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
  • C22C38/18—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
  • 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
  • C21D1/62—Quenching devices
  • C21D1/673—Quenching devices for die quenching
• • 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
  • C22C38/002—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
  • C22C38/02—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
  • 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
  • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
  • C23C10/28—Solid state diffusion of only metal elements or silicon into metallic material surfaces using solids, e.g. powders, pastes
• • 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
  • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
  • C23C10/60—After-treatment

Definitions

• the invention relates to a method for hot forming a part using a coated steel blank.
• Hot forming of coated steel blanks is much used in the automotive industry.
• the parts formed from these steels get high mechanical properties (such as a high strength) after the blanks are heated to a temperature above the Ac3 temperature, for instance a temperature between 850 °C and 950 °C, pressed in a hot forming press and quenched at a velocity above the critical quenching rate.
• these steels Before heating, these steels have a good formability and a tensile strength between 300 MPa and 500 MPa, for most grades.
• After the hot forming process these steels have a very high tensile strength, which can be above 1500 MPa, and nowadays up to 2000 MPa.
• the high tensile strength makes the hot formed products especially suitable for use in the body-in-white of automotive vehicles.
• a boron-alloyed steel is used, in particular steel grade 22MnB5.
• Hot forming is generally used for the direct hot forming process, but is also used in the indirect hot forming process.
• a general picture of hot forming (or hot stamping) is given by A. Naganathan and L. Penter, Chapter 7: Hot Stamping, in Sheet Metal Forming - Processes and Applications, (T. Altan and A. E. Tekkaya, editors), ASM International, 2012.
• At least one of these objects is reached using a method for hot forming a part, wherein the hot forming is performed starting with a metal coated steel blank, which is at least partially heated to a temperature between the Ac3-temperature of the steel and 1000° C in a furnace, after which the heated blank is transported to a hot forming press, and the heated blank is pressed in the hot forming press to form a part, wherein the steel has the following composition in weight%:
• V ⁇ 0.5
• the blank is cooled between the furnace and the hot forming press such that the blank has a temperature of at most 770° C and at least 450° C when placed in the hot forming press, and wherein the hot formed part is at least partially martensitic.
• the inventors In order to reduce the depth of micro-cracks that occur during the forming of zinc coated blanks, the inventors have found that is of significant importance to control the temperature at which the part is formed.
• the temperature of forming is preferably chosen such that any liquid components that exist in the coating solidify prior to forming.
• the inventors have found that it is beneficial to further reduce the forming temperature to at most 770°C and at least 450° C.
• the inventors found that it is complicated to obtain a fully martensitic substrate with the commonly used 22MnB5 steel.
• the inventors have come up with multiple alternative substrate compositions.
• the inventors claim that these compositions, which do not rely on the alloying with boron (B) for hardenability, are much less dependent on forming temperature to attain the required mechanical properties.
• the mechanical properties after forming and quenching remain high for a larger range of forming temperatures compared to 22MnB5.
• the invention will therefore provide more freedom to the hot former because he can select the forming temperature that fits best with his requirements regarding formability and microcrack depth without concerns with regard to mechanical properties.
• the inventors have found that the required formation of martensite will not be possible when a blank has a temperature below 450 °C when placed in the hot forming press.
• Mn and/or Cr can be used to create a steel type that has the same strength as a 22MnB5 steel, but having beneficial other properties.
• the inventors have found that it is beneficial to reduce the amount of other elements, apart from C, Mn and/or Cr, and Si as well.
• Non-metallic constituents reduce the homogeneity of the substrate and these inhomogeneities can lead to local stress concentrations and pre-mature failure of a mechanically loaded product, especially products with very high mechanical properties such as hot formed products with yield strengths >900MPa and tensile strengths >1400MPa.
• Typical non-metallic constituents in steel are TiN, BN, Fe 2 6(B,C) 6 , MnS, AIN, CaS, AI 2 O 3 , P, Fe 3 C etc.
• the invented steel composition is aimed to reduce the size and amount of all these non-metallic constituents by reducing the amount of B, Ti, S, Ca, Al, P and other required chemical elements.
