WO2019082326A1 - 低温用ニッケル含有鋼 - Google Patents
低温用ニッケル含有鋼Info
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- WO2019082326A1 WO2019082326A1 PCT/JP2017/038632 JP2017038632W WO2019082326A1 WO 2019082326 A1 WO2019082326 A1 WO 2019082326A1 JP 2017038632 W JP2017038632 W JP 2017038632W WO 2019082326 A1 WO2019082326 A1 WO 2019082326A1
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- 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
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- 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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- 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
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- 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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- 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
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- C—CHEMISTRY; METALLURGY
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- 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
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/32—Hydrogen storage
Definitions
- the present invention relates to a low temperature nickel containing steel, ie, a steel containing nickel (Ni) suitable for low temperature applications near -253 ° C.
- a steel plate used for a tank for storing and transporting liquefied gas such as liquid hydrogen is required to have excellent low temperature toughness, and austenitic stainless steel that is hard to break easily is used. Austenitic stainless steel has sufficient low temperature toughness, but the yield stress at room temperature of general-purpose materials is about 200 MPa.
- Ferritic 9% Ni steel and 7% Ni steel are used in tanks (sometimes referred to as LNG tanks) for liquefied natural gas (LNG), a typical liquefied gas. .
- LNG is at a higher temperature than liquid hydrogen
- 9% Ni steel and 7% Ni steel have excellent low temperature toughness. Therefore, conventionally, various 9% Ni steel and 7% Ni steel suitable for the LNG tank have been proposed (see, for example, Patent Documents 2 to 4).
- the 9% Ni steel can also have a yield stress at room temperature of 590 MPa or more, and can also be applied to large LNG tanks.
- Patent Document 2 contains 5 to 7.5% of Ni, the yield stress at room temperature is higher than 590 MPa, and the brittle fracture rate in the Charpy test at -233 ° C. is 50% or less.
- a low temperature steel having a thickness of 25 mm is disclosed.
- the low temperature toughness of the low temperature steel is secured by setting the volume fraction of retained austenite stable at -196 ° C to 2 to 12%.
- Patent Document 3 a low-temperature steel having a plate thickness of 6 to 50 mm containing 5-10% of Ni, having a yield stress at room temperature of 590 MPa or more, and excellent in low temperature toughness at -196 ° C. after strain aging It is disclosed.
- the volume fraction of retained austenite is 3% or more, the effective crystal grain size is 5.5 ⁇ m or less, and an appropriate defect is introduced into the structure in the grain, so that strain aging of the low temperature steel is performed. Low temperature toughness is secured.
- Patent Document 4 discloses a low-temperature thin nickel steel plate having a thickness of 6 mm which contains 7.5 to 12% of Ni and is excellent not only in the base material but also in the low temperature toughness of the weld heat affected zone. .
- the low temperature toughness at -196 ° C. of the steel sheet is secured by reducing the contents of Si and Mn so that island martensite is not generated in the weld heat affected zone.
- Japanese Patent No. 5709881 Japanese Patent Application Laid-Open No. 2014-210948 Japan JP 2011-219849 Japanese Patent Application Laid-Open No. 3-222342
- the austenitic high-Mn stainless steel disclosed in Patent Document 1 has a large thermal expansion coefficient as compared to a ferritic 9% Ni steel.
- 9% Ni steel which has a low coefficient of thermal expansion, is advantageous because of problems such as fatigue.
- 9% Ni steel and 7% Ni steel disclosed in Patent Documents 2 to 4 have difficulty in obtaining sufficient toughness at -253 ° C., which is the temperature of liquid hydrogen, as a result of studies by the present inventors. I understand.
- the present invention has a sufficient toughness at a low temperature around -253 ° C, and a low temperature nickel-containing steel having a yield stress at room temperature of 590 MPa or more and a tensile strength at room temperature of 690 MPa or more. Intended to provide.
- the present inventors examined the toughness at low temperatures around -253 ° C., the yield stress at room temperature, and the tensile strength of a steel having a Ni content higher than that of the conventional 9%.
- the content of Si is limited
- the content of Mn is strictly limited
- the volume of austenite It has been found that it is necessary to optimally control the fraction and the mean grain size and mean aspect ratio of the prior austenite grains.
- the low temperature nickel-containing steel according to one aspect of the present invention has a chemical composition of, by mass%, C: 0.040 to 0.080%, Si: 0.03 to 0.30%, Mn: 0 .20 to 0.80%, Ni: 10.5 to 12.4%, Mo: 0.10 to 0.50%, Al: 0.010 to 0.060%, N: 0.0015 to 0.0060 %, O: 0.0007 to 0.0030%, Cu: 0 to 0.50%, Cr: 0 to 0.50%, Nb: 0 to 0.020%, V: 0 to 0.080%, Ti : 0 to 0.020%, B: 0 to 0.0020%, Ca: 0 to 0.0040%, REM: 0 to 0.0050%, P: not more than 0.0070%, S: not more than 0.0040% , Remainder: Fe and impurities, and the metal structure at the center of the plate thickness contains 2.0 to 20.0% by volume of the austenite phase, and The average grain size of prior austenite grains measured
- the plate thickness is more than 16 mm
- Ni is 11.5% or more
- the plate thickness is 16 mm or less
- C 0.070% or less
- Si 0.19% or less
- Mn 0.40% or less
- Al 0.050% or less
- N 0.0050% or less
- Cr 0. 35% or less
- Nb 0.015% or less
- V 0.060% or less
- Ti 0.015% or less
- S 0.0030% or less
- the average particle diameter of the prior austenite grains is 6.0 ⁇ m It is below.
- Ni In the low-temperature nickel-containing steel described in the above (1), Ni may be 11.5% or more and Mn may be 0.50% or less. (3) In the low temperature nickel-containing steel according to the above (1) or (2), Ni may be 11.5% or more, and the average grain size of the prior austenite grains may be 9.0 ⁇ m or less . (4) In the low-temperature nickel-containing steel according to any one of the above (1) to (3), the average measured on the surface parallel to the rolling direction and the thickness direction of the thickness center The effective crystal grain size may be 2.0 to 7.0 ⁇ m. (5) In the low-temperature nickel-containing steel according to any one of the above (1) to (3), an average measured on the surface parallel to the rolling direction and the thickness direction of the thickness center The effective crystal grain size may be 2.0 to 4.0 ⁇ m.
- the present invention it is possible to provide a low temperature nickel-containing steel having sufficient toughness at a low temperature around -253 ° C. and having a tensile stress at room temperature of 590 MPa or more and a tensile strength at room temperature of 690 MPa or more. it can. Therefore, when the low temperature nickel-containing steel of the present invention is used, for example, in a liquid hydrogen tank, it is possible to make the thickness of the tank steel plate thinner than that of austenitic stainless steel.
- the low-temperature nickel-containing steel according to the present invention increases the size and weight of the liquid hydrogen tank, improves the heat insulating performance by reducing the surface area with respect to the volume, effectively uses the site where the liquid hydrogen tank is installed, and liquid hydrogen It is possible to improve the fuel efficiency of the carrier ship, etc.
- the low temperature nickel-containing steel according to the present invention has a smaller coefficient of thermal expansion as compared with austenitic stainless steel, the design of a large tank is not complicated, and the tank manufacturing cost can be reduced. Thus, the present invention is extremely significant for industrial contribution.
- the toughness of the conventional low-temperature steel was evaluated at -165 ° C. or -196 ° C.
- the low-temperature nickel-containing steel according to this embodiment (hereinafter referred to as "Ni steel") Toughness evaluation temperature is much lower than that of conventional steels. Note that, in order to be described briefly and distinctly from temperatures such as -165.degree. C. and -196.degree. C., a temperature near -253.degree. C. is conveniently described as "very low temperature" below.
- cryogenic toughness the content of components, the metal structure and the like that affect the toughness at a cryogenic temperature of Ni steel. According to conventional findings, it has been considered effective to increase the Ni content in order to improve the low temperature toughness. However, as a result of investigations by the present inventors, it was found that toughness at a cryogenic temperature was not sufficiently improved even if a change was simply made to increase the amount of Ni to the conventional 9% Ni steel.
- the present inventors examined a method for improving the toughness at low temperature of Ni steel.
- the present inventors particularly set the content of (a) C to 0.040 to 0.080%, and (b) the content of Si to 0.03 to 0.30%. And (c) to make the Mn content 0.20 to 0.80%, (d) to make the P content 0.0070% or less, and (e) the Mo content to be 0.10 to 0.
- the following conditions are satisfied: 50%, (f) controlling the grain size and aspect ratio of prior austenite grains, and (g) controlling the volume fraction of austenite phase, Ni steel It is found that it is necessary to improve the toughness at cryogenic temperatures.
- the effective grain size of the prior austenite grains (h) the low temperature toughness of the Ni steel at cryogenic temperatures is further improved. Furthermore, it was also found that, when the thickness of the Ni steel is 16 mm or less and the above-mentioned various conditions etc. are more strictly limited, the Ni content can be reduced to reduce the raw material cost.
- Ni steel according to the present embodiment will be described.
- the plate thickness when the plate thickness is 16 mm or less, securing of low temperature toughness through heat treatment becomes easy, so it is possible to provide sufficient low temperature toughness to Ni steel while suppressing the Ni content to less than 11.5%. It becomes.
- the plate thickness may be 16 mm or less, and the Ni content may be 11.5% or more.
- the plate thickness is 16 mm or less and the Ni content is less than 11.5% (hereinafter sometimes referred to as "the case where the Ni content is small")
- elements affecting low temperature toughness other than Ni content (The content of C, Si, Mn, Al, N, Cr, Nb, V, Ti, P, and S, and the average grain size of the prior austenite grains) is more than Ni: 11.5% or more It needs to be strictly controlled. With respect to the requirements that require further limitation according to the Ni content and the plate thickness due to the above-mentioned circumstances, this will be appropriately described.
- % of content means mass%, unless there is particular explanation.
