Effect of Nb Content on Strength and Toughness of 25MnB Crawler Steel and Its Microstructural Characterization

Abstract

To address the failure of 25MnB crawler track steel due to insufficient strength and toughness, this study designed a new type of 25MnB crawler track steel and investigated the effects of the Nb content on its mechanical properties, microstructure, dislocation evolution, and precipitation behavior. The experimental results show that, compared to 25MnB steel without Nb, the addition of 0.03% Nb significantly increased the yield strength of the experimental steel to 1450 MPa, with an elongation rate of 15.9%. The room-temperature impact energy showed a slight improvement, while the low-temperature impact absorption energy improved notably, reaching 36 J at −40 °C. The microstructural characterization and analysis revealed that increasing the Nb content refined the effective grain size of prior austenite and martensite. The (Nb, Ti)C precipitates provided a higher precipitation driving force, promoting the formation of precipitates and increasing the average dislocation density. Additionally, the increased proportion of high-angle grain boundaries (HAGBs) enabled grain boundaries and precipitates to jointly hinder crack propagation. This study demonstrates that Nb-Ti microalloying can effectively enhance the strength, toughness, and plasticity of track steel, demonstrating promising application prospects.

1. Introduction

Microalloyed steels represent an advanced class of high-strength materials derived from low-alloy high-strength steels [1]. By adding trace amounts of alloying elements, such as Nb, V, and Ti, the properties of steel can be significantly improved. These elements react with C and N to form fine carbides, nitrides, or carbonitrides, improving grain refinement, solid-solution strengthening, and precipitation strengthening, thereby enhancing the steel’s overall properties. Titanium (Ti) is a widely used element in microalloying, recognized for its strong affinity with N, C, and O. It combines with these elements to form finely dispersed carbonitrides in the matrix, contributing to both precipitation and grain refinement strengthening. Niobium (Nb) enhances the steel strength through precipitation strengthening, particularly affecting grain refinement. Additionally, the synergistic effect of Nb with other microalloying elements further enhances the steel’s performance. Nb combines with carbon (C) to form NbC, which consumes carbon at the grain boundaries, reducing the nucleation driving force for austenite formation [2,3]. In low-carbon bainitic steels, Nb atom segregation hinders the growth of austenite grains [4]. Nb(C, N) precipitates increase the incubation period for austenite recrystallization, delaying austenite growth and refining its grain size [5]. Furthermore, Nb atoms can dissolve in the iron matrix, causing lattice distortion and impeding dislocation movement, contributing to solid-solution strengthening [6]. NbC precipitates nucleate along dislocation lines, further hindering the dislocation motion. The dislocation density in Nb-containing steels is significantly higher than in steels without Nb [7]. The high dislocation density provides more nucleation sites for precipitates, increasing the number of NbC precipitates [8].

Crawler track steel is a crucial material in the construction machinery industry, particularly for large machinery that relies on tracked drives to traverse complex terrains. Therefore, crawler track steel must demonstrate both exceptional strength and toughness. Traditional crawler track steels, such as 22MnB, achieve high-strength martensitic microstructures through alloying, hot-forming, austenitizing, and quenching–tempering. The tensile strength of these steels typically exceeds 1000 MPa [9]. Building on 22MnB, the development of 25MnB crawler track steel through Ti microalloying treatment allows it to achieve a tensile strength of up to 1500 MPa after quenching and tempering [10,11]. Feng et al. [12] improved the impact energy absorption of 25MnB steel through Cr alloying treatment, while maintaining the same tensile strength. Zhu et al. [13] found that quenching at 840–900 °C and tempering at 200 °C could provide 30MnB steel with a good balance of strength and toughness.

As the material strength increases, the ductility and toughness often decrease, with the strength–ductility mismatch being a primary cause of premature failure in engineering components during service. To enhance the strength of 25MnB steel while improving its ductility and toughness, this study uses Nb-Ti composite microalloying for the composition and process design. The effect of the trace Nb addition on the microstructure and mechanical properties of 25MnB crawler track steel is investigated. Through experimental analysis and theoretical calculations, we clarify the mechanisms behind changes in its microstructure and mechanical properties due to different Nb addition levels.

2. Materials and Experimental Procedure

2.1. Materials and Thermo-Mechanical Controlled Processing

This study focuses on 25MnB crawler track steel with the addition of Nb to create Nb-Ti composite microalloyed steel. The chemical composition of the studied crawler track steels is shown in

Table 1. The chemical compositions of the investigated steels (mass fraction/%).

