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 the high strength hot rolled steel sheet with tensile strength of minimum 590 MPa, excellent stretch flangeability, and high ductility.

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), Chromium (Cr), etc. Because of its high tensile strength and low cost, steel may be considered as a major component in wide variety of applications. Some of the 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. 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 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 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 motor vehicles, impose conflicting requirements on materials used for the vehicle bodies. The vehicle bodies are required to be stronger yet lighter at the same time. In the recent past several technological advancements have been made, and such advancements may include commercialization of fuel cell vehicles and use of lighter materials like aluminium, composites etc. These materials meet the desired material properties but the associated problems like formability, reliability and recyclability and the higher cost make these materials commercially unattractive and hence, these are used only for specific types of components and vehicles. Thus, it becomes inevitable to re-look at advanced high strength steels to meet the desired properties, as usage of steel mitigates the above-mentioned issues with other materials.

There have been several developments in the field of advanced high strength steels, which poses good combination of tensile strength, elongation and stretch flangeability to address some of the afore-mentioned concerns. One such conventional process include development of Nb-V based SPFH590 steel. The method to manufacture 590 MPa strength steel with ferrite-bainite microstructure is disclosed. Though the steel has high strength and high ductility, the stretch flangeability may be lower due to ferrite-bainite two phase microstructure. Hardness difference exists between both phases, thereby reducing the hole expansion ratio. Similarly, one of the patent literatures disclose DP590 steel and a method to create steel with minimum tensile strength of 590 MPa and having a ferrite martensite microstructure. Due to significant difference in hardness between ferrite and martensite phases, the hole expansion ratio of dual phase steel is also low. Further, attempts have been made at developing steels with high strength and high stretch flangeability by addition of elements like Mo. However, this makes the steel expensive.

One more patent literature known in the art has focussed on developing fully ferritic microstructure by fixing the carbon with Titanium (Ti) (TiC precipitates). Also, in some publications it is disclosed that chemistry with high levels of silicon. However, surface quality of these steels may be poor as silicon promotes formation of scale during hot rolling.

In most of the conventional process as described above, primarily, precipitation strengthening has been utilized as the key mechanism to obtain strength. However, it will be difficult to replicate similar level of precipitation strengthening in every grains of the microstructure due to the variation in processing conditions across width and length of strip in actual plant scale production. This creates inhomogeneity in deformation behaviour inside the material during hole expansion test and hence, poses difficulty in obtaining high stretch flangeability in precipitation strengthened steels produced through large scale processing in hot strip mills.

Hence, there is a need for an economically attractive and technically viable way of developing high strength hot rolled steel with high ductility and high stretch flangeability grades without aforementioned limitations.

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, there is provided a method for producing high strength hot rolled steel sheet. The process starts from casting a steel slab of composition comprising in weight percentage of: carbon (C) at about 0.02% to about 0.06 %, manganese (Mn) at about 0.9% to about 1.5 %, silicon (Si) at about 0.05% to about 0.2 %, titanium (Ti) at about 0.05% to 0.10 %, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007%, the balance being Iron (Fe) optionally along with incidental elements. Then, heating, the steel slab to a first predetermined temperature for a first predetermined time. The steel slab is then subjected 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. After hot working, the steel sheet is cooled at a predetermined cooling rate. The method further includes coiling, the steel sheet, at a fourth predetermined temperature to obtain the high-strength hot rolled steel sheet. The high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure.

In an embodiment, the high-strength hot-rolled steel sheet exhibits tensile strength greater than 590 MPa, ductility ranging from about 19% to about 30%, and a hole expansion ratio of greater than 85%.

In an embodiment, the high-strength hot-rolled steel sheet comprises bainitic ferrite microstructure greater than 85% and the balance being polygonal ferrite and carbides.

In an embodiment, the casting is carried out by a continuous casting process. The continuous casting process is performed in at least one of continuous caster and a thin slab caster, and the temperature of the steel slab at exit of the thin slab caster is maintained above 1000 ?C.

In an embodiment, the first predetermined temperature is greater than 11500C, preferably ranging from about 12000C to 12500C, and the first predetermined time ranging from about 30 minutes to about three hours.

