Bainitic Ferrite Matrix Research Articles

Evaluation of Shear-Punched Surface Layer Damage in Ultrahigh-Strength TRIP-Aided Steels with Bainitic Ferrite and/or Martensite Matrix Structure

Evaluation of Shear-Punched Surface Layer Damage in Ultrahigh-Strength TRIP-Aided Steels with Bainitic Ferrite and/or Martensite Matrix Structure

Effects of Mean Normal Stress and Microstructural Properties on Deformation Properties of Ultrahigh-Strength TRIP-Aided Steels with Bainitic Ferrite and/or Martensite Matrix Structure.

The effects of mean normal stress on the deformation properties such as the strain-hardening, strain-induced martensite transformation, and micro-void initiation behaviors of low-carbon ultrahigh-strength TRIP-aided bainitic ferrite (TBF), bainitic ferrite/martensite (TBM), and martensite (TM) steels were investigated to evaluate the various cold formabilities. In addition, the deformation properties were related to the microstructural properties such as the matrix structure, retained austenite characteristics, and second-phase properties. Positive mean normal stress considerably promoted strain-induced martensite transformation and micro-void initiation, with an increased strain-hardening rate in an early strain range in all steels. In TM steel, the primary martensite matrix structure suppressed the micro-void initiation through high uniformity of a primary martensite matrix structure and a low strength ratio, although the strain-induced transformation was promoted, and a large amount of martensite/austenite constituent or phase was contained. A mixed matrix structure of bainitic ferrite/primary martensite in TBM steel also suppressed the micro-void initiation because of the refined microstructure and relatively stable retained austenite. Promoted micro-void initiation of TBF steel was mainly promoted by a high strength ratio.

Revealing intermetallic strengthening in bainitic steel designed for nickel aluminide formation

BainNiAlCu steel was designed to achieve a cementite-free bainitic structure and susceptibility to the formation of intermetallic phases at elevated temperatures. Based on Transmission Electron Microscopy (TEM) observations, the intermetallic strengthening occurring in the bainitic ferrite matrix was revealed. The B2-ordered phase are rich in Ni, Al, and Cu, which indicates the growth of NiAl/Cu co-precipitates. The intermetallic phase was characterized by a spherical morphology and nanometer size. The micro-segregation of Ni, Al and Cu was observed at the boundaries of cementite particles and the bainitic matrix. The redistribution of atoms during the growth of carbides forms a heterogeneous nickel aluminide nucleation site. Intermetallic strengthening is a promising strategy for controlling the thermal stability of bainitic structures. The results of this research constitute fundamental knowledge regarding the design of ultra-fine, cementite-free bainitic steels subjected to intermetallic strengthening.

Evaluation of Shear-Punched Surface Layer Damage in Three Types of High-Strength TRIP-Aided Steel

The damage properties in the shear-punched surface layer, such as the strain-hardening increment, strain-induced martensite fraction, and initiated micro-crack/void characteristics at the shear and break sections, were experimentally evaluated to relate to the stretch-flangeability in three types of low-carbon high-strength TRIP-aided steel with different matrix structures. In addition, the surface layer damage properties were related to the mean normal stress developed on shear-punching and microstructural properties. The shear-punched surface damage of these steels was experimentally confirmed to be produced under the mean normal stress of negative to 0 MPa. TRIP-aided bainitic ferrite (TBF) steel had the smallest surface layer damage, featuring a significantly suppressed micro-crack/void initiation. This was due to the fine bainitic ferrite lath matrix structure, a low strength ratio of the second phase to the matrix structure, and the high mechanical stability of the retained austenite. On the other hand, the surface layer damage of TRIP-aided annealed martensite (TAM) steel was suppressed next to TBF steel and was smaller than that of TRIP-aided polygonal ferrite (TPF) steel. The surface layer damage was also characterized by a large plastic strain, a large amount of strain-induced martensite transformation, and a relatively suppressed micro-crack/void formation, which resulted from an annealed martensite matrix and a large quantity of retained austenite. The excellent stretch-flangeability of TBF steel might be caused by the suppressed micro-crack/void formation and high crack propagation/void connection resistance. The next high stretch-flangeability of TAM steel was associated with a small-sized micro-crack/void initiation and high crack growth/void connection resistance.

