Ultralow Carbon IF Research Articles

Effect of strain ratio on hydrogen permeability properties of low carbon enamel steel

In this study, the effect of varying strain levels on hydrogen permeability properties were investigated. Distinct strain levels (10% and 40%) were carried out on the deep drawing test samples by using Marciniak die to simulate the forming process. Amount of strain on deep drawn material was calculated by GOM’s ARAMIS 3D deformation measurement system. Hydrogen diffusion coefficient and permeation time were calculated by using Helios II system. Light optical microscope (LOM) and scanning electron microscopy (SEM) were used for microstructure characterization. Automated inclusion/precipitation analysis was performed by Thermoscientific Explorer-4. By this study, it is aimed to understand the hydrogen permeation properties of ultra-low carbon IF steel material with varying strain values. Finally, it was determined that number of inclusion/precipitation per mm2 was significantly increased as a function of strain ratio, which improves hydrogen permeation properties.

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.

Thermodynamics and Industrial Trial on Increasing the Carbon Content at the BOF Endpoint to Produce Ultra-Low Carbon IF Steel by BOF-RH-CSP Process

Abstract Thermodynamic analysis was performed to obtain the relation between the carbon content at the BOF endpoint and the dissolved oxygen content in liquid steel and the (FeO + MnO) content in the slag with the help of thermodynamic calculation software FactSage. It finds that both the [O] and (FeO + MnO) content increase with decreasing the carbon content at the BOF endpoint and the increasing rate is larger when the carbon content is lower. In addition, in the case of the higher temperature at the BOF endpoint the [O] in liquid steel increase and the (FeO + MnO) in the slag increase as well. The consumption of O2 for decarbonization at the BOF endpoint is much more than that in RH degasser since the majority of the blowing O2 at the BOF endpoint will produce FeO into the slag, thus it increase the metal loss and deteriorate the steel cleanness during the consequent refining process. As a result, the carbon content at the BOF endpoint should be properly increased within the RH decarbonization ability. At last, industrial trials were carried out and confirmed that total oxygen consumption decrease obviously and the (FeO + MnO) of final BOF slag decline as well with increasing carbon content at BOF endpoint from 0.042% to 0.081%. In addition, it almost does not slow down the RH process and the carbon content in final steel all met the demand of the ultra-low carbon steel. In addition, mechanical properties of IF steel with higher carbon content at the endpoint of BOF are almost all more superior to those of heat with lower carbon content at BOF endpoint.

自動車外板用極低炭素IF鋼における加工後筋状欠陥の発生機構および熱延前再加熱温度とSb添加の影響

自動車外板用極低炭素IF鋼における加工後筋状欠陥の発生機構および熱延前再加熱温度とSb添加の影響

EFFECT OF GRAIN SIZE ON ATMOSPHERIC CORROSION RESISTANCE OF ULTRA--LOW CARBON IF STEEL

Three kinds of ultra-low carbon IF steel with different grain sizes,and same chemical composition were prepared by different rolling and heat treat process.The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test,cyclic immersion corrosion test,AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test,the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15μm to 220μm.The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15μm to 46μm.As grain size increase from 15μm to 220μm,the whole compactness of rust are increased,the rust resistance and the atmospheric corrosion resistance are increased.The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed.The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased.Meanwhile,the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased.The atmospheric corrosion resistance is influenced by the two factors simultaneously.

Microstructural Evolution with Annealing of Ultralow Carbon IF Steel Severely Deformed by Six-Layer Stack ARB Process

