US3926688A - Method of manufacturing a flat steel product having an oxidation-resistant coating - Google Patents

Abstract

A coating of oxidation-resistant material, having a given oxygen content, is deposited on mild steel sheet. The coated sheet is heat treated under a vacuum. Carbon already present in the coating and carbon diffusing into the coating combines with the oxygen in the coating, the products of this combination escaping because of the vacuum, so that the coating is de-oxidized and decarburized. To prevent chromium subliming because of the high temperature and low pressure, the sheet is heat treated as a compact stack or coil.

Description

United States Patent [191 Liesenborghs et a1.

[ Dec. 16, 1975 METHOD OF MANUFACTURING A FLAT STEEL PRODUCT HAVING AN OXIDATION-RESISTANT COATING Inventors: Roland Cesar Amand Liesenborghs,

Sc1essin; Vincent Marguerite Leroy, Liege, both of Belgium Assignee: Societe Anonyme Cockerill-Ougree-Providence et Esperance-Longdoz, Seraing, Belgium Filed: Jan. 30, 1974 Appl. No.: 437,917

Foreign Application Priority Data Jan. 30, 1973 Belgium 794758 US. Cl. l48/l2.l; 148/14; 148/3l.5 Int. Cl. C21D 7/14 Field of Search 148/14, 12.1, 127, 31.5

[56] References Cited UNlTED STATES PATENTS 2,470,988 5/1949 Kanter 148/14 3,116,179 12/1963 Carpenter et a1.... 148/12.1 3,356,528 12/1967 Gibson et a1 148/14 3,583,887 6/1971 Steger et a1 148/1 2.1

Primary ExaminerR. Dean Attorney, Agent, or FirmHo1man & Stern [57] ABSTRACT A coating of oxidation-resistant material, having a given oxygen content, is deposited on mild steel sheet. The coated sheet is heat treated under a vacuum. C arbon already present in the coating and carbon diffusing into the coating combines with the oxygen in the coating, the products of this combination escaping because of the vacuum, so that the coating is de-oxidized and decarburized. To prevent chromium subliming because of the high temperature and low pressure, the sheet is heat treated as a compact stack or coil.

" 4 Claims, No Drawings PRODUCT HAVING AN OXIDATION-RESISTANT COATING The present invention relates to a method of manu- 5 facturing a flat steel product having an oxidation-resistant coating, the product being, for example, ordinary mild steel sheet. The coating comprises an oxidationresistant material which may consist of one or more of the metals Cr, Ni Co and M or of an alloy of two or more of these metals with one another, or an alloy of one or more of these metals with iron or with other elements.

The most important use of oxidationresistant coatings concerns the protection of ferrous products from corrosion and oxidation, and it has already been-recommended to use for this purpose powders comprising one or more of the metals Cr, Ni, Co, and M0 or their alloys, whose protective properties are well-known.

Certain present-day processes consist in carefully cleaning the substrate, then depositing an alloy on the substrate by spray metallization or by spreading and compacting the powders or by metallizing through evaporation under vacuum, and finally subjecting the coated substrate to a heat treatment at high temperatures in a dry hydrogen atmosphere in order to cause diffusion of the alloy.

After this treatment, the coated product may be rolled, and recrystallization annealed, depending on the mechanical and structural properties required of the finished product.

Tests made on such products have shown that the surface finish of the coatings as well as the quality of the corrosion resistance may be impaired, owing to the two following phenomena.

In coatings provided on mild steel, for example by depositing an alloy such as ferro-chromium, contamination by oxidation of the deposited alloy has been ob served. The diffusion heat treatment carried out in dry hydrogen often proved unsuitable for reducing all the chromium oxides present in the plating, particularly when the coated product was in the form of a compact coil. The result is a deterioration in the surface finish of the product thus treated.

With mild steels having a carbon content of approximately 0.050%, during the diffusion heat treatment in dry hydrogen there is also observed on the one hand a migration into the substrate of the characteristic constituents of the coating and on the other a migration of the carbon of the substrate into the coating, in which it can form chromium carbides. These chromium carbides present in the coating reduce the corrosion resistance of the finished product. In order to avoid this migration of the carbon of the substrate into the coating it has been proposed to use steels which are stabilized with titanium. These additions of titanium to the base steel result in the formation of titanium carbides in the base steel, these carbides being stable at the temperature of the diffusion treatment. It is also possible to avoid this migration of the carbon by using decarburized steels.

The object of the present invention is to provide a method which allows these disadvantages to be removed without using titanium.

