Showing papers on "Bainite published in 2006"

Quenching and partitioning martensite-a novel steel heat treatment

Abstract: A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.

Effect of block size on the strength of lath martensite in low carbon steels

Abstract: The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels.

Mechanical stabilisation of austenite

Abstract: A theory has been developed for the mechanical stabilisation of plastically deformed austenite by balancing the force which drives the transformation interface against the resistance from dislocation debris in the austenite. The work has been used to explain why very large strains are required to mechanically stabilise certain stainless steels, and also to interpret the subunit mechanism of bainite growth.

Influence of alloying elements on the kinetics of strain-induced martensitic nucleation in low-alloy, multiphase high-strength steels

Abstract: Low-alloy multiphase transformation-induced-plasticity (TRIP) steels offer excellent mechanical properties in terms of elongation and strength. This results from the complex synergy between the different phases, i.e., ferrite, bainite, and retained austenite. The precise knowledge of the austenite-to-martensite transformation kinetics is required to understand the behavior of TRIP steels in a wide array of applications. The parameters determining the stability of the metastable austenite were reviewed and investigated experimentally, with special attention paid to the effect of the chemical composition, the temperature, and the size of the austenite particles. The results show that the stability and rate of transformation of the austenite particles in TRIP steels have a pronounced composition dependence: austenite particles transform at a faster rate in CMnSi TRIP steel than in TRIP steels in which Si is fully or partially replaced by Al and P. The results clearly support the view that (1) both a high C content and a submicron size are required for the room-temperature stability of the austenite particles and (2) the effect of the chemical composition on the transformation is due to its influence on the intrinsic stacking-fault energy. In addition, the composition dependence of the Md 30 temperature was derived by regression analysis of experimental data. The influence of the size of the retained austenite particles on their Ms σ temperature was studied by means of a thermodynamic model. Both the analysis of the transformation-kinetics data and the microstructural analysis by transmission electron microscopy revealed the very limited role of autocatalysis in the transformation.

Crystallography of upper bainite in Fe-Ni-C alloys

Abstract: The upper bainite structures were investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys transformed at temperatures between 723 and 623 K. A bainite packet similar to lath martensite is formed and is partitioned into blocks containing lath-shaped bainitic ferrite with the same parallel close-packed plane relationship of the K–S relationship. Its feature is categorized into three types: (A) a packet contains laths of two K–S variants with a small misorientation (sub-blocks), (B) a packet is divided by blocks largely misoriented and each block contains sub-blocks and (C) a packet is divided by blocks containing a single variant of laths. A packet of type (A) is formed at 723 K in each alloy whereas the packet type becomes (B) or (C) at lower temperatures. As a result, bainite blocks are refined with decreases in transformation temperature and carbon content. In contrast, blocks and packets of lath martensite are refined with increasing carbon content.

Effects of deformation and boron on microstructure and continuous cooling transformation in low carbon HSLA steels

Abstract: The continuous-cooling-transformation (CCT) diagram and continuous cooled microstructure were investigated for low carbon (0.05 wt.% C) high strength low alloy steels with/without boron. Microstructures observed in continuous cooled specimens were composed of pearlite, quasi-polygonal ferrite, granular bainite, acicular ferrite, bainitic ferrite, lower bainite, and martensite depending on cooling rate and transformation temperature. A rapid cooling rate depressed the formation of pearlite and quasi-polygonal ferrite, which resulted in higher hardness. However, hot deformation slightly increased transformation start temperature, and promoted the formation of pearlite and quasi-polygonal ferrite. Hot deformation also strongly promoted the acicular ferrite formation which did not form under non-deformation conditions. Small boron addition effectively reduced the formation of pearlite and quasi-polygonal ferrite and broadened the cooling rate region for bainitic ferrite and martensite.

