EP2465962B1 - Feuilles d'acier très résistantes et leurs procédés de production - Google Patents

Feuilles d'acier très résistantes et leurs procédés de production Download PDF

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Publication number
EP2465962B1
EP2465962B1 EP11193479.0A EP11193479A EP2465962B1 EP 2465962 B1 EP2465962 B1 EP 2465962B1 EP 11193479 A EP11193479 A EP 11193479A EP 2465962 B1 EP2465962 B1 EP 2465962B1
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EP
European Patent Office
Prior art keywords
steel sheet
phase
martensite phase
high strength
space factor
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Not-in-force
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EP11193479.0A
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German (de)
English (en)
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EP2465962A1 (fr
Inventor
Kenji Saito
Tomokazu Masuda
Masaaki Miura
Yoichi Mukai
Shushi Ikeda
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Kobe Steel Ltd
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Kobe Steel Ltd
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Publication date
Priority claimed from JP2007144705A external-priority patent/JP4291860B2/ja
Priority claimed from JP2007144466A external-priority patent/JP5201653B2/ja
Priority claimed from JP2007145987A external-priority patent/JP5234893B2/ja
Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Publication of EP2465962A1 publication Critical patent/EP2465962A1/fr
Application granted granted Critical
Publication of EP2465962B1 publication Critical patent/EP2465962B1/fr
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing 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
    • 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
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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 present invention relates to a high strength steel sheet for which high press formability is required, typically including steel sheets for automobiles, particularly to a high strength steel sheet with both elongation and stretch-flanging performance and a method for manufacturing the same.
  • High strength steel sheets which are generally used by being press-molded, are used in industrial product such as automobiles, electric devices and industrial machines. Since high strength steel sheets are used for the purpose of lightening industrial products, they need not only have high strength, but also have the ability to form various configurations of the products. Accordingly, it is required for high strength steel sheets to have excellent press formability. To meet this requirement, high-strength steel sheets having excellent elongation and stretch-flanging performance, which are necessary for improving press formability, are required.
  • Examples of known steels having such characteristics include dual phase steel (DP steel) whose metal structure is composed of a ferrite phase and a martensite phase, as described in Patent document 1. Since this DP steel can ensure ductility (elongation) due to its soft ferrite and strength due to its rigid martensite, it has both strength and elongation (in particular, uniform elongation). However, because of the coexistence of soft ferrite and rigid martensite, distortion (stress) is concentrated at the interface of the two phases when deformed, and therefore the interface is likely to serve as the starting point of rupture, thereby disadvantageously preventing ensuring stretch-flanging performance (local elongation).
  • Examples of steel sheets which expectedly have ductility (especially, uniform elongation) higher than those of DP steels include TRIP steels utilizing the TRIP (Transformation Induced Plasticity) phenomenon, as described in Patent document 2.
  • TRIP steel is a steel sheet in which uniform elongation is increased by transforming retained austenite into martensite during deformation (working-induced transformation).
  • martensite which has been transformed from retained austenite in the TRIP steel is extremely hard, it likely serves as the starting point of rupture, lowering the stretch-flanging performance of the steel sheet.
  • martensite single-phase structure steel sheet has a uniform structure, it is known as a steel sheet which has both strength and stretch-flanging performance.
  • the martensite single-phase structure steel sheet disadvantageously has low ductility, and insufficient elongation.
  • Patent document 3 discloses a high-stretch-strength cold-rolled steel sheet in which martensite single-phase structure is achieved by justifying the composition and heat treatment conditions of the steel sheet, and tensile strength is 880 to 1170 MPa. That is, the high-stretch-strength cold-rolled steel sheet of Patent document 3 is produced by heating and retaining a steel sheet having a predetermined composition range at 850°C, which is normally reachable temperature industrially, to transform the steel sheet into austenite, and then rendering it a martensite single-phase structure.
  • a steel sheet of a martensite single-phase structure produced by this invention has a tensile strength of 880 to 1170MPa, and thus has excellent stretch-flanging performance. However, it has elongation EL (%) lower than 8% and thus has low ductility. In the high strength steel sheet of the invention of Patent document 3, if ductility is improved, press formability can be further improved.
  • Patent document 4 discloses a method for manufacturing a high tensile strength steel sheet, in which a steel sheet in which the ratio by volume of a low-temperature transformation phase comprising a martensite phase and others and a retained austenite phase is 90% or higher of the entire metal structure is heated and retained to produce a two phase region: a ferrite phase and an austenite phase, a metal structure comprising a fine ferrite phase which has succeeded the laths of the low-temperature transformation phase and the austenite phase is provided, and finally the steel sheet is given such a metal structure that comprises ferrite and the low-temperature transformation phase finely dispersed in the form of laths.
