Pressure vessel steel plates

Abstract

The present invention provides a steel plate for pressure vessels that has excellent resistance to high-temperature post-weld heat treatment (PWHT), and whose mechanical properties do not deteriorate even after the PWHT process at high temperatures, and a method for manufacturing the same. [Solution] The alloy contains 0.10 to 0.16 weight percent C, 0.20 to 0.35 weight percent Si, 0.4 to 0.6 weight percent Mn, 7.5 to 8.5 weight percent Cr, 0.7 to 1.0 weight percent Mo, 0.005 to 0.05 weight percent Al, 0.015 weight percent or less P, 0.002 weight percent or less S, 0.001 to 0.025 weight percent Nb, 0.25 to 0.35 weight percent V, with the remainder being Fe and unavoidable impurities.

Description

[Technical field]

[0001] The present invention , pressure More particularly, the present invention relates to a steel plate for pressure vessels which has excellent tensile strength and low-temperature impact toughness even when PWHT is performed at high temperatures of 750 to 850°C, and a method for manufacturing the same. [Background technology]

[0002] When steel plates are welded, partial thermal expansion and contraction occur, forming residual stress inside the steel plate. The residual stress may cause deformation later, and may cause crack growth if a part of the base material breaks. Therefore, in order to stabilize the dimensions of the structure after welding and prevent deformation, it is necessary to perform a process to remove the residual stress. In order to remove residual stress inside the steel sheet, a post-weld heat treatment (PWHT) can be performed. However, the PWHT causes softening and growth of grain boundaries in the steel sheet and coarsening of carbides during the long heat treatment process, which causes problems in that mechanical properties are deteriorated. In particular, when the PWHT is performed at 700°C or higher, there is a problem that the deterioration of mechanical properties becomes more serious.

[0003] Among the materials used in parts, austenitic stainless steel has excellent elongation, which makes it advantageous for making complex shapes, and it is a type of steel that is used in a variety of fields due to its excellent work hardening ability. Such austenitic stainless steel can improve its strength by utilizing interstitial elements that hinder the movement of electric potential when stress is applied.

[0004] As a means for preventing the deterioration of mechanical properties after PWHT, Patent Document 1 discloses a steel sheet for medium to high temperatures containing C: 0.05-0.25%, Mn: 0.1-1.0%, Si: 0.1-0.8%, Cr: 1-3%, Cu: 0.05-0.3%, Mo: 0.5-1.5%, Ni: 0.05-0.5%, Al: 0.005-0.1%, and one or more of Ir: 0.005-0.10% and Rh: 0.005-0.10%, with the remainder being Fe and unavoidable impurities. However, this steel sheet has a problem that it is difficult to apply when the PWHT is at 700°C. In the following patent documents, no technology suitable for this situation could be found. Therefore, as steel materials become thicker and welding conditions become more severe, there is a demand for technology to manufacture steel plates that have excellent mechanical properties even after high-temperature PWHT. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Korean Patent Publication No. 10-2020-0064581 [Patent Document 2] JP 2015-018868 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above problems, and an object of the present invention is to provide a steel plate for pressure vessels having excellent resistance to high-temperature post-weld heat treatment (PWHT), which does not deteriorate its mechanical properties even after the PWHT process at high temperatures, and a manufacturing method thereof. [Means for solving the problem]

[0007] The present invention was made to achieve the above object. PressureThe steel plate for pressure vessels is characterized by containing 0.10 to 0.16 weight% C, 0.20 to 0.35 weight% Si, 0.4 to 0.6 weight% Mn, 7.5 to 8.5 weight% Cr, 0.7 to 1.0 weight% Mo, 0.005 to 0.05 weight% Al, 0.015 weight% or less P, 0.002 weight% or less S, 0.001 to 0.025 weight% Nb, 0.25 to 0.35 weight% V, with the remainder being Fe and unavoidable impurities.

[0008] The structure of the steel sheet is preferably a mixed structure of tempered martensite and tempered bainite. The area fraction of the tempered martensite is preferably 50 to 80%, with the remainder being composed of tempered bainite. The steel sheet can have a tensile strength of 650 MPa or more despite being subjected to a post-weld heat treatment (PWHT) at 750 to 850° C. for 10 to 50 hours. The above-mentioned steel plate for pressure vessels may have a Charpy impact energy (CVN@-30°C) value of 100J or more.