Effect of TiN inclusions on the impact toughness of low-carbon microalloyed steels

Abstract: Microalloying with various elements, including titanium, coupled with thermomechanically controlled processing, has become a major technology for the manufacture of high-quality steel plate. In this research, the influence of TiN inclusions on the impact toughness of low-carbon plate steels microalloyed with titanium, vanadium, and boron was investigated. The three experimental steels had Ti/N ratios of 2.44, 3.5, and 4.2, and all three had a granular bainite microstructure. However, Charpy V-notch testing showed that steel A had very high toughness at both room temperature and −20 °C, whereas steels B and C showed very low toughness at −20 °C and moderate toughness at room temperature. Scanning electron microscope fractography revealed that coarse TiN inclusions had acted as cleavage fracture initiation sites in steels B and C. The effect of Ti and N levels on TiN formation and growth is analyzed using alloy thermodynamics. It is shown that not only is the Ti/N ratio important, but also the product of total Ti and N plays a most important role in TiN formation and growth. It is concluded that the product of the total Ti and N contents should not be greater than the solubility product of TiN at the solidus temperature to prevent the precipitation of TiN particles before solidification. Furthermore, the ratio of Ti to N should also be maintained lower than the stoichiometric ratio of 3.42 to ensure a low coarsening rate for the TiN inclusions during soaking before rolling.

Effects of Hot Deformation and Subsequent Austempering on the Mechanical Properties of Si-Mn TRIP Steels

Abstract: In the present paper, effects of hot deformation and subsequent austempering on the mechanical properties of hot rolled Si–Mn TRIP steels were investigated. Thermomechanical controlled processing (TMCP) was conducted by using a laboratory hot rolling mill, in which three different kinds of finish rolling reduction, temperatures and austemperings with various isothermal holding duration were applied. The results have shown that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. Ultimate tensile strength, yield strength and total elongation increase with increasing the amount of deformation and decreasing finish rolling temperature due to the stabilization of retained austenite. Tensile strength and total elongation can reach the maximum values (791 MPa and 36%, respectively), and isothermal holding 20 min at 400°C after hot deformation has been proved to be the optimum treatment.

High strength microalloyed CMn(V–Nb–Ti) and CMn(V–Nb) pipeline steels processed through CSP thin-slab technology: Microstructure, precipitation and mechanical properties

Abstract: Compact strip production (CSP) technology is an important upcoming processing route to produce low cost microalloyed high strength pipeline steels that meet the API standards. Hot strips of CMn(VNbTi) and CMn(VNb) steel grades with fine-grained ferrite–pearlite microstructure and small volume fraction of lower transformation product (non-polygonal ferrite: acicular ferrite/bainite) were produced using CSP technology with high strength and excellent low-temperature toughness up to −60 °C. For strip thicknesses between 6 and 12.5 mm, yield strength levels of up to 590 MPa and tensile strength levels up to 680 MPa were achieved. The CMn(VNb) steel exhibited outstanding notch-toughness in the range of 200 and 400 J/cm 2 , in spite of its higher yield strength (∼100 MPa or greater) over the CMn(VNbTi) steel. The precipitates present in CMn(VNbTi) and CMn(VNb) steels were characterized in terms of morphology, size and chemistry, and crystallography. The microalloying elements, Ti, Nb, and V form M 4 C 3 type of carbides in the ferrite matrix of both the steels. The multi-microalloying approaches of CMn(VNbTi) and CMn(VNb) results in the formation of duplex and triplex carbonitrides, respectively. The results of the development effort are described.

Design of Advanced Bainitic Steels by Optimisation of TTT Diagrams and T0 Curves

Abstract: Cementite is responsible of the limited application of conventional bainitic steels, however it has been proof that cementite precipitation during bainite formation can be suppressed by the judicious use of silicon in medium carbon steels In this work, thermodynamic and kinetic models were used to design steels with an optimum bainitic microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite and some martensite Using these models, a set of seven carbide free bainitic steels with a 03 wt% carbon content were proposed for manufacturing The work presented here is concerned with the microstructural and mechanical characterisation of the steels manufactured Except for the steel with the highest content of alloying elements, all the grades present the same microstructure composed of carbide-free upper bainite and retained austenite after hot rolling and a two-steps cooling Theirs tensile strengths range from 1600 to 1950 MPa while keeping a uniform elongation equal to 4% and a total elongation over 10% Regarding toughness at room temperature, they match quenched and tempered martensitic steels

