Effect of interpass time on austenite grain refinement by means of dynamic recrystallization of austenite

A 0.06 pct C-0.3 pct Mn and a 0.07 pct C-0.6 pct Mn-0.028 pct Nb steel were deformed in torsion at a constant strain rate of 2/s. Two schedules were used. In schedule A, seven roughing passes executed between 1260°C and 1130°C were followed by a single large finishing pass with a strain of 3.5 at constant temperatures between 1010°C and 840°C. The time between roughing and finishing was 200 seconds. In schedule B, the seven roughing passes were followed by 10 finishing passes, again applied isothermally, with strains of 0.3 and interpass times of 0.6, 2, and 10 seconds. The results indicate that for the Nb steel, low rolling temperatures (870°C) and strains above 2 are required for complete dynamic recrystallization, which results in austenite grain sizes under 6μm. Cooled at a rate of 10°C/s, the dynamically recrystallized austenite grain structures transform into ferrite with grain sizes under 4 μ. Extrapolations from the present data suggest that at industrial strain rates and cooling rates, ferrite grain sizes under 2 μm should be achieved. Y.W. BOWDEN, formerly CSIRA Research Associate, Department of Mining and Metallurgical Engineering, McGill University     

Softening and microstructural change following the dynamic recrystallization of austenite

To characterize the dynamic recrystallization behavior of austenite, continuous-torsion tests were carried out on a Mo steel over the temperature range 950 ‡C to {dy1000} ‡C, and at strain rates of 0.02, 0.2, and 2 s^-1. Interrupted-torsion tests also were performed to study the characteristics of postdynamic recrystallization. Quenches were performed after increasing holding times to follow the development of the postdynamic microstructure. Finally, torsion simulations were carried out to assess the importance of metadynamic recrystallization in hot-strip mills. The postdynamic microstructure shows that the growth of dynamically recrystallized grains is the first change that takes place. Then metadynamically recrystallized grains appear and contribute to the softening of the material. The rate of metadynamic recrystallization and the meta-dynamically recrystallized grain size depend on strain rate and temperature and are relatively independent of strain, in contrast to the observations for static recrystallization. True dynamic recrystallization-controlled rolling (DRCR) is shown to require such short interpass times that it does not occur in isolation in hot-strip mills. As these schedules involve 20 to 80 pct softening by metadynamic recrystallization, a new concept known as metadynamic recrystallization-controlled rolling (MDRCR) is introduced to describe this type of situation. 1 C. ROUCOULES, formerly with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada     

Effect of Strain-Induced Precipitation on the Recrystallization Kinetics in a Model Alloy

The effects of Nb addition on the recrystallization kinetics and the recrystallized grain size distribution after cold deformation were investigated by using Fe-30Ni and Fe-30Ni-0.044 wt pct Nb steel with comparable starting grain size distributions. The samples were deformed to 0.3 strain at room temperature followed by annealing at 950 °C to 850 °C for various times; the microstructural evolution and the grain size distribution of non- and fully recrystallized samples were characterized, along with the strain-induced precipitates (SIPs) and their size and volume fraction evolution. It was found that Nb addition has little effect on recrystallized grain size distribution, whereas Nb precipitation kinetics (SIP size and number density) affects the recrystallization Avrami exponent depending on the annealing temperature. Faster precipitation coarsening rates at high temperature (950 °C to 900 °C) led to slower recrystallization kinetics but no change on Avrami exponent, despite precipitation occurring before recrystallization. Whereas a slower precipitation coarsening rate at 850 °C gave fine-sized strain-induced precipitates that were effective in reducing the recrystallization Avrami exponent after 50 pct of recrystallization. Both solute drag and precipitation pinning effects have been added onto the JMAK model to account the effect of Nb content on recrystallization Avrami exponent for samples with large grain size distributions.

     

An electron microscopy examination of primary recrystallization in TD-nickel

Primary recrystallization in TD-nickel 1 in. bar has previously been regarded as the process by which the initial fine grain structure (1μ average grain radius) is converted to a coarse grain size (increases in grain size by 500 times) under suitable deformation and annealing conditions. This process is dependent on deformation mode. While it occurs readily after rolling transverse to the bar axis and annealing (800°C), it is completely inhibited by longitudinal rolling and swaging deformations, even for very high (1320°C) annealing temperatures. A transmission electron microscopy examination of deformation and annealing substructures indicates that primary recrystallization in TD-nickel 1 in. bar actually occurs on the sub-light optical level, to produce a grain structure similar in size to the initial fine grained state. Coarse grain formation is the result of abnormal grain growth (or secondary recrystallization), which follows primary recrystallization.     

Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060 °C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-μm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750 °C and 925 °C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.     