[0009] The present invention , pressure The method for manufacturing steel plate for pressure vessels is to use a steel sheet containing 0.10-0.16% by weight of C, 0.20-0.35% by weight of Si, 0.4-0.6% by weight of Mn, 7.5-8.5% by weight of Cr, 0.7-1.0% by weight of Mo, 0.005-0.05% by weight of Al, 0-0.015% by weight of P, 0.002% by weight or less of S, 0.001-0.025% by weight of Nb, 0.25-0.35% by weight of V, and the remainder being Fe and unavoidable impurities. The method includes the steps of: reheating the slab at 1,070 to 1,250°C; hot rolling the reheated slab at a reduction rate of 2.5 to 35% per rolling pass; performing a primary heat treatment on the hot rolled steel sheet by holding it at 1,020 to 1,070°C; cooling the primarily heat-treated steel sheet at a rate of 1 to 30°C / sec; and performing a secondary heat treatment on the cooled steel sheet by holding it at 820 to 845°C.

[0010] The time (T1) for the first heat treatment can be defined by the following Relation 1. [Equation 1] 1.3×t+10≦T1≦1.3×t+30 (In the above Relational Formula 1, T1 means the time (min) for performing the first heat treatment, and t means the thickness of the hot-rolled steel sheet.)

[0011] The time (T2) for the second heat treatment can be defined by the following Relation 2. [Equation 2] 1.6×t+10≦T2≦1.6×t+30 (In the above Relational Formula 2, T2 means the time (min) for performing the second heat treatment, and t means the thickness of the hot-rolled steel sheet.) It is preferable to carry out a post-weld heat treatment (PWHT) process in which the steel sheet that has been subjected to the secondary heat treatment is held at 750 to 850° C. for 10 to 50 hours. Effect of the Invention

[0012] According to the present invention, the steel plate for pressure vessels having the above-mentioned composition retains its mechanical properties even after a long-term PWHT process at 750 to 850°C. Thus, the present invention can provide a steel plate for pressure vessels having excellent high-temperature PWHT resistance and a manufacturing method thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The features of the embodiments of the present invention and the methods of achieving the same will become clearer by reference to the embodiments described in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms, and the embodiments are provided merely to complete the disclosure of the present invention and to allow those skilled in the art to fully understand the scope of the invention. The scope of the present invention is defined by the following claims. The same reference numerals refer to the same elements throughout the specification. In describing the embodiments of the present invention, if it is determined that a detailed description of a known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted. Furthermore, the terms used are defined in consideration of the functions in the embodiments of the present invention, and may vary depending on the intention or practice of a user or operator. Therefore, the definition should be determined based on the contents of the entire specification. Hereinafter, the embodiments of the present invention will be described in detail.

[0014] The present invention provides a steel plate for pressure vessels containing 7.5 to 8.5 wt % Cr, which has high resistance to post-weld heat treatment (PWHT) performed at high temperatures of 700° C. or higher. The PWHT is a heat treatment process for removing residual stress generated in the base material during welding or rolling, and is characterized by being performed at high temperature for a long time. Although the PWHT removes residual stress in the steel sheet, it may induce softening and growth of grain boundaries in the base material and coarsening of carbides, which may reduce the mechanical properties of the steel sheet. In order to prevent this, by appropriately controlling the alloy composition and manufacturing conditions of the steel plate and providing the microstructure of the steel plate as a mixed structure with tempered martensite as the main phase, it is possible to provide a steel plate for pressure vessels whose mechanical properties do not decrease even when subjected to high temperature and long-term PWHT.

[0015] The steel plate for pressure vessels having excellent high-temperature PWHT resistance according to the embodiment of the present invention contains 0.10 to 0.16 wt% C, 0.20 to 0.35 wt% Si, 0.4 to 0.6 wt% Mn, 7.5 to 8.5 wt% Cr, 0.7 to 1.0 wt% Mo, 0.005 to 0.05 wt% Al, 0 to 0.015 wt% P, 0.002 wt% or less S, 0.001 to 0.025 wt% Nb, and the remainder being Fe and unavoidable impurities. The composition range of the present invention will be described in detail below. In the following, the unit is % by weight unless otherwise specified.

