Search Results (38)

Continuous cooling transformation (CCT) diagrams for two thermo-mechanically controlled processed (TMCP) steels were produced using a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, which accounted for the simultaneous transformation of multiple phases under non-isothermal conditions. A basin hopping algorithm was used to sequentially optimize the model parameters for each phase. Samples were prepared using a dilatometer which replicated the deformation and cooling rates experienced during TMCP. Scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) were used to identify and quantify the phases present in each steel. CCT diagrams illustrating the start and stop temperatures of each phase were constructed for both steel samples. Through inclusion of the stop temperatures of each phase transformation, the utility of the CCT diagrams were expanded. This was done by introducing the possibility of applying the Scheil additive principle with respect to the beginning and end of each phase transformation. With this modification, the CCT diagrams are now more appropriately suited to predict the phase transformations that occur on the ROT, where non-continuous cooling occurs. Full article

With the increase in strength of pipeline steels manufactured by thermomechanical control process (TMCP), the softening of the welding heat-affected zone (HAZ) becomes another important factor affecting the properties of welded steel pipes and the safety of pipeline operation. In this work, based on the actual welding process of steel pipes, the strength, phase transformation, and microstructure of the HAZ of six pipeline steels with different chemical compositions were studied by using a thermomechanical simulator, and the effect of chemical composition on the softening of HAZ was discussed. Results show that the strength of HAZs is significantly influenced by the peak temperature, and the softening zone mainly occurs in fine-grained HAZ (FGHAZ) when peak temperature is 900~1000 °C. Meanwhile, the degree of softening is also affected by the chemical composition of the steels. The effects of peak temperature and chemical composition of the steels on the strength of the HAZs when the peak temperature is over Ac₃ are attributed to their effect on the austenite transformation during the heating process, and then the effect on phase transformation during the cooling process and final microstructure. The strength of the HAZs is linearly related to the beginning phase temperature during the cooling process, and the strength of sub-HAZs at the same peak temperature is linearly related to the value of carbon equivalent (Ceq) of steels. Therefore, controlling the appropriate value of Ceq is necessary to improve the softening of HAZs for high-strength pipeline steels. Full article

Thick steel plates in bridges exhibit mechanical property gradients along their thickness, yet the underlying micro-mechanisms remain unclear. This study investigates an 80 mm thick 420 MPa-grade HSLA steel plate, and also quantitatively investigates the mechanism of its mechanical gradient behavior in the thickness direction through layered tensile tests combined with multi-scale microstructural characterization. The unique contribution of this work lies in establishing a quantitative correlation between the gradient in the dislocation density and the transition in yielding behavior. The results show that the surface layer area of the tested steel exhibited continuous yield characteristics, while all core layers exhibited pronounced discontinuous yielding. The mechanical properties showed a gradient distribution along the thickness direction, with the yield strength and tensile strength decreasing from 512.4 MPa and 545.9 MPa at the surface to 419.5 MPa and 520.4 MPa at the center (1/2t). Microstructural analysis shows that the full-thickness structure was composed of granular bainite (GB) and polygonal ferrite (PF). With respect to increases with depth, the average grain size increased from 6.86 µm at the surface to 11.57 µm at the center. Moreover, the surface region exhibited a broader grain size distribution range and higher size dispersity. The second-phase precipitates in the full thickness were mainly of two types, namely, Fe₃C and (Nb, Ti) (C, N) composite precipitates, and the precipitates in the surface layer had smaller sizes and higher distribution densities. Crucially, the dislocation density decreased sharply from the surface to 1/8t, then stabilized. While quantitatively elucidating the contributions of various strengthening mechanisms to the strength gradient, the mechanistic analysis also reveals a dislocation microstructure synergistic mechanism underlying the yield behavior differences. Full article