Effect of thermomechanical treatments on the room-temperature mechanical behavior of iron aluminide Fe3AI

The room-temperature hydrogen embrittlement (HE) problem in iron aluminides has restricted their use as high-temperature structural materials. The role of thermomechanical treatments (TMT),i.e., rolling at 500 °C, 800 °C, and 1000 °C, and post-TMT heat treatments,i.e., recrystallization at 750 °C and ordering at 500 °C, in affecting the room-temperature mechanical properties of Fe-25A1 intermetallic alloy has been studied from a processing-structure-properties correlation viewpoint. It was found that when this alloy is rolled at higher temperature, it exhibits a higher fracture strength. This has been attributed to fine subgrain size (28/μ) due to dynamic recrystallization occurring at the higher rolling temperature of 1000 °C. However, when this alloy is rolled at 1000 °C and then recrystallized, it shows the highest ductility but poor fracture strength. This behavior has been ascribed to the partially recrystallized microstructure, which prevents hydrogen ingress through grain boundaries and minimizes hydrogen embrittlement. When the alloy is rolled at 1000 °C and then ordered at 500 °C for 100 hours, it shows the highest fracture strength, due to its finer grain size. The alloy rolled at 500 °C and then ordered undergoes grain growth. Hence, it exhibits a lower fracture strength of 360 MPa. Fracture morphologies of the alloy were found to be typical of brittle fracture,i.e., cleavage-type fracture in all the cases.     

The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys

In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy (TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens, austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C, no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening mechanisms were also discussed. Formerly Graduate Student, Seoul National University.     

Recrystallization kinetics of microalloyed steels deformed in the intercritical region

A plain carbon and two microalloyed steels were tested under interrupted loading conditions. The base steel contained 0.06 pct C and 1.31 pct Mn, and the other alloys contained single additions of 0.29 pct Mo and 0.04 pct Nb. Double-hit compression tests were performed on cylindrical specimens of the three steels at 820 °C, 780 °C, and 740 °C within the α + γ field. A’softening curve was determined at each temperature by the offset method. In parallel, the progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. It was observed that, in the base steel, a recrystallizes more slowly thany. The addition of Mo retards recrystallization and has a greater influence on γ than on α recrystallization. This effect is in agreement with calculations based on the Cahn theory of solute drag. Niobium addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hour). The metallographic results indicate that the nucleation of recrystallization occurs heterogeneously in the microstructure, the interface between ferrite and austenite being the preferred site for nucleation.     

Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels

Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 μm at 600 °C to 5.85 μm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 μm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)₂₃C₆ carbides form within the grains and at the grain boundaries.     

Austenite grain growth kinetics in Al-killed plain carbon steels

Austenite grain growth kinetics have been investigated in three Al-killed plain carbon steels. Experimental results have been validated using the statistical grain growth model by Abbruzzese and Lücke, which takes pinning by second-phase particles into account. It is shown that the pinning force is a function of the pre-heat-treatment schedule. Extrapolation to the conditions of a hot-strip mill indicates that grain growth occurs without pinning during conventional processing. Analytical relations are proposed to simulate austenite grain growth for Al-killed plain carbon steels for any thermal path in a hot-strip mill.     

Fracture toughness of aisi M2 high-speed steel and corresponding matrix tool steel

The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4 pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated. In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit 2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples is a maximum. The above described results were explained through changes in the microstructure and the fracture modes. CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI.     

Formation of austenite in 1.5 pct Mn steels

The formation of austenite from different microstructural conditions has been studied in a series of 1.5 pct Mn steels that had been heated in and above the intercritical (α+ γ) region of the phase diagram. The influence of variables such as cementite morphology, initial structural state of the ferrite and the carbon content has been assessed in terms of their respective effects on the kinetics of austenite formation and final microstructure. Austenite was found to form preferentially on ferrite-ferrite grain boundaries for all initial structures. The results of this study have shown that the 1.5 pct Mn has lowered both the AC₃ and AC₁, lines causing large amounts of austenite to form in low carbon steel. The kinetics of austenite formation at 725 °C were not only very slow but also were approximately independent of the amount formed. Austenite appeared to form slightly more rapidly from cold rolled ferrite than from recrystallized ferrite or ferrite-pearlite structures.     

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.     

