| In recent years,polytetrafluoroethylene hoses have been widely used for pipeline compensation in the external pipelines of many engines under research and pre-research in China.However,polytetrafluoroethylene hose materials have high requirements on material quality,complex structure and complex manufacturing process,so they have always relied on imports.Stainless steel wire is the reinforcing layer material of polytetrafluoroethylene hose,which is the key to ensure the working pressure of hose.In order to break the current situation that the hose product depends on imports and realize the independent production of key parts in China,this paper develops stainless steel wire material for hose reinforcement layer based on traditional SUS304 stainless steel.Combined with thermodynamic theoretical calculation and high temperature experimental verification,the deoxidation mode of SUS304 stainless steel and the composition optimization design based on carbon and nitrogen regulation are studied.Based on thermodynamic theory and Factsage software,this paper calculates the dominant formation area of rare earth inclusions in SUS304 stainless steel,theoretically analyzes the formation and transformation conditions of rare earth inclusions,and simulates the precipitation behavior of inclusions during solidification.The calculation results show that:(1)When cerium is added to steel,the first generated rare-earth oxide,and then generate rare-earth sulfide oxide,only when the dissolved oxygen in the steel is reduced to a certain extent to meet the sulfur and oxygen ratio greater than 2004,rare-earth cerium will have a significant desulfurization effect,generating rare-earth sulfides;(2)The activity ratio of cerium to aluminum is 0.08 when CeAlO3 exists stably.At 1873K,when the mass fraction of acid-soluble aluminum in SUS304 stainless steel is in the range of 0.01%～0.06%,even if there is a small amount of cerium,Al2O3 will be modified into CeAlO3;(3)According to the thermodynamic calculation results,the main types of inclusions in steel are Ce2O2S and CeAlO3.In the experiments of aluminum pre-deoxidation and cerium deep deoxidation,Al2O3 inclusions were modified into CeAlO3,while in the local cerium-enriched area,cerium was deoxidized alone to form rare earth oxysulfide.The metallographic results show that when the mass fraction of rare earth is 0.002%,the average diameter and area of inclusions are the smallest.Compared with the steel without rare earth treatment,the diameter and area are reduced by 14.7%and 49.9%,respectively.The refinement effect of cerium on inclusions is obvious.In the experiments of silicon pre-deoxidation and cerium deep deoxidation,the inclusions have achieved good denaturation effect.The Fe-Cr-Ni-O inclusions in the initial molten steel are denatured into rare earth oxide Ce2O3 in the early stage of deoxidation,and then Ce2O3 combines with sulfur to form Ce2O2S.Metallographic results show that with the increase of Ce content in steel from 0 to 0.018%,the number of inclusions decreases by 50%and the aspect ratio decreases by 16%.The solidification process of SUS304 stainless steel with different carbon and nitrogen mass fractions was calculated and analyzed by Thermo-Calc software,in order to study the influence of composition change on solidification precipitates.The calculation results show that Sigma and M23C6 phases are the second phases with the largest precipitation in SUS304 stainless steel.At the same time,the increase of mass fraction of carbon and nitrogen can significantly inhibit the precipitation of harmful Sigma phase.When the mass fraction of carbon increases from 0.04%to 0.07%,the precipitation temperature of Sigma phase in steel decreases from 757℃ to 741℃,and the precipitation amount decreases by 7%.Sigma phase causes local brittleness of steel,so the increase of carbon can improve the strength and toughness of SUS304 stainless steel by inhibiting the precipitation of Sigma phase.When the mass fraction of nitrogen increases from 0.04%to 0.08%,the precipitation temperature of Sigma phase in steel decreased from 741℃ to 732℃,and the precipitation amount decreased by 5.4%.Composition optimization experiments show that the size and quantity of precipitated phases can be controlled by carbon and nitrogen regulation,thus adjusting the grain size.With the increase of carbon,the austenite grains of SUS304 stainless steel are obviously coarsening.When the carbon mass fraction in the steel increased from 0.039%to 0.066%,the grain size rating decreased from 8.7 to 7.2.Appropriate increase in nitrogen mass fraction can refine the grain refinement,however,when the nitrogen mass fraction is too high,the grain size deteriorates sharply.As the nitrogen content in the steel increases from 0.04%to 0.055%and 0.075%,the grain size rating number first increases and then decreases significantly.The effect of carbon and nitrogen on grain refinement is determined by the size and number of precipitated phases.When the average sizes of the precipitated phases in the experimental steel were 3.09 μm,4.71 μm,5.9 μm,and 6.93 μm,the grain size ratings were 6.7,7.2,8.7,and 7.8,respectively.When the size of the precipitation phase is less than 5.9μm,the presence of large size precipitation phase can nail the grain boundaries and prevent the growth of austenite grains,thus improving the mechanical properties of the steel. |
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