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ISIJ International Vol. 59 (2019), No. 7

ISIJ International
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Grid List Abstracts
オンライン版ISSN: 1347-5460
冊子版ISSN: 0915-1559
発行機関: The Iron and Steel Institute of Japan

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キーワードランキング

03 Dec. (Last 30 Days)

ISIJ International Vol. 59 (2019), No. 7

Understanding the Structure and Structural Effects on the Properties of Blast Furnace Slag (BFS)

Muhammad Sajid, Chenguang Bai, Muhammad Aamir, Zhixiong You, Zhiming Yan, Xueming Lv

pp. 1153-1166

DOI:

10.2355/isijinternational.ISIJINT-2018-453

抄録

Evolving knowledge of the structure and physical properties of metallurgical slags is summarized in current review. Slag structure, compositional effects, role of cations in structural modifications, parameters used to represent the structure, structural analysis techniques and effects of structure on properties of blast furnace slag (BFS) studied in details. The basicity, polymerization (Q) or depolymerization (NBO/T), optical basicity, Qn values, concentrations of bridging O’s (Oo), non-bridging O’s (O−) and free O’s (O2−) in slag are useful to represent the structure of slag. Methods and techniques utilized to study the slags are also discussed. The BFS is characterized by using X-ray Diffraction and Spectroscopy, Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS) and Nuclear Magnetic Resonance (NMR) Spectroscopy. The physical properties such as surface tension, viscosity, density, thermal expansion and diffusion, electrical conductivity and resistivity of slags are reviewed thoroughly which are heavily dependent on structure of slag. Viscosity is affected by polymerization or depolymerization of slag structure and cation size; electrical resistivity depends on Q, size of cations and number of available cations; thermal expansion depends on Q and cation field strength (i.e. z/r2); thermal conductivity is linked with rigidity of slag network which is also dependent on Q and metal-oxygen (M–O) bond strength. Degree of polymerization or depolymerization of slag structure also effect the surface and interfacial tension, it decreases as metal-oxygen (M–O) bond strengths (i.e. z/r2, cation field strength) decrease.

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Advances in Ladle Shroud as A Functional Device in Tundish Metallurgy: A Review

Jiangshan Zhang, Qing Liu, Shufeng Yang, Zhixin Chen, Jingshe Li, Zhengyi Jiang

pp. 1167-1177

DOI:

10.2355/isijinternational.ISIJINT-2019-044

抄録

Ladle shroud is a small but significant device in tundish metallurgy to facilitate both production process and steel quality. Past decades have witnessed its evolution from a simply shrouding tube to a multi-functional device in continuous casting processes. Advances in the functions of ladle shroud in tundish metallurgy have been reviewed in this work, including shrouding the teeming stream, fluid flow control, slag carry-over detection, and the potentials of heating and additive feeding. The features of various commercialized and novel ladle shrouds are discussed. The effect of practical operations, such as argon injection and misalignment, on the performance of ladle shrouds is also analyzed in this review.

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Effect of Temperature on Hexavalent Chromium Formation in (Al,Cr)2O3 with Calcium Aluminate Cement in Air

Yingjiang Wu, Shengqiang Song, Zhengliang Xue, Mithun Nath

pp. 1178-1183

DOI:

10.2355/isijinternational.ISIJINT-2018-699

抄録

Cr2O3 containing refractories castables are widely used as linings of various furnaces because of its remarkable corrosion resistance. However, Cr2O3 can be oxidized to toxic Cr(VI) by calcium aluminate cement phases (CA and CA2) which are used as hydraulic binder in castables. In the present research, one of the most stable Cr(III) phase viz., (Al,Cr)2O3 solid solution was synthesized and then reacted with cement phases in order to check the formation of Cr(VI). The (Al, Cr)2O3 solid solution and calcium aluminate cement (70% Al2O3) were mixed in the ratio 1:1 (mass%) and heated in the temperature range of 500–1500°C in air. The phase compositions and formation/leachability of Cr(VI) were investigated using XRD, XPS and the leaching tests. The (Al,Cr)2O3 solid solution didn’t react with calcium aluminate cement at 500–900°C, while it partially converted to a Cr(VI)-containing phase at 1100–1300°C. However, (Al,Cr)2O3 and cement phases react to form a ternary Cr(III) phase (Ca(Al,Cr)12O19) at 1500°C.

