ULTRA-HIGH STRENGTH STEEL WITH IMPROVED FRACTURE TOUGHNESS AND METHOD OF MANUFACTURING THEREOF

TECHNICAL FIELD
[0001] The present disclosure relates to ultra-high-strength steel with improved fracture toughness and method of manufacturing thereof.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed subject matter, or that any publication specifically or implicitly referenced is prior art.
[0003] There are many steels available which are having an excellent combination of strength and toughness such as maraging steels and aermet alloys. These steels contain very high alloying addition and are particularly used for aerospace application where catastrophic failure is to be avoided. However, at the surface application, such steel is highly expensive.
[0004] Steel for high-end applications, such as armor or excavator teeth, where very high strength and toughness are required, users have to go for coated steels which are highly expensive and not strong enough. Although advanced high strength steels are available in the steel market, all such advanced strength steels are either too expensive or do not meet the combination of required mechanical properties.
[0005] The limitations of the available advanced strength steels are shown in the following Table 1:

S. No Steel Grade Limitations Remarks
1 AISI 4340 Hardened and tempered
Low Strength and toughness Cannot be rolled
2 Maraging Steels Very High Alloyed and too expensive Used for aerospace applications
3 Armox Advance Low Fracture Toughness Widely used as armor Steel
4 Aermet alloys Too Expensive Used for aerospace applications
Table 1: Steels available in the market and their limitations
[0006] Accordingly, from Table 1, it is obvious to those skilled in the art that there is no ultra-high strength steel available in the market which is available at a cheaper price for high-end applications.
[0007] Therefore, in the state of the art, a need is felt to develop steel which is cheaper, and have lean chemistry, high strength > 2000 MPa, high fracture toughness > 70 MPavm, high impact toughness at -40OC >25 Joules, and high hardness >650 HV.

OBJECTS OF THE DISCLOSURE
[0008] Some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0009] A general object of the present disclosure is to produce steel with ultra-high strength > 2000 MPa and superior Fracture Toughness (K1C) > 70 MPavm.
[0010] An object of the present disclosure is to provide steel with hardness >650 HV.
[0011] These and other objects and advantages of the present disclosure will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present disclosure is illustrated.

SUMMARY
[0012] This summary is provided to introduce concepts related to ultra-high strength steel with improved fracture toughness and method of manufacturing thereof. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0013] The present disclosure relates to a method for making ultra-high strength steel with improved fracture toughness. The method includes producing an ingot with composition comprising, in weight, 0.25-0.50% Carbon (C), 1-3% Silicon (Si), 1-3% Chromium (Cr), 3-5% Nickel (Ni), 0.30-0.80% Molybdenum (Mo), 0.2-0.5% Vanadium (V), and reminder being iron (Fe) in a ladle sample; reheating the ingot to 1050 °C to 1200 °C in a reheating furnace; forging the ingot above 950 °C to 1000 °C by a forging press; finish rolling the ingot above 760 °C to 850 °C in a hot rolling mill; and annealing 170 °C to 220 °C for 4 to 6 hours in an annealing furnace.
[0014] The present disclosure further relates to ultra-high strength steel with improved fracture toughness is obtained, which has yield strength of 1200 to 1600 MPa, and fracture toughness (K1C) of 60 to 80 MPavm.
[0015] Further, in an aspect, ultimate tensile strength (UTS) of the ultra-high-strength steel is 1900-2300 MPa.
[0016] Yet further, in an aspect, elongation of the ultra-high-strength steel in the longitudinal direction is 14- 15% and in traverse direction is 10 - 11%.
[0017] Yet further, in an aspect, impact toughness of the ultra-high strength steel, at -40 °C, in the longitudinal direction is 28 and in traverse direction is 28.
[0018] Yet further, in an aspect, the microstructure of the ultra-high-strength steel is a mixture of bainite and martensite.
[0019] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
[0021] FIG. 1 illustrates a method for making ultra-high strength steel with improved fracture toughness, in accordance with an embodiment of the present disclosure; and
[0022] FIGS. 2A and 2B illustrate microstructures of the steel, developed in accordance with the present disclosure, showing a mixture of bainite and martensite.

