To investigate the effect of magnesium addition on the evolution of inclusions in Al–Ca deoxidized melts, both thermodynamic calculations and deoxidized experiments were carried out in the present work. The samples took from the melts were polished and analyzed by field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDX). The results showed that the utilization of magnesium significantly influenced the size of oxide inclusions in Al–Ca deoxidized melts. Superfluous MgO raised the melting point of the complex inclusions, and then worsen the steel castability. Therefore, the amount of magnesium addition should be under control. Based on thermodynamic calculations and experimental results, the recommend range of [Mg] is 1–5 ppm. All inclusions observed in the sample were nearly spherical except for solid calcium aluminates inclusions. Oxide inclusions were modified quickly after magnesium addition. Meanwhile, magnesium can also modify solid calcium aluminate inclusions in the melts similarly, but there are unreacted cores in the inclusions at the initial stage. The evolution mechanisms of inclusions were comprehensively discussed, and models for the formation of oxide inclusions were set up. As for the production practice, to achieve the full liquid inclusions in molten steel, the addition amount of magnesium and calcium should be considered simultaneously.
In order to promote utilization of scrap procured in the market and reduction of energy consumption in iron- and steelmaking, it is necessary to develop a practical method of Cu removal. In this study, Cu removal from hot metal by sulfidation of Cu was identified as a potential candidate for industrial use and was investigated in laboratory experiments using ferrous sulfide (FeS) and sodium carbonate (Na2CO3) as commercially-available fluxes. A laboratory experiment on removal of Cu from hot metal was carried out using a 10 kg-scale furnace. Cu removal occurred while feeding the Na2CO3–FeS fluxes, and stopped after the end of flux feed. In the experimental temperature range, lower temperatures were advantageous for Cu removal, reflecting the exothermic nature of Cu sulfidation. The dependence of Cu removal on the composition of the hot metal was also confirmed based on an analysis focusing on the Cu partition ratio. The difference in the Cu partition ratio in the present study and in the literature, in which pure Na2S was used as a Na source, was interpreted in terms of the optical basicities of the components in the melt.
In contemporary East Asian, high-basicity sinter is the predominant Fe-bearing material used for blast furnace iron making. The wettability of calcium ferrite (CF), which is the main adhesive in high-basic sinter, plays an important role in the assimilation process. In this study, an improved sessile drop technique was used to explore the wettability of a CF-based slag to solid MgO substrate at 1250°C. This investigation entailed adding 2% of Al2O3, MgO, SiO2, or TiO2. The interfacial microstructure of the slag-substrate and the spreading mechanisms for each system were discussed. The CF-based slag could melt and spread along the substrate. The spreading process was driven by the dissolution of MgO into the slag and the interfacial chemical reaction. The wetting process was divided into four stages. All of the slag systems attained a low apparent contact angle of approximately 3° to 11°, thus indicating that all slag systems could wet well with the substrate. Adding Al2O3, SiO2, or TiO2 into the CF increases the contact angle.
To efficiently recycle valuable metals such as chromium and nickel in stainless steel dust, a research was made to separate metal nugget included chromium and nickel from self-reduced products of coal composite stainless steel dust briquette, here is defined as a CCSB. Metal nuggets and slag were formed in self-reduction process of CCSB. After reduction, a large amount of C3S and a little C2S existed in slag due to its high basicity. However, C2S content was increased by solidification of oxide melt in slag, and then, it was much increased by decomposition reaction of C3S as reduced products were kept up at low temperature. The transformation of β C2S→γ C2S was occurred in slag during cooling as slag was shattered due to volume expansion, and then separation of metal nugget was achieved. The effect of basicity on separation of metal nugget was considered. Thermodynamics equilibrium calculations and non-equilibrium cooling calculations were carried out on chemical reactions in CCSB, and separation mechanism of metal nugget and the effects of holding time and temperature on separation were investigated. The optimum holding temperature and time were 1100°C and 15 min, respectively. Recovery ratio of iron, chromium and nickel were 92.5%, 92.0%, and 93.1, respectively. Metal nuggets were separated from self-reduced product of CCSB without using any auxiliary materials. This separation method can indicate one innovative process for stainless steel dust comprehensive utilization.
