Effect of Composition and Processing Parameters on the Formation of Nano-Bainite in Advanced High Strength Steels

The nano-bainitic microstructures were compared in a 0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al (wt%) steel after isothermal heat-treatment and a Fe-0.2C-1.5Mn-1.2Si-0.3Mo-0.6Al-0.02Nb (wt%) steel after controlled thermomechanical processing.The microstructure for both steels consisted of bainite.The microstructural characteristics of bainite,such as the morphology of the nano-bainite and thicknesses of bainitic ferrite and retained austenite layers,as a function of steel composition and processing was studied using transmission electron microscopy (TEM).It was found that the nano-bainitic structure can be formed in the low alloy steel through thermomechanical processing.Atom probe tomography (APT) was employed as a powerful technique to determine local composition distributions in three dimensions with atomic resolution.The important conclusions from the APT research were that the carbon content of bainitic ferrite is higher than expected from paraequilibrium level of carbon in ferrite for both steels and that Fe-C clusters and fine particles are formed in the bainitic ferrite in both steels despite the high level of Si.     

Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels

To develop a transformation-induced plasticity (TRIP)-aided bainitic ferrite steel (TBF steel) with high hardenability for a common rail of the next generation diesel engine, 0.2?pct C-1.5?pct Si-1.5?pct Mn-0.05?pct Nb TBF steels with different contents of Cr, Mo, and Ni were produced. The notch-fatigue strength of the TBF steels was investigated and was related to the microstructural and retained austenite characteristics. If Cr, Mo, and/or Ni were added to the base steel, then the steels achieved extremely higher notch-fatigue limits and lower notch sensitivity than base TBF steel and the conventional structural steels. This was mainly associated with (1) carbide-free and fine bainitic ferrite lath structure matrix without proeutectoid ferrite, (2) a large amount of fine metastable retained austenite, and (3) blocky martensite phase including retained austenite, which may suppress a fatigue crack initiation and propagation.     

Constitutive Modeling of the Mechanical Properties of V-added Medium Manganese TRIP Steel

In this study, medium Mn transformation-induced plasticity steel with the composition Fe-0.08 pct C-6.15 pct Mn-1.5 pct Si-2.0 pct Al-0.08 pct V was investigated. After intercritical annealing at 1013 K (740 °C), the steel contained coarse-grained ferrite and two ultrafine-grained (UFG) phases: ferrite and retained austenite. The material did not deform by localized Lüders band propagation: it did not suffer from this major problem as most UFG steels do. Localization of plastic flow was shown to be suppressed because of a combination of factors, including a bimodal grain size distribution, a multiphase microstructure, the presence of nanosized vanadium carbide precipitates, and the occurrence of the deformation-induced martensitic transformation of retained austenite. A constitutive model incorporating these effects was developed. The model was used to identify the factors which can lead to a further improvement of the mechanical properties of the UFG medium Mn TRIP steels.     

Anisotropy in Thermal Expansion of Bainitic Ferrite

The evolution of local d-spacings between lattice planes of bainitic ferrite in a high strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)), has been determined to calculate the thermal expansion behavior. For this purpose, in-situ continuous cooling tests have been carried out in a high-energy synchrotron X-ray diffractometer. The results indicate thermal anisotropy in the bainitic ferrite planes.     

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.     

In-Situ Synchrotron Diffraction Studies on Transformation Strain Development in a High Strength Quenched and Tempered Structural Steel—Part I. Bainitic Transformation

In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)) structural steel during continuous cooling under different mechanical loading conditions has been studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the formation of bainite from austenite and the effect of carbon on the lattice parameter of bainitic ferrite were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. The elastic strains increase with increase in the applied tensile stress.     

The Evolution of Ultrafine Grained Structure Formation Through Isothermal Static Phase Transformation

In the current study,a 0.3C-2Si-2Mn-0.28Mo (in wt%) steel with high hardenability was deformed at a relatively low temperature followed by isothermal static phase transformation.This novel thermomechanical processing made it possible to successfully produce an ultrafine ferrite grained structure (～2 μm) in the absence of both dynamic phase transformation and controlled cooling.The use of a model Ni-30Fe austenitic alloy showed that the low temperature deformation induced very fine intragranular defects throughout the microstructure,which would then act as fine spaced ferrite nucleation sites at an early stage of phase transformation.As a result,the coarsening of ferrite was extremely limited during isothermal phase transformation,resulting a very fine ferrite grained structure;even nanoscale in the region of the prior austenite grain boundary.     

