Methodology Of Establishing Flexible Rolling Technology Based On Prediction Of Microstructure And Properties

There was a contradiction occurring with development of science, technology and economy between mass demand of iron & steel product and manufacturing technique & systemic supervisor. Therefore, it is necessary to offer a new rolling technique with larger flexibility and compatibility in rolling process, called flexible rolling technology. In other words, flexible rolling technology involves that products with distinct properties level are manufactured with raw materials with identical chemical compositions, or that products with identical properties level are manufactured with raw materials with distinct chemical compositions. Successful development of super steels, multiphase steels and corresponding devices is the practical base for the realization of flexible rolling technology, and additionally, precise prediction of microstructure and properties of hot rolled production is the academic base for that. Mass data in practical hot rolling production were collected, and relationship between each factor and microstructure & properties was systemically analyzed. In addition, Temperature models, recrystallization models and phase transformation models in hot rolling was established to exactly describe microstructure evolution of the whole hot rolling process and the foundation and mechanism of mechanical properties changing was explored. From macroscopic and microcosmic point of view, the final mechanical properties of hot rolled product were predicted by prediction technology of microstructure & properties based on Artificial Neural Network (ANN) or physic-metallurgy models. On the basis of high-precision predicting ability, temperature schedule was constituted reversely, and relationship between each factor and microstructure & properties was analyzed, as well as that production of desire strength obtained by changing temperature schedule of hot rolling was realized. In addition, effect of hot rolling parameters on corresponding microstructure evolution was analyzed, and processing system was adjusted according to mechanism of mechanical properties changing so as to realize flexible rolling of steels. According to methodology of establishing flexible technology based on prediction of microstructure and properties, corresponding software was programmed. The innovation and main research content of this paper is as follows: (1) From macroscopic point of view, the function relationship between each factor and microstructure & properties was analyzed and created. The effect of each factor on mechanical properties was analyzed by meanly handling method, and regressed models and ANN models for mechanical properties of plain carbon steels and microalloyed steels were created. It is shown that both regressed models and ANN models can be used to precisely predict the mechanical properties of hot rolled product, and regressed models was taken as an academic guide for chemical compositions design, whereas the later model is more precise than the former one. Predicted result for the influence of each factor on the mechanical properties by ANN models is in good agreement with calculated value by meanly handling method.(2) Method for improving predicting ability of ANN models and constituting temperature schedule reversely based on high-precision ANN models was put forward. Considering that parameters such as neurons of hidden layer and training time, etc are important factors for ANN models, an optimizing ANN architecture method by Genetic Algorithm (GA) was put forward, so that predicting ability of ANN models was improved. Based on the optimized high-precision ANN models, temperature schedule including 1st starting temperature,2nd starting temperature, which exists in the controlled rolling processes, finishing temperature and final cooling temperature, which exists in the controlled cooling processes was constituted by GA, and mechanical properties calculated by the optimized temperature schedule was in good accordance with desire mechanical properties. Thus, theoretically, quantitatively calculating method for flexible rolling technology was established, realizing that products with different mechanical properties were manufactured by ingot with identical chemical compositions, and that products with identical mechanical properties were manufactured by ingot with different chemical compositions.(3) Models of the quantitative influence of roll gangs on temperature distribution were established, and method for improving the simulation precision of simulated temperature distribution was put forward. Considering that hot strip transfers heat to roll gangs, heat conductivity models for contact between roll gangs and hot strip was established. It is shown that during the whole rolling process, strip temperature dropping of about 14～18℃, and temperature at top surface of strip 4～16℃higher than that at bottom surface was affected by roll gangs, which is relatively little. So, these models can further contribute to improve the precision of simulated temperature distribution. Temperature distribution of hot strip was optimized within a narrow range by Monte Carlo algorithm. It is shown that, optimized temperature distribution was much closer to measured temperature distribution, and with this method, precision of the simulated temperature distribution can be improved, which helps to realize inline precise simulation of temperature distribution.(4) Microstructure evolution models of pipeline X65 during recrystallization process and mean flow stress (MFS) models were established. Moreover, technology of grain refinement was put forward. Ferrite grain refinement technology was put forward and corresponding microstructure evolution was simulated after simulating recrystallization process of pipeline and analyzing evolution of austenite grain size (AGS) and recrystallization fraction at each position of 2-D cross-section which vertical to the rolling direction, It is shown that increasing reduction at the frontal finishing mill and decreasing the inlet and exit finishing temperature contributed to austenite grain refinement before transformation and transformed ferrite grain refinement in the following cooling process. It's not good either for austenite grains refinement or for protection of manufacturing device if the inlet finishing temperature was too low. However, lots of ferrite nucleation site were supplied due to the occurrence of retained strain, and ferrite grain size still can be refined. Regressed models and ANN models of mean flow stress which was more precise than Misaka models was established.(5) Ferrite transformation models considering the effect of solute-drag effect,retained stress and AGS, etc, was created. Ferrite transformation process varying with temperature and time was determined by metallographic analysis method combined with thermal dilation curve obtained by corresponding thermal dilation experiment. And quantitative relationship between Nb content and ferrite start temperature was calculated with ferrite nucleation and growth models. It is shown that, limiting carbon concentration at the vicinity of ferrite nuclei at the moment of ferrite transformation didn't vary with temperature and cooling rate, but increased linearly with increase of Nb content in steels. Ferrite nucleation finish temperature decreased with increase of Nb content when Nb content was low, but it did not decrease when Nb content is above 0.023mass%. Solute Nb in austenite inhibited ferrite transformation. Ferrite start temperature decreased remarkably with increase of cooling rate.if cooling rate is small (<5℃·s-1), while decrease of ferrite start temperature in (direct) proportion to the increase of cooling rate if cooling rate beyond 5℃·s-1. Ferrite start temperature calculated by models agreed with the measured one. With the increasing of Nb content and Nb content beyond given values, the amount of retained stress and Nb-containing precipitations may be increased during the hot rolling process, which accelerated ferrite nucleation, improved ferrite nucleation rate and transformed volume fraction, but the increament of transformed ferrite volume fraction was small.(6) Prediction of microstructure & properties based on physic-metallurgy models was applied to hot rolling process of plates. According to datum in the whole production process, temperature distribution, recrystallization and phase transformation evolution of plates was simulated. It is shown that predicted final microstructure agreed well with practical metallurgical structure and models of microstructure evolution for microalloyed steels was precise to predict the hot rolling process. When rolling lay in the non-recrystallied region during the 2nd rolling processing, it was advantageous to increase ferrite nucleation rate if total reduction increases. When cooling rate is small at post ferrite growth stage, increasing the cooling rate at ferrite nucleation and earlier growth stage helps to increase total ferrite nucleation ratio, refine ferrite grain size, and improve mechanical properties of steels. According to the simulated result of hot rolled plates, proper adjustment of production technology contributes to get desire mechanical properties and to realize flexible rolling technology.(7) Main function of the developed software for flexible rolling technology setup based on prediction of microstructure & properties is as follows:prediction of microstructure evolution in hot rolling process, online prediction of mechanical properties, and inverse constitution of temperature schedule.