The aim of this study is to investigate the influence of Mo on the microstructure and mechanical properties of railway frog steel. To address the challenges of a coarse microstructure and alloy element segregation caused by the current casting method of railway frog steel, the application of thermal mechanical control process (TMCP) technology can achieve a uniform and refined microstructure and stable mechanical properties, which is progress for the development of high-manganese railway frog steel. The TMCP of four experimental steels with varying Mo contents of 0.02~1.01 wt.% was simulated using a Gleeble 3500. The mechanical properties were tested, and the microstructure of each sample was characterized. A single austenite formed in each Mo-containing steel. With the increased Mo content, the grain boundary carbides decreased due to the formation of carbides within the grains, and the austenite and twin sizes were refined. Moreover, grain boundary strengthening and dislocation strengthening increased, while solid solution strengthening and precipitation strengthening had little effect, leading to an increase in the final yield strength. The contribution of dislocation strengthening to the yield strength was 51~56%, indicating that dislocation strengthening was the most significant strengthening method in the high-manganese railway frog steel produced using the TMCP. The impact energy showed a trend of first increasing and then decreasing, and the impact energy reached the highest point when the Mo content was 0.30 wt.%. In addition, the mechanisms governing the effect of increased Si in controlling the final microstructure and mechanical properties are discussed. Full article

In this study, a strategy was adopted to promote the formation of NiAl precipitates with the aim of enhancing strength by incorporating a 0.2 wt.% Al into a traditional ultra-low carbon bainitic (ULCB) steel alloy. By integrating thermo-mechanical control processing (TMCP) and a tailored tempering process, a new-generation steel with an outstanding combination of properties has been successfully developed for shipbuilding and marine engineering equipment. It features a yield strength of 880 MPa, a yield ratio of 0.84, and an impact toughness of 175 J. The precipitation characteristics of nanoscale particles in this steel, including NiAl intermetallics and carbides, were systematically investigated. The results show that the alloy with low Al addition formed NiAl precipitates during tempering. The high-density distributions of NiAl, (Mo, V)C, and (Ti, V, Nb)C precipitates, which exhibit slow coarsening kinetics, played a dominant role in enhancing the strength of the tempered steel. In addition to precipitation, the microstructure before and after tempering was also analyzed. It was observed that a granular bainite morphology was favorable for decreasing the yield ratio. Additionally, the formation of reverse-transformed austenite during tempering was critical for retaining toughness despite substantial strength gains. Finally, theoretical modeling was employed to quantitatively assess the contributions of these microstructural modifications to yield strength enhancement of thermo-mechanical controlled processing (TMCP) and tempered steel. This study establishes a fundamental basis for subsequent industrial-scale development and practical engineering applications of novel products. Full article

Marine engineering thick plates are essential structural materials for large vessels and offshore platforms, and optimizing their manufacturing processes is critical for advancing marine equipment. This study examined the microstructural and property variations in 120 mm-thick S460 plates fabricated by thermo-mechanical controlled processing (TMCP). A finite element model was developed to simulate the cooling phase, enabling the prediction of the internal cooling path in the thick plate. An optimized cooling scheme was proposed, which was validated against the model and implemented. The following key results were obtained: (1) Under the initial cooling parameters (initial temperature: 715 °C, duration: 130 s), the 60 mm depth toughness was severely compromised, as evidenced by a low −40 °C impact energy of 59 J, significantly lower than values observed at the10 mm and 30 mm depth positions. Microstructural analysis revealed that the 60 mm depth region was dominated by ferritic bainite and pearlite, with a pearlite content of 8.7%. Numerical simulations further indicated a 60 mm depth cooling rate of 1.10 °C/s under these conditions. (2) Model predictions confirmed the original 60 mm depth cooling rate of 1.10 °C/s. The optimized process increased the initial cooling temperature to 725 °C and extended the cooling time to 160 s, achieving an enhanced 60 mm depth cooling rate of 1.36 °C/s. (3) The optimized process remarkably improved the 60 mm depth impact energy to 144 J, achieving near-complete elimination of pearlite, increased granular bainite content, refined M-A constituent size, and enhanced density of high-angle grain boundaries. This study demonstrates that enhancing internal temperature gradients and prolonging cooling durations can effectively inhibit microstructural degradation in 60 mm depth regions of thick plates, providing both theoretical foundations and practical methodologies for optimizing TMCP processes of extra-thick steel plates. Full article

