Development of ultra-high strength carbide-free bainitic cast steels (original) (raw)
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Influence of the Austempering Time on the Mechanical Properties of Carbide-Free Bainitic Cast Steels
International Journal of Metalcasting, 2020
Three medium-carbon, high-silicon cast steels with different alloy contents were austempered at 330°C for different holding times in order to obtain carbide-free bainitic microstructures. Aiming at evaluating the influence of holding time and microstructural features on strength and ductility, tensile properties were measured for each steel at selected austempering times. The results obtained indicate that it is possible to adjust holding time in order to obtain the best strength/ductility combination at determined austempering temperature. Moreover, it has been shown that the mechanical stability of retained austenite is the key factor in controlling tensile performance. Short austempering times result in low carbon enrichment of the austenite (low stability) and promote higher ultimate tensile strength and lower ductility. For longer austempering times, steels present a slight decrease in ultimate tensile strength but a marked increase in ductility. This work shows that it is possible to obtain cast steels with ultimate tensile strength of 1682 MPa, yield strength of 1493 MPa and total elongation of 12.5% by means of bainitic reaction. This strength/ductility combination and others reported in this study are remarkable for cast steels.
Development of New High-Strength Carbide-Free Bainitic Steels
Metallurgical and Materials Transactions A, 2011
An attempt was made to optimize the mechanical properties by tailoring the process parameters for two newly developed high-strength carbide-free bainitic steels with the nominal compositions of 0.47 pct C, 1.22 pct Si, 1.07 pct Mn, 0.7 pct Cr (S1), and 0.30 pct C, 1.76 pct Si, 1.57 pct Mn, and 0.144 pct Cr (S2) (wt pct), respectively. Heat treatment was carried out via two different routes: (1) isothermal transformation and (2) quenching followed by isothermal tempering. The results for the two different processes were compared. The bainitic steels developed by isothermal heat treatment were found to show better mechanical properties than those of the quenched and subsequently tempered ones. The effect of the fraction of the phases, influence of the transformation temperatures, the holding time, and the stability of retained austenite on the mechanical properties of these two steels was critically analyzed with the help of X-ray diffraction, optical metallography, scanning electron microscopy, and atomic force microscopy. Finally, a remarkable combination of yield strength of the level of 1557 MPa with a total elongation of 15.5 pct was obtained.
An attempt was made to optimize the mechanical properties by tailoring the process parameters for two newly developed high-strength carbide-free bainitic steels with the nominal compositions of 0.47 pct C, 1.22 pct Si, 1.07 pct Mn, 0.7 pct Cr (S1), and 0.30 pct C, 1.76 pct Si, 1.57 pct Mn, and 0.144 pct Cr (S2) (wt pct), respectively. Heat treatment was carried out via two different routes: (1) isothermal transformation and (2) quenching followed by isothermal tempering. The results for the two different processes were compared. The bainitic steels developed by isothermal heat treatment were found to show better mechanical properties than those of the quenched and subsequently tempered ones. The effect of the fraction of the phases, influence of the transformation temperatures, the holding time, and the stability of retained austenite on the mechanical properties of these two steels was critically analyzed with the help of X-ray diffraction, optical metallography, scanning electron microscopy, and atomic force microscopy. Finally, a remarkable combination of yield strength of the level of 1557 MPa with a total elongation of 15.5 pct was obtained.
Mechanical properties of a carbide-free bainitic cast steel with dispersed free ferrite
Materials Science and Technology, 2019
A high silicon, medium carbon cast steel was designed and heat-treated in order to develop microstructures composed of carbide-free bainite and small amounts of free ferrite, with the aim of obtaining high strength cast steels with improved ductility. Because of microsegregation, it was observed that ferrite present in partially austenitised samples is mostly present at the highly alloyed zones, creating an interconnected network even for low proportions of this phase. Despite the coarse solidification structure and marked microsegregation in the cast steel, the mechanical properties obtained for both fully bainitic and bainitic-ferritic microstructures largely satisfy the minimum standard requirements for high strength cast steels and are similar to those reported for wrought steels of similar microstructures.
Metals
The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350 °C and 370 °C, respectively), and three kinds of two-step routes (370 °C + 300 °C, 370 °C + 250 °C, and 350 °C + 250 °C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature.
