JP7272438B2 - Steel material, manufacturing method thereof, and tank - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material suitable for structural steel used in extremely low temperature environments, such as a liquefied gas storage tank, and a method for producing the same. The present invention also relates to a tank using this steel material.

In order to use a hot-rolled steel sheet as a material for a structure for a liquefied gas storage tank, it is required that the steel sheet have not only high strength but also excellent toughness at low temperatures because the operating environment becomes extremely low temperature. be. For example, when a hot-rolled steel plate is used for a storage tank for liquefied natural gas, it is necessary to secure excellent toughness at the boiling point of liquefied natural gas: -164°C or lower. If the low-temperature toughness of the steel material is inferior, it may become impossible to maintain the safety of the structure for cryogenic storage tanks. In the following description, the term “low temperature” includes the cryogenic range of −164° C. or lower.

To meet this requirement, conventionally, austenitic stainless steels, 9% Ni steels, or 5000 series aluminum alloys having an austenite steel structure that does not show brittleness at low temperatures have been used. However, due to high alloy costs and manufacturing costs, there is a demand for steel materials that are inexpensive and have excellent low-temperature toughness.

Therefore, as a new steel material to replace the conventional steel for low temperature, it is proposed to use a high Mn steel to which a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is added as a structural steel for a low temperature environment. has been proposed to

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

Japanese Patent Publication No. 2015-508452

In the austenitic steel material described in Patent Document 1, the cooling rate of the weld heat affected zone is limited to 10° C./s or more from the viewpoint of carbide suppression. When a steel sheet having a thickness of less than 10 mm is cooled at 10° C./s or more, the steel sheet is likely to be warped or distorted, and an extra step such as shape correction is required, which hinders productivity. 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 does not verify the low temperature toughness in the C direction.

A liquefied gas storage tank structure (for example, a liquefied gas storage tank) is manufactured by welding steel materials. Since the internal pressure from the liquefied natural gas is applied to the inner wall of a liquefied gas storage tank (hereinafter sometimes referred to as a tank), the steel materials that make up the tank have a rolling direction (L direction) and a plate width direction (C direction). ), tensile stress is generated not only in directions parallel to all the steel materials constituting the tank (hereinafter sometimes referred to as “all directions”). Furthermore, tensile stresses in the L and C directions are also generated in the welded portion of the tank. Therefore, when steel is used as a material for tanks, the base metal (base metal part) and welded parts must have characteristics that can withstand loads due to tensile stress in all directions, especially in the L and C directions. be. As described above, in the present invention, "all directions" refer to all directions including directions perpendicular to and parallel to the rolling direction.

It is known that the steel materials used for the above-mentioned applications deteriorate in toughness called strain aging embrittlement not only at the raw material stage but also when subjected to plastic deformation due to processing or unforeseen accidents. .

The present invention has been made in view of the above problems, and an object of the present invention is to provide a steel material excellent in low temperature toughness, a method for manufacturing the same, and a tank.

Here, the above-mentioned "weld heat affected zone" refers to the weld heat affected zone coarse-grained zone (CGHAZ), which is a portion where the toughness is reduced in general steel.

In addition, the above-mentioned "excellent low-temperature toughness" means that the absorbed energy (vE _-196 ) of the Charpy impact test at -196 ° C in all directions at the 1/2 plate thickness position in the steel material is 41 J or more. Point. Generally, the absorbed energy in the Charpy impact test in the C direction shows the lowest value compared to the L direction and Z direction (thickness direction). Therefore, in the present invention, when the absorbed energy (vE ₋₁₉₆ ) in the C direction is 41 J, it is called “excellent in low temperature toughness”. In addition, the above "41J" is a specification proposal for -196 ° C in the L direction of high Mn steel created by IACS (International Federation of Classification Societies) as of 2019, and 27J is proposed as the absorbed energy in the C direction. there is According to the present invention, the specifications in the L direction can be satisfied even in the Charpy impact test in the C direction.

In order to achieve the above objects, the present inventors have investigated the composition, microstructure, and manufacturing method of austenitic steel (e.g., high-Mn steel), and the properties of the welded part of this steel. Intensive research was conducted on various factors that determine As a result, the following findings a to d were obtained.

a. In order to improve the absorbed energy in the Charpy impact test at -196 ° C, the development of the (110) [001] texture, which has the lowest surface atomic density in the face-centered cubic structure (FCC), is suppressed and the hardness is increased. It is important to keep it below 300HV. Hot rolling under appropriate conditions to control the (110)[001] texture strength to less than 10.0 is effective in improving the absorbed energy. Preferably, the (110)[001] texture strength is less than 9.0.

b. High-Mn austenitic steel contains a large amount of Mn, and thus has a larger amount of sulfide-based inclusions than carbon steel. Furthermore, since sulfide-based inclusions extend in the rolling direction, the C-direction fracture surface in the Charpy impact test generally has a higher area ratio of sulfide-based inclusions than the L-direction fracture surface. Since sulfide-based inclusions are one of the factors that cause fracture initiation, if the cleanliness of sulfide-based inclusions is 1.0% or more after hot rolling, the low-temperature toughness is degraded. For this reason, it is effective to lower the cleanliness of sulfide inclusions in order to improve the low-temperature toughness of high-Mn steel.

c. In the hot rolling, if cross rolling is performed under appropriate conditions, the above b can be achieved also in the C direction.

d. Unlike carbon steel, high-Mn steel does not undergo transformation during welding, and therefore retains its pre-welding microstructure even after welding.

