MANUFACTURING THEREOF

The present invention relates to cold rolled and heat treated steel sheet which is suitable for use as a steel sheet for vehicles. 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:

US 9 074 272 describes steels that have the chemical composition: 0.1 - 0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn and the remainder consisting of iron and the inevitable impurities. The microstructure includes residual austenite between 5 to 20%, bainitic ferrite 40 to 65%, polygonal ferrite 30 to 50% and less than 5% martensite. US 9 074 272 refers to a cold rolled steel sheet with excellent elongation but the invention described in it fails to achieve the strength of 900 MPa which is a mandate for reducing the weight while keeping the complex automotive part robust. US2015/0152533 discloses a method for producing a high strength steel which contains C: 0.12 -0.18%, Si: 0.05 -0.2%, Mn: 1.9 -2.2%, Al: 0.2 -0.5%, Cr: 0.05 -0.2%, Nb: 0.01 -0.06%, P: £0.02%, S: £0.003%, N: £0.008%, Mo: £0.1 %, B: £0.0007%, Ti: £0.01 %, Ni: £0.1 %, Cu: £0.1 % and, as the remainder, iron and unavoidable impurities. The steel sheet produced by method described in patent application US2015/0152533 should have a microstructure that consists of 50-90% by volume ferrite, including bainitic ferrite, 5-40% by volume martensite, up to 15% by volume residual austenite and up to 10% by volume other structural constituents. Even though US2015/0152533 contains a substantial amount of martensite (i.e. up to 40%) still fail to achieve the tensile strength level of 900MPa.

The known prior art related to the manufacture of high strength and high formability steel sheets is inflicted by one or the other lacuna :hence there lies a need for a cold rolled steel sheet having strength greater than 1000MPa and a method of manufacturing the same.

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

- an ultimate tensile strength greater than or equal to 1000 MPa and preferably above 1 180 MPa, or even above 1220 MPa,

- a good phospahtability with at least 96% of the surface

In a preferred embodiment, the steel sheet according to the invention may have a yield strength value greater than or above 700 MPa.

Preferably, such steel can also have a good suitability for forming, in particular 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 cold rolled heat treated 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.

Figurel is not according to the present invention. Figure 1 is a micrograph illustrating the cracks formed due to internal oxides on the surface of the cold rolled steel sheet and an internal oxide layer formed thereof . To demonstrate the cracks one of the crack is marked as 10. The cold rolled steel sheet belongs to Steel Grade 7 of table 1.

Figure 2 is a micrograph illustrating the surface of the cold rolled steel sheet which is according to the present invention. The cold rolled steel sheet is free from internal- oxide layer. The cold rolled steel sheet belongs to Steel Grade 2 of table 1.

Figure 3 is not according to the present invention. Figure 3 is a micrograph illustrating the cracks formed due to internal oxides on the surface of the cold rolled and heat treated steel sheet and an internal oxide layer formed thereof. To demonstrate the cracks one of the crack is marked as 20. The cold rolled steel sheet belongs to Steel Grade 7 of table 1.

Figure 4 is a micrograph illustrating the surface of the cold rolled and heat treated steel sheet which is according to the present invention. The cold rolled steel sheet has an internal-oxide layer of less than 3 microns. The cold rolled steel sheet belongs to Steel Grade 2 of table 1.

Figure 5 is a micrograph demonstrating the phosphatation on a cold rolled and heat treated steel sheet not according to the invention. Figure 5 demonstrates the porosity in coverage. One of the porosity marks is highlighted as 30. Figure 5 belong to Steel grade 7 of table 1

Figure 6 is a micrograph demonstrating the phosphatation on a cold rolled and heat treated steel sheet according to the invention. The demonstrated steel sheet belongs to Steel Grade 2 of table 1 with 100% phosphatation coverage. Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

Carbon is present in the steel between 0.18% and 0.24%. Carbon is an element necessary for increasing the strength of a steel sheet by producing a low-temperature transformation phase such as martensite. Further carbon also plays a pivotal role in austenite stabilization. A content less than 0.18% would not allow stabilizing austenite, thereby decreasing strength as well as ductility. On the other hand, at a carbon content exceeding 0.24%, a weld zone and a heat- affected zone are significantly hardened, and thus the mechanical properties of the weld zone are impaired.

