Steel and its manufacturing methods, tanks and their manufacturing methods

Abstract

This invention provides a steel material and a method for manufacturing the same, as well as a can and a method for manufacturing the same. The steel material of this invention has the following composition: by mass % C: 0.200% to 0.700%, Si: 0.05% to 1.00%, Mn: 20.0% to 40.0%, P: less than 0.030%, S: less than 0.0050%, Al: less than 5.00%, Cr: less than 7.0%, N: less than 0.0500%, O: less than 0.0050%, Ti: less than 0.005%, Nb: less than 0.005%, and contains one or more elements selected from Ca: less than 0.0100%, Mg: less than 0.0100%, and REM: less than 0.0200%, with the remainder consisting of iron and unavoidable impurities; the microstructure is such that the maximum crystal grain size at a position 1 mm below the surface of the steel is less than 200 μm.

Description

Technical Field

[0001] This invention relates to structural steel suitable for use in extremely low-temperature environments, such as for storing liquid helium (primarily liquid hydrogen) and liquefied gases, and to a method thereof. Additionally, this invention relates to tanks using this steel and a method thereof. Background Technology

[0002] For hot-rolled steel sheets to be used as billets for structures used in storage tanks for liquid hydrogen, liquid helium, and liquefied gas, the extremely low temperatures necessitate excellent low-temperature toughness. For example, when using hot-rolled steel sheets for liquid helium storage tanks, excellent toughness below helium's boiling point of -269°C is required. Poor low-temperature toughness of the steel could compromise the safety of the structure used in cryogenic storage tanks; therefore, there is a high demand for improved low-temperature toughness in the applicable steel.

[0003] Previously, austenitic stainless steels, 9% Ni steel, or 5000 series aluminum alloys with an austenitic microstructure that does not exhibit brittleness at low temperatures could be used to meet this requirement. However, due to the high cost of alloys and manufacturing processes, inexpensive steels with excellent low-temperature toughness are needed.

[0004] Therefore, for example, Patent Document 1 proposes a new type of steel as an alternative to conventional low-temperature steel, using high-Mn steel with a large amount of relatively inexpensive austenite stabilizing element Mn as structural steel for low-temperature environments.

[0005] Patent document 1 proposes a technique to ensure the low-temperature toughness of the weld heat-affected zone by controlling the crystal grain size, carbide coverage, etc.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2016-196703 Summary of the Invention

[0009] For example, structures for liquefied gas storage tanks (e.g., tanks for liquefied gas storage tanks) are manufactured by linearly heating steel. Linear heating is a processing method that utilizes plastic deformation caused by localized thermal stress to form curved surfaces. In JSQS (Japanese Steel Shipbuilding Work Method Precision Standard, 2018), for high-tensile steel with a carbon equivalent (Ceq) of >0.38% used in shipbuilding, the linear heating condition is set to a maximum surface heating temperature of 650°C or less when water-cooled immediately after heating. The maximum surface heating temperature exceeding this temperature is set to 900°C or less, and it is specified that air cooling to 500°C followed by water cooling is performed. When carbides form after linear heating, low-temperature toughness decreases, but Patent Document 1 does not provide any verification regarding low-temperature toughness after linear heating.

[0010] The present invention was made in view of the above-mentioned problems, and its purpose is to provide a steel with excellent low-temperature toughness after linear heating and a method for manufacturing the same, as well as a tank using the steel and a method for manufacturing the same.

[0011] The aforementioned "excellent low-temperature toughness after linear heating" refers to the Charpy impact test energy of 41 J or more at a position 1 mm below the surface of the steel (1 mm from the steel surface along the thickness direction) at -269°C in a tank obtained by performing the linear heating treatment described later on the steel. The aforementioned "linear heating section" refers to the area of ​​the steel that is subjected to heat-affected zone after linear heating. It should be noted that the Charpy impact test energy absorbed by the linear heating section can be measured according to the method described in the examples described later.

[0012] In order to achieve the aforementioned objectives, the inventors conducted in-depth research on austenitic steel (e.g., high-Mn steel), focusing on the composition, microstructure, manufacturing method, and various factors determining the properties of the steel (steel sheet), as well as structures manufactured by linearly heating the steel. As a result, the following insights (a to c) were obtained. It should be noted that in this invention, "high-Mn steel" refers to steel with a Mn content of 20 to 40% by mass.

[0013] a. In the linear heating section of a structure manufactured by linearly heating high-Mn steel, in order to suppress the decrease in absorbed energy during Charpy impact tests above -269°C, it is important that the maximum crystal grain size during steel manufacturing is less than 200 μm. Preferably, the maximum crystal grain size is less than 180 μm.

[0014] b. High-Mn austenitic steel contains a large amount of carbon (C), resulting in more carbides than stainless steel. Furthermore, the formation of carbides at grain boundaries reduces grain boundary strength. When the C concentration at the grain boundaries of high-Mn steel after linear heating is less than 0.100%, the grain boundaries become the initiation point for fracture, leading to poor low-temperature toughness. Therefore, to suppress the deterioration of low-temperature toughness in high-Mn steel after linear heating, it is effective to increase the C concentration at the grain boundaries. Thus, in high-Mn steel billets, it is effective to ensure that the maximum grain size is less than 200 μm.

