Steel plate for pressure vessels with excellent cryogenic toughness and manufacturing method thereof

Abstract

The present invention provides a steel plate for a cryogenic pressure vessel having high strength and excellent cryogenic toughness, and a method for manufacturing the same. [Solution] The present invention relates to a method for producing a steel sheet, the method comprising the steps of reheating a slab containing, by weight %, 0.05 to 0.15% C, 0.20 to 0.35% Si, 0.5 to 1.5% Mn, 0.012% or less P, 0.015% or less S, 0.02 to 0.10% Al, 6.01 to 6.49% Ni, 0.2 to 0.4% Mo, 0.05 to 0.25% Cr, and the balance being Fe and unavoidable impurities; hot-rolling the reheated steel sheet and air-cooling it; and rolling the air-cooled steel sheet at 800 to 1200°C. The present invention provides a method for manufacturing a steel plate for a cryogenic pressure vessel, the method comprising the steps of: performing a primary heat treatment at 880°C for {2.4×t+(10-40)} minutes [t: slab thickness (mm)] and performing a primary water cooling; performing a secondary heat treatment on the primarily water-cooled steel plate at 700-780°C for {2.4×t+(10-40)} minutes [t: slab thickness (mm)] and performing a secondary water cooling; and tempering the secondary water-cooled steel plate. The present invention also provides a method for manufacturing a steel plate for a cryogenic pressure vessel, the method comprising the steps of:

Description

[Technical Field]

[0001] The present invention relates to a steel plate for pressure vessels having excellent cryogenic toughness and a method for manufacturing the same. [Background technology]

[0002] High strength thick steel plates for low temperature use must be able to be used as cryogenic structural materials when constructed, and therefore must have high strength and cryogenic toughness properties.

[0003] High-strength hot-rolled steel produced by a normalizing treatment has a mixed structure of ferrite and pearlite, and an example of the prior art in this regard is the invention described in Patent Document 1.

[0004] The above-mentioned Patent Document 1 discloses a high-strength steel material for 500 MPa-class LPG, characterized by a composition, in weight percent, of C: 0.08-0.15%, Si: 0.2-0.3%, Mn: 0.5-1.2%, P: 0.01-0.02%, S: 0.004-0.006%, Ti: more than 0% but not more than 0.01%, Mo: 0.05-0.1%, Ni: 3.0-5.0%, with the remainder being Fe and other unavoidable impurities, and characterized by the addition of Ni and Mo to the compositional components of the steel.

[0005] However, the invention described in Patent Document 1 above is a steel material produced by ordinary normalizing, and therefore has the problem that the cryogenic lateral expansion properties of the steel material are insufficient even if Ni or the like is added.

[0006] This has led to an increasing demand for the development of steel materials that have excellent cryogenic impact toughness and improved cryogenic lateral expansion properties. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 2012-0011289 Summary of the Invention [Problem to be solved by the invention]

[0008] The technical problem to be achieved by the present invention is to provide a steel plate for a cryogenic pressure vessel having high strength and excellent cryogenic toughness, and a method for manufacturing the same.

[0009] More specifically, the present invention relates to a steel plate for cryogenic pressure vessels that can secure a tensile strength of 750 MPa class while having strength and lateral expansion properties that allow stable use at cryogenic temperatures of -150°C or below, and a method for manufacturing the same.

[0010] The objects of the present invention are not limited to the above-mentioned objects, and other objects not mentioned will be clearly understood by those having ordinary skill in the art to which the present invention pertains from the following description. [Means for solving the problem]

