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-[0003] 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.
• DP steel dual phase steel
• 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).
• elongation due to its soft ferrite and strength due to its rigid martensite
• elongation in particular, uniform elongation
• distortion stress
• 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.
• 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.
• 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 the remainder comprising iron and inevitable impurities, has a space factor of a martensite phase which is a main component of a metal structure of 50% or higher, and has a tensile strength of 590 MPa or higher.
• 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.
• the high strength steel sheet of the present invention has a structure mainly comprising tempered martensite and finely dispersed annealed bainite, a space factor of the tempered martensite of 50 to 95%, a space factor of the annealed bainite of 5 to 30%, and a mean grain size of the tempered martensite in terms of the equivalent of a circle diameter of 10 ⁇ m or lower.
• 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.
• 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. Moreover, 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 inventors of the present invention have also invented a high strength steel sheet having a limited ratio by volume of the retained austenite phase of 3%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 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, and the space factor of the retained austenite phase in the metal structure is 3% or lower.
• 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 3% at the highest in the present invention, and therefore stretch-flanging performance is not lowered.
• the martensite phase is a tempered martensite phase; an annealed martensite phase is contained as the metal structure other than the martensite phase and the retained austenite phase; and that the space factor of the annealed martensite phase is 3 to 20%.
• Such features suppress combination between the crystal grains of the austenite phase and the growth of the same by the finely dispersed annealed martensite phase. As a result, the final structure is micronized, and the processability of the high strength steel sheet is ensured.
• 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-100°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 structure which is the main part of the metal structure is a martensite phase and a ferrite phase; the space factor of the martensite phase is 50 to 95% (meaning “% by volume", and so on); the space factor of the ferrite phase is 5 to 30%; and the mean grain size of the martensite phase is 10 ⁇ m or smaller in terms of the equivalent of a circle diameter.
• the ferrite phase is preferably an annealed martensite.
• 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, as a material steel sheet, a steel sheet in which the total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher and the grain size of the former austenite is 20 ⁇ m or smaller in terms of the equivalent of a circle diameter, 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 1 to 2400 seconds, then 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 subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550°C for 60 to 1200 seconds.
• a material steel sheet a steel sheet in which the total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher and the grain size of the former
• 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.
• this high strength steel sheet in which the space factor of the retained austenite phase is 3% or lower and the space factor of the fine 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 present invention it is also possible to achieve a high strength steel sheet which has excellent elongation and stretch-flanging performance at the same time by designing the steel sheet especially for a dual phase steel sheet mainly composed of a ferrite phase and martensite, ensuring high strength of the steel sheet as a whole, and appropriately controlling the space factors of especially the ferrite phase and martensite and the mean grain sizes of the same.
• a high strength steel sheet according to one embodiment of the present invention 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 50 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, and a tensile strength of 590 MPa or higher.
• 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 lower limit of the tempered martensite phase is 50%, and more preferably 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 90% or higher, and preferably 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 bymicronizing 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 maybe 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 annealed bainite and tempered martensite 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 ( ⁇ ) 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 ⁇ .
• 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.
• composition of constituents of the high strength steel sheet material of this embodiment will be described.
• 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 3% 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 3% 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 preferably 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 measurements.
• 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 o; a test steel sheet which meets all the conditions was rated O; 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, 17, 18, 20, 23, 24, 27, 28, 33, 34, 37, 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 Al 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.
• a very fine ferrite + martensite dual structure can be obtained by subjecting a steel sheet having a fine lath-shaped structure (martensite and/or bainite) as a material steel sheet (that is, as an initial structure) to annealing (hereinafter referred to as "dual-phase range annealing") in a dual-phase range (ferrite +austenite range).
• a steel sheet having such a structure has good elongation and stretch-flanging performance.
• ferrite produced by the dual-phase range annealing is finely dispersed, and the growth of austenite during the dual-phase range annealing is suppressed by its pinning effect. Accordingly, its structure after hardening becomes a very fine ferrite + martensite structure.
