WO2011154153A1 - Method of producing an austenitic steel - Google Patents

Method of producing an austenitic steel Download PDF

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Publication number
WO2011154153A1
WO2011154153A1 PCT/EP2011/002868 EP2011002868W WO2011154153A1 WO 2011154153 A1 WO2011154153 A1 WO 2011154153A1 EP 2011002868 W EP2011002868 W EP 2011002868W WO 2011154153 A1 WO2011154153 A1 WO 2011154153A1
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Prior art keywords
strip
process according
hot
metallic coating
temperature
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PCT/EP2011/002868
Other languages
French (fr)
Inventor
Basjan Berkhout
Marcus Cornelis Maria Cornelissen
Jayesh Ramjibhai Patel
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Tata Steel Ijmuiden Bv
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Application filed by Tata Steel Ijmuiden Bv filed Critical Tata Steel Ijmuiden Bv
Priority to KR1020127032074A priority Critical patent/KR101900963B1/en
Priority to CN2011800284065A priority patent/CN102939394A/en
Priority to EP11725880.6A priority patent/EP2580359B1/en
Priority to JP2013513585A priority patent/JP6009438B2/en
Priority to BR112012031466-1A priority patent/BR112012031466B1/en
Priority to US13/699,516 priority patent/US20130118647A1/en
Publication of WO2011154153A1 publication Critical patent/WO2011154153A1/en
Priority to ZA2013/00240A priority patent/ZA201300240B/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
    • C21D1/70—Temporary coatings or embedding materials applied before or during heat treatment while heating or quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0442—Flattening; Dressing; Flexing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/34—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tyres; for rims
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching

Definitions

  • the invention relates to a method of producing an austenitic steel sheet excellent in resistance to delayed cracking.
  • small amounts of hydrogen may be introduced at some stages of steel fabrication such as chemical or electrochemical pickling, annealing under special atmospheres, electroplating or hot dip galvanizing. Subsequent machining operations using lubricating oils and greases may also cause hydrogen production after decomposition of these substances at high temperatures.
  • the SFE of the steel increases. Any adverse effects of the elements that lower the SFE, such as silicon, are counteracted by the addition of aluminium. Additionally, aluminium lowers the activity and the diffusivity of carbon in austenite, which reduces the driving force for forming carbides.
  • the vanadium which is added as an essential alloying addition, forms carbides. These vanadium-carbides act as hydrogen sinks if and when the size and distribution of the vanadium-carbides is correct.
  • the increased aluminium content is therefore essential to control the vanadium-carbide precipitation because it prevents the vanadium-carbides from coarsening due to the reduced carbon activity and diffusivity as a result of the presence of aluminium .
  • the annealing treatment is crucial in that it controls the precipitation of the vanadium-carbides and ca uses the recrystall isation of the cold-deformed microstructure caused by the cold-rolling to result in a fine grain structure.
  • the silicon content is very low, i .e. at impurity level.
  • the aluminium content is limited only by the fact that the steel according to the i nventi on is a n a ustenitic steel.
  • the maximum aluminium content is 5%.
  • the aluminium content is at least 1.25 and/or at most 3.5%, more preferably at least 1.5 and/or at most 2.5%.
  • the maximum annealing temperature T a is 825°C or even 800°C.
  • the cooling rate V c is between 10 and 100 °C/s.
  • a preferable cooling rate is between 20 and 80 °C/s.
  • the heating rate is preferably between 3 and 60°C/s.
  • the annealing time t a is preferably between 15 and 300 seconds.
  • the maximum annealing temperature T a is from 775 to 795 °C (i.e. 785 ⁇ 10°C).
  • the steel strip material has been pickled before cold rolling.
  • Pickling is (often) necessary before cold rolling to remove oxides, to prevent rolling in of oxides.
  • the cold rolled strip material is produced from a hot rolled strip material or a belt cast strip material.
  • the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating .
  • the metallic coating can be any known common coating such as zinc or zinc alloy, wherein the zinc may be alloyed with elements such as aluminium and/or magnesium.
  • the strip is pickled after continuous annealing and wherein the strip is provided with a metallic coating by pickling after annealing followed by heati ng to a temperature below the conti nuous annealing temperature before the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating.
  • This alternative process is available if the economical process as described hereinabove is not preferred. There may be issues with adhesion with certain specific metallic coatings for which a pickling treatment may be necessary. After pickling, it is not necessary nor desirable to heat the strip above the T a . It is preferable that the heating temperature remain below T a .
  • the strip material is heated only to a temperature high enough to form a closed inhibition layer.
  • This temperature is lower than the normal continuous annealing temperature necessary for metallurgical reasons (such as recrystallisation to influence mechanical properties). The forming of oxides on the surface of the steel strip material is thereby reduced.
  • the temperature below the continuous anneal ing temperature is between 400 and 600 °C. In this temperature range the forming of oxides is considerably reduced a nd the stri p material is heated sufficiently for the subsequent hot dip galvanizing.
  • the Fe in the strip material is reduced during or after the heati ng to a temperature below the continuous annealing temperature and before the hot dip galvanising.
  • the Fe-oxides that are formed are reduced, and in this way the amount of oxides present on the su rface of the stri p material before hot di p galva n izing is decreased considerably.
  • the reduction is performed using H 2 N 2 , more preferably using 5 - 30% H 2 N 2 in the reducing atmosphere. It has been found that with the use of this atmosphere most oxides can be removed.
  • an excess amount of 0 2 is provided in the atmosphere during or after the heating of the strip material and before the reduction of the strip material .
  • the providing of an excess amount of oxygen improves the quality of the surface of the steel strip material before the hot dip galvanizi ng, and thus the quality of the zinc layer coated on the AHSS strip material. It is supposed that the oxygen binds the alloying elements in the AHSS strip material both at the surface of the strip material and internally, and that in this way the oxides formed cannot migrate to the surface of the strip material .
  • the reducing atmosphere that follows after the oxidation will then reduce the oxides at the surface of the strip material, and in this way the amount of oxides at the surface of the strip material is considerably reduced or even almost absent, as experiments have shown.
  • the excess amount of 0 2 is provided in an amount of 0,05 - 5 % 0 2 . This amount of oxygen has been found to suffice.
  • V-alloyed TWIP steel strip material accord i ng to the i nvention was hot rol led, pickled and cold rol led, and continuously annealed to a temperature according to the invention and pickled again . Then the strip material is heated to a temperature of 527 °C in an annealing line, and thereafter hot dip galvanised in a galvanising bath at approximately 450°C.
  • the strip material is reduced using approximately 5% H 2 N 2 . The reduction of the strip material removes the oxides from the surface, but the oxides formed under the surface remain where they are and cannot migrate to the surface.
  • the oxides are effectively removed and no new oxides can be formed at the surface.
  • These oxides when not removed, cause a bad adhesion of the zinc layer to the substrate, resulting in bare spots, flaking and the forming of cracks in the zinc layer when the material is bent.
  • the alloying elements migrate to the surface very fast at the alloying temperature and thus form oxides at the surface again before the hot dip galvanising takes place. Whatever the exact mechanism may be, it has been found that the use of this method diminishes or almost eliminates the amount of oxides found in a hot dip galvanised zinc layer on a V-alloyed TWIP steel.
  • the cold-rolling reduction is between 10 to 90%, more preferably between 30 and 85, even more preferably between 45 and 80%.
  • the annealed strip is temper rolled with a reduction of from 0.5 to 10% prior to or after the metallic coating has been provided to the strip.
  • the Vanadium content is between 0.06 and 0.22%.
  • a strip or sheet is provided produced by a method according to any one of claims 1 to 6, wherein the steel preferably is provided with a metallic coating.
  • the strip or sheet is used for the production of automotive inner or outer parts or wheels or for hydroforming applications.
  • the finish rolling temperature was chosen to ensure recrystallisation of the deformed microstructure and the coiling temperature was kept below 500°C to avoid carbide precipitation. Recrystallisation does not solely depend on the FRT but also depends on the time, on the rolling strain accumulated since the last recrystallisation event during hot rolling and on the strain rate.
  • the delayed cracki ng and stress corrosion cracking results on the V-alloyed grades show a lower susceptibility to crack formation as the material is annealed at a higher temperature.
  • V addition is clearly beneficial at an annealing temperature of 750°C, but also at higher annealing temperatures.
  • V-alloys were subjected to a resistance spot welding tests. Hot cracking in the weld was largely reduced compared to the Si-free non V-alloyed material.

