US3668022A - Method of treating an alloy steel for enameling - Google Patents

Abstract

A METHOD OF TREATING A COLD ROLLED SHEET OF LOW CARBON STEEL CONTAINING TITANIUM IN ORDER TO PREVENT THE OCCURRENCE OF OBJECTIONABLE SHADOW LINES NORMALLY APPEARING ON THE SURFACE THEREOF AFTER APPLYING A SINGLE OR MULTIPLE COATING OF A VITREOUS ENAMEL WHICH COMPRISES HEAT TREATING THE COLD ROLLED SHEET IN A REDUCING NON-OXIDIZING ATMOSPHERE CONTAINING CARBON MONOXIDE GAS HAVING A CONCENTRATION BETWEEN ABOUT 0.50% AND 1.50% BY VOLUME AND WITH A CONTROLLED AMOUNT OF MOISTURE SO THAT THE TREATINGH ATMOSPHERE REMAINS NON-OXIDIZING TO THE STEEL AT HEAT TREATING TEMPERATURE. THE SHEET IS TREATED IN EITHER A CONTINUOUS NORMALIZING APPARATUS OR IN AN OPENCOIL ANNEALING APPARATUS. THE REQUIRED AMOUNT OF CARBON MONOXIDE GAS CAN BE PROVIDED IN THE TREATING ATMOSPHERE AND THE TREATING ATMOSPHERE MAINTAINED NON-OXIDIZING TO THE STEEL BY PASSING AT LEAST A PORTION OF A REDUCING NONOXIDIZING TREATING ATMOSPHERE COMPRISED OF A MIXTURE OF HYDROGEN AND NITROGEN WITH A SMALL AMOUNT OF MOISTURE THROUGH A POROUS BED OF CHARCOAL DISPOSED WITHIN THE TREATING ZONE.

Description

June 6, 1972 J. P. NOVAK 3,668,022

METHOD OF TREATING AN ALLOY STEEL FOR ENAMELING Filed 001;. 26, 1970 11": "I, 1 u -c 2 47 1 i i I 47' 72 l I l l I I g I l 55 T 75 TY smug: 4s"

71 7O Inventor Jo kn RNova k 5 fiJM N gw fl-i-tornegps United States Patent 3,668,022 METHOD OF TREATING AN ALLOY STEEL FOR ENAMELING John P. Novak, Homewood, Ill., assignor to Inland Steel Company, Chicago, Ill.

Continuation-impart of application Ser. No. 686,618, Nov. 29, 1967. This application Oct. 26, 1970, Ser. No. 83,946

. rm. c1. czia 1/74 US. Cl. 148-16 7 Claims ABSTRACT OF THE DISCLOSURE A method of treating a cold rolled sheet of low carbon steel containing titanium in order to prevent the occurrence of objectionable shadow lines normally appearing on the surface thereof after applying a single or multiple coating of a vitreous enamel which comprises heat treating the cold rolled sheet in a reducing non-oxidizing atmosphere containing carbon monoxide gas having a concentration between about 0.50% and 1.50% by volume and with a controlled amount of moisture so that the treating atmosphere remains non-oxidizing to the steel at heat treating temperature; The sheet is treated in either a continuous normalizing apparatus or in an opencoil annealing apparatus. The required amount of carbon monoxide gas can be provided in the treating atmosphere and the treating atmosphere maintained non-oxidizing to the steel by passing at least a portion of a reducing nonoxidizing treating atmosphere comprised of a mixture of hydrogen and nitrogen with a small amount of moisture through a porous bed of charcoal disposed within the treating zone.

low-carbon steel sheet material, could be coated with a single coat of light-colored vitreous enamel without using a conventional dark-colored ground coat on the steel by incorporating in the steel melt a quantity of titanium, preferably in an amount sufiicient to combine with all of the uncombined carbon, nitrogen and oxygen in the steel melt (see Comstock et al. No. 2,495,835). The foregoing cold reduced-low carbon steel sheet material containing titanium, after coating with a single coat of light-colored vitreous enamel, is relatively free of blistering, specking and reboiling defects commonly found in single coat white enameled articles of conventional steel where no dark-colored ground coat is used. However, a serious defect is encountered as a result of incorporating the titanium in the steel melt; namely, longitudinal lines Which are called shadow lines or steel lines are formed on the cold reduced article or steel strip and are clearly visible after applying a coating of light vitreous enamel.

