KR20220131996A - Steel material and its manufacturing method, and tank - Google Patents

Abstract

A steel material, a method for manufacturing the same, and a tank are provided. In the steel material of the present invention, the microstructure is 95% or more FCC in terms of area ratio, the (110)[001] texture strength at the 1/2 plate thickness position is less than 10.0, and the hardness at the 1/2 plate thickness position is It is less than 300 HV, and the absorbed energy of the Charpy impact test at -196 degreeC in the C direction in the position of 1/2 plate thickness is 41 J or more.

Description

Steel and its manufacturing method, and tank

FIELD OF THE INVENTION The present invention relates to a steel material and a method for producing the same, suitable for application to structural steel used in very low temperature environments, such as, for example, tanks for liquefied gas storage. Moreover, this invention relates to the tank using this steel material.

In order to use a hot-rolled steel sheet as a raw material of a structure for liquefied gas storage, since the use environment becomes very low temperature, in addition to high strength, it is also calculated|required that the steel sheet is excellent in toughness at low temperature. For example, when using a hot-rolled steel sheet for a tank of liquefied natural gas, it is necessary to ensure the outstanding toughness at the boiling point of liquefied natural gas: -164 degreeC or less. If the low-temperature toughness of the steel is inferior, there is a possibility that the safety as a cryogenic storage structure cannot be maintained. Therefore, the demand for improving the low-temperature toughness of the applied steel is strong. In addition, in the following description, "low temperature" is generically called including the cryogenic region of -164 degrees C or less.

In response to this request, conventionally, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy which uses austenite which does not exhibit brittleness at low temperature as the structure of the steel sheet has been used. However, since alloy cost and manufacturing cost are high, it is cheap and there exists a request|requirement for the steel material excellent in low-temperature toughness.

Therefore, as a new steel material instead of conventional low-temperature steel, it is proposed in Patent Document 1, for example, that a high-Mn steel containing a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is used as structural steel in a low-temperature environment. has been

Patent Document 1 proposes a technique for ensuring low-temperature toughness in a weld heat affected zone by, for example, setting the area fraction of carbides to 5% or less.

Japanese Patent Publication No. 2015-508452

As for the austenitic steel material described in patent document 1, the cooling rate of a weld heat-affected zone is limited to 10 degreeC/s or more from a viewpoint of carbide suppression. When a steel plate with a plate thickness of less than 10 mm is cooled to 10° C./s or more, warpage or distortion tends to occur in the steel plate, an extra step such as shape correction is required, and productivity is impaired. In general, the low-temperature toughness in the rolling width direction (C direction) tends to be inferior to the low-temperature toughness in the rolling direction (L direction), but Patent Document 1 has not verified this low-temperature toughness in the C direction at all.

In addition, the structure for liquefied gas storage (for example, tank for liquefied gas storage) welds steel materials and is manufactured. Since the internal pressure from liquefied natural gas is applied to the inner wall of the tank for liquefied gas storage (hereinafter, sometimes referred to as a tank), the steel materials constituting the tank have not only the rolling direction (L direction) and the plate width direction (C direction) but also , a tensile stress is also generated in a direction parallel to all the steel materials constituting the tank (hereinafter, also referred to as “all directions”). In addition, tensile stresses in the L and C directions are also generated in the welded portion of the tank. Therefore, when a steel material is used as a raw material for a tank, it is necessary for the base metal (base metal part) and the welded part to have characteristics that can withstand the load caused by tensile stress in all directions, especially in the L direction and the C direction. In addition, as mentioned above, in this invention, the said "all directions" shall refer to all directions including a direction perpendicular|vertical to a rolling direction, and a parallel direction.

In addition, it is known that the toughness, called strain aging embrittlement, deteriorates when the steel material used for the above-mentioned purpose is subjected to plastic deformation due to processing or unexpected accidents as well as the material stage.

This invention has been made in view of the said subject, and an object of this invention is to provide the steel material excellent in low-temperature toughness, its manufacturing method, and a tank.

Here, the "welding heat affected zone" refers to a coarse grain heat affected zone (CGHAZ), which is a portion in which toughness decreases in general steel.

In addition, the said "it was excellent in low-temperature toughness" means that the absorbed energy (vE-196 ) of the Charpy impact test at _-196 °C in all directions in the 1/2 position of the plate thickness is 41J or more in steel materials. Usually, compared with the L direction and the Z direction (thickness direction), the absorbed energy of the Charpy impact test in the C direction shows the lowest value. Therefore, in the present invention, if the absorbed energy (vE ₋₁₉₆ ) in the C direction is 41 J, it is called “excellent low-temperature toughness”. In addition, the above "41J" is a specification of -196°C in the L direction of high Mn steel prepared by IACS (International Association of Classification Society) as of 2019, and 27J is proposed as the absorbed energy in the C direction. . According to the present invention, the specification in the L direction can be satisfied also in the Charpy impact test in the C direction.

MEANS TO SOLVE THE PROBLEM In order to achieve the said subject, the present inventors target an austenitic steel material (for example, high Mn steel material), the component composition of a steel material (steel sheet), microstructure, and a manufacturing method, and the characteristic of the welding part which welded this steel material. Conducted intensive research on various factors that determine As a result, the following recognitions a to d were obtained.

a. In order to improve the absorbed energy of the Charpy impact test at -196°C, the development of the texture of (110)[001], which has the smallest surface atomic density in the face-centered cubic structure (FCC), is suppressed, and the hardness is lowered to less than 300 HV. it is important to do It is effective to improve the absorbed energy by performing hot rolling under appropriate conditions and controlling the (110)[001] texture strength to be less than 10.0. Preferably, the (110)[001] texture strength is less than 9.0.

b. Since high Mn austenitic steel contains a large amount of Mn, sulfide-type inclusions exist more abundantly than carbon steel. In addition, since the sulfide-based inclusions extend in the rolling direction, in general, the C-direction fracture surface of the Charpy impact test has a higher area ratio of the sulfide-based inclusions than the L-direction fracture surface. Since sulfide inclusions are one factor of the origin of fracture, when the cleanliness of sulfide inclusions is 1.0% or more after hot rolling, deterioration of low-temperature toughness is caused. From this point of view, it is effective to lower the cleanliness of the sulfide inclusions to improve the low-temperature toughness of the high-Mn steel.

c. Hot rolling WHEREIN: If cross rolling is performed under suitable conditions, said b can be implement|achieved also in C direction.

d. Unlike carbon steel, high-Mn steel does not transform during welding, so it inherits the microstructure before welding even after welding.

