鉄と鋼
Online ISSN : 1883-2954
Print ISSN : 0021-1575
ISSN-L : 0021-1575
特集号: 鉄と鋼
102 巻 , 6 号
特集号「鋼脆性破壊の微視的機構と靭性のミクロ組織依存性」
選択された号の論文の11件中1~11を表示しています
巻頭言
論文
  • 吉村 信幸, 白幡 浩幸, 星野 学, 川畑 友弥, 粟飯原 周二
    2016 年 102 巻 6 号 p. 276-285
    発行日: 2016年
    公開日: 2016/05/31
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    Many studies on brittle fracture initiation mechanisms have focused on Ferrite-Pearlite steel. Recently, high-strength steel has attracted attention in order to achieve cost saving. Since the 1970’s, Bainite microstructure has been commonly used for structural high-strength steel instead of Ferrite-Pearlite steel. In engineering, it is important to understand the mechanism of brittle crack initiation of Bainite as well as that of conventional Ferrite-Pearlite. However, due in part to the complexity of the Bainite microstructure, its micro-mechanism has not been revealed. In this study, six laboratory-scale steels with a composition of 0.2%C-2%Mn-1.5%Ni were prepared in order to clarify the qualitative and quantitative effects of several candidate controlling factors on brittle crack initiation behavior, which are phase transformation, prior austenite grain size, Martensite-Austenite constituent amount and size, cementite shape and size and orientation information inside the prior austenite grain. Charpy impact tests were used for analysis of the brittle crack initiation conditions. The results suggest that the most effective key factor in degrading the brittle crack initiation behavior is the existence of the tempered Martensite-Austenite constituent on the prior austenite grain boundary.
  • 森戸 茂一, 林 泰輔, Pham Anh Hoang, 川畑 友弥
    2016 年 102 巻 6 号 p. 286-294
    発行日: 2016年
    公開日: 2016/05/31
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    Crack propagations of a lathlike upper bainite with martensite-austenite (MA) regions were examined, and a fracture unit of the crack propagation was determined by means of a scanning electron microscope equipped with an electron backscatter diffraction detector. Fracture surfaces of the cracks were parallel to {001} planes in not only bainite but also the MA regions, and the cracks were arrested at the MA regions. In tempered specimens, the crack planes were not always parallel to the {001} planes, additionally the MA regions were deformed by the cracks. Although the MA regions arrest the cracks, macroscopic crack planes were flat in the similar crystal orientated regions in the bainite. To consider the macroscopic fracture unit of the crack propagation, a crystallographic unit which is a single crystal grain and a morphological unit which corresponds to a single bain group region were measured. From the measurements, it was found that the crystallographic unit was better than the morphological unit in the crack propagation. One of the reasons was that the prior austenite grain boundaries change to low angle grain boundaries after bainitic transformation, variant selection rules constrain the bainitic ferrites to form in both the austenite grains with a similar crystal orientation.
  • 田川 哲哉, 高山 直樹, 今村 駿吾, 伊木 聡
    2016 年 102 巻 6 号 p. 295-303
    発行日: 2016年
    公開日: 2016/05/31
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    Microstructures at the cleavage triggers were investigated in Bainitic steels. Bainitic steels with different Austenite grain size and second phase were prepared, and the fracture absorbed energies in cleavage fracture were examined under static loading with Charpy type bend specimens. Cleavage trigger point was identified on each fracture surface by fractography, and was exposed into an etching process. The correspondence of cleavage facets around a trigger with Bainite microstructural unit was investigated. Bainite packet boundaries were frequently observed on a continuous cleavage facet. Fracture surfaces were sectioned, and the cross sections which included a trigger were polished. These cross sections were exposed onto EBSD observations, and the correspondence of cleavage facets with Bainite microstructure was crystallographically investigated. Optically observed microstructure boundary did not always act as a resistance against a cleavage cracking, and EBSD analyses suggested that the crystalline orientation units such as a Bain group controlled a cleavage facet unit.
