Claims:We claim:
1. An alloy steel composition comprising:
a) about 0.3 wt % to about 0.4 wt % of carbon; b) about < 1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0 wt. % of nickel; e) about 1.5 wt. % to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities less than 0.02 wt%.

2. The alloy steel composition as claimed in claim 1, wherein said composition is essentially free of cobalt.

3. The alloy steel composition as claimed in claim 1, wherein said composition is essentially free of tungsten.

4. The alloy steel composition as claimed in claim 1, wherein said composition is essentially free of Vanadium.

5. The alloy steel composition as claimed in claim 1, wherein said composition characterized in that said composition exhibits at least one of an ultimate tensile strength of =1850 MPa; a yield strength at 0.2% offset of = 1450 MPa; % strain to failure of = 14%; an impact toughness as measured by a Charpy V-notch test at room temperature of = 30 J, a Hardness Rockwell C-scale of = 52; and a fracture toughness of = 75 MPavm.

6. A method of thermally processing the alloy steel composition said method comprising the following steps:
i) austenitizing a sample of the alloy steel composition comprising about 0.3 wt % to about 0.4 wt % of carbon; b) about < 1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0% of nickel; e) about 1.5 wt. % to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities by heating the sample to an austenitizing temperature to form an austenite mixture;
ii) quenching the austenite mixture to a temperature below a martensitic-forming temperature in a quenching medium selected from the group consisting of oil, water, gas and polymer to obtain quenched alloy steel composition, and
iii) tempering the quenched alloy steel composition,
wherein said tempering comprising heating the quenched alloy steel composition to a tempering temperature, which is in a tempering temperature range that is less than about 280° C.

7. A method of manufacturing a steel alloy; said method comprising the following steps:
c. providing a steel composition comprising about 0.3 wt % to about 0.40 wt % of carbon; b) about <1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0 wt. % of nickel; e) about 1.5 to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities; and
d. subjecting said composition to a manufacturing technique selected from the group consisting of Electric Arc, Ladle Refined and Vacuum Treatment; Vacuum Induction Melting; Vacuum Arc Re-Melting, and Electro Slag Re-Melting to obtain said steel alloy.

8. A process for manufacturing a steel alloy article; said process comprising the following steps:
? providing a steel alloy made of a composition comprising about 0.3 wt % to about 0.40 wt % of carbon; b) about <1 wt% manganese; c) about 1.0 wt. % to about 2.5 wt % of silicon; d) about 1.5 wt. % to about 5.0 wt. % of nickel; e) about 1.5 wt. % to about 3.5 wt % of Chromium; f) about maximum 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities; and
? forging said steel alloy under predetermined temperature to obtain said steel alloy article, wherein the forging is selected from the group consisting of open die forging and closed die forging.

9. A process for manufacturing a steel alloy article; said process comprising the following steps:
? providing a steel alloy made of a composition comprising about 0.3 wt % to about 0.40 wt % of carbon; b) about <1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0 wt. % of nickel; e) about 1.5 wt% to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities; and
? obtaining the steel alloy article from said steel alloy.

10. The process as claimed in claim 9, wherein the step of obtaining the steel alloy article comprises a method comprising hollow extrusion, static or centrifugal casting, continuous casting, plate rolling and bar rolling under predetermined temperature.