[0015] c. In the hot rolling process of steel manufacturing, if rolling is performed at a temperature above 950°C with a total reduction rate of 40% or more, followed by at least one hot rolling pass at a temperature below 950°C, and then finishing at a temperature above 750°C, then the above-mentioned a and b can be achieved.

[0016] This invention is the result of further research into the above insights, and its main points are as follows.

[0017] [1] A steel having the following composition: by mass % containing C: 0.200% to 0.700%, Si: 0.05% to 1.00%, Mn: 20.0% to 40.0%, P: less than 0.030%, S: less than 0.0050%, Al: less than 5.00%, Cr: less than 7.0%, N: less than 0.0500%, O: less than 0.0050%, Ti: less than 0.005%, Nb: less than 0.005%, and containing one or more of Ca: less than 0.0100%, Mg: less than 0.0100%, and REM: less than 0.0200%, with the remainder consisting of iron and unavoidable impurities;

[0018] The maximum crystal grain size at a position 1 mm below the surface of the steel in the microstructure is less than 200 μm.

[0019] [2] The steel according to [1] above, wherein the above composition further contains, by mass %, one or more of the following: Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 2.0%, V: less than 2.0%, W: less than 2.0%.

[0020] [3] According to the steel described in [1] or [2] above, the number density of the crystal grains with a diameter of 50 μm or more at a position 1 mm below the surface of the steel is 1.0 grains / mm. 2 above.

[0021] [4] The steel according to any one of [1] to [3] above, wherein the first 10% of the inclusion particle size distribution of the microstructure at a position 1 mm below the surface of the steel has an inclusion particle size of 3.5 μm or less.

[0022] [5] A method for manufacturing steel, which is the method for manufacturing steel described in any one of [1] to [4] above.

[0023] The steel billet with the above-mentioned composition is heated to a temperature range of 1100℃ to 1300℃.

[0024] Hot rolling is performed under the following conditions: a total reduction rate of 40% or more at temperatures above 950°C, at least one hot rolling pass at temperatures below 950°C, and a finishing temperature of 750°C or higher.

[0025] Then it is cooled.

[0026] [6] A can is a can made by welding the steel described in any one of [1] to [4] above.

[0027] The carbon concentration at the grain boundaries 1 mm below the surface of the base material after linear heating is greater than 0.100%.

[0028] The energy absorbed in the Charpy impact test at a position 1 mm below the surface of the linearly heated section after linear heating is above -269°C, which is above 41 J.

[0029] [7] A method for manufacturing a can, which is the method for manufacturing a can described in [6] above.

[0030] The surface of the steel described in any one of [1] to [4] is heated to below 900°C, then the steel is air-cooled to below 500°C, and then water-cooled linear heating treatment is performed for surface machining.

[0031] Next, the steel pieces that have undergone surface curvature processing will be welded together.

[0032] [8] The manufacturing method of the tank according to [7] above, wherein the above welding uses solid welding wire as an electrode and is carried out under the conditions of interpass temperature: 100-150°C and protective gas: 80% Ar + 20% CO2.

[0033] According to the present invention, a steel with excellent low-temperature toughness after linear heating and a method for manufacturing the same can be provided. Furthermore, the steel of the present invention is suitable as a billet for steel structures (such as liquefied gas storage tanks) used in low-temperature environments, thus providing tanks with excellent low-temperature toughness even after linear heating and a method for manufacturing the same. Therefore, it can greatly contribute to improving the safety and lifespan of the aforementioned steel structures, having a particularly significant effect on industry. In addition, the manufacturing method of the present invention does not cause a decrease in productivity or an increase in manufacturing costs; therefore, it provides a manufacturing method that is also excellent in terms of economy. Attached Figure Description

[0034] Figure 1 This is a schematic diagram illustrating the linearly heated sample used in the embodiments of the present invention. Detailed Implementation

[0035] The present invention will now be described in detail. It should be noted that the present invention is not limited to the embodiments described below.

[0036] First, the technical concept of this invention will be explained in detail.

[0037] As mentioned above, austenitic steels (such as high-Mn steels) are inexpensive and have excellent low-temperature toughness. In order to use this high-Mn steel as a billet for steel structures (such as tanks) used in low-temperature environments, excellent low-temperature toughness is required even in the heat-affected parts during the linear heating process of the billet.

[0038] The inventors' research shows that, in the absence of carbides, the larger the crystal grain size of high-Mn steel, the higher the energy absorption. However, in the presence of carbides, a larger crystal grain size does not necessarily mean higher energy absorption. During the linear heating process, carbides form in areas subjected to heat effects of approximately 600–800°C, thus reducing low-temperature toughness.

[0039] Therefore, the inventors conducted an in-depth investigation into the cause and discovered that the C concentration at the grain boundaries is due to a decrease in absorbed energy. The relationship between the decrease in absorbed energy and the C concentration at the grain boundaries will be explained below.

[0040] Grain boundaries are one of the fracture initiation points in high-Mn steel. Reducing grain boundaries, i.e., coarsening the grains, improves low-temperature toughness. Normally, when carbides form at grain boundaries due to heat, the surrounding carbon is deficient, leading to decreased grain boundary strength. However, due to the high carbon content in high-Mn steel, during carbide formation and growth at grain boundaries, a rapid diffusion rate of carbon is achieved during annealing, ensuring sufficient supply from the grains far from the grain boundaries. This suppresses a sharp carbon deficiency at grain boundaries. However, when the grains become too coarse, carbon cannot be supplied from within the grains in time, resulting in insufficient carbon at the grain boundaries.

