WO2020184683A1 - Steel sheet and production method for same - Google Patents

Steel sheet and production method for same Download PDF

Info

Publication number
WO2020184683A1
WO2020184683A1 PCT/JP2020/010944 JP2020010944W WO2020184683A1 WO 2020184683 A1 WO2020184683 A1 WO 2020184683A1 JP 2020010944 W JP2020010944 W JP 2020010944W WO 2020184683 A1 WO2020184683 A1 WO 2020184683A1
Authority
WO
WIPO (PCT)
Prior art keywords
steel sheet
steel
less
content
ferrite
Prior art date
Application number
PCT/JP2020/010944
Other languages
French (fr)
Japanese (ja)
Inventor
啓介 中井
浩久 田邉
祥晃 新宅
中島　清孝
真吾 中村
Original Assignee
日本製鉄株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to CN202080021699.3A priority Critical patent/CN113574197B/en
Priority to JP2021505143A priority patent/JP7201068B2/en
Priority to KR1020217033103A priority patent/KR102376351B1/en
Publication of WO2020184683A1 publication Critical patent/WO2020184683A1/en

Links

Classifications

    • 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/12—Accessories for subsequent treating or working cast stock in situ
    • B22D11/128—Accessories for subsequent treating or working cast stock in situ for removing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/04—Removing impurities by adding a treating agent
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10—Handling in a vacuum
    • 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
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a steel sheet and a method for manufacturing the same.
  • crude oil tanks that transport or store crude oil, such as crude oil tankers or above-ground or underground crude oil tanks (hereinafter collectively referred to as "crude oil tanks"), for welded structures with excellent strength and weldability. Steel is used. Further, steel used as a crude oil tank is required to have excellent corrosion resistance against corrosive gas components, salts and the like contained in crude oil (see, for example, Patent Document 1).
  • Patent Document 1 describes for a welded structure that exhibits excellent total corrosion resistance and local corrosion resistance against crude oil corrosion that occurs in a steel oil tank, and can further suppress the formation of corrosion products (sludge) containing solid S.
  • the method for producing crude oil tank steel, the method for producing crude oil tank steel, the crude oil tank, and the method for preventing corrosion of the crude oil tank are disclosed.
  • the crude oil tank steel described in Patent Document 1 contains a predetermined amount or more of Mo and W in a solid solution state, and therefore has excellent corrosion resistance.
  • Patent Document 1 when the steel piece before hot rolling is heated, the steel piece is set to a heating temperature of 1200 to 1350 ° C., and the state is maintained for 2 to 100 hours by performing diffusion heat treatment.
  • the solid solution amount of Mo and W is secured. For this reason, a large amount of fuel gas for heating is used, and the heating time becomes long, which causes problems such as an increase in manufacturing cost and hindering productivity, and there is room for improvement.
  • An object of the present invention is to solve the above-mentioned problems and to provide a steel sheet having excellent corrosion resistance against corrosive gas components, salts and the like contained in crude oil, and a method for producing the same.
  • the present inventors have studied a method for improving corrosion resistance without performing diffusion heat treatment. As a result, by increasing the oxygen concentration during heating and forming a relatively thick scale on the steel surface, it becomes possible to concentrate Mo and W on the surface layer of the steel sheet directly under the scale, and on the surface of the steel sheet. It has been found that Mo and W in a solid solution state can be secured, and thereby the corrosion resistance can be significantly improved.
  • the present invention has been made based on the above findings, and the gist of the following steel sheet and its manufacturing method is.
  • the chemical composition is mass% C: 0.050 to 0.200%, Si: 0.100 to 1.000%, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.010% or less, Al: 0.002 to 0.050%, N: 0.0010 to 0.0060%, O: 0.0005 to 0.0060%, Ti: 0.003 to 0.020%, Cu: 0.01-1.50%, Ca: 0 to 0.0080%, Mg: 0 to 0.0080%, REM: 0-0.0080%, Mo: 0 to 0.20%, W: 0 to 0.50%, Nb: 0 to 0.030%, V: 0 to 0.050%, Ni: 0 to 1.00%, Cr: 0 to 0.50%, B: 0 to 0.0030%, Sb: 0 to 0.30%, Sn: 0 to 0.30%, Pb: 0 to 0.30%, As: 0 to 0.30%, Bi: 0 to 0.30%, Ta: 0 to 0.50%, Zr: 0
  • the average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet is 60 ⁇ m or less.
  • the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
  • the chemical composition is mass%. Si: 0.200 to 1.000%, P: 0.015% or less, S: 0.003% or less, And Satisfy the following equations (ii) and (iii),
  • the ferrite transformation start temperature Ar 3 obtained by the following equation (iv) is 760 to 820 ° C.
  • the metallographic structure at a position 1 / 4t from the surface of the steel sheet is in area%.
  • Perlite 5-20%, Bainite: 10% or less, Remaining: Ferrite,
  • the average aspect ratio of ferrite at a position 1 / 4t from the surface of the steel sheet is 1.0 to 1.5.
  • the average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet is 5 to 20 ⁇ m.
  • the average dislocation density in ferrite at a position 1 / 4t from the surface of the steel sheet is 7.0 ⁇ 10 12 / m 2 or less.
  • the average value of Vickers hardness in the region between the surface of the steel sheet and the position of 1 / 4t from the surface of the steel sheet is the position of 1 / 4t from the surface of the steel sheet.
  • the number density of inclusions having a length of 5 ⁇ m or more existing in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet is 10 pieces / mm 2 Is below,
  • the maximum concentration of P in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet is 0.02 to 0.20% in mass%.
  • a cooling step of cooling the steel sheet after hot rolling to room temperature is provided.
  • an O 2 concentration is not less than 1.0 vol% atmosphere, and held at a heating temperature of 950 ⁇ 1100 °C 30 ⁇ 60 minutes, the heating extraction temperature of 950 ⁇ 1100 ° C.
  • the surface temperature of the steel piece in a temperature range of Ar 3 -30 °C ⁇ T rex °C performs the finish rolling under conditions where the cumulative rolling reduction is 50-75%, Steel sheet manufacturing method.
  • Ar 3 is calculated by the following formula (iv)
  • Trex means the recrystallization start temperature at which the growth of crystal grains starts, and is calculated by the following formula (v).
  • the element symbol in the following formula represents the content (mass%) of each element.
  • Ar 3 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ...
  • T rex -91900 [Nb *] 2 +9400 [Nb *] + 770 ⁇ (v)
  • Nb * sol. Nb Nb ⁇ sol.
  • Nb * Nb
  • Nb (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 ⁇ N) ⁇ ⁇ ⁇ (vi)
  • T in the above formula represents a heating extraction temperature (° C.) of a steel piece.
  • the dissolved oxygen content of the molten steel is adjusted to 40 ppm or less by a vacuum degassing device, and then Al is added so that the final content of Al is 0.002 to 0.050%.
  • the total final content of Ca, Mg and REM of one or more selected from Ca, Mg and REM becomes 0.0005 to 0.0080%.
  • the gap between the casting rolls is 0.2 mm to 1 m in the casting progress direction. Casting while narrowing to 3.0 mm and reducing The method for manufacturing a steel sheet according to any one of (6) to (9) above.
  • C 0.050 to 0.200%
  • C is an element effective for forming pearlite and increasing its strength.
  • the C content is set to 0.050 to 0.200%.
  • the C content is preferably 0.070% or more or 0.100% or more, and preferably 0.180% or less or 0.160% or less.
  • Si 0.100 to 1.000%
  • Si is an inexpensive deoxidizing element, which is effective in strengthening solid solution and raises the transformation point to contribute to the reduction of dislocation density in ⁇ .
  • the Si content is set to 0.100 to 1.000%.
  • the Si content is preferably 0.200% or more, and more preferably 0.300% or more.
  • the Si content is preferably 0.800% or less, and preferably 0.500% or less.
  • Mn 0.50 to 2.00% Mn is effective as an element for improving the strength and toughness of the base material. On the other hand, if the Mn content is excessive, the weldability and joint toughness are deteriorated. Therefore, the Mn content is set to 0.50 to 2.00%.
  • the Mn content is preferably 0.80% or more, more preferably 0.90% or more.
  • the Mn content is preferably 1.60% or less, and more preferably 1.50% or less.
  • P 0.030% or less
  • P is an element contained in steel as an impurity, and it should be 0.030% or less in order to ensure corrosion resistance. Further, in order to ensure ductility and toughness, the smaller the P content, the more desirable, and preferably 0.015% or less.
  • S 0.010% or less
  • S is an element contained in steel as an impurity, and is 0.010% or less in order to ensure corrosion resistance. Further, in order to secure ductility and toughness, it is desirable that the S content is small, and the S content is preferably 0.003% or less.
  • Al 0.002 to 0.050%
  • Al is an important deoxidizing element.
  • the Al content is set to 0.002 to 0.050%.
  • the Al content is preferably 0.010% or more, and preferably 0.040% or less.
  • N 0.0010 to 0.0060% N forms a nitride together with Al to improve joint toughness.
  • the N content is set to 0.0010 to 0.0060%.
  • the N content is preferably 0.0020% or more, preferably 0.0050% or less, and more preferably 0.0040% or less.
  • O forms an oxide together with Ca, Mg, and REM described later. If the O content is excessive, the oxide becomes coarse and ductility and toughness decrease. On the other hand, the smaller the O content is, the better, but in order to reduce it excessively, for example, the reflux work in the RH vacuum degassing device takes a long time, which is not realistic. Therefore, the O content is set to 0.0005 to 0.0060%.
  • Ti 0.003 to 0.020%
  • Ti contains a small amount and contributes to the improvement of toughness through the microstructure of the base metal and welded parts. On the other hand, if the Ti content is excessive, the welded portion is hardened and the toughness is significantly deteriorated. Therefore, the Ti content is set to 0.003 to 0.020%.
  • the Ti content is preferably 0.006% or more, and preferably 0.013% or less.
  • the ratio of the Ti content to the N content is 0.5 or more, it is possible not only to reduce the solid solution N and improve the elongation characteristics, but also to prevent the occurrence of surface defects of the slab. Become. Further, by setting the ratio of the Ti content to the N content to 4.0 or less, the formation of TiC can be suppressed and the elongation characteristics can be improved. Therefore, when it is desired to obtain excellent ductility, it is preferable that the Ti content satisfies the following equation (iii) in relation to the N content. 0.5 ⁇ Ti / N ⁇ 4.0 ... (iii) However, the element symbol in the above formula represents the content (mass%) of each element.
  • Cu 0.01 to 1.50%
  • Cu is an element effective for improving not only overall corrosion resistance but also local corrosion resistance. Further, it is also effective in suppressing the formation of solid S.
  • the Cu content is set to 0.01 to 1.50%.
  • the Cu content is preferably 0.03% or more, preferably 0.50% or less, and more preferably less than 0.20%.
  • Mg and REM may be contained as necessary because they suppress the formation of coarse inclusions (stretched MnS and the like) by forming sulfides. On the other hand, if any content exceeds 0.0080%, the effect is saturated and coarse oxides or sulfides are formed to deteriorate toughness and elongation. Therefore, the contents of Ca, Mg and REM are all 0.0080% or less.
  • the total content of these elements is preferably 0.0005% or more. Further, from the viewpoint of preventing deterioration of toughness and elongation characteristics due to coarse oxides or sulfides, the total content of these elements is preferably 0.0080% or less.
  • the total content is more preferably 0.0010% or more, and further preferably 0.0015% or more. Further, the total content is more preferably 0.0060% or less, and further preferably 0.0040% or less. 0.0005 ⁇ Ca + Mg + REM ⁇ 0.0080 ... (ii) However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
  • REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements.
  • Lanthanoids are industrially added in the form of misch metal.
  • Mo and W are elements effective for improving local corrosiveness. Therefore, it is necessary to contain at least one of Mo and W, and the total content should be 0.01% or more. On the other hand, if Mo is contained in excess of 0.20% and W is contained in excess of 0.50%, the local corrosiveness is conversely lowered, and the weldability and toughness are deteriorated. Therefore, the Mo content is 0.20% or less, the W content is 0.50% or less, and the total content is 0.70% or less. That is, it is necessary to satisfy the following equation (i). 0.01 ⁇ Mo + W ⁇ 0.70 ... (i) However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
  • the Mo content is preferably 0.01% or more, more preferably 0.03% or more.
  • the Mo content is preferably 0.08% or less, more preferably 0.07% or less.
  • the W content is preferably 0.01% or more, and preferably less than 0.05%.
  • Nb 0 to 0.030% Since Nb is an element that contributes to microstructure miniaturization by adding a small amount and is effective in ensuring the strength of the base metal, it may be contained if necessary. On the other hand, if the Nb content is excessive, the welded portion is hardened and the toughness is significantly deteriorated. Therefore, the Nb content is set to 0.030% or less. When the above effect is desired, the Nb content is preferably 0.003% or more.
  • V 0 to 0.050% Since V contributes to the increase in strength by strengthening precipitation, it may be contained if necessary. On the other hand, if the V content is excessive, the toughness of the joint may be impaired. Therefore, the V content is set to 0.050% or less. When the above effect is desired, the V content is preferably 0.010% or more.
  • Ni 0 to 1.00% Since Ni is effective in ensuring strength and improving toughness, it may be contained if necessary. On the other hand, if the Ni content is excessive, the cost will increase. Therefore, the Ni content is set to 1.00% or less. When the above effect is desired, the Ni content is preferably 0.05% or more.
  • Cr 0 to 0.50% Since Cr is effective for improving hardenability and increasing strength, it may be contained if necessary. On the other hand, if the Cr content is excessive, the hardness of the joint may increase and the toughness may decrease. Therefore, the Cr content is set to 0.50% or less. When the above effect is desired, the Cr content is preferably 0.05% or more.
  • B 0 to 0.0030% Since B is added in a small amount to enhance the hardenability and contribute to the improvement of the strength of the base material, it may be contained as necessary. On the other hand, if the B content is excessive, the elongation and joint toughness are deteriorated. Therefore, the B content is set to 0.0030% or less. When the above effect is desired, the B content is preferably 0.0003% or more.
  • Sb 0 to 0.30% Sn: 0 to 0.30% Pb: 0 to 0.30% As: 0 to 0.30% Bi: 0 to 0.30% Sb, Sn, Pb, As and Bi have an effect of further suppressing the progress of local corrosion, and may be contained as necessary. On the other hand, even if any content exceeds 0.30%, the effect is saturated and there is a concern that it may adversely affect other characteristics. Therefore, in consideration of economic efficiency, the contents of Sb, Sn, Pb, As and Bi are all set to 0.30% or less. Moreover, the content of any element is preferably 0.15% or less. When the above effect is desired, Sb: 0.01% or more, Sn: 0.01% or more, Pb: 0.01% or more, As: 0.01% or more, and Bi: 0.01% or more. It is preferable to contain one or more selected species.
  • Ta and Zr are elements effective for increasing the strength of steel in a small amount, and may be contained as necessary mainly for strength adjustment. On the other hand, even if any of the contents exceeds 0.50%, the toughness deterioration becomes remarkable. Therefore, the contents of Ta and Zr are both set to 0.50% or less. When the above effect is desired, it is preferable to contain one or two kinds selected from Ta: 0.005% or more and Zr: 0.005% or more.
  • the balance is Fe and impurities.
  • the "impurity” is a component mixed with raw materials such as ore and scrap and various factors in the manufacturing process when the steel sheet is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.
  • Total content of solid solution Mo and solid solution W in the surface layer of the steel sheet 0.005% or more
  • Mo and W are concentrated in the surface layer of the steel sheet in order to improve corrosion resistance as described above. By doing so, the amount of Mo and W to be solid-solved is secured at a predetermined value or more.
  • the total content of the solid solution Mo and the solid solution W in the surface layer portion of the steel sheet is 0.005% or more in mass%.
  • the total content of the solid solution Mo and the solid solution W in the surface layer portion of the steel sheet is preferably 0.010% or more, and more preferably 0.020% or more.
  • the surface layer portion of the steel sheet refers to a region from the surface of the steel sheet to a position of 1 mm in the depth direction.
  • the total content (mass%) of the solid solution Mo and the solid solution W is measured by the following procedure. First, two test pieces having a thickness of 1 mm are cut out from the surface of the steel plate. Then, for one of the test pieces, the contents of W and Mo in the test piece are measured by using a known chemical analysis method (for example, ICP emission spectroscopy).
  • W and Mo in the test piece are considered to be W precipitate and Mo precipitate and solid solution W and solid solution Mo
  • W and Mo in the extraction residue are considered to be W precipitate and Mo precipitate.
  • the contents of the solid solution W and the solid solution Mo are obtained by obtaining the difference between the contents of the W and Mo in the extraction residue from the contents of W and Mo in the test piece.
  • Maximum concentration of P in the center of plate thickness 0.02 to 0.20% P may be centrally segregated during continuous casting to form an embrittlement region in the central portion of the plate thickness, causing cracks and deteriorating local elongation. Therefore, when it is desired to obtain excellent ductility, it is preferable that the maximum concentration of P in the center of the plate thickness is low. Specifically, the position is 2/5 t from the surface of the steel sheet and 3/5 t from the surface of the steel plate. It is preferable that the maximum concentration of P in the region between the positions is 0.20% or less. Further, since it is practically difficult to set the maximum concentration of P to less than 0.02%, 0.02% is set as the lower limit.
  • the maximum concentration of P is the accelerating voltage of the region between the position of 2/5 t from the surface of the steel sheet and the position of 3/5 t from the surface of the steel plate by an electron probe microanalyzer (EPMA). It is the maximum value of the concentration of P when measured at 15 kV, beam diameter: 20 ⁇ m, irradiation time: 20 ms, and measurement pitch: 20 ⁇ m.
  • the average crystal grain size of ferrite is set to 60 ⁇ m or less from the viewpoint of ensuring predetermined elongation characteristics.
  • the average crystal grain size of ferrite is preferably 50 ⁇ m or less.
  • the metallographic structure is 1/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet when the width and thickness of the steel sheet are W and t, respectively, in the rolling direction cross section of the steel sheet. It means the organization at the position of / 4t.
  • predetermined elongation characteristics means that the total elongation (t-EL) is 11% or more when the steel plate thickness is more than 4.5 mm and 5 mm or less, and the total elongation (t-EL) is when the steel plate thickness is more than 5 mm and 10 mm or less. ) Is 12% or more, the total elongation (t-EL) is 13% or more when the steel plate thickness is more than 10 mm and 15 mm or less, and the total elongation (t-EL) is 14% or more when the steel plate thickness is more than 15 mm and 20 mm or less.
  • Total elongation (t-EL) is 15% or more when the thickness is more than 20 mm and 25 mm or less, total elongation (t-EL) is 16% or more when the steel plate thickness is more than 25 mm and 30 mm or less, and total when the steel plate thickness is more than 30 mm and 40 mm or less. When the elongation (t-EL) is 17% or more and the steel plate thickness is more than 40 mm and 50 mm or less, the total elongation (t-EL) is 18% or more.
  • the other metallographic structure is not particularly limited, but if it is desired to obtain better ductility, it is preferable to have the metallographic structure shown below.
  • “%” means “area%”.
  • the area ratio of pearlite is preferably 5 to 20%.
  • the area ratio of pearlite is more preferably 10 to 15%.
  • Bainite 10% or less
  • the metal structure is mainly ferrite and preferably contains a predetermined amount of pearlite.
  • the area ratio of bainite is preferably 10% or less, and more preferably 5% or less. Bainite may not be included, that is, the area ratio of bainite may be 0%.
  • Ferrite Ferrite has a structure with excellent ductility. The higher the area ratio of ferrite, the better the elongation characteristic EL can be. Therefore, the structures other than pearlite and bainite are ferrite.
  • the area ratio of the metal structure is calculated as follows. As described above, first, the sample is taken from the position of 1/4 W from the end face of the steel sheet and 1 / 4t from the surface of the steel sheet. Then, the rolling direction cross section (so-called L direction cross section) of the sample is observed.
  • the sample is night-game-etched, and after etching, observation is performed with a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope. Then, image analysis is performed on the obtained tissue photograph, and the area ratio of each is determined by using ferrite as the white color, pearlite as the black color, and bainite as the gray color.
  • the average aspect ratio and average grain size of ferrite and the average dislocation density in ferrite are also within the ranges shown below.
  • Average aspect ratio of ferrite 1.0-1.5
  • the average aspect ratio is preferably 1.5 or less.
  • the lower limit of the average aspect ratio is 1.0, which makes the ferrite grains spherical.
  • Average grain size of ferrite 5 to 20 ⁇ m
  • the average crystal grain size is preferably 20 ⁇ m or less. Further, the finer the ferrite grain is, the more preferable it is, but since it is industrially difficult to realize a ferrite grain of less than 5 ⁇ m, the lower limit is set to 5 ⁇ m.
  • the average aspect ratio and average grain size of ferrite are measured by the above-mentioned microscopic observation. Specifically, each ferrite is ellipsically approximated by image analysis, and the aspect ratio of ferrite is obtained by dividing the major axis length by the minor axis length. Similarly, the area of each ferrite is obtained by image analysis, and the diameter of a circle equal to this area is obtained to obtain the crystal grain size of the ferrite. Then, the average aspect ratio and the average grain size are obtained by calculating the average values of the aspect ratio and the diameter of all the ferrites in the visual field, respectively.
  • the average dislocation density in ferrite is preferably 7.0 ⁇ 10 12 / m 2 or less.
  • the preferred upper limit of the average dislocation density is 6.0 ⁇ 10 12 / m 2 .
  • Number of coarse inclusions in the center of the plate thickness Density: 10 pieces / mm 2 or less
  • Coarse inclusions sulfides or oxides such as MnS and alumina (Al 2 O 3 )
  • voids ductile fractures
  • the number density of inclusions having a length of 5 ⁇ m or more and existing in the region between the 3 / 5t position is preferably 10 pieces / mm 2 or less.
  • the inclusions shall be measured by particle analysis using a scanning electron microscope (SEM).
  • the average value of Vickers hardness in the surface layer region is preferably 80 to 105% of the average value of Vickers hardness in the central region.
  • the average value of Vickers hardness in each region shall be obtained by a 1 mm pitch Vickers hardness test.
  • the test force is 10 kgf (98N).
  • the steel sheet according to the present invention preferably has the strength required for use as, for example, a crude oil tank. Specifically, it is preferable that the yield stress (YS) is 235 MPa or more and the tensile strength (TS) is 400 to 620 MPa.
  • the total elongation (t-EL) is 19% or more when the steel plate thickness is more than 4.5 mm and 5 mm or less, and the total elongation (t-EL) is when the steel plate thickness is more than 5 mm and 10 mm or less. ) Is 22% or more, the total elongation (t-EL) is 23% or more when the steel plate thickness is more than 10 mm and 15 mm or less, and the total elongation (t-EL) is 25% or more when the steel plate thickness is more than 15 mm and 20 mm or less.
  • the total elongation (t-EL) is 26% or more
  • the total elongation (t-EL) is 27% or more
  • the total elongation (t-EL) is 30% or more.
  • the tensile strength (TS), yield stress (YS), and total elongation (t-EL) were taken from the center of the plate thickness in the direction perpendicular to the rolling direction based on JIS Z 2241: 2011. It was measured using a test piece. Specifically, the yield stress (YS) is the proof stress of the permanent elongation method when the permanent elongation is 0.2%, and the total elongation (t-EL) is the total elongation at break.
  • (D) Steel Sheet Manufacturing Method The steel sheet manufacturing conditions according to the present invention are not particularly limited, but the refining step, continuous casting step, heating step, descaling step, hot rolling step, and cooling step described later are performed in this order. Can be manufactured at. Each process will be described.
  • (A) Refining process Molten steel is manufactured in the refining process.
  • a known method may be adopted for the refining process, and there is no particular limitation.
  • the amount of dissolved oxygen in the molten steel is adjusted to 40 ppm or less by a vacuum degassing device when adjusting the composition of the molten steel.
  • a vacuum degassing device When adjusting the dissolved oxygen content of the molten steel to 40 ppm or less, for example, the degree of vacuum of the RH vacuum degassing device is 1 to 5 torr, and the molten steel is refluxed for 1 to 3 minutes for adjustment.
  • Al is added so that the final content of Al is 0.002 to 0.050%, and the dissolved oxygen content of the molten steel is adjusted to 10 ppm or less.
  • one or more selected from Ca, Mg and REM are added so that the total final content of Ca, Mg and REM is 0.0005 to 0.0080%.
  • Ca, Mg, and REM are preferentially sulfided to suppress the formation of MnS. If the amount of dissolved oxygen exceeds 10 ppm, it will be oxidized when Ca, Mg, and REM are added, and sulfide control may not be sufficient.
  • the vacuum degree of the RH vacuum degassing device is 1 to 5 torr, the molten steel is refluxed for 10 to 60 minutes, and the dissolved oxygen content of the molten steel is adjusted to 10 ppm or less. To do.
  • the degree of vacuum is 1 to 5 torr and the molten steel is refluxed for 10 to 60 minutes, the amount of dissolved oxygen cannot be reduced to 10 ppm or less. Further, the smaller the dissolved oxygen amount is, the better, and it is not necessary to set the lower limit of the dissolved oxygen amount of the molten steel.
  • (B) Continuous Casting Step molten steel is continuously cast to produce steel pieces having the above-mentioned chemical composition.
  • a known method may be adopted, and there is no particular limitation.
  • the gap between the casting rolls is set in the range of 0.2 to 0.7 in the central solid phase ratio of the slab, which is the final stage of solidification of the slab. Is preferably narrowed to 0.2 mm to 3.0 mm per 1 m in the casting traveling direction and cast while being reduced.
  • the maximum concentration of P in the central portion of the plate thickness can be adjusted in the range of 0.02 to 0.20%.
  • the central solid phase ratio referred to here can be defined as the solid phase ratio of the molten portion in the slab width direction and in the central portion in the slab thickness direction, and can be obtained by heat transfer and solidification calculation. ing.
  • the gap between the casting rolls is more preferably narrowed to 0.5 to 2.0 mm per 1 m in the casting traveling direction and lightly reduced, and further narrowed to 0.7 to 1.5 mm per 1 m in the casting traveling direction and lightly reduced. preferable. It is preferable to reduce the pressure lightly, but in the case of a component having a low P content, the pressure reduction may not be applied.
  • (C) Heating step The steel pieces are heated in order to perform hot rolling on the steel pieces.
  • O 2 concentration in the atmosphere of 1.0% by volume, and held at a heating temperature of less than 950 ° C. or higher 1200 ° C. 30 ⁇ 120 minutes, thermal extraction The temperature is 950 ° C. or higher and lower than 1200 ° C.
  • an oxide scale mainly composed of Fe is formed on the surface of the steel sheet.
  • Mo and W which are elements nobler than Fe, are not contained in the oxidation scale, but can be concentrated in the surface layer portion of the steel sheet immediately below the scale to contain the solid solution Mo and the solid solution W.
  • the heating temperature is less than 950 ° C., or the holding time is less than 30 minutes, the oxide scale formed is thin and Mo and W are concentrated on the surface layer of the steel sheet. May become insufficient, and the total content of the solid solution Mo and the solid solution W may be insufficient.
  • the ferrite grains may become coarse.
  • the O 2 concentration is preferably 10.0% by volume or less, and more preferably 5.0% by volume or less.
  • heating is performed under the following conditions. Is preferable.
  • the above-mentioned heating temperature is 1100 ° C. or lower and the heating extraction temperature is 1100 ° C. or lower.
  • the heat extraction temperature is set to 1100 ° C. or lower, the austenite ( ⁇ ) particles are refined to make the ferrite ( ⁇ ) particles finer, the aspect ratio of the ferrite ( ⁇ ) particles is reduced, and the elongation characteristics are improved.
  • the holding time when heating the steel piece affects the miniaturization of ferrite grains.
  • the holding time is preferably 80 minutes or less, and when the average crystal grain size of ferrite is to be 20 ⁇ m or less, the holding time is 60 minutes or less. Is preferable.
  • the oxide scale mainly composed of Fe on the surface of the steel sheet is removed, and Mo and W directly under the oxide scale are hot-rolled in a state where the surface layer of the steel piece is concentrated.
  • the solid solution Mo and the solid solution W can be concentrated on the surface layer of the steel sheet.
  • the descaling method is not particularly limited as long as the above-mentioned oxidation scale can be removed, and a known method may be used.
  • (E) Hot rolling process steel pieces are hot-rolled to obtain steel sheets.
  • the hot rolling process includes rough rolling and finish rolling.
  • finish rolling conditions in addition to the optimization of the heating conditions as described above. More specifically, when subjected to hot rolling, after rough rolling, in the temperature range of the surface temperature of the steel strip is Ar 3 -30 °C ⁇ T rex °C , under the conditions cumulative rolling reduction is 50-75% It is preferable to perform finish rolling.
  • Ar 3 is the ferrite transformation start temperature when the steel is cooled, and is obtained by the following equation (iv).
  • Trex means the recrystallization start temperature at which the growth of crystal grains starts, and is calculated by the following formula (v).
  • Nb mass of solid solution obtained by the following formula (vi)
  • sol Nb
  • Nb *] sol. Nb Nb ⁇ sol.
  • [Nb *] Nb
  • Nb (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 ⁇ N) ⁇ ⁇ ⁇ (vi)
  • T in the above formula represents a heating extraction temperature (° C.) of a steel piece.
  • the finish rolling by performing at Ar 3 -30 ° C. or more, can suppress the formation of stretched ferrite. Further, by performing the finish rolling in the unrecrystallized region of Trex or less, it is possible to suppress the coarsening of ferrite.
  • the cumulative reduction rate is 50% or more, the number of ferrite nucleation sites in austenite increases, the ferrite can be granulated, and the ⁇ ⁇ ⁇ transformation temperature can be increased. On the other hand, if the cumulative reduction rate exceeds 75%, the productivity deteriorates. Therefore, the cumulative reduction rate is preferably 50 to 75%, more preferably 55 to 65%.
  • Cooling step The steel sheet after hot rolling is cooled to room temperature.
  • air cooling may be performed at an average cooling rate of 1 ° C./sec or less, or cooling may be performed with cooling water and water cooling may be performed at an average cooling rate of more than 1 ° C./sec.
  • the surface temperature of the steel sheet to a temperature of Ar 3 -150 °C ⁇ Ar 3 -50 °C, 1 °C / sec greater than the average cooling 20 ° C. / sec or less. It is preferable to perform water cooling at a rate, and after the water cooling, perform air cooling at an average cooling rate of 1 ° C./sec or less.
  • the cooling stop temperature in the water-cooling in a range of from Ar 3 -150 °C ⁇ Ar 3 -50 °C prevent lowering the transformation temperature, it is possible to suppress the dislocation density increases or bainite formation in ferrite grains.
  • the average cooling rate of water cooling is set to 20 ° C./sec or less, it is possible to prevent the transformation temperature from becoming low. Since water cooling is effective as long as it is equal to or higher than the cooling rate of air cooling, the lower limit of the average cooling rate of water cooling is set to more than 1 ° C./sec.
  • the average dislocation density in ferrite can be controlled to 7.0 ⁇ 10 12 / m 2 or less, and the ratio of Vickers hardness in the surface layer region to the central region is 80 to 105%. Can be a range.
  • a steel plate having a thickness of 5 to 50 mm was prototyped according to the manufacturing conditions shown in Tables 2 and 3.
  • a descaling step was carried out between the heating step and the rolling step to remove the Fe-based oxide scale formed on the surface of the steel sheet.
  • amount of oxygen before Ca, Mg, REM in Table 2 means the amount of dissolved oxygen before adding one or more selected from Ca, Mg and REM.
  • the cooling rate (° C./s) in the column of cooling conditions in Table 3 is a cooling rate at 1/2 thickness obtained by heat conduction analysis by a known difference method from the measured surface temperature.
  • Air cooling described in the cooling pattern column of Table 3 is an example of air cooling without water cooling (accelerated cooling), and “partially water cooling” is air cooling after rolling and then partially water cooling. This is an example of performing.
  • the metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, in the rolling direction cross section of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, it is 1/4 W from the end face of the steel sheet and 1/4 t from the surface of the steel sheet. A test piece for observing the metallographic structure was cut out from the position.
  • the rolling direction cross section (so-called L direction cross section) of the above test piece was subjected to nightl etching, and after etching, observation was performed with a visual field of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope.
  • the area ratios of ferrite, pearlite, and bainite were determined by performing image analysis on the obtained microstructure photograph.
  • the average aspect ratio and average grain size of ferrite were measured by the following procedure. Each ferrite identified in the field of view was ellipsically approximated by image analysis, and the aspect ratio of ferrite was obtained by dividing the major axis length by the minor axis length. Similarly, the area of each ferrite was determined by image analysis, and the diameter of a circle equal to this area was determined to determine the grain size of the ferrite. Then, the average aspect ratio and the average grain size were obtained by calculating the average values of the aspect ratio and the diameter of all the ferrites in the visual field, respectively.
  • the total content (mass%) of the solid solution Mo and the solid solution W on the surface layer of the steel sheet was measured by the following procedure. First, two test pieces having a thickness of 1 mm were cut out from the surface of the steel sheet, and the contents of W and Mo in the test pieces were measured by using ICP emission spectroscopy for one of the test pieces.
  • the contents of the solid solution W and the solid solution Mo were obtained by obtaining the difference between the contents of the W and Mo in the extraction residue from the contents of W and Mo in the test piece.
  • the number density of coarse inclusions in the center of the plate thickness was measured. Specifically, particle analysis by SEM is performed on the number density of inclusions having a length of 5 ⁇ m or more existing in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet. Measured by.
  • the region between the position of 2/5 t from the surface of the steel sheet and the position of 3/5 t from the surface of the steel sheet is measured by EPMA, the maximum value of P concentration is obtained, and the maximum concentration of P in the central portion is obtained.
  • the measurement conditions were an accelerating voltage: 15 kV, a beam diameter: 20 ⁇ m, an irradiation time: 20 ms, and a measurement pitch: 20 ⁇ m.
  • TS tensile strength
  • YS yield stress
  • t-EL total elongation
  • ⁇ Corrosion test 1, 2> A test piece having a length of 40 mm in the rolling direction, a length of 40 mm in the width direction, and a length of 4 mm in the thickness direction was collected from the surface of the steel sheet. The cut surface (other than the surface) was coated with paint, and the iron oxide (scale) on the surface of the steel sheet was removed by wet polishing of No. 600 to prepare a test piece in which the base iron was exposed only on the surface of the 40 mm ⁇ 40 mm steel sheet.
  • the immersion conditions were a liquid temperature of 30 ° C. and an immersion time of 24 hours to 4 weeks, and the corrosion weight loss was measured to evaluate the corrosion rate.
  • the corrosive liquid composition simulates the environmental conditions when local corrosion occurs in an actual steel structure, and the rate of local corrosion progresses in the actual environment as the rate of corrosion in the corrosion test decreases. Will be done.
  • ⁇ Corrosion test 3> A test piece having a length of 40 mm in the rolling direction, a length of 40 mm in the width direction, and a length of 4 mm in the thickness direction was collected from the surface of the steel sheet. The cut surface (other than the surface) was coated with paint, and the iron oxide (scale) on the surface of the steel sheet was removed by wet polishing of No. 600 to prepare a test piece in which the base iron was exposed only on the surface of the 40 mm ⁇ 40 mm steel sheet. Then, using the test piece, the corrosion rate and the sludge formation rate mainly composed of solid S were evaluated by the following procedure.
  • an aqueous NaCl solution was applied to the surface of the test piece so that the amount of NaCl adhered was 1000 mg / m 2 , dried, and horizontally placed on a constant temperature heater plate in the test chamber. Then, a gas adjusted to a constant dew point (30 ° C.) was sent into the test chamber. Gas used is, CO 2: 12 vol%, H 2 S: 500ppm, O 2: 5 by volume%, N 2: having a composition the balance.
  • test Nos. 1 to 16, 18, 20 to 32, 34 to 37, 39, 40 and 43 to 50 any corrosion It can be seen that the test also shows excellent corrosion resistance.
  • test No. which satisfies all the above-mentioned suitable conditions. It can be seen that 15, 16, 18, 21-24, 26-31, 35 and 37 have a particularly excellent strength-ductility balance.
  • Test No. In 17, 19, 33, 38, 41 and 42 the result was that the corrosion resistance was inferior.
  • the test No. In Nos. 17 and 38 the O 2 concentration in the atmosphere during heating before hot spreading was low
  • Test No. In 19 and 33 since the holding time at the time of heating was short, the formation of the scale was insufficient, and the concentration of Mo and W just below the scale was not sufficiently generated.
  • test No. 41 and 42 steel U containing neither Mo nor W was used.
  • the total contents of the solid solution Mo and the solid solution W in the surface layer portion of the steel sheet did not satisfy the provisions of the present invention, resulting in inferior corrosion resistance.
  • Test No. which is a comparative example. In No. 51, since the heating time in the heating step was too long, the crystal grains became coarse and the strength-ductility balance deteriorated.
  • the steel sheet according to the present invention it is possible to obtain a steel sheet having excellent corrosion resistance against corrosive gas components, salt and the like contained in crude oil. Therefore, the steel sheet according to the present invention can be suitably used for a crude oil tank.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Continuous Casting (AREA)