Dated this 29th January 2019

CHIRAG TANNA
OF NOVOIP
APPLICANT’S PATENT AGENT
, Description:FIELD OF THE INVENTION
[0001] The present invention relates to a lean alloy steel composition, its preparation and articles made therefrom. Particularly, the present invention relates to a lean alloy ultra-high strength steel composition, its preparation and articles made therefrom.
BACKGROUND & INTRODUCTION
[0002] Steel compositions having high strength, high fracture toughness and high performance are utilized in various fields. These compositions are used in critical applications such as aircraft components, automotive parts, pressure vessels, hydraulic and mechanical press components, die blocks, defense components, and the like.
[0003] The available steel compositions are known to be expensive to use due to the presence of high alloy content and its expensive manufacturing process. In general, the known prior art compositions have richer elements such as Nickel, Tungsten, vanadium and Cobalt to provide high strength and high performance to said composition. As these are expensive elements these contributes to the high overall cost of the final steel product. For instance, to produce strength and toughness combinations, traditional steels often contain significant amounts of nickel and cobalt, often totaling more than 15% of the alloy combinations. Other alloy steels contain difficult to process alloy additions, such as tungsten.
[0004] U.S. Pat. No. 4076525 discloses a high strength (1500 to 1860 MPa), high performance steel at an expensive cost due to the high weight percentage of nickel, which comprises about 9.5 to about 10.25 percent by weight of the entire steel composition.
[0005] Another alloy composition, described in U.S. Patent Appl. Publ. No. 2010/0018613, provides a high strength (about 1600 MPa), high toughness (32 J) FeCuNiCr alloy steel that includes 0.35% to 0.55% carbon, 0.5% to 0.6% copper, 3.5% to 7.0% nickel, and 0.75% to 2.0% chromium, as well as requiring specific weight ratios of silicon (Si), copper (Cu), Vanadium (V), and niobium (Nb).
[0006] US Patent No. 7537727 attempted to provide a low alloy, low to medium carbon content, high strength, and high ductility steel composition which comprises about 0.16% to about 0.35% carbon, about 0.85% maximum manganese, an amount of silicon up to about 1.25% maximum, about 1.50% to about 3.25% chromium, about 5.00% maximum nickel, about 0.55% maximum molybdenum, 1.17% to about 3.25% tungsten, about 0.05% to about 0.30% vanadium, about 0.50% maximum copper, about 0.015% maximum phosphorous, about 0.012% maximum sulfur, about 0.02% maximum calcium, about 0.14% maximum nitrogen, and about 0.05% maximum aluminum. However, said patent discloses the composition which contains up to 3.25% tungsten, which is expensive, limited in supply, and difficult to process due to the high melting point and very high density of the master alloy. Further, the higher levels of tungsten increase subsequent heat treatment process cost on account of higher austenizing temperatures.
[0007] Another alloy composition, described in CN103147020A, provides a low temperature tempered martensitic ultra-high strength steel (~ 2000 Mpa UTS and > 80 Mpa vm fracture toughness) with chemical components in wt. % of elements 0.35%-0.45% of C, 3%-6% of Ni, 2%-5% of Co, 1.5%-2.5% of Si, 1.5%-2.5% of Mn, 0.5%-1.25% of Cr, 0.5%-2.0% of Mo, 0.01%-0.05% of Nb, 0.01%-0.03% of Ti, not more than 0.005% of S, not more than 0.005% of P, not more than 20ppm of O, not more than 20ppm of N, and the balance being Fe. However, said chemical composition contains cobalt up to 5% which is costlier element.
[0008] Accordingly, there is still a need in the art for high strength steel composition that can be produced relatively inexpensively without compromising on desirable high performance characteristics.
OBJECT OF THE INVENTION
[0009] It is an object of the present invention to provide a lean steel composition having ultra-high strength as well as toughness.
[00010] It is another object of the present invention to provide a leaner alloy ultra-high tensile strength steel composition which provides higher fracture toughness.
