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ISIJ International Vol. 57 (2017), No. 7

ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559
Publisher: The Iron and Steel Institute of Japan

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21 Nov. (Last 30 Days)

ISIJ International Vol. 57 (2017), No. 7

Temperature Control Technology by Finite Difference Scheme with Thickness Unequally Partitioned Method in Gradient Temperature Rolling Process

JingGuo Ding, Zhong Zhao, ZhiJie Jiao, Jun Wang

pp. 1141-1148

DOI:

10.2355/isijinternational.ISIJINT-2016-679

Abstract

In order to solve the problem of low the computational speed in gradient temperature rolling (GTR) process and guarantee the accuracy of temperature distribution in thickness direction, temperature control technology by finite difference scheme with thickness unequally partitioned method was investigated. The thickness partitioned method is adopted by means of equidistance logarithmic value for thickness, which causes the distance of two neighboring nodes to be larger as thickness increasing and gets the effect of more detailed grid on surface and rough grid in the core. Finite difference scheme was used to study the formation law of the temperature field which considers effect of oxide scale thickness in thickness direction, then temperature distributed regularity with different heat transfer coefficients in thickness direction of plate was obtained. The temperature distributions of gradient temperature rolling process is calculated while rolling procedure running, temperature gradient change rules over time with certain water jet pressure as well as temperature gradient change rules with different water jet pressures are studied. The industrial testing result illustrates that calculated rolling force reference and rolling torque reference are slightly larger than neighboring values by about 10% when gradient temperature rolling mode is used in pass 1, pass 3, pass 5 and pass 7. From comparison of measured temperature and calculated temperature, it indicates that prediction difference can be controlled with ±5°C when node number equals to 16 with half thickness and deviation of calculated force and measured force can be controlled with ± 4.0%.

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Temperature Control Technology by Finite Difference Scheme with Thickness Unequally Partitioned Method in Gradient Temperature Rolling Process

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Intermittent Microscopic Observation of Structure Change and Mineral Reactions of High Phosphorus Oolitic Hematite in Carbothermic Reduction

Yuan Yuan Zhang, Qing Guo Xue, Haibin Zuo, Cheng Cheng, Guang Wang, Fei Han, Jing Song Wang

pp. 1149-1155

DOI:

10.2355/isijinternational.ISIJINT-2016-690

Abstract

Change of the typical structure of high phosphorous oolitic hematite during carbothermic reduction process have been studied using intermittent observation. It indicated that there were four change stages, including crack formation and growth, mineral diffusion, mineral crystalization, mineral crystal migration and reaction. Eventually, inherent oolitic structure was completely destroyed. Mineral reactions were also investigated and discussed along the structural change, giving much understanding of thermal behavior of the oolitic hematite during reduction. The results showed a variety of mineral reactions occurred, such as decomposition of CaCO3, reduction of iron oxide, dehydration and decomposition of chlorite, decomposition of fluorapatite, and transformation of gangues. SiO2 and Al2O3 decomposed from chlorite can not only combine with FeO to form Fe2SiO4 and FeAl2O4, which hindered the reduction of iron oxide, but also promoted the decomposition of fluorapatite to form Ca3(PO4)2 in the presence of SiO2. Then, Ca3(PO4)2 was reduced to form CaSiO3 and P2, which was the main process of dephosphorization. However, a small amount of fluorapatite diffused into iron oxides, which made it difficult to separate by mechanical crushing and screening.

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Intermittent Microscopic Observation of Structure Change and Mineral Reactions of High Phosphorus Oolitic Hematite in Carbothermic Reduction

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Melting Separation Process of High Chromium Vanadium-bearing Titanomagnetite Metallized Pellet and its Optimization by Multi-Index Synthetic Weighted Scoring Method

Jue Tang, Mansheng Chu, Cong Feng, Feng Li, Yating Tang, Zhenggen Liu

pp. 1156-1165

DOI:

