Patent Application
20230036084
The present invention relates to cold rolled steel sheet with high strength and high formability having tensile strength of 950 MPa or more and a hole expansion ratio of more than 56% which is suitable for use as a steel sheet for vehicles.
Automotive parts are required to satisfy two inconsistent necessities, namely ease of forming and strength. However 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 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 undertaken 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:
JP2012111978 is a patent application having a composition C: 0.05-0.3%, Si: 0.01-3.0%, Mn:0.5-3%, Al: 0.01-0.1%, and the balance Fe and incidental impurities and having a component composition consisting of, ferrite and tempered martensite as a main component of the high-strength cold rolled steel sheet but such steel is not able to reach more than 50% of hole expansion ratio.
EP2971209 is patent that relates to a high strength hot dip galvanised complex phase steel strip having improved formability to be used in the automotive industry having an mandatory elemental composition C: 0.13-0.19%, Mn:1.70-2.50% Si: 0-0.15%, Al: 0.40-1.00%, Cr: 0.05-0.25%, Nb: 0.01-0.05%, P: 0-0.10%, Ca: 0-0.004%, S: 0-0.05%, N: 0-0.007% the balance being Fe and inevitable impurities, wherein 0.40%<Al+Si<1.05% and Mn+Cr>1.90%, and having a complex phase microstructure, in volume percent, comprising 8-12% retained austenite, 20-50% bainite, less than 10% martensite, the remainder being ferrite but the granted patent is unable to reach the tensile strength beyond 900 MPa.
The known prior art related to the manufacture of high strength and high formability steel sheets is inflicted by one or the other problems: hence there lies a need for a cold rolled steel sheet having high strength and high formability and a method of manufacturing the same.
It is an object of the present invention to provide cold-rolled steel sheets that simultaneously have:
In a preferred embodiment, the steel sheet according to the invention may have a yield strength value greater than or above 750 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.
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 0.09% and 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. A content less than 0.09% would not allow securing adequate amount of martensite, thereby decreasing strength as well as ductility. 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. The preferred limit for Carbon is between 0.1% and 0.14%, more preferably 0.1% and 0.13%.
Manganese content of the steel of the present invention is between 1.8% and 2.5%. Manganese is an element that imparts strength by solid solution strengthening. An amount of at least about 1.8% by weight of manganese has been found in order to provide the strength and hardenability of the steel sheet. Thus, a higher percentage of Manganese such as 1.9% to 2.4% is preferred and more preferred limit is between 2.0% and 2.3%. But when manganese is more than 2.5%, this produces adverse effects such as slowing down the transformation of austenite to martensite during the cooling after annealing, leading to a reduction of strength. Moreover, a manganese content above 2.5% would also reduce the weldability of the present steel.
Silicon is an essential element for the steel of the present invention, Silicon is present between 0.2% and 0.7%. Silicon is added to the steel of the present invention to impart strength by solid solution strengthening. Silicon plays a part in the formation of the microstructure by preventing the precipitation of carbides and by promoting the formation of martensite. But whenever the silicon content is more than 0.7%, surface properties and weldability of steel is deteriorated, therefore the Silicon content is preferred between 0.3% and 0.7% and more preferably between 0.4% and 0.6%.
Aluminum content of the present invention is between 0.01% and 0.1%. Aluminum is added to de-oxidize the steel of the present invention. Aluminum is an alphageneous element and also retarding the formation of carbides. This can increase the formability and ductility of steel. In order to obtain such an effect, Aluminum content is required at 0.01% or more. However, when the Aluminum content exceeds 0.1%, Ac3 point increases beyond acceptable, austenite single phase is very difficult to achieve industrially hence hot rolling in complete austenite region cannot be performed. Therefore, Aluminum content must not be more than 0.1%. The preferable limit for the presence of Aluminum is between 0.01% and 0.08% and more preferably 0.01% and 0.05%.
Phosphorus content of the steel of the 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 at most 0.01%, 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% in order to avoid ageing of material, nitrogen forms the nitrides which impart strength to the steel of the present invention by precipitation strengthening with Vanadium and Niobium but whenever the presence of nitrogen is more than 0.09% it can form high amount of Aluminum Nitrides which are detrimental for the present invention hence the preferable upper limit for nitrogen is 0.01%.
Niobium is an optional element that can be added to the steel up to 0.1%, preferably between 0.001% and 0.1%. 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.
