KR20130111214A - Method of producing an austenitic steel - Google Patents

Abstract

The present invention relates to a method for producing an austenitic steel sheet having excellent resistance to delayed cracking and to steel produced thereby.

Description

Manufacturing method of austenitic steel {METHOD OF PRODUCING AN AUSTENITIC STEEL}

The present invention relates to a method for producing an austenitic steel sheet having excellent resistance to delayed cracking.

In terms of fuel economy and stability in the event of a crash, high strength steel is increasingly used in the automotive industry. This requires the use of structural materials that combine high ductility with high tensile strength. Austenitic alloys include iron, carbon and high levels of manganese as main elements, which can be hot rolled or cold rolled and have strengths that can exceed 1000 MPa. The deformation mode of such steels depends on the stacking fault energy: For sufficiently high stacking fault energy, the mode of observation of mechanical deformation is twinning, which results in high work hardening. By acting as an obstacle to the propagation of dislocations, the twin increases the flow stress. However, if the lamination defect energy exceeds certain limits, the slip of perfect dislocation is the main deformation mechanism, and work hardening is reduced. Since there is a tendency for high residual tensile stress to remain after deformation, the sensitivity to delayed cracking increases with mechanical strength, especially after certain cold-forming operations. By being associated with hydrogen atoms that may be present in the metal, these stresses tend to cause delayed cracking, ie cracking that occurs at a certain point after the deformation itself. Hydrogen collects gradually by diffusion to produce crystal lattice defects such as matrix / inclusion interfaces, twin boundaries, and grain boundaries. The latter becomes harmful when hydrogen reaches a critical concentration after a certain time. At constant grain size, the time required to achieve the critical level depends on the initial concentration of mobile hydrogen, the strength of the residual stress concentration field, and the kinetics of hydrogen diffusion.

In certain circumstances, small amounts of hydrogen may be introduced at some stages of steel fabrication, such as chemical or electrochemical pickling, annealing under a special atmosphere, electroplating or galvanizing. . Subsequent processing operations with lubricating oils and greases may also cause hydrogen production after decomposition of these materials at high temperatures.

An object of the present invention is to provide a method for producing an austenitic steel sheet having excellent resistance to delayed cracking.

It is also an object of the present invention to provide a process for producing austenite steel sheets with increased yield stress and good weldability.

It is a further object of the present invention to provide a method for producing an austenitic steel sheet which is energy efficient and simple when compared to conventional methods for this type of steel.

According to the present invention, one or more of these objects are achieved by providing a method for producing an austenitic steel sheet having excellent resistance to delayed cracks as described below.

Casting an ingot, or continuous casting slab, or continuous casting thin slab or strip-casting strip comprising the following composition as weight percent:

    0.50% to 0.80% C

    10 to 17% Mn

    -1.0% Al

    -Less than 0.5% Si

    0.020% or less

    Less than or equal to 0.050%

    50 to 200 ppm N

    0.050 to 0.25% V

    Residual iron and unavoidable impurities accompanying production;

Providing a hot rolled strip by hot rolling the ingot, continuous cast slab, continuous cast thin slab or strip-casting strip to a desired hot rolled thickness;

Cold rolling the hot rolled strip to the desired final thickness;

The cold rolling strip in a process comprising cooling to a cooling rate (V _c ) after heating of the strip at a heating rate (V _h ) to an annealing temperature (T _a ) of 750 to 850 ° C. for an annealing time (t _a ). Method for producing an austenitic steel sheet having excellent resistance to delayed cracking comprising the step of continuous annealing.

