FIELD OF INVENTION:

The present invention relates to steel alloys for structural applications in which enhanced strength and high corrosion resistance are desirable.

BACKGROUND:

Steel is one of the most widely used alloys in metallurgical applications. Different applications demand different material properties of steel. For example, use of steel in manufacturing tools demands high wear resistance, high toughness, high corrosion resistance and high compressive yield strength.

Corrosion resistance is one of the highly desirable properties in steel. Corrosion can cause a variety of problems, depending on the applications: Perforation of tanks and pipes, which allows leakage of fluids or gases; loss of strength where the cross section of structural members is reduced by corrosion, leading to a loss of strength of the structure and subsequent failure; degradation of appearance and surface finish. Finally, corrosion can produce scale or rust which can contaminate the material being handled; this particularly applies in the case of food processing equipment.

Typically corrosion occurs by oxidation of the metals. In iron or plain carbon steels the oxygen migrates into the underlying material resulting in corrosion. Chromium is generally recognized as an element that can reduce corrosion. When chromium is introduced, oxygen passivates the added chromium, forming a thin protective oxide surface layer. This layer is a spinel structure, only a few atoms thick. It is very dense, preventing diffusion of oxygen into the underlying material. Chromium is usually plated on top of a nickel layer which may first have been copper plated. Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. However, chromium suffers from nitrogen embrittlement - chromium reacts with nitrogen from air and forms brittle nitrides at temperatures necessary to work the metal parts.

As taught by Japanese Patent Public Disclosures No. Sho 63-143240 and 63-143241, steels containing 5-10% of Cr were developed for increasing the corrosion resistance of the steel base material. The corrosion and wear resistant tool steels that are commercially available include grades such as 440C, CPM S90V, M390, Elmax and HTM X235, among others. One of the practices that has been used to improve the combination of resistance to corrosion and wear, as exemplified by CPM S90V, is to add vanadium. This alloying addition forms hard vanadium-rich MC primary carbides and ties up a part of the carbon. The corrosion resistance of tool steels is further improved by the presence of molybdenum in the steel matrix. Another example is Crucible 154 CM grade, which is based on the Fe-1.05C-14Cr-4Mo system. U.K. Pat. No. 1053913 titled 'Hard wear-resistant ferrous alloy' proposes chromium-boron white cast irons containing molybdenum and vanadium, while U.K. Pat. No. 1152370 titled 'Hard, wear-resistant ferrous alloy' proposes nickel-boron cast irons containing molybdenum and vanadium. U.S. Pat. No. 5679908 titled 'Corrosion resistant, high vanadium, powder metallurgy tool steel articles with improved metal to metal wear resistance and a method for producing the same' describes a high vanadium, powder metallurgy cold work tool steel mainly comprising chromium, vanadium, carbon and nitrogen to achieve high corrosion resistance. U.S. Pat. No. 4,765,836 titled 'Wear and corrosion resistant articles made from PM alloyed irons' describes a powder-metallurgy alloy having a good combination of corrosion resistance and wear-resistance containing 15% to 30% chromium 15-30, 6% to 11% vanadium, 2% to 10% molybdenum along with nitrogen, nickel, sulfur, silicon and nickel.

AISI All (Trade name: CPM 10V) is one of the most widely used tool steel compositions. It was introduced in 1978 by Crucible Inc. in US patent number 4,249,945 titled 'Powder-metallurgy steel article with high vanadium-carbide content'. CPM 10V sets the standard for highly corrosion resistant industrial tooling. Percentage by weight of primary components of CPM 10V are Vanadium (9.75 %), (1.30 %), Chromium (5.25 %), and Carbon (2.45 %). Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures. Further, the elements Chromium (Cr), Molybdenum (IMo), Vanadium (V) are highly carbide forming elements. Carbides of these elements impart special characteristics to the steel. Tungsten (W), Titanium (Ti), Niobium (Nb), Tantalum (Ta), and Zirconium (Zr) also form hard (often complex) carbides, increasing steel hardness and strength. Carbide layers commonly produced include vanadium carbide (which is the most commonly used coating for tooling applications), niobium carbide and chromium carbide. Vanadium and niobium carbide layers exhibit superior peel strength and resistance to wear, corrosion and oxidation when compared to other processes. Chromium carbide has light wear resistance and high resistance to oxidation.