[0016] C is added in an amount of 0.1 to 0.16 wt %. The C is an element that improves strength, and if its content is less than 0.1 wt%, the strength of the matrix itself decreases, while if its content exceeds 0.16 wt%, the strength increases excessively, resulting in a problem of decreased toughness. Therefore, the C content is preferably added in the range of 0.1 to 0.16 wt%, with a more preferred lower limit being 0.12 wt% and a more preferred upper limit being 0.15 wt%.

[0017] Silicon is added in an amount of 0.2 to 0.35 weight percent. The above-mentioned Si is an element effective for deoxidization and solid solution strengthening, and is an element that is accompanied by an increase in impact transition temperature. If the Si content is less than 0.2 wt%, the strength of the above-mentioned steel plate for pressure vessels is insufficient and it is difficult to obtain sufficient mechanical properties, and if the Si content exceeds 0.35 wt%, there is a problem that the weldability of the above-mentioned steel plate for pressure vessels is deteriorated, the workability is deteriorated, and the impact toughness is deteriorated. Therefore, the above-mentioned Si is preferably added in the range of 0.2 to 0.35 wt%, with a more preferable lower limit being 0.25 wt% and a more preferable upper limit being 0.32 wt%.

[0018] Mn is added in an amount of 0.4 to 0.6 wt %. The Mn forms nonmetallic inclusions MnS together with S, which will be described later. The nonmetallic inclusions MnS have the effect of preventing dislocation movement inside crystal grains and increasing the strength of the base material, but cause a decrease in elongation at room temperature and toughness at low temperatures. For example, if the Mn content exceeds 0.6 wt%, the MnS is excessively formed, significantly decreasing the elongation and low-temperature toughness, while if the Mn content is less than 0.4 wt%, the amount of MnS formed is insufficient, making it difficult to ensure appropriate strength. For these reasons, the Mn content is preferably added in the range of 0.4 to 0.6 wt%, with a more preferred lower limit being 0.5 wt% and a more preferred upper limit being 0.58 wt%.

[0019] The Cr is added in an amount of 7.5 to 8.5% by weight. The Cr content increases the hardenability and forms a low-temperature transformation structure, thereby increasing the yield and tensile strength, and slows down the decomposition rate of cementite during tempering after quenching and PWHT, thereby preventing a decrease in strength. Furthermore, a tempered martensite structure is formed in the center of the steel sheet, thereby enhancing low-temperature strength. For these reasons, it is preferable to add 7.5 wt% or more of the Cr content. However, if the Cr content exceeds 8.5 wt%, the Cr-Rich M alloy, which is coarse in size, may be formed. 23 There is a risk that C6-type carbides will precipitate inside the tempered martensite structure. These carbides will significantly reduce the impact toughness of the steel plate and cause brittle fracture. In addition, if the Cr content is increased, problems will arise such as an increase in manufacturing costs and a decrease in weldability. For this reason, the Cr content is preferably added in an amount of 7.5 to 8.5 wt%, with a more preferred lower limit being 7.8 wt% and a more preferred upper limit being 8.3 wt%.

[0020] Mo is added in an amount of 0.7 to 1.0 wt%. Like Cr, Mo can increase the high temperature strength of the base material. It can also prevent cracks from occurring in the steel plate for pressure vessels due to sulfides. For this reason, Mo is preferably added in an amount of 0.7 wt% or more. However, since Mo is relatively expensive compared to other additive elements, if Mo exceeds 1.0 wt%, there is a risk that the production cost will increase excessively and the marketability will decrease. Therefore, Mo is preferably added in the range of 0.7 to 1.0 wt%, and the more preferable lower limit is 0.8 wt%.

