Ductility And Cracking Mechanism Of Low Ni Austenitic Stainless Steel During Hot Deformation

Hot ductility in austenitic stainless steel (ASS) slab is low and prone to suffer from cracking during hot deformation. The factors which affect the hot ductility in ASS, such as microstructure, chemical composition, deformation temperature, strain rate and deformation mode have been studied—a case study of two kinds of low Ni ASSs Cr17Mn6Ni4Cu2N and Cr15Mn9Cu2NiN. Combined with the above results, the cracking mechanism was discussed and the measures that avoid cracking during hot deformation were proposed.In the slab shell of Cr17Mn6Ni4Cu2N, the microstructure is dendrite8ferrite which distributing in columnar austenite grains (1#microstructure); in the core,δ ferrite distributes in the interior of austenite and on the austenite boundaries in the form of bar (2#microstructure); the solidification mode of this steel is FA.In the shell of Cr15Mn9Cu2NiN, as the distance from slab surface shorter than27.5mm, the microstructure is consist of dendrite8ferrite, austenite matrix and channel-like austenite (3#microstructure), the solidification mode is the co-existence of FA and A; as the distance longer than27.5mm, the microstructure is consist of ferrite and austenite; in core, the microstructure is coarse austenite columnar grains (4#microstructure), and the solidification mode is FA.Though the comparison of hot ductility in specimens which made from slab and hot-rolled plate, it is found as deforming temperature in the range of1000-1250℃, RA in slab decreased rapidly with temperature. Analysis suggested that, the as-cast microstructure and its deformation characters induce the cracking problem during hot deformation. Through the comparison of cracking in hot working and ductility-dip cracking in welding, it is found that they have the same characters and induced by the same mechanism. The concept of ductility-dip temperature range (DDTR) during hot deformation was proposed in this article. As the strain rate is0.1s-1, the cracking tendency of each microstructure from high to low is1#,3#,4#,2#, respectively.The influence of strain rate on hot ductility, cracks nucleating positions and DDTR were analyzed. As the strain rate is lower, RA in1#and3#microstructure is high, while increasing the strain rate would decrease the hot ductility and the positions of crack nucleuses were changed from δ ferrite dendrites to austenite GB. In2#microstructure, RA increases with strain rate, and the positions of cracks nucleuses are changed from GB ferrite to the co-existence of GB ferrite and austenite GB. As the materials deformation at higher strain rate, the strength can be improved both in austenite and ferrite, which will decrease the stress concentration on ferrite and transfer the weak location from ferrite to austenite GB, then induce the change of nucleating positions of cracks. As increasing the strain rate from0.1to10s-1will enlarge the DDTR in1#and4#microstructure, move the DDTR50℃to the low temperature side in3#microstructure, decrease the DDTR in2#microstructure.According to the hot rolling experiment results, in the range of1000-1250℃, in the two steels, the critical cracking strain deceases rapidly with deformation temperature, In multi-pass hot rolling, the microstructure in Cr15Mn9Cu2NiN slab easier suffer from cracking than the equiaxed grains which from recrystallization, all of this is consistent with the results from tensile tests.According to the above results, the cracking mechanism of as-cast austenitic stainless steel during hot working is proposed. The reason that as-cast ASSs are easier suffer from cracking problems during hot working, mainly due to the as-cast microstructure and its deformation characters. The as-cast microstructure is consisting of coarse columnar grains, which has obviously crystal orientation. During deformation, as decreasing deformation temperature, the columnar grains are easy to strengthen, leading to higher yield strength, the deformation coordination ability is poor, and under a small deformation amount, the cracks will nucleate at the boundaries of columnar grains. The majority of columnar grains boundaries are high angle GB. During deformation, once the cracks nucleate on columnar GBs, they will grow and propagate along them. The stacking faults energy in ASSs is low, and the soften mechanism in which is dynamic recrystallization during elevated-temperature deformation. As the strain lower than the critical strain that needed for dynamic recrystallization, and the dynamic recovery is suppressed, such as at lower deformation temperature or higher strain rate, the cracks will nucleate quickly.In the practical hot rolling produce of Cr15Mn9Cu2NiN, increasing the time in the furnace, preheating temperature and roughing rolling start temperature or adjusting the content of B will reduce the proportion of edge-cracking in strip. But increasing the content of N will deteriorate the hot ductility in slab, and increase the proportion of edge-cracking in strip.