CA1104910A

CA1104910A - Method of treating strip surfaces for metallic coating

Info

Publication number: CA1104910A
Application number: CA293,785A
Authority: CA
Inventors: Jerry L. Arnold; Frank C. Dunbar; Alan F. Gibson; Marvin B. Pierson
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1976-12-23
Filing date: 1977-12-22
Publication date: 1981-07-14
Also published as: AU3174177A; JPS53102236A; AU509460B2; FR2375334A1; BR7708572A; FR2375334B1; JPS5723748B2

Abstract

ABSTRACT OF THE DISCLOSURE
A method of preparing the surfaces of steel strip and sheet stock for fluxless hot dip metallic coating, comprising the steps of passing the stock through a first heating section, continuing the heating of the stock in a second heating section, the atmosphere in the first and second heating sections being isolated from one another, increasing the radiant energy absorptivity of the stock throughout the heating sections, and cooling the stock approximately to the temperature of the molten coating metal. The radiant energy absorptivity is increased by forming a visible iron oxide-containing layer or an iron-oxysulfide layer on the stock surfaces in the first heating section and preserving the oxide layer throughout the second heating section.

Description

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This invention relates to hot dip metallic coating of steel strip and sheet stock and more particularly to a method of preliminary treatment of the surfaces of the stock to develop initially an iron oxide or sulfur and oxygen rich film, to preserve this film during further heating, and to reduce the film while cooling the stock prior to immersion thereof in a molten coating metal bath. The invention has utility in the coating of carbon steels, low carbon rimmed steels, low carbon aluminum killed steels, and low alloy steels 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 low carbon steel as defined above containing from about 0.03%
` to about 0.06% acid-soluble aluminum.
In the fluxless hot dip metallic coating of steel strip and sheet stock it is necessary to subject the surfaces to a preliminary treatment which provides a clean surface free of iron oxide scale and other surface contaminants, and which is readily wettable by the molten coating metal in order to obtain good adherence. Two types of in-line anneal preliminary treatments are in common use in this country, one being the so-called Sendzimir process or oxidation-reduction practice ~disclosed in United States Patents 2,100,893 and 2,197,6221, and the other being the so-called Selas process or high intensity direct fired furance line (disclosed in United States Patent 3,320,085 to C.A. Turner, Jr.).
; In the Sendzimir process steel strip or sheet stock is heated in ; an oxidizing furnace ~which may be a direct fired furnace) to a temperature of about 37Q-485C without atmosphere control, withdrawn into air to form a controlled surface oxide layer varying in appearance from light yellow to -- 1 ~
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purple or even blue, introduced into a reduction furnace contalning a hydrogen and nitrogen atmosphere wherein the stock is heated to about 735-925C and the controlled oxide layer is completely reduced. The stock is then passed in-to a cooling section containing a hydrogen and nitrogen atmosphere, brought approximately to the temperature of the molten coating metal bath, and then led beneath the bath surface ~hile still surrounded by the protective atmosphere.
In the Selas process steel strip or sheet stock is passed through a direct fired preheat furnace section, heated to a temperature about 1315C
by direct combustion of fuel and air therein to produce gaseous products of combustion containing at least about 3% combustibles in the Eorm oF carbon monoxide and hydrogen, the stock reaching a temperature of about 425-705C
while nlaintaining bright steel sureaces completely free from oxidation. I'he stock is then passed into a reducing section which is in sealed relation to the preheat section and which contains a hydrogen and nitrogen atmosphere 9 wherein it may be further heated b~ radiant tubes to about 425-925C and/or cooled approximately ~o the molten coatlng metal bath temperature. The stock ; is then led beneath the bath surface w~ile surrounded by the protective atmosphere. The process may optionally include holding the stock at a selected tmperature in a reducing atmosphere after reaching maximum tempera--ture in the radiant tube section.
United States Patent 3~936,543 issued February 3, 1976, to F. Byrd et al, discloses an improvement in the Selas process, resulting in higher combustion eficienc~ and better product~on rates, wherein strip ~md sheet stock is heated to about 540-705~C in a direct fired preheat furnace section heated to at least about 12Q5C and containing gaseous products of combustion ranging from about 3% by volume oxygen to about 2% by volume ex-cess combustibles in the orm of carbon monoxide and hydrogen, followed by heating in a reducing section containi~ng at least about 5% hydrogen by volume 3Q to a temperature of at least about 675C. Preferably the preheat furnace

