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Research Papers: Friction & Wear

Frictional Behaviors of a Mild Steel and a TRIP780 Steel Under a Wide Range of Contact Stress and Sliding Speed

[+] Author and Article Information
Chongmin Kim

Graduate Institute of Ferrous Technology (GIFT), POSTECH,
Pohang 790-784, South Korea
e-mail: bzkhk5@naver.com

Jeong-Uk Lee

Graduate Institute of Ferrous Technology (GIFT), POSTECH,
Pohang 790-784, South Korea;
POSCO,
>Gwang yang, Gumho-dong,
Gwangyang 545-090, South Korea
e-mail: jeonguklee@posco.com

F. Barlat

Graduate Institute of Ferrous Technology (GIFT), POSTECH,
Pohang 790-784, South Korea
e-mail: f.barlat@postech.ac.kr

Myoung-Gyu Lee

Graduate Institute of Ferrous Technology (GIFT), POSTECH,
Pohang 790-784, South Korea
e-mail: mglee@postech.ac.kr

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received May 20, 2013; final manuscript received December 8, 2013; published online February 5, 2014. Assoc. Editor: Dae-Eun Kim.

J. Tribol 136(2), 021606 (Feb 05, 2014) (7 pages) Paper No: TRIB-13-1104; doi: 10.1115/1.4026346 History: Received May 20, 2013; Revised December 08, 2013

The application of advanced high-strength steels (AHSS) generally makes it necessary to use higher tool-sheet contact pressures compared with those used for forming low-strength steel, and it leads to significant changes in frictional behavior, which in turn change the final product characteristics. In order to understand frictional behaviors between steel sheets and tool materials under high contact stresses present in real stamping conditions, a novel friction tester was conceived, fabricated, and used. This tester can generate high normal loads, as high as 625 MPa, whereas traditional friction testers were limited to 10 MPa or less. A mild steel and a TRIP780 steel were paired with Cr-coated D2 tool steel, and friction behaviors were observed under various conditions, including the use of two lubricants, wide ranges of sliding speeds, and normal contact stresses. The coefficient of friction (COF) decreased at a low contact pressure as the sliding velocity increased. The contact pressure had a significant effect, albeit too complex to be explained by simple models. It was also evident that lubricant effects must be studied coupled with the contact pressure and sliding speed. In a nonlubricated condition at normal stresses roughly half of the steel’s yield strength, the friction event caused plastic deformation that reached up to 0.2 mm from the surface. In this deformed region, the amount of retained austenite in the TRIP steel decreased substantially, and significant residual compressive stress, reaching 350 MPa, also developed in the ferrite phase (plus a minor amount of martensite, which is undistinguishable from ferrite by the X-ray diffraction method used herein). The magnitude of change of friction constant due to changes in contact conditions was enough to significantly affect springback of automotive body panels.

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Figures

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Fig. 1

The new friction tester. Letters in the photo signify: S, sample holder; T, tool; HN, hydraulics generating normal contact pressure; and HS, hydraulics for actuating horizontal sliding. When the sliding table is the tool and the sheet specimen is mounted at the top, it represents the new testing method. The tester can be operated in the traditional mode, as well, by having the tool on the upper side and a large sheet sample is placed on the sliding table.

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Fig. 2

An example of friction data output from the GIFT friction tester. The TRIP780 sample was tested using a low-viscosity lubricant (explained later) at 50 MPa normal contact pressure.

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Fig. 3

Change of friction coefficient with sliding speed. The material was TRIP780; the low-viscosity lubricant was used; and the contact pressure was 12.5 MPa.

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Fig. 8

Residual stress in the ferrite phase in TRIP780 steel tested in friction under a high 450 MPa contact pressure in nonlubricated condition

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Fig. 7

Volume fraction of retained austenite below the surface of the TRIP780 steel sample tested under a high 450 MPa contact load

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Fig. 6

Subsurface microhardness profile in friction tested TRIP780 steel under a high contact stress of 450 MPa under nonlubricated condition

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Fig. 5

Dependence of friction coefficient under dry or lubricated conditions and a range of contact pressures. The sample was TR780 steel, and the sliding speed was 10 mm/s.

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Fig. 4

The dependence of friction coefficient for TRIP780 steel sheet on the lubrication condition, contact stress and sliding distance. The sliding speed was set at 10 mm/s. (a) No lubricant, (b) low-viscosity lubricant, and (c) high-viscosity lubricant. For every condition, the experiment was conducted in triplicates, and the three results were found to be reasonably similar. Therefore, one data curve is presented for each condition.

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