JP5741260B2 - Cryogenic steel material excellent in CTOD characteristics after imparting strain and method for producing the same - Google Patents

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. For cryogenic use, LPG (Liquefied petroleum gas:
It means use in a liquid temperature range such as liquefied petroleum gas (liquefied petroleum gas) and LNG (Liquefied Natural Gas), that is, 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 particular, assuming that a ground-based LNG tank is built in a country with frequent earthquakes such as Japan, it is stated that liquid deformation and confidentiality can be maintained even if it is subjected to an earthquake. . That is, the inner tank material of the tank cannot tolerate breakage that penetrates the plate thickness. When a large external force is applied due to the earthquake, it can be assumed that the inner tank material and the annular plate are subjected to a large plastic deformation, and a very high level of characteristics is required as the anti-destructive performance. An example of steel that meets this requirement is 9% Ni steel.

  Conventionally, for 9% Ni steel, reduction of impurities such as P and S, reduction of C, and further three-stage heat treatment, that is, “quenching (Q), two-phase region quenching (L) and tempering” Various improvements such as heat treatment (QLT) "(T)" have been made. Further, addition of Mo has been studied as an alloy element effective for improving the strength and toughness of Ni-containing steel.

  This is because the addition of QLT or Mo increases the amount of retained austenite which is the basis for improving toughness. There are the following patent documents as documents in which such techniques are described.

  Patent Document 1 discloses that a plate thickness of 40 mm manufactured by a three-stage heat treatment method (QLT) or a direct quenching-two-phase quenching method (DQ-LT) method in which Mo: 0.04 to 0.5% is added. The above 9Ni steel is disclosed.

  Patent Document 2 discloses a method for producing 9% Ni steel having a thickness of 40 mm or more by a quenching-tempering method (QT) or a direct quenching-tempering method (DQ-T) method.

  However, the price of steel materials has soared due to the recent increase in alloy element prices. In the 9% Ni steel to which a large amount of expensive alloy elements such as Ni must be added, the increase in the price of the alloy elements causes a further increase in the price of the steel material. Therefore, in order to suppress the price of steel materials, it has become necessary to develop steel materials having performance equivalent to or better than 9% Ni steel, for example, excellent toughness, with a Ni content with low cost reduction. As a prior art regarding such a low Ni type cryogenic steel, there is one disclosed in the following patent document.

  Patent Document 3 discloses a steel for cryogenic temperature containing 4.0 to 7.5% of Ni and having an Ms point of 370 ° C. or lower. Patent Document 4 discloses a steel containing 5.5 to 10% Ni and a continuous casting method thereof. Furthermore, Patent Document 5 and Patent Document 6 disclose steels containing 1.5 to 9.5% Ni and 0.02 to 0.08% Mo.

  Patent Document 5 and Patent Document 6 describe CTOD (Crack Tip Opening Displacement) characteristics of welded joints.

  Patent Document 7 describes the knowledge about the CTOD characteristic improvement of the weld toe part (Toe part). This document describes a method for improving the CTOD characteristics by promoting the precipitation of cementite in the heat-affected zone structure by adjusting chemical components with reduced Si, Al, and N.

Japanese Unexamined Patent Publication No. 4-371520 JP-A-6-184630 Japanese Patent Laid-Open No. 6-136483 JP 7-90504 A Japanese Patent Laid-Open No. 9-302445 JP 2002-129280 A WO2007 / 080645

  CTOD characteristics of welded joints are the amount of crack tip opening immediately before a phenomenon in which a crack develops rapidly (unstable fracture) when a cracked material is held at a predetermined temperature and a bending external force is applied. The measured value (limit CTOD value) is shown. It can be said that the larger this critical CTOD value is, the better the material is.

  Generally, 9% Ni steel welded joints use an austenitic weld material with extremely high low temperature toughness. Therefore, the CTOD characteristics of FL are preceded by plastic deformation in the molten metal structure at the crack tip region under load. The CTOD value is improved. For example, Sato et al .: “Plastic deformation behavior and crack opening displacement of a material with a notch in a sudden change in mechanical properties” (Journal of the Japan Welding Society, Vol. 52, No. 2, pp.86-93, (1983) is also revealed analytically as a general problem with undermatched joints.

  However, in the Toe part, there is almost no help of plastic deformation of the weld metal, and it is CTOD evaluation of HAZ toughness itself. In other words, this Toe notch CTOD test is the most severe in the evaluation of the brittle fracture occurrence characteristics of 9% Ni steel welded joints.

