TW202246537A - Steel material and method for producing same, and tank and method for producing same - Google Patents

Abstract

Provided are a steel material and a method for producing the same, and a tank and a method for producing the same. A steel material according to the present invention contains, in terms of mass%, 0.200%-0.700% C, 0.05%-1.00% Si, 20.0%-40.0% Mn, 0.030% or less P, 0.0050% or less S, 5.00% or less Al, 7.0% or less Cr, 0.0500% or less N, 0.0050% or less O, less than 0.005% Ti, and less than 0.005% Nb, and contains at least one selected from 0.0100% or less Ca, 0.0100% or less Mg, and 0.0200% or less REM, with the remainder having a component composition comprising iron and unavoidable impurities. The microstructure of the steel material is such that the maximum crystal grain size at a position 1 mm below the surface of the steel material is less than 200 [mu]m.

Description

Steel material and manufacturing method thereof, groove and manufacturing method thereof

The present invention relates to a steel material preferably used for structural steel used in extremely low temperature environments such as liquid helium, liquefied gas, etc. represented by tanks for storing liquid hydrogen, and a method for manufacturing the same. Moreover, this invention relates to the tank which used the said steel material, and its manufacturing method.

In order to use hot-rolled steel sheets as materials for storage tank structures for liquid hydrogen, liquid helium, and liquefied gas, since the temperature of the use environment is extremely low, hot-rolled steel sheets are required to have excellent toughness at low temperatures. For example, when using a hot-rolled steel sheet in a liquid helium storage tank, it is necessary to ensure excellent toughness at the boiling point of helium: -269°C or lower. If the low-temperature toughness of the steel material is poor, there is a possibility that the safety as a structure for cryogenic storage tanks cannot be maintained, and therefore there is a high demand for improvement of the low-temperature toughness of the steel material used.

In order to meet these requirements, conventionally used wastite-based stainless steel, 9% Ni steel, or 5000-series aluminum alloy, which uses wastten iron, which does not show brittleness at low temperatures, as the steel sheet structure. However, since alloy costs and manufacturing costs are high, inexpensive steel materials with excellent low-temperature toughness are required.

Therefore, as a new steel material replacing the conventional steel for low temperature, for example, Patent Document 1 proposes the use of high Mn steel to which a large amount of the relatively inexpensive wast iron stabilizing element Mn is added as structural steel for low temperature environments.

Patent Document 1 proposes a technique for ensuring low-temperature toughness in a welded heat-affected zone by controlling crystal grain size, carbide coverage, and the like. [Prior art literature] [Patent Document]

Patent Document 1: Japanese Patent Laid-Open No. 2016-196703

[Problem to be Solved by the Invention] For example, a structure for a liquefied gas storage tank (for example, a tank for a liquefied gas storage tank) is produced by heating a steel material in a linear shape. The so-called linear heating is a processing method that utilizes plastic deformation caused by local thermal stress to form a curved surface. In the Japan Shipbuilding Quality Standard (JSQS) (Japan Shipbuilding Quality Standard, 2018), for high-tensile steel with a carbon equivalent (Ceq) in shipbuilding of Ceq>0.38%, the linear The heating conditions were such that the maximum heating temperature on the surface when water cooling was performed immediately after heating was 650° C. or lower. When the temperature exceeds this temperature, the maximum heating temperature of the surface is set to be 900°C or lower, and water cooling is performed after air cooling to 500°C. When carbides are formed after linear heating, the low-temperature toughness decreases, and Patent Document 1 does not conduct any verification on the low-temperature toughness after linear heating.

The present invention was 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 after linear heating, a method for producing the same, a tank using the steel material, and a method for producing the same.

The term "excellent low-temperature toughness after linear heating" means that in the groove obtained by subjecting the steel material to the linear heat treatment described later, the position 1 mm below the surface of the steel material at the linear heating portion (1 mm from the surface of the steel material in the thickness direction) mm position) in the Charpy impact test above -269°C, the absorbed energy is above 41 J. The "linearly heated portion" refers to a region that is affected by heat after linearly heating the steel material. In addition, the absorbed energy of the Charpy impact test of a linear heating part can be measured by the method described in the Example mentioned later. [Means to solve the problem]

In order to achieve the above-mentioned problems, the inventors of the present invention focused on the composition of the steel (steel plate), the microstructure, the manufacturing method, and the factors that determine the properties of the steel material (such as high-Mn steel material). Efforts have been made to study various factors and structures produced by linearly heating the steel materials. As a result, the following findings a to c were obtained. In addition, in this invention, a "high Mn steel material" means the steel material whose Mn content is 20 mass % - 40 mass %.

a. In the linear heating part of the structure manufactured by linear heating of high Mn steel, in order to suppress the decrease in the absorbed energy of the Charpy impact test at -269°C or higher, it is important to reduce the maximum crystallinity of the steel when it is produced. The particle size is set to be less than 200 μm. Preferably, the maximum crystal grain size is less than 180 μm.

b. Since high-Mn washer steel contains a large amount of C, there are more carbides than stainless steel. Furthermore, since carbides are formed at the grain boundaries, the grain boundary strength decreases. When the C concentration in the grain boundary after linear heating of the high-Mn steel material is less than 0.100%, the grain boundary becomes the starting point of fracture, resulting in deterioration of low-temperature toughness. Accordingly, in order to suppress the deterioration of the low-temperature toughness after linear heating of the high-Mn steel, it is effective to increase the C concentration in the grain boundaries of the high-Mn steel. For this reason, it is effective to make the maximum crystal grain size smaller than 200 μm in the high-Mn steel material used as the material.

c. In hot rolling during steel manufacturing, if the total rolling reduction rate is 40% or more at 950°C or higher, one or more hot-rolling passes are carried out at less than 950°C, and the temperature at the end of finish rolling If it is carried out under the condition of 750° C. or higher, the above a and b can be realized.

