JP2013014811A - Steel material for very low temperature use and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for very low temperature use, which is excellent in fracture toughness; to provide a method for manufacturing the steel material; and to provide an LNG tank to which the steel material is applied.SOLUTION: The steel material for very low temperature includes, by mass, 0.01-0.12% of C, 0.01-0.3% of Si, 0.4-2.0% of Mn, ≤0.05% of P, ≤0.008% of S, >5.0 and <10.0% of Ni, 0.002-0.08% of Al, 0.0015-0.0040% of N, and the balance Fe with impurities, and is characterized in that the residual γ quantity at a position of (1/4)t of the plate thickness t is ≥3.0 vol.%, the value represented by formula (1) below is ≥1.3, and the reduction rate of the residual γ quantity is ≤25% when the steel material is subjected to a plastic strain of 1% under an environment of -165°C. (1): σ/σ, wherein, σrepresents yield strength [MPa] at -165°C, and σrepresents yield strength [MPa] at room temperature.

Description

  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. The term “for cryogenic use” means an application in a cryogenic environment of −60 ° C. or lower. The present invention is mainly intended for use in an LNG temperature environment of −165 ° C. in particular.

  Steel materials for producing cryogenic storage tanks for storing liquefied gases such as LPG and LNG are required to have excellent fracture toughness in terms of ensuring safety. In other words, in addition to the property that brittle fracture does not easily occur even at extremely low temperatures, in order to prevent the entire structure from collapsing even if a brittle crack occurs, the property that stops crack propagation ( It is required to have excellent arrest characteristics.

  Conventionally, steel containing about 9% Ni (9% Ni steel) has been used as a steel material used for the LNG tank. The 9% Ni steel is required to have properties such as brittle fracture propagation stop characteristics (hereinafter referred to as arrest characteristics) of the base material and the welded joint at the LNG temperature (−165 ° C.). In order to give such 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) Various improvements have been made.

  Furthermore, as seen in Patent Documents 1 to 3, steel materials having a performance equivalent to that of 9% Ni steel have been developed with a lower Ni content than 9% Ni steel due to the influence of the raw material height in recent years.

WO 2007/34576 WO 2007/80645 WO 2007/80646

  As described above, steel materials for producing cryogenic storage tanks that store liquefied gases such as LPG and LNG are required to have excellent fracture toughness in terms of ensuring safety. In particular, assuming that a ground type LNG tank is to be constructed in an earthquake-prone country such as Japan, the tank is required to be sound even after an earthquake. According to the LNG ground-type storage tank guidelines (JGA Finger-108-02, Japan Gas Association Gas Works Technical Standards Investigation Committee), as the target performance of the tank inner tank, level 2 earthquake motion (expected range) It is said that even if it is subjected to earthquakes, deformation remains, but liquid-tightness and confidentiality are maintained. That is, the inner tank material of the tank cannot tolerate breakage that penetrates the plate thickness. Further, when an external force as large as Level 2 earthquake motion is applied, it is possible to assume that the inner tank material and the annular plate are subjected to a large plastic deformation, and this extremely high level of characteristics is required as the anti-destructive performance.

  The present invention meets such a demand, and is a steel material for cryogenic temperature that has excellent fracture toughness so that the liquid tightness and confidentiality of the inner tank of the tank can be maintained even when subjected to level 2 earthquake motion. An object of the present invention is to provide an LNG tank to which the steel material is applied and the steel material.

  In order to solve the above-mentioned problems, the present inventors have recognized that it is extremely important to aim at a rational manufacturing method while maintaining the characteristics required for manufacturing a cryogenic steel material. We decided to clarify the fracture resistance that we wanted to secure, and to clarify the parameters on the metal structure that affected those characteristics. By clarifying the target metallographic structure, it becomes possible to freely design the minimum manufacturing method necessary to achieve them, and finally, a rational manufacturing method can be established. .

  The present inventors are required to perform a wide range of trial production tests within a range of chemical components considered necessary as a premise of the application, that is, within a range of Ni content of 5 to 10% particularly effective for low temperature toughness. Comparison with characteristics was conducted. As a result, the following findings (a) to (f) were obtained.

  (a) The characteristics required to maintain liquid-tightness and confidentiality when the cryogenic tank material is subjected to seismic force are (i) brittle fracture initiation characteristics and (ii) brittle crack propagation arrest characteristics. (Arrest characteristics).

  (b) In order to improve these properties, it is necessary to secure a structure of residual γ (austenite) in the steel material at 3.0% by volume or more under a specific range of components. However, the absolute amount needs to be secured. Since the residual γ structure has a very high brittle crack propagation stopping function, the brittle crack propagation stopping property of the material is remarkably enhanced by the dispersion of the residual γ structure. Here, when the residual γ is measured at a position (1/4) t of the thickness t of the steel material, evaluation at an average position in the entire thickness can be obtained. The content of residual γ can be evaluated by an X-ray diffraction method.

(c) In order to keep the amount of plastic deformation as small as possible during an earthquake, it is necessary to maintain the yield strength at a very low temperature, which is the operating temperature. It is effective if the yield strength at a cryogenic temperature is 1.3 times or more the yield strength at room temperature. That is, the value indicated by the following equation (1) may be 1.3 or more.
σ _{y, −165 ° C./σ y, RT} (1) equation where σ _{y, −165 ° C.} is the yield strength [MPa] at −165 ° C., and σ _{y, RT} is the normal temperature Yield strength [MPa] is shown respectively.

