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Predecessor

Transactions of the Iron and Steel Institute of Japan

ISIJ International Vol. 57(2017) No. 5

Fundamentals of High Temperature Processes

Regular Article

Alternative Reduction of Copper Matte in Reduction Process of Copper Slag

Bao-jing Zhang, Li-ping Niu, Ting-an Zhang, Zhi-qiang Li, Dong-liang Zhang, Chao Zheng

Released: May 16, 2017

p.775-781

In order to make full use of copper slag and solve the problem of its storage, the direct reduction of copper slag to antibacterial stainless steel was proposed. However, mixture of copper matte in the product reduces the value of the alloy. In this article, we studied the changes of Cu₂S and FeS in chemical reactions. The Fact-Sage software was used to calculate the ΔG of the reaction, the phase diagram of FeS–CaO–C system, Cu₂S–CaO–C system and FeS–Cu₂S–CaO–C system. Three reduction experiments have been done to verify the calculation results. Cu₂S and FeS in alloy can be alternatively reduced to elemental metal. In reduction of matte, the recovery of metal is 73.96%, and the removal rate of sulfur is 99.11%. In reduction of copper slag, the recovery of metal is 88.50%. In reduction of matte in metal, the removal rate of sulfur is 99.64%. The sulfur is with the presence of CaS in slag eventually.

Thermodynamic Modeling of Fe–C–S Ternary System

Fahmi Tafwidli, Youn-Bae Kang

Released: May 16, 2017

p.782-790

A CALPHAD type thermodynamic modeling of the Fe–C–S system was carried out, in particular, in order to provide accurate description of activity-composition relationship of Fe–C–S liquid phase. The liquid may be used as a solvent of ferrous scrap recycling. All available and reliable experimental data including phase equilibria and activity of S in the liquid Fe–C–S were analyzed, and a self-consistent set of Gibbs energy equations for the stable phases were obtained. In addition to the Gibbs energy equations, interaction parameters between S and C in the liquid were additionally estimated as functions of temperature:

which were derived from the optimized Gibbs energy of the liquid phase using the Modified Quasichemical Model in the pair approximation. Moreover, the present thermodynamic modeling can be used to calculate various phase diagrams in the Fe–C–S system, and can be further extended in order to develop a multicomponent thermodynamic database.

Ironmaking

Regular Article

Microwave Carbothermic Reduction of Oolitic Hematite

Ying Lei, Yu Li, Wen Chen, Rundong Wan

Released: May 16, 2017

[Advance Publication] Released: March 30, 2017

p.791-794

The microwave reduction of coke-bearing oolitic hematite composite pellets followed by magnetic separation was investigated in this work to explore the feasibility of microwave heating on utilization of oolitic hematite. The reduction was conducted in a temperature controllable microwave oven at 1273, 1373, 1423, 1473 and 1523 K for 5–60 min. The effects of temperature and holding time on reduction were studied. The highest metallization of 95.42% was achieved at 1523 K in 10 min. Then the air cooling metallic pellets were crushed and grinded and subsequent separated via magnetic separation. The effect of magnetic field intensity on recovery and grade of iron powder were investigated. The iron powder with iron content, recovery and phosphorus content of 92.89%, 85.28% and 0.24% respectively was achieved at the magnetic field intensity of 0.04 Wb/m². This will be a potential way to realize the comprehensive utilization of oolitic hematite in China.

