FIELD OF INVENTION
[0001] The present invention relates to a high strength hot rolled steel, and
more particularly to the high strength hot rolled steel having low yield ratio and
tensile strength greater than 540 MPa and method of manufacturing the high
strength hot rolled steel.

BACKGROUND

[0002] Due to raising global energy crisis, and environmental problems, automobile manufacturers are focusing on light weight technologies to improve the fuel efficiency of vehicles and to comply with the emission norms. The use of high-strength steels has become an inevitable trend for the sake of weight reduction, better fabricability, stretch flange ability and to improve safety. Automakers are pushing for the manufacturing of complex shapes with thinner high strength steels. One such important component is wheel rims and discs in which significant weight reduction can be achieved with high strength steel and complex forming [T. Irie, K. Tsunoyama, M. Shinozaki and T. Kato: SAE Paper No. 880695, 1988], and the corresponding energy savings is estimated to be 1.2 - 1.3 times the amount possible for non-rotating parts [M. Mizui, T. Sekine, S. Soneda and T. Herai: SAE Paper No. 850540, 1985].
[0003] The automotive wheel is composed of a disk and a rim. While the disc is press formed, the rim is flared and then roll formed after flash butt welding. From the forming point of view, the rim material needs to have good formability after forming and welding. The most important functional requirement from the point of view of application is durability which can be improved by improving fatigue resistance of the steel. There is a trend towards the use of hot rolled sheets of a minimum tensile strength of 540 MPa for wheel rim applications. Moreover, steel needs to have good ductility in terms of good uniform elongation and low yield strength (YS) to ultimate tensile strength (UTS) ratio, so that the wheel does not develop cracks after the wheel fabrication or welding.
[0004] European Patent EP2053139B1 discloses a method of hot rolled steel sheet manufacturing which is subjected to heat treatment after forming to achieve a tensile strength varying in the range of 440 to 640 MPa. However, the heat treatment after forming to regain the ductility, which is an essential part of the invention, is likely to add to the processing cost and hence, may not be suitable for mass production.
[0005] European Patent EP1201780B1 discloses a method of manufacturing a hot-rolled steel sheet with minimum tensile strength of 540 MPa and the microstructure consist ferrite and martensite. However, the steel sheet may not be suitable for forming after welding, as the welding results in tempering of martensite causing softening of the steel.
[0006] European Patent EP2243853A1 discloses a method of manufacturing a high-strength hot-rolled steel sheet of tensile strength varying in the range of 540 to 780 MPa. The proposed hot-rolled steel relies on precipitation strengthening of ferrite by Titanium. The dissolution temperature of Titanium is quite high and hence in conventional hot-rolling facilities the steel is expected to have Titanium carbonitride precipitates which may cause high yield ratio (YS/UTS) and may result in crack formation after forming or welding.
[0007] The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts
OBJECTIVE OF INVENTION
[0008] It is an object of the invention to solve the problems of the prior art and to provide a high strength hot rolled steel having minimum tensile strength of 540 MPa.
[0009] Another objective of the present invention is to develop a method of manufacturing the high strength hot rolled steel sheet having desired yield ratio and a uniform elongation.
[0010] Another objective of present invention is to provide a new easier manufacturing method combining thermomechanical, and heat treatment processes for the proposed chemical composition.
[0011] It is yet another objective of the present invention, to provide a high strength hot rolled steel sheet, having the following composition in weight%:0.05-0.07% of C, 1.4-1.6% of Mn, 0.10-0.3% of Si, 0.03-0.04 % of Nb, maximum 0.02% of S, maximum 0.030% of P, maximum 0.005% of N, maximum of 0.16 wt.% Cr, the remaining being substantially iron and incidental impurities.
SUMMARY OF INVENTION
[0012] This summary is provided to introduce concepts related to a high strength hot rolled steel and a method of manufacturing the high strength hot rolled steel sheet. 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.