  (d) In order to achieve both brittle fracture initiation characteristics and brittle crack propagation stopping characteristics, the reduction rate of the residual γ amount when subjected to 1% plastic strain in an environment of −165 ° C. should be 25% or less. That's fine. In order to leave much of the structure even if the residual γ undergoes plastic deformation, it is desirable that the geometric shape of the residual γ be close to a sphere. Specifically, it is desirable that the aspect ratio is 3.5 or less as a quantity rule.

(e) In addition, the manufacturing method for making the steel material having these structures is not particularly limited, but by appropriately managing the heating conditions and rolling conditions, the occupation time of the heating furnace is shortened. Control is possible. Shortening the heating time is important not only for economic efficiency but also for reducing greenhouse gas emissions. That is, it is desirable to heat the steel ingot at as low a temperature as possible, and it is desirable that the heating time be short. When the heating temperature is low, the heating time can be allowed even for a long time. Conversely, when the heating temperature is high, the long heating time is not allowed because the structure becomes coarse. As a result of various experiments, the inventors have found that the heating temperature Tr (° C.) and the heating time t (hr) of the steel ingot only have to satisfy the following expressions (2) and (3). It was.
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.

(f) Further, it is desirable that the rolling surface has a large rolling reduction in the final finishing pass. Thereby, refinement | miniaturization of steel material structure is achieved. The present inventors have found that the reduction amount can be defined as the shape ratio. That is, the steel ingot may be rolled at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the following expression (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.

  The present invention has been made on the basis of the above knowledge, and the following (1) to (6) cryogenic thick steel plate and (7) to (10) a method for producing a cryogenic steel material and (11 )-(12) LNG tank to which the steel material is applied.

(1) By mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.3%, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008% or less, Ni: more than 5.0% and less than 10.0%, Al: 0.002-0.08%, N: 0.0015-0.0040%, the balance being Fe and impurities The amount of residual γ at the (1/4) t position of the sheet thickness t is 3.0% by volume or more, and the value expressed by the following formula (1) is 1.3 or more, and 1 % Reduction in residual γ amount when subjected to a plastic strain of −165 ° C. in an environment of −165 ° C. is 25% or less.
σ _{y, −165 ° C./σ y, RT} (1) equation where σ _{y, −165 ° C.} is the yield strength [MPa] at −165 ° C., and σ _{y, RT} is the normal temperature Yield strength [MPa] is shown respectively.

  (2) Instead of a part of Fe, by mass%, Cu: 2.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V: 0.1% or less, and B: 0 The steel material for cryogenic temperature of said (1) characterized by containing 1 type or 2 types or more of 0.005% or less.

  (3) Instead of a part of Fe, the composition contains one or two of Nb: 0.1% or less and Ti: 0.1% or less in mass%. ) Or (2) Cryogenic steel.

  (4) The cryogenic steel material according to any one of the above (1) to (3), characterized by containing Sn: 0.50% or less in mass% instead of part of Fe.

  (5) Instead of a part of Fe, by mass%, it contains one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. The cryogenic steel material according to any one of (1) to (4) above,

  (6) The cryogenic steel material according to any one of (1) to (5) above, wherein the average aspect ratio of residual γ at the (1/4) t position of the sheet thickness t is 3.5 or less.

(7) A method for producing a steel material for cryogenic temperature characterized by subjecting a steel ingot having the chemical composition described in any one of (1) to (5) to the following steps.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3] A step of cooling the steel to room temperature at a cooling rate RA (° C./s) that satisfies the following equation (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling.

(8) A method for producing a cryogenic steel material, comprising subjecting the steel ingot having the chemical composition described in any one of (1) to (5) to the following steps.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3] A step of cooling the steel to room temperature at a cooling rate RA (° C./s) that satisfies the following equation (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling.
[Step 4] A step of tempering the steel at a temperature of [Ac ₁ point + 80 ° C.] or lower.

(9) A method for producing a steel material for cryogenic temperature, which comprises subjecting a steel ingot having the chemical composition described in any one of (1) to (5) above to the following steps.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3 ′] A step of cooling the steel material to room temperature.
[Step 3 ″] A step of reheating the steel material at a temperature of Ac ₁ point or higher and 900 ° C. or lower.
[Step 3 ′ ″] A step of quenching the steel material at a cooling rate RH (° C./s) that satisfies the following equation (6).
RH ≧ 3 (6) where RH represents the cooling rate (° C./s) at the center of the plate thickness during quenching.
[Step 4] A step of tempering the steel at a temperature of [Ac ₁ point + 80 ° C.] or lower.

(10) A method for producing a steel material for cryogenic temperature, which comprises subjecting a steel ingot having the chemical composition described in any of (1) to (5) above to the following steps.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3] A step of cooling the steel to room temperature at a cooling rate RA (° C./s) that satisfies the following equation (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling.
[Step 3 ″] A step of reheating the steel material at a temperature of Ac ₁ point or higher and 900 ° C. or lower.
[Step 3 ''''] A step of quenching the steel material.
[Step 4] A step of tempering the steel at a temperature of [Ac ₁ point + 80 ° C.] or lower.

  (11) An LNG tank, wherein the cryogenic steel material according to any one of (1) to (6) is applied to an inner tank member.

  (12) An LNG tank, wherein the cryogenic steel material according to any one of (1) to (6) is applied to an annular plate.

  Provided is a cryogenic steel material excellent in fracture toughness that can maintain the liquid-tightness and confidentiality of the inner tank of the tank even when subjected to level 2 earthquake motion, a manufacturing method thereof, and an LNG tank to which the steel material is applied. It becomes possible.

It is an example in the case of the full size of a three-surface slit Charpy test piece. A front view and a plan view are shown on the left side, and a sectional view (side view) of the slit portion is shown on the right side (unit: mm). The slit width is based on a grinder cut: 0.4 mm.

