Cold-rolled Low-carbon Steel Research Articles

Heat Treatment of Cold-Rolled Low-Carbon Si-Mn Dual Phase Steels

The effects of overageing (OA) and vanadium addition on microstructure and properties of cold-rolled low-carbon Si-Mn dual phase (DP) steel sheets are studied. The results show that the microstructure and mechanical properties of DP steels are greatly affected by OA temperature. When OA is done at low temperatures (473 — 573 K), the elongation rate increases sharply while both the yield strength and tensile strength decrease significantly. When OA is done at high temperatures (>573 K), the elongation does not further increase, but the yield strength increases and tensile strength decreases, which deteriorates the mechanical properties of the steel. The results also show that both the strength and the tempering stability can be significantly improved by vanadium microalloying. Finally, the optimum OA temperature for the traditional steel appears to be around 523 K, while it is around 573 K for the V-containing steel, resulting in the best overall mechanical properties.

Measurement of the residual stress tensor on the surface of a specimen by layer removal and interferometry: uncertainty analysis

A new method for measuring the principal residual stresses on the surface of a thin metallic specimen is presented. The novelty of the method resides in a combination of the layer removal technique by chemical etching and of electronic speckle pattern interferometry to measure the ensuing deformation. It is shown that the loci of constant deflection with respect to the original plane surface are conic sections. An interferometer sensitive to out-of-plane displacements was used, yielding fringe patterns that reproduced those curves. Immediate visualization of the orientation and relative magnitude of the principal stresses within the removed layer was then possible. A fit of the measured deformation to the theoretical shape, together with knowledge of the initial and final thickness of the specimen and of the elastic properties of the material (a cold-rolled low-carbon steel sheet), allowed estimating the principal stresses. It is shown that the critical factor that determines the uncertainty of these estimates is the thickness of the removed layer.

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.

On the use of the Theory of Critical Distances to predict static failures in ductile metallic materials containing different geometrical features

This paper summarises an attempt to use the Theory of Critical Distances (TCD) to predict static failures in notched specimens made of a commercial cold-rolled low-carbon steel. Over the last 80 years, such a theory has been successfully employed to predict both static and fatigue failures in notched components, exploring its accuracy and its reliability in different ambits of the structural integrity field. In most of these previous applications, the stress fields in the relevant region close to the notch could be assumed to be linear-elastic without much loss of accuracy. The aim of the present paper is to investigate whether this linear-elastic TCD can also be successful in predicting static failures in notched components when the final breakage is preceded by large-scale plastic deformations. Notched specimens containing different geometrical features were tested under both tensile loading and three-point bending and failures were predicted by post-processing the results of linear-elastic Finite Element Analysis (FEA). The predictions thus obtained were found to be highly accurate, falling within an error interval of about 15%, independent of specimen thickness, notch geometry and applied load type. A similar degree of accuracy was obtained when elasto-plastic stress analysis was used. This result is very interesting, because it supports the idea that the linear-elastic TCD can successfully be used in situations of practical interest, reducing the time and costs of the design process.

Effect of structural changes on hysteresis properties of steel

Several sets of steel samples, like cold-rolled low carbon steel with different rolling reductions and thermal aged Fe–1 wt% Cu alloys, are investigated with respect to magnetic parameters such as coercivity, remanence and shape of the hysteresis loop. Measurements are performed in a magnetic open system, which allows magnetic saturation of the material and in a magnetic closed system using ring or picture frame shaped samples. The results obtained for ring or picture frame shaped samples give a much better accuracy for determining the coercivity. The coercivity (remanence) of the cold-rolled carbon steel samples increases (decreases) systematically with the rolling reduction. The duration of the thermally aging procedure performed at 450 °C has nearly no influence on the coercivity value. However, if thermally aging is applied on a cold-rolled sample, the coercitivity decreases with increasing aging time.