[0021] Al is added in an amount of 0.005 to 0.05% by weight. The Al, together with the Si, is one of the powerful deoxidizers in the steelmaking process. The deoxidizer plays a role in injecting oxygen into the base metal and inducing it to be discharged in the form of CO. For this reason, if the Al content is less than 0.005 wt%, the oxygen in the base metal increases, and the quality of the steel sheet may deteriorate. On the other hand, if the Al content exceeds 0.05 wt%, the deoxidizing effect is more than necessary, and there is a possibility that the manufacturing cost increases and the marketability decreases. Therefore, the Al is preferably added in the range of 0.005 to 0.05 wt%, with a more preferable lower limit being 0.02 wt% and a more preferable upper limit being 0.04 wt%.

[0022] P is added in amounts up to 0.015 wt %. The P reduces the low-temperature toughness of the steel plate for pressure vessels and is the main cause of temper brittleness due to segregation at grain boundaries. In theory, it is advantageous to control the P content as low as possible to approach 0 wt%, but since P is an element that is inevitably contained in the manufacturing process, it is difficult to reduce the P content and the additional process increases production costs, so it is preferable to set an upper limit and control it. Therefore, it is recommended that the P content be controlled to 0.015 wt% or less.

[0023] S is added in an amount of 0.002% by weight or less. Like P, S is an element that reduces low-temperature toughness and forms MnS inclusions in the steel plate for pressure vessels, which reduces the toughness of the steel plate for pressure vessels. Like P, it is advantageous to control the S content as low as possible to approach 0% by weight, but considering the cost and time consumed for this, it is preferable to control the S content by setting an upper limit. Therefore, it is preferable to control S to 0.002% by weight or less.

[0024] Nb is added in an amount of 0.001 to 0.025% by weight. The Nb is an element effective in preventing softening of the matrix that forms fine carbides or nitrides in the steel plate for pressure vessels. For this reason, the Nb is preferably added in an amount of 0.001% by weight or more. However, if the Nb content exceeds 0.025% by weight, the cost of the steel plate increases and the marketability may decrease. Therefore, the Nb is preferably added in the range of 0.001 to 0.025% by weight, with a more preferable lower limit being 0.01% by weight and a more preferable upper limit being 0.023% by weight.

[0025] V is added at 0.25 to 0.35 wt %. Like Nb, V can easily form fine carbides and nitrides, and is an effective element for preventing softening of the matrix. For this reason, V is preferably added in an amount of 0.25% by weight or more. However, if V exceeds 0.35% by weight, the cost of the steel sheet increases and marketability may decrease. Therefore, V is preferably added in the range of 0.25 to 0.35% by weight, with a more preferred lower limit being 0.28% and a more preferred upper limit being 0.32% by weight.

[0026] The remaining components excluding the above-mentioned components are provided as Fe. However, in a normal manufacturing process, there is a possibility that unintended impurities are inevitably mixed in from the raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to any engineer in a normal manufacturing process, the contents of all of them will not be specifically mentioned in this specification. The composition, which is one feature of the present invention, has been described above. In the following, the microstructure, which is another feature of the present invention, will be described.

[0027] In the steel plate for pressure vessels having excellent high temperature PWHT resistance according to the embodiment of the present invention, the microstructure in the center of the steel plate may be a mixed structure of tempered martensite and tempered bainite, and more preferably, the area fraction of the tempered martensite is 50% or more, and the remaining portion is a mixed structure of tempered bainite. The tempered martensite structure means a martensite structure in which residual stress is relieved through a secondary heat treatment process described below, and has the effect of complementing brittleness while maintaining the strength of a normal martensite structure. For this reason, in order to manufacture a 650 MPa class steel plate as the object of the present invention, it is preferable that the area fraction of the tempered martensite structure is 50% or more.

[0028] However, when the area fraction of the tempered martensite structure in the steel plate exceeds 80%, the tempered martensite structure has coarse Cr-Rich M at the grain boundaries. 23 C6-type carbides may precipitate, resulting in a decrease in toughness. For this reason, the area fraction of the tempered martensite structure is preferably 50 to 80%. Meanwhile, the tempered bainite has a lower strength than the tempered martensite structure, but is relatively excellent in toughness and has a high impact absorption energy. As a result, the tempered bainite can complement the toughness of the steel plate for pressure vessels. For this reason, the steel plate for pressure vessels is preferably provided as a mixed structure of the tempered martensite structure and the tempered bainite, and more preferably, the area fraction of the tempered martensite is 50 to 80%, and the area fraction of the tempered bainite is 20 to 50%.