  Below, the steel material for cryogenic temperature concerning this invention, its manufacturing method, and the LNG tank to which the said steel material is applied are demonstrated in detail for every requirement. In addition, "%" regarding content means "mass%" unless otherwise indicated.

(A) Regarding chemical composition C: 0.01 to 0.12%
C is an element necessary for ensuring the strength of the base material. If the content is less than 0.01%, not only the required strength cannot be secured, but also lath formation in FL (Fusion Line) becomes insufficient, and the toughness of HAZ (Heat Affected Zone) near FL decreases. , C must be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.12%, the toughness deterioration of HAZ, particularly HAZ near FL, becomes remarkable. Therefore, the C content is set to 0.01 to 0.12%. The preferable lower limit of C is 0.03%, and the preferable upper limit is 0.09%.

Si: 0.01 to 0.3%
Si is an element necessary as a deoxidizer. In order to acquire this effect, it is necessary to contain 0.01% or more of Si. On the other hand, in the case of the steel according to the present invention, Si is greatly related to the tempering process of martensite as it is quenched, and if the Si content exceeds 0.3%, The toughness of the weld is reduced by suppressing the self-tempering by suppressing the decomposition and precipitation reaction from cement to cementite, or by increasing island martensite. Therefore, the Si content is set to 0.01 to 0.3%. From the viewpoint of improving the toughness of the weld zone, the Si content is preferably as low as possible. A preferable lower limit of Si is 0.02%, and a more preferable lower limit is 0.03%. A preferable upper limit of Si is 0.15%, and a more preferable upper limit is 0.10%.

Mn: 0.4 to 2.0%
Mn is an element necessary as a deoxidizer and for ensuring the strength and toughness of the base material and the hardenability of HAZ. If the Mn content is less than 0.4%, not only these effects can be obtained, but also ferrite side plates are formed in the HAZ, resulting in insufficient lath formation, and the toughness of the weld is reduced. The content is 0.4% or more. On the other hand, if the Mn content exceeds 2.0%, the base material characteristics in the thickness direction are uneven due to center segregation. Therefore, the Mn content is 0.4 to 2.0%. A preferable lower limit of Mn is 0.5%, and a more preferable lower limit is 0.6%. A preferable upper limit of Mn is 1.5%, and a more preferable upper limit is 1.1%.

P: 0.05% or less P is present in the steel as an impurity, and segregates at the grain boundary to cause a decrease in toughness. If the P content exceeds 0.05%, hot cracking is caused during welding, so the P content is 0.05% or less. In addition, it is good to make content of P as small as possible, and preferable content of P is 0.03% or less.

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. Since the S content should be as small as possible, there is no particular lower limit. In addition, the preferable content of S is 0.003% or less.

Ni: more than 5.0% and less than 10.0% Ni is the most basic element necessary for securing toughness as a low-temperature steel. In order to secure toughness as a low-temperature steel, a Ni content exceeding 5.0% 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 less than 10.0%. Therefore, the Ni content target is more than 5.0% and less than 10.0%. In addition, from the viewpoint of securing low temperature toughness and cost reduction, the preferable lower limit of Ni is 5.5%, and the more preferable lower limit is 6.0%. A preferable upper limit of Ni is 9.3%, and a more preferable upper limit is 8.0%.

Al: 0.002 to 0.08%
Al is an element generally contained as a deoxidizer, but in the case of the steel of the present invention, it has a function of delaying self-tempering of martensite, similar to Si. The content of is preferably as low as possible. However, if the Al content is less than 0.002%, a sufficient deoxidizing effect cannot be obtained. On the other hand, when the Al content exceeds 0.08% and becomes excessive, as in the case of Si described above, the decomposition and precipitation reaction from martensite, which is supersaturated with C to a solid solution, is suppressed in the welding cooling process. , Reduce the toughness of the weld. Therefore, the Al content is 0.002 to 0.08%. A preferable lower limit of Al is 0.005%, and a more preferable lower limit is 0.010%. A preferable upper limit of Al is 0.05%, and a more preferable upper limit is 0.04%.

N: 0.0015 to 0.0040%
N is an element that contributes to the stabilization of austenite. Moreover, it combines with Al to become AlN, which is effective for refining austenite grains during heating. In order to obtain these effects, it is necessary to contain 0.0015% or more. However, if the N content exceeds 0.0040%, the HAZ toughness is deteriorated. Therefore, the N content is set to 0.0015 to 0.0040%. A preferable lower limit of N is 0.0020%. A preferable upper limit of N is 0.0035%.

  The thick steel sheet for low temperature according to the present invention is composed of Fe and impurities in the balance in addition to the above components. Here, an impurity is a component that is mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially producing a thick steel plate, and has an adverse effect on the present invention. It means what is allowed in the range.

  The low-temperature steel plate according to the present invention further contains one or more of Cu, Cr, Mo, V, B, Nb, Ti, Sn, Ca, Mg, and REM in addition to the above components. May be.

Cu: 2.0% or less Cu can be contained as required. When Cu is contained, the strength of the base material can be improved. However, if this content exceeds 2.0%, the toughness of HAZ heated to a temperature of Ac ₃ points or less is deteriorated, so the Cu content is 2.0% or less. A preferable upper limit is 1.3%. In order to stably develop the strength improvement effect of the base material by Cu, it is preferable to contain Cu by 0.1% or more. A more preferable lower limit of Cu is 0.2%.

