Ultra Low Carbon Steel Research Articles

Non-uniform Deformation Behavior of Coarse-grained Ultralow Carbon Steel Measured Using Digital Image Correlation Method

Non-uniform Deformation Behavior of Coarse-grained Ultralow Carbon Steel Measured Using Digital Image Correlation Method

Thermodynamic and experimental study on CO2 injection in RH decarburization process of ultra-low carbon steel

In this work, carbon dioxide (CO2) was investigated as a lifting gas in the Ruhrstahl–Heraeus (RH) decarburization process of ultra-low carbon steel. The main challenge was to determine whether the local carburization reaction leads to the carburization of molten steel with ultra-low carbon content. Thermodynamic analysis of the reaction of the Fe-C-O melt with CO2 demonstrated that compared with traditional Ar injection, the decarburization rate can be improved during the early stage of the decarburization process. This occurs because oxygen that is present not only in molten steel but also produced by CO2 dissociation can react with dissolved carbon to remove the carbon from molten steel. However, it is difficult to determine whether the decarburization rate can be influenced during the later stage by injecting CO2. This is because the carburization reaction at the surface of CO2 bubbles in the up-snorkel and the decarburization reaction at other reaction sites can occur simultaneously. Industrial test results demonstrated that compared with Ar injection, the decarburization rate was not significantly affected by CO2 injection at the early stage, whereas it was reduced during the middle and later stages. Nevertheless, the average carbon content in molten steel after the RH treatment with CO2 injection was only 2.6 ppm higher compared with that in Ar injection. Therefore, Ar can potentially be replaced by CO2 in the RH decarburization process of ultra-low carbon steel based on economic and environmental considerations.

Finite Element Indentation Analysis of Automotive Roof Panel using Ultra Low Carbon Steel with and without Bake Hardening Effect

Ultra-low carbon (ULC) steels, which are lightweight, are used in fabricating steel sheets for many applications, particularly the automobile sector where good formability and surface quality are required. For mechanical properties development reasons, the bake hardening technique of ultra-low carbon (ULC) steels is adopted during the automotive paint baking process at elevated temperature. Automotive outer body with bake-hardenable steel ensure good indentation strength and resistance to fatigue. Usually, the mechanical properties of automotive steels depend both on the chemical composition and thermal treatment. Consequently, the present paper aimed to design a car roof panel using elastic-plastic ULC steel sheet with and without bake hardening. Finite element (FE) analysis was used to predict the static indentation response of automotive roof panels. First of all, the roof panel was modelled via CATIA V5 and evaluated by FE analysis using ABAQUS Software. The results showed that the bake-hardenable steel sheets present a good indentation strength and less plastic deformation.

STUDY OF THE DESIGNS OF BOTTOM BLOWING DEVICES FOR OXIDATIVE BLOWING IN TEEMING LADLES

It is discussed in the article the concept proposed for the production of ultra-low carbon steel, which involves the production of crude steel in basic oxygen furnace followed by oxidative blowing with an oxygen-argon mixture in a teeming ladle to decrease a carbon content in steel to less than 0.03%. High efficiency of the proposed technology is possible only under the intensive process of metal decarburization, which consists of the three stages: supply of reagents to the gas bubble, chemical interaction of reagents on the interfacial surface and removal of reaction products. At low carbon concentrations in the metal, the limiting link of the process is carbon mass transfer to the interfacial surface, which can be intensified by melt stirring. The objective of this article is to study the influence of design of the blowing devices, namely, the position and shape of the pores, on the efficiency of metal homogenization in the teeming ladle. Blowing devices with a circular hole, a slit and undirectional porosity were considered. To perform physical simulation by Buckingham's theorem, similarity numbers were chosen to describe the considered process. In particular, it is proposed to use dimensionless volume flow and a modified homochronicity number. Based on the physical simulation on the “water” model, it was found that the best results of homogenization of the chemical composition of the liquid metal in the teeming ladle show blowing devices with undirected porosity. They are ideal for oxidative purging in a crowded ladle with a mixture of argon and oxygen required for the production of ultra-low carbon steel with an oxygen content of less than 0.03%. The purpose of further research is to develop the design of the mixing chamber of the purge device, in which oxygen and argon are pre-mixed before injection into the liquid metal.

