Cold-rolled Low Carbon Research Articles

Microstructure evolution and quantitative analysis of mechanical properties of cold-rolled low carbon niobium steel

Based on the idea of “plain” of alloying elements, Fe-0.2C-2Nb (wt%) alloy steel was designed. The microstructure of the alloy was effectively controlled by means of severe plastic deformation at room temperature and reasonable heat treatment process. The microstructure evolution of the alloy was systematically characterized by TEM. After annealing at 600 °C for 0.5 h, the ultimate tensile strength of the sample reached 844 MPa, the total elongation reached 13%, which had an equilibrium state of mechanical properties. Thus a quantitative model between the microstructure parameters and mechanical properties was established to reveal the deformation mechanism and strengthening mechanism of the alloy.

Recrystallisation kinetics and yield-strength adjustment after annealing of cold-rolled microalloyed steel

After continuous annealing process (CAP) at 790°C, 85% of the coils of 50% cold-rolled low carbon microalloyed (LCM) steel did not exhibit yield-strength (YS) on the target range, while the 70% cold-reduced LCM coils did. In this context, the non-isothermal recrystallisation kinetics of ferrite for the above two full-hard LCM steel were investigated using differential scanning calorimetry and the Friedman differential isoconversional method. The recrystallisation kinetics of ferrite for the two deformed states showed different behaviour. Regarding a fixed degree of cold-rolling deformation, the soaking temperature was found as the manageable parameter to control YS during CAP. Consequently, a suitable YS of the 50% cold-rolled LCM steel was achieved by setting the soaking temperature at 773°C.

Epilamization of surface as a way to plasticize cold-rolled low carbon steel

Abstract. About 60 % of automobile and 40 % of tractor parts of various shapes and sizes are made from cold-rolled thin-sheet low-carbon steel by cold stamping. Automatic cutting lines for rolled sheet steel into blanks for stamping and automatic presses for the manufacture of deep-drawn parts of complex geometric shapes from a sheet result in increased demands on the quality of the metal: high technological ductility in order to improve stampability and obtain the required degree of deformation without fracture and ensuring sufficient strength after stamping to reduce weight, fuel economy and vehicle safety. In this regard, the search for new technological solutions relevant to ensure these conflicting requirements for the consumer properties of steel sheets is relevant. According to modern concepts, an approach that determines the influence of a thin surface layer on the deformation behavior and formation of product properties is promising for solving this problem. In this paper, we propose a method for significant improving of formability while maintaining the strength of low-carbon cold-rolled sheet steel by changing the state of the surface by epilating (EP). It is established that EP significantly reduces surface roughness, heals surface defects and significantly plasticizes the product, which greatly improves the stampability of 08кп steel blanks. Estimated by the Ereksen method, the ability of a sheet of 08kp steel to be drawn varies from the high-temperature group to values exceeding the requirements of WWTP, which according to GOST 9045-93 are regulated only for the best quality steel 08Yu. At the same time, sheet strength and resistance to brittle fracture increased. This effect has been obtained for the first time and is fixed by a patent for the invention.

Internal Friction Study of the Bake-Hardening Properties on Low Carbon Steel Influenced by the Degree of Pre-Deformation

This paper selected cold-rolled low carbon steel as the object of experimentation to better understand that the annealed sample was baked in different degree of pre-deformation span to measure its baking hardening property. The main results of the experiments are as follows: The sample was set in different pre-deformation. Firstly, the baking temperature was set as 170°C and the time span was 20 minutes. The results released that when the degree of pre-deformation was set between 2% and 5%, the BH value increased with the increasing degree of pre-deformation. Secondly, the degree of pre-deformation was set between 5% and 10%, the BH value decreased with the increasing degree of pre-deformation. Thirdly, when the degree of pre-deformation was set at 5%, the BH value reached the maximum. It is illustrated that the BH value increased along with the rising of SKK peak while decreased with the falling of SKK peak. It indicated that the increase of the BH value was mainly due to the intensification of the Cottrell Atmosphere. As the pre-deformation increased, the sum of hSnoek+hSKK decreased. It is worth noting that while under the same circumstances, the sum of hSnoek+hSKK increased after baking.

