Claims:We claim:
1. A steel comprising, composition in weight percentage (wt.%) of:

Carbon (C) at a concentration ranging from about 0.02% to about 0.06%;
Manganese (Mn) at a concentration ranging from about 0.8% to about 1.3 %;
Niobium (Nb) at a concentration ranging from about 0.06% to about 0.12%;
Chromium (Cr) at a concentration ranging from about 0.01% to about 0.25%;
Titanium (Ti) at a concentration ranging from about 0.015% to about 0.025%;
Aluminium (Al) at a concentration ranging from about 0.03% to about 0.05%;
Silicon (Si) at a concentration ranging from about 0.1% to about 0.5%;
Nitrogen (N) at a concentration ranging from about 0.0001% to about 0.0080%;
Sulphur (S) at a concentration ranging from about 0.0001% to about 0.0050%;
and Phosphorus (P) at a concentration ranging from about 0.0001% to about 0.015%,
wherein said steel composition exhibits excellent stretch flangeability, with HER ratio more than 115 % and ferrite potential of more than 1.05.

2. The steel as claimed in claim 1, wherein the steel is non-peritectic.

3. The steel as claimed in claim 1, wherein the carbon equivalence is less than 0.35.

4. The steel as claimed in claim 1, wherein the cumulative concentration of Nb, Ti , Mo and Cr is less than 0.5 % weight percentage.

5. The steel as claimed in claim 1, wherein the steel has yield strength (YS) ranging from about 490 MPa to about 650 MPa; ultimate tensile strength (UTS) ranging from about 550 MPa to about 800 MPa; and elongation values greater than 20 %.

6. The steel as claimed in claim 1, wherein the steel has polygonal ferrite and bainitic ferrite microstructure.

7. The steel as claimed in claim 1, wherein the steel has average grain size ranging from about 2 µm to about 5 µm.

8. The steel as claimed in claim 1, wherein the steel has an impact toughness ranging greater than 100J at -60°C temperature and greater than 180J at room temperature.

9. The steel as claimed in claim 1, wherein the steel has a hardness value ranging from about 185 Hv to about 250 Hv.

10. The steel as claimed in claim 1, wherein the steel has a HER value of at least 115 %.

11. A method for manufacturing the steel as claimed in claim 1, said method comprising steps of:
a. casting a steel slab with the steel composition of claim 1 followed by heating the slab to a temperature ranging from about 1100°C to about 1250°C;
b. hot rolling of the slab with about more than 80% reduction below recrystallization stop temperature (TNR) with finish hot rolling temperature ranging from about Ae3 - 50 (°C) to about Ae3 + 50 (°C); and
c. controlled cooling of the hot rolled steel sheet to a coiling temperature ranging from about 500°C to about 600°C to obtain said steel.

12. The method as claimed in claim 12, wherein the heating in step (a) is carried out for a duration ranging from about 20 minutes to about 2 hours; and wherein the cooling in step (c) is carried out at a rate ranging from about 20°C to about 50°C per second.

13. The method as claimed in claim 13, wherein the cooling at said temperature results in steel having polygonal ferrite and bainitic ferrite microstructure.
Dated this 21st March 2022

GOPINATH A S
IN/PA 1852
OF K&S PARTNERS
AGENT FOR THE APPLICANT
, Description:FORM 2

THE PATENT ACT, 1970
(39 of 1970)
&
THE PATENT RULES, 2003

COMPLETE SPECIFICATION
(See Section 10, Rule 13)

Title: “HIGH STRENGTH STEEL GRADE WITH EXCELLENT COMBINATION OF STRENGTH, DUCTILITY AND STRETCH FLANGEABILTY FOR MANUFACTURING OF AUTOMIBLE COMPONENTS”

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

The present disclosure relates to designing of steel composition and method of producing high strength steel grade with an excellent combination of strength, ductility, toughness and stretch flangeability. The developed steel of the present disclosure exhibits high strength with tensile strength greater than 600 MPa, % elongation in excess of 20 % and hole expansion ratio (HER) greater than 100 %.

