We Claim:
A cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics, the steel comprising the following composition expressed in weight %:
Carbon (C): 0.04% - 0.08%,
Manganese (Mn): 1.2% - 1.5%,
Sulphur (S): 0.003% - 0.004%,
Phosphorus (P): 0.014% or less,
Nitrogen (N): 70 ppm or less,
Silicon (Si): 0.25% - 0.5%,
Aluminium (Al): 0.02% - 0.04%,
Chromium (Cr): 0.5 - 0.75%,
Molybdenum (Mo): 0.002% or less,
Copper (Cu): 0.009% or less,
Vanadium (V): 0.001% or less,
Titanium (Ti): 0.01% or less,
Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% or less and the balance being Iron (Fe) and unavoidable impurities, wherein the steel comprises a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the cold rolled low carbon micro-alloyed steel comprises the composition expressed in weight %: Carbon (C): 0.053%, Manganese (Mn): 1.32%, Sulphur (S): 0.003%, Phosphorus (P): 0.014%, Nitrogen (N): 60 ppm, Silicon (Si): 0.35%, Aluminium (Al): 0.037%, Chromium (Cr): 0.61%, Molybdenum (Mo): 0.001%, Copper (Cu): 0.0083%, Vanadium (V): 0.001%, Niobium (Nb): 0.015%, Titanium (Ti): 0.001%, Calcium (Ca): 0.0016%, Boron (B): 0.005%, Nickel (Ni): 0.019%, and the balance being Iron (Fe) and unavoidable impurities.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 600 MPa.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 3, wherein the cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 600 MPa to 1301 MPa.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 4, wherein the cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 624 MPa to 1301 MPa.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 2, wherein the cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 19%.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 19% to 48%.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.10 to 0.3.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein the cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 1.07 to 1.85.
The cold rolled low carbon micro-alloyed steel as claimed in the claims 1 to 9, wherein the cold rolled low carbon micro-alloyed steel exhibits a unique combination of high r ¯ (Normal Anisotropy), high strength and ductility.
The cold rolled low carbon micro-alloyed steel as claimed in the claim 1, wherein a minimal carbon of about 0.053 wt.% and microalloying kept at a minimum of ~2.56% of the cold rolled low carbon micro-alloyed steel provides good weldability with minimum segregation effects.
A method (100) of manufacturing the cold rolled low carbon micro-alloyed steel sheet or strip or blank, as claimed in the claims 1 to 10, wherein the method (100) comprises:
producing a hot rolled steel sheet by hot rolling a slab having chemical composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% or less, and the balance being Iron (Fe) and unavoidable impurities at a rolling finishing temperature of 840oC to 880oC to obtain a hot rolled (HR) steel sheet;
cooling the hot rolled (HR) steel sheet till a coiling temperature (TCT) in the range 560 - 600°C is reached and coiling thereafter;
normalizing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 5 – 100 minutes;
cold rolling the hot rolled (HR) and normalized 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) of 750- 920 °C and soaking at the first predetermined temperature for a first predetermined time of 5-30 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 the cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
The method (100) as claimed in the claim 12, wherein the hot rolled (HR) and normalized steel sheet is cold rolled in five consecutive passes with rolling reduction per pass in the range of 25-34%, and with a total accumulated von-Mises strain of 2.1 to obtain the cold rolled (CR) steel sheet.
The method (100) as claimed in the claim 12, wherein the third predetermined temperature is below 40°C and the third cooling rate is between 70-120°C/s.
The method (100) as claimed in the claim 14, wherein the third predetermined temperature is room temperature and the third cooling rate of 100 °C/s.
The method (100) as claimed in the claims 12 to 15, wherein the first predetermined temperature is 750 °C and first predetermined time of 5 minutes.
The method (100) as claimed in the claim 16, wherein the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 750 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.15Martensite dual-phase microstructure with an average ferrite grain size of 6.4±2.2µm and avg. Martensite island size of 2.9±0.78µm, 13.8% CSL boundary and 86.2% RHAGB boundaries in the microstructure, an UTS of 624MPa with 48% ductility, yield strength of 401 MPa, a strain hardening exponent (n) of 0.30, isotropic properties of r ¯ of 1.80 with a low ?r of -0.13, a toughness of 260 J/m3.
