Effect of Pre-Formed Microstructure on Mechanical Properties of Bainitic Steel

Abstract

Achieving high strength, high toughness, and high plasticity in bainitic steel to meet the needs of economic and industrial development is the direction and driving force for researchers. The reinforcement technology of bainitic steel has become very well developed, with research conducted on microstructure, dislocation density, and alloying elements. While improving strength, some plasticity and toughness are inevitably sacrificed. Therefore, achieving high strength, high plasticity, and high toughness in bainitic steel has always been a research focus. Existing research has shown that pre-transformation before the bainitic transformation can help increase the bainitic transformation rate and thereby regulate the microstructure. This paper will conduct a detailed analysis of the influence of pre-phase transformation microstructure on the strength, toughness and plasticity of bainitic steel.

1. Introduction

Due to the absence of carbide precipitation in the microstructure, carbide-free bainitic steel exhibits superior comprehensive properties compared to traditional carbide-containing bainitic steels. This is mainly related to two composition phases. Retained austenite possesses good plasticity and is distributed between bainitic ferrite, which can effectively passivate the crack tip and hinder crack propagation. At the same time, under the stress field at the crack tip, it can absorb fracture energy through martensitic transformation, which is beneficial for improving the toughness of the microstructure. Bainitic ferrite is the main phase in carbide-free bainitic steel. Current research has shown that the size, dislocation density, and solid solution effect of alloying elements all affect the rheological stress of bainitic ferrite [1]. The formula for the total rheological stress (σ) is shown in (1):

$$\mathsf{\sigma }={\sigma }_{dis}+{\sigma }_{GB}+{\sigma }_{others}$$

(1)

$${\sigma }_{dis}=\mathsf{\alpha }\mathrm{MGb}\sqrt{\rho }$$

(2)

$${\sigma }_{GB}=K_{HP}/\sqrt{t}$$

(3)

where ${\sigma }_{dis}$ represents dislocation strengthening; ${\sigma }_{GB}$ represents grain boundary strengthening; and ${\sigma }_{others}$ represents solid solution strengthening. ${\sigma }_{dis}$ can be calculated according to Taylor’s formula [2], where α represents the Taylor constant, M is the average Taylor factor, b is the elastic modulus, and ρ is the dislocation density. K_HP represents the coefficient of the Hall Patch relationship [3], and t represents the thickness of bainitic ferrite plates.

Through linear fitting, t = A/√ρ + B [1] is obtained. The research indicates that the thickness of bainitic ferrite plates diminishes as the dislocation density increases. This relationship between dislocation density and bainitic ferrite plate thickness is likely attributed to the plastic deformation of austenite during the bainite transformation process. On the one hand, the increase in dislocations during strain adjustment inhibits further growth in the thickness of bainitic ferrite plates. On the other hand, the extent of the strain adjustment varies depending on the transformation conditions and the alloy composition. Since most dislocations accumulate in the original austenite rather than in the bainite phase, the number of inherited dislocations from the original austenite to the bainite matrix increases with rising dislocation density. A higher density of admissible dislocations can rapidly halt the migration of the bainite ferrite boundary, thereby reducing the thickness of the bainitic ferrite plates [1,4].

The above research indicates that size, dislocation density, and retained austenite stability in the microstructure all have an impact on carbide-free bainitic steel. Therefore, based on the influence of pre-formed martensite, pre-formed ferrite, and precipitated second-phase bainitic steel on the phase transformation characteristics and microstructure, combined with the literature and research conducted by our team, this article elaborates in detail on the influence of pre-formed phases on strength, toughness, and plasticity.

2. Experimental Procedures

Each section outlines the experimental steels and heat treatment processes, while this section describes the experimental methods which were used to test tensile and impact toughness properties. The tensile test was conducted at room temperature using the MTS universal hydraulic servo testing machine (MTS Systems (China) Co., Ltd., Shanghai, China). The tensile speed was 3 mm/min. The tensile sample was a round bar with a gauge diameter of 5 mm and a length of 25 mm. Charpy U-shaped samples were tested with the JB-300J pendulum impact tester (Yongbang Testing Instrument Co., Ltd., Jinan, China) at room temperature. The size of the samples was 10 mm × 10 mm × 55 mm. The TH501 Rockwell hardness tester (Laizhou Huayin Experimental Instrument Co., Ltd., Yantai, China) was used to measure the hardness. Three samples were taken from each process.

