Steel plate for pressure vessel having excellent low temperature toughness and method for manufacturing the same

Abstract

The present invention relates to a method of manufacturing a steel plate for a cryogenic pressure vessel and a steel plate for a cryogenic pressure vessel manufactured thereby, the method including the steps of: reheating a steel billet including, in wt%, 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; hot rolling the reheated steel plate, and then performing air cooling; performing primary heat treatment on the air-cooled steel plate at 800 to 880°C for (2.4 x t + (10 to 40)) minutes (t: thickness of the steel billet (mm)), and then performing primary water cooling; performing secondary heat treatment on the primary water-cooled steel plate at 700 to 780°C for (2.4 x t + (10 to 40)) minutes (t: thickness of the steel billet (mm)), and then performing secondary water cooling; and tempering the secondary water-cooled steel plate.

Description

Technical Field

[0001] This disclosure relates to steel plates for pressure vessels with excellent low-temperature toughness and methods for manufacturing the same. Background Technology

[0002] Since high-strength, low-temperature plate steel needs to be able to be used as a low-temperature structural material during construction, it needs to have both high strength and low-temperature toughness.

[0003] High-strength hot-rolled steel produced by normalizing has a mixed microstructure of ferrite and pearlite, and examples of prior art for high-strength hot-rolled steel may include the invention described in Korean Patent Publication No. 2012-0011289.

[0004] Korean Patent Publication No. 2012-0011289 discloses a high-strength steel for 500 MPa grade LPG, which is composed of the following by weight percent: 0.08% to 0.15% C, 0.2% to 0.3% Si, 0.5% to 1.2% Mn, 0.01% to 0.02% P, 0.004% to 0.006% S, more than 0% and less than 0.01% Ti, 0.05% to 0.1% Mo, 3.0% to 5.0% Ni, and the balance Fe and other unavoidable impurities, wherein Ni and Mo are added to the steel composition.

[0005] However, since the invention described in Korean Patent Publication No. 2012-0011289 is steel manufactured by a typical normalizing process, there may be a problem that the low-temperature lateral expansion characteristics of the steel are still insufficient even with the addition of Ni.

[0006] Therefore, there is a need to develop steels with excellent low-temperature impact toughness and improved low-temperature lateral expansion properties.

[0007] Existing technical documents

[0008] (Patent Document 0001) Korean Patent Publication No. 2012-0011289 (February 7, 2012) Summary of the Invention

[0009] Technical issues

[0010] This disclosure provides a steel plate for cryogenic pressure vessels with high strength and excellent low-temperature toughness, and a method for manufacturing the same.

[0011] More specifically, this disclosure provides steel plates for cryogenic pressure vessels and methods for manufacturing the same, wherein the steel plates for cryogenic pressure vessels have strength and lateral expansion characteristics that allow for stable use at cryogenic temperatures of -150°C or lower while ensuring a tensile strength of 750 MPa.

[0012] The purpose of this disclosure is not limited to the purposes described above, and other unmentioned purposes will be clearly understood by those skilled in the art from the following description.

[0013] Technical solution

[0014] In one aspect of this disclosure, a method for manufacturing steel plates for cryogenic pressure vessels includes: reheating a steel billet comprising, by weight percent: 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; hot-rolling the reheated steel plate and then air-cooling it; and subjecting the air-cooled steel plate to a single heat treatment at 800°C to 880°C (2.4 × t + (10 to 40) minutes (t: billet thickness (mm)), then water-cooled once: the water-cooled steel plate is subjected to a second heat treatment at 700°C to 780°C (2.4 × t + (10 to 40)) minutes (t: billet thickness (mm)), then water-cooled once: and the water-cooled steel plate is tempered.

[0015] In another aspect of this disclosure, the steel plate for cryogenic pressure vessels comprises, by weight percent: 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, wherein the steel microstructure has a three-phase mixed structure: 1% to 9.5% retained austenite based on area fraction, 40% to 80% tempered bainite, and the balance being tempered martensite.

[0016] Beneficial effects

[0017] According to the method for manufacturing steel plates for cryogenic pressure vessels disclosed herein, a steel plate for cryogenic pressure vessels with a steel microstructure consisting of the following three-phase mixed structure can be manufactured by performing a process of heat treatment twice after hot rolling, in which the air-cooled steel plate is subjected to a temperature of 800°C to 880°C and a temperature of 700°C to 780°C: 1% to 9.5% of retained austenite, 40% to 80% of tempered bainite, and the balance being tempered martensite.