The method (100) as claimed in the claims 12 to 15, wherein the first predetermined temperature is 800 °C and first predetermined time of 5 minutes.
The method (100) as claimed in the claim 18, wherein the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.31Martensite dual-phase microstructure with an average ferrite grain size of 6.8±2.6µm and avg. Martensite island size of 3.0±0.99µm, 18.4% CSL boundary and 81.6% RHAGB boundaries in the microstructure, a UTS of 850MPa with 31% ductility, a yield strength of 464 MPa, a strain hardening exponent (n) of 0.20, isotropic properties of r ¯ of 1.85 with a low ?r of 0.21, a toughness of 233 J/m3.
The method (100) as claimed in the claims 12 to 15, wherein the first predetermined temperature is 800 °C and first predetermined time of 30 minutes.
The method (100) as claimed in the claim 20, wherein the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 30 minutes exhibits a Ferrite-0.36Martensite dual-phase microstructure with an average ferrite grain size of 7.2±2.5µm and avg. Martensite island size of 3.7±0.78µm, 16.2% CSL boundary and 83.8% RHAGB boundaries in the microstructure, a UTS of 760MPa with 41% ductility, a yield strength of 398 MPa, a strain hardening exponent (n) of 0.24, isotropic properties of r ¯ of 1.1 with a low ?r of 0.13, a toughness of 276 J/m3.
The method (100) as claimed in the claims 12 to 15, wherein the first predetermined temperature is 840 °C and first predetermined time of 5 minutes.
The method (100) as claimed in the claim 22, wherein the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 840 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.39Martensite dual-phase microstructure with an average ferrite grain size of 6.0±2.6µm and avg. Martensite island size of 3.8±1.2µm, 13.1% CSL boundary and 86.9% RHAGB boundaries in the microstructure, a UTS of 959MPa with 30% ductility, a yield strength of 554 MPa, a strain hardening exponent (n) of 0.17, isotropic properties of r ¯ of 1.07 with a low ?r of 0.12, a toughness of 253 J/m3.
A component produced from the cold rolled low carbon micro-alloyed steel as claimed in the claims 1 to 23, wherein the component is used in structural as well as automotive applications.
, Description:FIELD OF INVENTION
The present invention relates to a cold rolled low carbon micro-alloyed steel and more particularly, to the cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics, 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). Of the advanced high strength steels (AHSS) family, dual-phase steels are important members as they provide high strength and high strain hardening ability at a very nominal cost. Lean alloying and simple thermo-mechanical treatments have been used in these kinds of steels. DP steels are produced industrially by hot rolling and/or cold rolling, followed by Intercritical annealing (IA).
Following papers describe the methodology used in the production of DP steels: “A novel route for the development of ultra-high strength dual phase steels” by Y. Mazaheri, A. Kermanpur, A. Najafizadeh in Mater. Sci. Eng. A, vol. 619, 2014, pp. 1-11; “High strength-elongation balance in ultrafine grained ferrite-martensite dual phase steels developed by thermomechanical processing” by Y. Mazaheri, A. H. Jahanara, M. Sheikhi, A. G. Kalashami in Mater. Sci. Eng. A, vol. 761, 2019, pp. 138021; “Microstructure and mechanical properties of V-Nb microalloyed ultrafine-grained dual phase steels processed through severe cold rolling and intercritical annealing” by M. P. Rao, V. S. Sharma, S. Sankaran in Metall. Mater. Trans. A, 48A, 2017, pp. 1176-1188, and “Development of a high strength and ductile Nb-bearing dual phase steel by cold-rolling and intercritical annealing of the ferrite-martensite microstructures” by A. G. Kalashami, A. Kermanpur, A. Najafizadeh, Y. Mazaheri in Mater. Sci. Eng. A, vol. 658, 2016, pp. 355-366.
