Classifications

• • 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
  • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • 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
• • 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

Definitions

• the invention relates to a substantially austenitic steel having high strength and good formability for cold rolling.
• the invention also relates to a method of producing said steel and the use thereof.
• Austenitic steels having a high strength such as Hadfield steels, comprising manganese (11 to 14%) and carbon (1.1 to 1.4%) as its main alloying elements, have been known for a long time.
• the original Hadfield steel containing about 1.2% C and 12% Mn, was invented by Sir Robert Hadfield in 1882. This steel combines high toughness and a reasonable ductility with high work-hardening capacity and, usually, good resistance to wear.
• Hadfield steels do not have good formability due to large amounts of brittle carbides. Due to the high work-hardening rate, the steels are difficult to machine.
• GB 297420 discloses a cast Hadfield-type steel with additions of aluminium to improve the machinability. The addition of aluminium results in the formation of particles which improve the machinability, particularly machinability by material detaching tools.
• U.S. Pat. No. 5,431,753 discloses a process for manufacturing a cold rolled steel having a manganese content of between 15 and 35%, up to 1.5% in carbon and between 0.1 and 3.0% of Aluminium. A lower manganese content is disclosed to be undesirable.
• At least one of these objects can be reached by a steel for cold rolling comprising (in weight percent)
• the remainder being iron and unavoidable impurities, wherein the microstructure comprises at least 75% in volume of austenite, and wherein (Ni+Mn) is from 11.0 to 15.9%.
• the carbon content of the steel according to the invention is much lower than the Hadfield steels, which is known to be about 1.2%.
• the contribution of the alloying elements is believed to be as follows hereinafter.
• SFE Stacking Fault Energy
• Stacking faults are precursors to ⁇ -martensite, so increasing the SFE decreases the tendency to form ⁇ -martensite.
• the lower carbon content results in a lower tendency to form embrittling phases and/or precipitates during cooling after rolling, and the lower carbon content in comparison to Hadfield steels is also beneficial for the weldability of the steel.
• carbon improves the stability of the austenite since carbon is an austenite stabilising element.
• the main deformation mechanisms in the austenitic steel according to the invention are strain induced twinning and transformation induced plasticity.
• Aluminium reduces the activity of carbon in austenite in steels according to the invention.
• the reduction in carbon activity increases the solubility of carbon in austenite, thereby decreasing the driving force for precipitation of carbides, particularly of (FeMn)-carbides, by reducing the carbon super-saturation.
• Aluminium also reduces the diffusivity of carbon in austenite and thereby reduces the susceptibility to dynamic strain ageing during deformation processes such as cold rolling.
• the lower diffusivity also leads to a slower formation of carbides, and thus prevents or at least hinders the formation of coarse precipitates. Since higher aluminium contents also lead to a higher SFE, the tendency for strain induced twinning is lowered at increasing Aluminium levels.
• aluminium is also a ferrite stabilising element
• the influence on the austenite stability of the aluminium additions has to be compensated for by manganese and other austenite stabilising elements.
• Manganese can, at least partly, be replaced by elements which also promote austenite stability such as nickel. It is believed that Nickel has a beneficial effect on the elongation values and impact strength.
• the austenite is meta-stable and the microstructure of the steel may not be fully austenitic.
• the microstructure in the steel according to the present invention as a function of composition may comprise a mixture of ferrite and austenite with components of martensite.
• a beneficial combination of the deformation mechanisms of plasticity induced by twinning and plasticity induced by transformation under the influence of deformation provides excellent formability, whereas the lower strain hardening and work hardening rate as compared to conventional Hadfield steel in combination with a lower susceptibility to dynamic strain ageing as a result of the aluminium addition and the absence of coarse and/or brittle carbides results in good cold-rolling and forming properties. It has been found that the favourable cold rolling and mechanical properties are already obtained when the microstructure comprises at least 75% in volume of austenite.
