Classifications

• • 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
  • 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
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • 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
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
  • C21D8/04—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
  • C21D8/041—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 involving a particular fabrication or treatment of ingot or slab
  • C21D8/0415—Rapid solidification; Thin strip casting
• • 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
  • C21D8/0426—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
  • C21D8/04—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
  • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• This invention relates to a method for producing a high strength steel, and a high strength steel produced by this method.
• the dedicated thermal treatment after hot rolling may comprise a controlled cooling after hot rolling, a quench and temper treatment, or an after-annealing of the finished hot-rolled products.
• the product quality is difficult to reproduce and each combination of strength and elongation properties necessitates another combination of chemical composition, thermo-mechanical treatment and dedicated thermal treatment after hot rolling. This makes the production process inflexible, increases the risk of not producing the desired product quality and each added chemical composition adds to the logistical burden.
• the elongation values of these high strength steels are insufficient to satisfy the high demands the designers of cars and earth moving equipment nowadays set.
• the object according to the invention can be achieved by a process for producing a high strength steel product, wherein the product is produced from a hot-rolled and/or cold-rolled and annealed TWIP steel and having an initial ratio of yield strength and tensile strength, R i , and wherein at least part of the TWIP steel is subsequently subjected to a cold reduction which is chosen such that the desired ratio of yield strength and tensile strength, R d , in the part is obtained.
• the cold reduction of the hot-rolled TWIP steel starting material or the cold-rolled and annealed TWIP steel starting material may be performed by a cold rolling step which is able to impose different cold reductions over length and/or width of the rolled product.
• the rolled product may be a strip, plate or sheet of steel. From this product blanks may be produced which can e.g. be used for a stamping or pressing operation.
• the TWIP-steel used in the process according to the invention preferably comprises 0.05-0.78% C , 11 to 23% Mn, at most 5% Al, at most 5% Cr, at most 2.5% Ni, at most 5% Si, up to 0.5% V, remainder iron and unavoidable impurities. All compositional percentages are given in weight percent unless indicated otherwise.
• the microstructure of the formed part comprises at least 75% in volume of austenite.
• the TWIP-steel also comprises up to 0.5% of Vanadium.
• the alloying with vanadium will promote the formation of VC-precipitates which contribute to the strength of the steel and the prevention of delayed cracking by providing sinks for hydrogen at the predominantly semi-coherent interface between the VC-precipitates and the matrix.
• at least 0.05% V is added to the steel to obtain this effect.
• a minimum vanadium content of 0.1 or even 0.2% is preferable.
• a preferable minimum aluminium content of the steel is at least 1 %.
• the aluminium content of the steel of at least 1% ensures an increased stability of the austenite.
• the chromium content is at most 0.5% and/or at least 0.05%.
• a suitable minimum chromium content is 0.10 or even 0.15%.
• TWIP steels in general and those having a composition in the claimed range in particular, have a high tensile strength and a relatively low initial yield strength in combination with high total elongation values.
• the process according to the invention uses these steels suitable for selectively tuning the properties, particularly the yield strength, i.e. the stress at which the steel starts to deform, by cold deforming the steel. The level of cold deformation determines the level of the yield strength which is obtained.
• the residual formability is sufficient to sustain significant post-cold deformation forming operations. No annealing is to take place after the cold deformation has taken place, because this will adversely affect the yield strength.
• the steel sheet comprises 0.05 to 0.78 % C, 11.0 to 18% Mn, 1.0 to 5.0% Al, 0 to 2.5% Ni, 0 to 5% Si, 0 to 0.5% Cr, 0 to 0.5% V, the remainder being iron and unavoidable impurities, wherein the microstructure of the formed part comprises at least 75% in volume of austenite.
• Ni and Mn contents are chosen such that (Ni+Mn) is from 11.0 to 23.
• a preferable maximum for Ni+Mn is 18%. More preferably the maximum is chosen to be 17 or even 15.9%.
• the microstructure of the formed part in particular after cooling, comprises at least 80%, preferably at least 85%, more preferably at least 90% and even more preferably at least 95% in volume of austenite.
• the inventor found that a further improvement of the cold rolling and mechanical properties could be obtained if the steel was chosen such that the austenite content in the microstructure comprises at least 80%, preferably at least 85%, more preferably at least 90% and even more preferably at least 95% in volume of austenite. Due to the meta-stability of the austenite, and the occurrence of transformation induced plasticity, the amount of austenite tends to decrease during subsequent processing steps. In order to ensure good formability and high strength, even during a later or its last processing step, it is desirable to have an austenite content which is as high as possible at any stage of the processing, but in particular after cold-rolling and annealing.
• the TWIP steel Prior to rolling the TWIP steel is provided into a shape and geometry suitable for further processing.
• the TWIP steel may be provided as a conventional thick slab and hot rolled to a thickness of about 1 to 5 mm before being cold-reduced to obtain the desired properties.
• the TWIP-steel may also be provided by thin slab casting and direct (hot) rolling, preferably using a soaking step between the casting and the rolling step to homogenise the temperature and the starting structure of the steel prior to hot-rolling.
