The present invention relates to hot rolled steel sheet suitable for use as structural steel or for manufacturing industrial machinery, yellow goods, green goods and for cryogenic applications.

In recent years, efforts have been actively made to reduce the weight of the equipment and structures by applying high-strength steel for the purpose of improving fuel efficiency as well as reducing the environmental impact. However, when the strength of the steel is increased, the toughness generally deteriorates. Therefore, in the development of high-strength steel, it is an important issue to increase the strength without deteriorating the toughness.

Intense Research and Development endeavors are put in reducing the amount of material utilized by increasing the strength of material. Conversely, an increase in strength of steel decreases toughness, and thus development of materials having both high strength and good toughness is necessitated.

Earlier Research and Developments in the field of high strength and good toughness steel have resulted in several methods for producing high strength steel, some of which are enumerated herein for conclusive appreciation of the present invention:

EP2392681 discloses thick-walled high-strength hot rolled steel sheet having composition which contains by mass% 0.02 to 0.08% C, 1.0% or less Si, 0.50 to 1.85% Mn, 0.03% or less P, 0.005% or less S, 0.1% or less Al, 0.03 to 0.10% Nb, 0.001 to 0.05% Ti, 0.0005% or less B, optionally one or two kinds or more selected from a group consisting of 0.010% or less Ca, 0.02% or less REM, 0.003% or less Mg, 0.5% or less V, 1.0% or less Mo, 1.0% or less Cr, 4.0% or less Ni, 2.0% or less Cu, other unavoidable impurities and Fe as balance. The steel sheet has the structure formed of bainitic ferrite phase or a bainite phase in which solid-solution C content in ferrite grains is 10ppm or more, and surface layer hardness is 230HV or less in terms of Vickers hardness but the steel of EP2392681 is unable to reach the tensile strength of 700MPa or more. EP2971211 discloses a method for fabricating a high manganese steel component having a composition consisting of: manganese ranging from about 9 to about 20 weight % of the total composition, carbon ranging from about 0.5 to about 2.0 weight % of the total composition, and the balance iron; and optionally: chromium ranging from 0.5 to 30 weight % of the total composition; nickel or cobalt ranging from 0.5 to 20 weight % of the total composition; aluminum ranging from 0.2 to 15 weight % of the total composition; molybdenum, niobium, copper, titanium or vanadium ranging from 0.01 to 10 weight % of the total composition; silicon ranging from 0.1 to 10 weight % of the total composition; nitrogen ranging from 0.001 to 3.0 weight % of the total composition; boron ranging from 0.001 to 0.1 weight % of the total composition; or zirconium or hafnium ranging from 0.2 to 6 weight % of the total composition; heating the composition to at least about 1000°C; cooling the composition at a rate of from about 2°C per second to about 60°C per second, followed by hot rolling the composition at a temperature in a range of about 700°C to about 1000°C; slowly cooling or isothermally holding the composition; and quenching or accelerated cooling or air cooling the composition from a temperature in a range of from 700°C to about 1000°C to a temperature in range of from 0°C to about 500°C at a rate of at least about 10°C per second. But EP2971211 is not able to reach an Impact toughness of 60J/cm2 or more when measured at -40°C

The purpose of the present invention is to solve these problems by making available hot-rolled steel that simultaneously have:

- a yield strength 715 MPa or more and preferably equal to or more than 725MPa,

- a tensile strength of 750 MPa or more and preferably 800MPa or more,

- a total elongation greater than or equal to 20%.

- an impact toughness of greater than or equal to 60 J/cm2 when measured at - 40°C.

In a preferred embodiment, the steel sheets according to the invention may also present a yield strength to tensile strength ratio of 0.5 or more Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability, bending.

Another object of the present invention is also to make available a method for the manufacturing of these steels that is compatible with conventional industrial processes while being robust towards manufacturing parameters shifts.

The hot rolled steel sheet of the present invention may optionally be coated with zinc or zinc alloys, to improve its corrosion resistance.

Carbon is present in the steel between 0.02% and 0.2%. Carbon is an element necessary for increasing the strength of the steel by assisting in the stabilization of austenite at room temperature. But Carbon content less than 0.02% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.2%, the steel exhibits poor weldability as well as it is detrimental for the impact toughness which limits its application for the structural parts of yellow or green goods. A preferable content for the present invention may be kept between 0.03% and 0.18%, and more preferably between 0.04% and 0.15%.

