WO2014203347A1 - Steel material, process for producing same, and lng tank - Google Patents
Steel material, process for producing same, and lng tank Download PDFInfo
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- WO2014203347A1 WO2014203347A1 PCT/JP2013/066825 JP2013066825W WO2014203347A1 WO 2014203347 A1 WO2014203347 A1 WO 2014203347A1 JP 2013066825 W JP2013066825 W JP 2013066825W WO 2014203347 A1 WO2014203347 A1 WO 2014203347A1
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/001—Heat treatment of ferrous alloys containing Ni
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—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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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- 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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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- 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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- 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
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- 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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- 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
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- 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
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- 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
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17C—VESSELS FOR CONTAINING OR STORING COMPRESSED, LIQUEFIED OR SOLIDIFIED GASES; FIXED-CAPACITY GAS-HOLDERS; FILLING VESSELS WITH, OR DISCHARGING FROM VESSELS, COMPRESSED, LIQUEFIED, OR SOLIDIFIED GASES
- F17C1/00—Pressure vessels, e.g. gas cylinder, gas tank, replaceable cartridge
- F17C1/12—Pressure vessels, e.g. gas cylinder, gas tank, replaceable cartridge with provision for thermal insulation
Definitions
- the present invention relates to a cryogenic steel material excellent in toughness, a manufacturing method thereof, and an LNG tank to which the steel material is applied.
- LPG Liquid Petroleum Gas: liquefied petroleum gas
- LNG Liquid Natural Gas: liquefied natural gas
- the present invention is mainly intended for use in the vicinity of ⁇ 165 ° C., which is a temperature environment for storing LNG in a liquid state.
- Steel materials for producing cryogenic storage tanks for storing liquefied gas such as LPG or LNG are required to have excellent fracture toughness from the viewpoint of ensuring safety.
- the LNG temperature environment is around ⁇ 165 ° C.
- Brittle fracture propagation stop characteristics (hereinafter referred to as “arrest characteristics”) of the base metal and the welded joint are required.
- the arrest characteristic is required as an important characteristic.
- the base material characteristics reduction of impurities including P and S, reduction of C, three-stage heat treatment method (quenching (Q), two-phase region quenching (L), tempering (T)), etc. It has been controlled and improved in various ways.
- Patent Document 1 discloses a steel material in which the Ni content is reduced to less than 8% for a low Ni type cryogenic steel.
- Patent Document 1 discloses that increasing the amount of residual ⁇ improves the brittle crack propagation stopping characteristics of a steel sheet. Therefore, a steel sheet having a good V-notch Charpy absorbed energy v E -196 described in Patent Document 1 (in particular, Test Nos. 1-a to 1-h, 4 to 12, and 22 to described in Patent Document 1). 35), various fracture toughness tests were carried out. As a result, each sample had a DT (Dynamic Tear) energy at -196 ° C.
- DT Dynamic Tear
- Patent Document 2 discloses that a large amount of highly stable residual ⁇ (residual austenite) is obtained by rolling at a specified cumulative reduction ratio and then performing offline QT (Quenching and Tempering) or DQT (Direct-Quenching and Tempering) treatment. The technology to be secured is described. However, in order to stabilize the residual ⁇ , Ni or Mn in the residual ⁇ needs to be concentrated as compared with the structure around the residual ⁇ . Also, if the cooling after tempering is slow, the fracture resistance properties of the steel are impaired. Patent Document 2 does not describe any of these findings. Moreover, although the Example shown by patent document 2 contains 9% or more of Ni by mass%, the amount of residual (gamma) prescribed
- Patent Document 3 discloses a so-called CR-DQT (Controlled Rolling, Direct-Quenching, and Tempering) or CR-DQLT (Controlled) which is subjected to low-temperature heating after low-temperature heating, water cooling to a temperature of 200 ° C. or less, and heat treatment. Rolling, Direct-Quenching, Lamellarizing, and Tempering processes are described. Although these processes are the same as those of the present invention, the invention described in Patent Document 3 has a small cumulative rolling amount, and does not include an embodiment having a cumulative rolling ratio of 50% or more defined in the present invention example. When the cumulative rolling reduction is low, the above-described concentration of Ni or Mn is not sufficiently performed, so that it is considered that the residual ⁇ is not stable.
- Patent Document 3 does not include a description relating to the cooling rate after tempering and the concentration of Ni or Mn in the residual ⁇ , which is greatly related to the characteristics of the steel material. These facts suggest that the invention described in Patent Document 3 does not have high fracture resistance.
- Patent Document 4 focuses on the segregation ratio of Ni, but it is essential in Patent Document 4 to perform soaking diffusion treatment in order to reduce this segregation ratio. This is not preferable from the viewpoint of economy or lead time.
- Patent Document 4 relates to a so-called CR-DQT or CR-DQLT process as in Patent Document 3. Although this process itself is the same as that of the present invention, in Patent Document 4, there is no description about the cooling rate after tempering, and there is no provision regarding the concentration of Ni and Mn in the residual ⁇ .
- the technique described in 4 cannot be said to be a technique that can stably satisfy extremely high fracture resistance.
- the present invention has been made in view of such a situation, and its purpose is to achieve both economic efficiency and fracture resistance characteristics by achieving both the provision of extremely high fracture resistance characteristics and the suppression of the price of steel materials.
- An object of the present invention is to provide an excellent steel material, a manufacturing method thereof, and an LNG tank.
- the present inventors have adopted a steel having a Ni content effective in securing low temperature toughness in a range of 6.6% to 8.0% by mass, A wide range of prototype tests were conducted, and the correspondence with the characteristics was examined. As a result, the following findings (a) to (h) were obtained.
- A Fracture resistance required to maintain liquid and air tightness when the cryogenic storage tank material is subjected to an earthquake force is brittle fracture occurrence characteristics and brittleness in the unlikely event that fracture occurs. It is a brittle crack propagation stop characteristic (arrest characteristic) that can stop the propagation of cracks due to fracture.
- Residual ⁇ is a structure having an extremely high brittle crack propagation stopping function. When this structure is finely dispersed, the arrest properties of the material are remarkably improved.
- the amount of residual ⁇ can be evaluated by an X-ray diffraction method.
- the particle size of residual ⁇ is an average major axis of 0.85 ⁇ m or less, the steel material exhibits good arrest characteristics.
- the major axis of the residual ⁇ indicates the length of the residual ⁇ along the longest direction of the residual ⁇ when the cross section is observed.
- the residual ⁇ is preferably measured by collecting a sample at a position (1/4) of the thickness t of the steel material.
- the residual ⁇ is in a metastable state, and the steel material easily undergoes martensitic transformation due to plastic deformation.
- the geometric shape of the residual ⁇ is close to a sphere, and when the quantity is specifically defined, the upper limit of the average aspect ratio of the residual ⁇ is 2 .5 is desirable.
- the aspect ratio of the residual ⁇ is the ratio of the longest diameter (L) of the residual ⁇ , that is, the longest diameter of the residual ⁇ to the shortest diameter (W), that is, the shortest diameter of the residual ⁇ , that is, L / W. .
- the lower limit of the aspect ratio is 1.
- the steel material is heated to a temperature range of 620 ° C. to 720 ° C. and then water-cooled, if necessary. Processing may be performed.
- the lower limit of the cooling rate until the steel surface temperature is reduced to 300 ° C. or less after the tempering treatment is set to 0.5 ° C./s in order to obtain good characteristics. Is necessary.
- annular plate is a part of the above ground LNG tank that is subjected to a large plastic strain during a severe earthquake.
- Such a steel material excellent in low-temperature toughness has high fracture safety and can be applied to the annular plate in the LNG tank.
- the present invention has been completed on the basis of the above-mentioned knowledge.
- the gist of the LNG tank is steel.
- the chemical components are mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.4 to 2 0.0%, Ni: 6.6 to 8.0%, Al: 0.002 to 0.08%, N: 0.0050% or less, P: 0.05% or less, S: 0.008% or less, Cu : 0-1.0%, Cr: 0-1.0%, Mo: 0-0.5%, V: 0-0.10%, B: 0-0.0050%, Nb: 0-0.
- the lower limit of the amount of residual ⁇ at the (1/4) t position of the steel sheet thickness t is 4.0% by volume
- the upper limit of the average value is 2.5
- the upper limit of the average value of the major axis is 0.85 ⁇ m
- the average Mn concentration and the average Ni concentration in the residual ⁇ satisfy the following formulas (A) and (B), respectively.
- [Mn] retained ⁇ is an average Mn concentration in the residual ⁇
- [Mn] ⁇ is an average Mn concentration in the ferrite phase
- [Ni] retained ⁇ is an average Ni concentration in the residual ⁇
- Ni] ⁇ represents the average Ni concentration in the ferrite phase.
- the chemical component is C: 0.02% to 0.07% Si: 0.01% to 0.10%, Mn: 0.6% to 1.0% Ni: 7.0 to 7.8% Cu: 0 to 0.30%, Cr: 0 to 0.80%, Mo: 0 to 0.20%, V: 0 to 0.05%, B may be 0 to 0.0005%, Nb may be 0 to 0.02%, Ti may be 0 to 0.02%, and Sn may be 0 to 0.01%.
- the chemical component may be Cr: 0.30 to 0.60% and Mo: 0.05 to 0.15% by mass%. .
- the steel material according to any one of the above (1) to (3) is a steel plate having a thickness of 3 mm to 100 mm, a yield stress of 585 MPa or more, and a tensile strength of 690 MPa to 885 MPa. Good.
- C 0.01 to 0.12% C is an element necessary for ensuring the strength of the base material.
- the C content is less than 0.01%, the required strength cannot be secured, and the formation of lath martensite in FL (Fusion Line) becomes insufficient during welding, and the HAZ near the FL Since the toughness of (Heat Affected Zone) also decreases, the lower limit of the C content needs to be 0.01%.
- the content of C exceeds 0.12%, the toughness of HAZ, particularly HAZ near FL, is significantly deteriorated. Therefore, the C content is 0.01% to 0.12%.
- the lower limit of the C content may be 0.02%, 0.03%, or 0.04%.
- the upper limit of the C content may be 0.10%, 0.08%, 0.07%, or 0.06%.
- Si 0.01 to 0.30%
- Si is an element necessary as a deoxidizer.
- the lower limit of the Si content needs to be 0.01%.
- Si and the tempering process of martensite as quenched are greatly related.
- Si content exceeds 0.30%, Si is supersaturated in the welding cooling process. This suppresses the decomposition and precipitation reaction of C from the martensite in the form of solid solution to cementite. By suppressing the decomposition and precipitation reaction of C, self-tempering is delayed and the toughness of the weld is lowered.
- the Si content is set to 0.01% to 0.30%. From the viewpoint of improving the toughness of the welded portion, the Si content should be as low as possible.
- the upper limit of the Si content is 0.20%, 0.15%, or 0.10. % May be used.
- the lower limit of the Si content may be 0.02%, 0.03%, or 0.04%.
- Mn 0.4 to 2.0%
- Mn is an element necessary as a deoxidizer, for ensuring the strength and toughness of the base material, and for ensuring the hardenability of the HAZ. If the Mn content is less than 0.4%, these effects cannot be obtained, and furthermore, ferrite side plates are generated in the HAZ, so that the formation of lath martensite becomes insufficient and the toughness of the welded portion is reduced. The lower limit of the content of is 0.4%. On the other hand, if the Mn content exceeds 2.0%, the base material characteristics may be uneven in the thickness direction due to the central segregation of Mn. Therefore, the Mn content is set to 0.4% to 2.0%.
- the lower limit of the Mn content may be 0.50%, 0.60%, or 0.70%.
- the upper limit of the Mn content may be 1.5%, 1.2%, 1.0%, or 0.9%.
- P 0.05% or less P is present in the steel as an impurity, segregates at the grain boundary, and causes toughness to decrease. If the P content exceeds 0.05%, hot cracking may occur during welding, so the P content is limited to 0.05% or less. In order to improve toughness, the P content should be as small as possible.
- the upper limit of the P content is 0.03%, 0.02%, 0.01%, 0.008%, or 0. It may be 0.006%. There is no need to specifically define the lower limit of the P content, and the lower limit is 0%. However, reducing P more than necessary leads to a cost increase during refining, so the lower limit of the P content may be 0.0001% or 0.0005%.
- S 0.008% or less S is present in the steel as an impurity, and if it is too much, it promotes center segregation or causes a large amount of stretched MnS that causes brittle fracture. If the S content exceeds 0.008%, the mechanical properties of the base material and the HAZ deteriorate. For this reason, content of S shall be 0.008% or less.
- the upper limit of the S content may be 0.006%, 0.004%, 0.003%, or 0.002%. Since the S content should be as small as possible, it is not necessary to define the lower limit of the S content, and the lower limit is 0%. From the problem of refining costs, the lower limit of the S content may be 0.0001% or 0.0003%.
- Ni 6.6 to 8.0%
- Ni is the most basic element necessary for securing toughness as a low-temperature steel material.
- a Ni content of 6.6% or more is required.
- the higher the Ni content the higher the low-temperature toughness.
- the cost increases accordingly, so the upper limit of the Ni content is 8.0%. Therefore, the target of Ni content is 6.6% to 8.0%.
