Texture development in dual-phase cold-rolled 18 pct Ni maraging steel

Austenite and martensite textures were studied in 18 pct Ni 350-maraging steel as a function of various degrees of cold rolling. The austenite phase in the samples was produced by repeated thermal cycling between ambient and 800 °C. The austenite phase thus formed was mechanically unstable and transformed to the martensite phase after 30 pct cold rolling. The texture developed as a result of cold rolling, and its effect upon microstructure and hardness has been studied.     

The tensile properties of an Fe-4 pct Mo-0.2 pct C martensite tempered under applied stress

In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure o°Curred in tensile test at 600 °C to 700°C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries. TAKATOSHI OGAWA, formerly with Kyoto University. YOSHIFUMI OHMURA, formerly with Kyoto University. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

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.     

Tempering characteristics of a vanadium containing dual phase steel

Dual phase steels are characterized by a microstructure consisting of ferrite, martensite, retained austenite, and/or lower bainite. This microstructure can be altered by tempering with accompanying changes in mechanical properties. This paper examines such changes produced in a vanadium bearing dual phase steel upon tempering below 500 °C. The steel mechanical properties were minimally affected on tempering below 200 °C; however, a simultaneous reduction in uniform elongation and tensile strength occurred upon tempering above 400 °C. The large amount of retained austenite (≅10 vol pct) observed in the as-received steel was found to be essentially stable to tempering below 300 °C. On tempering above 400 °C, most of the retained austenite decomposed to either upper bainite (at 400 °C) or a mixture of upper bainite and ferrite-carbide aggregate formed by an interphase precipitation mechanism (at 500 °C). In addition, tempering at 400 °C led to fine precipitation in the retained ferrite. The observed mechanical properties were correlated with these microstructural changes. It was concluded that the observed decrease in uniform elongation upon tempering above 400 °C is primarily the consequence of the decomposition of retained austenite and the resulting loss of transformation induced plasticity (TRIP) as a contributing mechanism to the strain hardening of the steel. B. V. N. RAO, formerly Senior Research Engineer, Analytical Chemistry Department, General Motors Research Laboratories     

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.     

Tempering of Mn and Mn-Si-V Dual-Phase Steels

Changes in the yield behavior, strength, and ductility of a Mn and a Mn-Si-V dual-phase (ferrite-martensite) steel were investigated after tempering one hour at 200 to 600 °C. The change in yield behavior was complex in both steels with the yield strength first increasing and then decreasing as the tempering temperature was increased. This complex behavior is attributed to a combination of factors including carbon segregation to dislocations, a return of discontinuous yielding, and the relief of residual stresses. In contrast, the tensile strength decreased continuously as the tempering temperature was increased in a manner that could be predicted from the change in hardness of the martensite phase using a simple composite strengthening model. The initial tensile ductility (total elongation) of the Mn-Si-V steel was much greater than that of the Mn steel. However, upon tempering up to 400 °C, the ductility of the Mn-Si-V decreased whereas that of the Mn steel increased. As a result, both steels had similar ductilities after tempering at 400 °C or higher temperatures. These results are attributed to the larger amounts of retained austenite in the Mn-Si-V steel (9 pct) compared to the Mn steel (3 pct) and its contribution to tensile ductility by transforming to martensite during plastic straining. Upon tempering at 400 °C, the retained austenite decomposes to bainite and its contribution to tensile ductility is eliminated.     

Steel Microstructure Effect on Mechanical Properties and Corrosion Behavior of High Strength Low Carbon Steel

Different thermomechanical treatments were applied to a high strength low carbon steel with a novel chemical composition. As a result, three different microstructures were produced with dissimilar mechanical and corrosion properties. Subsequently, a tempering heat treatment was applied to redistribute the phases in the steel. Microstructure A with 56 pct martensite and 32 pct bainite presented high strength but medium ductility; microstructure C with 95 pct ferrite and 3 pct martensite/austenite resulted in low strength and high ductility, and finally microstructure B with 98 pct bainite and 2 pct martensite/austenite resulted in high strength and ductility. Alternatively the corrosion behavior obtained by polarization curves was characterized in 0.1 M H₂SO₄, 3 M H₂SO₄, 3.5 wt pct NaCl, and NS4 solutions resulting in similar magnitudes, while the corrosion behavior acquired by electrochemical impedance spectroscopy had slightly differences in 3 M H₂SO₄.     

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.     

