THEREOF

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 1700 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:

The steel sheet of WO2017/065371 is manufactured through the steps of: rapidly heating a material steel sheet for 3 to 60 seconds to an Ac3 transformation point or higher and maintaining the material steel sheet, the material steel sheet containing 0.08 to 0.30 wt% of C, 0.01 to 2.0 wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P and 0.05 wt% or less of S and the remainder being Fe and other unavoidable impurities; rapidly cooling the heated steel sheet to 100°C/s or higher with water or oil; and rapidly tempering to 500°C to A1 transformation point for 3 to 60 seconds including heating and maintaining time. But the steel of WO2017/065371 not able to provide the hole expansion ratio 22% while having a tensile strength of 1700 MPa.

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 1700 MPa and preferably above 1750 MPa,

a yield strength greater than or equal to 1500 MPa and preferably above 1550 MPa

- a hole expansion ratio of at least 22% and preferably above 25%

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 between 0.3% and 0.4%. 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, Therefore, Carbon plays two pivotal roles, one is to increase the strength. But Carbon content less than 0.3% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.4%, the steel exhibits poor spot weldability which limits its application for the automotive parts. A preferable content for the present invention may be kept between 0.3% and 0.38% and more preferably between 0.3% and 0.36%.

Manganese content of the steel of present invention is between 0.5 % and 1 %. This element is gammagenous. Manganese provides solid solution strengthening and suppresses the ferritic transformation temperature and reduces ferritic transformation rate hence assist in the formation of martensite. An amount of at least 0.5% is required to impart strength as well as to assist the formation of Martensite. But when Manganese content is more than 1 % it produces adverse effects such as it retards transformation of Austenite to Martensite during cooling after annealing. Manganese content of above 1 % 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 between 0.5% and 0.9% and more preferably between 0.6% and 0.8%.

Silicon content of the steel of present invention is between 0.2% and 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 between 0.2% and 0.5% and more preferably between 0.25% and 0.45%.

Chromium content of the composite coil of steel of present invention is between 0.1 % and 1 %. 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 1 % impairs surface finish of steel. The preferred limit for the presence of Chromium is between 0.3% and 0.9 % and more preferably between 0.4% and 0.8%.

The content of the Aluminum is between 0.01 % and 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 1 %, increases Ac3 point to a high temperature thereby lowering the productivity. The preferred limit for the presence of Aluminium is between 0.01 % and 0.5%

Titanium is added to the Steel of present invention between 0.001 % to 0.1 %. It forms Titanium- nitrides appearing during solidification of the cast product. 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.

Molybdenum is an essential element that constitutes 0.01 % to 0.5% 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 0.01 %. Molybdenum also facilitate the formation of Ferrite and Pearlite microstructure during cooling after hot rolling, this ferrite and pearlite microstructure facilitate the cold rolling. 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 0.5%. The preferable limit for Molybdenum is between 0.1 % and 0.3%

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.

Niobium is present in the Steel of present invention between 0% and 0.1 % and suitable for forming carbo-nitrides to impart 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.

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.

Boron is an optional element for the steel of present invention and may be present between 0% and 0.05%. Boron forms boro-nitirides and impart additional strength to steel of present invention when added in an amount of at least 0.0001 %.

Calcium can 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.

Martensite constitutes at least 95% of the microstructure by area fraction. The martensite of the present invention can comprise both fresh and tempered martensite. However, fresh martensite is an optional microconstituent which is preferably limited in the steel at an amount of between 0% and 4%, preferably between 0 and 2% and even better equal to 0%. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and particularly after below Ms temperature and more particularly between Ms-10°C and 20°C.Such martensite is then tempered during the holding at a tempering temperature Temper between 150°C and 300°C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is between 96% and 99% and more preferably between 97% and 99%.

The cumulated amount of ferrite and bainite represents between 1 % and 5% of the microstructure. The cumulative presence of bainite and ferrite does not affect adversely to the present invention till 5% but above 5% the mechanical properties may get impacted adversely. Hence the preferred limit for the cumulative presence ferrite and bainite is kept between 1 % and 4% and more preferably between 1 % and 3%.

