Claims:CLAIMS
We Claim:
1. A method (100) for manufacturing ultra-high strength hot formed heat-treated steel component having high strength, high plasticity and oxidation resistance, the method (100) comprising:
casting molten steel having a composition expressed in weight %: Carbon (C): 0.22% - 0.32%, Manganese (Mn): 1.5% - 2.2%, Silicon (Si): 0.05% - 0.9%, Chromium (Cr): 0.1% - 0.9%, Aluminium (Al): 0.02% - 0.6%, Molybdenum (Mo): 0.01% - 0.6%, Niobium (Nb): 0.02% - 0.08%, Titanium (Ti): 0.01% - 0.06%, Boron (B): 0.0005% - 0.005%, Nickel (Ni): 0.0% - 0.5%, Sulphur (S): 0.01% - 0.02%, Phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium (Zr) and/or other rare earths: 0.0% - 0.02%, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab;
reheating the steel slab inside a furnace kept at a temperature greater than 1050°C and soaking the steel slab at the temperature greater than 1050°C for a time duration of in the range of 60-200 minutes;
hot rolling the steel slab to produce a hot rolled steel sheet such that finish rolling is done at a temperature TFRT in the range 800oC to 900oC;
cooling at a cooling rate 20-60oC/s till a coiling temperature (TCT) is reached and coiling thereafter, wherein TCT varies in the range 400 to 700oC;
cooling the coiled hot rolled steel to ambient temperature;
cold rolling the cooled hot rolled steel sheet to obtain a cold rolled steel sheet;
heating the cold rolled steel sheet to a temperature in the range of 800 to 880oC for a time duration in the range of 3 to 5 minutes;
hot forming the heated cold rolled steel sheet;
quenching the hot formed steel component to obtain semi-quenched hot formed steel component;
heat-treating the semi-quenched hot formed steel to obtain ultra-high strength hot formed heat-treated steel component, wherein the ultra-high strength hot formed heat-treated steel component comprises a microstructure of 85 – 98% martensite, and about 2 to 15% retained austenite, wherein the ultra-high strength hot formed heat-treated steel exhibits an ultimate tensile strength in the range of 1800 – 2300 MPa.
2. The method (100) as claimed in the claim 1, wherein the ultra-high strength hot formed heat-treated steel component comprises fine precipitates in the microstructure, wherein the fine precipitates include few amounts of ferrite with solute elements, carbide, nitride, oxide, and sulphide precipitates.
3. The method (100) as claimed in the claim 1, wherein the ultra-high strength hot formed heat-treated steel exhibits a hardness greater than 500 HV, a yield strength in the range of 1300 to 1800 MPa, total elongation of greater than 9%, and strength and ductility product is more than 20GPa%.
4. The method (100) as claimed in the claim 3, wherein the ultra-high strength hot formed heat-treated steel exhibits the hardness in the range of 650 to 750 HV.
5. The method (100) as claimed in the claim 1, wherein the obtained ultra-high strength hot formed heat-treated steel exhibits a low average layer thickness less than 20 micrometer.
6. The method (100) as claimed in the claim 1, wherein reheating of the steel slab is performed inside a furnace kept at a temperature in the range of 1050 - 1250 °C.
7. The method (100) as claimed in the claim 1, wherein during the hot rolling, reduction rate in thickness of the steel is high in initial six passes and reduction rate in thickness is less in subsequent passes.
8. The method as claimed in claims 1 to 7, wherein thickness of the steel sheet after the hot rolling is in the range of 2.5mm to 6mm, and thickness of the steel sheet after the cold rolling is 1mm to 1.8mm.
9. The method as claimed in claim 1, wherein the hot forming process is a hot stamping process, wherein the start temperature of hot forming is in the range between 600 to 850 oC.
10. The method as claimed in claim 9, wherein the quenching is a die quenching process, wherein the quenching is performed for a duration of 5-15 seconds based on initial quenching temperature of the hot formed component, wherein the die cooling rate varies in the range from 30 to 80oC/ sec and the final die quenching temperature is in the range of 100 to 600oC.
11. The method as claimed in claim 10, wherein the semi-quenched hot formed component at a temperature in the range of 100-600oC is transferred for heat-treatment.
12. The method as claimed in the claim 11, wherein heat-treatment is performed in the temperature range of 200oC to 600oC for a time duration in the range of 20 to 200 sec.
13. The method as claimed in the claim 1, wherein the cold rolled steel sheet is heated in an electric furnace having normal air atmosphere.