The present invention is based on further studies of the above findings, and its gist is as follows. [1] A steel material having the following composition, wherein in the microstructure, the maximum grain size at a position 1 mm below the surface of the steel material is less than 200 μm, said composition containing C in mass %: 0.200% 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: less than 0.0050%, Ti: less than 0.005%, Nb: less than 0.005%, and contain one or more of Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0200%, The remainder contains iron and unavoidable impurities. [2] The steel material as described in [1] above, wherein the composition further contains, by mass %, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less , W: one or more of 2.0% or less. [3] The steel according to [1] or [2] above, wherein in the microstructure, the number density of crystal grains having a diameter of 50 μm or more at a position 1 mm below the surface of the steel is 1.0 crystals/mm ² or more. [4] The steel according to any one of [1] to [3] above, wherein in the microstructure, inclusions in the top 10% of the particle size distribution of inclusions at a position 1 mm below the surface of the steel are The particle size is below 3.5 μm. [5] A method for producing a steel material, for producing the steel material according to any one of [1] to [4], wherein the steel material having the composition is heated to 1100°C In the temperature range above and below 1300°C, the total reduction rate above 950°C: 40% or more, the number of hot rolling passes at less than 950°C: one or more, and the finishing temperature at the end of rolling: 750°C or above. Rolling, followed by cooling. [6] A groove formed by welding the steel material according to any one of the above [1] to [4], wherein the grain boundary at a position 1 mm below the surface of the base material part heated linearly The concentration of C in is 0.100% or more, and the absorbed energy of the Charpy impact test at -269°C or higher at the position 1 mm below the surface of the linear heating part after linear heating is 41 J or more. [7] A method for manufacturing a trough, which manufactures the trough as described in [6] above, in which the steel material described in any one of [1] to [4] is prepared. The surface is heated to 900°C or lower, the steel is air-cooled to a surface temperature of 500°C or lower, and then water-cooled in a linear heat treatment to perform curved surface processing, and then the curved steel materials are welded to each other. [8] The manufacturing method of the tank as described in [7], wherein the welding uses a solid wire as an electrode, the temperature between passes: 100°C to 150°C, and the shielding gas: 80%Ar+20%CO ₂ conditions. [Effect of the invention]

According to the present invention, a steel material excellent in low-temperature toughness after linear heating and a method for producing the same can be provided. In addition, the steel material of the present invention is preferably used 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 even after linear heating and a method for manufacturing the same . Therefore, it can greatly contribute to the improvement of the safety and lifespan of the said steel structure, and has a remarkable effect in industry. In addition, since the production method of the present invention does not cause a decrease in productivity or an increase in production cost, it is possible to provide a production method that is also excellent in economical efficiency.

Hereinafter, the present invention will be described in detail. 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 are Worth steel materials (such as high-Mn steel materials) that are inexpensive and have excellent low-temperature toughness. In order to use such a high Mn steel material as a material of a steel structure (such as a tank) used in a low-temperature environment, it is required to have excellent low-temperature toughness even at a portion affected by heat in the step of linearly heating the material.

As a result of research conducted by the inventors of the present invention, it has been found that when there are no carbides, the larger the crystal grain size of the high-Mn steel material, the higher the absorbed energy. However, it can be seen that when carbide exists, the larger the grain size, the higher the absorbed energy. In the step of linear heating, carbides are formed in the parts affected by the heat of about 600°C to 800°C, so the low-temperature toughness decreases.

Therefore, the inventors of the present invention have diligently investigated the cause, and as a result, have newly found that the C concentration in the grain boundary is caused by a decrease in absorbed energy. Hereinafter, the relationship between the decrease in absorbed energy and the C concentration in the grain boundary will be described.

One of the starting points of destruction of high Mn steel is the grain boundary. By reducing grain boundaries, that is, making crystal grains coarse, low-temperature toughness is improved. Usually, when carbides are formed at the grain boundaries under the influence of heat, C around the carbides is deficient, and the grain boundary strength decreases. However, in high-Mn steels, due to the large amount of C added, during the formation and growth of carbides on the grain boundaries, C with a fast diffusion rate is sufficiently supplied from the inside of the grains away from the grain boundaries, and a healing phenomenon occurs. Thereby, rapid C deficiency at grain boundaries can be suppressed. However, when the crystal grains become too coarse, it is too late to supply C from the inside of the grains, and C on the grain boundaries is lacking.

Therefore, in the present invention, by setting the maximum crystal grain size to less than 200 μm in the hot rolling step described later, even when carbides are formed, the C concentration can be ensured to be 0.100% or more, and the loss of absorbed energy can be suppressed. reduce.

Next, the steel material of the present invention will be described. The steel material of the present invention has the composition described below, and in the microstructure, the maximum crystal grain size at a position 1 mm below the surface of the steel material is set to be less than 200 μm. Thereby, even after the steel material is linearly heated, the C concentration in the grain boundary can be set to 0.100% or more. In addition, the expression of "%" about C concentration means "mass %".

[ingredient composition] First, the component composition in the steel material (Worth steel material) of the present invention will be described. In the present invention, the steel material used for the Worth field iron steel material (for example, high Mn steel material) and its manufacture has the above-mentioned composition. The component composition of the washer steel material of the present invention and the reason for its limitation will be described. In addition, the expression of "%" concerning a component composition means "mass %" unless otherwise specified.

C: 0.200% or more and 0.700% or less C is an inexpensive wasted iron stabilizing element and is an important element for obtaining wasted iron. In order to prevent the lack of C at the grain boundaries, C is contained at 0.200% or more. On the other hand, if the C content exceeds 0.700%, Cr carbides are excessively formed, and the low-temperature toughness (low-temperature toughness after linear heating) decreases. Therefore, the content of C is set to not less than 0.200% and not more than 0.700%. The content of C is preferably at least 0.250%, more preferably at least 0.300%. In addition, the content of C is preferably at most 0.600%, more preferably at most 0.550%.

Si: 0.05% to 1.00% Si functions as a deoxidizing material, and is not only necessary for steel production, but also has the effect of solid-solution in steel and strengthening of steel sheets by solid-solution strengthening. In order to obtain such an effect, Si is contained in an amount of 0.05% or more. On the other hand, if the Si content exceeds 1.00%, the non-thermal stress increases excessively, so the low-temperature toughness deteriorates. Therefore, the content of Si is set to 0.05% or more and 1.00% or less. The content of Si is preferably at least 0.07%, more preferably at least 0.10%, and still more preferably at least 0.15%. In addition, the content of Si is preferably at most 0.80%, more preferably at most 0.75%, even more preferably at most 0.70%.