Furthermore, the United States patents US 2015/0071811 A1 (2015), US20100252149A1 (2010), US20090071574A1 (2009), US20080099109A1 (2008), US20080166257A1 (2008), US Patent US20060108035A1 (2006), US Patent Application No- 005139580A (1992), US Patent Application No- 4,609,410 (1986), US Patent Application No- 4,376,661 (1983); Canadian patent CA 2837049 A1 (2012); European patent EP 3 421 629 A1 (2019); International Patents WO 2020/ 002285 A1(2020), WO 2019/154819 A1 (2019), WO 2014/149505 A1 (2014), WO 2008/082134 A1 (2008), WO2007/051080A2 (2007) and WO96/17965 (1996) also contributed in the development of DP steels.
The other unconventional methods of production of DP steels include severe plastic deformation techniques such as equal channel angular pressing (ECAP) followed by Intercritical annealing. The papers entitled, “Ultrafine grained ferrite-martensite dual phase steels fabricated via equal channel angular pressing: microstructure and tensile properties by Y. II. Son, Y. K. Lee, K. T. Park, C. S. Lee, D. H. Shin in Acta Mater. 53 (2005) 3125–3134; and “Fabrication of ultrafine grained ferrite/martensite dual phase steel by severe plastic deformation, by K. T. Park, Y. K. Lee, D. H. Shin published in ISIJ Int. 45 (2005) 750–755 have successfully produced DP steels using ECAP and ECAP + Intercritical annealing, respectively, in a DP steel containing 0.15 C, 1.1 Mn, 0.25 Si. However, the properties observed (504-605 MPa YS, 648-1044 MPa UTS, 9.8-11.5% UE and TE 13.1-22%) were similar to the properties observed through conventional processing routes.
One of the highest reported strength in DP steels is 1914MPa with 4% ductility in the paper entitled “Effect of microvoid formation on the tensile properties of dual-phase steel” by E. Ahmed, T. Manzoor, K. L. Ali, J. I. Akhter, published in J. Mater. Eng. Perform, vol. 9, 2000, pp. 306-310. One of the highest ductility is reported in patent US20080166257A1, with 341MPa UTS, n=0.25 and r ¯=1.18. The highest reported r ¯= 1.94 was reported in a US Patent Application No- 4609410, in the year 1986, for a DP steel with 412 MPa UTS, n=0.258 and 33.7% total elongation. The highest reported n was also observed in this patent for a DP steel.
However, there has been increasing industrial demand to produce ductile, damage-tolerant DP steel with high UTS suitable for secondary sheet metal forming operations, such as deep drawing. Alloy designing and thermomechanical processing (TMP) parameters optimization are very important to be carried out in a DP to obtain a combination of high strength, high n, high normal anisotropy, and lower planer anisotropy for the best sheet metal 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 the combination of high r ¯ (Normal Anisotropy), high strength and ductility 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 cold-rolled (CR) fine grained low carbon micro-alloyed dual-phase steels having very high ductility, strength, high strain hardening exponent, and isotropy combination.
Another objective of the present invention is to provide a new DP steel with an ultimate tensile strength (UTS) in the range of 624-1301MPa, ductility in the range of 19-48%, n in the range of 0.17-0.30 and r ¯ in the ranges of 1.07-1.85 in a combination of low ?r.
It is yet another objective of the present invention, to develop a method of the manufacturing the cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics.
SUMMARY OF INVENTION
This summary is provided to introduce concepts related to a cold rolled low carbon micro-alloyed steel and method of manufacturing thereof. 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 cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics is provided. The steel comprising the following composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% or less and the balance being Iron (Fe) and unavoidable impurities. The steel comprises a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
In an embodiment, the cold rolled low carbon micro-alloyed steel comprises the composition expressed in weight %: Carbon (C): 0.053%, Manganese (Mn): 1.32%, Sulphur (S): 0.003%, Phosphorus (P): 0.014%, Nitrogen (N): 60 ppm, Silicon (Si): 0.35%, Aluminium (Al): 0.037%, Chromium (Cr): 0.61%, Molybdenum (Mo): 0.001%, Copper (Cu): 0.0083%, Vanadium (V): 0.001%, Niobium (Nb): 0.015%, Titanium (Ti): 0.001%, Calcium (Ca): 0.0016%, Boron (B): 0.005%, Nickel (Ni): 0.019%, and the balance being Iron (Fe) and unavoidable impurities.
In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 600 MPa. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 600 MPa to 1301 MPa. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits tensile strength ranging from about 624 MPa to 1301 MPa.