[00011] It is another object of the present invention to provide a leaner alloy ultra-high tensile strength steel composition which provides high strength to weight ratio favoring component weight reduction.
[00012] It is another object of the present invention to provide a leaner alloy ultra-high tensile strength steel composition which is devoid of costly elements such as tungsten, vanadium and cobalt.
[00013] It is another object of the present invention to provide a leaner alloy ultra-high tensile strength steel composition which is amenable to conventional heat treatment.
SUMMARY OF THE INVENTION
[00014] The present invention overcomes one or more of the existing needs in the prior art by providing a leaner alloy, medium carbon, low nickel and low chromium content steel composition without addition of expensive elements like Cobalt, Vanadium or tungsten. Said composition still provides desirable high performance characteristics similar to known high strength steel compositions.
[00015] The present invention provides a high strength, good impact toughness at high strength levels, and high fracture toughness. In one embodiment said composition comprises about 0.3 wt. % to about 0.40 wt. % of carbon, about <1.0 wt. % of manganese, about 1.0 wt. % to about 2.5 wt. % of silicon, about 1.5 wt. % to about 5.0 wt. % of nickel, about 1.5 wt. % to about 3.5 wt. % of Chromium and about 0.4 wt. % to 0.9 wt. % of Molybdenum, and balance consisting essentially of iron optionally with impurities less than 0.02 wt. %.
[00016] The alloy steel composition thermally processed has at least one of the following properties: an ultimate tensile strength of about 1850 MPa or more; a yield strength at 0.2% offset of about 1450 MPa or more; an % strain to failure of about 14 % or more; an impact toughness as measured by a Charpy V-notch test at room temperature of about 30 J or more, or a Hardness Rockwell C-scale of about 52 or more; a fracture toughness of about 75 MPavm or more.
[00017] In accordance with another aspect there is also provided a method of thermally processing the alloy steel composition said method comprising the following steps:
i) austenitizing a sample of the alloy steel composition to an austenitizing temperature to form an austenite mixture;
ii) quenching the austenite mixture to a temperature below a martensitic-forming temperature in a quenching media selected from the group consisting of oil, water, gas and polymer to obtain quenched alloy steel composition and
iii) tempering the quenched alloy steel composition,
wherein said tempering comprising heating the quenched alloy steel composition to a tempering temperature, which is in a tempering temperature range that is less than about 280° C.
[00018] In still another aspect there is provided a method of manufacturing a method of manufacturing a steel alloy which involves subjecting the steel composition to a manufacturing technique selected from the group consisting of Electric Arc, Ladle Refined and Vacuum Treatment; Vacuum Induction Melting; Vacuum Arc Re-Melting, and Electro Slag Re-Melting.
[00019] In yet another aspect there is provided a process for manufacturing a steel alloy article which involves forging the steel alloy under predetermined temperature to obtain said steel alloy article, wherein the forging is selected from the group consisting of open die forging and closed die forging.
[00020] In a further aspect there is provided a process for manufacturing a steel alloy article; said process comprising providing a steel alloy made of a composition and obtaining the steel alloy article from said steel alloy, wherein the step of obtaining the steel alloy article comprises a method comprising hollow extrusion, static or centrifugal casting, continuous casting, plate rolling and bar rolling under predetermined temperature.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[00021] Figure 1 illustrates fine tempered martensite with randomly oriented laths; and
[00022] Figure 2 TEM image showing fine lath martensitic structure with nano ? carbides.