10.2355/isijinternational.ISIJINT-2016-714

Abstract

Based on the gas-based direct reduction followed by melting separation process, the melting separation process of high chromium vanadium-bearing titanomagnetite metallized pellet and its optimization by multi-index synthetic weighted scoring method are studied in the present work. The optimal melting separation parameters include a melting temperature of 1650°C, a melting time of 45 min, and a basicity of 1.10. Under these conditions, the recoveries of Fe, V, Cr, and TiO2 reach 99.87%, 98.26%, 95.32%, and 95.04% respectively; the mass fraction of Fe, V, Cr, and TiO2 are 94.16%, 0.94%, 0.76%, and 38.21% respectively. The basicity has the strongest effect and its effect on the melting separation kinetic is more significant than thermodynamic. As increased basicity from 0.6 to 1.1, the slag viscosity decreases and surface tension increases, which are both attributed to smooth melting separation and improved indexes. But further increasing basicity to 1.2, the amount of CaTiO3 and slag melting point increase, and the slag amount is relatively excessive, then all the melting separation indexes decrease instead. The melting separation contains four key behaviors: Fe–C melt formation and Fe(l) generation; slag melting initiation and slag(l) generation; small iron droplets formation and start of iron-slag separation; continuous aggregation and growth of iron and accomplishment of iron-slag separation. The iron aggregation and growth should go through iron crystal nucleus formation, reaction interface formation and enlargement, and subsequent reaction interface decrease.

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Melting Separation Process of High Chromium Vanadium-bearing Titanomagnetite Metallized Pellet and its Optimization by Multi-Index Synthetic Weighted Scoring Method

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Dynamic Wettability of Liquids on Gasified Metallurgical Cokes

Tatsuya Kon, Sohei Sukenaga, Shigeru Ueda

pp. 1166-1172

DOI:

10.2355/isijinternational.ISIJINT-2017-008

Abstract

For blast furnace iron-making, understanding the flow behavior of molten iron and slag on the surface of metallurgical coke will lead to optimal design of burden material, which strongly influences the gas-permeability of the furnace. Since the surface structure of coke changes as the reaction of carbon with CO2 or iron oxide progresses, wettability and motion of liquid in the coke-packed bed may be influenced depending on the reaction ratio of coke, i.e., the surface structure. In the present study, to investigate the influence of the surface structure of coke on liquid flow behavior, the advancing and receding contact angles of mercury and water on coke were measured. Of the two liquids, mercury has the higher surface tension. The surface structure of the coke substrate was varied by heating in CO–CO2 or CO2 atmospheres at 1273 K. It was found that the fine irregularities of the coke surface formed by the gasification reaction of the carbon increases the receding contact angle of water but has little influence on the dynamic contact angle of mercury. Depending on the results, influence of surface property of coke on flows of molten iron and slag in the blast furnace is discussed.

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Dynamic Wettability of Liquids on Gasified Metallurgical Cokes

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Effect of the Main Feeding Belt Position on Burden Distribution during the Charging Process of Bell-less Top Blast Furnace with Two Parallel Hoppers

Wenxuan Xu, Shusen Cheng, Qun Niu, Guolei Zhao

pp. 1173-1180

DOI:

10.2355/isijinternational.ISIJINT-2017-003

Abstract

Based on an actual 4070 m3 bell-less top blast furnace with two parallel hoppers in China, a full model of bell-less top mainly consisting of a bunker, a main feeding belt, a switch chute, two parallel hoppers, a central throat tube, a rotating chute and a throat of blast furnace was established in this study. Specifically, the angle between the centerline of the main feeding belt and the symmetry plane of two parallel hoppers is 22 degrees. Then the processes of coke charging into and discharging from the left hopper and right hopper were calculated by the discrete element method (DEM) model, respectively. The calculation results show that the burden distribution of the left hopper and right hopper was not symmetric, which caused the asymmetric and uneven distribution of coke in the radial and circumferential direction of stock surface, respectively. In the case of coke was discharged from the left hopper, the total volume of coke in the radial region 1 to 9 was smaller than that in the radial region 12 to 20 of stock surface. And the total volume of coke in the circumferential region 1 to 4 was larger than that in the circumferential region 5 to 8 of stock surface. However, the burden distribution of stock surface was contrary when coke was discharged from the right hopper.

