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Title: Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process

Abstract

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. Themore » 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.0.« less

Authors:
;  [1]; ;  [2]
  1. Luleaa University of Technology, Dep of Applied Physics and Mechanical Engineering, SE-97187 Luleaa (Sweden)
  2. Gestamp Hartech AB, SE-97125 Luleaa (Sweden)
Publication Date:
OSTI Identifier:
21057346
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 908; Journal Issue: 1; Conference: NUMIFORM '07: 9. international conference on numerical methods in industrial forming processes, Porto (Portugal), 17-21 Jun 2007; Other Information: DOI: 10.1063/1.2740970; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; AUSTENITE; BORON; COMPUTERIZED SIMULATION; CRYSTAL STRUCTURE; DEFORMATION; FERRITE; FINITE ELEMENT METHOD; HARDNESS; L CODES; MARTENSITE; METALS; MICROSTRUCTURE; PEARLITE; PHASE TRANSFORMATIONS; PLASTICITY; SHEETS; SOLIDS; STEELS; THERMOMECHANICAL TREATMENTS

Citation Formats

Oldenburg, Mats, Salomonsson, Per, Aakerstroem, Paul, and Bergman, Greger. Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process. United States: N. p., 2007. Web. doi:10.1063/1.2740970.
Oldenburg, Mats, Salomonsson, Per, Aakerstroem, Paul, & Bergman, Greger. Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process. United States. doi:10.1063/1.2740970.
Oldenburg, Mats, Salomonsson, Per, Aakerstroem, Paul, and Bergman, Greger. Thu . "Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process". United States. doi:10.1063/1.2740970.
@article{osti_21057346,
title = {Simulation and Evaluation of Phase Transformations and Mechanical Response in the Hot Stamping Process},
author = {Oldenburg, Mats and Salomonsson, Per and Aakerstroem, Paul and Bergman, Greger},
abstractNote = {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.0.},
doi = {10.1063/1.2740970},
journal = {AIP Conference Proceedings},
number = 1,
volume = 908,
place = {United States},
year = {Thu May 17 00:00:00 EDT 2007},
month = {Thu May 17 00:00:00 EDT 2007}
}
  • Hot stamping, which combines forming and quenching in one process, produces high strength steels with limited ductility because the quenching is uncontrolled. A new processing technique has been proposed in which the hot stamping step is followed by a controlled quenching and partitioning process, producing a microstructure containing retained austenite and martensite. To investigate this microstructure, specimens were heated at a rate of 10 deg. C/s to the austenitizing temperature of 900 deg. C, held for 5 min to eliminate thermal gradients, and cooled at a rate of 50 deg. C/s to a quenching temperature of 300 deg. C, whichmore » is between the martensite start temperature and the martensite finish temperatures. The resulting microstructure was examined using optical microscope, scanning electron microscopy and transmission electron microscopy. The material produced contains irregular, fragmented martensite plates, a result of the improved strength of the austenite phase and the constraints imposed by a high dislocation density. - Research Highlights: {yields} A novel heat treatment of advanced high strength steels is proposed. {yields} The processing technique is hot stamping plus quenching and partitioning process. {yields} The material produced contains irregular, fragmented martensite plates. {yields} The reason is strength of austenite phase and constraint of dislocation density.« less
  • The hot stamping of boron steels is widely used to produce ultra high strength automobile components without any spring back. The ultra high strength of final products is attributed to the fully martensitic microstructure that is obtained through the simultaneous forming and quenching of the hot blanks after austenization. In the present study, a mathematical model incorporating both heat transfer and the transformation of austenite is presented. A FORTRAN program based on finite element technique has been developed which permits the temperature distribution and microstructure evolution of high strength steel during hot stamping process. Two empirical diffusion-dependent transformation models undermore » isothermal conditions were employed respectively, and the prediction capability on mechanical properties of the models were compared with the hot stamping experiment of an automobile B-pillar part.« less
  • The hot stamping of quenchable High Strength Steels offers the possibility of weight reduction in structural components maintaining the safety requirements together with enhanced accuracy and formability of sheets. The proper design of this technology requires a deep understanding of material behavior during the entire process chain, in terms of microstructural evolution and mechanical properties at elevated temperatures, in order to perform reliable FE simulations and obtain the desired characteristic on final parts. In particular, the analysis of technical-scientific literature shows that accurate data on material rheological behavior are difficult to find; while the lack of knowledge about anisotropic behaviormore » at elevated temperatures is even more evident. To overcome these difficulties, a new experimental set-up was developed to reproduce the thermo-mechanical conditions of the industrial process and evaluate the influence of temperature and strain rate on 22MnB5 flow curves through uniaxial tensile tests; an optical strain measurement system was utilized to evaluate the effective strain after necking. From the same data, plastic anisotropy evolution was determined by means of a specially developed procedure. The influence of different cooling rates was taken into account and the rheological properties were correlated with microstructural changes occurring during deformation, previously evaluated through a dilatometric analysis performed in the same range of temperatures.« less
  • Mechanical alloying followed by hot-pressing consolidation has been used to produce NbCr{sub 2} intermetallics under different conditions. High-purity Nb and Cr crystalline powders, in the relative (molar) ratio of 13:1, were milled for periods up to 100 h. This powder was vacuum-sintered at temperatures ranging from 1423 to 1573 K for 0.5 h under a pressure of 45 MPa. The phase transformations of the NbCr{sub 2} were investigated by X-ray diffraction and scanning electron microscopy; several different phase transformations were observed. Increasing the milling time up to 100 h transforms into a mixture of C14, Nb, Cr and C15. Themore » experimental results show that new evidence based on X-ray diffraction measurements further establishes the existence of a high-temperature C14 Laves polytype; an intermediate C36 structure for NbCr{sub 2}, reported in the literature, was not detected in this study. The relationship between the various phase transformations, based on the atomic radii and different preparation techniques, is discussed.« less
  • Hot stamping of boron alloyed steel is gaining more and more importance for the production of high strength automotive body parts. Within hot stamping of quenchenable steels the blank is heated up to austenitization temperature, transferred to the tool, formed rapidly and quenched in the cooled tool. To avoid scale formation during the heating process of the blank, the sheet metal can be coated with an aluminium-silicum alloy. The meltimg temperature of this coating is below the austenitization temperature of the base material. This means, that a diffusion process between base material and coating has to take place during heating,more » leading to a higher melting temperature of the coating.In conventional heating devices, like roller hearth furnaces, the diffusion process is reached by relatively low heating rates. New technologies, like induction heating, reach very high heating rates and offer great potentials for the application in hot stamping. Till now it is not proofed, that this technology can be used with aluminum-silicon coated materials. This paper will present the results of comparative heating tests with a conventional furnace and an induction heating device. For different time/temperature-conditions the phase formation within the coating will be described.« less