Al-killed Steel Research Articles

Effect of small amounts of cerium on impact toughness and tensile properties of industrial Al-killed steel Q355D

The influence of cerium (Ce) on the mechanical properties of industrial steel was meaningful. And inclusions, grains, impact toughness, and tensile properties of industrial aluminium-killed Q355D were investigated by adding varying amounts of Ce. It was found that (i) inclusion of the sample without Ce was mainly manganese sulphide (MnS) and Al2O3-Mn(Ca)S, inclusion of the sample with adding 0.0017% Ce was mainly MnS- and Ce-based inclusions of CeAlO3–Ce2O2S–CaS, and inclusion of the sample with adding 0.0025% Ce was mainly MnS- and Ce-based inclusions of CeAlO3–Ce2S3–Mn(Ca)S; (ii) the number of MnS inclusions had decreased and shortened by increasing Ce. Meanwhile, Ce-based inclusions moved in the direction of decreasing size by increasing Ce; (iii) the grains were refined by adding Ce. In particular, fine acicular ferrite was formed by adding 0.0017% Ce; and (iv) the low-temperature impact toughness was best, and yield strength was the highest for the sample with adding 0.0017% Ce, which was attributed to the contribution of fine acicular ferrite and Ce2O2S inclusion.

Effect of TiO2 Addition in Ladle Slag on Evolution of Nonmetallic Inclusions in Ti-Bearing Al-Killed Steel

Effect of TiO2 Addition in Ladle Slag on Evolution of Nonmetallic Inclusions in Ti-Bearing Al-Killed Steel

Formation mechanism and control of a kind of pit-shaped defect in Cf53 rolled products

Aiming at the frequent issue of pit-shaped surface defects in S-bearing and Al-killed steel products and the related poor castability due to submerged entry nozzle (SEN) clogging, the melting and casting process of a popular auto steel Cf53 has been studied based on in-situ investigation and experimental analysis. To clarify the generation of the pit-shaped defects, the local surface chemistry, evolution of steel inclusions throughout the melting process, SEN clogging substances, and the internal relationship among them were systematically analyzed. The results show that this type of defect in the Cf53 rolled products is caused by the large-sized composite inclusions rich in CaS, which originate from the molten steel during the refining process, gradually accumulate on the inner wall of the SEN during casting, and finally lead to SEN clogging. The loose structure of clogging inclusions makes them prone to fall off into the mold and get trapped by solidifying steel shell. The formation mechanism of the inclusions was studied through FactSage software calculation. The results show that the increase in total sulfur and total calcium content, and the decrease in total oxygen content and temperature in steel will lead to CaS inclusion formation, which agrees reasonably well with experimental results. An optimized refining process to reduce calcium addition into molten steel is proposed accordingly, which achieves a remarkably alleviation in casting SEN clogging and an elimination of the pit-shaped defects on the final rolled products as well.

Analysis of the Steelmaking Process via Data Mining and Pearson Correlation.

The continuous improvement of the steelmaking process is a critical issue for steelmakers. In the production of Ca-treated Al-killed steel, the Ca and S contents are controlled for successful inclusion modification treatment. In this study, a machine learning technique was used to build a decision tree classifier and thus identify the process variables that most influence the desired Ca and S contents at the end of ladle furnace refining. The attribute of the root node of the decision tree was correlated with process variables via the Pearson formalism. Thus, the attribute of the root node corresponded to the sulfur distribution coefficient at the end of the refining process, and its value allowed for the discrimination of satisfactory heats from unsatisfactory heats. The variables with higher correlation with the sulfur distribution coefficient were the content of sulfur in both steel and slag at the end of the refining process, as well as the Si content at that stage of the process. As secondary variables, the Si content and the basicity of the slag at the end of the refining process were correlated with the S content in the steel and slag, respectively, at that stage. The analysis showed that the conditions of steel and slag at the beginning of the refining process and the efficient S removal during the refining process are crucial for reaching desired Ca and S contents.

