Abstract Advanced High Strength Steels Research Articles

Recent Progress in Understanding the Nano/Micro-Mechanical Behavior of Austenite in Advanced High Strength Steels

Advanced high strength steels (AHSS) are developed to reduce vehicle weight without sacrificing passenger safety. The newly developed AHSS frequently incorporates the austenite as the intrinsic component with large amount and good stability, which is realized by carefully designed alloying elements and thermo-mechanical processing. To explore the great potential of austenite in enhancing the strain hardening behavior of AHSS, detailed information on the mechanical behavior of single austenite grain is a prerequisite, which can be collected by a small-scale test. The present work reviews the recent progress in understanding the nano/micro-mechanical behavior of austenite in varied AHSS. Three different plasticity modes including dislocation plasticity, martensitic transformation, and deformation twinning can be observed in the austenite grains during small-scale tests, given proper stacking fault energy and crystal orientation. The remaining issues concerned with the nano/micro-mechanical behavior of austenite are discussed. The present review advances the general understanding of the nano/micro-mechanical behavior of austenite grains in AHSS, which may shed light on the precise austenite engineering with the development of new AHSS, realizing the dream of high-performance steels at low cost.

Investigation of Liquid Metal Embrittlement during Resistance Spot Welding of Martensitic Steel with Zn Jet Vapor-Deposited Coating

Advanced high-strength steels protected by zinc coatings have contributed to a reduction in CO2 emissions in the automotive industry. However, the liquid metal embrittlement (LME) of the Fe/Zn couple induced by simultaneously acting stresses and high temperatures during resistance spot welding could be the cause of unexpected failure. We investigated the possible risk of LME in spot-welded martensitic steel with Zn jet vapor-deposited coating and its influence on weld strength. The weld nugget cross-sections were analyzed (optical microscopy, SEM-EDS), and their tensile shear strengths were compared with their uncoated counterparts. LME cracks were observed in all samples meeting the process window (6, 6.5, 7 kA) located at the edge of the sheet/electrode indentation area. The frequency and length of cracks increased with current, and the occurrence of Zn within cracks indicated the LME mechanism. The shear tests showed the Zn-coated sample underwent a decrease in tensile shear strength that was most evident at a welding current of 7 kA (13.2%). However, LME was excluded as a cause of lower strength. The decrease was attributed to the smaller nugget diameter and the thin slit of Zn coating remaining in the weld notch.

Investigation into the dephosphorization of ferromanganese alloys for the production of advanced high-strength steel

Advanced high-strength steels (AHSS) are sophisticated materials being developed by the steel industry to mitigate challenges related to the performance of motor vehicles. To meet the requirements of AHSS, the ferromanganese alloys (FeMn) utilized in the production of the steel are required to contain acceptable levels of unwanted impurities, i.e. P, S, N, H, and C. The focus of the current study was to investigate dephosphorization of ferromanganese to produce a low-P alloy that could be effectively utilized in the production of AHSS. The study involved conducting laboratory-scale testwork to study the efficiency of CaO-based slag systems to dephosphorize FeMn alloys. The addition of Na2O, CaF2, and BaO to MnO-CaO-SiO2 slag was considered. The test work was carried out in a 25 kW induction furnace at temperatures of 1350°C, 1400°C, and 1450°C. The P partition coefficient (Lp) remained small at <1, which is an indication that dephosphorization had not been achieved. The baseline slag, comprising 40%CaO-40%SiO2-20%MnO, reported higher Lpvalues. Addition of Na2O and CaF2 did not show any further benefit. Substituting half of the CaO by BaO, resulted in similar Lpvalues to those of the baseline slag under conditions of 1350°C and 1450°C at 30 minutes. In summary, based on the Lpvalues obtained, the conditions investigated with the CaO-based slags appeared to have been unfavourable for dephosphorization of FeMn alloys, as most of this impurity element remained in the alloy.

Special Issue on Recent Trends in Advanced High-Strength Steels

Advanced high-strength steels play an essential role in many industries and engineering applications because of their excellent combination of mechanical properties important for design, e [...]