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Effect of the Main Feeding Belt Position on Burden Distribution during the Charging Process of Bell-less Top Blast Furnace with Two Parallel Hoppers

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Reduction of CaO–Fe2O3 Series Compounds by CO

Chengyi Ding, Xuewei Lv, Gang Li, Chenguang Bai, Senwei Xuan, Kai Tang, Yun Chen

pp. 1181-1190

DOI:

10.2355/isijinternational.ISIJINT-2017-088

Abstract

The isothermal reduction kinetics of C2F, CF, and CF2 by 30% CO and 70% N2 at 1123 K (850°C), 1173 K (900°C), and 1223 K (950°C) were investigated by thermogravimetric analysis in this study. Results indicated that the maximum reduction degree increased and its corresponding reduction time decreased by the order of C2F, CF, and CF2. The reduction rate analysis by peak fitting based on the Gauss rule revealed that the C2F, CF, and CF2 reductions were typical one-step, two-step, and three-step reactions, respectively. Fe3O4 to FeO stage overlaps with the following FeO to Fe stage and tends to approach the previous Fe2O3 to Fe3O4 stage in reduction of CaO–Fe2O3 system with an increase in Fe2O3 content. The apparent activation energy of the sample C2F, CF, and CF2 reductions were 51.74, 46.89, and 34.37 kJ/mol, respectively, indicating that reduction proceeds more easily for them. The ln-ln and Sharp analysis implied that the C2F reduction was described by the Avrami–Erofeev function, thus appeared as a 2D reaction during the whole reaction, whereas the CF and CF2 reductions were expressed initially by a 2D reaction when α<0.5 and subsequently by a 3D reaction when α>0.5. In the new proposed reduction model, the Fe2O3 content increases in the CaO–Fe2O3 system from C2F to CF2 and the gradual promotion of the reduction rate leads to the reduction process of the C2F, CF, and CF2 samples occurring by a 2D tending to 3D reaction mechanism.

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Reduction of CaO–Fe2O3 Series Compounds by CO

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Cold Model Experiment and Numerical Simulation of Flow Characteristics of Multi-phase Slag

Masahiro Tsuboi, Kimihisa Ito

pp. 1191-1196

DOI:

10.2355/isijinternational.ISIJINT-2017-045

Abstract

In order to attain a highly efficient dephosphorization, multi-phase slags consisting of liquid and solid phases have been used for the hot-metal treatment processes.However, the viscosity of a multi-phase slag dramatically increases with the fraction of coexisting solid phase and transits to a Bingham fluid, which may cause difficulties in discharging the slag after the refining. In this study, in order to understand flow behaviors, the simulated multi-phase slag discharged from a dam was observed by a high-speed video system. The criterion for the free jet to wall flow was obtained. The travelling time of the simulated slag increased dramatically with the solid fraction in the Bingham fluid region. A three-dimensional smoothed-particle hydrodynamics simulation program for Bingham fluid was developed and its calculations were compared with the experimental observations. The simulation replicated the observed flow in the cold model experiment.

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Cold Model Experiment and Numerical Simulation of Flow Characteristics of Multi-phase Slag

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Effect of Slag Composition on Phosphorus Separation from Steelmaking Slag by Reduction

Kenji Nakase, Akitoshi Matsui, Naoki Kikuchi, Yuji Miki

pp. 1197-1204

DOI:

10.2355/isijinternational.ISIJINT-2017-071

Abstract

Steelmaking slag is recycled and reused, but its utilization is restricted due to its chemical properties. On the other hand, steelmaking slag is considered to be a promising resource of iron and phosphorus in Japan. In this work, reduction of (FetO) and (P2O5) in steelmaking slag and recovery of iron and phosphorus resources were investigated with the aim of developing a new recycling process for steelmaking slag. In order to investigate the effect of slag basicity (%CaO/%SiO2) and FetO content in slag on phosphorus separation from steelmaking slag, steelmaking slags were reduced by carbonaceous material at high temperature by using an induction furnace. From the results, the effects of slag composition, temperature and oxygen partial pressure on phosphorus separation from steelmaking slag were discussed.

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Effect of Slag Composition on Phosphorus Separation from Steelmaking Slag by Reduction

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Effects of Metal Droplets on Electromagnetic Field, Fluid Flow and Temperature Field in Electroslag Remelting Process

Jia Yu, Zhouhua Jiang, Fubin Liu, Kui Chen, Huabing Li, Xin Geng

pp. 1205-1212

DOI:

10.2355/isijinternational.ISIJINT-2017-084

Abstract

A transient 2D axisymmetry mathematical model is established to study the effect of the metal droplets on the electromagnetic field, fluid flow and temperature field. The electromagnetic field is solved with the electric potential method. The movement of droplet and phase boundary is tracked by the Volume of Fluid (VOF) model. The solidification is addressed by the enthalpy-porosity model. The governing equations are discretized based on the finite volume method and solved simultaneously using the commercial software FLUENT. The simulation results indicate that the falling process of droplet can be represented more exactly considering the effect of metal droplets on electromagnetic field. Otherwise the droplets can bridge between electrode and metal pool. The temperature and final velocity of the droplet passing through the slag/pool interface is 1992 K and 0.40 m/s when the effect of metal droplets on electromagnetic field is considered. On the other hand, that is 2019 K and 0.34 m/s correspondingly for the case ignoring the effect of metal droplets on electromagnetic field. Furthermore, the liquidus and solidus depth of the case ignoring the effects of metal droplets on electromagnetic field increases by 18% and 15% compared with that of the other case, respectively.

