TWI842982B - Steel material, method for manufacturing the same, and groove - Google Patents

Abstract

The present invention provides a steel material, a method for manufacturing the same, and a groove. The steel material of the present invention has a microstructure of 95% or more FCC by area ratio, a (110) [001] fabric strength of less than 10.0 at a position of 1/2 of the plate thickness, a hardness of less than 300 HV at a position of 1/2 of the plate thickness, and an absorbed energy of 41 J or more in a Charpy impact test at -196°C in a C direction at a position of 1/2 of the plate thickness.

Description

Steel material, method for manufacturing the same, and groove

The present invention relates to a steel material suitable for use as a structural steel in an extremely low temperature environment such as a tank for a liquefied gas storage tank and a method for manufacturing the same. In addition, the present invention relates to a tank using the steel material.

In order to use hot-rolled steel plates as materials for liquefied gas tank structures, the temperature of the use environment is extremely low, so in addition to high strength, the steel plates are also required to have excellent toughness at low temperatures. For example, when using hot-rolled steel plates in liquefied natural gas tanks, it is necessary to ensure excellent toughness below the boiling point of liquefied natural gas: -164°C. If the low-temperature toughness of the steel is poor, it may not be possible to maintain the safety of the structure for the extremely low-temperature tank, so there is a strong demand for the steel used to improve the low-temperature toughness. In addition, in the following description, the extremely low temperature area including below -164°C is collectively referred to as "low temperature".

In response to such requirements, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy, which uses austenitic steel that does not show brittleness at low temperatures as a steel plate structure, has been used in the past. However, since the alloy cost and manufacturing cost are high, there is a demand for inexpensive steel with excellent low-temperature toughness.

Therefore, as a new steel material to replace the conventional low-temperature steel, for example, Patent Document 1 proposes the use of a high-Mn steel to which a relatively cheap austenite stabilizing element Mn is added in large amounts as a structural steel for a low-temperature environment.

Patent document 1 proposes a technique for ensuring low-temperature toughness in the weld heat-affected portion by, for example, reducing the area fraction of carbides to 5% or less. [Prior art document] [Patent document]

Patent document 1: Japanese Patent Publication No. 2015-508452

[Problems to be Solved by the Invention] For the austenitic steel described in Patent Document 1, the cooling rate of the weld heat-affected portion is limited to 10°C/s or more from the viewpoint of suppressing carbides. When a steel plate with a thickness of less than 10 mm is cooled at a rate of 10°C/s or more, the steel plate is prone to warping or deformation, requiring unnecessary steps such as shape correction, which impedes productivity. Generally, 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.

In addition, structures for liquefied gas storage tanks (e.g., tanks for liquefied gas storage tanks) are manufactured by welding steel materials. Internal pressure from liquefied natural gas is applied to the inner wall of the tank for liquefied gas storage tanks (hereinafter, sometimes referred to as tanks), so tensile stress is generated not only in the rolling direction (L direction) and the plate width direction (C direction) but also in the direction parallel to all the steel materials constituting the tank (hereinafter, sometimes referred to as "all directions"). Furthermore, tensile stress is also generated in the welded portion of the tank in the L and C directions. Therefore, when steel is used as the material for the tank, the base material (base material portion) and the welded portion need to have characteristics that can withstand loads caused by tensile stress in all directions, especially in the L and C directions. Furthermore, as described above, in the present invention, the "all directions" refer to all directions including directions perpendicular to and parallel to the rolling direction.

Furthermore, it is known that the toughness of steel materials used for the above-mentioned applications deteriorates not only in the material stage but also when subjected to plastic deformation due to processing or accidents, which is called strain aging embrittlement.

The present invention is completed in view of the above-mentioned subject, and its purpose is to provide a steel material with excellent low-temperature toughness, a manufacturing method thereof, and a groove.

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

In addition, the "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 thickness position of the steel material is 41 J or more. Generally, the absorbed energy of the Charpy impact test in the C direction shows the lowest value compared to the L direction and the Z direction (plate thickness direction). 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". Furthermore, the "41 J" is the specification plan for -196°C in the L direction of high Mn steel formulated by the International Association of Classification Societies (IACS) in 2019, and 27 J is proposed as the absorbed energy in the C direction. According to the present invention, the specification in the L direction can also be met in the Charpy impact test in the C direction. [Methods of solving the problem]

In order to achieve the above-mentioned problem, the inventors of the present invention have conducted in-depth research on the composition, microstructure and manufacturing method of austenitic steel (e.g., high-Mn steel) and various factors that determine the characteristics of the welded portion of the austenitic steel (steel plate). As a result, the following findings a to d were obtained.

a. In order to increase the absorbed energy of the Charpy impact test at -196°C, it is important to suppress the development of the (110)[001] fabric with the lowest surface atomic density in the face-centered cubic (FCC) structure and to make the hardness less than 300 HV. It is effective to perform hot rolling under appropriate conditions and control the (110)[001] fabric strength to less than 10.0. Preferably, the (110)[001] fabric strength is less than 9.0.

b. High-Mn austenitic steel contains a large amount of Mn, so sulfide inclusions exist in larger amounts compared to carbon steel. Furthermore, since sulfide inclusions extend in the rolling direction, the area ratio of sulfide inclusions is generally higher in the C-direction section of the Charpy impact test than in the L-direction section. Sulfide inclusions are a factor that causes damage, so when the cleanliness of sulfide inclusions after hot rolling is 1.0% or more, it leads to deterioration of low-temperature toughness. Therefore, it is effective to reduce the cleanliness of sulfide inclusions in order to improve the low-temperature toughness of high-Mn steel.

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

d. Unlike carbon steel, high Mn steel does not undergo phase change during welding, so the microstructure before welding will be maintained after welding.

The present invention is completed by further studying the above-mentioned viewpoints, and its gist is as follows. [1] A steel material, wherein the microstructure is FCC in an area ratio of 95% or more, the (110) [001] fabric strength at 1/2 of the plate thickness is less than 10.0, the hardness at 1/2 of the plate thickness is less than 300 HV, and the absorbed energy of the Charpy impact test at -196°C in the C direction at 1/2 of the plate thickness is 41 J or more. [2] The steel material as described in [1], wherein the absorbed energy of the Charpy impact test at -196°C in the C direction at 1/2 of the plate thickness after strain aging is 41 J or more. [3] The steel material as described in [1] or [2], wherein the absorbed energy of the Charpy impact test at -196°C in the C direction in the coarse-grained region of the weld heat affected portion is 41 J or more. [4] The steel material as described in any one of [1] to [3] has the following composition, which contains, in terms of 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: less than 0.005%, and contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less, the remainder comprises iron and inevitable impurities, and the cleanliness of sulfide inclusions in the microstructure is less than 1.0%. [5] The steel material as described in [4], wherein the component composition further contains 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 in mass %. [6] The steel material as described in [4] or [5], wherein the sulfide inclusions are MnS. [7] A method for manufacturing a steel material, manufacturing the steel material as described in any one of [1] to [6], heating the steel material to a temperature range of 1100°C to 1300°C, hot rolling the steel material under the conditions of a cross rolling ratio of 20 or less calculated by formula (1), a final pass reduction of 30% or less, and a finishing temperature of 750°C or more, and then cooling the steel material; Cross rolling ratio = rolling direction rolling ratio/rolling right angle direction rolling ratio... (1). [8] A groove welded with a steel material as described in any one of [1] to [6], wherein the absorbed energy of the Charpy impact test at -196°C in the C direction in the coarse-grained region of the weld heat affected portion is 41 J or more. [Effect of the invention]