High-strength steel components produced by hot metal gas forming

Advanced high-strength steels are promising materials for a variety of applications. They can be used in the automotive industry to reduce the car body weight and thus the fuel consumption as well. Very good mechanical properties can be obtained in martensitic steels by the quenching and partitioning process. Making complex-shaped parts using a single forming step is now possible with hot metal gas forming. Several heat treatment sequences were designed. One involved plain hardening to a temperature below the martensite start temperature. Another sequence was intended to explore the influence of the quenching and partitioning process. In this steel with a carbon content of 0.42%, tensile strengths in excess of 1800 MPa with elongations above 15% can be achieved.

Fracture Characterisation by Butterfly-Tests and Damage Modelling of Advanced High Strength Steels

Advanced High Strength Steels (AHSS) are widely used in today's automotive structures for lightweight design purposes. FE simulation is commonly used for the design of forming processes in automotive industry. Therefore, besides the description of the plastic flow behaviour, also the definition of forming limits in order to efficiently exploit the forming potential of a material is required. AHSS are prone for crack appearances without prior indication by thinning, like exemplary shear fracture on tight radii and edge-fracture, which can not be predicted by conventional Forming Limit Curve (FLC). Stress based damage models are able to do this. However, the parameterisation of such models has not yet been standardised. In this study a butterfly specimen geometry, which was developed at the Institute for Forming Technology and Machines (IFUM), was used for a stress state dependent fracture characterisation. The fracture behaviour of two AHSS, CP800 and DP1000, at varied stress states between pure shear and uniaxial loading was characterised by an experimental-numerical approach. For variation of the stress state, the specimen orientation relative to the force direction of the uniaxial testing machine was orientated at different angles. In this way, the relevant displacement until fracture initiation was determined experimentally. Subsequently, the experimental tests have been numerically reproduced giving information about the strain and stress evolution in the crack impact area of the specimen for the experimentally identified fracture initiation. With the help of this testing procedure, two different stress-based damage models, Modified Mohr-Coulomb (MMC) and CrachFEM, were parameterised and compared.

Study on Microstructure Transformation of High Strength Hot Rolled Steel by Using Finite Element Simulation

Advanced High-strength steels (AHSS) has widely application in automotive due to their high tensile strength and remarkable ductility. These good mechanical performances are strongly influenced by the processing and final microstructure. This paper performed Deformation Dilatometer and finite element simulation to study the effect of hot rolling parameters such as strain, cooling rate, and holding time at constant temperature on the microstructure formation of Nb-V low carbon microalloyed steel grade. It found that increasing deformation degree increased the volume fraction of ferrite, both of deformation dilatometer and finite element simulation give a similar trend of effects of hot rolling parameters on evolution of volume fraction of ferrite. These results give an insight for industrial application.

Liquid Metal Embrittlement during Hot Forming of Hot-Dip Galvanized Advanced High-Strength Unalloyed Steels (AHSS)

Abstract Advanced High-Strength Steels (AHSS) are of interest as, owing to their increased strength, they are able to absorb more energy than conventional steels in a car crash. Thus, thinner metal sheets can be used for car design so that vehicle weight and fuel consumption are reduced. The elements Si, Mn, Al, and B are added in order to achieve the desired AHSS alloy properties. Corrosion protection for the automotive sheet metal is mainly ensured by hot-dip galvanizing. The sheets are further processed by hot forming. Hot-dip galvanized steel sheets made of 20MnB8 were examined after high temperature tensile tests. Metallographic examinations were performed on tensile test specimens which were tested at temperatures between 400 and 800 °C and strain rates between 0.05 and 5 s−1. Liquid metal embrittlement was observed at temperatures above 700 °C. Lower strain rates resulted in a smaller degree of liquid metal embrittlement. In order to avoid the occurrence of liquid metal embrittlement during hot forming of galvanized AHSS sheets, the relevant process parameters need to be further optimized.