- C (C: 0.040 to 0.080%) C is an element that raises the yield stress of Ni steel at room temperature, and also contributes to the formation of martensite and austenite. If the C content is less than 0.040%, the strength of the Ni steel can not be secured, and the formation of coarse bainite and inclusions may lower the cryogenic toughness of the Ni steel. Therefore, the lower limit of the C content is set to 0.040%. The lower limit of the preferred C content is 0.045%. On the other hand, if the C content exceeds 0.080%, cementite is likely to be precipitated at the prior austenite grain boundaries, and this cementite causes fracture at the grain boundaries to lower the cryogenic toughness of the Ni steel. Therefore, the upper limit of the C content is set to 0.080%. The upper limit of the C content is preferably 0.070%, more preferably 0.060%, and still more preferably 0.055%.
- the C content When the Ni content is low, the C content needs to be made 0.070% or less.
- the upper limit of the C content is preferably 0.065%, 0.060%, or 0.055%.
- the lower limit and the preferable lower limit of the C content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- Si is an element that raises the yield stress at room temperature of Ni steel. If the Si content is less than 0.03%, the effect of improving the yield stress at room temperature is small. Therefore, the lower limit of the Si content is 0.03%. The lower limit of the Si content is preferably 0.05%. On the other hand, if the Si content exceeds 0.30%, cementite in the prior austenite grain boundaries tends to be coarsened, this cementite causes fracture at the grain boundaries, and the cryogenic toughness of the Ni steel decreases. Therefore, limiting the upper limit of the Si content to 0.30% is extremely important in order to secure the toughness at low temperature of Ni steel. The upper limit of the Si content is preferably 0.20%, more preferably 0.15%, and still more preferably 0.10%.
- the Si content when the Ni content is small, Si: 0.19% or less
- the Si content needs to be 0.19% or less.
- the upper limit of the Si content is preferably 0.16%, 0.13%, or 0.10%.
- the lower limit and the preferred lower limit of the Si content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- Mn is an element that raises the yield stress at room temperature of Ni steel. If the Mn content is less than 0.20%, the strength of the Ni steel can not be secured, and the formation of coarse bainite, inclusions and the like may lower the cryogenic toughness of the Ni steel. Therefore, the lower limit of the Mn content is 0.20%. The lower limit of the preferred Mn content is 0.25% or 0.30%. On the other hand, when the Mn content exceeds 0.80%, fracture at grain boundaries is caused by Mn segregated in the prior austenite grain boundaries, MnS precipitated coarsely, and the like, and the cryogenic toughness of the Ni steel decreases.
- the upper limit of the Mn content is 0.70%, or 0.60%, more preferably 0.55%, or 0.50%.
- Mn 0.40% or less when Ni content is small
- the Mn content needs to be 0.40% or less.
- the upper limit of the preferable Mn content is 0.35% or 0.30%.
- the lower limit and the preferable lower limit of the Mn content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- Ni 10.5 to 12.4%
- Ni is an essential element for securing the cryogenic toughness of Ni steel. If the Ni content is less than 10.5%, the low temperature toughness of the Ni steel is insufficient. Therefore, the lower limit of the Ni content is 10.5%.
- the lower limit of the preferred Ni content is 10.8%, 11.0%, or 11.5%.
- Ni is an expensive element and containing more than 12.4% impairs economy. Therefore, the upper limit of the Ni content is limited to 12.4%.
- the upper limit of the Ni content may be 12.2%, 12.0%, or 11.8%. When the plate thickness is 16 mm or less, the upper limit of the Ni content may be 11.3%, 11.1%, or 10.9%.
- the plate thickness is more than 16 mm, Ni: 11.5% or more
- the Ni content needs to be 11.5% or more.
- the lower limit of the preferable Ni content is 11.8% or 12.0%.
- the upper limit and the preferable upper limit of the Ni content may be the same as those of Ni steel having a thickness of 16 mm or less.
- Mo is an element that raises the yield stress of Ni steel at room temperature, and also has the effect of suppressing intergranular embrittlement of Ni steel. Therefore, the lower limit of the Mo content is 0.10%. Preferably, the lower limit of the Mo content is 0.20%. On the other hand, Mo is an expensive element, and if the Mo content exceeds 0.50%, the economy is impaired. Therefore, the upper limit of the Mo content is limited to 0.50%. Preferably, the upper limit of the Mo content is 0.40%, 0.35%, or 0.30%.
- Al 0.010 to 0.060%
- Al is an element mainly used for deoxidation.
- Al is an element which forms AlN and contributes to the refinement of the metal structure and the reduction of solid solution N which lowers the cryogenic toughness of the Ni steel. If the Al content is less than 0.010%, the effect of deoxidation, the effect of refining the metal structure, and the effect of reducing solid solution N are small. Therefore, the lower limit of the Al content is set to 0.010%.
- the lower limit of the Al content is preferably 0.015%, and more preferably 0.020%.
- the upper limit of the Al content is set to 0.060%.
- the upper limit of the Al content is more preferably 0.040% or 0.035%.
- the Al content When the Ni content is low, the Al content needs to be 0.050% or less.
- the upper limit of the Al content is preferably 0.040%, 0.030%, or 0.020%.
- the lower limit and the preferable lower limit of the Al content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- N is an element that contributes to the formation of a nitride that refines crystal grains.
- the N content is reduced to less than 0.0015%, fine AlN which suppresses coarsening of austenite grain size during heat treatment may run short, austenite grains may coarsen and the cryogenic toughness of the Ni steel may decrease. Therefore, the lower limit of the N content is 0.0015%.
- the preferred N content is 0.0020%.
- the upper limit of the N content is set to 0.0060%.
- the preferred upper limit of the N content is 0.0050%, 0.0040%, or 0.0035%.
- the Ni content is low, N: 0.0050% or less
- the upper limit of the N content is preferably 0.0040% or 0.0030%.
- the lower limit and the preferable lower limit of the N content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- O is an impurity, and when the O content exceeds 0.0030%, Al 2 O 3 clusters may increase and the toughness at low temperature of Ni steel may decrease. Therefore, the upper limit of the O content is made 0.0030%.
- the upper limit of the O content is preferably 0.0025%, more preferably 0.0020%, and still more preferably 0.0015%. It is desirable to reduce the O content as much as possible, but reducing the O content to less than 0.0007% may be accompanied by cost increases. Therefore, the lower limit of the O content is set to 0.0007%.
- the lower limit of the preferable O content is 0.0008%, more preferably 0.0010%.
- P is an element that causes intergranular embrittlement at prior austenite grain boundaries, and is an element that is detrimental to the cryogenic toughness of Ni steel. Therefore, it is desirable to reduce the P content as much as possible.
- the cryogenic toughness of the Ni steel may be reduced. Therefore, the upper limit of the P content is limited to 0.0070%.
- the upper limit of the P content is preferably 0.0050%, more preferably 0.0040%, and still more preferably 0.0030%.
- P may be mixed into the molten steel as an impurity during molten steel production, but the lower limit thereof is not particularly limited, and the lower limit is 0%. However, if the P content is reduced to less than 0.0003%, the melting cost may increase. Therefore, the lower limit of the P content may be 0.0003%, 0.0005%, or 0.0010%.
- the upper limit of the P content is preferably 0.0040% or 0.0030%.
- the lower limit and the preferred lower limit of the P content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- S forms MnS, which may be a starting point of brittle fracture, and thus is an element harmful to cryogenic toughness.
- the upper limit of the S content is limited to 0.0040%.
- the upper limit of the S content is preferably 0.0030%, more preferably 0.0020%, and still more preferably 0.0010%.
- S may be mixed into the molten steel as an impurity during molten steel production, but the lower limit thereof is not particularly limited, and the lower limit is 0%. However, if the S content is reduced to less than 0.0002%, the melting cost may increase. Therefore, the lower limit of the S content may be 0.0002%, 0.0004%, or 0.0006%.
- the S content needs to be 0.0030% or less.
- the upper limit of the preferable S content is 0.0010%, 0.0015%, or 0.0010%.
- the lower limit and the preferred lower limit of the S content of the Ni steel having a small Ni content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- the Ni steel according to the present embodiment may contain Cu. However, if the Cu content exceeds 0.50%, the toughness of the Ni steel at extremely low temperatures decreases. Therefore, the upper limit of the Cu content is 0.50%.
- the upper limit of the Cu content is preferably 0.40%, more preferably 0.30%, and still more preferably 0.20%.
- Cu may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.
- the lower limit of the Cu content may be 0.02%, 0.05%, or 0.10%.
- the upper and lower limit values of the Cu content and the preferable upper and lower limit values are the above-mentioned values regardless of the plate thickness and the Ni content.
- the Ni steel according to the present embodiment may contain Cr. However, if the Cr content exceeds 0.50%, the toughness of the Ni steel at extremely low temperatures decreases. Therefore, the upper limit of the Cr content is set to 0.50%.
- the upper limit of the Cr content is preferably 0.30%, more preferably 0.20%, still more preferably 0.10%.
- the Cr may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof need not be particularly limited, and the lower limit is 0%.
- the lower limit of the Cr content may be 0.02%, 0.05%, or 0.10%.
- the Cr content When the Ni content is low, Cr: 0.35% or less, the Cr content needs to be 0.35% or less.
- the upper limit of the Cr content is preferably 0.30%, 0.25%, or 0.20%.
- the lower limit and the preferable lower limit of the Cr content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- Nb is an element that raises the yield stress of Ni steel at room temperature, and also has the effect of improving the cryogenic toughness due to the refinement of the metal structure, so the Ni steel according to the present embodiment may contain Nb.
- the upper limit of the Nb content is set to 0.020%.
- the upper limit of the Nb content is preferably 0.015%, more preferably 0.010%.
- Nb may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.
- the lower limit value of the Nb content may be set to 0.002%, 0.005%, or 0.010%.
- the Nb content needs to be 0.015% or less.
- the upper limit of the preferable Nb content is 0.012% or 0.010%.
