WO2023223694A1 - Steel sheet and method for producing same - Google Patents

Steel sheet and method for producing same Download PDF

Info

Publication number
WO2023223694A1
WO2023223694A1 PCT/JP2023/013709 JP2023013709W WO2023223694A1 WO 2023223694 A1 WO2023223694 A1 WO 2023223694A1 JP 2023013709 W JP2023013709 W JP 2023013709W WO 2023223694 A1 WO2023223694 A1 WO 2023223694A1
Authority
WO
WIPO (PCT)
Prior art keywords
less
steel sheet
content
steel
temperature
Prior art date
Application number
PCT/JP2023/013709
Other languages
French (fr)
Japanese (ja)
Inventor
茂樹 木津谷
俊一 村上
順平 釘屋
Original Assignee
Ｊｆｅスチール株式会社
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 Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to JP2023534743A priority Critical patent/JP7367896B1/en
Publication of WO2023223694A1 publication Critical patent/WO2023223694A1/en

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a steel plate, and particularly to a steel plate that has both high strength and excellent low-temperature toughness and can be suitably used as a structural steel used in a low-temperature environment such as a liquefied gas storage tank.
  • the present invention also relates to a method for manufacturing the steel plate.
  • thick steel plates used in low-temperature structures such as liquefied gas storage tanks are required to have excellent low-temperature toughness from the perspective of ensuring safety against brittle fracture at low temperatures.
  • liquefied natural gas LNG
  • the thick steel plates used in LNG storage tanks have excellent toughness at temperatures below -164°C. required.
  • thick steel plates that contain high concentrations of Ni of 7% or 9% and have excellent low-temperature toughness have been used for applications such as liquefied gas storage tanks.
  • Patent Document 1 proposes a method for producing 9% Ni steel with a thickness of 40 mm or more by sequentially subjecting a hot rolled steel plate to quenching treatment, two-phase region quenching treatment, and tempering treatment.
  • Patent Document 2 proposes a method that can easily produce a thick 9% Ni steel plate having excellent low-temperature toughness.
  • a method that can easily produce a thick 9% Ni steel plate having excellent low-temperature toughness by reducing the amount of Si in the steel and at the same time applying appropriate hot rolling to control the microstructure, a large amount of stable retained austenite ( ⁇ ) can be produced without the need for two-phase region quenching treatment. It is possible to obtain excellent toughness over a wide tempering temperature range.
  • Patent Document 3 proposes a Ni-containing steel plate with excellent toughness in which the average coarse grain size of prior austenite at the 1/4t position of the steel plate is 20 ⁇ m or less.
  • Patent Documents 1 to 3 have the following problems.
  • the present invention has been made in view of the above-mentioned circumstances, and aims to provide a steel plate that has both high strength and excellent low-temperature toughness, and is also excellent in manufacturability, and a method for manufacturing the same.
  • the present inventors conducted intensive research on the chemical composition and manufacturing method of steel sheets, targeting Ni-containing steel sheets suitable for structural steel used in low-temperature environments, and conducted the following research. I gained knowledge.
  • the present invention was made by further considering the above findings, and the gist thereof is as follows.
  • the component composition further includes, in mass%, Cu: 0.01 to 1.00%, Cr: 0.01-1.50%, Mo: 0.03-1.0%, Nb: 0.001-0.030%, V: 0.01 to 0.10%, Ti: 0.003 to 0.050%, B: 0.0003 to 0.0050%, Sn: 0.01-0.30%, Sb: 0.01 to 0.30%, W: more than 0%, less than 2.00%, Co: more than 0%, less than 2.00%, Ca: 0.0005-0.0050%, Mg: 0.0005-0.0100%, Zr: 0.0005 to 0.0050%, Ta: 0.01-0.20%, Y: 0.001 to 0.010%, and REM: 0.0010 to 0.0200%,
  • the steel plate according to 1 above containing at least one selected from the group consisting of.
  • a heating step of heating a steel material having the composition described in 1 or 2 above to a heating temperature of 900°C or more and 1200°C or less The steel material heated in the heating step is hot-rolled to a plate thickness of 40 mm under the conditions that the rolling reduction ratio is 5 or more and the number of passes is 2 or more in which the rolling reduction ratio per pass is 10% or more among the final 5 passes.
  • a hot rolling process to produce the following hot rolled steel sheet a cooling step of cooling the hot rolled steel sheet; a reheating and quenching step of reheating and quenching the hot rolled steel sheet after the cooling step to a reheating temperature of 3 Ac or more and 900° C. or less;
  • a method for manufacturing a steel sheet comprising a tempering step of tempering the hot rolled steel sheet after the reheating and quenching step at a tempering temperature of 500° C. or more and 650° C. or less.
  • the steel plate of the present invention has both high strength and excellent low-temperature toughness.
  • the steel sheet of the present invention can be manufactured by performing general reheating and quenching and tempering after the hot rolling process, other products cannot be simultaneously produced on the same line that manufactures the steel sheet of the present invention. It also has excellent manufacturability.
  • a steel plate in one embodiment of the present invention has a specific composition, microstructure, plate thickness, tensile strength, and yield strength. The reasons for each limitation will be explained below.
  • C 0.01-0.15%
  • C is an element that has the effect of increasing the strength of the steel plate.
  • the C content is set to 0.01% or more, preferably 0.03% or more.
  • the C content is set to 0.15% or less, preferably 0.10% or less, and more preferably 0.08% or less.
  • Si 0.01 ⁇ 1.00%
  • Si is an element that acts as a deoxidizing agent in the steel manufacturing process. Furthermore, Si has the effect of increasing the strength of the steel plate through solid solution strengthening. In order to obtain the above effect, the Si content is set to 0.01% or more. On the other hand, if the Si content is higher than 1.00%, low-temperature toughness decreases due to an increase in inclusions, and weldability and surface quality deteriorate. Therefore, the Si content is set to 1.00% or less, preferably 0.5% or less, and more preferably 0.3% or less.
  • Mn 0.10-2.00%
  • Mn is an element that has the effect of improving the hardenability of a steel plate and increasing its strength.
  • the Mn content is set to 0.10% or more, preferably 0.40% or more.
  • the Mn content is 2.00% or less, preferably 1.00% or less.
  • the P content exceeds 0.010%, low temperature toughness decreases. This is because P segregates at grain boundaries, reduces grain boundary strength, and becomes a starting point for fracture. Therefore, the P content is set to 0.010% or less.
  • the lower limit of the P content is not particularly limited and may be 0%.
  • excessive reduction leads to an increase in manufacturing costs and a decrease in productivity. Therefore, from the viewpoint of industrial production, it is preferable that the P content is 0.001% or more.
  • S 0.0050% or less S forms MnS in steel and significantly deteriorates low temperature toughness. Therefore, it is desirable to reduce S as much as possible, and the S content is 0.0050% or less, preferably 0.0020% or less. On the other hand, since it is desirable to reduce S as much as possible from the viewpoint of low-temperature toughness, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction causes an increase in manufacturing costs and a decrease in productivity, so from the viewpoint of industrial production, it is preferable to set the S content to 0.0001% or more.
  • Ni 5.0-10.0%
  • Ni is an element that has the effect of improving the strength of the steel plate. Further, Ni is an extremely effective element for improving the low-temperature toughness of steel sheets. If the Ni content is less than 5.0%, desired strength and low temperature toughness cannot be obtained. Therefore, the Ni content is set to 5.0% or more, preferably 6.5% or more, more preferably 6.8% or more, and even more preferably 8.0% or more. On the other hand, since Ni is an expensive element, the higher the Ni content, the higher the cost of the steel sheet. Therefore, the Ni content is 10.0% or less, preferably 9.5% or less.
  • Al 0.002-0.100%
  • Al is an element that acts as a deoxidizing agent and is commonly used in the molten steel deoxidizing process. Moreover, Al reacts with N in steel to form AlN. This reaction reduces solid solution N, resulting in improved low-temperature toughness.
  • the Al content is set to 0.002% or more, preferably 0.010% or more, and more preferably 0.020% or more.
  • the Al content is set to 0.100% or less, preferably 0.070% or less, and more preferably 0.060% or less.
  • N 0.0080% or less N forms nitrides and carbonitrides, thereby reducing low-temperature toughness. If the N content is higher than 0.0080%, desired low temperature toughness cannot be obtained. Therefore, the N content is set to 0.0080% or less, preferably 0.0040% or less. On the other hand, from the viewpoint of low-temperature toughness, it is desirable to reduce N as much as possible, so the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction causes an increase in manufacturing costs and a decrease in productivity, so from the viewpoint of industrial production, it is preferable to set the N content to 0.0010% or more.
  • a steel plate in an embodiment of the present invention has a composition containing the above elements, with the balance consisting of Fe and inevitable impurities.
  • the chemical composition of the steel sheet in another embodiment of the present invention may optionally further contain at least one of the elements listed below for the purpose of further improving the characteristics of the steel sheet.
  • Cu 0.01 ⁇ 1.00%
  • Cu is an element that has the effect of further increasing the strength of the steel sheet by improving hardenability.
  • the Cu content is set to 0.01% or more.
  • the Cu content is 1.00% or less, preferably 0.30% or less.
  • Cr 0.01 ⁇ 1.50% Cr is an element effective in further improving the strength of the steel sheet.
  • the Cr content is set to 0.01% or more in order to obtain the above effects.
  • Cr may precipitate as precipitates such as nitrides, carbides, and carbonitrides during rolling, and the precipitates serve as starting points for corrosion and fracture, reducing low-temperature toughness. Therefore, the Cr content is set to 1.50% or less, preferably 1.00% or less.
  • Mo 0.03 ⁇ 1.0%
  • Mo is an element that has the effect of suppressing the temper embrittlement susceptibility of steel sheets. Moreover, Mo has the effect of further improving the strength of the steel sheet.
  • Mo content is set to 0.03% or more, preferably more than 0.05%, in order to obtain the above effect.
  • the Mo content exceeds 1.0%, low temperature toughness decreases. Therefore, the Mo content is 1.0% or less, preferably 0.30% or less.
  • Nb 0.001-0.030%
  • Nb is an element that has the effect of further improving the strength of the steel plate.
  • the Nb content is set to 0.001% or more, preferably 0.005% or more, and more preferably 0.007% or more.
  • the Nb content is set to 0.030% or less, preferably 0.025% or less, and more preferably 0.022% or less.
  • V is an element that has the effect of further improving the strength of the steel plate.
  • the V content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more.
  • the V content is set to 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less.
  • Ti 0.003 ⁇ 0.050%
  • Ti is an element that precipitates as a nitride or carbonitride and has the effect of further refining the austenite grains in the steel sheet structure.
  • the Ti content is set to 0.003% or more, preferably 0.005% or more, and more preferably 0.007% or more.
  • the Ti content is set to 0.050% or less, preferably 0.035% or less, and more preferably 0.032% or less.
  • B 0.0003-0.0050%
  • B is an element that has the effect of further improving the strength of the steel plate.
  • the B content is set to 0.0003% or more in order to obtain the above effect.
  • the B content is set to 0.0050% or less, preferably 0.0030% or less.
  • Sn 0.01-0.30%
  • Sn is an element that has the effect of improving the corrosion resistance of steel sheets, and even when contained in a small amount, the effect is exhibited. Therefore, when adding Sn, the Sn content is set to 0.01% or more. On the other hand, if Sn is excessive, low-temperature toughness decreases. Therefore, the Sn content is 0.30% or less, preferably 0.25% or less.
  • Sb 0.01 ⁇ 0.30%
  • Sb is an element that has the effect of improving the corrosion resistance of steel sheets, and even if it is contained in a small amount, it exhibits the effect. Therefore, when adding Sb, the Sb content is set to 0.01% or more. On the other hand, if Sb is excessive, low-temperature toughness decreases and cost increases. Therefore, the Sb content is 0.30% or less, preferably 0.25% or less.
  • W More than 0%, 2.00% or less W, like Sn and Sb, is an element that has the effect of improving the corrosion resistance of steel sheets, and even if it is contained in a small amount, it is effective. Therefore, when adding W, the W content is set to more than 0%, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if W is in excess, the low-temperature toughness decreases and the cost increases. Therefore, the W content is set to 2.00% or less, preferably 0.50% or less.
  • Co more than 0%, 2.00% or less
  • Co is an element that has the effect of improving the corrosion resistance of steel sheets, and even if it is contained in a small amount, it is effective. Therefore, when adding Co, the Co content should be more than 0%, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if Co is in excess, the cost will increase. Therefore, the Co content is 2.00% or less, preferably 1.50% or less.
  • Ca 0.0005-0.0050%
  • Ca is an element effective in controlling the morphology of inclusions such as MnS. Controlling the form of inclusions means suppressing the formation of expanded sulfide-based inclusions to form granular inclusions. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance.
  • the Ca content is set to 0.0005% or more, preferably 0.0010% or more in order to obtain the above effects.
  • the Ca content is 0.0050% or less, preferably 0.0040% or less.
  • Mg 0.0005-0.0100% Mg, like Ca, is an element effective in controlling the morphology of inclusions such as MnS. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance.
  • the Mg content is set to 0.0005% or more, preferably 0.0010% or more.
  • the Mg content is set to 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0040% or less.
  • Zr 0.0005-0.0050% Zr, like Ca and Mg, is an element effective in controlling the morphology of inclusions such as MnS. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance.
  • the Zr content is set to 0.0005% or more, preferably 0.0010% or more in order to obtain the above effects.
  • the Zr content is 0.0050% or less, preferably 0.0040% or less.
  • Ta 0.01 ⁇ 0.20%
  • Ta is an element effective in further improving the strength of the steel sheet.
  • the Ta content is set to 0.01% or more in order to obtain the above effect.
  • the Ta content exceeds 0.20%, the low temperature toughness decreases due to the formation of precipitates. Therefore, the Ta content is set to 0.20% or less.
  • Y 0.001-0.010%
  • Y is an element effective in forming an oxide that is stable at high temperatures. The formation of the oxide makes it possible to effectively suppress coarsening of prior austenite grains in the weld heat affected zone, thereby improving the toughness of the weld zone.
  • the Y content is set to 0.001% or more in order to obtain the above effects.
  • the Y content exceeds 0.010%, the amount of inclusions increases and low-temperature toughness decreases. Therefore, the Y content is set to 0.010% or less.
  • REM 0.0010-0.0200% REM (rare earth metal), like Ca, Mg, and Zr, is an element effective in controlling the morphology of inclusions such as MnS. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance.
  • the REM content is set to 0.0010% or more, preferably 0.0020% or more in order to obtain the above effects.
  • the REM content is set to 0.0200% or less.
  • the steel plate in one embodiment of the present invention has a microstructure that satisfies the following conditions (1) to (3).
  • (1) The volume fraction of retained austenite at the 1/4 position of the plate thickness is less than 3.0%.
  • (2) The maximum grain size of prior austenite grains at the 1/2 position of the plate thickness is 100 ⁇ m or less.
  • (3) The ratio b/a of the average value b of the top 5% of the grain sizes of the prior austenite grains to the average grain size a of the prior austenite grains at the 1/2 plate thickness position is 4.5 or less.
  • prior austenite grains refers to ⁇ grains before transformation when viewed from a steel sheet that has undergone structural transformation as austenite ( ⁇ ) is cooled and has a different structure.
  • the amount of retained austenite at the 1/4 plate thickness position is set to be less than 3.0% in terms of volume fraction, preferably 2.8% or less, and more preferably 2.6% or less.
  • the lower limit of the volume fraction of retained austenite is not particularly limited, and may be 0% or 0.5% or more.
  • the microstructure is mainly composed of tempered martensite and bainite. Specifically, it is preferable that the total area ratio of tempered martensite and bainite is 90% or more. The upper limit of the total area ratio of tempered martensite and bainite is not particularly limited, but may be 100%.
  • the volume fraction of the retained austenite can be measured by X-ray diffraction. More specifically, it can be measured by the method described in Examples.
  • the maximum grain size of prior ⁇ grains at the 1/2 plate thickness position is set to 100 ⁇ m or less, preferably 80 ⁇ m or less.
  • the lower limit of the maximum grain size is not particularly limited, but in order to make the maximum grain size of the prior ⁇ grains 20 ⁇ m or less, it is necessary to control the quenching conditions very strictly, resulting in poor productivity.
  • the maximum particle size is preferably greater than 20 ⁇ m, more preferably 22 ⁇ m or more, and even more preferably 25 ⁇ m or more.
  • the equivalent circle diameter is used as the grain size of the prior austenite grains.
  • the maximum grain size of the prior austenite grains can be measured using an optical microscope. More specifically, it can be measured by the method described in Examples.
  • the ratio b/a of the average value b of the top 5% of the grain sizes of the prior austenite grains to the average grain size a of the prior austenite grains is set to 4.5 or less.
  • b/a is preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less.
  • the lower limit of b/a is not particularly limited, but the theoretical lower limit is 1.
  • b/a may be 1.2 or more, and may be 1.3 or more. Note that, as the values of a and b, the values at the plate thickness 1/2 position are used.
  • the average particle diameter a and the average value b can be measured using an optical microscope. More specifically, it can be measured by the method described in Examples.
  • the aspect ratio of the prior austenite grains at the 1/2 plate thickness position is not particularly limited, but is preferably 2.0 or less.
  • the aspect ratio is 2.0 or less, the anisotropy of mechanical properties, especially the anisotropy of low-temperature toughness, is improved.
  • the aspect ratio of the prior austenite grains can be measured using an optical microscope. More specifically, it can be measured by the method described in Examples.
  • Plate thickness 40 mm or less
  • the thickness of the steel plate should be 40 mm or less.
  • the plate thickness is 40 mm or less, the heat treatment time is shortened, so that the influence of temper embrittlement during tempering can be reduced. Therefore, in the present invention, restrictions on the Si content are small.
  • the lower limit of the plate thickness is not particularly limited, but is preferably 6 mm or more.
  • TS tensile strength
  • the tensile strength (TS) of the steel plate according to the present invention is 690 MPa or more. Since the steel plate of the present invention has a high tensile strength of 690 MPa or more, it can be suitably used for applications such as LNG tanks.
  • the upper limit of the tensile strength is not particularly limited, but may be, for example, 830 MPa or less, or 800 MPa or less.
  • yield strength (YS) of the steel plate according to the present invention is 585 MPa or more. Since the steel sheet of the present invention has a high yield strength of 585 MPa or more, it can be suitably used for applications such as LNG tanks. On the other hand, the upper limit of the yield strength is not particularly limited, and may be, for example, 790 MPa or less, or 770 MPa or less.
  • the above tensile strength and yield strength can be measured by a tensile test based on JIS Z 2204. More specifically, it can be measured by the method described in Examples.
  • the absorbed energy vE -196 at -196°C is 100 J or more.
  • the steel sheet of the present invention has high low temperature toughness with vE -196 of 100J or more, so it can be suitably used for applications such as LNG tanks.
  • the absorbed energy vE -196 is preferably 150J or more.
  • the upper limit of the absorbed energy vE -196 is not particularly limited, but may be, for example, 400 J or less, or 350 J or less.
  • the absorbed energy vE -196 can be measured by a Charpy impact test in accordance with JIS Z 2242. More specifically, it can be measured by the method described in Examples.
  • the steel plate can be manufactured by sequentially performing the following steps (1) to (5) on a steel material having the above-mentioned composition.
  • the temperature "°C" means the temperature at the 1/2 position of the plate thickness.
  • the temperature at the 1/2 plate thickness position is determined by differential calculation or the like.
  • the steel material may be, for example, a steel slab.
  • the method for manufacturing the steel material is not particularly limited, but, for example, it can be manufactured by melting molten steel having the above-mentioned composition by a conventional method and casting.
  • the melting can be performed by any method such as a converter, an electric furnace, an induction furnace, or the like.
  • the casting is preferably carried out by a continuous casting method, but may be carried out by an ingot-forming method.
  • Heating process In the heating step, the steel material is heated to a heating temperature of 900°C or more and 1200°C or less.
  • the heating may be performed after once cooling the steel material obtained by a method such as casting, or the obtained steel material may be directly subjected to the heating without cooling.
  • the purpose of heating the steel material is to dissolve the precipitates in the structure of the steel material.
  • the heating temperature is set to 900°C or higher.
  • the heating temperature of the steel material is 1200°C or lower, preferably 1150°C or lower.
  • the heating time is not particularly limited, but is preferably 2 hours or more and 8 hours or less.
  • the steel material heated in the heating step is hot rolled into a hot rolled steel plate having a thickness of 40 mm or less.
  • the rolling end temperature is not particularly limited, it is preferably 700° C. or higher, which is the austenite single phase region.
  • the upper limit of the rolling end temperature is not particularly limited, but is preferably 950°C or lower, more preferably 920°C or lower.
  • Reduction ratio 5 or more
  • the rolling reduction ratio in the hot rolling step is 5 or more, preferably 6 or more, and more preferably 10 or more.
  • the upper limit of the rolling reduction ratio is not particularly limited, but is preferably 50 or less. Note that the rolling ratio is defined here as (thickness of steel material/thickness of hot rolled steel sheet after hot rolling).
  • Number of passes with a rolling reduction of 10% or more among the final 5 passes 2 or more Recrystallization of austenite grains in the latter half of the hot rolling process is particularly effective for grain refinement and grain size regulation of the steel sheet structure. . Therefore, the number of passes in which the rolling reduction per pass is 10% or more among the final five passes of the hot rolling process is set to two or more. If the number of passes is less than 2, grain size regulation of austenite grains will not proceed sufficiently. As a result, b/a in the finally obtained steel plate becomes larger than 4.0, and low-temperature toughness deteriorates.
  • the number of passes in which the rolling reduction per pass is 10% or more among the final five passes of the hot rolling process is preferably 3 or more, and 4 or more. It is more preferable to set it to 5, and even more preferably to set it to 5.
  • the cooling process Next, the hot rolled steel sheet obtained in the above hot rolling process is cooled.
  • the cooling suppresses coarsening of precipitates and improves strength and toughness.
  • the cooling stop temperature in the cooling step is not particularly limited, but may be, for example, room temperature (20° C., etc.) or higher. Further, the cooling stop temperature is preferably 400°C or less.
  • the cooling is not particularly limited, and can be performed by any method such as air cooling or water cooling.
  • Water cooling such as spray cooling, mist cooling, laminar cooling, etc. may be performed to enhance necessary properties such as strength and low temperature toughness.
  • water cooling since the cooling is rapid, the elongated structure formed during rolling remains, and the anisotropy of the structure tends to increase. Therefore, from the viewpoint of reducing the anisotropy of the structure, it is preferable to perform air cooling.
  • the hot rolled steel sheet after the cooling step is heated to a reheating temperature of 3 Ac or more and 900° C. or less, and then quenched.
  • a reheating temperature 3 Ac or more and 900° C. or less
  • the structure of the entire hot rolled steel sheet undergoes reverse transformation to austenite.
  • the grain size of the steel plate structure is further refined, and low-temperature toughness is improved.
  • the reheating temperature is less than the Ac3 point, the microstructure of the heated steel sheet will contain a ferrite phase, making it impossible to make the microstructure uniform, resulting in a decrease in low-temperature toughness.
  • austenite grains grow and become coarse, resulting in a decrease in low-temperature toughness.
  • Ac3 points are calculated using the following formula (1).
  • Ac3(°C) 937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B...(1)
  • the element symbol in the above formula (1) represents the content (mass%) of each element, and is set to 0 when the element is not contained.
  • Quenching can be carried out under any conditions without particular limitations, but it is preferably carried out by water cooling.
  • the conditions for the quenching are not particularly limited, but it is preferable to cool to a cooling stop temperature of less than 200°C.
  • the cooling stop temperature is more preferably 100°C or lower, and even more preferably 50°C or lower.
  • the lower limit of the cooling stop temperature is not particularly limited, but for example, the cooling stop temperature may be room temperature or higher.
  • the tempering step the steel plate after the reheating and quenching step is tempered at a tempering temperature of 500° C. or more and 650° C. or less.
  • the tempering temperature is set at 500°C or more and 650°C or less.
  • the cooling method is not particularly limited, and any method such as air cooling or water cooling can be used.
  • Steel having the composition shown in Table 1 was melted to obtain a steel slab as a steel material.
  • the obtained steel slabs were sequentially subjected to a heating process, a hot rolling process, a cooling process, a reheating quenching process, and a tempering process under the conditions shown in Table 2 to produce a steel plate.
  • the steel plate was reheated to the reheating temperature shown in Table 2, and then cooled to a cooling stop temperature of less than 200°C.
  • the top 5 grain sizes of prior austenite grains are determined based on the amount of retained austenite (volume ratio), the maximum grain size of prior austenite grains, and the average grain size a of prior austenite grains using the following procedure.
  • the ratio b/a of the average value b in % was determined. The measurement results are shown in Table 3.
  • a test piece for X-ray diffraction was taken parallel to the plate surface so that the measurement plane was at 1/4 of the plate thickness of the obtained steel plate.
  • the test piece was subjected to mirror polishing and electrolytic polishing, and then subjected to X-ray diffraction.
  • the diffraction intensities of the (200) and (211) planes of ⁇ -Fe and the (200), (220) and (311) planes of ⁇ -Fe that appear in the symmetrical reflection X-ray diffraction pattern are determined, and the amount of retained austenite is calculated using the following formula.
  • V ⁇ was calculated.
  • V ⁇ 100/((I ⁇ R ⁇ /I ⁇ R ⁇ )+1)
  • V ⁇ volume fraction of retained austenite
  • I diffraction X-ray intensity
  • R theoretical intensity value per unit volume.
  • I the integrated intensity after background removal was used.
  • the amount of retained austenite is extremely small, sufficient measurement accuracy cannot be obtained, so if the calculated amount of retained austenite is 0.5% or less, the microstructure does not substantially contain retained austenite. (0%), and the volume percentage column of residual ⁇ in Table 3 was left blank (-).
  • the steel plates in all Examples and Comparative Examples had microstructures mainly composed of tempered martensite and bainite. Specifically, the total area ratio of tempered martensite and bainite was 90% or more.
  • the ratio obtained by dividing the long axis by the short axis when prior austenite grains are approximated to an ellipse was calculated as the aspect ratio.
  • a vE -196 of 100J or more was considered to be a pass.
  • a vE -196 of 50J or more was considered a pass.
  • the steel plates of the examples of the present invention satisfied the above characteristics and were excellent in strength (tensile strength and yield strength), low-temperature toughness, and manufacturability.
  • steel plates of comparative examples outside the scope of the present invention were inferior in at least one of strength (tensile strength and yield strength), low-temperature toughness, and manufacturability.