Increasing fatigue performance in AHSS thick sheet by surface treatments

Advanced High Strength Steels (AHSS) have been widely applied in the automotive industry as an affordable solution for car lightweighting, mainly in parts subjected to crash requirements. Heavy duty vehicle (HDV) can also benefit from the expertise learned in cars, but parts must be designed considering fatigue resistance, especially on trimmed areas, and stiffness. Mechanical surface treatments, as blasting or shot peening, help increasing fatigue life of AHSS in trimmed areas and will allow weight reduction in HDV through gauge downsizing. The expected decrease in stiffness through thickness reduction can be improved by design changes. However, scarce information about the effect of mechanical surface treatments on AHSS are available. Thus, the aim of this work is to evaluate the increment in fatigue life of two different steel grades (350 MPa, and 500MPa of yield strength) in thick sheet by means of mechanical surface treatment – sandblasting. High Cycle Fatigue [HCF] tests were conducted at alternating load [R=-1]. Residual stresses were measured by an X-ray tensometry prior fatigue tests. Also the surface roughness [Rz] and form is measured using an optical non-contact 3D microscope. On the other hand, the fracture surfaces of the test specimens were observed via scanning electron microscope (SEM) in order to determine the crack initiation points. The evaluation of fatigue life in terms of SN curves is also discussed, analysing how the sandblasting process modifies the surface roughness and introduce compressive residual stresses on the external layer of the material. Both phenomena enhance the fatigue strength of the evaluated steel grades.

High Rate Characterization of Three DP980 Steels

Advanced high strength steels (AHSS) are used extensively in the automotive industry in the ongoing effort to reduce vehicle weight. Their increased strength allows for the reduction of sheet thickness, and thus a reduction in mass, while offering formability and cost advantages when compared to other metal alloys typically considered for lightweight applications. DP980 steels are AHSS being considered for structural energy absorbing components; however, there is a lack of published information on their high rate behaviour. This paper presents the results of an experimental program that characterized three production DP980 steels from three different manufacturers at strain rates of 0.001, 1, 10, 100 and 1,000 s-1. An electro-mechanical frame was used for the quasi-static tests, the 1, 10, and 100 s-1 tests were carried out using a fast hydraulic apparatus and the 1,000 s-1 experiments were carried out using a tensile split Hopkinson bar. The quasi-static hardening response at strains higher than the uniform elongation of about 7% was obtained by using a shear test, thus avoiding the use of inverse modelling techniques. The results indicate that the DP980 steels are moderately rate sensitive, with one of the materials showing higher sensitivity than the others. One of the materials exhibited a yield point phenomenon that appears to affect the behaviour of the material at 100 and 1,000 s-1, however, the reasons for this behaviour remain an open question. The data was fit to modified Johnson-Cook and Cowper-Symonds model to account for rate sensitivity. The results presented in this paper provide a tool for modelling the dynamic behaviour of DP980 steels.

Applying 2<sup>k</sup> Factorial Design to Study on Parameters Affecting Springback of Forming of Advanced High Strength Steel Sheets (AHSS)

Advanced high strength steels (AHSS) are widely used in the automotive industry due to their appropriate strength to weight ratio. This alloy has unique hardening behavior and variable unloading elastic modulus; however, the unavoidable obstacle of AHSS sheet metal forming is springback. The springback is a result of elastic recovery and residual stress. The aim of this study is to determine the proper process parameters enabling the reduction of the springback defects in AHSS forming process. This work was divided into two parts, regarding to the effects of numerical parameters and process parameter on forming AHSS. In this paper, a U-shape forming was used to examine the springback behaviors, such as springback angle, sidewall curl, and thickness, through an experiment. To achieve this purpose, 2k factorial statistical experimental design has been employed to investigate the parameters affecting the springback of forming in AHSS to find out the main effect in the springback reduction focusing on using as a guideline for die design. It showed that the blank holder force is the most influential parameter. The second is the punch radius. However, the blank holder force and punch radius is not simple to adjust in die design, the die radius becomes the important parameter to be used to reduce the springback angle.

Reduction of Induced Central Damage in Cold Extrusion of Dual-Phase Steel DP800 Using Double-Pass Dies

Advanced High Strength Steels (AHSS) are a promising family of materials for applications where a high strength-to-weight ratio is required. Central burst is a typical defect commonly found in parts formed by extrusion and it can be a serious problem for the in-service performance of the extrudate. The finite element method is a very useful tool to predict this type of internal defect. In this work, the software DEFORM-F2 has been used to choose the best configurations of multiple-pass dies, proposed as an alternative to single-pass extrusions in order to minimize the central damage that can lead to central burst in extruded parts of AHSS, particularly, the dual-phase steel DP800. It has been demonstrated that some geometrical configurations in double-pass dies lead to a minimum value of the central damage, much lower than the one obtained in single-pass extrusion. As a general rule, the position of the minimum damage leads to choosing higher values of the contacting length between partial reductions (L) for high die semiangles (α) and to lower values of the reduction in the first pass (RA) for low total reductions (RT). This methodology could be extended to find the best configurations for other outstanding materials.

