Fracture strength and toughness of ultra high strength TRIP aided steels

Sugimoto, Koh Ichi

doi:10.1179/174328409x453307

Cited by 57 publications

(46 citation statements)

References 46 publications

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“…These properties are achieved by using hard phases such as martensite and bainite to increase strength and strain-induced transformation of the retained austenite phase to improve elongation. [1][2][3] When using martensite, bainite and retained austenite to improve the properties of steel, it is important to understand the relationship between the microstructure and carbon distribution with high accuracy and high spatial resolution. [4][5][6][7][8][9][10] The steel microstructure is formed through phase transformation.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Carbon Partitioning at an Early Stage of Proeutectoid Ferrite Transformation in a Low Carbon Mn–Si Steel by High Accuracy FE-EPMA

Yamashita¹,

Enomoto

Tanaka³

et al. 2018

ISIJ Int.

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To understand the γ→α transformation and control the carbon contents among the phases involved is most important in the design of high-strength and high-ductility low carbon steels. Using a previouslydeveloped field emission-electron probe microanalyzer (FE-EPMA) which is able to suppress hydrocarbon contamination to a very low level, the carbon concentrations at γ/α interfaces and within the austenite were analyzed in Fe-xC-2.0Si-yMn (x = 0.15 or 0.20, y = 1.5 or 2.0 in mass%) steels isothermally transformed at 750°C and 800°C. The paraequilibrium (PE) model gives a good accounting of carbon enrichment in the 1.5 mass% Mn steel held for 15 s, whereas the NPLE/PLE transition model of local equilibrium gives a much better accounting in the 2.0 mass% Mn steel than PE. However, the interfacial carbon concentration agrees with the composition shown by the NPLE/PLE transition line in both alloys when annealed for 1 800 s. In the 1.5 mass% Mn steel, the carbon concentration at the interface in austenite seems to have shifted from PE to NPLE during annealing.

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Section: Introductionmentioning

confidence: 99%

Analysis of Carbon Partitioning at an Early Stage of Proeutectoid Ferrite Transformation in a Low Carbon Mn–Si Steel by High Accuracy FE-EPMA

Yamashita¹,

Enomoto

Tanaka³

et al. 2018

ISIJ Int.

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“…The applied process is schematically shown in Figure 4. Given that there exists the possibility of forming bainite by isothermal transformation at temperatures below the Ms [26][27][28][29][30][31][32][33], this is also becoming an attractive alternative, not only to accelerate the bainitic transformation, but also to obtain a finer microstructure in later stages of transformation. Figure 5 shows an example for 0.15 and 0.28 C steels transformed to bainite below the Ms; the author reports a decrease in the bainitic ferrite plate thickness of almost 40 nm in both cases, the final plate thickness being around 140-200 nm [26].…”

Section: Heat Treatment Variationsmentioning

confidence: 99%

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

García‐Mateo

Paul

Somani

et al. 2017

Metals

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Abstract:The major strengthening mechanisms in bainitic steels arise from the bainitic ferrite plate thickness rather than the length, which primarily determines the mean free slip distance. Both the strength of the austenite from where the bainite grows and the driving force of the transformation, are the two factors controlling the final scale of the bainitic microstructure. Usually, those two parameters can be tailored by means of selection of chemical composition and transformation temperature. However, there is also the possibility of introducing plastic deformation on austenite and prior to the bainitic transformation as a way to enhance both the austenite strength and the driving force for the transformation; the latter by introducing a mechanical component to the free energy change. This process, known as ausforming, has awoken a great deal of interest and it is the object of ongoing research with two clear aims. First, an acceleration of the sluggish bainitic transformation observed typically in high C steels (0.7-1 wt. %) transformed at relatively low temperatures. Second, to extend the concept of nanostructured bainite from those of high C steels to much lower C contents, 0.4-0.5 wt. %, keeping a wider range of applications in view.Keywords: bainite; ausforming; kinetics; plate thickness Structural Refinement of Bainitic Steels: General ConsiderationsBainitic steels can be designed on the basis of the theory that predicts the highest temperature at which bainite (Bs) and martensite (Ms) can start to form in a steel of a given composition. These two temperatures constitute the upper and lower limits at which the isothermal heat treatment can be performed to generate bainite.It has been reported that bainitic ferrite plate thickness depends primarily on three parameters, i.e., (1) the strength of the austenite at the transformation temperature, (2) the dislocation density in the austenite and (3) the chemical free energy change accompanying transformation [1][2][3]. In accord, a strong austenite possessing a high dislocation density and a large driving force results in finer plates. Austenite strength and dislocation density refine the structure by increasing the resistance to interface motion, and the thermodynamic driving force refines the structure by increasing the nucleation rate. All three factors-austenite strength, dislocation density and driving force-increase

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“…In general, the carbon concentration of the retained austenite is mainly controlled by the T 0 or T 0 ′ temperatures, as well as by the morphology, size and carbon diffusion rate. 21) Figure 14 shows the T 0 and T 0 ′ lines calculated by ThermoCalc, and the carbon concentrations of the retained austenite in steels A-E measured by X-ray diffractometry. The carbon concentration in steel A agrees quite well with the T 0 line.…”

Section: Effects Of Chemical Composition On Microstructurementioning

confidence: 99%

Effects of the Addition of Cr, Mo and Ni on the Microstructure and Retained Austenite Characteristics of 0.2% C^|^ndash;Si^|^ndash;Mn^|^ndash;Nb Ultrahigh-strength TRIP-aided Bainitic Ferrite Steels

et al. 2012

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Cr, Mo and/or Ni were added to TRIP-aided bainitic ferrite (TBF) steel (0.2% C, 1.5% Si, 1.5% Mn and 0.05% Nb ultrahigh-strength TBF steel) in order to increase its hardenability. In addition, the effects of the alloying elements on the Vickers hardness, microstructure and retained austenite characteristics of the TBF steels were investigated. When the TBF steels were austempered at temperatures between MS and Mf, the Vickers hardness increased from HV300 to HV430 with increasing hardenability. The microstructure consisted of martensite and bainitic ferrite lath structures and retained austenite phases and the volume fraction of retained austenite increased with increasing hardenability. Conversely, the carbon concentration of the retained austenite decreased with increasing hardenability. Simultaneously, the quantity of the hard blocky martensite phase (M-A constituent) with refined interlath retained austenite films increased with increasing hardenability. These characteristics are mainly caused by the delayed bainite transformation during austempering through the addition of Cr, Mo and/or Ni. The addition of Ni lowered the T0 line further. The retained austenite phases of Cr-and/or Mo-bearing TBF steels were relatively stable against straining, despite their low carbon concentrations.

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Fracture strength and toughness of ultra high strength TRIP aided steels

Cited by 57 publications

References 46 publications

Analysis of Carbon Partitioning at an Early Stage of Proeutectoid Ferrite Transformation in a Low Carbon Mn–Si Steel by High Accuracy FE-EPMA

Analysis of Carbon Partitioning at an Early Stage of Proeutectoid Ferrite Transformation in a Low Carbon Mn–Si Steel by High Accuracy FE-EPMA

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Effects of the Addition of Cr, Mo and Ni on the Microstructure and Retained Austenite Characteristics of 0.2% C^|^ndash;Si^|^ndash;Mn^|^ndash;Nb Ultrahigh-strength TRIP-aided Bainitic Ferrite Steels

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