Claims:Abstract
ULTRA-HIGH STRENGTH AND TOUGH LOW ALLOY STEEL COMPOSITION

The present invention provides a low alloy steel composition which provides a combination of ultra-high strength, high ductility along with high impact toughness at RT as well as sub-zero temperatures and high fracture toughness. The present invention steel composition comprises 0.20 wt. % to 0.40 wt. % of carbon, 0.5 wt. % to 1.0 wt. % of manganese, 0.5 wt. % to 1.5 wt. % of silicon, 0.9 wt. % to 2.9 wt. % of nickel, 2.0 wt. to 3.5 wt. % chromium, 0.3 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 2.2 wt. % of tungsten, 0.05 wt.% to 0.15 wt. % of niobium, <0.030 wt. % of nitrogen and balance iron with incidental impurities less than 0.02 wt.%.
, Description:FIELD OF THE INVENTION
[0001] The present invention relates to a low alloy steel composition. Particularly, the present invention provides the low alloy steel composition which exhibits ultra-high strength, high fracture toughness and high impact toughness at room temperature as well as sub-zero temperatures.

BACKGROUND & INTRODUCTION

[0002] In recent years, substantial efforts are taken for component weight reduction in defense, aerospace and for some critical applications in automotive industry by using ultra-high strength steels (UTS < 1500MPa). There are various UHSS grades available which are currently being used for different applications in the auto industry (e.g. gears and auto bodies), in aerospace applications (e.g. landing gears, bolts, and fasteners) and defence applications (e.g. missile components like motor casings, armor plating, military aircraft frame). Many applications in these sectors mostly demands for combination of ultra-high strength, high fracture toughness and high ductility along with high impact toughness for making light weight components and improving the performance of the products. Moreover, for critical applications, steel needs to be used at subzero temperatures. Steel which is ductile at room temperature may become brittle at sub-zero temperatures. So ductility of steel at sub-zero temperatures is also important for steels being exposed to low temperature applications. Generally, development of such a high strength and tough material involves addition of high amount of costlier alloying elements like chromium, nickel etc. and special and expensive manufacturing technologies like Vacuum Induction meting (VIM) or Vacuum Arc Remelting (VAR) etc. These factors will contribute toward overall higher cost of such materials. So, there is still need in the art for a low cost, high Strength, high performance steel composition.

[0003] There are many steel compositions developed earlier for achieving ultra-high strength and high toughness combination.
[0004] U.S. Patent No 5268044, disclose a commercial ultra-high- strength, non-stainless steel with UTS of about 2000 MPa, Charpy V Notch impact toughness of 40 J at room temperature and KIC of about 120 MPa vm. However, these alloys contain very high levels of costlier alloying elements such as 13.4 % of cobalt, 11.1 % of nickel and 3.1 % of chromium which increases the overall cost of the alloy.
[0005] U.S. Pat. No. 4076525 discloses an ultra-high strength steel composition containing high weight percentage of alloying elements (> 22%). The composition contains costlier elements like nickel (9.5 to 10.25%), cobalt (12 to 14%) and provides strength of about 1500 to 1860 MPa, room temperature impact toughness of 40 to 70 joules and fracture toughness of about 126 MPa vm.
[0006] Another alloy composition, described in U.S. Patent Appl. Publ. No. 2010/0018613, provides a high strength of about 1600 MPa, high toughness of 32 J at the cost of higher alloying elements. The Fe-Cu-Ni-Cr alloy steel described 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.

[0007] Although various steel compositions have been disclosed in the prior art, there is still a need for high strength steel composition that can be produced relatively inexpensively without compromising on desirable high performance characteristics.

OBJECT OF THE INVENTION
[0008] It is an object of the present invention to provide an inexpensive low alloy steel composition with a combination of ultra-high strength and toughness.

[0009] It is yet another object of the present invention to provide a low alloy ultra-high strength and high tough steel composition with low amount of nickel and chromium.

[00010] It is still another object of the present invention to provide a low alloy steel composition which provides high fracture toughness.

[00011] It is another object of the present invention to provide a low alloy steel composition which provides higher subzero and room temperature impact toughness.

[00012] It is further object of the present invention to provide a low alloy ultra-high strength steel composition which provides a high strength to weight ratio favoring component weight reduction.

[00013] It is still further object of the present invention to provide a low alloy ultra-high strength steel composition which is amenable to conventional heat treatment.