[0018] The steel plate for cryogenic pressure vessels can possess strength and lateral expansion characteristics that allow for stable use at temperatures of -150°C or lower. Specifically, the steel plate for cryogenic pressure vessels can have a yield strength of 610 MPa or greater and a tensile strength of 750 MPa or greater, as well as excellent cryogenic toughness characteristics with a Charpy impact energy of 190 J or greater at -195°C.

[0019] Specifically, the steel plate for the cryogenic pressure vessel is composed of a three-phase mixed structure and has excellent transverse expansion characteristics with an elongation of 30% or greater: 1% to 9.5% retained austenite, 40% to 80% tempered bainite, and the balance being tempered martensite. Detailed Implementation

[0020] The following describes in detail a pressure vessel steel plate with excellent low-temperature toughness and a method for manufacturing the same, according to the present disclosure. The embodiments provided below are given by way of example so that the spirit of the present disclosure can be fully conveyed to those skilled in the art. Therefore, the present disclosure is not limited to the embodiments provided below, but can be modified in many different forms. Unless otherwise defined, the technical and scientific terms used in this specification have their general meanings as understood by one of ordinary skill in the art to which this disclosure pertains, and descriptions of known functions and configurations that unnecessarily obscure the essence of the present disclosure will be omitted in the following description.

[0021] Throughout this specification, unless explicitly stated otherwise, “include” any element shall be understood to mean that other elements are included rather than excluded.

[0022] According to one aspect of this disclosure, a method for manufacturing steel plates for cryogenic pressure vessels includes: reheating a steel billet, said billet comprising, by weight percent: 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; hot-rolling the reheated steel plate and then air-cooling it; subjecting the air-cooled steel plate to a single heat treatment at 800°C to 880°C (2.4 × t + (10 to 40) minutes (t: billet thickness (mm)), then water-cooled once; the water-cooled steel plate is subjected to a second heat treatment at 700°C to 780°C (2.4 × t + (10 to 40)) minutes (t: billet thickness (mm)), then water-cooled once; and the water-cooled steel plate is tempered.

[0023] As described above, the method for manufacturing steel plates for cryogenic pressure vessels according to this disclosure involves heat-treating the air-cooled steel plate twice after hot rolling at a temperature of 800°C to 880°C and at a temperature of 700°C to 780°C. This process produces a steel plate for cryogenic pressure vessels with a three-phase mixed microstructure consisting of: 1% to 9.5% retained austenite based on an area fraction, 40% to 80% tempered bainite, and the balance being tempered martensite.

[0024] The steel plate for cryogenic pressure vessels can possess strength and lateral expansion characteristics that allow for stable use at temperatures of -150°C or lower. Specifically, the steel plate for cryogenic pressure vessels can have a yield strength of 610 MPa or greater and a tensile strength of 750 MPa or greater, as well as excellent cryogenic toughness characteristics with a Charpy impact energy of 190 J or greater at -195°C.

[0025] Specifically, the steel plate for the cryogenic pressure vessel is composed of a three-phase mixed structure and has excellent transverse expansion characteristics with an elongation of 30% or greater: 1% to 9.5% retained austenite, 40% to 80% tempered bainite, and the balance being tempered martensite.

[0026] The reasons for limiting the numerical values ​​of alloy component content in the examples of this disclosure will be described below. Unless otherwise stated, all units below are by weight%.

[0027] In a cryogenic pressure vessel steel plate according to one embodiment of this disclosure, the carbon (C) content can be from 0.05% to 0.15%. When the C content is less than 0.05%, the strength of the matrix itself decreases, while when the C content exceeds 0.15%, the weldability of the steel plate is significantly impaired. A more preferred lower limit is 0.07%, and a more preferred upper limit is 0.13%.

[0028] In a cryogenic pressure vessel steel sheet according to one embodiment of this disclosure, the silicon (Si) content can be from 0.20% to 0.35%. Si is added for deoxidation, solid solution strengthening, and impact transition temperature enhancement, and is preferably added at 0.20% or more to achieve these effects. However, when Si content exceeds 0.35%, weldability deteriorates and a severe oxide film forms on the surface of the steel sheet; therefore, it is preferable to limit the Si content to 0.20% to 0.35%. A more preferred lower limit is 0.23%, and a more preferred upper limit is 0.32%.

