The present invention relates to a method of manufacturing of a cold rolled martensitic steel suitable for automotive industry and particularly to Martensitic steels having tensile strength 960 MPa or more.

Automotive parts are required to satisfy two inconsistent necessities, viz. ease of forming and strength but in recent years a third requirement of improvement in fuel consumption is also bestowed upon automobiles in view of global environment concerns. Thus, now automotive parts must be made of material having high formability in order that to fit in the criteria of ease of fit in the intricate automobile assembly and at same time have to improve strength for vehicle crashworthiness and durability while reducing weight of vehicle to improve fuel efficiency.

Therefore, intense Research and development endeavors are put in to reduce the amount of material utilized in car by increasing the strength of material. Conversely, an increase in strength of steel sheets decreases formability, and thus development of materials having both high strength and high formability is necessitated.

Earlier research and developments in the field of high strength and high formability steel sheets have resulted in several methods for producing high strength and high formability steel sheets, some of which are enumerated herein for conclusive appreciation of the present invention:

EP3901299 provides a cold rolled steel sheet having excellent workability, a hot-dip galvanized steel sheet, a hot-dip galvannealed steel sheet, and manufacturing methods therefor, the cold rolled steel sheet comprising, by wt%, 0.06-0.15% of carbon (C), 1.2% or less of silicon (Si) (excluding 0), 1.7-2.7% of manganese (Mn), 0.15% or less of molybdenum (Mo) (excluding 0), 1.0% or less of chromium (Cr) (excluding 0), 0.1% or less of phosphorus (P), 0.01 % or less of sulfur (S), 0.001 -0.04% of titanium (Ti), 0.001 -0.04% of niobium (Nb), 0.01 % or less of nitrogen (N), 0.01% or less of boron (B) (excluding 0), and the balance of Fe and other inevitable impurities, wherein the amounts of silicon (Si), carbon (C), manganese (Mn), molybdenum (Mo) and chromium (Cr) in the matrix structure at a point of thickness of 1/4t satisfy the following relation 1 , the microstructure comprises, by area%, 10-70% of ferrite, 10-50% of bainite and retained austenite as the sum thereof, and the balance of fresh martensite, and the ratio (Mb/Mt) of the fraction of the total fresh martensite (Mt) and the fraction of fresh martensite (Mb) that is adjacent to bainite is 60% or higher. [Relation 1] ([Si]+[C]x3)/([Mn]+[Mo]+[Cr]) > 0.18 ([Si], [C], [Mn], [Mo] and [Cr] are the wt% of silicon (Si), carbon (C), manganese (Mn), molybdenum (Mo) and chromium (Cr), respectively, at a point of thickness of 1/4t of the cold rolled steel sheet. However the steel of EP3901299 does not demonstrate the hole expansion ratio of 40% or more.

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

- an ultimate tensile strength greater than or equal to 960 MPa and preferably above 1000 MPa, a yield strength greater than or equal to 700 MPa and preferably above 720 MPa

- a hole expansion ratio of 40% or more and preferably 45% or more

Preferably, such steel can also have a good suitability for forming, for rolling with good weldability and coatability.

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

The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention.

The chemical composition of the cold rolled martensitic steel comprises of the following elements:

Carbon is present in the steel of present invention is from 0.07% to 0.12%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing a low- temperature transformation phases such as Martensite. Carbon content less than 0.07% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.12%, the steel exhibits poor spot weldability which limits its application for the automotive parts. A preferable content for the present invention may be kept from 0.08% to 0.1 1% and more preferably from 0.09% to 0.1 1 %.

Manganese content of the steel of present invention is from 1 .9% to 2.5%. This element promotes the Austenite formation. Manganese provides solid solution strengthening and reduces ferritic transformation rate hence assist in the formation of martensite. An amount of at least 1 .9% is required to impart strength as well as to assist the formation of Martensite. But when Manganese content is more than 2.5% it produces adverse effects such as it retards transformation of Austenite to Martensite during cooling after annealing. Manganese content of above 2.5% can get excessively segregated in the steel during solidification and homogeneity inside the material is impaired which can cause surface cracks during a hot working process. The preferred limit for the presence of Manganese is from 2% to 2.4% and more preferably from 2.1 % to 2.35%.

Silicon content of the steel of present invention is from 0.2% to 0.6%. Silicon is an element that contributes to increasing the strength by solid solution strengthening. Silicon is a constituent that can retard the precipitation of carbides during cooling after annealing, therefore, Silicon promotes formation of Martensite. But Silicon is also a ferrite former and also increases the Ac3 transformation point which will push the annealing temperature to higher temperature ranges that is why the content of Silicon is kept at a maximum of 0.6%. Silicon content above 0.6% can also temper embrittlement and in addition silicon also impairs the coatability. The preferred limit for the presence of Silicon is from 0.25% to 0.5% and more preferably from 0.25% to 0.4%.

