CLIAMS:1. A method and/or process of preparation and/or production of 50C-350 grade Cold Rolled Non-Oriented (CRNO) steels with CRNO steel melt comprising C: 0.02-0.025%, Mn: 0.25%, P: 0.02% max., S: 0.001 % max, Si: 2.5-2.7% (Si+Al <2.85%), Al: 0.15-0.20%, N2: 50 ppm max, said process comprising the steps of:
- treating the steel melt with calcium-silicon (Ca-Si) for globular inclusion shape and to reduce inclusion level by using Ca-Si core wire during secondary steel making;
- Vacuum Oxygen Refining (VOR) to lower the excess amount of Carbon and Sulphur level in the steel melt by VOR tank and steam injector comprising of programmable logic controller (PLC);
- continuous casting of the steel melt forming concast slabs wherein superheating temperature is maintained at about 20oC max and the casting speed is maintained from 0.6-0.70 m/min;
- sending the concast slabs to Hot Strip Mill (HSM) in hot condition by the additional hood fabricated to prevent slab cracking due to high silicon content wherein said slabs are slowly cooled under the hoods;
- reheating and hot rolling of said slabs forming hot bands wherein the slab soaking temperature is around 1180-1200 °C, slab retention time is around 4 hrs, roughing temperature around 1120-980 °C, finish rolling temperature is 860-880 °C and coiling temperature is maintained at around 680-700 °C; and
- processing of said hot bands in silicon mill to side trim the bands in Bust line, followed by pickling, cold rolling and decarb-annealing to generate the carbon level less than 0.003% wherein the decarb-annealing comprises different annealing zones in decarb line wherein maximum decarb temperature is maintained at around 840 °C, annealing temperature in Zone#8 is varied from 950-970 °C, maximum annealing temperature of around 990 °C is maintained in Zone#9 to Zone#11, and minimum decarb line speed is maintained at around 15 to 16 m/min.
2. The method as claimed in claim 1, wherein said hot rolling of the slabs is done within 24 hours of casting.
3. The method as claimed in claim 1, wherein final hot rolled coils gauge is around 2.5 ±0.1 mm.
4. The method as claimed in claim 1, wherein the optimum reheating temperature is selected by considering the required temperature for higher silicon content of steel melt and mill limitations ensuring complete homogenization.
5. The method as claimed in claim 1 wherein maintaining of coiling temperature for self annealing of said hot bands is achieved by switching-off the laminar cooling and avoiding descaling.
6. The method as claimed in claim 1, wherein said hot bands are cold rolled into 0.5 mm thickness.
7. The method as claimed in claim 1, wherein percentage reduction is maintained at around 83-85% during said cold rolling of said hot bands.
8. The method as claimed in claim 1, further comprises arresting of leakages points in decarb furnace to eliminate nitrogen ingress.

9. The method as claimed in claim 1-8, wherein core loss value of 50C-350 grade Cold Rolled Non-Oriented (CRNO) steels is less than 3.5 W/kg at 1.5 T.
,TagSPECI:TECHNICAL FIELD OF THE INVENTION
The present invention relates to the process or method of development for production of 50C-350 grade Cold Rolled Non-Oriented (CRNO) steels. More particularly, the invention relates to designing the alloy chemistry, hot rolling parameters and optimize decarb-annealing conditions to achieve low core loss value and high magnetic permeability in all directions of the Cold rolled non-oriented (CRNO) electrical steels strip.
BACKGROUND OF THE INVENTION
Cold Rolled Non-Oriented (CRNO) electrical steels finds a wide variety of applications in the core of electrical machines like rotors, motors, generators and small transformers, particularly where low core loss and high magnetic permeability in all directions of the strip are desired. CRNO steels posses good electrical conductivity which makes it a “material of choice” for use in refrigerators, fans, mixies, light fittings, etc. CRNO steels are generally supplied in fully processed or semi-processed condition. In fully processed steels, the magnetic properties are fully developed by the steel producer and are ready to use without any additional processing. In the case of semi-processed steels, the material is supplied without the decarb-annealing (DA) treatment necessary to develop final magnetic properties. The customer/ user punches laminations from the semi-processed material and subjects them to DA treatment before use.
Rourkela Steel Plant (RSP) is a leading producer of CRNO steels in the country, with an annual production of about 75,000 tons. RSP produces fully processed grades of CRNO, namely, M-47, M-45 and M-43. To meet the growing market requirements for higher end applications, efforts have been made towards development of M-36 (C-350) grade CRNO steels exhibiting superior magnetic properties and low core loss values.