Abstract

This invention relates to a method of producing an austenitic steel sheet excellent in resistance to delayed cracking and a steel produced thereby.

Description

METHOD OF PRODUCING AN AUSTENITIC STEEL
The invention relates to a method of producing an austenitic steel sheet excellent in resistance to delayed cracking.
In view of fuel economy and safety in the case of collisions, high strength steels are used more and more in the automobile industry. This requires the use of structural materials that combine a high tensile strength with high ductility. Austenitic alloys comprising as main elements iron, carbon and high levels of manganese, which can be hot-rolled or cold-rolled and have a strength that may exceed 1000 MPa. The mode of deformation of these steels depends on the stacking fault energy: for a sufficiently high stacking fault energy, the observed mode of mechanical deformation is twinning, which results in a high work hardenability. By acting as an obstacle to the propagation of dislocations, the twins increase the flow stress. However, when the stacking fault energy exceeds a certain limit, slip of perfect dislocations becomes the main deformation mechanism and the work hardening is reduced. It is known that the sensitivity to delayed cracking increases with the mechanical strength, in particular after certain cold-forming operations since high residual tensile stresses are liable to remain after deformation. In combination with atomic hydrogen possibly present in the metal, these stresses are liable to result in delayed cracking, that is to say cracking that occurs a certain time after the deformation itself. Hydrogen may progressively build u p by d iffusion to crystal lattice defects, such as matrix/inclusion interfaces, twin boundaries and grain boundaries. It is in the latter areas that hydrogen may become harmful when it reaches a critical concentration after a certain time. For a constant grain size, the time required to attain a critical level depends on the initial concentration of mobile hydrogen, the intensity of the residual stress concentration field and the kinetics of hydrogen diffusion.
In particular circumstances, small amounts of hydrogen may be introduced at some stages of steel fabrication such as chemical or electrochemical pickling, annealing under special atmospheres, electroplating or hot dip galvanizing. Subsequent machining operations using lubricating oils and greases may also cause hydrogen production after decomposition of these substances at high temperatures.
It is an object of this invention to provide a method of producing an austenitic steel sheet excellent in resistance to delayed cracking. It is also an object of this invention to provide a method of producing an austenitic steel sheet having an increased yield stress and excellent weldability.
It is a further object of this invention to provide a method of prod ucing an austenitic steel sheet which is energy efficient and simple in comparison to the conventional route for this type of steel.
According to the invention one or more of these objects is reached by providing a method of producing an austenitic steel sheet excellent in resistance to delayed cracking comprising
- casting an ingot, or a continuously cast slab, or a continuously cast thin slab or a strip-cast strip, the composition thereof comprising, in weight:
- 0.50% - 0.80 %C
- 10 - 17% Mn
- at least 1.0 % Al
- at most 0.5% Si
- at most 0.020% S
- at most 0.050% P
- 50 - 200 ppm N
- 0.050 - 0.25 % V
the remainder being iron and unavoidable impurities inherent to fabrication; - providing a hot-rolled strip by hot rolling the ingot, the continuously cast slab, the continuously cast thin slab or the strip-cast strip to the desired hot rolled thickness.
- cold-rolling the hot-rolled strip to the desired final thickness,
- continuous annealing of the cold-rolled strip in a process comprising heating up the strip at a heating rate Vh to an annealing temperature Ta for an annealing time ta followed by cooling at a cooling rate Vc and wherein Ta is from 750 to 850°C.
By using a high aluminium content the SFE of the steel increases. Any adverse effects of the elements that lower the SFE, such as silicon, are counteracted by the addition of aluminium. Additionally, aluminium lowers the activity and the diffusivity of carbon in austenite, which reduces the driving force for forming carbides. The vanadium, which is added as an essential alloying addition, forms carbides. These vanadium-carbides act as hydrogen sinks if and when the size and distribution of the vanadium-carbides is correct. The increased aluminium content is therefore essential to control the vanadium-carbide precipitation because it prevents the vanadium-carbides from coarsening due to the reduced carbon activity and diffusivity as a result of the presence of aluminium . The inventors found that at least 1.0 % Al and from 0.050 to 0.25 % V is