JP6009438B2 - Method for producing austenitic steel - Google Patents

Description

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

  Considering the fuel economy and safety in case of a collision, high strength steel is increasingly used in the automotive industry. This requires the use of structural materials that combine high tensile strength and high flexibility. Austenitic alloys contain iron, carbon, and high levels of manganese as main elements, and can be hot or cold rolled and have a strength of over 1000 MPa. The deformation mode of these steels depends on the stacking fault energy, and for sufficiently high stacking fault energy, the observed mode of mechanical deformation is twinned, resulting in high work hardening. Twins increase flow stress by acting as an obstacle to dislocation propagation. However, when the stacking fault energy exceeds a certain limit, slip of complete dislocation becomes the main deformation mechanism, and work hardenability decreases. Since high residual tensile stress tends to remain after deformation, it is known that the sensitivity to delayed cracking increases with mechanical strength, especially after certain cold forming operations. In combination with atomic hydrogen present in the metal, these stresses tend to lead to delayed cracks, ie cracks that occur after a certain time of deformation. Hydrogen may increase progressively due to dispersion into crystal lattice defects, such as matrix / inclusion interfaces, twin boundaries and grain boundaries. It is in the latter area that hydrogen can be detrimental when a critical concentration is reached after a certain time. For a given particle size, the time required to reach a critical level depends on the initial concentration of fluid hydrogen, the strength of the residual stress concentration field, and the hydrogen diffusion rate.

  In certain situations, a small amount of hydrogen may be introduced at some stage of steel production, such as chemical or electrochemical pickling, annealing in special environments, electroplating, or hot dip galvanizing . Subsequent machining operations using lubricants and greases can cause hydrogen evolution after these materials decompose at high temperatures.

  It is an object of the present invention to produce a sheet of austenitic steel having excellent resistance to delayed cracks.

  It is also an object of the present invention to provide a method for producing an austenitic steel sheet having increased yield stress and excellent weldability.

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

According to the present invention, a method of manufacturing a sheet of austenitic steel having excellent resistance to delayed cracks,
-An ingot, continuous cast slab, continuous cast thin slab, or strip cast strip, the composition of which is by weight,
-C 0.50% to 0.80%
-Mn 10-17%
-Al at least 1.0%
-Si at most 0.5%
-S at most 0.020%
-P at most 0.050%
-N 50-200 ppm
-V 0.050-0.25%
Casting an ingot, continuous cast slab, continuous cast thin slab, or strip cast strip comprising the balance iron and inevitable impurities associated with production;
-Hot rolling the ingot, the continuous cast slab, the continuous cast thin slab, or the strip cast strip to a desired hot rolled thickness to provide a hot rolled strip;
-Cold rolling the hot rolled strip to a desired final thickness;
- the strip, heating rate _{V h,} then heated to the annealing temperature _{T a} at annealing time _{t a,} then a method comprising cooling at a cooling rate _{V c, T a} is at 750 to 850 ° C. One or more of these objectives are achieved by providing a method comprising continuously annealing the cold-rolled strip in some manner.

  By using a high aluminum content, the SFE of the steel is increased. Any adverse effects of elements that reduce SFE, such as silicon, are neutralized by the addition of aluminum. In addition, aluminum reduces the carbon activity and diffusivity in austenite and reduces the driving force to form carbides. Vanadium added as an essential alloy additive forms carbides. If the size and dispersion of the vanadium carbides are correct, these vanadium carbides behave as hydrogen sinks. And the increased aluminum content is essential to control vanadium carbide precipitation because it prevents the coarsening of vanadium carbide due to the reduction of carbon activity and diffusivity due to the presence of aluminum. The inventors have found that at least 1.0% Al and 0.050-0.25% V are necessary to achieve this. The low aluminum content results in very coarse vanadium carbide, thereby eliminating the effect of vanadium carbide as a hydrogen sink, so to obtain a sufficiently small number of precipitates, the vanadium content is It is necessary to control in between. High V values cause premature nucleation of the precipitates, thereby inevitably coarsening and reducing the precipitates, while values of V below 0.050%, even if fine enough Results in less precipitation. The annealing treatment is extremely important in that it controls the precipitation of vanadium carbide and causes recrystallization of the microstructure deformed by cold rolling, resulting in a fine grain structure. In a preferred embodiment, the silicon content is very low, i.e. at the impurity level. In principle, the aluminum content is limited only by the fact that the steel according to the invention is an austenitic steel. In one embodiment, 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%.

In one embodiment, the maximum annealing temperature _{T a} is 825 ° C. or 800 ° C.. In one embodiment, the cooling rate V _c is 10-100 ° C./s. A preferable cooling rate is 20 to 80 ° C./s. The heating rate is preferably 3 to 60 ° C./s. 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. (i.e. 785 ± 10 ℃).