Microstructure and property evolution during tempering of a medium carbon bainitic steel

Tempering a medium carbon carbide-free bainite steel led to two distinct microstructures and properties below and above the bainitic start (Bs) temperature. Austenite decomposition during tempering, depended on carbon content, tempering temperatures and time, determining retained austenite (RA) stability. Up to 400 °C, most RA remained intact, but a significant reduction occurred above the Bs temperature. Film austenite primarily turned into carbides within the bainitic–ferrite matrix and form lines of carbide particles, while blocky austenite decomposed into troostite at 500 °C and sorbite at 600 °C. During tempering, on the other hand, bainitic ferrite lost tetragonality losing its carbon and resulting in the growth of bainitic–ferrite plates through grain boundary migration and misorientation annihilation. Above Bs, drastic microstructural changes caused significant alterations in tensile properties.

Effect of rapid tempering at high temperature on microstructure, mechanical properties and stability of retained austenite of medium carbon ultrafine bainitic steel

Rapid tempering at high temperature is beneficial to release residual stress. However, the evolution of microstructure and mechanical properties of ultrafine bainitic steel after rapid tempering at high temperature is still unclear. In this research, the effect of rapid tempering at high temperature on microstructure, mechanical properties and mechanical stability of retained austenite (RA) of a medium-carbon ultrafine bainitic steel were studied in detail. Results show that after a rapid tempering at 550 °C, the volume fraction of RA decreased gradually, and the carbon atoms in bainitic ferrite (BF) gradually diffused out of the lattice and finally formed carbides, and the BF plate gradually coarsened. When the tempering time is only 2 min, the carbon clusters formed, strengthening the matrix and stabilizing the RA, which result in the highest yield strength, and the highest impact toughness of the steel. With the extension of tempering time, nano-carbides precipitated and gradually grew up, and the dislocation density in steel decreased, resulting in a gradual decrease in the yield strength of the steel and the mechanical stability of RA. However, the deformation ability and strain hardening ability of the BF matrix gradually increase, so that the tensile strength of the steel gradually increases. The decomposition of RA and the formation of brittle martensite making the impact toughness of the steel gradually decrease. This study can provide theoretical support for the development of rapid tempering process of ultrafine bainitic steel.

Effects of Normalizing Temperature on the Precipitation of Fine Particles and Austenite Grain Growth during Carburization of Al- and Nb-Microalloyed Case-Hardening Steel

This paper deals with the precipitation behaviors of AlN and Nb(C, N) particles during normalizing of hot-forged Al- and Nb-microalloyed case-hardening steel, and investigates the effects of the number density and volume fraction of these particles on the grain growth behavior of austenite (γ) during the subsequent high-temperature carburization. When the sample was cooled from the hot forging temperature at 16°C/min, AlN did not precipitate at all and was completely supersaturated in the matrix, although fine Nb(C, N) particles precipitated during cooling. When reheated to the normalizing temperature, AlN particles started precipitation at approximately 620–640°C on the grain boundaries of the bainitic ferrite matrix and the interfaces between the cementite particles and the matrix, and the AlN particles contained Cr and Mn, and had an NaCl structure and Bain orientation relationship with the matrix. The AlN structure changed from NaCl type into wurtzite type at approximately 800°C. AlN particles grew during reheating to the normalizing temperature and holding at the temperature. Both the number density and volume fraction of the AlN particles were dramatically increased by reducing the normalizing temperature from 1070 to 900°C. The distribution of these particles strongly affected the γ grain structure formed during carburization. As the normalizing temperature was reduced from 1070 to 950 and 900°C, the γ grain structure became finer. Some abnormally large grains were formed near the carburized surface, when normalized at 1070°C. However, no abnormal grain was found when normalized below 950°C.

