Description:TECHNICAL FIELD
The present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a high strength hot rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing a high strength hot rolled steel sheet with yield strength of minimum 450 MPa. Strength of this steel (YS>450 MPa) is to be maintained after hot forming operation.
BACKGROUND OF THE DISCLOSURE
Steel is an alloy of iron, carbon and other elements such as Phosphorous (P), Sulphur (S), Nitrogen (N), Manganese (Mn), Silicon (Si), Aluminium (Al) and any other incidental elements. Because of its high tensile strength and low cost, steel may be considered as a major component in wide variety of applications. Some applications of the steel may include buildings, ships, tools, automobiles, machines, bridges and numerous other applications. The steel obtained from steel making process may not possess all the desired properties for all application and therefore, the steel may be subjected to secondary processes such as heat treatment for controlling material properties to meet various needs in the intended applications.
Generally, heat treatment may be carried out using techniques including but not limiting to annealing, normalising, hot rolling, quenching, and the like. During heat treatment process, the material of steel undergoes a sequence of heating and cooling operations, thus, the microstructures of the steel may be modified during such operation. As a result of heat treatment, the steel may undergo phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability etc. The purpose of heat treatment is to increase service life of a product by improving its strength, hardness etc., or prepare the material for improved manufacturability.
With rising concerns over global environmental problems and demand from automotive industry for higher collision safety of vehicles impose conflicting requirements on materials used for bodies of the vehicle. The vehicle bodies are required to be stronger yet lighter at the same time. Advancements in steel manufacturing may include commercialization of fuel cell vehicles and use of lighter materials like aluminium, composites etc. These materials meet the desired material properties, however, associated problems such as formability, reliability
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and recyclability and higher cost of manufacturing make such materials commercially unattractive and hence, usage of such material may be limited to specific types of components of the vehicles. Thus, it becomes inevitable to re-look at high strength steels to meet the desired properties, as usage of steel mitigates some of above-mentioned issues with other materials.
Application such as axle housing require thicker- and stronger material. The component is formed from a steel blank after forming the blank at high temperature (Hot forming). Typically, such a temperature varies between 800 C and 900 C. After hot forming, the component is allowed to cool in normal atmosphere. Material after hot forming has typical yield strength of ~410 MPa strength is being used. In order to reduce weight of the vehicle, strength greater than 450 MPa is being aimed to be achieved after hot forming applications. One of the patent literature[CN104213019A] discloses a vanadium micro alloyed steel for such applications and the steel contains carbon higher than 0.21 wt%. Additionally, nitrogen higher than 120 ppm was added in its chemistry. Such a high level of carbon and high nitrogen may affect weldability of the steel. This will negatively affect the later part of the manufacturing process.
The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.
SUMMARY OF THE DISCLOSURE
One or more shortcomings of the prior art are overcome by method as disclosed and additional advantages are provided through the method as described in the present disclosure.
Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.
In one non-limiting embodiment of the present disclosure, there is provided a high strength hot-rolled steel sheet. The steel sheet includes alloying composition comprising of carbon (C) at about 0.05 wt% to about 0.15 wt%, manganese (Mn) at about 1.2 wt% to about 1.7 wt%, silicon (Si) 0.2 to 0.6 wt%, Niobium (Nb) at about 0.02 wt% to 0.06 wt% titanium (Ti) up to 0.02 wt%, vanadium 0.07 wt% to 0.15wt.%, aluminium (Al) at about 0.02 wt% to 0.1 wt.%,
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sulphur (S) up-to 0.008 %, phosphorous (P) up-to 0.025 wt%, nitrogen (N) up-to 0.012 wt%, the balance being Iron (Fe) optionally along with incidental elements. The high strength hot-rolled steel sheet comprises ferrite, pearlite and/or bainitic microstructure with small amount of carbide, and/or carbonitride.
In an embodiment, the high strength hot-rolled steel sheet exhibits yield strength of at least 450 MPa, ductility greater than 20% and toughness (at 0 C) greater than 35 J.
In the embodiment, the hot formed steel exhibits yield strength of at least 450 MPa, ductility greater than 20% and toughness greater than 35 J at 0 C.
In an embodiment, the microstructure is mixture of ferrite, pearlite and/or bainitic microstructure with small amount of carbide, and/or carbonitride.
