Thermo-mechanical Control Process Research Articles

Brittle Fracture Avoidance Technology in Large Structures with Thick Steel Plates.

The 460-MPa-class steel was developed by thermomechanical control process for shipbuilding, and the maximum plate thickness was 100 mm, which has the fine grain size as 5-20 µm. The surfaces were studied in terms of micro and nano structures, surface roughness, and surface energy to evaluate the effect of fracture toughness in large steel structure. The thick steel plate has possibility to occur unstable fracture because the fracture toughness will be decrease with increase of thickness. The increase in the temperature in thermomechanical control process accelerated the surface energy and created both micro and nano structures on the surfaces more effectively. It was effective to avoid brittle fracture in the base metal when the brittle crack was deviated into base metal. The developed 460-MPa-class steel plate improves the brittle fracture safety despite being a thick steel plate through the fine grain size. They had to be designed in such a manner as to avoid crack initiation, especially in welded joints. In this study, brittle crack arrest designs were developed for large weld construction using arrest design concept and micro and nano structures in high strength steel plate.

Comparative study on the stress corrosion cracking of a new Ni-advanced high strength steel prepared by TMCP, direct quenching, and quenching & tempering

The stress corrosion cracking (SCC) behaviors of Ni-advanced steels prepared by thermo-mechanical control process (TMCP), direct quenching (DQ), and quenching & tempering (QT) are examined by microstructural characterization, electrochemical test, hydrogen permeation test, and slow strain rate tensile test under different applied potentials. The results show that the SCC susceptibility of these Ni-advanced steels is low at open circuit potential in the simulated environment, and the SCC mechanism is mainly controlled by the anodic dissolution process. However, the SCC sensitivity increases significantly at −1200 mV, and the SCC mechanism becomes hydrogen-induced cracking. The SCC sensitivity of the test steels decreases in the following order: TMCP steel > QT steel > DQ steel, which is primarily attributed to the microstructural difference.

Model of Layer-by-Layer Cooling Trajectory in Rolled Products by TMCP

The paper describes some aspects of a scientific and research project dedicated to mechanical property prediction in plates for pipe production rolled at PJSC MMK 5000 Mill. The prediction is based on billet thermal state modelling by thermo-mechanical control process (TMCP) of rolling and subsequent cooling. The central focus of the modelling is the calculation of billet layer-by-layer cooling trajectory. The paper presents some results of model verification by test rolling.

Effect of heat treatment on the mechanical properties and microstructure of HSLA steels processed by various technologies

This study presents a striking advance in investigating the influence of heat treatment on the microstructure and properties of high strength low-alloyed steels obtained using various technologies. In contrast to the normalization treatment, the application of the thermo-mechanical control process (TMCP) offers higher strength characteristics but less stable properties during consequent high-temperature heat treatment. It has been established that the mechanical properties of both the steels are stable up to 650 °C. With an increase in the treatment temperature, the mechanical properties of the TMCP steel (grade S460M) are strongly degraded, while the normalized steel (grade S355J2) remains stable up to 950 °C. This is attributed to intensive grain growth at a temperature higher than Ac3 for TMCP steel and to the microstructural stability of the normalized steel. It is shown that the structural stability during high-temperature heat treatment is controlled by a number of factors such as heating temperature and holding time, grain growth, accumulated strain, the presence of deformation texture, steel deoxidation, and dissolution/uncontrolled growth of precipitations. Data availabilityThe raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

On the structure-property relationship in a novel 1000 MPa hot-rolled TRIP steel with strain-assisted ferrite transformation

We describe here structure-property relationship in a novel 1000 MPa hot-rolled TRIP steel with strain-assisted ferrite processed by thermo-mechanical controlled processing (TMCP) and tuning of the coiling process. A wide range of experimental material characterization techniques, notably, electron microscopy, EBSD, TEM, and nanoindentation were uniquely combined to understand the structure-property relationship. The ultrafine ferrite (average size of ~2.2–3.4 μm) was obtained by strain-assisted ferrite transformation and carbon was enriched in untransformed austenite, which led to increase in the content of retained austenite. The results indicated that ferrite delayed granular bainite transformation, and a high content of ferrite resulted in high austenite hardenability and facilitated the formation of lath bainite and martensite. The volume fraction of retained austenite was initially increased with the decrease of rolling temperature, followed by a decrease. Both granular bainite and lath bainite contained blocky retained austenite, while film-like retained austenite mainly existed in lath bainite. A good combination of strength and plasticity in the experimental steel was obtained because of strong TRIP effect. The product of tensile strength and % elongation (PSE) of the experimental steel approached 20.4 GPa·%, which is significantly greater as compared to hot-rolled ferrite-free TRIP steels.