In an embodiment, wherein the hot working is a hot rolling process. The first hot working process is performed in a roughing mill, and the second predetermined temperature is greater than a recrystallization temperature of the steel slab. Further, the second hot working process is performed in four or more than four stands of a finishing mill.

In an embodiment, the third predetermined temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium.

In an embodiment, the predetermined cooling rate is greater than 300C/second, and the cooling is performed till the temperature of the steel sheet reaches the fourth predetermined temperature. The fourth predetermined temperature is ranging from about 450°C to 590°C, preferably 540°C to 580°C.

In an embodiment, the cooling is a laminar cooling, and the cooling is carried out on a run-out-table.

In an embodiment, the method further includes performing a pickling and skin pass treatment on the steel sheet after hot working.

In another non-limiting embodiment of the disclosure a high strength hot rolled steel sheet is disclosed. The steel comprising of a composition in weight percentage (wt%) of: carbon (C) at about 0.02% to about 0.06 %, manganese (Mn) at about 0.9% to about 1.5 %, silicon (Si) at about 0.05% to about 0.2 %, titanium (Ti) at about 0.05% to 0.10 %, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, the balance being Iron (Fe) optionally along with incidental elements.
In an embodiment, the high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure greater than 85% and the balance being polygonal ferrite and carbides.

In an embodiment, wherein the manganese (Mn) is preferably in the range of about 1.1% to about 1.4%. Further, the titanium (Ti) is preferably in the range of about 0.06% to about 0.09%.

In yet another non-limiting embodiment, a method for manufacturing high strength hot rolled steel sheet is disclosed. The method comprising: heating a steel slab of composition comprising in weight percentage of: carbon (C) at about 0.02% to about 0.06 %, manganese (Mn) at about 0.9% to about 1.5 %, silicon (Si) at about 0.05% to about 0.2 %, titanium (Ti) at about 0.05% to 0.10 %, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, the balance being Iron (Fe) optionally along with incidental elements, to a temperature greater than 11500C for a time ranging from about 30 minutes to about three hours. Then, hot rolling, the steel slab to produce a steel sheet. The hot rolling includes: deforming the steel slab in a roughing mill, at a temperature above the recrystallisation temperature of the steel slab, and deforming the steel in four or more stages of a finishing mill, at a temperature raging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium. The method further includes cooling, the steel sheet at a cooling rate greater than 300C/second to a coiling temperature; and coiling, the steel sheet, at the coiling temperature ranging from about 450 °C to 590 °C to obtain a high-strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure.

In an embodiment, the method further comprising: performing a pickling and skin pass treatment on the steel sheet after hot rolling. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.
In still another embodiment, automotive chassis part, hinges and suspension parts, comprising a high-strength hot-rolled steel sheet as described above is disclosed.

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.

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 the method for producing high strength hot rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figures. 3a-3c illustrates microstructure of steel sheet-sample-1 manufactured by method of present disclosure in which (3a) is an Optical micrograph, (3b) is a SEM micrograph showing chunky pearlite, and (3c) is EBSD image showing bainitic ferrite grains having features of low angle boundaries inside, according to an exemplary embodiment of the present disclosure.

Figures. 4a and 4b illustrates microstructure of steel sheet-sample-3 manufactured by method of present disclosure in which (4a) is an Optical micrograph, and (4b) is EBSD image showing bainitic ferrite grains having features of low angle boundaries inside grains, in accordance with some embodiments of the present disclosure.

Figure. 5a and 5b illustrates microstructure of steel sheet-sample-4 manufactured by method of present disclosure in which (5a) is an Optical micrograph, and (5b) is EBSD image showing bainitic ferrite grains having features of low angle boundaries inside grains, in accordance with some embodiments of the present disclosure.
Figures. 6a-6c illustrates a graphical representations of hardness distribution in the cross-section of steel sheets samples 1, 3 and 4 respectively, manufactured by the method of present disclosure.
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 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.