In an embodiment, after hot forming, most of precipitates are freshly formed during the cooling stage of the component. These precipitates are Niobium, and Vanadium carbides/carbonitrides precipitates.
To manufacture the steel sheet as per the current invention, the steel slab is subjected to heating at a first predetermined temperature for a first predetermined time, followed by subjecting the steel slab to a hot working.
In an embodiment, the casting is carried out in a continuous casting process. The continuous casting process is performed in a slab caster.
Then, subjecting the steel slab to heating in a first predetermined temperature for a first predetermined time and then subjecting, the steel slab to a hot working to produce a steel sheet. The hot working includes deforming, the steel slab in a first hot working process, at a second predetermined temperature, and deforming, the steel slab in a second hot working process, at a third predetermined temperature. The hot worked steel sheet is then subjected to cooling at a predetermined cooling rate to a fourth predetermined temperature. Further, the cooled steel sheet is coiled at the fourth predetermined temperature to obtain a high strength hot-rolled steel sheet/plate.
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In an embodiment, the first predetermined temperature is greater than 1150°C, preferably ranging from about 1200°C to 1250°C, and the first predetermined time ranging from about 30 minutes to about three hours.
In an embodiment, the hot working is a hot rolling process. The first hot working process is performed in a roughing mill, and the second predetermined temperature ranges from about 1050°C to 1170°C and the second hot working process is performed in four or more than four strands of a finishing mill.
In an embodiment the third predetermined temperature ranging from 840 C to 950 C.
In an embodiment, the predetermined cooling rate is greater than 15°C/second, and the cooling is performed with intensive or laminar water cooling on a run out table.
In an embodiment, the fourth predetermined temperature ranges from about 500°C to 600°C, preferably 550°C to 570°C.
Steel thus made was imposed with a hot forming heat treatment conditions: Rapid heating (upto 10 C/s) to maximum temperature (800 C/850 C/900 C), where forming happens and then, slowly cooling the material to room temperature.
It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the disclosure.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
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BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:
Figure.1 is a flowchart illustrating a method for producing high strength hot rolled steel sheet, according to an exemplary embodiment of the present disclosure.
Figure.2 is graphical flow diagram of cooling profile followed during cooling of the sheet as mentioned in block 104 as mentioned in Figure. 1.
Figures. 3a-3c illustrates microstructure of hot rolled steel plate manufactured by method of present disclosure in which (3a) is an Optical micrograph, (3b) is a SEM micrograph showing combination of equiaxed ferrite(polygonal), Bainitic ferrite, carbide. Figure 3c and 3d refer to corresponding optical and SEM micrographs of sample after heat treatment condition.
Figures (4a) and 4b show the image quality map as obtained from the EBSD measurement on as rolled sample and after Heat Treatment. Fraction of polygonal ferrite phase is calculated using grain orientation spread criteria (GOS<1) and area fraction of ferrite is in the range of 20 to 30% in as rolled sample.
Figures. 5a and 5b-c show transmission electron microscope (TEM) micrographs of as rolled steel sample and Heat Treatment sample respectively. TEM micrograph of as rolled sample show clearly presence of bainitic ferrite and polygonal ferrite. Precipitates were not clearly seen suggesting possibility of suppression of precipitates due to relatively lower Coiling Temperature. Figure 5b and 5c micrographs show presence of precipitates: both coarser (~ 20nm ) and finer precipitates (3 to 7nm) .
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative
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embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.
In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.
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Embodiments of the present disclosure discloses a high strength hot rolled steel sheet suitable for manufacturing axle halves and a method for manufacturing or producing a high strength hot rolled steel sheet. Strength, ductility and impact toughness are some of the important properties for the mass industrial application of high strength material like steel. The method of present disclosure discloses a production of high strength hot rolled steel sheet, with yield strength of minimum 450 MPa, > 20% ductility, impact toughness at 0 C greater than 35 J. The hot rolled steel may be widely employed to make automotive components through hot forming route. Henceforth, the present disclosure is explained with the help of figures for a method of manufacturing high strength-hot rolled steel sheet. However, such exemplary embodiments should not be construed as limitations of the present disclosure since the method may be used on other types of steels where such need arises. A person skilled in the art may envisage various such embodiments without deviating from scope of the present disclosure.