In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 19%. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 19% to 48%.
In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.10 to 0.3. In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 1.07 to 1.85.
In an embodiment, the cold rolled low carbon micro-alloyed steel exhibits a unique combination of high r ¯ (Normal Anisotropy), high strength and ductility.
In an embodiment, a minimal carbon of about 0.053 wt.% and microalloying kept at a minimum of ~2.56% of the cold rolled low carbon micro-alloyed steel provides good weldability with minimum segregation effects.
In an embodiment, a method of manufacturing the cold rolled low carbon micro-alloyed steel sheet or strip or blank is provided. The method comprising producing a hot rolled steel sheet by hot rolling a slab having chemical composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% or less, and the balance being Iron (Fe) and unavoidable impurities at a rolling finishing temperature of 840oC to 880oC to obtain a hot rolled (HR) steel sheet. The method also 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 further comprises normalizing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 5 – 100 minutes. The method comprising cold rolling the hot rolled (HR) and normalized steel sheet to obtain a cold rolled (CR) steel sheet. The method further comprising heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) of 750- 920 °C and soaking at the first predetermined temperature for a first predetermined time of 5-30 minutes. The method comprising quenching the cold rolled (CR) and heated steel sheet to a third predetermined temperature in a bath at a third cooling rate to obtain the cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
In an embodiment, the hot rolled (HR) and normalized steel sheet is cold rolled in five consecutive passes with rolling reduction per pass in the range of 25-34%, and with a total accumulated von-Mises strain of 2.1 to obtain the cold rolled (CR) steel sheet.
In an embodiment, the third predetermined temperature is below 40°C and the third cooling rate is between 70-120°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 first predetermined temperature is 750 °C and first predetermined time of 5 minutes.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 750 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.15Martensite dual-phase microstructure with an average ferrite grain size of 6.4±2.2µm and avg. Martensite island size of 2.9±0.78µm, 13.8% CSL boundary and 86.2% RHAGB boundaries in the microstructure, an UTS of 624MPa with 48% ductility, yield strength of 401 MPa, a strain hardening exponent (n) of 0.30, isotropic properties of r ¯ of 1.80 with a low ?r of -0.13, a toughness of 260 J/m3.
In an embodiment, the first predetermined temperature is 800 °C and first predetermined time of 5 minutes.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.31Martensite dual-phase microstructure with an average ferrite grain size of 6.8±2.6µm and avg. Martensite island size of 3.0±0.99µm, 18.4% CSL boundary and 81.6% RHAGB boundaries in the microstructure, a UTS of 850MPa with 31% ductility, a yield strength of 464 MPa, a strain hardening exponent (n) of 0.20, isotropic properties of r ¯ of 1.85 with a low ?r of 0.21, a toughness of 233 J/m3.
In an embodiment, the first predetermined temperature is 800 °C and first predetermined time of 30 minutes.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 30 minutes exhibits a Ferrite-0.36Martensite dual-phase microstructure with an average ferrite grain size of 7.2±2.5µm and avg. Martensite island size of 3.7±0.78µm, 16.2% CSL boundary and 83.8% RHAGB boundaries in the microstructure, a UTS of 760MPa with 41% ductility, a yield strength of 398 MPa, a strain hardening exponent (n) of 0.24, isotropic properties of r ¯ of 1.1 with a low ?r of 0.13, a toughness of 276 J/m3.
In an embodiment, the first predetermined temperature is 840 °C and first predetermined time of 5 minutes.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 840 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.39Martensite dual-phase microstructure with an average ferrite grain size of 6.0±2.6µm and avg. Martensite island size of 3.8±1.2µm, 13.1% CSL boundary and 86.9% RHAGB boundaries in the microstructure, a UTS of 959MPa with 30% ductility, a yield strength of 554 MPa, a strain hardening exponent (n) of 0.17, isotropic properties of r ¯ of 1.07 with a low ?r of 0.12, a toughness of 253 J/m3.
In an embodiment, a component is produced from the cold rolled low carbon micro-alloyed steel. The component is used in structural as well as automotive applications.