It has been found that addition of low proportions of alloying elements can impart special characteristics to the metal. This method is known as Micro-alloying. In steel production, minor additions of elements as Aluminum, Titanium, Niobium, Vanadium, Boron, Zirconium etc have been used to make a more fine-grained microstructure of the steel plate material. This process helped in increasing the properties like corrosion resistance and other material properties. The reason for improved mechanical properties on alloying with other elements lies in the fact that the micro-structure of steel gets modified. The micro-alloying technique has opened up a lot of possibilities for improving the mechanical properties of steel.
An object of the present invention is to present a composition of steel which is highly corrosion resistant as well as wear resistant.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention comprises a steel alloy, particularly suitable for tools, that comprises in the range of approximately 0.002% to 0.02% Boron, in the range of approximately 16% to 36% Chromium, in the range of approximately 3% to 5% vanadium, in the range of approximately 0.50% to 2.00% Molybdenum and in the range of approximately 0.02% to 0.05% Titanium, all by weight, with the balance comprising iron. In addition to these elements, the said alloy will also comprise elements commonly alloyed with iron to make steel. The range of these common elements include approximately 1.7% to 2.6% Carbon, 0.75% to 1.2 % Silicon, 0.35% to 0.65% [Manganese, up to 0.3% Nickel, up to 0.03% Sulphur and up to 0.03% Phosphorus. A person skilled in the art will recognize each element alloyed with the iron contributes to various characteristics of the resulting material.

Chromium: Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance, and to improve high-temperature strength. The chromium within the stainless steel alloy is one of the primary components which inhibit corrosion. The chromium on the stainless steel surface reacts with atmospheric oxygen to form chromium oxide which creates an almost impenetrable barrier between the iron within the stainless steel and the oxygen in the atmosphere. The chromium oxide film forms a very tight and strong bond with the stainless steel and is not easily removed. Additionally, Chromium is a strong carbide forming element thus imparting strength to the steel. However, Chromium carbides formed tend to mobilize and form carbide dendrite network. Formation of carbide dendrite networks could result in catastrophic damages to the material. Reduction in the percentage of Chromium could limit the problem of carbide dendrite network to an extent. Owing to the above mentioned reasons.

Chromium composition is maintained in the range of 16 % to 36 % by weight.

Vanadium: Vanadium forms stable nitrides and carbides. Vanadium carbides, unlike Chromium carbides do not form dendrite networks. Vanadium carbides are very stable and disperse uniformly in the steel matrix. Hence, addition of Vanadium increases the yield strength and the tensile strength of Carbon steel. However, addition of excess Vanadium can affect weldability and ductility of the steel at high temperatures. Owing to the above mentioned reasons, composition of Vanadium is maintained in the range of 8% to 12% by weight.

Molybdenum: flolybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. Molybdenum also combines with Carbon to form stable carbides which also contribute to the creep rupture strength. Molybdenum's ability to add to the strength at high temperatures reduces with increasing proportions. Owing to the reasons specified above, the composition of Molybdenum is maintained in the range of 0.50% to 2.00% by weight.

Boron: Boron settles in the inter-granular spaces and fills the gaps between the grains. As a result, the changes to the grain structure at high temperatures are prevented. Further, Boron prevents dendrite network formation of chromium carbides thereby increasing crack propagation resistance of the steel. Boron also limits the grain size of the steel and hence improves toughness. However, addition of Boron in high quantities results in lower workability of steel at elevated Temperatures. Owing to the above mentioned factors, composition of Boron is maintained in the range of 0.002% to 0.02% by weight.

Titanium: Titanium, like Boron, is used to retard grain growth and hence refines the grain structure. Finer grain structure results in improved toughness. However, Titanium is a strong oxide and Nitride forming element. The formation of these oxides and nitrides results in low ductility and toughness. Owing to the above specified factors, the composition of Titanium is maintained in the range of 0.02% to 0.05% by weight.