US 4898627 discloses about ultra-rapid annealing of non-oriented electrical steel which is conducted at a rate above 100° C. per second on prior to or as part of the strip decarburization and/or annealing process to provide an improved texture and, thereby, improved permeability and reduced core loss. During the ultra-rapid heating of cold-rolled strip, the recrystallization texture is enhanced by more preferential nucleation of {100} and {110} oriented crystals and reduced formation of {111} oriented crystals. The preferred practice has a heating rate above 262° C per second to a peak temperature between 750° C and 1150° C and held at temperature for 0 to 5 minutes.
The aforementioned patent application does not deal with 50C-350 grade Cold Rolled Non-Oriented (CRNO) steels. It suggests the texture-property correlation. The texture does not play important role in CRNO steel. The texture analysis is done in present work too and the inventors of the present invention found that, texture cannot be correlated with magnetic property and core loss value. Hence texture does not influence magnetic property of CRNO steel.

Hence there exists a need of systematic study for detailed characterization of C-350 grade steel to achieve low core loss value and high magnetic permeability in all directions of the steel strip which is carried out by the inventors of the present invention to design the alloy chemistry, hot rolling parameters and optimize decarb-annealing conditions to achieve core loss value <3.5 W/kg at 1.5 T as per specification. In the present invention analyses of all the major process parameters (from steel making to decarb annealing) and their influence on acceptance level in C-350 grade CRNO steel is done.
The present invention provides detailed characterization of C-350 grade steel and the influence of microstructure, inclusion level, and nitrogen pick-up are established on acceptance level in C-350 grade. Further, present work provides preparation of process chart for producing C-350 grade on regular basis.
OBJECT OF THE INVENTION
The prime object of the present invention is to overcome the drawbacks of the prior art.
An object of the present invention is to optimize the parameters and introduce certain measures which improves the acceptance level of processed coils in CRNO 50C-350 grade steel.
Another object of the present invention is to provide design of alloy chemistry, hot rolling parameters and other process parameters to achieve lower core loss value of CRNO steel.
Yet another object of the present invention is to provide a method of preparation and/or production of 50C-350 grade Cold Rolled Non-Oriented (CRNO) steels.
An object of the present invention is to provide low core loss value CRNO 50C-350 grade steel coils.
It is a further object of the present invention to find the effect of alloy content (Si with Al), inclusion level, nitrogen pick up during decarb annealing, coiling temperature of hot band and annealing temperature in deacrb line on the acceptance level of C-350 grade coil.
These and other objects of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY OF INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of the some aspects of the invention.

According to the one of the aspect of the present invention there is provided a method of preparation and/or production of 50C-350 grade Cold Rolled Non-Oriented (CRNO) steels with steel melt comprising the steps of:
- treating the steel melt with calcium-silicon (Ca-Si) for globular inclusion shape and to reduce inclusion level by using Ca-Si core wire during secondary steel making;
- Vacuum Oxygen Refining (VOR) to lower the excess amount of Carbon and Sulphur level in the steel melt by VOR tank and steam injector comprising of programmable logic controller (PLC);
- continuous casting of the steel melt forming concast slabs wherein superheating temperature is maintained at about 20oC max and the casting speed is maintained to 0.6-0.70 m/min;
- sending the concast slabs to Hot Strip Mill (HSM) in hot condition by the additional hood fabricated to prevent slab cracking due to high silicon content wherein said slabs are slow cooled under the hoods;
- hot rolling of the slabs within 24 hours of casting;
- reheating of the slabs forming hot bands ensuring complete homogenization wherein the optimum reheating temperature is selected by considering the required temperature for higher silicon content of steel melt and mill limitations;
- maintaining coiling temperature for self annealing of hot band wherein the said coiling temperature is achieved by switching-off the laminar cooling and avoiding descaling; and
- processing of said hot bands in silicon mill to side trim the bands in Bust line, followed by pickling, cold rolling and decarb-annealing to generate the desired carbon level (<0.003%) before coating wherein leakages points in decarb-annealing is arrested to eliminate nitrogen ingress in decarb furnace.
In another aspect of the present invention there is provided an alloy chemistry of CRNO steel melt comprising of C: 0.02-0.025%, Mn: 0.25%, P: 0.02% max., S: 0.001 % max, Si: 2.5-2.7% (Si+Al <2.85%), Al: 0.15-0.20%, N2: 50 ppm max.
In yet another aspect of the present invention there is provided a slab reheating and hot rolling parameters as per high silicon CRNO rolling practice wherein the slab soaking temperature is 1180-1200°C, slab retention time is 4 hrs, roughing temperature 1120-980°C, finish rolling temperature is 860-880°C and coiling temperature is maintained at around 680-700 °C.
