FIELD OF INVENTION

The present invention relates to a low carbon micro-alloyed steel and more particularly, to the low carbon micro-alloyed steel having good ductility, strength, high strain hardening exponent, strength and isotropy combination and method of manufacturing thereof.

BACKGROUND
Metals such as steel, aluminum are widely employed for applications such as, automobile parts, construction materials etc. Sheets and strips of steel compositions have been used in forming body structural members and body panels for automotive vehicles. In order to achieve the technical goal of lightweight and safety of automobiles, the automotive manufacturers, steel companies and major research institutes have developed the first generation, and the second generation of advanced high strength steels (AHSS). The development of the third generation of AHSS started with the work of Speer et al. (“Carbon partitioning into austenite after martensite transformation” by J. Speer, D.K. Matlock, B. C. De Cooman, J. G. Schroth in Acta Mater. 51 (2003) 2611–2622) in which after martensitic transformation, the thermodynamic feasibility of the partitioning of carbon under constrained para-equilibrium condition from the martensite phase to retained austenite was shown for the first time.
The third generation of advanced high strength steels (AHSS) are being developed to fulfil the demands of the automobile sector to overcome the difficulties of high alloying cost, alloy segregation effect, poor weldability, and complex thermo-mechanical processing (TMP) processing of the second generation of AHSS. To reduce the weight of the automotive parts in order to improve their fuel efficiency in view of the global environmental conservation it is desirable to have steels having improved strength-ductility balance and also good formability.
OBJECTIVE OF INVENTION
It is an object of the invention to overcome the drawbacks of second-generation AHSS, such as poor weldability, expensive alloying, alloy segregation and processing difficulties due to high alloying.
Another objective of the present invention is to obtain high strength, ductility, strain hardening exponent (n), and isotropy combination simultaneously in steel with minimum possible C and alloying additions (<3%), for usage in automobiles.
Another objective of the present invention is to provide a new hot-rolled (HR) and cold-rolled (CR) low carbon micro-alloyed dual-phase steels having very high ductility, strength, high strain hardening exponent, and isotropy combination.
It is yet another objective of the present invention, to develop a method of the manufacturing the low carbon micro-alloyed steel having very high ductility, strength, and isotropy combination.
SUMMARY OF INVENTION
This summary is provided to introduce concepts related to a hot rolled or cold rolled low carbon micro-alloyed steel. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
In one aspect of the present invention, a hot rolled or cold rolled low carbon micro-alloyed steel is provided. The hot rolled or cold rolled low carbon micro-alloyed steel comprises the following composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities. The steel comprises a structure including, by volume, at least 63% of ferrite, between 29% to 37 % of martensite, and a small fraction of retained austenite.
In an embodiment, the steel comprises a structure including the ferrite phase 63 – 71%, martensite phase 29 – 37 % and a small fraction of retained austenite.
In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel comprises the composition expressed in weight %: C - 0.053, Mn - 1.32, Si - 0.35, Cr - 0.61, Nb - 0.015, Al - 0.037, Ca - 0.0016, N - 60 ppm, B - 0.0005, and the balance being Iron (Fe) and unavoidable impurities.
In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 450 MPa. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 500 MPa to 600 MPa. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 530 MPa to 550 MPa.
In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength of at least 250 MPa. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 250 MPa to 450 MPa. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 260 MPa to 420 MPa.
In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 30%. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 30 % to 60 %. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 43 % to 52%.
In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.25 to 0.4. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 0.9 to 1.05.
In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 0.93 to 1.04. In an embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a unique combination of high ductility, high strain hardening exponent, strength and isotropy.
In an embodiment, a method for manufacturing hot rolled low carbon micro-alloyed steel is provided. The method (100) comprising casting steel having a composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities to obtain steel slab. The method also comprises reheating the steel slab to a temperature greater than 1200°C. The method further comprises hot rolling the steel slab to produce a hot rolled (HR) steel sheet such that finish rolling is done at a finish rolling temperature (TFRT) in the range 840oC to 880oC. The method comprises cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 - 600°C is reached and coiling thereafter. The method also comprises heating the hot rolled (HR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) and soaking at the first predetermined temperature for a first predetermined time of 30-90 minutes. The method further comprises quenching the hot rolled (HR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain a hot rolled quenched (HR+Q) steel. The method comprises performing a partitioning treatment by reheating the hot rolled quenched (HR+Q) steel sheet to a second predetermined temperature ranging below critical temperature (below Ac1) for a second predetermined time of less than 20 minutes. The method also comprises cooling the partitioned steel sheet in a bath at a fourth cooling rate to obtain a hot rolled low carbon micro-alloyed steel (HR + Q&P) sheet.
In an embodiment, a method for manufacturing the cold rolled low carbon micro-alloyed steel is provided. The method comprises casting steel having a composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab. The method also comprises reheating the steel slab to a temperature greater than 1200°C. The method further comprises hot rolling the steel slab to produce a hot rolled (HR) steel sheet such that finish rolling is done at a finish rolling temperature (TFRT) in the range 840oC to 880oC. The method comprises cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 - 600°C is reached and coiling thereafter. The method also comprises annealing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 40 – 100 minutes. The method comprises cold rolling the hot rolled (HR) and annealed steel sheet to obtain a cold rolled (CR) steel sheet. The method also comprises heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) and soaking at the first predetermined temperature for a first predetermined time of 30-90 minutes. The method further comprises quenching the cold rolled (CR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain a cold rolled quenched (CR+Q) steel sheet. The method comprises performing a partitioning treatment by reheating the cold rolled quenched (CR+Q) steel sheet to a second predetermined temperature ranging below critical temperature (below Ac1) for a second predetermined time of less than 20 minutes. The method also comprises cooling the partitioned steel sheet in a bath at a fourth cooling rate to obtain a cold rolled low carbon micro-alloyed steel (CR + Q&P) sheet.
