Study On Martensitic Transformation And Strengthening And Toughening Mechanism Of Ultra-low Carbon Fe-Mn-Ni-Cu-Al Alloy

In this paper,we study the martensitic transformation characteristics of an ultra-low carbon Fe-Mn-Ni-Cu-Al alloy,and microstructure evolution and precipitation strengthening of martensite during tempering.We also study the evolution and influence on mechanical properties of martensite and reverted austenite by one-step and two-step intercritical tempering heat treatment.The CCT curves of the steels were measured by phase change dilatometer,and the microstructure and morphology at different cooling rates were obtained:when the cooling rate between 100?/s to 1?/s only lath martensite forms.Lath martensite and bainite were obtained under the cooling rate of 0.1?/s,and bainite,lath martensite a small amount of polygonal ferrite structure were obtained at the cooling rate of0.02?/s.The K-M equation for the kinetics of martensitic transformation is obtained by fitting the experimental data.The parameter?is about 0.03,and the M_s point is about 380?,the M_s point(378?)obtained by the double tangent method has a small difference with it.The kinetics of phase transition?and M_s increase with the decrease of cooling rate.In the tempering process,the experimental steel has a strong resistance to temper softening ability,with the tempering temperature increases,the hardness of the quenched test steel continued to rise in the temperature of about 500?to reach the peak hardness,and in the two-phase area back When the second hardening peak of fire.The precipitation hardening effect at 450?is significant at 2-500 h and reaches the peak at about 100 h.In the low temperature tempering stage(450-550?),the matrix is mainly tempered martensite,and the formation of the dispersion of nano-rich copper precipitation and NiAl phase;in the high temperature(650-700?)tempering stage,a large number of reverted austenite appears in the tempered martensite.In the same original austenite grain,the reverted austenite maintains a certain position relationship with the matrix.The steels were heat treated by one-step QL and two-step QLT process at different temperatures and at different times.After tempering at a lower critical tempering temperature,it forms the tempering martensite,critical ferrite,secondary martensite and retained austenite.After the quenching at high temperature,the martensite,critical ferrite and a small amount of tempered martensite are obtained.The maximum retained austenite,about12%,can be obtained after intercritical tempering at 675?for 20 min.Increasing the intercritical tempering time and increasing the tempering at675?in the second step are not conducive to obtaining stable retained austenite.The choice of temperature and time for intercritical tempering has a large effect on the retained austenite content.The hardenability of the material itself is very good,easy to harden and form martensite,and in the process of tempering Cu produced a small dispersion of precipitation phase strengthening matrix,which makes hardness and strength values of the materials under various processes vary little.