Description:FORM 2
THE PATENTS ACT, 1970
[39 of 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See Section 10 and Rule 13]

TITLE: “A STEEL GRATE BAR”

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 relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a steel grate bars used in pellet and sinter making.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon, and other alloying element. Because of its high tensile strength and low cost, steel may be considered as a most viable choice for major components manufacturing in a wide variety of applications. Some of the applications of the steel may include buildings, ships, tools, automobiles, machines, bridges, steel grate bars and numerous other applications.

Pelletization is an agglomeration process. In pelletization fine-grained iron ore are transformed into small balls known as pellets. These pellets are charged in blast furnace for direct reduction processes and iron making.

The Pellet plants moving beds are made up of steel grate bars.
The steel grate bar material must be highly wear resistant, because steel grate-bars handle iron ore fine particles that are subjected to the pelletising process and operate under the conditions of the cyclic effect of high-temperature gas flow. The temperature at the grate bar reaches upto 1100-1200°C. Such combination of process condition makes them susceptible to corrosion and premature failure. That is why such steel grate bars demand high hardness and resistance to erosion and oxidation. To address resistance to erosion and oxidation, chromium content up to 28 wt% and nickel content up to 14% is added. For high hardness, Cr/C is maintained around 55-60. The chromium carbides which are present in the austenite matrix enhances hardness and wear properties. Usually some amount of molybdenum is also added to these alloys for pitting resistance. Such conventional steel grate bars have hardness of 184 BHN with life around 1 year.
The addition of chromium, nickel, and molybdenum considerably increase the cost of the steel grate bars. Therefore, it is widely being sought to reduce the cost of the steel grate bars with hardness and life substantially similar to the conventional ones.
The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the conventional arts.

SUMMARY OF THE DISCLOSURE

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

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

In one non limiting embodiment of the disclosure, a steel grate bar, comprising:
carbon (C) in range of 2-2.02 wt%,
manganese (Mn) in range of 0.66-0.68 wt%,
sulphur (S) in range of 0.05 to 0.15 wt%,
phosphorous (P) in range of 0.80 -0.81 wt%,
Silicon (Si) in range of 1.36-1.38 wt%,
Chromium (Cr) in range of 20.80-21.81 wt%,
Nickel (Ni) in range of 9.62-9.70 wt%,
Molybdenum (Mo) in range of 0.079-0.090 wt%,
Vanadium (V) in range of 0.060-0.063 wt%,
Copper (Cu) in range of 0.093-0.095 wt%,
Niobium (Nb) in range of 0.005-0.019 wt%,
balance being Iron (Fe) optionally along with incidental elements.
The provision of selected Chromium Carbide ratio increases the volume fraction and intrinsic hardness of the chromium carbide particles. This contributes to the hardness of the alloy.

In an embodiment, the microstructure of the steel grate bar is ferritic matrix with M23C6 carbides and iron phosphide embedded in ferritic matrix, where M being Chromium.

In an embodiment, hardness of steel grate bar is 217 – 225 BHN.

In an embodiment, Cr/C ratio is 10.297-10.9.

In an embodiment, Cr/C ratio increases volume fraction and intrinsic hardness of the chromium carbide particles.

In an embodiment, molten iron for the steel grate bar is produced in induction melting furnace or blast furnace or air-melting furnace or vacuum furnace.

In an embodiment, the steel grate bar is casted.

In an embodiment, Ferrite being 90 to 95% (by vol) and Chromium Carbide and Iron phosphide being 5-10% (by vol.).

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:

Figs 1shows an image of a steel grate bars deployed over moving pellet car.
Figs 2(a)-2(b) shows an optical microstructures of the steel grate bar samples according to an exemplary embodiment of the present disclosure.
Fig 3 shows an optical microstructure of conventional steel grate bar sample.
Fig 4 shows an image of the steel grate bar after one year of service.

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 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 form 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 inclusion, 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.

The present disclosure is explained with the help of images of a steel grate bars. However, such exemplary embodiments should not be construed as limitations of the present disclosure since the method may be used on other products where such need arises. A person skilled in the art may envisage various such embodiments without deviating from scope of the present disclosure.

Fig 1 shows an image of plurality of the steel grate bars deployed over a moving pellet car.

The steel grate bar produced by in accordance with present disclosure, includes a microstructure of mainly ferritic matrix with M23C6 carbides and iron phosphide embedded within the ferrite matrix. Here “M” may be referred to Chromium.
The Ferrite is 90 to 95% (by vol) and Chromium Carbide and Iron phosphide together being 5-10%.