Manganese content of the steel of present invention is between 3 % and 9%.

This element is gammagenous and therefore plays an important role in controlling the residual austenite fraction as well as enriching the residual austenite with Manganese to impart hardenability to the steel and impact toughness. An amount of at least 3% by weight of Manganese has been found in order to provide the strength and toughness to the steel. But when Manganese content is more than 9 % it produces adverse effects such as it stabilizes the austenite too much and devoid the steel of present invention from TRIP effect. In addition, the Manganese content of above 9% leads to excessive central segregation, hence reducingthe formability and also deteriorating the weldability of the present steel. A preferable content for the present invention may be kept between 3.5% and 8.5% and more preferably 4% and 8%.

Silicon content of the steel of present invention is between 0.2% and 1.2%. Silicon is solid solution strengthener for the steel of present invention. In addition, Silicon retards the precipitation of Cementite and also limits the formation of cementite although it often cannot completely eliminate cementite formation. Silicon keeps C in solid solution in austenite, as such lower the Ms temperature to below room temperature. As such, Silicon assists in the formation of Residual austenite at room temperature. However, a content of Silicon more than 1.2% leads to a problem such as surface defects which adversely effects the steel of present invention. Therefore, the concentration is controlled within an upper limit of 1 .2%. A preferable content for the present invention may be kept between 0.3% and 1 % and more preferably between 0.4% and 0.8%.

Aluminum is an essential element and is present in the steel between 0.9% and 2.5%. Aluminum is an alphagenous element a minimum of 0.9% of Aluminum is required to have a minimum Ferrite thereby imparting the elongation and toughness to the steel of present invention. Aluminum is also used for removing oxygen from the molten state of the steel to clean steel of present inventionand it also prevents oxygen from forming a gas phase. But whenever the Aluminum is more than 2.5% it is difficult to do casting because of the surface defects on the slabs such as breakouts. Therefore preferable range for the presence of the Aluminum is between 1 % and 2.3% and more preferably between 1 % and 2%.

Phosphorus content of the steel of present invention is between 0% and 0.03%. Phosphorus reduces the hot ductility and toughness, particularly due to its tendency to segregate at the grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.03% and preferably lower than 0.015%.

Sulfur is not an essential element but may be contained as an impurity in steel and from point of view of the present invention the Sulfur content is preferably as low as possible, but is 0.03% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese which is detrimental on the steel of present invention, therefore preferred below 0.01 %

Nitrogen is limited to 0.025% in order to avoid ageing of material and to minimize the precipitation of nitrides during solidification which are detrimental for mechanical properties of the Steel. Hence the preferable upper limit for nitrogen is 0.02% and more preferably 0.005%. Molybdenum is an optional element that constitutes 0% to 0.6% of the steel of present invention. Molybdenum increases the hardenability and allowing the steel of present invention to achieve targeted properties for thicker gauges . A minimum of 0.1 % of Molybdenum is required to be beneficial in increasing the hardenability. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.6%. Preferable limit for molybdenum is between 0 % and 0.4% and more preferably between 0 % and 0.3%.

Titanium is an optional element and present between 0% and 0.1 % in the steel of present invention. Titanium imparts the steel of present invention with the strength by forming carbides and control the grain size. But whenever Titanium is present more than 0.1 %, it imparts excess strength and hardness to the steel of present invention which diminishes the toughness beyond the targeted limits. The preferable limit for titanium is between 0 % and 0.09% and more preferred limit is 0% and 0.08%.

Boron is an optional element to the steel of present invention and may be present between 0.0001 % and 0.01 %. Boron imparts toughness to the steel of present invention when added along with Titanium.

Chromium is an optional element for the present invention. Chromium content may be present in the steel of present invention is between 0% and 0.5%. Chromium is an element that provides hardenability to the steel but content of Chromium higher than 0.5% leads to central co-segregation with Manganese.

Vanadium is an optional element that may be present between 0% and 0.2% of the steel of present invention. Vanadium is effective in enhancing the strength of steel by forming carbides, nitrides or carbo-nitrides and the upper limit is 0.2% due to economic reasons and even if Vanadium is present above 0.2% it does not bring any considerable benefit to the steel of present invention.

