2021 Volume 61 Issue 2 Pages 537-545
Fe-0.4C-1.2Si-0.8Mn (mass%) alloys austenitized at different temperatures, ranging from 1103 to 1473 K, were subjected to interrupted quenching (IQ) at 473 K and then maintained at that temperature to induce the partitioning of carbon from martensite to austenite (one-step quenching and partitioning (Q&P) process). The initial austenite grain size before the IQ was varied from 20 to 573 µm. As the initial austenite grain size becomes finer, the enrichment of carbon in the untransformed austenite during the partitioning treatment is enhanced, which leads to a greater increase in the volume fraction of retained austenite. The reasons for the increased carbon enrichment were explained by the effective carbon partitioning as well as the promoted bainitic transformation, which were both caused by the increase in the area of the martensite/austenite interface. Tensile tests of the specimens with different initial austenite grain sizes revealed that the mechanical properties of the one-step Q&P specimens improved in both strength and elongation by the refinement of the initial austenite grains.
Transformation-induced plasticity (TRIP)-aided composite-structured steels play an important role in the field of research and development of low-carbon steel sheets for automobile bodies, wherein the TRIP induced by retained austenite improves the strength–ductility balance. To obtain high strengths close to that of quenched martensite without losing sufficient ductility due to the TRIP effect, quenching-and-partitioning (Q&P),1) which is a type of heat treatment to disperse retained austenite grains within the martensite matrix, is regarded as a promising process for fabricating next generation advanced high-strength steels (AHSS).2,3) Therefore, many fundamental studies on Q&P have been conducted, mainly by the developer John G. Speer.1,4,5,6) The Q&P process consists of two treatments: interrupted quenching (IQ) technique, in which a part of austenite is transformed to martensite by quenching to a temperature between the martensitic transformation start temperature (Ms) and the finish temperature (Mf) after austenitization; and the other is the partitioning, in which the interrupt-quenched material is maintained at a relatively low temperature at which the diffusion of carbon can take occur. During the partitioning, carbon diffuses from martensite to austenite, leading to the formation of retained austenite with sufficient stability even at room temperature due to the enriched carbon. This process is termed as one-step Q&P when the isothermal holding is performed at the same temperature as the IQ temperature, and as two-step Q&P when the isothermal holding is performed at a temperature higher than the IQ temperature.5)
In recent years, the use of retained austenite has been gaining attention in the field of medium-carbon mechanical structural steels for undercarriage parts and bearings for automobiles to improve toughness7) and wear resistance.8) In the case of this type of steel, Q&P is the most promising process because the high hardness attributed to the martensitic structure is essential for improving the required properties.
In this study, as a fundamental study of the Q&P process for medium-carbon steel, we focused on the effect of the initial austenite grain size before IQ. It is generally known that the austenite grain size influences the Ms temperature9) and the size of the substructure, such as martensite blocks and packets.10) In addition, considering that the martensite nucleates at the austenite grain boundaries, it is expected that the initial austenite grain size influences the distribution and growth behavior of martensite. Therefore, in the Q&P-treated steels, the morphology, volume fraction, distribution, etc., of the retained austenite may also be affected by the initial austenite grain size. Furthermore, in the case of low-alloy steels, it is considered that bainitic transformation occurs during the partitioning treatment. If the distribution and size of bainitic ferrite are affected by the initial austenite grain size, the behaviors of carbon partitioning between the phases and the microstructure development should also change more unpredictably.
In this study, Fe-0.4C-1.2Si-0.8Mn alloy was used as a representative medium-carbon steel. The effect of the initial austenite grain size on the microstructure development and mechanical properties obtained by a one-step Q&P process with partitioning at 473 K was investigated from the above points of view.
