Effect Of Titanium And Niobium On Second Phase Precipitation During Solidification And As-Cast Structure Of High-Purity Ferritic Stainless Steel

Ridging,as a typical surface defect appeared in the forming process of ferritic stainless steel(FSS),has been widely concerned,and its formation is closely related to the columnar crystals in the as-cast structure.With the decrease of carbon and nitrogen content,the columnar crystal is getting more evident in the as-cast structure,usually resulting in a more severe ridging defect.Therefore,obtaining a high ratio of fine and dense equiaxed grains is the primary objective of high-purity FSS.It has been confirmed that the addition of titanium could increase the equiaxed zone ratio(EZR)and improve the ridging resistance.In recent years,although Ti and Nb dual-stabilized high-purity FSS have developed rapidly,there are few systematic researches about the independent and interactive effects of titanium and niobium on the second phase precipitation during solidification and as-cast structure.These researches will be of great significance for obtaining an ideal as-cast structure and improvement of ridging resistance.In this thesis,the effect of titanium and niobium on second phase precipitation during solidification and as-cast structure of high-purity FSS is carried out.The nucleus structure and the nucleation mechanism after Ti alloying were confirmed by Raman spectroscopy,electron backscatter diffraction(EBSD),and droplet experiments.A multi-component non-equilibrium solidification model was established to discuss the EZR increasing mechanism of Nb alloying.The interaction effect of Ti and Nb addition on refining as-cast grains was clarified.Combined with the industrial production results,the critical points for the production of Ti and Nb dual-stabilized high-purity FSS continuous casting slab with high EZR was proposed.The main findings are shown as follows:(1)The direct nucleation evidence by complex nucleus was found in the initially formed equiaxed dendrites by the rapid cooling droplet experiment.Raman spectroscopy confirmed that the oxide in the complex nucleus is Ti203.The specific orientation relationship between Ti2O3 and TiN was revealed to be{0001}Ti2O3//{111}TiN,resulting in an extremely low disregistry(0.6%),indicating that Ti2O3 will effectively promote the formation of TiN.The formation conditions of the effective nucleus were proposed,that is,the Ti content is higher than 0.14wt%,the Al content is less than 150ppm,and the dissolved oxygen content is 9-12ppm.(2)The increasing mechanism of EZR by Nb alloying was revealed.For every 0.1 wt%increase of Nb content in FSS,the EZR increases by 5.5%.Nb alloying could significantly influence the dendritic growth kinetics,hinder the crystal growth,and then expand the constitutional supercooling region at the solidification front.These effects result in the promotion of the homogeneous nucleation of FSS and finally improve the EZR.(3)The as-cast structure could be further refined when Ti and Nb were alloyed together.It was shown that the average grain diameter decreased by 8%for every 0.1 wt%increase in Nb content when the Ti content is around 0.19wt%.Ti alloying can promote the formation of the complex nucleus and induce heterogeneous nucleation.Nb alloying could expand the solidification range and promote the precipitation at the grain boundary,which improves the nucleation efficiency and hinder the grain merging and growth.The interactive effect of Ti and Nb is superior to Ti single stabilization process.(4)The dual-stabilized high-purity FSS slab with high EZR was manufactured in industrial production.After systematic nucleus statistics,it was found that the formation of(Ti,Nb)(CN)and heterogeneous nucleation could be effectively promoted when the Ti2O3 oxide contains a certain amount of Mg and Al.Under such circumstances,the oxide size is 1?1.5?m,and the oxide efficiency is about 20%.In order to form the effective nucleus,it is suggested that Mg content is 3-5ppm,and Al content is 50-75ppm when Ti content is 0.18wt%.(5)The distribution characteristics of the nucleus along the thickness direction of the FSS slab are statistically extracted.The results show that the nucleus density is closely related to the cooling rate.The nucleation undercooling could be increased by 35.7K for every 1K/s increase of cooling rate,providing favorable conditions for nucleus formation.A CET prediction model suitable for FSS is established based on the experimental data.The results show that the nucleus effectiveness and density are two key factors to improve the EZR.It was also suggested that CET and EZR could be reasonably predicted and controlled,which contributes to the fabrication of high-grade FSS products.