| The research and development of advanced high-strength steel with high strength and good formability is the focus of automobile industry in recent years.Air-hardening steel is a kind of advanced high-strength steel with excellent cold formability in annealed state and high strength martensite/bainite structure can be obtained by air cooling after austenitizing.Its processing process has the characteristics of short process,low cost,high precision and high efficiency.However,at present,the regulation mechanism of the formability of air-hardening steel and its air-hardening mechanism are not clear.The objective of this paper is to develop a kind of air-hardening steel which has good cold forming performance in annealed state and can obtain high strength and good plasticity after heat treatment.By studying the evolution of microstructure and mechanical properties of air cooling strengthened steel during recrystallization.The effect of Nb microalloying on the microstructure and properties during recrystallization annealing process was analyzed.To obtain the annealing process window of optimal cold forming performance of air-hardening steel.And reveal the regulation mechanism of its formability.On the basis of obtaining the best formability of the experimental steel.By studying the change rule of different austenitizing processes on the microstructure and mechanical properties of experimental steel.Analyze the influence of Nb microalloying on the microstructure and properties of experimental steel after austenitizing.The strengthening mechanism of air-hardening steel is clarified.Summarized as follows:Considering the influence of alloy elements on the hardenability,streng th and plasticity of air-hardening steel,four air-hardening steels Fe-1.95Mn0.26Si-0.1C-0.75Cr-0.16Mo-0.005B-0.06V-0.035Ti,Fe-1.96Mn-0.28Si-0.1C-0.76Cr-0.18Mo-0.006B-0.06V-0.033Ti-0.01 Nb,Fe-1.98Mn-0.27Si-0.1 C-0.79Cr-0.17Mo-0.006B-0.06V-0.037Ti-0.021Nb and Fe-1.95Mn-0.28Si-0.1C-0.73Cr-0.18 Mo-0.005B-0.06V-0.038Ti-0.049Nb were designed and prepared.Phase Trans formation Points and Precipitates of Four Experimental Steels Calculated by Thermal-Calc Thermodynamic Software.The results show that Nb has little effect on the transformation point and precipitation temperature of the exp erimental steel.The critical temperatures of Ac1,Ac3 and Ms of experiment al steels were measured by thermal dilatometer.The CCT curve of experim ental steel is drawn.The microstructure evolution and mechanical properties of experimental steel were studied by annealing process.The influence of Nb microalloying on microstructure and properties of experimental steel during recrystallization annealing was analyzed.The results show that:The experimental steel recovers and recrystallizes during annealing.After annealing at 600～700 ℃ or 700 ℃ for 0.25～4 h,ferrite+carbide can be obtained.The recrystallization process is accelerated with the increase of annealing temperature or the extension of holding time.The ferrite grain size increases,the nanocrystalline carbide in the grain decreases,and the coarse carbide in the grain boundary increases.The strength of the experimental steel decreases,the elongation increases,and the n and r values increase.Annealed at 700～750℃,the microstructure was composed of ferrite,martensite and carbide at room temperature.With the increase of annealing temperature,the nano-scale carbide disappears,the coarse carbide in the grain boundary decreases,and the volume fraction of martensite increases.The strength of the experimental steel increases,the elongation decreases,and the n and r values decrease.Based on the changes of grain size and the amount of nano-scale carbides in experimental steel,the nature of yield extension phenomenon in air-hardening steel was revealed.The formability of experimental steel can be controlled by controlling the ferrite grain size and the amount of nano-scale carbides.Finally,the key process parameters of the optimum cold forming performance of the steel are obtained.After annealing at 700℃ for 4 h,the yield strength of air-hardening steel is lower than 310 MPa,n value is higher than 0.245,r value is higher than 1.52.With the increase of niobium content,the yield strength of the experimental steel increases and the n and r values decrease.By researching the effect of austenitizing process on the microstructure and properties of experimental steel.Analyze the influence of Nb microalloying on the microstructure and properties of experimental steel after austenitizing.The effect of cold deformation on microstructure and properties of experimental steel after austenitizing was investigated.The results show that:Austenitizing at 750～800℃can obtain a dual phase structure of ferrite+martensite with 700 MPa tensile strength and high plasticity.950 MPa granular bainite+lath bainite with high strength and good plasticity can be obtained by austenitizing at 825～900℃.Austenitizing above 900℃,and the structure of the experimental steel remains unchanged at room temperature.The original austenite grain coarsens,and the strength and plasticity decrease.The addition of Nb can precipitate a large number of nano-sized niobium-containing carbide particles in the matrix.The grain size of the experimental steel was significantly refined.Through precipitation strengthening and fine grain strengthening,the strength of experimental steel is improved by nearly 100 MPa.The experimental steel is austenitized after about 10%cold deformation treatment,and its room temperature grain size is relatively large and its strength is low.Austenitized after 20%cold deformation treatment,its room temperature grain size is relatively small and its strength is high.The air-hardening steel should avoid 10%cold deformation as far as possible during the cold stamping process.The work hardening behavior of air-hardening steel was studied by Hollomon analysis and differential C-J analysis.The results show that,Hollomon analysis can better explain multi-stage work hardening behavior and plastic deformation mechanism of experimental steel.The air-hardening steel heat treated in the two-phase zone,and the various strengthening interactions occur.They jointly influence the yield strength.The strengthening model can be represented by modified root mean square superposition(RMS)relationship:σ’y=(?)-δγ.The strengthening model of austenite region heat treatment can be expressed by linear superposition of each strengthening contribution:σy=σ0+σs+σg+σp+σd.The main strengthening contributions of air-hardening steel come from fine grain strengthening,dislocation strengthening and precipitation strengthening. |
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