Niobium is an optional element for the present invention. Niobium content may be present in the steel of present invention between 0% and 0.1 % and is added in the steel of present invention for forming carbides or carbo-nitrides to impart strength to the steel of present invention by precipitation strengthening. Preferable limit is between 0% and 0.05% Nickel may be added as an optional element in an amount of 0% to 1 % to increase the strength of the steel present invention and to improve its toughness. A minimum of 0.01 % is preferred to get such effects. However, the content of Nickel is restricted to 1 % due to economic viability.

Copper may be added as an optional element in an amount of 0% to 1 % to increase the strength of the of Steel of present invention and to improve its corrosion resistance. A minimum of 0.01 % is preferred to get such effects. However, when its content is above 1 %, it can lead to problems such as copper hot shortness during the casting process.

Calcium content in the steel of present invention is below 0.005%. Calcium is added to steel of present invention in a preferable amount of 0.0001 to 0.005% as an optional element especially during the inclusion treatment, thereby, retarding the harmful effects of Sulfur.

Other elements such as, Magnesium can be added in the following proportions by weight Magnesium 0.0010%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.

The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing.

The microstructure of the Steels comprises:

Martensite is present in the steel of present invention is at least 60% wherein the martensite of present invention is comprising of tempered martensite and fresh martensite wherein tempered martensite is the matrix phase for the steel of present invention. The tempered martensite of the steel of present invention preferably has its aspect ratio between 4 and 12 preferably and more preferably between 5 and 11 . The aspect ratio is the ratio between the longest and the shortest dimension within a single grain. Tempered martensite is formed from the martensite which forms during the cooling after hot rolling. Such martensite is then tempered during the annealing process. The tempered martensite of the steel of present invention imparts ductility and strength. It is preferred that the content of tempered martensite is between 65% and 84% and more preferably between 70% and 80% by area fraction of total microstructure. Fresh martensite can also be optionally present in the steel of present invention. Fresh martensite may form during cooling after annealing from remaining unstable residual austenite. Fresh martensite can be present between 0% an 15%, preferably between 0 and 10% and even better no fresh martensite is present.

Residual Austenite is an essential microstructural constituent of the steel of present invention and is present between 15% and 40%. Residual Austenite of the present invention imparts toughness to the steel of present invention. Residual Austenite of the present invention can only be stable at room temperature when it is enriched with Manganese and Carbon. The percentage of Carbon inside the Residual Austenite higher than 0.8% and lower than 1.1 %. The percentage of Manganese in Residual Austenite is preferably more than 5% and more preferably more than 5.5%. However, when the Residual Austenite of present invention is not enriched with Carbon and Manganese it will not be stable at room temperature and will lead to formation of excess fresh martensite instead of adequate amount of Residual Austenite This effect provides excess strength to the steel and is also detrimental to elongation and toughness. The preferable limit for the presence of Austenite is between 18% and 35% and more preferably between 18% and 30% wherein the preferable Carbon content limit in austenite is preferred between 0.9% and 1.1 % and more preferably between 0.95% and 1.05%.

Polygonal Ferrite constitutes from 0% to 10% of microstructure by area fraction for the Steel of present invention. In the present invention Polygonal Ferrite imparts high strength as well as elongation to the steel of present invention. Polygonal Ferrite may be formed during the soaking and cooling after annealing in steel of present invention. But whenever polygonal ferrite content is present above 10% in steel of present invention the strength is not achieved.

Bainite and cementite may present in the steel of present invention between 0% and 5%. Till 5% bainite does not influence the target properties of the steel of present invention.

In addition to the above-mentioned microstructure, the microstructure of the hot rolled steel is free from microstructural components, such as Pearlite. Carbides of alloying elements might be present in the steel of present invention between 0% and 5% such as of Niobium, Titanim, Vanadium, Iron and others these carbides impart the steel of present invention with the strength by precipitation strengthening in but whenever the presence of carbides is 5% or more carbides consume partly the amount of Carbon, which is deterimental for the stablization of residual austenite.

A hot rolled steel according to the invention can be produced by any suitable method. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done either into ingots or continuously in form of thick slabs, thin slabs or thin strips, i.e. with a thickness ranging from approximately 220mm to 350mm for slabs up to several tens of millimeters for thin strip.

For example, a slab having the above-described chemical composition is manufactured by continuous casting wherein the slab optionally underwent the direct soft reduction during the continuous casting process to avoid central segregation. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.