The present invention was made by further studying the above knowledge, and the gist thereof is as follows.
[1] The microstructure has an area ratio of 95% or more FCC,
The (110) [001] texture strength at the 1/2 plate thickness position is less than 10.0,
The hardness at the plate thickness 1/2 position is less than 300 HV,
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.
[2] The steel material according to [1], which has an absorbed energy of 41 J or more in a Charpy impact test at −196° C. in the C direction at the half thickness position after strain aging.
[3] The steel material according to [1] or [2], wherein the absorbed energy in the Charpy impact test at −196° C. in the C direction in the coarse-grained region of the weld heat-affected zone is 41 J or more.
[4] in % by 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%,
Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less,
A component composition with the balance being iron and inevitable 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 further includes, in % by mass,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: The steel material according to [4], containing one or more selected from 2.0% or less.
[6] The steel material according to [4] or [5], wherein the sulfide-based inclusions are MnS.
[7] A method for manufacturing the steel material according to any one of [1] to [6],
The steel material is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and 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 A method for producing a steel material, comprising hot rolling under conditions of 750° C. or higher and then cooling.
Cross rolling ratio=rolling direction rolling ratio/rolling perpendicular direction rolling ratio (1)
[8] A tank welded with the steel material according to any one of [1] to [6],
A tank having an absorbed energy of 41 J or more in a Charpy impact test at −196° C. in the C direction in the coarse-grained region of the weld heat affected zone.

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 steel structures (such as tanks for liquefied gas storage tanks) used in low-temperature environments. A tank with toughness can be provided. Therefore, it can greatly contribute to the improvement of the safety and life of the steel structure, and has a remarkable industrial effect. In addition, since the production method of the present invention does not cause a decrease in productivity and an increase in production costs, it is possible to provide an economical production method.

The present invention will be described in detail below. In addition, this invention is not limited to the following embodiment.

First, the technical idea of the present invention will be described 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 excellent in low-temperature toughness. In order to use this high Mn steel material as a material for steel structures (for example, tanks) used in a low temperature environment, the inner wall and welded parts of the tank must have the property of withstanding the internal pressure of the stored gas, especially only in the L and C directions. However, it is required to have properties that can withstand loads due to tensile stress in all directions.

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

As a result of the research of the present inventors, when a high Mn steel material is subjected to a Charpy impact test at an ultra-low temperature of -196 ° C., although it is a ductile fracture, the absorbed energy in the L direction is about 100 J, and the absorbed energy in the C direction is about 100 J. It was found that there are cases where it falls below 41J. This means that base metals and welds of tanks manufactured by welding high Mn steel materials are likely to break when tensile impact stress acts in the direction perpendicular to the rolling direction.

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

In addition, "C direction" refers to a direction perpendicular to the rolling direction (L direction). "Charpy impact test in the C direction" means that the longitudinal direction of the Charpy impact test piece is parallel to the C direction and the notch is oriented in the rolling direction. The "rolling direction" in the present application refers to the rolling direction in which the total reduction amount is the largest among various directions in which the rolled material is rolled.

Therefore, as a result of further intensive investigation of the cause, the inventors of the present invention found that the rolling texture (texture due to rolling) is caused by such a difference in absorbed energy, that is, the relationship between ductile fracture and texture. newly found. The relationship between ductile fracture and texture will be described below.

In the present invention, attention is focused on the direction in which the Charpy test piece is hit in the Charpy impact test. An L-direction Charpy test piece (however, the notch is in the C direction) taken so that the longitudinal direction of the Charpy test piece is in the rolling direction of the steel plate, and a Charpy test piece whose longitudinal direction is perpendicular to the rolling direction of the steel plate Considering the direction of hitting the C-direction Charpy test piece (however, the notch faces the L direction) to be taken in the direction of .

As noted above, higher (110) texture tends to be less tough. Since the absorbed energy cannot be predicted from the texture, the reason is not clear. 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 with the notch in the C direction, but a bad value is obtained in the C-direction Charpy impact test with the notch in the L direction. The JIS standard specifies that the absorbed energy value in the C-direction Charpy impact test should be 27 J or more, and a low value is acceptable. However, when the tank is formed, as described above, since stress is applied in all directions, it is preferable that the C direction has the same level of absorbed energy as in the L direction.

