Showing papers on "Bainite published in 2007"

Effect of martensite plasticity on the deformation behavior of a low-carbon dual-phase steel

Abstract: An experimental study has been conducted to quantify the effects of martensite plasticity on the mechanical properties of a commercial low-carbon (0.06 wt pct) dual-phase steel. The volume fraction and morphology (banded and more equiaxed) of the martensite second phase were systematically varied by control of the intercritical annealing temperature and the heating rate to this temperature. It was observed that the yield and tensile strengths were dependent on the volume fraction of martensite but not on the morphology. In contrast, the true uniform strain, fracture strain, and fracture stress were found to have a significant dependence on martensite morphology. These results were rationalized by considering an Eshelby-based model, which allowed for the calculation of the stress in the martensite islands for different morphologies and volume fractions. By comparing the stress in the martensite with an estimate of its yield stress, it was possible to rationalize the conditions under which martensite plasticity occurs. The implications of martensite plasticity affect the work hardening of the steels but most importantly the fracture properties. For conditions where martensite codeforms with the ferrite matrix, void nucleation is suppressed and the final fracture properties are dramatically improved.

Microstructure and texture of a lightly deformed TRIP-assisted steel characterized by means of the EBSD technique

Abstract: The microstructural and textural changes after a tensile strain of 10% were observed by orientation contrast measurements in a TRIP-assisted steel. On the undeformed samples it was shown that the electron back scatter diffraction (EBSD) technique could be used successfully for determining the volume fraction of the microstructural constituents bainite, ferrite and austenite, whereas after deformation only the BCC and FCC phases could be separated. The results show that the tensile strain of 10% gave rise to a drop in residual austenite content from 10 to 4%, which was also confirmed by magnetic measurements. The texture data showed only minor orientation rotations after 10% tensile strain for the BCC ferrite and bainite grains, whereas the residual austenite did show a significant texture change. By meticulously monitoring the local intra-granular misorientations it was concluded that the BCC phases (ferrite and bainite) took up the larger part of the nominal strain whereas the residual austenite primarily responded to the mechanical load by a partial (stress-induced) martensite transformation. Hence, the texture change observed in the residual austenite could be attributed to the orientation selective character of the phase transformation.

Variant Selection of Reversed Austenite in Lath Martensite

Abstract: The microstructure of partially reversed lath martensite in 13%Cr–6%Ni steel was examined by electron backscatter diffraction, and the crystallographic character of the reversed austenite is discussed in relation to the mechanism of ‘austenite memory’. Most of the reversed austenite grains had the same orientation as the original austenite matrix before martensitic transformation. However, some austenite grains had a different orientation in a twin relationship to the other major austenite grains, although all the reversed austenite grains retained a Kurdjumov–Sachs relationship to the martensite matrix. On the basis of the crystallographic relationships among the habit plane, the close packed direction of austenite and the martensite lath boundary, we suggest that the austenite variants are theoretically limited to two kinds within one packet and five kinds within one original austenite grain. In addition, we found that internal stress introduced by martensitic transformation plays an important role in determining the austenite variant: internal stress operates so that reversed austenite selects the same variant as that present in the original austenite matrix before martensitic transformation. This phenomenon is understood as the austenite memory.

Microstructural and mechanical characterization of C–Mn–Al–Si cold-rolled TRIP-aided steel

Abstract: Retained austenite characteristics and tensile properties in a 021C–177Mn–12Al–0283Si, high strength cold-rolled TRIP steel were investigated The microstructure of this steel is comprised of ferrite, bainite and retained austenite, which is obtained by controlled cooling from the intercritical annealing temperature to the isothermal bainitic holding temperature The effects of cooling rate from intercritical annealing temperature to isothermal transformation temperature, as well as effect of isothermal transformation time on microstructure and mechanical properties, were studied using optical microscopy, SEM, TEM, XRD, dilatometry and mechanical testing These studies revealed that the microstructure of the steel mainly consisted of bainitic ferrite lath matrix, blocky martensites and stable retained austenite films of 7–8 vol% When austempered at temperatures above M S temperature, the steel possessed high tensile strength of 600 MPa, total elongation of 28–32% and n -value 02 The present paper deals with the effect of heat treatment on microstructure and mechanical properties of C–Mn–Al–Si cold-rolled TRIP steel