DETAILED DESCRIPTION
[0023] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0024] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0025] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0026] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0027] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0028] Embodiments and/or implementations described herein relate to a steel having low alloying additions with the chemical composition comprising, in weight, 0.25-0.50% Carbon (C), 1-3% Silicon (Si), 1-3% Chromium (Cr), 3-5% Nickel (Ni), 0.30-0.80% Molybdenum (Mo), 0.2-0.5% Vanadium (V), and reminder being iron (Fe). The steel proposed herein is produced through a vacuum induction melting route. The steel is subsequently hot forged and rolled to 6 mm thickness with the reheating temperature of 1150 °C and finish rolling temperature of >780 °C and air cooled to room temperature. After this, the steel is tempered at 190 °C. In an aspect, one steel plate is rolled to 16 mm thickness, as it is not possible to do valid fracture toughness test (K1C) in 6 mm thickness.
[0029] As can be appreciated that by those skilled in the art, the task of increasing toughness and strength is non-trivial. For instance, refinement of a microstructure is the one general mechanism which is known to improve both strength and toughness. In strong steels, the strengthening is achieved by refinement of the martensite or bainite plate size, both depending to a large degree on the transformation temperature. Toughness also improves with the refinement of the microstructure; in the past correlations have been reported to the size of bainite or martensite packets as well as the prior austenite grain size, and more recently to the crystallographic grain size. The refinement of remaining blocks of unstable retained austenite is of particular importance, as it is well established that large blocks of retained austenite can be detrimental to the toughness. The approach used in the past was to make changes which increased the toughness and strength simultaneously.
[0030] In an approach, microalloying with Vanadium (V) was identified as the austenite grain refiner during processing and therefore to reduce the maximum size of the martensite plates and decrease the size of bainite packets.
[0031] To this, in the present disclosure, Vanadium (V) is used due to its lower carbide formation energy, and segregation of elements during solidification is explicitly considered in the design process of the steel proposed herein.
[0032] Additionally, the steel proposed herein is designed to allow transformation to bainite at low temperature during continuous cooling. The formation of carbide-free bainite increases the fraction of retained austenite, which is able to improve toughness and ductility via a transformation induced plasticity effect. Considering all these factors, the steel with chemical composition comprising, in weight, 0.35% Carbon (C), 1.5% Silicon (Si), 1.5% Chromium (Cr), 3.5% Nickel (Ni), 0.40% Molybdenum (Mo), 0.3% Vanadium (V), and reminder being iron (Fe), is designed anticipating that a target mechanical properties >2000MPa strength with fracture toughness (K1C) value of >50 MPavm strength can be achieved.
[0033] As can be appreciated by those skilled in the art, Carbon (C) is for increasing hardness of the steel, forms carbide, and affects weldability; Molybdenum (Mo) promotes the formation of low-temperature carbides and increases hardenability; Nickel (Ni) is for solid solution hardening, increases the precipitate/matrix misfit by modifying the lattice spacing, is grain refiner, decreases Ductile-Brittle Transition Temperature (DBTT), and strongly decreases AC1 temperature; Chromium (Cr) retards softening from Fe3C, by forming M3C, and increases hardenability; Vanadium (V) promotes fine grain size and formation of low-temperature carbides and increases hardenability; and Silicon (Si) delays the decomposition of martensite, reduces the lattice spacing of the ferritic matrix, and promotes formation of carbide-free bainite.
[0034] Further, to make the clean steel and keep the impurities as low as possible the conventional vacuum induction melting followed by vacuum arc re-melting is followed. For instance, as shown in FIG. 1, a method 100 for making ultra-high strength steel with improved fracture toughness, in accordance with an embodiment of the present disclosure. The order in which the method 100 is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method 100, or an alternative method.
[0035] At block 102, the method 100 includes producing an ingot with composition comprising, in weight, 0.25-0.50% Carbon (C), 1-3% Silicon (Si), 1-3% Chromium (Cr), 3-5% Nickel (Ni), 0.30-0.80% Molybdenum (Mo), 0.2-0.5% Vanadium (V), and reminder being iron (Fe) in a ladle sample.
[0036] At block 104, the method 100 includes reheating the ingot to 1050 °C to 1200 °C in a reheating furnace.
[0037] At block 106, the method 100 includes forging the ingot above 950 °C to 1000 °C by a forging press. In an aspect, the forging is performed by reheating the ingot at a temperature ranging from 1000 °C to 1200 °C and finish the forging operations above 950 °C followed by pit cooling to avoid cracking due to thermal stresses.
[0038] As the mechanical properties are to be achieved by controlled rolling, the reheating temperature and finish rolling temperatures are designed considering thermodynamic principles. For instance, thermocalc simulation is performed for the chemistry designed for the steel proposed herein and the results obtained are shown in Tables 3 and 4:

