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US20180100213A1 - Hot-rolled steel sheet and method for producing the same - Google Patents

Hot-rolled steel sheet and method for producing the same Download PDF

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Publication number
US20180100213A1
US20180100213A1 US15/566,246 US201615566246A US2018100213A1 US 20180100213 A1 US20180100213 A1 US 20180100213A1 US 201615566246 A US201615566246 A US 201615566246A US 2018100213 A1 US2018100213 A1 US 2018100213A1
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Prior art keywords
steel sheet
hot
rolled steel
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content
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US15/566,246
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English (en)
Inventor
Yasuaki Tanaka
Mutsumi SAKAKIBARA
Takafumi Yokoyama
Hiroyuki Kawata
Natsuko Sugiura
Yasumitsu Kondo
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Publication date
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Assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION reassignment NIPPON STEEL & SUMITOMO METAL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KAWATA, HIROYUKI, KONDO, YASUMITSU, SAKAKIBARA, Mutsumi, SUGIURA, NATSUKO, TANAKA, YASUAKI, YOKOYAMA, TAKAFUMI
Publication of US20180100213A1 publication Critical patent/US20180100213A1/en
Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION CHANGE OF NAME Assignors: NIPPON STEEL & SUMITOMO METAL CORPORATION
Abandoned legal-status Critical Current

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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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/001—Heat treatment of ferrous alloys containing Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • 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
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
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    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
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    • C—CHEMISTRY; METALLURGY
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B1/24—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process
    • B21B1/26—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
    • B21B2001/225—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
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    • C21—METALLURGY OF IRON
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a hot-rolled steel sheet and a method for producing the hot-rolled steel sheet.
  • a high strength steel sheet contains a large amount of alloying elements to increase strength.
  • a high strength steel sheet having a tensile strength of 980 MPa or more contains a large amount of Si and Mn.
  • a high strength steel sheet is usually produced by the following method. First, a slab is hot rolled to produce a hot-rolled steel sheet, which is then coiled in a coil shape. Next, the hot-rolled steel sheet is pickled, cold-rolled, and annealed.
  • the temperature of the hot-rolled steel sheet when being coiled into a coil shape may be raised to enhance the cold workability of the hot-rolled steel sheet.
  • coiling temperature If the coiling temperature is high, an internal oxidized layer is formed in the vicinity of an outer layer of the hot-rolled steel sheet.
  • the internal oxidized layer is formed with a thickness of several tens of ⁇ m from the surface of the base metal of the hot-rolled steel sheet toward the center of the plate thickness.
  • the internal oxidized layer reduces the surface properties, formability and weldability of the steel sheet after cold rolling (cold-rolled steel sheet). Therefore, the internal oxidized layer is removed before cold rolling, by subjecting the hot-rolled steel sheet to a pickling treatment.
  • an oxide film (scale) is formed on the surface of the hot-rolled steel sheet.
  • the scale reduces the surface properties, formability and weldability of the steel sheet. Therefore, similarly to the internal oxidized layer, the scale is also removed by subjecting the hot-rolled steel sheet to a pickling treatment.
  • the internal oxidized layer or the scale is thick, an excessive workload is applied in the pickling treatment for the hot-rolled steel sheet.
  • the internal oxidized layer or the scale remains, as described above, the surface properties, formability and weldability of the cold-rolled steel sheet are reduced.
  • the internal oxidized layer or the scale peel off during forming of the cold-rolled steel sheet, and cause surface defects such as indentation defects.
  • the internal oxidized layer is formed as a result of an alloying element in the base metal being selectively oxidizing. Si and Mn are easily oxidized. Therefore, an internal oxidized layer is liable to arise in a hot-rolled steel sheet in which the content of Si and Mn is high. Scale is similarly liable to become thick on a hot-rolled steel sheet having a high Si and Mn content.
  • the thicknesses of the internal oxidized layer and scale increase. If the coiling temperature is raised in order to enhance the cold workability of the hot-rolled steel sheet as described above, an internal oxidized layer is more liable to arise, and is liable to be thick. This situation similarly applies with respect to scale.
  • Patent Literature 1 JP62-13520A
  • Patent Literature 2 JP2010-535946A
  • Patent Literature 3 JP2013-253301A
  • Patent Literature 4 JP2011-184741A
  • Patent Literature 4 JP2011-231391A
  • Patent Literature 5 JP2012-036483A
  • Patent Literature 6 JP2013-216961A
  • Patent Literature 8 JP2013-103235A
  • Patent Literature 9 JP2011-523441A
  • Patent Literature 10 JP2015-113505A
  • Patent Literature 12 JP2004-332099A
  • Patent Literature 13 JP2013-060657A
  • Patent Literature 14 JP2011-523443A
  • Patent Literature 1 an antioxidant agent is coated onto a steel sheet surface. It is described in Patent Literature 1 that by this means the formation of an internal oxidized layer and scale is suppressed.
  • Patent Literature 2 a hot-rolled steel sheet is coiled at a comparatively low temperature of 530 to 580° C. It is described in Patent Literature 2 that by this means the formation of an oxidized layer is suppressed.
  • Patent Literature 3 a hot-rolled steel sheet after rolling is coiled at a temperature between 750° C. and 600° C. to form a coil. After coiling, the coil is maintained for 10 to 30 minutes, and thereafter the hot-rolled steel sheet is cooled while dispensing the coil. Subsequently, when the temperature of the hot-rolled steel sheet reaches 550° C. or less, the hot-rolled steel sheet is coiled again to form a coil. It is described in Patent Literature 3 that in this case the oxidized layer can be thin.
  • Patent Literatures 4 to 6 a heat treatment or a cooling treatment is performed on a steel sheet after hot rolling or after coiling in an atmosphere in which the oxygen concentration is reduced. It is described in the aforementioned Patent Literatures 4 to 6 that the heat treatment or cooling treatment in the atmosphere in which the oxygen concentration is reduced is effective to reduce scale and an internal oxidized layer.
  • Patent Literature 7 descaling is performed prior to coiling on a hot-rolled steel sheet after hot rolling, to thereby remove oxide scale from the surface thereof.
  • an oxygen supply source that is utilized for formation of an internal oxidized layer during coil cooling decreases. It is described in Patent Literature 7 that, consequently, not only the scale but also the internal oxidized layer decreases.
  • Patent Literature 8 a cooling method is proposed for uniformizing an internal oxidation amount of a hot-rolled steel sheet within a preferable range across the width direction, in the longitudinal direction thereof.
  • Patent Literatures 9 to 14 technology that is different from the technology disclosed in the above described Patent Literatures is proposed.
  • internal oxidation is suppressed by appropriately controlling the alloy elements of a steel and the conditions for heat treatment of a hot-rolled steel sheet.
  • Sb of a content of 0.001 to 0.1% is contained in the steel, the steel sheet is reheated to 1100 to 1250° C. and subjected to hot rolling, and coiled at a temperature of 450 to 750° C. Thereafter, the hot-rolled steel sheet is subjected to pickling and cold rolling, and is annealed at 700 to 850° C.
  • formation of an internal oxidized layer is suppressed.
  • Patent Literature 10 technology is proposed to appropriately control alloy elements to suppress formation of an oxide, and thereby improve a plating property.
  • a steel slab is used that contains 0.005 to 0.1% of Sb, and in which the relation between the contents of Ni, Mn, Al and Ti is adjusted.
  • the steel slab is subjected to hot working, and hot rolled and coiled at 500 to 700° C.
  • the steel slab is subjected to pickling, cold rolling and annealing. By this means, internal oxidation is suppressed.
  • a slab containing 0.02 to 0.10% of Sb is subjected to hot rolling, pickling, cold rolling, annealing and cooling.
  • the finish rolling temperature for the hot rolling is 800 to 1000° C.
  • the draft during the cold rolling is set to 20% or more.
  • annealing is performed under conditions of being held for 60 seconds or more in a temperature range of 750 to 900° C. in an atmosphere in which a dew-point temperature is ⁇ 35° C. or less.