Manganese content of the steel of present invention is between 1.5% and 2.5%. Manganese is an element that imparts strength as well as stabilizes austenite to obtain residual austenite. An amount of at least about 1.5 % by weight of manganese has been found in order to provide the strength and hardenability of the steel sheet as well as to stabilize austenite. Thus, a higher percentage of Manganese such as 1.9 to 2.2% is preferred. But when manganese is more than 2.5 %, this produces adverse effects such as slowing down the transformation of austenite to bainite during the isothermal holding for bainite transformation, leading to a reduction of ductility. Moreover, a manganese content above 2.5% would also reduce the weldability of the present steel.

Silicon content of the steel of present invention is between 1.2% and 2%. Silicon as a constituent retards the precipitation of carbon from austenite. Therefore due to the presence of 1.2% of silicon, carbon-rich Austenite is stabilized at room temperature. However, adding more than 2% of silicon does not improve the mentioned effect and leads to problems such as hot rolling embrittlement. Therefore, the concentration is controlled within an upper limit of 2%.

The content of aluminum of the steel of the present invention is between 0.01 and 0.06%. Within such range, aluminum bounds nitrogen in the steel to form aluminum nitride so as to reduce the size of the grains. But, whenever the content of aluminum exceeds 0.06% in the present invention, it will increase the Ac3 point, thereby lowering the productivity. Chromium content of the steel of present invention is between 0.2% and 0.5%. Chromium is an essential element that provide strength and hardening to the steel, but when used above 0.5 % impairs surface finish of the steel.

Phosphorus content of the steel of present invention is limited to 0.02%.

Phosphorus is an element which hardens in solid solution and also interferes with formation of carbides. Therefore a small amount of phosphorus, of at least 0.002% can be advantageous, but phosphorus has its adverse effects also, such as a reduction of the spot weldability and the hot ductility, particularly due to its tendency to segregation at the grain boundaries or co-segregation with manganese. For these reasons, its content is preferably limited a maximum of 0.015%.

Sulfur is not an essential element but may be contained as an impurity in steel. The sulfur content is preferably as low as possible, but is 0.03% or less and preferably at most 0.003%, from the viewpoint of manufacturing cost. Further if higher sulfur is present in steel it combine to form sulfide especially with Mn and Ti and reduces their beneficial impact on the present invention.

Niobium is an optional element that can be added to the steel up to 0.06%, preferably between 0.0010 and 0.06%. It is suitable for forming carbonitrides to impart strength to the steel according to the invention by precipitation hardening. Because niobium delays the recrystallization during the heating, the microstructure formed at the end of the holding temperature and as a consequence after the complete annealing is finer, this leads to the hardening of the product. But, when the niobium content is above 0.06% the amount of carbo- nitrides is not favorable for the present invention as large amount of carbo nitrides tend to reduce the ductility of the steel.

Titanium is an optional element which may be added to the steel of the present invention up to 0.08%, preferably between 0.001 % and 0.08%. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form TiN appearing during solidification of the cast product. The amount of Ti is so limited to 0.08% to avoid coarse TiN detrimental for hole expansion. In case the titanium content is below 0.001 % it does not impart any effect on the steel of present invention.

Vanadium is an optional element which may be added to the steel of the present invention up to 0.1 %, preferably between 0.001 % and 0.01 %. As niobium, it is involved in carbo-nitrides so plays a role in hardening. But it is also involved to form VN appearing during solidification of the cast product. The amount of V is so limited to 0.1 % to avoid coarse VN detrimental for hole expansion. In case the vanadium content is below 0.001 % it does not impart any effect on the steel of present invention. Calcium is an optional element which may be added to the steel of present invention up to 0.005%, preferably between 0.001 % and 0.005%. 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 arresting the detrimental sulphur content in globularizing it.

Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination in the following proportions: Ce < 0.1 %, B < 0.01 %, Mg < 0.05% and Zr < 0.05%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification.