In the case of an austenitic steel, the base material does not undergo transformation even when the temperature is raised. Therefore, the texture of the weld obtained by welding the austenitic steel is substantially the same as that of the base material, that is, does not change. Therefore, it is important to create a texture during the production of the austenitic steel as the base material.

Therefore, in the hot rolling process described later in the present invention, the (110) [001] texture that is easily formed during normal rolling and the texture that develops other orientations by cross rolling that rotates 90 degrees and rolls The strength of the (110)[001] texture is reduced (ie, the (110)[001] texture is not developed) by mixing with the tissue as evenly as possible. Here, in the face-centered cubic structure (FCC), the (110) plane has the lowest surface atomic density, and the plane with the lowest surface atomic density is the most fragile plane. In ductile fracture, such a brittle surface is likely to be torn off, and the absorbed energy is considered to be low. Therefore, it is considered that the Charpy absorbed energy in the L direction and the C direction can be equalized by not developing the (110)[001] texture.

Furthermore, as a result of the research by the present inventors, it was found that when the hardness of the high Mn steel material is 300 HV or more, the absorbed energy in the Charpy impact test at −196 ° C. in the C direction after strain aging is less than 41 J. I found out. Although the detailed mechanism is unknown, it is considered that the dislocation density is higher in high hardness, and therefore more dislocations are anchored by C, which is contained in a large amount in the high Mn steel.

Next, the steel material of the present invention will be explained.

In the steel material of the present invention, the microstructure at normal pressure has an area ratio of 95% or more of the FCC structure, the (110) [001] texture strength at the 1/2 position of the plate thickness is less than 10.0, and the plate The hardness at the 1/2 thickness position is less than 300 HV, and the absorbed energy in the Charpy impact test at -196°C in the C direction at the 1/2 thickness position is 41 J or more.
In addition, the steel material of the present invention can have an absorbed energy of 41 J or more in a C-direction Charpy impact test at −196° C. in the coarse-grained region of the weld heat-affected zone after strain aging and welding.
The microstructure may also have a cleanliness of less than 1.0% sulfide inclusions.

The reasons for limiting the microstructure as described above in the present invention will be described below.

[Microstructure of steel]
Microstructure at normal pressure: 95% or more in area ratio is FCC structure In the present invention, "microstructure at normal pressure" refers to a microstructure in a temperature range from 1300 ° C. or less to -273 ° C. under a pressure of 1 atm. . In the case of high Mn steel, the microstructure in the temperature range of 1300° C. or lower (for example, 1250° C.) has an area ratio of 95% or more FCC.

As described above, when the crystal structure of the steel material is a body-centered cubic structure (BCC), the steel material may undergo brittle fracture in a low-temperature environment, so it is not suitable for use in a low-temperature environment. Therefore, assuming use in a low-temperature environment, the matrix phase of the steel material is required to have a face-centered cubic (FCC) crystal structure. In the present invention, "using austenite as a base phase" means that the austenite phase accounts for 95% or more of the entire microstructure in terms of area ratio. The austenitic phase is preferably 97% or more. The balance other than the austenite phase is ferrite phase and/or martensite phase. The remainder other than the austenite phase preferably has a total area ratio of 5% or less for each phase.

In addition, in the present invention, the area fraction of the austenite phase and the like can be measured by the method described in Examples described later.

(110) [001] texture strength: less than 10.0 In the present invention, as described above, hot rolling is performed under appropriate conditions in order to improve the low temperature toughness of the steel material (base material) and the weld heat affected zone. important to do. This reduces the strength of the microstructure, particularly the (110)[001] texture, and equalizes the Charpy absorbed energies in the C and L directions.

When the (110)[001] texture strength in the microstructure at the position of 1/2 plate thickness is 10.0 or more, cracks are likely to propagate. As a result, the absorbed energy is lowered. Therefore, the above (110)[001] texture strength is set to less than 10.0. It is preferably 9.0 or less. It is more preferably 6.0 or less. Since the absorbed energy in the L direction is reduced, the (110)[001] texture strength in the microstructure at the half thickness position is preferably 1.0 or more. It is more preferably 4.0 or more.

Hardness: Less than 300 HV If the hardness at the half thickness position is 300 HV or more, the ductility is lowered and the absorbed energy is lowered. Therefore, the above hardness is set to less than 300 HV. Preferably, it is 280 HV or less. More preferably, it is 260 HV or less. Since the strength of the steel material decreases, it is preferable that the hardness at the plate thickness 1/2 position is 200 HV or more. More preferably, it is 220HV or higher.

Cleanliness of sulfide-based inclusions: less than 1.0% (preferred conditions)
If the cleanliness of the sulfide-based 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 fracture. As a result, the absorbed energy may decrease. Therefore, the cleanliness of the sulfide-based inclusions is preferably less than 1.0%. More preferably, it is 0.8% or less. More preferably, it should be 0.6% or less. Although the lower limit of the cleanliness is not particularly specified, it is preferably 0.1% or more from the viewpoint of manufacturing cost.