Direct decomposition of austenite in two Fe-V-C alloys

Investigations of austenite decomposition have been undertaken in (1) Fe-0.5Mn-1V-0.2C and (2) Fe-0.5Mn-3Ni-1V-0.2C alloys. Isothermal transformation characteristics were determined using dilatometric and thermo-electric potential techniques. Also, micro-structural features were observed using optical and transmission electron microscopy for treatments of interest following isothermal austenite decomposition in the 550 to 750° C range. Associated mechanical properties were measured with emphasis being placed on Charpy impact behavior. Both alloys exhibited two temperature regions in which “C-curve” austenite decomposition occurred. In the upper region a combination of fibrous and fine particle VC precipitation was observed in both alloys. In the lower transformation region, bainitic microstructures resulted from the isothermal treatments. Additionally, the alloy containing 3 pct Ni exhibited VC precipitation in the austenite prior to ferrite formation. In both alloys, complete isothermal transformation produced microstructures with poor impact properties. However, a good combination of strength and toughness was produced in the 3 pct Ni alloy using the heat treatment that promoted VC precipitation in austenite but avoided total isothermal austenite decomposition. Formerly with University of California, Berkeley This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metal-lurgy Committee of The Metallurgical Society of AIME.     

Effects of alloying elements upon austenite decomposition in Low-C steels

The kinetics of austenite decomposition were studied in high-purity Fe-0.1C-0.4Mn-0.3Si-X (concentrations in weight percent;X represents 3Ni, 1Cr, or 0.5Mo) steels at temperatures between 500 °C and 675 °C. The transformation stasis phenomenon was found in the Fe-C-Mn-Si-Mo and Fe-C-Mn-Si-Ni alloys isothermally transformed at 650 °C and 675 °C but not in the Fe-C-Mn-Si and Fe-C-Mn-Si-Cr alloys at any of the temperatures investigated. The occurrence of transformation stasis was explained by synergistic interactions among alloying elements. The paraequilibrium model was applied to calculate the metastable fraction of ferrite in each alloy. This fraction was shown to coincide with cessation of transformation in the Mo alloy transformed at 600 °C. Transformation stasis was found in both the Ni and the Mo alloys isothermally reacted at 650 °C and 675 °C. The interactions among Mn, Si, and Mo, as well as interactions among Mn, Si, and Ni, appear to decrease the threshold concentrations for transformation stasis in Fe-C-Mn-Si systems. Segregation of Mn and Mo to the α/yγ boundary, assisted by the presence of Si, was suggested to enhance the solute draglike effect (SDLE) and lead to transformation stasis. In the Ni alloy, a lower driving force for ferrite formation resulting from the Ni addition could be responsible for the occurrence of transformation stasis.     

Phase transformational kinetics and hardenability of low-carbon,boron-treated steels

The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate (bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free) 0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action, molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation, thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are reflected in terms of higher hardenability.     

Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100 °C/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (>0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.     

Interrelations of cooling rate,microstructure, and mechanical properties in four HSLA steels

The present study was carried out on four steels containing 0.1 pct C-1.5 pct Mn-0.003 pct B* in common, with additions of 1 pct Cr, 0.5 pct Mo, 0.25 pct Mo + 1 pct Cr, 0.2 pct Ti + 1 pct Cr. They were designated, accordingly, as Cr, Mo, Mo-Cr, and Cr-Ti steels. All the steels exhibited a complete lath martensite microstructure with thin interlaths of retained austenite (≈0.05 pct) in the quenched condition. The normalized microstructures, granular bainite, contained massive areas of ferrite and granules of bainite laths. Both microconstituents contained a fine dispersion of cementite particles (size ≈50 Å) together with high dislocation densities. A mechanism explaining their for-mation has been given. The Cr steel, due to its low hardenability, showed in addition polygonal ferrite in the neighborhood of the so-called M-A constituent (twinned martensite and/or austenite). The annealed microstructure (using a cooling rate of 0.033 °C s^?1) of the Cr steel consisted of coarse ferrite-pearlite. Addition of 0.2 pct Ti to the Cr steel markedly refined the structure, whereas an addition of 0.25 pct Mo altered the microstructure to ferrite-lower bainite. In the 0.5 pct Mo steel, polygonal ferrite was found to be completely missing. The mechanical properties of the four steels after quenching, normalizing, and annealing were investigatedvia hardness and tensile test mea-surements. An empirical equation, relating the ultimate tensile strength to the steel composition, for steels that had granular bainite microstructures in the normalized condition, was proposed. The fracture surfaces exhibited cleavage and variable-size dimples depending on the microstructure and steel composition.     

