Patent Description:
In automotive sector reduction in fuel consumption, therefore, lowering emission and maintaining high standard of safety demands use of stronger steel. Both the needs could be fulfilled through use of advanced high strength steels (AHSS) with high elongation. Ultra high strength (UHSS) or AHSS are not new until now. However, major issue with the UHSS is poor forming capability and weak load bearing capability due to limited elongation. As strength and elongation behaves opposite way in metals and alloys, therefore, with the development of stronger or UHSS steel, quite naturally the elongation also reduces or decreases significantly. As a result the application scope of UHSS for various parts in motor vehicle gets limited as forming become increasingly difficult. Therefore, UHSS steel development equally demands high elongation and formability. The situation / scenario mentioned has necessitated development of a hot rolled ultra-high strength (UHSS) thin steel sheet with combination of high tensile strength and extraordinary uniform elongation and total elongation for various automotive component such as lower suspension, long and cross member and bumpers as well.

Strong and tough steel is one of the major contributors to control air pollution. Light-weight environmental friendly vehicle design is essential now-a-days to address the problems of environmental pollution. Effective light-weight motor vehicles require utilization of advanced high strength and ultra-high strength steel (UHSS) sheets. However, because of its poor formability, the UHSS sheet cannot be applied easily to a wide variety of motor vehicle components. Hence, the ductility and formability required for UHSS sheet becomes increasingly demanding. Therefore addressing the present scenario has necessitated development of a hot rolled steel strip with high tensile strength coupled with excellent elongation for various automotive components such as lower suspension, long and cross member and bumpers as well.

In the recent past many researchers have attempted to develop UHSS steels. First such steel sheet with very high strength was reported by <NPL>; <NPL>; <NPL>). The source of very high strength was attributed to presence of nanostructured bainite or nano bainite. Although, the steel developed by Bhadeshia et. has very high strength, however, the application scope in automotive and many other is very limited specially due to high alloy content, long production time (<NUM>-<NUM> days) , limited elongation (<<NUM>%). The first two factors make difficulty in real production line whereas the last one is not favourable in end user side. The higher carbon content (><NUM>. 7wt%) makes the steel difficult for welding. Overall the steel is expansive and has inadequate formability.

Another group of researcher [(<NPL>; <NPL>] came with slightly different thought and tried to address the issues mentioned in last paragraph. These researchers attempted to improve the overall elongation, lower the cost and accelerate the production in real line and easily weldable by adjusting the composition through lowering of carbon content. However, the steel has not been considered for continuous production line for various reasons and also the steels contain high amount of expensive alloying additions like Ni and Mo in their production.

In an effort to meet the demand of present day motor vehicle manufacturers, recent work (Ref. <CIT>) attempted to obtain high strength and ductility combination. This work has successfully achieved minimum <NUM> MPa tensile strength with <NUM> % total elongation. However, it has high Carbon (> <NUM> wt. %) and Silicon ( > <NUM> wt. High amount of Carbon decreases the weldability and high Silicon causes surface scales during the process of hot rolled steel sheets. These problems yet to be addressed.

Very recently, another team of researcher [<CIT>] has developed a high strength steel with tensile strength greater than 1000MPa with elongation <NUM>-<NUM>% with yield strength 615MPa. Although the steel has good combination of strength and ductility but it has quite high amount of silicon which is not desirable from surface and coating point of view. Steel with such high silicon content also offer greater difficulty during rolling process in terms of product quality.

From <CIT> an ultra-high strength steel with a tensile strength between <NUM> and <NUM> MPa and percentage elongation of <NUM> to <NUM> % is known.

In view of this it is the object of the invention to disclose an UHSS steel that has super high elongation, and preferably good weldability, commercial viability and preferably can be produced by the existing hot rolling mill facilities. Preferably, the hot rolled product shall have a thickness minimum <NUM> with YS: TS ratio above <NUM>.

Further, the hot rolled product preferably shall have a thickness minimum <NUM>% with tensile toughness in the range <NUM> GPa-<NUM> GPa.