  • cooling is performed to 300° C. or less at an average cooling rate of 30° C./sec or more, followed by tempering. By this means internal oxidation is suppressed.
  • Patent Literatures 12 to 14 describe suppressing scale by appropriately adjusting a content of Si, a heating temperature of a slab, a temperature of finish rolling and a coiling temperature and the like.
  • Patent Literatures 1 to 14 even when the respective technologies described in Patent Literatures 1 to 14 are implemented, in some cases a deep internal oxidized layer is formed or thick scale is formed.
  • Patent Literature 1 Japanese Patent Application Publication No. 62-13520
  • Patent Literature 2 National Publication of International Patent Application No. 2010-535946
  • Patent Literature 3 Japanese Patent Application Publication No. 2013-253301
  • Patent Literature 4 Japanese Patent Application Publication No. 2011-184741
  • Patent Literature 5 Japanese Patent Application Publication No. 2011-231391
  • Patent Literature 6 Japanese Patent Application Publication No. 2012-036483
  • Patent Literature 7 Japanese Patent Application Publication No. 2013-216961
  • Patent Literature 8 Japanese Patent Application Publication No. 2013-103235
  • Patent Literature 9 National Publication of International Patent Application No. 2010-503769
  • Patent Literature 10 National Publication of International Patent Application No. 2011-523441
  • Patent Literature 11 Japanese Patent Application Publication No. 2015-113505
  • Patent Literature 12 Japanese Patent Application Publication No. 2004-332099
  • Patent Literature 13 Japanese Patent Application Publication No. 2013-060657
  • Patent Literature 14 National Publication of International Patent Application No. 2011-523443
  • An objective of the present invention is to provide a hot-rolled steel sheet in which formation of an internal oxidized layer and scale is suppressed.
  • a hot-rolled steel sheet has a chemical composition consisting of, in mass %, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Mg:
  • FIG. 1 is a list view showing SEM images of cross-sections subjected to vital etching, oxygen mapping images obtained using EPMA at the SEM image regions, and Sb mapping images with respect to a steel with a high Si and high Mn content that does not contain Sb (steel not containing Sb), and a steel containing Sb that is a steel with a high Si and high Mn content that contains 0.1% of Sb.
  • FIG. 2 is a view illustrating the relation between an Sb content ( ⁇ 10 ⁇ 3 %) and the thickness ( ⁇ m) of an internal oxidized layer for a case where the amount of Sb contained in a steel with a high Si and high Mn content was varied and hot-rolled steel sheets were produced.
  • FIG. 3 shows SEM images of the vicinity of an outer layer in the above described steel not containing Sb and steel containing Sb.
  • the present inventors conducted investigations and studies regarding an internal oxidized layer and scale with respect to a steel with a high Si and high Mn content, and obtained the following findings.
  • an internal oxidized layer is formed within the base metal (ground metal), and scale is formed adjoining the surface.
  • the internal oxidized layer and scale are formed by the following mechanism. Oxygen ions penetrate into the hot-rolled steel sheet through grain boundaries of a near-surface portion of the hot-rolled steel sheet and the surface of the hot-rolled steel sheet. An internal oxidized layer is formed as a result of the oxygen ions that penetrated into the inside of the hot-rolled steel sheet oxidizing iron of the base metal. On the other hand, iron ions in the base metal move to the surface of the hot-rolled steel sheet through the grain boundaries. Scale is formed as a result of the Fe that moved to the surface being oxidized.
  • segregation elements are contained in a hot-rolled steel sheet, the segregation elements segregate at the surface and grain boundaries of the hot-rolled steel sheet and suppress movement of oxygen ions and iron ions. Therefore, penetration of oxygen ions into the inside of the hot-rolled steel sheet can be suppressed. In addition, movement of iron ions to the hot-rolled steel sheet surface can be suppressed. As a result, formation of an internal oxidized layer and scale can be suppressed.
  • the segregation elements are, for example, P, B, and Sb.
  • P and B segregate at grain boundaries and block the movement path of oxygen ions and iron ions, they also reduce the mechanical properties of the hot-rolled steel sheet.
  • the present inventors produced a hot-rolled steel sheet from steel with a high Si and high Mn content, which also contained Sb, and examined the thickness of scale and an internal oxidized layer.
  • FIG. 1 shows, with respect to a conventional steel with a high Si and high Mn content that does not contain Sb (hereunder, referred to as “steel not containing Sb”) and a steel containing Sb that is a conventional with a high Si and high Mn content, which also contains 0.10% of Sb, SEM images at a cross-section in the vicinity of the surface, oxygen mapping images obtained using EPMA at the SEM image regions, and Sb mapping images.
  • the steel not containing Sb contained, in mass %, C: 0.185%, Si: 1.8%, Mn: 2.6%, P: 0.01%, S: 0.002%, Al: less than 0.03%, N: 0.003%, O: 0.0009%, and Ti: 0.005%, with the balance being Fe and impurities.
  • the steel containing Sb was steel for which 0.10% of Sb was added to the chemical composition of the steel not containing Sb.
  • a hot-rolled steel sheet was formed by hot rolling in a similar manner to the conventional method. The aforementioned microstructure observation and EPMA mapping was performed with respect to the prepared hot-rolled steel sheets.
  • Sb mapping was performed using EPMA.
  • a layer 30 containing Sb (white region in the drawing; hereunder referred to as “Sb concentrated layer”) was observed at the interface between the scale 10 and the base metal.
  • an Sb concentrated layer is formed. It is considered that, because of the formation of the Sb concentrated layer, the situation is as follows. In a case where steel with a high Si and high Mn content contains a suitable amount of Sb, in a hot rolling process, an Sb concentrated layer is formed at the interface (surface of the hot-rolled steel sheet) between scale and the base metal. The Sb concentrated layer blocks the penetration of oxygen ions into the base metal. Consequently, iron in the base metal is not oxidized, and it is difficult for an internal oxidized layer to form. The Sb concentrated layer also suppresses movement of iron ions contained in the base metal to the scale. Consequently, growth of the scale is suppressed, and the thickness of the scale is thin.
  • the Sb concentrated layer functions as a so-called “barrier layer” that blocks the movement of oxygen ions and iron ions. Therefore, by formation of the Sb concentrated layer, the penetration of oxygen ions into the base metal from the scale after coiling of the hot-rolled steel sheet can be suppressed. In addition, movement of iron ions to the scale from the base metal can be suppressed. Therefore, the formation of an internal oxidized layer and scale is suppressed.
  • a barrier layer like the Sb concentrated layer is not formed even if P and B that are elements segregable at grain boundaries are contained in steel with a high Si and high Mn content. Accordingly, Sb is suitable for suppressing scale and an internal oxidized layer.
  • FIG. 2 is a view illustrating the relation between the Sb content ( ⁇ 10 ⁇ 3 %) and the thickness ( ⁇ m) of an internal oxidized layer in a case where the Sb amount contained in a steel with a high Si and high Mn content is varied and hot-rolled steel sheets are produced (coiling temperature of 750° C.).
  • the thickness of the internal oxidized layer decreases noticeably as the Sb content increases.
  • the margin of the decrease is not as great as when the Sb content is less than 0.03%. That is, in the relation between the thickness of the internal oxidized layer and the Sb content, an inflection point exists in the vicinity of an Sb content equal to 0.03%.
  • the Sb concentrated layer also suppresses movement of carbon contained in the base metal, and not just movement of oxygen ions and iron ions. Consequently, it is easy to maintain a uniform micro-structure in the plate thickness direction, and the obtainment of strength in a cold-rolled steel sheet after cold rolling and annealing is facilitated.
  • FIG. 3 shows SEM images of the vicinity of an outer layer in the above described steel not containing Sb and steel containing Sb.
  • a decarburization layer 40 is formed in the outer layer.
  • the Sb concentrated layer can also suppress the movement of carbon in the base metal, and not just suppress the movement of oxygen ions and iron ions.