Effect of electropulsing on microstructure and hardness of cold-rolled low carbon steel

The application of electropulsing treatment (EPT) is studied on the microstructure and hardness of cold-rolled low carbon steel. The effects of electrical pulses on the grain refinement, precipitate size evolution and hardness are investigated. It is found that applying electrical pulse decreases the grain and precipitate size and increases the hardness in the early stages of electropulsing, but the grain and precipitate size is increased and the hardness is decreased by continuing electropulsing. Also, the effect of electropulsing treatment variables as well as treated time and number of periods are discussed. It is demonstrated that increasing treated time can accelerate achieving maximum hardness in lower number of periods.

Exploring the microstructure and tensile properties of cold-rolled low and medium carbon steels after ultrafast heating and quenching

The aim of the present study is to evaluate the impact of heating rate on the microstructure and tensile properties of cold-rolled low and medium carbon steels. For this purpose, cold-rolled low and medium carbon steels were subjected to short peak-annealing experiments at 900 and 1100 °C under three heating rates (10, 450 and 1500 °C/s). The microstructure reveals a mixture of phases and microconstituents (ferrite, bainite, and as-quenched martensite) which are related to the carbon heterogeneities in austenite. The microstructural characterization suggests that the grain refinement achieved after ultrafast heating has a minor effect on the yield and ultimate tensile strength, compared to the relative microstructural distribution. It is suggested that the interplay of various strengthening mechanisms in samples subjected to ultrafast heating rates are responsible for the observed increase in strength and ductility.

Texture simulation of a severely cold rolled low carbon steel using polycrystal modeling

Characteristic textures of cold-rolled low carbon steel consist of the RD//〈110〉 fiber (α fiber) and ND//⟨111⟩ fiber (γ fiber). Although the development of the γ fiber has been successfully modeled by the relaxed constraints polycrystal plasticity model, modeling of the α fiber has been less successful. In this work, we investigate mechanisms underlying the development of a strong α fiber in a low carbon steel during cold rolling to large thickness reductions of 90%, 98%, and 99.8%. We compare the deformation textures measured using X-ray and electron backscatter diffraction methods with those simulated using polycrystal models, employing one of four homogenization schemes: the Taylor model, Secant model, Tangent model and an intermediate stiffness model between the Secant and Tangent models. The measured textures consist of an extremely strong α fiber and unusually weak γ fiber. The calculated textures achieve the best agreement with the measured ones when (1) the intermediate stiffness model is used to account for grain shape effects, (2) the flow rule exponent (n) is chosen to allow for multiple slip in grains, (3) latent hardening effects are suppressed, (4) ⟨111⟩ slip on {110}, {112} and {123} planes is made available for deformation, and (5) the grains shape is permitted to evolve individually. Taken together, these aspects represent indirectly the effects of cross slip on slip activity, suggesting that the strong α fiber in severely cold rolled steel results from cross slip. Although the cross slip is a well accepted mechanism in b.c.c. crystals, its connection to α fiber dominance in texture development during severe rolling deformation has not been established before.

Corrosion Inhibition of Cold-rolled Low Carbon Steel with Pulse Fiber Laser Ablation in Water

This study aims at the use of a fiber laser for modifying the surface properties of cold-rolled low carbon steel via a pulse laser ablation technique in water. The effect on the corrosion behavior of the fiber laser-treated metal surface was investigated in NaCl and HCl environments. Electrochemical tests showed significant improvement in the corrosion resistance of the laser-treated sample in NaCl, with an increase in open-circuit potential (OCP) from − 0.65 to − 0.60 V and an inhibition efficiency of 89.22% as obtained from the impedance study. Such improvement was less significant in an acidic environment. Lower corrosion rates of 20.9 mpy and 5.819 × 103 mpy were obtained for the laser-treated samples in neutral and acidic electrolytes, respectively, than the corrosion rates obtained for the as-received samples (33.2 mpy and 11.98 × 103 mpy). Morphological analysis indicated a passive film built by spherical grains of regular size on the metal surface after laser treatment. The corrosion inhibition effects in NaCl were evident by the nonexistence of the common corrosion products of lepidocrocite and crystalline structures that were seen on as-received samples; only polyhedral crystals with micrograins grown on them were seen covering the laser-treated surface. Therefore, the laser treatment using a fiber laser source improved the corrosion resistance of cold-rolled low carbon steel.