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Effect of Al2O3 Addition on the Phase Equilibria Relations of CaO-SiO2-5 wt%MgO-Al2O3-TiO2 System Relevant to Ti-bearing Blast Furnace Slag

Lifeng Sun, Junjie Shi

pp. 1184-1191

DOI:

10.2355/isijinternational.ISIJINT-2019-014

抄録

The influence of Al2O3 addition up to 30 wt% on equilibria phase relations of Ti-bearing slag system with w(CaO)/w(SiO2) of 1.00 was investigated at 1300°C and 1400°C by classical high temperature thermodynamic equilibria technique followed by XRD (X-Ray diffraction) and SEM-EDX (Scanning Electron Microscope and Energy Dispersive X-ray Fluorescence scope) analysis. Experimental results indicated that with the increase of Al2O3 addition the equilibria solid phases gradually change from melilite solid solution and perovskite to Al–Ti diopside solid solution, spinel, and pseudobrookite solid solution. Moreover, comparisons with the predictions by Factsage 7.2 revealed that the calculated equilibria phase relations were only well agreed with experimental results below 20 wt% Al2O3 and TiO2, while great deviations presented when Al2O3 content higher than 20 wt%. Based on experimental and calculated results, the 1300°C and 1400°C liquidus lines as well as the predicted primary crystal phase boundary lines were constructed for specific composition range of CaO-SiO2-5wt%MgO-Al2O3-TiO2 system.

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Melting Behavior and Interaction of Gangue Phase of Iron-containing Burden

Yu-zhu Pan, Xue-feng She, Guang Wang, Hai-bin Zuo, Jing-song Wang, Qing-guo Xue

pp. 1192-1197

DOI:

10.2355/isijinternational.ISIJINT-2018-822

抄録

The cohesive zone is an important location in the blast furnace due to decrease of permeability and sharp increase of pressure drop. Since the liquid phase has an important influence on the permeability, this study aimed to investigate the regulation of liquid phase formation in the cohesive zone. Through melting experiments of single and double slag tablets combined with thermodynamic calculations, some important phenomena were observed. The pellet slag melted prior to sinter slag, and the molten pellet slag caused the sinter slag to melt at a temperature below the melting temperature. The liquid phase of the cohesive zone originated due to the low melting point material of the gangue phase, and the interaction of different iron-containing burdens gangue phase. The production of the liquid phase was affected by the composition and temperature. The progress of the interaction seemed to be affected by the liquid phase ratio. The interaction was re-understood: interaction is physicochemical reactions of gangue phases of different iron-containing burdens which occurred during the formation of molten blast furnace slag with uniform composition. Thermodynamic calculations were consistent with the experimental results and showed that the interaction of the cohesive zone increased the liquid phase, which was disadvantageous for the permeability.

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Effect of Coke Particle Arrangement on Reduction and Gasification Reaction in Mixed Layer of Ore and Coke

Yusuke Kashihara, Yuki Iwai, Kiyoshi Fukada, Hiroshi Nogami

pp. 1198-1204

DOI:

10.2355/isijinternational.ISIJINT-2019-011

抄録

To reduce CO2 emissions from steel works, low reducing agent rate (low coke rate) operation of the blast furnace is desired. Mixing nut coke in the ore layer is one effective measure for realizing this type of operation. Therefore, the effect of coke mixing on the reduction reaction rate of ore and the gasification reaction rate of coke in the mixed layer of ore and coke was investigated. The reduction rate of the ore and the gasification rate of the coke in the mixed layer of ore and coke was estimated by a reduction and gasification experiment, and the packing structure of the coke in the mixed layer was estimated by a mathematical model analysis using the discrete element method. The reduction rate of the ore and the gasification rate of the coke in the mixed layer was affected by the degree of contact between the ore and coke. In addition, the reduction rate of the ore and the gasification rate of the coke in the mixed layer was accelerated by the effects of mutual utilization of the gases generated by the reactions.

