Thermo-mechanical Control Process Research Articles

Contributions of Grain Size and Crystal Orientation to Fatigue Crack Deflection and Branching Behavior in Low Carbon Steel Plates

In-situ observation of fatigue crack growth behavior under ΔK-constant cyclic loading in Paris regime for two Thermo-Mechanical Control Process (TMCP) steels with different grain size was carried out. It was found from the results that crack closure dominantly contributed to fatigue crack growth resistance in the fine grain material, while crack tip stress shielding induced by crack deflection and crack branching dominantly contributed to fatigue crack growth resistance in the coarse grain material. The electron back scatter diffraction (EBSD) analysis revealed that crack deflection and crack branching in the coarse grain material were induced in the grain with (101) crystal orientation intensively under co-existence of the grain with (001) crystal orientation and also along the grain boundary between the grains with (101) and (001) crystal orientations. Hence, higher content ratio of the grains with (001) crystal orientation could contribute to higher crack growth resistance.

Effect of Ta and Ti on Modified Reduced Activation Ferritic/Martensitic Steels with a Thermo‐mechanical Control Process

Modified reduced activation ferritic/martensitic steels with different amounts of Ta and Ti are fabricated with a thermo‐mechanical control process (TMCP). TMCP greatly enhances the refinement of both the precipitates and the martensite matrix. In addition, an increase in the dislocation density is achieved. The modified steels show excellent comprehensive mechanical properties compared with traditional reduced activation ferritic/martensitic steels. In addition, under the conditions of the TMCP, the effects of Ta and Ti on the microstructure and properties are analyzed. Simply increasing the amounts of Ta and Ti does not increase the phase fraction of MX, which is quite different from that derived from equilibrium thermodynamics theory or the typical hot rolling process for traditional reduced activation ferritic/martensitic steels. Without a high enough nucleation rate for MX, the excess Ta and Ti, which are not consumed by the formation of MX, promotes the formation of proeutectoid ferrite. This decreases the impact on toughness significantly. Herein, preliminary guidance is offered for a comprehensive design with a combination of both composition and rolling processing modification for the production of reduced activation ferritic/martensitic steels.

Microstructure, Mechanical Properties, and Strain‐Hardening Behavior of V–N Microalloyed Pipeline Steels Consisted of Polygonal Ferrite and Acicular Ferrite

A novel V–N microalloying method is adopted to produce pipeline steels using a two‐stage thermomechanical controlled processing involving relaxation process and accelerated cooling. The main objective of the current work is to study the novel practice in terms of microstructure, mechanical properties, and strain‐hardening behavior. The microstructure of the experimental steels consists of polygonal ferrite (PF), quasi‐polygonal ferrite (QF), acicular ferrite (AF), and granular bainite (GB). Mechanical properties including strength, ductility, and low‐temperature toughness of V–N microalloyed steels meet the requirements of X80 pipeline steels. Yield strength, tensile strength, total elongation, and −20 °C impact energy are 639 MPa, 761 MPa, 21.0%, and 283 J when the volume fraction of PF is 9.1%, and 603 MPa, 724 MPa, 24.1%, and 214 J when the volume fraction of PF increases to 30.4%. The V–N microalloyed steels exhibit high strain‐hardening ability, and the values of strain‐hardening exponent are 0.12 and 0.11 based on Hollomon analysis.