By using a high content of aluminum, the SFE of the steel increases. The reaction of elements that lower SFE, such as silicon, is offset by the addition of aluminum. In addition, aluminum reduces carbon activity and diffusion in austenite, which reduces the driving force for carbide formation. Vanadium is an essential alloy additive that is added to form carbides. If the size and distribution of vanadium-carbide are correct, the vanadium-carbide acts as a hydrogen sink. The increased aluminum content is therefore essential for controlling vanadium-carbide precipitation because it prevents coarsening of vanadium-carbide due to reduced carbon activity and diffusion as a result of the presence of aluminum. The inventors have found that at least 1.0% by weight of Al and 0.050 to 0.25% by weight V are required to obtain this. Low content aluminum results in too coarse vanadium-carbide, which makes them inefficiently acting as a hydrogen sink, and it is necessary to control the amount of vanadium between the above mentioned values in order to obtain a sufficient amount of small precipitates. Higher V-values lead to premature nucleation of precipitates, which inevitably leads to very small amounts of coarse precipitates, while V values below 0.050% by weight rarely produce precipitates despite being sufficiently fine. . The annealing treatment is very important in that it controls the precipitation of vanadium-carbide and allows fine grain structures to be produced by recrystallization of the cold-deformed microstructure caused by cold rolling. In a preferred embodiment, the silicon content is very low, for example at impurity levels. In principle, the aluminum content is limited only by the fact that the steel according to the invention is an austenitic steel. In an embodiment, the maximum aluminum content is 5% by weight. Preferably the aluminum content is at least 1.25% by weight and / or at most 3.5% by weight, more preferably at least 1.5% by weight and / or at most 2.5% by weight.

In an embodiment, the maximum annealing temperature T _a is 825 ° C or even 800 ° C. In an embodiment, the cooling rate V _c is 10 to 100 ° C./s. Preferred cooling rates are 20 to 80 ° C / s. Preferably the heating rate is 3 to 60 ° C / s. The annealing time t _a is preferably 15 to 300 seconds.

In a preferred embodiment, the maximum annealing temperature T _a is 775 to 795 ° C. (eg 785 ± 10 ° C.).

Preferably, the steel strip material is pickled prior to cold rolling. The pickling is necessary prior to cold rolling to remove the (often) oxide and prevent the oxide from rolling. Preferably, the cold rolled strip material is made from a hot rolled material or a belt cast strip material.

In a preferred embodiment of the present invention, during cooling at a cooling rate V _c after continuous annealing, the strip is provided with a metal coating via a melt immersion bath by melt dipping the strip into a molten bath of metal making a metal coating. This process is very economical and fast for producing metal coated steel strips. The metal coating may be of any known coating method, such as zinc or zinc alloy, which may be alloyed with elements such as aluminum and / or magnesium.

In another embodiment of the invention, the strip is pickled after continuous annealing, and before the strip is immersed into the metal melt bath to make the metal coating, the strip is annealed prior to providing the metal coating through the melt immersion bath. The metal coating is then provided by pickling and then heating to a temperature below the continuous annealing temperature. This alternative process can be used when the economic process described above is undesirable. There may be problems with adhesion to any special metal coating that may require pickling treatment. It is neither necessary nor desirable to heat the strip to a temperature above T _a after pickling. It is desirable to maintain the heating temperature below T _a .

In this way, the strip material is only heated to a temperature sufficient to form a closed inhibition layer. The temperature is lower than the typical continuous annealing temperature required for metallic reasons (such as recrystallization affecting mechanical properties). Oxide production on the steel strip material surface is thereby reduced.

Preferably, the temperature below the continuous annealing temperature is 400 to 600 ° C. Within this temperature range, oxide production is significantly reduced and the strip material is sufficiently heated for subsequent hot dip galvanizing.

   In a preferred embodiment, iron (Fe) in the strip material is reduced during or after heating to a temperature below the continuous annealing temperature, and prior to hot dip galvanizing. By reducing the strip material, the resulting Fe-oxide is reduced and in this way the amount of oxide present on the surface of the strip material prior to hot dip galvanizing is significantly reduced.

Preferably, the reduction is carried out using H ₂ N ₂ , more preferably 5 to 30% of H ₂ N ₂ in a reducing atmosphere. It has been found that the use of this atmosphere can remove most of the oxides.

