Hot-rolled austenitic stainless steel for low temperatures and its manufacturing method

Abstract

To provide a steel material that is excellent in low temperature toughness capable of applying to a low temperature vessel use and high in economic efficiency, while enjoying high intensity due to TMCP technology and alloy elements.SOLUTION: A low-temperature austenitic stainless hot-rolled steel material of the present invention is characterized by having a chemical composition comprising, in mass percentage, C: 0.070% or less, Si: 1.50% or less, Mn: 0.10 to 3.00%, and P: 0.045. % or less, S: 0.0001 to 0.0300%, Cr: 16.0 to 22.0%, Ni: 6.5 to 13.0%, N: 0.060 to 0.250%, Al: 0.100% or less, O: 0.0090% or less, the balance being Fe and inevitable impurities, where an amount of N [N] in the extraction residue is 0.014% or less, and a half-size impact absorption energy is 54 J or more at -100°C.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled austenitic stainless steel material that can be used as a container for low-temperature liquids such as LNG, liquefied ammonia, and liquefied carbon dioxide, has high strength, and is excellent in low-temperature impact properties, and does not require solution heat treatment, and a method for producing the same. [Background technology]

[0002] Austenitic stainless steels, typified by SUS304, have good low-temperature mechanical properties, especially low-temperature impact properties, and are used as low-temperature steels. Other low-temperature steels include nickel steels, such as 9% Ni steel, which is used at temperatures up to liquid nitrogen temperature (-196°C). Nickel steel plates for low-temperature pressure vessels (JIS G3127) include three types of Ni steel that can be used at temperatures down to -196°C: SL7N590 (7% Ni), SL9N520 (9% Ni), and SL9N590 (9% Ni), all of which have a strength of 690 to 830 N / mm 2 (MPa). Here, the low-temperature steel material referred to in the present invention is a steel material having a minimum usable temperature of -70 to -196°C, in accordance with the examples of minimum usable temperatures for steel materials specified in Table 1 of JIS G3127. On the other hand, SUS304 steel is a hot-rolled stainless steel plate and steel strip (JIS G4304) with a room-temperature tensile strength of 520 N / mm 2 (MPa) is the lower limit of the room temperature tensile strength, and it is characterized by relatively low strength.

[0003] One method for increasing the strength of austenitic stainless steel is to use a technology known as thermal processing control (hereinafter referred to as TMCP), which controls the processing and temperature during the hot rolling process. Patent documents 1, 2, 3, and 4 disclose inventions related to TMCP technology for austenitic stainless steel sheets.

[0004] Furthermore, one common method for increasing the strength of stainless steel using alloying elements is to add N, and austenitic N-containing steels such as SUS304N1, 304N2, 304LN, and 316LN are standardized as JIS steel grades. By combining the above two methods, it is possible to manufacture high-strength hot-rolled steel material, as disclosed in Patent Document 5, for example. High strength allows for structural designs that reduce the steel material weight by reducing the wall thickness, making it possible to propose more economical materials to customers.

[0005] However, there are no examples of hot-rolled austenitic stainless steels strengthened by TMCP technology being used for low-temperature applications such as vessels for storing cryogenic liquids. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 3-66368 [Patent Document 2] Special Publication No. 5-82453 [Patent Document 3] Special Publication No. 5-75809 [Patent Document 4] Patent No. 3000860 [Patent Document 5] Patent No. 6176208 Summary of the Invention [Problem to be solved by the invention]

[0007] In order to apply hot-rolled austenitic stainless steel sheets manufactured using TMCP technology to cryogenic vessel applications, the inventors evaluated the low-temperature properties of steels obtained by varying the chemical composition and hot-rolling method. Through this research, they discovered that the room-temperature tensile strength of hot-rolled steel increases with the addition of N. On the other hand, they also discovered that low-temperature toughness gradually decreases with increasing N content, and that hot-rolled steel manufactured using TMCP technology exhibits lower low-temperature toughness than solution-treated steel.