• the nowadays commonly used 22MnB5 substrate composition contains 20 to 40 ppm boron (B) to improve the hardenability during hot forming operations.
• the steelmaker adds titanium (Ti) to the cast to prevent B to form boron nitride (BN).
• Ti is normally added in an over-stochiometric ratio to the nitrogen (N) to maximize the efficiency of the added amount of B.
• Boron is also known to form fine Fe 26 (B,C) 6 complex precipitates that can lead to local stress concentrations in the matrix. Therefore the inventors have removed the B from the steel composition to limit the presence of B based non-metallic constituents.
• the inventors added manganese (Mn) and/or chromium (Cr).
• Mn is a favourable metallic component because of its compatibility with the iron matrix. Moreover, the addition of Mn reduces the Aci and Ac 3 temperature of the steel substrate. This means that a lower furnace temperature can be utilized to austenitize the substrate prior to hot forming.
• the invented compositions demonstrate a reduction in Ac 3 of approximately 25°C compared to the commonly used 22MnB5. Reducing the furnace temperature is economically and environmentally favourable and also opens up new process opportunities for Zn, Zn alloy or Al and Al alloy coatings. For Zn alloy coatings it is commonly known that an increased furnace temperature reduces the corrosion performance of the hot formed product. For Al or Al alloy coatings it is known that high furnace temperatures reduce the weldability of the component. A steel composition that enables the use of lower furnace temperatures is therefore favourable over the commonly used 22MnB5.
• Mn does strengthen the substrate by solid solution strengthening. Furthermore, Mn additions also lower the M s temperature, which means that less (auto-)tempering will occur and therefore the substrate will have a higher martensite strength at room temperature.
• the M s temperature of the invented compositions is approximately 25°C lower compared to the commonly used 22MnB5. Due to both strengthening mechanisms, the inventors claim that they can reduce the amount of carbon (C) in steel substrates for hot forming and obtain a similar strength level as achieved with 22MnB5. Reducing the amount of C is favourable to prevent Fe3C formation during (auto-)tempering during the hot forming process step. Fe3C precipitates can introduce local inhomogeneities and stress concentrations during mechanically loading, leading to premature failure of the product. Furthermore, the spot-weldability of hot-formed products will improve due to the lower C content in the inventive steel substrate.
• Cr increases the hardenability, and it also lowers the M s temperature. Furthermore, Cr contributes to the strength of the substrate by solid solution strengthening.
• Si also delivers a solid solution strengthening contribution.
• Si retards the (auto)tempering because of its weak solubility in carbides.
• Sulphur (S) is a common element found in steel substrates. Steelmakers use various desulphurization methods to reduce the amount of S because it could lead to hot-shortness during continuous casting. S can also precipitate with manganese (Mn) to form soft MnS inclusions. During hot rolling and subsequent cold rolling, these inclusions are elongated and form relatively large inhomogeneities that could lead to premature failure, especially when loaded in the tangential direction. Calcium (Ca) can be added to spherodize the S containing inclusions and to minimize the amount of elongated inclusions. However, the presence of CaS inclusions will still lead to inhomogeneities in the matrix. Therefore, it is best to reduce S.
• Mo manganese
• Ca calcium
• Aluminium (Al) is normally added to steel in an over-stoichiometric ratio to oxygen (O) to prevent carbon monoxide (CO) formation during continuous casting by reducing the available amount of free O through formation of aluminium oxide AI 2 O 3 .
• the formed AI 2 O 3 normally forms a slag on top of the liquid steel, but can be entrapped in the solidifying steel during casting. During subsequent hot and cold-rolling, this inclusion will become segmented and forms non-metallic inclusions that lead to premature fracture upon mechanically loading the product.
• the over-stoichometric Al precipitates as aluminium nitrides (AIN) which also leads to local inhomogeneities in the steel matrix.