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Solubility of Oxygen in Molten Ni–Cr Alloys in Equilibrium with Silicate Slag System Containing Cr Oxide

Yoshiharu Miyazaki, Fumiaki Kirihara, Hidekazu Todoroki

pp. 1205-1213

DOI:

10.2355/isijinternational.ISIJINT-2018-685

抄録

A study has been carried out to measure the solubility of oxygen in a molten Ni–Cr alloy contained in a MgO crucible in equilibrium with CaO–SiO2–MgO–Cr2O3 system slag. It was found that oxygen content increased with increasing Cr content in the range between 2 and 40 mass%.According to SEM observation along with EDS analysis, the oxide phases in the slag consisted of MgO·Cr2O3 crystals and CaO–SiO2–MgO slicate phases. In the crucible close to the surface in contact with the molten alloy and the slag, almost pure MgO phases were observed in addition to the above two phases. Therefore, it was confirmed that soluble oxygen in the molten Ni–Cr alloy was supplied by decomposition of the MgO·Cr2O3 phase as MgO·Cr2O3(s) = MgO(s) + 2Cr + 3O. Consequently, thermodynamic analysis led to the interaction coefficient of = −0.162±0.012 (Cr: 2–5 mass% in reference to Ni) and −0.044±0.001 (Cr: 5–40 mass% in reference to Ni-5%Cr) available in molten Ni.

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Numerical Simulation of Multiphase Flow and Mixing Behavior in an Industrial Single Snorkel Refining Furnace (SSRF): The Effect of Gas Injection Position and Snorkel Diameter

Weixing Dai, Guoguang Cheng, Shijian Li, Yu Huang, Guolei Zhang, Yunlong Qiu, Weifei Zhu

pp. 1214-1223

DOI:

10.2355/isijinternational.ISIJINT-2018-826

抄録

A three-dimensional mathematical model of an industrial Single Snorkel Refining Furnace (SSRF) was implemented in designed experiments to investigate the influence of injection position and snorkel diameter on mixing efficiency and circulation rate. The discrete phase model–volume of fluid coupled model was employed to describe the argon/steel/slag/air multiphase flow. The expansion behavior of argon bubbles was considered. The results indicated that eccentric injection is greatly beneficial for increasing circulation rate and decreasing mixing time. The effect of snorkel diameter was investigated in light of eccentric plug position. It was found that the oversized and undersized snorkel diameter are not desirable, as it contributes poor mixing at ladle bottom and periphery of snorkel, respectively. An optimum diameter was recommended according to a principle that the stirring energy of plume should be distributed reasonably for achieving homogeneous flow in the whole bath.

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Demystifying Underlying Fluid Mechanics of Gas Stirred Ladle Systems with Top Slag Layer Using Physical Modeling and Mathematical Modeling

Rodolfo D. Morales, Fabian Andres Calderon-Hurtado, Kinnor Chattopadhyay

pp. 1224-1233

DOI:

10.2355/isijinternational.ISIJINT-2018-797

抄録

Effects of slag layer thickness on the fluid dynamics of liquid steel in gas-stirred ladles by bottom injection of argon was studied through water modeling experiments and numerical simulations. Mixing times increase considerably with thicker slag layers and decrease of gas flow rates. The physical properties of the system have a smaller influence on mixing time. Slag Eye Opening (SEO) area is increased under thin slag layers, increase of gas flow rates, and denser and less viscous slags. The planes close to the metal-slag interface, under the presence of thick slag layers for a given gas flow rate, are split in subregions of small velocities with different orientations making the lower fluid to come close to a stagnant condition. The presence of, either, thick or thin slag layers does not influence the axial velocity along the plume height for a fixed flow rate of gas. The SEO area follows a linear relationship with the square root of the densiometric Froude number based on the slag layer thickness.

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Effect of Ultra-high Magnesium on SKS51 Liquid Steel Cleanliness and Microstructure

Zhouhua Jiang, Guang Xu, Yang Li, Huabing Li, Jianbo Lv, Qi Wang

pp. 1234-1241

DOI:

10.2355/isijinternational.ISIJINT-2018-804

抄録

This study emphasizes that ultra-high Mg ([%Mg]>0.03) content is very difficult to be achieved under conventional smelting conditions. The content of Mg in steel has significant influence on the cleanness of molten steel and microstructure. As the content of Mg increases, the content of O and S in steel decreases significantly, with O content as low as 0.0002% and S content as low as 0.0008%. Almost all inclusions in ultra-high Mg steel are magnesium-bearing oxide, sulfide, even carbide. As the content of Mg increases, the number of inclusions in the steel increases and the size decreases. But if too much Mg is added into the steel, the size and number of inclusions will rapid increase. The as-cast secondary dendrite spacing of steel decreased obviously with the increase of Mg content in steel. As the content of Mg increases, the as-cast microstructure changes from lamellar pearlite to granular pearlite. The phase diagram of SKS51 steel was calculated by Thermal-calc software. The calculated results showed that MgC2 was precipitated in the austenite before the austenite was converted into perlite. MgC2 may become the nuclear core, leading to perlite transformation. Suspected MgC2 was found in spheroidized annealing Fe3C core.