According to the present invention, a steel material having excellent low-temperature toughness and a method for manufacturing the same can be provided. In addition, the steel material of the present invention is suitable for use as a material for steel structures (tanks for liquefied gas storage tanks, etc.) used in low-temperature environments, thereby providing a tank having excellent low-temperature toughness in both the base material and the welded heat-affected portion after welding. Therefore, it can greatly contribute to improving the safety or life of the steel structure, and has a significant effect in the industry. In addition, the manufacturing method of the present invention does not cause a decrease in productivity and an increase in manufacturing costs, so a manufacturing method that is also excellent in economy can be provided.

The present invention is described in detail below. In addition, the present invention is not limited to the following embodiments.

First, the technical concept of the present invention is described in detail.

As mentioned above, austenitic steel (e.g., high Mn steel) is a cheap steel with excellent low-temperature toughness. In order to use this high Mn steel as a material for steel structures (e.g., tanks) used in low-temperature environments, the inner wall of the tank and the welded portion are required to have characteristics that can withstand the internal pressure of the gas in the tank, and in particular, characteristics that can withstand loads caused by tensile stress in all directions, not only in the L and C directions.

High Mn steel (here, steel plates with a Mn content of 20.0 mass% to 40.0 mass%) is austenitic steel, so brittle fracture basically does not occur, and most of the fracture is ductile. In contrast, in ordinary steel (here, low-carbon steel plates whose crystal structure at room temperature is body-centered cubic (BCC)), ductile fracture is not related to the structure, and the shelf energy (maximum absorption energy) of ordinary steel is more than 200 J, and sometimes exceeds 300 J depending on the conditions. In other words, the absorption energy of ordinary steel is sufficiently large, so in the case of ordinary steel, if a brittle fracture is not formed, the absorption energy does not need to be a problem.

The inventors of the present invention have found that when a high-Mn steel is subjected to a Charpy impact test at an ultra-low temperature of -196°C, although it is ductile failure, the absorbed energy in the L direction is about 100 J, and the absorbed energy in the C direction is sometimes less than 41 J. This means that the base material and welded portion of the groove produced by welding the high-Mn steel are easily broken when a tensile impact stress acts in a direction perpendicular to the rolling direction.

That is, the internal pressure of liquefied natural gas applied to the inner wall and weld of the groove is generated in the L direction, C direction and the direction parallel to the inner surface (inner wall) of all steel materials constituting the groove, so it is necessary to have sufficient toughness values in all directions. It is well known that the toughness of rolled materials is the lowest when a Charpy impact test piece is taken in the C direction relative to the rolling direction. Therefore, it is important to improve the toughness value of the Charpy impact test in the C direction.

Furthermore, the so-called "C direction" refers to the direction perpendicular to the rolling direction (L direction). The so-called "Charpy impact test in the C direction" means that the length direction of the Charpy impact test piece is parallel to the C direction, and the notch faces the rolling direction. The so-called "rolling direction" of this application refers to the rolling direction with the largest total reduction when rolling the rolled material in various directions.

Therefore, the inventors of the present invention have further investigated the cause and found that the rolled fabric (rolling-induced fabric) is caused by the difference in the absorbed energy, that is, the relationship between ductile failure and the fabric. The relationship between ductile failure and the fabric is described below.

In the present invention, attention is paid to the striking direction of the Charpy test piece in the Charpy impact test. Consider the striking directions of an L-direction Charpy test piece (where the notch faces the C direction) taken so that the length direction of the Charpy test piece becomes the rolling direction of the steel plate, and a C-direction Charpy test piece (where the notch faces the L direction) taken so that the length direction of the Charpy test piece becomes the direction perpendicular to the rolling direction of the steel plate.

As described above, if the (110) texture becomes higher, there is a tendency for the toughness to further decrease. Since the absorbed energy cannot be predicted based on the texture, the reason is unclear, but as described later, it is believed that it is probably affected by the (110) [001] texture. This texture has a (100) plane oriented in the C direction and a (110) plane oriented in the L direction. Therefore, a good value can be obtained in the L-direction Charpy impact test with a notch in the C direction, but a poor value is obtained in the C-direction Charpy impact test with a notch in the L direction. In the JIS standard, the absorbed energy value of the C-direction Charpy impact test is specified to be above 27 J, which can be a lower value. However, when a groove is formed, as described above, since stress is applied in all directions, it is better to have the same degree of absorbed energy in the C direction as in the L direction.

In the case of austenitic steel, the base material does not undergo phase change even when the temperature rises, so the texture of the weld obtained by welding austenitic steel is substantially the same as that of the base material, that is, it does not change. Therefore, it is important to prepare the texture in advance when manufacturing austenitic steel as the base material.

Therefore, in the present invention, in the hot rolling step described later, the strength of the (110) [001] fabric is reduced (i.e., the (110) [001] fabric is not developed) by mixing the (110) [001] fabric that is easily formed during normal rolling and the fabric that is developed in other directions during cross rolling in which the rolling is rotated 90 degrees as much as possible. Here, in the face-centered cubic structure (FCC), the surface atomic density in the (110) plane is the smallest, and the plane with a small surface atomic density is the most brittle plane. It is believed that in ductile failure, the brittle plane is easily torn and the absorbed energy is reduced. Therefore, it is believed that by not developing the (110) [001] fabric, the Charpy absorbed energy in the L direction and the C direction can be equalized.

Furthermore, the inventors of the present invention have found that when the hardness of high Mn steel is 300 HV or more, the absorbed energy of the Charpy impact test at -196°C in the C direction after strain aging is less than 41 J. Although the detailed mechanism is unclear, it is believed that the higher the hardness, the higher the dislocation density, so the C contained in high Mn steel fixes more dislocations.

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

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

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

[Microstructure of steel] Microstructure under normal pressure: 95% or more of the area ratio is FCC structure In the present invention, the so-called "microstructure under normal pressure" refers to the microstructure in the temperature range from 1300℃ or less to -273℃ under a pressure of 1 atm. In the case of high-Mn steel, the microstructure in the temperature range below 1300℃ (e.g. 1250℃) is FCC in an area ratio of 95% or more.