Mn: 20.0% or more and 40.0% or less Mn is a relatively cheap Stabilizing element of Worth field iron. In the present invention, it is an important element for achieving both strength and low-temperature toughness. In order to obtain the above-mentioned effects, Mn must be contained in an amount of 20.0% or more. On the other hand, when the Mn content exceeds 40.0%, the low-temperature toughness deteriorates. In addition, weldability and cuttability deteriorate. Furthermore, segregation is promoted, and the generation of stress corrosion cracking is promoted. Therefore, the content of Mn is set to 20.0% or more and 40.0% or less. The content of Mn is preferably at least 23.0%, more preferably at least 23.3%, and still more preferably at least 23.5%. The content of Mn is preferably at most 35.0%, more preferably at most 30.0%.

P: less than 0.030% When P content exceeds 0.030%, since excessive segregation at grain boundaries occurs, the low-temperature toughness decreases. Therefore, 0.030% is set as the upper limit, and it is desirable to reduce it as much as possible. Therefore, the content of P is made 0.030% or less. Furthermore, an excessive reduction of P increases the refining cost and is economically disadvantageous, so the content of P is desirably set at 0.002% or more. The content of P is more preferably at least 0.005%, more preferably at least 0.007%. The content of P is preferably at most 0.028%, more preferably at most 0.024%, still more preferably at most 0.020%.

S: less than 0.0050% S degrades the low-temperature toughness and ductility of the base material, so 0.0050% is set as the upper limit, and it is desirable to reduce it as much as possible. Therefore, the content of S is set to 0.0050% or less. Preferably, it is 0.0045% or less, More preferably, it is 0.0043% or less. Furthermore, an excessive reduction of S increases the refining cost and is economically disadvantageous, so the content of S is desirably set at 0.0010% or more. The content of S is more preferably at least 0.0012%.

Al: less than 5.00% Al functions as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel plates. In addition, the yield strength and local elongation in the tensile test are improved. In order to obtain such an effect, Al is preferably contained at 0.01% or more. On the other hand, if the Al content exceeds 5.00%, a large amount of inclusions will exist to degrade the low-temperature toughness, so the Al content is made 5.00% or less. The content of Al is preferably at least 0.01%, more preferably at least 0.02%. The Al content is preferably at most 4.00%, more preferably at most 3.00%.

Cr: less than 7.0% Cr improves grain boundary strength, and therefore is an effective element for improving low-temperature toughness. In order to obtain such an effect, Cr is preferably contained at 0.5% or more. On the other hand, if the Cr content exceeds 7.0%, the low-temperature toughness and stress corrosion cracking resistance may be lowered due to the formation of Cr carbides. Therefore, the content of Cr is set to 7.0% or less. The content of Cr is preferably at least 0.5%, more preferably at least 1.0%, and still more preferably at least 1.2%. The Cr content is preferably at most 6.7%, more preferably at most 6.5%. In addition, in order to further improve the stress corrosion cracking resistance, it is more preferable to set the Cr content to 2.0% or more and 6.0% or less.

N: 0.0500% or less N is an element for stabilizing ferrite, and is an element effective for improving low-temperature toughness. In order to obtain such an effect, N is preferably contained at 0.0050% or more. On the other hand, if the N content exceeds 0.0500%, the nitrides or carbonitrides will be coarsened, which may lower the low-temperature toughness. Therefore, the content of N is made 0.0500% or less. The content of N is preferably at least 0.0050%, more preferably at least 0.0060%. The content of N is preferably at most 0.0400%, more preferably at most 0.0300%.

O: less than 0.0050% O (oxygen) degrades the low-temperature toughness through the formation of oxides. Therefore, the content of O is set to a range of 0.0050% or less. Preferably, it is 0.0045% or less, More preferably, it is 0.0040% or less. Furthermore, an excessive reduction of O increases the refining cost and is economically disadvantageous, so the content of O is preferably 0.0010% or more. The content of O is more preferably at least 0.0012%.

Ti: less than 0.005%, Nb: less than 0.005% Ti and Nb form high-melting carbonitrides in steel, so low-temperature toughness decreases. Since Ti and Nb are unavoidably mixed components from raw materials, etc., they are usually mixed in the range of Ti: 0.005% to 0.010% and Nb: 0.005% to 0.010%. Therefore, it is necessary to avoid the unavoidable mixing of Ti and Nb according to the melting method described later, and to suppress the respective contents of Ti and Nb to less than 0.005%. By suppressing the contents of Ti and Nb to less than 0.005%, the adverse effects of the carbonitrides can be eliminated, and excellent low-temperature toughness and ductility can be ensured. It is preferable to set the contents of Ti and Nb to be 0.003% or less, more preferably 0.002% or less. Of course, the content of Ti and Nb may also be 0%.

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 metal) are elements useful for controlling the morphology of inclusions. The so-called shape control of inclusions refers to making stretched inclusions into granular inclusions. Ductility, low temperature toughness and stress corrosion cracking resistance are improved through the morphology control of the inclusions. In order to obtain such an effect, Ca and Mg are preferably contained at 0.0005% or more, and REM is preferably contained at 0.0010% or more. On the other hand, if a large amount of arbitrary elements is also contained, the amount of non-metallic inclusions increases, conversely leading to a decrease in ductility, low-temperature toughness, and stress corrosion cracking resistance. In addition, it is economically disadvantageous.

Therefore, when Ca and Mg are contained, they are each set to 0.0100% or less, and when REM is contained, they are set to 0.0200% or less. Preferably, Ca is 0.0005% to 0.0090%, Mg is 0.0005% to 0.0090%, and REM is 0.0010% to 0.0180%. More preferably, Ca is 0.0010% to 0.0080%, Mg is 0.0010% to 0.0080%, and REM is 0.0020% to 0.0150%. More preferably, Ca is 0.0015% to 0.0050%, Mg is 0.0015% to 0.0050%, and REM is 0.0030% to 0.0100%.

Regarding the Wurst iron steel material of the present invention, the balance other than the above-mentioned components is iron (Fe) and unavoidable impurities. H, B, etc. are mentioned as an unavoidable impurity here, and it is allowable as long as the sum total of each element is 0.01% or less.

Let the elements mentioned above be the basic composition. The characteristics targeted by the present invention can be obtained by the above-mentioned basic component composition. In the present invention, in order to further improve the strength and low-temperature toughness, the following elements may be contained as necessary in addition to the above-mentioned elements. In addition, since each component of Cu, Ni, Mo, V, and W shown below may be contained as needed, 0% of these components may be sufficient.

