Study On Effects Of Cold Rolling-Process On Microstructure And Properties Of Nb+Ti- Interstitial Free Steel

As the third generation steel for stamping, the IF steel has the broad application prospect. In this paper, Nb+Ti-IF steel double cold rolling was carried on after intermediate annealing, which was after the single cold rolling, the procedure as acid pickling, cold rolling, intermediate annealing, the finishing, double cold rolling, the recrystallization annealing has been completed. Sheet size as 200mm (T)×1000mm (L)×(0.325 2.16 mm) was attained.The basic mechanical properties of stamping have been obtained by tensile test and computation, which result indicated that double cold rolling obtained superior mechanics performance than the single cold rolling technics under the same reduction, the plastic strain ratio after annealing obtained by double cold rolling technics reaches as high as 3.08, enhanced 18% compared to single cold rolling. Limit stretch coefficient as a stimulant mechanical property of stamping has been predicted by the data of the basic mechanical properties. Three kinds of hardened curves have fitted by the non-linear parameter fitting method, in which the optimum work hardening model describing the Nb+Ti-IF steel using in this experiment is:σ=Bε^{N/(1+C₁ε+C₂ε2)}The work hardening properties of sheets under different cold rolling technics have been studied by this model, the result indicated that the sheet under the cold reduction of 60% in first time and 30% in second time can obtain the best work hardening property. The cold rolling and annealing microstructures were observed by the transmission electron microscope and the metallographic microscope. Along with the cold reduction increase, the deformed microstructures turn uniform; grains after recrystallization turn symmetrical, even under the double cold rolling technics. In the IF steel microstructure of high degree cold deformed, the mix dislocation which primarily belong to edge-dislocation distribute around the slip-bands. As a result of the existence of shearing stress concentrated in some narrow region, massive and very dense"shear bands"as shapes of parallel sliver have form.The texture distributions under different cold rolling technics were measured by using XRD. The deformation textures under double cold rolling are remarkable character with the <111> //ND texture even the {111} < 110> component obtained obvious enhancement after intermediate annealing. The evolution ofα({110} //RD) andγ({111} //ND) textures is not being the simple linear relationships with the second cold reduction, along the increase of which the {001} texture grows. After double cold rolling and annealing, the recrystallization textures have lower intensity {001} texture the higher {111} //ND texture compared to single cold rolling; The recrystallization texture content ratio of {111} and {001} is highest after double cold rolling which reduction assignment is cold rolling 70% in first time and 20% in second time, the result indicated the highest r value of the sheet after this cold rolling condition is accurate.