Hot Deformation Behavior of C-Mn Steel with Incomplete Recrystallization during Roughing Phase with and without Nb Addition

Abstract

The objective of the study is to improve understanding of the practical role of niobium (Nb) in the case of industrial inconsistent rolling processes such as the rolling of heavy gauge plates where a lower stored energy rolling practice will result in a less stable and less repeatable static recrystallization (SRX) activation that prevents complete recrystallization. In the current study, these variabilities are validated by comparing the mean flow stress (MFS) indirectly determined from the rolling force measured on a reverse four-high rolling mill stand. The material resistance to deformation and grain size evolution of a C-Mn steel during hot rolling was observed and validated with and without Nb addition. The pre-defined rolling schedule was predicted to exhibit incomplete recrystallization in the roughing phase due to the limited stored energy of deformation that resulted from low rolling loads and a higher number of rolling passes. The prior austenite grain size (PAGS) distribution was predicted and compared to the measured effective ferrite grain (FG) size distribution after the completion of hot rolling and phase transformation achieved using natural air plate cooling. Both the predicted PAG and measured FG distributions revealed the presence of multimodality, and both distributions were used for grain size reduction factor determination for γ → α transformation for the current study with 1.96 for 0 Nb and 1.70 for 240 Nb. The results presented in this paper are not only limited to the rolling schedule used in this paper because instabilities resulting in incomplete austenite conditioning are also observed when evaluating the cross-sections of other heavy plates and various steel grades utilizing different processing routes with comparable compositions such as modern lean abrasion-resistant steels, regular line pipe steels, and other similar grades.

The Effect of Niobium Addition and Pre-Annealing on the Tensile Properties of 52CrMoV4 Spring Steel

“…This study showed that when comparing the initial prediction to the experimental results, a strong correlation was found in terms of Nb efficiency and how the grain size evolution affected the ductility qualities. In their investigations, they discovered that the toughness increased, and the grain size decreased with the addition of Nb [23]. Su et al investigated the mechanical properties of carbon pearlitic steels with different rates of the Nb element.…”

Grain Size Distribution of DP 600 Steel Using Single-Pass Asymmetrical Wedge Test

“…A basic composition, typical for lean (C-Mn) structural steels, was set (with low carbon, high manganese grade) where manganese defined the proportion of ferrite and pearlite under ordinary air-cooled conditions [24,25]. No precipitation-strengthening carbide or nitride-forming elements were intentionally added to the given study to allow full softening kinetics (for close to equiaxed austenite shape formation) concerning the kinematic parameters of wedge rolling.…”

“…The cementite acts as a potential nucleation site of austenite [29]. When partial SRX is activated at sufficiently high temperatures, the starting new PAG can easily grow due to high HAGB mobility until sufficient roughing passes are introduced to limit/stop the HAGB mobility, and the continuous refining of PAG can again be observed with further passes [17,25]. By using a wedge rolling test, the notch positions are usually observed where sufficient ε (or e) is introduced for an effective through-section deformation, achieving the uniform cross-section dislocation density and promoting a repeatable dislocation-free grain formation to minimize any microstructural cross-section variation (microstructural non-uniformity).…”

The Performance of Niobium-Microalloying Ultra-High-Strength Bridge Cable Steel during Hot Rolling

“…Furthermore, grain refinement can improve the uniformity of the material during deformation, reducing the risk of performance degradation caused by temperature changes and stress concentration. Nb also affects the recrystallization behavior of materials, slowing down the recrystallization process, allowing for finer grain sizes at higher deformation temperatures [ 10 , 11 , 12 ]. Due to these benefits of Nb-microalloying, bridge cable steel in modern bridge engineering increasingly employs this approach [ 13 ].…”

“…This process, through plastic deformation at high temperatures, enables the material to achieve the desired geometric shape and size. Simultaneously, the internal microstructure of the material undergoes a complex evolution, including recrystallization, grain growth, and phase transformation [ 5 , 11 , 14 , 15 ]. Scientific research has shown that fine control of hot-rolling parameters, such as the heating temperature, degree of deformation, deformation speed, and cooling rate, can manipulate the material’s microstructure at the microscopic level, thus achieving targeted performance design at the macroscopic scale [ 16 , 17 ].…”