Microstructure Evolution and Mechanical Behavior of a CMnSiAl TRIP Steel Subjected to Partial Austenitization Along with Quenching and Partitioning Treatment

The present study investigated the microstructure evolution and mechanical behavior in a low carbon CMnSiAl transformation-induced plasticity (TRIP) steel, which was subjected to a partial austenitization at 1183 K (910 °C) followed by one-step quenching and partitioning (Q&P) treatment at different isothermal holding temperatures of [533 K to 593 K (260 °C to 320 °C)]. This thermal treatment led to the formation of a multi-phase microstructure consisting of ferrite, tempered martensite, bainitic ferrite, fresh martensite, and retained austenite, offering a superior work-hardening behavior compared with the dual-phase microstructure (i.e., ferrite and martensite) formed after partial austenitization followed by water quenching. The carbon enrichment in retained austenite was related to not only the carbon partitioning during the isothermal holding process, but also the carbon enrichment during the partial austenitization and rapid cooling processes, which has broadened our knowledge of carbon partitioning mechanism in conventional Q&P process.     

Effects of Annealing Treatment Prior to Cold Rolling on the Edge Cracking Phenomenon of Ferritic Lightweight Steel

Effects of annealing treatment from 923 K to 1023 K (650 °C to 750 °C) prior to cold rolling on the edge cracking phenomenon of a ferritic lightweight steel were investigated. The edge cracking was severely found in the hot-rolled and 923 K (650 °C)-annealed steels after cold rolling, whereas it hardly occurred in the 1023 K (750 °C)-annealed steel. As the annealing temperature increased, lamellar κ-carbides were dissolved and coarsened, and most of the κ-carbides continuously formed along boundaries between ferrite and κ-carbide bands disappeared. Microstructural observation of the deformed region of tensile specimens revealed that the removal of band boundary κ-carbides reduced the difference in tensile elongation along the longitudinal direction (LD) and transverse direction (TD), which consequently led to the reduction in edge cracking. The 1023 K (750 °C)-annealed steel showed fine ferrite grain size, weak texture, and decomposed band structure after subsequent cold rolling and intercritical annealing, because κ-carbides actively worked as nucleation sites of ferrite and austenite. The present annealing treatment prior to cold rolling, which was originally adopted to prevent edge cracking, also beneficially modified the final microstructure of lightweight steel.     

Volume Fractions of Proeutectoid Ferrite/Pearlite and Their Dependence on Prior Austenite Grain Size in Hypoeutectoid Fe-Mn-C Alloys

It has been generally believed that pearlite transformation in hypoeutectoid steels starts when the average carbon concentration in untransformed austenite reaches the A_cm line after the formation of proeutectoid ferrite. To test this concept experimentally, volume fractions of proeutectoid ferrite/pearlite and carbon contents in the austenite being transformed into pearlite were measured for the Fe-2Mn-0.3C alloy isothermally transformed in the temperature range 848 K to 898 K (575 °C to 625 °C). It was found that lamellar pearlite can form even when the average carbon content in untransformed austenite is much lower than the A_cm line. This peculiar observation is probably due to the two-dimensional diffusion of carbon, i.e., parallel to and normal to the austenite/pearlite interface, which enables lamellar cementite to grow continuously by supplying carbon atoms to its growth front. This results in proeutectoid ferrite fractions with respect to pearlite being much lower than those predicted by the lever rule. With decreasing prior austenite grain size, proeutectoid ferrite fractions with respect to pearlite were found to increase, but the thickness of proeutectoid ferrite was constant within the range of grain size investigated. This is due to the existence of the critical α/γ interface velocity only below which pearlite (actually cementite) can be nucleated at the migrating α/γ interface. Furthermore, the upper limit temperatures for pearlite formation in the Fe-1Mn-0.33C and Fe-2Mn-0.3C alloys were found to be well between the PLE/NPLE and PE A_e1 temperatures.     

Influence of Temperature and Grain Size on Austenite Stability in Medium Manganese Steels

With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation.     

Effect of Annealing Temperature on Microstructure and Mechanical Properties of Bulk 316L Stainless Steel with Nano- and Micro-crystalline Dual Phases

Microstructures and mechanical properties of 316L stainless steels with dual phases austenite prepared by an aluminothermic reaction casting were explored. It is found that the steels consist of nano- and micro-crystalline austenite phases, a little δ ferrite and contaminations. Before and after annealing at 1073 K and 1273 K (800 °C and 1000 °C), average grain sizes of the nanocrystalline austenite phase are about 32, 31, 38 nm, respectively. Tensile strength increases first from 371 to 640 MPa and then decreases to 454 MPa. However, elongation ratio increases gradually from 16 to 23 and then 31 pct after annealing. The results illustrate that the steel after annealing at 1073 K (800 °C) has better properties, also indicating that combination of dual nano- and micro-crystalline austenite phase is conductive to improving tensile properties of materials.     

Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS

The isothermal transformation vs time of a medium-carbon microalloyed steel at 450°C, following austenitization at 1250°C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.     

Upper acicular ferrite formation in a medium-carbon microalloyed steel by isothermal transformation: Nucleation enhancement by CuS

The isothermal transformation vs time of a medium-carbon microalloyed steel at 450 °C, following austenitization at 1250 °C for 45 minutes, has been investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). At short times, the fine microstructure of acicular ferrite is nucleated at MnS inclusions, which are covered by a shell of a hexagonal CuS phase. The special orientation between MnS and the CuS crystals of this shell enables the formation of a low-energy interface between the ferrite and the inclusion with, at the same time, the ferrite satisfying one of the 24 variants of the orientation relationship into the Bain region with austenite. As the treatment times are increased, the increase in the volume fraction of acicular ferrite being formed raises the carbon concentration of the austenite, such that some retained austenite instead of martensite is observed for these intermediate treatment times. This retained austenite transforms to ferrite plus carbides at long treatment times, resulting in a final microstructure of acicular ferrite, very similar in nature to those encountered in the case of upper bainite formation.     

Carbide refining heat treatments for 52100 bearing steel

The life of through-hardened 52100 anti-friction bearing components is improved if the excess carbides, undissolved during austenitization, are small and uniformly dispersed. One kind of carbide-refining heat treatment consists of 1) dissolving all carbides, 2) isothermally transforming the austenite to pearlite or bainite, and 3) austenitizing, quenching and tempering in the usual manner. Each step in this sequence of treatments was investigated, and the behavior of pearlitic and bainitic microstructures during subsequent austenitization was contrasted with the behavior of ferrite/spheroidized-carbide microstructures. It was shown that: 1) The usual hardening treatments given spheroidize-annealed bearing components result in an inhomogeneous microstructure, possibly due to the faster dissolution of carbides near austenite grain boundaries. 2) Austenitization of pearlite or bainite produces very uniform dispersions of ultra-fine carbides on the order of 0.1 μm diameter or less. 3) Specimens with ultra-fine carbides tend to have more retained austenite. 4) The rate of coarsening of ultra-fine carbides at austenitizing temperatures of 840°C and below, is slow enough so that conventional furnace heat treatments are satisfactory for producing this microstructure.     

Development of Ultrafine-Grained Dual-Phase Steels: Mechanism of Grain Refinement During Intercritical Deformation

Heavy deformation of metastable austenite (below A_e3) or both austenite and ferrite in the two-phase region (between A_r3 and A_r1) is known to develop an ultrafine ferrite grain structure with an average grain size of less than 3 μm. Different dynamic softening mechanisms, such as dynamic recovery, dynamic recrystallization, and dynamic strain-induced austenite→ferrite transformation (DSIT), are responsible for such grain refinement. However, the sequence of those metallurgical events and the temperature range over which any particular mechanism dominates is not yet well understood. The current study throws some light on this aspect by applying heavy, single-pass compressive deformation (with true strain of 1.0) on the microalloyed steel samples over a temperature range of 1173 K to 873 K (900 °C to 600 °C) using a Gleeble simulator (Dynamic Systems Inc., Poestenkill, NY) and water quenching the samples immediately after deformation. The current study showed the dominating effect of the following mechanisms with respect to the deformation temperature: (1) DSIT followed by conventional dynamic recrystallization (Conv-DRX) of ferrite at higher deformation temperatures (≥1073 K [800 °C]), (2) extended recovery and continuous dynamic recrystallization (Cont-DRX) of ferrite at intermediate deformation temperatures (~1023 K [750 °C]), and (3) simple dynamic recovery of ferrite at lower deformation temperatures (≤923 K [650 °C]).     

Formation of Austenite During Intercritical Annealing of Dual-Phase Steels

The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of 1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of most dual-phase steels. Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures (~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion in austenite. Diffusion models for the various steps were analyzed and compared with experimental results.     

