US20210115527A1 - Method for manufacturing a hot-formed article, and obtained article - Google Patents

Method for manufacturing a hot-formed article, and obtained article Download PDF

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US20210115527A1
US20210115527A1 US16/463,420 US201716463420A US2021115527A1 US 20210115527 A1 US20210115527 A1 US 20210115527A1 US 201716463420 A US201716463420 A US 201716463420A US 2021115527 A1 US2021115527 A1 US 2021115527A1
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temperature
article
hot
steel
blank
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Shangping Chen
Christiaan Theodorus LAHAYE
Radhakanta Rana
Guido Cornelis Hensen
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Tata Steel Ijmuiden BV
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • 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
    • 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • 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
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • 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
    • C21D2211/005—Ferrite
    • 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

Definitions

  • the invention relates to a method for hot forming a steel blank into an article having enhanced mechanical properties, such as an automotive part with improved ductility, bendability and impact toughness, and to a hot formed article obtained by said method.
  • hot forming also known as hot stamping, hot-forming, press-hardening and die-quenching
  • the basics of the hot forming technique and steel compositions adapted to be used were for the first time described in GB1490535.
  • 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. However the ductility of total elongation is less than 6%.
  • the blank is furnace-heated and austenitized at 900-950° C., transferred from furnace to forming tool, and stamped into the desired part geometry.
  • the blank has an ultimate tensile strength R m ⁇ 200 MPa and a total elongation A>50%.
  • the formed blank is finally die-quenched to 100-200° C. with a mean cooing rate of >30° C./s and a homogenous martensitic microstructure is obtained, while it is constrained in the forming tool.
  • the final martensitic part typically exhibits a proof strength Rp 0.2 >1100 MPa, an ultimate tensile strength R m >1500 MPa and a total elongation A ⁇ 8%.
  • Hot stamped boron steel parts typically include anti-intrusive structural body parts such as roof pillar-, door beam- and bumper beam-reinforcements that constitute the ‘safety-cell’ of a car.
  • the relatively soft and ductile high-temperature austenitic microstructure during forming permits down-gauging and lightweighting while not compromising forming limits and moreover, permits part-consolidation and in turn, increased structural strength and increased process efficiency compared to cold forming as joining/welding is reduced.
  • the martensitic transformation which releases forming stresses and geometric constraint on the part during quenching, eliminates springback giving rise to geometric accuracy.
  • the ultra high strength martensitic microstructure resulting in the final part permits down-gauging (lightweighting) simultaneously while improving anti-intrusive crashworthiness compared to the use of cold formed parts.
  • the supremacy of boron steel in hot stamping applications owes to the (quench) hardenability (owing to the boron addition) and in turn ultra high strength martensitic microstructure that can be obtained from the relatively lean chemical composition.
  • hot stamped martensitic boron steel provides excellent mechanical properties for anti-intrusive crashworthiness, the limited ductility, and in turn limited toughness give rise to very poor impact-energy absorptive crashworthiness.
  • EP1939308 A1 proposed a modification of the standard hot-forming method, wherein a complete martensitic transformation is avoided during hot-forming.
  • a steel blank was heated up to its intercritical temperature between Ac 1 and Ac 3 and then hot-formed to a temperature below Ms so that a dual-phase microstructure consisting of martensite and proeutectoid ferrite is obtained.
  • the hot-formed article obtained by said method showed improved elongation and tensile strength.
  • dual phase steels are sensitive to damage upon forming due to the large hardness difference between ferrite and martensite.
  • the damage tolerance and formability of said hot-formed article is relatively poor due to the microstructural inhomogeneities and microcracks may occur during forming.
  • the energy absorption capability of structural components in a car body is also limited.
  • Such parts are typically cold formed from ferritic or multiphase steels, such as Transformation Induced Plasticity (TRIP) steels.
