US20090126836A1 - High Carbon Hot Rolled Steel Sheet and method for manufacturing same - Google Patents

High Carbon Hot Rolled Steel Sheet and method for manufacturing same Download PDF

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US20090126836A1
US20090126836A1 US11/922,250 US92225006A US2009126836A1 US 20090126836 A1 US20090126836 A1 US 20090126836A1 US 92225006 A US92225006 A US 92225006A US 2009126836 A1 US2009126836 A1 US 2009126836A1
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hot
sheet
rolled
cooling
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Nobusuke Kariya
Norio Kanamoto
Hidekazu Ookubo
Yoshiharu Kusumoto
Takeshi Fujita
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: FUJITA, TAKESHI, KANAMOTO, NORIO, KARIYA, NOBUSUKE, KUSUMOTO, YOSHIHARU, OOKUBO, HIDEKAZU
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • the present invention relates to a high carbon hot-rolled steel sheet having excellent workability and a method for manufacturing thereof.
  • the high carbon steel sheets face ever-increasing request of workability to attain higher ductility than ever. Since some of the parts are often subjected to hole-expansion (burring) treatment after punching, they are wanted to have excellent stretch-flange formability.
  • these steel sheets are strongly requested to have homogeneous mechanical properties.
  • the homogeneity of hardness in the sheet thickness direction is keenly desired because large differences of hardness in the steel sheet thickness direction between the surface portion and the central portion significantly deteriorate the punching tool during punching.
  • JP-A-3-174909 (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), proposed a method for manufacturing stably a high carbon hot-rolled steel strip having excellent homogeneous mechanical properties in the longitudinal direction of coil by the steps of:
  • JP-A-9-157758 proposed a method for manufacturing high carbon workable steel strip having excellent structural homogeneity and workability (ductility) by the steps of:
  • JP-A-5-9588 proposed a method for manufacturing high carbon steel thin sheet having good workability by the steps of:
  • JP-A-2003-13145 proposed a method for manufacturing high carbon steel sheet having excellent stretch-flanging formability by the steps of:
  • JP-A-2003-73742 disclosed a technology for manufacturing high carbon hot-rolled steel sheet which satisfies the above requirements except for selecting the cooling-stop temperature of 620° C. or below.
  • the obtained steel sheet is what is called the “as hot-rolled” steel sheet without subjected to heat treatment after hot-rolling. Accordingly, the manufactured steel sheet not necessarily attains excellent elongation and stretch-flange formability.
  • a microstructure composed of pro-eutectoid ferrite and pearlite containing lamellar carbide is formed depending on the hot-rolling condition, and the succeeding annealing converts the lamellar carbide into fine spheroidal cementite.
  • formed fine spheroidal cementite becomes the origin of voids during hole-expansion step, and the generated voids connect with each other to induce fracture of the steel. As a result, no excellent stretch-flange formability is attained.
  • the steel sheet after hot-rolling is cooled under a specified condition, followed by reheating thereof by direct electric heating process and the like.
  • a special apparatus is required and a vast amount of electric energy is consumed.
  • the steel sheet coiled after reheating likely forms fine spheroidal cementite, there are often failed to obtain excellent stretch-flange formability owing to the same reason to that given above.
  • An object of the present invention is to provide a high carbon hot-rolled steel sheet having excellent stretch-flange formability and excellent homogeneity of hardness in the sheet thickness direction, and a method for manufacturing thereof.
  • the inventors of the present invention conducted detail study of the effect of microstructure on the stretch-flange formability and the hardness of high carbon hot-rolled steel sheet, and found that it is extremely important to adequately control the manufacturing conditions, specifically the cooling condition after hot-rolling, the coiling temperature, and the annealing temperature, thus found that the stretch-flange formability is improved and the hardness in the sheet thickness direction becomes homogeneous by controlling the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size to the total carbide in the steel sheet, determined by the method described later, to 15% or less.
  • the inventors of the present invention found that further excellent stretch-flange formability and homogeneous distribution of hardness are attained by controlling more strictly the cooling condition after hot-rolling and the coiling temperature, thereby controlling the volume percentage of the carbide to 10% or less.
