| 其他摘要 | Austenitic stainless steel is a familiar kind of stainless steel, and it can be used as high strength stainless steel. Austenitic stainless steel has good performance in corrosion resistance and in processability. However, this kind of stainless steel is often used in the state of cold worked in order to meet the requirement of high strength. Plasticity is not well in the state of cold worked, and this deficiency limits the application when austenitic stainless steel is used as high strength stainless steel. According to the features of process of the steel, microstructures and mechanical properties of austenitic stainless steel were investigated, and the relationship of microstructures and mechanical properties were analyzed.
This thesis selected ultralow carbon stainless steel 00Cr18Ni10 as a representation of austenitic stainless steel. In order to remedy the loss of strength, resulting from C reduction, little N added in 00Cr18Ni10. Microstructure and mechanical properties of 00Cr18Ni10N in the state of cold drawn and heat treated were studied.
Stainless steel (00Cr18Ni10N) wires with different area reductions of 10%, 36%, 45.1%, 67.3%, 75% and 84%were obtained through cold drawing. Tensile test, magnetic test and microstructure observation show that at the initial stage of deformation (ε≤40%), slipping and twinning are the main deformation mechanisms, more amounts of twin lamella and dislocation cell microstructure appear, tensile strength changes from 600 to 1200 MPa, and the elongation falls down severely from 67% to 5% while the tensile strength increases. At the large deformation stage (ε＞40%), strain induced martensites begin to take part in deformation, the microstructure exhibits a fibrous band--like character, whose substructure is dislocation cell and broken deformation twin with irregular shape, the tensile strength is above 1200 MPa, and the downtrend of elongation slows down.
Cold drawn stainless steel (00Cr18Ni10N) wires with different reductions of area (36%, 67.3% and 84%) were heated at different temperatures. Tensile test, magnetic test and microstructure observation show the results as follows. 1. For the slightly deformed wire(with area reduction of 36%, contenting little α’), after annealed at 550℃~650℃, deformation twins and the other deformation structures which are stable in recovery could keep the wire with a high strength (1000MPa) while an increased elongation (above 10%) can also be gained from the recovery. 2. For the severely deformed wire(with area reductions of 67.3% and 84%, containing much α’), after annealed at 650℃, the elongation can be improved to 30% when the tensile strength decreases to 900MPa, and this improvement owes to the fine equiaxed grains which form at the initial stage of recrystallization.
Microstructures of 00Cr18Ni10N with different area reductions were studied. For the slightly deformed 00Cr18Ni10N (with area reduction of 36%), recovery including point defects, dislocations and deformation twins occurs in 500~670℃, small deformation twins of dozens nm wide disappeared with increasing temperature or holding time, but the larger deformation twins of 100~200 nm wide are very stable even at 700℃, and the main evolution is recrystallization when the temperature is above 670℃. For the severely deformed 00Cr18Ni10N (with area reductions of 67.3% and 84%), which contains much α’, reverse transformation (α’→γ) occurs firstly in 500~650℃, recrystal grains appears in 600~630℃, and the processes of reversion, recovery and recrystallization could occur synchronously in 600~630℃. The evolution of fine twins firstly into subgrains and then into dislocation nets occurs with increasing temperature or holding time in reversed austenite, while the normal recovery and recrystallization exhibit in residual austenite.
Based on the studies of microstructure and mechanical properties of 00Cr18Ni10N, it is found that mechanical properties are affected mostly by strain induced martensite in the range of 1300~2000MPa, plasticity can’t be improved obviously by the reversion during annealing, and the elongation is below 5%, which is at the level of cold worked; in the range of 1000~1300MPa, microstructure undergoes a transition of reversion to recrystallization, and the elongation changes in 5% ~20%; in the range of 800~1000MPa, mechanical properties are affected mostly by recovery and recrystallization, plasticity can be improved obviously through annealing, and elongation is above 20%. |
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