Carbon: Carbon has a major effect on steel properties. Carbon is the primary hardening element in steel. Hardness and tensile strength increases as Carbon content increases However, ductility and weldability of steel decreases with increasing Carbon. Hence the proportion of Carbon has to be kept in check. Carbon forms carbides with Chromium, Vanadium, Molybdenum and these carbides settle in the inter-granular space. These carbides distribute uniformly in the steel matrix and impart toughness to the resultant product. Owing to the above mentioned reasons. Carbon composition is maintained in the range of 1.7% to 2.6% by weight.

Other Elements: Manganese is generally beneficial to surface quality especially in resulphurized steels. Manganese contributes to strength and hardness, but less than carbon. The increase in strength is dependent upon the carbon content. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel. Hence the composition of Manganese is maintained in the range of 0.35% to 0.65% by weight. Silicon is one of the principal deoxidizers used in steel making. In low-carbon steels, Silicon is generally detrimental to surface quality. The composition of silicon is maintained in the range of 0.75% to 1.2 % by weight. Nickel is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels but makes the steel embrittlement susceptible. Hence the composition of Nickel is limited to 0.3% by weight. Phosphorus increases strength and hardness and decreases ductility and notch impact toughness of steel. The adverse effects on ductility and toughness are greater in quenched and tempered higher-carbon steels. Thus, Phosphorous levels are normally controlled to low levels upto 0.03%. Similarly, Sulphur decreases ductility and notch impact toughness especially in the transverse direction. Further, weldability decreases with increasing sulphur content. Thus, Sulphur levels are normally controlled to low levels upto 0.03%.

The alloy of the present invention also comprises the standard proportion of elements that are commonly used when preparing steel like Carbon, Silicon, Manganese, Nickel, Sulphur and Phosphorus.

Although the alloy of the present invention has been described in connection with specific examples and embodiments, it will be understood by those skilled in the art the variations and modifications of which this invention is capable without departing from its broad scope.

It is to be understood that this invention is not limited to particularly exemplified apparatus, systems, structures or methods as such may, of course, vary*. Thus, although a number of apparatus, systems and methods similar or equivalent to those described herein can be used in the practice of the present invention, the preferred materials and methods are described herein.

We claim:

1. A steel alloy composition suitable for the manufacture of tools, comprising in percentage by weight: 0.5% to 2% Molybdenum, 16% to 36% Chromium and 3% to 5% Vanadium, 0.002% to 0.02% Boron, 0.02% to 0.05% Titanium, 0.75% to 1.2% Silicon, 0.35% to 0.65% Manganese, 1.7% to 2.6% Carbon, within 0.3% Nickel, within 0.03% Sulphur and within 0.03% Phosphorus, the remainder being iron and other incidental impurities.

2. The steel alloy composition as claimed in claim 1 is produced by means of molten metallurgy processes.

3. A steel alloy composition suitable for the manufacture of tools, comprising in percentage by weight: 0.5% to 2% Molybdenum, 16% to 24% Chromium and 3% to 5% Vanadium, 1.7% to 2.6% Carbon, 0.002% to 0.02% Boron, 0.02% 0.05% Titanium, the remainder being iron and incidental impurities.

4. The steel alloy composition as claimed in claim 3 is produced by means of molten metallurgy processes.

5. The steel alloy as claimed in claim 3 further comprises in percentage by weight:

0.75% to 1.2% Silicon, 0.35% to 0.65% Manganese, within 0.3% Nickel, within 0.03% Sulphur and within 0.03% Phosphorus.

6. A steel alloy composition suitable for the manufacture of tools, comprising in percentage by weight: 16% to 24% Chromium, 3% to 5% Vanadium, 0.002% to 0.02% Boron, 1.7% to 2.6% Carbon, the remainder being iron and other incidental impurities.

7. The steel alloy composition as claimed in claim 6 further comprises about 0.5% to 2% Molybdenum.

8. The steel alloy composition as claimed in claim 6 further comprises about 0.02% to 0.05% Titanium.

9. The steel alloy composition as claimed in claim 6 further comprises in percentage by weight: 0.75% to 1.2% silicon, 0.35% to 0.65% manganese, 0% to 0.3% nickel, upto 0.03% sulphur and upto 0.03% phosphorus.

10. The steel alloy composition as claimed in claim 6 is produced by means of molten metallurgy processes.