Hot Rolled High Strength Steel Research Articles

A comparison between tensile and hole expansion properties in 800 MPa tensile strength grade hot-rolled steels

Sheared edge formability can play a critical role in cold forming of high-strength hot-rolled steels due to susceptibility to edge cracking. Standardized ISO 16630 hole expansion test is currently the most widely used method to estimate sheared edge formability of steel sheets, however, there has been increasing interest in the industry to reduce the amount of required testing. The aim of this paper was to compare hole expansion performance of hot-rolled strip steels to various uniaxial tensile test results. The five investigated steels were selected to have 3 mm thickness and similar (838±10 MPa) longitudinal tensile strength level while consisting of different microstructures. Tensile properties were measured in 0°, 45°, and 90° angle relative to the rolling direction to investigate anisotropy. Plastic strain ratios were determined by digital image correlation measurement. Local ductility was evaluated with thickness strain measurements (εp_amin) on fracture surfaces of the tensile specimens. To investigate stretch-flangeability, ISO 16630 conforming hole expansion tests were performed. Test holes were produced by punching and by wire electrical discharge machining (W-EDM) to evaluate edge damage tolerance of the materials. The hole expansion ratios (HER) were not observed to correlate with majority of the investigated tensile test results, such as uniform elongation and yield strength. However, a good correlation between HER and planar anisotropy was observed. Planar anisotropy is seen to affect strain localization tendency around the hole. The HER values of neither punched nor W-EDM specimens correlated clearly with εp_amin measurement results. The results show that at a similar strength level, anisotropy is highly influencing the hole expansion performance of hot-rolled steels, and that the HER is not reliably predicted by a single local formability parameter.

Forming limit of hot-rolled high-strength steels considering sheet thickness effect

The forming limit curve (FLC) has been commonly used tool to predict the metal forming cracking in engineering. Whereas, based on the assumption of plane stress state, the formability prediction of thick sheet of hot-rolled high-strength steels is quite different from the experimental measurement. Finite element simulation was used to study the effect of sheet thickness. It is found that extra stress along thickness is introduced between inside and outside the instability region, which become larger increased. Additionally, the visco-plastic self-consistent (VPSC) model and the M-K model are combined to establish the VPSC-MK model for investigating the influence of thickness difference. With reasonable capture of thickness stress, accurate prediction of FLC under different sheet thickness of hot rolled high-strength steel (HR-HSS) is well realized via VPSC-MK model. That is, it is revealed that the sheet thickness effect for HR-HSS mainly origins from normal stress induced during necking process.

Influence of shear cut holes on the fatigue performance of hot-rolled 800 MPa automotive steel grades

Experimental fatigue tests were carried out for two t = 3 mm 800 MPa hot-rolled (HR) high-strength steel grades. These steels have good formability, and it is possible to use shear cutting to enable high productivity. The test campaign included unnotched base metal specimens to evaluate the fatigue performance of HR surface and notched specimens with shear-punched and electrical discharge-machined circular holes to evaluate the effect of cutting process and geometrical shape on the fatigue strength capacity. In this context, the properties of the shear-cut affected zone (SAZ) and resulting high tensile residual stresses, and their effects on the fatigue performance, were evaluated. The fatigue test results were first evaluated based on the nominal stress approach, followed by the applications of local approaches. The mean fatigue strength of the specimens with shear cut holes in the nominal stress system was 125 MPa (with a slope parameter of m = 3, stress range at the net cross section), which is at the comparable level to laser cut holes. Fractography analysis, conducted with scanning electron microscopy (SEM), showed local crack-like defects at the SAZ, and the characterization of local defect feature was utilized in the fatigue strength assessment using a multiparametric notch stress method applying effective stresses obtained by the critical distance approach.

Flexural capacity and local buckling half-wavelength of high strength steel tubular beams under moment gradients: An experimental study

