Method of treating aluminum-killed and low alloy steel strip and sheet surfaces, in sulfur-bearing atmosphere, for metallic coating

A method of surface treatment of aluminum-killed and low alloy steel strip and sheet for fluxless hot dip metallic coating which comprises heating the steel in a furnace atmosphere containing the hot gaseous combustion products of air with a sulfur bearing gaseous fuel including 5 to 1600 grains of sulfur per 100 cubic feet of fuel wherein the atmosphere includes sulfur compounds and from about 6% free oxygen to about 7% by volume excess combustibles whereby to form a sulfur and oxygen rich film on the steel surfaces, passing the steel into a further heating section wherein it is brought to a maximum temperature of about 593.degree. to about 927.degree. C. in a reducing atmosphere containing at least 10% hydrogen by volume, passing the steel into a cooling section having an atmosphere containing at least 10% hydrogen and the balance nitrogen whereby to reduce the sulfur and oxygen rich film to a metallic iron surface, and cooling the steel approximately to the temperature of the molten coating metal bath. Coke oven gas may be used as the fuel for the furnace.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to a process of hot dip metallic coating of aluminum 
killed and low alloy steel strip and sheet material and more particularly 
to the preliminary treatment of the strip and sheet surfaces in a 
sulfur-containing atmosphere whereby to enhance the wettability thereof by 
molten coating metals such as zinc, zinc alloys, aluminum, aluminum 
alloys, and terne. Low alloy steels which may be treated by the process of 
the invention contain up to about 3% aluminum, up to about 1% titanium, up 
to about 2% silicon, or up to about 5% chromium, and mixtures thereof, 
with the remainder of the composition typical of carbon steel, as defined 
by Steel Products Manual, Carbon Sheet Steel, page 7 (May 1970), published 
by American Iron and Steel Institute. Aluminum killed steels include 
typical carbon steel as defined above containing from about 0.03% to about 
0.06% acid-soluble aluminum. 
(2) Description of the Prior Art 
In the fluxless, hot dip metallic coating of steel strip and sheet, it is 
necessary to subject the strip and sheet surfaces to a preliminary 
treatment which provides a clean surface free of oxide scale which is 
readily wettable by the molten coating metal and to which the coating 
metal will adhere after solidification thereof. One of the principal types 
of anneal-in-line preliminary treatment, to which the present invention is 
applicable, is the so-called Selas process, a description of which is 
contained in U.S. Pat. No. 3,320,085, issued May 16, 1967 to C. A. Turner, 
Jr. 
The Turner patent discloses a method of treating carbon steel strip and 
sheet material which comprises passing the material through a furnace 
heated to a temperature of at least about 2200.degree. F. (1205.degree. 
C.) by direct combustion of fuel and air therein, the furnace containing 
an atmosphere of gaseous products of combustion having no free oxygen and 
at least about 3% excess combustibles in the form of carbon monoxide and 
hydrogen, the residence time of the material being sufficient to cause it 
to reach a temperature of about 800.degree. to 1300.degree. F. 
(427.degree. to 705.degree. C.), while maintaining bright steel surfaces 
completely free from oxidation, withdrawing the material from the furnace 
while still surrounded by gaseous products of combustion, introducing the 
material directly into a reducing section having a hydrogen and nitrogen 
atmosphere, in which the material may be further heated from 800.degree. 
to 1700.degree. F. (427.degree. to 927.degree. C.) and/or cooled to 
approximately molten coating bath temperature, and then leading the 
material beneath the surface of the bath while surrounded by the 
hydrogen-nitrogen protective atmosphere. 
U.S. Pat. No. 3,925,579 issued Dec. 9, 1975, to C. Flinchum et al, 
discloses a method of fluxless hot dip metallic coating of low alloy steel 
strip and sheet stock (as hereinabove defined) in which one or more 
alloying elements is present in an amount greater than the critical 
content thereof as hereinafter defined, wherein the surfaces of the stock 
are prepared for coating by heating to a temperature of about 593.degree. 
to about 913.degree. C. in an atmosphere oxidizing to iron whereby to 
produce a surface layer of iron oxide containing a uniform dispersion or 
solid solution of oxides of the alloying elements, followed by further 
heat treatment under conditions reducing to iron oxide. The method of this 
patent is applicable either to the Selas method, or to the so-called 
Sendzimir method of preliminary treatment (described in U.S. Pat. Nos. 