Characterization of the Carbon and Retained Austenite Distributions in Martensitic Medium Carbon, High Silicon Steel

The retained austenite content and carbon distribution in martensite were determined as a function of cooling rate and temper temperature in steel that contained 1.31 at. pct C, 3.2 at. pct Si, and 3.2 at. pct noniron metallic elements. Mössbauer spectroscopy, transmission electron microscopy (TEM), transmission synchrotron X-ray diffraction (XRD), and atom probe tomography were used for the microstructural analyses. The retained austenite content was an inverse, linear function of cooling rate between 25 and 560 K/s. The elevated Si content of 3.2 at. pct did not shift the start of austenite decomposition to higher tempering temperatures relative to SAE 4130 steel. The minimum tempering temperature for complete austenite decomposition was significantly higher (>650 °C) than for SAE 4130 steel (～300 °C). The tempering temperatures for the precipitation of transition carbides and cementite were significantly higher (>400 °C) than for carbon steels (100 °C to 200 °C and 200 °C to 350 °C), respectively. Approximately 90 pct of the carbon atoms were trapped in Cottrell atmospheres in the vicinity of the dislocation cores in dislocation tangles in the martensite matrix after cooling at 560 K/s and aging at 22 °C. The 3.2 at. pct Si content increased the upper temperature limit for stable carbon clusters to above 215 °C. Significant autotempering occurred during cooling at 25 K/s. The proportion of total carbon that segregated to the interlath austenite films decreased from 34 to 8 pct as the cooling rate increased from 25 to 560 K/s. Developing a model for the transfer of carbon from martensite to austenite during quenching should provide a means for calculating the retained austenite. The maximum carbon content in the austenite films was 6 to 7 at. pct, both in specimens cooled at 560 K/s and at 25 K/s. Approximately 6 to 7 at. pct carbon was sufficient to arrest the transformation of austenite to martensite. The chemical potential of carbon is the same in martensite that contains 0.5 to 1.0 at. pct carbon and in austenite that contains 6 to 7 at. pct carbon. There was no segregation of any substitutional elements.     

The microstructural evolution,crystallography, and thermal processing of ultrahigh carbon Fe-1.85 pct C melt-spun ribbon

The microstructural evolution, mechanisms of grain refinement, crystallography, and thermal processing of a rapidly solidified Fe-1.85 pct C alloy have been studied by transmission electron microscopy (TEM). Melt-spun ribbons quenched in liquid nitrogen consist of carbide-free highly twinned martensite plates between 0.5-and 2.0-μm long and 0.1-and 0.5 -μm thick, with approximately 40 pct retained austenite also present. Ribbons tempered at 600 °C for 10 seconds consist of ferrite of approximately the same grain size and both intragranular and intergranular cementite precipitates. The intragranular cementite particles are about 0.1 /um or less in size and exhibit a single variant of the Bagaryatskii orientation relationship with respect to a given ferrite grain; the intergranular particles are about 0.1 μm in thickness and can be as long as 0.5 μm due to growth and/or coalescence along ferrite grain boundaries. A heat-treatment cycle investigated with a view toward generating structures suited for superplastic consolidation of the rapidly solidified ribbons consists of quenching the ribbon in liquid nitrogen, tempering at 600 °C for 10 seconds, “upquenching” to 750 °C (austenitizing) for 10 seconds, and subsequently quenching again in liquid nitrogen. This treatment results in martensite grains highly misoriented with respect to one another and typically 0.5 μm or less in both length and thickness and cementite particles 0.4 μm or less in size. (Occasionally, longer martensite plates were observed; but they never exceeded 1 μm in length.) The microstructures produced here offer the potential for producing fine-grained ultrahigh carbon steels of very high strength without the brittleness associated with the formation of coarse carbide particles or the loss of strength due to graphite formation. This investigation has thus provided the basis for follow-on studies currently underway in ultrahigh carbon Fe-C-Cr and Fe-C-Cr-Si steels, with the intent of producing similar microstructures which will also exhibit enhanced high-temperature stability.     

Tempering of Mn and Mn-Si-V dual-phase steels

Changes in the yield behavior, strength, and ductility of a Mn and a Mn-Si-V d11Al-phase (ferrite-martensite) steel were investigated after tempering one hour at 200 to 600 °C. The change in yield behavior was complex in both steels with the yield strength first increasing and then decreasing as the tempering temperature was increased. This complex behavior is attributed to a combination of factors including carbon segregation to dislocations, a return of discontinuous yielding, and the relief of resid11Al stresses. In contrast, the tensile strength decreased continuously as the tempering temperature was increased in a manner that could be predicted from the change in hardness of the martensite phase using a simple composite strengthening model. The initial tensile ductility (total elongation) of the Mn-Si-V steel was much greater than that of the Mn steel. However, upon tempering up to 400 °C, the ductility of the Mn-Si-V decreased whereas that of the Mn steel increased. As a result, both steels had similar ductilities after tempering at 400 °C or higher temperatures. These results are attributed to the larger amounts of retained austenite in the Mn-Si-V steel (9 pct) compared to the Mn steel (3 pct) and its contribution to tensile ductility by transforming to martensite during plastic straining. Upon tempering at 400 °C, the retained austenite decomposes to bainite and its contribution to tensile ductility is eliminated. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

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.     