Simultaneous Analysis of Soluble and Insoluble Oxygen Contents in Steel Specimens Using Inert Gas Fusion Infrared Absorptiometry

In order to evaluate cleanliness of steel samples which contain oxygen in two different forms - chemically dissolved form and physically dispersed form in steel matrix, identification of each oxygen in the steel is important. A simple but promising method for simultaneous analysis of these two types oxygen in steel samples was developed in the present study using Inert Gas Fusion Infrared Absorption Method. By utilizing different carbothermic reaction temperature for each type of oxygen, the chemically dissolved oxygen (soluble oxygen) was first separated from the steel specimen at a low reaction temperature, while the physically dispersed oxygen (insoluble oxygen in the form of oxide inclusion) was separated at a higher reaction temperature. This idea was applied to a number of Al-killed ultra low carbon steel specimens, which contain alumina inclusions. It was shown that separation of the soluble oxygen and the insoluble oxygen was possible. The obtained oxygen content in this new method was independently validated by a conventional Inert Gas Fusion Infrared Absorption Method for the total oxygen content and by cross-sectional analysis of non-metallic inclusion for the insoluble oxygen. A possible supersaturation state of liquid steel after RH process was observed.

Observation of Inhomogeneous Deformation in a Cold-Rolled Ti-added Ultra-Low Carbon Steel using High-Precision Markers Drawn by Focused Ion Beam

The development of deformation inhomogeneities in ultra-low carbon steel due to cold rolling has been investigated by observation of the longitudinal plane at chosen sites of a rolled sheet using SEM and SEM-EBSD techniques. Particular attention has been paid to the development of strain distribution in the crystals. The microstructures and the orientation distribution in the longitudinal plane of a sheet rolled to a 60% reduction in thickness were observed, and some grains with preferred orientation such as α-fiber and γ-fiber were selected. In order to examine the local strain distribution of these grains due to cold rolling, high-precision dot markers, 0.3μm in diameter, were drawn using a focused ion beam. Then, before being subjected to additional rolling, the sheet was fitted into a frame made of the same steel under a plane strain condition. Using the marker method, the local displacement of the grains due to cold rolling has been directly measured.

Dual rolls equal channel extrusion as unconventional SPD process of the ultralow-carbon steel: finite element simulation, experimental investigations and microstructural analysis

The paper presents results of FEM modelling as well as properties and microstructure of the ultralow-carbon ferritic steel after the unconventional SPD process—DRECE (dual rolls equal channel extrusion). Based on the conducted numerical simulation information about the deformation behaviour of a steel strip during the DRECE process was obtained. The simulation results were experimentally verified. The influence of DRECE process on hardness distribution, fracture behaviour and microstructure evolution of the investigated steel was analysed. The increase of steel strength properties after subsequent deformation passes was confirmed. The microstructural investigations revealed that the processed strips exhibit the dislocation cell microstructure and subgrains with mostly low-angle grain boundaries. The grains after processing had relatively high dislocation density and intense microband formation was observed. It was also proved that this unconventional SPD method fosters high grain refinement.

Effect of Solute Carbon on the Characteristic Hardening of Steel at High Temperature

Small ball rebound hardness tests demonstrated characteristic hardening at 700 K in the ultra-low carbon and pearlitic steels. The equilibrium phase diagram of Fe-C binary alloy calculated using Thermo-Calc exhibited dissolving of cementite above 700 K. Moreover, in-situ heating neutron diffraction measurement demonstrated the increase of lattice parameter by dissolving of cementite above 700 K. Therefore, it can be concluded that the characteristic hardening above 700 K can be attributed to the solid solute carbon.