Transformation in Stir Zone of Friction Stir Welded Carbon Steels with Different Carbon Contents

Abstract: Five types of ferrite–pearlite structure carbon steels with different carbon contents (IF steel, S12C, S20C, S35C, S50C) were friction stir welded under various welding conditions, and the mechanical properties and microstructures of the FSW carbon steel joints were evaluated. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions. When the carbon content is less than or equal to 0.12 mass%, the welding produces ferrite–pearlite structures, and the strength slightly increases compared to the base metal due to the refined microstructure; when the carbon content is above 0.2 mass%, the welding produces ferrite–pearlite plus harder phases like the martensite and bainite microstructures, resulting in a significantly increased strength of the joints. These are dependent on each of the thermal-mechanical cycles.

Crystallographic texture of stress-affected bainite

Abstract: A method is presented for calculating both the macroscopic strains and the crystallographic bias which develop when a polycrystalline sample of austenitic steel is transformed into bainite or martensite under the influence of an applied stress or a system of stresses. Any texture present in the austenite prior to transformation is taken into account, as is the detailed crystallography of the transformation. Comparisons with experimental data are encouraging. A strong correlation has been observed between the proportion of the driving force for transformation that is attributed to stress and the extent of variant selection.

Transmission Electron Microscopy Characterization of the Bake-Hardening Behavior of Transformation-Induced Plasticity and Dual-Phase Steels

Abstract: The effect of prestraining (PS) and bake hardening (BH) on the microstructures and mechanical properties has been studied in transformation-induced plasticity (TRIP) and dual-phase (DP) steels after intercritical annealing. The DP steel showed an increase in the yield strength and the appearance of the upper and lower yield points after a single BH treatment as compared with the as-received condition, whereas the mechanical properties of the TRIP steel remained unchanged. This difference appears to be because of the formation of plastic deformation zones with high dislocation density around the “as-quenched” martensite in the DP steel, which allowed carbon to pin these dislocations, which, in turn, increased the yield strength. It was found for both steels that the BH behavior depends on the dislocation rearrangement in ferrite with the formation of cell, microbands, and shear band structures after PS. The strain-induced transformation of retained austenite to martensite in the TRIP steel contributes to the formation of a complex dislocation structure.

Dynamic response of aluminium containing TRIP steel and its constituent phases

Abstract: In the automotive industry a lot of effort is put into the development of lightweight car body structures. Therefore, complex multiphase steel grades have been developed with exceptional mechanical properties: they combine high strength values (yield strength, tensile strength, etc.) with an excellent ductility. TRansformation Induced Plasticity steels (TRIP steels) show these properties pre-eminently. To guarantee a controlled dissipation of the energy released during a crash it is essential to characterize and understand the impact-dynamic material properties. In this paper, results are presented of an extensive experimental program to investigate the strain rate dependent mechanical properties of TRIP steel and its constituent phases. These different phases (ferrite, bainite and austenite) were prepared separately to obtain a clear understanding of their individual behaviour within the multiphase steel. A split Hopkinson tensile bar set-up was used for the experiments and microstructural observation techniques such as SEM and XRD revealed the mechanisms governing the observed high strain rate behaviour. The results show clearly that the excellent mechanical properties are not only preserved at higher strain rates, but they are still improved. As in the static case the ferrite phase is responsible for the large deformation in the TRIP steel and bainite causes the high strength levels. The martensite/austenite constituent is responsible for the excellent combination of high stress and strain in the TRIP steel as well as for the important strain hardening.

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

Abstract: 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. Mossbauer 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.

Microstructural Features of Austenite Formation in C35 and C45 alloys

Abstract: The microstructural evolution during continuous heating experiments has been studied for two C-Mn steels with carbon contents in the range 0.35 to 0.45 wt pct using optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). It is shown that the formation of the austenitic phase is possible in pearlite as well as in ferrite regions. Thus, a considerable overlap in time of ferrite-to-austenite and pearlite-to-austenite transformations is likely to occur. Another observation that was made during the experiments is that, depending on the heating rate, the pearlite-to-austenite transformation can proceed in either one or two steps. At low heating rates (0.05 oC/s), ferrite and cementite plates transform simultaneously. At higher heating rates (20 oC/s), it is a two-step process: first ferrite within pearlite grains transforms to austenite and then the dissolution of the cementite lamellae takes place. Several types of growth morphologies were observed during the experiments. The formation of a finger-type austenite morphology was noticed only for low and intermediate heating rates (0.05 oC/s and 20 oC/s), but not for the heating rate of 300 oC/s. The formation of this fingertype austenite occurs on pearlite-ferrite grain boundaries and coincides with the direction of cementite plates. The carbon inhomogeneities in the microstructure affect the formation of martensitic/bainitic structures on cooling.

