JP7338792B2 - Steel material and manufacturing method thereof, tank and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material suitable for use in structural steels used in extremely low-temperature environments, such as tanks for storing liquid hydrogen, liquid helium, and liquefied gas, and a method for producing the same. The present invention also relates to a tank using this steel material and a method for manufacturing the same.

In order to use hot-rolled steel sheets as materials for storage tanks for liquid hydrogen, liquid helium, and liquefied gas, the operating environment is extremely cold, so hot-rolled steel sheets are required to have excellent toughness at low temperatures. be done. For example, when a hot-rolled steel plate is used for a liquid helium storage tank, it is necessary that the boiling point of helium is −269° C. or less and excellent toughness is ensured. If the low-temperature toughness of the steel material is inferior, it may become impossible to maintain the safety of the structure for cryogenic storage tanks.

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

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

Patent Literature 1 proposes a technique for ensuring low-temperature toughness in the weld heat-affected zone by controlling the crystal grain size, carbide coverage, and the like.

JP 2016-196703 A

For example, a liquefied gas storage tank structure (for example, a liquefied gas storage tank) is manufactured by linearly heating a steel material. Linear heating is a processing method that uses plastic deformation due to local thermal stress to form a curved surface. In JSQS (Japan Steel Ship Manufacturing Precision Standard, 2018), the carbon equivalent (Ceq) in shipbuilding is for high-strength steel with Ceq > 0.38%, and the surface when water cooling immediately after heating. The maximum heating temperature of is 650° C. or less. If the maximum temperature is exceeded, the maximum surface heating temperature should be 900°C or less, and water cooling should be performed after air cooling to 500°C. When carbides are formed after linear heating, the low-temperature toughness is lowered, but Patent Document 1 does not verify the low-temperature toughness after linear heating.

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

The above-mentioned "excellent low-temperature toughness after linear heating" means that in a tank obtained by subjecting a steel material to linear heating treatment described later, 1 mm below the surface of the steel material in the linear heating part (from the surface of the steel material in the plate thickness direction) 1 mm position) at -269°C or higher, the absorbed energy in the Charpy impact test is 41 J or higher. The above-mentioned "linearly heated portion" refers to a heat-affected area after linearly heating the steel material. The absorbed energy of the linear heating portion in the Charpy impact test can be measured by the method described in Examples below.

In order to achieve the above-mentioned problems, the present inventors focused on austenitic steel materials (for example, high-Mn steel materials), and investigated the chemical composition, microstructure, manufacturing method, and various factors that determine the characteristics of the steel materials. In addition, we conducted extensive research on structures manufactured by linearly heating this steel material. As a result, the following findings a to c were obtained. In the present invention, "high Mn steel material" refers to steel material having a Mn content of 20 to 40% by mass.

a. In order to suppress the decrease in absorbed energy in the Charpy impact test at -269°C or higher in the linear heating part of a structure manufactured by linearly heating high Mn steel, the maximum grain size at the time of steel production is less than 200 μm. Preferably, the maximum grain size is less than 180 μm.

b. A high Mn austenitic steel contains a large amount of C, and therefore has more carbides than stainless steel. Furthermore, since carbides are formed at grain boundaries, the grain boundary strength is lowered. If 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. Therefore, in order to suppress the deterioration of the low-temperature toughness after linearly heating the high-Mn steel, it is effective to increase the C concentration in the grain boundaries of the high-Mn steel. For this purpose, it is effective to set the maximum crystal grain size to less than 200 μm in the high-Mn steel used as the raw material.

c. In hot rolling at the time of steel production, after rolling is performed at 950 ° C. or higher with a total rolling reduction of 40% or more, hot rolling is performed one or more times at less than 950 ° C., and the finish rolling end temperature is 750 ° C. or higher. , the above a and b can be realized.

