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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
• • 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
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • 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
  • C23C2/0224—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 multiphase steel and to its production.
• High-strength steel can be obtained by using hardening mechanisms.
• known hardening mechanisms are ferrite grain refining, precipitation hardening (for example in high-strength low-alloy (HSLA) steel grades), structural hardening (for example in multiphase steel grades) and solid solution hardening (for example in phosphorus-containing (ultra)low carbon steel grades).
• the use of grain refining or solid solution hardening alone does not allow tensile strengths of 600 MPa or higher to be achieved.
• the drawback of precipitates and dissolved atoms is that the deformability of the steel is reduced.
• multiphase steel satisfies the above requirements of a high strength in combination with a relatively high deformability.
• Cold-rolled multiphase steel is produced by means of intercritical annealing in the two-phase austenite/ferrite range (the temperature range between Ac 1 and Ac 3 ), followed by cooling at a sufficiently high cooling rate to prevent the formation of pearlite, so that austenite is transformed into martensite or bainite during cooling and a ferritic/martensitic or ferritic/bainitic multiphase structure is obtained at room temperature.
• the cooling rate after the annealing in the ferrite/austenite range has to be sufficiently high to prevent the formation of pearlite and bainite during the cooling.
• Elements such as Mn and Mo can be added, since they delay the formation of pearlite and bainite.
• the cooling rate after annealing has to be high enough to prevent the formation of pearlite.
• a re-annealing treatment at generally 350-450°C is required for production of these steel grades, in order to transform austenite into bainite and to stabilize austenite.
• elements such as P, Si and Al should be added in order to stabilize austenite. The latter ferrite-forming and non-carbide-forming elements prevent precipitation of cementite and contribute to enriching the carbon content of austenite.
• the sheet steel For automotive applications, it is generally desirable for the sheet steel to be galvanized. Immersion galvanization is preferred to electrolytic galvanization in view of the (process) costs.
• electrolytic galvanization has to be used if the steel is annealed by means of conventional bell-type annealing or on a conventional continuous annealing line which is only used to produce uncoated steel sheet.
• One possible advantage of electrolytic galvanization is that a better surface quality is obtained.
• a multiphase steel having the following composition (in percent by weight unless otherwise indicated):
• the type and quantity of alloying elements are selected firstly on the basis of the expected effect on wetting during immersion galvanization and secondly on the basis of the maximum quantity possible under industrial process conditions (casting and rolling). Moreover, specific combinations have been taken into account: by way of example, a suitable combination of high Mn and Si contents may have a beneficial effect on the wetting during immersion galvanization.
• the most important alloying element is not molybdenum or chromium, but rather manganese.
• the quantity of manganese in the steel is no higher than 1.8%, which is too low to suppress the formation of pearlite under the cooling conditions encountered in many immersion galvanization lines. Therefore, small quantities of elements such as P, Al, Nb, V, Ti, B and N have been added.
• the choice of elements is based on the following metallurgical principles: (i) the prevention of the formation of pearlite during cooling after annealing, and/or (ii) the promotion of grain refining, precipitation hardening and solid solution hardening of ferrite.
• the following are present in the multiphase steel according to the invention:
• the following levels of the elements P, Al, Nb, V, Ti, B and N are present:
• the carbon content is preferably at least 0.08 - 0.10%, in order to form sufficient austenite during annealing. If there are elements which form carbide precipitates (Ti, Nb, V), the carbon content is preferably 0.16%.
• the manganese content selected serves to counteract the formation of pearlite during cooling. Silicon ensures that ferrite is formed during annealing and cooling, with carbon remaining in austenite. The latter results in the stabilization of austenite.
• Nitrogen can be added in order to stabilize austenite and for ferrite grain refining.
• Precipitation hardening of ferrite can be obtained by adding the elements Ti, Nb, V, Al and B with suitable quantities of C, S, N and P, in order to form precipitates.
• precipitation hardening is more effective in the hot-rolled state, it has an indirect effect on the strength after cold-rolling and annealing.
• the elements Ti and Nb are also added in order to form stable TiN and Nb(C, N) precipitates, so as to keep B and Al in solution, so that the latter elements can influence the transformation properties. Boron in solution prevents recrystallization of ferrite (resulting in grain refining) and is an element which promotes the formation of bainite.
• Grain refining of austenite during the hot-rolling process can be obtained by adding niobium, since this element delays the growth of recrystallization nuclei.
• a smaller austenite grain in turn leads to a smaller ferrite grain and a higher austenite stability in the cold-rolled and annealed end product which is ultimately obtained.
