Effects of ausforming temperature on carbide-free bainite transformation and its correlation to the transformation plasticity strain in a medium C- Si-rich steel

Zorgani, Muftah; García‐Mateo, C.; Jahazi, Mohammad

doi:10.1016/j.matchar.2021.111124

Cited by 10 publications

(2 citation statements)

References 56 publications

(92 reference statements)

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“…Instead of the typical structure of bainite, which includes discontinuous carbides (DC) and bainitic ferrite (BF) platelets, carbide‐free bainite consists of BF laths interwoven with thin films of untransformed retained austenite (RA). [ 25,26 ] In contrast, the microstructure of s4 was a combination of polygonal ferrite (PF) and pearlite (P) as can be seen at higher magnification in the inset of Figure 2d. Because all four steels were similarly air‐cooled after the hot‐rolling process, this difference in microstructure can be attributed to the absence of B in s4 (see Table 1).…”

Section: Resultsmentioning

confidence: 99%

Effect of Ti Addition on Yield Strength of Low‐Mo Fire‐Resistant Steel at Elevated Temperatures

Choi

Kim

Hocheng

et al. 2022

steel research int.

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Herein, a series of low‐Mo (0.2 wt%) fire‐resistant steels with varying amounts of Ti (0.008–0.13 wt%) are investigated to study the effects of Ti on yield strength at elevated temperatures. At room temperature (RT), precipitation strengthening by nanoscale TiC precipitates is found to be the major factor for the enhanced strength. The amount of TiC precipitates and both the yield and tensile strengths increase monotonically with the Ti content. However, the yield strength ratio (YS at 600 °C divided by YS at RT) of the steel with the highest Ti content (0.13 wt%) is significantly reduced to 0.6. In contrast, the YS ratio of the steels with Ti content in the range of 0.008–0.087 wt% remains above 0.7 and increases with Ti content. The difference between the steels lies in the B content and the resultant bainite volume fraction. The steel with 0.13 wt% Ti does not contain B and has only 4% bainite, whereas the other steels contain 20 ppm B and approximately 60% bainite. Hence, a microstructure with a sufficient fraction of bainite is required to ensure strength at elevated temperatures. The properties can be further improved by Ti precipitation strengthening.

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Section: Resultsmentioning

confidence: 99%

Effect of Ti Addition on Yield Strength of Low‐Mo Fire‐Resistant Steel at Elevated Temperatures

Choi

Kim

Hocheng

et al. 2022

steel research int.

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show abstract

“…When this time is over, it is quenched to room temperature. All processes are under vacuum conditions and the temperature is controlled by K-type thermocouples [9]. From this experiment, the following are obtained by analysing the dilatometric curves and the microstructure: the beginning and end of the transformation from ferrite-to-austenite (Ac1 and Ac3) during heating, the beginning and end of the transformation from austenite-to-ferrite (Ae1 and Ae3) during cooling, the beginning and end temperatures of the bainitic transformation (Bs and Bf) and the beginning and end temperatures of the martensitic transformation (Ms and Mf).…”

Section: Determination Of Critical Temperaturesmentioning

confidence: 99%

Crashworthiness evaluation of press hardened steels with different lath-like microstructures.

Aroca,

Philippot,

Pujante

et al. 2023

IOP Conf. Ser.: Mater. Sci. Eng.

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The deployment of Press Hardened Steels (PHSs) in the Body-In-White structures that took place during the two last decades is at the origin of a breakthrough weight saving on vehicles while guaranteeing the safety requirements of the automotive industry. The production of parts with complex shapes and a high strength between 1500-2000MPa was made possible by taking advantage of the hot rheology of austenite combined with the fast cooling obtained by in-die quenching leading to auto-tempered martensitic microstructures. In the present work, PHS1500 was hot stamped with different thermomechanical processes to promote different lath-like microstructures (bainite, tempered martensite…). An innovative approach is proposed to link the complex microstructures formed between thermoregulated dies to the mechanical behaviour and local ductility of these materials during crash-like solicitations. The microstructure has been characterised through a multi-characterization technique approach including light optical microscopy, scanning electron microscopy and X-ray diffraction. The local ductility is assessed with a combination of crack initiation tests (bending, notch tensile test) and crack propagation resistance tests (Essential work of fracture). Despite similar microstructural crystallographic features, lower bainite and auto-tempered martensite can lead to very different strength – local ductility compromise.

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Evaluation of a Processing Route and Microstructural Characteristics for the Development of Ultrafine Bainite in Low-Temperature Ausformed Medium-Carbon Steels

Kaikkonen

Somani

Pohjonen

et al. 2022

J. of Materi Eng and Perform

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A combination of physical simulation and laboratory rolling experiments, including thermomechanical rolling and low-temperature ausforming, was conducted for designing a suitable processing route to enable phase transformation from austenite to ultrafine bainite in a medium-carbon steel. Following low-temperature ausforming at 500-550 °C, two different cooling and holding paths were tried in the study: (1) water cooling close to martensite start temperature (300 °C), followed by isothermal holding (route A), and (2) air cooling to 350 °C followed by isothermal holding (route B). For reference, a third sample was directly water-cooled to 300 °C after hot rolling without ausforming treatment, followed by isothermal holding (route C). Field emission scanning electron microscopy and electron backscatter diffraction, as well as x-ray diffraction, were employed for microstructural analysis and correlations with the mechanical properties evaluated in respect of hardness and tensile properties. The low-temperature ausforming and subsequent cooling schedules resulted in the decomposition of austenite into ultrafine bainite and some martensite, while stabilizing a fraction of finely divided, carbon-enriched interlath austenite. Results suggested the development of a novel, multiphase bainite-martensite-austenite microstructure, achieved via low-temperature ausforming and subsequent air-cooling (route B), was beneficial in respect of mechanical properties. Most of the bainitic plates were in the range of 50-200 nm with the occasional presence of coalesced plates as wide as 2000 nm. Despite the differences in the microstructure, the mechanical behavior of non-ausformed samples was not significantly different. The technique paves way for developing medium-carbon nanostructured/ultrafine bainitic steels with high mechanical properties achieved via innovative modification of processing routes including low-temperature ausforming. Graphical Abstract

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Effects of ausforming temperature on carbide-free bainite transformation and its correlation to the transformation plasticity strain in a medium C- Si-rich steel

Cited by 10 publications

References 56 publications

Effect of Ti Addition on Yield Strength of Low‐Mo Fire‐Resistant Steel at Elevated Temperatures

Effect of Ti Addition on Yield Strength of Low‐Mo Fire‐Resistant Steel at Elevated Temperatures

Crashworthiness evaluation of press hardened steels with different lath-like microstructures.

Evaluation of a Processing Route and Microstructural Characteristics for the Development of Ultrafine Bainite in Low-Temperature Ausformed Medium-Carbon Steels

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