In addition, the above-described cleanliness is calculated by the following formula (2).
d=(n/p×f)×100 (2)
Here, in the above equation (2), let p be the total number of grid points in the visual field, f be the number of visual fields, and n be the number of grid point centers occupied by inclusions in the f visual fields.
Therefore, the cleanliness is a value obtained by calculating the area percentage occupied by sulfide-based inclusions at the plate thickness 1/2 position of the steel material, and indicates the sulfide-based inclusions in the C direction. Examples of sulfide-based inclusions include MnS.

The above (110) [001] texture strength: less than 10.0, hardness: less than 300 HV, and purity of sulfide inclusions: less than 1.0% are hot rolled according to the conditions described later. It can be realized by

In the present invention, the texture strength, hardness, and cleanliness of sulfide-based inclusions can be measured by the methods described in Examples below.

The steel material of the present invention having the above microstructure is excellent in low temperature toughness.

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

If the (110) [001] texture strength is less than 10.0 and the hardness is less than 300 HV, the microstructure at the 1/2 thickness position of the steel material is C direction and Absorbed energy (vE ₋₁₉₆ ): 41 J or more can be realized in all directions including the L direction. As a result, the absorbed energy (vE ₋₁₉₆ ) in the C direction of the coarse-grained region of the weld heat-affected zone (vE −196 ): 41 J or more can be realized even in the weld zone welded with the steel material of the present invention. In addition, the steel material of the present invention is subjected to pre-strain and aging treatment under predetermined conditions (for example, the conditions described in Examples below), and the absorbed energy in the C direction (vE _-196 ) after strain aging: 41 J or more. can be realized.
Welding conditions such as a preferable amount of heat are the same as preferable welding conditions for a tank, which will be described later, and therefore are omitted here.

In addition to the above texture strength and hardness, if the cleanliness of sulfide-based inclusions at the 1/2 plate thickness position of the steel material is less than 1.0%, even in the C direction, which shows a low value, Absorbed energy (vE ₋₁₉₆ ): 41 J or more can be obtained more effectively.

Next, a preferred range of the chemical composition of the steel material (austenitic steel material) of the present invention will be described. The austenitic steel material (for example, high Mn steel material) of the present invention is used as a raw material, and a structure (for example, a tank) obtained by welding this steel material has the same chemical composition and microstructure in the base material and the welded part. (However, the austenite grain size in the weld increases).

[Component composition]
In the present invention, the austenitic steel material and the steel material used for its production have the above-described chemical composition. The chemical composition of the austenitic steel material of the present invention and the reasons for its limitation will be described. In addition, the display of "%" regarding a component composition means "mass %" unless otherwise indicated.

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, the C content is preferably 0.100% or more. On the other hand, when the C content exceeds 0.700%, Cr carbides are excessively formed, which may reduce the low temperature toughness. Therefore, C is preferably 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, 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 dissolves in steel and has the effect of increasing the strength of the steel sheet by solid solution strengthening. . In order to obtain such an effect, the Si content is preferably 0.05% or more. On the other hand, when the Si content exceeds 1.00%, the non-thermal stress increases excessively, which may deteriorate the low-temperature toughness. Therefore, it is preferable that the Si content is 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, 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 the present invention, Ni is an important element for achieving both strength and low temperature toughness. In order to obtain the effect, the Mn content is preferably 20.0% or more. On the other hand, if the Mn content exceeds 40.0%, the low temperature toughness may deteriorate. Moreover, there is a possibility that weldability and cuttability may deteriorate. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, Mn is preferably 20.0% or more and 40.0% or less. Mn is more preferably 23.0% or more, more preferably 24.0% or more. It is more preferably 35.0% or less, still more preferably 30.0% or less.

P: 0.030% or less When the P content exceeds 0.030%, excessive segregation occurs at grain boundaries, resulting in a decrease in low-temperature toughness. Therefore, it is desirable to set the upper limit to 0.030% and reduce it as much as possible. Therefore, P is set to 0.030% or less. In addition, excessive reduction of P raises the refining cost and is economically disadvantageous, so it is desirable to make the P content 0.002% or more. P is more preferably 0.005% or more, more preferably 0.010% or more. It is more preferably 0.028% or less, still more preferably 0.024% or less.

S: 0.0050% or less S deteriorates the low-temperature toughness and ductility of the base metal, so it is desirable to reduce it as much as possible, with the upper limit set to 0.0050%. Therefore, S is set to 0.0050% or less. It is more preferably 0.0045% or less, still more preferably 0.0040% or less. Note that an excessive reduction of S increases the refining cost and is economically disadvantageous, so it is desirable that the S content is 0.0010% or more. More preferably, it is 0.0012% or more.