14. An ultra-high strength hot formed heat-treated steel comprises the following composition expressed in weight %:
Carbon (C): 0.22% - 0.32%,
Manganese (Mn): 1.5% - 2.2%,
Silicon (Si): 0.05% - 0.9%,
Chromium (Cr): 0.1% - 0.9%,
Aluminium (Al): 0.02% - 0.6%,
Molybdenum (Mo): 0.01% - 0.6%,
Niobium (Nb): 0.02% - 0.08%,
Titanium (Ti): 0.01% - 0.06%,
Boron (B): 0.0005% - 0.005%,
Nickel (Ni): 0.0% - 0.5%,
Sulphur (S): 0.01% - 0.03%,
phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium and/or other rare earth elements: 0.0% - 0.02% and the remaining being substantially iron and incidental impurities, wherein the ultra-high strength hot formed heat-treated steel comprises a microstructure of 85-98% martensite, small amount of 2 to 15% retained austenite, and fine precipitates.
15. The ultra-high strength hot formed heat-treated steel as claimed in the claim 14, wherein the ultra-high strength hot formed heat-treated steel component comprises fine precipitates in the microstructure, wherein the fine precipitates include few amounts of ferrite with solute elements, carbide, nitride, oxide, and sulphide precipitates.
16. The ultra-high strength hot formed heat-treated steel as claimed in the claims 1 and 14, wherein Zirconium is added to obtain minimum oxide scale thickness.
17. The ultra-high strength hot formed heat-treated steel as claimed in the claim 14, wherein the ultra-high strength hot formed heat-treated steel exhibits hardness greater than 500 HV, yield strength (YS) in the range of 1300 to 1800 MPa, ultimate tensile strength (UTS) in the range of 1800 – 2300 MPa, and minimum %Elongation – 9.
18. The ultra-high strength hot formed heat-treated steel as claimed in the claims 1 and 14, wherein rare earths includes at least one of Cerium (Ce), Ytterbium (Yb).
19. The ultra-high strength hot formed heat-treated steel as claimed in the claims 18, wherein rare earths comprising at least one of Cerium (Ce), Ytterbium (Yb) can be added along with zirconium, wherein total addition of rare earths and zirconium within the steel is restricted to 0.02 wt.%.
20. The ultra-high strength hot formed heat-treated steel as claimed in claim 1 to 19, wherein the carbon is preferably less than 0.3%.
21. The ultra-high strength hot formed heat-treated steel as claimed in claim 20, wherein total alloying content is preferably less than 5 wt%.
22. A component made of the ultra-high strength hot formed heat-treated steel as claimed in claims 1 to 21, wherein the component is used in automotive structural, safety, reinforcement support, external part, battery/fuel cell casing and protection covers or, high strength railway bogies/ wagon members, mechanized agricultural and mining equipment, or other immobile applications like road furniture, seashore structures, pre-engineered buildings, bridges.

Dated this 30th March 2022

GOPINATH A S
IN/PA 1852
OF K&S PARTNERS
AGENT FOR THE APPLICANT
, Description:FORM 2

THE PATENT ACT, 1970
(39 of 1970)
&
THE PATENT RULES, 2003

COMPLETE SPECIFICATION
(See Section 10, Rule 13)

METHOD OF MANUFACTURING ULTRA HIGH STRENGTH HOT FORMED HEAT-TREATED STEEL
APPLICANT: TATA STEEL LIMITED, JAMSHEDPUR – 831001 JHARKHAND, INDIA
Nationality: Indian

The following specification particularly describes the nature of the invention and the manner in which it is to be performed.

FIELD OF INVENTION
[0001] The present invention relates to an ultra-high strength hot formed heat-treated steel, and more particularly to the method of manufacturing the ultra-high strength hot formed heat-treated steel having high strength, high plasticity, and oxidation resistance.

BACKGROUND
[0002] Ultra-High strength steel is a preferred choice over conventional low strength and high strength steels due to is light weight and crash safety management design of the auto body parts. With the desired high strength requirement, the forming becomes difficult in cold condition. Therefore, the process of forming at higher temperature has been adopted and to increase the hardenability of these hot formed steel suitable alloying with carbon and boron becomes a popular grade of steel. The Carbon-Manganese-Boron alloys such as 22MnB5, 27MnCrB5, and 37MnB5 steels were hot formed and water cool die quenched to develop fully martensite structures which impart the strength. The formation of martensite is enables by different alloying elements which improves the hardenability at the achievable cooling rate under dies after hot forming. The tensile strength the steel having die quenched martensite microstructure is coarsely proportional to the carbon content in the steel. About 0.36 wt% of carbon in 38MnB5 steel is added to achieve a strength level of 2 GPa hot formed steel. Such high carbon content steel is difficult to fabricate by welding process in modern light weight vehicles. Therefore, the need to formulate a novel steel chemistry for hot forming grades with tensile strength > 2GPa has been realized in our work maintaining the required level of ductility (plasticity) of the hot forming components.
