EP1362930A1 - Dünnes stahlblech für autos mit hervorragender kerbdauerfestigkeit und verfahren zu seiner herstellung - Google Patents

Dünnes stahlblech für autos mit hervorragender kerbdauerfestigkeit und verfahren zu seiner herstellung Download PDF

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Publication number
EP1362930A1
EP1362930A1 EP02700640A EP02700640A EP1362930A1 EP 1362930 A1 EP1362930 A1 EP 1362930A1 EP 02700640 A EP02700640 A EP 02700640A EP 02700640 A EP02700640 A EP 02700640A EP 1362930 A1 EP1362930 A1 EP 1362930A1
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Prior art keywords
steel sheet
notch
ray diffraction
fatigue strength
temperature
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English (en)
French (fr)
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EP1362930A4 (de
Inventor
Tatsuo c/o NIPPON STEEL CORP. OITA WORKS YOKOI
Natsuko Sugiura
Naoki Yoshinaga
Koichi NIPPON STEEL CORP. OITA WORKS TSUCHIHASHI
Takehiro NIPPON STEEL CORP. OITA WORKS NAKAMOTO
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Nippon Steel Corp
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Nippon Steel Corp
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    • 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • 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
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips

Definitions

  • the present invention relates to a thin steel sheet for automobile use excellent in notch-fatigue strength, and a method for producing the steel sheet, and, more specifically, to a thin steel sheet for automobile use excellent in notch-fatigue strength and suitable as the material for undercarriage components of an automobile and the like to overcome the problem of the propagation of a fatigue crack from a site of stress concentration such as a blanked or welded portion, and a method for producing the steel sheet.
  • an undercarriage component such as a suspension arm is produced through the processes of blanking and boring by shearing and punching, thereafter press forming and, in some cases, welding. It is often the case with such a component that a crack propagates from a point near a sheared end face or a weld and causes fatigue fracture. In other words, a sheared end face or a weld acts as a stress concentration site like a notch and a fatigue crack propagates therefrom.
  • the fatigue limit of a material is lowered as a notch becomes acute.
  • a fatigue limit does not lower any further. This is because a fatigue limit shifts from being dominated by a crack initiation limit toward being dominated by a crack propagation limit as the acuteness of a notch increases.
  • the strength of a material increases, while a crack initiation limit increases, a crack propagation limit does not, and therefore the acuteness of a notch, at which a fatigue limit shifts from being dominated by a crack initiation limit toward being dominated by a crack propagation limit, moves toward an acuter side.
  • Thin steel sheets having strength of the 340 to 440 MPa class are presently used for the undercarriage members of an automobile.
  • the level of strength required of the steel sheets for those members is rising toward the 590 to 780 MPa class. Therefore, to satisfactorily respond to such a requirement, it is essential to develop a steel sheet with which the advantages of high strength can be secured even when an acute notch exists.
  • Japanese Unexamined Patent Publication No. H5-51695 discloses a technology wherein the occurrence of a burr is suppressed by reducing the addition amount of Si and forming precipitates of Ti, Nb and V for lowering breaking elongation and thereby the fatigue strength of an as-blanked or as-sheared steel sheet is enhanced.
  • Japanese Unexamined Patent Publication No. H5-179346 discloses a technology wherein the upper limit of the volume percentage of bainite is regulated by defining an upper limit of a finish rolling temperature and, thereby, the fatigue strength of an as-blanked or as-sheared steel sheet is enhanced.
  • Japanese Unexamined Patent Publication No. H8-13033 discloses a technology wherein the formation of martensite is suppressed by defining a cooling rate after rolling and, thereby, the fatigue strength of an as-blanked or as-sheared steel sheet is enhanced.
  • Japanese Unexamined Patent Publication No. H8-302446 discloses a technology wherein strain energy during blanking or shearing is reduced by regulating the hardness of the second phase of a dual phase steel to at least 1.3 times that of ferrite and, thereby, the fatigue strength of an as-blanked or as-sheared steel sheet is enhanced.
  • Japanese Unexamined Patent Publication No. H9-170048 discloses a technology wherein the occurrence of a burr during blanking or shearing is suppressed by regulating the length of intergranular cementite and thereby the fatigue strength of an as-blanked or as-sheared steel sheet is enhanced.
  • Japanese Unexamined Patent Publication No. H9-202940 discloses a technology wherein blanking performance is improved by regulating a parameter based on the addition amounts of Ti, Nb and Cr and thereby the fatigue strength of an as-blanked steel sheet is enhanced.
  • Japanese Unexamined Patent Publication No. H6-88161 discloses a technology wherein the X-ray diffraction strength ratio of a (100) plane parallel to the rolling surfaces in the texture at a steel sheet surface layer is regulated to 1.5 or more and, thereby, a fatigue crack propagation speed is lowered. Further, Japanese Unexamined Patent Publications No. H8-199286 and No.
  • H10-147846 disclose technologies wherein the area percentage of recovered or recrystallized ferrite is controlled in the range from 15 to 40% by regulating the X-ray diffraction strength ratio of a (200) plane in the thickness direction in the range from 2.0 to 15.0 and, thereby, a fatigue crack propagation speed is lowered.
  • the aforementioned technologies are ones wherein a fatigue crack propagation speed is controlled in a PARIS zone that is referred to in the fracture mechanics of a fatigue crack mainly propagating from a weld toe portion and therefore are insufficient as technologies to be employed in such a case as a thin steel sheet, for automobile use, where a crack propagation zone is not included in the PARIS zone because of the thickness of the steel sheet.
  • the present invention relates to a technology wherein the notch-fatigue strength of a thin steel sheet for automobile use is improved by controlling the texture of the steel sheet and thus enhancing the resistance to a fatigue crack propagating from a notch such as an end face formed after blanking or shearing, regardless of the conditions such as the clearance of tools during blanking or shearing.
  • the object of the present invention is to provide a thin steel sheet for automobile use excellent in notch-fatigue strength and a method for producing the steel sheet economically and stably.
  • the present inventors in consideration of the production processes of thin steel sheets presently produced on an industrial scale using generally employed production facilities, earnestly studied methods for enhancing the notch-fatigue strength of a thin steel sheet for automobile use.
  • the present invention has been established on the basis of a new discovery that the following conditions are very effective for enhancing notch-fatigue strength: that, on a plane at an arbitrary depth within 0.5 mm from the surface of a steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength is 4 or less; and that the thickness of the steel sheet is in the range from 0.5 to 12 mm.