Bainite forms during the reheating before tempering. In a preferred embodiment, the steel of present invention contains 1 to 3% of bainite. Bainite can impart formability to the steel but when present in a too big amount, it may adversely impact the tensile strength of the steel.

Ferrite may form during the first step of cooling after annealing but is not required as a microstructural constituent. Ferrite formation must be kept as low as possible and preferably less than 2% or even less than 1 %. Residual Austenite is an optional microstructure that can be present between 0% and 2% in the steel.

In addition to the above-mentioned microstructure, the microstructure of the cold rolled martensitic steel sheet is free from microstructural components such as 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 1280° 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 20°C/s to the coiling temperature which must be below 650°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 650°C to avoid ovalization and preferably between 475°C and 625°C to avoid scale formation, with an even prefererred range for such coiling temperature between 500°C and 625°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 be 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 then heating in a two steps heating process wherein the first step of heating starts from room temperature, the cold rolled steel sheet being heated, at a heating rate HR1 of at least 10°C/s, to a temperature HT1 which is in a range between 550°C and 750°C. In a preferred embodiment, the heating rate HR1 for such first step of heating is at least 15°C/s and more preferably at least 18°C/s. The preferred HT1 temperature for such first step is between 575°C and 725°C.

In the second step of heating, the cold rolled steel sheet is heated from HT1 to an annealing temperature Tsoak which is between Ac3 and Ac3 + 100°C, preferably between Ac3 +10°C and Ac3 + 100°C, at a heating rate HR2 which is between 1 °C/s and 50°C/s. In a preferred embodiment, the heating rate HR2 for the second step of heating is between 1 °C/s and 25°C/s and more preferably 1 °C/s and 20°C/s, wherein Ac3 for the steel sheet is calculated by using the following formula:

Ac3 = 910 - 203 [C] ^A (l/2) - 15.2 [Ni] + 44.7 [Si] + 10A[V] + 31.5 [Mo] + 13.1 [W] - 30 [Mn]

- 11 [Cr] - 20 [Cu\ + 700 [P] + 400 [Al] + 120[As] + 400[7T] wherein the elements contents are expressed in weight percentage of the cold rolled steel sheet.

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 in a two steps cooling process wherein the first step of cooling starts from Tsoak, the cold rolled steel sheet being cooled down, at a cooling rate CR1 between 30°C/s and 150°C/s, to a temperature T 1 which is in a range between 630°C and 750°C. In a preferred embodiment, the cooling rate CR1 for such first step of cooling is between 30°C/s and 120°C/s. The preferred T1 temperature for such first step is between 640°C and 725°C.

In the second step of cooling, the cold rolled steel sheet is cooled down from T1 to a temperature T2 which is between Ms-10°C and 20°C, at a cooling rate CR2 of at least 50°C/s. In a preferred embodiment, the cooling rate CR2 for the second step of cooling is at least 100°C/s and more preferably at least 150°C/s. The preferred T2 temperature for such second step is between Ms- 50°C and 20°C. Ms for the steel sheet is calculated by using the following formula:

Ms = 545 - 601.2 * (l - EXP(- 0.868 [C])) - 34.4[Mn] - 13.7[Si] - 9.2 [Cr] - 17.3[M]

- 15.4[Mo] + 10.8[7] + 4.7[Co] - 1A[AI] - 16.3 [Cu] - 361 [Mb] - 2.44[Gϊ]

- 3448 [B] Thereafter the cold rolled steel sheet is reheated to a tempering temperature Ttemper between 150°C and 300°C with a heating rate of at least 1 °C/s and preferably of at least 2°C/s and more of at least 5°C/s during 100 s and 600 s.The preferred temperature range for tempering is between 200°C and 300°C and the preferred duration for holding at Ttemper is between 200 s and 500 s.

Then, the cold rolled steel sheet is cooled down to room temperature to obtain a cold rolled martensitic steel.

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.

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

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 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

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