EP2627790B1 - Methode zum heissumformen eines stahl belches und das heiss umgeformte teil - Google Patents

Methode zum heissumformen eines stahl belches und das heiss umgeformte teil Download PDF

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Publication number
EP2627790B1
EP2627790B1 EP11772882.4A EP11772882A EP2627790B1 EP 2627790 B1 EP2627790 B1 EP 2627790B1 EP 11772882 A EP11772882 A EP 11772882A EP 2627790 B1 EP2627790 B1 EP 2627790B1
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Prior art keywords
steel
temperature
blank
article
hot
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English (en)
French (fr)
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EP2627790A1 (de
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Shangping Chen
Christiaan Theodorus Wilhelmus Lahaye
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Tata Steel Ijmuiden BV
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Tata Steel Ijmuiden BV
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    • 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
    • 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
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • 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
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • 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
    • C21D2211/008—Martensite
    • 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
    • C21D8/0226—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
    • C21D8/0221—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
    • 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

Definitions

  • the invention relates to a method of hot forming a steel blank into a hot formed article having very high mechanical properties, such as an automotive part, to an article thus hot formed and to a steel strip, sheet or blank for use in a thermomechanical treatment process, in particular for use in a hot forming method according to the invention.
  • AHSS advanced high strength steels
  • hot (press) forming (hot stamping or press hardening) has been developed.
  • hot (press) forming the strength of the steel article is improved through fast cooling after heating it to a temperature range where an austenitic phase exists and through the phase transformations of the austenite to harder phases such as bainite and martensite.
  • a typical steel used for the hot press forming is based on a composition system of 22MnB5, i.e. 0.22% of C, 1.2% of Mn, maximum 50 ppm of B, specified in EN10083.
  • Hot press forming of 22MnB5 steel can produce complex parts such as bumpers and pillars with ultrahigh strength, minimum springback, and reduced sheet thickness.
  • the tensile strength of boron steels is up to 1600 MPa, which is far above that of the highest-strength conventional cold stamping steels.
  • the ductility of total elongation A5 is less than 6%.
  • Advanced high strength steels with TRIP (transformation induced plasticity) added multiphase microstructure are a solution. These steels are characterized by a ferritic-bainitic microstructure with retained austenite, which undergoes martensitic transformation during forming of products, additionally contributing to the strengthening of a ready part.
  • US2008/0286603 A1 has disclosed a steel sheet that exhibits an ultra-high strength after hot press forming followed by rapid cooling, and an enhanced yield strength after painting.
  • the steel sheet has a composition (expressed in % by weight) comprising 0.1 to 0.5 C, 0.01 to 1.0 Si, 0.5 to 4.0 Mn, 0.1 or less P, 0.03 or less S, 0.1 soluble Al, 0.01 to 0.1 N, 0.3 or less W, and the balance being Fe and other inevitable impurities.
  • a hot-pressed part made of the steel sheet and a method for manufacturing the hot-pressed part are disclosed.
  • the hot-pressed part achieves a high increment in yield strength after heat treatment for painting while ensuring an ultra-high tensile strength.
  • the steel provided according to this US patent application has an ultimate tensile strength (UTS) of 1500 MPa with the total elongation being less than 8%.
  • a high-strength cold-rolled steel sheet having an uniform elongation including (in wt.%): 0.10 - 0.28 C; 1.0 - 2.0 Si; and 1.0 - 3.0 Mn.
  • the structure has the space factors: 30 - 65% of bainitic ferrite; 30 - 50% of polygonal ferrite; and 5 - 20% of residual austenite.
  • the desired microstructures were obtained according to the following process.
  • the steel sheet was heated for soaking up to a temperature which is equal to or higher than the A3 transformation point (A3), then cooled down temporarily to a temperature Tq expressed by the formula A3-250 (°C) ⁇ Tq ⁇ A3-20 (°C) at an average cooling rate of 1 - 10 °C/s, and then quenched from this temperature down into a bainitic transformation temperature range at an average cooling rate of 11 °C/s or faster.
  • US2008/0308194 A1 has disclosed a process for manufacturing a part made of steel having a multiphase microstructure.