Multiscaling in Molecular and Continuum Mechanics: Interaction of Time and Size from Macro to Nano

Abstract: Low-alloy TRIP steels are a new class of steels with excellent combinations of strength and formability, which offer a unique field for the study of multiscale effects in materials, in the sense for the understanding and the design of these steels. In the present work, models involving multiscale physical quantities are reported, which regard prediction of the stability of retained austenite and of the kinetics of its mechanically-induced transformation to martensite, optimization of the heat-treatment stages necessary for austenite stabilization in the microstructure, as well as prediction of the mechanical behaviour of these steels under deformation. Austenite stability depends on chemical composition, austenite particle size, strength of the matrix and stress state, i.e. on factors ranging from the nanoto the macro-scale. The stability of retained austenite against mechanically-induced transformation to martensite is characterized by the s M temperature, which can be derived as a function of the aforementioned multiscale factors by an appropriate model presented in this work. The kinetics of the mechanically-induced transformation of retained austenite to martensite are also dependent on multiscale factors, such as the population density of martensitic nucleation sites, the retained austenite particle size and the macroscopic level of plastic deformation. In the present work, a model describing the kinetics of this mechanically-induced transformation as a function of these factors is presented. Furthermore, the mechanical behaviour of TRIP steels also depends on the amount of retained austenite present in the microstructure, which is determined by the combinations of temperature and temporal duration of the heat-treatment stages undergone by the steel. Optimum amounts of retained austenite require optimization of the heat-treatment conditions. A physical model is presented in this work, which is based on the interactions between bainite and austenite during the heat-treatment of TRIP steels, which allows for the selection of treatment conditions leading to the maximization of retained austenite in the final microstructure. Finally, a constitutive micromechanical model is presented, which describes the mechanical behaviour of TRIP steels under deformation, taking into account the evolution of the microstructure during plastic deformation. This model is then used for the calculation of forming limit diagrams (FLD) for these complex steels, thus allowing for the optimization of stretch-forming and deep-drawing operations. * Corresponding author. E-mail address: hgreg@mie.uth.gr (G.N. Haidemenopoulo). G.C. Sih (ed.), Multiscaling in Molecular and Continuum Mechanics: Interaction of Time and Size from Macro to Nano, 161–178. 161 G.N. Haidemenopoulos*, A.I. Katsamas, N. Aravas that experimental observations and models referring to different scale levels have to be combined,

High-strength steel sheets and processes for production of the same

Abstract: A high strength steel sheet with both excellent elongation and stretch-flanging performance is provided The high strength steel sheet of the present invention comprises, in percent by mass, C: 005 to 03%, Si: 001 to 30%, Mn: 05 to 30%, Al: 001 to 01%, and Fe and inevitable impurities as the remainder, and has a structure mainly composed of tempered martensite and annealed bainite The space factor of the tempered martensite is 50 to 95%, the space factor of the annealed bainite is 5 to 30%, and the mean grain size of the tempered martensite is 10 µm or smaller in terms of the equivalent of a circle diameter The steel sheet has a tensile strength of 590 MPa or higher The high strength steel sheet of the present invention has a space factor of the martensite phase which is a main component of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 µm or smaller in terms of the equivalent of a circle diameter; in the martensite phase, the space factor of the martensite phase having a grain size of 10 µm or larger in terms of the equivalent of a circle diameter is 15% or lower; and the space factor of the retained austenite phase in the metal structure is 3% or lower The high strength steel sheet of the present invention is a dual phase steel sheet mainly composed of a ferrite phase and martensite, and the space factor of the ferrite phase is 5 to 30%, and the space factor of the martensite phase is 50 to 95% Moreover, the ferrite phase is annealed martensite

Influence of interface migration during annealing of martensite/austenite mixtures

Abstract: Tempering of martensite in the absence of carbide precipitation leads to carbon partitioning into retained austenite. If the martensite/austenite interface is assumed to remain stationary during this process, the phase compositions reach a condition that has recently been called constrained carbon equilibrium. If iron atoms are sufficiently mobile at the interface, longer partitioning times may lead to migration of the ferrite/austenite interface. The interface may be expected to move in either direction, depending on the specific details of the phase fractions and compositions controlling the chemical potential of iron at the interface. If interface migration occurs during carbon partitioning, the situation is more complicated and conditions could exist where the interface moves first in one direction and then the other.