Cr: 1.5% or less Cr can be contained as necessary. When Cr is contained, the carbon dioxide gas corrosion resistance and hardenability can be improved. However, if this content exceeds 1.5%, not only is it difficult to suppress the hardening of the HAZ, but the effect of improving the corrosion resistance to carbon dioxide gas is saturated, so the Cr content is 1.5% or less. . A preferable upper limit is 1.0%. In order to stably exhibit the effects of improving the carbon dioxide corrosion resistance and hardenability by Cr, it is preferable to contain 0.05% or more of Cr. A more preferable lower limit of Cr is 0.1%.

Mo: 0.5% or less Mo can be contained as necessary. Inclusion of Mo has an effect of improving the strength and toughness of the base material. However, if this content exceeds 0.5%, the hardness of the HAZ increases and the toughness and SSC resistance are impaired, so the Mo content is set to 0.5% or less. A preferable upper limit is 0.3%. In order to stably develop the effect of improving the strength and toughness of the base material due to Mo, it is preferable to contain Mo by 0.02% or more. A more preferable lower limit of Mo is 0.05%.

V: 0.1% or less V can be contained as necessary. Inclusion of V has an effect of improving the strength of the base material mainly due to carbonitride precipitation during tempering. However, if the content exceeds 0.1%, the performance improvement effect of the base material strength is saturated and the toughness is deteriorated, so the V content is set to 0.1% or less. A preferable upper limit is 0.08%. In order to stably develop the effect of improving the strength of the base material by V, it is preferable to contain V by 0.015% or more. A more preferable lower limit of V is 0.02%.

B: 0.005% or less B can be contained if necessary. Inclusion of B has an effect of improving the strength of the base material. However, if this content exceeds 0.005%, precipitation of coarse boron compounds is caused and the toughness is deteriorated, so the B content is made 0.005% or less. A preferable upper limit is 0.004%. In order to stably develop the effect of improving the strength of the base material by B, it is preferable to contain B by 0.0003% or more. A more preferable lower limit of B is 0.001%.

Nb: 0.1% or less 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 this content exceeds 0.1%, coarse carbides and nitrides are formed and the toughness is lowered, so the Nb content is made 0.1% or less. A preferable upper limit is 0.08%. In order to stably develop the effect of improving the low temperature toughness due to Nb, it is preferable to contain 0.01% or more of Nb. A more preferable lower limit of Nb is 0.02%.

Ti: 0.1% or less Ti can be contained as necessary. When Ti is contained, it is mainly used as a deoxidizing element, but has the effect of forming an oxide phase composed of Al, Ti, and Mn to refine the structure. However, when this content exceeds 0.1%, the oxide formed is Ti oxide or Ti-Al oxide, and the dispersion density is lowered. Therefore, the Ti content is 0.1% or less. A preferable upper limit is 0.07%. In order to stably develop the effect of refining the structure by Ti, it is preferable to contain Ti by 0.02% or more. A more preferable lower limit of Ti is 0.03%.

Sn: 0.50% or less Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly to reduce the Fe ^3+, by having an effect of reducing Fe ³⁺ concentration as oxidizing agent, since inhibit corrosion promoting effect of Fe ^3+, thereby improving the weather resistance in high airborne salt environments. Moreover, Sn has the effect | action which suppresses the anodic dissolution reaction of steel and improves corrosion resistance. In order to obtain this effect, Sn may be included. However, when Sn is contained exceeding 0.50%, the effect is saturated. For this reason, when it contains Sn, the content shall be 0.50% or less. A preferable upper limit is 0.30%. In addition, in order to make the said effect by Sn express stably, it is preferable to contain Sn 0.03% or more. A more preferable lower limit of Sn is 0.05%.
When Sn and Cu are simultaneously contained in the steel, rolling cracks are likely to occur when the steel plate is produced. In order to prevent this, when Sn is added, the Cu content is preferably less than 0.2% and the Cu content ratio (Cu / Sn ratio) is preferably 1.0 or less.

Ca: 0.004% or less Ca can be contained as necessary. When Ca is contained, it reacts with S in the steel to form oxysulfide (oxysulfide) in the molten steel. Unlike MnS, etc., this oxysulfide does not extend in the rolling direction during rolling, so it remains spherical after rolling, and has no weld cracks or hydrogen-induced cracks starting from cracks at the ends of the elongated inclusions. There is an inhibitory effect. However, if this content exceeds 0.004%, the toughness may be deteriorated, so the Ca content is set to 0.004% or less. A preferable upper limit is 0.003%. In order to stably develop the effect of suppressing weld cracking and hydrogen induced cracking due to Ca, it is preferable to contain 0.0003% or more of Ca. A more preferable lower limit of Ca is 0.0005%.

Mg: 0.002% or less Mg can be contained as necessary. When Mg is contained, a fine Mg-containing oxide is produced, which is effective in reducing the γ particle size. However, if this content exceeds 0.002%, the amount of oxide becomes too much and the ductility may be lowered, so the Mg content is set to 0.002% or less. A preferable upper limit is 0.001%. In order to stably develop the effect of refining the γ particle diameter by Mg, it is preferable to contain 0.0002% or more of Mg. A more preferable lower limit of Mg is 0.0004%.

REM: 0.002% or less REM (rare earth element) can be contained as required. When REM is contained, there is an effect of refining the structure of the weld heat affected zone and fixing S. When REM is excessively contained, inclusions are formed and the cleanliness is lowered. However, inclusions formed by the addition of REM have a relatively small influence on deterioration of toughness, so that the content of REM is 0.00. If it is 002% or less, a decrease in the toughness of the base material can be tolerated. Therefore, the content of REM is set to 0.002% or less. A preferable upper limit is 0.001%. In order to stably develop the effect of refining the structure of the heat affected zone by REM and the effect of fixing S, it is preferable to contain 0.0002% or more of REM. A more preferable lower limit of REM is 0.0003%.