BACKGROUND OF THE DISCLOSURE

The ever-increasing concerns related to increasing carbon footprint (i.e., CO2 emission) and depletion of non–renewable sources of energy have evoked the necessity of development of high strength steel grades for the automobiles. Thus, in last few decades, primary focus of the auto manufacturer has been the reduction in the weight of automobile by extending usage of high strength steel, to improve the fuel efficiency and enhance passenger’s safety in the vehicle. Although there are variety of high strength steel grades developed but these may not be deemed fit for the manufacturing of automobile components which requires forming the hot rolled steel grades into complex and intricate shapes i.e., the components requiring high stretch flangeability in addition to the combination of high strength and ductility.
Dual phase (DP) steels are one such class of high strength steels which offers the distinct combination of properties such as high strength, high ductility, higher strain hardening rate, low YS/UTS ratio. These steels are widely used in the automobile components such as, B pillars, body side inner, inner panels, rear rails, rear shock reinforcement etc. Patent JPA4329848 disclosed the development of high strength dual phase steel having excellent fatigue properties but its application was restricted because of poor stretch flangeability. Similar observations were also recorded in the development of dual phase steel reported in US20200123630A1.

Recent developments which are based on the precipitation strengthening in completely ferrite matrix by TiC/ (TiMo)C exhibited superior stretch flangeability. However, such developments were reported to result in a wide variation in the steel properties along the length of the coil during manufacturing of hot rolled steels. This resulted in the inhomogeneity in the steel properties.

In the light of the above discussed prior art, there is a need of a steel composition and the microstructure which overcomes the limitations of prior art and exhibits superior stretch flangeability, coupled with superior formability and weldability.

SUMMARY OF THE DISCLOSURE
The present disclosure relates to non-peritectic steel made of a composition which provides excellent stretch flangeability and has enhanced strength and toughness. The said composition comprises Carbon (C) at a concentration ranging from about 0.02wt% to about 0.06 wt%; Manganese (Mn) at a concentration ranging from about 0.8 wt% to about 1.30 wt%; Niobium (Nb) at a concentration ranging from about 0.06wt% to about 0.12 wt%; Molybdenum (Mo) at a concentration ranging from about 0.10 wt% to about 0.20 wt%; Chromium (Cr) at a concentration ranging from about 0.010 wt% to about 0.25 wt%; Titanium (Ti) at a concentration ranging from about 0.015wt% to about 0.025wt%; Aluminum (Al) at a concentration ranging from about 0.03wt% to about 0.05 wt%; Silicon (Si) at a concentration ranging from about 0.1wt% to about 0.4 wt%; Nitrogen (N) at a concentration ranging from about 0.0010wt% to about 0.060wt%; Sulphur (S) at a concentration ranging from about 0.0001wt% to about 0.0050wt%; and Phosphorus (P) at a concentration ranging from about 0.0001wt% to about 0.015wt%.

The steel as per the current invention possesses superior stretch flangeability exhibiting HER value in excess of 115 %. Also, the said steel possesses superior low temperature toughness. As per the current invention, the ferrite potential of said composition is more than 1.05, thereby making the resultant steel non-peritectic; whereas the carbon equivalence of the composition is less than 0.30, ensuring that the steel exhibits excellent weldability. The steel manufactured as per the current invention has polygonal ferrite and bainitic ferrite microstructure; and possesses tensile strength greater than 600 MPa.

The steel of the present disclosure is designed such that it is readily hot/cold formed, highly weldable to form automotive components with intricate and complex shapes, which demands the combination of high stretch flangeability, toughness and fatigue properties. The present invention also provides a method for manufacturing steel having composition as described above, wherein said method involves casting of the composition in steel slab, hot rolling of the steel slab at specific conditions, and controlled cooling of the hot rolled steel sheet to obtain the steel.
In embodiments of the present disclosure, recrystallization stop temperature (TNR) with finish hot rolling temperature (FRT), along with the coiling temperature are critical to arrive at the steel of the present disclosure.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 provides a schematic illustrating the designed thermo-mechanical processing for the manufacturing of high strength hot rolled steel grade for manufacturing of automotive components.
Figure 2 provides (a) optical and (b) SEM micrograph showing the polygonal ferrite and ferrite- bainite microstructure and (c) grain size distribution of steel coiled at 500°C.
Figure 3 provides (a) optical and (b) SEM micrograph showing predominantly polygonal ferrite with acicular or bainitic ferrite microstructure and (c) grain size distribution of steel coiled at 600°C.
Figure 4 provides bright field TEM micrographs revealing the bainitic ferrite laths in the steel coiled at 500°C.
Figure 5 provides (a) bright field TEM micrographs revealing the bainitic ferrite laths and (b) bright field TEM micrographs in the steel coiled at 600°C.