The chemical composition of the medium carbon steel (0.4C steel) specimen used in this study is shown in the upper row of Table 1. A 1.2 mass% of Si was added to suppress the precipitation of carbide (cementite). For the steel produced by vacuum melting and hot rolling, plate specimens with a thickness of 5 mm were cut out from the hot-rolled plates, and then one-step quenching and partitioning (one-step Q&P) was performed as follows: after holding at various austenitizing temperatures ranging 1103–1473 K for 1.8 ks, IQ was performed using a salt bath at a temperature between Ms and Mf, 473 K, which is a general low-temperature tempering temperature. After performing isothermal holding (partitioning treatment) for a maximum of 5.4 ks at that temperature, the specimen was water-cooled to room temperature (Q&Ped specimen). In addition, referential specimens that were directly quenched to room temperature were also prepared. The directly quenched specimens were tempered at the same temperature as that used for the partitioning (473 K) before the mechanical tests.
| Steel | C | Si | Mn | P | S | Ni | Cr | N | O | Al | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.4C | 0.38 | 1.22 | 0.8 | 0.004 | 0.002 | – | – | 0.0014 | 0.0006 | 0.016 | Bal. |
| SUS410 | 0.120 | 0.21 | 0.83 | 0.034 | 0.0053 | 0.01 | 12.13 | 0.0080 | 0.0110 | <0.001 | Bal. |
Martensitic stainless steel was fabricated as an additional steel specimen to visualize the distribution of untransformed austenite in the interrupt-quenched specimen and is shown in the lower row of Table 1 (SUS410). The following Greninger–Troiano (G-T) method11) was applied to SUS410 specimens with a thickness of 5 mm: after austenitizing at 1423 K for 10.8 ks or 1223 K for 0.3 ks, the SUS410 specimen was quenched using a salt bath maintained at a temperature between 553 and 623 K to transform austenite partially to martensite. Immediately thereafter, the temperature was raised to 873 K and then held for 0.06 ks to perform tempering, followed by water cooling to room temperature.
Each heat-treated sample was wet-polished with emery paper and buffed with a diamond suspension. The 0.4C steel was etched with a 3% nital solution (nitric acid:ethanol = 3:97), and SUS410 was performed using picric acid alcohol solution (picric acid:ethanol = 96:4) with a few drops of hydrochloric acid. The microstructure was observed using an optical microscope. However, it was difficult to measure the grain size of the prior austenite because the grain boundaries of 0.4C steel could not be clearly observed via the above etching method. Therefore, another etching method using a mixed solution of picric acid, cupric chloride, and sodium dodecylbenzenesulfonate12) was used to reveal the prior austenite grain boundaries. Figure 1 shows an optical micrograph of the 0.4C steel obtained by this method. The grain size was evaluated with the nominal grain size measured by the planimetric method13) using the micrographs shown in Fig. 1.
Example of optical micrograph of 0.4C steel etched with a mixed solution of picric acid, cupric chloride, and sodium dodecylbenzenesulfonate to reveal prior austenite grains.
For observation of the distribution of retained austenite, an electron backscattering diffraction (EBSD) method was used using an orientation image microscope mounted on a field-emission scanning electron microscope (Carl Zeiss Microscopy SIGMA500). In addition, dark-field image observation was also performed using a transmission electron microscope (JEOL JEM2010). The volume fraction of retained austenite at room temperature was determined by subtracting the martensite volume fraction quantified by the saturation magnetization method14) from the entire volume (100 vol.%).
The phase transformation temperatures and transformation rates of each specimen were evaluated by thermal expansion measurements using a dilatometer (Fujimaster EDP FTF-200 manufactured by Fuji Denpa Koki Co., Ltd. and Transmaster II manufactured by Advance Riko Co., Ltd.). The heating and cooling rates during the heat treatment controlled by these devices were 2 K/s and −100 K/s, respectively.
Tensile tests were performed using an Instron type testing machine (TCM-50kNB manufactured by Minebea) with a crosshead speed of 3.33 × 10−3 mm/s for small test pieces with a parallel part of 3 × 6 × 1 mm3 (w × l × t). They were conducted at room temperature under the conditions of a strain rate of 5.56 × 10−4/s.