Microstructure and Strengthening/Toughening Mechanisms of Heavy Gauge Pipeline Steel Processed by Ultrafast Cooling

Heavy gauge pipeline steels experience a low qualification in drop-weight-tear test properties because of the low cooling capability of conventional thermomechanical controlled processing. To solve this problem, a new-generation thermomechanical-controlled processing technology based on ultrafast cooling was applied to prepare heavy gauge pipeline steels. The microstructure, strengthening and toughening mechanisms of 25.4 mm X70 and 22 mm X80 pipeline steels that were processed by ultrafast cooling were studied. The microstructures of the 25.4 mm X70 and 22 mm X80 pipeline steels consisted of bainitic ferrite, M-A island and acicular ferrite with a large fraction above 85%. The grain size and high-angle grain boundary fraction of X70 pipeline steel were 2.7 μm and 43%, respectively, whereas those of the X80 pipeline steel were 2.4 μm and 45%, respectively. The strengthening and toughening mechanisms were studied for the ultrafast cooling method. The main strengthening mechanism for 25.4 mm X70 pipeline steel was solution and grain-refining strengthening and precipitation strengthening with contributions of ~456 MPa and ~90.5 MPa, respectively. In the 22 mm X80 pipeline steel, the main strengthening mechanism was the solution and grain-refining strengthening, and dislocation strengthening with contributions of ~475 MPa and ~109.8 MPa, respectively.

Microstructure Design and Properties of X100 Gas Line Pipe

The microstructure evolution process of X100 pipeline steel is analysed, and the influence of different alloy composition and accelerated cooling process on microstructure evolution is studied in this article. It is obtained that acicular ferrite microstructure can make the yield strength of steel reach 650 MPa through micro alloying and thermo-mechanical control process(TMCP) with the comprehensive reinforcement effect of grain refinement, micro-alloying element precipitates and dislocation substructure. Acicular ferrite has the characteristics of fine grain, cross distribution, large angle grain boundary and presence of sub structure and high density movable dislocation. Test results of product quality also show that X100 pipeline steel which adopts Mn-Nb-Ti component system with a small amount of Ni (V), Mo alloy addition and TMCP process has obvious characteristics of fine grain. The Matrix is consisted of granular bainite and appropriate amount of MA as dispersion phase, which makes the tensile strength reach more than 800 Mpa. At the same time, the X100 pipeline steel has the characteristics of high toughness and good weldability, which is beneficial to ensure the safe operation of high pressure natural gas pipeline.

The Impact of Interphase Precipitation on the Mechanical Behavior of Fire-Resistant Steels at an Elevated Temperature.

In this study, we address the challenge of obtaining high strength at ambient and elevated temperatures in fire-resistant Ti–Mo–V steel with ferrite microstructures through thermo-mechanical controlled processing (TMCP). Thermally stable interphase precipitation of (Ti, Mo, V)C was an important criterion for retaining strength at elevated temperatures. Electron microscopy indicated that interphase precipitation occurred during continuous cooling after controlled rolling, where the volume fraction of interphase precipitation was controlled by the laminar cooling temperature. The interphase precipitation of MC carbides with an NaCl-type crystal structure indicated a Baker–Nutting (B–N) orientation relationship with ferrite. When the steel was isothermally held at 600 °C for up to 3 h, interphase precipitation occurred during TMCP with high thermal stability. At the same time, some random precipitation took place during isothermal holding. The interphase precipitation increased the elastic modulus of the experimental steels at an elevated temperature. It is proposed that fire-resistant steel with thermally stable interphase precipitation is preferred, which enhances precipitation strengthening and dislocation strengthening at elevated temperatures.

Effects of finish rolling deformation on hydrogen-induced cracking and hydrogen-induced ductility loss of high-vanadium TMCP X80 pipeline steel

An optimum finish rolling deformation (FRD) of thermomechanical controlled processing (TMCP) is suggested to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel in this study. The results demonstrate that with increasing FRD the microstructure refines, the grain size of the steel decreases and the recrystallization degree deepens. The increase of FRD leads to the reduction of low angle grain boundaries (LAGBs) and the grains oriented with plane {100} parallel to normal direction ({100}//ND) fibres, which plays a significant role in improving the resistance of crack propagation. Besides, the differences of effective hydrogen diffusion coefficient and diffusible hydrogen concentration are negligible among four experimental steels with various FRD. However, the best hydrogen-induced ductility loss resistance is obtained in the steel with 40% FRD containing the most nano-scale precipitates acting as effective hydrogen traps.