To evaluate the viscosities of molten slags that are suitable for capillary refining, a viscometer has been developed based on the single sphere pulling method that enables high accuracy viscosity measurement of high temperature melts. Viscosity measurements using the sphere pulling method generally carry experimental uncertainties attributed to shear stress on the wire suspending the sphere, which is not taken into account in the conventional Stokes’ law when deriving the liquid viscosity from the viscous force applied to a moving sphere in a liquid. In this study, we first considered a modified Stokes’ law equation that describes the balance between the external force and the sum of forces applied to the sphere and the suspension wire including shear stress, ascending force and the surface tension of a liquid applied to the wire, and then determined the liquid viscosity from movement of a single sphere by measuring the external force when the sphere passed through a set position. Second, we designed a new viscometer that controls the liquid container’s velocity using an electrical actuator and measures viscous force on a single sphere statically suspended from an electrical valance. We confirmed that relative sphere velocity in the liquid reaches terminal velocity immediately. These treatments improved the viscosity evaluation accuracy using the single sphere pulling method, and this viscometer enables the viscosity of the standard reference material (SRM2) for high temperatures to be measured to within ±5% relative errors from the recommended values, which is adequate for viscosity measurement of high temperature melts.
In order to develop a process to effectively desulfurize liquid steel, a desulfurization of liquid steel was performed in a specially designed equipment. Reaction between a layer of liquid slag (CaO–MgO–Al2O3) and multiple steel droplets generated from a liquid steel was attempted in a two-zone heating furnace. The liquid steel containing S was melted in a crucible at an upper zone of the furnace, and droplets were generated by falling the liquid steel through a hole at the bottom of the upper crucible. The droplets were allowed to free-fall and passed through the liquid slag melted in the other crucible at a lower zone of the furnace. The liquid steel accumulated at the lower crucible was sampled and was analyzed for the S content. It was found that desulfurization took place in two distinctive modes: dynamic and static mode. A model equation was formulated in order to interpret the obtained experimental data, and apparent rate constants for the dynamic mode (kd) and the static mode (ks) were obtained. While the ks was in the order of 10−5 m sec−1 which is in agreement with the literature data obtained by usual metal-slag reaction, the kd was much higher than the ks, as high as in the order of 10−3 m sec−1. In order to maximize the desulfurization rate by the droplets/liquid slag, it is required to keep the slag having higher sulfide capacity while the desulfurization takes place. It further requires an even distribution of the droplets in the slag layer.
The change in aromatic CH (3030–2950 cm−1) and aliphatic CH (2920–2850 cm−1) of raw and heated coals in the thermoplastic region was investigated by Fourier transform infrared spectroscopy (FT-IR). It was confirmed that the aliphatic CH abundant in coals of high volatiles is closely related to the maximum fluidity of raw coals. The peak area ratio of aromatic CH to aliphatic CH (AaroCH/AaliCH) has close relationship with mean reflectance and aromaticity of raw coals. Aliphatic CH and aromatic CH decreased while AaroCH/AaliCH increased with increasing temperature in the thermoplastic range. However, the decomposition trend of each coal clearly showed different features. The coal of low fluidity consumed aliphatic hydrogen to release volatile even before entering thermoplastic region while the coal of high fluidity gradually consumed aliphatic hydrogen all through the thermoplastic region even though they have similar volatiles. The difference in decomposition trend of hydrocarbon resulted in different condensation pattern. In addition, according to the application of 2D correlation spectroscopy for analyzing the spectral change of each coal in thermoplastic region, the relations between R3CH and RCH3 (2910 cm−1, 2958 cm−1) and between R2CH2 and RCH3 (2923 cm−1, 2958 cm−1) were different. It was suggested that the coal properties was correlated with the different decomposition trend and fluidity.