[0041] Therefore, in the hot rolling process described later in this invention, by making the maximum crystal grain size less than 200 μm, even when carbides are formed, it is possible to ensure that the C concentration is above 0.100%, thereby suppressing the decrease in absorbed energy.

[0042] Next, the steel of the present invention will be described.

[0043] The steel of the present invention has the composition described later, and the maximum crystal grain size at a position 1 mm below the surface of the steel in its microstructure is less than 200 μm. Therefore, even after linear heating of the steel, the C concentration at the grain boundaries can be above 0.100%. It should be noted that the "%" in C concentration refers to "mass %".

[0044] [Ingredients]

[0045] First, the composition of the steel (austenitic steel) of the present invention will be described.

[0046] In this invention, austenitic steel (e.g., high-Mn steel) and the billet used in its manufacture have the above-described composition. The composition of the austenitic steel of this invention and the reasons for its limitation will be explained. It should be noted that, unless otherwise stated, the expression "%" in the composition refers to "mass %".

[0047] C: 0.200%~0.700%

[0048] C is an inexpensive austenite stabilizing element and an important element for obtaining austenite. To prevent insufficient C at the aforementioned grain boundaries, the C content is 0.200% or more. On the other hand, if the C content exceeds 0.700%, excessive Cr carbides are formed, resulting in a decrease in low-temperature toughness (low-temperature toughness after linear heating). Therefore, the C content is set to 0.200% to 0.700%. The C content is preferably 0.250% or more, more preferably 0.300% or more. Furthermore, the C content is preferably 0.600% or less, more preferably 0.550% or less.

[0049] Si: 0.05%~1.00%

[0050] Si acts as a deoxidizer, essential not only in steelmaking but also in strengthening steel sheets through solid solution treatment. To achieve this effect, the Si content is 0.05% or higher. Conversely, if the Si content exceeds 1.00%, non-thermal stress increases excessively, resulting in poor low-temperature toughness. Therefore, the Si content is set to 0.05% to 1.00%. The Si content is preferably 0.07% or higher, more preferably 0.10% or higher, and even more preferably 0.15% or higher. Furthermore, the Si content is preferably 0.80% or lower, more preferably 0.75% or lower, and even more preferably 0.70% or lower.

[0051] Mn: 20.0%~40.0%

[0052] Mn is a relatively inexpensive austenite stabilizing element. In this invention, it is an important element for balancing strength and low-temperature toughness. To achieve this effect, the content of Mn is 20.0% or more. On the other hand, if the content of Mn exceeds 40.0%, the low-temperature toughness deteriorates. Furthermore, weldability and shear strength deteriorate. In addition, it promotes segregation and the formation of stress corrosion cracks. Therefore, the Mn content is set to 20.0% to 40.0%. The Mn content is preferably 23.0% or more, more preferably 23.3% or more, and even more preferably 23.5% or more. The Mn content is preferably 35.0% or less, more preferably 30.0% or less.

[0053] P: below 0.030%

[0054] If the phosphorus (P) content exceeds 0.030%, excessive segregation occurs at grain boundaries, leading to decreased low-temperature toughness. Therefore, 0.030% is set as an upper limit, and it is preferable to reduce it as much as possible. Thus, the P content is set to 0.030% or less. It should be noted that excessively reducing P would increase refining costs and be economically disadvantageous; therefore, the P content is preferably 0.002% or more. More preferably, the P content is 0.005% or more, and even more preferably 0.007% or more. The P content is preferably 0.028% or less, more preferably 0.024% or less, and even more preferably 0.020% or less.

[0055] S: below 0.0050%

[0056] Since sulfur (S) deteriorates the low-temperature toughness and ductility of the base material, its content is set to an upper limit of 0.0050%, and is preferably reduced as much as possible. Therefore, the S content is set to 0.0050% or less, preferably 0.0045% or less, and more preferably 0.0043% or less. It should be noted that excessively reducing S would increase refining costs and be economically disadvantageous; therefore, the S content is preferably 0.0010% or more, and more preferably 0.0012% or more.

[0057] Al: Below 5.00%

[0058] Al acts as a deoxidizer, most commonly used in the deoxidation process of molten steel plates. Furthermore, it improves the yield strength and local elongation during tensile tests. To achieve this effect, an Al content of 0.01% or more is preferred. On the other hand, if the Al content exceeds 5.00%, a large number of inclusions are present, resulting in poor low-temperature toughness; therefore, the Al content is set to 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.

[0059] Cr: below 7.0%

[0060] Cr is an effective element for improving low-temperature toughness because it enhances grain boundary strength. To achieve this effect, Cr preferably contains 0.5% or more. On the other hand, if the Cr content exceeds 7.0%, the formation of Cr carbides may decrease low-temperature toughness and resistance to stress corrosion cracking. 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 even more preferably 1.2% or more. The Cr content is preferably 6.7% or less, more preferably 6.5% or less. Furthermore, to further improve resistance to stress corrosion cracking, the Cr content is even more preferably 2.0% to 6.0%.