Mechanical properties of TMCP Q690 high strength structural steel at elevated temperatures

The mechanical properties of high strength structural steel (HSSS) at elevated temperatures are the basis for fire-resistant design of HSSS members and structures. Previous studies on the mechanical properties of HSSS at elevated temperatures were mainly conducted on QT (Quenched and Tempered) HSSS, while few studies were conducted on TMCP (Thermo-Mechanical Controlled Process) HSSS. In order to identify the effect of manufacturing process on the mechanical properties of HSSS at elevated temperatures, an experimental study was carried out by steady-state method on TMCP Q690 HSSS, considering nine elevated temperatures ranging from 200 to 800 °C. Results showed that the elastic modulus and strength of TMCP Q690 HSSS decreased significantly as the temperature increased over 400 °C, while the ultimate elongation increased as the temperature increased above 500 °C. Formulas for calculating the reduction factors of mechanical properties of TMCP Q690 HSSS at elevated temperatures were proposed. Comparing the results of TMCP Q690 HSSS with those of QT Q690 HSSS previously studied, it can be found that the reduction on elastic modulus and strength was more severe for TMCP Q690 HSSS than QT Q690 HSSS in most cases, which was explained in the microstructure level of HSSS.

Effect of Cooling Rate and Finish Rolling Temperature on Structure and Strength of API 5LX70 Linepipe Steel Plate

The present study has investigated the effect of changes in accelerated cooling (ACC) and finish rolling temperature (FRT) on the mechanical properties of high-strength low-alloy (HSLA) Nb-V-Ti steel plate of non-sour API 5LX70 linepipe during the thermomechanical controlled process (TMCP). Tensile test results showed that increasing ACC or reducing FRT enhanced yield and tensile strengths of the subject steel, which was also confirmed by the Vickers hardness test. Microstructure examinations demonstrated that increasing ACC and reducing FRT resulted in a lower volume fraction and a finer size of pearlite. Moreover, an increase in the ACC enhanced the formation of granular ferrite (GF) and fine polygonal ferrite (FPF), while a decrease in the FRT mostly affected the formation of acicular ferrite (AF). In all microstructures, banded structures and particles with a size of approximately 6 μm were observed due to segregation during solidification, where changing the parameters did not affect their formations.

Effect of Tempering Temperature after Thermo-Mechanical Control Process on Microstructure Characteristics and Hydrogen-Induced Ductility Loss in High-Vanadium X80 Pipeline Steel.

In this study, an optimum tempering temperature after a thermo-mechanical control process (TMCP) was proposed to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel. The results showed that with increasing tempering temperature from 450 to 650 °C, the size and quantity of granular bainite decreased but the spacing of deformed lath ferrite and the fraction of massive ferrite increased. The number of fine vanadium carbides increased as well. However, as the tempering temperature increased to 700 °C, the microstructure of T700 steel completely converted to massive ferrite and the grain size became larger. Additionally, the amount of nanoscale precipitates decreased again, and the mean size of precipitates evidently increased in T700 steel. The steel tempering at 650 °C, containing the most vanadium precipitates with a size less than 20 nm, had the lowest hydrogen diffusion coefficient and the best resistance to hydrogen-induced ductility loss.

Effect of C-Mn ratio on the maximum hardness and toughness in TMCP steels with an identical carbon equivalent

In this study, the mechanical properties of two types of thermo-mechanically controlled processed (TMCP) steels with the same carbon equivalent but different alloying constituents of C and Mn were examined. Tensile, impact, hardness, and through-thickness tensile tests were employed for this examination. The taper hardness test was conducted in the weld heat-affected zones (HAZs) by using common onsite welding methods. The results revealed that the two distinct steel plates exhibited different microstructures in the base metal; however, their maximum hardness and microstructures in the HAZs were similar within the heat input range (10–30kJ/cm) that is often used at manufacturing sites. Moreover, the continuous cooling transformation diagrams for the HAZs and the base metal used for welding the TMCP steels were simulated to validate the experimental results for the microstructure. The drop weight test and weld bead bend test were employed to compare the toughness related with the weldability of the two steel plates. The steel with composition of 0.08% C-1.5% Mn exhibited lower nil-ductility transition temperature and better crack arrestability than that with the composition of 0.15% C-1.1% Mn.