Effect of Niobium Addition on the Texture Formation of High Strength Cold-Rolled Low Carbon Steel Sheets

The effect of niobium (Nb) addition on recrystallization texture formation in cold-rolled low carbon steel sheets containing 2% manganese (Mn) was investigated. The microstructures of hot-bands were significantly refined by Nb addition, which led to the development of the cold-rolling texture in both the γ-fiber (<111>//ND-fiber) and the α-fiber (<110>//RD-fiber). Recrystallization was retarded by Nb addition, in particular, the growth of <110>//ND grains was retarded. However, the γ-fiber and {112}<110> grains developed during annealing even in the intercritical (α-γ) region. Consequently, the r-value increased as the content of Nb was increased due to the development of the intensity ratio of the <111>//ND texture to the <100>//ND texture, which is desirable for deep-drawable high strength steel sheets.

Uncertainty of residual stresses measurement by layer removal

A model to evaluate the uncertainty in the measurement of the through-thickness residual stress distribution in plates by the layer removal technique is presented. Thin layers were chemically etched from a stripe on rectangular specimens cut from a low carbon cold-rolled steel sheet. Phase shifting laser interferometry was used to measure the ensuing curvature. Polynomials were least-squares adjusted to the curvatures as a function of the etched depth. The polynomials were inserted into an integro-differential equation relating the curvature to the residual stresses, which were assumed to be a function of depth only. A comparison with X-ray diffraction measurement of the surface residual stresses showed good agreement. The uncertainty was found to increase steeply at the surfaces and to depend mainly on the assumed value for the modulus of elasticity, on the curvature fit, and on the depth of etching.

The Snoek-Köster (SK) Relaxation and Dislocation-Enhanced Snoek Effect (DESE) in Deformed Iron

The Snoek-Köster (SK) relaxation and the dislocation-enhanced Snoek Effect (DESE) in deformed ultra-high purity Fe-C alloys and industrial low-carbon cold-rolled steel sheets as well as a quantitative comparison between experimental data and computer simulations have been presented.

THE EFFECT OF CIRCUIT LOADING ON DISTRIBUTION UNIFORMITY OF DEFORMATION DURING THE PLASTIC FLOW OF CARBON STEEL

Investigations of the substructure of cold-rolled low-carbon steels have shown that after 20 % pressurization the already big volumes of ferrite have periodic dislocation structures similar to dislocation cells. After two to four alternating bending cycles, an increase in the homogeneity of distribution of ferrite micro-hardness can be observed, along with a decrease of homogeneity of distribution of dislocations inside individual cells.

Conversion of Elongation Values for Cold-Rolled Low Carbon Steel Sheets

Abstract It is often difficult or impossible to prepare tension test pieces from flat steel products to specified standard size due to inadequate sample piece size. Hence several sizes of test pieces yielding different gage lengths are envisaged. However, the value of the total elongation beyond maximum stress in tension testing depends on the gage length due to the effect of necking. Acceptance values of elongation at fracture for flat steel products are therefore specified together with corresponding gage lengths, e.g., 50 or 80 mm (fixed gage lengths) and k So (proportional gage lengths) where k takes the value of 4.00, 5.65, 8.16, or 11.30 and k √SO is the original cross-sectional area of the test piece in mm2. Methods for converting room temperature percentage elongation at fracture obtained on various proportional and nonproportional gage lengths to other gage lengths exist for austenitic, carbon, and low alloy steels as hot-rolled, normalized, annealed, etc., but not as cold-reduced. These methods are also not valid where the gage length is greater than 25 times √SO or where the width-to-thickness ratio of the test piece is greater than twenty. This paper proposes a possible solution for cold-rolled low carbon steel sheets of thickness less than 2.0 mm, which continue to be, if not the major, one of the major raw materials in the automotive industry. The solution lies in the modification of the exponent in the Oliver formula used for flat steel products other than cold-rolled low carbon steel sheets.

