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EP2053140B1 - Hochfeste stahlbleche und herstellungsverfahren dafür - Google Patents

Hochfeste stahlbleche und herstellungsverfahren dafür Download PDF

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Publication number
EP2053140B1
EP2053140B1 EP07790799.6A EP07790799A EP2053140B1 EP 2053140 B1 EP2053140 B1 EP 2053140B1 EP 07790799 A EP07790799 A EP 07790799A EP 2053140 B1 EP2053140 B1 EP 2053140B1
Authority
EP
European Patent Office
Prior art keywords
steel sheet
conditions
martensite
mass
bainite
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
Application number
EP07790799.6A
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English (en)
French (fr)
Other versions
EP2053140A4 (de
EP2053140A1 (de
Inventor
Kenji c/o Kobe Corporate Research Lab. SAITO
Tomokazu c/o Kobe Corporate Research Lab. MASUDA
Masaaki c/o Kobe Corporate Research Lab. MIURA
yoichi c/o Kobe Corporate Research Lab. MUKAI
Shushi c/o Kobe Corporate Research Lab. IKEDA
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Kobe Steel Ltd
Original Assignee
Kobe Steel Ltd
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Priority claimed from JP2007145987A external-priority patent/JP5234893B2/ja
Priority claimed from JP2007144705A external-priority patent/JP4291860B2/ja
Priority claimed from JP2007144466A external-priority patent/JP5201653B2/ja
Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Priority to EP11193479.0A priority Critical patent/EP2465962B1/de
Priority to EP11193464.2A priority patent/EP2465961B1/de
Publication of EP2053140A1 publication Critical patent/EP2053140A1/de
Publication of EP2053140A4 publication Critical patent/EP2053140A4/de
Application granted granted Critical
Publication of EP2053140B1 publication Critical patent/EP2053140B1/de
Not-in-force legal-status Critical Current
Anticipated expiration legal-status Critical

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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 1170 MPa, 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.
  • JP 2004 091924 discloses a high strength steel sheet having the contents of C, Si+Al, Mn, P, S, Ca and rare earth elements as prescribed, optionally one or more kinds of the metals selected from Mo, Ni, Cu, Cr and optionally one or more metals selected from Ti, Nb and V, having a space factor of tempered martensite or tempered bainite or ferrite in a base phase structure to the whole structure and the space factor of retained austenite in a second phase structure to the whole structure, the C concentration in the retained austenite and the ratio of the lath-shaped retained austenite occupied in the while retained austenite are described.
  • EP 1 365 037 discloses a high strength steel sheet having a base phase structure, the base phase structure being tempered martensite or tempered bainite and accounting for 50% or more in terms of a space factor relative to the whole structure, or the base phase structure comprising tempered martensite or tempered bainite which accounts for 15% or more in terms of a space factor relative to the whole structure and further comprising ferrite, the tempered martensite or the tempered bainite having a hardness which satisfies the relation of Vickers hardness (Hv) ⁇ 500 [C] +30 [Si] +3 [Mn] +50 where [ ] represents the content (mass %) of each element; and a second phase structure comprising retained austenite which accounts for 3 to 30% in terms of a space factor relative to the whole structure and optionally further comprising bainite and/or martensite,; the retained austenite having a C concentration of 0.8% or more.
  • Hv Vickers hardness
  • US 2003/0084966 discloses a dual-phase steel sheet containing (on the mass% basis).
  • C 0.01-0.20%, Si: 0.5% or less, Mn: 0.5-3%, sol.
  • Al 0.06% or less (inclusive 0%), P: 0.15% or less (exclusive 0%), and S: 0.02% or less (inclusive 0%), and in which the matrix phase contains tempered martensite; tempered martensite and ferrite; tempered bainite; or tempered bainite and ferrite, and the second phase comprises 1 to 30% of martensite at an area ratio based on the entire structure.
  • JP-A Japanese Unexamined Patent Application Publication
  • Patent document 2 JP-A-S60-43425
  • Patent document 3 Japanese Patent No. 3729108
  • Patent document 4 JP-A-2005-272954
  • 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%, optionally further comprising at least an element selected from Ti, Nb, V and Zr in an amount of 0.01 to 1% by mass in total, optionally further comprising Ni and/or Cu in an amount of 1% by mass or lower in total, optionally further comprising Cr: 2% by mass or less and/or Mo: 1% by mass or less, optionally further comprising 0.0001 to 0.005% by mass of B, optionally further comprising Ca and/or REM in an amount of 0.003% by mass or lower in total, and the remainder comprising iron and inevitable impurities, wherein the structure which is a main part of the metal structure is the martensite phase, which is tempered martensite, and finely dispersed annealed bainite; the space factor of the tempered martensite is 70 to 95%; the space
  • the inventors of the present invention have studied various structures that can ensure high strength and improve elongation, especially stretch-flanging performance at the same time.