- the Ni content is preferably 6.7% or more, and the lower limit of the Ni content may be 6.8%, 6.9%, or 7.0% as necessary.
- the upper limit of Ni content may be 7.8%, 7.6%, or 7.4%. However, even if the Ni content is more than 8.0%, the characteristics required as a low-temperature steel material can be obtained.
- Al 0.002 to 0.080%
- Al is an element generally contained as a deoxidizer, but in the case of the steel material according to the present embodiment, it has a function of delaying self-tempering of martensite, similar to Si. . Therefore, it is desirable that the Al content be as small as possible.
- the Al content exceeds 0.080% and becomes excessive, the decomposition precipitation reaction of C into cementite from martensite, which is supersaturated with C in the welding cooling process, is suppressed as in the case of Si described above. In some cases, the toughness of the welded portion may be reduced.
- the Al content is less than 0.002%, a sufficient deoxidizing effect cannot be obtained. Therefore, the Al content is set to 0.002% to 0.080%.
- the lower limit of the Al content may be 0.005%, 0.010%, 0.015%, or 0.020%.
- the upper limit of the Al content may be 0.060%, 0.050%, or 0.040%.
- N 0.0050% or less N is present in steel as an impurity and causes deterioration of HAZ toughness through an increase in solid solution N or formation of precipitates. Therefore, N content is required to ensure HAZ toughness. The amount should be low. If the N content exceeds 0.0050%, the HAZ toughness may deteriorate significantly, so the N content is set to 0.0050% or less. In order to improve the HAZ toughness, the upper limit of the N content may be 0.0045% or 0.0040%. There is no need to define the lower limit of the N content, and the lower limit is 0%. However, the lower limit of the N content may be set to 0.0001% or 0.0010% from the problem of cost during refining.
- the steel material according to the present embodiment is composed of Fe and impurities in addition to the above components.
- the impurities are components mixed in due to various factors of the raw material, such as ore or scrap, or the manufacturing process when the steel material is industrially manufactured, and in a range that does not adversely affect the present invention. It means what is allowed.
- the steel material according to the present embodiment further includes one or more of Cu, Cr, Mo, V, B, Nb, Ti, Sn, Ca, Mg, and REM in addition to the above components. Also good. There is no particular need to define the lower limit of the content of these components, and the lower limit is 0%. Further, even if these alloy elements are intentionally added to the steel material according to the present embodiment or mixed as impurities, if the content is within the specified range, the steel material is claimed in the scope of the present invention. Interpreted as in.
- Cu 0 to 1.00% Cu can be contained as needed.
- the strength of the base material can be improved.
- the upper limit of the Cu content is 1.00%.
- the upper limit of the preferable Cu content is 0.80% or 0.60%, and the upper limit of the more preferable Cu content is 0.30%.
- the lower limit of the Cu content may be 0.10%.
- Cr 0 to 1.00% Cr can be contained as needed.
- the carbon dioxide gas corrosion resistance is improved, and the strength can be improved as a result of the improvement of the hardenability.
- the upper limit of the Cr content is 1.00%.
- the upper limit of the Cr content may be 0.80%, 0.60%, or 0.50%. There is no need to define the lower limit of the Cr content, and the lower limit is 0%.
- the lower limit of the Cr content may be 0.05%.
- the lower limit of the Cr content may be 0.10%.
- a more preferable lower limit of the Cr content is 0.20%. If necessary, the lower limit of the Cr content may be 0.30% or 0.40%.
- Mo 0 to 0.50% Mo can be contained as required.
- Mo When Mo is contained, there is an effect of improving the strength and toughness of the base material.
- the Mo content exceeds 0.50%, the hardness of the HAZ increases and the toughness and SSC resistance may be impaired, so the upper limit of the Mo content is 0.50%.
- the upper limit of the preferable Mo content is 0.30%.
- the upper limit of the Mo content may be 0.20%, 0.15%, or 0.12%.
- the lower limit is 0%.
- V 0 to 0.10%
- V can be contained as required. Inclusion of V has an effect of improving the strength of the base material mainly due to carbonitride precipitation during tempering. However, if the V content exceeds 0.10%, the effect of improving the strength of the base metal is saturated and the toughness may be deteriorated. Therefore, the upper limit of the V content is 0.10%. There is no need to define the lower limit of the V content, and the lower limit is 0%. In order to improve toughness, the upper limit of the V content may be 0.08%, 0.06%, or 0.04%. In addition, when obtaining the effect of improving the strength of the base material due to V, the lower limit of the V content may be 0.015% or 0.02%.
- B 0 to 0.0050% B can be contained as required. Inclusion of B has an effect of improving the strength of the base material. However, if the B content exceeds 0.0050%, precipitation of coarse boron compounds may be caused to deteriorate toughness, so the upper limit of the B content is set to 0.0050%. In order to prevent toughness deterioration, the upper limit of the B content may be 0.0040%, 0.0030%, or 0.0020%. There is no need to define the lower limit of the B content, and the lower limit is 0%. In addition, when obtaining the effect of improving the strength of the base material by B, it is preferable that the lower limit of the B content is 0.0003%. A more preferable lower limit of the B content is 0.0005% or 0.0010%. When the effect of improving the base material strength by B is not required, the upper limit of the B content may be 0.0010%, 0.0005%, 0.0003%, or 0.0002%.
- Nb 0 to 0.10% Nb can be contained as necessary.
- Nb When Nb is contained, there is an effect of refining the structure and improving the low temperature toughness. However, if the Nb content exceeds 0.10%, coarse carbides or nitrides may be formed and the toughness may be lowered, so the upper limit of the Nb content is 0.10%. There is no need to define the lower limit of the Nb content, and the lower limit is 0%. In order to prevent a decrease in toughness, the upper limit of the Nb content may be 0.08%, 0.06%, or 0.04%. In addition, when acquiring the effect of improving the low temperature toughness by Nb, it is good also considering the minimum of content of Nb as 0.01% or 0.02%.
- Ti 0 to 0.10% Ti can be contained as required. Ti is mainly used as a deoxidizing element, but has the effect of further miniaturizing the structure by forming an oxide phase containing Al, Ti, and Mn. However, if the Ti content exceeds 0.10%, the oxide formed will be Ti oxide or Ti-Al oxide, and the dispersion density will decrease, especially in the heat-affected zone of the small heat input weld zone. Since the ability to refine the structure may be lost, the upper limit of the Ti content is set to 0.10%. The upper limit of the preferable Ti content is 0.07% or 0.05%. There is no need to define the lower limit of the Ti content, and the lower limit is 0%. In addition, when obtaining the effect of refining the structure by Ti, the lower limit of the Ti content may be 0.02% or 0.03%.
- Sn 0 to 0.50% Sn can be contained as necessary.
- Sn When Sn is contained, it becomes Sn 2+ and dissolves in the steel material surface deposit, and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly reducing the Fe 3+, by having an effect of reducing the Fe 3+ concentration of the oxidizing agent, since inhibit corrosion promoting effect of Fe 3+, thereby improving the weather resistance in high airborne salt environments.
- the upper limit of the Sn content is 0.50%.
- the upper limit of preferable Sn content is 0.20%.
- the upper limit of the Sn content may be limited to 0.10%, 0.05%, or 0.01%. There is no need to define the lower limit of the Sn content, and the lower limit is 0%.
- the lower limit of Sn when obtaining the corrosion resistance and weather resistance effect by Sn, the lower limit of Sn may be set to 0.03% or 0.05%.
- Ca 0 to 0.004% Ca can be contained as needed.
- Ca When Ca is contained, it reacts with S in the steel to form oxysulfide (oxysulfide) in the molten steel. Since this oxysulfide does not extend in the rolling direction by rolling unlike MnS and the like, it is spherical after rolling. This spherical oxysulfide has an effect of suppressing weld cracking or hydrogen-induced cracking starting from the tip of an elongated inclusion.
- the upper limit of the Ca content is set to 0.004%. In order to surely avoid a decrease in toughness, the upper limit of the Ca content may be 0.003%.
- the lower limit of the Ca content there is no need to define the lower limit of the Ca content, and the lower limit is 0%.
- the lower limit of the Ca content may be 0.0003% or 0.0005%.
- Mg 0 to 0.0020% Mg can be contained as needed.
- Mg a fine Mg-containing oxide is generated, which is effective in reducing the ⁇ particle size.
- the upper limit of the Mg content is set to 0.0020%.
- the upper limit of the preferable Mg content is 0.0010%.
- the lower limit of Mg is preferably set to 0.0002%.
- a more preferable lower limit of the Mg content is 0.0004%.
- REM 0 to 0.0020% REM (rare earth element) can be contained as required.
- REM When REM is contained in steel, it has the effect of refining the structure of the weld heat affected zone and further binding S to fix S. If REM is excessively contained, inclusions may be formed and the cleanliness of the weld may be lowered. However, inclusions formed by the inclusion of REM have a relatively small influence on toughness degradation. If the content is 0.0020% or less, a reduction in the toughness of the base material due to the inclusion of REM is acceptable. Therefore, the upper limit of the content of REM is set to 0.0020%. The upper limit of the preferable REM content is 0.0010%.
- the lower limit of the REM content is preferably set to 0.0002%.
- a more preferable lower limit of the REM content is 0.0003%.
- REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained.
- the term “REM content” means the total content of these elements.
- the steel material according to the present embodiment contains the above components, and the balance contains iron and impurities.
- the welded steel material according to the present embodiment contains the following alloy elements in addition to the above components for the purpose of further improving the strength, toughness, etc. of the steel material itself, or as impurities from secondary materials such as scrap. May be. Since Sb impairs the toughness of HAZ, the upper limit of the Sb content may be 0.03%.
- the upper limit of the Sb content may be 0.01%, 0.005%, 0.003%, or 0.001%. Since As impairs the toughness of HAZ, the upper limit of the As content may be 0.02%. If necessary, the upper limit of the As content may be 0.005%, 0.003%, or 0.001%. Moreover, in order to improve strength and toughness, the upper limits of the Pb, Zr, Zn and W contents may be set to 0.1%, 0.01% or 0.005%. There is no particular need to determine the lower limit of the content of these elements, and it is 0%. Co may be contained as an impurity in Ni. Since Co impairs HAZ toughness, the upper limit of the Co content may be 0.5%, 0.3%, 0.1%, or 0.05%. There is no particular need to determine the lower limit of the Co content, and the lower limit is 0%.
- the lower limit of the residual ⁇ amount at the (1/4) t position of the sheet thickness t is 4.0% by volume.
- the residual ⁇ in the steel material is a brittle crack in the steel material. Contributes to improved propagation stop characteristics. As a result, an effect of improving toughness in a low temperature environment can be expected. In order to obtain this effect, it is necessary that the lower limit of the residual ⁇ amount at the (1/4) t position of the steel sheet thickness t is 4.0% by volume. In order to improve toughness, the lower limit of the residual ⁇ amount may be 4.5% by volume, 5.0% by volume, 5.5% by volume, 6.0% by volume, or 6.5% by volume.
- the upper limit of the residual ⁇ amount is not particularly specified, but if there is too much residual ⁇ , the yield strength may decrease, so the upper limit of the residual ⁇ amount is 20.0% by volume or 15.0% by volume. It is good.
- the reason why the residual ⁇ amount is evaluated at the (1/4) t position of the sheet thickness t is to perform evaluation at an average position in the entire sheet thickness.
- the tempering temperature T (° C.) satisfies the following formula (3)
- the lower limit of the residual ⁇ amount at the (1/4) t position of the plate thickness t can be set to 4.0 vol%.
- Ac 1 is defined by the following equation (4).
- FIG. 1 shows the steel No. described in Table 1.
- a slab having one chemical component is heated to 950 ° C., then rolled to achieve a cumulative reduction of 70% at 850 ° C. or lower, immediately cooled to room temperature immediately after rolling, and subsequently tempered at various tempering temperatures. It is a graph which shows the relationship between tempering temperature and residual (gamma) amount in the various steel materials manufactured by water-cooling after that.
- the cumulative rolling reduction is a percentage ((t1 ⁇ t2) / t1 ⁇ a value obtained by dividing the difference between the plate thickness t1 at the start of rolling and the plate thickness t2 at the end of rolling by the plate thickness t1 at the start of rolling. 100).
- the tempering temperature is too low, there are too few regions to reversely transform to ⁇ , so the amount of residual ⁇ is small.
- the tempering temperature is too high, the generated ⁇ becomes unstable and during cooling. Since it undergoes martensitic transformation, the amount of residual ⁇ decreases. Therefore, it can be understood that a large amount of residual ⁇ can be secured by satisfying the expression (3).
- the upper limit of the average value of the aspect ratio of residual ⁇ is 2.5 and the upper limit of the average value of the major axis is 0.85 ⁇ m.
- the residual ⁇ in the ⁇ structure (ferrite structure) is a metastable state. And easily undergoes martensitic transformation by undergoing plastic deformation. Residual ⁇ needs to be dispersed in order to improve the brittle fracture occurrence characteristics or propagation stop characteristics, and when it disappears after an earthquake, the desired fracture resistance characteristics are not exhibited.