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 a steel sheet by solid solution strengthening, but also improve the mobility of dislocations and also improve low-temperature toughness. In order to obtain such an effect, Cu and Ni are preferably contained at 0.01% or more. On the other hand, if Cu and Ni are contained in excess of 1.0%, in addition to deterioration of the surface properties during rolling, the burden on manufacturing costs will also increase. Therefore, when these alloy elements are contained, the content thereof is preferably 1.0% or less. Cu and Ni are each more preferably at least 0.03%, and more preferably at most 0.7%. Furthermore, it is more preferable to set it as 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 the washer and the improvement of the strength of the base material. In order to obtain such an effect, Mo, V, and W are preferably contained at 0.001% or more each. On the other hand, when each of Mo, V, and W is contained in excess of 2.0%, coarse carbonitrides are formed, which may become the starting point of destruction, and in addition to this, increase the burden on manufacturing costs. Therefore, when these alloy elements are contained, their content is preferably 2.0% or less. Mo, V, and W are each more preferably at least 0.003%, and more preferably at most 1.7%. Furthermore, it is more preferable to set it as 0.1 % or more, and it is still more preferable to set it as 1.5 % or less.

[Microstructure of steel] Next, the reason why the microstructure is limited as described above in the present invention will be described.

The maximum crystal grain size at 1 mm below the surface of the steel: less than 200 μm As described above, when the crystal grain size of the steel material (base material) is large, C is deficient at the time of carbide formation. By setting the maximum grain size at 1 mm below the surface of the steel to less than 200 μm, the C concentration in the grain boundary can be set to 0.100% or more even after the steel is linearly heated. That is, in the linear heating part of the structure obtained after linear heating (for example, a groove), a steel material having excellent low-temperature toughness with an absorbed energy of 41 J or more in the Charpy impact test at -269°C or higher can be produced.

The maximum crystal grain size is preferably at most 150 μm, more preferably at most 100 μm, and still more preferably at most 80 μm. The lower limit of the maximum crystal grain size is not particularly defined. In order to ensure the toughness of the hot-rolled steel sheet (steel), the maximum grain size is preferably at least 50 μm, more preferably at least 60 μm. Here, the crystal grains refer to particles that appear by etching. In the present invention, the maximum crystal grain size can be measured by the method described in Examples described later.

Furthermore, in the present invention, the maximum crystal grain size of the steel material can be controlled within the numerical range described above by performing hot rolling under the conditions described later. As a result, even after in-line heating, the C concentration in the grain boundary can be ensured, and the energy absorbed can be realized.

The number density of crystal grains at the position 1 mm below the surface of the steel is 50 μm or more (preferable condition) The starting point of destruction of high Mn steel is the grain boundary, and cracks propagate through the grain boundary, so there are coarse grains, thereby It can inhibit the propagation of cracks and further improve the low temperature toughness. For this reason, the number of wurst iron grains having a grain size of 50 μm or more is preferably 1.0 or more per 1 mm ² , more preferably 2.0 or more. On the other hand, when the number of the washer grains exceeds 10.0 per 1 mm ² , the strength decreases. Therefore, it is preferably 10.0 or less per 1 mm ² , more preferably 9.0 or less. In the present invention, the number per 1 mm ² (number density) of Worsfield iron crystal grains having a grain size of 50 μm or more can be measured by the method described in Examples described later. The said number density can be controlled within the said numerical range by performing the hot rolling mentioned later.

Inclusion particle size at 1 mm below the surface of the steel (better condition) When there are coarse inclusions 1 mm below the surface of the steel material, the stress corrosion cracking resistance decreases. It can be seen that the stress corrosion cracking resistance decreases when the inclusion particle size of the top 10% of the inclusion particle size distribution at the position 1 mm below the surface of the steel material (top 10% inclusion particle size) exceeds 3.5 μm. Accordingly, it is preferable to set the particle size of the top 10% inclusions below 3.5 μm, more preferably below 3.0 μm. On the other hand, the particle size of the top 10% inclusions is preferably as small as possible, and from the viewpoint of manufacturability, it is preferably 1.5 μm or more, more preferably 2.0 μm or more. Here, the "top 10% inclusion particle size" refers to the particle size at the 10% position when the inclusion particle sizes are sorted in order of size in the inclusion particle size distribution. In the present invention, the particle size of the inclusions can be measured by the method described in Examples described later.

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

[manufacturing method of steel materials] Next, the manufacturing method of the steel material which concerns on one Embodiment of this invention is demonstrated.

Regarding the steel material (Worth field iron steel material) of the present invention, molten steel having the above composition can be melted by a melting method such as a converter or an electric furnace. In addition, it can also be refined twice in a vacuum degassing furnace.

At this time, in order to limit Ti and Nb, which interfere with structure control, within the above numerical range, it is necessary to take measures to avoid unavoidable contamination of Ti and Nb from raw materials and the like and to reduce their content. For example, by reducing the basicity of the slag in the refining stage, these alloys are concentrated into slag and discharged, reducing the concentration of Ti and Nb in the final slab product. Alternatively, methods such as blowing oxygen to oxidize and floating and separating the alloy of Ti and Nb during reflow may be used.

Thereafter, it is preferable to manufacture steel materials such as slabs of predetermined dimensions by casting methods such as continuous casting, slab-block rolling, and the like.

Hereinafter, the production conditions for producing the above-mentioned steel material into a steel material (Worth steel material) excellent in low-temperature toughness after linear heating will be described in detail.

In order to obtain the Worth iron steel material with the above-mentioned structure, it is important to heat the steel material with the above-mentioned composition to a temperature range of 1100°C to 1300°C, and then to make the total reduction ratio of 950°C or higher to 40%. After the above rolling, one or more hot rolling passes are performed at less than 950° C., and the hot rolling is performed under the condition that the finish rolling finish temperature is 750° C. or higher. Furthermore, cooling is performed after completion|finish of the said hot rolling. The temperature control here is based on the surface temperature of the steel material.

In addition, in the following description of the manufacturing method, the expression "°C" concerning the temperature is the surface temperature of the steel material or the steel sheet, unless otherwise specified. The surface temperature can be measured with, for example, a radiation thermometer or the like. In addition, the temperature at the thickness center of the slab or steel plate can be measured, for example, by adding a thermocouple to the thickness center of the steel plate, or by calculating the temperature distribution in the steel plate section through heat transfer analysis, and correcting it according to the surface temperature of the steel plate Find the result.