  • a hot-rolled steel sheet according to the present embodiment that was completed based on the above described findings has a chemical composition consisting of, in mass %, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, Al: 0.01 to less than 1.0%, N: 0.01% or less, O: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0.5
  • the above described chemical composition may also contain one or more types of element selected from the group consisting of Ti: 0.005 to 0.15%, V: 0.001 to 0.30% and Nb: 0.005 to 0.15%.
  • the above described chemical composition may also contain one or more types of element selected from the group consisting of Cr: 0.10 to 1.0%, Ni: 0.10 to 1.0%, Mo: 0.01 to 1.0%, W: 0.01 to 1.0% and B: 0.0001 to 0.010%.
  • the above described chemical composition may also contain Cu: 0.10 to 0.50%.
  • the above described chemical composition may also contain one or more types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As in a total amount of 0.0001 to 0.30%.
  • the above described chemical composition may also contain one or more types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal in a total amount of 0.0001 to 0.50%.
  • a hot-rolled steel sheet according to the present embodiment includes an Sb concentrated layer having a thickness of 0.5 ⁇ m or more between the surface and scale.
  • a total area fraction of ferrite and pearlite may be 75% or more, and the tensile strength of the hot-rolled steel sheet may be 800 MPa or less.
  • a total area fraction of bainite and martensite may be 75% or more, and the tensile strength of the hot-rolled steel sheet may be 900 MPa or more.
  • a total area fraction of bainite and martensite may be 75% or more, and the tensile strength of the hot-rolled steel sheet may be 800 MPa or less.
  • the thickness of an internal oxidized layer of the hot-rolled steel sheet is 5 ⁇ m or less.
  • a scale thickness is 10 ⁇ m or less.
  • a decarburization layer thickness in an outer layer of the hot-rolled steel sheet is 20 ⁇ m or less.
  • a scale thickness is 7 ⁇ m or less.
  • a scale thickness is 7 ⁇ m or less.
  • a production method for producing the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of ferrite and pearlite is 75% or more and which has a tensile strength of 800 MPa or less includes: a process of preparing a steel material having the above described chemical composition; a process of heating the steel material to 1100 to 1350° C. and thereafter performing hot rolling so as to form the steel material into a steel sheet; and a process of coiling the steel sheet at a temperature of 600 to 750° C., preferably 650 to 750° C., and more preferably 700 to 750° C.
  • a production method for producing the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of bainite and martensite is 75% or more and which has a tensile strength of 900 MPa or more includes: a preparation process of preparing a steel material having the above described chemical composition; a hot rolling process of heating the steel material to 1100 to 1350° C., thereafter performing hot rolling so as to form the steel material into a steel sheet, and cooling the steel sheet to a coiling temperature; and a process of coiling the steel sheet after cooling, at a temperature of 150 to 600° C., preferably 350 to 500° C., and more preferably 400 to 500° C.
  • a production method for producing the above described hot-rolled steel sheet having a micro-structure in which a total area fraction of bainite and martensite is 75% or more and which has a tensile strength of 800 MPa or less includes: a preparation process of preparing a steel material having the above described chemical composition; a hot rolling process of heating the steel material to 1100 to 1350° C., thereafter performing hot rolling so as to form the steel material into a steel sheet, and cooling the steel sheet to a coiling temperature; a process of coiling the steel sheet after cooling, at a temperature of 150 to 600° C., preferably 350 to 500° C., and more preferably 400 to 500° C.; and a process of tempering the steel sheet after coiling at a temperature of 550° C. or more.
  • the chemical composition of the hot-rolled steel sheet according to the present embodiment contains the following elements.
  • the symbol “%” with respect to the chemical composition means “percent by mass” unless specified otherwise.
  • Carbon (C) forms retained austenite in the hot-rolled steel sheet and enhances the strength and formability of the steel. If the C content is too low, the aforementioned effect will not be obtained. On the other hand, if the C content is too high, the strength of the hot-rolled steel sheet will be too high and a cold rolling property will be reduced. If the C content is too high, the weldability of the steel will also decrease. Therefore, the C content is from 0.07 to 0.30%.
  • a preferable lower limit of the C content is 0.10%, more preferably is 0.12%, and further preferably is 0.15%.
  • a preferable upper limit of the C content is 0.25%, and more preferably is 0.22%.
  • Silicon (Si) suppresses the formation of iron-based carbides and facilitates formation of retained austenite. The strength and formability of the steel is improved by formation of retained austenite. If the Si content is too low, the aforementioned effect will not be obtained. On the other hand, if the Si content is too high, an internal oxidized layer will grow noticeably and the surface properties of the hot-rolled steel sheet will decrease. If the Si content is too high, the hot-rolled steel sheet will also become brittle and the ductility will decrease. Therefore, the Si content is from more than 1.0 to 2.8%. A preferable lower limit of the Si content is 1.3%, and more preferably is 1.5%. A preferable upper limit of the Si content is 2.5%, and more preferably is 2.0%.
  • Manganese (Mn) increases the strength of the steel sheet. If the Mn content is too low, a large amount of soft micro-structure is formed during cooling after annealing, and the strength is lowered. On the other hand, if the Mn content is too high, coarse Mn concentrated parts form at a central portion of the plate thickness and the steel becomes brittle. Consequently, a slab that is cast is liable to crack. If the Mn content is too high, the weldability of the steel also decreases. If the Mn content is too high, the hot-rolled steel sheet will also harden and a cold rolling property will decrease. Therefore, the Mn content is from 2.0 to 3.5%. A preferable lower limit of the Mn content is 2.2%, more preferably is 2.3%, and further preferably is 2.5%. A preferable upper limit of the Mn content is 3.2%, and more preferably is 3.0%.
  • Phosphorus (P) segregates at a central portion of the plate thickness of the steel sheet and embrittles a weld zone. Therefore, the P content is 0.030% or less. A low P content is preferable. However, making the P content low increases the production costs. Therefore, when taking the production cost into consideration, the lower limit of the P content is, for example, 0.0010%.
  • S Sulfur reduces the weldability of the steel. S also reduces the producibility during casting and heat rolling. S also combines with Mn to form MnS, and reduces the ductility and stretch flangeability of the steel. Therefore, the content of S is 0.010% or less.
  • a preferable upper limit of the S content is 0.005%, and more preferably is 0.0025%.
  • a lower limit of the S content is not particularly limited. However, when taking the production cost into consideration, the lower limit of the S content is, for example, 0.0001%.
  • Aluminum (Al) suppresses formation of iron-based carbides and facilitates formation of retained austenite.
  • the strength and formability of the steel is enhanced by the formation of retained austenite.
  • Al also deoxidizes the steel. If the Al content is too low, the aforementioned effects are not obtained. On the other hand, if the Al content is too high, the weldability of the steel decreases. Therefore, the Al content is from 0.01 to less than 1.0%.
  • a preferable lower limit of the Al content is 0.02%.
  • a preferable upper limit of the Al content is 0.8%, and more preferably is 0.5%.
  • the “Al” content means the content of “sol. Al” (acid-soluble Al).
  • N Nitrogen
  • the N content is 0.01% or less.
  • a preferable upper limit of the N content is 0.005%.
  • the lower limit of the N content is not particularly limited. However, when taking the production cost into consideration, the lower limit of the N content is, for example, 0.0001%.
  • Oxygen (O) forms oxides and reduces the toughness and stretch flangeability of the steel. Therefore, a low O content is preferable.
  • the O content is 0.01% or less.
  • a preferable upper limit of the O content is 0.008%, and more preferably is 0.006%.
  • the lower limit of the O content is not particularly limited. However, when taking the production cost into consideration, a preferable lower limit of the O content is, for example, 0.0001%.
  • Antimony is, as described above, an element that easily segregates at the surface of the steel.
  • Sb forms an Sb concentrated layer in the surface (interface between scale and base metal) of the hot-rolled steel sheet during hot rolling.
  • the Sb concentrated layer suppresses penetration of oxygen ions into the inside of the hot-rolled steel sheet from grain boundaries that are exposed on the surface of the hot-rolled steel sheet.