Among the alloying elements present in the steel of present invention Silicon, Manganese, Aluminum and Chromium are more oxidizable than iron and it is well known that the mentioned elements undergo selective oxidation in addition to iron during coiling, hot band annealing, annealing and also during other similar heat treatment processes thereby forming internal-oxides.

Figure 1 is a micrograph demonstrating the cold rolled steel sheet which is not in accordance of the present invention, the cold rolled steel sheet having a layer of internal-oxides, wherein these selective oxides are formed during coiling on the hot rolled steel sheet due to the reduced partial pressure of oxygen, these selective oxides also cause for crack generation at the grain boundaries during cold rolling on cold rolled steel sheet. In figure 1 crack (10) is also demonstrated on the surface of the cold rolled steel sheet. Figure 1 also shows the internal oxides on a surface of cold rolled steel sheet having a thickness of more than 1 micron. Similarly selective oxidation also takes place during annealing.

Figure 2 is a micrograph demonstrating the cold rolled steel sheet in accordance with the present invention, wherein the cold rolled steel sheet is free from the internal-oxides.

Figure 3 is a micrograph demonstrating the heat treated cold rolled steel sheet which is not in accordance of the present invention, the heat treated cold rolled steel sheet is having a layer of internal-oxides wherein these selective oxides are formed during coiling on the hot rolled steel sheet or during the hot band annealing on the hot rolled steel sheet or annealing on the cold rolled steel sheet due to the reduced partial pressure of oxygen, these selective oxides also cause for crack generation at the grain boundaries during cold rolling on cold rolled steel sheet which get aggravated during annealing. In figure 3 crack (20) is also demonstrated on the surface of the heat treated cold rolled steel sheet. Figure 3 also shows the internal oxides on a surface of cold rolled steel sheet of thickness more than 3 microns.

Figure 4 is a micrograph demonstrating the heat treated cold rolled steel sheet in accordance of the present invention, wherein the heat treated cold rolled steel sheet having layer of the internal-oxides and in accordance of the present invention a thickness of up to 3 microns oxide layer is acceptable on the heat treated cold rolled sheet.

Flence present invention envisages putting in place specific process parameters such as keeping the coiling temperature below 500°C and performing at least one mandatory pickling before cold rolling to control the formation of internal-oxides. The present invention keeps the internal-oxide layer up to 3 microns on the final cold rolled and heat treated steel sheet. In a preferred embodiment, such layer is being made of iron, silicon, manganese and chromium.

In another embodiment, the presence of a layer of internal oxides of a thickness of 1 micron or less on the cold rolled sheet after cold rolling is preferred. The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.

The microstructure of the steel sheet according to the invention comprises 0% to 15% of tempered martensite, 10% to 15% of residual austenite and optionally up to 30% of ferrite in area fractions, the balance being made of bainite, bainite content being at least 55%.

Bainite is the matrix of the steel and is contained in a minimum of 55%, preferably of 60%. In the frame of the present invention, bainite consists in lath bainite and granular bainite. Granular bainite is a bainite having a very low density of carbides, meaning that the steel includes less than 100 carbides per area unit of 100pm ² . Lath bainite is in the form of thin ferrite laths with carbide formed between the laths. The size of carbides present between the laths is such that the number of carbides bigger than 0.1 micron is below 50,000 / mm ² . The lath bainite provides the steel with adequate hole expansion whereas the granular bainite provides an improved elongation.

Tempered martensite is contained in an amount of 0 to 15%. It is preferred to have the content of tempered martensite to achieve the strength level of 1000 MPa or more and if the martensite amount reaches beyond 15%, it would have detrimental impact on ductility.

Residual Austenite is contained in an amount of 10 to 15%. It is known to have a higher solubility of carbon than bainite and hence acts as effective carbon trap, therefore retarding the formation of carbides in bainite. The retained austenite of the present invention preferably contains carbon between 0.9 and 1.15%, with an average content of carbon in austenite of 1.00%. Austenite also imparts ductility to the present steel.

Martensite and austenite can be present in the steel according to the invention, as isolated phases or under the form of martensite-austenite islands, which is preferred.