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)

Abstract

The present invention provides a steel sheet which has both high strength and excellent toughness at low temperatures, while having excellent productivity. The present invention provides a steel sheet which has a specific component composition and a microstructure wherein: the volume fraction of residual austenite at 1/4 the sheet thickness is less than 3.0%; the maximum particle grain size of prior austenite grains at 1/2 the sheet thickness is 100 µm or less; and the ratio b/a of the average b of the top 5% of the prior austenite grain sizes to the average grain size a of prior austenite grains at 1/2 the sheet thickness is 4.5 or less. This steel sheet has a sheet thickness of 40 mm or less, a yield strength of 585 MPa or more and a tensile strength of 690 MPa or more.

Description

鋼板およびその製造方法Steel plate and its manufacturing method
　本発明は、鋼板に関し、特に、高強度と優れた低温靭性とを兼ね備え、液化ガス貯蔵用タンクなどの低温環境で使用される構造用鋼として好適に用いることができる鋼板に関する。また、本発明は前記鋼板の製造方法に関する。 The present invention relates to a steel plate, and particularly to a steel plate that has both high strength and excellent low-temperature toughness and can be suitably used as a structural steel used in a low-temperature environment such as a liquefied gas storage tank. The present invention also relates to a method for manufacturing the steel plate.
　液化ガス貯蔵用タンク等の低温用構造物に用いられる厚鋼板には、強度に加えて、低温での脆性破壊に対する安全性確保の観点から、低温靱性に優れることが要求される。例えば、液化天然ガス（ＬＮＧ）は、ＬＮＧの沸点である－１６４℃以下の低温で貯蔵されるため、ＬＮＧ貯蔵用タンクに用いられる厚鋼板には、－１６４℃以下の温度において優れた靱性が要求される。 In addition to strength, thick steel plates used in low-temperature structures such as liquefied gas storage tanks are required to have excellent low-temperature toughness from the perspective of ensuring safety against brittle fracture at low temperatures. For example, liquefied natural gas (LNG) is stored at low temperatures below -164°C, the boiling point of LNG, so the thick steel plates used in LNG storage tanks have excellent toughness at temperatures below -164°C. required.
　そこで、従来、液化ガス貯蔵用タンク等の用途には、７％や９％と高濃度のＮｉを含むみ、低温靭性に優れる厚鋼板が使用されてきた。 Therefore, thick steel plates that contain high concentrations of Ni of 7% or 9% and have excellent low-temperature toughness have been used for applications such as liquefied gas storage tanks.
　例えば、特許文献１では、熱延鋼板に焼入れ処理、二相域焼入れ処理、および焼戻し処理を順次施すことにより、板厚４０ｍｍ以上の９％Ｎｉ鋼を製造する方法が提案されている。 For example, Patent Document 1 proposes a method for producing 9% Ni steel with a thickness of 40 mm or more by sequentially subjecting a hot rolled steel plate to quenching treatment, two-phase region quenching treatment, and tempering treatment.
　また、特許文献２では、優れた低温靱性を有する厚肉９％Ｎｉ鋼板を容易に製造することができる方法が提案されている。前記方法では、鋼中のＳｉ量を低減すると同時に、適切な加熱圧延を付与し、ミクロ組織を制御することで、二相域焼入れ処理を施さなくても安定な残留オーステナイト（γ）を多量に生成でき、広い焼戻し温度範囲で優れた靭性を得ることができる。 Further, Patent Document 2 proposes a method that can easily produce a thick 9% Ni steel plate having excellent low-temperature toughness. In the above method, by reducing the amount of Si in the steel and at the same time applying appropriate hot rolling to control the microstructure, a large amount of stable retained austenite (γ) can be produced without the need for two-phase region quenching treatment. It is possible to obtain excellent toughness over a wide tempering temperature range.
　特許文献３では、鋼板の１／４ｔ位置における旧オーステナイトの平均粗大粒径が２０μｍ以下である、靭性に優れたＮｉ含有鋼板が提案されている。 Patent Document 3 proposes a Ni-containing steel plate with excellent toughness in which the average coarse grain size of prior austenite at the 1/4t position of the steel plate is 20 μm or less.
特開平４－３７１５２０号公報Japanese Patent Application Publication No. 4-371520 特開平６－１８４６３０号公報Japanese Patent Application Publication No. 6-184630 国際公開第２０２０／１３６８２９号International Publication No. 2020/136829
　近年、ＳＯｘやＮＯｘの排出規制強化に伴い、船舶燃料を重油からＬＮＧへ転換することが検討されており、７％Ｎｉ鋼板や９％Ｎｉ鋼板などＮｉ含有量の高い鋼板を船舶燃料用タンクの本体に適用することが模索されている。この用途では、従来の地上貯蔵用タンクに比べ、板厚が薄いことが求められる。さらに、鋼材需要量が多いため、製造性に優れる鋼板が求められる。 In recent years, with the tightening of SOx and NOx emission regulations, consideration has been given to converting marine fuel from heavy oil to LNG. Application to the main body is being explored. This application requires thinner plates than conventional above-ground storage tanks. Furthermore, since the demand for steel materials is large, steel plates with excellent manufacturability are required.
　しかし、特許文献１～３で提案されているような従来の技術には、次の問題があった。 However, the conventional techniques proposed in Patent Documents 1 to 3 have the following problems.
　すなわち、特許文献１で提案されている９％Ｎｉ鋼の製造方法では、熱延鋼板に対して、焼入れ処理、二相域焼入れ処理、および焼戻し処理の３段階の熱処理を施す必要がある。そのため、製造コストおよび生産性の面で不利であった。また、前記二相域焼入れ処理は、通常の焼入れ処理とは異なる特殊な温度に設定された炉を用いて行う必要がある。そのため、前記方法を適用している製造ラインでは、他の製品を製造することができないという製造上の制約があった。 That is, in the method for manufacturing 9% Ni steel proposed in Patent Document 1, it is necessary to perform three stages of heat treatment on a hot rolled steel plate: quenching treatment, two-phase region quenching treatment, and tempering treatment. Therefore, it was disadvantageous in terms of manufacturing cost and productivity. Further, the two-phase region hardening process needs to be performed using a furnace set at a special temperature different from that of a normal hardening process. Therefore, there is a manufacturing restriction in that the manufacturing line to which the above method is applied cannot manufacture other products.
　また、特許文献２で提案されている厚肉９％Ｎｉ鋼板の製造方法では、Ｓｉ含有量を０．１０質量％以下に厳格に管理する必要があり、成分設計の自由度が低かった。 Furthermore, in the method for manufacturing a thick 9% Ni steel plate proposed in Patent Document 2, it was necessary to strictly control the Si content to 0.10% by mass or less, and the degree of freedom in component design was low.
　さらに、特許文献３で提案されているＮｉ含有鋼板を製造するためには、再加熱焼入れ時の特定の温度域において昇温速度を厳密に制御する必要があるため、製造性の観点で課題があった。 Furthermore, in order to manufacture the Ni-containing steel sheet proposed in Patent Document 3, it is necessary to strictly control the temperature increase rate in a specific temperature range during reheating and quenching, which poses a problem in terms of manufacturability. there were.
　本発明は上記の実状に鑑みてなされたものであり、高強度と優れた低温靭性とを兼ね備え、かつ製造性にも優れる鋼板、およびその製造方法を提供することを目的とする。 The present invention has been made in view of the above-mentioned circumstances, and aims to provide a steel plate that has both high strength and excellent low-temperature toughness, and is also excellent in manufacturability, and a method for manufacturing the same.
　本発明者らは、上記の課題を達成するため、低温環境で使用される構造用鋼に好適なＮｉを含有する鋼板を対象に、鋼板の成分組成、製造方法に関して鋭意研究を行い、以下の知見を得た。 In order to achieve the above-mentioned problems, the present inventors conducted intensive research on the chemical composition and manufacturing method of steel sheets, targeting Ni-containing steel sheets suitable for structural steel used in low-temperature environments, and conducted the following research. I gained knowledge.
　[１]　５．０～１０．０質量％のＮｉを添加した鋼素材を、圧下比が５以上、かつ最終５パスのうち１パス当たりの圧下率が１０％以上であるパス数が２以上の条件で熱間圧延して板厚４０ｍｍ以下の熱延鋼板とすることにより、オーステナイト粒を細粒化するとともに整粒化することができる。そしてその結果、前記熱延鋼板に再加熱焼入れおよび焼戻しを施した後に得られるミクロ組織においても、旧オーステナイト粒が細粒化および整粒化されるため、優れた低温靭性が得られる。 [1] Steel material to which 5.0 to 10.0 mass% of Ni is added has a reduction ratio of 5 or more, and the number of passes with a reduction ratio of 10% or more per pass among the final 5 passes is 2 or more By hot rolling the steel sheet under the following conditions to obtain a hot rolled steel sheet having a thickness of 40 mm or less, the austenite grains can be made finer and grain-sized. As a result, even in the microstructure obtained after subjecting the hot-rolled steel sheet to reheating quenching and tempering, the prior austenite grains are refined and sized, so excellent low-temperature toughness can be obtained.
[２]　従来、優れた低温靭性を実現するためには、二相域焼入れを行って残留オーステナイト量を高める必要があった。これに対して上記のプロセスでは、熱間圧延、再加熱焼入れ、および焼戻しによりオーステナイト粒を細粒化、整粒化することができる。そのため、二相域焼入れによる残留オーステナイト量の増加を行う必要がなく、残留オーステナイトの体積率が３．０％未満であっても、優れた低温靭性が得られる。したがって、上記プロセスで得られる鋼板は、製造性にも優れている。 [2] Conventionally, in order to achieve excellent low-temperature toughness, it was necessary to perform two-phase region quenching to increase the amount of retained austenite. On the other hand, in the above process, the austenite grains can be refined and sized by hot rolling, reheating and quenching, and tempering. Therefore, there is no need to increase the amount of retained austenite by two-phase region quenching, and even if the volume fraction of retained austenite is less than 3.0%, excellent low-temperature toughness can be obtained. Therefore, the steel plate obtained by the above process also has excellent manufacturability.
　本発明は、以上の知見にさらに検討を加えてなされたものであり、その要旨は以下のとおりである。 The present invention was made by further considering the above findings, and the gist thereof is as follows.
１．質量％で、
　　Ｃ　：０．０１～０．１５％、
　　Ｓｉ：０．０１～１．００％、
　　Ｍｎ：０．１０～２．００％、
　　Ｐ　：０．０１０％以下、
　　Ｓ　：０．００５０％以下、
　　Ｎｉ：５．０～１０．０％、
　　Ａｌ：０．００２～０．１００％、および
　　Ｎ　：０．００８０％以下、を含有し、
　　残部Ｆｅおよび不可避的不純物からなる成分組成と、
　板厚１／４位置における残留オーステナイトの体積率が３．０％未満であり、
　板厚１／２位置における旧オーステナイト粒の最大粒径が１００μｍ以下であり、かつ、
　板厚１／２位置における、旧オーステナイト粒の平均粒径ａに対する、旧オーステナイト粒の粒径の上位５％における平均値ｂの比ｂ／aが４．５以下であるミクロ組織とを有し、
　板厚が４０ｍｍ以下であり、
　降伏強度が５８５ＭＰａ以上、かつ、
　引張強度が６９０ＭＰａ以上である、鋼板。 1. In mass%,
C: 0.01-0.15%,
Si: 0.01-1.00%,
Mn: 0.10-2.00%,
P: 0.010% or less,
S: 0.0050% or less,
Ni: 5.0 to 10.0%,
Contains Al: 0.002 to 0.100%, and N: 0.0080% or less,
A component composition consisting of the remainder Fe and unavoidable impurities,
The volume fraction of retained austenite at the 1/4 plate thickness position is less than 3.0%,
The maximum grain size of prior austenite grains at the plate thickness 1/2 position is 100 μm or less, and
The microstructure has a microstructure in which the ratio b/a of the average value b in the top 5% of the grain size of the prior austenite grains to the average grain diameter a of the prior austenite grains at the plate thickness 1/2 position is 4.5 or less. ,
The plate thickness is 40 mm or less,
Yield strength is 585 MPa or more, and
A steel plate having a tensile strength of 690 MPa or more.
２．前記成分組成が、さらに、質量％で、
　　Ｃｕ：０．０１～１．００％、
　　Ｃｒ：０．０１～１．５０％、
　　Ｍｏ：０．０３～１．０％、
　　Ｎｂ：０．００１～０．０３０％、
　　Ｖ　：０．０１～０．１０％、
　　Ｔｉ：０．００３～０．０５０％、
　　Ｂ　：０．０００３～０．００５０％、
　　Ｓｎ：０．０１～０．３０％、
　　Ｓｂ：０．０１～０．３０％、
　　Ｗ　：０％超、２．００％以下、
　　Ｃｏ：０％超、２．００％以下、
　　Ｃａ：０．０００５～０．００５０％、
　　Ｍｇ：０．０００５～０．０１００％、
　　Ｚｒ：０．０００５～０．００５０％、
　　Ｔａ：０．０１～０．２０％、
　　Ｙ　：０．００１～０．０１０％、および
　　ＲＥＭ：０．００１０～０．０２００％、
　　からなる群より選択される少なくとも１つを含有する、上記１に記載の鋼板。 2. The component composition further includes, in mass%,
Cu: 0.01 to 1.00%,
Cr: 0.01-1.50%,
Mo: 0.03-1.0%,
Nb: 0.001-0.030%,
V: 0.01 to 0.10%,
Ti: 0.003 to 0.050%,
B: 0.0003 to 0.0050%,
Sn: 0.01-0.30%,
Sb: 0.01 to 0.30%,
W: more than 0%, less than 2.00%,
Co: more than 0%, less than 2.00%,
Ca: 0.0005-0.0050%,
Mg: 0.0005-0.0100%,
Zr: 0.0005 to 0.0050%,
Ta: 0.01-0.20%,
Y: 0.001 to 0.010%, and REM: 0.0010 to 0.0200%,
The steel plate according to 1 above, containing at least one selected from the group consisting of.
３．上記１または２に記載の成分組成を有する鋼素材を、９００℃以上１２００℃以下の加熱温度まで加熱する加熱工程と、
　前記加熱工程で加熱した鋼素材を、圧下比が５以上、かつ最終５パスのうち１パス当たりの圧下率が１０％以上であるパス数が２以上の条件で熱間圧延して板厚４０ｍｍ以下の熱延鋼板とする熱間圧延工程と、
　前記熱延鋼板を冷却する冷却工程と、
　前記冷却工程後の熱延鋼板を、Ａｃ３点以上９００℃以下の再加熱温度まで再加熱し、焼入れする再加熱焼入れ工程と、
　前記再加熱焼入れ工程後の熱延鋼板を、５００℃以上６５０℃以下の焼戻し温度で焼戻しする焼戻し工程とを有する、鋼板の製造方法。 3. A heating step of heating a steel material having the composition described in 1 or 2 above to a heating temperature of 900°C or more and 1200°C or less,
The steel material heated in the heating step is hot-rolled to a plate thickness of 40 mm under the conditions that the rolling reduction ratio is 5 or more and the number of passes is 2 or more in which the rolling reduction ratio per pass is 10% or more among the final 5 passes. A hot rolling process to produce the following hot rolled steel sheet,
a cooling step of cooling the hot rolled steel sheet;
a reheating and quenching step of reheating and quenching the hot rolled steel sheet after the cooling step to a reheating temperature of 3 Ac or more and 900° C. or less;
A method for manufacturing a steel sheet, comprising a tempering step of tempering the hot rolled steel sheet after the reheating and quenching step at a tempering temperature of 500° C. or more and 650° C. or less.