  Therefore, CTOD evaluation of HAZ toughness itself even when developing steel materials with low Ni steel material with less Ni content than 9% Ni steel and having performance equivalent to or better than 9% Ni steel, for example, excellent toughness. Therefore, it is necessary to evaluate the brittle fracture occurrence characteristics of welded joints by this Toe notch CTOD test. In other words, this low Ni steel material is required to have excellent CTOD characteristics after applying strain in a heat-affected zone that is not subjected to subsequent thermal cycles.

  However, Patent Document 3 discloses a method for improving the toughness of the weld heat affected zone (HAZ), but does not describe the CTOD characteristics of the HAZ.

  Patent Document 4 only discloses the invention of the continuous casting method, and does not disclose the design and manufacturing method of chemical components for obtaining the base material characteristics equivalent to 9% Ni steel, Further, not only the CTOD characteristics but also the base material characteristics themselves are not disclosed. Moreover, the minimum value of the Ni content specifically shown is 9.08%, and means for obtaining a base material performance equivalent to 9% Ni steel with low Ni is not disclosed.

  On the other hand, Patent Document 5 and Patent Document 6 describe CTOD characteristics of welded joints. However, this CTOD characteristic shows the limit CTOD value in the fusion line (FL) part, and does not consider the limit CTOD value in the weld toe part (Toe part).

  Patent Document 7 describes the knowledge about CTOD characteristic improvement of the Toe part. However, the object of evaluation here is a joint in a welded state, and not a welded joint that has undergone severe plastic deformation after a severe earthquake. In order to ensure the safety of an LNG tank that assumes an earthquake, it is necessary to confirm that it has sufficient fracture resistance even after undergoing this large plastic deformation.

  The present invention has been made in view of such a situation, and the object thereof is a low Ni steel material having a lower Ni content than 9% Ni steel, and is not subjected to a subsequent heat cycle. An object of the present invention is to provide a cryogenic steel material excellent in CTOD characteristics after imparting a strain and a manufacturing method thereof, and an LNG tank to which the steel material is applied.

  Here, the object of evaluation is not a welded state, but a welded joint after undergoing a large plastic deformation in an earthquake. Specifically, a joint that has received a tensile plastic strain of about 1% at its weld toe is targeted for evaluation.

  In order to solve the above-mentioned problems, the present inventors conducted metallurgical studies on the heat-affected zone structure of the Toe part for a cryogenic steel having a Ni content of 5.5 to 8.5%. The findings shown in (a) to (h) were obtained.

  (a) When a welded joint is produced, the heat-affected zone of the Toe portion tends to have a tempered structure, but a part of the upper bainite may be formed in a chemical component system with poor hardenability. In the case where this upper bainite is generated, the deterioration of fracture toughness in the heat-affected zone is remarkable. First, it is necessary to prevent the formation of this upper bainite.

  In addition, when the hardenability is excessive, a martensite structure is formed. However, the progress of autotempering to precipitate cementite during cooling is extremely poor, and this over-hardened martensite structure must be avoided.

  That is, there is an appropriate range for hardenability. In order to maintain the Toe CTOD characteristics at the time of earthquake at a high level, the chemical composition adjustment must be based on the premise of generating a matrix that becomes an appropriate quenched structure, and further, the martensite structure should be improved by autotempering. Further adjustments are needed to achieve this.

  (b) Normally, when manufacturing a low-temperature storage tank, a steel material having a thickness of 10 mm or more is used, so that welding by multiple passes is essential. Therefore, the base material structure heated to the vicinity of the melting point by welding also receives a history of heating at a relatively low temperature and subsequent cooling by the subsequent pass, and the base material is refined and tempered. . After this, the CTOD characteristics are improved due to the refinement of the structure and the temper effect during the thermal history of the subsequent pass.

  However, since the HAZ structure at the Toe position is a portion that has been affected by heat due to the final pass, miniaturization and temper effect due to the subsequent pass cannot be expected. In other words, the progress of the autotemper effect during welding cooling is decisive for the toughness. Metallurgically, it is extremely important to reduce the Si content in order to accelerate the tempering by the autotemper.

  (c) It is also known that minimizing impurity elements is generally effective in securing the toughness of the heat-affected zone. In the present invention, impurity elements such as P, S, and N are extremely reduced, and the heat affected zone toughness is reliably improved.

