The Key Technologies Of Ferrite Rolling And Softening Mechanism Of Steel Produced On CSP Process

Compact strip production(CSP)has been widely used in iron and steel enterprises at home and abroad for its short process,high automation level,energy saving,emission reduction,high output and stable production.Low carbon steel SPHC products are produced by hot rolling,cold rolling and subsequent annealing process,which can be used as stamping materials.At present,austenite rolling has been mainly used in CSP production line,which has relatively high yield strength but limits its application.Based on the production of low carbon steel SPHC by CSP process,the application of ferrite rolling process in hot rolling was studied in this paper.The basic characteristics of materials under ferrite rolling conditions were studied.The microstructure of ferrite rolling and austenite rolling under hot rolling,cold rolling,cover annealing and continuous annealing were also compared in detail.The key technologies of ferrite rolling and its softening mechanism were revealed in order to realize the application of low carbon steel SPHC ferrite rolling in CSP process.The basic characteristic parameters of materials were the main basis to formulate the process parameters of hot rolling process.In this paper,the cooling process and deformation process of SPHC after rough rolling were simulated by thermal simulation experiment.The results showed that the Ar3 and Ar1 of SPHC steel were 873℃ and 796℃ respectively,and the deformation resistance reached the minimum at the temperature of 820℃.In the temperature range from 850℃ to 775℃,as the low temperature region of two-phase region or the high temperature region of ferrite single-phase region,the dynamic recrystallization of ferrite was difficult to occur and the grain coarsening was obvious.The yield strength of hot rolling,cold rolling and annealing products of SPHC steel after ferrite rolling and austenite rolling were compared.The results showed that the yield strength of the hot rolling plate decreased by 72MPa,and the elongation and n value increased slightly when the finishing rolling temperature was about 780℃.The average yield strength and tensile strength of ferrite rolling cover annealing plate were 44 MPa and 28 MPa lower than those of austenite rolling respectively.The difference of elongation and n value was not obvious.The difference of strength mainly came from the difference of grain size.Compared with austenite rolling continuous annealing plate,the average yield strength and tensile strength of ferrite rolling continuous annealing plate were 15 MPa and 4 MPa lower,respectively.The elongation and n value had little difference,and the reduction of strength difference mainly came from the decrease of grain size difference.After ferrite rolling,a strong {001}<110>texture was formed in the SPHC hot rolling plate,therefore the average r value decreased from 0.96 to 0.67 compared with austenite rolling.After cold rolling,the orientation distribution density of favorable texture {112}<110>and unfavorable texture {001}<110>were significantly higher than that of hot rolling,and the unfavorable texture was inherited after cold rolling.After cold rolling and annealing,the texture was mainly{111}<110>and the texture of austenite rolling was stronger than that of ferrite rolling.and the unfavorable texture of hot rolling was less inherited after annealing.Thus,the average r value was higher than that of ferrite rolling.Compared with the cover annealing plate,the {111} texture was weaker in the continuous annealing plate,and {001} texture was still existed in the ferrte rolling continuous annealing plate,which led to the lower average r value than that of austenite rolling continuous annealing plate.Based on the research of hot rolling and cold rolling annealed products under ferrite rolling process,the softening mechanism of ferrite rolling on the material was clarified.Through theoretical calculation,it can be seen that the main contribution to the reduction of yield strength of ferrite rolling hot rolling plate was the coarsening of grain size and reached 86%.The reduction of dislocation density also contributed to 14%.At this temperature,the grain was difficult to be refined by dynamic recrystallization,the grain size of ferrite became coarser,and the deformation resistance at this temperature was also significantly reduced.After high temperature coiling,the recovery or static recrystallization and grain growth occurred in the deformed ferrite grains formed after rolling,which further increased the grain size and reduced the dislocation density.The mechanism of ferrite rolling on formability reduction was clarified.The deformation textures of SPHC steel at 900℃ and 870℃ were mainly {111}favorable texture and {001} unfavorable texture produced by dynamic recrystallization of austenite,but deformation from 850℃ to 800℃ was strong{001} unfavorable texture.At 750℃,there was a small amount of {001}unfavorable texture,which was formed due to partial dynamic recrystallization of ferrite and more {111} favorable textures were formed.The existence of unfavorable texture after hot rolling led to the reduction of r value,which will be inherited into subsequent cold rolling and annealing processes.The key parameter was the finishing rolling temperature of ferrite rolling process.The finishing rolling temperature should be in the high temperature section of double-phase region and ferrite single-phase region.The results showed that the strength of SPHC ferrite rolling hot rolling plate was obviously decreased compared with that of austenite rolling plate.The average Rp0.2 was 229Mpa,decreased by 24%.The average Rm was 331Mpa,decreased by 1 5%.The average elongation was 33%,increased by 20%.The average n value was 0.22,increased by 20%.The average r value was 0.72,decreased by 32%.At the same time,the oxide scale thickness decreased by 31 to 35%.The reduction of ultimate reduction rate of cold rolling was also higher than that of austenite rolling,and the rolling thickness was reduced from 0.44mm to 0.33mm.