Investigation of the thermo-mechanical properties of hot stamping steels

Investigation of the thermo-mechanical properties of hot stamping steels

Journal of Materials Processing Technology 177 (2006) 452–455 Investigation of the thermo-mechanical properties of hot stamping steels M. Merklein 1 ...

333KB Sizes 425 Downloads 311 Views

Journal of Materials Processing Technology 177 (2006) 452–455

Investigation of the thermo-mechanical properties of hot stamping steels M. Merklein 1 , J. Lechler ∗ Department of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstrasse 11, 91058 Erlangen, Germany

Abstract Within the innovative hot forming process for sheet metals, called hot stamping, it is possible to combine forming and quenching in one process step. This affords the opportunity to manufacture components with complex geometric shapes, high strength and a minimum of springback which currently find applications as crash relevant components in the automotive industry. As standard material for hot stamping the quenchenable high strength steel 22MnB5 is commonly used. With regard to the numerical modeling of the process, the knowledge of thermal and thermo-mechanical properties of the material is required. To determine the thermo-mechanical material characteristics, the flow behavior of the steel 22MnB5 in the austenitic state has been investigated by conductive, hot tensile tests with a Gleeble 1500 system dependent on the time–temperature characteristic of the hot stamping process. © 2006 Elsevier B.V. All rights reserved. Keywords: Hot stamping; High strength steels; 22mnB5; Thermo-mechanical properties; Flow behavior

1. Introduction One of the most important challenges for the automotive industry in the upcoming years is to meet the demand of reducing the fuel consumption with a contemporaneously increase of the safety properties. This can be primarily realized by reducing the weight of body in white components by using thinner materials with higher strength. Therefore more high and ultra-high strength steels are increasingly used in the automotive industry, due to their improved forming properties [1]. For example with the application of the quenchenable ultra-high strength steel 22MnB5, complex crash relevant components like reinforcement parts, front bumpers, etc. with a final strength of about 1500 MPa [2] can be manufactured by simultaneously reducing the materials thickness. But the use of high strength steel usually leads also to some disadvantages like a high impact on the tools, reduced formability and the tendency to springback. To improve the formability of such materials new forming technologies like for example the hot stamping process of quenchenable

∗ Corresponding author at: Department of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstrasse 11, D-91058 Erlangen, Germany. Tel.: +49 9131/85 28309; fax: +49 9131/930142. E-mail addresses: [email protected] (M. Merklein), [email protected] (J. Lechler). 1 Present address: Department of Manufacturing Technology, University of Erlangen-Nuremberg, Egerlandstrasse 11, D-91058 Erlangen, Germany. Tel.: +49 9131/8527961; fax: +49 9131/930142.

0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.03.233

steels had been developed. Hot stamping is a non-isothermal forming process for sheet metals, where forming and quenching takes place in one combined process step. In Fig. 1 the hot stamping process is schematically illustrated. As-delivered the base material 22MnB5 has a ferritic-pearlitic microstructure with a tensile strength of about 600 MPa. After passing through the hot forming process, the component finally exhibits a martensitic microstructure with strength of about 1500 MPa. A pre-condition for the desired final high strength martensitic microstructure, is that the blank must be austenitized first for about 5–10 min in a furnace at about 900–950 ◦ C. After having achieved a homogeneous austenitic microstructure the blank is transferred automatically to the water cooled die within three seconds, where forming and quenching takes place simultaneously. Thereby advantage is taken of the reduced flow stress due the elevated temperature. Through the occurring contact of the hot blank with the cold die, high cooling rates can be realized, and a non diffusional martensitic transformation occurs. With regard to a reliable process modeling, besides the tribological conditions and the mechanical characteristics like the Young modulus, the Poisson ratio, etc. the knowledge of thermal and thermo-mechanical material properties, in dependency of the time–temperature characteristic of the hot forming process, is required. In the following sections experimental results of investigations on the thermo-mechanical flow properties of 22MnB5, according to the hot stamping process requirements and the influencing parameters, will be presented. Due to that hot

M. Merklein, J. Lechler / Journal of Materials Processing Technology 177 (2006) 452–455

453

cooling rate of at least 27 K s−1 is essential for avoiding bainitic transformation and to achieve a full martensitic microstructure for hot stamped parts.