[0003] Another basic problem of hot forming process is the control of high temperature oxide layer formation. This problem is solved by using Al-Si alloy coating on the conventional hot forming grade of steel (22MnB5). But this coating is very brittle in nature and reduce the plasticity of the ultra-high strength hot forming steel specially when the strength is more than 1800 MPa. The micro cracks present in the brittle coating layers reduces the fatigue life, toughness, and ductility of the hot formed steel. Therefore, the performance of ultra-high strength hot forming components is not attractive with brittle nature of coating to avoid high temperature oxidation. Therefore, alternate method of reducing high temperature scale has been experimented (i) by reducing the hot forming temperature and (ii) by self-passivation of the hot forming steel with enriching alloys replicating the phenomenon of stain-less steel/ weather resistant steel with low-cost optimum alloying innovations. Therefore, the need for inventing a high temperature oxidation resistant hot forming steel without coating with improved strength and ductility for superior performance of automotive lightweight structural component is a state-of-the-art invention.
[0004] The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.
OBJECTIVE OF INVENTION
[0005] It is an object of the invention to solve the problems of the prior art and to provide 20GPa% low-alloy uncoated ultra-high strength hot formed heat-treated steel developed with leaner chemistry to achieve the required properties (ductility, low average oxide layer thickness (less than 20 micrometer)) by exploiting the existing hot rolling facilities in the integrated steel plants to get desired microstructure.
[0006] Another objective of the present invention is to develop an ultra-high strength hot formed steel that can be made into steel strips, sheets, and blanks, having tensile strength about 2000 MPa (2GPa) and total elongation 9 % and having microstructure consisting of majorly martensite and small amount of retained austenite.
[0007] Another objective of the present invention is to develop a grade of steel without excessive alloying to facilitate easy fabricability, printability without much difference in the cost from conventional hot forming grade of steel.
[0008] Another objective of present invention is to provide a new easier manufacturing method combining thermomechanical, hot rolling and heat treatment processes for the proposed chemical composition.
[0009] It is yet another objective of the present invention, to provide the ultra-high strength hot formed heat treated steel having strength and ductility product is more than 20GPa% comprises the following composition expressed in weight %: Carbon (C): 0.22% - 0.32%, Manganese (Mn): 1.5% - 2.2%, Silicon (Si): 0.05% - 0.9%, Chromium (Cr): 0.1% - 0.9%, Aluminium (Al): 0.02% - 0.6%, Molybdenum (Mo): 0.01% - 0.6%, Niobium (Nb): 0.02% - 0.08%, Titanium (Ti): 0.01% - 0.06%, Boron (B): 0.0005% - 0.005%, Nickel (Ni): 0.0% - 0.5%, Sulphur (S): 0.01% - 0.02%, Phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium and/or rare earths: 0.0% - 0.02%, and the balance being Iron (Fe) and unavoidable impurities.
SUMMARY OF INVENTION
[0010] This summary is provided to introduce concepts related to an ultra-high strength hot formed heat-treated steel, and a method of manufacturing the ultra-high strength hot formed heat-treated steel. 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.
[0011] In one aspect of the present invention, a method for manufacturing ultra-high strength hot formed heat-treated steel component having high strength, high plasticity and oxidation resistance is provided. The method comprises casting molten steel having a composition expressed in weight %: Carbon (C): 0.22% - 0.32%, Manganese (Mn): 1.5% - 2.2%, Silicon (Si): 0.05% - 0.9%, Chromium (Cr): 0.1% - 0.9%, Aluminium (Al): 0.02% - 0.6%, Molybdenum (Mo): 0.01% - 0.6%, Niobium (Nb): 0.02% - 0.08%, Titanium (Ti): 0.01% - 0.06%, Boron (B): 0.0005% - 0.005%, Nickel (Ni): 0.0% - 0.5%, Sulphur (S): 0.01% - 0.02%, Phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium (Zr) and/or other rare earths: 0.0% - 0.02%, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab. The method also comprises reheating the steel slab inside a furnace kept at a temperature greater than 1050°C and soaking the steel slab at the temperature greater than 1050°C for a time duration of in the range of 60-200 minutes. The method further comprises hot rolling the steel slab to produce a hot rolled steel sheet such that finish rolling is done at a temperature TFRT in the range 800oC to 900oC. The method comprises cooling at a cooling rate 20-60oC/s till a coiling temperature (TCT) is reached and coiling thereafter. TCT varies in the range 400 to 700oC. The method further comprises cooling the coiled hot rolled steel to ambient temperature. The method comprises cold rolling the cooled hot rolled steel sheet to obtain a cold rolled steel sheet. The method also comprises heating the cold rolled steel sheet to a temperature in the range of 800 to 880oC for a time duration in the range of 3 to 5 minutes. The method further comprises hot forming the heated cold rolled steel sheet. The method comprises quenching the hot formed steel component to obtain semi-quenched hot formed steel component. The method also comprises heat-treating the semi-quenched hot formed steel to obtain ultra-high strength hot formed heat-treated steel component. The ultra-high strength hot formed heat-treated steel component comprises a microstructure of 85 – 98% martensite, and about 2 to 15% retained austenite. The ultra-high strength hot formed heat-treated steel exhibits an ultimate tensile strength in the range of 1800 – 2300 MPa.
