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.     

Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Decomposition of fine-grained austenite (10-μm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanst?tten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanst?tten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanst?tten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanst?tten ferrite. Kinetic and microstruc-tural differences between Widmanst?tten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products. Formerly Graduate Student, Department of Metallurgical and Materials Engineering, Colorado School of Mines. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Copper precipitation during continuous cooling and isothermal aging of a710-type steels

Precipitation in copper-containing A710 (also referred to as HSLA-80) and modified-A710 steels was investigated by transmission electron microscopy. Isothermal aging of as-quenched specimens at 675 °C produced e-copper precipitates located primarily at α-iron matrix dislocations. The precipitates exhibited multiple variants of an orientation relationship (OR) consistent with that reported by Kurdjumov and Sachs, fine fault formation, and associated streaking in electron diffraction patterns. For reaustenitized and continuously cooled specimens, the primary precipitation event was associated with interphase precipitation of copper at ferrite/austenite interfaces. Interphase precipitates frequently displayed ORs other than that reported by Kurdjumov and Sachs, although a unique crystallographic variant was observed within any one region of interphase precipitation, faults were observed infrequently, and streaking was not observed in diffraction patterns. At high temperatures during cooling, precipitate-free ferrite formed, whereas at lower temperatures, nucleation of copper precipitates occurred at ferrite/austenite interfaces for crystals of polygonal ferrite and Widmanstätten ferrite. This latter feature precludes the formation of Widmanstätten ferritevia a displacive mechanism. Interphase precipitation was not observed for granular ferrite or acicular ferrite. Less-common precipitation events during continuous cooling included the formation of AIN and CuS.     

The effect of intercritical annealing temperature on the structure of niobium microalloyed dualphase steel

The structures produced in a Nb-microalloyed steel by oil quenching after intercritical anneals at 760 and 810 °C have been examined by light and transmission electron microscopy. After both anneals, the periphery of the austenite pool transforms on cooling to ferrite in the same orientation as the ferrite retained during intercritical annealing. Thus the ferrite forms by an epitaxial growth mechanism without the formation of a new interface or grain boundary. The new ferrite is precipitate-free in contrast to the retained ferrite which develops a very dense precipitate dispersion during intercritical annealing. In the carbonenriched interior of the austenite pool beyond the epitaxial ferrite only martensite forms in specimens annealed at 760 °C but various mixtures of ferrite and cementite form in specimens annealed at 810 °C. The latter structures include lamellar pearlite, a degenerate pearlite, and cementite interphase precipitation. All Nb is in solution in the austenite formed at 810 °C, and therefore the low hardenability of the specimens annealed at that temperature is best explained by the effect of low austenite carbon content.     

Comparative Study on Austenite Decomposition and Cu Precipitation During Continuous Cooling Transformation

The nature of Cu precipitation during continuous cooling of austenite is an essential issue in thermomechanical processing of Cu-bearing high-strength low-alloyed steel. The Cu precipitation behaviors and microstructural evolution are comparatively studied at two representative cooling rates of 0.1 and 10 K/s using dilatometry, optical microscopy, transmission electron microscope, and atom probe tomography. The microstructure developed at the cooling rate of 0.1 K/s contains a variety of microconstituents of polygonal ferrite, acicular ferrite, bainite, martensite, and retained austenite, which is attributed to the varying concentrations of untransformed austenite and consequently changing austenite decomposition kinetics. The Cu precipitation occurs only in association with polygonal ferrite transformation by interphase precipitation mechanism. No Cu precipitates are detected in acicular ferrite, retained austenite, and martensite. At the cooling rate of 10 K/s, the microstructure is dominated by acicular ferrite dispersed with the C-Ni precipitates that are assumed to be formed on the retained austenite. The Cu precipitation generally occurs from aging because of the high temperature and supersaturated austenite in association with acicular ferrite transformation. The nature of Cu precipitation in different cooling rates greatly depends on the initial composition or the extent of solute enrichment in the prior untransformed austenite and the associated phase transformation mechanisms, resulting in the differences in sizes, morphology, and compositions of Cu precipitation.     