- the strain applied to the residual ⁇ particles varies greatly depending on the distribution pattern of the residual ⁇ particles, and the more fine the residual ⁇ particles are in a spherical shape, the more the strain is distributed. The rate drops. Therefore, it is necessary to set the upper limit of the average value of the aspect ratio of the residual ⁇ particles obtained by cross-sectional observation to 2.5 and the upper limit of the average value of the major axis of the residual ⁇ particles obtained by cross-sectional observation to 0.85 ⁇ m. . Since the toughness is improved as the average aspect ratio of the residual ⁇ particles is smaller, the upper limit of the average aspect ratio may be 2.3 or 2.0.
- the upper limit of the average value of the major axis may be 0.80 ⁇ m or 0.75 ⁇ m.
- the lower limit of the average value of the major axis need not be specified, but is usually 0.05 ⁇ m.
- the lower limit of Mn concentration and the lower limit of Ni concentration in each residual ⁇ are set to 1. It is extremely important to make it 4 times.
- the lower limit of the cumulative reduction rate of 850 ° C. or less in the hot rolling process is set to 50%, and the cooling rate after tempering is 0.5 ° C./s. It is necessary to make it larger. 2 shows the steel No. described in Table 1. A slab having one chemical component is heated to 960 ° C., then rolled at various cumulative reduction rates, immediately cooled to room temperature immediately after rolling, and subsequently tempered at 570 ° C. (with water cooling after tempering).
- FIG. 5 is a graph showing the relationship between the cumulative reduction ratio of 850 ° C. or less and the concentration ratios of Ni and Mn ([M] ⁇ / [M] ⁇ ) in various steel materials manufactured by the above.
- the concentration ratios of Ni and Mn are values obtained by dividing [Mn] retained ⁇ and [Ni] retained ⁇ by [Mn] ⁇ and [Ni] ⁇ , respectively. From FIG. 2, it can be seen that a concentration ratio of 1.4 or more can be obtained by satisfying the expressions (1) and (2) by setting the lower limit of the cumulative rolling reduction to 50%.
- FIG. 3 shows the relationship between the concentration ratios of Ni and Mn and DT (Dynamic Tear) energy, which is a typical fracture characteristic evaluation parameter.
- the DT energy exceeds 1500 J.
- FIG. 3 shows that the DT energy exceeds 1500 J by setting the lower limit of the concentration ratios of Ni and Mn to 1.4.
- the lower limit of the concentration ratio is 1.5 or 1.6, a higher DT energy can be obtained, which is preferable.
- the steel material according to the present embodiment can be produced through the following steps. However, it is not limited to the following manufacturing method.
- the casting conditions are not stipulated.
- An ingot-splitting slab may be used, or a continuous cast slab may be used. From the viewpoint of production efficiency, yield, and energy saving, it is preferable to use a continuously cast slab.
- the thickness of the steel material to be manufactured is 3 mm to 100 mm, mainly 6 mm to 50 mm.
- the manufactured steel material may be a steel plate.
- the slab heating temperature is controlled to 920 ° C. to 980 ° C.
- the lower limit of the slab heating temperature is preferably 920 ° C.
- the upper limit of the heating temperature of the slab is set to 970 ° C. so that the ⁇ grains are not excessively coarsened and the fracture resistance is not impaired.
- (C-2) Rolling process In the hot rolling process, the heated slab is rolled. Specifically, the rolling may be divided into rough rolling and finish rolling.
- the slab thickness at the end of the rough rolling it is preferable to reduce the slab thickness at the end of the rough rolling until it becomes 3 to 8 times the product thickness (steel material thickness). If the slab thickness after rough rolling is reduced to 3 times or more of the product sheet thickness, it can be sufficiently reduced by subsequent finish rolling, and the toughness of the product steel can be improved. On the other hand, if the slab thickness after the rough rolling is reduced to 8 times or less of the product thickness, the finish rolling temperature in the subsequent finish rolling (temperature at which the finish rolling is finished) can be easily controlled to 700 ° C. or more. .
- finish rolling the slab subjected to rough rolling in this way is continuously reduced without cooling to a product with a predetermined thickness.
- the lower limit of the cumulative rolling reduction at 850 ° C. or lower is set to 50%. Increasing the amount of reduction at a relatively low temperature is effective in actively introducing a deformation zone, leaving a large amount of residual ⁇ finally produced, and further reducing the average aspect ratio of the residual ⁇ . . This is because the stretched residual ⁇ is divided when the amount of reduction is large. Further, in order to positively introduce deformation bands, it is desirable to lower the finish rolling start temperature as much as possible so that the final rolling temperature (finish rolling temperature) during finish rolling is 700 ° C. to 730 ° C.
- (C-3) Cooling step In the cooling step, it is desirable to accelerate cooling the steel material after finish rolling. In particular, as the plate thickness increases, it becomes more difficult to ensure the toughness of the steel material. Therefore, in a steel material with a thick plate thickness, it is better that the cooling rate of the accelerated cooling after the rolling process is faster. Specifically, when the plate thickness is 15 mm or less, the lower limit of the cooling rate at the central portion of the steel plate thickness t, that is, at the (1/2) t position of the plate thickness t is 3 ° C./s. When the plate thickness exceeds 15 mm, the lower limit of the cooling rate is 10 ° C./s.
- the upper limit of the cooling rate at the (1/2) t position of the plate thickness t is not particularly defined, but may be 50 ° C./s in consideration of the facility capacity.
- the lower limit of the cooling start temperature is set to 660 ° C. in order to make the steel metal structure sufficiently quenched and to obtain a fine residual ⁇ and a concentration ratio of 1.4 or more by subsequent tempering treatment or the like.
- Accelerated cooling is preferably performed until the surface temperature of the steel material is 250 ° C. or lower.
- this cooling stop temperature exceeds 250 ° C., a phenomenon occurs in which the transformation to the martensite structure becomes incomplete or the dislocation in the martensite structure is recovered by the autotemper effect, and as a result, Fine residual ⁇ is not effectively generated by the heat treatment, and the possibility that the strength is insufficient increases.
- the upper limit of the cooling stop temperature is preferably 200 ° C or 150 ° C.
- the lower limit of the cooling stop temperature is not particularly defined, but may be 50 ° C. or room temperature in consideration of the facility capacity.
- DQT Direct-Quenching and Tempering
- DQLT Direct-Quenching, Lamellarizing, and Tempering
- (C-4) L treatment process When a sufficiently hardened structure is obtained, the L treatment (steel material was heated to a temperature range of two phases of ferrite and austenite, which has been conventionally used in 9% Ni steel) It is not always necessary to perform the water-cooling process later in this embodiment, and a steel material exhibiting sufficient characteristics can be obtained by performing only the tempering process. However, if the steel material is heated to a two-phase temperature range of ferrite and austenite, the toughness can be improved by refining the metal structure and generating a stable austenite phase, so that the temperature ranges from 620 ° C to 720 ° C as necessary. You may implement the L process process which performs a water cooling process after heating.
- the temperature range of heating in the L treatment step is preferably 640 ° C to 700 ° C.
- tempering step is extremely important for realizing the present invention, and is an essential process that requires detailed control. If the tempering temperature is too low, the amount of ⁇ produced is insufficient, and the amount of residual ⁇ itself decreases. In addition, if the tempering temperature is too low, temper embrittlement may occur, resulting in a deterioration of the fracture resistance. Conversely, when the tempering temperature is too high, the amount of ⁇ during heating increases, but the concentration of Ni and Mn in the residual ⁇ decreases. In this case, a large amount of residual ⁇ is transformed during subsequent cooling, or even if transformation does not occur during cooling, it is transformed and disappears only by being exposed to a very low temperature.
- the range of the tempering temperature is governed by the thermodynamic equilibrium behavior, and has a property that varies depending on the chemical composition of the steel material. Specifically, it is necessary that the lower limit of the tempering temperature T is 3.8 ⁇ Ni ⁇ 33 + Ac 1 and the upper limit is 6.3 ⁇ Ni ⁇ 0.4 + Ac 1 . That is, it is necessary to satisfy the following expression (3).
- the coefficient described in Formula (3) was calculated
- the tempering temperature is preferably more than Ac 1 .
- the cooling rate after heating in the tempering process is low, the fracture resistance is impaired due to the reason that a partial bainite transformation proceeds due to the diffusion movement of carbon.
- the lower limit of the cooling rate in the central portion of the plate thickness until the surface temperature becomes 300 ° C. or lower needs to be 0.5 ° C./s.
- the upper limit of the cooling rate after heating in the tempering step is not particularly defined, it may be 50 ° C./s in consideration of the upper limit of the equipment capacity.
- the lower limit of the cooling stop temperature is not particularly defined, but may be 50 ° C. or room temperature in consideration of the facility capacity.
- V notch test piece full size test piece
- a V-notch test piece with a plate thickness and width of 10 mm could not be collected, so a sub-size test piece was collected.
- the criteria for determining the quality are YS: 585 MPa or more, TS: 690 MPa or more, V-notch Charpy absorbed energy value per unit area v E -196 : 150 J / cm 2 or more, absorbed energy DT -196 (J ): Accepting 1500J or more.
- a test piece having a plate thickness of less than 15 mm was subjected to a precrack Charpy test.
- the test piece for the pre-crack Charpy test has a crack depth of 2 mm with respect to a test piece width of 10 mm as in the case of a normal V-notch Charpy.
- the evaluation method of the residual ⁇ amount is as follows. A test piece for residual ⁇ measurement was taken from a (1/4) t position of the thickness t of the steel material, and the residual ⁇ amount (% by volume) was measured by X-ray diffraction. The measured cross section was an L cross section (a plane parallel to the rolling direction and perpendicular to the steel sheet surface). Furthermore, the shape of residual ⁇ was evaluated by thin film observation with a transmission electron microscope. 20 or more residual ⁇ particles were observed, the average aspect ratio of these particle samples and the average size of the major axis were measured, and the average value in the sample was calculated. Further, the concentration of Mn and Ni in the residual ⁇ was evaluated by the following method.
- the average Mn concentration and the average Ni concentration in the residual ⁇ were measured by EDX (Energy Dispersive X-ray spectroscopy) quantitative analysis, and compared with the average Mn concentration and the average Ni concentration in the ferrite phase, respectively. ) And Formula (2) are evaluated.
- the average Mn concentration and the average Ni concentration in the ferrite phase were the bulk values (chemical analysis results) of the steel material.
- [Mn] retained ⁇ average Mn concentration in residual ⁇
- [Mn] ⁇ average Mn concentration in ferrite phase
- [Ni] retained ⁇ average Ni concentration in residual ⁇
- [Ni] ⁇ ferrite
- the average Ni concentration in the phase is represented respectively.
- Test No. 1-c steel the water cooling stop temperature after rolling exceeds the specified range.
- Test No. 1-d steel the cooling rate after tempering falls below the specified range.
- Steel No. 1-e Test No. with heating temperature below specified range.
- Test No. 1 in which the finish rolling temperature was lowered due to the slab thickness after completion of the rough rolling with respect to the steel thickness of 1-j and the product thickness product exceeding the specified range.
- the 1-l steel material lacked strength (yield strength YS, tensile strength TS).
- Test No. 1-c steel, Test No. 1-d steel, Test No. Steel No. 1-e Test No. with tempering temperature below specified range.
- 1-f steel Test No.
- L processing temperature exceeds specified range.
- the 1-p steel material lacked arrest properties (DT absorbed energy: DT- 196 or absorbed energy at -196 ° C. obtained by a precrack Charpy test).
- Steel No. 34 has a C content of steel No. 34.
- Test No. 35 made of 35 steel.
- Steel No. 35 has a Si content and Test No. 36 made of 36 steel. Since the steel material No. 36 has an excessively high Mn content, the strength characteristics (yield strength and tensile strength) are not a problem, but the fracture characteristics (brittle crack initiation characteristics and arrest characteristics) are insufficient.
- steel no. Test No. 38 made of 38 steel. Steel No. 38 has an Al content of Steel No.
- Test No. 39 made of 39 steel. Since the steel No. 39 had an excessively high N content, the tensile strength was insufficient, and the residual ⁇ was not sufficient and the fracture characteristics were insufficient. Steel No. whose C content is lower than the specified value.
- Test No. 40 made of 40 steel. No. 40 steel, and No. whose Mn content is lower than the specified value.
- Test No. 42 made of 42 steel. Forty-two steel materials lacked both strength and fracture properties. Steel No. whose Si content is below the specified value.
- Test No. 41 made of 41 steel. Steel No. 41 and steel No. 1 with an Al content below the specified value.
- Steel No. 44-No. 56 is one of P, S, Cu, Cr, Mo, V, Nb, Ti, B, Sn, Al, Mg, and REM. 44-No. The steel material No. 56 has deteriorated fracture characteristics.
- the steel material having a Ni content of 6.6% to 8.0% by mass according to the present invention is excellent in economic efficiency and fracture resistance.
- This steel material is suitable for use as an inner tank member or an annular plate of an LNG tank.
