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

Present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a microalloyed steel strip. Further embodiments of the disclosure disclose a method for manufacturing the microalloyed steel strip, which exhibits a tensile strength greater than 600 MPa.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon, and other alloying elements. Because of its high tensile strength and low cost, 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 steel obtained from steel making process may not possess all the desired properties. Therefore, the steel may be subjected to secondary processes such as various heat treatment processes for controlling material properties to meet various needs in the intended applications.

With rising concerns over global environmental problems and demand from automotive industry for higher collision safety of vehicles, impose conflicting requirements on materials used for building vehicle. The vehicle bodies are required to be stronger, yet lighter at the same time. Conventionally, steels possessing tensile strength above 450 MPa has been adapted in automotive applications. With evolution and advancements, advanced vehicles in terms of load bearing capacity, crash worthiness and the like have been developed. To meet these advancements, demand towards steel possessing high strength (i.e., high tensile strength) has been in rise. To manufacture high strength steels typically having tensile strength above 600 MPa, it necessitates high alloying addition and carbon content above 0.5 wt.%. Carbon content above 0.5 wt.% leads to deterioration of weldability and decreases ductility. These undesired properties make the steels not suitable in manufacturing long members of the vehicle, which involves several bending and welding operations, as such steel possess poor ductile properties.

One of the patent publications IN201301370 discloses a steel exhibiting tensile strength of 540 MPa and microstructure having 10% to 30% bainite and remaining being ferrite. The steel composition of this patent comprises of 0.05-0.07% of Carbon, 1.0-1.5% of Manganese, 0.1-0.5% of Silicon, 0.03% of Niobium, maximum 0.005% of Sulphur.

Further, IN201400279 discloses a steel exhibiting tensile strength of 540 MPa and microstructure having 10% to 30% bainite and remaining being ferrite. The steel composition of this patent comprises 0.05- 0.07% of Cr 1.0-1.5% of Mn 0.3-0.5% of Si 0.03% or less of Nb 0.06% or less of V maximum 0.005% of S maximum 0.030% of P 0.007 - 0.01% of N.

Another patent publication, IN201931004464 discloses a steel exhibiting tensile strength of 540 MPa and microstructure having single phase bainitic ferrite.

However, in the known arts the steel produced posses lower strength.

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 disclosed 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.

In one non-limiting embodiment, there is provided a method for manufacturing a microalloyed steel strip is disclosed. The method starts with casting a steel slab of a composition in weight percentage including: carbon (C) at about 0.04% to 0.08%, manganese (Mn) at about 1% to about 1.8%, sulphur (S) at about 0.001- 0.006%, phosphorous (P) at about 0.01-0.02%, silicon (Si) at about 0.1% to 0.25%, aluminum (Al) at about 0.02% to 0.06%, titanium (Ti) at about 0.01% to 0.02%, niobium (Nb) at about 0.07% to 0.12%, vanadium (V) at about 0.005% to 0.06%, chromium (Cr) at about 0.1% to 0.6%, nickel (Ni) at about 0.1% to 0.3%, molybdenum (Mo) at about 0.1% to 0.2%, copper (Cu) about 0.001% to 0.2%, nitrogen (N) at about 30 ppm to 80 ppm, and balance being Iron (Fe) optionally along with incidental elements. After casting, the steel slab is cooled to ambient temperature by a first cooling process. The steel slab is then heated to a first predetermined temperature for a first predetermined time. The method further includes subjecting the steel slab to a first hot working process, to reduce thickness of the steel slab to about 30mm to 60mm. Further, the steel slab is subjected to a second hot working process, to induce at least 50% deformation below a second predetermined temperature, to reduce thickness of the steel slab to about 2mm to 8mm to form a steel strip. At least three passes in the second hot working process are performed below the second predetermined temperature. Further, the method includes cooling the steel strip by a second cooling process at a predetermined cooling rate and coiling the steel strip at a third predetermined temperature to form the microalloyed steel strip. The microalloyed steel strip comprises polygonal ferrite and bainite microstructure.

In an embodiment, the microalloyed steel strip exhibits tensile strength of about 600 MPa to 750 MPa and total elongation of about 25% to 33%.

In an embodiment, the microalloyed steel strip comprises microstructure represented by, in mass%, the bainite of about 30% to 80% and remaining being polygonal ferrite.