The continuous cooling transformation (CCT) curves of undercooled austenite serve as crucial references for obtaining desired microstructures and properties in metallic materials (particularly deformed metals) through heat treatment. In this study, static and dynamic CCT curves were constructed for experimental steels micro-doped with rare earth element Ce by combining temperature-dilatometric curves recorded after austenitization at 900 °C with microstructural characterization and microhardness measurements. Comparative analyses were conducted on the microstructures and microhardness of three experimental steels with varying Ce contents subjected to sizing (reducing) diameter deformation at 850 °C and 950 °C. The CCT experimental results revealed that the microhardness of the tested steels increased with cooling rates. Notably, dynamic CCT specimens cooled at 50 °C/s to room temperature following superheated deformation exhibited 56.7 HV5 higher microhardness than static CCT specimens, accompanied by increased martensite content. The reduction of deformation temperature from 950 °C to 850 °C resulted in the expansion of the bainitic phase region. The incorporation of trace Ce elements demonstrated a significant enhancement in the microhardness of 30MnNbRE steel. This research proposes an effective processing route for improving strength-toughness combination in microalloyed oil well tubes: introducing trace Ce additions followed by sizing (reducing) diameter deformation at 950 °C and subsequent ultra-fast cooling at 50 °C/s to room temperature. This methodology facilitates the production of high-strength/toughness steels containing abundant martensitic microstructures. Full article

This paper comprehensively examines three structural steel grades’ microstructural features and mechanical properties, evaluating their suitability for shipbuilding applications. The steels analyzed include quench and tempered (Q and T) steel, thermomechanical controlled processed (TMCP) steel, and hot rolled (HR) steel. A microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The analysis was complemented by extensive mechanical testing including assessments of hardness, tensile, and Charpy impact tests across a range of temperatures. Additionally, corrosion behavior was evaluated using the potentiodynamic polarization test. The findings revealed that Q and T grade steel exhibited the most refined microstructure, characterized by a complex mixture of ferrite, tempered martensite, upper bainite, and Fe₃C phases. In contrast, the TMCP grade steel demonstrated a balanced microstructure of polygonal ferrite and pearlite. Meanwhile, the HR grade steel contained polygonal ferrite and aligned pearlite. The tensile testing results demonstrated that the Q and T grade steel had superior hardness, yield strength (YS), and ultimate tensile strength (UTS), although it exhibited the lowest elongation % (El %). The TMCP grade steel met all ABS standards for marine steels, displaying optimal YS, UTS, and El %. Despite the superior YS of the HR grade steel, it did not meet the necessary criteria for UTS. Charpy impact tests revealed that the TMCP grade steel exhibited the highest impact energy absorption across a range of temperatures. As a result, the TMCP grade steel emerged as the optimal choice for ship construction, fulfilling all ABS requirements with a balanced combination of strength, ductility, and impact energy absorption. Additionally, the potentiodynamic polarization results revealed that the Q and T grade steel demonstrated the highest corrosion resistance. Following Q and T steel, the HR grade steel ranked second in corrosion resistance, with TMCP steel closely behind, showing only a slight difference. Full article

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. Full article

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. Full article

Nb-Ti high-strength low-alloy pipeline steel was subjected to a six-pass rolling process followed by the coiling process at different temperatures between 600 and 650 °C using the thermo-mechanical testing system Gleeble 3500 (Gleeble, New York, NY, USA). This experimental steel was subjected to 72% heavy reduction through a thermos-mechanical controlled process. Thereafter, the microstructures were observed using optical microscopy, scanning electron microscopy, electron backscatter scanning diffraction, and transmission electron microscopy coupled with energy dispersive spectrometry and selected area electron diffraction. For the selected three coiling temperatures of 600, 625, and 650 °C, acicular ferrite, polygonal ferrite, and pearlite were observed, and morphology and statistical analysis were adopted for the study of precipitates. Based on the estimation by the Ashby–Orowan formula, the incremental strength through precipitation strengthening decreases with coiling temperatures and reaches 26.67 Mpa at a coiling temperature of 600 °C. Precipitation-time-temperature curves were obtained to explain the transformation of precipitates. The (Nb, Ti)(C, N) particles tended to precipitate in the acicular ferrite with [011]_{(Nb, Ti)(C, N)}//[011]_α-Fe orientation. The lower coiling temperature provided enough driving force for the nucleation of precipitates while inhibiting their growth. Full article

