Low-carbon Aluminum-killed Steel Research Articles

Effect of calcium treatment on evolution of inclusions in low-carbon aluminum-killed steel

Based on the steel samples collected from low carbon aluminum-killed steel at ladle furnace start, end of ladle furnace heating, ladle furnace end and tundish pouring process for inclusions analysis, the influence of calcium treatment on the evolution of inclusions in steel from ladle furnace refining to continuous casting was investigated under the condition of using Al2O3-lining ladle. The results show that when calcium treatment is not used, the inclusions in steel are mainly Al2O3 and MgO-Al2O3 spinel, and the mass fraction of Al2O3 in the inclusions composition is ≥90% from ladle furnace refining to continuous casting. After calcium treatment is carried out, the types of inclusions in steel are mainly MgO-Al2O3-CaS (-CaO) inclusions, in which the content of Al2O3 and CaS is higher than the samples without calcium treatment, and low-melting point calcium-aluminate inclusions were not found in the samples. The average size of inclusions in the steel was similar for both processes during ladle furnace refining, but the number density of inclusions in the steel increased significantly after calcium treatment. At the end of LF, the number density of inclusions in the calcium-treated is about twice that of the non-calcium-treated.

Assessment of Tundish Process Performance at Industrial Scale

Air ingression through ladle shroud and consequent re-oxidation of dissolved aluminium in steelmaking tundish systems have been investigated at industrial scale. To this end, methods and measurements, reproducibility in measurements, relationship between rates of dissolution of nitrogen and oxygen, etc. have been analysed and discussed. Embodying measured soluble and total (= soluble + insoluble) aluminium content of LCAK (low carbon aluminium killed) steel in a material balance frame work, a formalism has been developed to quantify metallurgical performance of steelmaking tundish systems. Two dimensionless tundish process performance parameters namely, generation index, GI and floatation index, FI have been derived and represented as: $$G_{\text{I}} = \frac{{ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} }}{{\left( {{\text{wt}}\% {\text{Al}}_{{{\text{insol}},{\text{VD}}}} + [ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} ]} \right)}}$$ $$F_{\text{I}} = \frac{{ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} - \Delta {\text{wt}}\% {\text{Al}}_{\text{insol}} }}{{\left( {{\text{wt}}\% {\text{Al}}_{{{\text{insol}},{\text{VD}}}} + [ - \Delta {\text{wt}}\% {\text{Al}}_{\text{sol}} ]} \right)}}$$ in which, Δ represents composition differential between tundish exit and entry, respectively. To demonstrate the present work, operating data from six different domestic steels plants, including integrated and special steel mills, have been applied. It is shown, that while floatation of inclusions from tundishes is barely satisfactory, aluminium fading due to re-oxidation, and consequent generation of inclusions, are significant. In addition to such, on the basis of relevant theory and measurements, nitrogen and oxygen absorption from air during ladle to tundish transfer have been investigated at length and possible routes of dissolved aluminium re-oxidation in tundish have been examined further. It is shown that chemical reactions between slag and/or gunning material (viz., SiO2) and dissolved aluminium, rather than air ingression through shrouds, dominate [Al] re-oxidation in the tundish systems. The formalism developed in this study can also be conveniently extended to investigate equivalent phenomena in calcium treated LCAK steels, containing magnesium.

Physical Modelling and Numerical Simulation of the Deep Drawing Process of a Box-Shaped Product Focused on Material Limits Determination

Similitude theory helps engineers and scientists to accurately predict the behaviors of real systems through the application of scaling laws to the experimental results of a scale model related to the real system by similarity conditions. The theory was applied when studying the deep drawing process of a bathtub made from cold rolled low carbon aluminum-killed steel from the point of view of material limits. The bathtub model was created on the basis of geometric, physical, and mechanical similarity on a scale of 1:5. Thus, simulations and physical models were created. The simulation model was used to verify the combination yield locus/hardening law on the basis of comparing the thickness change. As a result, Hill 48/Krupkowski showed the minimal deviation by comparing data evaluated from numerical simulations and that measured on the physical model. Additionally, material anisotropy was modelled when virtual materials were defined from experimentally measured values of the plastic strain ratio. As an outcome, extra deep drawing quality steel with an average plastic strain ratio of rm ≥ 1.47 and an average strain hardening exponent of nm ≥ 0.23 must be used for the deep drawing of the bathtub.

