Fraction Of Bainite Research Articles

Influence of coiling temperature on mechanical properties in hot rolling C-Mn-Si-Al steel

A low carbon C-Mn-Si-Al steel was hot rolled with coiling temperatures ranged from 510 ºC to 710 ºC. The microstructure of specimens consisted of deformed ferrite, lath martensite, twin martensite and bainite. Cementite was only observed in sample coiled at 570 ºC. With increasing coiling temperature, deformed ferrite fraction decreased and recrystallized ferrite fraction increased. The ultimate tensile strength increased gradually from 1084 MPa to 1183 MPa due to the increasing fraction of martensite and bainite. The optimal mechanical properties were obtained when coiled at 570 ºC with an ultimate tensile strength of 1085 MPa and ultimate elongation of 19.1 %.

Effect of Austenitizing Temperature on Mechanical Properties of the Mixed Bainite - Martensite Microstructure in CrMoV Steel

The effect of austenitizing temperature on mechanical properties of the mixed bainite - martensite microstructure in CrMoV steel was studied in the present work. The result showed that at low austenitizing temperature (910°C - 1000°C), the mixed microstructures containing 12-28% volume fraction of lower bainite showed higher yield and tensile strength than fully martensitic microstructure. The partitioning of the prior austenite grain by lower bainite was found to cause a refinement of the martensite packet size. In addition the strength of the lower bainite in the mixed microstructure is enhanced by plastic constraint induced by the surrounding stronger martensite. By increasing the austenitizing temperature from 1000°C to 1200°C (40min), the YS, UTS, %EL, %RA and CVN impact energy decreased for all samples. This is attributed mainly to the increase in austenite grain size and width of bainite sheaves.

Misorientation, grain boundary, texture and recrystallization study in X90 hot bend related to mechanical properties

In the present study, electron backscattering diffraction (EBSD) was used to study the relationship between misorientation, grain boundary, texture, recrystallization and the mechanical properties for API (American Petroleum Institute) X90 hot induction bend. The experimental results show that the yield strength of the parent pipe is 748MPa, while it reduced about 40MPa after hot induction bending. The strength in the inner arc side is similar along the longitudinal and transverse direction, but it is anisotropic in the outer arc side. The texture of {113} <110> components and {112} <110> components are the main reason for anisotropy. After hot induction bending, the deformed and elongated grains exist in the bend zone of X90 bend, and the sub-grain boundaries exist in the grains. The grains in the neutral axis are equiaxed and without lath-like structure and sub-structure. A large fraction of lath bainite (LB) boundaries and Σ3 boundaries in the outer arc side resulted in a high strength and low impact toughness. Moreover, the deformed region and the recrystallization region have the same tendency for the tested specimens, the neutral axis of X90 bend has a lower stored energy in the grains that could resist the cracks propagation effectively.

The Effect of Isothermal Hold Temperature on Microstructure and Mechanical Properties of TRIP Steel

TRIP (transformation induced plasticity) steels are low alloyed low carbon steels with complex microstructures consisting of ferrite, bainite and retained austenite. This complex microstructure provides them with excellent strength to ductility balance, making them a member of advanced high strength steels (AHSS) group. Suitable microstructure can be obtained by either heat or thermo-mechanical treatment. A hold in bainite transformation region is an integral part of any form of commercial TRIP steel processing route, as it enables formation of sufficient volume fraction of bainite and also stabilization of retained austenite in the final microstructure. Various bainitic hold temperatures ranging from 350 °C to 500 °C were tested within thermo-mechanical treatment of 0.2C-1.5Mn-0.6S-1.5Al steel and the final microstructures were evaluated with regard to the suitability to TRIP effect and achieved mechanical properties. The microstructures were analyzed by scanning electron microscopy and mechanical properties measured by tensile test.