As described above, when the crystal structure of the steel is a body-centered cubic structure (BCC), the steel may cause brittle fracture in a low-temperature environment and is therefore not suitable for use in a low-temperature environment. Therefore, when it is assumed to be used in a low-temperature environment, the matrix phase of the steel needs to have a face-centered cubic structure (FCC) crystal structure. Furthermore, in the present invention, "using austenite as the matrix phase" means that the austenite phase accounts for 95% or more of the area ratio of the entire microstructure. The austenite phase is preferably 97% or more. The remaining portion other than the austenite phase is a ferrite phase and/or a matterite phase. The remaining part other than the austenite phase is preferably such that the total area ratio of each phase is 5% or less.

In the present invention, the area fraction of austenite can be measured by the method described in the examples described later.

(110) [001] fabric 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 portion, it is important to perform hot rolling under appropriate conditions. This can reduce the strength of the microstructure, especially the (110) [001] fabric, and equalize the Charpy absorbed energy in the C direction and the L direction.

If the (110) [001] fabric strength in the microstructure at the position of 1/2 of the plate thickness is 10.0 or more, the tortoise crack is easy to propagate. As a result, the absorbed energy is reduced. Therefore, the (110) [001] fabric strength is set to be less than 10.0. It is preferably set to be 9.0 or less. It is more preferably set to be 6.0 or less. Since the absorbed energy in the L direction is reduced, the (110) [001] fabric strength in the microstructure at the position of 1/2 of the plate thickness is preferably set to be 1.0 or more. It is more preferably set to be 4.0 or more.

Hardness: less than 300 HV If the hardness at the half-thickness position is 300 HV or more, the ductility decreases and the absorbed energy decreases. Therefore, the hardness is set to less than 300 HV. It is preferably set to 280 HV or less. It is more preferably set to 260 HV or less. Since the strength of the steel material decreases, the hardness at the half-thickness position is preferably set to 200 HV or more. It is more preferably set to 220 HV or more.

Cleanliness of sulfide inclusions: less than 1.0% (preferable condition) If the cleanliness of sulfide inclusions in the microstructure at the 1/2 thickness position is 1.0% or more, it becomes the starting point of damage. As a result, there is a possibility of reduced absorption energy. Therefore, the cleanliness of the sulfide inclusions is preferably set to less than 1.0%. It is more preferably set to 0.8% or less. It is further preferably set to 0.6% or less. There is no special lower limit for the cleanliness, but from the perspective of manufacturing cost, it is preferably set to 0.1% or more.

Furthermore, the cleanliness is calculated by the following formula (2). d = (n/p×f)×100···(2) Here, let p in the formula (2) be the total number of grid points in the field of view, f be the number of fields of view, and n be the number of grid point centers occupied by inclusions in f fields of view. Therefore, the cleanliness is the value of the percentage of the area occupied by sulfide inclusions at the position of 1/2 of the plate thickness of the steel material, which represents the sulfide inclusions in the C direction. As an example of sulfide inclusions, MnS can be listed.

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

Furthermore, in the present invention, the fabric strength, hardness and cleanliness of sulfide inclusions can be measured by the methods described in the examples described below.

The steel material of the present invention having the above microstructure has excellent low-temperature toughness.

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

With respect to the microstructure at the half-thickness position of the steel material, if the (110) [001] fabric strength is set to less than 10.0 and the hardness is set to less than 300 HV, an absorbed energy (vE _-196 ): 41 J or more can be achieved in all directions including the C direction and the L direction at the half-thickness position of the steel material. Thus, even in the welded portion of the steel material of the present invention, an absorbed energy (vE _-196 ): 41 J or more can be achieved in the coarse-grained region of the weld heat affected portion. In addition, when the steel material of the present invention is subjected to pre-straining and aging treatment under predetermined conditions (e.g., the conditions described in the examples described later), an absorbed energy (vE _-196 ): 41 J or more can be achieved in the C direction after strain aging. Furthermore, the preferred welding conditions such as heat are the same as the preferred welding conditions for the groove described later, so they are omitted here.

In addition to the above-mentioned structural strength and hardness, if the cleanliness of sulfide inclusions at the 1/2 position of the steel plate thickness is set to less than 1.0%, the absorbed energy (vE _-196 ): 41 J or more can be obtained more effectively in the C direction showing a low value.

Next, the preferred range of the component composition in the steel material (austenitic steel) of the present invention is described. In addition, regarding a structure (such as a groove) obtained by using the austenitic steel material (such as a high Mn steel material) of the present invention as a raw material and welding the steel material, the base material and the weld part also have the same component composition and microstructure (however, the austenitic grain size in the weld part becomes larger).

[Component composition] In the present invention, austenitic steel and the steel material used for its production have the above-mentioned component composition. The component composition of the austenitic steel of the present invention and the reasons for its limitation are explained. In addition, the expression of "%" in the component composition means "mass %" unless otherwise specified.

C: 0.100% or more and 0.700% or less C is a cheap austenite stabilizing element and an important element for obtaining austenite. In order to obtain the above effect, C is preferably contained in an amount of 0.100% or more. On the other hand, if C is contained in an amount exceeding 0.700%, Cr carbides are excessively generated, and there is a risk of reduced low-temperature toughness. Therefore, C is preferably set to 0.100% or more and 0.700% or less. C is more preferably set to 0.200% or more, and more preferably set to 0.600% or less. C is further preferably set to 0.250% or more, and further preferably set to 0.550% or less.

Si: 0.05% or more and 1.00% or less Si functions as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of being dissolved in steel and strengthening the steel plate by solid solution strengthening. In order to obtain the above effect, Si is preferably contained at 0.05% or more. On the other hand, if Si contains more than 1.00%, non-thermal stress increases excessively, so there is a risk of low-temperature toughness deterioration. Therefore, Si is preferably set to 0.05% or more and 1.00% or less. Si is more preferably set to 0.07% or more, and more preferably set to 0.80% or less. Si is further preferably set to 0.10% or more, and further preferably set to 0.60% or less.

Mn: 20.0% or more and 40.0% or less Mn is a relatively cheap austenite stabilizing element. In the present invention, it is an important element for achieving both strength and low-temperature toughness. In order to obtain the above-mentioned effect, Mn is preferably contained in an amount of 20.0% or more. On the other hand, when Mn contains more than 40.0%, there is a risk of deterioration of low-temperature toughness. In addition, there is a risk of deterioration of weldability and cutting properties. Furthermore, segregation is promoted, and the generation of stress corrosion cracks is promoted. Therefore, Mn is preferably set to 20.0% or more and 40.0% or less. Mn is more preferably set to 23.0% or more, and more preferably set to 24.0% or more. It is more preferably set to 35.0% or less, and more preferably set to 30.0% or less.

