Study On Mechanical Properties And Tensile Fracture Mechanism Of 15CrMo Steel Plates With Martensitic + Ferrite Lamellar Structure

In recent years,the excellent properties of duplex steels have gradually come to the attention of researchers.In a typical martensitic + ferritic duplex steel,the hard phase is martensitic,which improves the strength of the entire lamellar structure,but is poorly coordinated;the soft phase is ferritic,which has excellent tensile plasticity and is well coordinated.Therefore,reasonable control of the volume fraction and the size of the hard and soft layers in the lamellar structure is the key to improving the performance of dualphase steel.In this paper,martensitic + ferritic lamellar structure steel plates were prepared by sub-temperature quenching and cold rolling process of 15CrMo steel plates,and various volume fractions and lamellar sizes of martensite and ferrite were obtained by adjusting the quenching temperature and cold rolling deformation.The effect of lamellar thickness on the mechanical properties of micro-lamellar steel plates was investigated,and the mechanism of tensile fracture delamination was analyzed using finite element simulation techniques.The results show that:the martensitic + ferritic lamellar structure of 15CrMo steel was obtained after sub-temperature quenching and more than 50% cold rolling processing,and this steel plate has two characteristics: the martensitic + ferritic micro lamellar structure parallel to the plate surface and the layered tensile fracture;the martensite +ferrite layer structure is a direct factor affecting the mechanical properties of the material,and the tensile strength and hardness of the cold-rolled specimen are inversely proportional to the thickness of the martensite + ferrite layer.With the increase of cold rolling deformation,the martensite and ferrite gradually tend to be arranged parallel to the surface of the steel plate,which leads to the fracture delamination phenomenon,and the reduction of the thickness of the martensite + ferrite lamellae facilitates the expansion of the delamination cracks,and the fracture delamination phenomenon becomes more obvious.The finite element simulation results show that the maximum stress concentration and strain energy distribution are located inside the martensite,the narrower the martensite,the more stress concentration points and strain energy,and with the increase of quenching temperature,the more stress concentration points and strain energy,so the martensite is the main stress point to bear the martensite + ferrite lamellar structure.With the increase of cold rolling deformation,the lamellar structure is evenly distributed inside the steel plate,and the stress and strain energy distribution area increases.In the event of fracture,fracture occurs first within the martensite layer,and then with the continued plastic deformation of the ferrite layer,fracture occurs at the martensitic-ferritic interface,forming layered cracks,and finally the ferrite layer breaks to form a layered fracture.