Cold rolled and annealed steel sheet, method for the production thereof and use of such a steel sheet for the production of vehicle parts

Abstract

To provide a steel sheet having low density, high strength and high ductility.SOLUTION: A cold-rolled and annealed steel sheet comprising, by weight: 0.6 <C <1.3%, 15 ≤ Mn <35%, 6 ≤ Al <15%, Si ≤ 2.40%, S ≤ 0.03%, P ≤ 0.1%, N ≤ 0.1%, optionally up to 3% independently, one or more optional elements selected from Ni, Cr, and Cu; and optionally up to a cumulative 2.0% of one or more elements selected from B, Ta, Zr, Nb, V, Ti, Mo, and W; The remainder of the composition being made up of iron and unavoidable impurities from the smelting, the microstructure of the steel sheet comprising between 1% and 10% of ordered ferrite, optionally up to 10% of kappa carbides, the remainder being made of austenite, and the density of the steel sheet being less than or equal to 7.2 and the FWHM of the austenitic matrix being between 0,700 and 1,100.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a low density steel sheet exhibiting a microstructure comprising predominantly austenite.The steel sheet according to the invention is particularly well suited for the manufacture of safety or structural parts for vehicles, such as land vehicles. [Background technology]

[0002] Environmental constraints force automobile manufacturers to continually reduce the CO2 emissions from their vehicles. To do so, they have several options, the main ones being to reduce the weight of the vehicle or to improve the efficiency of the engine system. Progress is often achieved by combining these two approaches. This invention concerns the first option, i.e., reducing the weight of the vehicle. In this very specific field, there are two alternative procedures:

[0003] The first step consists in reducing the thickness of the steel while improving its level of mechanical strength. Unfortunately, this solution has its limitations due to the reduction in stiffness of certain car components and the appearance of acoustic problems that create discomfort for passengers, not to mention the inevitable loss of ductility that accompanies the increase in mechanical strength.

[0004] The second step consists in reducing the density of steel by alloying it with other lighter metals: the lower density of these alloys allows for significant weight savings while offering attractive mechanical and physical properties.

[0005] In particular, US Patent Application Publication No. 2003 / 0145911 discloses Fe-Al-Mn-Si lightweight steels with good formability and high strength, but the ultimate tensile strength of such steels does not exceed 800 MPa, preventing the full utilization of their low density for producing parts with any kind of shape. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US Patent Application Publication No. 2003 / 0145911 Summary of the Invention [Problem to be solved by the invention]

[0007] It is therefore an object of the present invention to provide a steel sheet that exhibits a density of less than 7.2, an ultimate tensile strength of at least 1000 MPa, and a tensile elongation of at least 15%. [Means for solving the problem]

[0008] In a preferred embodiment, the steel sheet according to the invention exhibits a density of 7.1 or less, or 7.0 or less, an ultimate tensile strength of at least 1000 MPa, a yield strength of at least 750 MPa, and a tensile elongation of at least 18%.

[0009] This object is achieved by providing a steel sheet as claimed in claim 1. The steel sheet also includes the features of claims 2 to 16. Another object is achieved by providing a method as claimed in claims 17 to 21. Another aspect is achieved by providing a component or a vehicle as claimed in claims 22 to 24.

[0010] Other features and advantages of the present invention will become apparent from the following detailed description of the invention.

[0011] Without wishing to be bound by any theory, the low density steel sheet according to the present invention allows for improved mechanical properties due to its specific microstructure.

[0012] With respect to the chemical composition of steel, carbon plays an important role in forming the microstructure and achieving the desired mechanical properties. Its primary role is to stabilize austenite, the dominant phase in the steel microstructure, and to provide strength. A carbon content below 0.6% will reduce the proportion of austenite, thereby reducing both the ductility and strength of the alloy.

[0013] Kappa carbide (Fe,Mn)3AlC x As a major constituent element, carbon promotes the precipitation of such carbides. However, a carbon content greater than 1.3% can promote the precipitation of coarse carbides at grain boundaries, leading to a decrease in the ductility of the alloy.

