EP1980635A1 - Feuille d'acier convenant parfaitement a un decoupage fin et son procede de production - Google Patents

Feuille d'acier convenant parfaitement a un decoupage fin et son procede de production Download PDF

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Publication number
EP1980635A1
EP1980635A1 EP07713798A EP07713798A EP1980635A1 EP 1980635 A1 EP1980635 A1 EP 1980635A1 EP 07713798 A EP07713798 A EP 07713798A EP 07713798 A EP07713798 A EP 07713798A EP 1980635 A1 EP1980635 A1 EP 1980635A1
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Prior art keywords
cementite
ferrite
steel sheet
performance
working
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EP07713798A
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German (de)
English (en)
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EP1980635B1 (fr
EP1980635A4 (fr
Inventor
Kazuhiro Seto
Takeshi Yokota
Nobuyuki Nakamura
Nobusuke Kariya
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JFE Steel Corp
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JFE Steel Corp
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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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • 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
    • C22C38/18—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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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

Definitions

  • the present invention is concerned with a steel sheet suitable for applications to automobile parts or the like and in particular, relates to a steel sheet excellent in fine blanking performance suitable for the uses to which fine blanking working (hereinafter also referred to as "FB working") is applied.
  • FB working fine blanking working
  • fine blanking working is an extremely advantageous working method as comparing with machining working.
  • a tool-to-tool clearance is from approximately 5 to 10 % of a thickness of a metal sheet as a material to be blanked.
  • the fine blanking working differs from the usual blanking working and is a blanking working method of not only setting up the tool-to-tool clearance extremely small as substantially zero (actually, not more than approximately 2 % of the thickness of the metal sheet as a material to be blanked) but also making a compression stress act on a material in the vicinity of a tool cutting blade. Then, the fine blanking working has the following characteristic features.
  • materials to which the fine blanking working is applied are required to not only have excellent fine blanking performance but also prevent a reduction in mold life.
  • Patent Document 1 proposes a high carbon steel sheet excellent in fine blanking performance, which has a composition containing from 0.15 to 0.90 % by weight of C, not more than 0.4 % by weight of Si and from 0.3 to 1.0 % by weight of Mn, has a microstructure with a cementite having a spheroidization ratio of 80 % or more and an average grain size of from 0.4 to 1.0 ⁇ m scattered in a ferrite matrix and has a notch tensile elongation of 20 % or more.
  • a technology described in Patent Document 1 it is described that the fine blanking performance is improved and that the mold life is also improved.
  • Patent Document 1 the high carbon steel sheet described in Patent Document 1 involved a problem that fabrication performance after the fine blanking working is inferior.
  • Patent Document 2 proposes a steel sheet for fine blanking prepared by applying proper hot rolling to a billet containing from 0.08 to 0.19 % of C and proper amounts of Si, Mn and Al and containing from 0.05 to 0.80 % of Cr and from 0.0005 to 0.005 % of B into a steel sheet. It is described that the steel sheet described in Patent Document 2 is a steel sheet which is low in a yield strength, high in an impact value, excellent in fine blanking performance, high in an n-value in a low strain region, excellent in combined formability and excellent in quenching property at short-time rapid heating. However, Patent Document 2 does not show concrete evaluation regarding the fine blanking performance. Also, the steel sheet described in Patent Document 2 involved a problem that fabrication performance after the fine blanking working is inferior.
  • Patent Document 3 proposes a high carbon steel sheet excellent in flow forming and fine blanking working, which has a composition containing from 0.15 to 0.45 % of C, with the contents of Si; Mn, P, S, Al and N being adjusted at proper ranges and has a structure having a fractional ratio of (pearlite + cementite) of not more than 10 % and an average grain size of ferrite grain of from 10 to 20 ⁇ m. It is described that the high carbon steel sheet described in Patent Document 3 is excellent in fine blanking performance and is improved in mold life in the fine blanking working. However, the high carbon steel sheet described in Patent Document 3 involved a problem that fabrication performance after the fine blanking working is inferior.