Effects of Initial Structure and Reversion Temperature on Austenite Nucleation Site in Pearlite and Ferrite–Pearlite

Austenite nucleation sites were investigated in near-eutectoid 0.8 mass pct C steel and hypoeutectoid 0.4 mass pct C steel samples with full pearlite and ferrite–pearlite initial structures, respectively. In particular, the prior austenite grain size had been coarsened to compare grain boundaries in the hierarchical pearlite structure, i.e., the low-angle pearlite colony and high-angle block boundaries with ferrite/pearlite interfaces in the austenite nucleation ability. When the full pearlite in 0.8 mass pct C steel underwent reversion at a relatively low temperature, austenite grains preferentially formed at pearlite block boundaries. Consequently, when the full pearlite with the coarse block structure underwent reversion just above the eutectoid temperature, the reversion took a long time due to the low nucleation density. However, austenite grains densely formed at the pearlite colony boundaries as well, as the reversion temperature became sufficiently high. On the other hand, when ferrite–pearlite in the 0.4 mass pct C steel underwent reversion to austenite, the ferrite/pearlite interface acted as a more preferential austenite nucleation site than the pearlite block boundary even in the case of low-temperature reversion. From these results, it can be concluded that the preferential austenite nucleation site in carbon steels is in the following order: ferrite/pearlite interface?>?pearlite block?>?colony boundaries. In addition, orientation analysis results revealed that ferrite restricts the austenite nucleation more strongly than pearlitic ferrite does, which contributes to the preferential nucleation at ferrite/pearlite interfaces. This suggests that austenite grains formed at a ferrite/pearlite interface tend to have an identical orientation even under high-temperature reversion. Therefore, it is thought that the activation of austenite nucleation within pearlite by increasing the reversion temperature may be effective for rapid austenitization and the grain refinement of austenite structure after the completion of reversion in carbon steels.     

Microstructural development and austempering kinetics of ductile iron during thermomechanical processing

A new method of refining the microstructure of austempered ductile iron (ADI) by thermome chanical processing is investigated. Refinement of microstructure is effected by grain refinement of parent austenite by hot deformation in the austenitizing temperature range, before the austempering treatment. The effects of austenite deformation on the kinetics of austempering reaction and the microstructure development were studied using metallography and X-ray diffraction (XRD), at different austempering temperatures and deformations. The process window for optimum microstructure was determined in terms of the parameters involved. Deformation of 40 to 60 pct could be imparted in the temperature range 900 °C to 1025 °C, resulting in a reduction in the prior austenite grain size by 35 to 50 pct and ferrite size in ausferrite by 70 to 75 pct. The effects of austenitization temperature on the austempered microstructure were also studied.     

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.     

Investigation of Austenite-to-Ferrite Transformation in Ultralow and Low-Carbon Steel Using High-Speed Quenching Dilatometry and Thermokinetic Simulation

The isothermal austenite decomposition kinetics is studied in 0.004 wt pct C ultralow carbon (ULC) and 0.11 wt pct C low-carbon (LC) steel using high-speed quenching dilatometry. Standard samples of these steels are heated to austenitization temperatures of 1223 K and 1373 K (950 °C and 1100 °C) and then quenched to testing temperatures between 1163 K and 933 K (890 °C and 660 °C). The measured and calculated austenite-to-ferrite phase fractions are compared with dilatation values to analyze the ferrite nucleation and growth conditions during austenite decomposition. Ferrite evolution profiles are assessed to investigate the underlying growth kinetics. The analysis in ULC steel shows regimes of partitionless, partitioning, and two-stage transformation kinetics. In contrast, LC steel shows only diffusion-controlled transformation kinetics. The experimental results are well reproduced with thermokinetic calculations, thus supporting our interpretation of governing mechanisms during transformation.     

Microstructural evolution during the austenite-to-ferrite transformation from deformed austenite

It is well established that the ferrite grain size of low-carbon steel can be refined by hot rolling of the austenite at temperatures below the nonrecrystallization temperature (T _nr ). The strain retained in the austenite increases the number of ferrite nuclei present in the initial stages of transformation. In this work, a C-Mn-Nb steel has been heavily deformed by torsion at temperatures below the determined T _nr for this steel. After deformation, specimens are cooled at a constant cooling rate of 1 °C/s, and interrupted quenching at different temperatures is used to observe different stages of transformation. The transformation kinetics and the evolution of the ferrite grain size have been analyzed. It has been shown that the stored energy due to the accumulated deformation is able to influence the nucleation for low undercoolings by acting on the driving force for transformation; this influence becomes negligible as the temperature decreases. At the early stages of transformation, it has been observed that the preferential nucleation sites of ferrite are the austenite grain boundaries. At the later stages, when impingement becomes important, ferrite coarsening accompanies the transformation and a significant reduction in the number of the ferrite grains per unit volume is observed. As a result, a wide range of ferrite grain sizes is present in the final microstructure, which can influence the mechanical properties of the steel.     