  • the inventors found the following: by annealed bainite, which is a fine lath-shaped structure, as an initial structure in a two phase temperature region of ferrite +austenite (hereinafter referred to as "two-phase region annealing".), fine annealed bainite produced in a base material acts in a manner of suppressing the growth of austenite, fine tempered martensite is produced from austenite by the following hardening and tempering, and the entire structure is formed from these microstructures. Therefore, elongation and stretch-flanging performance are improved.
  • the inventors accomplished the present invention based on these findings.
  • 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.
  • 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.
  • the method for manufacturing a high strength steel sheet with excellent elongation and stretch-flanging performance comprises using a steel sheet having a space factor of bainite in the entire metal structure of 90% or higher as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac 3 point -100°C) or higher but not higher than Ac 3 point for 0 to 2400 seconds (including 0 seconds), and then cooling to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10°C/sec. or higher, subsequently heating and retaining the steel sheet at a temperature of 300 to 550°C for 60 to 1200 seconds.
  • the high strength steel sheet of the present invention is thus produced.
  • the material steel sheet can be produced by hot rolling a steel piece having the above-mentioned chemical component or further by cold rolling the same.
  • 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 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.
  • a structure which is mainly composed of especially tempered martensite and finely dispersed annealed bainite is provided, wherein the space factors thereof are defined to have predetermined amounts, and the mean grain size of tempered martensite is defined 10 ⁇ m or smaller. Accordingly, a high strength steel sheet which has strength as high as 590 MPa or higher, excellent elongation and stretch-flanging performance, and thus excellent press formability can be provided.
  • a high strength steel sheet is designed to have a structure, as the main body, in which annealed bainite is finely dispersed in tempered martensite, a space factor of the tempered martensite of 70 to 95%, a space factor of the annealed bainite of 5 to 30%, a mean grain size of the tempered martensite of 10 ⁇ m or smaller in terms of the equivalent of a circle diameter, wherein the structure which is a main part of the metal structure is the martensite phase, which is tempered martensite, and finely dispersed annealed bainite; the space factor of the tempered martensite is 70 to 95%; the space factor of the annealed bainite is 5 to 30%; and the total space factor of tempered martensite and annealed bainite is 95% or higher; and a mean grain size of the tempered martensite is 10 ⁇ m or lower in terms of the equivalent of a circle diameter, and a tensile strength of 5
  • the space factor of the annealed bainite is 5% or lower, the pinning effect, which suppresses the growth of austenite, is weak, and austenite grains grow so that martensite grains become large, thereby preventing ensuring good elongation.
  • the space factor is higher than 30%, stretch-flanging performance is lowered.
  • the lower limit of annealed bainite is 5%, and preferably 7%, while its upper limit is 30%, and preferably 25%.
  • the space factor of tempered martensite is lower than 50%, strength and stretch-flanging performance are lowered. In contrast, when the space factor is higher than 95%, the steel sheet becomes too hard and thus elongation is lowered. For this reason, the lower limit of the tempered martensite phase is 70%, while its upper limit is 95%, and preferably 85%.
  • the mean grain size of the tempered martensite varies depending on the amount of annealed bainite finely dispersed.
  • the grain size is larger than 10 ⁇ m in terms of the equivalent of a circle diameter, elongation and stretch-flanging performance are lowered. For this reason, the upper limit is 10 ⁇ m.
  • the structure in which the tempered martensite and annealed bainite coexist constitutes the main part of the structure of the high strength steel sheet of the present invention.
  • the main part means 95% or higher, and other structures contained in an amount of less than about 10% are permitted because they hardly affect elongation, especially stretch-flanging performance.
  • examples of other structures include ferrite, pearlite, retained austenite and the like. Of course, the less these structures, the better.
  • Chemical component (unit: % by mass) which is preferable for obtaining the structure and strength of the steel sheet according to the present invention will be described now.
  • Examples of such a chemical component include that comprises the followings: C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0%, and Al: 0.01 to 0.1%, and Fe and inevitable impurities as the remainder. The reasons for component limitation will be described below.
  • the amount of C is an important element in producing martensite, and increasing the strength of the steel sheet.
