Showing papers on "Bainite published in 2010"

Effect of microstructure and inclusions on hydrogen induced cracking susceptibility and hydrogen trapping efficiency of X120 pipeline steel

Abstract: The susceptibility to hydrogen induced cracking (HIC) was evaluated for X120 steels containing different amounts of Mn and Al in a H2S environment. The hydrogen trapping efficiency was investigated by measuring the permeability (J∞L) and the apparent diffusivity (Dapp). The results demonstrated that larger amount of the inclusions, and larger area and volume fraction of the inclusions make steels more susceptible to HIC. The steel with a microstructure consisting of granular bainite and M/A (martensite/austenite) microconstituents is more susceptible to HIC. The ability of the microstructure and the inclusions to trap hydrogen was explained in terms of the apparent diffusivity (Dapp), permeability (J∞L), and solubility of hydrogen in steels (capp). The lower the values of Dapp and J∞L and the higher the value of capp are, the more the hydrogen trapping occurs in the steel, and the more the steel is susceptible to HIC.

New low carbon Q&P steels containing film-like intercritical ferrite

Abstract: In this work, the application of the Quenching and Partitioning (Q&P) process to two low-carbon steels has led to the development of a new kind of steel microstructure formed by laths of martensite separated by films of intercritical ferrite and retained austenite. The chemical compositions of the steels have been specially designed for this process, containing 3.5 wt.% Mn to retard the formation of bainite and combinations of Si and Al to avoid cementite precipitation. The microstructural changes occurring during the application of the heat treatments are discussed in terms of the current knowledge of the Q&P process and the experimental observations. A significant amount of retained austenite has been obtained in both steels after application of appropriate heat treatments, especially in the steel alloyed with higher amount of Si, in which the volume fraction of retained austenite reached values up to 0.19. Tensile tests in some selected specimens of both materials have shown outstanding combinations of strength and ductility, indicating that the designed Q&P steels are a promising candidate for the development of a new generation of advanced high strength steels.

Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn1-xCoGe alloys

Abstract: It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0≤x≤0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field–induced martensitic transformation and a giant magnetocaloric effect in the Mn1−xCoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.

Thermal stability of austenite retained in bainitic steels

Abstract: Steels with a microstructure consisting of a mixture of bainitic ferrite and carbon-enriched retained austenite are of interest in a variety of commercial applications because they have been shown to exhibit good combinations of strength, toughness and ductility. However, their use at temperatures moderately above ambient requires a knowledge of the thermal stability of the austenite. The changes that occur during the tempering of a mixture of bainitic ferrite, carbon-enriched retained austenite and martensite have been characterised. An analysis of the volume change due to transformation shows that it is possible to distinguish the decomposition of austenite from the tempering of martensite. The nature of the carbides that form during the heat treatment is discussed as are the implications on the development of mathematical models accounting for calculating the strain during austenite decomposition and martensite tempering. It is found that the early stages of tempering reactions where the austenite content is not greatly reduced, can dramatically influence the stability of the austenite as it is cooled to ambient temperature.

Delamination Effect on Impact Properties of Ultrafine-Grained Low-Carbon Steel Processed by Warm Caliber Rolling

Abstract: Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500 °C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong α-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 μm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100 °C to −40 °C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.

New Trends in Advanced High Strength Steel Developments For Automotive Application

Abstract: The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.

On the Significance of Nature of Strain-Induced Martensite on Phase-Reversion-Induced Nanograined/Ultrafine-Grained Austenitic Stainless Steel

Abstract: We recently described the reversal of strain-induced martensite to the parent austenite phase in the attempt to produce nanograins/ultrafine grains via controlled annealing of heavily cold-worked metastable austenite. The phase-reversion-induced microstructure consisted of nanocrystalline (d < 100 nm), ultrafine (d ≈ 100 to 500 nm), and submicron (d ≈ 500 to 1000 nm) grains and was characterized by high strength (800 to 1000 MPa)–high ductility (30 to 40 pct) combination, which was a function of cold deformation and temperature-time annealing sequence.[1] In this article, we demonstrate that the success of the approach in obtaining nanograined/ultrafine-grained (NG/UFG) structure depends on the predominance of dislocation-cell–type structure in the severely deformed martensite. Electron microscopy and selected area electron diffraction analysis indicated that with an increase in the degree of cold deformation there is transformation of lath martensite to dislocation-cell–type martensite, which is a necessary prerequisite to obtain phase-reversion-induced NG/UFG austenite. The transformation of lath-type to dislocation-cell–type martensite involves refinement of packet and lath size and break up of lath structure. Based on detailed and systematic electron microscopy study of cold-deformed metastable austenite (~45 to 80 pct deformation) and constant temperature-time annealing sequence, when the phase reversion kinetics is rapid, our hypothesis is that the maximization of dislocation-cell–type structure in lieu of lath-type facilitates NG/UFG structure with a high strength–high ductility combination. Interestingly, the yield strength follows the Hall–Petch relation in the NG/UFG regime for the investigated austenitic stainless steel.