[0029] In a cryogenic pressure vessel steel plate according to one embodiment of this disclosure, the manganese (Mn) content can be from 0.5% to 1.5%. Mn, together with sulfur, forms elongated nonmetallic inclusions MnS, thereby reducing room temperature elongation and low-temperature toughness; therefore, it is preferred to control the Mn content to 1.5% or less. However, because of the nature of the components in this disclosure, it is difficult to ensure sufficient strength when Mn is less than 0.5%, it is preferred to limit the amount of Mn added to 0.5% to 1.5%. A more preferred lower limit is 0.52%, and a more preferred upper limit is 1.2%.

[0030] In a cryogenic pressure vessel steel plate according to one embodiment of this disclosure, the aluminum (Al) content can be from 0.02% to 0.10%. Along with Si, Al is one of the strong deoxidizers in the steelmaking process. When the Al content is less than 0.02%, the effect is not significant, and when Al is added at 0.10% or more, the manufacturing cost increases. Therefore, it is preferable to limit the Al content to 0.02% to 0.10%. A more preferred lower limit is 0.025%, and a more preferred upper limit is 0.09%.

[0031] In a cryogenic pressure vessel steel plate according to one example of this disclosure, phosphorus (P) is an element that impairs low-temperature toughness. However, removing phosphorus (P) from the steelmaking process would be too costly. Therefore, it is preferable to control phosphorus (P) to 0.012% or less.

[0032] In a cryogenic pressure vessel steel plate according to one example of this disclosure, sulfur (S), along with P, is also an element that adversely affects cryogenic toughness. However, like P, removing sulfur (S) during the steelmaking process requires excessively high costs, so it is preferable to control sulfur (S) to 0.015% or less.

[0033] In a cryogenic pressure vessel steel plate according to one embodiment of this disclosure, the nickel (Ni) content can be from 6.01% to 6.49%. Ni is the most effective element for improving low-temperature toughness. However, when Ni is added to less than 6.01%, the low-temperature toughness decreases, while when Ni is added to more than 6.49%, the manufacturing cost increases. Therefore, it is preferable to add Ni in the range of 6.01% to 6.49%. A more preferred lower limit is 6.08%, and a more preferred upper limit is 6.45%.

[0034] In a cryogenic pressure vessel steel plate according to one example of this disclosure, molybdenum (Mo) is a very important element for improving hardenability and strength. When molybdenum (Mo) is added at less than 0.2%, the desired effect may not be expected, and molybdenum is an expensive element. Therefore, it is preferable to limit the molybdenum (Mo) content to 0.2% to 0.4%. More preferably, the molybdenum (Mo) content may be 0.32% or less.

[0035] In a cryogenic pressure vessel steel plate according to one example of this disclosure, chromium (Cr) is an important element that ensures strength even at low temperatures and room temperature. Since adding less than 0.05% chromium (Cr) may not produce the desired effect, and chromium (Cr) is an expensive element, it is preferable to limit the chromium (Cr) content to 0.05% to 0.25%. A more preferred upper limit may be 0.22%.

[0036] The remainder of the composition is iron (Fe). However, due to the unavoidable contamination from raw materials or the surrounding environment during normal manufacturing processes, it may be impossible to eliminate these unintended impurities. Since these impurities are known to those skilled in the art in ordinary manufacturing processes, not all impurities are specifically mentioned in this specification.

[0037] On the other hand, as described above, the cryogenic pressure vessel steel sheet according to this disclosure can be heat-treated twice to obtain a steel microstructure having the following three-phase mixed structure: 1% to 9.5% retained austenite, 40% to 80% tempered bainite, and the remainder tempered martensite. Therefore, cryogenic pressure vessel steel sheets with excellent strength and cryogenic toughness properties can be ensured. On the other hand, when the area fraction of tempered bainite is less than 40%, the amount of tempered martensite becomes excessive, and the cryogenic toughness of the steel sheet may deteriorate, making it difficult to ensure an elongation of 30% or greater. Furthermore, when the area fraction of tempered bainite exceeds 80%, it may be difficult to ensure the target strength of the steel sheet. Moreover, when the area fraction of retained austenite is less than 1.0%, the cryogenic toughness properties are compromised, and it may be difficult to ensure an elongation of 30% or greater. Conversely, when the area fraction of retained austenite exceeds 9.5%, the strength decreases; therefore, it is preferable to limit the area fraction of retained austenite to the range of 1.0% to 9.5%.