Applicability of Hybrid Built-Up Wide Flange Steel Beams

To accommodate growing demands on either heavy steel structures or unique buildings with irregular configurations, built-up wide-flange steel (BWS) beams are being popularly used in modern steel construction. In current fabrication practices of BWS members, high-performance steels produced in steelmaking factories under the thermo-mechanical control process (TMCP) are typically utilized to achieve proper welding performances. However, since its basic unit price is quite higher than typical hot-rolled steel materials, this study introduced a hybrid BWS section for cost saving with no performance degradation, where high-performance TMCP steel was used in flanges, and conventional hot-rolled steel was adopted in web plate. To verify the tensile performances of a hybrid BWS section with non-uniform properties, split T tension and Charpy impact tests were conducted, and flexural tests were also carried out on hybrid and homogeneous BWS beam members. On this basis, it was confirmed that the structural performance of the hybrid BWS member is comparable with that of the conventional one with a uniform section property.

Favorable Modulation of Microstructural Constituents in Governing Low Temperature Toughness Induced Through a Three-Stage Cooling Trajectory

Low temperature toughness continues to be a major challenge and significant efforts are being made in this regard. We address the challenge of scientific and technological gap through the design of a novel process involving a three-stage cooling trajectory during thermo-mechanical controlled processing to process high strength low alloy heavy plates with high low temperature toughness. Interestingly, in contrast to the conventional continuous cooling process, the three-stage cooling trajectory enabled us to obtain a favorable acicular ferrite toughening enhancing microstructure. On the other hand, martensite–austenite islands were characterized by small dot morphology in the three-stage cooling trajectory that restrict crack nucleation and prevent debonding from the matrix. The low temperature impact toughness at − 80 °C exceeded ~ 500% in relation to the conventional process. The mechanism of formation of acicular ferrite and its impact on toughness is critically analyzed experimentally via electron microscopy and electron back scattered diffraction analysis and theoretically discussed in the context of toughening and crack arresting mechanism. Low temperature toughness in heavy steel plates continues to be an aspect of significant concern. We address the challenge of scientific and technological gap through the design of a novel process involving a three-stage cooling trajectory during thermo-mechanical controlled processing (TMCP) to process high strength low alloy (HSLA) heavy plates.

Interactions of the process parameters and mechanical properties of laser butt welds in thin high strength low alloy steel plates

High strength low alloy steels subjected to the thermomechanical control process present excellent strength–toughness combination, high strength/weight ratio, and weldability. Therefore, they are widely used in structural components, such as pressure vessels, oil/gas transportation pipes, lifting equipment, vehicles, shipbuilding and offshore industries, and in the automotive industry where low thickness (0.8–3 mm thickness) is of great importance. Usually, these steels are welded by conventional gas metal arc welding, which creates wide heat-affected zones, large residual stresses, and distortion in the welded parts. Laser welding is nowadays an alternative process to weld high strength low alloy steel parts due to its advantages. The aim of this work is to understand the effect of process parameters on defects, weld bead geometry, microstructure, and mechanical properties, namely hardness and tensile strength. We identify the main laser welding parameters and their influence on the weld bead geometry and defects, for a 3 mm thick high strength low alloy steel welded under a maximum power of 2 kW. A cross section of the weld seam was optimized achieving a good geometry without porosity. The threshold value of the heat input to achieve complete penetration was determined for different focus diameters. The microstructure, size, and hardness of the heat-affected zone and of the fusion zone are strongly influenced by the heat input. The values of the tensile strength achieved in butt welds were close to the base metal by an appropriate selection of the laser welding parameters and the heat input.

