Ultra-low Carbon Interstitial Free Steel Research Articles

Research on the strengthening mechanism of Nb–Ti microalloyed ultra low carbon IF steel

Interstitial free (IF) steels are characterized by their interstitial free ferritic matrix due to addition of micro-alloying elements that draw carbon and nitrogen out of matrix to form carbonitrides. The IF steels typically have excellent deep drawing ability, certain mechanical strength, and high plastic deformation performance. In this paper, the microstructure evolution and carbides precipitation behavior during the hot rolling, cold rolling and annealing processes have been studied using both experimental and thermodynamic and kinetic modeling approaches. The strengthening mechanism of Nb–Ti microalloyed IF steel has been investigated. The results of the thermodynamic simulation indicates that the addition of Nb into Ti bearing IF steel promotes recrystallization at 900 °C, leading to fine recrystallized grains. Due to the recrystallization of austenite during the final rolling process, Nb–Ti microalloyed hot rolled plates exhibit finer grain size, fewer substructures, and lower dislocation density compared to the hot rolled plates containing only Ti. Building upon the initial fine and uniform structure, Nb–Ti microalloyed steel maintains a refined structure after undergoing cold rolling and annealing. The study of the carbides precipitation behavior suggests that TiN precipitates at high temperature above 1100 °C, while a substantial amount of (Nb,Ti)C and TiC precipitate at about 900 °C in Nb–Ti and Ti bearing steels, respectively. There is no significantly difference between the amount of carbides in steels hot deformed at 900 °C and in the hot rolled plates, indicating that the carbides mainly precipitate during the hot rolling process. There are negligible carbides precipitated during the coiling process at 600 °C. In Nb–Ti containing steel, Nb and Ti in solid solution are significantly reduced due to the large amount precipitation of (Nb,Ti)C during the final rolling process. The reduction of solid solution Nb effectively promotes the recrystallization of austenite. Therefore, the grains of hot-rolled, cold-rolled, and annealed strips are refined. In addition, there are more small size carbides in Nb–Ti bearing steel than that in Ti bearing steel. Therefore, the higher strength of Nb–Ti steel is mainly from the strengthening effect of grain refinement and carbides precipitation.

Effect of electrolyte composition on morphology and corrosion resistance of plasma electrolytic oxidation coatings on aluminized steel

Surface modification of the aluminized ultra-low carbon interstitial free steels by the process of plasma electrolytic oxidation (PEO) is reported in the current study. Coatings were formed by using a pulsating DC power source operating at a current density of 225 mA/cm2 over the period of 15 min. The effect of the electrolyte composition on the performance and properties of the coatings was studied. Three different electrolytes with varying concentrations of sodium silicate (Na2SiO3.9H2O) and potassium hydroxide (KOH) with their proportions as 2:1, 1:1 and 1:2 were used. The elemental composition, surface and cross section morphology and thickness of the coatings were analyzed by using scanning electron microscope (SEM) rigged with an energy dispersive X-ray spectroscope (EDS). The phase composition of the samples was assessed with X-ray diffraction (XRD). The surface profile and the roughness of the coatings were analyzed with an optical profilometer. The corrosion resistance of the substrate and the coated samples was determined by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) studies in a 3.5 wt% aqueous NaCl environment along with the immersion test in a 5 wt% aqueous NaCl environment. The practical adhesion strength of the PEO coating was assessed by the scratch testing. Among all the samples, the sample PEO treated with equal proportions of silicate and KOH concentration exhibited better corrosion resistance (icorr = 1.09 × 10−5 mA/cm2, inner barrier layer resistance of 1.32 × 108 Ω-cm2 and impedance (Ω-cm2) of 107 order) along with the better adhesion strength (Lc = 16.2 N). The better corrosion performance of the sample PEO treated with equal proportions of silicate and KOH concentrations can be imputed to the higher coating thickness along with the less porous surface morphology and higher inner barrier layer resistance. The higher adhesion strength of the sample, PEO treated with equi-concentrated silicate and KOH can be ascribed to the higher coating thickness along with the less porous surface morphology.

