High strength steel is one of the development trends of color steel products. At present, more and more 350 strength color coated steel plates are used abroad. There are usually two processes for producing this kind of strength steel plate. The first is to select carbon manganese steel, choose C, Mn and other strengthening elements in steel-making, and use of strong hot rolling and cold rolling capacity to produce relatively thin high-strength color coated steel plate. For example, Baosteel and other domestic and foreign steel plants (ore to product) have chosen such production technology. Another method is to reduce the annealing temperature of the hot-dip galvanizing line so that it can not be fully annealed, but also save some hard rolling state and improve the strength. At present, most domestic galvanizing plants and color coating plants have chosen this technology, because they do not have steel-making process, and there is no corresponding high-strength steel in the market to purchase hot-rolled coil or hot-rolled plate.
But there are three dangers:
① Due to the decrease of annealing temperature and the temperature imbalance of strip width, the annealing temperature is near the critical recrystallization temperature of the steel plate, which leads to the serious inhomogeneity of strip strength in the width direction of the steel plate, and the shape of the processed strip is difficult to be guaranteed.
② The reduction of annealing temperature will inevitably lead to poor cleaning effect, because the fine oxidation of the surface needs to be recovered in the high temperature recovery gas, which has a negative impact on the adhesion of the hot-dip galvanized layer.
③ When the annealing temperature decreases, the yield strength increases, but the tensile strength does not increase correspondingly, resulting in the increase of the yield strength ratio of the data, which has a great impact on the processing formability, seismic resistance and fire resistance of the data. For example, the strength of high-strength steel produced by reducing annealing temperature is greatly reduced in case of fire, which seriously affects the fire resistance rating of buildings.