In an embodiment, the first predetermined temperature ranges from about 1150 °C to 1280 °C and the first predetermined time is about 1 to 2 hours.
In an embodiment, the first hot working process is a roughing process performed in a roughing mill.
In an embodiment, the second hot working process is hot rolling process, which is performed in a finishing mill.

In an embodiment, in the roughing mill, roughing mill exit temperature ranges from about 1010 °C to 1080 °C

In an embodiment, the second predetermined temperature is austenite no-recrystallisation temperature.
In an embodiment, finish rolling exit temperature ranges from about 820 °C to 900 °C.
In an embodiment, the third predetermined temperature ranges from about 480 °C to 540 °C.
In an embodiment, the first cooling process is air cooling and the second cooling process is performed on a run-out table.
In an embodiment, the predetermined cooling rate during the second cooling process is about 10°C/s to 60°C/s.
In an embodiment, the second cooling process restricts grain growth, facilitating forming of bainite phase along with polygonal ferrite.
In another non-limiting embodiment of the disclosure, a microalloyed steel strip is disclosed. The microalloyed steel strip comprises a composition in weight percentage including: at about 0.04% to 0.08%, manganese (Mn) at about 1% to about 1.8%, sulphur (S) at about 0.001- 0.006%, phosphorous (P) at about 0.01-0.02%, silicon (Si) at about 0.1% to 0.25%, aluminum (Al) at about 0.02% to 0.06%, titanium (Ti) at about 0.01% to 0.02%, niobium (Nb) at about 0.07% to 0.12%, vanadium (V) at about 0.005% to 0.06%, chromium (Cr) at about 0.1% to 0.6%, nickel (Ni) at about 0.1% to 0.3%, molybdenum (Mo) at about 0.1% to 0.2%, copper (Cu) about 0.001% to 0.2%, nitrogen (N) at about 30 ppm to 80 ppm, and balance being Iron (Fe) optionally along with incidental elements. The microalloyed steel strip comprises polygonal ferrite and bainite microstructure.
In an embodiment, the microalloyed steel strip exhibits yield strength of about 500MPa to 650MPa.
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 together 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 is a flowchart illustrating a method for manufacturing a microalloyed steel strip, in accordance with an embodiment of the present disclosure.

Figures. 2a and 2b illustrates a micrograph obtained from scanning electron microscope (SEM) of microalloyed steel strip having composition A and B, respectively, in accordance with an embodiment of the present disclosure.

Figure. 2c illustrates micrograph obtained from scanning electron microscope (SEM) of microalloyed steel strip having composition B, in accordance with an embodiment of the present 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 characteristic 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 particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and 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.

Embodiments of the present disclosure discloses a microalloyed steel strip and a method for manufacturing or producing the microalloyed steel strip. With evolution and advancements, vehicles developed are advanced in terms of load bearing capacity, crash worthiness and the like. To manufacture high strength steels typically having tensile strength above 600 MPa, it necessitates high alloying addition and carbon content. Carbon content above 0.08 wt% leads to deterioration of weldability and decreases ductility. These undesired properties make the steels not suitable in manufacturing long members of the vehicle, which involves several bending and welding operations, as such steel possess poor weldability and ductile properties. Accordingly, the method for manufacturing microalloyed steel strip with tensile strength greater than 600 MPa, total elongation of about 25% to 33% and formed by a simpler method is described in the present disclosure. The microalloyed steel strip may be widely employed in automotive applications to manufacture long members and load bearing components, as the microalloyed steel strip possess desired ductility.