Continuous cooling transformation diagrams and properties of micro-alloyed TRIP steels

Abstract: Continuous cooling transformation (CCT) diagrams and properties of four kinds of low-silicon C–Mn–Si–Al transformation-induced plasticity (TRIP) steels with different carbon contents, with or without microalloy element Ti/V, as well as a reference TRIP steel containing 1.19 wt.% Si were studied. The CCT diagrams exhibited that as the carbon equivalent (CE) increased, it caused a shift of the ferrite forming and pearlite forming temperatures to the right side and the bainite forming and martensite forming to the lower temperatures of the diagram. The microstructural evolution obtained from the dilatometry samples revealed that the highest cooling rates produced fully martensitic microstructure in all cases except the reference TRIP steel. As the cooling rate decreased, more ferrite and bainite were formed. The increase of CE caused the increase of the amount of martesite in the microstructure. Tensile test and Erichsen test of the investigated steels showed an excellent mechanical strength and ductility combination, with tensile strength between 800 and 1000 MPa, total elongation of around 20%, and a quite good formability with a dome height of about 10 mm in all cases.

Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals: Part 1 – Effect of nickel content

Abstract: The effects of increasing the nickel content from 3 to 7 or 9 wt-% were investigated in high strength steel weld metals with 2 wt-% manganese. Nickel additions were beneficial to strength but detrimental to impact toughness. Significant segregation of nickel and manganese to interdendritic regions was observed at the two higher nickel contents. In these weld metals a mainly martensitic microstructure developed at interdendritic regions, whereas bainite was found at dendrite core regions. The microstructural inhomogeneity was due to segregation and the accompanying stabilisation of austenite in solute enriched regions to lower transformation temperatures. With 3 wt-% nickel the microstructure was found to be more homogeneous, with mainly bainite forming. The decrease in impact toughness with increasing nickel content was mainly attributed to the formation of coarse grained coalesced bainite.

The incomplete transformation phenomenon in steel

Abstract: The incomplete transformation (ICT) phenomenon is defined as the temporary cessation of ferrite formation (in the absence of carbide precipitation at α:γ boundaries) before the fraction of austenite transformed to ferrite predicted by the Lever rule is attained. The ICT phenomenon is central to the “overall reaction kinetics” definition of bainite but plays lesser roles in the quite different groups of phenomena comprising the “surface relief” and “generalized microstructural” definitions. Experimental generalizations that can be made about the ICT are briefly noted. Effects of alloying elements, X, upon various aspects of the nucleation and growth of ferrite are listed in order of apparently increasing strength. The ICT is seen to be one of the stronger effects in the latter spectrum. Theories of the ICT are then critically examined. The currently most promising theories involve (1) the cessation of growth induced by the coupled-solute drag effect (C-SDE), accentuated by the overlap of the carbon diffusion fields associated with adjacent ferrite crystals; and (2) the concepts of item (1) plus local alloying element partition between ferrite and austenite (LE-NP), thereby making any further ferrite growth require volume diffusion of X in austenite and thus to take place exceedingly slowly. Distinguishing between these theories will require use of an Fe-C-X system in which the temperature-carbon concentration paths of the paraequilibrium (PE) Ae3 and of the “no partition” boundary are well separated. Although the Fe-C-Mo system has proved convenient for studying many aspects of the ICT phenomenon, it does not fulfill this specification. Fe-C-Mn alloys do so and should be particularly useful subjects for future investigations of the ICT phenomenon.

Bimodal size-distribution of bainite plates

Abstract: There are two well-known phenomena associated with the bainite reaction, both of which have been exploited here to enhance the mechanical behaviour of steel. Firstly, the bainite plate size decreases as the transformation temperature is reduced. Secondly, it is bad to have large regions of untransformed austenite in the microstructure; this is because they can transform, under the influence of external stress, into corresponding large regions of untempered, brittle martensite. By adopting a two-stage heat treatment in which coarse bainite is produced by isothermal transformation at a high temperature, followed by isothermal transformation at a lower temperature, it has been possible to eliminate blocks of austenite. This induces a microstructure containing an organized dispersion of fine plates of bainitic ferrite in the regions between the coarse plates. The mechanical properties of this mixture are shown to be better than those of bainite obtained by transformation at any single temperature. The experiments have been conducted in the context of very strong steels, where the strength and hardness can exceed 2.5 GPa and 650 HV, respectively.

