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 parameterαis 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 transitionαand 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.