In another non limiting aspect of the present invention there is provided optimized decarb-annealing conditions comprising different annealing zones (Zone#8 to Zone#11) in decarb line to achieve low C level (<0.003%) and better magnetic properties of CRNO steel strip wherein maximum decarb temperature is maintained at around 840 °C, annealing temperature in Zone#8 is varied from 950-970 °C, maximum annealing temperature of 990 °C is maintained in Zone#9 to Zone#11, and minimum decarb line speed is maintained at around 15 to 16 m/min.

To enable the invention to be more clearly understood and carried into practice reference is now made to the accompanying drawing in which like references denote like parts throughout the description.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
An embodiment of the invention will now be described, by way of example with reference to the accompanying drawings, in which:
Figure 1 illustrates the schematic representation of rolling schedule for CRNO M-36 steel.
Figure 2 shows schematic representation of processing of CRNO steel.
Figure 3 (a-c) depicts micro-structural changes during processing of M-36 grade (passed) steel (a) hot rolled, (b) cold rolled and (c) decarb-annealed.
Figure 4 (a-c) illustrates micro-structural changes during processing of M-36 grade (failed) steel (a) hot rolled, (b) cold rolled and (c) decarb-annealed.
Figure 5 (a-b) illustrates orientation distribution of grain for (a) M-36 (passed) & (b) M-36 (failed) samples.
Figure 6 (a-b) depicts optical photomicrograph showing distribution of inclusions in (a) M-36 (passed) and (b) M-36 (failed) sample.
Figure 7 illustrates effect of Si with Al on acceptance level in C-350 grade.
Figure 8 depicts effect of coiling temperature on acceptance level in C-350 grade.
Figure 9 shows the effect of annealing temperature on acceptance level.
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic is intended to provide.
Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative experiment now to be described in detail in connection with accompanying drawings.
Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of known implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions should be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
In an embodiment of the present invention the process parameters are optimized based on the systematic studies and characterization of material. According to the present invention during the heat making and processing of steel melt based on the metallurgical requirement and target magnetic properties, the alloy chemistry (1) is designed in the present invention which is comprised of C: 0.025 max, Mn: 0.25 max, P: 0.02 max, S: 0.005 max, Si: 2.3-2.7 and Al: 0.15-0.20. Three experimental heats with different Si levels are made in a 60 Ton basic oxygen furnace (BOF) converter and processed through Vacuum Oxygen Refining-Laddle Heating Furnace (VOR-LHF) (2,3) secondary refining facility, before continuous casting (4) into 210 mm thick slabs. The alloy chemistry of the 3 heats are shown in Table 1.
Sl. No. Heat No. C Mn P S Si Al
1 A 0.025 0.22 0.024 0.004 2.37 0.158
2 B 0.025 0.20 0.030 0.003 2.53 0.123
3 C 0.022 0.14 0.020 0.004 2.73 0.230
Table-1
Alloy Chemistry of experimental M-36 CRNO grade heats (wt %) is depicted in Table 1. It may be noted that the Si level is varied from 2.3% (Heat A) to 2.7% (Heat C). The S level is controlled within 0.005%. The C level for all the 3 heats are controlled within 0.025% which ensures C<0.003% in the final product after decarb-anneal.
During the hot rolling (5) of steel melt at Hot Strip Mill (HSM) the design of the parameters concerning hot rolling (5) of steel melt at Hot Strip Mill (HSM) is proposed by the inventors of the present invention. The slabs are slow cooled under hoods, transported to the HSM and hot charged into the reheating furnace within 24 hours of its production. The slabs are reheated to a temperature of 1180-1200°C for 4 hours to ensure complete homogenisation. The optimum reheating temperature is selected by considering the required temperature for higher silicon content of steel & mill limitations. However, due to high silicon content, this steel is very soft and has tendency to melt at high temperature, soaking temperature is kept in the range of 1180-1200°C. The thermal and deformation schedule is designed to achieve the desired microstructure in the hot band. A schematic representation of the hot rolling (5) process is shown in Figure 1.
The slabs are subjected to 8 passes in the roughing stand from an input gauge thickness of 210 m to 21 mm. The exit temperature at R2 is maintained at 1080-1100°C. The intermediately rolled slabs are descaled, head end cropped and fed to the finishing mill. The slabs are rolled from 21 mm to a final gauge thickness of ~2.5 mm in finishing stands. The coiling temperature is maintained at 680-700°C for self annealing of hot band. The high coiling temperature is achieved by switching-off the laminar cooling at run out table.
