Coarse Austenite Research Articles

鍛造型板厚大圧下によるオーステナイト組織の微細化機構

To obtain fine and uniform microstructures, appropriate forging conditions were examined using high-speed large-reduction forging. Swing-type forging equipment was employed in hot-forging experiments using low-carbon steel (0.14%C-0.64%Mn). Also, a microstructure analytical system was developed by combining thermomechanical FE analysis with microstructural analysis to discuss the metallurgical phenomena occurring during forging. It was observed that coarse austenite grains can be refined to 20-30 μm uniformly using the proposed forging technology in the case of 59% reduction in thickness at 1273 K. In this case, dynamic recrystallization is a dominant factor for obtaining fine and uniform austenite grains. In the case of 43% reduction in thickness, the recrystallization is inhomogeneous, resulting in an inhomogeneous microstructure. Thus, about 60% reduction in thickness is necessary to obtain fine and uniform microstructures using the proposed technology when applied to the roughing of hot strip mills.

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (∼20 μm) and a coarse average grain size (∼260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ∼534°C.

Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels

Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure. Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions (0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite. Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However, an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary units.

Effect of Austenite Grain Size on the Fatigue Crack Growth and Arrest Behavior of Carburized Steel

The effect of austenite grain size on the fatigue crack growth and arrest behavior of carburized steel was studied. On the fatigue test of same stress amplitude, the fatigue crack initiation of fine austenite grain steels is delayed in comparison with coarse austenite grain steels, but the difference of crack initiation life between fine grain steels and coarse grain steels is very slight against 107 cycles. The improvement of crack arrest property is dominant for increasing fatigue limit by refinement of austenite grain size. The critical stress intensity factor for crack arrest under fatigue tests at the stress amplitude of the each fatigue limit of carburized steels is calculated by considering the effect of compressive residual stress. As a result, it becomes clear that the finer the austenite grain size, the larger becomes the critical stress intensity factor for crack arrest, and crack arrest property is improved remarkably by the refinement of austenite grain size.

Influence of Deformation on Transformation of Low-Carbon and High Nb-Containing Steel During Continuous Cooling

The effect of deformation in the non-recrystallization region on the phase transformation for a low-carbon and high Nb-containing steel with coarse austenite grain size was investigated by means of dilatometric measurement and microstructure observation. The results show that with increasing the cooling rate, both the transformation start and finish temperatures measured by dilatometer are decreased, and the corresponding microstructure is transformed from ferrite and pearlite to full granular bainite gradually. The dynamic continuous cooling transformation diagram was plotted according to the dilatometric measurements and microstructure observations. Dilatometric measurements also show that the transformation start and finish temperatures of the test steel rise with increasing strain, strain rate and deformation temperature, and the reasons are discussed.

Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ

Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ.

Abnormal α to γ Transformation Behavior of Steels with a Martensite and Bainite Microstructure at a Slow Reheating Rate

The same coarse austenite (γ) grains as those before austenitizing emerge when a martensite or bainite steel with coarse grains is reheated to an austenite region at a slow reheating rate. This is called abnormal ferrite (α) to austenite (γ) transformation or γ grain memory. In this paper, α to γ transformation behavior is investigated in order to clarify the mechanism of abnormal α to γ transformation from the viewpoint of the roles of cementite and retained γ. (1) Coarse γ grains and fine globular γ grains that nucleate along the coarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature. The size distribution of γ grain is the same as that before reheating. (2) Coarse γ grains are formed by the growth, impingement, and coalescence of acicular γ grains that corresponds to retained γ between laths. (3) Abnormal α to γ transformation is suppressed by decreasing the amount of retained γ and by increasing the amount of cementite before reheating. These results suggest that α to γ transformation behavior is governed by competition between the nucleation and growth of newly formed γ from the dissolution of cementite and the growth and coalescence of retained γ. Abnormal α to γ transformation occurs when the growth and coalescence of retained γ dominates rather than the nucleation and growth of globular γ grains.

Dynamic Recrystallization Behavior of Twin Roll Cast Low Carbon Steel Strip

Dynamic recrystallization behavior of twin roll cast low carbon steel strip was investigated in this paper in an attempt to provide guiding deformation parameters for the on line hot rolling. As cast strip was reheated and soaked with austenite grain size similar to the width level of the as cast columnar structure. Tensile test was used and the deformation temperature is in the range of 900 to 1100°C and the strain rates are 0.01, 0.1 and 1 s−1. Activation energy and stress exponent were determined by regression to be 306 kJ/mol and 4.69 respectively. The ratio of critical strain to the peak strain εc/εp is 0.65, and that of critical stress to the peak stress σc/σp is 0.92. Dependence of the peak strain εp on the initial grain size D0 and Zener–Hollomon parameter Z is εp=9.1×10−4×D00.48Z0.13. Kinetics of the dynamic recrystallization and the recrystallized grain size was predicted using models published. The as cast coarse austenite will be dramatically refined after complete dynamic recrystallization under low deformation temperature and slow strain rate according to the predicting results.