Effect of Hydrogen Trapping and Poisons on Diffusion Behavior of Hydrogen in Low Carbon Steel

Effect of hydrogen trapping and poisons on diffusion behavior of hydrogen in commercial cold-rolled low carbon steel was investigated by means of electrochemical hydrogen permeation techniques. The experimental results reveal that diffusion rate and diffusion flux of hydrogen in the materials gradually increase with increasing the number of hydrogen charging and outgassing, and lag time significantly shortens with them, therefore, hydrogen trapping impede diffusion behavior of hydrogen in the materials. Different poisons in the hydrogen charging solution have also resulted in a certain influence on the assessment of hydrogen diffusion behavior.

Scanning Three-Dimensional X-Ray Diffraction Microscopy with a High-Energy Microbeam at SPring-8

The grain-resolved residual stress (type II) in commercial-quality low carbon steel was observed using scanning three-dimensional X-ray diffraction (3DXRD) microscopy. In this method, grain orientations and lattice parameters are mapped using a monochromatic high-energy X-ray microbeam and 3DXRD-based polycrystalline indexing. Defining the reference lattice parameter a0 as the average value in the entire field of view, grain orientations and lattice parameters are converted into stress tensors, yielding a grain-resolved stress tensor map. The effectiveness of the scanning 3DXRD method was demonstrated by evaluating the residual stress in a cold-rolled low carbon steel sheet using a 50 keV microbeam at SPring-8. The area of the cross-sectional sample was 1×1 mm2, which was sufficiently larger than the grain size of about 20 μm. To produce a two-dimensional map of a circular region with a diameter of 160 μm at a pixel size of 1×1 μm2, the measurement time was about 1 h. From the stress tensor map, differences in residual stress of about 150–200 MPa between some neighboring grains were observed.

The effect of the pre-heating stage on the microstructure and texture of a cold rolled FeCMnAlSi steel under conventional and ultrafast heating

This work focuses on the effect of a pre-heating stage on the microstructure evolution during continuous heating of 50% and 75% cold-rolled low carbon steel under conventional and ultrafast heating rates. Peak annealing experiments under two heating rates (10°C/s and 400°C/s) are applied to the samples after a pre-heating stage of 10s. The selected pre-heating temperatures (300°C and 400°C) show an influence on the phase fraction of austenite formed after a short holding time in the intercritical (α+γ) phase field, and an effect on the intensity of recrystallized ferrite texture components. A refinement of ferritic average grain diameter is observed after increasing the heating rate from 10°C/s to 400°C/s. It is concluded that a pre-heating stage has a negligible impact on the microstructure and texture of cold-rolled low carbon steel. Therefore, a suitable technological window for the application of ultrafast heating rates in steel processing is enabled.

The Effect of Ultrafast Heating on Cold-Rolled Low Carbon Steel: Formation and Decomposition of Austenite

The effect of heating rate on the formation and decomposition of austenite was investigated on cold-rolled low carbon steel. Experiments were performed at two heating rates, 150 °C/s and 1500 °C/s, respectively. The microstructures were characterized by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Experimental evidence of nucleation of austenite in α/θ, as well as in α/α boundaries is analyzed from the thermodynamic point of view. The increase in the heating rates from 150 °C/s to 1500 °C/s has an impact on the morphology of austenite in the intercritical range. The effect of heating rate on the austenite formation mechanism is analyzed combining thermodynamic calculations and experimental data. The results provide indirect evidence of a transition in the mechanism of austenite formation, from carbon diffusion control to interface control mode. The resulting microstructure after the application of ultrafast heating rates is complex and consists of a mixture of ferrite with different morphologies, undissolved cementite, martensite, and retained austenite.

The Effect of Ultrafast Heating in Cold-Rolled Low Carbon Steel: Recrystallization and Texture Evolution

The microstructure and texture evolution of cold-rolled low carbon steel after ultrafast heating and quenching is investigated. Experiments were carried out at heating rates of 150 °C/s and 1500 °C/s. The recrystallization of ferrite is studied by scanning electron microscopy and electron backscattered diffraction techniques. The texture evolution of cold rolled steel during ultrafast heating was studied, making it possible to estimate the precise effect of heating rate on the orientations of newly formed grains. The experimental results showed that the recrystallization of ferrite was not completed before the full transformation of austenite. The noticeable increase in the fraction of recrystallized grains of diameter less than 1 µm, when the heating rate is increased from 150 °C/s to 1500 °C/s suggests that the increase of the heating rate enhances the nucleation of ferrite. The crystallographic orientations in recrystallized ferrite are strongly influenced by the heating rates. The effect of heating rate in the releasing of stored energy, carbon diffusion and spheroidization of cementite might explain some differences in textures observed in recrystallized ferrite.

