EP2246450B1 - Steel sheets and process for manufacturing the same - Google Patents

Steel sheets and process for manufacturing the same Download PDF

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Publication number
EP2246450B1
EP2246450B1 EP08861016.7A EP08861016A EP2246450B1 EP 2246450 B1 EP2246450 B1 EP 2246450B1 EP 08861016 A EP08861016 A EP 08861016A EP 2246450 B1 EP2246450 B1 EP 2246450B1
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EP
European Patent Office
Prior art keywords
graphite
cementite
present
reference example
ferrite
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EP08861016.7A
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German (de)
English (en)
French (fr)
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EP2246450A4 (en
EP2246450A1 (en
Inventor
Nobusuke Kariya
Kazuhiro Seto
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JFE Steel Corp
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JFE Steel Corp
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Priority claimed from JP2007326869A external-priority patent/JP5157417B2/ja
Priority claimed from JP2007326868A external-priority patent/JP5157416B2/ja
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Publication of EP2246450A1 publication Critical patent/EP2246450A1/en
Publication of EP2246450A4 publication Critical patent/EP2246450A4/en
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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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • 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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C21D2211/003—Cementite
    • 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
    • C21D2211/006—Graphite

Definitions

  • the present invention relates to a steel sheet suitable for automotive parts and the like, and particularly to a steel sheet with excellent formability and quench hardenability and a method for manufacturing the same.
  • a steel sheet for use in tools, automotive parts (gear, transmission), etc. is formed into a desired shape, and subjected to heat treatment, such as hardening annealing, for use.
  • heat treatment such as hardening annealing
  • Such a steel sheet is processed into various complicated shapes, and thus is required to have excellent formability.
  • reduction in manufacturing cost has been strongly demanded in such parts.
  • processing techniques in which omission of a processing process or alteration of a processing manner is intended e.g., a double-acting processing technique which allows thickening of automobile driving parts using a high carbon steel sheet, and achieves sharp reduction in the number of processes, have been developed, and some of them have been put into practical use.
  • the steel sheets for use in automotive parts have been strictly required to have high formability, and the steel sheets have been demanded to be softer and have high ductility.
  • the steel sheets have been demanded to be softer and have high ductility.
  • lower yield stress has been demanded.
  • hole expanding (burring) is performed after punching, excellent stretch-flangeability is desired.
  • Patent Document 1 discloses a steel sheet suitable as a tiller claw: containing, by mass %, C: 0.40 to 0.80%, Si: 0.20 to 2.00%, Mn: 0.20 to 1.50%, Al: 0.001 to 0.150%, P: 0.018% or lower, S: 0.010% or lower, N: 0.0050% or lower, balance Fe, and inevitable impurities; having a microstructure containing a ferrite phase and a graphite as a main body; has a soft material of TS ⁇ 60kgf/mm 2 ; and having excellent formability, tenacity, and quench hardenability, and a method for manufacturing the same.
  • Patent Document 2 discloses a method for manufacturing a medium carbon steel sheet with excellent formability, including: holding a hot rolled steel sheet containing, by mass%, C: 0.10 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.50%, Nb: 0.005 to 0.1%, Al: 0.01 to 1.00%, N: 0.002 to 0.010%, B:3 to 50 ppm, Ca: 0.001 to 0.01%, Ni: 0 to 2.00%, the balance being Fe and inevitable impurities, P in the impurities of 0.012% or lower, and S in the impurities of 0.008% or lower within a temperature range of from Ac 1 to Ac 3 for 0.1 to 10 hr; cooling the resultant at a cooling rate of from 20 to 100°C/hr; and box annealing the resultant within the temperature range of from 650 to 750°C to thereby graphitize 50 area% or more of cementite in the steel.