Determination of non-metallic inclusions in a continuous casting slab of ultra-low carbon interstitial free steel by applying of metallographic method, electrolytic method and RTO technique

Metallographic, electrolytic method and RTO(room temperature organic) technique were applied in the present study to more accurately determine non-metallic inclusion in a ultra-low carbon interstitial free(IF) steel and further to confirm their origination in a Compact Strip Production Process (CSP Process) continuous casting (CC) slab. Results show that inclusions detected by metallographic method usually appear relative smaller size and are mainly Al2O3 based or TiN based in composition, whereas those extracted by electrolytic method usually have larger size and are much more calcium-silicate based in composition. In addition, inner structures of extracted inclusions were detected by applying of RTO technique. The large size calcium-silicate based inclusions are confirmed high possibilities originating from mold flux and/or tundish flux entrapment, which are less affected by the liquid steel composition; while the smaller ones are generally endogenous inclusion precipitating during the refining or solidification process that strongly depending on the liquid steel composition and temperature.

Influence of the Drece Process of Severe Plastic Deformation on the Mechanical Properties of the Ultra-Low Carbon Interstitial Free Steel

Influence of the Drece Process of Severe Plastic Deformation on the Mechanical Properties of the Ultra-Low Carbon Interstitial Free Steel

Characteristic Transformation of Manganese-containing Inclusions during Al-killed Process in Ultra-low Carbon Interstitial-free Steel

Characteristic Transformation of Manganese-containing Inclusions during Al-killed Process in Ultra-low Carbon Interstitial-free Steel

Effect of plastic deformation on the structure and mechanical properties of an ultra-low carbon interstitial-free steel in the monolithic material and as a component of a sandwich composite

The structure and mechanical properties of ultra-low carbon interstitial-free (IF) steel in the annealed state, after warm and cold rolling, and as a component of seven-layer steel–aluminum composite have been studied. A comparative analysis of the results of structural studies using optical microscopy and scanning and transmission electron microscopy have revealed the possibility of the formation of an ultrafinegrained structure in a steel layer during rolling at temperatures ranging from room temperature to 520°C. It has been found that the seven-layer composite has higher strength properties as compared to monolithic samples of the IF steel after analogous regime of the warm rolling.

Effect of Deformation Temperature on Microstructure Evolution in ARB Processed Ultralow Carbon IF Steel

The effect of deformation temperature on the microstructure evolution was investigated in the range from room temperature to 600°C for an ultralow carbon interstitial free steel deformed to high strain by accumulative roll-bonding (ARB). In the whole temperature range, the microstructure was being subdivided by deformation-induced boundaries, and the spacing of such boundaries decreased with increasing the applied strain. The mechanism of this microstructure evolution is known as grain subdivision. In the warm-temperature ARB at 400°C and above, a quite uniform lamellar boundary structure elongated to the rolling direction was obtained after high strain, where the boundary spacing tended to be saturated above strain of approximately 4 and the saturated boundary spacing became smaller when the deformation temperature was lower. It is suggested that the microstructure after high strain deformation is determined by a balance of deformation-induced grain subdivision and restoration processes such as recovery and short-range boundary migration, and that the saturated boundary spacing is explained as a function of the Zener–Hollomon parameter. However, localized shear banding occurred at high strain in the room-temperature ARB, leading to inhomogeneities in the microstructure. It is therefore considered that a moderate deformation temperature as well strain rate has to be chosen to avoid shear localization and obtain homogeneous nanostructures.

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.

Friction Stir Welding of Ultrafine Grained Interstitial Free Steels

Friction stir welding (FSW) was applied to ultra low-carbon interstitial free steels with mean grain sizes ranging from 0.7 μm, prepared by accumulative roll-bonding, to 27 μm. The steel with the intermediate grain size (1.8 μm) is most preferable for obtaining the highest hardness in the stir zone with the smallest grain size.