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,, atmosphere contains 0% oxygen and 0% excess combustibles, i.e. perfect combustion.
In all prior art processes for preliminary treatment of steel strip and sheet surfaces which are exposed to atmc>spheres of direct fired furnaces, it has been considered that the presence of even small amounts of sulfur in the atmosphere ~ould be highly deleterious. Accordingly, sub-stantially sulfur-free fuel such as natural gas has ~een prescribed for use in such furnaces. However, natural gas shortages have made it necessary to consider alternative sources oE fuel. In a steel mill having coke ovens, the use of coke oven gas as a fuel source ~ould be an obvious choice except for the fact that ra~ coke oven gas ordinarily contains about 300 to 500 grains of sulfur per 100 cubic feet Oe gas, thc sulfur belng present primarily as hydrogen sulf:ide with a small amount of organic suleur coDIpounds. Al-though the gas can be easily scrubbed to a sulfur level of about 75 to 100 grains per 100 cublc feet, and more recently even to a level of about 25 to 4Q grains per lOQ cubic feet, it has nevertheless been generally considered that preli~inary treatment methods involving exposure of steel strip sur-faces to atmospheres containing products of combustion could not tolerate even the lo~er sulfur levels of scrubbed coXe oven gas. Accordingly, it has been feared that curtailment of natural gas supply would force the shut-do~n Oe coating lines equipped with direct fired Eurnaces Eor preliminary treat-ment of steel strip and sheet material.
The present inventlon 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 steel strip and sheet stock, and that greater increases in energy eficiency and/or production rates can be achieved in both the Sendzimir and Selas processes ~as modified by the above United States Patent 3,936,543) by increasing the radiant energy absorptivity of ; the steel stock. This absorptivity is increased by forming a visible film or layer oE lron oxide or an oxysulfide layer rich in sulfur and oxygen on

- 3 -~ -'~' 1 the stock surfaces in the initial direct fired ~or preheat~ furnace section, and by preserving this Eilm throughout the heating sections.
It has been found that a film rich in sulEur and oxygen, which is thin and uniform, can be readily formed on the stock surfaces, and that this film can be easily reduced in a subsequent cooling section to produce a fresh ferrous surface which is readily wetted by molten coating metal, with resultant excellent adherence after solidification of the coating.
Accordingly, the present invention provides a method of preparing the surfaces of steel strip or sheet stock for fluxless hot dip coating with molten metal, comprising the steps of passing the stock through a first heating section, continuing the heating of said stock in a ~urther heating section, and cooling the stock in a cooling section approximately to the tomperature of thc molten coating metal in a protective atmosphere, char-acterized by the steps of increasing the radiant energy absorptivity of said stock throughout said heating sections by forming a visible iron oxide-containing layer in said first heating section, preserving said layer through-out said further heating section, and reducing said oxide-containing layer completely in said cooling section in an atmosphere containing at least 10% by volume hydrogen.
`20 The term "iron oxide-containing layer" is to be construed as in-cluding either a visible iron oxide layer in the color range of dark straw through blue, or a -visible iron oxysulfide layer rich in sulfur and oxygen, the precise chemical composition of which has not been determined.
The temperature to ~hich the stock is heated in the successive heating zones is not crltical so long as the formation of a ~hick oxide or sulfur and oxygen containing scale is avoided. In general, the temperatures may be the same as those described above for conventional practice, i.e., for the Sendzimir process a range of about ~70 -~ ~85C in the oxid:izing furnace and about 735 - 925C in the further heating zone; and for the process of the Byrd et al patent a range o about 5~0 - 705C in the direct fired _ ~ , preheat section and at least about 67~C and up to about 925C in the radiant tube heating section. The stock may be held at a selected temperature, after passage through ~he radiant tube section, for a short period of time (in order -~o improve formability or to modify ths mechanical properties~, and ~he atmosphere in the holding section is preferab:Ly reducing, but may contain less than 5~ hydrogen.
Due to the dark coloration of the sulfur and oxygen rich layer~
the heat absorptivity o~ the stock i5 greatly increased, thereby decreasing the residence time in the heating zones if the radiant tube furnace tempera-tures are maintained at conventiona] levels. Hence this results in an increased production rate. Alternatively, the radiant tube furnace tempera-tures could be reduced somewhat, thereby maintaining the same production rate at lower euel requirements. Lt is oE course evident that a balance between increased productLon rate and Lower fuel requirements could also be effected.
It is a preferred feature oE the process that the further heating zone have an atmosphere containing less than 5% hdyrogen by volume and be substantially isolated from the atmospheres of the other zones. A gas inert to the sulfur and oxygen rich layer, preferably nitrogen, is used.
The residence times in the various zones are variable and depend upon strip thickness, speed and related factors. The temperature to which the stock is brought in each zone occurs at or near the exit therefrom, so that there is substantially no holding time at temperature, as is customary in continuous annealing practice.
BRIEF DESC~IPTI~N OF THE DRA~ING
Reference is made to the accompanying drawing wherein:
Figure l is a diagrammatic illustration of a Sendzimir line modi-~ied to practice the present mvention; and ~igure 2 is a diagrammatic ~llustration of a Selas line modiFied to practice the present invention.

