ISIJ International, Vol. 47 (2007), No. 8, pp. 1195–1203 (original) (raw)
Related papers
2012
Specific treatment used for TRIP steels, as well as quenching and partitioning (Q-P) processing rely on producing multi-phase microstructures with retained austenite in low-alloyed steels in order to achieve excellent mechanical properties. It is namely the values of the product of strength and elongation that reach outstanding levels. The desired microstructure for exploiting the TRIP effect consists of carbide-free bainite, ferrite and a small amount of retained austenite. It can be produced by intercritical annealing. Depending on the chemical composition of the steel, this microstructure may exhibit strengths between 800 and 1000 MPa and elongations up to 30 %. Microstructures with martensite matrix produced by Q-P process contain retained austenite in the form of films between martensite laths. They show strengths of up to 2000 MPa and elongations of about 10 %. In order to achieve such properties, suitable combinations of alloying elements must be chosen to prevent carbide pre...
SAE technical paper series, 2001
The recent increased severity in service conditions, such as frequent earthquakes, have further promoted the development of steel production technologies for many types of microstructural control. In the present paper, two-stage thermomechanical control process (TMCP) combined with accelerated cooling was employed to control the microstructural evolution and to study the microstructure-property relationship of low carbon bainitic steel. The main microstructure of hot rolled steel plates changed from granular bainite to lath bainite (or bainitic ferrite) when the final accelerated cooling temperature decreased from about 530 to 430 °C, accompanied with a notable increase in yield strength at the expense of slightly decreasing toughness. The strengthening mechanism was mainly attributed to dislocation strengthening and precipitation strengthening for this low carbon microalloyed steel. In addition, if the strain hardening exponent of hot rolled steel plate with the thickness of 13 mm is expected to be higher than 0.1, the final cooling temperature range should be maintained above 500 °C.
Thermomechanical modelling strategies for multiphase steels
International Journal of Mechanical Sciences, 2011
In multiphase steels, heat treatments such as quenching are usually applied to achieve a desired metallurgical composition to attain the expected mechanical properties. In these processes, residual thermal stresses arise during the cooling of the material which may induce a permanent deformation leading to dimensional instability. This deformation can be increased by the existence of phase transformations in the steel which should not be overlooked. In the current work, the thermomechanical modelling of multiphase materials is discussed. Firstly, a multiphase thermo-elasto-plastic-viscoplastic model is presented and applied to simulate several quenching heat treatments in a high carbon steel. The model uses the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation to describe the diffusion transformation and the Koistinen-Marburger model to characterise the diffusionless martensitic transformation in non-isothermal kinetics. This allows the observation of the evolution of the different steel phases (austenite, ferrite, pearlite, bainite and martinsite) during the cooling process. However, it is not possible to determine the residual stresses that arise in the intersection of the different phases. In the second part, the model that considers generalised multiphase transformation being compared with a multiphase homogenisation model for the case of a dual-phase steel during cooling and subsequent forming. The homogenisation micro-model operates over a periodic Representative Unit-Cell (RUC), detailing the heterogeneous material distribution due to the different metal phases. Therefore, it is possible to determine the residual stress fields in the intersection of the different phases. On the other hand, this model does not allow to reproduce the transformation process from austenite during cooling. Continuous cooling processes are studied in both parts. Following the heat treatment, tensile and shear test curves are presented and compared with experimental results for the second part.
Theoretical design and advanced microstructure in super high strength steels
Materials & Design, 2009
A theoretical design procedure based on phase transformation theory alone has been successfully applied to design steels with a microstructure consisting of a mixture of bainitic ferrite and retained austenite. Using thermodynamics and kinetics models, a set of four carbide free bainitic steels with a 0.3 wt-% carbon content were designed and manufactured following a thermo-mechanical treatment consisting of hot rolling and two-step cooling. The designed steels present significant combinations of strength and ductility, with tensile strengths ranging from 1500 to 1800 MPa and total elongations over 15%. However, a carbon content of 0.3 wt% is still high for in-use properties such as weldability. In this sense, a reduction in the average carbon content of advanced bainitic steels was proposed. Improved bainitic steels with a carbon content of 0.2 wt% reached combinations of strength and ductility comparable to those in TRIP assisted steels.
steel research international, 2013
The microstructural changes and hardness exhibited by ductile iron with dual matrix structure (DMS) are investigated. In particular, DMS microstructures are obtained by continuous cooling in the (ferrite þ austenite) region followed by quenching to transform the austenite into martensite or by austempering at 3758C, to transform the austenite into ausferrite. Additionally, two deformation steps are applied in the austenite region. The structure was produced in a thermomechanical simulator equipped with a dilatometry system. The dilatometry is used to monitor the structure development throughout the thermomechanical processes. The structure was investigated using light optical microscopy and scanning electron microscopy. The influence of introducing ferrite to the microstructure and the deformation magnitude on the structure development and hardness properties are explored.