• the lower limit for the quantity of P, Al, Nb, V, Ti, B and N is determined by the quantity at which these elements remain active, while the upper limit is determined by the quantity at which the production process becomes impossible or too expensive.
• the steel has a microstructure which comprises ferrite and at least 15% of hardening structures of martensite, residual austenite and/or bainite, specifically at most 50% martensite and/or bainite.
• At most 50% hard phase (martensite and/or bainite) is present, in order to maintain sufficient elongation.
• at most 5-10% pearlite is present, provided that sufficient martensite and bainite are also present.
• the quantity of pearlite should preferably be low, since otherwise the multiphase properties, such as low R p0.2% /R m ratio and high tensile strength, are lost.
• there is also possible for there to be up to at most 10% residual austenite. Residual austenite is desirable, since in the metastable state it is plastically deformed, resulting in additional strength and elongation.
• the steel has a tensile strength of 600 - 1100 MPa. Consequently, the steel complies with the demand for high-strength steel from the automotive industry.
• the steel preferably has a value of the product of tensile strength times total elongation (A 80 ) of from 12,000 - 25,000 MPa.%. This ensures a high strength and, at the same time, a high deformability.
• the multiphase steel preferably has a yield point/tensile strength (R p0.2% /R m ) ratio of 0.4 to 0.6. This is favourable for the working properties during, for example, presswork. For the same reason, the steel preferably has a work hardening index of at least 0.165 and at most 0.30 between 10% elongation and uniform elongation.
• the work hardening index remains high and the R p0.2% /R m ratio, measured at a high rate of elongation of approximately 50 s -1 , remains lower than 0.7.
• a low R p0.2% /R m ratio of this nature (which corresponds to a high work hardening index) is favourable for steel which has to absorb energy in the event, for example, of a crash.
• a high work hardening index during rapid deformation also ensures that the deformation is spread across the material.
• the deformation energy of the multiphase steel measured at a rate of elongation of approximately 50 s -1 and at elongation values of approximately 20%, is higher than the deformation energy of (ultra)low carbon steel (with or without solution hardening) and precipitation-hardened steel, and the deformation energy density is at least 250 mJ/mm 3 . Consequently, the steel will absorb relatively large amounts of energy in the event of rapid and high deformation, which is desirable under crash conditions in cars.
• the BH 2 index which is defined by the yield point R p0.2% , measured after 2% prestretching and heating at 170°C for 20 minutes, minus the strength at 2% elongation (without prestretching and heating), is at least 55 MPa. This is considerably higher than the BH 2 of conventional "bake-hardening" steel grades (at most 30-40 MPa).
• a high BH 2 value means that the yield point increases relatively strongly after paint curing, which can provide a further weight saving.
• the surface of the steel is galvanized, either electrolytically galvanized or immersion galvanized. Consequently, the zinc protects the steel from corrosion, which is also desirable with a view to further coating (organic coating).
• a method for producing multiphase steel as described above in which a slab of steel of the desired composition is produced, after which the slab is successively hot-rolled, finish-rolled, coiled and cold-rolled, and is then continuously annealed at a maximum annealing temperature of 760° - 820°C and is cooled at a rate of at most 100°C/s.
• the hot-rolling parameters are selected in such a way that the steel in the hot-rolled state is sufficiently soft to be able to be cold-rolled.
• the said annealing temperature lies within the austenite/ferrite range, so that austenite is formed during annealing.
• austenite is converted into bainite and/or martensite during the cooling, depending on the composition, and the formation of pearlite is prevented.
• the cooling is carried out at a rate of 20-100°C/s on a continuous annealing line, after which a re-annealing treatment is carried out at a temperature of 250 - 470°C.
• a continuous annealing line it is possible to achieve this cooling rate, while the re-annealing temperature is favourable for the transformation of austenite to bainite and for the stabilization of austenite.
• the steel is preferably electrolytically galvanized after continuous annealing.
• the cooling is carried out at a rate of 5 to 50°C/s on an immersion galvanization line, during which process the surface of the steel is galvanized.
• an immersion galvanization line With a conventional immersion galvanization line, the cooling can only be carried out at a lower rate than with a continuous annealing line, but immersion galvanization is more desirable than electrolytic galvanization in view of process costs.
• Hot-rolling blocks with dimensions of 60*100*40 mm were heated to 1250°C for 30 minutes. Hot rolling was carried out in six steps, starting from an initial thickness of 40 mm until a final thickness of 4 mm was reached. The finish-rolling temperature was 940-970°C and the simulated coiling temperature was 690 ⁇ 10°C. After pickling, the hot-rolled material was cold-rolled, with a reduction of approximately 75%, to a final thickness of 1.1-1.2 mm.