Influences of Quenching Temperature on the Microstructure Evolution and Strength−Toughness of a Novel Medium‐Carbon Ti−Mo‐Bearing Martensite Steel

The simultaneous improvement of strength and toughness in high‐strength steels is a challenging task. Herein, a novel medium‐carbon Ti−Mo‐bearing martensite steel is developed. The influences of quenching temperature on microstructure evolution and strength−toughness are investigated. Microstructure and precipitation analyses are conducted to clarify possible reasons for the improvement of strength−toughness in the as‐developed steel. After the addition of Ti and Mo, the martensite steel yields a high tensile strength of ≈1700 MPa and an impact energy of 43 J at –40 °C. The improved strength and toughness is attributed to better grain refinement strengthening and precipitation strengthening by combination of thermo−mechanical control process (TMCP) and quenching and tempering (Q−T) processes. Although no significant variation is observed in the strengths of Ti−Mo‐bearing martensite steels quenched at a temperature range of 850−910 °C, the impact energy of Ti−Mo‐bearing martensite steel obviously decreases at a higher quenching temperature of 910 °C. The decreased low‐temperature impact energy is caused by the coarsening of (Ti, Mo)C precipitates.

A machine-learning-based alloy design platform that enables both forward and inverse predictions for thermo-mechanically controlled processed (TMCP) steel alloys

Predicting mechanical properties such as yield strength (YS) and ultimate tensile strength (UTS) is an intricate undertaking in practice, notwithstanding a plethora of well-established theoretical and empirical models. A data-driven approach should be a fundamental exercise when making YS/UTS predictions. For this study, we collected 16 descriptors (attributes) that implicate the compositional and processing information and the corresponding YS/UTS values for 5473 thermo-mechanically controlled processed (TMCP) steel alloys. We set up an integrated machine-learning (ML) platform consisting of 16 ML algorithms to predict the YS/UTS based on the descriptors. The integrated ML platform involved regularization-based linear regression algorithms, ensemble ML algorithms, and some non-linear ML algorithms. Despite the dirty nature of most real-world industry data, we obtained acceptable holdout dataset test results such as R2 > 0.6 and MSE < 0.01 for seven non-linear ML algorithms. The seven fully trained non-linear ML models were used for the ensuing ‘inverse design (prediction)’ based on an elitist-reinforced, non-dominated sorting genetic algorithm (NSGA-II). The NSGA-II enabled us to predict solutions that exhibit desirable YS/UTS values for each ML algorithm. In addition, the NSGA-II-driven solutions in the 16-dimensional input feature space were visualized using holographic research strategy (HRS) in order to systematically compare and analyze the inverse-predicted solutions for each ML algorithm.

Thermal Effect of Welding on Mechanical Behavior of High-Strength Steel

AbstractIn this study, an experimental investigation of the thermal effect of welding on the mechanical behavior of thermomechanical control process (TMCP) high-strength steel (HSS) Q690CFD is carr...

Study of the impact properties and microstructure evolution in a high-strength low-alloy heavy steel

In this study, the effect of microstructure evolution on the impact properties of high-strength low-alloy (HSLA) steel plates was investigated. Two positions (surface and inner) throughout the thickness direction of a block of the selected steel were comprehensively investigated using mechanical tests and microstructural characterization techniques. Considering the combination of structural units, carbides, and dislocation density, the best performance of strength and toughness were found in the surface and inner samples, respectively. The exceptional ductile–brittle transition temperature (DBTT) behavior was attributed to the carbide spacing distribution caused by the adopted thermo-mechanically controlled processing (TMCP).