  • the Sb concentrated layer also suppresses movement of iron ions contained in the base metal to scale. Therefore, formation of an internal oxidized layer in the hot-rolled steel sheet and the growth of scale are suppressed.
  • Sb also restricts movement of C to suppress formation of a decarburization layer.
  • the Sb content is from 0.03 to 0.30%.
  • a preferable lower limit of the Sb content is 0.05%, more preferably is 0.07%, further preferably is 0.10%, and further preferably is 0.11%.
  • a preferable upper limit of the Sb content is 0.25%, and more preferably is 0.20%.
  • the lower limit of the total content of Si and Mn is 120%. In this case, the strength and ductility of the steel sheet will be high even after cold rolling and annealing.
  • the lower limit of the total content of Si and Mn is preferably 3.50%.
  • an upper limit of the total content of Si and Mn is preferably 5.0%, and more preferably is 4.5%.
  • the balance of the chemical composition of the hot-rolled steel sheet of the present embodiment is Fe and impurities.
  • impurities refers to elements that, during industrial production of the hot-rolled steel sheet, are mixed in from ore or scrap used as a raw material, or from the production environment or the like, and that are allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
  • the chemical composition of the hot-rolled steel sheet that is described above may contain optional elements that are described hereunder in addition to the above described essential elements.
  • the chemical composition need not contain optional elements.
  • the above described chemical composition may contain one or more types of element selected from the group consisting of Ti, V and Nb as a substitute for a part of Fe.
  • element selected from the group consisting of Ti, V and Nb as a substitute for a part of Fe.
  • Ti, V and Nb is an optional element, and each element increases the strength of the steel.
  • Titanium (Ti) is an optional element, and need not be contained in the steel. In a case where Ti is contained, Ti forms carbo-nitrides and increases the strength of the steel. Ti also contributes to fine-grain strengthening of the steel by suppressing growth of ferrite grains. Ti also contributes to dislocation strengthening of the steel through suppression of recrystallization. However, if the Ti content is too high, carbo-nitrides are excessively formed and the formability of the steel decreases. Therefore, the Ti content is 0 to 0.15%. A preferable upper limit of the Ti content is 0.10%, and more preferably is 0.07%. A preferable lower limit of the Ti content is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.
  • Vanadium (V) is an optional element, and need not be contained in the steel.
  • V is contained, similarly to Ti, V increases the strength of the steel by precipitation strengthening, fine-grain strengthening and dislocation strengthening.
  • the V content is from 0 to 0.30%.
  • a preferable upper limit of the V content is 0.20%, and more preferably is 0.15%.
  • a preferable lower limit of the V content is 0.001%, and more preferably is 0.005%.
  • Niobium (Nb) is an optional element, and need not be contained in the steel.
  • Nb is contained, similarly to Ti and V, Nb increases the strength of the steel by precipitation strengthening, fine-grain strengthening and dislocation strengthening.
  • the Nb content is from 0 to 0.15%.
  • a preferable upper limit of the Nb content is 0.10%, and more preferably is 0.06%.
  • a preferable lower limit of the Nb content is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.
  • the above described chemical composition may contain one or more types of element selected from the group consisting of Cr, Ni, Mo, W and B as a substitute for a part of Fe.
  • element selected from the group consisting of Cr, Ni, Mo, W and B as a substitute for a part of Fe.
  • Cr, Ni, Mo, W and B is an optional element, and each element increases the strength of the steel.
  • Chromium (Cr) is an optional element, and need not be contained in the steel. In a case where Cr is contained, Cr suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the Cr content is too high, the workability of the steel decreases and productivity is reduced. Therefore, the Cr content is from 0 to 1.0%. A preferable lower limit of the Cr content is 0.10%.
  • Nickel (Ni) is an optional element, and need not be contained in the steel. In a case where Ni is contained, Ni suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the Ni content is too high, the weldability of the steel decreases. Therefore, the Ni content is from 0 to 1.0%. A preferable lower limit of the Ni content is 0.10%.
  • Molybdenum (Mo) is an optional element, and need not be contained in the steel. In a case where Mo is contained, Mo suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the Mo content is too high, the hot workability of the steel decreases and productivity is reduced. Therefore, the Mo content is from 0 to 1.0%. A preferable lower limit of the Mo content is 0.01%.
  • Tungsten (W) is an optional element, and need not be contained in the steel. In a case where W is contained, W suppresses phase transformation at a high temperature and increases the strength of the steel. However, if the W content is too high, the hot workability of the steel decreases and productivity is reduced. Therefore, the W content is from 0 to 1.0%. A preferable lower limit of the W content is 0.01%.
  • B Boron
  • B is an optional element, and need not be contained in the steel.
  • B suppresses phase transformation at a high temperature and increases the strength of the steel.
  • the B content is from 0 to 0.010%.
  • a preferable upper limit of the B content is 0.005%, and more preferably is 0.003%.
  • a preferable lower limit of the B content is 0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
  • the above described chemical composition may contain Cu as a substitute for a part of Fe.
  • Copper (Cu) is an optional element, and need not be contained in the steel. In a case where Cu is contained, Cu precipitates in the steel as fine particles and increases the strength of the steel. However, the weldability of the steel will decrease if the Cu content is too high. Therefore, the Cu content is from 0 to 0.50%. A preferable lower limit of the Cu content is 0.10%.
  • the above described chemical composition may contain one or more types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As as a substitute for a part of Fe.
  • Sn, Bi, Se, Te, Ge and As is an optional element, and each element suppresses formation of an internal oxidized layer.
  • Tin (Sn), bismuth (Bi), selenium (Se), tellurium (Te), germanium (Ge) and arsenic (As) are optional elements, and need not be contained in the steel. If contained, these elements suppress formation of an internal oxidized layer by suppressing segregation of Mn and Si. However, if the content of these elements is too high, the formability of the steel decreases. Therefore, the Sn content is from 0 to 0.30%, the Bi content is from 0 to 0.30%, the Se content is from 0 to 0.30%, the Te content is from 0 to 0.30%, the Ge content is from 0 to 0.30% and the As content is from 0 to 0.30%.
  • a preferable upper limit of the Sn content is 0.25%, and more preferably is 0.20%.
  • a preferable upper limit of the Bi content is 0.25%, and more preferably is 0.20%.
  • a preferable upper limit of the Se content is 0.25%, and more preferably is 0.20%.
  • a preferable upper limit of the Te content is 0.25%, and more preferably is 0.20%.
  • a preferable upper limit of the Ge content is 0.25%, and more preferably is 0.20%.
  • a preferable upper limit of the As content is 0.25%, and more preferably is 0.20%.
  • a preferable lower limit of the Sn content is 0.0001%.
  • a preferable lower limit of the Bi content is 0.0001%.
  • a preferable lower limit of the Se content is 0.0001%.
  • a preferable lower limit of the Te content is 0.0001%.
  • a preferable lower limit of the Ge content is 0.0001%.
  • a preferable lower limit of the As content is 0.0001%. Note that, when two or more types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As are to be contained in the steel, it is preferable to make the total content thereof from 0.0001 to 0.30%.
  • the above described chemical composition may contain one or more types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal (REM) as a substitute for a part of Fe.
  • element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal (REM) as a substitute for a part of Fe.
  • REM rare earth metal
  • REM Rare earth metal
  • Ca Calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf) and rare earth metal (REM) are each an optional element and need not be contained in the steel. If contained, these elements enhance the formability of the steel. However, if the content of these elements is too high, the ductility of the steel decreases. Therefore, the Ca content is from 0 to 0.50%, the Mg content is from 0 to 0.50%, the Zr content is from 0 to 0.50%, the Hf content is from 0 to 0.50%, and the rare earth metal (REM) content is from 0 to 0.50%.
  • a preferable lower limit of the Ca content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%.
  • a preferable lower limit of the Mg content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%.
  • a preferable lower limit of the Zr content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%.
  • a preferable lower limit of the Hf content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%.