  • the gist of the present invention is as follows:
  • a fatigue crack of a steel sheet starts from the surface thereof; this is true also with the case where a stress concentration site such as a notch exists.
  • a stress concentration site such as a notch exists.
  • an end face formed by blanking or shearing exists, it is often observed that, under a repeated load including a loading mode in the out-of-plane bending direction, a fatigue crack starts and propagates from an end of a steel sheet surface. It is clear from this that, even in such a case, it is effective for enhancing notch-fatigue strength to increase resistance to crack propagation at the surface of a steel sheet or in the layer from the surface to a depth of several crystal grains or so.
  • the range of a steel sheet texture effective in enhancing fatigue strength is limited to the range from the surface to a depth of 0.5 mm in the thickness direction.
  • the range is, more adequately, to a depth of 0.1 mm.
  • the present inventors investigated the influences of the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength on a plane at an arbitrary depth in the range from the surface of a steel sheet to a depth of 0.5 mm in the thickness direction thereof over notch-fatigue strength.
  • the specimens for the investigation were prepared by melting a steel and adjusting the chemical components thereof so that the steel contained 0.08% C, 0.9% Si, 1.2% Mn, 0.01% P, 0.001% S, and 0.03% Al, casting it into a slab, hot rolling the slab to a thickness of 3.5 mm so that the finish rolling was completed at a temperature of not lower than the Ar 3 transformation temperature, and then coiling the hot-rolled steel sheet.
  • a test piece was prepared by cutting out a specimen sheet 30 mm in diameter from a position of 1/4 or 3/4 of the width of a steel sheet, grinding the surface of the specimen sheet to a depth of about 0.05 mm from the surface so that the surface might have the second finest finish, and then removing strain by chemical polishing or electrolytic polishing.
  • a crystal orientation component expressed as ⁇ hkl ⁇ uvw> means that the direction of a normal to the plane of a steel sheet is parallel to ⁇ hkl> and the rolling direction of the steel sheet is parallel to ⁇ uvw>.
  • the measurement of a crystal orientation with X-rays is conducted, for example, in accordance with the method described in pages 274 to 296 of the Japanese translation of Elements of X-ray Diffraction by B. D. Cullity (published in 1986 by AGNE Gijutsu Center, translated by Gentaro Matsumura).
  • the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is obtained from the X-ray diffraction strengths in the principal orientation components included in said orientation component group, namely ⁇ 100 ⁇ 011>, ⁇ 116 ⁇ 110>, ⁇ 114 ⁇ 110>, ⁇ 113 ⁇ 110>, ⁇ 112 ⁇ 110>, ⁇ 335 ⁇ 110> and ⁇ 223 ⁇ 110>, in the three-dimensional texture calculated either by the vector method based on the pole figure of ⁇ 110 ⁇ or by the series expansion method using two or more (desirably, three or more) pole figures out of the pole figures of ⁇ 110 ⁇ , ⁇ 100 ⁇ , ⁇ 211 ⁇ and ⁇ 310 ⁇ .
  • the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is the arithmetic average of the ratios in all the above orientation components.
  • the arithmetic average of the strengths in the orientation components of ⁇ 100 ⁇ 011>, ⁇ 116 ⁇ 110>, ⁇ 114 ⁇ 110>, ⁇ 112 ⁇ 110> and ⁇ 223 ⁇ 110> may be used as a substitute.
  • the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength can be obtained from the three-dimensional texture calculated in the same manner as explained above.
  • a test piece for fatigue test having the shape shown in Fig. 1(b) was cut out from a position of 1/4 or 3/4 of the width of the steel sheet so that the longitudinal direction of the test piece coincided with the rolling direction of the steel sheet, and was subjected to a fatigue test. It has to be noted here that, whereas a test piece for fatigue test shown in Fig. 1(a) is a common unnotched test piece for evaluating the fatigue strength of a steel material, a test piece for fatigue test shown in Fig. 1(b) is a notched test piece prepared for evaluating notch-fatigue strength.
  • a test piece for fatigue test was ground to a depth of about 0.05 mm from the surface so that the surface might have the second finest finish, and a fatigue test was carried out using an electro-hydraulic servo type fatigue tester and the methods conforming to JIS Z 2273-1978 and JIS Z 2275-1978.
  • Fig. 2 shows the results of an investigation of the influences of the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength over notch-fatigue strength.
  • the numeral in a circle in the figure indicates the fatigue limit (the fatigue strength for finite life after 10 7 cycles of repetition) obtained through a fatigue test using a notched test piece having the shape shown in Fig. 1(b); the numeral is hereinafter referred to as a notch-fatigue strength.
  • the present inventors have newly found that it is very important, for enhancing notch-fatigue strength, that, on a plane at an arbitrary depth within 0.5 mm from the surface of a steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength is 4 or less.
  • the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is 4 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ ⁇ 110> to random X-ray diffraction strength is 2.5 or less.
  • the fatigue limit of a material is determined by the crack propagation limit of the material, namely the degree of the resistance to the propagation of a crack for arresting the crack.
  • the propagation of a fatigue crack is caused by the repetition of small plastic deformation at the bottom of a notch or a stress concentration site, and it is presumed that, when a crack length is comparatively small and plastic deformation occurs within a range comparable to the size of a crystal grain, the crack propagation is significantly influenced by crystallographic slip planes and slip directions. Therefore, if the proportion of the crystal grains having slip planes and slip directions that show a high resistance to crack propagation is large in the crack propagation direction and on the plane of a crack, then the propagation of the fatigue crack is suppressed.
  • the thickness of a steel sheet is less than 0.5 mm, the conditions of allowing the occurrence of a small-scale yield are not satisfied regardless of the extent of stress concentration and therefore there is a danger that monotonic ductile fracture is caused.
  • the thickness of a steel sheet is 1.2 mm or more for maintaining the state of plane strain.
  • the thickness of a steel sheet exceeds 12 mm, on the other hand, the deterioration of fatigue strength resulting from thickness effect (size effect) becomes significant. Further, when the thickness of a steel sheet exceeds 8 mm, an excessive load may be required to be imposed on production facilities for achieving the conditions of hot or cold rolling that allow a texture effective for enhancing notch-fatigue strength to be obtained. For that reason, a desirable thickness is 8 mm or less. As a conclusion, the thickness of a steel sheet is limited to 0.5 to 12 mm, or desirably 1.2 to 8 mm, in the present invention.