Abstract

A steel sheet that has a chemical composition that is, by mass%, 0.050%–0.200% C, 0.100%–1.000% Si, 0.50%–2.00% Mn, no more than 0.030% P, no more than 0.010% S, 0.002%–0.050% Al, 0.0010%–0.0060% N, 0.0005%–0.0060% O, 0.003%–0.020% Ti, 0.01%–1.50% Cu, 0%–0.0080% Ca, 0%–0.0080% Mg, 0%–0.0080% REM, 0%–0.20% Mo, 0%–0.50% W, 0%–0.030% Nb, 0%–0.050% V, 0%–1.00% Ni, 0%–0.50% Cr, 0%–0.0030% B, 0%–0.30% Sb, 0%–0.30% Sn, 0%–0.30% Pb, 0%–0.30% As, 0%–0.30% Bi, 0%–0.50% Ta, and 0%–0.50% Zr, the remainder being Fe and impurities. The chemical composition also satisfies 0.01≤Mo+W≤0.70, and the total solid-solution Mo and solid-solution W content of a surface layer part of the steel sheet is, by mass%, at least 0.005%. The average ferrite crystal grain size at a 1/4t position is no more than 60 μm.

Description

鋼板およびその製造方法Steel plate and its manufacturing method
　本発明は、鋼板およびその製造方法に関する。 The present invention relates to a steel sheet and a method for manufacturing the same.
　原油タンカー、または地上もしくは地下原油タンクなどの、原油を輸送または貯蔵する鋼製油槽（以下、これらを総称して、「原油油槽」という。）には、強度および溶接性に優れた溶接構造用鋼が使用されている。また、原油油槽として使用される鋼には、原油中に含まれる腐食性ガス成分、塩分等に対する優れた耐食性が求められる（例えば、特許文献１を参照。）。 For steel oil tanks that transport or store crude oil, such as crude oil tankers or above-ground or underground crude oil tanks (hereinafter collectively referred to as "crude oil tanks"), for welded structures with excellent strength and weldability. Steel is used. Further, steel used as a crude oil tank is required to have excellent corrosion resistance against corrosive gas components, salts and the like contained in crude oil (see, for example, Patent Document 1).
　特許文献１には、鋼製油槽で生じる原油腐食に対して、優れた耐全面腐食性及び耐局部腐食性を示し、さらに固体Ｓを含む腐食生成物（スラッジ）の生成を抑制できる溶接構造用の原油油槽用鋼、原油油槽鋼の製造方法、原油油槽、および原油油槽の防食方法が開示されている。 Patent Document 1 describes for a welded structure that exhibits excellent total corrosion resistance and local corrosion resistance against crude oil corrosion that occurs in a steel oil tank, and can further suppress the formation of corrosion products (sludge) containing solid S. The method for producing crude oil tank steel, the method for producing crude oil tank steel, the crude oil tank, and the method for preventing corrosion of the crude oil tank are disclosed.
特開２００４－２０４３４４号公報Japanese Unexamined Patent Publication No. 2004-204344
　特許文献１に記載の原油油槽用鋼は、固溶状態のＭｏおよびＷを所定量以上含有するため、優れた耐食性を有している。しかしながら、特許文献１では、熱間圧延前の鋼片を加熱する際に、鋼片を１２００～１３５０℃の加熱温度とし、その状態を２～１００時間保持し続ける、拡散熱処理を行うことで、ＭｏおよびＷの固溶量を確保している。このため、加熱のための燃料ガスを多く使用することになり、加熱する時間が長くなるため、製造コストの増加と生産性を阻害する問題が生じてしまい、改善の余地が残されている。加えて、高温での長時間加熱により結晶粒が粗大化し、強度－延性バランスが劣化する問題がある。 The crude oil tank steel described in Patent Document 1 contains a predetermined amount or more of Mo and W in a solid solution state, and therefore has excellent corrosion resistance. However, in Patent Document 1, when the steel piece before hot rolling is heated, the steel piece is set to a heating temperature of 1200 to 1350 ° C., and the state is maintained for 2 to 100 hours by performing diffusion heat treatment. The solid solution amount of Mo and W is secured. For this reason, a large amount of fuel gas for heating is used, and the heating time becomes long, which causes problems such as an increase in manufacturing cost and hindering productivity, and there is room for improvement. In addition, there is a problem that the crystal grains become coarse due to long-term heating at a high temperature, and the strength-ductility balance deteriorates.
　本発明は、上記の課題を解決し、原油中に含まれる腐食性ガス成分、塩分等に対する耐食性に優れる鋼板およびその製造方法を提供することを目的とする。 An object of the present invention is to solve the above-mentioned problems and to provide a steel sheet having excellent corrosion resistance against corrosive gas components, salts and the like contained in crude oil, and a method for producing the same.
　本発明者らは上記課題に対して詳細な検討を行った結果、以下の知見を得るに至った。 As a result of detailed studies on the above problems, the present inventors have obtained the following findings.
　鋼中に固溶状態のＭｏおよびＷを確保することで、原油油槽で生じる原油腐食に対して、優れた耐食性を発揮する。一方、ＭｏおよびＷの固溶量を確保するためには、拡散熱処理を実施する必要があり、これが結晶粒の粗大化を招く要因となっていた。 By securing Mo and W in a solid solution state in steel, it exhibits excellent corrosion resistance against crude oil corrosion that occurs in crude oil tanks. On the other hand, in order to secure the solid solution amounts of Mo and W, it is necessary to carry out diffusion heat treatment, which has been a factor causing coarsening of crystal grains.
　そこで、本発明者らは拡散熱処理を行わずに、耐食性を向上させる方法について検討を行った。その結果、加熱時の酸素濃度を高くし、鋼表面に比較的厚いスケールを形成することで、スケールの直下の鋼板表層部にＭｏおよびＷを濃化させることが可能になり、鋼板表層部に固溶状態のＭｏおよびＷを確保できて、それにより耐食性を大幅に向上できることを見出した。 Therefore, the present inventors have studied a method for improving corrosion resistance without performing diffusion heat treatment. As a result, by increasing the oxygen concentration during heating and forming a relatively thick scale on the steel surface, it becomes possible to concentrate Mo and W on the surface layer of the steel sheet directly under the scale, and on the surface of the steel sheet. It has been found that Mo and W in a solid solution state can be secured, and thereby the corrosion resistance can be significantly improved.
　本発明は、上記知見に基づいてなされたものであり、下記の鋼板およびその製造方法を要旨とする。 The present invention has been made based on the above findings, and the gist of the following steel sheet and its manufacturing method is.
　（１）化学組成が、質量％で、
　Ｃ　：０．０５０～０．２００％、
　Ｓｉ：０．１００～１．０００％、
　Ｍｎ：０．５０～２．００％、
　Ｐ　：０．０３０％以下、
　Ｓ　：０．０１０％以下、
　Ａｌ：０．００２～０．０５０％、
　Ｎ　：０．００１０～０．００６０％、
　Ｏ　：０．０００５～０．００６０％、
　Ｔｉ：０．００３～０．０２０％、
　Ｃｕ：０．０１～１．５０％、
　Ｃａ：０～０．００８０％、
　Ｍｇ：０～０．００８０％、
　ＲＥＭ：０～０．００８０％、
　Ｍｏ：０～０．２０％、
　Ｗ　：０～０．５０％、
　Ｎｂ：０～０．０３０％、
　Ｖ　：０～０．０５０％、
　Ｎｉ：０～１．００％、
　Ｃｒ：０～０．５０％、
　Ｂ　：０～０．００３０％、
　Ｓｂ：０～０．３０％、
　Ｓｎ：０～０．３０％、
　Ｐｂ：０～０．３０％、
　Ａｓ：０～０．３０％、
　Ｂｉ：０～０．３０％、
　Ｔａ：０～０．５０％、
　Ｚｒ：０～０．５０％、
　残部：Ｆｅおよび不純物であり、
　下記（ｉ）式を満足し、
　鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量が、質量％で、０．００５％以上であり、
　鋼板の圧延方向断面において、前記鋼板の厚さをｔとした時に、前記鋼板の表面から１／４ｔの位置におけるフェライトの平均結晶粒径が６０μｍ以下である、
　鋼板。
　０．０１≦Ｍｏ＋Ｗ≦０．７０　　　・・・（ｉ）
　但し、上記式中の元素記号は各元素の含有量（質量％）を表し、含まれない場合はゼロとする。 (1) The chemical composition is mass%
C: 0.050 to 0.200%,
Si: 0.100 to 1.000%,
Mn: 0.50 to 2.00%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.002 to 0.050%,
N: 0.0010 to 0.0060%,
O: 0.0005 to 0.0060%,
Ti: 0.003 to 0.020%,
Cu: 0.01-1.50%,
Ca: 0 to 0.0080%,
Mg: 0 to 0.0080%,
REM: 0-0.0080%,
Mo: 0 to 0.20%,
W: 0 to 0.50%,
Nb: 0 to 0.030%,
V: 0 to 0.050%,
Ni: 0 to 1.00%,
Cr: 0 to 0.50%,
B: 0 to 0.0030%,
Sb: 0 to 0.30%,
Sn: 0 to 0.30%,
Pb: 0 to 0.30%,
As: 0 to 0.30%,
Bi: 0 to 0.30%,
Ta: 0 to 0.50%,
Zr: 0 to 0.50%,
Remaining: Fe and impurities,
Satisfy the following equation (i)
The total content of the solid solution Mo and the solid solution W in the surface layer of the steel sheet is 0.005% or more in mass%.
In the rolling direction cross section of the steel sheet, when the thickness of the steel sheet is t, the average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet is 60 μm or less.
Steel plate.
0.01 ≤ Mo + W ≤ 0.70 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
　（２）前記化学組成が、質量％で、
　Ｓｉ：０．２００～１．０００％、
　Ｐ　：０．０１５％以下、
　Ｓ　：０．００３％以下、
　であり、
　下記（ii）式および（iii）式を満足し、
　下記（iv）式で求められるフェライト変態開始温度Ａｒ３が７６０～８２０℃であり、
　前記鋼板の表面から１／４ｔの位置における金属組織が、面積％で、
　パーライト：５～２０％、
　ベイナイト：１０％以下、
　残部：フェライトであり、
　前記鋼板の表面から１／４ｔの位置におけるフェライトの平均アスペクト比が１．０～１．５であり、
　前記鋼板の表面から１／４ｔの位置におけるフェライトの平均結晶粒径が５～２０μｍである、
　上記（１）に記載の鋼板。
　０．０００５≦Ｃａ＋Ｍｇ＋ＲＥＭ≦０．００８０　　　・・・（ii）
　０．５≦Ｔｉ／Ｎ≦４．０　　　・・・（iii）
　Ａｒ３＝９１０－３１０×Ｃ＋６５×Ｓｉ－８０×Ｍｎ－２０×Ｃｕ－５５×Ｎｉ－１５×Ｃｒ－８０×Ｍｏ　　　・・・（iv）
　但し、上記式中の元素記号は各元素の含有量（質量％）を表し、含まれない場合はゼロとする。 (2) The chemical composition is mass%.
Si: 0.200 to 1.000%,
P: 0.015% or less,
S: 0.003% or less,
And
Satisfy the following equations (ii) and (iii),
The ferrite transformation start temperature Ar 3 obtained by the following equation (iv) is 760 to 820 ° C.
The metallographic structure at a position 1 / 4t from the surface of the steel sheet is in area%.
Perlite: 5-20%,
Bainite: 10% or less,
Remaining: Ferrite,
The average aspect ratio of ferrite at a position 1 / 4t from the surface of the steel sheet is 1.0 to 1.5.
The average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet is 5 to 20 μm.
The steel sheet according to (1) above.
0.0005 ≤ Ca + Mg + REM ≤ 0.0080 ... (ii)
0.5 ≤ Ti / N ≤ 4.0 ... (iii)
Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iv)
However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
　（３）前記鋼板の表面から１／４ｔの位置におけるフェライト中の平均転位密度が７．０×１０１２／ｍ２以下であり、
　１ｍｍピッチのビッカース硬さの試験で、前記鋼板の表面と、前記鋼板の表面から１／４ｔの位置との間の領域におけるビッカース硬さの平均値が、前記鋼板の表面から１／４ｔの位置と、前記鋼板の表面から３／４ｔの位置との間の領域におけるビッカース硬さの平均値の８０～１０５％である、
　上記（２）に記載の鋼板。 (3) The average dislocation density in ferrite at a position 1 / 4t from the surface of the steel sheet is 7.0 × 10 12 / m 2 or less.
In the 1 mm pitch Vickers hardness test, the average value of Vickers hardness in the region between the surface of the steel sheet and the position of 1 / 4t from the surface of the steel sheet is the position of 1 / 4t from the surface of the steel sheet. Is 80 to 105% of the average value of Vickers hardness in the region between the surface of the steel sheet and the position of 3/4 t.
The steel sheet according to (2) above.
　（４）前記鋼板の表面から２／５ｔの位置と、前記鋼板の表面から３／５ｔの位置との間の領域において存在する、長さ５μｍ以上の介在物の個数密度が１０個／ｍｍ２以下である、
　上記（１）から（３）までのいずれか１項に記載の鋼板。 (4) The number density of inclusions having a length of 5 μm or more existing in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet is 10 pieces / mm 2 Is below,
The steel sheet according to any one of (1) to (3) above.
　（５）前記鋼板の表面から２／５ｔの位置と、前記鋼板の表面から３／５ｔの位置との間の領域における、Ｐの最大濃度が、質量％で、０．０２～０．２０％である、
　上記（１）から（４）までのいずれか１項に記載の鋼板。 (5) The maximum concentration of P in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet is 0.02 to 0.20% in mass%. Is,
The steel sheet according to any one of (1) to (4) above.
　（６）溶鋼を製造する精錬工程と、
　前記溶鋼を連続鋳造して、上記（１）に記載の化学組成を有する鋼片を製造する連続鋳造工程と、
　得られた前記鋼片を加熱する加熱工程と、
　加熱後の鋼片にデスケーリングを施すデスケーリング工程と、
　デスケーリング後の鋼片に対して熱間圧延を施して鋼板とする熱間圧延工程と、
　熱間圧延後の前記鋼板を室温まで冷却する冷却工程と、を備え、
　前記加熱工程において、前記鋼片に対して、Ｏ２濃度が１．０体積％以上の雰囲気で、９５０℃以上１２００℃未満の加熱温度で３０～１２０分間保持し、加熱抽出温度を９５０℃以上１２００℃未満とする、
　鋼板の製造方法。 (6) Refining process for manufacturing molten steel and
A continuous casting step of continuously casting the molten steel to produce a steel piece having the chemical composition described in (1) above.
A heating step of heating the obtained steel piece and
A descaling process that descales the heated steel pieces,
The hot rolling process of hot rolling the descaled steel pieces to make a steel sheet,
A cooling step of cooling the steel sheet after hot rolling to room temperature is provided.
Wherein in the heating step, with respect to the steel strip, an O 2 concentration is not less than 1.0 vol% atmosphere, and held at a heating temperature of less than 950 ° C. or higher 1200 ° C. 30 ~ 120 minutes, the heating extraction temperature 950 ° C. or higher Keep below 1200 ° C,
Steel sheet manufacturing method.
　（７）溶鋼を製造する精錬工程と、
　前記溶鋼を連続鋳造して、上記（２）に記載の化学組成を有する鋼片を製造する連続鋳造工程と、
　得られた前記鋼片を加熱する加熱工程と、
　加熱後の鋼片にデスケーリングを施すデスケーリング工程と、
　デスケーリング後の鋼片に対して仕上圧延を含む熱間圧延を施して鋼板とする熱間圧延工程と、
　熱間圧延後の前記鋼板を室温まで冷却する冷却工程と、を備え、
　前記加熱工程において、前記鋼片に対して、Ｏ２濃度が１．０体積％以上の雰囲気で、９５０～１１００℃の加熱温度で３０～６０分間保持し、加熱抽出温度を９５０～１１００℃とし、
　前記熱間圧延工程において、前記鋼片の表面温度がＡｒ３－３０℃～Ｔｒｅｘ℃の温度範囲内で、累積圧下率が５０～７５％となる条件で前記仕上圧延を行う、
　鋼板の製造方法。
　但し、Ａｒ３は下記（iv）式で求められ、Ｔｒｅｘは結晶粒の成長が始まる再結晶開始温度を意味し、下記（ｖ）式で求められる。なお、下記式中の元素記号は各元素の含有量（質量％）を表す。
　Ａｒ３＝９１０－３１０×Ｃ＋６５×Ｓｉ－８０×Ｍｎ－２０×Ｃｕ－５５×Ｎｉ－１５×Ｃｒ－８０×Ｍｏ　　　・・・（iv）
　Ｔｒｅｘ＝－９１９００［Ｎｂ＊］２＋９４００［Ｎｂ＊］＋７７０　　　・・・（ｖ）
　但し、下記（vi）式で求められる固溶Ｎｂ量（質量％）を、ｓｏｌ．Ｎｂとした時に、
　Ｎｂ≧ｓｏｌ．Ｎｂの場合は、［Ｎｂ＊］＝ｓｏｌ．Ｎｂ
　Ｎｂ＜ｓｏｌ．Ｎｂの場合は、［Ｎｂ＊］＝Ｎｂ
　とする。
　ｓｏｌ．Ｎｂ＝（１０（－６７７０／（Ｔ＋２７３）＋２．２６））／（Ｃ＋１２／１４×Ｎ）　　　・・・（vi）
　なお、上記式中のＴは鋼片の加熱抽出温度（℃）を表す。 (7) Refining process for manufacturing molten steel and
A continuous casting step of continuously casting the molten steel to produce a steel piece having the chemical composition described in (2) above.
A heating step of heating the obtained steel piece and
A descaling process that descales the heated steel pieces,
A hot rolling process in which a steel piece after descaling is hot-rolled, including finish rolling, to form a steel sheet.
A cooling step of cooling the steel sheet after hot rolling to room temperature is provided.
In the heating step, with respect to the steel strip, an O 2 concentration is not less than 1.0 vol% atmosphere, and held at a heating temperature of 950 ~ 1100 ℃ 30 ~ 60 minutes, the heating extraction temperature of 950 ~ 1100 ° C. ,
In the hot rolling step, the surface temperature of the steel piece in a temperature range of Ar 3 -30 ℃ ~ T rex ℃ , performs the finish rolling under conditions where the cumulative rolling reduction is 50-75%,
Steel sheet manufacturing method.
However, Ar 3 is calculated by the following formula (iv), and Trex means the recrystallization start temperature at which the growth of crystal grains starts, and is calculated by the following formula (v). The element symbol in the following formula represents the content (mass%) of each element.
Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iv)
T rex = -91900 [Nb *] 2 +9400 [Nb *] + 770 ··· (v)
However, the amount of solid solution Nb (mass%) obtained by the following formula (vi) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 × N) ・ ・ ・ (vi)
In addition, T in the above formula represents a heating extraction temperature (° C.) of a steel piece.
　（８）前記冷却工程において、鋼板の表面温度がＡｒ３－１５０℃～Ａｒ３－５０℃の温度まで、１℃／秒超、２０℃／秒以下の平均冷却速度で水冷を行い、該水冷後、１℃／秒以下の平均冷却速度で空冷を行う、
　上記（７）に記載の鋼板の製造方法。 (8) In the cooling step, to a temperature of the surface temperature of the steel sheet Ar 3 -150 ℃ ~ Ar 3 -50 ℃, 1 ℃ / sec greater performs water cooling at an average cooling rate of 20 ° C. / sec or less, the water-cooling After that, air cooling is performed at an average cooling rate of 1 ° C./sec or less.
The method for manufacturing a steel sheet according to (7) above.
　（９）前記精錬工程において、真空脱ガス装置により溶鋼の溶存酸素量を４０ｐｐｍ以下に調整し、次いで、ＡｌをＡｌの最終含有量が０．００２～０．０５０％となるように添加して、溶鋼の溶存酸素量を１０ｐｐｍ以下に調整した後、Ｃａ、ＭｇおよびＲＥＭから選択される１種以上を、Ｃａ、ＭｇおよびＲＥＭの合計の最終含有量が０．０００５～０．００８０％となるように添加する、
　上記（６）から（８）までのいずれか１項に記載の鋼板の製造方法。 (9) In the refining step, the dissolved oxygen content of the molten steel is adjusted to 40 ppm or less by a vacuum degassing device, and then Al is added so that the final content of Al is 0.002 to 0.050%. After adjusting the dissolved oxygen content of the molten steel to 10 ppm or less, the total final content of Ca, Mg and REM of one or more selected from Ca, Mg and REM becomes 0.0005 to 0.0080%. To add,
The method for manufacturing a steel sheet according to any one of (6) to (8) above.
　（１０）前記連続鋳造工程において、鋳片の凝固末期である鋳片の中心固相率が０．２～０．７の範囲において、鋳造ロールの間隙を、鋳造進行方向１ｍにつき０．２ｍｍ～３．０ｍｍに狭めて圧下しながら鋳造する、
　上記（６）から（９）までのいずれか１項に記載の鋼板の製造方法。 (10) In the continuous casting step, in the range of the central solid phase ratio of the slab, which is the final stage of solidification of the slab, in the range of 0.2 to 0.7, the gap between the casting rolls is 0.2 mm to 1 m in the casting progress direction. Casting while narrowing to 3.0 mm and reducing
The method for manufacturing a steel sheet according to any one of (6) to (9) above.
　本発明によれば、原油中に含まれる腐食性ガス成分、塩分等に対する耐食性に優れる鋼板を得ることが可能になる。 According to the present invention, it is possible to obtain a steel sheet having excellent corrosion resistance against corrosive gas components, salts, etc. contained in crude oil.
　以下、本発明の各要件について詳しく説明する。 Hereinafter, each requirement of the present invention will be described in detail.
　（Ａ）化学組成
　各元素の限定理由は下記のとおりである。なお、以下の説明において含有量についての「％」は、「質量％」を意味する。 (A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".
　Ｃ：０．０５０～０．２００％
　Ｃは、パーライトを形成して強度を高めるのに有効な元素である。一方、Ｃ含有量が過剰であると、溶接性および継手靭性の確保が困難となる。そのため、Ｃ含有量は０．０５０～０．２００％とする。Ｃ含有量は０．０７０％以上または０．１００％以上であるのが好ましく、０．１８０％以下または０．１６０％以下であるのが好ましい。 C: 0.050 to 0.200%
C is an element effective for forming pearlite and increasing its strength. On the other hand, if the C content is excessive, it becomes difficult to secure weldability and joint toughness. Therefore, the C content is set to 0.050 to 0.200%. The C content is preferably 0.070% or more or 0.100% or more, and preferably 0.180% or less or 0.160% or less.
　Ｓｉ：０．１００～１．０００％
　Ｓｉは、安価な脱酸元素であり、固溶強化に有効であるとともに、変態点を上昇させてα中の転位密度低減に寄与する。一方、Ｓｉ含有量が過剰であると、溶接性および継手靭性を劣化させる。そのため、Ｓｉ含有量は０．１００～１．０００％とする。優れた延性を得たい場合には、Ｓｉ含有量は０．２００％以上であるのが好ましく、０．３００％以上であるのがより好ましい。また、Ｓｉ含有量は０．８００％以下であるのが好ましく、０．５００％以下であるのが好ましい。 Si: 0.100 to 1.000%
