Prior Austenite Grain Size Research Articles

Design and control the selection of martensitic variant to simultaneously improve strength and toughness of low-carbon martensitic steel

In this paper, the microstructure evolution and mechanical properties of low carbon steels during direct quenched and rolling followed by water-cooled processes were studied. Two experimental steels, which were directly quenched at 900 °C (Q900) and 1000 °C (Q1000), were compared with steels that were water-cooled after rolling at the same temperatures (R900 and R1000). Microstructural analyses using EBSD and TEM revealed that rolling reduced the size of prior austenite grains (PAGs), resulting in an average width of 3.6 μm, which influenced grain boundary distributions and variant selection. The best combination of strength, ductility and toughness was obtained in R900 steel, including tensile (with the yield strength of 1304 MPa, the total elongation of 22.95 %), Charpy impact (with the impact energy at 20 °C is 182 J), and fracture toughness evaluations (with the J1c is 326.28 KJ/m2), this demonstrates that R900 steel exhibited significantly enhanced strength and ductility compared to Q900 steel. Moreover, EBSD analysis of crack propagation paths highlighted the role of high-angle grain boundaries (HAGBs) in enhancing fracture toughness by deflecting cracks. These findings underscore the critical role of PAGs size in tailoring microstructures to achieve superior mechanical properties in low carbon martensitic steels, offering insights for advanced material design and application in demanding structural and industrial contexts.Keyworks.low-carbon martensitic steel; strength and toughness; martensitic transformation; ductile-to-brittle transition phenomenon; selection of martensitic variants.

Effect of austenitizing temperature on variant selection of martensite transformation in a 2 GPa grade steel

The correlation of prior austenite grain (PAG) size, mechanical properties, and the variant selection in martensite transformation of a 2 GPa grade automobile steel has been systematically investigated by using a combination of scaning electron microscopy (SEM), electron backscatter diffraction (EBSD), and uniaxial tesile tests. As the austenitizing temperature decreases from 970 °C to 870 °C, the as-quenched microstructure of the studied steel is fully lath martensite. Meanwhile, the average size of PAG is refined from 7.2 ± 3.9 μm to 2.1 ± 1.2 μm, and the volume fraction of high angle grain boundaries (HAGB) is increased from 59.8% to 72.3%, accompanied with an increased number fraction of grains belonging to closely spaced planes (CP) group. With the refinement of the PAGs, the martensitic variant selectivity becomes strong, and the Bain group variant selection discretely transites to the CP group, resulting in a dominate CP group. Consequently, a tensile strength of 2014 ± 33 MPa, a tensile elongtation of 12.9 ± 0.5% and a maximum strength × ductility production of 26.0 ± 1.4 GPa·%, are obtained when the austenitization temperature is at 920 °C.

Enhancing strength and toughness in a pearlitic rail steel via multi-alloying and accelerated cooling

The demand for higher speeds and heavier loads has emphasized the necessity to tackle the inherent trade-off between strength and toughness in pearlitic rail steels. Herein, the co-additions of Cr, Ni and Cu and accelerated cooling rate on the microstructure and mechanical properties of a medium carbon pearlitic steel were systematically investigated. In comparison with base alloy, the new steel exhibits smaller prior austenite grain size (PAGS) due to the drag effect of Cu and Ni solutes. The finer PAGS and lower C content increased the volume fraction of proeutectoid ferrite. The finer pearlitic nodule size (PNS) and pearlitic colony size (PCS) were attributed to the small PAGS and greater extent of undercooling due to the combined effect of multi-alloying and accelerated cooling. Additionally, the synergistic effects of multi-alloying and accelerated cooling rate decrease the pearlitic phase transformation temperature, accountable for the finer interlamellar spacing (IS). Compared with the base steel, the yield strength and ultimate tensile strength of the new steel increased from 587 MPa and 1069 MPa to 740MPa and 1178MPa, respectively, with a simultaneous increase of ductility from 12.4% to 14.7%. The strength increments are mainly attributed to the finer IS in the new steel. Meanwhile, the new steel possesses a higher impact toughness compared to the base steel due to the increased volume fraction of proeutectoid ferrite, and finer PNS and IS.