  Preferably, the steel strip material is pickled before cold rolling. Pickling is (often) necessary to remove oxides and prevent oxide contamination prior to cold rolling. Preferably, the cold rolled strip material is manufactured from a hot rolled strip material or a belt cast strip material.

In a preferred embodiment of the present invention, the strip during cooling at a cooling rate V _c after continuous annealing, into a molten bath of metal to create a metal coating, for applying a metal coating to strip hot dip plating to Is passed through a hot dipping bath. This process provides a very economical and quick process for producing steel strips coated with metal. The metal coating may be any commonly known coating such as zinc or a zinc alloy, where zinc may be alloyed with elements such as aluminum and / or magnesium.

In another embodiment of the present invention, the strip is pickled after continuous annealing, in which case the strip is hot dip plated to provide the metal coating by hot dip plating into the molten bath of metal to form the metal coating. Prior to passing through the bath, the strip is pickled after annealing, and then the metal coating is applied to the strip by heating to a temperature below the continuous annealing temperature. This alternative process can be used when the economic process described above is not desired. Adhesion problems with certain metal coatings can occur that may require pickling. After pickling, it is not necessary or desirable to heat the strip to _a temperature above Ta. The heating temperature is preferably _Ta or lower.

  In this method, the strip material is heated only to a temperature sufficiently high to form a clogged deterrent layer. This temperature is lower than the normal continuous annealing temperature required for metallurgical reasons (for example, recrystallization affecting mechanical properties). Oxide formation on the surface of the steel strip material is thereby reduced.

  Preferably, the temperature below the continuous annealing temperature is 400-600 ° C. Within this temperature range, oxide formation is greatly reduced and the strip material is fully heated for subsequent hot dip galvanizing.

  According to a preferred embodiment, the 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 steel strip, the Fe oxide formed is reduced and in this way the amount of oxide present on the surface of the steel strip before hot dip galvanization is greatly reduced.

Preferably, the reduction is carried out using _H 2 _{N 2,} and more preferably carried out using a 5-30% of _H 2 _{N 2} in a reducing atmosphere. It has been found that most oxides can be removed by using this atmosphere.

According to a preferred embodiment, an excess amount of O ₂ is fed into the atmosphere during or after heating of the strip material and before the reduction of the strip material. The supply of excess oxygen improves the surface quality of the steel strip before hot dip galvanization and improves the quality of the zinc layer coated on the AHSS strip material. In the AHSS strip material, oxygen is to be combined with the alloying elements both on the surface and inside the strip material, and in this way, the oxide formed can migrate to the surface of the strip material. It is supposed to be impossible.

The reducing atmosphere after oxidation reduces the oxide on the surface of the strip material, and in this way, the amount of oxide on the surface of the strip material is significantly reduced, as shown in the experiment, or Almost disappear. Preferably, an excess amount of O ₂ is supplied 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-alloyed TWIP steel strip material according to the invention is hot-rolled, pickled, cold-rolled and continuously annealed to the temperature according to the invention, and again pickled. did. Then, the strip material was heated to 527 ° C. in an annealing line, and then hot dip galvanized in a galvanizing bath at about 450 ° C.

While heating the strip material to 527 ° C., a 1% excess of O 2 is fed. Oxygen is supplied at such high temperatures not only to form oxides at the surface of the strip material, but also to bond with the alloy elements at some depth from the surface. After the supply of oxygen, the strip material is reduced using about 5% H ₂ N ₂ . Although the reduction of the strip material removes the oxide from the surface, the oxide formed below the surface remains in place and cannot move to the surface.

  Therefore, by reducing the surface, the oxide is effectively removed and no new oxide is formed on the surface. If these oxides are not removed, they will cause poor adhesion of the zinc layer to the substrate, which will be exposed, peeled off and cracked in the zinc layer when the material is bent. According to normal reduction, it is presumed that the alloy elements move to the surface very quickly at the alloy temperature and re-form oxides on the surface before hot dip galvanization takes place. Regardless of what the exact mechanism is, using this method reduces or almost eliminates the amount of oxide found in hot dip galvanized layers on V-alloyed TWIP steel. I understood that.

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

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

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

  In a second aspect of the present invention there is provided a strip or sheet produced by the method of any one of claims 1-6, wherein the steel is preferably provided with a metal coating. Is done. In a preferred embodiment of the invention, the strip or sheet is used for the production of internal or external parts of a motor vehicle or wheel or for hydroforming applications.

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

Table 1 shows the chemical composition of the materials used in this study.

  The finish rolling temperature (FRT) was selected to ensure recrystallization of the deformed microstructure and the coiling temperature was kept below 500 ° C. to avoid carbide precipitation. Recrystallization depends not only on FRT, but also on time, rolling strain accumulated from the last recrystallization event during hot rolling, and strain rate.