2,110,893 and 2,197,622) which need not be described herein since the 
present invention is not practicable with the Sendzimir method. The method 
of the Flinchum et al patent is also applicable to aluminum killed steels 
which contain sufficient acid-soluble aluminum to cause poor adherence of 
the solidified coating metal when subjected to conventional preliminary 
treatment by the method disclosed in the Turner patent. 
In all prior art processes for preliminary treatment of steel strip and 
sheet surfaces which are exposed to atmospheres of direct fired furnaces, 
including the methods of the above-mentioned Turner and Flinchum et al 
patents, it has been considered that the presence of even small amounts of 
sulfur, in the atmosphere would be highly deleterious. Accordingly, 
substantially sulfur-free fuel such as natural gas has been prescribed for 
use in such furnaces. However, natural gas shortages have made it 
necessary to consider alternative sources of fuel. In a steel mill having 
coke ovens, the use of coke oven gas as a fuel source would be an obvious 
choice except for the fact that raw coke oven gas ordinarily contains 
about 300 to 500 grains of sulfur per 100 cubic feet of gas, the sulfur 
being present primarily as hydrogen sulfide with a small amount of organic 
sulfur compounds. Although the gas can be easily scrubbed to a sulfur 
level of about 75 to 100 grains per 100 cubic feet, and with modern and 
more sophisticated equipment can be cleaned to a level of about 25 to 40 
grains per 100 cubic feet, it has nevertheless been generally considered 
that the Selas-type preliminary treatment methods for in-line hot dip 
metallic coating could not tolerate even the lower sulfur levels of 
scrubbed coke oven gas. Accordingly, it was believed that curtailment of 
natural gas supply would force the shut-down of coating lines equipped 
with direct fired furnaces for preliminary treatment of steel strip and 
sheet material. 
SUMMARY OF THE INVENTION 
The present invention constitutes a discovery that sulfur-bearing coke oven 
gas can be used as a fuel in direct fired furnaces for preliminary 
treatment of the surfaces of aluminum-killed and low alloy steel strip and 
sheet material, without deleterious effects. Surprisingly, it has been 
found that a film rich in sulfur and oxygen, which is thin and uniform, 
forms readily on the strip and sheet material surfaces, and that this film 
can be easily reduced in a subsequent reducing section to produce a fresh 
ferrous surface which is readily wetted by liquid coating metal, with 
resultant excellent adherence after solidification of the coating. This 
sulfur and oxygen rich film is both easier to form and easier to reduce 
than the iron oxide film (containing a uniform dispersion or solution of 
oxides of alloying elements) formed in the process of the Flinchum et al 
U.S. Pat. No. 3,925,570. Accordingly, considerable latitude in 
temperature, furnace atmospheres and steel compositions is permissible in 
the practice of this invention. Moreover, it has been found that the 
sulfur content of the furnace fuel can vary over a wide range without 
adverse effect on coating metal adherence. 
Accordingly, the present invention provides a method of preparing the 
surfaces of aluminum-killed and low alloy strip and sheet material for 
fluxless hot dip metallic coating, which comprises passing the material 
through a furnace heated by direct combustion therein of air with gaseous 
fuel containing sulfur compounds ranging from about 5 to about 1600 grains 
of sulfur per 100 cubic feet of fuel to produce an atmosphere of gaseous 
products of combustion including sulfur and from about 6% by volume free 
oxygen to about 7% by volume excess combustibles in the form of carbon 
monoxide and hydrogen, in which atmosphere the material is heated; to form 
a sulfur and oxygen rich film on the surfaces; passing the material into a 
further heating section wherein the material is brought to a maximum 
temperature of about 1100.degree. to about 1700.degree. F. (593.degree. to 
927.degree. C.) in a reducing atmosphere containing at least about 10% 
hydrogen by volume; passing the material into a cooling section having an 
atmosphere containing at least 10% hydrogen by volume and the balance 
essentially nitrogen wherein the sulfur and oxygen rich film is reduced; 
and cooling the material approximately to the temperature of the molten 
coating metal bath.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Exemplary coating metals include zinc, zinc alloys aluminum, aluminum 
alloys and terne. The coating process may be any of the conventional 
continuous operations currently used. 
Although not believed to be critical, the direct fired furnace section (or 
preheater) may be maintained at about 2200.degree. F. (1205.degree. C.) or 
higher, and the strip and sheet material exiting this section may be at a 
maximum temperature of about 800.degree. to about 1300.degree. F. 