The steel grate bar according to the present disclosure is casted from the molten steel. Molten iron for the molten steel may be produced from induction melting furnace, blast furnace, air-melting furnace, or vacuum furnace. The molten iron is added with necessary alloys to make the molten steel with desired composition (as stated in Table 1). Later, the molten steel can be casted in a form of the steel grate bars.

The steel grate bar post casting is further cooled to room temperature.

The molten steel may have composition of carbon (C) in range of 2.0-2.02 wt%, manganese (Mn) in range of 0.66-0.68 wt%, sulphur (S) in range of 0.05 to 0.15 wt%, Phosphorous (P) in range of 0.80 -0.81 wt%, Silicon (Si) in range of 1.36-1.38 wt%, Chromium (Cr) in range of 20.80-21.81 wt%, Nickel (Ni) in range of 9.62-9.70 wt%, Molybdenum (Mo) in range of 0.079-0.090 wt%, Vanadium (V) in range of 0.060-0.063 wt%, Copper (Cu) in range of 0.093-0.095 wt%, Niobium (Nb) in range of 0.005-0.019 wt%, balance being Iron (Fe) optionally along with incidental elements.

In the molten steel, Cr / C (10.297-10.9) ratio is decreased as opposed to Cr/C of conventional steel grate bar (55-60). Also, a deliberate addition of phosphorus is done (can be referred from Table 1). This may help in modifying the microstructure of the steel grate bar and increasing the hardness. The Cr/C (Chromium carbide) ratio is selected such that it increases the volume fraction and intrinsic hardness of the chromium carbide particles. This contributes to the hardness of the alloy. Due to rise in the hardness, the wear resistance and erosion resistance is expected to rise.
The reason for the improvement in wear/ erosion resistance properties and hardness is refined microstructure and formation of fine dispersion of complex phosphides along with carbides in the matrix (refer to Figs. 2a, 2b).

Furthermore, the material renders adequate high temperature oxidation resistance due to the presence of chromium as well as phosphorus in the material. Including some amount of nickel made the material amenable to thermal cycling with sufficient toughness.
The steel grate bar in accordance with an embodiment of disclosure is economical as the Chromium and Nickel content is reduced compared to the conventional (refer Table 1).

Example:

Further embodiments of the present disclosure will now be described with examples of composition of the steel. Experiments have been carried out on the steel grate bar by using method of the present disclosure.

Composition of three steel grate bar samples one conventional (Regular alloy) and other two in accordance with embodiment of the disclosure (New Alloy 1, New Alloy 2) were taken. The composition of all three samples is given in the following Table.

Table 1: Chemical composition of the new alloys and the regular material

For microstructural investigation, the samples were sectioned in proper size and prepared further by grinding paper followed by polishing. Finally, samples were etched using 2 vol% nital solution. An optical microscope was used to study the microstructure. Hardness of the samples are measured by Vickers Hardness and converted into Brinell Hardness number.

Microstructure
FIGS. 2(a) – 2(b) present the optical micrographs of the grate bar sample of composition in accordance with an embodiment of the disclosure.
FIG. 3 present the optical micrographs of the conventional steel grate bar sample. The optical micrographs of conventional steel grate bar sample show microstructure of M23C6 carbides in the ferrite matrix, M being chromium.
The microstructure of the steel grate bar samples is M23C6 carbides and/or iron phosphides, both being embedded within the ferrite matrix (FIGs 2a-2b). (M refers to chromium). Due to similar contrast, M23C6 carbides and iron phosphides cannot be segregated and shown together in black (FIGs. 2a & 2b). FIG. 2a shows the microstructure with M23C6 carbides and iron phosphide, both being black in color and ferrite matrix being grey in colour. Similarly, in FIG. 2b the microstructure with M23C6 carbides and iron phosphide embedded in ferrite matrix is shown.
On the contrary, FIG. 3 shows the microstructure with M23C6 carbide (black in color) and ferrite (white in color) and martensite (grey in color) for the conventional steel grate bars.
The volume of carbide and /or phosphide shown in FIGs. 2a and 2b seems to be high as compared to the one in FIG. 3. This may be the reason for more hardness as compared to the one conventional steel grate bars.