  • the present invention has been made to solve such a problem, and an object thereof is to provide a high strength steel sheet excellent in both elongation and stretch-flanging performance and a method for manufacturing the same.
  • Another object of the present invention is to provide a high strength steel sheet having a tensile strength of 780 MPa or higher, in which elongation and stretch-flanging performance are both improved, and a method for manufacturing the same.
  • the high strength steel sheet of the present invention is constituted of, in percent by mass, C: 0.05 to 0.3%, Si: 3% or less (not including 0%.), Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%, and optionally, at least an element selected from Ti, Nb, V and Zr in an amount of 0.01 to 1% in total, Ni and/or Cu in an amount of 1% or lower in total, Cr: 2% or less, Mo: 1% or less, B: 0.0001 to 0.005%, Ca and/or REM in an amount of 0.003% or lower in total, and the remainder comprising iron and inevitable impurities, has a space factor of a martensite phase which is a main part of the metal structure is 80% or higher, the mean grain size of the martensite phase is 10 ⁇ m or smaller in terms of the equivalent of a circle diameter; the space factor of the martensite phase having a grain size of 10 ⁇ m or larger in terms of the equivalent of a circle diameter in the martensite phase is 15% or
  • the term "equivalent of a circle diameter” means the diameter of an anticipated circle having the same area as the grains of tempered martensite, and is determined by subjecting a structure picture to image analysis.
  • the term "space factor” means the percentage by volume, and is determined by corroding a structure observation test piece with nital, observing the test piece with an optical microscope (1000 times), and by subjecting the observed structure picture to image analysis.
  • annealed bainite is observed as a body centered cubic structure in terms of a crystal structure.
  • Ac 3 point is a temperature at which a two-phase region comprising an austenite phase and a ferrite phase transforms into an austenite single-phase region that is stable at high temperatures in a temperature raising step.
  • the inventors of the present invention have invented a high strength steel sheet having a limited ratio by volume of the retained austenite phase of 2% or lower, which does not affect stretch-flanging performance, and a metal structure in which a large part of the metal structure is a fine tempered martensite phase.
  • the space factor of the martensite phase which is a main component of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 ⁇ m or smaller in terms of the equivalent of a circle diameter; the space factor of a martensite phase having a grain size of 10 ⁇ m or larger in terms of the equivalent of a circle diameter in the martensite phase is 15% or lower, wherein the martensite phase is a tempered martensite phase; and the space factor of the retained austenite phase in the metal structure is 2% or lower and the rest of the metal structure is mainly ferrite phase.
  • space factor means a ratio by volume of each phase constituting the metal structure in the steel material to the entire metal structure.
  • the space factors of the martensite phase and ferrite phase were determined by subjecting the steel material to repeller corrosion, observing the material by an optical microscope and an SEM (1000 times), and then subjecting the material to image analysis.
  • the space factor of the retained austenite phase was determined by the saturation magnetization method (refer to " Netsushori" (heat treatment), Vol.136, (1996 )).
  • the mean grain size of the martensite phase is the mean value of the crystal grain size of the martensite phase, and is determined by structure analysis using a FE/SEM-EBSP at step intervals of 100 nm in the present invention.
  • the space factor of the fine tempered martensite phase having a mean grain size of 10 ⁇ m or smaller is 80% or higher, and therefore a tensile strength of 780 MPa or higher and excellent ductility are ensured.
  • stretch-flanging performance is lowered.
  • the space factor of the retained austenite phase is limited to 2% at the highest in the present invention, and therefore stretch-flanging performance is not lowered.
  • martensite phase is a tempered martensite phase.
  • the method for manufacturing the high strength steel sheet according to the present invention is for manufacturing a high strength steel sheet of the present invention by using a steel sheet in which the total space factor of the martensite phase and/or of the retained austenite phase in the entire metal structure is 90% or higher as a material steel sheet, heating and retaining the steel sheet at a temperature of (Ac 3 point-50°C) or higher, Ac 3 point or lower for 30 to 1200 seconds, cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10°C/sec. or higher, and further conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 500°C for 60 to 1200 seconds.
  • the high strength steel sheet according to the present invention may comprise, in addition to the above-mentioned basic components, any of the element groups (a) to (e) described below, or one or more elements selected from a plurality of groups within a range defined for each element group.
  • this high strength steel sheet in which the space factor of the retained austenite phase is 2% or lower and the space factor of the fine tempered martensite phase is 80% or higher by a relatively simple heat treatment step. Since this high strength steel sheet has a tensile strength of 780 MPa or higher, and also has excellent elongation and stretch-flanging performance, it is excellent in press formability.
  • the best mode for carrying out the invention will be described below in detail.