Heating temperature of steel material: 1100°C or more and 1300°C or less 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 the washer can be ensured even in the Mn negative segregation portion. Thereby, the stability of the washer can be ensured also in the linear heating portion, and brittle fracture can be prevented. That is, absorbed energy at -269°C can be ensured. On the other hand, if the heating temperature exceeds 1300°C, the steel may start to melt, so the upper limit of the heating temperature is made 1300°C. The heating temperature of the steel material is preferably at least 1130°C, more preferably at most 1270°C. More preferably, it is 1150°C or higher, more preferably 1250°C or lower.

Total rolling reduction in hot rolling at 950° C. or higher: 40% or higher As described above, in the present invention, it is important to make the maximum crystal grain size at a position 1 mm below the surface of the steel material less than 200 μm. If equiaxed grains cannot be formed during rolling in the recrystallized region, they will remain as coarse grains during subsequent rolling in the non-recrystallized region, and the maximum grain size will be 200 μm or more. Furthermore, the number density of crystal grains having a diameter of 50 μm or more exceeds 10.0 crystal grains/mm ² . Therefore, it is effective to secure a total reduction ratio of 40% or more in the recrystallization region, that is, a temperature region of 950° C. or higher. The total reduction ratio in the recrystallized region is preferably at least 50%, more preferably at least 52%. The upper limit of the total reduction ratio in the recrystallized region is not particularly specified, but the total reduction ratio in the recrystallized region is preferably 85% or less, more preferably 70% or less, in order to ensure strength. .

The number of hot rolling passes at less than 950°C: more than once and the finish rolling temperature: 750°C or higher In order to make the equiaxed grains formed in hot rolling at 950°C or higher The number of times is set to more than once. Preferably it is two or more times. When there is no hot rolling pass at less than 950° C., the maximum crystal grain size becomes 200 μm or more. Furthermore, the number density of crystal grains having a diameter of 50 μm or more exceeds 10.0 crystal grains/mm ² . The upper limit of the number of hot rolling passes is not particularly specified. From the viewpoint of manufacturability, the number of hot rolling passes is preferably 10 or less, more preferably 8 or less. When hot rolling is performed at less than 750°C, the crystal grain size becomes too fine and the low-temperature toughness decreases, so the finishing temperature of finish rolling is made 750°C or higher. When the finish rolling finish temperature is 775°C or lower, the crystal grain size becomes finer, and as a result, the maximum crystal grain size may be smaller than 50 μm. Therefore, the finish rolling finish temperature is preferably set to exceed 775°C, more preferably 780°C above. There is no particular limitation on the upper limit of the finish rolling finish temperature. From the viewpoint of securing the strength, the finish rolling finish temperature is preferably set to 930°C or lower, more preferably 900°C or lower.

cool down After the hot rolling is finished, it is cooled. The cooling conditions are not particularly specified. In the present invention, it is preferable to cool to 600° C. or less at an average cooling rate of 1.0° C./s or more from a temperature above (temperature at the end of hot rolling −100° C.). Thereby, formation of carbides and grain boundary segregation of P are suppressed, and the properties of the steel material can be further improved. The "temperature at the end of hot rolling" refers to the temperature at the end of finish rolling. In addition, the upper limit of the said average cooling rate is not specifically defined. From the viewpoint of controlling the cooling stop temperature, it is preferably set to 30.0° C./s or less.

Next, a steel structure (for example, a tank) manufactured by using the steel material of the present invention as a material and linearly heating the material will be described.

The groove of the present invention is manufactured by linearly heating the steel material under specific linear heating conditions to form a curved surface, and welding the steel material processed into the curved surface. In the tank of the present invention manufactured as described above, the composition and microstructure of the base material portion are the same as those of the above-mentioned steel material (Worthfield steel material).

In addition, in the groove of the present invention, the C concentration in the grain boundary at a position 1 mm below the surface of the base material portion after linear heating is 0.100% or more. If the C concentration in the grain boundary at the position of the base material portion after linear heating is less than 0.100%, the grain boundary strength cannot be ensured. Therefore, the C concentration in the grain boundary at the position of the base material portion after linear heating is set to 0.100% or more. Preferably it is 0.200% or more, More preferably, it is 0.250% or more. The upper limit of the C concentration in the grain boundary at the position of the base material part after the linear heating is not particularly specified. From the viewpoint of the reduction in low-temperature toughness associated with excessive Cr carbide formation, it is preferably at most 0.600%, more preferably at most 0.550%.

In the tank of the present invention manufactured as described above, the absorbed energy in the Charpy impact test at -269° C. or higher at a position 1 mm below the surface of the linear heating portion after linear heating can be 41 J or more. In addition, the absorbed energy of the said Charpy impact test can be measured by the method described in the Example mentioned later. That is, the absorbed energy in the Charpy impact test at -269° C. or higher in the linear heating portion is 41 J or more in the full size, and 27 J or more in the 5 mm sub-size. In addition, according to the present invention, stress corrosion cracking resistance can also be provided.

Next, an example of a preferable manufacturing method of the groove will be described.

The groove of the present invention is manufactured by heating the steel material linearly under the following conditions to form a curved surface, and welding the steel material processed into a curved surface. In addition, since the manufacturing method of the steel material (Worth iron steel material) which is a material has already been demonstrated, it is abbreviate|omitted. Here, preferable linear heating conditions and welding conditions are demonstrated.

[Linear heating condition] The steel material is linearly heated under the condition that the target (heating target temperature) of the surface heating temperature of the steel material is 900° C. or lower. After heating, the steel material is air-cooled until the surface temperature is 500° C. or lower, and then water-cooled. The above-mentioned linear heat treatment of heating and air cooling may be performed once, or may be repeated (repeated) more than once. In order to change the microstructure, the number of repetitions is preferably one or more. Since the local thermal cycle history becomes complicated, the number of repetitions is preferably 5 or less. The heating temperature is preferably set to exceed 800°C.