Further, the hot rolled product preferably shall comprise microstructural constituents <NUM>-<NUM> % martensite, <NUM>-<NUM> % bainite and <NUM> % austenite.

This object is solved by a steel according to claim <NUM>. Preferred embodiments are subject of the dependent claims.

The amount of carbon and manganese is restricted below certain level for better weldability, silicon was also kept lower to address the scale problem during hot rolling process. The optimum cooling and coiling was identified to ensure the steel could be produced under conventional mill operating parameters in the same run out table to obtain thicker sheet with high strength and elongation. The high strength and elongation was achieved through formation of low temperature phases mixture of bainte and martensite with small amount of retained austenite in final microstructure. The above mentioned phase constituent ensured the steel invented has ultra high strength with tensile strength at least <NUM> MPa and elongation not less than <NUM>%.

Table <NUM>Tensile properties of the steel.

The present invention relates to an ultra-high-strength hot-rolled steel strip or sheet according to the appended claims.

The tensile properties of steel developed as per the current invention has the property as described in the table <NUM> below:.

Description of the primary components constitutes (in weight percentage) the hot rolled steel sheet are described below.

C: <NUM> - <NUM> wt. Amount of carbon content must be adjusted to achieve desired strengthening, proportion of phase fractions so that proper strength level can be obtained. Amount of carbon also determine stability of retained austenite which is key to obtained enhanced elongation. Carbon level must also be controlled to ensure good weldability. Preferable carbon content should be kept below <NUM>% to achieve desired strength and elongation and also weldability, therefore, should be restricted below <NUM>%.

Mn: <NUM>- <NUM> wt. % Manganese addition ensured presence of stable retained austenite. However, its amount should be <NUM> or more, preferably <NUM> or more, more preferably <NUM>% or more. The amount of Mn needs to be <NUM>% or more, preferably <NUM>% or more, more preferably <NUM>% or more. Manganese amount should preferably be less than <NUM>% to avoid welding and casting crack AI: <NUM> - <NUM> wt. % Al is a stronger ferrite stabilizer. It does not allow the carbon to come out easily from retained austenite, thereby, allow more amount of retained austenite to be formed during bainite reaction. Al addition is favourable over Silicon addition from galvanizing point of view. However, the amount should not be excessive, which might further create problem during casting. Excess Al might allow formation of hard oxides in the weld area, thereby, deteriorate weldability. Hence, Al content in the newly developed steel should be maintained <NUM> % or preferably above <NUM>. 1wt% or more preferably below <NUM>. To ensure beneficial effect of Al the addition must be above <NUM> wt%. Preferably, Al varies in the range of <NUM> to <NUM>.

Si: <NUM>- <NUM> wt. % Silicon is also a ferrite stabiliser. Silicon suppress carbide precipitation during bainite transformation during constant temperature holding / coiling and alloy formation of greater amount of retained austenite in the microstructure. Excess amount of silicon addition in steel is detrimental due to varieties of scale formation during hot rolling and cooling. Scale formation leads to surface deterioration and reduce coatability / gavanizibility. Hence, Si should be restricted within certain range as mentioned and more preferably below <NUM>. Preferably, Si varies in the range of <NUM> to <NUM>.

P: <NUM>% maximum: Phosphorus is considered detrimental in steel. Therefore, should be amount be restricted to <NUM>% maximum or preferably <NUM>% or less.

S: <NUM>% maximum: Like Phosphrous Sulphur is also considered detrimental. So sulphur content to be kept as low as possible, preferably below <NUM> wt%.

More preferably sulphurs content should be below <NUM> wt% to minimize the amount of inclusions which is potential sites for premature failure during forming operations.

N: <NUM> % maximum: Excess nitrogen in steels is also detrimental. Excess nitrogen may lead to hard inclusions such as TiN and AIN which deteriorate formability. Therefore, nitrogen content has to be restricted below <NUM>.

Nb: <NUM>% maximum: Niobium is added to increase the strength of the steel by various mechanism such as grain refinement, precipitation. Nb addition also useful to have larger amount of retained austenite in the microstructure. Nb should be added carefully and optimized to take advantage of economic advantage as Nb is costly. Therefore, Nb level should be below <NUM>% or more preferably, below <NUM>%.

