Study On Annealing Process Of Cold Rolled Steel Strip After Hydrogen Reduction Descaling

The hydrogen reduction is environmentally friendly. However, the decarbonization layer and reduction layer on the surface of strip steel and grain coarsening will produce after hydrogen reduction. These defects may be passed on to finished product after cold rolling and annealing strip steel. Therefore, the experimental study on annealing of the cold-rolled strip after hydrogen reduction is of great significance for the popularization and application of hydrogen reduction descaling process.The unleveled plain carbon steel was used as raw experimental materials. The oxide scale on the sample surface was reduced by hydrogen using homemade furnace, and the experiment was carried out by using the laboratory four roll mills to cold rolling, and then the cold-rolled sample was proceeded annealing experiment with different processes in the protective atmosphere. As a comparative study, the acid pickled sample was cold rolled and annealed under the same conditions. The surface morphology and the cross-section microstructure of the sample were observed by scanning electron microscopy and optical microscopy. The tensile properties of sample were tested by MTS810 testing machine. The microstructure and properties of the reduced and cold-rolled sample before annealing were analyzed, the kinetics during the reduction process and the hydrogen embrittlement phenomenon of the reduced sample after cold rolling were discussed by designing contrast experiment, during the annealing, the evolution of the microstructure and mechanical properties of the sample was studied, and detailed analyzed the effect of annealing temperature and annealing time on microstructure and mechanical properties of strip steel.The experimental results show that there are no obvious cracks on the surface of the unleveled hot-rolled strip after reduction, but there are still a lot of micro defects such as bubbles, pits, and asperities. Under the reduction temperature of 700-900 ℃, the reduction weight loss of pure iron oxide scale after hydrogen reduction is increased with the increased of reduction time, pure iron oxide scale with 80 μm thickness will completely transformed into metallic iron via hydrogen reduction at 800 in 1℃ 0 minutes, and the unit area weight loss reaches to 126 g?m-2. After cold rolling, the surface of the reduction sample is very smooth, there are almost no cracks exist on it, only a small amount of micro pits, the roughness value of the surface is lower than that of the acid pickled sample. After reduction and cold rolling, the microstructure of sample appears the phenomenon of work hardening, but compared with the pickled sample after cold rolling, under the same rolling condition, the tensile strength of reduced sample after cold rolling is higher than that of pickled sample after cold rolling approximately 15-25 MPa, the elongation rate decreased about 3%-0.6%, it is considered that might be caused by the hydrogen embrittlement phenomenon due to hydrogen diffusion to strip steel matrix. And after annealing at 680-800 ℃, the internal microstructure of the sample was changed to the phenomenon of recovery, recrystallization and grain growth. The higher the annealing temperature, the longer the annealing time, the grain growth is more serious. Different from acid pickled strip, the cold-rolled strip steel after hydrogen reduction descaling has a shorter recrystallization annealing time, under the same conditions, after annealing, the matrix grain size is relatively large, the grain growth within surface decarburization layer is more obvious. In this experiment, when the temperature is 760 ℃, the annealing time controlling in 3-5 min is the best, and when the temperature is 800 ℃, the annealing time should be less than 1-2 min. Compared with the picked and cold rolled sample, the tensile strength of reduced and cold-rolled sample after annealing decreases about 10-20 MPa, and the elongation rate increases by about 3%-5%, correspondingly. Comprehensive results of the above studies show that, Compared with the pickled sample after cold rolling and annealing, unleveled hot-rolled strip steel is descaled by hydrogen reduction process and then conducted to cold rolling and annealing, the surface finish is higher than that of acid pickled sample, internal grain size of the reduced and cold-rolled sample after annealing increases slightly, tensile strength decreased slightly, and elongation increased slightly, but the overall trend is not big. So, it showed that hydrogen reduction process can replace pickling descaling process used in cold rolling production.