  • TRIP Transformation Induced Plasticity
  • the proposed heat treatment comprises the steps of: a) heating a steel blank above the austenization temperature; b) quenching: controlling the quenching temperature to be between the Ms and Mf temperature of the steel to partially transform austenite into martensite; c) partitioning: raising the temperature slightly above the Ms temperature and holding it for a short time, whereby a thin layer of austenite with high content of carbon is formed around the martensite lath by repartitioning the carbon between martensite and austenite; d) tempering: tempering the steel at a temperature to induce precipitation hardening in martensite; and e) cooling: cooling down the steel from the tempering temperature at room temperature to transform part of retained austenite into martensite, while remain a thin layer of retained austenite with high content of carbon around the martensite
  • Q-P-T Quenching-Partitioning-Temping
  • TE total elongation
  • BA bending angle
  • the present invention provides a method for hot-forming a steel blank into an article comprising the steps of:
  • the inventors have found that through forming the heated blank into an article as described above, complex shaped articles with enhanced mechanical properties can be obtained.
  • the articles exhibit excellent impact-energy absorptive crashworthiness and in turn, down-gauging and lightweighting opportunities based on impact-energy absorptive crashworthiness compared to the use of conventional hot-formed boron steels and cold-formed multiphase steels.
  • the steel blank In the common hot forming process, the steel blank is simply formed into the desired geometry at and then die-quenched to near-ambient temperature in which a complete transformation to martensite takes place. Thus, the final part exhibits a fully or almost fully martensitic microstructure.
  • the inventors have surprisingly found that by interrupting the die-quench and integrating a Q-P-T heat treatment in the hot-forming cycle, hot formed steel articles having a distinctive microstructure and thus improved mechanical properties can be obtained.
  • the inventors have found that by interrupting the quench temperature of the hot-forming process at a temperature T 3 and/or a temperature T 4 between Ms and Mf and applying a Q-P-T heat treatment, it is possible to control the phases formed, as well as the amount of each phase formed, thus the microstructure, and consequently the mechanical properties of the article.
  • Said article shows improved mechanical properties, such as enhanced tensile strength, ductility and impact toughness for impact-energy absorptive crashworthiness, while exhibiting excellent formability to enable down-gauging and moreover exhibiting excellent dimensional accuracy (minimal or no springback).
  • a heated blank is formed in a forming tool into the desired part geometry having a partially or a fully homogenous austenitic microstructure and simultaneously quenched to a(n) (interrupt) temperature T 3 , and preferably at a cooling velocity V 2 of 25° C./s or more.
  • T 3 is too high and above Bs, and/or the cooling rate is too low, pearlite may form.
  • V 2 is less than 25° C./s, there is also a risk of carbide formation resulting in degradation of the mechanical properties of the final article.
  • the cooling rate V 2 is at least 50° C./s, preferably at least 100° C./s.
  • the cooling rate V 2 is at least 150° C./s for best results.
  • step (e) the article is brought to a temperature T 4 and isothermally held at that temperature T 4 for a time t 4 .
  • Isothermal holding time t 4 is the time required for austempering, partitioning or tempering and is in the range of 10 seconds to 10 minutes. When t 4 is more than 10 minutes then no further improvement of the article is achieved. If t 4 is less than 10 seconds then there is not enough time for the effect to be achieved.
  • T 3 is between Ms and Mf and T 4 is between Bs and Ms.
  • the article must be heated up from temperature T 3 , which is lower than Ms, to temperature T 4 , which is higher than Ms.
  • T 3 some martensitic transformation will take place.
  • the amount of the martensite formed is mainly controlled by the temperature T 3 : the lower T 3 , the higher the fraction of martensite. Since T 4 is higher than T 3 , the untransformed austenite will transform to bainite and retained austenite at T 4 and during following cooling.
  • the martensite will be tempered to some degree during the isothermal holding at T 4 .
  • T 3 is between Bs and Ms and T 4 is between Ms and Mf.
  • austenite will first partially transform into bainite at temperature T 3 .
  • the untransformed austenite will transform to martensite and retained austenite during isothermal holding at T 4 and the following cooling.
  • both T 3 and T 4 are between Ms and Mf, and preferably T 3 and T 4 are the same.
  • Some martensite will form at the beginning; as the isothermal holding time increases, element partitioning between martensite and untransformed austenite may occur, the Ms point of the untransformed martensite will be decreased and some untransformed austenite may transform to bainite. At the same time, the transformed martensite might be tempered to some degree.