	C	Si	Mn	P	S	B	Nb	Cr	Ti	Al
0 Nb	0.26	0.27	0.96	0.002	0.003	0.0021	—	0.23	0.02	0.022
0.01 Nb	0.26	0.27	0.97	0.002	0.003	0.0023	0.011	0.23	0.02	0.021
0.03 Nb	0.26	0.27	0.97	0.002	0.003	0.0023	0.033	0.23	0.02	0.021

. According to the national standard GB/T 3077-2015 [14] for alloy-structure steels, the Chinese standard for 25MnB track steel is shown in

Table 2. Mechanical properties of 25MnB steel according to the standard.

	UTS/MPa	Rt0.2/MPa	EL/%	Hard-Ness/HBW	RA/%	25 °C Impact Energy/J
25MnB	≥835	≥635	≥10	≤207	≥45	≥47

. The track steel used in this study meets the requirements of this standard. The experimental steels were categorized into two groups based on Nb content: a control group without Nb (0 Nb steel) and experimental groups with 0.01% Nb and 0.03% Nb (0.01 Nb and 0.03 Nb steels). The experimental samples were melted in a 200 kg vacuum induction furnace and cast into ingots using a vacuum pouring process. The ingots were then heated to 1200 °C, with the initial rolling temperature set at 1150 °C. After 10 rolling passes, the final pass was completed at 950 °C, and the material was cooled using laminar flow cooling to produce hot-rolled plates with a thickness of 17 mm.

The hot-rolled plates were cut into heat treatment samples measuring 250 × 150 × 10 mm using wire cutting. We used JMatPro API V7.0 software to calculate the Ac₃ temperature as 807 °C. To ensure complete austenitization, we selected a heat treatment temperature 50 °C higher than the Ac3 curve. The heat treatment process was as follows: heating to 860 °C with a 40 min holding time, followed by water quenching, tempering at 215 °C for 1 h, and then air cooling to room temperature. The heat treatment process is illustrated in Figure 1.

2.2. Microstructure Characterization

After grinding, polishing, and etching in a 4% nitric acid alcohol solution, the samples were examined for the microstructure using a BMM-420 (Shanghai Batuo Instrument Co., Ltd., Shanghai, China) metallographic microscope (OM) and an image acquisition system. Fine microstructure analysis was performed using a Gemmini SEM 300 (Carl Zeiss Company/Oxford Company, Oxford, UK) scanning electron microscope (SEM) coupled with an EDS energy-dispersive spectroscopy system at an accelerating voltage of 15 kV. Electron back scatter diffraction (EBSD) was also employed for grain boundary analysis. EBSD samples were prepared by electrolytic polishing in a 10% perchloric acid alcohol solution at −20 °C, with a scanning step size of 0.18 μm. The data were analyzed using Aztec Crystal 2.1.2 software. Precipitate analysis was conducted using an FEI Talos F200X (Thermo Fisher Scientific, Inc., Waltham, MA, USA) transmission electron microscope (TEM) at 200 kV. The samples were prepared using a focused ion beam to a size of 5 × 8 μm.

2.3. Mechanical Property Tests

The impact test was performed using the Charpy impact testing method. The impact samples measured 10 × 10 × 55 mm, with a 2 × 2 mm V-shaped notch on each sample. The tests were carried out using a JB-300B metal pendulum impact testing machine (Jinan Zhongzheng Testing Machine Manufacturing Co., Ltd., Jinan, China), equipped with a 150 J pendulum. Three impact tests were performed for each heat treatment process, and the average of the three tests was taken as the final result. Tensile tests were conducted using an AG-20I universal material testing machine (Changchun Institute of Machinery Science Co., Ltd., Changchun, China) with a strain rate of 2 mm/min during the experiment. The dimensions and testing procedure for the tensile samples strictly adhered to ASTM standards, with the final result being the average of three tests. The scale bar size is 30 mm. Hardness testing was carried out using an HRS-150 A Rockwell hardness tester (Tianjin Shunuo Instrument Technology Co., Ltd., Tianjin, China) with a diamond indenter, a load of 150 kg, and a holding time of 15 s. Ten points were measured per test and the average value was calculated. The wear resistance test was performed using an MPS-1G high-speed pin-on-disk friction and wear tester (Jinan Shenggong Testing Machine Co., Ltd., Jinan, China). The ball–disk friction pair test was conducted with a load of 50 N and a wear time of 3600 s. The surfaces were cleaned using ultrasound and alcohol before and after the test.