Titanomagnetite concentrate with high-TiO2 content, is endowed with poor sintering property due to the coarse granularity and poor hydrophilicity compared to the conventional TiO2-free iron concentrates. In this study, Composite agglomeration process (CAP), an innovative route for preparing blast furnace burden, was introduced to overcome the difficulties in titanomagnetite concentrate sintering. CAP results showed that the sinter yield of 75.52%, tumbler index of 62.87% and productivity of 1.633 t·(m2·h)−1 were achieved in a laboratorial sinter pot. They are 17.38%, 13.08% and 17.82% higher than those of traditional sintering process (TSP) respectively. Moreover, CAP lowered solid fuel consumption by 3.52 kgcoke/tproduct. In addition, the reduction disintegration index of the product (RDI+3.15) was improved by 20.61%, while reduction index (RI) was decreased by 4.34%. In the CAP process of preparing BF burdens from titanomagntite concentrate, very few perovskite was generated in the pelletized part of CAP product for low CaO content. In the matrix part of CAP product, a large number of SFCA generated after perovskite fully crystallized from sinter melting for ultra-high CaO containing, thus the negative effects of perovskite on sinter quality were offset by SFCA for its excellent bonding property. Some melting phases in matrix part permeated through the outer layer of pellets and formed an interweaving structure, which bonded the pelletized part and matrix part together. As a results of this kind of mineralization mechanism, the high strength and excellent RDI property of CAP product were obtained, which proves that CAP is an effective agglomeration measure treatment for titanomagnetite concentrate.
The heavy disintegration of lump ores would produce plenty of small particles in COREX shaft furnace, which would decrease the gas permeability and productivity of the shaft furnace, thus the proportion of lump ores in the burden of COREX shaft furnace is limited to a low level. In this work, the reduction disintegration behavior of lump ore samples was studied by simulating the reduction process of COREX shaft furnace. The influence of temperature, reduction time and gas composition on the reduction disintegration index (RDI-6.3) of lump ore samples were also evaluated. The results showed that the disintegration behavior of lump ores in COREX shaft furnace could be generally divided into three steps and the disintegration mainly occurred in the second step, which was in the temperature zone from 450°C to 650°C with low reduction degree. Meanwhile, the RDI-6.3 of lump ore samples all presented the tendency of “inverted V-shape” in the temperature range from 450°C to 650°C under different reduction time. However, the mutual promotion of reduction reaction and carbon deposition reaction (CDR) was attributed to the main reason for the heavy disintegration of lump ores in COREX shaft furnace. In addition, increasing H2 concentration in reducing gas and rapid reducing at higher temperature would decrease the disintegration degree of lump ores in COREX shaft furnace.
Reducible oxides containing iron in iron ore sinter are hematite, magnetite and quaternary calcium ferrite (abbreviated by CF), which is the complex crystalline mineral produced from Fe2O3, CaO, SiO2 and Al2O3. Equilibrium diagram for CF reduction with CO–CO2 gas mixture is a little but significantly different from the one for pure iron oxides. In previous analyses for reduction reaction of iron oxides in a blast furnace, however, sinter has been treated as pure iron oxides; existence of CF has been ignored. Reduction steps for CF can be written as
which are much the same as pure iron oxides, where ‘Fe2O3’, ‘Fe3O4’, ‘FeO’ and ‘Fe’ designate hematite, magnetite, wustite and iron stages of CF, respectively. However, a reported variation of gas composition with temperature measured in a blast furnace shows that the gas composition in the thermal reserve zone is a little higher than the wustite/iron equilibrium, the reduction potential of which is less than that of ‘FeO’/‘Fe’ equilibrium and hence ‘FeO’ cannot be reduced to ‘Fe’. In the present work, therefore, gaseous reduction model for sinter is developed in consideration of CF reaction process; unreacted-core shrinking model for six interfaces is proposed to take into account reaction processes of CF as well as pure iron oxides. Trial comparison of the calculated reduction curve with our previously reported experimental data under simulated blast furnace conditions shows rather good agreement.