An experimental investigation into the effect of moment gradients on the flexural behaviour of hot-rolled high strength steel square hollow section (SHS) beams is presented in this paper. In total, 20 beam specimens in steel grades S690 and S770, and with cross-sections spanning from Class 2 to Class 4 based on the Eurocode 3 slenderness limits, were tested under three- and four-point bending. In the three-point bending tests, the beam spans were varied to achieve a range of moment gradients; the influence of different stiffening arrangements at the loading point was also considered. Local geometric imperfections were measured by means of 3D laser-scanning prior to testing and digital image correlation (DIC) was adopted to monitor the displacement and strain fields at critical regions and to assess the local buckling half-wavelengths of the test specimens for which a consistent measurement approach was proposed. The measured local buckling half-wavelengths were compared against the elastic local buckling half-wavelengths calculated using the finite strip method. It was observed that while the measured local buckling half-wavelengths remained approximately constant up to first yield, a significant reduction in half-wavelength was observed with increasing moment due to the non-uniform spread of plasticity. The comparisons also revealed that the local buckling half-wavelengths reduced with both the presence of moment gradients and intermediate stiffeners, with a new parameter proposed to quantify the local moment gradient. It was shown from the tests that the specimens subjected to moment gradients, despite the presence of shear, exhibited higher ultimate moment capacities (up to 10.5% for stiffened specimens and 3.4% for unstiffened specimens) than those subjected to uniform moments. This is attributed to the delay in the local buckling of the critical cross-section of beams under moment gradients due to the restraint provided by the less heavily stressed adjacent cross-sections.

Development of Hot-Rolled High-Strength Coil Steel with a Thickness of 4 mm for Load-Lifting Equipment

Development of Hot-Rolled High-Strength Coil Steel with a Thickness of 4 mm for Load-Lifting Equipment

Investigation of Production Technology for Hot-Rolled High-Strength Low-Alloy Automobile Sheet Steels to Improve Properties and Quality Indices at Reduced Cost

Investigation of Production Technology for Hot-Rolled High-Strength Low-Alloy Automobile Sheet Steels to Improve Properties and Quality Indices at Reduced Cost

Effect of Cut-Off Side Stiffness on Stretch Flangeability in High-Strength Steel

In high-strength steel (HSS), shear edge failure due to stretch-flange formability is a serious problem. Shear edge failure is strongly affected by cutting methods. As countermeasures, cut-off punching and double punching of the entire circumference have conventionally been considered. However, it has not been shown whether these processes can be applied to the stretch-flange forming part of the outer circumference of automotive parts. In this study, the effect of the punching allowance and the method of improving the double punching process on the outer circumferences of automotive parts were investigated using high-strength hot-rolled steel sheets. The results of hole expansion tests and observations of shear edge faces and metal flow show that partial double punching reduces the rigidity of the cut-off side and increases the collapse probability. Also, the crack propagation caused by high tensile stress near the upper blade is accelerated. In comparison with double punching of the entire circumference of a 590 MPa or 780 MPa-class high-strength steel sheet, the effect of improving the stretch-flange formability is approximately 1.5 times the hole expansion test value, as indicated by the index, up to the area where the punching exceeds the size of the sheet thickness. However, it is considered that the stiffness of the cut-off side varies depending on the curvature and the thickness of the plane, and the effect of such variations will be confirmed in a wide range of the curvature and the thickness in the future.

Effect of low-temperature annealing on two-different severely cold-rolled steels

The present work is on the effect of low-temperature annealing (or tempering) treatment of two severely cold rolled ultra-high-strength steels. Cold rolling of hot rolled steels significantly enhanced the dislocation density in martensitic and bainitic steels, leading to increase in the YS and UTS of both the steels. Further, these hot rolled high strength steels were cold rolled to about 70% reduction. It is proposed that the cold rolling process might have improved the stability of retained austenite within a range of 15 – 20 mJm–2 through a dynamic interaction process between retained austenite and dislocations during each rolling pass. While the initial passes were associated with the mechanical stability of retained austenite in the steels, the final stage was influenced by the rolling strain and heat generated during cold rolling process. The carbon partitioning at the dislocation sites too had a similar effect on increasing YS and UTS during relatively low-temperature annealing in both the steels. There was a difference in the YS values of two highly deformed steels in post annealing condition, which was due to the intrinsic hardness of the phases present in those steels, nevertheless, both the martensitic and bainitic steels had similar work hardening rates at each annealing temperature. It is also proposed that activities like carbon-dislocation pinning, recovery, and precipitation during annealing in any highly deformed steel may be governed through a similar mechanism, which can work independently irrespective of the phases present in the microstructure.

Effect of Heat Treatment on Phase Transformation Texture and Mechanical Properties in Hot-Rolled High-Strength Steels

Effect of Heat Treatment on Phase Transformation Texture and Mechanical Properties in Hot-Rolled High-Strength Steels

Experimental and FE study on strengthened steel beam-column joints for progressive collapse robustness under column-loss event