• crystal grain micronizing elements such as Ti, Nb, V and Zr are added to the steel sheet as chemical components, whereby further micronization of the structure can be achieved. The thus-obtained dual phase steel sheet is imparted further improved elongation and stretch-flanging performance.
• the high strength steel sheet of the present invention is a dual phase steel sheet which is mainly composed of a ferrite phase and martensite.
• the space factors of these phases to the entire structure are adjusted appropriately. That is, in the high strength steel sheet of the present invention, the space factors of the ferrite phase and martensite are 5 to 30% and 50 to 95%, respectively.
• a preferable space factor of the ferrite phase is 7% or higher but not higher than 25%.
• a preferable space factor of the martensite phase is 70% or higher but not higher than 85%.
• space factor means the ratio (% by volume) of each phase constituting a metal structure in the steel material to the entire structure, and the space factors of the ferrite phase and martensite can be determined by corroding a steel material with nital, observing the material with an optical microscope (1000 times), and then subjecting the material to image analysis.
• the mean grain size of the above ferrite phase is 3 ⁇ m or smaller in terms of the equivalent of a circle diameter, and that the mean grain size of the martensite phase is 6 ⁇ m or smaller in terms of the equivalent of a circle diameter. If these sizes are increased, elongation and stretch-flanging performance are lowered.
• the "mean grain sizes" of these phases are determined, for example, by measuring the grain sizes of twenty grains by observing the structure using an optical microscope and FE/SEM-EBSP, and averaging the measurements.
• the dual phase steel sheet according to the present invention is composed of a ferrite phase and martensite as its main structure, but it is not necessarily 100% composed of these phases, and it is also allowed that at least the total sum is 70% or higher, preferably 80% or higher, in terms of space factor, due to the intention that it is merely the main part, and that bainite, pearlite, retained austenite and the like are contained.
• a preferable composition of constituents considering the strength (590 MPa or higher as tensile strength TS) and other points is as follows: C: 0-05 to 0.3%; Si: 0.01 to 3%; Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%; at least one element selected from the group consisting of Ti, Nb, V and Zr: 0.01 to 1% in total; and iron and inevitable impurities as the remainder.
• the reason for the definition of these preferable ranges is as follow:
• the amount of C contained is preferably 0.05% or higher. From the perspective of increasing strength, the higher the amount of C contained, the better. However, if the amount of C is excessively high, a large amount of retained austenite which deteriorates stretch-flanging performance is produced, and weldability is also adversely affected. Therefore, the amount is preferably 0.3% or lower. A more preferable lower limit of the amount of C contained is 0.07%, and a more preferable upper limit is 0.25%.
• Si is an element which effectively acts as a deoxidizing element when steel is melted, and effectively increases strength without deteriorating the ductility of steel. It also acts to suppress deposition of coarse carbide which deteriorates stretch-flanging performance. In order to perform these effects effectively, it is preferably contained in an amount of 0.01% or higher. However, since the effect of adding Si is saturated in an amount of about 3%, A preferable upper limit is set to 3%. A more preferable lower limit of the amount of Si contained is 0.1%, and a more preferable upper limit is 2.5%.
• Mn is an element useful in increasing the hardening characteristics of the steel sheet to ensure high strength. To perform such an effect, it is preferably contained in an amount of 0.5% or higher. However, when the amount of Mn contained is excessively high, ductility is lowered and therefore processability is adversely affected. For this reason, the upper limit is set to 3.0%. A more preferable amount of Mn contained is 0.7% or higher but not higher than 2.5% or lower.
• Al is an element having a deoxidation effect, and when Al deoxidation is performed, it needs to be added in an amount of 0.01% or higher.
• the upper limit is set to 0.1%.
• a more preferable amount of Al contained is 0.03% or higher but not higher than 0.08%.
• These elements have the action to form precipitates such as carbide, nitride and carbonitride together with C and N to contribute to increased strength, and micronize crystal grains during hot rolling to increase elongation and stretch-flanging performance.