  • Patent Document 3 discloses a high carbon steel sheet with excellent formability containing a chemical composition including, by mass %, C: 0.20 to 1.00%, Si: 0.20% or more and 1.20% or lower, Mn: 0.05 to 0.50%, N: 0.005 to 0.015%, B: 0.2 ⁇ N% to 0.8 ⁇ N%, and Al: lower than 0.05% and satisfying 1.0 ⁇ (N - B)% to 5.0 ⁇ (N - B)%, balance Fe, inevitable impurities, P in the impurities of 0.020% or lower, and S in the impurities being 0.010% or lower, and a microstructure containing ferrite, graphite, and cementite, and a method for manufacturing the same.
  • a chemical composition including, by mass %, C: 0.20 to 1.00%, Si: 0.20% or more and 1.20% or lower, Mn: 0.05 to 0.50%, N: 0.005 to 0.015%, B: 0.2 ⁇ N% to 0.8 ⁇
  • Patent Document 2 relates to a technique which is intended to graphitize 50% or more of cementite in steel.
  • the amount of Si is large and exceeds 0.20%.
  • the steel sheets described in Patent Documents 1 to 3 are soft and excellent in bending properties and stretching properties in a tensile test, graphite and cementite may not fully dissolve at the time of hardening treatment of a steel sheet depending on heat conditions, resulting in poor hardening in some cases.
  • the steel sheets described in Patent Documents 1 to 3 are soft, the steel sheets have had a problem that they are not always excellent in stretch-flangeability which is an index of hole expanding formability after punching.
  • the present invention aims to provide a steel sheet which is soft and has excellent formability and quench hardenability and a steel sheet with excellent formability having excellent stretch-flangeability, and a method for manufacturing the same.
  • the present inventors have conducted intensive studies on the above-described problems of the prior-art techniques. As a result, the present inventors found that, even in the case where the content of Si in a high carbon steel is very low, specifically 0.1% or lower, excellent formability can be achieved and excellent quench hardenability and stretch-flangeability can be secured by controlling the distributions of graphite and cementite even when a graphitization ratio is not always high. More specifically, the present inventors have conducted intensive studies on influences of the microstructure of a steel sheet containing C: 0.3 to 0.7 mass% on strength, quench hardenability, and stretch-flangeability thereof, and, as a result, found the following findings:
  • the present invention is given in the claims and has been made based on such findings, and provides a steel sheet, containing: a composition containing, by mass%, C: 0.3 to 0.7%, Si: 0.1% or lower, Mn: 0.20% or lower, P: 0.01% or lower, S: 0.01% or lower, Al: 0.05% or lower, N: 0.0050% or lower, 0.3 to 1.0% Ni balance Fe, and inevitable impurities and a microstructure containing ferrite, graphite, and cementite, in which the total volume ratio of ferrite, graphite, and cementite based on the whole microstructure is 95% or more, the volume ratio of graphite (ratio of graphite) based on the total of graphite and cementite is 5% or more, and the mean grain diameter of graphite and cementite is 5 ⁇ m or lower.
  • the steel sheet of the present invention prefferably contains at least one member selected from B: 0.005% or lower, and Cu: 0.1% or lower (by mass%).
  • the steel sheet of the present invention can be obtained by a method, including: hot rolling the steel having the above-described composition at a finishing temperature of from 800 to 950°C to manufacture a hot rolled sheet, cooling the hot rolled sheet at a mean cooling rate of 50°C/s or more to a cooling temperature of 500°C or lower, winding the resultant at a winding temperature of 450°C or lower, and then annealing the wound hot rolled sheet at an annealing temperature of 600 to 720°C for 8 to 100 hours.
  • the present invention provides a steel sheet, containing: a composition containing, by mass %, C: 0.3 to 0.7%, Si: 0.1% or lower, Mn: lower than 0.20%, P: 0.01% or lower, S: 0.01% or lower, Al: 0.05% or lower, N: 0.0050% 0.3 to 1.0 Ni or lower, balance Fe, and inevitable impurities; and a microstructure containing ferrite, graphite, and cementite, in which the total volume ratio of ferrite, graphite, and cementite based on the whole microstructure is 95% or more, the volume ratio of graphite (ratio of graphite) based on the total of graphite and cementite is 5% or more.