Acicular ferrite morphologies in a medium-carbon microalloyed steel

The influence of time and isothermal transformation temperature on the morphology of acicular ferrite in a medium-carbon microalloyed steel has been studied using optical and transmission electron microscopy (TEM). This study has been carried out with the analysis of the microstructures obtained with one- and two-stage isothermal treatments at 400 °C and 450 °C, following austenitization at 1250 °C. The heat treatments were interrupted at different times to observe the evolution of the microstructure at each temperature. The results show that a decrease in the isothermal transformation temperature gives rise to the development of sheaves of parallel ferrite plates, similar to bainitic sheaves, but intragranularly nucleated. These replace the face-to-edge nucleation that dominates the transformation at higher temperatures. The TEM observations reveal that the plates correspond to upper acicular ferrite and the sheaves to lower acicular ferrite. In this last case, cementite precipitates are present at the ferrite unit interiors and between the different platelets.     

On the Low Temperature Strain Aging of Bainite in the TRIP Steel

The aging behavior of a thermomechanically processed Mo-Al-Nb transformation-induced plasticity steel with ultrafine microstructure was investigated using transmission electron microscopy and atom probe tomography (APT). Strain aging at 73 K (200 °C) for 1800 seconds led to a significant bake-hardening response (up to 222 MPa). Moreover, aging for 1800 seconds at room temperature after 4 pct pre-strain also revealed a bake-hardening response (~60 MPa). The experimental results showed the formation of carbon Cottrell atmospheres around dislocations and the formation of carbon clusters/fine carbides in the bainitic ferrite during aging. It is proposed that this is associated with the high dislocation density of bainitic ferrite with formation of a complex dislocation substructure after pre-straining and its high average carbon content (~0.35 at. pct). The segregation of carbon and substitutional elements such as Mn and Mo to the retained austenite/bainitic ferrite interface during aging was observed by APT. This segregation is likely to be the preliminary stage for Mo-C particles’ formation. The aging after pre-straining also induced the decomposition of retained austenite with formation of ferrite and carbides.     

Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel

Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 °C and 600 °C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 °C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M _s ) and martensite finish (M _f ) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the M _s temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 °C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed.     

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.     

冷却速率对薄带连铸低碳钢微观组织的影响

摘要：为了探究冷却速率对薄带连铸低碳钢微观组织的影响,采用相变仪设备对铸带重新加热并进行不同冷却速率冷却,采用光学显微镜、场发射电子探针和电子背散射衍射等手段对微观组织和针状铁素体形核所利用的夹杂物进行了分析。结果表明,铸带经1200℃保温3min后,原奥氏体晶粒尺寸约为150～650μm,可以满足晶内针状铁素体形核对原奥氏体晶粒尺寸的需要。在2～5℃/s的冷却速率范围内,试样中得到了大量的针状铁素体组织,冷却速率为2℃/s的试样中大角度晶界所占比例约为60%;当冷却速率大于20℃/s,针状铁素体的形成受到抑制。铸带中针状铁素体形核所利用的夹杂物是Ti-Al-Si-Mn-O+MnS复合夹杂物。     

Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel

The influence of hot deformation on the microstructure of a hot-rolled Si-Mn transformation-induced-plasticity (TRIP) steel was evaluated in an effort to better control retained austenite content. In this study, axial compressive strains varying in amounts from 0 to 60 pct were imposed in the austenite phase field, and effects on the formation of polygonal ferrite, bainite, and retained austenite were determined. In addition, modifications in simulated coiling temperature from 420 °C to 480 °C and cooling rates from the rolling temperature, between 10 °C/s and 35 °C/s, were assessed. Fast cooling rates, low coiling temperatures, and low degrees of hot deformation were generally found to decrease the amount of polygonal ferrite and increase retained austenite fraction. Unexpectedly, a sharp increase in polygonal ferrite content and decrease in retained austenite content occurred when the fastest cooling rate, 35 °C/s, was coupled with extensive hot deformation and high coiling temperatures. This effect is believed to be due to insufficient time for full recovery and recrystallization of the deformed austenite, even in the absence of intentional microalloying additions to control recrystallization kinetics. The resultant decrease in hardenability allowed the ferrite transformation to continue into the holding time at high (simulated) coiling temperatures.     

