Research On Internal Quality And Its Influencing Factors Of Ferrite/Pearlite Steel Plate

The quickest developed and most invigorated steel grade in the steel industry is low alloy and high strength steel, especially the ferrite/pearlite steel with the yield strength below500MPa. This type steel has excellent strength and toughness properties, which is currently one of the most widely used steel grades. At present, the main production technology of ferrite/pearlite steel is adding trace Nb, V, Ti and other microalloying elements to C-Mn steel or C-Mn-Si steel. The precipitation behavior of microalloying carbonitrides, and the phase transition behavior of overcooling austenite were controlled by TMCP (ThermoMechanical Control Process) technology, in order to achieve the goal of optimizing microstructure and improving strength and toughness properties. The researchers at home and aboard are committed to the study of improving the internal quality of ferrite/pearlite steel plate for a long time. The so-called good or bad of internal quality of the steel plate, actually refers to the uniformity and integrity of microstructure of the steel plate. For the steel plate of a certain thickness, whatever the technology how to change, the microstructure will appear inconformity from the surface to the center along thickness direction of steel plate, such as the difference of grain size, the morphology difference of ferrite, also the distribution and size difference of precipitates, et al. In addition, if the composition design is unreasonable or the technology is improper, which may cause non ferrite/pearlite structure or other defects internal the steel plate.Through surveying some domestic large scale iron and steel enterprises, engineering machinery and coal machine manufacturing enterprises, also a large number of literatures, we found that some batches of ferrite/pearlite steel plate appeared internal quality defects, especially the lamination defect. Scholars from home and aboard, steelmakers and users have found the lamination defect of steel plates and its adverse influence on mechanical properties. However, at present, there is no unified understanding on the cause of lamination defect, particularly, there is no elaboration about the influence of central segregation of billets on lamination of hot rolled steel plates. We found that the content of Mn element of the steel plate with lamination defect was generally higher through comparatively study on the steel plate with lamination defect and without lamination defect. In addition, we found the steel plates from some steelmakers still used excess other microalloying elements.Pointing at these two problems above, the alloying composition design, TMCP technology and continuous casting technology were optimized; the microstructure and mechanical properties of steel plates from laboratory and industrial production were analyzed, with the means of Transmission Electron Microscopy (TEM), Scanning Auger Microprobe (SAM), Original Position Analyzer (OPA) and measurement of mechanical properties, et al.The inherited relation between central segregation of billets and lamination defect of hot rolled steel plates was researched, also the cause of abnormal segregation band was revealed from kinetics and thermodynamics. On this basis, we concluded the influences of the factors (such as, alloying composition, continuous casting technology, TMCP technology and heat treatment) on internal quality of steel plates. The research contents and conclusions are follows:1. The reducing composition design of ferrite/pearlite steel plate was derived through studying the role of C, Mn, Si, Nb, V, Ti and other alloying elements in the steel plate. The essential elements such as C, Mn, Si, moderate microalloying elements such as Nb, V, Ti, and trace B element was adding to this experimental steel, also minimizing P, S and other harmful elements. Through contrasting the alloying composition of domestic commercial steels at present, it is derived that the addition of alloying elements of this experimental steel is significantly lower than that of commercial steels of the same level. The recrystallization curves and CCT curves of the experimental steel were determinated with the method of Gleeble thermal/mechanical simulation, and then the TMCP process was optimized. The austenitizing temperature of the experimental steel is1200℃. The first four passes of roughing rolling stage can be set below8.5mm in order to increase the volume fraction of recrystallized grains, and big reduction should be taken at last passes in order to refine austenite grains. The finishing rolling temperature can be controlled at1020℃. In the finishing rolling stage, the finishing rolling temperature can be controlled at830～850℃, and the cooling rate can be controlled at5～10℃/s according to the thickness of steel plates. The finishing cooling temperature can be controlled at670～690℃, then directly air cooling to room temperature after controlled cooling. The industrial experimental of designed steel plate indicates that the new technology is smooth, and the alloying cost is lower than that of commercial steels of the same level at present, also the mass