  • the composition of steel comprises, in % by weight: C 0.01 - 0.50, Mn 0.5 - 3.0, Si 0.001 - 3.0, Al 0.005 - 3.0, Mo ⁇ 1.0, Cr ⁇ 1.5, P ⁇ 0.10, Ti ⁇ 0.20, V ⁇ 1.0 and optionally, one or more elements such as: Ni ⁇ 2.0, Cu ⁇ 2.0, S ⁇ 0.05 and Nb ⁇ 0.15, the balance of the composition being iron and inevitable impurities.
  • the steel sheet is heated to reach a soaking temperature Ts above Ac1 but below Ac3 and is held at this soaking temperature Ts for a soaking time ts so that the steel has an austenite content equal to or greater than 25 % by area; the steel blank is transferred into a forming tool for hot forming; and the part obtained is cooled within the tool at a cooling rate V to obtain a multiphase microstructure comprising ferrite, martensite or bainite and retained austenite, and ferrite is homogeneous in each of the regions.
  • JP2003193193 teaches steel sheet having a two phase microstructure comprising bainite/bainitic ferrite as a main phase and austenite as a second phase.
  • the manufacturing process comprises an isothermal heat treatment at 200 - 450 °C for 1 - 3000 seconds.
  • TBF TRIP-aided bainitic ferrite
  • Another object of the present invention is to provide a multiphase microstructured steel resulting in simultaneously improved strength and ductility.
  • the invention provides a method of hot forming a steel blank into an article, such as an automotive part, as defined in claim 1, wherein the method comprises the following steps:
  • the steel blank as a starting material for executing the method according to the invention can be obtained by standard casting processes, or indirectly from steel strip or sheet material.
  • the preheating temperature may be about 1100 - 1250 °C.
  • Traditional hot rolling passes and rolling conditions may be implemented to roll the steel ingot to a sheet product of about 3 to 5 mm in thickness.
  • the finish rolling temperature is about 850 - 880 °C.
  • the steel sheet is cooled at an average cooling rate of 5 - 50 °C/s to a coiling temperature between 550 and 700 °C. An excess of martensite or bainite is formed at a coiling temperature lower than 550 °C, resulting in an excessive increase in the strength of the hot-rolled steel sheet.
  • the excessively increased strength acts as a load during subsequent cold rolling for the production of a cold-rolled steel sheet causing problems, such as a poor appearance.
  • cold rolling the coiled hot-rolled sheet is pickled and cold-rolled.
  • Cold rolling can also be carried out under standard conditions at a reduction of about 30 - 75%.
  • the cold rolling reduction is preferably controlled to range from 40 to 70%.
  • the sheet has a thickness of about 1 to 2 mm according to product requirements.
  • the sheet may be decoiled and blanked to proper size suitable for hot forming according to the invention.
  • a cold rolled sheet is a preferred starting product in the method according to the invention.
  • the two-step cooling pattern and the interrupt cooling temperature are properly controlled.
  • the heated steel blank used in step d) has an austenite structure.
  • the method according to the invention also comprises the steps of
  • These austenitizing steps of the method for obtaining an austenite structure are performed by heating steel sheets, strips of blanks e.g. in a furnace or in a hot forming facility itself, to a temperature T1 above Ac3, more preferably in the range of Ac3 + 20 °C to Ac3 + 60 °C.
  • the heating rate is in the range of 10 - 25 °C/s.
  • This heating step is followed by soaking at a temperature above Ac3, preferably in the range of Ac3 + 20 °C to Ac3 + 60 °C, for a short period of time, preferably 1 - 5 min (Ac3 being the temperature at which transformation of ferrite into austenite is completed in hypoeutectoid steel, upon heating).
  • the austenitizing temperature (T1) and soaking time are selected to ensure complete dissolution of carbides. If applicable, then the steel blank thus soaked is transferred to the hot forming device such as a hot forming press. If applicable, the transferring time is controlled, usually to a short transfer time such as less than 10 seconds to prevent cooling of the blank below the starting temperature T2 of the next hot forming step.
  • the starting temperature T2 for hot forming and simultaneous cooling should be above the Ar3 point (typically in the range of 780-830 °C) to prevent any ferrite phase transformation, e.g. during transferring.
  • the steel blank is allowed to cool down further to room temperature at a further cooling rate V3 of 0.2 - 10 °C/s.
