Investigation On TMCP Technology And Optimization Of The Roll Application Process During Manufacturing X80Pipeline Steel By3500Mm Steckel Mill

In this investigation, based on the TMCP technology during producing X80pipeline steel by using3500mm Steckel Mill of Nanjing Iron&Steel group maker (hereinafter referred to as NanGang) and the optimization of the process for applying the roller, the parameters for controlled rolling and controlled cooling process were set down through measuring the characteristics on the phase transformation of X80pipeline steel. TMCP technology was optimized thrrough analyzing the microstructures and the mechanical properties of X80pipeline steel produced by different TMCP processes. By Finite Element Method (FEM), the temperature field during rolling X80pipeline steel was simulated, and the temperature field of roller on the Steckel Mill was analyzed. Based on the above results, a new system for cooling roller was set up. At the same time, the three-dimensional finite element analysis was used to simulate the elastic deformation of roller on Steckel Mill and set up the model for forecasting the abrasion of the working roller and the new procedure for changing roll which are propitious to the actual application.The dynamic CCT curves, the temperature at which the recrystallization of austenite stops and the temperature scope of phase transformation of X80pipeline steel were respectively measured by using Gleeble-3500thermal simulation test equipment and DTA. According to the measured phase transformation characteristics of X80pipeline steel, the parameters for TMCP technology were set up and the TMCP test was done in the laboratory. The configuration of dislocations as well as the strenthening and toughening actions of fine second precipites in the tested steel were investigated. During the optimization of the process for applying the roller on3500mm Steckel Mill, both of FEM and tests were together used to the research on the temperature field and the elastic deformation of roller, and the adaptive processes for applying roller on Steckel Mill were developed. As a result, the shortage in the primary process for applying roller was solved, and the ability of the shape control was improved and the abrasion of roller was decreased. Through the field industrial test, the process for applying roller developed in the study could satisfy the requirements during production of the steel by Steckel Mill and obvious economic benefits were obtained.The following main results were obtained from this investigation:(1) The parameters for controlled rolling and controlled cooling of X80pipeline steel with high content of Nb was gained, i.e. the continuous casting slab should be heated to1200℃and held for180minutes; in the first rolling stage, rolling should be begun at about1100℃and finished at a temperature higher than1050℃; and in the second rolling stage, the rolling should be begun at abot950℃, and finished at a temperature higher than850℃; cooling rate should be controlled in the range of20-30℃/s, and final cooling temperature is about500℃.(2) The plate/coil of X80pipeline steel with high strength and toughness was successfully produced on Steckel Mil by using the developed TMCP technology. The average yield strength of the steel is higher than575MPa, the average tentile strength is higher than665MPa, the absorbed-in-fracture energy at-20℃is above330J, FATT50CVN<-60℃.(3) The microstructure of the X80pipeline steel produced by Steckel Mill is the microstructure consist of the typical fine acicular ferrite, and there are a large number of lath frrite subgrains in acicular ferrite. The dislocations with higher density were found in the subgrains and their boundaries, and some dislocation lines tangled each other to form cellular subgrain structure and to make ferritic lath smash and obtain a fine-grain-strengening effect. At same time, nano-scale carbonitrides of Nb, Ti precipitated dispersively in ferrite matrix, including two types of precipitates:one is the fine and dispersive Nb(Ti)C which size are about20nm and the other is the complex precipitates of Ti, Nb (NC) which are about50-200nm in size and mainly TiN. Such microstructure provides an essencial base for the mechanical properties of X80pipeline steel produced by Steckel Mill. The actual application indicates that the ERW pipeline and SSAW pipeline manufactured by X80steel plate and coil produced by Steckel Mill have a higher strength, impact toughness and resistance to tearing-off.(4) The temperature rise process of working roll when3500mm Steckel Mill is used for hot rolling was dynamically analysised by FEM, and the transient temperature distribution of the working roll was obtained. Based on the results, the off-line test was carried out, and improved the cooling procedure of working roll was improved according the results of the off-line test.(5) Based on the results of analysis and calculation of elastic deformation of roll system, three-dimensional finite element models for roll was set up, and the regurarity of effects of the rolling force, the roll bending force, the breadth of rolled material, the convexity of working roll etc on roll system was discussed. The contact stress between the rollers was analysised, and a new type of chamfer for supporting roll was designed according to the analyzed results.(6) A new cooling procedure on working roller and the curve on roll system for Steckel3500mm Mill were set up. Through the new process for applying roller not only the crown of the plate was decreased and the plate shape was improved, but also the homogeneous attrition of rolls and slower wearing rate were obtained. The practical application indicates that the roll abrasion was decreased by20.67%on the original basis.