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Effects of CeO2 on Melting Temperature, Viscosity, and Structure of CaF2-bearing and B2O3-containing Mold Fluxes for Casting Rare Earth Alloy Heavy Rail Steels

Zeyun Cai, Bo Song, Zhanbing Yang, Longfei Li

pp. 1242-1249

DOI:

10.2355/isijinternational.ISIJINT-2018-760

抄録

Rare earth have been widely used in the heavy rail steels to improve the impact toughness, yield strength, and high-temperature plasticity by deforming inclusions and refining grains. However, amounts of rare earth oxides with high melting temperatures can be generated and enter the mold fluxes, which has a significant influence on the lubrication and the heat transfer capability of mold fluxes and even the smooth of the continuous casting process. Thus in this work, effects of different CeO2 contents on melting temperature, viscosity, and structure of the CaF2-bearing mold fluxes and the B2O3-containing mold fluxes for casting the rare earth alloy heavy rail steels were investigated systemically. Besides, the mechanisms of the viscosity and structure were discussed. Results show that CeO2 increases the melting temperature and decreases the viscosity at high temperature of both two mold fluxes. The break temperature of the CaF2-bearing mold fluxes increases heavily with the addition of CeO2. Analyses of Raman spectra and the 11B magic angle spinning nuclear magnetic resonance spectra (11B MAS NMR) show that CeO2 enhances the de-polymerization of the network structure of two mold fluxes, leading to the decrease of viscosity at each temperature.

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Numerical Investigation of Effect of Casting Speed on Flow Characteristics of Molten Steel in Multistrand Tundish

Fei He, Haijun Wang, Zhenghai Zhu

pp. 1250-1258

DOI:

10.2355/isijinternational.ISIJINT-2018-835

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Casting speed is one of the key factors affecting flow characteristics in the tundish. This paper gives a detailed numerical investigation for the effect of casting speed on flow characteristics in a five-strand tundish. Firstly, a quantitative evaluation method of flow characteristics (such as dead region, short-circuit flow and flow uniformity among multiple strands) is proposed. And then, by using the method, the effect of the cases (such as reducing to the same casting speed for each strand, closing one or two strands, etc.) on flow characteristics in the case of reducing throughput is analyzed. The results show that the case of closing strands 2 and 4 has the best flow characteristics, and not only dead region volume is smaller, but also the flow difference among multiple strands is also smaller. Meanwhile, in view of slower flow and bigger dead region near the far-strand of the tundish and large difference for flow characteristics between the far-strand and other strands, the effect of increasing the far-strand’s casting speed on improving the overall flow characteristic in the tundish is also analyzed. The results show that, increasing the far-strand’s casting speed can significantly reduce dead region proportion and flow difference among multiple strands, and in the case of the same throughput, the short-circuit flow does not change much. In the case of increasing the same throughput, flow characteristics in the tundish for only increasing the far-strand’s casting speed are better than that for increasing casting speed of each strand simultaneously.

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An in situ Study of the Formation of Rare Earth Inclusions in Arsenic High Carbon Steels

Hongpo Wang, Bin Bai, Silu Jiang, Lifeng Sun, Yu Wang

pp. 1259-1265

DOI:

10.2355/isijinternational.ISIJINT-2018-853

抄録

The application of rare earths is an effective way to stabilize residual elements in steel, such as As and P, so as to improve the performances of steel products. In situ methods were used to investigate the formation of inclusions and their stability at high temperatures in arsenic high carbon steels with additions of lanthanum. The results show that La2O3 and La2O2S started generating in molten steel and had significant difference of appearances and growth behaviors. La2O3 started with triangular particles and rapidly grew up like crystals; by contrast, La2O2S particles were always spherical or near-spherical and didn’t significantly grow up. Arsenic existed as LaAsO4 that turned out to be unstable under high temperatures. LaAsO4 decomposed and As dissolved into the matrix when the temperature was higher than 1200°C. The formation of LaAsO4 during solidification and the dissolution of As into the matrix during heat treatment can effectively avoid the local enrichment of As. Therefore, it is possible to control As distributed uniformly in steel by appropriate heat treatment process.