Micromechanical characterisation of TRIP-assisted multiphase steels by in situ neutron diffraction

Abstract: The flow behaviour of the constitutive phases in multiphase steels, possibly exhibiting a mechanically-induced phase transformation ( TRIP effect), is investigated using neutron diffraction conducted during uniaxial tensile loading. The BCC and FCC lattice strains of several specimens containing different amounts ferrite, bainite, martensite and metastable retained austenite are measured along elastic and plastic deformation. The validity of the measurements, as well as the strengthening resulting from the TRIP effect, are evaluated on the basis of overall mechanical equilibrium.

TRIP-assisted steels: cracking of high-carbon martensite

Abstract: Modern TRIP assisted steels contain retained austenite with carbon concentrations in excess of 1 wt-%. Some of their mechanical properties, in particular the toughness and ductility, rely on the diffusionless transformation of this austenite into high-carbon martensite, induced by stress and strain. The properties can be excellent in spite of the fact that freshly formed high-carbon martensite is brittle. This contradictory behaviour has yet to be explained. In the present paper, the authors propose and show experimentally that the tendency of the martensite to crack in a mixed microstructure of austenite and martensite depends on its absolute size. It is demonstrated that in these mixtures, it is more difficult to crack fine martensite. It is the fine scale of the retained austenite in TRIP assisted steels that permits the martensite to be tolerated without endangering their mechanical properties.

Microstructures and stability of retained austenite in TRIP steels

Abstract: Transformation induced plasticity (TRIP) steels exhibit a combination of high strength and ductility due to their multi-phase microstructure, including ferrite, bainite and retained austenite which transforms to martensite under the external stress. The characterization of microstructures is necessary for understanding the relationship between microstructure and property. In the present work, an effort to determine the volume fraction of various phases was made for the conventional TRIP steel containing silicon. The microstructures in the TRIP steel were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, especially, an effective method was developed to identify multi-phase microstructures by atomic force microscopy based on the height difference. Furthermore, the stability of retained austenite determining TRIP effect was evaluated by electrical resistance tests and tensile tests. The results show that retained austenite does not generate martensitic transformation at −80 °C and exhibits a good thermodynamic stability, and the transition temperature from stress-induced martensitic transformation to strain-induced martensitic transformation is determined as about −5 °C, and thus strain-induced martensitic transformation over −5 °C (somewhat lower than room temperature) is favorable for the application of TRIP steels in the automobile industry.

Quench and partitioning steel: a new AHSS concept for automotive anti-intrusion applications

Abstract: A new type of high strength, high toughness, martensitic steel, based on a newly proposed Quench and Partitioning (Q&P) process, is presented. This high strength martensitic grade is produced by the controlled low temperature partitioning of carbon from as-quenched martensite laths to retained inter-lath austenite under conditions where both low temperature transition carbide formation and cementite precipitation are suppressed. The contribution focuses on both the current understanding of the fundamental processes involved and includes a discussion of the technical feasibility of large-scale industrial production of these steels as sheet products. The Q&P process, which is carried out on steels with a lean composition, should be implemented easily on some current industrial continuous annealing and galvanizing lines. In addition, martensitic Q&P sheet steel is characterized by very favourable combinations of strength, ductility and toughness, which are particularly relevant for high strength anti-intrusion automotive parts.

Low alloy steel

Abstract: According to a low alloy steel of the present invention, compositional elements thereof are limited, and a metal structure thereof comprises bainite or martensite. Further, a proper amounts of Nd inclusions are formed by appropriately selecting timings of deoxidation and Nd addition in melting a steel. Consequently, compatibility between high-temperature creep strength and long-term creep ductility, which is hardly established in conventional steels, can be achieved even in hostile conditions. Accordingly, the low alloy steel of the present invention can be widely applied as the material for the heat-resistant structural member used for a long time under the high-temperature and high-pressure conditions such as power plant boilers, turbines, and nuclear power plants.

