Claims:We Claim:
1. A method for manufacturing a coil break mark free formable low-carbon hot rolled steel, the method comprising:
casting steel slab of a composition comprising in weight percentage (wt%) of:
carbon (C) 0.01% to 0.09%,
manganese (Mn) 0.1% to 0.5%,
sulphur (S) 0 to 0.02%,
phosphorus (P) 0 to 0.02%,
silicon (Si) 0 to 0.3%,
aluminium (Al) 0.02% to 0.06%,
copper (Cu) 0 to 0.01 %,
chromium (Cr) 0 to 0.03%,
nickel (Ni) 0 to 0.03%,
molybdenum (Mo) 0 to 0.02%,
vanadium (V) 0 to 0.02%,
niobium (Nb) 0 to 0.025%,
tin (Ti) 0.005% to 0.025%,
nitrogen (N) 0.001% to 0.006%,
boron (B) 0.0005% to 0.004%, and
the balance being Iron (Fe) optionally along with incidental elements;
reheating, the steel slab to a first temperature of about 1150 °C -1250 °C and maintaining the steel slab at the first temperature for 2-3.5 hours;
roughening, the steel slab for reducing thickness at a roughening temperature;
finish rolling, the steel slab with at a finish rolling temperature of 840°C to 900°C;
cooling, the finish rolled steel strip to a temperature of 650 °C to 700 °C; and
coiling, the steel slab at a coiling temperature to obtain the coil break mark free formable low-carbon hot rolled steel.
2. The method as claimed in claim 1, comprises slow cooling the coiled steel in an open atmosphere after coiling the steel.
3. The method as claimed in claim 1, wherein the roughening temperature is an exit temperature of the steel slab after roughening, which ranges from about 1040 °C to 1100 °C.
4. The method as claimed in claim 1, wherein the finish rolling temperature ranges from about 850 °C to 950 °C.
5. The method as claimed in claim 1, wherein the coiling temperature ranges from 650 °C to 700 °C.
6. The method as claimed in claim 1, wherein cooling of the steel slab is carried out on a run out table.
7. The method as claimed in claim 1, wherein cooling the coiled steel avoid solute carbon (C) and nitrogen (N) interstitial elements.
8. A coil break mark free formable low-carbon hot rolled steel, comprising:
composition in weight percentage of:
carbon (C) 0.01% to 0.09%,
manganese (Mn) 0.1% to 0.5%,
sulphur (S) 0 to 0.02%,
phosphorus (P) 0 to 0.02%,
silicon (Si) 0 to 0.3%,
aluminium (Al) 0.02% to 0.06%,
copper (Cu) 0 to 0.01 %,
chromium (Cr) 0 to 0.03%,
nickel (Ni) 0 to 0.03%,
molybdenum (Mo) 0 to 0.02%,
vanadium (V) 0 to 0.02%,
niobium (Nb) 0 to 0.025%,
tin (Ti) 0.005% to 0.025%,
nitrogen (N) 0.001% to 0.006%,
boron (B) 0.0005% to 0.004%, and
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the coil break mark free formable low-carbon hot rolled steel comprises bimodal microstructure.

Dated this 24th March 2022

GOPINATH A S
IN/PA 1852
OF K&S PARTNERS
AGENT FOR THE APPLICANT
, Description:FORM 2
THE PATENTS ACT, 1970
[39 of 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See Section 10 and Rule 13]

TITLE: “A METHOD FOR MANUFACTURING A COIL BREAK MARK FREE FORMABLE LOW-CARBON HOT ROLLED STEEL”

Name and Address of the Applicant:
TATA STEEL LIMITED, Jamshedpur, Jharkhand, India 831001.