[0061] N: below 0.0500%

[0062] Nitrogen (N) is an austenite stabilizing element and is effective in improving low-temperature toughness. To achieve this effect, N preferably contains 0.0050% or more. On the other hand, if the N content exceeds 0.0500%, nitrides or carbonitrides become coarser, which may reduce low-temperature toughness. 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.

[0063] O: below 0.0050%

[0064] Oxygen (O) deteriorates low-temperature toughness through the formation of oxides. Therefore, the O content is set to be 0.0050% or less. Preferably, it is 0.0045% or less, more preferably 0.0040% or less. It should be noted that excessive reduction of O will increase refining costs, which is economically disadvantageous; therefore, the O content is preferably 0.0010% or more. More preferably, the O content is 0.0012% or more.

[0065] Ti: less than 0.005%, Nb: less than 0.005%

[0066] Ti and Nb form high-melting-point carbonitrides in steel, thus reducing low-temperature toughness. Ti and Nb are components that inevitably mix in from raw materials, typically in the range of 0.005% to 0.010% for Ti and 0.005% to 0.010% for Nb. Therefore, according to the smelting method described later, it is necessary to avoid the unavoidable mixing of Ti and Nb by suppressing their contents to less than 0.005%. By suppressing the contents of Ti and Nb to less than 0.005%, the adverse effects of the aforementioned carbonitrides can be eliminated, ensuring excellent low-temperature toughness and ductility. The contents of Ti and Nb are preferably 0.003% or less, more preferably 0.002% or less. Of course, the contents of Ti and Nb can also be 0%.

[0067] Selected from one or more of the following: Ca: less than 0.0100%, Mg: less than 0.0100%, and REM: less than 0.0200%.

[0068] Ca, Mg, and REM (rare earth metals) are elements useful for controlling the morphology of inclusions. Inclusion morphology control refers to transforming elongated inclusions into granular inclusions. This morphology control improves ductility, low-temperature toughness, and resistance to stress corrosion cracking. To achieve this effect, Ca and Mg are preferably contained at least 0.0005%, and REM at least 0.0010%. On the other hand, if any element is present in large quantities, the amount of non-metallic inclusions increases, which in turn decreases ductility, low-temperature toughness, and resistance to stress corrosion cracking. Furthermore, this becomes economically disadvantageous.

[0069] Therefore, when Ca and Mg are present, their respective percentages are set to 0.0100% or less, and when REM is present, they are set to 0.0200% or less. Preferably, Ca is set to 0.0005% to 0.0090%, Mg to 0.0005% to 0.0090%, and REM to 0.0010% to 0.0180%. More preferably, Ca is set to 0.0010% to 0.0080%, Mg to 0.0010% to 0.0080%, and REM to 0.0020% to 0.0150%. Even more preferably, Ca is set to 0.0015% to 0.0050%, Mg to 0.0015% to 0.0050%, and REM to 0.0030% to 0.0100% or less.

[0070] The austenitic steel of the present invention, in addition to the above-mentioned components, consists of iron (Fe) and unavoidable impurities. Examples of unavoidable impurities include H, B, etc., and the total amount of all elements is permissible if it is less than 0.01%.

[0071] The above-mentioned elements constitute the basic composition. The properties intended for the present invention can be obtained through this basic composition. In this invention, to further improve strength and low-temperature toughness, in addition to the above-mentioned elements, the following elements may be included as needed.

[0072] It should be noted that the various components of Cu, Ni, Mo, V, and W shown below can be included as needed, and therefore these components can also be 0%.

[0073] Selected from one or more of the following: Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 2.0%, V: less than 2.0%, and W: less than 2.0%.

[0074] Cu: less than 1.0%, Ni: less than 1.0%

[0075] Cu and Ni not only enhance the strength of steel sheets through solid solution strengthening, but also improve dislocation mobility and low-temperature toughness. To achieve this effect, Cu and Ni are preferably contained at 0.01% or more. On the other hand, if the content of Cu and Ni exceeds 1.0%, the surface properties deteriorate during rolling, and manufacturing costs are increased. Therefore, when these alloying elements are present, their contents are preferably 1.0% or less. Cu and Ni are more preferably 0.03% or more, further preferably 0.7% or less, and even more preferably 0.5% or less.

[0076] Mo: below 2.0%, V: below 2.0%, W: below 2.0%

[0077] Mo, V, and W contribute to austenite stabilization and improve the strength of the base material. To achieve this effect, the content of Mo, V, and W is preferably 0.001% or more. On the other hand, if the content of Mo, V, and W exceeds 2.0%, coarse carbonitrides are formed, which can sometimes become the initiation point of fracture and put pressure on manufacturing costs. Therefore, when these alloying elements are included, their contents are preferably 2.0% or less. More preferably, the content of Mo, V, and W is 0.003% or more, more preferably 1.7% or less. Even more preferably, it is 0.1% or more, and even more preferably 1.5% or less.

[0078] [Microstructure of steel]

[0079] Next, the reasons for defining the microstructure as described above in this invention will be explained.

[0080] Maximum crystal grain size at 1 mm below the surface of the steel: less than 200 μm

[0081] As described above, when the grain size of the steel (base material) is large, insufficient carbon (C) is present during carbide formation. By reducing the maximum grain size at a position 1 mm below the surface of the steel to less than 200 μm, the C concentration at the grain boundaries can be maintained above 0.100%, even after linear heating of the steel. That is, in the linear heating section of a structure (e.g., a can) obtained after linear heating, it is possible to manufacture steel with a Charpy impact test energy absorption of 41 J or more at -269°C or higher and excellent low-temperature toughness.