3. Research Advance

3.1. Pre-Formed Ferrite

The volume fraction, morphology, and strength difference between the soft and hard phases of pre-formed ferrite have a significant impact on the strength and plasticity of bainitic steel [5,6]. Nie et al. [7] used post-rolling relaxation-controlled phase transformation for different contents of ferrite/bainite dual-phase steel, and the results showed that the coordinated deformation between ferrite and bainite is the main mechanism for improving the stress ratio and uniform elongation of dual-phase steel. A reasonable ratio between the two is conducive to improving the degree of work hardening. Qian et al. [8] used Fe-0.28C-1.96Mn-1.62Cr-0.67Si-1.19Al-0.34Ni-0.23Mo (wt.%) alloy steel as an initial martensitic structure. After annealing in the intermediate temperature zone and isothermal quenching in the bainite zone, a layered structure was obtained. The layered structure exhibits excellent strength and ductility. This is attributed to the layered structure that combines the enhancement of the TRIP effect with back stress hardening, ensuring more durable high-strain hardening and delaying the occurrence of tensile necking.

The author’s team designed a medium-carbon steel with the following specific composition: Fe-0.39C-1.25Mn-1.55Si-1.11Cr-0.34Mo (wt.%). The heat treatment process was formulated. The sample underwent step I, involving austenitizing at 930 °C, held for 30 min, followed by direct oil quenching to obtain a martensitic structure. Step II: The sample was heated to 930 °C, 840 °C, 805 °C, 795 °C, and 780 °C, and held for 45 min, then isothermal treated in a salt bath furnace at Ms + 10 °C. The performance indicators of the tested steel obtained are shown in Figure 1. It can be seen that as the high-temperature transformation temperature (Step II) decreases, the strength indicators of the tested steel decrease, and the strength reaches a stable state after 805 °C. As the temperature decreases from 930 °C to 795 °C, the plasticity and toughness indicators of the tested steel show an increasing trend. Overall, introducing some ferrite before the bainite transformation is beneficial for improving the product of strength and elongation (PSE). Even if there is only a small amount of ferrite in the microstructure under the 840 °C process, it still exhibits a composite structure with better comprehensive properties.

In addition, in tested steels with the same introduction of ferrite, the influence of the ferrite content on performance shows a certain pattern. There are also some relevant studies in current research. Varshney et al. [9] found that before the bainite phase transformation occurs at 300 °C, there is a pre-formed ferrite of about 25% in the microstructure. The resulting bainite/ferrite/retained austenite multiphase structure exhibits a higher PSE value than the bainite/retained austenite microstructure. The former is 25% higher than the latter, but the PSE value decreases when the ferrite content is 5.0%. Ishikawa et al. [10] established a local strain measurement method using electron lithography technology to print nano microgrids (microgrid method) on the surface of samples. The microdeformation behavior of ferrite/bainite steel with different volume fractions of ferrite (16% and 40%) was studied. The strain-hardening ability of 40%-ferrite steel was significantly reduced, and the uniform elongation was lower than that of 16% ferrite steel.

This study found that when the isothermal temperature is 795 °C in the two-phase zone, the elongation reaches its maximum value of 21.6%, and the PSE value also reaches its maximum value of 25.7 GPa%. Based on the corresponding tensile work hardening curves, it was found that the n curves of the samples under two processes, 805 °C and 795 °C, passed through the valley region. As the strain increased, the n curve slowly decreased, and work hardening continued. The true strain reached 0.1 before intersecting with the straight-line n = ε, exhibiting good tensile strength and uniform elongation. The reasons for the analysis are as follows: ① Good coordinated deformation ability—ferrite accounts for a certain proportion in the samples treated at 805 °C and 795 °C. The ferrite phase is the soft phase that begins to deform, and strain first aggregates at the interface between ferrite and bainite. The deformation is mainly concentrated inside the ferrite, which bears and accommodates the load, sharing a portion of the stress acting on the retained austenite and promoting its gradual transformation. The two have exhibit good coordinated deformation, and the ferrite phase plays a lubricating role. ② Retained austenite possesses good stability and transforms into martensite during the stretching process [11]. The fresh-formed high-strength martensite undergoes phase transformation strengthening in the undeformed parts, and the strength of the deformed parts exceeds that of the undeformed parts, suppressing the continuation of local deformation. ③ There is a strain gradient between ferrite and bainite, and bainite generates significant internal stress. The back stress caused by strain incompatibility increases the work hardening rate. According to the analysis of the microstructure, the introduction of ferrite changed its orientation relationship, increased the proportion of the large-angle mismatch with a supporting role, and effectively changed the crack growth path [11]. The influence mechanism of introducing ferrite on the strength and toughness of bainitic steel is summarized in Figure 2, which provides an important theoretical basis for improving the comprehensive performance of bainitic steel.