The present invention was made by further studying the above knowledge, and the gist thereof is as follows.
[1] in % by mass,
C: 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: 0.0050% or less,
Ti: less than 0.005%,
Nb: contains less than 0.005%,
Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less,
having a component composition with the balance being iron and inevitable impurities,
A steel material in which the microstructure has a maximum grain size of less than 200 μm at a position 1 mm below the surface of the steel material.
[2] The component composition further includes, in % by mass,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: The steel material according to [1] above, containing one or more selected from 2.0% or less.
[3] The steel material according to the above [1] or [2], wherein the microstructure has a number density of crystal grains of 50 μm or more at a position 1 mm below the surface of the steel material of 1.0 or more/mm ² .
[4] Any one of the above [1] to [3], wherein the microstructure has an inclusion grain size of 3.5 μm or less in the top 10% of the inclusion grain size distribution at a position 1 mm below the surface of the steel material. The steel material described in 1.
[5] A method for manufacturing a steel material according to any one of [1] to [4],
heating the steel material having the above chemical composition to a temperature range of 1100° C. or higher and 1300° C. or lower;
Hot rolling is performed under the following conditions: total rolling reduction at 950°C or higher: 40% or higher, number of hot rolling passes below 950°C: 1 or higher, and finish rolling finish temperature: 750°C or higher,
A method of manufacturing a steel material, followed by cooling.
[6] A tank made by welding the steel material according to any one of [1] to [4],
The C concentration at the grain boundary at a position 1 mm below the surface of the linearly heated base material is 0.100% or more,
A tank having an absorbed energy of 41 J or more in a Charpy impact test at −269° C. or higher at a position 1 mm below the surface of the linearly heated linear heating part.
[7] A method for manufacturing the tank of [6],
The surface of the steel material according to any one of the above [1] to [4] is heated to 900 ° C. or less, the steel material is air-cooled to a surface temperature of 500 ° C. or less, and then water-cooled. curved surface,
Next, a method of manufacturing a tank, in which the curved steel materials are welded together.
[8] The method of manufacturing a tank according to [7], wherein the welding is performed using a solid wire as an electrode under conditions of interpass temperature: 100 to 150° C. and shielding gas: 80% Ar+20% CO ₂ .

ADVANTAGE OF THE INVENTION According to this invention, the steel material excellent in the low-temperature toughness after linear heating, and its manufacturing method can be provided. In addition, the steel material of the present invention is suitably used as a material for steel structures (such as tanks for liquefied gas storage tanks) used in low-temperature environments. can provide a method. Therefore, it can greatly contribute to the improvement of the safety and life of the steel structure, and has a remarkable industrial effect. In addition, since the production method of the present invention does not cause a decrease in productivity and an increase in production costs, it is possible to provide an economical production method.

FIG. 1 is a schematic diagram illustrating a linear heating specimen used in Examples of the present invention.

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

First, the technical idea of the present invention will be described in detail.

As described above, there is an austenitic steel material (for example, a high Mn steel material) as a steel material that is inexpensive and excellent in low-temperature toughness. In order to use this high Mn steel material as a material for steel structures (for example, tanks) used in a low temperature environment, it is necessary to have excellent low temperature toughness even in the heat-affected areas in the process of linearly heating the material. is required.

As a result of research by the present inventors, it was found that the larger the grain size of the high-Mn steel material, the higher the absorbed energy in the absence of carbides. However, it has been found that when carbides are present, the absorbed energy does not necessarily increase as the crystal grain size becomes coarser. In the process of linear heating, carbides are formed at locations affected by heat of about 600 to 800° C., resulting in low-temperature toughness being lowered.

As a result of intensive investigation of the cause, the inventors of the present invention have newly discovered that the C concentration at the grain boundary is responsible for the decrease in the absorbed energy. The relationship between the decrease in absorbed energy and the C concentration at grain boundaries will be described below.

One of the origins of fracture in high Mn steel is the grain boundary. The low temperature toughness is improved by reducing the number of grain boundaries, that is, by coarsening the grains. Generally, when carbides are formed at grain boundaries due to thermal effects, C around the carbides is depleted and the grain boundary strength is lowered. However, since high Mn steel has a large amount of C added, in the process of forming and growing carbides on grain boundaries, C, which has a high diffusion rate, is sufficiently supplied from inside the grains away from the grain boundaries, resulting in a healing phenomenon. do. Thereby, sharp C depletion on the grain boundary can be suppressed. However, if the crystal grains become too coarse, the supply of C from within the grains cannot keep up with the demand, resulting in a shortage of C on the grain boundaries.

Therefore, in the present invention, by setting the maximum crystal grain size to less than 200 μm in the hot rolling process described later, it is possible to ensure a C concentration of 0.100% or more even when carbides are formed, and absorb energy. Decrease can be suppressed.