  • the total volume ratio of graphite and cementite present in ferrite grains based on the total of graphite and cementite is 15% or lower.
  • the steel sheet of the present invention prefferably contains at least one member selected from B: 0.005% or lower, and Cu: 0.1% or lower (by mass%).
  • the steel sheet of the present invention can be obtained by a method, including: hot rolling the steel having the above-described composition at a finishing temperature of from 800 to 950°C to manufacture a hot rolled sheet, cooling the hot rolled sheet at a mean cooling rate of 50°C/s or more to a cooling temperature of 600°C or lower, winding the resultant at a winding temperature of 450°C or lower, and then annealing the wound hot rolled sheet at an annealing temperature of 600 to 720°C for 8 to 100 hours.
  • the present invention has made it possible to manufacture a steel sheet which is soft and has excellent formability and quench hardenability.
  • the steel sheet of the present invention can be easily manufactured at low cost because components and cooling conditions after hot rolling may be merely controlled.
  • the steel sheet of the present invention is soft and excellent in formability, and thus is suitable for thickening of automobile driving parts. Even when applied to complicated-shaped parts, processing and welding of a plurality of parts become unnecessary, and thus an increase in productivity and cost reduction of automotive parts can be achieved.
  • poor hardening due to non-dissolution of graphite and cementite at the time of heating with high frequency does not occur.
  • the present invention has made it possible to manufacture a steel sheet which is soft and is excellent in formability, such as stretch-flangeability.
  • the steel sheet of the present invention can be easily manufactured at low cost because components and cooling conditions after hot rolling may be merely controlled.
  • the steel sheet of the present invention is soft and excellent in formability, such as stretch-flangeability, and thus is suitable for thickening of automobile driving parts. Even when applied to complicated-shaped parts, processing and welding of a plurality of parts become unnecessary, and thus an increase in productivity and cost reduction of automotive parts can be achieved.
  • C is an element forming graphite.
  • the amount of C is lower than 0.3%, hardness after quench hardening cannot be secured.
  • the amount of C exceeds 0.7%, a steel sheet is hardened, resulting in reduced formability, even when graphitized. Therefore, the amount of C is adjusted to 0.3 to 0.7%.
  • the amount of Si exceeds 0.1%, ferrite is hardened, resulting in reduced formability. Therefore, the amount of Si is adjusted to 0.1% or lower, and preferably 0.05% or lower.
  • Mn is adjusted to 0.20% or lower, and preferably 0.10% or lower.
  • the amount of P is preferably reduced as much as possible. Therefore, the amount of P is adjusted to 0.01% or lower, and preferably 0.008% or lower.
  • the amount of S is preferably reduced as much as possible. Therefore, the amount of S is adjusted to 0.01% or lower, and preferably 0.007% or lower.
  • Al is an element which is combined with solid solution N to form AlN, thereby rendering the adverse effects of solid solution N, which has an action of impeding graphite formation, harmless and which promotes graphite formation with AlN as the nucleus.
  • the amount of Al is 0.05% or lower, and preferably 0.04% or lower.
  • the amount of N exceeds 0.0050%, the action of solid solution N of stabilizing cementite becomes remarkable, and graphite formation is impeded. Therefore, the amount of N is adjusted to 0.0050%, and preferably 0.0040% or lower.
  • the balance contains Fe and inevitable impurities, and it is preferable that at least one member selected from Ni: 30% or lower, B: 0.005% or lower, and Cu: 0.1% or lower be contained for the following reasons.
  • Ni is an element which promotes graphite formation and which is effective in improvement in quench hardenability. In order to obtain such effects, it is preferable to contain 0.1% or more of Ni. However, when the amount of Ni exceeds 3.0%, the effects are saturated. Therefore, the amount of Ni is adjusted 0.3 to 1.0%.
