Thermo-mechanical Control Process Research Articles

Waveform and frequency effects on corrosion-fatigue crack growth behaviour in modern marine steels

The primary focus of this work is to investigate the sensitivity of cyclic waveform, frequency (f), load level and microstructure on the corrosion-fatigue crack growth rate (CFCGR) in modern normalised-rolled (NR) and thermomechanical control process (TMCP) ferrite-pearlite steels in the Paris Region of the da/dN vs. ΔK log-log plot. Constant amplitude sinewave (si) and trapezoid waveform (generally referred to here as hold-time (h-t)) were used under frequencies of 0.2 Hz, 0.3 Hz and 0.5 Hz and stress ratio of 0.1. Comparison is also made between the crack path in the S355 TMCP steel under si and h-t in seawater (SW). The role of microstructure in retarding or accelerating fatigue crack growth in SW is also discussed. Experimental results showed that the CFCGR corresponding to the si is higher than that of the h-t for all the load levels and frequencies examined. It was observed that reduction in the f and fatigue load level increased the CFCGR for the h-t but had little effect on the si. Generally, f in the range 0.2–0.5 Hz had little effect; and for a given f an increase in load led to a reduction in the CFCGR, in the Paris Region (PR) for both si and h-t in SW. Under both si and h-t, the CFCGR in the TMCP steels (e.g. S355G8 + M, S355G10 + M) is lower than that of the normalised steels (e.g. S355J2 + N). Metallurgical analyses on the fractured surface of corrosion-fatigue specimens show that the main active crack tip blunting process is the primary factor controlling the CFCGR of steel at high stress intensity factor range (SIFR) and low f in SW. The results obtained from this study have been discussed in terms of the potential impact on the structural design and integrity of offshore wind turbine foundations.

Experimental study to differentiate mechanical behaviours of TMCP and QT high strength steel at elevated temperatures

High strength steel (HSS) has gained its popularity in high-rise and large-span structures in which fire is one of the extreme scenarios that needs to be considered in structural design. In view of different types of HSS manufactured from different heat-treatment processes, their mechanical behaviours under fire would be different. However, previous studies and modern design codes show inadequacy of information on their temperature-dependent mechanical properties, especially their mechanical differences due to the different heat-treatment methods. With the research gap being identified, high-temperature coupon tests were conducted on two types of S690 steel respectively manufactured from thermo-mechanically controlled process (TMCP) and quenching and tempering (QT) process. Both steady- and transient-state test methods were adopted. Microstructure changes were discussed and temperature-dependent mechanical properties were compared with each other and with those of mild steel produced through hot-rolling process. The test results revealed that TMCP-S690 steel exhibited similar elastic modulus at elevated temperatures as QT-S690 steel but smaller effective yield strengths when the thermal creep was more significant above 400 °C. Through this study, the heat-treatment dependent thermal elongations, elastic moduli and effective yield strengths at elevated temperatures were recommended for fire resistant design of HSS structural elements. The stress–strain constitutive model was also provided for numerical modelling.

Promene mikrostrukture i mehaničkih svojstava u zoni uticaja toplote zavarenog spoja čeličnih traka klase S960MC

The TMCP (thermo-mechanically controlled processed) steels belong to the group of ultra-high strength steels, which exhibit exceptional combination of high tensile and yield strength, toughness and ductility. These steels were introduced in the heavy machinery constructions, such as heavy mobile cranes, chassis trucks and other to reduce their weight, what increases their loading capacity and ecology of transport. The high tensile and yield strength of this type of steels is obtained by the combination of the chemical composition, heat treatment and the mechanical processing. However, the heat input into the material during the welding significantly affect properties of the steel and the whole joint. In this paper are presented results of mechanical properties evaluation and structural analysis of the welds of the thin sheets made of the S960MC steel, which were welded using the GMAW procedure. The microstructural evaluation referred significant changes in the HAZ. This area contains the three sub-zones, coarse grain (CGHAZ), fine grain (FGHAZ) and intercritical zone (ICHAZ). Analysis of microhardness and the tensile tests results showed, that ICHAZ is the most critical area of the whole welded joint.