  • the present invention has been perfected on the basis of above findings, and the present invention provides a method for manufacturing high carbon hot-rolled steel sheet having excellent workability, by the steps of: hot-rolling a steel containing 0.2 to 0.7% C by mass at finishing temperatures of (A r3 transformation point ⁇ 20° C.) or above to prepare a hot-rolled sheet; cooling thus hot-rolled sheet to temperatures of 650° C. or below, (called the “cooling-stop temperature”), at cooling rates from 60° C./s or larger to smaller than 120° C./s; coiling the hot-rolled sheet after cooling at coiling temperatures of 600° C. or below; and annealing the coiled hot-rolled sheet at annealing temperatures from 640° C. or larger to A c1 transformation point or lower, (called the “annealing of hot-rolled sheet).
  • the cooling step and the coiling step are conducted by cooling the hot-rolled sheet to temperatures of 600° C. or below at cooling rates from 80° C./s or larger to smaller than 120° C./s, and then coiling the sheet at temperatures of 550° C. or below.
  • the coiled hot-rolled sheet is subjected to descaling such as pickling before applying annealing of hot-rolled sheet.
  • the above volume percentage of carbide having smaller than 0.5 ⁇ m in particle size is 10% or less, and that above ⁇ Hv is 8 or smaller.
  • FIG. 1 shows the relation between ⁇ Hv (vertical axis) and volume percentage (horizontal axis) of carbide having smaller than 0.5 ⁇ m of particle size.
  • Carbon is an important element of forming carbide and providing hardness after quenching. If the C content is less than 0.2% by mass, formation of pre-eutectoid ferrite after hot-rolling becomes significant, and the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size after annealing of hot-rolled sheet, (the volume percentage to the total carbide in the steel sheet), increases, thereby deteriorating the stretch-flange formability and the homogeneity of hardness in the sheet thickness direction. In addition, even after quenching, satisfactory strength as the machine structural parts cannot be attained.
  • the C content exceeds 0.7% by mass, sufficient stretch-flange formability cannot be attained even if the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size is 15% or less.
  • the hardness after hot-rolling significantly increases to result in inconvenience in handling owing to the brittleness of the steel sheet, and also the strength as the machine structural parts after quenching saturates. Therefore, the C content is specified to a range from 0.2 to 0.7% by mass.
  • the C content For the case that the hardness after quenching is emphasized, it is preferable to specify the C content to above 0.5% by mass. For the case that the workability is emphasized, it is preferable to specify the C content to 0.5% or less by mass.
  • elements other than C elements such as Mn, Si, P, S, Sol.Al, and N can be added within ordinary respective ranges. Since, however, Si likely converts carbide into graphite, thus interfering the hardenability by quenching, the Si content is preferably specified to 2% or less by mass. Since excess amount of Mn likely induces the decrease in ductility, the Mn content is preferably specified to 2% or less by mass. Since excess amount of P and S decreases ductility and likely induces cracks, the content of P and S is preferably specified to 0.03% or less by mass, respectively.
  • the Sol.Al content is preferably specified to 0.08% or less by mass. Since excess amount of N deteriorates ductility, the N content is preferably specified to 0.01% or less by mass. Preferable respective contents of these elements are: 0.5% or less Si, 1% or less Mn, 0.02% or less P, 0.05% or less Sol.Al, and 0.005% or less N, by mass.
  • the S content is preferably reduced.
  • the stretch-flange formability is further significantly improved by specifying the S content to 0.007% or less by mass.
  • the effect of the present invention is not affected by the addition of at least one of the elements such as B, Cr, Cu, Ni, Mo, Ti, Nb, W, V, and Zr within ordinarily adding ranges to the high carbon hot-rolled steel sheet.
  • the elements such as B, Cr, Cu, Ni, Mo, Ti, Nb, W, V, and Zr within ordinarily adding ranges to the high carbon hot-rolled steel sheet.