In the Bernick et al. Pat. No. 3,303,064 a method is disclosed whereby the objectionable shadow lines can be reduced by subjecting a cold rolled titanium-containing steel sheet material to a combination of treating steps comprising (1) a controlled heat treatment in a reducing non-oxidizing atmosphere following the cold reduction tered in burning ofi rolling lubricants, and (2) pickling the heat treated steel in an acid pickling bath. of any conventional composition to effect a pickle weight loss between about .60 and 2.2 grams per square foot of surface area.-The cold reduced steel treated in the foregoing manner can then be subjected to a conventional single coat of light-colored vitreous enameling treatment which preferably includes a conventional nickel flash coating prior to enameling to improve enamel adherence in accordance with good enameling practice.

In the practice of the foregoing method of Pat. No. 3,303,064, however, it has been found that the desired elimination of the shadow lines is not consistently obtained and that the enamel adherence properties of the treated sheet steel material could be improved.

Accordingly, it is an object of the present invention to provide an improved and more economical method of treating a cold reduced low-carbon titanium bearing steel sheet material which makes the steel sheet material better adapted to being coated with a single coatof light-colored or white vitreous enamel without exhibiting objectionable shadow lines.

It is a further object of the present'invention to provide a method of treating a cold reduced low-carbon titanium bearing steel sheet material which consistently removes objectionable shadow lines and improves the enamel adherence properties of the steel sheet material.

Other objects of the present invention will be apparent to those skilled in the art when the following detailed description is read in conjunction with the accompanying drawing wherein:

FIG. 1 is a vertical sectional view of an open coil annealing furnace suitable for use in the present invention; and

FIG. 2 is a schematic vertical sectional view partially in elevation of a modified form of open coil annealing apparatus suitable for use in the present invention.

The foregoing objects of the invention can be achieved by subjecting a cold reduced titanium bearing steel sheet material which normally exhibits objectionable shadow lines to a controlled treatment at an elevated temperature in a reducing atmosphere which is non-oxidizing to the steel and which contains a small amount of carbon monoxide gas. The inclusion of carbon monoxide gas in a concentration of between about .50'% and 1.50% by volume in the reducing non-oxidizing treating atmosphere accelerates the removal of the shadow lines from the cold reduced titanium-containing steel sheet material and improves the enamel adherence properties of the treated material. The carbon monoxide gas appears to act as a catalyst in effecting the removal of the shadow lines. Any suitable manner or means can be employed for providing-the carbon monoxide gas in the treating atmos- .phere.

' It is essential that the moisture content of the treating atmosphere, be carefully controlled to avoid excess water, since the ratio of hydrogen to water in the treating atmosphere comprised essentially of hydrogen and nitrogen at a particular treating temperature largely determines whether the treating atmosphere has a reducing or oxidiz- 1 ingaction on the surface of the steel strip, and this ratio thereof and before the cold reduced steel has been exposed to a highly oxidizing atmosphere, such as encounvaries with the treating temperature. During the initial or heating up period when the steel temperature is relatively low, the dew point of the treating atmosphere is kept 10W in the range of about 30 F. and is preferably maintained not greater than about -4 F. After the heating up period, the dew point of the treating atmosphere can vary as high as +40 F. or above, provided the temperature is sufficiently elevated that the treating atmosphere remains non-oxidizing to the steel and preferably remains reducing. In the preferred process, the maximum moisture content of the treating atmosphere measured at the point of withdrawal from the treating chamber is betweena .dew .pint of about 09.1 and a. maximum .ofl

about -+35 F. with the atmosphere preferably being maintained at a dew point of about F. as measured atrthez point of withdrawal from the treating chamber. The manner of controlling the moisture content of the input treating atmosphere gas is not critical and any means canbe employed which permits maintaining a relatively low dew point .so that the atmosphere remains non-oxidizing to the steel atthe temperature to which the steel is heated. i

