| Untilizing manganese and nitrogen substituting nickel, the cost of low nickel austenitic stainless steel is decreased and nickel is saved. The steel is extensively used in the field of energy, chemical, petroleum, aerospace, food, light industry, bioengineering and so on. Because the composition of this steel is different from the conventional Cr-Ni series austenitic stainless, during the rough rolling continuous casting slabs it showes bad hot ductility, and is prone to occur surface forming problems such as surface defects, damage, edge crack and so on.Hot ductility of austenitic stainless steel in slab shell has a direct effect on the occurrence of cracks during hot deformation, and the occurrence of strain localization has an influence on the surface quality. Solidification mode determines hot ductility, so casting microstructures and solidification mode in slab shell were studied in the first instance. Then, in Thermorestor-W thermal/mechanical simulator, elevated temperature tensile tests were performed on the specimens from different depth regions of slab shell to investigate hot ductility and elevated temperature deformation process. Deformation temperature was in the range from 950 to 1200℃, and the strain rate was 0.12s-1. In addition, melting experiment was designed by adjusting the content of B or N in Cr15Mn9Cu2Ni1N, so as to chang its microstructures and solidification mode and provide a theoretical reference to enhanced its hot ductility.Microstructures of Cr15Mn9Cu2Ni1N slab were analyzed, also microstructures of Cr17Mn6Ni4Cu2N slab were analyzed for contrast. Results show that, microstructures in Cr15Mn9Cu2Ni1N stainless steel slab shell depth less than 27mm were composed of the residualδferrite, austenite matrix and the lighter channel-shaped austenite. However, there is no lighter channel-shaped austenite in area at the distance of 27mm from the shell surface. The solidification mode of this steel is FA. Microstructures in Cr17Mn6Ni4Cu2N stainless steel slabs were only composed of the residualδferrite and austenite matrix, and their solidification mode was FA. When cooling rate reached 2℃/s, channel-shaped austenite occurred. As cooling rate increased, channel-shaped austente increased. With the decreasing of cooling rate, the morphologies ofδferrite were skeletal, lathy and vermiculate respectively. However, solidification mode of these two steel did not change as cooling rate changed. Hot ductility of Cr15Mn9Cu2Ni1N austenitic stainless steel in slab shell was studied. Results showed that when deformation temperature was higher than 1050℃, reduction of area (RA) of specimens cut from shell surface was lower, and it increased gradually from surface to inside, then reached to the top at the distance of 27mm from the shell surface. According to microstructures of the slab, it was found that the cooling rate of shell surface was so high during solidification that austenite was got direactly from the retained liquid between dendrites. The austenite got direactly from the retained liquid was most in surface layer, which decreased its hot ductility. Owing to coarse equiaxed structures in core, the hot ductility was lower than that in shell.Elevated temperature deformation characteristics of Cr15Mn9Cu2Ni1N austenitic stainless steel was investigated, resulted show that:with the increase of deformation temperature their strength were decreased and percent elongation was increased. Before the occurrence of necking or strain localization, specimens experienced two stages including uniform deformation and necking diffusion deformation, both of which caused deformation zone of specimen to obtain macroscopic uniform appearance. It is well know that during stage of uniform deformation the inhibition of strain localization was mainly depended on strain hardening; during stage of necking diffusion deformation strain rate hardening played a vital role in obtaining larger macroscopic uniform deformation.Results of melting experiments showed that when the boron content varied from 59 to 140ppm, microstructures of specimens were mainly composed of the residualδferrite, austenite matrix and the lighter channel-shaped austenite, and the solidification mode was FA. With the increase of boron content, the ferrite number (FN) increased, but the channel-shaped austenite reduced. As nitrogen content varied from 0.1428 to 0.1596, microstructures of specimens were also mainly composed of the residualδferrite, austenite matrix and the lighter channel-shaped austenite, and the solidification mode was FA. With the increase of nitrogen content, channel-shaped austenite increased, and the grain refined. In contrast to specimen melted without adjusting nitrogen content, FN of those specimens reduced by half. |
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