Next, the steel material of the present invention will be explained.
The steel material of the present invention has the chemical composition described later, and the microstructure has a maximum grain size of less than 200 μm at a position of 1 mm below the surface of the steel material. As a result, the C concentration at the grain boundaries can be 0.100% or more even after the steel is linearly heated. In addition, the display of "%" regarding C density|concentration points out that it is "mass %."

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

C: 0.200% or more and 0.700% or less C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. 0.200% or more of C is contained in order to prevent the depletion of C on the grain boundaries described above. On the other hand, when the C content exceeds 0.700%, Cr carbides are excessively formed and the low temperature toughness (low temperature toughness after linear heating) is lowered. Therefore, the content of C is set to 0.200% or more and 0.700% or less. The content of C is preferably 0.250% or more, more preferably 0.300% or more. Also, the C content is preferably 0.600% or less, more preferably 0.550% or less.

Si: 0.05% or more and 1.00% or less Si acts as a deoxidizer and is not only necessary for steelmaking, but also dissolves in steel and has the effect of increasing the strength of the steel sheet by solid solution strengthening. . In order to obtain such effects, Si contains 0.05% or more. On the other hand, when the Si content exceeds 1.00%, the non-thermal stress excessively increases, degrading the low-temperature toughness. Therefore, the content of Si is set to 0.05% or more and 1.00% or less. The Si content is preferably 0.07% or more, more preferably 0.10% or more, and still more preferably 0.15% or more. Also, the Si content is preferably 0.80% or less, more preferably 0.75% or less, and still more preferably 0.70% or less.

Mn: 20.0% or more and 40.0% or less Mn is a relatively inexpensive austenite stabilizing element. In the present invention, Ni is an important element for achieving both strength and low temperature toughness. In order to obtain the effect, Mn contains 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, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, the content of Mn is set to 20.0% or more and 40.0% or less. The Mn content is preferably 23.0% or more, more preferably 23.3% or more, and still more preferably 23.5% or more. The Mn content is preferably 35.0% or less, more preferably 30.0% or less.

P: 0.030% or less When the P content exceeds 0.030%, excessive segregation occurs at grain boundaries, resulting in a decrease in low-temperature toughness. Therefore, it is desirable to set the upper limit to 0.030% and reduce it as much as possible. Therefore, the P content should be 0.030% or less. In addition, since excessive reduction of P raises the refining cost and is economically disadvantageous, the P content is preferably 0.002% or more. The P content is more preferably 0.005% or more, more preferably 0.007% or more. The P content is preferably 0.028% or less, more preferably 0.024% or less, and still more preferably 0.020% or less.

S: 0.0050% or less S deteriorates the low-temperature toughness and ductility of the base metal, so it is desirable to reduce it as much as possible, with the upper limit set to 0.0050%. Therefore, the S content should be 0.0050% or less. The content is preferably 0.0045% or less, more preferably 0.0043% or less. In addition, excessive reduction of S increases the refining cost and is economically disadvantageous, so the S content is preferably 0.0010% or more. The S content is more preferably 0.0012% or more.

Al: 5.00% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process for steel sheets. In addition, the yield strength and local elongation during tensile tests are improved. In order to obtain such an effect, it is preferable to contain 0.01% or more of Al. On the other hand, if the Al content exceeds 5.00%, a large amount of inclusions will be present, degrading the low temperature toughness, so the Al content is made 5.00% or less. The Al content is preferably 0.01% or more, more preferably 0.02% or more. The Al content is preferably 4.00% or less, more preferably 3.00% or less.

Cr: 7.0% or less Cr is an element effective in improving low-temperature toughness because it improves grain boundary strength. In order to obtain such an effect, it is preferable to contain 0.5% or more of Cr. On the other hand, if the Cr content exceeds 7.0%, there is a risk that the low temperature toughness and stress corrosion cracking resistance will decrease due to the formation of Cr carbides. Therefore, the Cr content is set to 7.0% or less. The Cr content is preferably 0.5% or more, more preferably 1.0% or more, and still more preferably 1.2% or more. The Cr content is preferably 6.7% or less, more preferably 6.5% or less. Further, in order to further improve stress corrosion cracking resistance, it is even 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 austenite stabilizing element and is an element effective in improving low temperature toughness. In order to obtain such an effect, it is preferable to contain 0.0050% or more of N. On the other hand, if the N content exceeds 0.0500%, the nitrides or carbonitrides may become coarse and the low-temperature toughness may deteriorate. Therefore, the N content is set to 0.0500% or less. The N content is preferably 0.0050% or more, more preferably 0.0060% or more. The N content is preferably 0.0400% or less, more preferably 0.0300% or less.