[0008] Against this background, the objective of the present invention is to provide an economical steel material that has excellent low-temperature toughness and can be used for cryogenic vessels, while taking advantage of the high strength achieved by TMCP technology and alloying elements. [Means for solving the problem]

[0009] As a preliminary study, the inventors conducted an experiment to evaluate the low-temperature toughness of 316LN-based austenitic hot-rolled stainless steel, in which the oxygen content was reduced to 0.001% in a laboratory melt. In the experiment, a 44 mm-thick billet was soaked at 1250°C for 1 hour, then rolled to a 6.5 mm-thick steel plate at a hot-rolling finish temperature of 950°C and water-cooled from 800°C. In this experiment, the oxygen content was reduced by vacuum melting and casting to isolate the atmosphere, using MgO and CaO crucibles, and using high-basicity slag in some melting processes. In addition, Si and Al were added as deoxidizers. Deoxidation using Al was an effective method for controlling the oxygen content of steel to a low value.

[0010] Impact test specimens were taken from the obtained steel in the direction perpendicular to the rolling direction (the direction in which the fracture propagates in the rolling direction) and subjected to 2mm V-notch half-size Charpy impact tests at -100℃ in accordance with the Charpy impact test method for metallic materials (JIS Z2242). When the results were sorted by oxygen content, contrary to expectations, no clear relationship was obtained, and results were obtained showing that low-oxygen steels contained steels that showed low absorbed energy.

[0011] The inventors have conducted detailed studies on the low-temperature toughness of the above-mentioned Al-containing, low-oxygen, N-containing austenitic stainless steel material, and have found that there is data showing that impact properties deteriorate when the Al concentration is too high, that the data also varies depending on the hot rolling conditions, and that AlN precipitates in low-toughness steel material, and that this nitride deteriorates low-temperature impact properties.

[0012] The inventors quantified the content of nitrides in steel materials by extraction analysis, conducted laboratory research on the effects of the content of Al and other nitride-forming elements and hot rolling conditions, and carried out several actual manufacturing tests, thereby completing the present invention.

[0013] That is, the gist of the present invention is as follows.

[0014] (1) A hot-rolled austenitic stainless steel material for low temperature use, characterized in that it has a chemical composition containing, by mass%, C: 0.070% or less, Si: 1.50% or less, Mn: 0.10 to 3.00%, P: 0.045% or less, S: 0.0001 to 0.0300%, Cr: 16.00 to 22.00%, Ni: 6.50 to 13.00%, N: 0.060 to 0.250%, Al: 0.100% or less, O: 0.0090% or less, with the balance being Fe and unavoidable impurities, in which the amount of N [N] in the extraction residue is 0.014% or less, and has a half-size impact absorption energy of 54 J or more at -100°C.

[0015] (2) A hot-rolled austenitic stainless steel material for low temperature use, characterized in that it has a chemical composition containing, by mass%, C: 0.070% or less, Si: 1.50% or less, Mn: 0.10 to 3.00%, P: 0.045% or less, S: 0.0001 to 0.0300%, Cr: 16.00 to 22.00%, Ni: 6.50 to 13.00%, N: 0.060 to 0.250%, Al: 0.100% or less, O: 0.0090% or less, with the balance being Fe and unavoidable impurities, in which the amount of N [N] in the extraction residue is 0.010% or less, and the full-size impact absorption energy at -196°C is 54J or more.

[0016] (3) The hot-rolled austenitic stainless steel material for low temperature use according to (1) or (2), characterized in that it contains, in place of a portion of the Fe, one or more of Cu: 2.00% or less, Sn: 0.050% or less, Mo: 3.00% or less, W: 1.00% or less, Co: 1.00% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, and REM: 0.10% or less.

[0017] (4) A hot-rolled austenitic stainless steel material for low temperature use according to any one of (1) to (3), characterized in that it contains, in place of a part of the Fe, one or more of V: ​​0.30% or less, Nb: 0.060% or less, Ti: 0.030% or less, B: 0.0050% or less, Zr: 0.050% or less, Hf: 0.100% or less, and Ta: 0.100% or less.

[0018] (5) A method for producing austenitic stainless hot-rolled steel material for low temperature use according to any one of (1) to (4), comprising the steps of: heating a steel material before hot rolling to a temperature equal to or higher than T1 calculated by the following formula, followed by hot rolling; and cooling the hot-rolled steel material under conditions of a rolling finish temperature of 850°C or higher and an accelerated cooling start temperature of 700°C or higher. T1(℃)=9300 / (4.2-LOG(Al×N))-273 Here, Al and N in the formula are values ​​that represent the contents of Al and N in the steel material, respectively, expressed in mass %. [Effects of the Invention]