Si is an inexpensive deoxidizing element, which is effective in strengthening solid solution and raises the transformation point to contribute to the reduction of dislocation density in α. On the other hand, if the Si content is excessive, the weldability and joint toughness are deteriorated. Therefore, the Si content is set to 0.100 to 1.000%. When it is desired to obtain excellent ductility, the Si content is preferably 0.200% or more, and more preferably 0.300% or more. The Si content is preferably 0.800% or less, and preferably 0.500% or less.
　Ｍｎ：０．５０～２．００％
　Ｍｎは、母材の強度および靭性を向上させる元素として有効である。一方、Ｍｎ含有量が過剰であると、溶接性および継手靭性を劣化させる。そのため、Ｍｎ含有量は０．５０～２．００％とする。Ｍｎ含有量は０．８０％以上であるのが好ましく、０．９０％以上であるのがより好ましい。また、Ｍｎ含有量は１．６０％以下であるのが好ましく、１．５０％以下であるのがより好ましい。 Mn: 0.50 to 2.00%
Mn is effective as an element for improving the strength and toughness of the base material. On the other hand, if the Mn content is excessive, the weldability and joint toughness are deteriorated. Therefore, the Mn content is set to 0.50 to 2.00%. The Mn content is preferably 0.80% or more, more preferably 0.90% or more. The Mn content is preferably 1.60% or less, and more preferably 1.50% or less.
　Ｐ：０．０３０％以下
　Ｐは、不純物として鋼中に含まれる元素であり、耐食性を確保するためには、０．０３０％以下とする。また、延性および靭性を確保するためには、Ｐ含有量は少ないほど望ましく、０．０１５％以下であるのが好ましい。 P: 0.030% or less P is an element contained in steel as an impurity, and it should be 0.030% or less in order to ensure corrosion resistance. Further, in order to ensure ductility and toughness, the smaller the P content, the more desirable, and preferably 0.015% or less.
　Ｓ：０．０１０％以下
　Ｓは、不純物として鋼中に含まれる元素であり、耐食性を確保するためには、０．０１０％以下とする。また、延性および靭性を確保するためには、Ｓ含有量は少ないほど望ましく、Ｓ含有量は０．００３％以下であるのが好ましい。 S: 0.010% or less S is an element contained in steel as an impurity, and is 0.010% or less in order to ensure corrosion resistance. Further, in order to secure ductility and toughness, it is desirable that the S content is small, and the S content is preferably 0.003% or less.
　Ａｌ：０．００２～０．０５０％
　Ａｌは、重要な脱酸元素である。一方、Ａｌ含有量が過剰であると、鋼片の表面品位を損ない、靭性に有害な介在物を形成する。そのため、Ａｌ含有量は０．００２～０．０５０％とする。Ａｌ含有量は０．０１０％以上であるのが好ましく、０．０４０％以下であるのが好ましい。 Al: 0.002 to 0.050%
Al is an important deoxidizing element. On the other hand, if the Al content is excessive, the surface quality of the steel piece is impaired and inclusions harmful to toughness are formed. Therefore, the Al content is set to 0.002 to 0.050%. The Al content is preferably 0.010% or more, and preferably 0.040% or less.
　Ｎ：０．００１０～０．００６０％
　Ｎは、Ａｌと共に窒化物を形成し継手靭性を向上させる。一方、Ｎ含有量が過剰であると、固溶Ｎによる脆化および伸び特性の低下が生じる。そのため、Ｎ含有量は０．００１０～０．００６０％とする。Ｎ含有量は０．００２０％以上であるのが好ましく、０．００５０％以下であるのが好ましく、０．００４０％以下であるのがより好ましい。 N: 0.0010 to 0.0060%
N forms a nitride together with Al to improve joint toughness. On the other hand, if the N content is excessive, embrittlement and deterioration of elongation characteristics occur due to solid solution N. Therefore, the N content is set to 0.0010 to 0.0060%. The N content is preferably 0.0020% or more, preferably 0.0050% or less, and more preferably 0.0040% or less.
　Ｏ：０．０００５～０．００６０％
　Ｏは、後述するＣａ、Ｍｇ、ＲＥＭとともに酸化物を形成する。Ｏ含有量が過剰であると、酸化物が粗大化して延性および靭性が低下する。一方、Ｏ含有量は少ないほどよいが、過度に低減するためには、例えば、ＲＨ真空脱ガス装置での還流作業が長時間となり現実的ではない。そのため、Ｏ含有量は０．０００５～０．００６０％とする。 O: 0.0005 to 0.0060%
O forms an oxide together with Ca, Mg, and REM described later. If the O content is excessive, the oxide becomes coarse and ductility and toughness decrease. On the other hand, the smaller the O content is, the better, but in order to reduce it excessively, for example, the reflux work in the RH vacuum degassing device takes a long time, which is not realistic. Therefore, the O content is set to 0.0005 to 0.0060%.
　Ｔｉ：０．００３～０．０２０％
　Ｔｉは、微量の含有により母材および溶接部の組織微細化を通じて靭性向上に寄与する。一方、Ｔｉ含有量が過剰であると、溶接部を硬化させ著しく靭性を劣化させる。そのため、Ｔｉ含有量は０．００３～０．０２０％とする。Ｔｉ含有量は０．００６％以上であるのが好ましく、０．０１３％以下であるのが好ましい。 Ti: 0.003 to 0.020%
Ti contains a small amount and contributes to the improvement of toughness through the microstructure of the base metal and welded parts. On the other hand, if the Ti content is excessive, the welded portion is hardened and the toughness is significantly deteriorated. Therefore, the Ti content is set to 0.003 to 0.020%. The Ti content is preferably 0.006% or more, and preferably 0.013% or less.
　また、Ｎ含有量に対するＴｉ含有量の割合を０．５以上にすることにより、固溶Ｎを低減し、伸び特性を向上させるだけでなく、スラブの表面疵の発生を防止することが可能となる。さらに、Ｎ含有量に対するＴｉ含有量の割合を４．０以下にすることにより、ＴｉＣの生成を抑制し、伸び特性を向上させることができる。そのため、優れた延性を得たい場合には、Ｔｉ含有量はＮ含有量との関係において、下記（iii）式を満足することが好ましい。
　０．５≦Ｔｉ／Ｎ≦４．０　　　・・・（iii）
　但し、上記式中の元素記号は各元素の含有量（質量％）を表す。 Further, by setting the ratio of the Ti content to the N content to 0.5 or more, it is possible not only to reduce the solid solution N and improve the elongation characteristics, but also to prevent the occurrence of surface defects of the slab. Become. Further, by setting the ratio of the Ti content to the N content to 4.0 or less, the formation of TiC can be suppressed and the elongation characteristics can be improved. Therefore, when it is desired to obtain excellent ductility, it is preferable that the Ti content satisfies the following equation (iii) in relation to the N content.
0.5 ≤ Ti / N ≤ 4.0 ... (iii)
However, the element symbol in the above formula represents the content (mass%) of each element.
　Ｃｕ：０．０１～１．５０％
　Ｃｕは耐全面腐食性だけでなく、耐局部腐食性の向上に有効な元素である。さらに、固体Ｓの生成抑制にも効果がある。一方、Ｃｕ含有量が過剰であると、鋼片の表面割れの助長、継手靭性の劣化等、悪影響も顕在化する。そのため、Ｃｕ含有量は０．０１～１．５０％とする。Ｃｕ含有量は０．０３％以上であるのが好ましく、０．５０％以下であるのが好ましく、０．２０％未満であるのがより好ましい。 Cu: 0.01 to 1.50%
Cu is an element effective for improving not only overall corrosion resistance but also local corrosion resistance. Further, it is also effective in suppressing the formation of solid S. On the other hand, if the Cu content is excessive, adverse effects such as promotion of surface cracking of the steel piece and deterioration of joint toughness become apparent. Therefore, the Cu content is set to 0.01 to 1.50%. The Cu content is preferably 0.03% or more, preferably 0.50% or less, and more preferably less than 0.20%.
　Ｃａ：０～０．００８０％
　Ｍｇ：０～０．００８０％
　ＲＥＭ：０～０．００８０％
　Ｃａ、ＭｇおよびＲＥＭは、いずれも硫化物を形成することで粗大な介在物（延伸ＭｎＳ等）の生成を抑制するため、必要に応じて含有させてもよい。一方、いずれの含有量が０．００８０％を超えても効果は飽和し、粗大な酸化物または硫化物を形成して靭性および伸びを劣化させる。そのため、Ｃａ、ＭｇおよびＲＥＭの含有量は、いずれも０．００８０％以下とする。 Ca: 0 to 0.0080%
Mg: 0 to 0.0080%
REM: 0-0.0080%
Ca, Mg and REM may be contained as necessary because they suppress the formation of coarse inclusions (stretched MnS and the like) by forming sulfides. On the other hand, if any content exceeds 0.0080%, the effect is saturated and coarse oxides or sulfides are formed to deteriorate toughness and elongation. Therefore, the contents of Ca, Mg and REM are all 0.0080% or less.
　優れた延性を得たい場合には、これらの元素の合計含有量を０．０００５％以上とすることが好ましい。また、粗大な酸化物または硫化物による靭性および伸び特性の劣化を防止する観点からは、これらの元素の合計含有量を０．００８０％以下とすることが好ましい。 When it is desired to obtain excellent ductility, the total content of these elements is preferably 0.0005% or more. Further, from the viewpoint of preventing deterioration of toughness and elongation characteristics due to coarse oxides or sulfides, the total content of these elements is preferably 0.0080% or less.
　すなわち、下記（ii）式を満足することが好ましい。上記合計含有量は０．００１０％以上であるのがより好ましく、０．００１５％以上であるのがさらに好ましい。また、上記合計含有量は０．００６０％以下であるのがより好ましく、０．００４０％以下であるのがさらに好ましい。
　０．０００５≦Ｃａ＋Ｍｇ＋ＲＥＭ≦０．００８０　　　・・・（ii）
　但し、上記式中の元素記号は各元素の含有量（質量％）を表し、含まれない場合はゼロとする。 That is, it is preferable to satisfy the following equation (ii). The total content is more preferably 0.0010% or more, and further preferably 0.0015% or more. Further, the total content is more preferably 0.0060% or less, and further preferably 0.0040% or less.
0.0005 ≤ Ca + Mg + REM ≤ 0.0080 ... (ii)
However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
　ここで、本発明において、ＲＥＭはＳｃ、Ｙおよびランタノイドの合計１７元素を指し、前記ＲＥＭの含有量はこれらの元素の合計含有量を意味する。なお、ランタノイドは、工業的には、ミッシュメタルの形で添加される。 Here, in the present invention, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements. Lanthanoids are industrially added in the form of misch metal.
　Ｍｏ：０～０．２０％
　Ｗ　：０～０．５０％
　ＭｏおよびＷは、耐局部腐食性の向上に有効な元素である。そのため、ＭｏおよびＷの少なくともいずれかを含有し、かつ合計含有量を０．０１％以上とする必要がある。一方、Ｍｏは０．２０％、Ｗは０．５０％を超えて含有させると耐局部腐食性が逆に低下し、かつ溶接性や靭性を劣化させる。そのため、Ｍｏ含有量は０．２０％以下、Ｗ含有量は０．５０％以下とし、合計含有量を０．７０％以下とする。すなわち、下記（ｉ）式を満足する必要がある。
　０．０１≦Ｍｏ＋Ｗ≦０．７０　　　・・・（ｉ）
　但し、上記式中の元素記号は各元素の含有量（質量％）を表し、含まれない場合はゼロとする。 Mo: 0 to 0.20%
W: 0 to 0.50%
Mo and W are elements effective for improving local corrosiveness. Therefore, it is necessary to contain at least one of Mo and W, and the total content should be 0.01% or more. On the other hand, if Mo is contained in excess of 0.20% and W is contained in excess of 0.50%, the local corrosiveness is conversely lowered, and the weldability and toughness are deteriorated. Therefore, the Mo content is 0.20% or less, the W content is 0.50% or less, and the total content is 0.70% or less. That is, it is necessary to satisfy the following equation (i).
0.01 ≤ Mo + W ≤ 0.70 ... (i)
However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
　Ｍｏ含有量は０．０１％以上であるのが好ましく、０．０３％以上であるのがより好ましい。また、Ｍｏ含有量は０．０８％以下であるのが好ましく、０．０７％以下であるのがより好ましい。一方、Ｗ含有量は０．０１％以上であるのが好ましく、０．０５％未満であるのが好ましい。 The Mo content is preferably 0.01% or more, more preferably 0.03% or more. The Mo content is preferably 0.08% or less, more preferably 0.07% or less. On the other hand, the W content is preferably 0.01% or more, and preferably less than 0.05%.
　Ｎｂ：０～０．０３０％
　Ｎｂは、微量の添加により組織微細化に寄与し、母材強度確保に有効な元素であるため、必要に応じて含有させてもよい。一方、Ｎｂ含有量が過剰であると、溶接部を硬化させて著しく靭性を劣化させる。そのため、Ｎｂ含有量は０．０３０％以下とする。上記の効果を得たい場合には、Ｎｂ含有量は０．００３％以上であるのが好ましい。 Nb: 0 to 0.030%
Since Nb is an element that contributes to microstructure miniaturization by adding a small amount and is effective in ensuring the strength of the base metal, it may be contained if necessary. On the other hand, if the Nb content is excessive, the welded portion is hardened and the toughness is significantly deteriorated. Therefore, the Nb content is set to 0.030% or less. When the above effect is desired, the Nb content is preferably 0.003% or more.
　Ｖ：０～０．０５０％
　Ｖは、析出強化により強度上昇に寄与するため、必要に応じて含有させてもよい。一方、Ｖ含有量が過剰であると、継手靭性を損なうことがある。そのため、Ｖ含有量は０．０５０％以下とする。上記の効果を得たい場合には、Ｖ含有量は０．０１０％以上であるのが好ましい。 V: 0 to 0.050%
Since V contributes to the increase in strength by strengthening precipitation, it may be contained if necessary. On the other hand, if the V content is excessive, the toughness of the joint may be impaired. Therefore, the V content is set to 0.050% or less. When the above effect is desired, the V content is preferably 0.010% or more.
　Ｎｉ：０～１．００％
　Ｎｉは、強度確保および靭性向上に有効であるため、必要に応じて含有させてもよい。一方、Ｎｉ含有量が過剰であると、コストが上昇する。そのため、Ｎｉ含有量は１．００％以下とする。上記の効果を得たい場合には、Ｎｉ含有量は０．０５％以上であるのが好ましい。 Ni: 0 to 1.00%
Since Ni is effective in ensuring strength and improving toughness, it may be contained if necessary. On the other hand, if the Ni content is excessive, the cost will increase. Therefore, the Ni content is set to 1.00% or less. When the above effect is desired, the Ni content is preferably 0.05% or more.
　Ｃｒ：０～０．５０％
　Ｃｒは、焼入れ性を向上させ、高強度化に有効であるため、必要に応じて含有させてもよい。一方、Ｃｒ含有量が過剰であると、継手の硬さが上昇して靭性が低下することがある。そのため、Ｃｒ含有量は０．５０％以下とする。上記の効果を得たい場合には、Ｃｒ含有量は０．０５％以上であるのが好ましい。 Cr: 0 to 0.50%
Since Cr is effective for improving hardenability and increasing strength, it may be contained if necessary. On the other hand, if the Cr content is excessive, the hardness of the joint may increase and the toughness may decrease. Therefore, the Cr content is set to 0.50% or less. When the above effect is desired, the Cr content is preferably 0.05% or more.
　Ｂ：０～０．００３０％
　Ｂは、微量添加により焼き入れ性を高め母材強度向上に寄与するため、必要に応じて含有させてもよい。一方、Ｂ含有量が過剰であると、伸びおよび継手靭性を劣化させる。そのため、Ｂ含有量は０．００３０％以下とする。上記の効果を得たい場合には、Ｂ含有量は０．０００３％以上であるのが好ましい。 B: 0 to 0.0030%
Since B is added in a small amount to enhance the hardenability and contribute to the improvement of the strength of the base material, it may be contained as necessary. On the other hand, if the B content is excessive, the elongation and joint toughness are deteriorated. Therefore, the B content is set to 0.0030% or less. When the above effect is desired, the B content is preferably 0.0003% or more.
　Ｓｂ：０～０．３０％
　Ｓｎ：０～０．３０％
　Ｐｂ：０～０．３０％
　Ａｓ：０～０．３０％
　Ｂｉ：０～０．３０％
　Ｓｂ、Ｓｎ、Ｐｂ、ＡｓおよびＢｉは、局部腐食の進展をさらに抑制する効果を有するため、必要に応じて含有させてもよい。一方、いずれの含有量が０．３０％を超えても効果は飽和し、他の特性への悪影響の懸念もある。そのため、経済性も考慮して、Ｓｂ、Ｓｎ、Ｐｂ、ＡｓおよびＢｉの含有量は、いずれも０．３０％以下とする。また、いずれの元素の含有量も０．１５％以下であるのが好ましい。上記の効果を得たい場合には、Ｓｂ：０．０１％以上、Ｓｎ：０．０１％以上、Ｐｂ：０．０１％以上、Ａｓ：０．０１％以上およびＢｉ：０．０１％以上から選択される１種以上を含有させることが好ましい。 Sb: 0 to 0.30%
Sn: 0 to 0.30%
Pb: 0 to 0.30%
As: 0 to 0.30%
Bi: 0 to 0.30%
Sb, Sn, Pb, As and Bi have an effect of further suppressing the progress of local corrosion, and may be contained as necessary. On the other hand, even if any content exceeds 0.30%, the effect is saturated and there is a concern that it may adversely affect other characteristics. Therefore, in consideration of economic efficiency, the contents of Sb, Sn, Pb, As and Bi are all set to 0.30% or less. Moreover, the content of any element is preferably 0.15% or less. When the above effect is desired, Sb: 0.01% or more, Sn: 0.01% or more, Pb: 0.01% or more, As: 0.01% or more, and Bi: 0.01% or more. It is preferable to contain one or more selected species.
　Ｔａ：０～０．５０％
　Ｚｒ：０～０．５０％
　ＴａおよびＺｒは、微量で鋼の強度を高めるのに有効な元素であり、主に強度調整のため、必要に応じて含有させてもよい。一方、いずれの含有量が０．５０％を超えても、靭性劣化が顕著となる。そのため、ＴａおよびＺｒの含有量はいずれも０．５０％以下とする。上記の効果を得たい場合には、Ｔａ：０．００５％以上およびＺｒ：０．００５％以上から選択される１種または２種を含有させることが好ましい。 Ta: 0 to 0.50%
Zr: 0 to 0.50%
Ta and Zr are elements effective for increasing the strength of steel in a small amount, and may be contained as necessary mainly for strength adjustment. On the other hand, even if any of the contents exceeds 0.50%, the toughness deterioration becomes remarkable. Therefore, the contents of Ta and Zr are both set to 0.50% or less. When the above effect is desired, it is preferable to contain one or two kinds selected from Ta: 0.005% or more and Zr: 0.005% or more.
　本発明の鋼板の化学組成において、残部はＦｅおよび不純物である。 In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities.
　ここで「不純物」とは、鋼板を工業的に製造する際に、鉱石、スクラップ等の原料、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。 Here, the "impurity" is a component mixed with raw materials such as ore and scrap and various factors in the manufacturing process when the steel sheet is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.
　鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量：０．００５％以上
　本発明に係る鋼板においては、上述のように、耐食性の向上のため、ＭｏおよびＷを鋼板表層部に濃化させ、それにより、固溶するＭｏおよびＷの量を所定値以上確保する。具体的には、鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量を、質量％で、０．００５％以上とする。鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量は、０．０１０％以上であるのが好ましく、０．０２０％以上であるのがより好ましい。 Total content of solid solution Mo and solid solution W in the surface layer of the steel sheet: 0.005% or more In the steel sheet according to the present invention, Mo and W are concentrated in the surface layer of the steel sheet in order to improve corrosion resistance as described above. By doing so, the amount of Mo and W to be solid-solved is secured at a predetermined value or more. Specifically, the total content of the solid solution Mo and the solid solution W in the surface layer portion of the steel sheet is 0.005% or more in mass%. The total content of the solid solution Mo and the solid solution W in the surface layer portion of the steel sheet is preferably 0.010% or more, and more preferably 0.020% or more.
　なお、本発明において、鋼板表層部とは、鋼板の表面から深さ方向に１ｍｍ位置までの領域を指す。また、固溶Ｍｏおよび固溶Ｗの合計含有量（質量％）は、以下の手順により測定する。まず、鋼板の表面から厚さ１ｍｍの試験片を２つ切り出す。そして、そのうちの一方の試験片については、公知の化学分析方法（例えば、ＩＣＰ発光分光分析法）を用いることで、試験片中のＷおよびＭｏの含有量を測定する。 In the present invention, the surface layer portion of the steel sheet refers to a region from the surface of the steel sheet to a position of 1 mm in the depth direction. The total content (mass%) of the solid solution Mo and the solid solution W is measured by the following procedure. First, two test pieces having a thickness of 1 mm are cut out from the surface of the steel plate. Then, for one of the test pieces, the contents of W and Mo in the test piece are measured by using a known chemical analysis method (for example, ICP emission spectroscopy).
