| Recently, austenite stainless steel with different Al contents has been widely concerned due to its excellent corrosion resistance, high temperature oxidation resistance and creep resistance. In our previous work, 304, 316 L, 310 S stainless steel with different Al contents, ingot, and plate were prepared by using vacuum arc furnace. It was found that, by controlling the Al content, the mechanical properties of stainless steel do not appear to significantly decrease, while the corrosion resistance and oxidation resistance are obviously improved. In order to use in the industry,in present paper, the media frequency induction furnace were applied to prepare 304, 316 L, 310 S stainless steel with different Al content(1.5, 2, 3 wt%). Microstructures evolution of 304, 316 L, 310 S stainless steels with different Al content under the condition of as-cast, solid solution and hot-rolled, besides, the mechanism of effect of microstructure on mechanical properties were investigated.It was found that the addition of the Al content makes the matrix of 304 stainless steel transit from single-phase austenite to austenite+ferrite dual phase When the Al content reaches 3wt%, the stainless steel shows the single ferrite phase.The morphologies of the ferrite changes from bone to sharp in shape. The solidification mode of 304 stainless steel with Al contents are F. The existence of Al elements in the matrix is found to be in the form of solid solution, and the segregation of Al element occurs in the ferrite phase. After solid solution treatment the phase ratio of austenite to ferrite and morphologies of ferrite in the 304 stainless steel with different Al contents can be changed, and the segregation of the Al element within the ferrite phase is eliminated.The effect of rolling temperature, rolling reduction, Al content on the microstructure in the hot-rolled 304 stainless steel were analyzed. When the rolling reduction is equal to 50%, the austenite and ferrite phases with different morphology and size can be obtained by controlling the rolling temperature and Al contents. Increasing rolling temperature or Al content is benefit to refine the microstructure. The optimized refinement microstructure can be achieved in the 304 stainless steel with 2wt% Al subjected to rolling temperature of 1200 oC. As the rolling temperature remains 1100 oC, increasing rolling reduction results in a increase of the number of sub-boundaries in the ferrite in 304 stainless steel with 1.5wt% Al content. But the morphologies and size of austenite and ferrite have not significantly changed. Increasing rolling reduction is benefit to refine the microstructure of 304 stainless steel with 2wt% Al content, But the coarsening of the microstructure can be obtained when the rolling reduction is 70%.The effect of Al element on the microstructure and solidification mode of 316 L stainless steel was similar to that of 304 stainless steel. The difference is that, with the increase of Al content, ferrite phase distributes within the austenitic matrix in sequence of short bar, strip and island shape. When the Al content is 3wt%, ferrite phase turns to matrix phase. The Al element mainly exists in the form of solid solution, the segregation occurs in the ferrite phase for the alloys with 3wt% Al. After solid solution, the volume fraction of ferrite of the alloy with same Al content increase and the distribution of Al element in the ferrite and austenite reaches a balance.The effect of rolling temperature, rolling reduction, Al content on the microstructure in the hot-rolled 316 L stainless steel were analyzed. When the rolling reduction is equal to 50%, Increasing rolling temperature or Al content is benefit to refine the microstructure. The optimized refinement microstructure can be achieved in the 316 L stainless steel with 2wt% Al subjected to rolling temperature of 1150 oC.When the rolling temperature is 1150 oC, the refinement of the microstructures for the ferrite and austenite in 316 L stainless steel with 2 wt% Al occurs due to the increase of rolling reduction.But the coarsening of the microstructures can be obtained when the rolling reduction is 70%.The addition of the Al content makes the matrix of 310 S stainless steel transit from single-phase austenite to austenite and ferrite dual phase. The solidification mode of the 310 S stainless steel with 1.5, 2wt% Al is FA mode and the solidification mode of the 310 S stainless steel with 3 wt% Al is F mode. The shape of the ferrite change from dentrite, network to rod+ island in shape with the increase of Al content. The existence of Al elements in the matrix is found to be in the form of solid solution, and the segregation of Al element occurs in the ferrite phase for the alloys with 3wt%Al. After solid solution treatment the phase ratio of austenite to ferrite and morphologies of ferrite in the 310 S stainless steel with different Al contents can be changed, and the segregation of the Al element within the ferrite phase is eliminated. The distribution of Al element in the ferrite and austenite reaches a balance. The carbide precipitation is observed in the grain boundaries between the ferrite and austenite when the Al content is 2 and 3 wt%.The microstructures of ferrite are stretched and coarsening for the 310 S stainless steel with 1.5, 2wt% Al content with various rolling temperatures and 50% rolling reduction. For the 310 S stainless steel with 1.5 wt% Al content, needle-like and strip-like austenite phase could be separated from the ferrite. For 310 S stainless steel with 3 wt% Al content, the partly austenite phase with broken fine block and short-rod in shape distributes in the matrix. |
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