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Abstract
Description
[Mn]retained γ>[Mn]α×1.4 ・・・(A)
[Ni]retained γ>[Ni]α×1.4 ・・・(B)
ここで、[Mn]retained γは前記残留γ中の平均Mn濃度を、[Mn]αはフェライト相中の平均Mn濃度を、[Ni]retained γは前記残留γ中の平均Ni濃度を、[Ni]αは前記フェライト相中の平均Ni濃度を、それぞれ表す。 (1) In the steel material according to one embodiment of the present invention, the chemical components are mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.4 to 2 0.0%, Ni: 6.6 to 8.0%, Al: 0.002 to 0.08%, N: 0.0050% or less, P: 0.05% or less, S: 0.008% or less, Cu : 0-1.0%, Cr: 0-1.0%, Mo: 0-0.5%, V: 0-0.10%, B: 0-0.0050%, Nb: 0-0. 10%, Ti: 0 to 0.10%, Sn: 0 to 0.50%, Ca: 0 to 0.004%, Mg: 0 to 0.0020%, REM: 0 to 0.0020%, balance: Fe and impurities steel, the lower limit of the amount of residual γ at the (1/4) t position of the steel sheet thickness t is 4.0% by volume, the residual γ of the aspect ratio The upper limit of the average value is 2.5 And the upper limit of the average value of the major axis is 0.85 μm, and the average Mn concentration and the average Ni concentration in the residual γ satisfy the following formulas (A) and (B), respectively.
[Mn] retained γ > [Mn] α × 1.4 (A)
[Ni] retained γ > [Ni] α × 1.4 (B)
Here, [Mn] retained γ is an average Mn concentration in the residual γ, [Mn] α is an average Mn concentration in the ferrite phase, [Ni] retained γ is an average Ni concentration in the residual γ, Ni] α represents the average Ni concentration in the ferrite phase.
C:0.01~0.12%
Cは、母材の強度確保のために必要な元素である。Cの含有量が0.01%未満である場合、必要な強度が確保できず、さらに溶接の際にFL(Fusion Line)でのラス状マルテンサイトの形成が不十分になってFL近傍のHAZ(Heat Affected Zone)の靭性も低下するので、C含有量の下限を0.01%とする必要がある。一方、Cの含有量が0.12%を超えると、HAZ、なかでもFL近傍のHAZの靭性劣化が著しくなる。したがって、Cの含有量は0.01%~0.12%とする。確実に強度を確保するために、Cの含有量の下限を0.02%、0.03%又は0.04%としてもよい。HAZ靭性の改善のために、Cの含有量の上限を、0.10%、0.08%、0.07%又は0.06%としてもよい。 (A) Regarding chemical composition C: 0.01 to 0.12%
C is an element necessary for ensuring the strength of the base material. When the C content is less than 0.01%, the required strength cannot be secured, and the formation of lath martensite in FL (Fusion Line) becomes insufficient during welding, and the HAZ near the FL Since the toughness of (Heat Affected Zone) also decreases, the lower limit of the C content needs to be 0.01%. On the other hand, when the content of C exceeds 0.12%, the toughness of HAZ, particularly HAZ near FL, is significantly deteriorated. Therefore, the C content is 0.01% to 0.12%. In order to ensure the strength, the lower limit of the C content may be 0.02%, 0.03%, or 0.04%. In order to improve the HAZ toughness, the upper limit of the C content may be 0.10%, 0.08%, 0.07%, or 0.06%.
Siは、脱酸剤として必要な元素である。脱酸の効果を得るためには、Siの含有量の下限を0.01%とする必要がある。一方、本実施形態に係る鋼材の場合、Siと焼入れままマルテンサイトの焼戻し過程とは大いに関連があり、Siの含有量が0.30%を超えると、Siは、溶接冷却過程において過飽和にCを固溶しているマルテンサイト中からセメンタイトへのCの分解析出反応を抑制する。Cの分解析出反応の抑制により、自己焼戻し(Self-tempering)が遅延し、溶接部の靭性が低下する。あるいは、含有量が0.30%を超えるSiは、島状マルテンサイトを増加させることによって溶接部の靭性を低下させる。よって、Si含有量は0.01%~0.30%とする。なお、溶接部の靭性向上の観点からは、Si含有量はできるだけ少ない方がよく、溶接部の靭性改善のために、Si含有量の上限を0.20%、0.15%又は0.10%としてもよい。脱酸を確実に行うために、Si含有量の下限を0.02%、0.03%又は0.04%としてもよい。 Si: 0.01 to 0.30%
Si is an element necessary as a deoxidizer. In order to obtain the effect of deoxidation, the lower limit of the Si content needs to be 0.01%. On the other hand, in the case of the steel material according to this embodiment, Si and the tempering process of martensite as quenched are greatly related. When the Si content exceeds 0.30%, Si is supersaturated in the welding cooling process. This suppresses the decomposition and precipitation reaction of C from the martensite in the form of solid solution to cementite. By suppressing the decomposition and precipitation reaction of C, self-tempering is delayed and the toughness of the weld is lowered. Alternatively, Si whose content exceeds 0.30% decreases the toughness of the welded portion by increasing island martensite. Therefore, the Si content is set to 0.01% to 0.30%. From the viewpoint of improving the toughness of the welded portion, the Si content should be as low as possible. To improve the toughness of the welded portion, the upper limit of the Si content is 0.20%, 0.15%, or 0.10. % May be used. In order to reliably perform deoxidation, the lower limit of the Si content may be 0.02%, 0.03%, or 0.04%.
Mnは、脱酸剤として、また、母材の強度及び靭性の確保、ならびにHAZの焼入性確保のために必要な元素である。Mnの含有量が0.4%未満ではこれらの効果が得られず、さらにHAZにフェライトサイドプレートが生成してラス状マルテンサイトの形成が不十分になり溶接部の靭性が低下するので、Mnの含有量の下限は0.4%とする。一方、Mnの含有量が2.0%を超えると、Mnの中心偏析により、母材特性の板厚方向での不均一をもたらす場合がある。よって、Mnの含有量は0.4%~2.0%とする。焼入性確保と溶接部の靭性向上とのために、Mnの含有量の下限を0.50%、0.60%又は0.70%としてもよい。母材特性の板厚方向での不均一をさらに防ぐために、Mn含有量の上限を1.5%、1.2%、1.0%又は0.9%としてもよい。 Mn: 0.4 to 2.0%
Mn is an element necessary as a deoxidizer, for ensuring the strength and toughness of the base material, and for ensuring the hardenability of the HAZ. If the Mn content is less than 0.4%, these effects cannot be obtained, and furthermore, ferrite side plates are generated in the HAZ, so that the formation of lath martensite becomes insufficient and the toughness of the welded portion is reduced. The lower limit of the content of is 0.4%. On the other hand, if the Mn content exceeds 2.0%, the base material characteristics may be uneven in the thickness direction due to the central segregation of Mn. Therefore, the Mn content is set to 0.4% to 2.0%. In order to ensure hardenability and improve the toughness of the welded portion, the lower limit of the Mn content may be 0.50%, 0.60%, or 0.70%. In order to further prevent non-uniformity of the base material characteristics in the thickness direction, the upper limit of the Mn content may be 1.5%, 1.2%, 1.0%, or 0.9%.
Pは、不純物として鋼中に存在し、粒界に偏析して、靭性を低下させる原因となる。Pの含有量が0.05%を超えると、溶接時に高温割れを招く場合があるので、Pの含有量を0.05%以下に制限する。なお、靭性の向上のためには、Pの含有量はできるだけ小さくするのがよく、Pの含有量の上限を0.03%、0.02%、0.01%、0.008%又は0.006%としてもよい。P含有量の下限を特に規定する必要はなく、その下限は0%である。しかし、Pを必要以上に低減させることは、精錬時のコストアップにつながるので、Pの含有量の下限を0.0001%又は0.0005%としてもよい。 P: 0.05% or less P is present in the steel as an impurity, segregates at the grain boundary, and causes toughness to decrease. If the P content exceeds 0.05%, hot cracking may occur during welding, so the P content is limited to 0.05% or less. In order to improve toughness, the P content should be as small as possible. The upper limit of the P content is 0.03%, 0.02%, 0.01%, 0.008%, or 0. It may be 0.006%. There is no need to specifically define the lower limit of the P content, and the lower limit is 0%. However, reducing P more than necessary leads to a cost increase during refining, so the lower limit of the P content may be 0.0001% or 0.0005%.
Sは、不純物として鋼中に存在し、多すぎると、中心偏析を助長したり、脆性破壊の原因となる延伸形状のMnSが多量に生成したりする原因となる。Sの含有量が0.008%を超えると、母材およびHAZの機械的性質が劣化する。このため、Sの含有量は0.008%以下とする。母材およびHAZの機械的性質の改善のために、Sの含有量の上限を0.006%、0.004%、0.003%又は0.002%としてもよい。Sの含有量はできるだけ小さくするのがよいので、Sの含有量の下限を規定する必要はなく、その下限は0%である。精錬コストの問題から、Sの含有量の下限を0.0001%又は0.0003%としてもよい。 S: 0.008% or less S is present in the steel as an impurity, and if it is too much, it promotes center segregation or causes a large amount of stretched MnS that causes brittle fracture. If the S content exceeds 0.008%, the mechanical properties of the base material and the HAZ deteriorate. For this reason, content of S shall be 0.008% or less. In order to improve the mechanical properties of the base material and the HAZ, the upper limit of the S content may be 0.006%, 0.004%, 0.003%, or 0.002%. Since the S content should be as small as possible, it is not necessary to define the lower limit of the S content, and the lower limit is 0%. From the problem of refining costs, the lower limit of the S content may be 0.0001% or 0.0003%.
Niは、低温用鋼材として靭性を確保するために必要な最も基本的な元素である。低温用鋼材として靭性を確保するためには、6.6%以上のNiの含有量が必要である。Niの含有量が多ければ多いほど高い低温靭性が得られるが、その分コストアップの要因となるので、Niの含有量の上限は8.0%とする。したがって、Niの含有量のターゲットは6.6%~8.0%である。低温靭性の確保の観点から、Ni含有量は6.7%以上が好ましく、必要に応じて、Niの含有量の下限を6.8%、6.9%又は7.0%としてもよい。また、コスト抑制の観点から、Ni含有量の上限を、7.8%、7.6%又は7.4%としてもよい。ただし、Ni含有量が8.0%超であったとしても、低温用鋼材として求められる特性は得られる。 Ni: 6.6 to 8.0%
Ni is the most basic element necessary for securing toughness as a low-temperature steel material. In order to ensure toughness as a low-temperature steel material, a Ni content of 6.6% or more is required. The higher the Ni content, the higher the low-temperature toughness. However, the cost increases accordingly, so the upper limit of the Ni content is 8.0%. Therefore, the target of Ni content is 6.6% to 8.0%. From the viewpoint of securing low temperature toughness, the Ni content is preferably 6.7% or more, and the lower limit of the Ni content may be 6.8%, 6.9%, or 7.0% as necessary. From the viewpoint of cost reduction, the upper limit of Ni content may be 7.8%, 7.6%, or 7.4%. However, even if the Ni content is more than 8.0%, the characteristics required as a low-temperature steel material can be obtained.
Alは、一般的には脱酸剤として含有させる元素であるが、本実施形態に係る鋼材の場合には、Siと同様に、マルテンサイトの自己焼戻し(Self-tempering)を遅延させる働きを有する。したがって、Alの含有量はできるだけ少ない方が望ましい。Alの含有量が0.080%を超えて過剰になると、前述したSiと同様に、溶接冷却過程において過飽和にCを固溶したマルテンサイトからのCのセメンタイトへの分解析出反応を抑制し、溶接部の靭性を低下させる場合がある。しかしながら、Alの含有量が0.002%未満では十分な脱酸効果が得られない。したがって、Alの含有量は0.002%~0.080%とする。確実に脱酸をおこなうために、Alの含有量の下限を0.005%、0.010%、0.015%又は0.020%としてもよい。溶接部の靭性向上のために、Alの含有量の上限を0.060%、0.050%又は0.040%としてもよい。 Al: 0.002 to 0.080%
Al is an element generally contained as a deoxidizer, but in the case of the steel material according to the present embodiment, it has a function of delaying self-tempering of martensite, similar to Si. . Therefore, it is desirable that the Al content be as small as possible. When the Al content exceeds 0.080% and becomes excessive, the decomposition precipitation reaction of C into cementite from martensite, which is supersaturated with C in the welding cooling process, is suppressed as in the case of Si described above. In some cases, the toughness of the welded portion may be reduced. However, if the Al content is less than 0.002%, a sufficient deoxidizing effect cannot be obtained. Therefore, the Al content is set to 0.002% to 0.080%. In order to perform deoxidation reliably, the lower limit of the Al content may be 0.005%, 0.010%, 0.015%, or 0.020%. In order to improve the toughness of the welded portion, the upper limit of the Al content may be 0.060%, 0.050%, or 0.040%.
Nは、不純物として鋼中に存在し、固溶Nの増加又は析出物の生成を通してHAZ靭性の悪化の原因となるので、HAZ靭性の確保のためにはNの含有量は低い方がよい。Nの含有量が0.0050%を超えるとHAZ靭性の悪化が顕著になる場合があるので、Nの含有量を0.0050%以下とする。HAZ靭性の向上のために、Nの含有量の上限を0.0045%又は0.0040%としてもよい。Nの含有量の下限を規定する必要はなく、その下限は0%である。ただし、精錬時のコストの問題から、Nの含有量の下限を0.0001%又は0.0010%としてもよい。 N: 0.0050% or less N is present in steel as an impurity and causes deterioration of HAZ toughness through an increase in solid solution N or formation of precipitates. Therefore, N content is required to ensure HAZ toughness. The amount should be low. If the N content exceeds 0.0050%, the HAZ toughness may deteriorate significantly, so the N content is set to 0.0050% or less. In order to improve the HAZ toughness, the upper limit of the N content may be 0.0045% or 0.0040%. There is no need to define the lower limit of the N content, and the lower limit is 0%. However, the lower limit of the N content may be set to 0.0001% or 0.0010% from the problem of cost during refining.