[0012] In an embodiment, the ultra-high strength hot formed heat-treated steel component comprises fine precipitates in the microstructure. The fine precipitates include few amounts of ferrite with solute elements, carbide, nitride, oxide, and sulphide precipitates.
[0013] In an embodiment, the ultra-high strength hot formed heat-treated steel exhibits a hardness greater than 500 HV, a yield strength in the range of 1300 to 1800 MPa, and total elongation of greater than 9%.
[0014] In an embodiment, the ultra-high strength hot formed heat-treated steel exhibits the hardness in the range of 650 to 750 HV.
[0015] In an embodiment, the obtained ultra-high strength hot formed heat-treated steel exhibits a low average layer thickness less than 20 micrometer.
[0016] In an embodiment, reheating of the steel slab is performed inside a furnace kept at a temperature in the range of 1050 - 1250 °C.
[0017] In an embodiment, during the hot rolling, reduction rate in thickness of the steel is high in initial six passes and reduction rate in thickness is less in subsequent passes.
[0018] In an embodiment, thickness of the steel sheet after the hot rolling is in the range of 2.5mm to 6mm, and thickness of the steel sheet after the cold rolling is 1mm to 1.8mm.
[0019] In an embodiment, the hot forming process is a hot stamping process, wherein the start temperature of hot forming is in the range between 600 to 850 oC.
[0020] In an embodiment, the quenching is a die quenching process, wherein the quenching is performed for a duration of 5-15 seconds based on initial quenching temperature of the hot formed component. The die cooling rate varies in the range from 30 to 80oC/ sec and the final die quenching temperature is in the range of 100 to 600oC.
[0021] In an embodiment, the semi-quenched hot formed component at a temperature in the range of 100-600oC is transferred for heat-treatment.
[0022] In an embodiment, the hot rolled steel sheet is pickled with 10-20% HCL acid solution at a temperature in the range of 60-90°C for a duration of 5-30 mins.
[0023] In an embodiment, the cold rolled steel sheet is heated in an electric furnace having normal air atmosphere.
[0024] In another aspect of the present invention, an ultra-high strength hot formed heat-treated steel is provided. The ultra-high strength hot formed heat-treated steel comprises the following composition expressed in weight %: Carbon (C): 0.22% - 0.32%, Manganese (Mn): 1.5% - 2.2%, Silicon (Si): 0.05% - 0.9%, Chromium (Cr): 0.1% - 0.9%, Aluminium (Al): 0.02% - 0.6%, Molybdenum (Mo): 0.01% - 0.6%, Niobium (Nb): 0.02% - 0.08%, Titanium (Ti): 0.01% - 0.06%, Boron (B): 0.0005% - 0.005%, Nickel (Ni): 0.0% - 0.5%, Sulphur (S): 0.01% - 0.03%, Phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium and/or other rare earth elements: 0.0% - 0.02% and the remaining being substantially iron and incidental impurities. The ultra-high strength hot formed heat-treated steel comprises a microstructure of 85-98% martensite, and small amount of 2 to 15% retained austenite.
[0025] In an embodiment, the ultra-high strength hot formed heat-treated steel component comprises fine precipitates in the microstructure. The fine precipitates include few amounts of ferrite with solute elements, carbide, nitride, oxide, and sulphide precipitates.
[0026] In an embodiment, Zirconium is added to obtain minimum oxide scale thickness. In an embodiment, the ultra-high strength hot formed heat-treated steel exhibits hardness greater than 500 HV, yield strength (YS) in the range of 1300 to 1800 MPa, ultimate tensile strength (UTS) in the range of 1800 – 2300 MPa, and minimum %Elongation – 9.
[0027] In an embodiment, rare earths includes at least one of Cerium (Ce), Ytterbium (Yb). In an embodiment, rare earths comprising at least one of Cerium (Ce), Ytterbium (Yb) can be added along with zirconium. The total addition of rare earths and zirconium within the steel is restricted to 0.02 wt.%.
[0028] In an embodiment, the carbon is preferably less than 0.3%. In an embodiment, total alloying content is preferably less than 5 wt%.