The smelting reduction of high Al2O3 ores with petroleum coke containing various slags was investigated in the temperature range of 1673 to 1773 K with a special focus on the effect of slag composition on the desulfurization and dephosphorization of iron nugget. The efficient smelting reduction of the carbon composite agglomerate (CCA) would greatly be affected by the mineralogy of iron ore, coal and additives used in the preparation of CCA although the design of final slag composition is important for the ultimate separation of metal and slag. The separation of metal and slag in the reduction progress was retarded due to the delayed melting of CCA with high basicity slag of low SiO2 content. The content of sulfur in iron nugget decreased to about 0.07 mass% by incorporating high basicity slag to CCA at the temperatures above 1723 K, which is compatible with the minimum sulfur content in iron of the conventional ITmk3 process (0.05–0.07 mass%). About 80–90% of phosphorus in CCA was eliminated using high basicity slag (CaO–Al2O3) at 1723 K, which is believed to be much better dephosphorization efficiency compared with the conventional process.
Characteristics of flow field and stirring effects of top lance with various tilt angles (Tilt angles were 39°, 41°, 43°, 45° and 47°, respectively) on the molten bath were studied. The mixing time, impacting depth and impacting diameter were measured by water experiment. Flow field characteristics of three-phase flow were simulated by Fluent software. It was found that 43° oxygen lance could get the shortest mixing time being 56 s, the medium impacting depth being 36 mm and medium impacting diameter being 115 mm. When injecting by the 43° oxygen lance, average flow velocity of molten bath was the biggest being 0.0329 m/s, and volume of dead-zone was the minimum being 1.90 m3. In order to study metallurgical effects of oxygen lance with tilt angle, both 43° and 45° oxygen lances were experimented in a dephosphorization ladle furnace. The experiment showed that the oxygen lance with tilt angle can guarantee the normal smelting process. Compared with the 45° oxygen lance, the content of phosphorus in semi-steel B was decreased by 0.012 mass%, the dephosphorization rate was increased by 7.9% and the content of T. Fe loss was dropped by 1.9 mass%. It proved the oxygen lance with tilt angle could get a remarkable dephosphorization effect in a ladle furnace.
The wettability of liquid iron and steel in contact with TiN substrates was studied. Initially, Spark Plasma Sintering (SPS) was used to prepare the samples using different operational conditions. It was found that a relative density of 96% and surface roughness values smaller than 250 nm could be obtained by using the following settings: a 1873 K temperature, a 89.2 MPa pressure and a 5 min sintering time. Thereafter, the wettability of the liquid iron and steel in contact with the TiN substrates was measured based on video recordings, at the moment when the metals started to melt. The results show that the contact angle value for a TiN/pure Fe system (130 to 87.9 degrees for 900 s) is larger than the value for a TiN/steel system (110 to 50 degrees for 981 s). Therefore, it is concluded that TiN has good resistance to the corrosion of the liquid iron and steel. In the liquid iron case, its wetting behaviour occurs mainly due to the oxygen increase in liquid iron after a full melting. However, in the liquid steel case the contact angle decreases sharply due to both the effects of an oxygen increase in liquid steel and a precipitation of Ti(N,C,O) at the interface.
This paper studied the steel cleanness’ difference of grade SPHC by comparing two RH modes. Experiments were carried out at 210 ton BOF, RH and 60 ton tundish by sampling systematically. Under mode I, free oxygen was killed during BOF tapping; and inclusions had sufficient removal during RH vacuum treatment; under mode II, RH vacuum carbon de-oxidation was adopted firstly; and then residual oxygen was killed by grain aluminum. Results showed that, (1) total generation amount of inclusions in mode II was 1/2 of that in mode I due to low residual oxygen after vacuum carbon de-oxidation, but steel cleanness in mode I was better in tundish due to long inclusions removal time; (2) under mode I, total oxygen in tundish could be controlled below 20 ppm; and cast-ability in continuous casting process reached 15 heats; (3) under mode II, nitrogen could be controlled below <15 ppm, which was 1/2 of that in mode I; (4) by adopting mode II to reduce total generation amount of inclusions and nitrogen picking up, also to prolong 20 to 30 minutes calming time before casting to guarantee inclusions’ removal, low nitrogen, low total oxygen and good cast-ability could realize.