Embodiments of the present disclosure discloses a high strength hot rolled steel sheet and a method for manufacturing or producing a high strength hot rolled steel sheet. Strength, ductility and stretch flangeability are some of the important properties for the mass industrial application of high strength material like steel. As of now, high strength steel sheets with tensile strength more than 590MPa are produced by the methods in which precipitation strengthening has been utilized as the key mechanism to obtain strength. However, it will be difficult to replicate similar level of precipitation strengthening in every grains of the microstructure due to the variation in processing conditions across width and length of strip in actual plant scale production. Accordingly, the method of present disclosure, discloses a production of high-strength hot rolled steel sheet, with tensile strength of minimum 590 MPa with excellent stretch flangeability. The present disclosure is directed towards producing a low carbon hot rolled steel sheet with tensile strength greater than 590 MPa along with a hole expansion ratio more than 85%, and % elongation more than 19%. The hot rolled steel sheet may be widely employed to make automotive components requiring high strength, high ductility and high stretch flangeability.

In the method of manufacturing high strength hot rolled steel sheet, includes first step of producing the steel slab of composition including in weight percentage of 0.02 - 0.06 % of carbon, 0.9 – 1.5 % of manganese - preferably 1.1 -1.4 %, 0.05- 0.2 % of silicon, 0.05-0.10 % of titanium- preferably 0.065 – 0.095%, 0 – 0.06 % of aluminum, maximum up to 0.005 % of sulphur, maximum up to 0.025 % of phosphorous and up to 0.007 % of nitrogen, the balance being iron and impurities by any manufacturing process including but not limiting to casting. The steel slab is then reheated to a temperature greater than 1150 °C. The steel slab is then hot-rolled in roughing mill and finishing mill. The finish rolling temperature may vary in the range of Ae3 to Ae3 + 70 °C, where Ae3 is the temperature at which the transformation of austenite to ferrite starts at equilibrium. After the hot rolling step, the steel sheet may be cooled at a cooling rate of greater than 30 °C/s and then coiling the steel strip at coiling temperature (TCT). Coiling temperature TCT varies in the range 450°C to 590°C preferably in the range of 530°C to 580°C. The hot rolled steel strip according to the present disclosure may have a microstructure comprising of more than 85 % bainitic structures.

In an embodiment, high strength hot rolled steel sheet exhibits, tensile strength of more than 590MPa along with excellent stretch flangeability, hole expansion ration more than 85%, and ductility more than 19% to suit automotive applications and its manufacturing process.

In an embodiment, the single phase bainitic ferrite structure in the hot rolled steel sheet enable in obtaining a high stretch flangeability.

In an embodiment, the alloying composition of the hot rolled steel sheet includes very low amount of Si, this avoids the possibility of detrimental scales post hot rolling. Further, alloying composition of the hot rolled steel sheet includes Ti, which is less expensive microalloying element than Nb, V, Mo. Addition of Ti primarily achieves fully bainitic ferrite structures.

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 are 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, stretch flangeability, and hole expansion ratio of the final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes a bainitic-ferrite microstructure. 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 according to the present disclosure consists of a casting step followed by a hot rolling step, a controlled cooling step and a coiling step using a steel material which satisfies the component composition described below. The various processing steps are described in their respective order below:

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 0.02 - 0.06 % of carbon, 0.9 – 1.5 % of manganese - preferably 1.1 -1.4 %, 0.05- 0.2 % of silicon, 0.05-0.10 % of titanium- preferably 0.065 – 0.095%, 0 – 0.06 % of aluminum, maximum up to 0.005 % of sulphur, maximum up to 0.025 % of phosphorous and up to 0.007 % of nitrogen, the balance being iron and impurities, is first continuously cast either in a conventional continuous caster or a thin slab caster. When cast in a thin slab caster, the temperature of the cast slab may not be allowed to drop to a temperature below 1000 °C, as it might result in loss of ductility and cause formation of edge cracks. There is also a possibility of the micro alloying elements to precipitate out and it may then become difficult to completely dissolve the precipitates in the subsequent reheating process rendering them ineffective for precipitation strengthening.

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 should be above 1150 °C, to ensure complete dissolution of any precipitates that may have formed in the preceding processing steps. 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.

Once the steel slab is heated as per the block 102, it may be subjected for hot working as shown in block 103 to form a steel sheet. In an embodiment, the hot working process is hot rolling. 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 is above recrystallisation temperature of the steel. 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 strands of the finishing mill, and temperatures in all strands of the finishing mill are such that microstructure of the material consists of single phase austenite. In an embodiment, the third predetermined temperature is in the range of Ae3 to Ae3 + 70 °C., wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium.
After finish rolling, the rolled steel sheet may be subjected to laminar cooling at a predetermined cooling rate as shown in block 104. In an embodiment, the predetermined cooling rate is greater than 30 °C/s till a desired coiling temperature is reached. The laminar cooling is performed on a run-out-table. In an embodiment, the predetermined cooling rate may be higher than specified to prevent formation of polygonal ferrite and pearlite. Rather the specified cooling rate will result in formation of bainitic ferrite which is different in a way from the conventional polygonal ferrite.