Microstructure Refinement and Mechanical Properties of 304 Stainless Steel by Repetitive Thermomechanical Processing

Repetitive thermomechanical processing (TMP) was applied for evaluating the effect of strain-induced α′-martensite transformation and reversion annealing on microstructure refinement and mechanical properties of 304 austenitic stainless steel. The first TMP scheme consisted of four cycles of tensile deformation to strain of 0.4, while the second TMP scheme applied two cycles of tensile straining to 0.6. For both schemes, tensile tests were conducted at 173 K (? 100 °C) followed by 5-minute annealing at 1073 K (800 °C). The volume fraction of α′-martensite in deformed samples increased with increasing cycles, reaching a maximum of 98 vol pct. Examination of annealed microstructure by electron backscattered diffraction indicated that increasing strain and/or number of cycles resulted in stronger reversion to austenite with finer grain size of 1 μm. Yet, increasing strain reduced the formation of Σ3 boundaries. The annealing textures generally show reversion of α′-martensite texture components to the austenite texture of brass and copper orientations. The increase in strain and/or number of cycles resulted in stronger intensity of copper orientation, accompanied by the formation of recrystallization texture components of Goss, cube, and rotated cube. The reduction in grain size with increasing cycles caused an increase in yield strength. It also resulted in an increase in strain hardening rate during deformation due to the increase in the formation of α′-martensite. The increase in strain hardening rate occurred in two consecutive stages, marked as stages II and III. The strain hardening in stage II is due to the formation of α′-martensite from either austenite or ε-martensite, while the stage-III strain hardening is attributed to the necessity to break the α′-martensite-banded structure for forming block-type martensite at high strains.     

Intercritical Austenitization of Two Fe-Mn-C Steels

With the introduction of dual phase steels, it is increasingly becoming important to obtain a thorough understanding of intercritical austenitization phenomena. Quantitative microscopy techniques were used to study the process of intercritical austenitization (740°C) of two Fe-Mn-C steels, one of them being microalloyed with Nb. The two steels showed essentially the same kinetics,viz., three stages of intercritical austenitization: (i) austenite growth into pearlite until complete pearlite dissolution, (ii) growth of austenite into ferrite, and (iii) equilibration of ferrite and austenite. However, compared to data published by other researchers, the maximum amount of austenite, in our case, was reached much faster. Ferrite-ferrite interface processes and preferred nucleation at particles in the ferrite boundaries accelerated the austenite growth. Austenite growth out of pearlite colonies was asymmetric due to the fast ferrite-ferrite interface processes.     

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.     

Austenitization during intercritical annealing of an Fe-C-Si-Mn dual-phase steel

Partial austenitization during the intercritical annealing of an Fe-2.2 pct Si-1.8 pct Mn-0.04 pct C steel has been investigated on four kinds of starting microstructures. It has been found that austenite formation during the annealing can be interpreted in terms of a carbon diffusion-limited growth process. The preferential growth of austenite along the ferrite grain boundaries was explained by the rapid carbon supply from the dissolving carbide particles to the growing fronts of austenite particles along the newly formed austenite grain boundaries on the prior ferrite grain boundaries. The preferential austenitization along the grain boundaries proceeded rapidly, but the austenite growth became slowed down after the ferrite grain boundaries were site-saturated with austenite particles. When the ferrite grain boundaries were site-saturated with austenite particles in a coarse-grained structure, the austenite particles grew by the mode of Widmanstätten side plate rather than by the normal growth mode of planar interface displacement.     

Evolution of Microstructure and Texture During Warm Rolling of a Duplex Steel

The effect of warm rolling on the evolution of microstructure and texture in a duplex stainless steel (DSS) was investigated. For this purpose, a DSS steel was warm rolled up to 90 pct reduction in thickness at 498 K, 698 K, and 898 K (225 °C, 425 °C, and 625 °C). The microstructure with an alternate arrangement of deformed ferrite and austenite bands was observed after warm rolling; however, the microstructure after 90 pct warm rolling at 498 K and 898 K (225 °C and 625 °C) was more lamellar and uniform as compared to the rather fragmented and inhomogeneous structure observed after 90 pct warm rolling at 698 K (425 °C). The texture of ferrite in warm-rolled DSS was characterized by the presence of the RD (〈011〉//RD) and ND (〈111〉//ND) fibers. However, the texture of ferrite in DSS warm rolled at 698 K (425 °C) was distinctly different having much higher fraction of the RD-fiber components than that of the ND-fiber components. The texture and microstructural differences in ferrite in DSS warm rolled at different temperatures could be explained by the interaction of carbon atoms with dislocations. In contrast, the austenite in DSS warm rolled at different temperatures consistently showed pure metal- or copper-type deformation texture which was attributed to the increase in stacking fault energy at the warm-rolling temperatures. It was concluded that the evolution of microstructure and texture of the two constituent phases in DSS was greatly affected by the temperature of warm rolling, but not significantly by the presence of the other phase.