Determination of Non-Recrystallization Temperature for Niobium Microalloyed Steel

In the present investigation, the non-recrystallization temperature (TNR) of niobium-microalloyed steel is determined to plan rolling schedules for obtaining the desired properties of steel. The value of TNR is based on both alloying elements and deformation parameters. In the literature, TNR equations have been developed and utilized. However, each equation has certain limitations which constrain its applicability. This study was completed using laboratory-grade low-carbon Nb-microalloyed steels designed to meet the API X-70 specification. Nb- microalloyed steel is processed by the melting and casting process, and the composition is found by optical emission spectroscopy (OES). Multiple-hit deformation tests were carried out on a Gleeble® 3500 system in the standard pocket-jaw configuration to determine TNR. Cuboidal specimens (10 (L) × 20 (W) × 20 (T) mm3) were taken for compression test (multiple-hit deformation tests) in gleeble. Microstructure evolutions were carried out by using OM (optical microscopy) and SEM (scanning electron microscopy). The value of TNR determined for 0.1 wt.% niobium bearing microalloyed steel is ~ 951 °C. Nb- microalloyed steel rolled at TNR produce partially recrystallized grain with ferrite nucleation. Hence, to verify the TNR value, a rolling process is applied with the finishing rolling temperature near TNR (~951 °C). The microstructure is also revealed in the pancake shape, which confirms TNR.

Static Softening Behavior of Low Carbon High Niobium Microalloyed Steel

The MMS-200 thermal simulation testing machine was used to study the static softening behavior of low carbon high niobium microalloyed steel. The effect of niobium to the static recrystallization softening behavior of the microalloy steel had been analyzed by establishing the kinetics model of static recrystallization and the micro-morphology of precipitates. The results indicated that: the static softening behavior of the tested steel significantly influenced by the deformation temperature and the interval pass time of the rolling processing. At relatively high deformation temperature and long interval pass time, the ratio of static softening was increased. Then the deformation temperature was lower to 950°C, and the static softening behavior of the test steel was ceased. But when the deformation temperature was higher than 1000°C, the static softening behavior of the test steel completely occurred. The activation energy of the test steel was 325·mol-1 by the established model calculated.

The Interdependence of the Degree of Precipitation and Dislocation Density during the Thermomechanical Treatment of Microalloyed Niobium Steel

In this paper, thermomechanical processing of niobium microalloyed steel was performed with the purpose of determining the interaction between niobium precipitates and dislocations, as well as determining the influence of the temperature of final deformation on the degree of precipitation and dislocation density. Two variants of thermomechanical processing with different final rolling temperatures were carried out. Samples were studied using electrochemical isolation with an atomic absorption spectrometer, transmission electron microscopy, X-ray diffraction analysis, and universal tensile testing with a thermographic camera. The results show that the increase in the density of dislocations before the onset of intense precipitation is insignificant because the recrystallization process takes place simultaneously. It increases with the onset of strain-induced precipitation. In this paper, it is shown that niobium precipitates determine the density of dislocations. The appearance of Lüders bands was noticed as a consequence of the interaction between niobium precipitates and dislocations during the subsequent cold deformation. In both variants of the industrial process performed on the cold deformed strip, Lüders bands appeared.

Influence of Finite Mobilities of Triple Junctions on the Grain Morphology and Kinetics of Grain Growth

Grain boundary networks composed of equal microstructural elements were investigated in a recent paper. In this work a more complicated artificial grain topology consisting of one four-sided, two six-sided and one eight-sided grain is designed to further investigate the influence of grain boundary and triple junction mobilities on the kinetics of the system in more detail. Depending on the value of the equal mobility of all triple junctions, the initially square-shaped four-sided grain changes its shape to become more or less rectangular. This indicates that the grain morphology is influenced by the value of the mobility of the triple junctions. It is also demonstrated that a grain arrangement with low mobility triple junctions controlling the kinetics of grain growth enhances growth of the large eight-sided grains. In addition, grain growth is investigated for different values of mobilities of triple junctions and grain boundaries. A strong elongation of several grains is predicted by the modeling results for reduced mobilities of the microstructural grain boundary elements. The two-dimensional modeling results are compared to micrographs of a heat-treated titanium niobium microalloyed steel. This feature, namely the evolution of elongated grains, is observed in the micrograph due to the pinning effect of (Ti, Nb)C precipitates at elevated soaking temperatures of around 1100 °C. Furthermore, the experiments show that a broader distribution of the grain sizes occur at 1100 °C compared to soaking temperatures, where pinning due to precipitates plays a less prominent role. A widening of the distribution of the grain sizes for small triple junction mobilities is also predicted by the unit cell model.

Through-Thickness Microstructure and Strain Distribution in Steel Sheets Rolled in a Large-Diameter Rolling Process

The rolling condition for fabricating a low-carbon niobium-microalloyed steel sheet with an ultrafine-grained (UFG) structure was examined through rolling experiments and finite element analysis. A large-diameter rolling process was proposed to create a UFG structure. The rolling was conducted near the transformation point, Ar3, from austenite to ferrite. The Ar3 was measured at the surface and the center of the sheet. First, the through-thickness microstructure and equivalent strain distribution in a 1-pass rolled sheet 2.0 mm thick were examined. In the rolling experiments, the embedded pin method was employed to understand through-thickness deformation. The magnitude of the equivalent strain to obtain a UFG structure was estimated to be 2.0. Based on these results, the fabrication of a 2 mm UFG steel sheet by 3-pass rolling for an initial thickness of 14.5 mm was attempted by the proposed large-diameter rolling process.

Identification of Lüders bands using digital image correlation

This paper presents the characterization of Lüders bands by digital image correlation on niobium microalloyed steel during a static tensile testing. Digital image correlation with the qualitative and quantitative analysis of the Lüders bands on the microalloyed steel was proved as a very precise and suitable method for determining the strain amount in the deformation zone. In this research was determined that the strain amount is the highest in the area behind the Lüders band front and the lowest in the area in front of the Lüders band.