Microstructure and deformation behavior of austenitic Fe-Mn-Al-C alloy have been studied to find optimum properties for automotive applications. Austenitic Fe-25Mn-1Al-0.2C alloy with fine strain-induced twins showed high strength, excellent formability and good weldability. The alloy possessed three times higher strength with comparable formability and better weldability compared with the current existing automotive materials. The applications of the austenitic alloy for automotive structural materials could provide a considerable weight reduction for fuel economy.
The mechanism, the reason for excellent properties of the alloy, has been investigated in this thesis ; phase transformation and mechanical property variation depending on aluminum contents and manganese contents of Fe-Mn-Al-C alloys in chapter 2 and chapter 3, nucleation mechanism of fine deformation twins in chapter 4, effects of austenite grain refinement on mechanical properties in chapter 5, formability behavior in chapter 6 and weldability in chapter 7.
Aluminum addition to the Fe-Mn-C alloy increased the stacking fault energy of the austenite phase and brought the formation of deformation twins instead of the formation of strain induced ε martensite and/or α‘ martensite during the deformation. When the aluminum contents were 0 %(wt.), 1 % and 3 %, the volume fractions of strain induced ε plus α martensite were 85 %, 75 %, and 0 %, respectively, in Fe-20Mn-0.2C alloys. Also, for the same aluminum contents, the volume fraction of strain induced ε martensite was 65 %, 5 % and 0 %, respectively, in Fe-25Mn-0.2C alloys. Decreasing of the volume fractions of strain induecd ε and/or α’ phases with aluminum addition decreased the tensile strengths of the alloys. However, uniform elongations of the alloys were maximized at 3 % aluminum addition to Fe-20Mn-0.2C alloys and 1 % aluminum addition to Fe-25Mn-0.2C alloys which strain-induced twins formed instead of any other phases. Strain-induced twins gave birth to the optimum combination of high strength and good ductility.
Fine reticulate shaped deformation twins brought high strength and excellent ductility due to high work hardening rate (dσ/dε) and high strain hardening exponent (n-value). Uniformly distributed fine AIN precipitates in austenite grains induced the fine reticulate shaped deformation twins during deformation. Nucleation sites of deformation twins were at interface of coherent AIN precipitates which implied the extended dislocations due to low stacking fault energy under $5mJ/m^2$ Partial dislocations occurred from hep structured AIN precipitates moved to austenite matrix and made the deformation twins by external stresses. Fe-25Mn-1Al-0. 2C-0.044N alloy with many AIN precipitates showed high strength and excellent ductility.
Decreasing of annealing temperature in the cold rolled Fe-Mn-Al-C alloys reduced the austenite grain size from 40㎛ at 1000℃ annealing temperature to 4㎛ at 600℃. Austenite grain refinement increased the yield strength and the tenite phase. High nitrogen grain refinement increased the yield strength and the austenite phase. High nitrogen (440 PPM) contained alloy with uniformly distributed AIN precipitates in austenite grains showed higher k value of Hall-Petch equation, higher uniform elongation and higher n-value than low nitrogen (33 PPM) contained alloy with few AIN precipitates in Fe-Mn-Al-C alloys. The effects of grain refinement of Fe-Mn-Al-C alloy on mechanical properties were stimulated by fine reticulate shaped deformation twins occured from AIN precipitates in austenite grains.
Austenitic Fe-Mn-Al-C alloy with strain-induced twins showed better formability than current existing automotive ferritic steels. Formability evaluations were conducted in all the testing methods ; r-value, cupping test (LDR), n-value, FLD and LDH. Excellent formability of Fe-25Mn-1Al-0. 5C alloy was brought by high uniform elongation. Uniform elongation of the alloy was 47 %, twice higher than the automotive ferritic steels. Work hardening characteristics of this alloy, low strain hardenig exponent at lower plastic strain and high strain hardening exponent at higher plastic strain, were obtained mainly by deformation twins and gave birth to good formability.
The austenitic Fe-Mn-Al-C alloys showed excellent weldability due to no phase transformation after the welding heat cycle. Brittle phases were not shown in heat affected zone of the weldments.