Study on the utilization of supersaturation degree in Al deoxidation to improve cleanliness and control of Al2O3 inclusions in ultra-low carbon steel

Effects of supersaturation degree (S) in Al deoxidation on cleanliness and alumina inclusions in steel were studied. If enough Al was added to steel, higher initial dissolve oxygen before Al deoxidation resulted in higher steel cleanliness. In the experiments, with the rise of [mass% O] from 102 ppm to 340 ppm and 931 ppm, logS in steel melts were increased from 4.8, to 7.0 and 8.8, respectively. As a result, total oxygen (T.O) was decreased to 25 ppm, 22 ppm and 21 ppm, respectively. Based on the results, important influence of supersaturation degree on the formation of Al2O3 was revealed. At lower S, Al2O3 would be singular and smaller particles ≤ 5 µm that was difficult to be removed. However, Al2O3 would exist as large clusters in steel at higher S, which can be efficiently removed. By utilizing the findings in laboratory, RH refining of ultra-low-carbon steel was optimized. Steel cleanness and control of inclusions were improved. As a result, surface defects in cold rolling sheets caused by Al2O3 inclusions were effectively minimized.

Microstructure and mechanical properties of ultralow carbon high-strength steel weld metals with or without Cu-Nb addition

Two types of ultralow carbon steel weld metals (with and without added Cu-Nb) were prepared using gas metal arc welding (GMAW) to investigate the correlation between the microstructure and mechanical properties of weld metals. The results of microstructure characterization showed that the weld metal without Cu-Nb was mainly composed of acicular ferrite (AF), lath bainite (LB), and granular bainite (GB). In contrast, adding Cu-Nb to the weld metal caused an evident transformation of martensite and grain coarsening. Both weld metals had a high tensile strength (more than 950 MPa) and more than 17% elongation; however, their values of toughness deviated greatly, with a difference of approximately 40 J at −50°C. Analysis of the morphologies of the fracture surfaces and secondary cracks further revealed the correlation between the microstructure and mechanical properties. The effects of adding Cu and Nb on the microstructure and mechanical properties of the weld metal are discussed; the indication is that adding Cu-Nb increases the hardenability and grain size of the weld metal and thus deteriorates the toughness.

Effect of Heating in the Intercritical Temperature Range on Formation of Austenite and Structure of Ultralow-Carbon Steel

Using micrographic, dilatometric, and high-temperature XRD methods, the kinetics of formation of austenite is studied under continuous heating of ultrafine-grained 08G2B steel in two initial states—after quenching and normalization.

Effect of oxidizing slag on the decarburization of ultra-low-carbon steel during the ruhrstahl-heraeus vacuum process

In this study, the effect of oxidizing slag on the decarburization of ultra-low carbon steel during the RH vacuum process was investigated. Based on the experimental results under different operating conditions, it was concluded that high oxidazibility in the top slag could enhance the decarburization rate, particularly in the rapid decarburization stage during the first 4 min of the vacuum process. The highly efficient decarburization for liquid steel depends on two parts of oxygen source. Internal oxygen source is the free oxygen in the liquid steel, and the external oxygen source is the FeO that existed in the RH slag or formed due to the re-oxidation of the splashed scrap and steel skull on the wall of the vacuum chamber. The external oxygen sources participated in decarburization reaction, and promoted the mass transfer of oxygen from the slag to molten steel in the early stage of the vacuum process to further improve the decarburization rate. The decarburization mechanism considering the influence of the oxidizing slag was proposed to explain the reaction sites at different stages of RH decarburization.

Thermal desorption spectroscopy evaluation of hydrogen-induced damage and deformation-induced defects

ABSTRACT Thermal desorption spectroscopy (TDS) was performed on ultra-low carbon (ULC) steel with various degrees of hydrogen-induced damage and deformation-induced defects. First, the extent to which hydrogen-induced damage manifests itself in TDS measurements was evaluated. Application of multiple test conditions on cold deformed ULC steel with and without the presence of hydrogen-induced damage showed that such damage did not appear as a hydrogen signal on the TDS spectra. Second, interesting features observed on the TDS spectra of cold deformed ULC steel were further investigated by assessing the TDS spectra of recrystallised and annealed ULC steel. As such, the four peaks in the TDS spectra were linked to microstructural features: interstitial lattice positions, grain boundaries, dislocations and microvoids.

Cu Precipitation Behaviors and Microscopic Mechanical Characteristics of a Novel Ultra-Low Carbon Steel.