[0029] The steel sheet having the above-mentioned composition and microstructure can effectively maintain a tensile strength of 650 MPa or more even if it is subjected to additional heat treatment at a high temperature range of 750 to 850°C for a maximum of 50 hours after welding. In addition, the steel plate having the above composition and microstructure can have excellent low-temperature toughness even after the above PWHT, specifically, a Charpy impact energy value of 100 J or more at -30°C. It can be seen that the steel plate for pressure vessels manufactured according to the embodiments of the present invention can maintain excellent tensile strength and low temperature toughness even when subjected to PWHT at high temperatures.

[0030] In addition to the above description of the steel plate for pressure vessels having excellent high-temperature PWHT resistance according to the present invention, a method for manufacturing the steel plate for pressure vessels having excellent high-temperature PWHT resistance according to the present invention will be described below. According to the embodiment, the steel plate for pressure vessels having excellent high temperature PWHT resistance includes at least one of the following steps: reheating a slab having the above-mentioned composition at 1,070 to 1,250°C; hot rolling the reheated slab at a rolling reduction of 2.5 to 35% per rolling pass; performing a primary heat treatment on the hot rolled steel plate at 1,020 to 1,070°C; cooling the primary heat treated steel plate at 1 to 30°C; and performing a secondary heat treatment on the cooled steel plate at 820 to 845°C.

[0031] First, in the present invention, a step of reheating the slab having the above compositional components can be performed. The reheating is preferably performed at 1,070 to 1,250°C. If the reheating temperature is less than 1,070°C, the solute atoms do not dissolve as intended, making it difficult to ensure strength, and if the reheating temperature exceeds 1,250°C, the austenite phase in the steel material grows excessively, which may reduce the mechanical properties of the steel sheet. Therefore, the reheating temperature is preferably 1,070 to 1,250°C, with a more preferred lower limit of 1,100°C and a more preferred upper limit of 1,170°C. The reheated slab can then be hot rolled to produce steel plate.

[0032] According to the embodiment, the hot rolling can be performed in the recrystallization region, which is a temperature range higher than the recrystallization finish temperature. The hot rolling is preferably performed with a reduction rate of 2.5 to 35% per rolling pass. If the reduction rate is less than 2.5%, the reduction amount is insufficient, and the tempered martensite and tempered bainite structures formed in the cooling process described below become coarse, which may reduce the strength of the steel sheet. On the other hand, if the reduction rate exceeds 35%, the load on the rolling machine becomes heavy, which may reduce the productivity. Therefore, the reduction rate per rolling pass is preferably controlled to 2.5 to 35%, with a more preferable lower limit being 5% and a more preferable upper limit being 25%.

[0033] The hot-rolled steel sheet may be subjected to a first heat treatment process. The first heat treatment process refers to a heat treatment in which the steel sheet is held at 1,020 to 1,070° C. for a time (T1) that satisfies the following Relational Expression 1: [Equation 1] 1.3×t+10≦T1≦1.3×t+30 (In the above Relational Formula 1, T1 means the time (min) for performing the first heat treatment, and t means the thickness of the hot-rolled steel sheet.)

[0034] According to the examples, if the temperature of the first heat treatment is less than 1,020°C or if T1 is less than 1.3×t1+10 minutes, the structure in the steel sheet may not be sufficiently homogenized. This may cause segregation in the steel sheet. In addition, it may become difficult for solute elements that were dissolved in the steel sheet to re-dissolve, which may cause a decrease in the mechanical properties of the steel sheet.

[0035] Conversely, if the primary heat treatment temperature exceeds 1,070° C. or T1 exceeds 13×t1+30 minutes, crystal grains in the steel sheet may grow, resulting in a decrease in the strength of the steel sheet. Thereafter, a cooling step may be performed to cool the steel sheet that has been subjected to the primary heat treatment. Specifically, the cooling step may involve cooling the steel sheet that has been subjected to the primary heat treatment to 20 to 40°C at a rate of 1 to 30°C / sec, and may be performed by water cooling (DQ) treatment. If the cooling rate is less than 1°C / sec, the ferrite in the steel sheet may not transform into martensite, and the area fraction of the tempered martensite structure in the steel sheet may decrease. Furthermore, the tempered martensite and tempered bainite structures may become coarse. This may cause a decrease in the strength of the steel sheet. If the cooling rate exceeds 30°C / sec, additional equipment is required to improve the cooling rate, and a large amount of cooling water is required. Therefore, there is a risk that the manufacturing cost of the steel sheet may increase. Therefore, the cooling rate is preferably 1 to 30°C / sec, with a more preferable lower limit being 1.5°C / sec and a more preferable upper limit being 25°C / sec.

