Press Hardening Research Articles

Microstructural Evaluation and Property Change of 5 Wt Pct Al-Zn Coating on Press Hardening Steel During Austenitization

Microstructural evolution of a 5 wt pct Al-Zn coating on press hardening steel was studied in comparison to hot-dip galvanized (GI) and galvannealed (GA) coatings. The results show that the presence of 5 wt pct Al effectively suppresses oxidation during austenitization; meanwhile, the presence of Fe resulting from galvannealing accelerates oxidation. Alloying with Al or Fe in the coating prior to austenitization reduces the susceptibility to liquid metal embrittlement (LME). The presence of Al in the as-coated Zn coating enhances the corrosion resistance in HCl solution and reduces the cathodic kinetics in NaCl solution. However, for sacrificial protection, the austenitized GI steel outperforms the other austenitized-coated steels. Nevertheless, the 5 wt pct Al-Zn coating exhibits better overall performance including high-temperature oxidation resistance, less LME susceptibility, and cathodic protection.

Numerical investigation of effects on blanks for press hardening process during longitudinal flux heating

With the induction heating technology, it is possible to heat up blanks for the press hardening process in 20 s or less. Furthermore, the dimension of an induction system is small and easy to control in comparison to conventional heating systems. To bring the induction heating technology to warm forming industry it is necessary to analyze the process under the view of induction. This paper investigates the edge- and end-effects of a batch heated blank. The results facilitate the later design of induction heating systems for the batch process.

The Influence of Aluminum Nitrate Pre-Treatment on High Temperature Oxidation Resistance of Dip-Coated Silica Coating on Galvanized Steel

Hot-dip galvanization is one of the common methods for anti-corrosion treatments on steels nowadays. Zinc coating on galvanized steel enhances the corrosion resistance of the steel substrate through barrier protection and sacrificial protection. However, its application on press hardening steel (PHS) is facing some challenges. During the austenitization treatment of PHS, the use of zinc coating results in severe liquid metal induced embrittlement and high temperature oxidation. This present study focused on the effect of aluminum nitrate pre-treatment on the properties of the subsequent silica coating, including the adhesion and high temperature oxidation resistance of the coating. Aluminum nitrate pre-treatment was employed to precipitate aluminum oxide and hydroxide on the substrate in order to provide a homogenous surface and improve the oxidation resistance. The silica coating was synthesized by a sol-gel method from tetraethoxysilane (TEOS) and 3-glicidoxipropyl-trimetoxisilano (GPTMS) precursor solution. The silica coating was applied via dip coating on a commercial hot-dip galvanized steel received from the China Steel Corporation, Kaohsiung, Taiwan, and was subsequently cured to confer high temperature oxidation resistance. The high temperature oxidation resistance was evaluated using the thermal gravimetric analysis (TGA, TA Q50) and the microstructural evolution during austenitization. Experimental results showed that the silica coating with pre-treatment had better high temperature oxidation resistance and hindered the liquid phase evaporation. Therefore, it is evident that the aluminum nitrate pre-treatment had positive effects to suppress high temperature oxidation. The Auger spectroscopy analysis further revealed that the surface of the hot-dip Zn coating became chemically homogeneous after the aluminum nitrate pre-treatment, signifying the necessity of a homogeneous aluminum hydroxide surface layer to enhance the adhesion and oxidation resistance of the silica coating on hot-dip galvanized steel during austenitization. Keywords: Press hardening steel; Hot-dip galvanized steel; Silanes; Sol-Gel; Pre-treatment

The effects of temperature on friction and wear mechanisms during direct press hardening of Al-Si coated ultra-high strength steel

Direct press hardening is a non-isothermal sheet metal forming method which combines forming and heat treatment in a single process. However, due to the high temperatures during the forming phase, tool wear is severe and friction is high. In this paper, hot strip draw tests are utilised to assess the influence of the forming temperature on the coefficient of friction (COF) and active wear mechanisms during sliding of Al-Si coated press hardening steel (PHS) strip in contact with uncoated tools under typical hot forming process conditions.The COF is found to be temperature dependent during initial sliding against a virgin tool surface. Whereas, for 10 consecutive strip draws, COF is only temperature dependent for the first samples over the temperature range from 400 °C to 750 °C. This would be due to the tribolayers which form in the tool-sheet contact during the test series. Conversely, the wear mechanisms active in this temperature range are temperature dependent: at higher temperatures (> 600 °C) an area of severe abrasive wear is found that precedes a thick layer of compaction galling while at lower temperatures, (< 600 °C) adhesive wear is dominant. Furthermore, the results show that particles leading to compaction galling are predominantly generated from the Al-Si coating and their size depends on temperature and are related to the fracture of the Al-Si coating.