The relationship between the austenitizing conditions and the austenite grain size was first investigated, which is the information required for controlling the initial austenite grain size before the IQ in one-step Q&P. Figure 2 shows the grain size measurements for the water-quenched 0.4C steel after holding for 1.8 ks at various austenitizing temperatures. The grain size increased markedly with increasing austenitizing temperature, and the grain size could be widely changed from 20 to 573 μm in the temperature range of 1103 to 1473 K. As a result of hardness tests for these specimens, it was confirmed that all the specimens have an identical hardness at approximately 720 HV. In other words, the 0.4C steel used in this study possesses sufficient hardenability to undergo martensitic transformation by water cooling, regardless of its austenite grain size.
Relation between prior austenite grain size and austenitizing temperature in water-quenched 0.4C steel.
In the one-step Q&P process, since the amount of untransformed austenite left after the first IQ is important to control, it is necessary to grasp the Ms and Mf temperatures and determine the appropriate temperature of IQ. It should be noted that the Ms temperature is generally known to depend on the austenite grain size, and it decreases as the grain size decreases.9) Therefore, to clarify the grain size dependence in this steel, the Ms temperatures measured by the thermal expansion tests were plotted as a function of the initial austenite grain size for the abovementioned specimens, with different grain sizes ranging from 20 to 573 μm (Fig. 3). As a result, it was confirmed that the Ms temperature of this steel tends to slightly decrease with decreasing initial austenite grain size, similar to previous findings. However, the difference is only about 10 K at maximum, and thus, the Ms temperature could be regarded to be almost unchanged in the range of the initial austenite grain size varied in this study. In addition, the volume fraction of the transformed martensite when cooled to an IQ temperature of 473 K was estimated at approximately 82 vol.% in all the specimens by the thermal expansion tests. Therefore, the volume fraction of untransformed austenite immediately after IQ at 473 K will be treated as identical at 18 vol.% in all the one-step Q&P processes applied to the 0.4C steel.
Relation between Ms temperature and prior austenite grain size in water-quenched 0.4C steel.
In order to investigate the phase transformation behavior during the partitioning treatment at 473 K and the effect of the initial austenite grain size on it in the IQed specimens, thermal expansion curves were obtained for the two specimens austenitized at 1103 K and 1373 K (initial austenite grain sizes of 20 μm and 284 μm, respectively) and then rapidly cooled to 473 K, followed by partitioning treatment in the thermal expansion tester. As is clearly shown in Fig. 4, both specimens exhibited significant expansion as the phase transformation from FCC to BCC during isothermal holding; i.e., bainitic transformation or isothermal martensite transformation (hereinafter referred to as bainitic transformation in this paper) occurs during the partitioning treatment at 473 K. Therefore, this heat treatment cannot be considered to be a pure Q&P process to be precise, and a mechanism similar to that of general low-alloy TRIP steel is also superposed to cause carbon enrichment to untransformed austenite. Here, focusing on the influence of the initial austenite grain size on the bainitic transformation, the 1103 K-austenitized specimen with the smaller austenite grain size obviously has a higher transformation rate. As will be described later, it is extremely difficult to reveal the morphology of bainite formed at such a low temperature. Thus, there is no sufficient evidence for this theory, but it is considered that this was because the area of the martensite/austenite interface, which becomes the nucleation sites of bainite,15) increases as the initial austenite grain size decreases.
Change in volume fraction of during partitioning treatment at 473 K after IQ in 0.4C steels with large and small grains.