The complete manufacturing process, will normally comprise the various standard phases of cleaning the steel, depositing the selected metal or alloy, heat treating the deposit, and then, if required, rolling and re- 2 crystallization annealing. For sheet steel of a given carbon content and thickness the processinc'ludes:

1. making. a deposit of known oxygen content, to a thickness so that the total amount of oxygen is in i stoichiometric proportion to the amount of carbon available in the substrate and in the deposit, with a view to obtaining by a reaction between these .two elements during the diffusion heat treatment, substantially complete de-oxidation of the coating and de-carburization of the substrate and coating so that the formation of, for example, chromium carbide in the coating is negligible;

2. selecting a temperature and time for the diffusion heat treatment which are capable of ensuring not only the above physico-chemical reactions, but

also the desired diffusion of constituents of the deposit into the steel substrate;

3. carrying out this heat treatment under a sufficient vacuum in order to ensure: the elimination from the coating of the products of the reaction of carbon with oxygen;

4. optionally, carrying out the heat treatment under vacuum on a compact coil or stack of the coated sheet with a view to avoiding for example the removal of chromium from. the plating by sublimation at high temperature, a phenomenon which has sometimes been observed as very significant.

It is preferable for the diffusion heat treatment to be carried out at a temperature of 950C to 1,300C for a time of 48 hours to 1/2 hour under a residual pressure of 10 to 10 Torr.

EXAMPLE As a comparative example of a known technique, a

deposit of a Fe-72Cr alloy containing 0.020% C was formed on a steel sheet having a carbon content of 0.055% C and a thickness of 2 mm. The thickness of the deposit was approximately l00 micrometers and its oxygen content was 1.5%. According to the standard technique, a diffusion heat treatment of 16 hours at l,l50C in dry hydrogen of a compact stack of sheets resulted in the formation of a superficially oxidized coating having a carbon content of 0.0160% and a chromium content of around 20%.

On the other hand, in accordance with the present invention, a deposit of an Fe-72Cr alloy containing 0.020% carbon, having a thickness of approximately 100 micrometers and an oxygen content of 1.5% was formed on a steel sheet containing 0.075% C and having a thickness of 2 mm. After heat treatment at l,l50C for 12 hours under vacuum of 10 mm Hg of a compact stack of sheets, a bright or brilliant coating was formed which had a carbon content of between 0.030 and 0.040% and a chromium content of around tion-resistant material selected from the group consisting essentially of the metals Cr, Ni, Co, and Mo, alloys of at least two of the metals with each other, and alloys of at least one of the metals with iron, the coating having an oxgen content of 1.5%; tightly winding the coated sheet to form a compact coil or stack of the coated sheet; and heat treating the compact coil or stack of the coated sheet at a temperature of 950-1300C for /2 hour to 48 hours under a vacuum so 4 prising rolling the coated sheet.

4. The method as claimed in claim 3, further comprising recrystallization annealing the rolled coated sheet.

Claims (4)

1. A METHOD OF MANUFACTURING A STEEL PRODUCT HAVING AN OXIDATION-RESISTANT COATING, COMPRISING THE STEPS OF: DEPOSITING ON A MILD STEEL SHEET A COATING OF AN OXIDATION-RESISTANT MATERIAL SELECTED FROM THE GROUP CONSISTING ESSENTIALLY OF THE METALS CR, NI, CO, AND MO, ALLOYS OF AT LEAST TWO OF THE METALS WITH EACH OTHER, AND ALLOYS OF AT LEAST ONE OF THE METLS WITH IRON, THE COATING HAVING AN OXYGEN, CONTENT OF 1.5%; TIGHTLY WINDING THE COATED SHEET TO FORM A COMPACT COIL OR STACK OF THE COATED SHEET; AND HEAT TREATING THE COMPACT COIL OR STACK OF THE COATED SHEET AT A TEMPERATURE OF 950-1300*C FOR 1/2 HOUR TO 48 HOURS UNDER A VACUUM BEING SUFFICIENT SO THAT THE COATING IS COATING, THE VACUUM BEING SUFFICIENT SO THAT THE COATING IS DE-OXIDIZED AND DECARBURIZED.

2. The method as claimed in claim 1, in which the vacuum has a residual pressure of 10 2 to 10 6 Torr.

3. THE METHOD AS CLAIMED IN CLAIM 1, FURTHER COMPRISING ROLLING THE COATED SHEET.

4. The method as claimed in claim 3, further comprising recrystallization annealing the rolled coated sheet.