3.2. Pre-Formed Martensite

The application of the pre-formed martensite process in bainitic steels has been studied, and the performance indexes of the bainite/martensite complex steels formed by pre-formed martensite have become a hotspot of interest. Avishan et al. [12] investigated the effect of pre-formed martensite on nanostructured low-temperature bainite fabric characteristics and mechanical properties in high-carbon steels. They found that pre-formed martensite prior to the bainite transformation increased the number of bainite laths, as well as increasing the dislocation density and carbon content within the retained austenite. Samples containing pre-formed martensite possessed higher yield and tensile strengths than those directly transformed to bainite. Their study concluded that the retained austenite possesses stable mechanical stability, which effectively prevents strain localization within the bainite–martensite–austenite composite structural organization, and the final specimens did not fracture in the early stages of tensile. Qian et al. [13] investigated the role of pre-formed martensite in isothermal processes below the M_S temperature in low-carbon, silicon-rich bainitic steels. The pre-formed martensite refines the bainite blocks/laths and retained austenite, and the samples show a relatively homogeneous strain distribution among the phases. The samples with pre-formed martensite had higher yield strength and elongation, although the tensile strength decreased slightly. The impact absorption energy and toughness of the sample were significantly higher than those without pre-formed martensite. The best toughness is obtained in the isothermal quenching temperature range of 30–60 °C below the M_S temperature, especially at 350 °C isothermal quenching, where the total absorption energy and toughness obtained reach ~150 J and ~190 J/cm², respectively, which are about 2.5 times higher than those obtained when isothermal quenching is performed above the M_S temperature.

In a medium-carbon steel, the team tested the Ms temperature of the test steel using a DIL expander at 285 °C. Based on the relationship equation between phase transition temperature and martensite volume fraction: VM = 1 − EXP(−0.011 °C⁻¹ × (Ms − T1)), 220, 235, and 250 °C were designed as the first-stage process for obtaining martensite, and the second-stage process for the occurrence of the bainite phase transition was chosen to be 295 °C. Comparing the carbide-free bainite process at 295 °C for the single-stage isothermal, the process diagram is shown in Figure 3. The microstructural parameters of the samples after different processes were tested and calculated, as shown in

Table 1. Microstructural parameters of the samples after different processes in the tested steel.

Parameter	295 °C	220–295 °C	235–295 °C	250–295 °C
VRA, %	15.5 ± 1.8	20.8 ± 2.2	18.5 ± 2.1	17.4 ± 2.5
CRA, wt.%	1.20 ± 0.14	1.30 ± 0.16	1.51 ± 0.19	1.33 ± 0.17
VM, %	0	51.1	42.3	31.9
VBF, %	84.5 ± 1.8	28.1 ± 2.2	39.2 ± 2.1	50.7 ± 2.5

[14].

Figure 4 compares the performance indicators of the test steel under the pre-formed martensitic process and carbide-free bainitic process. From the perspective of strength indicators, compared with single-stage bainitic steel, the strength of the test steel under the pre-formed martensitic process is slightly reduced, which is consistent with the conclusion in the literature. Based on Table 1 and Figure 4a, the relationship between microstructure phase composition and strength under pre-formed martensitic processes at 220, 230, and 250 °C was studied, and Formula (1) was roughly obtained. It is not difficult to see that the volume fraction of bainite ferrite dominates, despite the presence of martensite in the microstructure. Pre-formed martensite undergoes self-tempering during the second isothermal stage and belongs to low-carbon martensite. The strength of martensite increases with the increase in the carbon content. In fact, under different processes, the carbon content of martensite has already diffused into austenite after long-term insulation, and the strength change is only caused by the change in its content. Based on Figure 4a, the following inference can be drawn: in the pre-formed martensitic process, the influence of the microstructure and phase structure on strength is relatively small. It is considered as a “weak phase” retained austenite, which exhibits a strengthening effect in this microstructure due to its high carbon content and high stability. This study indicates that pre-formed partial martensite, within a certain temperature range, has little effect on the strength of the multiphase structure.