DESIGN PARAMETERS SELECTION OF SPRINGBACK EFFECT IN AIR-V BENDING USING TAGUCHI APPROACH ON ADVANCE HIGH STRENGTH STEEL-DP590

Advanced High Strength Steels (AHSS) are increasingly utilized especially in automotive industry. However, forming of AHSS is challenging particularly in prediction of springback effect caused by material properties, tools and dies parameters, work material and bending technique factors. An air V-bending process was chosen as an evaluation problem because it showed larger springback effect. This paper presents an optimization to predict the influence of various parameters on springback of sheet metal in air V-bending process using Taguchi method (TM). The experimental study was conducted on DP590 sheets with plate thickness of 1 and 2 mm under different process parameters such as punch radius, die radius, die gap and punch travel. A significant level of springback parameters was further described by using the analysis of variance (ANOVA). It showed that the contribution percentage of each factor to springback was calculated to optimum level and the significant levels of entire factor were observed. The thickness of material, die width, punch travel and punch radius were found to be the most significant factor affecting springback while die radius is insignificant.

Determination of Damage Criterion Using a Hybrid Analysis for Advanced High Strength Steel

Advanced High Strength (AHS) steels have been increasingly applied in the automotive industries due to their distinguished mechanical properties. Microstructures of these steels play an important role and are designed by constituent phases with distinct characteristics. AHS steels exhibit sophisticated damage mechanisms that complicate the prediction of material formability. In this work, Ductile Crack Initiation Locus (DCIL) was developed for describing failure behavior of dual phase steel sheet. A hybrid experimental and numerical analysis was used to determine the DCIL. Tensile tests of various sample geometries were experimentally carried out and crack initiation occurred during forming was identified by the Direct Current Potential Drop (DCPD) method. Then, FE simulations of the corresponding tests were performed to evaluate local stress triaxialities and equivalent plastic strains of the critical area. The damage curves for both crack initiation and localized necking were obtained. Additionally, the von Mises, Hill48 and Yld2000-2d yield criterion were defined in the calculations in order to examine effect of yield model on the resulted curves. To verify applicability of the damage curves, Nakazima test of uniaxial sample was taken into account.

Integration of High Strength Steel (HSS) Elements in Automotive Industry

High Strength Steels (HSS) and Advanced High Strength Steels (AHSS) are most commonly used nowadays in the automotive industry to achieve a weight reduction of the vehicle while assuring that all the safety requirements are fulfilled. These steels are characterized by very high ultimate stresses, reaching up to 1500MPa with yield strengths of up to 1000MPa. It is mainly because of these exceptional mechanical properties that it is becoming very usual to find these steels in sheets. This study presents a design methodology for integrated security sub-modules (constituting the suspension and steering modules) in the car manufacturing industry. The sub-modules are made up of a steel structure and different anchorage elements (rubber-metal or plastic-metal), which undergo separate surface treatments to avoid corrosion. Afterwards, the elements are traditionally joined by means of adhesives and screws. This process involves a great number of stages, low quality union methods and generation of corrosion areas that shorten its service life. The first step in this study has been the construction of a metallic structure in high strength low alloy steel (HSLA), instead of in traditional steel, in order to benefit from its better safety performance and lower weight at affordable cost, moreover the industrial manufacturing process was simulate with F.E.M in order to reduce the cost in forming manufacturing process. The second step, the improvement proposed is based on the use of laser welding as joining method, a high quality way to reduce the number of anchorage points and to increase the resistance of the sub-module. The last step is the surface treatment on the integrated piece in order provide an unaltered coating. Overcoming the currently need of assembly processes, which usually causes serious damages on the protective surface, this treatment will suppose an increase in the response against corrosion of the pieces. Regarding this aspect, the development of an organic treatment with lower required temperatures (max. 100oC) has been proposed, paying particularly attention to the removal of pollutant elements (Cr (VI), Zn and Ni) involved in traditional methods. This methodology provides automotive suppliers an additional added value and cost reduction, allowing them to increase their competitiveness in a sector that faces up to the transition from the traditional supply chain to a strategic value chain.