  • the article that is formed is held at temperature T 3 during a holding time t 3 of 1 to 180 seconds.
  • the length of the holding time t 3 determines the amount of the microstructural elements that are first formed at temperature T 3 . Which microstructural elements are first formed depends on temperature T 3 .
  • T 3 When T 3 is between Ms and Mf and T 4 is between Bs and Ms, usually the article is heated from temperature T 3 to temperature T 4 at a velocity V 3 of at least 15° C./s, preferably at least 20° C./s, more preferably at least 30° C./s.
  • the heating velocity V 3 depends on the temperature difference between T 3 and T 4 , and on the sheet thickness.
  • the article is cooled from temperature T 3 to temperature T 4 at a velocity t 3 of at most 15° C./s, preferably at most 10° C./s, more preferably at most 8° C./s.
  • the cooling velocity V 4 depends on the temperature difference between T 3 and T 4 , and on the sheet thickness.
  • the article is cooled from temperature T 4 to room temperature at a cooling velocity V 4 , in the range of 0.1-20° C./s, preferably in the range of 1-15° C./s, more preferably in the range of 2-10° C./s.
  • This cooling can be performed by natural cooling in air or by using an air blower, for instance.
  • T 1 is in the range of Ac 1 to Ac 3 +150, preferably Ac 1 to Ac 3 +100, more preferably in the range of Ac 3 ⁇ 50 to Ac 3 +50, and wherein t 1 is preferably at most 12 minutes, more preferably t 1 is in the range of 2-8 min, and wherein the steel blank is heated with a heating velocity V 1 in the range of 10-25° C./s.
  • Temperature T 1 determines how much ferrite is transformed into austenite in the steel blank, which also depends on the holding time t 1 .
  • a heating velocity V 1 above 25° C./s can be detrimental to the properties of the blank.
  • step (e) the article undergoes fine forming.
  • a steel strip or sheet is provided as an intermediate for the subsequent steps.
  • the steel strip or sheet can be obtained by standard casting processes.
  • the steel strip or sheet is cold-rolled.
  • the steel strip or sheet is cut to a steel blank and then heated to a temperature T 1 for a time period t 1 .
  • a preformed steel blank may also be used.
  • the preformed blank may be partially or entirely formed into the desired geometry, preferably at ambient temperature.
  • the heating apparatus may be an electric or gas powered furnace, electrical resistance heating device, infra-red induction heating device or any other heating device.
  • T 1 is in the range of Ac 1 to Ac 3
  • the blank is austenitized at an intercritical temperature.
  • the steel has at said temperature range a ferritic austenitic microstructure. Under this conditions preferably a small amount of the ferrite is retained, wherein the ferrite fraction is preferably less than 20 vol. %.
  • press hardening takes place wherein the structure of the steel part is a two-phase mixture of ferrite and austenite. After subsequent press hardening and Q-P-T heat treatment, a mixed structure of ferrite, bainite or martensite, tempered martensite and a portion of residual austenite is produced in the component.
  • T 1 is above the chemistry specific Ac 3 temperature to produce a fully or almost fully homogenous austenitic microstructure with uniform distribution of carbon.
  • the microstructure is a homogenous austenitic microstructure the formability is enhanced.
  • austenization time t 1 may be chosen in combination with the temperature T 1 and the blank thickness in order to control austenitic grain growth and associated quench hardenability of said steel.
  • the degree of quench hardenability will then influence the microstructural evolution during cooling. Therefore, t 1 is at most 12 min.
  • t 1 is in the range of 2-8 minutes, more preferably in the range of 2-6 minutes in view of process and energy efficiency.
  • the steel blank is transferred from the heating apparatus to a forming tool in a time period t 2 (step b).
  • Time t 2 is the time needed to transport the heated blank from the heating apparatus to the press forming tool and till the hot-forming apparatus is closed.
  • the blank may cool from temperature T 1 to temperature T 2 by the act of natural air-cooling and/or any other available cooling method.
  • the heated blank may be transferred from the heating apparatus to the forming tool by an automated robotic system or any other transfer method.
