This thesis is focused on three main topics, i.e. the austenite/ferrite phase transformation behavior by applying heavy deformation, the effect of austenite conditioning on the austenite/ferrite transformation, and the effect of nano sized precipitates on the austenite/ferrite transformation by heavy deformation in ultra fine grained HSLA steel.
High strength low alloy (HSLA) steels need good toughness, high tensile strength, and low weld cracking susceptibility for structural applications. In order to improve those properties, the refinement of ferrite grain size is a key technology because it is a unique method to increase both strength and toughness. Grain size refinement is achieved by the ferrites that are nucleated just after hot working because those ferrites are finer and exhibit lower grain growth rate than the conventional ferrites which are nucleated during cooling in the conventional thermo-mechanical controlled process(TMCP). These characteristics have been explained by the higher nucleation rate due to the stored strain energy and the fine carbides formed during the austenite/ferrite transformation. Therefore, researches on those ferrites have been investigated widely due to the their advantages and are defined as SIDT(strain induced dynamic transformation) ferrites. However, because austenite/ferrite phase transformation during hot working is too fast, it is very difficult to clarify the mechanism for it. Therefore, studies about the effect of hot working variables on austenite/ferrite transformation and investigation on the mechanism for austenite/ferrite transformation during hot working are needed.
The ingots used in this study was manufactured by vacuum induction melting (VIM) and followed by the sizing process for the steel plate with a thickness of 15 mm at Pohang Iron and Steel Co. (POSCO). Thermomechanical simulations were carried out by the Gleeble 1500, with which is possible to simulate complete rolling schedules, changing the applied strain, the number of temperatures of individual deformations and the subsequent cooling. The dimension of the specimen for hot working simulation was φ10×15mm.
Austenite/ferrite phase transformation behavior of ultra fine grained HSLA steel has been investigated by applying heavy deformation from 600℃ to 775℃ with a strain from 0 to 0.7 under the strain rate of $1s^{-1}$. Volume fraction of the SIDT ferrites increased and the average grain size of SIDT ferrites decreased with an increase in the amount of strain, and grain growth rate of SIDT ferrites is slower than that of static ferrites. Maximum volume fraction and minimum grain size of SIDT ferrites are strongly related with the $Ar_3$ temperature. Effect of hot working on austenite/ferrite phase transformation of ultra fine grained steel was explained by the diffusion and nucleation rate increase as the strain of hot working increases with comparing the static austenite/ferrite phase transformation.
Effect of austenite conditioning on austenite/ferrite phase transformation of ultra fine grained HSLA steel has been investigated by the relationship between SIDT ferrites and effective interface area per unit volume which acts as the nucleation sites. Effective interface area per unit volume were controlled by applying different austenite conditionings, i.e. controlling austenitizing temperature, hot working at recrystallization, and hot working at non-recrystallization region. The first method is to control the initial grain size of austenite by changing the austenitizing temperature from 1100℃ to 1200℃. The second method is to recrystallize the austenite grains by applying a fixed strain of 0.6 from 1050℃ to 1150℃ in the ecrystallization region. The last method is to cause the austenite grains to become elongated and produce deformation bands within grain by applying the strain from 0.2 to 0.6 at 900℃ in the non-recrystallization region. Interface area per unit volume of grain boundaries is inversely proportional to the grain size for the equi-axed prior austenite grains, while, it increases with increasing the applied strain in the non-recrystallization region for the elongated prior austenite grains. In addition, interface area per unit volume of deformation bands is proportional to the square of strain. Volume fraction of SIDT ferrites increases with an increase in the effective interface area pre unit volume of grain boundaries and deformation bands. The volume fraction of SIDT ferrite nucleated from the elongated prior austenite grains is higher than that nucleated from equi-axed prior austenite grains at the same effective interface area per unit volume. The volume fraction of SIDT ferrite nucleated from the elongated prior austenite grains, however, was higher than that nucleated from equi-axed prior austenite grains at the same effective interface area per unit volume. From the experiment results and discussions, it was concluded that diffusional austenite/ferrite phase transformation of HSLA steel by the interface nucleation and growth mechanism is accelerated by the deformation bands in the non-recrystallization region.
Effect of nano sized precipitates on austenite/ferrite phase transformation of ultra fine grained HSLA steel has been investigated by the relationship between SIDT ferrites and precipitation behavior. Because nucleation of SIDT ferrites is strongly related with effective interface area per unit volume and $Ar_3$ temperature, hot working temperatures were selected at 40℃ over $Ar_3$ temperature and controlling austenitizing temperature ranged from 1200℃ to 1250℃ makes effective interface area per unit volume same before hot working in all conditions. While precipitate distribution of Nb-containing steel is unimodal ranged from 20nm to 40nm, that of Ti-containing steel is bimodal ranged from 20nm to 40nm and from 500nm to 900nm. Size and distribution of nano sized precipitates do not affect the size and volume fraction of SIDT ferrite. However, the grain size and volume fraction of SIDT ferrite in Nb-containing steel is finer and smaller than those in Ti-containing steel because Nb element makes prior austenite grains elongated and cause higher strain energy than any other element.