The reducing atmosphere is preferably formed of HNX "gas which is comprised of a mixture of betweenabout 4% to 14% hydrogen and the balance being nitrogen with a small amount of water. Other common reducing nonoxidizing gaseousi atmospheres which can be used in the present. invention are DX gas and disassociated ammonia which comprises about 75% hydrogen and 25% nitrogen. It has been found that a very satisfactory method of supplying the required amount of carbon monoxide gas in a relatively dry. reducing non-oxidizing treating atmosphere comprised mainly of hydrogen and nitrogen and forsimultaneously controlling the water vapor content sothatthe treating atmosphere remains non-oxidizing to the steel is to pass the reducing gas through a porous bed of charcoal heated to an elevated temperature of between about 1600" F. and ,1950" F., and preferably having a temperature between about 1600 F. and 1700" F. When a treating. atmosphere comprised mainly of hydrogen and nitrogen with a small amount of moisture is passed through a porous bed of charcoal heated to an annealing temperature, the moisture in the treating gas reacts with the charcoal to produce an amount of carbon monoxide within the range of about 0.50% and 1.50% by volume which effects rapid removal of shadow lines from the sheet material. The carbon bed reaction simultaneously removes from the treating atmosphere 'a portion of the water which tends to build up when the at- .mosphere is recycled in the treating zone. The amount of charcoal required for effecting the foregoing reaction is about two pounds per ton of steel being treated with a minimum quantity of about ten pounds for treating the usual open coil in a conventional open coil annealing furnace. In each instance the quantity of charcoal used in the bed is that required to insure intimate contact between the treating atmosphere and carbon. It should be understood, however, that the entire quantity of charcoal in the bed is not converted into carbon monoxide during the heat treatment process.

If preferred, the water vapor which tends to accumulate when the treating atmosphere is recycled can be removed by a treatment other than that by which the carbon monoxide gas is provided in the treating atmosphere. For example, water can be removed from the treating atmosphere by any known physical process, such as by absorption, adsorption, transformation, or chilling to efiect precipitation, or by any chemical means. The carbon monoxide gas can also be provided by introducing carbon monoxide gas directly into the treating chamber or zone from a suitable supply source or by utilizing any other known chemical reactions to provide carbon monoxide gas in the reducing treating atmosphere without depart- :ing from the scope of the present invention.

As an alternate method of providing the required amount of carbon monoxide in a dry HNX treating atmosphere without causing the atmosphere to become oxidizing to the steel at the treating temperature, carbon dioxide gas in a concentration ranging between about 0.50%

to 1.50% by volume can be added to the treating atmos- F., and the dew point of the treating atmosphere should not exceed-a maximunrvalue of about 409 F. at the point of withdrawal from the treating zone so as to prevent the treating atmosphere oxidizing the surface of the steel.

The temperature and time of heating used in the treating step of the present invention can be varied 'considerably without detracting from the improved results. Genorally," the temperature and time of heat treating the cold reduced low carbon titanium-containing steel in a closed chamber are adjusted to provide the steel with the desired mechanical properties. For example, an enameling steel sheet material must have the necessary strain and aging characteristics for deep drawing, and the heat treating temperature is adjusted accordingly. In every instance, however, the atmosphere in the heating chamber is carefully controlled so that the atmosphere in which the steel is heated remains reducing during the heat treating period and doesnot become oxidizing to the steel while the exposed surface of the strip or sheet-is, at an elevated temperature.