• Such effects are effectively performed when they are added in an amount of 0.01% or higher in total (of one or more members).
• a more preferable amount of these elements contained is 0.03% or higher. however, when the amount is excessively high, elongation and stretch-flanging performance are deteriorated rather than improved. Therefore, the amount is to be limited to 1% or lower, and more preferably 0.7% or lower.
• Preferable basic components in the dual phase steel sheet of the present invention are as stated above, and the remainder is iron and inevitable impurities.
• inevitable impurities include steel raw materials, and P, S, N, O and others which can get into steel during the manufacturing process of the materials, among others.
• Ni and/or Cu 1% or lower (not including 0%) in total
• Cr 2% or lower (not including 0%) and/or Mo: 1% or lower (not including 0%)
• B 0.0001 to 0.005%
• Ca and/or REM 0.003% or lower (not including 0%) in total, among others.
• the characteristics of the steel sheet are further improved depending on the types of the components contained. The reason for setting the range of these elements when contained is as follows.
• Both Cr and Mo are elements effective in stabilizing the austenite phase, and facilitating the generation of the low-temperature transformation phase in the course of cooling. Although their effects increase as their amount contained is increased, if they are contained in an excessively high amount, ductility is deteriorated. Therefore, the amount of Cr is to be limited to 2% or lower (more preferably 1.5% or lower), and the amount of Mo is to be limited to 1% or lower (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 exhibit such an effect, it is preferably contained in an amount of 0.0001% or higher. However, when the amount of B contained is excessively high and is higher than 0.005%, crystal grain boundaries may be embrittled and cracks may occur during rolling.
• Ca and REM are elements effective in controlling the form of sulfide in the steel and improving processability.
• the material steel sheet used in the present invention has a space factor of the low-temperature transformation phase of 90% or higher.
• This low-temperature transformation phase may be constituted only by martensite or bainite.
• the space factor of the low-temperature transformation phase is lower than 90%, and the material steel sheet is heated to a 2-phase range of the ferrite phase and austenite phase (dual-phase range annealing) in the annealing step (final annealing step) described later, a coarse ferrite phase and an austenite phase are produced. Therefore, the fine ferrite phase and martensite mentioned above cannot be obtained in the final structure. As a result, stretch-flanging performance cannot be improved.
• a material steel sheet having space factor of the low-temperature transformation phase of 90% or higher can be produced by the following steps: First, a steel slab adjusted to meet the composition of chemical constituents as mentioned above is hot-rolled in such a manner that the finishing rolling temperature is higher than Ac 3 point. Second, the material steel sheet is cooled to a temperature lower than a martensite transformation start temperature, Ms point (temperature at which the austenite phase starts to transform into martensite), at an average cooling rate of 10°C/sec. or higher, and is then wound up, giving a material steel sheet having a space factor of martensite of 90% or higher.
• Ms point temperature at which the austenite phase starts to transform into martensite
• a material steel sheet which is mainly composed of bainite and has a space factor of the low-temperature transformation phase of 90% or higher is obtained by cooling the material steel sheet to a bainite transformation temperature after the hot rolling, at an average cooling rate of 10°C/sec. or higher and winding up the same.
• the finishing rolling temperature is Ac 3 point or lower or the cooling rate after the hot rolling is 10°C/sec. or lower, a ferrite phase is likely to be produced during cooling after the hot rolling, and therefore the space factor of the low-temperature transformation phase after the hot rolling is not 90% or higher.
• the grain size of austenite is micronized by utilizing the pinning effect by finely depositing a micro-alloy (Ti, Nb, V, Zr, etc.). In order to do so, it is necessary to re-solutionize the deposition of the coarse micro-alloy produced prior to the hot rolling step.
• the heating temperature and its retaining time is preferably 1000°C or higher, and 600 seconds or longer, respectively, to perform the solutionization effect of the micro-alloy (Ti, Nb, V, Zr and the like).
• the heating temperature is 1400°C or higher and its retaining time is longer than 1000 seconds, the grain size of austenite becomes undesirably coarse.