Now referring to block 105, the method further includes the step of coiling. Coiling may be carried out at a coiling temperature or a fourth predetermined temperature. In an embodiment, the fourth predetermined temperature ranges from about 450°C to 590°C. It is preferable to keep the fourth predetermined temperature at 540°C to 580°C in order to achieve optimized ductility and stretch flangeability. Coiling below 450°C may be avoided to prevent the formation of martensite microstructure in the steel, and coiling above 590°C, may create second phase such as pearlite. It also generates significant amount of precipitates thereby increasing the strength to much higher value. Precipitation in bainitic ferrite may also lead to variation in distribution of precipitates throughout the microstructure. Both second phases and inhomogeneity in precipitates distribution will reduce stretch flangeability. A schematic diagram of the cooling profile is shown in Figure. 2. This ensures that the microstructure consists of bainitic ferrite only.

The method optionally comprises cleaning the steel sheet by acid pickling and skin pass treatment. In an embodiment, the hot rolled sheet may be uncoiled and pickled in a pickling line and then skin passed in a skin pass mill and then coiled. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.
In an embodiment, the high-strength hot rolled steel sheet exhibits tensile strength greater than 590 MPa along with a hole expansion ratio more than 85%, and elongation more than 19%. In order to achieve the required mechanical properties as proposed in the disclosure, it is required to obtain a very homogenous microstructure. In the present disclosure, the microstructure consists of low carbon bainitic structures without any significant amount of polygonal ferrite. Carbide formation may be minimized as it is a very hard phase and often acts as a source for void formation during deformation and results in premature failure and very low stretch flangeability. Low carbon content is hence maintained in the desired range so as to obtain very low amount of carbide.

Strength may be primarily obtained from strength of bainitic structures. It is intended to restrict formation of polygonal ferrite in the microstructure as large hardness difference between polygonal ferrite and bainitic ferrite will be detrimental. Hence, a homogenous microstructure may be desired so that the stretch flangeability remains good. Inhomogeneity in microstructure causes inhomogeneous strain partitioning and affects local ductility. Hence a microstructure consisting of more than 85% bainitic ferrite along with very less cementite has been developed to achieve strength more than 590 MPa along with a HER more than 85%.

The following portions 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 0.02 wt% to about 0.06 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 0.02 wt%. But carbon content in steel beyond 0.06 wt% may have detrimental effects like it will lead to formation of more amount of undesirable second phases such as carbides and thereby, ductility and hole expansion ratio may get deteriorated.

Manganese (Mn) may be added in the range of about 0.91 wt% to about 1.5 wt%. Manganese not only imparts solid solution strengthening to the ferrite but it also lowers the austenite to ferrite transformation temperature. However, the Mn level cannot be increased to beyond 1.5 wt% as at such high levels it enhances centerline segregation during continuous casting and hence will create inhomogeneity in microstructure which will have detrimental effect on stretch flangeability. Also, it will have cost implications.

Silicon (Si) may be added in the range of about 0.05wt% to about 0.2 wt%. Silicon is a very cheap solid solution strengthening element and it has more solid solution strengthening potential than like manganese. However, Si content beyond 0.2 wt% promotes formation of scales during high temp soaking and which is often undesirable by the end customers.

Phosphorus (P) may be added about 0.025wt% maximum. Phosphorus content should 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 may be added about 0.005wt% maximum. Sulphur content has to be limited otherwise it results in a very high inclusion level that deteriorates formability.

Nitrogen (N) may be added 0.007wt% maximum. Higher nitrogen may fix up higher amount of titanium through formation of TiN, which may not be effective strengthener. It reduces availability of Ti and hence reduces the effectiveness of Ti which is utilized for strengthening purposes through transformation strengthening and TiC precipitates. Also, increase in nitrogen content increases size of the TiN. Larger sized TiN will reduce both ductility and hole expansion ratio.

