# Patent

## Title
Low temperature microcrystalline zinc phosphate coatings, compositions,

## Abstract

A compound steel material of high corrosion resistance is manufactured by alloying a carrier material of deep-drawing grade with a carbide and nitride forming substance, compounding the alloyed carrier material with a chromium steel material of normal carbon content, followed by annealing the compound steel material at a temperature and for a time period sufficient for the carbon content of the ferritic chromium steel coating layer to be reduced to between 0.001 and 0.003% so as to increase the corrosion resistace of the chromium steel material to that of a superferritic material. Prior to annealing, the compound steel material may be rolled to a fine sheet. The carrier material used in the method of the invention is a deep-drawing steel of at most 0.12% of carbon, while the ferritic chromium steel material has at most 0.1% of carbon prior to the annealing. The carbide and nitride forming substance may be titanium whose content in the alloyed carrier material is preferably between 0.5 and 2%. The thickness of the ferritic chromium steel coating layer is between 50 and 500 .mu.m, the annealing temperature between 650 and 900.degree. C., and the annealing time period between 8 and 72 hours.

## Background
The present invention relates to a method of manufacturing a compound material, and more particularly to a method of manufacturing such a material which has a high corrosion resistance.

The corrosion resistance of ferritic chromium steel materials is, in addition to the chromium content and the addition of, for instance, molybdenum, determined by the carbon and nitrogen content in the steel material. While the conventionally manufactured ferritic chromium steel AISI 430 corresponding to SAE 51430 has a lower resistance to oxidizing and reducing acids, bases, and sulfur dioxide and chlorine containing atmosphere than an austenitic chromium-nickel steel AISI 304 corresponding to SAE 30304, its corrosion resistance at the same chromium content improves with the reduction of the carbon and nitrogen content. When the carbon content is between 0.001 and 0.003% and the nitrogen content is approximately 0.01%, this chromium steel (superferrite) has corrosion resistance values which are higher than those of the austenitic chromium-nickel steel. In particular, the resistance of this superferritic steel to strain fissure, intercrystalline, pitting and contact corrosion is better than the resistance of the austenitic alloyed steel. Also, the corrosion-resistance in oxidizing and reducing acids, as well as in the atmosphere, is higher than that of the austenitic chromium-nickel steel.

Ferritic chromium steel materials with reduced carbon contents of 0.001 to 0.003% can, according to the present state of the melting and refining techniques, be manufactured only in electron beam vacuum ovens. The other improved manufacturing methods, such as the vacuum oxygen blow-refining method, the vacuum inductive melting method (VIM) and the argon-oxygen decarburization method (AOD) produce steel materials of carbon contents of between 0.01 and 0.02%. In order to give the "extra low carbon" steel materials having 0.01 to 0.02% of carbon content, a corrosion resistance which equals or exceeds the corrosion resistance of the austenitic steel materials, the carbon and nitrogen contents of the steel material must be chemically reacted, that is, stabilized. In most instances, titanium is used for the stabilization, but other alloying elements, such as niobium and tantalum can also be used for the same purpose.

Experience and tests with ferritic chromium steel materials which are stabilized with titanium have shown that such materials have a substantially higher corrosion resistance than non-stabilized chromium steel materials having a normal carbon and nitrogen content (such as SAE 51430 steel). The titanium content must be at least six times higher than the sum of the carbon and nitrogen contents. The ferritic chromium steel materials which are stabilized with titanium, however, are disadvantageous in comprison with the non-stabilized materials in that they have a lower purity and have a surface of worse quality (the surface cannot be annealed or ground to give it a mirror-like appearance).

Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.

More particularly, it is an object of the present invention to devise a method of producing a compound material which is provided with a chromium steel coating layer at one or more of its surfaces, wherein the corrosion resistance of the chromium steel layer corresponds to that of a superferritic material.

A further object of the present invention is to present a method of manufacturing a corrosion-resistant compound material without resorting to the use of nonstabilized superferritic materials.

In pursuance of these objects and others which will become apparent hereafter, one feature of the present invention resides, briefly stated, in a method of manufacturing a compound steel material having at least one chromium steel coating layer, which comprises the steps of alloying a carrier material of deep-drawing grade with a carbide and nitride forming substance; compounding the alloyed carrier material with a chromium steel material of normal carbon content; and increasing the corrosion resistance of the chromium steel material to that of a superferritic material, including annealing the compound steel material at a temperature and for a time period sufficient for the carbon content of the ferritic chromium steel coating layer to be reduced to between 0.001 and 0.003%. Preferably, the method of the present invention further comprises the step of rolling the compound steel material to a fine sheet prior to the increasing step.