Effect of ladle-lining materials on inclusion evolution in Al-killed steel during LF refining

Abstract The effect of lining materials (Al2O3 and Al2O3–MgO·Al2O3) of ladle on evolution of non-metallic inclusions in aluminum-killed (Al-killed) steel during ladle furnace refining without Ca treatment was investigated through industrial experiments. The results showed that non-metallic inclusions experienced the changes from Al2O3 → MgO–Al2O3 → CaO–Al2O3. During the refining process using either of the two ladle lining materials, for all non-metallic inclusions, the vast majority are distributed in the high Al2O3 area of the CaO–Al2O3–MgO phase diagram, with very little or none in the low melting point zone. Non-metallic inclusions are mainly smaller than 3 μm, while those larger than 3 μm consisted primarily of MgO·Al2O3 and CaO–Al2O3 inclusions. The use of an Al2O3–MgO·Al2O3-lining ladle is more effective in reducing the number density of inclusions in the steel. However, during the refining process, the Al2O3-lining ladle does not have a significant impact on the presence of MgO–Al2O3 and CaO–Al2O3 inclusions in the molten steel. The Al2O3–MgO·Al2O3-lining ladle does not have a significant effect on MgO–Al2O3 inclusions, but it does promote the formation of CaO–Al2O3 and CaS inclusions in the molten steel.

Discriminating the Fe Peak in Al-Killed Steel Specimens Using an Energy Dispersive Spectrometer: From Matrix or from Oxide Inclusions?

Discriminating the Fe Peak in Al-Killed Steel Specimens Using an Energy Dispersive Spectrometer: From Matrix or from Oxide Inclusions?

Mechanism of cerium-based inclusion formation and influence on impact toughness in industrial Al-killed steel

Mechanism of cerium-based inclusion formation and influence on impact toughness were researched in industrial Al-killed steel. In the final product, actual inclusions were complex, consisting of Al-O-Ce (Ce-poor), Al-O-Ce (Ce-rich), Al-O-Ce-S(Al-poor) and S-Ca (Mn-rich) from inside to outside. Results of thermodynamic balance calculation were completely inconsistent with the actual inclusions. Considering decomposing or disappearing of formed inclusions in steel difficult, new computing results showed that inclusions were formed by firstly Al2O3 (exist in molten steel before adding Ce), AlCeO3, Ce2O2S, CaS, Ca (Mn)S and MnS in turn, in better agreement with the actual inclusions. Outside of the actual inclusions, Ce2O2S and CaS had a smaller disregistry with ferrite, which could offer effective nucleation sites. And small inclusion was surrounded in a grain and bigger inclusion was surrounded in two grains at least. Microstructures were observed that grains with Ce were finer than grains without Ce. Moreover, V-notch impact toughness of samples was tested at 0 and −60°C, respectively. Compared with samples without Ce, the values of impact toughness were increased by 161% at 0°C and 225% at −60°C, respectively. And fracture surface with Ce had more and finer dimples and finer cleavages than that without Ce.

Estimation of Changes in Content and Characteristics of Mold Flux during Continuous Casting

Estimation of changes in the composition and physical properties of mold flux during continuous casting was investigated for control of slab surface quality. In this study, 0.7 mass% Al steel and normal Al-killed steel were cast with three kinds of mold flux having Al2O3 contents of 1.3 to 6.0 mass% and different basicities. The results can be summarized as follows:(1) The Al2O3 content of these mold fluxes increased to 30 mass% during continuous casting. The composition change of the mold flux can be reproduced by the Equilibrium Effective Reaction Zone Model (EERZM) by fitting parameters, referring to the casting results.(2) The analysis by EERZM revealed that the viscosity of the mold flux and the throughput of the molten steel affect the rate of increase of the Al2O3 content in the mold flux.(3) The change of the physical properties of the mold flux was estimated based on the changes in the flux composition. The change of the crystallization temperature and main crystal can be estimated by FactSage.(4) Mold flux viscosity can be estimated by revising the modified Iida’s equation, which considers the effect of Al2O3 as an amphoteric oxide.