This invention is made|formed by adding further examination to the above recognition, The summary is as follows.

[1] In the microstructure, 95% or more of the area ratio is FCC,

The (110) [001] texture strength of the plate thickness 1/2 position is less than 10.0,

The hardness at 1/2 of the plate thickness is less than 300HV,

A steel material having an absorbed energy of 41 J or more in a Charpy impact test at -196°C in the C direction at the plate thickness 1/2 position.

[2] The steel material according to [1], wherein the absorbed energy of the Charpy impact test at -196°C in the C direction at the 1/2 plate thickness position after strain aging is 41 J or more.

[3] The steel material according to [1] or [2], wherein the absorbed energy of the Charpy impact test at -196°C in the C direction in the granulation zone of the weld heat affected zone is 41 J or more.

[4] in mass %,

C: 0.100% or more and 0.700% or less;

Si: 0.05% or more and 1.00% or less;

Mn: 20.0% or more and 40.0% or less;

P: 0.030% or less;

S: 0.0050% or less;

Al: 5.00% or less;

Cr: 7.0% or less;

N: 0.0500% or less;

O: 0.0050% or less;

Ti: less than 0.005%;

Nb: contains less than 0.005%,

Ca: 0.0100% or less, Mg: 0.0100% or less, REM: contains one or two or more selected from 0.0200% or less,

a component composition in which the balance consists of iron and unavoidable impurities;

The steel material according to any one of [1] to [3], wherein the microstructure has a cleanliness of less than 1.0% of sulfide inclusions.

[5] The component composition is further, in mass%,

Cu: 1.0% or less;

Ni: 1.0% or less;

Mo: 2.0% or less;

V: 2.0% or less;

W: 2.0% or less

The steel material as described in [4] containing 1 type or 2 or more types selected from.

[6] The steel material according to [4] or [5], wherein the sulfide inclusion is MnS.

[7] The method for producing a steel material according to any one of [1] to [6],

The steel material is heated to a temperature range of 1100°C or more and 1300°C or less, the cross rolling ratio calculated by the formula (1) is 20 or less, the reduction ratio of the final pass of finish rolling is 30% or less, and the finish rolling end temperature is 750°C The manufacturing method of steel materials which performs cooling, after performing hot rolling on the conditions used as the above.

Cross rolling ratio = rolling direction rolling ratio / rolling right angle direction rolling ratio... (One)

[8] A tank in which the steel materials according to any one of [1] to [6] are welded,

The tank in which the absorbed energy of the Charpy impact test at -196 degreeC in C direction in a welding heat affected zone granulation area is 41 J or more.

ADVANTAGE OF THE INVENTION According to this invention, the steel material excellent in low-temperature toughness and its manufacturing method can be provided. In addition, the steel material of the present invention is suitably used as a material for a steel structure (liquid gas storage tank, etc.) used in a low-temperature environment, and accordingly, the base metal after welding and the weld heat affected zone together provide a tank having excellent low-temperature toughness. can do. Therefore, it can greatly contribute to the improvement of the safety and life of the steel structure, brings a special effect in the industry. Moreover, since the manufacturing method of this invention does not produce the fall of productivity and the increase of manufacturing cost, it can provide the manufacturing method excellent also in economical efficiency.

(Form for implementing the invention)

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail. In addition, this invention is not limited to the following embodiment.

First, the technical idea of this invention is demonstrated in detail.

As described above, there is an austenitic steel material (for example, a high Mn steel material) as a steel material that is inexpensive and has excellent low-temperature toughness. In order to use this high-Mn steel material as a material for a steel structure (for example, a tank) used in a low-temperature environment, the inner wall and welding part of the tank have characteristics that can withstand the low pressure of the gas, especially in the L direction and C direction, as well as It is required to have a characteristic that can withstand the load caused by tensile stress in all directions.

Since the high-Mn steel material (here, the Mn content points out the steel plate of 20.0-40.0 mass %) is an austenitic steel material, brittle fracture does not occur fundamentally, and most are ductile fractures. On the other hand, in ordinary steel (herein, it refers to a low-carbon steel sheet having a crystal structure of BCC at room temperature), ductile fracture is independent of the texture, and the shelf energy (maximum absorbed energy) of ordinary steel is 200 J or more, and may exceed 300 J depending on conditions. That is, since the absorbed energy of ordinary steel is sufficiently large, in the case of ordinary steel, there is no need to consider the absorbed energy as a problem unless a brittle fracture front is formed.

As a result of the studies of the present inventors, high Mn steel is ductile fracture when subjected to a Charpy impact test at an ultra-low temperature of -196° C., but the absorbed energy in the L direction is about 100 J, and the absorbed energy in the C direction is less than 41 J It was found that there are cases where This means that the base metal and welded portion of a tank manufactured by welding high Mn steel materials are easily destroyed when a tensile impact stress is applied in a direction perpendicular to the rolling direction.

That is, the internal pressure of liquefied natural gas applied to the inner wall and welded portion of the tank occurs in the L direction, the C direction, and the direction parallel to the inner surface (inner wall) of all steel materials constituting the tank, so that in all directions It is necessary to have a sufficient toughness value. It is known that a rolled material has the lowest toughness when a Charpy impact test piece in the C direction is taken with respect to the rolling direction. Therefore, it is important to improve the toughness value of the Charpy impact test in the C direction.

In addition, "C direction" refers to the direction perpendicular|vertical with respect to a rolling direction (L direction). "C-direction Charpy impact test" refers to that the longitudinal direction of the Charpy impact test piece is parallel to the C direction, and the notch faces the rolling direction. The "rolling direction" of this application refers to the rolling direction with the largest total rolling-reduction|draft amount among those which rolled the rolling material in various directions.

Then, as a result of further earnestly investigating this cause, the present inventors discovered that the rolling texture (texture by rolling) originates in this difference in absorbed energy, ie, the relationship between a ductile fracture and a texture, newly discovered. Below, the relationship between a ductile fracture and a texture is demonstrated.

In the present invention, attention was paid to the direction of hitting the Charpy test piece in the Charpy impact test. L-direction Charpy test specimens taken so that the longitudinal direction of the Charpy specimen is in the rolling direction of the steel sheet (however, the notch is in the C direction), and C collected so that the longitudinal direction of the Charpy specimen is perpendicular to the rolling direction of the steel sheet Direction The Charpy test piece (however, the notch is facing the L direction) was considered about the direction of hitting.