[0013] In one aspect of the present invention, a high strength hot rolled steel is provided. The high strength hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.05% - 0.07%, Manganese (Mn): 1.4% - 1.6%, Sulphur (S): maximum 0.02%, Phosphorus (P): maximum 0.030%, Nitrogen (N): maximum 0.005%, Silicon (Si): 0.10 - 0.30%, Chromium (Cr): maximum of 0.16%, Niobium (Nb): 0.03 - 0.04%, and the balance being Iron (Fe) and unavoidable impurities. The high strength hot rolled steel comprises a microstructure of 60-80% ferrite and 20-40% bainite.
[0014] In an embodiment, the ferrite is precipitation strengthened and has a grain size of 3 to 10 µm. In an embodiment, the high strength hot rolled steel has a tensile strength = 540 MPa. In an embodiment, the high strength hot rolled steel has a yield strength = 450 MPa.
[0015] In an embodiment, the high strength hot rolled steel a yield ratio (YS/UTS) = 0.85. In an embodiment, the high strength hot rolled steel has a uniform elongation = 10%.
[0016] In an embodiment, the high strength hot rolled steel having Mn/Si ratio between 5.0 to 15.0 to achieve a good flash-butt weldability. In an embodiment, the Si content of the high strength hot rolled steel is kept below 0.35 wt.% to avoid the scale formation.
[0017] In an embodiment, the C content of the high strength hot rolled steel is kept below 0.07 wt.% to avoid center-line segregation and to avoid peritectic reaction.
[0018] In an embodiment, the Cr content of the high strength hot rolled steel is kept below 0.16 wt.% to avoid any adverse effect during welding due to Cr-oxide formation.
[0019] In an embodiment, the high strength hot rolled steel comprises the composition expressed in weight %: C - 0.06, Mn - 1.5, S - 0.019, P - 0.018, Si - 0.11, Cr - 0.16, Nb - 0.04, N – 45 ppm and the balance being Iron (Fe) and unavoidable impurities.
[0020] In an embodiment, the high strength hot rolled steel has a yield strength in the range 468 – 481 MPa. In an embodiment, the high strength hot rolled steel has a tensile strength in the range 574 – 583 MPa. In an embodiment, the high strength hot rolled steel has a yield ratio in the range 0.81 – 0.83.
[0021] In another aspect of the present invention, a method for manufacturing high strength hot rolled steel sheet is provided. The method comprises casting steel slab having a composition expressed in weight %: Carbon (C): 0.05% - 0.07%, Manganese (Mn): 1.4% - 1.6%, Sulphur (S): maximum 0.02%, Phosphorus (P): maximum 0.030%, Nitrogen (N): maximum 0.005%, Silicon (Si): 0.10 - 0.30%, Chromium (Cr): maximum of 0.16%, Niobium (Nb): 0.03 - 0.04%, and the balance being Iron (Fe) and unavoidable impurities. The method also comprises reheating the steel slab to a temperature greater than 1100oC. The method further comprises hot rolling the steel slab to produce a steel sheet such that finish rolling is done at a temperature (TFRT). The temperature TFRT varies in the range 830oC to 890oC. The method comprises cooling the steel sheet at a cooling rate of 30-60oC/s till an intermediate temperature (TINT) is reached. The temperature TINT varies in the range 670oC to 700oC. The method further comprises isothermal holding at TINT for predetermined time. The predetermined time varies between 5 seconds – 10 seconds. The method comprises performing cooling at a cooling rate of 30-60oC/s till a coiling temperature (TCT) is reached. The temperature TCT varies in the range 500 to 560oC. The method comprises coiling the steel sheet at the coiling temperature TCT.
[0022] In an embodiment, the high strength hot rolled steel sheet comprises a microstructure of 60-80% ferrite and 20-40% bainite. In an embodiment, the ferrite is precipitation strengthened and has a grain size of 3 to 10 µm. In an embodiment, the high strength hot rolled steel has a tensile strength = 540 MPa and yield strength = 450 MPa. In an embodiment, the high strength hot rolled steel has a yield ratio (YS/UTS) = 0.85 and a uniform elongation = 10%.