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Effects of Metal Droplets on Electromagnetic Field, Fluid Flow and Temperature Field in Electroslag Remelting Process

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High Dimensional Data-driven Optimal Design for Hot Strip Rolling of C–Mn Steels

Siwei Wu, Guangming Cao, Xiaoguang Zhou, Naian Shi, Zhenyu Liu

pp. 1213-1220

DOI:

10.2355/isijinternational.ISIJINT-2017-014

Abstract

Recently, hot strip rolling processes are required to be agile and accurate in order to meet more and more diverse market demands. Under such circumstances, the traditional processing optimization by time-consuming pilot experiments becomes difficult. To realize this target, core models of processing and mechanical properties are often established by neural network methods, which are used to handle non-linear multi-variant systems. In modeling processing and mechanical properties, we found that the abnormal values in industrial big data could result in wrong predictions for the relationships between processing and properties. In the present work, therefore, data processing has been developed to prevent misleading predictions, which was performed by eliminating the redundant, abnormal, and imbalanced data before modeling. The Bayesian neural network was used to construct the modeling of mechanical properties for hot rolled C–Mn steels, which demonstrated that the accuracies between the measured and predicted values were within ±10% and ±5% for strength and elongation, respectively, providing a reliable model for the optimal process design. By applying the multi-objective optimization algorithm named Strength Pareto Evolutionary Algorithm 2 (SPEA2), the hot strip rolling processes for C–Mn steels were optimized in order for either stabilizing variations of properties or upgrading mechanical properties. Industrial trials were extensively carried out, showing good agreements with the optimized processes.

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High Dimensional Data-driven Optimal Design for Hot Strip Rolling of C–Mn Steels

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Furnace Charging Comprehensive Optimization Technology of the Bell-type Annealing Process

Zhen-hua Bai, Ya-ya Cui, Jun Zhang, Qi-lin Li, Bo-yang Li, Yu Xing

pp. 1221-1227

DOI:

10.2355/isijinternational.ISIJINT-2016-476

Abstract

Sticking defect in the bell-type annealing process is closely related to furnace charging technology. Unsuitable furnace charging technology increases the additional radial pressure of some coils in the furnace and leads to sticking defect. A suitable furnace charging comprehensive optimization technology of the bell-type annealing process is introduced, which aims at sticking prevention. This technology was developed after field experiments and theoretical analyses based on defining the concept of sticking index, considering the influences of coiling technology, coil size, product specifications, and annealing technology on the steel internal stress. The coil work-base quantity was as close as possible to the theoretical minimum value, and the average and peak sticking index value of the same batch of coils as the target were minimized, balanced with production efficiency and defect prevention. This technology was applied to the production practice of a cold-rolled sheet plant, achieving good using effect with furnace charging efficiency. Production increased, and sticking defect decreased. Considerable economic benefit for the enterprise was created with further popularization and application values.

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Furnace Charging Comprehensive Optimization Technology of the Bell-type Annealing Process

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Effect of Post-weld Heat Treatment on Fatigue Reliability of Super-duplex-stainless-steel Weldments

Ryujiro Udo, Hiroshi Numakura

pp. 1228-1232

DOI:

10.2355/isijinternational.ISIJINT-2017-079

Abstract

To assess the reliability of weldments of a super duplex stainless-steel (UNS S32750), we have studied how the microstructure and fatigue strength of a welded joint is affected by post-weld heat treatment, focusing on the formation of the intermetallic σ phase during cooling. The microstructure of the welded zone of arc-welded single-V joints was examined by optical microscopy and SEM-EBSD. It revealed that the σ phase appears as the tertiary constituent when cooled from the solution-treatment temperature of 1323 K at rates below 101 K s−1, with its fraction reaching 20% at rates as low as 10−1 K s−1. Fatigue properties were evaluated by plane-bending tests, and were found to be deteriorated correspondingly to the amount of the σ phase. Specimens without heat treatment performed better than any of those subjected to the solution treatment and cooled at a rate in the range between 0.1 to 50 K s−1. To secure fatigue reliability it is advisable that weldments of the material should not be subjected to post-weld heat treatment. If the treatment is necessary for some reasons, the condition should be carefully selected.