  • a preferable lower limit of the rare earth metal (REM) content is 0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. Note that, when two or more types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal (REM) are to be contained in the steel, it is preferable to make the total content thereof from 0.0001 to 0.50%.
  • REM refers to one or more types of element selected from Sc, Y, and lanthanoids (elements with atomic numbers 57 through 71 from La to Lu).
  • REM content refers to the total content of these elements.
  • the micro-structure of the hot-rolled steel sheet of the present embodiment is not particularly limited.
  • the micro-structure of the hot-rolled steel sheet of the present embodiment for example, mainly consists of ferrite and pearlite.
  • the combined area fraction of ferrite and pearlite is 75% or more.
  • a region (balance) other than ferrite and pearlite is one or more types of micro-structure selected from the group consisting of bainite (including tempered bainite), martensite (including tempered martensite) and retained austenite.
  • the total area fraction of ferrite and pearlite in the micro-structure is 75% or more, the strength of the hot-rolled steel sheet can be suppressed. In this case, the cold workability is enhanced.
  • the area fraction of the respective phases can be determined by the following methods.
  • the hot-rolled steel sheet is cut along a plane perpendicular to the rolling direction.
  • the cut surface is mirror polished.
  • a width-wise central portion of the hot-rolled steel sheet (range of ⁇ 10 mm in the width direction from the center in the width direction) that is a range of ⁇ 5 mm from a position that is equal to 1 ⁇ 4 of the plate thickness from the surface is defined as an observation region.
  • the observation region is corroded with a nital etching reagent. After corrosion, an arbitrary range of 200 ⁇ m ⁇ 150 ⁇ m of the observation region is photographed using a scanning electron microscope (SEM).
  • Ferrite and pearlite are identified using the image of the region that was photographed (hereunder, referred to as “photographing region”). The total of the areas of ferrite and pearlite that are identified is determined, and the determined total area is divided by the sum total of the areas of the entire photographing region to obtain the total area fraction (%) of ferrite and pearlite. The areas of ferrite and pearlite are measured using a mesh method or image processing software (product name: ImagePro).
  • a method for measuring the area fraction of bainite and martensite is as follows.
  • a photographing region (200 ⁇ m ⁇ 150 ⁇ m) that is the same as in the above described method for measuring an area fraction of ferrite and pearlite is photographed using an electron backscattering diffraction method (EBSD method) to generate a photographic image.
  • EBSD method electron backscattering diffraction method
  • a portion excluding pearlite and retained austenite is extracted from the photographic image by image processing.
  • 15 degrees is defined as a threshold value of an orientation difference with adjacent grains, and grains are identified.
  • GAIQ Geographic Average Image Quality
  • a histogram of area fractions is created with respect to the GAIQ values that were converted into numerical values. In a case where the created histogram has two peaks, a distribution on a side on which the GAIQ is high is taken as originating from bainite, and a distribution on a side on which the GAIQ is low is taken as originating from martensite.
  • the total area fraction of grains having a GAIQ identified as originating from bainite is defined as a bainite area fraction.
  • grains that are identified by the GAIQ up to a boundary at which the distributions overlap are defined as bainite, and the area fraction of the bainite is determined.
  • a value (%) obtained by deducting the sum total (%) of the aforementioned ferrite area fraction, pearlite area fraction and bainite area fraction as well as a retained austenite area fraction that is described later from 100(%) is defined as the area fraction of martensite.
  • the area fraction of retained austenite is determined by X-ray diffractometry. Specifically, in a photographing region (200 ⁇ m ⁇ 150 pin) that is the same as in the above described method for measuring an area fraction of ferrite and pearlite, the proportion of retained austenite is determined experimentally by X-ray diffractometry using the property that the reflection surface intensity differs between austenite and ferrite.
  • a retained austenite area fraction V ⁇ is determined using the following formula based on an image obtained by X-ray diffractometry using the K ⁇ ray of Mo:
  • V ⁇ (2 ⁇ 3) ⁇ 100/(0.7 ⁇ (211)/ ⁇ (220)+1) ⁇ +(1 ⁇ 3) ⁇ 100/(0.78 ⁇ (211)/ ⁇ (311)+1) ⁇
  • ⁇ (211) represents the reflection surface intensity at a (211) surface of ferrite
  • ⁇ (220) represents the reflection surface intensity at a (220) surface of austenite
  • ⁇ (311) represents the reflection surface intensity at a (311) surface of austenite
  • a preferable tensile strength of the hot-rolled steel sheet of the present embodiment is 800 MPa or less, and more preferably is 700 MPa or less. Because the tensile strength is low, the cold workability is enhanced. Although a lower limit of the tensile strength is not particularly limited, for example, the lower limit is 400 MPa.
  • the tensile strength can be determined by a tensile testing method for metal materials in accordance with JIS Z 2241 (2011).
  • an Sb concentrated layer is formed at an interface between a base metal surface and scale of a hot-rolled steel sheet.
  • the existence or non-existence of the Sb concentrated layer can be observed by electron probe microanalysis (EPMA).
  • EPMA electron probe microanalysis
  • the hot-rolled steel sheet is cut along a plane perpendicular to the rolling direction, and of the entire cut surface, an arbitrary region of 50 ⁇ m in the width direction ⁇ 45 ⁇ m in the depth direction of the hot-rolled steel sheet among a width-wise central portion (range of ⁇ 10 mm in the width direction from the center in the width direction) that includes the surface is defined as an observation region.
  • a sample including the observation region is extracted. Mapping analysis using EPMA is conducted with respect to the observation region.
  • a portion at which an Sb concentration is 1.5 times or more greater than the region average is defined as an Sb concentrated layer.
  • an Sb concentrated layer is confirmed in 90% or more of the width (50 ⁇ m) of the observation region, it is determined that an Sb concentrated layer is formed.
  • the thickness of the identified Sb concentrated layer is measured at a pitch of 5 ⁇ m in the width direction of the observation region, and an average value thereof is defined as the thickness of the Sb concentrated layer.
  • a preferable thickness of the Sb concentrated layer is 0.5 ⁇ m or more, more preferably is 1.0 ⁇ m or more, and further preferably is 1.5 ⁇ m or more.
  • the finish rolling temperature is preferably a high temperature.
  • a preferable thickness of the internal oxidized layer is 5 ⁇ m or less.
  • the internal oxidized layer is measured by the following method.
  • a small piece that includes a part of the surface of the hot-rolled steel sheet is cut out from an arbitrary position within a width-wise central portion (range of ⁇ 10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet.
  • a cross-section that is perpendicular to the rolling direction hereunder, referred to as “observation face” is mirror polished. The observation face is subjected to C vapor deposition.
  • a portion in the vicinity of the surface of the observation face is photographed for arbitrary visual fields at a magnification of ⁇ 1000 using a field-emission scanning electron microscope (FE-SEM) to obtain images (each visual field is 200 ⁇ m ⁇ 180 ⁇ m).
  • the thickness ( ⁇ m) of the internal oxidized layer is determined based on the obtained images. Oxides of Si and Mn arise in the base metal in the internal oxidized layer. Therefore, the scale, the internal oxidized layer and the base metal can be easily distinguished by means of a backscattered electron image obtained by a backscattered electron detector that is normally mounted in a common SEM.
  • a distance from the interface of the scale and base metal to the lowest edge of the internal oxidized layer is determined at each interval of 10 ⁇ m in the rolling direction. This measurement is performed for an arbitrary three visual fields, and the average value of the obtained distances is defined as the internal oxidized layer thickness ( ⁇ m).
  • a preferable thickness of the scale is 10 ⁇ m or less.
  • the scale thickness is measured by the following method. Images are obtained using an FE-SEM, similarly to when measuring the thickness of the internal oxidized layer. In the obtained images (it is sufficient to use the same images as when measuring the internal oxidized layer), the scale is identified, and a distance between an uppermost edge of the scale and the interface is determined at each interval of 10 ⁇ m in the rolling direction. This measurement is performed for an arbitrary three visual fields, and the average value of the obtained distances is defined as the scale thickness ( ⁇ m).
  • the thickness of a decarburization phase is also suppressed.