[0023] In an embodiment, the steel slab is reheated to a temperature in the range of 1100oC -1200oC for a duration of 20 minutes to 2 hours depending on the slab thickness.
[0024] In an embodiment, the thickness of the steel sheet is in the range of 2.9 mm - 5.5 mm.
[0025] In yet another aspect of the present invention, a high strength hot rolled steel having a tensile strength = 540 MPa is provided. The high strength hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.05% - 0.07%, Manganese (Mn): 1.4% - 1.6%, Sulphur (S): maximum 0.02%, Phosphorus (P): maximum 0.030%, Nitrogen (N): maximum 0.005%, Silicon (Si): 0.10 - 0.30%, Chromium (Cr): maximum of 0.16%, Niobium (Nb): 0.03 - 0.04%, and the balance being Iron (Fe) and unavoidable impurities. The high strength hot rolled steel comprises a microstructure of 60-80% ferrite and 20-40% bainite. The high strength hot rolled steel a yield ratio (YS/UTS) = 0.85.
[0026] An automobile component selected from any one of a lower arm, a wheel rim and a wheel disk produced from the high strength hot rolled steel.
[0027] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Figure 1 illustrates a flowchart of a method of manufacturing a high strength hot rolled steel sheet, according to an embodiment of the present invention;
[0029] Figure 2 illustrates a schematic diagram of cooling technique followed after rolling (in the run-out table of the hot-strip mill) during manufacture of the high strength hot rolled steel sheet, according to an embodiment of the present invention;
[0030] Figure 3a illustrates a final microstructure of high strength hot rolled steel sheet held at TINT for 5 s during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0031] Figure 3b illustrates a final microstructure of high strength hot rolled steel sheet held at TINT for 7 s during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0032] Figure 3c illustrates a final microstructure of high strength hot rolled steel sheet held at TINT for 10 s during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0033] Figure 4a illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength hot rolled steel held at TINT for 5 s during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention;
[0034] Figure 4b illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength hot rolled steel held at TINT for 7 s during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention; and
[0035] Figure 4c illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength hot rolled steel held at TINT for 10 s during manufacture of the high strength hot rolled steel sheet, according to an embodiment of present invention.
[0036] 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
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] The high strength hot rolled steel having a minimum tensile strength of 540 MPa according to the present invention comprises the following composition expressed in weight %: 0.05-0.07% of C, 1.4-1.6% of Mn, 0.10-0.3% of Si, 0.03-0.04 % of Nb, maximum 0.02% of S, maximum 0.030% of P, maximum 0.005% of N, maximum of 0.16 wt.% Cr, the remaining being substantially iron (Fe) and incidental impurities. The high strength hot rolled steel comprises a microstructure of 60-80% ferrite and 20-40% bainite. In the illustrated example, the ferrite is precipitation strengthened and has a grain size of 3 to 10 µm. The high strength hot rolled steel has a yield strength = 450 MPa, a yield ratio (YS/UTS) = 0.85 and a uniform elongation = 10%.
[0043] The high strength hot-rolled steel sheet with a minimum 540 MPa tensile strength consisting of ferrite + bainite microstructure with a low yield ratio (YS/UTS = 0.85) and good uniform elongation (%UEL = 10%) is suitable for producing automobile components such as a lower arm, a wheel rim and/or a wheel disk.
[0044] Referring to Figures 1 and 2, the method (100) of manufacturing a high strength hot rolled steel sheet of the desired composition is illustrated. At step (102), the method (100) comprises casting molten steel having composition expressed in weight %: 0.05-0.07% of C, 1.4-1.6% of Mn, 0.10-0.3% of Si, 0.03-0.04 % of Nb, maximum 0.02% of S, maximum 0.030% of P, maximum 0.005% of N, maximum of 0.16 wt.% Cr, the remaining being substantially iron (Fe) and incidental impurities is casted in a casting apparatus to obtain steel slabs (cast ingots). In the illustrated example, the steel is cast either in a conventional continuous caster or in a thin-slab caster. In the case of thin slab casting, the temperature of the thin-slab is not allowed to drop below 1000oC to avoid the Nb-carbonitride precipitation. In case, the precipitation occurs, it then becomes difficult to completely dissolve the precipitates in the subsequent reheating process rendering them ineffective for precipitation strengthening.