- the lower limit and the preferable lower limit of the Nb content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- V (V: 0 to 0.080%) Since V is an element that raises the yield stress of Ni steel at room temperature, the Ni steel according to the present embodiment may contain V. However, when the V content exceeds 0.080%, the toughness of the Ni steel at cryogenic temperatures is reduced. Therefore, the upper limit of the V content is set to 0.080%.
- the upper limit of the V content is preferably 0.060%, more preferably 0.040%.
- V may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof need not be particularly limited, and the lower limit is 0%.
- the lower limit of the V content may be 0.002%, 0.005%, or 0.010%.
- V 0.060% or less when Ni content is small
- the V content needs to be 0.060% or less.
- the upper limit of the V content is preferably 0.050% or 0.040%.
- the lower limit and the preferable lower limit of the V content of the Ni steel having a low Ni content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- the Ni steel according to the present embodiment may contain Ti. .
- the upper limit of the Ti content is set to 0.020%.
- the upper limit of the Ti content is preferably 0.015%, and more preferably 0.010%.
- Ti may be mixed into Ni steel as an impurity at the time of production of molten steel, the lower limit thereof need not be particularly limited, and the lower limit is 0%.
- the lower limit of the Ti content may be 0.001%, 0.002%, or 0.005%.
- the Ti content When the Ni content is small, the Ti content needs to be 0.015% or less.
- the upper limit of the Ti content is preferably 0.012% or 0.010%.
- the lower limit and the preferred lower limit of the Ti content may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- Upper and lower limit values of the content of B, Ca, REM, Sb, Sn, As, Co, Zn, and W described below and preferable upper and lower limit values are the same regardless of the plate thickness and the Ni content. .
- B is an element that raises the yield stress of Ni steel at room temperature, and also contributes to the reduction of solid solution N that forms BN and lowers the cryogenic toughness of Ni steel, according to the present embodiment.
- Ni steel may contain B.
- the upper limit of the B content is set to 0.0020%.
- the upper limit of the B content is preferably 0.0015%, more preferably 0.0012%, and still more preferably 0.0010%.
- B may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof need not be particularly limited, and the lower limit is 0%.
- the lower limit of the B content may be 0.0001%, 0.0002%, or 0.0005%.
- Ca is an element to spheroidize MnS, which is an inclusion that tends to increase the harmfulness to cryogenic toughness by drawing by hot rolling, as CaS, and is an element effective to improve the cryogenic toughness of Ni steel . Therefore, the Ni steel according to the present embodiment may contain Ca. However, when the Ca content exceeds 0.0040%, the acid sulfide containing Ca is coarsened, and this acid sulfide lowers the toughness of the Ni steel at the cryogenic temperature. Therefore, the upper limit of the Ca content is limited to 0.0040%. The upper limit of the Ca content is preferably 0.0030%.
- Ca may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.
- the lower limit of the Ca content may be 0.0005%, 0.0010%, or 0.0015%.
- REM 0 to 0.0050%
- REM rare earth metal element
- the content of REM means the total content of these 17 elements.
- the Ni steel according to the present embodiment may contain REM.
- the REM content exceeds 0.0050%, the acid sulfide containing REM is coarsened, and this acid sulfide lowers the toughness of the Ni steel at the cryogenic temperature. Therefore, the upper limit of the REM content is limited to 0.0050%.
- the upper limit of the REM content is preferably 0.0040%.
- REM may be mixed into Ni steel as an impurity at the time of production of molten steel, but the lower limit thereof need not be particularly limited, and the lower limit is 0%.
- the lower limit of the REM content may be 0.0005%, 0.0010%, or 0.0015%.
- the Ni steel according to the present embodiment contains or restricts the above components, and the balance contains iron and impurities.
- the impurities are components that are mixed due to various factors of the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing steel, and the Ni steel according to the present embodiment Means what is acceptable as long as it does not adversely affect the
- P and S need to define upper limits as described above.
- the Ni steel according to the present embodiment is made to contain the following alloying elements as an impurity from the auxiliary material such as scrap or the like for the purpose of further improving the strength, cryogenic toughness and the like of the steel itself.
- auxiliary material such as scrap or the like for the purpose of further improving the strength, cryogenic toughness and the like of the steel itself.
- Sb is an element that reduces the cryogenic toughness of Ni steel. Therefore, the Sb content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.
- Sn is an element that reduces the cryogenic toughness of Ni steel. Therefore, the Sn content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.
- the As content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.
- Ni steel In order to fully exhibit the above-mentioned effect of the above-mentioned ingredient, in Ni steel concerning this embodiment, it is preferred to limit Co, Zn, and W content to 0.01% or less or 0.005% or less, respectively.
- the inventors of the present invention have newly found that fracture occurs at the former austenite grain boundaries at cryogenic temperatures, and the toughness tends to decrease.
- the Ni steel according to the present embodiment is manufactured by subjecting it to hot rolling, water cooling, and then heat treatment called intermediate heat treatment and tempering.
- the prior austenite grain boundaries are mainly austenite grain boundaries which existed after hot rolling and before the start of water cooling. After hot rolling, the prior austenite grain boundaries existing before the start of water cooling are often coarse. Mn, P, and Si are segregated in the coarse prior austenite grain boundaries, and these elements are considered to lower the bonding strength of the prior austenite grain boundaries and impair the toughness of the Ni steel at cryogenic temperatures.
- prior austenite grain boundaries are newly formed also at the time of intermediate heat treatment, since the temperature of the intermediate heat treatment of the Ni steel according to the present embodiment is as low as 610 to 650 ° C., new austenite generated at the time of intermediate heat treatment There are very few coarse ones. The amounts of Mn, P, and Si segregated to the new coarse austenite grain boundaries are smaller than the amount diffused during hot rolling. Therefore, among the prior austenite grain boundaries, fracture from relatively non-coarse prior austenite grain boundaries (many of which are prior austenite grain boundaries generated during intermediate heat treatment) is relatively unlikely to occur.
- the prior austenite grain boundaries are coarse is determined based on whether or not the grain size of the prior austenite grains is 2.0 ⁇ m or more. That is, it is judged that the prior austenite grain having a grain size of less than 2.0 ⁇ m is a prior austenite grain which does not impair the low temperature toughness of the Ni steel, and the prior austenite grain is excluded excluding the prior austenite grain having a grain size less than 2.0 ⁇ m.
- Measure the average particle size and average aspect ratio of By this method, the average grain size and average aspect ratio of prior austenite grains are obtained.
- average grain size of prior austenite grains means an average value of grain sizes of respective prior austenite grains having a grain size of 2.0 ⁇ m or more
- average aspect of prior austenite grains The “ratio” means the average value of the aspect ratio of each of the prior austenite grains having a grain size of 2.0 ⁇ m or more.
- the inventors conducted a number of studies on means for suppressing fracture at prior austenite grain boundaries at cryogenic temperatures. As a result, the present inventors set the content of (A) C to be 0.040 to 0.080% (provided that the Ni content is 11.5% or more, (B) to The same applies to (G), setting the content of (B) Si to 0.03 to 0.30%, setting the content of (C) Mn to 0.20 to 0.80%, (D ) P content of 0.0070% or less, (E) Mo content of 0.10% or more and 0.50% or less, (F) control of grain size and aspect ratio of prior austenite grains It has been found that it is necessary to simultaneously improve the toughness of the Ni steel at cryogenic temperatures by simultaneously satisfying the seven conditions of (A) and (G) controlling the volume fraction of the austenite phase.
- the cryogenic toughness is sufficient even if the Ni content is less than 11.5%. It has also been found that it is possible to reduce the Ni content and to reduce the raw material cost by making it possible to apply Ni to Ni steel.
- the average grain size of the prior austenite grains to be measured in a plane parallel to the rolling direction and the thickness direction at the center of the thickness needs to be 3.0 to 12.0 ⁇ m.
- the average grain size of prior austenite grains refers to the one measured in the plane parallel to the rolling direction and thickness direction of the thickness central portion.
- Precipitation of coarse cementite at prior austenite grain boundaries and concentration of Mn and P may weaken the bonding strength of prior austenite grain boundaries and cause fracture at prior austenite grain boundaries.
- the place where coarse cementite precipitates in the prior austenite grain boundary, and the place where Mn and P are concentrated may be a starting point of occurrence of brittle fracture.
- an increase in the average grain size of the prior austenite grains reduces the cryogenic toughness of the Ni steel. Therefore, the upper limit of the average grain size of the prior austenite grains is set to 12.0 ⁇ m.
- the upper limit of the average grain size of prior austenite grains may be 10.0 ⁇ m, 9.0 ⁇ m, 8.0 ⁇ m, 7.0 ⁇ m, or 6.0 ⁇ m.
- the lower limit is 3.0 ⁇ m.
- the lower limit of the average grain size of prior austenite grains may be 3.5 ⁇ m, 4.0 ⁇ m, or 5.0 ⁇ m.
- the average grain size of the prior austenite grains When the Ni content is low, the average grain size of the prior austenite grains: 6.0 ⁇ m or less, the average grain size of the prior austenite grains needs to be 6.0 ⁇ m or less. If necessary, the upper limit may be 5.5 ⁇ m or 4.0 ⁇ m. When the Ni content is small, the lower limit and the preferred lower limit of the average grain size of the prior austenite grains may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- the average aspect ratio of prior austenite grains refers to those measured in a plane parallel to the rolling direction and the thickness direction at the central portion of the thickness.
- the average aspect ratio of prior austenite grains refers to the ratio of the length and thickness of prior austenite grains in a plane (L-plane) parallel to the rolling direction, ie, the length in the rolling direction of prior austenite grains / plate thickness of prior austenite grains Thickness in the direction.
- the prior austenite grain size may exceed 12.0 ⁇ m and the cryogenic toughness of the Ni steel may decrease due to excessive rolling in the unrecrystallized region.
- cementite tends to be coarsened at the former austenite grain boundary along the rolling direction.
- coarse cementite precipitates at the prior austenite grain boundaries, the stress acting on the prior austenite grain boundaries becomes high, and fracture at the prior austenite grain boundaries tends to occur. Therefore, the upper limit of the average aspect ratio of the prior austenite grains is set to 10.0.