  • Time t 2 may also be chosen in combination with T 1 , t 1 and T 2 in order to control the microstructural evolution of steel at the commencement of forming and quenching.
  • Time period t 2 is preferably chosen such to ensure that the steel blank when placed in the hot-forming tool has a temperature T 2 above the temperature Ar 1 of the steel to exhibit an austenitic-ferritic microstructure and prevent the formation of pearlite.
  • shorter transport times are beneficial in view of corrosion, process efficiency and hardenability.
  • the inventors have further found that when t 2 is equal or less than 10 s good results have been also achieved.
  • t 2 is equal or less than 8 s, more preferably equal or less than 6 s.
  • the steel blank comprises, in weight % (wt %) following elements:
  • Mn 0.50-4.00, preferably 1.00-3.00 more preferably 1.00-2.50,
  • Si ⁇ 2.0; preferably 0.1-2.0, more preferably 0.1-1.6,
  • Al ⁇ 2.0, preferably ⁇ 1.0, more preferably ⁇ 0.5,
  • Nb ⁇ 0.10, preferably ⁇ 0.05, more preferably 0.001-0.05,
  • V ⁇ 0.2, preferably ⁇ 0.1
  • N ⁇ 0.005, preferably ⁇ 0.003,
  • C 0.10-0.50.
  • C is an essential element for securing a high strength.
  • C is added in an amount of 0.10% or more to form required complex phase microstructures and achieve high strength and high ductility. More particularly, together with the intercritical austenization temperature, the C content determines the volume of ferrite and the carbon content of the remaining austenite, which controls the Bs and Ms points and the bainite formation kinetics. When the C content exceeds 0.50, there is a great possibility that the toughness and weldability of the steel sheet will deteriorate.
  • C amount is preferably present in the range of 0.15-0.40, more preferably in the range of 0.20-0.35.
  • Mn 0.50-4.00
  • the functions of the Mn is to stabilize the austenite and to obtain the desired multiphase microstructure.
  • the manganese content is at least 0.50 in order to provide adequate substitutional solid solution strengthening, adequate quench hardenability and adequate stabilisation of austenite at ambient temperature, while minimising segregation of Mn during casting and while maintaining sufficiently low carbon equivalent for automotive resistance spot-welding techniques. If the Mn content is less than 0.50 is difficult to attain an intended high strength of the steel by heat treating the steel in a ferrite-austenite two-phase region. When the Mn content exceeds 4%, there is the dangers that the steel sheet suffers from poor weldability and hot rolling characteristics.
  • Mn is an element that is useful in lowering the Ac 3 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% to 3.0%.
  • manganese is in the range of 1.00-2.50.
  • Si ⁇ 2.0.
  • Si is an element effective for reinforcing a solid solution, and is useful for suppressing production of carbide due to decomposition of retained austenite.
  • a certain amount of Si suppresses the precipitation of brittle cementite during bainite formation, and hence leads to an improvement in formability and toughness.
  • Si is further known to widen the intercritical temperature range of a steel.
  • Si is also known that in high amount the coatability deteriorates due to the formation of oxides adherent to the steel substrate. Therefore, the Si amount is equal or less than 2.0, preferably in the range of 0.1-2.0.
  • the Si content is preferably in the range of 0.1-1.0.
  • the Si content is preferably in the range of 1.0-2.0.
  • Al ⁇ 2.0 Al limited to a maximum of 2.0 in order to preserve weldability and minimise ‘nozzle blockage’ during steelmaking and casting.
  • Al is an element useful for suppressing production of carbide due to decomposition of, particularly, retained austenite. Partial replacement of Si by same amount of Al has been shown to effectively retard carbide formation without a detrimental effect on hot-dip coatability in steels.
  • a high concentration of Al leads to higher possibility of the polygonal ferrite to be created, which is less effective than fine plate ferrite on strength.
  • Preferably Al is limited to a maximum of 1.0, more preferably Al is at most 0.5.
  • Si and Al in sufficient amount can completely resist formation of carbide.