The slab is reheated to a temperature between at least Ac3 + 50° C and 1300°C. In case the temperature of the slab is lower than least Ac3 + 50° C, excessive load is imposed on the rolling mill. Therefore, the temperature of the slab is sufficiently high so that hot rolling can be completed fully in the austenitic range. Reheating at temperatures above 1300°C must be avoided because it causes productivity loss and is also industrially expensive and some segregated parts may melt which may lead to breaking of slabs or cracking of slabs. Therefore, the preferred reheating temperature is between least Ac3 + 100° C and 1280°C.

Hot rolling finishing temperature for the present invention is at least Ac3 and preferably between Ac3 and Ac3 + 100° C, more preferably between 840°C and 1000°C and even more preferably between 850°C and 990°C.

The hot rolled strip obtained in this manner is then cooled from hot roll finishing temperature to a temperature range between Ms and 20°C at a cooling rate between 1 °C/s and 50°C/s. In a preferred embodiment, the cooling rate for this step of cooling is between 1 °C/s and 20°C/s and more preferably between 5°C/s and 20°C/s. During this step the martensite is formed which will be tempered during the soaking done under annealing process to form tempered martensite.

Then the hot rolled strip may optionally be coiled wherein coiling temperature is between Ms and 20°C or may optionally be cut to sheets

The hot rolled steel strip, plate or sheet is being heated from a temperature between Ms and 20°C up to the annealing temperature Tsoak which is between 550°C and Ac3, preferably between 600°C and Ac3 -40°C, such heating being performed at a heating rate HR1 of at least 1 °C/s.

The hot rolled steel strip, plate or sheet is held at Tsoak during 5 seconds to 1000 seconds to ensure the targeted transformation to austenite from the initial structure.

Then, the hot rolled steel is cooled wherein the cooling starts from Tsoak at a cooling rate CR1 between 0.1 °C/s and 150°C/s, to a cooling stop temperature T1 which is in a range between Ms-10°C and 20°C. In a preferred embodiment, the cooling rate CR1 for such cooling is between 0.1 °C/s and 120°C/s. During this cooling the fresh martensite may form from some remaining unstable austenite.

The hot rolled steel thus obtained preferably has a thickness between 2mm and 100mm and more preferably between 2 mm and 80 mm and even more preferably between 2mm and 50 mm.

EXAMPLES

The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only, and will display the advantageous features of the present invention.

Steel sheets made of steels with different compositions are gathered in Table 1 , where the steel sheets are produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the steel sheets obtained during the trials and table 4 gathers the result of evaluations of obtained properties. Ac3 and Ms temperature are determined through thermodynamic calculations done with the use of a software like Thermo-Calc®. Table 1

I = according to the invention; R = reference; underlined values: not according to the invention. Table 2

Table 2 gathers the process parameters implemented on steels of Table 1 .

Table 3

Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as SEM, EPMA, EBSD, XRD or any other microscope for determining microstructural composition of both the inventive steel and reference trials. The area fractions for the carbides is measured on polished samples after etching them in 2% Nital etching solution for 10 seconds and observed by an SEM. Polygonal Ferrite and tempered martensite are measured using EBSD wherein Electron backscattered diffraction (EBSD) is a SEM based technique to measure crystal orientations with a sub-micron resolution. An electron beam is focused on the 70° tilted specimen in the scanning electron microscope (SEM). Electrons that satisfy the Bragg condition for a family of planes are channelled and induce kikuchi bands. Electrons strike a phosphor screen and produce light, which is detected and digitized by a camera. The resulting EBS pattern is analyzed and indexed. This process is realized for each point analysed. For a given steel sample, an EBSD analysis of at least 4 images corresponding to a magnification of 1000 allows to identify the polygonal ferrite and tempered martensite microconstituents, their location and area percentage. The Residual Austenite area fraction is measured using XRD which are demonstrated in table 3.

The results are stipulated herein:

11 and I2 samples include niobium carbides, I3 sample includes titanium carbides and R1 sample includes iron carbides (cementite). No samples were containing any fresh martensite or bainite constituents. Table 4

Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength and total elongation, tensile tests are conducted in accordance of NBN EN ISO6892-1 standards with tensile samples having A25 . The toughness is tested by a Charpy test performed according to ISO 148-1 .The results of the various mechanical tests conducted in accordance to the standards are gathered

I = according to the invention; R = reference; underlined values: not according to the invention.