A Formability of Two Copper-Bearing, Cold Rolled, Box Annealed and Temper Rolled Extra-Deep Drawing Low Carbon Aluminium-Killed Steel Sheets of Nagarjuna Steel Limited (Private, Andhra), and a Comparison with that of a Typical Steel Sheets Made by Bokaro Steel Plant (Steel Authority of India Limited, Ministry of Steel, Government of India)

A Formability of Two Copper-Bearing, Cold Rolled, Box Annealed and Temper Rolled Extra-Deep Drawing Low Carbon Aluminium-Killed Steel Sheets of Nagarjuna Steel Limited (Private, Andhra), and a Comparison with that of a Typical Steel Sheets Made by Bokaro Steel Plant (Steel Authority of India Limited, Ministry of Steel, Government of India)

Distribution of macro-inclusions in low carbon aluminium-killed steel slabs

ABSTRACTMacro-inclusions in low carbon, aluminium-killed steel slabs were characterised by step-machining within a 10 mm zone from the slab surface using an ASPEX automatic inclusion analyzer. Dendritic structures within the cross-section of slabs were examined. The results show that alumina clusters and alumina associated with bubbles are the dominant macro-inclusions. Along the slab width direction, macro-inclusions were mostly found at the slab centre because of the deeper hooks and freezing meniscus surrounding the submerged entry nozzle. In terms of slab thickness, inclusions were mainly concentrated within the zone 3.5–6 mm from the top of the slab surface, where the columnar dendrites showed a relatively small inclination angle, indicating small cross-flow velocities at the solidification front. The number density of macro-inclusions were strongly dependent on the washing effect produced by the flow velocity. High speed casting promotes this behaviour and improves the surface quality of the slabs.

Influence of Boron Addition on the Hot Ductility of Low-Carbon Aluminum-Killed Steel

Influence of Boron Addition on the Hot Ductility of Low-Carbon Aluminum-Killed Steel

Analysis of Earring in Circular-shell Deep-drawing of bcc and hcp Sheet Metals

The occurrences of earrings in circular-shell deep-drawing tests for two different types of crystal materials were investigated. The simulation was conducted with whole 360̊circumference analysis. As the first result, the earring height is proportional not only to ΔR/Rave but also to ΔTLave ((R90-R00)/Rave). Secondly, the optimum m-value in yield function for the pure titanium and the low-carbon aluminum-killed steel, which led to the fairly good reproduction of experimental results, is 8 and 2 respectively. Furthermore, the blank mesh pattern is assumed to clarify the influence of R-values of three directions.

Analysis of Earring in Circular-Shell Deep-Drawing Test

A circular-shell deep-drawing test and an FEM simulation utilizing LS-DYNA with Barlat-Lian anisotropic yield locus (1989) were conducted in order to investigate the occurrence of earring for low-carbon aluminum-killed steel sheet and pure titanium sheet. The simulation method was developed that can accurately estimate the earring pattern of whole 360° circular wall height, where R-value is supposed to be measured within the range of uniform elongation, in particular for the case of pure titanium. The well known opinion that the earring value is proportional to ΔR/Rave does not always hold for the whole sheet materials. Despite the fact that the value of ΔR/Rave of pure titanium is nearly zero, it exhibits large earring. This is considered to stem from the characteristic planer anisotropy of R-value of pure titanium, namely large ΔTLave ((R90 - R00))/Rave) together with very small ΔR. Furthermore, it is interesting to note that the earring in 45° direction was observed experimentally and was reproduced by simulation, despite the fact that pure titanium shows the straight planer anisotropy with the maximum in 90° direction. The optimum m-values in yield locus for pure titanium and low-carbon aluminum-killed steel, which led to the fairly good reproduction of experimental results, is 8 and 2, respectively. This difference in optimum m-value between the two metals is presumably related to the difference in the number of slip system; namely the number of active slip system is very limited for pure titanium.

Cleanliness of Low Carbon Aluminum‐Killed Steels during Secondary Refining Processes

AbstractEfficient secondary refining process is necessary for massive and stable production of low carbon aluminum‐killed (LCAK) steels. Plant trials were performed to investigate the cleanliness of steels. Characteristics of composition, cleanliness, and inclusions of LCAK steel during different secondary refining processes, including LF, CAS, RH‐LIT, and RH, were studied and compared. The results showed that CAS, RH‐LIT, and RH processes had better control of low carbon, silicon, and nitrogen than LF process. High cleanliness of LCAK steels could be achieved by all the mentioned refining processes. It was concluded that the total oxygen (T.O.) should be <35 ppm to reduce the amount of inclusions and reach the level of clean steels. The removal rate of inclusions during RH‐LIT and RH processes was much higher than that of LF and CAS. The T.O. content and the amounts of inclusions during CAS, RH‐LIT, and RH could be quickly decreased to a low value within 10 min. The results showed that CAS and RH‐LIT as well as RH refining processes can produce LCAK steels that meet the requirements of high efficient, low cost, clean, and stable production, while LF is more suitable for the heats with poor control of end point of BOF, and for the process with calcium treatment to control sulfur content and lower the clogging of Submerged Entry Nozzle during thin slab continuous casting.