In the method of manufacturing the microalloyed steel strip, a first step may include casting the steel of composition in weight percent including carbon (C) at about 0.04% to 0.08%, manganese (Mn) at about 1% to about 1.8%, sulphur (S) at about 0.001- 0.006%, phosphorous (P) at about 0.01-0.02%, silicon (Si) at about 0.1% to 0.25%, aluminum (Al) at about 0.02% to 0.06%, titanium (Ti) at about 0.01% to 0.02%, niobium (Nb) at about 0.07% to 0.12%, vanadium (V) at about 0.005% to 0.06%, chromium (Cr) at about 0.1% to 0.6%, nickel (Ni) at about 0.1% to 0.3%, molybdenum (Mo) at about 0.1% to 0.2%, copper (Cu) about 0.001% to 0.2%, nitrogen (N) at about 30 ppm to 80 ppm, and balance being Iron (Fe) optionally along with incidental elements, in form of a steel slab. The casted steel slab is then cooled to ambient temperature by a first cooling process, which is air cooling. Further, the method includes heating the steel slab to a first predetermined temperature ranging from about 1150 °C to 1280°C for a first predetermined time of about 1 to 2 hours. Upon heating the steel slab, the steel slab may be subjected to a first hot working process, which is a roughing process to reduce thickness of the steel slab to about 30mm to 60mm. Upon completion of the first hot working process, the steel slab may be subjected to a second hot working process which is a hot rolling process. During hot rolling process, at least 50% deformation may be induced below a second predetermined temperature, to reduce thickness of the steel slab to about 2 mm to 8mm, to form a steel strip. The second predetermined temperature may be the austenite no-recrystallisation temperature. Upon completion of the second hot working process, the steel strip may be subjected to second cooling process, which is a performed on a run-out table (ROT) at a predetermined cooling rate. In an embodiment, the predetermined cooling rate may range from about 10 °C/s to 60 °C/s. After cooling, the steel strip may be subjected to coiling at a third predetermined temperature which ranges from 480 °C to 540 °C, to form a microalloyed steel strip.

In an embodiment, the microalloyed steel strip exhibits, tensile strength of about 600 MPa to 750 MPa, total elongation of about 25% to 33% and yield ratio (yield strength/ultimate tensile strength) below 0.9. The microalloyed steel strip includes bainite microstructure of about 30% to 80% and remaining being polygonal ferrite. These properties favour the microalloyed steel strip to be used in manufacturing automotive components, such as but not limiting to long member chassis components.

Henceforth, the present disclosure is explained with the help of figures for a method of manufacturing a microalloyed steel strip. However, such exemplary embodiments should not be construed as limitations of the present disclosure since the method may be used on other types of steels where such need arises. A person skilled in the art may envisage various such embodiments without deviating from scope of the present disclosure.

Figure. 1 is an exemplary embodiment of the present disclosure illustrating a flowchart depicting a method for manufacturing a microalloyed steel strip. In the present disclosure, mechanical properties such as strength, tensile strength, and total elongation (ductility) of the final microstructure of the microalloyed steel strip may be improved. The microalloyed steel strip produced by the method of the present disclosure, includes less alloying content and microstructure includes bainite and polygonal ferrite.

The method is now described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject matter described herein.
At block 101, a steel of desired alloying composition may be made by mixing alloys in a furnace, such as a vacuum induction furnace. In an embodiment, the steel may have composition in weight percent [wt.%] including carbon (C) at about 0.04% to 0.08%, manganese (Mn) at about 1% to about 1.8%, sulphur (S) at about 0.001- 0.006%, phosphorous (P) at about 0.01-0.02%, silicon (Si) at about 0.1% to 0.25%, aluminum (Al) at about 0.02% to 0.06%, titanium (Ti) at about 0.01% to 0.02%, niobium (Nb) at about 0.07% to 0.12%, vanadium (V) at about 0.005% to 0.06%, chromium (Cr) at about 0.1% to 0.6%, nickel (Ni) at about 0.1% to 0.3%, molybdenum (Mo) at about 0.1% to 0.2%, copper (Cu) about 0.001% to 0.2%, nitrogen (N) at about 30 ppm to 80 ppm, and balance being Iron (Fe) optionally along with incidental elements.

At block 102, the method may include casting the steel in form of but not limiting to a slab. In an embodiment, liquid steel with the above-mentioned composition and range of alloying elements may be continuously casted into a slab. The liquid steel of the above-mentioned composition may be continuously casted either in a conventional continuous caster or a thin slab caster. Further, the method includes a step or stage of cooling of the steel slab by a first cooling process [as shown in block 103]. In an embodiment, the first cooling process may be air cooling.