  • a preferable thickness of the decarburization layer is 20 ⁇ m or less.
  • the decarburization layer is measured by the following method.
  • a small piece that includes a part of the surface of the hot-rolled steel sheet is cut out from an arbitrary position within a width-wise central portion (range of ⁇ 10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet.
  • Line analysis of the C-K ⁇ line is performed by means of EPMA with respect to the surface of the small piece, and C strengths (line analysis results) in the depth direction from the surface of the steel sheet are obtained.
  • a distance from a position at which the C strength is smallest in the steel sheet to a depth position at which a difference between the average C strength (C strength of the base metal) of the steel sheet and the smallest C strength in the steel sheet is 98% is defined as the thickness ( ⁇ m) of the decarburization layer.
  • an Sb concentrated layer suppresses formation of an internal oxidized layer.
  • the Sb concentrated layer also suppresses formation of scale. Furthermore, the Sb concentrated layer suppresses formation of a decarburization layer.
  • a total area fraction of ferrite and pearlite in the micro-structure is 75% or more. Therefore, the tensile strength is suppressed to 800 MPa or less, preferably 700 MPa or less, and the hot-rolled steel sheet is thus excellent in cold workability.
  • the hot-rolled steel sheet of the present embodiment may also be subjected to descaling that is described later.
  • descaling that is described later.
  • an area fraction of the surface of island-shaped scale that is caused by fayalite and formed on the surface is lowered. Consequently, a pickling property is further enhanced.
  • the production method includes a preparation process, a hot rolling process and a coiling process.
  • a steel material having the above described chemical composition is prepared. Specifically, molten steel having the above described chemical composition is produced. A slab as the steel material is produced using the molten steel. The slab may be produced by a continuous casting process. Alternatively, an ingot may be produced using the molten steel, and the ingot may be subjected to blooming to produce the slab.
  • the prepared steel material (slab) is heated.
  • the heating temperature is from 1100 to 1350° C.
  • the heating time period is set to 30 minutes or more.
  • the heated slab is subjected to hot rolling using a roughing mill and a finish rolling mill and made into a steel sheet.
  • the roughing mill includes a plurality of roll stands that are arranged in a single row, with each roll stand having a pair of rolls.
  • the roughing mill may be a reverse-type roughing mill.
  • the finish rolling mill includes a plurality of roll stands that are arranged in a single row, with each roll stand having a pair of rolls.
  • descaling may also be performed on the steel sheet that is being subjected to rolling, by means of one or a plurality of high-pressure water descaling devices that are installed between the plurality of roll stands (roughing mill or finish rolling mill).
  • the descaling is preferably performed on the steel sheet having a temperature of 1050° C. or more.
  • Fe 2 SiO 4 (fayalite) arising on the surface of a steel with a high Si and high Mn content, as in a steel sheet having the chemical composition of the present embodiment, can be effectively removed. If fayalite remains, island-shaped scale will be formed on the surface of the hot-rolled steel sheet.
  • a steel sheet (rough bar) in a state after rough rolling and before finish rolling is heated to 1050° C. or more by a heating apparatus arranged in the vicinity of the entrance side of the initial roll stand of the finish rolling mill.
  • the method of heating the rough bar is not particularly limited.
  • the rough bar is heated by an induction heating apparatus or a reflow furnace.
  • the surface temperature of the steel sheet of the exit side of the final stand of the finish rolling mill is defined as a finish rolling temperature FT (° C.).
  • a preferable finish rolling temperature FT (° C.) is the A r3 transformation temperature +50° C. or more. If the finish rolling temperature FT is less than the A r3 transformation temperature +50° C., rolling resistance of the steel sheet increases and productivity decreases.
  • the steel sheet is rolled in a two-phase region of ferrite and austenite. In this case, the micro-structure of the steel sheet forms a layered micro-structure and the mechanical properties decrease. Therefore, the finish rolling temperature FT is the A r3 transformation temperature +50° C. or more.
  • a preferable finish rolling temperature FT is more than 920° C., and more preferably is 950° C. or more.
  • the cooling method is not particularly limited.
  • the cooling methods are, for example, water cooling, forced air-cooling and allowing cooling.
  • the hot-rolled steel sheet produced in the hot rolling process is coiled to form a coil.
  • a surface temperature (hereunder, referred to as “coiling temperature”) CT of the hot-rolled steel sheet when starting coiling of a coil is preferably from 600° C. to 750° C.
  • the coiling temperature CT is too high, formation of an internal oxidized layer in the hot-rolled steel sheet is promoted. On the other hand, if the coiling temperature CT is too low, in steel that contains a large amount of Si, as in the case of the hot-rolled steel sheet of the present embodiment, the strength of the hot-rolled steel sheet will be too high and the cold rolling property will decrease.
  • the coiling temperature CT is made a temperature in the range of 600° C. to 750° C., an increase in the strength of the hot-rolled steel sheet can be suppressed, and formation of an internal oxidized layer in the steel composition defined in the present embodiment is suppressed.
  • the coiling temperature CT is preferably from 650° C. to 750° C., and more preferably is from 700° C. to 750° C.
  • the hot-rolled steel sheet of the present embodiment can be produced by the above described processes.
  • the above described production method is one example of a method for producing a hot-rolled steel sheet in which a total area fraction of ferrite and pearlite is 75% or more, and a method for producing the hot-rolled steel sheet of the present embodiment is not limited thereto.
  • the micro-structure of the hot-rolled steel sheet may be a micro-structure that consists mainly of bainite and martensite. Specifically, a combined area fraction of bainite and martensite may be 75% or more.
  • the chemical composition of a hot-rolled steel sheet according to the present embodiment is the same as the chemical composition of the hot-rolled steel sheet of the first embodiment, and satisfies Formula (1). If the chemical composition does not satisfy Formula (1), the ductility of the cold-rolled steel sheet may decrease. If Formula (1) is satisfied, excellent ductility is obtained in a cold-rolled steel sheet after annealing also.
  • the micro-structure of the hot-rolled steel sheet of the present embodiment is different from the first embodiment.
  • a combined area fraction of bainite and martensite is 75% or more.
  • a region (balance) other than bainite and martensite is one or more types of micro-structure selected from the group consisting of ferrite, pearlite and retained austenite.
  • tempering is performed on the hot-rolled steel sheet after coiling. By this means, the strength of the steel sheet can be reduced to a certain extent, and the cold workability can be enhanced while maintaining a certain degree of strength.
  • the bainite is mainly tempered bainite
  • the martensite is mainly tempered martensite. Methods for measuring the area fractions of the respective phases in the micro-structure are the same as in the first embodiment.
  • the hot-rolled steel sheet of the present embodiment has the above described chemical composition and micro-structure.
  • the tensile strength of the hot-rolled steel sheet of the present embodiment is 900 MPa or more.
  • the tensile strength of the hot-rolled steel sheet is 800 MPa or less. In this case the cold workability can be enhanced, and the load placed on the equipment system during cold rolling can be reduced.
  • the lower limit of the tensile strength is not particularly limited, for example, the lower limit is 400 MPa.
  • the tensile strength is determined by a method that is in accordance with JIS Z 2241 (2011).
  • the production method includes a preparation process, a hot rolling process and a coiling process.
  • the coiling temperature CT in the coiling process differs from the first embodiment.
  • tempering is also performed after the coiling process.
  • the other processes are the same as in the first embodiment.
  • the steel sheet produced in the hot rolling process is coiled to form a coil. If the surface temperature (coiling temperature) of the steel sheet when starting coiling of a coil is too low, the strength of the steel sheet increases and the load placed on a coiling apparatus becomes large. Therefore, the surface temperature (coiling temperature) CT of the steel sheet when starting coiling is from 150 to 600° C., preferably from 350 to 500° C., and more preferably from 400° C. to 500° C.
  • the coiling temperature CT of the hot-rolled steel sheet of the present embodiment is 600° C. or less, preferably 500° C. or less, the hardness of the hot-rolled steel sheet is high. Therefore, tempering may be performed to lower the strength and enhance the cold rolling property.
