Strength Steel Sheets Research Articles

Influence of Final Heat Treatment Modes on Structure and Properties of High Strength Sheet Steel

Influence of Final Heat Treatment Modes on Structure and Properties of High Strength Sheet Steel

Investigation of tool damage in shear cutting of ultra-high strength sheet steels

Proportions of high- and ultra-high strength steels are constantly increasing in modern car bodies. In the press shop there are uncertainties concerning the application of steels with strength higher than 1000MPa particularly concerning shear cutting processes. To support the steel user regarding a tool material and coating and the wear behavior, long-term shear cutting test had been carried out. The effects of various shear cutting conditions on tool damage were investigated by continuous shear cutting test with an amount of cutting cuts up to 100,000 times. Tested sheet materials were 780, 1’180 and 1’470MPa grade steels. The changed cutting conditions were sheet steel strength, tool material, cutting clearance, and rake angle. Measurements by laser and confocal microscopes were carried out in order to reveal tool damage mechanisms. The features of tool damage were classified as wear, chipping, and plastic deformation. The amount of plastic deformation rises with the increase of sheet steel strength. Not only plastic deformation but also chipping in the tested tool was observed in the cutting test of 1180MPa sheet steel by standard tool steel (DIN 1.2379) after 40’000 cuts. Large plastic deformation in the tested tool was observed in a cutting test of 1470MPa sheet steel by high-speed steel (DIN 1.3343) after 100’000 cuts although no chipping was detected. Under a rake angle of 5°, serious chipping and tool wear were observed especially near the final sheet-tool contact position. Regarding clearance, the amount of plastic tool deformation increased with the decrease of the clearance. This effect of the clearance can be explained by vertical force and by sheet rotation during cutting.

Insights on early recovery kinetics in ferrite - pearlite cold rolled high strength sheet steels

This study presents the results of temperature and composition dependent recovery behavior of ferrite-pearlite high strength steels. Annealing of cold rolled sheets at 300 °C with varying pearlite fractions results in an increase in hardness that scales with the pearlite content. In contrast, annealing at 500 °C results in a decrease in hardness, independent of pearlite content. By coupling complementary techniques such as high resolution nanoindentation mapping and electron microscopy (EBSD and TEM), microstructure-property correlations at the micrometer length scale are established to provide new insights on the driving force for temperature and composition dependent recovery kinetics. Simple recovery models are also discussed to complement the microstructural observations. Collectively, these results offer insights into the role of individual phases in the early recovery of ferrite-pearlite high strength steels that are critical for subsequent processing steps including inter-critical annealing.

Stating Failure Modelling Limitations of High Strength Sheets: Implications to Sheet Metal Forming.

This article discusses the fracture modelling accuracy of strain-driven ductile fracture models when introducing damage of high strength sheet steel. Numerical modelling of well-known fracture mechanical tests was conducted using a failure and damage model to control damage and fracture evolution. A thorough validation of the simulation results was conducted against results from laboratory testing. Such validations show that the damage and failure model is suited for modelling of material failure and fracture evolution of specimens without damage. However, pre-damaged specimens show less correlation as the damage and failure model over-predicts the displacement at crack initiation with an average of 28%. Consequently, the results in this article show the need for an extension of the damage and failure model that accounts for the fracture mechanisms at the crack tip. Such extension would aid in the improvement of fracture mechanical testing procedures and the modelling of high strength sheet metal manufacturing, as several sheet manufacturing processes are defined by material fracture.

Precipitation Behavior of AlN Inclusions in Fe-0.5Al-2.0Mn Alloy Under Continuous Unidirectional Solidification Process

Medium Manganese Transformation Induced Plastic (Mn-TRIP) steels are expected to be a new generation of advanced high strength sheet steels due to their excellent balance between material cost and mechanical properties. During the solidification process, AlN precipitates at the grain boundary, which leads to the serious deterioration of hot ductility. However, the precipitation of AlN in Mn-TRIP steel has not been clear. In this study, the chemical compositions, morphology, size distribution, and the precipitation behavior of AlN inclusion in an Fe-0.5Al-2.0Mn alloy were studied under the continuous unidirectional solidification process. The results show that there are two types of nitride inclusions in the Fe-0.5Al-2.0Mn alloy: AlN inclusion and complex inclusion of Al2O3-AlN. The planar sections of most AlN particles are hexagonal. Based on the thermodynamic calculation, it was found that the content of Al has a large effect on the stability of Al2O3 and AlN. When the content of Al increases, the molten iron can be changed from saturated by Al2O3 to saturated by AlN. During the solidification process, the precipitation of Al2O3 inclusions occurred at the beginning of the solidification process. The precipitation of AlN inclusions occurred when the contents of Al and N exceeded the equilibrium value and grew until the end of the solidification. The precipitation conditions of AlN inclusion in the Fe-0.5Al-2.0Mn alloy during the solidification process were discussed. The precipitation and the amount of precipitate of AlN inclusions depend on the initial contents of Al, N, and O. It was found that the precipitation of AlN inclusions can be controlled by reducing the initial content of N to less than 0.0072 mass%.