A Method to Estimate the Microstructures of Low‐Alloy Cold‐Rolled TRIP Steels

A method is implemented in the present study to estimate the microstructures of low‐alloy cold‐rolled TRIP steels. This method includes simulations of intercritical annealing (IA) process and calculations of isothermal bainitic transformation (IBT) process. The evolutions of volume fractions and elements concentration profiles with temperatures and holding times for ferrite phase and austenite phase could be acquired assisted by Thermo‐Calc and DICTRA during IA process. Bainitic transformation from austenite, with chemical compositions determined at the end of IA, takes place during the subsequent IBT process. By taking into account the martensitic transformation during cooling from IA to IBT and/or from IBT to room temperature (RT) and avoiding the complex discussions of bainitic transformation kinetics, the volume fractions of bainite, martensite and retained austenite could be approximately determined after IBT and subsequent water quench. At the same time, the experimental validation of this method is conducted with well agreements. This implemented method is expected to relate chemical compositions and heat treatment parameters to the final microstructures of low‐alloy cold‐rolled TRIP steels and be beneficial for optimizing TRIP microstructures without amounts of experiments.

Modeling Bainite Transformation and Retained Austenite in Hot Rolled TRIP Steel by Instantaneous Carbon Enrichment

The rate model inherited from Leblond and Devaux's concept has been applied to multiphase transformations in hot‐rolled TRIP steel. The phase fraction is collected from different laboratory scale process simulations, both multi‐pass rolling for repeated recrystallization and single‐pass cycles. The latter is also investigated by in situ synchrotron X‐ray diffraction (XRD) to acquire instantaneous data for the phase fraction and carbon content. The different cycles allow different austenite conditioning inputs for the model to enlarge the range of thermomechanical parameters. Bainite transformation model is a function of the instantaneous average carbon content in the remaining austenite, which is measured from the shift in the 1‐dimensional diffraction peaks. The modeling can map different process windows for a specific required microstructure of TRIP steel. The bainite formation is remarkably favored by low carbon content. But, the promotion of ferrite formation in the former step increases the carbon content dramatically and yields smaller bainite fraction that consequently larger fraction of retained austenite is allowed. A modified Koistinen–Marburger equation has been established from the in situ XRD data to model the martensite formation during the final cooling to reveal the stability of austenite. Above an average carbon content of 1.475 mass%, the retained austenite becomes stable.

Method to Evaluate the Kinetics of Bainite Transformation in Low-Temperature Nanobainitic Steel Using Thermal Dilatation Curve Analysis

The aim of this work was to develop a method to evaluate the kinetics of bainite transformation by theoretical deduction and thermal dilatation curve analysis. A Gleeble-3500 thermomechanical simulator and dilatometer (DIL805A) were employed to study the isothermal transformation in deformed (360 ∘ C , 600 ∘ C , and 860 ∘ C ) and undeformed conditions. The thermal dilatation information during isothermal transformation was recorded, and the dilatation curves were well smoothed. By taking a derivative of the dilation curve with respect to the transformation time, the peak time of transformation rate (PTTR) was obtained, which can serve as the essence of isothermal transformation time. The relative change of length ( Δ L / L ) due to phase transformation was theoretically deduced, and the effect of temperature was taken into consideration. Combing experimental data, the volume fraction of bainite in isothermal transformation was calculated. Making a graph of volume fraction versus PTTR was a good method to evaluate the kinetics of bainitic transformation clearly and concisely which facilitated optimization of the preparation technique for low-temperature nanobainitic steel.

Effect of composition and isothermal holding temperature on the precipitation hardening in Vanadium-microalloyed steels

The beneficial effect of increasing the C and N contents on precipitation strengthening in V-microalloyed steels is well known. However both C and N levels cannot be increased simultaneously as that can hamper the ductility and weldability of the steel. Therefore either higher C level or higher N level needs to be maintained. The present study is undertaken to understand the effect of C and N contents on the microstructure and precipitation in flat-rolled strips of V-microalloyed steels. During industrial processing, hot-rolled strips are subjected to a coiling treatment where V-precipitation primarily occurs. Therefore, besides steel composition, the effect of coiling temperature on the evolution of microstructure and precipitation in V-microalloyed steels needs to be studied. In order to achieve these objectives, two different V steels (0.05wt% V), one having higher C and lower N content (HCLN steel) and the other having lower C and higher N content (LCHN steel) were prepared and subjected to hot-deformation followed by isothermal holding treatments over a temperature range of 500–750°C inside Gleeble® thermo-mechanical simulator. The purpose of isothermal holding study was to simulate the industrial coiling treatment. Simulated samples were characterized in detail and the maximum precipitation strengthening from fine V(C,N) precipitates was obtained at an intermediate range of isothermal holding temperatures (600–650°C) in both the steels. In spite of the relatively coarser ferrite grain size and smaller fraction of pearlite and bainite, the yield strength of LCHN steel was close to HCLN steel and LCHN steel also showed superior ductility.