P: 0.030% or less If P contains more than 0.030%, the grain boundaries are excessively segregated, so the low-temperature toughness decreases. Therefore, 0.030% is set as the upper limit, and it is ideal to reduce it as much as possible. Therefore, P is set to 0.030% or less. Furthermore, excessive reduction of P will increase the refining cost, which is economically disadvantageous, so it is ideal to set it to 0.002% or more. P is more preferably set to 0.005% or more, and more preferably to 0.010% or more. It is more preferably set to 0.028% or less, and more preferably to 0.024% or less.

S: 0.0050% or less S deteriorates the low-temperature toughness or ductility of the base material, so 0.0050% is set as the upper limit, and it is ideal to reduce it as much as possible. Therefore, S is set to 0.0050% or less. It is more preferably set to 0.0045% or less, and it is even more preferably set to 0.0040% or less. In addition, excessive reduction of S will increase the refining cost and is economically disadvantageous, so S is ideally set to 0.0010% or more. It is more preferably set to 0.0012% or more.

Al: 5.00% or less Al acts as a deoxidizing agent and is most commonly used in the steel liquid deoxidation process of steel plates. In addition, the yield strength and local elongation during the tensile test are improved. In order to obtain the above effects, Al is preferably contained in an amount of 0.01% or more. On the other hand, if Al contains more than 5.00%, inclusions are present in large quantities and the low-temperature toughness is deteriorated, so it is set to 5.00% or less. Al is more preferably set to 0.01% or more, and more preferably set to 0.02% or more. Al is more preferably set to 4.00% or less, and more preferably set to 3.50% or less.

Cr: 7.0% or less Cr is an effective element for increasing grain boundary strength and low-temperature toughness. To achieve the above effects, Cr is preferably contained at 0.5% or more. On the other hand, if Cr is contained at more than 7.0%, there is a risk of reduced low-temperature toughness and stress corrosion cracking resistance due to the formation of Cr carbides. Therefore, Cr is preferably set to 7.0% or less. Cr is preferably set to 0.5% or more, more preferably to 1.0% or more, and more preferably to 1.2% or more. Cr is more preferably set to 6.7% or less, and more preferably to 6.5% or less. In addition, in order to further improve stress corrosion cracking resistance, it is more preferably 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 an effective element for improving low-temperature toughness. In order to obtain the above effect, N is preferably contained at 0.0050% or more. On the other hand, if N is contained in an amount exceeding 0.0500%, nitrides or carbonitrides will coarsen, and there is a risk of reduced toughness. Therefore, N is preferably set to 0.0500% or less. N is preferably set to 0.0050% or more, more preferably set to 0.0060% or more, and further preferably set to 0.0070% or more. N is more preferably set to 0.0400% or less, and further preferably set to 0.0300% or less.

O: 0.0050% or less O deteriorates low-temperature toughness due to the formation of oxides. Therefore, O is set to a range of 0.0050% or less. It is preferably set to 0.0045% or less, more preferably set to 0.0040% or less, and further preferably set to 0.0035% or less. Furthermore, excessive reduction of O will increase the refining cost and is economically disadvantageous, so O is preferably set to 0.0010% or more. It is more preferably set to 0.0012% or more.

Ti: less than 0.005%, Nb: less than 0.005% Ti and Nb form high-melting-point carbonitrides in steel, which reduces low-temperature toughness. Since Ti and Nb are components that are inevitably mixed from raw materials, they are usually mixed in the range of Ti: more than 0.005% and less than 0.010% and Nb: more than 0.005% and less than 0.010%. Therefore, it is necessary to avoid the inevitable mixing of Ti and Nb according to the melting method described below, and suppress the contents of Ti and Nb to less than 0.005%, respectively. By suppressing the contents of Ti and Nb to less than 0.005%, respectively, the adverse effects of the carbonitrides can be eliminated, ensuring excellent low-temperature toughness and ductility. It is better to set the contents of Ti and Nb to less than 0.003%. Of course, the contents of Ti and Nb can also 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 Ca, Mg and REM (rare earth metals) are useful elements for the morphology control of inclusions. The so-called morphology control of inclusions refers to the process of converting the extended sulfide inclusions into granular inclusions. The ductility, toughness and resistance to sulfide stress corrosion cracking are improved by the morphology control of the inclusions. In order to obtain the above effects, Ca and Mg are preferably contained in an amount of 0.0005% or more, and REM is preferably contained in an amount of 0.0010% or more. On the other hand, if a large amount of any element is also contained, the amount of non-metallic inclusions increases, which in turn leads to a decrease in ductility, toughness, and resistance to sulfide stress corrosion cracking. In addition, it is economically disadvantageous.

Therefore, when Ca and Mg are contained, they are preferably set to 0.0100% or less, and when REM is contained, they are preferably set to 0.0200% or less. It is preferred that Ca is set to 0.0005% or more, Mg is set to 0.0005% or more, and REM is set to 0.0010% or more. It is more preferred that Ca is set to 0.0010% or more and 0.0080% or less, Mg is set to 0.0010% or more and 0.0080% or less, and REM is set to 0.0020% or more and 0.0150% or less. It is further preferred that Ca is set to 0.0050% or less, and Mg is set to 0.0050% or less.

Regarding the austenitic steel of the present invention, the remainder other than the above-mentioned components is iron (Fe) and inevitable impurities. As inevitable impurities here, H, B, etc. can be listed, and it is allowed as long as the total amount of each element is 0.01% or less.

It is preferred to set the above elements as the basic component composition. By the above basic component composition, the target characteristics in the present invention can be obtained. In the present invention, in order to further improve the strength and low temperature toughness, in addition to the above elements, the following elements can be contained as needed.

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 steel sheets by solid solution strengthening, but also increase the migration rate of dislocations and low-temperature toughness. In order to obtain the above effects, Cu and Ni are preferably contained in an amount of 0.01% or more. On the other hand, if Cu and Ni are contained in an amount exceeding 1.0%, in addition to deterioration of the surface properties during rolling, the burden of manufacturing costs will also increase. Therefore, when these alloying elements are contained, their contents are preferably set to 1.0% or less, respectively. It is more preferably set to 0.03% or more, and more preferably set to 0.7% or less. It is further preferably set to 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 stabilization of austenite and to the improvement of the strength of the base material. In order to obtain the above-mentioned effects, Mo, V, and W are preferably contained in an amount of 0.001% or more, respectively. On the other hand, if Mo, V, and W are contained in an amount of more than 2.0%, coarse carbonitrides will be generated, which may sometimes become the starting point of damage, and will also increase the burden of manufacturing costs. Therefore, when these alloying elements are contained, their contents are preferably set to 2.0% or less, respectively. It is more preferably set to 0.003% or more, and more preferably set to 1.7% or less. It is further preferably set to 1.5% or less.