Ferrite may be present between 0% and 30 % in the steel. Such ferrite may comprise polygonal ferrite, lath ferrite, acicular ferrite, plate ferrite or epitaxial ferrite. The presence of ferrite in the present invention may impart the steel with formability and elongation. Presence of ferrite has negative impacts due to the fact that ferrite increases the gap in hardness with hard phases such as martensite and bainite and reduces local ductility. If ferrite presence is above 30% the targeted tensile strength is not achieved. A steel sheet 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 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 will be considered as a semi-finished product. A slab having the above-described chemical composition is manufactured by continuous casting wherein the slab preferably underwent a direct soft reduction during casting to ensure the elimination of central segregation and porosity reduction. 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 is preferably at least 1000°C, preferably above 1200°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. Further, the temperature must not be above 1280°C as there would be a risk of formation of rough ferrite grains resulting in coarse ferrite grain which decreases the capacity of these grains to re-crystallize during hot rolling. The larger the initial ferrite grain size, the less easily it re-crystallizes, which means that reheat temperatures above 1280°C must be avoided because they are industrially expensive and unfavorable in terms of the recrystallization of ferrite. The temperature of the slab is preferably sufficiently high so that hot rolling can be completed entirely in the austenitic range, the finishing hot rolling temperature remaining above 850°C and preferably above 900°C. It is necessary that the final rolling be performed above 850°C, because below this temperature the steel sheet exhibits a significant drop in rollability .A final rolling temperature between 900 and 950° C is preferred to have a structure that is favorable to recrystallization and rolling.

The sheet obtained in this manner is then cooled at a cooling rate above 30°C/s to a temperature which is below 500°C.The cooling temperature is kept below 500°C to avoid selective oxidation of alloying elements such as manganese, silicon and chromium. Preferably, the cooling rate will be less than or equal to 65°C/s and above 35°C/s. Thereafter the hot rolled steel sheet is coiled and during the time which hot rolled sheet remains coiled the transformation of austenite into bainite takes place and the temperature of the coiled hot rolled sheet rises due to recalescence. The temperature of the coiled hot rolled steel sheet must be kept below 570°C to avoid selective internal oxidation of Silicon, Manganese, Aluminum and Chromium on the surface of hot rolled coil as these oxides forms cracks on the surface of the hot rolled steel sheet. Thereafter the coiled hot rolled steel sheet is allowed to cool down to room temperature. Then the hot rolled sheet is subjected to scale removal process such as pickling to remove scale formed during hot rolling and ensure that there is no scale on the surface of hot rolled steel sheet before subjecting it to hot band annealing.

The hot rolled sheet is then subjected to hot band annealing at a temperature between 350°C and 750°C during 1 to 96 hours. The temperature and time of such hot band annealing is selected to ensure softening of the hot rolled sheet to facilitate the cold rolling of the hot rolled steel sheet. Further the atmosphere of the Hot band annealing is controlled to avoid oxidation during hot band annealing. The scale removal process before hot band annealing is not mandatory, if hot band annealing is done between temperature range 350°C and 500°C during 1 to 96 hours as in this temperature range there is very less possibility of increasing the thickness of oxide layer. However, if the hot band annealing is performed between 500 and 750°C, the scale removal process to be done before such annealing is mandatory.

The Hot rolled steel sheet is then cooled down to room temperature to obtain the annealed hot rolled sheet. Thereafter, the annealed hot rolled sheet may be subjected to an optional scale removal process. In accordance with the present invention, at least one scale removal process must be performed before cold rolling.

The annealed hot rolled sheet is then cold rolled with a thickness reduction between 35 to 70% to obtain a cold rolled steel sheet. The obtained cold rolled steel sheet is substantially free from internal-oxides.

Figure 2 is a micrograph demonstrating the cold rolled steel sheet in accordance of the present invention, wherein the cold rolled steel sheet is free from the internal oxides but according to the present invention a layer of oxides up to a thickness of 1 micron on the cold rolled sheet after pickling and HBA is acceptable.

The cold rolled steel sheet is then subjected to annealing to impart the steel of present invention with targeted microstructure and mechanical properties.