  • B in amounts of about 0.005% or less by mass, Cr about 3.5% or less by mass, Ni about 3.5% or less by mass, Mo about 0.7% or less by mass, Cu about 0.1% or less by mass, Ti about 0.1% or less by mass, Nb about 0.1% or less by mass, and W, V, and Zr, as the total, about 0.1% or less by mass.
  • Cr and/or Mo it is preferable to add Cr in amounts of about 0.05% or more by mass and Mo about 0.05% or more by mass.
  • Balance of above composition is preferably iron and inevitable impurities.
  • elements such as Sn and Pb entered the steel composition as impurities during the manufacturing process they do not affect the effect of the present invention.
  • the finishing temperature is below (A r3 transformation point ⁇ 20° C.)
  • the ferrite transformation proceeds in a part, which increases the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size, thereby deteriorating both the stretch-flange formability and the homogeneity of hardness in the sheet thickness direction.
  • the finishing temperature of hot-rolling is specified to (A r3 transformation point ⁇ 20° C.) or above.
  • the A r3 transformation point may be the actually determined value, and may be the calculated value of the following formula (1).
  • [M] designates the content (% by mass) of the element M.
  • correction terms such as ( ⁇ 1.[Cr]), (+31.5[Mo]), and ( ⁇ 15.2[Ni]) may be added to the right-hand member of the formula (1).
  • the cooling rate after hot-rolling is smaller than 60° C./s, the supercooling of austenite becomes small, and the formation of pre-eutectoid ferrite after hot-rolling becomes significant.
  • the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size exceeds 15% after annealing of hot-rolled sheet, thereby deteriorating both the stretch-flange formability and the homogeneity of hardness in the sheet thickness direction.
  • the cooling rate after hot-rolling is specified to a range from 60° C./s or larger to smaller than 120° C./s. Furthermore, if the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size is to be brought to 10% or less, the cooling rate is specified to a range from 80° C./s or larger to smaller than 120° C./s. It is more preferable to specify the upper limit of the cooling rate to 115° C./s or smaller.
  • the cooling-stop temperature is specified to 650° C. or below, and more preferably to 600° C. or below.
  • the cooling rate in a range from 80° C./s or larger to 120° C./s or smaller, (preferably 115° C./s or smaller), and the cooling-stop temperature of 600° C. or below.
  • the cooling-stop temperature is preferably specified to 500° C. or above.
  • the hot-rolled steel sheet after cooling is coiled. If the coiling temperature exceeds 600° C., pearlite containing lamella carbide is formed. As a result, the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size exceeds 15% after annealing of hot-rolled sheet, thereby deteriorating the stretch-flange formability and the homogeneity of hardness in the sheet thickness direction. Therefore, the coiling temperature is specified to 600° C. or below. The coiling temperature is selected to a temperature below the above cooling-stop temperature.
  • the above cooling-stop temperature is specified to 600° C. or below, and that the coiling temperature is specified to 550° C. or below.
  • the cooling rate to a range from 80° C./s or larger to 120° C./s or smaller, (preferably 115° C./s or smaller), the cooling-stop temperature to 600° C. or below, and the coiling temperature to 550° C. or below.
  • the coiling temperature is preferably specified to 200° C. or above, and more preferably to 350° C. or above.
  • the hot-rolled steel sheet after coiling is generally subjected to descaling before applying annealing of hot-rolled sheet.
  • the scale-removal method it is preferably to adopt ordinary pickling.
  • Temperature of annealing of hot-rolled sheet The hot-rolled sheet after pickling is subjected to annealing of hot-rolled sheet to spheroidize the carbide. If the temperature of annealing of hot-rolled sheet is below 640° C., the spheroidization of carbide becomes insufficient or the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size increases, which deteriorates the stretch-flange formability and the homogeneity of hardness in the sheet thickness direction.
  • the temperature of annealing of hot-rolled sheet is specified to a range from 640° C. to (A c1 transformation point).
  • the temperature of annealing of hot-rolled sheet is preferably specified to 680° C. or above.
  • the A c1 transformation point may be the actually determined value, and may be the calculated value of the following formula (2).