Strengthening of connections in steel framed structures is often required to mitigate the risk of progressive collapse initiated by column removal event. Steel framed buildings with simple shear connections between beams and columns are vulnerable to progressive collapse. Therefore, strengthening such structural systems for existing buildings as well as the designing of robust connections in the new structures is vital. This study was undertaken with the prime objective of investigating experimentally and numerically the efficient strengthening schemes for simple shear beam-column connections in order to minimize the risk of collapse in the incidence of sudden column loss. The experimental and numerical study includes testing a control test frame of a one-third scale single story shear-connection sub-assemblage consisting of two bays. A steel frame with an intermediate moment frame (IMF) connection was used as a target frame. Two schemes of strengthening beam-column connections were adopted for upgrading the control frame. The first scheme employed welded double side plates within the connection region, and the second scheme used pretensioned high-strength hot-rolled steel rods within the connection region. All specimens were subjected to dynamic loading in the vertical direction which essentially simulates extreme load cases like an explosion in which columns are removed. The efficacy of the strengthening schemes was evaluated based on the results of experiments and numerical simulation of the tested specimens.

Study on the interface reaction layer of hydrogen reduction hot-rolled high-strength steel hot-dip galvanizing

In this paper, the instability mechanism of the Fe-Al inhibition layer produced by the interfacial reaction in the Al-containing zinc melts on the hydrogen-reduced steel is studied in detail. The results show that the surface morphology of hydrogen-reduced hot-rolled steel at different temperatures is mainly divided into porous and dense states. Due to the different surface activity of the steel after hydrogen reduction, the reaction degree of the galvanized interface is also significantly different. When the Al content is 0.2 wt%, no Fe-Al inhibition layer is detected at the interface. After that, the influence of Al content on the interface inhibition layer was explored. The research results showed that under the same reduction temperature and inhibition time, the higher the Al content, the more obvious the inhibition layer formed at the interface. Finally, by extending the dipping time, it was confirmed that the Fe-Al inhibition layer had undergone a destabilization and cracking transformation process, that is, the zinc solution permeated the Fe-Al inhibition layer, and the iron interface and the liquid zinc were in direct contact, resulting in the formation of the Fe-Zn outburst structure, which was also aggravated the process of breaking the Fe-Al inhibition layer.

Numerical Analysis on Flexural Buckling of Hot-rolled High Strength Steel SHS Columns

Numerical Analysis on Flexural Buckling of Hot-rolled High Strength Steel SHS Columns

Hole Expansion Test and Characterization of High-Strength Hot-Rolled Steel Strip

Hole Expansion Test and Characterization of High-Strength Hot-Rolled Steel Strip

Analysis and Optimization of Springback during the V-bending of Hot-Rolled High Strength Steels (JSH440)

Influences of thickness, width, bend angle, applied load and holding time are evaluated over the springback in hot-rolled, high-tensile strength sheet-metals (JSH-440). Blanks' thickness, width and bend angles are considered the geometric parameters, whereas applied load and holding time are the process parameters considered in the research. Analysis of variance (ANOVA) and sensitivity analysis are applied to evaluate the significance of the factors over the springback magnitude. Analytical models are developed to predict the springback in the sheet metals for the desired geometric and process parameters. Simplified analytical models are also developed for different geometries of sheet metals. Finally, the Genetic Algorithm has also been applied to determine the optimal process parameters for the minimum springback with varying geometries of the sheet metals. Finally, the influence of parameters and optimized results are discussed in detail.

Influence of coiling temperature on microstructure and mechanical properties of a hot-rolled high-strength steel microalloyed with Ti, Mo and V

Influence of coiling temperature on microstructure and mechanical properties of a hot-rolled high-strength steel microalloyed with Ti, Mo and V

Experimental investigation on the creep behavior of G550 cold-formed steel at elevated temperatures

Creep behavior is an important parameter of the material properties of cold-formed steel (CFS) at elevated temperatures and the corresponding investigation is still very limited. In this paper, an experimental study with the creep duration of 4 h was performed for 1.0 mm thick G550 CFS. Forty-one test conditions were selected. The results showed that the creep curves of G550 CFS were distinctly different from that of Q550 and Q690 hot-rolled high-strength steel at elevated temperatures. In addition, the G550 CFS coupon specimens which demonstrated tertiary creep during the experiments would experience creep rupture. For the fire safety of steel structures, a critical temperature of G550 CFS which represented the onset of tertiary creep was given. When the stress ratio was less than 0.7, the critical temperature was lower than the temperature corresponding to the high-temperature yield strength of G550 CFS. Furthermore, a steady-state creep rate expression of G550 CFS was proposed. When the primary creep stage of G550 CFS was insignificant, the proposed expression could be used to give the estimation of the high-temperature creep curves of G550 CFS. Finally, the transient-state test strain of G550 CFS at 500 °C was predicted and compared with previous experimental results. It was shown that the creep strain was an essential part of the total strain of G550 CFS under transient-state conditions, and also a key factor leading to the difference of strain results between the transient-state and steady-state tests.