[0014] To obtain sufficient strength, the carbon content is preferably between 0.8% and 1.3% by weight, more preferably between 0.8% and 1.0%.

[0015] Manganese is an important alloying element in this system primarily due to the fact that alloying with very large amounts of manganese and carbon stabilizes austenite down to room temperature, allowing the inclusion of large amounts of aluminum without destabilizing it and transforming it to ferrite or martensite. The manganese content must be 15% or greater to allow the alloy to have good ductility. However, if the manganese content exceeds 35%, β-Mn phases will precipitate, reducing the ductility of the alloy.

[0016] Therefore, the manganese content should be controlled to be not less than 15% but not more than 35%. In a preferred embodiment, the manganese content is not less than 15.5%, or even more than 16%. This amount is more preferably between 18% and 30%, and even between 18% and 25%.

[0017] The addition of aluminum to high-manganese austenitic steels can effectively reduce the density of the alloy. Furthermore, it significantly increases the stacking fault energy (SFE) of austenite, which in turn changes the strain-hardening behavior of the alloy. Aluminum is a nano-sized kappa carbide (Fe,Mn)3AlC x Therefore, the addition of aluminum significantly enhances the formation of such carbides. The concentration of aluminum in the present alloy should be adjusted to ensure austenite stabilization and kappa carbide precipitation, while controlling ferrite formation. Therefore, the aluminum content should be controlled to be greater than or equal to 6% but less than or equal to 15%. In a preferred embodiment, the aluminum content is between 6% and 12%, preferably between 6% and 10%.

[0018] Silicon is a common alloying element found in high-manganese and high-aluminum steels. Silicon has a very strong effect on the formation of ordered ferrite DO3. Furthermore, silicon has been shown to enhance the activity of carbon in austenite and increase the partitioning of carbon into kappa carbide. Furthermore, silicon has been described as an effective alloying element that can be used to delay or prevent the precipitation of the brittle β-Mn phase. However, at concentrations above 2.40%, elongation decreases and there is a tendency for undesirable oxides to form during certain assembly processes; therefore, the silicon content must be kept below this limit. Preferably, the silicon content is less than 2.0%, and advantageously less than 1.0%.

[0019] Sulfur and phosphorus are impurities that embrittle grain boundaries and must not exceed 0.03% and 0.1%, respectively, to maintain sufficient heat-sensitive ductility.

[0020] The nitrogen content must be 0.1% or less to prevent the formation of volume defects (bubbles) during precipitation and solidification of AlN.

[0021] Nickel has a positive effect on the penetration of hydrogen into steel and can therefore be used as a diffusion barrier to hydrogen. Nickel can also be used as an effective alloying element, as it promotes the formation of ordered compounds such as B2 in ferrite, providing additional strength. However, for cost reasons, it is desirable to limit the addition of nickel to a maximum of 4% or less relative to the other elements, and preferably between 0.1% and 2.0%. In other embodiments, the amount of nickel is less than 0.1%.

[0022] Chromium can be used as an optional element to increase the strength of the steel through solid solution hardening. Chromium also enhances the hot corrosion resistance of the steel according to the present invention. However, since chromium reduces stacking fault energy, its content should not exceed 4%, preferably between 0.1% and 2.0%, or between 0.1% and 1.0%. In other embodiments, the amount of chromium is less than 0.1%.

[0023] Similarly, the optional addition of copper, at levels not exceeding 4%, is one means of hardening the steel by precipitation of copper-rich precipitates. However, above this level, copper can be a source of surface defects in hot-rolled steel. Preferably, the amount of copper is between 0.1% and 2.0%, or between 0.1% and 1.0%. In other embodiments, the amount of copper is less than 0.1%.

[0024] Boron has very low solid solubility and a strong tendency to segregate at grain boundaries due to strong interactions with lattice defects. Boron can therefore be used to limit the precipitation of kappa carbide between grains. Preferably, the amount of boron is less than 0.1%.