- the upper limit of the average aspect ratio of the prior-austenite grains may be 9.0 or 8.0.
- the lower limit may be 2.8, 3.2, 3.6, 4.1, 4.6 or 5.1.
- the upper and lower limit values and the preferred upper and lower limit values of the average aspect ratio of the prior austenite grains are the values described above regardless of the plate thickness and the Ni content.
- the measurement of the average grain size and the average aspect ratio of prior austenite is carried out using a plane (L plane) parallel to the rolling direction in the central portion of the plate thickness and the plate thickness direction as an observation plane.
- the former austenite grain boundaries are revealed by corroding the observation surface with a picric acid saturated aqueous solution.
- a magnified photograph of the thickness center portion of the corrosion-treated L surface is photographed with a scanning electron microscope (SEM) at a magnification of 1000 times or 2000 times for five or more fields of view.
- the equivalent circle particle size (diameter) of the prior austenite grain of at least 20 equivalent circle diameters (diameter) of 2.0 ⁇ m or more included in these SEM photographs is determined by image processing, and the plate is calculated by calculating the average value thereof.
- the average grain size of prior austenite grains measured in a plane parallel to the rolling direction and thickness direction of the thickness center portion is obtained. If prior austenite grains with a grain size of less than 2.0 ⁇ m are included, the above measurements are performed excluding this.
- the ratio (aspect ratio) of the length in the rolling direction to the thickness in the thickness direction of the prior austenite grains of at least 20 equivalent circle diameters (diameter) of 2.0 ⁇ m or more included in the above-mentioned SEM photograph is measured.
- the average value of the aspect ratio obtained by measurement the average aspect ratio of the prior austenite measured in the plane parallel to the rolling direction and thickness direction of the thickness central portion is obtained.
- the Ni steel according to the present embodiment contains 2.0% by volume or more of an austenite phase in the metallographic structure at the center of the thickness of the Ni steel at room temperature in order to enhance the toughness at a cryogenic temperature.
- the volume fraction of the austenite phase indicates one measured at the center of the plate thickness, unless otherwise specified. Note that this austenite phase is different from prior austenite and is an austenite phase present in a tempered Ni steel. The volume fraction of the austenite phase is measured by X-ray diffraction.
- austenite phase When 2.0 to 20.0% by volume of austenite phase is contained at the center of thickness of Ni steel at room temperature, the Ni steel is necessary for securing toughness at very low temperature even if it is cooled to very low temperature It is believed that a significant amount of stable austenite phase is present. In the presence of an austenite phase that is stable even at cryogenic temperatures, the applied stress and strain are alleviated by the plastic deformation of austenite, and it is believed that the toughness of the Ni steel is improved. In addition, the austenite phase is relatively uniformly and finely generated in the former austenite grain boundary, the block boundary of tempered martensite, the lath boundary, and the like.
- the lower limit of the volume fraction of the austenite phase in the metal structure at the central portion of the plate thickness is 2.0% by volume.
- the lower limit of the volume fraction of the austenite phase in the metal structure at the thickness center may be 3.0% by volume or 4.0% by volume.
- the upper limit of the volume fraction of the austenite phase in the metal structure at the center of the plate thickness is 20.0% by volume.
- the upper limit of the volume fraction of the austenite phase in the metal structure at the center of the thickness may be 15.0% by volume, 12.0% by volume, 10.0% by volume, or 6.0% by volume.
- the volume fraction of the austenite phase preferably 6.0 volume% or less
- the volume fraction of the austenite phase it is preferable to set the volume fraction of the austenite phase to 6.0 volume% or less.
- the upper limit of the volume fraction of the austenite phase may be 5.0% by volume, 4.5% by volume, or 4.0% by volume.
- the remainder of the metallographic structure of the Ni steel according to the present embodiment is mainly tempered martensite.
- tempered martensite In order to produce a Ni steel in which the average grain size and average aspect ratio of prior austenite grains are within the above-mentioned range, it is necessary to carry out water cooling, intermediate heat treatment and tempering after hot rolling.
- the balance (i.e., matrix phase) of the resulting metal structure inevitably becomes tempered martensite.
- Ni steel which concerns on this embodiment may contain the phase (for example, coarse inclusion etc.) in which the remainder of metal structure is not classified into neither austenite and tempered martensite.
- the total volume fraction of the austenitic phase and the tempered martensite phase in the metal structure at the thickness center portion is 99% or more, the inclusion of other phases is acceptable.
- the area fraction measured by structure observation which used nital as a corrosion liquid is a volume integral as it is Rate.
- the volume fraction of austenite phase in the center of thickness is measured by taking a sample having a plane parallel to the plate surface of Ni steel from the center of thickness of Ni steel and applying X-ray diffraction method to this sample Do.
- the volume fraction of the austenite phase is determined from the ratio of the integrated strength of austenite (face-centered cubic structure) to tempered martensite (body-centered cubic structure) at the X-ray peak.
- volume fraction of the austenite phase may be measured from the ratio of the integrated intensity of the (200) plane and the (220) plane.
- the process of cooling the test piece to a very low temperature is unnecessary.
- the volume fraction of the austenite phase may be measured by the test piece after the deep cooling treatment.
- the average effective crystal grain size (hereinafter abbreviated as “average effective crystal grain size”) measured in a plane parallel to the rolling direction and thickness direction of the thickness center portion is preferably 2.0 to 7.0 ⁇ m.
- the effective crystal grain size is defined as the equivalent circle diameter of a region (effective crystal grain) surrounded by the boundary of the metal structure having an orientation difference of 15 ° or more.
- the average effective crystal grain size refers to one measured in a plane parallel to the rolling direction and thickness direction of the thickness central portion.
- the effective grain size When the effective grain size is refined, the resistance to the propagation of fractures increases, and the toughness of the Ni steel is improved.
- the lower limit of the average effective crystal grain size is 2 It is preferable to set it as .0 micrometer.
- the lower limit of the average effective crystal grain size may be 3.0 ⁇ m, 4.0 ⁇ m, or 5.0 ⁇ m.
- the average effective crystal grain size exceeds 7.0 ⁇ m, the prior austenite grain boundary which is a hard phase which becomes a starting point of occurrence of brittle fracture, coarse cementite in tempered martensite, and coarse AlN, MnS, alumina, etc.
- the stress acting on the inclusions is increased, and the cryogenic toughness of the Ni steel is lowered. Therefore, it is preferable to set the upper limit of the average effective crystal grain size to 7.0 ⁇ m.
- the upper limit of the average effective crystal grain size may be 6.0 ⁇ m, 5.0 ⁇ m, or 4.0 ⁇ m.
- the average effective grain size preferably 4.0 ⁇ m or less
- the upper limit of the average effective crystal grain size is preferably 4.0 ⁇ m.
- the lower limit and the preferable lower limit of the average effective crystal grain size may be the same as those of the Ni steel having a Ni content of 11.5% or more.
- the average effective crystal grain size is determined by using a plane (L-plane) parallel to the rolling direction and thickness direction at the center of the thickness as an observation surface, and electron backscattered electron diffraction (Electron Back Scatter Diffraction: EBSD) Measure using an analyzer.
- the observation is performed at five or more fields of view at a magnification of 2000 times, and the boundary of the metal structure having a misorientation of 15 ° or more is regarded as a grain boundary.
- the crystal grains surrounded by the grain boundaries are regarded as effective crystal grains, and the equivalent circle diameter (diameter) of the effective crystal grains is determined by image processing. By calculating the average value of the equivalent circle grain size, the average effective crystal grain size measured on the plane parallel to the rolling direction and the plate thickness direction of the plate thickness central portion is obtained.
- the Ni steel according to the present embodiment is mainly a Ni steel plate, and the thickness thereof is set to 4.5 to 20 mm.
- the Ni steel having a thickness of less than 4.5 mm is hardly used as a material of a huge structure such as a liquid hydrogen tank, for example, so 4.5 mm is set as the lower limit of the thickness.
- the plate thickness is more than 20 mm, the cooling rate at the time of reheating and quenching becomes extremely slow, so securing of low temperature toughness becomes extremely difficult in the component range (particularly, the amount of Ni) of the Ni steel according to the present embodiment.
- the lower limit of the plate thickness may be 6 mm, 8 mm, 10 mm, or 12 mm
- the upper limit of the plate thickness may be 16 mm, 12 mm, or 11 mm.
- yield stress at room temperature: 590 to 710 MPa Yield stress at room temperature: 590 to 710 MPa
- tensile strength at room temperature: 690 to 810 MPa The yield stress at room temperature of the Ni steel according to the present embodiment is 590 to 710 MPa.
- the tensile strength at room temperature of the Ni steel according to the present embodiment is 690 to 810 MPa.
- the lower limit value of the yield stress at room temperature may be 600 MPa, 620 MPa, or 640 MPa.
- the upper limit value of the yield stress at room temperature may be 690 MPa, 670 MPa, or 650 MPa.
- the lower limit value of the tensile strength at room temperature may be 700 MPa, 720 MPa, or 740 MPa.
- the upper limit value of the tensile strength at room temperature may be 780 MPa, 760 MPa, or 750 MPa.
- room temperature is 20 ° C.
- the Ni steel according to the present embodiment can be stably obtained.
- the method of manufacturing Ni steel according to the present embodiment is The element content is adjusted in a state where the molten steel temperature is 1650 ° C. or less, the molten steel O concentration is 0.01% or less, and the molten steel S concentration is 0.02% or less, and then steel slabs are manufactured by continuous casting.
- the process to Heating the obtained billet to 950-1160 ° C. and holding it for 20-180 minutes; A step of hot rolling a steel piece under conditions of a cumulative rolling reduction of 90 to 95% at 950 ° C. or less at the time of hot rolling and an end temperature of 680 to 850 ° C.