The cooled steel slab may be then heated to a first predetermined temperature for a first predetermined time, for solutionising the alloying elements [as shown in block 104]. In an embodiment, the first predetermined temperature may range from about 1150 °C to 1280 °C and the first predetermined time is about 1 to 2 hours.
Once the steel slab is heated as per block 104, the steel slab may be subjected to a first hot working process [as shown in block 105]. In an embodiment, the first hot working process may be a roughing process which is performed in a roughing mill. In roughing process, thickness of the steel slab may be reduced to about 30mm to 60mm. After, subjecting the steel slab to the first hot working process, the steel slab may be subjected to a second hot working process [as shown in block 106]. In an embodiment, the second hot working process may be a hot rolling process, which may be performed in a finish mill. The hot rolling process may be performed such that, at least 50% of deformation may be induced below a second predetermined temperature, to reduce thickness of the steel slab to about 2mm to 8mm, to form a steel strip. In an embodiment, the second predetermined temperature is the austenite no-recrystallisation temperature. During deformation in the second hot working process below the non-recrystallization temperature, substantial deformation of austenite may occur, resulting in formation of austenite pancaking, which aids in increasing strength of the steel.
The heated steel slab may be initially passed through the roughing mill, which may consist of one or two roughing stands in which the steel slab may be hot rolled back and forth few times repeatedly between at least two rollers, to deform the steel slab i.e., to increase length of the steel slab by reducing thickness and without altering width of the steel slab. In an embodiment roughing mill exit temperature is maintained from about 1010 °C to 1080 °C. Upon deforming the steel slab in the roughing mill, the steel slab may be transferred into the finish mill, where the steel slab is continuously rolled in each of the seven stands, to further thickness reduction, preform surface finishing and dynamic recrystallization. The finish rolling exit temperature ranging from about 820 °C to 900 °C. In an embodiment, in the finishing mill at least last three passes may be performed below the second predetermined temperature.
Upon subjecting the steel strip to the second hot working process, the steel strip may be subjected to cooling by a second cooling process to a third predetermined temperature at a predetermined cooling rate [as seen in block 107]. In an embodiment, cooling may be performed on a run-out table and the predetermined cooling rate may be about 10 °C/s to 60 °C/s. Cooling the steel strip at high cooling rates restrict grain growth and facilitates formation of bainite phase along with polygonal ferrite and, also controls bainite volume fraction.

After cooling the steel strip, at block 108 the steel strip may be subjected to coiling at a third predetermined temperature, to form a microalloyed steel strip. In an embodiment, the third predetermined temperature may be about 480 °C to 540 °C. Coiling is the process of winding the steel in its hot state while it is still hot, immediately after it has been formed.

In an embodiment, the steel processed by the method of the present disclosure, results in microstructural changes to form the microalloyed steel strip. The microalloyed steel strip includes bainite of about 30% to 80% and remaining being polygonal ferrite. The bainite and ferrite microstructure results in excellent combination of strength and toughness.

In an embodiment, microstructure of the microalloyed steel strip possess grain size of about 3-4 microns. This contributes to increase in strength of the microalloyed steel strip.

In an embodiment, the microalloyed steel strip includes low carbon composition, suitably alloyed with Mn and other microalloying elements.

In an embodiment, the microalloyed steel strip exhibits, tensile strength of about 600 MPa to 750 MPa, total elongation of about 25% to 33% and yield strength of about 500MPa to 650MPa.
In an embodiment, the microalloyed steel strip exhibits a yield ratio (yield strength/ultimate tensile strength) below 0.9. This improves formability property of the microalloyed steel strip.

In an embodiment, the microalloyed steel strip can be used to manufacture automotive components such as but not limiting to long member, weight bearing components for heavy automobiles.

Figures. 2a and 2b are exemplary embodiments of the disclosure, which illustrates micrograph obtained by scanning electron microscope (SEM) of steels having compositions A and B respectively as shown in Table 1 below, and Figure. 2c and formed using method of the present disclosure. As seen in Figure. 2a, the micrograph confirms a polygonal ferrite + bainite type microstructure and in Figure. 2b, the micrograph confirms higher volume fraction of bainite. Further, in Figure. 2c, the micrograph obtained by Transmission electron microscopy [TEM] of Steel having composition B. The micrograph depicts how precipitates restrict the movements of dislocation, thereby effectively promoting precipitation strengthening, in addition to transformation strengthening.

Hence, the final microstructure of the microalloyed steel strip formed by the method described above, exhibits high combination of yield strength, tensile strength, and total elongation. This makes the steel strip to exhibit high strength. These properties make the microalloyed steel strip suitable for, but not limiting to automobile applications for manufacturing long members and weight bearing components for heavy vehicles.