  • Patent Document 1 it is hard to say that all of the steel sheets described in Patent Document 1, Patent Document 2 and Patent Document 3 are not provided with satisfactory and thorough fine blanking performance in the fine blanking working under a recent severe working condition. Also, problems that the mold life is not thoroughly improved and that fabrication performance after the fine blanking working is inferior still remained.
  • Patent Document 4 proposes a wear resistant hot rolled steel sheet excellent in stretch flanging property, which has a composition containing from 0.20 to 0.33 % of C, with the contents of Si, Mn, P, S, sol. Al and N being adjusted at proper ranges and further containing from 0.15 to 0.7 % of Cr and has a ferrite-bainite mixed structure which may contain pearlite.
  • a hole expansion ratio becomes high, whereby the stretch flanging property is improved.
  • Patent Document 5 proposes a high carbon steel sheet excellent in stretch flanging property, which has a composition containing from 0.2 to 0.7 % of C and has a structure in which a cementite average particle size is 0.1 ⁇ m or more and less than 1. 2 ⁇ m and a volume ratio of a cementite-free ferrite grain is not more than 15 %.
  • a cementite average particle size is 0.1 ⁇ m or more and less than 1. 2 ⁇ m and a volume ratio of a cementite-free ferrite grain is not more than 15 %.
  • Patent Document 6 proposes a high carbon steel sheet excellent in blanking performance and quenching property, which has a composition containing 0.2 % or more of C and has a structure composed mainly of ferrite and a cementite and having a cementite particle size of not more than 0.2 ⁇ m and a ferrite grain size of from 0.5 to 1 ⁇ m. It is described that according to this, both blanking performance and quenching property which are determined by a burr height and mold life are improved.
  • Patent Document 4 and Patent Document 5 all of the technologies described in Patent Document 4 and Patent Document 5 are those made on the assumption that the conventional blanking working is applied but not those made taking into consideration the application of fine blanking working in which the clearance is substantially zero. Accordingly, it is difficult to ensure similar stretch flanging property after the severe fine blanking working, and even when the stretch flanging property can be ensured, there is encountered a problem that the mold life is short.
  • the ferrite grain size is in the range of from 0.5 to 1 ⁇ m; and it is difficult to stably manufacture a steel sheet having such a ferrite grain size on an industrial scale, resulting in a problem that the product yield is reduced.
  • the invention has been made, and an object thereof is to provide a steel sheet excellent in fine blanking performance and also excellent in fabrication performance after fine blanking working and a manufacturing method of the same.
  • FB performance fine blanking performance
  • the material In the FB working, the material is worked in a state of zero clearance and compression stress. For that reason, after receiving large deformation, a crack is generated in the material. When a number of cracks are generated during large deformation, the FB performance is largely reduced. In order to prevent the generation of a crack, it is said that spheroidization of a cementite or miniaturization of a cementite particle size is important. However, in the FB working, in the case where even a 100 % spheroidized fine cementite is present in the ferrite grain, the generation of a fine crack is unavoidable.
  • the present inventors thought that in the case where stretch flanging working is further applied after the FB working, fine cracks generated at the time of the FB working are connected to each other, leading to a reduction in the stretch flanging property. Also, with respect to the mold life, the present inventors assumed that when a number of cementites are present in the ferrite grain, wear of a cutting blade is accelerated, leading to a reduction in the mold life.
  • a high steel slab (corresponding to S35C) containing 0.34 % of C, 0.2 % of Si and 0.8 % of Mn in terms of % by mass was heated at 1,150°C and then subjected to hot rolling consisting of rough rolling of 5 passes and finish rolling of 7 passes, thereby preparing a hot rolled steel sheet having a thickness of 4.2 mm.
  • a rolling termination temperature was set up at 860°C; a coiling temperature was set up at 600°C; and after the finish rolling, the steel sheet was cooled while changing a cooling rate from 5°C/s to 250°C/s.
  • a cooling stopping temperature was set up at 650°C.
  • the hot rolled steel sheet was subjected to pickling and then to batch annealing (at 720°C for from 5 to 40 hours) as hot rolled sheet annealing.
