Reverse Hall–Petch Effect of Nano-Bainite in a High-Carbon Silicon-Containing Steel

Abstract

High-strength steels are widely used in various mechanical production and construction industries for their low cost, high strength and high toughness. Among these, bainitic steels have better comprehensive performance relative to martensite and ferrite. In this paper, from the point of view of its microscopic fine structure and mechanical properties, the high-carbon silicon-containing steel Fe-0.99C-1.37Si-0.44Mn-1.04Cr-0.03Ni was austenitized at high temperature after a brief isothermal treatment at 280 °C and is briefly reviewed. We have used EBSD, TEM and 3D-APT to observe a unique transformation in which high-carbon silicon-containing steels form nanostructured bainite with nanometer widths. Intriguingly, as the isothermal duration decreases, the beam bainite width becomes increasingly finer. When the beam bainite width falls below 50 nm, there is a sudden shift in defect type from the conventional edge-type dislocations to a defect characterized by the insertion of a semi-atomic surface in the opposite direction, which leads to different degrees of reduction in the micro- and macro-mechanical properties of high-carbon silicon-containing steels from 1754 MPa to 1667 MPa. This sudden change in the sub-structural properties is typical of the reverse Hall–Petch effect.

1. Introduction

The iron and steel industry provide important material foundations for national economic construction, social progress and national defense security, and it is of strategic significance to carry out basic research on iron and steel materials and develop new iron and steel materials for the development of national major engineering projects. With ultra-high-strength and excellent plastic toughness, steel materials have broad application prospects, can be used in vehicle engineering, marine vessels, machinery manufacturing, bridge construction and other fields, and can also be used for large transport aircraft landing gear, heavy train bogies and armored vehicles and other special areas. As we all know, it is difficult to match the strength and plasticity of steel materials, and increasing the strength usually reduces the plasticity. In the development of ultra-high-strength steel materials, good plasticity and toughness at the same time is an important issue, and both ultra-high-strength and good plastic toughness of high-strength steel are development goals for the generation of new steel materials.

In the 1930s, Devenport and Bain discovered and studied bainitic organization [1], and subsequently, bainitic steel has been the focus of research in the steel materials community because of its high strength and good toughness. In 2002, Bhadeshia and colleagues [2,3,4,5,6,7] achieved an unprecedented nanostructured bainite microstructure in high-carbon Si-bearing steel by subjecting it to a prolonged isothermal hold near the M_s temperature. This novel microstructure is devoid of carbides and comprises elongated bainitic ferrite laths, each spanning over a dozen micrometers, interspersed with residual austenite films. The remarkable feature of these laths is their nanometer-scale widths, which significantly enhance the material’s mechanical properties. Consequently, the tensile strength and toughness of steel with nanostructured bainite can be elevated to 2.5 GPa and 30 MPa/m^1/2, respectively. Nano-bainite organization has been found in the isothermal quenching organization of many steel grades, which has attracted significant attention from the engineering and academic communities because of its superior comprehensive mechanical properties, and its mechanical properties will be significantly improved after the alloying element has been modified. Gupta obtained a nano-bainite organization from steel after isothermal quenching at 250° [8], which has a high toughness and good wear resistance and can be well adapted to service environments. Niu obtained a nano-bainite organization by hot rolling and isothermally treating a medium-carbon steel of composition Fe-0.43C-1.05Cr-1.01Mo-0.73-0.17Si in varying degrees [9]. Sun significantly improved the hardness and wear resistance of the steel by using a two-step isothermal treatment to obtain a thinner nano-bainite organization than that obtained after a one-step treatment [10]. Bainitic steels have the potential to achieve a good combination of strength, ductility and edge stretchability compared to other alloys, which is a highly desirable characteristic for the automotive industry [11].