The effect of prior austenite grain size on crystallography and variant selection in carbide-free nano-structured bainite

Prior austenite grain size (PAGS) significantly affects the microstructure and in turn the mechanical properties of nano-structured bainite. In this study, information about crystallographic variants/blocks as well as packets has been extracted using electron backscattered diffraction (EBSD) technique for two blocks of nano-baintic steels with different PAGS obtained by austenitization at two different temperatures of 900°C and 1000°C respectively. However, the bainitic fraction has been maintained to be similar by choosing the same isothermal temperature of 250°C for transformation to bainite. The child-parent orientation relationship (OR) was determined to be close to Kurdjumov-Sachs (K-S) using EBSD for larger PAGS specimen while smaller PAGS specimen displayed closeness to K-S OR predominantly and Nishiyama-Wassermann (N-W) OR in some regions. It was observed that a higher PAGS leads to finer bainitic lath thickness, packet size and block width size. Stricter variant selection was found in smaller PAGS sample even though variant pairing was observed between variants of higher misorientation. This hints towards small number of nucleation sites for a grain and space constraint, rather than the tendency for pairing as the reason for selection.

Influence of prior austenite grain size on martensite-austenite islands morphology and decomposition characteristics during two-step tempering in Mn-Ni-Mo steel

Martensite-austenite (M/A) islands are typical in the quenched microstructure of Mn-Ni-Mo steel, frequently utilized for fabricating reactor pressure vessels (RPV). These islands can decompose into harmful long rod-shaped Fe3C carbides during tempering above 600°C, quantifying the negative effect of M/A island decomposition on mechanical properties and indicating a potential issue in RPV fabrication. To address this issue, the present study investigates a two-step tempering process on three different prior austenite grain size (PAGS) specimens (5 µm, 47 µm, and 119 µm). The effect of PAGS on M/A island decomposition characteristics during sub-600°C tempering and their impact on precipitated carbide morphology is investigated. A preliminary pre-tempering treatment at 200°C, 350°C, and 500°C for 2 hours was followed by a 3-hour final tempering step at 650°C. The findings revealed unique temperature-dependent decomposition characteristics of M/A islands. For example, at 200°C, partial M/A island decomposition took place, resulting in long rod-shaped carbides during final tempering. Meanwhile, the 350°C pre-tempering specimen revealed aggregated carbide precipitates of varying sizes in former M/A island locations. In contrast, the 500°C pre-tempering specimen showed coarse globular carbides and the absence of carbide precipitate clustering. This result can be attributed to the decrease in surface energy associated with the carbide-matrix interface as size increases. After tempering, all specimens produced three types of precipitates: rod-shaped Fe3C carbides, rhombus-shaped Mn7C3, and globular-shaped Mn23C6 carbides. The formation of long rod-shaped carbides during M/A island decomposition reduces the critical cleavage stress for micro-crack initiation, jeopardizing steel mechanical properties. Aggregated carbides cause the simultaneous formation of numerous micro-voids in close proximity, which eventually merge to form a large crack, resulting in material failure. When compared to other pre-tempered and directly tempered specimens, pre-tempering at 500°C produced the most favorable microstructure in terms of impact properties.

The effect of network cementite dissolution on the nucleation and growth of prior austenite grains in high carbon low alloy steels

The state of austenite grains has an essential role in the microstructure transformation and mechanical properties of alloy steel during heat treatment or subsequent hot working. The initial microstructure of alloy steel is inseparable from the nucleation and growth of austenite grains. Hence, in this work, high carbon low alloy steel with the initial microstructure of lamellar pearlite and grain boundary cementite was selected, and the effect of cementite dissolution on austenite grain growth was analyzed from the perspective of the phase transition dilatometer curve. Three nucleation sites in high carbon low alloy steel were found by multi-perspective characterization, forming two types of austenite grains: pearlite-austenite grain and cementite-austenite grain. Under cementite pinning and solute drag effect, the growth of the prior austenite grains was retarded during the isothermal temperatures process. The average size of prior austenite grains increased only from 65.1 μm (1050 °C) to 69.8 μm (1150 °C). This work provides a foundation for optimizing the prior austenite grains state by utilizing grain boundary cementite dissolution.

