A study of HSLA steels microalloyed with vanadium and titanium during simulated controlled rollling [i.e. rolling] cycles
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Thermomechanical treatments involving recrystallisation controlled rolling process of hot rolling strip mills were simulated in a Gleeble-1500 testing machine. Five vanadium and one vanadium-titanium microalloyed HSLA steels were used for these simulations. Specimens of vanadium steels were heated to 1170°C to simulate slab reheating, prior to a 50% upset reduction at 1060°C, The specimens were then cooled rapidly to the simulated coiling temperatures of 950°C to 600°C and held for half an hour then air cooled to room temperature. Nitrogen analysis established that the maximum volume fraction of VN coincided with the minimum ferrite grain size at a simulated coiling temperature of 700°C. The resultant ferrite grain size indicated that the VN precipitated in the ferrite restricted the ferrite grain growth. The maximum VN precipitation that was observed at this temperature was considered to be a function of the vanadium diffusion in the ferrite and the temperature at which the austenite to ferrite transformation is completed. Quenched specimens following deformation at 1060°C showed that strain induced VN precipitation was detected when the equilibrium solubility product calculations predicted the formation of VN. The measured VN precipitation based upon nitrogen analysis, was less than that predicted. Comparison of detected VN at coiling temperatures of 600°C to 950°C for specimens subjected to 50% deformation at 1060°C with specimens without deformation showed that the deformation increased the VN precipitation. Using the experimentally determined ferrite grain size, volume fraction and mean particle size of the precipitated VN, the corresponding yield strength has been calculated for the range of coiling temperatures examined. These calculated yield strengths lie within the range determined experimentally on similar vanadium micro alloyed HSLA steels. The reheating behaviour of the vanadium-titanium steel was investigated by quenching specimens from 900°C to 1500°C after holding for half an hour at the respective temperatures. Insoluble nitrogen analysis indicated that VN completely dissolved below 1100°C and TiN started to dissolve in austenite at approximately 1300°C. The measured insoluble nitrogen content indicated the existence of TixV₁-xN. The measurement of size distribution of precipitates showed that the dissolution of precipitates of less than 10 nm resulted in abnormal austenite grain growth. It was thought that the results for AIN detected using the Beeghly method [Beeghly49, United Steel 62] were influenced by dissolution of the finer sized VN and TiN precipitates. This was because the detected AIN was in excess of that calculated from the equilibrium solubility for the remaining nitrogen content based upon the measured acid insoluble nitrogen content being combined as TiN. The nitrogen content detected as being associated with AIN was greater than that detected as the acid soluble nitrogen content that is defined as the total amount of nitrogen in the form of aluminium nitride, iron nitrides and interstitial nitrogen. X-ray diffraction of residues separated from specimens reheated at 900°C and 1350°C using 17% v/v dilute sulphuric acid showed that VN and TiN precipitates were present in the specimen reheated at 900°C, while there was only TiN detected at 1350°C, Precipitates extracted from reheated specimens using carbon replicas were identified using a convergent beam electron diffraction method. The indexing of dual spot diffraction patterns established that VN had precipitated on the surfaces of existing TiN precipitates with the same crystal orientation as the initial TiN. These dual spot diffraction patterns were not observed in specimens reheated above 11OO°C. The thermomechanical treatment of a vanadium-titanium steel in hot rolling strip mills was simulated using the Gleeble-1500 testing machine. The rolling was simulated by carrying out four passes each of 20% deformation on specimens at finish rolling temperatures of 1050°C to 850°C. An additional experiment involved a final deformation that varied from 10% to 30% for a finish rolling temperature of 1000°C. For all these finish rolling simulations, the specimens were rapidly cooled to a range of temperatures between 750°C and 600°C. The rapid cooling occurred at a rate of 10°C/s and having reached the simulated coiling temperature the specimens were subsequently slow cooled to represent the thermal behaviour in the coil. Insoluble nitrogen analysis showed that the quantity of nitrides decreased with the decreasing coiling temperature. While the finish rolling temperature and deformation percentage had no measurable effect on the final insoluble nitrogen content after coiling, the size of precipitates decreased with the decreasing coiling temperature and with increasing percentages of deformation. The final ferrite grain size decreased with the decreasing finish rolling temperature and with increasing percentages of deformation. The average ferrite grain size of specimens subjected to 30% final deformation at 1000°C, coiled at 750°C, 700°C, 650°C and 600°C was less than 10 µm. Finish rolling at 1000°C, 950°C and 900°C in the austenite recrystallisation region with four passes each of 20% deformation also achieved a fine ferrite grain size of under 10 µm provided that the coiling was performed at temperatures of 650°C or less. Interphase precipitation with the planar or non-planar morphologies was not observed in the thin foils or carbon replicas from the specimens subjected to the simulated thermomechanical treatments for the steels containing either vanadium, or the combination of vanadium and titanium. The observed precipitates in the ferrite phase in these steels were distributed on dislocations, within the ferrite and on the ferrite grain boundaries. The calculations based on the Hall-Petch equation showed that the lower yield strength for the specimens subjected to 20% and 30% deformation respectively at a finish rolling temperature of 1000°C and coiled at 750°C, 700°C, 650°C and 600°C increased as the coiling temperature decreased from 750°C to 600°C. The lower yield strength for specimens subjected to 30% final deformation was higher than that for 20% and the maximum lower yield strength occurred at the coiling temperature of 600°C for both 20% and 30% final deformations. The present experimental results showed that with appropriate coiling temperatures and a accelerated cooling rate the recrystallisation controlled rolling process for vanadium and vanadium-titanium steels can be used to produce a hot strip steel with a fine ferrite grain size of less than 10 µm. This means that the 70% to 80% deformation at around 800°C in the low temperature controlled rolling process was not necessary to obtain a fine ferrite grain size. Thus a fine grained strip steel can be produced in the existing hot strip rolling mills without exceeding the load limitation of a strip rolling equipment.