Precipitation of AlN and MnS in Low Carbon Aluminium-Killed Steel

The precipitation kinetics of AlN and MnS in low carbon aluminium-killed steel was calculated. Transmission electron microscopy (TEM), energy disperse spectroscopy (EDS) and phase analyses have been used to investigate the morphology, compositions and particle size distribution of AlN and MnS precipitates in three positions of the coil. The particles of AlN and MnS precipitates in the ferrite region after coiling and distributes along and adjacent to the ferrite grain boundaries. The shapes of AlN are plate-like, the precipitates size is about 10 to 60 nm; the shapes of MnS are spherical, the precipitates size is about 200 to 600 nm. The precipitation behavior of AlN is sensitive to the isothermal temperature and holding time, the precipitation quantity and particle size distribution of AlN in different positions of coil are unequal.

Investigation on Non-Metallic Inclusions in LCAK Steel Produced by BOF-LF-FTSC Production Route

The behavior of non-metallic inclusions in LCAK (low carbon aluminum killed) steel produced by BOF (basic oxygen furnace)-LF (ladle furnace) refining-FTSC (flexible thin slab continuous caster) production route was investigated. The results showed that, LF refining for LCAK steel could decrease the ωT[o] significantly, and the inclusions were modified by Ca treatment, which prevented nozzle clogging efficiently. However, owing to the unstable casting condition in the earlier stage of casting, a severe reoxidation occurred, accompanied with mold slag entrapment. The transformation of non-metallic inclusions during the steelmaking process was Al2O3 → MgO-Al2O3 type inclusion→MgO-Al2O3-CaO type inclusion with a CaS ring, and the mechanism of the transformation was proposed and discussed via thermodynamic and kinetic analysis. Besides, to avoid CaS precipitation, the product of ω2[Al] × ω[s] in steel should be less than 2.0 × 10−10 at 1873 K, which remands higher desulfurization ratio during LF refining.

Research on Characteristics of Deposition Precipitation Process in Production of Low Carbon Aluminum-Killed Steel in FTSR Technology

Precipitation characteristics of second phase in low-carbon steel produced by FTSR technology have been researched by TEM and EDS in this article. And preliminary research of precipitation conditions of second phase particles in thermodynamics and dynamics have been took. The results indicate that: there’re second phase particles precipitated dispersively in hot rolled low-carbon steels by FTSR technology. These particles mainly contain particles of Al2O3, MnS and AlN, and the precipitate size is about tens to hundreds of nano. Thermodynamical analysis declares that most of the Al2O3 and all of the MnS, AlN particles are precipitated in solid state. That’s why the precipitation process is slowed down by the diffusion velocity of the elements in solid, and thinner particles are precipitated while the material is in solid state than in liquid state. Different from the traditional plate rolling production of Al-killed steel, the precipitation of AlN particles are mainly controlled by the continuous casting process in thin slab casting and rolling process, but almost no precipitation of AlN particles in the rolling process.

Prediction of carbon segregation on the surface of continuously annealed hot-rolled LCAK steel

An attempt has been made to predict the segregation of carbon in continuously annealed hot-rolled low-carbon aluminum-killed steel (LCAK) through thermodynamic calculations. It is found that the surface carbon content calculated by the Guttmann model is comparable with the experimental results for inert atmosphere condition, whereas a large discrepancy is observed for the conventional continuous annealing furnace atmosphere. The main reason for this difference is the water–gas reaction that takes place on the surface of the steel sheet. It is proposed that this reaction, along with furnace atmosphere, determines the extent of carbon content on the sheet's surface. Based on this, the surface carbon content for various furnace conditions could be predicted.

Indirect Methods of Detecting and Evaluating Inclusions in Steel—A Review

Indirect methods to detect and evaluate inclusions in steel are extensively reviewed, which include total oxygen content measurement, nitrogen pickup, steel and slag composition measurement, lining refractory observation, tracer studies of determining exogenous inclusions from slag and lining erosion, SEN clogging and final product tests. There is no single ideal method of detecting and evaluating inclusions in steel, so it is better to use several methods simultaneously to give a more accurate evaluation. Total oxygen content and nitrogen pickup in low carbon aluminum-killed steel as criterion of clean steel in many plants are summarized.