  • the microstructure of a steel sheet it is not necessary to specify the microstructure of a steel sheet for the purpose of enhancing the notch-fatigue strength of the steel sheet.
  • the effect of enhancing notch-fatigue strength in the present invention is obtained as far as a texture falls in the range specified in the present invention (a texture showing the ratios of the X-ray diffraction strength in specific orientation components to random X-ray diffraction strength falling in the ranges specified in the present invention) in the structures of ferrite, bainite, pearlite and martensite forming in a commonly used steel material. Therefore, it is desirable to regulate the microstructure of a steel sheet in consideration of other required material properties.
  • a microstructure is a specific microstructure, for example, a compound structure containing retained austenite by 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite, a compound structure containing ferrite as the phase accounting for the largest volume percentage and mainly martensite as the second phase, or the like.
  • the ferrite mentioned here includes bainitic ferrite and acicular ferrite.
  • a structure which is not a bcc crystal structure, such as retained austenite is included in a compound structure composed of two or more phases, such a compound structure does not pose any problem insofar as the ratios of the X-ray diffraction strength in the orientation components and orientation component groups to random X-ray diffraction strength converted by the volume percentage of the other structures are within the relevant ranges according to the present invention.
  • the volume percentage of the pearlite containing coarse carbides may act as a starting point of a fatigue crack and remarkably deteriorate fatigue strength, it is desirable that the volume percentage of the pearlite containing coarse carbides is 15% or less. When still better fatigue properties are required, it is desirable that the volume percentage of the pearlite containing coarse carbides is 5% or less.
  • the volume percentage of ferrite, bainite, pearlite, martensite or retained austenite is defined as the area percentage thereof in a microstructure observed with an optical microscope under a magnification of 200 to 500 at a position in the depth of 1/4 of the steel sheet thickness on a section surface along the rolling direction of a specimen which is cut out from a position of 1/4 or 3/4 of the width of the steel sheet, the section surface being polished and etched with a nitral reagent and/or the reagent disclosed in Japanese Unexamined Patent Publication No. H5-163590.
  • the volume percentage may also be calculated in the following manner.
  • the microstructure of a steel sheet is a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage.
  • the present invention allows the compound structure to contain unavoidably included martensite, retained austenite and pearlite.
  • a good burring workability a hole expansion ratio
  • the microstructure of a steel sheet is a compound structure containing retained austenite by 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite.
  • the present invention allows the compound structure to contain unavoidably included martensite and pearlite as far as their total volume percentage is less than 5%.
  • the microstructure of a steel sheet is a compound structure containing ferrite as the phase accounting for the largest volume percentage and mainly martensite as the second phase.
  • the present invention allows the compound structure to contain unavoidably included bainite, retained austenite and pearlite as far as their total volume percentage is less than 5%. Note that, for securing a low yield ratio of 70% or less, it is desirable that the volume percentage of ferrite is 50% or more.
  • C is an indispensable element for obtaining a desired microstructure.
  • a C content exceeds 0.3%, however, workability deteriorates and, for this reason, a C content is limited to 0.3% or less.
  • a C content exceeds 0.2%, weldability tends to deteriorate and, for this reason, it is desirable that a C content is 0.2% or less.
  • a C content is less than 0.01%, steel strength decreases and, therefore, a C content is limited to 0.01% or more. Further, for the purpose of obtaining retained austenite stably in an amount sufficient for realizing a good ductility, it is desirable that a C content is 0.05% or more.
  • Si is a solute-strengthening element and, as such, it is effective for enhancing strength.
  • An Si content has to be 0.01% or more for obtaining a desired strength, but, when an Si content exceeds 2%, workability deteriorates. Therefore, an Si content is limited in the range from 0.01 to 2%.
  • Mn is also a solute-strengthening element and, as such, it is effective for enhancing strength.
  • An Mn content has to be 0.05% or more for obtaining a desired strength.
  • elements such as Ti, which suppress hot cracking induced by S, are not added in a sufficient amount in addition to Mn, it is desirable to add Mn so that the expression Mn/S ⁇ 20 is satisfied in terms of mass percentage.
  • Mn is an element that stabilizes austenite and, therefore, in order to stably obtain a sufficient amount of retained austenite in an attempt to secure a good ductility, it is desirable that an Mn addition amount is 0.1% or more.
  • Mn is added in excess of 3%, on the other hand, cracks occur to a slab. For this reason, an Mn content is limited to 3% or less.
  • P is an undesirable impurity, and the lower the P content, the better.
  • a P content exceeds 0.1%, workability and weldability are adversely affected, and so are fatigue properties. Therefore, a P content is limited to 0.1% or less.
  • S is also an undesirable impurity, and the lower the S content, the better.
  • S content is too high, the A type inclusions detrimental to local ductility and burring workability are formed and, for this reason, an S content has to be minimized.
  • a permissible content of S is 0.01% or less.
  • Al must be added by 0.005% or more for deoxidizing molten steel, but its upper limit is set at 1.0% to avoid a cost increase. Al increases the formation of non-metallic inclusions and deteriorates elongation when added excessively and, for this reason, a desirable content of Al is 0.5% or less.
  • Cu is added as occasion demands, since Cu has an effect of improving fatigue properties when it is in the state of solid solution. No tangible effect is obtained when a Cu addition amount is less than 0.2%, but the effect is saturated when a Cu content exceeds 2%. Thus, the range of a Cu content is determined to be from 0.2 to 2%. It has to be noted that, when a coiling temperature is 450°C or higher and Cu is added in excess of 1.2%, Cu may precipitate after coiling, drastically deteriorating workability. For this reason, it is desirable to limit a Cu content to 1.2% or less.
  • B is added as occasion demands, as B has an effect of raising fatigue limit when added in combination with Cu.
  • An addition of B by less than 0.0002% is not enough for obtaining the effect, but, when B is added in excess of 0.002%, cracks occur in a slab. For this reason, the addition amount of B is limited to 0.0002 to 0.002%.
  • Ni is added as occasion demands for preventing hot shortness caused by the presence of Cu.
  • An Ni addition amount of less than 0.1% is not enough for obtaining the effect, but, even when it is added in excess of 1%, the effect is saturated. For this reason, an Ni content is limited in the range from 0.1 to 1%.
