Showing papers on "Bainite published in 2001"

Bainite in Steels

Abstract: The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.

On the Influence of Interactions between Phases on the Mechanical Stability of Retained Austenite in Transformation-Induced Plasticity Multiphase Steels

Abstract: The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M s temperature induced by carbon enrichment.

The Developments of Cold-rolled TRIP-assisted Multiphase Steels. Al-alloyed TRIP-assisted Multiphase Steels

Abstract: The influence of heat-treating conditions on the retention of carbon-enriched austenite of TRIP-assisted multiphase steel grades containing different amounts of silicon and/or aluminium is investigated. The ensuing mechanical properties resulting from the TRIP effect are also scrutinised. The bainite transformation kinetics was followed by dilatometry whereas a detailed characterisation of the microstructures led to the construction of transformation maps giving the volume fractions of the different phases and the carbon content of austenite. The role of silicon and aluminium additions (i) on the retention of austenite by partial bainite transformation and (ii) on the mechanical properties is enlightened. A strong influence of the solid-solution strengthening effect of silicon is highlighted. Aluminium seems to be an effective alloying element for the retention of austenite in TRIP-aided steels even if lower strength levels can be attained. A mixed Al-Si TRIP-aided steel seems to be a very good compromise between the processing needs, the required mechanical properties and the industrial constraints.

Estimation of the amount of retained austenite in austempered ductile irons using neural networks

Abstract: Many of the properties of austempered ductile iron depend on the austenite which is retained following the bainite reaction. A neural network model within a Bayesian framework has been created using published data to model the retained austenite content. The model allows the quantity of retained austenite to be estimated as a function of the chemical composition and heat treatment parameters. The computer programs associated with the work have been made freely available ( http //www.msm.cam.ac.uk/map/mapmain.html )

Effect of Mo and W on the Phase Stability of Precipitates in Low Cr Heat Resistant Steels

Abstract: Effects of Mo and W on creep strength of low Cr heat resistant steels have been investigated from the stand points of the phase stability of precipitates during long-term exposure at the elevated temperatures. The study on 2.25%Cr-Mo-V-Nb (Mo-steel) and 2.25%Cr-W-V-Nb (W-steel) with a same amount of Mo-equivalent has shown that the substituting W for Mo retards the evolution of microstructure, and thus remarkably improves the creep rupture strength. TEM observations have suggested that the most important precipitates strongly affecting the stability of microstructure are M 6 C type carbide and MC type carbide in both Mo-steel and W-steel. It is found that the M 6 C carbide precipitates with concentration of Mo and W during long-term aging. Therefore, the amounts of solute Mo and W supersaturated in matrix have reduced to the thermal equilibrium levels with changing the substructure from bainite lath structure to sub-grain liked structure. The kinetics of Mo- and W-partitioning between matrix and M 6 C carbide has been successfully expressed by a Johnson-Mehl-Avrami type equation and estimated the activation energy for diffusion of solutes; 125 kJ/mol in Mo-steel and 219 kJ/mol in W-steel. Thus the diffusion rate of W is suggested to be definitely lower than that of Mo. Furthermore, Mo- and W-partitioning to MC carbides have also been confirmed to affect the coherency and the growth rate of MC carbides. TEM observations on the strain image have shown that the MC carbide in Mo-steel has already lost the coherency with matrix. On the other hand, the finer MC carbide in W-steel has kept the coherent relationship with matrix even after long-term aging. It is concluded that W-steel is superior to Mo-steel in the stabilization of bainite lath structure and precipitates at elevated temperatures, resulting in the higher creep strength.

A new empirical formula for the bainite upper temperature limit of steel

Abstract: The definition of the practical upper temperature limit of the bainite reaction in steels is discussed. Because the theoretical upper temperature limit of bainite reaction, B 0, can neither be obtained directly from experimental measurements, nor from calculations, then, different models related to the practical upper temperature limit of bainite reaction, B S, are reviewed and analyzed first in order to define the B 0 temperature. A new physical significance of the B S and B 0 temperatures in steels is proposed and analyzed. It is found that the B 0 temperature of the bainite reaction in steels can be defined by the point of intersection between the thermodynamic equilibrium curve for the austenite→ferrite transformation by coherent growth (curve Z $$\gamma \to \overrightarrow \alpha $$ ) and the extrapolated thermodynamic equilibrium curve for the austenite→cementite transformation (curve ES in the Fe-C phase diagram). The B S temperature for the bainite reaction is about 50–55 °C lower than the B 0 temperature in steels. Using this method, the B 0 and B S temperatures for plain carbon steels were found to be 680 °C and 630 °C, respectively. The bainite reaction can only be observed below 500 °C because it is obscured by the pearlite reaction which occurs prior to the bainite reaction in plain carbon steels. A new formula, B S(°C) =, 630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W, is proposed to predict the B S temperature of steel. The effect of steel composition on the B S temperature is discussed. It is shown that B S is mainly affected by alloying elements other than carbon, which had been found in previous investigations. The new formula gives a better agreement with experimental results than for 3 other empirical formulae when data from 82 low alloy steels from were examined. For more than 70% of these low alloy steels, the B S temperatures can be predicted by this new formula within ±25°C. It is believed that the new equation will have more extensive applicability than existing equations since it is based on data for a wide range of steel compositions (7 alloying elements).