3. Results

3.1. Microstructure

Figure 2a–c illustrate the hot-rolled metallographic microstructures of crawler track steels with varying Nb contents. The microstructures of all three steels primarily consist of ferrite and pearlite. The 0 Nb, 0.01 Nb, and 0.03 Nb steels were measured by ImageJ 2.4 software. The average ferrite grain sizes’ values of 22.3 μm, 18.9 μm, and 17.5 μm correspond to refinement rates of 15.2% and 21.5%. These results indicate that Nb addition effectively refined the grain size of the hot-rolled microstructure. Figure 2d–f illustrate the microstructures after heat treatment. The results indicate that the microstructure after heat treatment consists of tempered martensite, appearing as elongated, plate-like, or needle-shaped structures. These plates are arranged in a crossed pattern. Under SEM observation in Figure 2g–i, with the addition of Nb, the width of the martensite further refines and the distribution of the martensite block structures becomes denser.

EBSD analysis was performed on the three types of steel to further quantify the phase content and examine the microstructural characteristics. Figure 3a–c display the grain boundary (GB) angle map, band contrast (BC), and composition. Grains with boundary angles between 10° and 50° were identified as prior austenite grains using Aztec Crystal and ImageJ software. The prior austenite grain sizes for the 0 Nb, 0.01 Nb, and 0.03 Nb steels were 15.16 μm, 12.13 μm, and 11.05 μm, respectively. The refinement rates of prior austenite grains in the 0.01 Nb and 0.03 Nb steels, compared to the 0 Nb steel, were 20% and 27.1%, respectively. Figure 3d–f show the inverse pole figure (IPF) and {100} pole figure (PF). The experimental data indicate that martensite does not exhibit a preferred orientation, displaying anisotropy with similar crystal properties in all directions. Using Aztec Crystal software, pixel point statistics were performed on the EBSD images, which allows for the calculation of the martensite phase grain size. The martensite grain sizes for the 0 Nb, 0.01 Nb, and 0.03 Nb steels were 2.84 μm, 1.71 μm, and 1.65 μm, respectively. The refinement rates of martensite grains in the 0.01 Nb and 0.03 Nb steels, compared to the 0 Nb steel, were 39.8% and 41.9%, respectively. The results indicate that Nb addition significantly refines the austenite grain size. During heat treatment, the smaller the original austenite grain size, the finer the martensitic structure formed after cooling and transformation.

3.2. Mechanical Properties

Table 3. Summary of mechanical properties.

	UTS/MPa	Rt0.2/MPa	EL/%	Hard-Ness/HRC	Wear Loss/mg	25 °C Impact Energy/J	0 °C Impact Energy /J	−20 °C Impact Energy /J	−40 °C Impact Energy /J
0 Nb	1515	1300	13	55.2	6.8	47	40	34	28
0.01 Nb	1650	1431	16.1	58.8	4.5	52	48	46	35
0.03 Nb	1661	1450	15.9	60.2	4.2	54	50	44	36

presents the results of the tensile tests, impact tests, wear tests, and hardness measurements for the 0 Nb, 0.01 Nb, and 0.03 Nb steels. Figure 4a illustrates the stress–strain curves for the three steels. The experimental results indicate that, compared to the 0 Nb steel, the yield strength of the 0.01 Nb and 0.03 Nb steels increased by approximately 9.2% and 9.9%, respectively. The tensile strength increased by 8.2% and 8.8%, and elongation improved by 19.3% and 18.2%, respectively. This indicates that the addition of Nb significantly enhanced the strength of 25MnB crawler track steel while also improving its plasticity. Figure 4b presents the ductile-to-brittle transition temperature (DBTT) curves for the three steels. The experimental results indicate that, compared to the 0 Nb steel, the impact absorption energy of the 0.01 Nb and 0.03 Nb steels increases with the Nb content, showing a particularly excellent low-temperature performance. The fitting calculation was performed using the Boltzmann function, as shown in the following formula:

$$E=E_l+{\displaystyle \frac{E_h-E_l}{1+e^{\left(T-T_0\right)dT}}}$$

(1)

In the formula, E represents the ductile-to-brittle transition energy, E_h and E_l are the maximum and minimum impact energies, respectively, T₀ is the transition temperature, and dT is the slope parameter of the transition zone. Based on the fitting calculation results, we found that the DBTT for the 0 Nb, 0.01 Nb, and 0.03 Nb steels are −50 °C, −55 °C, and −57 °C, respectively, which significantly improved the performance of the track steel under low-temperature conditions.

Table 3 also presents the hardness and wear resistance performance of the three steels. The results indicate that, compared to the 0 Nb steel, the hardness of the 0.01 Nb and 0.03 Nb steels increased by 6.1% and 8.3%, respectively, while the wear rate decreased by 33.9% and 38.2%, respectively. Wear resistance is generally linked to both hardness and toughness. The addition of Nb significantly enhanced both the hardness and impact toughness of the 0.01 Nb and 0.03 Nb steels, leading to a greater than 30% improvement in the wear resistance.