Inclusions and microstructures in coarse-grained heat-affected zone of Al–Ti–Ca deoxidized shipbuilding steels with different Al contents after high-heat input welding

The inclusions, microstructure evolution and impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Al–Ti–Ca deoxidized shipbuilding steel plates with different Al contents after high-heat input welding of 400 kJ cm−1 are investigated. The characteristics of inclusions and their influence on the refinement of microstructure and impact toughness in CGHAZ was elucidated. Increasing the Al content from 0.016 wt% to 0.022 wt% and 0.043 wt% results in an increase in the average size of prior austenite grain and the brittle phase fractions, while decreasing the content of intragranular acicular ferrites (IAFs). The results of the thermodynamic calculations, high-temperature laser scanning confocal microscopy and differential scanning calorimetry all show that increasing the Al content can diminish the austenitic stable region, elevate the transition temperature from austenite to ferrite and suppress the nucleation of IAF. In the present steels, the IAF formation is successfully induced by the specific inclusions, namely Al2O3–CaO–Ti2O3–MnS(-CaS), in which the calcium aluminate inclusions are embedded with Ti-oxides and CaS–MnS precipitates. The effective inclusions have the number density of 93 mm−2 in 16Al steel and 27 mm−2 in 22Al steel, but they are rarely found in 43Al steel. On average, an effective inclusion can induce 3.4 IAFs in 16Al steel and 2.8 IAFs in 22Al steel. Thus, as the Al content increases from 0.016 wt% to 0.22 wt% and 0.043 wt%, the impact toughness of CGHAZ at −40 °C decreases from 134 J to 78 J and 24 J with the welding heat input of 400 kJ cm−1.

Postweld Heat Treatment on Toughness of Electric- Resistance Welded X70 Steel

The effect of postweld heat treatment (PWHT) on Charpy V-notch impact toughness of a highfrequency electric-resistance welded (HF-ERW) grade X70 pipeline steel is investigated. PWHT thermal cycles are simulated using the Gleeble on the as-welded specimens. Microstructure, along with crystallographic texture and microhardness, is characterized in specimens that are impact tested at –5°C (23°F), –30°C (–22°F), and –45°C (–49°F). The impact toughness values show a wide scatter band and decrease with the increasing peak temperature of the PWHT. For higher peak temperatures, the microstructure of the heat-affected zone (HAZ) gradually changed from equiaxed ferrite to bainitic ferrite. Furthermore, the prior austenite grain size (PAGS) increases with the increasing peak temperature. The density of high-angle grain boundaries decreases, and the fraction of cleavage planes {100} parallel to the impact fracture plane increases for higher peak temperatures of the PWHT.

Prior austenite grain measurement: A direct comparison of EBSD reconstruction, thermal etching and chemical etching

It is well known that the prior austenite grain (PAG) microstructure of steels has a significant impact on their microstructure evolution and mechanical properties. Many PAG measuring techniques have been developed over many decades, with the effectiveness of each varying alloy-to-alloy. Some of the more common techniques, specifically for bainitic and martensitic grades, are thermal etching and picric acid etching, which both involve the preferential etching of PAG boundaries in order to reveal the austenitic microstructure. More recently, parent grain reconstruction techniques using EBSD (electron backscatter diffraction) mapping have shown good promise in recreating PAG microstructures from BCC-FCC orientation relationships, and can now be deployed at speed with the advent of rapid detectors (1000s of pixels per second). This study aims to compare the accuracy and relative advantages/disadvantages of picric acid etching, thermal etching and EBSD reconstruction methods. A TESCAN and NewTec In-Situ Testing (TANIST) capability was used to directly observe and measure the true high-temperature PAG structure during the austenitisation of SA-540 B24 low alloy steel. These high-temperature PAG measurements were then compared to measurements from the same area obtained using thermal etching, picric acid etching and EBSD parent grain reconstructions after quenching to room temperature. Reconstructing the parent austenite grains from EBSD data resulted in the closest measurement of PAG size. PAG boundaries were delineated well by thermal etching but surface effects (such as surface relief and ghost traces) created complexities when identifying the exact position of boundaries. Picric acid etching, which produced the least accurate measurement, was found to reveal PAG boundaries well, however it was limited by its ability to sufficiently etch annealing twin boundaries and its susceptibility to microsegregational effects. EBSD reconstruction and thermal etching were more consistent at reconstructing/revealing these boundaries, although inaccuracies with the techniques were still observed.

