Ultra Low Carbon Steel Research Articles

Quantitative analysis of three-dimensional morphology of martensite packets and blocks in iron-carbon-manganese steels

Abstract Lath martensite is an important structure in steels; however, its quantitative volume fractions have not been measured to date. In this study, we seek to clarify the morphologies and volume fractions of the structural units of lath martensite-packets and blocks—by using three-dimensional observations with local crystallographical analyses. Lath martensite structures in ultra-low carbon and high-carbon steels contain both coarse and small packets, whose threshold of volume fraction in prior austenite grain is 10%. The volume fractions of the coarse packets in prior austenite grain in the specimens are greater than 70%, although the number of packets is fewer than five. In ultra-low carbon steel, the coarse packets have flat packet boundaries and contain three types of blocks with a plate-like morphology. On the other hand, in high carbon steel the morphology of the coarse packets is sponge-like and contains six types of blocks. There are more small packets than coarse packets in the observed specimens, and the volume fraction of the small packets is lower. The number of small packets in high carbon martensite is more than that in the ultra-low carbon martensite. Volume measurements show a distribution of volume fractions of Kurdjumov–Sachs (K–S) variants. In high carbon martensite, all the K–S variants appear continuously. The volume fractions of some K–S variants are relatively low in ultra-low carbon martensite.

Effect of carbon contents and Ti addition on microstructure of ultralow carbon steel

The microstructures of a series of ultralow carbon steels with various carbon contents (50–200 ppm) and Ti addition (<500 ppm) were studied by optical microscope, scanning electron microscope and transmission electron microscope. The specimens were fabricated under the process of hot rolling, cold rolling and two-step continuous annealing. The experimental results showed that the increase in carbon content decreased the grain size, whereas the addition of Ti resulted in a larger grain size compared with those without Ti addition. Dislocation (cell) structures were introduced by cold rolling, but it was removed by two-step annealing. Only MnS precipitated from the Ti free specimens. However, a larger amount of fine TiN, a few coarse TiS and an extremely low number of much coarser Ti3AlC were observed in the Ti added specimens. In addition, the reduction in MnS inclusion was obtained by the addition of Ti.

Production of rolled sheet from high-strength steel with ultralow carbon content

The influence of the chemical composition and production parameters (during hot and cold rolling, heat treatment, and temper rolling) on the mechanical properties of high-strength ultralow-carbon steel (including steel with a BH effect) is considered.

A method for observing the three-dimensional morphologies of inclusions in steel

A method for observing the three-dimensional morphologies of inclusions by deeply eroding a steel sample with a kind of organic solution composed of bromine water, acetone, and HCl (volume ratio, 45:45:10) was introduced. Four different kinds of inclusions in ultra low carbon steel were compared by metallographic observation and erosion observation. The results show that the three-dimensional morphologies of different kinds of inclusions could be observed clearly and simply after erosion. The method is useful for the observation and analysis of inclusions made by deep erosion. It also provides a new way to control and remove inclusions based on the true morphologies of inclusions.

Effect of Dislocation Density on the Initiation of Plastic Deformation on Fe&amp;ndash;C Steels

The effect of pre-existing dislocations and interstitial carbon on the initiation of plastic deformation in interstitial free (IF) steel and ultra low carbon (ULC) steel were investigated by the nanoindentation technique. The critical load, Pc, of the pop-in phenomenon, which corresponds to plasticity initiation, appeared clearly on the loading curve. The Pc in high dislocation density materials was smaller than that in low dislocation density materials with no difference between IF and ULC while the Pc in low dislocation density materials was remarkably higher in ULC than in IF. These results indicate that the interstitial carbon in the matrix does not affect the pop-in phenomenon when there are pre-existing dislocations or dislocation sources, and we discuss the reason for their occurrence occurs in high dislocation density materials.