Figure 5a–c show the wear tracks of three different microalloyed steels. The surface of the 25MnB base material exhibits typical adhesive wear features, including a large number of blocky spallation fragments and separated pits, indicating insufficient resistance to plastic deformation, with localized material tearing occurring under the action of frictional shear forces. In contrast, the wear surface of the 0.03% Nb steel, while still exhibiting some debris accumulation, does not show obvious separated pits. This is due to the dispersed distribution of the hard NbC phase, which delays the overall detachment of the surface material and transitions the wear process from severe spallation to a progressive micro-cutting process, thereby significantly reducing wear loss. The energy-dispersive X-ray spectroscopy (EDS) analysis in Figure 5d shows that, in addition to the Fe matrix, there is a significant enrichment of O and Nb on the wear surface of the 0.03% Nb steel. The distribution of Nb aligns with the characteristic absence of separated pits in the microstructure, confirming that NbC has delayed material detachment by hindering crack propagation. The enrichment of O suggests the presence of an oxidative wear mechanism, with localized oxide films possibly contributing to the debris formation.

Figure 6a–c presents SEM images of the fracture surfaces under impact testing at room temperature for steels with varying Nb contents. The fracture modes of the three steels are predominantly ductile, with some characteristics of a brittle fracture. The fracture surface of 0 Nb steel exhibits numerous smooth cleavage facets, a typical feature of cleavage fracture. Additionally, the surface displays various sizes of dimples and tear ridges, indicating resistance to crack propagation. Compared to 0 Nb steel, both 0.01 Nb and 0.03 Nb steels show an increase in the number of dimples, a decrease in their size, and a higher density, thus demonstrating superior impact toughness. At −20 °C, the impact energy of 0 Nb steel decreases significantly. The fracture images in Figure 6d–f show a marked increase in cleavage facets and a sharp reduction in dimples, leading to embrittlement at low temperatures and ineffective crack propagation resistance, which results in a brittle fracture. However, at low temperatures, both the 0.01 Nb and 0.03 Nb steels maintain a higher number of dimples, despite a reduction in their quantity and density. Based on the data in Figure 4b, it can be concluded that Nb elements effectively delay the low-temperature brittleness transition in steels.

4. Analysis and Discussion

4.1. Effect of Nb Content on the Precipitation Behavior of Microalloying Elements

Figure 7a,c,e show the TEM images of precipitates in steels with different Nb contents after quenching at 860 °C and tempering at 215 °C. The corresponding EDS spectra in Figure 7b,d,f indicate that the precipitates in the 0 Nb steel consist of TiC particles, which have a rectangular shape and range in size from 7 to 30 nm. In the Nb-containing steels, the precipitates are (Nb, Ti)C, exhibiting both rectangular and spherical morphologies, with particle sizes ranging from 4 to 30 nm. No Mn or Fe elements were detected in the precipitates. After the addition of Nb, the number of precipitates increased and their size slightly decreased. High-resolution TEM images and diffraction patterns in Figure 7g,h show that the (Nb, Ti)C precipitates (Figure 7e yellow circle) have a face-centered cubic (FCC) structure, with (200) interplanar spacing of 0.2366 nm. The calculated lattice constant is 0.4732 nm, which is similar to the theoretical value [15].

The precipitates in 25MnB crawler track steel can be classified into two types: The first type consists of particles typically smaller than 20 nm, which nucleate at the γ/α phase boundaries and dislocation lines, enhancing the yield strength through the Orowan mechanism. Furthermore, these nanoscale precipitates exhibit a semi-coherent relationship with the matrix, creating strong interfacial bonding that effectively inhibits crack initiation and propagation, significantly improving the plasticity and toughness of the microalloyed steel [16]. The second type of precipitates has sizes ranging from 20 to 100 nm and primarily forms during the rolling deformation process. Due to their higher nucleation temperature, large diffusion coefficient, and high diffusion efficiency, these particles tend to coarsen to some extent during subsequent cooling. However, they still play a significant role in refining the austenite grain size and enhancing the overall mechanical properties of microalloyed steel [17].