A Model for Predicting the Austenite Grain Size at the Surface of Continuously-Cast Slabs

During the continuous-casting process, retarded cooling of the strand surface below oscillation marks and surface depressions results in the formation of coarse austenite grains. These coarse grains have proven to dramatically reduce the ductility of steel within the second ductility trough, and thus increase the risk of surface crack formation. In addition to the thermal history the composition of the steel, in particular the content of carbon and precipitation-forming elements, plays a decisive role in the development of the austenite grain size. The present work addresses the development and validation of an experimental and numerical model for predicting the austenite grain size in the continuous-casting process. In a first step, the previous austenite grain size on the surface of slabs was determined by metallographic examinations for several slabs with various carbon content. Next, a solidification experiment was adjusted in order to simulate the cooling conditions in the mold of a slab caster, but also to suppress the precipitation of nitrides and carbo-nitrides by subsequent accelerated cooling. Thus, it was possible to study the influence of steel composition on austenite grain growth at temperatures close to the solidus temperature, unaffected by precipitates. The results of both the plant and laboratory experiments point to a maximum austenite grain size with a carbon content of approximately 0.17 mass pct.

Size distribution of austenite grains and disperse particles in steel

The physicomechanical properties of steels depend substantially on the austenite grain size and the uniformity of the size distribution of grains. Variation in the grain size of steel degrades many of its properties. In particular, this variation can cause nonuniform phase transformations in steel, when bainite forms in more stable coarse austenite grains upon supercooling and pearlite forms in less stable fine austenite grains. The criterion for variation in grain size is taken to be the coefficient of variation V (%), determined as the ratio of the root-mean-square deviation of the grain size σ from the arithmetical average of this size to this arithmetic average:

Comparison of recrystallization behaviour of cold deformed acicular ferrite produced by conventional processing and direct strip casting

The effect of initial microstructure on the recrystallization behaviour of low carbon (LC) steel was investigated. Steel strip (0.05 wt.% C) of 2-mm thickness was heat treated to produce an acicular ferrite (AF) microstructure from coarse austenite. The AF material and a strip-cast (SC) material of similar chemistry produced by twin roll casting were cold rolled to 50%, 70% and 90% reduction, and annealed for various times in the temperature range 580–680 °C. The evolution of microstructure during recrystallization was studied by optical microscopy and electron backscatter diffraction (EBSD) in the SEM. The initial microstructure was found to have a substantial influence on the recrystallization behaviour. AF recrystallized at a substantially faster rate than SC due to a higher level of solute Mn causing substantial solute drag effects.

Recrystallization Behaviour of Cold Rolled Low Carbon Steel Strip with Various Starting Microstructures

The effect of initial microstructure (acicular ferrite (AF), polygonal ferrite (PF) and strip cast (SC)) on the recrystallization behaviour of low carbon (LC) steel was investigated. Steel strip samples (0.05 wt.% C) of 2 mm in thickness were heat treated to produce an AF and PF microstructure from coarse austenite. The AF, PF and a similar chemistry SC sample manufactured from a twin roll caster were cold rolled to 50, 70 and 90% reduction, and annealed for various times in the temperature range 580-680 °C. The evolution of microstructure during recrystallization was studied by optical microscopy and electron backscatter diffraction (EBSD) in the SEM. The initial microstructure was found to have a substantial influence on the recrystallization behavior. PF recrystallized more rapidly than AF with SC showing extremely sluggish recrystallization behaviour. The recrystallizing grains in these initial microstructures have a lognormal distribution and the recrystallized number density (grains/mm2) decreased during annealing, with the initial microstructures affecting the degree of this decrease in number density.