Influence of temperature and holding time on metallization (upper left: schematic diagram of microwave oven; upper middle: hematite powder before reduction; upper right: pellets after reduction). Fullsize Image

Effect of Coarse-grain and Low-grade Iron Ores on Sinter Properties

Zhixin Xiao, Lingkun Chen, Yindong Yang, Xiangcai Li, Mansoor Barati

Released: May 16, 2017

[Advance Publication] Released: April 05, 2017

p.795-804

In sintering of iron ore, small particles act as a binder that joins larger particles through diffusion and melting. In order to understand the effect of gangue and structure of coarse ore on sinter properties, melt formation in three coarse-grain, low-grade iron ores was investigated. The melt fractions at the sintering temperature were estimated using the phase diagrams and melt fluidity was quantified by testing ore compacts in an infrared furnace. The result indicates that SiO₂ can significantly increase the quantity and fluidity of melt during sintering, whereas the effects of Al₂O₃ and MgO are small. Excessive quantity and fluidity of liquid phase result in merging of micro pores, leaving large pores behind. The impact of blending three ores with a base ore on sinter structure were tested in a mini-sinter pot. The result shows that bonding and pore structure of sinter are dominantly affected by the melt behavior and pore structure of the coarse ore. Relationships between the fluidity index of sinter feed and product properties were explored, showing meaningful and predictable trends. It was demonstrated that fluidity index can be used as parameter to link sinter mix composition and sintering conditions to the properties of the produced sinter.

Steelmaking

Regular Article

Motion of Single Bubble and Interactions between Two Bubbles in Liquid Steel

Guocheng Wang, Haichen Zhou, Qianren Tian, Xingang Ai, Lifeng Zhang

Released: May 16, 2017

[Advance Publication] Released: March 23, 2017

p.805-813

The bubbling in liquid steel can improve steel cleanliness effectively. To understand the single bubble behavior and the interactions between two bubbles, which are basis to analyze the mechanism of gas-liquid two-phase flow in metallurgical vessel, the bubbling at a laminar flow has been systematically studied by both water-model experiment and three-dimensional numerical simulation. Nozzle diameters of 1.5, 2, and 2.5 mm were investigated within a certain range of gas flow rate. The images of bubbles emerging from the nozzle were observed by the water-model experiment. The volume of fluid (VOF) model in conjunction with continuum surface force (CSF) model was used to describe the interface between the gas and liquid on the software platform of Fluent 14.5. The phenomena of bubbling, rising motion, coaxial bubbles coalescence and parallel bubbles bounce with one- and two-nozzle were found in air-water and argon-steel system. The initial bubble sizes increase with increasing gas flow rate or nozzle size, whereas is relatively independent of the nozzle size when gas flow rate is 0.975 L/min. The terminal velocities of all bubbles are around 0.32 m/s. The evolution history of bubble shape changes from non-deformed (spherical) to the deformed shapes (spherical cap, ellipsoidal, wobbly or ellipsoidal cap) without coalescence. Calculated velocity and pressure distributions indicated that the rising velocity of the trailing bubble is larger than that of the leading bubble, which result in two coaxial bubbles coalescence. The small eddies on the inner edge of two parallel bubbles eventually lead to bubbles bounced each other.

Casting and Solidification

Regular Article

Numerical Modeling of Macrosegregation in Round Billet with Different Microsegregation Models

Qipeng Dong, Jiongming Zhang, Liang Qian, Yanbin Yin

Released: May 16, 2017

p.814-823

A macrosegregation model, coupling fluid flow, heat and solute transport model, was developed based on continuum model to predict the evolution of macrosegregation in continuous round billet casting, as well as the influence of microsegregation model choice on prediction of macrosegregation. Evolution and characteristics of macrosegregation corresponding to predicted solidification were revealed. As solidification proceeds, solutes are ejected from solid phase to liquid at solidification front. The resulting mushy zone is enriched by solutes, due to the low velocity and limited diffusion, which produces segregation at the billet center as the liquid available for dilution diminishes near the end of solidification. Predicted and experimental results for surface temperature and centerline segregation compare agreeably, which indicates the validity of the coupled macrosegregation model in this work. A detailed analysis was performed to investigate the influence of microsegregation model on prediction of macrosegregation, demonstrating that choice of model affects predicted segregation degree of solutes, which effect varies with type of solute, due to the solute back-diffusion coefficient.