　また、もう一方については、１０％アセチルアセトン－１％テトラメチルアンモニウムクロライド／メタノールにて、２０ｍＡ／ｃｍ２の電流密度で約０．４ｇ電解する。その電解に用いた溶液を孔径０．２μｍのフィルターでろ過し、フィルター上に捕集した抽出残渣について、公知の化学分析方法（例えば、ＩＣＰ発光分光分析法）を用いることで、抽出残渣中のＷおよびＭｏの含有量を測定する。 For the other, about 0.4 g is electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride / methanol at a current density of 20 mA / cm 2 . The solution used for the electrolysis is filtered through a filter having a pore size of 0.2 μm, and the extraction residue collected on the filter is contained in the extraction residue by using a known chemical analysis method (for example, ICP emission spectroscopic analysis). The W and Mo contents are measured.
　試験片中のＷおよびＭｏは、Ｗ析出物およびＭｏ析出物と固溶Ｗおよび固溶Ｍｏであると考え、抽出残渣中のＷおよびＭｏは、Ｗ析出物およびＭｏ析出物と考える。そして、試験片中のＷおよびＭｏの含有量から抽出残渣中のＷおよびＭｏの含有量の差分を求めることで、固溶Ｗおよび固溶Ｍｏの含有量を求める。 W and Mo in the test piece are considered to be W precipitate and Mo precipitate and solid solution W and solid solution Mo, and W and Mo in the extraction residue are considered to be W precipitate and Mo precipitate. Then, the contents of the solid solution W and the solid solution Mo are obtained by obtaining the difference between the contents of the W and Mo in the extraction residue from the contents of W and Mo in the test piece.
　板厚中心部のＰの最大濃度：０．０２～０．２０％
　Ｐは、連続鋳造時に中心偏析して板厚中心部に脆化域を形成し、割れを生じさせて局部伸びを劣化させるおそれがある。そのため、優れた延性を得たい場合には、板厚中心部におけるＰの最大濃度は低い方が好ましく、具体的には、鋼板の表面から２／５ｔの位置と、鋼板の表面から３／５ｔの位置との間の領域における、Ｐの最大濃度を０．２０％以下とすることが好ましい。また、Ｐの最大濃度を０．０２％未満とすることは現実的に困難であるので、０．０２％を下限とする。 Maximum concentration of P in the center of plate thickness: 0.02 to 0.20%
P may be centrally segregated during continuous casting to form an embrittlement region in the central portion of the plate thickness, causing cracks and deteriorating local elongation. Therefore, when it is desired to obtain excellent ductility, it is preferable that the maximum concentration of P in the center of the plate thickness is low. Specifically, the position is 2/5 t from the surface of the steel sheet and 3/5 t from the surface of the steel plate. It is preferable that the maximum concentration of P in the region between the positions is 0.20% or less. Further, since it is practically difficult to set the maximum concentration of P to less than 0.02%, 0.02% is set as the lower limit.
　なお、Ｐの最大濃度は、鋼板の表面から２／５ｔの位置と、鋼板の表面から３／５ｔの位置との間の領域について、電子線マイクロアナライザー（Ｅｌｅｃｔｒｏｎ　Ｐｒｏｂｅ　ＭｉｃｒｏＡｎａｌｙｓｅｒ：ＥＰＭＡ）により、加速電圧：１５ｋＶ、ビーム径：２０μｍ、照射時間：２０ｍｓ、測定ピッチ：２０μｍで測定したときのＰの濃度の最大値である。 The maximum concentration of P is the accelerating voltage of the region between the position of 2/5 t from the surface of the steel sheet and the position of 3/5 t from the surface of the steel plate by an electron probe microanalyzer (EPMA). It is the maximum value of the concentration of P when measured at 15 kV, beam diameter: 20 μm, irradiation time: 20 ms, and measurement pitch: 20 μm.
　（Ｂ）鋼板の金属組織
　本発明の鋼板の金属組織において、所定の伸び特性を確保する観点から、フェライトの平均結晶粒径を６０μｍ以下とする。フェライトの平均結晶粒径は５０μｍ以下であることが好ましい。なお、本発明において金属組織は、鋼板の圧延方向断面において、鋼板の幅および厚さをそれぞれＷおよびｔとしたときに、該鋼板の端面から１／４Ｗで、かつ、該鋼板の表面から１／４ｔの位置における組織をいうものとする。 (B) Metallic structure of steel sheet In the metal structure of the steel sheet of the present invention, the average crystal grain size of ferrite is set to 60 μm or less from the viewpoint of ensuring predetermined elongation characteristics. The average crystal grain size of ferrite is preferably 50 μm or less. In the present invention, the metallographic structure is 1/4 W from the end face of the steel sheet and 1 from the surface of the steel sheet when the width and thickness of the steel sheet are W and t, respectively, in the rolling direction cross section of the steel sheet. It means the organization at the position of / 4t.
　また、上記「所定の伸び特性」とは、鋼板板厚が４．５ｍｍ超５ｍｍ以下では全伸び（ｔ－ＥＬ）が１１％以上、鋼板板厚が５ｍｍ超１０ｍｍ以下では全伸び（ｔ－ＥＬ）が１２％以上、鋼板板厚が１０ｍｍ超１５ｍｍ以下では全伸び（ｔ－ＥＬ）が１３％以上、鋼板板厚が１５ｍｍ超２０ｍｍ以下では全伸び（ｔ－ＥＬ）が１４％以上、鋼板板厚が２０ｍｍ超２５ｍｍ以下では全伸び（ｔ－ＥＬ）が１５％以上、鋼板板厚が２５ｍｍ超３０ｍｍ以下では全伸び（ｔ－ＥＬ）が１６％以上、鋼板板厚が３０ｍｍ超４０ｍｍ以下では全伸び（ｔ－ＥＬ）が１７％以上、鋼板板厚が４０ｍｍ超５０ｍｍ以下では全伸び（ｔ－ＥＬ）が１８％以上の特性である。 The above-mentioned "predetermined elongation characteristics" means that the total elongation (t-EL) is 11% or more when the steel plate thickness is more than 4.5 mm and 5 mm or less, and the total elongation (t-EL) is when the steel plate thickness is more than 5 mm and 10 mm or less. ) Is 12% or more, the total elongation (t-EL) is 13% or more when the steel plate thickness is more than 10 mm and 15 mm or less, and the total elongation (t-EL) is 14% or more when the steel plate thickness is more than 15 mm and 20 mm or less. Total elongation (t-EL) is 15% or more when the thickness is more than 20 mm and 25 mm or less, total elongation (t-EL) is 16% or more when the steel plate thickness is more than 25 mm and 30 mm or less, and total when the steel plate thickness is more than 30 mm and 40 mm or less. When the elongation (t-EL) is 17% or more and the steel plate thickness is more than 40 mm and 50 mm or less, the total elongation (t-EL) is 18% or more.
　その他の金属組織については、特に限定はないが、より優れた延性を得たい場合には、以下に示す金属組織を有することが好ましい。以下の説明において「％」は、「面積％」を意味する。 The other metallographic structure is not particularly limited, but if it is desired to obtain better ductility, it is preferable to have the metallographic structure shown below. In the following description, "%" means "area%".
　パーライト：５～２０％
　強度特性である降伏応力および引張強さと伸び特性とは、相反する性質であって、両者を同時に向上させることは一般に困難とされている。伸び特性を確保しつつ、強度特性を確保するためには、パーライトの面積率は５～２０％であることが好ましい。パーライトの面積率は１０～１５％であるのがより好ましい。 Perlite: 5-20%
Yield stress and tensile strength and elongation characteristics, which are strength characteristics, are contradictory properties, and it is generally considered difficult to improve both at the same time. In order to secure the strength characteristics while ensuring the elongation characteristics, the area ratio of pearlite is preferably 5 to 20%. The area ratio of pearlite is more preferably 10 to 15%.
　ベイナイト：１０％以下
　本発明において、金属組織はフェライトが主体であって、所定量のパーライトを含むことが好ましい。しかしながら、１０％以下のベイナイトが含まれていても上述した効果を阻害しない。そのため、ベイナイトの面積率は１０％以下であることが好ましく、５％以下であることがより好ましい。ベイナイトは含まれていなくてもよく、すなわち、ベイナイトの面積率は０％であってもよい。 Bainite: 10% or less In the present invention, the metal structure is mainly ferrite and preferably contains a predetermined amount of pearlite. However, even if it contains 10% or less of bainite, it does not inhibit the above-mentioned effects. Therefore, the area ratio of bainite is preferably 10% or less, and more preferably 5% or less. Bainite may not be included, that is, the area ratio of bainite may be 0%.
　残部：フェライト
　フェライトは延性に優れた組織である。フェライトの面積率が高いほど、伸び特性ＥＬを向上させることが可能となる。したがって、パーライトおよびベイナイト以外の組織はフェライトとする。 Remaining: Ferrite Ferrite has a structure with excellent ductility. The higher the area ratio of ferrite, the better the elongation characteristic EL can be. Therefore, the structures other than pearlite and bainite are ferrite.
　ここで、本発明において、金属組織の面積率は以下のように求める。上述のように、まず鋼板の端面から１／４Ｗで、かつ、鋼板の表面から１／４ｔの位置から試料を採取する。そして、該試料の圧延方向断面（いわゆるＬ方向断面）を観察する。 Here, in the present invention, the area ratio of the metal structure is calculated as follows. As described above, first, the sample is taken from the position of 1/4 W from the end face of the steel sheet and 1 / 4t from the surface of the steel sheet. Then, the rolling direction cross section (so-called L direction cross section) of the sample is observed.
　具体的には、試料をナイタールエッチングし、エッチング後に光学顕微鏡を用いて３００μｍ×３００μｍの視野で観察を行う。そして得られた組織写真に対し、画像解析を行い、白色に見えるものをフェライト、黒色に見えるものをパーライト、灰色に見えるものをベイナイトとして、それぞれの面積率を求める。 Specifically, the sample is night-game-etched, and after etching, observation is performed with a field of view of 300 μm × 300 μm using an optical microscope. Then, image analysis is performed on the obtained tissue photograph, and the area ratio of each is determined by using ferrite as the white color, pearlite as the black color, and bainite as the gray color.
　また、優れた延性を得たい場合には、フェライトの平均アスペクト比および平均結晶粒径、ならびにフェライト中の平均転位密度についても、以下に示す範囲内とすることが好ましい。 Further, when it is desired to obtain excellent ductility, it is preferable that the average aspect ratio and average grain size of ferrite and the average dislocation density in ferrite are also within the ranges shown below.
　フェライトの平均アスペクト比：１．０～１．５
　鋼板の表面から１／４ｔの位置におけるフェライトの平均アスペクト比は、低いほど好ましい。平均アスペクト比を１．５以下にすることで、転位密度を低下させ、伸びを向上させることができるようになる。そのため、平均アスペクト比は１．５以下であるのが好ましい。平均アスペクト比の下限は、フェライト粒が球状となる１．０である。 Average aspect ratio of ferrite: 1.0-1.5
The lower the average aspect ratio of ferrite at the position 1 / 4t from the surface of the steel sheet, the more preferable. By setting the average aspect ratio to 1.5 or less, the dislocation density can be reduced and the elongation can be improved. Therefore, the average aspect ratio is preferably 1.5 or less. The lower limit of the average aspect ratio is 1.0, which makes the ferrite grains spherical.
　フェライトの平均結晶粒径：５～２０μｍ
　鋼板の表面から１／４ｔの位置におけるフェライトの平均結晶粒径を２０μｍ以下とすることで、強度－延性バランスを向上させることができるようになる。そのため、平均結晶粒径は２０μｍ以下であるのが好ましい。また、フェライト粒は細粒であるほど好ましいが５μｍ未満は工業上実現が難しいため、下限を５μｍとした。 Average grain size of ferrite: 5 to 20 μm
By setting the average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet to 20 μm or less, the strength-ductility balance can be improved. Therefore, the average crystal grain size is preferably 20 μm or less. Further, the finer the ferrite grain is, the more preferable it is, but since it is industrially difficult to realize a ferrite grain of less than 5 μm, the lower limit is set to 5 μm.
　なお、フェライトの平均アスペクト比および平均結晶粒径は、前述した顕微鏡観察において測定する。具体的には、各フェライトを画像解析により楕円近似し、長軸長さを短軸長さで割ることによって、フェライトのアスペクト比を求める。同様に、各フェライトを画像解析により面積を求め、この面積に等しい円の直径を求めることで、フェライトの結晶粒径を求める。そして、視野内の全フェライトのアスペクト比および直径の平均値をそれぞれ算出することによって、平均アスペクト比および平均結晶粒径を求める。 The average aspect ratio and average grain size of ferrite are measured by the above-mentioned microscopic observation. Specifically, each ferrite is ellipsically approximated by image analysis, and the aspect ratio of ferrite is obtained by dividing the major axis length by the minor axis length. Similarly, the area of each ferrite is obtained by image analysis, and the diameter of a circle equal to this area is obtained to obtain the crystal grain size of the ferrite. Then, the average aspect ratio and the average grain size are obtained by calculating the average values of the aspect ratio and the diameter of all the ferrites in the visual field, respectively.
　フェライト中の平均転位密度：７．０×１０１２／ｍ２以下
　鋼板の表面から１／４ｔの位置におけるフェライト中の平均転位密度を低下させることで、フェライトが軟化し、より優れた伸び特性が得られるようになる。そのため、フェライト中の平均転位密度は７．０×１０１２／ｍ２以下とすることが好ましい。転位密度は低ければ低いほどよいが、通常１．０×１０１２／ｍ２を下回ることはほとんどない。平均転位密度の好ましい上限は６．０×１０１２／ｍ２である。 Average dislocation density in ferrite: 7.0 × 10 12 / m 2 or less By reducing the average dislocation density in ferrite at a position 1/4 t from the surface of the steel sheet, the ferrite softens and has better elongation characteristics. You will be able to obtain it. Therefore, the average dislocation density in ferrite is preferably 7.0 × 10 12 / m 2 or less. The lower the dislocation density, the better, but it is usually less than 1.0 × 10 12 / m 2 . The preferred upper limit of the average dislocation density is 6.0 × 10 12 / m 2 .
　フェライト中の平均転位密度は、以下のように求める。まず、鋼板の表面から１／４ｔの位置から薄膜試料を採取し、透過電子顕微鏡（ＴＥＭ）を用いて倍率を４００００倍として明視野の観察撮影を行う。得られたＴＥＭ像から任意の直線と転位線との交切点の数を測定する。そして、以下の式（vii）により平均転位密度を算出する。
　ρ＝２Ｎ／Ｌｄ　　　・・・（vii）
　但し、上記式中の各記号の意味は以下のとおりである。
　ρ：平均転位密度（／ｍ２）
　Ｌ：任意の直線の長さ（ｍ）
　Ｎ：任意の直線と転位線との交切点の数
　ｄ：薄膜試料の厚さ（ｍ） The average dislocation density in ferrite is calculated as follows. First, a thin film sample is taken from the surface of the steel sheet at a position of 1/4 t, and a bright field observation image is taken using a transmission electron microscope (TEM) at a magnification of 40,000 times. From the obtained TEM image, the number of intersections between an arbitrary straight line and a dislocation line is measured. Then, the average dislocation density is calculated by the following formula (vii).
ρ = 2N / Ld ・ ・ ・ (vii)
However, the meaning of each symbol in the above formula is as follows.
ρ: Average dislocation density (/ m 2 )
L: Length of arbitrary straight line (m)
N: Number of intersections between arbitrary straight lines and dislocation lines d: Thickness of thin film sample (m)
　板厚中心部の粗大介在物の個数密度：１０個／ｍｍ２以下
　長さ５μｍ以上の粗大な介在物（ＭｎＳ、アルミナ（Ａｌ２Ｏ３）等の硫化物または酸化物）は延性破壊（ボイド）の起点となり、局部伸びを劣化させることがある。そのため、優れた延性を得たい場合には、板厚中心部における粗大介在物の個数密度を低減することが好ましく、具体的には、鋼板の表面から２／５ｔの位置と、鋼板の表面から３／５ｔの位置との間の領域において存在する、長さ５μｍ以上の介在物の個数密度を１０個／ｍｍ２以下とすることが好ましい。なお、介在物の測定は走査電子顕微鏡（ＳＥＭ）による粒子解析によって行うものとする。 Number of coarse inclusions in the center of the plate thickness Density: 10 pieces / mm 2 or less Coarse inclusions (sulfides or oxides such as MnS and alumina (Al 2 O 3 )) with a length of 5 μm or more are ductile fractures (voids). ), Which may deteriorate local elongation. Therefore, when it is desired to obtain excellent ductility, it is preferable to reduce the number density of coarse inclusions in the center of the plate thickness, specifically, from the position of 2 / 5t from the surface of the steel sheet and from the surface of the steel sheet. The number density of inclusions having a length of 5 μm or more and existing in the region between the 3 / 5t position is preferably 10 pieces / mm 2 or less. The inclusions shall be measured by particle analysis using a scanning electron microscope (SEM).
　（Ｃ）鋼板の機械的特性
　中央領域に対する表層領域のビッカース硬さの割合：８０～１０５％
　厚鋼板の冷却時において、鋼板の表面付近は板厚中心付近に比べて相対的に冷却速度が速くなり、硬質化しやすい。一方、表層部近傍の硬さを低く抑えることで伸び特性を改善することが可能となる。ここで、以下の説明では、鋼板の表面付近である、鋼板の表面と、鋼板の表面から１／４ｔの位置との間の領域を表層領域と呼び、板厚中心付近である、鋼板の表面から１／４ｔの位置と、鋼板の表面から３／４ｔの位置との間の領域を中央領域と呼ぶ。 (C) Mechanical properties of steel sheet Ratio of Vickers hardness in the surface layer region to the central region: 80 to 105%
When the thick steel sheet is cooled, the cooling rate near the surface of the steel sheet is relatively faster than that near the center of the sheet thickness, and it is easy to harden. On the other hand, it is possible to improve the elongation characteristics by keeping the hardness in the vicinity of the surface layer portion low. Here, in the following description, the region between the surface of the steel plate and the position 1/4 t from the surface of the steel plate, which is near the surface of the steel plate, is called a surface layer region, and the surface of the steel plate near the center of the plate thickness. The region between the position of 1 / 4t and the position of 3/4t from the surface of the steel sheet is called the central region.
　板厚全体の伸び特性を考慮した場合、鋼板の表層領域の硬質化の影響はある程度は許容できるが、表層領域と中央領域との硬度差が大きくなると影響を無視できなくなってくる。そのため、優れた延性を得たい場合には、表層領域におけるビッカース硬さの平均値を、中央領域におけるビッカース硬さの平均値の８０～１０５％とすることが好ましい。なお、各領域におけるビッカース硬さの平均値は、１ｍｍピッチのビッカース硬さの試験で求めるものとする。また、試験力は１０ｋｇｆ（９８Ｎ）とする。 Considering the elongation characteristics of the entire plate thickness, the effect of hardening the surface layer region of the steel sheet can be tolerated to some extent, but when the hardness difference between the surface layer region and the central region becomes large, the effect cannot be ignored. Therefore, when it is desired to obtain excellent ductility, the average value of Vickers hardness in the surface layer region is preferably 80 to 105% of the average value of Vickers hardness in the central region. The average value of Vickers hardness in each region shall be obtained by a 1 mm pitch Vickers hardness test. The test force is 10 kgf (98N).
　その他の機械的特性については特に制限はないが、本発明に係る鋼板は、例えば、原油油槽として用いるのに必要な強度を有することが好ましい。具体的には、降伏応力（ＹＳ）が２３５ＭＰａ以上で、引張強度（ＴＳ）が４００～６２０ＭＰａであることが好ましい。 There are no particular restrictions on other mechanical properties, but the steel sheet according to the present invention preferably has the strength required for use as, for example, a crude oil tank. Specifically, it is preferable that the yield stress (YS) is 235 MPa or more and the tensile strength (TS) is 400 to 620 MPa.
　また、優れた延性を得たい場合には、鋼板板厚が４．５ｍｍ超５ｍｍ以下では全伸び（ｔ－ＥＬ）が１９％以上、鋼板板厚が５ｍｍ超１０ｍｍ以下では全伸び（ｔ－ＥＬ）が２２％以上、鋼板板厚が１０ｍｍ超１５ｍｍ以下では全伸び（ｔ－ＥＬ）が２３％以上、鋼板板厚が１５ｍｍ超２０ｍｍ以下では全伸び（ｔ－ＥＬ）が２５％以上、鋼板板厚が２０ｍｍ超２５ｍｍ以下では全伸び（ｔ－ＥＬ）が２６％以上、鋼板板厚が２５ｍｍ超３０ｍｍ以下では全伸び（ｔ－ＥＬ）が２７％以上、鋼板板厚が３０ｍｍ超４０ｍｍ以下では全伸び（ｔ－ＥＬ）が２９％以上、鋼板板厚が４０ｍｍ超５０ｍｍ以下では全伸び（ｔ－ＥＬ）が３０％以上を有することが好ましい。 If you want to obtain excellent ductility, the total elongation (t-EL) is 19% or more when the steel plate thickness is more than 4.5 mm and 5 mm or less, and the total elongation (t-EL) is when the steel plate thickness is more than 5 mm and 10 mm or less. ) Is 22% or more, the total elongation (t-EL) is 23% or more when the steel plate thickness is more than 10 mm and 15 mm or less, and the total elongation (t-EL) is 25% or more when the steel plate thickness is more than 15 mm and 20 mm or less. When the thickness is more than 20 mm and 25 mm or less, the total elongation (t-EL) is 26% or more, when the steel plate thickness is more than 25 mm and 30 mm or less, the total elongation (t-EL) is 27% or more, and when the steel plate thickness is more than 30 mm and 40 mm or less, the total is total. When the elongation (t-EL) is 29% or more and the steel plate thickness is more than 40 mm and 50 mm or less, it is preferable that the total elongation (t-EL) is 30% or more.