Experimental investigation of the transformation texture in hotrolled ferritic stainless steel using single orientation determination

Two ferritic stainless steels (≈16.5 mass pct Cr) were hot-rolled using seven subsequent passes. The first sample was rolled within the range 1280 °C to 750 °C,i.e., the deformation started in the ferritic region. The second sample was rolled within the range 1080 °C to 770 °C,i.e., the deformation started in the ferritic-austenitic region. In both cases, up to 40 vol pct of the ferrite transformed into austenite during hot rolling. During the last passes, the austenite transformed into cubic martensite. After hot rolling, these former austenitic regions were identified using a selective etching technique and examined using single orientation determination in the scanning electron microscope. The regions which remained ferritic throughout the hot-rolling process were investigated as well. Whereas the texture of the martensite considerably depended on the hot-rolling conditions, especially on the temperature and on the intervals between the rollings, the texture of the ferrite was less affected. The textures of the martensite were interpreted in terms of the crystallographic transformation rules between austenite and martensite. The textures of the ferrite were discussed in terms of recovery and recrystallization. M. YLITALO, formerly with the Department of Mechanical Engineering, University of Oulu, 90570 Oulu, Finland     

Retained carbide distribution in intercritically austenitized 52100 steel

Two 52100 steels, one containing 0.009 pct P, the other 0.023 pct P, were homogenized at 1150 °C, slowly cooled to form proeutectoid carbides and pearlite, partially spheroidized, austenitized at 850 °C for one hour, oil quenched, and tempered at 200 °C. Light microscopy and transmission electron microscopy of carbon extraction replicas showed that cementite particles were retained as three different morphologies in the fine-grained austenite formed during the 850 °C intercritical austenitizing treatment. The morphologies are characterized as follows: (1) closely spaced intragranular carbides most of which are less than 0.25 μm in diameter, (2) carbides about 1 μm in diameter, located on austenite grain boundaries, and (3) branched proeutectoid carbides arranged in networks corresponding to the coarse, 130 μm diameter austenite grains formed during homogenizing. The major effect of high phosphorus content was to retard the spheroidization of the retained carbides.     

Quantitative Analysis of Microstructural Constituents in Welded Transformation-Induced-Plasticity Steels

A quantitative analysis of retained austenite and nonmetallic inclusions in gas tungsten arc (GTA)–welded aluminum-containing transformation-induced-plasticity (TRIP) steels is presented. The amount of retained austenite in the heat-affected and fusion zones of welded aluminum-containing TRIP steel with different base metal austenite fractions has been measured by magnetic saturation measurements, to study the effect of weld thermal cycles on the stabilization of austenite. It is found that for base metals containing 3 to 14 pct of austenite, 4 to 13 pct of austenite is found in the heat-affected zones and 6 to 10 pct in the fusion zones. The decomposition kinetics of retained austenite in the base metal and welded samples was also studied by thermomagnetic measurements. The decomposition kinetics of the austenite in the fusion zone is found to be slower compared to that in the base metal. Thermomagnetic measurements indicated the formation of ferromagnetic ε carbides above 290 °C and paramagnetic η(ε′) transient iron carbides at approximately 400 °C due to the decomposition of austenite during heating.     

Microstructure and Mechanical Properties in Hot-Rolled Extra High-Yield-Strength Steel Plates for Offshore Structure and Shipbuilding

Key parameters for a thermomechanically controlled processing and accelerated cooling process (TMCP-AcC) were determined for integrated mass production to produce extra high-yield-strength microalloyed low carbon SiMnCrNiCu steel plates for offshore structure and bulk shipbuilding. Confocal scanning microscopy was used to make in-situ observations on the austenite grain growth during reheating. A Gleeble 3800 thermomechanical simulator was employed to investigate the flow stress behavior, static recrystallization (SRX) of austenite, and decomposition behavior of the TMCP conditioned austenite during continuous cooling. The Kocks–Mecking model was employed to describe the constitutive behavior, while the Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach was used to predict the SRX kinetics. The effects of hot rolling schedule and AcC on microstructure and properties were investigated by test-scale rolling trials. The bridging between the laboratory observations and the process parameter determination to optimize the mass production was made by integrated industrial production trials on a set of a 5-m heavy plate mill equipped with an accelerated cooling system. Successful production of 60- and 50-mm-thick plates with yield strength in excess of 460 MPa and excellent toughness at low temperature (213 K (–60 °C)) in the parent metal and the simulated coarse-grained heat affected zone (CGHAZ) provides a useful integrated database for developing advanced high-strength steel plates via TMCP-AcC.     

Microstructures and Mechanical Properties of As-Cast and Hot-Rolled Mg-8.43Li-0.353Ymm (Y-riched mischmetch) Alloy

Mg-8.43Li-0.353Ymm (Y-riched mischmetch) alloy was prepared using the metal model casting method and then hot-rolled at 573 K (300 °C) to total reduction of 81 pct, and microstructures and mechanical properties were evaluated. Results show that the rolling procedure introduces great dynamic recrystallization, and β-Li grains in the rolled alloy present an ultra-low average size of 2.18 μm. The ultimate tensile strength and yield strength of the rolled alloy are 158–167 MPa and 124–130 MPa, respectively, 42–63 pct higher than that of the as-cast alloy. The elongation of the rolled alloy is improved to 31–40 pct, as about four to five times that of the as-cast alloy, which is ascribed to the great grain refinement of the β-Li phase.