[Welding Conditions] From the standpoint of ensuring high strength and high ductility and excellent low-temperature impact toughness, welding is performed using a solid wire (diameter 1.2 mm) as an electrode, without preheating, and in a downward position, at The temperature between passes: 100 ℃ ~ 150 ℃, protective gas: 80% Ar + 20% CO ₂ under the conditions of implementation. In addition, the expression of "%" concerning the shielding gas means "volume %". [Example]

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

Steel slabs having the composition shown in Table 1 were produced by a converter-bucket refining-continuous casting method. In addition, "-" shown in Table 1 indicates that no element is added intentionally, and includes not only the case where the element is not contained (%) but also the case where the element is unavoidably contained. Next, the obtained slabs were hot-rolled under the conditions shown in Table 2-1, and then cooled to produce steel materials (hot-rolled steel sheets) with a plate thickness of 6 mm to 40 mm.

Using the obtained hot-rolled steel sheets (steel sheets), evaluations of crystal grain sizes and inclusion particle sizes were carried out in the following manner.

Next, the obtained steel sheets were linearly heated, and the C concentration, low-temperature toughness, and stress corrosion cracking resistance were evaluated by the following procedure using the linearly heated steel sheets. Here, the linear heating will be described. As the linear heating, the plate linear heating shown in FIG. 1 was performed. As shown in Figure 1, a linear heating test body with a length of 1000 mm and a width of 500 mm was made from the obtained steel plate, and the test body was fixed with a constraining plate at 1/2 position in the width direction (rolling direction). Plate linear heating was performed under the following conditions. The condition is to set the target surface heating temperature of the steel material at 900°C, heat to this temperature, air-cool until the surface temperature of the steel material is 500°C or lower, and then perform water cooling. Linear heating in the same area was repeated under the conditions shown in Table 2-2. In addition, the welding of the steel plates after the linear heat treatment was performed using a solid wire (1.2 mm in diameter) as an electrode, without preheating, in a downward posture, and under the welding conditions shown in Table 2-2.

(1) Evaluation of Microstructure [Crystal Grain Size] The cross section in the rolling direction of the obtained hot-rolled steel sheet was polished and then etched, and then the position 1 mm below the surface of the steel sheet was photographed with an optical microscope at a magnification of 200 times. From the captured image, 100 crystal grains that appeared by etching were randomly selected, and the circle-equivalent diameter of the crystal grains was defined as the crystal grain diameter, and the maximum crystal grain diameter (μm) at a position 1 mm below the steel plate surface was obtained. In addition, the total area of 100 crystal grains and the number of crystal grains of 50 μm or more were obtained, and the number density (mm ² /piece) of crystal grains having a diameter of 50 μm or more per 1 mm ² was obtained. In addition, aqua regia was used as the corrosion solution.

[Inclusion Particle Size] The obtained hot-rolled steel sheet was investigated for inclusion particle size using a scanning electron microscope (Scanning Electron Microscope, SEM). The evaluation area is set to 200 mm ² , and the particle size (μm) of the top 10% inclusions at the position 1 mm below the steel plate surface is calculated.

[C density] A transmission electron microscope (Transmission Electron Microscope, TEM) sample of 12 mm×10 mm was prepared from the obtained hot-rolled steel sheet after linear heating. For this sample, the composition analysis was performed across grain boundaries without carbides using an Energy Dispersive Spectrometer (EDS) detector attached to a TEM (Transmission Electron Microscope), and the obtained C concentration was evaluated. The position 1 mm below the surface of the steel plate was taken as the observation object. 10 grain boundaries were analyzed, and the average value was calculated|required.

(2) Low temperature toughness Evaluation of the low-temperature toughness of the linear heating portion was performed as follows. A linearly heated test body as shown in FIG. 1 was produced from the obtained hot-rolled steel sheet, and the low temperature toughness of the linearly heated portion was evaluated using the steel sheet obtained by linearly heating the test body under the above conditions. According to Japanese Industrial Standards (Japanese Industrial Standards, JIS) Z 2242 (2005), Charpy V-notch test pieces were collected from the linear heating part with a thickness of 10 mm or more (full-scale Charpy V-notch test piece). The Charpy impact test was implemented at -196 degreeC and -269 degreeC using three Charpy V-notch test pieces. The average value of three absorbed energies at each temperature was calculated|required. In this example, in the case of the full-scale Charpy V-notch test piece, it was judged that the average value of three absorbed energies at -269° C. was 41 J or more was excellent in low-temperature toughness.

In addition, for the linear heating portion with a plate thickness of less than 10 mm, Charpy V-notch test pieces with a small size of 5 mm were collected in accordance with JIS Z 2242 (2005). The Charpy impact test was implemented at -196 degreeC and -269 degreeC using three Charpy V-notch test pieces. The average value of three absorbed energies at each temperature was calculated|required. In Table 2, in the samples implemented using the small-sized Charpy V-notch test piece, "*1" is indicated in the item of absorbed energy. In the case of a small-sized Charpy V-notch test piece, it was judged that the average value of three absorbed energies at -269° C. was 27 J or more was excellent in low-temperature toughness.

(3) Stress Corrosion Cracking Resistance The stress corrosion cracking resistance was evaluated based on the stress corrosion cracking test based on American Society for Testing and Materials (ASTM) G36. A test piece having a size of 2.5 mm in thickness, 20 mm in width, and 80 mm in length was collected from a position 1 mm below the surface of the obtained hot-rolled steel sheet. The solution is set to boiling MgCl ₂ chloride, and the bending radius is set to 5 mm. The presence or absence of cracks was confirmed after immersing the test piece in which the stress was applied to the solution for 400 hours. The case where no cracks occurred was evaluated as "◯ (pass)" shown in Table 2-2, and the case with cracks was evaluated as "× (failure)" shown in Table 2-2.

The results obtained above are shown in Table 2-1 and Table 2-2.