Nationality: INDIAN

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

Present disclosure in general relates to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a coil break mark free formable low-carbon hot rolled steel. Further embodiments of the disclosure discloses a method for manufacturing the coil break mark free formable low-carbon hot rolled steel.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron (Fe), carbon (C), and other alloying elements such as Phosphorous (P), Sulphur (S), Manganese (Mn), Silicon (Si), Chromium (Cr), Magnesium (Mg) etc. Because of its high tensile strength and low cost, steel may be considered as a major component in wide variety of applications. Some of the applications of the steel may include buildings, ship building, tools, automobiles, machines, bridges, and numerous other applications. Steel obtained from steel making process may not possess all the desired properties. Therefore, steel may be subjected to secondary processes such as heat treatment for controlling material properties to meet various needs in the intended applications.

Generally, heat treatment may be carried out using techniques including but not limiting to annealing, normalising, hot rolling, quenching, and the like. During heat treatment process, the material undergoes a sequence of heating and cooling operations, thus the microstructure of the steel may be modified during such operations. As a result of heat treatment, the steel undergoes phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability and the like. The purpose of heat treatment is to increase service life of a product by improving its strength, hardness etc., or prepare the material for improved manufacturability.

In recent past, low carbon hot rolled grade steel sheets have been used in automotive applications, over conventional cold rolled grade steels. These steel sheets are adapted for light drawability, stretching and bending applications. During the manufacturing processes, the product quality is not maintained for all batches of hot rolled coils due to the aging of the steel. The propensity of yield point elongation and aging index are found different in different batches of hot rolled steel production. These steels possess maximum yield strength of 300 MPa and minimum 25% of total elongation, have carbon content of 0.02 to 0.09 wt%. In some applications like wheel disk and precision tubes, portions that included the coil break marks of the hot rolled steels were salvaged before use.

Further, due to various intermediate secondary processing of the steel for multiple coiling and uncoiling operations, wrinkle marks on the steel surface gets exaggerated, making the steel not suitable for use. This is a perennial problem in low carbon steel and required high amount of segregation to fit in the input quality requirement. One of the conventional techniques to reduce coil break mark, includes light skin pass rolling with high tension. Such a method is complex and involves high cost, which is undesired.

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 the 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 coil break mark free formable low-carbon hot rolled steel. The method initially starts from casting steel comprising composition in weight percentage (wt%) of carbon (C) 0.01% to 0.09%, manganese (Mn) 0.1% to 0.5%, sulphur (S) 0 to 0.02%, phosphorus (P) 0 to 0.02%, silicon (Si) 0 to 0.3%, aluminium (Al) 0.02% to 0.06%, copper (Cu) 0 to 0.01 %, chromium (Cr) 0 to 0.03%, nickel (Ni) 0 to 0.03%, molybdenum (Mo) 0 to 0.02%, vanadium (V) 0 to 0.02%, niobium (Nb) 0 to 0.025%, tin (Ti) 0.005% to 0.025%, nitrogen (N) 0.001% to 0.006%, boron (B) 0.0005% to 0.004% and balance being Iron (Fe) optionally along with incidental elements. The casted steel slab is then reheated to a first temperature of about 1150 °C -1250 °C and maintaining the steel slab at the first temperature for 2-3.5 hours. The method further includes roughening, the steel slab for reducing thickness at a roughening temperature. Upon roughening the steel slab, the steel slab is subjected to finish rolling with a finish rolling temperature of 840 to 900°C. The method furthermore includes cooling the finished rolled steel strip to a temperature of 650 to 700°C and coiling the steel strip at a coiling temperature, to obtain break mark free formable low-carbon hot rolled steel.
In an embodiment, the method comprises slow cooling the coiled steel in an open atmosphere.
In an embodiment, the roughening temperature is an exit temperature of the steel slab after roughening, which ranges from about 1040 °C to 1100 °C.
In an embodiment, the finish rolling temperature ranges from about 850 °C to 950 °C.
In an embodiment, the coiling temperature ranges from 650 °C to 700 °C.
In an embodiment, the cooling of the steel slab is carried out on a run out table. Cooling of the coiled steel avoid solute carbon (C) and Nitrogen (N) interstitial elements.
In another exemplary embodiment of the present disclosure, a manufacturing a coil break mark free formable low-carbon hot rolled steel. The coil break mark free formable low-carbon hot rolled steel comprises a composition in weight percentage (wt%) of carbon (C) 0.01% to 0.09%, manganese (Mn) 0.1% to 0.5%, sulphur (S) 0 to 0.02%, phosphorus (P) 0 to 0.02%, silicon (Si) 0 to 0.3%, aluminium (Al) 0.02% to 0.06%, copper (Cu) 0 to 0.01 %, chromium (Cr) 0 to 0.03%, nickel (Ni) 0 to 0.03%, molybdenum (Mo) 0 to 0.02%, vanadium (V) 0 to 0.02%, niobium (Nb) 0 to 0.025%, tin (Ti) 0.005% to 0.025%, nitrogen (N) 0.001% to 0.006%, boron (B) 0.0005% to 0.004% and balance being Iron (Fe) optionally along with incidental elements. The coil break mark free formable low-carbon hot rolled steel comprises bimodal microstructure.
It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:
Figure. 1 is a flowchart illustrating a method for producing a coil break mark free formable low-carbon hot rolled steel, according to an exemplary embodiment of the present disclosure.
Figure. 2 is a graphical representation of stress versus elongation, obtained during tensile test of the coil break mark free formable low-carbon hot rolled steel, according to an exemplary embodiment of the present disclosure.