　本発明の鋼板は、高強度と優れた低温靭性とを兼ね備えている。加えて、本発明の鋼板は、熱間圧延工程後に一般的な再加熱焼入れと焼戻しを行うことで製造できるため、本発明の鋼板を製造しているラインで他の製品を一緒に生産することもでき、製造性に優れている。 The steel plate of the present invention has both high strength and excellent low-temperature toughness. In addition, since the steel sheet of the present invention can be manufactured by performing general reheating and quenching and tempering after the hot rolling process, other products cannot be simultaneously produced on the same line that manufactures the steel sheet of the present invention. It also has excellent manufacturability.
　以下、本発明の実施形態について具体的に説明する。なお、以下の説明は、本発明の好適な実施形態の例を示すものであって、本発明はこれに限定されない。 Hereinafter, embodiments of the present invention will be specifically described. Note that the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.
［鋼板］
　本発明の一実施形態における鋼板は、特定の成分組成、ミクロ組織、板厚、引張強度、および降伏強度を備えている。以下、それぞれの限定理由について説明する。 [Steel plate]
A steel plate in one embodiment of the present invention has a specific composition, microstructure, plate thickness, tensile strength, and yield strength. The reasons for each limitation will be explained below.
[成分組成]
　まず、鋼板の成分組成の適正範囲およびその限定理由について説明する。なお、以下の説明において、含有量の単位としての「％」は、とくに断らない限り「質量％」を表す。 [Component composition]
First, the appropriate range of the composition of the steel sheet and the reason for its limitation will be explained. In addition, in the following description, "%" as a unit of content represents "mass %" unless otherwise specified.
Ｃ：０．０１～０．１５％
　Ｃは、鋼板を高強度化する効果を有する元素である。前記効果を得るために、Ｃ含有量は０．０１％以上、好ましくは０．０３％以上とする。一方、Ｃ含有量が０．１５％より高いと低温靭性が低下する。これは、鋼板の、特に中心偏析部においてＣｒ炭化物やＮｂ、Ｖ、Ｔｉ系炭化物が過度に析出するためである。そのため、Ｃ含有量は０．１５％以下、好ましくは０．１０％以下、より好ましくは０．０８％以下とする。 C: 0.01-0.15%
C is an element that has the effect of increasing the strength of the steel plate. In order to obtain the above effect, the C content is set to 0.01% or more, preferably 0.03% or more. On the other hand, when the C content is higher than 0.15%, low temperature toughness decreases. This is because Cr carbides and Nb, V, and Ti-based carbides precipitate excessively in the steel sheet, particularly in the center segregation area. Therefore, the C content is set to 0.15% or less, preferably 0.10% or less, and more preferably 0.08% or less.
Ｓｉ：０．０１～１．００％
　Ｓｉは、製鋼プロセスにおいて脱酸剤として作用する元素である。また、Ｓｉは、固溶強化により鋼板を高強度化する効果を有する。前記効果を得るために、Ｓｉ含有量を０．０１％以上とする。一方、Ｓｉ含有量が１．００％より高いと、介在物の増大により低温靭性が低下することに加え、溶接性および表面性状が劣化する。そのため、Ｓｉ含有量は１．００％以下、好ましくは０．５％以下、より好ましくは０．３％以下とする。 Si: 0.01~1.00%
Si is an element that acts as a deoxidizing agent in the steel manufacturing process. Furthermore, Si has the effect of increasing the strength of the steel plate through solid solution strengthening. In order to obtain the above effect, the Si content is set to 0.01% or more. On the other hand, if the Si content is higher than 1.00%, low-temperature toughness decreases due to an increase in inclusions, and weldability and surface quality deteriorate. Therefore, the Si content is set to 1.00% or less, preferably 0.5% or less, and more preferably 0.3% or less.
Ｍｎ：０．１０～２．００％
　Ｍｎは、鋼板の焼き入れ性を高め、高強度化させる効果を有する元素である。前記効果を得るため、Ｍｎ含有量を０．１０％以上、好ましくは０．４０％以上とする。一方、Ｍｎ含有量が２．００％より高いと、中心偏析を助長し、低温靭性の低下を引き起こす。そのため、Ｍｎ含有量は２．００％以下、好ましくは、１．００％以下とする。 Mn: 0.10-2.00%
Mn is an element that has the effect of improving the hardenability of a steel plate and increasing its strength. In order to obtain the above effect, the Mn content is set to 0.10% or more, preferably 0.40% or more. On the other hand, when the Mn content is higher than 2.00%, it promotes center segregation and causes a decrease in low temperature toughness. Therefore, the Mn content is 2.00% or less, preferably 1.00% or less.
Ｐ：０．０１０％以下
　Ｐ含有量が０．０１０％を超えると、低温靭性が低下する。これは、Ｐが粒界に偏析して粒界強度を低下させ、破壊起点となるためである。そのため、Ｐ含有量は０．０１０％以下とする。一方、低温靭性の観点からは、Ｐを可能な限り低減することが望ましいため、Ｐ含有量の下限は特に限定されず、０％であってよい。しかし、過度の低減は製造コストの上昇および生産性の低下を招く。そのため、工業的な生産の観点からは、Ｐ含有量を０．００１％以上とすることが好ましい。 P: 0.010% or less When the P content exceeds 0.010%, low temperature toughness decreases. This is because P segregates at grain boundaries, reduces grain boundary strength, and becomes a starting point for fracture. Therefore, the P content is set to 0.010% or less. On the other hand, from the viewpoint of low-temperature toughness, it is desirable to reduce P as much as possible, so the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction leads to an increase in manufacturing costs and a decrease in productivity. Therefore, from the viewpoint of industrial production, it is preferable that the P content is 0.001% or more.
Ｓ：０．００５０％以下
　Ｓは、鋼中でＭｎＳを形成し、低温靭性を著しく劣化させる。そのため、Ｓは可能な限り低減することが望ましく、Ｓ含有量は０．００５０％以下、好ましくは０．００２０％以下とする。一方、低温靭性の観点からも、Ｓを可能な限り低減することが望ましいため、Ｓ含有量の下限は特に限定されず、０％であってよい。しかし、過度の低減は製造コストの上昇および生産性の低下を招くため、工業的な生産の観点からは、Ｓ含有量を０．０００１％以上とすることが好ましい。 S: 0.0050% or less S forms MnS in steel and significantly deteriorates low temperature toughness. Therefore, it is desirable to reduce S as much as possible, and the S content is 0.0050% or less, preferably 0.0020% or less. On the other hand, since it is desirable to reduce S as much as possible from the viewpoint of low-temperature toughness, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction causes an increase in manufacturing costs and a decrease in productivity, so from the viewpoint of industrial production, it is preferable to set the S content to 0.0001% or more.
Ｎｉ：５．０～１０．０％
　Ｎｉは、鋼板の強度を向上させる効果を有する元素である。また、Ｎｉは、鋼板の低温靭性の向上に極めて有効な元素である。Ｎｉ含有量が５．０％未満であると、所望の強度および低温靭性を得ることができない。そのため、Ｎｉ含有量は５．０％以上、好ましくは、６．５％以上、より好ましくは６．８％以上、さらに好ましくは８．０％以上とする。一方、Ｎｉは高価な元素であるため、その含有量が高いほど鋼板コストが高騰する。そのため、Ｎｉ含有量は１０．０％以下、好ましくは９．５％以下とする。 Ni: 5.0-10.0%
Ni is an element that has the effect of improving the strength of the steel plate. Further, Ni is an extremely effective element for improving the low-temperature toughness of steel sheets. If the Ni content is less than 5.0%, desired strength and low temperature toughness cannot be obtained. Therefore, the Ni content is set to 5.0% or more, preferably 6.5% or more, more preferably 6.8% or more, and even more preferably 8.0% or more. On the other hand, since Ni is an expensive element, the higher the Ni content, the higher the cost of the steel sheet. Therefore, the Ni content is 10.0% or less, preferably 9.5% or less.
Ａｌ：０．００２～０．１００％
　Ａｌは、脱酸剤として作用する元素であり、溶鋼脱酸プロセスにおいて汎用的に使用されている。また、Ａｌは、鋼中のＮと反応してＡｌＮを形成する。この反応により固溶Ｎが低減される結果、低温靭性が向上する。前記効果を得るために、Ａｌ含有量を０．００２％以上、好ましくは０．０１０％以上、より好ましくは０．０２０％以上とする。一方、Ａｌ含有量が０．１００％より高いと、鋼中の介在物が増加し、かえって低温靭性が劣化する。そのため、Ａｌ含有量は０．１００％以下、好ましくは０．０７０％以下、より好ましくは０．０６０％以下とする。 Al: 0.002-0.100%
Al is an element that acts as a deoxidizing agent and is commonly used in the molten steel deoxidizing process. Moreover, Al reacts with N in steel to form AlN. This reaction reduces solid solution N, resulting in improved low-temperature toughness. In order to obtain the above effect, the Al content is set to 0.002% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Al content is higher than 0.100%, inclusions in the steel will increase, and the low-temperature toughness will deteriorate on the contrary. Therefore, the Al content is set to 0.100% or less, preferably 0.070% or less, and more preferably 0.060% or less.
Ｎ：０．００８０％以下
　Ｎは、窒化物や炭窒化物を形成し、それにより低温靭性を低下させる。Ｎ含有量が０．００８０％より高いと所望の低温靭性を得ることができない。そのため、Ｎ含有量は０．００８０％以下、好ましくは０．００４０％以下とする。一方、低温靭性の観点からは、Ｎを可能な限り低減することが望ましいため、Ｎ含有量の下限は特に限定されず、０％であってよい。しかし、過度の低減は製造コストの上昇および生産性の低下を招くため、工業的な生産の観点からは、Ｎ含有量を０．００１０％以上とすることが好ましい。 N: 0.0080% or less N forms nitrides and carbonitrides, thereby reducing low-temperature toughness. If the N content is higher than 0.0080%, desired low temperature toughness cannot be obtained. Therefore, the N content is set to 0.0080% or less, preferably 0.0040% or less. On the other hand, from the viewpoint of low-temperature toughness, it is desirable to reduce N as much as possible, so the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction causes an increase in manufacturing costs and a decrease in productivity, so from the viewpoint of industrial production, it is preferable to set the N content to 0.0010% or more.
　本発明の一実施形態における鋼板は、上記元素を含み、残部Ｆｅおよび不可避的不純物からなる成分組成を有する。 A steel plate in an embodiment of the present invention has a composition containing the above elements, with the balance consisting of Fe and inevitable impurities.
　また、本発明の他の実施形態における鋼板の成分組成は、鋼板の特性をさらに向上させることを目的として、任意に、以下に挙げる元素の少なくとも１つをさらに含有することができる。 Furthermore, the chemical composition of the steel sheet in another embodiment of the present invention may optionally further contain at least one of the elements listed below for the purpose of further improving the characteristics of the steel sheet.
Ｃｕ：０．０１～１．００％
　Ｃｕは、焼入れ性向上により鋼板強度をさらに高める効果を有する元素である。Ｃｕを添加する場合、前記効果を得るために、Ｃｕ含有量を０．０１％以上とする。一方、Ｃｕ含有量が１．００％より高いと、鋼板の低温靭性が低下することに加え、鋼素材表面の性状が悪化する。そのため、Ｃｕ含有量は１．００％以下、好ましくは、０．３０％以下とする。 Cu: 0.01~1.00%
Cu is an element that has the effect of further increasing the strength of the steel sheet by improving hardenability. When adding Cu, in order to obtain the above effect, the Cu content is set to 0.01% or more. On the other hand, when the Cu content is higher than 1.00%, the low-temperature toughness of the steel sheet decreases, and the surface properties of the steel material deteriorate. Therefore, the Cu content is 1.00% or less, preferably 0.30% or less.
Ｃｒ：０．０１～１．５０％
　Ｃｒは、鋼板強度のさらなる向上に有効な元素である。Ｃｒを添加する場合、前記効果を得るために、Ｃｒ含有量を０．０１％以上とする。一方、Ｃｒは圧延中に窒化物、炭化物、炭窒化物などの析出物として析出する場合があり、前記析出物は腐食や破壊の起点となって低温靭性を低下させる。そのため、Ｃｒ含有量は１．５０％以下、好ましくは１．００％以下とする。 Cr: 0.01~1.50%
Cr is an element effective in further improving the strength of the steel sheet. When adding Cr, the Cr content is set to 0.01% or more in order to obtain the above effects. On the other hand, Cr may precipitate as precipitates such as nitrides, carbides, and carbonitrides during rolling, and the precipitates serve as starting points for corrosion and fracture, reducing low-temperature toughness. Therefore, the Cr content is set to 1.50% or less, preferably 1.00% or less.
Ｍｏ：０．０３～１．０％
　Ｍｏは、鋼板の焼戻し脆化感受性を抑制する効果を有する元素である。また、Ｍｏは、鋼板強度をさらに向上させる効果を有する。Ｍｏを添加する場合、前記効果を得るために、Ｍｏ含有量を０．０３％以上、好ましくは０．０５％超とする。一方、Ｍｏ含有量が１．０％を超えると低温靭性が低下する。そのため、Ｍｏ含有量は１．０％以下、好ましくは０．３０％以下とする。 Mo: 0.03~1.0%
Mo is an element that has the effect of suppressing the temper embrittlement susceptibility of steel sheets. Moreover, Mo has the effect of further improving the strength of the steel sheet. When Mo is added, the Mo content is set to 0.03% or more, preferably more than 0.05%, in order to obtain the above effect. On the other hand, when the Mo content exceeds 1.0%, low temperature toughness decreases. Therefore, the Mo content is 1.0% or less, preferably 0.30% or less.
Ｎｂ：０．００１～０．０３０％
　Ｎｂは、鋼板の強度をさらに向上させる効果を有する元素である。Ｎｂを添加する場合、前記効果を得るために、Ｎｂ含有量を０．００１％以上、好ましくは０．００５％以上、より好ましくは０．００７％以上とする。一方、Ｎｂ含有量が０．０３０％より高いと粗大な炭窒化物が生成し、低温靱性が低下する。そのため、Ｎｂ含有量は０．０３０％以下、好ましくは０．０２５％以下、より好ましくは０．０２２％以下とする。 Nb: 0.001-0.030%
Nb is an element that has the effect of further improving the strength of the steel plate. When adding Nb, in order to obtain the above effects, the Nb content is set to 0.001% or more, preferably 0.005% or more, and more preferably 0.007% or more. On the other hand, when the Nb content is higher than 0.030%, coarse carbonitrides are generated, and low-temperature toughness is reduced. Therefore, the Nb content is set to 0.030% or less, preferably 0.025% or less, and more preferably 0.022% or less.
Ｖ：０．０１～０．１０％
　Ｖは、鋼板の強度をさらに向上させる効果を有する元素である。Ｖを添加する場合、前記効果を得るために、Ｖ含有量を０．０１％以上、好ましくは０．０２％以上、より好ましくは０．０３％以上とする。一方、Ｖ含有量が０．１０％より高いと、粗大な炭窒化物が析出し、破壊の起点となる。また、析出物が粗大化し、低温靱性を劣化させることがある。そのため、Ｖ含有量を０．１０％以下、好ましくは０．０９％以下、より好ましくは０．０８％以下とする。 V:0.01~0.10%
V is an element that has the effect of further improving the strength of the steel plate. When adding V, in order to obtain the above effects, the V content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, when the V content is higher than 0.10%, coarse carbonitrides precipitate and become the starting point of fracture. In addition, the precipitates may become coarse and deteriorate low-temperature toughness. Therefore, the V content is set to 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less.
Ｔｉ：０．００３～０．０５０％
　Ｔｉは、窒化物もしくは炭窒化物として析出し、鋼板組織中のオーステナイト粒をさらに細粒化させる効果を有する元素である。Ｔｉを添加する場合、前記効果を得るために、Ｔｉ含有量を０．００３％以上、好ましくは０．００５％以上、より好ましくは０．００７％以上とする。一方、Ｔｉ含有量が０．０５０％より高いと、析出物が粗大化し、低温靱性が低下する。そのため、Ｔｉ含有量は０．０５０％以下、好ましくは０．０３５％以下、より好ましくは０．０３２％以下とする。 Ti: 0.003~0.050%
