High Strength Steel Sheets Research Articles

Experimental Investigation of Springback in Forming of High Strength Steel Using Split-Ring Test

Light weight and high strength steel has been used as a substitute for conventional steel in sheet metal forming operations. High strength steel is less formable than conventional steel. A thorough understanding of the factors limiting the formability of high strength steel is important from engineering as well as economic viewpoint. In this work three different materials, galvanic coated steel, EN42J steel and stainless steel (Grade 304) are taken for analysis. The influence of blank holding force on the formation of cups is investigated. The springback behavior of high strength steel sheet is investigated by conducting split ring tests.

Transformation of localized necking of strain space into stress space for advanced high strength steel sheet

Normally, Forming Limit Curves (FLCs) can’t explain for shear fracture better than Damage Curve, this article aims to show the experimental of Forming Limit Curve (FLC) for Advanced High Strength Steel (AHSS) sheets grade JAC780Y with the Nakazima forming test and tensile tests of different sample geometries. From these results, the Forming Limit Curve (strain space) was transformed to damage curve (stress space) between plastic strain and stress triaxiality. Therefore, Stress space transformed using by Hill-48 and von-Mises yield function. This article shows that two of these yield criterions can use in the transformation.

Microstructures and mechanical properties of the welded AA5182/HC340LA joint by magnetic pulse welding

Magnetic pulse welding (MPW) is an innovative joining technology for dissimilar materials. The MPW with high joining qualities could be realized between high strength steel and 5× aluminum alloys. In this work, the MPW joints between aluminum alloy (AA 5182) sheet and high-strength low alloy steel (HC340LA) sheet were prepared under various discharge energies. The mechanical properties and microstructure observations were employed to evaluate the joint performance. Results showed that a lower discharge voltage tended to cause joints with poor quality. When the discharge energy increased, the failure position gradually changed from the weld zone to the weaker base metal (Al) during quasi-static tensile tests. In addition, the effect of discharge energies on microstructure evolution in the intermediate layer was revealed by high-resolution transmission electron microscopy (HRTEM) and scanning electron microscope (SEM). Atomic-scale diffusion was found to be the mainly welding mechanism of the Al/Fe joint. The intermetallic compound (IMC) FeAl2 and ultra-fine grains (UFG) were found in the intermediate layer under high discharge energy.

Anisotropic fracture of advanced high strength steel sheets: Experiment and theory

Sheet metals such as advanced high strength steels often exhibit varying degrees of anisotropy due to the existence of preferred orientation of their grain structures from the rolling process. While extensive research has been devoted to characterize and model anisotropic plasticity behavior over the past decades, only recently people began to tackle their anisotropic fracture behavior. The focus of these studies is almost always on the in-plane anisotropy, and ignores out-of-plane fracture behavior. The reasons are two-folds: (1). Sheet metal deformation during a stamping or crash application is mostly thought as in a plane stress state and modeled as such, so there is no need to consider out-of-plane fracture behavior; and (2). It is very challenging to develop experimental techniques to reliably measure out-of-plane fracture strengths for sheet metals with a thickness only about 1 mm. In this paper, we theorize, based on strong empirical evidence, that the fracture strength of a thin sheet metal in out-of-plane shear is inherently weaker than that under in-plane shear, analogous to the interlaminar properties of composite laminates. This weakness is responsible for the so-called “shear fracture” behavior, often observed during the stamping operations of advanced high strength steels when the sheet metal flows around a tight radius. A new double-notched shear specimen with shear plane perpendicular to the thickness direction is carefully designed and tested to accurately measure the out-of-plane shear fracture strength. Simulations are executed to validate the rationality of the out-of-plane shear specimen design. Test results of a DP980 sheet show that the out-of-plane shear fracture strength is indeed about 15% lower than that measured under the in-plane shear condition. A new anisotropic ductile fracture model based on linear transformation of stresses is proposed to account for the out-of-plane fracture strength for AHSS sheets. The model is calibrated with experimental data, and the fracture surface is compared to the isotropic Mohr-Coulomb (M-C) fracture model to illustrate its effectiveness.