  • the higher cooling rates above 5 °C/s within said range are advantageous for achieving a high total elongation (At).
  • the range of 0.5 - 5 °C/s is preferred in view of high Ultimate Tensile Strength (UTS).
  • UTS Ultimate Tensile Strength
  • T3 is too high, generally above 550 °C, some pearlite structure may form during the following slow cooling step at cooling rate V3. If T3 is too low, below Ms point, bainitic structure may be obtained in an insufficient amount in the final microstructure. If V3 is too fast, too much martensite is obtained in the final microstructure, and it is impossible to ensure sufficient elongation. If V3 is too small, too less martensite is obtained in order to guarantee the high strength. Moreover, the combination of T3 and V3 is selected such that the article obtained comprises a multiphase microstructure comprising by volume fraction 55 - 90% of bainitic ferrite, 5 - 15% retained austenite and 5 - 30% martensite.
  • a relatively high UTS is achieved if the multiphase microstructure comprises 55 - 85% bainitic ferrite, 5 - 15% retained austenite and 10 - 30% martensite.
  • the steel, in particular TBF steel, in the article as manufactured is characterized by a multiphase structure comprising, more preferably consisting of, carbide free bainitic ferrite, carbon enriched retained austenite and a relatively small amount of martensite.
  • the mother-phase structure comprises fine plates of essentially carbide free bainitic ferrite.
  • the formation of such a microstructure is due to the fact that the precipitation of cementite during bainitic transformation is suppressed by alloying the steel with a sufficient amount of Si and/or Al, which have very low solubility in cementite and greatly retards growth of cementite from austenite. Carbon that is rejected from the bainitic ferrite enriches the residual austenite, thereby transforming to martensite during cooling after bainitic transformation or stabilizing it down to room temperature.
  • the advantages of this type of microstructure are manifold.
  • the bainitic ferrite in TBF steel is present in the form of plates with an ultra fine grain size, usually the length ranging up to about 15 micrometer at most and the thickness ranging up to 0.3 micrometer at most (typically ⁇ 10 ⁇ m long and ⁇ 0.2 ⁇ m thick). Furthermore the strength and ductility are improved simultaneously.
  • the high flow stresses are due to the small thickness of the bainitic laths and to the absence of proeutectoid polygonal ferrite contrary to existing TRIP (transformation induced plasticity) steels. It is assumed that the retained austenite offers an additional TRIP effect, and that it is useful for improving total elongation.
  • the constituents of the various phases in the final microstructure preferably comprise, by volume fraction, bainitic ferrite: 55 - 90%, retained austenite: 5 - 15% and martensite: 5 - 30%.
  • the multiphase microstructure according to the invention can be obtained by designing steel with a specific composition and by applying careful control of the thermomechanical treatment process as discussed above.
  • a preferred embodiment of the present invention relates to a method of hot forming a steel blank into an article, wherein the method comprises heating the steel blank to an austenitizing temperature T1, advantageously in the range of Ac3 + 20 °C and Ac3 + 60 °C, and soaking the steel blank in the austenitizing temperature range, such that carbides are completely dissolved; introducing the heated and soaked blank in a hot forming device, such as a hot press, while preventing ferrite phase transformation; hot forming the steel blank to form a shaped article starting at a starting temperature T2 above Ar3 and cooling the article thus being hot formed to an interrupt temperature T3 in the bainitic transformation range at a cooling rate such that formation of proeutectoid ferrite and pearlite is avoided; and further cooling the article from the interrupt temperature T3 to ambient temperature at a cooling rate such that the volume fraction of bainitic ferrite is 55 - 90%, of retained austenite 5 - 15%, and of martensite
  • the steel comprises the following elements (in wt. %):C 0.15 - 0.45, Si 0.6 - 2.5, Mn 1.0 - 3.0, Mo 0 - 0.5, Cr 0 - 1.0, P 0.001 - 0.05, S ⁇ 0.03, Ca ⁇ 0.003, Ti 0.1 or less and V 0.1 or less and the balance Fe and other inevitable impurities.
  • the steel may also contain Al less than 1.5%, partially replacing the same amount of Si, provided that the sum of Si and Al is in the range of 1.2 - 2.5%.