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Mathematical Modeling of Flow and Heat Transfer Behavior of Liquid Slag in Continuous Casting Mold with Argon Blowing

Changgui Cheng, Haibiao Lu, Yang Li, Xufeng Qing, Yan Jin

pp. 1266-1275

DOI:

10.2355/isijinternational.ISIJINT-2018-832

抄録

The bubbles generated by argon blowing in a nozzle have important effects on the flow and heat transfer behavior of mold slag. To determine the effect of argon blowing on the flow and heat transfer behavior of liquid slag in the mold, we developed three-dimensional mathematical models coupled the volume of fluid and discrete phase models. The results showed a small circulation flow of liquid mold slag occurred near the nozzle side face and the mold narrow face respectively at the center plane between the mold wide faces. Additionally, we identified a larger circulation of liquid mold slag in the middle region of the mold. With increased argon flow rate, the flow velocity peak at the liquid steel and slag interface decreased, the temperature of the liquid mold slag increased. A moderate flow rate of argon improved the uniformity of flow velocity and temperature distribution of liquid slag and reduced the flow velocity peak at the interface of the liquid steel and slag. To avoid the solidification of liquid steel at the steel-slag interface near the mold face, moderately high casting speed and argon flow rate and larger inclination angle and immersion depth of the submerged entry nozzle may be beneficial. These results provide a theoretical basis to optimize the parameters of the argon blowing process and improve slab quality.

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A Novel Process Modeling Method for Steel Sulphur Content Soft Sensing during Ladle Furnace Steel Refining

Wu Lv

pp. 1276-1286

DOI:

10.2355/isijinternational.ISIJINT-2018-542

抄録

Steel sulphur content soft sensing is of great importance for optimal control of the desulphurization process during ladle furnace (LF) steel refining. However, the soft sensing models in the literature at present are not able to capture the multi-stage characteristics. For addressing this problem and thereby obtaining satisfactory performance, stage-based modeling is proposed by virtue of sub-models ensemble. The central idea of this method is to establish several individual sub-models in order to focus on the local process property of each stage during desulphurization. Furthermore, soft partition strategy using nonparametric regression is developed for realizing soft handoff among the sub-models of successive stages, by which the close and changing process properties in the stage-to-stage transition region can be accurately described. Finally, the effectiveness of the presented method is validated by practical data. It can be concluded from experiments that the proposed stage-based modeling approach is able to significantly improve the sulphur content soft sensing performance, which makes it helpful in both process monitoring and operations optimization for LF process.

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Ensemble Prediction of Tundish Open Eyes Using Artificial Neural Networks

Alvin Ma, Saikat Chatterjee, Kinnor Chattopadhyay

pp. 1287-1294

DOI:

10.2355/isijinternational.ISIJINT-2018-637

抄録

As global steelmakers are feeling the economical pinch, the need for improving quality and quantity using what is already readily available, increases. This gap in achievement can be bridged by innovation and perforation of already existing techniques and methodologies from other fields. Steel quality, an important issue, is often not associated with a phenomenon known as tundish open eyes. However, recently researchers have shown the detrimental effects of reoxidation and the deterioration of the final product (slabs/billets). Understanding the formation of this event, and mitigating the formation will be an important issue to solve. Current models investigating the former have existed largely in the computational fluid dynamics modelling domain. However, the solution for the former, can only provide static recommendations thus are less useful in a dynamic environment. Hence, development of a reliable model which has the ability to “learn on the fly” is very much needed. In the current study, artificial neural network models have been used to predict non-dimensional open eye sizes in the tundish. The dataset has been compiled from previous regression formulations. The performance of the models is determined based on the following metrics 1) coefficient of multiple determination (R2), 2) and root mean square error (RMSE). The ANN based models, show significant promise, in particular the ensemble variants, which have shown increased accuracy and stability across all domain and range.

