Experimental Characterization of Heat Transfer Coefficients During Hot Forming Die Quenching of Boron Steel
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ONE of the many challenges facing the automotive industry is to reduce the weight of its vehicles without compromising on passenger safety. A lightweight construction allows an increase in fuel efficiency and a reduction in CO2 emissions,[1–3] and can be realized not only with such materials as polymers, magnesium, aluminum, but also high- and ultra-high-strength steels (UHSS).[4] The advantage of UHSS over aluminum alloys is their lower cost and superior strength-to-weight ratio.[5] A quenchable boron steel such as 22MnB5 can be hardened to a tensile strength of 1600 MPa and a Vickers microhardness above 480 HV.[6,7] Compared to cold-stamped high-strength steel parts, which are limited to 1200 MPa, hardened boron steels can fulfill vehicle crash resistance requirements with thinner cross sections.[6,8] Since the introduction of boron steel components in the Saab 9000 in 1984, UHSS structural parts have been used increasingly in automobile construction—from 3 million parts per year in 1987 to over 100 million parts per year in 2007.[6,8] Crash-resistant parts include A-pillars, B-pillars, door beams, bumper beams, and roof and side rails.[8,9] However, the hard microstructure responsible for the high strength of UHSS is associated with low values of ductility ( 5.4 seconds) are in better agreement with the blank heat flux. The die heat fluxes Fb and Ft start to increase at an earlier time and reach a higher maximum. However, the duration of the corrected heat flux peaks is shorter than the one for the blank heat flux. The corresponding die surface temperatures Tb,s and Tt,s are not shown in Figure 9, but were also modified by the correction of the temperature signal used as input in the IHC analysis. Both the corrected heat fluxes Fd and surface temperatures Td,s are used to
i Pn1 h Dtðiþ1Þ 1 exp T T T T TC;n TC;0 sub;ni sub;ni1 i¼1 s Dt Tsub;n Tsub;n1 ¼ 1 exp s
in which Dt is the data acquisition period (0.1 seconds). The use of the corrected temperature Tsub instead of TTC
METALLURGICAL AND MATERIALS TRANSACTIONS B
½19
calculate the HTC at the blank/die interface according to Eq. [12].
VOLUME 44B, APRIL 2013—339
Fig. 8—Normalized heat balance deficit criterion J as function of die thermocouple response time sd (du = 2.4 mm, p = 8.0 MPa).
are 655 and 153 lm. Such wide air gaps cannot be caused by the roughness asperities on the blank and die surfaces. Moreover, the approach phase HTC was found to be higher for 2.4-mm-thick blanks (60 to 170 W/m2 K) than for 1.2-mm-thick blanks (40 to 77 W/m2 K). The HTC for the approach phase was also found to increase as blank samples were reused for successive hot stamping tests. Both of these observations indicate that the air gap thickness between the blank and the bottom die is caused by a macroscopic warping of the blank. Thinner blanks have a lower stiffness and thus a greater tendency to deform, whereas blank samples repeatedly hot stamped tend to be flatter. Blank warping could be observed visually during the approach phase of expe
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