  • the amounts of Mn and Cr satisfy Mn + Cr ⁇ 3%, while also preferably, the amounts of C and Mo satisfy C + 1/3Mo ⁇ 0.45% for better control of the multiphase microstructure.
  • Content is represented as % by weight of the total composition.
  • C is an element for securing high strength, and for securing retained austenite.
  • C is added in an amount of 0.15% or more to form the desired multiphase microstructure to achieve ultra-high strength and ductility. Meanwhile, when the C content exceeds 0.45%, it is difficult to obtain the multiphase microstructure through the method according to the invention comprising a contiguous cooling subprocess. Moreover, there is a great possibility that the toughness and weldability of the steel sheet will be deteriorated.
  • C is preferably present in an amount of 0.2 - 0.4%, more preferably 0.2 - 0.35%.
  • Mn is one of the main elements in the steel composition according to the invention.
  • the functions of Mn include stabilizing the austenite and obtaining the desired multiphase microstructure. If Mn is less than 1.0%, the effects are not sufficiently marked. Whereas if the content exceeds 3%, a fully martensite structure is easily created. As a result, the steel is hardened and embrittled during press forming.
  • Mn is an element that is useful in lowering the Ac3 temperature. A higher Mn content is advantageous in lowering the temperature necessary for hot press forming.
  • the Mn content is limited to the range of 1.0 - 3.0%, preferably 1.5 - 2.5% and more preferably 1.6 - 2.5%, even more preferably 1.7 - 2.4%.
  • Si is an element effective for reinforcing a solid solution, and is useful for suppressing production of carbide due to decomposition of retained austenite.
  • carbides either transition carbide or cementite
  • Si suppresses the precipitation of brittle cementite during bainite formation, and hence results in an improvement in formability and toughness.
  • a minimum of 1.0% Si is needed to form carbide free bainite.
  • Si is also known to deteriorate galvanizability due to the formation of oxides adherent to the steel substrate. Therefore, the upper limit of Si is controlled below 2.5%.
  • the Si content is advantageously limited to the range of 1.2 - 2.5% and more preferably to the range of 1.4 - 2.0%.
  • Al is also an element useful for suppressing production of carbide due to decomposition of, particularly, retained austenite. Partial replacement of Si by a same amount of Al has been shown to effectively retard cementite formation without a detrimental effect on hot-dip coatability in TBF steels. However, a high concentration of Al leads to higher possibility of polygonal ferrite to be generated, which is less effective than fine plate ferrite in view of strength. A full substitution of Si by an equivalent amount of Al leads to a marked deterioration of the strength-ductility balance. If added, the amount of Al is limited to 1.5% or smaller.
  • P is an element useful for maintaining desired retained austenite, and its effect is exerted by an amount of P of 0.001 % or larger, more preferably 0.005% or larger, but P may deteriorate the workability of the steel when it is added in an excess amount. Accordingly, the P content is preferably limited to 0.05% or less.
  • S is a harmful element which forms sulfide based inclusions such as MnS, which initiates crack formation, and deteriorates processibility. Therefore, it is desirable to reduce the amount of S as much as possible. Accordingly, S is controlled to 0.03% or smaller.
  • Mo and Cr serve to improve the hardenability of the steel and facilitate the formation of bainite ferrite. At the same time, they are elements having similar effectiveness useful for stabilizing retained austenite. Therefore, Mo and Cr are very effective for process control. It is advantageous that each of them is contained at 0.05% or larger. However, when each of them is added excessively, the effect is saturated and a higher addition is not economical. Therefore, the amount of Mo is 0.5% or smaller, and the amount of Cr is 1% or smaller.
  • Ti and V have the effect of forming strengthening precipitates and refining microstructure.
  • the amount of each of them is 0.1% or smaller, preferably 0.05% or smaller.
  • Ca is an element effective for controlling a form of sulfide in the steel, and improving processibility. It is recommended that Ca is contained at 0.0003% or more. However, when it is added excessively, the effect is saturated. Therefore, the preferred amount is 0.0003 - 0.003%.
  • the heat treatment described above may be carried out by heating and cooling in a continuous annealing facility (CAL), hot press forming facility, salt bath or the like.
  • CAL continuous annealing facility
  • hot press forming facility salt bath or the like.
  • the first combined deformation/cooling step d) is performed in a hot forming facility.