Figure 6 demonstrates the stress-strain curve for the steel coiled at 600°C.

DETAILED DESCRIPTION
In view of the problems of the prior art highlighted above, the present disclosure aims to provide a steel made of a composition which provides excellent stretch flangeability (High HER), is non-peritectic and has enhanced strength, ductility. toughness and fatigue performance. The steel as per the present disclosure discloses a micro alloyed steel composition and thermo-mechanical processing to manufacture the hot rolled sheets which exhibits tensile strength greater than 600 MPa, with an excellent combination of properties including high HER, low temperature toughness and fatigue performance, capable of forming automobile components involving complex shapes, automotive wheel disc and wheel rim in specific.

The present disclosure provides a steel composition comprising Carbon (C) at a concentration ranging from about 0.02wt% to about 0.06 wt%; Manganese (Mn) at a concentration ranging from about 0.8 wt% to about 1.3 wt%; Niobium (Nb) at a concentration ranging from about 0.06 wt% to about 0.12 wt%; Molybdenum (Mo) at a concentration ranging from about 0.10 wt% to about 0.20 wt%; Titanium (Ti) at a concentration ranging from about 0.015wt% to about 0.025wt%; Chromium (Cr) at a concentration ranging from about 0.01 wt% to about 0.25 wt% ; Aluminium (Al) at a concentration ranging from about 0.03wt% to about 0.05wt%; Silicon (Si) at a concentration ranging from about 0.1wt% to about 0.5wt%; Nitrogen (N) at a concentration ranging from about 0.0001wt% to about 0.080 wt.%; Sulphur (S) at a concentration ranging from about 0.0001wt% to about 0.0050 wt.%; and Phosphorus (P) at a concentration ranging from about 0.0001wt% to about 0.015wt%.

The steel composition as per the current invention limits the presence of segregation prone elements like Mn and C which promotes microstructural banding/segregation in mid-section thickness of hot rolled steels. The lower carbon and manganese content in conjunction with accelerated cooling at run-out table (ROT) to produce segregation free hot rolled steel enabling the attainment of high stretch flangeability.

The said composition of the present disclosure is specifically designed to ensure that the concentrations of the constituent elements provide optimum desired results. Thus, it is important to understand the role of each of the critical elements with respect to their concentrations as provided below:

Carbon: The carbon in the steel as per the current invention is present in the range of about 0.02wt% to about 0.06 wt%. C is added to derive the strength in steel through solid solution strengthening, second phase formation along with the formation of precipitates in the form of carbides/carbonitrides. The carbon content in the current steel composition is limited to a range which limits the segregation of carbon in steel which causes the formation of martensite or martensite/austenite (MA) constituents in the steel microstructure. The presence of martensite and MA constituents is detrimental to stretch flangeability and toughness of steel. Also, the increased carbon content decreases the weldability of steel. The lower carbon content results in the formation of bainitic ferrite which has relatively lower hardness as compared to the conventional bainite. This results in a lower hardness difference between the microstructural constituents (polygonal ferrite and bainitic ferrite) resulting in the improved stretch flangeability of the hot rolled steel strip. In addition, the lower carbon content also allows the designing of non-peritectic steel composition. Preferably, carbon is present in the range of 0.02 wt% to about 0.04 wt%.

Manganese: Mn in the steel of the current invention varies in the range of about 0.8 wt.% to about 1.30 wt.%. Preferably, Mn is present less than 1.20 weight%. Mn, apart from imparting solid solution strengthening, also lowers the austenite to ferrite transformation temperature and helps in refining the ferrite grain size. Higher manganese levels enhances the centerline segregation during the process of continuous casting. Additionally, it leads to a higher number of MnS inclusions which are detrimental to ductility and stretch flangeability of a steel grade. Higher level of manganese in steel also increases the carbon equivalence and impairs the weldability of steel.