As mentioned above, it is an object of the present invention to obtain a material which has the same or a higher corrosion resistance than an austenitic chromium-nickel steel SAE 30 304, which has the following composition: C     Si      Mn      P     S     Cr     Ni ______________________________________ 0.07  1.00    2.00    0.045 0.030 17-20  8.50-10 ______________________________________

To achieve this objective, the present invention starts with a ferritic chromium steel according to a SAE 51 430, which has the following composition: C      Si      Mn      P      S      Cr     Ni ______________________________________ 0.010  1.00    1.00    0.045  0.030  15.50- 17.50 ______________________________________

This ferritic chromium steel material is compounded with a carrier material which is alloyed with carbide and nitride forming elements, in particular titanium, whereupon the compound material is rolled into fine sheets and subsequently annealed until the carbon content in the ferritic chromium steel coating layer is reduced to between 0.001 and 0.003%.

The carrier material which is used to form the compound material is a deep-drawing steel of carbon contents of at most 0.12%. On the other hand, the carbon content of the ferritic chromium steel material which is to be decarburized should not be above 0.1%.

The titanium content in the alloyed carrier material is preferably in the range between 0.5 and 2%. Preferably, the thickness of the chromium steel coating layer to be decarburized is between 50 and 500 .mu.m.

The applicable annealing temperatures are preferably in the region between 650.degree. and 900.degree. C., while the annealing time periods can be between 8 and 72 hours depending on the titanium content of the carrier material, the thickness of the coating layer, and the selected temperature. Under certain circumstances, even substantially shorter annealing time periods are necessary.

The decarburization of the ferritic chromium steel coating layer takes place on the basis of diffusion. The advantage of this decarburizing procedure resides in the fact that only a very thin layer of the coating material is to be treated and that the treatment is accomplished after the termination of the fine sheet manufacturing process. After the annealing, the fine sheet is dressed and adjusted as usual in the customary alloyed steel manufacturing process. According to this method, there can be obtained carbon contents in the plating layer of 0.001 to 0.003%, which gives the ferritic chromium steel a corrosion resistance which is higher than that of the austenitic chromium-nickel steel. The obtained decarburized, non-stabilized chromium steel layer is devoid of carbide inclusions and impurities and thus the exposed surface thereof can be polished to high luster (highly lustrous polished surfaces possess an even better corrosion resistance). The decarburization of thin alloyed steel layers in accordance with the diffusion process is also possible in connection with sheets of intermediate and large thicknesses. The decarburization is independent of the chromium content of the plating layer. The corrosion resistance of the plating layer is improved with an increasing chromium content and by addition of molybdenum.

Preferably, a material is used as the starting compound material in which the carrier material alloyed with titanium, in its molten state, is poured on a ferritic chromium steel sheet in a compounding process, the chromium sheet having been introduced into an ingot mold prior to the pouring or molding of the carrier material.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of processes differing from the types described above.

While the invention has been illustrated and described as embodied in a method of manufacturing a compound steel material of high corrosion resistance, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

## Claims

forth in the appended claims:

1. A method of manufacturing a compound steel material having a corrosion-resistant coating, comprising the steps of alloying a carrier steel material of deep-drawing grade which has a carbon content of at most 0.12 with at least one of titanium, niobium or tantalum adapted to form carbides or nitrides; pouring the alloyed carrier steel material in molten state on a ferritic chromium steel sheet of a carbon content of at most 0.1; and increasing the corrosion resistance of the ferritic chromium steel sheet to that of a superferritic material by annealing the compound steel material at a temperature of substantially between 650.degree. and 900.degree. C. for a time period amounting to substantially between 8 and 72 hours whereby the carbon content of the ferritic chromium steel layer is reduced to between 0.001 and 0.003%.

2. A method as defined in claim 1 wherein the carrier steel material is alloyed with titanium so as to obtain a titanium content in the alloyed carrier material of between 0.5 and 2%.

3. A method as defined in claim 1, and further comprising the step of rolling the compound steel material to a fine sheet prior to said increasing step.

4. A method as defined in claim 1, wherein the thickness of the ferritic chromium steel coating layer is between 50 and 500 .mu.m.

5. A method as defined in claim 1, wherein the alloyed carrier material is an SAE 1006 steel stabilized with titanium.