O: 0.0050% or less O (oxygen) deteriorates the low temperature toughness due to the formation of oxides. For this reason, the content of O is made in the range of 0.0050% or less. The content is preferably 0.0045% or less, more preferably 0.0040% or less. In addition, excessive reduction of O raises the refining cost and is economically disadvantageous, so the O content is preferably 0.0010% or more. The O content is more preferably 0.0012% or more.

Ti: less than 0.005%, Nb: less than 0.005% Ti and Nb form carbonitrides with a high melting point in the steel, which lowers the low-temperature toughness. Since Ti and Nb are components that are unavoidably mixed from raw materials, etc., Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less are mixed. It is customary. Therefore, it is necessary to avoid unavoidable contamination of Ti and Nb and suppress the contents of Ti and Nb to less than 0.005%, respectively, according to the melting method described later. By suppressing the contents of Ti and Nb to less than 0.005% each, the adverse effects of the carbonitrides described above can be eliminated, and excellent low-temperature toughness and ductility can be ensured. Preferably, the Ti and Nb contents are each 0.003% or less, more preferably 0.002% or less. Of course, the content of Ti and Nb may be 0%.

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

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% or more and 0.0090% or less, Mg is 0.0005% or more and 0.0090% or less, and REM is 0.0010% or more and 0.0180% or less. More preferably, Ca is 0.0010% or more and 0.0080% or less, Mg is 0.0010% or more and 0.0080% or less, and REM is 0.0020% or more and 0.0150% or less. More preferably, Ca is 0.0015% or more and 0.0050% or less, Mg is 0.0015% or more and 0.0050% or less, and REM is 0.0030% or more and 0.0100% or less.

In the austenitic steel material of the present invention, the balance other than the above components is iron (Fe) and unavoidable impurities. The unavoidable impurities here include H, B, etc., and if the total content of each element is 0.01% or less, it is permissible.

Let the above elements be the basic component composition. This basic component composition provides the desired properties of the present invention. In the present invention, for the purpose of further improving strength and low-temperature toughness, in addition to the above elements, the following elements can be contained as necessary.
In addition, since each component of Cu, Ni, Mo, V, and W shown below can be contained as needed, these components may be 0%.

One or more selected from Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less Cu : 1.0% or less, Ni: 1.0% or less Cu and Ni are elements that not only increase the strength of the steel sheet by solid-solution strengthening, but also improve the mobility of dislocations and improve the low-temperature toughness. . In order to obtain such effects, Cu and Ni are preferably contained in an amount of 0.01% or more. On the other hand, if the content of Cu and Ni exceeds 1.0%, the surface quality deteriorates during rolling and the manufacturing cost is increased. Therefore, when these alloying elements are contained, the content thereof is preferably 1.0% or less. Cu and Ni are each more preferably 0.03% or more and more preferably 0.7% or less. More preferably, it is 0.5% or less.

Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less Mo, V, and W contribute to stabilizing austenite and improving base metal strength. In order to obtain such effects, each of Mo, V and W is preferably contained in an amount of 0.001% or more. On the other hand, if each of Mo, V, and W is contained in an amount exceeding 2.0%, coarse carbonitrides are formed, which may become starting points of fracture, and also press the production cost. Therefore, when these alloying elements are contained, the content thereof is preferably 2.0% or less. Mo, V and W are each more preferably 0.003% or more and more preferably 1.7% or less. It is more preferably 0.1% or more, more preferably 1.5% or less.

[Microstructure of steel]
Next, the reasons for limiting the microstructure as described above in the present invention will be explained.