Investigation on the genesis of the shape deformation of plates in New Plate mill, Rourkela Steel Plant

New Plate Mill (NPM), a state of the art facility has been commissioned at Rourkela Steel Plant (RSP). Facilities like roll bending, roll shifting and plan view rolling are integral part of the mill. High pressure de-scaling, accelerated cooling control (ACC) with high capacity hot plate leveller (HPL) & cold plate leveller (CPL) are other process facilities of the NPM. A special feature of the NPM (New Plate Mill) is its high roll separating force of 90MN. This enables high shape factor rolling that leads to the rolling of metallurgically sound thicker steel plate. Therefore it is possible to apply modern thermo-mechanically processing routes for the development of tailor made properties like high strength, toughness, close dimensional tolerance and excellent surface finish in plate.Trials were carried out for the achieving target mechanical properties with leaner chemistry by thermo-mechanical control processing (TMCP). After rolling, plates are passed through the Accelerated Cooling Control (ACC) unit for achieving desired mechanical properties. After ACC, waviness is visible in the plates and hence HPL is used for flattened the plate profile. Plates get flattened after hot plate leveling but after several minutes some waviness again occurs in the plates on the cooling bed. Initially several factors were considered for possible reason and thermal images of the plates were taken for understanding thermal variations in the plates. Finally, improper descaling and temperature inhomogeneity was found major factors affecting plates shape.

HIGH TOUGHNESS, 510 MPA STRENGTH CLASS MICROALLOYED STEEL PLATES WITH PRODUCED BY T.M.C.P. IN GERDAU OURO BRANCO

The Gerdau Ouro Branco Works has started to develop high complexity products three years after the implementation of its new heavy plate rolling mill. The evolution of its learning curve is increasingly challenging since products are being developed to their highest grades. This paper aims to present the first results of the development of steels of the strength class of 510 MPa produced by TMCP (Thermomechanical Controlled Processing). These results show the potential and advantages of the accelerated cooling process after rolling for the production of steel plates with high mechanical strength and high toughness, associated with a low equivalent carbon.

Microstructure and mechanical properties of high strength TMCP steel hybrid disk laser-MAG welded joint

Hybrid laser-arc welding (HLAW) was carried out on high strength thermo-mechanical controlled process (TMCP) steel of 14 mm thickness using single-pass welding (SPW) and multi-pass welding (MPW) processes. Well-formed weld joints with good performance were successfully obtained. The results showed that the microstructure of weld was pre-eutectoid ferrite, acicular ferrite and some granular bainite. There were mainly lath bainite, granular bainite and acicular ferrite in the heat-affected zone. The grain sizes of SPW were thicker than MPW, and the bainite content of MPW was higher than SPW. The average hardness value of the fusion zone was lower than that of the Heat affected zone, but higher than that of the base material. The weld was divided into upper arc domain zone (ADZ) and lower laser domain zone (LDZ). The ADZ was harder than the LDZ. The average absorbed energy of SPW and MPW in the fusion zone at −40[Formula: see text]C was 125.5 and 92 J/cm2, respectively. The influences of microstructure on impact toughness were summarized by analyzing the hardness distribution and microstructure of the welded joint. The brittle transition temperatures of the two processes obtained by the energy criterion and the morphology criterion were close to each other.

Development and Application of Thermo-mechanical Control Process Involving Ultra-fast Cooling Technology in China

Development and Application of Thermo-mechanical Control Process Involving Ultra-fast Cooling Technology in China

Texture Inheritance in the Ferrito-Martensite Structure of Low-Alloy Steel after Thermomechanical Controlled Processing

The electron back-scatter diffraction (EBSD) method was used to study the structural and textural states of low-carbon low-alloy steel after thermomechanical controlled processing (TMCP) and subsequent heat treatment (HT) including austenitization, holding at the temperature of the decomposition of the γ phase by the diffusion mechanism for different times, and quenching. As a result of the HT, structures were obtained consisting of polygonal ferrite and martensite in various ratios. The crystallographic textures of both martensite and ferrite after HT reproduced the texture of bainite formed as a result of the TMCP. The orientations that form this texture are related to the main orientations of deformed austenite grains upon the TMCP via the orientation relationships (ORs) intermediate between the Kurdjumov–Sachs and Nishiyama–Wassermann ORs. In the case of both ferrite and martensite, the orientational connection between the components of the texture components is explained by the onset of the realization of the phase transformation on crystallographically stipulated (including special) boundaries.

Structure-property relationship in novel low carbon hot-rolled TRIP steels via thermo-mechanical controlled processing and coiling

A novel process involving thermo-mechanical controlled processing (TMCP) and coiling process was adopted to obtain multiple-phase mixed microstructure. The coiling temperature governed microstructural evolution, stability of retained austenite (RA) and microstructure-properties relationships. The results indicated that the optimum properties were obtained at a coiling temperature of 485 °C with combination of tensile strength of 1006 MPa, elongation of 13.3%, hole-expansion ratio of 28% and impact toughness of 14.7 J (1/4-Charpy size sample at −20 °C). A small amount of RA was present when the coiling temperature was 430 °C (close to the Ms point), resulting in low elongation. The size of blocky RA located at grain boundary was ~450 nm, whereas, the film-like RA embedded in bainite laths was ~40–110 nm. The film-like RA was more stable than blocky RA, which enabled superior formability. Both types of RA provided positive and continuous TRIP effect during deformation. But the presence of a large amount of high-carbon martensite adversely affected impact toughness and formability because it acted as a source for crack initiation as a hard phase in the matrix.