To continuously anneal the cold rolled steel sheet, it is first heated at a heating rate between 1 and 20°C/s, preferably greater than 3°C/s, to a soaking temperature between Ac1 and Ac3+ 50° C during at least 100 s and preferably not more than 1000s to ensure an adequate re-crystallization and transformation to obtain a minimum of 70 % Austenite microstructure. Ac1 for the steel according to the invention is usually between 680 and 750°C. Ac3 for the steel according to the invention is usually between 820 and 900°C.

The sheet is then cooled at a cooling rate of more than 10°C/s between a cooling temperature range between Ms-20°C and Ms + 40°C wherein Ms is calculated according to the following formula :

Ms=565-(31 ^* [Mn]+13 ^* [Si]+10 ^* [Cr]+18 ^* [Ni]+12 ^* [Mo])-600 ^* (1 -EXP(-0,96 ^* [C]))

In a preferred embodiment, the cooling rate is greater than 30° C/s.

Then the temperature of cold rolled steel sheet is brought to a temperature range between Ms + 10 and Ms +100°C which is generally between 350°C and 450°C and held there for a time of at least 200 s but not more than 1000 s. This isothermal overaging stabilizes the carbon rich austenite and contributes to the formation and stabilization of low density carbide bainite, conferring the steel of present invention with targeted mechanical properties.

The cold rolled steel sheet is then cooled to room temperature at a cooling rate not more than 200°C/s. During this cooling unstable residual austenite may transform to fresh martensite in form of MA islands.

An optional skin pass or leveler operation with a reduction rate below 0.8% may be performed at that stage.

Figure 4 is a micrograph showing the heat treated cold rolled steel sheet in accordance of the present invention the sheet belong to the Steel Sample 2, wherein the heat treated cold rolled steel sheet having layer of the internal oxides of less than 3 microns which is in accordance of the present invention.

The heat treated cold rolled sheet may then be optionally coated by electro-deposition or vacuum coating or any other suitable process.

A post batch annealing, preferably done at 170 to 210°C during 12h to 30h can be done optionally after annealing on uncoated product or after coating on coated product in order to reduce hardness gradient between phases and ensure degasing for coated products. EXAMPLES

The following tests and examples 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 and expound the significance of the parameters chosen by inventors after extensive experiments and further establish the properties that can be achieved by the steel according to the invention.

Samples of the steel sheets according to the invention and to some comparative grades were prepared with the compositions gathered in table 1 and the processing parameters gathered in table 2. The corresponding microstructures of those steel sheets were gathered in table 3 and the properties in table 4. Table 1 depicts the steels with the compositions expressed in percentages by weight.

Table 1 : composition of the trials

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

Table 1 also shows Bainite transformation Bs and Martensite transformation Ms temperatures of inventive steel and reference steel. The calculation of Bs and Ms is done by using Van Bohemen formula published in Materials Science and Technology (2012) vol 28, n°4, pp487-495, which is as follows:

Bs=839-(86 ^* [Mn]+23 ^* [Si]+67 ^* [Cr]+33 ^* [Ni]+75 ^* [Mo])-270 ^* (1 -EXP(-1 ,33 ^* [C]))

Ms=565-(31 ^* [Mn]+13 ^* [Si]+10 ^* [Cr]+18 ^* [Ni]+12 ^* [Mo])-600 ^* (1 -EXP(-0,96 ^* [C]))

Further, before performing the annealing treatment on the steels of invention as well as reference, the samples were heated to a temperature between 1000° C and 1280°C and then subjected to hot rolling with finish temperature above 850° C. Table 2 : process parameters of the trials

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

5 HBA : hot band annealing of steel sheet

Table 3 gathers the results of test conducted in accordance of standards on different microscopes such as Scanning Electron Microscope for determining microstructural composition of both the inventive steel and reference trials.

Table 3 : microstructures of the trials

Table 4 gathers the mechanical and surface properties of both the inventive steel and reference steel. The tensile strength and yield strength are conducted in accordance with JIS Z2241 standards.

Table 4 : mechanical and surface properties of the trials

I = according to the invention; R = reference; underlined values: not according to the invention. The examples show that the steel sheets according to the invention are the only one to show all the targeted properties thanks to their specific composition and microstructures.