[0045] At step (104), the method (100) comprises reheating the steel slab (steel casting) to a temperature greater than 1100°C. In the illustrated example, the slab is reheated to temperature ranging between 1100 to 1200oC for a duration of 20 minutes to 2 hours depending on the slab thickness. The reheating temperature must be above 1100oC, to ensure complete dissolution of any precipitates of Nb that may have formed in the preceding processing steps. A reheating temperature greater than 1200oC is also undesirable because it may lead to excessive grain coarsening of austenite and/or scale loss. In one example, the casted steel may be heated in a furnace.
[0046] At step (106), the method (100) comprises hot rolling the steel slab to produce a steel sheet such that finish rolling is done at a finish rolling temperature (TFRT) (also shown in Figure 2). The TFRT varies in the range 830oC to 890oC. After the steel slab is cast in the specified composition and reheated, it is hot-rolled. In a conventional hot-strip mill, the slab is rough-rolled in the roughing stands above the recrystallization temperature and then subsequently hot-rolled below the recrystallization temperature. In the case of a thin-slab caster along with continuous strip mill, where there is no separate roughing mill, the deformation schedule of the steel is designed in such a manner that the cast structure is destroyed in the initial stands and finishing is done below the recrystallization temperature. More specifically the finish rolling in either set up should be done at a temperature, TFRT given by such that 830 = TFRT = 890oC. The above range of the finish rolling temperature (TFRT) range is chosen to finish the hot rolling in the austenitic range.
[0047] At step (108), the method (100) comprises cooling the steel sheet at a cooling rate of 30-60oC/s till an intermediate temperature (TINT) is reached. The TINT varies in the range 670oC to 700oC. After the hot rolling, the rolled sheet is subjected to laminar cooling on the Run-Out-Table (RoT) at a cooling rate of 30 - 60°C/s till the desired intermediate temperature (TINT) is reached. The cooling rate should be higher than 30oC/s to prevent formation of pearlite. High cooling rate also results in lowering the ferrite start temperature which leads to refinement of the ferrite grain size. It also prevents the growth of the ferrite. The cooling rate should be restricted to maximum of 60°C/s for a desired amount of ferrite formation. This fast cooling is continued up to the intermediate temperature, TINT below the ferrite start temperature (Ar1). More specifically, this temperature is given by 670 = TINT = 700 (°C).
[0048] At step (110), the method (100) comprises isothermally holding the cooled steel sheet at TINT for predetermined time. The predetermined time varies between 5 seconds – 10 seconds. After cooling the steel sheet to the intermediate temperature (TINT), the cooled steel sheet is isothermally held at the intermediate temperature(TINT) for a time duration varying between 5 seconds to 10 seconds. The intermediate temperature (TINT) and the holding time of 5-10 s is chosen to promote austenite to ferrite formation and to arrive at the required amount of ferrite in the final microstructure.
[0049] At step (112), the method (100) comprises performing cooling at a cooling rate of 30-60oC/s till a coiling temperature (TCT) is reached. The TCT varies in the range 500 to 560oC. After the intermediate temperature(TINT) is attained and the steel sheet is held at the intermediate temperature(TINT) for a time duration varying between 5 seconds to 10 seconds, the steel strip is subjected to again rapid cooling at a cooling rate of 30 - 60°C/s to the coiling temperature (TCT) of 500 = TCT = 560 (°C).