- the heating temperature of the billet during hot rolling is 950 to 1160.degree. When the heating temperature is lower than 950 ° C., the temperature may be lower than a predetermined end temperature of hot rolling. If the heating temperature exceeds 1160 ° C., the austenite grain size may become coarse during heating of the billet, and the cryogenic toughness of the Ni steel may be lowered.
- the holding time of heating is 20 to 180 minutes. If the holding time of heating is less than 20 minutes, the austenite transformation in the steel may not progress sufficiently. On the other hand, when the holding time of heating exceeds 180 minutes, the austenite grain in steel may be coarsened.
- the heating temperature is set to 950 to 1100.degree.
- the heating retention time is set to 20 to 180 minutes.
- the upper limit of the cumulative rolling reduction at 950 ° C. or less during hot rolling is 95%.
- the cumulative rolling reduction at 950 ° C. or less during hot rolling is set to 90 to 95%.
- End temperature of hot rolling 680 to 850 ° C
- the finish temperature of the hot rolling falls below 680 ° C.
- the water cooling start temperature falls below 580 ° C.
- the yield stress and tensile strength at room temperature of the Ni steel may be lowered. Therefore, the lower limit of the end temperature of hot rolling is set to 680 ° C.
- the finish temperature of hot rolling exceeds 850 ° C.
- the dislocations introduced by rolling may be reduced due to recovery and the cryogenic toughness of the Ni steel may be insufficient.
- the finish temperature of hot rolling exceeds 850 ° C.
- the yield stress and tensile strength at room temperature of the Ni steel may be insufficient. Therefore, the upper limit of the termination temperature of hot rolling is set to 850 ° C.
- the end temperature of the hot rolling is set to 680 to 770 ° C.
- Water-cooling start temperature 580 to 850 ° C
- Cooling after hot rolling is performed by water cooling.
- the water cooling end temperature is 200 ° C. or less.
- the water cooling start temperature is 580 to 850 ° C.
- the lower limit of the water cooling start temperature is set to 580 ° C.
- the upper limit of the water cooling start temperature does not have to be particularly limited, and the water cooling is started immediately after the end of the hot rolling. Since the upper limit of the end temperature of hot rolling is 850 ° C., this is taken as the upper limit of the water cooling start temperature.
- the average cooling rate during water cooling is 10 ° C./sec or more.
- the upper limit of the termination temperature of hot rolling is 770 ° C., and this is taken as the upper limit of the water cooling start temperature. Therefore, when the Ni content is small, the water cooling start temperature is set to 580 to 770.degree. Further, even when the Ni content is small, water cooling is performed to 200 ° C. or less. The average cooling rate during water cooling is 10 ° C./sec or more.
- the intermediate heat treatment is a heat treatment in which the water-cooled hot-rolled steel sheet is heated to an intermediate heat treatment temperature, held at the intermediate heat treatment temperature, and then cooled.
- the intermediate heat treatment is effective for the refinement of the effective crystal grain size contributing to the improvement of the cryogenic toughness and for securing the austenite phase.
- the intermediate heat treatment temperature is set to 610 to 650.degree. If the intermediate heat treatment temperature is below 610 ° C., austenite transformation may be insufficient. Also, if the intermediate heat treatment temperature is below 610 ° C., the fraction of tempered martensite excessively tempered may increase, and the base material strength may decrease. Furthermore, if the intermediate heat treatment temperature is below 610 ° C., the cryogenic toughness of the Ni steel may be reduced. Therefore, the lower limit of the intermediate heat treatment temperature is set to 610 ° C.
- the austenite transformation progresses excessively in the hot rolled steel sheet. As a result, it may be difficult to maintain austenite in a stable state, and it may be difficult to secure an austenite phase of 2.0% or more by volume fraction.
- the intermediate heat treatment temperature exceeds 650 ° C.
- the cryogenic toughness of the Ni steel may decrease. Therefore, the upper limit of the intermediate heat treatment temperature is set to 650 ° C.
- the holding time of the intermediate heat treatment is set to 20 to 180 minutes. If the holding time of the intermediate heat treatment is less than 20 minutes, the austenite transformation may not progress sufficiently. On the other hand, if the holding time of the intermediate heat treatment exceeds 180 minutes, carbides may precipitate.
- the cooling method during the intermediate heat treatment is water cooling to prevent temper embrittlement and water cooling to 200 ° C. or less.
- the average cooling rate during water cooling is 8 ° C./second or more.
- the tempering is a heat treatment in which the hot rolled steel sheet after intermediate heat treatment is heated to a tempering temperature, held at the tempering temperature, and then cooled. Tempering is effective in securing the austenite phase.
- the tempering temperature is set to 530 to 570 ° C.
- the lower limit of the tempering temperature is set to 530 ° C.
- the tempering temperature exceeds 570 ° C.
- the austenitic phase at room temperature of the Ni steel exceeds 20.0% by volume.
- the upper limit of the tempering temperature is 570 ° C.
- the retention time of tempering is 20 to 180 minutes. If the tempering holding time is less than 20 minutes, the austenite stability may not be sufficiently secured. On the other hand, if the tempering holding time exceeds 180 minutes, carbides that adversely affect the toughness of the Ni steel may be precipitated, and the tensile strength of the Ni steel may be significantly reduced.
- the cooling method at the time of tempering is water cooling to avoid temper embrittlement, and is cooled to 200 ° C. or less.
- the average cooling rate during water cooling is 5 ° C./s or more.
- Example 1 Ni steel having a Ni content of 11.5% or more
- the steel was melted by a converter, and a slab with a thickness of 100 to 360 mm was manufactured by continuous casting.
- Tables 1 and 2 show chemical components of steel types A1 to A25. These slabs were heated, subjected to controlled rolling, water-cooled as they were, and subjected to intermediate heat treatment and heat treatment of tempering to produce steel plates.
- the holding time of heating at the time of hot rolling was 30 to 120 minutes.
- the holding time of the intermediate heat treatment and the heat treatment of tempering was set to 20 to 60 minutes. Water cooling after hot rolling was performed to 200 ° C. or less.
- the cooling means in the heat treatment of the intermediate heat treatment and the tempering was water cooling, and the water cooling was performed from the treatment temperature in each heat treatment to 200 ° C. or less. Samples were taken from the steel plate and the metallographic structure, tensile properties and toughness were evaluated.
- the average grain size of the prior austenite grains (hereinafter sometimes referred to as the average grain size of the prior austenite) measured in a plane parallel to the rolling direction and thickness direction of the thickness center is the rolling of the thickness center
- a plane (L plane) parallel to the direction and thickness direction was measured as the observation plane.
- the grain size of prior austenite was measured in accordance with JIS G 0551. First, the austenite grain boundaries were revealed by corroding the observation surface of the sample with a picric acid saturated aqueous solution. An enlarged photograph of the corrosion-treated L surface was photographed with a scanning electron microscope (SEM) at a magnification of 1000 or 2000 at five or more fields of view.
- SEM scanning electron microscope
- the equivalent-circle particle size (diameter) of the prior austenite grain of at least 20 equivalent circle diameters (diameter) of 2.0 ⁇ m or more included in these SEM photographs was determined by image processing.
- the average grain size of prior austenite was determined by calculating the average value of the circle equivalent diameters.
- the grain boundaries of the prior austenite grains are reduced and the P content is suppressed so that the grain boundaries of the old austenite are not easily broken. It may be difficult to identify austenite grain boundaries.
- the sample was heated to 450 to 490 ° C. and heat treated to be held for 1 hour or more, and then the average grain size of the prior austenite was measured by the method described above.
- the average aspect ratio of prior austenite grains (hereinafter sometimes referred to as the average aspect ratio of prior austenite grains) measured in a plane parallel to the rolling direction and thickness direction of the thickness center part is the above-mentioned SEM photograph Measure the ratio (aspect ratio) of the length in the rolling direction and the length in the thickness direction of the prior austenite grains of at least 20 equivalent circle diameters (diameter) of 2.0 ⁇ m or more included, and average their values as old austenite It was the average aspect ratio of grains.
- the volume fraction of austenite phase contained in the metal structure at the center of thickness (hereinafter sometimes referred to as the volume fraction of austenite phase) has a plane parallel to the plate surface from the thickness center of Ni steel A sample was taken and measured by applying X-ray diffraction to this sample.
- the volume fraction of the austenite phase was determined from the ratio of the integrated strength of austenite (face-centered cubic structure) to tempered martensite (body-centered cubic structure) at the X-ray peak.
- the average effective grain size (hereinafter, sometimes referred to as average effective grain size) measured in a plane parallel to the rolling direction and thickness direction of the thickness center portion is the rolling direction of the thickness central portion of Ni steel
- the surface (L surface) parallel to plate thickness direction was made into the observation surface, and it measured using the EBSD analyzer attached to the scanning electron microscope.
- the observation of five or more fields of view was performed at a magnification of 2000 ⁇ , and the boundary of the metal structure having a misorientation of 15 ° or more was regarded as a grain boundary.
- the crystal grains surrounded by the grain boundaries were regarded as effective crystal grains, and the circle equivalent grain size (diameter) was determined from the area of the crystal grains by image processing.
- the average effective crystal grain size was determined by calculating the average value of the circle equivalent grain sizes.
- yield stress and tensile strength For strength (yield stress and tensile strength) at room temperature, a tensile test specimen with a thickness of 1A specified in JIS Z 2241 whose longitudinal direction is the direction parallel to the rolling direction (L direction) is collected, and JIS Z 2241 It evaluated at room temperature by a prescribed method.
- the target value of yield stress is 590 to 710 MPa
- the target value of tensile strength is 690 to 810 MPa.
- the yield stress is the lower yield stress, but in many cases a clear lower yield stress can not be seen, in which case the 0.2% proof stress is taken as the yield stress.
- the cryogenic toughness is obtained by collecting in the direction (C direction) perpendicular to the rolling direction the CT test piece of the full thickness obtained by grinding the front and back surfaces of the sample by 0.5 mm each, and in liquid hydrogen (-253 ° C.) A J-R curve was created in accordance with the unloading compliance method prescribed in Standard E1820-13, and the J value was converted to a K IC value.