[0019] The low-temperature austenitic hot-rolled stainless steel obtained by this invention has a high tensile strength of 600-900 MPa and excellent low-temperature toughness, which enables economical design and manufacturing of cryogenic containers with reduced wall thickness compared to conventional solution-heat-treated materials. In the effort to convert fuels for various transport ships to LNG and ammonia, which emit fewer greenhouse gases, and in application to large above-ground storage tanks, the steel of this invention is an ideal material for cryogenic fuel tanks, contributing to industrial aspects that prioritize both the environment (prevention of global warming) and economy. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a graph showing the relationship between the absorbed energy in a half-size impact test conducted at −100° C. and the amount of N [N] in the extraction residue for the hot-rolled austenitic stainless steel material for low temperature use of the present invention. [Figure 2] FIG. 1 is a graph showing the relationship between the absorbed energy in a full-size impact test conducted at −196° C. and the amount of N [N] in the extraction residue for the hot-rolled austenitic stainless steel material for low-temperature use of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] The reasons for limiting the chemical components of the present invention will be explained below.

[0022] To ensure the corrosion resistance of stainless steel, the C content is limited to 0.070% or less. If the C content exceeds 0.070%, Cr carbides may form during hot rolling, potentially deteriorating corrosion resistance and toughness. To reliably suppress the formation of Cr carbides, it is preferable to limit the C content to 0.030% or less.

[0023] Si is added for deoxidation. Although it is not necessary for Si to be contained in the final product, austenitic stainless hot-rolled steel for low temperature use, Si usually remains and is contained in a certain amount, so the lower limit may be set to 0.01% or 0.05%. If the Si content exceeds 1.50%, there is a possibility that toughness will deteriorate. Therefore, the upper limit is set to 1.50%. The preferred range is 0.20 to 1.00%.

[0024] Mn has the effect of increasing the austenite phase and improving toughness. For this reason, the content is set to 0.10% or more. On the other hand, Mn is an element that reduces the corrosion resistance of stainless steel, so in the present invention, the upper limit of Mn is set to less than 3.00%. The preferred content is 0.30 to 2.50%.

[0025] P is an element that is inevitably mixed in from the raw materials, and may deteriorate the hot workability and toughness, so its content is limited to 0.045% or less, preferably 0.035% or less.

[0026] S is an element that is inevitably mixed in from raw materials, and because it also deteriorates hot workability, toughness, and corrosion resistance, the upper limit is set to 0.0300%. Reducing S to an extremely low level increases the cost of refining. For this reason, the lower limit of S content was set at 0.0001% to keep costs in line.

[0027] Cr is contained in an amount of 16.00% or more to ensure the basic corrosion resistance of the steel of the present invention. On the other hand, if the Cr content exceeds 22.00%, the ferrite phase fraction increases, which may impair hot workability and toughness. For this reason, the Cr content is set to 16.00% or more and 22.00% or less. The preferred Cr content is 17.00 to 20.00%.

[0028] Ni stabilizes the austenite structure and improves corrosion resistance to various acids and low-temperature toughness, so it is contained in an amount of 6.50% or more. However, since Ni is an expensive alloy, the content of the steel of the present invention is limited to 13.00% or less from the viewpoint of cost. The preferred content is 7.00 to 12.50%.

[0029] N is an effective element that dissolves in the austenite phase and enhances the strength and corrosion resistance of the stainless steel material of the present invention. For this reason, it is contained at 0.060% or more. To further enhance strength and corrosion resistance, it is preferable to contain 0.100% or more. While N increases strength at room temperature and low temperatures, increasing the N content gradually reduces low-temperature impact absorption energy. Furthermore, in the steel material of the present invention, the N and Al concentrations are high, and as the hot-rolling temperature conditions become lower, AlN is formed in the steel material, impairing low-temperature toughness. In the steel material of the present invention, if the N content exceeds 0.250%, the low-temperature toughness may be impaired due to the effects of solute nitrogen and nitrides, and Cr nitrides may precipitate, reducing corrosion resistance. Therefore, the upper limit of the N content is set to 0.250%. The preferred N content in the steel material of the present invention is 0.100 to 0.230%.