required to achieve this. Lower aluminium contents led to too coarse vanadium-carbides, thereby rendering them ineffective as hydrogen sinks, and the amount of Vanadium needs to be controlled between the mentioned values to achieve a sufficient number of small precipitates. Higher V-values lead to early nucleation of the precipitates and thereby inevitably to coarse and fewer precipitates, whereas values below 0.050% V simply result in to few precipitates, even if they are fine enough. The annealing treatment is crucial in that it controls the precipitation of the vanadium-carbides and ca uses the recrystall isation of the cold-deformed microstructure caused by the cold-rolling to result in a fine grain structure. In a preferable embodiment the silicon content is very low, i .e. at impurity level. In principle the aluminium content is limited only by the fact that the steel according to the i nventi on is a n a ustenitic steel. In an embodiment the maximum aluminium content is 5%. Preferably the aluminium content is at least 1.25 and/or at most 3.5%, more preferably at least 1.5 and/or at most 2.5%.
I n a n embodiment the maximum annealing temperature Ta is 825°C or even 800°C. In an embodiment the cooling rate Vc is between 10 and 100 °C/s. A preferable cooling rate is between 20 and 80 °C/s. The heating rate is preferably between 3 and 60°C/s. The annealing time ta is preferably between 15 and 300 seconds.
In a preferable embodiment, the maximum annealing temperature Ta is from 775 to 795 °C (i.e. 785 ± 10°C).
Preferably, the steel strip material has been pickled before cold rolling. Pickling is (often) necessary before cold rolling to remove oxides, to prevent rolling in of oxides. Preferably, the cold rolled strip material is produced from a hot rolled strip material or a belt cast strip material.
In a preferred embodiment of the invention during the cooling at a cooling rate Vc after the continuous annealing the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating . This process leads to a very economical and quick process to produce a metallically coated steel strip. The metallic coating can be any known common coating such as zinc or zinc alloy, wherein the zinc may be alloyed with elements such as aluminium and/or magnesium. In another embodiment of the invention the strip is pickled after continuous annealing and wherein the strip is provided with a metallic coating by pickling after annealing followed by heati ng to a temperature below the conti nuous annealing temperature before the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating. This alternative process is available if the economical process as described hereinabove is not preferred. There may be issues with adhesion with certain specific metallic coatings for which a pickling treatment may be necessary. After pickling, it is not necessary nor desirable to heat the strip above the Ta. It is preferable that the heating temperature remain below Ta.
With this method, the strip material is heated only to a temperature high enough to form a closed inhibition layer. This temperature is lower than the normal continuous annealing temperature necessary for metallurgical reasons (such as recrystallisation to influence mechanical properties). The forming of oxides on the surface of the steel strip material is thereby reduced.
Preferably, the temperature below the continuous anneal ing temperature is between 400 and 600 °C. In this temperature range the forming of oxides is considerably reduced a nd the stri p material is heated sufficiently for the subsequent hot dip galvanizing.
According to a preferred embodiment, the Fe in the strip material is reduced during or after the heati ng to a temperature below the continuous annealing temperature and before the hot dip galvanising. By reducing the strip material, the Fe-oxides that are formed are reduced, and in this way the amount of oxides present on the su rface of the stri p material before hot di p galva n izing is decreased considerably.
Preferably, the reduction is performed 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.
According to a preferred embodiment, an excess amount of 02 is provided in the atmosphere during or after the heating of the strip material and before the reduction of the strip material . The providing of an excess amount of oxygen improves the quality of the surface of the steel strip material before the hot dip galvanizi ng, and thus the quality of the zinc layer coated on the AHSS strip material. It is supposed that the oxygen binds the alloying elements in the AHSS strip material both at the surface of the strip material and internally, and that in this way the oxides formed cannot migrate to the surface of the strip material . The reducing atmosphere that follows after the oxidation will then reduce the oxides at the surface of the strip material, and in this way the amount of oxides at the surface of the strip material is considerably reduced or even almost absent, as experiments have shown. Preferably, the excess amount of 02 is provided in an amount of 0,05 - 5 % 02. This amount of oxygen has been found to suffice.
