Effect Of Cooling Temperature Field On Formation Of Shelf-like Bainite In High Carbon Silicon Steel

In this paper,the structural characteristics of mesophilic isothermal structure of Fe-0.88C-1.36Si-1.01Cr-0.43Mn(wt.%)steel with different thickness is researched by optical microscope observation,transmission electron microscopy and X-ray diffraction phase analysis,and the effects of temperature gradient field on the formation characteristics of phase transition products after high temperature austenitizing and high temperature isothermal are discussed.It is found that,the shelf-like bainite which is composed of nearly parallel bainitic ferrite bars and austenite between the bars is formed in the experimental steel under the condition of austenitizing at 930℃ and isothermal at 270℃.The structure and spatial distribution of shelf-like bainite are closely related to the temperature gradient field in the sample.For the specimen with thickness of 2mm,bainite ferrite strip has a subunit structure,and the shelf-like bainite is preferred orientation distribution in the direction of thickness where has a larger temperature gradient.With the increase of thickness,the characteristics of preferred orientation of bainite in thickness direction gradually weaken.When the thickness is up to 5mm,the shelf-like bainite does not have the preferred orientation,and the bainite ferrite strips do not have the subunit structure.The morphological structure and distribution characteristics of the mesophilic phase transition tissue,which is in the Fe-0.88C-1.36Si-1.01Cr-0.43Mn(wt.%)samples with different thickness at 880℃ austenitizing and isothermal heat treatment at 380℃and 270℃are studied respectively.The results show that,for the plate sample with thickness of 2mm and after isothermal at 380℃,the upper bainite without preferred orientation distribution is formed in the direction of thickness where has a larger temperature gradient.For plate sample with a thickness of 2mm isothermal at 270℃,the lower bainite which has no preferred orientation distribution is also formed in the direction of thickness where has a larger temperature gradient.The microhardness test of sample after heat treatment results show that the microhardness of the shelf-like bainite with a subunit structure can reach 630.2HV,and the microhardness of the bainite without the subunit structure is only 571.6HV.As tthe shelf-like bainite in the samples with thickness of 2mm have preferred orientation distribution,therefore,this paper cut samples for the tensile test in the non thickness direction,that is,in the direction of non shelf-like bainite preferred orientation.Although the tensile strength is 1500 MPa,it is similar to the cubic sample.