MANUFACTURING THEREOF

The present invention relates to cold rolled steel sheet with high strength and high formability having tensile strength of 980 MPa or more and a total elongation of more than 14% 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 high 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 9074 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 9074272 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. 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 high strength and high formability 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 950 MPa and preferably above 980 or even above 1000 MPa,

- a total elongation greater than or equal to 14% and preferably greater than or equal to 15%.

In a preferred embodiment, the steel sheet according to the invention may have a yield strength value greater than or above 540 MPa or even better above 550 MPa.

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

Another objective 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.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

Carbon is present in the steel from 0.05% to 0.15%. 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 0.05% would not securing the formation of martensite, thereby decreasing strength. On the other hand, at a carbon content exceeding 0.15%, a weld zone and a heat-affected zone are significantly hardened, and thus the mechanical properties of the weld zone are impaired. Hence the preferable limit is from 0.07% to 0.12% and more preferably from 0.08% to 0.11%.

Manganese content of the steel of present invention is from 1.8% to 2.7%. Manganese is an element that imparts strength to the steel by solid solution strengthening. An amount of at least about 1.8 % by weight of manganese is needed in order to provide the strength and hardenability of the steel sheet as well as to form ferrite. Thus, a higher percentage of Manganese such as 1.9% to 2.5% is preferred and more preferably 2.1% to 2.5%. But when manganese is more than 2.7%, this produces adverse effects such as slowing down the transformation of austenite during cooling after annealing, leading to a reduction of ductility. Moreover, a manganese content above 2.7% would also reduce the weldability of the present steel.

Silicon content of the steel of present invention is from 0.1% to 1%. Silicon imparts the strength to the steel of present invention by solid solution strengthening. Silicon promotes the ferrite transformation. However, adding more than 1% 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 1%. A preferable limit for the presence of silicon is kept from 0.2% to 0.9% and more preferably from 0.3% to 0.7%.

The content of aluminum of the steel of the present invention is from 0.01 to 0.8%. 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 0.8% in the present invention, it will increase the Ac3 point, thereby lowering the productivity. Hence the preferable range for aluminum is kept from 0.01% to 0.7 % and more preferably from 0.01% to 0.6%.

In a preferred embodiment, the cumulated amounts of Silicon and Aluminum is at least 0.6% because both the elements are ferrite phase-generating element, thereby participating to the formation of ferrite that is favorable for both the elongation and ductility. Chromium content of the steel of present invention is from 0.1% to 0.9%. Chromium is an essential element that provide strength and hardening to the steel, but when used above 0.9% impairs surface finish of the steel. Hence to achieve the effects of chromium optimally the preferred limit is between 0.2% and 0.8% and more preferably from 0.2% to 0.7%.

Titanium is an essential element which may be added to the steel of the present invention from 0.0001% to 0.1% and preferably from 0.01% to 0.08%, Similarly to 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 0.1% to avoid coarse TiN detrimental for hole expansion. In case the titanium content is below 0.0001% it does not impart any effect on the steel of present invention.

Boron is an essential element for the present invention and is added in very small amount and is added from 0.0005% to 0.003%. Boron imparts hardenability and strength to the steel of present invention. However, when boron is added more than 0.003% the reliability of the steel sheet is found to be significantly lowered. Further boron the segregation may happen at grain boundaries which is detrimental for the formability.

Niobium is an essential element that can be added to the steel from 0.01% to 0.1%, preferably from 0.01% to 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 0.1% 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.

Vanadium is an optional element which may be added to the steel of the present invention up to 0.2%, preferably from 0.001% to 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 0.2% 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.

Phosphorus content of the steel of present invention is limited to 0.09%. 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 0.02%.

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 0.09% or less and preferably less than 0.03%, from the viewpoint of manufacturing cost. Further if higher sulfur is present in steel it combines to form sulfide especially with Mn and Ti and reduces their beneficial impact on the present invention.

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.

