Study On The Effect Of Process On Microstructure And Properties Of SCM435 Cold Heading Wire Rod

The cold heading steel SCM435 is mainly used to produce high strength fasteners and standard parts.In the process of use,in addition to the higher requirements for the strength property,the requirements for surface quality,cold heading performance and anti-delayed destructive performance are constantly increasing.However,the quality of domestic high strength cold heading wire rod is not stable enough to meet the high standard of manufacturing demand,resulting in a high defective rate of products.Therefore,improving the microsturcture and properties of high strength wire rod products represented by SCM435 has become a research hotspot in domestic major steel plants.Based on the actual production situation of SCM435 in Shagang,this paper studies the effects of steelmaking and rolling processes on the microsturcture,mechanical properties and cold heading performance of finished steel.The key process parameters of steelmaking and rolling process is clarified to ensure that the properties of products meet the requirements of standards.The main research results are as follows:(1).The steelmaking and casting process of SCM435 cold heading steel is studied.The composition of liquid steel is adjusted in time by means of intensified temperature measurement and component analysis to ensure that the composition of finished products meet the requirements.From the actual steelmaking results,the desulfurization effect of desulfurization station is relatively stable.The end-point sulfur is 0.007%,which meets the technological requirements.The end-point carbon,silicon and phosphorus is respectively 0.36%,0.228%and 0.014%,which meets the technological requirements.The other elements such as Cr and Mo also meet the requirements.By using 0.25L/kg specific water and 13mm reduction,the quality of billet can be guaranteed to meet the technological requirements.(2).Multi-batch trial rolling of SCM435 cold heading steel is carried out,and the fracture of tensile samples under different rolling processes is analyzed.The results show that there are considerable amounts of bainite and martensite in the fractures with higher tensile strength,which are brittle fracture;The fractures with lower tensile strength are mostly sorbite and ferrite.That fractures are ductile and have better toughness,general strength..The higher the content of martensite and bainite,the higher the tensile strength.(3).In the rolling process,SCM435 cold heading steel blank is cooled at different cooling rates.The results show that when the cooling rate is high,the mechanical properties of the wire rod are difficult to control and change greatly.There are great differences in the microstructure and properties of the materials at different positions in the same circles.In the same batch of tests,the maximum range of tensile strength is 213MPa and the maximum range of reduction of area is 29%.The mechanical properties of samples after quenching and tempering can meet the requirements of internal control standard(tensile strength is 985 MPa,reduction of area is 45%).Where the quenching temperature is 850? and the tempering temperature is 550?.The tensile strength of the samples after heat treatment is 1041.5 MPa and the reduction of area is 62.5%.(4).By analyzing the effects of grain size and uniformity on the microstructure and mechanical properties of the wire rod products,it can be seen that the larger number of bainite and martensite in the microstructure,the higher the hardness and the cold heading cracking rate.It can effectively improve the deformation in the non-recrystallization zone,control the phase transformation of the material,form ferrite+pearlite microstructure,reduce the hardness and improve the cold heading performance by reducing the billet start-rolling temperature,lengthening the holding time in the middle section of the roller and reducing the cooling rate to 0.24?/s.(5).The samples of Bao-steel and Sha-steel are comparative analyzed.The microstructure on 1/2 radius and hearting of Bao-steel's samples are mainly B+F.The microstructure is uniform.The resistance to dislocation is small and the resistance to deformation is low under stress,which is helpful to improve the cold heading performance of the material.The microstructures of our company's samples are P+B+F+little M.The pearlite volume in the heart is more than 1/2 radius.The microstructure is uneven.The hardness of the material is larger than that of Bao-steel,which will lead to stress concentration of the wire rod in the process of cold heading,cause cold heading cracking.Uniform microstructure can reduce the hardness of the material and improvethe cold heading deformation ability.