Titanium is an optional element which may be added to the steel of the present invention up to 0.1%, preferably between 0.001% and 0.1%. 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 0.1% 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 the present invention.
Chromium content of the steel of the present invention is between 0% and 1%. Chromium is an optional element that provide strength and hardening to the steel, but when used above 1% impairs surface finish of the steel.
Molybdenum is an optional element that constitutes between 0% and 1% of the Steel of the present invention; Molybdenum increases the hardenability of the steel of the present invention and influences the transformation of austenite to Ferrite and Bainite during cooling after annealing. However, the addition of Molybdenum excessively increases the cost of the addition of ahoy elements, so that for economic reasons its content is limited to 1%.
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 the present invention.
Calcium is an optional element which may be added to the steel of the present invention up to 0.005%, preferably between 0.001% and 0.005%. Calcium is added to steel of the 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. Present invention does not intend to add Copper and Nickel but these elements may be present as residuals up to 0.1% either severaly or cumulatively.
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 fractions 65% to 85% of Tempered Martensite, 0% and 5% of residual austenite and cumulative amount of bainite and ferrite between 15% and 35%. Tempered martensite constitutes the matrix phase for the steel of the present invention
Tempered Martensite constitutes between 65% and 85% of the microstructure by area fraction. Tempered martensite is formed from the martensite which forms during the second step of cooling after annealing and particularly below Ms temperature and more particularly between Ms-50° C. and 20° C. Such martensite is then tempered during the holding at a tempering temperature Temper between 150° C. and 400° C. Tempered martensite of the present invention does not contain coarse Iron Carbides specifically iron based carbide having a grain size aspect ratio of 3.5 or more because these iron carbides inhibits the present invention to reach the target hole expansion ratio. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is between 65% and 80% and more preferably between 68% and 78%.
Bainite and Ferrite are cumulatively present in the steel between 15% and 35%. In a preferred embodiment, the range for cumulated amount of ferrite and bainite is between 20% and 35% and more preferably between 22% and 32%.
Ferrite constituent improves the properties of the steel of the present invention, in particular regarding elongation and hole expansion ratio as ferrite is a soft and intrinsically ductile constituent. This ferrite is mainly formed during the first step of cooling after annealing. In a preferred embodiment, ferrite can be present at least in an amount of 15%.
Bainite can impart strength to the steel but when present in a large amount it may adversely impact the hole expansion ratio and elongation of the steel. Bainite forms during the reheating before tempering. In a preferred embodiment, the bainite content is kept between 0% and 10% more preferably below 8% and even more preferably below 5%.
Residual Austenite is an optional phase that can be present between 0% and 5% in the steel, but is preferably not present.
In a preferred embodiment the steel sheet according to the invention may be obtained by any appropriate method. It is however preferred to use the process according to the preferred embodiments of the invention, which comprises the following successive steps:
Such process includes providing a semi-finished product of steel with a chemical composition according to the invention. The semi-finished product can be casted either into ingots or continuously in form of thin slabs or thin strips, i.e. with a thickness ranging from approximately 220 mm for slabs up to several tens of millimeters for thin strip, for example.
For the purpose of simplification of the present invention, 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 at least 1000° C., preferably above 1100° C. and must be below 1250° 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 1250° 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 1250° 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 Ac3 and preferably above Ac3+50° C. It is necessary that the final rolling be performed above Ac3, because below this temperature the steel sheet exhibits a significant drop in rollability. A final rolling temperature is preferably above Ac3+50° C. to have a structure that is favorable to recrystallization and rolling.
The sheet obtained in this manner is then cooled down at a cooling rate of at least 30° C./s to the coiling temperature which is below 600° C. Preferably, the cooling rate will be less than or equal to 65° C./s and above 35° C./s. The coiling temperature is preferably of at least 350° C. to avoid the transformation of austenite into ferrite and pearlite and to contribute in forming an homogenous bainite and martensite microstructure.
The coiled hot rolled steel sheet may be cooled down to room temperature before subjecting it to an optional hot band annealing or may be send to an optional hot band annealing directly.
Hot rolled steel sheet may be subjected to an optional pickling to remove the scale formed during the hot rolling, if needed. The hot rolled sheet is then subjected to an optional hot band annealing at a temperature between 400° C. and 750° C., preferably during 1 to 96 hours.