  • B is a useful element which is combined with N to form BN, and acts as the nucleus of graphite formation and which effectively acts in improvement in quench hardenability. In order to obtain such effects, it is preferable to contain 0.0005% or more of B. When the amount of B exceeds 0.005%, the effects are saturated. Therefore, the amount of B is adjusted to 0.005% or lower, preferably 0.0005 to 0.005%, and more preferably 0.0010 to 0.0040%.
  • Cu is an element which promotes graphite formation and which is effective in improvement in quench hardenability.
  • Cu is contained in a proportion of 0.01% or more, and more preferably 0.02% or more.
  • the amount of Cu exceeds 0.1%, the effects are saturated. Therefore, the amount of Cu is adjusted to 0.1% or lower, and preferably 0.07% or lower.
  • the present invention includes the case where the ratio of graphite is 100%, i.e., cementite being thoroughly graphitized, because the same effects are obtained.
  • the volume ratio of ferrite, graphite, and cementite is determined as follows. More specifically, a steel sheet is ground at 1/4 position of the sheet thickness of a through-thickness section in the rolling direction of the steel sheet, and subjected to nital corrosion. Then, the resultant is observed under an optical microscope (400x magnification) for 5 parts per visual field, i.e., 10 visual fields (Total: 50 parts). These images are subjected to image analysis with an image-analysis software "Image Pro Plus ver. 4.0" manufactured by Media Cybernetics.
  • the present inventors have conducted various studies in order to obtain excellent quench hardenability. Hereinafter, an example of the studies will be described. More specifically, a steel slab containing C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe, and inevitable impurities is heated to 1,150°C. Then, the resultant is subjected to rough rolling of 5 passes, and then subjected to finish rolling of 7 passes at a finishing temperature of 880°C to form a hot rolled sheet with a sheet thickness of 4.0 mm.
  • the hot rolled sheet is wound at a winding temperature of 430°C, washed with acid, and then subjected to batch annealing at 720°C for 40 hr.
  • cooling is performed after finish rolling while controlling the temperature range to the winding temperature at a mean cooling rate of from air-cooling (5°C/(s)) to 200 °C/s.
  • the microstructure and quench hardenability are examined as follows.
  • a steel sheet is ground at 1/4 position of the sheet thickness of a cross section parallel to the rolling direction of the steel sheet, and subjected to nital corrosion. Then, the cross section is observed under a scanning electron microscope (1,500x magnification) for 5 parts per visual field, i.e., 10 visual fields (Total: 50 parts).
  • the diameter passing through two points on the outer circumference of cementite or graphite and the center of gravity of a substantially oval shape of cementite or graphite (ellipse having the same area as cementite and graphite and having the same primary and second moments as cementite and graphite) is measured twice, and then averaged to thereby determine each grain diameter. Then, grain diameters of cementite and graphite measured by observing 50 visual fields are averaged to be used as mean grain diameters of cementite and graphite.
  • Fig. 1 shows the relationship between the mean grain diameter d and ⁇ Hv of cementite and graphite.
  • ⁇ Hv becomes 8 or lower, which shows that excellent quench hardenability is obtained.
  • the present inventors have conducted various studies based on the above studies, and as a result, found that, in order to secure excellent quench hardenability, the mean grain diameter of cementite and graphite needs to be 5 ⁇ m or lower, and preferably 3 ⁇ m or lower.
  • a reason why excellent quench hardenability is obtained by specifying a microstructure is considered as follows. More specifically, it is considered that, when the mean grain diameter of cementite and graphite become 5 ⁇ m or lower, cementite and graphite nearly thoroughly dissolve at the time of high frequency heating, and thus hardness after quench hardening is equalized.
  • Finishing temperature at the time of hot rolling 800 to 950°C
  • finishing temperature at the time of hot rolling is lower than 800°C, a rolling load sharply increases.