• the steel according to the invention also has a good galvanisability as a result of the absence of silicon as an alloying element, i.e.
• the steel not only has excellent cold-rollability, but that similar excellent properties in terms of strength and formability are obtained in its pre-cold rolling state, i.e. for instance in its as-hot-rolled state, but also in the recrystallised state after cold-rolling and annealing.
• Ni+Mn is at most 14.9%. This embodiment allows the steel to be produced in a more economical way, because the amount of expensive alloying elements is reduced.
• the microstructure in particular after cold-rolling and annealing, 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 amount of austenite is favourably influenced by selecting the carbon content to be at least 0.10% or at least 0.15%, but preferably to be at least 0.30% and more preferably at least 0.50%.
• the carbon content of the steel is at most 0.78%, preferably at most 0.75%, more preferably at most 0.70%. It was found that the weldability of the steel is improved by limiting the carbon content. It was found that a steel having a carbon content of at most 0.78%, preferably at most 0.75%, more preferably at most 0.70% or even more preferably of at most 0.65% provides a good balance between the mechanical properties and the risk of martensite formation. In an embodiment of the invention, the carbon content is between 0.15 and 0.75%, preferably between 0.30 and 0.75%. From an economic point of view, the properties point of view, and a process control point of view, this range provides stable conditions.
• the nickel content is at most 1.25%. It is believed that nickel has a beneficial effect on the elongation values and impact strength. It has been found that at Nickel additions exceeding 2.5% the effect saturates. Since Nickel is also an expensive alloying element, the amount of Nickel is to be kept as low as possible if the demands to elongation values and/or impact strength are somewhat relaxed. In an embodiment of the invention the Nickel content is at most 0.10%, preferably at most 0.05%.
• the aluminium content is at most 4.0%. This embodiment limits the increase in stacking-fault energy by the addition of Aluminium, whilst still maintaining favourable properties.
• the manganese content is at least 11.5%, preferably at least 12.0%. This embodiment allows a more stable austenite to be formed.
• the manganese content is at most 14.7%. This embodiment allows a further reduction in costs of the steel according to the invention.
• the steel according to the invention is provided in the form of a continuously cast slab with a typical thickness of between 100 and 350 mm, or in the form of a continuously cast thin slab with a typical thickness of between 50 and 100 mm.
• the steel according to the invention is provided in the form of a continuously cast and/or hot rolled strip, preferably with a typical thickness between 0.5 and 20 mm, more preferably between 0.7 and 10 mm. Even more preferably the strip thickness is at most 8 mm or even at most 6 mm.
• the steel according to the invention is provided in the form of a hot rolled steel having a thickness between 0.5 and 20 mm, preferably between 0.7 and 10 mm, more preferably the strip thickness is at most 8 mm, or even more preferably between 0.8 and 5 mm.
• the steel according to the invention is provided in the form of a cold-rolled strip, or in the form of a cold-rolled and annealed (continuously or batch-annealed) strip which may be coated with a coating system comprising one or more metallic and/or organic layer or layers.
• the metallic coating may be provided in a hot-dip line, an electro-coating line, but also in a CVD or PVD process, or even by cladding.
• the microstructure of the cold rolled steel microstructure after rolling and annealing, and the optional coating comprises at least 80%, preferably at least 85%, more preferably at least 90%, and even more preferably at least 95% in volume of austenite. It was found that the cold rolled steel after rolling and annealing has optimal formability when the microstructure of the cold rolled steel microstructure after rolling and annealing, and the optional coating, comprises only or substantially only austenite.
• a method of producing a substantially austenitic steel strip, having an austenite content as described above comprising the steps of:
• the molten steel will most likely be provided by an EAF-process.
• the molten steel is then subsequently cast in a mould so as to obtain a solidified steel in a form suitable for hot rolling.