[0011] To achieve the above object, the present invention provides a method for producing a slab containing, by weight, 0.05 to 0.15% C, 0.20 to 0.35% Si, 0.5 to 1.5% Mn, 0.012% or less P, 0.015% or less S, 0.02 to 0.10% Al, 6.01 to 6.49% Ni, 0.2 to 0.4% Mo, 0.05 to 0.25% Cr, and the balance being Fe and unavoidable impurities, comprising the steps of: reheating a slab containing, by weight, 0.05 to 0.15% C, 0.20 to 0.35% Si, 0.5 to 1.5% Mn, 0.012% or less P, 0.015% or less S, 0.02 to 0.10% Al, 6.01 to 6.49% Ni, 0.2 to 0.4% Mo, 0.05 to 0.25% Cr, and the balance being Fe and unavoidable impurities; Slabs By hot rolling Manufacture steel plates, A step of air-cooling the steel sheet and heating the air-cooled steel sheet at 800 to 880°C for {2.4 × t + (10 to 40)} minutes [t: steel plate the steel sheet is subjected to a primary heat treatment for a thickness (mm)] and a primary water cooling, and the primary water cooling is performed at 700 to 780°C for 2.4 x t + (10 to 40) minutes [t: steel plate a second heat treatment for a thickness (mm) and a second water cooling step; and tempering the second water-cooled steel plate.

[0012] The present invention also relates to a steel plate for use in cryogenic pressure vessels, which comprises, by weight%, C: 0.05 to 0.15%, Si: 0.20 to 0.35%, Mn: 0.5 to 1.5%, P: 0.012% or less, S: 0.015% or less, Al: 0.02 to 0.10%, Ni: 6.01 to 6.49%, Mo: 0.2 to 0.4%, Cr: 0.05 to 0.25%, and the balance being Fe and unavoidable impurities, and the microstructure of the steel comprises, in terms of area fraction, a three-phase mixed structure of 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the balance being tempered martensite. [Effects of the Invention]

[0013] The method for producing a steel plate for cryogenic pressure vessels according to the present invention involves a step of heat treating a hot-rolled and air-cooled steel plate twice, at temperatures of 800 to 880°C and 700 to 780°C, thereby producing a steel plate for cryogenic pressure vessels having a microstructure of a three-phase mixed structure of, in terms of area fraction, 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite.

[0014] The steel plate for a cryogenic pressure vessel may have strength and lateral expansion properties that allow stable use at cryogenic temperatures of -150° C. or lower. Specifically, the steel plate for a cryogenic pressure vessel may have excellent strength properties, such as a yield strength of 610 MPa or higher and a tensile strength of 750 MPa or higher, and excellent cryogenic toughness properties, such as a Charpy impact energy of 190 J or higher at -195° C.

[0015] In particular, the above-mentioned steel plate for cryogenic pressure vessels has a three-phase mixed structure of 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite, and can have excellent lateral expansion properties with an elongation of 30% or more. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a steel plate for a pressure vessel having excellent cryogenic toughness and a manufacturing method thereof according to the present invention will be described in detail. The drawings introduced below are provided as examples so that the concept of the present invention can be fully conveyed to those skilled in the art. Therefore, the present invention is not limited to the drawings presented below and may be embodied in other forms, and the drawings presented below may be exaggerated to clarify the concept of the present invention. In this regard, unless otherwise defined, the technical and scientific terms used have the meanings commonly understood by those skilled in the art to which the present invention belongs. In the following description and the accompanying drawings, descriptions of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

[0017] Throughout the specification, when a part is said to "comprise" certain elements, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0018] The present invention relates to a method for producing a slab comprising, by weight, 0.05 to 0.15% C, 0.20 to 0.35% Si, 0.5 to 1.5% Mn, 0.012% or less P, 0.015% or less S, 0.02 to 0.10% Al, 6.01 to 6.49% Ni, 0.2 to 0.4% Mo, 0.05 to 0.25% Cr, and the balance being Fe and unavoidable impurities, and a method for producing a slab comprising the steps of: reheating the slab; Slabs By hot rolling Manufacture steel plates, A step of air-cooling the steel sheet and heating the air-cooled steel sheet at 800 to 880°C for {2.4 × t + (10 to 40)} minutes [t: steel plate the thickness (mm)] and then performing a primary heat treatment on the steel sheet after the primary water cooling at 700 to 780°C for {2.4 × t + (10 to 40)} minutes [t: steel plate The present invention relates to a method for manufacturing a steel plate for a cryogenic pressure vessel, the method comprising the steps of: subjecting the steel plate to a secondary heat treatment for a thickness (mm) of 10 ...