As described above, the higher the texture of (110), the lower the toughness tends to be. Since the absorbed energy cannot be predicted from the texture, the reason is not clear, but as will be described later, it is thought that the (110)[001] texture probably has an influence. In this texture, the (100) plane is oriented in the C direction and the (110) plane is oriented in the L direction, respectively. Therefore, a good value is obtained in the L-direction Charpy impact test having a notch in the C direction, but a bad value in the C-direction Charpy impact test having a notch in the L direction. In the JIS standard, the absorbed energy value of the C-direction Charpy impact test is prescribed to be 27 J or more, and a low value may be sufficient. However, when the tank is formed, as described above, since the stress is applied in the entire direction, it is preferable to have the same absorbed energy in the L direction also in the C direction.

In the case of an austenitic steel, since there is no transformation even when the temperature is raised, the texture of the weld zone obtained by welding the austenitic steel is substantially the same as that of the base material, that is, it does not change. Therefore, it becomes important to make a texture at the time of manufacture of the austenitic steel material used as a base material.

Therefore, in the present invention, in the process of hot rolling to be described later, a (110)[001] texture that is easily formed during normal rolling and a texture in which different orientations are developed by cross rolling that rotates 90 degrees and rolls are possible. By mixing by the same amount, the strength of the (110)[001] texture is reduced (ie, the (110)[001] texture is not developed). Here, in a face-centered cubic structure (FCC), the surface with the smallest surface atomic density in the (110) plane, and the surface with a small surface atomic density is the weakest surface. In ductile fracture, it is thought that such a fragile surface is brittle and the absorbed energy is low. Therefore, it is considered that the L-direction and C-direction Charpy absorbed energy can be equalized by not developing the (110)[001] texture.

Moreover, as a result of the research of the present inventors, it was recognized that the absorbed energy of a Charpy impact test at -196 degreeC in C direction in C direction after strain aging becomes less than 41 J of high Mn steel materials when hardness is 300 HV or more. Although the detailed mechanism is unclear, it is thought that the high hardness has a higher dislocation density, so that C, which is contained in a large amount in the high Mn steel, fixed more dislocations.

Next, the steel materials of this invention are demonstrated.

In the steel material of the present invention, the microstructure at normal pressure has an FCC structure of 95% or more in terms of area ratio, the (110)[001] texture strength at the position of 1/2 of the plate thickness is less than 10.0, and the plate thickness is 1/ The hardness at position 2 is less than 300 HV, and the absorbed energy of the Charpy impact test at -196°C in the C direction at the position of 1/2 of the plate thickness is 41 J or more.

Moreover, in the steel materials of this invention, the absorbed energy of the C direction Charpy impact test at -196 degreeC in the welded heat affected zone granulation zone after strain aging can be 41 J or more.

In addition, in the microstructure, the cleanliness of sulfide inclusions can be less than 1.0%.

Hereinafter, the reason for limiting the microstructure as mentioned above in this invention is demonstrated.

[Micro organization of steel]

Microstructure at normal pressure: 95% or more FCC structure by area ratio

In this invention, "microstructure in normal pressure" refers to the microstructure in the temperature range from the temperature of 1300 degrees C or less to -273 degreeC under the pressure of 1 atm. In the case of high Mn steel materials, the microstructure in a temperature range of 1300°C or lower (eg, 1250°C) is 95% or more FCC in terms of area ratio.

As described above, when the crystal structure of the steel material is a body-centered cubic structure (BCC), since the steel material may cause brittle fracture in a low-temperature environment, it is not suitable for use in a low-temperature environment. Therefore, assuming use in a low-temperature environment, the base phase of steel is required to have a face-centered cubic structure (FCC). In addition, in this invention, "use austenite as a matrix phase" means that the austenite phase is 95% or more in area ratio with respect to the whole microstructure. Austenite phase becomes like this. Preferably it is 97 % or more. The remainder other than the austenite phase is a ferrite phase and/or a martensite phase. As for the remainder other than the austenite phase, it is preferable that the total area ratio of each phase is 5% or less.

In addition, in this invention, the area fraction of an austenite phase etc. can be measured by the method described in the Example mentioned later.

(110)[001] texture strength: less than 10.0

In the present invention, as described above, in order to improve the low-temperature toughness of the steel material (base material) and the weld heat affected zone, it is important to perform hot rolling under appropriate conditions. Accordingly, the strength of the microstructure, particularly the (110)[001] texture, can be reduced, and the Charpy absorbed energy in the C direction and the L direction can be equalized.

When the (110)[001] texture strength in the microstructure at the position 1/2 of the plate thickness is 10.0 or more, cracks are more likely to propagate. As a result, absorbed energy falls. For this reason, the (110)[001] texture strength is set to less than 10.0. Preferably, it is set to 9.0 or less. More preferably, it is set to 6.0 or less. Since the absorbed energy in the L direction decreases, the (110)[001] texture strength in the microstructure at the 1/2 thickness position is preferably set to 1.0 or more. More preferably, it is set to 4.0 or more.

Hardness: less than 300HV

When the hardness at the position of 1/2 of the plate thickness is 300 HV or more, the ductility decreases and the absorbed energy decreases. For this reason, said hardness shall be less than 300 HV. Preferably, it is set as 280 HV or less. More preferably, it is set as 260 HV or less. Since the strength of steel materials falls, it is preferable that the hardness at the position of 1/2 plate thickness shall be 200 HV or more. More preferably, it is 220 HV or more.

Purity of sulfide inclusions: less than 1.0% (suitable conditions)

When the degree of cleanliness of the sulfide inclusions in the microstructure at the position of 1/2 of the plate thickness is 1.0% or more, it becomes the starting point of destruction. As a result, there exists a possibility that absorbed energy may fall. For this reason, it is preferable that the cleanliness of the said sulfide-type inclusion shall be less than 1.0 %. More preferably, it is made into 0.8 % or less. More preferably, it is set as 0.6 % or less. Although the lower limit in particular of said cleanliness is not prescribed|regulated, It is preferable to set it as 0.1 % or more from a viewpoint of manufacturing cost.