  • batch annealing at 720°C for from 5 to 40 hours
  • the metallurgical structure was observed by a scanning electron microscope (SEM), thereby measuring a ferrite grain size and a spheroidization ratio of a cementite.
  • SEM scanning electron microscope
  • the ferrite grain size an area of each ferrite grain was measured, and a circle-corresponding size was determined from the resulting area and defined as a grain size of each ferrite grain.
  • the thus obtained respective ferrite grain sizes were arithmetically averaged, and its value was defined as a ferrite average grain size of that steel sheet.
  • the number of measured ferrite grains was 5,000 for each.
  • a maximum length a and a minimum length b of each cementite were determined in each field of the structure observation (magnification: 3,000 times) by using an image analyzer; its ratio a/b was computed; and the number of cementite grains with a/b of not more than 3 was expressed by a proportion (%) against the total number of measured cementites, thereby defining it as a spheroidization ratio (%) of cementite.
  • the number of measured cementites was 9,000 for each.
  • the area of the cementite particle was measured in 30 fields (magnification: 3,000 times) for each.
  • a specimen (size: 100 ⁇ 80 mm) was collected from the obtained steel sheet and subjected to a fine blanking test (FB test).
  • the FB test was carried out by blanking a sample having a size of 60 mm ⁇ 40 mm (corner radius R: 10 mm) from the specimen by using a 110t hydraulic press machine under a lubricious condition of a clearance of 0.060 mm (1.5 % of the sheet thickness) and a working pressure of 8.5 tons.
  • a surface roughness (ten-point average roughness Rz) was measured, thereby evaluating the FB performance.
  • the both surfaces were equally ground in advance, thereby regulating the sheet thickness at 4.0 ⁇ 0.010 mm.
  • Rz ave (Rz 1 + Rz 2 + Rz 3 + Rz 4)/4 (wherein Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface) was computed.
  • the case where the appearance of the fracture surface on the blanked surface is not more than 10 % is defined as "excellent in FB performance".
  • excellent in FB performance the case where the average surface roughness Rz ave is small as 10 ⁇ m or less is defined as "excellent in FB performance”.
  • the measurement may be carried out by repeatedly performing scanning in a pitch of 100 ⁇ m in a sheet thickness direction in a region within a range of approximately ⁇ (sheet thickness (mm)) - 0.1 mm ⁇ in the sheet thickness direction of 0.5 mm from the surface and 10 mm in parallel to the surface to determine Rz on each surface, thereby determining Rz ave from Rzs of the respective surfaces.
  • the gist of the invention is as follows.
  • a steel sheet which is not only excellent in FB performance but also excellent in fabrication property after the FB working can be easily and cheaply manufactured, thereby giving rise to remarkable effects in view of the industry. Also, according to the invention, there are brought effects that a steel sheet excellent in FB performance is provided; an end surface treatment after the FB working is not necessary; a time of completion of manufacture can be shortened; the productivity is improved; and the manufacturing costs can be reduced.
  • C is an element influencing the hardness after hot rolling annealing and quenching, and in the invention, C is required to be contained in an amount of 0.1 % or more.
  • the content of C is less than 0.1 %, the hardness required as automobile parts cannot be obtained.
  • the content of C was limited to the range of from 0.1 to 0.5 %.
  • Si is an element not only acting as a deoxidizing agent but also increasing the strength (hardness) due to solution hardening.
  • Si when Si is contained in a large amount exceeding 0.5 %, ferrite becomes hard, thereby reducing the FB performance.
  • Si when Si is contained in an amount exceeding 0.5 %, a surface defect called as red scale is generated at the hot rolling stage. For that reason, the content of Si was limited to not more than 0.5 %. Incidentally, the content of Si is preferably not more than 0.35 %.
  • Mn from 0.2 to 1.5 %
  • Mn is an element not only increasing the strength of steel due to solution hardening but also acting effectively in improving the quenching property. In order to obtain such an effect, it is desirable that Mn is contained in an amount of 0.2 % or more. However, when Mn is contained excessively in an amount exceeding 1.5 %, the solution hardening becomes excessively strong so that the ferrite becomes hard, thereby reducing the FB performance. For that reason, the content of Mn was limited to the range of from 0.2 to 1.5 %. Incidentally, the content of Mn is preferably from 0.2 to 1.0 %, and more preferably from 0.6 to 0.9 %. P: not more than 0.03 %
  • the content of P of up to 0.03 % is tolerable. For such a reason, the content of P was limited to not more than 0.03 %.