  • Ca and REM are the elements that modify the shape of non-metallic inclusions, which serve as the starting points of fractures and/or deteriorate workability, and, by so doing, render them harmless. But no tangible effect is obtained when either of them is added at less than 0.0005%. When Ca is added in excess of 0.002% or REM in excess of 0.02%, the effect is saturated. Thus, it is desirable to add Ca by 0.0005 to 0.002% and REM by 0.0005 to 0.02%.
  • precipitation-strengthening and solute-strengthening elements may be added for enhancing strength.
  • precipitation-strengthening and solute-strengthening elements namely Ti, Nb, Mo, V, Cr and Zr
  • Sn, Co, Zn, W and/or Mg may be added at 1% or less in total to a steel containing aforementioned elements as the main components.
  • Sn may cause surface defects during hot rolling, it is desirable to limit an Sn content to 0.05% or less.
  • a steel sheet according to the present invention can be produced through any of the following process routes: casting, hot rolling and cooling; casting, hot rolling, cooling, pickling, cold rolling and annealing; heat treatment of a hot-rolled or cold-rolled steel sheet in a hot dip plating line; or, further, surface treatment applied separately to a steel sheet produced through any of the above process routes.
  • the present invention does not specify production methods prior to hot rolling. That is, a steel may be melted and refined in a blast furnace, an electric arc furnace or the like, then the chemical components may be adjusted in one or more of various secondary refining processes so that the steel may contain desired amounts of the components, and then the steel may be cast into a slab through a casting process such as an ordinary continuous casting process, an ingot casting process and a thin slab casting process. Steel scraps may be used as a raw material. Further, in the case of a slab cast through a continuous casting process, the slab may be fed to a hot-rolling mill directly while it is hot, or it may be hot rolled after being cooled to room temperature and then heated in a reheating furnace.
  • No limit is particularly set to the temperatures of reheating, but it is desirable that a reheating temperature is lower than 1,400°C, since, when it is 1,400°C or higher, the descale amount becomes large and the product yield decreases. It is also desirable that a reheating temperature is 1,000°C or higher, since a reheating temperature lower than 1,000°C remarkably deteriorates the operation efficiency of a rolling mill in terms of rolling schedule.
  • P (MPa) 5.64 x P 0 x V/H 2 , where, P 0 (MPa) is a pressure of liquid, V (l/min.) a liquid flow rate of a nozzle, and H (cm) a distance between a nozzle and the surface of a steel sheet.
  • L (l/cm 2 ) V/(W x v), where, V (l/min.) is a liquid flow rate of a nozzle, W (cm) the width of liquid when the liquid blown from a nozzle hits a steel sheet surface, and v (cm/min.) a traveling speed of a steel sheet.
  • the product of the impact pressure P and the flow rate L it is preferable that the product is 0.02 or less because, when the liquid flow rate of a nozzle is raised, problems such as violent nozzle wear occur.
  • the maximum roughness height Ry of a steel sheet after finish rolling is 15 ⁇ m (15 ⁇ m Ry, l 2.5 mm, ln 12.5 mm) or less.
  • the fatigue strength of an as-hot-rolled or as-pickled steel sheet correlates with the maximum roughness height Ry of the steel sheet surface, as stated, for example, in page 84 of Metal Material Fatigue Design Handbook edited by the Society of Materials Science, Japan.
  • the subsequent finish hot rolling is done within 5 sec. after high-pressure descaling so that scales may be prevented from forming again.
  • finish rolling may be carried out continuously by welding sheet bars together after rough rolling or the subsequent descaling.
  • the rough-rolled sheet bars may be welded together after being coiled temporarily, held inside a cover having a heat retention function as occasion demands, and then uncoiled.
  • the finish rolling is done at a total reduction ratio of 25% or more in the temperature range of the Ar 3 transformation temperature + 100°C or lower during the latter half of the finish rolling.
  • the total reduction ratio in the temperature range of the Ar 3 transformation temperature + 100°C or lower is less than 25%, the rolled texture of austenite does not develop sufficiently and, as a result, the effects of the present invention are not obtained, no matter how the steel sheet is cooled thereafter.
  • the total reduction ratio in the temperature range of the Ar 3 transformation temperature + 100°C or lower is 35% or more.
  • the present invention does not specify a lower limit of the temperature range in which rolling at a total reduction ratio of 25% or more is carried out.
  • a work-induced structure remains in ferrite having precipitated during the rolling, and, as a result, ductility falls and workability deteriorates.
  • it is desirable that a lower limit of the temperature range in which rolling at a total reduction ratio of 25% or more is carried out is not lower than the Ar 3 transformation temperature.
  • a rolling temperature lower than the Ar 3 transformation temperature is acceptable.
  • the present invention does not specify an upper limit of the total reduction ratio in the temperature range of the Ar 3 transformation temperature + 100°C or lower.
  • a total reduction ratio exceeds 97.5%, the rolling load becomes too high and it becomes necessary to increase the rigidity of a rolling mill excessively, resulting in economical disadvantage.
  • the total reduction ratio is, desirably, 97.5% or less.
  • lubrication may be applied for reducing the friction between a hot-rolling roll and a steel sheet as occasion demands.
  • the present invention does not specify an upper limit of the friction coefficient between a hot-rolling roll and a steel sheet.
  • a friction coefficient exceeds 0.2, crystal orientations mainly composed of ⁇ 110 ⁇ planes develop conspicuously, deteriorating notch-fatigue strength.
  • a friction coefficient between a hot-rolling roll and a steel sheet is the value calculated from a forward slip ratio, a rolling load, a rolling torque and so on on the basis of the rolling theory.
  • the present invention does not specify a temperature at the final pass (FT) of finish rolling, but it is desirable that the final pass is completed at a temperature not lower than the Ar 3 transformation temperature. This is because, if a rolling temperature is lower than the Ar 3 transformation temperature during hot rolling, a work-induced structure remains in ferrite having precipitated before or during the rolling, and, as a result, ductility lowers and workability deteriorates. However, when a heat treatment for recovery or recrystallization is applied during or after the subsequent coiling process, a temperature at the final pass (FT) of finish rolling is allowed to be lower than the Ar 3 transformation temperature.
  • the present invention does not specify an upper limit of a finishing temperature, but, if a finishing temperature exceeds the Ar 3 transformation temperature + 100°C, it becomes practically impossible to carry out rolling at a total reduction ratio of 25% or more in the temperature range of the Ar 3 transformation temperature + 100°C or lower. For this reason, it is desirable that an upper limit of a finishing temperature is the Ar 3 transformation temperature + 100°C or lower.