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Effect of Deformation on Microcrack Depth in Hot Press Formed Part with Galvanized Steel Sheet

Jong Won Choi, Kyung Seok Oh, Hong Gee Kim, Dong Jin Kim

pp. 1295-1303

DOI:

10.2355/isijinternational.ISIJINT-2018-734

抄録

Surface cracks or microcracks have been reported as one source of fatigue life deterioration. Propagation of microcracks into the steel substrate may appear on a deformed part when zinc-coated steels are used for direct hot press forming. This work investigated crack occurrence in galvanized steel sheets with coupled thermo-mechanical finite element simulations and realistic HPF experiments. This study used a two-dimensional hat-shaped tool called Conventional–Forming to effectively feature a cross-section of hot press formed parts in a vehicle. Microcracks occurred on the product wall-outside when the Conventional-Forming die moved vertically and the tool contacted the material. The depth of microcracks in the deformed zone where compressive-bending and unbending followed by a straightening occur is highly dependent on the magnitude of effective strain. Therefore, it is presumed that smaller magnitudes of effective strain from forming can reduce the microcrack depth on the workpiece. To identify ways to lower the effective strain, the study focused on the tool contact and bending curvature of the workpiece during forming. Consequently, Side-Forming, in which a material undergoes deformation behavior from a die entering from an oblique direction, was proposed. With other conditions unchanged, the numerical calculations and experimental results indicated a lowered effective strain and a subsequent significant reduction in microcrack depth. Moreover, the required qualities for hot formed products such as hardness and shape accuracy could be also achieved.

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Photographic Observations of the Transient Contact Behavior of Oil-in-Water Emulsion Droplets Impinging on a Hot Substrate

Hitoshi Fujimoto, Hiroaki Takeshita, Masayoshi Ashida, Wataru Obana, Takayuki Hama, Hirohiko Takuda

pp. 1304-1313

DOI:

10.2355/isijinternational.ISIJINT-2018-790

抄録

Oil-in-water (O/W) emulsions are widely used as the working fluids in various metalworking processes. In this study, the transient liquid/solid contact behavior of O/W emulsion droplets impinging on a hot sapphire prism was experimentally investigated in order to gain insight on spray jet impingement in industrial applications. Three-directional flash photography method (with three digital cameras and four flashlights) was used to obtain the time evolutions of the droplet shape and liquid/solid contact behavior. Water, base oil, and O/W emulsions (oil concentration: 5, 15 mass%) were chosen as the test liquids. The experiments were conducted under a droplet impact velocity of 1.0 m/s and the pre-impact diameter of the droplets was ~2.3 mm. The temperature of the substrate was varied from 150°C to 400°C. The results showed that there was less direct liquid/solid contact at higher substrate temperatures because the boiling of water was more intense. There was no direct liquid/solid contact at a substrate temperature of 400°C. In addition, the direct liquid/solid contact areas were smaller for the O/W emulsion with higher oil concentration because of the vapor trapped between the solid surface and oil-rich liquid with high viscosity. Furthermore, it was found that the oil adhesion on the test substrate rarely occurred except at lower substrate temperatures.

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Work Roll Surface Profile Design and Optimization for Hot-core Heavy Reduction Rolling Process

Tianxiang Li, Haijun Li, Ruihao Li, Zhaodong Wang, Guodong Wang

pp. 1314-1322

DOI:

10.2355/isijinternational.ISIJINT-2018-809

抄録

Hot-core Heavy Reduction Rolling (HHR2) is an innovative technology designed for eliminating center defects of blooms, which provides heavy reduction to blooms with two-high mill after solidification at the end of the strand. This works mainly focus on design and optimization of work roll profile that apply specifically to HHR2 process to obtain the best effect on shrinkage closing. Firstly, hot rolling experiment and corresponding finite element calculation were carried out. Based on the experiment and FEM results, the void closure model was established to describe the behavior of shrinkage closing. Secondly, this model was used in analyzing the effects of different roll profiles on void closure during HHR2 process. The result shows that the convex profile and box groove profile had better effects than flat profile and parabolic profile, which can provide greater value of effective strain and smaller value of stress triaxiality respectively. Finally, a new roll profile for HHR2 was designed by combining both geometrical features of convex profile and box groove profile. The rational value scope of convex width coefficient θ and convex height coefficient γ were optimized to achieve a better effect on eliminating shrinkage cavities.