Research has demonstrated that Nb refines the prior austenite grain size, primarily because the precipitates formed by Nb addition exhibit a small lattice mismatch with Fe (the lattice constant of (Nb, Ti)C is 0.4732 nm). The reduction in lattice mismatches helps to reduce the nucleation barrier for precipitates, leading to a high density and fine-sized nanoscale precipitates [18]. When Nb is added as an alloying element to iron-based materials, it replaces some positions in the iron lattice. Due to the size difference, this substitution causes local lattice distortion, which affects the precipitation of carbonitrides in austenite. It may also delay the austenite transformation through the solute drag effect, thus hindering the growth of austenite grains [19]. Nanoscale precipitates effectively impede the dislocation motion, contributing to precipitation strengthening. The degree of precipitation strengthening primarily depends on the size and volume fraction of the nanoscale precipitates [18]. The solubility and volume fraction of (Nb, Ti)C in γ-Fe can be calculated using the following formula [15,20]:

$${lg\left({\displaystyle \frac{\left[Nb\right]·\left[c\right]}{x}}\right)}_{\gamma }=2.96-7510/T$$

(2)

$${lg\left({\displaystyle \frac{\left[Ti\right]·\left[c\right]}{1-x}}\right)}_{\gamma }=2.75-7000/T$$

(3)

$${\displaystyle \frac{Nb-\left[Nb\right]}{c-\left[c\right]}}={\displaystyle \frac{{Nb}_m·x}{C_m}}$$

(4)

$${\displaystyle \frac{Ti-\left[Ti\right]}{c-\left[c\right]}}={\displaystyle \frac{{Ti}_m·\left(1-x\right)}{C_m}}$$

(5)

In the formulas, T denotes the temperature (K), while (Nb, Ti, C) represent the weight percentages of the elements (wt%) in the steel. The terms ([Nb], [Ti], [C]) indicate the solid solubility concentrations, and (Nb_m, Ti_m, C_m) correspond to the atomic masses. Using numerical iteration methods in Matlab, the solid solubility at 950 °C for the 0.01 Nb steel was obtained as [Nb] = 0.00121, [Ti] = 0.00246, [C] = 0.11334, x = 0.34129, and for the 0.03 Nb steel, it was [Nb] = 0.01025, [Ti] = 0.00632, [C] = 0.12398, x = 0.62129. The volume fraction and solubility product formulas for (Nb, Ti)C are provided in Equations (5) and (6) [15,20].

$${ƒ}_v=\left(Nb-\left[Nb\right]+Ti-\left[Ti\right]\right)\cdot {\displaystyle \frac{x{A}_{Nb}+\left(1-x\right)A_{Ti}+A_C}{x{A}_{Nb}+\left(1-x\right)A_{Ti}}}·{\displaystyle \frac{{\rho }_{Fe}}{100\left[x{\rho }_{NbC}+\left(1-x\right){\rho }_{TiC}\right]}}$$

(6)

$$\begin{array}{c}{\mathit{lg}{(\left[Nb\right]}_x{\left[Ti\right]}_{\left(1-x\right)}\left[C\right])}_{\gamma }=x\left(2.96+\mathit{lgx}\right)+\left(1-x\right)\left[2.75+\mathit{lg}\left(1-x\right)\right]-7510x+7000(1-x)/T\\ =A^{\prime }-B^{\prime }/T\end{array}$$

(7)

(Nb, Ti, C) refer to the weight percentages of the elements (wt%) added to the steel, while ([Nb], [Ti], [C]) denote the solid solubility concentrations. (A_Nb, A_Ti, A_C) represent the molecular masses of the solute elements and carbon, ρ_Fe and ρ_MC refer to the densities of iron and the solid solution, and ƒ_v denotes the volume fraction. Based on Equations (1)–(5), the theoretical volume fractions of TiC and (Nb, Ti)C during deformation in austenite at 950 °C were calculated as ƒ_TiC = 0.031%, ƒ_0.01Nb = 0.038%, and ƒ_0.03Nb = 0.041%. By simplifying the solubility product of (Nb, Ti)C in Equation (6), the phase transformation free energy during the cooling process can be calculated using Equation (7).

$${∆G}_{\left(Nb,Ti\right)C}=-19.1446B^{\prime }+19.1446T\left\{A^{\prime }-\mathit{lg}({\left[Nb\right]}_x[{Ti]}_{\left(1-x\right)})\right\}$$

(8)

Figure 8a shows the volume fractions of various precipitates. The results indicate that the volume fraction of each type decreases as the temperature increases. The volume fraction of (Nb, Ti)C precipitation is significantly higher than that of NbC and TiC alone, which indicates that more Nb and Ti elements combine with carbon to precipitate, leading to an increase in the number of precipitates. Figure 8b shows the phase transformation free energy for the three types of precipitates. The results indicate that the absolute value of the phase transformation free energy increases with the temperature. The absolute phase transformation free energy for the 0.01 Nb and 0.03 Nb steels is higher than that for the 0 Nb steel, and increases further with a higher Nb content. Therefore, the 0.01 Nb and 0.03 Nb steels have a greater precipitation driving force, making it easier for precipitation to occur during the cooling phase’s transformation process. Nb-Ti composite microalloying exhibits a higher level of carbide-phase transformation free energy than single microalloying. This promotes precipitate formation during cooling, increasing the volume fraction and number of precipitates, and significantly enhancing precipitation strengthening.