• This form may be an ingot which after slabbing and reheating is suitable for hot rolling. It may also be a continuously cast thick or thin slab having a typical thickness of between 50 and 300 mm.
• the form suitable for hot rolling may be a continuously cast strip, such as obtained after strip casting using some form of strip-casting device, such as twin-roll casting, belt-casting or drum casting. In order to convert the cast microstructure into a wrought microstructure, hot deformation such as rolling of the solidified steel is required.
• This method comprises a rolling process wherein the steel product is passed between a set of rotating rolls of a rolling mill stand in order to roll the steel product, characterised in that the rolls of the rolling mill stand have different peripheral velocities such that one roll is a faster moving roll and the other roll is a slower moving roll, in that the peripheral velocity of the faster moving roll is at least 5% higher and at most 100% higher than that of the slower moving roll, in that the thickness of the steel product is reduced by at most 15% per pass, and in that the rolling takes place at a maximum temperature of 1350° C.
• the hot-rolled strip is cold-rolled to the desired final thickness, preferably wherein the cold-rolling reduction is between 10 to 90%, more preferably between 30 and 85, even more preferably between 45 and 80%.
• the cold-rolled strip is annealed after cold rolling to the desired final thickness in a continuous or batch annealing process. This annealing treatment results in a substantially recrystallised product.
• the cold-rolled strip is galvanized.
• the absence of silicon as an alloying element, i.e. in the sense of a deliberate addition of silicon for alloying purposes, is beneficial for the galvanisability of the austenitic steel.
• the adherence of the zinc layer to the substrate is thereby greatly improved.
• the steel according to the invention may be annealed at annealing temperatures between 550 to 1100° C., preferably between 650 to 1100° C. either in a batch annealing process, in which case the maximum annealing temperature is preferably between 550 and 800° C., preferably between 650 and 800° C., more preferably at least at 700 and/or below 780° C., or in a continuous annealing process, in which case the maximum annealing temperature is at least 600° C., preferably wherein the maximum annealing temperature is between 700 and 1100° C., more preferably below 900° C.
• the strip may be subjected to a temper rolling process.
• an austenitic steel strip or sheet is provided as described above, produced according to a process as described above. These steels provide excellent strength and good formability in any process stage.
• the resulting steel strips may be processed to blanks for further processing such as a stamping operation or a pressing operation in a known way.
• the steel may be used to produce parts for automotive applications, both in the load bearing parts, such as chassis parts or wheels, but also in the outer parts, such as body parts.
• the steel is also suitable for the production of tubes and pipes, particularly for low temperature application. Due to its large forming potential, the steel is very well suited for shaping by hydroforming or similar processes. Its high work hardening potential and work hardening rate makes the steel suitable for producing products wherein the steel is subjected to impact loads.
• the tensile properties in the rolling direction for steel 1 and steels 9-12 are shown in tables 3 and 4. Different levels of cold reduction appear to have little effect on the driving force for recrystallisation. Fluctuations in coiling temperature between 600° C. and 680° also appear to have little effect.
• the tensile tests were performed on a standard tensile specimen and a gauge length of 80 mm was used, except for steel 12, where a gauge length of 50 mm was used. The tensile tests were performed according to EN 10002-1 in the longitudinal direction.

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• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Metal Rolling (AREA)
• Heat Treatment Of Steel (AREA)

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• 2006
  • 2006-02-01 JP JP2007553560A patent/JP5318421B2/ja not_active Expired - Fee Related
  • 2006-02-01 KR KR1020077020024A patent/KR20070099684A/ko active Search and Examination
  • 2006-02-01 WO PCT/EP2006/001034 patent/WO2006082104A1/en active Application Filing
  • 2006-02-01 CN CN2006800038854A patent/CN101111622B/zh active Active
  • 2006-02-01 US US11/815,087 patent/US20090165897A1/en not_active Abandoned
  • 2006-02-01 RU RU2007132863/02A patent/RU2401877C2/ru active
  • 2006-02-01 EP EP06706689.4A patent/EP1846584B2/en active Active

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