[0050] At step (114), the method (100) comprises coiling the steel sheet at the coiling temperature. Coiling is carried out at a temperature 500 = TCT = 560 (°C). Coiling below 500oC is avoided to prevent the formation of martensite. The steel is cooled rapidly and coiled at the coiling temperature (TCT) to allow the remaining austenite to transform to the required amount of bainite in the final microstructure. The obtained high strength hot rolled steel sheet has microstructure represented by, in area%, the 60-80% ferrite and 20-40% bainite. The high strength hot rolled steel sheet exhibits tensile strength greater than 540 MPa, uniform elongation greater than 10%, yield strength greater than 450 MPa and a yield ratio (YS/UTS) = 0.85. In the illustrated example, the thickness of the steel sheet is in the range of 2.9 mm - 5.5 mm.
[0051] According to the disclosed invention method (100), it is possible to manufacture the high strength hot-rolled steel sheet with a minimum 540 MPa tensile strength consisting of ferrite + bainite microstructure with a low yield ratio and good uniform elongation. Such a steel sheet is suited for automotive industry and other machinery industry where good formability and weldability is a requirement along with the high tensile strength.
[0052] Following portions of the present disclosure, provides details about the proportion of each element in a composition of the high strength hot rolled steel sheet and their role in enhancing properties.
[0053] C: 0.05-0.07%: Carbon is present in any commercial iron and steel making process. The presence of C ensures strength in the phases present i.e. ferrite and bainite. Also, the C content in austenite determines the austenite to ferrite and bainite transformation kinetics. Presence of more C causes a slower kinetics of transformation but with good strength in the phases present. Moreover, a higher C content causes peritectic reaction during the continuous casting stage which is not desirable. Also, higher C causes poor weldability in the final product. Therefore, the C content in the present invention is restricted to 0.05-0.07 wt.% to achieve optimum austenite to ferrite/bainite transformation kinetics after the hot-rolling along with good strength in the final microstructure without any adverse effect during welding. The C content of the high strength hot rolled steel is kept below 0.07 wt.% to avoid center-line segregation and to avoid peritectic reaction.
[0054] Si: 0.1-0.3%: Si is very efficient solid solution strengthening element in steel. Si is also very cheap and efficient ferrite strengthener. Moreover, higher Si content causes a faster austenite to ferrite transformation kinetics. However, presence of Si also causes surface scale formation during the hot-rolling stage. Therefore, the Si content is restricted to 0.1-0.3 wt.%.
[0055] Mn: 1.4-1.6%: Mn imparts solid solution strengthening in ferrite. Mn is also an austenite stabilizer and therefore reduces austenite to ferrite transformation temperature which is helpful is finishing the hot-rolling in the austenite stage at lower temperature and helps reduce the ferrite grain size. However, at higher Mn content there is a chance of centerline segregation in the hot-rolled steel sheet, which is undesirable. Moreover, for a good flash-butt weldability Mn/Si ratio should be within 5.0 to 15.0. Therefore, in the present invention Mn content is restricted to 1.40-1.60 wt.%.
[0056] Nb: 0.03% maximum: Nb is a very effective alloying element for grain refinement. Nb forms the Nb(CN) precipitates during the hot-rolling process, which pins the grain boundaries. This phenomenon creates fine austenite grains during the hot-rolling stage and subsequently the austenite transforms to fine ferrite grains in the final microstructure. Further, Nb in austenite solid-solution lowers the austenite-ferrite transformation temperature which further refines the ferrite grain size. Moreover, Nb in austenite promotes lower temperature transformation product like bainite. However, Nb must remain in austenite solid solution for these beneficial effects to take place and the Nb-precipitation should not take place before the hot-rolling commences. The Nb shall remain dissolved in austenite in the slab-reheating stage. Therefore, in the present invention the Nb content is restricted within 0.03-0.04 wt.%.
[0057] P: 0.03% maximum: P has a deleterious effect on the toughness and weldability of the steel by segregating at the grain-boundaries during the steel making and hot-rolling stages. P is an undesired element. Therefore, the P content is being restricted to a maximum of 0.03 wt.%.