2.2. Experimental set-up and procedure

Fig. 1. Schematic illustration of the direct hot stamping process.

tensile tests have be performed with a modified, servo hydraulic mechanical Gleeble 1500 testing system. 2. Materials and experimental proceeding 2.1. Material characteristics In the automotive industry for direct and indirect hot stamping the quenchenable, ultra-high strength steel 22MnB5 is commonly used. Within the scope of this paper a cold-rolled strip with a material thickness of 1.75 mm produced by Arcelor is used. The boron/manganese micro-alloyed steel, so-called USIBOR 1500P, exhibits a ferritic-pearlitic microstructure with a hardness of 171 HV10, a yield strength of 400 MPa and tensile strength of approximately 600 MPa [3]. Regarding the base material flow properties in dependency of the rolling direction and the strain rate, it should be referred to Merklein et al. [4]. In [4] it is shown, that USIBOR 1500P exhibits no significant sensitivity on the flow behavior with respect to the rolling direction and the deformation velocity as-delivered. In order to prevent the blank from oxidation and decarburization during the heat treatment and the transfer from the furnace to the die, USIBOR 1500P is precoated with an aluminum-based layer. The thickness of the coating usually is between 23 and 32 ␮m according to the supplier. To achieve the required homogeneous austenitic microstructure before quenching, according to [5], a furnace dwell time of at least 3.5 min for a 1.75 mm thick blank is essential. Following the continuous time temperature transformation (TTT) diagram in Fig. 2, a

Fig. 2. TTT diagram of USIBOR 1500P according to Arcelor [3].

According to the microstructural transformations during the hot stamping process, the temperature window for the actual forming process is limited to the austenitic phase of 22MnB5. Due to the martensite start temperature (MS) at approximately 400 ◦ C and the transfer dependent cooling on air, the forming of the blank occurs usually between 850 and 400 ◦ C. Pre-condition for a numerical modeling of the hot forming process, the determination of material thermo-mechanical characteristics in dependency of the influencing parameters like temperature, heating and cooling rate, true strain and strain rate, etc. is essential. With conventional mechanical testing systems this challenge is difficult to meet. Therefore a Gleeble 1500 testing system had been modified to be capable to reproduce the hot stamping process for characterizing the flow properties of USIBOR 1500P in conductive hot tensile test, in dependency of the relevant process parameters. The adjusted servo hydraulic Gleeble 1500 system is schematically displayed in Fig. 3. For receiving more precise force or stress data, respectively, due to the reduced stress values at elevated temperatures, an external, more sensitive 50 kN load cell, with an according self-constructed clamping device, was implemented in the test chamber of the machine. In order to realize cooling rates higher than 27 K s−1 , two compressed-air nozzles have been integrated. As a consequence of these modifications hot tensile tests with time–temperature characteristics, that fit to the hot stamping process, and cooling rates up to 80–90 K s−1 can be achieved. The measuring of the specimen elongations was realized using an optical deformation system, ARAMIS (GOM, Germany). Within this work uniaxial, conductive hot tensile tests have been performed to determine the flow properties of USIBOR 1500P in dependency of rolling direction, temperature (500, 650, 700 and 800 ◦ C) and strain rate (0.01, 0.1 and 1 s−1 ), according to the DIN EN10 002 Part 5 guideline. Therefore the specimens have been imposed to the following thermo-mechanical test program: With regard to the recommendation of the steel supplier the test samples had been heated up to an austenitization temperature of 950 ◦ C not faster than approximately 16 K s−1 . After leaving the specimen at 950 ◦ C for 180 s to guarantee a complete, homogenous austenitization [5], rapid cooling and stabilization at a temperature between 800 and 500 ◦ C for five seconds took place. Afterwards the tensile test was carried out under isothermal conditions. The measurement of the temperature is realized using Ni/Cr–Ni thermocouples spot welded onto the sample at half of the length (compare Fig. 3). The deformation of the specimens were detected using the optical measuring system ARAMIS. The specimen’s geometry follows the recommendation of EN482-2 [6], pictures of the deformation process have been taken with a frequency of 10 Hz. For each investigated

Fig. 3. Schematic sketch of the modified test chamber of the servo hydraulic Gleeble 1500 mechanical system.

