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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/001—Heat treatment of ferrous alloys containing Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C21D8/0205—
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a steel plate and a method of producing the same.
• the present disclosure particularly relates to a steel plate having high strength, low yield ratio, and excellent low-temperature toughness, and a method of producing the same.
• the steel plate according to the present disclosure can be suitably used for steel structures that are used in low-temperature environments, such as liquefied gas storage tanks for ships.
• stress relief after welding can be performed mechanically instead of by post weld heat treatment.
• JP 2016-507649 A discloses “A high strength steel plate comprising, in wt %, carbon (C): 0.02% to 0.12%, manganese (Mn): 0.5% to 2.0%, silicon (Si): 0.05% to 0.5%, nickel (Ni): 0.05% to 1.0%, titanium (Ti): 0.005% to 0.1%, aluminum (Al): 0.005% to 0.5%, phosphorus (P): 0.015% or less, and sulfur(S): 0.015% or less with a balance consisting of Fe and other inevitable impurities, wherein a microstructure contains 70% to 90% of ultrafine ferrite and 10% to 30% of MA (martensite/austenite) microstructure in area fraction and has a yield ratio (YS/TS) of 0.8 or less.”
• JP 2013-57105 A discloses “A steel plate for low-yield-ratio thick circular steel pipes or tubes having a tensile strength of 780 MPa or more, containing C:
• DI ⁇ ( inches ) ⁇ 1.16 ⁇ ( [ C ] / 10 ) ⁇ 1 / 2 ⁇ ⁇ ( 0.7 ⁇ [ Si ] + 1 ) ⁇ ⁇ 5.1 ⁇ ( [ Mn ] - 1.2 ) + 5 ⁇ ⁇ ( 0.35 ⁇ [ Cu ] + 1 ) ⁇ ( 0.36 ⁇ [ Ni ] + 1 ) ⁇ ( 2.16 ⁇ [ Cr ] + 1 ) ⁇ ( 3 ⁇ [ Mo ] + 1 ) ⁇ ( 1.75 ⁇ [ V ] + 1 ) ⁇ ( 200 ⁇ [ B ] + 1 ) ( 1 )
• JP 2019-119934 A discloses “An ultra-low yield ratio and high tensile strength steel plate comprising: a chemical composition containing, in mass %, C: 0.03% to 0.20%, Si: 0.01% to 0.50%, Mn: 0.5% to 3.0%, P: 0.015% or less, S: 0.0050% or less, Al: 0.005% to 0.1%, and N: 0.0015% to 0.0065% with a balance consisting of Fe and inevitable impurities; and a microstructure that contains bainite including martensite austenite constituent, martensite, and cementite and in which cementite is contained in either or both of bainite and martensite microstructures, a total area fraction of bainite and martensite is 50.0% or more and less than 95.0%, an area fraction of martensite austenite constituent is 5% to 20%, an average equivalent circular diameter of martensite austenite constituent is less than 5.0 ⁇ m, an area fraction of cementite is more than 0% and
• the TS of the steel plate described in PTL 1 is 620 MPa at a maximum.
• PTL 1 fails to provide a steel plate with TS of 690 MPa or more.
• toughness in cryogenic environments of ⁇ 50° C. to ⁇ 70° C. (hereafter also simply referred to as low-temperature toughness) is not taken into consideration.
• nickel steel, such as 9% Ni steel may be able to achieve the desired properties mentioned above, high material costs are required.
• An effective way of achieving desired property improvement is to control the microstructure at a depth position of 1 ⁇ 4 of the plate thickness from the surface of the steel plate in the plate thickness direction (hereafter also referred to as a depth position of 1 ⁇ 4 of the plate thickness of the steel plate) as follows:
• a steel plate having high strength, low yield ratio, and excellent low-temperature toughness specifically, a steel plate having TS of 690 MPa or more, YR of 0.8 or less, and vE ⁇ 70° C. of 100 J or more, which can replace expensive nickel steel.
• the steel plate can be used for steel structures that are used in low-temperature environments, for example, large liquefied gas storage tanks such as liquefied CO2 tanks and LPG tanks for ships, and contributes to significantly lower production costs than when nickel steel is used. This yields industrially great advantageous effects.
• a steel plate according to an embodiment of the present disclosure will be described in detail below.
• the following description shows a preferred embodiment of the present disclosure, and the present disclosure is not limited to such.
• the steel material used in the method of producing the steel plate according to an embodiment of the present disclosure also has the below-described chemical composition.
• “%” as a unit of content of each element denotes “mass %” unless otherwise specified.
• the C content is 0.02% or more.