4.2. The Effect of Nb Content on Dislocations

Figure 9a–d shows the geometrically necessary dislocation density (GND density) for steels with different Nb contents. The results indicate that the average dislocation densities of the 0 Nb, 0.01 Nb, and 0.03 Nb steels are 10.04 × 10¹⁴ m², 11.66 × 10¹⁴ m², and 11.74 × 10¹⁴ m², respectively. The dislocation density of the 0.01 Nb and 0.03 Nb steels is 13.9% and 14.5% higher than that of the 0 Nb steel, respectively. Figure 9e,f presents the dislocation and carbide morphology in the martensite laths of the 0 Nb and 0.01 Nb steels, where the light areas represent the martensite laths and the dark areas indicate dislocation pile-ups. The results indicate that carbides mainly precipitate at grain boundaries and dislocation lines. The 0.01 Nb and 0.03 Nb steels exhibit more dislocation pile-up areas and finer martensite lath widths compared to the 0 Nb steel.

During the martensitic transformation, Nb and C atoms dissolve into the iron matrix in a supersaturated state, contributing to solid solution strengthening. As cooling progresses, the precipitated (Nb, Ti)C particles effectively impede the dislocation motion and coalescence. After tempering, these supersaturated Nb atoms and (Nb, Ti)C particles tend to accumulate near dislocation lines, resulting in segregation. This segregation further impedes dislocation slip during material deformation. Dislocations that can no longer move accumulate in localized areas, causing dislocation pile-ups and a stress concentration, which significantly increase the material’s strength. When dislocations attempt to bypass these carbide particles or segregated carbon atom clusters, they require more energy. When the stress reaches a critical value, dislocations bypass the carbides, leaving behind dislocation loops and generating new dislocations [21,22]. The interaction and accumulation of dislocations increase their resistance to movement, thereby enhancing the material’s yield strength and hardness.

On the one hand, the addition of Nb promotes the formation of clusters of Nb, C, and N atoms, which form precipitates in steel, impeding the dislocation movement and causing dislocation pile-ups. On the other hand, solute Nb atoms strongly attract dislocations, with an interaction energy of about 42.4 ± 0.7 kJ/mol, which is higher than that of other elements [23], drawing more dislocations to the pinned regions. Solute Nb also reduces dislocation line energy, slowing its movement during recovery [24]. When the Nb content reaches 0.08%, solute Nb completely inhibits the dislocation motion during recovery, significantly improving strength while reducing ductility [25]. As a result, compared to 0 Nb steel, Nb-containing steel shows an increase in both the number and volume fraction of precipitates, along with a higher number of pinned regions. Solute Nb attracts more dislocations to accumulate in these regions, resulting in an increase in the average dislocation density with a higher Nb content.

4.3. The Effect of Nb Content on Grain Boundary Characteristics

Figure 10 displays the EBSD grain boundary orientation and distribution maps of the martensitic structures in 25MnB crawler track steel after heat treatment. To differentiate the grain boundary angles, the band contrast (BC) image is overlaid with the grain boundary (GB) image. In this map, red lines represent low-angle grain boundaries (LAGBs) with angles between 2° and 15°, black lines represent high-angle grain boundaries (HAGBs) with angles between 15° and 45°, and blue lines indicate HAGBs greater than 45°. Figure 10a–c present the distribution and proportion of grain boundary angles. The results indicate that the fraction of low-angle grain boundaries (LAGBs) in 0.01 Nb and 0.03 Nb steels is 37.3% and 36.3%, while the fraction of high-angle grain boundaries (HAGBs) is 62.72% and 63.66%, respectively. Compared to the 0 Nb steel, the fraction of low-angle grain boundaries in the 0.01 Nb and 0.03 Nb steels decreased by 5.7% and 6.7%, while the fraction of high-angle grain boundaries increased by 5.72% and 6.66%, respectively. Furthermore, the fraction of high-angle grain boundaries greater than 45° increased by 6.49% and 4.68%. Regarding the influence of grain boundary angles, research by Lan et al. [26] suggests that HAGBs can effectively block and deflect brittle cracks during crack propagation, whereas LAGBs are less effective at hindering crack growth. Gourgues et al. [27] pointed out that when the grain boundary misorientation exceeds 45°, its resistance to crack propagation is significantly increased. Numerous studies consistently agree that HAGBs play a crucial role in blocking and deflecting cracks. The addition of trace amounts of Nb has altered the grain boundary distribution characteristics, particularly by increasing the proportion of HAGBs in Nb-containing steels. Notably, the proportion of HAGBs greater than 45° has significantly increased, which plays an important role in enhancing the toughness of the material.