  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. Note that the content of REM means the total content of these elements.

(B) Regarding the metal structure (B-1) The residual γ amount at the (1/4) t position of the sheet thickness t is 3.0% by volume or more. The residual γ in the thick steel sheet is the brittle crack propagation stopping property of the thick steel sheet. It contributes to the improvement. As a result, an effect of improving toughness in a cryogenic environment can be expected. In order to obtain this effect, it is necessary that the amount of residual γ in the steel is 3.0% by volume or more.
The upper limit of the amount of residual γ is not particularly specified, but if there is too much residual γ, the yield stress may decrease, so the amount of residual γ is preferably 15.0% by volume or less. More preferably, it is 10.0 volume% or less. Here, the amount of residual γ at the (1/4) t position of the sheet thickness t is evaluated in order to evaluate at an average position in the entire sheet thickness.

(B-2) The reduction rate of residual γ is 25% or less even when subjected to 1% plastic strain in an environment of −165 ° C. Generally, residual γ in α (ferrite) structure is metastable and plastic. It is in a situation where martensite transformation is easily caused by deformation. Since the residual γ is dispersed for improving the brittle fracture occurrence characteristics and propagation stop characteristics, if it disappears after an earthquake, it means that the desired fracture resistance characteristics are not exhibited. If the decrease rate of the residual γ amount is within 25%, it can be considered that the initial characteristics can be substantially maintained. Note that the lower the reduction rate, the better.
The stability of the residual γ varies depending on its shape, and the more stable the remaining γ is, the more stable the state is. Therefore, the aspect ratio of the residual γ is preferably small, and the average aspect ratio of the residual γ is preferably 3.5 or less.

(C) Regarding the yield strength ratio When the yield strength test is performed at a low temperature, the yield point at low temperature rises compared to the yield point at room temperature. As the degree of increase increases, sufficiently high toughness can be obtained in a cryogenic environment. That is, when the ratio of the yield point in the cryogenic environment to the yield point at room temperature is large, a steel material having excellent fracture safety in the cryogenic environment can be obtained.
Specifically, based on the data shown in Table 3 of Examples described later, the toughness (V-notch Charpy absorbed energy) at the liquid nitrogen temperature (−196 ° C.) using the value represented by the following equation (1) as a parameter: to investigate the correlation between the vE _-196). As a result, when the value represented by the following equation (1) is 1.3 or more, the liquid nitrogen temperature (−196 ° C.) is lower than the LNG storage tank temperature (−165 ° C.). 196 ° C.) showed a high toughness (V-notch Charpy absorbed energy vE ₋₁₉₆ ) and was found to be excellent in fracture resistance.
σ _{y, −165 ° C./σ y, RT} (1) equation where σ _{y, −165 ° C.} is the yield strength [MPa] at −165 ° C., and σ _{y, RT} is the normal temperature Yield strength [MPa] is shown respectively.

  Moreover, since this fact can be used as a criterion for determining whether or not it is excellent in fracture resistance, the present invention can also be grasped as a method for determining the fracture resistance of a steel material for cryogenic temperature.

That is, “mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.3%, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008% or less, Ni: more than 5.0% and less than 10.0%, Al: 0.002-0.08%, N: 0.0015-0.0040%, the balance being Fe and impurities The amount of residual γ at the (1/4) t position of the thickness t is 3.0% by volume or more, and the residual when subjected to 1% plastic strain in an environment of −165 ° C. A method for judging the conformity of a cryogenic steel material in which the rate of decrease in the amount of γ is 25% or less, characterized in that it is judged as conforming when the value expressed by the following equation (1) is 1.3 or more. This is a method for determining the conformity of low temperature thick steel plates with excellent fracture safety.
σ _{y, −165 ° C./σ y, RT} (1) equation where σ _{y, −165 ° C.} is the yield strength [MPa] at −165 ° C., and σ _{y, RT} is the normal temperature Yield strength [MPa] is shown respectively. "
As such, the present invention can be grasped. Of course, in addition to the above components, one or more of Cu, Cr, Mo, V, B, Nb, Ti, Sn, Ca, Mg, and REM may be further contained.

(D) Production Method The cryogenic steel material according to the present invention can be produced through the following steps. However, it is not limited to the following manufacturing method.

  In addition, about a steel ingot, the casting conditions are not prescribed | regulated exceptionally. An ingot-splitting slab may be used as a steel ingot, or a continuously cast slab may be used. From the viewpoint of production efficiency, yield, and energy saving, it is preferable to use a continuously cast slab.

(D-1) Steel ingot heating step (step 1)
A step of heating the ingot such that the heating temperature Tr (° C.) and the heating time t (hr) of the ingot satisfy the following formulas (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.

Specifically, the heating temperature Tr (° C.) of the steel ingot may be a soaking temperature in the heating furnace, and the heating time t (hr) is the time during which the ingot is in the soaking zone. That's fine. Incidentally, Ac ₃ point may be used calculated values based on the following equation (a).
Ac ₃ points = 897.3−271.1 × C + 43.7 × Si−17 × Mn + 117.8 × P + 15.95 × S-40.8 × Cu-22.3 × Ni−6.5 × Cr + 6.5 × Mo + 65.8 × V + 145.2 × Nb + 56. 9 × Al + 88.5 × Ti-17968.4 × B + 121.8 × N (a) Formula Here, the element symbol in the formula represents the content (% by mass) of each element.