• the cold-rolled material was cut into sheets of approximately 550 x 120 mm, which were annealed on a continuous annealing simulator.
• the annealing process conditions annealing temperature and time, heating rate and cooling rate, re-annealing treatment
• the annealing process conditions were selected to correspond to those of conventional industrial immersion galvanization lines and continuous annealing lines (cf. the examples below).
• Example 1 Multiphase steel produced on an immersion galvanization line with a low cooling rate
• the steel grades given in Table 1 were annealed using parameters of an immersion galvanization line, with annealing carried out at a top temperature of 770-830°C, followed by slow cooling to 620-720°C, then cooling, at a rate V Q of 7°C/s, to the temperature of the zinc bath.
• annealing carried out at a top temperature of 770-830°C, followed by slow cooling to 620-720°C, then cooling, at a rate V Q of 7°C/s, to the temperature of the zinc bath.
• V Q rate
• Schedule 1 Process parameters (temperature, time, heating/cooling rate) of the immersion galvanization simulation of Example 1
• V heat heating rate; T top : top temperature during annealing; t anneal : annealing time; t sc : period of slow cooling between top temperature and T Q ; T Q : temperature at which the rapid cooling begins; V Q : cooling rate between T Q and temperature of the zinc bath; T Zn : temperature of the zinc bath; t Zn : period for which the steel remains in the zinc bath; V cool : cooling rate after the zinc bath (to 250°C).
• the maximum annealing temperature T top of 805°C lies within the austenite/ferrite range for all steel grades. This ensures that cementite and pearlite are transformed into austenite during the annealing.
• Table 2 gives the mechanical properties for the annealed materials and compares them with those of the reference material. Mechanical properties parallel to the rolling direction for material which has been annealed in accordance with the temperature/time profile of an immersion galvanization line with a cooling rate of 7°C/s.
• R p yield stress at 0.2% elongation; R eH : upper yield point; R eL : lower yield point; A e : elastic elongation; R m : tensile strength; A u : uniform elongation; A 80 : total elongation (elongation at break measured on a 20 x 80 mm European standard tensile specimen); n: work hardening index between 10% elongation and the uniform elongation Code
• Table 3 shows the structural properties of the annealed materials. In this context, is should be pointed out that a relatively small ferrite grain was found for steel E, which is to be expected for this Nb-containing variant. Structural properties of the annealed materials.
• Type of second phase present and surface factions of pearlite + bainite + any carbides (P+B), martensite and/or residual austenite (M+RA), and residual austenite (RA).
• steel grades with a second phase comprising virtually exclusively martensite (steel codes B and C) have the highest tensile strength (> 670 MPa) and the lowest R p0.2% /R m ratio (0.4-0.5). These steel grades show a continuous yielding behaviour. The absence of a yield point elongation is typical for ferritic/martensitic multiphase steel.
• the product of total strength times total elongation of at least 12,000 MPa.% is not achieved for steel A, although it is to be expected that for steel A produced under industrial conditions the total elongation A 80 will be higher; this is because the structure for industrially produced steel is generally higher than for laboratory-produced steel.
• Example 1a Properties during ageing and paint curing
• the steel grades display a relatively high increase in the yield stress after a paint curing simulation following annealing as described in Example 1.
• a simulation of this nature comprises 2% prestretching of the annealed material followed by "paint curing" (annealing) for 20 minutes at 170°C. From this example too, it is clear that steel E does not have typical multiphase properties, which is the direct consequence of the ferritic/pearlitic structure of this steel.
• the BH 2 index is given in Table 4.
• the low tendency to natural ageing and the good bake-hardening properties are typical of in particular ferritic/martensitic multiphase steel grades which contain little carbon in solution.
• Example 1b Sensitivity to rate of elongation (IDEM)
• the mechanical properties were determined at different rates of elongation between 0.001 and 100 s -1 .
• the deformation energy absorbed as a function of the elongation can be calculated by determining the area below the tension elongation curves. In this way, the deformation energy absorbed was determined up to an elongation of 5% and 20% both with a low rate of elongation (0.001 s -1 ) and with a high rate of elongation (50 s -1 ).
• Fig. 1 plots the deformation energy against the static yield stress.