- the target value of cryogenic toughness is 150 MPa ⁇ ⁇ m or more.
- Tables 3 and 4 show the plate thickness, manufacturing method, base material characteristics, and metal structure of steel plates (steel materials No. a1 to a36) manufactured using slabs having chemical compositions of steel types A1 to A25.
- manufacture No. which is a comparative example. Since a15 has low C content and low volume fraction of austenite phase, yield stress and tensile strength at room temperature, and cryogenic toughness decreased. Production No. Since a18 had few Mn content and the volume fraction of the austenitic phase was low, cryogenic toughness fell.
- Production No. a27 to a36 are examples in which manufacturing conditions deviating from the preferable range are adopted.
- Production No. In a27 since the heating temperature was high, the average grain size of the prior austenite grains increased, and the cryogenic toughness decreased.
- Example 2 Ni steel having a Ni content of less than 11.5%
- the steel was melted by a converter, and a slab of 100 to 300 mm in thickness was manufactured by continuous casting.
- Tables 5 and 6 show chemical compositions of steel types B1 to B25. These slabs were heated, subjected to controlled rolling, water-cooled as they were, and subjected to intermediate heat treatment and heat treatment of tempering to produce steel plates.
- the holding time of billet heating during hot rolling was 30 to 120 minutes.
- the holding time of the intermediate heat treatment and the heat treatment of tempering was set to 20 to 60 minutes. Water cooling after hot rolling was performed to 200 ° C. or less.
- the cooling means in the heat treatment of the intermediate heat treatment and the tempering was water cooling, and the water cooling was performed from the treatment temperature in each heat treatment to 200 ° C. or less. Samples were taken from the steel plate and the metallographic structure, tensile properties and toughness were evaluated.
- Tables 7 and 8 show plate thicknesses, manufacturing methods, base material characteristics, and metal structures of steel products (Production Nos. B1 to b36) produced using slabs having chemical components of steel types B1 to B25.
- the production No. The b1 to b14 had room temperature yield stress, tensile strength, and cryogenic toughness satisfying the target values.
- manufacture No. which is a comparative example. Since the b15 content was low in C content and the volume fraction of austenite phase was low, the yield stress and tensile strength at room temperature, and the cryogenic toughness decreased. Production No. Since b18 had a small Mn content and a small volume fraction of the austenite phase, the cryogenic toughness decreased.
- Production No. b27 to b36 are examples in which manufacturing conditions deviating from the preferable range are adopted.
- Production No. In b27 since the heating temperature was high, the average grain size and the average effective grain size of the prior austenite grains increased, and the cryogenic toughness decreased.
Abstract
Description
(2)上記(1)に記載の低温用ニッケル含有鋼では、Ni:11.5%以上であり、Mn:0.50%以下であってもよい。
(3)上記(1)又は(2)に記載の低温用ニッケル含有鋼では、Ni:11.5%以上であり、前記旧オーステナイト粒の前記平均粒径が9.0μm以下であってもよい。
(4)上記(1)~(3)の何れか一態様に記載の低温用ニッケル含有鋼では、前記板厚中心部の前記圧延方向及び前記板厚方向に平行な前記面において測定される平均有効結晶粒径が2.0~7.0μmであってもよい。
(5)上記(1)~(3)の何れか一態様に記載の低温用ニッケル含有鋼では、前記板厚中心部の前記圧延方向及び前記板厚方向に平行な前記面において測定される平均有効結晶粒径が2.0~4.0μmであってもよい。
なお、本実施形態に係るNi鋼のNi含有量は、板厚に応じて変更する必要がある。板厚が大きい場合(即ち板厚が16mm超の場合)、圧延直後の水冷時の冷却速度などが遅くなるので、熱処理を通じた低温靱性の確保が困難となる。そのため、板厚が16mm超の場合、低温靱性を確保するための元素であるNi含有量を11.5%以上にしなければならない。
上述の事情により、Ni含有量、及び板厚に応じた一層の限定が必要とされる要件に関しては、その旨を適宜説明する。
Cは、Ni鋼の室温での降伏応力を上昇させる元素であり、マルテンサイトやオーステナイトの生成にも寄与する。C含有量が0.040%未満ではNi鋼の強度が確保できず、粗大なベイナイト及び介在物等の生成によってNi鋼の極低温靭性が低下することがある。そのため、C含有量の下限を0.040%とする。好ましいC含有量の下限は0.045%である。一方、C含有量が0.080%を超えると、旧オーステナイト粒界にセメンタイトが析出しやすくなり、このセメンタイトが粒界で破壊を引き起こし、Ni鋼の極低温靭性を低下させる。そのため、C含有量の上限を0.080%とする。好ましいC含有量の上限は0.070%であり、より好ましくは0.060%であり、更に好ましくは0.055%である。
Ni含有量が少ない場合、C含有量を0.070%以下とする必要がある。Ni含有量が少ない場合、好ましいC含有量の上限は0.065%、0.060%、又は0.055%である。Ni含有量が少ない場合、C含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Siは、Ni鋼の室温での降伏応力を上昇させる元素である。Si含有量が0.03%未満では室温での降伏応力の向上効果が小さい。そのため、Si含有量の下限は0.03%とする。好ましいSi含有量の下限は0.05%である。一方、Si含有量が0.30%を超えると、旧オーステナイト粒界のセメンタイトが粗大化しやすくなり、このセメンタイトが粒界で破壊を引き起こし、Ni鋼の極低温靭性が低下する。そのため、Si含有量の上限を0.30%に制限することは、Ni鋼の極低温での靱性を確保するために、極めて重要である。好ましいSi含有量の上限は0.20%であり、より好ましくは0.15%であり、更に好ましくは0.10%である。
Ni含有量が少ない場合、Si含有量を0.19%以下とする必要がある。Ni含有量が少ない場合、好ましいSi含有量の上限は0.16%、0.13%、又は0.10%である。Ni含有量が少ない場合、Si含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Mnは、Ni鋼の室温での降伏応力を上昇させる元素である。Mn含有量が0.20%未満ではNi鋼の強度が確保できず、粗大なベイナイト及び介在物等の生成によって、Ni鋼の極低温靭性が低下することがある。そのため、Mn含有量の下限を0.20%とする。好ましいMn含有量の下限は0.25%、又は0.30%である。一方、Mn含有量が0.80%を超えると、旧オーステナイト粒界に偏析したMn及び粗大に析出したMnS等により、粒界での破壊が引き起こされ、Ni鋼の極低温靭性が低下する。そのため、Mn含有量の上限を0.80%に制限することも、Ni鋼の極低温での靱性を確保するために、極めて重要である。好ましいMn含有量の上限は0.70%、又は0.60%、より好ましくは0.55%、又は0.50%である。
Ni含有量が少ない場合、Mn含有量を0.40%以下とする必要がある。Ni含有量が少ない場合、好ましいMn含有量の上限は0.35%、又は0.30%である。Ni含有量が少ない場合、Mn含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Niは、Ni鋼の極低温靭性を確保するために必須の元素である。Ni含有量が10.5%未満であると、Ni鋼の極低温での靭性が不足する。そのため、Ni含有量の下限を10.5%とする。好ましいNi含有量の下限は10.8%、11.0%、又は11.5%である。しかし、Niは高価な元素であり、12.4%を超えて含有させると経済性を損なう。そのため、Ni含有量の上限を12.4%に制限する。Ni含有量の上限を12.2%、12.0%、又は11.8%としてもよい。板厚が16mm以下である場合、Ni含有量の上限を11.3%、11.1%、又は10.9%としてもよい。