• the more limited amounts of the elements according to claim 2 or 3 are used. It will be clear that a more limited amount of the elements as specified in claims 2 and 3 provides a steel in which the number of non- metallic constituents in the steel substrate are further reduced. For instance, the over-stochiometric amount of Ti will form titanium nitrides, which are known as hard, non-deformable inclusions. By limiting the amount of Ti and N, the TiN inclusions are limited. Claim 3 shows that it is possible to use a steel for hot forming in which no boron is added, such that the boron in the steel will be only present as an unavoidable impurity.
• the amount of boron that will be present as an impurity will depend on the raw materials used in the ironmaking process and also depends on the steelmaking process, the inventors have found that the impurity level for boron that is nowadays obtained has a maximum of 0.0001 weight% or 1 ppm.
• the steel contains Mn + Cr > 2.5 weight%, preferably Mn + Cr > 2.6 weight%.
• Mn and Cr in combination should be high enough to provide the desired strength of the hot formed part.
• the reduced forming temperature of the blank in the press reduces the need for advanced cooling techniques in the press.
• the reduced forming temperature also lowers the thermal load on the forming tools and will therefore improve the tool life.
• the metal coated steel blank has a metal coating comprising a layer of zinc or a zinc based alloy.
• Zinc or zinc based alloy coated blanks are preferred in the automotive industry to improve in-service corrosion performance.
• the metal coating is an iron-zinc diffusion coating obtained by heat treating a zinc layer, the zinc layer comprising Al ⁇ 0.18 wt% and Fe ⁇ 15 wt%, the remainder being zinc and traces of other elements, the metal coating preferably having a thickness between 5 and 15 ⁇ per side, more preferably a thickness between 6 and 13 ⁇ per side. This zinc pre-coating provides good corrosion properties.
• the metal coating comprises 0.5 to 4 wt% Al and 0.5 to 3.2 wt% Mg, the remainder being zinc and traces of other elements, the metal coating preferably having a thickness between 5 and 15 ⁇ per side, more preferably a thickness between 6 and 13 ⁇ per side. This pre-coating provides even better corrosion properties.
• the metal coated steel blank is at least partially heated to a temperature between 850 and 950° C, more preferably between 850 and 900° C. At these temperatures the steel blanks are austenitised fast enough.
• the coated steel blank is heated in a furnace during a time period in which at least part of the blank reaches a desired austenitising temperature plus an additional 10 to 600 seconds. This time period is suitable to heat steel blanks having the usual thicknesses for automotive purposes.
• the blank has a temperature of at least 480 °C when placed in the hot forming press, preferably a temperature of at least 500 or 520 or 550 or 580 or 600 or 620 °C.
• the required formation of martensite will not be possible when a blank has a temperature below 450 °C when placed in the hot forming press. It is possible to use temperatures that are higher then 450 °C, such as 480 °C, but the inventors have found that the higher the temperature is at which a zinc coated steel blank is placed in the hot forming press, the bigger the depth of the micro-cracks will be in the deformed portions of the hot formed parts.
• the blank has a temperature of at most 750 °C when placed in the hot forming press, preferably a temperature of at most 725 or 700 or 680 or 650 °C.
• a maximum temperature for the steel blank that is placed in the hot forming press provides a guarantee that the micro-cracks in the hot formed part will not be too deep.
• the blank is transported from the furnace to the hot forming press in a time period of at most 20 seconds, preferably in a time period of at most 15 seconds, more preferably in a time period of at most 12 seconds, even more preferably in a time period of at most 10 seconds, most preferably in a time period of at most 8 seconds.
• Transportation times that are that short provide a zinc coated steel blank that is oxidised only to a limited extent.
• the blank is at least partially cooled between the furnace and the hot forming press by forced cooling, preferably with an average cooling velocity of at least 20 °C/s during the forced cooling, more preferably with an average cooling velocity of at least 30 °C/s during the forced cooling, even more preferably with an average cooling velocity of at least 40 °C/s during the forced cooling, still more preferably with an average cooling velocity of at least 50 °C/s during the forced cooling, most preferably with an average cooling velocity of at least 60 °C/s during the forced cooling.
• forced cooling is usually performed using forced air or metal cooling plates, but other ways of forced cooling are also possible.