Cuは、必要に応じて含有させることができる。Cuを含有させると、母材の強度を向上させることができる。しかしながら、Cuの含有量が1.00%を超えると、Ac3点以下の温度に加熱されたHAZの靭性が劣化する場合があるので、Cuの含有量の上限は1.00%とする。好ましいCu含有量の上限は0.80%又は0.60%であり、さらに好ましいCu含有量の上限は0.30%である。なお、Cuによる母材の強度向上効果を得たい場合には、Cuの含有量の下限を0.10%としてもよい。 Cu: 0 to 1.00%
Cu can be contained as needed. When Cu is contained, the strength of the base material can be improved. However, if the Cu content exceeds 1.00%, the toughness of HAZ heated to a temperature of Ac 3 points or less may deteriorate, so the upper limit of the Cu content is 1.00%. The upper limit of the preferable Cu content is 0.80% or 0.60%, and the upper limit of the more preferable Cu content is 0.30%. In addition, when obtaining the strength improvement effect of the base material by Cu, the lower limit of the Cu content may be 0.10%.
Crは、必要に応じて含有させることができる。Crを含有させると、耐炭酸ガス腐食性を向上させるとともに、焼入れ性の向上により結果として強度を向上させることができる。しかしながら、Crの含有量が1.00%を超えると、HAZの硬化の抑制が難しくなり、さらに耐炭酸ガス腐食性向上効果が飽和するので、Crの含有量の上限は1.00%とする。HAZの硬化の抑制のために、Cr含有量の上限を0.80%、0.60%又は0.50%としてもよい。Crの含有量の下限を規定する必要はなく、その下限は0%である。Crによる耐炭酸ガス腐食性及び焼入性の向上効果を得たい場合には、Crの含有量の下限を0.05%としてもよい。焼入性の向上効果を確実に得るために、Cr含有量の下限を0.10%としてもよい。より好ましいCr含有量の下限は0.20%である。必要に応じて、Cr含有量の下限を、0.30%又は0.40%としてもよい。 Cr: 0 to 1.00%
Cr can be contained as needed. When Cr is contained, the carbon dioxide gas corrosion resistance is improved, and the strength can be improved as a result of the improvement of the hardenability. However, if the Cr content exceeds 1.00%, it becomes difficult to suppress the hardening of the HAZ, and further the effect of improving the corrosion resistance of carbon dioxide gas is saturated, so the upper limit of the Cr content is 1.00%. . In order to suppress the hardening of the HAZ, the upper limit of the Cr content may be 0.80%, 0.60%, or 0.50%. There is no need to define the lower limit of the Cr content, and the lower limit is 0%. When it is desired to obtain the effect of improving the carbon dioxide corrosion resistance and hardenability by Cr, the lower limit of the Cr content may be 0.05%. In order to reliably obtain the effect of improving hardenability, the lower limit of the Cr content may be 0.10%. A more preferable lower limit of the Cr content is 0.20%. If necessary, the lower limit of the Cr content may be 0.30% or 0.40%.
Moは、必要に応じて含有させることができる。Moを含有させると、母材の強度と靭性とを向上させる効果がある。しかしながら、Moの含有量が0.50%を超えると、HAZの硬度が高まり、靭性と耐SSC性とを損なう場合があるので、Moの含有量の上限は0.50%とする。好ましいMo含有量の上限は0.30%である。靭性及び耐SSC性の改善のために、Mo含有量の上限を0.20%、0.15%又は0.12%としてもよい。Moの含有量の下限を規定する必要はなく、その下限は0%である。Moによる母材の強度及び靭性を向上させる効果を得たい場合には、Moの含有量の下限を0.05%とした方が望ましい。必要に応じて、Moの含有量の下限を、0.06%又は0.07%としてもよい。 Mo: 0 to 0.50%
Mo can be contained as required. When Mo is contained, there is an effect of improving the strength and toughness of the base material. However, if the Mo content exceeds 0.50%, the hardness of the HAZ increases and the toughness and SSC resistance may be impaired, so the upper limit of the Mo content is 0.50%. The upper limit of the preferable Mo content is 0.30%. In order to improve toughness and SSC resistance, the upper limit of the Mo content may be 0.20%, 0.15%, or 0.12%. There is no need to define the lower limit of the Mo content, and the lower limit is 0%. In order to obtain the effect of improving the strength and toughness of the base material due to Mo, it is desirable to set the lower limit of the Mo content to 0.05%. If necessary, the lower limit of the Mo content may be 0.06% or 0.07%.
Vは、必要に応じて含有させることができる。Vを含有させると、主に焼戻し時の炭窒化物析出により母材の強度を向上させる効果がある。しかしながら、Vの含有量が0.10%を超えると、母材強度向上の効果が飽和するとともに、靭性劣化を招く場合があるので、Vの含有量の上限は0.10%とする。Vの含有量の下限を規定する必要はなく、その下限は0%である。靭性向上のために、V含有量の上限を、0.08%、0.06%又は0.04%としてもよい。なお、Vによる母材の強度を向上させる効果を得たい場合には、Vの含有量の下限を0.015%又は0.02%としてもよい。 V: 0 to 0.10%
V can be contained as required. Inclusion of V has an effect of improving the strength of the base material mainly due to carbonitride precipitation during tempering. However, if the V content exceeds 0.10%, the effect of improving the strength of the base metal is saturated and the toughness may be deteriorated. Therefore, the upper limit of the V content is 0.10%. There is no need to define the lower limit of the V content, and the lower limit is 0%. In order to improve toughness, the upper limit of the V content may be 0.08%, 0.06%, or 0.04%. In addition, when obtaining the effect of improving the strength of the base material due to V, the lower limit of the V content may be 0.015% or 0.02%.
Bは、必要に応じて含有させることができる。Bを含有させると、母材の強度を向上させる効果がある。しかしながら、Bの含有量が0.0050%を超えると、粗大な硼素化合物の析出を招いて靭性を劣化させる場合があるので、Bの含有量の上限は0.0050%とする。靭性の劣化を防止するために、B含有量の上限を0.0040%、0.0030%又は0.0020%としてもよい。Bの含有量の下限を規定する必要はなく、その下限は0%である。なお、Bによる母材の強度を向上させる効果を得たい場合には、Bの含有量の下限を0.0003%とすることが好ましい。より好ましいB含有量の下限は0.0005%又は0.0010%である。Bによる母材強度の向上効果が必要ない場合には、Bの含有量の上限を0.0010%、0.0005%、0.0003%又は0.0002%としても差し支えない。 B: 0 to 0.0050%
B can be contained as required. Inclusion of B has an effect of improving the strength of the base material. However, if the B content exceeds 0.0050%, precipitation of coarse boron compounds may be caused to deteriorate toughness, so the upper limit of the B content is set to 0.0050%. In order to prevent toughness deterioration, the upper limit of the B content may be 0.0040%, 0.0030%, or 0.0020%. There is no need to define the lower limit of the B content, and the lower limit is 0%. In addition, when obtaining the effect of improving the strength of the base material by B, it is preferable that the lower limit of the B content is 0.0003%. A more preferable lower limit of the B content is 0.0005% or 0.0010%. When the effect of improving the base material strength by B is not required, the upper limit of the B content may be 0.0010%, 0.0005%, 0.0003%, or 0.0002%.
Nbは、必要に応じて含有させることができる。Nbを含有させると、組織を微細化させて低温靭性を向上させる効果がある。しかしながら、Nbの含有量が0.10%を超えると、粗大な炭化物又は窒化物を形成し、靭性を低下させる場合があるので、Nbの含有量の上限は0.10%とする。Nbの含有量の下限を規定する必要はなく、その下限は0%である。靭性の低下を防止するために、Nb含有量の上限を0.08%、0.06%又は0.04%としてもよい。なお、Nbによる低温靭性を向上させる効果を得たい場合には、Nbの含有量の下限を0.01%又は0.02%としてもよい。 Nb: 0 to 0.10%
Nb can be contained as necessary. When Nb is contained, there is an effect of refining the structure and improving the low temperature toughness. However, if the Nb content exceeds 0.10%, coarse carbides or nitrides may be formed and the toughness may be lowered, so the upper limit of the Nb content is 0.10%. There is no need to define the lower limit of the Nb content, and the lower limit is 0%. In order to prevent a decrease in toughness, the upper limit of the Nb content may be 0.08%, 0.06%, or 0.04%. In addition, when acquiring the effect of improving the low temperature toughness by Nb, it is good also considering the minimum of content of Nb as 0.01% or 0.02%.
Tiは、必要に応じて含有させることができる。Tiは主に脱酸元素として利用するが、さらにAl、Ti、Mnを含む酸化物相を形成して組織を微細化する効果がある。しかしながら、Tiの含有量が0.10%を超えると、形成される酸化物がTi酸化物、あるいはTi-Al酸化物となって分散密度が低下し、特に小入熱溶接部熱影響部の組織を微細化する能力が失われる場合があるので、Tiの含有量の上限を0.10%とする。好ましいTiの含有量の上限は0.07%又は0.05%である。Tiの含有量の下限を規定する必要はなく、その下限は0%である。なお、Tiによる組織を微細化する効果を得たい場合には、Tiの含有量の下限を0.02%又は0.03%としてもよい。 Ti: 0 to 0.10%
Ti can be contained as required. Ti is mainly used as a deoxidizing element, but has the effect of further miniaturizing the structure by forming an oxide phase containing Al, Ti, and Mn. However, if the Ti content exceeds 0.10%, the oxide formed will be Ti oxide or Ti-Al oxide, and the dispersion density will decrease, especially in the heat-affected zone of the small heat input weld zone. Since the ability to refine the structure may be lost, the upper limit of the Ti content is set to 0.10%. The upper limit of the preferable Ti content is 0.07% or 0.05%. There is no need to define the lower limit of the Ti content, and the lower limit is 0%. In addition, when obtaining the effect of refining the structure by Ti, the lower limit of the Ti content may be 0.02% or 0.03%.
Snは、必要に応じて含有させることができる。Snを含有させると、Sn2+となって鋼材表面付着物に溶解し、酸性塩化物溶液中でのインヒビター作用により腐食を抑制する作用を有する。また、Fe3+を速やかに還元し、酸化剤としてのFe3+濃度を低減させる作用を有することにより、Fe3+の腐食促進作用を抑制するので、高飛来塩分環境における耐候性を向上させる。しかしながら、Snの含有量が0.50%を超えると、これらの効果は飽和するので、Snの含有量の上限は0.50%とする。好ましいSn含有量の上限は、0.20%である。合金コスト削減のために、Snの含有量の上限を0.10%、0.05%又は0.01%に制限してもよい。Snの含有量の下限を規定する必要はなく、その下限は0%である。なお、Snによる耐食性および耐候性効果を得たい場合には、Snの下限を0.03%又は0.05%としてもよい。 Sn: 0 to 0.50%
Sn can be contained as necessary. When Sn is contained, it becomes Sn 2+ and dissolves in the steel material surface deposit, and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly reducing the Fe 3+, by having an effect of reducing the Fe 3+ concentration of the oxidizing agent, since inhibit corrosion promoting effect of Fe 3+, thereby improving the weather resistance in high airborne salt environments. However, if the Sn content exceeds 0.50%, these effects are saturated, so the upper limit of the Sn content is 0.50%. The upper limit of preferable Sn content is 0.20%. In order to reduce the alloy cost, the upper limit of the Sn content may be limited to 0.10%, 0.05%, or 0.01%. There is no need to define the lower limit of the Sn content, and the lower limit is 0%. In addition, when obtaining the corrosion resistance and weather resistance effect by Sn, the lower limit of Sn may be set to 0.03% or 0.05%.
Caは、必要に応じて含有させることができる。Caを含有させると、鋼中のSと反応して溶鋼中で酸硫化物(オキシサルファイド)を形成する。この酸硫化物は、MnSなどと異なって圧延加工によって圧延方向に伸びることがないので、圧延後も球状である。この球状の酸硫化物は、延伸した介在物の先端などを割れの起点とする、溶接割れ又は水素誘起割れを抑制する効果がある。しかしながら、Caの含有量が0.004%を超えると、靭性の劣化を招くことがあるので、Caの含有量の上限は0.004%とする。靭性の低下を確実に避けるために、Caの含有量の上限を0.003%としてもよい。Caの含有量の下限を規定する必要はなく、その下限は0%である。なお、Caによる溶接割れ又は水素誘起割れを抑制する効果を得たい場合には、Caの含有量の下限を0.0003%又は0.0005%としてもよい。 Ca: 0 to 0.004%
Ca can be contained as needed. When Ca is contained, it reacts with S in the steel to form oxysulfide (oxysulfide) in the molten steel. Since this oxysulfide does not extend in the rolling direction by rolling unlike MnS and the like, it is spherical after rolling. This spherical oxysulfide has an effect of suppressing weld cracking or hydrogen-induced cracking starting from the tip of an elongated inclusion. However, if the Ca content exceeds 0.004%, the toughness may be deteriorated, so the upper limit of the Ca content is set to 0.004%. In order to surely avoid a decrease in toughness, the upper limit of the Ca content may be 0.003%. There is no need to define the lower limit of the Ca content, and the lower limit is 0%. In addition, when obtaining the effect of suppressing weld cracking or hydrogen-induced cracking due to Ca, the lower limit of the Ca content may be 0.0003% or 0.0005%.