Fullsize Image

Control of Columnar to Equiaxed Transition in Solidification Macrostructure of Austenitic Stainless Steel Castings

Simon Naumovich Lekakh, Ron O’malley, Mark Emmendorfer, Brenton Hrebec

Released: May 16, 2017

p.824-832

Solidification macrostructure is of great importance for the properties and the quality of castings made from austenitic grade stainless steels (ASS) because there are limited options to change as-cast macrostructure in the solid condition. A typical cast macrostructure of ASS has a fine surface chilled zone followed by an elongated dendrite zone, columnar to equiaxed transition (CET) zone, and centrally located equiaxed crystals. Several castings from ASS were produced to determine the effects of casting geometry, chilling, and grain refinement on CET. The transient thermal field in solidified heavy castings was simulated and used to determine an isotherm velocity (V) and the thermal gradient (G) in mushy zone at 50% solid fraction. The critical value of the parameter Gⁿ/V was determined from the macrostructure of the cylindrical casting. Using this value, the location of CET was predicted in the heavy rectangular casting and this prediction was in agreement with experimental macrostructure. Two methods of controlling casting macrostructure by using a chilled mold to stimulate extensive columnar zone and by using melt grain refinement to produce fine equiaxed crystals were experimentally verified and simulated.

Experimental and Numerical Investigations of the Multi-scale Thermoelectromagnetic Convection on the Microstructure during Directionally Solidified Sn-5wt%Pb Alloys

Dafan Du, Yves Fautrelle, Zhongming Ren, Rene Moreau, Xi Li

Released: May 16, 2017

[Advance Publication] Released: March 24, 2017

p.833-840

In this paper, the effect of multi-scale thermoelectromagnetic convection (TEMC) on the microstructure in directionally solidified Sn-5wt%Pb alloys under a transverse magnetic field was studied experimentally and numerically. The experiments are conducted within sample diameters ranging from 0.8 to 12 mm and with various magnetic field intensities. Experimental results show that the transverse magnetic field tilts solid/liquid interface shape and causes the channel segregation. The sloping degree first increases to a maximum value at a critical magnetic field intensity (B_max), and then decreases with the increase of the magnetic field intensity. The critical magnetic field intensity (B_max) decreases with the increase of sample diameter. Finite-element modeling is performed to simulate the multi-scale TEMC by using COMSOL software. Numerical results indicate that the value of the TEMC increases to a maximum and then decreases with the increase of the magnetic field intensity. The tendency of the simulated TEMC agrees with the evolution process of solid/liquid interface morphology by experimental results. The inter-dendritic TEMC increases monotonically with the increasing of magnetic field intensity in the present study (B ≤ 2 T). The modification of the solid/liquid interface and the channel segregation under the magnetic field should be attributed to the TEMC at the sample and inter-dendritic scales, respectively.

Maximum velocity of simulated TEMC at various scales as a function of applied magnetic field intensity. (Online version in color.) Fullsize Image

Instrumentation, Control and System Engineering

Regular Article

A New AdaBoost.IR Soft Sensor Method for Robust Operation Optimization of Ladle Furnace Refining

Hui-Xin Tian, Yu-Dong Liu, Kun Li, Ran-Ran Yang, Bo Meng

Released: May 16, 2017

[Advance Publication] Released: April 04, 2017

p.841-850

LF (Ladle Furnace) refining plays an important role during secondary metallurgic process. The traditional LF refining operation relies on the workers’ experience, it is disadvantageous to ensure the stable production, high-quality products and energy saving. A new robust operation optimization method of molten steel temperature based on AdaBoost.IR soft sensor is proposed in LF refining process. Firstly, an intelligent model based on BP (Back Propagation) neural network is established by analyzing the changes of energy during whole refining process of LF as sub intelligent model. Then an AdaBoost.IR is designed for the characters of industrial data, and is suitable for industrial soft sensor modeling. The ensemble soft sensor model is established for realizing the online real time measurement of molten steel temperature with better accuracy by using AdaBoost.IR. Secondly the robust operation optimization model is described by analyzing the process of LF refining based on above AdaBoost.IR soft sensor. And the HPOS-GA is used to solve the optimal operation solution of robust optimization model. The new robust operation optimization of temperature based on AdaBoost.IR is used in 300 t LF of the Baosteel Company. The results of experiments demonstrate the soft sensor can predict the temperature more accuracy and the end temperature of LF refining after robust optimization become more stable.

