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Article

In Situ Observation of the Austenite Grains Growth Behavior in the Austenitizing Process of Nb–Ti Micro-Alloyed Medium Manganese Steel

by
Guangpeng Yuan
1,
Yu Du
1,*,
Chao Sun
1,
Xiuhua Gao
2,
Hongyan Wu
2 and
Linxiu Du
2
1
Nanjing Iron & Steel Co., Ltd., Nanjing 210035, China
2
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(10), 1144; https://doi.org/10.3390/coatings15101144
Submission received: 22 August 2025 / Revised: 6 September 2025 / Accepted: 25 September 2025 / Published: 2 October 2025
(This article belongs to the Special Issue Research in Laser Welding and Surface Treatment Technology)
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Abstract

In this paper, the austenite grains growth behavior in the austenitizing process of Nb–Ti micro-alloyed medium manganese steel was studied through in situ observation by high temperature laser confocal microscope. The results show that the average austenite grain sizes change from about 3 μm at 1050 °C to over 50 μm at 1250 °C. When the grain boundary is a small-angle grain boundary, one grain boundary will split into several dislocations. With the extension of heating time, the lattice orientation difference further decreases, and the remaining dislocations may merge into new grain boundaries. The most suitable heating temperature for the medium manganese steel in this paper is from 1100 °C to 1150 °C, taking into account influences such as grain size, grain boundary damage, and deformation resistance.

1. Introduction

In recent years, manganese steel has emerged as a prominent research area owing to its potential to achieve a wide range of mechanical properties [1,2,3,4,5]. Medium manganese steel, containing manganese in the range of 3% to 12%, has become a leading candidate for the next generation of advanced high-strength steels due to its optimal balance between mechanical performance and cost efficiency [6,7,8,9,10]. The superior balance of high strength, toughness, and ductility in medium manganese steels originates from the transformation-induced plasticity effect, which is activated by the transformation of metastable austenite to martensite during deformation. The microstructures of medium manganese steel primarily comprise tempered martensite and retained austenite (RA). Enhanced ductility is achieved by introducing metastable austenite through the enrichment of C and Mn during intercritical annealing [11,12,13,14,15]. From the perspective of the rolling process, hot rolling temperature significantly influences microstructure and properties, which are essentially caused by grain boundaries. Yu et al. found that the decrease in hot rolling temperature from 1150 °C to 900 °C resulted in a decrease in average austenite grain size from 40 µm to 20 µm and a refinement of the average martensite lath width from 400 nm to 250 nm [16]. Therefore, it is very important to study the austenite grain size control of medium manganese steel at high temperature.
Grain boundary engineering is an advanced materials science approach aimed at optimizing the properties of polycrystalline metals through deliberate modification of their grain boundaries [17,18,19]. Kokkula et al. investigated the effect of grain boundary engineering on the microstructures of high-Mn steel, revealing that specimens processed with grain boundary engineering exhibited a higher fraction of Σ3 boundaries and their variants (Σ9 and Σ27), larger twin-related domain sizes, and a greater number of grains compared to conventional specimens [20]. This phenomenon is attributed to enhanced multiple twinning activated by strain-induced grain boundary migration. Controlling austenite grain size during the hot rolling heating process constitutes a critical initial step for grain boundary engineering, as the size and distribution of austenite grains significantly influence microstructure evolution and properties during subsequent rolling and cooling stages [21,22,23]. This is particularly critical for medium manganese steels, which exhibit a low phase transformation temperature, high austenite grain growth tendency, and susceptibility to grain boundary damage. Hanamura et al. investigated the influence of austenite grain size on the transformation behavior, microstructure, and mechanical properties of 0.1C–5Mn martensitic steel [24]. They found that austenite refinement substantially improves the true fracture strength and increases the true fracture strain of martensite, thereby challenging the conventional strength–ductility trade-off paradigm. Thus, low-carbon 5Mn martensitic steel produced from fine austenite demonstrates potential for an excellent balance of strength, ductility, and toughness.
Grain growth is a significant research focus in materials science, with existing studies [25,26,27] establishing that its driving force originates primarily from the reduction in grain boundary energy. Rohrer highlighted the critical role of grain boundary energy in complexion transitions, demonstrating that changes in grain boundary energy alter the grain boundary character distribution [28]. Evidence further indicates that high-energy grain boundaries transform into lower-energy complexions at lower temperatures compared to low-energy boundaries. Recently, solute segregation at grain boundaries and its impact on grain boundary energy have attracted substantial attention. Lejček and Hofmann investigated grain boundary segregation using thermodynamic models based on standard enthalpy and entropy [29]. Their predictions for iron-based binary systems showed strong consistency with published data. Current research on grain growth kinetics predominantly emphasizes the temperature-driven acceleration and the precipitation-induced retardation during the steady-state growth stage. However, mechanisms governing the early growth stage—immediately following austenite phase transformation and grain impingement—remain insufficiently explored. To predictively control microstructures with coexisting multiple complexions, a deeper theoretical and experimental understanding is imperative.
This study employs a high-temperature laser confocal microscope to conduct in situ observations of austenite grain growth behavior during the austenitizing process of Nb–Ti micro-alloyed medium manganese steel. Thermodynamic calculations were performed to quantify the content and average diameter of Nb–Ti precipitates. Statistical analysis of grain diameter frequency and area percentage under varying heating temperatures and durations was conducted to investigate recrystallization, grain growth, and grain coalescence. The growth mechanisms of austenite grains in medium manganese steel are comprehensively discussed, with emphasis on the role of micro-alloying elements in pinning grain boundaries.

