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
  • C22C38/06—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
• • 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 steel with a pearlite-free, predominantly ferritic structure and a method for its production.
• Dual phase steels for short "DP steels” are characterized by a strong hardening especially with small plastic strains and a low yield strength ratio. Thus, even small degrees of deformation lead to higher component strength, which can be increased further after pre-forming due to the high bake hardening potential.
• “Bake hardening” is understood to mean artificial aging as a result of stove enamelling, which leads to a further increase in component strength.
• DP steels therefore make a contribution to weight-optimized construction, particularly from the point of view of energy saving and passive safety.
• the processing properties of DP steels can be assessed as very favorable due to the low yield strength ratio and high work hardening capacity.
• the forming process is positively influenced by a lower springback compared to other high-strength steels.
• the ductility loss that always occurs with conventional high-strength steels Conventional softer steels, which manifests itself, for example, in a decrease in the uniform elongation, is significantly lower with DP steel.
• the structure of conventional DP steels consists of 70 to 90 vol .-% ferrite, the rest of martensite.
• the hard martensite is embedded in the island in the soft ferritic matrix.
• other carbon-rich transformation structures bainite
• Smaller quantities, particularly when silicon is added to the alloy, which inhibits carbide formation, may also contain thermodynamically metastable residual austenite. Metastable residual austenite improves the forming properties during cold forming.
• DP steels can be produced both by hot rolling with a special rolling strategy and by cold rolling with subsequent heat treatment.
• hot strip DP steel analyzes are necessary, the conversion behavior of which is characterized by strong pre-eutectoid ferrite formation and pearlite formation which has been postponed for long periods.
• alloy compositions are sensible in which a high carbon activity and a shift of the GOS line in the iron-carbon diagram to the right, ie to higher carbon contents, is observed in order to favor the carbon enrichment of the austenite during annealing in the two-phase ferrite-austenite region .
• the annealing time required for segregation is reduced with increasing carbon activity.
• the critical cooling rate decreases as the carbon content of the austenite increases. So there can be fewer after annealing in the two-phase area Cooling rates are used to set a predominantly ferritic-martensitic structure.
• the formation of ferrite after hot forming can be promoted by silicon.
• With manganese, pearlite formation can be suppressed both after hot forming and during continuous annealing.
• red scale is formed, which is associated with the risk of scale rolling.
• surface inhomogeneities may also be present on the strip surface after pickling.
• the red scale which cannot be removed even with very high injection pressures in the hot strip mill, also leads to a reduction in the pickling speed. This is associated with a significant drop in productivity.
• DP-steel containing silicon cannot be galvanized in continuous hot-dip galvanizing lines because the zinc only very poorly wets the steel. For this reason, it is also not possible to manufacture silicon-containing DP steel in the galvannealed version.
• the temperature cycle of a galvannealing hot-dip coating would in principle offer the possibility for Si-alloyed DP steel to produce metastable residual austenite, which further improves the cold formability.
• DP cold strip of the galvannealed surface finish by means of a continuous hot-dip galvanizing system is also possible with other alloy concepts known to date for DP cold strip, including the concept with Si, not reliable because the pearlite formation under the process conditions of most z. Z. existing systems is not sufficiently suppressed.
• the formation of pearlite is associated with the loss of the dual-phase steel characteristic.
• DP steels with a predominant ferrite content contain 0.03 to 0.12% C, up to 0.8% Si and 0.8 to 1.7% Mn (DE 29 24 340 C2) or 0.02 to 0.2% C, 0.05 to 2.0% Si, 0.5 to 2% Mn, 0.3 to 1.5% Cr and 1% Cu, Ni and Mo (EP 0 072 867 B1). Both steels only contain aluminum in amounts that result from calming down with aluminum. However, DP steels of this composition are not suitable for hot-dip galvanizing for the reason mentioned above.
• DP steels that can be represented as cold strip contain 0.03 to 0.12% C, at most 0.8% Si and 0.8 to 1.7% Mn (DE 29 24 340 C2).
• Such DP steels are generally very sensitive to changes in the annealing parameters, mainly to changes in the cooling rate in the rapid cooling part. As the cooling rate decreases, the mechanical properties, in particular the yield ratio, often deteriorate.