In order to retain austenite and achieve high ductility, it is important to find the optimal holding time that maximizes the amount of retained austenite. For example, Fig. 5 shows the change in volume fraction of each microstructure as a function of the holding time at 473 K in the specimen austenitized at 1173 K (the grain size is approximately 40 μm) and then IQed at 473 K. (Note that the horizontal axis is logarithmic, unlike Fig. 4.) First, as already mentioned, approximately 82% of martensite is formed just after IQ at 473 K (AsQ in Fig.), and this value should be constant during the subsequent isothermal holding for partitioning, although tempering proceeds to some extent. The volume fraction of bainite estimated from the volume expansion ratio increased rapidly in the initial stage and then gradually increased. In addition, the amount of fresh martensite generated by the final cooling decreases, and simultaneously, the amount of retained austenite increases, reaching the maximum volume when holding for about 5.4 ks, and turning down when holding for more than that. The above-mentioned increase in retained austenite is explained by the enrichment of carbon into the untransformed austenite, but as described above, this is not a pure Q&P process, so the carbon scavenging due to the bainitic transformation also contributes to the carbon enrichment in addition to carbon partitioning due to long-range diffusion from martensite.
Change in volume fraction of each microstructure as a function of the partitioning treatment time at 473 K in the specimen that was austenitized at 1173 K.
The effect of the initial austenite grain size on the microstructure obtained by the one-step Q&P process is shown under the condition of an IQ temperature of 473 K and holding time for partitioning of 5.4 ks. Figure 6 shows the relationship between the volume fraction of retained austenite measured at room temperature and the initial austenite grain size. For comparison, the amount of retained austenite in specimens directly quenched to room temperature after austenitization at various temperatures is also presented. In the directly quenched specimen, the volume fraction of retained austenite is only 2%, regardless of the initial austenite grain size. On the other hand, in the Q&Ped specimens, even the lowest volume fraction is approximately 7.7%, which demonstrates the effectiveness of the one-step Q&P process. In addition, the volume fraction of retained austenite tends to increase with the refinement of the initial austenite grains, and a retained austenite of approximately 10% was obtained when the initial austenite grain size was refined to 20 μm. Figure 7 shows the carbon concentration estimated from the lattice parameters in the retained austenite16) in Q&Ped specimens with various initial austenite grain sizes. The point to be noted here is that the carbon concentration in the retained austenite is not uniform and should be varied depending on the grain size and the formation history.17,18) Thus, these values measured by X-ray diffraction are the average values in the specimens. It is found from this data that the carbon concentration in the retained austenite is higher than the initial composition (0.38 mass%) of the 0.4C steel in any initial austenite grain size, and the carbon was effectively enriched into the untransformed austenite by the one-step Q&P process. Furthermore, it was confirmed that the carbon concentration in the retained austenite increases significantly with the refinement of the initial austenite grains. This is in good agreement with the tendency shown in Fig. 6 that the retained austenite volume fraction increases with the refinement of the initial austenite grains, indicating that the carbon concentration greatly contributes to the stabilization of austenite. However, the generally known composition dependency of Ms temperature19) predicts that the carbon enrichment of at most 0.4% confirmed here would lower Ms by only 170 K. Since the Ms of the initial austenite is 600 K, as shown in Fig. 3, the Ms temperature of the retained austenite after Q&P is unlikely to drop to less than 430 K, which is higher than room temperature. This suggests that the carbon may be localized in the untransformed austenite, as described later, and that the Ms temperature may be additionally lowered due to the grain refining effect.9,20)
Relation between the volume fraction of retained austenite measured at room temperature and the initial austenite grain size in 0.4C steels with Q&P and direct quenching.
Relation between carbon concentration in retained austenite and initial austenite grain size in Q&Ped specimens.