  • the content of P is preferably not more than 0.02 %. S: not more than 0.02 %
  • S is an element which forms a sulfide such as MnS and exists as an inclusion in the steel, thereby reducing the FB performance, and it is desirable that S is reduced as far as possible.
  • the content of S of up to 0.02 % is tolerable. For such a reason, the content of S was limited to not more than 0.02 %. Incidentally, the content of S is preferably not more than 0.01 %.
  • the foregoing components are a basic composition.
  • Al and/or one or two or more members selected from Cr, Mo, Ni, Ti and B can be contained.
  • Al is an element not only acting as a deoxidizing agent but also binding with N to form AlN, thereby contributing to prevention of an austenite grain from coarseness.
  • Al fixes N and B forms BN, thereby bringing an effect for preventing a reduction of the content of B effective for improving the quenching property.
  • Such effects become remarkable when the content of Al is 0.02 % or more.
  • the content of Al exceeds 0.1 %, an index of cleanliness of steel is reduced. For that reason, when Al is contained, it is preferable that the content of Al is limited to not more than 0.1 %.
  • the content of Al as an unavoidable impurity is not more than 0.01 %.
  • All of Cr, Mo, Ni, Ti and B are an element contributing to an improvement in quenching property and/or an improvement in resistance to temper softening and can be selected and contained as the need arises.
  • Cr not more than 3.5 %
  • Cr is an element effective for improving the quenching property. In order to obtain such an effect, it is preferable that Cr is contained in an amount of 0.1 % or more. However, when the content of Cr exceeds 3.5 %, not only the FB performance is reduced, but also an excessive increase of the resistance to temper softening is brought. For that reason, when Cr is contained, it is preferable that the content of Cr is limited to not more than 3.5 %. Incidentally, the content of Cr is more preferably from 0.2 to 1.5 %. Mo: not more than 0.7 %
  • Mo is an element acting to effectively improve the quenching property. In order to obtain such an effect, it is preferable that Mo is contained in an amount of 0.05 % or more. However, when the content of Mo exceeds 0.7 %, the steel becomes hard, thereby reducing the FB performance. For that reason, when Mo is contained, it is preferable that the content of Mo is limited to not more than 0.7 %. Incidentally, the content of Mo is more preferably from 0.1 to 0.3 %. Ni: not more than 3.5 %
  • Ni is an element effective for improving the quenching property. In order to obtain such an effect, it is preferable that Ni is contained in an amount of 0.1 % or more. However, when the content of Ni exceeds 3.5 %, the steel becomes hard, thereby reducing the FB performance. For that reason, when Ni is contained, it is preferable that the content of Ni is limited to not more than 3.5 %. Incidentally, the content of Mo is more preferably from 0.1 to 2.0 %. Ti: from 0.01 to 0.1 %
  • Ti is easy to bind with N to form TiN and is an element effectively acting to prevent coarseness of a ⁇ grain at the time of quenching. Also, when Ti is contained together with B, since Ti reduces N which forms BN, it has an effect for minimizing the addition amount of B necessary for improving the quenching property. In order to obtain such effects, it is required that the content of Ti is 0.01 % or more. On the other hand, when the content of Ti exceeds 0.1 %, the ferrite is subjected to precipitation strengthening due to precipitation of TiC or the like and becomes hard, thereby reducing the mold life. For that reason, when T is contained, it is preferable that the content of Ti is limited to the range of from 0.01 to 0.1 %. Incidentally, the content of Ti is more preferably from 0.015 to 0.08 %. B: from 0.0005 to 0.005 %
  • B is an element which segregates on an austenite grain boundary and when contained in a trace amount, improves the quenching property.
  • the case where B is compositely added together with Ti is effective.
  • the content of B is 0.0005 % or more.
  • the content of B is limited to the range of from 0.0005 to 0.005 %.
