Researches On Strengthening And Plasticizing Mechanism Of Martensite-austenite Composite Steel With High Strength And Plastic

Composited the high-strength steel and high-ductility steel is an effective method to improve the ductility of high-strength steel,however,the strength of high-strength steel will be reduced.Breaking the trade-off relationship between strength-ductility of steel is one of directions that researchers are persistently pursuing.In the present study,a strategy was innovatively proposed,which controlled the diffusion of small atomic carbon between neighboring layers in composite steels to balance the strength of the strengthening unit and the coordinated deformability of the ductility unit in composite steel,to prepared the martensite-austenite composite steel with both high strength and high ductility.The study started from the composition design of martensitic steel and the process of composite steel,simulating by Thermo-calc software,combined the detailed microstructure characterization of TEM,EBSD,XRD and others,methods of component analysis,tensile properties and other test methods.The method of controlling the carbon diffusion and the influence of compound parameters on the microstructure and mechanical properties of each layer in composite steel were systematically studied,and the strengthen and plasticize mechanism of martensite-austenite composite steel was revealed.Firstly,through the theoretical analysis,an idea was proposed to control the diffused content of carbon in neighboring layers in composite steel by Si element coordinated control the activity of carbon atoms.And the effect of introducing and no introducing the diffusion carbon element on the strength and ductility of the composite steel with the thickness ratio of martensite-austenite layer of 2:1 was compared.It was proved that the tensile strength and uniform elongation of composite steel,which introduce the diffusion carbon element and control the content of diffused carbon,were increased by 143 MPa and 7.4%,respectively.It was found that the diffused content of carbon element between neighboring layers gradually increased with the thickness ratio of martensite layer to austenite layer increased from 1:1 to 6:1,resulting in more twin martensite appeared in the lath martensite and the dislocation density gradually increased.The strengthening of martensite layer caused by the increase of carbon content and the dislocation strengthening introduced by the austenite layer due to the coordination deformation improved the strength of each layer,which is the main reason for the strength increment of composite steel.The TRIP effect of the transition layer in martensite layer and the generated GNDs near the interface was effectively ensure the ductility of composite steel.The optimized best layer thickness ratio of the composite steel was 2:1,the tensile strength reached 1736 MPa,the uniform elongation 10.5%,and the total elongation 22.5%.Furthermore,it was found that an obvious effect of holding time after compounding on the composite steel.With the holding time increasing from 0.5 minutes to 20 minutes,the carbon content,the size of grain and dislocation density in austenite layer gradually increased;the carbon content in the martensite layer increased gradually,and the distribution is more uniform,and the thickness of martensite lath and dislocation density decreased.Comprehensive factors resulting that the hardness difference between neighboring layers increases first and then decreases,the largest hardness difference at 5 minutes,and generated more GNDs,the HDI stress and hardening effect were the strongest during the deformation process,so that the composite steel can obtained the best comprehensive performance.Finally,the effect of the diffused content of carbon between neighboring layers on the distribution of element,microstructures and mechanical properties of composite steel was studied.The diffused content of carbon between neighboring layers gradually increased with the decreased of silicon content in martensite layer,and the dislocation density decreased slightly.This result in that the hardness increment increased from 114 HV to 138 HV.The increased of diffused carbon content resulted in the decrease of staking fault energy,and the strain-induced martensite transformation occurred during the deformation of the austenite layer,which improves the hardening ability of the austenite layer.Meanwhile,austenite layer coordinated the martensite transformation and result in that the hardness increment increased from 2HV to 39 HV.The comprehensive comparison shows that the greater the diffused content of carbon between neighboring layers,the greater the strength increment of composite steel.However,the ductility will be decreased.Conversely,the ductility of martensite-austenitic composite steel can be improved by decreasing the diffused content of carbon without decreasing the strength.In the present study,a strategy for preparation of martensite-austenite composite steels with both high-strength and high-ductility by controlling the diffusion of carbon was proposed successfully,which provides a new path for the development of high strength and ductility steels.At the same time,the principle was not only prepared martensite-austenite composite steel,but also can be used to prepare other high performance composite materials.