Deformation of metastable austenite and resulting properties during the ausform-finishing of 1 pct carburized AlSl 9310 steel gears

The process of ausform-finishing in gears involves the deformation of metastable austenite. A critical step in optimizing the deformation process is to determine the link between material deformation behavior and final material properties, such as hardness and microstructure. To this end, uniaxial compression testing was carried out on 1 pct carburized AISI 9310 steel specimens in the low-temperature ausforming regime (85 °C to 230 °C). The work-hardening response of metastable austenite and its relation to the hardness and microstructure was determined from these experiments. High work-hardening rates (work-hardening exponent n=0.4 to 0.7) were caused by deformation-induced transformation of metastable austenite to either martensite or bainite or both. It is postulated that, at the ausforming temperatures in the neighborhood of 230 °C, bainite formed at the highest achievable strains of 50 pct while oriented martensite (loading induced) was detectable at lower strains of 20 pct. The hardness of the resulting ausformed microstructure increased with degree of straining and with reduction in temperature of ausforming. An X-ray determination of the retained austenite content showed that austenite tends to stabilize even after minimal ausforming. A transmission electron microscopy study on ausformed specimens showed the presence of microtwinning and high-dislocation densities. The effect of processing parameters on fatigue response under rolling contact conditions is discussed given current understanding and available fatigue data.     

The effect of tempering and aging on a low activation martensitic steel

Tempering and aging studies were carried out on a martensitic stainless steel which was designed to have reduced long-life activation after exposure to neutrons. Nickel, molybdenum, and niobium additions were restricted in these low activation alloys. The composition of the steel in weight percent was 12 pct Cr, 0.1 pct C, 0.3 pct V, 0.9 pct W, 6.4 pct Mn, and 0.1 pct Si, where manganese is used to stabilize the steel against delta ferrite and tungsten is used for tempering resistance. The tempering conditions studied were 2 hours at 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, and 900 °C and 24 hours at 500 °C and 700 °C. The steel was aged for 1000 and 5000 hours at 365 °C, 420 °C, 520 °C, and 600 °C. Microhardness, optical metallography, and transmission electron microscopy (TEM) were used to characterize the samples. The results indicated that the Ac₁ in this steel lies between 700 °C and 800 °C. During the 2-hour tempers at 400 °C and 500 °C, M₃C formed. After 24 hours at 500 °C, the M₃C was starting to be replaced by M₂₃C₆. At higher tempering temperatures and in all the aged samples, M₂₃C₆ was the only carbide found. A manganese-rich chi phase was also seen in the samples aged at 420 °C and 520 °C. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, Illinois, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

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

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

Tempering behavior of iron-carbon sputter deposits with 0 to 5 wt pct C

Iron-carbon sputter deposits with 0.06, 0.18, 0.66, 2, 3, and 5 wt pct C were tempered at temperatures from 100° to 550°C. The 0.06, 0.18, and 0.66 wt pct C sputter deposits were similar to severely cold-worked martensite, both as-deposited and in tempering response. Specifically, these three deposits were much harder than water quenched steels of the same composition, and the deposit hardnesses decreased less than martensite hardnesses on tempering below 400°C. The hardnesses of the 2 and 3 wt pct C deposits increased 40 to 50 Dph units upon tempering at low temperatures (150° to 250°C) and decreased for higher tempering temperatures. The hardness of the 5 wt pct C deposit remained constant (920 Dph) after tempering at 150°C, but increased to 1170 Dph upon tempering at 250°C when monoclinic Hägg carbide (Fe₅C₂) formed. Cementite (Fe₃C) was the only other carbide detected in the tempered deposits, and it formed only at 475°C and above. The columnar grains of the sputter deposits transformed to equiaxed grains upon tempering above 250°C. This change in grain structure was due to recovery and not recrystallization. Some grain growth occurred in the 0.06, 0.18, 0.66, and 2 wt pct C deposits above 300°C, but the grain size of the 3 and 5 wt pct C deposits remained submicron. The hardnesses of the deposits after tempering at 550°C increased with carbon content, the 5 wt pct C deposit having the highest hardness (960 Dph) and the 0.06 wt pct C deposit the lowest (360 Dph).     