Influence of Heat Treatment Schedule on the Tensile Properties and Wear Behavior of Dual Phase Steels

The aim of present study is to investigate the influence of heat treatment schedule on the microstructure, tensile properties, and wear behavior of dual phase (DP) steels. In this regard, a cold-rolled low carbon steel (0.18% C, 1.25% Mn) is subjected to intercritical annealing (IA), step quenching (SQ), and intermediate quenching (IQ) heat treatment cycles to produce DP steels with different microstructures. The results show that the tensile strength, strength-elongation balance, and wear resistance of DP steels change in the order of SQ < IA < IQ. Variation of work hardening rate (θ = dσ/dϵ) with stress for the IA treated DP steel shows a one stage behavior corresponding to stage III, where θ linearly decreases with increasing flow stress. In contrast, for IQ and SQ treated specimens, a two stage behavior corresponding to an initial rapid decrease of θ with stress (transition stage) followed by stage III hardening are observed. The dominant wear mechanism for DP steels produced by IA and IQ treatments is a combination of abrasion and oxidation, while for the SQ treated specimen, a combination of oxidation and delamination is observed.

An experimental investigation of the formability of low carbon steel tailor-welded blanks of different thickness ratios

Cold-rolled low carbon steel sheets of thicknesses ranging from 0.8 to 2 mm were sheets welded together to form a single part before the formability tests. In this study, the formability of the tailor-welded blanks (TWBs) was evaluated in terms of three measures which are limited dome height (LDH), forming limit diagram (FLD), and minimum major strain. In addition to the mechanical properties, the welded specimens were analyzed experimentally under the effect of thickness ratios. The influence of welding orientation and its relation with the rolling direction on formability and mechanical properties were also studied. The mechanical properties of the welded specimens were investigated by tensile testing and micro-hardness measurements. The metallographic studies were investigated using optical microscope and scanning electron microscope (SEM). The experimental findings in this study showed that the higher the thickness ratio of the TWBs, the lower the forming limits curve level and the lower formability. The minimum major strain was clearly inversely proportional to the thickness ratio of the TWBs. On the other hand, the results of uniaxial tests clearly illustrated that there was no significant difference between the tensile strengths of the TWBs and base metal. The micro-hardness measurements indicated that the hardness in the weld bead increased by about 55 % of the base metal.

Ferrite recrystallization and austenite formation at the early stage of annealing in cold-rolled low-carbon steels

High strength low carbon steels used in the automobile industry are often annealed at the intercritical temperature, and recrystallization of ferrite and formation of austenite can occur at the same time during the intercritical annealing. However, detailed studies on the competition between recrystallization of ferrite and formation of austenite at the early stage of annealing have not been carried. Therefore, ferrite recrystallization and austenite formation in cold-rolled low carbon steels at an early stage of annealing have been observed. The chemical composition of the steel (in mass %) was 0.1C-2.0Mn. After hot and cold rolling, the specimens were heated from room temperature to 1003, 1013 and 1023 K at a rate of 30 K s-1, and then gas-quenched to room temperature at a rate of 50 K s-1. We observed the microstructure of the annealed specimens. Fractions of recrystallized ferrite and austenite gradually increased with increasing annealing temperature, whereas the fraction of non-recrystallized ferrite gradually decreased with increasing annealing temperature. A large portion of austenite was formed at the grain boundaries of recrystallized ferrite and at the interface between recrystallized and non-recrystallized ferrite grains, whereas formation of austenite was scarcely confirmed at subgrain boundaries inside non-recrystallized ferrite grains. The grain size of the recrystallized ferrite and the subgrains formed in the non-recrystallized ferrite hardly changed during the annealing. Ferrite recrystallization was able to proceed during intercritical annealing when the austenite fraction was less than at least 10%. A large portion of austenite was formed at the grain boundaries of the recrystallized ferrite and the interface between the recrystallized ferrite and non-recrystallized ferrite at an early stage of annealing at the intercritical temperature.