  • the amount of C is lower than 0.05%, such an effect is excessively lowered.
  • the higher the amount of C the more preferable.
  • the lower limit of the amount of C is 0.05%, and preferably 0.07%, while its upper limit is 0.3%, and preferably 0.25%.
  • Si acts as a deoxidizing element when steel is melted, and is an element effective in increasing strength without deteriorating the ductility of steel. Si also acts to suppress deposition of coarse carbide which deteriorates stretch-flanging performance.
  • the amount of Si is lower than 0.01%, these actions are excessively lowered, while addition of the same in an amount higher than about 3.0% saturates the effect.
  • the lower limit of the amount of Si is 0.01%, and preferably 0.1%, while its upper limit is 3.0%, and preferably 2.5%.
  • Mn is an element useful in increasing the hardening characteristics of steel to ensure high strength, but when its amount is lower than 0.5%, such an action is excessively lowered. In contrast, when its amount is higher than 3%, ductility is lowered and processability is thus adversely affected. For this reason, the lower limit of the amount of Mn is 0.5%, and preferably 0.7%, while its upper limit is 3%, and preferably 2.5%.
  • Al is an element which has a deoxidation effect, and needs to be added in an amount of 0.01% or higher to perform the effect. In contrast, even if it is added in an amount higher than 0.1%, the deoxidation effect is saturated, and it becomes a source of non-metallic mediators to deteriorate physical properties and surface properties. For this reason, the lower limit of the amount of Al is 0.01%, and preferably 0.03%, while its upper limit is 0.1%, and preferably 0.08%.
  • Preferable chemical components of the steel sheet the present invention include, in addition to the above-mentioned basic components, Fe and impurities which inevitably get in, for example, P, S, N and O.
  • any of the auxiliary element groups (a) to (e) described below, or one or more element selected from a plurality of groups may be added within the additional permissible range of each group.
  • These elements form precipitates such as carbides, nitrides, and carbonitrides together with C and N, and contribute to the improvement of strength. They also have an action to increase elongation and stretch-flanging performance by micronizing crystal grains during hot rolling. When the total amount of these elements added is 0.01%, such an action is excessively lowered. In contrast, when the amount is higher than 1%, elongation and stretch-flanging performance are lowered rather than increased. For this reason, the lower limit of the total amount of one or more of these elements is 0.01%, and preferably 0.03%, while its upper limit is 1.0%, and preferably 0.7%.
  • these elements are effective in maintaining the balance of strength and ductility high and realizing high strength at the same time.
  • the higher the amount of these elements contained the higher the above-mentioned effect, but when the total amount of one or more of these elements is higher than 1%, such an effect is saturated, and cracks may occur during hot rolling.
  • the upper limit of the total amount of these elements is 1.0%, and preferably 0.7%.
  • the amount of Cr is 2.0% or lower, and more preferably 1.5% or lower, while the amount of Mo is 1.0% or lower, and more preferably 0.7% or lower.
  • B is an element effective in improving hardening characteristics, and increasing the strength of the steel sheet when added in a minute amount. To perform such an effect, it is preferable that the element is contained in an amount of 0.0001% or higher. However, when the amount of B contained is excessive and higher than 0.005%, crystal grain boundaries may be embrittled and cracks may occur during rolling. For this reason, the upper limit of the amount of B is 0.005%.
  • a material steel sheet which has the above-mentioned chemical components and a space factor of bainite to the entire structure of 90% or higher is prepared.
  • this material steel sheet is retained at a temperature of (Ac 3 point-100) °C or higher but not higher than Ac 3 for 0 sec. or longer but nor longer than 2400 sec., and then an annealing heat treatment is carried out, in which the material steel sheet is cooled to the martensite transformation start temperature, Ms point, or lower at an average cooling rate of 10°C/sec. or higher.
  • a tempering heat treatment is carried out, in which the material steel sheet is retained at 300°C or higher but not higher than 550°C for 60 sec. or longer but not longer than 1200 sec., whereby a microstructure steel sheet mainly composed of the tempered martensite and annealed bainite and having a tensile strength of 590 MPa or higher is obtained.
  • the material steel sheet can be produced by the steps described below. First, steel having the above-mentioned chemical components is melted, By using the steel slab, hot rolling is terminated in such a manner that the finishing temperature is not lower than Ar 3 point. Second, the steel slab is cooled at an average cooling rate of 10°C/sec. or higher to the bainite transformation temperature (about 350 to 450°C), and is wound up at the same temperature. When the finishing temperature is lower than Ar 3 point or the cooling rate after the hot rolling is lower than 10°C/sec., a ferrite phase is likely to be produced in the hot-rolled steel sheet, and the space factor of bainite of the material steel sheet becomes lower than 90%.