  • the steel sheet after coiling is tempered at a temperature of 550° C. or more (Ac1 transformation temperature or less). If the tempering time period is too short, it is difficult to obtain the above described effect. On the other hand, if the tempering time period is too long, the effect saturates. Therefore, a preferable tempering time period is 0.5 to 8 hours in a temperature range of 550° C. or more.
  • the hot-rolled steel sheet of the second embodiment can be produced by the above described processes.
  • tempering need not be performed.
  • a combined area fraction of bainite and martensite is 75% or more, and the balance is one or more types of micro-structure selected from the group consisting of ferrite, pearlite and retained austenite.
  • the bainite micro-structure and martensite micro-structure in a case where tempering is not performed are not micro-structures that are mainly composed of tempered bainite and tempered martensite, but rather are micro-structures mainly composed of bainite and martensite that contain some tempered bainite and tempered martensite formed during the coiling process.
  • the tensile strength of the hot-rolled steel sheet is 900 MPa or more.
  • a hot-rolled steel sheet that has not been subjected to tempering is useful in particular in a case where a high tensile strength is required as a hot-rolled steel sheet and the like.
  • the above described production method is one example of a method for producing a hot-rolled steel sheet in which the total area fraction of bainite and martensite is 75% or more, and a method for producing the hot-rolled steel sheet of the present embodiment is not limited to the above described method.
  • the micro-structure is defined.
  • the micro-structure of a hot-rolled steel sheet of the present embodiments is not particularly limited.
  • an Sb concentrated layer can be formed and formation of an internal oxidized layer and/or scale can be suppressed while maintaining the necessary workability and strength.
  • the hot-rolled steel sheets of the first and second embodiments can also be produced by other production methods.
  • ADFT average cooling rate
  • steel types A to O are within the range of the chemical composition of the steel material of the present embodiments.
  • the chemical compositions of steel types P to U are outside the range of the chemical composition of the steel material of the present embodiments.
  • Hot-rolled steel sheets were produced by hot rolling the steel materials under the hot rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 2 using a hot rolling mill for testing that was composed of a plurality of hot rolling stands.
  • a heat history equivalent to a coil that was coiled at a coiling temperature CT (° C.) shown in Table 2 was imparted to the respective hot-rolled steel sheets after hot rolling by an N 2 -purged annealing furnace.
  • a reheating furnace simulating a rough bar heater was installed on the entrance side of the finish rolling mill, and reheating of the hot-rolled steel sheets was conducted under the conditions shown in Table 2. Further, a high-pressure water descaling device was disposed between roll stands of the finish rolling mill, and descaling was performed with respect to steel sheets undergoing finish rolling. The surface temperatures (descaling temperatures) of the respective steel sheets immediately before performing descaling were as shown in Table 2.
  • a small piece was cut out from a width-wise central portion (range of ⁇ 10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet of each test number.
  • a cross-section hereunder, referred to as “observation face” perpendicular to the rolling direction was mirror polished.
  • the observation face was subjected to C (carbon) vapor deposition. After the C vapor deposition, a portion in the vicinity of the surface of the observation face was photographed using a field-emission scanning electron microscope (FE-SEM) and images were obtained.
  • the thickness ( ⁇ m) of the internal oxidized layer and the scale thickness ( ⁇ m) were determined by the above described method using the obtained images.
  • a small piece was cut out from a width-wise central portion (range of ⁇ 10 mm in the width direction from the center in the width direction) of the hot-rolled steel sheet of each test number.
  • Line analysis of C-K ⁇ line was performed by means of EPMA with respect to the surface of the small piece.
  • a distance from a position at which the C strength was smallest in the steel sheet to a depth position at which a difference between the average C strength (C strength of the base metal) of the steel sheet and the smallest C strength in the steel sheet was 98% was defined as the thickness ( ⁇ m) of the decarburization layer.
  • the presence or absence of an Sb concentrated layer and the thickness ( ⁇ m) of an Sb concentrated layer were measured by the measurement method described in the first embodiment.
  • a test specimen including part of the steel sheet surface was taken from a width-wise central portion of the steel sheet of each test number. In each test specimen, a region including part of the steel sheet surface was 50 mm in width ⁇ 70 mm in length.
  • a pickling test was performed on each test specimen. In the pickling test, the test specimen was immersed in an 8% hydrochloric acid aqueous solution heated to 85° C., and scale on the surface of the test specimen was removed. A time period (pickling completion time period) taken to remove scale from the entire surface of the test specimen was measured. The pickling property was determined as being excellent if the pickling completion time period was within 60 seconds.
  • the item “micro-structure” shows the micro-structure at a depth position at 1 ⁇ 4 of the plate thickness at a width-wise central portion (range of ⁇ 10 mm in the width direction from the center in the width direction) of the steel sheet.
  • an area fraction (%) of ferrite in the respective micro-structures is described in an “F” column
  • an area fraction of pearlite is described in a “P” column.
  • the area fractions of phases other than ferrite and pearlite are described in an “Other” column.
  • the character “B” denotes the area fraction (%) of bainite (including tempered bainite), and the character “M” denotes the area fraction (%) of martensite (including tempered martensite). “Rg” denotes the area fraction (%) of retained austenite. The area fraction of each phase was measured by the above described measurement method.
  • the item “tensile strength TS” shows the tensile strength TS (MPa) at a width-wise central portion of the steel sheet.
  • the cold rolling property was determined as excellent if the tensile strength was 800 MPa or less.
  • An “O” mark in the “pickling property” column in Table 3 indicates that the pickling completion time period was within 60 seconds.
  • An “x” mark indicates that the pickling completion time period was more than 60 seconds.
  • the chemical compositions of test numbers 1 to 19 were appropriate, and also satisfied Formula (1). Therefore, an Sb concentrated layer was confirmed.
  • the thickness of each of the Sb concentrated layers was 0.5 ⁇ m or more.
  • the scale thickness was 10 ⁇ m or less, and the thickness of an internal oxidized layer was 5 ⁇ m or less.
  • the pickling property of the respective chemical compositions of test numbers 1 to 19 was excellent.
  • the thickness of the decarburization layer was 20 ⁇ m or less.
  • test numbers 1 to 5 test number 7 and test numbers 9 to 19, the production conditions were suitable for formation of ferrite and pearlite. Therefore, in the micro-structure of the hot-rolled steel sheet of each of these test numbers, the total area fraction of ferrite and pearlite was 75% or more. Therefore, the tensile strength was 800 MPa or less.
  • test number 6 and test number 8 because the coiling temperature CT was from 150 to 600° C., the total area fraction of bainite and martensite in the micro-structure was 75% or more, and the tensile strength was 900 MPa or more.
  • the Sb content was too low. Therefore, the thickness of the Sb concentrated layer was less than 0.5 ⁇ m, and the thickness of the internal oxidized layer was more than 5 ⁇ m. Consequently, the pickling property was poor.
  • the C content was too high.
  • Sb was not contained therein.
  • the coiling temperature CT was also too low. Consequently, the micro-structure mainly consisted of bainite and martensite, and did not contain ferrite and pearlite. Therefore, the tensile strength was more than 800 MPa.
  • the thickness of the internal oxidized layer was more than 5 ⁇ m, and the scale thickness was more than 10 ⁇ m. Therefore, the pickling property was poor.
  • the Si content was too high, and Sb was not contained therein. Consequently, an Sb concentrated layer was not formed, and the thickness of the internal oxidized layer was more than 5 ⁇ m and the scale thickness was more than 10 ⁇ m. Therefore, the pickling property was poor. In addition, the decarburization layer thickness was more than 20 ⁇ m.
  • steel materials were produced by an ingot-making process.
  • Steel sheets were produced by hot rolling the steel materials under the hot rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 5 using a hot rolling mill for testing.
  • a heat treatment that simulated coiling at a coiling temperature CT (° C.) shown in Table 5 was performed on the respective steel sheets after hot rolling.
