Ultra Low Carbon Steel Sheets Research Articles

Effects of Excess C on New Grain Formation and Static Recrystallization Behavior at Shear Bands in Cold-Rolled Ultra-Low Carbon Steel Sheets

Effects of Excess C on New Grain Formation and Static Recrystallization Behavior at Shear Bands in Cold-Rolled Ultra-Low Carbon Steel Sheets

Microstructural Characterization and Softening Mechanism of Ultra-Low Carbon Steel and the Control Strategy in Compact Strip Production Process

In this paper, the microstructures and properties of hot rolled ultra-low carbon steel sheet produced by different compact strip production (CSP) processes were investigated. The softening mechanism was also discussed and the control strategy was proposed in order to obtain optimum properties. Result showed that the average ferrite grain sizes of austenite rolling sheet and multiphase rolling sheet were 31.0 μm and 74.6 μm, respectively. The sheet after austenite rolling had a slightly higher yield and tensile strength while had a 6.3% higher elongation than that of the sheet after multiphase rolling. The higher dislocation in the sheet after multiphase rolling increased the strength while decreased the elongation. The softening mechanism of the sheet after multiphase rolling was the coarsening of ferrite grain. The combined role of {001} and {111} orientation resulted in a slight increase of the r and $${\bar{\text{r}}}$$ value in the sheet after multiphase rolling. It was a wise choice to conduct rolling at the Ac1 temperature in CSP process to increase the grain size and decrease the dislocation density. Then, the strength of the sheets could be further reduced and the elongation could also be improved. The microstructures and properties of hot rolled ultra-low carbon steel sheet produced by different compact strip production (CSP) processes were investigated. The average ferrite grain sizes of austenite rolling sheet and multiphase rolling sheet were 31.0 μm and 74.6 μm, respectively. The sheet after austenite rolling had a slightly higher yield and tensile strength while had a 6.3% higher elongation than that of the sheet after multiphase rolling. The higher dislocation in the sheet after multiphase rolling increased the strength while decreased the elongation. The softening mechanism of the sheet after multiphase rolling was the coarsening of ferrite grain. It was a wise choice to conduct rolling at the Ac1 temperature in CSP process to increase the grain size and decrease the dislocation density. Then, the strength of the sheets could be further reduced and the elongation could also be improved.

Fracture Prediction for Mild Steel Sheet and High-Strength Steel Sheet Subjected to Draw Bending Using Forming Limit Stress Criterion

Abstract A fracture criterion for sheet metal subjected to draw bending is proposed based on the concept of the forming limit stress criterion. An experimental apparatus that is capable of draw bending sheet specimens with forming speeds (approximately 100 mm·s−1) comparable to those for commercial press forming machines was designed and built. The test materials were an ultralow-carbon steel sheet and a high-strength (590 MPa) steel sheet. Specimens underwent bending and unbending under tension when passing over a die. The magnitude of the true stress, σ D B , at which the stretch-drawn specimens fractured was precisely determined from the measured drawing force and the cross-sectional area of the specimen after fracture. Moreover, multiaxial tube expansion tests were performed to measure the forming limit stress, σ P T , for the test materials under plane-strain tension. It is found that σ D B is consistent with σ P T when the strain rate is properly taken into account in evaluating σ P T .

Effects of Excess C on New Grain Formation and Static Recrystallization Behavior at Shear Bands in Cold-Rolled Ultra-Low Carbon Steel Sheets

Effects of Excess C on New Grain Formation and Static Recrystallization Behavior at Shear Bands in Cold-Rolled Ultra-Low Carbon Steel Sheets

Roles of Solute C and Grain Boundary in Strain Aging Behaviour of Fine-grained Ultra-low Carbon Steel Sheets

Roles of Solute C and Grain Boundary in Strain Aging Behaviour of Fine-grained Ultra-low Carbon Steel Sheets

Characterization of 2D and 3D morphology of Al2O3 inclusion in hot rolled ultra-low carbon steel sheets

ABSTRACTIn this work, the two-dimensional (2D) morphology of Al2O3 inclusions with oval and polygon was observed. The non-aqueous electrolysis extraction method was employed for extraction and characterization of the real three-dimensional (3D) morphology and size of inclusions in hot rolled ultra-low carbon steel sheets. The integrated morphology and size of Al2O3 inclusion were obtained through scanning electron microscopy along with an energy-dispersive spectrometer. Compared with 3D morphology and size of Al2O3 inclusions, the 2D morphology and size are hardly accurate to describe the real morphology and size. Additionally, the mechanical property of Al2O3 inclusion and steel matrices were tested by nano-indentation. The real morphology, size and mechanical properties of Al2O3 inclusions will help to establish, develop and validate the damage model or criteria, as well as to improve the quality of rolled productions.