• the deformation energy (with respect to the static yield stress) of steel grades A to F is compared with that of other industrially produced (ultra)low carbon steel (IF, LC in Fig. 1), HSLA steel and steel grades hardened by means of solution hardening (P-IF and P-LC in Fig. 1), the deformation energy of steels A-F and for other ferritic/martensitic multiphase steel grades (DP and ref3 in Fig. 1) at 20% deformation proves to be higher to a greater or lesser extent than for the other steel grades. This applies in particular when high rates of elongation (50 s -1 ) are used.
• Fig. 1 Deformation energy density at a low rate of elongation of 0.001 s -1 (on the left) and a high rate of elongation of 50 s -1 (on the right). Top: 5% elongation; bottom: 20% elongation.
• the difference between steels A-F (and DP) and the other steel grades must be attributed to the difference in microstructure: the multiphase steel grades contain relatively large amounts of second phase structures, such as bainite and martensite, while the other steel grades consist only of ferrite or ferrite with a little pearlite.
• the deformation energy is lower than for ferritic/martensitic and ferritic/bainitic steel grades.
• the major advantage of multiphase steel compared to the other steel grades is that the work hardening index is relatively high and remains high even for high rates of elongation. This is expressed by the R p0.2% /R m ratio, which for the multiphase steel grades examined remains below 0.7 even at high rates of elongation, while the R p0.2% /R m ratio increases considerably at an increasing rate of elongation for the other steel grades.
• Example 2 Multiphase steel produced on an immersion galvanization line with a cooling rate of 20°C
• the second example relates to a simulation of an immersion galvanization line in which, unlike in Example 1, there is no slow cooling after annealing: the material is cooled directly from the maximum annealing temperature (820°C) to the temperature of the zinc bath (480-450°C).
• the cooling rate during cooling to the galvanization temperature is 20°C/s, i.e. considerably higher than the 7°C/s used in Example 1 (cf. Schedule 2 below).
• the mechanical properties are summarized in Table 5 for steels A to F.
• Schedule 2 Process parameters (temperature, time, heating/cooling rate) of the immersion galvanization simulation from Example 2.
• V heat heating rate; t anneal : top temperature during annealing; T Q : temperature at which the rapid cooling begins, in this case equal to T top ; V Q : cooling rate between T Q and the temperature of the zinc bath; T Zn : temperature of the zinc bath; t Zn : time for which the steel remains in the zinc bath; V cool : cooling rate to 250°C after the zinc bath.
• the higher cooling rate means that the mechanical properties are considerably improved compared to Table 1: in this case, multiphase properties are obtained for all the steel grades examined, with the exception of steel E (Table 5). This is clear from the relatively high tensile strength, the lower R p0.2% /R m ratio and the decrease in the elastic limit compared to Example 1 (compare Tables 2 and 5). These mechanical properties correspond to a second phase which is characterized by substantially martensite and bainite. The higher cooling rate ensures that less pearlite is formed during cooling.
• Example 3 Continuous annealing line with re-annealing treatment
• the last example simulated an existing continuous annealing line in which the steel is annealed for 30 seconds at a maximum temperature of 760-840°C, followed by slow cooling to a temperature T Q of 620-720°C. This slow cooling is followed by rapid cooling at approximately 50-100°C/s to the re-annealing temperature (250-470°C) and then a re-annealing treatment for 1-3 minutes.
• a re-annealing treatment of this type is favourable for transforming austenite into bainite.
• austenite is enriched with carbon, resulting in stabilization of austenite.
• a lower re-annealing temperature or with a shorter re-annealing period in relative terms more hard second phase (martensite and bainite) will be obtained, while at high re-annealing temperatures (470°C) or with longer re-annealing periods, pearlite may also be formed and cementite may be precipitated. Pearlite and cementite are undesirable, since they are unfavourable for the mechanical properties, as is also clear from the above examples.
• Schedule 3 Process parameters (temperature, time, heating/cooling rate) of the continuous annealing simulation of Example 3.
• the sensitivity of ref to the rate of elongation is also considerably improved (ref3 in Fig. 1). Therefore, the steel comprises predominantly ferrite and martensite, with a little bainite.
• the mechanical properties (R p0.2% /R m ratio, tensile strength) of the remaining steel grades A-F which are even better than those of the reference material (cf. Table 6), it can be expected that the sensitivity of steels A-F to the rate of elongation will be higher than that of steel ref, since, after all, they have the same basic composition.
• a possible drawback of a continuous annealing line is that the end product is not directly galvanized, as is the case on an immersion galvanization line after annealing.
• the steel can be electrolytically galvanized after the annealing.

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