[0036] The steel sheet manufactured by carrying out the above-mentioned primary heat treatment and cooling processes is required to have a tensile strength of 650 MPa or more, and at the same time, a Charpy impact energy value of 100 J or more at -30°C. In order to achieve these conditions, it is preferable to carry out the secondary heat treatment and PWHT processes. The secondary heat treatment process refers to a heat treatment in which the steel sheet is held at 820 to 845° C. for a time (T2) that satisfies the following Relational Expression 2, or in other words, can be defined as a tempering heat treatment. [Equation 2] 1.6×t+10≦T2≦1.6×t+30 (In the above Relational Formula 2, T2 means the time (min) for performing the second heat treatment, and t means the thickness of the hot-rolled steel sheet.)

[0037] As described above, the secondary heat treatment step is preferably performed at 820 to 845°C for 1.6×t+10 to 1.6×t+30 minutes. This is because if the secondary heat treatment step is performed at less than 820°C or for less than 1.6×t+10 minutes, the dislocation recovery effect is reduced, the toughness of the steel sheet is reduced, and it becomes difficult to obtain a tempered martensite structure. On the other hand, if the secondary heat treatment step is performed at more than 845°C or the heat treatment time exceeds 1.6×t+30 minutes, precipitates grow excessively, causing an overaging phenomenon, which may result in a decrease in strength.

[0038] According to the embodiment, a PWHT process can be further performed after the second heat treatment process. The PWHT process is a process of performing heat treatment for a long time in a high temperature environment to remove residual stress inside the steel sheet, as described above, and specifically means a process of holding the steel sheet after the second heat treatment at 750 to 850°C for 10 to 50 hours. If the PWHT process temperature is less than 750°C or the PHWT process time is less than 10 hours, annealing is insufficient and residual stress may remain in the steel sheet. This may cause deformation of the steel sheet and shorten its life. Conversely, if the PWHT process temperature exceeds 850°C or the PWHT process is performed for more than 50 minutes, excessive heat energy may be injected into the steel sheet. This may promote recrystallization of the steel sheet and reduce the tensile strength to less than 650 MPa. For these reasons, the PWHT step is preferably carried out at 750 to 850°C for 10 to 50 hours, with a more preferred lower limit temperature being 780°C, a more preferred upper limit temperature being 820°C, and a more preferred lower limit time being 20 hours. The present invention will now be described in more detail with reference to examples. EXAMPLES

[0039] The following alloy slabs were reheated at 1,120°C for 300 minutes, and then hot rolled in the recrystallized region with a rolling reduction of 15% per rolling pass to produce steel plates.

[0040] [Table 1]

[0041] The steel sheets were air-cooled to room temperature of 25°C, and then heated to 1,050°C and subjected to a primary heat treatment process with the time adjusted according to the thickness of each steel sheet. Then, the steel was water-cooled to 25°C based on the temperature at the center of the steel material. The thickness of each steel sheet, the primary heat treatment holding time, and the cooling time are shown in Table 2 below. Finally, the steel sheets that had been subjected to the first heat treatment and cooling steps were subjected to a secondary heat treatment under the conditions shown in Table 2 below, and then further to a PWHT step.

[0042] [Table 2]

[0043] The tempered martensite fraction (%) and mechanical properties of the steel sheets manufactured according to Table 2 above were measured and are shown in Table 3 below. The mechanical properties measured were yield strength (YS), tensile strength (TS), elongation (EL) and low-temperature toughness (J). The low-temperature toughness was evaluated based on the Charpy impact energy (CVN@-30°C) value obtained by performing a Charpy impact test on a test piece having a V notch at -30°C.