Mo: <NUM> wt. % maximum: Molybdenum is added to enhance the hardenability in steel, thereby, favours easy formation of bainite. Due to excess hardenability softer ferrite and relatively harder pearlite phase formation could be suppressed during bainitic reaction. As Mo is costly, therefore, its amount should be restricted below <NUM> wt% to make the steel economical and taking processing advantage during hot rolling. Preferably, Mo varies in the range of <NUM> to <NUM> weight percentage.

Cr: <NUM> wt. % maximum: Function of Chromium very much similar to Mo, avoids formation of polygonal ferrite and pearlite. Cr addition is more economical in advanced high strength steel. However, Cr could be harmful if added excessive amount as Cr form various kind of carbides. Preferably, Cr varies in the range (weight percentage) of <NUM> to <NUM>.

Ti: <NUM> wt% maximum: Ti is beneficial to restrict austenite grain growth. In addition, Ti also form very fine carbonitride in the presence of Nb, V and increase strength. Excess amount of Ti could be harmful as Ti has tendency to form hard TiN inclusions. Therefore, amount of Ti should be restricted below <NUM>. 1wt% and more preferably, below <NUM> wt%. Preferably, Ti varies in the range of <NUM> to <NUM>. The developed ultra high strength hot rolled steel comprising mainly banitic ferrite phase <NUM>-<NUM>% and remaining retained austenite phase (<NUM>-<NUM>%). Small amount of hard martensite phase of <NUM>-<NUM>% is also present in the steel at ambient temperature. Preferably austenite phase is present in the range of <NUM>-<NUM>%.

Bainite: The bainite present (<NUM> -<NUM>%) in the microstructure is essentially carbide free with high dislocation density. The bainite is typically lath in nature. Higher dislocation density, therefore, results in higher strength and good ductility. Retained Austenite: Retained austenite (<NUM>-<NUM>%) is one of the important constituents of the microstructure of the steel developed. Retained austenite helps to enhance the ductility. To get beneficial effect microstructure should have at least <NUM>% and preferably <NUM>% or higher austenite. Small amount of retained austenite present in the developed steel is good for enhancing ductility.

Martensite: The hot rolled steel strip produced according to the present invention has also some amount of martensite, preferably, not exceed <NUM>-<NUM>%.

The method adapted to develop the steel product with the specified composition consists of following steps: alloy melting or heat making, casting, hot rolling, accelerated cooling and coiling and cooling to ambient temperature. Each and every processing steps involved are derailed below:.

The alloy was melted in induction furnace and subsequently cast in the form of <NUM>-<NUM> thick bar or ingot. The ingot was homogenized by keeping the steel in the austenite for sufficient time and subsequently reducing the temperature to deform in the austenite and forged to break the cast structure and reduce the thickness suitable for rolling process and subsequently air cooled to ambient temperature. The homogenized steel was prepared for hot rolling. Prior to hot rolling the steel was soaked at high temperature above <NUM>°Cfor <NUM>-<NUM> hours and subsequently hot rolled to thickness minimum <NUM> with finish rolling temperature keeping in the austenite region and subsequently coiling was done into salt bath or similar kind of arrangement at predetermined temperature above Ms bit below Bs and hold for few hours. Coiled steel samples were then transferred to air and allowed to cool to ambient temperature. Specimens for microstructure and mechanical properties were taken from the hot rolled sheet. Microstructural characterization was carried out using optical, scanning electron microscope and orientation imaging microscopy. Mechanical properties were evaluated by Vickers hardness method and tensile tests were performed as per ASTM standard. X-Ray diffraction was employed to confirm the microstructural constituents.

