| Interstitial free (IF) steel is one of the third generation deep-drawing steels. It is widely used in automobile and machinery and so on. In this paper, in order to obtain the relationship between the hot working process in ferrite state and the microstructure and properties, the compressive deformation data of Nb+Ti-IF steel at high temperature of ferrite area have been fit into an express of peak stressσ. Through the power dissipation map and the instability map, the process map for hot working was eventually obtained.In order to obtain the relationship between the cold working process and the microstructure and properties of Nb+Ti-IF steel, single cold rolling and double cold rolling with an intermediate annealing were performed. IF steel sheets with the dimension of 200mm×1000mm×(0.325 ~ 2.16 mm) were attained via the following manufacturing procedure: acid pickling, primary cold rolling, intermediate annealing, finishing, secondary cold rolling and recrystallization annealing,.The basic mechanical properties of deep-drawing sheet have been evaluated by tensile test and computation. The results indicated that double cold rolling process resulted in better mechanical properties than the single cold rolling process with the same total reduction did. After annealing, the plastic strain ratio resulting from double cold rolling techniques was 3.08, seeing an increment of 18% in comparison to that resulting from single cold rolling. The ultimate stretch coefficient, a simulated mechanical property for deep stamping, was predicted using the basic mechanical properties data. Three kinds of hardened curves were ploted via the non-linear simulation, in which the optimum work hardening model applicable to the Nb+Ti-IF steel was also obtained.The work hardening properties of sheets under different cold rolling processes were studied using this model. The result indicated that the sheet with the cold reduction of 60% in the first time and 30% in the second time had the best work hardening properties. The microstructures after cold rolling and annealing were observed using a transmission electron microscope (TEM) and an opticle metallographic microscope. As the cold reduction increased, the deformed microstructures became more uniform, and the recrystallized grains became more symmetrical. Moreover, the microstructure resulting from the double cold rolling was more homogeneous than that resulting from the single cold rolling. In the IF steel sample heavily cold deformed, a lot of mixing dislocations, most of which were edge-dislocations, distributed around the slip-bands. As a result of the shearing stress concentration, a lot of very dense"shear bands"with the shape of parallel sliver were formed in some areas.The textures resulting from different cold rolling processes were measured using XRD. The deformation textures resulting from double cold rolling were remarkably charactered by the enhenced <111>//ND texture after intermediate annealing, among which the {111} < 110> component was the most enhanced. The evolution ofαandγtextures were not linearly related to the second cold reduction, whereas the {001} texture incerased with the increase of the reduction. After double cold rolling and annealing, the recrystallization textures included lower intensity of {001} texture and higher intensity of <111> //ND texture in comparison to the textures resulting from the single cold rolling; The highest recrystallization texture content of {111} and {001} was measured after double cold rolling when the reduction assignment was 70% in the first time and 20% in the second time, suggesting that the highest r value of the sheet after this cold rolling condition was correct.The process map for hot working in ferrite region, stress-strain relationship and double cold rolling process were established for IF steel. The evolution of the textures during cold rolling was systemically analyzed. These results not only added new ideals to the theoretic system of deep stamping steels, but provided guideance to the production of such steels also.
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