The required treatment of a cold reduced titaniumcontaining steel strip can be continuouslycarried out by employing a continuous normalizing furnace or con- :tinuous annealing furnace wherein the temperature is kept within the herein specified limits and the treating atmosphere is maintained in a reducing non-oxidizing state, or can also be performed batchwise in an open'coil annealing furnace. Accordingly, when continuously heat treat ing a steel strip in a continuous normalizer of conventional design, the steel strip is heated continuously by passing through a heated zone having a reducing nonoxidizing atmosphere which contains between about 0.5% and 1.5% by volume CO with the dew point of the treating atmosphere before contacting the steel strip preferably ranging between about 30" F. and --10 F. and wherein the steel strip is heated to a temperature of at least 1000 F. for a controlled period of time. For example, the strip can be treated for about two minutes in the continuous normalizer when the temperature is about 1650 F. to effect removal of the shadow 'lines. a a The desired-beneficial effects on the enameling properties and mechanical properties of the steel can also be achieved by heat treating the cold reduced titaniumbe'aring steel sheet material'batchwise at a temperature within the temperature range herein specified in an open coil annealing'furnace. In the open coil annealing treatment, the steel strip is placed in an open coil form to prov de spaces between the laps of the coil for the passage of gas over the surface of the strip which'is slowly heatedto "a temperature of about 1620 F. over a period of between 2 and 4 hours, is held at the temperature of about 1620 F. fora period of between 2 and 3 hours, and is then heated to a temperature of between about .1675? F. to 1695 F. and held atthe latter temperature for between about 30 minutes and one hour. The atmosphere in the open coil annealing furnace, is preferably HNX gas, but can be DX gas 'or disassociated ammonia, with the gas always having about 0.5-1.5 percent 'by volume CO and a sufiiciently low dew point toremain non-oxidizing to the steel at the temperature of the gas in contact with the steel. The dew point of the HNX gas as introduced into the treating zone at the beginning of the' treating cycle when the temperature of the steel may be relatively low can be as low as 40 F. and is preferably not greater than about 4 F. The dew point of the treating atmosphere as withdrawn from the treating chamber when the temperature of the'steel is relatively high'can reach'abou t +35 -F. Without being oxidizing to the steel. e a Following the heat treatment of the cold reduced titanium-bearing steel strip, the strip must be pickled by immersingin'an acid pickling bath until a critical pickle weight loss iseffected in order to obtain a satisfactory single coat light-colored vitreous enamel product. Thus, ithas been found necessary to maintain the heat treated steel strip or article in an acid pickling bath, such'as an aqueous sulfuric acid solution containing 6%10% by weight sulfuric acid, until the pickle loss is at least about .60 gram per square foot and not greater than about 2.2 grams per square foot. The preferred amount of pickle loss effected ranges between .75 gram per square foot and 2.0 grams per square foot. Within the latter range of pickle loss the single coat light-colored vitreous enamel strip exhibits excellent enamel adherence and is free of shadow lines.

In the practicing of the improved process of the present invention for treating a coil of titanium-bearing coldrolled steel sheet material, the material in the form of an open coil is placed in an open coil annealing apparatus of the type shown in FIG. 1 of the drawing, wherein the foundation 40 supports a furnace base 45 which in turn supports an outer furnace bell 46 and an inner cover 47 which may be of any suitable design and typeJA' centrifugal fan or blower 51 is supported ona vertical shaft 52 which projects down through the bottom portion53 of the base. The fan or blower 51 can be driven by any suitable means, such as the belt 541'- A diffuser 55 surrounds the fan 51 and directs the discharge there of outwardly and upwardly through the annular opening 56 at the rim of the diffuser. Included in the diffuser 55 are a plurality of radially extending vanes 57, the upper edges of which lie in a common horizontal plane.

The plenum chamber and charge support P is supported on the top edges of the vanes 57 of the difiiuser 55 and includes an annular base plate 58 having a plurality of circumferentially spaced radially extending vertical webs 59 secured thereto. The upper edges of webs 59 all lie in a common horizontal plane and serve to engage and support the open coil C disposed thereon. An annular baflie wall 60 has its outer edge secured to the cylindrical outer Wall 61 of the plenum chamber P and is inclined inwardly and downwardly so that its inner periphery is secured to the annular base plate 58 at the center opening therein. Bathe wall 60, in addition to directing the atmosphere which has been forced downwardly through the spaces between the laps of the coil C into the inlet of fan 51, reinforcesthe vertical webs 59 and assists in giving the necessary rigidity to the entire plenum upper open end of the axial passage at the centerof the lower section of the inner chamber of the annealing apparatus and conveying the treating atmosphere to a separate carbonaceous gas mixing and demoisturizer chamber 72- in which additional carbon monoxide gas is mixed withthe treating atmosphere and the dew point of the atmosphere lowered to an acceptable level. The treating gasor" atmosphere from chamber 72 is then conveyed through conduit 75 to open coil annealing apparatus where it enters the inner chamber thereof and again comes in-contact with the surface of the open coiled steel strip being treated. While the preferred method of adding therequired amount of carbon monoxide gas and control- ,ling the moisture content of the treating atmosphere within the chamber 72 is by passing the atmosphere through a bed of heated charcoal disposed in chamber 72, it is possible to inject into chamber 72 the required quantity of carbon monoxide gas or another carbonaceous gas, such as carbon dioxide or a hydrocarbon gas, which will react with a.compone nt of the treating atmosphere, such as hydrogen or water vapor, at the temperature of the treating chamber or which will be decomposed at the elevated treating temperature to provide at equilibrium a suitable quantity of carbon monoxide gas. And, moisture can be removed from the treating atmosphere within chamber 72 by reacting the moisture in the treating atmosphere with a suitable chemical reagent or by known physical treatments for removing moisture before returning the atmosphere to the open coil annealing apparatus. The secondary treating chamber 72 as shown in FIG. 2 can be adapted for use in combination with a continuous normalizing or continuous annealing furnace for maintaining the treating atmosphere thereof in the required reduced no-oxidizing condition for continuous treatment of an endless strip of titanium bearing steel, if desired.