Mgは、必要に応じて含有させることができる。Mgを含有させると、微細なMg含有酸化物が生成されるので、γ粒径の微細化に効果がある。しかしながら、Mgの含有量が0.0020%を超えると、酸化物が多くなりすぎて延性低下をもたらすことがあるので、Mgの含有量の上限は0.0020%とする。好ましいMgの含有量の上限は0.0010%である。Mgの含有量の下限を規定する必要はなく、その下限は0%である。なお、Mgによるγ粒径の微細化効果を得たい場合には、Mgの下限を0.0002%とすることが好ましい。より好ましいMg含有量の下限は0.0004%である。 Mg: 0 to 0.0020%
Mg can be contained as needed. When Mg is contained, a fine Mg-containing oxide is generated, which is effective in reducing the γ particle size. However, if the Mg content exceeds 0.0020%, the amount of oxides may increase and ductility may be lowered, so the upper limit of the Mg content is set to 0.0020%. The upper limit of the preferable Mg content is 0.0010%. There is no need to define the lower limit of the Mg content, and the lower limit is 0%. In order to obtain the effect of refining the γ particle diameter by Mg, the lower limit of Mg is preferably set to 0.0002%. A more preferable lower limit of the Mg content is 0.0004%.
REM(希土類元素)は、必要に応じて含有させることができる。REMは、鋼中に含有させることにより、溶接熱影響部の組織を微細化し、さらにSと結合してSを固定する効果がある。REMを過剰に含有させると、介在物が形成されて溶接部の清浄度が低下する場合があるが、REMの含有によって形成される介在物は比較的靭性劣化への影響が小さいので、REMの含有量が0.0020%以下であれば、REM含有による母材の靭性の低下は許容できる。したがって、REMの含有量の上限を0.0020%とする。好ましいREMの含有量の上限は0.0010%である。REMの含有量の下限を規定する必要はなく、その下限は0%である。なお、REMによる溶接熱影響部の組織の微細化効果とSの固定効果とを得たい場合には、REMの含有量の下限を0.0002%とすることが好ましい。より好ましいREM含有量の下限は0.0003%である。 REM: 0 to 0.0020%
REM (rare earth element) can be contained as required. When REM is contained in steel, it has the effect of refining the structure of the weld heat affected zone and further binding S to fix S. If REM is excessively contained, inclusions may be formed and the cleanliness of the weld may be lowered. However, inclusions formed by the inclusion of REM have a relatively small influence on toughness degradation. If the content is 0.0020% or less, a reduction in the toughness of the base material due to the inclusion of REM is acceptable. Therefore, the upper limit of the content of REM is set to 0.0020%. The upper limit of the preferable REM content is 0.0010%. There is no need to define the lower limit of the REM content, and the lower limit is 0%. In addition, when it is desired to obtain the effect of refining the structure of the weld heat affected zone by REM and the effect of fixing S, the lower limit of the REM content is preferably set to 0.0002%. A more preferable lower limit of the REM content is 0.0003%.
本実施形態に係る鋼材は、上記成分を含有し、残部が鉄および不純物を含む。しかしながら、本実施形態に係る溶接鋼材には、上記成分の他に、鋼材自体の強度、靭性等を一段と改善する目的で、あるいはスクラップ等の副原料からの不純物として、以下の合金元素を含有してもよい。
SbはHAZの靭性を損なうので、Sbの含有量の上限を0.03%としてもよい。HAZ靭性を向上させるために、Sbの含有量の上限を、0.01%、0.005%、0.003%又は0.001%としてもよい。
AsはHAZの靭性を損なうので、Asの含有量の上限を0.02%としてもよい。必要に応じて、Asの含有量の上限を、0.005%、0.003%又は0.001%としてもよい。
また、強度及び靭性の向上のために、Pb,Zr、Zn及びW含有量それぞれの上限を、0.1%、0.01%又は0.005%としてもよい。これら元素の含有量の下限を特に決める必要はなく、0%である。
Coは、Niの中に不純物として含まれる場合がある。CoはHAZ靭性を損なうので、Coの含有量の上限を、0.5%、0.3%、0.1%又は0.05%としてもよい。Co含有量の下限を特に決める必要はなく、その下限は0%である。 Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. The term “REM content” means the total content of these elements.
The steel material according to the present embodiment contains the above components, and the balance contains iron and impurities. However, the welded steel material according to the present embodiment contains the following alloy elements in addition to the above components for the purpose of further improving the strength, toughness, etc. of the steel material itself, or as impurities from secondary materials such as scrap. May be.
Since Sb impairs the toughness of HAZ, the upper limit of the Sb content may be 0.03%. In order to improve the HAZ toughness, the upper limit of the Sb content may be 0.01%, 0.005%, 0.003%, or 0.001%.
Since As impairs the toughness of HAZ, the upper limit of the As content may be 0.02%. If necessary, the upper limit of the As content may be 0.005%, 0.003%, or 0.001%.
Moreover, in order to improve strength and toughness, the upper limits of the Pb, Zr, Zn and W contents may be set to 0.1%, 0.01% or 0.005%. There is no particular need to determine the lower limit of the content of these elements, and it is 0%.
Co may be contained as an impurity in Ni. Since Co impairs HAZ toughness, the upper limit of the Co content may be 0.5%, 0.3%, 0.1%, or 0.05%. There is no particular need to determine the lower limit of the Co content, and the lower limit is 0%.
(B-1)板厚tの(1/4)t位置での残留γ量の下限が4.0体積%であること
鋼材中の残留γは、鋼材の脆性き裂伝ぱ停止特性の向上に寄与する。この結果、低温環境下での靭性の向上効果が期待できる。この効果を得るには、鋼材の板厚tの(1/4)t位置での残留γ量の下限が4.0体積%であることが必要である。靭性向上のために、残留γ量の下限を4.5%体積%、5.0体積%、5.5%体積%、6.0%体積%又は6.5体積%としてもよい。残留γ量の上限は特に規定するものではないが、残留γが多く存在しすぎると降伏強さが低下するおそれがあるので、残留γ量の上限は20.0体積%又は15.0体積%としてもよい。ここで、板厚tの(1/4)t位置で残留γ量を評価するのは、板厚全域の平均的な位置での評価をするためである。
ここで、焼戻し温度T(℃)が以下の式(3)を満足すると、板厚tの(1/4)t位置での残留γ量の下限を4.0体積%とすることができる。
3.8×Ni-33+Ac1≦T≦6.3×Ni-0.4+Ac1 ・・・(3)
ここで、Ac1は次の式(4)によって定義される。
Ac1=712+20.1×Si-17.8×Mn-19.1×Ni+11.9×Cr-9.8×Mo ・・・(4)
ここで、式中の元素記号は鋼材中の各元素の含有量(質量%)を表す。
図1は、表1に記載された鋼No.1の化学成分を有するスラブを950℃に加熱した後、850℃以下で70%の累積圧下率を達成する圧延を行い、圧延後直ちに常温まで水冷し、引き続き種々の焼戻し温度にて焼戻しを行い、その後水冷することにより製造された種々の鋼材における、焼戻し温度と残留γ量との関係を示すグラフである。ここで、累積圧下率とは、圧延開始時の板厚t1と圧延終了時の板厚t2との差を圧延開始時の板厚t1で除した値の百分率((t1-t2)/t1×100)である。図1に示されるように、焼戻し温度が低すぎると、γに逆変態する領域が少なすぎるので残留γ量が少なく、逆に焼戻し温度が高すぎると、生成したγが不安定化し冷却中にマルテンサイト変態するので残留γ量は少なくなる。従って、式(3)を満足することにより残留γを多く確保できることがわかる。 (B) Regarding the metal structure (B-1) The lower limit of the residual γ amount at the (1/4) t position of the sheet thickness t is 4.0% by volume. The residual γ in the steel material is a brittle crack in the steel material. Contributes to improved propagation stop characteristics. As a result, an effect of improving toughness in a low temperature environment can be expected. In order to obtain this effect, it is necessary that the lower limit of the residual γ amount at the (1/4) t position of the steel sheet thickness t is 4.0% by volume. In order to improve toughness, the lower limit of the residual γ amount may be 4.5% by volume, 5.0% by volume, 5.5% by volume, 6.0% by volume, or 6.5% by volume. The upper limit of the residual γ amount is not particularly specified, but if there is too much residual γ, the yield strength may decrease, so the upper limit of the residual γ amount is 20.0% by volume or 15.0% by volume. It is good. Here, the reason why the residual γ amount is evaluated at the (1/4) t position of the sheet thickness t is to perform evaluation at an average position in the entire sheet thickness.
Here, when the tempering temperature T (° C.) satisfies the following formula (3), the lower limit of the residual γ amount at the (1/4) t position of the plate thickness t can be set to 4.0 vol%.
3.8 × Ni−33 + Ac 1 ≦ T ≦ 6.3 × Ni−0.4 + Ac 1 (3)
Here, Ac 1 is defined by the following equation (4).
Ac 1 = 712 + 20.1 × Si−17.8 × Mn−19.1 × Ni + 11.9 × Cr−9.8 × Mo (4)
Here, the element symbol in a formula represents content (mass%) of each element in steel materials.
FIG. 1 shows the steel No. described in Table 1. A slab having one chemical component is heated to 950 ° C., then rolled to achieve a cumulative reduction of 70% at 850 ° C. or lower, immediately cooled to room temperature immediately after rolling, and subsequently tempered at various tempering temperatures. It is a graph which shows the relationship between tempering temperature and residual (gamma) amount in the various steel materials manufactured by water-cooling after that. Here, the cumulative rolling reduction is a percentage ((t1−t2) / t1 × a value obtained by dividing the difference between the plate thickness t1 at the start of rolling and the plate thickness t2 at the end of rolling by the plate thickness t1 at the start of rolling. 100). As shown in FIG. 1, if the tempering temperature is too low, there are too few regions to reversely transform to γ, so the amount of residual γ is small. Conversely, if the tempering temperature is too high, the generated γ becomes unstable and during cooling. Since it undergoes martensitic transformation, the amount of residual γ decreases. Therefore, it can be understood that a large amount of residual γ can be secured by satisfying the expression (3).
一般にα組織(フェライト組織)中の残留γは、準安定状態にあり、塑性変形を受けることでマルテンサイト変態しやすい。残留γは、脆性破壊発生特性又は伝ぱ停止特性の向上のためには分散している必要があり、地震を受けた後に消失する場合、所望の耐破壊特性が発揮されない。マクロな塑性変形付加量が一定であっても、残留γ粒子に付加される歪は残留γの分布形態によって大いに変化し、残留γ粒子がより微細で球形に近い形状であるほど、歪の分配率は低下する。したがって、断面観察により得られる残留γ粒子のアスペクト比の平均値の上限を2.5、かつ断面観察により得られる残留γ粒子の長径の平均値の上限を0.85μmとすることが必要である。残留γ粒子の平均アスペクト比が小さいほど靭性が向上するので、アスペクト比の平均値の上限を2.3又は2.0としてもよい。また、長径の平均値が小さいほど靭性が向上するので、長径の平均値の上限を0.80μm又は0.75μmとしてもよい。長径の平均値の下限は、規定する必要はないが、通常0.05μmとなる。 (B-2) The upper limit of the average value of the aspect ratio of residual γ is 2.5 and the upper limit of the average value of the major axis is 0.85 μm. Generally, the residual γ in the α structure (ferrite structure) is a metastable state. And easily undergoes martensitic transformation by undergoing plastic deformation. Residual γ needs to be dispersed in order to improve the brittle fracture occurrence characteristics or propagation stop characteristics, and when it disappears after an earthquake, the desired fracture resistance characteristics are not exhibited. Even if the macro plastic deformation addition amount is constant, the strain applied to the residual γ particles varies greatly depending on the distribution pattern of the residual γ particles, and the more fine the residual γ particles are in a spherical shape, the more the strain is distributed. The rate drops. Therefore, it is necessary to set the upper limit of the average value of the aspect ratio of the residual γ particles obtained by cross-sectional observation to 2.5 and the upper limit of the average value of the major axis of the residual γ particles obtained by cross-sectional observation to 0.85 μm. . Since the toughness is improved as the average aspect ratio of the residual γ particles is smaller, the upper limit of the average aspect ratio may be 2.3 or 2.0. Moreover, since the toughness improves as the average value of the major axis is smaller, the upper limit of the average value of the major axis may be 0.80 μm or 0.75 μm. The lower limit of the average value of the major axis need not be specified, but is usually 0.05 μm.