SUMMARY OF THE INVENTION
[00014] The present invention is related to a low alloy steel composition which provides combination of ultra-high strength, high ductility along with high impact toughness at RT as well as sub-zero temperatures and high fracture toughness. The present invention steel composition provides high performance despite having low amount of alloying elements like chromium- and nickel. In one embodiment, the steel composition comprises 0.20 wt. % to 0.40 wt. % of carbon, 0.5 wt. % to 1.0 wt. % of manganese, 0.5 wt. % to 1.5 wt. % of silicon, 0.9 wt. % to 2.9 wt. % of nickel, 2.0 wt. to 3.5 wt. % chromium, 0.3 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 2.2 wt. % of tungsten, 0.05 wt.% to 0.15 wt. % of niobium, <0.030 wt. % of nitrogen and balance iron, optionally with incidental impurities less than 0.02 wt%.

[00015] In another embodiment, the present invention also provides an alloy steel characterized by an ultimate tensile strength of =1650 MPa; a yield strength at 0.2% offset of = 1340 MPa; % strain to failure of = 15%; an impact toughness as measured by a Charpy V-notch test at room temperature of = 50 J, sub-zero CVN Impact toughness at - 40° C is about =40 Joules; and a fracture toughness of = 100 MPavm.

[00016] In another aspect the present invention also provides a method of manufacturing a low alloy steel using a 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.

[00017] In still another aspect the present invention also provides a process for manufacturing a low alloy steel article which involves forging a low alloy steel under predetermined temperature to obtain said low alloy steel article.

[00018] In another embodiment, the process for manufacturing a low alloy steel article comprises obtaining the low alloy steel article using a technique selected from hollow extrusion, static or centrifugal casting, continuous casting, plate rolling and bar rolling under pre-determined temperature.

[00019] The present invention also provides a method of thermally processing a low alloy steel article which involves steps such as normalizing the low alloy steel article, austenitizing the alloy steel article; quenching, and tempering.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS:
[00020] Figure 1 illustrates SEM Microstructure of the specimen of the present invention with fine tempered martensite laths; and

[00021] Figure 2 illustrates TEM Images showing fine lath martensitic structure with nano ? carbides of the specimen of the present invention.

DETAILED DESCRIPTION
[00022] The present invention is focused on providing a low alloy composition with low cost which can provide ultra-high strength, higher toughness at room temperature (RT) as well as at subzero temperatures and higher fracture toughness. The alloy in the present invention contains low levels of nickel, chromium molybdenum, tungsten still it provides high performance characteristics.

[00023] In one embodiment, the steel composition comprises 0.20 wt. % to 0.40 wt. % of carbon, 0.5 wt. % to 1.0 wt. % of manganese, 0.5 wt. % to 1.5 wt. % of silicon, 0.9 wt. % to 2.9 wt. % of nickel, 2.0 wt. to 3.5 wt. % chromium, 0.3 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 2.2 wt. % of tungsten, 0.05 wt.% to 0.15 wt. % of niobium, <0.030 wt. % of nitrogen and balance iron, optionally with incidental impurities less than 0.02 wt%.

[00024] In one embodiment, the low alloy steel composition comprises 0.25 wt. % to 0.35 wt. % of carbon, 0.7 wt. % to 0.9 wt. % of manganese, 0.9 wt. % to 1.2 wt. % of silicon, 0.9 wt. % to 2.0 wt. % of nickel, 2.5 wt. to 3.0 wt. % chromium, 0.4 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 1.5 wt. % of tungsten, 0.05 wt.% to 0.10 wt. % of niobium, <0.030 wt. % of nitrogen and balance iron with incidental impurities less than 0.02 wt.%.

[00025] Carbon (C) is added to increase strength and hardenability of the material and contribute in the precipitation of carbides (Fe2.4C, Fe3C, M6C, M23C6, NbC, and WC) which can both positively and negatively impact alloy performance. Lowering carbon content will reduce the strength while higher carbon level can negatively affect weldability and quench cracking. Therefore, in one embodiment, carbon is present in an amount of about 0.20 % but not more than about 0.4 %.

[00026] Silicon (Si) helps to delay and inhibit the precipitation of cementite and allows precipitation of ? carbides which will improve the performance of the steel. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains silicon in an amount of about 0.5 % to 1.5 %.

[00027] Tungsten (W) is added in this alloy to produce a fine distribution of tungsten carbides between martensitic laths. The carbides serve as sites for dislocation pinning to produce significant work hardening. Also it helps to improve the wear resistance of the material. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains tungsten in of about 0.9 wt. % to 2.2 wt. %.

[00028] Nickel (Ni) helps to improve the low temperature toughness and hardenability of the material. Thus, in accordance with one embodiment of the present invention, nickel is restricted about 0.9 wt. % to 2.9 wt. %.