Development of Low-Carbon Cold-Rolled Steel Sheets for Enameling Application

Development of Low-Carbon Cold-Rolled Steel Sheets for Enameling Application

Effect of citrate ions on the electrochemical behaviour of low-carbon steel in borate buffer solutions

The electrochemical behaviour of cold-rolled low carbon steel was studied on both active and passive potential regions in borate buffer solutions with and without the addition of sodium citrate (NaCit). In the active region anodic charges increased significantly and R CT values decreased with citrate, due to the formation of soluble complexes. In the passive potential region the film formed at +0.4 V in borate buffer solution with and without 0.010 M NaCit is probably enriched by Fe 3O 4 oxide, while films formed at +0.8 V are probably enriched by γ-Fe 2O 3. The equivalent circuit [ R s( R CT Q)] fitted all experimental impedance data.

Orientation Related Microstructure Evolution during Continuous Annealing of a Cold-Rolled Low Carbon Steel

Orientation Related Microstructure Evolution during Continuous Annealing of a Cold-Rolled Low Carbon Steel

Recrystallization of a cold-rolled low-carbon steel by cold-plasma-discharge rapid annealing

A new annealing technology has been developed by the authors in order to conduct fast steel annealing. This process consists of steel heating by cold-plasma discharge. It allows the opportunity for new annealing cycles with higher heating rates (up to 300 K/s), shorter soaking times, and controlled cooling rates, so that well-recrystallized samples have been achieved in less than several seconds of total process time. This article reports the influence of various parameters of the annealing cycle (heating rate, maximum annealing temperature, and cooling rate) on the recrystallization and properties of a cold-rolled low-carbon steel. This study shows that the annealing time can be significatively reduced using this new technology, compared to the industrial continuous annealing technology used today, to obtain equivalent metallurgical properties.

The computer-based bootstrap method as a tool to select a relevant surface roughness parameter

The aim of this paper is to present how to make the most of the recent and powerful statistical computer-based bootstrap method (CBBM) in roughness studies. This work shows that this statistical method can help to determine quantitatively, and without preconception, the most relevant roughness parameter that characterises the surface morphology of a manufactured product as far as a correlation with a particular function, property or application is concerned. The efficiency of this statistical method is illustrated in this paper describing the relationships between the brightness level and the surface roughness of cold-rolled low carbon steel strips; the relevance of 100 or so roughness parameters was studied via a computer software we have been upgrading for a few years.

Changes of microstructure and mechanical properties of cold-rolled low carbon steel due to its surface treatment by Nd:glass pulsed laser

Cold-rolled low-carbon rimming steel (i.e. not killed with Al or Si and not continuous but cast in a form) was surface-treated using a Nd:glass pulsed laser. The changes of the microstructure parameters were analysed by X-ray diffraction and optical microscopy. The modification of mechanical properties was evaluated by the changes of the residual macrostresses and Vickers microhardness values. It has been indicated that during the laser pulse the surface material of up to 80 μm thickness becomes melted and then resolidifies in a strongly non-equilibrium condition. This causes scattering of the crystallographic texture and introduction of significant tensile residual macrostresses in the steel surface mainly transverse to the rolling direction. At the same time, the residual macrostresses in the rolling direction of the investigated steel are not changed by the laser treatment and this fact is considered to be due to the anisotropy of the thermal conductivity in the both investigated directions. The performed laser treatment causes some surface oxidation and also an increase of the Vickers microhardness in the surface region of approximately 30%. Some particularities of the estimation of residual macrostresses in textured materials via the sin 2ψ X-ray diffraction method realised in grazing incidence beam geometry are discussed. It is shown that for cubic materials (and particularly for the investigated steel) with well pronounced crystallographic texture, it is advisable to apply the sin 2ψ method to more elastically isotropic crystallographic directions such as 〈100〉. It is also demonstrated that for the experiments performed in grazing incidence geometry the refraction correction is compulsory.