Deformation behaviour of novel medium carbon bainitic steels with different retained austenite characteristics designed by the sparse mixed regression model

A new concept for the alloy design of advanced structural steels was figured out with the experimental elaboration of deformation behaviour in two novel medium carbon bainitic steels proposed sparse mixed regression model. Fe-0.53C-1.55Si-0.48Mn-0.59Cr-0.05Ni (wt%, Steel-A) and Fe-0.64C-1.74Si-2.11Mn-0.2Cr-0.15Ni (Steel-B) were studied. These steels were austempered to get bainite structure with retained austenite of different fraction and morphology. The chemical composition and heat treatment conditions were designed by sparse mixed regression modelling. In the Steel-A, the retained austenite fraction is 0.07 and it is in film form. In the Steel-B, the retained austenite fraction is 0.37 and most of them are in blocky form. Both the samples showed high tensile strength (>1.6 GPa) and good elongation (>12%). The Steel-A showed poor uniform elongation, but large post-uniform elongation. In the Steel-A, the retained austenite is stable during deformation, so inherent ductility of bainitic ferrite matrix largely contributed to higher post-uniform elongation. On the other hand, in Steel-B, the occurrence of deformation-induced martensitic transformation leads to large uniform elongation, through increasing the work hardening rate and delaying the onset of plastic instability.

Synergistic effects of hydrogen and deformation temperature on mechanical properties of TRIP-aided bainitic ferrite steel

Mechanical properties of TRIP-aided steel with bainitic ferrite matrix in the presence of hydrogen were investigated at the deformation temperature range of −100 to 100 °C. In hydrogen-uncharged condition, the yield strength (YS) first increased and then remained almost unchanged, while the highest uniform elongation (UEl) was observed at a deformation temperature of 21 °C. Hydrogen charging slightly increased the YS and deteriorated the UEl at all the temperature ranges. In the hydrogen-uncharged and hydrogen-charged specimens, the transformation ratio of retained austenite increased with a decrease in the deformation temperature; however, the hydrogen charging decreased the transformation ratio. The fracture behavior drastically changed when hydrogen was introduced, owing to which deformation-induced martensitic transformation occurred and hydrogen played dual roles in accelerating brittle-like cracking.

Temperature-Stimulated Morphological Features of Advanced High-Strength Medium-Mn TRIP Steel.

Advanced High-Strength Steels (AHSSs) are one of the most rapidly developing group of Fe-based metallic materials. Their excellent combination of high strength, ductility and formability is due to their complex microstructure and strain-induced martensitic transformation of metastable retained austenite (RA), which favors extra ductility of the sheet steels. A deformation temperature is one of the most important factors affecting the phase transformation behavior in these Fe–C–Mn–Al–Si systems. Therefore, the present study aimed at understanding the temperature-dependent phase transformations and structural phenomena in an advanced medium-Mn–Al-alloyed steel. The 3Mn steel was thermomechanically processed and subjected to tensile testing in a temperature range from 20°C to 200°C. The different extent of the strain-induced martensitic transformation and some softening phenomena of bainitic ferrite matrix were revealed using transmission electron microscopy and electron backscatter diffraction techniques. It was found that the thermal stability of RA is strongly dependent on the deformation temperature. Moreover, the dynamic recovery and carbide precipitation play a key role when the deformation temperature is increased to 140°C and higher temperatures.

Morphological and crystallographic features of granular and lath-like bainite in a low carbon microalloyed steel

In this work, different heat treatments were performed in order to study two different bainite morphologies, granular and lath-like, obtained at different cooling rates in a low carbon microalloyed steel. For that purpose, advanced crystallographic characterization and quantitative metallography using SEM, EBSD and TEM were carried out. The evolution of the volume percentage, size, aspect ratio, retained austenite volume percentage and crystallography of the M/A constituents was studied as a function of the cooling rate and classified according to a machine learning algorithm. For the characterization of the crystallography and morphology of the bainitic ferrite matrix, two microstructures were selected as representative of granular and lath-like bainite. The results show that the main differences between granular and lath-like bainite are the size and shape of the M/A constituents, that change progressively from a mixture of large grains, coarse blocks and fine roughly equiaxed grains at low cooling rates to short and long films at high cooling rates. The crystallographic and morphological differences between both microstructures can be explained based on the different driving force and amount of bainitic ferrite formed at the different temperature ranges. These results confirm that the mechanisms by which granular bainite forms are not different from the transformation mechanisms of lath-like bainite.