[0082] 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. There is no particular specification for the lower limit of the maximum crystal grain size. To ensure the toughness of the hot-rolled steel sheet (steel), the maximum crystal grain size is preferably 50 μm or more, more preferably 60 μm or more. Here, the aforementioned grains refer to the grains exposed by etching. In this invention, the aforementioned maximum crystal grain size can be determined by the method described in the examples described later.

[0083] It should be noted that in this invention, by hot rolling according to the conditions described later, the maximum grain size of the steel can be controlled within the aforementioned numerical range. As a result, even after linear heating, the C concentration at the grain boundaries can be ensured, enabling the aforementioned energy absorption.

[0084] The number density of crystal grains with a diameter of 50 μm or larger at a position 1 mm below the surface of the steel (preferred condition).

[0085] The fracture initiation point of high-Mn steel is the grain boundary, and cracks propagate along the grain boundaries. Therefore, the presence of coarse grains can inhibit crack propagation and further improve low-temperature toughness. Thus, the preferred number of austenite grains with a size of 50 μm or larger is approximately [number missing] per 1 mm. 2 The number is 1.0 or more, more preferably 2.0 or more. On the other hand, when the number of the above-mentioned austenite grains per 1 mm... 2 Strength decreases when the number of elements exceeds 10.0. Therefore, it is preferable to use a strength of 1 mm per element. 2 The number is 10.0 or less, and more preferably 9.0 or less.

[0086] In this invention, the austenite grains with a diameter of 50 μm or more per 1 mm 2 The number (number density) can be determined according to the method described in the examples below. The number density can be controlled within the above-mentioned numerical range by performing hot rolling as described below.

[0087] Inclusion particle size at a position 1 mm below the surface of the steel (preferred condition)

[0088] When coarse inclusions are present at a position 1 mm below the surface of the steel, the resistance to stress corrosion cracking decreases. It is known that the resistance to stress corrosion cracking decreases when the particle size of the top 10% of inclusions at a position 1 mm below the surface of the steel (the top 10% inclusion particle size) exceeds 3.5 μm. Therefore, the aforementioned top 10% inclusion particle size is preferably 3.5 μm or less, more preferably 3.0 μm or less. On the other hand, the smaller the aforementioned top 10% inclusion particle size, the more preferred, but from a manufacturing point of view, it is preferably 1.5 μm or more, more preferably 2.0 μm or more.

[0089] Here, "the top 10% of inclusion particle size" refers to the particle size corresponding to the 10% position when the inclusion particle sizes are arranged in descending order in the inclusion particle size distribution. In this invention, the above-mentioned inclusion particle size can be determined according to the method described in the embodiments described later.

[0090] It should be noted that in this invention, "steel (austenitic steel)" refers to steel plates with a thickness of 6 mm or more. From the viewpoint of using billets suitable for structural steel used in extremely low temperature environments, a plate thickness of more than 9 mm is preferred, and more preferably 12 mm or more. There is no particular upper limit to the plate thickness, and it can be any thickness, but it is preferably 40 mm or less.

[0091] [Steel Manufacturing Methods]

[0092] Next, a method for manufacturing steel according to one embodiment of the present invention will be described.

[0093] The steel (austenitic steel) of the present invention can be smelted into molten steel with the above-mentioned composition by means of smelting methods such as converters and electric furnaces. In addition, it can be refined twice in a vacuum degassing furnace.

[0094] At this point, in order to limit the Ti and Nb content, which hinder microstructure control, to the aforementioned numerical range and to avoid unavoidable contamination of Ti and Nb from raw materials, measures need to be taken to reduce their content. For example, by reducing the basicity of the slag during the refining stage, these alloys can be concentrated in the slag and discharged, thus reducing the concentration of Ti and Nb in the final slag product. Alternatively, oxygen can be blown in to oxidize the slag, and the Ti and Nb alloys can be separated by flotation during reflux.

[0095] Then, steel billets such as slabs of specified dimensions are preferably produced by casting methods such as continuous casting or ingot-rolling.

[0096] The following details the manufacturing conditions for producing the above-mentioned steel billet into a steel (austenitic steel) with excellent low-temperature toughness after linear heating.

[0097] To obtain the austenitic steel with the above-described composition, it is important to heat the steel billet to a temperature range of 1100°C to 1300°C, then perform rolling at a total reduction rate of 40% or more at a temperature above 950°C, followed by at least one hot rolling pass at a temperature below 950°C, and finally hot rolling at a finishing temperature of 750°C or higher. After this hot rolling, cooling is then performed. The temperature control here is based on the surface temperature of the steel billet.

[0098] It should be noted that, unless otherwise specified, the use of "°C" to indicate temperature in the following description of the manufacturing method refers to the surface temperature of the steel billet or steel plate. The surface temperature can be measured using, for example, a radiation thermometer. Furthermore, the temperature at the center of the slab or steel plate thickness can be measured, for example, by installing a thermocouple at the center of the slab thickness, or by calculating the temperature distribution within the cross-section of the steel plate through heat transfer analysis, and then correcting the result using the surface temperature of the steel plate.