Silicon: Silicon is present in the range of about 0.10 wt.% to about 0.50 wt.%. Silicon imparts the solid solution strengthening effect like Mn. Si is also being employed as a deoxidizing element. However, in order to prevent the formation of surface scales, the Si content in the steel is restricted to a maximum content of 0.5%. Also, higher Si content impairs the weldability of the steel by increasing carbon equivalence.

Niobium: Nb in the steel of the current invention varies in the range of about 0.06 wt.% to about 0.12 wt%. Preferably, Nb is present less than 0.12 weight%. Nb in steel helps in the grain refinement because of its solute drag effect and allows lowering the carbon content of the steel. Niobium significantly increases the recrystallization stop temperatures and allows the higher amount of deformation below recrystallization stop temperature (TNR) during the hot rolling of the steel. This allows significant reduction in grain size and remarkably increases the toughness of steel. The role of Niobium in the present disclosure is also extended to increase the hardenability of austenite to form the bainitic ferrite or acicular ferrite at relatively lower cooling rates. However, Niobium content more than 0.12 % can significantly increase the mill load which may drastically reduce the life of the rolls in the rolling mill or in some cases it may be beyond the capacities of the rolling mills.

Molybdenum: Mo in the steel of the current invention varies in the range of about 0.10 wt.% to about 0.20 wt.%. Preferably, Mo is present less than 0.15 wt.%. The presence of Mo promotes the formation of acicular ferrite/bainitic ferrite because of it increases the hardenability of steel. Its presence also significantly suppresses the pearlite formation, which is an undesirable phase in the present invention. Presence of Mo also contributes towards strengthening of steel through precipitation by limiting the coarsening of fine precipitates. These fine precipitates increase the strength of polygonal ferrite, thereby minimizing the hardness difference between the polygonal ferrite and bainitic ferrite, which significantly influences the stretch flangeability of the developed steel grade. The upper limit of Mo in the present steel is restricted to 0.20 by weight percent. Further increase in the Mo content causes the weld embrittlement by the formation of phases like martensite or martensite-austenite (MA) which not only deteriorates the toughness of weld heat affected zone but significantly impairs the stretch flangeability in heat affected region.

Chromium: The Cr content in the current invention is about 0.01 wt.% to 0.25 wt.%. The preferable content is less than 0. 20 wt.%. The chromium in the present steel increases the hardenability and helps in retarding the formation of pearlite. Thus, enhances the formation of second phase i.e., bainitic ferrite in the present invention. The higher content beyond the preferable range increase the carbon equivalence and thus impairs the weldability of the steel.

Nitrogen: The preferable range for the nitrogen in the steel is about 0.0040wt% to about 0.0080wt%. Nitrogen combines with Titanium and Niobium to form nitrides/carbonitirdes. Accordingly, Ti/N ratio should be maintained at or less than (=) 3.14, to limit the grain coarsening when material is subjected to end application process of welding. However, increasing the nitrogen content above 0.0080wt% may lead to the embrittlement of the heat affected zone (HAZ) of weld joints. The higher level of nitrogen may result in the increase in the number of coarse TiN particles which are detrimental to the stretch flangeability and other properties like toughness.

Titanium: The preferable range of titanium in the steel is 0.015-0.025 wt%. Titanium in steel combines with nitrogen to form TiN precipitates which inhibits the austenite grain coarsening when the steel is reheated prior to rolling. Also, the presence of TiN restricts the prior austenite grain coarsening in the heat affected zone, when the steel is subjected to the welding operation, this prevents the deterioration of toughness in the heat affected zone of the welded steel

Aluminum: The preferable range of aluminum 0.03-0.05 wt.%. Aluminum in steel is used for de-oxidation of steel. The content of Al was limited to restrict the content of aluminum oxide, the presence of which may deteriorate the stretch flangeability. However, increased levels of Al leads to casting issues and thus it should be restricted to a maximum content of 0.05 wt.%.