  • the content of B is more preferably from 0.0008 to 0.004 %.
  • the remainder other than the foregoing components is Fe and unavoidable impurities.
  • the unavoidable impurities for example, not more than 0.01 % of N, not more than 0.01 % of O and not more than 0.1 % of Cu are tolerable.
  • the steel sheet of the invention has a structure composed mainly of ferrite and a cementite.
  • the "structure composed mainly of ferrite and a cementite” as referred to herein means a structure in which ferrite and a cementite account for 95 % or more in terms of a volume ratio.
  • the grain size of ferrite is from 1 to 10 ⁇ m in terms of an average crystal grain size.
  • the average ferrite crystal grain size is less than 1 ⁇ m, not only the steel sheet is remarkably hardened, but also the cementite amount in ferrite grain increases, whereby the fabrication property such as hole expansion property after the FB working as well as the FB performance and the mold life are reduced.
  • the grain size of ferrite exceeds 10 ⁇ m, though the steel sheet is softened, thereby improving the mold life, the FB performance is reduced as shown in Fig. 3 .
  • the average ferrite crystal grain size was limited to the range of from 1 to 10 ⁇ m.
  • the average ferrite crystal grain size is preferably from 1 to 5 ⁇ m.
  • a spheroidization ratio of the cementite is 80 % or more.
  • the spheroidization ratio is less than 80 %, not only the steel sheet becomes hard, but also the deformability is small and the FB performance is reduced.
  • the spheroidization ratio is less than 80 %, Rz ave exceeds 10 ⁇ m and becomes large, and the FB performance is abruptly reduced.
  • the spheroidization ratio of a cementite was limited to 80 % or more.
  • the spheroidization ratio is preferably from 80 to 85 %.
  • an amount S gb of a ferrite intergranular cementite is 40 % or more.
  • the amount S gb of a ferrite intergranular cementite is less than 40 %, the amount of the cementite present in the ferrite grain is large; Rz ave exceeds 10 ⁇ m and becomes large as shown in Fig.
  • the amount S gb of a ferrite intergranular cementite was limited to 40 % or more.
  • the amount S gb of a ferrite intergranular cementite is preferably 50 % or more.
  • the cementite present on the crystal grain boundary of ferrite has an average grain size of not more than 5 ⁇ m. This is because it has been newly found that in the case where the amount S gb of a ferrite intergranular cementite is 40 % or more, with respect to the cementite present on the ferrite grain boundary, the smaller the particle size, the more improved the FB working and the larger the contribution to an improvement in mold life. Also, when the particle size of the cementite, in short-time heating in high-frequency quenching, it is possible to easily dissolve the cementite in the austenite, whereby it is easy to ensure a desired quenching hardness. For these reasons, it is preferable that the average particle size of a cementite present on the ferrite crystal grain boundary is limited to not more than 5 ⁇ m.
  • a molten steel having the foregoing composition is molten by a common melting method using a converter or the like and formed into a raw steel material (slab) by a common casting method such as a continuous casting method.
  • the obtained raw steel material is subjected to hot rolling to form a hot rolled sheet by heating and rolling.
  • the hot rolling is preferably a treatment in which a termination temperature of finish rolling is set up at from 800 to 950°C, after completion of the finish rolling, cooling is carried out at an average cooling rate of 50°C/s or more, the cooling is stopped at a temperature in the range of from 500 to 700°C, and coiling is carried out at from 450 to 600°C.
  • the hot rolling in the invention is characterized by adjusting the termination temperature of finish rolling and the subsequent cooling condition.
  • Termination temperature of finish rolling from 800 to 950°C
  • the termination temperature of finish rolling is a temperature in the range of from 800 to 950°C, which is a termination temperature region of usual finish rolling.
  • the termination temperature of finish rolling exceeds 950°C and becomes high, not only a generated scale becomes thick so that the pickling property is reduced, but also a decarburized layer may possibly be formed in the steel sheet surface layer.
  • the termination temperature of finish rolling is lower than 800°C, an increase in the rolling load becomes remarkable, and an excessive load against a rolling mill becomes problematic. For that reason, it is preferable that the termination temperature of finish rolling is a temperature in the range of from 800 to 950°C.