Transmission electron microscopy of martensite/austenite islands in pipeline steel X70

Abstract: The fine structure of martensite/austenite (M/A) islands in pipeline steel X70 was investigated using transmission electron microscopy. It was shown that the microstructure of the steel undergoing thermo-mechanical controlled processing, comprises a majority of irregular blocky ferrite and acicular ferritic constituents and a small amount of so-called M/A islands which in turn consisted of retained austenite and martensite plates different in size and orientation. Microtwins and midrib are usually formed within a martensite plate, which is an indication of the occurrence of high-carbon martensite. It was in addition found that the amount of the M/A islands decreased slightly with increasing the cooling rate after the hot deformation, and the morphology of the M/A islands changed from thick, irregular long strip to thin, short rods. The holding temperature after the fast cooling had no evident effect on the amount of the M/A islands, though lowering the holding temperature could yield a finer and more dispersive M/A islands.

Study of the austenite quantification by X-ray diffraction in the 18Ni-Co-Mo-Ti maraging 300 steel

Abstract: In this paper, quantifications of the austenitic phase in a maraging 300 steel heat treated at different temperatures and periods of time were carried out using the direct comparison method by X-ray diffraction. The influence of taking into account the chemical compositions of austenite and martensite phases in the results by the direct comparison method was evaluated. In order to analyze the instability of austenite under plastic deformation, the quantifications were carried out with and without previous grinding of the samples. The behavior of the austenite volume fraction against aging time at 560°C, 600°C and 650°C were determined. The variation of the martensite lattice parameter with aging time was also analyzed. The results show an increase of the austenite content with aging time at 560°C and 600°C. At 650°C, however, the austenite content present at room temperature decreases and the martensite parameter increases with the aging time above 1 h.

Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals: Part 3 – Increased strength resulting from carbon additions

Abstract: Neural network predictions suggested that the strength of a high strength steel weld metal with 7 wt-% nickel and 0·5 wt-% manganese could be increased significantly at moderate expense to impact toughness by additions of carbon. Based on this, three experimental weld metals were produced with carbon contents between 0·03 and 0·11 wt-%. Mechanical test results were in agreement with predictions. At low carbon content the microstructure was largely bainitic in dendrite core regions whereas martensite was found at interdendritic regions. From microstructural studies and dilatometry experiments it was found that carbon stabilised austenite to lower transformation temperatures and that the microstructure became more martensitic in nature. Effects on strength and impact toughness were explainable in terms of a refinement of the microstructure and tempering behaviour.

Tensile behavior of functionally graded steels produced by electroslag remelting

Abstract: Tensile behavior of functionally graded steels produced by electroslag refining has been studied. Functionally graded steels containing layers of ferrite, austenite, bainite, and martensite may be fabricatedvia diffusion of alloying elements during electroslag remelting. Tensile strength of the composites depends on the composition and number of layers and those have been modeled based on the tensile behavior of individual phases. The yield stress of each element in the composites is related to the microhardness value of that element. Considering the Holloman relation for the true stress-strain behavior of each element, the tensile strength of an individual element is also related to the microhardness value of that element. By applying the rule of mixtures, the tensile strengths of the composites were determinedvia the numerical method. The obtained results are in good agreement with the experimental results.

Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals Part 2 - Impact toughness gain resulting from manganese reductions

Abstract: Two experimental high strength steel weld metals were produced with 7 wt-% nickel and either 2 or 0·5 wt-% manganese. Neural network predictions that it is advantageous to reduce the manganese concentration in high nickel alloys have been confirmed, with impact energy increasing from 32 to 113 J at −40°C. High resolution microstructural investigations showed that both weld metals contained mainly martensite at interdendritic regions and predominantly bainite at dendrite core regions, as a consequence of manganese and nickel segregation. In the high manganese weld metal significant amounts of coarse grained coalesced bainite formed whereas mainly upper bainite was seen with 0·5 wt-% manganese. Reducing manganese content increased the transformation temperature, promoting fine upper bainite at the expense of coarse coalesced bainite. Increased toughness was attributed to the finer grain size of bainite constituents and a more effectively tempered microstructure.