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Effect of Post-weld Heat Treatment on Fatigue Reliability of Super-duplex-stainless-steel Weldments

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Microstructural Evolution and Carbides in Quenched Ultra-low Carbon (Fe–C) Alloys

Dehai Ping, Tianwei Liu, Masato Ohnuma, Takahito Ohmura, Taichi Abe, Hidehiro Onodera

pp. 1233-1240

DOI:

10.2355/isijinternational.ISIJINT-2016-624

Abstract

Binary Fe–C alloys are basic for various carbon steels in view of chemical compositions, thus, understanding the microstructural evolution in Fe–C alloys is fundamentally necessary. Three Fe-xC (x = 0.02, 0.05 and 0.1 (wt.%)) alloys were selected and the microstructure changes during water-quenching process has been studied using transmission electron microscopy (TEM).Fine carbides with a size of tens of nanometer, mainly θ-Fe3C cementite, are observed in quenched Fe-0.02C and Fe-0.05C samples. Lath martensite has been commonly observed in all the quenched alloys. Body-centered cubic (bcc) {112}<111>-type twinning structure with fine ω particles on the twinning boundaries has been frequently observed in the quenched Fe-0.1wt.%C samples; while the fine cementite are absent in the twinned region.A novel formation mechanism for the quenched microstructure has been proposed as follows: austenite → twinned martensite (bcc + ω on twinning boundaries) → lath martensite + carbides on lath boundaries → carbides + ferrite (α-Fe). The first transition corresponds to the martensitic transformation; while the second and third transitions are the auto-tempering results of the first one. In the ultra-low carbon sample (Fe-0.02C), the third transition will be the observed microstructure; Slight high carbon (Fe-0.05C) will result in the second transition, not to the third one. The product of the first transition will be the twinned martensite (Fe-0.1C), and the final microstructure will not experience the second and third transitions. Of course, the quenching condition can also affect the final microstructure.

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Microstructural Evolution and Carbides in Quenched Ultra-low Carbon (Fe–C) Alloys

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In-situ Heating EBSD Study of Effects of Cold Reduction Ratio on Recrystallization and Grain Growth Behaviors in 3% Si Electrical Steels

Hyunwoo Mun, Seil Lee, Yang Moo Koo

pp. 1241-1245

DOI:

10.2355/isijinternational.ISIJINT-2016-564

Abstract

Crystallographic texture development during recrystallization in samples cold-rolled to 78, 87 and 96% thickness reduction R were measured using EBSD on an in-situ heating stage inside a field emission scanning electron microscope. Nucleation, recrystallization and grain growth of grains having {411}<148> orientation occurred intensely and steadily inside the deformed region. Through this phenomenon, grains having {411}<148> showed larger average grain sizes than other textures at R = 87% and 96%. Due to the {411}<148> texture component, the {111}<112> texture component, which is known as a high reduction emerging texture, weakened. At R = 76%, grains having Goss orientation were frequently observed. However, at R = 87% and 96%, the {111}<112> and the {411}<148> texture components became dominant and the α* fiber was strengthened.

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In-situ Heating EBSD Study of Effects of Cold Reduction Ratio on Recrystallization and Grain Growth Behaviors in 3% Si Electrical Steels

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Crystallographic Analysis of Transformation Behavior of Acicular Ferrite from B1-type Compounds in Steels

Shoichi Nambu, Yoshihiko Takizawa, Mayumi Ojima, Junya Inoue, Toshihiko Koseki

pp. 1246-1251

DOI:

10.2355/isijinternational.ISIJINT-2017-017

Abstract

Crystallographic analysis was carried out on acicular ferrites (AFs) of steel that nucleated on B1-type compounds embedded in steels during the isothermal transformation from austenite (γ) to ferrite (α), and the orientation relationship between α, γ, and the compounds and the AF formation behavior were correlated by varying the B1-type compound, steel, and transformation temperature. The results demonstrated that all the AFs satisfy the Kurdjumov–Sachs orientation relationship with the prior γ, and in addition, some of the AFs have specific low-index orientation relationships with the B1-type compounds and nucleate earlier than others. Among the specific low-index orientation relationships, furthermore, AFs satisfying the Baker–Nutting orientation relationship, which gives the lowest interfacial energy between α and a compound, nucleate preferentially.