Fracture and Crack Propagation Behavior of 560 MPa Microalloyed Pipeline Steel under Different Cooling Schedules

Three kinds of pipeline steel with different microstructures were fabricated by varying cooling schedules during thermo-mechanical controlled processing (TMCP). Charpy impact property of the pipeline steels were obtained, and the fracture and crack-arrest mechanisms were further studied. The results indicated that the steels were classified into two kinds according to their microstructures, the mixture of acicular ferrite (AF), quasi-polygonal ferrite (QF), granular bainite (GB) and small fraction of degenerate pearlite (DP), and the mixed microstructure of AF and GB, respectively. The processed steel with microstructure of AF and GB exhibited more excellent low-temperature toughness and crack-arrest properties with upper shelf energy of ~281 J and energy transition temperature of ~-76°C. The mixed microstructure (AF + GB) possessing smaller effective grain size hindered the propagating of crack and consumed large amount of energy during fracture. The effective grain size of microstructure was the dominant factor controlling low-temperature toughness and crack-arrest properties of pipeline steel, which increased the high-angle boundary length per unit area and further increased the crack propagation energy during fracture.

Effect of Pre-Existing Martensite on Bainitic Transformation in Low-Temperature Bainite Steel

The effect of different volume fractions of pre-existing martensite on the low-temperature bainitic transformation and microstructures was quantitatively analyzed by dilatometer, optical microscope and scanning electron microscope. The results showed that pre-existing martensitic transformation accelerated the subsequent low-temperature bainitic transformation, and the incubation period and completion time of bainitic reaction were significantly shortened. This phenomenon was attributed to the increasing nucleation sites caused by the introduced dislocations in austenite due to the formation of pre-existing martensite. However, it was noteworthy that, because of the increased bainitic plates adjacent to the pre-existing martensitic plates, the probability of the impingement of bainitic plates during growth was increased, which resulted in a decrease in the maximum attainable volume fraction of bainite.

MIG-Welding of Dissimilar Advanced High Strength Steel Sheets

The need for steel materials with increasing strength is constantly growing. The main application of such advanced high strength steels (AHSS) is the automobile industry, therefore the welding process of different types of AHSSs in dissimilar welding joint was investigated. To simulate the mass production of thin steel sheet constructions (such as car bodies) automated metal inert gas (MIG) welding process was used to weld the TWIP (twinning induced plasticity) and TRIP (transformation induced plasticity) steel sheets together. The welding parameters were successfully optimized for butt welded joints. The joints were investigated by visual examination, tensile testing, quantitative metallography and hardness measurements. The TRIP steel side of the joints showed increased microhardness up to (450-500 HV0.1) through increased fraction of bainite and martensite. Macroscopically the tensile specimen showed ductile behaviour, they broke in the austenitic weld material.

Impact of quenching temperature and isothermal holding time during austempering on bainite content in high-silicon steel

Silicon plays an important role in the manufacture and processing of steel. It is involved in metallurgical processes in the melt, improves castability, and, together with aluminium, belongs to the elements which suppress cementite formation during heat treatment of steels. The last-named aspect offers a great potential for developing high-strength steels with excellent ductility. The amount of bainite in the microstructure has a substantial impact in this respect. Bainite fraction depends mainly on temperature and on the isothermal holding time in the bainitic transformation region. In steels with low silicon levels, isothermal bainitic transformation continues until all austenite has transformed to bainite. At higher silicon levels, approximately 2 weight percent, the bainitic transformation is expected to be incomplete. The resulting bainite fraction would then be dictated by the thermodynamic equilibrium in the austenite-bainite system. As a result, one could control microstructural evolution, and thus the mechanical properties in high-strength bainitic-martensitic steels