Flat product, such as sheet, made of steel having a high yield strength and exhibiting good ductility and process for manufacturing this product

Abstract: A flat product made of multiphase steel and a method for preparing the steel includes hot rolling at a temperature at which the austenite phase is stable, then tempering to form a C and Mn-enriched phase in a matrix, followed by a heat treatment to form austenite islands and/or enriching in Mn the already formed austenite and cooling down to room temperature so as to obtain a final product with a ferrite matrix containing residual austenite, bainite and/or martensite islands

Effects of Alloying Elements on Mechanical Properties and Phase Transformation of Cold Rolled TRIP Steel Sheets

Abstract: Tensile and dilatation tests were conducted in order to study the effects of Mn, Si and Nb on the transformation behavior in the 0.14C-(2.1∼2.4)Mn-(0.5∼1.0)Si-(0.00∼0.02)Nb TRIP steels. All the specimens for dilation tests were annealed under the condition to form the same austenite fraction, followed by cooling to the austempering temperature. The volume fraction of bainite transformed during austempering, ΔV B , and the martensite start temperature, M s , were measured. ΔV B was dependent on the austempering temperature and a minimum in ΔV B was obtained at 450°C for all the examined steels. The M s temperature was linearly decreased as ΔV B was increased. The decreasing rate of M s to ΔV B was not dependent on the steel composition, but on the austempering temperature, exhibiting the maximum value when the austempering temperature was 450°C. It was also observed that volume fraction of retained austenite was linearly increased with the decrease of M s for the steels austempered at various conditions. The highest elongation was obtained when the rate of M s to ΔV B was the maximum. M s was increased when the amount of Mn was increased from 2.1 % to 2.4 %. It was because Mn retarded the ferrite transformation rate, so that the soluble C diffused from the ferrite to the austenite was decreased. The addition of Si decreased M s because it activated the diffusion of C to austenite during the cooling to the austempering temperature. The addition of Nb was observed to give little influence on ΔV B and the M s .

Thick steel plate having excellent fatigue strength and production method therefor

Abstract: PROBLEM TO BE SOLVED: To obtain a thick steel plate for a welded structure which has excellent fatigue crack propagation resistance of a base material without depending on the addition of a large quantity of special or expensive alloy elements, or depending on a production method which is inferior in productivity or is complicated, or without the placement of large restrictions on tensile strength and steel plate thickness. SOLUTION: In the thick steel plate having a composition containing proper amounts of C, Si, Mn, Al and N, and, if required, containing Ni, Cu, Cr, Mo, W, Ti, V, Nb, Zr, Ta, B, Mg, Ca and rare earth metals, and having a structure consisting of ferrite and a hard secondary phase consisting of bainite or martensite or a mixed structure of both, by making the hard secondary phase in the cross-section parallel to the surface of the steel plate satisfy all conditions of a fraction of 20 to 80%, a mean Vickers hardness of 250 to 800, a diameter of the equivalent circle of 10 to 200 μm, and the maximum spacing between the hard secondary phases of ≤500 μm, the fatigue crack propagation of the base material is remarkably delayed, so that the fatigue strength of a joint is improved. COPYRIGHT: (C)2003,JPO

Texture development in cold rolled and annealed C-Mn-Si and C-Mn-Al-Si TRIP steels

Abstract: The texture of two transformation induced plasticity steels has been studied by means of crystallographic orientation mapping. Texture measurements were carried out on ferrite, bainite, and austenite. The polygonal ferrite and the bainite texture, both bcc, could be distinguished based on the image quality parameter of the electron backscattering diffraction measurement. Both bcc textures were very similar, the main difference being the more pronounced 111alpha ND ND=normal direction and 110alpha RD RD=rolling direction fibre textures in the polygonal ferrite. The fcc texture was a strong gamma deformation texture, characterised by the beta fibre. The presence of the alpha fibre confirmed the strong 110gamma ND direction, which was previously detected by means of X ray diffraction XRD. The measured fcc and bcc textures were used to calculate orientation distribution function transformations according the Bain, Kurdjumov-Sachs, and Nishiyama-Wasserman orientation relationships. The predicted cube c...

Bainite transformation during continuous cooling of low carbon microalloyed steel

Abstract: The evolution of the final microstructure for a low carbon Nb–Ti microalloyed plate steel was studied during a simulation of thermomechanical processing for hot rolling following by accelerated cooling. The effects of austenite deformation below the non-recrystallisation temperature T NR, cooling rate, and interrupt temperature on the formation of conventional (intergranular) bainite (CB), acicular ferrite (intragranular) (AF), and martensite–austenite (MA) constituents were determined. With increases in austenite deformation and cooling rate, and decrease in the interrupt temperature, the final microstructure changed from a mixture of CB+MA through CB+AF+MA to a dual phase AF+MA.