Correlation of Microstructure Feature with Impact Fracture Behavior in a TMCP Processed High Strength Low Alloy Construction Steel

The present article aims at elucidating the effect of thermo-mechanical controlled processing (TMCP), especially the finish cooling temperature, on microstructure and mechanical properties of high strength low alloy steels for developing superior low temperature toughness construction steel. The microstructural features were characterized by scanning electron microscope equipped with electron backscatter diffraction, and the mechanical behaviors in terms of tensile properties and impact toughness were analyzed in correlation with microstructural evolution. The results showed that the lower finish cooling temperature could lead to a considerable increase in impact toughness for this steel. A mixed microstructure was obtained by TMCP at lower finish cooling temperature, which contained much fine lath-like bainite with dot-shaped M/A constituent and less granular bainite and bainite ferrite. In this case, this steel possesses yield and ultimate tensile strengths of ~ 885 MPa and 1089 MPa, respectively, and a total elongation of ~ 15.3%, while it has a lower yield ratio of ~ 0.81. The superior impact toughness of ~ 89 J at −20 °C was obtained, and this was resulted from the multi-phase microstructure including grain refinement, preferred grain boundaries misorientation, fine lath-like bainite with dot-shaped M/A constituent.

Microstructure and mechanical properties of a Cr–Ni–W–Mo steel processed by thermo-mechanical controlled processing

Experiments were conducted to evaluate the microstructure and tensile properties of a medium carbon Cr–Ni–W–Mo steel processed by thermo-mechanical controlled processing (TMCP) with cooling at different conditions in water, oil, air or lime followed by low tempering. Compared to normal heat-treatment processing, TMCP with water-cooling after deformation enhances the yield strength and tensile strength of the steel by ~ 323 MPa and ~ 251 MPa, respectively, due to higher dislocation strengthening and grain boundary strengthening. Meanwhile, it increases the elongation by ~ 1.76% attributed to the increase in volume percentage of the retained austenite and the refined laths of tempered martensite. Slowing the cooling rate after deformation during TMCP leads to a decrease in the strength. This results from the coupling effects by the reduction in dislocation density and volume fraction of tempered martensite together with the coarseness in martensite sizes. However, cooling rate decreasing has less influences on ductility because the improved elongation from the increase in the volume fractions of both retained austenite and lower bainite together with dislocation density decreasing is compensated by the reduced elongation from coarsened grains.

Processing and cooling effects on post-fire mechanical properties of high strength structural steels

High strength structural steel (HSSS) can be divided into TMCP (Thermo-Mechanical Controlled Process) and QT (Quenched and Tempered) types according to the manufacturing process. Most previous studies on the post-fire mechanical properties of HSSS were conducted on QT HSSS, while few studies were conducted on TMCP HSSS. In order to identify the effect of manufacturing process on the post-fire mechanical properties of HSSS, experimental studies were carried out on TMCP Q550 and TMCP Q690 HSSS and the results were compared with the previous studies on corresponding QT Q550 and QT Q690 HSSS. The static tensile tests were conducted at ambient temperature on TMCP Q550 and TMCP Q690 HSSS specimens heated up to nine pre-selected temperatures ranging from 200 to 900 °C and then cooling in air and water respectively to simulate various post-fire situations. It can be found that the heating and cooling process had little influence on elastic modulus but significant effect on strength and ultimate elongation once the heated temperature exceeded 600 °C, and the cooling way and manufacturing process became critical once the heated temperature exceeded 700 °C. In general, the post-fire strength and ultimate elongation was always less for TMCP HSSS than QT HSSS.

Toughening mechanism in 5%Mn and 10%Mn martensitic steels treated by thermo-mechanical control process

A thermomechanical control process (TMCP) was performed for two types of medium-Mn martensitic steels to investigate the effect of microstructure control on fracture behavior. The TMCPed 5%Mn-0.1%C steel (mass%) exhibited a slight improvement in toughness, and the partially occurring intergranular fracture was suppressed completely owing to the elongated prior austenite grains. On the other hand, the TMCPed 10%Mn-0.1%C steel exhibited a remarkably enhanced low-temperature toughness with separation, in which a large number of sub-cracks were observed parallel to the impact direction on the main fracture surface. The separation induced by the elongated prior austenite grains reduced the triaxial stress at the crack tip, which in turn contributed to improved toughness. Furthermore, the ultra-refined microstructure obtained by the combination of the γ → ε → α’ martensitic transformation and TMCP without recrystallization improved the toughness of 10%Mn-0.1%C steel.

