Description:FIELD OF DISCLOSURE
[0001] The present disclosure relates to a process for production of high strength steel plate, say for high temperature forming applications.

BACKGROUND OF THE DISCLOSURE
[0002] The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Lightweight design of vehicles using higher strength steels has been the focus in recent days. This is owing to stricter air pollution control requirements in addition to the energy crisis. The high-strength steel enhances performance of vehicles and its parts due to enhanced strength to weight ratio. The drive axle housing is an important part of the heavy truck drive axle, which supports and protects the main reducer and fixed drive wheels’ relative position, and supports the frame and the total mass above it together with the front axle.
[0004] However, many problems such as cracking and rebounding may occur during manufacturing the axle housing at room temperature using high-strength steel plate. To avoid this, hot forming/stamping of the part is carried out at high temperature. However, as heating and cooling involve phase transformation, it may lead to change in the originally envisaged mechanical properties.
[0005] There is therefore a need in the art for a process involving steel making and hot rolling to retain originally envisaged mechanical properties even after forming at high temperature.
[0006] There is therefore a need in the art for a system that helps overcome the issues and disadvantages of the prior art mechanisms.
[0007] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0008] In some embodiments, the numbers expressing quantities or dimensions of items, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0009] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[00010] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[00011] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.

OBJECTS OF THE INVENTION
[00012] Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as listed herein below.
[00013] It is an object of the present disclosure to provide for a process involving steel making and hot rolling to retain originally envisaged mechanical properties even after forming at high temperature.

SUMMARY
[00014] In an aspect, the present disclosure relates to a high strength steel composition (in wt.%) comprising: Carbon (C) in the range of 0.17 – 0.23, Magnesium (Mn) in the range of 0.90 – 1.60, Silicon (Si) in the range of 0.25-0.45, Sulphur (S) in the range of 0.002 – 0.01, Aluminum (Al) in the range of 0.01 – 0.40, Phosphorus (P) in the range of 0.004-0.0.05, Titanium (Ti) in the range of 0.005-0.04, Vanadium (V) in the range of 0.005-0.05; and Niobium (Nb) in the range of 0.01-0.06.
[00015] In another aspect, the present disclosure relates to a method for production of high strength steel for forming at high temperature, wherein said method uses the above composition, and wherein the method can include the steps of: providing a slab having the above composition; heating the slab in a reheating furnace at a temperature between 1220-12300C; retaining discharging temperature at a range between 1200-12100C while maintaining rolling start temperature between a range of 1165-11750C to obtain hot steel plate; holding, for a time duration between 1 to 5 minutes, the steel plate after rough pass to bring down the temperature below highest temperature at which recrystallization can take place (Tnr); and maintaining finish rolling temperature between 800-8600C.
[00016] In an aspect, the hot steel plate can be passed along to cool in an air cooling bed. In another aspect, the step of holding can be done for 120 seconds. In another aspect, when the finish rolling temperature is between 800-8100C, the step can be followed by accelerated cooling of 3-40C/s.
[00017] Various objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.
[00018] Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

DETAILED DESCRIPTION
[00019] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered 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.
[00020] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.
[00021] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[00022] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[00023] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. These exemplary embodiments are provided only for illustrative purposes and so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. The invention disclosed may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure). Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[00024] Thus, for example, it will be appreciated by those of ordinary skill in the art that the diagrams, schematics, illustrations, and the like represent conceptual views or processes illustrating systems and methods embodying this invention. The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing associated software. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the entity implementing this invention. Those of ordinary skill in the art further understand that the exemplary hardware, software, processes, methods, and/or operating systems described herein are for illustrative purposes and, thus, are not intended to be limited to any particular named element.
[00025] The ensuing description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It should be understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the disclosure as set forth in the appended claims.
[00026] Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits, systems, networks, processes, and other components may be shown as components in block diagram form in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.
[00027] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[00028] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[00029] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[00030] In an aspect, the present disclosure relates to a high strength steel composition (in wt.%) comprising: Carbon (C) in the range of 0.17 – 0.23, Magnesium (Mn) in the range of 0.90 – 1.60, Silicon (Si) in the range of 0.25-0.45, Sulphur (S) in the range of 0.002 – 0.01, Aluminum (Al) in the range of 0.01 – 0.40, Phosphorus (P) in the range of 0.004-0.0.05, Titanium (Ti) in the range of 0.005-0.04, Vanadium (V) in the range of 0.005-0.05; and Niobium (Nb) in the range of 0.01-0.06.
[00031] In another aspect, the present disclosure relates to a method for production of high strength steel for forming at high temperature, wherein said method uses the above composition, and wherein the method can include the steps of: providing a slab having the above composition; heating the slab in a reheating furnace at a temperature between 1220-12300C; retaining discharging temperature at a range between 1200-12100C while maintaining rolling start temperature between a range of 1165-11750C to obtain hot steel plate; holding, for a time duration between 1 to 5 minutes, the steel plate after rough pass to bring down the temperature below highest temperature at which recrystallization can take place (Tnr); and maintaining finish rolling temperature between 800-8600C.
[00032] In an aspect, the hot steel plate can be passed along to cool in an air cooling bed. In another aspect, the step of holding can be done for 120 seconds. In another aspect, when the finish rolling temperature is between 800-8100C, the step can be followed by accelerated cooling of 3-40C/s.
[00033] In an exemplary aspect, the present disclosure relates to a process for production of high strength steel plate, say for high temperature forming applications. In an aspect, as part of the process, heat making can be carried out through BOF (Basic Oxygen Furnace) -LHF (Ladle Heating Furnace)-RHOB-CCM (Continuous Casting Method) route. An exemplary chemical composition is provided in Table 1. RHOB-Ruhrstahl Heraeus oxygen blowing, R & H are German terms used for degassing of mainly hydrogen and nitrogen to some extent while OB is for reducing carbon content by oxygen blowing.