  • [M] designates the content (% by mass) of the element M.
  • correction terms such as (+17.13[Cr]), (+4.51[Mo]), and (+15.62[V]) may be added to the right-hand member of the formula (2).
  • the annealing time is preferably between about 8 hours and about 80 hours.
  • the carbide treated by spheroidizing annealing gives about 5.0 or smaller average aspect ratio, (determined at a depth of about one fourth in the sheet thickness direction).
  • either converter or electric furnace can be applied.
  • high carbon steel is formed into slab by ingoting and blooming or by continuous casting.
  • the slab is normally heated, (reheated), and then treated by hot-rolling.
  • the slab manufactured by continuous casting may be treated by hot direct rolling directly from the slab or after heat-holding to prevent temperature reduction.
  • the slab heating temperature is preferably specified to 1280° C. or below to avoid the deterioration of surface condition caused by scale.
  • the hot-rolling can be given only by finish rolling eliminating rough rolling.
  • the material being rolled may be heated during hot-rolling using a heating means such as sheet bar heater.
  • a heating means such as sheet bar heater.
  • the coiled sheet may be thermally insulated by a slow-cooling cover or other means.
  • the thickness of the hot-rolled sheet is not specifically limited if only the manufacturing conditions of the present invention are maintained, a particularly preferable range of the thickness thereof is from 1.0 to 10.0 mm from the point of operability.
  • the annealing of hot-rolled sheet can be done either by box annealing or by continuous annealing. After annealing of hot-rolled sheet, skin-pass rolling is applied, at need. Since the skin-pass rolling does not affect the hardenability by quenching, there is no specific limitation of the condition of skin-pass rolling.
  • Steel sheets Nos. 1 to 10 are Examples of the present invention, and Steel sheets Nos. 11 to 19 are Comparative Examples.
  • the following methods were adopted to determine the particle size and volume percentage of carbide, the hardness in the sheet thickness direction, and the hole-expansion rate ⁇ .
  • the hole-expansion rate ⁇ was adopted as an index to evaluate the stretch-flange formability.
  • a cross section of steel sheet parallel to the rolling direction was polished, which section was then etched at a depth of one fourth of sheet thickness using a Picral solution (picric acid+ethanol).
  • the microstructure on the etched surface was observed by a scanning electron microscope ( ⁇ 300 magnification).
  • the particle size and volume percentage of carbide were quantitatively determined by image analysis using the image analyzing software “Image Pro Plus ver.4.0TM” manufactured by Media Cybernetics, Inc. That is, the particle size of each carbide was determined by measuring the diameter between two point on outer peripheral circle of the carbide and passing through the center of gravity of an equivalent ellipse of the carbide, (an ellipse having the same area to that of carbide and having the same first moment and second moment to those of the carbide), at intervals of 2 degrees, and then averaging thus measured diameters.
  • the area percentage of every carbide to the measuring visual field was determined, which determined value was adopted as the volume percentage of the carbide.
  • the sum of volume percentages, (cumulative volume percentage) was determined, which was then divided by the cumulative volume percentage of all carbides, thus obtained the volume percentage for every visual field.
  • the volume percentage was determined on 50 visual fields, and those determined volume percentages were averaged to obtain the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size.
  • the average aspect ratio (number average) of carbide was also calculated, and the spheroidizing annealing was confirmed.
  • the cross section of steel sheet parallel to the rolling direction was polished.
  • the hardness was determined using a micro-Vickers hardness tester applying 4.9 N (500 gf) of load at nine positions: 0.1 mm depth from the surface of the steel sheet; depths of 1 ⁇ 8, 2/8, 3 ⁇ 8, 4/8, 5 ⁇ 8, 6/8, and 7 ⁇ 8 of the sheet thickness; and 0.1 mm depth from the rear surface thereof.
  • the steel sheet was punched using a punching tool having a punch diameter of 10 mm and a die diameter of 12 mm (20% of clearance). Then, the punched hole was expanded by pressing-up a cylindrical flat bottom punch (50 mm in diameter and 8 mm in shoulder radius). The hole diameter d (mm) at the point of generating penetration crack at hole-edge was determined. Then, the hole-expansion rate ⁇ (%) was calculated by the formula (3).