Al: 5.00% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process for steel sheets. In addition, the yield strength and local elongation during tensile tests are improved. In order to obtain such an effect, it is preferable to contain 0.01% or more of Al. On the other hand, if the Al content exceeds 5.00%, a large amount of inclusions will be present, deteriorating the low temperature toughness, so the content is made 5.00% or less. Al is more preferably 0.01% or more, more preferably 0.02% or more. Al is more preferably 4.00% or less, more preferably 3.50% or less.

Cr: 7.0% or less Cr is an element effective in improving low-temperature toughness because it improves grain boundary strength. In order to obtain such an effect, it is preferable to contain 0.5% or more of Cr. On the other hand, if the Cr content exceeds 7.0%, there is a risk that the low temperature toughness and stress corrosion cracking resistance will decrease due to the formation of Cr carbides. For this reason, Cr is preferably 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, more preferably 6.5% or less. Moreover, in order to further improve the resistance to stress corrosion cracking, it is even more preferable to make the Cr content 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 in improving low temperature toughness. In order to obtain such an effect, it is preferable to contain 0.0050% or more of N. On the other hand, if the N content exceeds 0.0500%, the nitrides or carbonitrides may coarsen and the toughness may decrease. Therefore, it is preferable that N be 0.0500% or less. N is preferably 0.0050% or more, more preferably 0.0060% or more, still more preferably 0.0070% or more. N is more preferably 0.0400% or less, more preferably 0.0300% or less.

O: 0.0050% or less O deteriorates the low temperature toughness due to the formation of oxides. Therefore, the content of O is set to 0.0050% or less. The content is preferably 0.0045% or less, more preferably 0.0040% or less, still more preferably 0.0035% or less. In addition, excessive reduction of O raises the refining cost and is economically disadvantageous. More preferably, it is 0.0012% or more.

Ti: less than 0.005%, Nb: less than 0.005% Ti and Nb form carbonitrides with a high melting point in the steel, which lowers the low-temperature toughness. Since Ti and Nb are components that are unavoidably mixed from raw materials, etc., Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less are mixed. It is customary. Therefore, it is necessary to avoid unavoidable contamination of Ti and Nb and suppress the contents of Ti and Nb to less than 0.005%, respectively, according to the melting method described later. By suppressing the contents of Ti and Nb to less than 0.005% each, the adverse effects of the carbonitrides described above can be eliminated, 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 0.001% or more.

One or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less It is a useful element. Controlling the morphology of inclusions means turning expanded sulfide-based inclusions into granular inclusions. Ductility, toughness and resistance to sulfide stress corrosion cracking are improved through morphology control of inclusions. In order to obtain such effects, it is preferable to contain Ca and Mg at 0.0005% or more and REM at 0.0010% or more. On the other hand, if any element is contained in a large amount, the amount of non-metallic inclusions increases, and ductility, toughness, and resistance to sulfide stress corrosion cracking decrease. Moreover, it becomes disadvantageous economically.

Therefore, when Ca and Mg are contained, it is preferably 0.0100% or less, and when REM is contained, it is preferably 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 material of the present invention, the balance other than the above components is iron (Fe) and unavoidable impurities. The unavoidable impurities here include H, B, etc., and if the total content of each element is 0.01% or less, it is permissible.

It is preferable to use the above elements as a basic component composition. This basic component composition provides the desired properties of the present invention. In the present invention, for the purpose of further improving strength and low-temperature toughness, in addition to the above elements, the following elements can be contained as necessary.

One 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 increase the strength of the steel sheet by solid-solution strengthening, but also improve the mobility of dislocations and improve the low-temperature toughness. . In order to obtain such effects, Cu and Ni are preferably contained in an amount of 0.01% or more. On the other hand, if the content of Cu and Ni exceeds 1.0%, the surface quality deteriorates during rolling and the manufacturing cost is increased. Therefore, when these alloying elements are contained, the content thereof is preferably 1.0% or less. More preferably 0.03% or more, more preferably 0.7% or less. More preferably, it is 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 stabilizing austenite and improving base metal strength. In order to obtain such effects, each of Mo, V and W is preferably contained in an amount of 0.001% or more. On the other hand, if each of Mo, V, and W is contained in an amount exceeding 2.0%, coarse carbonitrides are formed, which may become starting points of fracture, and also press the production cost. Therefore, when these alloying elements are contained, the content thereof is preferably 2.0% or less. More preferably 0.003% or more, more preferably 1.7% or less. More preferably, it is 1.5% or less.

In the present invention, "steel material (austenitic steel material)" refers to a steel plate having a thickness of 6 mm or more. From the viewpoint of suitable use as a material for structural steel used in extremely low temperature environments, the plate thickness is preferably more than 9 mm, more preferably 12 mm or more. The upper limit of the plate thickness is not particularly limited and may be any thickness, but is preferably 40 mm or less.

[Manufacturing method of steel]
Next, a method for manufacturing a steel material according to one embodiment of the present invention will be described.

The steel material (austenitic steel material) of the present invention can be produced by melting molten steel having the chemical composition described above by a known melting method such as a converter or an electric furnace. Secondary refining may also be performed in a vacuum degassing furnace.