In addition, said cleanliness is computed by the following formula (2).

d=(n/p×f)×100… (2)

Here, in the formula (2), p: the total number of grid points in the field of view, f: the number of fields of view, and n: the number of grid point centers occupied by inclusions in the f fields.

Accordingly, the cleanliness is a value obtained by calculating the area percentage occupied by the sulfide inclusions at the 1/2 position of the sheet thickness of the steel, and represents the sulfide inclusions in the C direction. As a sulfide-type inclusion, MnS is mentioned, for example.

The aforementioned (110)[001] texture strength: less than 10.0, hardness: less than 300 HV, and cleanliness of sulfide inclusions: less than 1.0% can be realized by performing hot rolling according to the conditions described below.

In the present invention, the above-described texture strength, hardness, and cleanliness of sulfide-based inclusions can be measured by the method described in Examples to be described later.

The steel material of this invention which has the above microstructure is excellent in low-temperature toughness.

Here, in addition to the steel material (base material) having the above-described microstructure, the absorbed energy of the Charpy impact test after strain aging and at -196°C of the weld heat affected zone was measured.

The microstructure at the plate thickness 1/2 position of the steel material is (110) [001] When the texture strength is less than 10.0 and the hardness is less than 300 HV, in the plate thickness 1/2 position of the steel material, the C direction and L Absorbed energy (vE ₋₁₉₆ ): 41 J or more can be realized in all directions including the direction. Thereby, also in the welding part which welded the steel materials of this invention, the absorbed energy ( _vE -196) of the C direction of a welding heat affected zone granulation region: 41 J or more can be implement|achieved. Further, the absorbed energy in the C direction (vE-196 ) after strain aging in which the steel of the present invention was subjected to an aging treatment by giving a pre-strain under predetermined conditions (for example, the conditions described in the examples described later): _41J more can be realized.

In addition, since welding conditions, such as a preferable calorie|heat amount, are the same as the suitable welding conditions of the tank mentioned later, they are abbreviate|omitted here.

In addition to the above-mentioned texture strength and hardness, when the cleanliness of the sulfide-based inclusions at the 1/2 position of the steel material is less than 1.0%, the absorbed energy ( vE _-196 ): 41J or higher can be obtained.

Next, the preferable range of the component composition in the steel material (austenitic steel material) of this invention is demonstrated. In addition, the structure (eg, tank) obtained by using the austenitic steel of the present invention (eg, high Mn steel) as a material and welding this steel has the same composition and microstructure as the base material and the welded portion. (However, the austenite grain size at the weld becomes large).

[Ingredient composition]

In this invention, an austenitic steel material and the steel raw material used for its manufacture have the above-mentioned component composition. The component composition of the austenitic steel material of this invention and the reason for its limitation are demonstrated. In addition, unless otherwise indicated, the display of "%" regarding a component composition means "mass %".

C: 0.100% or more and 0.700% or less

C is an inexpensive austenite stabilizing element, and is an important element for obtaining austenite. In order to obtain this effect, it is preferable to contain 0.100% or more of C. On the other hand, when C is contained in an amount exceeding 0.700%, Cr carbides are excessively formed, and there is a fear that the low-temperature toughness may decrease. For this reason, it is preferable to make C into 0.100% or more and 0.700% or less. C is more preferably 0.200% or more, and more preferably 0.600% or less. C is more preferably 0.250% or more, and still more preferably 0.550% or less.

Si: 0.05% or more and 1.00% or less

Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of being dissolved in steel and strengthening the steel sheet by solid solution strengthening. In order to obtain such an effect, it is preferable to contain 0.05% or more of Si. On the other hand, since non-thermal stress rises excessively when Si contains exceeding 1.00 %, there exists a possibility that low-temperature toughness may deteriorate. For this reason, it is preferable to make Si into 0.05 % or more and 1.00 % or less. Si is more preferably 0.07% or more, and more preferably 0.80% or less. Si is more preferably 0.10% or more, and still more preferably 0.60% or less.

Mn: 20.0% or more and 40.0% or less

Mn is a relatively inexpensive austenite stabilizing element. In this invention, in order to make strength and low-temperature toughness compatible, it is an important element. In order to acquire the effect, it is preferable to contain 20.0% or more of Mn. On the other hand, when Mn is contained exceeding 40.0%, there exists a possibility that low-temperature toughness may deteriorate. Moreover, there exists a possibility that weldability and cutability may deteriorate. In addition, it promotes segregation and promotes the occurrence of stress corrosion cracking. For this reason, it is preferable to make Mn into 20.0 % or more and 40.0 % or less. Mn is more preferably 23.0% or more, and still more preferably 24.0% or more. More preferably, it is set as 35.0 % or less, More preferably, it is set as 30.0 % or less.

P: 0.030% or less

When P is contained in an amount exceeding 0.030%, it excessively segregates at grain boundaries, so that the low-temperature toughness decreases. For this reason, it is preferable to make 0.030 % as an upper limit, and to reduce as much as possible. Therefore, P is made into 0.030% or less. Moreover, since excessive reduction of P increases refining cost and becomes economically disadvantageous, it is preferable to set it as 0.002 % or more. P is more preferably 0.005% or more, still more preferably 0.010% or more. More preferably, you may be 0.028 % or less, More preferably, you may be 0.024 % or less.

S: 0.0050% or less

Since S deteriorates the low-temperature toughness and ductility of a base material, it is preferable to make 0.0050% as an upper limit and to reduce it as much as possible. Therefore, S is made into 0.0050% or less. More preferably, it is set as 0.0045 % or less, More preferably, it is set as 0.0040 % or less. In addition, since excessive reduction of S increases the refining cost and becomes economically disadvantageous, S is preferably set to 0.0010% or more. More preferably, it is set as 0.0012% or more.

Al: 5.00% or less

Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of a steel sheet. Moreover, the yield strength and local elongation at the time of a tensile test improve. In order to acquire such an effect, it is preferable to contain 0.01 % or more of Al. On the other hand, when Al is contained in excess of 5.00%, inclusions are present in a large amount to deteriorate the low-temperature toughness, so it is set to 5.00% or less. Al is more preferably 0.01% or more, and still more preferably 0.02% or more. Al is more preferably made into 4.00% or less, More preferably, it is made into 3.50% or less.