The boron hardenability effect in thermomechanically processed,direct-quenched 0.2 Pct C steels

The segregation and precipitation of boron have been studied in thermomechanically processed 0.2C-0.6Mn-0.5Mo steels containing nominally 0, 10, 20, 50, and 100 ppm B. These steels were hot-rolled in the laboratory (in simulation of production multipass rolling), and their transformation behavior during subsequent water quenching was examined for different finish-rolling temperatures (980 °C and 870 °C) and quenching temperatures (730 °C to 950 °C). The results showed that in general, a “free” boron content of 10 to 20 ppm (which is similar to the levels used for conventional quenched-and-tempered steels) will provide a boron hardenability increment similar to that for conventional quenched-and-tempered steels. The delay time prior to quenching (over the range of 10 to 100 seconds) did not have a significant effect on hardenability except in the steels containing 50 or more ppm B. In these higher B steels, precipitation of borocarbides occurred along austenite grain boundaries with a resultant decrease in hardenability.     

Nucleation sites for ultrafine ferrite produced by deformation of austenite during single-pass strip rolling

An austenitic Ni-30 wt pct Fe alloy, with a stacking-fault energy and deformation characteristics similar to those of austenitic low-carbon steel at elevated temperatures, has been used to examine the defect substructure within austenite deformed by single-pass strip rolling and to identify those features most likely to provide sites for intragranular nucleation of ultrafine ferrite in steels. Samples of this alloy and a 0.095 wt pct C-1.58Mn-0.22Si-0.27Mo steel have been hot rolled and cooled under similar conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction, and X-ray diffraction. Following a single rolling pass of ∼40 pct reduction of a 2mm strip at 800 °C, three microstructural zones were identified throughout its thickness. The surface zone (of 0.1 to 0.4 mm in depth) within the steel comprised a uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation cells, with an average diameter of 0.5 to 1.0 μm. The scale and distribution and, thus, nucleation density of the ferrite grains formed in the steel were consistent with the formation of individual ferrite nuclei on cell boundaries within the austenite. In the transition zone, 0.3 to 0.5 mm below the surface of the steel strip, discrete polygonal ferrite grains were observed to form in parallel, and closely spaced “rafts” traversing individual grains of austenite. Based on observations of the equivalent zone of the rolled Ni-30Fe alloy, the ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within the deformed austenite during rolling. Within the central zone of the steel strip, a bainitic microstructure, typical of that observed after conventional hot rolling of this steel, was observed following air cooling. In this region of the rolled Ni-30Fe alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions.     

Bainite morphologies in a 0.2 C–1.5 Mn steel

The isothermal transformation products of austenite over a wide range of temperatures and times in the bainitic range in a 0.2 wt.% C–1.5 wt.% Mn steel have been studied by transmission electron microscopy in order to characterise the bainitic microstructures in low-carbon low-alloy steels. Widmanstätten ferrite has formed with alternate layers of austenite (martensite) as a transition product at 600 and 500°C that has finally transformed on further isothermal transformation to either pearlite (at 600°C) or upper bainite (at 500°C). This type of transformation product was referred to as BI bainite by earlier investigators, but on the basis of the present investigation it is concluded that such ferrite-austenite (martensite) structures are not bainitic as this is not the final transformation product either at 600 or 500°C. Both upper bainite and lath-type lower bainite are formed at 450°C while the transformation product has been only lath-type lower bainite at 400°C.     