According to a preferred embodiment, an excess amount of O ₂ is provided to the atmosphere during or after heating of the strip material and before the reduction of the strip material. Providing excess oxygen improves the quality of the steel strip material surface prior to hot dip galvanizing, thereby improving the quality of the zinc layer coated on the AHSS strip material. Oxygen combines with alloying elements in the AHSS strip material both on and within the surface of the strip material, and it is likely that oxides formed in this way cannot migrate to the surface of the strip material.

The reducing atmosphere after oxidation will then reduce the oxide at the surface of the strip material, in this way the amount of oxide on the strip material surface is considerably reduced or even almost disappeared as shown in the experiment. Preferably, excess O ₂ is provided in an amount of 0.05-5% O ₂ . This amount of oxygen was found to be sufficient.

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

During heating the strip material to a temperature of 527 ° C., an excess of 1% O ₂ is provided. Oxygen is provided at these high temperatures to produce oxides at the surface of the strip material as well as to combine with alloying elements at any depth below the surface. After oxygenation, the strip material was reduced using approximately 5% H ₂ N ₂ . Reduction of the strip material removes oxides from the surface, but oxides formed below the surface remain intact and cannot migrate to the surface.

Thus, reducing the surface effectively removes the oxide, and no new oxide can be formed on the surface. If these oxides are not removed, this leads to poor adhesion of the zinc layer to the substrate, resulting in bare spots, flaking and crack formation in the zinc layer when the material is bent. It is believed that by the general reduction method, the alloying elements move to the surface very quickly at the alloy temperature, producing oxides on the surface again before hot dip galvanizing takes place. Whatever the exact mechanism, this method can be used to reduce or nearly eliminate the amount of oxide found in the hot dip galvanized zinc layer on the V-alloy TWIP steel.

In an embodiment of the invention, the cold rolling reduction is from 10 to 90%, more preferably from 30 to 85%, even more preferably from 45 to 80%.

In an embodiment of the invention, the annealed strip is temper rolled at a rolling reduction of 0.5 to 10% before or after providing the metal coating to the strip.

In an embodiment of the present invention, the vanadium content is from 0.06 to 0.22%.

In a second aspect of the invention, the strip or sheet is produced and provided by the method according to any one of claims 1 to 6, wherein the steel is preferably provided with a metal coating. In a preferred embodiment of the invention, the strip or sheet is used for the manufacture of internal or external parts or wheels for automobiles, or for hydroforming applications.

The invention is further illustrated by the following non-limiting examples.

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

The finish rolling temperature (FRT) is chosen to ensure recrystallization of the modified microstructure, and the winding temperature is kept below 500 ° C. to avoid carbide precipitation. Recrystallization depends not only on the FRT but also on the rolling strain, time and strain rate accumulated since the final recrystallization of the hot rolling process.

All hot rolled materials are 50% cold rolled and then recrystallized annealed. Different annealing cycles are applied to determine the optimum annealing parameters. Note that elongation is 45% to 50% except for materials that do not recrystallize (36-45%) and materials annealed at 920 ° C. (65%). Since strength is considered more important, the following discussion will focus on it.

For annealing temperatures up to 750 ° C., the material softens due to the growth of the increased portion of recrystallized material and possibly some crystals. At such a temperature, the precipitation effect is limited. The difference between the materials annealed (fully recrystallized) at 775 ° C. and 800 ° C. is insignificant because it is thought that it is optimal to precipitate in this temperature range to minimize crystal growth. Based on this observation, the recommended annealing temperature is 785 ± 10 ° C.

The results of delayed cracking and stress corrosion cracking on the V-alloy grade show that the material is less susceptible to crack formation when annealed at higher temperatures. For stress corrosion cracking sensitivity, V addition is certainly more advantageous at 750 ° C. as well as higher annealing temperatures.

Resistance spot welding tests were performed on the V-alloys. Thermal cracking during welding is greatly reduced compared to non-silicon free V-alloy materials.