  • the finishing temperature exceeds 950°C, a scale to be generated is thickened, pickling properties decrease, and a decarburized layer is manufactured on a steel sheet surface layer in some cases.
  • the finishing temperature at the time of hot rolling is adjusted to 800 to 950°C.
  • a steel sheet after hot rolling is immediately cooled to a cooling stop temperature mentioned later at a mean cooling rate of 50°C/s or more.
  • the mean cooling rate is lower than 50°C/s, ferrite grains easily grow during cooling to form large ferrite grains. It is considered that, at the time of annealing performed thereafter, graphite or cementite is formed with ferrite grain boundaries, inclusions, etc., as the nucleus. Thus, when ferrite grains are large, graphite or cementite which is formed with grain boundaries as the nucleus is coarsened, resulting in reduced quench hardenability. When the mean cooling rate is low, pearlites with coarse carbides are generated.
  • the mean cooling rate is adjusted to 50°C/s or higher, rolling distortion introduced into austenite by hot rolling easily remains in a microstructure after modification to increase dislocation density, and graphite formation with such dislocation as the nucleus is promoted at the time of annealing.
  • the mean cooling rate is 50°C/s or higher, and preferably 80°C/s or higher.
  • the upper limit of the mean cooling rate is not necessary specified, and is preferably 200°C/s or lower so as to suppress deterioration of the shape of a steel sheet to secure the shape of the steel sheet.
  • Cooling stop temperature during cooling after hot rolling 500°C or lower
  • cooling stop temperature When the lowest temperature which needs to be cooled at the above-mentioned cooling rate, i.e., cooling stop temperature, exceeds 500°C, pro-eutectoid ferrite generates during cooling until winding and a coarse pearlite generates. Thus, cementite or graphite is coarsened at the time of annealing after winding, reducing quench hardenability.
  • the cooling stop temperature is adjusted to 500°C or lower, and preferably 470°C or lower.
  • the lower limit of the cooling stop temperature is not necessary specified, and is preferably 200°C or higher so as to secure the shape of a steel sheet.
  • Winding temperature 450°C or lower
  • a hot rolled sheet after cooling is immediately wound.
  • the winding temperature is adjusted to 450°C or lower.
  • the winding temperature is preferably lower than the cooling stop temperature so as to sufficiently obtain the above-described cooling effects after hot rolling.
  • the winding temperature is preferably adjusted to 200°C or higher.
  • Annealing temperature 720°C or lower
  • a hot rolled sheet after winding is washed with acid or the like to remove scales, and is annealed so as to promote spheroidizing or graphitization of cementite for softening.
  • the annealing temperature exceeds 720°C, a coarse pearlite generates during cooling, resulting in reduced quench hardenability.
  • the annealing temperature is adjusted to 720°C or lower.
  • the annealing temperature is lower than 600°C, annealing time is excessively prolonged.
  • the annealing temperature is adjusted to 600°C or higher.
  • the annealing time is 8 hr or more so as to form graphite or 100 hr or lower because there is a possibility that ferrite grains may be excessively coarsened, resulting in reduced ductility.
  • both a converter and an electric furnace are usable.
  • the steel thus melted is formed into a slab by ingot making-slabbing or continuous casting.
  • a slab is generally hot rolled after heating (reheating).
  • reheating in the case of a slab manufactured by continuous casting, the slab can be used as it is or may be subjected to direct rolling in which rolling is performed while maintaining heat so as to suppress reduction in temperature.
  • reheating a slab for hot rolling it is preferable to adjust a slab heating temperature to 1,280°C or lower so as to avoid deterioration of the surface condition due to scales.
  • the hot rolling can be carried out merely by finish rolling while omitting rough rolling.
  • a material to be rolled may be heated with a heating member, such as a sheet bar heater, during hot rolling.