  • the second cooling step e) is performed in air or in stock outside the hot forming facility.
  • the steel of the formed article has an ultimate tensile strength (UTS) of at least 1400 MPa, advantageously at least 1500 MPa, more preferably at least 1600 MPa, and most preferably at least 1700 MPa.
  • UTS ultimate tensile strength
  • the steel of the formed article has a total elongation of at least 8%, preferably at least 10%, more preferably at least 12%, and most preferably at least 14%.
  • the heat treatment processes can be simply performed by applying the hot (press) forming in a standard hot forming facility with only modification of the simultaneous cooling process, including cooling interrupt temperature and velocities.
  • the heat blank is inserted into the die sets of a hot press, in which the blank is shaped and cooled.
  • the transfer time e.g. 5 to 10 s
  • the cooling rate V2 of the steel part in the forming tool depends on the deformation and on the quality of the contact between the tool and the steel blank.
  • the forming tools may be cooled for example by using circulation of a liquid to ensure that the cooling rate is high enough (>25 °C/s) during quenching in the press mould.
  • the interrupt cooling temperature can be controlled by the time for separation of the pressing tools.
  • the formed article is then removed from the mould and cooled to room temperature in air.
  • the formed articles, e.g. sheets, may also be stacked and then cooled to ambient temperature in air.
  • the present invention provides a steel article, preferably formed according to the method of the present invention, wherein the steel has a microstructure comprising by volume fraction:
  • the steel has an Ultimate Tensile Strength of at least 1400 MPa, advantageously at least 1500 MPa, preferably at least 1600 MPa, more preferably at least 1700 MPa and/or a total elongation of at least 8%, preferably at least 10%, more preferably at least 12%, most preferably at least 14%.
  • compositions of the steel strip, sheet or blank according to the third aspect of the invention comprises, the alloying elements are present in, expressed in weight %,: C: 0.2 - 0.4, preferably 0.2 - 0.35 and/or Si: 0.8 - 2.0, preferably 1.2 - 1.8 and/or Mn: 1.5 - 2.5, preferably 1.7 - 2.4 and/or Mo: 0.05 - 0.5 and/or Cr: 0.05 - 1:0 and/or P: 0.005 - 0.05 and/or Ca: 00003 - 0.003.
  • the articles obtained from the strip, sheet or blank exhibit a high tensile strength by rapid cooling after heat treatment and achieve a high increment in yield strength after heat treatment, especially for painting. Based on these advantages, excellent impact properties of the steel article according to the present invention are attained. In addition, the steel article according to the present invention advantageously exhibits good adhesion to a coating layer. Furthermore, other advantages of the steel article according to the present invention are good surface appearance and superior corrosion resistance after painting.
  • FIG. 1 A schematic representation of a practical embodiment of the method according to the invention is shown in Fig. 1 , showing a temperature vs. time plot.
  • a steel blank is heated at a heating rate of 15 °C to the austenitizing temperature T1 in the range of Ac3 + 20 °C to Ac3 + 60 °C and soaked at that temperature during a soaking time t1.
  • the thus heated and soaked blank is transferred from the furnace to the hot forming facility, during which cooling by air occurs to some extent. Care is taken that the temperature T2 is not decreased below Ar3 before hot press forming of the blank.
  • After hot press forming the blank thus formed is cooled down to the interrupt temperature T3 at a rate of more than 25 °C. Then air cooling is carried out.
  • the steel plates contain a microstructure consisting of ferrite and pearlite and have a tensile strength less than 700 MPa.
  • the plate was then cold rolled from 4 to 1 mm sheet. Table 1. Chemical compositions (in wt.
  • the critical temperatures such as Ac3 and Ms were determined by applying standard dilatometric analysis to facilitate the determination of the temperatures in the process according to the invention.
  • the resulting cold rolled sheets were subjected to a heat treatment by using a CASIM simulator. Specifically, the steel sheets were heated to 870 to 920°C at a rate of 15 °C/s, holding the sheets at this temperature for 2 min, then cooling at a rate of 50 °C/s to an interrupt temperature between 400 and 550 °C, thereafter, cooling at a rate of 0.2 to 10 °C/s to simulate different air cooling conditions.