Metals
Bainitic microstructures obtained in high-carbon (HC) and high-silicon (HSi) steels are currently of great interest. Microstructural evolution and the bainitic transformation kinetics of a high-carbon and high-silicon cast steel held at 280, 330, and 380 °C was analyzed using dilatometry, X-ray diffraction, optical and scanning electron microscopy, and electron backscatter diffraction (EBSD). It is shown that the heterogeneous distribution of silicon (Si), manganese (Mn), and chromium (Cr) associated to microsegregation during casting has a great impact on the final microstructure. The transformation starts in the dendritic zones where there is a lower Mn concentration and then expands to the interdendritic ones. As Mn reduces the carbon activity, the interdendritic areas with a higher Mn concentration are enriched with carbon (C), and thus, these zones contain a greater amount of retained austenite plus martensite, resulting in a heterogeneous microstructure. Higher transformation ...
2013
Advanced high strength steels for automotive applications were designed to achieve a carbide-free bainitic microstructure after conventional thermo-mechanical processing and a continuous annealing treatment. The microstructure obtained consists of ferrite laths interwoven with thin films of untransformed retained austenite. The sufficiently tough matrix and the control of the heterogeneity in the microstructure will allow an optimum combination of strength, ductility, and formability to be achieved. The designed steels reached far higher uniform elongations than that in commercial dual phase steels and martensitic steels with the same range of ultimate tensile strengths. Their formability was found to be appropriate for the production of final parts after cold-stamping or cold-forming. On the other hand, the yield strength/ ultimate tensile strengths ratio was found to remain roughly constant (∼0.7). The reduction of area value did not seem to change as a function of overaging temperature, but the V-bending angle and the hole expansion ratio (cut-edge stretching ability) decreased significantly at the bainite holding temperature increases.
Advanced Ultrahigh Strength Bainitic Steels
Materials and Manufacturing Processes, 2007
The addition of about 2 wt.% of silicon to steel enables the production of a distinctive microstructure consisting of a mixture of bainitic ferrite, carbon-enriched retained austenite, and some martensite. With careful design, impressive combinations of strength and toughness have been reported for high-silicon bainitic steels. More recently, it has been demonstrated experimentally that models based on phase transformation theory can be applied successfully to the design of carbide-free bainitic steels. Toughness values of nearly 130 MPa m 1/2 were obtained for strength in the range of 1600-1700 MPa. However, the concepts of bainite transformation theory can be exploited even further to design steels that transform to bainite at temperatures as low as 150 C. Microstructure obtained is so refined that it is possible to achieve a strength in excess of 2.5 GPa in a material which has considerable toughness (30 MPa m 1/2 . Such properties have never been achieved before with bainite. It is intended to provide a description of the characteristics and significance of this remarkable microstructure in the context of the mechanism of transformation.
New advanced ultra high strength bainitic steels: ductility and formability (DUCTAFORM)
The main objective of this proposal is to develop AHSS both hot rolled and annealed cold rolled bainitic steels with an optimal ductility and work hardening comparable to drawing steels, and/or keeping a suitable bending and stretching behaviour. In addition, forming processes such as roll-forming and hardening, and press-hardening were optimised to achieve a carbide free bainitic microstructure in final products such as structural safety components in the car body i.e. A-beam, roof-beams, car bumpers and side impact beams. Hot rolling of carbide free bainitic steels with 1100 MPa of yield strength and high toughness (KV(-40ºC)>30 J) was proved to be challenging. Apart of a high carbon content (~0.3wt.%), chemical composition requires of a high manganese content (~2wt.%), which leads to a high risk of banding. By contrast, annealed cold rolled bainitic steels designed for continuous annealing line achieved far higher uniform elongation, better stretching ability and formability t...
Materials Today Communications, 2020
Rail manufacture processing techniques, such as roller straightening, tempering and air-forced quenching, have shown to make a great influence on the microstructure and mechanical properties of rail steels. In this study, five carbide-free bainitic rail steels with same chemical composition were developed from the above rail processing routes in an industrial production line. In general, the carbide-free bainitic steels consisted of bainitic ferrite (BF) plates with distinct morphologies of retained austenite (RA) and minor occurrence of martensite in some conditions. Electron microscopy confirmed the orientation relationship between BF and RA was in line with the Nishiyama-Wassermann (N-W) orientation relationship, i.e., {111} γ //{110} α , < 112 > γ // < 110 > α. The roller straightening and tempering processes could facilitate the transformation of unstable RA. The small volume fraction of stable RA contributed to an obvious improvement of yield strength, hardness and impact toughness, but at the cost of ductility and strain hardening ability. In addition, the in-line heat treatment resulted in the refinement of bainitic ferrite plates, leading to an increase in the tensile strength of carbide-free bainitic steels. In summary, there was a clear positive relationship between the rail processing techniques and the mechanical properties of the carbide-free bainitic steels. This study can benefit the development of new bainitic steels with a combination of wear and rolling contact fatigue resistance by appropriate selection of processing techniques.