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

[Method for manufacturing steel material] Next, a method for manufacturing 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 above-mentioned composition by a known melting method such as a converter or an electric furnace. Alternatively, the steel material can be refined twice in a vacuum degassing furnace.

At this time, in order to limit Ti and Nb, which hinder the control of the structure, to the above numerical range, it is necessary to take measures to avoid the inevitable mixing of Ti and Nb from the raw materials and reduce their content. For example, by reducing the alkalinity of the slag in the refining stage, these alloys are concentrated and discharged as slag, and the concentration of Ti and Nb in the final slab product is reduced. Alternatively, it is also possible to inject oxygen to oxidize it, and float and separate the alloy of Ti and Nb during reflux.

Thereafter, it is preferred that a steel material such as a slab of a predetermined size is manufactured by a known casting method such as a continuous casting method or a block-segmented rolling method.

The following is a detailed description of the manufacturing conditions for manufacturing the steel material into a steel material (austenitic steel material) having excellent low-temperature toughness.

In order to obtain the above-mentioned austenitic steel, it is important to heat the steel material to a temperature range of 1100°C to 1300°C, perform a specified cross rolling, and perform hot rolling under the conditions of a final pass reduction ratio of 30% or less and a finishing temperature of 750°C or more. The temperature control here is based on the surface temperature of the steel material.

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

Heating temperature of steel material: 1100°C or higher and 1300°C or lower In order to diffuse Mn by 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 area. In this way, the stability of austenite can be ensured in the coarse-grained area of the weld heat affected area obtained during welding, and brittle fracture can be prevented. On the other hand, if the heating temperature exceeds 1300°C, there is a risk that the steel will begin to melt, so the upper limit of the heating temperature is set to 1300°C. It is preferably 1130°C or higher and 1270°C or lower.

Cross-rolling ratio calculated by formula (1): less than 20 Cross-rolling ratio = rolling direction rolling ratio / rolling ratio at right angles to rolling... (1) Here, the so-called "rolling direction rolling ratio" refers to the rolling ratio in the rolling direction relative to the total rolling. The so-called "rolling ratio at right angles to rolling" refers to the rolling ratio in the right angles to the total rolling. Therefore, "rolling direction rolling ratio / rolling ratio at right angles to rolling" means the rolling ratio in the rolling direction relative to the rolling at right angles to the rolling.

As described above, the (110)[001] structure is easily developed during the rolling of austenitic steel. Therefore, by adding rolling in different directions, the ratio of the (110)[001] structure is reduced, and the strength of the (110)[001] structure can be reduced. In order to set the (110)[001] structure strength to less than 10.0, the cross rolling ratio calculated by formula (1) is set to less than 20.

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, and more preferably 15 or less.

Furthermore, since the (110) [001] fabric is developed by repeatedly rolling in the same direction, it is preferred to repeat rolling in the rolling direction and rolling in a direction perpendicular to the rolling alternately to make the fabric uniform. It is preferred to repeat the rolling process two or more times. It is preferred to repeat the rolling process three or less times.

Reduction rate of the final pass of finishing rolling: 30% or less, finishing temperature of finishing rolling: 750°C or more If the reduction rate of the final pass of finishing rolling exceeds 30%, the dislocation density becomes too high and the low-temperature toughness deteriorates. If the finishing temperature of finishing rolling is less than 750°C, the (110) [001] structure is overdeveloped and the low-temperature toughness deteriorates. Therefore, the reduction rate of the final pass of finishing rolling is set to 30% or less. The reduction rate is preferably set to less than 25%, and more preferably set to less than 20%. The finishing temperature of finishing rolling is set to 750°C or more. The finishing temperature of finishing rolling is preferably set to 780°C or more, and more preferably set to 800°C or more. The upper limit of the finishing temperature of the finish rolling is not particularly specified, but from the viewpoint of ensuring strength, it is preferably 950°C or lower, and more preferably 920°C or lower. The lower limit of the reduction ratio in the final pass of the finish rolling is not particularly specified, but from the viewpoint of ensuring strength, it is preferably 5% or more, and more preferably 10% or more.

Furthermore, in the present invention, in order to further improve the strength and toughness, it is preferred to further control the following conditions during cross rolling.

Rolling start temperature (optimal conditions) The rolling start temperature is preferably 1100℃～1250℃. If it is less than 1100℃, the rolling temperature is less than 780℃, and there is a risk of excessive growth of the fabric. If it exceeds 1250℃, there is a risk that the fabric will not change.

The rolling temperature (temperature during rolling) is preferably 780°C to 1250°C. If it is less than 780°C, there is a risk that the fabric will overdevelop. If it exceeds 1250°C, there is a risk that the fabric will not change.

Reduction (preferable conditions) The reduction in the temperature range of 780℃ to 1250℃ is preferably 60% to 98%. If the reduction is less than 60%, there is a risk that the fabric will not change. If the reduction exceeds 98%, there is a risk that the fabric will over-grow. The reduction represents the total reduction rate in the temperature range of 780℃ to 1250℃.

Cooling After hot rolling is completed, cooling is performed. There is no particular regulation for cooling conditions. It is preferred to cool from a temperature above (temperature at the end of hot rolling - 100°C) to a temperature below 600°C at an average cooling rate of 1.0°C/s or more. This can suppress the formation of carbides and grain boundary segregation of P, and further improve the properties of the steel. In addition, the "temperature at the end of hot rolling" refers to the temperature at the end of finish rolling.

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

The groove of the present invention is a groove produced by welding the steel material. As described in the above-mentioned view d, the steel material of the present invention maintains the microstructure before welding even after welding. Therefore, the component composition and microstructure in the base material of the groove of the present invention are the same as those of the above-mentioned steel material (austenitic steel). By defining the component composition and microstructure of the base material (steel material) as described above, a groove 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 can be obtained. In addition, the absorbed energy of the Charpy impact test at -196°C in the coarse-grained area of the welding heat-affected portion of the groove can be set to 41 J or more. Furthermore, the absorbed energy of the Charpy impact test at -196°C after strain aging can be set to 41 J or more.

Since the cell of the present invention has the above-mentioned characteristics, it can be used in an extremely low temperature environment such as a cell for liquefied gas storage, for example.

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

The groove of the present invention is manufactured by welding the above-mentioned steel material. In addition, the manufacturing method of the steel material (austenitic steel material) as the raw material has been described, so it is omitted. Here, the preferred welding conditions are described.

[Preferable welding conditions] The preferred type of welding is gas metal arc welding.

The heat input range is preferably 3.0 kJ/mm or less. In addition, 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° C. to 800° C. is preferably set to 10° C./s or more. If the average cooling rate in this temperature range is less than 10° C./s, carbides are generated and the absorbed energy decreases.