　なお、引張強さ（ＴＳ）、降伏応力（ＹＳ）、全伸び（ｔ－ＥＬ）は、ＪＩＳ　Ｚ　２２４１：２０１１に基づき、板厚中心部から圧延方向と直角の方向に採取した、１Ｂ号引張試験片を用いて測定した。詳細には、降伏応力（ＹＳ）は永久伸び０．２％時の永久伸び法の耐力であり、全伸び（ｔ－ＥＬ）は破断時全伸びである。 The tensile strength (TS), yield stress (YS), and total elongation (t-EL) were taken from the center of the plate thickness in the direction perpendicular to the rolling direction based on JIS Z 2241: 2011. It was measured using a test piece. Specifically, the yield stress (YS) is the proof stress of the permanent elongation method when the permanent elongation is 0.2%, and the total elongation (t-EL) is the total elongation at break.
　（Ｄ）鋼板の製造方法
　本発明に係る鋼板の製造条件について特に制限はないが、後述する精錬工程、連続鋳造工程、加熱工程、デスケーリング工程、熱間圧延工程、および冷却工程を順に行うことで製造することができる。各工程について説明する。 (D) Steel Sheet Manufacturing Method The steel sheet manufacturing conditions according to the present invention are not particularly limited, but the refining step, continuous casting step, heating step, descaling step, hot rolling step, and cooling step described later are performed in this order. Can be manufactured at. Each process will be described.
　（ａ）精錬工程
　精錬工程において溶鋼を製造する。精錬工程については公知の方法を採用すればよく、特に制限はない。しかしながら、鋼板の延性を向上させるためには、溶鋼を以下の手順により製造することが好ましい。 (A) Refining process Molten steel is manufactured in the refining process. A known method may be adopted for the refining process, and there is no particular limitation. However, in order to improve the ductility of the steel sheet, it is preferable to manufacture the molten steel by the following procedure.
　まず、鋳造前処理として、溶鋼から炭素を除く１次精錬を行った後、溶鋼の成分調整をするにあたり、真空脱ガス装置により溶鋼の溶存酸素量を４０ｐｐｍ以下に調整する。溶鋼の溶存酸素量を４０ｐｐｍ以下に調整するには、例えば、ＲＨ真空脱ガス装置の真空度が１～５ｔｏｒｒで、溶鋼を１～３分間還流して調整する。 First, as a pre-casting treatment, after performing primary refining to remove carbon from the molten steel, the amount of dissolved oxygen in the molten steel is adjusted to 40 ppm or less by a vacuum degassing device when adjusting the composition of the molten steel. To adjust the dissolved oxygen content of the molten steel to 40 ppm or less, for example, the degree of vacuum of the RH vacuum degassing device is 1 to 5 torr, and the molten steel is refluxed for 1 to 3 minutes for adjustment.
　次いで、ＡｌをＡｌの最終含有量が０．００２～０．０５０％となるように添加して、溶鋼の溶存酸素量を１０ｐｐｍ以下に調整する。その後、Ｃａ、ＭｇおよびＲＥＭから選択される１種以上を、Ｃａ、ＭｇおよびＲＥＭの合計の最終含有量が０．０００５～０．００８０％となるように添加する。 Next, Al is added so that the final content of Al is 0.002 to 0.050%, and the dissolved oxygen content of the molten steel is adjusted to 10 ppm or less. Then, one or more selected from Ca, Mg and REM are added so that the total final content of Ca, Mg and REM is 0.0005 to 0.0080%.
　これにより、Ｃａ、Ｍｇ、ＲＥＭを優先的に硫化物化して、ＭｎＳの生成を抑制する。溶存酸素量が１０ｐｐｍ超であると、Ｃａ、Ｍｇ、ＲＥＭを添加した場合に酸化物化してしまい、硫化物制御が十分にできないことがある。溶鋼の溶存酸素量を１０ｐｐｍ以下に調整するには、例えば、ＲＨ真空脱ガス装置の真空度が１～５ｔｏｒｒで、溶鋼を１０～６０分間還流して、溶鋼の溶存酸素量を１０ｐｐｍ以下に調整する。真空度が１～５ｔｏｒｒで、溶鋼を１０～６０分間還流しなければ、溶存酸素量を１０ｐｐｍ以下とすることができない。また、溶存酸素量は少ないほどよく、溶鋼の溶存酸素量の下限については設定する必要はない。 As a result, Ca, Mg, and REM are preferentially sulfided to suppress the formation of MnS. If the amount of dissolved oxygen exceeds 10 ppm, it will be oxidized when Ca, Mg, and REM are added, and sulfide control may not be sufficient. To adjust the dissolved oxygen content of the molten steel to 10 ppm or less, for example, the vacuum degree of the RH vacuum degassing device is 1 to 5 torr, the molten steel is refluxed for 10 to 60 minutes, and the dissolved oxygen content of the molten steel is adjusted to 10 ppm or less. To do. Unless the degree of vacuum is 1 to 5 torr and the molten steel is refluxed for 10 to 60 minutes, the amount of dissolved oxygen cannot be reduced to 10 ppm or less. Further, the smaller the dissolved oxygen amount is, the better, and it is not necessary to set the lower limit of the dissolved oxygen amount of the molten steel.
　精錬工程を上記の条件で行うことで、板厚中心部の粗大介在物の個数密度を１０個／ｍｍ２以下に抑制することが可能となる。 By performing the refining step under the above conditions, it is possible to suppress the number density of coarse inclusions in the center of the plate thickness to 10 pieces / mm 2 or less.
　（ｂ）連続鋳造工程
　連続鋳造工程において溶鋼を連続鋳造し、上述した化学組成を有する鋼片を製造する。連続鋳造工程についても公知の方法を採用すればよく特に制限はない。しかしながら、鋼板の延性を向上させるためには、溶鋼を連続鋳造する際に、鋳片の凝固末期である鋳片の中心固相率が０．２～０．７の範囲において、鋳造ロールの間隙を、鋳造進行方向１ｍにつき０．２ｍｍ～３．０ｍｍに狭めて圧下しながら鋳造することが好ましい。 (B) Continuous Casting Step In the continuous casting step, molten steel is continuously cast to produce steel pieces having the above-mentioned chemical composition. As for the continuous casting process, a known method may be adopted, and there is no particular limitation. However, in order to improve the ductility of the steel sheet, when the molten steel is continuously cast, the gap between the casting rolls is set in the range of 0.2 to 0.7 in the central solid phase ratio of the slab, which is the final stage of solidification of the slab. Is preferably narrowed to 0.2 mm to 3.0 mm per 1 m in the casting traveling direction and cast while being reduced.
　上記の条件で鋳造を行うことで、Ｐ等の濃化溶鋼を上流側に排出させる。それにより、有害な中心偏析を低減することが可能となる。すなわち、板厚中心部のＰの最大濃度を０．０２～０．２０％の範囲に調整することが可能となる。ここでいう中心固相率とは、鋳片厚み方向の中心部で、かつ、鋳片幅方向の溶融部分の固相率と定義でき、伝熱、凝固計算によって求めることができること等が知られている。 By casting under the above conditions, concentrated molten steel such as P is discharged to the upstream side. This makes it possible to reduce harmful central segregation. That is, the maximum concentration of P in the central portion of the plate thickness can be adjusted in the range of 0.02 to 0.20%. It is known that the central solid phase ratio referred to here can be defined as the solid phase ratio of the molten portion in the slab width direction and in the central portion in the slab thickness direction, and can be obtained by heat transfer and solidification calculation. ing.
　鋳造ロールの間隙は、鋳造進行方向１ｍにつき０．５～２．０ｍｍに狭めて軽圧下することがより好ましく、鋳造進行方向１ｍにつき０．７～１．５ｍｍに狭めて軽圧下することがさらに好ましい。なお、軽圧下をすることが好ましいが、Ｐ含有量が低い成分の場合には軽圧下をしなくてもよい。 The gap between the casting rolls is more preferably narrowed to 0.5 to 2.0 mm per 1 m in the casting traveling direction and lightly reduced, and further narrowed to 0.7 to 1.5 mm per 1 m in the casting traveling direction and lightly reduced. preferable. It is preferable to reduce the pressure lightly, but in the case of a component having a low P content, the pressure reduction may not be applied.
　（ｃ）加熱工程
　鋼片に対して熱間圧延を施すために、鋼片を加熱する。加熱工程においては、上述した化学組成を有する鋼片に対して、Ｏ２濃度が１．０体積％以上の雰囲気において、９５０℃以上１２００℃未満の加熱温度で３０～１２０分間保持し、加熱抽出温度を９５０℃以上１２００℃未満とする。 (C) Heating step The steel pieces are heated in order to perform hot rolling on the steel pieces. In the heating step, against a steel strip having a chemical composition as described above, O 2 concentration in the atmosphere of 1.0% by volume, and held at a heating temperature of less than 950 ° C. or higher 1200 ° C. 30 ~ 120 minutes, thermal extraction The temperature is 950 ° C. or higher and lower than 1200 ° C.
　上記の条件で加熱することによって、鋼板の表面にＦｅを主体とする酸化スケールが形成される。この際、Ｆｅより貴な元素であるＭｏおよびＷは、酸化スケール中には含まれず、スケール直下の鋼板表層部に濃化し、固溶Ｍｏおよび固溶Ｗを含有させることができる。 By heating under the above conditions, an oxide scale mainly composed of Fe is formed on the surface of the steel sheet. At this time, Mo and W, which are elements nobler than Fe, are not contained in the oxidation scale, but can be concentrated in the surface layer portion of the steel sheet immediately below the scale to contain the solid solution Mo and the solid solution W.
　Ｏ２濃度が１．０体積％未満であるか、加熱温度が９５０℃未満であるか、保持時間が３０分間未満では、形成される酸化スケールが薄く、鋼板表層部にＭｏおよびＷの濃化が不十分となり、固溶Ｍｏおよび固溶Ｗの合計含有量が不十分となるおそれがある。 When the O 2 concentration is less than 1.0% by volume, the heating temperature is less than 950 ° C., or the holding time is less than 30 minutes, the oxide scale formed is thin and Mo and W are concentrated on the surface layer of the steel sheet. May become insufficient, and the total content of the solid solution Mo and the solid solution W may be insufficient.
　一方、加熱温度が１２００℃以上であるか、保持時間が１２０分間を超えると、フェライト粒が粗大化するおそれがある。 On the other hand, if the heating temperature is 1200 ° C. or higher or the holding time exceeds 120 minutes, the ferrite grains may become coarse.
　Ｏ２濃度に上限は特に設けないが、Ｏ２濃度が高すぎると温度が上がらず、操業上の問題が発生するおそれがある。そのため、Ｏ２濃度は１０．０体積％以下とするのが好ましく、５．０体積％以下とするのがより好ましい。 Limit the O 2 concentration is not particularly provided, the O 2 concentration is not increased too high temperature, there is a risk that operational problems may occur. Therefore, the O 2 concentration is preferably 10.0% by volume or less, and more preferably 5.0% by volume or less.
　さらに、金属組織中の各相の面積率を上述した範囲とし、フェライトの平均アスペクト比および平均結晶粒径を制御することで、優れた延性を得たい場合には、以下の条件で加熱を行うことが好ましい。 Further, if it is desired to obtain excellent ductility by setting the area ratio of each phase in the metal structure within the above range and controlling the average aspect ratio and the average grain size of ferrite, heating is performed under the following conditions. Is preferable.
　まず、上述した加熱温度を１１００℃以下とし、かつ加熱抽出温度を１１００℃以下にすることが好ましい。加熱抽出温度を１１００℃以下にすると、オーステナイト（γ）粒を微細化してフェライト（α）粒を細粒化するともに、フェライト（α）粒のアスペクト比が小さくなり、伸び特性が向上する。 First, it is preferable that the above-mentioned heating temperature is 1100 ° C. or lower and the heating extraction temperature is 1100 ° C. or lower. When the heat extraction temperature is set to 1100 ° C. or lower, the austenite (γ) particles are refined to make the ferrite (α) particles finer, the aspect ratio of the ferrite (α) particles is reduced, and the elongation characteristics are improved.
　また、鋼片を加熱する際の保持時間は、フェライト粒の微細化に影響する。例えば、フェライトの平均結晶粒径を５０μｍ以下にしたい場合には、保持時間は８０分間以下とすることが好ましく、フェライトの平均結晶粒径を２０μｍ以下にしたい場合には、保持時間は６０分間以下とすることが好ましい。 In addition, the holding time when heating the steel piece affects the miniaturization of ferrite grains. For example, when the average crystal grain size of ferrite is to be 50 μm or less, the holding time is preferably 80 minutes or less, and when the average crystal grain size of ferrite is to be 20 μm or less, the holding time is 60 minutes or less. Is preferable.
　（ｄ）デスケーリング工程
　加熱した鋼片に対して、デスケーリングを行った後に、後述する熱間圧延を施す。デスケーリングを行うことで、鋼板の表面のＦｅを主体とした酸化スケールを除去し、酸化スケール直下のＭｏおよびＷを、鋼片表層部に濃化させた状態で熱間圧延を施すことで、鋼板表層部に固溶Ｍｏおよび固溶Ｗを濃化させることができる。デスケーリング方法については上記の酸化スケールを除去可能な限りにおいて特に制限はなく、公知の方法を用いればよい。 (D) Descaling step After descaling the heated steel piece, hot rolling described later is performed. By descaling, the oxide scale mainly composed of Fe on the surface of the steel sheet is removed, and Mo and W directly under the oxide scale are hot-rolled in a state where the surface layer of the steel piece is concentrated. The solid solution Mo and the solid solution W can be concentrated on the surface layer of the steel sheet. The descaling method is not particularly limited as long as the above-mentioned oxidation scale can be removed, and a known method may be used.
　（ｅ）熱間圧延工程
　熱間圧延工程において、鋼片に対して熱間圧延を施して鋼板とする。熱間圧延工程は、粗圧延および仕上圧延を含む。 (E) Hot rolling process In the hot rolling process, steel pieces are hot-rolled to obtain steel sheets. The hot rolling process includes rough rolling and finish rolling.
　優れた延性を得たい場合には、上述のような加熱条件の適正化に加えて、仕上圧延条件を適正化することが好ましい。具体的には、熱間圧延を施す時に、粗圧延した後、鋼片の表面温度がＡｒ３－３０℃～Ｔｒｅｘ℃の温度範囲内で、累積圧下率が５０～７５％となる条件で仕上圧延を行うことが好ましい。 When it is desired to obtain excellent ductility, it is preferable to optimize the finish rolling conditions in addition to the optimization of the heating conditions as described above. More specifically, when subjected to hot rolling, after rough rolling, in the temperature range of the surface temperature of the steel strip is Ar 3 -30 ℃ ~ T rex ℃ , under the conditions cumulative rolling reduction is 50-75% It is preferable to perform finish rolling.
　Ａｒ３は鋼を冷却する際のフェライト変態開始温度であり、下記（iv）式で求められる。ここで、鋼組成としてのＡｒ３の値が大きいほど高温でフェライト変態するため、フェライト粒内の転位密度が低下し、伸び特性が向上する。すなわち、Ａｒ３の値が小さすぎると、ベイナイトを形成し伸び特性が劣化する。一方、Ａｒ３の値が大きすぎると、フェライトが粗大化し強度が低下する。そのため、Ａｒ３は７６０～８２０℃であるのが好ましい。
　Ａｒ３＝９１０－３１０×Ｃ＋６５×Ｓｉ－８０×Ｍｎ－２０×Ｃｕ－５５×Ｎｉ－１５×Ｃｒ－８０×Ｍｏ　　　・・・（iv）
　但し、上記式中の元素記号は各元素の含有量（質量％）を表す。 Ar 3 is the ferrite transformation start temperature when the steel is cooled, and is obtained by the following equation (iv). Here, the larger the value of Ar 3 as the steel composition, the higher the ferrite transformation, so that the dislocation density in the ferrite grains decreases and the elongation characteristics improve. That is, if the value of Ar 3 is too small, bainite is formed and the elongation characteristics deteriorate. On the other hand, if the value of Ar 3 is too large, the ferrite becomes coarse and the strength decreases. Therefore, Ar 3 is preferably at 760 to 820 ° C.
Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iv)
However, the element symbol in the above formula represents the content (mass%) of each element.
　また、Ｔｒｅｘは結晶粒の成長が始まる再結晶開始温度を意味し、下記（ｖ）式で求められる。（ｖ）式は実験式であり、低温加熱することで、固溶していないＮｂもあるので固溶Ｎｂ量（ｓｏｌ．Ｎｂ量）を、固溶Ｎｂと再結晶温度の関係から求めた式である。
　Ｔｒｅｘ＝－９１９００［Ｎｂ＊］２＋９４００［Ｎｂ＊］＋７７０　　　・・・（ｖ） Further, Trex means the recrystallization start temperature at which the growth of crystal grains starts, and is calculated by the following formula (v). The formula (v) is an empirical formula, and since some Nb is not solid-solved by heating at a low temperature, the solid-solved Nb amount (sol.Nb amount) is obtained from the relationship between the solid-solved Nb and the recrystallization temperature. Is.
T rex = -91900 [Nb *] 2 +9400 [Nb *] + 770 ··· (v)
　但し、下記（vi）式で求められる固溶Ｎｂ量（質量％）を、ｓｏｌ．Ｎｂとした時に、
　Ｎｂ≧ｓｏｌ．Ｎｂの場合は、［Ｎｂ＊］＝ｓｏｌ．Ｎｂ
　Ｎｂ＜ｓｏｌ．Ｎｂの場合は、［Ｎｂ＊］＝Ｎｂ
　とする。
　ｓｏｌ．Ｎｂ＝（１０（－６７７０／（Ｔ＋２７３）＋２．２６））／（Ｃ＋１２／１４×Ｎ）　　　・・・（vi）
　なお、上記式中のＴは鋼片の加熱抽出温度（℃）を表す。 However, the amount of solid solution Nb (mass%) obtained by the following formula (vi) is determined by sol. When it is Nb,
Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
Nb <sol. In the case of Nb, [Nb *] = Nb
And.
sol. Nb = (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 × N) ・ ・ ・ (vi)
In addition, T in the above formula represents a heating extraction temperature (° C.) of a steel piece.
　仕上圧延をＡｒ３－３０℃以上で行うことで、延伸したフェライトの形成を抑制できる。また、仕上圧延をＴｒｅｘ以下の未再結晶域で行うことで、フェライトの粗大化を抑制することが可能となる。 The finish rolling by performing at Ar 3 -30 ° C. or more, can suppress the formation of stretched ferrite. Further, by performing the finish rolling in the unrecrystallized region of Trex or less, it is possible to suppress the coarsening of ferrite.
　また、累積圧下率が５０％以上であると、オーステナイト中のフェライト核生成サイトが増え、フェライトを細粒化するとともにγ→α変態温度を高めることができる。一方、累積圧下率が７５％を超えると生産性が劣化する。そのため、累積圧下率を５０～７５％とするのが好ましく、５５～６５％とするのがより好ましい。 Further, when the cumulative reduction rate is 50% or more, the number of ferrite nucleation sites in austenite increases, the ferrite can be granulated, and the γ → α transformation temperature can be increased. On the other hand, if the cumulative reduction rate exceeds 75%, the productivity deteriorates. Therefore, the cumulative reduction rate is preferably 50 to 75%, more preferably 55 to 65%.
　（ｆ）冷却工程
　熱間圧延後の鋼板は、室温まで冷却する。耐食性に優れる鋼板を得るためには、冷却条件については特に制限はない。例えば、１℃／秒以下の平均冷却速度での空冷でもよいし、冷却水による冷却を行い、１℃／秒超の平均冷却速度での水冷をしてもよい。 (F) Cooling step The steel sheet after hot rolling is cooled to room temperature. In order to obtain a steel sheet having excellent corrosion resistance, there are no particular restrictions on the cooling conditions. For example, air cooling may be performed at an average cooling rate of 1 ° C./sec or less, or cooling may be performed with cooling water and water cooling may be performed at an average cooling rate of more than 1 ° C./sec.
　さらに、鋼板の延性を向上させるためには、冷却工程において、鋼板の表面温度がＡｒ３－１５０℃～Ａｒ３－５０℃の温度まで、１℃／秒超、２０℃／秒以下の平均冷却速度で水冷を行い、該水冷後、１℃／秒以下の平均冷却速度で空冷を行うことが好ましい。 Furthermore, in order to improve the ductility of the steel sheet, in the cooling step, the surface temperature of the steel sheet to a temperature of Ar 3 -150 ℃ ~ Ar 3 -50 ℃, 1 ℃ / sec greater than the average cooling 20 ° C. / sec or less It is preferable to perform water cooling at a rate, and after the water cooling, perform air cooling at an average cooling rate of 1 ° C./sec or less.