Structural topology optimization of a stamping die made from high-strength steel sheet metal based on load mapping

Even though high-strength steel is a favourable material because of its high strength and good formability, this material poses new challenges on the structure of stamping dies owing to potential damages of the die during production. Stamping dies are conventionally produced according to design manuals and standard manufacturing guidelines, where a high safety factor is specified to ensure that the stamping die is heavy and thick. A structural topology optimization method for a stamping die is presented in this study based on finite element simulations of the sheet metal stamping process. The finite element model of the stamping die is first established. Next, the boundary forces acting on the sheet metal are obtained from simulations and these forces are applied to the punch surface by means of load mapping during topology optimization. These forces are equivalent to the interaction between the blank and die. The objective function is to maximize the static stiffness under multi-conditions, which is defined by using the compromise programming approach. The analytic hierarchy process method is used to determine the weight ratio of the body stiffness in various load conditions and conduct topology optimization of the comprehensive objective function. The results show that the weight of the optimal punch is reduced while its performance is improved. More importantly, the reconstructed punch can be manufactured practically.

Advance on friction of stamping forming

Stamping forming is a method of pressure processing that has been widely applied to automotive manufacturing, household electrical appliance production, aviation, and other fields. Stamping friction plays a substantial role in the quality of stamping forming and the service life of the die. However, the problem of friction in stamping forming has not yet been solved due to several critical difficulties. In particular, with the application of hot forming technology and new lightweight materials in stamping forming, such as advanced high-strength steel sheets, aluminum alloy sheets, and carbon fiber materials, stamping friction has become more complex. Therefore, it is of paramount practical significance to study stamping friction. In this paper, state of the art research concerning the mechanism and factors influencing friction in stamping forming is reviewed. Investigations of the friction mechanism are described, starting by introducing the development process of traditional and modern friction theory. The present friction mechanism fails to adequately describe the frictional process occurring in stamping forming due to complex and variable factors. A survey of existing works reveals several gaps in the study of factors that influence stamping forming: the friction model is very fragile; the accuracy is not high because only a few factors are considered; and more importantly, some important factors are often ignored, such as temperature and coating. Therefore, different dynamic friction models with multifactor coupling should be established for different stamping processes and materials to fully reflect the characteristics and mechanisms of stamping friction. The present review is aimed at providing an insight into the shortcomings of the existing research background, and it demonstrates the enormous potential for further investigation and innovation in the field of friction of stamping forming.

Anisotropic fracture forming limit diagram considering non-directionality of the equi-biaxial fracture strain

This paper is concerned with modeling of anisotropic fracture forming limit diagram considering non-directionality of the equi-biaxial fracture strain. A new anisotropic ductile fracture criterion is developed based on the Lou–Huh ductile fracture criterion (Lou et al., 2012). In an attempt to predict the forming severity of advanced high-strength steel (AHSS) sheets, the proposed fracture criterion is converted into a Fracture Forming Limit Diagram (FFLD) and anisotropic fracture locus considering the sheet metal orientation. Tensile tests of the DP980 steel sheet with the thickness of 1.2 mm are conducted using various specimen geometries including pure shear, dog-bone, and flat grooved specimens. With Digital Image Correlation (DIC) method, equivalent plastic strain distribution on the specimen surface is computed until surface crack initiates. The fracture predictability of the proposed fracture criterion is evaluated with the experimental results which cover a wide range of stress states in various loading directions. By comparing fracture strains obtained from the experiments with the ones predicted from the proposed fracture criterion, it is clearly confirmed that the fracture criterion proposed is capable of predicting the equivalent plastic strain at the onset of fracture with great accuracy over a wide range of stress states while keeping non-directionality of the equi-biaxial fracture strain.

Thickened holes edge including compressed rollover for improving tensile fatigue strength of thick sheet

Body-in-white parts are generally punched to make many holes for joining, attachment and reduction in weight. The static strength of high strength steel sheets is almost proportional with the strength of the sheet, whereas the fatigue strength becomes gradually smaller. In addition, onset and progress of fatigue cracks of the punched high strength steel sheets are accelerated around holes due to concentration of stress, particularly for rough fracture surface and sharp burr of the sheared edge. For the purpose of improving fatigue strength of the steel sheet, the edge of the hole was thicken. Thus the fatigue strength was improved as compared to the normal piercing hole. However, in the thickening of the holes edge using punching process, rollover occurs at the top of holes edge and it lead to the occurrence crack due stretch of the sheets during the process. In order to further improve the stiffness and tensile fatigue strength, the shape of holes edge was altered by compressing the rollover of the holes edge of the thickening sheet. In this study, the effect the holes with thickening edge including compressed rollover formed by a punching on the tensile fatigue strength of sheets was investigated. It was found that the thickened and compressed holes edge of thick steel sheet are effective in improving tensile fatigue strengths.