Effect of ausferrite volume fraction and morphology on tensile properties of partially austenitised and austempered ductile irons with dual matrix structures

In the present study, an unalloyed ductile iron containing 3·50%C, 2·63%Si, 0·318%Mn and 0·047%Mg was intercritically austenitised (partially austenitised) in the two phase region (α + γ) at temperatures of 795 and 815°C for 20 min, and then was quenched into a salt bath held at an austempering temperature of 365°C for various times to obtain various ausferrite volume fractions. Fine and coarse dual matrix structures were obtained from the two different starting conditions. Some specimens were also conventionally austempered from 900°C for comparison. Results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high carbon retained or stabilised austenite) was developed. The phase previously described as new ferrite (also called epitaxial ferrite) was observed to form following heat treatment only in specimens with coarse austenite dispersion after austempering from the (α + γ) temperature range. It was observed that the parent austenite dispersion present at the intercritical austenitising temperature has an effect on the volume fraction of high carbon austenite following austempering. Finer austenite dispersions produced more stabilised high carbon austenite than coarse ones for a given austempering time. The volume fractions of proeutectoid ferrite, new ferrite, ausferrite can be controlled to influence the strength and ductility. Specimens austempered from the (α + γ) temperature range exhibited much greater ductility than conventionally austempered samples. Within each of the series austempered from the (α + γ) temperature range, the tensile strength increased and ductility decreased with increasing ausferrite volume fraction and decreasing proeutectoid ferrite volume fraction. However, specimens having a fine ausferritic structure without new ferrite exhibited lower ductility and higher strength than specimens with a coarse ausferritic structure under the same austempering conditions. The specimen with ∼60% AFVF (coarse structure) and ∼65% AFVF (fine structure) exhibited the best combination of high strength and ductility compared to pearlitic grades, but their ductility was slightly lower than ferritic grades. These materials satisfy the requirements for the strength of quenched and tempered grades and their ductility superior to that of this grade.

The Effect of Isothermal Heat Treatment on the Rolling Contact Fatigue of Carburized Low Carbon Microalloyed Steel

In case of the low carbon chromium steel that have widely been used for the carburized gears or rolling bearings, the austenite grain coarsening may be occasionally occurred during carburizing. To restrain this phenomenon, most of hot forged parts have been given to an isothermal heat treatment or normalizing immediately after hot forging and/or prior to carburizing. Therefore, their production cost includes unexpected additional expenses caused by such a non-value added process. To confirm the possibility of an energy saving, as well as attaining a superior fatigue life, in the production of automotive parts to be carburized, the austenite grain coarsening with the manufacturing process of the microalloyed steel, containing Nb and B as a grain refining elements, was investigated. The heat treatment characteristics and rolling contact fatigue behavior of the carburized specimens with the isothermal heat treatment were also investigated. In spite of omitting the isothermal heat treatment after hot forging, the abnormally coarse austenite grains were not found out in the carburized specimens. However, the rolling contact fatigue life of the carburized specimens, in which the isothermal heat treatment was omitted, was shorter than that of isothermally heat-treated specimens.

Mechanism of Cracking Resistance of Hardfacing Specimens of Steel 5CrNiMo Improved by Rare Earth Oxide

The cracking morphology of the hardfacing specimens taken from steel 5CrNiMo was observed. Meanwhile, the residual stress fields were measured and simulated. Based on experiment mentioned above, the improved structure and modified inclusion in hardfacing metal with rare earth (RE) oxide were analyzed. The results show that, the hardfacing crack is initiated from the coarse dendritic crystal grain boundary, inclusions and coarse austenite grain boundary in the HAZ and propagated by the residual stress existing in the center of the hardfacing metal and HAZ. The primary columnar grain structure can be refined by adding RE oxide in the coating of the electrode. The inclusion in the hardfacing metal can be modified as well. Meanwhile, if the martensite transformation temperature is decreased, the largest value of the residual tensile stress in the dangerous region can be reduced.

STUDY OF DYNAMIC RECRYSTALLIZATION OF LOW CARBON STEEL IN THIN SLAB CONTINUOUS ROLLING PROCESS

Combined with the technological characteristics of thin slab continuous rolling process (TSCR), dynamic recrystallization of an extremely coarse austenite of low carbon steel is studied by Thermecmaster-Z hot simulator. By the analysis of true stress-strain curves and the observation of microstructures at different deformation stages, the critical stress and critical strain are determined under different deformation conditions. The effect of Z parameter on dynamic recrystallization of coarse austenite is studied. The microstructure evolution in real production is also discussed.