　水冷における冷却停止温度をＡｒ３－１５０℃～Ａｒ３－５０℃の範囲とすることで、変態温度の低温化を防ぎ、フェライト粒内の転位密度上昇またはベイナイト形成を抑制することができる。同様に、水冷の平均冷却速度を２０℃／秒以下とすることで、変態温度の低温化を防ぐことができる。水冷は、空冷の冷却速度以上であれば効果があるので、水冷の平均冷却速度の下限は１℃／秒超とする。 The cooling stop temperature in the water-cooling in a range of from Ar 3 -150 ℃ ~ Ar 3 -50 ℃, prevent lowering the transformation temperature, it is possible to suppress the dislocation density increases or bainite formation in ferrite grains. Similarly, by setting the average cooling rate of water cooling to 20 ° C./sec or less, it is possible to prevent the transformation temperature from becoming low. Since water cooling is effective as long as it is equal to or higher than the cooling rate of air cooling, the lower limit of the average cooling rate of water cooling is set to more than 1 ° C./sec.
　水冷工程を上記の条件で行うことで、フェライト中の平均転位密度を７．０×１０１２／ｍ２以下に制御でき、かつ中央領域に対する表層領域のビッカース硬さの割合を８０～１０５％の範囲にすることができる。 By performing the water cooling step under the above conditions, the average dislocation density in ferrite can be controlled to 7.0 × 10 12 / m 2 or less, and the ratio of Vickers hardness in the surface layer region to the central region is 80 to 105%. Can be a range.
　以下、実施例によって本発明をより具体的に説明するが、本発明はこれらの実施例に限定されるものではない。 Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
　表１の化学組成を有する鋼片を用いて、表２および３の製造条件により板厚５～５０ｍｍの鋼板を試作した。なお、加熱工程と圧延工程との間でデスケーリング工程を実施し、鋼板の表面に形成したＦｅを主体とした酸化スケールを除去した。 Using the steel pieces having the chemical composition shown in Table 1, a steel plate having a thickness of 5 to 50 mm was prototyped according to the manufacturing conditions shown in Tables 2 and 3. A descaling step was carried out between the heating step and the rolling step to remove the Fe-based oxide scale formed on the surface of the steel sheet.
　また、表２の「Ｃａ、Ｍｇ、ＲＥＭ前酸素量」は、Ｃａ、ＭｇおよびＲＥＭから選択される１種以上を添加する前の溶存酸素量を意味する。表３の冷却条件の欄の冷却速度（℃／ｓ）は、実測された表面温度から、公知の差分法による熱伝導解析により求めた１／２厚部での冷却速度である。表３の冷却パターン欄で記載の「空冷」は、水冷（加速冷却）を行わずに空冷を行った例であり、そして、「一部水冷」は圧延後、一部水冷を行った後に空冷を行った例である。 Further, "amount of oxygen before Ca, Mg, REM" in Table 2 means the amount of dissolved oxygen before adding one or more selected from Ca, Mg and REM. The cooling rate (° C./s) in the column of cooling conditions in Table 3 is a cooling rate at 1/2 thickness obtained by heat conduction analysis by a known difference method from the measured surface temperature. "Air cooling" described in the cooling pattern column of Table 3 is an example of air cooling without water cooling (accelerated cooling), and "partially water cooling" is air cooling after rolling and then partially water cooling. This is an example of performing.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　得られた鋼板の金属組織観察を行い、各組織の面積率の測定を行った。具体的には、まず鋼板の圧延方向断面において、鋼板の幅および厚さをそれぞれＷおよびｔとしたときに、該鋼板の端面から１／４Ｗで、かつ、該鋼板の表面から１／４ｔの位置から金属組織観察用の試験片を切り出した。 The metallographic structure of the obtained steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, in the rolling direction cross section of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, it is 1/4 W from the end face of the steel sheet and 1/4 t from the surface of the steel sheet. A test piece for observing the metallographic structure was cut out from the position.
　そして、上記の試験片の圧延方向断面（いわゆるＬ方向断面）をナイタールエッチングし、エッチング後に光学顕微鏡を用いて３００μｍ×３００μｍの視野で観察を行った。得られた組織写真に対し、画像解析を行うことによって、フェライト、パーライトおよびベイナイトのそれぞれの面積率を求めた。 Then, the rolling direction cross section (so-called L direction cross section) of the above test piece was subjected to nightl etching, and after etching, observation was performed with a visual field of 300 μm × 300 μm using an optical microscope. The area ratios of ferrite, pearlite, and bainite were determined by performing image analysis on the obtained microstructure photograph.
　また、フェライトの平均アスペクト比および平均結晶粒径は、以下の手順で測定した。視野内で特定された各フェライトを画像解析により楕円近似し、長軸長さを短軸長さで割ることによって、フェライトのアスペクト比を求めた。同様に、各フェライトを画像解析により面積を求め、この面積に等しい円の直径を求めることで、フェライトの結晶粒径を求めた。そして、視野内の全フェライトのアスペクト比および直径の平均値をそれぞれ算出することによって、平均アスペクト比および平均結晶粒径を求めた。 The average aspect ratio and average grain size of ferrite were measured by the following procedure. Each ferrite identified in the field of view was ellipsically approximated by image analysis, and the aspect ratio of ferrite was obtained by dividing the major axis length by the minor axis length. Similarly, the area of each ferrite was determined by image analysis, and the diameter of a circle equal to this area was determined to determine the grain size of the ferrite. Then, the average aspect ratio and the average grain size were obtained by calculating the average values of the aspect ratio and the diameter of all the ferrites in the visual field, respectively.
　次に、以下の方法により、フェライト中の平均転位密度を求めた。まず、鋼板の表面から１／４ｔの位置から薄膜試料を採取し、ＴＥＭを用いて倍率を４００００倍として明視野の観察撮影を行い、得られたＴＥＭ像から任意の直線と転位線との交切点の数を測定した。そして、以下の式（vii）により平均転位密度を算出した。
　ρ＝２Ｎ／Ｌｄ　　　・・・（vii）
　但し、上記式中の各記号の意味は以下のとおりである。
　ρ：平均転位密度（／ｍ２）
　Ｌ：任意の直線の長さ（ｍ）
　Ｎ：任意の直線と転位線との交切点の数
　ｄ：薄膜試料の厚さ（ｍ） Next, the average dislocation density in ferrite was determined by the following method. First, a thin film sample is taken from the surface of the steel sheet at a position of 1 / 4t, and a bright field observation image is taken using a TEM at a magnification of 40,000 times. From the obtained TEM image, an arbitrary straight line intersects with a dislocation line. The number of cut points was measured. Then, the average dislocation density was calculated by the following formula (vii).
ρ = 2N / Ld ・ ・ ・ (vii)
However, the meaning of each symbol in the above formula is as follows.
ρ: Average dislocation density (/ m 2 )
L: Length of arbitrary straight line (m)
N: Number of intersections between arbitrary straight lines and dislocation lines d: Thickness of thin film sample (m)
　さらに、鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量（質量％）の測定を以下の手順により行った。まず、鋼板の表面から厚さ１ｍｍの試験片を２つ切り出し、そのうちの一方の試験片については、ＩＣＰ発光分光分析法を用いることで、試験片中のＷおよびＭｏの含有量を測定した。 Furthermore, the total content (mass%) of the solid solution Mo and the solid solution W on the surface layer of the steel sheet was measured by the following procedure. First, two test pieces having a thickness of 1 mm were cut out from the surface of the steel sheet, and the contents of W and Mo in the test pieces were measured by using ICP emission spectroscopy for one of the test pieces.
　また、もう一方については、１０％アセチルアセトン－１％テトラメチルアンモニウムクロライド／メタノールにて、２０ｍＡ／ｃｍ２の電流密度で約０．４ｇ電解し、その電解に用いた溶液を孔径０．２μｍのフィルターでろ過し、フィルター上に捕集した抽出残渣について、ＩＣＰ発光分光分析法を用いることで、抽出残渣中のＷおよびＭｏの含有量を測定した。 For the other, about 0.4 g was electrolyzed with 10% acetylacetone-1% tetramethylammonium chloride / methanol at a current density of 20 mA / cm 2 , and the solution used for the electrolysis was filtered with a pore size of 0.2 μm. The contents of W and Mo in the extraction residue were measured by using ICP emission spectroscopic analysis for the extraction residue collected on the filter.
　そして、試験片中のＷおよびＭｏの含有量から抽出残渣中のＷおよびＭｏの含有量の差分を求めることで、固溶Ｗおよび固溶Ｍｏの含有量を求めた。 Then, the contents of the solid solution W and the solid solution Mo were obtained by obtaining the difference between the contents of the W and Mo in the extraction residue from the contents of W and Mo in the test piece.
　続いて、板厚中心部の粗大介在物の個数密度の測定を行った。具体的には、鋼板の表面から２／５ｔの位置と、鋼板の表面から３／５ｔの位置との間の領域において存在する、長さ５μｍ以上の介在物の個数密度を、ＳＥＭによる粒子解析により測定した。 Subsequently, the number density of coarse inclusions in the center of the plate thickness was measured. Specifically, particle analysis by SEM is performed on the number density of inclusions having a length of 5 μm or more existing in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet. Measured by.
　また、鋼板の表面から２／５ｔの位置と、鋼板の表面から３／５ｔの位置との間の領域について、ＥＰＭＡにより測定し、Ｐ濃度の最大値を求め、中心部のＰの最大濃度とした。測定条件は、加速電圧：１５ｋＶ、ビーム径：２０μｍ、照射時間：２０ｍｓ、測定ピッチ：２０μｍとした。 Further, the region between the position of 2/5 t from the surface of the steel sheet and the position of 3/5 t from the surface of the steel sheet is measured by EPMA, the maximum value of P concentration is obtained, and the maximum concentration of P in the central portion is obtained. did. The measurement conditions were an accelerating voltage: 15 kV, a beam diameter: 20 μm, an irradiation time: 20 ms, and a measurement pitch: 20 μm.
　次に、１ｍｍピッチのビッカース硬さの試験を行い、表層領域における平均ビッカース硬さ（表層領域Ｈｖ）および中央領域における平均ビッカース硬さ（中央領域Ｈｖ）を測定した。試験力は１０ｋｇｆ（９８Ｎ）とした。そして、表層領域Ｈｖ／中央領域Ｈｖ（％）を算出した。 Next, a 1 mm pitch Vickers hardness test was performed, and the average Vickers hardness in the surface layer region (surface layer region Hv) and the average Vickers hardness in the central region (central region Hv) were measured. The test force was 10 kgf (98N). Then, the surface layer region Hv / central region Hv (%) was calculated.
　さらに、引張強さ（ＴＳ）、降伏応力（ＹＳ）および全伸び（ｔ－ＥＬ）は、ＪＩＳ　Ｚ　２２４１：２０１１に基づき測定した。試験片は、板幅をＷとした時に、板の片端から板幅方向に１／４Ｗの位置において、板厚中心部から圧延方向に直行する方向（幅方向）を長手方向として採取した、１Ｂ号引張試験片を用いて測定した。降伏応力（ＹＳ）は永久伸び０．２％時の永久伸び法の耐力であり、全伸び（ｔ－ＥＬ）は破断時全伸びとした。 Furthermore, the tensile strength (TS), yield stress (YS) and total elongation (t-EL) were measured based on JIS Z 2241: 2011. The test piece was taken at a position of 1/4 W in the plate width direction from one end of the plate when the plate width was W, and the direction perpendicular to the rolling direction (width direction) from the center of the plate thickness was taken as the longitudinal direction 1B. It was measured using a No. tensile test piece. The yield stress (YS) is the proof stress of the permanent elongation method when the permanent elongation is 0.2%, and the total elongation (t-EL) is the total elongation at break.
　そして、鋼板の耐食性を評価するため、以下に示す３種類の腐食試験を実施した。 Then, in order to evaluate the corrosion resistance of the steel sheet, the following three types of corrosion tests were carried out.
　＜腐食試験１、２＞
　圧延方向に長さ４０ｍｍ、幅方向に長さ４０ｍｍ、厚さ方向に長さ４ｍｍの試験片を鋼板の表面から採取した。切断面（表面以外）は塗料で被覆し、表面は６００番の湿式研磨により、鋼板表面の酸化鉄（スケール）を取り除き、４０ｍｍ×４０ｍｍの鋼板の表面だけ地鉄が露出した試験片とした。該試験片を、塩酸でｐＨを０．２に調整した２０質量％ＮａＣｌ水溶液（腐食試験１）、およびｐＨを０．５に調整した２０質量％ＮａＣｌ水溶液（腐食試験２）の２種類の腐食液中に浸漬した。 <Corrosion test 1, 2>
A test piece having a length of 40 mm in the rolling direction, a length of 40 mm in the width direction, and a length of 4 mm in the thickness direction was collected from the surface of the steel sheet. The cut surface (other than the surface) was coated with paint, and the iron oxide (scale) on the surface of the steel sheet was removed by wet polishing of No. 600 to prepare a test piece in which the base iron was exposed only on the surface of the 40 mm × 40 mm steel sheet. Two types of corrosion of the test piece, a 20 mass% NaCl aqueous solution (corrosion test 1) whose pH was adjusted to 0.2 with hydrochloric acid and a 20 mass% NaCl aqueous solution (corrosion test 2) whose pH was adjusted to 0.5. Immersed in liquid.
　浸漬条件は、液温３０℃、浸漬時間２４時間～４週間で実施し、腐食減量を測定し、腐食速度を評価した。該腐食液組成は、実際の鋼構造物で局部腐食が発生する際の環境の条件を模擬したもので、該腐食試験での腐食速度の低減に応じて実環境で局部腐食の進展速度が低減される。 The immersion conditions were a liquid temperature of 30 ° C. and an immersion time of 24 hours to 4 weeks, and the corrosion weight loss was measured to evaluate the corrosion rate. The corrosive liquid composition simulates the environmental conditions when local corrosion occurs in an actual steel structure, and the rate of local corrosion progresses in the actual environment as the rate of corrosion in the corrosion test decreases. Will be done.
　＜腐食試験３＞
　圧延方向に長さ４０ｍｍ、幅方向に長さ４０ｍｍ、厚さ方向に長さ４ｍｍの試験片を鋼板の表面から採取した。切断面（表面以外）は塗料で被覆し、表面は６００番の湿式研磨により、鋼板表面の酸化鉄（スケール）を取り除き、４０ｍｍ×４０ｍｍの鋼板の表面だけ地鉄が露出した試験片とした。そして、当該試験片を用いて、腐食速度および固体Ｓを主体とするスラッジの生成速度を以下の手順で評価した。 <Corrosion test 3>
A test piece having a length of 40 mm in the rolling direction, a length of 40 mm in the width direction, and a length of 4 mm in the thickness direction was collected from the surface of the steel sheet. The cut surface (other than the surface) was coated with paint, and the iron oxide (scale) on the surface of the steel sheet was removed by wet polishing of No. 600 to prepare a test piece in which the base iron was exposed only on the surface of the 40 mm × 40 mm steel sheet. Then, using the test piece, the corrosion rate and the sludge formation rate mainly composed of solid S were evaluated by the following procedure.
　まず、腐食試験前に、ＮａＣｌの付着量が１０００ｍｇ／ｍ２となるように、試験片の表面にＮａＣｌ水溶液を塗布、乾燥させ、試験チャンバー内の恒温ヒーター板の上に水平に設置した。その後、一定の露点（３０℃）に調整したガスを試験チャンバー内に送った。使用したガスは、ＣＯ２：１２体積％、Ｈ２Ｓ：５００ｐｐｍ、Ｏ２：５体積％、Ｎ２：残部である組成を有する。 First, before the corrosion test, an aqueous NaCl solution was applied to the surface of the test piece so that the amount of NaCl adhered was 1000 mg / m 2 , dried, and horizontally placed on a constant temperature heater plate in the test chamber. Then, a gas adjusted to a constant dew point (30 ° C.) was sent into the test chamber. Gas used is, CO 2: 12 vol%, H 2 S: 500ppm, O 2: 5 by volume%, N 2: having a composition the balance.
　そして、２０℃×１時間と４０℃×１時間の計２時間／サイクルの温度サイクルを与え、試験片表面で乾湿繰り返しが生じるようにした。７２０サイクル後に腐食減量から腐食速度を、試験片表面に生成した生成物質量からスラッジ生成速度を評価した。なお、生成物は化学分析およびＸ線分析で、オキシ水酸化鉄（鉄さび）および固体Ｓであることは予備試験により確認している。 Then, a temperature cycle of 20 ° C. x 1 hour and 40 ° C. x 1 hour for a total of 2 hours / cycle was given so that dry and wet repetition occurred on the surface of the test piece. After 720 cycles, the corrosion rate was evaluated from the corrosion weight loss, and the sludge formation rate was evaluated from the amount of the product produced on the surface of the test piece. It has been confirmed by a preliminary test that the products are iron oxyhydroxide (iron rust) and solid S by chemical analysis and X-ray analysis.
　これらの測定結果を表４～６に示す。なお、表６における「相対腐食速度」および「相対スラッジ生成速度」とは、それぞれ比較例である試験Ｎｏ．４１の腐食速度およびスラッジ生成速度を１００とした時の相対値を意味する。 The results of these measurements are shown in Tables 4-6. The “relative corrosion rate” and “relative sludge generation rate” in Table 6 are the test Nos. It means a relative value when the corrosion rate and sludge formation rate of 41 are set to 100.
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
　表４～６から分かるように、本発明の規定を満足する本発明例（試験Ｎｏ．１～１６、１８、２０～３２、３４～３７、３９、４０および４３～５０）では、いずれの腐食試験においても、優れた耐食性を示していることが分かる。また、なかでも上述した好適な条件を全て満足する試験Ｎｏ．１５、１６、１８、２１～２４、２６～３１、３５および３７では、特に優れた強度－延性バランスを有することが分かる。 As can be seen from Tables 4 to 6, in the examples of the present invention (test Nos. 1 to 16, 18, 20 to 32, 34 to 37, 39, 40 and 43 to 50) that satisfy the provisions of the present invention, any corrosion It can be seen that the test also shows excellent corrosion resistance. In addition, among them, the test No. which satisfies all the above-mentioned suitable conditions. It can be seen that 15, 16, 18, 21-24, 26-31, 35 and 37 have a particularly excellent strength-ductility balance.
　これらに対して、比較例のうち、試験Ｎｏ．１７、１９、３３、３８、４１および４２では、耐食性が劣る結果となった。具体的には、試験Ｎｏ．１７および３８では、熱延前の加熱時における雰囲気中のＯ２濃度が低く、試験Ｎｏ．１９および３３では、加熱時の保持時間が短かったため、スケールの形成が不十分となり、スケール直下でのＭｏおよびＷの濃化が十分に生じなかった。 In contrast to these, among the comparative examples, Test No. In 17, 19, 33, 38, 41 and 42, the result was that the corrosion resistance was inferior. Specifically, the test No. In Nos. 17 and 38, the O 2 concentration in the atmosphere during heating before hot spreading was low, and Test No. In 19 and 33, since the holding time at the time of heating was short, the formation of the scale was insufficient, and the concentration of Mo and W just below the scale was not sufficiently generated.
　また、試験Ｎｏ．４１および４２では、ＭｏおよびＷのいずれも含有しない鋼Ｕを用いた。その結果、これらの例では、鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量が本発明の規定を満足せず、耐食性が劣る結果となった。さらに、比較例である試験Ｎｏ．５１では、加熱工程における加熱時間が長すぎたため、結晶粒が粗大化し、強度－延性バランスが劣化する結果となった。 Also, test No. In 41 and 42, steel U containing neither Mo nor W was used. As a result, in these examples, the total contents of the solid solution Mo and the solid solution W in the surface layer portion of the steel sheet did not satisfy the provisions of the present invention, resulting in inferior corrosion resistance. Further, Test No. which is a comparative example. In No. 51, since the heating time in the heating step was too long, the crystal grains became coarse and the strength-ductility balance deteriorated.
　本発明によれば、本発明によれば、原油中に含まれる腐食性ガス成分、塩分等に対する耐食性に優れる鋼板を得ることが可能になる。したがって、本発明に係る鋼板は、原油油槽用として好適に用いることができる。 According to the present invention, according to the present invention, it is possible to obtain a steel sheet having excellent corrosion resistance against corrosive gas components, salt and the like contained in crude oil. Therefore, the steel sheet according to the present invention can be suitably used for a crude oil tank.