Ti is an element that precipitates as a nitride or carbonitride and has the effect of further refining the austenite grains in the steel sheet structure. When adding Ti, in order to obtain the above effects, the Ti content is set to 0.003% or more, preferably 0.005% or more, and more preferably 0.007% or more. On the other hand, if the Ti content is higher than 0.050%, the precipitates become coarse and the low temperature toughness decreases. Therefore, the Ti content is set to 0.050% or less, preferably 0.035% or less, and more preferably 0.032% or less.
Ｂ：０．０００３～０．００５０％
　Ｂは、鋼板の強度をさらに向上させる効果を有する元素である。Ｂを添加する場合、前記効果を得るために、Ｂ含有量を０．０００３％以上とする。一方、Ｂ含有量が０．００５０％より高いと、粗大な析出物を生成し、低温靭性が低下する。そのため、Ｂ含有量は０．００５０％以下、好ましくは、０．００３０％以下とする。 B: 0.0003-0.0050%
B is an element that has the effect of further improving the strength of the steel plate. When B is added, the B content is set to 0.0003% or more in order to obtain the above effect. On the other hand, when the B content is higher than 0.0050%, coarse precipitates are formed and low-temperature toughness is reduced. Therefore, the B content is set to 0.0050% or less, preferably 0.0030% or less.
Ｓｎ：０．０１～０．３０％
　Ｓｎは、鋼板の耐食性を向上させる効果を有する元素であり、少量の含有でも効果を発揮する。そのため、Ｓｎを添加する場合、Ｓｎ含有量は０．０１％以上とする。一方、Ｓｎが過剰であると低温靱性が低下する。そのため、Ｓｎ含有量は０．３０％以下、好ましくは０．２５％以下とする。 Sn: 0.01-0.30%
Sn is an element that has the effect of improving the corrosion resistance of steel sheets, and even when contained in a small amount, the effect is exhibited. Therefore, when adding Sn, the Sn content is set to 0.01% or more. On the other hand, if Sn is excessive, low-temperature toughness decreases. Therefore, the Sn content is 0.30% or less, preferably 0.25% or less.
Ｓｂ：０．０１～０．３０％
　Ｓｂは、Ｓｎと同様に、鋼板の耐食性を向上させる効果を有する元素であり、少量の含有でも効果を発揮する。そのため、Ｓｂを添加する場合、Ｓｂ含有量は０．０１％以上とする。一方、Ｓｂが過剰であると低温靱性が低下することに加え、コストが上昇する。そのため、Ｓｂ含有量は０．３０％以下、好ましくは０．２５％以下とする。 Sb: 0.01~0.30%
Sb, like Sn, is an element that has the effect of improving the corrosion resistance of steel sheets, and even if it is contained in a small amount, it exhibits the effect. Therefore, when adding Sb, the Sb content is set to 0.01% or more. On the other hand, if Sb is excessive, low-temperature toughness decreases and cost increases. Therefore, the Sb content is 0.30% or less, preferably 0.25% or less.
Ｗ：０％超、２．００％以下
　Ｗは、ＳｎやＳｂと同様に、鋼板の耐食性を向上させる効果を有する元素であり、少量の含有でも効果を発揮する。そのため、Ｗを添加する場合、Ｗ含有量は０％超、好ましくは０．０１％以上、より好ましくは０．０５％以上とする。一方、Ｗが過剰であると低温靱性が低下することに加え、コストが上昇する。そのため、Ｗ含有量は２．００％以下、好ましくは０．５０％以下とする。 W: More than 0%, 2.00% or less W, like Sn and Sb, is an element that has the effect of improving the corrosion resistance of steel sheets, and even if it is contained in a small amount, it is effective. Therefore, when adding W, the W content is set to more than 0%, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if W is in excess, the low-temperature toughness decreases and the cost increases. Therefore, the W content is set to 2.00% or less, preferably 0.50% or less.
Ｃｏ：０％超、２．００％以下
　Ｃｏは、Ｓｎ、Ｓｂ、Ｗと同様に、鋼板の耐食性を向上させる効果を有する元素であり、少量の含有でも効果を発揮する。そのため、Ｃｏを添加する場合、Ｃｏ含有量は０％超、好ましくは０．０１％以上、より好ましくは０．０５％以上とする。一方、Ｃｏが過剰であるとコストが上昇する。そのため、Ｃｏ含有量は２．００％以下、好ましくは１．５０％以下とする。 Co: more than 0%, 2.00% or less Co, like Sn, Sb, and W, is an element that has the effect of improving the corrosion resistance of steel sheets, and even if it is contained in a small amount, it is effective. Therefore, when adding Co, the Co content should be more than 0%, preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if Co is in excess, the cost will increase. Therefore, the Co content is 2.00% or less, preferably 1.50% or less.
Ｃａ：０．０００５～０．００５０％
　Ｃａは、ＭｎＳ等の介在物の形態制御に有効な元素である。介在物の形態制御とは、展伸した硫化物系介在物の生成を抑制し粒状の介在物とすることをいう。この介在物の形態制御を介して、低温靭性をさらに向上させるとともに、耐硫化物応力腐食割れ性を向上させることができる。Ｃａを添加する場合、前記効果を得るために、Ｃａ含有量を０．０００５％以上、好ましくは０．００１０％以上とする。一方、Ｃａを多く含有させると、非金属介在物量が増加し、低温靭性が低下する場合がある。そのため、Ｃａ含有量は０．００５０％以下、好ましくは０．００４０％以下とする。 Ca: 0.0005-0.0050%
Ca is an element effective in controlling the morphology of inclusions such as MnS. Controlling the form of inclusions means suppressing the formation of expanded sulfide-based inclusions to form granular inclusions. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance. When adding Ca, the Ca content is set to 0.0005% or more, preferably 0.0010% or more in order to obtain the above effects. On the other hand, when a large amount of Ca is contained, the amount of nonmetallic inclusions increases and low-temperature toughness may decrease. Therefore, the Ca content is 0.0050% or less, preferably 0.0040% or less.
Ｍｇ：０．０００５～０．０１００％
　Ｍｇは、Ｃａと同様に、ＭｎＳ等の介在物の形態制御に有効な元素である。この介在物の形態制御を介して、低温靭性をさらに向上させるとともに、耐硫化物応力腐食割れ性を向上させることができる。Ｍｇを添加する場合、前記効果を得るために、Ｍｇ含有量を０．０００５％以上、好ましくは０．００１０％以上とする。一方、Ｍｇを多く含有させると、非金属介在物量が増加し、低温靭性が低下する場合がある。そのため、Ｍｇ含有量は０．０１００％以下、好ましくは０．００５０％以下、より好ましくは０．００４０％以下とする。 Mg: 0.0005-0.0100%
Mg, like Ca, is an element effective in controlling the morphology of inclusions such as MnS. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance. When adding Mg, in order to obtain the above effects, the Mg content is set to 0.0005% or more, preferably 0.0010% or more. On the other hand, when a large amount of Mg is contained, the amount of nonmetallic inclusions increases and low-temperature toughness may decrease. Therefore, the Mg content is set to 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0040% or less.
Ｚｒ：０．０００５～０．００５０％
　Ｚｒは、ＣａやＭｇ同様、ＭｎＳ等の介在物の形態制御に有効な元素である。この介在物の形態制御を介して、低温靭性をさらに向上させるとともに、耐硫化物応力腐食割れ性を向上させることができる。Ｚｒを添加する場合、前記効果を得るために、Ｚｒ含有量を０．０００５％以上、好ましくは０．００１０％以上とする。一方、Ｚｒを多く含有させると、非金属介在物量が増加し、低温靭性が低下する場合がある。そのため、Ｚｒ含有量は０．００５０％以下、好ましくは０．００４０％以下とする。 Zr: 0.0005-0.0050%
Zr, like Ca and Mg, is an element effective in controlling the morphology of inclusions such as MnS. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance. When adding Zr, the Zr content is set to 0.0005% or more, preferably 0.0010% or more in order to obtain the above effects. On the other hand, when a large amount of Zr is contained, the amount of nonmetallic inclusions increases and low-temperature toughness may decrease. Therefore, the Zr content is 0.0050% or less, preferably 0.0040% or less.
Ｔａ：０．０１～０．２０％
　Ｔａは、鋼板強度のさらなる向上に有効な元素である。Ｔａを添加する場合、前記効果を得るために、Ｔａ含有量を０．０１％以上とする。一方、Ｔａ含有量が０．２０％を超えると、析出物生成によって低温靭性が低下する。そのため、Ｔａ含有量は０．２０％以下とする。 Ta: 0.01~0.20%
Ta is an element effective in further improving the strength of the steel sheet. When adding Ta, the Ta content is set to 0.01% or more in order to obtain the above effect. On the other hand, when the Ta content exceeds 0.20%, the low temperature toughness decreases due to the formation of precipitates. Therefore, the Ta content is set to 0.20% or less.
Ｙ：０．００１～０．０１０％
　Ｙは、高温で安定な酸化物の形成に有効な元素である。前記酸化物の形成により、溶接熱影響部の旧オーステナイト粒の粗大化を効果的に抑制することが可能となり、溶接部の靭性を向上させることができる。Ｙを添加する場合、前記効果を得るために、Ｙ含有量を０．００１％以上とする。一方、Ｙ含有量が０．０１０％を超えると、介在物量が増加し、低温靭性が低下する。そのため、Ｙ含有量は０．０１０％以下とする。 Y: 0.001-0.010%
Y is an element effective in forming an oxide that is stable at high temperatures. The formation of the oxide makes it possible to effectively suppress coarsening of prior austenite grains in the weld heat affected zone, thereby improving the toughness of the weld zone. When adding Y, the Y content is set to 0.001% or more in order to obtain the above effects. On the other hand, when the Y content exceeds 0.010%, the amount of inclusions increases and low-temperature toughness decreases. Therefore, the Y content is set to 0.010% or less.
ＲＥＭ：０．００１０～０．０２００％
　ＲＥＭ（希土類金属）は、Ｃａ、Ｍｇ、Ｚｒと同様に、ＭｎＳ等の介在物の形態制御に有効な元素である。この介在物の形態制御を介して、低温靭性をさらに向上させるとともに、耐硫化物応力腐食割れ性を向上させることができる。ＲＥＭを添加する場合、前記効果を得るために、ＲＥＭ含有量を０．００１０％以上、好ましくは０．００２０％以上とする。一方、ＲＥＭを多く含有させると、非金属介在物量が増加して低温靭性が低下する場合がある。そのため、ＲＥＭ含有量は０．０２００％以下とする。 REM: 0.0010-0.0200%
REM (rare earth metal), like Ca, Mg, and Zr, is an element effective in controlling the morphology of inclusions such as MnS. By controlling the morphology of these inclusions, it is possible to further improve low-temperature toughness and improve sulfide stress corrosion cracking resistance. When REM is added, the REM content is set to 0.0010% or more, preferably 0.0020% or more in order to obtain the above effects. On the other hand, when a large amount of REM is contained, the amount of nonmetallic inclusions increases and low-temperature toughness may decrease. Therefore, the REM content is set to 0.0200% or less.
［ミクロ組織］
　次に本発明の鋼板のミクロ組織について説明する。本発明の一実施形態における鋼板は、ミクロ組織が下記（１）～（３）の条件を満たす。
（１）板厚１／４位置における残留オーステナイトの体積率が３．０％未満である。
（２）板厚１／２位置における旧オーステナイト粒の最大粒径が１００μｍ以下である。
（３）板厚１／２位置における旧オーステナイト粒の平均粒径ａに対する、旧オーステナイト粒の粒径の上位５％における平均値ｂの比ｂ／aが４．５以下である。 [Microstructure]
Next, the microstructure of the steel plate of the present invention will be explained. The steel plate in one embodiment of the present invention has a microstructure that satisfies the following conditions (1) to (3).
(1) The volume fraction of retained austenite at the 1/4 position of the plate thickness is less than 3.0%.
(2) The maximum grain size of prior austenite grains at the 1/2 position of the plate thickness is 100 μm or less.
(3) The ratio b/a of the average value b of the top 5% of the grain sizes of the prior austenite grains to the average grain size a of the prior austenite grains at the 1/2 plate thickness position is 4.5 or less.
　なお、旧オーステナイト粒とは、オーステナイト（γ）が冷却されて組織変態が起こり、別の組織となった鋼板から見た際の変態前のγ粒を指す用語である。 Note that the term "prior austenite grains" refers to γ grains before transformation when viewed from a steel sheet that has undergone structural transformation as austenite (γ) is cooled and has a different structure.
残留γ：３．０％未満
　従来の技術では、残留オーステナイト量を高めることによって低温靭性を向上させていた。しかし、残留オーステナイト量を増加させるためには二相域焼入れが必要となり製造性が低下する。そのため、本発明では板厚１／４位置における残留オーステナイト量を体積率で３．０％未満、好ましくは２．８％以下、より好ましくは２．６％以下とする。一方、残留オーステナイトの体積率の下限はとくに限定されず、０％であってよく、０．５％以上であってもよい。 Retained γ: less than 3.0% In conventional technology, low-temperature toughness was improved by increasing the amount of retained austenite. However, in order to increase the amount of retained austenite, two-phase region quenching is required, which reduces productivity. Therefore, in the present invention, the amount of retained austenite at the 1/4 plate thickness position is set to be less than 3.0% in terms of volume fraction, preferably 2.8% or less, and more preferably 2.6% or less. On the other hand, the lower limit of the volume fraction of retained austenite is not particularly limited, and may be 0% or 0.5% or more.
　なお、残留オーステナイト以外の組織については特に限定されないが、上記ミクロ組織は、焼戻しマルテンサイトとベイナイトを主体とすることが好ましい。具体的には、焼戻しマルテンサイトとベイナイトの合計面積率が９０％以上であることが好ましい。焼戻しマルテンサイトとベイナイトの合計面積率の上限はとくに限定されないが、１００％であってよい。 Although there are no particular limitations on structures other than retained austenite, it is preferable that the microstructure is mainly composed of tempered martensite and bainite. Specifically, it is preferable that the total area ratio of tempered martensite and bainite is 90% or more. The upper limit of the total area ratio of tempered martensite and bainite is not particularly limited, but may be 100%.
　前記残留オーステナイトの体積率は、Ｘ線回折により測定することができる。より具体的には、実施例に記載した方法で測定することができる。 The volume fraction of the retained austenite can be measured by X-ray diffraction. More specifically, it can be measured by the method described in Examples.
旧γ粒の最大粒径：１００μｍ以下
　粗大な旧オーステナイト粒が存在すると、その粗大な旧オーステナイト粒に応力が集中し、破壊の起点となるため、低温靭性が低下する。そのため、本発明では板厚１／２位置における旧オーステナイト粒の最大粒径を１００μｍ以下、好ましくは８０μｍ以下とする。一方、前記最大粒径の下限はとくに限定されないが、旧γ粒の最大粒径を２０μｍ以下とするためには焼入れ条件などを非常に厳格に制御する必要があるため、製造性に劣る。そのため、工業的な生産の観点からは、前記最大粒径を２０μｍ超とすることが好ましく、２２μｍ以上とすることがより好ましく、２５μｍ以上とすることがさらに好ましい。なお、本発明では、旧オーステナイト粒の粒径として円相当径を用いるものとする。 Maximum grain size of prior γ grains: 100 μm or less If coarse prior austenite grains exist, stress concentrates on the coarse prior austenite grains, which become the starting point of fracture, resulting in a decrease in low-temperature toughness. Therefore, in the present invention, the maximum grain size of prior austenite grains at the 1/2 plate thickness position is set to 100 μm or less, preferably 80 μm or less. On the other hand, the lower limit of the maximum grain size is not particularly limited, but in order to make the maximum grain size of the prior γ grains 20 μm or less, it is necessary to control the quenching conditions very strictly, resulting in poor productivity. Therefore, from the viewpoint of industrial production, the maximum particle size is preferably greater than 20 μm, more preferably 22 μm or more, and even more preferably 25 μm or more. Note that, in the present invention, the equivalent circle diameter is used as the grain size of the prior austenite grains.