Figures.1 and 2 is exemplary embodiments of the present disclosure illustrating a flowchart depicting a method for manufacturing high strength-hot rolled steel sheet, and a graphical flow diagram of cooling profile, respectively. In the present disclosure, mechanical properties such as strength, ductility, impact toughness and final microstructure of the steel are improved. The steel produced by the method of the present disclosure, includes a microstructure which is primarily a mixture of polygonal ferrite and bainitic ferrite. A very small fraction of carbide or pearlite (< 7%) also may be retained. The method is now described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject matter described herein. The method is particularly applicable to high-strength hot rolled steel sheet and it may also be extended to other type of steels as well.
The method of manufacturing the high strength hot-rolled steel sheet/plate according to the present disclosure consists of a casting step followed by a hot working and controlled cooling, where the steel sheet is then coiled to satisfy component composition described below. It should not be construed that the following steps to be limited, rather, coiling may be performed after hot working and prior cooling, based on variation in temperature and cooling rate, respectively. The various processing steps of Figure 1 respective, are described below:
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As shown in block 101, the method starts with the process of casting. In the method of present disclosure, the steel of the specified composition including in weight percentage of the balance being iron and impurities by any manufacturing process including but not limiting to casting. In an embodiment, the casting process is a continuous casting process.
The method then includes the step of heating as shown in block 102. After casting the steel slab with the specified composition, the slabs are heated to a first predetermined temperature for a first predetermined time. In an embodiment, the first predetermined temperature is greater than 1150 °C, preferably in the range of 1200 °C to 1250 °C, and the first predetermined time ranges from 30 minutes to 3 hours. In an embodiment, the first predetermined temperature is above 1150 °C, to ensure complete dissolution of precipitates that may have formed in the preceding processing step. A first predetermined temperature greater than 1250 °C is also not desirable because it may lead to grain coarsening of austenite and/or excessive scale loss. In some embodiment, heating of the steel slab may be carried out in a furnace.
Once the steel slab is heated as per the block 102, it is subjected for hot working as shown in block 103 to form a steel sheet. In an embodiment, the hot working process is a hot rolling process. As shown in block 103, after casting and heating the steel slab with the specified composition, it is hot rolled. The hot rolling may constitute two steps of deformation via the first hot working process and the second hot working process. In an embodiment, the first hot working process is a deformation process of steel slab in a roughing mill at the second predetermined temperature.
In an embodiment, the second predetermined temperature range from about 1050°C to 1170°C. In the roughing stage, the cast structure may be broken down, and the new structures may be formed. Further, the second hot working process is carried out at a third predetermined temperature. In an embodiment, in the second hot working process is a further deformation process of the steel slab carried out in four or more stands of the finishing mill, and temperatures in all rolling stands of the finishing mill are such that microstructure of the material consists of austenite phase. In an embodiment, the third predetermined temperature is in the range of 840 C to 950 C.
After finish rolling, the steel sheet is subjected to intensive cooling or laminar cooling at a predetermined cooling rate as shown in block 104. In an embodiment, the predetermined
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cooling rate is greater than 15 °C/s till a fourth predetermined temperature (T4) is reached. In an embodiment, the cooling is performed on a run-out-table. The steel sheet is held at the fourth predetermined temperature as shown in block 105, which is also called as coiling temperature in the range of 500 C to 600 C.
It is preferable to keep the fourth predetermined temperature at 540°C to 580°C to achieve optimized strength, ductility and toughness in the as rolled state and as well as after hot forming conditions. Coiling below 500 °C may be avoided to prevent the formation of harder phases such as martensite microstructure in the steel, and coiling above 600°C, precipitate out all precipitates apart from forming larger grain size. In this case, fresh precipitates may not form and existing precipitates may grow in the as heat treated condition. Further, dislocation strengthening will not be there and, grain size may become higher. A schematic diagram of the cooling profile is shown in Figure. 2. This ensures that the microstructure consists of primarily ferrite-bainitic ferrite with small amounts of pearlite, carbides and carbonitrides
In an embodiment, the high strength hot rolled steel sheet exhibits yield strength greater than 450 MPa along with a impact toughness greater than 35J at 0 C , and % elongation more than 20%. In order to achieve the required mechanical properties in the heat-treated condition without significant addition of nitrogen as proposed in the disclosure, it may be required to suppress precipitates to some extent and they will form in the heat treated condition. Carbide or pearlite content may be minimized as they are hard phase and often are detrimental for impact toughness. Low carbon content is hence maintained in the desired range to obtain very low amount of carbide.