[Table 1] Steel No. Composition (mass%) C Si mn P S al Cr N o Ti Nb Ca Mg REM Cu Ni Mo V W 1 0.358 0.10 32.6 0.013 0.0018 0.03 4.3 0.0187 0.0017 0.001 0.001 0.0020 - - - - - - - 2 0.550 0.07 24.0 0.014 0.0023 0.20 3.6 0.0098 0.0016 0.002 0.001 - 0.0006 - - - - - - 3 0.402 0.38 26.6 0.014 0.0045 0.12 5.0 0.0203 0.0043 0.001 0.002 - - 0.0010 - - - - - 4 0.555 0.25 22.9 0.012 0.0025 0.04 6.5 0.0150 0.0015 0.003 0.003 - 0.0006 - - 1.0 - - - 5 0.253 0.73 24.2 0.025 0.0019 4.03 2.7 0.0215 0.0018 0.001 0.001 - - 0.0015 - - 1.8 - - 6 0.204 0.98 39.6 0.012 0.0016 0.05 7.0 0.0490 0.0014 0.001 0.002 0.0015 - - - - - 0.1 - 7 0.698 0.15 20.2 0.011 0.0018 0.04 5.9 0.0102 0.0013 0.001 0.002 - 0.0011 - - - - - 0.1 8 0.195 0.80 22.2 0.020 0.0023 0.36 1.5 0.0364 0.0021 0.002 0.001 0.0011 - - - - - - - 9 0.707 0.31 35.0 0.022 0.0030 0.60 5.8 0.0235 0.0023 0.002 0.002 - 0.0007 - - - - - - 10 0.630 1.05 23.5 0.019 0.0020 0.03 4.7 0.0319 0.0020 0.001 0.001 - - 0.0009 - - - - - 11 0.211 0.26 19.6 0.019 0.0019 0.12 1.9 0.0162 0.0022 0.003 0.002 0.0013 - - - - - - - 12 0.300 0.32 40.3 0.023 0.0025 0.05 0.7 0.0123 0.0025 0.002 0.003 - 0.0009 - - - - - - 13 0.524 0.44 23.4 0.032 0.0018 3.50 6.6 0.0417 0.0019 0.003 0.003 - - 0.0012 - - - - - 14 0.499 0.84 39.1 0.025 0.0051 2.35 0.9 0.0253 0.0021 0.002 0.001 0.0024 - - - - - - - 15 0.610 0.58 33.3 0.020 0.0021 5.02 3.4 0.0194 0.0028 0.002 0.001 - 0.0010 - - - - - - 16 0.672 0.43 21.0 0.026 0.0019 0.21 7.2 0.0334 0.0020 0.001 0.002 0.0019 - - - - - - - 17 0.498 0.60 22.6 0.017 0.0022 2.27 5.5 0.0505 0.0037 0.002 0.002 - - 0.0014 - - - - - 18 0.395 0.57 30.2 0.023 0.0020 4.00 2.3 0.0361 0.0053 0.003 0.002 0.0018 - - - - - - - 19 0.222 0.86 20.4 0.025 0.0040 0.04 0.8 0.0299 0.0030 0.006 0.003 - 0.0009 - - - - - - 20 0.514 0.28 38.5 0.024 0.0035 3.48 1.1 0.0426 0.0041 0.003 0.006 - - 0.0013 - - - - - twenty one 0.305 0.68 23.8 0.010 0.0014 2.90 6.0 0.0300 0.0013 0.002 0.001 0.0015 - - 0.5 0.5 - - - twenty two 0.541 0.17 29.5 0.019 0.0040 0.05 2.0 0.0094 0.0039 0.001 0.002 - 0.0015 0.0030 - - 1.5 - - twenty three 0.210 0.31 39.3 0.020 0.0039 3.97 6.5 0.0223 0.0036 0.001 0.001 - - - - - - - -

[table 2-1] Sample No. Steel No. plate thickness Manufacturing method of steel Microstructure at 1 mm below the steel plate surface exam preparation Slab heating temperature Total reduction ratio above 950℃ Number of hot rolling passes at less than 950°C finishing temperature average cooling rate cooling stop speed Maximum grain size The number density of crystal grains with a diameter of 50 μm or more Particle size of the top 10% inclusions (mm) (℃) (%) (Second-rate) (℃) (°C/s) (℃) (μm) (pcs/mm ² ) (μm) 1 1 25 1100 75 1 878 15.0 546 87 3.2 2.0 Invention example 2 2 20 1200 65 3 842 16.2 530 76 2.4 2.3 Invention example 3 3 15 1150 70 4 799 16.6 499 51 1.4 2.7 Invention example 4 4 30 1250 55 4 873 12.5 541 164 6.7 2.0 Invention example 5 5 10 1230 75 4 855 18.5 535 60 1.8 3.1 Invention example 6 6 6 1250 85 2 891 1.2 air cooling 79 2.7 2.6 Invention example 7 7 40 1270 40 4 900 10.2 568 190 8.3 2.8 Invention example 8 8 twenty one 1130 65 3 808 15.7 505 96 4.2 2.0 comparative example 9 9 twenty one 1130 70 2 819 15.8 524 102 5.0 2.3 comparative example 10 10 twenty four 1100 65 3 757 15.5 458 49 0.0 2.9 comparative example 11 11 18 1090 75 2 810 16.4 515 83 3.2 3.3 comparative example 12 12 18 1170 70 3 806 16.2 507 97 4.2 2.8 comparative example 13 13 12 1150 75 3 781 17.3 477 59 1.5 3.1 comparative example 14 14 12 1150 85 1 894 17.7 555 181 7.7 2.6 comparative example 15 15 15 1270 70 3 832 16.8 530 102 5.3 2.2 comparative example 16 16 15 1200 75 3 811 16.6 512 90 3.6 2.0 comparative example 17 17 9 1170 80 3 783 1.4 air cooling 51 1.6 2.7 comparative example 18 18 9 1170 85 2 768 1.4 air cooling 48 0.0 3.2 comparative example 19 19 6 1200 80 4 745 1.0 air cooling 36 0.0 2.1 comparative example 20 20 6 1200 85 3 775 1.1 air cooling 45 0.0 2.6 comparative example twenty one 1 40 1130 35 5 800 10.0 501 205 10.4 3.1 comparative example twenty two 3 40 1270 70 0 953 10.4 578 213 10.9 2.9 comparative example twenty three 2 15 1220 75 3 845 16.7 532 80 2.9 3.0 Invention example twenty four twenty one 20 1200 70 4 828 15.2 520 65 1.8 2.7 Invention example 25 twenty two 25 1150 65 6 800 14.7 504 53 1.4 2.6 Invention example 26 twenty three 15 1150 70 3 815 15.8 509 72 2.2 3.9 comparative example 27 1 25 1100 70 3 780 15.5 499 52 1.1 2.6 Invention example

