Study On The Phase Transformations And Phase Equilibria Of Fe-Mn-Al-Si TRIP/TWIP Steels

In the 21st century, lightweight design has been the main trend to automotive industry. Therefore many countries have developed AHSS (Advanced High Strength Steels, recorded as AHSS). Typical AHSS, including TRIP (transformation induced plasticity steels) and TWIP (twinning induced plasticity steels), have a high strength together with a good ductility. The austenite transformations during fast-cooling, low temperature heat treatments and deformations result in high strength combining with remarkable ductility of the AHSS. Therefore it is valuable both in theory and applications to study the phase transformations of the austenite, however, there is still lack of such systemic and in-depth study.In this paper, different experimental techniques including optical microscope, scanning electronic microscope, energy-dispersive X-ray analysis, X-ray diffraction analysis and electronic probe analysis have been used to study the phase equilibria at 1000’C and the phase transformations of austenite in the Fe-XMn-4Si-2Al, Fe-18.5Mn-XSi-Al and Fe-XMn-4Si-YAl alloys. The results are as follows:(1) Make sure the two-phase(a and y) region on the isothermal section graph of Fe-Mn-Al-Si quaternary alloys with the composition of 10～30%Mn,≤4%A1,≤8%Si at 1000℃.(2) The study on the Fe-XMn-4Si-2Al alloys show that the martensitic transformation changes from γâ†'α’ to γâ†'ε with the increase of Mn content. In details, â‘  The austenite transforms into a’martensite during quenching for the alloys with Mn content less than 14.3 wt.%. â‘¡ The austenite transforms into α’martensite and ε martensite during quenching for the alloys with Mn content from 14.3 to 18.5 wt.%. â‘¢ The austenite transforms into s martensite during quenching for the alloys with Mn content from 18.5 to 25 wt.%. â‘£ The austenite can remain to room temperature without transforming into martensite during quenching, for the alloys with Mn content more than 25 wt.%.(3) Study on Fe-18.5Mn-XSi-Al alloys has shown that Si has an important effect on austenitic transformation during quenching. In details, â‘  The austenite transforms into a’ martensite and ε martensite during quenching for the alloy with Si content less than 2.5 wt.%, and the e martensite is more than a’martensite. â‘¡ The austenite transforms into ε martensite during quenching for the alloy with Si content form 2.5 to 6 wt.%. â‘¢ There is only ferrite and it does not transforms into martensite during quenching for the alloy with Si content more than 6 wt.%.(4) Study on Fe-14.3Mn-Si-YAl alloys has shown that Al has a similar effect on austenitic transformation with Si. In details, â‘  The austenite transforms into ε martensite during quenching for the alloy with Al content less than 0.5 wt.%. â‘¡ The austenite transforms into a’martensite and ε martensite during quenching for the alloy with Si content form 0.5 to 1 wt.%. â‘¢ The austenite transforms into a’martensite during quenching for the alloy with Si content more than 1 wt.%.