Evolution of non-uniform grain structure during hot defornnation of a Nb-Ti microalloyed steel

Recrystallisation and the evolution of the abnormally grown austenite grains were investigated for a continuously cast slab of a 0.13 C-1.41 Mn-0.027 Nb-0.012 Ti steel during reheating and after the subsequent deformation. The stability of the recrystallised structure and the uniformity of the final microstructure were also studied. The abnormally grown grains appear in a few minutes at reheating temperatures around 1200 °C. All grains in the bimodal grain structure recrystallised at 1100 °C for strains > 0.2 within about 40 s. The coarse grains are refined, while the fine grains become slightly larger. Some abnormal grain growth can occur again in the recrystallised structure within 10 min. After cooling at 1° C/s the coarse austenite grains transform into large areas of upper bainite, while the finer grains transform to fine ferrite and pearlite. The transformed microstructure in specimens cooled at l°C/s consists of large upper bainitic areas corresponding to the prior coarse austenite grains, surrounded by fine ferrite-pearlite grains.

The evolution of ultrafine ferrite formation through dynamic strain-induced transformation

Abstract Hot torsion testing of a C–Mn–V steel was used to study the evolution of ultrafine ferrite (UFF) formation by dynamic strain-induced transformation (DSIT) in conjunction with air-cooling for two prior austenite grain sizes. This study evaluated not only the evolution of DSIT ferrite during straining, but also the grain growth behaviour of DSIT ferrite grains during post-deformation cooling. For both austenite grain sizes, the DSIT ferrite initially nucleated on/or near prior austenite grain boundaries at an early stage of transformation followed by the grain interiors. The prior austenite grain size affected the distribution of DSIT ferrite nucleation sites at an early stage of transformation and the subsequent coarsening behaviour of the grain boundary (GB) and the intragranular ferrite (IG) grains during post-deformation cooling. For the fine prior austenite grain size, the distribution of DSIT ferrite grains was more homogenous compared with the coarse austenite and the coarsening occurred not only in the GB ferrite grains but also in the IG ferrite grains. However, the ferrite coarsening mostly occurred for the IG ferrite rather than the GB ferrite grains in the coarse austenite. The result suggests that normal grain growth occurred during the overall transformation in the GB ferrite grains for the coarse initial austenite grain size.

Effect of strain on formation of ultrafine ferrite in surface of hot rolled microalloyed steel

The phenomenon of ultra grain refinement of ferrite in surface layers of hot rolled strip has been studied in a low carbon, niobium microalloyed steel. Wedge specimens were used, to vary the nominal equivalent strain applied during rolling from zero to approximately unity, and the cooling rate after rolling was varied from ~ 20 to 1 K s -1. In contrast with previous work, which contended that a very coarse austenite grain size and a low rolling temperature near the Ar3 were essential to obtain ultrafine ferrite in surface layers, such ultrafine layers were observed after rolling coarse austenite at up to 150 K above the Ar3 and after rolling fine grained austenite near the Ar3. In the case of coarse grained austenite, a critical nominal rolling strain needed to be exceeded to trigger the surface layer phenomenon, upon which cooling rate had little effect on the surface layer's grain size. Refining the prior austenite grain size had the further beneficial effect of refining the grain size at the centre of the rolled product, for example to 2·6 μm, while the surface layer was refined to 0·7 μm.

The Evolution of Microstructure during Thin Slab Direct Rolling Processing in Vanadium Microalloyed Steels.

The evolution of microstructure during a simulation of the thin slab direct rolling process has been studied on two low carbon steels, microalloyed with V-N and V-Ti-N. The steels were examined using optical microscopy, analytical transmission electron microscopy (TEM) and energy dispersive X-ray (EDAX).After the 4th rolling pass, in a five pass schedule, the initial coarse austenite grain size (≈1 mm) was reduced to about 50 μm in Steel V-N and 22μm in Steel V-Ti-N. The average ferrite grain size in the final strip was slightly smaller in Steel V-Ti-N (4.8-6.6 μm) than in Steel V-N (5.3-7.2 μm). For Steel V-N, VN was only observed after 1 050°C equalization, but it was not found after 1 200°C and 1 100°C equalisation. For Steel V-Ti-N, V-Ti(N) particles formed during casting and during equalization for all the equalization temperatures (1 200°C, 1 100°C and 1 050°C). AlN particles precipitated in Steel V-N only during 1 050°C equalization and were often associated with MnS or MnS and VN. No AlN was detected in Steel V-Ti-N. Fine V containing precipitates (<10 nm) were observed in the final strip for both of the steels, but the frequency of the fine particles was lower in Steel V-Ti-N than in Steel V-N. The fine precipitates in the final strip make a major contribution to dispersion strengthening. High strength (LYS≈460-560 MPa) with good toughness and good ductility were achieved in the steels, which are competitive to similar products made by conventional controlled rolling. However, the addition of Ti to the V-N steel decreased the yield strength due to formation of V-Ti(N) particles in austenite, which reduced the amounts of V and N available for subsequent V rich fine particle precipitation in ferrite.