Molybdenum is an optional element that constitutes from 0% to 0.2% of the Steel of present invention; Molybdenum improves hardenability and hardness, delays the appearance of Bainite hence promote the formation of Martensite, when added in an amount of at least 0.01%. Molybdenum also facilitate the formation of Ferrite. Flowever, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.2%. The preferable limit for Molybdenum is from 0.01% to 0.2%

Nickel may be added as an optional element in an amount of 0% to 2% 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. Flowever, when its content is above 2%, Nickel causes ductility deterioration. Copper may be added as an optional element in an amount of 0% to 2% 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 2%, it can degrade the surface aspects.

Calcium is an optional element which may be added to the steel of present invention up to 0.005%, preferably from 0.0001% to 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, magnesium or zirconium can be added individually or in combination in the following proportions: Ce < 0.1 %, 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.

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 in area fraction, 40% to 60% of martensite, 5% to 40% of Inter-critical Ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.

Martensite constitutes 40% to 60% of microstructure by area fraction. Martensite can notably be formed during the cooling after annealing and particularly after crossing the Ms temperature and particularly between Ms-10°C and 20°C or during cooling after overaging. Martensite imparts strength to the present invention. Preferable limit for Martensite is between 42% and 58% and more preferably between 43% and 56%.

Inter-critical ferrite constitutes between 5% and 40% of microstructure by area fraction of the steel of present invention. This inter-critical ferrite imparts the steel of present invention with a total elongation of at least 14%. The intercritical ferrite results from the annealing at a temperature below Ac3. The intercritical ferrite is different from the ferrite that could be created after the annealing, named hereinafter “transformed ferrite”, that will be described below. Contrarily to the transformed ferrite, the intercritical ferrite is polygonal. Besides, the transformed ferrite is enriched in carbon and manganese, i.e. has carbon and manganese contents which are higher than the carbon and manganese contents of the intercritical ferrite. The intercritical ferrite and the transformed ferrite can therefore be differentiated by observing a micrograph with a SEM microscope using secondary electrons, after etching with 2% Nital etching agent. On such micrograph, the intercritical ferrite appears in medium grey, whereas the transformed ferrite appears in dark grey, owing to its higher carbon and manganese contents

The total amount of transformed ferrite and bainite constitutes from 10% to 35% of microstructure by area fraction for the steel of present invention. Transformed Ferrite of present invention constitutes of Ferrite formed during the cooling after annealing. Transformed Ferrite imparts high strength as well as elongation to the steel of present invention. T ransformed Ferrite of the present steel is rich in carbon and Manganese as compared to the inter-critical ferrite. Bainite forms during the overaging holding especially between 40Q°C and 480°C. To ensure an elongation of 14% it is necessary to have 10% of transformed ferrite and bainite. But whenever the total amount is present above 35% in steel of present invention it is not possible to have both tensile strength and the total elongation at the same time. The preferred limit for transformed ferrite and bainite for the present invention is between 15% and 30%. Residual Austenite is an optional microstructure and can be present between 0% and 5% in the steel.

In addition to the above-mentioned microstructure, the microstructure of the cold rolled and heat treated steel sheet is free from microstructural components, such as pearlite, tempered martensite and cementite without impairing the mechanical properties of the steel sheets. 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 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 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, is 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. Reheating at temperatures above 1280°C must be avoided because they are industrially expensive. Therefore, the finish rolling temperature of the slab is above Ac3 and preferably sufficiently high so that hot rolling can be completed in the temperature range of Ac3 + 150°C to Ac3+250°C

A final rolling temperature range between Ac3 to Ac3+200°C is necessary to have a structure that is favorable to recrystallization and rolling. It is preferred that the final rolling pass to be performed at a temperature greater than 850°c and better of at least 950°C.

The hot rolled steel obtained in this manner is then cooled at a cooling rate of at least 30°C/s to the coiling temperature. Preferably, the cooling rate will be less than or equal to 200° C/s. The hot rolled steel is then coiled at a temperature between 475°C and 650°C to avoid ovalization and preferably between 475°C and 625°C to avoid scale formation. A more preferred range for such coiling temperature is between 500°C and 625°C. The coiled hot rolled steel is then cooled down to room temperature before subjecting it to optional hot band annealing.