`~ ` l DESCRIPTION OF THE PREFERRED EMBODI~ENTS
Referring to Figure 1, steel strip to be treated is indicated at 10, the direction of travel being shown by arrows. An oxidi~ing ~urnace is shown at 12, which is heated to a temperature of, e.g., about 870C by com-bustion of scrubbed coke oven gas. A second heating section, which may be a radiant tube furnace, is shown at 14. An inlet :for nitrogen into the second heating section is provided as shown at 16. Baffle means 18 are provided between heating section 14 and cooling section 20, which isolate the atmospheres in each section from one another. A hydrogen inlet into cooling section 20 is shown at 22~ and a stack for flaring hydrogen is pro-vided as shown at 24. A protective snout 26 extends downwardly beneath the surface o~ a coating metal bath 28, which surrounds strip 10 as it is con-ducted beneatll the surface o~ the bath, around a reversing roll 30 and vertically upwardly O~lt oE the bath. Any conventional finishing means (not shown~ may be used for metering and solidi~ying the metal coating.
Since the oxidizing furnace 12 and heating section 14 are separate, with heated strip 10 exposed to atmosphere therebetween, it is evident that the atmospheres of each are isolated from one another.
Referring to Figure 2, s-trip 10 to be treated is shown as in Figure 1. A direct fired preheat furnace is shown at 32, which is heated to a temperature, e.g. of about 120S~C, by direct combustion of scrubbed coke oven gas and air. A further heating section, which is preferably a radiant tube furnace is shown at 34, and baffle means 36, 36a are provided between the preheat furnace 32 and radiant tube furnace 34, thus isolating one from the other. An inle* for nitrogen into furnace 34 is sho~n a~ 38, and baffle means 42 is provided isolating cooling section 40 from the radiant ~ube ~urnace 34. An inlet for hydrogen or a hydrogen-nitrogen mixture into cooling section 40 is shown at 44. The protective snout 26, coating metal bath 2 and reversing roll 3Q are the same as described above in Figure 1.
As an alternat~ve arrangement, isola~ion of the atmosphere in the - 6 =