(d) Based on the findings in (a) to (c) above, it is appropriate for improving the fracture toughness of the Toe zone heat-affected structure of low Ni steel with Ni content of more than 5.5% to less than 8.5%. As a result of experiments on steel materials composed of various chemical components, P _hardening defined by the following equation (1) can evaluate the hardenability in this region. It was found to be a parameter. And when this value satisfied 0.54-0.65, it turned out that hardenability becomes appropriate and the fracture toughness of the Toe part heat influence structure of low Ni steel improves.
P _hardening = 0.075Si + 0.217Mn + 0.042Ni + 0.25Cr + 0.32Mo (1) Formula Here, the element symbol in the formula represents the content (mass%) of each element.

  (e) Furthermore, it has been found that the refinement of the front structure of the Toe part before welding is effective. Since the Toe part always becomes the surface of the steel plate, it is only necessary to refine the metal structure of this surface. Γ (austenite) transformed from a fine previous structure naturally becomes a fine structure, and a martensite structure retransformed from the γ becomes fine. Specifically, the average effective crystal grain size of the steel plate surface region (0 to 0.2 mm) may be 5.0 μm or less.

  (f) As described in detail above, in order to retain sufficiently high CTOD characteristics in the Toe part even after plastic deformation during a severe earthquake, a number of limiting conditions are necessary. However, it has been found that it is sufficient to satisfy all of these limiting conditions.

(g) 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.

(h) Furthermore, it is desirable that the rolling surface has a large reduction amount in the final finishing pass of rolling. 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 completed on the basis of the above knowledge, and the following (1) to (5) cryogenic steel materials and (6) to (9) methods for producing cryogenic steel materials and (10) The gist of the LNG tank to which the steel material of (11) is applied.

(1) By mass%, C: 0.01 to 0.12%, Mn: 0.4 to 1.1% , Ni: 5.5 to 8.5%, Al: 0.002 to 0.05% , N: 0.0015 to 0.004%, the balance being Fe and impurities, of which Si: 0.15% or less, P: 0.05% or less, and S: 0.008% or less And the P _hardening value defined by the following formula (1) is 0.54 to 0.65, and the average effective grain size in the region 0.2 mm or less from the steel surface is 5.0 μm or less. A steel material for cryogenic temperatures that has excellent CTOD characteristics after straining in the heat-affected zone that is not subjected to subsequent thermal cycles.
P _hardening = 0.075Si + 0.217Mn + 0.042Ni + 0.25Cr + 0.32Mo (1) Formula Here, the element symbol in the formula represents the content (mass%) of each element.

  (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) Strain imparting in a heat-affected zone that is not subjected to a subsequent thermal cycle, characterized in that the steel ingot having the chemical composition described in any one of (1) to (5) above is subjected to the following steps: A method of manufacturing steel for cryogenic use with excellent CTOD characteristics.
[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 at a cooling rate RA (° C./s) satisfying the following expression (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling.

(7) Strain imparting in a heat-affected zone that is not subjected to a subsequent thermal cycle, characterized in that the following steps are performed on the steel ingot having the chemical composition described in any one of (1) to (5) above: A method of manufacturing steel for cryogenic use with excellent CTOD characteristics.
[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 at a cooling rate RA (° C./s) satisfying the following expression (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.

(8) Applying strain in a heat-affected zone that is not subjected to a subsequent thermal cycle, characterized by performing the following steps on the steel ingot having the chemical composition described in any one of (1) to (5) above: A method of manufacturing steel for cryogenic use with excellent CTOD characteristics.
[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.

(9) Strain imparting in a heat-affected zone that is not subjected to a subsequent thermal cycle, characterized in that the steel ingot having the chemical composition described in any of (1) to (5) above is subjected to the following steps: A method of manufacturing steel for cryogenic use with excellent CTOD characteristics.
[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 at a cooling rate RA (° C./s) satisfying the following expression (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.

  (10) An LNG tank, wherein the cryogenic steel material according to any one of (1) to (5) is applied to an inner tank member.

  (11) An LNG tank, wherein the cryogenic steel material according to any one of (1) to (5) is applied to an annular plate.

  A low-Ni steel material having a lower Ni content than 9% Ni steel and having excellent CTOD characteristics after imparting strain in a heat-affected zone that is not subjected to subsequent thermal cycles, a method for producing the same, and the steel material It is possible to provide an applied LNG tank.

Indicates the processing position for the Toe notch CTOD test (displayed as a marking position).

  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%.

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%.

Ni: 5.5 to 8.5%
Ni is the most basic element necessary for securing toughness as a low-temperature steel. In order to ensure toughness as a low-temperature steel, a Ni content of 5.5% 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 8.5%. Therefore, the Ni content target is set to 5.5 to 8.5%. In addition, from the viewpoint of securing low temperature toughness and cost reduction, the preferable lower limit of Ni is 6.0%, and the more preferable lower limit is 6.5%. A preferable upper limit of Ni is 8.3%, and a more preferable upper limit is 8.0%.