In an embodiment, the first predetermined temperature is about 840 °C and the first predetermined time is about 60 minutes.
In an embodiment, the second predetermined temperature is about 600 °C and the second predetermined time is about 15 minutes.
In an embodiment, the third predetermined temperature is below 40°C and the third cooling rate is between 20-120°C/s.
In an embodiment, the third predetermined temperature is room temperature and the third cooling rate of 100 °C/s.
In an embodiment, the fourth cooling rate is between 20-120°C/s, more preferably at 100 °C/s.
In an embodiment, the hot rolling process carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the hot rolled (HR) steel sheet to about 2 - 5 mm.
In an embodiment, the hot rolled (HR) and annealed steel sheet is cold rolled in five consecutive passes with a total von-Mises strain of 2.1 to obtain the cold rolled (CR) steel sheet.
In an embodiment, the hot rolled quenched (HR+Q) low carbon micro-alloyed steel exhibits a ferrite grain size of 6.5±2.5 µm, a lath martensite volume fraction of 33.2±3.8%, an interlath distance of 0.6±0.15µm, a ductility of 37%, an ultimate tensile strength of 541MPa, isotropic properties r ¯ of 1.04, ?r of -0.19 and strain hardening exponent (n) of 0.26.
In an embodiment, the hot rolled quenched and partitioned (HR+Q &P) low carbon micro-alloyed steel exhibits a ferrite grain size of 6.4±3.4µm, a lath martensite volume fraction of 33.6±1.2%, an interlath distance of 0.65±0.21µm, a ductility of 46%, an ultimate tensile strength of 544MPa, isotropic properties r ¯ of 0.99, ?r of -0.09, and strain hardening exponent (n) of 0.32.
In an embodiment, the cold rolled quenched (CR+Q) low carbon micro-alloyed steel exhibits a ferrite grain size of 8.9±3.6µm, island martensite volume fraction of 32.1±1.0%, average martensite island size of 4.2±1.1µm, a ductility of 34%, an ultimate tensile strength of 543MPa, and strain hardening exponent (n) of 0.18.
In an embodiment, the cold rolled quenched and partitioned (CR+Q &P) low carbon micro-alloyed steel exhibits a ferrite grain size of 5.5±3.4µm, island martensite volume fraction of 34.2±1.1%, average martensite island size of 4.1±0.7µm, a ductility of 51%, an ultimate tensile strength of 544MPa, isotropic properties r ¯ of 0.93, ?r of 0.35 and strain hardening exponent (n) of 0.35.
In an embodiment, the hot rolled quenched (HR+Q) low carbon micro-alloyed steel exhibits 18.6% CSL boundaries and 81.4% RHAGB boundaries in the microstructure.
In an embodiment, the hot rolled quenched and partitioned (HR+Q&P) low carbon micro-alloyed steel exhibits 25.8% CSL boundaries and 74.2% RHAGB boundaries in the microstructure.
In an embodiment, the cold rolled quenched (CR+Q) low carbon micro-alloyed steel and the cold rolled low carbon micro-alloyed (CR + Q&P) steel exhibits 12.3% CSL boundaries and 87.7% RHAGB boundaries in the microstructure.
In an embodiment, the bath is water bath. In an embodiment, the steel having a composition expressed in weight %: C - 0.053, Mn - 1.32, Si - 0.35, Cr - 0.61, Nb - 0.015, Al - 0.037, Ca - 0.0016, N - 60 ppm, B - 0.0005, and the balance being Iron (Fe) and unavoidable impurities is used during casting.
In an embodiment, the quenching and partitioning of the steel leads to transformation induced plasticity (TRIP) effect.
In an embodiment, the presence of the CSL boundaries in the random high angle boundaries (RHAGB) network weakens the crystallographic texture. In an embodiment, CR+Q&P stabilizes retained austenite by Mn and C partitioning from the nearby martensitic phases. In an embodiment, HR+Q&P stabilizes retained austenite by Mn and C partitioning from the nearby martensitic phases.
Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1a illustrates a preferred example of production line for manufacturing hot rolled low carbon micro-alloyed (HR+Q, and HR+Q&P) steel, according to an embodiment of the present invention
Figure 1b illustrates a flowchart of method for manufacturing hot rolled low carbon micro-alloyed (HR+Q, and HR+Q&P) steel, according to an embodiment of the present invention;
Figure 2a illustrates a preferred example of production line for manufacturing cold rolled low carbon micro-alloyed (CR+Q, and CR+Q&P) steel, according to an embodiment of the present invention;
Figures 2b and 2c illustrate a flowchart of method for manufacturing cold rolled low carbon micro-alloyed (CR+Q, and CR+Q&P) steel, according to an embodiment of the present invention;
Figure 3a illustrates engineering stress-strain curves of HR+Q, CR+Q, HR+Q&P and CR+Q&P steel samples, according to an embodiment of the present invention;
Figure 3b illustrates inset of engineering stress-strain curve highlighting TRIP effect in HR+Q&P and CR+Q&P steel samples, according to an embodiment of the present invention;
Figure 4a illustrates TEM Microstructure of HR+Q&P steel revealing the microstructural features inside the island structure, according to an embodiment of the present invention;
Figure 4b illustrates SADP pattern marked from island martensitic structure showing the presence of retained austenite, according to an embodiment of the present invention;
Figures 5a, 5b, and 5c illustrate (a) EBSD IQ Map, (b) Grain Size distribution and (c) 110 Pole Figure of HR+Q steel, according to an embodiment of the present invention;
Figures 6a, 6b, and 6c illustrate (a) EBSD IQ Map, (b) Grain Size distribution and (c) 110 Pole Figure of HR+Q&P steel, according to an embodiment of the present invention;
Figures 7a, 7b, and 7c illustrate (a) EBSD IQ Map, (b) Grain Size distribution and (c) 110 Pole Figure of CR+Q steel, according to an embodiment of the present invention; and
Figures 8a, 8b, and 8c illustrate (a) EBSD IQ Map, (b) Grain Size distribution and (c) 110 Pole Figure of CR+Q&P steel, according to an embodiment of the present invention.
The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.

DETAILED DESCRIPTION
The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The present disclosure provides a hot rolled or cold rolled low carbon micro-alloyed steel having high ductility, high strength and isotropy combination. The low carbon micro-alloyed steel comprises the following composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities. In the preferred embodiment, the low carbon micro-alloyed steel comprises the composition expressed in weight %: as shown in Table 1.
Element C Mn Si Cr Nb Al Ca B N
Wt. % 0.053 1.32 0.35 0.61 0.015 0.037 0.0016 0.0005 60 ppm
Table 1. Chemical Co¬¬mposition of low carbon micro alloyed steel
The hot rolled or cold rolled low carbon micro-alloyed steel comprises a structure including, by volume, at least 63% of ferrite, between 29% to 37 % of martensite, and a small fraction of retained austenite. More preferably, the steel comprises a structure including the ferrite phase 63 – 71%, martensite phase 29 – 37 % and a small fraction of retained austenite.
The hot rolled or cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 450 MPa. More preferably, the low carbon micro-alloyed steel exhibits tensile strength ranging from about 500 MPa to 600 MPa. In the preferred embodiment, the low carbon micro-alloyed steel exhibits tensile strength ranging from about 530 MPa to 550 MPa. The hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength of at least 250 MPa. More preferably, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 250 MPa to 450 MPa. In the preferred embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 260 MPa to 420 MPa.
In some embodiments, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 30%. More preferably, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 30 % to 60 %. In the preferred embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 43 % to 52%.
In some embodiments, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.25 to 0.4. In some embodiments, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 0.9 to 1.05. In the preferred embodiment, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 0.93 to 1.04. The hot rolled or cold rolled low carbon micro-alloyed steel exhibits a unique combination of high ductility, high strain hardening exponent, strength and isotropy
Referring to Figures 1a and 1b, a preferred productive line and an exemplary method (100) of manufacturing the hot rolled low carbon micro-alloyed steel is illustrated. The method (100) initiates at step (102). At step (102), the method (100) comprises casting steel having a composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities to obtain steel slab. In the illustrated example, the steel is cast either in a conventional continuous caster or in a thin-slab caster. In the preferred embodiment, the steel having a composition expressed in weight %: C - 0.053, Mn - 1.32, Si - 0.35, Cr - 0.61, Nb - 0.015, Al - 0.037, Ca - 0.0016, N - 60 ppm, B - 0.0005, and the balance being Iron (Fe) and unavoidable impurities is used.
At step (104), the method (100) comprises reheating the steel slab to a temperature greater than 1200°C. In the preferred embodiment, the steel slab is reheated to temperature ranging between 1200 to 1250oC for a duration of 2.75 hours to 3.25 hours depending on the steel slab thickness.
At step (106), the method (100) comprises hot rolling the steel slab to produce a hot rolled (HR) steel sheet such that finish rolling is done at a finish rolling temperature (TFRT). The TFRT varies in the range 840oC to 880oC. In a conventional hot-strip mill, the steel slab is rough-rolled in the roughing stands above the austenite recrystallization temperature and then subsequently hot-rolled or finish rolled below the recrystallization temperature. The hot rolling process may be carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the hot rolled (HR) steel sheet to about 2 - 5 mm.
At step (108), the method (100) comprises cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 to 600oC is reached and coiling thereafter.
At step (110), the method (100) comprises heating the hot rolled (HR) steel sheet obtained in step (108) to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) and soaking at the first predetermined temperature for a first predetermined time of 30-90 minutes. In the preferred embodiment, the hot rolled (HR) steel sheet is heated to the first predetermined temperature of 840°C and soaked at the first predetermined temperature of 840°C for the first predetermined time of 60 minutes.