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 cold rolled low carbon micro-alloyed steel, according to an embodiment of the present invention
Figure 1b illustrates a flowchart of method for manufacturing cold rolled low carbon micro-alloyed steel, according to an embodiment of the present invention;
Figures 2a-2d illustrates (2a) Optical microstructure, (2b) Grain size distribution, (2c)? f?_2=45° ODF section after CR and IHT at 750°C for 5 minutes, and (2d) Ideal texture components formed during CR and IHT of BCC steel, according to an embodiment of the present invention;
Figures 3a-3c illustrates (3a) Optical microstructure, (3b) Grain size distribution, and (3c)? f?_2=45° ODF section after CR and IHT at 800°C for 5 minutes, according to an embodiment of the present invention;
Figures 4a-4c illustrates (4a) Optical microstructure, (b) Grain size distribution, and (c)? f?_2=45° ODF section after CR and IHT at 800°C for 30 minutes, according to an embodiment of the present invention;
Figures 5a-5c illustrates (5a) Optical microstructure, (5b) Grain size distribution, and (5c)? f?_2=45° ODF section after CR and IHT at 840°C for 5 minutes, according to an embodiment of the present invention; and
Figures 6a-6c illustrates (6a) Optical microstructure, (6b) Grain size distribution, and (6c)? f?_2=45° ODF section after CR and IHT at 920°C for 5 minutes, 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 number 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 cold rolled low carbon micro-alloyed steel having very high ductility, strength, high strain hardening exponent, and isotropy combination. The cold rolled low carbon micro-alloyed steel is used to produce components which may used in structural as well as automotive applications. The cold rolled low carbon micro-alloyed steel having excellent deep drawing and hydroforming characteristics comprises the following composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% 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 S P Si Cr Ni Mo
Wt. % 0.053 1.32 0.003 0.014 0.35 0.61 0.019 0.001
Element V Nb Ti Al Cu Ca N B
Wt. % 0.001 0.015 0.001 0.037 0.0083 0.0016 60 ppm 0.005
Table 1. Chemical Co¬¬mposition of low carbon micro alloyed steel
The cold rolled low carbon micro-alloyed steel comprises a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% - 54% of martensite.
The cold rolled low carbon micro-alloyed steel exhibits a tensile strength of at least 600 MPa. More preferably, the low carbon micro-alloyed steel exhibits tensile strength ranging from about 600 MPa to 1301 MPa. In the preferred embodiment, the low carbon micro-alloyed steel exhibits tensile strength ranging from about 624 MPa to 1301 MPa. The cold rolled low carbon micro-alloyed steel exhibits yield strength of at least 350 MPa. More preferably, the cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 350 to 770 MPa. In the preferred embodiment, the cold rolled low carbon micro-alloyed steel exhibits yield strength ranging from about 398 MPa to 770 MPa.
In some embodiments, the cold rolled low carbon micro-alloyed steel exhibits a ductility of at least 19%. In the preferred embodiment, the cold rolled low carbon micro-alloyed steel exhibits a ductility ranging from 19 % to 48 %.
In some embodiments, the cold rolled low carbon micro-alloyed steel exhibits strain hardening exponent (n) ranging from 0.10 to 0.3. In some embodiments, the hot rolled or cold rolled low carbon micro-alloyed steel exhibits the Lankford parameter of normal anisotropy (r ¯) ranging from 1.07 to 1.85. The cold rolled low carbon micro-alloyed steel exhibits a unique combination of high r ¯ (Normal Anisotropy), high strength and ductility. The minimal carbon of about 0.053 wt.% and microalloying kept at a minimum of ~2.56% of the cold rolled low carbon micro-alloyed steel provides good weldability with minimum segregation effects.