(427.degree. to about 705.degree. C.). In the further heating section the 
material is preferably brought to a maximum temperature of about 
1100.degree. F. to about 1450.degree. F. (593.degree. to 788.degree. C.), 
for the so-called anneal cycle. It is preferred to maintain a hydrogen 
content in the subsequent cooling section of at least about 20% by volume 
if the material is heated to a maximum strip temperature of about 
1100.degree. to about 1200.degree. F. (about 593.degree. to about 
650.degree. C.). The temperature of the further heating section may be 
maintained at about 1300.degree. to about 2000.degree. F. (705.degree. to 
1093.degree. C.). 
As is well known to those skilled in the art, the resident times in the 
various sections are variable and depend upon strip thickness, speed, heat 
absorptivity and related factors. The maximum temperature to which the 
material is brought in each section occurs at or near the exit therefrom, 
so that there is substantially no holding time at temperature, as is 
customary in continuous annealing practice. 
In the Flinchum et al U.S. Pat. No. 3,925,579 an equation is disclosed from 
which it is possible to calculate the critical content of an alloying 
element (in a low alloy steel). When the critical content is exceeded, 
preliminary treatment by the conventional Selas method results in 
"external oxidation", i.e., the formation of a surface layer of alloy 
oxide which cannot be reduced under ordinary treatment conditions, and 
which thus will not be wetted by the molten coating metal. The aluminum 
content of an aluminum-killed steel is also governed by the same equation. 
The present invention is similarly applicable to aluminum-killed and low 
alloy steels wherein alloying elements more readily oxidizable than iron 
are present in amounts greater than the critical contents thereof as 
defined in the above Flinchum et al patent. It will also be recognized 
that the atmosphere in the cooling section must be controlled so as to be 
reducing to iron oxide (and hence, a fortiori, reducing to the sulfur and 
oxygen rich film), but it will not be reducing to the oxides of the 
alloying elements, which remain as a uniform dispersion in the iron matrix 
at the surface. Within the temperature range of about 1100.degree. to 
about 1700.degree. F. (593.degree. to 927.degree. C.), an atmosphere 
containing at least about 10% hydrogen, balance substantially nitrogen, 
and a dew point not higher than about +20.degree. F., will readily meet 
these requirements. 
The sulfur in coke oven gas is primarily hydrogen sulfide with small 
amounts of organic sulfides, the latter being unstable. Upon combustion 
with air, the hydrogen sulfide and organic sulfur compounds are believed 
to be converted to sulfur oxides in the gaseous combustion products of a 
direct fired furnace. 
Full scale plant trials were conducted on a zinc coating line having a 
direct fired preheat furnace, a radiant tube furnace, and a cooling 
furnace as illustrated in FIG. 1. Although not shown, the cooling furnace 
comprised a jet cooling section and a slow cooling section. The direct 
fired preheat furnace was maintained at about 2300.degree. F. 
(1260.degree. C.), with strip temperature exiting therefrom ranging 
between 1000.degree. and 1300.degree. F. (538.degree. and 705.degree. C.). 
The amount of hydrogen sulfide was maintained at about 100 grains per 100 
cubic feet. In order to ascertain the effects of sulfur introduction at 
various zones within the preheat furnace, natural gas was used as the fuel 
with arrangements for introduction of hydrogen sulfide into the natural 
gas feed at selected zones of the preheat furnace, including the final 
zone which is the most critical in proper strip preparation. 
The first trial was designed to ascertain the effects of sulfur at various 
strip annealing temperatures, the effects of sulfur in the final zone of 
the furnace, and the effects of sulfur on aluminum-killed steel as 
compared to rimmed steel. 
The initial tests resulted in the following empirical observations: 
A definite visually detectable stain appeared on the surfaces of the strip 
upon the introduction of sulfur into the preheat furnace, the stain being 
a combined oxide and sulfide film. 
Firing the final zone with natural gas containing no hydrogen sulfide did 
not completely remove the visible stain. 
Aluminum-killed steel exhibited a much darker stain than rimmed steel. 
While the strip was definitely stained at the exit of the preheat furnace, 
complete removal was obtained in the radiant tube furnace, so that good 
coating metal adherence was obtained. No discernable difference in 
adherence occurred between samples processed in the preheat furnace with 
natural gas containing no sulfur and samples processed in the preheat 
furance with natural gas containing about 100 grains of sulfur per 100 
cubic feet. 
