Thermo-mechanical Control Process Research Articles

Comparison of Novel 1 GPa Low‐Carbon, Low‐Alloyed Steel Produced with Simulated and Laboratory‐Scale Thermomechanical Controlled Processes

A laboratory‐scale hot‐rolled Ti–Mo–V–Nb steel with 1 GPa tensile strength is produced, and its microstructure and tensile properties are characterized using advanced analysis techniques and uniaxial tensile testing. A Gleeble 3800 thermomechanical simulator is used to determine a process window for the thermomechanical controlled processing (TMCP) procedure. Although the simulated TMCP specimens are fully ferritic at coiling temperatures (CT) of 590 and 630 °C, the bainitic and mixed (bainitic + ferritic) microstructure is formed in the hot‐rolled steels. The variation in the microstructure causes variations in the dislocation density through the sheet thickness, which significantly reduces the steel's ductility properties, whereas a 16% elongation is achieved with the fully bainitic microstructure. Another significant difference between the simulated TMCP and hot‐rolled specimens is the precipitation behavior. No nanosized interphase‐precipitated (IP) carbides are formed in the hot‐rolled steel during the austenite‐to‐ferrite phase transformation, although the formation of the nanosized spherical IPs is observed within the polygonal ferrite grains of the simulated TMCP specimens at the CT of 630 °C. Relatively coarse (5–20 nm) spherical (V,Mo,Ti,Nb)C carbides do not strongly affect the tensile properties of the hot‐rolled Ti–Mo–V–Nb steel. The results show that the dislocation and grain boundary strengthening mainly contribute to the strength properties of this steel.

A Physically Based Mean Field Model for Strain‐Induced Precipitation and Recrystallization in High‐Strength Low‐Alloy Steels

A physically based mean field model developed to predict the microstructural evolution during the thermomechanical control process of X70 high‐strength low‐alloy (HSLA) steels is presented. The physically based mean field model incorporates a new integrated precipitation and recrystallization model developed to describe the interaction between strain‐induced precipitation of niobium and titanium carbonitrides and static recrystallization of austenite. The integrated model considers an effective Zener pinning force for the multimodal particle size distribution (PSD) of precipitates, an effective grain‐boundary mobility for the solute drag effect of niobium, and an inhomogeneous stored energy for austenite recrystallization. Given a processing route, the model predicts the variation of austenite grain size, recrystallized and precipitated fractions, and evolution of PSDs of precipitates. Model predictions reveal an excellent agreement with experimental grain size measurements and a final average ferrite grain size of 3.81 μm is achieved. The proposed model considers the heterogeneous nature of recrystallization and precipitation and can contribute to the process design of the HSLA and microalloyed steels.

Study on the evolution of multistage and multiscale Ti-bearing precipitation and microstructure in ultrahigh-strength titanium microalloyed weathering steels

In this study, 900 MPa ultrahigh-strength weathering steels were successfully developed through thermomechanical controlled processing (TMCP). Advanced microstructure characterization, combined with precipitation thermodynamics and kinetics models, elucidated the evolution of Ti-bearing precipitation and microstructure. The results showed that coiling temperature (CT) significantly impacts phase fractions, grain boundary density, and misorientation angles, while both CT and finishing rolling temperature (FRT) influence grain sizes in acicular ferrite (AF) and granular bainite. The lower coiling temperature resulted in a higher dislocation density of the test steel, which provided more nucleation sites for AF and TiC, favoring a higher number of TiC particles and a higher proportion of AF. Above 1050 °C, the addition of nitrogen changed the shape of the precipitation kinetics curve, reduced the nucleation energy barrier of TiC, decreased the critical nucleation size, and improved the nucleation rate. Meanwhile, the addition of nitrogen accelerated the precipitation transformation of TiC, which promoted the formation of Ti(C, N). Furthermore, increasing the deformation stored energy (DSE) further accelerated the precipitation of Ti(C, N) and significantly increased the nucleation rate. The formation mechanism of large-size Ti(C, N) and the transformation mechanism of Ti(C, N) to TiC are revealed by precipitation thermodynamics and kinetics. The completion time of TiC precipitation on dislocations is shorter than that on grain boundaries, which results in the TiC on dislocations being prone to coarsening during prolonged coiling. These findings provide crucial insights for optimizing the industrial production of ultrahigh-strength titanium microalloyed weathering steels.