[0025] Niobium is an effective grain refiner, which can simultaneously increase the strength and toughness of steel. In addition, tantalum, zirconium, niobium, vanadium, titanium, molybdenum, and tungsten are also optional elements that can be used to achieve hardening and strengthening through the precipitation of nitrides, carbonitrides, or carbides. However, if their cumulative amount exceeds 2.0%, preferably 1.0%, excessive precipitation can cause a decrease in toughness, which must be avoided.

[0026] The microstructure of the steel sheet according to the invention comprises between 1% and 10% ferrite, optionally up to 10% kappa carbide, the balance being austenite.

[0027] The austenite matrix is ​​present as the primary phase in the steel of the present invention, and is present in the steel at a volume fraction of at least 90%, preferably between 90% and 98%. The austenite of the present invention preferably has an average grain size of less than 12 μm, more preferably less than 10 μm. The strain state of the austenite of the present invention is evaluated by X-ray diffraction through measurement of the full width at half maximum (FWHM) of the diffraction peak corresponding to the {311} plane. X-ray diffraction is a non-destructive analytical technique that provides detailed information about the internal lattice of crystalline materials, including lattice dimensions, bond lengths, bond angles, and details of lattice order. It is directly related to the refinement of single crystals, and data obtained from X-ray analysis are interpreted and processed to obtain the crystal structure. Typically, an X-ray diffractometer is used to identify such a crystal structure. According to the present invention, the steel sheet has an austenite matrix, which has a face-centered cubic lattice system. Therefore, the diffraction peak corresponding to the {311} plane, which is believed to be the most sensitive to the strain state of the austenite lattice and therefore the most representative of the effect on dislocation density, was analyzed and the full width at half maximum (FWHM) was measured. The FWHM of the austenite of the present invention is between 0.700° and 1,100°.

[0028] In the microstructure of the steel according to the present invention, ferrite is present in a volume fraction between 1% and 10%, preferably between 2% and 10%, more preferably between 3% and 9%. However, the ferrite of the present invention has a restricted granular shape, and band-like ferrite is excluded, as it significantly reduces the ductility and formability of the steel. Preferably, the ferrite granules have an average grain size of less than 5 μm, more preferably less than 1 μm. Such ferrite may be in the form of normal disordered ferrite α, or may be ordered as a B2 structure with a (Fe,Mn)Al composition, or as a D03 structure with a (Fe,Mn)Al composition. Thus, in steels according to the invention, the α, B2 and D03 structures can generally be observed.

[0029] Kappa carbide (Fe,Mn)3AlC x may be present in the microstructure of the steel sheet according to the present invention in a volume fraction of up to 10%, preferably less than 5%, more preferably less than 4%, and advantageously more than 1%. The kappa carbides of the present invention include both intragranular kappa carbide (e.g., precipitated within austenite grains, so-called intragranular kappa carbide) and intergranular kappa carbide (e.g., precipitated between austenite grains, so-called intergranular kappa carbide). The uniform and homogeneous precipitation of nano-sized kappa carbides increases the strength of the alloy.

[0030] In order to protect the steel sheet according to the invention from corrosion, in a preferred embodiment the steel sheet is coated with a metallic coating, which may be an aluminum-based coating or a zinc-based coating.

[0031] Preferably, the aluminum-based coating contains less than 15% Si, less than 5.0% Fe, optionally 0.1% to 8.0% Mg, and optionally 0.1% to 30.0% Zn, the balance being Al.

[0032] Advantageously, the zinc-based coating comprises 0.01% to 8.0% Al, optionally 0.2 to 8.0% Mg, the balance being Zn.