  • a heating member such as a sheet bar heater
  • the sheet thickness of a hot rolled sheet is not limited insofar as the manufacturing conditions of the present invention can be maintained, and is preferably from 1.0 to 10.0 mm.
  • the steel sheet after annealing can be subjected to temper rolling as required. A working example will be described in Example 1.
  • the total volume ratio of cementite and graphite present in ferrite grains needs to be adjusted to 15% or lower in order to secure excellent stretch-flangeability. More preferably, the total volume ratio thereof is adjusted to 10% or lower.
  • a steel slab containing C: 0.55%, Si: 0.01%, Mn: 0.10%, P: 0.003%, S: 0.0006%, Al: 0.005%, N: 0.0018%, Ni: 0.50%, B: 0.0013%, balance Fe, and inevitable impurities is heated to 1,150°C, subjected to rough rolling of 5 passes, subjected to finish rolling of 7 passes at a finishing temperature of 870°C to manufacture a hot rolled sheet having a sheet thickness of 4.0 mm.
  • the hot rolled sheet is wound at a winding temperature of 520°C, washed with acid, and subjected to batch annealing at 720°C for 40 hr.
  • cooling is performed after finish rolling while changing the temperature range to the winding temperature at a mean cooling rate of from air-cooling (5°C/(s)) to 200°C/s.
  • the microstructure and stretch-flangeability are examined as follows.
  • a steel sheet is ground at 1/4 position of the sheet thickness of a cross section parallel to the rolling direction of the steel sheet, and subjected to nital corrosion. Then, the cross section is observed under an optical microscope (400x magnification) for 5 parts of the cross section, i.e., 10 visual fields (Total: 50 parts).
  • cementite and graphite present on ferrite grain boundaries and cementite and graphite present in ferrite grains are distinguished.
  • the occupation area S on of cementite and graphite present on ferrite grain boundaries and the occupation area S in of cementite and graphite present in ferrite grains are measured.
  • each cementite grain or each graphite grain is measured as an occupation area of cementite grains or graphite grains present on ferrite grain boundaries.
  • the area of cementite grains or graphite grains not having a part present on ferrite grain boundaries is measured as an occupation area of cementite grains or graphite grains present in ferrite grains.
  • Fig. 2 represents the relationship between the volume ratio S and the mean ⁇ of cementite and graphite present in ferrite grains. It is revealed that when the volume ratio S of cementite and graphite present in ferrite grains becomes 15% or lower, the mean ⁇ becomes 60% or more, and excellent stretch-flangeability is obtained.
  • the present inventors have conducted various studies based on the above studies, and, as a result, fount that, in order to secure excellent stretch-flangeability, the total volume ratio of cementite and graphite present in ferrite grains may be adjusted to 15% or lower, and preferably 10% or lower.
  • the reason why excellent stretch-flangeability is obtained by specifying the microstructure as described above is considered as follows. More specifically, when a large amount of cementite or graphite is present in ferrite grains, fine cracks are likely to form at the interfaces between cementite or graphite and ferrite at the time of punching, and propagation and coalescence of cracks occur from the first stage of a hole expanding test, easily resulting in the formation of through thickness cracks.
  • cementite or graphite on ferrite grain boundaries is likely to coarsen rather than cementite or graphite in ferrite grains, and the gap between each cementite grain and each graphite grain is likely to become broad. Therefore, cementite or graphite on ferrite grain boundaries slows down crack propagation compared with cementite or graphite in ferrite grains.
  • Finishing temperature at the time of hot rolling 800 to 950°C
  • finishing temperature at the time of hot rolling is lower than 800°C, a rolling load sharply increases.
  • finishing temperature at the time of hot rolling exceeds 950°C, a scale to be generated becomes thick, pickling properties decrease, and a decarburized layer may be formed on a steel sheet surface layer.
  • the finishing temperature at the time of hot rolling is adjusted to 800 to 950°C.
  • Mean cooling rate after hot rolling 50°C/s or more
  • the mean cooling rate is adjusted to 50°C/s or more, and preferably 80°C/s or more.