Thereafter, pickling of this hot rolled steel sheet may be performed if necessary to remove the scale.
The hot rolled steel sheets are then cold rolled with a thickness reduction between 35 to 90%. The cold rolled steel sheet is then subjected to annealing to impart the steel of the present invention with targeted microstructure and mechanical properties.
To anneal the cold rolled steel sheet, the cold rolled steel sheet is heated in a two-step heating process, in step one the cold rolled sheets is heated to a temperature HT1 between 600° C. and 650° C. at a heating rate HR1 of at least 10° C./s. Then, in step two, the cold rolled sheet is heated from HT1 to an annealing temperature between Ac3 and Ac3+200° C. at a heating rate HR2 of at least 1° C./s and preferably at least 2.0° C./sHR1 is always higher than HR2.
The preferred HR1 is at least 15° C./s and the preferred HT1 temperature range is between 600° C. and 630° C. The preferred range for annealing temperature is between Ac3+10° C. and Ac3+150° C. and more preferably between Ac3+20° C. and Ac3+100° C.
Then the cold rolled steel sheet is held at the annealing temperature during at least 5 s and not more than 1000 s. The temperature and time are selected to ensure 100% re-crystallization i.e. to obtain a percentage of 100% austenite at the end of the annealing.
The sheet is then cooled in a three-step cooling process. In step one, the cold rolled sheet is cooled from the annealing temperature to a temperature CT1 between 675° C. and 725° C. at a cooling rate CR1 of 10° C./s or less. Then, in step two, the cold rolled sheet is cooled from CT1 to CT2 between 450° C. and 550° C. at a cooling rate CR2 of at least 30° C./s. Then, in step three, the cold rolled sheet is cooled from CT2 to CT3 between Ms-50° C. and 20° C. at a cooling rate CR3 which is at least 200° C./s.
In a preferred embodiment, the cooling rate CR1 is 5° C./s or less and CT1 is preferably between 685° C. and 720° C. and more preferably 685° C. and 700° C. The preferred range for CR2 is at least 40° C./s and the preferred range for CT2 is between 450° C. and 525° C. and more preferably between 460° C. and 510° C. The preferred range for CR3 is at least 300° C./s and more preferably at least 400° C./s. Preferred limit for CT3 is between Ms-80° C. and 20° C. and more preferably between Ms-100° C. and 20° C.
Then the cold rolled steel sheet at a heating rate of at least 10° C./s, or better of at least 20° C./s and to a tempering temperature between 300° C. and 380° C. and held at tempering temperature during at least 100 s but not more than 1000 s. to obtain tempered martensite, conferring the steel of the present invention with good mechanical properties. The preferred tempering temperature range is between 320° C. and 360° C. and more preferably is 330° C. and 350° C.
The cold rolled steel sheet is then cooled to room temperature, preferably at a cooling rate of 200° C./s or less.
An optional skin pass operation with a reduction rate below 1% may be performed at that stage or an optional tension leveling operation.
The heat treated cold rolled sheet may then be optionally coated by electrodeposition or vacuum coating or any other suitable process.
An optional post batch annealing, preferably done at 170 to 210° C. during 12 h to 30 h 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.
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 2 gathers the annealing process parameters implemented on steels of Table 1.
Table 2 also shows Ac3 and Martensite transformation Ms temperatures of the steel samples. The calculation of Ac3 and Ms is done by using following formulas:
Further, the samples were heated to a temperature between 1000° C. and 1250° C. and then subjected to hot rolling with finish temperature 890° C. and thereafter were coiled at a temperature below 600° C. The hot rolled coils were then processed as claimed and cold rolled with a thickness reduction between 35 to 90%.
Table 2: Process Parameters of the Trials
590
440
660
680
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 4 gathers the mechanical properties of both the inventive steel and reference steel. The tensile strength, yield strength and total elongation test are conducted in accordance with ISO 6892 standards, whereas to estimate hole expansion, a test called hole expansion is applied according the standard IS016630:2009. In this test, sample is subjected to punching to form a hole of 10 mm (=Di) and deformed. After deformation, the hole diameter Df was measured and the hole expansion ratio (HER) is calculated using the under formula:
HER %=100*(Df−Di)/Di
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/IB2019/060741 | Dec 2019 | IB | international |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/061765 | 12/10/2020 | WO |