Softening Kinetics in the Subcritical Heat-Affected Zone of Dual-Phase Steel Welds

Abstract: Welds in dual-phase (DP) steels exhibit heat-affected zone (HAZ) softening in which the tempered or subcritical HAZ exhibits a lower hardness vs that of the parent material. The rate of this softening reaction with respect to welding heat input was determined for four DP steels by making several bead-on-plate laser welds using a variety of heat inputs and measuring the resulting minimum hardness. The reduction in hardness was then fit to the Avrami equation, enabling a comparison of the relative heat needed to soften each steel. It was found that the heat input required for HAZ softening decreased as the C content of the martensite within the DP structure increased. However, the presence of carbide forming alloying elements such as Cr and Mo was able to increase resistance to softening.

Processing Opportunities for New Advanced High-Strength Sheet Steels

Abstract: Currently there is considerable interest in developments leading to new advanced high-strength sheet steels (AHSS) for automotive and other transportation applications that demand high strength, light weight materials. Design requirements will involve material properties with strengths greater than currently available dual phase and transformation-induced plasticity (TRIP) steels (steels in this group are referred to as the “First Generation” of AHSS), with good ductility and formability, but produced at a cost less than the high ductility stainless steels or high manganese TWIP steels (materials referred to as the “Second Generation” of AHSS). Recent studies have shown that materials that satisfy the required property/cost combinations will include complex microstructures containing high amounts of retained austenite in combination with a high strength constituent that may be ultrafine grained ferrite, martensite, bainite, and/or combinations of ferrite-based constituents. In this article, selected metho...

Bainitic transformation during the two-step quenching and partitioning process in a medium carbon steel containing silicon

Abstract: A study of 40SiMnNiCr steel subjected to a two-step quenching and partitioning process (Q&P) is presented. The result suggests that strength variation of Q&P steels during the two-step Q&P process was a cumulative effect of increase of retained austenite fraction, decrease of carbon supersaturation of virgin martensite, and particularly much of lower bainite formation. A trade-off between high strength and good ductility of two-step Q&P steels can be tailored and adjusted by controlling lower bainite fraction. The final amount of austenite was influenced by the transformation kinetics of lower bainite during the partitioning process.

Effects of Cooling Conditions on Tensile and Charpy Impact Properties of API X80 Linepipe Steels

Abstract: In this study, four API X80 linepipe steel specimens were fabricated by varying the cooling rate and finish cooling temperature, and their microstructures and crystallographic orientations were analyzed to investigate the effects of the cooling conditions on the tensile and Charpy impact properties. All the specimens consisted of acicular ferrite (AF), granular bainite (GB), and martensite-austenite (MA) constituents. The volume fraction of MA increased with an increasing cooling rate, and the volume fraction and size of MA tended to decrease with an increasing finish cooling temperature. According to the crystallographic orientation analysis data, the effective grain size and unit crack path decreased as fine ACs having a large amount of high-angle grain boundaries were homogeneously formed, thereby leading to the improvement in the Charpy impact properties. The specimen fabricated with the higher cooling rate and lower finish cooling temperature had the highest upper-shelf energy (USE) and the lowest energy transition temperature (ETT), because it contained a large amount of MA homogeneously distributed inside fine AFs, while its tensile properties remained excellent.

Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel

Abstract: The effect of tempering on the microstructure and mechanical properties of a non-quenched (NQ) bainitic steel was investigated by optical microscopy, X-ray diffractometry, scanning electronic microscope (SEM) and transmission electronic microscopy (TEM). The results show that the NQ steel which has been investigated is granular bainite composed of bainitic ferrite lath, retained austenite film and island of austenite and martensite (M/A island) before tempering. The amount of retained austenite decreases with the rise of tempering, slowly before 400 °C, sharply at 450 °C, and it is close to 0 at 600 °C. When tempered at 350 °C, this kind of NQ steel has the optimum mechanical properties because of M/A islands partly decomposition, especially for the martensite in the M/A islands. However, when tempered at 450 °C, it appears brittleness, which results from carbides distributing along prior austenite grains because M/A island is largely decomposed.

An approach to define the effective lath size controlling yield strength of bainite

Abstract: A fully bainitic microstructure with negligible carbide precipitation is obtained in two ultra-low carbon steels. The size and misorientation of bainite laths are analysed by Electron Back Scattering Diffraction (EBSD). It is found that the yield stress of bainite is proportional to the inverse lath size defined with low boundary misorientation (2–7°). This can be explained by a theory predicting the flow stress of deformed metals, assuming that both lath boundary and dislocation cell boundary have similar capability of being dislocation obstacles.