Research on the influence of finish rolling technique parameters on H2S corrosion resistance behavior of pipeline steel in the thickness direction

Abstract Natural gas was an important energy for human society’s development, and there was still a great demand for this type of energy. It required pipeline steel for energy transport and had excellent hydrogen-induced cracking resistance. Thermomechanical controlled processing (TMCP) was a frequently used manufacturing method for pipeline steel. It was necessary to research the effect of finish rolling technique parameters on the H2S corrosion resistance behavior of pipeline steel in the thickness direction, especially concerning microstructure and hardness in the thickness direction. So, for the research, optical microscope (OM), hydrogen-induced cracking tests (HIC), and hardness tests were used to study the effect of finish rolling technique parameters on H2S corrosion resistance. As finish rolling deformation increases, deformation can effectively conduct from surface to center and reduce the average hardness gap between the surface and the center. It can also generate finer microstructure and smaller hardness differentials from the thickness direction. All these can enhance the hydrogen-induced cracking resistance.

Evolution of microstructure and mechanical properties along the thickness direction of 500 MPa HSLA steel heavy plates

High strength heavy plates with large thickness are demanded for the construction of large-span bridge projects. However, there is a lack of understanding on the microstructure evolution and mechanical properties through the entire thickness of heavy plates with yield strength over 500 MPa, especially manufactured by the thermo-mechanical control processing (TMCP) and tempering (T) process. In this research, the microstructure and mechanical property evolution through the thickness was investigated for the 85 mm thick, 500 MPa grade heavy steel plate manufactured by TMCP + T. The results show that the microstructure throughout the thickness predominantly consists of granular bainite (GB) and polygonal ferrite (PF). The yield strength and tensile strength at 1/8t, 1/4t, 3/8t and 1/2t (t, the thickness) are 659.4 MPa, 520.6 MPa, 500.3 MPa and 494.0Mpa, and 739.2 MPa, 642.9 MPa, 627.8 MPa and 625.2 MPa, respectively. Yield and tensile strength both significantly decline from 1/8t to 1/2t. The strengthening mechanisms at different thickness were analyzed, and grain refinement has the most significant contribution. In addition, the mechanical properties of ferrite/bainite heterostructure were analyzed by Cyclic load-unload-reload (LUR) tests. The hetero-deformation induced (HDI) stress increases from 1/8t to 1/2t. At 1/8t, HDI hardening is higher but occurs over a narrower strain range compared to that at 1/2t. The impact energy at −40 °C for 1/8t, 1/4t, 3/8t and 1/2t are 182.7 J, 213.0 J, 242.2 J and 253.7 J, respectively. It was discerned that PF undergoes plastic deformation and absorbs a large amount of energy, constituting the primary rationale for the enhanced impact toughness at 1/2t compared to 1/8t. Through this research, not only the microstructure and mechanical property evolution through the thickness was revealed, but also the ferrite/bainite microstructure was found a promising heterostructure to obtain high strength, high toughness and low yield ratio for heavy plates.

A prediction model of brittle crack arrest toughness in TMCP heavy plates

Micro-alloyed low carbon MnCrNiCuMo steels were proposed to produce heavy plates with a maximum thickness of 100 mm. An integrated industrial route including a thermo-mechanical control process (TMCP) based on a study on continuous cooling transformation was applied in industrial production. Comprehensive mechanical properties tests including the measurement on resistance against crack initiation using crack tip opening displacement (CTOD) approach were conducted. Large scale wide-plate temperature gradient double tension (DT) technique was employed to evaluate the crack arrest toughness Kca in the heavy plates under different conditions. Nil-ductility transition temperature (NDTT) was also tested across the entire thickness section of the plates. The results showed that heavy steel plates were qualified being brittle crack arrest (BCA) products at EH47 grade with yield strength high than 460 MPa. High crack initiation toughness was exhibited by both Charpy-V notch impact property and CTOD values. Crack arrest toughness at −10 °C greater than 253 MPa √m was achieved in a plate even with thickness of 100 mm. The mean value of crack arrest toughness at different temperature measured by the DT approach can be indexed by the highest NDTT in the interior section of the heavy plates leading to an empirical prediction model. Further, fracture mechanics based master curve approach was used to statistically predict crack arrest toughness distribution over a wide range of temperatures showing improved predictability over the empirical model and an existing model in the literature.