Determination of the Temperature-Dependent Thermal Material Properties in the Cooling Process of Steel Plates

Accelerated cooling (ACC) is a key technology in producing thermomechanically controlled processed (TMCP) steel plates. In a TMCP process, hot plates are subjected to a strong cooling resulting in a complex microstructure leading to increased strength and fracture toughness. The microstructure, residual stresses, and flatness deformations are strongly affected by the temperature evolution during the cooling process. Therefore, the full control (quantification) of the temperature evolution is essential regarding plate design and processing. It can only be achieved by a thermophysical characterization of the material and the cooling system. In this paper, the focus is on the thermophysical characterization of the material properties which govern the heat conduction behavior inside of the plates. Mathematically, this work considers a specific inverse heat conduction problem (IHCP) utilizing inner temperature measurements. The temperature evolution of a heated steel plate passing through the cooling device is modeled by a 1D nonlinear partial differential equation with temperature-dependent material parameters which describe the characteristics of the underlying material. Usually, the material parameters considered in IHCPs are often defined as functions of the space and/or time variables only. Since the measured data (the effect) and the unknown material properties (the cause) depend on temperature, the cause-to-effect relationship cannot be decoupled. Hence, the parameter-to-solution operator can only be defined implicitly. By proposing a parametrization approach via piecewise interpolation, this problem can be resolved. Lastly, using simulated measurement data, the presentation of the numerical procedure shows the ability to identify the material parameters (up to some canonical ambiguity) without any a priori information.

Influence of Welding Parameters and Filler Material on the Mechanical Properties of HSLA Steel S960MC Welded Joints

This article provides an overview of the influence of welding parameters and filler material on changes in the heat-affected zone (HAZ) of thermo-mechanically controlled processed (TMCP) steel welded joints. The research focused on evaluating the effect of heat input and cooling rate on the width of the soft zone, which significantly affects the mechanical properties of welded joints. The negative effect of the soft zone is more pronounced as the thickness of the material decreases. Therefore, the object of this research was a 3-mm-thick sheet of S960MC steel welded by gas metal arc welding (GMAW) and metal-cored arc welding (MCAW) technology. Variable welding parameters were reflected in different heat input and cooling rate values, which led to a change in the properties of the HAZ and thus the mechanical properties of the welded joints. The changes in the HAZ were analyzed by microscopic analysis and mechanical testing. The measured results showed a significant effect of heat input on the cooling rate, which considerably affected the width of the soft zone in the HAZ and thus the overall mechanical properties of the welded joints.

Welding Thermal Cycle Impact on the Microstructure and Mechanical Properties of Thermo–Mechanical Control Process Steels

Steels obtained by the thermo–mechanical control process (TMCP) possess a higher level of strength than conventional normalized steels due to the fine‐grained microstructures generated during TMCP. Such enhancement opens ways to decrease metal construction weight. At the same time, TMCP steels have less thermal stability, in connection with a lot of factors affecting and governing grain growth during the heating of TMCP steels. Herein, the effects of the welding thermal cycle (WTC) on the structure and properties of TMCP steels of ferritic–perlite (S460M) and bainitic (alform 620M) types are systematically investigated.

Effects of Induction Heating in the ESP Line on Microstructural Evolution and Nb Dissolution Behavior of Nb-Ti Micro-Alloyed Steel

The thermo-mechanical control processing of Nb-Ti micro-alloyed steel by induction heating in the endless strip production (ESP) line was analyzed to better understand the microstructural evolution and Nb precipitation and dissolution behavior in austenite during rapid heating to high temperatures. The Nb-Ti micro-alloyed steel consisting of 0.05 wt% C and 0.05 wt% Nb was processed through simulated rough rolling at 1050 °C followed by rapid isothermal reheating at 1150 °C. The austenite coarsening behavior and the Nb dissolution behavior at different holding times were compared, and the coarsening kinetics of austenite grains and the dissolution kinetics of precipitates were investigated. It was found that during induction heating, the size of austenite grains gradually increased with the isothermal time, and the amounts of precipitates were greatly reduced. Round precipitates of (Ti, Nb) (C, N) and square precipitates of Ti (C, N) gradually dissolved into the austenite matrix with the holding time. The Nb content in the solution increased from 0.0137 to 0.0299 wt% as the holding time increased from 1 to 40 s; therefore, about 59.8% of the total Nb content dissolved into the austenite matrix during the induction heating process.