Cr: 7.0% or less

Cr is an element effective in improving the low-temperature toughness in order to improve the grain boundary strength. In order to acquire such an effect, it is preferable to contain 0.5% or more of Cr. On the other hand, when Cr is contained in an amount exceeding 7.0%, there is a fear that the low-temperature toughness and stress corrosion cracking resistance may decrease due to the formation of Cr carbides. For this reason, it is preferable to make Cr into 7.0 % or less. Cr is preferably 0.5% or more, more preferably 1.0% or more, and still more preferably 1.2% or more. Cr is more preferably 6.7% or less, and still more preferably 6.5% or less. In addition, in order to further improve the stress corrosion cracking resistance, it is still more preferable to set Cr to 2.0% or more and 6.0% or less.

N: 0.0500% or less

N is an austenite stabilizing element, and is an element effective for improving low-temperature toughness. In order to acquire such an effect, it is preferable to contain 0.0050% or more of N. On the other hand, when N is contained exceeding 0.0500 %, nitride or carbonitride coarsens, and there exists a possibility that toughness may fall. For this reason, it is preferable to make N into 0.0500 % or less. N is preferably 0.0050% or more, more preferably 0.0060% or more, and still more preferably 0.0070% or more. N is more preferably 0.0400% or less, and still more preferably 0.0300% or less.

O: 0.0050% or less

O deteriorates low-temperature toughness by formation of an oxide. For this reason, O is made into the range of 0.0050% or less. Preferably it is 0.0045 % or less, More preferably, it is set as 0.0040 % or less, More preferably, it is set as 0.0035 % or less. Moreover, since reduction of excessive O increases refining cost sharply and becomes economically disadvantageous, it is preferable to make O into 0.0010% or more. More preferably, it is set as 0.0012% or more.

Ti: less than 0.005%, Nb: less than 0.005%

Ti and Nb form carbonitrides having a high melting point in steel, so that the low-temperature toughness decreases. Since Ti and Nb are components that are unavoidably mixed from raw materials and the like, it is customary to mix them in the range of Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less. Then, according to the method of the solvent mentioned later, it is necessary to avoid unavoidable mixing of Ti and Nb, and to suppress content of Ti and Nb to less than 0.005 %, respectively. By suppressing the content of Ti and Nb to less than 0.005%, respectively, the above-described adverse effect of carbonitride is excluded, and excellent low-temperature toughness and ductility can be ensured. Preferably, the content of Ti and Nb is 0.003% or less. Of course, the content of Ti and Nb may be 0%. More preferably, it is made into 0.001 % or more.

Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less 1 type or 2 or more types selected from

Ca, Mg, and REM (rare earth metal) are elements useful for morphological control of inclusions. The shape control of inclusions refers to using elongated sulfide inclusions as particulate inclusions. Through control of the shape of these inclusions, ductility, toughness and resistance to sulfide stress corrosion cracking are improved. In order to acquire such an effect, it is preferable to contain 0.0005% or more of Ca and Mg, and 0.0010% or more of REM. On the other hand, when a large amount of any element is contained, the amount of non-metallic inclusions increases, and on the contrary, ductility, toughness, and sulfide stress corrosion cracking resistance decrease. Also, it becomes economically disadvantageous.

For this reason, when Ca and Mg are contained, it is preferable to set it as 0.0100 % or less, respectively, and when it contains REM, it is preferable to set it as 0.0200 % or less. Preferably, Ca is 0.0005% or more, Mg is 0.0005% or more, and REM is 0.0010% or more. More preferably, Ca is 0.0010% or more and 0.0080% or less, Mg is 0.0010% or more and 0.0080% or less, and REM is 0.0020% or more and 0.0150% or less. More preferably, Ca is 0.0050% or less and Mg is 0.0050% or less.

In the austenitic steel of the present invention, the remainder other than the above-described components is iron (Fe) and unavoidable impurities. As unavoidable impurities here, H, B, etc. are mentioned, and if it is 0.01 % or less in total of each element, it is permissible.

It is preferable to make the said element into a basic component composition. By this basic component composition, the characteristic aimed at in the present invention is obtained. In the present invention, for the purpose of further improving strength and low-temperature toughness, in addition to the above elements, the following elements may be contained as necessary.

One or two or more selected from Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less

Cu: 1.0% or less, Ni: 1.0% or less

Cu and Ni are elements that not only strengthen the steel sheet by solid solution strengthening, but also improve dislocation mobility and low-temperature toughness. In order to acquire such an effect, it is preferable to contain Cu and Ni in 0.01 % or more. On the other hand, when Cu and Ni contain exceeding 1.0 %, a surface property will deteriorate at the time of rolling, and manufacturing cost will be pressed. For this reason, when these alloy elements are contained, it is preferable to make the content into 1.0 % or less, respectively. More preferably, it is set as 0.03 % or more, More preferably, it is set as 0.7 % or less. More preferably, it is made into 0.5 % or less.

Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less

Mo, V, and W contribute to the improvement of the strength of the base metal while contributing to the stabilization of austenite. In order to acquire such an effect, it is preferable to contain 0.001% or more of Mo, V, and W, respectively. On the other hand, when Mo, V, and W each contain exceeding 2.0 %, a coarse carbonitride may generate|occur|produce and it may become a starting point of destruction, and manufacturing cost will be pressed. For this reason, when these alloying elements are contained, it is preferable to make the content into 2.0 % or less, respectively. More preferably, you may be 0.003 % or more, More preferably, you may be 1.7 % or less. More preferably, it is made into 1.5 % or less.

In addition, in this invention, "steel materials (austenitic steel materials)" shall refer to the steel plate of 6 mm or more of plate|board thickness. From the viewpoint of suitably used as a raw material for structural steel used in a very low temperature environment, the plate thickness is preferably set to more than 9 mm, more preferably set to 12 mm or more. The upper limit of plate|board thickness is not specifically limited, Although it can be set as arbitrary thickness, it is preferable to set it as 40 mm or less.

[Method of manufacturing steel]

Next, the manufacturing method of the steel materials in one Embodiment of this invention is demonstrated.

The steel material (austenitic steel material) of this invention can melt|melt molten steel which has an above-described component composition by well-known melting methods, such as a converter and an electric furnace. Moreover, you may perform secondary refining in a vacuum degassing furnace.

At that time, in order to limit Ti and Nb, which interfere with the control of the structure, to the above-mentioned numerical range, it is necessary to avoid unavoidable mixing of Ti and Nb from raw materials and the like, and to take measures to reduce these contents. For example, by lowering the basicity of the slag in the refining step, these alloys are concentrated into slag and discharged, and the concentration of Ti and Nb in the final slab product is reduced. Alternatively, a method such as oxidizing by blowing in oxygen, and flotation separation of an alloy of Ti and Nb during refluxing may be used.