As described above, according to the present invention, 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, thereby reducing the orientation dependence of the impact characteristics of the steel material (base material) and the weld. This improves the reliability of the material. [Example]

Hereinafter, the present invention will be described in more detail based on the embodiments. Furthermore, the following embodiments represent preferred examples of the present invention, and the present invention is not limited to the embodiments.

Steel slabs having the composition shown in Table 1 were produced by converter-barrel refining-continuous casting. In addition, "-" shown in Table 1 indicates that no intentional addition is made, which includes not only the case of no inclusion (0%) but also the case of inevitable inclusion. Next, the obtained steel slabs were hot rolled under the conditions shown in Table 2, and then cooled to produce steel materials (steel plates) with a thickness of 6 mm to 40 mm. In addition, in cross-rolling, the temperature during rolling was appropriately controlled and carried out in such a way that the temperature was 780℃ to 1250℃, the reduction at 780℃ to 1250℃ was 60% to 98%, and the cooling condition after the completion of rolling was 1.0℃/s or more. The "cooling condition after the completion of rolling" refers to the average cooling rate from a temperature higher than (temperature at the completion of hot rolling - 100°C) to a temperature lower than 600°C.

In addition, joint test plates (size: 250 mm × 500 mm) were taken from the obtained steel plates and welded together in the L direction and in the C direction to produce welded joints. Here, welding was performed under the welding conditions of groove shape: レ shape, backing material: ceramic, shielding gas: Ar-30% CO ₂ , and welding gun retreat angle: 5° to 10°.

The obtained steel plates and welded joints were used to evaluate the tensile test properties, low temperature toughness and microstructure of the steel plates according to the following procedures, and the low temperature toughness of the coarse grain region of the welded joint was evaluated.

(1) Tensile test characteristics Using the obtained steel plates, the tensile test pieces shown below were taken from the center of the length direction and width direction of the steel plates at the 1/2 thickness position. For steel plates with a thickness of more than 15 mm, JIS No. 4 tensile test pieces were taken, and for steel plates with a thickness of less than 15 mm, round bar tensile test pieces were taken. Using each tensile test piece, a tensile test in accordance with the provisions of JIS Z2241 (2011) was carried out to evaluate the tensile strength (TS) and yield stress (YS). In this embodiment, those having a yield stress of 400 MPa or more were judged as "excellent parent material strength".

(2) Low temperature toughness The low temperature toughness of the steel plate was evaluated as follows.

Using the obtained steel plate, a Charpy V-notch test piece in the C direction was taken from a position 1/2 of the plate thickness from the surface of the steel plate in a direction perpendicular to the rolling direction. In addition, a Charpy V-notch test piece in the L direction was taken from a position 1/2 of the plate thickness from the surface of the steel plate obtained in a direction parallel to the rolling direction. Furthermore, a tensile test piece with a point spacing of 200 mm was taken from the L direction and the C direction respectively from a position 1/2 of the plate thickness from the surface of the steel plate obtained, and Charpy V-notch test pieces in the L direction and the C direction were taken from a tensile test piece that was subjected to an aging treatment at 250°C for 1 hour after a tensile prestrain of 5%.

Next, three Charpy impact tests were performed on each steel plate in accordance with JIS Z 2242 (2005), and the absorbed energy at -196°C was obtained to evaluate the toughness of the steel material (parent material). As described above, the steel plate showed a low value of toughness in the C direction. Therefore, in this embodiment, the average value of the absorbed energy (vE _-196 ) of three steel plates in the C direction: 41 J or more was judged as "excellent parent material toughness".

Furthermore, for steel plates with a thickness of less than 10 mm, a sub-size (5 mm) Charpy V-notch test piece was made in the C direction, and three Charpy impact tests were performed on each test piece at -196°C. In Table 3, the absorbed energy item for the samples tested using the sub-size Charpy V-notch test piece is indicated with "*1". In the case of the sub-size, the average value of the absorbed energy (vE _-196 ) of the three pieces in the C direction: 27 J or more was judged as "excellent base material toughness".

The low temperature toughness of the welded joint was evaluated as follows.

From each welded joint with a plate thickness exceeding 10 mm, a Charpy V-notch test piece was taken 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 embodiment, the average value of the absorbed energy of the three pieces was 41 J or more, which was judged as "excellent toughness of the welded portion".

Furthermore, for each welded joint with a plate thickness of less than 10 mm, a 5 mm small-sized Charpy V-notch test piece was used 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 Table 3, for samples tested using a small-sized Charpy V-notch test piece, the absorbed energy item is indicated as "*1". In the case of a small size, the average value of the absorbed energy of three pieces is 27 J or more, which is judged as "excellent toughness of the welded part". Here, as described above, the measured value in the C direction of the steel plate showing the lowest value is used for evaluation.

(3) Organization evaluation [Observation of microstructure] The area ratio of each phase in the microstructure is calculated from the phase map of electron backscatter diffraction (EBSD) analysis.

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

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

[Textile strength] Using the obtained steel plate, test specimens were taken from the 1/2 thickness position at the center of the length direction and width direction of the steel plate. Using each test specimen, the textile strength of the ND surface was measured by X-ray diffraction. The maximum value of the textile strength was calculated based on the obtained orientation distribution function (ODF (Orientation Distribution Function): three-dimensional crystal orientation distribution function). In addition, ODF can be obtained from the polar diagram ((110) [001], (100) [011], (100) [010], (110) [112], (112) [111]) measured by X-ray diffraction (internal standardization) after removing the residual stress on the steel plate surface by chemical polishing.

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

[Cleanliness of sulfide inclusions] Using the obtained steel plate, an optical microscope sample of a cross section in the rolling direction was cut from the center of the length direction and width direction of the steel plate at the 1/2 position of the plate thickness, and the cleanliness was calculated according to the "Microscopic test method for non-metallic inclusions based on point calculation method" in Appendix 1 of JIS G 0555. Here, the cleanliness of sulfide inclusions in the C direction was calculated. 60 fields of view were measured at a magnification of the microscope × 400, and the cleanliness (%) was calculated using the following formula. d = (n/p × f) × 100··· (2) Here, p in the above formula (2) is the total number of grid points in the field of view, f is the number of fields of view, and n is the number of grid point centers occupied by inclusions in f fields of view. Furthermore, the cleanliness of MnS as a sulfide-based impurity was calculated.

The above results are shown in Table 3.