  • the steel sheets were stacked and charged into a furnace that was set to the coiling temperature CT (° C.).
  • the inside of the furnace was a nitrogen atmosphere, and the steel sheet surface was in a state in which the surface was blocked-off from the atmosphere.
  • the surface state of the steel sheet was equal to the surface state of a coil obtained by actual production. After holding the steel sheet inside the furnace for 30 minutes at the coiling temperature CT (° C.), the steel sheet was gradually cooled to room temperature at 20° C./hour.
  • the thickness of an internal oxidized layer in the steel sheet after hot rolling (hot-rolled steel sheet) of each test number was measured by the same method as in Example 1. Specifically, a small piece including a part of the surface of the hot-rolled steel sheet was cut out from a width-wise central portion of the hot-rolled steel sheet. Of the entire surface of the small piece, a cross-section (hereunder, referred to as “observation face”) perpendicular to the rolling direction was mirror polished. The observation face was subjected to C (carbon) vapor deposition. After the C vapor deposition, a portion in the vicinity of the surface of the observation face was photographed at an observation magnification of ⁇ 1000 using a field-emission scanning electron microscope (FE-SEM) and images were obtained. The thickness of the internal oxidized layer was measured by the above described method based on the obtained images. The results are shown in Table 5.
  • scale is a layer that is formed when iron ions at the exterior of the hot-rolled steel sheet are oxidized.
  • an internal oxidized layer is a layer that contains oxides of Si and Mn and is formed inside the hot-rolled steel sheet. Therefore, scale, an internal oxidized layer and the base metal can be easily distinguished using a common SEM.
  • the tensile strength TS of the hot-rolled steel sheet of each test number was measured by a method in accordance with JIS Z 2241 (2011). The results are shown in Table 5.
  • Table 5 the symbol “-” in the “TS (MPa)” column indicates that cracking occurred at an edge of the hot-rolled steel sheet and measurement was not possible.
  • the hot-rolled steel sheet of each test number was cold-rolled at a draft of 50%. After cold rolling, each steel sheet was subjected to annealing. The annealing was performed under the following conditions. The steel sheet was heated to an HC temperature (Ae 3 temperature +10° C.) at an average heating rate of 5° C./second, and the steel sheet was subjected to annealing for 90 seconds at this HC temperature. Thereafter, the steel sheet was gradually cooled at a cooling rate of 2° C./sec to an AC temperature (HC temperature ⁇ 120° C.). In addition, the steel sheet was rapidly cooled at 80° C./sec from the AC temperature to 420° C. After being held at 420° C.
  • test numbers 1 to 10 were appropriate.
  • the production conditions of test numbers 1 to 10 were appropriate. Therefore, in the micro-structure of the hot-rolled steel sheets of test numbers 1 to 10, the total area fraction of ferrite and pearlite was 75% or more. Further, in the hot-rolled steel sheets of test numbers 1 to 10, an Sb concentrated layer with a thickness of 0.5 ⁇ m or more was formed. Furthermore, the thickness of an internal oxidized layer was 5 ⁇ m or less, and thus formation of an internal oxidized layer was suppressed.
  • the tensile strength of the hot-rolled steel sheets of test numbers 1 to 10 was 800 MPa or less, and the workability during cold rolling was excellent.
  • the uniform elongation of the cold-rolled steel sheets of test numbers 1 to 10 was 10.0% or more, indicating excellent workability after cold rolling also.
  • Steel type K used for test number 11 did not contain Sb. Consequently, in the hot-rolled steel sheet of test number 11, an Sb concentrated layer was not formed and an internal oxidized layer had a thick thickness of 47 ⁇ m.
  • the Si content was a low value of 0.93%.
  • the total content of Si and Mn was 3.04%, and thus Formula (1) was not satisfied. Therefore, the uniform elongation of the cold-rolled steel sheet of test number 15 was 8.7%, which was low in comparison to test numbers 1 to 10 in which, similarly to test number 15, the total area fraction of ferrite and pearlite was 75% or more.
  • the Sb content was a low value of 0.02%. Consequently, in the hot-rolled steel sheet of test number 19, the thickness of the Sb concentrated layer was less than 0.5 ⁇ m, and the internal oxidized layer had a thick thickness of 25 ⁇ m.
  • steel materials were produced by an ingot-making process.
  • Steel sheets were produced by hot rolling the steel materials under the hot rolling conditions (heating temperature (° C.) and finish rolling temperature FT (° C.)) shown in Table 6 using a hot rolling mill for testing.
  • a heat treatment that simulated coiling at a coiling temperature CT (° C.) shown in Table 6 was performed on the respective steel sheets after hot rolling.
  • the steel sheets were stacked and charged into a furnace that was set to the coiling temperature CT (° C.).
  • the inside of the furnace was a nitrogen atmosphere, and the steel sheet surface was in a state in which the surface was blocked-off from the atmosphere.
  • the surface state of the steel sheet was equal to the surface state of a coil obtained by actual production. After holding the steel sheet inside the furnace for 30 minutes at the coiling temperature CT (° C.), the steel sheet was gradually cooled to room temperature at 20° C./hour.
  • tempering was performed at a tempering temperature (° C.) and for a tempering time period (hr) as shown in Table 6.
  • Table 6 the column “tempering time period (hr)” shows the time period for which the relevant steel sheet was kept at the tempering temperature shown in Table 6.
  • the tensile strength TS (MPa) of each test number was measured by the same method as in Example 1. The results are shown in Table 6.
  • the symbol “-” in the “tensile strength” column indicates that cracking occurred at an edge of the hot-rolled steel sheet and measurement was not possible.
  • test numbers 1 to 15 were appropriate.
  • the production conditions of test numbers 1 to 15 were appropriate. Consequently, in the micro-structure of the hot-rolled steel sheets of test numbers 1 to 15, the total area fraction of bainite and martensite was 75% or more.
  • an Sb concentrated layer having a thickness of 0.5 ⁇ m or more was also confirmed. As a result, the thickness of the internal oxidized layer was 5 ⁇ m or less, and formation of an internal oxidized layer was suppressed.
  • the scale thickness of the hot-rolled steel sheets of test numbers 1 to 15 was 7 ⁇ m or less, and scale was suppressed.
  • test numbers 1 3, 4, 6, 8 to 12 and 14, tempering was performed. Consequently, the tensile strength TS was 800 MPa or less and the uniform elongation EL was 10% or more, and excellent workability was obtained after cold rolling. On the other hand, in test numbers 2, 5, 7, 13 and 15, tempering was not performed. Consequently, the tensile strength was 900 MPa or more and excellent strength was obtained.
  • steel type K used in test number 16 did not contain Sb. Consequently, an Sb concentrated layer was not formed. As a result, the thickness of an internal oxidized layer was more than 5 ⁇ m, and the scale thickness was more than 7 ⁇ m.
  • the Sb content was 0.004%, which was too low. Therefore, an Sb concentrated layer was not formed in the hot-rolled steel sheet of test number 18. Consequently, the thickness of the internal oxidized layer was more than 5 ⁇ m, and the scale thickness was more than 7 ⁇ m.
  • the Si content was a low value of 0.93%.
  • the total content of Si and Mn was 3.04%, and thus Formula (1) was not satisfied. Consequently, even though tempering was performed, the uniform elongation EL was less than 10%.
  • the Mn content was a low value of 1.55%. Consequently, in the micro-structure, the area fraction of ferrite was 30%, and the combined area fraction of martensite and bainite was less than 75%. As a result, even though tempering was performed, the uniform elongation EL was less than 10%.
  • the Si content was a high value of 2.96%. Consequently, even though tempering was performed, the uniform elongation EL was less than 10%.
  • the Mn content was a high value of 3.99%. Consequently, even though tempering was performed, the uniform elongation EL was less than 10%.
  • the Sb content was a low value of 0.02%. Therefore, the thickness of an Sb concentrated layer was less than 0.5 ⁇ m. Consequently, the thickness of an internal oxidized layer was more than 10 ⁇ m, and the scale thickness was more than 7 ⁇ m.

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