Use of the Press Hardening Technology for Treatment of TRIP Steel

Multiphase TRIP steels are able to absorb impact-deformation energy and therefore they are used as safety components in vehicle construction. One of the ways how to produce these precision sheet metal parts is the press hardening technology. Besides specific alloying which in most cases is done with manganese and silicon, the properties of TRIP steels are also determined by the appropriate heat treatment parameters. The key processing parameter is the holding time at the temperature of the bainitic transformation, at which not only the formation of bainite but also the stabilization of retained austenite take place. This holding time is technologically demanding for use in industry. Therefore, the possibility of processing this type of steel by press hardening technology has been experimentally verified. Material-technology modelling was used to build models corresponding to the press hardening temperature profile. These models were tested on low-alloy CMnSi steels. In the experimental program, the effect of the tool temperature in the range from RT to 550°C, as well as the influence of different cooling rates on the structure development and mechanical properties was investigated. Mixed structures from ferritic-martensitic to ferritic-martensitic-bainitic with different volume fraction of retained austenite were obtained. Tensile strengths ranged from 730 to 1300 MPa with elongations from 24 to 9%.

Effect of Pre-strain and High Stresses on the Bainitic Transformation of Manganese-boron Steel 22MnB5

During the last decade, the use of press-hardened components in the automotive industry has grown considerably. The so-called tailored tempering, also known as partial press hardening, employs locally heated tools seeking to obtain bainitic transformations. This leads to (seamless) zones within the formed parts with higher ductility. Due to the intrinsic nature of this process, phase transformations happen under the influence of high loads and in pre-deformed austenite. The austenite pre-strain state and applied stresses affect the kinetics of the bainitic transformation. Moreover, stresses have an additional relevant effect in this process, the so-called transformation plasticity. Linear transformation plasticity models have been successfully used to predict the behavior in the presence of low stresses. Nonetheless, because of the process’s severe conditions, these tend to fail. A strong nonlinearity of the transformation plasticity strain is observed for applied stresses above the austenite yield strength. Using thermomechanical tests on sheet specimens of a manganese-boron steel (22MnB5), widely utilized in the industry, the effect on the bainitic transformation of various degrees of deformation in the range of 0 to 18 pct, applied stresses in the range of 0 to 250 MPa and the transformation plasticity effect are investigated in this work.

Effect of niobium precipitation behavior on microstructure and hydrogen induced cracking of press hardening steel 22MnB5

The effect of different content of Nb on the microstructure, mechanical properties and hydrogen trapping of press hardening steel (PHS) 22MnB5 was investigated. The function of Nb addition on the microstructure was studied by SEM, TEM and XRD, etc. which indicated that the microstructure of the steels containing Nb are finer than that of steel without Nb, what's more, the lath martensite is narrower with the increase of Nb content. Certainly, the tensile strength and elongation of hot-rolled steels, hot stamping steels also were measured. To understand the influence of adsorbed hydrogen on the mechanical property of PHS, Slow Strain Rate Test (SSRT) of smooth specimen was conducted. The results of precipitation behavior and its impact on hydrogen induced cracking manifested that hydrogen embrittlement susceptibility decreased obviously with the addition of Nb content due to reduced hydrogen diffusivity.

Effect of molybdenum addition on hardenability of chromium-boron steels used for press hardening applications

Two laboratory heats of low-Carbon low-alloy steels, one with boron and chromium additions and other with boron, chromium and molybdenum additions were made and cast into pencil ingots. These ingots were hot-rolled to 2 mm sheets suitable for press hardening application in an experimental rolling mill. A detailed study was made on the effect of various alloying elements on the microstructure and mechanical properties of these steels. A significantly higher hardness (~496 VHN), yield strength (~1137 MPa) and ultimate tensile strength (~1673 MPa) with comparable elongation (~6%) could be achieved in the as rolled and water-quenched Mo-Cr-B steel in comparison to Cr-B steel under similar processing condition. Moreover, a reduction of critical cooling rate by10?C/s was observed in this steel to obtain fully martensitic structure. The combined addition of boron, chromium and molybdenum was found to be more effective than that of boron and chromium in enhancing the microstructure and mechanical properties of boron-added steels. Additionally, a few hot compression tests were carried out to simulate the press hardening process of these experimental steels. It was found that one of the press hardening process parameters, viz., prior strain had a greater impact on the transformation behaviour of Cr-B and Mo-Cr-B steels during cooling.