In impact tests, the region near the fracture site experiences a stress concentration due to intense internal stresses. This stress concentration phenomenon promotes the formation of crack initiation sites and further drives the propagation of secondary cracks. Figure 11 illustrates two distinct modes of secondary crack propagation. In Figure 11a,c, the crack propagates across grain boundaries. The yellow line represents the trajectory of the secondary crack, while the red region indicates the crack deflection zone. When the crack crosses a high-angle grain boundary (HAGB), the difference in the crystallographic orientation between the adjacent grains requires additional energy for the crack to traverse the boundary. Due to the large misorientation angle, the crack energy dissipates, causing part of the energy to spread and redirect the propagation path from the slip plane of one grain to that of the adjacent grain. This repeated process significantly disperses the crack propagation energy, leading to the premature arrest of the crack. In Figure 11b,d, the crack encounters precipitates. The yellow circles highlight (Nb, Ti)C particles, which are surrounded by numerous pinned dislocations. The dislocation pile-ups generate a strong stress field that interacts with the stress field at the crack tip, either attracting or repelling the crack and causing its propagation direction to deviate. As the crack bypasses these particles, its energy continuously dissipates, ultimately causing the crack to arrest before reaching its original endpoint.

4.4. Strengthening Mechanisms of Nb-Ti Microalloying

An analysis of Table 3 indicates that, after the same heat treatment, the tensile and yield strengths of the Nb-Ti microalloyed 0.01 Nb steel are significantly higher than those of the original 0 Nb steel. The yield strength of martensitic steel can be expressed using the following formula: [28,29].

$${\sigma }_y={\sigma }_0+{\sigma }_{ss}+{\sigma }_{pcpt}+{\sigma }_{\rho }+{\sigma }_g$$

(9)

σ₀ = 54 MPa represents the lattice friction stress of pure Fe [30], while σ_ss refers to solid solution strengthening, σₚ_cpt to precipitation strengthening, σ_ρ to dislocation strengthening, and σ_g to grain boundary strengthening based on the Hall–Petch effect.

According to Yong’s study [15], the contribution of solid solution strengthening σ_ss is given by the following formula:

$${\sigma }_{ss}=4570\left[C\right]+4570\left[N\right]+470\left[P\right]+83\left[Si\right]+37\left[Mn\right]+80[Nb+Ti]-30\left[Cr\right]$$

(10)

[X] represents the percentage of the alloying elements dissolved in steel. It is generally considered that alloying elements such as Si, Mn, and Cr are completely dissolved in steel. Hutchinson [29] and others have pointed out that the carbon content dissolved in martensite is very low, as carbon atoms tend to segregate at dislocations and plate boundaries, enhancing martensite strength through precipitation effects. Carbon tends to precipitate again during tempering, and as a result, the influence of carbon is typically excluded when calculating solid solution strengthening in martensite. Substituting the solid solution amounts into Equation (9), the solid solution strengthening values were calculated: the 0 Nb, 0.01 Nb, and 0.03 Nb steels exhibited strengthening values of 52 MPa, 52 MPa, and 53 MPa, respectively. The increase in the solid solution strengthening Δσ_ss for Nb-containing steels was 0 MPa for the 0.01 Nb steel and 1 MPa for the 0.03 Nb steel. The results indicate that solid solution strengthening has a minimal effect on the strength of quenched martensitic steels.