氮对含铌20MnSi钢组织演变的影响

 研究了不同冷速条件下氮对铌微合金化20MnSi钢组织演变的影响。试验钢经1 200 ℃全固溶处理后快冷至[Ac3,]然后分别以200、100 ℃/h速度冷却至室温。对试样进行了OM、SEM和TEM观察。结果表明：钢中细小Nb(C,N)粒子在原奥氏体晶内高密度位错区的密集析出导致贫碳区的形成,进而激发针状铁素体的形核长大。铌微合金钢增氮后能有效抑制钢中针状铁素体的生成,促进等轴铁素体的生成和珠光体球团的细小均匀化,同时珠光体退化倾向减弱或消失。     

Electron microstructure and mechanical properties of silicon and aluminium ductile irons

Samples of unalloyed silicon and aluminium spheroidal graphite cast iron have been studied in the austempered condition. Austempering times of up 3 h at 400°C for Al SG and 1 h at 350°C for Si SG gives a typical ADI microstructure consising of carbide-free banitic ferrite and stable, high carbon enriched, retained austenite. This has an attractive combination of elongation and strength. For longer austempering times transition carbides are precipitated in the bainitic ferrite, η-carbide in the upper bainitic range, i.e.400°C for Al SG and 350°C for Si SG, and ϵ-carbide in the lower bainite range. Increasing amounts of transition carbide reduce the ductility and produce a mixed model of fracture. For longer austempring times _X-carbide is precipitated at the ferrite/austenite boundaries leading to a more brittle fracture mode.     

The isothermal austenite-ferrite transformation in some deformed vanadium steels

The effect of vanadium on the isothermal austenite-ferrite transformation, between 725 °C and 775 °C, of a hot-deformed microalloyed steel has been studied by examination of the microstructure and measurement of the volume fraction of ferrite in specimens quenched from the reaction temperature. The accompanying precipitation was studied by transmission electron microscopy of thin foils and carbon extraction replicas and by electron energy-loss spectroscopy. Very early in the transformation a continuous band of fine-grained ferrite forms at austenite grain boundaries. After some time some of these grains coarsen to form large equiaxed ferrite grains. It is found that vanadium has no effect on the time to the start of coarsening but thereafter accelerates the rate of formation of ferrite. Interphase precipitation of VN occurs throughout the transformation in the vanadium steels and this is thought to influence the rate at which the ferrite coarsens at the lower temperatures (750 ° and 725 °C) in the range studied.     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo,and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M₂₃C₆ carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.     

On the Austenite Stability of a New Quality of Twinning Induced Plasticity Steel, Exploring New Ranges of Mn and C

A new high-manganese, low-silicon TWIP steel was studied to evaluate austenite stability after different heat treatment conditions. To determine the phase transformations, dilatometric experiments were performed, and the microstructure was characterized by light optical microscopy, X-ray diffraction, and transmission electron microscopy. Precipitation of lamellar cementite was observed in the microstructure for extended treatment times at 823 K (550 °C). Long isothermal holding at this temperature also caused epsilon martensite formation during cooling, resulting from a decrease in austenite stability due to carbon depletion in the matrix when a quantifiable amount of cementite is formed.     

20MnSi中夹杂物对针状铁素体形成的影响

 通过热模拟试验对20MnSi连续冷却过程中的相变规律进行了测定,通过电石、硅钙线脱氧及热处理得到了含有晶内针状铁素体试样,利用显微硬度仪对针状铁素体聚集区进行了显微硬度的测定,利用光学显微镜对晶内针状铁素体进行了形貌观察,利用扫描电镜和能谱仪对诱导针状铁素体生成的夹杂物的性质进行了分析。结果表明,20MnSi中可以形成晶内针状铁素体的冷却速度范围为5～20 ℃/s;能够诱发针状铁素体组织形核的夹杂物主要为MnS夹杂,其次为MnO·SiO2和MnS·SiO2夹杂,并且3类夹杂物的尺寸主要在小于3 μm的区间内;MnS夹杂促进针状铁素体形核是由应力-应变能和惰性界面能等原因共同造成的;高温加热和等温保温有利于使贫锰区减弱或消失,不利于针状铁素体的形成;高熔点夹杂物有利于诱导针状铁素体的形核,复合夹杂物和镶嵌存在的夹杂物可以为针状铁素体的形核提供多个合适的形核区,有利于促进多个针状铁素体的同时形核、长大。     

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming. We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing. First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure ‐ nature of the constituents, size and shape ‐ and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X‐ray diffraction measurements. Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this “non partitioned” composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium. Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.