Heat Treatment of Thixo-Formed Hypereutectic X210CrW12 Tool Steel

Steel is a particularly challenging material to semisolid process because of the high temperatures involved and the potential for surface oxidation. Hot-rolled X210CrW12 tool steel was applied as a feedstock for thixoforming. The samples were heated up to 1525?K (1250?°C) to obtain 30?pct of the liquid phase. They were pressed in the semisolid state into a die preheated up to 473?K (200?°C) using a device based on a high-pressure die casting machine. As a result, a series of main bucket tooth thixo-casts for a mining combine was obtained. The microstructure of the thixo-cast consisted of austenite globular grains (average grain size 46 ??m) surrounded by a eutectic mixture (ferrite, austenite, and M₇C₃ carbides). The average hardness of primary austenite grains was 470?HV_0.02 and that of eutectic 551?HV_0.02. The X-ray analysis confirmed the presence of 11.8?pct ??-Fe, 82.4?pct ??-Fe, and 5.8?pct M₇C₃ carbides in the thixo-cast samples. Thermal and dilatometric effects were registered in the solid state, and the analysis of curves enabled the determination of characteristic temperatures of heat treatment: 503?K, 598?K, 693?K, 798?K, 828?K, 903?K, and 953?K (230?°C, 325?°C, 420?°C, 525?°C, 555?°C, 630?°C, 680?°C). The thixo-casts were annealed at these temperatures for 2?hours. During annealing in the temperature range 503?K to 693?K (230?°C to 420?°C), the hardness of primary globular grains continuously decreased down to 385HV_0.02. The X-ray diffraction showed a slight shift of peaks responsible for the tension release. Moreover, after the treatment at 693?K (420?°C), an additional peak from precipitated carbides was observed in the X-ray diffraction. Thin plates of perlite (average hardness 820?HV_0.02) with carbide precipitates appeared at the boundaries of globular grains at 798?K (525?°C). They occupied 17?pct of the grain area. Plates of martensite were found in the center of grains, while the retained austenite was observed among them (average hardness of center grains was 512?HV_0.02). A nearly complete decomposition of metastable austenite was achieved after tempering at 828?K (555?°C) due to prevailing lamellar pearlite structure starting at grain boundaries and the martensite located in the center of the grains. The X-ray analysis confirmed the presence of 3.4?pct ??-Fe, 84.6?pct ??-Fe, and 12?pct M₇C₃ carbides. The dilatometric analysis showed that the transformation of metastable austenite into martensite took place during cooling from 828?K (555?°C). The additional annealing at 523?K (250?°C) for 2?hours after heat treatment at 828?K (555?°C) caused the precipitation of carbides from the martensite. After tempering at 903?K (630?°C), the thixo-cast microstructure showed globular grains consisting mainly of thick lamellar perlite of the average hardness 555?HV_0.02.     

Enhancement of the mechanical properties of a low-carbon, low-silicon steel by formation of a multiphased microstructure containing retained Austenite

Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.     

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

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

Effect of Tempering Time on Microstructure, Tensile Properties, and Deformation Behavior of a Ferritic Light-Weight Steel

In the present study, a ferritic light-weight steel was tempered at 973 K (700 °C) for various tempering times, and tensile properties and deformation mechanisms were investigated and correlated to microstructure. ??-carbides precipitated in the tempered band-shaped martensite and ferrite matrix, and the tempered martensite became more decomposed with increasing tempering time. Tempering times for 3 days or longer led to the formation of austenite as irregular thick-film shapes mostly along boundaries between the tempered martensite and the ferrite matrix. Tensile tests of the 1-day-tempered specimen showed that deformation bands were homogeneously spread throughout the specimen, and that the fine carbides were sufficiently deformed inside these deformation bands resulting in high strength and ductility. The 3-day-tempered specimen showed a small amount of boundary austenite, which readily developed voids or cracks and became sites for fracture. This cracking at boundary austenites became more prominent in the 7- and 15-day-tempered specimens, as the volume fraction of boundary austenites increased with increasing tempering time. These findings suggested that, when the steel was tempered at 973 K (700 °C) for an appropriate time, i.e., 1 day, to sufficiently precipitate ??-carbides and to prevent the formation of boundary austenites, that the deformation occurred homogeneously, leading to overall higher mechanical properties.