[0019] As described above, the method for manufacturing a steel plate for a cryogenic pressure vessel according to the present invention performs a step of heat treating the hot-rolled and air-cooled steel plate twice at temperatures of 800 to 880°C and 700 to 780°C, thereby manufacturing a steel plate for a cryogenic pressure vessel having a microstructure of a three-phase mixed structure of 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite, based on area fraction.

[0020] The steel plate for a cryogenic pressure vessel may have strength and lateral expansion properties that allow stable use at cryogenic temperatures of -150° C. or lower. Specifically, the steel plate for a cryogenic pressure vessel may have excellent strength properties, such as a yield strength of 610 MPa or higher and a tensile strength of 750 MPa or higher, and excellent cryogenic toughness properties, such as a Charpy impact energy of 190 J or higher at -195° C.

[0021] In particular, the above-mentioned steel plate for cryogenic pressure vessels has a three-phase mixed structure of 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite, and can have excellent lateral expansion properties with an elongation of 30% or more.

[0022] The reasons for the numerical limitations on the alloy component contents in one example of the present invention will be explained below. Unless otherwise specified, the units are % by weight.

[0023] In the steel plate for cryogenic pressure vessels of the present invention, the carbon (C) content may be 0.05 to 0.15%. If the C content is less than 0.05%, the strength of the matrix itself decreases, and if it exceeds 0.15%, the weldability of the steel plate is significantly impaired. A more preferable lower limit may be 0.07%, and a more preferable upper limit may be 0.13%.

[0024] In the steel plate for cryogenic pressure vessels of the present invention, the silicon (Si) content may be 0.20 to 0.35%. Si is an element added for deoxidizing effects, solid solution strengthening effects, and the effect of increasing the impact transition temperature, and to achieve these effects, it is preferable to add 0.20% or more. However, if added in excess of 0.35%, weldability deteriorates and an oxide film is severely formed on the steel plate surface, so the silicon content is preferably limited to 0.20 to 0.35%. A more preferable lower limit may be 0.23%, and a more preferable upper limit may be 0.32%.

[0025] In the steel plate for cryogenic pressure vessels of the present invention, the manganese (Mn) content may be 0.5 to 1.5%. Since Mn, together with S, forms elongated non-metallic inclusions called MnS, which reduces room-temperature elongation and low-temperature toughness, it is preferable to control the Mn content to 1.5% or less. However, due to the compositional characteristics of the present invention, if the Mn content is less than 0.5%, it becomes difficult to ensure appropriate strength, so the Mn content is preferably limited to 0.5 to 1.5%. A more preferable lower limit may be 0.52%, and a more preferable upper limit may be 1.2%.

[0026] In the steel plate for cryogenic pressure vessels of the present invention, the aluminum (Al) content may be 0.02 to 0.10%. Along with Si, Al is one of the powerful deoxidizers in the steelmaking process, and its effect is minimal at less than 0.02%, while the addition of 0.10% or more increases production costs. Therefore, the content is preferably limited to 0.02 to 0.10%. A more preferred lower limit may be 0.025%, and a more preferred upper limit may be 0.09%.

[0027] In the steel plate for cryogenic pressure vessels of the present invention, phosphorus (P) is an element that impairs low-temperature toughness. However, since removing it in the steelmaking process requires excessive costs, it is preferable to control the content within a range of 0.012% or less.

[0028] In the steel plate for cryogenic pressure vessels according to one example of the present invention, sulfur (S) is an element that, together with P, adversely affects low-temperature toughness. However, similar to P, removing S in the steelmaking process can require excessive costs, so it is appropriate to control the S content within a range of 0.015% or less.

