Research On Thermodynamic Calculation Of Second Phase Precipitation And Mechanical Properties In Ti Microalloyed 443 Stainless Steel

Ferritic stainless steel（FSS）has magnetic properties,high thermal conductivity,low coefficient of thermal expansion and low stress corrosion cracking sensitivity.And because it does not contain nickel,the production cost is significantly reduced compared to 300 series stainless steel.However,the low toughness and high ductile-brittle transition temperature of ferrite stainless steel limit its application in various structural components.Due to the low content of C and N（less than 200 ppm）,the corrosion resistance and formability of 443 FSS are significantly improved.Especially,resistance of stress corrosion cracking and pitting corrosion in Cl^–environment are outstanding.At the same time,it has the advantages of low linear thermal expansion coefficient and high thermal conductivity coefficient.They are used widely in elevators,building curtain walls,and kitchenware facilities fields.After microalloying of Ti element,it usually exists in two forms,solid solution and second phase in the matrix steel.When the size of the second phase reaches the nanoscale,a strong dispersion strengthening effect will occur.Titanium carbonitrides have high thermal stability and can inhibit grain growth during high-temperature processes,which is beneficial for fine grain strengthening and welding performance improvement.Study on the alloying mechanism and microstructure evolution of titanium alloyed 443 FSS,selection of reasonable alloying element additions,cold and hot processing and heat treatment parameters,which are of great significance for optimizing the production process of ultra-pure ferrite stainless steel and breaking through bottleneck technologies,achieving stable production of high-quality,specialized,and serialized products.Based on the calculation of thermodynamic software JMat Pro under equilibrium and non equilibrium solidification conditions,the relationship between the second phase precipitation and temperature during the solidification process of 443 FSS was predicted under five different Ti contents（0.23%,0.27%,0.31%,0.35%,0.38%）.443FSS with Ti content of 0.31%was selected as the experimental steel,and the effects of different hot rolling annealing temperatures and cold rolling deformation on the characteristics and evolution of the second phase and average particle size of the steel were studied using optical microscopy（OM）and scanning electron microscopy（SEM）.Based on the calculation of thermodynamic software JMat Pro under equilibrium and non equilibrium solidification conditions,the relationship between the second phase precipitation and temperature during the solidification process of 443 FSS was predicted under five different Ti contents（0.23%,0.27%,0.31%,0.35%,0.38%）.443FSS with Ti content of 0.31%was selected as the experimental steel,and the effects of different hot rolling annealing temperatures and cold rolling deformation on the characteristics and evolution of the second phase and average particle size of the steel were studied using optical microscopy（OM）and scanning electron microscopy（SEM）.The mechanical properties corresponding to different structures were tested by tensile tests,hardness tests,impact tests.The precipitation of Ti containing second phase in steel under different heat treatment conditions was observed and analyzed by X-ray diffraction（XRD）and transmission electron microscopy（TEM）.The density,specific heat capacity and coefficient of thermal expansion of the experimental steel were determined by JMat Pro performance calculation software.The research results indicate that,The 443 FSS with Ti contents of 0.23%,0.27%,0.31%,0.35%,and 0.38%respectively,was used as our reaserch object.the equilibrium phase composition of the experimental steel（0.31%Ti）was simulated and calculated using JMat Pro software.The phase composition of the experimental steel at room temperature reached 80%α-Fe,20.6%α’,0.6%Laves,0.38%Cu,0.27%Mg₂Si,0.25%G-phase,0.09%Ti（c,N）,and 0.0784%M₃P.There is no Ti N precipitation in experimental steel liquid,and solute segregation and enrichment occur at the front of the solid-liquid interface during solidification,which promotes Ti N to start precipitation at a solid phase ratio fs of 0.11.The size of Ti N precipitated is influenced by the initial content of Ti and N elements in the molten steel and the solidification cooling rate.Reducing the initial content of Ti and N and increasing the solidification cooling rate can both reduce the Ti N radius.N element content has a more sensitive effect on the size of Ti N,and can significantly delay the precipitation of Ti N particles during the solidification process by reducing the initial N element content in the molten steel.In actual production,when the N element content in 443 FSS ranges from 0.006%to0.013%,Ti element content should be controlled within the range of 0.19%to 0.41%.443 FSS exhibited structural stability with different annealing temperatures.As the annealing temperature increases,the grain size of the experimental steel first decreases and then increases,the grain size conforms to the normal distribution curve and static recrystallization occurs during the annealing process.The second phase particles present in steel are Ti CN and Ti N.When the temperature rises to 930℃,the second phase particles precipitated in the matrix begin to dissolve in the matrix,and the effect of solid solution strengthening decreases,but the effect of dispersion strengthening increases.At annealing temperatures of 930℃and 980℃,the grain size is uniform,polygonal,and tends towards equiaxed grains.When the annealing temperature is 1030℃and 1080℃,there is a significant difference in grain size,resulting in mixed crystal phenomenon.With the increase of annealing temperature,the tensile strength of 443 FSS hot-rolled plate first increases and then decreases,while the fracture elongation shows a decreasing trend.The hardness performance stabilizes and increases to 1030℃before decreasing.The strengthening effect of 443FSS hot-rolled plate is a composite strengthening effect of fine grain strengthening,dispersion strengthening,and solid solution strengthening.Charpy impact value shows a tortuous decrease and significant variation with increasing temperature,because ferritic stainless steel has a（110）crystal plane of bcc,which is prone to brittle fracture.Using the commercial finite element analysis software ABAQUS,numerical simulations were conducted on the temperature field,stress field,and strain changes during the annealing process of 443 FSS hot-rolled plate.The simulation results reflected the uniformity of the overall temperature field distribution of the experimental steel plate and the deformation trend and magnitude of the stress generated after heating.The 443 FSS with different cold rolling deformations has a single bcc structure and the（110）crystal plane has the highest crystallinity.After squeezing,the grains are elongated along the rolling direction,refined and broken,and the grain boundaries become blurry and discontinuous.The tensile strength and hardness of both 930℃and 980℃annealing exhibit a strong linear relationship.The decrease in elongation due to dislocation accumulation and the strengthening of fine grains lead to an increase in strength and hardness.The best surface roughness after cold rolling is the annealing temperature of 980℃and the rolling deformation of 30%.Using ABAQUS for numerical solution,the 443 sample with a rolling deformation of 30%has the highest equivalent plastic strain value.