Influence Of Grain Refiners On The Microstructure Of 430L Ferritic Stainless Steel

Ferritic stainless steel basically is Fe-Cr or Fe-Cr-Mo alloy with body centered cubic structure which is the ferritic structure in the using state and is widely used in automobile manufacturing, household appliances, chemical equipment, etc. However, The columnar crystal of this kind of steel is relatively developed and is easy to cause segregation. Surface defects, fold often appears in the following process which has bad effect on mechanical properties, such as yield strength and tensile strength. Through consulting literatures, it can be got that refining the grain and improving the proportion of equiaxed grains can solve this problem.From this point of view, the edge-to-edge matching model derived the orientation of BCC/FCC system and predicted the grain refinement of 430L ferritic stainless steel. Then the theory was verified by smelting 1 kg 430L stainless steel in MoSi2 about 10 heats. The mass fraction of TiN micron powder were 0.01%,0.02%, 0.03% and 0.05% and added in 1 to 4 heats respectively. The mass fraction of TiC micron powder were 0.01%,0.02%,0.03% and 0.05% and added in 5 to 8 heats respectively. The mass fraction of MgO·Al2O3 micron powder were 0.03% and 0.05% and added in 9 to 10 heats respectively. The influence on the refinement effect was explored by the different grain refiner types and the different additions with the same grain refiner.Deriving the orientation relationship of TiN/430L and TiC/430L system by using edge-to-edge matching model as follows: [100]B/[110]F [100]B/[110]F (001)B/(110)F (011)B/(002)F That is to say, there is orientation in this two systems from crystal perspective and the TiN and TiC can be regarded as grain refiners. However, there is no orientation relationship between MgO·Al2O3 and 430L ferritic stainless steel.Under the condition of laboratory, the experiments of adding TiN, TiC and MgO·Al2O3 micron powders in molten steel were done. The reuslts showed that TiN has better grain refinement effect than TiC, while MgO Al2O3 has no obvious effect.With the increasing of the additions of grain refiner, acicular ferrite increased after quenched. When the addition of grain refinement reach to 0.03%, the amount of acicular ferrite reached maximum, while added more refiners, it remained unchangeable.The results showed that the grain size decreased with the mass fraction of grain refinement increasing after anylysied the level of grain size. When the additions of grain refiners reach to 0.03%, the grain size is the smallest, while added more refiners, it remained unchangeable.Inclusion detected showed that the typical inclusion is Al2O3-SiO2-MnO compound inclusion, when the refiners were added, a certain amount of TiN or TiC inclusions were found in steel.Summarizing all of the results in the experiments, TiN is the best grain refiner with the best matching relationship to the 430L ferritic stainless steel, and the most suitable addition is 0.03%.