Microstructural Refinement during Annealing of Plastically Deformed Austenitic Stainless Steels

Abstract: The phenomena of strain hardening, strain induced martensite formation, recovery, martensite reversion and recrystallization have been studied in austenitic stainless steels of the AISI 304L and 316L types, after solution annealing, followed by rolling at different temperatures (-196, 25, 100 and 200°C) and subsequent annealing of the worked samples. Strain hardening and the percentage of α’ martensite formed showed strong dependency with the deformation temperature and with the austenite chemical composition. As expected, both strain hardening as well as the amount of the martensite formed was higher in the 304L steel and for lower temperatures. Reversion temperature of the α’ martensite was close to 550°C for both steels, independent of the amount of martensite. The 316L steel presented a higher resistance to recrystallization when compared to the 304L steel. The recrystallization temperature of both steels was about 150°C higher than the α’ phase reversion temperature. Rolling temperature did not influence significantly the recrystallization temperature. Proper thermal and mechanical treatments lead to interesting combinations of mechanical properties in both steels with values such as yield strength YS of about 1000 MPa, with an elongation around 10%.

Prototype evaluation of transformation toughened blast resistant naval hull steels: Part II

Abstract: Application of a systems approach to computational materials design led to the theoretical design of a transformation toughened ultratough high-strength plate steel for blast-resistant naval hull applications. A first prototype alloy has achieved property goals motivated by projected naval hull applications requiring extreme fracture toughness (C v > 85 ft-lbs or 115 J corresponding to K Id≥ 200 ksi.in1/2 or 220 MPa.m1/2) at strength levels of 150–180 ksi (1,030–1,240 MPa) yield strength in weldable, formable plate steels. A continuous casting process was simulated by slab casting the prototype alloy as a 1.75′′ (4.45 cm) plate. Consistent with predictions, compositional banding in the plate was limited to an amplitude of 6–7.5 wt% Ni and 3.5–5 wt% Cu. Examination of the oxide scale showed no evidence of hot shortness in the alloy during hot working. Isothermal transformation kinetics measurements demonstrated achievement of 50% bainite in 4 min at 360 °C. Hardness and tensile tests confirmed predicted precipitation strengthening behavior in quench and tempered material. Multi-step tempering conditions were employed to achieve the optimal austenite stability resulting in significant increase of impact toughness to 130 ft-lb (176 J) at a strength level of 160 ksi (1,100 MPa). Comparison with the baseline toughness–strength combination determined by isochronal tempering studies indicates a transformation toughening increment of 65% in Charpy energy. Predicted Cu particle number densities and the heterogeneous nucleation of optimal stability high Ni 5 nm austenite on nanometer-scale copper precipitates in the multi-step tempered samples was confirmed using three-dimensional atom probe microanalysis. Charpy impact tests and fractography demonstrate ductile fracture with C v > 80 ft-lbs (108 J) down to −40 °C, with a substantial toughness peak at 25 °C consistent with designed transformation toughening behavior. The properties demonstrated in this first prototype represent a substantial advance over existing naval hull steels. Achieving these improvements in a single design and prototyping iteration is a significant advance in computational materials design capability.

Evolution and thermal stability of retained austenite in SAE 52100 bainitic steel

Abstract: In this work, the evolution of retained austenite during isothermal bainitic heat treatment in SAE 52100 steel, 1.01C–1.36Cr–0.32Mn–0.25Si (wt.%), is investigated with optical microscopy, X-ray diffraction and thermo-magnetic measurements. A significant amount of austenite is retained in the material. The dependence of the volume fraction of retained austenite on bainitic holding time at 503 K shows a maximum at 45 min. It has been demonstrated that the increase of volume fraction of retained austenite occurs as a result of the increased carbon concentration. The thermal stability of austenite is investigated. The temperature at which retained austenite starts to decompose to ferrite and carbides upon heating varies with bainitic holding time. The transformation of austenite to martensite during cooling till 10 K is found to be not complete, and a large amount of austenite remains untransformed.