The alloy/liquid Fe interaction behaviour at early stages of deoxidation/alloying have considerable influence on the mechanical properties of steel products. The interfacial reaction between FeMnSi and liquid Fe, as well as the inclusion formation, were experimentally studied at the addition early stages based on quenched FeMnSi–Fe diffusion couples. The microstructure and composition of the quenched diffusion couple were evaluated and the inclusions in the reaction zone were characterised. Five regions were distinguished in the quenched reaction zone, i.e. (1) a solidified Fe shell, (2) a α1 phase consisting of a solid α1 phase and a Fe–Mn–Si liquid melt at the experimental temperature, (3) eutectic α1+Fe5Si3 and (4) eutectic Mn5Si3+MnSi phases, which are liquid at the experimental temperature and (5) the original FeMnSi alloy which did not melt during the interaction. Plenty of oxysulphide inclusions are observed in the reaction zone: Fe(O,S) inclusions formed in the bulk iron and an inclusion poor zone is observed in the inner part of the Fe shell. Large amounts of tiny (Mn,Si)O inclusions formed in the α1 phase near the Fe/α1 interface and large (Mn,Si)(O,S) inclusions precipitated in the liquid phase after solidification.
The paper presents a method for computing the parameters of an electromagnetic mould for a horizontal continuous casting of thin aluminium alloys strip. The method is based on the calculation of the distribution of the magnetic field, which is based on the interaction between the inductor and the surface of the shaped molten metal. The interaction of fields of the inductor and its mirror image in the molten metal moves the centres of the electrical current flow closer together and increases the current density at the inductor’s face adjacent to the liquid metal. The operating frequency of alternating current flowing through the inductor, which depends on the density, the surface tension, and specific electrical conductivity of the liquid metal, and the pool thickness, is determined. The mould was designed for the electromagnetic levitation a free surface of 150 × 20 mm strip from AlSi8.5Cu and AlSn20 alloys. It consists of a system of special inductors which are of the different configurations for shaping the upper and lower surfaces. At the inductor’s width of 28 mm, the uniform magnetic field ensuring the steady air gap between the inductor and the liquid metal can be expected over the length of 24 mm.
The accumulation of inclusions in the initial solidified shell, which may cause longitudinal cracks, is studied in this paper. The composition, morphology and distribution of large inclusions in thin slab are investigated by inclusion measurements. Based on the experiment, the quantity and size distribution of inclusions are obtained as reference in predicting inclusion behavior in CSP mold through numerical simulation. Due to the funnel-shaped structure of mold, inclusions injected through SEN are mostly captured by the solidifying shell in the funnel region nearby the parallel part of mold, which cause the accumulation of inclusions in thin slab within this region. Moreover, the simulation results are consistent with experimental data that large inclusions concentrate at the position of 1/4 slab width, which suggests a lower product quality there. In addition, the distribution of inclusions in the solidified shell is closely related to their motion in the mold. Specifically, the transport and removal of inclusions entrained from mold slag are calculated, since K/Na-contained inclusions and excessive surface velocity demonstrate that slag entrainment appears in CSP mold. These inclusions can float up easily, while those entrapped by the shell are detrimental to thin slab quality.
Flow structures were investigated in a dissipative ladle shroud (DLS) and a tundish using Large Eddy Simulation. The numerical results were validated inside the DLS and the tundish with PIV experiments. Velocity distribution, vorticity islands and strain rate were analyzed in the DLS respectively, compared with that of a bell-shaped ladle shroud (BLS). The results showed that the three chambers of the DLS gave rise to velocity differences, fluctuating strain rates and vortices, and promoted an increase on turbulence dissipation rate; and the average velocity of outflow ranged from 0.25 to 0.5 m/s when the inlet velocity was 0.708 m/s. In the BLS, the stream flowed straightforward with relatively consistent velocity; apparent vortices were only formed in the bell end; and the outflow went down with high speed and turbulent kinetic energy. The dissipative effect of the DLS was also validated by the flow structure in the tundish. When the stream left the outlet of the DLS, it swung, got twisted and was mixed with more surrounding fluid in the tundish which decreased the mean skin friction coefficient of tundish wall and the velocity of free surface, and finally contributed to a better tundish performance.