  • 郭 光植, 眞山 剛, 峯 洋二, 高島 和希
    2016 年 102 巻 6 号 p. 304-310
    発行日: 2016年
    公開日: 2016/05/31
    [早期公開] 公開日: 2016/01/29
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    Micro-tensile testing combined with analysis using a crystal plasticity finite element method (CPFEM) was employed to elucidate the deformation behaviour of bainite/martensite structures of a low alloy steel. The bainite single phase specimens exhibited habit-plane-orientation-dependent yielding similar to the martensite single phase specimens. In the bainite/martensite specimen, deformation concentrated in the bainite region oriented favourably for the in-habit-plane slip, leading to low ductility fracture. With consideration of the habit-plane-orientation-dependent yielding, the present CPFEM analysis successfully reproduced the anisotropic plastic deformation behaviour of the single phase steels obtained in the experiments. The numerical result for the bainite/martensite specimen showed slip localization in the bainite region and stress concentration near the interphase boundary. This suggests that the interphase boundary can be a site for the fracture origin.
  • 本間 祐太, 茅野 林造, 長井 寿
    2016 年 102 巻 6 号 p. 311-319
    発行日: 2016年
    公開日: 2016/05/31
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    The steels with intermediate stage transformation microstructures (Zw) show a good balance of strength and toughness. Especially, fracture appearance transition temperature (FATT) in Charpy impact test is one of the most important properties of steel to assure material reliability. As microstructural factors to govern FATT, the effective grain size (dEFF) on cleavage fracture surfaces is well known in ferrite-pearlitic and martensitic steels. However, the relationship between FATT and dEFF have not yet been clarified for the steels with Zw. Low-alloy heat-resistant steels for pressure vessels such as Cr-Mo steels are the type with Zw and demands for better low temperature toughness are becoming severer in recent years. The present study aims to determine the relationship between FATT and dEFF, and to clarify the microstructural factor to govern toughness in quenched and tempered Cr-Mo steel with a Zw microstructure by varying the prior austenite grain size. Not the prior austenite grain size but the bainite block section width corresponded to dEFF in the size distribution. Accordingly, FATT decreased with decreasing block section width.
  • 川田 樹, 平出 隆志, 柴沼 一樹, 川畑 友弥, 粟飯原 周二
    2016 年 102 巻 6 号 p. 320-329
    発行日: 2016年
    公開日: 2016/05/31
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    The present authors propose a cleavage fracture initiation model for bainite steels. The authors considered three stages of fracture initiation in the model: stage I; micro crack initiation in martensite-austenite constituent (MA) in low carbon bainite, stage II; propagation of the micro crack into low carbon bainite and stage III; propagation of the cleavage crack across grain boundary. Stage I is described as probabilistic event; cracking probability is formulated based on the experimental results. Stage II and Stage III are formulated by the fracture stress theory. In this model, multiple volume elements are defined around a notch tip and microstructure is arranged for each volume element. In each time step, Stage I, II and III are judged with stress and strain at each volume element obtained by finite element method. The authors assume that cleavage fracture is initiated when the conditions of these three stages are simultaneously satisfied in any one of the volume elements. The present model is validated by comparison between simulation results and experimental results of notched three point bend tests. The simulation results and the experimental results show good agreement with regard to fracture toughness and fracture initiation points.
  • 柴沼 一樹, 根本 義規, 平出 隆志, 鈴木 克幸, 粟飯原 周二
    2016 年 102 巻 6 号 p. 330-339
    発行日: 2016年
    公開日: 2016/05/31
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    This study presents investigations of micro-crack initiation in pearlite microstructure as a trigger of unstable cleavage crack propagation in ferrite-pearlite steel. In order to clarify the micro-crack initiation mechanism, a trace analysis was conducted to compare the direction of crack surface with those of cleavage and slip planes of ferrite phase in pearlite structure. It was found that any directions of crack surface were not coincident with those of {100} planes. On the other hand, all of the directions of crack surfaces showed good agreement with those of {110} planes. The result showed a possibility that micro-cracks in pearlite structure are formed by shear fracture on slip planes of the ferrite phase. The condition of unstable propagation from a micro-crack in pearlite structure into a neighbor ferrite grain was investigated. Effective surface energy was estimated by crack length obtained by SEM observation and local stress calculated by finite element analysis. The result showed the estimated effective surface energy is larger than that of cleavage crack propagation across boundary between ferrite grains. A probability of micro-crack initiation in pearlite structure was quantified by measurement of micro-cracks in steels having various ferrite-pearlite microstructures and finite element analysis. As the result, the probability of micro-crack initiation could be effectively estimated as a function of only the equivalent plastic strain, independently from temperature, volume fraction of pearlite and loading condition.