  • the present invention it is not necessary to specify the microstructure of a steel sheet for only the purpose of enhancing the notch-fatigue strength thereof and, therefore, no specific limitation is set forth regarding the cooling process after the completion of finish rolling until the coiling at a prescribed coiling temperature. Nevertheless, a steel sheet is cooled, as occasion demands, for the purpose of securing a prescribed coiling temperature or controlling the microstructure.
  • the present invention does not specify an upper limit of a cooling rate, but, as thermal strain may cause a steel sheet to warp, it is desirable to control a cooling rate to 300°C/sec. or lower.
  • a cooling rate here is, desirably, 150°C/sec. or lower. No lower limit of a cooling rate is specifically set forth, either.
  • the cooling rate in the case where a steel sheet is left to cool by air without any intentional cooling is 5°C/sec. or higher.
  • the microstructure of a steel sheet is a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage.
  • the present invention does not specify the conditions of the process after the completion of finish rolling until the coiling at a prescribed coiling temperature, except for the cooling rate applied during the process.
  • a hot-rolled steel sheet may be retained for 1 to 20 sec.
  • the retention of a hot-rolled steel sheet is carried out for accelerating ferrite transformation in the two-phase zone.
  • a retention time is less than 1 sec.
  • ferrite transformation in the two-phase zone is insufficient and a sufficient ductility is not obtained.
  • pearlite forms and an intended microstructure having a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage is not obtained.
  • the temperature range in which a steel sheet is retained for 1 to 20 sec. is from the Ar 1 transformation temperature to 800°C.
  • the retention time which has been defined earlier as in the range from 1 to 20 sec., is 1 to 10 sec. For satisfying all those requirements, it is necessary to reach said temperature range rapidly at a cooling rate of 20°C/sec. or higher after completing finish rolling.
  • the present invention does not specify an upper limit of a cooling rate, but, in consideration of the capacity of cooling equipment, a reasonable cooling rate is 300°C/sec. or lower.
  • a cooling rate here is, desirably, 150°C/sec. or lower.
  • a steel sheet is cooled at a cooling rate of 20°C/sec. or higher from the above temperature range to a coiling temperature (CT).
  • CT coiling temperature
  • a cooling rate is lower than 20°C/sec.
  • pearlite or bainite containing carbides forms and an intended microstructure having a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage is not obtained.
  • the effects of the present invention can be enjoyed without specifying an upper limit of the cooling rate down to the coiling temperature but, to avoid warping caused by thermal strain, it is desirable to control a cooling rate to 300°C/sec. or lower.
  • the microstructure of a steel sheet is a compound structure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite.
  • a hot-rolled steel sheet has to be retained for 1 to 20 sec. in the temperature range from the Ar 3 transformation temperature to the Ar 1 transformation temperature (the ferrite-austenite two-phase zone) in the first process after completing finish rolling.
  • the retention of a hot-rolled steel sheet is carried out for accelerating ferrite transformation in the two-phase zone.
  • ferrite transformation in the two-phase zone is insufficient and a sufficient ductility is not obtained.
  • a retention time exceeds 20 sec. pearlite forms and an intended microstructure containing retained austenite by 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite is not obtained.
  • the temperature range in which a steel sheet is retained for 1 to 20 sec. is from the Ar 1 transformation temperature to 800°C.
  • the retention time which has been defined earlier as in the range from 1 to 20 sec., is 1 to 10 sec.
  • the present invention does not specify an upper limit of a cooling rate, but, in consideration of the capacity of cooling equipment, a reasonable cooling rate is 300°C/sec. or lower.
  • a cooling rate is, desirably, 150°C/sec. or lower.
  • a steel sheet is cooled at a cooling rate of 20°C/sec. or higher from the above temperature range to a coiling temperature (CT).
  • CT coiling temperature
  • a cooling rate is lower than 20°C/sec.
  • pearlite or bainite containing carbides forms and a sufficient amount of retained austenite is not secured and, as a result, an intended microstructure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite is not obtained.
  • the effects of the present invention can be enjoyed without bothering to specify an upper limit of the cooling rate down to the coiling temperature but, to avoid warping caused by thermal strain, it is desirable to control a cooling rate to 300°C/sec. or lower.
  • the microstructure of a steel sheet is a compound structure containing ferrite as the phase accounting for the largest volume percentage and mainly martensite as the second phase.
  • a hot-rolled steel sheet has to be retained for 1 to 20 sec. in the temperature range from the Ar 3 transformation temperature to the Ar 1 transformation temperature (the ferrite-austenite two-phase zone) in the first process after completing finish rolling.
  • the retention of a hot-rolled steel sheet is carried out for accelerating ferrite transformation in the two-phase zone.
  • ferrite transformation in the two-phase zone is insufficient and a sufficient ductility is not obtained.
  • a retention time exceeds 20 sec. pearlite forms and an intended compound structure containing ferrite as the phase accounting for the largest volume percentage and mainly martensite as the second phase is not obtained.
  • the temperature range in which a steel sheet is retained for 1 to 20 sec. is from the Ar 1 transformation temperature to 800°C.
  • the retention time which has been defined earlier as in the range from 1 to 20 sec., is 1 to 10 sec.
  • the present invention does not specify an upper limit of a cooling rate, but, in consideration of the capacity of cooling equipment, a reasonable cooling rate is 300°C/sec. or lower.
  • a cooling rate is, desirably, 150°C/sec. or lower.
  • a steel sheet is cooled at a cooling rate of 20°C/sec. or higher from the above temperature range to a coiling temperature (CT).
  • CT coiling temperature
  • the effects of the present invention can be enjoyed without specifying an upper limit of the cooling rate down to the coiling temperature but, to avoid distortion caused by thermal strain, it is desirable to control the cooling rate to 300°C/sec. or lower.
  • the present invention it is not necessary to specify the microstructure of a steel sheet only for the purpose of enhancing the notch-fatigue strength thereof and, therefore, the present invention does not specify an upper limit of a coiling temperature.
  • the present invention in order to carry over the texture of austenite obtained by finish rolling at a total reduction ratio of 25% or more in the temperature range of the Ar 3 transformation temperature + 100°C or lower, it is desirable to coil a steel sheet at the coiling temperature T 0 shown below or lower. Note that it is unnecessary to set the temperature T 0 to room temperature or lower.
  • T 0 is the temperature defined thermodynamically as that at which austenite and ferrite having the same chemical components as the austenite have the same free energy.