MgAl2O4-TiN complex inclusion found in GCr15SiMn ESR ingot with MgAl2O4 as the core and TiN as the periphery is of great harm to steel. The characteristics of this complex inclusion precipitation and growth during solidification of metal molten pool have been theoretically investigated with the help of thermodynamics and kinetics. The results demonstrate that during solidification of metal molten pool, TiN will precipitate on the existed core MgAl2O4 due to the microsegregation of solutes Ti and N. The effect of high cooling rate on the size of complex inclusion with the core MgAl2O4 from the consumable electrode is not significant, particularly for larger size of the core, while with the core MgAl2O4 formed during solidification, its size is greatly affected by the cooling rate, which will possibly provide a useful method of distinguishing the source of the core MgAl2O4 under high cooling rate. At the same cooling rate, the growing extent of periphery TiN decreases with the increasing size of the core MgAl2O4. Meanwhile, decreasing C content in steel within the upper and lower limit will decrease the percentage of this complex inclusion.
A new catalytic combustion-type CO gas sensor was devised by using the precious metal-free CO oxidizing catalyst of 15.9 wt% La0.87Co1.13O3-loaded Ce0.67Zr0.18Sn0.15O2.0. Since the 15.9 wt% La0.87Co1.13O3-loaded Ce0.67Zr0.18Sn0.15O2.0 catalyst oxidizes CO completely at 130°C, the sensor showed smooth and reproducible response to CO gas at ca. 130°C. Moreover, the sensor exhibited a linear relationship between response and CO gas concentration, along with short 50% response time of 20–40 s.
The cooling process of a moving hot steel plate after hot rolling was numerically investigated. Many important parameters affect the cooling process. This study focuses on residual water, which accumulates on the surface of the moving plate by the continuous supply of cooling water. This residual water may significantly interfere with the impingement of water jets and the cooling of the steel plate. In this study, we devised a concept of artificial sidewalls to apply a realistic residual water level in a numerical simulation of a cooling hot steel plate. The necessity for this concept of an artificial sidewall for simulations of moving plate cooling was investigated by comparing the cooling history of the plate for three different longitudinal domain cases with and without artificial sidewalls.
In electric arc furnace (EAF) steelmaking the control of the overall process has traditionally been in the hands of the operator. The current level of automation has not allowed for full dynamic control of EAF. In this study new information was sought with a recently tested method of on-line in situ optical emission spectroscopy. Optical emission spectra were measured from Outokumpu Stainless Oy, Tornio works, electric arc furnace 2 during the course of one month. The focus of the study was the analysis of slag surface conditions and alkali emission lines during different process stages. The results show that alkali excitation lines are different in different process periods. According to the properties and appearance the optical emission of alkali metals, in early periods of EAF process the alkali transitions are caused by volatilization of contaminants in the EAF charge. In the later periods the optical emissions of alkali metals are linked to occasional high gas generation, which is likely caused by oxidation-reduction reactions occurring inside the melt. The thermal radiation of the heat was analyzed during different process periods. According to the scrap melting indicator calculated from the measured spectra, the melting conditions vary significantly between heats. The scrap melting indicators have a correlation to the metal yield, but it is obstructed by many different factors affecting the metal yield. The results indicate that on-line in situ optical emission spectrometry can be used in the analysis of scrap melting and slag surface conditions in EAF steelmaking.
Grazing incidence X-ray fluorescence analysis with a collodion film sample holder was performed by using a portable total reflection X-ray fluorescence spectrometer. The spectral background was low when using this sample holder, and a Cr detection limit obtained with this sample holder was comparable to that obtained with a diamond-like carbon coated quartz glass sample holder which was usually used as sample holder for the portable spectrometer. A river water sample was also measured with the collodion film sample holder, and 10.0 μg/L of Cr and 5.1 μg/L of Mn were detected. Collodion film sample holder can be used as a disposable sample holder for trace elemental analysis using the portable spectrometer.
Hot forging characteristics of chrome steel and carbon steel covered with oxide scale formed in steam atmosphere were investigated with the ring compression test. In oxidation of the steels in steam atmosphere, the oxide scale with high porosity was formed in oxidation of the steels, while the oxide scale with thick FeCr2O4 layer around the oxide scale–chrome steel interface was formed in oxidation of the chrome steel. The nominal coefficient of shear friction of the steels covered with oxide scale was derived from the plastic deformation behavior of the steels covered with oxide scale during the ring compression test. The oxide scale of the chrome steel formed in steam atmosphere provided low friction characteristic, compared with the oxide scale formed in air atmosphere. The mechanism of reduction of friction of the steels covered with oxide scale was discussed from the viewpoints of heat transfer and interfacial friction at oxide scale–chrome steel interface by the experiment and finite element analysis.