Maximum crystal grain size at a position 1 mm below the surface of the steel material: less than 200 μm As described above, when the crystal grain size of the steel material (base material) is coarse, C is depleted during carbide formation. By setting the maximum crystal grain size at a position 1 mm below the surface of the steel material to less than 200 μm, the C concentration at the grain boundaries can be 0.100% or more even after the steel material is linearly heated. That is, it is possible to manufacture a steel material having excellent low-temperature toughness, such as an absorbed energy of 41 J or more in a Charpy impact test at -269°C or higher in a linearly heated portion of a structure (for example, a tank) obtained after linear heating. can.

The maximum crystal grain size is preferably 150 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less. The lower limit of the maximum crystal grain size is not particularly specified. The maximum crystal grain size is preferably 50 μm or more, more preferably 60 μm or more, in order to secure the toughness of the hot-rolled steel sheet (steel material). Here, the crystal grains refer to grains exposed by etching. In the present invention, the maximum crystal grain size can be measured by the method described in Examples below.

In addition, in the present invention, the maximum grain size of the steel material can be controlled within the above numerical range by performing hot rolling according to the conditions described later. As a result, the C concentration at the grain boundaries can be ensured even after linear heating, and the absorption energy described above can be realized.

Number density of crystal grains of 50 μm or more at a position 1 mm below the surface of the steel material (preferred condition)
The origin of fracture in high-Mn steel is the grain boundary, and cracks propagate along the grain boundaries. can be done. For this purpose, the number of austenite grains having a grain size of 50 μm or more is preferably 1.0 or more, more preferably 2.0 or more per 1 mm ² . On the other hand, when the number of austenite grains exceeds 10.0 per 1 mm ² , the strength is lowered. Therefore, it is preferably 10.0 or less, more preferably 9.0 or less, per 1 mm ² .
In the present invention, the number of austenite grains having a grain size of 50 μm or more per 1 mm ² (number density) can be measured by the method described in Examples below. This number density can be controlled within the above numerical range by performing hot rolling, which will be described later.

Inclusion grain size at 1 mm below the surface of steel (preferred conditions)
If coarse inclusions are present at a position of 1 mm below the surface of the steel material, stress corrosion cracking resistance is lowered. If the grain size of the top 10% inclusions in the grain size distribution of inclusions (grain size of the top 10% inclusions) exceeds 3.5 μm at a position 1 mm below the surface of the steel material, stress corrosion cracking resistance may decrease. Do you get it. For this reason, the grain size of the top 10% inclusions is preferably 3.5 μm or less, more preferably 3.0 μm or less. On the other hand, the grain size of the top 10% inclusions is preferably as small as possible, but from the viewpoint of manufacturability, it is preferably 1.5 μm or more, more preferably 2.0 μm or more.
Here, the "upper 10% inclusion grain size" is the grain size corresponding to the 10% position when the inclusion grain sizes are sorted in descending order in the inclusion grain size distribution. In the present invention, the grain size of inclusions can be measured by the method described in Examples below.

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

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

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

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

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

Manufacturing conditions for building the above steel material into a steel material (austenitic steel material) having excellent low-temperature toughness after linear heating will be described in detail below.

In order to obtain the austenitic steel material having the above-described composition, the steel material having the above-described chemical composition is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and then rolled at a temperature of 950 ° C. or higher to a total reduction of 40% or more. After performing the above, it is important to perform one or more hot rolling passes at a temperature of less than 950°C and perform hot rolling under the condition that the finishing temperature of finish rolling is 750°C or higher. After this hot rolling is completed, cooling is performed. The temperature control here is based on the surface temperature of the steel material.

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

Heating temperature of steel material: 1100° C. or higher and 1300° C. or lower In order to diffuse Mn during hot rolling, the heating temperature of the steel material before hot rolling is set to 1100° C. or higher. By diffusing Mn, the stability of austenite can be ensured even in the Mn negative segregation part. As a result, the stability of austenite can be ensured even in the linear heating portion, and brittle fracture can be prevented. That is, the absorbed energy at -269°C can be secured. On the other hand, if the heating temperature exceeds 1300.degree. The heating temperature of the steel material is preferably 1130° C. or higher and preferably 1270° C. or lower. It is more preferably 1150° C. or higher, and more preferably 1250° C. or lower.