　前記旧オーステナイト粒の最大粒径は、光学顕微鏡により測定することができる。より具体的には、実施例に記載した方法で測定することができる。 The maximum grain size of the prior austenite grains can be measured using an optical microscope. More specifically, it can be measured by the method described in Examples.
ｂ／ａ≦４．０
　本発明においては、旧オーステナイト粒の平均粒径ａに対する、旧オーステナイト粒の粒径の上位５％における平均値ｂの比ｂ／aを４．５以下とする。ｂ／ａが４．５より高い場合、旧オーステナイト粒の整粒化が不十分であり、部分的に粗大な結晶粒が存在することから靭性が低下する。ｂ／ａは４．０以下であることが好ましく、３．５以下であることがより好ましく、３．０以下であることがさらに好ましい。一方、ｂ／ａの下限は特に限定されないが、理論上の下限値は１である。ｂ／ａが１に近いほど、粗大な結晶粒の生成が抑制され、整粒化が進行していることを意味するため、ｂ／ａは１に近いほど好ましい。工業的な生産の観点からは、ｂ／ａは１．２以上であってよく、１．３以上であってもよい。なお、前記ａおよびｂの値としては、板厚１／２位置における値を用いる。 b/a≦4.0
In the present invention, the ratio b/a of the average value b of the top 5% of the grain sizes of the prior austenite grains to the average grain size a of the prior austenite grains is set to 4.5 or less. When b/a is higher than 4.5, prior austenite grains are not sufficiently regulated and coarse grains are present in some areas, resulting in a decrease in toughness. b/a is preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less. On the other hand, the lower limit of b/a is not particularly limited, but the theoretical lower limit is 1. The closer b/a is to 1, the more suppressed the formation of coarse crystal grains and the more progressed in grain size regulation; therefore, the closer b/a is to 1, the more preferable it is. From the viewpoint of industrial production, b/a may be 1.2 or more, and may be 1.3 or more. Note that, as the values of a and b, the values at the plate thickness 1/2 position are used.
　前記平均粒径ａおよび平均値ｂの値は、光学顕微鏡により測定することができる。より具体的には、実施例に記載した方法で測定することができる。 The average particle diameter a and the average value b can be measured using an optical microscope. More specifically, it can be measured by the method described in Examples.
　また、板厚１／２位置における旧オーステナイト粒のアスペクト比は、特に限定されないが、２．０以下であることが好ましい。前記アスペクト比が２．０以下であると、機械的特性の異方性、特に低温靭性の異方性が改善される。 Furthermore, the aspect ratio of the prior austenite grains at the 1/2 plate thickness position is not particularly limited, but is preferably 2.0 or less. When the aspect ratio is 2.0 or less, the anisotropy of mechanical properties, especially the anisotropy of low-temperature toughness, is improved.
　前記旧オーステナイト粒のアスペクト比は、光学顕微鏡により測定することができる。より具体的には、実施例に記載した方法で測定することができる。 The aspect ratio of the prior austenite grains can be measured using an optical microscope. More specifically, it can be measured by the method described in Examples.
［板厚］
板厚：４０ｍｍ以下
　鋼板の板厚が４０ｍｍを超える場合、熱間圧延工程において、オーステナイト粒の細粒化および整粒が不十分となる。そしてその結果、再加熱焼入れおよび焼戻しを施した後の鋼板組織の細粒化および整粒化が不十分となり、低温靭性が低下する。そのため、鋼板の板厚は４０ｍｍ以下とする。さらに、板厚が４０ｍｍ以下である場合、熱処理時間が短くなるため、焼戻し時に焼戻し脆化の影響を低下させることができる。そのため、本発明においてはＳｉ含有量の制約は小さい。一方、板厚の下限については特に限定されないが、６ｍｍ以上とすることが好ましい。 [Plate thickness]
Plate thickness: 40 mm or less When the plate thickness of the steel plate exceeds 40 mm, the austenite grains are insufficiently refined and sized in the hot rolling process. As a result, the grain refinement and grain size regulation of the steel sheet structure after reheating and quenching and tempering become insufficient, resulting in a decrease in low-temperature toughness. Therefore, the thickness of the steel plate should be 40 mm or less. Furthermore, when the plate thickness is 40 mm or less, the heat treatment time is shortened, so that the influence of temper embrittlement during tempering can be reduced. Therefore, in the present invention, restrictions on the Si content are small. On the other hand, the lower limit of the plate thickness is not particularly limited, but is preferably 6 mm or more.
［引張強度］
ＴＳ：６９０ＭＰａ以上
　本発明に係る鋼板の引張強度（ＴＳ）は、６９０ＭＰａ以上とする。本発明の鋼板は、６９０ＭＰａ以上という高い引張強度を有しているため、ＬＮＧタンクなどの用途に好適に用いることができる。一方、前記引張強度の上限はとくに限定されないが、例えば、８３０ＭＰａ以下であってよく、８００ＭＰａ以下であってもよい。 [Tensile strength]
TS: 690 MPa or more The tensile strength (TS) of the steel plate according to the present invention is 690 MPa or more. Since the steel plate of the present invention has a high tensile strength of 690 MPa or more, it can be suitably used for applications such as LNG tanks. On the other hand, the upper limit of the tensile strength is not particularly limited, but may be, for example, 830 MPa or less, or 800 MPa or less.
［降伏強度］
ＹＳ：５８５ＭＰａ以上
　本発明に係る鋼板の降伏強度（ＹＳ）は、５８５ＭＰａ以上とする。本発明の鋼板は、５８５ＭＰａ以上という高い降伏強度を有しているため、ＬＮＧタンクなどの用途に好適に用いることができる。一方、前記降伏強度の上限はとくに限定されないが、例えば、７９０ＭＰａ以下であってよく、７７０ＭＰａ以下であってもよい。 [Yield strength]
YS: 585 MPa or more The yield strength (YS) of the steel plate according to the present invention is 585 MPa or more. Since the steel sheet of the present invention has a high yield strength of 585 MPa or more, it can be suitably used for applications such as LNG tanks. On the other hand, the upper limit of the yield strength is not particularly limited, and may be, for example, 790 MPa or less, or 770 MPa or less.
　上記引張強度および降伏強度は、ＪＩＳ　Ｚ　２２０４に準拠した引張試験により測定することができる。より具体的には、実施例に記載した方法で測定できる。 The above tensile strength and yield strength can be measured by a tensile test based on JIS Z 2204. More specifically, it can be measured by the method described in Examples.
[低温靭性]
　本発明に係る鋼板の低温靭性は、－１９６℃における吸収エネルギーｖＥ－１９６が１００J以上であることが好ましい。本発明の鋼板は、ｖＥ－１９６が１００J以上という高い低温靭性を有しているため、ＬＮＧタンクなどの用途に好適に用いることができる。前記吸収エネルギーｖＥ－１９６は１５０J以上であることが好ましい。一方、前記吸収エネルギーｖＥ－１９６の上限はとくに限定されないが、例えば、４００Ｊ以下であってよく、３５０Ｊ以下であってもよい。 [Low temperature toughness]
Regarding the low-temperature toughness of the steel plate according to the present invention, it is preferable that the absorbed energy vE -196 at -196°C is 100 J or more. The steel sheet of the present invention has high low temperature toughness with vE -196 of 100J or more, so it can be suitably used for applications such as LNG tanks. The absorbed energy vE -196 is preferably 150J or more. On the other hand, the upper limit of the absorbed energy vE -196 is not particularly limited, but may be, for example, 400 J or less, or 350 J or less.
　上記吸収エネルギーｖＥ－１９６は、ＪＩＳ　Ｚ　２２４２に準拠したシャルピー衝撃試験により測定することができる。より具体的には、実施例に記載した方法で測定できる。 The absorbed energy vE -196 can be measured by a Charpy impact test in accordance with JIS Z 2242. More specifically, it can be measured by the method described in Examples.
［製造方法］
　次に、本発明の一実施形態における鋼板の製造方法について説明する。前記鋼板は、上述した成分組成を有する鋼素材に対して、下記の（１）～（５）の工程を順次施すことによって製造することができる。
（１）加熱工程
（２）熱間圧延工程
（３）冷却工程
（４）再加熱焼入れ工程
（５）焼戻し工程 [Production method]
Next, a method for manufacturing a steel plate according to an embodiment of the present invention will be described. The steel plate can be manufactured by sequentially performing the following steps (1) to (5) on a steel material having the above-mentioned composition.
(1) Heating process (2) Hot rolling process (3) Cooling process (4) Reheating and quenching process (5) Tempering process
　以下、各工程における条件について説明する。なお、以下の説明において、温度「℃」は、板厚１／２位置における温度を意味するものとする。板厚１／２位置における温度は、差分計算などにより求められる。 Hereinafter, the conditions in each step will be explained. In addition, in the following description, the temperature "°C" means the temperature at the 1/2 position of the plate thickness. The temperature at the 1/2 plate thickness position is determined by differential calculation or the like.
（鋼素材）
　上記鋼素材としては、任意の形態の素材を使用することができる。前記鋼素材は、例えば、鋼スラブであってよい。鋼素材の製造方法は、とくに限定されないが、例えば、上記した成分組成を有する溶鋼を常法により溶製し、鋳造して製造することができる。前記溶
製は、転炉、電気炉、誘導炉等、任意の方法により行うことができる。また、前記鋳造は、生産性の観点から連続鋳造法で行うことが好ましいが、造塊法により行ってもよい。 (Steel material)
Any form of material can be used as the steel material. The steel material may be, for example, a steel slab. The method for manufacturing the steel material is not particularly limited, but, for example, it can be manufactured by melting molten steel having the above-mentioned composition by a conventional method and casting. The melting can be performed by any method such as a converter, an electric furnace, an induction furnace, or the like. Further, from the viewpoint of productivity, the casting is preferably carried out by a continuous casting method, but may be carried out by an ingot-forming method.
（加熱工程）
　加熱工程では、上記鋼素材を９００℃以上１２００℃以下の加熱温度まで加熱する。前記加熱は、鋳造などの方法によって得た鋼素材を一旦冷却した後に行ってもよく、また、得られた鋼素材を冷却することなく直接、前記加熱に供することもできる。 (Heating process)
In the heating step, the steel material is heated to a heating temperature of 900°C or more and 1200°C or less. The heating may be performed after once cooling the steel material obtained by a method such as casting, or the obtained steel material may be directly subjected to the heating without cooling.
　鋼素材を加熱するのは、鋼素材の組織中の析出物を固溶させるためである。加熱温度が９００℃未満の場合、未固溶の析出物の影響が大きくなり、混粒となるなど均一な組織が得られない。そのため、鋼素材の加熱温度は９００℃以上とする。一方、加熱温度が１２００℃を超えると、逆変態したオーステナイト粒が著しく粗大化し、次の熱間圧延および熱処理工程を経ても鋼板組織の十分な細粒化が図れない。また、過大なエネルギーが必要となり、製造性が低下する。そのため、鋼素材の加熱温度は１２００℃以下、好ましくは１１５０℃以下とする。なお、加熱時間は特に限定されないが、２時間以上、８時間以下とすることが好ましい。 The purpose of heating the steel material is to dissolve the precipitates in the structure of the steel material. When the heating temperature is less than 900° C., the influence of undissolved precipitates increases, and a uniform structure cannot be obtained, such as mixed grains. Therefore, the heating temperature of the steel material is set to 900°C or higher. On the other hand, if the heating temperature exceeds 1200° C., the reversely transformed austenite grains become extremely coarse, and the steel sheet structure cannot be sufficiently refined even after the subsequent hot rolling and heat treatment steps. Furthermore, excessive energy is required, resulting in reduced productivity. Therefore, the heating temperature of the steel material is 1200°C or lower, preferably 1150°C or lower. Note that the heating time is not particularly limited, but is preferably 2 hours or more and 8 hours or less.
（熱間圧延工程）
　熱間圧延工程では、前記加熱工程で加熱された鋼素材を熱間圧延して板厚が４０ｍｍ以下の熱延鋼板とする。圧延終了温度は特に限定されないが、オーステナイト単相域となる７００℃以上とすることが好ましい。前記圧延終了温度の上限はとくに限定されないが、９５０℃以下であることが好ましく、９２０℃以下であることがより好ましい。 (Hot rolling process)
In the hot rolling step, the steel material heated in the heating step is hot rolled into a hot rolled steel plate having a thickness of 40 mm or less. Although the rolling end temperature is not particularly limited, it is preferably 700° C. or higher, which is the austenite single phase region. The upper limit of the rolling end temperature is not particularly limited, but is preferably 950°C or lower, more preferably 920°C or lower.
圧下比：５以上
　鋼板組織の細粒化と整粒化を図るためには、熱間圧延工程において十分な加工を加え、オーステナイト粒の再結晶を促すことが必要である。前記熱間圧延工程における圧下比が５未満の場合、加工量が不足し、粗大なオーステナイト粒が残存し、その結果、低温靭性が低下する。また、圧下比が５未満であると、ポロシティと呼ばれる内部微小空孔等の鋳造欠陥の無害化が不十分となり、低温靭性が低下する。そのため、熱間圧延工程における圧下比は５以上、好ましくは６以上、より好ましくは１０以上とする。一方、圧下比の上限は特に限定されないが、５０以下とすることが好ましい。なお、ここで前記圧下比は、（鋼素材の板厚／熱間圧延後の熱延鋼板の板厚）と定義される。 Reduction ratio: 5 or more In order to refine and regularize the steel sheet structure, it is necessary to apply sufficient processing in the hot rolling process to promote recrystallization of austenite grains. When the reduction ratio in the hot rolling step is less than 5, the amount of processing is insufficient, coarse austenite grains remain, and as a result, low-temperature toughness decreases. Furthermore, if the rolling reduction ratio is less than 5, casting defects such as internal microvoids called porosity will not be rendered harmless, and low-temperature toughness will deteriorate. Therefore, the rolling reduction ratio in the hot rolling step is 5 or more, preferably 6 or more, and more preferably 10 or more. On the other hand, the upper limit of the rolling reduction ratio is not particularly limited, but is preferably 50 or less. Note that the rolling ratio is defined here as (thickness of steel material/thickness of hot rolled steel sheet after hot rolling).
最終５パスのうち圧下率が１０％以上であるパス数：２以上
　鋼板組織の細粒化と整粒化には、特に、熱間圧延工程の後半においてのオーステナイト粒の再結晶が有効である。そのため、前記熱間圧延工程の最終５パスのうち１パス当たりの圧下率が１０％以上であるパス数を２以上とする。前記パス数が２未満であると、オーステナイト粒の整粒化が十分に進行しない。そしてその結果、最終的に得られる鋼板におけるｂ／ａが４．０より大きくなり、低温靭性が劣化する。ｂ／ａをさらに低減するという観点からは、前記熱間圧延工程の最終５パスのうち１パス当たりの圧下率が１０％以上であるパス数は、３以上とすることが好ましく、４以上とすることがより好ましく、５とすることがさらに好ましい。 Number of passes with a rolling reduction of 10% or more among the final 5 passes: 2 or more Recrystallization of austenite grains in the latter half of the hot rolling process is particularly effective for grain refinement and grain size regulation of the steel sheet structure. . Therefore, the number of passes in which the rolling reduction per pass is 10% or more among the final five passes of the hot rolling process is set to two or more. If the number of passes is less than 2, grain size regulation of austenite grains will not proceed sufficiently. As a result, b/a in the finally obtained steel plate becomes larger than 4.0, and low-temperature toughness deteriorates. From the viewpoint of further reducing b/a, the number of passes in which the rolling reduction per pass is 10% or more among the final five passes of the hot rolling process is preferably 3 or more, and 4 or more. It is more preferable to set it to 5, and even more preferably to set it to 5.