A Morphological Comparison between Inclusions in Aluminium Killed Steels and Deposits in Submerged Entry Nozzle

A study was carried out to compare the inclusions present in low carbon aluminium killed (LCAK) steel with oxide particles in a clogging deposit formed during casting of LCAK steel. Steel pin samples were taken in the ladle and in the tundish. It was observed that oxide particles in the clogging deposit are mainly coral‐shaped clusters, dendritic clusters and irregular plates. From the comparison with inclusions in LCAK steel, it is concluded that the particles in the clogging deposits are formed during reoxidation of the steel, at least after the ladle metallurgy process and most probably in the submerged entry nozzle. They are not the result of sintering of small preexisting inclusions.

Kinetic and microstructural study of aluminium nitride precipitation in a low carbon aluminium-killed steel

The precipitation kinetics of free nitrogen present in a low carbon aluminium-killed steel was quantitatively followed at temperatures ranging from 600 to 700 °C using a methodology based on a succession of thermoelectric power (TEP) measurements. These were compared with the precipitation kinetics of aluminium nitrides determined by conventional TEP measurements. This comparison showed a difference between the two types of kinetics. In particular, it was observed that after the complete elimination of nitrogen from the solid solution (SS), the microstructure of the steel keeps evolving. Transmission electron microscopy (TEM) observations, combined with chemical X-ray energy dispersive spectroscopy (EDS), allowed to attribute this result to a phase transition between metastable (Al, Cr)N precipitates of cubic structure and equilibrium AlN nitrides of hexagonal structure. The presence of chromium in the initial precipitates was interpreted by a preferential segregation of this element on the former grain or sub-grain boundaries of austenite during or after hot-rolling. Then, during the precipitation of the nitrides, clusters of segregated chromium atoms could serve as nucleation sites for the (Al, Cr)N nitrides.

Change in Earing of Low-carbon Aluminum-killed Steel Sheets with Hot- and Cold-rolling Conditions.

Experiments have been conducted to investigate the effect of hot-and cold-rolling conditions on earing. It was shown that the earing of cold-rolled and annealed sheets increases with an increase in cold-rolling reduction. On the other hand, the earing decreases with a finishing temperature below Ar3 temperature in hot rolling, the effect of the finishing temperature below Ar3 being minimal in connection with a hot-rolling reduction. The earing increases as the hot-rolling reduction increases and a finishing temperature below Ar3 decreases. With a lower coiling temperature, a deformed structure remains in hot-rolled band and the earing is increased. These increases or decreases in earing depend on the growth or suppression of 45° earing arising from the texture of RD//<110>±20°. The texture development is affected by the initial texture and initial grain size before cold-rolling, and the earing is controlled by the effect of a balance between the same.

Modelling recrystallization texture development during slow heating of cold-rolled aluminium-killed steel

A model of recrystallization texture formation during slow heating of low-carbon aluminium-killed steel was developed. The evolution of three main texture components, having , and planes parallel to the sheet plane, is considered. The model is based on the oriented nucleation mechanism. The nucleation rate is treated as dependent on the stored energy of cold deformation, the peculiarities of deformation substructure and the texture. The effect of aluminium nitride precipitation on the grain nucleation is considered as the problem of selection of subgrains able to grow. A simple model of aluminium nitride precipitation is suggested describing the sequence of precipitation and particle coalescence. The recrystallization model was calibrated using available experimental data for a low-carbon aluminium-killed steel heated at . The simulated effect of heating rate on the texture is in qualitative agreement with experimental data. In particular, the optimum heating rate is predicted providing the peak drawability of the steel. This rate corresponds to the maximum overlapping of recrystallization and precipitation processes.

Texture, Yield Locus and Plastic Anisotropy in Steel Sheets

A comprehensive analysis of texture in the commercially produced low carbon aluminium-killed steel sheets of different thicknesses, by orientation distribution functions (ODF) technique, has been carried out. To characterise the plastic anisotropy behaviour of these sheets, studies about the evolution of Lankford parameter and yield locus prediction are performed, on the basis of Taylor theory using the {hkl} pencil glide {112} glide and {110} glide. The predicted r-values in the sheet plane are compared with the experimentally determined r-values.

Structure and properties of continuously cast low-carbon steel as a function of the reeling temperature

Investigation of continuously cast low-carbon aluminum-killed steel has great practical importance. Its structure and properties depend on the reeling temperature after hot rolling. After reeling at a low temperature the steel exhibits a capacity for deep drawing, whereas after reeling at a high temperature it is suitable for enameling.