Effect of magnesium addition in low-carbon steel part 1: behaviour of austenite grain growth

To reveal the effect of Mg addition on the austenite grain growth in low-carbon steel, the steels containing different Mg contents were refined with a vacuum induction furnace. First, the steels were subjected to the temperature range of 1000–1300°C for a holding time of 30?min. Moreover, using a confocal scanning laser microscope, the growth of austenite grains was investigated under isothermal holding conditions (1400°C), and the γ–α phase transformation was also identified after the samples were subjected to a cooling rate of 5°C?s^?1. It reveals that the grain growth is inhibited by Mg addition after increasing the temperature to 1300°C. The kinetic equations of austenite grain growth were further established by regression analysis based on the experimental results. Furthermore, a significant increase in the proportion of intra-granular ferrite takes place in 0.0026%Mg-added steel at the initial stage of γ?→?α with a cooling rate of 5°C?s^?1. This is mainly attributed to the plenty of Mg-containing inclusions, which are demonstrated to be effective nuclei for acicular ferrite, being in the Mg-added steel.     

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part I. transformation,microstructure, and room-temperature mechanical properties

Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300 °C and 500 °C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450 °C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.     

The effect of nitrogen on precipitation and transformation kinetics in vanadium steels

The isothermal decomposition of austenite has been studied in a series of vanadium steels containing varying amounts of carbon and nitrogen, (in approximately stoichio-metric proportions), in the temperature range 700 to 850°C. In the basic alloy, Fe-0.27V–0.05C (composition in wt pct), below 810°C the austenite to polygonal ferrite trans-formation is accompanied by interphase precipitation of vanadium carbide, the finer dis-persions being associated with the lower transformation temperatures. However, below 760°C there is an additional precipitation reaction where dislocation precipitation of vanadium carbide predominates; this is shown to occur in association with Widmanstätten ferrite. Above 810° C, a proeutectoid ferrite reaction results, the ferrite being void of precipitates; evidence is provided to show that partitioning of vanadium from ferrite to austenite occurs during the transformation. In the two steels containing nitrogen, namely Fe-0.26V-0.022N-0.020C and Fe-0.29V-0.032 N the basic interphase precipitation re-action is unchanged, but the resultant precipitate dispersions are finer at a given trans-formation temperature. The temperature range over which interphase precipitation oc-curs is expanded by the presence of nitrogen, since the Widmanstätten start tempera-ture is depressed and the proeutectoid ferrite reaction is inhibited. Precipitation in austenite prior to transformation and twin formation during transformation are both en-couraged by the presence of nitrogen.     

Effects of Start and Finish Cooling Temperatures on Microstructure and Mechanical Properties of Low-Carbon High-Strength and Low-Yield Ratio Bainitic Steels

The effects of start and finish cooling temperatures on microstructure and mechanical properties of low-carbon high-strength and low-yield ratio bainitic steels were investigated in this study. Four kinds of low-carbon high-strength and low-yield ratio bainitic steels were fabricated by varying the start and finish cooling temperatures and cooling rates, and their microstructure and mechanical properties such as tensile and Charpy impact properties were measured. In the steels cooled down from the high start cooling temperature above Ar₁ [978 K (705 °C)], the volume fraction of acicular ferrite is lower than in the steels cooled down from the low start cooling temperature below Ar₁ [978 K (705 °C)]. The finish cooling temperatures and cooling rates affect the formation of bainitic ferrite, granular bainite, and martensite–austenite (MA) constituents. According to the correlation between microstructure and mechanical properties, the tensile strength increases with increasing the volume fractions of bainitic ferrite and MA constituents, whereas the elongation decreases. The yield ratio decreases as the volume fraction of MA constituents increases. Charpy impact absorbed energy is proportional to the volume fraction of acicular ferrite, and is inversely proportional to the volume fraction of granular bainite.     

Analyses of Transformation Kinetics of Carbide-Free Bainite Above and Below the Athermal Martensite-Start Temperature

The isothermal transformation kinetics of austenite decomposition in Fe-0.4C-2.78Mn-1.81Si was analyzed by an electrical resistivity technique in the temperature interval 723 K to 418 K (450 °C to 145 °C). The analysis of transformation kinetics of the bainite transformation was performed using the Johnson–Mehl–Avrami–Kolgomorov (JMAK) and Austin–Rickett (AR) approaches. The kinetic parameters, the reaction constant n, rate constant k = k(T), and apparent activation energy Q were evaluated for isothermal transformations below and above the martensite-start temperature M _S  = 548 K (275 °C), which was determined experimentally. The formation of strain-induced martensite, which starts to accompany the bainite transformation at just above M _S , increases the rate of transformation and decreases the apparent activation energy of austenite decomposition.