Processing conditions for the initial tests are summarized in Table I. By 
way of explanation, Example 1 was a drawing quality rimmed steel of 0.043 
inch thickness and 311/8 inches width, while Example 2 was an 
aluminum-killed drawing quality steel of 0.055 inch thickness and 30 3/8 
inches width. The aluminum content of Example 2 was 0.040%-0.043%. 
The adherence test was the ball impact test. A rating of one indicates 
light crazing; a rating of two indicates heavy crazing; a rating of three 
indicates some detachment of the coating; and a rating of four indicates 
complete peeling of the coating. For prime applications a rating of one or 
two is considered satisfactory. 
It will be apparent from the data of Table I that the presence of sulfur in 
the preheat furnace atmosphere was not detrimental, regardless of the zone 
in which it was introduced. 
A second trial was conducted in order to determine whether possible 
adherence difficulties would occur with wider strip material, the reason 
for a more pronounced film on aluminum-killed steel than on rimmed steel 
at the same sulfur level, whether film was completely removed at the exit 
of the radiant tube (reducing) furnace, and the effect of lower 
temperature. 
These further test results are summarized in Table II. These tests were 
conducted at a sulfur level of 150 grains per 100 cubic feet in the 
preheater furnace fuel. By way of further explanation, Example 3 was a 
"CQ" rimmed steel of 0.075 inch thickness and 60 inches width, while 
Example 4 was a drawing quality aluminum-killed steel of 0.038 inch 
thickness and 51 3/16 inches width containing 0.040%-0.043% aluminum. 
It is evident from the data of Table II that satisfactory adherence was 
obtained at somewhat higher sulfur level, that the wide material presented 
no coating problems, that the "CQ" annealing temperature caused no 
adherence problems, and that the film was completely removed at the exit 
of the reducing furnace. These tests also confirmed that aluminum-killed 
steel developed a heavier film than rimmed steel, but no explanation for 
this can be given at the present time. 
Further laboratory scale tests have resulted in the following empirical 
determinations: 
Sulfur levels ranging between 60 and 1570 grains per 100 cubic feet 
resulted in a substantially constant surface discoloration at the exit of 
the direct fired furnace. 
When operating under an anneal cycle where the strip temperature reached a 
maximum of 1450.degree. F. (788.degree. C.), it was found that the 
hydrogen content of the reducing furnace atmosphere was not critical, and 
that excellent coating adherence was obtained at hydrogen levels ranging 
between 15% and 40% by volume with sulfur levels of 100 to 200 grains per 
100 cubic feet. 
TABLE I 
__________________________________________________________________________ 
Strip Temp. at Exit 
Radiant Tube 
Furnace.degree. F 
Sample 
Preheat 
(25% H.sub.2, 75% N.sub.2 
Amount Sulfur 
Zone of Adherence 
and Coil 
Furnace .degree. F 
atmosphere) 
Preheat (grains/100 ft.sup.3) 
Sulfur Addition 
Test 
__________________________________________________________________________ 
Example 1 
Coil 1 
1115.degree. 
1550.degree. 
100 Intermediate 
1/1 
Coil 2 
1115.degree. 
1550.degree. 
100 Final 1/1 
Coil 3 
1115.degree. 
1550.degree. 
100 Final 1/1 
Coil 4 
1115.degree. 
1550.degree. 
0 2/2 
Coil 5 
1115.degree. 
1550.degree. 
0 1/1 
Example 2 
Coil 1 
1100.degree. 
1620.degree. 
100 Intermediate 
2/2 
Coil 2 
1100.degree. 
1620.degree. 
200 Intermediate 
1/1 
Coil 3 
1100.degree. 
1620.degree. 
100 Final 1/1 
Coil 4 
1100.degree. 
1620.degree. 
0 1/2 
Coil 5 
1100.degree. 
1620.degree. 
0 1/1 
Coil 6 
1100.degree. 
1620.degree. 
0 1/1 
Coil 7 
1100.degree. 
1620.degree. 
0 1/1 
Coil 8 
1100.degree. 
1620.degree. 