• the invention also encompasses a product produced using the method as described above.
• This product has the mechanical properties provided by the hot forming method, as needed for automotive or other purposes.
• a product as described above is used in a motor vehicle.
• other properties besides mechanical properties are have to be taken into account, such as the weldability of the product.
• Figure 1 shows the depth of micro-cracks as a function of time and temperature of the blanks in the hot forming press.
• Figure 2 shows the depth of micro-cracks for different circumstances.
• Figure 3 shows the dimensions of pre-cooled area and location of cross- section (dashed line).
• Figure 4 shows the cracks in the outer vertical wall of a B-pillar.
• Figure 5 shows that the inventive steel ID0083 does not show ferrite formation just below the coating in contrast to comparative material ID0322.
• Figure 6 shows the depletion of boron just underneath the coating in regular 22MnB5 material
• Figure 7 shows the hardness of the hot formed parts using different substrates and temperatures.
• the inventors have demonstrated the effect of forming temperature on microcrack depth by performing various hot forming trials with zinc coated 22MnB5 and forced cooling of the heated blank prior to forming.
• a 1 .65mm gauge blank with 130 gr/m2 ZnFe coating was heated in a furnace to 900°C for a number of minutes, taken from the furnace and transported to a cooling station that cooled the blank to the required temperature with compressed air.
• the forced cooling the blank was transported to the press and formed into a top hat geometry with vertical walls, a draw depth of 50mm and a die radius of 2.1 mm.
• the draw gap and spacers used were equal to the blank thickness + 0.15mm.
• the forced cooling took place in 1 -3 seconds depending on the required forming temperature.
• the total transport time from furnace to press was 10 seconds.
• the process was monitored with pyrometers during the entire transportation.
• the maximum microcrack depth found in cross-section for each tested sample has been recorded and is plotted in Figure 1 .
• Table 2 Composition of used steel material Due to size limitations of the pre-cooling installation, only part of the B- pillar was pre-cooled as indicated in figure 3. This part is the most sensitive to micro-crack formation due to the high deformations.
• the samples were heated in a furnace at 900°C for a total of 3 minutes. After this, the samples were transported to the press within 10 seconds and formed into shape. Dependent on the desired pressing temperature, the part was pre-cooled in a pre-cooling station for 2-3 seconds during this transport.
• the inventors have casted multiple compositions into 25kg ingots. These ingots were subsequently hot rolled with a finish temperature of 900°C, a coiling temperature of 630°C and a hot rolled gauge of 4mm. Subsequently the strips were pickled and cold rolled to 1 .5mm gauge. Using dilatometry the Ac 3 temperature, the M s temperature and critical cooling rate of the compositions have been determined. For these tests, samples were heated in a Bahr 805A Dilatometer to a temperature of 900°C with an average heating rate of 15°C/s from room temperature up to 650°C and with an average heating rate of 3°C/s from 650-900°C.
• Table 3 compositions with Ms and Ac1 temperatures and CCR.
• test samples produced under laboratory condition show to contain 1 to 3 ppm B when no boron has been added to the steel. This variation in the amount of boron can be accounted for by a small contamination of the steelmaking equipment with previously produced boron containing steels.
• Commercial full-scale production of such types of steel to which no boron has been added contain an amount of less then 2 ppm boron; usually an amount of less then 1 ppm boron is measured.
• the inventors conducted high temperature deformation trials in a Gleeble ® 3800 to assess the influence of forming temperature on mechanical properties. These trials simulate the deformation of the steel that can take place during the hot forming process. 1 .5mm thick samples of different chemical composition were heated to 900°C with an average heating rate of 7.5°C/s and held at that temperature for 3 minutes. Subsequently the sample was cooled to the required forming temperature and strained to 20% elongation with a strain rate of 2s "1 . After the deformation was finished, the samples were quenched with a cooling rate of 40-70°C/s. The resulting hardness of the samples was measured and these results are plotted in Figure 7.

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• 2015
  • 2015-03-23 EP EP15719148.7A patent/EP3122486A1/en not_active Withdrawn
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