[0030] Al is an important element for deoxidizing the steel of the present invention and is included together with Si to reduce the oxygen content in the steel. Reducing the oxygen content is essential to ensuring the low-temperature toughness of the steel. While Si is not required in the final product, austenitic stainless hot-rolled steel for low-temperature use, Al typically remains and is contained in a certain amount. To ensure the deoxidizing effect, it is preferable to add Al to the final product so that 0.0001% or more of Al is contained. To reduce the oxygen content, a content of 0.001% or more is more preferable, and a content of 0.003% or more is even more preferable. However, Al has a strong affinity with N, and excessive addition will form Al nitrides, which will impair the low-temperature toughness of the steel of the present invention. The extent of this degradation depends not only on the N content but also on the heating temperature, rolling temperature, and accelerated cooling start temperature in the hot-rolling process. In the hot-rolled steel of the present invention, the upper limit of Al content is set to 0.100% because a significant decrease in toughness occurs when the Al content exceeds 0.100%. It is more preferably 0.080% or less, and even more preferably 0.050% or less.

[0031] O is an unavoidable impurity and an element that impairs the hot workability, toughness, and corrosion resistance of stainless steel. In the steel material of the present invention, its reduction is necessary to improve low-temperature toughness. Since the desired low-temperature toughness is ensured by keeping the O content at 0.0090% or less, the upper limit is set at 0.0090%. A lower O content is preferable to improve toughness, and it is more desirable to reduce it to 0.0040% or less. O does not need to be contained, and the lower limit is 0. However, if O is reduced too much, extremely high refining costs are required. Therefore, considering economic efficiency, the lower limit of the O content may be set at 0.0001% or 0.0005%.

[0032] The amount of N in the extraction residue [N] is a value obtained by quantifying the N content in the total amount of nitrides contained in the steel material using a method called extraction residue analysis, and is expressed as mass % obtained by dividing the total mass of N in the precipitates by the unit steel material mass. In the present invention, samples for extraction analysis measuring approximately 5 mm × 10 mm × 25 mm were cut out from hot-rolled steel material, and subjected to a current density of 20 mA / cm using a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution. 2 Approximately 0.8 g of steel material is electrolyzed by constant current electrolysis at 1000 kJ / s, and the resulting extraction residue is collected on a Nuclepore filter with a 0.2 μm pore size. After acid decomposition and distillation separation, the N content [N] in the extraction residue is determined by titration.

[0033] The inventors evaluated the low-temperature toughness of steel materials in which the content of Al and various nitride-forming elements was varied while limiting the oxygen content to 0.009% or less, and found that the low-temperature toughness tended to decrease with the value of [N].

[0034] Figure 1 shows the relationship between absorbed energy and [N] in a half-size impact test conducted at -100°C for hot-rolled steel material with a plate thickness of less than 12 mm. It has been discovered that good low-temperature toughness of 54 J or more can be ensured by setting [N] to 0.014% or less. For this reason, in the case of the hot-rolled austenitic stainless steel material for low-temperature use of the present invention, the upper limit of [N] is set to 0.014% for hot-rolled steel material with a plate thickness of less than 12 mm. A low [N] is preferable, and no lower limit is set.

[0035] For hot-rolled steel materials with a thickness of 12 mm or more, full-size impact tests were conducted at -196°C. The results are shown in Figure 2. In this case, it was discovered that good low-temperature toughness of 54 J or more can be ensured by setting [N] to 0.010% or less. For this reason, in the case of the hot-rolled austenitic stainless steel materials for low-temperature use according to the present invention, the upper limit of [N] is set to 0.010% for hot-rolled steel materials with a thickness of 12 mm or more.

[0036] The room-temperature tensile strength is an important characteristic because it is the design standard for the manufacture of cryogenic vessels and structures. The steel material of the present invention can achieve a tensile strength of 600 MPa or more by adding N and applying TMCP technology. Furthermore, in the case of the steel material of the present invention containing N in the range of 0.06 to 0.25%, the tensile strength can be increased to 690 MPa or more, or even 780 MPa or more, as shown in the examples, although this depends on the thickness and C content of the steel material.

[0037] The hot-rolled austenitic stainless steel material for low temperature use according to the present invention may contain the following elements in place of a portion of Fe. Since the following elements are not essential, the lower limit of their content is 0.

[0038] Cu is an element that additionally enhances the corrosion resistance of stainless steel to acids and also has the effect of improving toughness. This effect can be obtained even with the addition of a small amount, but to ensure the effect, it is preferable to add 0.01% or more. If the Cu content exceeds 2.0%, there is a possibility that the hot workability may be impaired. For this reason, the upper limit is set at 2.0%. If added, the preferred content is 0.1 to 1.5%. The more preferred content is 0.2 to 1.0%.