  • Average cooling rate after completion of finish rolling 50°C/s or more
  • the subject average cooling rate is an average cooling rate of from the termination temperature of finish rolling to a stopping temperature of the subject cooling (forced cooling).
  • the average cooling rate is less than 50°C/s, cementite-free ferrite is formed during cooling, and the structure after cooling is a heterogeneous structure of (ferrite + pearlite), whereby a homogeneous structure composed of substantially 100 % pearlite cannot be ensured.
  • the hot rolled sheet structure is a heterogeneous structure of (ferrite + pearlite)
  • the amount of the cementite present in the grain increases, and the amount of the cementite present on the grain boundary decreases.
  • the FB performance is reduced.
  • the average cooling rate after completion of finish rolling is limited to 50°C/s or more.
  • the average cooling rate after completion of finish rolling is not more than 120°C/s.
  • Cooling stopping temperature from 500 to 700°C
  • a temperature at which the foregoing cooling (forced cooling) is stopped is from 500 to 700°C.
  • the cooling stopping temperature is lower than 500°C, there are caused problems in operation such as a problem that hard bentonite or martensite is formed, whereby the hot rolled sheet annealing takes a long time; and the generation of a crack at the time of coiling.
  • the cooling stopping temperature exceeds 700°C and becomes high, since a ferrite transformation noise is present in the vicinity of 700°C, ferrite is formed during standing for cooling after stopping of cooling, whereby a homogeneous structure composed of substantially 100 % pearlite cannot be ensured. From these matters, it is preferable that the cooling stopping temperature is limited to a temperature in the range of form 500 to 700°C.
  • the cooling stopping temperature is more preferably from 500 to 650°C, and further preferably from 500 to 600°C.
  • the hot rolled sheet After stopping the cooling, the hot rolled sheet is immediately coiled in a coil state.
  • the coiling temperature is preferably from 450 to 600°C, and more preferably from 500 to 600°C.
  • hot rolled sheet (hot rolled steel sheet) is then subjected to removal of an oxidized scale of the surface by pickling or shot blasting and subsequently to hot rolled sheet annealing.
  • hot rolled sheet annealing By applying proper hot rolled sheet annealing to the hot rolled sheet having a substantially 100 % pearlite structure, not only the spheroidization of a cementite is accelerated, but also the grain growth of ferrite is inhibited, whereby a large amount of the cementite can be made present on the ferrite crystal grain boundary.
  • the annealing temperature is a temperature in the range of from 600 to 750°C.
  • the annealing temperature is lower than 600°C, spheroidization of the cementite cannot be sufficiently achieved.
  • the annealing temperature exceeds 750°C and becomes high, pearlite is regenerated during cooling, and the fine blanking performance and other fabrication property are reduced.
  • a holding time of the hot rolled sheet annealing is not required to be particularly limited, in order to sufficiently spheroidize the cementite, it is preferable that the holding time is 8 hours or more. Also, when it exceeds 80 hours, since the ferrite grain becomes excessively coarse, the holding time is preferably not more than 80 hours.
  • a raw steel material (slab) having a composition as shown in Table 1 was subjected to hot rolling and hot rolled sheet annealing as shown in Table 2, thereby forming a hot rolled steel sheet (thickness: 4.3 mm).
  • the obtained hot rolled steel sheet was examined with respect to the structure, FB performance and stretch flanging property after the FB performance.
  • the examination methods are as follows.
  • a specimen for structure observation was collected from the obtained steel sheet.
  • a cross section parallel to a rolling direction of the specimen was polished and corroded with nital; and with respect to a position of 1/4 of the sheet thickness, a metallurgical structure was observed (field number: 30 places) by a scanning electron microscope (SEM) (magnification, ferrite: 1,000 times, cementite: 3,000 times); and a volume ratio of ferrite and a cementite, a ferrite grain size, a spheroidization ratio of a cementite, an amount of ferrite intergranular cementite and an average particle size of a cementite on the ferrite grain boundary were measured.