A still further modified form of the present invention in which the reducing non-oxidizing treating atmosphere containing carbon monoxide gas is used can be carried out in a conventional open coil annealing furnace by "double-stacking an open coil of high carbon steel a-bove the open coil of low carbon titanium steel which is to be treated to prevent formation of shadow lines. The open coil of carbon steel double-stacked with the open coil of -.t itanium. bearing steel contains a sufiicient quantity of of carbon monoxide gas and simultaneously efiect's lowering the "moisture content thereof to the required low levels.

The relatively high carbon content of the carbon steel is coil C. If desired a special charcoal chamber can be mounted adjacent the opening 56 for retaining a-- loose bed of porous charcoal material through which'all 'of the gas being recycled passes before'contacting the surface of'the open coil. 7 v

A modified form of open coil annealing apparatus which can be used in the present invention is shown in FIG. 2 of the drawing, wherein at least a portion of the treating atmosphere of the open coil annealing chamber, after passing over the surface of the steel strip being treated, is

preferably continuously.withdrawnfrom the open coil the moisture content of the treatingatmosphere is adjusted so as to provide the required dew point in' the treating chamber. In FIG. 2 the open coil annealing apparatus parts correspond with the similarly numbered parts described in FIG. 1, and differ therefrom by having a gas outlet conduit 70 which has a blower 71 associated therewith for drawingthe treatingatmo'sphere from the equivalent to having a quantity of coke within the open coil annealing apparatus.

In treating the steel sheet material with open coil annealing'; apparatus in accordance with the present invention it will be understood that it is advisable to employ good opencoil annealing practices in order to obtain the desired results. For example, it is important to correlate the lap gap between the coil winding and the rate of flow of the treating atmosphere in order to insure adequate gas-flow over the surface of the sheet material and to avoid pockets of high moisture content. Thus, if the lap gap is narrow, a higher gas flow rate is required. Also, the vv idth of the coil has an important effect on the flow of gas, with the wider coils requiring a wider lap gap or a higher gas flow rate to provide proper treatment thereof. In coils having a Width of 30 to 40 inches and above a single wire spacer between the coil laps is generally inadequate, and it is preferable to use multiple spacer wires to promote a more uniform gas flow between the laps of the coil. A treating gas flow rate of at least about 1200 cubic feet per hour of HNX gas is used. Higher gas flow rates in excess of 2000 cubic feet per hour can also be used and are preferred for all coils except the very heavy gauge narrow coils.

The following specific example is for the purpose of further illustrating a treatment of steel sheet material in invention.

accordance with the preferred process of the present EXAMPLE I Coils of low carbon titanium bearing steel having a gauge of .0359 inch and having the following approximate composition: .04% C, 35% 'Mn, 34%. T;-'.048% Al and .03 Cu with the remainder being essentially iron, were rewound as open coils using two-.075 inch wires twisted on a .075 inch wire to provide a lap gap of .225 inches. The coils were single stacked (or double stacked where indicated) on-an open coil annealing stand of the type shown in FIG. 1, with an open mesh tray containing about-20 pounds of porous charcoal briquettes supported on the exposed upper end of. a coil being heat treated within the inner cover of the annealing apparatus. At equilibrium, the recirculating treating atmosphere comprised of HNX gas H and 90% N). cohtainsbetween 0.5% and 1.5% by volume carbon monoxide. The temperature within the apparatus and the treating atmosphere was regulated to provide the following heat treat- 1 ment cycle and operating conditions: 1

(1) Temperature of heat treating cycle.

(a) 1 620/1640 F./6 hour soak, single stack (12 hour soak when double stacked).

(b) 1660/1680 1 1/2 hour soak.

(2) AtmOsphere-HNX gas. f

(a) Flow rate2000 cfh. v p (b) Dew point: Inflowing gas, 30 F; tit-10F. Dew point: Exhaust readings, ;+9 F. to +351 F.