2. Experimental Materials and Methods

The forged billet of medium manganese steel with a size of 120 × 130 × 130 mm3 was heated to 1200 °C and held isothermally for 120 min. Then, the steel billet was hot rolled to 30 mm thick in a Φ450 mm hot rolling mill and on-line quenched to room temperature, with the initial and finish rolling temperature being 1050 °C and 900 °C, respectively. The chemical composition of the medium manganese steel is listed in Table 1. The phase zone and temperature at which precipitates dissolve were calculated using Thermo-Calc 2024a software.
To study the austenite grain growth during heating and isothermal processes of medium manganese steel, ultra high temperature confocal scanning laser microscope (UHT, VL3000DX-SVF17SP/15FTC, Yonekura Mfg Co., Ltd., Yokohama, Japan) was used for in situ observing the dynamic migration and growth of austenite grains at high temperature. The UHT specimens were cut from the 30 mm thick plate to a size of Φ5 × 3 mm3. The specimens were manually and mechanically polished to ensure the flatness and cleanliness of the observed surface. The specimens were heated to 1050 °C, 1100 °C, 1150 °C, 1200 °C, and 1250 °C and held isothermally for 600 s at a heating rate of 10 °C/s, respectively. Throughout the heating and isothermal stages, continuous in situ imaging was performed using UHT. Representative micrographs were selected for statistical quantification of average grain size and area percentage as functions of temperature and time, enabling analysis of grain growth kinetics.