[0058] S: 0.005% maximum: The S content is restricted to a maximum of 0.02 wt.% to limit the deleterious effect of sulphide inclusions on formability.
[0059] N: 0.005% maximum: Presence of high N content causes formation of Nb(CN) at higher temperature and raises the dissolution temperature of the same during the slab-reheating stage. Moreover, at higher N content the ageing stability and toughness of the heat-affected zone in weld seam reduces. Therefore, N content is restricted to a maximum of 0.005 wt.%.
[0060] Cr: 0.16% maximum: Cr improves the hardenability as well as acts as the solid solution strengthening element for ferrite. Presence of Cr helps in avoiding the austenite to pearlite formation during continuous cooling from the finish rolling temperature. However, presence of higher amount Cr may cause formation of Cr-oxides during flash-butt welding of the wheel rims. Therefore, the Cr content is restricted to a maximum of 0.16 wt.%.
[0061] Microstructure: The final set of desired properties in the hot-rolled steel is achieved by the presence of ferrite and bainite with grain refinement along with the strengthening contributions from the alloying elements, described above. All the hot-rolling, controlled cooling and coiling conditions have significance in achieving the final microstructure and properties. The contribution of the each of the phases i.e. ferrite and bainite is described below.
Ferrite: The final hot-rolled microstructure contains 60-80% ferrite, which is strengthened by the contributions from the alloying elements mentioned above. Ferrite is a softer phase than the bainite. The presence of the ferrite as the major phase constituent in the microstructure ensures low yield strength as subsequently low yield ratio (YS/UTS <= 0.85).
Bainite: The bainite is present in the microstructure in 20-40%. Bainite is a harder phase. Presence of the above specified quantity of bainite ensures the minimum tensile strength of 540 MPa is achieved in the final hot-rolled steel sheet.
Examples
[0062] Further embodiments of the present disclosure will be now described with an example of a particular composition of the high strength hot rolled steel, which is illustrated in Table 1. Various experiments and tests were conducted on a laboratory scale in order to evaluate various conditions. The molten steel having composition shown in table 1 is cast into the steel slab which is then hot rolled following the schedule as mentioned in Figure 2. Subsequently, the high strength hot rolled sheet is immediately transferred after the finish rolling into a salt bath treatment (herein the salt bath furnace is used to emulate the ROT conditions in lab scale) kept at the intermediate temperature (700oC). The high strength hot-rolled sheet is then held at intermediate temperature for three different holding times i.e. 5s, 7 s and 10 s to achieve the austenite to ferrite transformation. Thereafter, the sheets were transferred to another salt-bath facility kept at the coiling temperature (550oC) to emulate the coiling simulation.
Composition 1:

Elements C Mn S P Si Cr N
Wt.% 0.06 1.50 0.019 0.018 0.11 0.16 45 ppm

Table: 1
[0063] Referring to Figures 3a, 3b, and 3c, final achieved microstructures of the high strength hot-rolled steel according to the present invention is shown. (a) 5 s holding at TINT, (b) 7 s holding at TINT, and (c) 10 s holding at TINT are illustrated. Subsequently, dog bone type tensile test samples were prepared from the high strength hot rolled steel sheets according to the ASTM E8M standard with 25 mm gauge length. Three samples were prepared from each of the heat treatment conditions to check for the repeatability and reproducibility. The Tensile plots of the high strength hot rolled steel sheets disclosed in the present invention for each of the heat-treatment conditions; (a) 5 s holding at TINT, (b) 7 s holding at TINT, and (c) 10 s holding at TINT are illustrated in the Figures 4a, 4b, and 4c. The tensile results are tabulated in Table 2. As an example, the tensile testing may be performed using tensile testing machines with samples, prepared following the ASTM E8/E8M standard and scanning electron microscopy (SEM) was conducted to investigate microstructure of the high strength hot rolled steel sheets.