  • the upper limit of the mean cooling rate does not need to be specified, and is preferably adjusted to 200°C/s or lower in order to suppress deterioration of the shape of a steel sheet and secure the shape of a steel sheet.
  • Cooling stop temperature during cooling after hot rolling 500°C or lower
  • cooling stop temperature The lowest temperature which needs to be cooled at the above-mentioned cooling rate, i.e., cooling stop temperature, exceeds 500°C, a pro-eutectoid ferrite generates during cooling to winding, a pearlite generates, cementite or graphite present in ferrite grains increases at the time of annealing after winding, and stretch-flangeability deceases.
  • the cooling stop temperature is adjusted to 500°C or lower.
  • the lower limit of the cooling stop temperature does not need to be specified, and is preferably adjusted to 200°C or higher in order to secure the shape of a steel sheet.
  • Winding temperature 450°C or lower
  • a hot rolled sheet after cooling is immediately wound.
  • the winding temperature exceeds 450°C, a pearlite generates, cementite or graphite present in ferrite grains at the time of annealing increases, and stretch-flangeability decreases. Therefore, the winding temperature is adjusted to 450°C or lower. It should be noted that, in order to fully obtain the effects of cooling after hot rolling, it is preferable for the winding temperature to be lower than the cooling stop temperature. Since the shape of a hot rolled sheet is likely to deteriorate, in view of securing the shape of a steel sheet, the winding temperature is adjusted to preferably 200°C or higher.
  • Annealing temperature 720°C or lower
  • a hot rolled sheet after winding is washed with acid to remove scales, and then is annealed in order to promote spheroidizing and graphitization of cementite for softening.
  • the annealing temperature exceeds 720°C, a pearlite generates during cooling and stretch-flangeability deceases.
  • the annealing temperature is adjusted to 720°C or lower.
  • the annealing temperature is lower than 600°C, there is a tendency that cementite or graphite present in ferrite grains increases and stretch-flangeability deteriorates.
  • the annealing temperature is adjusted to 600°C or higher.
  • the annealing time does not need to be specified, and is preferably 8 hr or more for forming graphite and reducing cementite or graphite present in ferrite grains. Moreover, there is a possibility that ferrite grains are excessively coarsened to reduce ductility, and thus the annealing time is 100 hr or lower.
  • both a converter and an electric furnace are usable.
  • the steel thus melted is formed into a slab by ingot making-slabbing or continuous casting.
  • a slab is generally hot rolled after heating (reheating).
  • reheating heating
  • the slab can be used as it is, or may be subjected to direct rolling in which rolling is performed while maintaining heat so as to suppress reduction in temperature.
  • the slab heating temperature is 1,280°C or lower so as to avoid deterioration of the surface condition due to scales.
  • the hot rolling can be carried out merely by finish rolling while omitting rough rolling.
  • a material to be rolled may be heated with a heating member, such as a sheet bar heater, during hot rolling.
  • a heating member such as a sheet bar heater
  • the sheet thickness of a hot rolled sheet is not limited insofar as the manufacturing conditions of the present invention can be maintained, and is preferably from 1.0 to 10.0 mm.
  • the hot rolled sheet is washed with acid or subjected to shot blasting to remove scales on the surface, and then annealed.
  • the steel sheet after annealing can be subjected to temper rolling as required. A working example will be described in Example 2.