[0099] Heating temperature of steel billets: 1100℃~1300℃

[0100] To facilitate Mn diffusion during hot rolling, the heating temperature of the steel billet before hot rolling is set to 1100°C or higher. By allowing Mn diffusion, the stability of austenite is ensured even in the Mn negative segregation region. This ensures austenite stability in the linear heating region, preventing brittle fracture. In other words, energy absorption at -269°C is guaranteed. On the other hand, if the heating temperature exceeds 1300°C, there is concern that the steel will begin to melt; therefore, the upper limit of the heating temperature is set at 1300°C. The heating temperature of the steel billet is preferably 1130°C or higher, preferably 1270°C or lower, more preferably 1150°C or higher, and even more preferably 1250°C or lower.

[0101] Hot rolling

[0102] Total reduction rate above 950℃: 40% or more

[0103] As described above, in this invention, it is important to ensure that the maximum grain size at a position 1 mm below the surface of the steel is less than 200 μm. If equiaxed grains cannot be formed during rolling in the recrystallization zone, coarse grains will remain during subsequent rolling in the non-recrystallization zone, resulting in a maximum grain size of 200 μm or more. Furthermore, the number density of grains with a diameter of 50 μm or more exceeds 10.0 grains / mm. 2 Therefore, in the temperature range of 950°C or higher, which is the recrystallization zone, it is effective to ensure a total reduction rate of 40% or more. The total reduction rate in the recrystallization zone is preferably 50% or more, more preferably 52% or more. There is no particular upper limit for the total reduction rate in the recrystallization zone, but for the sake of ensuring strength, the total reduction rate in the recrystallization zone is preferably 85% or less, more preferably 70% or less.

[0104] Hot rolling passes below 950℃: at least one, and finishing rolling temperature above 750℃.

[0105] To ensure fine equiaxed grains formed during hot rolling at temperatures above 950°C, it is important to have at least one hot rolling pass at temperatures below 950°C. Two or more passes are preferred. In the absence of hot rolling passes below 950°C, the maximum grain size is 200 μm or more. Furthermore, the number density of grains with a diameter of 50 μm or more exceeds 10.0 grains / mm. 2There is no specific upper limit to the number of hot rolling passes. From a manufacturing point of view, the number of hot rolling passes is preferably 10 or less, more preferably 8 or less. When hot rolling is performed at temperatures below 750°C, the grain size becomes excessively fine, and the low-temperature toughness decreases; therefore, the finishing rolling end temperature is set to 750°C or higher. When the finishing rolling end temperature is 775°C or lower, the grain size becomes fine, and as a result, the maximum grain size is sometimes less than 50 μm; therefore, the finishing rolling end temperature is preferably higher than 775°C, more preferably 780°C or higher. There is no specific upper limit to the finishing rolling end temperature. From the viewpoint of ensuring strength, the finishing rolling end temperature is preferably 930°C or lower, more preferably 900°C or lower.

[0106] cool down

[0107] After hot rolling, cooling is performed. The cooling conditions are not specifically specified. In this invention, it is preferable to cool from a temperature above (the temperature at the end of hot rolling - 100°C) to below 600°C at an average cooling rate of 1.0°C / s or higher. This suppresses carbide formation and P grain boundary segregation, further improving the properties of the steel. The "temperature at the end of hot rolling" mentioned above refers to the finishing rolling temperature.

[0108] It should be noted that there is no specific upper limit for the above-mentioned average cooling rate. From the viewpoint of controlling the cooling stop temperature, it is preferably 30.0°C / s or less.

[0109] Next, a steel structure (e.g., a tank) manufactured by linearly heating the steel of the present invention as a blank will be described.

[0110] The can of the present invention is manufactured by linearly heating the aforementioned steel under specific linear heating conditions to form a curved surface, and then welding the curved steel. The composition and microstructure of the base material of the can of the present invention manufactured in this way are the same as those of the aforementioned steel (austenitic steel).

[0111] Furthermore, the carbon concentration (C) at the grain boundaries of the linearly heated base material portion in the can of the present invention at a position 1 mm below the surface is 0.100% or higher. If the C concentration at the grain boundaries at the aforementioned position of the linearly heated base material portion is less than 0.100%, grain boundary strength cannot be ensured. Therefore, the C concentration at the aforementioned position of the linearly heated base material portion is set to 0.100% or higher. Preferably, it is 0.200% or higher, more preferably 0.250% or higher. There is no particular upper limit for the C concentration at the aforementioned position of the linearly heated base material portion. From the viewpoint that low-temperature toughness decreases with excessive formation of Cr carbides, it is preferable to be 0.600% or lower, more preferably 0.550% or lower.

[0112] In the can of the present invention manufactured in this way, the energy absorbed in a Charpy impact test at a position 1 mm below the surface of the linear heating section after linear heating is -269°C or higher, which is 41 J or higher. It should be noted that the absorbed energy in the Charpy impact test can be measured according to the method described in the embodiments described later. That is, the absorbed energy in a Charpy impact test at -269°C or higher for the linear heating section is 41 J or higher in the full-size case and 27 J or higher in the 5 mm small-size case.

[0113] In addition, the present invention can also possess resistance to stress corrosion cracking.