Sulphur: Sulphur needs to be limited to about 0.0050wt% to avoid high level of MnS inclusions, as they cause severe deterioration in stretch flangeability and toughness properties.

Phosphorous: Phosphorus content as per the current invention needs to be restricted to a maximum of 0.015wt% as higher phosphorus levels can lead to reduction in stretch flangeability, toughness and weldability due to segregation of P at grain boundaries.

In an embodiment of the present disclosure, the total micro alloying content of the composition is restricted to less than 0.25wt%. Particularly, in the composition of the present disclosure, the cumulative concentration of Nb,Mo, Ti and Cr does not exceed 0.5 wt%.

This specific concentration of the components in the composition of the present disclosure lead to specific microstructure formation, that helps in providing the desired HIC properties to the steel. In embodiments of the present disclosure, the steel sheet according to the present disclosure has 50-85 % ferrite. The ferrite is strengthened by solid solution strengthening contributions from Mn and Si. With the application of high Nb and Mo coupled with controlled thermo-mechanical processing conditions, the average grain size is restricted to about 1.9 and 2.3 ?m for coiling temperature of 500 and 600°C, respectively. This grain refinement significantly increases the strength of ferrite governed by the Hall-Petch relationship. Also, the finer grain size results in remarkable toughness of the steel at room temperature and at sub-zero temperatures. The dispersion of fine precipitates Niobium rich carbides, which are few nanometers in size, also contribute towards the strength of the ferrite. This can be seen from figure 4 which shows bright and dark field TEM micrographs revealing the dispersion of fine precipitates of niobium carbide/carbonitride for the steel coiled at 600°C (as provided by example 1 below). In various embodiments of the present disclosure, the steel having the said composition has average grain size ranging from about 1.5 µm to about 5µm.
Accordingly, in embodiments of the present disclosure, the steel having the said composition has polygonal ferrite and bainitic ferrite microstructure. The amount of bainitic ferrite /acicular ferrite in the microstructure ranges between about 10% to about 50%. The strengthening from bainite/acicular ferrite is derived from its fine structure and higher dislocation density.

This microstructure, formed by the composition, lends enhanced strength and quality to the resultant steel of the present disclosure. More particularly, the steel of the present disclosure possesses high yield strength (YS) and ultimate tensile strength (UTS),. In various embodiments of the present disclosure, the steel having the composition of the present disclosure has yield strength ranging from about 490 MPa to about 650 MPa; ultimate tensile strength (UTS) ranging from about 550 MPa to about 800 MPa; and elongation value of at least 20%. Accordingly, the YS/UTS ratio of the steel is also kept below 0.90.

In addition to YS and UTS, in embodiments of the present disclosure, the steel having the said composition has an impact toughness greater than 100 J at -60°C and greater than 180 J at room temperature d. Further, the steel also has a hardness value ranging from about 185 Hv to about 250 Hv.

The steel of the present disclosure has a ferrite potential of either less than 0.85 or greater than 1.05, thereby making the steel non-peritectic. This ferrite potential (FP) is calculated by the following empirical formula:
FP = 2.5 * (0.5 - Ceq),
where Ceq is carbon equivalence of the composition, and defined by the following equations:
Ceq = C + 0.04*Mn + 0.1*Ni + 0.7*N - 0.14*Si - 0.04*Cr - 0.1*Mo - 0.24*Ti - 0.7*S;

whereas the critical metal parameter (Pcm) for weld cracking is calculated by:
Pcm = + + + + + + +B

On the other hand, the formula based on International Institute of Welding (IIW) is:
CE =
In embodiments of the present disclosure, the carbon equivalence of the composition is less than 0.35. Said carbon equivalence ensures that the steel exhibits excellent weldability during the process of tube manufacturing and other end applications.

Various embodiments covered in the current invention are precise synergistic interplay of elements at specific concentrations, that allow the steel of the present disclosure to exhibit excellent stretch flangeability (High HER) along with the tensile and toughness properties, and capable of being used for automotive components that needs to be formed in complex shapes and experiences the cyclic loading at end applications (e.g., wheel disc /wheel rim).