• the material steel sheet used in the present invention needs to have the grain size of the former austenite of 20 ⁇ m or smaller. This is from the perspective of improvement of elongation and stretch-flanging performance due to the micronization of the structure. That is, by subjecting a basis steel sheet having a grain size of the former austenite of 20 ⁇ m or smaller to the final annealing step and tempering step, the final structure becomes finer than in the case where the grain size is larger than 20 ⁇ m, and elongation and stretch-flanging performance are significantly improved.
• This preliminary annealing is a treatment in which the above steel sheet is retained in a temperature range of Ac 3 point or higher for 5 seconds or longer, and is then cooled at an average cooling rate of 10°C/sec. or higher to a temperature of Ms point or lower or to the bainite transformation temperature range and retained.
• the retaining temperature of the steel sheet is lower than Ac 3 point, a ferrite phase is likely to be produced, and a space factor of the low-temperature transformation phase of 90% or higher is not attained.
• the retaining time is shorter than 5 seconds, transformation of the metal structure into austenite is insufficient, and a space factor of 90% or higher is not attained.
• a material steel sheet is subjected to a heat treatment in which it is heated to and retained at a temperature range of (Ac 3 point -100°C) or higher but not higher than Ac 3 point for 1 second or longer but not longer than 2400 seconds, and is then cooled at a cooling rate of 10°C/sec. or higher to Ms point or lower (cooling stop temperature).
• a steel sheet having the structure (the space factor of ferrite: 5 to 30%, the space factor of martensite: 50 to 95%) mentioned above is obtained.
• the mean crystal grain diameters of the ferrite phase and martensite in the high strength steel sheet which is finally obtained are determined by the sizes of the crystal grains of the ferrite phase and austenite produced when the material steel sheet is heated to and retained at a temperature range of (Ac 3 point -100°C) or higher but not higher than Ac 3 point. That is, in order to obtain a fine dual phase steel sheet in which the mean grain size of the ferrite phase is 3 ⁇ m or smaller and the mean grain size of martensite is 6 ⁇ m or smaller, it is necessary to heat the material steel sheet to a temperature range of (Ac 3 point -100°C) or higher but not higher than Ac 3 point and retain at the same.
• the basis steel sheet has a structure form mainly composed of a highly micronized lath-shaped low-temperature transformation phase due to the micronization effect of the micro-alloy.
• a finely dispersed ferrite phase having a low space factor is produced.
• the term "ferrite phase” used in this invention denotes annealed martensite or annealed bainite produced when martensite or bainite is annealed at a high temperature (dual-phase range).
• the final structure obtained in the following hardening and tempering steps becomes a structure mainly composed of a very fine ferrite phase and martensite.
• a temperature lower than (Ac 3 point -100°C) transformation into austenite does not proceed sufficiently, and the space factor of martensite after the heat treatment becomes lower than 50%, thereby lowering the stretch-flanging performance of the steel sheet.
• the heating and retaining time in the final annealing needs to be in the range of 1 second or longer but not longer than 2400 seconds. It is preferably 5 seconds or longer but shorter than 1200 seconds.
• the cooling rate after heating and retaining is 10°C/sec. or lower or the cooling stop temperature is higher than Ms point, generation of bainite, retained austenite phase and pearlite, generation of more ferrite phase than necessary, and deposition of a cementite phase are caused, and structures other than martensite are formed in large amounts. Therefore, the space factor of martensite is lowered, and the space factor and mean crystal grain size of the ferrite phase are excessively increased, leading to lowered elongation and stretch-flanging performance.
• the higher the cooling rate at this time, and the lower the cooling stop temperature the higher the space factor of martensite is likely to be. However, since the temperature and time of the above dual-phase range annealing are suitably controlled, the space factor becomes no greater than 95%.
• tempering reheating treatment
• fine (ferrite phase +martensite) is formed in its metal structure.
• martensite in an annealed state is very hard, which lowers elongation.