Understanding and exploring anisotropy mechanism of mechanical properties for ferrous alloy under different cooling paths

To understand and illuminate the anisotropy mechanism of mechanical properties with respect to typical ferrous alloy, two different kinds of microstructures were fabricated by adopting thermo-mechanical controlled processing (TMCP) with identical rolling schedule and different cooling paths, named high coiling temperature process (HC) and low coiling temperature process (LC). The influences of microstructure together with distribution of crystallographic orientation on the anisotropy discipline were investigated, and the anisotropy mechanism were further explored on the basis of plastic deformation theory. Results indicated that the microstructure processed by HC was primarily composed of quasi-polygonal ferrite (QPF) and small fraction of bainitic ferrite (BF). While the LC-processed microstructure predominantly consisted of acicular ferrite (AF), BF as well as martensite-austenite island (M/A). The LC-processed alloy exhibited more obvious anisotropy performance indicated by tensile properties along different tensile directions compared to the HC-processed alloy. There was a phenomenon of uniform distribution of macro-texture and uneven distribution of micro-texture with respect to the both achieved microstructures, and the uneven distribution of micro-texture together with high density of dislocation for BF were responsible for the obvious anisotropy performance of the studied ferrous alloy.

Investigation of the effects of beam oscillations in electron beam–welded S1100M TMCP steel

The development of thermomechanically controlled processed (TMCP) high-strength steel (HSS) has significantly contributed to designing and developing the intricate structural components. It has broader applications in the cranes and lifting process industry (base frame, crane jibs, and crane columns), trailers, agricultural and forestry machinery, earth-moving equipment, etc. However, the development of new-grade steels with higher tensile strength led to higher requirements for welded joints, and the associated weldability issues have inspired detailed studies on electron beam welding (EBW) with different beam oscillations. Beam oscillation application with EBW processes improves the welding efficiency, weld quality, weld geometry, keyhole, etc., affecting the welded joints mechanical and microstructural properties. Thus, the present study investigates the impact and comparison of various beam oscillations on the microstructural and mechanical properties of EB-welded S1100M steel. The influence of welding parameters on the microstructure of welded joints was analyzed using a scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). The analysis focused on evaluation of grain sizes, morphologies, distributions, and crystallographic orientations of different phase constituents in fusion zone (FZ) and heat-affected zone (HAZ). The mechanical properties were analyzed using hardness, tensile, and Charpy V-notch impact tests. The texture in the FZ is typically random, while the HAZ typically exhibits a strong rolling texture. In general, the cooling rate in EBW is very fast, possibly resulting in a fine-grained structure and reduced formation of coarse second-phase particles in the weld zone. The elliptical beam oscillation showed the highest hardness in HAZ 450 HV10. Elliptical beam oscillation slightly improves the welded joint’s tensile strength, and the impact test showed mixed fracture behavior.

Enhancing the Tensile Properties and Ductile-Brittle Transition Behavior of the EN S355 Grade Rolled Steel via Cost-Saving Processing Routes.

Structural rolled steels are the primary products of modern ferrous metallurgy. Consequently, enhancing the mechanical properties of rolled steel using energy-saving processing routes without furnace heating for additional heat treatment is advisable. This study compared the effect on the mechanical properties of structural steel for different processing routes, like conventional hot rolling, normalizing rolling, thermo-mechanically controlled processing (TMCP), and TMCP with accelerating cooling (AC) to 550 °C or 460 °C. The material studied was a 20 mm-thick sheet of S355N grade (EN 10025) made of low-carbon (V+Nb+Al)-micro-alloyed steel. The research methodology included standard mechanical testing and microstructure characterization using optical microscopy, scanning and transmission electronic microscopies, energy dispersive X-ray spectrometry, and X-ray diffraction. It was found that using different processing routes could increase the mechanical properties of the steel sheets from S355N to S550QL1 grade without additional heat treatment costs. TMCP followed by AC to 550 °C ensured the best combination of strength and cold-temperature resistance due to formation of a quasi-polygonal/acicular ferrite structure with minor fractions of dispersed pearlite and martensite/austenite islands. The contribution of different structural factors to the yield tensile strength and ductile-brittle transition temperature of steel was analyzed using theoretical calculations. The calculated results complied well with the experimental data. The effectiveness of the cost-saving processing routes which may bring definite economic benefits is concluded.