The analysis of the plasticity index (elongation) reveals that samples processed with pre-formed martensite exhibit higher elongation values. Combined with the study of work hardening rates across different processes, as illustrated in Figure 5, the work hardening rate curve can be divided into three distinct stages:

Stage I: All three samples demonstrate high work hardening rates, corresponding to the elastic and yielding stages of the tested steel.

Stage II The: work hardening rate of the three samples enters a significant decline phase, though the samples continue to harden. In this stage, samples with a higher volume fraction of pre-formed martensite (230 °C) exhibit slightly lower work hardening rates.

Stage III: When the strain exceeds 0.034, the samples processed with pre-formed martensite show a high work hardening rate. This behavior is attributed to the high-volume fraction and stability of retained austenite in the microstructure, which enhances the transformation-induced plasticity (TRIP) effect as the strain increases. The TRIP effect continues to contribute to the hardening behavior due to the retained austenite transformation.

σ_b-Pre-M = (13.11 MPa)V_BF + (7.13 MPa)V_M + (2.38 MPa)V_RA

(4)

For toughness indicators, the pre-formed martensite/bainite mixed microstructure exhibits high impact toughness, essentially changing the overall microstructure orientation, distribution, etc. due to the formation of pre-formed martensite [14]. According to the EBSD test results, the block size in the microstructure was calculated. The average grain size of the block in the pre-formed martensitic process was 2.9 ± 0.5 μm, and the carbide-free bainite was 3.5 ± 0.8 μm. The pre-formed martensite phase provides numerous nucleation sites for the bainite transformation, thereby limiting the growth of bainite plates due to constraints from adjacent variants. As a result, the bainite block size tends to be smaller in these microstructures. To further analyze the microstructural differences, the distribution of misorientation angles between samples processed with pre-formed martensite and those with carbide-free bainite was compared, as shown in Figure 6. The results indicate that the proportion of low-angle grain boundaries (LAGBs) is significantly lower in the pre-formed martensitic samples compared to the carbide-free bainitic samples. This reduction in LAGBs is attributed to the self-tempering effect of the pre-formed martensite during isothermal quenching. The self-tempering process promotes dislocation recovery, which effectively decreases the density of low-angle boundaries in the microstructure. This mechanism contributes to the observed differences in boundary characteristics between the two types of samples [15]. In addition, the proportion of high-angle grain boundaries (HAGB) in pre-formed martensitic samples is greater than that in carbide-free bainitic samples. It is generally agreed that HAGB can serve as an effective barrier to crack propagation. When encountering HAGB, the crack tip will change its propagation direction. In addition, small block sizes can effectively increase impact absorption energy [16,17,18,19,20,21]. Therefore, existing research results have found that under the pre-formed martensitic process, the microstructure of the sample contains a high proportion of HAGBs and small block sizes. In addition, the retained austenite in the microstructure of the pre-formed martensitic process exhibits the highest carbon content and stability. The above factors are beneficial for improving toughness.

It is worth noting that introducing martensite to regulate the strength and toughness of bainitic steel requires controlling the proportion of martensite, especially in terms of the influence on plasticity, toughness, and strength of plasticity products. It can be seen that introducing martensite at 235 °C exhibits the best comprehensive performance, as shown in Figure 4.

3.3. Precipitated Particles

The addition of microalloying elements (mainly Nb, Ti, V) to steel can directly affect the microstructure, phase transformation process, strength, toughness, and plasticity of bainitic steel, as well as the morphology, size, and quantity of the second-phase precipitation. Hu et al. [22] reported the effect of Nb and Mo on the strength of low-carbon bainitic steel. The addition of Nb refined the original austenite grains, and improved the strength of the steel, but hindered the transformation of bainite. Mo effectively improves the strength of low-carbon bainitic steel by promoting the transformation of bainite and increasing the amount of bainite. Wang et al. [23] studied the influence of microalloyed Nb element on improving mechanical properties and found homogeneous and fine precipitates increase the strength and hardness. The study of Kong et al. [24] showed that the combined addition of Ti and B significantly improved the yield strength and tensile strength of low-carbon bainitic steel, and the elongation and strain-hardening index n values were reduced. After adding Ti to low-carbon bainitic steel, the fine Ti (C, N) particles precipitated through controlled rolling and cooling can nail the grain boundaries of the original austenite and constrain grain growth [25].