Roles of Solute C and Grain Boundary in Strain Aging Behaviour of Fine-grained Ultra-low Carbon Steel Sheets

The roles of solute C and the grain boundary in the strain aging phenomenon of polycrystalline ferritic steel were investigated using Nb-bearing ULC steel sheets with a relatively low solute C content of 1–3 ppm and ferrite grain sizes of 9.5 µm and 183 µm at aging temperatures from 70 to 400°C. The steels exhibited two definite hardening stages. The 1st hardening stage appeared in both fine- and coarse-grained specimens, in which the increase in YP (ΔYP) became saturated at around 30 MPa. From the apparent activation energy and hardening kinetics, the hardening mechanism was assumed to be dislocation pinning by solute C atoms. The 2nd hardening stage, significantly appeared in fine-grained specimens accompanying a large increase in the Hall-Petch coefficient; ΔYP was quite large, reaching 90 MPa. Fine precipitates were not detected in aged specimens observed by TEM and 3DAP. Segregation of solute C to the grain boundaries and diffusion of Fe atoms in the grain boundaries were proposed as possible mechanisms of this 2nd hardening. Grain-boundary hardening was assumed to be one of the hardening mechanisms in the strain aging in polycrystalline ferritic steel.

Opposite Relationship between Orientation Selection and Texture Memory in the Deformed Electrical Steel Sheets during α→γ→α Transformation

The undesired {111} texture component for the magnetic properties mainly exists in the sheets of electrical steels by the conventional process, whereas the sheets with the non-{111} texture can be obtained by α→γ→α transformation. In this paper, we mainly investigate the opposite relationship between orientation selection and texture memory in the deformed ultra-low carbon steel sheet during α→γ→α transformation annealing. A 0.5 mm thick hot-rolled sheet is directly subjected to transformation. The result shows that the specific transformation textures are not possible to generate in the sheets without deformation. Besides, transformation annealing is conducted on the recrystallized sheets in hydrogen and vacuum, respectively. The near {100} and {110} grains have the growth advantage at the atmosphere/metal interface, and the initial ferrite textures are retained in vacuum. Cold-rolled sheets with different thicknesses are annealed for transformation in vacuum, hydrogen and nitrogen, respectively. The near {100} and {110} textures are still the preferential orientations at the atmosphere/metal interface. When the surface grains have sufficiently large growth advantage, the {111} grains developed by texture memory effect will be annexed. Otherwise, the {111} grains at the center layer of the sheets are hard to be replaced, and they are retained after α→γ→α transformation cycle. The results of deformed sheets annealed with different heating rates in hydrogen show that the growth of initial recrystallization grains has a great effect on variant selection.

Effect of the r-value change on the forming limit analysis for a ultra-low carbon steel sheet

The effect of the r-value change on the accuracy of the forming limits predicted using the Marciniak-Kuczynski-type (M-K) forming limit analysis for a cold rolled interstitial- free (IF) steel sheet is investigated. Uniaxial tensile tests with a digital image correlation system are used to measure the r-value change. A tube subjected to tension-expansion loading under linear paths in the first quadrant of the stress space are performed to measure the multiaxial plastic deformation behavior and the forming limits of the test material. The observed differential hardening (DH) behavior is approximated by changing the material parameters of the Yld2000-2d yield function (Barlat et al, 2003) as functions of the reference plastic strain. The M-K analyses are performed using the r-value change and r-value constant DH model. It is concluded that the DH model considering the r-value change leads to the more accurate predicted forming limits.

Nb添加極低炭素鋼板の再結晶核の成長挙動

Nb添加極低炭素鋼板の再結晶核の成長挙動

Fracture Prediction for Ultralow Carbon Steel Sheet Subjected to Draw-Bending Using Forming Limit Stress Criterion

A fracture criterion for sheet metals subjected to draw-bending is investigated using the concept of forming limit stress criterion. The test material used is a 1.0-mm-thick ultralow carbon steel sheet. Draw-bending experiment of a wide specimen is performed for a die profile radius of 4mm. A specimen undergoes bending-unbending under tension when passing over the die profile radius. The drawing speed was set to 5mm/s. The magnitude of true stress when a specimen fractured has been precisely determined from the measured data of a drawing force and the cross sectional area of the draw-bent specimen after fracture. Moreover, multiaxial tube expansion tests of the test material are performed to measure the forming limit stress of the test material under plane strain tension. It is found that the is larger than by approximately 10 %. Therefore, it is concluded that the forming limit stress criterion is effective as a fracture criterion for a mild steel sheet subjected to draw-bending.