Aluminium (Al) may be added in the range of 0 – 0.05 wt %. Aluminium is used as a deoxidizer and killing of steel. It limits growth of austenite grains. Higher amount of Al causes casting issues, hence should be restricted.

Titanium (Ti) may be added in the range of about 0.05 wt% to about 0.10 wt%. Titanium improves strength by limiting austenite grain size. More importantly, Ti addition in disclosure is to create close to single phase bainitic ferrite structures, as it restricts diffusional transformation (ferrite) before commencement of bainitic transformation (which occurs at lower temperature). Further, titanium forms carbides which when finely dispersed promotes strengthening.

Example:

Further embodiments of the present disclosure will now be described with an example of a particular composition of the steel. 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 N S P
A 0.037 1.37 0.14 0.045 0.08 0.0054 0.003 0.013
B 0.032 1.18 0.085 0.05 0.09 0.0052 0.004 0.018

Table – 1

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
Steel
Composition
Thickness, mm
Third predetermined temperature (°C)
Coiling or fourth predetermined temperature (°C)
Steel 1 A 3.2 900 620
Steel 2 A 2 920 618
Steel 3 A 2 900 580
Steel 4 B 3.2 910 570
Steel 5 A 3.2 920 470

Table-2
As evident from Table-2 above, for steel sheet samples 3-5, the processing was done in accordance with the present invention, whereas for steel sheet samples 1 and 2, a higher coiling temperature of more than 600°C was used. Steel sheet samples 1 and 2 did not meet the target properties in both hot rolled and skin passed hot rolled sheets.
The microstructures of steel sheet samples 1, 3 and 4 are shown in Figures 3a-3c, 4a-4b and 5a-5b respectively. All steels show microstructure which have been obtained primarily through bainitic transformation. As the carbon content in their chemistry is very low, the start temperature of bainitic transformation is high. The EBSD maps shown in Figure 3(c), 4(b), 5(b) confirm presence of low angle boundaries inside ferrite grains which suggest they have formed at low temperature (w.r.t. diffusional transformation i.e. polygonal ferrite transformation) and most of these ferrite are bainitic ferrite. If the coiling is done at higher temperature, the bainitic ferrite may have less dislocation density as completion of bainitic ferrite formation is occurring at higher temperature where dislocation annihilation is possible. Such bainitic ferrite will give higher ductility. But as the coiling temperature reaches 620°C, strength of steel becomes higher.
As shown in figures 6a-6c, the hardness of steel sheet-sample 1 has not only increased but also there is a significant variation in hardness suggesting development of inhomogeneity in microstructure. However, steel sheet-samples 3 and 4 show bainitic ferrite microstructure but the hardness variation is comparatively less. Hence, these steels i.e. steel sheet-samples 3 and 4 are more homogenous in terms of microstructure and hence more stretch flangeability may be observed. But as the coiling temperature becomes lower, the bainitic ferrite formed at lower temperature may have higher dislocation density and hence, lower ductility may be observed. Steel sheet sample 5 is coiled at very low temperature of 470°C and it shows very high stretch flangeability. However, the ductility is relatively lower than steel sheet samples- 3 and steel 4, though the ductility is higher than minimum requirement of 19%. Hence, in order to obtain optimum ductility and stretch flangeability, the coiling temperature or third predetermined temperature is to be preferably kept between 540 °C to 570 °C so that both high ductility and high stretch flangeability may be obtained.
Thus it is clear from the mechanical properties and the microstructures achieved, that the target properties cannot be achieved when the coiling temperature or third predetermined temperature cooling parameters do not conform to the processing parameters of the present disclosure.