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Relationship between Alloy Element and Weld Solidification Cracking Susceptibility of Austenitic Stainless Steel

Seidai Ueda, Kota Kadoi, Shun Tokita, Hiroshige Inoue

pp. 1323-1329

DOI:

10.2355/isijinternational.ISIJINT-2019-035

抄録

The effect of alloy elements such as niobium, titanium, and zirconium on the weld solidification cracking susceptibility in fully austenitic stainless steel was investigated. Niobium, titanium, or zirconium was added as an alloy element to Fe-24 mass%Cr-26 mass%Ni stainless steel. The cracking susceptibility was evaluated by crack length, number of cracks, and brittle temperature range (BTR) corresponding to results of the Trans-Varestraint test. Depending on the addition of the alloy element, the crack length increased; the length ordering tendencies between the total crack length (TCL) and the maximum crack length (MCL) differed with the alloy addition. The BTR was obtained by corresponding the MCL to the temperature range using the measured temperature history of the weld metal and was increased by the addition of the alloy element. The maximum BTR for the specimen with titanium was 266.9°C, which was three times that of the specimen without the alloy element. The MC carbide and the Laves phase formed at the dendrite cell boundaries as secondary phases. Solidification calculation based on the Scheil model was used to investigate the effect of the type of the alloy element on the solidification temperature range. Depending on the type of the alloy element, the solidification temperature range varied. A significant difference was found between the solidification temperature range and BTR in the case of the specimen with niobium.

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Quantitative Prediction of Sensitization in Austenitic Stainless Steel Accounting for Multicomponent Thermodynamic and Mass Balance Effects

Satish Kolli, Thomas Ohligschläger, David Porter

pp. 1330-1336

DOI:

10.2355/isijinternational.ISIJINT-2018-715

抄録

Double Loop Electrochemical Potentiokinetic Reactivation testing has been employed to experimentally determine the degree of sensitization (DOS) of an austenitic stainless steel subjected to isothermal heat treatment for various times in the temperature range 700–820°C. For the different heat treatment conditions, the chromium concentration profiles across grain boundaries were calculated using the diffusion module in Thermo-Calc® based on the assumptions that sensitization is caused by grain boundary M23C6 precipitates and that local multicomponent equilibrium and flux balance exist at the carbide - matrix interface. Comparison of the experimental DOS values and the details of the chromium concentration profiles was used to establish a quantitative depletion factor that predicts sensitization at short annealing times.

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Effect of Carbon Content on Toughness of Tempered Martensitic Steels Analyzed by Toughness Prediction Model

Tetsuya Namegawa, Manabu Hoshino, Masaaki Fujioka, Masanori Minagawa

pp. 1337-1343

DOI:

10.2355/isijinternational.ISIJINT-2018-654

抄録

To evaluate the effect of carbon content on the toughness of tempered martensite, the critical crack tip opening displacement (CTOD) was evaluated for 0.1/0.3/0.5C-1.5Mn-1.0Mo (mass%) steels. The critical CTOD was the highest for 0.5C steel because of grain refinement rather than strengthening and cementite coarsening. The results were analyzed by a toughness prediction model that considered the strength, grain size, and cementite size. This model incorporated the microstructure information, stress distribution calculated using the finite element method (FEM), and fracture process criteria. It calculated the fracture point at which the local stress and local strength of the material correspond. The fracture process was divided into the following three stages: Stage I, cementite cracking; Stage II, microcrack propagation into the cementite and ferrite boundary by stress concentration caused by dislocation pileup along the major axis of the martensite block; and Stage III, crack propagation into the first intersecting 15°-oriented boundary with the crack length of the minor axis of the martensite block. The model calculation reflected experimental trends, revealing that the bottleneck in the fracture process was Stage III. Therefore, the refinement of the minor axis of the block was effective for toughness improvement.

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Effect of Pearlite Volume Fraction on Two-step Ductile to Brittle Transition in Ferrite + Pearlite Structure Steel Sheets

Hiroyuki Kawata, Osamu Umezawa

pp. 1344-1353

DOI:

10.2355/isijinternational.ISIJINT-2018-764

抄録

Multi structural steels exhibit high strength and good formability; however, their performance depends on the volume fraction of the secondary phase. In our previous study, ferrite + pearlite structural steel sheet showed the characteristic two-step ductile-to-brittle transition (DBT) with decreasing temperature, and the absorbed energy curve exhibited a distinct middle shelf. In this study, we evaluated the effect of pearlite volume fraction (VP) on the DBT behavior by the Charpy impact test with sub-size specimens. For specimens without pearlite, the absorbed energy directly dropped from the upper shelf to the lower shelf with decreasing temperature. For samples with 2–3% pearlite, the absorbed energy corresponding to the transition temperature range was dispersed between the two shelves, and the transition behavior seemed to be the typical DBT behavior. When VP was increased to 21%, the absorbed energy just above the transition-finish temperature became stable at a middle level between the two shelves; thus, the existence of a distinct middle shelf was confirmed. Although the transition-start temperature increased with increasing VP, VP did not affect the transition-finish temperature and the absorbed energy at the middle shelf. These results were analyzed with a simple model based on the Yoffee diagram.