Pressurization System in Low Pressure Tube Hydroforming

Advanced high strength steels are the group of material with high strength and good formability, because high strength lesser gauge thickness can be used without compromising the function of component. In terms of economic forming process, hydroforming is the manufacturing option which uses a fluid medium to form a component by using high internal pressure. This process gained steep interest in the automotive and aerospace industries because of its many advantages such as part consolidation, good quality of the formed part etc. The main advantage is that the uniform pressure can be transferred to whole projected part at the same time. Low pressure tube hydroforming considered an inexpensive option for forming these advanced high strength steel. This paper investigates the pressurization system used during the low pressure tube hydroforming cycle. It is observed that the usage of ramp pressure cycle during forming the part from low pressure tube hydroforming results in lesser die holding force. Also, the stress, strain and thickness distribution of the part during low pressure tube hydroforming are critically analysed.

Optimisation of Die Radius Geometry in Sheet Metal Stamping

Advanced high strength steels (AHSS) are increasingly utilised in sheet metal stamping in the automotive manufacture. In comparison with conventional steels, AHSS stampings produce higher contact pressures at the interface between the tool-workpiece interface, leading to more severe wear conditions, particularly at the draw die radius. To minimise tool wear using this approach it would be necessary to optimise the shape for a particular combination of circular and high elliptical profiles. This paper presents a methodology to optimise a die radius profile. For this, a specialised software routine is developed and compiled for optimisation of die radius profiles to minimise or achieve uniform contact pressure (wear distribution) using Python computer programming language supported by Abaqus software. A detailed algorithm for the optimisation is explained. A case study based on the algorithm is also discussed.

A Study on Tool Wear of Sheet Metal Stamping Die Using Numerical Method

Advanced high strength steels (AHSS) are increasingly used in sheet metal stamping in the automotive industry. In comparison with conventional steels, advanced high strength steel (AHSS) stampings produce higher contact pressures at the interface between draw die and sheet metal blank, resulting in more severe wear conditions, particularly at the draw die radius. The prediction of tool wear patterns for sheet metal stamping die is a highly challenging task as there are many control parameters involved in the production. This paper presents a numerical simulation methodology to analyse the influences of various control parameters on tool wear patterns of a sheet metal stamping die with different die radius arc profiles. The results of tool wear patterns provide informative guidelines for on-site production.

Experimental Comparison of Fatigue Characterizations between TRIP and DP Steels

Advanced high strength steels are increasingly utilized to realize the lightweight of automotive body for economical and ecological considerations. The fatigue durability of notched components is one of the significant evaluation parameters for reasonable material selection. Strain-controlled fatigue experiments of low-alloy TRIP steel and DP steel with 590MPa grade were performed at room temperature in this study. Experimental results indicate that both fatigue life and cyclic stress amplitude of TRIP steel are superior to those of DP steel at the same strain amplitude. Furthermore, local strain-life models of two steels were determined with linear regression method to predict the fatigue life of notched body structures with finite element method. It can be concluded that TRIP steel can provide more excellent potential than DP steel for the lightweight design of notched automotive structures from the viewpoint of fatigue resistance.

Design of Modern Steels for Automotive Application

Advanced high-strength steels offer a great potential for the further development of automobile bodies-in-white due to their combined mechanical properties of high formability and strength. New types of grades – multi-phase steels, superductile steels and density reduced steels – are under development at ThyssenKrupp Steel with tensile strength levels of up to 1000 MPa in combination with excellent formability for the high demands of cold formed structural automobile components. New forming technologies at increased temperatures – hot forming, semi-hot forming and superplastic forming - enable the processing of complex parts with extreme high strength. ThyssenKrupp Steel identifies potential future steels and technology concepts by technology monitoring and evaluates their potential for future applications in pre-development projects. University research institutions are significantly involved in this essential future oriented challenge. Seminal concepts are being implemented together with automotive manufactures by simultaneous engineering processes with coordinated phases of production and testing.