  • SEM scanning electron microscope
  • the metallurgical structure was observed (field number: 30 places) by SEM (magnification: 3,000 times); an area ratio obtained by dividing an area resulting from summing up an area of ferrite and an area of a cementite by a total field area; and this value was judged as a volume ratio of ferrite and a cementite.
  • ferrite grain size an area of each ferrite grain was measured, and a circle-corresponding size was determined from the resulting area and defined as a grain size of each ferrite grain.
  • the thus obtained respective ferrite grain sizes were arithmetically averaged, and its value was defined as a ferrite average grain size of that steel sheet.
  • a maximum length a and a minimum length b of each cementite were determined in each field (field number: 30 pieces) of the structure observation (magnification: 3,000 times) by using an image analyzer; its ratio a/b was computed; and the number of cementite grains of a/b with not more than 3 was expressed by a proportion (%) against the total number of measured cementites, thereby defining it as a spheroidization ration (%) of cementite.
  • a diameter passing through two points on the periphery of the cementite and a center of gravity of a corresponding oval of the cementite was measured at every 2° to determine a circle-corresponding size, thereby defining it as a grain size of each cementite.
  • the thus obtained respective cementite particle sizes were averaged, and its value was defined as a cementite average particle size in ferrite grain.
  • a specimen (size: 100 ⁇ 80 mm) was collected from the obtained steel sheet and subjected to an FB test.
  • the FB test was carried out by blanking a sample having a size of 60 mm ⁇ 40 mm (corner radius R: 10 mm) from the specimen by using a 110t hydraulic press machine under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5 % of the sheet thickness) and a working pressure of 8.5 tons.
  • a surface roughness (ten-point average roughness Rz) was measured, thereby evaluating the FB performance.
  • the both surfaces were equally ground in advance, thereby regulating the sheet thickness at 4.0 ⁇ 0.010 mm.
  • Rz ave ( Rz 1 + Rz 2 + Rz 3 + Rz 4 ) / 4 (wherein Rz 1, Rz 2, Rz 3 and Rz 4 each represents Rz on each surface.)
  • the life of the used tool was evaluated.
  • a surface roughness (ten-point average roughness Rz) of the sample end surface (blanked surface) at the point of time when the number of blanking in the FB working reached 30,000 times was measured, thereby evaluating the mold life.
  • the measurement method the surface roughness was the same as described above.
  • the case where the average surface roughness Rz ave of the sample end surface is not more than 10 ⁇ m is defined as " ⁇ "; the case where it is more than 10 ⁇ m and not more than 16 ⁇ m was defined as " ⁇ "; and the case where it is more than 16 ⁇ m was defined as " ⁇ ".
  • a specimen (size: 100 mm ⁇ 100 mm) was blanked from the obtained steel sheet by FB working, thereby examining a stretch flanging property.
  • the FB working was carried out under a lubricious condition of a tool-to-tool clearance of 0.060 mm (1.5 % of the sheet thickness) and a working pressure of 8.5 tons.
  • the stretch flanging property was evaluated by carrying out a hole expansion test to determine a hole expansion ratio ⁇ .
  • the hole expansion test was carried out by a method in which a punch hole of 10 mm ⁇ (do) was blanked in a specimen and expanding the subject hole by a tool; a hole size d at the point of time when a through thickness crack was generated in a flange of the punch hole was determined; and a hole expansion ratio ⁇ (%) as defined by the following expression was determined.
  • ⁇ ( % ) ( d - d 0 ) / d 0 ⁇ 100
  • the average surface roughness Rz ave on the blanked surface is not more than 10 ⁇ m; the FB performance is excellent; the blanked surface at the time of 30,000 times in blanking number is smooth (evaluation: ⁇ ); and a reduction in mold life is not acknowledged.
  • the examples of the invention are excellent in the stretch flanging property after FB working.
  • the volume ratio of ferrite and a cementite was confirmed in the foregoing method.
  • the sum of volume ratio of the ferrite and cementite is 95 % or more, thereby forming a structure composed mainly of ferrite and a cementite.
  • the particle size of a cementite present on the ferrite crystal grain boundary was confirmed by the foregoing method.
  • the average particle size was not more than 5 ⁇ m.
