MEHTOD OF TREATING FERROUS STRAND BY HOT DIP COATING PROCEDURE

METHODE DE GALVANISATION PAR IMMERSION A CHAUD DE SUPPORTS FERREUX EN DEFILEMENT

English 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.

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,

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



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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.


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