[Mn]retained γ>[Mn]α×1.4 ・・・(1)
[Ni]retained γ>[Ni]α×1.4 ・・・(2)
ここで、[Mn]retained γ:残留γ中の平均Mn濃度、[Mn]α:フェライト相中の平均Mn濃度、[Ni]retained γ:残留γ中の平均Ni濃度、[Ni]α:フェライト相中の平均Ni濃度を、それぞれ表す。
オーステナイトフォーマーであるNiおよびMnは、γ→α変態点を下げる元素であり、残留γを安定化させる作用を有することが知られている。塑性歪を受けた後に残留γ量を多く確保するためには、個々の残留γ中のMn濃度の下限およびNi濃度の下限を、それぞれ、フェライト相中におけるMn濃度およびNi濃度に対して1.4倍にすることが極めて重要である。
上述の式(1)及び式(2)を満足するためには、熱間圧延工程にて850℃以下の累積圧下率の下限を50%とし、焼戻し後の冷却速度を0.5℃/sより大きくすることが必要である。図2は、表1に記載された鋼No.1の化学成分を有するスラブを960℃に加熱した後、種々の累積圧下率にて圧延を行い、圧延後直ちに常温まで水冷し、引き続き570℃にて焼戻し(焼戻し後の水冷あり)を行うことにより製造された種々の鋼材における、850℃以下の累積圧下率とNi及びMnの濃化率([M]γ/[M]α)との関係を示すグラフである。ここで、Ni及びMnの濃化率とは、[Mn]retained γおよび[Ni]retained γを[Mn]αおよび[Ni]αでそれぞれ除することにより得られる値である。図2から、特に累積圧下率の下限を50%とすることにより、1.4以上の濃度比が得られ、式(1)及び式(2)を満足できることがわかる。
Ni及びMnの濃化率と、代表的な破壊特性評価パラメータであるDT(Dynamic Tear)エネルギーとの関係を図3に示す。DTエネルギーが高い場合、アレスト特性が良いと判断される。LPG又はLNGなどの液化ガスを貯蔵する極低温貯槽タンクを製造するための鋼材では、DTエネルギーが1500Jを上回ることが好ましい。図3から、Ni及びMnの濃化率の下限をいずれも1.4とすることにより、DTエネルギーが1500Jを上回ることがわかる。これらの濃化率の下限を1.5又は1.6とすると、更に高いDTエネルギーを得ることができ、好ましい。
Ni及びMnの濃化率の上限を特に規定する必要はない。しかし、Ni及びMnの濃化率が10又は5を超えることは殆どないので、それらの上限を10又は5としてもよい。 (B-3) The average Mn concentration and the average Ni concentration in the residual γ satisfy the following equations (1) and (2), respectively: [Mn] retained γ > [Mn] α × 1.4・ (1)
[Ni] retained γ > [Ni] α × 1.4 (2)
Here, [Mn] retained γ : average Mn concentration in residual γ, [Mn] α : average Mn concentration in ferrite phase, [Ni] retained γ : average Ni concentration in residual γ, [Ni] α : ferrite The average Ni concentration in the phase is represented respectively.
Ni and Mn, which are austenite formers, are elements that lower the γ → α transformation point and are known to have the effect of stabilizing residual γ. In order to secure a large amount of residual γ after being subjected to plastic strain, the lower limit of Mn concentration and the lower limit of Ni concentration in each residual γ are set to 1. It is extremely important to make it 4 times.
In order to satisfy the above-mentioned formulas (1) and (2), the lower limit of the cumulative reduction rate of 850 ° C. or less in the hot rolling process is set to 50%, and the cooling rate after tempering is 0.5 ° C./s. It is necessary to make it larger. 2 shows the steel No. described in Table 1. A slab having one chemical component is heated to 960 ° C., then rolled at various cumulative reduction rates, immediately cooled to room temperature immediately after rolling, and subsequently tempered at 570 ° C. (with water cooling after tempering). 5 is a graph showing the relationship between the cumulative reduction ratio of 850 ° C. or less and the concentration ratios of Ni and Mn ([M] γ / [M] α ) in various steel materials manufactured by the above. Here, the concentration ratios of Ni and Mn are values obtained by dividing [Mn] retained γ and [Ni] retained γ by [Mn] α and [Ni] α , respectively. From FIG. 2, it can be seen that a concentration ratio of 1.4 or more can be obtained by satisfying the expressions (1) and (2) by setting the lower limit of the cumulative rolling reduction to 50%.
FIG. 3 shows the relationship between the concentration ratios of Ni and Mn and DT (Dynamic Tear) energy, which is a typical fracture characteristic evaluation parameter. When the DT energy is high, it is determined that the arrest characteristic is good. In a steel material for producing a cryogenic storage tank for storing a liquefied gas such as LPG or LNG, it is preferable that the DT energy exceeds 1500 J. FIG. 3 shows that the DT energy exceeds 1500 J by setting the lower limit of the concentration ratios of Ni and Mn to 1.4. When the lower limit of the concentration ratio is 1.5 or 1.6, a higher DT energy can be obtained, which is preferable.
There is no need to particularly define the upper limit of the concentration ratio of Ni and Mn. However, since the concentration ratios of Ni and Mn rarely exceed 10 or 5, their upper limit may be 10 or 5.
本実施形態に係る鋼材は、以下に示す工程を経て製造することができる。ただし、以下の製造方法に限定されるものではない。 (C) Regarding production method The steel material according to the present embodiment can be produced through the following steps. However, it is not limited to the following manufacturing method.
加熱工程は、スラブの加熱温度を920℃~980℃にコントロールする。AlNの固溶を進め、後続の熱処理時に結晶粒の粗大化を抑制し所望の耐破壊特性を得るためには、スラブの加熱温度の下限を920℃とするのがよい。また、γ粒が粗大化しすぎず、耐破壊特性が損なわれないようにするためには、スラブの加熱温度の上限を970℃とする。 (C-1) Slab Heating Step In the heating step, the slab heating temperature is controlled to 920 ° C. to 980 ° C. In order to advance the solid solution of AlN and suppress the coarsening of the crystal grains during the subsequent heat treatment to obtain the desired fracture resistance, the lower limit of the slab heating temperature is preferably 920 ° C. Further, the upper limit of the heating temperature of the slab is set to 970 ° C. so that the γ grains are not excessively coarsened and the fracture resistance is not impaired.
熱間圧延工程では、加熱したスラブの圧延を行う。具体的には、粗圧延と仕上圧延とに分けて圧延すればよい。 (C-2) Rolling process In the hot rolling process, the heated slab is rolled. Specifically, the rolling may be divided into rough rolling and finish rolling.
冷却工程では、仕上圧延をした圧延後の鋼材を加速冷却することが望ましい。特に、板厚が大きくなるほど鋼材の靭性確保が困難となるので、板厚が厚い鋼材では、圧延工程後の加速冷却の冷却速度は速い方がよい。具体的には、板厚15mm以下の場合、鋼材の板厚tの中心部、すなわち、板厚tの(1/2)t位置での冷却速度の下限を3℃/sとする。板厚15mm超の場合、冷却速度の下限を10℃/sとする。これは圧延工程後の加速冷却時の冷却速度が遅くなることにより、最終組織の有効結晶粒径が粗大化することを防ぐためである。板厚tの(1/2)t位置での冷却速度の上限は特に規定されないが、設備能力を考慮して、50℃/sとしてもよい。加速冷却を行う場合、鋼材の金属組織を十分な焼入れ組織とし、その後の焼戻し処理等により微細な残留γと1.4以上の濃化比とを得るために、冷却開始温度の下限を660℃とする。 (C-3) Cooling step In the cooling step, it is desirable to accelerate cooling the steel material after finish rolling. In particular, as the plate thickness increases, it becomes more difficult to ensure the toughness of the steel material. Therefore, in a steel material with a thick plate thickness, it is better that the cooling rate of the accelerated cooling after the rolling process is faster. Specifically, when the plate thickness is 15 mm or less, the lower limit of the cooling rate at the central portion of the steel plate thickness t, that is, at the (1/2) t position of the plate thickness t is 3 ° C./s. When the plate thickness exceeds 15 mm, the lower limit of the cooling rate is 10 ° C./s. This is to prevent the effective crystal grain size of the final structure from becoming coarse due to the slow cooling rate during accelerated cooling after the rolling process. The upper limit of the cooling rate at the (1/2) t position of the plate thickness t is not particularly defined, but may be 50 ° C./s in consideration of the facility capacity. In the case of accelerated cooling, the lower limit of the cooling start temperature is set to 660 ° C. in order to make the steel metal structure sufficiently quenched and to obtain a fine residual γ and a concentration ratio of 1.4 or more by subsequent tempering treatment or the like. And
十分な焼入れ組織が得られた場合には、従来から9%Ni鋼で多く用いられてきたL処理(鋼材をフェライト及びオーステナイトの二相域の温度範囲に加熱した後に水冷処理すること)は、本実施形態にて必ずしも実施する必要はなく、焼戻し処理のみを行うことで、充分な特性を示す鋼材を得ることができる。ただし、鋼材をフェライト及びオーステナイトの二相域温度に加熱すると、金属組織の微細化、および安定なオーステナイト相の生成により靭性を改善できるので、必要に応じて、620℃~720℃の温度範囲に加熱した後に水冷処理をするL処理工程を実施してもよい。加熱温度の下限を620℃とすることによって残留γの増加を見込むことができ、加熱温度の上限を720℃とすることによって組織の粗大化を防止できる。L処理工程における加熱の温度範囲は、好ましくは640℃~700℃である。 (C-4) L treatment process When a sufficiently hardened structure is obtained, the L treatment (steel material was heated to a temperature range of two phases of ferrite and austenite, which has been conventionally used in 9% Ni steel) It is not always necessary to perform the water-cooling process later in this embodiment, and a steel material exhibiting sufficient characteristics can be obtained by performing only the tempering process. However, if the steel material is heated to a two-phase temperature range of ferrite and austenite, the toughness can be improved by refining the metal structure and generating a stable austenite phase, so that the temperature ranges from 620 ° C to 720 ° C as necessary. You may implement the L process process which performs a water cooling process after heating. By setting the lower limit of the heating temperature to 620 ° C., an increase in residual γ can be expected, and by setting the upper limit of the heating temperature to 720 ° C., coarsening of the structure can be prevented. The temperature range of heating in the L treatment step is preferably 640 ° C to 700 ° C.
焼戻し工程は、本発明を実現するために極めて重要であり、詳細な制御が必要な必須プロセスである。焼戻し温度が低すぎる場合には、生成するγの量が不足するので、残留γ量自体が少なくなる。また、焼戻し温度が低すぎる場合には、焼戻し脆化が起こる可能性があり、これにより耐破壊特性を損なう結果となってしまう。逆に焼戻し温度が高すぎる場合には、加熱時のγ量は多くなるものの、残留γ中のNiおよびMn濃度が低下する。この場合残留γは、その後の冷却時に多くが変態してしまうか、あるいは冷却時に変態が生じなかったとしても極低温にさらされるだけで変態し、消失してしまうことになる。焼戻し温度の範囲は、熱力学的な平衡挙動が支配するものであり、鋼材の化学成分によって変動する性質を持つ。具体的には、焼戻し温度Tの下限を3.8×Ni-33+Ac1とし、そして、上限を6.3×Ni-0.4+Ac1とすることが必要である。すなわち、次の式(3)を満足する必要がある。ここで、式(3)に記載されている係数は、実験結果の重回帰により求められた。焼戻し温度は、Ac1超とすることが好ましい。
また、焼戻し工程の加熱後の冷却速度が遅い場合には、炭素の拡散移動による一部ベイナイト変態が進行してしまうなどの理由によって、耐破壊特性が損なわれる。加えて、焼戻し工程の冷却速度が遅い場合には、焼戻し中に生成したγからセメンタイトとして一部の炭素を吐き出す反応が進むことによりγが不安定化し、室温に冷却した後に、総量としての残留γが減少する傾向を示すと考えられる。さらに、焼き戻し中に生成したγの一部は、焼き戻し後の冷却の間にマルテンサイト変態を起こす。冷却速度の下限を0.5℃/sにすることにより、このマルテンサイト変態により生じたマルテンサイト内の転位密度を増加させることができ、さらに、このマルテンサイトに隣接した残留γに力学的拘束作用を加えて残留γの安定性を向上させることができると推定できる。したがって、焼戻し工程の加熱の実施後、表面温度が300℃以下になるまでの板厚中心部における冷却速度の下限を0.5℃/sとする必要がある。焼戻し工程の加熱後の冷却速度の上限は特に規定されないが、設備能力の上限を考慮すると、50℃/sとしてもよい。冷却停止温度の下限は特に規定されないが、設備能力を考慮して、50℃又は室温としてもよい。
3.8×Ni-33+Ac1≦T≦6.3×Ni-0.4+Ac1 ・・・(3)
ここで、Ac1は次の式(4)によって定義される。
Ac1=712+20.1×Si-17.8×Mn-19.1×Ni+11.9×Cr-9.8×Mo ・・・(4)
ここで、式中の元素記号は鋼材中の各元素の含有量(質量%)を表す。
なお、特許文献1においてVノッチシャルピー吸収エネルギーvE-196が良好な鋼板など(特に、Test No.1-a~1-h、4~12、22~35)のDT試験およびプレクラックシャルピー試験が低下した原因は、いずれも焼戻し後に加速冷却を行わなかったから、又は冷却速度を0.5℃/s未満としたからである。 (C-5) Tempering Step The tempering step is extremely important for realizing the present invention, and is an essential process that requires detailed control. If the tempering temperature is too low, the amount of γ produced is insufficient, and the amount of residual γ itself decreases. In addition, if the tempering temperature is too low, temper embrittlement may occur, resulting in a deterioration of the fracture resistance. Conversely, when the tempering temperature is too high, the amount of γ during heating increases, but the concentration of Ni and Mn in the residual γ decreases. In this case, a large amount of residual γ is transformed during subsequent cooling, or even if transformation does not occur during cooling, it is transformed and disappears only by being exposed to a very low temperature. The range of the tempering temperature is governed by the thermodynamic equilibrium behavior, and has a property that varies depending on the chemical composition of the steel material. Specifically, it is necessary that the lower limit of the tempering temperature T is 3.8 × Ni−33 + Ac 1 and the upper limit is 6.3 × Ni−0.4 + Ac 1 . That is, it is necessary to satisfy the following expression (3). Here, the coefficient described in Formula (3) was calculated | required by multiple regression of the experimental result. The tempering temperature is preferably more than Ac 1 .