Al: 0.002 to 0.05%
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 0.05% and becomes excessive, as in the case of Si described above, the decomposition precipitation reaction from martensite, which is supersaturated with C to solid solution, in the welding cooling process to cementite is suppressed. , Reduce the toughness of the weld. Therefore, the Al content is 0.002 to 0.05%. 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 0.045%, and a more preferable upper limit is 0.04%.

N: 0.0015 to 0.004%
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 0.004%, the HAZ toughness is deteriorated. Therefore, the N content is set to 0.0015 to 0.004%. 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. However, in the present invention, among impurities, Si, P, and S, it is necessary to define an upper limit as follows.

Si: 0.15% or less Si is unavoidably present in steel as an impurity, but if it is contained, it acts as a deoxidizer. However, when the Si content exceeds 0.15%, the autotemper is delayed to suppress the decomposition and precipitation reaction to cementite from martensite that is supersaturated with C in the welding cooling process. Or to increase island martensite, the toughness of the weld is reduced. Therefore, the Si content is 0.15% or less.

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.

  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.

P _hardening : 0.54-0.65
P _hardening has been found to require appropriate hardenability to improve the fracture toughness of the Toe heat-affected structure of low Ni steel with a Ni content of 5.5 to 8.5%. It is a parameter that can be evaluated for the hardenability in this region, obtained as a result of experiments on steel materials composed of components, and is defined by the following equation (1). If the value of P _hardening is less than 0.54, the hardenability is low and the toughness is insufficient, and if the value of P _hardening exceeds 0.65, the hardenability is excessive and the toughness is similarly insufficient. Therefore, P _hardening was defined as 0.54 to 0.65.
P _hardening = 0.075Si + 0.217Mn + 0.042Ni + 0.25Cr + 0.32Mo (1) Formula Here, the element symbol in the formula represents the content (mass%) of each element.

(B) Metal structure The metal structure is refined so that the average effective crystal grain size in the region of 0.2 mm or less from the steel material surface is 5.0 μm or less. The refinement of the steel structure has the function of refining the heat-affected structure of the Toe part through the inheritance of the structure and promoting the tempering effect of the martensite. This is because it is effective in improving the quality. The refinement of the structure means that a lot of old γ grain boundaries and packet boundaries, which are cementite precipitation sites, are included, and has the function of promoting the precipitation of cementite. The numerical critical point was experimentally determined to have an average effective crystal grain size of 5.0 μm or less. A more preferable average effective crystal grain size is 3.0 μm or less. Here, the reason why the region of 0.2 mm or less from the steel material surface is evaluated is that the Toe part is necessarily the surface of the steel plate.

(C) Regarding 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.

(C-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.

(C-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.

(C-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 the 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) 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.

(C-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.

(C-5) Quenching process (process 3 ''', process 3'''')
A quenching process can be implemented as needed. The steel is hardened after the reheating process described above, 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.

(C-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, in order to evaluate the normal temperature strength, a No. 10 tensile test piece or a No. 5 tensile test piece prescribed in JISZ2201 or a No. 4 test piece from the position of (1 / 4t) t of the plate thickness t. Collected. The direction is the direction perpendicular to rolling. Also, the same steel materials are joined together by TIG welding under the condition of maximum heat input of 30 kJ / cm, and as-is (As Weld) or 1% tensile strain is applied from the welded portion, then specified in BS7448-1991. A B × B test piece was collected and a CTOD test was performed in an environment of −165 ° C. Specimens shall have a notch in the Toe part, and three of each are sampled. The average value of the measured values of these three specimens is used as the critical CTOD value (δ- ₁₆₅ (As Weld) and δ). _-165 (after applying 1% tensile strain)).

  Further, the “average effective crystal grain size” was measured as a crystal grain size of a portion surrounded by a structure boundary having an orientation difference of 15 ° or more when evaluated by EBSP. That is, EBSP (Electron Backscatter Diffraction Pattern) method is used to observe five or more fields of view at a magnification of 2000 times, and a structure boundary having an azimuth difference of 15 ° or more is regarded as a grain boundary. The area inside the crystal was determined, and the average of the areas converted into equivalent circle diameters was evaluated as the average effective crystal grain size. The evaluated plate thickness is a surface area of 0 to 0.2 mm, and the direction is a direction perpendicular to rolling.

  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
Joint limit CTOD value: 0.3mm or more.