[0029] In the steel plate for cryogenic pressure vessels of the present invention, the nickel (Ni) content may be 6.01 to 6.49%. Ni is the most effective element for improving low-temperature toughness. However, if the amount of Ni added is less than 6.01%, low-temperature toughness will be reduced, and if the amount of Ni added exceeds 6.49%, manufacturing costs will increase. Therefore, it is preferable to add Ni in the range of 6.01 to 6.49%. A more preferable lower limit may be 6.08%, and a more preferable upper limit may be 6.45%.

[0030] In the steel plate for cryogenic pressure vessels of the present invention, molybdenum (Mo) is an extremely important element for improving hardenability and strength, but if added in an amount of less than 0.2%, the effect cannot be expected, and since Mo is an expensive element, it is preferable to limit the content to 0.2 to 0.4%, and more preferably, it may be 0.32% or less.

[0031] In the steel plate for cryogenic pressure vessels of the present invention, chromium (Cr) is an important element that ensures strength at low and normal temperatures. Since the effect cannot be expected if added in an amount of less than 0.05%, and chromium is an expensive element, it is preferable to limit the Cr content to 0.05 to 0.25%. A more preferable upper limit may be 0.22%.

[0032] The remaining component is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed in from the raw materials or the surrounding environment, and it is not possible to eliminate these. These impurities are known to any engineer working in a normal manufacturing process, and therefore, the contents of all of them will not be specifically mentioned in this specification.

[0033] As described above, by undergoing two heat treatment processes, the steel plate for cryogenic pressure vessels according to the present invention can have a microstructure consisting of a three-phase mixed structure of 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite, based on area fraction. This allows for a steel plate for cryogenic pressure vessels with excellent strength and low-temperature toughness. On the other hand, if the area fraction of tempered bainite is less than 40%, the amount of tempered martensite becomes excessive, which may deteriorate the low-temperature toughness of the steel plate and make it difficult to ensure an elongation of 30% or more. Conversely, if the area fraction of tempered bainite exceeds 80%, it may become difficult to ensure the target strength of the steel plate. Furthermore, if the area fraction of retained austenite is less than 1.0%, it may impair low-temperature toughness and make it difficult to ensure an elongation of 30% or more. Conversely, if the area fraction of retained austenite exceeds 9.5%, strength decreases, so it is preferably limited to the range of 1.0 to 9.5%.

[0034] In order to manufacture a steel plate for cryogenic pressure vessels having a three-phase mixed structure that satisfies such an area fraction, it is particularly important to undergo two heat treatment steps, one after hot rolling and one before tempering.

[0035] As described above, the method for manufacturing a steel plate for a cryogenic pressure vessel includes the steps of reheating a slab, hot-rolling the reheated steel plate and air-cooling it, and rolling the air-cooled steel plate at 800 to 880°C for {2.4 × t + (10 to 40)} minutes [t: steel plate the thickness (mm)] and then performing a primary heat treatment on the steel sheet after the primary water cooling at 700 to 780°C for {2.4 × t + (10 to 40)} minutes [t: steel plate The method includes the steps of: performing a secondary heat treatment for a thickness (mm) of the steel sheet; performing a secondary water cooling; and tempering the secondary water cooled steel sheet.

[0036] First, a slab having the above-described composition is prepared. In the steelmaking step, the molten steel having the above-described composition can be produced into a slab through continuous casting. The composition and content of the slab have been described above, so a repeated description will be omitted.

[0037] The produced slab is then reheated. Reheating facilitates the subsequent hot rolling process and allows the slab to be homogenized. The slab reheating temperature may be 1000 to 1200°C. If the reheating temperature is less than 1000°C, it is difficult to achieve solid solution of solute atoms. On the other hand, if the reheating temperature exceeds 1200°C, the austenite grain size becomes too coarse, which impairs the physical properties of the steel, and this is not preferable.

[0038] The heated slab is then hot rolled to produce a hot rolled steel sheet. Specifically, the hot rolling is performed at a reduction rate of 5 to 30% per pass, and the rolling can be completed at a temperature of 780°C or higher.