[00029] Niobium (Nb) acts as a grain refiner and helps to improve the toughness of the alloy. Thus, in accordance with one embodiment of the present invention, Niobium is restricted about 0.05 wt. % to 0.15 wt. %.

[00030] In one embodiment, Niobium can be replaced with other micro-alloying elements like Vanadium or Titanium etc. in the appropriate proportion to get similar effect as described above. In one embodiment, the low alloy steel composition comprises 0.20 wt. % to 0.40 wt. % of carbon, 0.5 wt. % to 1.0 wt. % of manganese, 0.5 wt. % to 1.5 wt. % of silicon, 0.9 wt. % to 2.9 wt. % of nickel, 2.0 wt. to 3.5 wt. % chromium, 0.3 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 2.2 wt. % of tungsten, 0.05 wt.% to 0.15 wt. % of vanadium or titanium, <0.030 wt. % of nitrogen and balance iron with incidental impurities less than 0.02 wt.%.

[00031] Chromium (Cr) acts as a solid solution strengthener and helps to improve corrosion resistance and hardenability of the material. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains chromium in an amount of about 2.0 wt. % to 3.5 wt. %.
[00032] Molybdenum (Mo) and Manganese (Mn) plays important role in the solid solution strengthening. Molybdenum acts as a precipitation strengthener also which enhances fracture toughness. However, excessive molybdenum and manganese leads to segregation and lowers the toughness. Thus, in accordance with one embodiment of the present invention, the alloy steel composition contains molybdenum in of about 0.3 wt. % to 0.6 wt. % and manganese of about 0.5 wt. % to 1.0 wt. %.

[00033] The alloy may content small amount of impurities such as P, S, etc. < 0.04%.
[00034] The present invention also provide a low alloy steel obtained from the steel composition described herein above. The alloy steel is characterized in that it exhibits an ultimate tensile strength of =1650 MPa; a yield strength at 0.2% offset of = 1340 MPa; % strain to failure of = 15%; an impact toughness as measured by a Charpy V-notch test at room temperature of = 50 J, sub-zero CVN Impact toughness at - 40° C is about =40 Joules; and a fracture toughness of = 100 MPavm.

[00035] The alloy steel 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 manufacturing process selection is based on the criticality of the end product. As the liability and number of manufacturing processes increase, the cost also increases. End products made from steel can be produced using deformation processes such as open die forging, close die forging, solid or hollow extrusion methods, plate rolling, bar rolling and the cast products can be made by static or centrifugal castings, continuous casting, ingot casting, or other conventional methods.

[00036] In one embodiment, the method of manufacturing an alloy steel comprising the following steps:
- providing a steel composition comprising 0.20 wt. % to 0.40 wt. % of carbon, 0.5 wt. % to 1.0 wt. % of manganese, 0.5 wt. % to 1.5 wt. % of silicon, 0.9 wt. % to 2.9 wt. % of nickel, 2.0 wt. to 3.5 wt. % chromium, 0.3 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 2.2 wt. % of tungsten, 0.05 wt.% to 0.15 wt. % of niobium, <0.030 wt. % of nitrogen and balance iron optionally with incidental impurities less than 0.02 wt%; and

- 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 alloy steel.

[00037] In another embodiment, the process for manufacturing an alloy steel article; said process comprising the following steps:
? providing a steel composition comprising 0.20 wt. % to 0.40 wt. % of carbon, 0.5 wt. % to 1.0 wt. % of manganese, 0.5 wt. % to 1.5 wt. % of silicon, 0.9 wt. % to 2.9 wt. % of nickel, 2.0 wt. to 3.5 wt. % chromium, 0.3 wt. % to 0.6 wt. % of molybdenum, 0.9 wt. % to 2.2 wt. % of tungsten, 0.05 wt.% to 0.15 wt. % of niobium, <0.030 wt. % of nitrogen and balance iron with incidental impurities less than 0.02 wt.%; and

? forging said alloy steel under predetermined temperature to obtain said low alloy steel article, wherein the forging is selected from the group consisting of open die forging and closed die forging.

[00038] In one embodiment, the step of obtaining the alloy steel article comprises a technique selected from hollow extrusion, static or centrifugal casting, continuous casting, plate rolling and bar rolling under pre-determined temperature.

[00039] In one embodiment, the process further comprises thermal processing the low alloy steel article.