3. Results

3.1. Growth Behavior of Austenite Grains During Heating Process

Figure 1 shows the austenite grains heated to 1050 °C and then held isothermally for 600 s at a heating rate of 10 °C/s. The average austenite grain size is very small, only about 3 μm. The reason why the grains are so small under these conditions is that the phase transformation has just ended, and the grains have collided, so they did not have time to grow at this temperature.
Figure 2 shows the austenite grain image at higher temperatures, which was heated to 1100 °C, 1150 °C, 1200 °C, and 1250 °C and then held isothermally for 200 s, 300 s, 400 s, and 600 s at a heating rate of 10 °C/s. The four images in Figure 2a,e,i,m show the austenite grain size after 200 s, and the austenite grain boundaries become clear only when the temperature reaches 1200 °C. However, when the isothermal time increases to 300 s, the austenite boundaries become clear even at lower temperatures, although the grain size at 1100 °C is still very small, as shown in Figure 2b,f,j,n. When the isothermal time is increased to 400 s, the austenite grain boundaries fully emerge, as shown in Figure 2e,g,k,o, and obvious coarsening phenomena appear at the austenite grain boundaries when the insulation time is increased to 600 s, as shown in Figure 2d,h,i,p. From the results of UHT, it can be seen that the austenite grain boundaries become increasingly clear and coarse with the extension of isothermal time, and the austenite grain size continuously increases. It can also be seen in Figure 2d,h,i,p that the austenitic grain size increases with the isothermal temperature and changes significantly at 1100 °C, and this change is reflected in the transformation from small grains to large equiaxed grains. Meanwhile, th traces of austenite grain merging and grain boundary migration become increasingly apparent above 1150 °C, while the austenite grain boundaries gradually become straight.
To investigate the influence of isothermal temperature on the austenite grain size distribution, the austenite grain size distributions and equivalent circular area percentage of different specimens were statistically analyzed, as shown in Figure 3 and Figure 4. Figure 3 shows the specimens at 1100 °C and 1150 °C, where the grain size of the specimens at these two temperatures has significantly increased. Figure 3a,b show that the grain sizes of the specimen held isothermally at 1100 °C for 200 s and 300 s are all smaller than 30 μm. Meanwhile, some grains larger than 30 μm appeared, and less than 3% grain sizes are above 80 μm when held isothermally at 1100 °C for 400 s and 600 s, as shown in Figure 3c,d. Although some large grains appeared, the reason why the average grain size did not increase significantly is that the proportion of large grains is small, and the grain size is mainly determined by small grains. Figure 3e shows that the grain size of the specimen isothermal at 1150 °C for 200 s is smaller than 80 μm. As for Figure 3f–h, the grain size distributions of the specimen held isothermally at 1150 °C for 300 s, 400 s, and 600 s are similar, mainly between 20 μm and 50 μm, with a few exceeding 80 μm.
Figure 4 shows the specimens at 1200 °C and 1250 °C, as the grain size of the specimens at these two temperatures remains stable without significant increase. It is shown in Figure 4a that the austenite grain size is mainly concentrated in the range of 30–35 μm of the specimen held isothermally at 1200 °C for 200 s, and the grain size is between 20 μm and 80 μm when the isothermal time were 300 s, 400 s, and 600 s, as shown in Figure 4b–d. Finally, when the isothermal temperature is 1250 °C, the range of grain size concentration is similar, but the dispersion is more uniform, as shown in Figure 4e–h. This indicates that the rapid grain growth stage was completed within 200 s at temperatures of 1200 °C and 1250 °C, after which the grain size became relatively stable.
Gaussian fitting was performed on the austenite grain size distribution from Figure 3 and Figure 4, and the average grain size and its standard deviation are listed in Table 2. The average austenite grain sizes at 1100 °C are about 10 μm within 400 s and about 20 μm at 600 s. However, the average austenite grain sizes of 1150 °C are about 25 μm at 200 s and about 35 μm between 300 s and 600 s. In addition, the average austenite grain sizes at 1200 °C are about 35 μm at 200 s and about 45 μm between 300 s and 600 s. Finally, for the specimen held isothermally at 1250 °C, the average austenite grain size remain stable at around 50 μm.
From the results of average austenite grain size, it can be seen that 80 μm is a clear demarcation point to distinguish between large and small grains. Therefore, we calculated the proportion of grain sizes and the percentage of grain area greater than 80 μm, and listed them in Table 3. It can be seen that there are almost no grains larger than 80 μm at 1100 °C, and both the proportion of grain sizes and the percentage of grain area larger than 80 μm increase with isothermal time, which are from 0% to 13.3% and to 49.7%, respectively. However, the proportion of grain sizes and the percentage of grain area larger than 80 μm are almost the same for 1200 °C and 1250 °C, which are about 20% and 50%, respectively.

3.2. Thermodynamics Calculation of Phase Zones and Carbides Average Diameter

Figure 5a shows the equilibrium phase zone of the steel in the temperature range from 0 °C to 1400 °C. FCC phase exists between 0 and 296 °C and 430–1400 °C, while the BCC phase can exist stably below 752 °C, and cementite dissolves above 605 °C. Furthermore, the weight fraction of precipitates below 1000 °C is about 0.065%, and these precipitates gradually dissolve from about 1000 °C to 1200 °C, as shown in Figure 5b. The original specimen was on-line quenched to room temperature at 900 °C after hot rolling; therefore, most of the precipitates have already precipitated at this temperature and will remain in the microstructure.
Figure 6 shows the average diameter of precipitates during heating process at different isothermal temperatures. The curve of 1050 °C, 1100 °C, 1150 °C, 1200 °C, and 1250 °C need 105, 110, 115, 120, and 125 s to reach the isothermal temperature by setting a heating rate of 10 °C/s through Thermal-Calc software. Figure 5 shows that the dissolution temperature of precipitates is approximately around 1200 °C. At a holding temperature of 1050~1150 °C, a portion of the precipitates nucleate and grow during the heating and holding stages, resulting in a continuous increase in their average diameter. However, during the cooling process, fine precipitates begin to precipitate extensively, leading to a rapid decrease in the average diameter. When the holding temperature exceeds 1200 °C, the precipitates dissolve completely during heating, and no precipitation occurs during subsequent cooling.