Measurement and Analysis of the Differential Hardening of Ultralow Carbon Steel Sheets

Multiaxial tube expansion tests were performed to precisely measure the work hardening behavior of mild steel sheets with different r-values for a range of strain from initial yield to fracture. The testing machine is capable of applying an arbitrary linear stress path to a tubular specimen using an electrical, closed-loop servo-control system for the axial force and internal pressure applied to the tubular specimen. Tubular specimens with an inner diameter of 44.6 mm were fabricated from the as-received sheet sample by roller bending and laser welding. Nine linear stress paths, σx(rolling direction) :σy (transverse direction) =1:0, 4:1, 2:1, 4:3, 1:1, 3:4, 1:2, 1:4, and 0:1, in the first quadrant of the principal stress space were applied to the tubular specimens to measure the contours of plastic work and the directions of the plastic strain rates. It was found that the shapes of the measured work contours changed with increasing plastic work, or equivalently with increasing the reference plastic strain ; the test materials exhibited differential hardening (DH). The general trend of the DH appeared to be affected by the average r-value, or equivalently by the texture, of the materials.

Lubrication Effects during Biaxial Stretch Forming of Galvanized Steel Compared to Interstitial-Free Steel

AbstractIn this study, the effect of lubrications in biaxial forming zone was investigated and their performances in stretch forming process were compared. Two different ultra low carbon steel sheets which are extensively used in the industries, interstitial-free steel and galvanized steel, were tested. Three different lubrications were tested and changes in the right hand of the forming limit diagrams of the materials with different lubricants were determined by using out-of-plane formability test as well as limiting dome heights were obtained for each lubricant. Moreover, strain distributions of samples, which were stretched with dry and different lubricants, were evaluated experimentally and the effectiveness of lubricants is compared. As a result, it was observed that deep-drawing oil has the highest performance among all lubricants. Furthermore, galvanized steel has lower formability under all conditions when compared to interstitial-free steel.

Development of formable cold rolled steel sheets for automotive use

Cold rolled steel sheets with excellent formability have been developed by using the RH–degassing process and hot rolling technology. These steel sheets contribute to lower alloying and lower energy consumption, and meet the needs of the automotive industry. Recently, for DDQ/EDDQ products, ultra–low carbon steel sheets with titanium or niobium added have been used as a substitute for decarburized annealing steel sheets. In this paper, the technical background and mechanical properties of new steels will be introduced, based on the recent developments in steel–making and hot rolling technology in Nippon Kokan, Kawasaki.

Tensile flow behavior of ultra low carbon, low carbon and micro alloyed steel sheets for auto application under low to intermediate strain rate

This paper is concerned with tensile characteristics of auto grade low carbon, ultra low carbon and micro alloyed steel sheets under low to intermediate strain rates ranging from 0.0007 to 250s−1. Experimental investigation reveals two important aspects of these steels under intermediate strain rate deformation. Firstly, the yield stress increases with strain rate in all these steels. Of course yield stress increment is higher for low carbon and ultra low carbon steel sheets. Secondly, the strain hardening rate drastically decreases with strain rate for low carbon and ultra low carbon steel sheets, whereas it remains steady for micro alloyed steel sheets. Based on these observations, a constitutive model has been proposed which predicts the strain rate sensitive flow behavior of all these grades within the strain rate range of automotive crash event.

Effectiveness of Stamping Lubricants in Erichsen Test

In the present work, formability of ultra low carbon steel sheets (uncoated interstitial free and galvanized steel sheet) has been characterized and the effectiveness of dry and liquid lubricants on formability of these steel sheets was evaluated by using the standard Erichsen test. Primarily, some important mechanical properties of materials like strain hardening coefficient (n), normal anisotropy (r) were determined using the tensile test and formability of these steel sheets has been correlated with these mechanical properties. Then these steel sheets were subjected to the Erichsen test using four different lubricants. Teflon, PVC, Polyethylene films and mineral oil were used in the experiments. Finite element simulations were done using various friction coefficients. Erichsen tests were conducted at 1 kN blank holder force and 2.4 mm/min punch velocity. Values of the Erichsen index and punch force-displacement curves were determined for each lubricant. PVC film proved to be the most effective lubricant for both materials.