  • Tensile tests were conducted by applying JIS 5 tensile test specimen to measure tensile strength and elongation of the specimens with the required microstructures. The volume fraction of bainite and/or martensite in the microstructures was estimated by using metallographic characterisation in combination with dilatametric analysis. The volume fraction of the retained austenite was determined by using TEM. Other measurements are standard.
  • the tensile test results and the invented alloys with the required the microstructure constituents are given in Table 2.
  • the experimental results indicate that the microstructures and the properties are essentially independent from the austenitizing temperature T1 as long as the T1 is above Ac3, advantageously between Ac3 + 20 °C and Ac3 + 60 °C. It is also proven that the transferring temperature T2 and the cooling rate V2 do not significantly affect the microstructures and properties as well provided that T2 is higher than Ar3 and V2 is equal to or more than 25 °C/s and preferably less than 100 °C/s. However, the microstructures and the properties are strongly dependent on the interrupt temperature T3 and the cooling rate V3. The C content and alloying element contents have a large effect on the selection of T3 and V3.
  • the conditions for the required multiphase microstructures of the alloys according to the invention can be obtained by careful adjustment of T3 and V3.
  • the cooling rate V3 can be controlled in a lower range 0.25 to 2 °C/s; for alloys containing lower C or lower Mo, the cooling rate V3 can be controlled in a higher range 2 to 10° C/s.
  • higher cooling rate will result in less bainitic ferrite, but relatively more martensite. Therefore higher strength will be obtained.
  • Lower cooling rate will result in more bainitic ferrite, but relatively less martensite, then higher elongation will be achieved.

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Claims (15)

  1. Verfahren zum Warmumformen eines Stahlrohlings zu einem Artikel, wobei das Verfahren die folgenden Schritte umfasst:
    d) Abkühlen eines erhitzten Stahlrohlings, um während dem Warmumformen einen Artikel zu bilden, beginnend bei einer Ausgangstemperatur T2 oberhalb von Ar3 bis auf eine Unterbrechungstemperatur T3 im Bereich von 400 - 550 °C mit einer Kühlrate V2 von mindestens 25 °C/s,
    wobei der Rohling die folgende Zusammensetzung in Gew.-% aufweist: C: 0,15 - 0,45 Si: 0,6 - 2,5 Mn: 1,0 - 3,0 Al: 0 - 1,5 Mo: 0 - 0,5 Cr: 0 - 1,0 P: 0,001 - 0,05 S: < 0,03 Ca: < 0,003 Ti: < 0,1 V: < 0,1
    wobei es sich bei dem Rest um Fe und unvermeidliche Unreinheiten handelt, wobei Si + Al = 1,2 - 2,5 %;
    e) ohne den Rohling über einen vorbestimmten Zeitraum auf einer Temperatur von T3 zu halten, unmittelbares Abkühlen des Artikels von der Unterbrechungstemperatur T3 auf Umgebungstemperatur mit einer Kühlrate V3 von 0,2 - 10 °C/s,
    wobei die Unterbrechungstemperatur T3 und die Kühlrate V3 unter Verwendung der Relation ausgewählt werden, dass je höher T3 ist, desto niedriger ist V3, so dass der auf diese Weise erhaltene Artikel eine Mehrhasen-Mikrostruktur aufweist, welche nach Volumenanteilen folgendes umfasst:
    55 - 90 % bainitischen Ferrit;
    5 - 15 % Restaustenit;
    5 - 30 % Martensit.
  2. Verfahren nach Anspruch 1, wobei der Rohling aus einem Stahlstreifen oder Blech hergestellt wird.
  3. Verfahren nach Anspruch 1 oder 2, wobei Mn + Cr ≤ 3 %, und wobei C + 1/3 Mo ≤ 0,45 %.
  4. Verfahren nach einem der vorstehenden Ansprüche, wobei die Zusammensetzung in Gew.-% folgendes umfasst: C: 0,2 - 0,4 und/oder Si: 0,8 - 2,0 und/oder Mn: 1,5 - 2,5 und/oder Mo: 0,05 - 0,5 und/oder Cr: 0,05 - 1,0 und/oder P: 0,005 - 0,05 und/oder Ca: 0,0003 - 0,003.