Evaluation and review of simultaneous transformation model in high strength low alloy steels
Materials Science and Technology, 2002
This work brie¯y describes and evaluates one of the most complete transformation models, which deals with the nonisothermal decomposition of austenite. The model, that does not consider the effect of precipitation on phase transformations, has been experimentally validated in high strength low alloy steels in order to evaluate how it works for microalloyed steels, where precipitation may play an important role. It has been found that the simultaneous transformation model is able to predict with an excellent agreement in microalloyed steels the formation of microstructures consisting of ferrite plus pearlite. However, the bainite formation is not successfully described by the model. The calculations incorrectly predict the formation of martensite instead of bainite in many situations.
Development of Advanced High Strength Steels by Physical Simulation and Laboratory Testing
As a response to the demand of high-performance steels, processing of advanced steel types such as DP, TRIP, CP and TWIP, and high-strength low-carbon bainitic and martensitic DQ-T steels, has been developed based on physical simulation and modelling studies. Physical simulation has been used by employing a Gleeble thermo-mechanical simulator to reveal the phenomena occurring in the hot rolling stage (strain hardening, flow resistance, recrystallization kinetics and microstructure evolution), and in the cooling stage (CCT diagrams). Connecting these data with microstructures examined in optical and electron microscopes and resultant mechanical properties have improved the understanding of those phenomena and the role of numerous process variables in the optimization of enhanced mechanical properties. Based on a well-designed experimental plan, new regression models have been developed for the start of bainite/martensite transformation temperature and also for the hardenability for boron-containing steels for the direct quenching route. Processing and properties of Ti-microalloyed TRIP steels were studied. For multi-phase steels, realtime phase fractions as a function of heating rate and holding in the intercritical annealing for optimized processing have been estimated, and for TWIP steels, high-temperature deformation and recrystallization have been investigated.
Metals
In the present work, a Cr+Mo+Si low-alloyed low-carbon steel was fabricated at laboratory scale and processed to produce multiphase advanced high-strength steels (AHSS), under thermal cycles similar to those used in a continuous annealing and galvanizing process. Cold-rolled steel samples with a microstructure constituted of pearlite, bainite, and martensite in a matrix ferrite, were subjected to an intercritical annealing (817.5 °C, 15 s) and further isothermal bainitic treatment (IBT) to investigate the effects of time (30 s, 60 s, and 120 s) and temperature (425 °C, 450 °C, and 475 °C) on the resulting microstructure and mechanical properties. Results of an in situ phase transformation analysis show that annealing in the two-phase region leads to a microstructure of ferrite + austenite; the latter transforms, on cooling to IBT, to pro-eutectoid ferrite and bainite, and the austenite-to-bainite transformation advanced during IBT holding. On final cooling to room temperature, auste...
High-strength steels by microalloying and thermomechanical treatment
Steels with higher strength, ductility and improved fatigue behavior are required for light-weight structures in the transportation industry. It is shown that for martensitic steels the combination of microalloying and an optimized thermomechanical treatment (TMT) results in increased strength and improved ductility. Proper conditioning of the austenite by deformation either refines the austenitic grains or generates a dislocation substructure that is inherited to the martensite structure. In contrast to simply quenched and tempered martensite with no prior deformation, the thermomechanically processed martensite exhibits a more refined structure with refined blocks and is free of grain boundary carbides. Addition of vanadium is beneficial in controlling the austenite grain size during austenitization and for the stabilization of the austenite defect structures that are produced by deformation. It enables to use higher deformation temperatures for TMT, i.e. lower rolling forces can be applied in an industrial process. It is possible to increase the strength and ductility of conventionally heat treated Si-Cr steel by addition of vanadium or by TMT, but the highest improvement is achieved through the combination of both. In this study, an increase of more than 600 MPa in the ultimate tensile strength and an improvement of 40% in the reduction area are reported.
A moderately high carbon (0.61%) high silicon steel was subjected to a newly designed heat treatment cycle consisting of continuous cooling for different duration after austenitization followed by austempering at 300, 350 and 400 °C to form a very high strength and highly ductile multiphase steels with microstructures consisting of varied amounts of ferrite (formed during continuous cooling), bainite (formed during austempering) and retained austenite. Steels with very high strength up to (tensile strength ~1100–2000 MPa) along with excellent ductility (elongation ~10–32%) were obtained. Effect of continuous cooling duration on ferrite content, amount of carbon diffused in the prior austenite grains, variation of carbon content in the retained austenite (c γ) and its volume fraction (V γ) have been analyzed. Finally, structure property correlation has been established.