Energy saving by manufacturing technology

There are different ways of saving energy in production engineering and thereby reducing the environmental stress. Here, different possibilities of saving energy by manufacturing technology are presented.On the one hand, this is possible through lightweight components. The production of components for the automotive industry with low weight while maintaining the same mechanical properties helps to reduce the fuel consumption of the vehicles. This can be achieved by means of load-adjusted components. Furthermore, suitable materials can be selected for the respective function of the part. This is in interaction with the selected manufacturing process of the component. Press hardening can be mentioned as an example. Here, the production of complex components with high strength gets only possible by the forming process.On the other hand, energy can be saved directly during the production of the components. This is possible through energy efficient production and begins with the selection of the production process. A huge part of the energy used for the production of a component results from the manufacturing of the raw materials. Consequently, a production process should be selected which produces as little as possible excess material. For example, the production of a screw by forming results in a high utilization of the material. If this part would be produced by machining, there would be relatively much waste of material in form of chips. This in turn would have a negative impact on the environmental stress. However, the production of the forming tools must also be considered. Therefore, this example is mainly valid for large-scale production. This consideration is also important for the sector of additive manufacturing where the entire material has to be melted and therefore a large amount of energy is required. Nevertheless, if advantages with regard to lightweight components are obtained, this increased energy expenditure can be justified.

Microstructure Evolution and Simulation in 22MnB5 Steel during Hot Stamping

Hot stamping components with 1500 MPa ultra-high strength are obtained by press hardening during hot stamping, and the properties depend on the microstructures. It is very important that the microstructure evolution rule is found out during hot stamping process. To characterize the microstructure evolution during hot stamping, a method combining finite element and experiment is carried out. Samples were heated to 950°C and held for 300 second at a induction heating furnace, then taken out from the furnace and stayed in the air at different time (7 s, 11 s, 13 s, 22 s), respectively, finally the specimens were formed and quenched at a die. Microstructural observation as well as surface hardness profiling of formed specimens was performed. And the numerical simulation to predict the austenite transformation into ferrite, pearlite, bainite, and martensite and the volume fraction of each phase during the hot stamping process was made with ABAQUS software. The results show that the ferrite is observed when the specimen stays in the air for 22 s, and the temperature drops to 325°C when the dwell time increases from 7 s to 22 s. The results of numerical simulation and experimental results are in good agreement. So the method finite element can be used to guide the optimization of hot stamping process parameters.

Investigation of masking concepts for influencing the austenitization process during press hardening

One possibility to adjust tailored properties in hot stamping is the application of a masking concept to prevent a complete austenitization. In this study different concepts were investigated in order to use them as a suitable masking. A simulation model of the heating phase was developed for the purpose of predicting the resulting microstructure and hardness. Experimentally determined temperature profiles were used for the numerical model. The numerical results of the temperature profile and hardness were validated by experimental investigations and non-destructive hardness measurements. The simulated hardness is in adequate agreement with the measured hardness.

Coating effects on contact conditions in resistance spot weldability

An experimental apparatus was designed to reproduce a resistance spot welding operation and allow observation via infrared camera. A specific ex situ device was used to measure the variations of electrical contact resistances, versus pressure and temperature, for the electrode-sheet and sheet–sheet interfaces. The high values of the electrical contact resistances associated to the Al-Si coated Press Hardened Steel sheet explains the spark effect observed at these interfaces by infrared thermography. The initiation and the growth of the nugget from the Al-Si coated sheet to the galvanized low carbon thin sheet are also due to these high contact resistances. The different observation methods and measurements concur to explain the nugget formation.

Press hardening of alternative materials: conventional high- strength steels

The increase in strength of new high strength steels (HHS) and advanced high strength steels (AHHS) has led to forming issues, such as high springback, low formability, increase of forming forces and tool wear. These problems increase thecosts of manufacturing and maintaining stamping tools in the automotive industry. The aim of this research was to analyse the advantages of applying the press- hardening process toconventional HSS and AHSS steel to increase their formability and therefore reduce the number of forming steps and productioncosts. With this aim in mind, the press-hardening process was used to manufacture an industrialcomponent using four different automotive steel grades: dual phase (DP),complex phase (CP), transformation- induced plasticity (TRIP) and martensitic (MS) grade. Springback measurements werecarried out, together with an analysis of the obtained final mechanical properties and microstructures. The results showed that the formability of all the materials increased. The mechanical properties of theCP800 and TRIP700 materials were maintained or even improved, whereas those of the MS1200 and HCT980X materials were significantly reduced. Weconclude that press hardening is a suitable manufacturing process forCP800 and TRIP700components.