Precipitation strengthening σ_pcpt can be calculated using the Ashby–Orowan equation [30,31]:

$${\sigma }_{pcpt}=\left(0.538Gb\sqrt{{ƒ}_v}/X\right)\mathit{ln}\left(X/2b\right)$$

(11)

G = 80 GPa is the shear modulus [32], b = 0.248 is the Burgers vector, ƒᵥ represents the volume fraction of precipitates, and X denotes the average diameter of the precipitates. According to the Orowan precipitation strengthening theory, only precipitates smaller than 20 nm effectively contribute to strengthening [33]. The average precipitate diameter was measured as 10 nm by TEM images. The volume fractions of ƒ_TiC, ƒ_0.01Nb, and ƒ_0.03Nb are 0.031%, 0.038%, and 0.041%, respectively. The precipitation strengthening was calculated as follows: the strengthening values for 0 Nb, 0.01 Nb, and 0.03 Nb steels were 564 MPa, 625 MPa, and 649 MPa, respectively. The increase in precipitation strengthening Δσ_pcpt is 61 MPa and 85 MPa, respectively, accounting for a significant portion of the yield strength improvement. Dislocation strengthening σ_ρ can be calculated using the following equation [34,35]:

$${\sigma }_{\rho }=M\alpha Gb\sqrt{\rho }$$

(12)

For BCC metals, M = 3 is the Taylor factor; for high-dislocation-density martensite, α = 0.3 is the strengthening constant [30]; G = 80 GPa is the shear modulus [32]; b = 0.248 nm is the Burgers vector; and ρ is the dislocation density. The average dislocation density is provided in Section 4.3. The precipitation strengthening was calculated as follows: the strengthening values for the 0 Nb, 0.01 Nb, and 0.03 Nb steels were 565 MPa, 610 MPa, and 612 MPa, respectively. The dislocation strengthening increments Δσ_ρ were 45 MPa and 47 MPa, respectively, which are another major contributor to the yield strength improvement.

After the addition of Nb, the martensitic grain size is refined. The grain refinement strengthening σ_g can be calculated using the Hall–Petch equation [36,37].

$${\sigma }_g=k_y{d}^{-1/2}$$

(13)

where d is the average grain size and k_y is a constant. Due to tempering precipitation, the solubility of carbon in the martensitic lath is very low, and thus k_y = 0.12 MPa·m^1/2 [24]. Quenched martensitic laths possess a substructure composed of martensite blocks and packets. Therefore, Daigne et al. [37] suggested that the effective grain size for grain refinement strengthening should be based on the width of martensitic laths or the packet size. Wang et al. [38] experimentally verified that the strength increment derived from the prior austenite grain size and the effective martensitic grain size was nearly identical, indicating that either grain size can serve as the basis for grain refinement strengthening. The grain refinement strengthening values for the 0 Nb, 0.01 Nb, and 0.03 Nb steels are 71 MPa, 92 MPa, and 93 MPa, respectively. The contribution of Nb to grain refinement strengthening Δσ_g is 21 MPa and 22 MPa, respectively.

Table 4. Summary and comparison of strength increment contributions.

	σ0/MPa	σss/MPa	σpcpt/MPa	σρ/MPa	σg/MPa	Total/MPa	Measurement /MPa
0 Nb	54	52	564	565	71	1306	1300
0.01 Nb	54	53	625	610	92	1433	1431
0.03 Nb	54	53	649	612	93	1443	1450
△0.01 Nb	0	0	61	45	21	127
△0.03 Nb	0	1	85	47	22	155

and Figure 12 presents the theoretical calculations and experimental measurements of the contributions of various strengthening mechanisms to the yield strength of the three steels. The data indicate a good agreement between the calculated and measured values, effectively reflecting the strengthening mechanisms of the crawler track steels. This study indicates that precipitation strengthening and dislocation strengthening are the primary strengthening mechanisms for 25MnB crawler track steels, and Nb-Ti composite microalloying is an effective approach for developing new types of crawler track steels.

5. Conclusions

This study examines the impact of Nb addition on the microstructure and mechanical properties of 25MnB high-strength crawler track steel. The main conclusions are summarized as follows.

(1) Nb-Ti composite microalloyed steel demonstrates excellent mechanical properties. As the Nb content increases to 0.033%, the tensile strength, yield strength, and elongation reach 1661 MPa, 1450 MPa, and 15.9%, respectively. The impact energy at −40 °C is 36 J and the hardness reaches 60 HRC.

(2) The addition of Nb increases the free energy of phase transformation during cooling, promoting precipitation at dislocations, which leads to dislocation pile-up, impedes dislocation movement, and increases the dislocation density.

(3) The addition of Nb significantly refined the prior austenite grain size and the effective grain size of martensite, while markedly increasing the proportion of high-angle grain boundaries (HAGBs), thereby further enhancing the resistance to crack propagation. The primary modes of crack propagation were influenced by both precipitates and grain boundaries. The crack propagation energy was dissipated by HAGBs and impeded by precipitates, leading to the premature arrest of the crack extension.

(4) The strengthening mechanisms of 25MnB crawler track steel are primarily precipitation strengthening and dislocation strengthening, with the strengthening effect increasing as the Nb content increases.