This work is concerned with a development of low-cost strong-tough cryogenic steels of Fe-30Mn-5Al-0.3C-X by adding microalloying elements and applying controlled rolling process. The microalloying elements studied were Nb and V. The purposes of the microalloying elements were two folds; the first was to introduce precipitation hardening and the second was to restrict grain growth during controlled rolling to produce fine grain size. The presence of the precipitates also restricts grain growth in the heat affected zone in the welding of the alloy. Refinement of grain size is very important in cryogenic alloys since it can increase strength as well as toughness.
It was found that the yield strength of the controlled rolling condition at -196℃ was higher by about 40% over that of cold rolled and recrystallized material. The application of controlled rolling greatly reduced the grain size from ASTM No.4 to ASTM No. 10. The addition of Nb was more effective in increasing yield strength than that of V in the Fe-30Mn-5Al-0.3C-X alloy steel.
Impact toughness of Fe-30Mn-5Al-0.3C-0.10Nb at -196℃ was about twice higher than that of 9\% Ni steel, the standard commercial cryogenic steel while possessing about the same yield strength as 9% Ni steel.
A specially interested observation was the increase of elongation with decreasing temperature. This unusual mechanical behavior is ideal for cryogenic material. The anomalous behavior in the alloy steel is related to the formation of stress induced phase during deformation. Electron microscopic study revealed that fine mechanical twinning was formed during deformation. The formation of twin at the crack tip restricts propagation of the crack, thereby preventing continuous growth of necking at the region. Further uniform elongation without premature necking is possible by the stress induced twinning formation. As the temperature lowers, formation of twinning is favored.
It was also found that the newly developed steel exhibited a strong resistance to hydrogen embrittlement. The addition of extra alloying elements such as Cu and Si to the Fe-30Mn-5Al-0.3C-0.05Nb also enhanced corrosion resistance without impairing mechanical properties.