Influence of Microstructure Constituents on the Hydrogen‐Induced Mechanical Degradation in Ultra‐High Strength Sheet Steels

Influence of Microstructure Constituents on the Hydrogen‐Induced Mechanical Degradation in Ultra‐High Strength Sheet Steels

Third generation of advanced high strength sheet steels for the automotive sector : A literature review

The modern vehicles demand a better fuel economy, decrease in ozone harming substance outflows, and superior safety requirements led to new developments of steel grades with higher strength and good formability. Third generation of advanced high strength steels are the next stage for the automotive companies in steel sheets development. The principal concept of third generation of AHSS is to reap the mechanical properties benefits from first and second generation of AHSS at cost neither too high nor too low. This literature review summarizes the results achieved in a previous paper of the Third Generation of Advanced High Strength Sheet steels literature published by D. Krizan et al. Where we intend to focus on, the recent developments and future trends of the third generation of advanced high strength sheet steels (3-GEN AHSSs) including quenching and partitioning (Q&P), TRIP bainitic ferrite (TBF), medium manganese, density reduced TRIP (δ-TRIP) and nano steels for the modern automotive industry, with emphasis on their main characteristics, processing, and applications.

Net section tension strength of bolted connections in ultra-high strength sheet steel during and after fire

This paper investigates the net section tension strength of bolted connections in ultra-high strength sheet steel (yield stress greater than 1000 MPa) under various fire scenarios through laboratory testing. The specimens are tested at room (ambient) temperature and elevated temperatures up to 700 °C, and are subjected to air or water cooling from temperatures as high as 1000 °C. The degradations of the tensile strength at elevated temperatures are much more severe than design code predictions. Substantial recovery of the tensile strength is feasible using water cooling if the steel has been exposed to 1000 °C, by almost 90%. Based on finite element analyses incorporating tensile fracture simulation, the paper determines that elongation at fracture is the most relevant parameter of material ductility for a structural steel connection. Tension fractures in very low ductility materials tend to be inclined to the loading direction of the bolted connection. This paper also examines the alternative formulae for determining the net section tension strength of a stagger bolted connection. Their accuracy is verified through the use of a certain geometry, which eliminates any potential errors due to the use of incorrect material tensile strength in the calculations. Cochrane's original formula is confirmed to be significantly more accurate than the simplified version used in the literature and some design codes, for the tested specimens.

Identification of fracture toughness parameters to understand the fracture resistance of advanced high strength sheet steels

The fracture toughness of four advanced high strength steel (AHSS) thin sheets is evaluated through different characterization methodologies, with the aim of identifying the most relevant toughness parameters to describe their fracture resistance. The investigated steels are: a Complex Phase steel, a Dual Phase steel, a Trip-Aided Bainitic Ferritic steel and a Quenching and Partitioning steel. Their crack initiation and propagation resistance is assessed by means of J-integral measurements, essential work of fracture tests and Kahn-type tear tests. The results obtained from the different methodologies are compared and discussed, and the influence of different parameters such as specimen geometry or notch radius is investigated. Crack initiation resistance parameters are shown to be independent of the specimen geometry and the testing method. However, significant differences are found in the crack propagation resistance values. The results show that, when there is a significant energetic contribution from necking during crack propagation, the specific essential work of fracture (we) better describes the overall fracture resistance of thin AHSS sheets than JC. In contrast, energy values obtained from tear tests overestimate the crack propagation resistance and provide a poor estimation of AHSS fracture performance. we is concluded to be the most suitable parameter to describe the global fracture behaviour of AHSS sheets and it is presented as a key property for new material design and optimization.

Methods for the Definition of the Destruction Deformation of High Strength Sheet Steels Depending on the Deformation Rate

The complex of methods necessary to experimentally study the effect of the deformation rate on the high strength sheet steels (as exemplified by DP780, DP980, and BH240 steels) on the increase in their plasticity during uniaxial and biaxial stretching is determined. The deformation rates of steels were measured in a wide range from 10–4 to 17 × 103 s–1. An FP 10/1 HECKERT tensile testing machine, a P6324 hydraulic press with special tools, a Hopkinson–Kolsky machine and a T1226B electro-hydraulic press were used. The deformations of the materials destructed during the biaxial stretching were defined using a control grid on their surfaces. Dependences of the destruction deformations of DP780, DP980, and ВН240 steels vs. the deformation rate and their stress-strain state showing an increase in plasticity of DP780 steel up to 2.9 were determined.

