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  • 1.
    Oldenburg, Mats
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Åkerström, Paul
    Bergman, G.
    Salomonsson, Per
    Modelling of microstructure and material response in the press hardening process2008Inngår i: Conference Best in Class Stamping, June 16 - 18, 2008, Olofström, Sweden: [proceedings] / IDDRG, International Deep Drawing Research Group / [ed] Nader Asnafi, Olofström: Industriellt utvecklingscentrum i Olofström AB , 2008, s. 463-474Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The use of ultra-high strength components in automotive structures is rapidly increasing due to strong driving forces to reduce weight in order to minimise fuel consumption. This should also be accomplished with maintained or increased passenger safety. Components manufactured with the press hardening process meet most of the requirements and the market for such products is currently growing very fast. The quenching results in a material with a very high yield and tensile strength falling into the category of martensitic ultra high strength steels. Simulation of the complete press hardening process requires coupled thermo-mechanical transient analysis with the possibility to account for mechanical end thermal contact conditions between the tool and the blank. In addition, one sided or two sided contact areas may occur. Thus, large temperature variations through the thickness of the blank may be present during the process. In an earlier work, Bergman and Oldenburg formulated a thermal shell element with quadratic temperature interpolation through the thickness of the shell while there is a linear interpolation in the plane of the element. This formulation is implemented in the LS-Dyna code and is used in coupled thermomechanical analysis of hot forming processes. The modelling of the press hardening process has been developed in several steps. The model development involves e.g. determination of the flow stress, austenite decomposition modelling, constitutive modelling, experimental studies and evaluation of simulation results. The presented model accounts for the most significant phenomena occurring in the thermo-mechanical press hardening process. The mechanical response and the micro-structure evolution as well as the final material state can be predicted with good accuracy.

  • 2.
    Oldenburg, Mats
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Åkerström, Paul
    Gestamp Hardtech AB.
    Bergman, Greger
    Gestamp Hardtech AB.
    Salomonsson, Per
    Microstructure evolution and mechanical response in the hot stamping process2007Inngår i: 6th European LS-Dyna Users' Conference: Gothenburg, Sweden, ERAB , 2007, s. 135-144Konferansepaper (Fagfellevurdert)
  • 3.
    Oldenburg, Mats
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Åkerström, Paul
    Bergman, Greger
    Gestamp Hardtech AB, Luleå.
    Salomonsson, Per
    Simulation and evaluation of phase transformations and mechanical response in the hot stamping process2007Inngår i: Materials Processing and Design: Modeling, Simulation and Applications; NUMIFORM 2007: Proceedings of the 9th International Conference on Numerical Methods in Industrial Forming Processes / [ed] Jose M.A.Cesar de Sa; Abel D. Santos, Melville, NY: American Institute of Physics (AIP), 2007, Vol. 908, s. 1181-1186Konferansepaper (Fagfellevurdert)
    Abstract [en]

    When producing thin ultra high strength steel components with the hot stamping process it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components. Often the desirable microstructure consists of a mix of martensite and bainite. Therefore, it is of great importance to accurately predict the final microstructure of the component early in the product development process. In this work a model to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths for thin sheet boron steel is used. The decomposition model is based on Kirkaldy's rate equations and later modifications by Li et al. The modified model accounts for the effect from the added boron. The model is implemented as part of a material subroutine in the Finite Element Program LS-DYNA 970. Both the simulated volume fractions of micro-constituents and hardness profiles show good agreement with the corresponding experimental observations. The phase proportions affect both the thermal and the mechanical properties during the process of continuous cooling and deformation of the material. A thermo-elastic-plastic constitutive model including effects from changes in the microstructure as well as transformation plasticity is implemented in the LS-DYNA code. The material model is used in combination with a thermal shell formulation with quadratic temperature interpolation in the thickness direction to simulate the complete process of simultaneous forming and quenching of sheet metal components. The implemented model is used in coupled thermo-mechanical analysis of the hot stamping process and evaluated by comparing the results from hot stamping experiments. The results from simulations such as local thickness variations, hardness distribution and spring-back in the component show good agreement with experimental results.

