Toughening of Low-Carbon Steel by Ultrafine-Grained Structure

Inoue, Tatsuo; Kimura, Yuuji

doi:10.1299/kikaia.79.1226

Cited by 11 publications

(9 citation statements)

References 5 publications

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“…[4][5][6][7]21) Brittle fracture occurs when the maximum tensile stress (σt) at the notch root or crack tip exceeds the brittle fracture stress (σBF). The σt is proportional to the σy.…”

Section: Occurrence Mechanism Of Inverse Temperaturementioning

confidence: 99%

“…Therefore, the stress state in the vicinity of the notch and/or crack tip changes from a highly triaxial tension state into an uniaxial one; the impact specimen virtually behaves as an un-notched specimen. Additionally, it can be expected that the stress fielding effect from the formation of microcracks along the RD will reduce the driving force for fracture 21) As a result, the vE is enhanced. Furthermore, in addition to the delamination crack along the RD, the cleavage crack initiation and propagation along the 45° direction is facilitated below T2, in which the σt//45° exceeds the σc//45°.…”

Section: Occurrence Mechanism Of Inverse Temperaturementioning

confidence: 99%

“…Figure 15 summarizes the vE at room temperature as a function of the yield strength × total elongation (σy·EL) balance. Data for the conventional steels 24,25) and ultrafine grained steels 21) are also shown. The σy·EL balance is improved with decreasing carbon content in both of the QT and TF samples.…”

Section: Influence Of Carbon Content On Inverse Temperature Dependencmentioning

confidence: 99%

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Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

Kimura

Inoue

2015

ISIJ International

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(0.2-0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and deformed by multipass caliber rolling (i.e. warm tempforming) with a rolling reduction of 78%, in order to obtain ultrafine elongated grain (UFEG) structures. The tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to determine the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with strong <110>//rolling direction (RD) fiber textures. The transverse grain size and aspect ratio in the UFEG structure tended to reduce as the carbon content increased, whilst the carbide particle size became slightly larger. The increase in the carbon content resulted in an increase in the yield strength from 1.68 to 1.95 GPa at room temperature; however, it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness. This was due to delaminations, where cracks were observed to branch in the longitudinal direction (//RD) of the impact test bars. The upper-shelf energy of the TF sample was enhanced as the carbon content decreased, and higher absorbed energy was also achieved as delamination occurred at lower temperatures. The delamination was found to be controlled not only by the transverse grain size, the grain shape, and the <110>//RD fiber texture but also by carbide particle distribution in the UFEG structure.

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Section: Occurrence Mechanism Of Inverse Temperaturementioning

confidence: 99%

Section: Occurrence Mechanism Of Inverse Temperaturementioning

confidence: 99%

Section: Influence Of Carbon Content On Inverse Temperature Dependencmentioning

confidence: 99%

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Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

Kimura

Inoue

2015

ISIJ International

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“…25) The stress fielding effect from the formation of microcracks along the RD can also reduce the driving force for fracture. 27) As a result, the vE is enhanced through the occurrence of crack arrester-type delamination. Figure 11 displays the representative fracture surfaces of the TF samples that were tested at a temperature of 293 K, where the marked difference in the delamination toughening was observed.…”

Section: Discussionmentioning

confidence: 99%

Influence of Prior-Austenite Grain Structure on the Mechanical Properties of Ultrafine Elongated Grain Structure Steel Processed by Warm Tempforming

Kimura

Inoue

2015

ISIJ International

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0.4%C-2%Si-1%Cr-1%Mo steels with different austenite grain structures were quenched and tempered at 773 K and deformed by multi-pass caliber rolling with a rolling reduction of 78% (i.e. warm tempforming, (WTF)). The microstructures, in addition to the tensile and Charpy impact properties of the warm tempformed (TF) steels, were investigated in relation to the prior-austenite grain (PAG) structures. The TF samples demonstrated hierarchical microstructures consisting of "packet bands" and "ultrafine elongated grains (UFEGs)". The PAG structure had little influence on the transverse grain size, grain shape, <110>// rolling direction (RD) fiber texture, and carbide particle distribution in the UFEG structure, while the size of the packet bands decreased as the PAG size decreased. The PAG size had little influence on the strength of the TF sample. However, the effect of the WTF on the tensile ductility and toughness was enhanced as the PAG size decreased. It was demonstrated that as the PAG size decreased, the inverse temperature dependence of impact toughness (resulting from delamination) became significant at lower temperatures. The delamination toughening mechanism was discussed in relation to the UFEG and packet band structures.

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“…In the SM490 sample, the average size of the ferrite grain in Fig. 2(d) was approximately 20m (Inoue and Kimura (2013b)]. Figure 3 shows the yield strength,  ys , and the fracture energy, J, as a function of the test temperature for all samples and Figure 4 shows the P  u curves and appearances of the samples at ambient temperature.…”

Section: Methodsmentioning

confidence: 99%

Strength-toughness balance of low-alloy steel by fail-safe design

Inoue¹,

Kimura²,

Qiu³

2015

Mechanical Engineering Letters

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Microstructural design for improving the strength-toughness balance was studied in low-alloy steel. High-strength medium-carbon steels with an ultrafine elongated grain (UFEG) structure with an average transverse grain size of 0.3 m and an ultrafine equiaxed grain (UFG) structure with a grain size of 0.7 m were fabricated by multi-pass caliber rolling at a warm working temperature and subsequent annealing. For comparison, conventionally quenched and tempered steel with a martensitic structure and a 480 MPa-class low-carbon steel with a ferrite (grain size 20m)-pearlite structure were also prepared. A quasi-static three-point bending test was conducted at a temperature range from 200°C to -196°C. The steels, except for the UFEG steel, exhibited a typical energy transition curve, in which the fracture energy decreases with decreasing temperature. In the UFEG steel, the fracture energy increased with the occurrence of delaminating cracks as the temperature decreased from 200°C, reached a maximum at ambient temperature, and then decreased. In other words, the steel showed inverse temperature dependence of the toughness. As a result, the strengthtoughness balance of the UFEG steel was excellent compared with that of all other steels. For stronger, tougher steel, it is important to design a heterogeneous microstructure rather than a homogeneous microstructure.

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Toughening of Low-Carbon Steel by Ultrafine-Grained Structure

Cited by 11 publications

References 5 publications

Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

Influence of Carbon Content on Toughening in Ultrafine Elongated Grain Structure Steels

Influence of Prior-Austenite Grain Structure on the Mechanical Properties of Ultrafine Elongated Grain Structure Steel Processed by Warm Tempforming

Strength-toughness balance of low-alloy steel by fail-safe design

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