Influence of Hot Rolling Process on the Microstructure and Mechanical Properties of a Press Hardening Steel

Abstract In this article, microstructure and mechanical properties of a press hardening steel prepared with different rolling parameters are studied, and the effects of the rolling processes are analyzed. The results show that when the finishing rolling temperatures are 870°C and 900°C and the coiling temperature rises from 640°C to 700°C, the strength of the press hardening steel increases and the elongation decreases. The yield strength of the steel prepared with low finishing rolling temperature is high, but the tensile strength has little difference, and the elongation decreases quickly. The microstructures consist of ferrite and pearlite, but as the coiling temperature rises, the amount of the alloy-carbide precipitates in the ferrite, and grain boundary is reduced significantly. The interlamellar spacing of pearlite gradually becomes narrow and the density of dislocation lines in the ferrite increases slightly. The press hardening steel rolled at the finishing temperature of 900°C and coiled at 670°C has excellent balance of mechanical properties. The yield strength, ultimate tensile strength, and elongation are 370 MPa, 598 MPa, and 19 %, respectively.

Press-Hardening Simulation - the next Level of Maturity

Press hardening has been fully established in the automotive industry during the last years. Forming simulation is an important tool to make the whole potential of press hardening available in both, the mass production and further developments. One of the usual simulation packages is AutoForm. It has been known as a reliable software tool for the sheet metal forming application area as well as for the press hardening for years. AutoForm-Thermo Solverplus supports direct and indirect hot forming, which are followed by quenching and cooling processes. It takes into account phase transformation during quenching and thermal distortion after cooling. The recent version AutoFormplus R7 brings further advances in process modeling for hot forming as this release allows users to take into account cooling channels. Based on this, the heat conduction in tools can be calculated. Cooling channel modelling affects press hardening simulation in two ways: it can be used to optimize the cooling channel design in the press hardening tools and it increases the accuracy level of the simulation in general. The resulting benefits coming from these new features will be presented in this paper; based on a B-pillar which was produced with direct press-hardening process.

Modeling of multi-phase microstructures in press hardened components: plastic deformation and fracture in different stress states

Hot stamping or press hardening is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production stage. Within the field of press hardening the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. This paper presents an approach for modeling the mechanical response of mixed microstructures under different stress states. A homogenization method is used to predict the hardening of the material; the strain decomposition provides the possibility of applying a fracture criterion per phase. To validate the modeling approach for different stress states a set of samples with different notch and hole geometries as well as microstructural composition are produced. The combination of a homogenization method and a fracture criterion show good agreement with experimental results. The homogenization method is suitable to predict the hardening of the material with good accuracy. Fracture for different microstructural compositions is well predicted over a range of stress triaxialities relevant for sheet metal applications. It is concluded that the use of a homogenization method combined with a fracture model can be used to predict the mechanical response of mixed microstructures for a range of different stress states.

Microstructural and Mechanical Study of Press Hardening of Thick Boron Steel Sheet

Press hardening has become a staple in the production of automotive safety components, due to the combination of high mechanical properties and form complexity it offers. However, the use of press hardened components has not spread to the truck industry despite the advantages it confers, namely affordable weight reduction without the use of exotic materials, would be extremely attractive for this sector.The main reason for this is that application of press hardened components in trucks implies adapting the process to the manufacture of thick sheet metal. This introduces an additional layer of complexity, mainly due to the thermal gradients inside the material resulting in though-thickness differences in austenitization and cooling, potentially resulting in complex microstructure and gradient of mechanical properties.This work presents a preliminary study on the press hardening of thick boron steel sheet. First of all, the evolution of the sheet metal during austenitization is studied by means of dilatometry tests and by analysing the effect of furnace dwell time on grain size. Afterwards, material cooled using different cooling strategies, and therefore different effective cooling rates, is studied in terms of microstructure and mechanical properties. Initial results from finite element simulation are compared to experimental results, focusing on the phase composition in through thickness direction.Results show that industrial-equivalent cooling conditions do not lead to gradient microstructures, even in extreme scenarios involving asymmetrical cooling.