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Crystallographic Analysis of Transformation Behavior of Acicular Ferrite from B1-type Compounds in Steels

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Microstructures and Mechanical Properties of TaC Added to Vanadis 4 Tool Steel through Vacuum Sintering and Heat Treatments

Kuo-Tsung Huang, Shih-Hsien Chang, Po-Ting Yeh

pp. 1252-1260

DOI:

10.2355/isijinternational.ISIJINT-2017-033

Abstract

In this study, different amounts of TaC powders (10, 15 and 20 mass%) are mixed and added to Vanadis 4 tool steel powder. The composite powders are sintered at 1250, 1300 and 1325°C for 1 h, respectively. After that, a series of heat treatments is performed to determine the optimal parameters of the Vanadis 4 composites. The experimental results show the optimal vacuum sintering temperature for the Vanadis 4 composites to be 1300°C. The Vanadis 4 specimens with a 10% TaC added possess the highest transverse rupture strength (TRS) value of 1974.2 MPa and a suitable hardness (82.6 HRA). When the optimally sintered specimens of the Vanadis 4 composites then undergo a heat treatment, the TRS shows a significant increase to 2069.4 MPa, while the hardness declines slightly to 81.5 HRA. In addition, the microstructural evaluation reveals that the plate-shaped carbides (M7C3) located on the grain boundaries disappear after the TaC powders are added and that clustered TaC carbides appear in the grain boundaries instead. Meanwhile, with TaC as the nucleation sites of the VC carbides, the result is the generating of VC carbides in the grain boundaries. Moreover, the VC carbides decompose and re-precipitate the refined carbides (Fe3C), which are uniformly distributed within the grains. The results also show that heat treatment effectively improves the microstructure and strengthens the Vanadis 4 composites.

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Microstructures and Mechanical Properties of TaC Added to Vanadis 4 Tool Steel through Vacuum Sintering and Heat Treatments

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Role of Lanthanum Addition on Acicular Ferrite Transformation in C–Mn Steel

Mingming Song, Bo Song, Shenghua Zhang, Zhengliang Xue, Zhanbing Yang, Runsheng Xu

pp. 1261-1267

DOI:

10.2355/isijinternational.ISIJINT-2017-037

Abstract

Acicular ferrite formation in C–Mn steel after rare earth La addition and the potency of different La-containing inclusions inducing the formation of acicular ferrite have been investigated. It was found that there would be a large amount of acicular ferrite in quenched steel after 0.020 mass% La additions. The size of effective inclusion for acicular ferrite nucleation is mainly 1–4 µm. The best single La-containing effective inclusion is La2O2S. When MnS attached to the surface of La2O2S forming complex inclusion, the potency of inclusion on nucleating acicular ferrite is enhanced significantly. Low lattice mismatch between α–Fe and La2O2S plays an important role on acicular ferrite nucleation. When patches of MnS precipitated on the surface La2O2S forming the complex inclusion, the main mechanism for that inclusion is the combine of low degree of lattice mismatch and existing of Mn-depleted zone.

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Role of Lanthanum Addition on Acicular Ferrite Transformation in C–Mn Steel

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Difference in Reheat Cracking Susceptibility of 2.25Cr-W and 9Cr-W Heat-resistant Steels

Hyun Je Sung, Nam Hoe Heo, Sung-Joon Kim

pp. 1268-1272

DOI:

10.2355/isijinternational.ISIJINT-2016-737

Abstract

This research has been performed to evaluate the reheat cracking susceptibility of 9Cr T/P92 heat-resistant steels and find the difference in susceptibility between 2.25Cr T/P23 and 9Cr T/P92 heat-resistant steels. After the welding simulation at 1200°C and 1300°C, the prior austenite grain size is much smaller in the T/P92 steel than the T/P23 steel. This is attributed to the undissolved Nb-rich carbo-nitrides in the T/P92 during the welding simulation which inhibit the growth of prior austenite grains. After tensile test at 750°C for the evaluation of cracking susceptibility, the fracture mode is typically intergranular in the T/P23 steel, while that of the T/P92 is ductile. Therefore, the insensitivity to intergranular cracking of the T/P92 steel is mainly attributed to the much lower phosphorus segregation concentration at grain boundaries which is caused by the smaller prior austenite grain size. Calculation results finally confirm that the grain boundary segregation concentration of phosphorus decreases with decreasing grain size.