Chemical and Physical Analysis

Regular Article

Evaluation of Local Creep Strain in Face-centred Cubic Heat-resistant Alloys Using Electron Backscattered Diffraction Analysis

Shigeto Yamasaki, Masatoshi Mitsuhara, Hideharu Nakashima, Mitsuharu Yonemura

Released: May 16, 2017

[Advance Publication] Released: March 28, 2017

p.851-856

Creep strain in SUS347HTB austenitic heat-resistant steel and Ni-based heat-resistant alloys was evaluated by electron backscattered diffraction (EBSD). Localized crystallographic misorientations in the crept samples were quantified by using misorientation indicators such as kernel average misorientation and grain reference orientation deviation. In most crept samples, the misorientation indicators increased with creep deformation. However, this trend was not observed for alloys with dense dispersions. We proposed a method to extract and evaluate data only near the grain boundary from the total EBSD data. For Ni-based alloys, the misorientation indicators tended to increase preferentially near grain boundaries. Conversely, there was no substantial difference between the misorientation indicators near grain boundaries and the intergranular region for SUS347HTB. Consequently, although it is necessary to limit the region for evaluating the misorientation indicators according to the dispersion density of the reinforcing phase in the materials, the misorientation indicators, such as kernel average misorientation or grain reference orientation deviation, are useful for evaluating the creep strain in face-centred cubic heat-resistant alloys.

EBSD maps around the HAGBs in the ruptured sample of SUS347. (a) Normal crystallographic orientation map, (b) extracted region around the HAGBs, (c) KAM map obtained by extracting the area around the HAGBs, and (d) GROD map obtained by extracting the area around the HAGBs. Colour indices are shown in each map and HAGBs are indicated by black lines. Fullsize Image

Note

Reduction of Spectral Interference between X-ray Peaks Originating from an X-ray Tube and X-ray Fluorescence Peaks in Total Reflection X-ray Fluorescence Analysis

Shinsuke Kunimura, Yugo Sugawara, Shumpei Kudo

Released: May 16, 2017

p.953-955

This paper describes a method for improving the detection limit for zinc by a portable total reflection X-ray fluorescence (TXRF) spectrometer with a tungsten target X-ray tube. Measuring a TXRF spectrum of a small amount of sample in a vacuum remarkably reduced the intensity of the W Lα line (8.40 keV) that originated from the X-ray tube and that partially overlapped with the Zn Kα line (8.63 keV), leading to an improvement in the detection limit for zinc. A detection limit of 0.003 mg/L was achieved when a TXRF spectrum of a 0.05 mg/L zinc standard solution was measured in a vacuum. A TXRF spectrum of a river water sample containing 0.05 mg/L of zinc was also measured, and the Zn Kα line measured in a vacuum was more clearly observed than that measured in air.

Forming Processing and Thermomechanical Treatment

Regular Article

Effect of Phase Transformation and Latent Heat on Hot Rolling Deformation Behavior of Non-oriented Electrical Steel