Evolution of inclusion populations in Al-killed steel during the steelmaking process

The increasing demand for higher steel inclusion cleanliness has motivated control over the formation and evolution of inclusions during the production process. To generate knowledge of the formation and evolution of inclusions during the production of Al-killed steel slabs, the evolution of the chemical composition and size distribution of inclusions throughout the process, including ladle furnace (LF) treatment and continuous casting (CC), was studied. Liquid steel sampling was conducted throughout the process. The chemical composition was measured using the EDS technique, and the size distribution analysis was performed with the population density function (PDF) approach. The upper tail of the size distributions was analyzed by generalized extreme value (GEV) theory. Evolution of the chemical composition of inclusions involved the following path: solid Al2O3 solid Al2O3-MgO partly liquid Al2O3–MgO–CaO fully liquid Al2O3–MgO–CaO. The PDF analysis accounted for the evolution of the size distribution, which was supposed to be driven by the balance of the coalescence/growth and collision/breakage of inclusions and their removal. Finally, the GEV approach allowed us to describe the evolution of large inclusions and compare inclusion populations in terms of survival probability.

Effect of Cerium and Magnesium Addition on Evolution and Particle Size of Inclusions in Al-killed Molten Steel

To investigate the effect of Ce and Mg addition on the evolution of inclusions in Al-killed molten steel, both thermodynamic calculations and laboratory-scale experiments were carried out in the present work. The samples taken at different time after various addition amounts and order of Ce and Mg were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The results show that Al2O3 was firstly modified to MgAl2O4 and then the transformation from MgAl2O4 to CeAlO3 with the addition order of Al → Mg → Ce. Under the addition order of first Ce followed by Mg, CeAlO3 was eventually modified to MgAl2O4 in the case of low Mg contents, and was modified to MgO with high Mg contents. The thermodynamic conditions were performed to discuss the mechanism of inclusions formation and transformation. More importantly, under the condition of Ce addition followed by Mg addition, due to the transformation from large-sized RE-inclusions to disperse MgO-containing ones, the average sizes of inclusions reduced from 3.12 µm and 3.28 µm to 1.99 µm and 1.83 µm, respectively. This evolution behavior of inclusions was firstly observed in-situ by a novel experiment, in which the large-sized CeAlO3 clusters were disaggregated to MgO–Al2O3 particles with smaller sizes. The control strategy of inclusions proposed in this study is expected to improve the casting process and the product quality of Al-killed steel containing rare earth element.

Simultaneous Analysis of Soluble and Insoluble Oxygen Contents in Al-Killed Steels of Various C Contents and Supersaturation Phenomena in the Steel

Soluble and insoluble O contents ([S. O] and [I. O], respectively) in Al-killed steels of various C contents (0 to ~2 mass pct.) were simultaneously measured using “two-stage” inert gas fusion infrared absorptiometry, where majority of the inclusions was alumina. Several steel specimens were used for the O content analysis where the specimens were either taken from a steel plant or prepared in this laboratory. In addition to this, several pretreatment methods of the specimens were tested in order to provide reliable analysis results. It was found that mechanical grinding of the specimen’s surface should be carefully carried out in order not to induce unwanted surface oxidation. This resulted in overestimation of the O content of the specimen. [S. O] and [I. O] in the steel specimens were successfully analyzed using the “two-stage” gas fusion method in the context of inert gas fusion infrared absorptiometry. Considerable portion of total O content ([T. O] = [S. O] + [I. O]) was the [S. O], which was higher than the equilibrium O content of the known Al deoxidation equilibria. Therefore, the supersaturation in the Al deoxidation was confirmed. The analyzed [I. O] was independently validated by measuring area of the exposed alumina inclusions on the polished section of the specimen using SEM. It was also found that increasing C content in the steel lowered the [S. O] while [I. O] hardly changed. It is concluded that C in the steel, mechanical stirring, and remelting, could relieve the supersaturation.