V-Bendability of Ultrahigh-Strength Low Alloy TRIP-Aided Steel Sheets with Bainitic Ferrite Matrix

V-Bendability of Ultrahigh-Strength Low Alloy TRIP-Aided Steel Sheets with Bainitic Ferrite Matrix

Effect of Carbon Content and Intercritical Annealing on Microstructure and Mechanical Tensile Properties in FeCMnSiCr TRIP-Assisted Steels

Four TRIP (Transformation Induced Plasticity) assisted steels, three TBF (TRIP Bainitic Ferrite) steels and one TPF (TRIP Polygonal Ferrite) steel, were manufactured from three different carbon contents (0.2, 0.3 and 0.4 wt.% C), to study the evolution of their microstructure and tensile mechanical properties in 15 mm thick plates. TBF steels were subjected to the same austenitization heat treatment and subsequent bainitization isothermal treatment. The TPF steel was subjected to an intercritical annealing and subsequent isothermal bainitization treatment. All were microstructurally characterized by optical, scanning electron and atomic force microscopy, as well as X-ray diffraction. Mechanically, they were characterized by the ASTM E8 tensile test and fractographies. For the TBF steels, the results showed that when the carbon content increased, there were an increase in volume fraction of retained austenite, of the microconstituent “martensite/retained austenite” and in the tensile strength; and a decrease in the volume fraction of bainitic ferrite matrix and elongation; with an improvement in TRIP behavior due to the increase in retained austenite. The TPF steel presented around 50% ductile polygonal ferrite developing better TRIP behavior than the TBF steels. The evolution of the fractographies was ductile to brittle for TBF steels with an increase in carbon content, and for TPF, the appearance of the fracture surface was ductile.

Thermal Stability of Retained Austenite in Advanced TRIP Steel with Bainitic Ferrite Matrix for Automotive Industries

Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Microstructure characterization and tensile properties of hot isostatic pressed China ultrahigh strength steel

Powder metallurgy hot isostatic pressing (HIP) is a promising technology for producing complex ultrahigh strength steel (UHSS) components. In the present study, a kind of China UHSS, 30CrMnSiNi2A steel, was prepared by HIP with different processing temperatures. The microstructure and tensile properties of HIPed compacts were analyzed in depth via scanning electron microscopy, X–ray diffraction, electron back–scattered diffraction, transmission electron microscopy, tensile testing and nanoindentation experiment. Results show that all HIPed compacts are granular bainitic structure composed of bainitic ferrite matrix and M–A islands. With the HIP temperature increasing, the volume fraction of M–A islands decreases, and the average grain size increases. Besides, a harmful structure (that is prior particle boundaries, PPBs), which is composed of a great number of Mn–rich and Si–rich oxide inclusions, is formed in the HIPed compacts. Notably, increasing HIP temperature is beneficial to eliminate PPBs. The volume fraction of M–A islands, grain size and the state of PPBs together determine the tensile properties of HIPed compacts, in which the state of PPBs has a most significant impact. With the HIP temperature rising, the tensile fracture mode changes from inter–particle debonding to transgranular quasi–cleavage, which benefits from the elimination of PPBs.

The correlation between martensite-austenite islands evolution and fatigue behavior of SA508-IV steel

The correlation between microstructural evolution and low cycle fatigue behavior of SA508-IV steel were investigated. The fatigue tests of specimens with different tempering times were performed under the strain amplitudes of ±0.45% and ±0.6% at 300 °C, respectively. The experimental results showed that the more and more islands which were martensite/austenite islands (M/A islands) in granular bainite decomposed into carbides and bainitic ferrite matrix with the increase of the tempering time. The fatigue life decreased with increasing the strain amplitude under the same tempering time. However, the fatigue life increased with the increase of the tempering time under the same strain amplitude. After 30 min tempering, the crack initiation preferentially occurred near the M/A islands. However, the crack initiation occurred at the trigeminal grain boundary with the tempering time increasing to 15 h and 45 h. Moreover, the decomposition products of M/A islands – carbides were beneficial to increase the fatigue life of SA508-IV steel due to its resistance against crack propagation.

Effects of Cr and Mo on Mechanical Properties of Hot-Forged Medium Carbon TRIP-Aided Bainitic Ferrite Steels

In this study, the effects of Cr and Mo additions on mechanical properties of hot-forged medium carbon TRIP-aided bainitic ferrite (TBF) steel were investigated. If 0.5%Cr was added to the base steel with a chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0.5%Al, and 0.05%Nb in mass%, the developed steel achieved the best combination of strength and total elongation. The best combination of strength and impact toughness was attained by multiple additions of 0.5%Cr and 0.2%Mo to the base steel. The excellent combination of strength and impact toughness substantially exceeded those of quenched and tempered JIS-SCM420 and 440 steels, although it was as high as those of 0.2%C TBF steels with 1.0%Cr and 0.2%Mo. The good impact toughness was mainly caused by uniform fine bainitic ferrite matrix structure and a large amount of metastable retained austenite.