Study of factors affecting Springback in Sheet Metal Forming and Deep Drawing Process

Spring back is the most important phenomenon that affects the accuracy of the sheet metal parts. the important problem because spring back is major forming defect in sheet metal forming using high strength steel sheets. Spring back occurs in metal forming after removal of the load due to elastic recovery of material. It is difficult to develop analytical formula for spring back the formed parts it is highly recommended to use such process parameter, tool geometry that are allow a significantly dimension of the spring back amount of the major defects in sheet metal forming processed. Spring back is influenced by many factors including properties of the sheet materials and processing conditions, Obtaining the desired size, shape depends on the prediction of spring back. By analyzing these parameters, the defects like wrinkling, earing, tearing and spring back is reduced and also we can get the very good quality product.

Shearing of ultra-high strength steel sheets with step punch

The punch with a step was applied to improve the punch life in punching of ultra-high strength steel sheets. The contact pressure in punching with the step was reduced due to the early separation of the sheet under tension. Punching with the step punch was carried out for 7000 times in stroke, whereas chipping in the punch occurred at 3000 times in punching without the step. Then, punching with the step punch was applied to the quenched steel sheet to prevent the occurrence of the delayed cracking of the sheared edge. The delayed cracking occurred, although the residual stress in the sheared edge decreased with increasing the step height. Furthermore, shearing with the step punch was applied to improve the stretch-flangeability of the ultra-high strength steel and high strength steel sheets.

Effect of sheet thickness on facture behavior of resistance spot welding joints in high strength steel sheets and dominant factors for fracture toughness

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out by using 980MPa 0.13%C high strength steel sheets of 1.0mm, 1.6mm and 2.0mm thickness. At the 5mm of nugget diameter full plug fracture were observed in cross tension resistance spot welded joint of 1.0mm, on the other hand partial plug fracture were recognized in cross tension resistance spot welded joint of 1.6mm and interface fracture were indicated in cross tension resistance spot welded joint of 2.0mm. These test results imply that it is necessary to consider the thickness effect in order to prevent deterioration of strength in resistance spot welded joint. In case of interface fracture of cross tension resistance spot welded joint of 2.0mm, circumferential crack initiation due to separation of corona bond arose. The crack opening process without propagation (plateau crack length) was recognized until just before fracture and then the crack propagated to nugget immediately in brittle manner around CTS. Fracture toughness (Mode I stress intensity factor and crack tip opening displacement) at CTS of nugget diameter of 3√t, 3.5√t, 4√t and 5√t in cross tension resistance spot welded joint of 2.0mm thickness were almost the same to the previous study of nugget diameter of 3√t in cross tension resistance spot welded joint of 980MPa 0.13%C 1.6mm thickness. Average fracture facet size of interface fracture and average packet size of nugget (fully martensite) were almost the same with meaningful scatter. These experimental results imply that fracture toughness were almost same independent of nugget diameter, sheet thickness and strength of steel sheet because the nugget of same C contents showed same hardness and same packet size with meaningful scatter as long as brittle chemical elements (P and S) were extremely changed.

Wear resistance evaluation of hard-coatings for sheet blanking die

Stamping of advanced high strength steel (AHSS) sheet blanks lead to elevated level of wear, surface quality issues and consequently production interruptions. In order to address wear-oriented problems in stamping dies several countermeasures have been developed. The aim of current study was set to investigate the wear resistance performance of 8 different hard-coatings applied on a tool steel material AISI D2. Roller-like shaped and coated die samples were subjected to non-reciprocating wear tests against two different AHSS sheet blanks (DP 780, LCE 1000) for 2-4 km long. Wear performances were evaluated by means of specific wear rate, microscopic examination as well as 3-D surface roughness parameters. Results showed slightly higher wear resistance for TiAlN and CrN coated samples. It was also noted that the sheet blank material, and its surface properties affected the wear resistance performance.

