HOT-DIP GALVANISING ALLOY AND PROCESS

Abstract: This invention relates to a bath for hot-dip galvanising consisting of alloyed zinc, that is particularly useful for batch-wise galvanising low reactivity steel articles. The aim of the invention is to provide a bath which results in a coating thickness of 30 to 50 mu m. Such a relatively thin coating provides increased resistance to stress induced crack formation. The bath contains, by weight, 0.1 to less than 1 % Bi, 0.01 to 0.05 % A1, 0.04 to 0.12 % Ni, up to 0.05 % Mn, the rest being zinc and unavoidable impurities. Preferably, the bath contains 0.3 to 0.8 % Bi, 0.02 to 0.04 % A1, 0.06 to 0.10 % Ni and up to 0.05 % Mn. A bath composition with 0.5 % Bi, 0.03 % A1 and 0.08 % Ni is especially preferred.

Description: Hot-dip galvanising alloy and process The present invention relates to a bath for hot-dip galvanising consisting of alloyed zinc, that is particularly useful for batch- wise galvanising steel articles. Conventional galvanisation practice results in a coating thickness of 70 to 120 urn. A drawback of such thick coatings is the reduced resistance to stress induced crack formation. The aim of the present invention is therefore to provide a bath composition for hot-dip galvanising, which allows for the formation of a coating thickness of only 30 to 50 μm. Also, the thickness of the intermetallic layer is reduced to approximately 25 to 35 μm. This results in an increased resistance to stress induced cracks, as required e.g. for critical parts in the automotive industry. According to the invention, the bath contains, by weight, 0.1 to less than 1 % Bi, 0.01 to 0.05 % Al, 0.04 to 0.12 % Ni, up to 0.05 % Mn, the rest being zinc and unavoidable impurities. Preferably, the bath contains 0.3 to 0.8 % Bi, 0.02 to 0.04 % Al, 0.06 to 0.10 % Ni and up to 0.05 % Mn. A bath composition with 0.5 % Bi, 0.03 % Al and 0.08 % Ni is especially preferred. The galvanising process using the alloy of the invention is particularly suitable for coating low reactivity steel . By low reactivity steel is meant steel comprising, by weight, Si and P in such amounts that % Si < 0.03 and % Si + 2.5 x % P < 0.09. An illustration of the criticality of the nickel content in the bath is given in the following example: steel samples measuring 100 x 50 x 2.5 mm were immersed in a galvanising bath during 5 minutes, after being prepared in a traditional way by degreasing, pickling, fluxing and rinsing. The following bath compositions were used: - an alloy according to the invention containing 0.53 % Bi, 0.046 % Ni, 0.028 % Al, 0.021 % Mn, and - a comparative alloy containing 0.51 % Bi, 0% Ni, 0.026 % Al, 0.003 % Mn. After hot-dip coating, the coating thickness was determined with a MiniTest 3100 (ElektroPhysik) . The test was performed on different types of steel alloys: - HE 320 DR low reactivity steel containing 0.011 % Si and 0.008 % P; - HE 360 D low reactivity steel containing 0.005 % Si and 0.009 % P; and - CS steel containing 0.22 % Si and 0.07 % P. The results are summarised in the following table . The results show that, by using the alloys according to the invention, the coating thickness can be reduced by at least 10 %, and even by more than 60 % according to the type of steel used.

Claims: Claims 1. An alloy for hot-dip galvanising steel comprising, by weight, 0.1 to less than 1 % Bi, 0.01 to 0.05 % Al, 0.04 to 0.12 % Ni, up to 0.05 % Mn, the rest being zinc and unavoidable impurities 2. An alloy according to claim 1, comprising, by weight, 0.3 to 0.8 % Bi, 0.02 to 0.04 % Al, 0.06 to 0.10 % Ni, up to 0.05 % Mn, the rest being zinc and unavoidable impurities 3. A process for galvanising steel by immersion in a zinc alloy galvanising bath comprising the step of immersing the steel articles in a molten bath of a zinc alloy comprising, by weight, 0.1 to less than 1 % Bi, 0.01 to 0.05 % Al, 0.04 to 0.12 % Ni, up to 0.05 % Mn, the rest being zinc and unavoidable impurities 4. A process according to claim 3, characterised in that the steel articles comprise, by weight, Si and P in such amounts that % Si < 0.03 and % Si + 2.5 x % P < 0.09

Inventors: GILLES MIK, GILLES, MIK
Applicants: GILLES MIK, UMICORE, GILLES, MIK
Application year: 2001
Main classification: C23C2
IPC codes: C23C2