| Basing on a low carbon silicon-manganese series high strength steel containing Cr and P, the effect of rolling process parameters on the microstructures and mechanical properties of the experimental steels was investigated through thermal simulation and thermal mechanical control processing (TMPC) in laboratory. And got hot-rolled high strength wheel steels with different complex phases through industrial hot rolling experiments. Since the wheels work under random load, the damage form of the wheels is given priority to strength damage and fatigue damage, more than80%caused by fatigue failure, the fatigue life of wheels is the most important performance indicator. So far, few domestic studies about fatigue performance of car wheel steel have been carried out, especially forfatigue performance difference of high strength wheel steel with different microstructure. On the basis of the hot rolling experiments, the axial tension and compression fatigue test for different complex phases high strength wheel steel were carried out by using the high-frequency fatigue testing machine. To research the differences of fatigue performance of different complex phaseshigh strength steelthe fatigue life curves were mapped and fracture micro morphologies were analyzed. The main conclusions are as follows:(1) The static CCT curve and dynamic CCT curve of experimental steel whose composition was0.12C-0.52Si-1.40Mn-0.51Cr-0.08P were measured and mapped. Pearlite microstructure could not be found in the experimental steel under various cooling speed. With the increase of cooling speed, the start temperature of ferritic phase transformation declined, and ferrite phase transformation interval narrowed. Deformation increased the start temperature of ferritic phase transformation, expanded the polygonal ferrite formation area. When isothermal temperature respectively is630℃,660℃and700℃, corresponding to the ferritic phase transformation time is900s,400s and300s respectively. With the isothermal temperature increasing, the ferrite content decreased from81%to42%.(2) When finish cooling temperature was630℃, finish rolling temperature increased from766℃to820℃, the Rm of experimental steels improved from720MPa to748MPa, the Rp0.2improved from463MPa to525MPa, elongations were all around20.0%. When finish rolling temperature was870℃, Rm was lower to655MPa, Rp0.2was415MPa, but the elongation was higher to26.98%. When finish rolling temperature was820℃, the experimental steel showed obvious TRIP effect. Retained austenite was mainly distributed in the interior of bainite and the border of ferrite and bainite.(3) When finish rolling temperature was770℃, finish cooling temperature increased from630℃to660℃, the Rm of experimental steels improved from720MPa to790MPa, the Rp0.2improved from463MPa to513MPa, elongation change was not obvious, comprehensive mechanical properties was improved. When the final cooling temperature continued to rise to700℃, Rm was780MPa, RP0.2was478MPa,δ was19.03%, Rp0.2/Rm was0.61. Tensile fracture types are all ductile fracture.(4) When finish rolling temperature was770℃, finish cooling temperature was630℃, and bainite isothermal time were5min,15min,30min respectively, retained austenite content of experimental steels were8.26%,7.26%and7.87%respectively, the carbon content of the retained austenite were1.288%,1.299%and1.344%respectively. As the extension of isothermal time, the carbon content of the retained austenite shows a trend of increasing.(5) Under the condition of hot rolling test in the industrial production, for the experimental steel whose composition was0.12C-0.519Si-1.4Mn-0.507Cr, when the finish rolling temperature was810℃, coiling temperature was379～392℃, microstructure was composed of ferrite, bainite and degenerated pearlite; when coiling temperature was382～420℃, the microstructure of experimental steel was composed of ferrite and bainite; when coiling temperature was310～330℃, the microstructure of experiment steel was composed of ferrite and martensite. For the experimental steel whose composition was0.08C-0.19Si-1.54Mn-0.016P-0.038Al-0.023Ti, when the finish rolling temperature was845～866℃, coiling temperature was562～614℃, microstructure was composed of ferrite, pearlite and dispersed M-A island.(6) When cyclic stress ratio r was-1, the end of the cycle number was1×107, the fatigue limit σ-1of experimental steel whose microstructure was composed of ferrite, bainite and degenerated pearlite was327.5MPa; the fatigue limit σ-1of experimental steel whose microstructure was composed of ferrite, pearlite and dispersed M-A island was412.5MPa, the spacing of fatigue striations when stress amplitude was415MPa was less than that when stress amplitude was460MPa, and tyre pattern could be found in the fracture microstructure; the fatigue limit σ-1of experimental steel whose microstructure was composed of ferrite, bainite and retained austenite was462.5MPa. Fatigue fracture microstructure has significant fatigue striations and its direction was perpendicular to the direction of crack propagation.(7) The fatigue source area was usually in the specimen surface of experimental steel. When there was a big inclusion, fatigue crack initiation at inclusion and the fatigue life was reduced. The secondary cracks could be found in all of instantaneous fracture regions of experimental steels, and it could be also found in the fatigue crack propagation region of experimental steel whose microstructure was composed of ferrite, bainite and retained austenite. |
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