Effect of Substructure on Toughness of Lath Martensite/Bainite Mixed Structure in Low-Carbon Steels

Abstract: The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM), in order to research the effect on toughness. The test results indicate that the toughness of the steel is enhanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38 µm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstructural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is “the effective grain size” for controlling toughness.

High-strength steel sheet, hot-dipped steel sheet, and alloy hot-dipped steel sheet that have excellent fatigue, elongation, and collision characteristics, and manufacturing method for said steel sheets

Abstract: This high-strength steel sheet includes: in terms of percent by mass, 0.03 to 0.10% of C; 0.01 to 1.5% of Si; 1.0 to 2.5% of Mn; 0.1% or less of P; 0.02% or less of S; 0.01 to 1.2% of Al; 0.06 to 0.15% of Ti; and 0.01% or less of N; and contains as the balance, iron and inevitable impurities, wherein a tensile strength is in a range of 590 MPa or more, and a ratio between the tensile strength anda yield strength is in a range of 0.80 or more, a microstructure includes bainite at an area ratio of 40% or more and the balance being either one or both of ferrite and martensite, a density of Ti(C,N) precipitates having sizes of 10 nm or smaller is in a range of 1010 precipitates/mm3 or more, and a ratio (Hvs/Hvc) of a hardness (Hvs) at a depth of 10 [mu]m from a surface to a hardness (Hvc) ata center of a sheet thickness is in a range of 0.85 or more.

Effect of Martensite in Initial Structure on Bainite Transformation

Abstract: Lath-shaped upper bainite structures play a very important role in many high-strength steels (HSSs) and ultra high-strength steels (UHSSs). Although bainite transformation is strongly affected by the initial structure, the effect of the second phase in a multi-phase structure is yet to be clearly understood. It is significant for the advancement of UHSS to study this effect. The aim of this study is to clarify the effect of martensite, which forms before bainite, in Fe-0.2C-8Ni alloy. The bainite transformation from an austenite and martensite dual-phase structure is faster than that from single-phase austenite and the nucleation of bainitic ferrite laths are accelerated around martensite. This effect of martensite on bainite kinetics is equivalent to that of polygonal ferrite when their volume fractions are almost the same. This suggests that the boundary between martensite and austenite is a prior nucleation site of bainitic ferrite. Martensite also affects the crystallographic features of bainite. The orientations of bainitic ferrite laths tend to belong to the same block with martensite adjacent. This tendency intensifies with an increase of the transformation temperature of bainite, resulting in the formation of huge blocks consisting of bainitic ferrite and martensite laths at high temperatures (693K and 723K). In contrast, at a low temperature (643K), bainitic ferrite laths belong to same packet as martensite and have several orientations. This change of crystallographic features with transformation temperature can explain with the driving force of the nucleation of bainitic ferrite.

HIGH-STRENGTH STEEL SHEET HAVING EXCELLENT HYDROGEN EMBRITTLEMENT RESISTANCE AND MAXIMUM TENSILE STRENGTH OF 900 MPa OR MORE, AND PROCESS FOR PRODUCTION THEREOF

Abstract: A high-strength steel sheet having excellent hydrogen embrittlement resistance and a maximum tensile strength of 900 MPa or more, characterized in that (a) 10 to 50 vol% of ferrite, 10 to 60 vol% of bainitic ferrite and/or bainite, and 10 to 50 vol% of tempered martensite exist in the structure of the steel sheet and (b) an iron-containing carbide containing 0.1% or more of Si or both of Si and Al exists in an amount of 4 × 10 8 (particles/mm 3 ) or more in the structure of the steel sheet.

Martensite start temperature and the austenite grain-size

Abstract: The dependence of the martensite start temperature on the austenite grain-size is modeled based upon aspects of martensite nucleation and validated by data reported in the literature. The results obtained support the view that grain boundaries provide an environment for the formation of martensite embryos in annealed austenite. The model can account for a two-stage dependence of martensite start temperature on the austenite grain-size, considering that grain boundaries provide a suitable environment for martensite nucleation, and grain-size also has a stabilizing effect by limiting the volume of individual martensite units.

The kinetics of bainite and martensite formation in steels during cooling

Abstract: The kinetics of bainite and martensite formation in steel 41Cr4 has been determined using dilatometry for a range of cooling rates. It is demonstrated that all fraction curves can be adequately described with a previously proposed model using a single set of input parameters. This validation shows that the model can give accurate predictions of the CCT diagram of bainitic steels provided the model can be tested against a single experimental fraction curve to determine the model parameters for the given composition.