• a steel with 0.08 to 0.20% C, 1.5 to 3.5% Mn, 0.1 to 0.5% Cr and 0.010 to 0.1% Nb also allows this Representation of a DP steel as a cold strip, but makes welding more difficult due to the increased carbon equivalent.
• the structure After cold rolling with subsequent heat treatment in a hot-dip galvanizing plant or in a continuous annealing furnace, the structure consists of a ferritic matrix in which island-like martensite is embedded. Depending on the manufacturing conditions, proportions of intermediate and residual austenite can also be set.
• Aluminum ensures extensive ferrite formation during annealing between the conversion temperatures Ac 1 and Ac 3 without loss of productivity in the claimed content range.
• the formation of perlite is postponed at significantly longer times to such an extent that it is sufficiently suppressed for cooling rates that are easy to implement on an industrial scale.
• the galvannealing process can be carried out under customary conditions, it being possible to improve the phase characteristics by adjusting residual austenite.
• Manganese also delays pearlite formation.
• the solid solution strengthening effect increases the strength of the steel.
• treatment of the melt with calcium makes sense in order to convert stretched manganese sulfides and other sulfides into a globular form that is less detrimental to forming.
• the carbon content should be at least 0.05%.
• the steel should not contain more than 0.3% C.
• Titanium up to 0.05% leads to an increase in strength through grain refinement and precipitation hardening and improves cold formability.
• Chromium increases the strength and improves the temper resistance of the martensite and thus enables the bake hardening potential to be fully exploited. However, more than 0.8% Cr is not required and would only increase the price.
• Molybdenum up to 0.5% lowers the critical cooling rate and thus reduces the risk of third-party residual stresses, since the hot-dip galvanizing process can be carried out with a lower cooling capacity. This offers greater security against band ripple due to third-party residual stresses.
• Nickel serves to increase the strength through solidification and to lower the Transition temperatures and the cooling rates required for diffusion-free conversion. Nickel also has an austenite-stabilizing effect in an amount of up to 0.5%.
• niobium increases the strength through grain refinement and precipitation hardening in quantities of up to 0.05% and improves the hardenability.
• Phosphorus up to 0.08% can be added to increase the strength by solid-solution strengthening.
• the steel according to the invention is particularly insensitive to changes in the annealing parameters.
• a steel of this composition can be very reliable, i. H. regardless of fluctuations in production conditions. It can also be coated very well, especially galvanized. Red scale does not form in the preliminary hot strip.
• the structure After cold rolling with a degree of cold rolling ⁇ ⁇ 40%, the structure recrystallizes between 740 and 850 ° C.
• the two-phase ferrite-austenite area is subsequently cooled to the zinc bath temperature.
• the cooling rates are between 10 and 50 K / s.
• the zinc bath temperatures are between 450 and 485 ° C.
• Slow cooling down to temperatures of 650 ° C before rapid cooling is also permitted and offers the possibility of Controlling the enrichment of austenite with carbon. Even with this slow cooling there is no risk of pearlite formation because aluminum shifts pearlite formation at significantly longer times.
• the steel After galvanizing, the steel is immediately cooled in a hot-dip galvanizing line, or when a cold strip with a zinc-iron alloy layer is produced in the "galvannealed" version, the steel is reheated to temperatures between 480 and 580 ° C.
• the new alloy concept allows the production of a high-strength, good cold-formable, surface-finished, i.e. coated, weldable cold strip in the "galvanized” versions and a higher-strength, good cold-formable, surface-finished cold-rolled strip in the "galvannealed” version with improved spot weldability, which is particularly required in automated welding lines is.
• a special feature of the steel according to the invention is its pronounced insensitivity to fluctuations in the annealing parameters, which leads to a high degree of production reliability.
• the steel was heated to 750 ° C at 6 K / s and then further heated to 830 ° C at 1.2 K / s. From the two-phase area, there was first a slow cooling at 4 K / s to 680 ° C, followed by an accelerated cooling at 20 K / s to 470 ° C. After passing through the 470 ° C. warm zinc strip, the mixture was cooled to room temperature at 10 K / s. Steel A was immediately rolled in line with a skin pass level of 0.8%.
• this dual-phase steel has a ferritic matrix in which martensite islands are evenly embedded.
