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Research Papers

Mechanical Behavior of a Rephosphorized Steel for Car Body Applications: Effects of Temperature, Strain Rate, and Pretreatment

[+] Author and Article Information
Yu Cao1

Department of Materials and Manufacturing Technology, Chalmers University of Technology, Gothenburg SE-41296, Swedenyu.cao@chalmers.se

Johan Ahlström, Birger Karlsson

Department of Materials and Manufacturing Technology, Chalmers University of Technology, Gothenburg SE-41296, Sweden

1

Corresponding author.

J. Eng. Mater. Technol 133(2), 021019 (Mar 22, 2011) (11 pages) doi:10.1115/1.4003491 History: Received September 21, 2010; Revised January 20, 2011; Published March 22, 2011; Online March 22, 2011

Temperature and strain rate effects on the mechanical behavior of commercial rephosphorized, interstitial free steel have been investigated by uniaxial tensile testing, covering temperatures ranging from 60°C to +100°C and strain rates from 1×104s1 to 1×102s1 encompassing most conditions experienced in automotive crash situations. The effect of prestraining to 3.5% with or without successive annealing at 180°C for 30 min has also been evaluated. These treatments were used to simulate pressing of the plates and the paint-bake cycle in the production of car bodies. Yield and ultimate tensile strengths, ductility including uniform and total elongation and area reduction, thermal softening effect at high strain rate, and strain rate sensitivity of stress were determined and discussed in all cases. It was found that the Voce equation [σ=σs(σsσ0)exp(ε/ε0)] can be fitted to the experimental true stress-true plastic strain data with good precision. The parameter values in this equation were evaluated and discussed. Furthermore, temperature and strain rate effects were examined in terms of thermal and athermal components of the flow stresses. Finally, a thermal activation analysis was performed.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Microstructure of the studied steel. Longitudinal section perpendicular to the rolling plane. Rolling direction vertical.

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Figure 2

An example of the fracture area measurement

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Figure 3

Engineering stress-strain curves at different temperatures and strain rates for the (a) AR, (b) PS, and (c) PSA material conditions

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Figure 4

Yield strength, ultimate tensile strength and strain hardening ratio as a function of strain rate and temperature

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Figure 5

Uniform and total elongation as a function of strain rate and temperature

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Figure 6

Strain hardening rate for AR steel. Isothermal true stress-strain curve (cf. Fig. 8) was used for calculation at 1×102 s−1.

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Figure 7

RA at fracture as a function of (a) temperature and (b) strain rate

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Figure 8

(a) Determination of the thermal softening function, Eq. 4 and (b) effect of thermal softening on stress-strain curve for a strain rate of 1×102 s−1

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Figure 9

(a) Variation of Vickers microhardness (300 g) as a function of the distance from the fracture surface. AR condition at largest strain rate investigated, 1×102 s−1 at 20°C. (b) Schematic development of the stress after necking.

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Figure 10

Strain rate sensitivity β for AR material. In the strain rate range of 1×10−4–1×10−1 s−1 at −60°C, the average value is given as only two data points were available. For the largest strain rate 1×102 s−1, the transformed isothermal data were used.

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Figure 11

Examples of the Voce fitting for AR material. The fitting was performed for the nominally recorded stress-strain data, except for the largest strain rate 1×102 s−1, where the transformed isothermal data were used.

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Figure 12

Voce parameters for the different material conditions

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Figure 13

Comparison of the Voce parameter σ0 with yield stress Rp0.2

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Figure 14

(a) Relationship between stress and strain rate compensated temperature Tsrc at different strain levels. For the largest strain rate 1×102 s−1, however, transformed isothermal stress data were used. (b) Variation of athermal stress component σath with plastic strain. Material in AR and PS conditions, respectively.

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Figure 15

Variation of thermal activation volume with Tsrc. Only average values are given for −60°C (AR and PS) and 100°C(PS⊗) in the strain rate range of 1×10−4–1×10−1 s−1.

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