  • the elements which constitute the composition of the high strength steel sheet of this embodiment are C, Si, Mn, Al, Cr, Mo, Nb, Ti and V, and the remainder is Fe and inevitable impurities.
  • Cr, Mo, Nb, Ti and V are not essentially necessary constituent elements, but are the elements which are added to further increase the effect of the present invention.
  • the actions of the elements will be described below. In the description provided below, the proportions of compositional ranges are indicated by % by mass.
  • the compositional range of C is limited within the range of 0.05% to 0.3%.
  • C is an element effective in producing the tempered martensite phase, and increasing the strength of the steel sheet material.
  • the lower limit value i.e., 0.05% is an amount which is minimally necessary to obtain desired strength.
  • the upper limit value i.e. 0.3%, is limited for the following reason.
  • C is added in an amount higher than the upper limit value, 0.3%, the concentrations of C in the tempered martensite phase and the retained austenite phase are increased, and the strength of these phases is increased. A difference in strength between these phases and the ferrite phase having a low concentration of C is increased. Since rupture is likely to occur at the interface of these phases having a difference in strength, stretch-flanging performance is lowered. Meanwhile, when the concentration of C in the steel sheet is increased, weldability is significantly deteriorated.
  • the compositional range of Si is limited within the range higher than 0% but not higher than 3%.
  • Si has the action to inhibit the generation of relatively coarse carbide which lowers stretch-flanging performance, and also improve ductility. However, this action to improve ductility is saturated in an amount of Si added of about 3%.
  • si has the action to retard softening by tempering of the tempered martensite phase, when the amount of Si contained is high, the tempered martensite phase is not sufficiently tempered and thus strength is retained high, whereby a difference in strength between the martensite phase and the ferrite phase is increased and stretch-flanging performance is lowered. Accordingly, the upper limit of the amount of Si added is 3%.
  • the compositional range of Mn is limited within the range of 0.5% or higher but not higher than 3%.
  • Mn has the effect to increase the tensile strength of the steel sheet by solid solubility reinforcement, improve the hardening characteristics of the steel sheet, and promote generation of the martensite phase.
  • Such an action of Mn is found in steel having containing Mn in an amount of 0.5% or higher.
  • the amount of Mn contained is 1% or higher.
  • the amount of Mn contained is preferably 2.5% or lower.
  • the compositional range of Al is limited within the range of 0.01% or higher not higher than 0.1%.
  • Al is used for deoxidation of steel in the steelmaking process. When there is no solid solution of Al present in the metal structure of steel, deoxidation of steel may not be completed. When oxygen is remaining in steel, remaining oxygen is bonded to Si and Mn. Since these oxidation products of Si and Mn are likely to separate and float from the cast, the composition of steel becomes non-uniform and processability is lowered. Moreover, when the amount of Al solutionized in the metal structure of steel is higher than 0.1%, deoxidation products are reduced by Al again, and metal-like Al is produced. This metal-like Al serves as a relatively large mediator, and creates material defects or surface flaws. Therefore, the upper limit value of Al is 0.1%.
  • Cr and Mo are not elements essential to the high strength steel sheet of the embodiment, but their addition acts effectively. Cr and Mo act to inhibit the generation of carbide which lowers stretch-flanging performance, and promote the generation of the martensite phase in the metal structure of the steel sheet. Therefore, they can be added as needed.
  • the compositional range of Cr and Mo is such that at least one or more elements selected from Cr and Mo is contained, and the total compositional ratio of these elements is 0.5% or lower. In order to effectively perform the action of Cr and Mo, it is recommended that the compositional proportions of Cr and Mo are 0.05% or higher (more preferably 0.1% or higher), respectively. However, even if Cr and Mo are added, whether singly or in combination of both, in an amount higher than 0.5%, the action mentioned above is saturated, and an action which is worth the amount of Cr and Mo contained cannot be obtained.
  • Nb, Ti and V is an element which is essential to the high strength steel sheet of this embodiment, but their addition acts effectively.
  • Nb, Ti and V have the action to form carbonitride, increase the tensile strength of steel by enhancing deposition, and micronize crystal grains in the metal structure of the steel sheet. Accordingly, these elements are added.as needed.
  • the action of Nb, Ti and V mentioned above is not effective.
  • the upper limit of the total amount added mentioned above is 0.1%.
  • the high strength steel sheet of this embodiment may be composed to contain Ni or Cu in an amount of 1% by mass or lower in place of Cr, Mo, Nb, Ti and V. Moreover, it may be composed to contain B in an amount of 0.0001% by mass or higher but 0.0010% by mass or lower. Further, it may be composed to contain 0.003% by mass or less of Ca and/or REM in total.