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Mechanical Properties and Microstructures of Mo2C Strengthened Vanadis 4 Extra Alloy Steel by Powder Metallurgy and Heat Treatments

Shih-Hsien Chang, Chan-Yu Chuang, Kuo-Tsung Huang

pp. 1354-1361

DOI:

10.2355/isijinternational.ISIJINT-2018-781

抄録

In this work, different ratios of molybdenum carbide (1, 3 and 5 mass% Mo2C) powders were added to Vanadis 4 extra alloy steel powders and then mixed by ball milling for 6 h. The composite powders underwent vacuum sintering at 1200, 1220, 1240, 1260 and 1280°C for 1 h, respectively. The results showed that the optimal sintering temperature for the addition of 5 mass% Mo2C powders was 1220°C. It also represented that the apparent porosity was 0.18%, and that a transverse rupture strength (TRS) value of 2281.3 MPa and a hardness value of 79.2 HRA were obtained, respectively. Additionally, the TRS value was obviously enhanced to 2437.6 and 2491.4 MPa by the addition of 5 mass% Mo2C powders after heat treatment and sub-zero plus heat treatments, respectively. Meanwhile, the hardness value also increased to 80.6 and 81.3 HRA, respectively, whereas the Mo-rich M6C carbides distributed in the grain boundaries, and V-rich MC carbides appeared in the grain and grain boundaries after sub-zero plus heat treatments. Significantly, a series of heat treatment processes is effective in improving the microstructure and strengthening the mechanical properties of the sintered Vanadis 4 extra composites.

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Void Nucleation, and Growth during Tensile Deformation of Nanoscale Precipitated Steel and Bainitic Steel

Yasutaka Mugita, Masatoshi Aramaki, Masayuki Yamamoto, Akihisa Takeuchi, Miyuki Takeuchi, Takeshi Yokota, Yoshimasa Funakawa, Osamu Furukimi

pp. 1362-1368

DOI:

10.2355/isijinternational.ISIJINT-2018-762

抄録

A local elongation of 8% for nanoscale precipitated steel was observed via tensile testing, which is higher than that of 5% for bainitic steel. To determine the factor underlying this difference, void nucleation, growth, and coalescence mechanisms in the nanoscale precipitated steel and the bainitic steel were examined using electron backscattering diffraction and subsequent observation by synchrotron radiation X-ray laminography during tensile testing. Synchrotron radiation X-ray laminography analysis of void growth and coalescence revealed that the critical strain and the critical void volume fraction for fracture in the bainitic steel were smaller than those for the nanoscale precipitated steel. Secondary-ion mass spectrometry analyses revealed that C atoms were segregated at grain boundaries in the bainitic steel. Void nucleation sites in the nanoscale precipitated steel were nanoscale precipitates inside the grain and at grain boundaries and coarse precipitates; however, in the bainitic steel, void nucleation sites were entirely at grain boundaries. Nanoindentation hardness measurements showed a larger plastic strain gradient between the grain boundary and matrix in the bainitic steel than in the nanoscale precipitated steel. From these results, the high local elongation exhibited by the nanoscale precipitated steel was concluded to be due to the reduced plastic strain gradient with a uniform hardness distribution between the grain boundary and the grain interior.

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Internal Friction Behavior Associated with Martensitic Decomposition in Low-carbon Dual-phase Steel

Jinfeng Zhang, Xiaochun Wu, Na Min, Shungui Zuo, Mingjiang Jin

pp. 1369-1374

DOI:

10.2355/isijinternational.ISIJINT-2018-718

抄録

In this study, the martensitic decomposition behaviors of a low-carbon dual-phase steel were investigated by the low-frequency internal friction method, combined with various structural analysis techniques including X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Two internal friction peaks were observed at 418.4°C and 448.1°C, and were attributed to desolventization of supersaturated carbon atoms from martensite and formation of Cr3C7 precipitates, respectively. The results indicated that the two-step process during the martensitic decomposition, involving carbon atom diffusion and carbide precipitation, could be well explained by the internal friction technique. The microstructural mechanisms associated with the generation of the two internal friction peaks during the martensitic decomposition are discussed.

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