Thereafter, it is preferable to use a known casting method such as a continuous casting method or an ingot making and slabbing method to obtain a steel material such as a slab having a predetermined size.

Hereinafter, manufacturing conditions for forming the steel material into a steel material having excellent low-temperature toughness (austenitic steel material) will be described in detail.

In order to obtain an austenitic steel material having the above-described configuration, the steel material is heated to a temperature range of 1100°C or higher and 1300°C or lower, a predetermined cross rolling is performed, and the rolling reduction in the final pass of finish rolling is 30% or less, finishing It is important to perform hot rolling under the condition that the rolling end temperature is 750°C or higher. The temperature control here is based on the surface temperature of the steel material.

In addition, in the following description of a manufacturing method, the "degreeC" indication regarding temperature is the surface temperature of a steel raw material or a steel plate, respectively, unless otherwise stated. The surface temperature can be measured, for example, with a radiation thermometer or the like. In addition, the temperature at the plate thickness center position of the slab or steel plate is measured by attaching a thermocouple to the plate thickness center of the steel plate, or the temperature distribution in the steel plate cross section is calculated by electrothermal analysis, and the result is calculated on the surface of the steel plate It can be calculated|required by correct|amending by temperature.

Heating temperature of steel material: 1100℃ or more and 1300℃ or less

In order to diffuse Mn in hot rolling, the heating temperature of the steel material before hot rolling is made into 1100 degreeC or more. By diffusing Mn, it is possible to ensure the stability of austenite even in the Mn negative segregation portion. Thereby, the stability of austenite can be ensured also in the granulation area|region of a welding heat affected zone obtained when welding is carried out, and a brittle fracture can be prevented. On the other hand, since there exists a possibility that melting of steel may start when heating temperature exceeds 1300 degreeC, the upper limit of heating temperature shall be 1300 degreeC. Preferably, it is 1130 degreeC or more and 1270 degrees C or less.

Cross rolling ratio calculated by formula (1): 20 or less

Cross rolling ratio = rolling direction rolling ratio / rolling right angle direction rolling ratio... (One)

Here, the "rolling direction rolling ratio" refers to the rolling ratio of the rolling direction with respect to total rolling. A "rolling right angle direction rolling ratio" refers to the rolling ratio of the rolling right angle direction with respect to total rolling. Therefore, "rolling direction rolling ratio/rolling right angle direction rolling ratio" represents the rolling ratio of the rolling direction with respect to rolling right angle direction rolling.

As described above, in the rolling of austenitic steel, the (110)[001] texture tends to develop. Therefore, the ratio of the (110)[001] texture is reduced by applying the rolling in different directions, and the strength of the (110)[001] texture can be reduced. (110) [001] In order to make the texture strength less than 10.0, the cross rolling ratio calculated by the formula (1) is set to 20 or less.

It is also effective to reduce the area fraction of sulfide inclusions in the C direction by performing cross rolling in which rolling is performed in the C direction at the time of hot rolling and making the cross rolling ratio 20 or less. Cross-rolling ratio becomes like this. Preferably it is 18 or less, More preferably, it is 15 or less.

In addition, since the (110)[001] texture is developed by repeating rolling in the same direction, it is preferable to alternately repeat rolling in the rolling direction and rolling in the direction perpendicular to the rolling for uniform texture. Preferably, it is preferable to repeat two or more times. Preferably, it is set as 3 times or less.

Rolling reduction in the final pass of finish rolling: 30% or less, finish rolling end temperature: 750°C or more

When the reduction ratio in the final pass of finish rolling exceeds 30%, the dislocation density becomes excessively high and the low-temperature toughness deteriorates. When the finish rolling end temperature is less than 750° C., the (110) [001] texture is excessively developed, and the low-temperature toughness deteriorates. For this reason, the reduction ratio of the final pass of finish rolling is set to 30% or less. The reduction ratio is preferably less than 25%, more preferably 20% or less. The finish rolling end temperature is 750°C or higher. It is preferable to set it as 780 degreeC or more, and, as for the finish rolling completion|finish temperature, it is more preferable to set it as 800 degreeC or more. Although the upper limit of the finish rolling completion temperature is not particularly specified, from the viewpoint of securing strength, it is preferably set to 950°C or lower, and more preferably set to 920°C or lower. Although the lower limit is not specifically prescribed|regulated as for the rolling-reduction|draft ratio of the final pass of finish rolling, 5 % or more is preferable from a viewpoint of ensuring strength, and it is more preferable to set it as 10 % or more.

Further, in the present invention, for the purpose of further improvement in strength and toughness, it is preferable to further control under the following conditions in cross rolling.

Rolling start temperature (suitable conditions)

As for rolling start temperature, 1100-1250 degreeC is preferable. If it is less than 1100 degreeC, a rolling temperature becomes less than 780 degreeC, and there exists a possibility that a texture may develop excessively. If it exceeds 1250 degreeC, there exists a possibility that a texture may not change.

Rolling temperature (suitable conditions)

As for rolling temperature (temperature during rolling), 780-1250 degreeC is preferable. If it is less than 780 degreeC, there exists a possibility that a texture may develop excessively. If it exceeds 1250 degreeC, there exists a possibility that a texture may not change.

Amount of reduction (suitable condition)

As for the amount of rolling reduction in the temperature range of 780-1250 degreeC, 60 to 98 % is preferable. If the reduction amount is less than 60%, there is a fear that the texture does not change. If the reduction amount is more than 98%, there is a possibility that the texture may develop excessively. The said reduction amount represents the total reduction ratio in the temperature range of 780-1250 degreeC.

Cooling

After the hot rolling is finished, cooling is performed. Cooling conditions are not specifically prescribed. It is preferable to cool to 600 degrees C or less at a temperature of (temperature at the time of completion|finish of hot rolling - 100 degreeC) or more at an average cooling rate of 1.0 degreeC/s or more. Thereby, carbide formation and grain boundary segregation of P are suppressed, and the characteristic of steel materials becomes higher. In addition, said "temperature at the time of completion|finish of hot rolling" refers to the finish-rolling completion temperature.

Next, the tank of this invention is demonstrated.