DETAILED DESCRIPTION
[00023] The present invention particularly focused on providing a leaner alloy steel composition which utilizes moderate amount of molybdenum, chromium and nickel and strictly avoids addition of strong carbide forming elements like tungsten, Vanadium and cobalt yet achieves desired high strength and high performance.
[00024] In one embodiment the present invention provides an alloy steel composition comprising a) about 0.3 wt % to about 0.4 wt % of carbon; b) < 1.0 wt. % of manganese; c) about 1.0 wt. % to about 2.5 wt. % of silicon; d) about 1.5 wt. % to about 5.0 wt. % of nickel; e) about 1.5 wt. % to about 3.5 wt. % of Chromium; f) about 0.4 wt. % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities < 0.02 wt%.
[00025] Carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases sensitivity to quench cracking and also reduces weldability. Therefore, in one embodiment, carbon is present in an amount of about 0.3 % but not more than about 0.4 %.
[00026] Chromium primarily increases hardenability of steel and increase the corrosion resistance as well as the yield strength of the steel. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains chromium in an amount of about 1.5 wt. % to 3.5%.
[00027] Molybdenum plays important role in the solid solution strengthening and precipitation strengthening which enhances fracture toughness. However, excessive molybdenum leads to segregation and forms detrimental MC carbides (M-metal) at temperatures just below the austenitization transformation temperature. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains molybdenum in of about 0.4 wt. % to 0.9 wt.%.
[00028] Manganese helps to improve the strength and hardenability of the steel. However excessive manganese can lead to segregation, banding and lower toughness due to undesirable secondary phases. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains manganese in maximum amount of 1 %.
[00029] Nickel enhances the low-temperature behavior of the material by improving the fracture toughness and favorable for increase hardenability. Thus, in accordance with one embodiment of the present invention, nickel is restricted about 1.5 wt. % to 5.0 wt. %.
[00030] Silicon benefits the hardenability and temper resistance of this alloy. Silicon is added to suppress cementite formation. However, too much silicon adversely affects the hardness, strength, and ductility of the alloy steel composition through over-stabilization of bainitic phases. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains silicon in an amount of about 1.0% to 2.5%.
[00031] The alloy may contain small amount of impurities such as P, S, etc. < 0.02%.
[00032] The alloy steel composition is essentially free of cobalt, tungsten and Vanadium.
[00033] The term “essentially free” refers to no external addition of metal element. However, it may be present as a fractional impurity less than 0.02%.
[00034] The steel alloy of the present invention can be manufactured by the following processes: (i) Electric Arc, Ladle Refined and Vacuum Treated; (ii) Vacuum Induction Melting; (iii) Vacuum Arc Re-Melting, and/or (iv) Electro Slag Re-Melting. The use of the end product will dictate the manufacturing process that should be applied. As the liability and number of manufacturing processes increase, the cost also increases. End products made from steel (steel alloy articles) can be produced using open die forging, close die forging, solid or hollow extrusion methods, static or centrifugal castings, continuous casting, ingot casting, plate rolling, bar rolling or other conventional methods under predetermined conditions like temperature.
[00035] In accordance with another embodiment of the present invention, a method of thermally processing of the alloy steel composition is provided. The method includes austenitising a sample of the alloy steel composition by heating the sample to an austenitising temperature of 900° C to 1100° C that is above a critical temperature to form an austenite mixture followed by quenching in one of the quench media such as oil, polymer, water, gas etc. to a temperature below a martensitic-finish forming temperature to obtain quenched alloy steel composition; and tempering the quenched austenitized alloy steel composition. Based on the Quenchant media like oil or water or gaseous or polymer, the steel properties may improve. The tempering step includes heating the austenitized alloy steel composition to a tempering temperature, which is in a tempering temperature range 160° C to 280° C.
[00036] The alloy steel composition thermally processed in accordance with the foregoing method has at least one of the following properties:
? an ultimate tensile strength of about 1850 MPa or more;
? a yield strength at 0.2% offset of about 1450 MPa or more;
? % strain to failure of about 14% or more;
? an impact toughness as measured by a Charpy V-notch test at room temperature of about 30 J or more, or
? a Hardness Rockwell C-scale of about 52 or more; a fracture toughness of about 75 MPavm or more.
? The microstructural features of the present alloy show the presence of fine laths of martensite along with the fine precipitates of ? carbides. The size of the ? carbides ranges from 100 nm to 150 nm in length and about 5-15 nm in width. The distribution of these carbides provides high strength and toughness to the alloy in the present invention.

