Patent Application
20230287531
The present invention relates to cold rolled steel sheet with high strength and high formability.
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 has also been required for automobiles in view of global environment concerns. Thus, now automotive parts must be made of material having high formability for the criteria of ease of fit in the intricate automobile assembly and at same time improving 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 automobiles 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 appreciation of the present invention:
EP3187608 is high-strength hot-dip galvanized steel sheet having a tensile strength (TS) of 1,300 MPa or more and excellent in ductility and in-plane uniformity of material properties is provided, and a method for manufacturing the steel sheet is also provided. The high-strength hot-dip galvanized steel sheet has a specific composition including C, Si, Mn, etc. In this chemical composition, the content of Ti [Ti] and the content of N [N] satisfy [Ti]>4[N]. The high-strength hot-dip galvanized steel sheet has a microstructure including martensite at an area fraction of 60% or more and 90% or less, polygonal ferrite at an area fraction of more than 5% and 40% or less, and retained austenite at an area fraction of less than 3% (including 0%). The average hardness of the martensite is 450 or more and 600 or less in terms of Vickers hardness, and the average crystal grain diameter of the martensite is 10 μm or less. The standard deviation of the crystal grain diameters of the martensite is 4.0 μm or less. EP3187608 is able to provide the tensile strength above 980 MPa but does not have an elongation of 8% or more.
EP3473741 is a steel sheet having a tensile strength of 950 MPa or more and good toughness and a method for manufacturing the same. The steel sheet has a specific composition and a metallographic structure containing: a ferrite area fraction of 30% or less (including 0%), a tempered martensite area fraction of 70% or more (including 100%), and a retained austenite area fraction of 4.5% or less (including 0%), wherein the average aspect ratio of an iron based carbide, precipitated in tempered martensite grains, having a grain size in the largest 10% is 3.5 or more. But the steel of EP3473741 is not able to provide the ultimate tensile strength of 950 or more in both the rolling as well as the transversal direction.
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.
It is an object of the present disclosure to provide cold-rolled steel sheets that simultaneously have:
In a preferred embodiment, the total elongation of the steel sheet is greater than or equal to 8%.
In a preferred embodiment, a yield strength from 700 MPa to 850 MPa in both the transverse direction as well as the rolling direction and preferably from 720 MPa to 850 MPa in both the transverse direction as well as the rolling direction.
Preferably, such steel can also have a good suitability for forming, in particular for rolling with good weldability and coat ability.
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 from 0.1% to 0.2%. 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 0.1% would not allow the formation of martensite there by tempered martensite, thereby decreasing strength as well as ductility. On the other hand, at a carbon content exceeding 0.2%, 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 from 0.12 to 0.19% and more preferably is from 0.13 to 0.17%.
Manganese content of the steel of the present invention is from 1.2% to 2.2%. Manganese is an element that imparts strength. An amount of at least about 1.2% 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 1.3% to 2.1% is preferred. But when manganese is more than 2.2%, this produces adverse effects such as slowing down the transformation of austenite to ferrite during the slow cooling after annealing, leading to a reduction of ductility. Moreover, a manganese content above 2.2% would also reduce the weldability of the present steel. Hence the preferred limit for the steel of the present invention is from 1.3% to 2.1% and more preferably from 1.6% to 2.0%.
Silicon is an essential element for the steel of the present invention. Silicon is present from 0.05% to 0.6%. 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 0.6%, surface properties and weldability of steel are deteriorated, therefore the Silicon content is preferred from 0.1% to 0.5% and more preferably 0.1% to 0.4%.
Aluminum content of the present invention is from 0.001% to 0.1%. Aluminum is added to de-oxidise the steel of the present invention. Aluminum is an alphageneous element. This can increase the formability and ductility of steel. In order to obtain such an effect, Aluminum content is required at 0.001% 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 from 0.001% to 0.09% and more preferably 0.001% to 0.06%.
Chromium content of the steel of the present invention is from 0.01% to 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. The preferred limit for Chromium is from 0.1% to 0.4% and more preferably 0.1% to 0.3%.
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 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 0.09%.