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radiant tube furnace 34 from the atmospheres in the preheat furnace 32 and in the cooling section 40 can be effected by providing a sufficiently large flow of nitrogen through inlet 38 into furnace 32 so as to block entry of hydrogen into furnace 34 from section 409 thereby eliminating the need :For baffles 36, 36a and 42.
The remaining elements shown in Figures 1 and 2 are conventional and require no discussion since the functions thereof are well known to those skilled in the art.
~Ihile it is preferred rom the standpoint of economics to operate the preheat furnace zone under perfect combustion conditions (0% excess combustibles and 0% oxygen), it is within the scope of the invention to operate ~ith an atmosphere ranging Erom up to about 2% free oxygen by volume up to abo~lt 2% hy volume excess combustibles in the form of carbon monoxide and hydrogen.
- Preferakly the atmosphere in the cooling zone is a mixture of about 20% to 40% hydrogen by -volume, and balance nitrogen; a minimum of 10%
hydrogen by volume is belleved to be essential.
hlllen practicing an anneal cyc]e the stock is brought to a tem-perature of about 427 to about 705C in the preheat zone and to a maximum temperature of about 788C in the radiant tube zone. Then practicing a full hard cycle the stock is brought to a maximum temperature of about 565C in the preheat zone and a maximum of about 538C in the radiant tube zone. In the full hard cycle the hydrogen contact in the cooling zone is preferably increased to about 40% by volume.
The amount Oe sulfur present in the coke oven fuel and in the atmosphere of the preheat furnace has been found to have little effect on the nature of the sulfur and oxygen rich film formed on the strip surfaces and may be varied from about 5 to abou~ 1600 grains per 100 cubic feet in the coke oven gas (a~out O.OQ7% to about 2.6~ hydrogen sulfide by volume at standard temperature and pressurel. Similarly, variations in sulfur content have little influence on coating metal adherence except in the practice of a full hard cycle wherein the maximum strip temperature is about 565C. Under these conditions an increase in the hydrogen content in the cooling zone to about 40% by volume will result in improvement in coating adherence, as indicated above.
The method of the invention is applicable to any type of generally used coating metal including, but not limited to, aluminum, alloys o~
aluminum, zinc, alloys of ~inc, and terne.
As indicated above, this invention has utility in the coating of any type of steel strip and sheet stock in thicknesses generally used for hot dip metallic coating, including carbon steel, low carbon rimmed steel, low carbon alumim~n kil]ed steel, low carbon columbillm and/or titanium treated stecls, and low alloy steels oE the type disclosed in lJnlted States Patent 3,905,780 to ~. C. Jasper et al. Low alloy steels of this type, containing alloying elements more readily oxidizable than iron in an amount greater than a critical content3 previously could be successfully prepared ~or hot dip coating only by a process disclosed in United States Patent 3,92~,579 to C. Flinchum et al, which included subjecting the steel to strongly oxidizing conditions in the initial heating stage. Since the sulEur and oxygen rich ~ilm oE the present process is more easily formed than an oxide film, it is an advantage of the present invention that the steel need not be subjected to oxidizing condltions as strong as those required in the Flinchum et al patent, for steels containing alloying elements in amounts greater than the critical content thereof, as defined in that patent.
Although not sho~n in the dra~ing, a holding section may be pro-vided between the radiant tube section 14 and the cooling section 20 of Figure 1 or radian tube section 34 and cooling section 40 of Figure 2. As indicated above, some installations include such a holding section in order to maintain the stock at some predetermined temperature, after reaching the peack tem-perature in the radiant tube section, for the purpose of improving the , `' !

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formability or genera] mechanical properties of the stock. This is consider-ed to be within the scope of the present invention, and such a control zone will preferably be supplied with an atmosphere containing at least 10%
hydrogen by volume, although the atmosphere could be non-reducing in order to preserve the oxygen and sulfur rich film until the stock reaches the cooling section.
As is customary in continuous annealing practice, the temperature ~ `
to which the stock is brought in each section occurs at or near the exit there~rom, so that there is substantially no holding time at temperature.
The residence times in various sections are variable and are dependent on strip thickness, heat absorptivity and related factors. In the present invention an oxide layer is maintained on the surfaces of the stock until the maximum temperature is reached, which is usually at or near the exit Eronl thc second heating zone or radiant tu~e furnace.
Tests have been conducted on a production Selas-type line using natural gas as a fuel to compare con~entlonal practice with that of the present invention. Conventional practice involved maintaining 3% by volume excess combustibles in the preheat furnace atmosphere (having five heating zones2 and a 5% hydrogen - 95% nitrogen atmosphere in the radiant tube heating section and cooling section. ~hen conditions were altered to the practice of the invention) i.e., approximately perfect combustion in the first Eour zones of the preheat furnace ~0% excess combustibles and 0% oxygen), 1.5%
oxygen in the fifth and final zone of the prehea-t :furnace, less than 5%
hydrogen in the radiant heating sectlon, and 25% hydrogen 75% nitrogen by volume in the cooling section, it was found that the production rate was in-creased 10 to 2Q% in comparlson to conventlonal practice. Complete removal of the oxide layer was effected in the cooling section in thls test.
~urther mill trials were conducted on low carbon rimmed steel and aluminum-killed steel strip ranging in thickness from 0.05~ inch to 0.099 inch ~ to 2.5mm~. In the preheat furnace conventional operating conditions _ 9 _ ,,~''i I