Microstructural Evolutions with Precipitation of Carbides in Steels

Abstract: The studies on the crystallographic features of diffusional transformation of austenite in various steels have been reviewed. A primary ferrite forming at an austenite grain boundary is related to at least one of the austenite grains separated by the boundary with the Bain correspondence. In the case of the microstructures containing both ferrite and carbides such as pearlite, degenerate pearlite, interphase precipitation of alloy carbides, and upper and lower bainites, the crystallographic relationships between ferrite and carbides provide further information to elucidate the transformation mechanisms. In the present report, therefore, the orientation relationships between ferrite, carbide and austenite as well as the habit planes of platelike transformation products and the mechanisms of transformations are discussed.

The macro- and micromechanics of TRIP-assisted multiphase steels, experiments and modeling

Abstract: TRIP-assisted multiphase steels exhibit enhanced strength and ductility properties. These properties result from the unique combination of various work-hardening and damage mechanisms taking place in a multiphase microstructure consisting of an intercritical ferrite matrix with a dispersion of bainite + martensite + metastable retained austenite grains. Martensite plays a crucial role in these mechanisms for the improvement of plastic properties (through the TRIP effect, i.e. the mechanically-induced martensitic transformation), and also, in the damaging process. This study aims at establishing how martensitic transformation influences the mechanical properties (plasticity and fracture) when occurring in a multiphase microstructure. On the one side, macro- and micromechanical tests, SEM and TFM, Mossbauer spectroscopy and x-ray diffraction are used to characterise the mechanisms of deformation, transformation, and fracture at the various relevant scales. On the other side, computational unit cell models are employed for assisting (i) the development of micromechanically-based constitutive models, (ii) the interpretation of experimental results.

Method for manufacturing high strength bolt excellent in resistance to delayed fracture and to relaxation

Abstract: A method for manufacturing a high strength bolt excellent in the resistance to delayed fracture and to relaxation, characterized in that a steel product having a chemical composition, wherein C: 0.50 to 1.0 % , Si: 0.5 % or less, Mn: 0.2 to 1 %, P; 0.03 % or less and S: 0.03 % or less, and having a structure, wherein the total area percentage of pro-eutectoid ferrite, pro-eutectoid cementite, bainite and martensite is 20 % or less and the balance is pearlite structure, is subjected to heavy wire drawing, the resulting wire is subjected to a cold heading to make a product having a bolt form, and the product is subjected to a blueing treatment in a temperature range of 100 to 400°C, thereby to manufacture a bolt form product having a tensile strength of 1200N/mm2 or higher and also being excellent in the resistance to delayed fracture and to relaxation.

High strength steel sheet having excellent weldability, hole expansibility and ductility and production method therefor

Abstract: PROBLEM TO BE SOLVED: To provide a high strength steel sheet in which the weldability, hole expansibility and ductility of a high strength steel sheet having a tensile strength of ≥800 MPa are simultaneously improved, and to provide a production method therefor. SOLUTION: The high strength steel sheet having excellent weldability, hole expansibility, and ductility, and having a tensile strength of ≥800 MPa has a microstructure containing bainite or bainitic ferrite as the main phase of 50 to 97% by an area fraction, and containing austenite as a second phase of 3 to 50% in total by an area fraction. COPYRIGHT: (C)2003,JPO

Relationship between hardness and tensile properties in various single structured steels

Abstract: An attempt has been made to establish a relationship between hardness and tensile properties for various single structured steels: ferrite, pearlite, bainite, and martensite. It is found that the proportionality constant A Y of hardness to yield strength changes from 5.79 to 3.17 and is highest for the ferrite steel and lowest for the tempered martensitic steels. A less pronounced change was found in the proportionality constant A T of hardness to tensile strength (from 3.97 to 2.72). A dependence on microstructure of the proportionality constant at 8% strain A 0.08 was found as well. This difference in A was found to be attributable mostly to the effect of different work hardening behaviours owing to different microstructures. Regression analysis shows that hardness can be expressed as a function of accessible material parameters such as composition, grain size, and transformation temperatures for various single structured steels within a certain degree of accuracy.

Body-centred tetragonal martensite formed from Au7Cu5Al4 β phase

Abstract: The Al–Au–Cu system contains a ternary β electron compound with a nominal stoichiometry of Au7Cu5Al4. At elevated temperatures this compound has a predominantly B2 structure, with some additional next-nearest neighbour ordering of the L21 type. The high-temperature phase transforms displacively and reversibly to martensite, with nominal Ms and As temperatures of 23 and 79°C, respectively. It is shown here that the martensite phase is body-centred tetragonal with c/a

Microstructure and mechanical properties of C-Si-Mn(-Nb) TRIP steels after simulated thermomechanical processing

Abstract: Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s -1 . The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.

Nano-composite martensitic steels

Abstract: Carbon steels of high performance are disclosed that contain dislocated lath structures in which laths of martensite alternate with thin films of austenite, but in which each grain of the dislocated lath structure is limited to a single microstructure variant by orienting all austenite thin films in the same direction. This is achieved by careful control of the grain size to less than ten microns. Further improvement in the performance of the steel is achieved by processing the steel in such a way that the formation of bainite, pearlite, and interphase precipitation is avoided.