Claims:We claim:
1. A process for producing a low alloy steel comprising:
casting the low alloy steel in a casting unit with composition (all in wt. %):
0.10 to 0.20 Carbon, 1.0 to 1.5 Manganese, 0.20 to 0.50 Silicon, 0.1 to 0.30 Aluminium, 0.1 to 0.30 Chromium, 0.10 to 0.30 Molybdenum, 0.40 to 0.60 Nickel, 0.01 to 0.05 Vanadium, and balance being Iron and residual impurities;
first homogenizing the low alloy steel in a first furnace for austenitizing to a first homogenizing temp for a first homogenizing time;
rolling the low alloy steel at Hot rolling mill for about 85-95% thickness reduction;
first quenching the low alloy steel over a first quenching unit to a first quenching temp at a first quenching rate;
first cooling the low alloy steel in a first cooling unit to room temperature;
second homogenizing the low alloy steel in a second furnace to a second homogenizing temp for a second homogenizing time;
second quenching the low alloy steel in a second quenching unit to a second quenching temperature at a second quenching rate; and
second cooling the low alloy steel in a second cooling unit to room temperature.

2. The process as claimed in claim 1, wherein the first furnace is reheating furnace.

3. The process as claimed in claim 1, wherein the first homogenizing temp is 1150-1200°C.

4. The process as claimed in claim 1, wherein the first homogenizing time is 100-120 minutes.

5. The process as claimed in claim 1, wherein the Finish rolling temperature (FRT) is 850-900 º. C.

6. The process as claimed in claim 1, wherein the first quenching unit is Run Out Table (ROT) or salt bath.

7. The process as claimed in claim 1, wherein the first quenching temperature is 200-250 º C.

8. The process as claimed in claim 1, wherein the first quenching rate is (50-70°C/s).

9. The process as claimed in claim 1, wherein the first cooling unit is open atmosphere.

10. The process as claimed in claim 1, wherein the second furnace is reheating furnace

11. The process as claimed in claim 1, wherein the second homogenizing temp is 1000-1050°C.

12. The process as claimed in claim 1, wherein the second homogenizing time is 30-45 mins.

13. The process as claimed in claim 1, wherein the second quenching unit is Run Out Table (ROT) or salt bath.

14. The process as claimed in claim 1, wherein the second quenching temperature is 500-600°C.

15. The process as claimed in claim 1, wherein the second quenching rate is (10-20°C/s).

16. The process as claimed in claim 1, wherein the second cooling is done at cooling rate 0.001-0.007°C/s

17. A low alloy steel comprising (all in wt. %):
0.10 to 0.20 Carbon, 1.0 to 1.5 Manganese, 0.20 to 0.50 Silicon, 0.1 to 0.30 Aluminium, 0.1 to 0.30 Chromium, 0.10 to 0.30 Molybdenum, 0.40 to 0.60 Nickel, 0.01 to 0.05 Vanadium, and balance being Iron and residual impurities with
Ultimate Tensile Strength of the low alloy steel is 550-750 MPa,
total elongation of the low alloy steel is 22-30 % and
charpy impact toughness of the low alloy steel is 60-120 J (for sub size sample with 5×10 mm2 cross section).

18. The low alloy steel as claimed in claim 17, wherein the microstructure of the low alloy steel is comprised of bainite-ferrite (85-90 vol%), carbides (1-10 vol%) and retained austenite (1- 5 vol.%).
19. The low alloy steel as claimed in claim 17, wherein hardness of the low alloy steel (at 1 kg load) is 180 Hv-220 Hv.

20. The low alloy steel as claimed in claim 17, wherein Yield Strength of the low alloy steel is 350-550 MPa.

21. The low alloy steel as claimed in claim 17, wherein uniform elongation of the low alloy steel is 7-12 %.
, Description:TECHNICAL FIELD

Present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure the present disclosure relates to a steel with high toughness and ductility.
BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon, and other elements such as Phosphorous (P), Sulphur (S), Nitrogen (N), Manganese (Mn), Silicon (Si), Chromium (Cr), etc. Because of its high tensile strength, low cost, toughness and ductility, steel is considered as a major component in wide variety of applications. Some of the applications of the steel includes construction, ship building tools, automobiles, machines, bridges and numerous other applications.

The industrial sectors, such as earthmoving, agriculture, mining, excavation, mineral processing, transportation etc., requires the use of steel which can provide the desired resistance to the wear process. The relative motion between active and counter body involves mechanical actions or chemical reactions, which as a result, leads to loss of material from the surface of a component. Therefore, these components require maintenance or replacement, leading to increased costs and safety concerns for the industries.