The present disclosure thus also relates to the designing of the chemical composition of steel coupled with the controlled thermo-mechanical processing and accelerated cooling method to develop an automotive steel grade with excellent stretch flangeability, superior low temperature toughness along with an excellent weldability and formability.

Figure 1 shows the schematic that defines the thermo-mechanical processing employed for the production of hot rolled strips of the designed chemistry used for manufacturing of high strength automotive steel grade.

The steel as per the current invention is manufactured by casting a steel slab as per the above specified steel composition followed by heating the slab to a temperature ranging from about 1100°C to about 1250°C. The hot rolling of the slab with more than 80 % reduction is controlled below recrystallization stop temperature (TNR) with predefined finish hot rolling temperature (FRT) from about Ae3 - 50 (°C) to about Ae3 + 50 (°C). Thereafter, hot rolled steel sheet is cooled in controlled fashion to a coiling temperature ranging from about 500°C to about 600°C to obtain the said steel.

In further embodiments of the present disclosure, the heating of the slab as aforementioned is carried out for a duration ranging from about 20 minutes to about 2 hours; and wherein the cooling in the step is carried out at a rate ranging from about 20°C to about 50°C per second.

In embodiments of the present disclosure, the said method is carried out under specific conditions and parameters, which help achieve the desired steel of the present disclosure. Initially, the specified composition is first casted either through conventional continuous caster or a thin slab casting route. The non-peritectic steel composition of the present disclosure ensures smooth casting of steel through either route. After casting the slab with the specified composition, the slabs are reheated to a temperature greater than 1100°C (preferably in the range of about 1100°C to 1250°C) for a duration of about 20 minutes to about 2 hours. The reheating temperature is above 1100°C to ensure complete dissolution of any precipitates Niobium and Molybdenum carbide/carbonitrides may have formed in the preceding processing steps. A reheating temperature greater than about 1250°C is also undesirable as it may lead to grain coarsening of austenite and lead to yield loss due to excessive scale formation.

After casting and reheating the steel slab with the specified composition, hot rolling of the slab is carried out. The hot rolling constitutes a roughing step above the recrystallization temperature and a finishing step below the recrystallization temperature, when rolling is done in a conventional hot strip mill. The recrystallization stop temperature (TNR in degree centigrade) is a critical parameter in defining the final microstructure of the developed steel in terms of grain size and second phase formation. The rolling is done with percentage reduction greater than 80% below TNR with specific finish rolling temperature (FRT). In an embodiment, where a CSP (compact strip processing)/TSCR (thin slab casting) is used for producing the steel (where there is no separate roughing mill) the deformation schedule should be designed in order to break the cast structure during the initial stands of hot rolling, and finishing must be done below the recrystallization temperature such that the percentage reduction below TNR is greater than 80% with FRT ranging from about Ae3 - 50 (°C) to about Ae3 + 50 (°C).

Thereafter, the hot rolled steel sheet is subjected to accelerated cooling strategy on the Run-Out-Table (ROT), at a cooling rate ranging from about 20°C/s to about 50°C/s to a coiling temperature (CT) ranging from about 500°C to about 600°C, in order to suppress the pearlite formation and encourage the formation of bainitic ferrite or acicular ferrite in the microstructure. Higher coiling temperature of around 600°C allows increase in the strength of steel by the precipitation of fine carbides in supersaturated ferrite.

In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

EXAMPLES

Example 1: Manufacturing of steel of the present disclosure and analysis of the Microstructure thereof
Steel with composition as defined in Table 1 below was cast into two billets. The cast billets were then reheated to a temperature of 1200°C for a period of 1 hour to ensure the complete dissolution of microalloying precipitates in particular niobium precipitates. Both the billets were hot rolled with the identical deformation schedule, with the 80% reduction below TNR (1030-1040°C) and finish rolled to a temperature of 860-880°C. Post hot rolling, the two hot rolled sheets processed from the two billets were cooled to a coiling temperature of 500°C and 600°C, respectively at cooling rate of 25-30°C/s.

