| The market demand for high strength low alloy wear resistant steel(HSLAWRS)is becoming more extensive with the development of economy and society.Furthermore,the development trend of HSLAWRS is not just bound to the pursuit of high performance with the increasing pressure of environment pollution and resource depletion.The research on resource-saving and environment-friendly HSLAWRS with excellent comprehensive performance is one of the important fields of steel materials nowadays.However,there are still many problems in the production of wear resistant steel,such as very single microstructure of products(tempering martensite),outdated production technology(off-line quenching + tempering after rolling),small selection range of the thickness of the product(20 ～ 40 mm)high probability of warp and delayed cracking.Therefore,further works should be done to improve the quality of HSLAWRS,including optimization of composition and processing,solution of cracking due to internal stress,etc.In this study,steels with different composition were designed to investigate the effect of alloying elements Chromium(Cr)and Nickel(Ni)on the transformation,mechanical properties and wear properties of HSLAWRS.Moreover,to clarify the influence of microstructure on the mechanical and wear properties,various microstructural types were obtained by changing heat treatment after controlled rolling and cooling.Furthermore,HSLAWRS which was mainly consisted of bainite was prepared,and the effect of heat treatment on mechanical and wear properties of bainitic wear resistant steel was studied.In addition,the direct quenching(DQ)and tempering processes of production of martensitic wear resistant steel were designed.The main conclusions are as follows:(1)The addition of Ni and/or Cr can effectively improve the yield strength,tensile strength,cryogenic impact toughness and hardness of the experimental steel after the identical rolling and cooling process.The improvement of yield strength and cryogenic impact toughness by the single addition of Ni is the most significant among the three addition modes.The enhancement in tensile strength and hardness through adding Cr is more considerable than that of single addition of Ni.The composite addition of Ni and Cr greatly improved the tensile strength and hardness of the sample,whereas the increment of yield strength and cryogenic impact toughness by composite addition are inferior to that by single Ni addition.(2)The addition of Cr can increase the amount of bainitic transformation,while the addition of Ni reduced the amount of bainitic transformation.Both Cr and Ni reduce the rate of bainitic transformation,and the inhibition effect of Ni on the bainite transformation rate was greater than that of Cr.Moreover,the combination of adding Ni and Cr can further reduce the rate of bainitic transformation.(3)Continuous cooling treatment contributes to the improvement of low temperature impact toughness,but isothermal treatment reduces the low temperature impact toughness due to carbide precipitation.In addition,the addition of Ni can effectively improve the low temperature impact toughness during the continuous cooling process,while the composite addition of Ni and Cr can increase the hardness and reduce the low temperature impact toughness.Moreover,the effect of Ni addition on the low temperature impact toughness was not obvious during the isothermal cooling process,but the composite addition of Ni and Cr can effectively improve the low temperature impact toughness hardness.(4)With different addition of Ni during the continuous cooling process,the volume fractions of bainite and martensite are 20.63% and 79.37%,26.41% and 73.59%,35.26% and 64.74%,respectively.The addition of Ni can reduce the spalling wear and improve the wear resistance of the tested steel.In addition,the weight loss of the tested steel with composite addition of Cr and Ni was small in the early wear stage due to the high hardness and strength,but the wear rate increased in the late wear stage due to the deterioration of low temperature impact toughness.(5)The 68.72% and 82.06% volume fractions of bainite were obtained in Ni steel during isothermal holidng at 340℃ for 200 s and 400 s.In addition,the impact toughness,elongation and yield ratio increase with the increase of bainite content in the bainite/martensite dual phase steel,while the hardness,tensile strength and yield strength decrease.Furthermore,the bainite/martensite dual phase steel with small bainitic volume fraction show better wear resistance under low impact load wear condition,while the bainite/martensite dual phase steel with high bainitic volume fraction shown better wear resistance under large impact load wear condition.(6)Low alloy bainitic wear resistant steel was successfully prepared by continuous cooing treatment after rolling.Moreover,it was found that the comprehensive mechanical properties and wear properties of the tested steel are the best during tempering treatment at 200 ℃ for 30 min.The wear resistance of the steel can be effectively improved using isothermal quenching at 320 ℃for different times and 400 ℃ for 3min and 6min,while the wear resistance decreases using isothermal quenching at 360 ℃.(7)The influences of quenching processes on the microstructure,hardness and residual stress of low alloy martensitic wear resistant steel were studied.It was found that the residual stress on the surface of the tested steel increased gradually with the increase of quenching temperature.Moreover,the hardness of the experimental steel increased with the increase of quenching temperature which is below 940℃,but the hardness of the tested steel decreased obviously when the quenching temperature is 1150℃.In addition,the residual stress and hardness of the tested steel increased gradually with the decrease of quenching termination temperature.Furthermore,the on-line direct quenching process and the industrial trial production of low alloy martensitic wear resistant steel were designed based on the results of different quenching experiments. |
PDF Full Text Request