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. Full article

Thermo-mechanical controlled processing (TMCP) is employed to obtain the required level of mechanical properties of contemporary high-strength low-alloy (HSLA) steel plates utilized for gas and oil pipeline production. The strength, deformation behavior and resistance to the formation and propagation of running fractures of the pipeline steel are mainly determined by its microstructure and crystallographic texture. These are formed as a result of austenite deformation and consequent γ→α-transformation. This present study analyses the crystallographic regularities of the structural and textural state formation in a steel plate that has been industrially produced by means of TMCP. The values of the mechanical properties that have been measured in different directions demonstrate the significance of the crystallographic texture in the deformation and failure of steel products. An electron backscatter diffraction (EBSD) method and crystallographic analysis were utilized to establish the connection between the main texture components of the deformed austenite and α-phase orientations. This paper demonstrates that the crystallographic texture that is formed due to a multipath γ→α-transformation results from the α-phase nucleation on the special boundaries between grains with γ-phase orientations. The analysis of the spectra of the α-γ-interface boundary angle deviations from the Kurdjumov–Sachs (K–S), Nishiyama–Wassermann (N–W), and Greninger–Troiano (G–T) orientation relationships (ORs) allows to suggest that the observed austenite particles represent a secondary austenite (not retained) that precipitates at intercrystalline α-phase boundaries and correspond to the ORs with regard to only one adjacent crystallite. Full article

In this study, the effects of Si on the microstructure and tensile properties of weathering bridge steel were elucidated. The thermal mechanical control process (TMCP), containing two stages of controlled rolling and accelerated cooling process, was simulated using a thermo-mechanical simulator for four experimental steels with varying Si contents (0.15–0.77 wt.%). Micro-tensile tests were performed, and the microstructures were observed via optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and electron back-scattered diffraction (EBSD). Furthermore, the tensile properties and microstructures of these steels were analyzed. The results show that a mixed microstructure comprising granular bainitic ferrite (GBF), quasi-polygonal ferrite (QF), and martensite/austenite (M/A) constituent was formed in each sample. With an increase in Si content, the GBF content decreased, QF content increased, mean equivalent diameter (MED) of the QF+GBF matrix increased, and the fraction and average size of the M/A constituent increased. With a rise in Si content from 0.15 to 0.77 wt.%, the contributions of dislocation strengthening, grain boundary strengthening, and precipitation strengthening decreased from 149, 220, and 21 MPa to 126, 179, and 19 MPa, respectively. However, the combined contribution of solution strengthening, lattice strengthening, and M/A strengthening increased from 41 to 175 MPa, which augmented the final yield strength from 431 to 499 MPa. The decreasing yield ratio shows that strain hardening capacity is enhanced due to an increase in the fraction of the M/A constituent as well as in the MED of the QF+GBF matrix. Furthermore, the mechanisms by which Si content controls the microstructure and mechanical properties of weathering bridge steel were also discussed. Full article

Fine-grained steels, which belong to the HSLA (High-Strength Low-Alloy) group of steels, are increasingly used for parts of statically and dynamically loaded constructions. Due to the thermal effect of welding, combined with the inherent stiffness and clamping stiffness of the part, residual stresses are generated in the HAZ (heat-affected zone) which affect the fatigue life of the sub-weld and the entire construction. In this article, a specific temperature cycle measured during welding is used, which, together with a defined clamping stiffness, produces residual stresses of a defined shape and value in the sample. Subsequently, the effect of these stresses on the fatigue life on the change of the S–N curve compared to the annealed material, is assessed. Temperature cycles were applied using a Gleeble 3500 and the residual stresses were analyzed by X-ray diffraction (XRD). It was found that the effect of residual stresses decreased the fatigue strength by 33% compared to the annealed material. It was further found that by using annealing to reduce the residual stresses, it is possible to restore the fatigue life of S700MC steel to the original value of the base material. Full article