The method of present disclosure is simple with changes in deformation in the second working process, which make the production efficient.
The following portion of the present disclosure provides details about the proportion of each alloying element in a composition of the steel and their role in enhancing properties.

Carbon (C) may be added at about 0.04% to 0.08%. Carbon is one of the most effective and economical strengthening elements. Carbon aids in generating bainite phase on cooling and carbon content below 0.5% is preferable.

Manganese (Mn) may be added in the range of about 1 wt.% to 1.8 wt.%. Manganese promotes solid solution strengthening and stabilizes austenite also. However, excessive amount of Mn is not recommended as it can deteriorate weldability of the steel, and lead to banding in microstructure.

Sulphur (S) and Phosphorus (P) may be added at about 0.001- 0.006 wt% and at about 0.01-0.02 wt%, respectively. Phosphorus and sulphur are considered detrimental in steel. Phosphorous content of less than 0.02wt% and sulphur less than 50 ppm are preferable.
Titanium (Ti), Niobium (Nb) may be added in the range of 0.01 wt.% to 0.02 wt.% and 0.07 wt.% to 0.12 wt.%, respectively and vanadium (V) may be added at about 0.005% to 0.06%. These elements aid in a) maintaining strength at room temperature, (b) restrict transverse cracking, (c) raise Tnr temperature, etc. Vanadium aids in improving strength due to carbonitride formation. The addition of these elements also should be optimum, as excessive addition will unnecessarily increase the strength and cost of the material.
Chromium (Cr)) may be added at about 0.1 wt.% to 0.6 wt.%. of Cr in an optimum quantity can improve hardenability of the steel and facilitate bainite formation

Example:

Embodiments of the present disclosure will now be described with an example of a particular compositions 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 1.
Alloying Elements Steel A Steel B
C % 0.045 0.040
Mn % 1.20 1.23
P % 0.018 0.021
Si % 0.20 0.25
Al % 0.06 0.033
Ti % 0.02 0.015
Nb % 0.073 0.09
V % -- 0.045
Cr % 0.50 0.31
Ni % 0.19 0.193
Mo % 0.15 0.15
Cu % 0.10 0.103
N (ppm) 70 67

Table 1

The processed steel may be subjected to testing to determine mechanical properties of the microalloyed steel strip. As an example, tensile testing may be performed using Instron machine as per ASTM standard. Accordingly, table 2 illustrates mechanical properties of steel having compositions A and B.

Steel YS
MPa UTS
MPa %
Elongation
A 589 684 28
B 576 676 29.5

Table-2

As seen in table 2, the steel demonstrates a high stability in the mechanical properties, when processed by the method of the present disclosure. The steel exhibits tensile strength of greater than 600 MPa and total elongation greater than 28%. Therefore, microalloyed steel strip will have a great potential for automobile applications.

It should be understood that the experiments are carried out for particular compositions of the steel and the results brought out in Table 2 are for the compositions shown in Table 1. 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.

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.

Referral Numerals:

Referral Numerals Description
101-108 Flowchart blocks
101 Making stage
102 Casting stage
103 First cooling stage
104 Heating stage
105 First hot working stage
106 Second hot working stage
107 Second cooling stage
108 Coiling stage

Claims:We Claim:

1. A method for manufacturing a microalloyed steel strip, the method comprising:
casting a steel slab of a composition comprising in weight percentage (wt.%) of:
carbon (C) at about 0.04% to 0.08%,
manganese (Mn) at about 1% to about 1.8%,
sulphur (S) at about 0.001% to 0.006%,
phosphorous (P) at about 0.01% to 0.02%,
silicon (Si) at about 0.1% to 0.25%,
aluminum (Al) at about 0.02% to 0.06%,
titanium (Ti) at about 0.01% to 0.02%,
niobium (Nb) at about 0.07% to 0.12%,
vanadium (V) at about 0.005% to 0.06%,
chromium (Cr) at about 0.1% to 0.6%,
nickel (Ni) at about 0.1% to 0.3%,
molybdenum (Mo) at about 0.1% to 0.2%,
copper (Cu) about 0.001% to 0.2%,
nitrogen (N) at about 30 ppm to 80 ppm, and
balance being Iron (Fe) optionally along with incidental elements;
cooling, the steel slab to an ambient temperature;
heating, the steel slab to a first predetermined temperature for a first predetermined time;
subjecting, the steel slab to a first hot working process, to reduce thickness of the steel slab to about 30-60 mm;
subjecting the steel slab to a second hot working process, to induce at least 50% deformation below a second predetermined temperature, and reduce thickness of the steel slab to around 2-8 mm to form a steel strip, wherein at least last three passes in the second hot working process is performed below the second predetermined temperature;
cooling the steel strip by a second cooling process at a predetermined cooling rate; and
coiling the steel strip at a third predetermined temperature, to form the microalloyed steel strip;
wherein, the microalloyed steel strip comprises polygonal ferrite and bainite microstructure.
2. The method as claimed in claim 1, wherein the microalloyed steel strip exhibits ultimate tensile strength of about 600 MPa to 750 MPa.
3. The method as claimed in claim 1, wherein the microalloyed steel strip exhibits total elongation of about 25% to 33%.
4. The method as claimed in claim 1, wherein the microstructure of the microalloyed steel strip represented by, in mass%, the bainite of about 30% to 80% and remaining being polygonal ferrite.
5. The method as claimed in claim 1, wherein the first predetermined temperature ranges from about 1150 °C to 1280 °C and the first predetermined time is about 1 to 2 hours.
6. The method as claimed in claim 1, wherein the first hot working process is a roughing process, which is performed in a roughing mill.
7. The method as claimed in claim 1, wherein the second hot working process is hot rolling process, which is performed in a finishing mill.
8. The method as claimed in claim 6, wherein in the roughing mill, roughing mill exit temperature ranges from about 1010 °C to 1080 °C.
9. The method as claimed in claim 1, wherein the second predetermined temperature is the austenite no-recrystallisation temperature.
10. The method as claimed in claims 7, wherein the finish rolling exit temperature ranging from about 820 °C to 900 °C.
11. The method as claimed in claim 1, wherein the third predetermined temperature ranges from about 480 °C to 540 °C.
12. The method as claimed in claim 1, wherein the first cooling process is air cooling.
13. The method as claimed in claim 1, wherein the second cooling process is performed on a run-out table.
14. The method as claimed in claim 1, wherein the predetermined cooling rate during the second cooling process is about 10°C/s to 60°C/s.
15. The method as claimed in claim 1, wherein the second cooling process restricts grain growth, facilitating forming of bainite phase along with polygonal ferrite.
16. A microalloyed steel strip, comprising:
composition in weight percentage [wt%] of:
carbon (C) at about 0.04% to 0.08%,
manganese (Mn) at about 1% to about 1.8%,
sulphur (S) at about 0.001% to 0.006%,
phosphorous (P) at about 0.01% to 0.02%,
silicon (Si) at about 0.1% to 0.25%,
aluminum (Al) at about 0.02% to 0.06%,
titanium (Ti) at about 0.01% to 0.02%,
niobium (Nb) at about 0.07% to 0.12%,
vanadium (V) at about 0.005% to 0.06%,
chromium (Cr) at about 0.1% to 0.6%,
nickel (Ni) at about 0.1% to 0.3%,
molybdenum (Mo) at about 0.1% to 0.2%,
copper (Cu) at about 0.001% to 0.2%,
nitrogen (N) at about 30 ppm to 80 ppm, and
balance being Iron (Fe) optionally along with incidental elements;
wherein, the microalloyed steel strip comprises polygonal ferrite and bainite microstructure.
17. The microalloyed steel strip as claimed in claim 16, wherein the microstructure of the microalloyed steel strip represented by, in mass%, the bainite of about 30% to 80% and remaining being polygonal ferrite.
18. The microalloyed steel strip as claimed in claim 16, wherein the microalloyed steel strip exhibits ultimate tensile strength of about 600 MPa to 750 MPa.
19. The microalloyed steel strip as claimed in claim 16, wherein the microalloyed steel strip exhibits total elongation of about 25% to 33%.
20. The microalloyed steel strip as claimed in claim 16, wherein the microalloyed steel strip exhibits yield strength 500-650 MPa.
21. An automotive component made of a microalloyed steel strip as claimed in claim 16.