Study On Bending And Wear Resistance Of High Strength Low Alloy Steel

High strength low alloy steel is widely used in engineering machinery,mining machinery,automobile manufacturing and other fields.Domestic high-strength steel will crack and wrinkle in the bending process,while foreign high-strength steel has superior mechanical properties and bending properties.Therefore,the microstructure and mechanical properties of domestic NM450 high strength steel during continuous casting,billet reheating and hot rolling are studied,and the fracture causes of NM450 steel after bending are analyzed.In this paper,the microstructure,tensile fracture and wear fracture of high strength low alloy steel were studied by quasi-static tension,wear test,OM,SEM,EBSD and TEM,so as to lay a foundation for the large-scale application of the steel.The research results are as follows:(1)There is composition segregation in the thickness direction of NM450 steel billet,and the volume fraction of pearlite on the surface is less.The closer to the thickness center,the higher the volume fraction of pearlite.Pearlite belongs to hard phase structure,so the hardness in the thickness direction is low at both ends and high in the middle.The TiN distribution in the thickness direction shows a trend of more in the middle and less at both ends,larger in the middle and smaller at both ends.Size 2-6 μm has the largest number of TiN,up to 64%,and the largest tin size can reach 38μm or so.(2)When the heating temperature is fixed,the grain grows gradually when the holding time increases from 15 min to 35 min and 55 min;when the holding time is fixed and the heating temperature increases from 1100 ℃ to 1125 ℃,1150 ℃ and1175 ℃,the grain size also increases gradually.When the heating temperature is 1100 ℃and the holding time is 15 min,the average diameter of austenite grain is the smallest,and its diameter is 53.52 μm;when the heating temperature is 1175 ℃ and the holding time is 55 min,the average diameter of austenite grain is the largest,and its diameter is 116.07 μm.In the process of grain growth,serious mixed crystal phenomenon occurs.(3)Compared with NM450 steel,the martensite structure in Strenx1300 steel is finer,evenly distributed,the grains are finer,the hardness is strengthened,the yield strength is high,and the mechanical properties in TD and RD directions are more stable.Strenx1300 steel has larger fiber area diameter,better plasticity,fewer cleavage surfaces and smaller area in the shear lip area.The wear amount of Strenx1300 steel and NM450 steel increases with the increase of wear times,but the wear amount of Strenx1300 steel changes slightly,while the wear amount of NM450 steel changes greatly.After 15000 times of impact wear,it is found that Strenx1300 steel has better wear resistance.(4)After bending of NM450 steel plate,the degree of microstructure deformation at different positions is different.Severe surface deformation and work hardening lead to the increase of surface hardness and tensile strength of the steel plate,but the elongation and area shrinkage after fracture are significantly reduced,dislocation accumulation,dislocation density in the center becomes larger compared with the thickness,and the proportion of low angle grain boundary increases.The fracture mechanism of TiN inclusion on the surface can be divided into the following four types:(1)homogeneous nucleation TiN inclusion forms multiple cracks perpendicular to the outer arc surface under the tensile stress;(2)Under the tensile stress,microcracks initiate at the interface between oxide core and TiN,and expand along the tensile direction to form holes;(3)Under the compressive stress,the tin inclusion with homogeneous nucleation initiates cracks from the center of TiN and propagates in a divergent manner to the surrounding;(4)Under the compressive stress,the crack originates from the interface between oxide core and TiN,and expands around to form a divergent crack.