| 304 austenitic stainless steel has many outstanding features like good corrosion resistance, plasticity and thermal processing performance etc. However, the final microstructure of 304 stainless steel is controlled by joint action of peritectic reaction during solidification and subsequent solid state phase transformation. The final solidification structure is consisted of ferrite and austenite with different shape and distribution. The solidification structure is mainly affected by the chemical composition and solidification behavior, especially by the solidification rate when the chemical composition is the same. Thus, it is particularly important to investigate the effect of solidification rate on solidification structure.In this paper, ordinary metal molding, laser re-melting and single roller-spinning method are used to investigate the solidification behavior of 304 austenitic stainless steel. The formation and evolution of solidification structure are studied in a wide range of solidification rate. The solidification structures distribution of 304 stainless steel are investigated by optical microscopy(OM), scanning electron microscopy(SEM), electron backscatter diffraction(EBSD) and transmission electron microscopy(TEM). Furthermore, the formation mechanism of ferrite in the solidification structure and phase transition mechanism during solidification are analyzed.The solidification structure of 304 stainless steel are studied under low solidification with solidification rate of 25℃/s-206℃/s by common metal mold casting. The results showed that the mold casting microstructure was closely related to solidification rate. When solidification rate is high(206℃ /s),δ-ferrite as the primary phase precipitated directly from the liquid phase in the initial solidification stage, the phenomena of CR-rich and Ni-depletion was found at the center of δ-ferrite. In the following solid state phase transformation (δ→γ) is not complete, which leads to the center zone of high temperature 8-ferrite preserving on the austenite matrix with skeleton-like clusters. When the solidification rate is low with 25℃/s, the solute elements diffuse uneven in the ferrite and austenite during solid phase transformation process (δ→γ), and segregation is serious, which leads to Cr, Ni element accumulation seriously in γ- austenite generated form solid phase transformation. As the temperature decreases, the stability decreased. The phase transformation γ→α occurs. Ambient lath a-ferrite is transited from austenite, and there exit specific crystal orientation relationship between ambient a-ferrite and austenite (K-S/N-W relations):(011)α//(111)γ, [111]α//[011]γ; (110)α//(111)γ,[001]α//[011]γ.The formation and evolution of solidification structure for 304 stainless steel under rapid solidification condition are studied by laser re-melting and quenching with single roller spin technique. The laser re-melting solidification microstructure of 304 austenitic stainless steel in the fusion zone is dense fine ferrite grain layer, lath ferrite, bone-shaped ferrite, vermicular iron ferrite and pit point shaped ferrite from the bottom to the top. A layer of fine ferrite grains appears at the bottom of the fusion zone in contact with matrix, the new ferrite phase directly attached to this fine ferrite layer epitaxial growth along the crystal orientation of the original. Since the solute diffusion, austenite will be generated directly between the lath ferrite during the solidification growth process, austenite and ferrite phase coupling grow together. Lath ferrite formed at the bottom of the fusion zone, and having a specific crystal orientation relationship (N-W relations) between the lath ferrite and austenite:(110)α// (111)γ, [001]α//[011]γ。With the decrease of the rate of solidification, solidification structure transited from lath-like to bone-like, worm-like, point pit-like in fusion zone and the number of residual ferrite increased. The formation mechanism of 304 austenitic stainless steel solidification structure is same to that in molding technique.In the process of rapid solidification for 304 stainless steel strip, the side near the roller cool fast, and undercooling is larger than austenite nucleation undercooling, austenite precipitated directly from the liquid phase as a primary phase. Because the degree of solute segregation is inconsistent, solidification mode directly changes to AF or A mode. In the subsequent solid phase transformation process, and the final microstructure is either fully austenite or ferrite strip remaining on the austenite matrix. With the increasing of the solidification rate, the solidification structure of 304 austenitic stainless steel is mainly determined by the solidification mode. So the lath ferrite maybe not a product of FA solidification mode, they may grow directly from the substrate in epitaxial direction without new nucleation. When the cooling rate is larger than the critical undercooling, there will be a fully austenitic structure. With the decreasing of cooling rate, ferrite will grow directly from the primary austenite phase in epitaxial direction and finally develop into the strips-like microstructure. there exit specific crystal orientation relationship between strips ferrite and austenite N-W relations:(110)α//(111)γ [111]α//[101]γ...
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