Referring to Figures 1a and 1b, a preferred productive line and an exemplary method (100) of manufacturing the cold rolled low carbon micro-alloyed steel is illustrated. The method (100) initiates at step (102). At step (102), the method (100) comprises producing a hot rolled steel sheet by hot rolling a slab having chemical composition expressed in weight %: Carbon (C): 0.04% - 0.08%, Manganese (Mn): 1.2% - 1.5%, Sulphur (S): 0.003% - 0.004%, Phosphorus (P): 0.014% or less, Nitrogen (N): 70 ppm or less, Silicon (Si): 0.25% - 0.5%, Aluminium (Al): 0.02% - 0.04%, Chromium (Cr): 0.5 - 0.75%, Molybdenum (Mo): 0.002% or less, Copper (Cu): 0.009% or less, Vanadium (V): 0.001% or less, Titanium (Ti): 0.01% or less, Niobium (Nb): 0.02% or less, Calcium (Ca): 0.003% or less, Boron (B): 0.003% - 0.005%, Nickel (Ni): 0.02% or less, and the balance being Iron (Fe) and unavoidable impurities at a rolling finishing temperature (TFRT) of 840oC to 880oC to obtain a hot rolled (HR) steel sheet.
In the preferred embodiment, the steel having a composition expressed in weight %: Carbon (C): 0.053%, Manganese (Mn): 1.32%, Sulphur (S): 0.003%, Phosphorus (P): 0.014%, Nitrogen (N): 60 ppm, Silicon (Si): 0.35%, Aluminium (Al): 0.037%, Chromium (Cr): 0.61%, Molybdenum (Mo): 0.001%, Copper (Cu): 0.0083%, Vanadium (V): 0.001%, Niobium (Nb): 0.015%, Titanium (Ti): 0.001%, Calcium (Ca): 0.0016%, Boron (B): 0.005%, Nickel (Ni): 0.019%, and the balance being Iron (Fe) and unavoidable impurities is used. Before hot rolling in a conventional hot-strip mill, a steel slab cast in a conventional continuous caster or in a thin-slab caster 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 (104), 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 (106), the method (100) comprises normalizing the hot rolled (HR) steel sheet at temperature ranging between 800°C - 1000°C for a time duration of 5 – 100 minutes. In the preferred embodiment, the hot rolled (HR) steel sheet is normalized at temperature of 900°C for a duration of 5 minutes.
At step (108), the method (100) comprises cold rolling the hot rolled (HR) and normalized 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 (110), the method (100) comprises heating the cold rolled (CR) steel sheet to a first predetermined temperature ranging between inter-critical temperature (between Ac1 and Ac3) of 750 - 920°C and soaking at the first predetermined temperature of 750 - 920°C for a first predetermined time of 5-30 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 5 minutes. In another embodiment, the first predetermined temperature is 750 °C and first predetermined time of 5 minutes. In yet another embodiment, the first predetermined temperature is 800 °C and first predetermined time of 5 minutes. In an embodiment, the first predetermined temperature is 800 °C and first predetermined time of 30 minutes. In another embodiment, the first predetermined temperature is 920 °C and first predetermined time of 30 minutes.
At step (112), the method (100) 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 cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite.
In an embodiment, 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 third cooling rate of 70-120°C/s to obtain the cold rolled low carbon micro-alloyed steel sheet or strip or blank having a fine grain ferrite-martensite dual phase structure including, by volume, between 46% - 85 % of ferrite, and between 15% to 54% of martensite. In 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.
In an embodiment, the hot rolled (HR) and normalized steel sheet is cold rolled in five consecutive passes with rolling reduction per pass in the range of 25-34%, and with a total accumulated von-Mises strain of 2.1 to obtain the cold rolled (CR) steel sheet.
Table 3 provides Cold rolling and Intercritical heat treatment schedules that have been used to produce the disclosed steels. A high rolling reduction of ~25-34% per pass was carried out during the present investigations.
Cold Rolling Heat Treatment
25-34% CR reduction per pass up to 5 stages
(e_VM = ~2.1) holding temperature (750°C-840°C)
holding time (5min-30min)
WQ @>100°C/s
Table 3. Cold Rolling schedule and heat treatments parameters used in the development of dual phase steel.