The hot bands are next processed in the silicon mill where they are side trimmed in BUST line, pickled (6), cold rolled (7) into 0.5% thickness and decarb-annealed (8) to generate the desired C level before oraganic coating (9). A schematic representation of the entire process is shown in Figure 2.
In the present invention the decarb annealing parameters are further optimized to get the desired object of the present invention. The Decarb-Annealing parameters, as shown in Table-2, are optimized by trials at Decarb Line to achieve low C level (<0.003%) and better magnetic properties.
Decarb Temp( C ) Annealing Temp ( C ) Line Speed (m/min)
Zone #8 Zone #9,10,11
840 950-970 990 15-16
Table-2
To achieve the results the decarb-annealing comprising different annealing zones (Zone#8 to Zone#11) in decarb line are optimized by having maximum decarb temperature maintained at around 840 °C, annealing temperature in Zone#8 varied from 950-970 °C, maximum annealing temperature of 990 °C maintained in Zone#9 to Zone#11, and minimum decarb line speed maintained which is around 15 to 16 m/min. further the leakages points in decarb-annealing aree arrested to eliminate N2 ingress in the furnace.
In the present invention the core loss values of finally processed CRNO coils are evaluated. It is observed that ~50% of the coils conformed to M-36/ IS-648-50-C-350 specification while the others are diverted to M-43/ IS-648-50-C-400 grade. The detailed metallurgical study is carried out to find out the genesis of diversion to M-43 grade. To understand the genesis of the problem, chemical, metallography, inclusion volume fraction and texture analysis studies are carried out on samples from 1 passed (coil No.C5) and 1 failed (coil No.C4) coil.
Figure 3(a-c) shows microstructural changes associated during processing of passed sample through (a) hot rolling, (b) cold rolling and (c) final decarb-annealing. The Hot rolled (HR) coils exhibited a typical ferrite structure with average grain size of 32 µm (Figure 3a).
A typical cold deformed structure is obtained upon imparting ~84% cold reduction (Figure 4b), which further transformed into recrystallized large strain free grains (Figure 4c) of ~111 µm average ferrite grain size (GS) on decarb-annealing. In the case of the failed sample, the microstructural changes during hot rolling, cold rolling and decarb annealing are depicted in Fig.5 (a-c). It is observed that the average grain size of the final product is significantly lower (~81 µm) than the GS of passed sample (~111 µm). This is attributed to the presence of fine unrecrystallized grains at the edge of the sample (Figure 4c).
Electron back scattered diffraction (EBSD) studies are carried out to determine the orientation distribution among the grains and to ascertain if there is any texture effect contributing to lower core loss values in the M-36 steels (Figure 5 a-b). It may be noted that as expected, both the passed (Figure 5a) and failed samples (Figure 5b) exhibited a random orientation distribution of {100}, {110} and {111} grains. Thus it is concluded that CRNO steels do not exhibit a strong texture.
To eliminate the possibility of nitrogen pick up during annealing process, nitrogen analysis is carried out on samples taken from M-36 (passed) and M-36 (failed) coils. Nitrogen pick-up during the final annealing process leads to increased chances of precipitation of AlN, which impedes the grain growth and orientation process leading to higher core loss and lower magnetic permeability. The results of the analysis have shown a significantly higher N level (172 ppm) in the failed samples as compared to the passed sample (49 ppm).
Inclusion analysis is carried out for the passed and failed samples. The volume fraction of inclusions is found to be higher (0.43%) for the failed sample as compared to the passed sample (0.35%). This is corroborated through optical microscopy, as shown in Figure 6 (a-b). Further details inclusion volume fraction is given in Table-3.
Coil No Inclusion Vol fraction (%) Remark
Max Min Avg
A 0.59 0.03 0.35 Pass
B 0.69 0.27 0.43 Fail
Table-3
To reduce the inclusion level in 50C-350 grade steel and improve the steel cleanliness, Ca-Si treatment is adopted during secondary steel making. The active element in CaSi is calcium. During calcium treatment, the alumina and silica inclusions are converted to molten calcium aluminates and silicate which are globular in shape because of the surface tension effect. The calcium aluminates inclusions retained in liquid steel suppress the formation of MnS stringers during solidification of steel. This change in the composition and mode of precipitation of sulphide inclusion during solidification of steel is known as sulphide morphology or sulphide shape control.