  • a coiling temperature is not lower than 50°C.
  • the microstructure of a steel sheet is a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage.
  • the coiling temperature has to be restricted to 450°C or higher. This is because, when a coiling temperature is lower than 450°C, retained austenite or martensite considered detrimental to burring workability may form in a great amount and, as a consequence, an intended microstructure having a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage is not obtained.
  • a cooling rate after coiling is 30°C/sec. or higher to a temperature of 200°C. Otherwise, when Cu is added by 1.2% or more, it precipitates after coiling and, as a result, not only workability is deteriorated but also solute Cu effective for improving fatigue properties may be lost.
  • the microstructure of a steel sheet is a compound structure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite.
  • the coiling temperature is restricted to lower than 450°C. This is because, when a coiling temperature is 450°C or higher, bainite containing carbides forms and a sufficient amount of retained austenite is not secured and, as a result, an intended microstructure containing retained austenite at 5 to 25% in terms of volume percentage, and having the balance mainly consisting of ferrite and bainite, is not obtained.
  • a coiling temperature is not higher than 350°C, on the other hand, a great amount of martensite forms and a sufficient amount of retained austenite is not secured and, as a result, an intended microstructure containing retained austenite by 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite is not obtained. For this reason, a coiling temperature is limited to higher than 350°C.
  • a cooling rate after coiling is 30°C/sec. or higher up to a temperature of 200°C. Otherwise, when Cu is added at 1% or more, it precipitates after coiling and, as a result, not only is the workability deteriorated but also solute Cu effective for improving fatigue properties may be lost.
  • the microstructure of a steel sheet is a compound structure containing ferrite as the phase accounting for the largest volume percentage and mainly martensite as the second phase.
  • a coiling temperature has to be restricted to 350°C or lower. This is because, when a coiling temperature exceeds 350°C, bainite forms and a sufficient amount of martensite is not secured and, as a result, an intended microstructure containing ferrite as the phase accounting for the largest volume percentage and martensite as the second phase is not obtained. It is not necessary to specify a lower limit of a coiling temperature but, to avoid a poor appearance caused by rust when a coil is kept wet with water for a long period of time, it is desirable that a coiling temperature is not lower than 50°C.
  • a steel sheet After completing a hot rolling process, as occasion demands, a steel sheet may be subjected to pickling and then skin pass rolling at a reduction ratio of 10% or less or cold rolling at a reduction ratio up to 40% or so, either on-line or off-line.
  • the present invention does not specify the conditions of finish hot rolling.
  • a total reduction ratio in the temperature range of the Ar 3 transformation temperature + 100°C or lower, is 25% or more.
  • the temperature at the final pass (FT) of finish rolling is allowed to be lower than the Ar 3 transformation temperature, in such a case, since an intensively work-induced structure remains in ferrite having precipitated before or during the rolling, it is desirable that the work-induced structure is recovered and recrystallized through the subsequent coiling process or a heat treatment.
  • a total reduction ratio at subsequent cold rolling after pickling must be less than 80%. This is because, when a total reduction ratio at cold rolling is 80% or more, the ratios of the integrated X-ray diffraction strengths in ⁇ 111 ⁇ and ⁇ 554 ⁇ crystallographic planes parallel to the plane of a steel sheet, the crystallographic planes having a texture usually obtained through cold rolling and recrystallization, tend to rise.
  • a preferable total reduction ratio at cold rolling is 70% or less.
  • a steel sheet is subjected to a heat treatment for 5 to 150 sec. in the temperature range of the Ac 3 transformation temperature + 100°C or lower.
  • an upper limit of a heat treatment temperature exceeds the Ac 3 transformation temperature + 100°C, ferrite having formed through recrystallization transforms into austenite, the texture formed by the growth of austenite grains is randomized, and the texture of ferrite finally obtained is also randomized.
  • an upper limit of a heat treatment temperature is set at the Ac 3 transformation temperature + 100°C or lower.
  • the Ac 1 and Ac 3 transformation temperatures mentioned herein can be expressed in relation to steel chemical components using, for example, the expressions according to p. 273 of the Japanese translation of The Physical Metallurgy of Steels by W. C. Leslie (published by Maruzen in 1985, translated by Hiroshi Kumai and Tatsuhiko Noda).
  • a lower limit of a heat treatment temperature it is acceptable if the temperature is equal to or higher than the recovery temperature, because it is not necessary to specify the microstructure of a steel sheet for the purpose of enhancing the notch-fatigue strength thereof.
  • a heat treatment temperature is lower than the recovery temperature, however, a work-induced structure is retained and formability is significantly deteriorated. For this reason, a lower limit of a heat treatment temperature is set to be equal to or higher than the recovery temperature.
  • a retention time in the above temperature range when a retention time is shorter than 5 sec., it is insufficient for having cementite completely dissolve again. However, when a retention time exceeds 150 sec., the effect of the heat treatment is saturated and, what is worse, productivity is lowered. For this reason, a retention time is determined to be in the range from 5 to 150 sec.
  • the present invention does not specify the conditions of cooling after a heat treatment. However, for the purpose of controlling the microstructure of a steel sheet, cooling or the combination of retention at an arbitrary temperature and cooling as explained later may be employed as deemed necessary.
  • the microstructure of a steel sheet is a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage.
  • a lower limit of a heat treatment temperature is set at a temperature of the Ac 1 transformation temperature or higher.
  • a heat treatment temperature When it is intended to obtain both a good burring workability and a high ductility without sacrificing the burring workability too much, a heat treatment temperature must be in the range from the Ac 1 transformation temperature to the Ac 3 transformation temperature (the ferrite-austenite two-phase zone) in order to increase the volume percentage of ferrite. Further, for the purpose of obtaining a still better burring workability, it is desirable that the heat treatment temperature is in the range from the Ac 3 transformation temperature to the Ac 3 transformation temperature + 100°C in order to increase the volume percentage of bainite.
  • the present invention does not specify the conditions of a cooling process in heat treatment.
  • a heat treatment temperature is in the range from the Ac 1 transformation temperature to the Ac 3 transformation temperature
  • a cooling end temperature is 350°C or lower, martensite, which is considered detrimental to burring properties, may form in a great amount and, as a result, an intended microstructure having a compound structure containing bainite or ferrite and bainite as the phase accounting for the largest volume percentage is not obtained. For this reason, it is desirable that a cooling end temperature is higher than 350°C. In addition, in order to carry over the texture obtained to the previous process, it is desirable that a cooling end temperature is not higher than T 0 .