Claims (15)

In the method for producing an austenitic steel sheet having excellent resistance to delayed cracking,
Casting an ingot, or continuous casting slab, or continuous casting thin slab or strip-casting strip comprising, by weight%:
0.50% to 0.80% C
10 to 17% Mn
-1.0% Al
0.5% Si or less
0.020% or less
Less than or equal to 0.050%
-50 to 200 ppm N
0.050 to 0.25% V
Residual iron and impurities which are inevitably added during the production process;
Providing a hot rolled strip by hot rolling the ingot, continuous cast slab, continuous cast thin slab or strip-cast strip to a desired hot roll thickness;
Cold rolling the hot rolled strip to the desired final thickness; And
The cold rolling in a process comprising heating the strip at a heating rate (V _h ) at an annealing temperature (T _a ) of 750 to 850 ° C. for an annealing time (t _a ) followed by cooling to a cooling rate (V _c ). A method for producing austenite steel sheet comprising a continuous annealing step of strips. The method of claim 1,
A process for producing austenitic steel sheets having an aluminum content of at least 1.25% and / or at most 3.5%. The method of claim 1,
While cooling at a cooling rate (V _c ) after the continuous annealing, a method of producing an austenitic steel sheet in which the metal coating is provided through the melt immersion bath by melt immersing the strip into a melt bath of the metal to which the metal coating is applied. . The method of claim 1,
The strip is pickled after continuous annealing, and after the strip is annealed and pickled and then below the continuous annealing temperature before the metal coating is provided through the melt immersion bath by dipping the strip into a molten bath of the metal to which the metal is coated. A method for producing an austenitic steel sheet provided with a metal coating by heating to a temperature of. The method according to any one of claims 1 to 4,
The cold rolling reduction rate is 10 to 90%, more preferably 30 to 85%, even more preferably 45 to 80% method for producing austenite steel sheet. 6. The method according to any one of claims 1 to 5,
And wherein said annealed strip is temper rolled to a rolling reduction of 0.5 to 10% before or after providing said metal coating to said strip. 7. The method according to any one of claims 1 to 6,
The vanadium content of 0.06 to 0.22% of austenite steel sheet manufacturing method. 8. The method according to any one of claims 1 to 7,
The cooling rate (V _c ) is from 10 to 100 ° C./s, and / or
The heating rate (V _h ) is preferably 3 to 60 ° C./s, and / or
The annealing time (t _a ) is a manufacturing method of the austenite steel sheet is preferably 15 to 300 seconds. The method according to any one of claims 1, 2 or 4 to 8,
The strips are pickled after continuous annealing, and after the continuous annealing and pickling of the strips before the metal coating is provided through the melt dip bath by dipping the strips into a molten bath of the metal to which the metal coating is applied, then 400 to 600 A method of producing an austenitic steel sheet by providing a metal coating by heating to a temperature of < RTI ID = 0.0 > The method of claim 9,
Fe in the strip material is reduced during or after heating to a temperature below the continuous annealing temperature and before hot dip galvanizing, preferably the reduction is 5 to 30% under H ₂ N ₂ , more preferably in a reducing atmosphere A process for producing austenite steel sheet carried out using H ₂ N ₂ . The method of claim 10,
An excess amount of O ₂ is provided in the atmosphere during or after heating of the strip material and before the reduction of the strip material, preferably the excess amount of 0 ₂ is 0.05 to 5% of 0 ₂ austenite steel Manufacturing method of the sheet. 12. The method according to any one of claims 1 to 11,
Maintained at the hot coiling temperature is less than 500 ℃ after rolling and / or the annealing temperature (T _a) is 785 ℃ ± 10 ℃ the austenite method of producing a steel sheet night. A strip or sheet produced by the method according to claim 1, wherein
Strip or sheet, wherein the steel is preferably provided with a metal coating. Use of a strip or sheet according to claim 13 for the manufacture of internal or external parts or wheels of a motor vehicle. The method of claim 13,
Method of use of strips or sheets used in hydraulic forming.