  • 4.
    Oldenburg, Mats
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Åkerström, Paul
    Bergman, Greger
    Salomonsson, Per
    Simulation of the micro structure evolution in a press hardened component2008Inngår i: Hot Sheet Metal Forming of High-Performance Steel: Proceedings, 1st International Conference, Kassel, Germany, October 22-24, 2008 / [ed] Kurt Steihoff; Mats Oldenburg; Braham Prakash, Bad Harzburg: GRIPS media , 2008, s. 3-13Konferansepaper (Fagfellevurdert)
  • 5.
    Salomonsson, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Experimental and numerical evaluation of heat transfer in the press hardening process2009Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    In recent decades, the use of high- and ultra high strength steels in modern car bodies has increased drastically. This is due to both severe legislation of passenger passive safety and the recent year's effort to reduce fuel consumption to decrease environmental harmful emissions. Many new materials and technologies have been developed to meet these new demands, where press hardening, or also called hot stamping, has been one of the most successful technologies to produce complex components with superior mechanical properties. In the product development process, thermo-mechanical coupled forming simulations are used to predict the final components properties. In order to obtain accurate results, correct models of the physics involved in the simultaneous forming and quenching is needed. The objective of current work is to investigate the heat transfer between the hot blank and the cold tools in the press hardening process. The transfer of heat is the key process that affects the products formability, final geometry, residual stresses and the development of mechanical properties. The two most important contact criteria are investigated in this thesis, heat transfer at a thin contact gap and heat transfer at mechanical contact under an applied contact pressure. An experimental setup to investigate these two contact criteria is developed. It consists of an upper and lower cylindrically shaped tool were the hot blank is quenched between cold tool surfaces. The results from experiments consist of measured temperature histories in the tools. A finite element model of the experiments in combination with an inverse simulation algorithm is used to predict the heat transfer at each contact condition. The inverse technique called improved advance retreat and golden section method is used to solve the ill conditioned inverse heat conduction problem that arise when solving for the heat transfer at the contact interface. The result from inverse simulations in combination with regression analysis is used to develop a general model of the heat transfer coefficient. The outcome is two regression models, one for each contact criterion, were the parameters affecting the heat transfer coefficient are identified. It is found, that a heat transfer coefficient depending on contact pressure or contact gap as well as contacting surface temperatures provide a good match between experimental and simulated temperature response in the tools. The regression model captures the main characteristics of the heat transfer coefficient and has been implemented as a subroutine in the finite element code LS-DYNA.

    Fulltekst (pdf)
    FULLTEXT01
  • 6. Salomonsson, Per
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Investigation of heat transfer in the press hardening process2009Inngår i: Hot sheet metal forming of high-performance steel, CHS2: 2nd international conference, June 15-17 2009, Luleå, Sweden. Proceedings / [ed] Mats Oldenburg; Kurt Steinhoff; Braham Parkash, Auerbach: Verlag Wissenschaftliche Scripten , 2009, s. 239-246Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The use of thin ultra high strength steel automotive passive safety components manufactured with the press hardening process has increased drastically the last two decades. The application and complexity in design has increased and thermo mechanical forming simulation is an important tool to optimize the final component properties early in the product development process. The transfer of heat is a key process that affects formability and the evolution of the mechanical properties in the product. Therefore it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulation code. In this study, an experimental setup and inverse simulation approach is used to predict the interfacial heat transfer coefficient (IHTC) in the press hardening process. Different process conditions such as contact pressure, air gaps in the contact and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation approach, the improved-advance-retreat and golden section method is used to solve the ill-posed inverse heat conduction problem (IHCP). In the finite element analysis, a thermo-mechanical coupled simulation model is used including effects from changes in the microstructure during quenching. The results from simulations give the variations of the heat transfer coefficient in time for best match to experimental results. It is found that the heat transfer coefficient is dependent of the contact pressure and air gap and varies in time. The variation in time is assumed to be the temperature dependence. When heat transfer is under mechanical contact, the variation of the heat transfer coefficient is higher for higher contact pressures compared to lower. If there is heat transfer over a thin gap of air, the heat transfer coefficient also varies but tend to be close to constant for bigger air gaps. The present work will be used as a base in a future model of the heat transfer coefficient influence at different contact conditions in press hardening process.