At step (112), the method (100) comprises quenching the hot rolled (HR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain a hot rolled quenched (HR+Q) steel. In some embodiments, the hot rolled (HR) and heated steel sheet is quenched to the third predetermined temperature below 40°C in either a water bath or an oil bath at 20-120°C/s third cooling rate to obtain the hot rolled quenched (HR+Q) steel. In the preferred embodiment, the hot rolled (HR) and heated steel sheet is quenched to room temperature in water bath at 100°C/s third cooling rate to obtain the hot rolled quenched (HR+Q) steel sheet.
At step (114), the method (100) comprises performing a partitioning treatment by reheating the hot rolled quenched (HR+Q) steel to a second predetermined temperature ranging below critical temperature (below Ac1) for a second predetermined time of less than 20 minutes. In the preferred embodiment, partitioning treatment is performed by reheating the hot rolled quenched (HR+Q) steel to the second predetermined 600°C for a second predetermined time of 15 minutes.
At step (116), the method (100) comprises cooling the partitioned steel in a bath at a fourth cooling rate to obtain a hot rolled low carbon micro-alloyed steel (HR + Q&P) sheet. In some embodiments, the partitioned steel is cooled in in either a water bath or an oil bath at 20-120°C/s fourth cooling rate to obtain hot rolled low carbon micro-alloyed steel (HR + Q&P) sheet. In the preferred embodiment, the partitioned steel is cooled to room temperature in water bath at 100°C/s third cooling rate to obtain the hot rolled quenched and partitioned (HR+Q &P) steel sheet.
In the preferred embodiment, the hot rolled quenched (HR+Q) steel sheet exhibits a tensile strength of 541 MPa with 37% elongation. The hot rolled quenched (HR+Q) steel sheet exhibits a strain hardening exponent of 0.26. The hot rolled quenched (HR+Q) steel sheet exhibits isotropic properties, r ¯ of 1.04 and ?r of -0.19. The hot rolled quenched (HR+Q) steel sheet exhibits a ferrite grain size of 6.5±2.5µm, a lath martensite volume fraction of 33.2±3.8% and an interlath distance of 0.60±0.15µm. The hot rolled quenched (HR+Q) steel sheet exhibits high 18.6% coincident site lattice (CSL) boundaries in the microstructure. The hot rolled quenched (HR+Q) steel sheet exhibits high 81.4% RHAGB (random high angle grain boundaries) in the microstructure.
In the preferred embodiment, the hot rolled quenched and partitioned (HR+Q &P) steel sheet exhibits a tensile strength of 544 MPa with 46% elongation. The hot rolled quenched and partitioned (HR+Q &P) steel sheet exhibits a strain hardening exponent of 0.32. exhibits isotropic properties, r ¯ of 0.99 and ?r of -0.09. The hot rolled quenched and partitioned (HR+Q &P) steel sheet exhibits a ferrite grain size of 6.4±3.4µm, a lath martensite volume fraction of 33.6±1.2% and an interlath distance of 0.65±0.21µm. The hot rolled quenched and partitioned (HR+Q &P) steel sheet exhibits high 25.8% CSL boundaries in the microstructure. The hot rolled quenched and partitioned (HR+Q &P) steel sheet exhibits high 74.2% RHAGB in the microstructure. The hot rolled quenched and partitioned (HR+Q &P) steel sheet stabilizes retained austenite by Mn and C partitioning from the nearby martensitic phases. The method (100) (i.e. hot rolling and quenching and partitioning (HR+Q &P) leads to transformation induced plasticity (TRIP) effect (Figure 3a and 3b) and weakens the crystallographic texture (Figures 5a-5c, 6a-6c). The unique combination of strength-ductility-n-isotropy is attributed to TRIP effect, presence of CSL boundaries in the RHAGB network in the microstructure and weak crystallographic texture.
Referring to Figures 2a, 2b and 2c, a preferred production line and an exemplary method (200) of manufacturing the cold rolled low carbon micro-alloyed steel is illustrated. The method (200) initiates at step (202). At step (202), the method (200) comprises casting steel having a composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab. In an example, the steel is cast either in a conventional continuous caster or in a thin-slab caster. In the preferred embodiment, the steel having a composition expressed in weight %: C - 0.053, Mn - 1.32, Si - 0.35, Cr - 0.61, Nb - 0.015, Al - 0.037, Ca - 0.0016, N - 60 ppm, B - 0.0005, and the balance being Iron (Fe) and unavoidable impurities is used.
At step (204), the method (200) comprises reheating the steel slab to a temperature greater than 1200°C. In the preferred embodiment, the steel slab is reheated to temperature ranging between 1200 to 1250oC for a duration of 2.75 hours to 3.25 hours depending on the steel slab thickness.
At step (206), the method (200) comprises hot rolling the steel slab to produce a hot rolled (HR) steel sheet such that finish rolling is done at a finish rolling temperature (TFRT). The TFRT varies in the range 840oC to 880oC. In a conventional hot-strip mill, the steel slab is rough-rolled in the roughing stands above the austenite recrystallization temperature and then subsequently hot-rolled or finish rolled below the recrystallization temperature. The hot rolling process may be carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the hot rolled (HR) steel sheet to about 2 - 5 mm.
At step (208), the method (200) comprises cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 to 600oC is reached and coiling thereafter.
At step (210), the method (200) comprises annealing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 40 – 100 minutes. In the preferred embodiment, the hot rolled (HR) steel sheet is annealed at temperature of 900°C for a duration of 60 minutes.
