| Super bainitie steels not only meet the requirements of high strength in mechanical properties but also have enough plastic toughness due to its microstructure,morphology and distribution characteristics.So many researchers are devoted to study it recent years.Researchers represented by Bhadeshia,University of Cambridge United Kingdom,designed the alloy steel with Mn,Si,Ni,Co and Al elements,and the super bainitie steel was obtained after isothermal treatment at low temperature(above martensitic transformation temperature)for a long time.Notwithstanding possessing good mechanical properties,the application of the steel was restricted by the following factors: the transformation period is long(usually takes dozens of hours or even weeks to complete the transformation of super bainite structure),and the expensive cost of various kinds of alloy elements.Can super bainite structure be obtained by using low-cost alloy based steel? Can the mechanical properties of super bainite steel be further improved by the transformation of its microstructure? And can we shorten the time of microstructural transformation by controlling the temperature of microstructural transformation? The research are aimed at solving the above problems.60MnSi2 Cr and 70Mn2Si2 CrMo experimental steels were designed with Mn and Si as the main components and a few Cr and Mo and other alloy elements.The super bainite structure was obtained by optimizing the isothermal quenching process.Both microstructure transformation and strengthening and toughening mechanism for steels were analysed.We determined TRIP effect of super bainite with fatigue test.In order to shorten the transformation time of super bainite,the Q & P process was introduced into its preparation process,and the influence of Q & P process on microstructure transformation process and mechanical property was studied.We researched the influence factors of carbide precipitate and the effect of that on microstructure and performance in super bainite.Based on above work,following conclusions can be drawn from the present study.Super bainite structure(bainite ferrite BF + retained austenite AR)was obtained using60MnSi2 Cr and 70Mn2Si2 CrMo designed with Mn and Si as the main alloy elements and a spot of Cr and Mo through heat treatment.When the temperature was a little higher than the Ms temperature,AR film was distributed between BF bands.With the decrease ofisothermal treatment temperature,the bainite ferrite is refined and the content of retained austenite is in a decreasing trend.After complete austenitizing at 900℃,60MnSi2 Cr steel was isothermal treated at 250 ℃,260 ℃ and 270 ℃ for 12 h,respectively.The content of retained austenite was 5.5%,9.4% and 9.3% respectively.The tensile strength,elongation,contraction of area and impact toughness of the specimen after isothermal treatment at260 ℃ were 1816 MPa,7.37%,24.58% and 22.82J/cm2,respectively.When the isothermal quenching temperature of 70Mn2Si2 CrMo reaches 245 ℃ for48 h,there is(Fe,M)7C3 phase in the structure besides super bainite(M is a metal element other than Fe).The precipitation of this phase results in the decrease of retained austenite content(7.73%)and carbon content in AR(0.88%);However,there is no carbide phase in the sample after isothermal treatment at 220 ℃,the content of AR and C are 8.42% and1.02%,respectively.The mechanical properties were decreased due to the decrease of carbide precipitation and AR content.The tensile strength decreased from 2347 MPa to2082 MPa,the elongation decreased from 7.64% to 7.24%,and the number of fatigue fracture decreased from 184 × 103 to 83 × 103,compared with the specimen which has no precipitates after isothermal treatment at 220 ℃.It can be found that the temperature of isothermal treatment to obtain super bainite is suitable when there is no obvious carbide precipitation,so as to avoid the decrease of its mechanical properties.When the content of retained austenite in the super bainite obtained using 60MnSi2 Cr by isothermal treatment is less than 5.5%.Although the load is applied,the martensitic transformation will not take place,that is to say,there is no trip effect.When there is 9.9%retained austenite in the structure,the content of retained austenite in the specimen is 4.6%after applying load that is 60% yield strength(pull pull)for 72 hours.It is found that the51.06% of retained austenite has transformited to Martensite.Simultaneously,the tensile strength and elongation of the sample are 2000 MPa and 8.8%,respectively.While the retained Austenite that has no transformation to martensite has a high carbon content(>1.3%),and its stability is increased,which leads to no phase transformation when the applied load is not enough.The results show that when the content of retained austenite in super bainite reaches a certain content(higher than 5.5% under experimental conditions),trip effect can be generated,and the mechanical properties of super bainite can be further improved.Q & P process of heat treatment technology was introduced into the experiment,and the method combined Q&P process and salt bath isothermal treatment were employed totreat 60MnSi2 Cr steel.The results manifest that the mechanical performance that takes 12 hours to process using previous heat treatment technique,which can be achieved in 6 hours after Q & P process and 260 ℃ isothermal treatment.In the former specimen,the tensile strength,elongation and the impact toughness are 1948 Mpa,9.96%,40.34J/cm2,increased about 7.26%,35.12% and 76.77% than the latter,respectively;The main reason is that the nuclear driving forces is larger,and the quenching temperature in Q & P process is lower,which is positive to the formation of more α phase nucleation.Meanwhile,a small amount of martensite formed results in a numerous of crystal defects,which promotes the heterogeneous nucleation,not only accelerates the transformation of bainite ferrite but refines the super bainite microstructure during the subsequent isothermal treatment.Consequently,the microstructure was refined and the mechanical properties were improved. |
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