[Table 1] Steel No. Ingredient composition (mass %) C Si Mn P S Al Cr N O Ti Nb Ca Mg REM Cu Ni Mo V W 1 0.455 0.31 24.2 0.017 0.0023 0.03 3.6 0.0160 0.0020 0.002 0.002 0.0005 - - - - - - - 2 0.528 0.24 25.5 0.016 0.0020 0.11 3.1 0.0201 0.0019 0.001 0.002 - 0.0005 - - - - - - 3 0.290 0.98 34.8 0.014 0.0050 0.05 5.2 0.0332 0.0017 0.002 0.001 - - 0.0010 - - - - - 4 0.334 0.58 32.4 0.030 0.0018 0.04 4.6 0.0164 0.0016 0.001 0.002 0.0010 - - 0.8 - - - - 5 0.489 0.39 23.7 0.016 0.0018 0.57 6.0 0.0173 0.0015 0.002 0.003 - 0.0007 - - 0.9 - - - 6 0.616 0.15 20.4 0.013 0.0019 3.30 0.9 0.0090 0.0015 0.003 0.002 - - 0.0012 - - 1.8 - - 7 0.198 0.87 33.6 0.015 0.0020 0.07 5.8 0.0427 0.0016 0.002 0.002 0.0030 - - - - - 0.2 - 8 0.421 0.36 26.7 0.016 0.0018 0.04 2.7 0.0245 0.0018 0.002 0.001 - 0.0010 - - - - - 0.2 9 0.092 0.43 20.5 0.020 0.0025 0.04 1.8 0.0189 0.0017 0.002 0.001 0.0035 - - - - - - - 10 0.713 0.89 38.7 0.028 0.0041 1.06 0.7 0.0373 0.0034 0.003 0.002 - 0.0005 - - - - - - 11 0.644 1.04 21.6 0.026 0.0031 0.05 6.0 0.0390 0.0035 0.002 0.002 - - 0.0011 - - - - - 12 0.201 0.56 19.6 0.019 0.0019 0.03 1.3 0.0175 0.0026 0.002 0.003 0.0018 - - - - - - - 13 0.357 0.34 40.5 0.022 0.0027 0.04 0.6 0.0193 0.0020 0.002 0.001 - 0.0008 - - - - - - 14 0.669 0.58 22.2 0.033 0.0018 0.71 6.8 0.0214 0.0041 0.002 0.002 - - 0.0013 - - - - - 15 0.582 0.73 37.6 0.021 0.0054 2.47 1.0 0.0172 0.0018 0.001 0.003 0.0022 - - - - - - - 16 0.453 0.53 35.1 0.017 0.0021 5.05 3.7 0.0168 0.0032 0.002 0.001 - 0.0011 - - - - - - 17 0.652 0.30 23.0 0.026 0.0034 0.08 7.4 0.0457 0.0028 0.001 0.001 0.0025 - - - - - - - 18 0.632 0.61 21.6 0.019 0.0022 3.50 5.7 0.0519 0.0043 0.002 0.002 - - 0.0010 - - - - - 19 0.524 0.84 20.2 0.020 0.0032 4.71 2.1 0.0230 0.0053 0.003 0.001 0.0016 - - - - - - - 20 0.119 0.56 20.5 0.025 0.0025 0.05 0.6 0.0152 0.0038 0.006 0.002 - 0.0009 - - - - - - twenty one 0.642 0.79 23.1 0.028 0.0037 0.06 6.6 0.0475 0.0035 0.002 0.006 - - 0.0015 - - - - -

[Table 2] Sample No. Steel No. Thickness Steel manufacturing method Welding conditions Slab heating temperature Rolling start temperature Cross rolling ratio The number of times rolling in the rolling direction and rolling in the right-angle direction are performed alternately Reduction rate of the final pass of finishing rolling 780℃～1250℃ Pressing amount Finishing rolling end temperature Cooling Cooling rate in the range of 500°C to 800°C (mm) (℃) (℃) (-) (-) （%） （%） (℃) (℃/s) (℃/s) 1 1 12 1250 1230 15 3 20 95 851 15.0 8 2 2 15 1230 1210 13 2 18 94 807 10.0 11 3 3 18 1200 1180 17 3 17 93 835 12.0 13 4 4 twenty one 1180 1160 12 2 16 92 812 9.0 10 5 5 twenty four 1160 1140 15 2 17 90 826 7.0 19 6 6 27 1140 1120 11 2 15 89 833 8.0 15 7 7 30 1120 1100 10 1 16 86 750 5.0 17 8 8 9 1270 1250 20 3 25 93 870 Air Cooling 5 9 9 twenty one 1180 1160 15 3 18 92 813 11.0 18 10 10 twenty one 1180 1160 16 3 17 92 828 10.0 10 11 11 12 1230 1210 18 1 19 95 782 8.0 16 12 12 25 1150 1130 12 3 18 90 831 14.0 15 13 13 25 1150 1130 14 3 17 90 840 8.0 17 14 14 12 1230 1210 18 2 twenty one 94 773 12.0 14 15 15 20 1200 1180 17 2 19 92 795 9.0 13 16 16 20 1200 1180 18 1 18 92 809 13.0 10 17 17 30 1100 1080 13 3 15 88 816 10.0 10 18 18 12 1200 1180 16 2 20 95 800 7.0 11 19 19 15 1200 1180 18 2 17 94 797 11.0 12 20 20 7 1260 1250 17 2 twenty two 94 783 Air Cooling 8 twenty one twenty one 7 1260 1250 18 2 twenty three 93 771 Air Cooling 9 twenty two 1 30 1090 1060 15 1 14 88 804 6.0 12 twenty three 2 30 1100 1080 twenty one 1 16 88 780 8.0 11 twenty four 3 6 1180 1160 15 2 twenty four 94 740 Air Cooling 6 25 4 15 1120 1100 - 0 18 94 785 12.0 9 26 6 27 1140 1120 11 2 17 87 754 10.0 11 27 6 27 1140 1120 11 2 16 89 783 9.0 13 28 7 30 1120 1100 10 2 17 86 755 7.0 18 29 7 30 1120 1100 10 3 18 86 758 8.0 19 30 6 12 1200 1180 15 2 30 95 946 6.0 10 31 6 12 1200 1180 15 2 31 95 950 5.0 10 32 1 30 1100 1080 18 3 15 57 770 0.8 16