Process simulation and experimental validation of Hot Metal Gas Forming with new press hardening steels

One field in the work of the Fraunhofer Institute for Machine Tools and Forming Technology IWU in Chemnitz is industry applied research in Hot Metal Gas Forming, combined with press hardening in one process step. In this paper the results of investigations on new press hardening steels from SSAB AB (Docol®1800 Bor and Docol®2000 Bor) are presented. Hot tensile tests recorded by the project partner (University of West Bohemia, Faculty of Mechanical Engineering) were used to create a material model for thermo-mechanical forming simulations. For this purpose the provided raw data were converted into flow curve approximations of the real stress-real strain-curves for both materials and afterwards integrated in a LS-DYNA simulation model of Hot Metal Gas Forming with all relevant boundary conditions and sub-stages. Preliminary experimental tests were carried out using a tool at room temperature to permit evaluation of the forming behaviour of Docol 1800 Bor and Docol 2000 Bor tubes as well as validation of the simulation model. Using this demonstrator geometry (outer diameter 57 mm, tube length 300 mm, wall thickness 1.5 mm), the intention was to perform a series of tests with different furnace temperatures (from 870 °C to 1035 °C), maximum internal pressures (up to 67 MPa) and pressure build-up rates (up to 40 MPa/s) to evaluate the formability of Docol 1800 Bor and Docol 2000 Bor. Selected demonstrator parts produced in that way were subsequently analysed by wall thickness and hardness measurements. The tests were carried out using the completely modernized Dunkes/AP&T HS3-1500 hydroforming press at the Fraunhofer IWU. In summary, creating a consistent simulation model with all relevant sub-stages was successfully established in LS-DYNA. The computation results show a high correlation with the experimental data regarding the thinning behaviour. The Hot Metal Gas Forming of the demonstrator geometry was successfully established as well. Different hardness values could be achieved depending on the furnace temperatures and the investigated material. Hardness up to 620 HV could be measured on the component with a complete martensitic structure.

Microstructure and performance evaluations on Q&amp;P hot stamping parts of several UHSS sheet metals

Hot stamping (press hardening) is widely used to fabricate safety components such as door beams and B pillars with increased strength via quenching. However, parts that are hot-stamped from ultra-high-strength steel (UHSS) have very limited elongation, i.e., low ductility. In the present study, a novel variant of hot stamping technology called quenching-and-partitioning (Q&P) hot stamping was developed. This approach was tested on several UHSS sheet metals, and it was confirmed that this method can be used to overcome the drawbacks associated with conventional hot stamping. The applicability of Q&P hot stamping to each of these steels was also assessed. The part properties and performances of three widely used ultra-high-strength sheet metals, B1500HS, 27SiMn, and TRIP780, were evaluated through tensile testing and microstructural observations. The results demonstrated that the ductility of Q&P hot-stamped sheet metals was notably higher than that of the conventionally hot-stamped parts because Q&P hot stamping gives rise to a dual-phase structure of both martensite and austenite. Further, material tests demonstrated that the Q&P treatment had positive effects on all three selected materials, of which TRIP780 had the best ductility and the highest value of the product of strength and plasticity. Scanning electron microscopy images indicated that the silicon in the steels could limit the formation of cementite and would, therefore, improve the mechanical properties of Q&P hot-stamped products.

Tribological Behavior of Tool Steel Under Press Hardening Conditions Using Simulative Tests

Press hardening is employed in the automotive industry to produce advanced high-strength steel components for safety and structural applications. This hot forming process depends on friction as it controls the deformation of the sheet. However, friction is also associated with wear of the forming tools. Tool wear is a critical issue when it comes to the dimensional accuracy of the produced components and it reduces the service life of the tool. It is therefore desirable to enhance the durability of the tools by studying the influence of high contact pressures, cyclic thermal loading, and repetitive mechanical loading on tool wear. This is difficult to achieve in conventional tribological testing devices. Therefore, the tribological behavior of tool–workpiece material pairs at elevated temperatures was studied in a newly developed experimental setup simulating the conditions prevalent during interaction of the hot sheet with the tool surface. Uncoated 22MnB5 steel and aluminum–silicon (Al–Si)-coated 22MnB5 steel were tested at 750 °C and 920 °C, respectively. It was found that higher loads led to lower and more stable friction coefficients independent of sliding velocity or surface material. The influence of sliding velocity on the coefficient of friction was only marginal. In the case of Al–Si-coated 22MnB5, the friction coefficient was generally higher and unstable due to transfer of Al–Si coating material to the tool. Adhesion was the main wear mechanism in the case of uncoated 22MnB5.