The Research Of Microstructure And Thermal Stress In 2101 Duplex Stainless Steel Of The Continuous Catsing Slab

2101 duplex stainless steel production has huge economic benefits and good prospects for development caused by the low raw material costs and integrated performance. Currently, the vertical continuous casting was widely used by more industrialized countries to produce 2101 duplex stainless steel. But vertical continuous casting production technology is not yet mature in our country, and all aspects of the continuous casting processis still in the exploratory stage. Therefore, there are many defects in the casting strand.Normally, the crack defects are generated by unreasonable continuous casting technology parameters which will generate unreasonable microstructure and thermal stress in the strand. Based on the actual production, the 2101 duplex stainless steel continuous casting slab, which was produced by Bao Special Steel, was studied in this thesis. The influence of the technology parameters of continuous casting on the microstructure formation and thermal stress distribution in the slab was studied. The main contents and conclusions are listed as follows:（1） Cellular Automaton-Finite Element（CAFE） model, which is used to simulate the grains growth of 2101 duplex stainless steel, was established and confirmed.The commercial finite element software Pro CAST was used to study the grains growth of 2101 duplex stainless steel, and found that the order of importance of the nucleation and growth parameters effect on Columnar-to-Equiaxed Transition（CET） is n_max>ΔT_max>a2>ΔT_σ>a3. The nucleation and growth parameters, which are applicable for simulation the grains growth in 2101 duplex stainless steel, are determined by the experiment. These parameters were confirmed by experimental results. And the accuracy of the established CAFE model was fully verified by thermal experimental results under different superheat and numerical simulation.（2） The impact mechanism of the continuous casting technological parameters on the hot cracking tendency of 2101 duplex stainless steel slab.Firstly, the hot cracking tendency of slab was analysed from the microstructrue aspect under different technological parameters. The grains growth was simulated by CAFE model. The results indicated that the zone in front of liquid-solid inerface, where is meet the requiment of the undercooling for the nucleation, is broadened with the reducing of the superheat and increasing of the cating speed. The number of nucleation is easily over the critical value of CET. The equiaxial crystal ratio is increased approximately 8% with the superheat decreasing 10 ℃ and increased approximately 6.5% with the casting speed increasing 0.2 m/min. The temperature gradient in front of liquid-solid inerface is increased with the cooling rate increasing before CET, and caused the equiaxial crystal ratio reducing. The anisotropy of the mechanical property at the subsurface and corner of slab is decreased by the increasing of the equiaxial crystal ratio, and reducing the hot cracking tendency.Secondly, the experimental analysis of microstructure of the actual production found that the austenite grain refinement and generation as block or short strip dispersed on the substrate during the solid phase transformation and reducing the size and number of Widmanstatten with decreasing the cooling rate after CET. This will improve the mechanical properties of the slab. Therefore, the cooling intensity should be reduced in the mold and the first, second secondary cooling zone to increase the equiaxial crystal ratio; The cooling intensity should increase after CET to refine the γ-Fe phase and reducing the volume of Widmanstatten. Therefore, it can decrease the hot cracking tendency from the macrostructure and microstructure.（3） The numerical simulation of the thermal stress distribution and analysis the generation mechanism and the influence of thermal stress on the hot cracking tendency of the slab.The results of thermal stress simulation indicated that there are third zones, where the thermal stress serious dramatic changes, along the casting direction on the slab. It is caused by temperature difference along the thickness of slab, liquid-solid transformation and the variation of the solid shrinkage rate with the temperature change, respectively. The hot cracking is easily generated near the corner at 1.13 m from the meniscus and around the solid end of slab.The temperature returning on the strand surface is controlled, and caused the phase translation shrinkage and solid shrinkage is gradually released with decreasing of superheat and increasing of casting speed. The thermal stress at the first and second zone is decreased approximately 8.1% with the superheat decreasing 10 ℃. And the thermal stress at the center of wide surface is decreased approximately 60% and the third thermal stress concentration zone is decreased approximately over 50% with the casting speed increasing 0.2 m/min.In order to reduce the thermal stress, the temperature returning on the strand surface will be controlled when the cooling intensity is reduced in the mold and increased at the footroller zone. It can reduce or eliminate the subsurface crack of slab; The thermal stress of the residual liquid phase between the dendrites of the liquid-solid interface could be decreased when the temperature is unform before the solid end of slab. Thus, it can reduce the hot crack between the dendrics; And after the solidification completion, the cooling intensity of slab should increase to reduce the thermal stress inside of the slab and reduce the hot cracks at the center of slab. Finally, it can reduce the hot crack generation and improve the quility of slab from the macrostructure, microstructure and thermal stress.