  • the surface roughness Rz on the blanked surface exceeds 10 ⁇ m and becomes coarse, whereby a reduction of the FB performance is confirmed, the mold life is reduced, and the stretch flanging property is reduced.
  • the hot rolled sheet annealing and subsequent treatments were not carried out.

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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)
EP07713798A 2006-01-31 2007-01-29 Feuille d'acier convenant parfaitement a un decoupage fin et son procede de production Active EP1980635B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2006023104 2006-01-31
PCT/JP2007/051835 WO2007088985A1 (fr) 2006-01-31 2007-01-29 Feuille d'acier convenant parfaitement a un decoupage fin et son procede de production

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EP1980635A1 true EP1980635A1 (fr) 2008-10-15
EP1980635A4 EP1980635A4 (fr) 2010-12-01
EP1980635B1 EP1980635B1 (fr) 2012-01-11

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US (1) US20090173415A1 (fr)
EP (1) EP1980635B1 (fr)
KR (1) KR101023633B1 (fr)
CN (1) CN101379208B (fr)
WO (1) WO2007088985A1 (fr)

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EP2103697A4 (fr) * 2006-12-25 2015-03-11 Jfe Steel Corp Tôle en acier laminé à chaud, à teneur en carbone élevée, et son procédé de fabrication
EP3282032A4 (fr) * 2015-04-10 2018-09-12 Nippon Steel & Sumitomo Metal Corporation Tôle d'acier ayant une excellente aptitude au façonnage à froid lors du formage et son procédé de production
US11365460B2 (en) 2018-02-23 2022-06-21 Jfe Steel Corporation High-carbon cold rolled steel sheet and method for manufacturing same

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CN103084386B (zh) * 2011-10-31 2015-02-25 上海汇众汽车制造有限公司 优化热轧钢板表面质量的方法
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
BR112017024692A2 (pt) * 2015-05-26 2018-07-24 Nippon Steel & Sumitomo Metal Corporation placa de aço e método de produção da mesma
JP6515332B2 (ja) * 2015-05-26 2019-05-22 日本製鉄株式会社 被切削性及び焼入れ焼戻し後の耐摩耗特性に優れる低炭素鋼板及びその製造方法
JP6160783B2 (ja) 2015-05-26 2017-07-12 新日鐵住金株式会社 鋼板及びその製造方法
EP3305930A4 (fr) * 2015-05-26 2018-12-05 Nippon Steel & Sumitomo Metal Corporation Tôle d'acier et son procédé de production
WO2016204288A1 (fr) * 2015-06-17 2016-12-22 新日鐵住金株式会社 Tôle d'acier et procédé de fabrication
CN105624567B (zh) * 2016-01-13 2017-06-13 东北大学 一种纳米级球形渗碳体强化的铁素体钢板及其制备方法
KR102348549B1 (ko) * 2019-12-20 2022-01-06 주식회사 포스코 가공성이 우수한 강재 및 그 제조방법
CN114058941A (zh) * 2020-07-31 2022-02-18 宝山钢铁股份有限公司 一种冷轧钢板及制造方法和汽车用冲裁件

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2103697A4 (fr) * 2006-12-25 2015-03-11 Jfe Steel Corp Tôle en acier laminé à chaud, à teneur en carbone élevée, et son procédé de fabrication
EP3282032A4 (fr) * 2015-04-10 2018-09-12 Nippon Steel & Sumitomo Metal Corporation Tôle d'acier ayant une excellente aptitude au façonnage à froid lors du formage et son procédé de production
US11365460B2 (en) 2018-02-23 2022-06-21 Jfe Steel Corporation High-carbon cold rolled steel sheet and method for manufacturing same

Also Published As

Publication number Publication date
EP1980635B1 (fr) 2012-01-11
KR20080077254A (ko) 2008-08-21
CN101379208B (zh) 2012-06-20
KR101023633B1 (ko) 2011-03-22
CN101379208A (zh) 2009-03-04
US20090173415A1 (en) 2009-07-09
WO2007088985A1 (fr) 2007-08-09
EP1980635A4 (fr) 2010-12-01

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