Experimental behaviour and modelling of screwed connections of high strength sheet steels in shear

Abstract Screws are commonly used as connectors in cold-formed steel structures as they can be drilled easily through thin sheet steels, allowing on-site assembly. However, in a connection undergoing shear and bending, while a screw may fail, other screws may be still in the elastic state. Therefore, investigation in the load-deflection response of screw connectors before failure is required for further understanding of complex screwed connections. Experiments and simulation are being carried out at the University of Sydney in order to investigate the behaviour of cold-formed steel connections with screw fasteners in shear. The screw model is built with solid deformable elements containing the screw threads. A plasticity model is applied to the steel sheets to represent different stress states of steel during failure. The model successfully simulates the different behaviour occurring simultaneously in the screwed connection tests: tearing and piling up of the steel sheets, end plate failure, tilting and fracture of the screws.

Developing smart multi-sensor monitoring for tool wear in stamping process

Tool wear and galling are of significant concern in the automotive stamping industry, due to the increase in use of higher strength sheet steels in automotive structures and reduced lubrication during stamping production. There are many methods explored in the literature and applied in industry to combat wear in stamping, including new die materials and coatings, alternative lubrication systems and better predictive models. However, smart condition monitoring will continue to be relevant in conjunction with these methods because it can provide further opportunities for production quality and cost improvements, despite the advancements of these other methods. This paper explores the use of multiple sensors and multiple signal processing techniques, aimed at developing a smart multi-sensor method to monitor galling wear. The three main sensors and corresponding signal processing techniques examined are: (i) measurement of punch force signatures analyzed via Principal Component Analysis (PCA); (ii) acoustic emissions signals measured via wideband sensors and examined using time and frequency domain features; (iii) measurement of audio signals in the audible frequency range analyzed via blind signal separation techniques. For all techniques, a semi-industrial stamping test was used to provide realistic production-type conditions, albeit with accelerated wear rates. The relationship between the key outputs from the three sensor/analysis methods were directly compared to a new quantitative measure of galling wear severity. Based on these results, it was observed that a multi-sensor approach for wear condition monitoring provides an opportunity for the development of a smart monitoring tool that can actively track the progression of wear.

Effects of material strength levels and nugget sizes on fatigue behaviour of resistance spot welded steel sheets

ABSTRACT Tensile–shear test blanks were fabricated using high strength and low carbon steel sheets with three different strength levels by a resistance spot welding (RSW) procedure. The nugget sizes were changed by controlling the welding process parameters, resulting in the nugget sizes of 3√t, 4√t and 4.7√t, where it was the sheet thickness. Subsequently, tensile-shear static and fatigue tests were conducted to investigate the effects of strength levels of steels and nugget sizes on the mechanical properties of the welds. The tensile strength increased with increasing strength levels and nugget sizes. However, the fatigue strengths of the welds with the nugget sizes of 4√t and 4.7√t were nearly comparable irrespective of the strength levels of steels. That was because fatigue crack propagation life was dominant in the total fatigue life. In the welds with 3√t nugget size, the steel with higher strength level exhibited lower fatigue strength. It could be attributed to the lower bonding strength along the corona bond in the high-strength steel.

Innovative Development of High Strength Sheet Steel for Automobile Weight Reduction

Innovative Development of High Strength Sheet Steel for Automobile Weight Reduction

Effect of Cooling Rate on the Microstructure and Mechanical Properties of C-Mn-Al-Si-Nb Hot-Rolled TRIP Steels

A novel thermomechanical process to manufacture hot-rolled TRIP steels has been proposed based on dynamic transformation of undercooled austenite (DTUA). The cooling rate between DTUA and isothermal bainitic treatment in the novel process is important. In the present study, effect of this cooling rate on the final microstructures and mechanical properties of a C-Mn-Al-Si-Nb TRIP steel was investigated. The results showed that the volume fractions of acicular ferrite and retained austenite were increased with the increment of cooling rate. As a consequence, higher yield strength and larger total elongation were obtained for the investigated steel with higher cooling rate. In addition, a value of 30.24 GPa% for the product of tensile strength and total elongation was acquired when the cooling rate was 25 K/s. This value has met the standard of the “Third Generation” of advanced high strength sheet steels.