  • the microstructure of a steel sheet is a compound structure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite.
  • a steel sheet must be subjected to a heat treatment for 5 to 150 sec. in the temperature range from the Ac 1 transformation temperature to the Ac 3 transformation temperature + 100°C, as described earlier. In this case, when a temperature is too low within the above temperature range and when cementite has precipitated in an as-hot-rolled state, it takes too long for the cementite to dissolve again.
  • bainite transformation for stabilizing retained austenite is insufficient and, as a consequence, unstable retained austenite may transform into martensite at the end of the subsequent cooling, and, as a result, an intended microstructure containing retained austenite at 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite is not obtained.
  • a retention time in the above temperature range must be from 5 to 600 sec.
  • a cooling rate up to the end of cooling is lower than 5°C/sec.
  • bainite transformation may overshoot during the cooling and a required amount of stable retained austenite is not formed, and, as a consequence, an intended microstructure containing retained austenite by 5 to 25% in terms of volume percentage and having the balance mainly consisting of ferrite and bainite may not be obtained.
  • a cooling rate is set at 5°C/sec. or higher.
  • a cooling end temperature when a cooling end temperature is higher than 200°C, aging properties may deteriorate and, for this reason, a cooling end temperature must be 200°C or lower.
  • the present invention does not specify a lower limit for a cooling end temperature. However, if water cooling or mist cooling is applied and a coil is kept wet with water for a long period of time, it is desirable, to avoid a poor appearance caused by rust, that a cooling end temperature is not lower than 50°C.
  • the microstructure of a steel sheet is a compound structure containing ferrite as the phase accounting for the largest volume percentage and mainly martensite as the second phase.
  • a steel sheet must be subjected to a heat treatment for 5 to 150 sec. in the temperature range from the Ac 1 transformation temperature to the Ac 3 transformation temperature + 100°C as described before. In this case, when the temperature is too low within the above temperature range and when cementite has precipitated in an as-hot-rolled state, it takes too long for the cementite to dissolve again.
  • a cooling rate after retention is lower than 20°C/sec.
  • the temperature history of steel is likely to pass through the transformation nose of bainite or pearlite containing much carbide, and, for this reason, a cooling rate must be 20°C/sec. or higher.
  • a cooling end temperature is higher than 350°C, an intended microstructure containing ferrite as the phase accounting for the largest volume percentage and martensite as the second phase is not obtained. For this reason, the cooling must be continued down to a temperature of 350°C or lower.
  • the present invention does not specify a lower limit of a cooling end temperature. However, if water cooling or mist cooling is applied and a coil is kept wet with water for a long period of time, it is desirable, to avoid a poor appearance caused by rust, that a cooling end temperature is not lower than 50°C.
  • skin pass rolling may be applied, if required.
  • the steel sheet When galvanizing is applied to a hot-rolled steel sheet after pickling or a cold-rolled steel sheet after completing the above annealing for recrystallization, the steel sheet is dipped in a zinc-plating bath. After that, it may be subjected to an alloying treatment, if required.
  • Example 1 The present invention is further explained hereafter based on Example 1.
  • Table 2 shows the details of the production conditions.
  • SRT means the slab reheating temperature
  • FT the finish rolling temperature at the final pass
  • reduction ratio the total reduction ratio in the temperature range of the Ar 3 transformation temperature + 100°C or lower. Note that, in the case where a hot-rolled steel sheet is cold rolled, it is not necessary to restrict the reduction ratio of hot rolling and, for this reason, the space of "reduction ratio” is filled with a dash meaning “not applicable.” Further, “lubrication” indicates if or not lubrication is applied in the temperature range of the Ar 3 transformation temperature + 100°C or lower.
  • ⁇ means that the coiling temperature (CT) is equal to or lower than T 0 , and ⁇ that the coiling temperature is higher than T 0 . Note that, in the case of a cold-rolled steel sheet, the space is filled with a dash meaning “not applicable,” because it is not necessary to restrict the coiling temperature as one of the production conditions.
  • the thickness of the cold-rolled steel sheets ranged from 0.7 to 2.3 mm.
  • “cold reduction ratio” means the total reduction ratio of the cold rolling, and "time” the time of annealing.
  • ⁇ means that the annealing temperature is within the range from the recovery temperature to the Ar 3 transformation temperature + 100°C, and ⁇ that it is outside the range.
  • Steel L was subjected to descaling under the conditions of an impact pressure of 2.7 MPa and a flow rate of 0.001 l/cm 2 after the rough rolling. Further, among the steels mentioned above, steels G and F-5 were subjected to zinc plating.
  • the hot-rolled steel sheets thus prepared were subjected to a tensile test in accordance with the test method specified in JIS Z 2241, after forming the specimens into No. 5 test pieces according to JIS Z 2201.
  • the yield strength ( ⁇ Y), tensile strength ( ⁇ B) and breaking elongation (El) of the steel sheets are shown also in Table 2.
  • test piece 30 mm in diameter was cut out from a position of 1/4 or 3/4 of the width of each of the steel sheets, the surfaces were ground to a depth of about 0.05 mm so that the surfaces might have the three-triangle grade finish (the second finest finish) and, subsequently, strain was removed by chemical polishing or electrolytic polishing.
  • the test pieces thus prepared were subjected to X-ray diffraction strength measurement in accordance with the method described in pages 274 to 296 of the Japanese translation of Elements of X-ray Diffraction by B. D. Cullity (published in 1986 by AGNE Gijutsu Center, translated by Gentaro Matsumura).
  • the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is obtained from the X-ray diffraction strengths in the principal orientation components included in the orientation component group, namely ⁇ 100 ⁇ 011>, ⁇ 116 ⁇ 110>, ⁇ 114 ⁇ 110>, ⁇ 113 ⁇ 110>, ⁇ 112 ⁇ 110>, ⁇ 335 ⁇ 110> and ⁇ 223 ⁇ 110>, in the three-dimensional texture calculated either by the vector method based on the pole figure of ⁇ 110 ⁇ or by the series expansion method using two or more (desirably, three or more) pole figures out of the pole figures of ⁇ 110 ⁇ , ⁇ 100 ⁇ , ⁇ 211 ⁇ and ⁇ 310 ⁇ .
  • the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is the arithmetic average of the ratios in all the above orientation components.