Phase Transformation and Precipitation Mechanism of Nb Microalloyed Bainite–Martensite Offshore Platform Steel at Different Cooling Rates

The phase transformation and precipitation mechanism of Nb microalloyed offshore platform steel during continuous cooling are studied by the thermal expansion method and transmission electron microscopy. The results show that a large amount of NbC is precipitated in the investigated steel during phase transformation and is classified into three types according to its size: Type I (>100 nm) is randomly generated in the austenitization stage; type II (20–100 nm) is precipitated by stress induction during thermal deformation; and type III (≈10 nm) is formed by the combination of segregated C and Nb during the cooling process after the formation of bainitic ferrite matrix. In addition, herein, a model of phase transition and precipitation behavior at different cooling rates is designed. At a low cooling rate (0.5 °C s−1), the microstructure consists of a granular structure formed by diffusion and granular bainite formed by shearing, which is a diffusion phase transformation. At a medium cooling rate (3 °C s−1), the orientation relationship of ferrite and austenite is [001]α//[011]γ and (110)α//(−1−11)γ, which is semidiffusion and a half‐shear mechanism. At a high cooling rate (15 °C s−1), there are a large number of intertwined dislocations in martensite, which is a shear mechanism.

Structure-properties relationship in TRIP type bainitic ferrite steel austempered at different temperatures

BackgroundAttractive properties of TRIP-type bainitic ferrite (TBF) steel ascribe to its unique microstructure of lath structure bainitic ferrite matrix and interlath retained austenite films. This work is concerned with obtaining ultra high-strength hot forged TBF steel with high elongation and excellent strength-elongation balance.MethodsThe effect of austempering temperature on the microstructure along with its retained austenite characteristics and tensile properties of a hot forged TBF steel was studied. A detailed investigation correlating the steel structure and its tensile properties was carried out.ResultsTensile strength ranging from 1058 to 1552 MPa was achieved when the hot forged steel was austempered at (325 - 475 °C).ConclusionsUltra high tensile strength of 1058 MPa, large total elongation of 29% and excellent strength-elongation balance of 30 GPa % were attained when the steel was austempered at 425 °C. The large total elongation of this steel is mainly due to the uniform fine lath structure matrix and the pronounced TRIP effect of a large amount of retained austenite films which prevents a rapid decrease of strain hardening rate at low strain and leads to a relatively high strain hardening at high strain level. Rapid transformation of blocky retained austenite at low strain in the hot forged TBF steel austempered at higher temperatures results in a rapid increase of initial strain hardening. In addition, the coarse microstructure that contains large blocks of retained austenite / martensite and the insufficient numbers of bainitic ferrite lathes and retained austenite films deteriorate the total elongation and the strength-elongation balance of the TBF austempered at 475 °C.

Carbon concentration measurements by atom probe tomography in the ferritic phase of high-silicon steels

Recent studies using atom probe tomography (APT) show that bainitic ferrite formed at low temperature contains more carbon than what is consistent with the paraequilibrium phase diagram. However, nanocrystalline bainitic ferrite exhibits a non-homogeneous distribution of carbon atoms in arrangements with specific compositions, i.e. Cottrell atmospheres, carbon clusters, and carbides, in most cases with a size of a few nanometers. The ferrite volume within a single platelet that is free of these carbon-enriched regions is extremely small. Proximity histograms can be compromised on the ferrite side, and a great deal of care should be taken to estimate the carbon content in regions of bainitic ferrite free from carbon agglomeration. For this purpose, APT measurements were first validated for the ferritic phase in a pearlitic sample and further performed for the bainitic ferrite matrix in high-silicon steels isothermally transformed between 200 °C and 350 °C. Additionally, results were compared with the carbon concentration values derived from X-ray diffraction (XRD) analyses considering a tetragonal lattice and previous APT studies. The present results reveal a strong disagreement between the carbon content values in the bainitic ferrite matrix as obtained by APT and those derived from XRD measurements. Those differences have been attributed to the development of carbon-clustered regions with an increased tetragonality in a carbon-depleted matrix.