production can be achieved.2. Through thermodynamic and kinetic calculations, combined with TEM observation and EDS analysis, the soluting and precipitating behaviors of microalloying carbonitrides during TMCP process were researched. The strengthening and toughening mechanism of the steel plate was analyzed from grain refining, solid solution strengthening and precipitation strengthening. Results indicate that, microalloying carbonitrides of steel plates are mainly TiN、VC and Nb(C,N). The soluting and precipitating temperature of TiN is high. TiN experiences the growth process from sphericity to square, and forms diphase precipitate with VC and Nb(C,N), which can effectively suppress the coarsening of recrystallized grains during the austenitizing and roughing rolling stage. VC with shape of disk, mainly precipitating coherently and randomly at the ferrite interior during cooling after γ�'α transformation, can cause the precipitation strengthening of steel plate. Nb(C,N) with shape of disk, its size is about10nm, mainly nucleates at grain boundaries and dislocations, and precipitates during γ�'α transformation. They can effectively pin the dislocations motion, strongly discourage the recrystallization of deformed austenite, and have certain extent of grain refinement. According to the Orowan mechanism of dislocations rounding the precipitates, it is calculated that the strength increment from precipitation strengthening is143.9MPa, the strength increment from grain refining is261.9MPa, the strength increment from solid solution strengthening is92MPa, and the calculated strength of designed steel plate is497.8MPa. Due to the strength increment from grain refining is over40%of the total strength, the designed steel plate has well plasticity and toughness.3. The composition distribution at different positions along thickness direction of the continuous casting billet was researched by Original Position Analyzer, also the relationship between the technology (molten steel superheat, second cooling intensity, casting speed, electromagnetic stirring, light reduction technology) and internal quality, microstructure along thicknees direction of the continuous casting billet were discussed by theoretical calculations and statistical analysis. Results indicate that, the center of some batches of billets appear obvious C, Mn, S and P element segregation. Gleeble thermal/mechanical simulation experiment shows that, after hot deformation of the billet with central segregation, banded bainitic structure appears at the original segregation region. TEM observation shows that the quantity of sulfide inclusions increases from the surface to the center, and the inclusions appear aggregated distribution at the center. Controlling reasonable continuous casting technology parameters, such as controlling the molten steel superheat below30℃, reasonable stronger second cooling intensity, properly choosing the casting speed, controlling the frequency of electromagnetic stirring and choosing rational position with modest reduction, these measures can obviously improve the internal quality of continuous casting billets.4. The microstructure of the surface,1/4thickness and center of the steel plate from industrial production was observed by SEM and SAM. The mechanical properties of local region were measured with the method of cutting thin specimens from corresponding thickness. Then the formation mechanism of abnormal segregation band was disclosed from thermodynamic and kinetics analysis. Results indicate that, the granular bainitic abnormal segregation band and a certain degree of mixed grain appear at the center of the steel plate with lamination defect, simultaneously, some sulphide inclusions appear at this region. These three factors lead to lamination defect of the steel plate. Obvious C-Mn segregation appears at the abnormal segregation region of the steel plate, and the Mn segregation of the steel plate inherites from the Mn segregation of the billet. The formation of the bainite at abnormal segregation region has immediate relationship with the central segregation of the billet. The Fe-Mn-C clusters segregation region caused by the C-Mn segregation at the center of the billet can effectively bind the atomic motion, and increase the lattice reconstruction resistance of austenite transformation, which provide thermodynamic conditions for the bainite. The C-Mn segregation causes the relative displacement of pearlite transformation curve and bainite transformation curve of the corresponding C curve at the center of the billet, so the original C curve of the center of the billet becomes bay-like shape, which creates the kinetics conditions for the transformation from undercooled austenite to bainite with lower cooling rate at this region. The best normalizing technology of the steel plate with lamination defect is900℃×3min/mm, this technology can not eliminate Mn segregation, but can decrease C content of the abnormal segregation band region by diffusing. This technology can also make the bainite of the abnormal segregation band region to transform into pearlite completely, simultaneously, achieve the purpose of eliminating the lamination defect of the steel plate.