At that time, in order to limit Ti and Nb, which hinder structure control, to the numerical ranges described above, measures are taken to avoid unavoidable contamination of Ti and Nb from raw materials, etc., and to reduce their content. There is a need. For example, by reducing the basicity of the slag during the refining stage, these alloys are concentrated in the slag and discharged, reducing the concentration of Ti and Nb in the final slab product. Alternatively, a method such as blowing oxygen to oxidize the Ti and Nb alloys and separating the alloy of Ti and Nb during reflux may be used.

After that, it is preferable to form a steel material such as a slab having a predetermined size by a known casting method such as a continuous casting method, an ingot casting-blooming rolling method, or the like.

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

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

In the following description of the manufacturing method, unless otherwise specified, "°C" regarding temperature indicates the surface temperature of the steel material or steel plate. The surface temperature can be measured with, for example, a radiation thermometer. In addition, the temperature at the thickness center position of the slab or steel plate can be measured, for example, by attaching a thermocouple to the thickness center of the steel plate, or by calculating the temperature distribution in the steel plate cross section by heat transfer analysis, and using the results as the steel plate can be obtained by correcting with the surface temperature of

Heating temperature of steel material: 1100° C. or higher and 1300° C. or lower In order to diffuse Mn during hot rolling, the heating temperature of the steel material before hot rolling is set to 1100° C. or higher. By diffusing Mn, the stability of austenite can be ensured even in the Mn negative segregation part. As a result, the stability of austenite can be secured even in the coarse-grained region of the weld heat-affected zone obtained when welding, and brittle fracture can be prevented. On the other hand, if the heating temperature exceeds 1300.degree. Preferably, it is 1130°C or higher and 1270°C or lower.

Cross rolling ratio calculated by formula (1): 20 or less Cross rolling ratio=rolling direction rolling ratio/rolling perpendicular direction rolling ratio (1)
Here, the "rolling direction rolling ratio" refers to the rolling direction rolling ratio to the total rolling. "Cross-rolling roll ratio" refers to the ratio of cross-rolling to total rolling. Therefore, the "rolling direction rolling ratio/rolling direction perpendicular to rolling ratio" indicates the rolling ratio of the rolling direction to the rolling direction perpendicular to the rolling direction.

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

Furthermore, it is also effective to reduce the area fraction of sulfide inclusions in the C direction by performing cross rolling in the C direction during hot rolling and setting the cross rolling ratio to 20 or less. The cross rolling ratio is preferably 18 or less, more preferably 15 or less.

By repeating rolling in the same direction, the (110) [001] texture develops, so it is preferable to alternately repeat rolling in the rolling direction and rolling in the direction perpendicular to the rolling for homogenization of the texture. . Preferably, it is repeated two or more times. Preferably 3 times or less.

Rolling reduction of final pass of finish rolling: 30% or less, finishing temperature of finish rolling: 750° C. or higher When the rolling reduction of the final pass of finish rolling exceeds 30%, the dislocation density becomes excessively high and the low temperature toughness deteriorates. If the finishing temperature of finish rolling is lower than 750°C, the (110)[001] texture develops excessively and the low temperature toughness deteriorates. Therefore, the draft of the final pass of finish rolling is set to 30% or less. The rolling reduction is preferably less than 25%, more preferably 20% or less. The finishing temperature of finish rolling shall be 750° C. or higher. The finishing temperature of finish rolling is preferably 780° C. or higher, more preferably 800° C. or higher. Although the upper limit of the finish rolling finish temperature is not particularly specified, it is preferably 950° C. or lower, more preferably 920° C. or lower, from the viewpoint of ensuring strength. Although the lower limit of the rolling reduction in the final pass of finish rolling is not specified, it is preferably 5% or more, more preferably 10% or more, from the viewpoint of ensuring strength.

In addition, in the present invention, it is preferable to further control the cross rolling to the following conditions for the purpose of further improving the strength and toughness.

Rolling start temperature (preferred condition)
The rolling start temperature is preferably 1100 to 1250°C. If it is less than 1100°C, the rolling temperature will be less than 780°C, and the texture may develop excessively. If the temperature exceeds 1250°C, the texture may not change.

Rolling temperature (preferred conditions)
The rolling temperature (temperature during rolling) is preferably 780 to 1250°C. If the temperature is less than 780°C, the texture may develop excessively. If the temperature exceeds 1250°C, the texture may not change.

Reduction amount (preferred condition)
The reduction amount in the temperature range of 780 to 1250° C. is preferably 60 to 98%. If the reduction amount is less than 60%, the texture may not change. If the reduction amount exceeds 98%, the texture may develop excessively. The reduction amount means the total reduction ratio in the temperature range of 780 to 1250°C.