A sample of ultra low carbon IF steel was processed by six-layer stack accumulative roll-bonding (ARB) and annealed. The ARB was conducted at ambient temperature after deforming the as-received material to a thickness of 0.5 mm by 50% cold rolling. The ARB was performed for a six-layer stacked, i.e. a 3 mm thick sheet, up to 3 cycles (an equivalent strain of ~7.0). In each ARB cycle, the stacked sheets were, first, deformed to 1.5 mm thickness by 50% rolling and then reduced to 0.5 mm thickness, as the starting thickness, by multi-pass rolling without lubrication. The specimen after 3 cycles was then annealed for 0.5 h at various temperatures ranging from 673 to 973 K. The microstructural evolution with the annealing temperature for the 3-cycle ARB processed IF steel was investigated in detail by transmission electron microscopy observation. The ARB processed IF steel exhibited mainly a dislocation cell lamella structure with relatively high dislocation density in which the subgrains were partially observed. The selected area diffraction (SAD) patterns suggested that the misorientation between neighboring cells or subgrains was very small. The thickness of the grains increased in a gradual way up to 873 K, but above 898 K it increased drastically. As a result, the grains came to have an equiaxed morphology at 898 K, in which the width and the thickness of the grains were almost identical. The grain growth occurred actively at temperatures above 923 K.

Effect of Dynamic Baking on BH Value of Ultra Low-Carbon IF Steel

In this paper, the ultra-low carbon IF steel was studied by dynamic baking and the influence of dynamic baking process parameters on bake-hardening performance was analysed .The result is: The maximum bake-hardening value occur at 5 min. After 5 min ,the bake-hardening value gradually decreased and stabilized finally. The BH value gradually increases along with the increase of temperature. The BH value reaches to the maximum with the stretch rate of 5.64*10-4mm/s and 0.00169mm/s.The BH value decreases along with the increase of pre-strain increases.

High-Temperature Severe Plastic Deformation of Ferritic Steel by Torsion

An ultra-low carbon IF steel was heavily deformed up to an equivalent strain of 36 at various high temperatures of ferrite single-phase region and various strain rates. Effects of temperature and strain rate on the microstructures evolved in torsion deformation were clarified. On the other hand, it was found that homogeneous ultrafine grained structures were not obtained by the present torsion deformation though very high strain was applied. The coarser grain sizes than those obtained by conventional severe plastic deformation (like ARB) were due to the deformation at higher temperature and lower strain rate, but lower fraction of high-angle grain boundaries in the torsion specimen was suggested to be attributed to the characteristics of monotonic torsion (or simple shear) deformation including the way of strain evaluation.

Development of low-phosphorus slag-forming mixtures for gunited tundishes on continuous slab casters

This article describes the compositions of slag-forming mixtures (SFM) developed for continuous slab casters. The mixtures are to be used in tundishes with a gunited lining and monoblocks equipped with corundum-graphite stoppers. The goal is to reduce wear of the tundish lining and the stoppers. The physicochemical properties of the mixtures are described and their performance in the tundish is evaluated. The mixtures have been put to use at the Magnitogorsk combine for casting ultralow-carbon IF steels (SFM-11) and other grades of steel (SFM-12).

Production of ultrafine-grained sheet from ultralow-carbon steel by pack rolling

A method of multistage pack rolling (MPR) is investigated, which yields an ultrafine-grained (up to nanocrystalline) structure in ultralow-carbon IF steels used in the production of automobile steel sheets. The results of estimating the effect of temperature and the number of passes in MPR on structure and mechanical properties are given. The effect of MPR on technological plasticity is determined using the normal plastic anisotropy index (the Lankford coefficient).

Effect of redundant shear strain on microstructure and texture evolution during accumulative roll-bonding in ultralow carbon IF steel