The tank of the present invention is a tank manufactured by welding the above-described steel materials. As described in the above recognition d, the steel material of the present invention inherits the microstructure before welding even after welding. For this reason, the component composition and microstructure in the base material of the tank of this invention are the same as that of the above-mentioned steel material (austenitic steel material). By prescribing the component composition and microstructure of the base material (steel) as described above, a tank having an absorbed energy of 41 J or more in a Charpy impact test at -196°C at a position of 1/2 of the plate thickness of the base material is obtained. Moreover, the absorbed energy of the Charpy impact test at -196 degreeC in the welding heat affected zone assembly area of a tank can be made into 41 J or more. Moreover, the absorbed energy of the Charpy impact test at -196 degreeC after strain aging can be made into 41 J or more.

Since the tank of the present invention has the above characteristics, it can be used in a very low temperature environment such as a tank for storing liquefied gas.

Next, a suitable example of the manufacturing method of the said tank is demonstrated.

The tank of this invention is manufactured by welding said steel materials. In addition, since the manufacturing method of the steel material (austenitic steel material) which is a raw material is already demonstrated, it is abbreviate|omitted. Here, suitable welding conditions are demonstrated.

[suitable welding conditions]

As for the kind of welding, gas metal arc welding is preferable.

The heat input range is preferably 3.0 kJ/mm or less. Moreover, Preferably it is 0.5 kJ/mm or more. By satisfying this heat input range, the above characteristics can be satisfied.

The average cooling rate in the temperature range of 500 to 800°C is preferably 10°C/s or more. If the average cooling rate in this temperature range is less than 10°C/s, carbides are formed and the absorbed energy decreases.

As described above, according to the present invention, since the absorbed energy of the Charpy impact test in all directions of the steel material, especially in the L direction and the C direction, can be equalized, the orientation dependence of the impact characteristics of the steel material (base material) and the welded portion is small. can do. Thereby, the reliability of the material (material) improved.

Example

Hereinafter, the present invention will be described in more detail based on Examples. In addition, the following examples show suitable examples of the present invention, and the present invention is not limited to these examples.

By the converter-ladle refining-continuous casting method, steel slabs having the component compositions shown in Table 1 were produced. In addition, "-" shown in Table 1 has shown that it does not add intentionally, and means not only the case where it does not contain (0%), but also includes the case where it contains unavoidably. Next, the obtained steel slab was hot-rolled under the conditions shown in Table 2, it cooled after that, and the plate|board thickness produced the steel materials (steel plate) of 6-40 mm.

In the cross rolling, the temperature during rolling: 780 to 1250°C, the rolling reduction of 780 to 1250°C: 60 to 98%, and the cooling conditions after rolling: 1.0°C/s or more were appropriately controlled. The said "cooling conditions after completion|finish of rolling" refers to the average cooling rate from the temperature above (temperature at the time of completion|finish of hot rolling - 100 degreeC) to the temperature of 600 degrees C or less.

Furthermore, a weld joint was produced by extracting a joint test plate (size: 250 mm x 500 mm) from the obtained steel plate, and welding those L-directions and C-directions. Here, the shape of the groove: B-shaped, backing material: ceramics, shield gas: Ar-30%CO ₂ , torch drag angle: Welding was performed under welding conditions of 5 to 10°.

Using the obtained steel sheet and welded joint, the evaluation of tensile test properties, low-temperature toughness, and microstructure was performed for the steel sheet, and the evaluation of low-temperature toughness was performed for the weld heat-affected zone granulation region of the welded joint in the following manner, respectively. .

(1) Tensile test properties

Using the obtained steel plate, the tensile test piece shown next was sampled from the plate-thickness 1/2 position in the center position of the longitudinal direction and the width direction of a steel plate. For steel sheets exceeding 15 mm in thickness, JIS No. 4 tensile test pieces were taken, and from steel sheets having a thickness of 15 mm or less, round bar tensile test pieces were taken. Using each tensile test piece, the tensile test based on the prescription|regulation of JIS Z2241 (2011) was done, and tensile strength (TS) and yield stress (YS) were evaluated. In this Example, it was determined that the yield stress had a characteristic of 400 MPa or more as "excellent in the base metal strength".

(2) low temperature toughness

Evaluation of the low-temperature toughness of the steel sheet was performed as follows.

Using the obtained steel sheet, a Charpy V-notch test piece in the C direction was taken from the surface of the steel sheet at a position of 1/2 of the sheet thickness from the direction perpendicular to the rolling direction. Further, a Charpy V-notch test piece in the L direction was taken from the steel sheet surface of the obtained steel sheet at a position of 1/2 of the sheet thickness from a direction parallel to the rolling direction. In addition, at a position 1/2 of the plate thickness from the steel plate surface of the obtained steel plate, tensile test specimens with a distance between the gage points of 200 mm were taken from the L direction and the C direction, respectively, after 5% tensile pre-strain, at 250° C. for 1 hour Charpy V-notch test pieces in the L direction and in the C direction were taken from the tensile test pieces subjected to the aging treatment.

Next, based on the regulations of JIS Z 2242 (2005), three Charpy impact tests were performed on each steel sheet, the absorbed energy at -196°C was determined, and the steel (base material) toughness was evaluated. As described above, the steel sheet C direction exhibits a low toughness value. Therefore, in the present Example, the average value of the three absorbed energies (vE _-196 ) determined that the C direction: 41 J or more was "excellent in the base metal toughness".

In addition, for a steel plate with a plate thickness of 10 mm or less, a sub-size (5 mm) Charpy V-notch test piece was produced in the C direction, and three Charpy impact tests were performed on each test piece at -196°C. In Table 3, in the sample implemented using the subsize Charpy V-notch test piece, "*1" is shown in the item of absorbed energy. In the case of sub-size, the average value of three absorbed energies (vE _-196 ) determined that "it was excellent in the base metal toughness" in C direction: 27J or more.

Evaluation of the low-temperature toughness of a welded joint was performed as follows.

From each weld joint having a plate thickness exceeding 10 mm, a Charpy V-notch test piece was taken in accordance with the regulations of JIS Z 2242 (2005), and three Charpy impact tests were performed for each weld joint at -196 ° C. . In the present Example, the average value of the three absorbed energies judged 41 J or more as "the toughness of a weld part is excellent".