Evolution and deformability of inclusions in Al-killed steel with rare earth-alkali metals (Ca or Mg) combined treatment

The quality of stainless steel is closely related to the deformability of inclusions, which is significantly affected by their compositions. The present study first investigated the evolution of inclusion compositions in Al-killed steel with rare earth-alkali metals (Ca or Mg) combined treatment through four laboratory-scale experiments. The Ce contents in the final steel are 0.0080 wt%, 0.015 wt%, 0.016 wt% and 0.010 wt%, respectively. The Mg content is 0.0014 wt% in Ce–Mg combined treated steel, and the Ca content is 0.0015 wt% in Ce–Ca combined treated steel. The deformability of inclusions in both Ce2O3–Al2O3–CaO and Ce2O3–Al2O3–MgO systems was subsequently evaluated by calculating their Young's modulus at low temperature. The results show that irregular Al2O3 and MgAl2O4 with poor deformability are modified to CeAlO3 and Ce2O3 by Ce treatment, resulting in the decrease of Young's modulus of inclusions. The deformability of inclusions is further improved due to the transformation from lumped-like CeAlO3 to spherical CaO–Al2O3–Ce2O3 caused by Ca treatment, and some of these inclusions are the ones with low liquidus temperature. Thermodynamic analysis was used to discuss the control condition of the formation and evolution of inclusions. Accordingly, the appropriate addition amounts of Al, Mg, Ce, and Ca are expected to control inclusion compositions and properties, including deformability and liquidus temperature, thereby improving the steel performance.

Effect of the La2O3 Content in Slag on Inclusions in Al-Killed Steels

Effect of the La2O3 Content in Slag on Inclusions in Al-Killed Steels

Deformation mechanism of ferrite in a low carbon Al-killed steel: Slip behavior, grain boundary evolution and GND development

The forming quality and performance of double walled brazed tube formed by steel sheet are seriously affected by the deformation behavior of the initial sheet. In this work, the deformation mechanism of ferrite in a low carbon Al-killed steel sheet was investigated by in situ tensile and electron backscatter diffraction (EBSD) tests. It is found that ferrite presents single slip, double slip and cross slip as well as slip transfer behavior in the deformation process. The activation of the {110}<111> slip system is the main deformation mode, the less activation number of {112}<111> and {123}<111> is due to the orientation dependence and high critical resolved shear stresses (CRSS), respectively. A high Schmid factor (SF) or geometric compatibility factor m' will contribute to slip transfer. Meanwhile, the evolution of low angle grain boundaries (LAGBs) is the main deformation feature for ferrite polycrystals, and the fraction of 2–5° LAGBs in the whole LAGBs shows a strong linear relationship with strain. The subdivision of grains and rotation of different parts lead to the gradual evolution of LAGBs into high angle grain boundaries (HAGBs). Furthermore, small grains are more likely to accumulate the geometrically necessary dislocation (GND) because the microstructural unit size restricts the movement of dislocation, while the less effect of grain orientation is related to the high SF for all grains.

Influence of Acicular Ferrite Microstructure on Toughness of Ti-Rare Earth Metal (REM)-Zr Killed Steel

Novel oxide particles composed of Ti, rare-earth metal and Zr was found by the present authors to promote the formation of acicular ferrite (AF), but their effect on toughness has not yet been clarified. Therefore, influence of AF formation on toughness of low carbon steel has been investigated using Ti-Rare earth metal-Zr-killed (TRZ) steel and Al-killed (AL) steels in this study. Dominant microstructure of the AL steel is bainite forming block/packet structure elongated from prior austenite grain boundaries. In contrast, a large amount of AF which grows from intragranular inclusions is formed in the TRZ steel. Toughness of the TRZ steel is better than the AL steel. In addition, at higher austenitizing temperature, AF formation is promoted in the TRZ steel, leading to lowering ductile-brittle transition temperature. Cracks in Charpy impact specimens propagate along {001}α plane and deflect at Bain unit boundary across which {001}α planes are largely misoriented, which means the effective grain size for crack propagation is Bain unit size. Bain unit size is smaller in the TRZ steel than in the AL steel since bainite structure is subdivided by pre-formed AFs belonging to different Bain groups from bainite. Martensite-austenite constituent (MA) in AF is less-elongated in comparison to those in bainite and thus the amount of elongated MA in the TRZ steel is less than in the AL steel. These results clarify that AF formation in the TRZ steel improves toughness due to the refinement of Bain unit size and reduction of the amount of elongated MA.

