US20130192726A1 - Method of hot forming a steel blank and the hot formed part - Google Patents

Method of hot forming a steel blank and the hot formed part Download PDF

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US20130192726A1
US20130192726A1 US13/819,281 US201113819281A US2013192726A1 US 20130192726 A1 US20130192726 A1 US 20130192726A1 US 201113819281 A US201113819281 A US 201113819281A US 2013192726 A1 US2013192726 A1 US 2013192726A1
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steel
temperature
blank
article
hot forming
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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 is 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 is 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 ⁇ 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 is 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 is 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 TBF-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 T 1 above Ac3, more preferably in the is 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 (T 1 ) 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 T 2 of the next hot forming step.
  • the starting temperature T 2 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 V 3 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
  • T 3 is too high, generally above 550° C., some pearlite structure may form during the following slow cooling step at cooling rate V 3 . If T 3 is too low, below Ms point, bainitic structure may be obtained in an insufficient amount in the final microstructure. If V 3 is too fast, too much martensite is obtained in the final microstructure, and it is impossible to ensure sufficient elongation. If V 3 is too small, too less martensite is obtained in order to guarantee the high strength. Moreover, the combination of T 3 and V 3 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. Generally the higher the interrupt temperature T 3 , the lower V 3 has to be in order to get this multiphase microstructure.
  • 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 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 T 1 , advantageously in the range of Ac3+20° C.
  • 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 ⁇ 3 Mo ⁇ 0.45% for better control of the multiphase microstructure.
  • 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 is 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 V 2 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%.
  • the present invention provides a steel strip, sheet or strip for use in a thermomechanical treatment, in particular a hot forming method according to the invention described above, having a composition, in weight %:
  • 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 %:
  • 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 is 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 T 1 in the range of Ac3+20° C. to Ac3+60° C. and soaked at that temperature during a soaking time t 1 .
  • 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 T 2 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 T 3 at a rate of more than 25° C. Then air cooling is carried out.
  • Steels having compositions A-F as specified in Table 1 were cast to ingots of about 25 kg (100 ⁇ 110 ⁇ 330 mm). The ingots were reheated and then roughly hot rolled to slabs having a thickness of 40 mm. Then finish hot rolling was applied with the following process parameters: preheating at 1200° C. for 30 min; multi-pass rolling to thickness 4 mm (40-27-18-12-8-6-4 mm); and finishing rolling temperature 860 ⁇ 20° C. Controlled cooling at a rate 30° C./s to 600° C. in a run-out-table was carried out in order to simulate the commercially used coiling process. After the coiling process 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.
  • 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 conditions for the required multiphase microstructures of the alloys according to the invention can be obtained by careful adjustment of T 3 and V 3 .
  • the cooling rate V 3 can be controlled in a lower range 0.25 to 2° C./s; for alloys containing lower C or lower Mo, the cooling rate V 3 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.
  • the higher T 3 temperature is, the relatively more bainitic ferrite will be in the final microstructure for a given cooling rate, resulting in high strength but lower elongation.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Steel (AREA)
US13/819,281 2010-10-12 2011-10-10 Method of hot forming a steel blank and the hot formed part Abandoned US20130192726A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
EP10013537 2010-10-12
EP10013537.5 2010-10-12
PCT/EP2011/005053 WO2012048841A1 (en) 2010-10-12 2011-10-10 Method of hot forming a steel blank and the hot formed part

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US20160130675A1 (en) * 2013-05-28 2016-05-12 Salzgitter Flachstahl Gmbh Method for producing a component by hot forming a pre-product made of steel
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JP2013545890A (ja) 2013-12-26
WO2012048841A8 (en) 2013-04-25
EP2627790B1 (de) 2014-10-08
WO2012048841A1 (en) 2012-04-19
CN103154279B (zh) 2015-09-23
CN103154279A (zh) 2013-06-12

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