ESTUDO DA VARIAÇÃO DA TAXA DE DESCARBURAÇÃO EM FUNÇÃO DA VAZÃO DE ARGÔNIO UTILIZADA NA PERNA DE SUBIDA DO DESGASEIFICADOR A VÁCUO RH

This work investigates the variation of the apparent decarburization constant (kc) as function of the argon flow rate injected in a RH degasser. It allows optimization of the flow rate at each decarburization stage. For this, 97 ultra low carbon steel heats were analyzed using three different argon flow rates (80 Nm3/h, 110 Nm3/h e 140 Nm3/h). Steel samples were taken at 0, 6 and 12 minutes of each heat. In the first decarburization stage, the kc values were similar for the three tested flow rates. In this case, this flow rate was reduced from 110 Nm3/h to 80 Nm3/h and the argon consumption decreased in 18.900 Nm3/year. With this change, the temperature drop during the treatment and the gas volume to be sucked by the vacuum system were reduced. In the second decarburization stage, the flow rate of 140 Nm3/h remained due to the highest kc value

フェライト鋼へのNi添加に伴う転位易動度の変化と脆性‐延性遷移挙動

In order to elucidate the mechanism behind the decrease in the brittle-to-ductile transition (BDT) temperature with the addition of Ni, impact tests and tensile tests were performed at various test temperatures from 130K to 320K with Ni added ultra-low carbon steels. The dependence of absorbed impact energy on temperature and the Ni content indicates that the BDT temperature was decreased with the increasing Ni content, which suggests that the dislocation mobility at low temperatures was increased with the Ni content. The yield stress which is also influenced by the dislocation mobility was decreased at low temperatures while it was increased at room temperature with the Ni content. The values of the activation volume and the effective stress were measured at several temperatures, and then the dependence of the activation energy for dislocation gliding on Ni content was obtained by extrapolating the relation between temperature and the multiplication of the activation volume and the effective stress to 0K. The activation energy was found to decrease with the increasing Ni content, which suggests that the dislocation mobility was increased with the addition of Ni. Discrete dislocation dynamics simulation was also performed in order to calculate the dependence of fracture toughness on temperature and the Ni content, and it was clarified that the BDT temperature is decreased by increasing dislocation mobility.

鋼板での表面欠陥発生機構解明のための再現ラボ実験

To understand the generation mechanism of surface defects appeared on the final sheet product, the defects found in cast production slabs were investigated.Defects located in a cast production slab of ultra-low carbon steel were detected by using an ultrasonic test. After that, using laboratory mill, hot rolling, cold rolling and galvanized coating were applied to the samples to simulate actual production conditions. And cross section of the sample was observed at the location corresponding to the original location of defects in the cast slab.Some samples showed similar defect shapes like defects of scabs in the steel sheet. And foreign materials which consist of Al, Fe, and O were observed at the location of the scab-like defects.Various defects are shown by the diameter and depth from the surface to the top of defects in cast slabs with their reproduction. It is clearly divided by a boundary related to the diameter and depth into two regions.Based on the information on scabs observed in steel sheet product, the size and depth of defects from the surface existed in the cast slab were estimated.Boundary plotted by the diameter and depth of the defects estimated from the surface defects of sheet products also shows the similar tendency of the threshold of possibilities of the generation of defects in the reproduction experiment.

Dry Sliding Wear Properties of Sub-Microcrystalline Ultra-Low Carbon Steel Produced by High-Pressure Torsion Straining

This study is the first to show dry sliding wear properties of sub-microcrystalline ultra-low carbon steel produced by high-pressure torsion (HPT) straining. Effects of number of turns in HPT process and counter materials in wear tests were investigated using a ball-on-disc friction method. Wear tests were carried out in a normal laboratory atmosphere of air at a sliding speed of 0.042m/s and an applied load of 39.6N. When the ball material was cemented carbide (WC­Co), the wear depth of HPT-processed discs decreased with increasing the number of turns in HPTstraining. This was explained by the hardness variation, i.e. the HPT-processed discs in which large strain was introduced exhibit high anti-wear resistance due to the high hardness. On the other hand, when the ball material was high carbon-chromium bearing steel (SUJ2), the wear depth of disc specimens increased with increasing the number of turns. The unusual behavior of the wear depth variation was thought to be attributed to the intensive adhesion between the HPT-processed steel disc and the SUJ2 ball because the coefficient of friction was considerably high for the ultra fine-grained specimens. [doi:10.2320/matertrans.MD201118]