Microtexture and Rolling Deformation Behavior Analysis of the Formation Mechanism Fe3O4 at the Interface Formed on Hot-Rolled High-Strength Steel

In order to better understand the formation mechanism of tertiary oxide scale in high-strength steel during hot rolling, the microtexture of the oxide layer has been characterized and analyzed by the electron backscatter diffraction (EBSD) method. The results show that the Fe3O4 phase in the oxide layer has a two-phase heterogeneous morphology, Fe3O4 in the oxide layer comprises columnar grains, and Fe3O4 near the substrate comprises spherical grains. As the reduction rate increases, the Fe2O3 layer is gradually wedged into the surface of the Fe3O4 layer. Fe3O4 forms a <110> fiber texture at a reduction rate of 10%. The inner layer of the oxide scale comprises spherical grains, and Fe3O4 is preferentially nucleated and precipitated in the direction of Fe surface grains <110> texture. With the increase in the reduction rate, the {112}<−1−21> directional slip system shows the lowest Schmidt factor value, so the grains with a low Schmidt factor exhibit higher stored strain energy. The formation of the spherical Fe3O4 seam layer close to the steel matrix is the result of the combined effect of the stress state at the matrix and ion diffusion.

Effect of austenitizing and tempering on impact resistance of a hot rolled high strength steel

The aim of this study is to investigate the effect of heat treatments on the impact properties of hot rolled high strength steel and describes the effect of tempering temperature and quenching media on the microstructure, hardness, and impact resistance of plates. In the present study a high strength steel was austenitized at 900 °C with different quenching medium and followed by tempering at 300 °C, 500 °C. After thermal treatments, the values of Charpy impact resistance, hardness, and microscopic structure were evaluated from mechanical and metallographic analysis of metals respectively. The change of mechanical properties and microstructure of the metal with the existence of heat treatment with the ballistic performance of high-strength steel. Experimental results showed that tempering at 500 °C for 2 hours after water quenching medium it provides the best mechanical properties in conjunct on with an improved in microstructure.

Effects of Solid-Solute V on the Phosphatability of Hot-Rolled Steel Sheets

The phosphatability of hot-rolled steel sheets has become increasingly important with application of higher strength and thinner steel sheets to automotive parts. Although vanadium (V) is an alloying element which is often added to high strength hot-rolled steel sheets, the effect of V on phosphatability was still unclear. This study investigated the phosphatability of V-added hot-rolled steel sheets by using V-free, 0.20% V, and 0.47% V steel sheets as test specimens. After phosphate treatment, phosphate crystals covered the whole surface of the V-free and 0.20% V steel sheets, but no phosphate crystals were observed on the surface of the 0.47% V steel sheets. In order to clarify this difference, potentiostatic polarization measurement was carried out in the phosphate treatment solution. Phosphate crystals were found on the surface of the V-free steel sheet after both cathodic and anodic polarization. In contrast, no phosphate crystals were found on the surface of the 0.47% V steel sheet after anodic polarization, but similarly to the V-free steel sheet, phosphate crystals had formed after cathodic polarization. A surface analysis by XPS revealed that V oxides had precipitated on the surface of the 0.47% V steel sheet after anodic polarization. V oxidation reaction involves the generation of hydrogen ions and prevent the steel/solution interfacial pH from rising. This reaction identified as the deterioration mechanism of the phosphatability of V-added hot-rolled steel sheets.

A Study on Potential Strip Shape Control of High‐Strength Steel during Hot Pinch Rolling Process

Hot‐rolled high‐strength steel (HSS) strips always present side‐camber after they are cut into subsections along the rolling direction. To deal with the potential strip shape defect of hot‐rolled HSS, the nonlinear finite element (FE) software is applied to establish a 3D model of pinch roll and strips to simulate the pinch rolling process before the hot rolling coiler. The effects of temperature distribution, roll wear, friction coefficient, initial strip flatness defects, and tension on the strip longitudinal extension distribution are studied along the width direction. The results indicate that the pinch roll wear presents a great influence on both the tendency and quantitative value of the strip longitudinal extension distribution. The maximum unevenness value is 178%, and the maximum differences of longitudinal strain between the center and edge of the strips reach to 51.43%. A newly pinch roll contour is proposed to improve the potential strip shape defects. After the industrial application, the average pinch roll wear thickness decreases from 180 to 120 μm. The hot rolled strips are cut into substrips and the maximum side‐camber value decreases from 12 to 3 mm, which meets the requirement from the auto part manufacturers.