Investigation On Precipitation Law Of TiN In Ultra Clean Ferritic Stainless Steel

In recent years, the price of nickel is substantially fluctuated in international market, andthis produce adverse impact on production and use of austenitic stainless steel which take up ofabout70%of global stainless steel annual output, while as excellent cost performance, savingnickel and other features the ultra-clean ferritic stainless steel is taken more and more attentionby corporation. The TiN inclusion have an important effect on smelting, capability and use ofultra-pure ferritic stainless steel, this article using theoretical and experiment investigate theprecipitation law of TiN inclusion.The content of theoretical research: calculate the formation thermodynamic of oxideinclusions which can promote the precipitation of TiN; in1873K, calculate the influence ofchrome content in the precipitation of TiN; calculate the critical titanium nitrogen’s product ofgeneration TiN in liquidus temperature （1782K） and tundish temperature （1823K） in ultra-pureferritic stainless steel which contains Cr16mass%; calculate the segregation of Ti, N and thefinal grain size of TiN in ultra-pure ferritic stainless steel which contains Cr22mass%. Thecontent of experiment research: smelting the B445R ultra-pure ferritic stainless steel ininduction furnace with argon protecting, and adding ferrotitanium, ferrochrome nitride to adjustthe content of Ti （[Ti]=0.1%,[Ti]=0.2%,[Ti]=0.3%）, N （[N]=0.010%,[N]=0.013%） andchanging the cooling rate （furnace cooling, water cooling, air cooling） to acquire the samplewith different content of Ti, N and cooling rate, then to analyse the sample with electronmicroscope and energy spectrum analysis, observing the feature, compose and counting the sizedistribution of TiN in each sample in order to investigate TiN inclusion precipitation law withdifferent of nitrogen content, titanium content and cooling rate.The results of theoretical research are as follows:（1） The silicon aluminium composite of deoxidizing products of ultra pure ferriticstainless steel is Al₂O₃; even trace amount of Mg in molten steel, can react with Al to formmagnesia-alumina spinel; after titanium alloying, at the experimental conditions of the contentof titanium （[Ti]=0.1%～0.3%）, the oxide product of titanium is Ti2O3, not Ti3O5.（2） In the titanium stabilization ultra-pure ferritic stainless steel which contains Cr16mass%, when the concentration of [%Ti][%N] is less than0.0025can avoid generating TiN intundish （1823K）; when the concentration of [%Ti][%N] is less than0.0014can avoidgenerating TiN above iquidus temperature （1782K）.（3） In smelting process, as the content of [Cr] is increased in liquid steel, theconcentration product of titanium nitrogen to form TiN inclusion is increasing; as solidificationprocess going, the temperature and titanium nitrogen balance concentration product ofsolidification front increase, the titanium nitrogen actual concentration product and titanium,nitrogen content in molten steel of solidification front decrease; during solidification process,the titanium enrichment degree is a little less than nitrogen, at the end of solidification, thecontent of [Ti] in residual liquid is added2.5times, the content of [N] in residual liquid is added more than3times; to reduce the content of [N], can delay the precipitation time of TiN.（4） when the cooling rate is separately10k/s,20k/s,30k/s, TiN separate out at thesolidification rate of about0.66, and the final size of TiN is separately7.1μm,5μm,4.1μm, soalong with the increase of the cooling rate, the final size of TiN decrease, the amount of TiNincrease, and the cooling rate have litte influence of precipitation time of TiN; When the initialnitrogen is separately0.005%,0.007%,0.01%, the solidification rate of TiN precipitation isseparately about0.62,0.42,0.32, and the final size of TiN is separately4.2μm,5.8μm,6.6μm,so as initial nitrogen content increased in liquid steel, the precipitation time of TiN in steelbring forward continuously and the final size of TiN increase; When the initial titaniumseparately is0.1%,0.15%,0.2%, the solidification rate of TiN precipitation separately is about0.58,0.37,0.17, and the final size of TiN is separately6.3μm,6.6μm,6.8μm, so as initialtitanium content increased in liquid steel, the precipitation time of TiN in steel bring forwardcontinuously but the final size of TiN increase but not obviously.The results of crucible experimental research are as follows:（1） The2D image of TiN is square or triangle in the electron microscope, as the rawmaterial of smelting containing Nb （0.35mass%）, it is observed that there is NbC dissolvingout around TiN, and it appears irregular bright white stripes in the electron microscope, bothsingle and compound TiN are observed in the electron microscope.（2） Along with the increase of the content of titanium, the sum of unit area TiN increasein each sample; in the sample with furnace cooling, the amount and percentage of the total ofTiN which grain size are more than2μm are increasing; in the sample with water-cooling, theamount of TiN which grain size are less than2μm is increasing; in the sample with air-cooling,the amount of TiN which grain size are1μm～2μm is increasing.（3） Along with the increase of the content of nitrogen, the sum of unit area TiN decrease;in the sample with furnace cooling, the amount of TiN which grain size are less than1μmdecrease; in the sample with water-cooling and air-cooling, the amount of TiN which grain sizeare1μm～2μm increase, other particle size range of the amount of TiN are almost the same.（4） Along with the increase of the intensity of cooling, the sum of unit area TiN and theamount of TiN which grain size are less than1μm increasing in each sample, the amount ofTiN which grain size are more than2μm decrease; the sum of unit area TiN which grain sizeare1μm～2μm, the sample with furnace cooling is least, the sample with air-cooling is most,and the sample with water-cooling is in the middle.This Experiment using alumina crucible and adding the titanium alloys, with theoreticalcalculation and energy spectrum analysis indicating that: the nucleus of compound type TiN isTi2O3or Ti2O3and Al₂O₃composite oxide inclusions; the influence of cooling rate, initialtitanium and nitrogen content to the amount and grain size of TiN in experiment are in goodagreement with theoretical calculation.