Holding at
TINT Sl. No YS (MPa) UTS (MPa) Yield ratio
(YS/UTS) TEL (%) UEL (%)
5 s holding at TINT (Fig. 3a) 1 468 574 0.81 26 13
2 479 582 0.82 29 14
3 475 576 0.82 29 14
7 s holding at TINT (Fig. 3b) 4 480 581 0.83 29 14
5 477 578 0.83 29 13
6 479 581 0.82 28 14
10 s holding at TINT (Fig. 3c) 7 481 583 0.83 26 14
8 480 577 0.83 25 14
9 468 574 0.82 26 14
Table: 2
[0064] It can be clearly noted that the inventive examples have achieved the minimum tensile strength of 540 MPa, with yield ratio less than 0.85 and uniform elongation more than 10%.
[0065] The high strength hot rolled steel demonstrates a minimum tensile strength of 540 MPa and a good ductility in terms of good uniform elongation and low yield strength (YS) to ultimate tensile strength (UTS) ratio. Such high strength hot-rolled steel sheet has an excellent formability and weldability. Therefore, the high strength hot rolled steel will have a great potential for automobile applications such as wheel applications. The high strength hot rolled steel having good uniform elongation (more than 10% in all the cases) and low yield ratio (less than 0.85) restricts development of cracks during wheel fabrication or welding.
[0066] The present invention provides the high strength hot rolled steel and method of manufacturing high strength hot rolled steel having low yield ratio. The high strength hot rolled steel makes an important contribution towards durable, cost effective, futuristic and strategic light weight application of steel with greater factor of safety. A superior factor of safety may be obtained by achieving greater than 540 MPa tensile strength with reasonable ductility, specifically required for the automotive applications.
[0067] It should be understood that the experiments are carried out for particular compositions of the high strength hot rolled steel sheet and the results brought out in the previous paragraphs are for the composition shown in Table 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the high strength hot rolled steel strip, as well.
[0068] 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.
[0069] 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.
[0070] 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:1. A high strength hot rolled steel comprising the following composition expressed in weight %:
Carbon (C): 0.05% - 0.07%,
Manganese (Mn): 1.4% - 1.6%,
Sulphur (S): maximum 0.02%,
Phosphorus (P): maximum 0.030%,
Nitrogen (N): maximum 0.005%,
Silicon (Si): 0.10 - 0.30%,
Chromium (Cr): maximum of 0.16%,
Niobium (Nb): 0.03 - 0.04%, and the remaining being substantially iron and incidental impurities, wherein the high strength hot rolled steel comprises a microstructure of 60-80% ferrite and 20-40% bainite.
2. The high strength hot rolled steel as claimed in the claim 1, wherein the ferrite is precipitation strengthened and has a grain size of 3 to 10 µm.
3. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength hot rolled steel has a tensile strength = 540 MPa.
4. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength hot rolled steel has a yield strength = 450 MPa.
5. The high strength hot rolled steel as claimed in the claims 1 to 4, wherein the high strength hot rolled steel a yield ratio (YS/UTS) = 0.85.
6. The high strength hot rolled steel as claimed in the claims 1 to 5, wherein the high strength hot rolled steel has a uniform elongation = 10%.
7. The high strength hot rolled steel as claimed in the claim 5, wherein the high strength hot rolled steel having Mn/Si ratio between 5.0 to 15.0 to achieve a good flash-butt weldability.
8. The high strength hot rolled steel as claimed in the claim 5, wherein the Si content of the high strength hot rolled steel is kept below 0.35 wt.% to avoid the scale formation.
9. The high strength hot rolled steel as claimed in the claim 5, wherein the C content of the high strength hot rolled steel is kept below 0.07 wt.% to avoid center-line segregation and to avoid peritectic reaction.
10. The high strength hot rolled steel as claimed in the claim 5, wherein the Cr content of the high strength hot rolled steel is kept below 0.16 wt.% to avoid any adverse effect during welding due to Cr-oxide formation.