As per available data, the cost for UK industries with wear problems has been estimated to be about 0.25% of their turnover. Interestingly, among different wear mechanisms, abrasive wear alone contributes to about 63% of this total loss. Therefore, the development of a new grade of steel is required to minimize or eliminate these losses, as well as for the safety of the people working in these industries.

At present, these components are mostly dominated by the use of the medium to high carbon steels with martensitic or tempered martensitic microstructure. This tempering process is performed to relieve the brittleness of martensite, thereby, improving the ductility and toughness with a compromise on the strength. Moreover, the tempering process requires an additional heat treatment furnace after hot rolling, adding to the cost, reduced productivity and additional energy requirements.
Reference has been made to patent EP3492611, where inventors claims a hot rolled steel of minimum tensile strength of 950 MPa with 70 % bainite and remaining martensite/ferrite. The steel is produced by hot rolling and quenching in the bainite range, followed by hot rolled coil cooling simulations. However, the total elongation is claimed to be at least 8%.

Reference has been made to patent CN109554622B, wherein the hot-rolled steel is quenched in the bainite region, followed by furnace cooling to achieve tensile strength higher than 1050 MPa and elongation of more than 20%. The steel contains carbon in a higher range, i.e. 0.22-0.25%, along with 2.8-3.2% of Mn, 1.8-2.2% of Al and the balance of Fe. The composition of the steel is quite rich in terms of Aluminium and manganese hence the cost is on higher side.

Reference has been made to patent CN103160680A, wherein the hot rolled steel is produced by quenching below Ms temperature, followed by partitioning at temperatures above Ms and cooling to room temperature. Although the low-carbon silicon-manganese-series steel achieves the product of strength and elongation of more than 30 GPa%, the partitioning at higher temperature requires additional furnace.
However, most of the prior arts as described above either exhibits low figure of merit or require complex process to improve the figure of merit.
The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.

SUMMARY OF THE DISCLOSURE
One or more shortcomings of the prior art are overcome by method as claimed and additional advantages are provided through the method as described in the present disclosure.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