Transformation Temperatures, oC
Ae¬1 Ae3
667 769
Table3: Equilibrium Transformation temperatures
Precipitate Composition Precipitate Type Precipitation Start, oC Precipitate Finish, oC
(V, Cr, Mo)C FCC 1060 -
(Mo, V)C MC_ETA 790 -
(Fe, Cr, Mo, V, C) M7C3 750 462
(Cr, V, Mo)C M3C2 465 -
Table4: Precipitate dissolution temperatures
[0039] Using Tables 3 and 4, it is found from thermocalc simulation that all the precipitates go into the solution at 1060 °C. Therefore, the reheating temperature may be selected to be 1100 °C.
[0040] At block 108, the method 100 includes finish rolling the ingot above 760 °C to 850 °C in a hot rolling mill. The finish rolling temperature is kept just above the two-phase region, i.e., >780 °C. In an aspect, the rolling is done in several passes as shown in Table 5:
Process Steps Rolling Sequence Remark
Heating 2 Hrs at 750 °C/Raise to 1150 °C in 4-5 Hrs/ Held at 2-3 Hrs at 1150 °C

Furnace atmosphere: Neutral to slightly oxidizing

Sheet Mill (Roughing Stand)
Deformation passes
(Approx Roll gap in mm) 80 mm
?
70 mm
?
60 mm
?
52 mm
?
45 mm
?
37 mm
?
30 mm
Reheating 0.5 - 1 Hr at 1150OC
Sheet Mill
(Roughing Stand)
Deformation passes
(Approx Roll gap in mm) 30 mm
?
20 mm
?
15 mm
?
11 mm

(Finishing Mill)
Deformation passes Reheat to 1100 °C( 1-3 hours)
11 mm
?
6 mm
In 4 to five passes Rolling to be finished around 790OC
Table 5: Flow chart for Hot rolling
[0041] Accordingly, one slab can be rolled to 16 mm thickness.
[0042] Finally, at block 110, the method 100 includes annealing 170 °C to 220 °C for 4 to 6 hours in an annealing furnace.
[0043] Thus, with the method 100 proposed herein, ultra-high strength steel with improved fracture toughness is obtained, which has a yield strength of 1200 to 1600 MPa, and fracture toughness (K1C) of 60 to 80 MPavm.
[0044]
[0045] Further, in an aspect, ultimate tensile strength (UTS) of the ultra-high-strength steel is 1900-2300 MPa.
[0046] Yet further, in an aspect, elongation of the ultra-high-strength steel in the longitudinal direction is 14- 15% and in traverse direction is 10 - 11%.
[0047] Yet further, in an aspect, impact toughness of the ultra-high strength steel, at -40 °C, in the longitudinal direction is 28 and in traverse direction is 28.
[0048] Yet further, in an aspect, a microstructure of the ultra-high-strength steel is a mixture of bainite and martensite. FIGS. 2A and 2B illustrate microstructures of the steel, developed in accordance with the present disclosure, showing a mixture of bainite and martensite.
[0049] Furthermore, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0050] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0051] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

We claim:

1. A method for making ultra-high strength steel with improved fracture toughness, the method comprising:
producing an ingot with composition comprising, in weight, 0.25-0.50% Carbon (C), 1-3% Silicon (Si), 1-3% Chromium (Cr), 3-5% Nickel (Ni), 0.30-0.80% Molybdenum (Mo), 0.2-0.5% Vanadium (V), and reminder being iron (Fe) in a ladle sample;
reheating the ingot to 1050 °C to 1200 °C in a reheating furnace;
forging the ingot above 950 °C to 1000 °C by a forging press;
finish rolling the ingot above 760 °C to 850 °C in a hot rolling mill; and
annealing 170 °C to 220 °C for 4 to 6 hours in an annealing furnace.
2. An ultra-high-strength steel with improved fracture toughness, comprising:
yield strength is 1200 to 1600 MPa, and
fracture toughness (K1c) is 60 to 80 MPavm.
3. The ultra-high strength steel as claimed in claim 2, wherein ultimate tensile strength (UTS) of the ultra-high-strength steel is 1900-2300 MPa.
4. The ultra-high strength steel as claimed in claim 2, wherein the elongation of the ultra-high-strength steel in the longitudinal direction is 14- 15% and in traverse direction is 10 - 11%.
5. The ultra-high strength steel as claimed in claim 2, wherein impact toughness of the ultra-high strength steel, at -40 °C, in the longitudinal direction is 28 and in traverse direction is 28.
6. The ultra-high strength steel as claimed in claim 2, wherein microstructure of the ultra-high-strength steel is a mixture of bainite and martensite.