The microstructure of the one-step Q&Ped specimen was observed by SEM-EBSD to understand the morphology of each structure and the distribution of retained austenite grains. Figure 8 shows low-magnification and high-magnification orientation imaging maps and shows only the FCC phase extracted from the high-magnification images for the one-step Q&Ped specimens with initial austenite grain sizes of 20 μm and 284 μm. The 20 μm specimen contains multiple prior austenite grains in the field of view, and martensite blocks with various crystal orientations exist, while in the 284 μm specimen, parallelly arranged martensite blocks with a pair of crystal orientations occupy a large part of the image, which corresponds to a packet. As mentioned above, it can be confirmed that the distribution of the martensite substructures changes depending on the initial austenite grain size, but there seems to be no noticeable difference in the width of the blocks from these observations. Here, note that the Q&Ped specimens contain bainite in the microstructure, as mentioned above. However, the lower bainite formed at a low temperature of 473 K is difficult to distinguish from martensite in the orientation imaging maps because of their similarity in morphology and even crystallographic character.21) Focusing on the distribution and amount of FCC phase (retained austenite) in Figs. 8(e) and 8(f), it was confirmed in the 20 μm specimen (f) that the retained austenite grains with a size of several microns are dispersed in the microstructure. Among them, jagged-shaped austenite is also present, as indicated by the arrow, which indicates that the carbon is concentrated at the tip of the bainite laths or martensite laths and stabilizes the austenite adjacent to them. On the other hand, in the 284 μm specimen (e), an image of the retained austenite, which should be present in approximately 8%, is hardly observed. The austenite likely transformed to martensite during the preparation of the EBSD specimens owing to its low stability. The difference in austenite stability between both specimens, which will be discussed in the next chapter, would be related to the difference in size as well as carbon concentration. In addition to the retained austenite shown in Fig. 8, much finer film-like austenite was also observed with TEM. Figure 9 shows the TEM images and diffraction patterns obtained in the Q&Ped specimens with initial austenite grain sizes of 39 μm. The tiny particles shown in the dark field image (b) are the retained austenite grains existing at lath boundaries, and their thickness is only several nanometers to several tens of nanometers. Judging from their morphology and size, they are recognized to be ‘film-like austenite’ retained along martensite laths existing even in as-quenched martensite,22) or austenite retained between bainite laths stabilized during the partitioning treatment. It was confirmed that these retained austenite grains have the Kurdjumov–Sacks crystallographic orientation relationship, described by the following equation with the surrounding martensite:
| (1) |
EBSD orientation imaging maps with low-magnification (a) (b) and high-magnification (c) (d) and that showing only FCC phase extracted from the high-magnification images (e) (f) for the Q&Ped specimens with initial austenite grain sizes of 284 μm (a) (c) (e) and 20 μm (b) (d) (f). (Online version in color.)
TEM images and diffraction pattern obtained in the Q&Ped specimens with initial austenite grain sizes of 39 μm, which shows the distribution of retained austenite at lath boundaries.
On the other hand, as shown in the TEM image in another field of view of Fig. 10, needle-shaped precipitates of several tens of nanometers are observed within the laths. By analyzing the selected area diffraction pattern, the precipitates observed in Fig. 10 were identified as the transition carbides generally reported in carbon steels: namely, η-carbide with an orthorhombic structure. Although 1.2% Si was added to this specimen to suppress cementite precipitation in this specimen, Speer et al. reported that Si did not contribute much to the suppression of transition carbides.5) Thus, η-carbide was thought to be precipitated within laths during the partitioning treatment at 473 K.
TEM images and diffraction pattern obtained in the Q&Ped specimens with initial austenite grain sizes of 39 μm, which shows the distribution of transition carbide particles within laths.
Figure 11 shows nominal stress–nominal strain curves (a) and the true stress–true strain as well as work hardening rate curves (b) of Q&Ped specimens with initial austenite grain sizes of 20 μm and 284 μm. Comparing the mechanical properties of the two stress–strain curves (a) reveals that the 20-μm specimen has a higher tensile strength as well as a larger elongation than the 284-μm specimen. It is found from the work hardening rate curves (b) that the refinement of initial austenite grains results in a larger work hardening rate up to the high-strain region, leading to the larger uniform elongation. The reasons for the improved mechanical properties by the initial austenite grain refinement could be explained mainly by the higher carbon concentration in the retained austenite grains, which leads to a sufficient stability of the austenite that realizes the prolonged TRIP effect to a higher strain region. The higher strength of the deformation-induced martensite due to the high carbon concentration would also contribute to more significant stress partitioning effect,23) increasing the work hardening rate.