  All the rolled materials were cold rolled 50%, followed by recrystallization annealing. Various annealing cycles were applied to determine the optimal annealing parameters. Note that the elongation for all samples is 45% to 50%, except for the non-recrystallized sample (36-45%) and the material annealed at 920 ° C. (65%) It is. Since strength is considered more important, the following discussion focuses on it.

For annealing temperatures below 750 ° C., the material becomes soft due to the increased percentage of recrystallized material and possibly some grain growth. At these temperatures, the effect of precipitation is limited. Since precipitation is believed to be optimal for minimizing grain growth in this temperature range, the difference between materials annealed at 775 ° C. and 800 ° C. (fully recrystallized) is small. Based on these observations, the recommended annealing temperature is 785 ° C. ± 10 ° C.

  Delayed cracks and stress corrosion cracks that occur in V-alloyed grades indicate a low susceptibility to crack formation as the material is annealed at higher temperatures. In terms of stress corrosion cracking susceptibility, V addition is clearly beneficial not only at an annealing temperature of 750 ° C., but also at higher annealing temperatures.

  The V alloy was subjected to a resistance spot welding test. Hot cracks at the joints were greatly reduced compared to non-Si alloyed non-V alloy materials.

Claims (17)

A method for producing a sheet of austenitic steel having excellent resistance to delayed cracks, comprising:
-An ingot, continuous cast slab, continuous cast thin slab, or strip cast strip, the composition of which is by weight,
-C 0.50% to 0.80%
-Mn 10-17%
-Al 1.0% to 5%
-Si at most 0.5%
-S at most 0.020%
-P at most 0.050%
-N 50-200 ppm
-V 0.050-0.25%
Casting an ingot, continuous cast slab, continuous cast thin slab, or strip cast strip consisting of the balance iron and inevitable impurities associated with production;
-Hot rolling the ingot, the continuous cast slab, the continuous cast thin slab, or the strip cast strip to a desired hot rolled thickness to provide a hot rolled strip;
-Cold rolling the hot rolled strip to a desired final thickness;
- said strip, 3 to 60 ° C. / s heating rate _V h of, heated to the annealing temperature _{T a} at annealing time _{t a} of 15 to 300 seconds, then cooling at a cooling rate _{V c} of 10 to 100 ° C. / s a method comprising that, in the method T _a is 750 to 850 ° C., comprising the step of annealing the said cold rolled strip continuous method.   The method of claim 1, wherein the aluminum content is at least 1.25% and / or at most 3.5%. During continuous annealing and cooling at a cooling rate V _c , the strip is molten into a hot dipping bath to apply the metal coating by hot dipping the strip into a metal hot bath to form a metal coating. The method of claim 1, wherein:   A method in which the strip is pickled after continuous annealing, before the strip is passed through a hot dipping bath for hot dipping and applying the metal coating into a metal hot bath to form a metal coating. The method of claim 1, wherein the strip is pickled after annealing and then the metal coating is applied to the strip by heating to a temperature below the continuous annealing temperature.   The method according to any one of claims 1 to 4, wherein the cold rolling reduction is 10 to 90%.   The method according to claim 1, wherein the cold rolling reduction is 30 to 85%.   The method according to any one of claims 1 to 6, wherein the cold rolling reduction is 45 to 80%. The method according to claim 3 or 4 , wherein the annealed strip is temper rolled at a rolling reduction of 0.5 to 10% before or after applying the metal coating to the strip.   The method as described in any one of Claims 1-8 whose vanadium content is 0.06-0.22%. The method according to claim 1, wherein the cooling rate V _c is 20 to 80 ° C./s.   A method in which the strip is pickled after continuous annealing, before the strip is passed through a hot dipping bath for hot dipping and applying the metal coating into a metal hot bath to form a metal coating. 11. A method according to any one of claims 1, 2 or 4-10, wherein the strip is pickled after annealing and then heated to 400-600 [deg.] C. to apply a metal coating to the strip.   The method according to claim 11, wherein the Fe in the strip material is reduced during or after heating to a temperature below the continuous annealing temperature and before hot dip galvanizing. The method according to claim 12, wherein the reduction is performed using H ₂ N ₂ . The reduction is carried out using 5-30% of _H 2 _{N 2} in a reducing atmosphere, The method of claim 12. The method according to any one of the prior reduction of the strip the strip material A during or after heating the heating of the material, and supplies the excess of O ₂ in the atmosphere, claims 12 to 14. The method according to claim 15, wherein the excess amount of O ₂ is supplied in an amount of 0.05 to 5% O ₂ .   The method according to any one of claims 1 to 16, wherein a coiling temperature after the hot rolling is maintained at 500 ° C or lower and / or the annealing temperature Ta is 785 ° C ± 10 ° C.