New insights into crystallographic correlations between ferrite and cementite in lamellar eutectoid structures, obtained by SEM–FEG/EBSD and an indirect two-trace method

Abstract: Four different ferrite/cementite orientation relationships (ORs) in near-eutectoid steel are derived using SEM–FEG/EBSD (scanning electron microscopy–field emission gun/electron back-scatter diffraction) and an indirect two-trace method. They show a common feature of close-packed plane parallelism between ferrite and cementite. Their crystallographic compatibility with habit planes shows a variety of possible habit planes and excludes the existence of the exact conventional Bagaryatsky and Pitsch–Petch ORs. Each of these new ferrite/cementite ORs is correlated with a different edge-to-edge matching condition between austenite and pearlitic ferrite, and between austenite and pearlitic cementite, and possesses specific morphological features. The present results may give deep insight into the crystallography of pearlitic transformation and provide useful information for materials design through interface tailoring in steels.

Ultra-fine Bainite Structure in Hypo-eutectoid Steels

Abstract: Ultra-fine, carbide-free bainitic structure of plate thickness between 34 and 116 nm has been obtained by low temperature austempering process of two hypo-eutectoid steels with 0.42 and 0.57% C. Decreasing the carbon content results in accelerating the bainite transformation reaction together with decreasing the retained austenite content, which is known to be detrimental to the mechanical properties. Furthermore, lowering the carbon content below the eutectoid composition allowed intercritical annealing of the material which resulted in a wider window for heat treatment parameters and consequently in a spread field for mechanical properties. Dilatometric measurements were used to design the suitable heat-treatment parameters including an estimation of the required time frames for the cessation of the bainitic reaction. The structure was characterized using light optical microscopy (LOM), scanning electron microscopy (SEM) and X-ray diffractometry. In order to investigate the effect of the microstructure parameters on the materials mechanical properties, compression tests had been conducted at room temperature.

High-strength cold-rolled steel sheet and high strength hot-dip galvanized steel sheet both excellent in deep-drawability and strength-ductility balance, and producing method of the both

Abstract: PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet excellent in deep-drawability and strength-ductility balance. SOLUTION: To a hot-rolled coil having composition composed by mass% of 0.05-0.2% C, 0.1-2.0% Si, 0.005-1.5% Al, and the suitable contents of Mn, P, S, N and adjusting to 0.5-2.5% Si+Al and having bainite and martensite as the main structure: a batch hot-rolled sheet annealing process for holding the coil in the range of 550°C to Ac 1 point for ≥1 hr, a cold-rolling process and a recrystallizing annealing process, in which a continuous recrystallizing annealing for heating the cold-rolled sheet to Ac 1 to Ac 3 temperature at ≥5°C/s average temperature-raising speed in the Ac 1 point to (Ac 1 point+50°C) and holding it for ≥5s, and successively, austemper-treatment for cooling the sheet to 350-500°C at ≥5°C/s cooling speed and holding the sheet at this temperature for 10-600s, are applied, are applied in this order. In this way, the high strength cold-rolled steel sheet having dual structure composed of the ferrite having ≥70% volume ratio as the main phase and a second phase having ≤3 μm average grain diameter containing a retained austenite having at least ≥3% volume ratio and ≥1.1 r-value and ≥22,000 MPa % strength-ductility balance TS×EL, is obtained. COPYRIGHT: (C)2009,JPO&INPIT

Effect of Martensite on the Mechanical Behavior of Ferrite–Bainite Dual Phase Steels

Abstract: This paper reports study on correlations obtained between the microstructure and mechanical properties of four hot rolled ferrite–bainite dual phase steels containing 2–6% of martensite phase. It has been observed in these steels that a small amount of martensite (2% and above) is adequate to produce the continuous yielding behavior, characteristic of conventional dual phase steels. These dual phase steel contain substantial amounts of bainite (47 to 74%) and can achieve high mechanical strength coupled with adequate ductility. The value of the strain hardening exponents of such steels is however rather low. The addition of substitutional alloying elements such as Cr, Mo or V has been found to increase significantly the strength levels of such steels over that of the C–Mn–Si base composition.