In addition, for each weld joint having a plate thickness of less than 10 mm, in accordance with JIS Z 2242 (2005), a Charpy V-notch test piece with a sub-size of 5 mm is taken, and three Charpy impact tests are performed for each weld joint. was carried out at -196 °C. In Table 3, in the sample implemented using the subsize Charpy V-notch test piece, "*1" is shown in the item of absorbed energy. In the case of a sub-size, the average value of three absorbed energies judged 27 J or more as "it is excellent in the toughness of a welding part".

Here, it evaluated using the measured value in the steel plate C direction which shows the lowest value similarly to the above.

(3) Organizational evaluation

[Observation of micro-organisms]

The area ratio of each phase of the microstructure was calculated|required from the phase map of EBSD analysis.

A test piece for EBSD analysis is taken from a section parallel to the rolling direction at a position of 1/2 of the sheet thickness of the obtained steel sheet, and EBSD analysis is performed in a field of view of 500 μm × 200 μm with a measurement step of 0.3 μm, and the phase map is The values described were taken as the area ratios of the austenite phase, ferrite phase, and martensite phase.

In addition, in Table 3, the remainder other than austenite phase, ie, the total area ratio of a ferrite phase and/or a martensite phase, is shown for "other phases".

[aggregate tissue strength]

Using the obtained steel plate, the test piece for a measurement was extract|collected from the plate-thickness 1/2 position in the center position of the longitudinal direction and the width direction of a steel plate. Using each test piece for measurement, the texture strength of the ND surface was measured by X-ray diffraction. The maximum value of the texture strength was calculated|required from the obtained ODF (Orientation Determination Function: three-dimensional crystal orientation distribution function). In addition, the ODF has the pole viscosity ((110)[001], (100)[011], (100)[010] measured by X-ray diffraction (internal normalization) after removing the residual stress on the surface of the steel sheet by chemical polishing. ], (110)[112], (112)[111]).

[Hardness]

Using the obtained steel plate, 100 points|pieces were measured by HV 10kg in the center position of the longitudinal direction and the width direction of a steel plate at 1/2 plate|board thickness position. Its maximum value was used as the highest hardness value.

[Cleanliness of sulfide inclusions]

Using the obtained steel plate, an optical microscope sample of the cross section in the rolling direction is cut out from the plate thickness 1/2 position at the central position in the longitudinal direction and the width direction of the steel plate, and the "point counting method (point counting method) of JIS G 0555 Annex 1 method) was computed by the microscopic test method of non-metallic inclusions". Here, the cleanliness of the sulfide-based inclusions in the C direction was calculated. 60 fields of measurement were performed at the magnification of a microscope x 400, and cleanliness (%) was computed using the following formula|equation.

d=(n/p×f)×100… (2)

Here, in the formula (2), p: the total number of grid points in the field of view, f: the number of fields of view, and n: the number of grid point centers occupied by inclusions in the f fields.

Further, the cleanliness of MnS as a sulfide inclusion was calculated.

The results obtained by the above are shown in Table 3.

As shown in Table 3, in the austenitic steel of the present invention, the above-described target performance ((110)[001] texture strength: less than 10.0, hardness: less than 300 HV, Charpy impact test at 1/2 the sheet thickness of steel It was confirmed that the absorbed energy (vE ₋₁₉₆ ) satisfies 41 J or more). In addition, it was confirmed that the weld joint obtained by welding the austenitic steel material of the present invention satisfies the above-mentioned target performance (absorbed energy (vE ₋₁₉₆ ) of the Charpy impact test in the welded heat affected zone granulation region is 41 J or more). Moreover, it was confirmed that the above-mentioned performance (absorbed energy (vE ₋₁₉₆ ) of the Charpy impact test after strain aging was 41 J or more) was also satisfied even after the strain aging treatment.

On the other hand, in the comparative example out of the scope of the present invention, the austenitic steel could not satisfy the above target performance. In addition, in the obtained weld joint, the absorbed energy could not satisfy the above-mentioned target performance. In addition, it was confirmed that the above-mentioned target performance was satisfied after the distortion aging treatment.

Claims (8)

The microstructure is 95% or more FCC by area ratio,
The (110) [001] texture strength of the plate thickness 1/2 position is less than 10.0,
The hardness at 1/2 of the plate thickness is less than 300HV,
A steel material having an absorbed energy of 41 J or more in a Charpy impact test at -196°C in the C direction at the plate thickness 1/2 position. According to claim 1,
A steel material having an absorbed energy of 41 J or more in a Charpy impact test at -196°C in the C direction at the 1/2 plate thickness position after strain aging. 3. The method of claim 1 or 2,
A steel material having an absorbed energy of 41 J or more in a Charpy impact test at -196°C in the C direction in a welded heat-affected zone granulation region. 4. The method according to any one of claims 1 to 3,
in mass %,
C: 0.100% or more and 0.700% or less;
Si: 0.05% or more and 1.00% or less;
Mn: 20.0% or more and 40.0% or less;
P: 0.030% or less;
S: 0.0050% or less;
Al: 5.00% or less;
Cr: 7.0% or less;
N: 0.0500% or less;
O: 0.0050% or less;
Ti: less than 0.005%;
Nb: contains less than 0.005%,
Ca: 0.0100% or less, Mg: 0.0100% or less, REM: contains one or two or more selected from 0.0200% or less,
a component composition in which the balance consists of iron and unavoidable impurities;
The said microstructure is a steel material whose cleanliness of a sulfide-type inclusion is less than 1.0%. 5. The method of claim 4,
The component composition is further, in mass%,
Cu: 1.0% or less;
Ni: 1.0% or less;
Mo: 2.0% or less;
V: 2.0% or less;
W: 2.0% or less
Containing one or two or more selected from, steel. 6. The method of claim 4 or 5,
The sulfide-based inclusion is MnS, steel. As a method for manufacturing the steel according to any one of claims 1 to 6,
The steel material is heated to a temperature range of 1100°C or more and 1300°C or less, the cross rolling ratio calculated by the formula (1) is 20 or less, the reduction ratio of the final pass of finish rolling is 30% or less, and the finish rolling end temperature is 750°C The manufacturing method of steel materials which performs cooling, after performing hot rolling on the conditions used as the above.
Cross rolling ratio = rolling direction rolling ratio / rolling right angle direction rolling ratio... (One) A tank in which the steel according to any one of claims 1 to 6 is welded, comprising:
The tank in which the absorbed energy of the Charpy impact test at -196 degreeC in C direction in a welding heat affected zone granulation area is 41 J or more.