Claims (10)

  1. 　化学組成が、質量％で、
    　Ｃ　：０．０５０～０．２００％、
    　Ｓｉ：０．１００～１．０００％、
    　Ｍｎ：０．５０～２．００％、
    　Ｐ　：０．０３０％以下、
    　Ｓ　：０．０１０％以下、
    　Ａｌ：０．００２～０．０５０％、
    　Ｎ　：０．００１０～０．００６０％、
    　Ｏ　：０．０００５～０．００６０％、
    　Ｔｉ：０．００３～０．０２０％、
    　Ｃｕ：０．０１～１．５０％、
    　Ｃａ：０～０．００８０％、
    　Ｍｇ：０～０．００８０％、
    　ＲＥＭ：０～０．００８０％、
    　Ｍｏ：０～０．２０％、
    　Ｗ　：０～０．５０％、
    　Ｎｂ：０～０．０３０％、
    　Ｖ　：０～０．０５０％、
    　Ｎｉ：０～１．００％、
    　Ｃｒ：０～０．５０％、
    　Ｂ　：０～０．００３０％、
    　Ｓｂ：０～０．３０％、
    　Ｓｎ：０～０．３０％、
    　Ｐｂ：０～０．３０％、
    　Ａｓ：０～０．３０％、
    　Ｂｉ：０～０．３０％、
    　Ｔａ：０～０．５０％、
    　Ｚｒ：０～０．５０％、
    　残部：Ｆｅおよび不純物であり、
    　下記（ｉ）式を満足し、
    　鋼板表層部における固溶Ｍｏおよび固溶Ｗの合計含有量が、質量％で、０．００５％以上であり、
    　鋼板の圧延方向断面において、前記鋼板の厚さをｔとした時に、前記鋼板の表面から１／４ｔの位置におけるフェライトの平均結晶粒径が６０μｍ以下である、
    　鋼板。
    　０．０１≦Ｍｏ＋Ｗ≦０．７０　　　・・・（ｉ）
    　但し、上記式中の元素記号は各元素の含有量（質量％）を表し、含まれない場合はゼロとする。     The chemical composition is% by mass,
    C: 0.050 to 0.200%,
    Si: 0.100 to 1.000%,
    Mn: 0.50 to 2.00%,
    P: 0.030% or less,
    S: 0.010% or less,
    Al: 0.002 to 0.050%,
    N: 0.0010 to 0.0060%,
    O: 0.0005 to 0.0060%,
    Ti: 0.003 to 0.020%,
    Cu: 0.01-1.50%,
    Ca: 0 to 0.0080%,
    Mg: 0 to 0.0080%,
    REM: 0-0.0080%,
    Mo: 0 to 0.20%,
    W: 0 to 0.50%,
    Nb: 0 to 0.030%,
    V: 0 to 0.050%,
    Ni: 0 to 1.00%,
    Cr: 0 to 0.50%,
    B: 0 to 0.0030%,
    Sb: 0 to 0.30%,
    Sn: 0 to 0.30%,
    Pb: 0 to 0.30%,
    As: 0 to 0.30%,
    Bi: 0 to 0.30%,
    Ta: 0 to 0.50%,
    Zr: 0 to 0.50%,
    Remaining: Fe and impurities,
    Satisfy the following equation (i)
    The total content of the solid solution Mo and the solid solution W in the surface layer of the steel sheet is 0.005% or more in mass%.
    In the rolling direction cross section of the steel sheet, when the thickness of the steel sheet is t, the average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet is 60 μm or less.
    Steel plate.
    0.01 ≤ Mo + W ≤ 0.70 ... (i)
    However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
  2. 　前記化学組成が、質量％で、
    　Ｓｉ：０．２００～１．０００％、
    　Ｐ　：０．０１５％以下、
    　Ｓ　：０．００３％以下、
    　であり、
    　下記（ii）式および（iii）式を満足し、
    　下記（iv）式で求められるフェライト変態開始温度Ａｒ３が７６０～８２０℃であり、
    　前記鋼板の表面から１／４ｔの位置における金属組織が、面積％で、
    　パーライト：５～２０％、
    　ベイナイト：１０％以下、
    　残部：フェライトであり、
    　前記鋼板の表面から１／４ｔの位置におけるフェライトの平均アスペクト比が１．０～１．５であり、
    　前記鋼板の表面から１／４ｔの位置におけるフェライトの平均結晶粒径が５～２０μｍである、
    　請求項１に記載の鋼板。
    　０．０００５≦Ｃａ＋Ｍｇ＋ＲＥＭ≦０．００８０　　　・・・（ii）
    　０．５≦Ｔｉ／Ｎ≦４．０　　　・・・（iii）
    　Ａｒ３＝９１０－３１０×Ｃ＋６５×Ｓｉ－８０×Ｍｎ－２０×Ｃｕ－５５×Ｎｉ－１５×Ｃｒ－８０×Ｍｏ　　　・・・（iv）
    　但し、上記式中の元素記号は各元素の含有量（質量％）を表し、含まれない場合はゼロとする。     When the chemical composition is mass%,
    Si: 0.200 to 1.000%,
    P: 0.015% or less,
    S: 0.003% or less,
    And
    Satisfy the following equations (ii) and (iii),
    The ferrite transformation start temperature Ar 3 obtained by the following equation (iv) is 760 to 820 ° C.
    The metallographic structure at a position 1 / 4t from the surface of the steel sheet is in area%.
    Perlite: 5-20%,
    Bainite: 10% or less,
    Remaining: Ferrite,
    The average aspect ratio of ferrite at a position 1 / 4t from the surface of the steel sheet is 1.0 to 1.5.
    The average crystal grain size of ferrite at a position 1/4 t from the surface of the steel sheet is 5 to 20 μm.
    The steel plate according to claim 1.
    0.0005 ≤ Ca + Mg + REM ≤ 0.0080 ... (ii)
    0.5 ≤ Ti / N ≤ 4.0 ... (iii)
    Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iv)
    However, the element symbol in the above formula represents the content (mass%) of each element, and if it is not included, it is set to zero.
  3. 　前記鋼板の表面から１／４ｔの位置におけるフェライト中の平均転位密度が７．０×１０１２／ｍ２以下であり、
    　１ｍｍピッチのビッカース硬さの試験で、前記鋼板の表面と、前記鋼板の表面から１／４ｔの位置との間の領域におけるビッカース硬さの平均値が、前記鋼板の表面から１／４ｔの位置と、前記鋼板の表面から３／４ｔの位置との間の領域におけるビッカース硬さの平均値の８０～１０５％である、
    　請求項２に記載の鋼板。     The average dislocation density in ferrite at a position 1 / 4t from the surface of the steel sheet is 7.0 × 10 12 / m 2 or less.
    In the 1 mm pitch Vickers hardness test, the average value of Vickers hardness in the region between the surface of the steel sheet and the position of 1 / 4t from the surface of the steel sheet is the position of 1 / 4t from the surface of the steel sheet. Is 80 to 105% of the average value of Vickers hardness in the region between the surface of the steel sheet and the position of 3/4 t.
    The steel plate according to claim 2.
  4. 　前記鋼板の表面から２／５ｔの位置と、前記鋼板の表面から３／５ｔの位置との間の領域において存在する、長さ５μｍ以上の介在物の個数密度が１０個／ｍｍ２以下である、
    　請求項１から請求項３までのいずれか１項に記載の鋼板。     The number density of inclusions having a length of 5 μm or more existing in the region between the position 2/5 t from the surface of the steel sheet and the position 3/5 t from the surface of the steel sheet is 10 pieces / mm 2 or less. ,
    The steel sheet according to any one of claims 1 to 3.
  5. 　前記鋼板の表面から２／５ｔの位置と、前記鋼板の表面から３／５ｔの位置との間の領域における、Ｐの最大濃度が、質量％で、０．０２～０．２０％である、
    　請求項１から請求項４までのいずれか１項に記載の鋼板。     The maximum concentration of P in the region between the position of 2/5 t from the surface of the steel sheet and the position of 3/5 t from the surface of the steel sheet is 0.02 to 0.20% in mass%.
    The steel sheet according to any one of claims 1 to 4.
  6. 　溶鋼を製造する精錬工程と、
    　前記溶鋼を連続鋳造して、請求項１に記載の化学組成を有する鋼片を製造する連続鋳造工程と、
    　得られた前記鋼片を加熱する加熱工程と、
    　加熱後の鋼片にデスケーリングを施すデスケーリング工程と、
    　デスケーリング後の鋼片に対して熱間圧延を施して鋼板とする熱間圧延工程と、
    　熱間圧延後の前記鋼板を室温まで冷却する冷却工程と、を備え、
    　前記加熱工程において、前記鋼片に対して、Ｏ２濃度が１．０体積％以上の雰囲気で、９５０℃以上１２００℃未満の加熱温度で３０～１２０分間保持し、加熱抽出温度を９５０℃以上１２００℃未満とする、
    　鋼板の製造方法。     The refining process for manufacturing molten steel and
    A continuous casting step of continuously casting the molten steel to produce a steel piece having the chemical composition according to claim 1.
    A heating step of heating the obtained steel piece and
    A descaling process that descales the heated steel pieces,
    The hot rolling process of hot rolling the descaled steel pieces to make a steel sheet,
    A cooling step of cooling the steel sheet after hot rolling to room temperature is provided.
    Wherein in the heating step, with respect to the steel strip, an O 2 concentration is not less than 1.0 vol% atmosphere, and held at a heating temperature of less than 950 ° C. or higher 1200 ° C. 30 ~ 120 minutes, the heating extraction temperature 950 ° C. or higher Keep below 1200 ° C,
    Steel sheet manufacturing method.
  7. 　溶鋼を製造する精錬工程と、
    　前記溶鋼を連続鋳造して、請求項２に記載の化学組成を有する鋼片を製造する連続鋳造工程と、
    　得られた前記鋼片を加熱する加熱工程と、
    　加熱後の鋼片にデスケーリングを施すデスケーリング工程と、
    　デスケーリング後の鋼片に対して仕上圧延を含む熱間圧延を施して鋼板とする熱間圧延工程と、
    　熱間圧延後の前記鋼板を室温まで冷却する冷却工程と、を備え、
    　前記加熱工程において、前記鋼片に対して、Ｏ２濃度が１．０体積％以上の雰囲気で、９５０～１１００℃の加熱温度で３０～６０分間保持し、加熱抽出温度を９５０～１１００℃とし、
    　前記熱間圧延工程において、前記鋼片の表面温度がＡｒ３－３０℃～Ｔｒｅｘ℃の温度範囲内で、累積圧下率が５０～７５％となる条件で前記仕上圧延を行う、
    　鋼板の製造方法。
    　但し、Ａｒ３は下記（iv）式で求められ、Ｔｒｅｘは結晶粒の成長が始まる再結晶開始温度を意味し、下記（ｖ）式で求められる。なお、下記式中の元素記号は各元素の含有量（質量％）を表す。
    　Ａｒ３＝９１０－３１０×Ｃ＋６５×Ｓｉ－８０×Ｍｎ－２０×Ｃｕ－５５×Ｎｉ－１５×Ｃｒ－８０×Ｍｏ　　　・・・（iv）
    　Ｔｒｅｘ＝－９１９００［Ｎｂ＊］２＋９４００［Ｎｂ＊］＋７７０　　　・・・（ｖ）
    　但し、下記（vi）式で求められる固溶Ｎｂ量（質量％）を、ｓｏｌ．Ｎｂとした時に、
    　Ｎｂ≧ｓｏｌ．Ｎｂの場合は、［Ｎｂ＊］＝ｓｏｌ．Ｎｂ
    　Ｎｂ＜ｓｏｌ．Ｎｂの場合は、［Ｎｂ＊］＝Ｎｂ
    　とする。
    　ｓｏｌ．Ｎｂ＝（１０（－６７７０／（Ｔ＋２７３）＋２．２６））／（Ｃ＋１２／１４×Ｎ）　　　・・・（vi）
    　なお、上記式中のＴは鋼片の加熱抽出温度（℃）を表す。     The refining process for manufacturing molten steel and
    A continuous casting step of continuously casting the molten steel to produce a steel piece having the chemical composition according to claim 2.
    A heating step of heating the obtained steel piece and
    A descaling process that descales the heated steel pieces,
    A hot rolling process in which a steel piece after descaling is hot-rolled, including finish rolling, to form a steel sheet.
    A cooling step of cooling the steel sheet after hot rolling to room temperature is provided.
    In the heating step, with respect to the steel strip, an O 2 concentration is not less than 1.0 vol% atmosphere, and held at a heating temperature of 950 ~ 1100 ℃ 30 ~ 60 minutes, the heating extraction temperature of 950 ~ 1100 ° C. ,
    In the hot rolling step, the surface temperature of the steel piece in a temperature range of Ar 3 -30 ℃ ~ T rex ℃ , performs the finish rolling under conditions where the cumulative rolling reduction is 50-75%,
    Steel sheet manufacturing method.
    However, Ar 3 is calculated by the following formula (iv), and Trex means the recrystallization start temperature at which the growth of crystal grains starts, and is calculated by the following formula (v). The element symbol in the following formula represents the content (mass%) of each element.
    Ar 3 = 910-310 x C + 65 x Si-80 x Mn-20 x Cu-55 x Ni-15 x Cr-80 x Mo ... (iv)
    T rex = -91900 [Nb *] 2 +9400 [Nb *] + 770 ··· (v)
    However, the amount of solid solution Nb (mass%) obtained by the following formula (vi) is determined by sol. When it is Nb,
    Nb ≧ sol. In the case of Nb, [Nb *] = sol. Nb
    Nb <sol. In the case of Nb, [Nb *] = Nb
    And.
    sol. Nb = (10 (-6770 / (T + 273) + 2.26) ) / (C + 12/14 × N) ・ ・ ・ (vi)
    In addition, T in the above formula represents a heating extraction temperature (° C.) of a steel piece.
  8. 　前記冷却工程において、鋼板の表面温度がＡｒ３－１５０℃～Ａｒ３－５０℃の温度まで、１℃／秒超、２０℃／秒以下の平均冷却速度で水冷を行い、該水冷後、１℃／秒以下の平均冷却速度で空冷を行う、
    　請求項７に記載の鋼板の製造方法。     Wherein in the cooling step, to a temperature of the surface temperature of the steel sheet Ar 3 -150 ℃ ~ Ar 3 -50 ℃, 1 ℃ / sec greater performs water cooling at an average cooling rate of 20 ° C. / sec or less, after the water cooling, 1 Air cooling at an average cooling rate of ° C / sec or less,
    The method for manufacturing a steel sheet according to claim 7.
  9. 　前記精錬工程において、真空脱ガス装置により溶鋼の溶存酸素量を４０ｐｐｍ以下に調整し、次いで、ＡｌをＡｌの最終含有量が０．００２～０．０５０％となるように添加して、溶鋼の溶存酸素量を１０ｐｐｍ以下に調整した後、Ｃａ、ＭｇおよびＲＥＭから選択される１種以上を、Ｃａ、ＭｇおよびＲＥＭの合計の最終含有量が０．０００５～０．００８０％となるように添加する、
    　請求項６から請求項８までのいずれか１項に記載の鋼板の製造方法。     In the refining step, the dissolved oxygen content of the molten steel is adjusted to 40 ppm or less by a vacuum degassing device, and then Al is added so that the final content of Al is 0.002 to 0.050% to obtain the molten steel. After adjusting the dissolved oxygen content to 10 ppm or less, one or more selected from Ca, Mg and REM are added so that the total final content of Ca, Mg and REM is 0.0005 to 0.0080%. To do,
    The method for manufacturing a steel sheet according to any one of claims 6 to 8.
  10. 　前記連続鋳造工程において、鋳片の凝固末期である鋳片の中心固相率が０．２～０．７の範囲において、鋳造ロールの間隙を、鋳造進行方向１ｍにつき０．２ｍｍ～３．０ｍｍに狭めて圧下しながら鋳造する、
    　請求項６から請求項９までのいずれか１項に記載の鋼板の製造方法。     In the continuous casting step, in the range of the central solid phase ratio of the slab, which is the final stage of solidification of the slab, in the range of 0.2 to 0.7, the gap between the casting rolls is 0.2 mm to 3.0 mm per 1 m in the casting progress direction. Casting while narrowing down to
    The method for manufacturing a steel sheet according to any one of claims 6 to 9.
PCT/JP2020/010944 2019-03-14 2020-03-12 Steel sheet and production method for same WO2020184683A1 (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
CN202080021699.3A CN113574197B (en) 2019-03-14 2020-03-12 Steel sheet and method for producing same
JP2021505143A JP7201068B2 (en) 2019-03-14 2020-03-12 Steel plate and its manufacturing method
KR1020217033103A KR102376351B1 (en) 2019-03-14 2020-03-12 Steel plate and manufacturing method thereof