At step (212), the method (200) comprises cold rolling the hot rolled (HR) and annealed steel sheet in five consecutive passes with a total von-Mises strain of 2.1 (equivalent to ~80% CR thickness reduction) to obtain a cold rolled (CR) steel sheet.
At step (214), the method (200) comprises heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) and soaking at the first predetermined temperature for a first predetermined time of 30-90 minutes. In the preferred embodiment, the cold rolled (CR) steel sheet is heated to the first predetermined temperature of 840°C and soaked at the first predetermined temperature of 840°C for the first predetermined time of 60 minutes.
At step (216), the method (200) comprises quenching the cold rolled (CR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain a cold rolled quenched (CR+Q) steel. In some embodiments, the cold rolled (CR) and heated steel sheet is quenched to the third predetermined temperature below 40°C in either a water bath or an oil bath at 20-120°C/s third cooling rate to obtain the cold rolled quenched (CR+Q) steel. In the preferred embodiment, the cold rolled (CR) and heated steel sheet is quenched to room temperature in water bath at 100°C/s third cooling rate to obtain the cold rolled quenched (CR+Q) steel sheet.
At step (218), the method (200) comprises performing a partitioning treatment by reheating the cold rolled quenched (CR+Q) steel to a second predetermined temperature ranging below critical temperature (below Ac1) for a second predetermined time of less than 20 minutes. In the preferred embodiment, partitioning treatment is performed by reheating the cold rolled quenched (CR+Q) steel to the second predetermined 600°C for a second predetermined time of 15 minutes.
At step (220), the method (200) comprises cooling the partitioned steel sheet in a bath at a fourth cooling rate to obtain a cold rolled quenched and partitioned (CR+Q&P) steel. In some embodiments, the partitioned steel is cooled in in either a water bath or an oil bath at 20-120°C/s fourth cooling rate to obtain cold rolled low carbon micro-alloyed steel (CR + Q&P) sheet. In the preferred embodiment, the partitioned steel is cooled to room temperature in water bath at 100°C/s third cooling rate to obtain the cold rolled quenched and partitioned (CR+Q &P) steel sheet.
In the preferred embodiment, the cold rolled quenched (CR+Q) steel sheet exhibits a tensile strength of 543 MPa with 34% elongation. The cold rolled quenched (CR+Q) steel sheet exhibits a strain hardening exponent of 0.18. The cold rolled quenched (CR+Q) steel sheet exhibits a ferrite grain size of 8.9±3.6µm, a lath martensite volume fraction of 32.1% and average island martensite size of 4.2±1.1µm. The cold rolled quenched (CR+Q) steel sheet exhibits ~12.3% CSL boundaries in the microstructure. The cold rolled quenched (CR+Q) steel sheet exhibits ~87.7% RHAGB in the microstructure.
In the preferred embodiment, the cold rolled quenched and partitioned (CR+Q&P) steel sheet exhibits a tensile strength of 544 MPa with 51% elongation. The cold rolled quenched and partitioned (CR+Q&P) steel sheet exhibits a strain hardening exponent of 0.35. The cold rolled quenched and partitioned (CR+Q&P) steel sheet exhibits isotropic properties, r ¯ of 0.93 and ?r of 0.35. The cold rolled quenched and partitioned (CR+Q&P) steel sheet exhibits a ferrite grain size of 5.5±3.4µm, a lath martensite volume fraction of 34.2±1.1% and average island martensite size of 4.1±0.7µm. The cold rolled quenched and partitioned (CR+Q&P) steel sheet exhibits ~12.3% CSL boundaries in the microstructure. exhibit ~87.7% RHAGB in the microstructure. The cold rolled quenched and partitioned (CR+Q&P) steel sheet stabilizes retained austenite by Mn and C partitioning from the nearby martensitic phases. The method (200) (i.e. cold rolling and quenching and partitioning (CR+Q&P)) leads to TRIP effect (Figures 3a and 3b) and weakens the crystallographic texture (Figure 7a-7c and 8a-8c).
The method (100) (i.e. hot rolling and quenching and partitioning (HR+Q&P)) and the method (200) (i.e. cold rolling and quenching and partitioning (CR+Q&P)) leads to TRIP effect in very low carbon grade steel. Due to which, a very significant improvement in ductility to 46%, n=0.32 and strength ~ 544MPa was observed after hot rolling (HR) and Q & P. Further improvement in ductility to ~51% and n=0.35 was observed in the cold-rolled Q & P steel with the same UTS. A high strain hardening exponent (n) obtained is helpful in enhancing the malleability/formability of the steel. After Q & P process, co-incidence site lattice (CSL) boundaries were observed, which are known to follow Kurdjamov-Sachs (K-S) transformation relation. The presence of the CSL boundaries in the random high angle boundaries (RHAGB) network weakens the crystallographic texture. Therefore, a weak texture was observed, and this adds to the isotropy of the material. The isotropy effect leads to normal anisotropy (r ¯) of ~1.0 in this low carbon micro-alloyed steel after Q & P.
Table 2 provides processing schedule followed during manufacture of the hot rolled and cold rolled low carbon micro alloyed steel.
Processing Schedule IA Quenching (Q) Partitioning (P)
840°C, 60min WQ @>100°C/s 600°C, 15min, WQ
Hot Rolling (HR) IA + Q
IA +Q & P
Cold Rolling (CR) IA + Q
IA +Q & P
Table 2. Intercritical annealing, quenching and partitioning processing schedule for hot rolled and cold rolled low carbon micro alloyed steel.