Mechanical properties of the new developed steel are evaluated by tensile testing. Tensile stress-strain curve of the invented steel is depicted in <FIG>. Figure shows the steel has very high tensile strength and tensile ductility. Ultimate tensile strength (UTS) and elongation of the steel is at least 1100MPa and <NUM>% respectively. The strain hardening exponent value is in the range <NUM> to <NUM>. The uniform elongation is in the range <NUM>-<NUM>%. The optical micrograph of the newly developed steel is presented in <FIG>. The micrograph confirms that the developed steel has predominantly banitic ferrite with small amount of retained austenite and some martensite. The scanning electron micrographs presented in <FIG> further confirmed that the invented steel contains mainly bainite with small amount of other phases such as retained austenite and martensite. EBSD micrograph further ensured the observation made in <FIG>. Thickness of banitie sheaves determines the strength and toughness of the steel. The thickness of sheaves was found below submicron level. X-ray diffraction carried is out on the developed steel showed presence diffraction peaks from body centre cubic (BCC) indicated by αbcc and face centre cubic (FCC)austenite indicated by γfcc peak in the plot shown in <FIG>. The intensity of the BCC phase peak is several times higher than the intensity of the FCC peak clearly confirmed the amount of BCC bainite phase is the major phase in the developed steel. This confirms that the newly steel developed has mainly the bainite structure and some amount of martensite along with little amount of retained austenite. The amount of retained austenite determined at least <NUM>-<NUM>%. Electron back scatter diffraction (EBSD) presence of small amount of retained austenite.

Examples: The following examples relate to comparative embodiments of invention.

The steels developed are shown in table <NUM> and designated as example <NUM>, example <NUM> and example <NUM>. The processing conditions involved for these examples are described below:
The developed steel given in example <NUM> was soaked in the temperature <NUM>-<NUM> using heating rate <NUM>-<NUM>/. The steel was cooled and subjected to rough rolling in the temperature range <NUM>-<NUM> applying deformation in the range <NUM>-<NUM>%. The rough rolled steel was further cooled and subjected to hot rolling with finishing rolling temperature in the range <NUM>-<NUM> applying deformation <NUM>-<NUM>%. The steel finished rolled steel was cooled using cooling rate not less than <NUM>/s and coiled in the temperature range <NUM>-<NUM> followed by air cooling to room temperature.

The steel given in example <NUM> was processed by soaking in the temperature <NUM>-<NUM>. The heating rate employed during soaking was <NUM>-<NUM>/s. The soaked steel was cooled and subjected to roughening deformation by compression in the temperature range <NUM>-<NUM> applying deformation in the range <NUM>-<NUM>%. The rough rolled steel was further cooled and subjected to hot rolling with finishing rolling temperature in the range <NUM>-<NUM> applying deformation <NUM>-<NUM>%. The steel finished rolled steel was cooled using rate at least <NUM>-<NUM>/s and coiled in temperature range <NUM>-<NUM> followed by air cooling to room temperature.

The steel given in example <NUM> was reheated in the temperature <NUM>-<NUM>. The heating rate employed during soaking was <NUM>-<NUM>/s. The soaked steel was cooled and subjected to rough deformation around <NUM>-<NUM>% in the temperature range. The rough rolled steel was cooled and subjected to hot rolling deformation <NUM>-<NUM>% using several passes and the steel was finish rolled in the temperature <NUM>-<NUM>. The steel finished rolled steel was cooled using rate not less than <NUM>-<NUM>/s and coiled in temperature range <NUM>-<NUM> followed by air cooling to room temperature.

The steel produced has excellent combination of tensile strength and ductility to make it useful for automotive structural application and several other areas where good combination of tensile strength and elongation properties is needed. Also, the presence of low silicon in the developed product allows the steel to be rolled in conventional hot strip mill. Further, low silicon in the steel reduces scale formation issues during hot rolling. The product developed with relatively low silicon is expected to improve coat ability and surface texture. Also, low carbon equivalent of the steel will allow easily weldable and presence of Aluminium in the developed product increases the castability.

Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

Claim 1:
An ultra-high-strength hot-rolled steel strip or sheet with tensile strength of at least <NUM> MPa and total elongation not less than <NUM>%, comprising in weight percentage:

<TAB>

wherein the remainder is Fe with inevitable impurities;
wherein the steel comprises by volume <NUM> to <NUM>% of bainitic ferrite phase, <NUM> to <NUM>% of martensite phase, and retained austenite phase as a remainder, measured with method according to the description.