The present invention provides a process for producing a low alloy steel comprising:
casting the low alloy steel in a casting unit with composition (all in wt. %):
0.10 to 0.20 Carbon, 1.0 to 1.5 Manganese, 0.20 to 0.50 Silicon, 0.1 to 0.30 Aluminium, 0.1 to 0.30 Chromium, 0.10 to 0.30 Molybdenum, 0.40 to 0.60 Nickel, 0.01 to 0.05 Vanadium, and balance being Iron and residual impurities; first homogenizing the low alloy steel in a first furnace for austenitizing to a first homogenizing temp for a first homogenizing time; rolling the low alloy steel at Hot rolling mill for about 85-95% thickness reduction; first quenching the low alloy steel over a first quenching unit to a first quenching temp at a first quenching rate; first cooling the low alloy steel in a first cooling unit to room temperature; second homogenizing the low alloy steel in a second furnace to a second homogenizing temp for a second homogenizing time; second quenching the low alloy steel in a second quenching unit to a second quenching temperature at a second quenching rate; and second cooling the low alloy steel in a second cooling unit to room temperature.
The first furnace is reheating furnace, with the first homogenizing temp is 1150-1200°C. and the first homogenizing time is 100-120 minutes.
In an embodiment, the Finish rolling temperature (FRT) is 850-900 º. C.
In a preferred embodiment, the first quenching unit is Run Out Table (ROT) or salt bath.
In an embodiment, the first quenching temperature is 200-250 º C., the first quenching rate is (50-70°C/s).
In an embodiment, the first cooling unit is open atmosphere.
In an embodiment, the second furnace is reheating furnace
In an embodiment, the second homogenizing temp is 1000-1050°C. the second homogenizing time is 30-45 mins.
In an embodiment, the second quenching unit is Run Out Table (ROT) or salt bath, with the second quenching temperature is 500-600°C and the second quenching rate is (10-20°C/s).
In an embodiment, the second cooling is done at cooling rate 0.001-0.007°C/s, and second cooling unit is a salt bath furnace atmosphere.
In another embodiment, a low alloy steel comprising (all in wt. %):
0.10 to 0.20 Carbon, 1.0 to 1.5 Manganese, 0.20 to 0.50 Silicon, 0.1 to 0.30 Aluminium, 0.1 to 0.30 Chromium, 0.10 to 0.30 Molybdenum, 0.40 to 0.60 Nickel, 0.01 to 0.05 Vanadium, and balance being Iron and residual impurities with
Ultimate Tensile Strength of the low alloy steel is 550-750 MPa, total elongation of the low alloy steel is 22-30 % and charpy impact toughness of the low alloy steel is 60-120 J (for sub size sample with 5×10 mm2 cross section).
In an embodiment, the microstructure of the low alloy steel is comprised of bainite-ferrite (85-90 vol%), carbides (1-10 vol%) and retained austenite (1- 5 vol.%), the hardness of the low alloy steel (at 1 kg load) is 180 Hv-220 Hv, Yield Strength of the low alloy steel is 350-550 MPa and uniform elongation of the low alloy steel is 7-12 %.
It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Figure.1 illustrates flowchart to produce a low alloy steel in accordance with an embodiment of a disclosure.
Figure.2 illustrates flowchart to produce a low alloy steel in accordance with an exemplary embodiment of the disclosure.
Figure3 illustrates XRD profile of the low alloy steel in accordance with an exemplary embodiment of the disclosure.
Figure. 4a and 4b illustrates secondary electron micrographs of samples in accordance with the experimental analysis of the low alloy steel at (a) low magnification and (b) high magnification.
Figure. 5 illustrates an engineering stress-strain curve of the low alloy steel in accordance with an exemplary embodiment of the disclosure.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristics of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusions, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.
Shown in Figure.1 is a process (100) for producing a low alloy steel. The process (100) comprises Step 104 at which casting of the low alloy steel is done in a casting unit with composition (all in wt. %): 0.10 to 0.20 Carbon, 1.0 to 1.5 Manganese, 0.20 to 0.50 Silicon, 0.1 to 0.30 Aluminium, 0.1 to 0.30 Chromium, 0.10 to 0.30 Molybdenum, 0.40 to 0.60 Nickel, 0.01 to 0.05 Vanadium, and balance being Iron and residual impurities. The casting unit in an embodiment is steel caster.
The carbon addition has been done in the alloy for improving the hardenability, increasing the strength through interstitial solid solution strengthening and most importantly for the stabilization of austenite phase at room temperature. However, the amount of carbon in the alloy has been kept at lower due to its detrimental effects on weldability and impact toughness.
Manganese, Nickle & Molybdenum: The bainite-based quenching and nonisothermal partitioning processes requires the elimination of ferrite and pearlite formation during quenching. This can be achieved by alloying the steel with austenite stabilizers, mainly Ni and Mn. However, a very high addition of Mn leads to the possibility of banding in the microstructure and an increase in Ni additions is associated with a higher cost. Therefore, only a small amount of Mn and Ni was added to the steel.
As a combined addition of Ni and Mo is beneficial for high hardenability, a small addition of Mo was also done to the steel.
Silicon & Aluminium: The steel was also alloyed with a balanced addition of Si and/or Al to prevent the carbide precipitation. However, a higher addition of these elements was restricted to avoid smelting difficulties, eliminate the formation of rolled-in-scale during hot rolling from surface oxides and prevent issues in hot-dip galvanizing.
Chromium: A small amount of Cr is added for solid solution strengthening.
Vanadium & Nitrogen: The additional grain refinement and precipitation strengthening from microalloying additions has shown to improve the strength in TRIP steel. Therefore, a small amount of Vanadium is added, which also consumes the detrimental free N from the steel by forming its precipitates.
The preferred composition of the low steel alloy is shown below in Table 1
Table 1
