Wear-resistant steel final acid process system
The metallurgical function of pickling process is to effectively remove the oxide layer and chrome-poor layer, achieve surface passivation, and improve the surface quality. After pickling, the surface of the wear-resistant steel plate still has silica rich phase oxide layer and chromium poor layer, which need to be removed by the final pickling process. In order to improve the pickling efficiency, the final pickling system consists of at least one acid and one oxidizer.
1. Design of nitric acid-hydrofluoric acid pickling system
Nitro-hydrofluoric acid is the most widely used final pickling system for wear-resistant steel plates, and has good pickling effect on austenitic wear-resistant steel plates, ferritic wear-resistant steel plates and biphase wear-resistant steel plates. Among them, nitric acid can not only provide H+, dissolve the oxide layer, but also act as an oxidizer to enhance the REDOX potential of the pickling solution, so as to improve the pickling efficiency. In addition to providing a small amount of H+, HF can ionize F- to form stable complexes with Cr3+ and Fe3+, play a depassivation role, and improve the dissolution rate of chrome-poor layer.
It is found that 85% of the acid is used to dissolve the metal and only 15% of the acid is used to dissolve the oxide layer. In the pickling process, the removal of the oxide layer mainly depends on the dissolution of the chrome-poor layer and the base metal, so that the oxide layer attached to the surface is peeled off.
2, nitric acid-hydrofluoric acid pickling system improvement
Under the synergistic action of HNO3, HF and metal ions, the nitric acid-hydrofluoric acid pickling system can basically completely remove the residual oxide layer and improve the surface quality of wear-resistant steel plates, but the pickling process will cause serious NOx and nitrite pollution. For how to solve the problem of environmental pollution caused by nitric acid, there are two ideas: one is to add oxidizing agents, NO, NO2 will be reoxidized to nitric acid, not only reduce pollution, but also realize the recycling of nitric acid; One is to replace nitric acid with other acids and oxidants.
More research work has focused on the direct substitution of nitric acid with acids and oxidants. Oxidizing agents such as H2O2 began to introduce the pickling process of wear-resistant steel plates to replace the oxidation of HNO3, thereby reducing environmental pollution. In the early 1990s, BianchiM et al. first proposed that H2SiO4-HF-H2O2 system could replace HNO3 / HF system to meet the quality requirements of final pickling, and MadiVN also studied the H2SiO4-HF-H2O2 pickling system successively to further improve the pickling system. Henkel's Cleanox352 environmentally-friendly pickling process realizes the industrial application of nitron-free pickling process.
Nitric acid-hydrofluoric acid mixed acid pickling process has more serious NOx and nitrite pollution, which increases the environmental pressure and social pressure of wear-resistant steel plate manufacturers, which also shows from the side that the environmental protection pickling process of hot-rolled wear-resistant steel plate will have broad prospects for development.
