Study On Turbulent Acid Pickling Of Hot-rolled Steel Strip

Steel acid pickling is an improtant technique in the process of cold-rolled strip production and the quality of the strip surface is heavily reliant upon the pickling process. In the process of steel strip production, oxide scale on the strip surface should be removed in turbulent acid pickling process so as to improve the quality of strip surface. The quality of acid pickling is determined by the parameters such as acid temperature, acid concentration, the strip speed, acid injection flow rate, the incident angle, and the peeling behaviors of the scale.The turbulent acid pickling theory relates to jet impinging theory, chemical reaction kinetics, turbulent diffusion theory and convective heat transfer theory. However, previous studies on acid pickling were mainly aimed at testing whose results were fit to empirical formula or a acid pickling control model set up on the basis of a large number of trials and flow simulation in the acid pickling tank. Mathematical expression for turbulent acid pickling, the peeling effect of the hydrogen bubble generated by the reaction between the steel base and hydrochloric acid and the heat and mass transfer characteristic of the impingment moving strip, which play a great role in improving pickling efficiency, have not been investigated. Therefore, the investigation of each pickling parameter is essential to master the mechanism of heat and mass transfer and the bubble growth.In this research, turbulent acid pickling for steel strip is investigated. The method of first zero potential differential value which base on electrochemistry is put forward to determin the endpoint of acid pickling. The pickling time determined by the method of first zero potential differential value is well consonant with the emprical formula done by Hudson, which verifies the reliability and feasibility of the new method. Besides, the precise control of the pickling can avoid overpickling. The overpickling of the steel will increase the penetrability of hydrogen to the steel matrix and change the material properties of the steel strip, even to cause hydrogen embrittlement.Morphology change of the typical oxide scale with time is analysed to observe the change of the cross section of the oxide scale. It follows that the acid penetrates the inner of the scale from the position of weak or soluble surface. Besides, the number of initial pickling core is also an important factor to affect the picling efficiency. Based on this, the theory of chemical reaction dynamics and heat transfer combines with the parameters such as acid temperature, acid concentration, chemical reaction rate, concentration diffusion upon the strip and the contact area between the acid and oxide scale to build a mathematical model of the dissolution of oxide scale. The results show that for scale-breaking strip, the pickling time is proportional to initial area of the scale and reciprocal sum of the mass transfer coefficient and chemical reaction coefficient, and is inversely proportional to the acid concentration and the degree of the sacle breaking. Apart from above paremeters, for the unscale-breaking strip, the pickling time even has a relationship with the formation of the pickling nucleus. Laws of the hydrogen bubble growing are investigated under different acid temperature, acid concentration and the degree of the scale breaking to obtain the initial time of bubble formation, changes in the number of bubbles, initial time of oxide removal and size of the detached scale for the scale which is difficult to dissolve in acid.Assumed that the number of moles of strip base diffusion is equal to the number of moles of bydrogen bubbles generation, a theoretical bubble growing and peeling model which bases on the state equation of ideal gas is established to determine the relationship among the diameter of the bubble, thickness of the scale, dissolution rate of the scale, dissolution rate of the strip base, acid temperature and acid concentration. Based on this, the requirements of the bubble detachment and the scale removal are theoretically determined.The turbulent kinetic energy distribution on the moving strip surface which represents the hydrogen ion diffusion coefficient is computed under different pickling parameters. Moreover, the effect of pickling parameters on the turbulent kinetic energy distribution upon the strip surface are computed to measure the degree of those parameters on the convective mass transfer of strip surface. The simulation revels that the maximal turbulent kinetic energy is obtained under the condition that the jet angle is45°when the moving direction of the strip opposites to the jet inclination. Flow rate and jet angle determine the flow state and turbulent kinetic energy in the entrance of the pickling tank, however, the speed of the moving strip determines flow state and turbulent kinetic energy in the inner of the tank.Finally, the initial temperature of the steel strip is found to have a great effect on the acid pickling efficiency. Temperature rise of the steel strip in still acid is theoretically analyzed. The Reynolds stress turbulence model and VOF model solved sequentially by the SIMPLIC method are used to do the solid-liquid coupling calculation of the heat transfer upon the steel strip and temperature rise of the strip in the turbulent pickling tank to obtain the influence of pickling process parameters on the Nusselt number distribution and temperature rise of the strip.The achievements of this research are beneficial to turbulent pickling process, and supply helpful theoretical and experimental guides to further process development and optimization.