4. Discussion

4.1. Effect of Isothermal Temperature on Austenite Grain Growth Behavior

It is well established that temperature exerts a dominant influence on grain growth kinetics, wherein elevated temperatures accelerate growth rates. This phenomenon primarily stems from enhanced atomic diffusion at higher temperatures, which increases grain boundary mobility. Concurrently, the thermal energy reduces the free energy disparity between impurities in the grain interior and those segregated at boundaries, thereby attenuating solute segregation. Consequently, the solute drag effect restraining boundary migration is diminished. Both mechanisms elevate effective grain boundary energy, destabilizing the microstructure and driving grain growth [30,31,32,33]. Consistent with these principles, our results demonstrate that variations in isothermal temperature induce substantially greater changes in austenite grain size than equivalent variations in isothermal time. For instance, the average austenite grain sizes change from about 3 μm at 1050 °C to over 50 μm at 1250 °C, whereas increasing the isothermal time from 200 s to 600 s did not result in a significant increase in austenite grain size, as shown in Figure 3i and Figure 4i. At 1200 °C and 1250 °C, the proportion of large grains with a diameter greater than 80 μm did not increase significantly, indicating that the large grains became relatively stable after 200 s for the specimens heating over 1200 °C.
Refining grain size is one of the most successful processing strategies to improve the properties of polycrystalline solids, and the fact that precipitates play a significant role in grain refinement has also been extensively studied by many researchers. The current research results indicate that the interaction between precipitates and grain boundaries hinders grain boundary movement, thereby hindering grain growth. Assuming that the dispersed phase particles are spherical, the maximum tensile force Fmax exerted by the precipitates in the opposite direction of grain boundary movement is:
F m a x = π r γ b
Assuming that precipitates are uniformly distributed in the metal with a density of N per unit volume, which means the grain boundary intersects with 2rN precipitates per unit area. The total constraint force F′max acting on grain boundary movement per unit area is:
F ′ m a x = 2 r N F m a x = 3 f γ b / 2 r
where r is the radius of precipitates, γb is the grain boundary energy per unit area, and f is the volume percentage of precipitates [34,35].
It can be clearly seen from Equation (2) that the total constraint force increases with the increase in volume percentage of precipitates f and with the decrease in precipitates radius r. According to the calculation of precipitates content and diameter in this paper, the volume percentage of precipitates decrease with the increase in isothermal temperature from 1050 °C to 1250 °C (Figure 5), and the precipitates radius at 1050 °C is smaller than 1100 °C and 1150 °C (Figure 6). From these two aspects, the pinning effect of the precipitate is strongest at 1050 °C, weakened at 1100 °C and 1150 °C, and basically lost at 1200 °C and 1250 °C. This conclusion also corresponds to the phenomenon that the grain size increases linearly between 1050 °C and 1200 °C, while the grain size between 1200 °C and 1250 °C remains basically unchanged. Moreover, the grain boundary width increases with the temperature, especially for 1250 °C, indicating that grain boundaries are damaged at high temperatures. Although the size of grains increases with temperature, the proportion of large grains stabilizes at around 50% at 1150 °C. This indicates that under this micro-alloying composition condition, the optimal isothermal temperature range for medium manganese steel is between 1100 °C and 1150 °C.
The current research results show that the precipitates not only have an impact on grain boundary migration, which means they not only influence grain size but also affect the inhomogeneity of grain size. If the initial grain size is small while the density of precipitates is insufficient, grain growth tends to occur rapidly. Bignon and Bernacki studied particle pinning during grain growth in a 2D polycrystalline context [36]. The results show that particles not only impede grain growth but also prevent—to some extent—shrinking grains from vanishing. Particles thus inhibit grain growth and grain contraction, possibly leading to a heterogeneous distribution of grain sizes within microstructures, characterized by a mixture of small and large grains. In this study, the dissolution of precipitates at about 1200 °C resulted in different influencing factors at different isothermal temperatures. When the isothermal temperature is below 1200 °C, the grain size and its distribution are jointly influenced by precipitates and grain boundary energy, whereas they are mainly determined by the grain boundary energy when the isothermal temperature is above 1200 °C.

