Research On Optimal Setting Method Of Second Cooling Water In Continuous Casting For Billet Quality Index

As the central link of steelmaking production,the quality of slab has a profound impact on the quality of the rolling products and the performance of the final product.The internal quality problems in the casting process are most common and difficult to handle among the quality problems of billet.And cracks and shrinkage are relatively common internal quality defects.The strength and distribution of second cooling water in continuous casting process is the key to determine the solidification process and then affect the quality defects of billet.In the process of steady pouring,reasonable second cooling water is conducive to the uniform distribution of temperature field of billet,and is the basis to ensure the quality of slab.But at present,the decision of second cooling water mainly takes the temperature system into consideration,and does not optimize the quality of billet directly.In view of the above problems,this paper takes the cracks and shrinkage quality problems as the research object in continuous casting.The specific works are as follows:1)The causes and influencing factors of cracks and shrinkage quality problems of billet are analyzed,and the quality index models of crack index and shrinkage radius are established.Firstly,the temperature field and the stress field numeric calculation model of the slab are established,and the boundary conditions required for the model calculation are determined.Then,based on the heat transfer model in continuous casting process,the shrinkage quality model is established,and the shrinkage cavity radius is defined as the quantitative index of shrinkage quality problem;Meanwhile,the crack quality model is established on the basis of the finite element stress field model,and the crack index is defined as the quantitative index of the internal crack quality problem.2)Based on the slab quality index model,a new second cooling water optimization model and off-line solution method are established.The objective function of the optimization problem is established based on the quality index model of cracks and shrinkage,and the water quantity of second cooling zone is taken as the decision variable.Meanwhile,the metallurgical rule of the continuous casting process is taken as the constraint condition.In view of the multi-objective optimization problem,the weights of each optimization target are determined by the analytic hierarchy process(AHP)method,and the constrained conditions are treated.Finally,the particle swarm optimization algorithm with chaos mechanism is used to solve this problem.In addition,for a set of process variables(such as casting speed and casting temperature)in the process of billet production,the optimal water quantity corresponding to these process variables can be obtained by this optimization model.So we can get an optimal solution function of the second cooling water for variables in continuous casting process through this new water optimization model,which will provide training samples for the data driven method mentioned below.3)According to the problem that the optimization model of second cooling water has high computational complexity and long calculation cycle,which is hard to use online,a fitting model of the optimal solution function of second cooling water based on M-SVR is established,taking casting speed,superheat and off-line optimization of water as samples.And it is used as an online optimizer to realize the online setting of second cooling water.Firstly,for each set of process variables(casting speed,casting temperature)second cooling water optimization model built above can be used to calculate the corresponding optimal solution of the water quantity,which can be used to generate the training samples(X:casting speed,superheat,Y:two the optimal water cooling zone).Then,the data set is divided into training set and test set,and the training of M-SVR is completed.Finally,using the trained M-SVR model,the experimental simulation of on-line setting of second cooling water is carried out.The experimental results show that the optimal setting model of the second cooling water based on M-SVR has a good estimation accuracy.4)Based on the Q235 production data of a steel mill,the optimal setting method of the second cooling water in continuous casting is verified by experiments.In order to verify the effect of the proposed method on slab quality,the Q235 temperature field,stress field,crack index and shrinkage radius quality index were calculated before and after water optimization by using the simulation model established in advance.The experimental results show that the water in the first section of second cooling zone is reduced by 21.8%,and the water in the second and third section of second cooling zone are increased by 16.11%and 50%respectively compared with the water of Q235 in every section used before the optimization.Meanwhile,the temperature change of the surface of Q235 is slow,the temperature rise and the maximum stress obviously decreases in every second cooling zone after the optimization.The radius of shrinkage decreased from 2.7mm to 2.4mm,and the crack index in every second cooling zone decreased from 0.86,4.98 and 1.07 to 9.19,18.52 and 4.54,the crack and shrinkage quality problem in Q235 were improved.