[0039] Like Cu, Sn is an element that additionally enhances the corrosion resistance of stainless steel to acids. This effect can be obtained even with the addition of a small amount, but to ensure the effect, it is preferable to add 0.001% or more. On the other hand, Sn is an element that is harmful to hot workability, and if it is added in an amount of 0.050% or more, it may impair hot workability. For this reason, the upper limit is set at 0.050%. If added, the preferred content is 0.003 to 0.030%.

[0040] Mo is a very effective element for improving the corrosion resistance of stainless steel. This effect can be obtained even with the addition of a small amount, but to ensure the effect, it is preferable to contain 0.01% or more. However, adding a large amount increases costs, so the upper limit of the content in the steel material of the present invention is set to 3.0%. If added, the preferred content is 0.1 to 2.5%.

[0041] Like Mo, W is an element that improves the corrosion resistance of stainless steel. This effect can be obtained even with the addition of a small amount, but to ensure the effect, it is preferable to add 0.01% or more. However, adding a large amount increases costs, so the upper limit of the content in the steel material of the present invention is 1.0%. If added, the preferred content is 0.1 to 0.5%.

[0042] Co is an element that is effective in improving the toughness and corrosion resistance of steel. This effect can be obtained even with the addition of a small amount, but to ensure the effect, it is preferable to add 0.01% or more. Since Co is an expensive element, if it is contained in an amount exceeding 1.0%, the effect will not be commensurate with the cost, so the upper limit was set at 1.0%. If Co is added, the preferred content is 0.1 to 0.5%.

[0043] Ca, Mg, and REM are elements that improve the hot workability of steel and are added as needed. Here, REM refers to rare earth elements, and the REM content is the sum of the contents of all rare earth elements. These elements are also strong oxide-forming elements, forming relatively large oxides in steel and impairing the toughness of the steel. Although these effects can be obtained even with trace additions, to ensure the desired effects, the respective amounts are preferably 0.0003-0.005% Ca, 0.0001-0.003% Mg, and 0.005-0.10% REM, and more preferably 0.0010-0.004% Ca, 0.0003-0.002% Mg, and 0.010-0.06% REM.

[0044] V is an element that has an affinity with N and C and has the effect of forming nitrides and carbides. Its nitride-forming effect is slightly weaker than that of the other nitride-forming elements specified in the present invention, and a content of up to 0.30% is permitted. However, if the content exceeds 0.30%, V nitrides and carbides will precipitate in large quantities, which may impair toughness, so the upper limit has been set at 0.30%.

[0045] Nb is an element that has a stronger affinity with N and C than V and is likely to form nitrides and carbides. If the content exceeds 0.06%, a large amount of Nb carbonitrides will precipitate, which may impair toughness, so the upper limit has been set at 0.06%.

[0046] Ti has a very strong affinity with N, forming Ti nitrides in steel. For this reason, if Ti is added, it must be contained in a very small amount. If it is added in excess of 0.03%, Ti nitrides may impair toughness, so the upper limit has been set at 0.03%.

[0047] B is an element that has a very strong affinity with N, and if it is added in large quantities, nitrides of B may precipitate, which may impair toughness. For this reason, the upper limit of B content has been set at 0.0050%.

[0048] Zr and Hf are elements that have a strong affinity with N, and Ta is an element that has a strong affinity with both N and C. If these elements are added in large amounts, their carbonitrides may precipitate and impair toughness. For this reason, the upper limits of their contents have been set at 0.05%, 0.10%, and 0.10%, respectively.

[0049] Next, a method for producing the hot-rolled austenitic stainless steel material for low temperature use according to the present invention will be described.

[0050] In the steel material of the present invention, the value of [N] and the precipitation behavior of carbides change depending on the chemical composition and hot rolling conditions, so the steel material is manufactured by the manufacturing method described below.

[0051] This steel may contain various nitrides due to the presence of nitride-forming elements. Examples of nitrides that are affected by the hot rolling process include AlN, NbN, TaN, and VN. These nitrides are characterized by the fact that some of them are solid-solutioned during the heating process of hot rolling, and the amount of precipitation increases as the hot rolling process progresses.

