US3377162A - Stainless steel - Google Patents

Description

United States Patent 3,377,162 STAINLESS STEEL William Bleloeh, Johannesburg, Transvaal, Republic of South Africa, assignor to Rand Mines Limited, Johannesburg, Transvaal, Republic of South Africa No Drawing. Filed Aug. 30, 1965, Ser. No. 483,826 Claims priority, application Republic of South Africa, Sept. 10, 1964, 4,319/ 64 5 Claims. (Cl. 75-125) ABSTRACT OF THE DISCLOSURE A stainless steel is disclosed whose essential composition is from 1% to 30% chromium; 0.3% to 2.0% vanadium, 2.5% to 4% copper, up to 0.03% carbon and the balance iron.

This invention relates to a stainless steel and more particularly to a stainless steel having good rust resistant a properties but which does not require a large proportion of expensive alloying elements such as molybdenum.

Various stainless steel compositions have been used in the production of steels to meet particular requirements and, in general, the mechanical properties and rust and corrosion resistance are increased only with increased costs of production thus restricting the applications to which the steels can be used economically.

The costs of production are generally increased because of the expensive alloying elements necessary to give the additional rust and corrosion resistant property.

Stainless steel has become generally recognized as a steel which includes more than of chromium but the term is used in this specification to include steels which have any effective amount of chromium in their composition and it is to be understood that an effective amount of chromium as far as rust resistance is concerned is 1% or more. Where percentages of constituents are referred to hereinafter it will be appreciated that these are given as percentages by weight.

It is the object of this invention to provide a stainless steel having at least good rust resistance and other desirable physical properties which may be produced in large quantities and more economically than steels at present available with similar characteristics.

According to this invention there is provided a stainless steel having a constituent composition of at least iron, an effective quantity of chromium, a minimum vanadium content of 0.3% and a carbon content which is below a maximum 0.03%.

It has been appreciated that the inclusion of copper in stainless steel has decided advantages in the resistance of such steels to rusting and to certain other types of cor rosive agents, in increased cold workability and in improved other mechanical properties. Copper is also an alloying element which gives these properties at no great increase in cost of material. However, it has been ditficult to introduce copper into commercially available steels in quantities substantially in excess of 3% unless large proportions of the expensive nickel and/ or molybdenum constituents are also included.

The iron-copper phase diagram indicates that up to 8% of copper is soluble in austenite, and therefore under equilibrium conditions iron-copper alloys up to this composition and of low carbon content should be forgeable in the normal industrial range of 1l001200 C. Alloys containing larger amounts of copper have a duplex structure; the copper rich phase is molten above 1094 C. and incipient melting of the alloys takes place if they are exposed to temperatures above this. In practice, iron-copper alloys containing very considerably less than 8% of copper are subject to incipient melting when exposed to temperatures above 1094 C. and even in some cases below this temperature. A widely held view is that the tendency to incipient melting in alloys which should be free from it is due to the ordinary phenomenon of coring, aggravated by the presence of other unintentional alloying elements which either lower the melting point of the copper-rich phase, reduce the range of solid solubility of copper in iron, or both.

Once produced in large amounts, the copper-rich phase is very difficult to remove because the rate of diffusion of copper in iron is low below 1100 C. Hot-working helps, but if the amount of copper-rich phase is large the metal disintegrates, so that the alternatives are either to prevent the formation of the low melting point phase by adjusment of composition or hot-work only below its melting point.

Further there is the phenomenon of copper penetration from the surfacedue to the oxidation and reduction mechanism; this can take place with very low percentages of 7 copper and leads to relatively shallow surface cracking during hot-work as compared with the complete disintegration which takes place due to general bodily structure.

Special treatment of commercial stainless steel or other than usual processes and temperatures also increases the costs of the final product. It has been found on pilot plant scale tests that copper in excess of 2.5% can be included in the stainless steel composition specified above and result in a steel which can be readily worked at normal steel rolling temperatures, that is, at temperatures of between 1100 C. and 1200" C.

It is accordingly a further feature of this invention that the stainless steel of the type specified includes copper as an alloying element in a quantity in excess of 2.5 and for the percentage of vanadium in the steel to increase with increased copper content.

The invention also provides for the vanadium to be obtained and introduced directly into the steel from an ore containing vanadium such as a Transvaal type titaniferous magnetite.

A still further feature of this invention provides for the steel to be made by the process set forth in our patent application Ser. No. 355,895.

Pilot scale quantities of stainless steel were made in the manner set out in our patent referred to above with a copper content in excess of 2.5% and having a nickel content of:

(a) 10.7%, and (b) Nil Both these melts included a vanadium content of 0.4- 0.6% which was derived directly from the iron ore used for the manufacture of the steel, and the carbon content was maintained at a level of below 0.03%.

In both cases the ingots subsequently removed from the moulds were rolled at a temperature of 1200 C. in a normal commercial steel rolling mill. The ingots were easily rolled and the resultant bars were quite satisfactory.

From the above it will be appreciated that this invention provides a stainless steel which includes appreciable quantities of vanadium and also the very low carbon content which features enable a product to be obtained which has desirable rust resistant properties and at a highly competitive price when made in accordance with the process of our previous patent application Ser. No. 355,895. It will also be clear to those skilled in the art that for certain purposes, the advantageous properties obtainable through the inclusion in very low carbon content stainless steel of vanadium in the range 0.3% to 2.0% and copper in the tectural cladding purposes the steel will have a composition consisting essentially of:

. Percent Chromium 11 Copper 3.0 Vanadium 0.6

the balance being iron and the carbon content being below 0.03%.

What I claim as new and desire to secure by Letters Patent is:

1. A stainless steel consisting essentially of 1% to 30% chromium; 0.3% to 2.0% vanadium; 2.5% to 4% copper; up to 0.03% carbon, the balance being iron, all percentages being by weight of the composition.

2. A stainless steel as claimed in claim 1 wherein the chromium content is at least 11%.

3. A stainless steel as claimed in claim 1 wherein the chromium content is between 11% and 30%.

- i 4. A stainless steel as claimed in claim 1 in which the vanadium is introduced as an oxide in an ore containing vanadium.

5. A stainless steel as claimed in claim 1 in which the vanadium is introduced into the steel directly from a Transvaal type titaniferous magnetite.

References Cited UNITED STATES PATENTS 2,120,554 6/1938 Franks 75-125 2,315,156 5/1943 Larrabee 75-125 2,845,345 7/1958 Bauscher 75-126 2,854,330 9/1958 Tanczyn 75-125 2,193,222 3/1940 Browne 75-128 2,187,525 1/1940 Schafmeister 75-128 XR 1,803,467 5/1931 Driver 75-128 2,968,549 1/1961 Brady 75-126 2,850,380 9/1958 Clarke 75-125 2,797,992 7/ 1957 Eberle 75-125 2,848,323 8/1958 Harris 75-125 XR 2,865,741 12/1958 Smith 75-125 2,894,867 7/1959 Smith 75-128 XR 5 HYLAND BIZOT, Primary Examiner.