The heating process greatly affects the structure of the steel material. As described above, the higher the heating temperature, the more coarse the structure, so a high heating temperature is not preferable. Normally, in the heating process, the steel ingot temperature gradually rises after being inserted into the heating furnace, and after the temperature of the soaking zone is exceeded, so-called overshoot in which the ingot temperature becomes steady at the soaking zone temperature may occur. When the steel ingot temperature exceeds 50 ° C. from the soaking zone temperature due to the occurrence of overshoot, the steel ingot structure becomes coarser and the intended structure may not be obtained. For this reason, it is preferable to control the overshooting temperature to 50 ° C. or lower. That is, in the heating step, it is preferable steel ingot is suppressed to below Tr the highest temperature T _OS steel ingot before stabilizing at (℃) [Tr + 50] (℃).

The heating temperature needs to be Ac ₃ points or higher in order to transform the structure into austenite. In addition, it is preferable that heating temperature shall be 850 degreeC or more. This is because a steel ingot of 850 ° C. or higher has low deformation resistance, and the load on the roll used in the subsequent hot rolling process is not so large. On the other hand, the heating temperature is preferably 1000 ° C. or lower. This is because if the heating is performed at 1000 ° C. or less, a sufficient heating time can be secured, and a more uniform steel ingot can be obtained.

  Thus, since the heating process is the process that most affects the steel structure, strict control is required.

(D-2) Rolling process (process 2)
In the hot rolling process, the heated steel ingot is rolled. Specifically, the rolling may be divided into rough rolling and finish rolling.

  In rough rolling on a heated steel ingot, it is preferable to reduce the steel ingot thickness until the thickness of the steel ingot at the end of the rough rolling is 3 to 8 times the product thickness (steel material thickness). When the steel ingot thickness after rough rolling is reduced to 3 times or more of the product thickness, sufficient reduction can be performed in the subsequent finish rolling, so that the toughness of the product thick steel plate can be improved. On the other hand, when the ingot thickness after rough rolling is reduced to 8 times or less than the product thickness, the finishing temperature in the subsequent finishing rolling (temperature at which finishing rolling ends) can be easily controlled to 750 ° C. or more. .

In finish rolling, the steel ingot subjected to rough rolling in this way is continuously reduced without cooling to a product with a predetermined plate thickness. In this finish rolling, the finish temperature is 750 ° C. or lower. The reason why the finish rolling temperature is set to 750 ° C. or less is to refine the effective crystal grain size of the final structure by positively introducing a deformation band into the structure during rolling. The finishing temperature is preferably 650 ° C. or higher. This is because if the finishing temperature is 650 ° C. or higher, the deformation resistance is small and rolling is easy. In addition, what is necessary is just to measure the surface temperature of the steel ingot which is a to-be-rolled material, or steel materials as the temperature in rolling. The most important factor in the rolling process is the regulation of the amount of reduction in the final finishing pass. Increasing the amount of reduction in the final finishing pass is effective in actively introducing the deformation band, leaving a large amount of finally generated residual γ and reducing the aspect ratio. Therefore, rolling is performed so that the shape ratio Γ of the final finishing pass satisfies the expression (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.

(D-3) Cooling step (step 3, step 3 ′)
In the cooling step, the rolled steel material that has been finish-rolled is cooled. A faster cooling rate of the steel material after rolling is better. Specifically, in order to prevent the effective crystal grain size of the final structure from becoming coarse due to a slow cooling rate during cooling after rolling, a cooling rate RA (° C./°C) that satisfies the following equation (5): It is preferable to cool the steel to room temperature in s).
RA ≧ 3 ······················································································································ (5) where RA is the cooling center (° C / s) at the (1/2) t position of the thickness t at the time of cooling Represents.

  In addition, when the quenching step (D-5) described later is performed, if the cooling rate RH at the center of the plate thickness at the time of quenching is 3 ° C./s or more, a sufficient amount of residual γ as a steel material for cryogenic use Therefore, the cooling rate RA at the center of the plate thickness in this cooling step may be less than 3 ° C./s.

(D-4) Reheating step (step 3 ″)
When performing the hardening process (D-5) mentioned later, it is necessary to reheat steel materials to hardening temperature. The quenching temperature is preferably a temperature of Ac ₁ point or higher and 900 ° C. or lower. The increase in residual γ can be expected by setting Ac to ₁ point or more, but the structure becomes coarse when heated at a temperature exceeding 900 ° C. Here, Ac ₁ point is a temperature at which transformation from pearlite to austenite is completed. In addition, when not performing a hardening process, a reheating process is unnecessary.

(D-5) Quenching process (process 3 ''', process 3'''')
A quenching process can be implemented as needed. After the reheating process described above, quenching is performed, but in order to refine the structure and increase the initial residual γ, the steel material at a cooling rate RH (° C./s) that satisfies the following equation (6): It is preferable to quench.
RH ≧ 3 (6) where RH represents the cooling rate (° C./s) at the center of the plate thickness during quenching.

  In the cooling in the cooling process, the cooling rate RA may not be 3 ° C./s or more. In such a case, the cooling rate RH in the quenching process is 3 ° C./s or more. This is an effective means for increasing the amount of residual γ.

  On the other hand, in the cooling in the cooling process, when the cooling rate RA at the center of the plate thickness is 3 ° C./s or more, a sufficient amount of residual γ can be obtained as a cryogenic steel material. The cooling rate RH at the center of the plate thickness in this quenching step may be less than 3 ° C./s.

  The quenching treatment method may be any means such as a spray method. The cooling stop temperature is preferably 200 ° C. or lower.