Effect of Coiling Temperature on Microstructure and Properties of Ferritic-Bainitic Dual-Phase Steels

In this study, a 780 MPa grade ferritic-bainitic dual-phase steel with excellent matching of strength-plasticity and formability was developed using thermomechanical control processing. Optical microscopy, Scanning electron microscopy, and Electron Backscatter Diffraction techniques were used to characterize the microstructure comprehensively, and the effects of coiling temperature on the microstructure, the strength-plasticity, and hole-expansion ratio of the test steels were thoroughly investigated. The results showed that the test steel had an excellent combination of ferrite and bainite at the coiling temperature of 520 °C, 23.7 and 76.3%, respectively, with a hole expansion ratio of 58.5 ± 2.8%. The uniformity of the microstructure was the key to obtaining a high expansion ratio in ferrite-bainite dual-phase steels. The test steels formed granular bainite at low-temperature coiling, while polygonal ferrite was promoted at high-temperature coiling. The effect of coiling temperature on grain size is small. Dislocations were redistributed during high-temperature coiling, resulting in a decrease in dislocation density. The higher elongation and hole expansion rate at higher coiling temperatures were attributed to increased polygonal ferrite content, reduced grain size, and enhanced TRIP effect. When coiling at low temperatures, the agglomeration of polygonal ferrite or granular bainite tends to result in a non-uniform distribution of the soft and hard phases of the matrix. At the same time, the strong texture parallel to the rolling direction has a significant difference in plasticity in different directions, leading to non-uniform deformation, which is liable to stress concentration, causing crack nucleation and extension in the hole expanding process, thus reducing the hole expansion performance.

The Fabrication of Ultrahigh-Strength Steel with a Nanolath Structure via Quenching-Partitioning-Tempering.

A novel low-alloy ultrahigh-strength steel featuring excellent mechanical properties and comprising a nanolath structure was fabricated in this work using a quenching-partitioning-tempering (Q-P-T) process. The Q-P-T process comprised direct quenching and an isothermal bainitic transformation for partitioning after thermo-mechanical control processing (online Q&P) and offline tempering (reheating and tempering). The ultrafine nanolath martensite/bainite mixed structure, combined with residual austenite in the form of a thin film between the nanolaths, was formed, thereby conferring excellent mechanical properties to the steel structures. After the Q-P-T process, the yield and tensile strengths of the steels reached 1450 MPa and 1726 MPa, respectively. Furthermore, the Brinell hardness and elongation rate were 543 HB and 11.5%, respectively, with an average impact energy of 20 J at room temperature.

Microstructural Evolution in a 0.09% Niobium Low Carbon Steel during Controlled Hot Deformation

A series of plane strain compression tests were carried out in order to simulate the thermomechanical controlled processing of a 0.09wt% Nb low carbon steel, in a scheme of multipass finish rolling at 950 °C with interpass times of 10 s. It was observed that after the first two finishing passes a remarkable grain refinement can be achieved, since the recrystallisation was fully suppressed and abundant ultrafine ferrite was transformed dynamically during the deformation. The addition of a third finishing pass however, led to partial recrystallisation. A deep characterisation of the dynamic ferrite was carried out by diverse methods conducting to relevant findings that contribute to a better elucidation of the dynamic transformation. The results obtained indicated that the dynamic formation of a colony of Widmanstätten ferrite plates during deformation, initiates with the formation of a pair of self-accommodating plates followed by face-to-face sympathetic nucleation of new plates at one of the faces of the pairs of plates already formed. Furthermore, the crystal orientation within the dynamic ferrite phase was analysed with EBSD, it was observed that during the coalescence of plates, prior to the full polygonisation of grains, the ferrite adopts a transitory morphology which possesses particular crystallographic characteristics.

Neural network prediction of the effect of thermomechanical controlled processing on mechanical properties

The as-rolled mechanical properties of microalloyed steels result from their chemical composition and thermomechanical processing history. Accurate predictions of the mechanical properties would reduce the need for expensive and time-consuming testing. At the same time, understanding the interplay between process variables and alloy composition will help reduce product variability and facilitate future alloy design. This paper provides an artificial neural network methodology to predict lower yield strength (LYS) and ultimate tensile strength (UTS). The proposed method uses feature engineering to transform raw data into features typically used in physical metallurgy to better utilize the artificial neural network model in understanding the process. SHAP values are used to reveal the effect of thermomechanical controlled processing, which can be rationalized by physical metallurgy theory.