A hot rolled steel sheet of ~6mm thickness was used in normalized condition (1193K(900°C) for 5 mins followed by air cooling) for the CR process. Intercritical heat treatment was carried out by heating the sample at 750°C, 800°C, 840°C and 920°C, holding there for 5 minutes, followed by room temperature water quenching. Another sample at 800°C was held for 30 minutes, followed by room temperature water quenching.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 750 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.15Martensite dual-phase microstructure with an average ferrite grain size of 6.4±2.2µm and avg. Martensite island size of 2.9±0.78µm, 13.8% CSL boundary and 86.2% RHAGB boundaries in the microstructure, an UTS of 624MPa with 48% ductility, yield strength of 401 MPa, a strain hardening exponent (n) of 0.30, isotropic properties of r ¯ of 1.80 with a low ?r of -0.13, a toughness of 260 J/m3.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 5 minutes exhibits a Ferrite-0.31Martensite dual-phase microstructure with an average ferrite grain size of 6.8±2.6µm and avg. Martensite island size of 3.0±0.99µm, 18.4% CSL boundary and 81.6% RHAGB boundaries in the microstructure, a UTS of 850MPa with 31% ductility, a yield strength of 464 MPa, a strain hardening exponent (n) of 0.20, isotropic properties of r ¯ of 1.85 with a low ?r of 0.21, a toughness of 233 J/m3.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 800 °C and first predetermined time at 30 minutes exhibits a Ferrite-0.36Martensite dual-phase microstructure with an average ferrite grain size of 7.2±2.5µm and avg. Martensite island size of 3.7±0.78µm, 16.2% CSL boundary and 83.8% RHAGB boundaries in the microstructure, a UTS of 760MPa with 41% ductility, a yield strength of 398 MPa, a strain hardening exponent (n) of 0.24, isotropic properties of r ¯ of 1.1 with a low ?r of 0.13, a toughness of 276 J/m3.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 840 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.39Martensite dual-phase microstructure with an average ferrite grain size of 6.0±2.6µm and avg. Martensite island size of 3.8±1.2µm, 13.1% CSL boundary and 86.9% RHAGB boundaries in the microstructure, a UTS of 959MPa with 30% ductility, a yield strength of 554 MPa, a strain hardening exponent (n) of 0.17, isotropic properties of r ¯ of 1.07 with a low ?r of 0.12, a toughness of 253 J/m3.
In an embodiment, the cold rolled low carbon micro-alloyed steel manufactured by keeping the first predetermined temperature at 920 °C and first predetermined time at 5 minutes exhibits a Ferrite 0.54Martensite dual-phase microstructure with an average ferrite grain size of 6.1±1.3µm and avg. Martensite lath spacing of 0.7±0.3µm , 26.6% CSL boundary and 73.4% RHAGB boundaries in the microstructure, a UTS of 1301MPa with 19% ductility, a yield strength of 770 MPa, a strain hardening exponent (n) of 0.10, isotropic properties of r ¯ of 1.11 with a low ?r of -0.14, a toughness of 191 J/m3.
This exceptional combination of mechanical and anisotropic properties is reported for the first time in fine-grained Ferrite-Martensite (Dual-phase) microstructure in a low carbon micro-alloyed category steel during CR and IHT processing. All these samples produced fine-grained ferrite-martensite dual phase microstructures.
Table 4 provides mechanical properties of different types of the low carbon micro-alloyed steel produced.
Sample YS
(MPa) UTS
(MPa) Total Elongation Toughness
(J/m3) n r ¯ ?r
CR 750°C IHT 5 min 401 624 0.48 260 0.3 1.80 -0.13
CR 800°C IHT 5 min 464 850 0.31 233 0.20 1.85 0.21
CR 800°C IHT 30 min 398 760 0.41 276 0.24 1.10 0.13
CR 840°C IHT 5 min 554 959 0.30 253 0.17 1.07 0.12
CR 920°C IHT 5 min 770 1301 0.19 191 0.10 1.11 -0.14
Table 4. Mechanical Properties after the processing the cold rolled low carbon micro-alloyed steel.
Ferrite, F-Martensite, M: Phase Fractions Ceq in
Martensite Avg. Ferrite Grain Size
(µm) Avg. Martensite Island Size
(µm) % CSL
Boundaries % RHAGB
CR 750°C IHT 5 min F-0.15M (I) 0.213 6.4±2.2 2.9±0.78 13.8 86.2
CR 800°C IHT 5 min F-0.31M (I) 0.116 6.8±2.6 3.0±0.99 18.4 81.6
CR 800°C IHT 30 min F-0.36M (I) 0.103 7.2±2.5 3.7±0.78 16.2 83.8
CR 840°C IHT 5 min F-0.39M (I) 0.096 6.0±2.6 3.8±1.20 13.1 86.9
CR 920°C IHT 5 min F-0.54M (L) 0.076 6.1±1.3 0.7±0.3 (Lath spacing) 26.6 73.4
*I - Island Morphology and *L - Lath Morphology
Table 5. Microstructural features after the processing of the cold rolled low carbon micro-alloyed steel.