4.2. Austenite Grain Growth Mechanism During Isothermal

Medium manganese steel forms martensite upon quenching due to the shear transformation of austenite. During subsequent heating, this martensite transforms back to austenite. Understanding the relationship between these two transformations is therefore essential. Both Dong et al. [37,38] and Yu et al. [16] observed that austenite nucleates between martensite laths and at grain boundaries during heating in medium manganese steel, resulting in numerous nucleation sites and extremely fine austenite grains. Figure 1 further confirms these very small austenite grain sizes, indicative of nucleation site saturation during the phase transformation. Additionally, Yang et al. found that martensite blocks transform into austenite at heating rates ranging from 10 °C/s to 50 °C/s [39]. Moszner et al. extensively studied Mn redistribution’s role in the reverse transformation of binary Fe-10Mn alloy, observing that austenite formation proceeds via an interface-dominated mechanism during rapid heating [40]. Under slow heating conditions, however, austenite reversion occurs through a dual-step process: diffusional mechanisms dominate the initial stage, followed by interface migration control in the second stage. For the specimens in this study, the orientation difference between the austenite formed by these transformations and the adjacent martensitic matrix is expected to be small. Consequently, newly formed grain boundaries are predominantly low-angle boundaries. This prevalence of low-angle grain boundaries, especially prominent at lower heating temperatures, dictates austenite growth characteristics during isothermal holding and facilitates grain merging.
Isothermal time significantly influences austenite grain growth, and distinct grains exhibit different growth mechanisms. Figure 2b clearly shows the initial microstructure composed entirely of fine austenite grains, while Figure 2c reveals the appearance of numerous large austenite grains after 100 s, indicating that grain merging is a primary austenite growth mechanism. However, not all grains merge; successful coalescence into a single large grain requires a small orientation difference between the grains prior to merging. An alternative growth mechanism occurs when one grain engulfs another through grain boundary migration, as illustrated in Figure 2n,o. This phenomenon arises when the orientation difference between grains is relatively large, necessitating reliance on boundary migration rather than merging. Schematic representations of both mechanisms are provided in Figure 7. Small-angle grain boundaries possess a strong tendency for multiple grains to merge into a single larger grain. Within this merged grain, the original boundary position generates numerous dislocations—effectively splitting one grain boundary into several dislocation arrays—causing it to appear as a unified grain under observation. With prolonged heating time, the lattice orientation difference further diminishes, yet dislocations may persist. These remaining dislocations can subsequently coalesce to form new, low-angle sub-boundaries, manifesting as faint grain boundaries within the large grains. Conversely, large-angle grain boundaries exclusively facilitate growth through grain consumption, driven by boundary movement.
From the specimens at 1200 °C, it can be seen more clearly that both mechanisms coexist simultaneously, as shown in Figure 8. As the isothermal time extends, some austenite grains become larger through mutual annexation, and traces of grain boundary migration (Arrow in Zone 2) and disappearance can also be seen from Zone 1, 2, and 3. However, it can also be observed from Zone 4, 5, and 6 that recrystallization nucleation occurs within some austenite grains as the isothermal time increases. Also, a small grain is gradually annexed by adjacent grains over the isothermal time, as shown in Zone 7, 8, and 9. With the migration of grain boundaries, they eventually reach an equilibrium state, that is characterized by flat grain boundaries and hexagonal grains.
According to the above results, large-sized grains are relatively stable because large grain boundaries have low energy, while small grain boundaries have high energy, and small grains are far away from large grain boundaries. It is possible that not every boundary surrounding the large grain has transformed into the high-mobility complexion. Furthermore, boundaries around the smaller grains might have transformed but did not create an abnormally large grain, either because not enough time elapsed for the grain to increase in size to differentiate it from the others, or not enough of the boundaries surrounding that grain had transformed and allowed it to grow abnormally large. The increase in average grain size is due to the growth of small grains, while the size of large grains remains largely unchanged.