We studied the effect of Cu addition on the hardness of ultra-low carbon steels heat treated with different cooling rates using thermal simulation techniques. The microstructural evolution, Cu precipitation behaviors, variations of Vickers hardness and nano-hardness are comparatively studied for Cu-free and Cu-bearing steels. The microstructure transforms from ferritic structure to ferritic + bainitic structure as a function of cooling rate for the two steels. Interphase precipitation occurs in association with the formation of ferritic structure at slower cooling rates of 0.05 and 0.2 °C/s. Coarsening of Cu precipitates occurs at 0.05 °C/s, leading to lower precipitation strengthening. As the cooling rate increases to 0.2 °C/s, the interphase and dispersive precipitation strengthening effects are increased by 63.9 and 50.0 MPa, respectively. Cu precipitation is partially constrained at cooling rate of 5 °C/s, resulting in poor nano-hardness and Young’s Modulus. In comparison with Cu-free steel, the peak Vickers hardness, nano-hardness and Young’s Modulus are increased by 56 HV, 0.61 GPa and 55.5 GPa at a cooling rate of 0.2 °C/s, respectively. These values are apparently higher than those of Cu-free steel, indicating that Cu addition in steels can effectively strengthen the matrix.

Slag Pool Depth Effectiveness of Molten Mold Flux Feeding Technology

POSCO’s advanced CASting Technology (PoCAST) is an innovative molten mold flux feeding technology which has been developed to ensure quality of continuously-cast Twinning-Induced Plasticity (TWIP) steel and Ultra-low carbon steels (ULCSs). Stable qualities of ULCS slabs could be achieved by reducing mold flux entrapments by optimizing the range of slag pool depth; this result matches well with entrapment characteristics during 0.6 scale water model tests. Also, uniform solidification at the meniscus with shallower depth of oscillation mark and subsurface hook depth was performed in optimum range of slag pool depth. Thermal plasma melting furnace feeding facility has been developed to control the slag pool depth in the mold by increasing the melting rate. The proposed optimized range of slag pool depth was verified in a commercial trial by realizing thickness control of slag pool depth during ULCS production.

Prediction of re-oxidation behaviour of ultra-low carbon steel by different slag series

A kinetic model was developed using FactSage Macro Processing to simulate the re-oxidation of ultra-low carbon steel via different oxidising slags. The calculated results show good agreement with experimental laboratory thermal simulation data. Therefore, the model can be used to predict the change behaviour of slag-metal-inclusion in the re-oxidation reaction of liquid steel. It can provide prediction and guidance for an accurate secondary oxidation control process. During the slag re-oxidation process, when the oxygen in the steel is supersaturated and the slag is low in oxidation, it can easily form stick-like and dendritic shape inclusions of Al2O3 in steel. As the (FeO) content increases in slag, the oxygen transfer from slag to steel is evident, and the inclusion size increases, showing clusters and spherical shapes. In addition, supersaturated oxygen in steel easily forms unstable Al2O3-TiOx inclusions with [Ti]. As the components of liquid steel tend to be uniform, the Al2O3-TiOx inclusions will decompose and disappear, forming stable Al2O3 and TiO2 inclusions. The number of inclusions can be reduced by increasing the basicity and the ratio of CaO to Al2O3 in the initial slag.

Criteria for achieving the BH effect in ultra-low carbon steels for deep drawing

The authors have investigated key technological parameters of stable production of IF steel sheet with BH-effect. The features of ultralow carbon IF steels and IF steels with BH effect are described. Scheme of IF-BH steel hardening during hot drying of the car body after painting is considered. Data on the chemical composition of IF and IF-BH steels produced at Russian and worldwide enterprises are presented. The authors have analyzed the reasons of appearance of such a defect in the steel sheet as slip bands that arise when the yield area appears on the tensile diagram of steel samples. The requirement on the “shelf life” presented to the IF-BH steel grades is given and disassembled. The article considers the formula for calculating effective carbon content in steel based on the total carbon content in steel, niobium, titanium and nitrogen. The range of carbon effective in steel is given to achieve the optimal value of the BH effect based on previously published works. The results of industrial IF-BH steel production have been analyzed for the conditions of Russian enterprise and recommendations were given on criteria such as the optimum carbon content range in solid solution, the recommended maximum total concentrations of carbon and nitrogen in steel, and the BH effect value guaranteeing a high yield of IF-BH steel sheet in the production. Calculations of various options of microalloying by titanium and niobium for IF-BH steel were done. Criteria are formulated that allow stably obtaining a given value of the BH effect in cold-rolled ultralow-carbon steels. A two-stage scheme for microalloying by titanium and niobium is proposed. The influence of grain size in sheet steel on the presence and value of the BH effect is described as well as on the presence/absence of a yield plateau on the tensile diagram in the sheet metal in the initial state.

