Intergranular zinc embrittlement and its inhibition by phosphorus in 55 pct Al-Zn-coated sheet steel
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I.
INTRODUCTION
Zn-coated sheet steel becomes embrittled after exposure to elevated temperature for prolonged periods. Sebisty reported that the tensile ductility of galvanized sheet was reduced following heating for several weeks in the temperature range 335 to 400 ~ He attributed embrittlement to penetration of Zn from the coating into the grain boundaries of the steel substrate. Although Sebisty's tests were run at temperatures below the melting point of Zn (420 ~ this phenomenon is similar to liquid metal embrittlement as reviewed in detail by Rostoker, et al. 2 Galvalume sheet* is a relatively new coated steel product *Galvalume sheet is Bethlehem Steel Corporation's tradename for steel sheet coated with the 55 pet A1-Zn alloy.
with a unique coating comprised of 55 pct A1, 43.5 pct Zn, and 1.5 pet Si,** referred to hereafter as the 55 pct AI-Zn **All percentages are by weight.
coating. This coating provides a favorable combination of the galvanic corrosion characteristics of Zn coatings with the durability and heat resistance of AI coatings, a'4 This work deals with the phenomenology and mechanism of high-temperature Zn embrittlement of the 55 pct A1-Zn alloy coated sheet. We also describe how Zn embrittlement is prevented in steel substrates containing greater than 0.04 pct P.
II.
PROCEDURE
The experirriental steels in Table I are grouped according to three areas of study: the phenomenology of intergranular Zn embrittlement, the inhibiting effect of P, and the mechanism of embrittlement and its inhibition by P. Steels A1 to A9 are commercially produced cold-rolled sheets typically used as substrates for continuous galvanizing in accordance with the chemical requirements of ASTM A526-71. The 55 pct AI-Zn coating was applied on a commercial hot-dip coating line employing a Selas-type L. ALLEGRA, Supervisor, R.G. HART, Engineer, and H.E. TOWNSEND, Supervisor, are all with Sheet Steels and Coated Products Division, Research Department, Bethlehem Steel Corporation, Bethlehem, PA 18016. Manuscript submitted July 21, 1982. METALLURGICALTRANSACTIONS A
continuous annealing section. The controlled variable within this group is steel thickness. The coated sheets were cut into 2.5 by 5.0 cm coupons, heated in laboratory ovens at specific temperatures between 316 and 510 ~ at intervals up to 5000 hours, and cooled in still air to room temperature. Zero-thickness bends t were made in each coupon, and the *A zero-thickness bend is a 180-degree bend with a maximum radius of curvature equal to the sheet thickness.
degree of steel fracture along the tensile side of the bend was noted. A coupon was characterized as brittle if fracture occurred along at least half the length of the bend. Optical metallography and electron microprobe analyses were performed on selected coupons. The second group of steels, B1 to B15, was produced in the laboratory to determine the effect of P on Zn embrittlement susceptibility. The steels were air-melted and teemed with controlled ferrophosphorus additions into 135 kg ingots. The ingots w
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