Figures 2a-2c represents optical microstructure, area fraction grain size distribution and crystallographic texture in terms of ?_2= 45° orientation distribution functions (ODF) for CR 750°C IHT 5 Min Steel. Figure 2d shows the ideal ?_2= 45° ODF section for CR body centered cubic (BCC) steels. It could be observed that CR 750°C IHT 5 Min Steel produced Ferritic-0.15 Martensite having an average grain size of 5±2.1µm, with grain size distribution in the range of 0.7-12µm (Fig. 2b). The average ferrite grain size and martensite island size were 6.4±2.2µm and 2.9±0.78µm, respectively (Figure 2a).
Figures 3a-3c, 4a-4c, 5a-5c, and 6a-6c represent similar information for CR 800°C IHT 5 Min, CR 800°C IHT 30 Min, CR 840°C IHT 5 Min and CR 920°C IHT 5 Min steel samples, respectively. CR 800°C IHT 5 min steel sample produced Ferritic-0.31 Martensite microstructure, with an average ferrite grain size of 6.8±2.6µm, martensite island size of 3±0.99µm and grain size distribution in the range of 0.5-9µm (Figures 3a-3b).
Further CR 800°C IHT 30 min steel sample produced Ferritic-0.36 Martensite microstructure, with an average ferrite grain size of 7.2±2.5µm, martensite island size of 3.7±0.78µm and grain size distribution in the range of 0.7-11µm (Figures 4a-4b). On further CR 840°C IHT 5 min steel sample produced Ferritic-0.39 Martensite microstructure, with an average ferrite grain size of 6±2.6µm, martensite island size of 3.8±1.2µm and grain size distribution in the range of 0.7-9µm (Fig. 5a-5b). The high r ¯ and low ?r makes these steels very suitable for deep drawing and hydroforming operations.
CR 920°C IHT 5 min steel sample produced Ferrite 0.54Martensite dual-phase microstructure with an average ferrite grain size of 6.1±1.3µm and avg. Martensite lath spacing of 0.7±0.3µm , 26.6% CSL boundary and 73.4% RHAGB boundaries in the microstructure (Fig 6a-6c). The CR 920°C IHT 5 min steel sample has high strength and can be used for applications such as tool steel etc.
Apart from the microstructural investigation, the crystallographic texture in terms of ?_2= 45° ODF sections (Fig. 2c, 3c, 4c, and 5c) for all these samples show the presence of a fiber: RD? <110>, ?-fiber: ND? <111>, ?-fiber: ND? <001>. Steel samples IHT treated at 750°C and 800°C for 5 minutes shows strong intensity for the most desired ?-fiber texture and weak intensity for a and ?-fibers. Steel samples IHT treated at 800°C for 30 minutes and 840°C for 5 minutes shows stronger intensity for all the texture fibers, i.e., a-, ?- and ?-fibers. The high r ¯ are mostly associated with the crystallographic texture i.e., formation of strong ?-fiber texture in this steel (Figures 2-4). Beyond 800°C IHT the intensity of ?-fiber reduces (Figure 5) and high r ¯ could not be achieved.
The present invention relates to the 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 produced cold rolled low carbon micro-alloyed steel exhibits very high ductility, strength, high strain hardening exponent, and isotropy combination. The ductile, damage-tolerant cold rolled low carbon micro-alloyed steel with high UTS is suitable for secondary sheet metal forming operations, such as deep drawing. Components made of the 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 cold-rolled (CR) low carbon micro-alloyed steel lies facilitates in overcoming the drawbacks, such as poor weldability, expensive alloying, alloy segregation and processing difficulties due to high alloying. The most important feature of the present invention is that it could be easily incorporated into an existing industrial production line without any major modifications.
The method of manufacturing may include additional processes such as descaling, pickling which are well known in the prior art and thus will not be described herein detail.
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.