Improving the Weld Heat-Affected-Zone (HAZ) Toughness of High-Strength Thick-Walled Line Pipes

The low-temperature fracture toughness of double-V weld seams is a well-known challenge due to the essential increased heat input for heavy-wall pipelines. A thorough investigation was conducted to explore the impact of the heat input on the grain size and precipitate coarsening, correlating the microstructure with the heat-affected-zone (HAZ) toughness. The results indicated that the actual weldments showed a toughness transition zone at −20 °C, with considerable scattering in Charpy V-notch (CVN) tests. Gleeble thermal simulations confirmed the decreased toughness of the coarse-grained HAZ (CGHAZ) with increasing heat input and prior austenite grain size (PAGS). A specially designed thermal treatment demonstrated its potential for enhancing the toughness of the CGHAZ, with the recommended thermal cycle involving peak temperatures of 700 and 800 °C, holding for 1 s, and rapid cooling. The toughness of the intercritically reheated CGHAZ (ICCGHAZ) improved with higher intercritical reheating temperatures and the removal of necklace-type M–A constituents along the PAG. Despite various thermal treatments, no significant improvements were observed in the toughness of the ICCGHAZ. Future work was suggested for optimising the use of tack welds to reduce the effective heat input (HI) associated with double-sided submerged arc welding (SAW).

In-situ heating-stage EBSD validation of algorithms for prior-austenite grain reconstruction in steel

Prior-austenite grain (PAG) structures play a critical role in determining steel microstructures and properties. Over recent years, the use of algorithms that reconstruct PAGs from electron backscatter diffraction (EBSD) measurements of martensitic or bainitic microstructures have become commonplace. These appear to work well, but there has yet to be a comparison made of the reconstructed PAG microstructure to the true PAG microstructure of a steel measured above Ae3 before transformation. Here, in-situ EBSD was used to record the austenite grain structure (at 1000 °C) before transformation, as well as the progress of subsequent martensitic and bainitic transformations. The resulting dataset, which we have made available to the community here: https://doi.org/10.5281/zenodo.8348372, was used to assess of two popular algorithms for PAG reconstruction. They were found to be accurate enough for PAG size measurement and general texture analysis, although the exact morphologies of some PAG and twin boundaries are sometimes lost.

Study on the materials flow, microstructure and mechanical properties of ultrasonic vibration-assisted friction stir weld of 1500 MPa martensitic steel

In this study, friction stir welding (FSW) was conducted on 1500 MPa martensitic steel with a thickness of 2.3 mm at 500 rpm and 60 mm/min with and without the application of ultrasonic vibration. The utilization of pure nickel as a marker material could help to visualize the material flow behavior throughout the whole FSW process. In addition, the width change of the marker material was used to distinguish the welding stages. At the same time, the microstructure was also characterized to study the effect of ultrasonic vibration on the welding process. The conventional FSW (C-FSW) joint had tunnel defects due to poor material flow, while the ultrasonic vibration-assisted FSW (U-FSW) joint was defect-free. To study the microstructure and texture evolution during the process, a fully austenitized welding zone was reconstructed from martensite to prior austenite grain (PAG) based on the Kurdjumov-Sachs orientation relationship. The strain rate increased in U-FSW compared to that in C-FSW, resulting in a reduction of the PAG size and an alteration of its shear texture components from type-A to type-B. The uniaxial tensile tests showed that the ultimate tensile strength (UTS) of the stir zone (SZ) of U-FSW was significantly increased compared with that of C-FSW.

Microstructural characterization and mechanical properties in resistance spot welding of Q&P980 steel involving “effective softening” at the fusion boundary

This study proposes a welding process for Q&P980 steel that involves prolonged post-weld pulses. The process effectively softens the nuggets and results in welded joints with excellent comprehensive mechanical properties. The relationship between microstructural characteristics, element distribution and mechanical properties of weldments was elucidated for the samples subjected to different heat input histories. The microstructure evolution was analyzed using electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), and the mechanical properties of the spot welds were evaluated at room temperature by tensile shear (TS) and cross tension (CT) tests. The findings indicate that the softened area expands from the fusion boundary towards the opposite direction of the nugget as the heat input increases. A mixed microstructure of martensite and retained austenite, characterized by a larger size of prior austenite grains and lower kernel average misorientation, is formed in the “effective softening” region. This region provides serves as the primary location for crack nucleation and governs the propagation path of cracks within the halo ring-like softening zone, which has a significant impact on the fracture mode of the weld. The energy absorption in the TS test was increased by 95.40% compared to the single pulse weld (SPW), while the best combination of strength and energy absorption of the weldment in CT test was 7.2 kN and 64.24 J, respectively.