[0029] In an embodiment, the component is used in automotive structural, safety, reinforcement support, external part, battery/fuel cell casing and protection covers or, high strength railway bogies/ wagon members, mechanized agricultural and mining equipment, or other immobile applications like road furniture, seashore structures, pre-engineered buildings, bridges.
[0030] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Figure 1 illustrates a flowchart of a method of manufacturing an ultra-high strength hot formed heat-treated steel, according to an embodiment of the present invention;
[0032] Figure 2 illustrates a table depicting the chemical composition of the ultra-high strength hot formed heat-treated steel, according to the embodiment of the present invention;
[0033] Figure 3 illustrates a table depicting ultra-high strength hot formed heat-treated steel having different compositions, according to the embodiment of the present invention;
[0034] Figure 4 illustrates an SEM image of the ultra-high strength hot formed heat-treated steel having composition of sample 5, according to the embodiment of the present invention;
[0035] Figure 5a illustrates a graphical representation of stress versus elongation, obtained during tensile test of the ultra-high strength hot formed heat-treated steel having composition of sample 4, according to the embodiment of the present invention;
[0036] Figure 5b illustrates a graphical representation of stress versus elongation, obtained during tensile test of the ultra-high strength hot formed heat-treated steel having composition of sample 5, according to the embodiment of the present invention; and
[0037] Figure 5c illustrates a graphical representation of stress versus elongation, obtained during tensile test of the ultra-high strength hot formed heat-treated steel having composition of sample 6 according to the embodiment of the present invention.
[0038] The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.

DETAILED DESCRIPTION
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] The present disclosure provides a method (100) of manufacturing ultra-high strength hot formed heat-treated steel having high strength, high plasticity and oxidation resistance that may be used to produce components for automobile applications. The ultra-high strength hot formed heat-treated steel comprises the following composition expressed in weight %: Carbon (C): 0.22% - 0.32%, Manganese (Mn): 1.5% - 2.2%, Silicon (Si): 0.05% - 0.9%, Chromium (Cr): 0.1% - 0.9%, Aluminium (Al): 0.02% - 0.6%, Molybdenum (Mo): 0.01% - 0.6%, Niobium (Nb): 0.02% - 0.08%, Titanium (Ti): 0.01% - 0.06%, Boron (B): 0.0005% - 0.005%, Nickel (Ni): 0.0% - 0.5%, Sulphur (S): 0.01% - 0.02%, Phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium (Zr): 0.0% - 0.02%, and the balance being Iron (Fe) and unavoidable impurities. The ultra-high strength hot formed heat-treated steel comprises a structure including a martensite phase, a retained austenite phase and fine precipitates. In an embodiment, the ultra-high strength hot formed heat-treated steel comprises a microstructure 85 – 98% martensite, about 2 to 15% retained austenite and fine precipitates. The fine precipitates include few amounts of ferrite with solute elements, carbide, nitride, oxide, and sulphide precipitates.
[0045] The ultra-high strength hot formed heat-treated steel exhibits an ultimate tensile strength in the range of 1800 – 2300 MPa, a hardness greater than 500 HV, a yield strength in the range of 1300 - 1800 MPa, and total elongation of greater than 9%. In an embodiment, the ultra-high strength hot formed heat-treated steel exhibits the hardness in the range of 650 to 750 HV.
[0046] The chemical composition of the proposed alloys are shown in Table 1. In an embodiment, the carbon is preferably less than 0.3%. In an embodiment, total alloying content is preferably less than 4 wt%. In an embodiment, rare earths comprising at least one of Cerium (Ce), Ytterbium (Yb) may be added along with zirconium, however the total addition of rare earths and zirconium within the steel should be restricted to 0.02 wt.%.
[0047] In one embodiment, the ultra-high strength hot formed steel comprises composition expressed in weight %: C: 0.35, Mn: 2.2, S: 0.007, P: 0.02, Si: 1.2, Al: 0.18, Cu: 0.67, Cr: 0.75, Ni: 0.43, Mo: 0.31, V: 0.005, Nb: 0.08, Ti: 0.002, N (ppm): 90, B: 0.0048, Zr and/or RE: 0, and the balance being Iron (Fe) and unavoidable impurities.
[0048] In another embodiment, the ultra-high strength hot formed steel comprises composition expressed in weight %: C: 0.3, Mn: 1.84, S: 0.004, P: 0.019, Si: 0.5, Al: 0.47, Cu: 0.56, Cr: 0.52, Ni: 0.48, Mo: 0.28, V: 0.007, Nb: 0.057, Ti: 0.004, N (ppm): 106, B: 0.0034, Zr and/or RE: 0.013, and the balance being Iron (Fe) and unavoidable impurities.