Effect of deformation parameters in unrecrystallization range on microstructural characteristics in Al-bearing hot-rolled TRIP steel

The scanning electron microscope, transmission electron microscope, optical microscope, X-ray diffraction and hardness tests were used to investigate the effect of deformation parameters in unrecrystallization range on microstructural characteristics in Al-bearing hot-rolled transformation-induced plasticity steel. The thermomechanical-controlled processing was carried out with thermomechanical simulation machine, and the samples were compressed to compression strains of 0, 0.15, 0.25 and 0.35 at compression temperatures of 850, 900 and 950 °C. The results showed that the volume fraction of polygonal ferrite increased with the increasing compression strain, while the volume fraction of retained austenite reached the maximum value at compression strain of 0.25. The volume fraction of polygonal ferrite decreased with the increasing compression temperature, whereas the volume fraction of retained austenite possessed the maximum value at compression temperature of 850 °C. Some granular retained austenite was present in uncompressed samples, and some pearlite appeared at large compression strain, while the hardness of the samples exhibited the similar variation tendency to the volume fraction of retained austenite.

Texture and structure formation of low-carbon low-alloy pipe steel after TMCP and heat treatment

The structure and texture of low-carbon low-alloy pipe steel after Thermo-Mechanical Controlled Processing (TMCP) and subsequent isothermal quenching were studied by the method of Electron Backscatter Diffraction (EBSD) analysis. The texture, obtained after TMCP, was formed mainly by two strong scattered orientations from {112}<110> and two weaker scattered orientations from {110}<223> and one weak orientation (001)[110]. Complex multicomponent textures were observed after heat treatment. The regions with “ferrite” and martensitic structures were identified and analyzed using Oxford Instruments HKL software. Orientations, constituting textures for both selected areas types, are associated with the basic orientations of the initial deformed austenite grains, formed during the hot deformation by orientation relationships (OR), transitory between Kurdjumov-Sachs OR and Nishiyami-Wasserman OR.

Дослідне освоєння виробництва товстолистового прокату з марки сталі EH36 із застосуванням термомеханічного контрольованого процесу на стані 3600

For the first time, technology has been developed, and experimental batch of shipbuilding steel plates (Grade ЕН36, 25 × 2150 × 8000 mm) has been produced according to the requirements of classification societies (Bureau Veritas Rules), at the rolling mill 3600, PJSC “AZOVSTAL IRON &amp; STEEL WORKS”. The technology is developed using general requirements for the manufacture of rolled products with the thermo-mechanical controlled process and mathematical model of the rolling process, on the basis of which the calculations of deformation schedules have been performed. The technical possibility of shipbuilding steel plates (Grade ЕН36, 25 × 2150 × 8000 mm) producing using thermo-mechanical controlled process (TMCP) instead of heat treatment accessing by normalization in accordance with Bureau Veritas Rules has been conformed at the rolling mill 3600. Introduction of the thermo-mechanical controlled process instead of normalization will reduce the production cost of the rolled products by eliminating natural gas consumption for heat treatment (normalization). To prepare the certification, in accordance with Bureau Veritas Rules, it is necessary to conduct an additional series of investigations for the rolled products (Grade EH36) of different thicknesses

On the Significant Impact of Deformation Coupled with Cooling Rate during Thermo‐Mechanical Controlled Processing (TMCP) on Microstructure‐Property Relationship in 25Mn2Si2Cr High Strength Steel

To elucidate the effect of deformation and cooling rate on microstructure‐property relationship in 25Mn2Si2Cr high strength steel during TMCP process, the dynamic continuous cooling transformation (CCT) curves are first determined by thermal simulation experiments with different degree of deformation (engineering strain: 0.35, 0.50, 0.65) and cooling rate. At the same time, direct cooling after rolling experiments are carried out with different degree of deformation and cooling rate. The results indicate that dynamic CCT curves are changed by increasing the degree of deformation. The high temperature phase transformation zone moves to left. The start temperature of martensitic transformation (Ms) is decreased. The microstructure is composed of granular bainite (GB), lath bainite (LB), and martensite/austenite (M/A) islands after air cooling (cooling rate: about 1 °C s−1). When the cooling method was wind cooling (cooling rate: about 2.5 °C s−1), microstructure was a combination of lath bainite and martensite. The microstructure on water cooling (cooling rate: about 30 °C s−1) was primarily martensite. Accelerating the cooling rate, the strength was increased, but toughness was decreased. While increasing the degree of deformation increased strength and toughness simultaneously.