  • the material steel sheet used may be a cold-rolled steel sheet produced by hot rolling steel and then subj ecting the steel to an acid cleaning process and cold rolling.
  • the steel types which contain Ti, Nb, V and Zr, to re-solutionize precipitates containing the elements produced before hot rolling it is preferable to heat and retain the steel piece at a relatively high temperature during hot rolling.
  • the space factor of bainite can be made 90% or higher by subjecting a hot-rolled steel sheet which does not meet the above hot rolling condition and cooling condition to preliminary annealing.
  • This preliminary annealing is a heat treatment in which a hot-rolled steel sheet is retained in a temperature range of Ac 3 point or higher for about 5 seconds, and then the steel sheet is cooled at an average cooling rate of 10°C/sec. or higher to the bainite transformation temperature.
  • the retaining temperature is lower than Ac 3 point, the ferrite phase is likely to be produced in the steel sheet, and the space factor of bainite is lowered.
  • the material steel sheet is retained at a temperature (Ac 3 point -100) °C or higher but not higher than Ac 3 for 0 sec. or longer (including 0 sec.) but not longer than 2400 sec., and then two-phase region annealing is carried out, in which the material steel sheet is cooled to the martensite transformation start temperature, Ms point, or lower at an average cooling rate of 10°C/sec. or higher, followed by tempering.
  • the structure of the high strength steel sheet according to the present invention is obtained.
  • the conditions of the two-phase region annealing will be described below.
  • the annealing temperature of the two-phase region annealing is set to (Ac 3 point -100) °C or higher but not higher than Ac 3 is as follows:
  • the annealing temperature is set to a temperature range higher than Ac 3 point in which the austenite single phase is stable, the crystal grains of austenite grow in the material steel sheet and combine with each other to become coarse, and the growth inhibitory effect (pinning effect) of austenite by finely dispersed annealed bainite cannot be obtained. For this reason, a fine dual phase steel sheet cannot be obtained, and the stretch-flanging performance of the high strength steel sheet is lowered.
  • the annealing time heat and retaining time
  • austenite having a space factor of about 50% and thus martensite can be obtained simply by heating the steel sheet to the annealing temperature, but the time is preferably 1 sec. or longer, and more preferably 5 seconds or longer.
  • the retaining time is limited to 2400 sec. or shorter, and preferably 1200 sec. or shorter.
  • Tempering is carried out after the two-phase region annealing, which is a process for improving elongation and stretch-flanging performance by softening hard martensite, and decomposing retained austenite which produces martensite by working-induced transformation.
  • Tempering conditions are as follows: the material steel sheet is retained at a temperature of 300°C or higher but not higher than 550°C for 60 sec. or longer but not longer than 1200 sec.
  • the cooling rate after retaining is not especially limited.
  • tempering temperature When the tempering temperature is lower than 300°C, softening of martensite is insufficient, and the elongation and stretch-flanging performance of the steel sheet are lowered. In contrast, when the temperature is higher than 550°C, a coarse cementite phase is deposited, and the stretch-flanging performance of the steel sheet is lowered. For this reason, tempering is carried out at a temperature of 300°C or higher but not higher than 550°C.
  • the lower limit of the retaining time during tempering is 60 sec., preferably 90 sec. or longer, and more preferably 120 sec.
  • the upper limit is 1200 sec., preferably 900 sec., and more preferably 600 sec.
  • the structures (space factors of annealed bainite, space factors and mean grain sizes of tempered martensite), and mechanical characteristics (tensile strength TS, elongation EL and stretch-flanging performance) of the sample steel sheets were determined in the manner described below.
  • Test pieces for observing structures were collected from the sample steel sheets, and the space factors of annealedbainite and temperedmartensite were determined by subjecting microscope structure pictures after being corroded with natal to image analysis.
  • the mean grain sizes of tempered martensite were determined by measuring the areas of the grains by structure analysis using FE/SEM-EBSP, determining the diameters of circles corresponding to the grains, and averaging the diameters.
  • tensile strength and elongation were determined by using a universal tensile tester manufactured by Instron and JIS No. 5 tensile test piece. Stretch-flanging performance was determined by measuring a hole expansion rate (A) by using a 20-ton hole expansion tester manufactured by Tokyo Koki, according to The Japan Iron and Steel Federation standard (JFST 1001-1996), and was evaluated based on this. The results of these measurements are also shown in Table 4.