In this study, plain C–Mn steels with different ferrite grain size were obtained through various thermo-mechanical processing. Fast cooling suppressed ferrite transformation at high temperature, but promoted ferrite nucleation at low temperature and restricted ferrite grain growth. The microstructure and mechanical properties were governed by the combined effects of cooling rate and cooling temperature. A new thermo-mechanical controlled processing (TMCP) based on ultra fast cooling (UFC) technology was adopted to increase the strength of plain C–Mn steel strips. The new TMCP approach involving UFC enabled significantly finer grain size to be obtained in relation to conventional TMCP. The grain size of Fe-0.17C-0.33Mn steel strips obtained by the new TMCP was refined to ~3–4 µm, and both yield strength and tensile strength were increased by 60–100 MPa over the conventional TMCP.
The influence of Ti addition on inclusion characteristics and their effects on the nucleation of acicular ferrite has been studied using as-deposited bainitic-type GMA weld metals prepared with oxygen content constant. TEM analysis was carried out to identify the inclusion phases and to investigate the local variation in chemical composition around the inclusions. It was found that the Ti-containing phase changes from (Mn,Ti)-spinel oxide to Ti2O3 at ~0.07 wt.% Ti while Mn-depleted zone (MDZ) starts to form at ~0.02 wt.% Ti. This indicates that MDZs are induced by the formation of Ti2O3 as well as a large amount of (Mn,Ti)-spinel oxide. Due to MDZ formation, acicular ferrite can nucleate to a great extent, even from the amorphous surface of the Mn-silicate phase in the 0.02% Ti weld.
Wet and dry corrosion tests using a NaCl solution were performed on Cr bearing steels in concrete. The nano-scale structure and electrochemical behavior of the rust were analyzed by TEM (Transmission Electron Microscopy) and EIS (Electrochemical Impedance Spectroscopy). The carbon steel (SM) had large amount of corrosion and produced cracking in the concrete after corrosion test. On the other hand, the Cr bearing steel showed much less corrosion, and had no concrete cracking. After corrosion test, EIS measurements were performed on the samples to determine rust resistance (Rrust) and corrosion resistance (Rt) of the different steel types. The Rrust and Rt of Cr bearing steel were much larger than those of SM after corrosion test. SEM (Scanning Electron Spectroscopy) with EDS (Energy Dispersive Spectroscopy) showed that Cr and Si were enriched in inner rust of the Cr and Si bearing steel. TEM showed that nano-scale complex iron oxides containing Cr and Si were formed in inner rust. It was found that the Cr bearing steel formed nano-scale iron complex oxides containing Cr and Si in inner rust, which increased Rrust and Rt, and suppressed the corrosion by chloride ions in the concrete.
The deformation twinning behavior in Fe–17Mn–0.6C, Fe–17Mn–0.8C, and Fe–18Mn–1.2C (wt.%) twinning-induced plasticity (TWIP) steels was investigated by atomic force microscopy (AFM) and electron backscatter diffraction pattern (EBSD) analyses. AFM-based surface relief analysis combined with EBSD measurements was employed to determine the active twinning direction and deformation twin fraction for specific crystallographic orientations. The addition of carbon is known to increase the stacking fault energy. The <111> tensile orientation grains revealed suppression of deformation twinning with increasing carbon concentration; however, the deformation twin fraction in the <144> tensile orientation did not change as a function of the carbon concentration. These results imply that another factor in addition to stacking-fault-energy-based criteria is required to interpret the deformation twinning behavior of carbon-added TWIP steels.
