Microstructure,Texture And Forming Properties Of 00Cr19Mo2NbTi Cold Rolled Strips

With the rapid development of modern society,the automobile has become an indispensable transportation means for the modern people.The new requirements of safety,environmental protection and energy saving for the automobile have been put forward.Therefore,the investigation on the automobile exhaust system components with high performance and low cost steels has been a research hotpot of the current automotive materials.Due to the advent of ultrapure ferritic stainless steel,the corrosion resistance and formability of ferritic stainless steels have been significantly improved and widely used in automotive exhaust systems.Due to the complexity of automobile exhaust manifold processing technology,the excellent formability of materials has been required.High-performance and low-cost ferritic stainless steels with niobium,titanium and low carbon and nitrogen,are automotive exhaust system components steels with promising developing prospect.In this thesis,the microstructure,texture and formability of 00Cr19Mo2NbTi stainless steels were studied based on the national natural science foundation project "Research on the Alloying Mechanism and Key Performance of High Temperature Ferritic Stainless Steels for Next Generation Automotive(U16602001 8)".The main contents of this thesis are summarized as follows.(1)The hot rolling and heat treatment process of 00Cr19Mo2NbTi stainless steels were studied,and the effects of different heat treatment processes on microstructure and texture of hot rolled sheets were analyzed.With the increase of annealing temperature,the grain size of hot rolled and annealed sheets increased gradually and the grains tend to be uniform.The α-fiber texture of hot rolled and annealed sheets became stronger and the y-fiber texture became weaker.(2)The effects of cold rolling reduction on the microstructure and texture of 00Cr19Mo2NbTi stainless steels were analyzed.With the rolling reduction rate increasing,the grain boundary area and shear zone density of cold-rolled structure increased gradually,and the recrystallized grain size decreased gradually.As the cold rolling reduction increased from 60%to 73%,the γ-fiber texture density of cold-rolled strips increased from 9.00 to 12.50.The cold rolled and annealed strips with a reduction of 73%had a strong point orientation density of 10.64 and a secondary strong point orientation density of 10.18.(3)The effects of different heat treatment processes after hot rolling on the texture evolution of cold rolled strips were studied.With the increase of hot rolling annealing temperature,the γ-fiber texture density in cold rolled and annealed strips decreased significantly that the surface layer decreased from 8.12 to 4.72;the center layer decreased from 9.78 to 6.63.When the heat treatment was carried out at 950℃ for 8 min,the strengthened y fiber texture was produced in the cold rolled and annealed strips,which was favorable for the formability of the cold rolled and annealed strips.(4)The effects of annealing temperature on the microstructure,texture and formability of 00Cr19Mo2NbTi cold-rolled strips were studied.In the range of 850～1100℃,the grain sizes gradually increased with the increase of annealing temperatures,and the overall strength of recrystallized y-fiber texture firstly increased and then decreased.When the annealing temperature was 1050℃,the density of the {111}<112>texture in the annealed strips reached the maximum of 11.31 and the value of corresponding "r" was 1.69,indicating that cold rolled and annealed strips can obtain good forming properties with the texture of {111}<112>.(5)The types of precipitates under different heat treatment conditions were studied.The annealing temperature was in the range of 850～950℃,the precipitates in the steel were the finer Laves phase of(Fe,Cr)2(Nb,Ti)and(Fe,Cr,Si)2(Mo,Nb).At 1050℃,the precipitates were coarsed(Nb,Ti)(C,N).