  • a tensile test was carried out, and a yield stress YP, a tensile strength Ts, and elongation El were measured. It should be noted that the same test was carried out twice for every test piece to obtain the mean value. Then, the mean value was defined as a property value of the steel sheet.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
EP08861016.7A 2007-12-19 2008-11-20 Steel sheets and process for manufacturing the same Not-in-force EP2246450B1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
JP2007326869A JP5157417B2 (ja) 2007-12-19 2007-12-19 鋼板およびその製造方法
JP2007326868A JP5157416B2 (ja) 2007-12-19 2007-12-19 鋼板およびその製造方法
PCT/JP2008/071597 WO2009078261A1 (ja) 2007-12-19 2008-11-20 鋼板およびその製造方法

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CN101906597A (zh) * 2010-08-14 2010-12-08 武汉钢铁(集团)公司 一种环保型高性能石墨化易切削钢
JP5594226B2 (ja) * 2011-05-18 2014-09-24 Ｊｆｅスチール株式会社 高炭素薄鋼板およびその製造方法
JP5338873B2 (ja) * 2011-08-05 2013-11-13 Ｊｆｅスチール株式会社 引張強度４４０ＭＰａ以上の加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法
US10323293B2 (en) 2012-01-05 2019-06-18 Jfe Steel Corporation High-carbon hot rolled steel sheet with excellent hardenability and small in-plane anistropy and method for manufacturing the same
JP6479538B2 (ja) * 2015-03-31 2019-03-06 株式会社神戸製鋼所 機械構造部品用鋼線
CN106048179B (zh) * 2016-07-15 2017-09-15 北京科技大学 一种石墨化热轧钢板的制备方法
CN113862609B (zh) * 2021-09-03 2022-05-27 北京科技大学 利用渗碳、表面石墨化提高中低碳钢工件耐磨减摩的方法

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JPH0830241B2 (ja) 1987-07-20 1996-03-27 川崎製鉄株式会社 加工性及び靭性に優れ、かつ焼入性の良好な鋼板と、その製造方法
JPH02107742A (ja) * 1988-10-14 1990-04-19 Kawasaki Steel Corp 加工性、焼入性に優れた鋼材
JPH04124216A (ja) * 1990-09-12 1992-04-24 Sumitomo Metal Ind Ltd 成形性の良好な高炭素薄鋼板の製造方法
JPH04202744A (ja) 1990-11-30 1992-07-23 Sumitomo Metal Ind Ltd 成形性の良好な高炭素薄鋼板とその製造方法
JPH0913142A (ja) * 1991-01-17 1997-01-14 Kawasaki Steel Corp 曲げ加工性及び熱処理性に優れた黒鉛析出熱間圧延鋼 板ならびにその製造方法
JP3241748B2 (ja) * 1991-04-11 2001-12-25 川崎製鉄株式会社 加工性と焼入れ性に優れた鋼材及びその製造方法
JPH06323399A (ja) * 1992-06-30 1994-11-25 Sumitomo Metal Ind Ltd 自動車用ギヤおよびその製造方法
JPH07258743A (ja) 1994-03-18 1995-10-09 Sumitomo Metal Ind Ltd 加工性に優れた中炭素鋼板の製造方法
JPH08246051A (ja) * 1995-03-07 1996-09-24 Sumitomo Metal Ind Ltd 加工性に優れた中炭素鋼板の製造方法
JPH08291362A (ja) * 1995-04-21 1996-11-05 Sumitomo Metal Ind Ltd 冷間加工性に優れた鋼材
JP3848444B2 (ja) * 1997-09-08 2006-11-22 日新製鋼株式会社 局部延性および焼入れ性に優れた中・高炭素鋼板
JP3879459B2 (ja) * 2001-08-31 2007-02-14 Ｊｆｅスチール株式会社 高焼入れ性高炭素熱延鋼板の製造方法
JP5011846B2 (ja) * 2005-06-29 2012-08-29 Ｊｆｅスチール株式会社 高炭素熱延鋼板およびその製造方法
EP1905851B1 (en) * 2005-06-29 2015-11-04 JFE Steel Corporation High-carbon hot-rolled steel sheet and process for producing the same

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CN101903547B (zh) 2012-05-23
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WO2009078261A1 (ja) 2009-06-25
KR20100076073A (ko) 2010-07-05
EP2246450A1 (en) 2010-11-03
CN101903547A (zh) 2010-12-01

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