Hot rolling Total rolling reduction 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 1 mm below the surface of the steel material less than 200 μm. If equiaxed grains cannot be obtained by rolling in the recrystallized region, coarse grains remain in subsequent rolling in the non-recrystallized region, and the maximum grain size becomes 200 μm or more. Moreover, the number density of crystal grains of 50 μm or more exceeds 10.0/mm ² . Therefore, it is effective to ensure a total rolling reduction of 40% or more in the temperature range of 950° C. or higher, which is the recrystallization region. The total rolling reduction in the recrystallization zone is preferably 50% or more, more preferably 52% or more. Although the upper limit of the total rolling reduction in the recrystallization zone is not particularly specified, the total rolling reduction in the recrystallization zone is preferably 85% or less, more preferably 70% or less, for the purpose of ensuring strength. .

Number of hot rolling passes at less than 950°C: 1 or more, and finish rolling temperature: 750°C or higher It is important that the number of hot rolling passes is one or more. Preferably, it is twice or more. If there is no hot rolling pass below 950°C, the maximum grain size will be 200 μm or more. Moreover, the number density of crystal grains of 50 μm or more exceeds 10.0/mm ² . The upper limit of the number of hot rolling passes is not 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 excessively fine and the low-temperature toughness decreases. If the finish rolling end temperature is 775°C or less, the grain size becomes finer, and as a result, the maximum grain size may be less than 50 µm. More preferably, it is 780°C or higher. The upper limit of the finish rolling end temperature is not particularly specified. From the viewpoint of ensuring strength, the finishing temperature of finish rolling is preferably 930° C. or lower, more preferably 900° C. or lower.

Cooling Cooling is performed after hot rolling is completed. No particular cooling conditions are specified. In the present invention, it is preferable to cool from a temperature of (temperature at the end of hot rolling - 100°C) or higher to 600°C or lower at an average cooling rate of 1.0°C/s or higher. This suppresses the formation of carbides and grain boundary segregation of P, and further enhances the properties of the steel material. The above "temperature at the end of hot rolling" refers to the finishing temperature of finish rolling.
The upper limit of the average cooling rate is not particularly defined. From the viewpoint of cooling stop temperature control, it is preferably 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 raw material and linearly heating the raw material will be described.

The tank of the present invention is manufactured by linearly heating the steel material described above under specific linear heating conditions to form a curved surface, and welding the steel materials processed into the curved surface. The tank of the present invention manufactured in this way has the same component composition and microstructure in the base metal portion as those of the above steel material (austenitic steel material).

In addition, in the tank of the present invention, the C concentration at the grain boundary at a position of 1 mm below the surface of the base material after linear heating is 0.100% or more. Grain boundary strength cannot be ensured if the C concentration in the crystal grain boundary at the above position of the base material portion after linear heating is less than 0.100%. Therefore, the C concentration at the crystal grain boundary at the above position of the base material portion after linear heating is set to 0.100% or more. It is preferably 0.200% or more, more preferably 0.250% or more. The upper limit of the C concentration at the crystal grain boundary at the above position of the base material portion after linear heating is not particularly defined. From the viewpoint of deterioration of low-temperature toughness due to excessive Cr carbide formation, the content is preferably 0.600% or less, more preferably 0.550% or less.

The tank of the present invention manufactured in this way can absorb energy of 41 J or more in a Charpy impact test at -269°C or higher at a position 1 mm below the surface of the linear heating portion after linear heating. The absorbed energy in the Charpy impact test can be measured by the method described in Examples below. 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 for the full size, and 27 J or more for the 5 mm sub-size.
Moreover, according to the present invention, stress corrosion cracking resistance can also be provided.

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

The tank of the present invention is manufactured by linearly heating the above steel material under the following conditions to form a curved surface, and welding the steel material processed into the curved surface. Since the manufacturing method of the steel material (austenitic steel material), which is the raw material, has already been described, it will be omitted. Here, preferred linear heating conditions and welding conditions will be described.

[Linear heating conditions]
The steel material is linearly heated under the condition that the target heating temperature (heating target temperature) of the surface of the steel material is 900° C. or less. After heating, the steel material is air-cooled to a surface temperature of 500° C. or less, and then water-cooled. The above linear heat treatment of heating and air cooling may be performed once, or may be repeated (repeated) one or more times. The number of times of repetition is preferably one or more in order to change the microstructure. The number of iterations is preferably 5 or less because the local thermal cycle history becomes complicated. The heating temperature is preferably above 800°C.