Correlation between microstructure, mechanical properties, and slurry erosion behavior of hot-rolled dual-phase steel

A low alloy dual-phase steel was produced via a thermo-mechanical control process to improve production efficiency and reduce costs. The study investigates the influence of martensite volume fractions on the mechanical properties and slurry erosion behavior of the hot-rolled dual-phase steel, and the correlation between the mechanical properties and slurry erosion behavior was revealed to provide insights into the material's wear resistance. Microstructure of dual-phase steel with varying martensite were characterized, and nanohardness, Vickers hardness, tensile propertiy, and slurry erosion were tested. Result indicates that martensite content significantly affects the mechanical propertiy of the hot-rolled dual-phase steel. Hardness and strength increase with increased martensite content, exhibiting a linear relationship. The main strengthening mechanisms are dislocation and solid solution strengthening. Slurry erosion result reveals that matrix hardness, martensite content/size, and work hardening ability significantly impact the slurry erosion performance of hot-rolled dual-phase steel. Martensite plays a crucial role in resisting wear failure, with higher content and larger size providing better protection to the soft ferrite and reducing erosion weight loss. Work hardening ability also leads to a decrease in periodic erosion weight loss.

Buried Arc Application in Hybrid Laser-Arc Welding of Thermo-Mechanical Control Process Steels

Buried Arc Application in Hybrid Laser-Arc Welding of Thermo-Mechanical Control Process Steels

Achieving 2.2 GPa Ultra-High Strength in Low-Alloy Steel Using a Direct Quenching and Partitioning Process.

Advanced high-strength steels (AHSS) have a wide range of applications in equipment safety and lightweight design, and enhancing the strength of AHSS to the ultra-high level of 2 GPa is currently a key focus. In this study, a new process of thermo-mechanical control process followed by direct quenching and partitioning (TMCP-DQP) was developed based on Fe-0.4C-1Mn-0.6Si (wt.%) low-alloy steel, and the effects of microstructure evolution on mechanical properties under TMCP-DQP process and conventional hot rolled quenched and tempered process (HR-QT) were comparatively studied. The results show that the TMCP-DQP process not only shortened the processing steps but also achieved outstanding comprehensive mechanical properties. The TMCP-DQP steel exhibited a tensile strength of 2.23 GPa, accompanied by 11.9% elongation and a Brinell hardness of 624 HBW, with an impact toughness of 28.5 J at -20 °C. In contrast, the HR-QT steel exhibited tensile strengths ranging from 2.16 GPa to 1.7 GPa and elongations between 5.2% and 12.2%. The microstructure of TMCP-DQP steel primarily consisted of lath martensite, containing thin-film retained austenite (RA), nanoscale rod-shaped carbides, and a minor number of nanoscale twins. The volume fraction of RA reached 7.7%, with an average carbon content of 7.1 at.% measured by three-dimensional atom probe tomography (3DAP). Compared with the HR-QT process, the TMCP-DQP process resulted in a finer microstructure, with a prior austenite grain (PAG) size of 11.91 μm, forming packets and blocks with widths of 5.12 μm and 1.63 μm. The TMCP-DQP process achieved the ultra-high strength of low-alloy steel through the synergistic effects of grain refinement, dislocation strengthening, and precipitation strengthening. The dynamic partitioning stage stabilized the RA through carbon enrichment, while the relaxation stage reduced a small portion of the dislocations generated by thermal deformation, and the self-tempering stage eliminated internal stresses, all guaranteeing considerable ductility and toughness. The TMCP-DQP process may offer a means for industries to streamline their manufacturing processes and provide a technological reference for producing 2.2 GPa grade AHSS.

Grain refinement, strain hardening and fracture in thermomechanically processed ultra-strong microalloyed steel

The current research in microalloyed steel has witnessed a continuous improvement in strength-ductility combination. The intention is to reduce the gauge thickness of automotive body sheets without compromising formability, thereby enhancing fuel efficiency. In this light, the present work has devised a novel thermomechanical controlled processing (TMCP) for a low-carbon automotive-grade (Nb+V)-stabilized microalloyed steel. The microstructural modification by TMCP has resulted in an excellent combination of yield strength (782 MPa), ultimate tensile strength (1059 MPa) and ductility (21%). The critical factor translating the investigated alloy towards advanced high strength steel is the ferrite grain refinement by dynamic recrystallization (DRX) and deformation-induced ferrite transformation (DIFT). The DRX predominated during TMCP at the intercritical temperature domain of 800–700 °C, imparting the average recrystallized grain size of 2 ± 0.8 µm. In contrast, the DIFT starts well above the upper intercritical temperature regime (i.e., around 810 °C), but it contributes less to the ferrite grain refinement. The thermodynamic reason has been analyzed keeping in view the changes in grain boundary energy and dislocation energy. The plastic deformation additionally leads to precipitation of cubic (Nb, V)-rich carbides in ferrite. The incoherency, coarse precipitate size (70–80 nm), and a poor phase fraction (⁓0.5%) are the key factors that the strengthening mechanism is governed by ferrite grain refinement majorly, followed by dislocation and solid solution strengthening. The segregation of Mn, Nb and S leads to MnS-type inclusion formation, which acts as potential sites for crack initiation due to plasticity mismatch with ferrite matrix during the tensile deformation.