板厚が16mm超である場合、Ni含有量を11.5%以上とする必要がある。板厚が16mm超である場合、好ましいNi含有量の下限は11.8%、又は12.0%である。板厚が16mm超である場合、Ni含有量の上限、及び好ましい上限は、板厚が16mm以下のNi鋼のものと同じとしてもよい。
Moは、Ni鋼の室温での降伏応力を上昇させる元素であり、Ni鋼の粒界脆化を抑制する効果も有する。そのため、Mo含有量の下限を0.10%とする。好ましくはMo含有量の下限を0.20%とする。一方、Moは高価な元素であり、Mo含有量が0.50%を超えると経済性を損なう。そのため、Mo含有量の上限を0.50%に制限する。好ましくはMo含有量の上限を0.40%、0.35%、又は0.30%とする。
Alは、主に脱酸に使用する元素である。また、Alは、AlNを形成し、金属組織の微細化や、Ni鋼の極低温靱性を低下させる固溶Nの低減にも寄与する元素である。Al含有量が0.010%未満では脱酸の効果、金属組織の微細化効果及び固溶N低減効果が小さい。そのため、Al含有量の下限を0.010%とする。Al含有量の下限は0.015%が好ましく、0.020%がより好ましい。一方、Al含有量が0.060%を超えると、Ni鋼の極低温における靭性が低下する。そのため、Al含有量の上限を0.060%とする。より好ましいAl含有量の上限は0.040%、又は0.035%である。
Ni含有量が少ない場合、Al含有量を0.050%以下とする必要がある。Ni含有量が少ない場合、好ましいAl含有量の上限は0.040%、0.030%、又は0.020%である。Ni含有量が少ない場合、Al含有量の下限、及び好ましい下限値は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Nは、結晶粒を微細化する窒化物の形成に寄与する元素である。N含有量を0.0015%未満に低減すると、熱処理時にオーステナイト粒径の粗大化を抑制する微細なAlNが不足し、オーステナイト粒が粗大化してNi鋼の極低温靭性が低下する場合がある。そのため、N含有量の下限を0.0015%とする。好ましいN含有量は0.0020%である。一方、N含有量が0.0060%を超えると、固溶Nの増加、及びAlNの粗大化が生じるため、Ni鋼の極低温での靭性が低下する。そのため、N含有量の上限を0.0060%とする。N含有量の好ましい上限は0.0050%、0.0040%、又は0.0035%である。
Ni含有量が少ない場合、N含有量を0.0050%以下とする必要がある。Ni含有量が少ない場合、好ましいN含有量の上限は0.0040%、又は0.0030%である。Ni含有量が少ない場合、N含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Oは、不純物であり、O含有量が0.0030%を超えるとAl2O3のクラスターが増加し、Ni鋼の極低温での靭性が低下する場合がある。そのため、O含有量の上限を0.0030%とする。好ましいO含有量の上限は0.0025%であり、より好ましくは0.0020%であり、更に好ましくは0.0015%である。O含有量は可能な限り低減することが望ましいが、0.0007%未満へのO含有量の低減はコスト上昇を伴う場合がある。そのため、O含有量の下限を0.0007%とする。好ましいO含有量の下限は0.0008%であり、更に好ましくは0.0010%である。
Pは、旧オーステナイト粒界での粒界脆化を引き起こす元素であり、Ni鋼の極低温靭性に有害な元素である。そのため、P含有量は可能な限り低減することが望ましい。P含有量が0.0070%を超えると、Ni鋼の極低温での靭性が低下する場合がある。したがって、P含有量の上限を0.0070%に制限する。P含有量の上限は、好ましくは、0.0050%、より好ましくは0.0040%、更に好ましくは0.0030%である。Pは溶鋼製造時に不純物として溶鋼に混入する場合があるが、その下限を特に制限する必要はなく、その下限は0%である。ただし、P含有量を0.0003%未満に低減すると、溶製コストが上昇する場合がある。そのため、P含有量の下限を0.0003%、0.0005%、又は0.0010%としてもよい。
Ni含有量が少ない場合、P含有量を0.0050%以下とする必要がある。Ni含有量が少ない場合、好ましいP含有量の上限は0.0040%、又は0.0030%である。Ni含有量が少ない場合、P含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Sは、MnSを形成し、このMnSが脆性破壊の発生起点となる場合があるので、極低温靭性に有害な元素である。S含有量が0.0040%を超えると、Ni鋼の極低温での靭性が低下する場合がある。そのため、S含有量の上限を0.0040%に制限する。S含有量の上限は、好ましくは0.0030%、より好ましくは0.0020%、更に好ましくは0.0010%である。Sは溶鋼製造時に不純物として溶鋼に混入する場合があるが、その下限を特に制限する必要はなく、その下限は0%である。ただし、S含有量を0.0002%未満に低減すると、溶製コストが上昇する場合がある。そのため、S含有量の下限を0.0002%、0.0004%、又は0.0006%としてもよい。
Ni含有量が少ない場合、S含有量を0.0030%以下とする必要がある。Ni含有量が少ない場合、好ましいS含有量の上限は0.0010%、0.0015%、又は0.0010%である。Ni含有量が少ないNi鋼のS含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Cuは、Ni鋼の室温での降伏応力を上昇させる元素であるので、本実施形態に係るNi鋼はCuを含有してもよい。ただし、Cu含有量が0.50%を超えると、Ni鋼の極低温における靭性が低下する。そのため、Cu含有量の上限を0.50%とする。Cu含有量の上限は、好ましくは0.40%、より好ましくは0.30%、更に好ましくは0.20%である。
Crは、Ni鋼の室温での降伏応力を上昇させる元素であるので、本実施形態に係るNi鋼はCrを含有してもよい。ただし、Cr含有量が0.50%を超えると、Ni鋼の極低温における靭性が低下する。そのため、Cr含有量の上限を0.50%とする。Cr含有量の上限は、好ましくは0.30%、より好ましくは0.20%、更に好ましくは0.10%である。
Ni含有量が少ない場合、Cr含有量を0.35%以下とする必要がある。Ni含有量が少ない場合、好ましいCr含有量の上限は0.30%、0.25%、又は0.20%である。Ni含有量が少ない場合、Cr含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Nbは、Ni鋼の室温での降伏応力を上昇させる元素であり、金属組織の微細化による極低温靭性の向上効果も有するので、本実施形態に係るNi鋼はNbを含有してもよい。ただし、Nb含有量が0.020%を超えると、Ni鋼の極低温における靭性が低下する。そのため、Nb含有量の上限を0.020%とする。Nb含有量の上限は、好ましくは0.015%、より好ましくは0.010%である。
Ni含有量が少ない場合、Nb含有量を0.015%以下とする必要がある。Ni含有量が少ない場合、好ましいNb含有量の上限は0.012%、又は0.010%である。Ni含有量が少ない場合、Nb含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Vは、Ni鋼の室温での降伏応力を上昇させる元素であるので、本実施形態に係るNi鋼はVを含有してもよい。しかし、V含有量が0.080%を超えると、Ni鋼の極低温における靭性が低下する。そのため、V含有量の上限を0.080%とする。V含有量の上限は、好ましくは0.060%、より好ましくは0.040%である。
Ni含有量が少ない場合、V含有量を0.060%以下とする必要がある。Ni含有量が少ない場合、好ましいV含有量の上限は0.050%、又は0.040%である。Ni含有量が少ないNi鋼のV含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Tiは、TiNを形成し、金属組織の微細化や、Ni鋼の極低温靱性を低下させる固溶Nの低減にも寄与するので、本実施形態に係るNi鋼はTiを含有してもよい。しかし、Ti含有量が0.020%を超えると、Ni鋼の極低温における靭性が低下する。そのため、Ti含有量の上限を0.020%とする。好ましいTi含有量の上限は0.015%であり、より好ましい上限は0.010%である。
Ni含有量が少ない場合、Ti含有量を0.015%以下とする必要がある。Ni含有量が少ない場合、好ましいTi含有量の上限は0.012%、又は0.010%である。Ni含有量が少ない場合、Ti含有量の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
Bは、Ni鋼の室温での降伏応力を上昇させる元素であり、また、BNを形成し、Ni鋼の極低温靱性を低下させる固溶Nの低減にも寄与するので、本実施形態に係るNi鋼はBを含有してもよい。しかし、B含有量が0.0020%を超えると、Ni鋼の極低温での靭性が低下する。そのため、B含有量の上限を0.0020%とする。B含有量の上限は、好ましくは0.0015%であり、より好ましくは0.0012%、更に好ましくは0.0010%である。
Caは、熱間圧延により延伸して極低温靭性への有害性が高まりやすい介在物であるMnSを、CaSとして球状化させる元素であり、Ni鋼の極低温靭性の向上に有効な元素である。そのため、本実施形態に係るNi鋼はCaを含有してもよい。しかし、Ca含有量が0.0040%を超えると、Caを含有する酸硫化物が粗大化して、この酸硫化物が、Ni鋼の極低温における靭性を低下させる。そのため、Ca含有量の上限を0.0040%に制限する。Ca含有量の上限は、好ましくは0.0030%とする。
REM(希土類金属元素)は、Sc、Yおよびランタノイドからなる合計17元素を意味する。REMの含有量とは、これらの17元素の合計含有量を意味する。REMは、Caと同様に、熱間圧延によって延伸して極低温靭性への有害性が高まりやすい介在物であるMnSを、REMの酸硫化物として球状化させるので、Ni鋼の極低温靭性の向上に有効である。そのため、本実施形態に係るNi鋼はREMを含有してもよい。しかし、REM含有量が0.0050%を超えると、REMを含有する酸硫化物が粗大化して、この酸硫化物が、Ni鋼の極低温における靭性を低下させる。そのため、REM含有量の上限を0.0050%に制限する。REM含有量の上限は、好ましくは0.0040%とする。
板厚中心部の圧延方向及び板厚方向に平行な面において測定される旧オーステナイト粒の平均粒径は3.0~12.0μmとする必要がある。本実施形態において、特に断りが無い限り、旧オーステナイト粒の平均粒径とは板厚中心部の圧延方向及び板厚方向に平行な面において測定されるものを示す。旧オーステナイト粒の平均粒径が12.0μmを超えると、旧オーステナイト粒界に粗大なセメンタイトが析出する。また、旧オーステナイト粒の平均粒径が12.0μmを超えると、粒界におけるMn及びPの濃度が上昇する。
Ni含有量が少ない場合、旧オーステナイト粒の平均粒径を6.0μm以下とする必要がある。必要に応じて、その上限を5.5μm、又は4.0μmとしてもよい。Ni含有量が少ない場合、旧オーステナイト粒の平均粒径の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
本実施形態では、上述の化学成分を有する鋼に後述する製造方法を適用すると、板厚中心部の圧延方向及び板厚方向に平行な面において測定されるオーステナイト粒の平均アスペクト比は2.6~10.0となる。
本実施形態に係るNi鋼は、極低温における靭性を高めるために、室温のNi鋼の板厚中心部の金属組織において、オーステナイト相を2.0体積%以上含有する。本実施形態において、特に断りが無い限り、オーステナイト相の体積分率とは板厚中心部において測定されるものを示す。なお、このオーステナイト相は旧オーステナイトとは異なり、焼戻し後のNi鋼に存在するオーステナイト相である。オーステナイト相の体積分率は、X線回折法で測定する。