  • the material of the high strength steel sheet of this embodiment is composed of Fe and inevitable impurities, in addition to the above-mentioned components.
  • P and S are present as inevitable impurities, but they do not adversely affect the characteristics of the high strength steel sheet of this embodiment as long as the amount of P is 0.05% or lower (not including 0%) and the amount of S is 0.02% or lower (including 0%).
  • the less the amount of P and S contained the better the processability of the steel sheet.
  • MnS which serves as a mediator is increased in steel, whereby the stretch-flanging performance of the steel sheet is significantly lowered.
  • the metal structure of the high strength steel sheet of this embodiment comprises a tempered martensite phase having a space factor of 80% or higher and a retained austenite phase having a space factor of 2% or lower, and the rest is mainly composed of a ferrite phase.
  • the tempered martensite phase will be described first.
  • the space factor of the tempered martensite phase is 80% or higher, combination between austenite crystal grains and growth of the same can be suppressed by the annealed martensite phase remaining finely in part of the ferrite phase after the annealing step employed in the method for manufacturing the high strength steel sheet of the embodiment described later.
  • the space factor of the tempered martensite phase is lower than 80%, the tempered martensite phase is divided into ferrite phases, and therefore stretch-flanging performance is lowered.
  • the phase becomes a substantially single-phase structure of tempered martensite having a space factor of the tempered martensite phase of 100%, ductility is lowered. For this reason, the case where the space factor is 100% is not included in the present invention.
  • the mean grain size is 10 ⁇ m or smaller, and the space factor of the tempered martensite phase having a grain size larger than 10 ⁇ m is 15% or lower.
  • the mean grain size is larger than 10 ⁇ m, or when the space factor of the tempered martensite phase having a grain size larger than 10 ⁇ m is higher than 15%, the interfaces of the tempered martensite phase which act as the starting point of rupture are unevenly distributed, and therefore sufficient stretch-flanging performance cannot be obtained.
  • the space factor of the retained austenite phase is 2% or lower.
  • the retained austenite phase undergoes induced transformation in which it transforms into a tempered martensite phase during processing. Accordingly, the retained austenite phase lowers stretch-flanging performance. Therefore, in order to improve stretch-flanging performance, the space factor of the retained austenite phase needs to be limited to a low level.
  • the space factor of the retained austenite phase is 2% or lower, and more preferably 1% or lower.
  • the high strength steel sheet of the embodiment as described above has a fine tempered martensite phase formed therein and has a sufficiently low space factor of the retained austenite phase. Hence, it has excellent characteristics: it not only has high tensile strength, but also high elongation and stretch-flanging performance at the same time.
  • the method for manufacturing the high strength steel sheet of this embodiment will be now described. First, materials of the high strength steel sheet of this embodiment will be described.
  • the high strength steel sheet of this embodiment is obtained by subjecting a steel sheet material which meets predetermined conditions to a heat treatment comprising a predetermined annealing step and a tempering step.
  • the steel sheet material of the high strength steel sheet of this embodiment needs to meet the conditions of the composition of constituents mentioned above and also the conditions of the following metal structure.
  • the steel sheet material of the high strength steel sheet of this embodiment needs to have a space factor of the martensite phase and the retained austenite phase of 90% or higher.
  • the space factor of the martensite phase and the retained austenite phase is 95% or higher.
  • the steel sheet material of the high strength steel sheet of this embodiment having a space factor of the martensite phase and the retained austenite phase of 90% or higher is produced in the manner described below.
  • the process for manufacturing a steel sheet material of the high strength steel sheet of this embodiment comprises the following steps: A steel slab adjusted to meet the composition of constituents of the high strength steel sheet material mentioned above is hot-rolled at such a temperature that the finishing rolling temperature is Ac 3 point or higher. This hot-rolled steel sheet is then cooled at a cooling rate of 10°C/sec. or higher to a cooling stop temperature, which is lower than Ms point at which the austenite phase starts to transform into the martensite phase (about 350°C or lower), and is wound up.
  • the finishing rolling temperature is Ac 3 point or lower or the cooling rate after the hot rolling is 10°C/sec. or lower, the ferrite phase is likely to be produced during cooling after the hot rolling, and the space factor of the low-temperature transformation phase after the hot rolling does not become 90% or higher.
  • a steel sheet produced under conditions which do not meet those of the hot rolling and cooling rate mentioned above from a steel slab adjusted to meet the composition of constituents of the steel sheet material can be modified into a steel sheet material having a space factor of its low-temperature transformation phase of 90% or higher by carrying out the following preliminary annealing.