Effect of Cooling Rate and Chemical Composition on Microstructure and Properties of Naturally Cooled Vanadium-Microalloyed Steels

Samples from two V-microalloyed steels (0.05 wt pct V) having different C and N levels, namely high-C low-N steel, HCLN (0.22 wt pct C, 0.007 wt pct N) and low-C high-N steel, LCHN (0.06 wt pct C, 0.013 wt pct N) were naturally cooled from 1373 K (1100 °C) to room temperature over a range of cooling rates (0.07 to 3.33 K/s). Samples from a plain C-Mn steel (0.06 wt pct C, 0.007 wt pct N) were also subjected to the same heat treatment for comparison. The effect of cooling rate and steel composition on microstructures, precipitates, and tensile properties has been investigated. Due to the presence of large fraction of harder constituents, like pearlite and bainite, HCLN steel showed higher strength and lower ductility than LCHN steel. LCHN steel, on the other hand, showed good combination of strength and ductility due to its predominantly ferrite matrix with precipitation strengthening. The V-precipitate size was more refined and the precipitate density was higher in HCLN steel than that in LCHN steel. This observation confirms the importance of C content in V-microalloyed steel in terms of precipitation strengthening. An intermediate cooling rate (~1.4 K/s) has been found to be the optimum choice in order to maximize the precipitation strengthening in V-containing steels.

Tensile deformation behavior of high strength anti-seismic steel with multi-phase microstructure

To investigate the tensile deformation behavior of high strength anti-seismic steel with multi-phase microstructure, tensile tests with strains of 0.05, 0.12 and 0.22 were performed at room temperature. Microstructure of tested steels was observed by means of optical microscopy (OM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Tensile mechanical properties of tested steels were obtained, and the influence of bainite content on deformation behavior was also discussed. Meanwhile, the deformation mechanism of steel with three kinds of microstructures of bainite, pearlite and ferrite was analyzed. Results show that tested steel with high volume fraction of bainite exhibits a continuous deformation behavior, and this may be attributed to a higher bainite volume fraction and a lower mobile dislocation density. The morphology of microstructure will influence the mechanical properties of tested steels. An increasing content of bainite can improve the tensile strength, but reduce the plasticity and toughness of the tested steels. In the deformation process of 0.039Nb steel, the ferrite and bainite have priorities to deform, and the deformation exhibits co-deformation of all microstructures in the later stage of deformation. In the deformation process of 0.024Nb-0.032V steel, the ferrite and pearlite have priorities to deform, and the deformation exhibits co-deformation of all microstructures in the later stage of deformation.

Effect of bainite in microstructure on hydrogen diffusion and trapping behavior of ferritic steel used for sour service application

Abstract

Effect of Red Scale on the Bendability of Ultrahigh-Strength Strip Steel

The effect of red scale on the bendability of a thermomechanically rolled and direct quenched pilot-scale strip steel has been studied by comparing the bending behaviour of adjacent areas with and without red scale. The yield strength of the studied 8 mm thick strip was 960 MPa. The local microstructure and texture below the different scale surfaces were characterized using FESEM and FESEM-EBSD, chemical compositions were determined using GDOES, microhardness profiles were measured and bendability was determined using three-point brake press bending. Red scale was found to significantly affect bendability especially when the bend axis is transverse to the rolling direction. The minimum usable punch radius for defect-free bends in the absence of red scale was 12 mm (1.5 x thickness) while under red scale it was 30 mm (3.75 x thickness). Beneath the red scale the microstructure 50 to 400 μm below the surface was clearly different to that in the absence of red scale. Without the red scale the microstructure was mainly granular bainite with small fraction of upper bainite and polygonal ferrite. Below the red scale the microstructure was a mixture of upper bainite and granular bainite. As a result of the microstructural differences, the subsurface hardness changed substantially from 360 HV in the absence of red scale to 410 HV with red scale. The chemical composition did not change as a result of the presence or absence of red scale, which rules this factor out as possible cause of differences in bendability or final microstructure. Possible explanations for the observed effects of red scale on subsurface microstructure, and microstructure on bendability, are discussed in the paper.