• the C content is preferably 0.03% or more. If the C content is more than 0.15%, the amount of martensite austenite constituent in the steel plate is excessive and low-temperature toughness decreases. The C content is therefore 0.15% or less.
• the C content is preferably 0.12% or less.
• Si 0.01% or More and 0.50% or Less
• Si is an element that acts as a deoxidizer.
• the Si content is 0.01% or more.
• the Si content is preferably 0.03% or more. If the Si content is excessively high, toughness decreases.
• the Si content is therefore 0.50% or less.
• the Si content is preferably 0.30% or less.
• the B is an element that enhances hardenability when added in a small amount.
• the B content is preferably 0.0003% or more. If the B content is more than 0.0030%, toughness may degrade. Accordingly, in the case where B is added, the B content is preferably 0.0030% or less. The B content is more preferably less than 0.0025%.
• Ca is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel. If the Ca content is excessively high, the cleanliness of the steel may be impaired and low-temperature toughness, in particular Charpy absorbed energy at low temperatures, may decrease. Accordingly, in the case where Ca is added, the Ca content is preferably 0.007% or less. The Ca content is more preferably 0.004% or less. Although no lower limit is placed on the Ca content, the Ca content is preferably 0.001% or more in order to achieve the foregoing effect.
• REM rare earth metal
• the REM content is preferably 0.010% or less.
• the REM content is more preferably 0.008% or less.
• the REM content is preferably 0.001% or more in order to achieve the foregoing effect.
• REM is a generic term for 17 elements including 15 lanthanoid elements and Y and Sc. These 17 elements may be contained singly or in combination. The REM content is the total content of these 17 elements.
• Mg is an element that has the effect of improving the low-temperature toughness of the steel plate by controlling the form of inclusions in the steel, as with Ca and REM. If the Mg content is excessively high, the cleanliness of the steel may be impaired and low-temperature toughness, in particular Charpy absorbed energy at low temperatures, may decrease. Accordingly, in the case where Mg is added, the Mg content is preferably 0.007% or less. The Mg content is more preferably 0.004% or less. Although no lower limit is placed on the Mg content, the Mg content is preferably 0.001% or more in order to achieve the foregoing effect.
• microstructure of the steel plate according to an embodiment of the present disclosure will be described.
• the microstructure is measured at a depth position of 1 ⁇ 4 of the plate thickness of the steel plate as described later.
• the tensile strength can be measured by the method described in the EXAMPLES section below.
• the cooling treatment in the quenching may be performed by any method without limitation.
• one or both of air cooling and water cooling may be used.
• water cooling any cooling method (for example, spray cooling, mist cooling, laminar cooling, etc.) using water may be used.
• the cooling end temperature is more than 500° C., martensite austenite constituent decomposes during cooling to room temperature after the end of the cooling, for example, during cooling by air cooling (hereafter also simply referred to as air cooling), and the desired low yield ratio cannot be achieved. If the cooling end temperature is less than 200° C., the desired tempering effect cannot be achieved during air cooling and toughness degrades. The cooling end temperature is therefore 500° C. or less and 200° C. or more.
• the temperature herein is the temperature at a depth position of 1 ⁇ 4 of the plate thickness of the hot-rolled steel plate.
• molten steel having the chemical composition shown in Table 1 (with the balance consisting of Fe and inevitable impurities) was prepared by steelmaking using a converter, and subjected to continuous casting to produce a steel slab (thickness: 200 mm) as a steel material.
• Ac 1 point (C) calculated according to the foregoing formula (1) and Ac 3 point (° C.) calculated according to the foregoing formula (2) are shown in Table 1.
• microstructure analysis was conducted by electron backscatter diffraction measurement (hereafter also referred to as EBSD measurement) using the foregoing test piece.
• EBSD measurement electron backscatter diffraction measurement
• the step size was 0.1 ⁇ m
• the measurement region was 1 mm ⁇ 1 mm in total.
• each crystal grain was defined with large-angle grain boundaries with an orientation difference of 15 degrees or more being set as crystal grain boundaries.
• the equivalent circular diameter of each crystal grain was then calculated from the area of the crystal grain. After this, the number of crystal grains having an equivalent circular diameter of more than 30 ⁇ m was counted, and the number was divided by the total area of the measurement region to determine the number density of coarse crystal grains.
• a JIS No. 4 tensile test piece perpendicular to the rolling direction was collected from a depth position of 1 ⁇ 4 of the plate thickness of the steel plate.
• a tensile test was conducted in accordance with JIS Z 2241 to measure the tensile strength (TS) and yield stress (YS) of the steel plate.
• TS tensile strength
• YS yield stress
• YR yield ratio

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