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2019047573 2019-03-14
JP2019-047573 2019-03-14

Publications (1)

Publication Number Publication Date
WO2020184683A1 true WO2020184683A1 (en) 2020-09-17

Family

ID=72426115

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2020/010944 WO2020184683A1 (en) 2019-03-14 2020-03-12 Steel sheet and production method for same

Country Status (4)

Country Link
JP (1) JP7201068B2 (en)
KR (1) KR102376351B1 (en)
CN (1) CN113574197B (en)
WO (1) WO2020184683A1 (en)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111485174A (en) * 2020-04-13 2020-08-04 攀钢集团攀枝花钢铁研究院有限公司 Steel rail for subway and preparation method thereof
CN112267073A (en) * 2020-09-30 2021-01-26 东北大学 Corrosion-wear-resistant steel plate with excellent low-temperature toughness and welding performance and preparation method thereof
WO2022145069A1 (en) * 2020-12-28 2022-07-07 日本製鉄株式会社 Steel material
WO2023008163A1 (en) * 2021-07-27 2023-02-02 日本製鉄株式会社 Steel sheet and method for manufacturing same
JP7323090B1 (en) * 2022-03-03 2023-08-08 Ｊｆｅスチール株式会社 Steel plate and steel plate manufacturing method
JP7323091B1 (en) * 2022-03-03 2023-08-08 Ｊｆｅスチール株式会社 Steel plate and its manufacturing method
WO2023162896A1 (en) * 2022-02-22 2023-08-31 日本製鉄株式会社 Steel material for crude oil tank
WO2023166934A1 (en) * 2022-03-03 2023-09-07 Ｊｆｅスチール株式会社 Steel sheet and method for manufacturing same
WO2023166935A1 (en) * 2022-03-03 2023-09-07 Ｊｆｅスチール株式会社 Steel sheet and method for manufacturing steel sheet

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004204344A (en) * 2002-06-19 2004-07-22 Nippon Steel Corp Steel for crude oil tank, its production method, crude oil tank and its corrosion prevention method
JP2006137963A (en) * 2004-11-10 2006-06-01 Sumitomo Metal Ind Ltd Welded joint for crude oil tank, and crude oil tank
JP2010196113A (en) * 2009-02-25 2010-09-09 Jfe Steel Corp Steel for ship having excellent corrosion resistance

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP4898543B2 (en) * 2007-05-02 2012-03-14 株式会社神戸製鋼所 Steel sheet with excellent pit resistance and method for producing the same
JP6007968B2 (en) * 2014-12-26 2016-10-19 新日鐵住金株式会社 High-strength and highly ductile steel plate and its manufacturing method
JP6641875B2 (en) 2015-10-21 2020-02-05 日本製鉄株式会社 Low yield ratio steel sheet and method of manufacturing the same
CN110536973B (en) * 2018-03-16 2020-08-18 日本制铁株式会社 Steel plate for cargo hold of coal/ore transport ship

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004204344A (en) * 2002-06-19 2004-07-22 Nippon Steel Corp Steel for crude oil tank, its production method, crude oil tank and its corrosion prevention method
JP2006137963A (en) * 2004-11-10 2006-06-01 Sumitomo Metal Ind Ltd Welded joint for crude oil tank, and crude oil tank
JP2010196113A (en) * 2009-02-25 2010-09-09 Jfe Steel Corp Steel for ship having excellent corrosion resistance

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111485174A (en) * 2020-04-13 2020-08-04 攀钢集团攀枝花钢铁研究院有限公司 Steel rail for subway and preparation method thereof
CN112267073A (en) * 2020-09-30 2021-01-26 东北大学 Corrosion-wear-resistant steel plate with excellent low-temperature toughness and welding performance and preparation method thereof
WO2022145069A1 (en) * 2020-12-28 2022-07-07 日本製鉄株式会社 Steel material
WO2023008163A1 (en) * 2021-07-27 2023-02-02 日本製鉄株式会社 Steel sheet and method for manufacturing same
JP7277862B1 (en) * 2021-07-27 2023-05-19 日本製鉄株式会社 Steel plate and its manufacturing method
WO2023162896A1 (en) * 2022-02-22 2023-08-31 日本製鉄株式会社 Steel material for crude oil tank
JP7323090B1 (en) * 2022-03-03 2023-08-08 Ｊｆｅスチール株式会社 Steel plate and steel plate manufacturing method
JP7323091B1 (en) * 2022-03-03 2023-08-08 Ｊｆｅスチール株式会社 Steel plate and its manufacturing method
WO2023166934A1 (en) * 2022-03-03 2023-09-07 Ｊｆｅスチール株式会社 Steel sheet and method for manufacturing same
WO2023166935A1 (en) * 2022-03-03 2023-09-07 Ｊｆｅスチール株式会社 Steel sheet and method for manufacturing steel sheet

Also Published As

Publication number Publication date
CN113574197A (en) 2021-10-29
KR102376351B1 (en) 2022-03-18
CN113574197B (en) 2022-09-06
JPWO2020184683A1 (en) 2021-12-16
JP7201068B2 (en) 2023-01-10
KR20210129236A (en) 2021-10-27

Similar Documents

Publication Publication Date Title
JP7201068B2 (en) Steel plate and its manufacturing method
US9109275B2 (en) High-strength galvanized steel sheet and method of manufacturing the same
US20200087764A1 (en) High-strength steel sheet
KR102284770B1 (en) Steel sheet hot-dip-coated with zn-al-mg-based system having excellent workability and method for manufacturing same
TWI486459B (en) High strength hot-rolled steel sheet and method for manufacturing the same
TWI486458B (en) High strength hot-rolled steel sheet and method for manufacturing the same
KR20180120210A (en) Thin steel plate and coated steel sheet, method of manufacturing hot-rolled steel sheet, manufacturing method of cold-rolled full-hard steel sheet, manufacturing method of thin steel sheet and manufacturing method of coated steel sheet
JP6418361B1 (en) Steel plate for holding coal and ore carrier
EP2952603B1 (en) High-strength hot-rolled steel sheet and method for manufacturing the same
CN116694988A (en) Steel sheet, plated steel sheet, method for producing steel sheet, and method for producing plated steel sheet
WO2020158065A1 (en) High-strength steel sheet and method for manufacturing same
JPWO2020026594A1 (en) High-strength hot-rolled steel sheet and manufacturing method thereof
JP6275510B2 (en) High strength hot rolled steel sheet and method for producing the same
JP5272412B2 (en) High strength steel plate and manufacturing method thereof
TW202012649A (en) Steel sheet
KR20180120715A (en) Thin steel plate and coated steel sheet, method of manufacturing hot-rolled steel sheet, manufacturing method of cold-rolled full-hard steel sheet, manufacturing method of thin steel sheet and manufacturing method of coated steel sheet
CN116018416A (en) Steel sheet and method for producing same
JP5124865B2 (en) High tensile cold-rolled steel sheet and method for producing the same
CN115443344B (en) Steel sheet and method for producing same
JP6610788B2 (en) High strength hot rolled galvanized steel sheet
JP6320792B2 (en) High strength hot rolled steel sheet and method for producing the same
JP2001355043A (en) High strength hot dip galvannealed steel sheet and high strength steel sheet excellent in hole expandability and ductility and their production method
JP5971281B2 (en) Method for producing high-strength hot-rolled steel sheet with excellent workability and toughness
JP7277862B1 (en) Steel plate and its manufacturing method
WO2024053663A1 (en) Plated steel sheet

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 20769056

Country of ref document: EP

Kind code of ref document: A1

ENP Entry into the national phase

Ref document number: 2021505143

Country of ref document: JP

Kind code of ref document: A

NENP Non-entry into the national phase

Ref country code: DE

ENP Entry into the national phase

Ref document number: 20217033103

Country of ref document: KR

Kind code of ref document: A

122 Ep: pct application non-entry in european phase

Ref document number: 20769056

Country of ref document: EP

Kind code of ref document: A1