[0038] In order to manufacture steel plates for cryogenic pressure vessels with a three-phase mixed microstructure that satisfies such an area fraction, it is particularly important to perform a heat treatment process twice, after hot rolling and before tempering.

[0039] As described above, the method for manufacturing steel plates for cryogenic pressure vessels includes: reheating a steel billet; hot rolling the reheated steel plate and then air cooling; subjecting the air-cooled steel plate to a first heat treatment at 800°C to 880°C for (2.4 × t + (10 to 40)) minutes (t: billet thickness (mm)), and then water cooling; subjecting the water-cooled steel plate to a second heat treatment at 700°C to 780°C for (2.4 × t + (10 to 40)) minutes (t: billet thickness (mm)), and then water cooling; and tempering the water-cooled steel plate.

[0040] First, prepare a steel billet that meets the above composition. The steel billet can be manufactured by continuous casting from molten steel whose composition has been adjusted to the above specifications during the steelmaking process. The composition and content of the steel billet have already been described above, therefore, a repetition of that description will be omitted.

[0041] The prepared steel billet is then reheated. Reheating facilitates subsequent hot rolling processes and homogenizes the billet. The reheating temperature can be between 1000°C and 1200°C. When the reheating temperature is below 1000°C, it is difficult to dissolve solute atoms; however, when the reheating temperature exceeds 1200°C, the austenite grain size becomes too coarse, which is undesirable as it impairs the physical properties of the steel.

[0042] The heated billet is then hot-rolled to produce hot-rolled steel sheet. Specifically, hot rolling can be performed at a reduction rate of 5% to 30% per pass, and the rolling process can be terminated at a temperature of 780°C or higher.

[0043] When the reduction rate per pass during hot rolling is less than 5%, there is a problem of increased manufacturing costs due to the reduced rolling productivity. On the other hand, a reduction rate exceeding 30% may load the rolling mill and have significant adverse effects on the equipment, which is not preferred. Rolling is preferably terminated at a temperature of 780°C or higher. Rolling to a temperature of 780°C or lower loads the rolling mill, which is not preferred. There is no particular upper limit to the rolling termination temperature, but it can be 900°C.

[0044] Hot-rolled steel sheets can be air-cooled after hot rolling. In this case, there are no particular restrictions on the air-cooling method, as long as it is carried out under the conditions used in this field.

[0045] Afterward, the air-cooled steel sheet can undergo a heat treatment, specifically heating at 800°C to 880°C for (2.4 × t + (10 to 40)) minutes (t: billet thickness (mm)), followed by a water cooling. When the heat treatment temperature before water cooling is below 800°C, it is difficult to ensure the target strength and elongation due to the lack of austenitization, while when the heat treatment temperature exceeds 880°C, the grain size is too coarse and toughness is compromised.

[0046] Within the above temperature range, when the holding time during a single heat treatment is less than {(2.4×t) + 10} minutes, it is difficult to homogenize the tissue. However, when the holding time exceeds {(2.4×t) + 40} minutes, productivity is impaired, which is not preferred.

[0047] On the other hand, water cooling at 150°C or lower may reduce the strength of the steel plate when the water cooling temperature exceeds 150°C.

[0048] Afterward, the water-cooled steel sheet can undergo a secondary heat treatment, specifically heating at 700°C to 780°C for (2.4 × t + (10 to 40)) minutes (t: billet thickness (mm)), followed by a second water cooling. When the heat treatment temperature before water cooling is below 700°C, it is difficult to redissolve the solid solution elements, thus making it difficult to ensure the target strength and elongation. However, when the temperature exceeds 780°C, there is a risk of grain growth occurring, which could impair low-temperature toughness.

[0049] Within the above temperature range, when the holding time during the secondary heat treatment is less than {(2.4×t) + 10} minutes, it is difficult to homogenize the tissue. However, when the holding time exceeds {(2.4×t) + 40} minutes, productivity is impaired, which is not preferred.

[0050] On the other hand, secondary water cooling is also performed at a temperature of 150°C or lower. When the water cooling temperature exceeds 150°C, the strength of the steel plate may decrease.

[0051] Next, the steel sheet that has undergone secondary water cooling can be tempered, specifically within a temperature range of 600°C to 750°C for {2.4 × t + (10 to 40)} minutes [t: billet thickness (mm)]. When the temperature during tempering is below 600°C, it is difficult to ensure the target strength due to the difficulty in precipitating fine precipitates. However, when the temperature exceeds 750°C, there is a risk that precipitates may grow, thereby impairing strength and low-temperature toughness.