  • This preliminary annealing is a heat treatment in which the hot-rolled steel sheet is retained in a temperature range of Ac 3 point or higher for 5 seconds or longer, and is cooled at a cooling rate of 10°C/sec. or higher to a cooling stop temperature of 350°C or lower.
  • a ferrite phase is produced, and the space factor becomes no greater than 90%.
  • the heat treatment step of the high strength steel sheet of this embodiment will be described now.
  • the high strength steel sheet of this embodiment is obtained by subjecting a steel sheet material to a heat treatment comprising a predetermined annealing step and a tempering step.
  • This annealing step is a heat treatment in which the steel sheet material is heated to a temperature of Ac 3 point or lower but not lower than AC 3 point -50°, retained for 30 seconds or longer but not longer than 1200 seconds, and is then cooled at a cooling rate of 10°C/sec. or higher to Ms point or lower.
  • the above-mentioned martensite phase having a space factor of 80% or higher is formed.
  • the size of austenite crystal grains produced when the steel sheet material is heated to and retained at a temperature of Ac 3 point or lower but not lower than Ac 3 point -50° affects the crystal grain size of the tempered martensite phase of the high strength steel sheet of the embodiment. That is, to obtain a fine tempered martensite having a mean grain size of 10 ⁇ m or smaller and the space factor of the tempered martensite phase having a grain size larger than 10 ⁇ m is 15% or lower phase as the high strength steel sheet of this embodiment, the steel sheet material needs to be heated to and retained at a temperature of Ac 3 point or lower but not lower than Ac 3 point -50°.
  • a steel sheet having the metal structure in which such a fine tempered martensite phase is formed is characterized by high strength and high ductility.
  • this annealing step when the steel sheet material is retained in a temperature range higher than Ac 3 point at which the austenite single-phase is stable, crystal grains of austenite grow and combine with each other to be coarse. Therefore, the steel sheet material cannot be imparted a metal structure having a fine tempered martensite phase as the high strength steel sheet of this embodiment. As a result, the stretch-flanging performance of the high strength steel sheet is lowered.
  • the steel sheet material is retained at a temperature lower than Ac 3 point -50°C, transformation into austenite does not proceed sufficiently, and the space factor of the tempered martensite phase of the high strength steel sheet after the heat treatment becomes lower than that of the high strength steel sheet of this embodiment. As a result, the stretch-flanging performance of the high strength steel sheet is lowered. Therefore, the retaining temperature was set to Ac 3 point or lower but not lower than Ac 3 point -50°C.
  • the retaining time in this annealing step. is shorter than 30 seconds, the austenite phase is not sufficiently produced, and thus a fine martensite phase cannot be obtained after this annealing step.
  • the retaining time is longer than 1200 seconds, produced austenite crystal grains become coarse, and therefore the fine tempered martensite phase mentioned above cannot be obtained.
  • the retaining time is to be in the range of 30 seconds or longer but not longer than 1200 seconds, Preferably, it is in the range of 120 seconds or longer but not longer than 600 seconds.
  • this annealing step when the cooling rate is 10°C/sec. or lower, or the cooling stop temperature is higher than Ms point at which the transformation from the austenite phase into the tempered martensite phase starts, generation of a bainite phase, retained austenite phase, pearlite phase and ferrite phase and deposition of a cementite phase are caused, and a number of phases other than the martensite phase are formed, whereby the space factor of the martensite phase cannot be increased. Accordingly, the stretch-flanging performance of the steel sheet is lowered. The higher the cooling rate, and the lower the cooling stop temperature, the higher the space factor of the tempered martensite phase can be.
  • the tempering step will be described now.
  • the steel sheet material which has undergone the annealing step is retained at a temperature of 300°C to 550°C for 60 seconds to 1200 seconds.
  • a fine martensite phase is formed in the metal structure of the steel sheet material which has undergone the annealing step.
  • the steel sheet material is softened by tempering this martensite phase to reduce a difference in hardness from the annealed martensite phase and ferrite phase, whereby excellent stretch-flanging performance, as well as ductility, can be obtained.
  • the retaining temperature in this tempering step is lower than 300°C, the hardness of the tempered martensite phase is too high, and the stretch-flanging performance of the steel sheet is thus lowered.
  • the retaining temperature is higher than 550°C, the cementite phase produced by the decomposition of the retained austenite phase becomes coarse, whereby the stretch-flanging performance of the steel sheet is lowered.
  • the retaining time in this tempering step is shorter than 60 seconds, the hardness of the tempered martensite phase is too high, and therefore the elongation and stretch-flanging performance of the steel sheet are lowered.
  • the retaining time in this tempering step is 60 seconds or longer but not longer than 1200 seconds, but it is preferably 90 seconds or longer but not longer than 900 seconds, and more preferably 120 seconds or longer but not longer than 600 seconds.