Cooling Cooling is performed after hot rolling is completed. No particular cooling conditions are specified. It is preferable to cool from a temperature of (temperature at the end of hot rolling - 100°C) or higher to 600°C or lower at an average cooling rate of 1.0°C/s or higher. This suppresses the formation of carbides and grain boundary segregation of P, and further enhances the properties of the steel material. The above-mentioned "temperature at the end of hot rolling" refers to the finish rolling end temperature.

Next, the tank of the present invention will be explained.

The tank of the present invention is a tank manufactured by welding the steel materials described above. As described in the knowledge d above, the steel material of the present invention retains the microstructure before welding even after welding. Therefore, the chemical composition and microstructure of the base material of the tank of the present invention are the same as those of the above steel material (austenitic steel material). By specifying the chemical composition and microstructure of the base metal (steel) as described above, a tank having an absorbed energy of 41 J or more in a Charpy impact test at -196°C at the half thickness position of the base metal can be obtained. be done. In addition, the absorbed energy in the Charpy impact test at -196°C in the coarse-grained region of the welded heat-affected zone of the tank can be made 41 J or more. Furthermore, the absorbed energy in the Charpy impact test at -196°C after strain aging can be increased to 41 J or more.

Since the tank of the present invention has the above properties, it can be used in an extremely low temperature environment such as a liquefied gas storage tank.

Next, a preferred example of the method for manufacturing the tank will be described.

The tank of the present invention is manufactured by welding the above steel materials. Since the manufacturing method of the steel material (austenitic steel material), which is the raw material, has already been described, it will be omitted. Here, suitable welding conditions will be described.

[Preferred Welding Conditions]
The preferred type of welding is gas metal arc welding.

The heat input range is preferably 3.0 kJ/mm or less. Moreover, it is preferably 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 is reduced.

As described above, according to the present invention, the absorbed energy in 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 characteristics can be reduced. This improved the reliability of the material (raw material).

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

A steel slab having the chemical composition shown in Table 1 was produced by a converter-ladle refining-continuous casting method. In addition, "-" shown in Table 1 indicates that it is not added intentionally, and means that it includes not only the case of no content (0%) but also the case of unavoidable inclusion. Next, the obtained steel slabs were hot rolled under the conditions shown in Table 2 and then cooled to produce steel materials (steel sheets) having a thickness of 6 to 40 mm.
In cross rolling, the temperature during rolling: 780 to 1250 ° C., the amount of reduction at 780 to 1250 ° C.: 60 to 98%, and the cooling condition after rolling: 1.0 ° C./s or more. I did. The "cooling condition after the completion of rolling" refers to the average cooling rate from (the temperature at the end of hot rolling - 100°C) to a temperature of 600°C or lower.

In addition, joint test plates (size: 250 mm×500 mm) were taken from the obtained steel plates, and welded joints were produced by welding them together in the L direction and the C direction. Here, welding was performed under welding conditions of groove shape: square shape, backing material: ceramics, shield gas: Ar-30% CO ₂ , and torch receding angle: 5 to 10°.

Using the obtained steel sheets and welded joints, the tensile test properties, low temperature toughness, and microstructure of the steel sheets were evaluated, and the low temperature toughness of the weld heat-affected coarse grain region of the welded joints was evaluated according to the following procedures. was carried out.

(1) Tensile test properties Using the obtained steel sheet, the following tensile test specimens were taken from the 1/2 sheet thickness position at the central position in the longitudinal direction and width direction of the steel sheet. A JIS No. 4 tensile test piece was taken from the steel plate with a thickness of more than 15 mm, and a round bar tensile test piece was taken from the steel plate with a thickness of 15 mm or less. Using each tensile test piece, a tensile test was performed in accordance with JIS Z2241 (2011) to evaluate tensile strength (TS) and yield stress (YS). In this example, those having a yield stress of 400 MPa or more were judged to be "excellent in base material strength".

(2) Low-temperature toughness Evaluation of the low-temperature toughness of steel sheets was performed as follows.

Using the obtained steel sheet, a C-direction Charpy V-notch test piece was taken from the direction perpendicular to the rolling direction at a position of 1/2 of the thickness from the surface of the steel sheet. In addition, a L-direction Charpy V-notch test piece was sampled from the direction parallel to the rolling direction at a position of 1/2 of the plate thickness from the steel plate surface of the obtained steel plate. Furthermore, tensile test pieces with a gauge length of 200 mm were taken from the L direction and the C direction at 1/2 of the plate thickness from the steel plate surface of the obtained steel plate, and after 5% tensile prestrain, at 250 ° C. L-direction and C-direction Charpy V-notch test pieces were taken from the tensile test pieces that had been aged for 1 hour.

Then, three Charpy impact tests were performed on each steel plate in accordance with the provisions of JIS Z 2242 (2005) to determine the absorbed energy at -196°C to evaluate the toughness of the steel material (base material). As described above, the C direction of the steel sheet exhibits a low value of toughness. Therefore, in this example, the average value of the absorbed energies (vE ₋₁₉₆ ) of the three lines in the C direction of 41 J or more was judged to be “excellent in base material toughness”.