In-situ SEM/EBSD investigation of tensile deformation-driven microstructural changes in low-carbon HSLA steels with ferrite-bainite microstructures

In this study, we aimed to investigate how microstructural constituents affect the plastic deformation behavior of low-carbon HSLA steels during a uniaxial tensile test using the in-situ SEM/EBSD technique. We fabricated three types of low-carbon HSLA steels, each with distinct ferrite-bainite microstructures, by altering the thermomechanical control processing conditions. All the steels were composed primarily of polygonal ferrite (PF) and bainitic microstructures. As the finish rolling temperature and the cooling rate decreased, the volume fraction of PF increased. The in-situ SEM and EBSD analysis during deformation revealed a greater concentration of plastic strain and an increased mobile dislocation density in the PF than in the bainitic microstructures. Most notably, as the volume fraction of the bainitic microstructure increased, so did the degree of strain gradient between the soft PF and the hard bainitic microstructure. This suggests that at the initial yield stage the soft PF plays a more significant role in mitigating the yield point phenomenon than the hard bainitic microstructure. Our findings offer a deeper understanding of the microstructural factors that influence plastic deformation, and may serve as a guide for designing optimal microstructures to ensure the mechanical properties of low-carbon HSLA steels for a range of applications.

Evaluation of Strengthening Mechanisms in Novel Fully Ferritic Advanced High‐Strength Steels

New steel alloying concepts are designed in order to produce a fully ferritic, low‐alloy steel with high (1 GPa) ultimate tensile strength (TS). A simulated hot‐deformation process of the Ti–Mo–V–Nb and Ti–Mo–V steels is designed for that purpose, and the strengthening mechanisms of the steels are evaluated after the isothermal dwell at three different temperatures (590, 630, and 680 °C). The TS and the yield strength (YS) of the test alloys are estimated via hardness measurements. Results show that the estimated TS of over 1000 MPa and YS of over 900 MPa can be achieved in both steels, although the contribution of different strengthening mechanisms to the YS varies between the steels. The effect of the dislocation strengthening can especially compensate the reduced effect of the precipitation strengthening at all tested coiling temperatures (CTs). Based on the results, a CT range of 590–630 °C with the 1800 s dwell time seems to be a potential process window for the studied steels after the present thermomechanically controlled processing (TMCP) route.

Insight into the Role of Mo Content on the Microstructure and Impact Toughness of X80 Thick-Walled Low-Temperature Pipeline Steel

In this manuscript, the effects of Mo content on the microstructure and impact toughness of X80 thick-walled low-temperature pipeline steel were studied. Two test steels with different Mo content (0.25% and 0.40%) were prepared by the thermo-mechanical control process. The impact properties were measured at −45 °C, and the microstructure evolution was observed via an optical microscope (OM), a scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and a transmission electron microscope (TEM). Each steel showed the formation of a mixed microstructure consisting of polygonal ferrite (PF), granular bainite (GB), and lath bainite (LB). Increasing Mo content resulted in the rise of LB at the expense of PF and GB. At the same time, the morphology of martensite/austenite (M/A) constituents changed from blocky to slender. The dislocation density in the ferrite matrix around the M/A constituents enhanced with an increase in Mo content. This also led to an increase in the microstrains around the M/A constituents. Also, the number fraction of the high angle grain boundary (HAGB) (MTA > 15°) decreased with the addition of more Mo content. Furthermore, with an increase in Mo content from 0.25% to 0.40%, the low-temperature impact toughness decreased from 206 to 57 J. Both an increase in the slender M/A constituents and a decrease in the HAGB number fraction deteriorated the low-temperature impact toughness of the X80 thick-walled low-temperature pipeline steel.