Table 3 provides mechanical Properties of different types of the low carbon micro-alloyed steel produced using methods (100, 200).
YS
(MPa) UTS
(MPa) Total Elongation Toughness
(J/m3) n r ¯ ?r
1. HR+Q 262 541 37 175 0.26 1.04 -0.19
2. HR+Q & P 413 544 46 227 0.32 0.99 -0.09
3. CR+Q 326 543 34 161 0.18 - -
4. CR+Q & P 396 544 51 242 0.35 0.93 0.35
Table 3. Mechanical Properties after the Q & P of low carbon micro-alloyed steel.
Figure 3a represents engineering stress-strain curves for HR+Q, CR+Q, HR+Q & P and CR+Q & P steel samples. In this figure, the TRIP effect was evident in CR+Q & P and HR+Q & P steels samples, which is shown clearly in the inset of Figure 3b (the portion of engineering stress-strain curve showing TRIP effect was rescaled). The TRIP effect was not observed in engineering stress-strain curve for HR+Q and CR+Q steel samples (Figure 3a). Figure 4a represents the TEM Microstructure of HR+Q & P steel revealing the inside of the island structure, and Figure 4b shows the selected area diffraction pattern (SADP) of the marked island martensitic structure in Figure 4a, confirming the presence of retained austenite. Partitioning treatment at 600°C for 15min is sufficient to stabilize the retained austenite by partitioning of Mn and C into retained austenite from the nearby martensitic phases in this new low carbon micro alloyed steel. During deformation, the retained austenite transforms to martensite and produces additional ductility.
Further electron backscattered diffraction (EBSD) analysis was carried out on all these samples to obtain quantitative microstructural and micro-textural information’s during Q and Q & P. Figures 5a-5c represents the EBSD image quality (EBSD-IQ) Map, Grain Size distribution, 110 Pole Figure (PF) for HR+Q steel, respectively. Figures 6a-6c, 7a-7c and 8a-8c represents similar information for HR+Q & P, CR+Q and CR+Q & P steels, respectively. The HR+Q and HR+Q & P have produced ferrite-lath martensite microstructure (Figures 5a-5c, 6a-6c), while CR+Q and CR+Q & P produced ferrite-island martensite microstructure (Figures 7a-&c, 8a-8c).
The HR+Q steel was reported to have a ferrite grain size of ~6.5±2.5 µm, lath martensite morphology with vol fraction of 33.2±3.8% and the martensite interlath spacing of 0.60±0.15 µm (Table 4 and Figures 5a-5c). The values of average ferrite grain size, martensite volume fraction and martensite interlath spacing were ~6.4±3.4 µm, 33.6±1.2% and 0.65±0.21 µm, for HR+Q & P steel, respectively (Table 4 and Figure 6a-6c). The grain size distribution in HR+Q and HR+Q & P steels were 0.7-13µm and 0.7-16µm, sequentially (Figures 5a-5c, 6a-6c).
The EBSD scan for CR+Q steel was taken on TD-RD plane instead of ND-RD plane for the rest of the samples. The average ferrite grain and martensite island sizes were reported to be ~8.9±3.6µm and 4.20±1.1µm with a martensite volume fraction of 32.1% (Table 4 and Figures 7a-7c). The grain size distribution were 0.4-14µm (Figures 7a-7c).
In CR+Q & P steel, average ferrite grain size was 5.5±3.4µm with martensite island size of 4.1±0.7µm (Table 4 and Figure 8a-8c). Martensite volume fraction was ~34.2±1.1% in this case. The grain size distribution was 0.4-15 µm.
The HR+Q, HR+Q & P, CR+Q and CR+Q & P process has produced 18.6%, 25.8%, 12.3% and 12.3% CSL boundary fractions in the microstructures, respectively (Table 4). The RHAGB in these processed steel samples were 81.4%, 74.2%, 87.7% and 87.7%, respectively (Table 4). The presence of these CSL boundaries in the RHAGB network leads to overall weakening in crystallographic texture (Table 4, Figures 5c, 6c, 7c, and 8c). Therefore, a weak 110 PF was observed for all the cases in this processing schedule of Q & Q & P of HR an CR steel (Figures 5c, 6c, 7c, and 8c). This weakening in texture is evident by a low multiple of random distribution (mrd) of 3.3 (Figures 5c, 6c, and 7c) in HR+Q, HR+Q & P and CR+Q steel; and 2.7 mrd in CR+Q & P steel (Figure 8c). The weak crystallographic texture incorporates isotropic behavior. Therefore, the Lankford parameter of normal anisotropy (r ¯) was very close to ~1 for isotropic material (Table 3).

%CSL %RHAGB Martensite
Vol % Type Inter-lath
/ island size (µm) Ferrite Grain Size (µm)
1. HR+Q 18.6 81.4 33.2±3.8 lath 0.60±0.15 6.5±2.5
2.HR+Q&P 25.8 74.2 33.6±1.2 lath 0.65±0.21 6.4±3.4
3. CR+Q 12.3 87.7 32.1 island 4.20±1.1 8.9±3.6
4. CR+Q&P 12.3 87.7 34.2±1.1 island 4.10±0.70 5.5±3.4
Table 4. Microstructural data after the Q and Q & P of low carbon micro-alloyed steel.