  • the steel sheet material which has been subjected to the annealing step and this tempering step becomes the high strength steel sheet of this embodiment, and is characterized by high stretch-flanging performance, in addition to high tensile strength and high ductility. Accordingly, this high strength steel sheet is used for various industrial products typically including automobiles as a steel sheet having excellent press formability.
  • the steel slabs having the compositions of constituents of A to Y, B, C, E, F, I, J, L, N to Y are the steel slabs having the compositions of constituents which fall within the Examples of the embodiment. Steel slabs having other compositions of constituents do not fall within the compositions of constituents of this embodiment.
  • the test steel sheets prepared from these steel slabs are Comparative Examples. The steel slabs having the compositions of constituents of these A to Y, respectively, were hot-rolled at a finishing temperature of 850°C to give 56 types of test steel sheets having a thickness of 3 mm (Nos.1 to 56), which were then wound up at predetermined temperatures shown in Table 6.
  • test steel sheets No.1 to 45 were washed with acid to remove scales, and were cold-rolled to a thickness of 1.2 mm.
  • the test steel sheets excluding test steel sheets 2 and 11 were then subjected to preliminary annealing under predetermined conditions shown in Table 6. Thereafter, test steel sheets Nos.1 to 56 were subjected to the heat treatment comprising the annealing step and the tempering step under predetermined conditions shown in Table 7, and were used as test steel sheets for measurement.
  • the 56 types of the test steel sheets prepared by these steps were tested for their tensile strength and stretch-flanging performance.
  • the tensile strength test was performed in such a manner that the direction perpendicular to the rolling direction of each of the test steel sheets is the direction of pulling during testing by using a JIS No. 5 test piece collected from each test steel sheet according to JIS Z2241. In this test, yield strength YS, tensile strength TS and elongation EL were determined.
  • the stretch-flanging performance test was performed according to Japan Iron and Steel Federation standard (JFST 1001-1996), and hole expansion rates ⁇ were determined.
  • Test steel sheets which meet all the following conditions are considered to correspond to high strength steel sheets according to the present invention: tensile strength: TS ⁇ 780 MPa, elongation: EL ⁇ 10%, hole expansion rate: ⁇ 80%.
  • a test steel sheet which meets all these three conditions and has especially good hole expansion rate ( ⁇ 100%) was rated ⁇ ; a test steel sheet which meets all the conditions was rated ⁇ ; a test steel sheet which meets two conditions out of three was rated ⁇ ; and a test steel sheet which meets only one condition or less out of three conditions was rated ⁇ .
  • test steel sheets No.3, 5, 7, 8, 11, 13, 14, 18, 20, 23, 24, 27, 28, 33, 34, 38 and 40 to 45 are all prepared from steel slabs (B, C, E, F, I, J, L, N to T in Table 5) corresponding to the compositions of constituents of the high strength steel sheet of this embodiment.
  • the space factor of the martensite phase and the retained austenite phase of the metal structures of these test steel sheets before the annealing step, the annealing step and the tempering step correspond to the conditions of the high strength steel sheet of this embodiment. All of these test steel sheets meet the conditions of the tensile strength, elongation and stretch-flanging performance of the present invention.
  • test steel sheets (Nos.46 to 56) of Table 9 all meet the conditions of the tensile strength, elongation and stretch-flanging performance of the present invention.
  • test steel sheets which correspond to the high strength steel sheet of the embodiment, Nos. 3, 5, 8, 14 and 20 have especially good stretch-flanging performance.
  • the space factor of the retained austenite phase of these test steel sheets is 0%.
  • the tempered martensite phases of these test steel sheets have relatively small mean grain sizes, and the space factor of the tempered martensite phase having a crystal grain size of 10 ⁇ m or larger is relatively low.
  • test steel sheet No-1 was prepared from steel slab A having low level of C, it has low tensile strength.
  • Test steel sheet No.2 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it is yet to be annealed were low. Therefore, crystal grains of the tempered martensite phase became coarse, and the strength and stretch-flanging performance were lowered.
  • Test steel sheet No.4 was subjected to the preliminary annealing at a temperature lower than Ac 3 point, and therefore the space factor of the low-temperature transformation phase in the metal structure in a state that it is yet to be annealed was lowered. This caused crystal grains of the tempered martensite phase to be coarse, whereby ductility and stretch-flanging performance are low.
  • the test steel sheet No.6 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it was yet to be annealed because the retaining time in the preliminary annealing was short, and therefore crystal grains in the tempered martensite phase became coarse. As a result, it had low elongation and stretch-flanging performance.
  • the test steel sheet No.9 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it was yet to be annealed because cooling after the preliminary annealing was delayed, and therefore the tempered martensite phase became coarse. As a result, it had low elongation and stretch-flanging performance.