Assessment of the Effect of Laser Welding on the Properties and Structure of TMCP Steel Butt Joints.

The research work and related tests aimed to identify the effect of filler metal-free laser beam welding on the structure and properties of butt joints made of steel 700MC subjected to the TMCP (thermo-mechanically controlled processed) process. The tests involved 10-mm thick welded joints and a welding linear energy of 4 kJ/mm and 5 kJ/mm. The inert gas shielded welding process was performed in the flat position (PA) and horizontal position (PC). Non-destructive testing enabled classification of the tested welded joints as representing the quality level B in accordance with the requirements set out in standard 13919-1. Destructive tests revealed that the tensile strength of the joints was 5% lower than S700MC steel. The results of tensile tests and changes in structure were referred to joints made using the MAG (Metal Active Gas) method. The tests of thin films performed using a high-resolution scanning transmission electron microscope revealed that, during laser beam welding, an increase in dilution was accompanied by an increase in the content of alloying microadditions titanium and niobium, particularly in the fusion area. A significant content of hardening phases in the welded joint during cooling led to significant precipitation hardening by fine-dispersive (Ti,Nb)(C,N) type precipitates being of several nanometres in size, which, in turn, resulted in the reduction of plastic properties. An increase in the concentration of elements responsible for steel hardening, i.e., Ti and Nb, also contributed to reducing the weld toughness below the acceptable value, which amounts to 25 J/cm2. In cases of S700MC, the analysis of the phase transformation of austenite exposed to welding thermal cycles and the value of carbon equivalent cannot be the only factors taken into consideration when assessing weldability.

Improved Toughness of a High‐Strength Low‐Alloy Steel for Arctic Ship by Ni and Mo Addition

The effect of Ni and Mo addition on the microstructures of a high strength low alloy (HSLA) steel under the thermo‐mechanical control processing (TMCP) process is currently unknown, which is herein explored by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). As the Ni increase from 0.6 to 0.9 wt%, the volume fraction of bainitic ferrite increase from 30% to 50%, the average size decrease from 3 to 2 μm, and the size of the martensite–austenite (M–A) constituent decrease from 1.2 to 0.7 μm. The volume fraction of bainitic ferrite increases from 30% to 85%, and grain size increases from 3 to 5 μm with increasing Mo from 0 to 0.2 wt%. The size of M–A constituent decreases from 1.2 to 0.3 μm. Impact absorbed energy at −100 °C increases from 50 to 70 J as the Ni increase from 0.6 to 0.9 wt%, which decreases from 50 to 10 J with increasing Mo from 0 to 0.2 wt%. The improved impact toughness mainly results from the decreasing size of the grain and the M–A constituent with the increase in Ni from 0.6 to 0.9 wt%.

Failure Mechanisms of Mechanically and Thermally Produced Holes in High-Strength Low-Alloy Steel Plates Subjected to Fatigue Loading

High-strength low-alloy steels (HSLA) are gaining popularity in structural applications in which weight reduction is of interest, such as heavy duty machinery, bridges, and offshore structures. Since the fatigue behavior of welds appears to be almost independent of the base material and displays little improvement when more resistant steel grades are employed, the use of bolted joints is an alternative joining technique which can lead to an increased fatigue performance of HSLA connections. Manufacturing a hole to allocate the fastener elements is an unavoidable step in bolted elements and it might induce defects and tensile residual stresses that could affect its fatigue behavior. This paper studies and compares several mechanical (punching, drilling, and waterjet-cut) and thermal (plasma and laser-cut) hole-making procedures in HSLA structural plates. A series of 63 uniaxial fatigue tests was completed, covering three HSLA grades produced by thermomechanically controlled process (TMCP) with yield strength ranging from 500 to 960 MPa. Samples were tested at single load level, which was considered representative in HSLA typical applications, according to the input received from end users. The manufactured holes were examined by means of optical and electron microscopy, 3D point measurement, micro hardness tests, X-ray diffraction, and electron backscatter diffraction (EBSD). The results give insight on cutting processes in HSLA and indicate how the fatigue failure is dominated by macro defects rather than by the steel grade. It was shown that the higher yield strength of the HSLA grades did not lead to a higher fatigue life. Best fatigue results were achieved with laser-cut specimens while punched samples withstood the lowest amount of cycles.