Formation of abnormal structures and their effects on the ductility of eutectoid steel

The formation of abnormal structures and their effects on reduction of area (RA) were investigated in eutectoid steels transformed at different temperatures ranging from 560 °C-650 °C. The occurrence of abnormal structures, such as upper bainite, degenerate pearlite, free ferrite, and grain boundary cementite, was confirmed. The volume fraction of upper bainite and degenerate pearlite decreased on increasing the transformation temperature, while the amount of free ferrite increased. As the transformation temperature increased, RA increased, reached a maximum, and then decreased, while the tensile strength continuously decreased. The crack formations during the tensile test could be classified into three types: tearing, shear cracking, and void formation/ coalescence. The decrease of the ductility at low transformation temperatures was attributed to the increased amount of upper bainite and degenerate pearlite, since the formation of cracks occurred by tearing interfaces or by void formation at abnormal structures during the tensile test. Meanwhile, the decrease in RA at high transformation temperatures was attributed to the occurrence of shear cracking rather than the presence of abnormal structures.

Effect of Mo Addition on the Transformation Stasis Phenomenon During the Isothermal Formation of Bainitic Ferrite

The effect of Mo addition on the transformation stasis (TS) phenomenon during bainitic transformation has been investigated in Fe-0.2C-1.5Mn-1.5Si-(0.2, 1.5, 3)Mo (wt pct) alloys. Both diffusionless and diffusional models failed in predicting the TS that is strongly affected by Mo concentration in experiment. The Gibbs energy balance approach could capture TS in alloys with 0.2 and 1.5 wt pct Mo, but significantly underestimate bainite fraction at stasis for alloy with 3 wt pct Mo, which is deduced to arise from the so-called anti-coupled solute drag effect.

Study on the constitutive model of boron steel 22MnB5 with different phase fractions

The mechanical properties of boron steel 22MnB5 is determined by phase fractions. In order to obtain the mechanical properties and service performance of car component by simulation accurately, a microstructure based constitutive model needs to be developed. In this paper, different area fractions of martensite, bainite and ferrite in boron steel 22MnB5 were obtained via control of cooling rate and the different microstructures were determined quantitatively by metallographic image processing. A modified Katsuro Inoue’s constitutive model was then proposed, which is a function of effective plastic strain, strain rate and the phase fractions of martensite, bainite and ferrite. On the other hand, crash simulation of B-pillar with tailored mechanical properties was conducted as a case study to investigate the relationship between microstructure and property configuration by using the modified Katsuro Inoue’s model. The relationship among phase fraction, the height of tailored segment, and the internal energies absorbed by different segments, the maximum acceleration and displacement of B-pillar during the collision process was studied, and the empirical equations between microstructure and dynamic response of crash performance were also established and formulated.

Effects of aging treatment on the microstructure and superelasticity of columnar-grained Cu71Al18Mn11 shape memory alloy

The effect of aging treatment on the superelasticity and martensitic transformation critical stress in columnar-grained Cu71Al18Mn11 shape memory alloy (SMA) at the temperature ranging from 250°C to 400°C was investigated. The microstructure evolution during the aging treatment was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that the plate-like bainite precipitates distribute homogeneously within austenitic grains and at grain boundaries. The volume fraction of bainite increases with the increase in aging temperature and aging time, which substantially improves the martensitic transformation critical stress of the alloy, whereas the bainite only slightly affects the superelasticity. This behavior is attributed to a coherent relationship between the bainite and the austenite, as well as to the bainite and the martensite exhibiting the same crystal structure. The variations of the martensitic transformation critical stress and the superelasticity of columnar-grained Cu71Al18Mn11 SMA with aging temperature and aging time are described by the Austin–Rickett equation, where the activation energy of bainite precipitation is 77.2 kJ·mol−1. Finally, a columnar-grained Cu71Al18Mn11 SMA with both excellent superelasticity (5%–9%) and high martensitic transformation critical stress (443–677 MPa) is obtained through the application of the appropriate aging treatments.