[0114] Next, an example of a preferred manufacturing method for the aforementioned can will be described.

[0115] The can of the present invention is manufactured by linearly heating the aforementioned steel under the following conditions to form a curved surface, and then welding the curved steel. It should be noted that the method for manufacturing the steel (austenitic steel) used as the blank has already been described, and therefore is omitted here. Preferred linear heating and welding conditions will be described here.

[0116] [Linear heating conditions]

[0117] For steel, linear heating is performed with a target surface temperature (target heating temperature) of 900°C or lower. After heating, the steel is air-cooled until the surface temperature is below 500°C, and then water-cooled. This linear heating and air-cooling process can be performed once or repeated more than once. Due to changes in microstructure, the number of repetitions is preferably more than once. Because it complicates the local thermal cycling process, the number of repetitions is preferably 5 times or less. The heating temperature is preferably above 800°C.

[0118] [Welding conditions]

[0119] From the perspective of ensuring high strength, high ductility, and excellent cryogenic impact toughness, welding is performed using solid welding wire (1.2 mm in diameter) as the electrode, under the following conditions: no preheating, downward orientation, inter-pass temperature: 100–150 °C, and shielding gas: 80% Ar + 20% CO2. It should be noted that the "%" in the shielding gas description refers to "volume %".

[0120] Example

[0121] The present invention will now be described in more detail based on embodiments. It should be noted that the following embodiments are preferred examples of the invention, and the invention is not limited to these embodiments.

[0122] Steel slabs with the compositions shown in Table 1 were produced using a converter-ladle refining-continuous casting method. It should be noted that "-" in Table 1 indicates that no element was intentionally added; this includes not only cases where the element is absent (0%) but also cases where unavoidable elements are present. Next, the resulting steel slabs were hot-rolled under the conditions shown in Table 2-1, and then cooled to produce steel products (hot-rolled steel plates) with a thickness of 6–40 mm.

[0123] Using the obtained hot-rolled steel sheet, the evaluation of crystal size and inclusion size is carried out according to the following guidelines.

[0124] Next, the obtained steel plate is linearly heated, and the linearly heated steel plate is used to evaluate C concentration, low temperature toughness and stress corrosion cracking resistance according to the following guidelines.

[0125] Here, the linear heating described above will be explained. As linear heating, the process is as follows: Figure 1 The plate shown is linearly heated. (As shown...) Figure 1 As shown, a linear heating specimen with a longitudinal diameter of 1000 mm and a transverse diameter of 500 mm was prepared from the obtained steel plate. The specimen was fixed with a constraint plate at the 1 / 2 position in the width direction (rolling direction), and linear heating of the plate was performed under the following conditions: The target heating temperature of the steel surface was set to 900°C, heated to this temperature, air-cooled until the surface temperature of the steel was below 500°C, and then water-cooled. The linear heating of the same region was repeated under the conditions shown in Table 2-2.

[0126] In addition, the welding of the steel plates after linear heat treatment was carried out using solid welding wire (1.2 mm in diameter) as electrodes, under the welding conditions shown in Table 2-2, without preheating, in a downward orientation.

[0127] (1) Micro-organism evaluation

[0128] [Crystal Grain Size]

[0129] The obtained hot-rolled steel sheet was ground along the rolling direction, then etched. Next, an optical microscope was used to photograph the area 1 mm below the steel sheet surface at 200x magnification. From the photographed images, 100 grains exposed by the etching were randomly selected. The equivalent circle diameter of each grain was used as the grain size to calculate the maximum grain size (μm) at 1 mm below the steel sheet surface. Additionally, the total area of ​​the 100 grains and the number of grains larger than 50 μm were calculated, and the area per 1 mm was determined. 2 Number density of crystals with a diameter of 50 μm or larger (crystals / mm) 2 It should be noted that aqua regia was used as the corrosive solution.

[0130] [Inclusion particle size]

[0131] The particle size of inclusions in the obtained hot-rolled steel sheet was investigated using SEM (scanning electron microscopy). The evaluation area was set to 200 mm. 2 Find the particle size (μm) of the first 10% of inclusions located 1 mm below the surface of the steel plate.

[0132] [C concentration]

[0133] A 12mm × 10mm TEM sample was prepared from the hot-rolled steel sheet after linear heating. Compositional analysis was performed on this sample using an EDS detector attached to a TEM (transmission electron microscope), traversing the carbide-free grain boundaries, to evaluate the carbon concentration. The observation area was 1mm below the surface of the steel sheet. Ten grain boundaries were analyzed, and their average values ​​were calculated.

[0134] (2) Low temperature toughness

[0135] The low-temperature toughness of the linear heating section was evaluated as follows.

[0136] The hot-rolled steel sheet obtained is used to make such Figure 1 The linearly heated specimen shown was prepared using a steel plate that had undergone linear heating under the conditions described above, and the low-temperature toughness of the linearly heated section was evaluated. According to JIS Z 2242 (2005), Charpy V-notch specimens (full-size Charpy V-notch specimens) were taken from the linearly heated section with a plate thickness of 10 mm or more. Charpy impact tests were performed using three Charpy V-notch specimens at -196°C and -269°C. The average absorbed energy of the three specimens at each temperature was calculated. In this embodiment, with full-size Charpy V-notch specimens, an average absorbed energy of 41 J or more from the three specimens at -269°C was considered excellent low-temperature toughness.