Figure. 3 illustrates microstructure images of the coil break mark free formable low-carbon hot rolled steel, having mixed grain bimodal microstructure of the strip with no coil break marks, according to an exemplary 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 for 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 embodiment thereof has 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 alternatives falling within the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusions, such that a method that includes 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.

Henceforth, method for manufacturing the coil break mark free formable low-carbon hot rolled steel of the present disclosure is explained with the help of figures. 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 can envisage various such embodiments without deviating from scope of the present disclosure.

Figure. 1 is an exemplary embodiment of the present disclosure which illustrates a flowchart depicting a method for manufacturing a coil break mark free formable low-carbon hot rolled steel [hereinafter interchangeably used as steel]. The steel formed by the method for the present disclosure includes a bimodal type of microstructure, which has low response to the coil break marks during secondary processing such as, but not limiting to high angle bending, forming, drawing, pickling, trimming, blanking and the like. Further, this method is simple and economical unlike conventional methods which includes light skin pass rolling with high tension, which is complex and involving high cost. The method is now described with reference to the schematic representation and flowchart blocks. 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 comprising composition in weight percentage (wt%) carbon (C) 0.01% to 0.09%, manganese (Mn) 0.1% to 0.5%, sulphur (S) 0 to 0.02%, phosphorus (P) 0 to 0.02%, silicon (Si) 0 to 0.3%, aluminium (Al) 0.02% to 0.06%, copper (Cu) 0 to 0.01 %, chromium (Cr) 0 to 0.03%, nickel (Ni) 0 to 0.03%, molybdenum (Mo) 0 to 0.02%, vanadium (V) 0 to 0.02%, niobium (Nb) 0 to 0.025%, tin (Ti) 0.005% to 0.025%, nitrogen (N) 0.001% to 0.006%, boron (B) 0.0005% to 0.004% and balance being Iron (Fe) optionally along with incidental elements, may be casted. As an example, the steel of the above composition may be casted into a steel slab, in a continuous caster or a thin slab caster which is known in the art.

At block 102, , the method includes re-heating the steel slab to a first temperature of about 1150 °C -1250 °C and maintaining the steel slab at the first temperature for about 2-3.5 hours. As an example, casted steel slab may be heated in a furnace.

Once, the steel is heated and soaked as per block 102, the steel may be subjected to roughening process for reducing thickness at a roughening temperature [as per block 103]. During roughening the steel is rolled to reducing thickness to a great extent. In an embodiment, the roughening temperature may be same as that of an exit temperature of the steel slab after completion of the roughening process, which may range from about 1040 °C to 1100 °C. Upon roughening the steel slab, the steel slab may be subjected to finish rolling at a finish rolling temperature [as per block 104]. In an embodiment, the finish rolling temperature ranges from about 850 °C to 950 °C. The finish rolling process results in formation of mixed grain size due to chemical inhomogeneity.