Investigating the Properties of Coil Tail in Ti–Nb–Mo Microalloyed Hot‐Rolled Strip

In the present study, the thermo‐mechanical controlled processing (TMCP) technology was applied to produce Ti–Nb–Mo microalloyed hot‐rolled steels. The strength at the tail of a strip coil is often found to be non‐uniform. The microstructural and the mechanical properties of the specimens in the outermost circle as well as in the third circle and the fifth circle from the outermost circle were analyzed in detail. The optical microscopy (OM) and the electron backscatter diffraction (EBSD) techniques are employed for the characterization of ferrite grains and grain boundaries. The precipitates were revealed by high‐resolution transmission electron microscopy (HR‐TEM) and energy dispersive spectroscopy (EDS). It is found that the precipitation strengthening induced by nano‐scale (Ti, Nb, Mo)C particles was the dominant factor for the disparity in properties in different positions of the coil tail. The volume fraction of nano‐scale (Ti, Nb, Mo)C particles increases from 0.220% in the outermost circle to 0.343% in the fifth circle. In addition, the lattice planes of (110)α and (100)(Ti, Nb, Mo)C followed a Baker‐Nutting orientation relationship. Nano‐scale (Ti, Nb, Mo)C particles mainly precipitated in ferrite grains during coiling and subsequent cooling. Furthermore, different cooling rates along the strip had inconspicuous influences on the grain size and lengths of ferrite grain boundaries.

Optimization of TMCP strategy for microstructure refinement and flow-productivity characteristics enhancement of low carbon steel

Improving the mechanical properties of low carbon steel through the microstructural refinement without adding alloying elements is a complicated scientific and technical mission. This challenge can be overcome by using thermo-mechanical controlled processing (TMCP) technology. Most researches have examined the influence of TMCP on the microstructure development without shed light on the flow characteristics. The current work aims at studying the effect of applying different TMCP strategies on the microstructure refinement, flow-productivity characteristics, i.e. peak stress, flow stress, strain hardening index, Zener–Hollomon parameter and total strategy time, of low carbon steel. These TMCP strategies involve applying heavy deformation in single-pass TMCP strategy versus light deformation/pass in multi-pass TMCP strategy through applying different plane-strain hot compression regimes using Gleeble 3500 thermo-mechanical simulator. The proposed TMCP strategies have been involved application of heavy accumulative strain of 45–57% through: (1) single pass TMCP strategy at various strain rates (0.01, 0.1 and 1.0s−1), (2) double pass TMCP strategy at different strain rates and (3) multi-pass TMCP strategy at strain rate of 0.1s−1. Microstructures after the diverse TMCP strategies reveal ferrite grain refinement and the flow curves show typical strain hardening. This denotes that the working grain refinement mechanism during all TMCP is dynamic strain-induced transformation of austenite to ferrite. It has been found that the strain rate plays a considerable role in determining the flow characteristics regardless the applied TMCP strategy type. Double-pass TMCP strategy is the optimum TMCP strategy that promotes the ferrite refinement and enhances the flow and productivity characteristics.

Effect of Cooling Rate on the Structure of Low-Carbon Low-Alloy Steel After Thermomechanical Controlled Processing

The method of orientation microscopy (EBSD) is used to study the structure and texture of low-carbon low-alloy pipe steel after thermomechanical controlled processing (TMCP) and subsequent quenching at cooling rates 100 – 700 K/sec.

Thermomechanical Controlled Rolling of Hot Coils of Steel Grade S355MC at the Wide-Strip Rolling Mill 1700

Тhere has been developed technology, and pilot batch of hot rolling coils (6×1500 mm, steel grade S355MC) has been produced using thermo-mechanical controlled process (TMCP) for the wide-strip rolling mill 1700. The integrated technology for TMCP coil production (steel grade S355MC) has been firstly developed for the rolling mill 1700 in accordance with EN 10149-2. Air cooling for coils to 450°C after coiling has been firstly used in the developed technology, which provides for decrease in air scale and improvement of surface quality for the customers. It is possible to manufacture rolled products up to 6×1500 mm (steel grade S355MC) in accordance with EN 10149-2 using the existing equipment without exceeding the existing process constraints during its operation and without upgrading. It is possible to further master the rolled products, which are manufactured according to the TMCP technology.