[table 3] Sample No. Steel No. Steel Structure Characteristics of Steel Characteristics of coarse grain area in weld heat affected area Notes (110) [001] Fabric strength Cleanliness of sulfide inclusions (C direction) Vostian Tiexiang Other phases TS YS hardness Absorption energy at -196°C (L direction) Absorbed energy at -196°C (C direction) Absorbed energy at -196°C (L direction after strain aging) Absorbed energy at -196°C (C direction after strain aging) Absorbed energy at -196°C (CGHAZ L direction) Absorbed energy at -196°C (CGHAZ C direction) (-) (%) (Area %) (Area %) (MPa) (MPa) (HV) (J) (J) (J) (J) (J) (J) 1 1 4.0 0.4 100 0 950 560 253 83 55 73 48 71 49 Invention Example 2 2 5.9 0.3 100 0 956 571 252 102 56 85 46 99 56 Invention Example 3 3 4.8 0.6 100 0 856 484 242 92 62 89 60 87 63 Invention Example 4 4 5.6 0.1 100 0 887 502 240 120 60 114 57 113 60 Invention Example 5 5 5.2 0.3 100 0 920 555 243 131 66 117 59 138 74 Invention Example 6 6 5.1 0.2 100 0 966 580 243 137 69 109 55 137 71 Invention Example 7 7 9.9 0.1 100 0 862 458 221 142 57 143 58 146 62 Invention Example 8 8 4.0 0.9 100 0 936 551 280 94 ^*1 50 ^*1 73 ^*1 34 ^*1 55 ^*1 30 ^*1 Invention Example 9 9 5.5 0.2 85 15 963 398 235 47 31 50 33 53 36 Comparison Example 10 10 5.0 0.5 100 0 809 590 247 45 32 33 twenty four 44 32 Comparison Example 11 11 6.6 0.7 100 0 948 578 257 87 36 68 27 89 39 Comparison Example 12 12 4.7 0.2 88 12 970 460 234 48 32 46 31 50 34 Comparison Example 13 13 4.3 0.4 100 0 800 511 234 50 33 47 31 55 37 Comparison Example 14 14 8.0 0.6 100 0 947 582 266 67 35 51 26 65 37 Comparison Example 15 15 6.3 0.5 100 0 835 574 236 67 35 56 29 68 36 Comparison Example 16 16 5.9 0.6 100 0 851 562 236 85 35 76 31 82 35 Comparison Example 17 17 5.5 0.2 100 0 852 581 237 48 32 38 25 42 32 Comparison Example 18 18 5.9 0.5 100 0 960 586 261 67 35 52 27 62 35 Comparison Example 19 19 6.2 0.6 100 0 963 569 249 69 36 58 30 67 36 Comparison Example 20 20 8.2 0.6 90 10 955 408 255 37 ^*1 26 ^*1 38 ^*1 27 ^*1 32 ^*1 23 ^*1 Comparison Example twenty one twenty one 8.5 0.6 100 0 948 577 273 34 ^*1 24 ^*1 24 ^*1 16 ^*1 28 ^*1 23 ^*1 Comparison Example twenty two 1 6.0 0.4 100 0 960 573 234 99 40 89 36 87 40 Comparison Example twenty three 2 8.2 1.0 100 0 955 570 238 114 39 96 33 108 39 Comparison Example twenty four 3 10.1 0.4 100 0 870 445 260 42 ^*1 26 ^*1 37 ^*1 22 ^*1 32 ^*1 24 ^*1 Comparison Example 25 4 10.1 0.3 100 0 893 506 248 120 40 114 38 110 40 Comparison Example 26 6 7.5 0.2 95 5 970 599 242 82 41 82 42 81 43 Invention Example 27 6 6.9 0.2 97 3 968 594 240 90 45 89 46 91 48 Invention Example 28 7 9.7 0.1 100 0 856 450 230 120 60 120 60 123 64 Invention Example 29 7 9.5 0.1 100 0 850 441 233 93 62 90 60 99 67 Invention Example 30 6 5.9 0.1 100 0 960 590 298 100 56 79 43 95 50 Invention Example 31 6 6.1 0.1 100 0 964 595 300 95 50 77 40 84 45 Comparison Example 32 1 8.9 0.4 100 0 828 420 233 67 43 65 42 69 44 Invention Example *1.5 mm small size

As shown in Table 3, it was confirmed that the austenitic steel of the present invention satisfied the target performance ((110) [001] fabric strength: less than 10.0, hardness: less than 300 HV, and the absorbed energy (vE _-196 ) of the Charpy impact test at the position of 1/2 of the plate thickness of the steel material was 41 J or more). In addition, it was confirmed that the welded joint obtained by welding the austenitic steel of the present invention satisfied the target performance (the absorbed energy (vE _-196 ) of the Charpy impact test in the coarse-grained region of the weld heat affected portion was 41 J or more). Furthermore, it was confirmed that the performance was satisfied even after strain aging treatment (the absorbed energy (vE _-196 ) of the Charpy impact test after strain aging was 41 J or more).

In contrast, in the comparative example outside the scope of the present invention, the austenitic steel material did not satisfy the target performance. In addition, the absorbed energy in the obtained welded joint did not satisfy the target performance. Furthermore, it was confirmed that the target performance was satisfied after strain aging treatment.

without

Claims (10)

A steel material, wherein the microstructure is a face-centered cubic structure in an area ratio of 95% or more, the (110) [001] fabric strength at a position of 1/2 of the plate thickness is less than 10.0, the hardness at a position of 1/2 of the plate thickness is less than 300 HV, and the absorbed energy of the Charpy impact test at -196°C in the C direction at a position of 1/2 of the plate thickness is 41 J or more, the steel material having the following component composition, the component composition containing, 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%, and Nb: less than 0.005%, and contain one or more selected from Ca: less than 0.0100%, Mg: less than 0.0100%, and REM: less than 0.0200%, and the remainder contains iron and unavoidable impurities. The steel material as described in claim 1, wherein the absorbed energy of the Charpy impact test at -196°C in the C direction at the 1/2 thickness position after strain aging is 41 J or more. The steel material as described in claim 1, wherein the absorbed energy of the Charpy impact test at -196°C in the C direction in the coarse-grained area of the weld heat affected portion is 41 J or more. The steel material as described in claim 2, wherein the absorbed energy of the Charpy impact test at -196°C in the C direction in the coarse-grained area of the weld heat affected portion is 41 J or more. The steel material as described in any one of claim 1 to claim 4, wherein the cleanliness of sulfide inclusions in the microstructure is less than 1.0%. The steel material as described in claim 5, wherein the component composition further contains 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, and W: 2.0% or less in terms of mass %. The steel material as described in claim 5, wherein the sulfide-based impurity is MnS. The steel material as described in claim 6, wherein the sulfide inclusions are MnS. A method for manufacturing a steel material, manufacturing a steel material as described in any one of claim 1 to claim 8, heating the steel material to a temperature range of 1100°C or higher and 1300°C or lower, hot rolling the steel material under the conditions of a cross rolling ratio of 20 or lower, a final pass reduction of 30% or lower, and a finishing temperature of 750°C or higher calculated by formula (1), and then cooling the steel material; wherein the steel material has a component composition as described in claim 1 or claim 6; cross rolling ratio = rolling direction rolling ratio / rolling right angle direction rolling ratio ... (1). A groove welded with a steel material as described in any one of claim 1 to claim 8, wherein the absorbed energy of the Charpy impact test at -196°C in the C direction in the coarse-grained area of the weld heat affected portion is 41 J or more.