  5. Verfahren nach einem der Ansprüche 1 bis 4, wobei die Zusammensetzung in Gew.-% folgendes umfasst: C: 0,2 - 0,35 und/oder Si: 1,2 - 1,8 und/oder Mn: 1,7 - 2,4.
  6. Verfahren nach einem der vorstehenden Ansprüche, wobei der bainitische Ferrit im Wesentlichen karbidfrei ist, und wobei der Restaustenit mit Kohlenstoff angereichert ist.
  7. Verfahren nach einem der vorstehenden Ansprüche, wobei Körner des bainitischen Ferrits eine Länge von höchstens 15 µm und eine Dicke von höchstens 0,3 um aufweisen.
  8. Verfahren nach einem der vorstehenden Ansprüche, wobei dieses ferner vor dem Schritt des Warmumformens d) die folgenden Schritte umfasst:
    a) Erhitzen des Stahlrohlings auf eine Austenisierungstemperatur T1 von über Ac3, vorzugsweise im Bereich von Ac3 + 20 °C - Ac3 + 60 °C, vorzugsweise mit einer Erhitzungsrate von 10 - 25 °C/s;
    b) Einweichen des Stahlrohlings innerhalb des Bereichs, vorzugsweise während einer Einweichzeit von 1 - 5 Minuten;
    c) optionales Übertragen des erhitzten und eingeweichten Stahlrohlings an eine Warmumformeinrichtung.
  9. Verfahren nach Anspruch 8, wobei Schritt a) in einer Durchlaufglüheinrichtung oder in einer Warmumformeinrichtung oder einem Salzbad oder einer äquivalenten Einrichtung ausgeführt wird.
  10. Verfahren nach einem der vorstehenden Ansprüche, wobei Schritt d) in einer Warmumformeinrichtung ausgeführt wird.
  11. Verfahren nach einem der vorstehenden Ansprüche, wobei Schritt e) außerhalb einer Warmumformeinrichtung in Luft ausgeführt wird.
  12. Stahlartikel, der gemäß dem Verfahren nach einem der vorstehenden Ansprüche warmumgeformt worden ist, wobei der Stahl eine Mikrostruktur aufweist, die nach Volumenanteilen folgendes umfasst:
    55 - 90 % bainitischen Ferrit;
    5 - 15 % Restaustenit;
    5 - 30 % Martensit, und
    wobei der Stahlartikel die folgende Zusammensetzung in Gew.-% aufweist: C: 0,15 - 0,45 Si: 0,6 - 2,5 Mn: 1,0 - 3,0 Al: 0 - 1,5 Mo: 0 - 0,5 Cr: 0 - 1,0 P: 0,001 - 0,05 S: < 0,03 Ca: < 0,003 Ti: < 0,1 V: < 0,1
    wobei es sich bei dem Rest um Fe und unvermeidliche Unreinheiten handelt, wobei Si + Al = 1,2 - 2,5 %.
  13. Stahlartikel nach Anspruch 12, wobei der Stahl eine Bruchfestigkeit von mindestens 1400 MPa, in vorteilhafter Weise von mindestens 1500 MPa, vorzugsweise von mindestens 1600 MPa, darüber hinaus bevorzugt von mindestens 1700 MPa aufweist und/oder eine Gesamtdehnung von mindestens 8 %, vorzugsweise von mindestens 10 %, darüber hinaus bevorzugt von mindestens 12 % und am meisten bevorzugt von mindestens 14 %.
  14. Stahlartikel nach Anspruch 12 oder 13, wobei Mn + Cr ≤ 3 %, und wobei C + 1/3 Mo ≤ 0,45 %.
  15. Stahlartikel nach einem der Ansprüche 12 bis 14, wobei ausgedrückt in Gew.-% folgendes gilt: C: 0,20 - 0,4, vorzugsweise 0,2 - 0,35 und/oder Si: 0,8 - 2,0, vorzugsweise 1,2 - 1,8 und/oder Mn: 1,5 - 2,5, vorzugsweise 1,7 - 2,4 und/oder Mo: 0,05 - 0,5 und/oder Cr: 0,05 - 1,0 und/oder P: 0,005 - 0,05 und/oder Ca: 0,0003 - 0,003.
EP11772882.4A 2010-10-12 2011-10-10 Methode zum heissumformen eines stahl belches und das heiss umgeformte teil Revoked EP2627790B1 (de)

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