Chromium content of the steel of present invention is from 0.1 % to 0.5%. Chromium is an essential element that provide strength to the steel by solid solution strengthening and a minimum of 0.1 % is required to impart the strength but when used above 0.5% impairs surface finish of steel. The preferred limit for the presence of Chromium is from 0.15% to 0.4% and more preferably from 0.15% to 0.35%.

The content of the Aluminum is from 0.01 % to 0.1%. in the present invention Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process. Aluminum also fixes Nitrogen in the steel to form Aluminum nitride to reduce the size of the grains. Higher content of Aluminum, above 0.1%, increases Ac3 point to a high temperature thereby lowering the productivity. The preferred limit for the presence of Aluminium is from 0.01 % to 0.09%

Molybdenum is an essential element that constitutes 0.2% to 0.6% of the Steel of present invention; Molybdenum plays an effective role in improving hardenability and hardness, delays the appearance of Bainite hence promote the formation of Martensite, in particular when added in an amount of at least 0.2%. 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%. The preferred limit for the presence of Molybdenum from 0.25% to 0.5% and more preferably from 0.25% to 0.4%. Niobium is present in the Steel of present invention from 0.001 % to 0.1% and suitable for forming carbo-nitrides to impart additional strength of the Steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus, finer microstructure formed at the end of the holding temperature and as a consequence after the complete annealing will lead to the hardening of the product. However, Niobium content above 0.1 % is not economically interesting as a saturation effect of its influence is observed this means that additional amount of Niobium does not result in any strength improvement of the product.

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 0.09% or less from the viewpoint of manufacturing cost. Further if higher Sulfur is present in steel it combines to form Sulfides especially with Manganese and reduces its beneficial impact on the present invention.

Phosphorus constituent of the Steel of present invention is between 0% and 0.09%, Phosphorus reduces the spot weldability and the hot ductility, particularly due to its tendency to segregate at the grain boundaries or co-segregate with Manganese. For these reasons, its content is limited to 0.09 % and preferably lower than 0.06%.

Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the precipitation of Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel.

Titanium is an alloying element and may be added to the Steel of present invention from 0.001 % to 0.1 %. It forms Titanium-nitrides and protects B into solution during steelmaking. The amount of Titanium is so limited to 0.1 % to avoid the formation of coarse Titanium-nitrides detrimental for formability. In case the Titanium content below 0.001 % does not impart any effect on the steel of present invention.

Boron is an alloying element for the steel of present invention and can be present between 0.0005% and 0.005%. Boron can enhance hardenability of steel during cooling, which can avoid soft phase formation and promotes the formation of hard martensite in the final microstructure, when added in an amount of at least of 0.0005%.

Vanadium is an optional element for the steel of present invention. Vanadium is effective in enhancing the strength of steel by forming carbides or carbo-nitrides and the upper limit is 0.1% from economic points of view.

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, when its content is above 1 %, Nickel causes ductility deterioration.

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 degrade the surface aspects.

Calcium may be added to the steel of present invention in an among between 0.001 % and 0.01 %%. Calcium is added to steel of present invention as an optional element especially during the inclusion treatment. Calcium contributes towards the refining of the Steel by binding the detrimental Sulfur content in globular form thereby retarding the harmful effect of Sulfur.

Other elements such as Sn, Pb or Sb can be added individually or in combination in the following proportions: Sn ^0.1 %, Pb ^0.1% and Sb ^0.1 %. 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 martensitic steel sheet will now be described in details, all percentages being in area fraction.

Tempered Martensite constitutes from 80% to 94% of the microstructure by area fraction. Tempered martensite is formed from the martensite which forms during the cooling after annealing and particularly in a temperature from Ms-100°C to Ms and more particularly from Ms- 70°C to Ms. Such martensite is then tempered during the holding at a tempering temperature Ttemper from 400°C to 550°C. The tempered martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of tempered martensite is from 84% to 94% and more preferably from 85% to 94%.

Fresh Martensite constitutes from 6% to 20% of microstructure by area fraction. Steel of present invention form fresh martensite due to the cooling after tempering holding of cold rolled steel sheet. Martensite imparts ductility and strength to the Steel of present invention. However, when fresh martensite presence is above 20% it imparts excess strength but diminishes the hole expansion ratio beyond acceptable limit for the steel of present invention. As fresh martensite contains high amount of carbon it is brittle and hard therefore preferred limit for fresh martensite for the steel of present invention is from 6% to 18% and more preferably from 6% to 15%.