Now referring to Tables 3 and 4 below, mechanical properties of of hot rolled sheet, and of hot rolled and skin passed sheet has been illustrated respectively.
Steel
Composition Thickness, mm YS
(MPa) TS
(MPa) Elongation
(%) HER
(%)
Steel 1 A 3.2 599 644 30 60
Steel 2 A 2 560 611 26 70
Steel 3 A 2 560 620 25 88
Steel 4 B 3.2 555 604 27 106
Steel 5 A 3.2 554 608 22 115

Table-3
Steel
Composition Thickness,mm YS
(MPa) TS
(MPa) Elongation
(%) HER
(%)
Steel 1 A 3.2 590 650 26 78
Steel 2 A 2 593 654 22 78
Steel 3 A 2 569 621 21 93
Steel 4 B 3.2 570 621 23 103

Table-4
From the Tables 3 and 4, it is evident that, though mechanical properties like yield strength, tensile strength, and ductility of steel sheet samples 1 and 2 is in comparable limits with that of steel sheet samples 3-5. However, the hole elongation ratio of steel sheet samples 3-5 is much superior when compared to steel sheet samples 1 and 2. Thus, it is evident that, in order to obtain optimum ductility and stretch flangeability, the coiling temperature or fourth predetermined temperature is to be preferably kept between 540°C to 570°C so that high ductility, remarkable hole expansion ratio and high stretch flangeability may be obtained.

It should be understood that the experiments are carried out for particular compositions of the steel and the results brought out in the previous paragraphs are for the compositions shown in Table – 1. However, the said compositions should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the steel as well.

In another embodiment, the present disclosure discloses, a method for manufacturing high strength hot rolled steel sheet is disclosed. The method comprising: heating a steel slab of composition comprising in weight percentage of: carbon (C) at about 0.02% to about 0.06 %, manganese (Mn) at about 0.9% to about 1.5 %, silicon (Si) at about 0.05% to about 0.2 %, titanium (Ti) at about 0.05% to 0.10 %, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, the balance being Iron (Fe) optionally along with incidental elements, to a temperature greater than 11500C for a time ranging from about 30 minutes to about three hours. Then, hot rolling, the steel slab to produce a steel sheet. The hot rolling includes: deforming the steel slab in a roughing mill, at a temperature above the recrystallisation temperature of the steel slab; and deforming the steel slab in one or more stages of a finishing mill, at a temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium. The method further includes cooling, the steel sheet at a cooling rate greater than 300C/second to a coiling temperature; and coiling, the steel sheet, at the coiling temperature ranging from about 450 °C to 590 °C to obtain a high-strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure.

In an embodiment, the method further comprising: performing a pickling and skin pass treatment on the steel sheet after hot rolling. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.
In an embodiment of the present disclosure, the high strength-high hot rolled steel sheet of the present disclosure may be used any application including but not limiting to automotive applications to manufacture structural components like chassis, pillars, outer and inner panels, suspension parts and the like. The high strength-hot rolled steel sheet may be used in any other industrial structural applications.

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 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, 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
101-108 Flowchart blocks
101 Forming stage
102 Heating stage
103 Hot working stage
104 Cooling stage
105 Coiling stage

Claims:

1.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 of:
carbon (C) at about 0.02% to about 0.06 %,
manganese (Mn) at about 0.9% to about 1.5 %,
silicon (Si) at about 0.05% to about 0.2 %,
titanium (Ti) at about 0.05% to 0.10 %,
aluminium (Al) up-to 0.06 %,
sulphur (S) up-to 0.005 %,
phosphorous (P) up-to 0.025 %,
nitrogen (N) up-to 0.007 %,
the balance being Iron (Fe) optionally along with incidental elements;
heating, the steel slab to a first predetermined temperature for a first predetermined time;
hot working, on the steel slab 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;
cooling, the steel sheet at a predetermined cooling rate; and
coiling, the steel sheet, at a fourth predetermined temperature to obtain the high-strength hot rolled steel sheet;
wherein, the high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure.

2. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet exhibits tensile strength greater than 590 MPa.

3. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet exhibits ductility ranging from about 19% to about 30%.

4. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet exhibits hole expansion ratio of greater than 85%.

5. The method as claimed in claim 1, wherein the high-strength hot-rolled steel sheet comprises bainitic ferrite microstructure greater than 85% and the balance being polygonal ferrite and carbides.

6. The method as claimed in claim 1, wherein the casting is carried out in a continuous casting process.

7. The method as claimed in claim 6, wherein the continuous casting process is performed in at least one of continuous caster and a thin slab caster.

8. The method as claimed in claim 7, wherein the temperature of the steel slab at exit of the thin slab caster is maintained above 1000 ?C.

9. The method as claimed in claim 1, wherein the first predetermined temperature is greater than 11500C, preferably ranging from about 12000C to 12500C, and the first predetermined time ranging from about 30 minutes to about three hours.