Comparative Study on the Cleanliness of Ultra-low Carbon Al-Killed Steel by Different Refining Processes

Cleanliness of aluminum-killed ultra-low carbon steel produced through BOF–RH–CC and BOF–LF–RH–CC was comparatively investigated by nitrogen–oxygen analysis, SEM analysis and thermodynamic calculation to seek for an optimum refining process. The results show that the non-metallic inclusions with area above 8 μm2 could be removed obviously in both the refining processes. Compared with BOF–RH–CC process, in RH refining stage, the size of inclusion and total oxygen control were better than the original process. Furthermore, BOF–LF–RH–CC process reduced the inclusion area ratio, the proportion of single inclusion area of < 8 μm2 increased from 85 to 95%, the proportion of Al2O3 inclusion less than 5 μm2 increased from 50 to 70%. Meanwhile, the desulfurization effect became pronounced and the temperature control of the double refining process was more stable than that of the original process.

Criterial Parameters for Achieving the BH Effect in Ultralow-Carbon Steels for Deep Drawing

Key technological parameters of the stable production of interstitial free (IF) grade steel sheet with the bake hardening (BH) effect have been studied. The features of ultralow-carbon IF grade steels and IF steels with the BH effect, as well as the process flowchart for hardening IF-BH steel during hot drying of a car body after painting are described. Data concerning the chemical composition of IF and IF-BH steels produced at Russian and foreign enterprises are presented. The causes of slip band defects in steel sheet occurring when the yield plateau is observed in the tensile diagram of steel samples are analyzed. Requirements for the shelf life of IF-BH steel grades are presented and analyzed. A calculation formula of effective carbon content in steel is proposed based on the content of total carbon, niobium, titanium and nitrogen in steel. The range of effective carbon content in steel required for achieving the optimal BH effect magnitude is presented based on the papers published earlier. The industrial IF-BH steel production results have been analyzed for Russian enterprise conditions. In addition, the following criteria are recommended: the optimum range of carbon content in solid solution, the recommended maximum total concentrations of carbon and nitrogen in steel, and the BH effect magnitude that provides a high yield of IF-BH steel sheet fit for production. The calculations of various IF-BH steel microalloying with titanium and niobium have been performed. Criteria have been formulated that for obtaining a given BH effect magnitude in cold-rolled ultralow-carbon steels. A two-stage scheme for microalloying with titanium and niobium is proposed. The grain size influence in sheet steel upon the presence and BH effect magnitude, as well as upon the presence or absence of a yield plateau observed in the sheet metal’s tensile diagram in the initial state, is described.

Effect of Boron on Microstructure and Properties of Ultra-Low Carbon Steel

The effect of boron on microstructure, mechanical property and conductivity of ultra-low carbon steel wire rod with diameter 6.5 mm was studied. The results show that boron has little influence on the grain size and mechanical property of ultra-low carbon steel wire rod when adding 0.0020% boron alone and under the same process conditions; but when 0.0055% boron is added, the ASTM grain size number of the wire rod is coarsened from the 7.5 grade to 6 grade, at the same time, the tensile strength is reduced by 20 MPa and the electrical conductivity is increased from 16.2%IACS to 16.3%IACS. The effect of Boron on tensile strength is mainly caused by coarsening grain diameter. The effect of boron on grain diameter is related to [B]/[N], when the [B]/[N] is relatively high(for example 1.34), the grain diameter growth is obvious, conversely, when the [B]/[N] is relatively low(for example 0.44), the grain diameter growth is not obvious.