[0049] In yet another embodiment, the ultra-high strength hot formed steel comprises composition expressed in weight %: C: 0.29, Mn: 1.83, S: 0.021, P: 0.012, Si: 0.59, Al: 0.29, Cu: 0.58, Cr: 0.49, Ni: 0.42, Mo: 0.31, V: 0.004 Nb: 0.065, Ti: 0.007, N (ppm): 113, B: 0.0035, Zr and/or RE: 0.006, and the balance being Iron (Fe) and unavoidable impurities.
[0050] The ultra-high strength hot formed heat-treated steel exhibits yield strength (YS) in the range of 1200 to 1800 MPa, ultimate tensile strength (UTS) in the range of 1800 to 2300 MPa and minimum %Elongation 9.
[0051] Referring to Figure 1, an exemplary thermo-mechanical method (100) of manufacturing the ultra-high strength hot formed heat-treated steel component is illustrated. Each step shown in figure 1 represents one or more process, method or subroutine steps carried out in the method. Furthermore, the order of blocks is illustrative only and the blocks can change in accordance with the present disclosure. Additional blocks can be added, or fewer blocks can be utilized, without departing from this disclosure. The method (100) for manufacturing the ultra-high strength hot formed heat-treated steel component begins at step (102). At step (102), molten steel having composition expressed in weight %: Carbon (C): 0.22% - 0.32%, Manganese (Mn): 1.5% - 2.2%, Silicon (Si): 0.05% - 0.9%, Chromium (Cr): 0.1% - 0.9%, Aluminium (Al): 0.02% - 0.6%, Molybdenum (Mo): 0.01% - 0.6%, Niobium (Nb): 0.02% - 0.08%, Titanium (Ti): 0.01% - 0.06%, Boron (B): 0.0005% - 0.005%, Nickel (Ni): 0.0% - 0.5%, Sulphur (S): 0.01% - 0.02%, Phosphorus (P): 0.01% - 0.03%, Vanadium (V): 0.0% - 0.02%, Copper (Cu): 0.01-0.6%; Nitrogen (N): 0.0005% - 0.02%, Zirconium and/or Rare earths: 0.0% - 0.02%, and the balance being Iron (Fe) and unavoidable impurities is cast in a casting apparatus to obtain a steel slab.
[0052] At step (104), the steel slab (cast ingots) is reheated inside a furnace kept at a temperature greater than 1050 °C and soaked at the temperature greater than 1050 °C for a time duration of in the range of 60-200 minutes. In one example, the furnace is an electrical furnace with normal air environment. In the preferred embodiment, the steel slab is reheated to a temperature in the range of 1050 - 1250 oC.
[0053] At step (106), the reheated steel slab is hot rolled to produce a hot rolled steel sheet such that finish rolling is done at a temperature (TFRT) to obtain a hot rolled steel sheet or strip. TFRT varies in the range 800oC to 900oC. The hot rolling process may be carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the steel to required size in the range of 2.5-6 mm. In an embodiment, the hot rolling process is performed by passing the steel through a pair of rolls and rolling is carried out for at least 5 times. During the hot rolling, reduction rate in thickness of the steel is high in initial six passes and reduction rate in thickness is less in subsequent passes.
[0054] At step (108), the hot rolled steel strip is cooled at a cooling rate 20-60oC/s till a coiling temperature (TCT) is reached and is coiled thereafter to obtain a coiled hot rolled steel sheet or strip. TCT varies in the range 400 to 700oC.
[0055] At step (110), the coiled hot rolled steel sheet or strip is cooled to ambient temperature (less than 100oC).
[0056] At step (112), the cooled hot rolled steel sheet or strip is cold rolled to obtain a cold rolled steel sheet or strip. In an embodiment, reduction during cold rolling is in the range of 30-80%. In an embodiment, thickness of the steel strip or sheet after the cold rolling is in the range of 1mm to 1.8mm. In an embodiment, the cold rolled steel strip or sheet is heated in an electric furnace having normal air atmosphere.
[0057] At step (114), the cold rolled steel strip or sheet is heated to a temperature in the range of 800 to 880oC for a time duration in the range of 3 to 5 minutes.
[0058] At step (116), the heated cold rolled steel strip or sheet is hot formed to obtain hot formed steel component. The hot forming process is a hot stamping process. The heated cold rolled steel strip or sheet is transferred to a press for hot forming between two dies. The start temperature of hot forming is in the range between 600 to 850oC.
[0059] At step (118), the hot formed steel component is quenched to obtain semi-quenched hot formed steel component. In an embodiment, the quenching is a die quenching process performed for a duration of 5-15 seconds based on sample thickness and initial quenching temperature (600 to 850oC) of the hot formed component. In the preferred embodiment, the die quenching time duration is less than the normal die quenching time of normal hot forming process (which is generally kept between 15 to 25 second per stroke).