極低炭素 Nb 添加鋼の高温での引張特性とその機構

The aim of this study is to clarify the high-temperature strengthening mechanism of Nb-bearing ultra-low carbon steel, which is well-known as a superior steel for high-temperature applications. Observations by 3DAP suggested that the Nb atoms are either distributed in a solid solution within the grain or segregated at the grain boundary after hot rolling. The strength at 600℃ increases significantly upon the addition of Nb, and the corresponding dominant strengthening mechanism is considered to consist of the following: the resistance for the dislocation gliding motion due to solute Nb, the retardation of the dislocation climbing-up motion due to solute Nb and Nb-C dipoles, and the resistance of the dislocation motion caused by the Nb-C(N) clusters formed when the materials are heated up to 600℃ within 10 s and then held for 600 s. Further, compared with Nb-free steel or 0.1% Nb-bearing steel, 0.3% Nb-bearing steel has considerably reduced ductility at 600℃. This is attributed to the retardation of recovery due to the Nb addition. TEM observations imply that the dynamic recovery takes place easily during the tensile deformation at 600℃ in Nb-free steel or 0.1% Nb-bearing steel, whereas the tensile stress increases significantly because of the work hardening presumably caused by the retardation of the restoration process by further addition of Nb. Hence, a rupture followed by necking is thought to occur easily. Moreover, there is a possibility that the segregated Nb at the ferrite grain boundary might affect the dislocation behavior resulting in an increase in the steel strength at a high temperature and a retardation of the recovery process. This possibility will be investigated in a future work.

The Effect of Pre-Strain on the Bake Hardening Property for Ultra-Low Carbon Steel

The bake-hardening experiment was carried out for the Ultra-low carbon steel under different pre-strains, and the bake-hardening property is determined. The results are as follows: when the pre-strain is in the range of 0% ~4%, the BH value increases with the increase of the pre-strain; while in the range of 4% ~ 8%, the BH value decreases with the increase of the pre-strain, and when the pre-strain is 4% , the corresponding BH value is higher than others. When the pre-strain is from 8% to 15%, The BH value gets improved with the increase of the pre-strain, and the BH minimum value is corresponding to 8% pre-strain. While the results of Transmission electron microscopy(TEM) analysis shows that the dislocation density of 4% pre-strain is higher than that of 8% obviously, which improves that the dislocation density is the main factor that affects BH value.

Development of Recrystallization Texture Prediction Method Linking with Deformation Texture Prediction Model

T.M.C.P. (Thermo Mechanical Control Process) has been widely used in the steel industry. We have produced ultra low-carbon steel strip and ferritic stainless-steel strip through T.M.C.P. rolling method. Next, we analyze rolling texture and recrystallization texture of the ultra low-carbon strip and the ferritic stainless- steel strip using our developed texture prediction model. We can predict rolling texture accurately using the conventional Taylor model. Moreover, we precisely predict recrystallization texture classifying the total number of microscopic slips which are calculated using the Taylor model. We consider that these calculated results prove nucleation-oriented model and two types of recrystallization and grain growth mechanisms exit in our studies. One mechanism is that grains which had the lowest total number of microscopic slips are preferred orientation for the hot rolled and annealed ferritic stainless-steel strip. The other mechanism is that grains which had the highest total number of microscopic slips are preferred orientation for the cold rolled and annealed ultra low-carbon strip.