[Table 2-2] Sample No. Steel No. groove exam preparation linear heating condition Welding conditions Element concentration after linear heating Characteristics of the linear heating part heating target temperature Linear heating times Temperature between passes Protective gas C Absorbed energy at -196°C Absorbed Energy at -269°C Stress corrosion cracking resistance (℃) (Second-rate) (℃) (%) (J) (J) (-) 1 1 900 5 100 80%Ar+20%CO ₂ 0.247 80 65 ○ Invention example 2 2 900 5 150 80%Ar+20%CO ₂ 0.441 77 63 ○ Invention example 3 3 900 5 130 80%Ar+20%CO ₂ 0.298 78 60 ○ Invention example 4 4 900 5 120 80%Ar+20%CO ₂ 0.303 70 56 ○ Invention example 5 5 900 5 140 80%Ar+20%CO ₂ 0.150 68 52 ○ Invention example 6 6 900 5 110 80%Ar+20%CO ₂ 0.100 45 ^*1 29 ^*1 ○ Invention example 7 7 900 5 130 80%Ar+20%CO ₂ 0.443 68 45 ○ Invention example 8 8 900 5 130 80%Ar+20%CO ₂ 0.065 49 30 ○ comparative example 9 9 900 5 120 80%Ar+20%CO ₂ 0.497 50 35 ○ comparative example 10 10 900 5 140 80%Ar+20%CO ₂ 0.519 53 38 ○ comparative example 11 11 900 5 100 80%Ar+20%CO ₂ 0.105 51 30 ○ comparative example 12 12 900 5 130 80%Ar+20%CO ₂ 0.170 55 31 ○ comparative example 13 13 900 5 150 80%Ar+20%CO ₂ 0.417 52 36 ○ comparative example 14 14 900 5 120 80%Ar+20%CO ₂ 0.248 51 37 ○ comparative example 15 15 900 5 130 80%Ar+20%CO ₂ 0.410 43 30 ○ comparative example 16 16 900 5 110 80%Ar+20%CO ₂ 0.532 50 32 ○ comparative example 17 17 900 5 110 80%Ar+20%CO ₂ 0.394 35 ^*1 24 ^*1 ○ comparative example 18 18 900 5 120 80%Ar+20%CO ₂ 0.293 34 ^*1 24 ^*1 ○ comparative example 19 19 900 5 140 80%Ar+20%CO ₂ 0.115 35 ^*1 23 ^*1 ○ comparative example 20 20 900 5 120 80%Ar+20%CO ₂ 0.408 38 ^*1 26 ^*1 ○ comparative example twenty one 1 900 5 130 80%Ar+20%CO ₂ 0.058 46 32 ○ comparative example twenty two 3 900 5 130 80%Ar+20%CO ₂ 0.092 45 30 ○ comparative example twenty three 2 850 4 130 80%Ar+20%CO ₂ 0.475 84 70 ○ Invention example twenty four twenty one 900 5 120 80%Ar+20%CO ₂ 0.209 69 53 ○ Invention example 25 twenty two 900 5 110 80%Ar+20%CO ₂ 0.438 77 64 ○ Invention example 26 twenty three 900 5 130 80%Ar+20%CO ₂ 0.114 50 39 x comparative example 27 1 900 5 120 80%Ar+20%CO ₂ 0.258 61 44 ○ Invention example *1: 5 mm small size

As shown in Table 2-1 and Table 2-2, it was confirmed that the washer steel material of the present invention satisfies the target performance of the maximum crystal grain size in the microstructure: less than 200 μm. It was confirmed that the carbon concentration in the grain boundary satisfying the above-mentioned target performance at the portion where the washer iron steel material of the present invention is linearly heated: 0.100% or more, and the absorbed energy (vE _-269 ) of the Charpy impact test was 41 More than J, 27 J or more for a small size of 5 mm.

On the other hand, in the comparative example which deviated from the scope of the present invention, the target performance could not be satisfied.

1: Thermocouple buried hole 2: Constraint plate A: rolling direction

FIG. 1 is a schematic diagram illustrating a linear heating test body used in an example of the present invention.

Claims (8)

A steel material having the following composition, and in the microstructure, the maximum crystal grain size at the position 1 mm below the surface of the steel material is less than 200 μm, The composition contains in mass % C: 0.200% or more and 0.700% or less, Si: 0.05% to 1.00%, Mn: 20.0% or more and 40.0% or less, P: less than 0.030%, S: less than 0.0050%, Al: less than 5.00%, Cr: less than 7.0%, N: 0.0500% or less, O: less than 0.0050%, Ti: less than 0.005%, Nb: less than 0.005%, and Contains one or two or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less, and the remainder contains iron and unavoidable impurities. The steel as claimed in item 1, wherein the composition further contains in mass % selected from Cu: 1.0% or less, Ni: 1.0% or less, Mo: less than 2.0%, V: less than 2.0%, W: less than 2.0% one or more of them. The steel material according to claim 1 or claim 2, wherein in the microstructure, the number density of crystal grains with a diameter of 50 μm or more at a position 1 mm below the surface of the steel material is 1.0 grains/mm ² or more. The steel according to any one of claim 1 to claim 3, wherein in the microstructure, the particle size of the first 10% of the inclusion particle size distribution at the position 1 mm below the surface of the steel is 3.5 μm or less. A method of manufacturing steel, manufacturing the steel described in any one of Claim 1 to Claim 4, in the manufacturing method of the steel, heating the steel material having the above composition to a temperature range of not less than 1100°C and not more than 1300°C, Hot rolling is carried out under the conditions of the total reduction ratio above 950°C: 40% or above, the number of hot rolling passes at less than 950°C: more than one, and the finishing temperature at the end of rolling: above 750°C. Afterwards, cooling is performed. A trough, which is a trough formed by welding the steel described in any one of claim 1 to claim 4, The C concentration in the grain boundary at a position 1 mm below the surface of the base metal part heated in a linear manner is 0.100% or more, The absorbed energy in the Charpy impact test at -269° C. or higher at a position 1 mm below the surface of the linear heating portion heated in the linear shape was 41 J or higher. A method for manufacturing a groove, manufacturing the groove as described in claim 6, in the method for manufacturing the groove, Curved surfaces are formed by performing a linear heat treatment in which the surface of the steel material according to any one of claims 1 to 4 is heated to 900°C or lower, air-cooled to a surface temperature of 500°C or lower, and then water-cooled. processing, Next, the curved steels are welded to each other. The manufacturing method of the tank as claimed in item 7, wherein the welding uses a solid welding wire as an electrode, and is carried out under the conditions of a temperature between passes: 100°C to 150°C, and a shielding gas: 80%Ar+20%CO ₂ .

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