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Difference in Reheat Cracking Susceptibility of 2.25Cr-W and 9Cr-W Heat-resistant Steels

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Roles of Solute C and Grain Boundary in Strain Aging Behaviour of Fine-grained Ultra-low Carbon Steel Sheets

Yoshihiko Ono, Yoshimasa Funakawa, Kaneharu Okuda, Kazuhiro Seto, Naoki Ebisawa, Koji Inoue, Yasuyoshi Nagai

pp. 1273-1281

DOI:

10.2355/isijinternational.ISIJINT-2016-622

Abstract

The roles of solute C and the grain boundary in the strain aging phenomenon of polycrystalline ferritic steel were investigated using Nb-bearing ULC steel sheets with a relatively low solute C content of 1–3 ppm and ferrite grain sizes of 9.5 µm and 183 µm at aging temperatures from 70 to 400°C. The steels exhibited two definite hardening stages. The 1st hardening stage appeared in both fine- and coarse-grained specimens, in which the increase in YP (ΔYP) became saturated at around 30 MPa. From the apparent activation energy and hardening kinetics, the hardening mechanism was assumed to be dislocation pinning by solute C atoms. The 2nd hardening stage, significantly appeared in fine-grained specimens accompanying a large increase in the Hall-Petch coefficient; ΔYP was quite large, reaching 90 MPa. Fine precipitates were not detected in aged specimens observed by TEM and 3DAP. Segregation of solute C to the grain boundaries and diffusion of Fe atoms in the grain boundaries were proposed as possible mechanisms of this 2nd hardening. Grain-boundary hardening was assumed to be one of the hardening mechanisms in the strain aging in polycrystalline ferritic steel.

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Roles of Solute C and Grain Boundary in Strain Aging Behaviour of Fine-grained Ultra-low Carbon Steel Sheets

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Two Step Ductile to Brittle Transition Behavior on Ferrite + Pearlite Structure Steel Sheet

Hiroyuki Kawata, Osamu Umezawa

pp. 1282-1288

DOI:

10.2355/isijinternational.ISIJINT-2017-026

Abstract

Although a few studies on the fracture behavior of dual phase (DP) steels have been conducted, the mechanism of impact fracture in DP steels has not yet been clarified. In this study, the ductile to brittle transition behavior in ferrite + pearlite DP steel sheets was evaluated using the Charpy impact test with a sub-size specimen. The absorbed energy of DP steel decreased with decreasing temperature and the transition curve had a clear “middle shelf” existing between upper and lower shelves. The fracture surface at the middle shelf was flat and contained few dimples, similar to that at the lower shelf. Ferrite grains just below the fracture surface obviously received plastic strain with fracture on the middle shelf. This result indicated that the fracture mode on the middle shelf was quasi-cleavage fracture (QCF). Furthermore, on the lower shelf, ferrite grains received very small plastic strain with fracture. The two step ductile to brittle transition in this study corresponded to the fracture mode that changed from microvoid coalescence fracture to QCF and from QCF to cleavage fracture.

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Two Step Ductile to Brittle Transition Behavior on Ferrite + Pearlite Structure Steel Sheet

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Void Generation in Cold-rolled Dual-Phase Steel Sheet Having Excellent Stretch Flange Formability

Katsutoshi Takashima, Kohei Hasegawa, Yuki Toji, Yoshimasa Funakawa

pp. 1289-1294

DOI:

10.2355/isijinternational.ISIJINT-2017-086

Abstract

The aim of this research is to investigate generation of micro voids affecting stretch flange formability in cold-rolled Dual-Phase (DP) steel sheets having a low volume fraction of martensite. The volume fraction of martensite was the dominant factor of the hole expanding ratio instead of the difference in hardness between ferrite and martensite when the difference in hardness between ferrite and martensite was large. On the other hand, the difference in hardness became the dominant factor of the hole expanding ratio in DP steel sheets having a low difference in hardness. Micro voids around the punched hole and the fracture edge of tensile deformation were observed in order to understand these different results. The void density is able to be associated with the hole expanding ratio under the same strain condition and the void density depends on the martensite spacing.

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Void Generation in Cold-rolled Dual-Phase Steel Sheet Having Excellent Stretch Flange Formability

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