  • the test steel sheet No.10 had low space factors of the martensite phase and the retained austenite phase in the metal structure in a state that it was yet to be annealed because the cooling stop temperature after the preliminary annealing was high, and therefore the tempered martensite phase became coarse. As a result, it had low elongation and stretch-flanging performance.
  • the test steel sheet No.12 has a metal structure after the tempering step corresponding to that of the high strength steel sheet of the embodiment, the difference in strength between the annealed martensite phase which is a part of the ferrite phase and the tempered martensite phase has not been sufficiently reduced because this test steel sheet was prepared from steel slab D having a high level of C. As a result, it had low stretch-flanging performance.
  • test steel sheet No.15 has a metal structure after the tempering step corresponding to that of the high strength steel sheet of the embodiment, this test steel sheet was prepared from steel slab G having a high level of Si. Accordingly, the tempered martensite phase was not sufficiently tempered, and the difference in strength between the annealed martensite phase which is a part of the ferrite phase and the tempered martensite phase has not been sufficiently reduced. As a result, it had low stretch-flanging performance. Since the test steel sheet No. 16 was prepared from steel slab H having a low level of Mn, it has insufficient hardening characteristics, and therefore a large amount of retained austenite remained after the annealing step.
  • test steel sheet No. 19 was prepared from steel slab K having a high level of Mn, uneven distribution of Mn occurred although the space factors and the size of the martensite phase and the retained austenite phase in the metal structure after the tempering step correspond to that of the high strength steel sheet of the embodiment. As a result, it had low elongation and stretch-flanging performance.
  • the test steel sheet No. 21 was prepared from steel slab M having a high amount of A1 added. Accordingly, it had a number of flaws on the surface of the steel material. As a result, it had low stretch-flanging performance.
  • the test steel sheet No. 22 had coarse crystal grains of the austenite phase since it was heated to a temperature higher than AC 3 point in the annealing step. As a result, its ductility was lowered.
  • the austenite phase was not sufficiently produced because the heating and retaining temperature in the annealing step was lower than Ac 3 point -50°C. As a result, it had low space factor of the tempered martensite phase, and low stretch-flanging performance.
  • the austenite phase was not sufficiently produced because the retaining time at a temperature of Ac 3 point or lower but not higher than Ac 3 point -50°C in the annealing step was too short.
  • the tempered martensite phase was not sufficiently generated because cooling after the annealing step was too late and thus tempered phases other than the martensite phase were produced. As a result, it had low tensile strength.
  • the martensite phase was not sufficiently produced because the cooling stop temperature after the annealing step was higher than Ms point. As a result, it had low space factor of the tempered martensite phase, and low stretch-flanging performance.
  • the dislocation density of the tempered martensite phase was not lowered because the heating and retaining temperature in the tempering step was lower than the lower limit value, and distortion was not sufficiently mitigated. As a result, it had low elongation and stretch-flanging performance.
  • the high strength steel sheet according to the present invention has excellent elongation and stretch-flanging performance at the same time, and thus has excellent press formability. Therefore, the high strength steel sheet according to the present invention can be processed by press molding to be used for various industrial products such as automobiles, especially for industrial products where weight reduction is necessary.

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Claims (2)

  1. Feuille d'acier à haute résistance qui comprend, en pour cent en masse, C : 0,05 à 0,3 % ; Si : 3 % ou moins (sans inclure 0 %) ; Mn : 0,5 à 3,0 % ; AI : 0,01 à 0,1 % ; et facultativement, au moins un élément choisi parmi Ti, Nb, V et Zr dans une quantité de 0,01 à 1 % au total, Ni et/ou Cu dans une quantité de 1 % ou moins au total, Cr : 2 % ou moins, Mo : 1 % ou moins, B : 0,0001 à 0,005 % ; Ca et/ou des métaux de terres rares dans une quantité de 0,003 % ou moins au total, et le reste comprenant du fer et des impuretés inévitables, la feuille d'acier à haute résistance ayant un facteur d'espacement d'une phase de martensite qui est une partie principale de la structure métallique de 80 % ou plus ; la taille moyenne de grain de la phase de martensite est de 10 µm ou moins en termes de l'équivalent d'un diamètre de cercle ; le facteur d'espacement de la phase martensite ayant une taille de grain de 10 µm ou plus en termes de l'équivalent d'un diamètre de cercle dans la phase de martensite est de 15 % ou moins, dans laquelle la phase de martensite est une phase de martensite revenue ; et le facteur d'espacement de la phase d'austénite résiduelle dans la structure métallique est de 2 % ou moins, et le reste de la structure métallique est principalement une phase de ferrite ; et une résistance à la traction de 590 MPa ou plus.