In a preferred embodiment of the invention V-alloyed TWIP steel strip material accord i ng to the i nvention was hot rol led, pickled and cold rol led, and continuously annealed to a temperature according to the invention and pickled again . Then the strip material is heated to a temperature of 527 °C in an annealing line, and thereafter hot dip galvanised in a galvanising bath at approximately 450°C.
During the heating of the strip material to the temperature of 527 °C, an excess amount of 1% 02 is provided. The oxygen is provided at such a high temperature so that not only forms oxides at the surface of the strip material, but also at some depth under the surface binds the alloying elements. After the providing of the oxygen, the strip material is reduced using approximately 5% H2N2. The reduction of the strip material removes the oxides from the surface, but the oxides formed under the surface remain where they are and cannot migrate to the surface.
Thus, by reducing the surface the oxides are effectively removed and no new oxides can be formed at the surface. These oxides, when not removed, cause a bad adhesion of the zinc layer to the substrate, resulting in bare spots, flaking and the forming of cracks in the zinc layer when the material is bent. It is presumed that by normal reduction, the alloying elements migrate to the surface very fast at the alloying temperature and thus form oxides at the surface again before the hot dip galvanising takes place. Whatever the exact mechanism may be, it has been found that the use of this method diminishes or almost eliminates the amount of oxides found in a hot dip galvanised zinc layer on a V-alloyed TWIP steel.
In an embodiment of the invention the cold-rolling reduction is between 10 to 90%, more preferably between 30 and 85, even more preferably between 45 and 80%.
In an embodiment of the invention the annealed strip is temper rolled with a reduction of from 0.5 to 10% prior to or after the metallic coating has been provided to the strip.
In an embodiment of the invention the Vanadium content is between 0.06 and 0.22%. In a second aspect of the invention a strip or sheet is provided produced by a method according to any one of claims 1 to 6, wherein the steel preferably is provided with a metallic coating. In a preferred embodiment of the invention the strip or sheet is used for the production of automotive inner or outer parts or wheels or for hydroforming applications.
The invention will now be further explained by means of the following, non- limitative examples: The chemical compositions of the materials used in this study are shown in Table 1.
Table 1 Chemical composition of the materials studied (incl. typical semi- industrial Si-free reference material) (all in wt%, Balance: Fe and impurities)
Figure imgf000007_0001
* Low = impurity level
The finish rolling temperature (FRT) was chosen to ensure recrystallisation of the deformed microstructure and the coiling temperature was kept below 500°C to avoid carbide precipitation. Recrystallisation does not solely depend on the FRT but also depends on the time, on the rolling strain accumulated since the last recrystallisation event during hot rolling and on the strain rate.
All hot rolled materials were 50% cold rolled and subsequently recrystallisation annealed. Different annealing cycles were applied to determine the optimal annealing parameters. Note that the elongations were between 45% and 50% for all samples, except for those that were not recrystallised (36-45%) and the material annealed 920°C (65%). Since the strength is considered more important, the following discussion will focus on that.
For annealing temperatures up to 750°C, the material softens due to an increased fraction of recrystallised material and probably some grain growth. At these temperatures, the effect of precipitation is limited. The difference between the (fully recrystallised) materials annealed at 775°C and 800°C is small because the precipitation is considered optimal in this temperature region for minimising grain growth. Based on these observations, the recommended annealing temperature is 785°C ± 10°C. Table 1 Mechanical properties of the materials after 50% cold rolling and
Figure imgf000008_0001
The delayed cracki ng and stress corrosion cracking results on the V-alloyed grades show a lower susceptibility to crack formation as the material is annealed at a higher temperature. For the stress corrosion cracking sensitivity, V addition is clearly beneficial at an annealing temperature of 750°C, but also at higher annealing temperatures.
The V-alloys were subjected to a resistance spot welding tests. Hot cracking in the weld was largely reduced compared to the Si-free non V-alloyed material.