454

M. Merklein, J. Lechler / Journal of Materials Processing Technology 177 (2006) 452–455

parameter at least five test runs had been carried out. For the calculation of the flow curves the essential stress and strain data had been received from the 50 kN load cell and the Aramis system, respectively. The final calculation of the true stress strain values followed [7,8].

3. Experimental results 3.1. Influence of the rolling direction on the flow properties in the austenitic state The influence of the rolling direction on the flow behavior of 22MnB5 had been investigated for different temperatures in the austenitic state following the test path mentioned in the section before. For the lower and upper test limit temperatures, 500 and 800 ◦ C, the impact of the rolling direction on flow properties of USIBOR 1500P, is representatively shown in Fig. 4. For both test temperatures an exemplarily flow curve is illustrated for each of the three rolling direction 0◦ , 45◦ and 90◦ , after rapid cooling with a cooling rate of approximately 80 K s−1 and an exemplarily strain rate of 0.1 s−1 . Furthermore for a test temperature of 650 ◦ C the flow curves in dependency of the rolling direction are shown. The good accordance of all curves shows, that the material exhibits no dependency on the rolling direction in the austenitic phase. Further experiments with various strain and cooling rates confirmed this results. Based on this, further tensile tests have been carried out without taking the rolling direction in account. Thus only specimens with an orientation parallel to the rolling direction had been used.

shown in Figs. 5 and 6, the strain rate seems to influence as well the flow properties of 22MnB5 at elevated temperature besides the temperature, and has thus to be considered regarding the characterization of the material’s forming behavior.

3.2. Influence of temperature on the flow properties in the austenitic state

3.3. Influence of the strain rate on the flow properties in the austenitic state

The influence of the temperature on the flow properties of the test material USIBOR 1500P has been investigated for different temperatures and strain rates in the austenitic state after rapid cooling. Fig. 5 shows the temperature-sensitivity of the material. For various temperatures between 500 and 800 ◦ C after rapid cooling, representative true stress–strain curves are displayed for an exemplarily strain rate of 1 s−1 . The flow curve characteristics show, that the temperature has a significant influence on the forming behavior of the quenchenable steel. Increasing the temperature leads to a significant reduction of the flow stress and a decreasing work hardening exponent, resulting in a noticeable decrease of the slope of the true stress–strain curves. For lower strain rates like, e.g., of 0.1 s−1 the material shows the same dependency on temperature, but with additional, simultaneously occurring dynamic annihilation and recovery processes at temperatures above 650 ◦ C (compare Fig. 6) during deformation. This leads to an increasing tendency of the sheet metal to exhibit almost a plane flow curve characteristic after the initial strain hardening with increasing temperature. Due to the time–temperature dependency this effect is more apparent the higher the temperature and the lower the deformation velocity are. According to the results

The dependency of the material’s flow behavior on the strain rate has been investigated at three different strain rates 0.01, 0.1 and 1 s−1 , after rapid cooling in the austenitic phase. Exemplarily for the sensitivity on this influencing parameter, in Fig. 7 the strain hardening function is shown in dependency of the various deformation velocities at a temperature of 650 ◦ C. For each strain rate a representative flow curve is displayed. According to the curve characteristics, it is obvious that the strain rate has an significant impact on the forming behavior of USIBOR 1500P. Increasing the strain rate leads to appreciable increase of the stress level and the slope of the curve as a consequence of an enforced work hardening of the material. Further it could be seen, that with a decreasing testing velocity and thus with increasing deformation time, the flow curves exhibit a tendency to achieve a steady state after the initial strain hardening. This arises in the approach of an almost asymptotic trend of the strain hardening function with progressive elongation. This effect can related to occurring diffusional dependent microstructural recovery processes balancing the strain hardening [9,10]. For higher temperatures up to 800 ◦ C, a comparable material sensitivity on the deformation velocity could be detected [4].

Fig. 4. Dependency on rolling direction of USIBOR 1500P, sheet thickness t0 = 1.75 mm.