[0052] To heat a steel material before hot rolling to dissolve nitrides such as AlN, the higher the heating temperature, the more preferable. For the steel material of the present invention, rolling experiments were conducted using steel material in which nitrides such as AlN may precipitate during hot rolling, with varying heating temperatures, and heating conditions were determined for heating at a temperature equal to or higher than T1, which is determined based on the Al and N contents. In the following formula, LOG is a common logarithm (base 10). Heating the steel material before hot rolling at a temperature equal to or higher than T1, calculated based on the solution temperature calculated by formula (1) obtained from an Arrhenius plot of the solubility product of Al and N for the steel material of the present invention, reduces undissolved nitrides and improves the low-temperature toughness of the steel material of the present invention.

[0053] T1=9300 / (4.2-LOG(Al×N))-273(℃) … (1)

[0054] Here, the Al and N contents (mass%) in the steel are substituted for equation (1). The element contents in steel are usually measured after deoxidation and before casting. Since the element contents in steel do not change after casting, the values ​​measured here are the same as the element contents in the final product.

[0055] In this invention, the end temperature of hot rolling is defined as the entry temperature of the final rolling pass. The rolling temperature conditions of hot rolling have a significant effect on the tensile strength of hot-rolled steel material, and the lower the end temperature, the higher the tensile strength obtained. On the other hand, if this end temperature is too low below 850°C, the tensile strength becomes excessively high and low-temperature toughness is impaired. Therefore, the end temperature of hot rolling is set to 850°C or higher.

[0056] In addition, the temperature range in austenitic stainless steels where carbide precipitation begins and corrosion resistance begins to decline varies depending on the amount of C, N, and carbonitride-forming elements, but is approximately 700°C or below. When steel is air-cooled at temperatures below 700°C, the cooling rate becomes slow in thick steel, and carbonitride precipitation may occur during cooling. Based on this precipitation behavior in stainless steel, the amount of carbonitride precipitation can be reduced by performing accelerated cooling, which increases the cooling rate from temperatures above 700°C after hot rolling using methods such as spray cooling. Therefore, the accelerated cooling start temperature was set at 700°C or above.

[0057] As described above, in the steel material of the present invention, the manufacturing conditions are specified by relating the contents of alloying elements to the hot rolling conditions. [Example]

[0058] Examples are described below.

[0059] Table 1 shows the chemical composition of the test steels. These steels were either hot-rolled from ingots or cut out and hot-rolled from actual manufactured materials. The components listed in Table 1 are Fe and unavoidable impurity elements. Furthermore, for the components shown in Table 1, the absence of a content indicates that the content is at the impurity level, and REM stands for lanthanide rare earth elements, with the content indicating the total of these elements.

[0060] The melted material was hot forged to produce a rolled material measuring 60 mm thick x 110 mm wide x 150 mm long, which was then soaked at 1050 to 1200°C for 10 to 120 minutes to produce a hot-rolled steel plate having a thickness of 12 mm, with the hot-rolling end temperature set to 800 to 1050°C and the accelerated cooling start temperature set to 600 to 850°C. In addition, hot-rolled steel plates having a thickness of 12 to 40 mm were produced in the same manner from actual slabs having a thickness of 140 to 200 mm and rough-rolled slabs rolled to a thickness of 120 to 80 mm.

[0061] [Table 1]

[0062] The [N] in the extraction residue was determined by the following procedure.

[0063] [1] A 5mm thick x 25mm wide x 10mm long sample is taken from 1 / 4 of the thickness of the hot-rolled steel plate. [2] Wet polish the entire surface of the sample with #600 grit. [3] Approximately 0.8 g of the matrix is ​​dissolved by constant current electrolysis (current density 20 mA / cm2) in a non-aqueous solution (10% acetylacetone + 1% tetramethylammonium chloride + balance methanol). [4] Filter the residue (= precipitate) using a Nuclepore filter with a 0.2 μm pore diameter and extract the precipitate. [5] The residue is acid-decomposed and separated by distillation, and the nitrogen content [N] in the extracted residue is determined by titration.

[0064] The mechanical properties were evaluated as follows.

[0065] Tensile tests (n=2) at room temperature and impact tests (n=3) at -196°C were conducted on steel plates with thicknesses of 12 to 40 mm. All test specimens were taken perpendicular to the rolling direction. Tensile test specimens were prepared with a parallel section of 8 mm diameter x 50 mm and a gauge distance of 40 mm. They were taken from the center of the plate for materials with a thickness of 20 mm or less, and from the quarter-thickness section for materials with a thickness of 25 mm or more. Impact test specimens were JIS No. 4 full-size test specimens taken from the center of the plate for materials with a thickness of 20 mm or less, and from the quarter-thickness section for materials with a thickness of 25 mm or more. Tensile tests were conducted in accordance with the Tensile Test Method for Metallic Materials (JIS Z2241), and impact tests were conducted in accordance with the Charpy Impact Test Method for Metallic Materials (JIS Z2242).

