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Research Papers

Influence of the Duration of Transfer From the Furnace to the Press During the Hot Stamping Process

[+] Author and Article Information
Alexandre Blaise

LTN,
UMR CNRS 6607,
La Chantrerie,
BP 50609,
Nantes Cedex 3 44306, France;
ArcelorMittal Global R&D Montataire,
1, Route de Saint Leu,
BP 30109,
Montataire 60761, France
e-mail: Alexandre.Blaise@univ-nantes.fr; Alexandre.Blaise@arcelormittal.com

Brahim Bourouga

LTN,
UMR CNRS 6607,
La Chantrerie,
BP 50609,
Nantes Cedex 3 44306, France

Christine Dessain

ArcelorMittal Global R&D Montataire,
1, Route de Saint Leu,
BP 30109,
Montataire 60761, France

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received January 8, 2014; final manuscript received July 25, 2014; published online November 5, 2014. Assoc. Editor: Samuel Sami.

J. Thermal Sci. Eng. Appl 7(1), 011007 (Mar 01, 2015) (10 pages) Paper No: TSEA-14-1007; doi: 10.1115/1.4028358 History: Received January 08, 2014; Revised July 25, 2014

During the hot stamping process a blank is first heated up 900 °C for 6 min 30 s. Then, the blank is transferred to the press where it is hot stamped. During the transfer, the blank is slowly cooled down by convection and radiation with the ambient air and metallurgical transformations can occur and modify the mechanical properties after hot stamping. Using samples with welded thermocouples, a method is presented to estimate the kinetic of the metallurgical transformations. The results are compared with the metallurgical analysis of samples that are quenched in the water after different transfer times. The influence of the thickness of the blank is also investigated. It is highlighted that using different thicknesses leads to different cooling rates and different resulting microstructures. The influence of the transfer time on the mechanical properties after hot stamping is highlighted with hardness measurements and tensile tests. Finally, a coupled heat transfer/metallurgical model is built to simulate the kinetic of the transformations and a diagram can be plotted that presents the evolution of the metallurgy as a function of the thickness and the transfer time. These results show the high importance of the transfer time from the furnace to the press and a tool is proposed to help the engineers to define a process window.

Copyright © 2015 by ASME
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References

Figures

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Fig. 2

A section of a sample presenting the thermocouple wires welded in the bottom of the hole

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Fig. 1

The Johnson–Mehl–Avrami model compared to the experimental data from dilatometer

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Fig. 4

Temperature measurements during the whole cycle

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Fig. 7

Metallographic analysis of the different samples with a picral + metabisulfite etching

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Fig. 11

Identification of the kinetic of the austenite to pearlite transformation from the inverse method and from the direct computation using splines

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Fig. 12

Comparison between the estimated kinetic from different methods and the metallographic analysis

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Fig. 8

Microstructure as a function of the target transfer duration

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Fig. 9

Released heat from the metallurgical transformations as a function of time

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Fig. 15

Influence of the ferrite/pearlite proportion on the ultimate tensile strength and the ultimate elongation

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Fig. 10

Identification of the heat released by the austenite to ferrite and austenite to pearlite transformations by the inverse method

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Fig. 5

Biot number value as a function of time

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Fig. 6

Metallographic analysis of the different samples with a nital etching

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Fig. 13

Hardness measurements as a function of the transfer duration

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Fig. 14

Stress–strain curves for different transfer times

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Fig. 16

Temperature measurements for the different sample thickness

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Fig. 17

Kinetics of the transformations for the samples with different thickness

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Fig. 18

Microstructure from the samples with different thicknesses (etching nital)

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Fig. 19

The temperature evolution of the samples with different thicknesses from experiments and from the model

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Fig. 20

The comparison between the predicted transformed austenite from the model and the experimental results

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Fig. 21

Evolution of the microstructure and the temperature as a function of the thickness and the time during an air cooling

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