Chao Liu, Anrui He, Yi Qiang, Defu Guo

Released: May 16, 2017

[Advance Publication] Released: April 04, 2017

p.857-865

In order to make up for the deficiency of the traditional austenite deformation resistance model and the existing rolling theory in the gage and shape control of hot rolled non-oriented electrical steel, the mathematical models related to phase transformation are established by regression analysis and then written to be ABAQUS subroutines, which are subsequently embedded into the heat transfer model and rolls-strip coupling model. The finite element models are used to accurately predict the transverse distribution of the temperature field and phase field, and analyze the effect of phase transformation and latent heat on the total roll force, distribution of per unit roll force and roll gap profile. The transverse transformation difference is induced by the temperature difference along strip width direction. The latent heat contributes to the uniformity of temperature field and phase field. The total roll force and center gage are reduced due to the phase transformation, but the adjustment of roll force on the crown is enhanced and the rolling pressure spikes are eliminated due to the appearance of ferrite phase in the edge of strip. The roll force distribution and roll gap profile are weakly dependent on the latent heat, but primarily influenced by the transformation.

Crystal Plasticity Finite-element Simulation on Development of Dislocation Structures in BCC Ferritic Single Crystals

Takayuki Hama, Keisuke Kojima, Masahiro Kubo, Hitoshi Fujimoto, Hirohiko Takuda

Released: May 16, 2017

p.866-874

The role of {112} slip activity on the deformation of bcc ferritic single crystals with different crystallographic orientations was studied numerically using a crystal plasticity finite-element method. Peeters model [Peeters et al., Acta Mater., 49 (2001), 1607] was utilized to predict development of dislocation structures as well as work-hardening behavior. To examine the effect of the {112} slip activity in detail, the simulation was carried out using original Peeters model in which development of cell-block boundaries (CBBs) along the {112} planes was not taken into account, Peeters model in which development of CBBs along the {112} planes was taken into account (extended-1 model), and Peeters model in which {112} slip activity was not taken into consideration (extended-2 model). The predicted stress-strain curves were in qualitatively good agreement with the experimental results for all cases when the original and extended-1 models were used, whereas two-stage work hardening observed for the crystal with {100} <011> was not predicted when the extended-2 model was used. Concerning development of CBBs, the extended-1 and extended-2 models gave better prediction as compared to the original model. The abovementioned results suggested that the extended-1 model gave the most appropriate predictions among the models in terms of work-hardening behavior and development of CBBs, showing that it was more reasonable to take into account both {110} and {112} slip systems and development of CBBs along not only the {110} planes but also the {112} planes.

Welding and Joining

Regular Article

Prediction of the Temperature Distribution and Microstructure in the HAZ of SA508Gr4 Reactor Pressure Vessel Steel

Qingwei Bai, Yonglin Ma, Shuqing Xing, Zhongyi Chen, Xiaolan Kang

Released: May 16, 2017

[Advance Publication] Released: April 04, 2017

p.875-882

SA508Gr4 is a newly developed steel for reactor pressure vessel and its behavior data is rare, especially its welding characters attract researchers and industries attention. In this paper, a finite element method (FEM) is employed to investigate the thermal process and phase transformation process of arc welding of SA508Gr4 steel, where a body heat source model and element birth and death technique were used in this simulation process. This study focuses on the effect of peak temperature (t_m) and cooling rate (t_8/5) on microstructural transformation in HAZ, which is to be overcome by combining SH-CCT diagram with numerical thermal cycle curves. Through conversion of thermal cycle data into microstructural information, it is found that the quenched zone is developed rapidly whose width is 1560 µm, while the inter-critical HAZ region is three times smaller than the quenched zone dimension, which is 450 µm. Post weld heat treatment was suggested to be adopted for improving the property of HAZ. In addition, the metallographic analysis confirms the expected microstructural evolutions. The microstructure is martensite (M) + some bainite (B) and the microhardness is between 429 HV and 432 HV.