Effect of Crystallographic Texture on Anisotropy of Mechanical Properties in High Strength Martensitic Steel

The thermomechanical control process (TMCP) is widely applied as one of the effective processes for improving the strength and toughness of steel plates. In actual application, the anisotropies of mechanical properties arising from the crystallographic texture developed during the controlled rolling process are important issues. In this study, the effect of texture on the anisotropies of mechanical properties in an experimentally manufactured YP960 MPa class steel plate was investigated. Strength varied through the plate thickness from the surface to mid-thickness. At the surface, the strength in the longitudinal direction was higher than that in the transverse direction, and in contrast, at mid-thickness, the strength in the transverse direction was higher than that in the longitudinal direction. The major components of the texture at the plate surface were {110}<111> and {112}<111>, whereas those at mid-thickness were {332}<113> and {211}~{311}<011>. It is considered that the texture of the plate surface was formed by shear strain in the austenite region, whereas that at mid-thickness was formed by plane strain compressive. A crystal plasticity analysis based on the initial texture information obtained experimentally revealed that the anisotropies of mechanical properties were strongly affected by the crystallographic orientation.

Tracing Microalloy Precipitation in Nb-Ti HSLA Steel during Austenite Conditioning

The microalloying with niobium (Nb) and titanium (Ti) is standardly applied in low carbon steel high-strength low-alloy (HSLA) steels and enables austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. In that respect, it is important to better understand the precipitation kinetics as well as the precipitation sequence in a typical Nb-Ti-microalloyed steel. Various characterization methods were utilized in this study for tracing microalloy precipitation after simulating different austenite TMCP conditions in a Gleeble thermo-mechanical simulator. Atom probe tomography (APT), scanning transmission electron microscopy in a focused ion beam equipped scanning electron microscope (STEM-on-FIB), and electrical resistivity measurements provided complementary information on the precipitation status and were correlated with each other. It was demonstrated that accurate electrical resistivity measurements of the bulk steel could monitor the general consumption of solute microalloys (Nb) during hot working and were further complemented by APT measurements of the steel matrix. Precipitates that had formed during cooling or isothermal holding could be distinguished from strain-induced precipitates by corroborating STEM measurements with APT results, because APT specifically allowed obtaining detailed information about the chemical composition of precipitates as well as the elemental distribution. The current paper highlights the complementarity of these methods and shows first results within the framework of a larger study on strain-induced precipitation.

Improvement of microstructure and properties of air‐cooled 46MnVS5 forging steel rod for fracture splitting connecting by thermomechanical control process (TMCP) and by niobium micro alloying

AbstractMicrostructure control and properties of air‐cooled high‐strength 46MnVS5 forging steel rod for fracture splitting connecting were systematically investigated by quantitative metallographic analysis, thermomechanical simulation and industrial trials etc. The results indicate that the critical cooling rate for bainite transformation in the steel is about 1.5 °C/s–2 °C/s during continuous cooling. Cooling parameters are optimized in two sections. That is, the specimen is firstly cooled to 750 °C at a rate of 4 °C/s, then continuously cooled to 570 °C at 1 °C/s, 1.5 °C/s. Addition of 0.024 wt.% niobium improves the fracture splitting performance by 10 %–20 % reduction of decarburization sensitivity. Finally, the optimized parameters for the niobium micro alloyed steel in the laboratory were successfully applied to produce the connecting rod with excellent fracture splitting performance on the traditional production line.