  • Table 4 as for "evaluation”, tensile strength (TS) of 590 MPa or higher, elongation (EL) of 10% or higher, and hole expansion rate ( ⁇ ) of 80% or higher were rated excellent characteristics. The samples which were excellent in all three characteristics were rated o; those which were excellent in two characteristics out of three were rated ⁇ ; and those which were excellent in only one characteristic out of three were rated x.
  • sample steel sheets sample Nos. 1, 2, 4, 5, 7, 8, 11, 12, 14, 15 and 17 to 27 in which the conditions of the present invention were met in terms of all of chemical components, material steel sheet structures, final annealing conditions and tempering conditions all have tensile strengths as high as 590 MPa or higher, elongations of 10% or higher, and stretch-flanging performances of hole expansion rates of 80% or higher. That is, it can be seen that these samples have high strength and yet excellent elongation and stretch-flanging performance, and excellent press formability.
  • 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. Hochfestes Stahlblech, das in Masseprozent umfasst: C: 0,05 bis 0,3 %; Si: 3 % oder weniger (ausgenommen 0 %); Mn: 0,5 bis 3,0 %; Al: 0,01 bis 0,1 %; optional weiterhin umfassend wenigstens ein Element ausgewählt aus Ti, Nb, V und Zr in einer Menge von insgesamt 0,01 bis 1 Masse%, optional weiterhin umfassend Ni und/oder Cu in einer Menge von insgesamt 1 Masse% oder weniger, optional weiterhin umfassend Cr: 2 Masse% oder weniger und/oder Mo: 1 Masse% oder weniger, optional weiterhin umfassend 0,0001 bis 0,005 Masse% B, optional weiterhin umfassend Ca und/oder Seltenerdmetalle in einer Menge von insgesamt 0,003 Masse% oder weniger, und wobei der Rest Eisen und unvermeidbare Verunreinigungen umfasst, wobei die Struktur, die ein Hauptbestandteil der Metallstruktur ist, die Martensitphase, bei der es sich um angelassenen Martensit handelt, und fein verteilter geglühter Bainit ist; der Raumfaktor des angelassenen Martensits 70 bis 95 % beträgt; der Raumfaktor des geglühten Bainits 5 bis 30 % beträgt; und der Gesamt-Raumfaktor des angelassenen Martensits und des geglühten Bainits 95 % oder mehr beträgt; und eine mittlere Korngröße des angelassenen Martensits, bezogen auf das Äquivalent eines Kreisdurchmessers, 10 µm oder weniger beträgt, und eine Zugfestigkeit von 590 MPa oder mehr.
  2. Verfahren zur Herstellung eines hochfesten Stahlblechs nach Anspruch 1, wobei das Verfahren umfasst: Verwenden eines Stahlblechs mit einem Raumfaktor von Bainit in der gesamten Metallstruktur von 90 % oder mehr als Material-Stahlblech; Erhitzen und Halten des Material-Stahlblechs auf/bei eine(r) Temperatur von (Ac3-Punkt - 100 °C) oder höher, jedoch nicht höher als der Ac3-Punkt, für 0 bis 2400 Sekunden (einschließlich 0 Sekunden); dann Abkühlen des Material-Stahlblechs auf eine Umwandlungsstarttemperatur von Martensit, den Ms-Punkt, oder niedriger mit einer durchschnittlichen Abkühlgeschwindigkeit von 10 °C/s oder höher; und anschließend Durchführen einer Wärmebehandlung, bei der das Stahlblech 60 bis 1200 Sekunden lang auf/bei eine(r) Temperatur von 300 bis 550 °C erhitzt und gehalten wird.
EP07790799.6A 2006-07-14 2007-07-13 Hochfeste stahlbleche und herstellungsverfahren dafür Not-in-force EP2053140B1 (de)

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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
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
PCT/JP2007/064019 WO2008007785A1 (fr) 2006-07-14 2007-07-13 Feuilles d'acier très résistantes et procédés de production de celles-ci

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CN101460647B (zh) 2015-05-20
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US20090277547A1 (en) 2009-11-12
EP2053140A4 (de) 2011-06-29
KR101082680B1 (ko) 2011-11-15
CN101460647A (zh) 2009-06-17
EP2053140A1 (de) 2009-04-29
EP2465962B1 (de) 2013-12-04
KR20090018166A (ko) 2009-02-19
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