Formability Analysis of Fukui Stretch-Drawing and Square Cup Drawing Using Strain and Stress Based Forming Limit Curves

In this work, the experimental and numerical analyses of Forming Limit Curve (FLC) and Forming Limit Stress Curve (FLSC) for Advanced High Strength Steel (AHSS) sheet, grade JAC780Y, are performed. Initially, the FLC is experimentally determined by means of the Nakazima Stretch forming test. Subsequently, the FLSC of investigated steel was plastically calculated using the experimental FLC data. Different yield criteria including Hill48, and Yld89, are applied to describe plastic flow behavior of the AHS steel and Swift hardening law is taken into account. Hereby, influences of the constitutive yield models on the numerically determined FLSCs are evaluated regarding to those results from the experimental data. The obtained stress based forming limits are affected significantly by the yield criteria. Finally, the experimental and numerical formability analyses of Fukui stretch-drawing and square cup drawing tests are studied through FLC and FLSCs. It is observed that all stress based curves can be used very well to describe material formability of the examined steel compared to the strain based FLC. The strain based FLC depend on forming history and strain paths change. In the other hand, the stress based FLC do not depend on these issue. In this study, it can be concluded that the FLSCs could predict failure more realistically and better than the strain based FLC.

Numerical Simulation on the Hot Stamping Process of an Automobile Protective Beam

As an important method of plastic forming, sheet metal forming was widely applied to automobile manufactory. High strength sheet steels were used in automobile stamping parts recently, but traditional cold stamping method was hard to produce body parts with complex structure and shape as the steel sheet strength is greater than 1000 MPA, and then the hot stamping forming process arose at the historic moment. In this paper, the hot stamping process of automotive protective beam was studied. The numerical simulation and analysis were conducted for the forming technological property, temperature variation of sheet metal and possibility of spring back and wrinkling by using finite element analysis software DYNAFORM. Possible defects could be predicted, which provided a new theoretical method and technical support to reduce the production cost of similar products and shorten the development cycle.

Position deviation of bending point in asymmetric V-die bending process with HSS trapezoid sheet

The prediction of the deviation of bending point in asymmetric V-die bending is extremely needed for practical application. An analytical model and a series of experiments are performed to characterize high strength steel (HSS) sheet metal parts fabricated by asymmetric V-die bending dies and trapezoid sheet. The proposed strategy uses an elementary bending theory to develop a basic model to predict the position deviation of bending point of asymmetric V-die bending process and to test its viability for a trapezoid sheet. Accordingly, a series of experiments obtained good agreement with the calculations. The effects of die radius on deviation in the bending point were experimentally tested to identify the behavior of position deviation in sheet metal bending processes. The experimental results show the important role of contact friction on deviation change; however, its value, i.e., friction coefficient, is always difficult to be determined in calculation. In conclusion, the interactive effects of process parameters are complex and show the important roles of die radius and sheet width on deviation change. The results of this study can be considered when developing process design guidelines for asymmetric processes in HSS sheets.

Material property of metal skin – sheet molding compound laminate structures for the production of lightweight vehicles body frame

Abstract The present research work focusses on both the material characterization and the preliminary test applications for hybrid structures made of high strength steel skin and sheet molding compound (SMC) core, aimed for the production of lightweight automotive body frames. A full material characterization has been carried out in order to determine the properties of the single materials, high strength steel and sheet molding compound. In addition, laminate specimens made of steel skin and SMC core have been manufactured utilizing the compression molding process and the relevant stress-strain behavior have been tested. By means of shear test, the shear strength resistance of the adhesive, utilized for the cohesion between these two materials, have been also characterized. In order to test the proposed hybrid material for the application in the automotive industry, the case of a commercial vehicle B-pillar component have been considered and tested by means of FEM analysis. By simulating the severe load conditions to which the B-pillar has to stand, the numerical analysis implemented in ANSYS will show how the developed material is able to withstand the same load conditions of the original full-steel one while granting a considerable reduction of weight.

Numerical modeling and experimental verification of ductile damage in boron steel hot stamping process

Abstract High Strength Steel (HSS) sheets formed by hot stamping is more and more widely used in automotive industry. Compared to cold formed components, hot stamped parts show significantly improved material formability as well as the desired microstructure and mechanical property distribution. However, the description of the material formability by means of Forming Limit Diagrams (FLDs), which are usually implemented in FEM dedicated codes, appears strongly limiting when applied to hot stamping, as the material behavior is dependent on temperature and strain rate. A general Lemaitre based damage model was developed and implemented in the FE model of the hot stamping process to describe the fracture onset of 22MnB5 sheets. Tensile tests were carried out at elevated temperatures, in a range between 550 oC and 750 oC, at different strain rates, to identify the necessary parameters for the calibration of the Lemaitre damage model. Nakajima-type experiments were conducted and numerically simulated to test the validity of the proposed criterion. The comparison between numerical and experimental results in terms of critical load at crack growth initiation shows that the proposed criterion is able to predict the ductile fracture onset in hot stamping.