Ni含有量が少ない場合、オーステナイト相の体積分率を6.0体積%以下とすることが好ましい。必要に応じて、オーステナイト相の体積分率の上限を5.0体積%、4.5体積%、又は4.0体積%としてもよい。
なお、本実施形態においては、オーステナイト相の体積分率の測定の前に、試験片を極低温に冷却する処理(いわゆる深冷処理)は、不要である。しかしながら、深冷処理後の試験片しかないなどの場合、深冷処理後の試験片でオーステナイト相の体積分率を測定してもよい。
板厚中心部の圧延方向及び板厚方向に平行な面において測定される平均有効結晶粒径(以下「平均有効結晶粒径」と略す)は2.0~7.0μmとすることが好ましい。本実施形態において、有効結晶粒径は、15°以上の方位差を有する金属組織の境界で囲まれた領域(有効結晶粒)の円相当径であると定義する。本実施形態において、特に断りが無い限り、平均有効結晶粒径とは板厚中心部の圧延方向及び板厚方向に平行な面において測定されるものを示す。
Ni含有量が少ない場合、平均有効結晶粒径の上限を4.0μmとすることが好ましい。Ni含有量が少ない場合、平均有効結晶粒径の下限、及び好ましい下限は、Ni含有量が11.5%以上のNi鋼のものと同じとすればよい。
本実施形態に係るNi鋼は主にNi鋼板であり、その板厚は、4.5~20mmとする。板厚が4.5mm未満であるNi鋼は、例えば液体水素タンクのような巨大構造物の材料として用いることは殆どないため、4.5mmを板厚の下限とした。板厚が20mm超である場合、再加熱焼入れ時の冷却速度が極めて遅くなるので、本実施形態に係るNi鋼の成分範囲(特に、Ni量)では低温靱性の確保が非常に難しくなる。必要に応じて、板厚の下限を6mm、8mm、10mm、又は12mmとしてもよく、板厚の上限を16mm、12mm、又は11mmとしてもよい。
(室温での引張強さ:690~810MPa)
本実施形態に係るNi鋼の室温での降伏応力は590~710MPaである。また、本実施形態に係るNi鋼の室温での引張強さは690~810MPaとする。室温での降伏応力の下限値を600MPa、620MPa、又は640MPaとしてもよい。室温での降伏応力の上限値を690MPa、670MPa、又は650MPaとしてもよい。室温での引張強さの下限値を700MPa、720MPa、又は740MPaとしてもよい。室温での引張強さの上限値を780MPa、760MPa、又は750MPaとしてもよい。なお、本実施形態において室温とは、20℃である。
溶鋼温度を1650℃以下として、溶鋼O濃度を0.01%以下、溶鋼S濃度を0.02%以下とした状態で、元素の含有量の調整を行った後、連続鋳造により鋼片を製造する工程と、
得られた鋼片を950~1160℃に加熱し、20~180分保持する工程と、
鋼片を、熱間圧延時の950℃以下での累積圧下率が90~95%であり、終了温度が680~850℃である条件で熱間圧延して、熱延鋼板を得る工程と、
熱延鋼板を、冷却開始温度を580~850℃として室温まで水冷する工程と、
熱延鋼板を、中間熱処理温度を610~650℃とし、保持時間を20~180分として中間熱処理する工程と、
熱延鋼板を、焼戻し温度を530~570℃とし、保持時間を20~180分として焼戻す工程とを備える。これら製造条件は、Ni含有量等に応じて、更に限定することが好ましい。
(加熱の保持時間:20~180分)
熱間圧延の際の再結晶による旧オーステナイト粒の均質な細粒化は、本実施形態に係るNi鋼の極低温靭性を確保する上で特に重要である。そのため、熱間圧延における温度及び圧下率を厳格に規制することが好ましい。熱間圧延時の鋼片の加熱温度は、950~1160℃である。加熱温度が950℃を下回ると、所定の熱間圧延の終了温度を下回る場合がある。加熱温度が1160℃を上回ると、鋼片の加熱時にオーステナイト粒径が粗大となり、Ni鋼の極低温靭性が低下することがある。加熱の保持時間は20~180分である。加熱の保持時間が20分未満であると、鋼中のオーステナイト変態が十分に進展しない場合がある。一方、加熱の保持時間が180分を超えると、鋼中のオーステナイト粒が粗大化する場合がある。
熱間圧延時の950℃以下での累積圧下率が90%を下回ると、圧延中の鋼片においてオーステナイトの再結晶によるオーステナイト粒の細粒化が不十分となり、圧延後のオーステナイト粒の一部が粗大化し、Ni鋼の極低温靭性が低下する場合がある。そのため、熱間圧延時の950℃以下での累積圧下率の下限は90%である。
熱間圧延の終了温度が680℃を下回ると、水冷開始温度が580℃を下回り、Ni鋼の極低温靭性が低下する場合がある。また、熱間圧延の終了温度が680℃を下回ると、水冷開始温度が580℃を下回り、Ni鋼の室温での降伏応力及び引張強さが低下する場合がある。そのため、熱間圧延の終了温度の下限を680℃とする。
熱間圧延後の冷却は、水冷によって行う。水冷終了温度は、200℃以下とする。
(中間熱処理の保持時間:20~180分)
中間熱処理は、水冷後の熱延鋼板を中間熱処理温度まで加熱し、中間熱処理温度で保持し、次いで冷却する熱処理である。中間熱処理は、極低温靭性の向上に寄与する有効結晶粒径の細粒化、及びオーステナイト相の確保に有効である。
(焼戻しの保持時間:20~180分)
焼戻しは、中間熱処理後の熱延鋼板を焼戻し温度まで加熱し、焼戻し温度で保持し、次いで冷却する熱処理である。焼戻しは、オーステナイト相の確保に有効である。焼戻し温度は530~570℃とする。
転炉により鋼を溶製し、連続鋳造により厚さが100~360mmのスラブを製造した。表1、表2に鋼種A1~A25の化学成分を示す。これらのスラブを加熱し、制御圧延を行い、そのまま水冷し、中間熱処理、焼戻しの熱処理を施して鋼板を製造した。熱間圧延時の加熱の保持時間は30~120分とした。中間熱処理、及び焼戻しの熱処理の保持時間は20~60分とした。熱間圧延後の水冷は、200℃以下まで行った。中間熱処理、及び焼戻しの熱処理における冷却手段は水冷とし、水冷は各熱処理における処理温度から200℃以下まで行った。鋼板からサンプルを採取し、金属組織、引張特性、靱性を評価した。
転炉により鋼を溶製し、連続鋳造により厚さが100~300mmのスラブを製造した。表5、表6に鋼種B1~B25の化学成分を示す。これらのスラブを加熱し、制御圧延を行い、そのまま水冷し、中間熱処理、焼戻しの熱処理を施して鋼板を製造した。熱間圧延時の鋼片加熱の保持時間は30~120分とした。中間熱処理、及び焼戻しの熱処理の保持時間は20~60分とした。熱間圧延後の水冷は、200℃以下まで行った。中間熱処理、及び焼戻しの熱処理における冷却手段は水冷とし、水冷は各熱処理における処理温度から200℃以下まで行った。鋼板からサンプルを採取し、金属組織、引張特性、靱性を評価した。
Claims (5)
- 化学組成が、質量%で、
C:0.040~0.080%、
Si:0.03~0.30%、
Mn:0.20~0.80%、
Ni:10.5~12.4%、
Mo:0.10~0.50%、
Al:0.010~0.060%、
N:0.0015~0.0060%、
O:0.0007~0.0030%、
Cu:0~0.50%、
Cr:0~0.50%、
Nb:0~0.020%、
V:0~0.080%、
Ti:0~0.020%、
B:0~0.0020%、
Ca:0~0.0040%、
REM:0~0.0050%、
P:0.0070%以下、
S:0.0040%以下、及び
残部:Fe及び不純物であり、
板厚中心部の金属組織が、2.0~20.0体積%のオーステナイト相を含有し、
前記板厚中心部の圧延方向及び板厚方向に平行な面において測定される旧オーステナイト粒の平均粒径が3.0~12.0μmであり、
前記板厚中心部の前記圧延方向及び前記板厚方向に平行な前記面において測定される前記旧オーステナイト粒の平均アスペクト比が2.6~10.0であり、
板厚が、4.5~20mmであり、
室温での降伏応力が、590~710MPaであり、
前記室温での引張強さが、690~810MPaであり、
前記板厚が16mm超である場合、Ni:11.5%以上であり、
前記板厚が16mm以下且つNi:11.5%未満である場合、C:0.070%以下、Si:0.19%以下、Mn:0.40%以下、Al:0.050%以下、N:0.0050%以下、Cr:0.35%以下、Nb:0.015%以下、V:0.060%以下、Ti:0.015%以下、P:0.0050%以下、及びS:0.0030%以下であり、且つ前記旧オーステナイト粒の前記平均粒径が6.0μm以下であることを特徴とする低温用ニッケル含有鋼。 - Ni:11.5%以上であり、
Mn:0.50%以下である
ことを特徴とする請求項1に記載の低温用ニッケル含有鋼。 - Ni:11.5%以上であり、
前記旧オーステナイト粒の前記平均粒径が9.0μm以下である
ことを特徴とする請求項1又は2に記載の低温用ニッケル含有鋼。 - 前記板厚中心部の前記圧延方向及び前記板厚方向に平行な前記面において測定される平均有効結晶粒径が2.0~7.0μmである
ことを特徴とする請求項1~3の何れか一項に記載の低温用ニッケル含有鋼。 - 前記板厚中心部の前記圧延方向及び前記板厚方向に平行な前記面において測定される平均有効結晶粒径が2.0~4.0μmである
ことを特徴とする請求項1~3の何れか一項に記載の低温用ニッケル含有鋼。
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Cited By (4)
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US11371127B2 (en) | 2017-10-26 | 2022-06-28 | Nippon Steel Corporation | Nickel-containing steel for low temperature |
US11371126B2 (en) | 2017-10-26 | 2022-06-28 | Nippon Steel Corporation | Nickel-containing steel for low temperature |
US11384416B2 (en) | 2017-10-26 | 2022-07-12 | Nippon Steel Corporation | Nickel-containing steel for low temperature |
US11578394B2 (en) | 2017-10-26 | 2023-02-14 | Nippon Steel Corporation | Nickel-containing steel for low temperature |
Also Published As
Publication number | Publication date |
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EP3702484A4 (en) | 2021-03-03 |
US11371121B2 (en) | 2022-06-28 |
CN111263828A (zh) | 2020-06-09 |
EP3702484B1 (en) | 2022-01-26 |
JP6852807B2 (ja) | 2021-03-31 |
EP3702484A1 (en) | 2020-09-02 |
JPWO2019082326A1 (ja) | 2020-11-05 |
CN111263828B (zh) | 2021-08-17 |
US20200291506A1 (en) | 2020-09-17 |
US11578391B2 (en) | 2023-02-14 |
KR102307707B1 (ko) | 2021-10-01 |
KR20200058488A (ko) | 2020-05-27 |
US20220282359A1 (en) | 2022-09-08 |
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