As can be seen from the characteristic evaluation results shown in Table 3, the steel No. whose chemical composition (including P _hardening ) is within the range defined by the present invention. 1-27 and using 29 to 35 of the steel Test was performed the appropriate manufacturing No.1-a to 1-e, steel 2 to 27 and 29 to 35 are an average effective crystal grain size near the surface 5.0 It can be adjusted to μm or less, and the critical CTOD value (δ- ₁₆₅ (after applying 1% tensile strain)) of the joint Toe part after undergoing strength and plastic deformation of 1% satisfies the target range. . Steel No. 28 and Test No. 28 are reference examples.

  On the other hand, the steel of Test No. 1-f was not suitable for the manufacturing method and the shape ratio of the final finishing pass was less than 1, so the average crystal grain size near the steel plate surface exceeded 5 μm. For this reason, the critical CTOD value of the joint Toe portion after receiving 1% plastic deformation was below the target range.

  Since the steels of Test No. 1-g and Test No. 1-h were not suitable for production and did not satisfy the formula (2), the average crystal grain size in the vicinity of the steel sheet surface exceeded 5 μm. For this reason, the critical CTOD value of the joint Toe portion after receiving 1% plastic deformation was below the target range.

The steel of Test No.36 has a large P _hardening , the steel of Test No.37 has a small P _hardening , and the steel of Test No.38 has a high Mn content. After that, the critical CTOD value of the joint Toe was below the target range.

  The steel of Test No. 39 had a low Ni content and a high finish rolling temperature, so the average crystal grain size near the steel plate surface exceeded 5 μm. For this reason, although there was no problem in strength characteristics, the critical CTOD value of the joint Toe portion after being subjected to 1% plastic deformation was below the target range.

  Test No. 40 steel has a high Al content, and Test No. 41 steel has a high N content, so there is no problem in strength characteristics, but the critical CTOD value of the joint Toe after undergoing 1% plastic deformation Fell below the target range.

  Even if the steel for cryogenic temperature according to the present invention is welded as a base material, a steel having excellent CTOD characteristics of the weld heat affected zone including the Toe portion can be obtained. Furthermore, this good characteristic is ensured even after undergoing a large plastic deformation due to a strong earthquake. Since this steel has a lower Ni content than 9% Ni steel, it is inexpensive and excellent in low-temperature toughness. Therefore, it is suitable as a structural material for storage tanks of low-temperature substances such as LNG.

Claims (11)

In mass%, C: 0.01 to 0.12%, Mn: 0.4 to 1.1% , Ni: 5.5 to 8.5%, Al: 0.002 to 0.05%, N: 0.0015-0.004% is contained, the balance is made of Fe and impurities, of which impurities are Si: 0.15% or less, P: 0.05% or less and S: 0.008% or less, And the P _hardening value defined by the following formula (1) is a steel material of 0.54 to 0.65, and the average effective crystal grain size in the region of 0.2 mm or less from the steel surface is 5.0 μm or less. Cryogenic steel with excellent CTOD characteristics after applying strain in the heat-affected zone that is not subjected to subsequent thermal cycles.
P _hardening = 0.075Si + 0.217Mn + 0.042Ni + 0.25Cr + 0.32Mo (1) Formula Here, the element symbol in the formula represents the content (mass%) of each element.   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. A CTOD characteristic after applying strain in a heat-affected zone that is not subjected to a subsequent thermal cycle, wherein the steel ingot having the chemical composition according to any one of claims 1 to 5 is subjected to the following steps: An excellent method for producing cryogenic steel.
[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 at a cooling rate RA (° C./s) satisfying the following expression (5).
RA ≧ 3 (5) where RA represents the cooling rate (° C./s) at the center of the plate thickness during cooling. A CTOD characteristic after applying strain in a heat-affected zone that is not subjected to a subsequent thermal cycle, wherein the steel ingot having the chemical composition according to any one of claims 1 to 5 is subjected to the following steps: An excellent method for producing cryogenic steel.
[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 at a cooling rate RA (° C./s) satisfying the following expression (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. A CTOD characteristic after applying strain in a heat-affected zone that is not subjected to a subsequent thermal cycle, wherein the steel ingot having the chemical composition according to any one of claims 1 to 5 is subjected to the following steps: An excellent method for producing cryogenic steel.
[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 at a cooling rate RA (° C./s) satisfying the following expression (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 characterized by applying the cryogenic steel material according to any one of claims 1 to 5 to an inner tank member. An LNG tank characterized by applying the cryogenic steel material according to any one of claims 1 to 5 to an annular plate.

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