Some iron phosphides also present at grain boundary of the steel grate bars because phosphorous segregates into areas, which solidify late in the freezing process which may add upto the hardness of the steel grate bar. Furthermore, grain refinement is observed in the microstructure as phosphorous content increased. The iron phosphide particles are present as small particles in the microstructure along with the marked M23C6 carbides.

Also, the Phosphorous addition and the change in the Cr/C ratio (conventional being 59.125 and claimed (10.297-10.9)) altered the microstructure characteristics of the steel grate bars and affected its properties. The Cr/C ratio is so chosen that appropriate Chromium is present with Carbon for bonding. If there are insufficient Carbon atoms for bonding with Chromium, lesser bonds will be formed. Similarly, if there are lesser Chromium for bonding with Carbon, again the number of bonds may be less. So appropriate Chromium atoms may be presented for Carbon in accordance with disclosure.

Table 2 below shows the effect of the change in the chemistry on hardness of the steel grate bar samples. It seems the hardness values are marginally raised by increasing the phosphorous content. This increase may be related to the reinforcement of the matrix with the hard-complex phosphide phase, which are harder than chromium carbide. The refinement obtained by phosphorous additions as well as increase in the volume fraction of chromium carbide precipitates may have also contributed to increase in the hardness.

Fig. 1 shows the pellet car with plurality of the steel grate bars (fresh). Fig. 4 shows the steel single grate bar after one year of service. Generally, as per the SOP the grate bars are pulled out from service after one year of service. One may witness the steel grate bar had developed rust on its surface but didn’t worn out and is devoid of cracks. The appropriate service life is provided by the claimed steel grate bar.

Advantage

From the disclosure following can be deduced for the steel grate bars in accordance with an embodiment:
1) the steel grate bar shows higher hardness and good wear/ erosion properties as compared to conventional steel grate bars even on decreasing chromium and nickel content.
2) on increasing the phosphorous and carbon level simultaneously, the hardness and wear/ erosion properties improved even at the reduced chromium and nickel content.
3) Phosphorous addition caused an increase in hardness due to reinforcement of the matrix with hard complex phosphide phase.
4) Grain refinement also caused the improvement in hardness and wear/ erosion properties.
5) steel grate bar has higher hardness value showed better wear/erosion resistance properties as compared to the conventional grate bar sample having lower hardness value.
6) reduction in the amounts of the costly alloying elements such as chromium, nickel and molybdenum led to a significant drop in the cost of the steel grate bars.
7) the service life provided by the steel grate bar is in comparison with the conventional.

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.
, Claims:We Claim:

1. A steel grate bar, comprising:
carbon (C) in range of 2-2.02 wt%,
manganese (Mn) in range of 0.66-0.68 wt%,
sulphur (S) in range of 0.05 to 0.15 wt%,
phosphorous (P) in range of 0.80 -0.81 wt%,
Silicon (Si) in range of 1.36-1.38 wt%,
Chromium (Cr) in range of 20.80-21.81 wt%,
Nickel (Ni) in range of 9.62-9.70 wt%,
Molybdenum (Mo) in range of 0.079-0.090 wt%,
Vanadium (V) in range of 0.060-0.063 wt%,
Copper (Cu) in range of 0.093-0.095 wt%,
Niobium (Nb) in range of 0.005-0.019 wt%, and
balance being Iron (Fe) optionally along with incidental elements.

2. The steel grate bar as claimed in claim 1 wherein, the microstructure of the steel grate bar is ferritic matrix with M23C6 carbides and iron phosphide embedded in ferritic matrix, where M being Chromium

3. The steel grate bar as claimed in claim 1 wherein, hardness of steel grate bar is 217 – 225 BHN.

4. The steel grate bar as claimed in claim 1 wherein, Cr/C ratio is 10.297-10.9.

5. The steel grate bar as claimed in claim 4 wherein, Cr/C ratio increases volume fraction and intrinsic hardness of the chromium carbide particles.

6. The steel grate bar as claimed in claim 1 wherein, molten iron for the steel grate bar is produced in induction melting furnace or blast furnace or air-melting furnace or vacuum furnace.

7. The steel grate bar as claimed in claim 6 wherein, the steel grate bar is casted.

8. The steel grate bar as claimed in claim 1 wherein, the Ferrite being 90 to 95% (by vol) and Chromium Carbide and Iron phosphide being 5-10% (by vol.).

Dated this 15th day of July 2022


Gopinath Arenur Shankararaj
IN/PA-1852
of K&S Partners
Agent for the Applicant