It is well known that, when the grain size of a metallic material is smaller than a certain limit, the substructure and deformation mechanism of the material undergoes a sudden change, and mechanical properties such as strength and hardness no longer increase with the reduction in grain size. When the grain size is smaller than 30 nm, Li et al. found in their study that the Vickers hardness of nanometallic copper and palladium does not increase but decreases with the reduction in metal grain size [12]. Li et al. believe that this ultrafine metal nanocrystal undergoes the phenomenon of grain boundary diffusion creep at room temperature which leads to this phenomenon. Exhibiting the reverse Hall–Petch effect, this phenomenon has been confirmed in many nanocrystalline metallic materials [13,14,15,16,17]. Currently, in the known literature on nanostructured bainite, the scientific explanation of the nano-bainite phase transition mechanism and its high strength and toughness coordination properties is still controversial and the width of the bainite beam is mostly concentrated in the range of 60–90 nm [9,17]. No studies have been carried out in which the widths of the beam bainite are less than 50 nm and there are also few reports on the reverse Hall–Petch effect in nanostructured bainite. In view of the above reasons, in this paper, a high-carbon silicon-containing steel was selected to be austenitized at a temperature much higher than the Ac_cm temperature and then isothermally treated for a short period of time at a temperature much higher than the M_s temperature to form a nanostructured bainite intermediate-temperature phase-transition organization of even finer dimensions, to study the effect of the width of the beam bainite on the microstructure and mechanical properties of high-carbon silicon-containing steel, and to elucidate the critical size of performance mutation in nanostructured bainite and the generation mechanism of reverse Hall–Petch effect.

2. Materials and Methods

The experimental steel was a high-carbon silicon-containing steel, vacuum-melted in a medium-frequency induction furnace, with a nominal composition of Fe-0.99C-1.37Si-0.44Mn-1.04Cr-0.03Ni (wt.%). Si can allow carbon atoms to exist in the lattice gap in the solid solution instead of precipitating in the form of carburite or other carbides, in order to improve the stability of the austenite, to ensure that in the medium-temperature phase transition stage, carbide-free bainite organization is generated. In addition, Si solidly dissolved in the ferrite lattice can play a certain role in strengthening the bainite, and Cr can effectively improve the hardenability of the steel to ensure that transformation products can be obtained in the specimen from the edge to the center of the medium-temperature phase. In addition, Cr can increase the range of bainite transformation in the CCT curve of experimental steel, which is conducive to the generation of bainite organization in a wider isothermal range. Mn can improve the toughness of experimental steel and, at the same time, is helpful in improving its hardenability.

In order to formulate the heat treatment process, the phase change point of the experimental steel was determined. In this paper, the surface oxidation was removed by sandpaper, and the thermal analyzer TG/DTA6300 produced by Netzsch, Selb, Germany was used to determine the absorption and exothermic reaction of the sample during the heating process (the heating rate was 10 °C/min). The test temperature range was 20~1000 °C and the phase change point was determined by this analysis. The Ac₁ and Ac_cm points of the experimental steel were 774 °C and 814 °C, respectively. The temperature above the Ac_cm point was used to heat the experimental material, to test the M_s point temperature of the material under the heating condition above the Ac_cm temperature. The Formastor-Digital 2 thermal dilatometer was used to measure the M_s point temperature of the material. The test process included slowly heating the material to the experimental temperature at a speed of 0.35 K/s, holding it for 30 min, and then cooling it to room temperature at a speed of 0.7 K/s. The measured Ac1 temperature of the experimental steel is 773 °C, the Ac_cm temperature is 816 °C, which is consistent with the DTA data, and the M_s point temperature is 158 °C.

The heating temperature selected in this paper was higher than the Ac_cm point of experimental steel (814 °C). The purpose was to obtain the parent phase with uniform structure and composition distribution, considering that too high a heating temperature will put more stringent requirements on the heating equipment and waste energy and is not conducive to the actual engineering application which means that the heating temperature should not be too high. Considering various factors, the austenitizing temperature of experimental steel was set at 950 °C. For the selection of isothermal temperature, the temperature range should be higher than the M_s point temperature. On the one hand, it should be considered that an isothermal temperature too close to the M_s point temperature will make the isothermal transition of the material require an extremely long incubation period. On the other hand, the phase transition products with temperatures higher than M_s point can be compared with those obtained by Bhadeshia et al. Moreover, the isothermal temperature should not be higher than the upper limit of the bainite transition temperature range, so as to avoid a diffusion phase transition which interferes with the formation of bainite. Considering various factors, we chose to set the isothermal temperature of the experimental steel at 250 °C. The molten high-carbon silicon-containing steel specimens were first put into a FURNACE 1200 tubular resistance furnace (Selb, Germany), and argon gas was injected into the furnace and heated to austenitizing temperature (950 °C) at a rate of 10 °C/min and held for 30 min for austenitizing. After that, the sample was quickly transferred to molten salt (280 °C) for isothermal treatment at different times. The salt bath equipment used was a self-made salt bath furnace, in which the molten salt used as the quenching medium was a mixture of 55% sodium nitrite and 45% potassium nitrate, and the cooling rate was achieved at about 40 °C/min. Isothermal times of was 20 min, 40 min and 60 min were used, and finally, the water was cooled to room temperature.