In addition to the above-mentioned microstructure, the microstructure of the cold rolled martensitic steel sheet is free from microstructural components such as ferrite, bainite, residual austenite, pearlite and cementite.

The steel according to the invention can be manufactured by any suitable methods. It is however preferable to use the method according to the invention that will be detailed, as a non-limitative example.

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

For example, a slab having the chemical composition according to the invention is manufactured by continuous casting wherein the slab optionally underwent a direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1 .10. 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 temperature of the slab, which is subjected to hot rolling, must be at least 1000° C and must be below 1280°C. In case the temperature of the slab is lower than 1000° C, excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the slab must be high enough so that hot rolling should be completed in the temperature range of Ac3 to Ac3+100°C. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive.

The sheet obtained in this manner is then cooled at a cooling rate of at least 10°C/s to the coiling temperature which must be below 675°C. Preferably, the cooling rate will be less than or equal to 200° C/s.

The hot rolled steel sheet is then coiled at a coiling temperature below 675°C to avoid ovalization and preferably between 475°C and 675°C to avoid scale formation, with an even prefererred range for such coiling temperature between 500°C and 660°C. The coiled hot rolled steel sheet is then cooled down to room temperature before subjecting it to optional hot band annealing.

The hot rolled steel sheet may be subjected to an optional scale removal step to remove the scale formed during the hot rolling before optional hot band annealing. The hot rolled sheet may then have subjected to an optional hot band annealing. In a preferred embodiment, such hot band annealing is performed at temperatures between 400°C and 750°C, preferably for at least 12 hours and not more than 96 hours, the temperature preferably remaining below 750°C to avoid transforming partially the hot-rolled microstructure and therefore, possibly losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel sheet may be performed through, for example, pickling of such sheet.

This hot rolled steel sheet is then subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%.

Thereafter the cold rolled steel sheet is being heat treated which will impart the steel of present invention with requisite mechanical properties and microstructure.

The cold rolled steel sheet is heated at a heating rate HR1 which is of at least 1 °C/s and preferably greater than equal to 2°C/s, to a soaking temperature Tsoak between Ac3 and Ac3+100° C and preferably between Ac3°C and Ac3+50°C, wherein Ac3 for the steel sheet is calculated by a dilatometry study conducted according to the ASTM A1033 -18 standards. The cold rolled steel sheet is held at Tsoak during 10 seconds to 500 seconds to ensure a complete recrystallization and full transformation to austenite of the strongly work hardened initial structure.

The cold rolled steel sheet is then cooled from Tsoak, at a cooling rate CR1 from 1 °C/s to 150°C/s, to a temperature T1 which is in a range from Ms -100°C to Ms. In a preferred embodiment, the cooling rate CR1 for such step of cooling is from 20°C/s to 120°C/s. The preferred T 1 temperature for such first step is from Ms -70°C to Ms. The cold rolled steel sheet is then held at T1 for a time from 5 seconds to 100 seconds and preferably from 5 seconds to 50 seconds and more preferably 5 seconds to 30 seconds.

Ms for the steel sheet is calculated by using the dilatometry study according to the ASTM A1033 -18 standards.

Thereafter the cold rolled steel sheet is reheated to a tempering temperature Ttemper from 400°C to 550°C with a heating rate of at least 1 °C/s and preferably of at least 2°C/s and more preferably of at least 10°C/s during 10s and 600s. The preferred temperature range for tempering is from 420°C to 500°C and the preferred duration for holding at Ttemper is from 20 s to 300 s.

The cold rolled martensitic steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or with aluminum or aluminum alloys to improve its corrosion resistance. Then, the cold rolled steel sheet is cooled down to room temperature to obtain a cold rolled martensitic steel.

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. Table 1

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

Table 2

Table 2 gathers the hot rolling and annealing process parameters implemented on cold rolled steel sheets to impart the steels of table 1 with requisite mechanical properties to become a cold rolled martensitic steel.

The table 2 is as follows:

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

Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of both the inventive and reference steels in terms of area fraction. The results are stipulated herein: Table 3 :

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

Table 4

The results of the various mechanical tests conducted in accordance to the standards are gathered. For testing the ultimate tensile strength and yield strength are tested in accordance of JIS-Z2241 . To estimate hole expansion, a test called hole expansion is applied, in this test sample is subjected to punch a hole of 10mm and deformed after deformation we measure the hole diameter and calculate HER%= 100*(Df-Di)/Di wherein the Df is the diameter measured in millimetre after deformation and Di is the diameter measured in millimetre before deformation. he invention; R = reference; underlined values: not according to the invention.