[0060] The hot formed steel component is cooled by die quenching (conductive quenching of the sheet to the dies). The die can be cooled by water or other cooling medium depending on the hot forming press setup, without any limitations.
[0061] The die cooling rate varies in the range from 30 to 80oC/ sec and the final die quenching temperature is in the range of 100 to 600oC. The cooled hot formed steel component (at Temp between 100 to 600oC) is transferred from die to a heat treatment facility.
[0062] At step (120), the semi-quenched hot formed steel component is heat-treated to obtain ultra-high strength hot formed heat-treated steel component. In an embodiment, the semi-quenched hot formed steel component is heat-treated in the temperature range of 200oC to 600oC for a time duration in the range of 20 to 200 sec.
[0063] The obtained ultra-high strength hot formed heat-treated steel component comprises a microstructure of 85 – 98% martensite, about 2 to 15% retained austenite and fine precipitates. The fine precipitates include few amounts of ferrite with solute elements, carbide, nitride, oxide, and sulphide precipitates. The ultra-high strength hot formed heat-treated steel exhibits an ultimate tensile strength in the range of 1800 – 2300 MPa.
[0064] The obtained ultra-high strength hot formed heat-treated steel exhibits a hardness greater than 500 HV. The obtained ultra-high strength hot formed heat-treated steel exhibits a low average layer thickness less than 20 micrometer. The obtained ultra-high strength hot formed heat-treated steel exhibits high strength, high plasticity, and oxidation resistance.
[0065] Additionally, the hot rolled steel sheet or strip may be pickled with 10-20% HCL acid solution at a temperature in the range of 60-90°C for a duration of 5-30 mins before cold rolling. Similarly, cold rolled strip may be cleaned with alkali / electrolyte treatment/ brushing/ water rinsing/ compressed air (or Nitrogen air) to reduce iron fines and emulsions. Annealing heat treatment of the full hard cold rolled material can be done for easy handling and blanking before hot forming. Cold rolled sheets can be cut with laser cutter for precision blanking. Various optional post hot forming processes like cleaning, sand blasting, welding, laser trimming, sectioning, coating, painting, bake hardening, heat treating, re-hot forming, sandwiching, cladding, texturing etc. can be performed as per the requirement, without limiting the scope of the invention.
[0066] A component made of the ultra-high strength hot formed heat-treated steel is used in automotive structural, safety, reinforcement support, external part, battery/fuel cell casing and protection covers or, high strength railway bogies/ wagon members, mechanized agricultural and mining equipment, or other immobile applications like road furniture, seashore structures, pre-engineered buildings, bridges without limitations.
[0067] Following portions of the present disclosure provides details about the proportion of each element in a composition of the ultra-high strength hot formed heat-treated steel and their role in enhancing properties.
[0068] Carbon (C) may be added in the range of about 0.22% to about 0.32%. Carbon is a main strengthening element in the steel. Carbon in this range increases hardenability and hardness of martensite phase.
[0069] Manganese (Mn) may be added in the range of about 1.5% to about 2.2%. Manganese may improve hardenability and reduce austenite transformation temperature. This helps in lowering hot forming temperature. At low hot forming temperature, the grain growth after transformation will be low. With lower grain size, the tensile strength and toughness of steel is increased.
[0070] Silicon (Si) may be added in the range of about 0.05% to about 0.9%. Silicon helps in reducing bonding during hot rolling.
[0071] Chromium (Cr) may be added in the range of about 0.1% to about 0.9%. Chromium helps in increasing hardenability and helps in avoiding unwanted phases during martensite transformation.
[0072] Aluminium (Al) may be added in the range of about 0.02% to about 0.6%. Aluminium reduces oxygen and nitrogen from steel melt.
[0073] Molybdenum (Mo) may be added in the range of about 0.01% to about 0.6%. Molybdenum may increase hardenability and aids in forming precipitate to increase strength.
[0074] Niobium (Nb) may be added in the range of about 0.02% to about 0.08%. Niobium’s carbide precipitation help in controlling the grain size during hot rolling.
[0075] Titanium (Ti) may be added in the range of about 0.01% to about 0.06%. Titanium form precipitates to control grain size, reduce solute nitrogen level in steel.
[0076] Boron (B) may be added in the range of about 0.0005% to about 0.005%. Boron helps in increasing hardenability.
[0077] Nickel (Ni) may be added in the range of about 0.0 to 0.5%. Nickel helps in improving toughness of the steel.
[0078] Sulphur (S) may be added in the range of about 0.01% to about 0.03%. Sulphur increases brittleness of the steel and low level of Sulphur is desired.
[0079] Nitrogen (N) may be added in the range of about 0.0005% to about 0.02%. Nitrogen increases brittleness of the steel and low level of Nitrogen is desired.