In addition, when the cooling rate after heating in the tempering process is low, the fracture resistance is impaired due to the reason that a partial bainite transformation proceeds due to the diffusion movement of carbon. In addition, when the cooling rate of the tempering process is slow, γ is destabilized by the reaction of exhaling part of carbon from γ generated during tempering as cementite, and after cooling to room temperature, the total amount remains It is considered that γ tends to decrease. Furthermore, a part of γ generated during tempering causes martensitic transformation during cooling after tempering. By setting the lower limit of the cooling rate to 0.5 ° C./s, it is possible to increase the dislocation density in the martensite caused by this martensite transformation, and further to mechanically restrain the residual γ adjacent to this martensite. It can be estimated that the stability of the residual γ can be improved by adding an action. Therefore, after the heating in the tempering step, the lower limit of the cooling rate in the central portion of the plate thickness until the surface temperature becomes 300 ° C. or lower needs to be 0.5 ° C./s. Although the upper limit of the cooling rate after heating in the tempering step is not particularly defined, it may be 50 ° C./s in consideration of the upper limit of the equipment capacity. The lower limit of the cooling stop temperature is not particularly defined, but may be 50 ° C. or room temperature in consideration of the facility capacity.
3.8 × Ni−33 + Ac 1 ≦ T ≦ 6.3 × Ni−0.4 + Ac 1 (3)
Here, Ac 1 is defined by the following equation (4).
Ac 1 = 712 + 20.1 × Si−17.8 × Mn−19.1 × Ni + 11.9 × Cr−9.8 × Mo (4)
Here, the element symbol in a formula represents content (mass%) of each element in steel materials.
In
[Mn]retained γ>[Mn]α×1.4 ・・・(1)
[Ni]retained γ>[Ni]α×1.4 ・・・(2)
ここで、[Mn]retained γ:残留γ中の平均Mn濃度、[Mn]α:フェライト相中の平均Mn濃度、[Ni]retained γ:残留γ中の平均Ni濃度、[Ni]α:フェライト相中の平均Ni濃度を、それぞれ表す。 The evaluation method of the residual γ amount is as follows. A test piece for residual γ measurement was taken from a (1/4) t position of the thickness t of the steel material, and the residual γ amount (% by volume) was measured by X-ray diffraction. The measured cross section was an L cross section (a plane parallel to the rolling direction and perpendicular to the steel sheet surface). Furthermore, the shape of residual γ was evaluated by thin film observation with a transmission electron microscope. 20 or more residual γ particles were observed, the average aspect ratio of these particle samples and the average size of the major axis were measured, and the average value in the sample was calculated. Further, the concentration of Mn and Ni in the residual γ was evaluated by the following method. The average Mn concentration and the average Ni concentration in the residual γ were measured by EDX (Energy Dispersive X-ray spectroscopy) quantitative analysis, and compared with the average Mn concentration and the average Ni concentration in the ferrite phase, respectively. ) And Formula (2) are evaluated. The average Mn concentration and the average Ni concentration in the ferrite phase were the bulk values (chemical analysis results) of the steel material.
[Mn] retained γ > [Mn] α × 1.4 (1)
[Ni] retained γ > [Ni] α × 1.4 (2)
Here, [Mn] retained γ : average Mn concentration in residual γ, [Mn] α : average Mn concentration in ferrite phase, [Ni] retained γ : average Ni concentration in residual γ, [Ni] α : ferrite The average Ni concentration in the phase is represented respectively.
Test No.1-cの鋼材、Test No.1-dの鋼材、Test No.1-eの鋼材、焼戻し温度が規定範囲を下回るTest No.1-fの鋼材、L処理温度が規定範囲を上回るTest No.1-hの鋼材、加熱温度が規定範囲を上回るTest No.1-iの鋼材、Test No.1-jの鋼材、成品厚さ製品の板厚に対する粗圧延終了後のスラブ厚さが規定範囲を下回るTest No.1-kの鋼材、Test No.1-lの鋼材、焼戻し温度が規定範囲を上回るTest No.1-mの鋼材、L処理温度が規定範囲を下回るTest No.1-nの鋼材、オフラインQTを行ったTest No.1-oの鋼材、オフラインQLTを行ったTest No.1-pの鋼材は、脆性き裂発生特性(単位面積当たりのVノッチシャルピー吸収エネルギーvE-196)が不足した。
850℃以下の累積圧下率が規定範囲を下回るTest No.1-bの鋼材、並びにTest No.1-c~1-f、Test No.1-h、Test No.1-i、及びTest No.1-k~Test No.1-pの鋼材は、アレスト特性(DTの吸収エネルギー:DT-196、またはプレクラックシャルピー試験によって得られる-196℃での吸収エネルギー)が不足した。 On the other hand, Test No. in which the cooling rate after rolling falls below the specified range. Test No. 1-c steel, the water cooling stop temperature after rolling exceeds the specified range. Test No. 1-d steel, the cooling rate after tempering falls below the specified range. Steel No. 1-e, Test No. with heating temperature below specified range. Test No. 1 in which the finish rolling temperature was lowered due to the slab thickness after completion of the rough rolling with respect to the steel thickness of 1-j and the product thickness product exceeding the specified range. The 1-l steel material lacked strength (yield strength YS, tensile strength TS).
Test No. 1-c steel, Test No. 1-d steel, Test No. Steel No. 1-e, Test No. with tempering temperature below specified range. 1-f steel, Test No. L processing temperature exceeds specified range. 1-h steel, Test No. with heating temperature exceeding specified range. 1-i steel, Test No. Test No. 1 in which the slab thickness after the rough rolling with respect to the steel thickness of the 1-j steel product and the product thickness product is less than the specified range. 1-k steel, Test No. 1-l steel, Test No. with tempering temperature exceeding specified range. Test No. 1-m steel, L treatment temperature is below specified range. 1-n steel, Test No. in which offline QT was performed. No. 1-o steel, Test No. in which offline QLT was performed. The steel material of 1-p lacked brittle crack initiation characteristics (V-notch Charpy absorbed energy v E -196 per unit area).
Test No. in which the cumulative rolling reduction at 850 ° C. or lower is below the specified range. 1-b steel, and Test No. 1-c to 1-f, Test No. 1-h, Test No. 1 1-i and Test No. 1-k to Test No. The 1-p steel material lacked arrest properties (DT absorbed energy: DT- 196 or absorbed energy at -196 ° C. obtained by a precrack Charpy test).
Claims (6)
- 化学成分が、質量%で、
C:0.01~0.12%、
Si:0.01~0.30%、
Mn:0.4~2.0%、
Ni:6.6~8.0%、
Al:0.002~0.08%
N:0.0050%以下、
P:0.05%以下、
S:0.008%以下、
Cu:0~1.00%、
Cr:0~1.00%、
Mo:0~0.50%、
V:0~0.10%、
B:0~0.0050%、
Nb:0~0.10%、
Ti:0~0.10%、
Sn:0~0.50%、
Ca:0~0.004%、
Mg:0~0.0020%、
REM:0~0.0020%、
残部:Feおよび不純物
である鋼材であって、
前記鋼材の板厚tの(1/4)t位置での残留γの量の下限が4.0体積%であり、
前記残留γは、そのアスペクト比の平均値の上限が2.5かつその長径の平均値の上限が0.85μmである形態を有するとともに、前記残留γ中の平均Mn濃度および平均Ni濃度がそれぞれ、次の式(1)および式(2)を満足することを特徴とする鋼材。
[Mn]retained γ>[Mn]α×1.4 ・・・(1)
[Ni]retained γ>[Ni]α×1.4 ・・・(2)
ここで、[Mn]retained γは前記残留γ中の平均Mn濃度を、[Mn]αはフェライト相中の平均Mn濃度を、[Ni]retained γは前記残留γ中の平均Ni濃度を、[Ni]αは前記フェライト相中の平均Ni濃度を、それぞれ表す。 Chemical composition is mass%,
C: 0.01 to 0.12%,
Si: 0.01 to 0.30%,
Mn: 0.4 to 2.0%,
Ni: 6.6 to 8.0%,
Al: 0.002 to 0.08%
N: 0.0050% or less,
P: 0.05% or less,
S: 0.008% or less,
Cu: 0 to 1.00%,
Cr: 0 to 1.00%,
Mo: 0 to 0.50%,
V: 0 to 0.10%,
B: 0 to 0.0050%,
Nb: 0 to 0.10%,
Ti: 0 to 0.10%,
Sn: 0 to 0.50%,
Ca: 0 to 0.004%,
Mg: 0 to 0.0020%,
REM: 0 to 0.0020%,
The balance: steel that is Fe and impurities,
The lower limit of the amount of residual γ at the (1/4) t position of the steel sheet thickness t is 4.0% by volume,
The residual γ has a form in which the upper limit of the average value of the aspect ratio is 2.5 and the upper limit of the average value of the major axis is 0.85 μm, and the average Mn concentration and the average Ni concentration in the residual γ are respectively A steel material characterized by satisfying the following formulas (1) and (2):
[Mn] retained γ > [Mn] α × 1.4 (1)
[Ni] retained γ > [Ni] α × 1.4 (2)
Here, [Mn] retained γ is an average Mn concentration in the residual γ, [Mn] α is an average Mn concentration in the ferrite phase, [Ni] retained γ is an average Ni concentration in the residual γ, Ni] α represents the average Ni concentration in the ferrite phase. - さらに、前記化学成分が、質量%で
C:0.02%~0.07%
Si:0.01%~0.10%、
Mn:0.6%~1.0%
Ni:7.0~7.8%
Cu:0~0.30%、
Cr:0~0.80%、
Mo:0~0.20%、
V:0~0.05%、
B:0~0.0005%、
Nb:0~0.02%、
Ti:0~0.02%、
Sn:0~0.01%であることを特徴とする請求項1に記載の鋼材。 Further, the chemical component is in mass% C: 0.02% to 0.07%
Si: 0.01% to 0.10%,
Mn: 0.6% to 1.0%
Ni: 7.0 to 7.8%
Cu: 0 to 0.30%,
Cr: 0 to 0.80%,
Mo: 0 to 0.20%,
V: 0 to 0.05%,
B: 0 to 0.0005%,
Nb: 0 to 0.02%,
Ti: 0 to 0.02%,
The steel material according to claim 1, wherein Sn: 0 to 0.01%. - さらに、前記化学成分が、質量%で
Cr:0.30~0.60%、
Mo:0.05~0.15%であることを特徴とする請求項1または2に記載の鋼材。 Further, the chemical component is Cr: 0.30-0.60% by mass,
The steel material according to claim 1 or 2, wherein Mo: 0.05 to 0.15%. - 前記鋼材は、板厚が3mm~100mmの鋼板であり、降伏強さの下限が585MPaであり、引張強さが690MPa~885MPaであることを特徴とする請求項1~3のいずれか一項に記載の鋼材。 The steel material is a steel plate having a thickness of 3 mm to 100 mm, a lower limit of yield strength is 585 MPa, and a tensile strength is 690 MPa to 885 MPa. Steel materials described.
- 請求項1~4のいずれか一項に記載の鋼材が内槽部材に適用されたことを特徴とするLNGタンク。 An LNG tank, wherein the steel material according to any one of claims 1 to 4 is applied to an inner tank member.
- 請求項1~4のいずれか一項に記載の鋼材がアニュラープレートに適用されたことを特徴とするLNGタンク。 An LNG tank, wherein the steel material according to any one of claims 1 to 4 is applied to an annular plate.
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Also Published As
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JPWO2014203347A1 (en) | 2017-02-23 |
KR101572786B1 (en) | 2015-11-27 |
JP5561442B1 (en) | 2014-07-30 |
KR20150020258A (en) | 2015-02-25 |
CN104520461B (en) | 2016-06-15 |
EP2871255A4 (en) | 2016-01-06 |
EP2871255A1 (en) | 2015-05-13 |
EP2871255B1 (en) | 2017-05-03 |
CN104520461A (en) | 2015-04-15 |
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