Effects of cooling rate and strain rate on phase transformation, microstructure and mechanical behaviour of thermomechanically processed pearlitic steel

In the present investigation, thermomechanical controlled processing of a high carbon steel and a Nb microalloyed high carbon steel have been conducted in a Gleeble 3800 simulator. Different microscopic techniques have been utilised for the characterisation of the microstructure and hardness data has been used for the evaluation of mechanical properties. In order to suppress the transformation enthalpy, experiments are performed under varying cooling rate and strain rate. The effect of niobium microalloying leads to the lowering of recalescence and suppresses austenite to pearlite transformation start and finish temperatures at every cooling rate which leads to the refinement of interlamellar spacing and thereby improve hardness and predicted yield strength values. It is evident that a higher strain rate accelerates the kinetics of pearlite transformation and elevates the pearlite start temperature. The increase of strain rate in the range of 1s–1 to 100s–1 followed by a constant cooling rate (free cooling) leads to the refinement of interlamellar spacing as well as improves mechanical properties. The true stress-true strain diagram at a lower strain rate indicates higher strain hardening with sharp yield point, whereas the same at a higher strain rate indicates the sudden occurrence of strain softening. The variation in recalescence due to the alternation of the cooling rate and strain rate has been correlated with the final microstructure and mechanical properties.

Critical assessment of the fatigue crack growth rate sensitivity to material microstructure in ferrite-pearlite steels in air and marine environment

This paper presents a critical assessment of the effect of microstructure on the fatigue crack growth rate (FCGR) in ferrite-pearlite (α-P) steels in air and marine environment. This was done by evaluating the resistance of three subgrades of S355 steel, produced by normalized-rolling, NR (S355J2+N) and thermo-mechanical control process, TMCP (S355G8+M and S355G10 + M), to fatigue crack growth (FCG) in air and seawater. The data from the tests were then compared with results of several studies on α-P steels of similar compositions and microconstituents in the literature. It was found that microstructure strongly affected the Paris region of the da/dN vs. ΔK sigmoidal curve in both media. This is contrary to the current belief that microstructure has little or a negligible effect in the linear region of the FCGR curve in air. TMCP and quenched-and-tempered (QT) steels offered better resistance to FCG than NR steel in both media. TMCP steel offered the highest resistance to FCG. The crack path was non-planar with a complex crack front. The tendency for crack deviation, branching, wedging action by metal crumbs and perhaps interlocking majorly explained why the FCGRs of the TMCP and QT steels are lower than that of NR. The result also suggests that ductility and yield stress do not have a strong retarding effect on FCG in α-P steels at low stress intensity factor range (SIFR). This finding has a fundamental implication for analytical and numerical models that wholly depend on the mechanical properties of the steel and assumes plane horizontal crack propagation front in predicting FCGR in steels. Such models are likely to fail in predicting FCGR even in a subgrade of the same grade of steel.

Numerical and Experimental Investigation of the Heat Input Effect on the Mechanical Properties and Microstructure of Dissimilar Weld Joints of 690-MPa QT and TMCP Steel

The study evaluates numerically and experimentally the effect of welding heat input parameters on the microstructure and hardness of the heat-affected zone (HAZ) of quenched and tempered (QT) and thermo-mechanically controlled process (TMCP) 690-MPa high-strength steel. Numerical analyses and experimental comparisons were applied using three heat input values (10, 14, and 17 kJ/cm) in order to predict the thermal fields during welding. Experimental analysis was carried out of the microstructure and microhardness behavior in different HAZ areas. The numerical values indicate that the maximum respective values of temperature measured in QT steel and TMCP steel were about 1300 and 1200 °C for a heat input of 10 kJ/cm, 1400 and 1300 °C for a heat input of 14 kJ/cm, and 1600 and 1450 °C for a heat input of 17 kJ/cm. The cooling times resulted, for a heat input of 10 kJ/cm, in numerical t8/5 (14.5 s) and experimental (18.84 s) increases in hardness in the coarse-grain heat-affected zone (CGHAZ) of the QT steel (317 HV0.1), due to the formation of bainite and lath martensite structures with grain growth. Decreased hardness in the CGHAZ of TMCP steel (240 HV0.1) was caused by primary recrystallization of the microstructure and the formation of more equilibrium products of austenite decomposition. Increasing the heat input (14 to 17 kJ/cm) led to numerical t8/5 (29 s) and experimental (36 s) decreases in hardness in the CGHAZ of QT steel (270 HV0.1) due to the full austenite (thermal weld cycle), and maintained the relative value of TMCP steel (235 HV0.1).