(D-6) Tempering step (step 4)
A tempering process can be implemented as needed. The distortion in the martensite produced in the steel material by tempering can be removed. When tempering is performed, it is performed at a temperature of [Ac ₁ point + 80 ° C.] or lower. The reason why the tempering is performed in this temperature range is because the martensite structure can be made tough and the amount of residual γ can be increased. In order to effectively obtain the distortion removal effect, the temperature is preferably 500 ° C. or higher.

A steel ingot (slab) with a thickness of 300 mm consisting of 41 types of steel having the chemical composition shown in Table 1 is prepared and finished under the conditions shown in Table 2 by heating, rolling, accelerated cooling, etc. The heat treatment is carried out. The plate thickness is a steel material of 6 to 50 mm. From each steel material obtained, No. 10 tensile test piece or No. 5 tensile test piece prescribed in JISZ2201 or No. 4 test piece was taken from the position of (1/4) t of the plate thickness t. The direction is the direction perpendicular to rolling. Moreover, the V notch test piece prescribed | regulated to JISZ2202 was extract | collected from the rolling direction. The sub-size test was used for the plate thickness of 10 mm or less. Perform Charpy impact test in a tensile test and -196 ° C. at normal temperature and -165 ° C., a tensile strength (TS: MPa), yield strength (YS: MPa) and the absorbed energy (vE _-196: J (3 pieces of The average value)) was examined. Absorbed energy was converted to absorbed energy per unit area in order to facilitate comparison between the sub-size test piece and the full-size test piece.

  Further, in order to evaluate the brittle crack propagation characteristics, a three-surface slit Charpy test piece shown in FIG. 1 was taken from the position of (1/2) t of the plate thickness t. The material with a plate thickness of 10 mm or less was processed into a sub-size test piece similar to the V notch. The absorbed energy is converted to a numerical value per unit area, as with the V-notch specimen.

  In order to investigate the brittle fracture occurrence characteristics, CTOD specimens according to BS7448 part 1 were also collected from the L direction. The plate thickness is a full-thickness test piece, a B × 2B test piece is used, and the test method conforms to the BS standard.

  For residual γ, a test piece for residual γ measurement was taken from the (1/4) t position of the plate thickness t, and the residual γ amount (% by volume) was measured by X-ray diffraction. Further, the aspect ratio is a plane parallel to the L direction and the Z direction and perpendicular to the C direction. Using an optical microscope, the size of the residual γ grains is averaged in the Z direction and the L direction of the steel material, respectively. And obtained from “average intercept length in L direction / average intercept length in Z direction”. The aspect ratio can be calculated by capturing residual γ grains as an electrophotographic photograph and processing the image with an image analysis apparatus.

  The above test results are summarized in Table 3.

The criteria for determining whether the characteristics are good or bad are as follows.
YS: 590MPa or more,
TS: 690 MPa or more
V-notch Charpy absorbed energy per unit area: 1.25J / mm ² or more,
Per unit area trihedral slit Charpy absorbed energy: 0.125J / mm ² or more,
Limit CTOD value: 0.1mm or more.

  As can be seen from the results of characteristic evaluation shown in Table 3, the steel No. having a chemical composition within the range defined by the present invention. Steel Nos. 1-a to 1-e and 2-35 manufactured under appropriate conditions using 1 to 35 have a large increase in yield strength at cryogenic temperatures relative to yield strength at room temperature. The residual γ amount can be adjusted to 3.0% by volume or more, and the decrease rate of the residual γ amount is small even when subjected to plastic deformation. For this reason, strength, brittle crack initiation characteristics (V-notch Charpy absorbed energy, critical CTOD value), and arrest characteristics (three-plane slit Charpy absorbed energy) satisfy the target ranges.

  On the other hand, the steel No. 1-f was not manufactured properly and the final finish pass had a shape ratio of less than 1. Therefore, the amount of residual γ was small and the amount of decrease in residual γ after deformation was large Furthermore, the rate of increase in yield strength at cryogenic temperatures relative to the yield strength at room temperature was reduced. For this reason, the strength could not be satisfied, and the arrest characteristics (three-plane slit Charpy absorbed energy) and brittle crack generation characteristics (limit CTOD value) were also low.

  Test No. 1-g steel material is not suitable for manufacturing, and the value shown in equation (1) is less than 1.3, so the amount of residual γ after deformation is large and the cryogenic temperature at room temperature is very low. The rate of increase in yield strength underneath was reduced. For this reason, the yield strength could not be satisfied, and the brittle crack initiation characteristics (limit CTOD value) were also low.

  Test No. 1-h steel is not suitable for manufacturing, and the value shown in equation (1) is less than 1.3, so the amount of residual γ is small, the amount of decrease in residual γ after deformation is large, and the room temperature The rate of increase in yield strength at very low temperatures relative to the lower yield strength decreased. For this reason, the brittle crack initiation characteristic (limit CTOD value) was low.

  Test No. 36 steel has C content, Test No. 37 steel has Si content, and Test No. 38 steel has Mn content which is too high. The increase in yield strength at cryogenic temperatures relative to the yield strength at room temperature was small. For this reason, arrest characteristics (three-surface slit Charpy absorbed energy) and brittle crack initiation characteristics (limit CTOD value) were also low.

  The steel of Test No.39 has too high Ni content, so the finish rolling temperature is also high, the amount of decrease in residual γ after deformation is large, and the rate of increase in yield strength at cryogenic temperatures relative to the yield strength at room temperature Became smaller. For this reason, arrest characteristics (three-surface slit Charpy absorbed energy) and brittle crack initiation characteristics (limit CTOD value) were also low.