For steel sheets with a thickness of 10 mm or less, sub-sized (5 mm) Charpy V-notch test pieces were prepared in the C direction, and three Charpy impact tests were performed on each test piece at -196°C. In Table 3, "*1" is shown in the item of absorbed energy for the samples performed using sub-sized Charpy V-notch test pieces. In the case of the subsize, the average value of the absorbed energies (vE ₋₁₉₆ ) of the three wires in the C direction of 27 J or more was judged to be “excellent in base material toughness”.

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

A Charpy V-notch test piece was collected from each welded joint with a plate thickness exceeding 10 mm in accordance with the provisions of JIS Z 2242 (2005), and three Charpy impact tests were performed on each welded joint at -196 ° C. . In this example, when the average value of the absorbed energies of the three wires was 41 J or more, it was determined that the toughness of the weld zone was excellent.

In addition, for each welded joint with a plate thickness of less than 10 mm, a 5 mm sub-sized Charpy V notch test piece was collected in accordance with the provisions of JIS Z 2242 (2005), and three Charpy impact tests were performed for each welded joint. was carried out at -196°C. In Table 3, "*1" is shown in the item of absorbed energy for the samples performed using sub-sized Charpy V-notch test pieces. In the case of the sub-size, the average value of the absorbed energy of the three wires was determined to be 27 J or more as being excellent in "excellent toughness of the weld zone."
Here, similarly to the above, the evaluation was performed using the measured value in the direction of steel sheet C, which exhibits the lowest value.

(3) Structure evaluation [Observation of microstructure]
The area ratio of each phase of the microstructure was determined from the phase map of EBSD analysis.

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

In Table 3, "other phases" indicates the total area ratio of the remainder other than the austenite phase, that is, the ferrite phase and/or the martensite phase.

[Texture strength]
Using the obtained steel plate, a test piece for measurement was taken from the 1/2 plate thickness position at the central position in the longitudinal direction and width direction of the 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 texture strength was obtained from the obtained ODF (Orientation Determination Function: three-dimensional crystal orientation distribution function). The ODF is a pole figure ((110) [001], (100) [011], (100) [ 010], (110) [112], (112) [111]).

[Hardness]
Using the obtained steel sheet, 100 points were measured at HV 10 kg at the 1/2 sheet thickness position at the central position in the longitudinal direction and width direction of the steel sheet. The maximum value was used as the highest hardness value.

[Cleanliness of sulfide-based inclusions]
Using the obtained steel plate, an optical microscope sample of a cross section in the rolling direction is cut from the 1/2 position of the plate thickness at the center position in the longitudinal direction and width direction of the steel plate. Microscopic Test Method for Metal Inclusions". Here, the cleanliness of sulfide-based inclusions in the C direction was calculated. 60 visual fields were measured at a microscope magnification of 400, and the cleanliness (%) was calculated using the following formula.
d=(n/p×f)×100 (2)
Here, in the above equation (2), let p be the total number of grid points in the visual field, f be the number of visual fields, and n be the number of grid point centers occupied by inclusions in the f visual fields.
The cleanliness of MnS was calculated as sulfide inclusions.

Table 3 shows the results obtained as described above.

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

On the other hand, in Comparative Examples outside the scope of the present invention, the austenitic steel materials could not satisfy the above target performance. In addition, the resulting welded joint did not satisfy the above target performance in terms of absorbed energy. Furthermore, it was confirmed that the above target performance was satisfied after the strain aging treatment.

Claims (6)

in % by 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%,
Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less,
having a component composition with the balance being iron and inevitable impurities,
The microstructure has an area ratio of 95% or more FCC,
the cleanliness of the sulfide-based inclusions is less than 1.0%, and the sulfide-based inclusions are MnS;
The (110) [001] texture strength at the 1/2 plate thickness position is 4.0 or more and less than 10.0,
The hardness at the plate thickness 1/2 position is 220 HV or more and less than 300 HV,
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. 2. The steel material according to claim 1, wherein the absorbed energy in a Charpy impact test at −196° C. in the C direction of the strain-aged 1/2 thickness position is 41 J or more. 3. The steel material according to claim 1 or 2, wherein the absorbed energy in a Charpy impact test at −196° C. in the C direction in the coarse-grained region of the welded heat affected zone is 41 J or more. 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: The steel material according to any one of claims 1 to 3, containing one or more selected from 2.0% or less. A method for manufacturing a steel material according to any one of claims 1 to 4,
A steel material having the above chemical composition is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and 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 A method for producing a steel material, comprising performing hot rolling under the condition that the finishing temperature of finish rolling is 750° C. or higher, and then cooling.
Cross rolling ratio=rolling direction rolling ratio/rolling perpendicular direction rolling ratio (1) A tank welded with the steel material according to any one of claims 1 to 4,
A tank having an absorbed energy of 41 J or more in a Charpy impact test at −196° C. in the C direction in the coarse-grained region of the weld heat affected zone.