    Fulltekst (pdf)
    FULLTEXT01
  • 7. Salomonsson, Per
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Åkerström, Paul
    Bergman, Greger
    Experimental and numerical evaluation of the heat transfer coefficient in press hardening2008Inngår i: Hot Sheet Metal Forming of High-Performance Steel: Proceedings, 1st International Conference, Kassel, Germany, October 22-24, 2008 / [ed] Kurt Steihoff; Mats Oldenburg; Braham Prakash, Bad Harzburg: GRIPS media , 2008, s. 267-274Konferansepaper (Fagfellevurdert)
    Abstract [en]

    When producing thin ultra high strength steel components with the press hardening process it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components were it is of great importance to accurately predict the final component properties early in the product development process. The transfer of heat is a key process that affects the evolution of the mechanical properties in the product and it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulations. In this study an experimental setup is developed combined with an elementary inverse simulation approach to predict the interfacial heat transfer coefficient (IHTC) when the hot blank and cold tool are in mechanical contact. Different process conditions such as contact pressure and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation, a thermo-mechanical coupled simulation model is used with a thermo-elastic-plastic constitutive model including effects from changes in the microstructure during quenching. The results from simulations give the variations of the heat transfer coefficient in time for best match to experimental results. It is found that the pressure dependence for the two materials is different and the heat transfer coefficient is varying during quenching. This information together with further testing will be used as a base in a future model of the heat transfer coefficient influence at different conditions in press hardening process.

    Fulltekst (pdf)
    FULLTEXT01
  • 8. Salomonsson, Per
    et al.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Åkerström, Paul
    Gestamp Hardtech AB, Luleå.
    Bergman, Greger
    Gestamp Hardtech AB, Luleå.
    Experimental and numerical evaluation of the heat transfer coefficient in press hardening2009Inngår i: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 80, nr 11, s. 841-845Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When producing thin ultra high strength steel components with the press hardening process, it is essential that the final component achieves desirable material properties. This applies in particular to passive automotive safety components where it is of great importance to accurately predict the final component properties early in the product development process. The transfer of heat is a key process that affects the evolution of the mechanical properties in the product and it is essential that the thermal contact conditions between the blank and tool are properly described in the forming simulations. In this study an experimental setup is developed combined with an elementary inverse simulation approach to predict the interfacial heat transfer coefficient (IHTC) when the hot blank and cold tool are in mechanical contact. Different process conditions such as contact pressure and blank material (22MnB5 and Usibor 1500P) are investigated. In the inverse simulation, a thermo-mechanical coupled simulation model is used with a thermo-elastic-plastic constitutive model including effects from changes in the microstructure during quenching. The results from simulations give the variations of the heat transfer coefficient in time for best match to experimental results. It is found that the pressure dependence for the two materials is different and the heat transfer coefficient is varying during quenching. This information together with further testing will be used as a base in a future model of the heat transfer coefficient influence at different conditions in press hardening process.

  • 9. Åkerström, Paul
    et al.
    Bergman, Greger
    Gestamp R&D.
    Oldenburg, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Material- och solidmekanik.
    Salomonsson, Per
    Utveckling av mikrostruktur och mekanisk respons vid presshärdning2007Inngår i: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, s. 98-Konferansepaper (Annet vitenskapelig)
    Fulltekst (pdf)
    FULLTEXT01
1 - 9 of 9
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