BR112012031466B1 - METHOD OF PRODUCING AN EXCELLENT AUSTENTIC STEEL SHEET IN RESISTANCE TO DELAYED FRACTURE AND STRIP OR SHEET - Google Patents

Abstract

method of producing an austenitic steel. The present invention relates to a method of producing an excellent austenitic steel plate in delayed fracture resistance and to a steel thereby producing.

Description

[001] The present invention relates to a method of producing an austenitic steel sheet excellent in delayed fracture resistance.

[002] Aiming at fuel economy and safety in the event of collisions, high strength steels are more and more used in the automobile industry. This requires the use of structural material that combines high tensile strength with high ductility. Austenitic alloys comprising as main elements iron, carbon and high levels of manganese, which can be hot-rolled and cold-rolled and have a strength that can exceed 1,000 MPa. The deformation mode of these steels depends on the stack failure energy: for a sufficiently high stack failure energy, the observed mode of mechanical deformation is maclation, which results in a high hardening capacity. Acting as an obstacle to the propagation of displacements, the thighs increase the stress for plastic deformation. However, when stacking failure energy exceeds a certain limit, sliding from perfect displacements becomes the main deformation mechanism and hardening is reduced. It is known that the sensitivity to delayed fracture increases with mechanical strength, in particular after certain cold forming operations, since a high residual stress is likely to remain after deformation. In combination with the atomic hydrogen possibly present in the metal, these stresses are likely to result in delayed fractures, that is, fractures that occur a certain time after deformation. Hydrogen can progressively accumulate by diffusion

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2/11 in the defects of the crystalline lattice, such as matrix / inclusions interfaces, skin contour and grain contour. It is in these latter areas that hydrogen can become harmful when it reaches a critical concentration after a period of time. For a constant grain size, the time required to reach a critical level depends on the initial concentration of the moving hydrogen, the intensity of the residual stress concentration field and the hydrogen diffusion kinetics. [003] In particular circumstances, small amounts of hydrogen can be introduced in some stages of steelmaking such as chemical or electrochemical pickling, annealing under special atmospheres, electroplating or hot-dip galvanizing. Subsequent machining operations using lubricating oils and greases can also cause the production of hydrogen after the decomposition of these substances at high temperatures.

[004] It is an objective of this invention to provide a method of producing an austenitic steel sheet excellent in delayed fracture resistance.

[005] It is also an objective of this invention to provide a method of producing an austenitic steel sheet having an increased yield limit and excellent weldability.

[006] It is another objective of this invention to provide a method of producing an austenitic steel sheet that is energy efficient and simple compared to the conventional way for this type of steel.

[007] According to the invention, one or more of these objectives is achieved by providing an excellent austenitic steel sheet production method in resistance to delayed fracture comprising

- ingot an ingot, or a continuous ingot plate

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3/11 bare, or a thin continuous caster plate, or a caster strip, its composition comprising, in% by weight:

- 0.50% - 0.80% of C

- 10 - 17% Mn

- at least 1.0% Al

- maximum 0.5% Si

- at most 0.020% S

- maximum 0.050% P

- 50 - 200 ppm N

- 0.050 - 0.25% of V, the balance being iron and the inevitable impurities inherent in manufacturing;

- supply a hot-rolled strip by laminating the ingot, the continuously cast ingot, the thin continuously cast ingot, or the ingot strip to the desired thickness in the hot lamination.

- cold-laminate the hot-rolled strip to the desired final thickness.

- carrying out the continuous annealing of the cold rolled strip in a process comprising heating the strip at a heating rate Vh to an annealing temperature T _a for an annealing time t _a followed by cooling at a cooling rate Vc and where T _a is between 750 ° C and 850 ° C.

[008] Using a high aluminum content, the stacking failure energy (SFE) of the steel increases. Any adverse effects of the elements that decrease SFE, such as silicon, are neutralized by the addition of aluminum. Additionally, aluminum decreases the activity and the diffusion capacity of carbon in austenite, which reduces the driving force for carbide formation. Vanadium, which is added as an addition of an essential alloying element, forms carbides.