[0137] It should be noted that for linear heating sections with a plate thickness of less than 10 mm, 5 mm Charpy V-notched specimens were used according to JIS Z 2242 (2005). Charpy impact tests were conducted using three Charpy V-notched specimens at -196 °C and -269 °C. The average absorbed energy of the three specimens at each temperature was calculated. In Table 2-2, the absorbed energy item is marked "*1" for samples tested using small-sized Charpy V-notched specimens. In the case of small-sized Charpy V-notched specimens, an average absorbed energy of 27 J or higher from the three specimens at -269 °C is considered excellent low-temperature toughness.

[0138] (3) Resistance to stress corrosion cracking

[0139] The evaluation of stress corrosion cracking resistance is based on the stress corrosion cracking test according to ASTM G36. A specimen with a thickness of 2.5 mm, a width of 20 mm, and a length of 80 mm is taken from 1 mm below the surface of the obtained hot-rolled steel sheet. The solution is boiling MgCl2 chloride, and the bending radius is 5 mm. After immersing the stressed specimen in the above solution for 400 hours, it is checked whether cracks have formed. Cases without crack formation are evaluated as "〇 (Acceptable)" as shown in Table 2-2, and cases with crack formation are evaluated as "× (Unacceptable)" as shown in Table 2-2.

[0140] The results obtained above are shown in Tables 2-1 and 2-2.

[0141]

[0142]

[0143]

[0144] As shown in Tables 2-1 and 2-2, it was confirmed that the austenitic steel of the present invention satisfies the target performance requirement of a maximum grain size of less than 200 μm in the microstructure. It was also confirmed that the austenitic steel of the present invention, in the linearly heated portion, satisfies the target performance requirement of a C concentration of 0.100% or more at the grain boundaries, and a Charpy impact test absorption energy (vE) of [missing value]. -269 The value is 41J or higher, and 27J or higher for a small size of 5mm.

[0145] In contrast, comparative examples outside the scope of this invention fail to meet the aforementioned target performance.

Claims

1. A type of steel, comprising the following components: It contains, by mass%, C: 0.200%–0.700%, Si: 0.05%–1.00%, Mn: 20.0%–40.0%, P: less than 0.030%, S: less than 0.0050%, Al: 0.12%–5.00%, Cr: less than 7.0%, N: less than 0.0500%, O: less than 0.0050%, Ti: less than 0.005%, Nb: less than 0.005%, and contains one or more elements selected from Ca: less than 0.0100%, Mg: less than 0.0100%, and REM: less than 0.0200%, with the remainder consisting of iron and unavoidable impurities; It has a linear heating section that implements linear heating. In the microstructure excluding the linear heating section, the maximum crystal grain size at a position 1 mm below the surface of the steel is less than 200 μm. In the linear heating section, the C concentration at the grain boundary 1 mm below the surface is 0.100% or higher.

2. The steel according to claim 1, wherein, The composition further contains, by mass%, a component selected from Cu: One or more of the following: less than 1.0%, Ni: less than 1.0%, Mo: less than 2.0%, V: less than 2.0%, and W: less than 2.0%.

3. The steel according to claim 1, wherein, The number density of crystal grains with a diameter of 50 μm or larger at a position 1 mm below the surface of the steel in the microstructure, excluding the linear heating section, is 1.0 grains / mm. 2 above.

4. The steel according to claim 2, wherein, The number density of crystal grains with a diameter of 50 μm or larger at a position 1 mm below the surface of the steel in the microstructure, excluding the linear heating section, is 1.0 grains / mm. 2 above.

5. The steel according to any one of claims 1 to 4, wherein, The top 10% of the inclusion particle size distribution at a position 1 mm below the surface of the steel in the microstructure excluding the linear heating section has an inclusion particle size of less than 3.5 μm.

6. The steel according to any one of claims 1 to 4, wherein, In the microstructure excluding the linear heating section, the maximum crystal grain size at a position 1 mm below the surface of the steel is less than 80 μm.

7. The steel according to claim 6, wherein, The maximum crystal grain size at a position 1 mm below the surface of the steel in the microstructure, excluding the linear heating section, is 51–80 μm.

8. A method for manufacturing steel, as described in any one of claims 1 to 7. The steel billet having the aforementioned composition is heated to a temperature range of 1100°C to 1300°C. Hot rolling is performed under the following conditions: a total reduction rate of 40% or more at temperatures above 950°C; at least one hot rolling pass at temperatures below 950°C; and a finishing rolling temperature of 750°C or higher. Then it is cooled. Next, the surface of the steel billet is heated to below 900°C, then air-cooled to below 500°C, and then subjected to linear heating treatment with water cooling to form a curved surface.

9. A can, which is welded from the steel according to any one of claims 1 to 7. The energy absorbed in the Charpy impact test at a position 1 mm below the surface of the linear heating section at a temperature above -269°C is 41 J or more.

10. A method for manufacturing a can, which is the method for manufacturing the can according to claim 9. Weld the steels according to any one of claims 1 to 7 together.

11. The method for manufacturing a can according to claim 10, wherein, The welding is performed using solid welding wire as the electrode, under the conditions of inter-pass temperature of 100-150℃ and shielding gas of 80%Ar + 20%CO2.

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