Strength in as rolled condition may be primarily obtained through grain size strengthening, dislocation strengthening and to some extent precipitation strengthening. It is intended to have finer structures in as rolled state so that good impact toughness is obtained. Precipitates are mostly suppressed in the as rolled state by keeping a low coiling temperature. During subsequent hot forming process, fresh precipitates are allowed to form. Hot forming process involves rapid heating of steel blank and a very slow cooling of the formed component. Annihilation of dislocations are likely to occur during the cooling process and more is the hot forming temperature, less will be the retained dislocations and hence less dislocations strengthening in the microstructure after component cools down to room temperature. It is
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expected that some strength contribution in as formed component is due to dislocations strengthening. Finer structure in as rolled steel ensures finer grains in as formed component as well. So, some contribution from strength is due to grain size strengthening.
The following portion of the present disclosure provides details about the proportion of each alloying element in a composition of the steel and their role in enhancing properties.
Carbon (C) may be added in the range of about 0.05 wt% to about 0.15 wt%. Carbon is an inherent component in steel, carbon helps in strengthening phases, and is often considered as a cheaper element to increase strength. Strength will not be achieved with carbon less than 0.05 wt%. But carbon content in steel beyond 0.15 wt% may have detrimental effects like it will lead to formation of more amount of undesirable second phases such as carbide, pearlite martensite/austenite islands and thereby, ductility and impact toughness may get deteriorated.
Manganese (Mn) may be added in the range of about 1.2 wt% to about 1.7 wt%. Manganese not only imparts solid solution strengthening to ferrite but also lowers the austenite to ferrite transformation temperature. However, Mn level cannot be increased beyond 1.7 wt% as at such high levels Mn may enhance centerline segregation during continuous casting and hence may create inhomogeneity in microstructure which may cause detrimental effect on impact toughness as well as result in increase in cost implications.
Silicon is a very cheap solid solution strengthening element and it has more solid solution strengthening potential than manganese. Si is added in the range of 0.2 – 0.6 wt%. However, Si content beyond 0.6 wt% may promote formation of excessive scales during high temperature soaking, which may be undesirable.
Phosphorus content may be restricted to 0.025 wt% maximum as higher phosphorus levels may lead to reduction in toughness and weldability due to segregation of phosphorus into grain boundaries.
Sulphur content may be limited (<0.008%) otherwise it results in a very high inclusion level that deteriorates formability.
Nitrogen (N) may be kept 120 ppm (0.012 wt.%) maximum. Higher nitrogen in vanadium micro alloyed steel is detrimental from weldability point of view. Also, increase in nitrogen
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content increases size of the TiN. Larger sized TiN will reduce both ductility and impact toughness.
Aluminium (Al) may be added in the range of 0.02 – 0.1 wt %, preferable below 0.06 wt %. Aluminium is used as a deoxidizer and killing of steel. It limits growth of austenite grains.
Titanium (Ti) may be added less than 0.02 wt%. Such small amount of Ti ensures formation of fine TIN precipitates, which act as inhibitors to growth of austenite grains during reheating and subsequent hot rolling stages. Thus, titanium indirectly ensures finer austenite grain size. Thus, finer and uniform ferrite grains are created during subsequent processing stages. This helps in improving impact toughness.
Niobium (Nb) may be added in the range of about 0.02 wt% to about 0.06 wt%. Nb improves strength by grain refinement. More importantly, Nb forms carbides which also promotes strengthening.
Vanadium (V) may be added in the range of about 0.07 wt% to about 0.15 wt%. V improves strength through formation of fine carbides which also promotes strengthening.
Example:
A few embodiments of the present disclosure have been described below to illustrate the claimed invention. Experiments have been carried out for a specific composition of the steel formed by using method of the present disclosure. The composition of the steel for which the tests are carried out is as shown in below table 1.
Chemical composition (wt. %)
C
Mn
Si
Al
Ti
Nb
V
N
S
P
A
0.092
1.47
0.37
0.037
0.015
0.045
0.11
0.005
0.002
0.018
B
0.069
1.54
0.38
0.034
0.005
0.042
0.11
0.008
0.0034
0.011
C
0.082
1.47
0.39
0.036
0.016
0.047
0.115
0.0107
0.003
0.014
Table – 1
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The compositions of table-1 were continuously casted in a slab caster and the slabs were hot-rolled in a hot rolling mill. However, varying processing parameters were used in the mill and cooling as shown in Table 2 for single step cooling process- which shows hot rolling process parameters used for hot rolling.