The present invention relates to the hot-rolled (HR) and cold-rolled (CR) low carbon micro-alloyed (C~0.05 and total alloying <3%) steels having improved strength-ductility balance and good formability to be used in automobile applications. The present invention also relates to the methods (100, 200) of manufacture the hot-rolled (HR) and cold-rolled (CR) low carbon micro-alloyed steel having high ductility, strength, high strain hardening exponent, and isotropy combination. The Q & P of the low carbon micro-alloyed steel has led to the stabilization of retained austenite, introducing TRIP effect for the first time in very low carbon grade steel. Components made of the hot-rolled (HR) and cold-rolled (CR) low carbon micro-alloyed steel of the present disclosure provides good crashworthiness and damage tolerance in the automobile sector. The strength-ductility combination obtained in the hot-rolled (HR) and cold-rolled (CR) low carbon micro-alloyed steel lies in the domain of the third generation of AHSS in the strength-elongation diagram.
Information relating to the processes such as descaling, pickling process as shown in the process Figures 1a, 2a are well known in the prior art and thus will not be described herein detail. In the above description, the unavoidable impurities may include, but are not limited to sulfur, phosphorus, other elements etc., without any limitations.
Furthermore, the terminology used herein is for describing embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims:CLAIMS

We Claim:
A hot rolled or cold rolled low carbon micro-alloyed steel comprising the following composition expressed in weight %:
Carbon (C): 0.03% - 0.08%,
Manganese (Mn): 1.00% - 1.6%,
Nitrogen (N): 70 ppm or less,
Silicon (Si): 0.2% - 0.6%,
Aluminium (Al): 0.02% - 0.06%,
Chromium (Cr): 0.3 - 0.9%,
Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities, wherein the steel comprises a structure including, by volume, at least 63% of ferrite, between 29% to 37 % of martensite, and a small fraction of retained austenite.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the steel comprises a structure including the ferrite phase 63 – 71%, martensite phase 29 – 37 % and a small fraction of retained austenite.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the hot rolled or cold rolled low carbon micro-alloyed steel comprises the composition expressed in weight %: C - 0.053, Mn - 1.32, Si - 0.35, Cr - 0.61, Nb - 0.015, Al - 0.037, Ca - 0.0016, N - 60 ppm, B - 0.0005, and the balance being Iron (Fe) and unavoidable impurities.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claims 1 to 3, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 450 MPa.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claim 4, wherein hot rolled or cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 500 MPa to 600 MPa.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claim 5, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 530 MPa to 550 MPa.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claims 1 to 4, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength of at least 250 MPa.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claim 7, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 250 MPa to 450 MPa.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in the claim 8, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 260 MPa to 420 MPa.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claims 1 to 7, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 30%.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claim 10, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 30 % to 60 %.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claim 11, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 43 % to 52%.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claims 1 to 10, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.25 to 0.4.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claims 1 to 13, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 0.9 to 1.05.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claim 14, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 0.93 to 1.04.
The hot rolled or cold rolled low carbon micro-alloyed steel as claimed in claims 1 to 14, wherein the hot rolled or cold rolled low carbon micro-alloyed steel exhibits a unique combination of high ductility, high strain hardening exponent, strength and isotropy.
A method (100) for manufacturing the hot rolled low carbon micro-alloyed steel as claimed in the claims 1 to 16, the method (100) comprising:
casting steel having a composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities to obtain steel slab;
reheating the steel slab to a temperature greater than 1200°C;
hot rolling the steel slab to produce a hot rolled (HR) steel sheet such that finish rolling is done at a finish rolling temperature (TFRT) in the range 840oC to 880oC;
cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 - 600°C is reached and coiling thereafter;
heating the hot rolled (HR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) and soaking at the first predetermined temperature for a first predetermined time of 30-90 minutes;
quenching the hot rolled (HR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain a hot rolled quenched (HR+Q) steel;
performing a partitioning treatment by reheating the hot rolled quenched (HR+Q) steel sheet to a second predetermined temperature ranging below critical temperature (below Ac1) for a second predetermined time of less than 20 minutes; and
cooling the partitioned steel sheet in a bath at a fourth cooling rate to obtain a hot rolled low carbon micro-alloyed steel (HR + Q&P) sheet.
A method (200) for manufacturing the cold rolled low carbon micro-alloyed steel as claimed in the claims 1 to 16, the method (200) comprising:
casting steel having a composition expressed in weight %: Carbon (C): 0.03% - 0.08%, Manganese (Mn): 1.00% - 1.6%, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.2% - 0.6%, Aluminium (Al): 0.02% - 0.06%, Chromium (Cr): 0.3 - 0.9%, Niobium (Nb): 0.04% or less, Calcium (Ca): 0.0030% or less, Boron (B): 0.003% or less, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab;
reheating the steel slab to a temperature greater than 1200°C;
hot rolling the steel slab to produce a hot rolled (HR) steel sheet such that finish rolling is done at a finish rolling temperature (TFRT) in the range 840oC to 880oC;
cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 - 600°C is reached and coiling thereafter;
annealing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 40 – 100 minutes;
cold rolling the hot rolled (HR) and annealed steel sheet to obtain a cold rolled (CR) steel sheet;
heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) and soaking at the first predetermined temperature for a first predetermined time of 30-90 minutes;
quenching the cold rolled (CR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain a cold rolled quenched (CR+Q) steel sheet;
performing a partitioning treatment by reheating the cold rolled quenched (CR+Q) steel sheet to a second predetermined temperature ranging below critical temperature (below Ac1) for a second predetermined time of less than 20 minutes; and
cooling the partitioned steel sheet in a bath at a fourth cooling rate to obtain a cold rolled low carbon micro-alloyed steel (CR + Q&P) sheet.
