FIELD OF INVENTION:

The present invention relates to steel alloys for structural applications that require enhanced strength and wear resistance.

BACKGROUND:

Steel is a high strength material which is employed in a wide variety of applications. Each of these applications demands a special property in steel. For example, steel is used to manufacture various machine tools and material processing elements. When steel is used as a machine tool, it should have high abrasion resistance to prevent quick wear-down. Apart from wear resistance, fracture strength and high impact toughness are highly desirable qualities when the steel is used as a cutting tool. Similarly, various mining, forestry, manufacturing and construction applications require a good level of abrasion resistance. In yet another example, steel is used to manufacture screws employed in extruders. In this case, steel is subjected to corrosive chemicals like high temperature thermoplastics, acidic substances etc. At times, when the working material in the extruder is glass reinforced thermoplastics the steel has to be abrasion resistant as well. To cater to these varied applications, various steel compositions have been proposed. 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 has helped in increasing the properties like yield and tensile strengths combined with improved toughness.

AISI All (Trade name: CPM lOV) is one of the 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 lOV sets the standard for highly wear resistant industrial tooling. Percentage by weight of primary components of CPM lOV are Vanadium (9.75 %), Molybdenum (1.30 %), Chromium (5.25 %), and Carbon (2.45 %). Vanadium is one of the primary contributors to strengthening in micro-alloyed steels. 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 (Mo), Vanadium (V) have high tendency to form carbides. 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.

Certain modifications and additions have been proposed to increase abrasion resistance. US Patent 4,863,515 titled 'Tool Steel', discloses a composition with relatively higher Chromium and lower Vanadium when compared to CPM lOV. US patent 6,057,045 titled 'High-speed steel article' discloses a composition with high proportions of Molybdenum and Tungsten.

In late 80s low alloy steels have been found to have high wear resistance. European patent EP0531589 titled, "Low-alloy steel, clad with stainless steel by overlay welding and resisting delamination at the interface caused by hydrogen" discloses a low alloy composition. The composition disclosed has very low proportions of Boron (0.0005 to 0.010% wt). Titanium (0.010 to 0.040% wt) and Niobium (0.010 to 0.040% wt) added to Chromium (0.8 to 5.5% wt) and Molybdenum (0.5 to 1.5% wt). It has to be noticed that the European patent keeps Vanadium to very low levels (0.35 % wt max). US Patent 4381940 titled 'Low alloy heat-resisting steel for high temperature use' discloses another low alloy steel composition with low levels of Boron, Titanium, and Vanadium.

The micro-alloying technique has opened up a lot of possibilities for improving the mechanical properties of steel. The reason for improved mechanical properties on alloying with other elements lies in the fact that the micro-structure of steel gets modified.

An object of the present invention is to present a steel composition which has enhanced abrasive resistance.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention provides a steel alloy that comprises in the range of approximately 3 % to 6 % Chromium, in the range of approximately 8% to 12% Vanadium, in the range of approximately 0.50% to 2.00% Molybdenum and in the range of approximately 0.02% to 0.05% Titanium, in the range of approximately 0.002% to 0.02% Boron, in the range of approximately 1.7% to 2.6% Carbon all by weight, with the balance primarily comprising iron and other incidental impurities. In addition to these elements, the said alloy also comprises elements commonly alloyed with iron to make steel. The range of these common elements include, 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. A person ordinarily 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. Chromium is a strong carbide forming element. Chromium carbides settle at the grain boundaries of steel and increase the strength of the material. 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. Carbide dendrite network forms a least resistance path for the crack propagation. Cracks can propagate through the carbide dendrite at very high speeds and hence result in breakage of the material. Reduction in the percentage of Chromium could limit the problem of carbide dendrite network to an extent. However, owing to its high corrosion resistance property of Chromium, it cannot be taken below a certain minimum proportion.
Owing to the above mentioned reasons, Chromium composition is maintained in the range of 3% to 6% 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. Vanadium in high Carbon steels forms hard primary carbides during casting sometimes in combination with other alloying elements present in the steel. These carbides are stable at high temperatures and mostly remain in primary form throughout subsequent forging and heat treatment operations. They are the basis of the wear resistance and cutting performance of cold pressing dies and tools. The stability of these carbides at high temperatures makes the steels containing them suitable for high speed machining operations and gives wear resistance to hot forging and pressing dies. In addition to the primary carbides, some Vanadium remains in solution and contributes to the hard enability, strength and toughness of the tools and dies. During heat treatment some additional hardening occurs to give increased wear resistance as a result of the precipitation of very fine particles of secondary Vanadium carbides. 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.
Boron: A very small amount of Boron (about 0.001%) has a strong effect on hardenability. 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 (creep embrittlement) 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.

Molybdenum: Molybdenum increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low-alloy steels at elevated temperatures. 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.

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. 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. Hardness and tensile strength increases as Carbon content increases. However, ductility and weldability of steel decreases with increasing Carbon content. Hence, the proportion of Carbon has to be kept in control. 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 of the steel. Manganese also contributes to strength and hardness of the steel. Increasing the Manganese content beyond certain limit decreases ductility and weldability. 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 decreases ductility and notch impact toughness of steel. Thus, phosphorous levels are normally controlled to low levels. The composition of phosphorus is limited to 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 levels below 0.03%.

An advantage of the present invention is that the abrasive resistance of the steel alloy may be improved up to 300% over AISI All steel. In the ASTM G65 Test, the steel according to the present indention shows a wear resistance of 0.35 grams in 30 minutes. In comparison, AISI All steel displays a wear resistance of 1.2 grams in 30 minutes for the same test.

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 tine manufacture of tools, comprising in percentage by weight: 0.5% to 2% Molybdenum, 3% to 6% Chromium and 8% to 12% 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, 3% to 6% Chromium and 8% to 12% Vanadium, 1.7% to 2.6% Carbon, 0.002% to 0.02% Boron, 0.02% to 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: 3% to 6% Chromium, 8% to 12% 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: 1.7% to 2.6% carbon, 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.