Table 2: Processing parameters imposed during hot rolling.
Table 3 provides the properties of selected steels when hot formed at given temperatures. The steels as per the current invention when subjected to hot forming heat treatment with temperature preferably in the range of 800 -950 C, exhibit excellent properties as depicted in table 3 below.
Steel
Composition
Thickness,
mm
FRT
(°C)
CT
(°C)
YS (MPa)
UTS, (MPa)
%El
Impact Toughness at 0 C
(J)
Steel 1
A
15.7
900
620
548
651
32
189
Steel 2
A
15.7
885
585
587
691
31
115
Steel 3
A
15.7
930
550
537
652
32
186
Steel 4
A
13.7
930
550
565
678
24
198
Steel 5
B
15.7
895
560
536
641
23
259
Steel 6
C
15.7
891
580
595
693
24
131
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Table-3
Referring to the above table of single step cooling process, abbreviation for FRT refers to finish rolling temperature which corresponds to third predetermined temperature and abbreviation for CT refers to coiling temperature which corresponds to fourth predetermined temperature.
Steel
Thickness, mm
Hot forming Temperature, C
YS, MPa
UTS, MPa
%El
Impact Toughness at 0 C
(J)
Steel 1
15.7
800
454
554
38
230
15.7
850
-
-
-
-
15.7
900
428
533
41
242
Steel 2
15.7
800
554
654
30
204
15.7
850
467
590
36
232
15.7
900
458
562
38
235
Steel 3
15.7
800
592
674
31
238
15.7
850
496
610
34
244
15.7
900
477
565
38
258
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Referring to the above table of single step cooling process, abbreviation for YS refers to yield strength and abbreviation for TS refers to tensile strength.
The compositions of table-1 were continuously cast in a slab caster and the slabs were hot-rolled in a hot rolling mill. However, varying processing parameters were used in the mill and cooling as shown in Table 2 which shows hot rolling process parameters used for hot rolling.
The microstructures of the as rolled steel 3 is shown in Figures 3a-3b. Figure 4 show the image quality map as obtained from EBSD measurement. Both confirm presence of polygonal ferrite and bainitic ferrite in the microstructure. Apart from these, some carbides or small amount of pearlite are also seen. TEM micrograph as shown in Figure 5 also confirm presence of bainitic ferrite and polygonal ferrite. Bainitic ferrite is identified as the region with comparatively higher dislocation density and look comparatively darker than polygonal ferrite region which appear bright in the micrographs. In the as rolled condition, precipitates are not clearly visible in TEM and are considered as mostly suppressed, though some precipitates could be present in the microstructure. Size of the grains is in the range of 3 to 4 micron. Strength in as rolled condition is primarily due to finer grain size (EBSD and SEM images), dislocation strengthening (TEM images).
The microstructure of as heat-treated condition (850 C) for steel 3 is shown in Figure 3c-d. The average grain size was 7 micron, which was higher than the one obtained from EBSD map of as rolled sample. The grain size became a little higher. TEM micrographs show grains with relatively lower dislocation density. Major difference is in the precipitation conditions. Both large (~20nm) and smaller precipitates (3-10 nm) have formed. Retention of yield strength (>450 MPa) after treatment at this temperature is resulted due to such precipitates. Increased YS at 800 C HT condition compared to as rolled condition suggests that the higher strength is a result of these finer precipitates.
High impact toughness of as rolled steel is due to suppressed precipitates and finer grain size. In the heat-treated conditions, impact toughness properties became higher than the as rolled condition. Grains are relatively cleaner w.r.t. dislocations density.
Equivalents:
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the
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plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings,
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should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."
While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.