Study of the kinetics of the recrystallization of cold-rolled low-carbon steel

The suitability of the differential scanning calorimetry (DSC) technique for the quantitative study of the recrystallization process in cold-rolled steels is demonstrated, the recrystallization kinetics was described with a semiempirical model, and the transformation of a cold-rolled steel submitted to an industrial-batch thermal cycle was appropriately simulated. It was found that the activation energy for the recrystallization process depends on the heating-rate range, having values of 522 ± 13 and 259 ± 12 kJ/mole for low and high heating rates, respectively. It was concluded that the smallest value corresponds to the recrystallization process alone, while the largest one contains an additional contribution from the aluminium nitride precipitation. It is also shown that the X-ray diffraction (XRD) line-width measurement is a useful complementary method to determine the temperature regions where recovery and recrystallization occur.

Russian Federation Patents and Certificates for Useful Models in the Year 2000

A method has been proposed for making low-carbon cold-rolled steel strip.The method provides for cold rolling with a prescribed amount of reduction per pass, tension on the strip between the stands, and cutting of its sides by disk shears. The reduction in the first pass is 32‐33% for the rolling of strip having the smallest cross section. In this case, the initial yield point σ y = 215‐415 MPa and the total relative reduction after four passes is 68‐72%. After the first pass, the size of the cross-section of the strip is restricted by the concave surface of the rolls. The amount of tension between the stands is established in relation to the average theoretical yield point σ y ′ = σ y + 33.5·e 0.6 MPa. After the nth pass, the yield point is equal to σ n = k·σ y ′. Here, the tension on the strip during the cutting of the edges is σ 0 = (0.34‐0.46)σ y . The disk shears are installed so that they have a radial overlap of Δ = 0.5h, where e is the total relative reduction in the nth pass, %; k is a proportionality factor; σ y  is the average yield point of the metal being cut, MPa; h is the thickness of the strip, mm. The method expands the capabilities of continuous cold-rolling mills by improving the quality of strip having the smallest cross-sections. A. A. Morozov, R. S. Takhautdinov, A. I. Antipenko, et al. Patent No. 2147943 MPK 7 B 21 B 1/28, published 2000.27.04. Patent-holder Magnitogorsk Metallurgical Combine. A method has been proposed for using rolling-mill rolls. The method can be used on mills designed for the cold rolling of steel sheet. It entails repeating a sequence of operations numerous times: grinding of the body of the roll; assembly of the roll with its chocks on bearings; operation of the roll in the stand; disassembly. After the roll has been used, i t and the chocks are exposed to jets of water with a temperature of 65‐95°C. The water is delivered at a pressure of 25‐45 MPa for 2‐7 min. While the water is being delivered, the roll is moved back and fourth at a rate of 1.2‐1.8 m/min within the range of the jets. The method improves the durability of rolling-mill rolls and their bearings. V. P. Nastich, A. A. Ugarov, V. P. Vinogradov, et al. Patent No. 2147944 MPK 7 B 21 B 28/02, published 2000.27.04. Patent-holder Novolipetsk Metallurgical Combine.

Surface segregation of phosphorus, carbon, and sulfur in commercial low-carbon grades of steel

Surface segregation behavior of solute atoms has been studied on low-carbon steels used in producing galvannealed sheet steels for automotive body panel applications. Samples of cold-rolled low-carbon steels with different amounts of carbon and phosphorus in solution were heated in a vacuum chamber and their surface chemistries analyzed by Auger electron spectroscopy. For the steels studied here, one or more of the elements carbon, phosphorus, and sulfur accumulated significantly at the surface within a temperature window of 300 to 973 K. As the temperature was increased, carbon appeared on the surface first, followed by phosphorus, and then sulfur. Each succeeding segregating element displaced the previous one from the surface. The free solute concentration in the bulk and the temperature were critical factors controlling the amount of solute accumulation at the surface. Once segregated, the solute atoms remained on the surface as the samples cooled. Carbon and/or boron in steels retarded the transport of phosphorus to the surface. The implications of these findings in understanding the galvannealing behavior of these steels are discussed.

Cu析出強化型極低炭素冷延鋼板の再結晶集合組織におよぼす Cu,Ti 系析出物の影響

Cu析出強化型極低炭素冷延鋼板の再結晶集合組織におよぼす Cu,Ti 系析出物の影響