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were initially established as follows: zone 1 had 0.7% ~by volume excess combustibles; zone 2 had 0.~% excess combustibles; zone 3 had 0.6% excess combustibles; zone 4 had 0.2% free oxygen; zone 5 had 0.3% combustibles. The strip had a light straw color exiting zone 5, indicating an extremely thin oxide film, i.e., less than 10 5 inch thickness.
Gas flow to the radiant tube furnace was then changed to pure nitrogen rather than the conventional nitrogen-hydrogen mixture, and zone 5 of the preheat furnace was adjusted to 1.3% excess oxygen in accordance with the method of the invention. The strip exiting the preheat furnace then exhibited a reddish purple to blue iridescent color, indicating an oxide layer on the order of 10 5 inches thickness. With this change in conditions the radiant tube furnace temperature began to drop (even though the ~iring rate was maintained at 100%~, thus indicating greater heat tr~msfer to the str:ip due to greater radiant energy absorptivity.
The strip exiting the radiant tube furnace was oxidized and ranged in temperature from 1350 to 1420F (732 to 770C), depending upon strip thickness and speed.
The cooling section was supplied ~ith a 30% hydrogen- 70% nitrogen mixture, and the o~ide layer was reduced in the cooling section.

The llne speed was increased to 110% o~ scheduled speed (as a conservative measure) because of the higher strip temperature in the radian*
tube section.
The trials were concluded by returning to conventional practice, and it was observed that the radiant tube section temperature increased and the strip temperature~decreased therein, with the firing rate maintained constant at 100%.
The abo~e trials show that close control of the initial heating zones in the preheat ~urnace is not essential so long as an oxidizing atmosphere (greater than 0% and up to 2% free oxygen) is maintained in the final zone or inal t~o zones. The initial zones may thus be operated at - lQ --~ I
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perfect combus~ion or with up to about 1% by volume excess combustibles.
In the appended claims the term "substantially no excess combustibles and from O to 2% free oxygen by volume" is intended to cover this mode of operation (i.e., up to 1% excess combusti.bles in the initial heating zones but not in the final zone~

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preparing the surfaces of steel strip or sheet stock for fluxless hot dip coating with molten metal, comprising the steps of passing the stock through a first heating section, continuing the heating of said stock in a further heating section, and cooling the stock in a cooling section approximately to the temperature of the molten coating metal in a protective atmosphere, characterized by the steps of increasing the radiant energy absorptivity of said stock throughout said heating sections by form-ing a visible iron oxide-containing layer in said first heating section, preserving said layer throughout said further heating section, and reducing said oxide-containing layer completely in said cooling section in an atmos-phere containing at least 10% by volume hydrogen.

2. The method claimed in claim 1, characterized by forming a visible iron-oxysulfide layer rich in sulfur and oxygen on said stock surfaces in said first heating section.

3. The method claimed in claim 1 or 2, characterized in that said first heating section contains an atmosphere of gaseous products of combus-tion comprising up to 2% by volume free oxygen and from 0 to 1600 grains of sulfur per 100 cubic feet, that said further heating section is isolated from said first heating section and contains an atmosphere having less than 5% by volume hydrogen, and that said cooling section is isolated from said heating sections.

4. The method according to claim 1, characterized in that said first heating section is heated by the direct combustion of fuel and air therein to a temperature of at least about 1205°C and contains an atmosphere of gaseous products of combustion comprising substantially no excess combustibles and from 0 to 2% by volume free oxygen, and that a visible iron oxide layer is formed on the stock surfaces in the color range of dark straw through blue.

5. The method according to claim 2, characterized in that said first heating section is heated by the direct combustion of sulfur-bearing gaseous fuel and air therein to a temperature of at least about 1205°C, and that the atmosphere therein contains from about 2% free oxygen by volume up to about 2% by volume excess combustibles in the form of carbon monoxide and hydrogen, and from 5 to 1600 grains of sulfur per 100 cubic feet, thereby causing the formation of a visible oxygen and sulfur rich layer on the stock surfaces.

6. The method according to claim 4 or 5, characterized in that said stock is heated to 540° to 705°C in said first heating section and to 675° to 925°C in said further heating section.

7. The method according to claim 1, characterized in that said first heating section is heated to a temperature of about 870°C without atmosphere control by the direct combustion of gaseous fuel and air therein, and that said stock is passed from said first heating section into outside atmosphere before passing into said further heating section.

8. The method according to claim 5 or 7, characterized in that said fuel is sulfur-bearing coke oven gas.

9. The method according to claim 7, characterized in that said stock is heated to 370° to 485° in said first heating section and to 735° to 925°C
in said further heating section.

10. The method according to claim 1 or 2, characterized in that said further heating section is a radiant tube furnace.

11. The method according to claim 1 or 2, characterized by the addi-tional step of holding said stock at a predetermined temperature in a control zone between said further heating section and said cooling section in a reducing atmosphere containing at least 10% by volume hydrogen.