Reduction of Diffusion Bonding Temperature with Recrystallization at Austenitic Stainless Steel

Masahito Katoh, Naoko Sato, Tomomi Shiratori, Yohei Suzuki

Released: May 16, 2017

p.883-887

Diffusion bonding process in low temperature is desirable for the manufacturing method of metal MEMS (Micro-Electronic-Mechanical Systems) such as metal micro-pump that the high proof stress is required. Severe plastic deformed metals having high grain boundary mobility are expected to bond in low temperature. Then, we tried to perform recrystallization and solid phase diffusion bonding at the same time. In this paper, we confirmed that the reduction of diffusion bonding temperature in severe plastic deformed SUS304 and SUS316L as compared to solution heat treated one. Especially the bonding temperature was decreased prominently in SUS304 having strain-induced martensite.

Surface Treatment and Corrosion

Regular Article

Electrochemical Characterization on the Potential Dependent Stress Corrosion Cracking Mechanism of 10Ni8CrMoV High Strength Steel

Lin Fan, Kangkang Ding, Penghui Zhang, Weimin Guo, Kun Pang, Likun Xu

Released: May 16, 2017

p.888-894

Stress corrosion cracking (SCC) of 10Ni8CrMoV high strength steel influenced by applied potentials in simulated seawater are investigated by combining slow strain rate tensile (SSRT) tests with electrochemical measurements. The potential region corresponding to different cracking mechanism is divided, and the SCC susceptibility is also discussed. The results show that the potential dependent SCC mechanism can be given theoretically by using potentiodynamic polarization at different scanning rate, cyclic voltammetry and dynamic electrochemical impedance spectroscopy (DEIS) measurements, which includes ductile fracture through slip separation with serious uniform corrosion, transgranular SCC (TGSCC) under anodic dissolution (AD), retarded crack growth under the cathodic protection, and intergranular SCC (IGSCC) under hydrogen embrittlement (HE). With the negative shift of the potential, the SCC susceptibility of the steel increases firstly above the open circuit potential (OCP) and decreases later under the cathodic protection, then increases again below the hydrogen evolution potential. The bainite lath grain boundary of 10Ni8CrMoV steel has opposite effects on TGSCC and IGSCC.

Fullsize Image

Effect of Al-based Oxide Layer on Frictional Properties of Hot-dip Galvanized Steel Sheets

Katsuya Hoshino, Masayasu Nagoshi, Wataru Tanimoto, Yuji Yamasaki, Shinichi Furuya, Akira Matsuzaki, Naoto Yoshimi

Released: May 16, 2017

p.895-904

The effect of the Al-based oxide layer which segregates on the surface of hot-dip galvanized steel sheets (GI) with aging on the frictional properties of the GI was investigated.

Conventional GI with the Zn coating weight of 67 g/m² including 0.36 mass% Al were used as test specimens. It was found that an Al-based oxide layer grew on the GI surface with aging after production, and the friction coefficient tended to decrease due to the existence of these Al-based oxides. However, this tendency was clearer under the sliding conditions of shorter tool length and higher contact pressure than of longer tool length and lower contact pressure.

In order to understand this behavior, surface observation and analysis of both the test specimens and the tools after sliding were carried out by SEM, EDX and EPMA. Both Al-based oxides and metallic Zn were detected as adhered materials on the tool surface after sliding, and the surfaces of adhered materials were covered with Al-based oxides. This suggests that the adhered materials on the tool have the effects of reducing the adhesion force between the metallic Zn of the GI and the tool and reducing the tool roughness. These effects led to a lower friction coefficient because both shearing and plowing resistance were decreased. In addition, the area on the tool which were covered by the adhered materials depended on the tool length. This is thought to be the reason why the effect of the Al-based oxide layer depended on the sliding conditions.

Schematic illustration of friction model for GI. Fullsize Image

Corrosion Resistance of Cu-modified 3Cr Low-alloy Steel in 3.5%NaCl Solution

Qianlin Wu, Zhonghua Zhang, Yaoheng Liu, Yameng Qi

Released: May 16, 2017

p.905-912

The effect of Cu addition on the corrosion behavior of 3Cr steel in 3.5 wt.% NaCl solution was investigated by immersion tests and electrochemical measurements. Cu was selected to partially replace Cr in Cu-modified 3Cr steel. Cu addition can improve corrosion resistance of 3Cr steel at the initial stage, and then the phenomenon gradually weakens with increasing immersion time. The reduced corrosion resistance of 3Cr steel caused by Cr decrease suitably offset by the presence of Cu, resulting a reduced-cost Cu-modified 3Cr steel without sacrificing ether its mechanical properties or corrosion resistance.