4.3. Consideration of Heating Process for Medium Manganese Steel Continuous Casting Billet

From the perspective of toughness requirements, strict control of the heating process is needed for medium manganese steel. Medium manganese steel has high hardenability and is also martensitic when slowly cooled, so the microstructure of the continuous casting billet is martensitic. Medium manganese steel has special characteristics during the heating process due to the low austenite transformation temperature, because the increase in manganese content can significantly reduce the austenite transformation temperature. During the heating process, carbon and manganese tend to segregate at the grain boundaries, further reducing the austenite transformation temperature.
From the experimental results, it can be seen that 1050 °C is composed entirely of small grains, at 1100 °C to 1150 °C it is composed of a mixture of large and small grains, while grain sizes above 1200 °C are relatively large. Moreover, an increase in grain boundary width indicates damage to the grain boundary when the heating temperature exceeds 1200 °C. The heating temperature also affects processing difficulty due to low temperature and high deformation resistance, and it is not possible to choose a temperature that generates very small grains. In summary, the most suitable heating temperature for the medium manganese steel in this paper is from 1100 °C to 1150 °C.

5. Conclusions

(1).
The average austenite grain sizes of the specimen isothermally held at 1050 °C, 1100 °C, 1150 °C, 1200 °C, and 1250 °C for 600 s are 3.2, 19.8, 37.7, 53.4, and 52.7 μm, respectively. The average austenite grain sizes change from about 3 μm at 1050 °C to over 50 μm at 1250°C.
(2).
When the grain boundary is a small-angle grain boundary, one grain boundary will split into several dislocations. With the extension of heating time, the lattice orientation difference further decreases, and the remaining dislocations may merge into new grain boundaries. When the grain boundary is a large-angle grain boundary, only grain boundary movement can occur.
(3).
The weight fraction of precipitates below 1000 °C is about 0.065%, and precipitates gradually dissolve from about 1000 °C to 1200 °C. A portion of the precipitates nucleate and grow during the heating and holding stage at a holding temperature of 1050~1150 °C, and the precipitates dissolve completely during heating when the holding temperature exceeds 1200 °C.
(4).
The most suitable heating temperature for the medium manganese steel in this paper is from 1100 °C to 1150 °C, taking into account influences such as grain size, grain boundary damage, and deformation resistance.

Author Contributions

Methodology, C.S.; Software, H.W.; Validation, Y.D.; Formal analysis, Y.D.; Investigation, G.Y., C.S. and X.G.; Resources, G.Y. and C.S.; Data curation, X.G. and H.W.; Writing—original draft, G.Y.; Writing—review & editing, Y.D. and H.W.; Supervision, L.D.; Project administration, L.D.; Funding acquisition, L.D. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully appreciate the financial support from the National High-tech R&D Program (863 Program) [No. 2015AA03A501].

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Guangpeng Yuan, Yu Du, and Cao Sun are employees of Nanjing Iron & Steel Co., Ltd. Authors Xiuhua Gao, Hongyan Wu, and Linxiu Du declare no conflicts of interest.