  • a carbide free bainite i.e. bainitic ferrite
  • Si+Al Si+Al ⁇ 5 ⁇ C
  • traditional bainite consisting of bainitic ferrite+cementite
  • P is known to widen the intercritical temperature range of a steel. P is also an element useful for maintaining desired retained austenite. However, P may deteriorate the workability of the steel when it is added in an excess amount. Therefore, P cannot be added deliberately beyond its usual permissible limit in steel ⁇ 0.015 wt. %.
  • S S ⁇ 0.03. Sulphur needs to be minimised for minimisation of harmful non-metallic inclusions. S forms a sulfide based inclusions such as MnS, which initiates crack, and deteriorates processability. Therefore, it is desirable to reduce the S amount as much as possible. Therefore the sulphur content is limited to a maximum of 0.03, preferably to a maximum of 0.01.
  • Mo and Cr are elements which improve the hardenability of the steel and facilitate the formation of bainite ferrite, and at the same time, are elements having similar effectiveness useful for stabilizing retained austenite. Therefore, Mo and Cr are very effective for the process control.
  • Mo and Cr are each present in an amount of 0.001% or higher. However, when each of them is added excessively, the effect is saturated and the process is not economical. Therefore, the amounts of Mo is limited to a maximum of 0.5, preferably at most 0.05.
  • Cr is limited to a maximum of 1.5, preferably 1.2.
  • Advantageous Cr is in the range of 0.001-1.1
  • Ti ⁇ 0.1; Nb ⁇ 0.1; V ⁇ 0.2: Ti, Nb and V have the effect of forming strengthening precipitates and refining microstructure.
  • the steel should have sufficient strength level. Due to the soft phases such as proeutectoid and bainitic ferrite, the strength of the final product can be lower than standard hot formed material. To compensate for this drop in strength, microalloying elements such as Nb, Ti and V might be added individually or in combination. Said elements increase the strength by means of grain refinement and precipitation hardening. Grain refinement results in a more homogeneous microstructure improving the hot-forming behaviour, in particular when high localized strains are being introduced. Therefore the amount of Ti and Nb is each limited to a maximum of 0.1, preferably to a maximum of 0.05.
  • Advantageously Ti is at most 0.04 and/or Nb is in the range of 0.001-0.05.
  • the amount of V is limited to a maximum of 0.2%, preferably to a maximum of 0.10.
  • Ca is an element effective for controlling a form of sulfide in the steel, and improving processability. However, it is recommended that Ca is contained at 0.0003% or more. However, when it is added excessively, the effect is saturated. Therefore, it is better to suppress an amount thereof to 0.003% or less.
  • B is an important element for increasing the hardenability of 22MnB type steel sheets and for further increasing the effect of stably guaranteeing strength after hardening.
  • B is an optional element for the relatively high carbon contented steel.
  • the B amount is preferably limited to a maximum of 0.008, preferably at most 0.005.
  • the B amount is in the range of 0.0005-0.0025%.
  • the steel microstructural composition is a multiphase steel, preferably a complex phase (CP) like steel that exhibits the capacity for stress/strain induced transformation effect. More particularly the steel is an advanced high strength steel product suitable for hot press forming as a boron steel alternative for automotive applications.
  • CP complex phase
  • the inventors have surprisingly found that applying the present method to a CP like steel composition results in hot-formed articles exhibiting homogeneous mechanical properties.
  • the relative fraction of the existing phases in the microstructures depends essentially on the initial chemical composition of the steel and on the method conditions under which the various step are performed.
  • a CP microstructure comprising bainite, martensite, tempered martensite and/or retained austenite can be obtained, wherein the phases are homogeneously distributed in each of the regions of the part.
  • the articles obtained by the present invention show vastly improved mechanical properties comparable to those of the traditional CP steel prior to cold forming, and thus impact-energy absorptive crashworthiness can be achieved.
  • the present invention provides an improved method of introducing during hot-forming operation the desired bainitic gradient into the steel microstructure while with the standard hot-forming methods this is not possible. Further, the standard hot-forming processes can not provide sufficient amount of tempered martensite and retained austenite in the steel.
  • the bainite and tempered martensite portions in the structure of the formed steel part at the end of the Q-P-T heat treatment should be in total at least 80%, while the amount of the other phases should be less than 20%.
