The If Steel, Ultra-deep Red Hot Galvanized Plate Annealing Process

The mechanical properties of Extra-deep drawing grade IF steel hot-dip galvanized sheet are remarkably affected by its microstructure and recrystallization textures, which, at the same time, are controlled by chemical compositions and production process. First of all, the microstructure and textures in the Pangang produced typical IF steel hot-rolled strip were measured. And then, according to the processes of the No.1 and No.2 continuous hot-dip galvanizing lines at Pangang, Gleeble-1500 annealing simulation and salt bath annealing experiments were conducted to investigate the effect of annealing process on the microstructure, recrystallization textures and properties systematically by means of optical microscopy, X-ray texture measurement and mechanical evaluation in order to provide the basis for the determination of annealing process of extra-deep drawing grade IF steel hot-dip galvanized sheet at No.l and No.2 continuous hot-dip galvanizing lines at Pangang.Microstructure and texture analyses were conducted on the hot-rolling strip of the experimental steels. The results showed that all the steels exhibit fine-grained ferrite measured to be approximately ASTM grade 11.0. In addition, the hot-rolling strip shows obvious textures. The main texture components were {115} < 110>, {223}<110>ang {332} <110> along α fiber. The chemical compositions and coiling temperatures during hot rolling seems to have little effect on the microstructures and textures of experimental steels.Gleeble-1500 annealing simulation experiment of the cold-rolled specimen showed that the full recrystallization temperature for all experimental steels is below 800℃. It decreases as the coiling temperature is increased or the carbon content in the steels decreases. At higher carbon content, the full recrystallization temperature is lower for the process of the line 2 in comparison with that of the line 1.When the annealing temperature is below 880℃, the annealing temperature has little influence on the recrystallization grain size of the Ti-IF steel and the Ti-Nb-IF steel containing higher carbon content . The grain size is measured to be above ASTM grade 11.0 for low temperature coiled Ti-IF steel as well as Ti-Nb-IF steel containing higher carbon content or to be about ASTM grade 10.5 for high temperature coiled Ti-IF steel, respectively. The grain size of low temperature coiled Ti-IF steel coarsens by half grade under the process of line 2 in comparison with that of line 1 while the grain size of the Ti-Nb-IF steel containing low carbon content can grow to about ASTM grade 8.5 at higher annealing temperatureWhen the annealing temperature is risen to 920 ℃,the grain remarkably coarsens. Strong {223} < 110> and {114} <110> texture components were found in all experimental steels after annealing. The intensity of them is little affected by coiling temperature and annealing process while the intensity of {111} recrystallization texture tends to increase when the annealing temperature is increased, especially at higher annealing temperature or when the carbon content in the steels is lower. When the annealing temperature is further increased to 920℃,the intensity of {111} recrystallization texture increases obviously. The processing conditions of the line 2 are beneficial to the evolution of {111} recrystallization texture in contrast to those of the line 1 .The Ti-Nb-IF steel containing lower carbon content exhibits the strongest {111} texture followed by the high temperature coiled Ti-IF steel and Ti-Nb-IF steel while the low temperature coiled Ti-IF steel containing higher carbon content shows weakest {111} texture. The holding time at 840℃ annealing temperature has little effect on microstructure and recrystallization texture.In designing industrial pilot scheme, Ti-IF was selected for the production of DX53 and DX54D grades whereas Ti-Nb-IF steel was used for the DX56D grade. The carbon content and the total elongation of skinpassing and tension leveling should be decreased whereas the coiling temperature during hot rolling should be increased in the sequence of DX53D,DX54D and DX56D in order to realize the gradation of mechanical properties and to meet the application requirement.More than four thousands of DX53D, DX54D and DX56D deep drawing and extra-deep drawing series hot-dip galvanized steel sheet was commercially produced by applying the alloying scheme and production parameters that were determined according to experimental results. The mechanical properties of these products conform the EN10142-2000 standard and meet the application requirement of appliance-making customers.