Surface Microstructure Evolution during Phase Transformation on Mn, Al and Si Alloyed Ultra Low Carbon Steel

This paper investigates the surface texture evolution after a short phase transformation annealing in low vacuum on ultra low carbon steel sheets alloyed with high Mn and Al and the cold rolled steel sheets of industrial composition alloyed with silicon. The ultra low carbon steel sheets with high Mn and Al show surface monolayer which has a characteristic surface texture components <100>//ND texture and microstructure with special grain morphology. Contrastingly, the industrial composition alloyed with silicon does not show specific surface texture components inspired by surface energy anisotropy at the surface. The composition depth profiling investigations performed on the all steel sheet surface shows that oxidation characteristics of alloying elements at the metal vapour interface have played a decisive influence on surface texture evolution. Further, transformation annealing in higher vacuum reveals that surface texture can be obtained in an industrial composition alloyed with silicon.

Surface Microstructure and Texture Evolution during Interrupted Annealing in Ultra Low Carbon Steels

The austenite-to-ferrite phase transformation, which is an inherent feature of low-alloyed ultra low carbon steels, has scarcely been investigated to control surface texture and microstructure evolution. This paper investigates the systematic evolution of texture and microstructure at the metal-vapour interface during interrupted annealing in vacuum. Interrupted annealing experiments were carried out on three ultra low carbon steel sheets alloyed with Mn, Al and Si. The texture and microstructures have been investigated by X-ray diffraction and SEM-EBSD techniques. These results reveal a very clear variation in the surface texture components as well as in the surface microstructure after BCC recrystallisation and double  transformation interrupted annealing. The recrystallisation texture consists mainly of a <111>//ND fibre, while the transformation texture at the surface exhibits a <100>// ND fibre in combination with components of the <110> //ND fibre. It has been revealed that the latter specific surface texture was present in a monolayer of outer surface grains which were in direct contact with the vapour atmosphere. This observed phenomenon could be explained by considering the role of surface energy anisotropy occurring during phase transformation annealing.

Change in Yield Strength of Ultra Low Carbon Steels with Cr Addition

The relationship between ferrite grain diameter and upper yield strength was investigated for cold-rolled and annealed ultra low carbon steel sheets containing various chromium contents from 0 to 23%. The upper yield strengths were dropped with annealing temperature and the tangent of the change in the yield strength was decreased with increasing in chromium content. The coefficient in Hall–Petch equation decreased linearly with chromium content. This is attributed to lowering solute carbon content at grain boundaries with precipitation of chromium carbide since the decrease was not due to ferrite region hot-rolling nor to α′ phase precipitation in high chromium steel. While friction stress (σ0) in Hall–Petch equation of air-cooled sheets after annealing showed the minimum at chromium content of 5%, σ0 of the lower chromium content steels increased and became to show linear relationship for chromium content after 150°C aging. This result indicates that σ0 increases with chromium content by solid solution strengthening. The reason why the minimum σ0 is exhibited in the 5% chromium steel air-cooled after annealing is that the amount of decrease in σ0 by retardation of aging during air-cooled process is larger than that of solid-solution strengthening by chromium.

Experimental and numerical studies on the forming behavior of tailor welded steel sheets in biaxial stretch forming

Stretch forming is an important process in making complex stampings for autobody components. In the present work formability of three different types of tailor welded blanks (TWBs) in biaxial stretch forming modes has been studied by conducting limiting dome height (LDH) tests. The TWBs are laser-welded samples of low carbon and ultra low carbon steel sheets with difference in thickness, grade and surface conditions. In TWBs with difference in thickness, the LDH decreases as the thickness ratio increases and the thickness of the thinner side is also crucial. A high thickness ratio causes two major strain peaks on thinner side and fracture takes place due to strain localization at the peak close to the pole. The weld ductility and the extent of difference in properties are the two crucial parameters for formability in TWBs with difference in properties. In both these TWBs, the fracture takes place perpendicular to the weld line and propagates towards the stronger side. Significant weld line movement occurs towards the thicker/stronger side in biaxial stretch forming. The maximum weld line movement occurs at the pole and it increases with increase in thickness ratio and becomes constant beyond a certain thickness ratio. The peak load required to deform the TWB specimens is less compared to the corresponding parent sheets. In case of TWBs with difference in thickness, as the thickness ratio increases, the peak load reduces due to decreasing punch-blank contact area.