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Coatings 15 01144 g001
Figure 1. Images of austenite grains isothermal at 1050 °C for 600 s.
Figure 1. Images of austenite grains isothermal at 1050 °C for 600 s.
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Figure 2. Influence of isothermal time on austenite grain size: (a–d) 200 s, 300 s, 400 s, and 600 s for 1100 °C; (e–h) 200 s, 300 s, 400 s, and 600 s for 1150 °C; (i–l) 200 s, 300 s, 400 s, and 600 s for 1200 °C; (m–p) 200 s, 300 s, 400 s, and 600 s for 1250 °C.
Figure 2. Influence of isothermal time on austenite grain size: (a–d) 200 s, 300 s, 400 s, and 600 s for 1100 °C; (e–h) 200 s, 300 s, 400 s, and 600 s for 1150 °C; (i–l) 200 s, 300 s, 400 s, and 600 s for 1200 °C; (m–p) 200 s, 300 s, 400 s, and 600 s for 1250 °C.
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Figure 3. Austenite grain size distribution at 1100 °C and 1150 °C: (a) 1100-200, (b) 1100-300, (c) 1100-400, (d) 1100-600, (e) 1150-200, (f) 1150-300, (g) 1150-400, (h) 1150-600, (i) average grain size.
Figure 3. Austenite grain size distribution at 1100 °C and 1150 °C: (a) 1100-200, (b) 1100-300, (c) 1100-400, (d) 1100-600, (e) 1150-200, (f) 1150-300, (g) 1150-400, (h) 1150-600, (i) average grain size.
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Figure 4. Austenite grain size distribution at 1200 °C and 1250 °C: (a) 1200-200, (b) 1200-300, (c) 1200-400, (d) 1200-600, (e) 1250-200, (f) 1250-300, (g) 1250-400, (h) 1250-600, (i) average grain size.
Figure 4. Austenite grain size distribution at 1200 °C and 1250 °C: (a) 1200-200, (b) 1200-300, (c) 1200-400, (d) 1200-600, (e) 1250-200, (f) 1250-300, (g) 1250-400, (h) 1250-600, (i) average grain size.
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Figure 5. Equilibrium phase zones of the steel calculated by Thermal-Calc: (a) Full range of the steel, (b) Weight fraction range of (Nb,Ti)C precipitates.
Figure 5. Equilibrium phase zones of the steel calculated by Thermal-Calc: (a) Full range of the steel, (b) Weight fraction range of (Nb,Ti)C precipitates.
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Figure 6. Average diameter of precipitates during heating process at different isothermal temperatures: (a) Specimens of all isothermal temperatures, (b) magnification at 1200 °C and 1250 °C.
Figure 6. Average diameter of precipitates during heating process at different isothermal temperatures: (a) Specimens of all isothermal temperatures, (b) magnification at 1200 °C and 1250 °C.
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Figure 7. The schematic diagrams of two grain growth mechanisms: (a) Multiple grains merge into one grain, (b) One grain annexes another grain.
Figure 7. The schematic diagrams of two grain growth mechanisms: (a) Multiple grains merge into one grain, (b) One grain annexes another grain.
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Figure 8. Austenite grain growth at 1200 °C with the extension of isothermal time. (a) 200 s, (b) 300 s, (c) 400 s.
Figure 8. Austenite grain growth at 1200 °C with the extension of isothermal time. (a) 200 s, (b) 300 s, (c) 400 s.
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Table 1. Chemical composition of the experimental steel (wt.%).
Table 1. Chemical composition of the experimental steel (wt.%).
CSiMnPSNbTi
0.080.254.520.0110.00190.0180.036
Table 2. Average austenite grain size of different specimens (μm).
Table 2. Average austenite grain size of different specimens (μm).
200 s300 s400 s600 s
1100 °C11.3 ± 0.510.3 ± 0.911.8 ± 0.619.8 ± 1.2
1150 °C25.9 ± 1.741.4 ± 2.231.1 ± 1.137.7 ± 1.2
1200 °C34.9 ± 1.549.2 ± 3.141.6 ± 2.153.4 ± 2.7
1250 °C55.3 ± 2.948.1 ± 2.553.7 ± 2.352.7 ± 4.1
Table 3. The diameter frequency and area percentage of the grain size larger than 80 μm (%).
Table 3. The diameter frequency and area percentage of the grain size larger than 80 μm (%).
200 s300 s400 s600 s
Diameter
Frequency		Area
Percentage		Diameter
Frequency		Area
Percentage		Diameter
Frequency		Area
Percentage		Diameter
Frequency		Area
Percentage
1100 °C0.00.00.00.02.722.62.011.0
1150 °C0.00.06.928.05.331.113.349.7
1200 °C21.353.719.051.116.049.317.546.5
1250 °C17.541.120.053.522.553.321.351.8
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MDPI and ACS Style

Yuan, G.; Du, Y.; Sun, C.; Gao, X.; Wu, H.; Du, L. In Situ Observation of the Austenite Grains Growth Behavior in the Austenitizing Process of Nb–Ti Micro-Alloyed Medium Manganese Steel. Coatings 2025, 15, 1144. https://doi.org/10.3390/coatings15101144

AMA Style

Yuan G, Du Y, Sun C, Gao X, Wu H, Du L. In Situ Observation of the Austenite Grains Growth Behavior in the Austenitizing Process of Nb–Ti Micro-Alloyed Medium Manganese Steel. Coatings. 2025; 15(10):1144. https://doi.org/10.3390/coatings15101144

Chicago/Turabian Style

Yuan, Guangpeng, Yu Du, Chao Sun, Xiuhua Gao, Hongyan Wu, and Linxiu Du. 2025. "In Situ Observation of the Austenite Grains Growth Behavior in the Austenitizing Process of Nb–Ti Micro-Alloyed Medium Manganese Steel" Coatings 15, no. 10: 1144. https://doi.org/10.3390/coatings15101144

APA Style

Yuan, G., Du, Y., Sun, C., Gao, X., Wu, H., & Du, L. (2025). In Situ Observation of the Austenite Grains Growth Behavior in the Austenitizing Process of Nb–Ti Micro-Alloyed Medium Manganese Steel. Coatings, 15(10), 1144. https://doi.org/10.3390/coatings15101144

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