10. The method as claimed in claim 1, wherein the hot working is a hot rolling process.

11. The method as claimed in claim 1, wherein the first hot working process is performed in a roughing mill, and the second predetermined temperature is greater than a recrystallization temperature of the steel slab.

12. The method as claimed in claim 1, wherein the second hot working process is performed in four or more than four stands of a finishing mill.

13. The method as claimed in claim 1, wherein the third predetermined temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium.

14. The method as claimed in claim 1, wherein the predetermined cooling rate is greater than 300C/second, and the cooling is performed till the temperature of the steel sheet reaches the fourth predetermined temperature.

15. The method as claimed in claim 1, wherein the fourth predetermined temperature is ranging from about 450°C to 590°C, preferably 540°C to 580°C.

16. The method as claimed in claim 1, wherein the cooling is a laminar cooling, and the cooling is carried out on a run-out-table.

17. The method as claimed in any of the preceding claims, further comprising: performing a pickling and skin pass treatment on the steel sheet after hot working.

18. A high-strength hot-rolled steel sheet with a tensile strength greater than 590 MPa, comprising:
composition in weight percentage of:
carbon (C) at about 0.02% to about 0.06 %,
manganese (Mn) at about 0.9% to about 1.5 %,
silicon (Si) at about 0.05% to about 0.2 %,
titanium (Ti) at about 0.05% to 0.10 %,
aluminium (Al) up-to 0.06 %,
sulphur (S) up-to 0.005 %,
phosphorous (P) up-to 0.025 %,
nitrogen (N) up-to 0.007 %,
the balance being Iron (Fe) optionally along with incidental elements.

19. The high-strength hot-rolled steel sheet as claimed in claim 18, wherein the high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure greater than 85% and the balance being polygonal ferrite and carbides.

20. The high-strength hot-rolled steel sheet as claimed in claim 18, wherein the high-strength hot-rolled steel sheet exhibits tensile strength greater than 590MPa.

21. The high-strength hot-rolled steel sheet as claimed in claim 18, wherein the high-strength hot-rolled steel sheet exhibits ductility ranging from about 19% to about 30%.

22. The high-strength hot-rolled steel sheet as claimed in claim 18, wherein the high-strength hot-rolled steel sheet exhibits hole expansion ratio of greater than 85%.

23. The high-strength hot-rolled steel sheet as claimed in claim 18, wherein the manganese (Mn) is preferably in the range of about 1.1% to about 1.4%.

24. The high-strength hot-rolled steel sheet as claimed in claim 18, wherein the titanium (Ti) is preferably in the range of about 0.06% to about 0.09%.

25. A method for manufacturing a high-strength hot-rolled steel sheet, the method comprising:
heating, a steel slab of a composition comprising in weight percentage of:
carbon (C) at about 0.02% to about 0.06 %,
manganese (Mn) at about 0.9% to about 1.5 %,
silicon (Si) at about 0.05% to about 0.2 %,
titanium (Ti) at about 0.05% to 0.10 %,
aluminium (Al) up-to 0.06 %,
sulphur (S) up-to 0.005 %,
phosphorous (P) up-to 0.025 %,
nitrogen (N) up-to 0.007 %,
the balance being Iron (Fe) optionally along with incidental elements, to a temperature greater than 11500C for a time ranging from about 30 minutes to about three hours;
hot rolling, the steel slab to produce a steel sheet, wherein, the hot rolling includes:
deforming the steel slab in a roughing mill, at a temperature above the recrystallisation temperature of the steel slab; and
deforming the steel slab in one or more stages of a finishing mill, at a temperature raging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium;
cooling, the steel sheet at a cooling rate greater than 300C/second to a coiling temperature; and
coiling, the steel sheet, at the coiling temperature ranging from about 450 °C to 590 °C to obtain a high-strength hot-rolled steel sheet;
wherein, the high-strength hot-rolled steel sheet comprises primarily a single phase bainitic ferrite microstructure.

26. The method as claimed in claim 25, further comprising: performing a pickling and skin pass treatment on the steel sheet after hot rolling.

27. The method as claimed in claim 26, wherein pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.

28. Automotive chassis part, hinges, and suspension parts comprising a high-strength hot-rolled steel sheet as claimed in claim 18.