[0039] During the hot rolling, if the reduction rate per pass is less than 5%, there is a problem of increased production costs due to reduced rolling productivity. On the other hand, if it exceeds 30%, it is not preferable because it may cause a load on the rolling mill and have a fatal adverse effect on the equipment. It is preferable to finish rolling at a temperature of 780°C or higher. Rolling to a temperature below 780°C is not preferable because it will cause a load on the rolling mill. The upper limit of the rolling finish temperature is not particularly limited, but it may be 900°C.

[0040] After hot rolling, the hot-rolled steel sheet can be air-cooled. In this case, the air-cooling method is not particularly limited, and the cooling can be carried out under conditions commonly used in the industry.

[0041] Thereafter, the air-cooled steel sheet can be subjected to the first heat treatment, specifically, at 800 to 880°C for {2.4 × t + (10 to 40)} minutes [t: steel plate The steel is then heated for a thickness of 1 / 4 mm, followed by a primary water cooling process. If the heat treatment temperature before water cooling is below 800°C, austenitization does not occur, making it difficult to achieve the desired strength and elongation. If it exceeds 880°C, the grain size becomes too coarse, hindering toughness.

[0042] During the primary heat treatment within the above-mentioned temperature range, if the holding time is less than {(2.4 × t) + 10} minutes, it is difficult to homogenize the structure, while if it exceeds {(2.4 × t) + 40} minutes, productivity is impaired, which is not preferable.

[0043] The primary water cooling is carried out at a temperature of 150°C or less. If the water cooling temperature exceeds 150°C, the strength of the steel plate may decrease.

[0044] Thereafter, the water-cooled steel sheet can be subjected to a secondary heat treatment, specifically, at 700-780°C for {2.4 × t + (10-40)} minutes [t: steel plate If the heat treatment temperature before water cooling is below 700°C, it becomes difficult to redissolve the solute elements, making it difficult to achieve the desired strength and elongation. On the other hand, if the temperature exceeds 780°C, grain growth occurs, which may impair low-temperature toughness.

[0045] During the secondary heat treatment within the above-mentioned temperature range, if the holding time is less than {(2.4 × t) + 10} minutes, it is difficult to homogenize the structure, while if it exceeds {(2.4 × t) + 40} minutes, productivity is impaired, which is not preferable.

[0046] The secondary water cooling is also carried out at a temperature of 150°C or less. If the water cooling temperature exceeds 150°C, the strength of the steel plate may decrease.

[0047] Next, the second water-cooled steel sheet can be tempered, specifically, in the temperature range of 600 to 750°C for {2.4 × t + (10 to 40)} minutes [t: steel plate If the tempering temperature is less than 600°C, it becomes difficult to form fine precipitates, making it difficult to achieve the desired strength. On the other hand, if the tempering temperature exceeds 750°C, the precipitates will grow, which may impair the strength and low-temperature toughness.

[0048] When tempering is performed in the above-mentioned temperature range, if the holding time is less than {(2.4 × t) + 10} minutes, it is difficult to homogenize the structure, while if it exceeds {(2.4 × t) + 40} minutes, productivity is impaired, which is not preferable.

[0049] Hereinafter, the present invention will be described in more detail with reference to examples, in which the present invention relates to a steel plate for a pressure vessel having excellent cryogenic toughness and a method for manufacturing the same. However, the following examples are merely a reference for explaining the present invention in detail, and the present invention is not limited thereto, and may be realized in various forms.

[0050] Furthermore, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In this application, the terms used in the description are merely for the purpose of effectively describing specific embodiments and are not intended to limit the present invention. Unless otherwise specified in the specification, the percentage unit of additives is weight percent, and 1 ppm is 0.0001 weight percent.

[0051] [Invention Examples 1 to 6 and Comparative Examples 1 to 8] Steel slabs having the alloy compositions and contents shown in Table 1 below were prepared, and then reheated at 1,100°C for 2 hours. The reheated steel sheets were then hot-rolled at a cumulative reduction of 30%, and the rolling was completed at the temperature shown in Table 2, followed by air-cooling at room temperature.