Fig. 6. Influence of the temperature on the flow curve properties of USIBOR 1500P, strain rate dε/dt = 0.1 s−1 .

Fig. 5. Influence of the temperature on the flow curve properties of USIBOR 1500P, strain rate dε/dt = 1 s−1 .

M. Merklein, J. Lechler / Journal of Materials Processing Technology 177 (2006) 452–455

455

direction, temperature and strain rate after rapid cooling down from 950 ◦ C to an isothermal test temperature. The results show, that the material exhibits a high sensitivity on temperature and strain rate. For rolling direction no significant influence could be determined. Regarding the generation of a numerical model to describe the material’s flow behavior, the temperature and the strain rate have to be considered as well. In future work the impact of the heating and the cooling rate on the flow behavior will be investigated with regard to a reliable material model for the numerical process design. Additionally an experimental solution will be developed for determination of a fracture criterion according to the significant characteristics of the hot stamping process. Fig. 7. Influence of the strain rate on the flow properties of USIBOR 1500P at 650 ◦ C.

4. Conclusion In this paper the thermo-mechanical flow properties of precoated 22MnB5 had been investigated in dependency of the time–temperature characteristic of the hot stamping process. The data receiving from these tests are necessary regarding the numerical modeling of the material’s forming behavior during the hot stamping process. The results in chapter three show, that for the mathematically description of the material’s flow behavior at elevated temperatures in the austenitic state, the rolling direction has not to be taken in account. In opposite the temperature and the strain rate as well have an influence on the forming behavior of the ultra-high strength steel 22MnB5. Increasing the temperature leads to significant decrease of the flow stress values and the slope of the initial strain hardening. For the sensitivity of the material’s forming properties regarding the strain rate, an increase of the deformation velocity leads to a significant increase of the stress levels and the work hardening. 5. Summary and outlook Within this work the thermo-mechanical flow properties of the quenchenable ultra-high strength steel USIBOR 1500P manufactured by Arcelor was investigated. Hence a servo hydraulic Gleeble 1500 system was modified to be capable to characterize the forming behaviour of 22MnB5 in the austenitic state, following the time–temperature characteristics of the hot stamping process. Therefore conductive hot tensile test have been carried out in dependency of the influencing parameters like rolling

Acknowledgement The authors would like to thank the German Research Foundation DFG for their financial support of the project, which is part of the DFG founded research unit “Principles of sheet metal hot forming of quenchenable steels”. Furthermore the authors would like to thank Arcelor for their support regarding the sufficient supply with USIBOR 1500P for free. References [1] G. Schießl, T. Possehn, T. Heller, S. Sikora, Manufacturing a roof frame from ultra high strength steel materials by hot stamping, in: IDDRG International Deep Drawing Research Group 2004 Conference, Sindelfingen, Germany, 2004. [2] P. Hein, A global approach of the finite elemente simulation of hot stamping, in: Tagungsband SheMet’05, Erlangen, Germany, pp. 763–769. [3] N.N.: USIBOR 1500 precoated, Arcelor, 2003. [4] M. Merklein, J. Lechler, M. Geiger, Characterization of the flow properties of the quenchenable ultra high strength steel 22MnB5, Ann. CIRP 55 (1) (2006). [5] M. Merklein, C. Hoff, J. Lechler, Einflussgr¨oßen im Pressh¨artprozess, in: Tagungsband 4. Chemnitzer Karosseriekolloquium, Chemnitz, Germany, 2005, pp. 137–153. [6] Europ¨aische Norm EN485-2. [7] S. Novotny, Innenhochdruck-Umformen bei erh¨ohter Temperatur, PhD dissertation, University of Erlangen-Nuremberg, 2003. [8] H. Hoffmann, Chr. Vogl, Determination of true stress–strain-curves and anisotropy in tensile test with optical deformation measurement, Ann. Cirp 52 (1) (2003) 217–220. [9] G. Gottstein, Physikalische Grundlagen der Materialkunde, Springer, Berlin, 1998. [10] M. Eriksson, M. Oldenburg, M.C. Somani, L.P. Karjalanien, Testing and evaluation of material data for analysis forming and hardening of boron steel components, Simul. Mater. Sci. Eng. 10 (2002) 277–294.