[Welding conditions]
From the viewpoint of securing excellent cryogenic impact toughness with high strength and high ductility, welding is performed using a solid wire (diameter 1.2 mm) as an electrode, without preheating, in a downward position, interpass temperature: 100 to 150 ° C. , shield gas: 80% Ar+20% CO ₂ . In addition, the display of "%" regarding the shield gas indicates "% by volume".

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

A steel slab having the chemical composition shown in Table 1 was produced by a converter-ladle refining-continuous casting method. In addition, "-" shown in Table 1 indicates that the element is not intentionally added, and means that not only the case where the element is not contained (0%) but also the case where the element is unavoidably contained is included. do. Next, the obtained steel slabs were hot rolled under the conditions shown in Table 2-1 and then cooled to produce steel materials (hot rolled steel sheets) having a thickness of 6 to 40 mm.

Using the obtained hot-rolled steel sheets (steel sheets), the crystal grain size and inclusion grain size were evaluated in the following manner.

Subsequently, the obtained steel sheets were linearly heated, and the steel sheets after the linear heating were evaluated for C concentration, low-temperature toughness, and stress corrosion cracking resistance in the following manner.
Here, the above linear heating will be described. As the linear heating, plate linear heating shown in FIG. 1 was performed. As shown in FIG. 1, a linear heating test specimen with a length of 1000 mm and a width of 500 mm was prepared from the obtained steel plate, and the specimen was fixed with a restraining plate at a half position in the width direction (rolling direction). Then, sheet wire heating was performed under the following conditions. As for the conditions, the target heating temperature of the surface of the steel material was set to 900° C., the steel material was heated to that temperature, air-cooled to a surface temperature of the steel material of 500° C. or less, and then water-cooled. Linear heating of the same area was repeated under the conditions shown in Table 2-2.
Welding of the steel sheets after the linear heat treatment was performed using a solid wire (1.2 mm in diameter) as an electrode, without preheating, in a downward position, and under the welding conditions shown in Table 2-2.

(1) Microstructure evaluation [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. 100 crystal grains exposed by etching were randomly selected from the photographed image, and the maximum crystal grain size (μm) at a position of 1 mm below the surface of the steel sheet was determined using the equivalent circle diameter of the crystal grains as the crystal grain size. Also, the total area of 100 crystal grains and the number of crystal grains of 50 μm or more were determined, and the number density of crystal grains of 50 μm or more per mm ² (mm ² /piece) was determined. Aqua regia was used as the corrosive liquid.

[Inclusion particle size]
The obtained hot-rolled steel sheets were examined for grain size of inclusions using an SEM (scanning electron microscope). The evaluation area was set to 200 mm ² , and the grain size (μm) of the top 10% inclusions at the position 1 mm below the surface of the steel sheet was determined.

[C concentration]
A 12 mm×10 mm TEM sample was produced from the obtained hot-rolled steel sheet which had been linearly heated. The sample was subjected to compositional analysis across carbide-free grain boundaries using an EDS detector attached to a TEM (transmission electron microscope), and the resulting C concentration was evaluated. A position 1 mm below the surface of the steel plate was used as an observation target. The analysis was performed on 10 grain boundaries and the average value was obtained.

(2) Low-temperature toughness Evaluation of the low-temperature toughness of the linear heating portion was performed as follows.
A linear heating specimen shown in FIG. 1 was produced from the obtained hot-rolled steel sheet, and the steel sheet after linear heating was performed on the specimen under the above-described conditions was used to heat the linearly heated part to a low temperature. An evaluation of toughness was performed. A Charpy V-notch test piece (full-size Charpy V-notch test piece) was taken from a linear heating portion having a plate thickness of 10 mm or more in accordance with JIS Z 2242 (2005). Charpy impact tests were performed at -196°C and -269°C using three Charpy V-notch specimens. An average value of three absorbed energies at each temperature was obtained. In this example, in the case of a full-size Charpy V-notch test piece, an average value of 41 J or more of the absorbed energy of three specimens at -269°C was judged to be excellent in low temperature toughness.