[00037] In one embodiment, the method of thermally processing the alloy steel composition comprising the following steps:
i) austenitizing a sample of the alloy steel composition comprising about 0.3 wt % to about 0.4 wt % of carbon; b) about < 1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0% of nickel; e) about 1.5 wt. % to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities by heating the sample to an austenitizing temperature to form an austenite mixture;
ii) quenching the austenite mixture to a temperature below a martensitic-forming temperature in a quenching medium selected from the group consisting of oil, water, gas and polymer to obtain quenched alloy steel composition, and
iii) tempering the quenched alloy steel composition,
wherein said tempering comprising heating the quenched alloy steel composition to a tempering temperature, which is in a tempering temperature range that is less than about 280° C.

[00038] In one embodiment, the method of manufacturing a steel alloy comprising the following steps:
a. providing a steel composition comprising about 0.3 wt % to about 0.40 wt % of carbon; b) about <1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0 wt. % of nickel; e) about 1.5 to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities; and
b. subjecting said composition to a manufacturing technique selected from the group consisting of Electric Arc, Ladle Refined and Vacuum Treatment; Vacuum Induction Melting; Vacuum Arc Re-Melting, and Electro Slag Re-Melting to obtain said steel alloy.

[00039] In one embodiment, the process for manufacturing a steel alloy article; said process comprising the following steps:
? providing a steel alloy made of a composition comprising about 0.3 wt % to about 0.40 wt % of carbon; b) about <1 wt% manganese; c) about 1.0 wt. % to about 2.5 wt % of silicon; d) about 1.5 wt. % to about 5.0 wt. % of nickel; e) about 1.5 wt. % to about 3.5 wt % of Chromium; f) about maximum 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities; and
? forging said steel alloy under predetermined temperature to obtain said steel alloy article, wherein the forging is selected from the group consisting of open die forging and closed die forging.

[00040] In one embodiment, the process for manufacturing a steel alloy article comprising the following steps:
? providing a steel alloy made of a composition comprising about 0.3 wt % to about 0.40 wt % of carbon; b) about <1 wt % of manganese; c) about 1.0 wt % to about 2.5 wt % of silicon; d) about 1.5 wt % to about 5.0 wt. % of nickel; e) about 1.5 wt% to about 3.5 wt % of Chromium; f) about 0.4 wt % to about 0.9 wt. % of Molybdenum; and g) balance consisting essentially of iron optionally with impurities; and
? obtaining the steel alloy article from said steel alloy, wherein the step of obtaining the steel alloy article comprises a method comprising hollow extrusion, static or centrifugal casting, continuous casting, plate rolling and bar rolling under predetermined temperature.

[00041] The invention is now illustrated with the help of following non-limiting examples. The examples are provided merely for illustration purpose and should not be construed as limitation to scope of present invention.
[00042] Example 1: Invention Composition & Article
[00043] An ingot of 400 Kg, 325mm X 325mm X 600mm was produced by Air-Induction melting. Inert atmosphere was created above liquid metal by argon shielding. The Chemical composition of the ingot produced from the above method is given in Table 1. Further, ingot was hot forged in open die forging to size of 90 X 90 mm2 with reduction ratio greater than 12. Further, forged bars were subjected to typical heat treatment as given below;
[00044] Heat treated steps includes:
i) Charging the specimen at temperature of 300° C in to furnace
ii) Heating the specimen to temperature of 940° +/-10° C with heating rate of 70°C/Hr. max
iii) Holding the specimen at 940° C for 2 Hrs.(depending on section thickness.)
iv) Oil quenching the specimen to room temperature
(v) Quenching followed by immediate tempering to 250° C ± 10° C.
(vi) Holding the specimen for 4 Hrs. at this temperature
(vii) Cooling the specimen in air to room temperature
[00045] Heat treated specimens were analyzed for microstructural features through Scanning electron microscopy and Transmission Electron microscopy. Specimens were prepared and tested conforming to ASTM E8 for tensile properties and Charpy V Notch impact properties tested conforming to ASTM E23. Further fracture toughness specimens were prepared and tested as per ASTM E399.
[00046] Mechanical Properties such as UTS, YS, Impact, Hardness & % strain to failure (The results are shown in Table No.2)
[00047] SEM Microstructure: It showed fine tempered martensitic structure (Figure 1)
[00048] TEM Micro-structure showed presence of fine ? carbides within martensitic laths (Figure 2)
[00049] Table 1: Chemical composition
C Mn Si S P Cr Ni Mo
0.33 0.76 1.5 0.02 max 0.02 max 3.0 2.5 0.4

[00050] Table 2 – Mechanical Properties
UTS
(MPa) YS
(MPa) % STF Impact Toughness, J K1C, Mpa vm
Room Temperature 1877 1421 14 34 87.85

[00051] It is found that the lean steel composition, steel alloy and steel alloy article of the presently claimed invention exhibit ultra-high strength as well as toughness and higher fracture toughness compared to known and out of scope steel compositions, steel alloys and steel alloy articles.

[00052] The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
[00053] Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
[00054] The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
[00055] While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.