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4/11

These vanadium carbides act as hydrogen absorbers if and when the size and distribution of the vanadium carbides is correct. The increased aluminum content is therefore essential to control vanadium carbide precipitation because it prevents the vanadium carbide from thickening due to the reduced activity of the carbon and the diffusion capacity as a result of the presence of aluminum. The inventors found that at least 1.0% Al and 0.050% to 0.25% V are needed to achieve this. Lower aluminum contents lead to very greasy vanadium carbides, thus rendering them ineffective as hydrogen sinks, and the amount of vanadium needs to be controlled within the mentioned values to reach a sufficient number of small precipitates, Higher values of V lead prior nucleation of the precipitates and thus inevitably less precipitates and thicker ones, with which values below 0.050% of V simply result in few precipitates, even if they are sufficiently fine. The annealing treatment is crucial in that it controls the precipitation of vanadium carbides and causes the cold deformed microstructure to be recrystallized by cold rolling to result in a fine grain structure. In a preferred configuration, the silicon content is very low, that is, at the level of impurity. In principle, the aluminum content is limited only by the fact that the steel of the invention is an austenitic steel. In one configuration, the maximum aluminum content is 5%. Preferably the aluminum content is at least 1.25% and / or at most 3.5%, more preferably at least 1.5% and / or at most 2.5%.

[009] In a configuration the maximum annealing temperature Ta is 825 ° C or even 800 ° C. In one configuration, the cooling rate Vc is between 10 and 100 ° C / s. The preferred cooling rate is between 20 and 80 ° C / s. Heating rate is preferable

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5/11 between 3 and 60 ° C / s. The annealing time t _a is preferably between 15 and 300 seconds.

[0010] In a preferred configuration, the maximum annealing temperature T _a is 775 ° C to 795 ° C (ie 785 ± 10 ° C).

[0011] Preferably the steel strip material was stripped before cold rolling. Pickling is (often) necessary before cold rolling to remove oxides, to prevent lamination of oxides. Preferably, the cold-rolled strip material is produced from a hot-rolled strip material or a strip of strip material.

[0012] In a preferred embodiment of the invention, during cooling to a cooling rate Vc after continuous annealing, the strip is fed through a hot dip bath to provide a hot dip metal coating of the strip in a bath molten metal making the metallic coating. This process leads to a very fast and economical process to produce a metal-coated steel strip. The metallic coating can be any common known coating such as zinc or zinc alloy, where the zinc can be bonded with elements such as aluminum and / or magnesium.

[0013] In another embodiment of the invention, the strip is stripped after continuous annealing and where the strip is provided with a metallic coating by pickling after annealing followed by heating to a temperature below the temperature of continuous annealing before the strip is taken through a hot dip bath to provide a metallic coating by immersing the metallic strip in a molten metal bath making the metallic coating. This alternative process is available if the economic process as described above is not preferred. There may be adhesion problems with certain specific metallic coatings for which a tra

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6/11 paint stripping may be required. After blasting, it is neither necessary nor desirable to heat the strip above temperature Ta. It is preferable that the heating temperature remains below Ta.

[0014] With this method, the strip material is heated only to a temperature high enough to form a closed inhibition layer. This temperature is less than the normal continuous annealing temperature required for metallurgical reasons (such as recrystallization to influence mechanical properties). The formation of oxides on the surface of the steel strip material is thus reduced.

[0015] Preferably, the temperature below the continuous annealing temperature is between 400 ° C and 600 ° C. In this temperature range the formation of oxides is considerably reduced, and the strip material is sufficiently heated for subsequent hot dip galvanizing.

[0016] According to a preferred configuration, the Fe in the strip material is reduced during or after heating to a temperature below the temperature of continuous annealing and before hot dip galvanizing.

[0017] By reducing the strip material, the iron oxides that are formed are reduced, and in this way the amount of oxides present on the surface of the strip material before hot dip galvanizing is considerably reduced.

[0018] Preferably, the reduction is carried out using H2N2, more preferably using 5 - 30% H2N2 in the reducing atmosphere. It has been found that with the use of this atmosphere, most oxides can be removed.

[0019] According to a preferred configuration, an excess amount of O2 is supplied in the atmosphere during or after the

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7/11 heating the strip material and before reducing the strip material. The supply of an excess amount of oxygen improves the surface quality of the steel strip material before hot dip galvanizing, as well as the quality of the zinc layer coated on the AHSS strip material. It is assumed that oxygen is bound to the alloying elements in the AHSS strip material both on the surface of the strip material and internally, and that thus the oxides formed cannot migrate to the surface of the strip material.

[0020] The reducing atmosphere that follows after oxidation will then reduce the oxides on the surface of the strip material, and thus the amount of oxides on the surface of the strip material is considerably reduced or even absent, as experiments have shown. Preferably, the excessive amount of O2 is supplied in an amount of 0.05 - 5% O2. That amount of oxygen was found to be sufficient.

[0021] In a preferred embodiment of the invention, TWIP steel strip material bonded with V according to the invention was hot rolled, pickled and cold rolled, and continuously annealed to a temperature according to the invention, and pickled again. The strip material is then heated to a temperature of 527 ° C in an annealing line, and subsequently hot-dip galvanized in a galvanizing bath at approximately 450 ° C.