0 1/1 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
Strip Temp. at Exit 
Radiant Tube 
Furnace .degree. F 
Sample 
Preheat 
(25% H.sub.2, 75% N.sub.2 
Amount Sulfur 
Zone of Adherence 
and Coil 
Furnace .degree. F 
atmosphere) 
Preheater (Grains/100ft.sup.3) 
Sulfur Addition 
Test 
__________________________________________________________________________ 
Example 3 
Coil 1 
1030.degree. 
1450.degree. 
150 Intermediate 
1/2 
Coil 2 
1030.degree. 
1450.degree. 
150 Final 1/1 
Coil 3 
1030.degree. 
1450.degree. 
150 Final 1/1 
Coil 4 
1030.degree. 
1450.degree. 
150 Final 2/2 
Coil 5 
1030.degree. 
1450.degree. 
0 1/1 
Coil 6 
1030.degree. 
1450.degree. 
0 1/1 
Example 4 
Coil 1 
1075.degree. 
1580.degree. 
150 Intermediate 
2/2 
Coil 2 
1075.degree. 
1580.degree. 
150 Final 1/1 
Coil 3 
1075.degree. 
1580.degree. 
150 Final 1/1 
Coil 4 
1075.degree. 
1580.degree. 
0 2/4 
__________________________________________________________________________ 
Auger spectra were obtained by means of an Auger Spectrometer, made by 
Physical Electronics, Inc., for the surface of aluminum-killed steel 
samples subjected to treatment in a direct fired preheater furnace 
containing about 100 grains of sulfur per 100 cubic feet of furnace 
atmosphere. These samples were taken from strip exiting the preheat 
furnace. It was found that both oxides and sulfur compounds were present 
in the surface scale. The oxide concentration was greatest at the surface 
and declined gradually with distance inwardly therefrom, whereas the 
sulfur content increased in a rather irregular manner inwardly from the 
surface to a maximum and then decreased. 
A number of literature references deal with the oxidation and sulfidation 
of iron and suggest theoretical explanations of the mechanism of formation 
of iron sulfide and the concentration thereof at the scale-metal 
interface. Such theoretical considerations form no part of the present 
invention and hence are not discussed herein. 
The relatively dark color film resulting from sulfur compounds has high 
heat absorptivity and hence is initially heated efficiently in the radiant 
tube section. Accordingly, the present invention provides the option of 
increasing strip speed and hence production, or operating at a lower 
furnace temperature in order to save fuel costs and reduce refractory 
wear. A combination of these two advantages could of course also be 
obtained. 
From what has been said above with respect to processing aluminum-killed 
steel in accordance with the present invention containing more than a 
critical content of aluminum (as defined in the above Flinchum et al 
patent), it will be recognized that the process may be carried out to even 
greater advantage for low alloy steels containing up to about 3% aluminum, 
up to about 1% titanium, up to about 2% silicon, and/or up to about 5% 
chromium. Since alloy steels are relatively difficult to oxidize, the more 
easily formed sulfur and oxygen rich film makes it unnecessary to subject 
the material to oxidizing conditions as strong as those required in the 
Flinchum et al U.S. Pat. No. 3,925,579. 
As indicated previously, the process of the invention is operative at 
levels ranging from about 5 to about 1600 grains of sulfur per 100 cubic 
feet of coke oven gas (about 0.007% to about 2.6% by volume hydrogen 
sulfide at standard temperature and pressure). A sulfur and oxygen rich 
film will be formed in a preheat furnace atmosphere containing up to 7% by 
volume excess combustibles, although perfect combustion conditions are 
preferred from the standpoint of fuel ecomony. As little as 10% hydrogen 
by volume in the radiant tube and cooling sections will reduce the sulfur 
and oxygen rich film in an anneal cycle wherein the maximum temperature is 
about 788.degree. C., while at least about 20% hydrogen by volume is 
preferred if the maximum strip temperature is about 593.degree. to about 
650.degree. C. 
While the invention has been described in its preferred embodiments, it 
will be evident that modifications may be made without departing from the 
spirit and scope of the invention. Thus, in some Selas-type installations 
a holding section is provided between the radiant tube section and the 
cooling section, in which the strip may be held at some selected 
temperature (usually for a short period of time) after reaching a maximum 
temperature in the radiant tube furnace, in order to improve the 
formability or modify the mechanical properties of the steel strip. 
Preferably a reducing atmosphere containing at least 10% hydrogen by 
volume is maintained within such a control zone, although an inert 
atmosphere such as nitrogen could be provided. It is to be understood that 
the provision of such a control zone or holding step is within the scope 
of the present invention.