Nominal stress–nominal strain curves (a) and the true stress–true strain and work hardening rate curves (b) of Q&Ped specimens with initial austenite grain sizes of 20 μm and 284 μm.
In addition, the initial austenite grain refinement also results in the refinement of the retained austenite grains formed through the Q&P process, as described later. This also might have caused a positive effect on the mechanical properties, but there is no sufficient evidence yet to further explain it.
As shown in Fig. 7, the average carbon concentration in the retained austenite increased from 0.62% to 0.73%, accompanied by a decrease in the initial austenite grain size from 284 μm to 20 μm. Because the average chemical composition of the steel used is 0.38%, the carbon concentration in the retained austenite increases by a factor of 1.5 to 2. As described above, the carbon enrichment into austenite could be explained by two mechanisms: one is carbon partitioning due to long-distance diffusion from martensite that is originally assumed in the Q&P process, and the other is carbon scavenging due to bainitic transformation, which is generally confirmed to occur in commercial low-alloy TRIP steels. First, let us consider the latter contribution. The IQ at 473 K retains 18 vol.% untransformed austenite, and the subsequent partitioning transforms 6 to 7 vol.% austenite into bainite, as shown in Fig. 5. If the carbon in the austenite part that existed before the bainitic transformation was fully extruded to the austenite side by the transformation, the carbon concentration of the untransformed austenite should be enriched to 0.57% to 0.62% because of the mass balance. Most of the enriched carbon concentration can be explained by the contribution of the bainitic transformation. However, that value is slightly below the experimental one, and it has been suggested that some amount of solute carbon remains in bainite in the case of low-temperature transformation,24) and thus, it should be considered that carbon partitioning from martensite also occurs. On the other hand, if it is assumed that the partitioning from martensite occurs up to equilibrium, the carbon concentration in austenite can be estimated by the calculation using the CCEθ model25) proposed by Toji et al. The CCEθ model predicts the carbon concentration in each phase, such that the carbon potential in the ferrite and cementite (θ) in the martensite is identical. In addition, the carbon potential in the austenite is the same as that in ferrite. This condition is expressed by the following equations:
| (2) |
| (3) |
As mentioned in the last paragraph in Section 4.1, the increase in average carbon concentration due to the refinement of the initial austenite grain size can be reasonably explained if we consider that the initial austenite grain refinement leads to the refinement of the untransformed austenite grains formed after Q&P. However, it is difficult to distinguish among the three types of microstructures: the initially formed martensite by IQ, bainite formed during partitioning, and fresh martensite generated on the final cooling after partitioning. Therefore, it is extremely difficult to experimentally observe the difference in the size and distribution of untransformed austenite in this steel. To solve this problem, Tsuchiyama et al. used high-alloy steel that does not cause bainitic transformation during partitioning treatment,6) and also applied the G-T heat treatment method11) to visualize the difference between the initial martensite and fresh martensite. The high-alloy steel used here is a conventional martensitic stainless steel, Fe-12%Cr-0.12%C (SUS410). The Ms temperature of this steel is around 573 K, which is close to that of the 0.4C steel used in the above experiments. By setting the austenitizing temperatures at 1223 K and 1423 K, the initial austenite grain size was controlled to be approximately 30 μm and 120 μm, respectively. The G-T method was then performed as follows: First, approximately 80% of martensite was produced by IQ at 473 K, immediately heated to a high temperature of 873 K, held for 60 s, and then rapidly cooled again. By this heat treatment, the martensite produced via IQ undergoes tempering by heating for a short time to precipitate carbides, but the fresh martensite produced during the final quenching has a supersaturated carbon in the solid solution. Since the etching conditions of both microstructures differ depending on the state of carbon, they become distinguishable. Figure 12 shows optical micrographs of the SUS410 with different initial austenite grain sizes treated with the G-T method. The strongly etched lath-like structure was determined to be the