• a difference in hardness between martensite and soft ferrite is large, which leads to lowered stretch-flanging performance.
• the hardness of martensite needs to be reduced than in an annealed state, which is why it is subjected to the tempering step.
• the retaining temperature in this tempering step is lower than 300°C, softening of martensite is insufficient, and therefore elongation and stretch-flanging performance of the steel sheet are lowered.
• the retaining temperature is higher than 550°C, a coarse cementite phase is deposited, whereby the stretch-flanging performance of the steel sheet is lowered.
• the retaining time of the tempering step is shorter than 60 seconds, softening of martensite is insufficient, and therefore elongation and stretch-flanging performance of the steel sheet are lowered.
• the retaining time is longer than 1200 seconds, martensite is too softened so that ensuring strength is made difficult, and the stretch-flanging performance of the steel sheet is lowered by the deposition of cementite.
• This retaining time 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.
• a steel sheet in which the space factors and grain sizes of the ferrite phase and martensite are suitably adjusted can be obtained, and a tensile strength as high as 590 MPa and excellent elongation and stretch-flanging performance are achieved.
• Such a high strength steel sheet can be used as a steel sheet with excellent press formability as a material for various steel products typically including automobiles.
• Tables 10 and 11 steel slabs having compositions of chemical constituents shown in Tables 10 and 11 below were prepared, and material steel sheets were prepared from the steel slabs under the hot rolling conditions and preliminary annealing conditions shown in Tables 12 and 13 below.
• Tables 10 and 11 also show the Ac 3 point (Ac 3 transformation point) and martensite transformation start temperature, Ms point, for each steel type determined by equations (1) and (2).
• the material steel sheets obtained were subjected to the final annealing and reheating (tempering) under the conditions shown in Tables 14 and 15 below to prepare test steel sheets, and the structures (space factor of ferrite ⁇ , mean grain size of ferrite ⁇ , space factor of martensite M, and mean grain size of martensite M) and mechanical characteristics (tensile strength TS, elongation EL, hole expansion rate ⁇ ) of the test steel sheets were determined by the methods described below.
• Tables 14 and 15 below also show the structures [phase constitution, space factor of low-temperature transformation phase, grain size of former austenite ( ⁇ )] of the test steel sheets before the final annealing.
• the space factors of ferrite ⁇ and martensite M were determined by subjecting the structure pictures of the test steel sheets after being corroded with nital to image analysis.
• the mean grain sizes of ferrite ⁇ and martensite M were measured by structure analysis using FE/SEM-EBSP, and the measurements were converted into "the equivalent of a circle diameter" described above to determine their mean value.
• test pieces of No.1 to 3, 6, 9, 10, 13, 16 to 18 and 33 to 36 are not provided with satisfactory characteristics as the followings because at least one requirement of their composition of chemical constituents and manufacturing conditions falls outside the scope defined in the present invention.
• the test piece of Experiment No.3 has low tensile strength TS since the amount of C contained does not fall with in the preferable range defined in the present invention.
• the test piece of Experiment No. 6 has strength higher than necessary because the amount of C contained is higher than the preferable range defined in the present invention, so that ductility is lowered and elongation characteristics are deteriorated.
• the amount of Si contained is higher than the preferable range defined in the present invention, and therefore its ductility is lowered, and elongation and stretch-flanging performance are deteriorated.
• the amount of Mn contained does not fall with in the preferable range defined in the present invention, and therefore the space factor of ferrite is increased, deteriorating tensile strength and stretch-flanging performance.
• the amount of Mn contained is higher than the preferable range defined in the present invention, and therefore its ductility is lowered, deteriorating elongation and stretch-flanging performance.
• the heating temperature in the final annealing is much below the range defined in the present invention. Therefore, the space factor and mean grain size of ferrite, the space factor and mean grain size of martensite in the final structure fall outside the range defined in the present invention, and desired tensile strength and stretch-flanging performance have not been obtained.
• 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.
• the present invention further relates to the following embodiments:

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• 2007
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