Diffusion Reactivity between Zinc Plate Layer and Nanocrystalline Surface Layer of IF Steel by Near Surface-Severe Plastic Deformation

The effect of near surface-ultrafine grain (NSUFG) layer with grain size of about 90nm on the solid reaction between Ti-added ultra-low carbon interstitial free steel sheet and electroplated zinc layer (ZP) was basically investigated at 473K and 573K, in comparison with the reactions of the coarse grains IF steel sheet (CG-IF) and the ZP. The NSUFG structure dramatically changes the reactions between ZP and IF steel, as follows; (1) the incubation times for the formation of reaction layers, pseudo-z-phase, are quite shorter for the NSUFG/ZP reactions than the CG/ZP ones, (2) in the early stage of annealing the former has the smooth interfaces of pseudo-z-phase layers but the latter has wavier ones like a stone wall, (3) the thickness of the pseudo-z-phase layer of the former is thicker than that of the latter, (4) some cracks are observed in NSUFG layers while no crack in coarse grain IF steel, and (5) the subsequent layer appears in the reaction between pseudo-z-phase and NSUFG layers former after long annealing by the blast-like break of the interface.

Microstructure and texture through thickness of ultralow carbon IF steel sheet severely deformed by accumulative roll-bonding

Ultralow carbon interstitial free (IF) steel was severely deformed up to a strain of 5.6 by the Accumulative Roll-bonding (ARB) process at 773 K. Crystallographic analysis by electron back-scattering diffraction (EBSD) technique in a field-emission type scanning electron microscope (FE-SEM) was carried out for the ARB processed IF steel throughout thickness of the sheet. Microstructural parameters, such as grain size, grain boundary misorientation and crystal orientation, through thickness of the ARB processed specimen were quantitatively clarified by the EBSD analysis. The ARB processed material was homogeneously filled with the lamellar or pancake-shaped ultrafine grains whose mean grain thickness were about 200–300 nm. More than 80% of the boundaries surrounding the ultrafine grains were high-angle grain boundaries. The ARB processed sheet had unique and complex textural distribution through thickness. The region near the thickness center has the conventional but quite weak rolling texture mainly composed of 〈110〉//RD and 〈111〉//ND. On the other hand, the surface region had the sharp shear texture, ND//〈110〉. Such a textural distribution is due to the redundant shear strain induced by high-friction between the sheet and roll during rolling. The correspondence between the textural and microstructural distribution and the shear strain distribution throughout thickness of the sheet was discussed.

Formation of nanocrystalline surface layers in various metallic materials by near surface severe plastic deformation

The surface of the various kinds of metallic materials sheets were severely deformed by wire-brushing at ambient temperature to achieve nanocrystalline surface layer. The surface layers of the metallic materials developed by the near surface severe plastic deformation (NS-SPD) were characterized by means of TEM. Nearly equiaxed nanocrystals with grain sizes ranging from 30 to 200 nm were observed in the near surface regions of all the severely scratched metallic materials, which are Ti-added ultra-low carbon interstitial free steel, austenitic stainless steel (SUS304), 99.99 wt.%Al, commercial purity aluminum (A1050 and A1100), Al–Mg alloy (A5083), Al-4 wt.%Cu alloy, OFHC-Cu (C1020), Cu–Zn alloy (C2600) and Pb-1.5%Sn alloy. In case of the 1050-H24 aluminum, the depth of the surface nanocrystalline layer was about 15 μm. It was clarified that wire-brushing is an effective way of NS-SPD, and surface nanocrystallization can be easily achieved in most of metallic materials.

Strengthening of low-interstitial steels by strain-ageing treatments

Formable low-carbon automotive sheet steels can be designed to achieve a 30--50 MPa strength increment during paint baking. This so-called bake hardening process is a form of static strain ageing, whereby dissolved carbon or nitrogen atoms segregate to the dislocations generated during temper rolling and press forming. In ultra-low carbon interstitial-free steels, the concentration of C and N in solution is determined by the equilibrium partitioning between ferrite and precipitates of Ti- or Nb carbonitrides. Thus, during continuous annealing the concentration of interstitial atoms could increase due to precipitate dissolution. The reverse process could occur during cooling due to NbC precipitation at 800--700 C or Fe{sub 3}C precipitation at 400--300 C. The purpose of the present study was to determine the effects of the annealing process variables (temperature, time and cooling rate) on the solute carbon concentration in a series of interstitial free (IF) steels.