Sulfur is not an essential element but may be contained as an impurity in steel up to 0.09%. The sulfur content is preferred as low as possible, but between 0.001% and 0.03% is preferred 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 can form nitrides or carbonitrides together with carbon, that can 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 a high amount of Aluminum Nitrides which are detrimental for the present invention hence the preferable limit for the nitrogen is between 0.001% and 0.01%.
Molybdenum is an optional element that constitutes from 0% to 0.5% 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 alloy elements, so that for economic reasons its content is limited to 0.5%.
Niobium is an optional element that can be added to the steel up to 0.1%, preferably between 0.0010 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.
Vanadium is an optional element which may be added to the steel of the present invention up to 0.1%, preferably from 0.001% to 0.01%. As with 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.
Nickel may be added as an optional element in an amount of 0% to 1% to increase the strength of the steel and to improve its toughness. A minimum of 0.01% is required to produce 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 steel and to improve its corrosion resistance. A minimum of 0.01% is required to produce such effects. However, when its content is above 1%, copper causes hot ductility deterioration during hot rolling.
Calcium is an optional element which may be added to the steel of the present invention up to 0.005%, preferably from 0.001% to 0.005%. Calcium is added to the 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≤0.05%, 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 60% to 85% of tempered martensite, 0% to 5% of residual austenite, 0% to 5% of fresh martensite and cumulative amount of ferrite and bainite of 15% to 38% in area fractions. Tempered Martensite constitutes the matrix phase for the steel of the present invention.
Tempered Martensite constitutes from 60% to 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 when the temperature drops below Ms temperature and more particularly from Ms−10° C. to 15° C. Such martensite is then tempered during the holding at a tempering temperature Temper from 150° C. to 300° C. The martensite of the present invention imparts ductility and strength to such steel. Preferably, the content of martensite is from 62% to 80% and more preferably from 62% to 75%.
Fresh martensite is an optional microconstituent which is limited in the steel at an amount of from 0% to 5%, preferably from 0 to 2% and even better equal to 0%. Fresh martensite may form during the final cooling after tempering.
The cumulated amount of ferrite and bainite represents from 15% to 38% of the microstructure. The cumulated amounts of bainite and ferrite is greater than 15% is mandatory to ensure a balance between strength and elongation in which presence of Bainite impart tensile strength of 980 MPa and Ferrite ensure the elongation. Bainite forms during the reheating before tempering. Bainite can impart strength to the steel but when present in a too big amount, it may adversely impact the yield strength of the steel. Ferrite imparts elongation as well as formability to the steel of the present invention. To ensure an elongation of 8% and preferably 9% or more it is preferred to have 10% of Ferrite. Ferrite is formed during the first step of cooling after annealing. But when the cumulative presence of bainite and ferrite are present above 38% the mechanical properties may get impacted adversely specifically the tensile strength and yield strength in transverse direction. Hence the preferred limit for the cumulative presence ferrite and bainite is kept from 20% to 37% and more preferably from 25% to 36%.
Residual Austenite is an optional microstructure that can be present from 0% to 5% in the steel. The presence of Residual austenite till 5% is not detrimental to the mechanical properties. Up to 5% Residual austenite imparts ductility and elongation to the steel. It is preferred residual austenite between 0% and 3% and more preferably from 0% to 2%.
In addition to the above-mentioned microstructure, the microstructure of the cold rolled 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 220 mm for slabs up to several tens of millimeters for thin strip.
For example, a slab having the chemical composition according to the invention is manufactured by continuous casting wherein the slab optionally underwent a direct soft reduction during the continuous casting process to avoid central segregation and to ensure a ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by continuous casting process can be used directly at a high temperature after the continuous casting or may be first cooled to room temperature and then reheated for hot rolling.
The temperature of the slab, which is subjected to hot rolling, must be at least 1000° C. and must be below 1280° C. In case the temperature of the slab is lower than 1000° C., excessive load is imposed on a rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing rolling, whereby the steel will be rolled in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the slab must be high enough so that hot rolling should be completed in the temperature range of Ac3 to Ac3+100° C. Reheating at temperatures above 1280° C. must be avoided because they are industrially expensive.