  Test No. 40 steel has an Al content, and Test No. 41 steel has an N content that is too high. Therefore, the amount of residual γ is small, and the amount of residual γ after deformation is large. The rate of increase in yield strength at very low temperatures relative to the yield strength was reduced. For this reason, the tensile strength could not be satisfied, and the arrest characteristics (three-plane slit Charpy absorbed energy) and brittle crack generation characteristics (limit CTOD value) were also low.

  The cryogenic steel material according to the present invention has excellent fracture toughness that can maintain the liquid tightness and confidentiality of the inner tank of the tank even when subjected to level 2 earthquake motion. In other words, in addition to the property that brittle fracture does not easily occur even at extremely low temperatures, in order to prevent the entire structure from collapsing even if a brittle crack occurs, the property that stops crack propagation ( Arrest characteristics). And the steel material for cryogenic temperature concerning this invention can be used conveniently for the inner tank member and annular plate of an LNG tank, especially an LNG tank, and can raise the reliability of an LNP tank.

Claims (12)

In mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.3%, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008 %, Ni: more than 5.0% and less than 10.0%, Al: 0.002 to 0.08%, N: 0.0015 to 0.0040%, the balance being Fe and impurities steel The amount of residual γ at the (1/4) t position of the thickness t is 3.0% by volume or more, and the value expressed by the following formula (1) is 1.3 or more, and further 1% plasticity. A steel material for cryogenic temperature characterized in that the reduction rate of the residual γ amount when strain is received in an environment of −165 ° C. is 25% or less.
σ _{y, −165 ° C./σ y, RT} (1) equation where σ _{y, −165 ° C.} is the yield strength [MPa] at −165 ° C., and σ _{y, RT} is the normal temperature Yield strength [MPa] is shown respectively.   Instead of a part of Fe, by mass%, Cu: 2.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% The steel material for cryogenic temperature according to claim 1, comprising one or more of the following.   It replaces with a part of Fe and contains 1 type or 2 types of Nb: 0.1% or less and Ti: 0.1% or less in the mass%, The claim 1 or 2 characterized by the above-mentioned. Steel material for cryogenic described.   The steel material for cryogenic temperature according to any one of claims 1 to 3, characterized by containing Sn: 0.50% or less in mass% instead of part of Fe.   Instead of a part of Fe, by mass%, it contains one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. The steel material for cryogenic temperature according to any one of claims 1 to 4.   The steel material for cryogenic temperature according to any one of claims 1 to 5, wherein the average aspect ratio of the residual γ at the (1/4) t position of the plate thickness t is 3.5 or less. The manufacturing method of the steel material for cryogenic temperature characterized by performing the following process to the steel ingot which has the chemical composition as described in any one of Claim 1-5.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3] A step of cooling the steel to room temperature at a cooling rate RA (° C./s) that satisfies the following equation (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling. The manufacturing method of the steel material for cryogenic temperature characterized by performing the following process to the steel ingot which has the chemical composition as described in any one of Claim 1-5.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3] A step of cooling the steel to room temperature at a cooling rate RA (° C./s) that satisfies the following equation (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling.
[Step 4] A step of tempering the steel at a temperature of [Ac ₁ point + 80 ° C.] or lower. The manufacturing method of the steel material for cryogenic temperature characterized by performing the following process to the steel ingot which has the chemical composition as described in any one of Claim 1-5.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3 ′] A step of cooling the steel material to room temperature.
[Step 3 ″] A step of reheating the steel material at a temperature of Ac ₁ point or higher and 900 ° C. or lower.
[Step 3 ′ ″] A step of quenching the steel material at a cooling rate RH (° C./s) that satisfies the following equation (6).
RH ≧ 3 (6) where RH represents the cooling rate (° C./s) at the center of the plate thickness during quenching.
[Step 4] A step of tempering the steel at a temperature of [Ac ₁ point + 80 ° C.] or lower. The manufacturing method of the steel material for cryogenic temperature characterized by performing the following process to the steel ingot which has the chemical composition as described in any one of Claim 1-5.
[Step 1] A step of heating the ingot such that the ingot heating temperature Tr (° C.) and the heating time t (hr) satisfy the following equations (2) and (3).
^{t × exp (Tr 3/270000000} ) ≦ 580 ········ (2) formula Ac ₃ point ≦ Tr ·········· (3) formula, where, Tr is the steel ingot The heating temperature (° C.), t represents the heating time (hr) of the steel ingot, and Ac ₃ represents the temperature at which the transformation from ferrite to austenite is completed.
[Step 2] A step of rolling a steel ingot to obtain a steel material at a finishing temperature of 750 ° C. or less so that the shape ratio Γ of the final finishing pass satisfies the formula (4).
Γ = 2 × (R × (H1−H2)) ^1/2 /(H1+H2)≧1.0 (4) where Γ is the shape ratio of the final finishing pass, and R is the lower work roll. The radius, H1 represents the incoming wall thickness, and H2 represents the outgoing wall thickness.
[Step 3] A step of cooling the steel to room temperature at a cooling rate RA (° C./s) that satisfies the following equation (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling.
[Step 3 ″] A step of reheating the steel material at a temperature of Ac ₁ point or higher and 900 ° C. or lower.
[Step 3 ''''] A step of quenching the steel material.
[Step 4] A step of tempering the steel at a temperature of [Ac ₁ point + 80 ° C.] or lower. An LNG tank, wherein the cryogenic steel material according to any one of claims 1 to 6 is applied to an inner tank member. An LNG tank, wherein the cryogenic steel material according to any one of claims 1 to 6 is applied to an annular plate.

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