[0052] The air-cooled steel plates were subjected to primary heat treatment, secondary heat treatment, and tempering at the temperatures and times shown in Table 2 below to obtain steel plates for cryogenic pressure vessels. After the primary and secondary heat treatments, water cooling was performed at 150°C or less.

[0053] [Table 1]

[0054] [Table 2]

[0055] The steel plates manufactured above were subjected to tests for yield strength (YS, Yield Strength, MPa), tensile strength (TS, Tensile Strength, MPa), and elongation (EL, Elongation, %). Low-temperature toughness was evaluated by performing a Charpy impact test on a V-notched test piece at -195°C and measuring the Charpy impact energy (Ec, charpy impact energy, J) value. The impact and tensile tests were conducted in accordance with the standard ASTM A370 for the test piece, and the test methods were ASTM E23 and ASTM E8, respectively.

[0056] [Table 3]

[0057] As shown in Tables 1 to 3 above, in the case of Invention Examples 1 to 6, in which the steel composition and manufacturing process conditions satisfied the ranges of the present invention, the microstructure of the steel after tempering treatment contained an area fraction of 1.0 to 9.5% retained austenite (RO), and a three-phase mixed structure of 40 to 80% tempered bainite (TB) and the remainder tempered martensite (TM) was obtained, and the yield strength and tensile strength were higher by about 100 MPa compared to the comparative examples, and the elongation was also improved by 5% or more, and the cryogenic impact energy at -195°C was also increased by 150 J or more.

[0058] In contrast, when the primary heat treatment temperature or the secondary heat treatment temperature was changed, as shown in Table 3, it was found that the area fraction of the microstructure fell outside the range proposed in the present invention, and it was confirmed that this resulted in a decrease in strength or a decrease in elongation or low-temperature toughness properties.

[0059] The present invention has been described above through specific and limited examples, but these are provided to facilitate a more comprehensive understanding of the present invention. The present invention is not limited to the above examples, and various modifications and variations can be made from such descriptions by those skilled in the art to which the present invention pertains.

[0060] Therefore, the concept of the present invention should not be limited to the described embodiments, and it can be said that not only the scope of the claims described below, but also all modifications that are equivalent or similar to the scope of the claims fall within the scope of the concept of the present invention.

Claims

1. reheating a slab consisting of, by weight percent, 0.05 to 0.15% C, 0.20 to 0.35% Si, 0.5 to 1.5% Mn, 0.012% or less P, 0.015% or less S, 0.02 to 0.10% Al, 6.01 to 6.49% Ni, 0.2 to 0.4% Mo, 0.05 to 0.25% Cr, and the balance being Fe and unavoidable impurities; hot rolling the reheated slab to produce a steel plate and air cooling it; The air-cooled steel sheet is subjected to a primary heat treatment at 800 to 880°C for {2.4 x t + (10 to 40)} minutes (t: thickness of the steel sheet (mm)), followed by a primary water cooling; The primarily water-cooled steel sheet is subjected to a secondary heat treatment at 700 to 780°C for {2.4 x t + (10 to 40)} minutes (t: thickness of the steel sheet (mm)), followed by a secondary water cooling; and tempering the second water-cooled steel sheet, a microstructure of the steel plate for a cryogenic pressure vessel manufactured in the tempering step, which comprises, based on area fraction, a three-phase mixed structure of 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite.

2. The alloy consists, in weight percent, of C: 0.05 to 0.15%, Si: 0.20 to 0.35%, Mn: 0.5 to 1.5%, P: 0.012% or less, S: 0.015% or less, Al: 0.02 to 0.10%, Ni: 6.01 to 6.49%, Mo: 0.2 to 0.4%, Cr: 0.05 to 0.25%, and the balance being Fe and inevitable impurities; A steel plate for use in cryogenic pressure vessels, characterized in that the microstructure of the steel comprises a three-phase mixed structure of, based on area fraction, 1 to 9.5% retained austenite, 40 to 80% tempered bainite, and the remainder tempered martensite.

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