  • the final microstructure consist of (vol. %) (sum should be add 100).
  • the metastable retained austenite fraction ensures the balanced combination of strength and ductility properties.
  • Retained austenite enhances ductility partly through the stress/strain induced transformation effect, which manifests itself in an observed increase in uniform elongation and total elongation as the work hardening exponent increases to higher plastic strains. Above 20 vol. % the desired level of ductility and/or uniform elongation according to the present invention will not be achieved.
  • Part or the totality of the process according to the present invention may be conducted in a controlled inert atmosphere of hydrogen, nitrogen, argon or any other inert gas in order to prevent oxidation and/or decarburisation of said steel.
  • the hot-formed article obtained by the present invention has ultimate tensile strength (UTS) of at least 1000 MPa, preferably at least 1100 MPa and/or yield strength of at least 600 MPa, preferably at least 700 MPa, and/or a total elongation (TE) of at least 6% and/or a bending angle of at least 45°, preferably 50° C., the obtained product exhibits improved impact-energy absorptive crashworthiness.
  • UTS ultimate tensile strength
  • TE total elongation
  • the steel strip, sheet, blank, preformed blank, or article is provided with a coating.
  • a coating may be performed prior to the hot-forming process or after the hot-forming process and has the purpose of minimising oxidation of the steel when exposed to an oxidising atmosphere at high temperature and/or providing cathodic corrosion protection of the final hot formed part.
  • the steel strip, sheet, blank, preformed blank, or article is coated with a zinc based coating, aluminium-silicon based coating or an organic based coating.
  • the zinc based coating is a galvanized or galvannealed coating.
  • the coating can be applied in various ways, hot dip galvanising is preferred using a standard GI coating bath.
  • Other Zn coatings may also be applied.
  • An example comprises a Zn alloy coating according to WO 2008102009, in particular a zinc alloy coating layer consisting of 0.3-4.0 wt % Mg and 0.05-6.0 wt % Al and optionally at most 0.2 wt % of one or more additional elements along with unavoidable impurities and the remainder being zinc.
  • An additional element typically added in a small amount of less than 0.2 wt % could be selected from the group comprising Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.
  • Pb, Sn, Bi and Sb are usually added to form spangles.
  • the total amount of additional elements in the zinc alloy is at most 0.2%.
  • These small amounts of an additional element do not alter the properties of the coating nor the bath to any significant extent for the usual applications.
  • the articles according to the present invention exhibit good adhesion to a coating layer, have good surface appearance and superior corrosion resistance after coating.
  • FIG. 1 shows a schematic representation of a first embodiment of the method according to the invention.
  • FIG. 2 shows a schematic representation of a second embodiment of the method according to the invention.
  • FIG. 2 shows a schematic representation of a third embodiment of the method according to the invention.
  • the horizontal axis represents the time t
  • the vertical axis represents the temperature T.
  • the hot forming of the blank into the article in the hot forming press is indicated by HF.
  • the time t and temperature T are indicated diagrammatically in the Figures, and so are the cooling and heating velocities V. No values can be derived from the Figures.
  • a steel blank is heated up to the austenitizing temperature T 1 above Ac 1 at a heating rate of 15° C./s and held at aT 1 for a time period t. Then the heated blank is transferred from the furnace to the hot forming press, during which cooling of the blank to temperature T 2 by air occurs to some extend. Care is taken that the temperature T 2 does not decrease below the Ar 1 temperature before the blank is placed in the hot-forming press.
  • the blank is then hot-formed into an article and cooled down to temperature T 3 at a cooling rate of >25° C./s and the cooling is interrupted and the article is held at T 3 for a time period t 3 . Thereafter, the three embodiments follow different routes. Finally, the formed article is cooled down to room temperature at a cooling rate V 4 .
  • the values for the temperatures T 3 , T 4 and cooling or heating velocities V 3 and V 4 , and the holding time t 3 and t 4 depend on the different embodiments as shown in the FIGS. 1-3 .
  • Ar 1 The temperature at which transformation of austenite to ferrite is completed during cooling.
  • Ar 3 The temperature at which austenite begins to transform to ferrite during cooling.