（冷却工程）
　次いで、上記熱間圧延工程で得られた熱延鋼板を冷却する。前記冷却により析出物の粗大化が抑制され、強度および靭性が向上する。前記冷却工程における冷却停止温度はとくに限定されないが、例えば、常温（２０℃など）以上であってよい。また、前記冷却停止温度は、４００℃以下とすることが好ましい。 (cooling process)
Next, the hot rolled steel sheet obtained in the above hot rolling process is cooled. The cooling suppresses coarsening of precipitates and improves strength and toughness. The cooling stop temperature in the cooling step is not particularly limited, but may be, for example, room temperature (20° C., etc.) or higher. Further, the cooling stop temperature is preferably 400°C or less.
　前記冷却は、特に限定されることなく、例えば、空冷、水冷など、任意の方法で行うことができる。強度、低温靭性など必要な特性を高めるために、スプレー冷却、ミスト冷却、ラミナー冷却などの水冷を実施してもよい。ただし、水冷を行った場合、急速に冷却されるため、圧延時に形成された細長い組織が残留し、組織の異方性が大きくなる傾向がある。そのため、組織の異方性を低減するという観点からは、空冷を行うことが好ましい。 The cooling is not particularly limited, and can be performed by any method such as air cooling or water cooling. Water cooling such as spray cooling, mist cooling, laminar cooling, etc. may be performed to enhance necessary properties such as strength and low temperature toughness. However, when water cooling is performed, since the cooling is rapid, the elongated structure formed during rolling remains, and the anisotropy of the structure tends to increase. Therefore, from the viewpoint of reducing the anisotropy of the structure, it is preferable to perform air cooling.
［再加熱焼入れ工程］
　再加熱焼入れ工程では、前記冷却工程後の熱延鋼板を、Ａｃ３点以上９００℃以下の再加熱温度に加熱した後に焼き入れする。Ａｃ３点以上に再加熱することにより、熱延鋼板全体の組織がオーステナイトに逆変態する。その結果、鋼板組織の細粒化がさらに生じ、低温靭性が向上する。前記再加熱温度がＡｃ３点未満であると、加熱後の鋼板のミクロ組織中にフェライト相が含まれるため、ミクロ組織を均一にできず低温靭性が低下する。一方、前記再加熱温度が９００℃を超えると、オーステナイト粒が成長して粗大になるため、低温靭性が低下する。 [Reheating and quenching process]
In the reheating and quenching step, the hot rolled steel sheet after the cooling step is heated to a reheating temperature of 3 Ac or more and 900° C. or less, and then quenched. By reheating to Ac3 point or higher, the structure of the entire hot rolled steel sheet undergoes reverse transformation to austenite. As a result, the grain size of the steel plate structure is further refined, and low-temperature toughness is improved. If the reheating temperature is less than the Ac3 point, the microstructure of the heated steel sheet will contain a ferrite phase, making it impossible to make the microstructure uniform, resulting in a decrease in low-temperature toughness. On the other hand, when the reheating temperature exceeds 900°C, austenite grains grow and become coarse, resulting in a decrease in low-temperature toughness.
　なお、Ａｃ３点は下記（１）式で計算される。
Ａｃ３（℃）＝９３７．２－４３６．５Ｃ＋５６Ｓｉ－１９．７Ｍｎ－１６．３Ｃｕ－２６．６Ｎｉ－４．９Ｃｒ＋３８．１Ｍｏ＋１２４．８Ｖ＋１３６．３Ｔｉ－１９．１Ｎｂ＋１９８．４Ａｌ＋３３１５Ｂ…（１）
ただし、上記（１）式中の元素記号は、各元素の含有量（質量％）を表し、当該元素が含有されていない場合は０とする。 Note that the Ac3 points are calculated using the following formula (1).
Ac3(℃)=937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B...(1)
However, the element symbol in the above formula (1) represents the content (mass%) of each element, and is set to 0 when the element is not contained.
　焼入れは、特に限定されることなく、任意の条件で行うことができるが、水冷で行うことが好ましい。 Quenching can be carried out under any conditions without particular limitations, but it is preferably carried out by water cooling.
　前記焼入れの条件は特に限定されないが、２００℃未満の冷却停止温度まで冷却することが好ましい。前記冷却停止温度は１００℃以下であることがより好ましく、５０℃以下であることがさらに好ましい。一方、前記冷却停止温度の下限はとくに限定されないが、例えば、前記冷却停止温度は室温以上であってもよい。 The conditions for the quenching are not particularly limited, but it is preferable to cool to a cooling stop temperature of less than 200°C. The cooling stop temperature is more preferably 100°C or lower, and even more preferably 50°C or lower. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but for example, the cooling stop temperature may be room temperature or higher.
（焼戻し工程）
　焼戻し工程では、再加熱焼入れ工程後の鋼板を５００℃以上６５０℃以下の焼戻し温度で焼戻しする。焼戻し温度が５００℃未満の場合、降伏強度が低下する。一方、焼戻し温度が６５０℃を超える場合は、鋼板組織の再結晶により強度が著しく低下する。したがって、焼戻し温度は、５００℃以上６５０℃以下とする。 (Tempering process)
In the tempering step, the steel plate after the reheating and quenching step is tempered at a tempering temperature of 500° C. or more and 650° C. or less. When the tempering temperature is less than 500°C, the yield strength decreases. On the other hand, when the tempering temperature exceeds 650° C., the strength of the steel sheet is significantly reduced due to recrystallization of the steel sheet structure. Therefore, the tempering temperature is set at 500°C or more and 650°C or less.
　上記焼戻し工程の後は、鋼板を冷却することが好ましい。前記冷却の方法は特に限定されず、例えば、空冷、水冷など、任意の方法で行うことができる。 After the above-mentioned tempering process, it is preferable to cool the steel plate. The cooling method is not particularly limited, and any method such as air cooling or water cooling can be used.
　以下、本発明の作用効果について、実施例を用いて説明する。なお、本発明は以下の実施例に限定されない。 Hereinafter, the effects of the present invention will be explained using Examples. Note that the present invention is not limited to the following examples.
　表１に示す成分組成の鋼を溶製し、鋼素材としての鋼スラブを得た。得られた鋼スラブに対して、表２に示した条件で、加熱工程、熱間圧延工程、冷却工程、再加熱焼入れ工程、および焼戻し工程を順次施して鋼板を製造した。なお、再加熱焼入れ工程においては、鋼板を表２に示した再加熱温度まで再加熱した後、２００℃未満の冷却停止温度まで冷却した。 Steel having the composition shown in Table 1 was melted to obtain a steel slab as a steel material. The obtained steel slabs were sequentially subjected to a heating process, a hot rolling process, a cooling process, a reheating quenching process, and a tempering process under the conditions shown in Table 2 to produce a steel plate. In addition, in the reheating and quenching step, the steel plate was reheated to the reheating temperature shown in Table 2, and then cooled to a cooling stop temperature of less than 200°C.
　次いで、得られた鋼板のそれぞれについて、以下の手順で残留オーステナイト量（体積率）、旧オーステナイト粒の最大粒径、および旧オーステナイト粒の平均粒径ａに対する、旧オーステナイト粒の粒径の上位５％における平均値ｂの比ｂ／aを測定した。測定結果を表３に示す。 Next, for each of the obtained steel plates, the top 5 grain sizes of prior austenite grains are determined based on the amount of retained austenite (volume ratio), the maximum grain size of prior austenite grains, and the average grain size a of prior austenite grains using the following procedure. The ratio b/a of the average value b in % was determined. The measurement results are shown in Table 3.
（残留オーステナイト量）
　得られた鋼板の板厚１／４位置が測定面となるよう、板面に平行にＸ線回折用試験片を採取した。前記試験片に鏡面研磨および電解研磨を施した後、Ｘ線回折に供した。対称反射Ｘ線回折パターンに現れるα－Ｆｅの（２００）、（２１１）面、γ－Ｆｅの（２００）、（２２０）、（３１１）面の回折強度を求め、次式にて残留オーステナイト量Ｖγを算出した。
　Ｖγ＝１００／（（ＩαＲγ／ＩγＲα）＋１）
ここで、Ｖγ：残留オーステナイトの体積率、Ｉ：回折Ｘ線強度、Ｒ：単位体積当たりの理論強度値である。前記回折Ｘ線強度Ｉとしては、バックグラウンド除去後の積分強度を使用した。なお、残留オーステナイト量が極めて少ない場合には十分な測定精度が得られないため、算出された残留オーステナイト量が０．５％以下の場合についてはミクロ組織に残留オーステナイトが実質的に含まれていない（０％）と見なし、表３の残留γの体積率欄は空欄（－）とした。 (Amount of retained austenite)
A test piece for X-ray diffraction was taken parallel to the plate surface so that the measurement plane was at 1/4 of the plate thickness of the obtained steel plate. The test piece was subjected to mirror polishing and electrolytic polishing, and then subjected to X-ray diffraction. The diffraction intensities of the (200) and (211) planes of α-Fe and the (200), (220) and (311) planes of γ-Fe that appear in the symmetrical reflection X-ray diffraction pattern are determined, and the amount of retained austenite is calculated using the following formula. Vγ was calculated.
Vγ=100/((IαRγ/IγRα)+1)
Here, Vγ: volume fraction of retained austenite, I: diffraction X-ray intensity, R: theoretical intensity value per unit volume. As the diffraction X-ray intensity I, the integrated intensity after background removal was used. In addition, if the amount of retained austenite is extremely small, sufficient measurement accuracy cannot be obtained, so if the calculated amount of retained austenite is 0.5% or less, the microstructure does not substantially contain retained austenite. (0%), and the volume percentage column of residual γ in Table 3 was left blank (-).
　表３には残留γ量のみを記載したが、全ての実施例及び比較例における鋼板は、焼戻しマルテンサイトとベイナイトを主体とするミクロ組織を有していた。具体的には、焼戻しマルテンサイトとベイナイトの合計面積率が９０％以上であった。 Although only the residual γ amount is listed in Table 3, the steel plates in all Examples and Comparative Examples had microstructures mainly composed of tempered martensite and bainite. Specifically, the total area ratio of tempered martensite and bainite was 90% or more.
（旧γ粒の結晶粒径）
　得られた鋼板から、板厚１／２位置が観察位置となるように、組織観察用の試験片を採取した。前記試験片を、圧延方向（Ｌ方向）断面を観察面とするよう樹脂に埋め、鏡面研磨した。次いで、ピクリン酸腐食を実施した後、倍率２００倍の光学顕微鏡で観察した。撮影した１０視野分の画像を解析し、旧オーステナイト粒の最大粒径と、旧オーステナイト粒の平均粒径ａに対する、旧オーステナイト粒の粒径の上位５％における平均値ｂの比ｂ／aを求めた。ここで、旧オーステナイト粒の粒径としては、円相当径を使用した。 (Grain size of prior γ grains)
A test piece for microstructure observation was taken from the obtained steel plate so that the observation position was at 1/2 the thickness of the plate. The test piece was buried in resin so that the cross section in the rolling direction (L direction) was the observation surface, and mirror-polished. Next, after performing picric acid corrosion, it was observed with an optical microscope at a magnification of 200 times. Analyze the images taken for 10 fields and calculate the ratio b/a of the average value b of the top 5% of the grain sizes of the prior austenite grains to the maximum grain size of the prior austenite grains and the average grain size a of the prior austenite grains. I asked for it. Here, the equivalent circle diameter was used as the grain size of the prior austenite grains.
　また、旧オーステナイト粒を楕円近似した場合における長軸を短軸で除して得られる比率をアスペクト比として算出した。 In addition, the ratio obtained by dividing the long axis by the short axis when prior austenite grains are approximated to an ellipse was calculated as the aspect ratio.
　さらに、得られた鋼板のそれぞれについて、以下の手順で機械的特性を評価した。評価結果を表３に示す。 Furthermore, the mechanical properties of each of the obtained steel plates were evaluated using the following procedure. The evaluation results are shown in Table 3.
（強度）
　鋼板の板幅方向（Ｃ方向）が引張方向と一致するように、ＪＩＳ　Ｚ　２２０１に記載の５号試験片を採取し、ＪＩＳ　Ｚ　２２０４に準拠して引張試験を行い、降伏強度（ＹＳ）及び引張強度（ＴＳ）を求めた。 (Strength)
A No. 5 test piece described in JIS Z 2201 was taken so that the width direction (C direction) of the steel plate coincided with the tensile direction, and a tensile test was conducted in accordance with JIS Z 2204 to determine yield strength (YS) and Tensile strength (TS) was determined.
（低温靭性）
　低温靭性を評価するためにシャルピー衝撃試験を行った。具体的には、まず、前記鋼板から、該鋼板の圧延方向（Ｌ方向）が長辺となるように２ｍｍＶノッチを有するシャルピー衝撃試験片を採取した。前記試験片を液体窒素中で－１９６℃に冷却し、ＪＩＳ　Ｚ　２２４２に準拠してシャルピー衝撃試験を行い、－１９６℃における吸収エネルギーｖＥ－１９６を測定した。３本の試験片で同様の測定を行い、得られた吸収エネルギーｖＥ－１９６の平均値を表３に示した。なお、板厚が１２ｍｍ以下の場合は、サブサイズ試験片を用いて評価した。 (Low temperature toughness)
Charpy impact test was conducted to evaluate low temperature toughness. Specifically, first, a Charpy impact test piece having a 2 mm V notch was taken from the steel plate so that the rolling direction (L direction) of the steel plate was the long side. The test piece was cooled to -196°C in liquid nitrogen and subjected to a Charpy impact test in accordance with JIS Z 2242 to measure absorbed energy vE -196 at -196°C. Similar measurements were performed on three test pieces, and the average value of the obtained absorbed energy vE -196 is shown in Table 3. In addition, when the plate thickness was 12 mm or less, evaluation was performed using a sub-size test piece.
　通常の試験片を用いた場合、ｖＥ－１９６が１００Ｊ以上を合格とした。また、サブサイズ試験片を用いた場合、ｖＥ－１９６が５０Ｊ以上を合格とした。 When a normal test piece was used, a vE -196 of 100J or more was considered to be a pass. In addition, when a sub-size test piece was used, a vE -196 of 50J or more was considered a pass.
　表３に示した結果から分かるように、本発明例の鋼板は、上記特性を満足し、強度（引張強度及び降伏強度）、低温靭性、製造性ともに優れていた。一方、本発明の範囲を外れる比較例の鋼板は、強度（引張強度及び降伏強度）、低温靭性、製造性の少なくとも１つが劣っていた。 As can be seen from the results shown in Table 3, the steel plates of the examples of the present invention satisfied the above characteristics and were excellent in strength (tensile strength and yield strength), low-temperature toughness, and manufacturability. On the other hand, steel plates of comparative examples outside the scope of the present invention were inferior in at least one of strength (tensile strength and yield strength), low-temperature toughness, and manufacturability.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003