• the TWIP-steel may also be provided by strip casting, optionally followed by a hot-rolling step in one or more reduction steps, for instance in a tandem rolling device comprising two or more rolling stands.
• Strip casting may for instance be performed in a twin-roll casting device or in a belt casting device.
• Belt casting allows thicker strips to be cast, leaving potential for a larger hot rolling reduction after solidification, and thus potentially influencing the microstructure and texture of the hot-rolled strip.
• the cold deformation is chosen such that the ratio of the yield strength to the tensile strength after cold deformation is below 0.98. If the ratio is higher than this value, the yield strength practically attains the same level as the tensile strength, leading to immediate failure if the yield strength is exceeded.
• the ratio after cold deformation is at least 0.60, 0.70 or 0.80. More preferably the minimum ratio is 0.85. For higher ratios such as 0.90, the remaining ductility is reduced to an extent insufficient for the majority of applications. However, for some specific applications a ratio exceeding 0.85, such as 0.90 may be applicable.
• the cold reduction is achieved by cold rolling. This ensures a homogeneous deformation over the entire product, and hence homogenous properties.
• the cold rolling reduction can be brought about in conventional cold rolling mills. The selection of the cold rolling reduction will, starting from the properties of the hot-rolled steel or cold-rolled and annealed steel, determine the level of the final cold deformed product.
• the cold reduction is not higher than 35%. It was found that when cold deforming the hot-rolled steel or cold-rolled and annealed steel, to a degree substantially higher than 35%, that the residual formability becomes too low. More in particular, the yield strength attains the same level as the tensile strength, leading to immediate failure if the yield strength is exceeded. Preferably, the cold reduction is not higher than 20%.
• the cold reduction is at least 2%, preferably at least 3%, more preferably at least 5%.
• the cold reduction may be applied before or after the TWIP steel is provided with a metallic and/or organic coating.
• a process to produce a steel product wherein the cold reduction is a step in the production of a tailor-rolled blank, and wherein the cold reduction of the TRB is chosen such that in the cold-reduced part or parts of the TRB the desired value of R d is obtained.
• a steel product having an initial set of mechanical properties, is subjected to a rolling process allowing to change the degree of reduction over the length of the strip (laterally) and/or over the width of the strip (longitudinally).
• the degrees of reduction are chosen such so as to produce the TRB having the desired geometrical properties, but more importantly, to have the desired mechanical properties in each part of the TRB.
• Parts which have been subjected to a higher degree of cold reduction will possess a higher yield strength than the parts which have not been cold deformed to a smaller or not extent.
• the undeformed parts still have a large formability potential and can be used on the location where the TRB will be subjected to a large deformation during the production of the finished part for of the TRB, whereas the more heavily cold deformed parts can be located such that they coincide with the location of the finished part where a high yield strength is required.
• This tuning of the mechanical properties prior to the stamping or production of the final part allows a larger flexibility of designing the final part, whereas the cold-deformation process is a more accurate way of tuning the properties than the stamping or production process of the final part.
• metal sheets of uniform thickness are used for stamping or forming vehicle structural parts.
• a metal sheet with varying thickness is desirable. It not only saves material but also increases design flexibility. For example, some areas of a cross member require thicker thickness to support localized, larger loading, while for other areas, where there is no localized loading, thinner thickness can be used to save material.
• TWB Tailor Welded Blank
• the problem with TWB is that the heat input of the welding operation may adversely affect the mechanical properties of the parts welded together, thereby affecting the properties of the TWB as a whole.
• TRB is a manufacturing technology which allows engineers to change blank thickness continuously within a sheet metal, virtually eliminating the need for welding local reinforcements in the part. TRB also provides simpler structural design due to smooth, rolled transitions, which prevent stress concentrations in the finished part. In addition, there is little cost penalty for multiple thickness transitions, due to the nature of the rolling process.
• the use of TWIP steel for producing this TRB has the added advantage that for a different thickness different properties are obtained without an annealing treatment after the production of the TRB.
• a TRB made from a low carbon steel always needs to be annealed after the rolling step.
• a process is provided wherein the cold reduction is a step in the production of a tailor-rolled blank, and wherein the cold reduction of the TRB is chosen such that in the cold-reduced part or parts of the TRB the desired value of R or yield strength is obtained.
• a TWIP-steel tailor rolled blank is produced by the method according to the invention having a desired value of R d in the, or in each cold-reduced part. This not only allows to tailor the thickness of the blank, but also the mechanical properties.
• Table 1 shows that up to cold reductions of 15%, the level of ductility in the deformed product is still very high, resulting in a deformed material with a significant deformation potential. At higher levels of cold reduction such as 20%, the remaining ductility is reduced, but still at a very high level when compared to conventional steels of similar strength.

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• Chemical & Material Sciences (AREA)
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• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
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• Heat Treatment Of Sheet Steel (AREA)

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Cited By (18)

Citations (11)

• 2008
  • 2008-01-30 EP EP08101118A patent/EP2090668A1/fr not_active Ceased

Patent Citations (11)

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