  • Ms Temperature at which, during cooling, transformation of the austenite into martensite starts.
  • Mf Temperature at which, during cooling, transformation of the austenite into martensite ends.
  • Critical phase transformation temperatures are determined by dilatometer experiments.
  • Table 1 shows the steel composition used in a method according to the present invention.
  • the tables 2, 3 and 4 give the process parameters and mechanical properties for the different steel types of Table 1, and for the process types of the three embodiments. The results are discussed below.
  • Steel blanks with dimensions of 200 mm ⁇ 110 mm ⁇ 1.5 mm have been prepared from a cold-rolled steel sheet having the composition B or C as given in Table 1.
  • the Ac 3 and Ms temperatures were determined using dilatation tests.
  • the blanks were first heated at 880° C. (T 1 ) in a box furnace for 6 min and then transported to a hot-forming apparatus.
  • the hot-forming was performed in laboratory scale by Schuler SMG company, Germany (hereafter SMG press).
  • the SMG press tools were preheated to a temperature of 450° C. or 300° C. (T 3 ).
  • the blanks were transferred to the SMG press in 10 s and press-quenched to T 3 temperature into an article.
  • the article was transferred to a muffle furnace preheated at a temperature of 350° C. or 400° C. (T 4 ) and austempering or partitioning and/or tempering heat treatment was applied (Q-P-T treatment).
  • the experimental results with respect to the yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UL) total elongation (TL) and banding angle (BA), are given in Table 2 and Table 3.
  • the tensile properties were measured in the samples with the stress direction being parallel to the rolling direction.
  • the 3-point “guided bending tests” were conducted on samples with dimensions 40 mm ⁇ 3025 mm.
  • the length direction of the samples was parallel to the rolling direction of steel sheets.
  • Parallel bending tests where the bending axis is perpendicular to the rolling direction of the sheets were carried out.
  • a former and two supporting cylinders were used in order to bend the steel sheets.
  • the cylinders and the punch were mounted in a tensile testing machine.
  • the load cell is used to measure the punch force and the displacement of the crosshead gives the punch displacement.
  • the experiments were stopped at different bending angles and the bent surface of the specimen was inspected for identification of failure in order to determine the bending angle.
  • the present inventive method has been applied to a steel composition E, as shown in Table 2, similar to the known 22MnB5, but with a C content higher than from this of 22MnB5.
  • the Ac 1 , Ac 3 , and Ms temperatures were determined using dilatation tests.
  • a cold-rolled steel sheet having the composition E and a thickness of 1.5 mm has been prepared and blanks with dimensions of 600 mm ⁇ 110 mm were cut.
  • a thermal cycle according to the invention was applied to simulate the hot press forming process using a continuous annealing simulator (CASIM). The blanks were first heated to a variable soak temperature of 900° C. or 810° C. (T 1 ) and held at said temperature for a soak time of 2 min (t 1 ).
  • the transfer of the heated blanks from the furnace to the press forming apparatus was simulated by cooling down slowly the blanks to 750° C. (T 2 ) and at a cooling rate of 3° C./s.
  • the blanks are cooled down to a temperature T 3 at a cooling rate of 40° C./s (V 2 ) and isothermal held for t 3 and subsequently cooled down to room temperature at a cooling rate of 3.5° C./s (V 4 ).
  • T 3 T 4 .
  • the process parameters, the tensile properties and the bending angle are given in Table 2.
  • Steel compositions F, G, H, I and J according to table 1 have be used.
  • Steel blanks with dimensions of 600 mm ⁇ 110 mm ⁇ 1.5 mm or 230 mm ⁇ 110 mm ⁇ 1.5 mm have been prepared from a cold-rolled steel sheets having the composition F, G and H, I, J respectively as given in Table 1.
  • the blanks of steel F and G were heat treated in a Continuous Annealing Simulator (CASIM), those of steels H, I and J in a Hot Dip Annealing Simulator (HDAS). Whatever be the apparatus used for heat treatments, it was ensured that the thermal cycles were simulated accurately.
  • CASIM Continuous Annealing Simulator
  • HDAS Hot Dip Annealing Simulator

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EP3548641A1 (fr) 2019-10-09

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