11. The high strength hot rolled steel as claimed in the claim 1, wherein the high strength hot rolled steel comprises the composition expressed in weight %: C - 0.06, Mn - 1.5, S - 0.019, P - 0.018, Si - 0.11, Cr - 0.16, Nb - 0.04, N – 45 ppm and the balance being Iron (Fe) and unavoidable impurities.
12. The high strength hot rolled steel as claimed in the claim 11, wherein the high strength hot rolled steel has a yield strength in the range 468 – 481 MPa.
13. The high strength hot rolled steel as claimed in the claims 11 and 12, wherein the high strength hot rolled steel has a tensile strength in the range 574 – 583 MPa.
14. The high strength hot rolled steel as claimed in the claims 11 to 13, wherein the high strength hot rolled steel has a yield ratio in the range 0.81 – 0.83.
15. A method (100) for manufacturing high strength hot rolled steel sheet, the method (100) comprising:
casting steel slab having a composition expressed in weight %: Carbon (C): 0.05% - 0.07%, Manganese (Mn): 1.4% - 1.6%, Sulphur (S): maximum 0.02%, Phosphorus (P): maximum 0.030%, Nitrogen (N): maximum 0.005%, Silicon (Si): 0.10 - 0.30%, Chromium (Cr): maximum of 0.16%, Niobium (Nb): 0.03 - 0.04%, and the balance being Iron (Fe) and unavoidable impurities;
reheating the steel slab to a temperature greater than 1100oC;
hot rolling the steel slab to produce a steel sheet such that finish rolling is done at a temperature (TFRT), wherein TFRT varies in the range 830oC to 890oC;
cooling the steel sheet at a cooling rate of 30-60oC/s till an intermediate temperature (TINT) is reached, wherein TINT varies in the range 670oC to 700oC;
isothermal holding at TINT for predetermined time, wherein predetermined time varies between 5 seconds – 10 seconds;
performing cooling at a cooling rate of 30-60oC/s till a coiling temperature (TCT) is reached, wherein TCT varies in the range 500 to 560oC; and
coiling the steel sheet at the coiling temperature TCT.
16. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 15, wherein the high strength hot rolled steel sheet comprises a microstructure of 60-80% ferrite and 20-40% bainite.
17. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 16, wherein the ferrite is precipitation strengthened and has a grain size of 3 to 10 µm.
18. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claims 15 to 17, wherein the high strength hot rolled steel has a tensile strength = 540 MPa and yield strength = 450 MPa.
19. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claims 15 to 18, wherein the high strength hot rolled steel has a yield ratio (YS/UTS) = 0.85 and a uniform elongation = 10%.
20. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claim 15, wherein the steel slab is reheated to a temperature in the range of 1100oC -1200oC for a duration of 20 minutes to 2 hours depending on the slab thickness.
21. The method (100) for manufacturing high strength hot rolled steel sheet as claimed in the claims 15 to 20, wherein the thickness of the steel sheet is in the range of 2.9 mm - 5.5 mm.
22. A high strength hot rolled steel having a tensile strength = 540 MPa comprising the following composition expressed in weight %:
Carbon (C): 0.05% - 0.07%,
Manganese (Mn): 1.4% - 1.6%,
Sulphur (S): maximum 0.02%,
Phosphorus (P): maximum 0.030%,
Nitrogen (N): maximum 0.005%,
Silicon (Si): 0.10 - 0.30%,
Chromium (Cr): maximum of 0.16%,
Niobium (Nb): 0.03 - 0.04%, and the remaining being substantially iron and incidental impurities, the high strength hot rolled steel comprises a microstructure of 60-80% ferrite and 20-40% bainite, wherein the high strength hot rolled steel a yield ratio (YS/UTS) = 0.85.
23. An automobile component produced from the high strength hot rolled steel as claimed in the claims 1 to 22.
24. The automobile component as claimed in the claim 23, wherein the automobile component is selected from any one of a lower arm, a wheel rim and a wheel disk.