[0052] Within the above temperature range, when the holding time during tempering is less than {(2.4×t) + 10} minutes, it is difficult to homogenize the tissue. However, when the holding time exceeds {(2.4×t) + 40} minutes, productivity is impaired, which is not preferred.

[0053] The following describes in more detail a pressure vessel steel plate with excellent low-temperature toughness according to embodiments of the present disclosure and a method for manufacturing the same. However, the following examples are merely reference examples for describing the present disclosure in detail, and the present disclosure is not limited thereto and can be implemented in various forms.

[0054] 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 pertains. The terminology used in this description is for the purpose of effectively describing particular embodiments only and is not intended to limit the invention. Additionally, % of additives not specifically described in the specification is by weight, and 1 ppm is 0.0001 wt%.

[0055] Invention Embodiments

[0056] [Examples 1 to 6 and Comparative Examples 1 to 8]

[0057] After preparing steel billets that meet the alloy composition and content shown in Table 1, these billets are reheated at 1100°C for 2 hours. After hot rolling the reheated steel sheets with a cumulative reduction rate of 30%, rolling is terminated at the temperatures shown in Table 2, and the sheets are air-cooled at room temperature.

[0058] The air-cooled plates are 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. In this case, water cooling is performed at 150°C or lower after the primary and secondary heat treatments.

[0059] [Table 1]

[0060]

[0061] [Table 2]

[0062]

[0063] The prepared steel plates were tested for yield strength (YS, MPa), tensile strength (TS, MPa), and elongation (EL, %). Low-temperature toughness was assessed by Charpy impact energy (Ec, J) values ​​from Charpy impact tests performed on specimens with V-notches at -195°C. Impact and tensile tests conformed to the standard ASTM A370 for the test specimens, and the test methods were performed according to ASTM E23 and ASTM E8, respectively.

[0064] [Table 3]

[0065]

[0066] As shown in Tables 1 to 3, in Invention Examples 1 to 6 where the steel composition and manufacturing process conditions meet the scope of this disclosure, it was found that the microstructure of the tempered steel can contain 1.0% to 9.5% of retained austenite (RO) in area fraction, and obtain a three-phase mixed structure of 40% to 80% tempered bainite (TB) and the balance being tempered martensite (TM). Therefore, the yield strength and tensile strength are about 100 MPa higher than those of the comparative example, the elongation is increased by more than 5%, and the low-temperature impact energy at -195°C is also increased by more than 150 J.

[0067] On the other hand, when the primary heat treatment temperature or the secondary heat treatment temperature is different, as shown in Table 3, it is found that the area fraction of the microstructure is outside the range proposed in this disclosure. Therefore, it is determined that the strength is reduced or the elongation or low-temperature toughness is reduced.

[0068] As described above, while this disclosure has been described by way of specific details such as the components and exemplary embodiments, these are provided only to aid in the overall understanding of this disclosure. Therefore, this disclosure is not limited to the exemplary embodiments. Various modifications and changes can be made by those skilled in the art based on this description.

[0069] Therefore, the spirit of this disclosure should not be limited to these exemplary embodiments, but rather the claims and all modifications equivalent to or related to the claims are intended to fall within the scope and spirit of this disclosure.

Claims

1. A method for manufacturing steel plates for cryogenic pressure vessels, comprising: The steel billet is reheated, and the steel billet contains, by weight percent: 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 is Fe and unavoidable impurities; The reheated steel billet is hot-rolled to produce steel plate, and then air-cooled. The air-cooled steel plate is subjected to a heat treatment at 800℃ to 880℃ for (2.4×t + (10 to 40)) minutes, where t is the thickness of the steel plate in mm, and then subjected to a water cooling. The steel plate that has been water-cooled once is subjected to a second heat treatment at 700℃ to 780℃ for (2.4×t + (10 to 40)) minutes, where t is the thickness of the steel plate in mm, and then subjected to a second water cooling. as well as The steel plate that has undergone secondary water cooling is tempered at a temperature range of 600℃ to 750℃ for (2.4×t + (10 to 40)) minutes, where t is the thickness of the steel plate in mm.

2. A steel plate for cryogenic pressure vessels, comprising, by weight percent: 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. The microstructure of the steel plate is a three-phase mixed structure: 1% to 9.5% of retained austenite, 40% to 80% of tempered bainite, and the balance being tempered martensite.

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