Claims

A method of producing an austenitic steel sheet excellent in resistance to delayed cracking, comprising
- casting an ingot, or a continuously cast slab, or a continuously cast thin slab or a strip-cast strip, the composition thereof comprising, in weight:
- 0.50% - 0.80 %C
- 10 - 17% Mn
- at least 1.0 % Al
- at most 0.5% Si
- at most 0.020% S
- at most 0.050% P
- 50 - 200 ppm N
- 0.050 - 0.25 % V
the remainder being iron and unavoidable impurities inherent to fabrication;
- providing a hot-rolled strip by hot rolling the ingot, the continuously cast slab, the continuously cast thin slab or the strip-cast strip to the desired hot rolled thickness.
- cold-rolling the hot-rolled strip to the desired final thickness,
- continuous annealing of the cold-rolled strip in a process comprising heating up the strip at a heating rate Vh to an annealing temperature Ta for an annealing time ta followed by cooling at a cooling rate Vc and wherein Ta is from 750 to 850°C.
Process according to claim 1 wherein the aluminium content is at least 1.25 and/or at most 3.5%.
Process according to claim 1 wherein during the cooling at a cooling rate Vc after the continuous annealing the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating.
Process according to claim 1 wherein the strip is pickled after continuous annealing and wherein the strip is provided with a metallic coating by pickling after annealing followed by heating to a temperature below the continuous annealing temperature before the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating.
Process according to any one of the preceding claims wherein the cold-rolling reduction is between 10 to 90%, more preferably between 30 and 85, even more preferably between 45 and 80%.
Process according to any one of the preceding claims wherein the annealed strip is temper rolled with a reduction of from 0.5 to 10% prior to or after the metallic coating has been provided to the strip.
Process according to any one of the preceding claims wherein the Vanadium content is between 0.06 and 0.22%.
Process according to any one of the preceding claims wherein :
■ the cooling rate Vc is between 10 and 100°C/s, and/or wherein
■ the heating rate Vh is preferably between 3 and 60°C/s, and/or wherein
■ the annealing time ta is preferably between 15 and 300 seconds.
Process according to any one of the claims 1, 2 or 4 to 8 wherein the strip is pickled after continuous annealing and wherein the strip is provided with a metallic coating, by pickling after continuous annealing followed by heating to a temperature between 400 and 600 °C before the strip is led through a hot dip bath for providing a metallic coating by hot dipping the strip into a molten bath of the metal making up the metallic coating.
Process according to claim 9 wherein the Fe in the strip material is reduced during or after the heating to a temperature below the continuous annealing temperature and before the hot dip galvanising, preferably wherein the reduction is performed using H2N2, more preferably using 5 - 30% H2N2 in the reducing atmosphere.
Process according to claim 10 wherein an excess amount of 02 is provided in the atmosphere during or after the heating of the strip material and before the reduction of the strip material, preferably wherein the excess amount of 02 is provided in an amount of 0,05 - 5 % 02.
Process according to any one of the preceding claims wherein the coiling temperature after hot rolling is kept below 500°C and/or wherein rge annealing temperature Ta is 785°C ± 10°C.
13. Strip or sheet produced by a method according to any one of claims 1 to 12, wherein the steel preferably is provided with a metallic coating.
14. Use of a strip or sheet according to claim 13 for the production of automotive inner or outer parts or wheels.
15. Use of a strip or sheet according to claim 13 for hydroforming applications.
PCT/EP2011/002868 2010-06-10 2011-06-10 Method of producing an austenitic steel WO2011154153A1 (en)

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