Micromechanical analysis and finite element modelling of laser-welded 5-mm-thick dissimilar joints between 316L stainless steel and low-alloyed ultra-high-strength steel

As base metals (BMs), plates of 5-mm-thick low-alloyed ultra-high-strength carbon steel (LA-UHSS) with a tensile strength of 1.3 GPa and 5-mm-thick 316L austenitic stainless steel were laser-welded at two different energy inputs (EIs; 60 and 100 J/mm). The microstructural characteristics of the fusion zones (FZs) in the welded joints were examined using electron backscattering diffraction (EBSD) and transmission electron microscopy. The fine microstructural components, such as the prior austenite grain size (PAGS) and effective grain size of the fresh martensite promoted during welding, were analysed by processing the EBSD maps using MATLAB software. The micromechanical performance of the weldments was investigated using microindentation hardness (HIT) to display the mechanical responses of different zones. Uniaxial tensile testing was conducted to explore the joint strength and plasticity failure. The dominant phase structures promoted in the FZs at low and high EIs were similar, that is, martensite with a small fraction of austenite. The HIT values displayed a distinct variation in strength between different zones. The HIT values of 316L, LA-UHSS, and FZ were 1.95, 5.55, and 4.63 GPa, respectively. The PAGS increased from 45 to 70 μm with an increasing EI, and a finer martensitic grain structure with an average size of 2.62 μm was observed at high EIs. The mechanical tensile properties of the dissimilar joints at the studied EIs closely matched those of the BM 316L, demonstrating comparable yield and tensile strengths of 225 MPa and 650 MPa, respectively. This similarity can be attributed to the localized plastic tensile deformation occurring primarily within the relatively softer BM 316L, ultimately resulting in joint failure. The flow behaviour of the dissimilar joints under uniaxial tensile testing was analysed using finite element modelling to determine the stress and strain distributions. The plastic strain was mainly localised within the soft metal 316L owing to enhanced dislocation-mediated plasticity.

Optimizing bendability of AlSi-coated 2GPa-grade press-hardened steel by double austenitization

The present work, for the first time, demonstrates that a new heat treatment process involving double austenitization can effectively optimize the coating structure and prior austenite grain size (PAGS) of an AlSi-coated 2GPa-grade press hardened steel (PHS), resulting in an improvement of bendability while keeping similar ultimate tensile strength (UTS). It is found that the interdiffusion ferrite layer in the coating structure is responsible for the improvement of bendability while the controlled PAGS keeps similar UTS. More specifically, it shows that the interdiffusion ferrite layer with high Al content is brittle and detrimental to bendability. Consequently, a double austenitization process is employed to produce an interdiffusion ferrite layer with an Al content lower than 4 wt.%, resulting in an improvement of bending angle over 60° while keeping similar UTS.

Unforeseen influence of the prior austenite grain size on the mechanical properties of a carbide-free bainitic steel

The reasons behind the limited impact toughness of medium manganese, medium carbon carbide-free bainitic steels has been discussed in this manuscript. Various hot-rolling schedules have been employed to obtain microstructures with three different prior austenite grain sizes (PAGSs): coarse (CG = 45 ± 5 μm), medium (MG = 18 ± 4 μm) and fine (FG = 8 ± 2 μm) grain sizes. Although the coarse and middle grain size microstructures present similar levels of strength (tensile strength ∼1000 MPa) and ductility (36% and 41%, respectively), the impact toughness measured at room temperature improves significantly from 17 J to 83 J in the middle grain size one because of the lower ductile-to-brittle transition temperature (DBTT) temperature. Compared to the other microstructures, the fine grain size one contains higher contents of blocky unstable retained austenite, which are responsible for the enhanced strain hardening behavior under tensile testing but can be harmful for the toughness. Besides, during the first stages of straining these blocky austenite islands transform effortlessly, deteriorating the ductility (30%) and toughness (58 J) in spite of keeping a similar DBTT as the microstructure with the medium grain size.