Strain Localizations in Ultra Low Carbon Steel

Ultra low carbon (ULC) steel samples were deformed in near plane-strain compression mode with different strains, strain rates and temperatures. Different aspects of microstructural developments, for deformed γ (ND//) and θ (ND//) fibre grains, were investigated using X-ray line profiles and high resolution electron diffraction. The study clearly showed increase in grain interior strain localizations and in-grain misorientation at the intermediate deformation temperature. This effect was more apparent in γ-fibre and can best be explained through orientation sensitive recovery. γ-fibre also demonstrated higher potential for increase in dislocation density. This was observed experimentally and simulated through discrete dislocation dynamic simulations. Higher textural softening with stronger increase in dislocation density and possible effects of orientation sensitive recovery appears to define the orientation dependent recovery in low carbon steels.

Orientation Relationships of Coarse Pearlite Islands Formed in Tundish Well Clogging Deposits

It is well known that clogging constitutes one of the main issues encountered during casting in the steel industry. To clarify how clogging deposits are formed and to minimize their impact in the process, their microstructure and relationship between the different constituents need to be studied. In the tundish well the studied deposits consist predominantly of Al2O3 inclusion fans but are found interspersed with islands of pearlite along the margins. These islands of coarse pearlite, i.e. thick cementite/ferrite plates surrounded by large ferrite grains are formed even in Ti-rich ultra low carbon steels and the orientation relationships are used to track the initial composition of the deposit when it was formed. The crystallographic orientation relationship (ORS) between ferrite and cementite in pearlite can follow Isaichev (I), Bagaryatskii (B) or Pitsch-Petch (PP). Results showed that the pearlite islands are formed with ORS relatively close to PP and are intersected by alumina particles which show a specific relation with the islands and with the ferrite matrix. It was concluded that an external carbon source and temperatures just above the eutectoid are needed to form those islands.

The effect of interstitial carbon on the initiation of plastic deformation of steels

Abstract Pop-in behavior in nanoindentation was studied as a mechanism for the initiation of plastic deformation in two kinds of steels with different interstitial carbon contents; interstitial free (IF) and ultra low carbon (ULC) steels. The critical load P c at which the pop-in occurs is higher in ULC than in IF, and the P c decreases with decreasing loading rate, indicating that the pop-in mechanism is based on a thermal activation process. The interstitial carbon is thought to yield higher friction stress against dislocation movement and have an influence on the critical stress for the activation of the dislocation source formed underneath the indenter.

Ultra-low carbon (ULC) steel modified by triode plasma nitriding and PVD coating: Effects on micro-abrasive wear behavior

Ultra-low carbon (ULC) steels exhibit low yield strength and excellent formability. Plasma Assisted Physical Vapor Deposition (PAPVD) is a potential coating method for enhancing the strength at the surface of these steels [1]. However, when deposited onto low strength alloys PAPVD coatings may undergo premature failure if the substrate plastically deforms under heavy load. Extra load support is usually required for hard coatings to perform satisfactorily. Combined treatments involving plasma nitriding and PAPVD coating have been used to improve the load-bearing capacity of hard films [2]. This work describes the characterization and micro-abrasive wear behavior of Ti-stabilized ULC steels after surface modification by D.C Triode Plasma Nitriding (DC-TPN) and sequential coating with CrAlN by Electron Beam Plasma Assisted Physical Vapor Deposition (EB-PAPVD). Ti-ULC steel, plasma nitrided Ti-ULC steel and Ti-ULC duplex system were characterized by SEM, EDS, XRD analyses, micro-hardness and instrumented indentation hardness measurements, and stylus profilometry. Micro-abrasive wear tests were performed in fixed-ball configuration up to 1350 revolutions using SiC abrasive slurry and a 25mm diameter — AISI 52100 steel ball. Micro-abrasion mechanisms are presented and discussed. Nitrided steel and duplex systems were, respectively, 2.6 and 3.5 times harder than the untreated Ti-ULC steel. Wear coefficient of nitrided steel was 36% lower than that of the parent Ti-ULC steel. Regression analyses were used to calculate substrate (ks) and coating (kc) wear coefficients for the duplex system, the latter being 6.5 times lower than that of the nitrided steel. Coating thickness (3μm max.) was determined from inner and outer diameter measurements of the wear scars. Results indicate that it is feasible to manufacture duplex Ti-ULC steel via PAPVD, as significant improvements in wear resistance were recorded for both nitrided and duplex-treated steels. Duplex treatment clearly was the most effective method to enhance the wear resistance of ULC steels.