• the martensite is located both on the triple points of the ferrite grains and along the ferrite grain boundaries.
• the ferrite grain size is around 60 ⁇ m 2 . Bainite or other structural components are not present.
• Galvanized dual-phase steel is quasi-isotropic.
• the planar isotropy ⁇ r is - 0.02.
• the cold strip was heated to 750 ° C at 6 K / s and then further heated to 830 ° C at 1.2 K / s. From the two-phase area, there was first a slow cooling at 4 K / s to 720 ° C, followed by an accelerated cooling at 20 K / s to 470 ° C. After passing through the 470 ° C warm zinc bath, induction heating at 12 K / s followed by the galvannealing temperature of 520 ° C followed by cooling at 10 K / s to room temperature. The galvanneal cold strip from steel B was immediately cold rolled in line with a skin pass of 1.1%.
• the galvanneal cold strip After the annealing treatment, the galvanneal cold strip has a pearlite-free ferritic matrix with a ferrite grain size of around 60 ⁇ m 2 , in which martensite islands are evenly embedded.
• the martensite islands concentrate on the triple points of the ferrite grains, but also occur along the ferrite grain boundaries, accompanied by traces of bainite.
• Another steel C according to the invention alloyed with 0.21% C, 1.50% Mn, 1.03% Al was melted in an induction furnace.
• the cast block was forged and hot-rolled after mechanical processing. The last rolling pass took place between 920 and 950 ° C.
• a cold strip sample was then conductively heated to 740 ° C. at 7 K / s in the ambient atmosphere and then heated further to 820 ° C at 1.2 K / s. From the two-phase area, there was then an accelerated cooling at 35 K / s to 550 ° C., followed by a milder cooling at 4 K / s to a temperature of 450 ° C., corresponding to a customary zinc bath temperature. The sample was then heated to a temperature of 500 ° C. at 7 K / s, kept at 500 ° C. for 5 s, then cooled to 350 ° C. at 35 K / s and finally cooled to room temperature at 10 K / s. The cycle corresponds to a common galvannealing process.
• the sample made of steel C according to the invention which was heat-treated like galvanneal cold strip, has a pearlite-free ferritic matrix after the annealing treatment, in which martensite islands and bainite areas with 8.5 vol.% Residual austenite are uniformly embedded. These embedded phases are found along the grain boundaries, concentrating on the triple points of the ferrite grains.
• the ferrite grain size is approximately 70 ⁇ m 2 .
• This steel according to the invention has the mechanical properties given in Table 2.
• a steel D according to the invention alloyed with 0.21% C, 1.49% Mn, 1.99% Al, was melted in an induction furnace.
• the cast block was forged and hot-rolled after mechanical processing. The last rolling pass took place between 920 and 950 ° C.
• a cold strip sample was then conductively heated to 760 ° C. at 7 K / s in the ambient atmosphere and then further heated to 840 ° C. at 1.2 K / s. From the two-phase area there was then an accelerated cooling at 35 K / s to 550 ° C, followed by a milder cooling at 4 K / s to a temperature of 450 ° C, corresponding to a typical zinc bath temperature.
• the sample was then heated to a temperature of 500 ° C. at 7 K / s, held at 500 ° C. for 5 s, then cooled to 350 ° C. at 35 K / s and finally cooled to 10 K / s to room temperature. This cycle corresponds to a common galvannealing process.
• this steel D according to the invention has a pearlite-free ferritic matrix, in which martensite islands and bainite areas with 11% by volume of austenite are uniformly embedded. These embedded phases are found along the grain boundaries, concentrating on the triple points of the ferrite grains.
• the ferrite grain size is approximately 80 ⁇ m 2 .
• Samples of the coated cold strip produced in this way have mechanical properties as indicated in Table 2. Plate 1 Chemical composition (in mass -%) stole C. Mn Si Al Cr P S A, B 0.073 1.44 0.052 1.27 0.35 0.02 0.001 V 0.092 1.24 0.035 0.04 0.47 0.014 0.014 C.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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• 1996
  • 1996-03-19 DE DE19610675A patent/DE19610675C1/de not_active Expired - Lifetime
  • 1996-06-18 EP EP96109744A patent/EP0796928A1/fr not_active Withdrawn

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