Study On Relationship Between Recrystallized Structure And Properties Of Ti+P-IF Steel

Besides chemical composition control of high strength IF steel, annealing process is a critical process of deciding property of high strength IF steel during complicated production process. Studying on relationship between annealing parameters and structure property of high strength IF steel not only helps to reveal IF steel recrystallization rules, but also provides the theoretical basis for annealing process optimization of manufacturing process.In this paper, the relationship between annealing temperature, soaking time, and structure property of the Ti+P-IF steel was studied by single stage annealing, four stage temperature-increasing annealing, and four stage temperature-decreasing annealing. With research methods of metallurgical structure observation, texture analysis, TEM analysis, and mechanical test, etc., relationship between recrystallized structure and property of Ti+P-IF steel was discussed. The results show:1 ) The single-stage-annealed samples and the four-stage-annealed samples of Ti+P-IF steel have same microstructural features: equiaxed grain structure, grain finess is 9.1～9.5; FeTiP precipitation in initial recrystallization stage, uneven distribution of small and dense second phase particles of average size of 20nm; uniform tubular distribution ofγfibre texture, but its intensity changed greatly.2) Microstructural changes of the four-stage-annealed Ti+P-IF steel were caused by combined effects of the annealing temperature and soaking time. With gradual increasing or decreasing annealing temperature, it was shown thatγfibre texture became stronger, recrystallized grains were larger and became more uniformly equiaxed with precipitation of small and dense second phase particles. Under certain annealing temperature in every stage, recrystallized grains kept growing, component {111} of recrystallization texture further developed, and second phase particles gathered and developed. However, over soaking time had no significant effects on grain structures and second phase particles, but it had important effects ofγfibre texture.3 ) Although the four-stage-temperature-increasing annealed steel provided relatively smaller changing range of strength index and higher changing range of plasticity index, property of the four-stage-temperature-decreasing annealed Ti+P-IF steel changed greatly.4) Property of the single-stage-annealed Ti+P-IF steel was R_p0.2=177MPa,R_m=361 MPa,A_80mm=34.2%,r=2.16,n=0.28,Δr=-0.44. For the four-stage-temperature-increasing annealed Ti+P-IF steel, the best property (R_p0.2=181MPa,R_m=373MPa,A_80mm=40.1%,r=2.13,n=0.27,Δr=-0.77) were achieved under process parameters of 680℃(30min)→700℃(60min)→720℃(90min )→740℃(60min). For the four-stage-temperature-decreasing annealed steel, the best property (R_p0.2=164MPa,R_m=361MPa,A_80mm=34.0%,r=2.03,n=0.27,Δr=-0.03) were achieved under process parameters of 740℃(90min)→720℃(90min)→700℃(30min)→680℃(30min).To summarize, the four-stage-temperature-increasing annealing procedure is recommended for manufacturing process of the Ti+P-IF steel.