Referral Numerals:
Referral Numerals
Description
101-105
Flowchart blocks
101
Forming stage
102
Heating stage
103
Hot working stage
104
Cooling stage , Claims:We claim:
1. A high-strength hot-rolled steel plate/sheet, comprising:
composition in weight percentage (wt.%) of:
carbon (C) at about 0.05 wt% to about 0.15 wt%,
manganese (Mn) at about 1.2 wt% to about 1.7 wt%,
silicon (Si) 0.2 to 0.6 wt%,
Niobium (Nb) at about 0.02 wt% to 0.06 wt%
titanium (Ti) upto 0.02 wt%, vanadium 0.07 wt% to 0.15 wt.%, aluminium (Al) at about 0.02 to 0.1 wt.%, Sulphur (S) up-to 0.008%, phosphorous (P) up-to 0.025 wt%, nitrogen (N) up-to 0.012 wt%, the balance being Iron (Fe) optionally along with incidental elements. the balance being Iron (Fe) along with incidental elements,
wherein, the high-strength hot-rolled steel sheet comprises of ferrite, pearlite and/or bainitic microstructure with small amount of carbide, and/or carbonitride.
2. The high strength hot-rolled steel sheet as claimed in claim 1 wherein, the high strength hot-rolled steel sheet exhibits yield strength of at least 450 MPa in as hot rolled condition.
3. The high strength steel sheet as claimed in claim 3, wherein Niobium, and Vanadium are present in hot rolled steel as Carbide precipitate and/or Carbonitrides precipitate and/or dissolved solute element.
4. The high strength steel sheet as claimed in claim 1, wherein the high strength steel sheet exhibits total elongation greater than 20%.
5. The high strength steel sheet as claimed in claim 1, wherein the high strength steel sheet exhibits impact toughness > 35 J at 0 C.
6. The high strength steel sheet as claimed in claim 1, wherein nitrogen is restricted upto 120 ppm (0.012 wt.%)
7. The high strength steel sheet as claimed in claim 1, wherein the steel sheet exhibits YS > 450 MPa and impact toughness > 35 J at 0 C after hot forming heat treatment.
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8. A method for manufacturing a high-strength hot-rolled steel sheet, the method comprising:
casting a steel slab of a composition comprising in weight percentage (wt%) of: carbon (C) at about 0.05 wt% to about 0.15 wt%,
manganese (Mn) at about 1.2 wt% to about 1.7 wt%,
silicon (Si) 0.2 to 0.6 wt%,
Niobium (Nb) at about 0.02 wt% to 0.06 wt%
titanium (Ti) upto 0.02 wt%,
vanadium 0.07 wt% to 0.15 wt.%,
aluminium (Al) at about 0.02 wt% to 0.1 wt.%,
sulphur (S) up-to 0.008%,
phosphorous (P) up-to 0.025 wt%,
nitrogen (N) up-to 0.012 wt%,
the balance being Iron (Fe) along with incidental elements;
heating the steel slab to a first predetermined temperature for a predetermined time;
subjecting the steel slab to the hot working to produce a steel sheet, wherein, the hot working includes:
deforming, the steel slab in a first hot working process, at a second predetermined temperature; and,
deforming, the steel slab in a second hot working process, at a third predetermined temperature, to form a steel sheet;
cooling, the steel sheet at a predetermined cooling rate to a fourth predetermined temperature; and
coiling, the steel sheet, at the fourth predetermined temperature to obtain a high-strength hot-rolled steel sheet;
wherein, the high-strength hot-rolled steel sheet comprises ferrite- bainitic ferrite and or pearlite microstructure.
9. The method as claimed in claims 8, wherein casting is a continuous casting process.
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10. The method as claimed in claims 8, wherein the first predetermined temperature is greater than 1150 ℃, preferably ranging from about 1200 ℃ to 1250 ℃, and the first predetermined time ranging from about 30 minutes to about three hours.
11. The method as claimed in claim 8, wherein the hot working is a hot rolling process.
12. The method as claimed in claims 8 and 11, wherein the first hot working process is performed in a roughing mill, at the second predetermined temperature ranges from about 1050 ℃ to 1170 ℃.
13. The method as claimed in claims 8 and 11, wherein the second hot working process is performed in a finishing mill including at least four finishing mill stands.
14. The method as claimed in claims 8 and 11, wherein the third predetermined temperature ranges from 840 C to 950 °C,
15. The method as claimed in claim 8, wherein the first predetermined cooling rate is greater than 15°C/second.
16. The method as claimed in claim 10, wherein the fourth predetermined temperature ranges from 500 C to 600 C.
17. An automotive component manufactured from a high-strength steel sheet as claimed in claim 1.