Element C Mn Si Al Cr Mo Ni V
Amount 0.15 1.15 0.30 0.18 0.20 0.10 0.50 0.030
Element P S N Fe CE Ms (°C) Bs (°C) Ae3 (°C)
Amount 0.006 0.001 0.002 bal. 0.43 428.6 645.2 834.8
At Step 108, first homogenizing of the low alloy steel is done in a first furnace for austenitizing to a first homogenizing temp for first homogenizing time. In an embodiment, the first homogenizing temp is 1150-1200°C. The first homogenizing time is 100-120 minutes.
At the instant step, the entire phase of steel is converted into austenite structure.
In an embodiment of the disclosure, the first furnace is reheating furnace.
At Step 112, rolling of the low alloy steel is done at Hot rolling mill for about 85-95% thickness reduction. The finish rolling temperature (FRT) is 850-900 º. C. The reduction of the thickness to the desired levels to break the as cast dendritic structure. The initial thickness of the low alloy steel before rolling is 85 mm.
At Step 116, first quenching of the low alloy steel is done over a first quenching unit. The first quenching unit is being a run out table (ROT) or salt bath. The first quenching is done to a first quenching temp and at a first quenching rate. The first quenching temperature is 200-250 º C with the first quenching rate maintained at 50-70°C/s. The quenching medium in an embodiment is water.
At the instant step Ferrite, martensite and austenite is obtained.
At Step 120, first cooling of the low alloy steel is done in a first cooling unit to room temperature. At the instant step, ferrite, martensite, carbides and retained austenite is obtained. In an embodiment, the first cooling unit is open atmosphere where the low alloy steel allowed to cool to room temperature.
At Step 124, second homogenizing of the low alloy steel is performed in a second furnace to a second homogenizing temp for a second homogenizing time.
In an embodiment, the second homogenizing temp is maintained at 1000-1050°C with the second homogenizing time at 30-45 mins. The instant step achieves a complete austenitic structure of the steel.
In an embodiment, the second furnace is reheating furnace.
At Step 128, second quenching of the low alloy steel is performed in the second quenching unit to a second quenching temperature at a second quenching rate. The quenching medium is preferably water. In some embodiment, oil or molten salt may also be used. The second quenching unit in an embodiment is Run Out Table (ROT) or salt bath.
In an embodiment, the second quenching temperature is maintained at 500-600°C with the second quenching rate at (10-20°C/s). At the instant step, some amount of bainite with remaining austenite is formed.
At Step 132, the low alloy steel is second cooled at a second cooling unit to room temperature. The low alloy steel is coiled and kept in the second cooling unit. The second cooling unit in an embodiment is open atmosphere or salt bath furnace. the cooling rate is maintained in the salt bath is 0.001-0.007°C/s. The said process is the nonisothermal partitioning condition during the hot rolled coil cooling for carbon partitioning. At this step, a lot of carbon would be rejected out of the bainitic-ferrite phase causing partitioning.
The final microstructure obtained for the low alloy steel is comprised of bainite-ferrite (85-90 vol%), carbides (1-10 vol%) and retained austenite (1-5 vol%).
The hardness of the low alloy steel (at 1 kg load) is 180 Hv-220 Hv.
The Yield Strength of the low alloy steel is 350-550 MPa.
The Ultimate Tensile Strength of the low alloy steel is 550-750 MPa.
The total elongation of the low alloy steel is 22-30 %.
The uniform elongation of the low alloy steel is 7-12 %.
The charpy impact toughness of the low alloy steel is 60-120 J (for sub size sample with 5×10 mm2 cross section).
Experimental Analysis:
A sample of size 120 mm × 120 mm × 85 mm was cut from the as-cast ingot (60 kg) with composition as mentioned below in Table 1 for first homogenization at 1200°C for 2 h.
After homogenization, the steel plate was hot-rolled to 6 mm from 85 mm, followed by quenching to 200-250°C at a quenching rate of 50°C/s and thereafter cooled to room temperature (27°C).
As shown in Figure. 2, the hot-rolled steel plate was again austentized at 1000-1050°C for 30-45 mins to achieve a complete austenitic structure and then quenched to the temperature range of 500-600°C at 10-20°C/s for the formation of bainite with remaining austenite. Thereafter, the steel is cooled slowly at a cooling rate of 0.001-0.007 °C/s to room temperature.
The X-ray diffraction profile of the sample is shown in Figure 3. The results show the presence of reflections corresponding to a BCC/BCT structure, which can be ferrite/bainite/martensite. In addition, some reflections of retained austenite were also observed, the intensity of which indicates its content to be less than or equal to 5 vol.%.
The microstructure of steel is shown in Figure. 4a & 4b, which depicts the presence of bainite-ferrite laths and carbides. The microstructure comprises retained austenite also, but due to small proportion, it could not be seen in Figure. 4a & 4b, but in Figure. 3. The engineering stress-strain curve and the mechanical properties of the developed low alloy steel are mentioned in Figure. 5 and Table 2 below, respectively.
Table 2
Hardness (HV-1) Tensile Strength (MPa) Uniform elongation (%) Total elongation (%) YS (MPa) Charpy Impact toughness (J)*
199-203 625-633 8.6-10.5 26-27 440-470 74-96
* - Measured on the sub-size sample with 10×5 mm2 cross-section
The low alloy steel shows excellent mechanical properties as mentioned in the Table 2.
Advantages
• Achievement of high ductility and impact toughness in hot-rolled low alloy steel, while maintaining the high strength, through an energy-efficient bainite-based quenching and nonisothermal partitioning process.
• Higher uniform elongation of steel, therefore better formability for making the components with complicated shapes.
• The elimination of isothermal holding as required in conventional austempering or tempering or quenching and isothermal partitioning processes.

Equivalents:

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.