[0044] [Table 3]

[0045] Referring to Tables 1 to 3, it can be seen that Examples 1 to 9, which simultaneously satisfy the alloy composition and manufacturing conditions proposed by the present invention, have an area fraction of tempered martensite of 50% or more, and therefore have a high yield strength of 650 MPa or more, more preferably 656 MPa or more, even after a PWHT process is performed for 50 hours. At the same time, it can be seen that the Charpy impact energy value at -30°C is 100 J or more, more preferably 215 J or more, and thus has excellent low-temperature toughness. Specifically, even if the PWHT process is extended from 20 hours to 50 hours, the yield strength (YS) is reduced by 0.5 to 3%, and the tensile strength (TS) is reduced by about 1 to 4.5%. This is because, as described above, the tempered martensite structure in the steel sheet is formed in 50% or more of the area fraction, which compensates for the reduction in strength caused by the softening of grain boundaries and the coarsening of carbides after PWHT.

[0046] In contrast, in the above Comparative Examples 1 to 6, it can be seen that the mechanical properties are significantly reduced when the PWHT process time is increased from 20 hours to 50 hours. Specifically, in the Comparative Examples 1 to 3 in which the Comparative Steel A containing 2.29 wt% of Cr was heat-treated in the same manner as in the Examples, when the PWHT process time is increased by 30 hours from 20 hours to 50 hours, the yield strength (YS) and tensile strength (TS) are both reduced by 7 to 10%, and the Charpy impact energy is reduced by 45 to 55%. In the Comparative Examples 4 to 6 produced from the Comparative Steel B containing 5.21 wt% of Cr, the yield strength is reduced by 15 to 20%, the tensile strength is reduced by 10 to 15%, and the Charpy impact energy is reduced by 45 to 55%. Unlike Examples 1 to 9, the reason why the mechanical properties rapidly deteriorate in Comparative Examples 1 to 6 is that when the Cr content in the steel plate is less than 7.5 wt %, the austenite region increases and the retained austenite is generated, which relatively reduces the fraction of the tempered martensite and tempered bainite structures.

[0047] On the other hand, when the Cr content is 7.5 wt% or more, the austenite region is reduced, and no unnecessary austenite structure remains even after the cooling step, and all of the austenite is transformed into the martensite or bainite. As a result, it can be confirmed that in Examples 1 to 9 containing 7.5 wt% or more of Cr, the tempered martensite is 50% or more, and in Comparative Examples 1 to 6 containing less than 7.5 wt% of Cr, the tempered martensite is less than 25%. Furthermore, the residual austenite structure has coarse grains and low stability, which increases the brittleness of the steel sheet. For this reason, it was confirmed that the low temperature toughness of Comparative Examples 1 to 6 was also reduced.

[0048] Specifically, in the above-mentioned Examples 1 to 9, even after a PWHT process at 800°C for 50 hours, a tensile strength of 650 MPa or more and a low-temperature toughness of 200 J or more are maintained, whereas in the above-mentioned Comparative Examples 1 to 6, the area fraction of the tempered martensite structure formed inside the steel plate is less than 20%, so that the strength of the base material is relatively low. This is because, in the above-mentioned Examples 1 to 9, the martensite structure having relatively excellent strength is formed at an area fraction of 50% or more, and the strength is maintained even after heat treatment, whereas in the above-mentioned Comparative Examples 1 to 6, the martensite structure is insufficient, so that the decrease in strength caused by the softening of the grain boundaries and the coarsening of carbides after the high-temperature PWHT cannot be compensated for.

[0049] In contrast, in Comparative Examples 7 to 9, which contain 9.54% by weight of Cr, the yield strength is excellent at an average of 715 MPa, but the elongation is extremely low at an average of 15.3%, and the low-temperature toughness is very low at an average of 44 J. This is because the tempered bainite structure is formed in an excessively small amount, making it difficult to complement the toughness of the steel plate. Furthermore, coarse Cr-Rich M phases are formed at the tempered martensite grain boundaries. 23 This is because C6-type carbides precipitated, greatly increasing the brittleness of the steel sheet. For this reason, when considering both the strength and toughness of the steel sheet, it is preferable that the tempered martensite structure is formed in an area fraction of 50 to 80%.