[0033] The steel sheet according to the invention can be manufactured by any suitable manufacturing method, which will be clear to the person skilled in the art. However, it is preferred to use a manufacturing method for the steel sheet according to the invention, which comprises the following steps: - providing a slab having a composition according to the invention, - reheating such slabs to a temperature above 1000°C and hot rolling them to a final rolling temperature of at least 800°C; - coiling the hot-rolled steel sheet at a temperature below 600°C; - performing a first cold rolling of the hot-rolled steel sheet at a rolling reduction comprised between 30% and 80%; - first annealing of the steel sheet thus cold-rolled by heating it to an annealing temperature comprised between 700 ° C and 1000 ° C, maintaining it at that temperature for less than 5 minutes, and cooling it at a rate of at least 30 ° C / s, - second cold rolling of the thus annealed steel sheet with a reduction comprised between 10% and 50%; - second annealing of the steel sheet so cold rolled by heating it to a temperature comprised between 700°C and 880°C, maintaining it at that temperature for at least 1 minute to 150 hours, and cooling it at a rate of at least 30°C / s.

[0034] The steel sheet according to the invention is preferably produced by casting a semi-finished product such as a slab, thin slab or strip made from the steel according to the invention having the above composition, and heating the cast feedstock to a temperature above 1000°C, preferably above 1050°C, more preferably between 1100°C and 1150°C, and using it directly at that temperature after casting without intermediate cooling.

[0035] The hot rolling step is carried out at a temperature above 800° C. The final rolling temperature is preferably above 850° C. to avoid any cracking problems through lack of ductility due to the formation of banded ferrite.

[0036] After hot rolling, the strip must be coiled below 600° C., preferably above 350° C. In a preferred embodiment, coiling is carried out between 350° C. and 450° C. to avoid excessive kappa carbide precipitation.

[0037] The hot-rolled product obtained by the above process is subjected to a pickling operation in the usual manner, preferably by sandblasting, before being cold-rolled.

[0038] The first cold rolling step is carried out with a reduction between 30% and 80%, preferably between 40% and 70%.

[0039] After this rolling step, a first annealing of the steel sheet is carried out by heating it to an annealing temperature comprised between 700°C and 1000°C, maintaining it at such temperature for less than 5 minutes, and cooling it at a rate of at least 30°C / s, more preferably at least 50°C / s and even more preferably at least 70°C / s. Preferably, this annealing is carried out continuously.

[0040] By controlling the annealing temperature and time, a fully austenitic or two-phase structure having the above characteristics can be obtained.

[0041] After this first annealing step, the blank was pre-strained by means of a second cold rolling step with a rolling reduction of between 10% and 50%, preferably between 15% and 40%, which allowed the steel sheet to gain strength through strain hardening.

[0042] After this second rolling step, the steel sheet is subjected to a second annealing by heating it to an annealing temperature comprised between 700°C and 880°C, maintaining it at such temperature for 1 minute to 150 hours, and then cooling it at a rate of at least 30°C / s, more preferably at least 50°C / s, and even more preferably at least 70°C / s. Preferably, this annealing is performed continuously. During this second annealing, the austenite matrix recovers and recrystallizes, possibly reducing the dislocation density. An indirect measure of this progression is given by the full width at half maximum (FWHM) measured by X-ray diffraction for the diffraction peak corresponding to the {311} plane. During this time, precipitation of hard phases such as kappa carbide and ferrite occurs. Furthermore, ferrite may undergo an ordering reaction, promoting the formation of DO3 and B2. The combination of these trace elements provides a steel that combines ultra-high strength and ultra-high ductility.

[0043] After these two annealing steps, the steel sheet is optionally subjected to a metallic coating operation to improve its protection against corrosion. The coating process used can be any process applicable to the steel of the invention. Mention can be made of electrolytic or physical vapor deposition, with particular attention being paid to jet vapor deposition. The metallic coating can be, for example, zinc-based or aluminum-based. [Example]

[0044] Two grades having the compositions listed in Table 1 were cast into slabs and processed according to the process parameters listed in Table 2.

[0045] [Table 1]

[0046] [Table 2]

[0047] The obtained samples were analyzed and the corresponding microstructural components and mechanical properties are listed in Tables 3 and 4, respectively.

[0048] [Table 3]

[0049] [Table 4]

[0050] This example shows that the steel sheet according to the invention is the only steel that exhibits all of the targeted properties due to its specific composition and microstructure.