[0066] Table 2 shows the plate thickness, heating temperature, soaking time, rolling end temperature, accelerated cooling start temperature, and N content in the extracted residue [N] of the steel material explained in the previous section, as well as the tensile strength in tensile tests at room temperature and the average absorbed energy in impact tests at -196°C.

[0067] As shown in Table 2, all of the hot-rolled steel materials according to the present invention have high tensile strengths of 600 MPa or more at room temperature and high absorbed energy values ​​of 54 J or more at -196°C. On the other hand, the comparative examples either had tensile strengths of less than 600 MPa or low absorbed energy values ​​of less than 54 J at -196°C.

[0068] [Table 2]

[0069] As can be seen from the above examples, it has become clear that the present invention makes it possible to obtain high-strength, N-containing, hot-rolled austenitic stainless steel material that is excellent in low-temperature impact properties. [Industrial Applicability]

[0070] The low-temperature austenitic hot-rolled stainless steel material obtained by this invention has high tensile strength and excellent low-temperature toughness, which enables economical design and manufacturing of cryogenic containers with reduced wall thickness compared to conventional solution-heat-treated materials. In the effort to convert fuels for various transport ships to LNG and ammonia, which emit fewer greenhouse gases, and in application to large above-ground storage tanks, the steel material of this invention is an ideal material for cryogenic fuel tanks, and will make a significant contribution to industrial areas that emphasize both the environmental aspect of preventing global warming and economic efficiency.

Claims

1. In mass%, C: 0.070% or less, Si: 1.50% or less, Mn: 0.10-3.00%, P: 0.045% or less, S: 0.0001-0.0300%, Cr: 16.00-22.00%, Ni: 6.50-13.00%, N: 0.060-0.250%, Al: 0.100% or less, O: 0.0090% or less and the balance being Fe and unavoidable impurities, The amount of N in the extraction residue [N] is 0.014% or less, It has a half-size impact absorption energy of 54J or more at -100°C, Tensile strength at room temperature is 600 MPa or more A hot-rolled austenitic stainless steel material for low temperature use.

2. In mass%, C: 0.070% or less, Si: 1.50% or less, Mn: 0.10-3.00%, P: 0.045% or less, S: 0.0001-0.0300%, Cr: 16.00-22.00%, Ni: 6.50-13.00%, N: 0.060-0.250%, Al: 0.100% or less, O: 0.0090% or less and the balance being Fe and unavoidable impurities, The amount of N in the extraction residue [N] is 0.010% or less, It has a full-size impact energy absorption characteristic of 54J or more at -196°C. Tensile strength at room temperature is 600 MPa or more A hot-rolled austenitic stainless steel material for low temperature use.

3. Instead of a part of the Fe, Cu: 2.00% or less, Sn: 0.050% or less, Mo: 3.00% or less, W: 1.00% or less, Co: 1.00% or less, Ca: 0.0050% or less, Mg: 0.0030% or less, REM: 0.10% or less 3. The hot-rolled austenitic stainless steel material for low temperature use according to claim 1, further comprising one or more of the following:

4. Instead of a part of the Fe, V: 0.30% or less, Nb: 0.060% or less, Ti: 0.030% or less, B: 0.0050% or less, Zr: 0.050% or less, Hf: 0.100% or less, Ta: 0.100% or less 4. The hot-rolled austenitic stainless steel material for low temperature use according to claim 1, further comprising one or more of the following:

5. A method for producing the hot-rolled austenitic stainless steel material for low temperature use according to any one of claims 1 to 4, comprising: A method for producing austenitic stainless hot-rolled steel for low temperature use, comprising the steps of heating a steel material before hot rolling to a temperature equal to or higher than T1 calculated by the following formula, followed by hot rolling, and cooling the hot-rolled steel material under conditions of a rolling finish temperature of 850°C or higher and an accelerated cooling start temperature of 700°C or higher. T1(℃)=9300 / (4.2-LOG(Al×N))-273 Here, Al and N in the formula are values ​​that represent the contents of Al and N in the steel material, respectively, expressed in mass %.

Images