initial martensite formed by IQ because it was confirmed that its volume fraction increased with a decrease in IQ temperature. The remaining lightly etched area is fresh martensite, corresponding to the untransformed austenite. In both specimens, it was observed that the initial austenite grains were divided by many martensite plates, and the untransformed austenite grains exist in the space between these martensite plates. Comparing the two specimens revealed that the initial austenite grains were more finely divided in the specimen with smaller initial austenite grains, resulting in the finer untransformed austenite grains. In the specimen with smaller initial austenite grains, the size of most of the untransformed austenite grains is very fine, at approximately 1 to 3 μm, while in the specimen with larger initial austenite grains, a wide range of grain sizes were found to be mixed, and a very large grain was also observed, with a size of approximately 20 μm. The diffusion distance of the carbon in the untransformed austenite was roughly estimated using the following equation:
| (4) |
Optical micrographs of the SUS410 with different initial austenite grain size treated with G-T method. The specimens with large grain size (a) and small grain size (b) were austenitized at 1423 K for 10.8 ks and 1223 K for 0.3 ks, respectively. The strongly etched lath-like structure corresponds to initial martensite formed by interrupted quenching.
Based on the above hypothesis, Fig. 13 schematically shows the microstructure development that occurs through the one-step Q&P process when the initial austenite grain size is large and small. The size of the untransformed austenite after the IQ becomes finer by the refinement of the initial austenite grains, which leads to a larger area of the martensite/austenite interface (a). This promotes both the carbon scavenging due to bainitic transformation and carbon partitioning from martensite; thus, the stabilization by carbon enrichment becomes more pronounced when the initial grain size is smaller (b). When the initial grain size is large, it is expected that martensite transformation will occur in the central part of the large grains upon the final cooling after the partitioning treatment (c). Because the TRIP effect is more effectively exerted by the increased volume fraction and the enhanced stability of the retained austenite, the mechanical properties of the Q&Ped steel are improved by the refinement of the initial austenite grains.
Schematic illustrations showing the microstructure development that occurs trough the one-step Q&P process in 0.4C steels with large and small initial austenite gran size. The γ, α’, B, and F α’ denote austenite, martensite, bainite, and fresh martensite, respectively.
Fe-0.4C-1.2Si-0.8Mn (mass%) alloys with different austenite grain sizes ranging from 20 to 573 μm (initial austenite grain size) were subjected to the one-step Q&P process, where the IQ and partitioning were performed at the same temperature of 473 K. After investigating the influence of the initial austenite grain size on the microstructure development and mechanical properties, the following findings were obtained.
(1) As the initial austenite grain size before the IQ becomes finer, the enrichment of carbon in the untransformed austenite during the partitioning treatment is enhanced, which leads to a greater increase in the volume fraction of retained austenite. However, carbon partitioning within the untransformed austenite did not reach an equilibrium state under the condition of the 473 K–5.4 ks adopted in this study.
(2) The enhancement of carbon by the refinement of the initial austenite grains is explained by the following two mechanisms: (i) the effect of promoting carbon scavenging caused by the enhanced bainitic transformation due to the increase in nucleation sites, and (ii) the effect of promoting carbon partitioning from martensite to the untransformed austenite during the partitioning treatment. Both mechanisms are derived from an increase in the area of the martensite/austenite interface due to the refinement of the initial austenite grains.
(3) The mechanical properties of the composite microstructured steel obtained by the one-step Q&P treatment improved with the refinement of the initial austenite grains in both strength and elongation. We speculate that this is because the TRIP effect is more effectively exerted by the increase in the volume fraction of retained austenite, the increase in its stability, and the increase in the strength of martensite produced by the deformation-induced transformation of the retained austenite.
This study was supported by JSPS KAKENHI Grant Number JP17H01333.