Particle encapsulation techniques for atom probe tomography of precipitates in microalloyed steels

Atom probe tomography (APT) provides sub-nm resolution in the analysis of complex industrial steels. It can resolve the carbonitride precipitates in Nb-Ti microalloyed high-strength low-alloy (HSLA) steels that strongly affect material performance and illuminate the complex precipitation sequence before and during the thermo-mechanical controlled process (TMCP). However, the precipitate concentration is low in HSLA steels during austenite conditioning, especially at temperatures > 850 °C, so that the probability of detecting precipitates via APT is below 5%. Here, we demonstrate two encapsulation-based approaches that increase the precipitate concentration in the APT sample volume sufficiently to enable the analysis of sparse precipitates. The first method is based on metallographic etching and direct targeting of precipitates in the steel. A focused ion beam was used to mark precipitation sites. Encapsulation with nickel-phosphorus (Ni-P) enabled localized APT and increased the yield by a factor of 10. The second method relies on the chemical extraction of precipitates and subsequent encapsulation in a silicon oxide (SiOx) network at a very high particle density. Analysis of tips cut from the encapsulated particles increased the yield by a factor of >15. We discuss and compare the spatial and chemical accuracy obtained in the analysis of pure Nb-, Ti- and mixed Nb-Ti carbonitrides.

Application of High Tensile (E-550 Grade) Structural Plates for Built-up Columns and Beams in Boiler Structure

In the past Boiler Structure has been designed using mainly carbon steel plates of Grade E250 or E350. This paper presents and analyses the application of High Tensile steel (E550 Grade as per IS: 2062) in Heavy structure like Boiler Supporting Steel Structure. Boiler supporting Structure consists of major weight contribution in Columns and Beams which are stress governed, the objective is to optimize the structural quantity through application of High Tensile Steel with higher yield stress. This paper describes the complete analytical approach adopted in analysis & design, conclusion and recommendation. Latest structural plate rolling technology provides excellent combination of high strength, Toughness and weldability for High Tensile Steel, these properties of steel plate are most critical mechanical properties for steel structure design. The key microstructural control is obtained in the plate rolling mills through on-line heat treatment immediately after accelerated cooling during Thermo-Mechanical Controlled Processing (TMCP). Thermo-Mechanical Controlled Processing technology is recent development applied in latest steel structural plate rolling mills for rolling with thickness up to 120mm. Niobium (Nb) as a micro alloy in the steel enables to achieve a substantial grain refinement, very fine-grained microstructure improves toughness and increases the yield strength. The TMCP process enables the required tensile properties in structural plates with leaner chemical composition. Leaner chemical composition, especially lower carbon content and lower carbon equivalents improves weldability. Improved weldability benefits the fabricator for fabrication of plate fabricated built-up profiles which is the main requirement in Heavy Structure like Boiler Supporting Steel Structure. Detailed analysis and design of typical 660 MW coal fired Boiler structure has resulted in 25% structural weight saving in case of High Tensile steel application (E550 Grade) as compared with commonly used carbon steel (E250/E350 Grade). Also, this analysis concludes that use of high tensile steel should be limited to Columns and Beams only which are stress governed and where yield strength is the main design criteria.

Developing of X65 steel coils making at Steckel mill using thermo-mechanical control process

The process of thermo-mechanically controlled rolling in the conditions of Steckel mill of Ferriera Valsider plant was developed for the first time for hot-rolled coils in the sizes of 12 mm × 1510 mm from steel of X65 grade for further production of electric welded pipes according to the API-5L standard. The developed process provides carrying out of the roughing-rolling in two stages to facilitating the forming of a smaller austenite grain and therefore more dispersed and homogeneous inherited ferrite structure in the finished rolled product. At the implementation of the process, the holddown of the temperature at the desired level at 1780 Steckel rolling mill has been carried out by adjusting the number of burners in furnace coilers. The process of controlled air cooling of rolled coils to a temperature of 400°C after winding in the developed technology for Steckel’s mill condition of Ferriera Valsider plant is applied for the first time that allowed to reduce the thickness of the air scale layer and to improve the surface quality. The performed research has been allowed to evaluate the feasibility of the operating equipment and to determine the possibility of coils fabrication related to the nowadays world requirements and satisfied to the demand of European electricwelded pipes manufacturers.