The tested steel used in this study is V-microalloyed medium-carbon bainitic steel, with a specific chemical composition (wt.%) of Fe-0.40C-1.53Mn-1.53Si-1.09Cr-0.31Mo-0.15V. According to previous research results, V was completely solid-solved in the experimental steel during isothermal treatment at 950 °C [26]. In the second stage of isothermal treatment at 870 °C and 848 °C, some of the V precipitated. Based on this, a high-temperature two-stage isothermal heat treatment process for the experimental steel was designed as follows: all samples and heat treatments were uniformly annealed at 1050 °C for 3 h and oil quenched to room temperature. Double-stage high temperature was followed by a bainitic transformation heat treatment process: After heating the sample to 950 °C and holding for 30 min, it was quickly cooled down to 870 °C and 848 °C and held for 30 min to precipitate different amounts of VC particles. Subsequently, the bainitic phase transition was carried out via isothermal treatment in a 320 °C salt bath furnace for 2 h.

The mechanical properties of the experimental steel after different heat treatment processes are shown in Figure 7. The different amounts of the second phase have varying effects on mechanical performance. With the decrease in the phase transition temperature in the second stage, the precipitation content of the second phase increased. In this study, the strength and plasticity increased slightly with the increase in the precipitation content of the second phase. Especially under the 950 °C–848 °C process, the elongation and strength are improved. This is related not only to the amount of second-phase precipitation, but also to its influence on the volume fraction, morphology, and crystallographic orientation of retained austenite in the microstructure. Consistent with the literature, the precipitation of V particles can improve strength to a certain extent [27]. Therefore, it is feasible to use microalloying to regulate the strength and toughness of bainitic steel.

In fact, as early as 1988, Inoue’s team [28] showed that V alloy could simultaneously improve the toughness and hardness of bainitic steel. Adding V can strengthen medium- and low-carbon steel with a bainite structure, with an increase in tensile strength between 50 and 150 MPa. Some scholars attribute the beneficial effect of V to the pinning of dislocations by V (C, N) precipitates, resulting in the slow recovery of dislocation entanglement [29]. Steel with added V has a higher proportion of low-angle grain boundaries and a smaller bainite grain structure [30]. In another study, the authors quantified the increased volume fraction and refined complex TiNbV carbonitride precipitates resulting from V microalloying, and estimated that the additional 20 MPa contribution of precipitates to overall strength may be reasonable [31]. However, the decrease in the amount of precipitated carbides in this study did not fully demonstrate the strengthening of precipitation to a certain extent.

3.4. Other Factors

The width of bainitic ferrite laths and the strength of the austenite phase have a direct influence on the properties of bainitic steel. Singh et al. [32] pointed out that the changes in austenite strength and chemical-free energy are the main factors affecting bainitic ferrite thickness. Cornide et al. [33] studied the influence of austenite phase strength on bainitic ferrite laths and found that the increase in austenite strength will reduce the thickness of bainitic ferrite laths. These studies show that high-stability austenite is more conducive to obtaining thinner bainitic ferrite laths and improving the strength of bainitic steel. During the stretching process, the TRIP effect is also strongly influenced by retained austenite. Chiang et al. [34] reported that the work hardening behavior is directly related to the transformation rate of retained austenite, which in turn depends on the C content and grain size of retained austenite in the microstructure. At the same time, blocky retained austenite is prone to transform into martensite and has high deformability [35]. A highly stable thin filmy retained austenite is beneficial for preventing crack propagation. Guo et al. [36] investigated the mechanical properties of ultrafine bainitic steels by introducing a deformation quenching process and found that the synergistic effect of austenite formation and martensitic phase transformations effectively refined the bainitic laths and greatly reduced the size of the deleterious massive retained austenite, thus providing an improved phase transformation-induced plasticity effect.

4. Conclusions and Expectation

In this study, we comprehensively explored the influence of pre-formed microstructures on the mechanical properties of bainitic steel. Our results indicate that introducing pre-formed ferrite significantly enhances the strength and plasticity by promoting coordinated deformation between ferrite and bainite, thereby improving the stress ratio and uniform elongation. Meanwhile, pre-formed martensite refines the bainite structure, increases dislocation density, and stabilizes retained austenite, leading to higher yield strength, elongation, and impact toughness. Additionally, the addition of microalloying elements and their precipitates, such as V, plays a crucial role in strengthening the steel while maintaining ductility. Overall, the mechanical properties of bainitic steel are not determined by a single pre-formed phase but are influenced by the collective effects of microstructure composition, morphology, and size. These findings provide valuable insights for optimizing the mechanical properties of bainitic steel through process control and pave the way for future research on the evolution of bainitic steels during fatigue and wear, which are critical in engineering applications.