The sheet obtained in this manner is then cooled at a cooling rate of at least 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 from 475° C. to 625° C. to avoid scale formation, with an even prefererred range for such coiling temperature from 500° C. to 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 from 400° C. to 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 from 35 to 90%.
Thereafter the cold rolled steel sheet is heat treated which imparts the steel of the present invention with requisite mechanical properties and microstructure.
The cold rolled steel sheet is then heated in a two step 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 from 550° C. to 750° C. In a preferred embodiment, the heating rate HR1 for such first step of heating is at least 12° C./s and more preferably at least 15° C./s. The preferred HT1 temperature for such first step is from 575° C. to 725° C. and more preferably from 575° C. to 700° C.
In the second step of heating, the cold rolled steel sheet is heated from HT1 to an annealing temperature Tsoak which is from Ac3 to Ac3+100° C., preferably from Ac3+10° C. to Ac3+100° C., at a heating rate HR2 which is from 1° C./s to 15° C./s. In a preferred embodiment, the heating rate HR2 for the second step of heating is from 1° C./s to 8° C./s and more from 1° C./s to 4° C./s, wherein Ac3 for the steel sheet is calculated by using the following formula:
Ac3=910−203[C]{circumflex over ( )}(½)−15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]−30[Mn]−11[Cr]−20[Cu]+700[P]+400[Al]+120[As]+400[Ti]
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 step 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 1° C./s and 15° C./s, to a temperature T1 which is in a range from 630° C. to 685° C. In a preferred embodiment, the cooling rate CR1 for such first step of cooling is from 1° C./s to 10° C./s and more preferably from 1° C./s to 4° C./s. The preferred T1 temperature for such first step is from 640° C. to 685° C. and more preferably from 650° C. to 685° C.
In the second step of cooling, the cold rolled steel sheet is cooled down from T1 to a temperature T2 which is from Ms−10° C. to 15° C., at a cooling rate CR2 of at least 100° C./s. In a preferred embodiment, the cooling rate CR2 for the second step of cooling is at least 200° C./s and more preferably at least 300° C./s. The preferred T2 temperature for such second step is from Ms−20° C. to 20° C. and more preferably from Ms−50° C. to 20° C.
Ms for the steel sheet is calculated by using the following formula:
Ms=545−601.2*(1−EXP(−0.868[C]))−34.4[Mn]−13.7[Si]−9.2[Cr]−17.3[Ni]−15.4[Mo]+10.8[V]+4.7[Co]−1.4[Al]−16.3[Cu]−361[Nb]−2.44[Ti]−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 5° C./s and preferably of at least 10° C./s and more preferably 12° C./s or more during 100 s to 600 s. The preferred temperature range for tempering is from 175° C. to 280° C. and the preferred duration for holding at Ttemper is from 200 s to 500 s.
According to the present invention the tempering temperature is selected such that the difference between T1 and Ttemper is from 415° C. to 455° C. AT is determined as follows:
ΔT=(T1−Ttemper)
When ΔT is less than 415° C. then the cumulative amount of bainite and ferrite exceeds 38% which is detrimental for the mechanical properties specifically the tensile strength in transversal direction. When ΔT is greater than 455° C. then the amount of tempered martensite is too high, thereby the steel of the present invention in the rolling direction exceeds 1150 MPa. The preferred ΔT is between 420° C. and 440° C.
Then, the cold rolled steel sheet is cooled down to room temperature to obtain a heat treated cold rolled steel sheet.
The heat treated cold rolled 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.
The heat treated cold rolled steel sheet can also be coated by any of the known industrial processes such as Electro-galvanization, JVD, PVD, etc.,
Then an optional post batch annealing may be done at a temperature between 150° C. and 300° C. during 30 minutes to 120 hours.
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.
All the examples and counter examples are reheated to a temperature of 1200° C. and then hot rolled wherein the hot rolled finishing temperature is 890° C. thereafter the hot rolled steel strip is cooled at a rate of 80° C./s and coiled at 530° C. and cold rolled reduction for all examples and counter examples is 50%.
690
460
407
410
410
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.
87
13
39
53
47
57
43
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 NF EN ISO 6892 standards,
1177
1159
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/056787 | 7/20/2020 | WO |