In a previous study, the carbon concentration dependence of the deformation twinning behavior in twinning-induced plasticity steels was investigated, which clarified that the deformation twin fraction in the <144> tensile orientation did not change with the carbon concentration. In this study, twinning deformation occurred in the Fe–18Mn–1.2C steel at 473 K with a relatively high stacking fault energy of 55 mJ/m2. To explain these experimental results, dynamic strain aging of Shockley partials dislocations was proposed as an additional contributing factor to assist the deformation twinning in high-carbon-added austenitic steels. Most abnormalities concerning deformation twinning such as the high stacking fault energy in Fe–Mn–C austenitic steels were interpreted by considering the effect of dynamic strain aging.
0.4%C-2%Si-1%Cr-1%Mo steels with different austenite grain structures were quenched and tempered at 773 K and deformed by multi-pass caliber rolling with a rolling reduction of 78% (i.e. warm tempforming, (WTF)). The microstructures, in addition to the tensile and Charpy impact properties of the warm tempformed (TF) steels, were investigated in relation to the prior-austenite grain (PAG) structures. The TF samples demonstrated hierarchical microstructures consisting of “packet bands” and “ultrafine elongated grains (UFEGs)”. The PAG structure had little influence on the transverse grain size, grain shape, <110>//rolling direction (RD) fiber texture, and carbide particle distribution in the UFEG structure, while the size of the packet bands decreased as the PAG size decreased. The PAG size had little influence on the strength of the TF sample. However, the effect of the WTF on the tensile ductility and toughness was enhanced as the PAG size decreased. It was demonstrated that as the PAG size decreased, the inverse temperature dependence of impact toughness (resulting from delamination) became significant at lower temperatures. The delamination toughening mechanism was discussed in relation to the UFEG and packet band structures.
The role of the dynamic interactions between hydrogen and the strain-induced martensite transformation in the hydrogen embrittlement of type 304 stainless steel has been investigated by fractographic observations after a modified hydrogen charging. The modified charging is cathodically conducted in 3.5% NaCl solution at 80°C under aerated conditions while preventing the dissolution of chlorine and oxygen gases evolving on the platinum counter electrode, thus increasing the amount of hydrogen thermally desorbed at low temperatures. Upon tensile testing at 160°C, plastic deformation of the austenite phase in the stainless steel occurs, but no strain-induced martensite transformation occurs. The fracture surface of the hydrogen-charged specimen exhibits the double-cup mode and consists of microscopic shallow dimples. Upon tensile testing at 25°C, the martensite transformation and plastic deformation both occur and are intricately related; a brittle area is observed on the outer part of the fracture surface that exhibits both transgranular and intergranular fracture. At −196°C, the martensite transformation increases, but the amount of plastic deformation decreases and the amount of intergranular fracture increases. It is found that, when the martensite transformation occurs before hydrogen charging, the amount of intergranular fracture decreases. Moreover, when charged hydrogen is trapped in defects in the austenite phase, the amount of intergranular fracture also decreases. The present study indicates that the dynamic interactions between hydrogen and the martensite transformation play an important role in the hydrogen embrittlement of type 304 stainless steel.
This paper reports the effect of nano-precipitation strengthening of ferrite on the tensile behavior of ferrite-martensite dual phase (DP) steels. Samples of ferrite-martensite DP steel containing a dispersion of nano-sized vanadium carbides (VCs) in the ferrite phase were produced by interphase precipitation and quenching of a V-added low carbon steel, and the mechanical properties are compared with those of conventional ferrite-martensite DP samples without VC particles. Both the yield stress and the ultimate tensile strength are significantly increased by nano-VC precipitates. For ferrite volume fractions of 20–50% a dispersion of VCs results in only a small change in the elongation, whereas for ferrite volume fractions of above 50% both uniform and post-uniform elongations are decreased by a VC dispersion. It is suggested that dispersion of nano-precipitates in ferrite is an effective approach to simultaneously improve the strength and the strength-ductility balance of DP steels. Digital image correlation (DIC) analysis demonstrates that the ferrite phase is more deformed than the martensite phase in both VC-free and VC-dispersed DP samples, but that such strain partitioning is less pronounced in the VC dispersion-hardened samples. It is found that the stress-strain relationship of DP samples can reasonably be explained based on a law of mixtures using partitioned strain and stress values as estimated from the DIC analysis.