Accumulative roll-bonding (ARB) is a severe plastic deformation process that can effectively produce ultrafine grained (UFG) structures in metals and alloys. In previous investigations, the ARB process has often been carried out under high-friction conditions without any lubricant between materials and rolls, which may cause a large amount of redundant shear strain near the sheet surface. Owing to repetition of cutting, stacking and roll-bonding in the ARB, a complicated redundant shear strain distribution is expected through the sheet thickness. The purpose of the present study is to clarify the effect of the redundant shear strain on the microstructure and texture evolution during ARB. A Ti-added ultralow carbon interstitial free steel was deformed by up to seven cycles of ARB (a thickness reduction of 99.2%) at 500 °C, with or without lubrication, in order to investigate the effect of shear strain. Microstructural characterization by electron backscatter diffraction analysis was carried out at various thickness locations of the ARB processed sheets. The sheet processed by one cycle of ARB with good lubrication showed typical deformation microstructures uniformly throughout the thickness. In contrast, the specimen processed by one ARB cycle without lubrication had an inhomogeneous microstructure, and the fraction of deformation-induced high-angle boundaries increased close to the surface. Non-lubricated ARB caused through-thickness microstructural heterogeneity in low numbers of cycles, but repetition of ARB above five cycles finally produced quite uniform UFG structures. It was established that the microstructural parameters of the deformation structures can be basically understood in terms of the total equivalent strain, taking account of the redundant shear strain.

ARBによる加工後焼鈍された極低炭素IF鋼の組織と機械的性質に及ぼすひずみ量の影響

ARBによる加工後焼鈍された極低炭素IF鋼の組織と機械的性質に及ぼすひずみ量の影響

連続せん断変形加工された極低炭素IF鋼板の集合組織と微視組織

連続せん断変形加工された極低炭素IF鋼板の集合組織と微視組織

Microstructures and Mechanical Properties of Ultra Low Carbon IF Steel Processed by Accumulative Roll Bonding Process

An ultra low carbon IF steel was severely strained by accumulative roll-bonding (ARB) process and subsequently annealed for 0.5 h at various temperatures ranging from 673 to 1073 K for strengthening by grain refinement. The ARB process was carried out up to 5 cycles (an apparent equivalent strain of 4.0) at ambient temperature without lubrication. The as-ARBed specimen exhibited a dislocation cell structure with relatively high dislocation density, rather than a well-defined ultrafine grained structure. However, the subsequent annealing at 673 or 773 K resulted in the formation of ultrafine grains in the structure. This indicates that the heating (recovery) is necessary for the formation of ultrafine grains in the present IF steel severely deformed by ARB process. The tensile strength of the as-ARB processed IF steel increased with strain, reached a maximum of 813 MPa, which is about 3 times higher than the initial value. The elongation dropped largely from 60 to below 10% at the 1st cycle, but it hardly changed from the 2nd cycle even if the ARB cycle increased. The strength decreased gradually and the elongation increased with annealing temperature. Changes in the mechanical properties with the annealing temperatures corresponded well with the changes in the microstructures.

繰り返し重ね接合圧延により強ひずみ加工された極低炭素鋼における超微細結晶粒

繰り返し重ね接合圧延により強ひずみ加工された極低炭素鋼における超微細結晶粒

Macroscopic and microscopic subdivison of a cold–rolled aluminium single crystal of cubic orientation

An aluminium single crystal of cube orientation has been rolled to 15, 30 and 50% reductions under controlled homogeneous rolling conditions. The deformation structure of the rolled specimens was investigated by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) over several scales of magnification. The local crystallographic orientations have been measured by an automatic electron back scattering patterns (EBSP) technique and a semiautomatic TEM method. Orientation image maps based on the local orientation data have been used to reveal the evolution of the deformation structure during rolling. It is observed that by an opposite rotation around transverse direction (TD) the crystal was subdivided into four macroscopic bands, termed matrix bands in the present paper, which are parallel to the rolling plane. Between the four bands there are three transition bands in which the orientation changes continuously from that of a matrix band to that of the adjoining one. A model based on the idea of location–dependent shear strain caused by geometric and friction effects together with a plasticity analysis has been used to explain the macroscopic subdivision of the crystal. In addition to the macroscopic subdivision, a microscopic subdivision by the formation of cell–blocks within the matrix bands and a cell structure within transition bands has also been observed. A difference related to shear amplitude difference between the active slip systems changing continuously across the crystal has been observed. Both the macroscopic orientation of the dislocation boundaries and the misorientation angles and axes across dislocation boundaries are analysed and it is found that Frank9s formula is a useful tool in analysing the dislocation boundaries formed during deformation.