Microstructural and substructural analyses were conducted using Electron Backscatter Diffraction (EBSD) Zeiss Gemini SEM 450 with SUPRA^TM (Oberkochen, Germany) and Transmission Electron Microscopy (TEM) FEI Tecnai (G2 Spirit TWIN) (Cambridge, MA, USA). For TEM observation, a sheet specimen with a thickness of 1.5 mm was cut from the heat-treated specimen using an EDM wire-cutting machine model DK7720 (Taizhou, Jiangsu, China), and the thin-foil specimens were mechanically ground to a thickness of 15 μm. A TEMATCH’s TJ100BE-type (Wuxi, Jiangsu, China) double-jet electrolytic device was used to reduce the thickness of the thinning (double-jet solution for the concentration of 5% perchloric acid solution in methanol; the operating temperature was −20 °C, the operating voltage was 20 V, and the operating current was 20 mA) to the perforation and then quickly placed in alcohol to wash away the residual acid, followed by the use of Gatan 691-type ion thinning instrument near the perforation site (Pleasanton, CA, USA). A voltage of 20 V and a current of 20 mA were applied to the perforation and then it quickly put it into alcohol to wash away the residual acid, and then further thinning was carried out near the perforation using a Gatan 691 ion thinning instrument (working voltage 3 V, ion gun angle 4°, thinning time 1 h) to enlarge the thin area for transmission electron microscopy observation. Fine structural observations (bright-field phase, selected area electron diffraction and high-resolution image) were performed using a Philip’s TECNAI G2 transmission electron microscope (Hillsboro, OR, USA). The transmission electron microscope images and data were processed and analyzed using Gatan Digital Micrograph 3.0 software. Recipro software (v4.892) was used to calculate and calibrate the constituent electron diffraction data.

Due to the small size and structure of nano-bainite, it is difficult to use conventional elemental analysis methods (such as EDS and EELS, etc.) to accurately analyze the elemental distribution. This paper therefore chose the more advanced 3D Atom Probe Tomography (hereinafter referred to as APT) to analyze it. APT analysis was performed on the Cameca Instruments LEAP 5000 Atom Probe Tomography Analyzer (Gennevilliers, France), employing the voltage mode with the sample tip temperature set at 50 K, a pulse frequency of 200 kHz, and a pulse parameter value of 0.2. APT samples were selected from nano-bainite regions using the Zeiss Auriga dual-beam scanning electron microscope equipped with a Focused Ion Beam (FIB) system (Oberkochen, Germany). Micro-mechanical properties were assessed using a TriboIndenter nanoindentation tester (Billerica, MA, USA) with a diamond cubic cone indenter, applying a maximum load of 5 mN, loading and unloading rates of 5 μN/s and a hold time of 5 s. Tensile properties were evaluated using an Instron universal testing machine (Norwood, MA, USA) at a crosshead speed of 1 mm/min. The specific dimensions of the tensile specimen are shown in Figure 1. The thickness is 1 mm. For the reproducibility of the data, the samples were tested 3 times.

3. Results

3.1. Subsections on Microstructure

Figure 2 presents the EBSD test outcomes for high-carbon silicon-containing steel subjected to different isothermal holding times. Figure 2a–c show bond contrast plots (BC), where brighter areas are structurally perfect and darker areas represent certain defects, strains, etc. Figure 2d–f show the phase diagram (Ph) and the grain boundary diagram (Gb). Figure 2g–i show the inverse pole figure (IPF), a diagram that expresses the effect of the three-dimensional distribution of crystalline phases on the crystal surface by means of polarized ruddy projection of the crystallographic coordinates of the grains to achieve a two-dimensional diagram. The BC plots reveal that the steel exhibits the formation of numerous organizational bundles post-isothermal treatment, a characteristic indicative of the nanostructured bainite morphology [18]. The Ph diagrams distinctly delineate two phases: a body-centered cubic (bcc) structure, depicted in red, corresponding to the beam-like organization in the BC diagrams, represents the bainite ferrite phase developed during the isothermal process. Additionally, a face-centered cubic (fcc) structure is observed in the blue region, which signifies the residual austenite from the high-temperature transformation. The residual austenite content diminishes with increasing isothermal time, dropping from 5.4% to 3.7%. This reduction is attributed to the enhanced capacity of austenite to fully transform into bainite during extended isothermal periods, which also results in an expansion of the bundled bainite width and a significant decrease in grain boundary count. The IPF plots reveal that the bainite organization post-isothermal treatment is randomly oriented and uniformly dispersed, with the beam bainite broadening as the isothermal time lengthens.