Following the foregoing heat treatment cycle the coils were pickled in a 9% by wt. aqueous sulfuric acid bath at a temperature of 170 F. for a period of 5 minutes and thereafter coated in a conventional manner Witha single coat of light-colored vitreous enamel.

The porcelain enamel test results of coils processed in the foregoing manner are shown in the following Table I:

8 (a) disposing in a treating zone a cold reduced titanium-containing steel sheet material having an amount of titanium at least sufiicient to combine with all the carbon in said steelsheetmaterial and whose surface has not been subjected to an oxidizing treatment after cold reduction,

(b) heating said steel sheet material in said treating zone to a temperature between about 1600 F. and

195N1 2, and i A (e) maintaining in intimate contact with said heated steel sheetmaterial in said treating zone a reducing atmosphere which is non-oxidizing" to' said' steel sheet material consisting essentially of hydrogenand.. ni-

trog en with suificient (water vapor to ielfecfadw point of about F. or less during the initial heat up periodand whichpcontains a concentration of between about 0.50% and 1.50% by volume of carbon monoxide gas.' 7 j 2. A method as in claim 1, wherein said carbon monoxide is formed by bringinga said reducing atmosphere having a maximum dew'point of about F. into intimate contact with a bed of porous charcoal at a temperature between about 1600F. and 1950 F.

3. A method as in claim 2', wherein said bed of porous charcoal is disposed within said treating zone containing said steel sheet material. I v

4. A method as in claim 2, wherein said treating zone consists of, an open-coil annealing chamber, and said atmosphere is cycled in contact with a coil of said steel sheet material at a temperature between about 1600 F. and 170.0: F. for a period of about-.8 hours; whereby said coil is simultaneously heat, treated while removal of shadow lines iseffected. V I

5. A method as in claim 1, wherein said atmosphere is passed through a regenerating zone spaced from said treating zone containing said steel sheet material and carbon dioxide gas is supplied to said atmosphere and TABLE I.SUMMARY 0F PoRCELAlNvgfiNAMEL TEST RESULTS OF TI-NAMEL COILS PROCESSED THROUGH THE 00A ITH AND WITHOUT CHARCOAL BBIQUETS Pickle loss, grn./sq.'t't., 5 Appearance, shadow line rating, min. in 170 F., 9% H2804 Adherence, percent of tests percent of tests N 0. of Aver- No. of Excel- No. of Very Modermentity tests agev Range tests lent Good Fan Poor tests None lite Lite ate 40lcoils; processed as in Example 7 "14 single stacked 2s 1. 66 0. 77 2.40 56 20 double stacked 56 1. 46 0. 63-3. 44 283 83 11 3 3 144 20 coils, processed as in Example I g 1: 20 single stacked 70 1. 25 0.66-2.00 410 73 I 19 7 1 205 40 coils, processed as in Example 1, without charcoal: 19 and 20 single stacked 80 1. 01 0. 60-2. 01 v t 478 4 8 33 239 Y 61 I 20 17 2 Open Coil Annealer.

It .will be evident from the foregoing data th at substantial improvements were obtained where the treating gas is passed through a bed of porous charcoal as compared with the results obtained when an identical .treating cycle was employed but without passing the treating gases through a bed of porous charcoal.

In each of the embodiments of the present invention in which the treating atmosphere is brought into; contact with a source of solid carbon, the solid carbon performs the dual functions in the reducing non-oxidizing atmosphere of reducing the amount of moisture in the treating atmosphere to a relatively low level so that the treating atmosphere remains non-oxidizing to the steel and providing a quantity of carbon monoxide gas for the reducingnonoxidizing treating atmosphere so as to readily etfect'removal of the material in the steel which is responsible for the formation of objectionable shadow 'lines'and without adversely effecting the surface or internal properties of the steel. I claim: I 1. In a method of removing objectionable shadow lines from cold reduced titanium-containing steel sheet mate"- rial, the improvement comprising;

moisture removed from said atmosphere in said regenerating zone. and thereafter returning said atmosphere to said treating zone. V

6; A method as in claim 1,wherein said treating zone containing said' sheet material is a continuous normalizing chamber and wherein said atmosphere containing said carbon monoxide gas is brought into' contact with said steel sheet material at a temperature of about 1650f F.

for a period of about 2 minutes.

7. method as claim 1, wherein said carbon monoxide gas content of said atmosphere 'isp'rovided by adding a hydrocarbon gas to said atmosphere.

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