Effect of lining refractory and high-basicity slag on non-metallic inclusions in a high carbon Al-killed steel

Laboratory experiments on the effect of lining refractory and high-basicity slag on non-metallic inclusions in a high carbon Al-killed steel were carried out. Alumina inclusions in the steel could hardly be affected by the Al2O3 refractory, however, would be transformed into MgO · Al2O3 when the MgO refractory was used. After the steel-slag-MgO lining-inclusion reaction, the high-basicity slag was saturated with MgO due to the dissolution of MgO from the refractory into the slag, meanwhile, original Al2O3 inclusions were transformed into MgO via MgO · Al2O3, regardless of the slag basicity. After the steel-slag-Al2O3 lining-inclusion reaction, the CaO/Al2O3 ratio of slag decreased significantly due to the dissolution of Al2O3 refractory into the slag, resulting in the slight increase of the magnesium content in steel and the transformation of Al2O3 inclusions into MgO · Al2O3. The reduction of the MgO in the lining refractory and top slag by the dissolved aluminum ([Al]) in molten steel occurred independently, and a higher CaO/Al2O3 ratio of slag would result in a higher activity of MgO, which was beneficial for the reduction of MgO. The CaO in the slag was hardly reduced by the [Al] in the molten steel, thus, it was proposed that CaO-Al2O3 type inclusions could hardly be generated from the steel-slag reaction during the production of high carbon Al-killed steels.

Effect of Medium Basicity Refining Slag on the Cleanliness of Al-killed Steel

Effect of Medium Basicity Refining Slag on the Cleanliness of Al-killed Steel

Effect of the inclusion features on mechanical properties of steel

ABSTRACT Polar steel requires good strength and excellent toughness for breaking icy surfaces in low-temperature sea water environments. In this study, the effect of inclusions and metallurgical structure on the mechanical property of polar steel was examined. The composition and morphology of the inclusions in steel were formed using different deoxidation methods during steel refining, and steel plates were rolled by the same rolling process. The results show that there are lower inclusion densities and more globular inclusions in steel deoxidized using Zr–Ti than deoxidized using Al. The mean diameters of grains in Zr–Ti-killed steel from different thicknesses are smaller than that in Al-killed steel, the impact toughness and yield strength of Zr–Ti-killed steel are substantially higher than those of Al-killed steel. It is determined that the use of the Zr–Ti deoxidizer has a positive effect on the ductility of the steel.

Effect of initial aluminium-oxygen concentration product on alumina-based inclusions in high carbon Al-killed steels during the ladle refining process

ABSTRACT The effect of different aluminium-oxygen concentration products at the start of deoxidation on the evolution of alumina-based inclusions in high carbon Al-killed steels during the ladle refining process was investigated. Results showed that the higher aluminium-oxygen concentration product at the start of deoxidation, the higher area fraction and the number density of large-sized inclusions before the slag melting, then clustered alumina-based inclusions gradually became ripening. Thermodynamic equilibrium results indicated that initial contents of the [O] were 100, 200, 300 and 400 ppm in the molten steel, then contents of the [O] at the final reaction equilibrium were 4.6, 4.9, 5.8 and 9.5 ppm, respectively. The relationship between oversaturation and size of Al2O3 inclusions showed that the larger the critical radius of nucleus, the lower the oversaturation. Meanwhile, the evolution mechanism of alumina-based inclusions during the LF refining process was also revealed.

Transformation of inclusions in Al-killed steels with different calcium contents during the heat treatment

ABSTRACT The transformation of inclusions in solid Al-killed steels with different total calcium (T.Ca) contents during the heat treatment process at 1473 K was investigated through laboratory experiments. Calcium aluminate inclusions transformed to Al2O3-CaS during the heat treatment due to reactions between dissolved aluminum and sulfur in the steel matrix and CaO in inclusions. With the T.Ca content in steel reached to 0.003 wt-%, CaO inclusions can hardly transform to CaS inclusions completely due to the insufficient sulfur content. After the heat treatment, the content of CaO in inclusions in steel with 0.0002 wt-% T.Ca was less than 5 wt-%, while it was roughly 30 wt-% with 0.003 wt-% T.Ca in molten steel. The effect of T.Ca content on the transformation of inclusions was calculated using FactSage. Moreover, a kinetic model was developed to predict the transformation of inclusions with different T.Ca contents at heat treatment temperature.