  2. Procédé de fabrication d'une feuille d'acier à haute résistance selon la revendication 1, le procédé comprenant l'utilisation d'une feuille d'acier dans laquelle le facteur d'espacement total de la phase de martensite et/ou de la phase d'austénite résiduelle dans la totalité de la structure métallique est de 90 % ou plus en tant que feuille d'acier de matériau ; le chauffage et le maintien de la feuille d'acier à une température de (point Ac3-50 °C) ou plus mais non supérieure au point Ac3 pendant 30 à 1200 secondes, le refroidissement de la feuille d'acier à une température de démarrage de transformation de la martensite, point Ms, ou moins à une vitesse de refroidissement moyenne de 10 °C/s ou plus ; et la réalisation en outre d'un traitement thermique dans lequel la feuille d'acier est chauffée et maintenue à une température de 300 à 500 °C pendant 60 à 1200 secondes.
EP11193479.0A 2006-07-14 2007-07-13 Feuilles d'acier très résistantes et leurs procédés de production Not-in-force EP2465962B1 (fr)

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JP2006194056 2006-07-14
JP2007144705A JP4291860B2 (ja) 2006-07-14 2007-05-31 高強度鋼板およびその製造方法
JP2007144466A JP5201653B2 (ja) 2007-05-31 2007-05-31 伸びおよび伸びフランジ性に優れた高強度鋼板およびその製造方法
JP2007145987A JP5234893B2 (ja) 2007-05-31 2007-05-31 伸びおよび伸びフランジ性に優れた高強度鋼板およびその製造方法
EP07790799.6A EP2053140B1 (fr) 2006-07-14 2007-07-13 Feuilles d'acier très résistantes et procédés de production de celles-ci

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US7090731B2 (en) * 2001-01-31 2006-08-15 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) High strength steel sheet having excellent formability and method for production thereof
FR2830260B1 (fr) * 2001-10-03 2007-02-23 Kobe Steel Ltd Tole d'acier a double phase a excellente formabilite de bords par etirage et procede de fabrication de celle-ci
JP4306202B2 (ja) * 2002-08-02 2009-07-29 住友金属工業株式会社 高張力冷延鋼板及びその製造方法
JP4062616B2 (ja) * 2002-08-12 2008-03-19 株式会社神戸製鋼所 伸びフランジ性に優れた高強度鋼板
JP4085826B2 (ja) * 2003-01-30 2008-05-14 Ｊｆｅスチール株式会社 伸びおよび伸びフランジ性に優れた二相型高張力鋼板およびその製造方法
JP4457681B2 (ja) * 2004-01-30 2010-04-28 Ｊｆｅスチール株式会社 高加工性超高強度冷延鋼板およびその製造方法
JP4396347B2 (ja) 2004-03-25 2010-01-13 Ｊｆｅスチール株式会社 延性および伸びフランジ性に優れる高張力鋼板の製造方法
JP2005336526A (ja) * 2004-05-25 2005-12-08 Kobe Steel Ltd 加工性に優れた高強度鋼板及びその製造方法
JP4445365B2 (ja) * 2004-10-06 2010-04-07 新日本製鐵株式会社 伸びと穴拡げ性に優れた高強度薄鋼板の製造方法
CA2531616A1 (fr) * 2004-12-28 2006-06-28 Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) Tole mince d'acier a haute resistance mecanique possedant une resistance elevee a la fragilisation par l'hydrogene et une grande aptitude a l'usinage
JP2006194056A (ja) 2005-01-11 2006-07-27 Hisashi Kawajiri 網戸の網の均一な張りを得るための網張りローラー用特殊具
JP4714010B2 (ja) 2005-11-25 2011-06-29 トッパン・フォームズ株式会社 ラベル製造装置
JP2007145987A (ja) 2005-11-28 2007-06-14 Kaneka Corp 環状オレフィンコポリマー樹脂押出発泡体およびその製造方法
JP4561614B2 (ja) 2005-11-29 2010-10-13 Ｊｆｅスチール株式会社 圧延材の保持装置

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EP2465962A1 (fr) 2012-06-20
EP2053140A4 (fr) 2011-06-29
CN101460647B (zh) 2015-05-20
CN101460647A (zh) 2009-06-17
US20090277547A1 (en) 2009-11-12
EP2465961A1 (fr) 2012-06-20
KR20090018166A (ko) 2009-02-19
EP2465961B1 (fr) 2013-12-04
EP2053140B1 (fr) 2013-12-04
EP2053140A1 (fr) 2009-04-29
WO2008007785A1 (fr) 2008-01-17
KR101082680B1 (ko) 2011-11-15

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