At block 105, the method includes cooling the finished rolled steel strip to a temperature ranging from about 650 °C to 700°C. In an embodiment, the steel slab may be cooled on a run out table. Further, the cooled steel slab may be coiled at a coiling temperature [as per block 106]. In an embodiment, the coiling temperature may range from about 650 °C to 700 °C. This coiling temperature may facilitate slow cooling of the steel in a coil form and helps in avoiding solute Carbon (C) and Nitrogen (N) interstitial elements present. In other words, this coiling temperature mitigates fast cooling of the coiled steel, as a result of which chances of solute interstitial Carbon (C) and Nitrogen (N) interstitial elements may be minimum. This may result in forming more precipitate (carbides) and pinning of dislocation will also be at a minimum. Hence, the yield point phenomenon (creation of coil break marks) is minimum or zero during high angle bending and unbending of the coiled steel. Further, the coiling temperature along with the above mentioned composition of the steel, aids in developing bi-modal microstructure. In other words, addition of Ti ( micro alloy) will partially stabilize some Carbon (C)and Nitrogen (N), and few precipitates formed during reheating and roughening temperature may restrict some nearby grains to grow and some grains will grow normally after recrystallisation. Boron (B) will minimize the grain boundary energy and help the normal grains to grow farther, thereby forming bimodal microstructure, which has low response to the coil break marks at high angle bending.

Upon coiling, the coiled steel may be slow cooled in an open atmosphere to ambient temperature [as per block 107].

Example:

Further embodiments of the present disclosure will be now described with an example of particular composition of the coil break mark free formable low-carbon hot rolled steel, which is illustrated in Table 1. Experiments have been carried out on the coil break mark free formable low-carbon hot rolled steel formed by using the method for the present disclosure.
C Mn S P Si Al Cu Cr Ni Mo V Nb Ti N B
Min 0.02 0.10 0 0 0 0.03 0 0 0 0 0 0 0.01 0.001 0.001
Max 0.08 0.40 0.02 0.02 0.3 0.05 0.008 0.03 0.03 0.005 0.005 0.005 0.02 0.006 0.003

Table 1
In an embodiment of the present disclosure, various experiments were carried out on the coil break mark free formable low-carbon hot rolled steel with the composition as mentioned in Table-1. For conducting the experiment, coil break mark free formable low-carbon hot rolled steel samples were prepared for conducting microstructural investigation and conducting tensile test. As an example, tensile testing may be performed using Instron machine as per ASTM standard and XRD, SEM tests were conducted to investigate microstructure of coil break mark free formable low-carbon hot rolled steel.

Accordingly, Figure. 2 illustrates graphical representation of stress vs elongation obtained from the tensile test of coil break mark free formable low-carbon hot rolled steel. From Figure. 2 it is evident that, the difference between an upper yield point and a lower yield point is below 20 MPa. This contributes to minimizing the coil break mark when the steel is subjected to high angle bending.

Referring to Figure. 3 that depicts microstructure images of the coil break mark free formable low-carbon hot rolled steel produced by the method for the present disclosure. From Figure. 3 it is clear that, the coil break mark free formable low-carbon hot rolled steel exhibits heterogeneous bimodal microstructure. The bimodal microstructure is developed due to Tin (Ti)-Boron (B) chemistry in the composition. The bimodal microstructure is low severity to the coil break mark, since the bimodal microstructure increases the residence time, resulting in a decarburized soft surface, which prevents formation of the coil break mark.
In an embodiment of the present disclosure, the coil break mark free formable low-carbon hot rolled steel of the present disclosure may be used in any application including but not limiting to automotive applications.

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.

Referral Numerals

Referral Numerals Description
101-106 Flowchart blocks
101 Casting stage
102 Re-heating stage
103 Roughening stage
104 Finish rolling stage
105 Cooling stage
106 Coiling stage
107 Cooling stage to ambient temperature