Identification of the parameters controlling the grain refinement of ultra-rapidly annealed low carbon Al-killed steels

Ultra-rapid annealing (URA) cycles were used in order to refine the microstructure of cold-rolled low carbon Al-killed steel sheets. It was shown that the degree of grain refinement is controlled by the parameters (heating and cooling rates, annealing temperature) of the URA cycles and by the size, distribution and number of iron carbides formed during the coiling of these steels. In order to obtain a strong refinement, four conditions have to be fulfilled: (i) the initial microstructure of the steels before annealing has to be composed of numerous iron carbides finely distributed in the ferritic matrix, (ii) intercritical annealing leading to the formation of about 80% of austenite has to be performed, (iii) the heating rate ( R H) has to be higher than 500 °C/s and (iv) the cooling has to be rapid between 800 and 650 °C. From a microstructural point of view, the grain refinement is the result of three phenomena: (i) the inhibition of the grain growth of the primary ferrite grains by the iron carbides (below A C1 ) and by the austenite nuclei resulting from these carbides (above A C1 ), (ii) the development of the austenite grains above A C1 and (iii) the γ → α transformation during cooling.

Sensitiveness of Magnetic Inductive Parameters for the Characterization of Recovery and Recrystallization in Cold-Rolled Low-Carbon Steel

The efficacy of some magnetic inductive parameters and of the maximum differential permeability derived from magnetic hysteresis loop measurements is studied in terms of nondestructively monitoring recovery and the onset and the progress of recrystallization in a cold-rolled low carbon steel. The remnant induction, which is known to be affected by the dislocation density, proved useful to monitor recovery, whereas the induction values measured at higher magnetic fields are not sensitive enough. In order to differentiate between contributions from crystal orientation and from changes in microstructure, the effect of texture evolution during recrystallization is analyzed in terms of the average magnetocrystalline energy. All the magnetic inductive parameters considered have proven to be able to characterize the recrystallized fraction when the usual weakening in the intensity of α fiber components and the enhancement of γ fiber components took place during recrystallization.

Study on process parameters and its analytic application for nonaxisymmetric rectangular cup of multistage deep drawing process using low carbon thin steel sheet

Multistage deep drawing process is widely used to obtain various nonaxisymmetric rectangular cups. This deep drawing scheme including drawing and ironing processes consists of several tool sets to carry out a continuous production within one progressive press. To achieve the successive production, design and fabrication of the necessary tools such as punch, die, and other auxiliary devices are critical, therefore, a series of process parameters play an important role in performing the process design. This study focuses on the tool design and modification for developing the rectangular cup with an aspect ratio of 5.7, using cold-rolled low carbon thin steel sheet with the initial thickness of 0.4 mm. Based on the design results for the process and the tools, finite element analysis for the multistage deep drawing process is performed with thickness control of the side wall in intermediate blanks as the first approach. From the results of the first approach, it is shown that the intermediate blanks could experience failures such as tearing, wrinkling, and earing by excessive thinning and thickening. To solve these failures, the modifications for the deep drawing punches are carried out, and the modified punches are applied to the same process. The simulation results for the multistage rectangular deep drawing process are compared with the thickness distributions before and after the punch shape modifications, and with the deformed shape in each intermediate blank, respectively. The results of finite element reanalysis using the modified punches show significant improvement compared with those by using the original designed punch shapes.

Microstructures and fracture features of cold-rolled low carbon steel sheet after annealing and mechanical stress concurrently loaded

The microstructure and fracture feature of a cold-rolled low carbon steel sheet after annealing under a stress were investigated by using the optical microscope (OM), scanning-electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that a commixed grain structure was formed when concurrently stressed and annealed at 650 °C, and that both the tensile strength and hardness of the steel are still higher while the elongation is increased markedly. The dimple like fracture feature with delamination cracks was observed after tensile deformation. These superior mechanical properties were derived from the commixed grain structure composing of cold-rolled lamellar grains, fine equiaxed grains, and sub-grains as well as ultrafine carbide precipitates.