[00040] In one embodiment, method of thermal processing of the low alloy steel article includes three steps:
(a) Normalizing - Normalizing step involves heating the sample to a temperature above 900° C, holding the material at that temperature for certain time and then cooling the material in air to room temperature. Normalizing is used to dissolve any residual stresses that may have persisted through the production cycles. The potential for the formation of bainite and W rich M23C6 necessitates the use of higher normalization temperatures (> 900°C).
(b) Austenitising and quenching – This involves heating the normalized sample to an austenitising temperature of 900° C to 1050° C that is above a critical temperature to form an austenite followed by quenching in one of the quench media such as oil, polymer, water, gas etc. to a temperature below a martensitic-finish temperature. Based on the quenchant media like oil or water or gaseous or polymer the steel properties may improve.
(c) Tempering – Tempering step includes heating the hardened alloy steel composition to a tempering temperature in a range of 160° C to 300° C, hold the alloy at this temperature for certain time followed by air cooling.

[00041] In one embodiment, the step of austenitizing comprises charging the article at temperature of 300° C in to furnace; heating the article to temperature of 900 to 1050°C with heating rate of <70°C/hr., and holding the article for a predetermined soaking time basis section thickness.

[00042] The alloy steel thermally processed in accordance with the above method has the following properties:
? an ultimate tensile strength of about 1650 MPa or more;
? a yield strength at 0.2% offset of about 1350 MPa or more;
? % strain to failure of about 15% or more;
? an impact toughness as measured by a Charpy V-notch test at room temperature of about 50 J or more;
? an impact toughness as measured by a Charpy V-notch test at Sub-zero at - 40° is about 40 Joules or more; and
? Fracture toughness KIC of about 100 MPa vm or more.

[00043] Strengthening mechanism of the present invention steel composition is by solid solution strengthening with addition of alloying elements; by transformation hardening with formation of fine martensitic laths via proper heat treatment and by precipitation strengthening by precipitation of fine nano-size ? carbides. The precipitation of ? carbide creates barriers for dislocation movement, increasing material strength, and provides a sink for C that softens the martensitic matrix enhancing ductility and toughness. Niobium carbides benefit for grain refinement which helps to improve strength and toughness.

[00044] The microstructural features of the present alloy shows presence of fine laths of martensite along with the fine precipitates of ? carbides. The size of the ? carbides ranges from 50 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.

[00045] 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.

[00046] EXAMPLE 1
[00047] A steel ingot of 400 Kg was prepared by melting through air induction melting furnace. Argon shielding was used to avoid oxidation during air induction melting. The steel chemistry was confirmed using ARL 3460 spectrometer as given in Table 1. Further the ingot was hot forged using an open die forging by providing reduction ratio of 13. The sample from forged bar were taken for further thermal processing;
[00048] Thermal Processing included the following steps:
(a) Normalizing –
(i) Charging the specimen at a temperature of 300° C in to a furnace.
(ii) Heating the specimen to a temperature of 950° +/-10° C with a heating rate of 70°C/Hr. max.
(iii) Holding the specimen at 950° C for 2 hrs. (1hr/inch depends on section thickness).
(iv) Air cooling the specimen to room temperature.
(b) Austenizing and Quenching –
(i) Charging the specimen at a temperature of 300° C in to the furnace
(ii) Heating the specimen to a temperature of 925° +/-10° C with heating rate of 70°C/hr. max
(iii) Holding the specimen at 925° C for 2 hrs. (1hr/inch depends on section thickness.)
(iv) Oil quenching the specimen to room temperature to obtain hardened specimen.
(c) Tempering -
(i) tempering the hardened specimen to 260° C ± 10° C.
(ii) Holding the specimen for 4 hrs. at this temperature
(iii) Cooling the specimen in air to room temperature

[00049] Heat treated specimens were analyzed for microstructural features through Optical and Scanning electron microscopy and Transmission Electron microscopy. Further, specimens were prepared and tested conforming to ASTM E8 for tensile properties and Charpy V Notch impact properties tested conforming to ASTM E23 and fracture toughness specimens were prepared and tested as per ASTM E399.

[00050] The representative chemical composition is shown in table 1.

[00051] Table 1: Chemical composition
C Mn Si Cr Ni Mo W Nb
0.29 0.82 1.1 3.0 2.0 0.46 0.9 0.08

[00052] The mechanical properties such as UTS, YS, impact, hardness & % strain to failure are shown in Table No.2.

[00053] Table 2: Mechanical Properties
UTS
(MPa) YS
(MPa) % STF CVN, J
@ RT CVN, J
@-40° C K1C, Mpa vm
1675 1350 16.5 50 40 100

[00054] SEM Microstructure showed fine tempered martensitic structure which is illustrated in Figure 1.
[00055] TEM Micro-structure showed the presence of fine ? carbides within martensitic laths which is illustrated in Figure 2.

[00056] 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.

[00057] 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.

[00058] 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.

[00059] 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.