Claims (3)

  1. 　質量％で、
    　　Ｃ　：０．０１～０．１５％、
    　　Ｓｉ：０．０１～１．００％、
    　　Ｍｎ：０．１０～２．００％、
    　　Ｐ　：０．０１０％以下、
    　　Ｓ　：０．００５０％以下、
    　　Ｎｉ：５．０～１０．０％、
    　　Ａｌ：０．００２～０．１００％、および
    　　Ｎ　：０．００８０％以下、を含有し、
    　　残部Ｆｅおよび不可避的不純物からなる成分組成と、
    　板厚１／４位置における残留オーステナイトの体積率が３．０％未満であり、
    　板厚１／２位置における旧オーステナイト粒の最大粒径が１００μｍ以下であり、かつ、
    　板厚１／２位置における、旧オーステナイト粒の平均粒径ａに対する、旧オーステナイト粒の粒径の上位５％における平均値ｂの比ｂ／aが４．５以下であるミクロ組織とを有し、
    　板厚が４０ｍｍ以下であり、
    　降伏強度が５８５ＭＰａ以上、かつ、
    　引張強度が６９０ＭＰａ以上である、鋼板。     In mass%,
    C: 0.01-0.15%,
    Si: 0.01-1.00%,
    Mn: 0.10-2.00%,
    P: 0.010% or less,
    S: 0.0050% or less,
    Ni: 5.0 to 10.0%,
    Contains Al: 0.002 to 0.100%, and N: 0.0080% or less,
    A component composition consisting of the remainder Fe and unavoidable impurities,
    The volume fraction of retained austenite at the 1/4 plate thickness position is less than 3.0%,
    The maximum grain size of prior austenite grains at the plate thickness 1/2 position is 100 μm or less, and
    The microstructure has a microstructure in which the ratio b/a of the average value b in the top 5% of the grain size of the prior austenite grains to the average grain diameter a of the prior austenite grains at the plate thickness 1/2 position is 4.5 or less. ,
    The plate thickness is 40 mm or less,
    Yield strength is 585 MPa or more, and
    A steel plate having a tensile strength of 690 MPa or more.
  2. 　前記成分組成が、さらに、質量％で、
    　　Ｃｕ：０．０１～１．００％、
    　　Ｃｒ：０．０１～１．５０％、
    　　Ｍｏ：０．０３～１．０％、
    　　Ｎｂ：０．００１～０．０３０％、
    　　Ｖ　：０．０１～０．１０％、
    　　Ｔｉ：０．００３～０．０５０％、
    　　Ｂ　：０．０００３～０．００５０％、
    　　Ｓｎ：０．０１～０．３０％、
    　　Ｓｂ：０．０１～０．３０％、
    　　Ｗ　：０％超、２．００％以下、
    　　Ｃｏ：０％超、２．００％以下、
    　　Ｃａ：０．０００５～０．００５０％、
    　　Ｍｇ：０．０００５～０．０１００％、
    　　Ｚｒ：０．０００５～０．００５０％、
    　　Ｔａ：０．０１～０．２０％、
    　　Ｙ　：０．００１～０．０１０％、および
    　　ＲＥＭ：０．００１０～０．０２００％、
    　　からなる群より選択される少なくとも１つを含有する、請求項１に記載の鋼板。     The component composition further includes, in mass%,
    Cu: 0.01 to 1.00%,
    Cr: 0.01-1.50%,
    Mo: 0.03-1.0%,
    Nb: 0.001-0.030%,
    V: 0.01 to 0.10%,
    Ti: 0.003 to 0.050%,
    B: 0.0003 to 0.0050%,
    Sn: 0.01-0.30%,
    Sb: 0.01 to 0.30%,
    W: more than 0%, less than 2.00%,
    Co: more than 0%, less than 2.00%,
    Ca: 0.0005-0.0050%,
    Mg: 0.0005-0.0100%,
    Zr: 0.0005 to 0.0050%,
    Ta: 0.01-0.20%,
    Y: 0.001 to 0.010%, and REM: 0.0010 to 0.0200%,
    The steel plate according to claim 1, containing at least one selected from the group consisting of.
  3. 　請求項１または２に記載の成分組成を有する鋼素材を、９００℃以上１２００℃以下の加熱温度まで加熱する加熱工程と、
    　前記加熱工程で加熱した鋼素材を、圧下比が５以上、かつ最終５パスのうち１パス当たりの圧下率が１０％以上であるパス数が２以上の条件で熱間圧延して板厚４０ｍｍ以下の熱延鋼板とする熱間圧延工程と、
    　前記熱延鋼板を冷却する冷却工程と、
    　前記冷却工程後の熱延鋼板を、Ａｃ３点以上９００℃以下の再加熱温度まで再加熱し、焼入れする再加熱焼入れ工程と、
    　前記再加熱焼入れ工程後の熱延鋼板を、５００℃以上６５０℃以下の焼戻し温度で焼戻しする焼戻し工程とを有する、鋼板の製造方法。     A heating step of heating a steel material having the composition according to claim 1 or 2 to a heating temperature of 900 ° C. or more and 1200 ° C. or less,
    The steel material heated in the heating step is hot-rolled to a plate thickness of 40 mm under the conditions that the rolling reduction ratio is 5 or more and the number of passes is 2 or more in which the rolling reduction ratio per pass is 10% or more among the final 5 passes. A hot rolling process to produce the following hot rolled steel sheet,
    a cooling step of cooling the hot rolled steel sheet;
    a reheating and quenching step of reheating and quenching the hot rolled steel sheet after the cooling step to a reheating temperature of 3 Ac or more and 900° C. or less;
    A method for manufacturing a steel sheet, comprising a tempering step of tempering the hot rolled steel sheet after the reheating and quenching step at a tempering temperature of 500° C. or more and 650° C. or less.
PCT/JP2023/013709 2022-05-19 2023-03-31 Steel sheet and method for producing same WO2023223694A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
JP2023534743A JP7367896B1 (en) 2022-05-19 2023-03-31 Steel plate and its manufacturing method

