alloy layer remains stable and is not affected by its own thickness.The growth of the multi-element alloy layer obeys the parabolic law and is controlled by the volume diffusion,and its activation energy is 54.35kJ/mol.Along the thickness direction of multi-element alloy layer,from the stainless steel side to the carbon steel side,the nano hardness and elastic modulus of the multi-element alloy layer decreases monotonously and increases monotonously,respectively,and these above trend is not affected by the thickness of multi-element alloy layer.The greater the thickness of the multi-element alloy layer is,the greater the composite strength of the clad plate specimen is.(3)The thermal stability of hot rolling bonded stainless/carbon steel clad plate is studied by experiments,and that the effect of temperature on the chemical composition,microstructure and mechanical properties of the clad plate are revealed.The experimental results shows that when the temperature is below 500℃,the microstructure and the element distribution of the composite zone can keep stable;when the temperature is higher than 500℃,the obvious atomic diffusion occurs again in the clad plate.The carbon element in carbon steel would diffuse into stainless steel gradually,which makes the thickness of the decarburization layer of carbon steel increase and the thickness of the carbon steel decrease,and increases the carbon element content of the stainless steel until it is fully carburized.The increase of carbon content in stainless steel would gradually change its grain boundary morphology,and its yield strength,tensile strength and nano hardness will gradually increase,while its elongation and surface corrosion resistance will gradually decrease.The yield strength,tensile strength and composite strength of the whole clad plate would decrease with the increase of temperature.(4)The experimental study on the deformation and failure behavior of hot rolling bonded stainless/carbon steel clad plate under axial tension and thick compression is carried out,and the effect of interface layer on the whole deformation and failure of the clad plate is revealed.The experimental and simulation results shows that the interface between multi-element alloy layer and carburization layer of stainless steel will be destroyed and expanded first with the increase of unidirectional tension.When the interface stress difference reaches 500 MPa,the interface between the multi-element alloy layer and the carburized layer of stainless steel will be destroyed.The compatibility of the continuous deformation of the clad plate determines that the failure of the clad plate under uniaxial tension does not first occur at the place with the weakest strength,but occurs at the interface with the largest difference in mechanical properties.When the clad plate is deformed,the difference of mechanical properties between the multi-element alloy layer and the carburized layer of stainless steel is the largest,which would lead to the occurrence of great stress difference at the interface between the multi-element alloy layer and the carburized layer of stainless steel,causing the interface would be destroyed first.(5)Based on the new delamination structure and its mechanical model,the finite element simulation model of the reduction rolling treatment of clad plate is established,which reveals the elastic-plastic deformation behavior and the law of shape generation and change during the reduction rolling treatment process of clad plate.The FEM results shows that the clad plate can be thinned without interface damage through the reduction rolling treatment,but the large warpage would appear on the the thinned clad plate.The optimization of the related process parameters could not obviously improve the shape defects,but the straightening treatment could effectively eliminate the shape defects of the the clad plate after reduction rolling treatment.(6)Based on the new delamination structure and its mechanical model,the finite element simulation model of the stamping process of clad plate is established,which reveals the elastic-plastic deformation behavior and the formability of clad plate.The FEM results shows that there is no interface damage of clad plate during the stamping process.The effect of stamping direction on the forming limit and springback of clad plate is relatively small.With the other conditions unchanged,the forming limit and springback of clad plate would increase with the increase of stainless steel(cladding)thickness,and the springback of clad plate will decrease with the increase of stamping. |
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