Achieving ultrahigh strength by tuning the hierarchical structure of low-carbon martensitic steel

The high strength of martensitic steel can be attributed to its fine hierarchical structure and high dislocation density. To further improve the mechanical properties of low-carbon martensitic steel, a nano-lamellar structure in size of 28 nm was first produced by warm rolling on the dual-phase structure. Subsequent annealing at 870 °C for different times was performed to tune the hierarchical structures of the martensitic steels with different prior austenite grain (PAG) sizes. The WRQ-15 steel sample, which underwent a 15-min annealing process, exhibits an outstanding combination of strength and elongation with a yield strength (YS) of 1.55 GPa, ultimate tensile strength (UTS) of 2.0 GPa, and elongation to failure (ETF) of 8.7%. It shows higher strength than the martensitic sample annealed from undeformed steel (UQ-15 steel), which has a YS of 1.31 GPa and UTS of 1.68 GPa. The WRQ-15 sample has a finer PAG size (7.2 μm) than the UQ-15 sample (9.5 μm). The results reveal that the refined PAGs lead to further refinement of packets, blocks, and laths, as well as increased dislocation density, contributing to the superior mechanical properties of the WRQ-15 sample. Moreover, the refined hierarchical structures provide high-density boundaries, impeding the propagation of cleavage cracks to improve the toughness. However, the excessive refinement of the hierarchical structures, increase of dislocation densities, and the undissolved carbides in the WRQ-1 sample are harmful for the strength-toughness balance.

Influence of hydrogen on the hydraulic fracture behavior of a 42CrMo4 steel welds: Effect of the prior austenite grain size

The influence of hydrogen on the mechanical behavior of a quenched and tempered 42CrMo4 steel has been evaluated by means of high internal pressure fracture tests carried out on hydrogen precharged notched cylindrical specimens. The notched cylindrical specimens were precharged in a 1 M H2SO4 + 0.25 g/l As2O3 solution for 3 h with 1.2 mA/cm2. Hydraulic fracture tests were performed at different loading rates. Hydrogen embrittlement resistance increased with grain size refinement although the fine grained specimen had a higher hydrogen content than the coarse grained one. Fractographic analysis showed hydrogen enhanced decohesion fracturewas less pronounced with decreasing grain size. Hydrogen embrittlement susceptibility is discussed in terms of the prior austenite grain size (PAGS) and the operative fracture mechanisms.

Microstructural insights into EUROFER97 batch 3 steels

Extensive analytical electron microscopical analyses were carried out from the micrometer scale down to the nanometer scale to characterize three variants of the 9% reduced activation ferritic martensitic (RAFM) steel EUROFER97/3. No huge microstructural differences were observed between the three grades. Electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM) was used to determine prior austenite grain (PAG) and lath sizes of the martensite matrix. The PAG size varied between 4.5 µm and 6.5 µm depending on the reconstruction algorithm. Furthermore, the martensitic lath sizes determined by SEM-EBSD are only half or 1/3 of that determined manually from transmission electron microscopy (TEM) images, which might be related to the limited statistics in this type of TEM data evaluations. The SEM-EDX shows that M23C6-type phases are preferentially located on lath and grain boundaries due to preferential diffusion of elements like Cr, W, and C to and along grain boundaries, which agrees with TEM-EDX measurements. TEM techniques like STEM-EDX and high-resolution TEM were used to describe the occurring precipitates i.e., M23C6, VN, TaC morphologically, structurally, and chemically. In addition, the thermodynamic calculations were carried out to explain phase formation, phase fraction and phase composition. The results are in good agreement with the experimentally determined values. These results will provide a profound basis to explain the mechanical performance of these materials. Furthermore, it will lay a good reference basis of comparison for the material after neutron irradiation.

The austenite to polygonal ferrite transformation in low-alloy steel: multi-phase-field simulation

The austenite to ferrite phase transformation is a critical structural transformation in steel production, where the morphology and grain size of ferrite substantially influence the mechanical properties of steel materials. In this work, the influences of cooling rate, prior austenite grain size (PAGS), and Mn content on the microstructure evolution and component distribution of austenite-to-polygonal ferrite phase transformation are investigated by a multi-phase-field model. It is found that higher cooling rates intensify the driving force for austenite to polygonal ferrite phase transformations and delay the phase transformation process. As PAGS decrease, the increased proportion of austenite grain boundary offers more nucleation sites for polygonal ferrite and thus refines the polygonal ferrite grain. Additionally, increased Mn content results in significant grain refinement due to a reduction in the transformation temperature of austenite to polygonal ferrite. This work provides valuable insights into adjusting and designing desired microstructures of polygonal ferrite for enhancing the mechanical performance of steel.