[0080] Phosphorous (P) may be added in the range of about 0.01% to about 0.03%. Phosphorous increases brittleness of the steel and low level of Phosphorus is desired.
[0081] Zirconium (Zr) may be added in the range of 0.0% - 0.02%. Zirconium is added to obtain minimum oxide scale thickness. It prevents diffusion of oxygen beyond the characteristic passivation layers and retain the low temperature passivation layer during heating and hot forming.
[0082] Copper (Cu) may be added in the range of 0.01% to 0.6%. It helps to form a passivation layer at lower temperature at normal/ hydrated atmospheric environment.
[0083] Vanadium (V) may be added in the range of 0.0% to 0.02%. I helps to form fine VN precipitates for reducing solute nitrogen by forming fine VN precipitates and strengthening ferrite and transformed martensite matrix.
[0084] Embodiments of the present disclosure will now be described with an example of a particular composition of the steel. Experiments have been carried out for a specific composition of the steel formed by using method of the present disclosure. The composition of the steel for which the tests are carried out is as shown in below table 2 (shown in Figure 3).
[0085] Figure 4 illustrates an SEM image of the ultra-high strength hot formed heat-treated steel having composition of sample 5. As can be observed from this image, the ultra-high strength hot formed heat-treated steel has Martensite, Retained Austenite (lamellar) and fine precipitates microstructure.
[0086] The steel (having compositions of samples 4, 5, and 6), processed by the method (100) of the present disclosure, may be subjected to testing to determine mechanical properties of the ultra-high strength hot formed heat-treated steel component. As an example, tensile testing may be performed as per ASTM standards with E8 sub-size sample having gauge length of 25 mm. Tensile test may be performed in a rigid servo hydraulic 100 T capacity universal tensile testing machine. Results from the tensile test have been illustrated in Figures 5a, 5b, and 5c.
[0087] Figure 5a illustrates tensile test results of steel having composition of sample 4 and processed by the method (100) of the present disclosure. Figure 5b illustrates tensile test results of steel having composition of sample 5 and processed by the method (100) of the present disclosure. Figure 5c illustrates tensile test results of steel having composition of sample 6 and processed by the method (100) of the present disclosure. As seen in Figure 5a, the yield strength obtained is about 1522 MPa, ultimate Tensile strength obtained is about 2053 MPa, total elongation is 10.2%, and strength and ductility product is about 20.9 GPA%. Further as seen in Figure 5b, the yield strength obtained is about 1497 MPa, ultimate tensile strength obtained is about 2073 MPa, total elongation is 10.2 %, and strength and ductility product is about 21.1 GPA%. Further as seen in Figure 5c, the yield strength obtained is about 1526 MPa, ultimate tensile strength obtained is about 2054 MPa, total elongation is 10.6 % and strength and ductility product is about 21.8 GPA%. The hot forming process step in the method of the present disclosure aids in producing martensitic microstructure in the steel.
[0088] In the developed steels, martensitic transformation can occur at very low cooling rates, therefore, simple dies can be used for hot forming. Some retained austenite may be present after hot forming, which will improve the elongation and energy absorption during crash.
[0089] It should be understood that the experiments are carried out for particular compositions of the steel and the results brought out in Figures 5a, 5b and 5c are for the compositions 4, 5, and 6 as shown in Table – 2. However, the said compositions should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the steel as well.
[0090] The present invention provides ultra-high strength hot formed heat-treated steel and the method (100) of manufacturing the ultra-high strength hot formed heat-treated steel having about 2 GPA strength with bare minimum addition of alloying, without coating, with improved strength and ductility for superior performance. The combination of high strength and ductility gives high impact toughness and crush resistance property. The total oxide sale thickness formation of this steel during heating to the required hot forming temperature at normal air atmosphere, transferring the hot sample to the die and after forming and die quenching is found significantly lower than the conventional steel. The average oxide layer thickness is about 20 micrometer and it is easily removable by compressed air cleaning.
[0091] Ultra-high strength hot formed heat-treated steels makes an important contribution towards the cost effective, futuristic, and strategic light weight application of steel with greater factor of safety. A superior factor of safety may be obtained by achieving the sharp increase of yield strength with reasonable ductility, specifically required for the automotive and structural applications. The thermomechanical / hot-rolling process is quite simple and does not require huge energy consumption. Therefore, the method (100) of the present disclosure aids in reducing energy consumption and thus a cost-effective steel manufacturing process. Further, the method (100) provides a new steel developed with leaner chemistry to achieve the required properties by exploiting the existing hot rolling facilities in the integrated steel plants to get desired microstructure.
[0092] Furthermore, the terminology used herein is for describing 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.
[0093] 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.
[0094] 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.