Effect of Dilution on Wear Performance of Plasma Transferred Arc Deposited Layers

Byoung-Hyun Yoon, Chang-Hee Lee, Hyung-Jun Kim

Released: May 16, 2017

p.913-920

The effect of dilution on the wear behavior of PTA (Plasma Transferred Arc) Inconel 625, Inconel 718, and Stellite 6 overlays on Nimonic 80A substrate were investigated. In order to evaluate the wear performance, two-body and three-body abrasive wear tests, and pin-on-disk dry sliding wear test were performed. Microstructural development during the solidification of deposits is also discussed. Wear test results show that the wear rate of deposits with 30% dilution is higher than that of deposits with 10% dilution by around 10%. The sliding wear resistance of overlay deposits follows a similar trend to the abrasive wear resistance. The wear rate of pins for the pin-on-disk dry sliding wear testing is increased as the applied stress is increased. However, the wear rate of pins initially increases sharply with increasing the sliding speed, then stabilizes with further increase in sliding speed at around 0.84 m/s. Cross sectional examinations of the worn surface of pin specimens implies that the plastic deformation near worn surface has occurred during the wear testing.

Transformations and Microstructures

Regular Article

Recrystallization Behavior and Texture Evolution in Severely Cold-rolled Fe-0.3mass%Si and Fe-0.3mass%Al Alloys

Miho Tomita, Tooru Inaguma, Hiroaki Sakamoto, Kohsaku Ushioda

Released: May 16, 2017

p.921-928

The effect of Si and Al additions on the recrystallization behavior of severely cold-rolled Fe by 99.8% reduction was investigated in comparison with a previous study on pure Fe.⁶⁾ In Fe-0.3mass%Si alloy, recrystallized grain with {411}<011> and {411}<148>preferentially nucleated at an early stage of recrystallization, and the texture did not changed substantially with the progress of recrystallization, which supports the oriented nucleation theory. The {411}<148> texture significantly increased at the expense of recrystallized grains with {100}<023> and ND//<111> during normal grain growth. In Fe-0.3mass%Al alloy, dynamic recovery during heavy cold-rolling and substantial subgrain growth during low temperature annealing (350°C) occurred, similar to the case of pure Fe and different from that of Fe-0.3mass%Si alloy. This is presumably because of the subtle influence of Al addition on cross-slip frequency and smaller solute-dislocation/vacancy interaction as compared with Si addition. Furthermore, at the early stage of recrystallization, the tendency of oriented nucleation became weaker in Fe-0.3mass%Al alloy than that in Fe-0.3mass%Si alloy. With the progress of recrystallization, {100}<012> and {111}<112> orientations intensified. In the following normal grain growth, {100}<012> texture intensified. However, the change in the texture during growth cannot be explained only by the size effect. A rigorous grain growth simulation model is required to explain the experimental facts by considering the dependency of grain boundary mobility and energy on grain boundary characteristics.

Retransformation Behavior of Dynamically Transformed Ferrite during the Simulated Plate Rolling of a Low C and an X70 Nb Steel

Samuel Filgueiras Rodrigues, Clodualdo Aranas Jr., John Joseph Jonas

Released: May 16, 2017

[Advance Publication] Released: March 29, 2017

p.929-936

Plate rolling simulations were carried out on an X70 Nb and a low C steel by means of torsion testing. A seven-pass rolling schedule was employed where the last pass was always applied above the respective Ae₃ temperature of the steel. Interpass intervals of 10 and 30 s were employed, which corresponded to cooling rates of 1.5 and 0.5 C/s. The mean flow stresses (MFS`s) applicable to each schedule increased less rapidly than expected from the decreases in temperature due to the dynamic transformation (DT) that took place during straining. The amounts of ferrite that retransformed into austenite during holding were determined by optical metallography. These increased with length of the interpass intervals and were reduced in the X70 steel due to the presence of Nb. The holding times after rolling required to increase the amount of austenite available for microstructure control on subsequent cooling were also determined for the two steels.