Table1: Chemical composition of experimental steels (in wt. %)
C Mn P S Si Al Ti V Nb
0.19 1.35 0.016 0.005 0.350 0.33 0.011 0.035 0.040

[00034] In an exemplary implementation of the present invention, the continuous cast slabs were hot rolled in a high power plate mill. Soaking temperature was kept at 1220-12300C and discharge temperature was kept at 1200-12100C while rolling start temperature was at 1165-11750C. Holding was done for 120 seconds after finish rolling to bring down the temperature below Tnr (the highest temperature at which recrystallization can take place). Finish rolling temperature can be kept at 820-8300C initially, followed by air cooling and later the finish rolling temperature was decreased to 800-8100C followed by accelerated cooling of 3-40C/s.
[00035] As part of assessment of efficiency of the invention, Charpy V-notch impact test at 00C and tensile testing were carried out of standard samples as per ASTM A370 specification to assess the materials properties, results of which are mentioned below in Table 2. Mechanical properties before forming and after forming are provided in Table 2.

Table 2: Mechanical Properties
YS, (MPa) UTS, (MPa) %El CIE, J
Actual (before forming) 470-500 640-680 23-25 50-70 at RT
Actual (after forming at 7500 C) 445 - 447 581 - 585 23-24 80,76, 74 at 0 °C

[00036] Based on the evaluated properties after forming, the material can be used for the forming application at 7500C, or below with minimum YS requirement of 440 MPa or less, UTS 580 MPa or below and similarly for elongation and CIE as provided in Table 2.
[00037] As used herein, and unless the context dictates otherwise, the term “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each otheror in contact with each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “coupled to” and “coupled with” are used synonymously. Within the context of this document terms “coupled to” and “coupled with” are also used euphemistically to mean “communicatively coupled with” over a network, where two or more devices are able to exchange data with each other over the network, possibly via one or more intermediary device.
[00038] Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C ….and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.
[00039] While some embodiments of the present disclosure have been illustrated and described, those are completely exemplary in nature. The disclosure is not limited to the embodiments as elaborated herein only and it would be apparent to those skilled in the art that numerous modifications besides those already described are possible without departing from the inventive concepts herein. All such modifications, changes, variations, substitutions, and equivalents are completely within the scope of the present disclosure. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims.

ADVANTAGES OF THE INVENTION
[00040] The present invention provides for a process involving steel making and hot rolling to retain originally envisaged mechanical properties even after forming at high temperature.

Claims:1. A high strength steel composition (in wt.%) comprising:
Carbon (C) in the range of 0.17 – 0.23
Magnesium (Mn) in the range of 0.90 – 1.60
Silicon (Si) in the range of 0.25-0.45
Sulphur (S) in the range of 0.002 – 0.01
Aluminum (Al) in the range of 0.01 – 0.40
Phosphorus (P) in the range of 0.004-0.0.05
Titanium (Ti) in the range of 0.005-0.04
Vanadium (V) in the range of 0.005-0.05; and
Niobium (Nb) in the range of 0.01-0.06.
2. A method for production of high strength steel for forming at high temperature, wherein said method uses the composition as claimed in claim 1, and wherein said method comprises the steps of:
providing a slab having composition as claimed in claim 1;
heating the slab in a reheating furnace at a temperature between 1220-12300C;
retaining discharging temperature at a range between 1200-12100C while maintaining rolling start temperature between a range of 1165-11750C;
holding, for a time duration between 1 to 5 minutes, after rough pass to bring down the temperature below highest temperature at which recrystallization can take place (Tnr); and
maintaining finish rolling temperature between 800-8600C.
3. The method as claimed in claim 2, wherein hot plate was passed along to cool in an air cooling bed.
4. The method as claimed in claim 2, wherein the step of holding is done for 120 seconds.
5. The method as claimed in claim 2, wherein when the finish rolling temperature is between 800-8100C,the step is followed by accelerated cooling of 3-40C/s.
6. The method as claimed in claim 2, wherein the forming is performed through BOF-LHF-RHOB route.
7. The method as claimed in claim 2, wherein the forming is performed through BOF (Basic Oxygen Furnace) -LHF (Ladle Heating Furnace) – RHOB-CCM (Continuous Casting Method) route.