  • the arithmetic average of the strengths in the orientation components of ⁇ 100 ⁇ 011>, ⁇ 116 ⁇ 110>, ⁇ 114 ⁇ 110>, ⁇ 112 ⁇ 110> and ⁇ 223 ⁇ 110> may be used as a substitute.
  • the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength can be obtained from the three-dimensional texture calculated in the same manner as explained above.
  • strength ratio 1 under “ratios of X-ray diffraction strength to random X-ray diffraction strength” means the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength
  • strength ratio 2 the average of the ratios of the X-ray diffraction strength in the above three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength.
  • a test piece for fatigue test having the shape shown in Fig. 1(b) was cut out from a position of 1/4 or 3/4 of the width of each of the steel sheets so that the longitudinal direction of the test piece coincided with the rolling direction of the steel sheet, and subjected to a fatigue test.
  • the surfaces of the test pieces for fatigue test were ground to a depth of about 0.05 mm so that the surfaces might have the second finest finish, and the fatigue test was carried out using an electro-hydraulic servo type fatigue tester and methods conforming to JIS Z 2273-1978 and Z 2275-1978.
  • the notch-fatigue limit ( ⁇ WK) and notch-fatigue limit ratio ( ⁇ WK/ ⁇ B) of each of the steel sheets are shown also in Table 2.
  • the samples according to the present invention are 11 steels, namely steels A, E, F-1, F-2, F-5, G, H, I, J, K and L.
  • the steel sheet contains prescribed amounts of chemical components; on a plane at an arbitrary depth within 0.5 mm from the surface of the steel sheet in the thickness direction thereof, the average of the ratios of the X-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength is 4 or less; and the thickness of the steel sheet is in the range from 0.5 to 12 mm.
  • Example 2 The present invention is hereafter explained in more detail based on Example 2.
  • Table 3 shows the details of the production conditions.
  • SRT means the slab reheating temperature
  • FT the finish rolling temperature at the final pass
  • reduction ratio the total reduction ratio in the temperature range of the Ar 3 transformation temperature + 100°C or lower. Note that, in the case where a hot-rolled steel sheet is cold rolled, it is not necessary to restrict the reduction ratio of hot rolling and, for this reason, the space “reduction ratio” is filled with a dash meaning “not applicable.”
  • “lubrication” indicates if or not lubrication is applied in the temperature range of the Ar 3 transformation temperature + 100°C or lower.
  • CT indicates the coiling temperature.
  • cold-rolled steel sheet the space is filled with a dash meaning "not applicable,” because it is not necessary to restrict the coiling temperature as one of the production conditions.
  • Some of the steel sheets were subjected to pickling, cold rolling and heat treatment after the hot rolling.
  • the thickness of the cold-rolled steel sheets ranged from 0.7 to 2.3 mm.
  • cold reduction ratio means the total reduction ratio of the cold rolling, "ST” the temperature of the heat treatment and “time” the time thereof.
  • the hot-rolled and cold-rolled steel sheets thus prepared were subjected to a tensile test in the same manner as described earlier.
  • Table 4 shows the microstructures of the steel sheets, too.
  • “others” accounts for pearlite and any other phase than ferrite, bainite, retained austenite and martensite, which are listed individually in Table 4.
  • the volume percentage of ferrite, bainite, retained austenite, pearlite or martensite is defined as the area percentage thereof in the microstructure of each of the steel sheets observed with an optical microscope under a magnification of 200 to 500 at a position in the depth of 1/4 of the steel sheet thickness on a section surface along the rolling direction of a specimen which is cut out from a position of 1/4 or 3/4 of the width of the steel sheet, the section surface being polished and etched with a nitral reagent and the reagent disclosed in Japanese Unexamined Patent Publication No. H5-163590.
  • the measurement result of the volume percentage of retained austenite was substantially the same either by the optical microscope observation or the X-ray diffraction method, and, thus,
  • the X-ray diffraction strength was measured by the same method as described earlier.
  • notch-fatigue limit ( ⁇ WK) and notch-fatigue limit ratio ( ⁇ WK/ ⁇ B) of the steel sheets are shown also in Table 4.
  • the samples according to the present invention are 9 steels, namely steels g-1, g-2, g-3, g-5, g-6, g-7, h-1, h-2 and h-3.
  • each of the steel sheets being characterized in that: the steel sheet contains prescribed amounts of chemical components; on a plane at an arbitrary depth within 0.5 mm from the surface of the steel sheet in the thickness direction thereof, the average of the ratios of the x-ray diffraction strength in the orientation component group of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> to random X-ray diffraction strength is 2 or more and the average of the ratios of the X-ray diffraction strength in the three orientation components of ⁇ 554 ⁇ 225>, ⁇ 111 ⁇ 112> and ⁇ 111 ⁇ 110> to random X-ray diffraction strength is 4 or less; the thickness of the steel sheet is in the range from 0.5 to 12 mm
  • the present invention relates to a thin steel sheet, for automobile use, excellent in notch-fatigue strength, and a method for producing the steel sheet.
  • the use of a thin steel sheet according to the present invention makes it possible to expect a significant improvement in notch-fatigue strength that is one of the essential properties of such a structural member including an undercarriage component of an automobile to overcome the problem of generating the propagation of a fatigue crack from a site of stress concentration including a blanked or welded portion and thus to require durability. For this reason, the present invention is of a high industrial value.

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EP02700640A 2001-02-23 2002-02-20 Dünnes stahlblech für autos mit hervorragender kerbdauerfestigkeit und verfahren zu seiner herstellung Withdrawn EP1362930A4 (de)

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JP2001049012 2001-02-23
JP2001049012 2001-02-23
JP2001247306 2001-08-16
JP2001247306A JP3927384B2 (ja) 2001-02-23 2001-08-16 切り欠き疲労強度に優れる自動車用薄鋼板およびその製造方法
PCT/JP2002/001498 WO2002066697A1 (fr) 2001-02-23 2002-02-20 Feuille mince d'acier a resistance de fatigue d'entaille excellente, destinee a une automobile, et procede de production

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KR20030077018A (ko) 2003-09-29
US20040069382A1 (en) 2004-04-15
EP1362930A4 (de) 2004-11-24
CA2438393A1 (en) 2002-08-29
CN1221680C (zh) 2005-10-05
JP3927384B2 (ja) 2007-06-06
CN1492938A (zh) 2004-04-28
KR100572762B1 (ko) 2006-04-24
WO2002066697A1 (fr) 2002-08-29
JP2002322533A (ja) 2002-11-08

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