Finite element based analysis of two-stage forming for advanced high strength steel part

Advanced high strength (AHS) steel sheets have been increasingly used for producing various automotive structural parts. The components of new generation vehicles mostly exhibit very complex shapes, for which multiple step forming procedures are required in order to achieve the desired geometry. By sheet metal forming of such parts with large deformation, fracture and springback phenomenon are significantly crucial, especially, in case of the AHS steels. In this work, a two-stage forming process of an automotive part made of AHS steel grade 780 was investigated. Initially, the forming limit of steel was experimentally determined according to the Nakajima test. Subsequently, the specific two-step forming process and its corresponding FE simulations were conducted. In the simulations, the forming limit curve (FLC) was used as failure criterion and calculated formabilities of deformed parts were evaluated. Additionally, the Yoshida-Uemori kinematic hardening model and that coupled with the anisotropic Yld89 yield criterion were applied. Then, the springback occurrences of parts formed after each forming step were examined, in which the measured and computed results were compared. It was thus found that the obtained FLC could precisely describe the material formability, while the Yoshida-Uemori model more acceptably predicted the springback angles of AHS parts in the two-stage forming process.

Innovative Development of High Strength Sheet Steel for Automobile Weight Reduction

Innovative Development of High Strength Sheet Steel for Automobile Weight Reduction

Integration process of stamping and welding for DP600 advanced high strength steel sheets

The springback behavior of AHSS (Advanced High Strength Steels) is more serious than ordinary mild steels. Currently, deformations due to springback can be compensated by modifying the tool geometry in the industry, which often consumes for a long time. In addition, the stamping parts usually need welding and assembly, and the welded part also has deformation. Therefore, an integrated process combines stamping and welding is presented in this paper, including stamping, placing welding parts and spot welding a total of 3 processes. This paper takes Ω-shaped parts as the object of study. The dynamic explicit method is adopted in the stamping process, and the static implicit method is used for the analysis of the stress release process. The effect of welding location, thickness of welding plate and width of welding plate on geometrical dimension are considered. Finally, the results of finite element analysis is evaluated by the validation experiment. The result shows that the combination of stamping and welding process can effectively reduce the springback value, and the deformation of the stamping part is less than 1mm. After the stress of the welded part is released, the stress on the inner and outer surface of the Ω-shaped part can be greatly reduced along the thickness direction.

Bendability and failure mechanisms of dual phase steel under air-bending at elevated temperatures

In this study, process parameters such as the bending angle and bending temperature of the metal sheets for pure bending of advanced high strength steel sheet DP980 were investigated using the air-bending method. This investigation focused on bendability and failure mechanisms. Experimental studies were carried out with various bending temperatures and the bending angle parameters to identify effect on springback angle, initial crack, and the evolution of the microstructure. Prior to testing, the DP steel sheets were heattreated at different temperatures (room temperature, 200 °C, 400 °C, and 600 °C) with a heating rate of 2 °C/S and 5 minutes of holding time in an electric furnace. As a result of the experiments performed at different temperature values and bending angle values, it was determined that an increase in the bending angle reduces the size of the springback angle. It was also determined that an increase in air-bending-temperature reduces the amount of springback. Air-bending-temperature also affected the microstructure evolution and slowed the cracking of the bent surface.

Control of rollover of sheared edge in punching of high strength steel sheets

The rollover along the contour line on the separated edge of a punched product is not homogeneous especially when the counter shape of the punched product is locally concave or convex. This variety of rollover is adverse for the following processes such as hole expanding or deep drawing. Therefore, it is necessary to develop a punching technology to assure the magnitude of the rollover homogeneous along the counter line. An improvement of the punch profile in its axial direction as a method to control rollover was adopted and the performance of the punch was tested by the experimental approach with high strength steels.

Strengthening mechanism of weldbonded high strength steel joints

The strengths of weldbonded joints of high strength steel sheets against two load conditions, tensile shearing and peeling are evaluated by experiments. The strengthening mechanism of the weldbonded joint is revealed by numerical calculations. The weldbonded joints are not strengthened drastically in comparison with spot welded joints and adhesive bonded joints against peeling, whereas the weldbonded joints are stronger against tensile shearing. The strengthening mechanism of the weldbonded joints is mutual prevention of the strain concentration on the nugget of the spot welding and the adhesive.

Effect of Strain Rate of High-Strength Sheet Steel on Its Strain Fracture under Uniaxial and Biaxial Loading

Effect of Strain Rate of High-Strength Sheet Steel on Its Strain Fracture under Uniaxial and Biaxial Loading

Effects of Tempering Treatment on Fatigue Strength of Resistance Spot Welded High-strength Steel Sheets

Effects of Tempering Treatment on Fatigue Strength of Resistance Spot Welded High-strength Steel Sheets