    EBSD images near micro-cracks together with {100} plane or {110} plane traces. (Online version in color.) Fullsize Image
  • 田中 將己, 安井 隼人, 東田 賢二
    2016 年 102 巻 6 号 p. 340-346
    発行日: 2016年
    公開日: 2016/05/31
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    Low temperature temper embrittlement of low carbon steels was investigated. 0.2 mass % carbon steels with fully martensitic structure were employed. The specimen was tempered at 473 K, 623 K and 873 K for 7.2 ks. Impact absorbed energy tested at room temperature shows the lowest value for the specimen tempered at 623 K while the value of Vickers hardness decreases with the increasing in the temperature tempered. Temperature dependence of impact absorbed energy was also measured, exhibiting the brittle-to-ductile transition temperature was highest in the specimen tempered at 623 K, which is low temperature temper embrittlement. Inter-granular fracture was observed in the specimens tempered at 623 K and 873 K, which indicates that tempering at not less than 623 K tends to increase the BDT temperature due to the inter-granular fracture. The temperature dependence of yield stress was also measured, showing that the athermal stress in the specimen tempered at 873 K was drastically decreased, which tends to decrease the BDT temperature. These opposite temperature dependences against BDT temperature induce the low temper embrittlment.
  • 柴沼 一樹, 山本 悠貴, 柳本 史教, 鈴木 克幸, 粟飯原 周二, 白幡 浩幸
    2016 年 102 巻 6 号 p. 347-355
    発行日: 2016年
    公開日: 2016/05/31
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    A new multiscale model is proposed by a “model synthesis” approach, as the first attempt to clarify the relationship between microstructures of steel and macroscopic brittle crack propagation and arrest behavior. The first part of the present paper shows the model presentation. The multiscale model consists of two models: (1) a microscopic model to simulate cleavage fracture in the grain scale and (2) a macroscopic model to simulate brittle crack propagation and arrest behavior in the steel plate scale. In both the models, we utilize the same framework, where a simple two-dimensional domain discretization is employed but a three-dimensional crack propagation can be effectively modeled. The discretized unit cells in the microscopic model correspond to the grain size. On the other hand, the discretized unit cells in the macroscopic model correspond to the entire domain of the microscopic model. The microscopic model proposed by Aihara and Tanaka is basically employed except the integration with the macroscopic model. The effective surface energy, which is used for the integration between microscopic and macroscopic models, is assumed as the plastic work to form tear-ridge. The proposed model synthesis for multiscale model as an integrated macroscopic model is performed by systematically incorporating (1) the preparatory macroscopic finite element analysis and (2) the Monte Carlo simulation of microscopic analysis into (3) the macroscopic analysis for brittle crack propagation and arrest in steel plate. The integration procedure is implemented by the assignment of physical quantities based on interpolation methods as a one-way coupling algorithm for simplification.
    Procedure of the integrated macroscopic model by model synthesis in the multiscale model. (Online version in color.) Fullsize Image
  • 山本 悠貴, 柴沼 一樹, 柳本 史教, 鈴木 克幸, 粟飯原 周二, 白幡 浩幸
    2016 年 102 巻 6 号 p. 356-364
    発行日: 2016年
    公開日: 2016/05/31
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    The second part of the present paper shows an application of the proposed multiscale model to the temperature gradient crack arrest test of the steel plates having nonhomogeneous distributions of microstructures in thickness direction. The multiscale model is developed as the integrated macroscopic model composed of the three-staged analyses. The first stage is a preparatory macroscopic finite element analysis, where the nodal force release method is employed to simulate fast crack propagation under the dynamic elastic-plastic condition without considering non-linearity of geometry. The second stage is the Monte Carlo simulation of microscopic analysis for cleavage fracture at the discrete evaluation points. The results of local fracture toughness and direction of fracture surface show large scatters even at the same evaluation point. The final stage is the integrated macroscopic analysis, which is composed of the two parts: (a) assignment of parameters obtained in the previous analyses in each unit cell, and (b) simulation of brittle crack propagation/arrest behavior. As a result, the proposed multiscale model successfully simulated the complicated brittle crack propagation/arrest behavior. In particular, not only the arrested crack length but also the characteristic fracture surface such as “split nails” were accurately simulated. It is therefore found that the proposed model has been validated by the comparison with experiment. That is, the proposed model in the present study has a potential basis of the framework to establish the theory for the clarification of the relationship between microstructures of steel and macroscopic arrest toughness of steel plate.
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