CA1083437A

CA1083437A - Mehtod of treating ferrous strand by hot dip coating procedure

Info

Publication number: CA1083437A
Application number: CA294,010A
Authority: CA
Inventors: Laurence B. Caldwell; James J. Connolly
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1977-12-28
Filing date: 1977-12-28
Publication date: 1980-08-12

Abstract

Abstract of the Disclosure This application describes an improved method of forming a zinc-aluminum coating a ferrous base which comprises passing the ferrous base through a protective hood and into a molten bath consisting essentially of from 25% to 70%
by weight aluminum, balance substantially zinc, continuously subjecting the ferrous base and the portion of the surface of said molten bath within said protective hood to a hot gas which when introduced into said protective hood has a temperature of at least 750°F, the gas comprising at least 6%
by volume of hydrogen, balance essentially nitrogen, and having a dewpoint no greater than -100°F, and wherein a portion at least of said hot gas is first caused to sweep across the surface of the bath adjacent the region of entry of the ferrous base and then to flow countercurrent to the direction of movement of said ferrous base.

Description

~LCI 839~37 - Ir~ OV~ E~lT IM TE~E M~3THOD OF TRF.ATING ' `
F:l;.RR()U~; STRA~ BY l~IOT DIP COATING PROCEDURE
'' ' . ' ' This invention relates to a process for coating the surface of a ferrous metal or strip with a metallic alloy comprised of between 25% and 70% by weight alum~num, balance substantially zinc., and comprises an improvement in ..
or modification of the invention disclosed in Canadian Patent i~o. 1,016,423. .
According to the prior invention there is disclosed a process for applying an aluminum-zinc alloy coating to a ferrous metal strand by the steps of moving the strand through a protective hood and into a molten metal coating bath comprised of said aluminum-zinc alloy into which the protective hood extends in a manner to enclose at least a portion of the surface of said alloy coating bath within the : protective hood. A reducing environment within the hood is continuously replenished by the injection of a hot protective gas which when introduced into said protective hood has a -temperature.of at least 750F, comprising at least 20% by a volume hydrogen, balance essentially nitrogen, and a dewpoint ~o greater than 0F.
As will hereinafter be explained, subsequent tests performed on a commerical hot dip coating line have shed additional light on the correlation of the hydrogen content and dewpoint of the reducing, protective atmosphere. Thus, according to the present invention there is provided a process whereby a ferrous metal strand is coated with an aluminum-zinc coating alloy, by a hot dip coatlng method, wherein the reducing protective atmosphere utilized in said process is correlated in such a manner as to eliminate bath 10~33437 surface oxidation within the protective hood through which the ferrous metal strand passes into the coated bath.
Specifically, the reducing, protective atmosphere comprises a gas heated to a temperature of at least 750F, containing a minimum of at least 6% by volume hydrogen, balance essentially nitrogen and a dewpoint no greater than about -100F, when introduced into said protective hood. A sub-stantial portion, at least, of fresh protective gas is arranged to contact or sweep aeross the portion of the molten coating bath within the hood prior to passing eountercurrently to the movement of the strand along the -strand.
It has now been discovered that the eorrelation of the hydrogen eontent and dewpoint of the hot redueing atmosphere may be expanded beyond the limits of the prior inventlon to as low as 6 to 15% hydrogen, with dewpoints not less than about -100F. Such discovery was partieularly -surprising as heretofore it was believed that high hydrogen contents for the reducing atmosphere were needed in the 20 presence of a high aluminum eontalning molten coating bath, ~;
such as the aluminum-zinc coating bath of this invention.
Specifically, for example, on a hot dip coating line operating at speeds up to 500 fpm., using gas flow rates of 300 m3/hr., aluminum-zinc coated strip, free of pinhole and ferrous spot defects, was produced using a reducing atmos- ~ !
phere (l) preheated to at least 750F, (2) comprising an average hydrogen eontent of 10%, balance nitrogen, and (3) oompr~sing a dewpoint of about -112P.

., ~.

-2- ~
.,,:

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:

1. A method of forming a zinc-aluminum coating on a ferrous base which comprises passing the ferrous base through a protective hood and into a molten bath consisting essentially of from 25% to 70% by weight aluminum, balance substantially zinc, continuously subjecting the ferrous base and the portion of the surface of said molten bath within said protective hood to a hot gas which when introduced into said protective hood has a temperature of at least 750°F, the gas comprising at least 6% by volume of hydrogen, balance essentially nitrogen, and having a dewpoint no greater than -100°F, and wherein a portion at least of said hot gas is first caused to sweep across the surface of the bath adjacent the region of entry of the ferrous base and then to flow countercurrent to the direction of movement of said ferrous base.

2. The method according to claim 1 wherein the gas comprises from 6% to 15% of hydrogen by volume,