For the linear heating portion with a plate thickness of less than 10 mm, a 5 mm sub-sized Charpy V-notch test piece was taken in accordance with JIS Z 2242 (2005). Charpy impact tests were performed at -196°C and -269°C using three Charpy V-notch specimens. An average value of three absorbed energies at each temperature was obtained. In Table 2, "*1" is shown in the item of absorbed energy for the samples performed using the sub-sized Charpy V-notch test pieces. In the case of the sub-sized Charpy V-notch test piece, an average value of 27 J or more of the absorbed energy of three specimens at -269°C was judged to be excellent in low temperature toughness.

(3) Stress Corrosion Cracking Resistance For evaluation of stress corrosion cracking resistance, a stress corrosion cracking test was performed based on ASTM G36. A test piece having a thickness of 2.5 mm, a width of 20 mm, and a length of 80 mm was taken from a position 1 mm below the surface of the obtained hot-rolled steel sheet. The solution was boiling MgCl ₂ chloride and the bending radius was 5 mm. After immersing the test piece to which stress was applied in the above solution for 400 hours, the presence or absence of cracking was confirmed. A case where no cracks occurred was evaluated as "○ (accepted)" shown in Table 2-2, and a case where cracks occurred was evaluated as "× (failed)" shown in Table 2-2.

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

As shown in Tables 2-1 and 2-2, it was confirmed that the austenitic steel material of the present invention satisfies the aforementioned target performance of maximum grain size in the microstructure: less than 200 μm. At the location where the austenitic steel material of the present invention is linearly heated, the C concentration at the grain boundary, which is the above-mentioned target performance: 0.100% or more, the absorbed energy (vE _-269 ) in the Charpy impact test is 41 J or more, and the 5 mm sub-size was confirmed to satisfy 27J or more.

On the other hand, in Comparative Examples outside the scope of the present invention, the above target performance could not be satisfied.

Claims (8)

in % by mass,
C: 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: 2.0% or more and 7.0% or less,
N: 0.0500% or less,
O: 0.0050% or less,
Ti: less than 0.005%,
Nb: contains less than 0.005%,
Contains one or more selected from Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0200% or less,
having a component composition with the balance being iron and inevitable impurities,
A steel material in which the microstructure has a maximum grain size of 50 μm or more and less than 200 μm at a position 1 mm below the surface of the steel material. The component composition is further, in mass %,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 2.0% or less,
V: 0.1 % or less,
W: The steel material according to claim 1, containing one or more selected from 0.1 % or less. The steel material according to claim 1 or 2, wherein the microstructure has a number density of crystal grains of 50 µm or more at a position of 1 mm below the surface of the steel material of 1.0/mm ² or more. The steel material according to any one of claims 1 to 3, wherein the microstructure has an inclusion particle size of 3.5 µm or less in the top 10% of the inclusion particle size distribution at a position of 1 mm below the surface of the steel material. A method for manufacturing a steel material according to any one of claims 1 to 4,
heating the steel material having the above chemical composition to a temperature range of 1100° C. or higher and 1300° C. or lower;
Hot rolling is performed under the following conditions: total rolling reduction at 950°C or higher: 40% or higher, number of hot rolling passes below 950°C: 1 or higher, and finish rolling finish temperature: 750°C or higher,
Thereafter, cooling is performed from a temperature of (said finish rolling end temperature - 100°C) or higher to 600°C or lower at an average cooling rate of 1.0°C/s or higher and 30.0°C/s or lower. A tank made by welding the steel material according to any one of claims 1 to 4,
The C concentration at the grain boundary at a position 1 mm below the surface of the linearly heated base material is 0.100% or more,
A tank having an absorbed energy of 41 J or more in a Charpy impact test at −269° C. or higher at a position 1 mm below the surface of the linearly heated linear heating part. A method for manufacturing the tank of claim 6,
The surface of the steel material according to any one of claims 1 to 4 is heated to 900 ° C. or less, the steel material is air-cooled to a surface temperature of 500 ° C. or less, and then water-cooled. death,
Next, a method of manufacturing a tank, in which the curved steel materials are welded together. 8. The method of manufacturing a tank according to claim 7, wherein the welding is performed using a solid wire as an electrode under conditions of interpass temperature: 100 to 150° C. and shield gas: 80% Ar+20% CO ₂ .

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