Influence of Martensite Morphology on Sheared-Edge Formability of Dual-Phase Steels

Oscar Rene Terrazas, Kip Owen Findley, Chester John Van Tyne

Released: May 16, 2017

[Advance Publication] Released: March 30, 2017

p.937-944

The automotive industry is increasingly demanding the availability of advanced high-strength steels (AHSS) with both higher strength and higher ductility. While there have recently been major improvements in the trade-off between ductility and strength, the sheared-edge formability of AHSS remains a critical issue. AHSS sheets exhibit cracking on sheared edges during stamping and forming operations, below the predictions of forming limit diagrams. The current study investigates the effects of microstructure on sheared edge formability measured through hole expansion experiments. Five commercially produced dual-phase (DP) steels with different microstructural configurations, but with equivalent tensile strengths of approximately 1 GPa, were evaluated. Quantitative stereological measurements of martensite size, contiguity, mean free distance, and number of colonies per unit area were made. The results showed the number of martensite particles per unit area, which represents microstructure homogeneity, correlates directly with the hole expansion ratio (HER). The overall trend is that HER increases as the number of martensite colonies per unit area increases; i.e. HER increases with a fine, evenly dispersed distribution of martensite. Additionally, HER decreases as the calculated carbon content in the martensite of these steels increases, which is attributed to the effect of carbon on martensite strength.

Mechanical Properties

Regular Article

Effect of Microstructure before Intercritical Annealing on Mechanical Properties of 5% Mn Steel

Hirokazu Natsumeda, Amane Kitahara, Shunichi Hashimoto

Released: May 16, 2017

p.945-952

Recently, 5% Mn steel has been focused on as one of the promising candidates for third generation AHSS by showing an excellent TS (Tensile Strength)-El (Elongation) relationship. The excellent TS-El relationship is brought about by a large volume fraction of retained austenite through the enrichment of austenite stabilizing elements such as C and Mn in retained austenite. The effect of the microstructure of mother hot band on the changes in microstructure and mechanical properties was compared with the intercritical annealing time in this study. The steel containing about 10% of retained austenite in a mother hot band exhibited a higher volume fraction of retained austenite and higher strength after intercritical annealing. On the other hand, the steel which did not contain retained austenite in a mother hot band exhibited excellent TS-El combination. The difference of work hardening behavior in these steels was analyzed and thought to be brought about by the difference of transformation behavior during deformation determined by the stability of retained austenite affected by Mn concentration. Although the precipitation of cementite was intended to effectively act as a nucleus of reverted austenite formation and to accelerate its formation, this affirmative result was not obtained. Since the volume fraction of cementite in a short annealing time is nearly the same in all hot rolling conditions, the higher volume fraction in hot band did not act to increase retained austenite during intercritical annealing.

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Volume And Issue List ISIJ International

Predecessor

ISIJ International Vol. 57(2017) No. 5

Fundamentals of High Temperature Processes

Regular Article

Ironmaking

Regular Article

Steelmaking

Regular Article

Casting and Solidification

Regular Article

Instrumentation, Control and System Engineering

Regular Article

Chemical and Physical Analysis

Regular Article

Note

Forming Processing and Thermomechanical Treatment

Regular Article

Welding and Joining

Regular Article

Surface Treatment and Corrosion

Regular Article

Transformations and Microstructures

Regular Article

Mechanical Properties

Regular Article

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ISIJ International