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2022-082485 2022-05-19
JP2022082485 2022-05-19

Publications (1)

Publication Number Publication Date
WO2023223694A1 true WO2023223694A1 (en) 2023-11-23

Family

ID=88834972

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2023/013709 WO2023223694A1 (en) 2022-05-19 2023-03-31 Steel sheet and method for producing same

Country Status (2)

Country Link
TW (1) TWI841339B (en)
WO (1) WO2023223694A1 (en)

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009074123A (en) * 2007-09-19 2009-04-09 Nippon Steel Corp METHOD FOR MANUFACTURING Ni-CONTAINING STEEL HAVING EXCELLENT SURFACE QUALITY
CN110129685A (en) * 2019-05-22 2019-08-16 南京钢铁股份有限公司 A kind of manufacturing method of ultra-low temperature container 7Ni steel thick plate
WO2020136829A1 (en) 2018-12-27 2020-07-02 日本製鉄株式会社 Nickel-containing steel sheet
WO2020237975A1 (en) * 2019-05-27 2020-12-03 南京钢铁股份有限公司 7ni steel plate for lng storage tank and production process
WO2022118592A1 (en) * 2020-12-03 2022-06-09 Ｊｆｅスチール株式会社 Steel plate

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103882264A (en) * 2012-12-19 2014-06-25 海恩斯国际公司 Acid And Alkali Resistant Ni-cr-mo-cu Alloys With Critical Contents Of Chromium And Copper
TWI662138B (en) * 2018-03-31 2019-06-11 日商新日鐵住金股份有限公司 High-strength steel sheet with excellent ductility and hole expansion

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009074123A (en) * 2007-09-19 2009-04-09 Nippon Steel Corp METHOD FOR MANUFACTURING Ni-CONTAINING STEEL HAVING EXCELLENT SURFACE QUALITY
WO2020136829A1 (en) 2018-12-27 2020-07-02 日本製鉄株式会社 Nickel-containing steel sheet
CN110129685A (en) * 2019-05-22 2019-08-16 南京钢铁股份有限公司 A kind of manufacturing method of ultra-low temperature container 7Ni steel thick plate
WO2020237975A1 (en) * 2019-05-27 2020-12-03 南京钢铁股份有限公司 7ni steel plate for lng storage tank and production process
WO2022118592A1 (en) * 2020-12-03 2022-06-09 Ｊｆｅスチール株式会社 Steel plate

Also Published As

Publication number Publication date
TW202407115A (en) 2024-02-16
TWI841339B (en) 2024-05-01

Similar Documents

Publication Publication Date Title
JP5846311B2 (en) Thick high-strength steel excellent in welding heat affected zone CTOD characteristics and method for producing the same
WO2015088040A1 (en) Steel sheet and method for manufacturing same
JP2019504210A (en) Steel for pressure vessels excellent in resistance to hydrogen induced cracking (HIC) and method for producing the same
KR100920536B1 (en) High tensile and fire-resistant steel excellent in weldability and gas cutting property and method for production thereof
WO2014132968A1 (en) HIGH-STRENGTH HOT-ROLLED STEEL SHEET HAVING MAXIMUM TENSILE STRENGTH OF 980 MPa OR ABOVE, AND HAVING EXCELLENT AND BAKING HARDENABILITY AND LOW-TEMPERATURE TOUGHNESS
WO2010087512A1 (en) Heavy gauge, high tensile strength, hot rolled steel sheet with excellent hic resistance and manufacturing method therefor
WO2012002566A1 (en) High-strength steel sheet with excellent processability and process for producing same
CN113166897B (en) Ultra-high strength steel having excellent cold workability and SSC resistance and method for manufacturing the same
JP6492862B2 (en) Low temperature thick steel plate and method for producing the same
EP3730659A1 (en) High-strength steel material for polar region environment having excellent anti-fracture characteristics at low temperatures and method for manufacturing same
CN112236539B (en) High-tensile thick steel plate for extremely low temperature and method for producing same
JP2022548144A (en) High-strength extra-thick steel material with excellent low-temperature impact toughness and its manufacturing method
JP7221476B6 (en) Steel material excellent in resistance to hydrogen-induced cracking and its manufacturing method
CA3108674C (en) Steel for pressure vessel having excellent surface quality and impact toughness, and method for manufacturing same
CN112877591A (en) High-strength and high-toughness steel for hardware tool and chain and manufacturing method thereof
WO2022129995A1 (en) Coated steel sheet and high strength press hardened steel part and method of manufacturing the same
JP7367896B1 (en) Steel plate and its manufacturing method
KR102508128B1 (en) Steel plate having excellent low temperature impact toughness of heat affeected zone and manufacturing mehtod for the same
JP7251512B2 (en) Steel plate and its manufacturing method
JP7291222B2 (en) High-strength steel sheet with excellent ductility and workability, and method for producing the same
WO2023223694A1 (en) Steel sheet and method for producing same
JP3077568B2 (en) Method of manufacturing steel for low-temperature rebar
CN113166896A (en) Steel material for pressure vessel having excellent hydrogen-induced cracking resistance and method for producing same
CN114341386B (en) Steel material excellent in strength and low-temperature impact toughness and method for producing same
JPWO2018168618A1 (en) High strength cold rolled steel sheet and method of manufacturing the same

Legal Events

Date Code Title Description
ENP Entry into the national phase

Ref document number: 2023534743

Country of ref document: JP

Kind code of ref document: A

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

Ref document number: 23807309

Country of ref document: EP

Kind code of ref document: A1

WWE Wipo information: entry into national phase

Ref document number: 2023807309

Country of ref document: EP

ENP Entry into the national phase

Ref document number: 2023807309

Country of ref document: EP

Effective date: 20240906