The method (100, 200) as claimed in the claims 17 and 18, wherein the first predetermined temperature is about 840 °C and the first predetermined time is about 60 minutes.
The method (100, 200) as claimed in the claims 17 and 18, wherein the second predetermined temperature is about 600 °C and the second predetermined time is about 15 minutes.
The method (100, 200) as claimed in the claims 17 and 18, wherein the third predetermined temperature is below 40°C and the third cooling rate is between 20-120°C/s.
The method (100, 200) as claimed in the claim 21, wherein the third predetermined temperature is room temperature and the third cooling rate of 100 °C/s.
The method (100, 200) as claimed in the claim 29, wherein the fourth cooling rate is between 20-120°C/s, more preferably at 100 °C/s.
The method (100, 200) as claimed in the claims 17 and 18, wherein the hot rolling process carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the hot rolled (HR) steel sheet to about 2 - 5 mm.
The method (200) as claimed in the claim 18, wherein the hot rolled (HR) and annealed steel sheet is cold rolled in five consecutive passes with a total von-Mises strain of 2.1 to obtain the cold rolled (CR) steel sheet.
The method (100) as claimed in the claim 17, wherein the hot rolled quenched (HR+Q) low carbon micro-alloyed steel exhibits a ferrite grain size of 6.5±2.5 µm, a lath martensite volume fraction of 33.2±3.8%, an interlath distance of 0.6±0.15µm, a ductility of 37%, an ultimate tensile strength of 541MPa, isotropic properties r ¯ of 1.04, ?r of -0.19 and strain hardening exponent (n) of 0.26.
The method (100) as claimed in the claim 17, wherein the hot rolled quenched and partitioned (HR+Q &P) low carbon micro-alloyed steel exhibits a ferrite grain size of 6.4±3.4µm, a lath martensite volume fraction of 33.6±1.2%, an interlath distance of 0.65±0.21µm, a ductility of 46%, an ultimate tensile strength of 544MPa, isotropic properties r ¯ of 0.99, ?r of -0.09, and strain hardening exponent (n) of 0.32.
The method (200) as claimed in the claim 18, wherein the cold rolled quenched (CR+Q) low carbon micro-alloyed steel exhibits a ferrite grain size of 8.9±3.6µm, island martensite volume fraction of 32.1±1.0%, average martensite island size of 4.2±1.1µm, a ductility of 34%, an ultimate tensile strength of 543MPa, and strain hardening exponent (n) of 0.18.
The method (200) as claimed in the claim 18, wherein the cold rolled quenched and partitioned (CR+Q &P) low carbon micro-alloyed steel exhibits a ferrite grain size of 5.5±3.4µm, island martensite volume fraction of 34.2±1.1%, average martensite island size of 4.1±0.7µm, a ductility of 51%, an ultimate tensile strength of 544MPa, isotropic properties r ¯ of 0.93, ?r of 0.35 and strain hardening exponent (n) of 0.35.
The method (100) as claimed in the claims 17, wherein the hot rolled quenched (HR+Q) low carbon micro-alloyed steel exhibits 18.6% CSL boundaries and 81.4% RHAGB boundaries in the microstructure.
The method (100) as claimed in the claims 17, wherein the hot rolled quenched and partitioned (HR+Q&P) low carbon micro-alloyed steel exhibits 25.8% CSL boundaries and 74.2% RHAGB boundaries in the microstructure.
The method (200) as claimed in the claims 18, wherein the cold rolled quenched (CR+Q) low carbon micro-alloyed steel and the cold rolled low carbon micro-alloyed (CR + Q&P) steel exhibits 12.3% CSL boundaries and 87.7% RHAGB boundaries in the microstructure.
The method (100, 200) as claimed in the claims 17 and 18, wherein the bath is water bath.
The method (100, 200) as claimed in the claims 17 and 18, wherein the steel having a composition expressed in weight %: C - 0.053, Mn - 1.32, Si - 0.35, Cr - 0.61, Nb - 0.015, Al - 0.037, Ca - 0.0016, N - 60 ppm, B - 0.0005, and the balance being Iron (Fe) and unavoidable impurities is used during casting.
The method (100, 200) as claimed in the claims 17 and 18, wherein quenching and partitioning of the steel leads to transformation induced plasticity (TRIP) effect.
The method (100, 200) as claimed in the claims 30 to 32, wherein the presence of the CSL boundaries in the random high angle boundaries (RHAGB) network weakens the crystallographic texture.
The method (100) as claimed in the claim 17, wherein the HR+Q&P stabilizes retained austenite by Mn and C partitioning from the nearby martensitic phases.
The method (200) as claimed in the claim 18, wherein the CR+Q&P stabilizes retained austenite by Mn and C partitioning from the nearby martensitic phases.