To further examine the substructure of the specimen’s organization, transmission electron microscopy was performed on the high-carbon silicon-containing steel subjected to isothermal treatments of varying durations, as illustrated in Figure 3 and Figure 4. At an isothermal time of 40 min, the ferrite width within the formed beam bainite is approximately 100 nm, comparable to the size of reported nanostructured bainite. The electron diffraction spots (Figure 3b) within the red-boxed area do not exhibit significant deviation from the standard Bragg dot matrix, indicating the absence of pronounced spot distortion. The high-resolution images (Figure 3c) of the red box region reveal the insertion of a semi-atomic surface into the lattice, which is characteristic of an edge-type dislocation, similar to defects found in conventional bainite ferrite of steels with analogous compositions [19,20,21,22,23,24]. This observation suggests that the beam bainite ferrite with a width of around 100 nm does not fundamentally differ from conventional bainite.

Correspondingly, when the sample undergoes isothermal treatment for 20 min, the width of the ferrite bars in the resulting beam bainite is significantly less than 50 nm. A high-resolution analysis of the content within the yellow box area suggests a mutation in the ferrite substructure at this stage, with defects of a different nature compared to conventional bainite ferrite. In the high-resolution photographs, seven atomic surfaces are marked with numbers 1 through 7, each approximately 2 nm thick, and are superimposed on one another. Atomic surfaces labeled 1, 3, 5 and 7 exhibit some distortion but remain continuous. In contrast, atomic surfaces labeled 2, 4 and 6 are half-atomic surfaces, inserted in opposite directions on either side of the same intact atomic surface, with an insertion length of about 1 nm, creating a novel type of defect distinct from conventional edge-type dislocations.

As the width of the bunched bainite in high-carbon silicon-containing steels reduces to less than 50 nm, the nature of its crystal defects undergoes a significant transformation, shifting from traditional edge-type dislocations to defects composed of multiple overlapping semi-atomic surfaces. The latter is superior in terms of defect complexity, lattice deformation extent, and coverage area, regardless of the intricacy of the defect structure. This suggests that ultrafine nanostructured bainite and coarsened nanostructured bainite represent distinct crystalline types, with the critical size for nano-bainite to undergo such substructural mutation being approximately 50 nm.

To facilitate a more detailed examination of the elemental distribution within bainitic ferrite with widths below 50 nm in high-carbon silicon-containing steels and to elucidate the origin of the observed novel defects, three-dimensional atom probe tests were conducted, as depicted in Figure 5. The carbon atom distribution revealed a band-shaped region (e.g., region A in Figure 5a) with relatively low carbon content, approximately 20 nm to 30 nm in width, situated between two regions (B and C) with higher average carbon content. This region’s dimensional characteristics closely resembled those of the bunched bainitic ferrite observed under the electron microscope, leading to the inference that it represents nano-bainitic ferrite in the low-carbon region and residual austenite in the high-carbon region. Subsequently, we conducted an elemental analysis of a carbon atom-partitioned region within area A (the cyan-colored columnar region in Figure 5b), which is approximately 2 nm in length and 1 nm in diameter. Our findings indicated that the carbon atom concentration in this region surpassed the average value of area A. The dimensions of this carbon atom-partitioned region, such as its length and diameter, align with the observed new type of crystal defect, suggesting that these defects are in a state of carbon atom segregation. Despite the observation of carbon atom segregation, it is not definitively established that the defects are dislocations. Recent atom probe experiments suggest that the carbon atom segregation in nanometallic crystals is not a result of dislocations [25,26,27]. Therefore, we speculate that the observed defects may represent a novel type of defect distinct from traditional dislocations.