  • Table 3 shows the result.
  • Steel sheets Nos. 1 to 10 which are Examples of the present invention, gave 15% or smaller volume percentage of carbide having smaller than 0.5 ⁇ m of particle size, and, compared with Steel sheets Nos. 11 to 19, which are Comparative Examples with the same chemical compositions, respectively, the hole-expansion rate ⁇ was large, and the stretch-flange formability was superior.
  • a presumable cause of the high hole-expansion rate ⁇ is that, as described above, although the fine carbide having smaller than 0.5 ⁇ m of particle size acts as the origin of voids during hole-expansion step, which generated voids connect with each other to induce fracture, the quantity of that fine carbide decreases to 15% or less by volume.
  • FIG. 1 shows the relation between the ⁇ Hv (vertical axis) and the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size, (horizontal axis).
  • ⁇ Hv vertical axis
  • FIG. 1 shows the relation between the ⁇ Hv (vertical axis) and the volume percentage of carbide having smaller than 0.5 ⁇ m of particle size, (horizontal axis).
  • Steel sheets Nos. 2, 4, 6, 8, and 10 which are Examples of the present invention, having 10% or less of volume percentage of carbide having smaller than 0.5 ⁇ m of particle size, prepared under the conditions of 600° C. or below of cooling-stop temperature and 550° C. or below of coiling temperature, provided not only more excellent stretch-flange formability but also more excellent homogeneity of hardness, of ⁇ Hv of 8 or smaller, in sheet thickness direction.
  • Example 9 E 752 65 600 570 700° C. ⁇ 40 hr
  • Example 10 E 772 100 540 490 720° C. ⁇ 40 hr
  • Example 11 A 801 80 680 580 700° C. ⁇ 40 hr Comparative example 12 A 751 100 610 570 700° C. ⁇ 40 hr Comparative example 13 B 798 110 620 560 600° C. ⁇ 40 hr Comparative example 14 B 793 90 600 630 690° C. ⁇ 40 hr Comparative example 15 C 816 150 580 520 720° C.
  • Steel H (0.32% C, 1.2% Si, 1.5% Mn, 0.025% P, 0.010% S, 0.06% Sol.Al, and 0.0070% N, by mass; 804° C. of A r3 transformation point; and 746° C. of A c1 transformation point); Steel I (0.35% C, 0.20% Si, 0.68% Mn, 0.012% P, 0.0038% S, 0.032% Sol.Al, 0.0033% N, 0.98% Cr, and 0.17% Mo, by mass; 773° C. of A r3 transformation point; and 754° C. of A c1 transformation point); and Steel E given in Table 1.
  • Example 5 To thus prepared hot-rolled steel sheets, similar method to that in Example 1 was applied to determine the particle size and volume percentage of carbide, the hardness in the sheet thickness direction, and the hole-expansion rate ⁇ . The results are given in Table 5.
  • Steel sheets Nos. 20 to 26 in which the conditions other than the cooling rate were kept constant Steel sheets Nos. 21 to 25 in which the cooling rate was within the range of the present invention showed significantly excellent stretch-flange formability and homogeneity of hardness in the sheet thickness direction. Steel sheets Nos. 22 to 25 showed further significant improvement in these characteristics, giving maximum values thereof at around 100° C. (for Steel sheets Nos. 23 to 25).
  • the present invention has realized the manufacture of high carbon hot-rolled steel sheet which gives excellent stretch-flange formability and excellent homogeneity of hardness in the sheet thickness direction without adding special apparatus.

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EP1905851B1 (en) 2015-11-04
US8071018B2 (en) 2011-12-06
CN101208442A (zh) 2008-06-25
CN101208442B (zh) 2011-07-20
US20100266441A1 (en) 2010-10-21
EP1905851A1 (en) 2008-04-02
KR20080012942A (ko) 2008-02-12

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