Table 1 – Steel composition
Elements/Properties Concentrations (wt%)/Values
C 0.024
Mn 1.25
Si 0.30
Nb 0.10
Mo 0.12
Cr 0.15
Ti 0.017
Al 0.03
S <0.0010
P 0.005
N 0.0050
Ferrite Potential (Fp) 1.24
Pcm 0.10
C (IIW) 0.28
Total Micro alloying content
(Nb +Mo+Cr+ Ti ) 0.39

The resulting hot rolled sheets coiled at 520°C and 600°C are analysed for their microstructure details, grain sizes and hardness values, and the results are provided in Table 2 below:

Table 2 - Microstructural details
Coiling Temperature
(CT) Microstructure Average Grain size
(µm) Volume fraction of bainitic ferrite
(%) Hardness
(Hv)
500°C Ferrite + Bainitic ferrite 1.9 ±1 30-50 200±15
600°C Ferrite+ Bainitic ferrite 2.3±1.5 15-30 230±10

Results: For both cases of steel coiled at 500°C and 600°C, the microstructure, grain size, volume fraction of bainitic ferrite and hardness correspond to the values required by the steel of the present disclosure.

While figure 2 provides optical (2a) and SEM micrograph (2b) showing polygonal ferrite and ferrite-bainite microstructure of steel coiled at 500°C; figure 3 provides optical (3a) and SEM micrograph (3b) showing predominantly polygonal ferrite with acicular or bainitic ferrite microstructure of steel coiled at 600°C.

Further, while figure 4 shows bright field TEM micrographs revealing the presence of
Bainitic ferrite laths for the steel coiled at 500°C , figure 5 (a) demonstrates the presence of bainitic ferrite in the polygonal ferrite matrix, with figure 5(b) revealing the dispersion of fine precipitates of niobium rich carbide/carbonitride for the steel coiled at 600°C.

Analysis of the Tensile properties of steel of the present disclosure
Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 520°C and 600°C are analysed for their yield strength (YS), ultimate tensile strength (UTS), % Elongation and YS/UTS ratio, and the results are provided in Table 3 below:

Coiling Temperature Tensile properties
YS (MPa) UTS (MPa) % El. (YS/UTS) Ratio
CT 500°C 554 640 26 0.86
CT 600°C 640 720 21 0.88

Results: For both cases of steel coiled at 500°C and 600°C, the tensile properties correspond to the values required by the steel of the present disclosure. Further, figure 6 shows stress- strain curve for the steel coiled at 600°C

Analysis of the Impact Toughness of steel of the present disclosure
Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 500°C and 600°C are analysed for their impact toughness as per ISO148:1983/ ASTM A370-17a, and the results are provided in Table 4 below:

Table 4 - Impact toughness of hot rolled strips coiled at 500°C and 600°C
Impact Toughness
(In Joules) CT-500°C CT-600°C
Temperature
25°C 205 190
0°C 180 165
-60°C 160 140
Results: For both cases of steel coiled at 500°C and 600°C, the impact toughness was found to be ranging between 140 -160 J at -60°C and 190- 205 J at room temperature, as required by the steel of the present disclosure.

Analysis of stretch flangeability of steel of the present disclosure
Steel with composition and process details as defined in Example 1 was manufactured, and the resulting hot rolled sheets coiled at 500°C and 600°C are tested as per standard ISO 16630:2009. Stretch-flangeability is a measure of steel’s ability to be formed into complex shapes without edge bursting or splitting and is measured in terms of hole expansion ratio. The hole expansion ratio is the key indicator for assessing the stretch-flanging properties by expanding a punched hole (10 mm in diameter) through a conical punch until a through thickness crack is visible. The hole expansion value is estimated using the following expression:
Hole Expansion Ratio (?, %) =((df-do)/do) *100, where do and df are the initial and the final diameter of the hole.

Table 5 – Hole expansion ratio (%) of hot rolled strips coiled at 500°C and 600°C
Hole expansion ratio (%) CT-500°C CT-600°C
148 118

Results: For both cases of steel coiled at 500°C and 600°C, the HER value which is a measure of stretch flangeability was found to be ranging between 118-148 %, as required by the steel of the present disclosure.

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

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.

Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.