[0022] During the heating of the strip material to a temperature of 527 ° C, an excessive amount of 1% O2 is supplied. Oxygen is supplied at such a high temperature so that it not only forms oxides on the surface of the strip material, but also at some depth under the surface and binds the alloying elements. After delivery of oxygen, the strip material is reduced using approximately 5% H2N2. Reducing the strip material removes

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8/11 the oxides on the surface, but the oxides formed under the surface remain where they are and cannot migrate to the surface.

[0023] Thus, by reducing the surface oxides are effectively removed and no new oxides can be formed on the surface. These oxides, when not removed, cause poor adhesion of the zinc layer to the substrate, resulting in bright spots, flaking and the formation of fractures in the zinc layer when the material is bent. It is assumed that, by normal reduction, the alloying elements migrate to the surface very quickly at bonding temperatures and thus form oxides on the surface again before hot dip galvanizing occurs. Whatever the exact mechanism may be, it has been found that using this method decreases or almost eliminates the amount of oxides discovered in a hot dip galvanized zinc layer in a TWIP steel bonded with Vanadium.

[0024] In a configuration of the invention, the reduction of cold rolling is between 10 and 90%, more preferably between 30 and 85, even more preferably between 45 and 80%.

[0025] In an embodiment of the invention, the annealed strip undergoes hardening lamination with a reduction of 0.5 to 10% before, or after, the metallic coating has been supplied to the strip.

[0026] In one embodiment of the invention, the vanadium content is between 0.06 and 0.22%.

[0027] In a second aspect of the invention, a strip or sheet produced by a method according to any one of claims 1 to 6 is provided, where the steel is preferably provided with a metallic coating. In a preferred embodiment of the invention, the strip or sheet is used for the production of internal or external automotive parts or wheels or for hydraulic forming applications;

[0028] The invention will now also be explained through the

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9/11 non-limiting examples to follow.

[0029] The chemical compositions of the materials used in this study are shown in Table 1.

Table 1 - Chemical composition of the materials studied (including semi-industrial free reference material if Si) (all in% by weight, Balance: Fe and impurities)

Code Ç Si Mn Al V s N P 8V-1 0.74 0.36 13.8 2.2 0.080 0.011 0.011 Low* 8V-2 0.74 0.36 13.8 2.3 0.086 0.011 0.009 Low 8V-3 0.74 0.31 13.7 2.3 0.084 0.011 0.007 Low 10V 0.71 0.22 13.4 2.4 0.106 0.010 0.011 Low 16V 0.72 0.25 14.5 2.3 0.160 0.001 0.006 0.008 21V 0.69 0.21 14.9 2.6 0.213 0.010 - 0.005 Self-free 0.70 <0.20 14.5 2.5 Impurity 0.004 0.007 0.030

* Low = level of impurity [0030] The rolling end temperature (FRT) was chosen to guarantee the recrystallization of the deformed microstructure and the winding temperature was kept below 500 ° C to prevent carbide precipitation. Recrystallization not only depends on the FRT, but also depends on the time, the rolling tension accumulated since the last recrystallization event during hot rolling and the stress rate.

[0031] All hot rolled materials were cold rolled 50% and subsequently annealed for recrystallization. Different annealing cycles were applied to determine the optional annealing parameters. Note that the stretches were between 45% and 50% for all samples, except for those that were not recrystallized (36-45%) and the material annealed at 920 ° C (65%). Since resistance is considered to be more important, the following discussion will focus on this aspect.

[0032] For annealing temperatures up to 750 ° C, the material softens due to an increased fraction of recrystallized material and

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11/10 probably some grain growth. At these temperatures, the effect of precipitation is limited. The difference between the materials (fully recrystallized) annealed at 775 ° C and 800 ° C is small because precipitation is considered optimal in this temperature region to minimize grain growth. Based on these observations, the recommended annealing temperature is 785 ° C ± 10 ° C.