To elucidate the underlying cause of the observed phenomenon, we examined the Fe atom content in the vicinity of the novel defects and discovered a reduction of approximately 10% in Fe atom concentration compared to the surrounding area, indicating a deficiency of Fe atoms. This is associated with the high-temperature austenitizing conditions at 950 °C. Under such extreme heating, a significant number of Fe atomic vacancies are inevitably formed in the γ phase. As the γ → α transition occurs, these vacancies migrate to the α phase lattice. To minimize the system’s thermodynamic energy, vacancy combinations, such as double vacancy cluster combinations [28], are formed. These combinations result in an incomplete atomic surface due to the structure of the vacancy arrangement. Consequently, the characteristics displayed by the atomic planes identified as 2, 4, or 6 in Figure 3, which are closely adjacent and inserted into the lattice from opposite directions, are closely linked to the presence of double vacancy cluster combinations.

3.2. Mechanical Properties

To assess the impact of substructural transformations on the mechanical properties of high-carbon silicon-containing steel, nanoindentation tests were conducted on the area highlighted in yellow in Figure 2, while macroscopic tensile property tests were performed on the specimens. Figure 6 displays the nanoindentation load-displacement curves for high-carbon silicon-containing steel subjected to different isothermal holding times. The microhardness of the specimen can be calculated based on the nanoindentation load–displacement curves. The hardness of the beam bainite after isothermal treatments of 20 min, 40 min and 60 min is 18.14 GPa, 19.73 GPa and 16.94 GPa. The microhardness of the steel after a 40 min isothermal treatment exceeds that of the steel treated for 20 min and 60 min. In conjunction with the observed changes in beam bainite width in the previous section, it is evident that when the width is less than 50 nm, the defect structure of nano-bundled bainite undergoes a mutation, leading to an increase in microhardness with grain size, which exhibits a pronounced reverse Hall–Petch effect. Conversely, when the grain width reaches approximately 100 nm, the crystal defect type in the beam bainite reverts to traditional dislocations, and the microhardness of high-carbon silicon-containing steel decreases with increasing grain size, aligning with the Hall–Petch relationship. The pattern of change of microscopic mechanical properties also affects the macroscopic mechanical properties. Figure 7 shows the true stress/strain curves of steel specimens after different isothermal times, and the true stress/strain curves have a more practical application in characterizing the material behavior. The true tensile strength of specimens after isothermal treatments of 20 min, 40 min, and 60 min is 1667 MPa, 1754 MPa, and 1672 MPa, respectively, which parallels the trend in microhardness. The macroscopic mechanical properties of the specimens exhibit the same reverse Hall–Petch relationship when the bainite width is less than 50 nm.

Combined with the above results, when the beam bainite organization size is larger than 50 nm, the microstructure and mechanical properties of silicon-containing steels are consistent with the Hall–Petch effect, i.e. the volume fraction of grain boundaries is larger in materials with smaller grain sizes, and the grain boundaries, as a kind of surface defects, prevent the dislocations from normal sliding and climbing, which leads to dislocations tangling and plugging out of the boundaries, and this results in the stresses required for deformation to occur increasing. When the size of the beam bainite organization is lower than 50 nm, a new type of defect is generated, and dislocations are no longer the main mode of deformation, the nano-sized structure mainly affects deformation by changing the grain boundary structure, grain rotation, and grain boundary migration throughout the deformation process, absorbing most of the deformation energy and reducing the yield strength, so that the nanostructured material shows the reverse Hall–Petch effect [29].

4. Conclusions

The effect of the size of nano-bainite formed by isothermal quenching of Fe-0.99C-1.37Si-0.44Mn-1.04Cr-0.03Ni (wt%) high-carbon silicon-containing steels on the crystal defects and mechanical properties was investigated.

• When the isothermal temperature was shortened from 60 min to 40 min, the beam bainite structures were refined and were all greater than 50 nm. The main mechanism of deformation was dislocation, and the tensile strength was increased from 1672 MPa to 1754 MPa, which was in line with the Hall–Petch effect.
• When the isothermal temperature is further shortened to 20 min, the beam bainite is refined to less than 50 nm scale. Different from the edge dislocation in conventional beam nanoribbons, a new defect structure exists in nanoribbons with a width of less than 50 nm, in which multiple adjacent half-atom surfaces are stacked in opposite directions within a few nanometers. The defect range includes saturated carbon atoms, and the tensile strength of silicon-containing high-carbon steel is reduced from 1754 MPa to 1667 MPa. This new type of defect results in the reverse Hall–Petch effect.

In the future, experiments will deepen the study of corrosion resistance of high-carbon steel with silicon and the stability of nano-bainite, and the experimental results will provide a theoretical basis for the subsequent process improvement and popularization of its application.