Inclusion Characterization of Titanium Stabilized Ultra Low Carbon Steels: Impact of Oxygen Activity before Deoxidation

AbstractAt an ArcelorMittal steel‐producing plant, heats deoxidized with initial oxygen activity greater or equal to 750 ppm were automatically downgraded. After analyzing liquid steel samples using automated SEM and remelt button techniques, it was concluded that higher oxygen levels before killing do not necessarily correlate to an increase in inclusion content of the heats. Results from both automated scanning electron microscope (SEM) and remelt button technique showed that the frequency of inclusions decreases with increasing inclusion size regardless of oxygen levels. The cleanliness of the heats with temperature correction following deoxidation at the RH degasser was also determined. It was observed that the number density of inclusions significantly increased in the ladle samples and more than doubled in the tundish samples for the heats with late temperature correction at the degasser and, therefore, this practice should be controlled.

The Influence of Critical Turning Point of Argon Flowrate Ascension on Decarburization Rate of RH Treatment for Ultra-Low Carbon Steel

Through the mathematical model of RH vacuum decarburization, the decarburization mechanism of argon gas bubble surface was studied. By the RH equipment of Shougang Qian’an Iron and Steel Co. Ltd for background, the calculated results of this model showed that the decarburization quantity of argon gas bubble surface was about 8% that of overall decarburization. In the initial conditions was constant, the ascension of argon flowrate will promote the decarburization quantity of argon gas bubble surface, free surface and splash droplet surface as well as reducing the decarburization quantity of internal liquid steel decarburization. Based on the commercial production data, it was found the best opportunity of ascension blow argon flowrate to acquire lower carbon content in relatively shorter time.

Surface cleanliness evaluation in Ti stabilised ultralow carbon (Ti-IF) steel

The surface cleanness of Ti-IF steel, including head, transition, end and normal slabs, was studied by original position analyser. The content, number, size and porosity of inclusions on the surface were obtained. The results showed that: the contents of inclusions in head and end slabs were significantly higher than in normal or transition slabs and fluctuated significantly in non-steady state slabs within 3.5 mm of the surface; the porosity in 3.5 mm of slab surface was lower than other positions; the surface cleanliness order was normal slab>transition slab>end slab>head slab; a reasonable surface peeling thickness for non-steady state slabs was 3.5 mm. The energy dispersive spectroscopy showed that more lump Al2O3 existed in the head slabs, and slag entrapment inclusions also appeared frequently in the end slabs. The main effect factors for cleanness of the head and end slabs were reoxidation and slag entrapment.

Heterogeneous Phase Transformation Texture Evolution in Low Alloyed ULC Steel Sheets

AbstractIn the present study the cold rolled sheet of ultra low carbon steel alloyed with manganese and aluminium has been investigated after α − γ − α transformation annealing in vacuum. The texture analysis was carried out on the surface, sub‐surface and in the bulk of the annealed sheet. A &lt;100&gt; // ND and &lt;110&gt;//ND texture with weak intensity is observed on the sheet surface and a strong &lt;111&gt;//ND fibre in the bulk of the transformed sheet. In depth micro‐texture analysis by orientation contrast microscopy reveals that two different types of orientation selection mechanisms could be active. The texture at the metal‐vapour interface is controlled by surface energy anisotropy (producing a local &lt;100&gt;, &lt;110&gt;//ND texture at the surface) and a texture memory mechanism generating strong &lt;111&gt;//ND fibre is active in the bulk of the sheets.