Table 1 - Mechanical properties of materials after cold rolling at 50% and annealing

Free Si degree Heating rate (° C / s) OK (° C) OK (s) Rp (MPa) Rm (MPa) Ag (%) Atot (%) Free Si 5 700 270 580 920 43 Free Si 5 750 270 545 900 46 Free Si 5 800 270 490 875 47 Free Si 5 850 270 450 850 48 Free Si 5 920 270 410 870 65 8V-1 5 785 270 584 962 44 46 8V-2 5 785 270 595 970 45 49 8V-3 5 785 270 585 964 46 50 10V 5 785 270 575 939 45 49 16V 5 700 270 764 1072 32 36 16V 5 725 270 687 1012 37 41 16V 5 750 270 636 977 41 45 16V 5 775 270 614 964 42 46 16V 5 800 270 590 946 43 48 16V 5 750 60 670 1004 30 43 16V 5 775 60 626 973 41 45 16V 5 800 60 597 955 44 48 21V 30 700 60- 655 985 45 21V 30 750 60 600 950 43 21V 30 920 60 425 835 53

[0033] The results of the delayed fracture and the stress corrosion fracture in the V-bonded degree show a lower susceptibility to fracture formation as the material is annealed at a higher temperature. For stress corrosion fracture sensitivity, the addition of V is clearly beneficial at an annealing temperature

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11/11 at 750 ° C, but also at higher annealing temperatures. [0034] V alloys were subjected to spot welding tests. The hot fracture in the weld was greatly reduced compared to the Si free material not bonded with V.

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Claims (13)

1. Method of producing an austenitic steel sheet excellent in resistance to delayed fracture, characterized by the fact that it comprises: ingot an ingot, or a continuously cast ingot, or a thin continuously cast ingot, or a cast strip, its composition comprising, in% by weight: - 0.50% - 0.80% of C - 10% - 17% Mn - 1% - 5% of Al - maximum 0.5% Si - at most 0.020% S - maximum 0.050% P - 50 - 200 ppm N - 0.050% - 0.25% V the remainder being iron and the inevitable impurities inherent in manufacturing; - supply a hot-rolled strip by hot rolling of the ingot, the continuously cast ingot, the thin continuously cast ingot or the ingot strip to the desired thickness of the hot lamination, - cold-laminate the hot-rolled strip to the desired final thickness, - continuously annealing the cold-rolled strip in a process comprising heating the strip at a heating rate Vh between 3 and 60 ° C / s to an annealing temperature T _a for an annealing time t _a between 15 and 300 s followed by cooling at a cooling rate Vc between 10 and 100 ° C / s and where T _a is 750 ° C to 850 ° C. 2. Method according to claim 1, characterized Petition 870190028860, of 03/26/2019, p. 15/22 2/3 due to the fact that the aluminum content is at least 1.25 and / or at most 3.5%. 3. Method according to claim 1, characterized by the fact that, during cooling to a cooling rate Vc after continuous annealing, the strip is carried through a hot dip bath to provide a metallic coating by immersion the strip in a molten metal bath producing the metallic coating. 4. Method according to claim 1, characterized by the fact that the strip is stripped after continuous annealing and in which the strip is provided with a metallic coating by pickling after annealing followed by heating to a temperature below the temperature of continuous annealing before the strip is taken through a hot dip bath to provide a metallic coating by hot dip the strip in a molten metal bath making the metallic coating. Method according to any one of claims 1 to 4, characterized in that the reduction of cold rolling is between 10 to 90%, more preferably between 30 and 85%, even more preferably between 45 and 80%. Method according to any one of claims 1 to 5, characterized in that the annealed strip undergoes hardening lamination with a reduction of 0.5 to 10% before or after the metallic coating has been supplied to the strip. Method according to any one of claims 1 to 6, characterized in that the vanadium content is between 0.06 and 0.22%. 8. Method according to any one of claims 1 to 7, characterized by the fact that the cooling rate Vc is between 20 and 80 ° C / s. Petition 870190028860, of 03/26/2019, p. 16/22 3/3 Method according to any one of claims 1, 2, or 4 to 8, characterized in that the strip is stripped after continuous annealing and where the strip is provided with a metallic coating, by stripping after continuous annealing followed by heating to a temperature between 400 and 600 ° C before the strip is taken through a hot dip bath to provide a metallic coating by hot dip the strip in a molten metal bath producing a metallic coating. 10. Method according to claim 9, characterized in that the Fe in the strip material is reduced during or after heating to a temperature below the temperature of continuous annealing and before hot dip galvanizing, preferably where the reduction is carried out using H2N2, more preferably using 5 - 30% H2N2 in the reducing atmosphere. 11. Method according to claim 10, characterized in that an excessive amount of O2 is supplied to the atmosphere during or after heating the strip material and before the reduction of the strip material, preferably where the excess amount of O2 is supplied in an amount of 0.05 - 5% O2. Method according to any one of claims 1 to 11, characterized in that the winding temperature after hot rolling is kept below 500 ° C and / or where the annealing temperature T _a is 785 ° C ± 10 ° C. 13. Strip or sheet, characterized by the fact that it is produced by the method as defined in any one of claims 1 to 12, wherein the steel is preferably supplied with a metallic coating.