The hot rolled steel 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 subjected to an optional Hot Band Annealing at, for example, temperatures between 400°C and 750°C for preferably at least 12 hours and not more than 96 hours, the temperature remaining below 750°C to avoid transforming partially the hot-rolled microstructure and, therefore, losing the microstructure homogeneity. Thereafter, an optional scale removal step of this hot rolled steel may performed through, for example, pickling of such sheet.

This hot rolled steel is subjected to cold rolling to obtain a cold rolled steel sheet with a thickness reduction between 35 to 90%. The cold rolled steel sheet obtained from cold rolling process is then subjected to annealing to impart the steel of present invention with microstructure and mechanical properties.

To anneal the said cold rolled steel sheet, it is heated up to the soaking temperature between Ac1 +60°C and Ac3, preferably at a heating rate of at least 3°C/s, then the annealing is performed at that temperature during 5 to 500 seconds, preferably during 50 to 250 seconds. In a preferred embodiment, the heating is at least 10°C/s and more preferably at least 15°C/s. During this annealing Inter-critical ferrite forms.

The preferred annealing soaking temperature is between Ac1 + 70°C and Ac3 and more preferably between Ac1 + 80°C and Ac3 - 30°C.

In a preferred embodiment, the time and temperature of soaking are selected to ensure that the microstructure of the steel sheet at the end of the soaking contains at least 50% of Austenite and more preferably at least 60% of austenite. Then the cold rolled steel is cooled in a two-step cooling process wherein the first step starts from soaking temperature to a temperature T1 between 550°C and 650°C, at a cooling rate CR1 which is least 3°C/s, preferably at least 5°C/s and more preferably at least 10°C/s. During this step the transformed ferrite forms. The cold rolled steel is then held at T 1 during 1 s to 20 s and preferably between 2 s and 15 s and more preferably between 5 s and 12 s.

Thereafter the second step starts from cooling further the cold rolled steel sheet from T 1 to overaging temperature T2 between 400°C and 480°C, at a cooling rate CR2 of at least 3°C/s, preferably at least 5°C/s and more preferably at least 7°C/s..

Then overaging is being performed at T2 during 5 to 100 seconds. During overaging, some bainite gets formed. The preferred temperature T2 for overaging is between 420°C and 475°C. The preferred time for overaging temperature during 15 to 75 seconds and more preferably 20 and 75 seconds. The cold rolled steel sheet can then either be cooled down to room temperature or can be brought to the temperature of a hot dip coating bath between 420°C and 680°C, depending on the nature of the coating, to facilitate hot dip coating of the cold rolled steel sheet.

In either case, the final cooling down to room temperature is done at a cooling rate of at least 5°C/s and preferably at least 9°C/s to ensure the formation of fresh martensite in the steel of present invention.

The cold rolled steel sheet can also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, etc, which may not require bringing it to the above-mentioned temperature range before coating. 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 5 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 10 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 : composition of the trials

Table 1 depicts the steels with the compositions expressed in percentages by is weight.

Table 1 also shows Ac1 and Ac3 temperature points which are calculated by dilatometry. Table 2 : Process parameters

Table 2 gathers the annealing process parameters implemented on steel samples of Table 1 , that were all reheated at 1230°C, hot-rolled with a finish rolling temperature of 875°C, coiled at 550°C and cold rolled with a 50% reduction rate 5 before undergoing annealing and two-steps cooling scheme including overaging: underlined values: not according to the invention.

Table 3 gathers the results of test conducted in accordance of standards on 10 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 underlined values: not according to the invention. Table 4 gathers the mechanical properties of both the inventive steel and reference steel. The tensile strength, yield strength and total elongation tests are conducted in accordance with JIS Z2241 standards. Table 4 : mechanical properties of the trials 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.