In the present invention analysis of process parameters is done by way of exemplary embodiments of the present invention. The influence of alloy chemistry (Si & Al), coiling temperature at Hot Strip Mill and zone#8 annealing temperature in Decarb line on acceptability of coils as per IS-648-50-C-350 grade are studied.
Figure 7 shows the effect of alloy chemistry (Si+Al) on acceptability in C-350 grade. It may be seen that increase in Si+Al content beyond 2.6% leads to a significant improvement in acceptance of coils. Si at the level of 2.53 resulted in highest acceptance level. Based on the analysis Si is selected in the range of 2.5-2.6 and Al in the range of 0.12-0.20. Increasing Si beyond 2.7 does not increase the acceptance level in C-350 grade steel.
Figure 8 shows the effect of coiling temperature at Hot Strip Mill on acceptance in C-350 grade. Coiling temperature does not show any clear cut trend on acceptance level in C-350 grade. However, it is observed that, high coiling temperature leads to self annealing of hot band and thus desirable to achieve better magnetic properties and lower core loss value in CRNO grade steel. During rolling, it is observed that, heat loss is very high in C-350 grade steel due to higher Si content. Achieving, high coiling temperature (>680 C) is difficult with normal laminar cooling pattern. Thus to achieve high coiling temperature, laminar cooling is not used. Further, descaling is also avoided to reduce temperature losses during processing.
There are 4 annealing zones (Zone#8 to Zone#11) in Decarb line. Normal annealing temperature for processing regular grade in Decarb line is 940 °C in zone#8 and 970 °C in other zones. Initially, it is increased to 950 °C in Zone #8 for processing C-350 grade. Maximum annealing temperatures of 990 °C are maintained in other zones. Based on the evaluation of core loss value, temperature in Zone#8 is increased in steps from 950 to 970 C. 970 °C is the maximum temperature that can be achieved in zone#8.
Figure 9 shows the effect of annealing temperature on acceptance level. Increasing the annealing temperature to 970 °C had marked improvement in core loss value and hence in acceptance level in C-350 grade.
In a non-limiting embodiment of the present invention there is provided a process chart or steps for processing 50C-0350 grade steel. Based on the material characterization and process parameter analysis, the processing parameters are further optimized / modified as given below to achieve higher acceptance level of the processed coils in M-36 grade.
A. Steel Chemistry & Casting
· Suggested steel chemistry
C: 0.02-0.025%
Mn: 0.25%
P: 0.02% max.
S: 0.001 % max
Si: 2.5-2.7% (Si+Al <2.85%)
Al: 0.15-0.20%
N2: 50 ppm max.
· Ca-Si treatment during heat making for globular inclusion shape
· Improving vacuum in VOR to lower Carbon & Sulphur level
· Following casting parameters will be maintained:
Superheat: 20oC max.
Casting speed: 0.6-0.70 m/min.
· Concast slabs will be sent to HSM in hot condition under hood. Rolling of the slabs will be carried out within 24 hours of casting.

B. Hot Strip Rolling
· Two hot slabs is selected for rolling in HSM.
· Slab reheating and hot rolling parameters will be as per high silicon CRNO rolling practice i.e.
Slab soaking temperature: 1180-1200C
Slab retention time: 4 hrs.
Roughing temperature 1120-980 C
Finish rolling temperature: 860-880 C
Coiling Temperature: >680C
· Final hot rolled coils gauge- 2.5 ±0.1 mm
C. Processing at SSM
· The hot bands were side trimmed in BUST line, pickled & cold rolled into 0.5 mm thickness
· % Reduction maintained at 83-85% during cold rolling at Reversing mill
· The following parameters ensured at Decarb Line to achieve low C level (<0.003%) & better magnetic properties as depicted in Table-2:
§ Max Decarb temp maintained
§ Annealing temp in Zone#8 varied from 950-970C
§ Max annealing temp maintained in Zone#9-11
§ Min. line speed maintained
· Leakages points in Decarb-annealing were arrested to eliminate N2 ingress in the furnace ingress and reduce nitrogen level in the processed coils.
Although the invention has been described with reference to particular experimental results of the invention, it should be appreciated that it may be exemplified in other forms. A similar approach can be adopted to develop higher grades of CRNO steels. The invention qualifies to be adopted in a variety of other embodiments such modifications and alternatives obtaining the advantages and the benefits of the present invention will be apparent to those skilled in the art. All such modifications and alternatives will be obvious to a person skilled in art.
The description herein contains many specifics, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the embodiments of the invention. One of ordinary skill in the art will appreciate that elements and materials other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such elements and materials are intended to be included in this invention. Numerous variations, changes and substitutions may be made without departing from the invention herein.