Claims

1. Cold rolled and annealed steel sheet, expressed by weight, having the following properties: 0.6<C<1.3%, 15≦Mn<35%, 6≦Al<15%, Si≦2.40%, S≦0.03%, P≦0.1%, N≦0.1%, and may contain up to 3% individual amounts of any one or more elements selected from Ni, Cr and Cu, and up to 2.0% cumulative amounts of one or more elements selected from B, Ta, Zr, Nb, V, Ti, Mo and W, with the remainder of the composition being iron and unavoidable impurities derived from smelting; the microstructure of the steel sheet comprises between 1% and 10% ordered ferrite, optionally up to 10% kappa carbide, the remainder being made up of austenite, and the density of the steel sheet is 7.2 or less, and the FWHM of the austenite matrix is ​​between 0,700 and 1,100; steel plate.

2. The steel sheet according to claim 1, wherein the carbon content is between 0.8% and 1.0%.

3. 3. The steel sheet according to claim 1, wherein the manganese content is between 18% and 30%.

4. 4. Steel sheet according to any one of claims 1 to 3, wherein the aluminum content is comprised between 6% and 10%.

5. The steel sheet according to any one of claims 1 to 4, wherein said steel sheet has an ultimate tensile strength of at least 1000 MPa and a yield strength of at least 700 MPa.

6. 6. Steel sheet according to any one of claims 1 to 5, wherein the austenitic component is comprised between 90% and 98%.

7. A steel sheet according to any one of the preceding claims, wherein the austenitic constituent has an average grain size of less than 12 microns.

8. A steel sheet according to any one of the preceding claims, wherein the austenitic constituent has an average grain size of less than 10 microns.

9. A steel sheet according to any one of the preceding claims, wherein the ferrite content is between 2% and 10% with an average grain size of less than 5 microns.

10. A steel sheet according to any one of the preceding claims, wherein the ferrite content is between 3% and 9% with an average grain size of less than 1 micron.

11. 11. The steel sheet according to any one of claims 1 to 10, wherein the kappa carbide content is less than 5%.

12. 12. The steel sheet according to any one of claims 1 to 11, wherein the kappa carbide content is less than 4%.

13. The steel sheet according to any one of claims 1 to 12, wherein the steel sheet is coated with a metallic coating.

14. 14. The steel sheet according to claim 13, wherein the steel sheet is coated with an aluminum-based coating or a zinc-based coating.

15. A method for manufacturing a steel sheet, comprising the steps of: - providing a slab having a composition according to claims 1 to 4, - reheating such slabs to a temperature above 1000°C and hot rolling them to a final rolling temperature of at least 800°C, - coiling the hot-rolled steel sheet at a temperature below 600°C, - carrying out a first cold rolling of the steel sheet thus hot-rolled with a reduction comprised between 30% and 80%; - carrying out a first annealing of the steel sheet so cold-rolled by heating it to an annealing temperature comprised between 700°C and 1000°C, keeping it at that temperature for less than 5 minutes and cooling it at a rate of at least 30°C / s, - carrying out a second cold rolling of the steel sheet thus annealed with a reduction comprised between 10% and 50%; - carrying out a second annealing of the steel sheet so cold rolled by heating it to a temperature comprised between 700°C and 800°C, keeping it at that temperature for at least 1 minute to 150 hours, and cooling it at a rate of at least 30°C / s.

16. The method according to claim 15, wherein the first annealing temperature is comprised between 800°C and 950°C.

17. 17. The method according to claim 15, wherein the coiling temperature is comprised between 350°C and 500°C.

18. The method according to any one of claims 15 to 17, wherein the duration of the second annealing is between 2 minutes and 10 hours.

19. The method according to any one of claims 15 to 18, wherein the pickling is carried out by sandblasting.

20. The method of any one of claims 15 to 19, further comprising a final coating step.

21. Use of a steel sheet according to any one of claims 1 to 14 or obtainable by the method according to any one of claims 15 to 20 for the manufacture of structural or safety parts of a vehicle.

22. 16. The part according to claim 15, obtained by flexible rolling of the steel sheet.

23. A vehicle comprising a component according to any one of claims 15 to 23.

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