[0050] On the other hand, the above Comparative Examples 10 to 17 were produced by changing the heat treatment time using the invention steel A that satisfies the alloy composition proposed by the present invention. As a result, it can be confirmed that the mechanical properties were decreased compared to the above Production Examples 1 to 3. Specifically, in Comparative Examples 10 to 11, in which the primary heat treatment was performed for 50 minutes less than T1, the yield strength (YS) was 427 MPa on average, and the tensile strength (TS) was 512 MPa, which was confirmed to be 15 to 25% lower than in Examples 1 to 3. Furthermore, the Charpy impact energy was also 35 to 45% lower than in Examples 1 to 3. This is because, as described above, the primary heat treatment time was insufficient, and therefore the stress inside the steel material was not sufficiently relieved, resulting in the formation of unstable martensite and bainite structures.

[0051] In addition, in Comparative Examples 12 to 13, in which the primary heat treatment was carried out for 50 minutes longer than T1, the yield strength (YS) was 4005 MPa on average, and the tensile strength (TS) was 529 MPa, which was reduced by 15 to 25% compared to Examples 1 to 3. Furthermore, the Charpy impact energy was also 141.5 J on average, which was reduced by 15 to 25% compared to Examples 1 to 3. This proves that the crystal grains in the steel sheet grew, causing a decrease in the strength of the steel sheet. In addition, in Comparative Examples 14 to 15, in which the secondary heat treatment was performed for 50 minutes less than T2, the yield strength (YS) was 4175 MPa on average, and the tensile strength (TS) was 487.5 MPa on average, which was reduced by 15 to 25% compared to Examples 1 to 3. Furthermore, the Charpy impact energy was also 161 J on average, which was reduced by 25 to 35% compared to Examples 1 to 3.

[0052] Finally, in Comparative Examples 16 to 17, in which the secondary heat treatment was performed for 50 minutes longer than T2, the yield strength (YS) was 404 MPa on average, and the tensile strength (TS) was 543.5 MPa on average, which was reduced by 20 to 30% compared to Examples 1 to 3. It can also be confirmed that the Charpy impact energy was 172.5 J on average, which was reduced by 25 to 35% compared to Examples 1 to 3. This confirms that if the secondary heat treatment time is insufficient or exceeds the time, the mechanical properties of yield strength, tensile strength, elongation, and low temperature toughness are reduced.

[0053] In the above description, various embodiments of the present invention have been presented and described, however, the present invention is not necessarily limited thereto, and a person having ordinary knowledge in the technical field to which the present invention pertains can easily understand that various substitutions, modifications and alterations are possible within the scope that does not deviate from the technical concept of the present invention.

Claims

1. A steel plate containing, by weight, C: 0.10-0.16%, Si: 0.20-0.35%, Mn: 0.4-0.6%, Cr: 7.5-8.5%, Mo: 0.7-1.0%, Al: 0.005-0.05%, P: 0.015% or less, S: 0.002% or less, Nb: 0.001-0.025%, V: 0.25-0.35%, and the remainder being Fe and unavoidable impurities, The structure of the steel plate is a mixed structure of tempered martensite and tempered bainite, The steel plate for pressure vessels is characterized in that the area fraction of the tempered martensite is 50 to 80%, with the remainder being composed of tempered bainite.

2. 2. The steel plate for pressure vessels according to claim 1, wherein the steel plate has a tensile strength of 650 MPa or more despite a post-weld heat treatment (PWHT) at 750 to 850° C. for 10 to 50 hours.

3. A steel plate for pressure vessels as described in claim 2, characterized in that even after the PWHT process is performed for 50 hours, the steel plate has a Charpy impact energy (CVN @ -30°C) value of 100 J or more.

4. reheating a slab containing, by weight, 0.10-0.16% C, 0.20-0.35% Si, 0.4-0.6% Mn, 7.5-8.5% Cr, 0.7-1.0% Mo, 0.005-0.05% Al, 0-0.015% P, 0.002% or less S, 0.001-0.025% Nb, 0.25-0.35% V, with the remainder being Fe and unavoidable impurities, at 1,070-1,250°C; hot rolling the reheated slab at a reduction of 2.5 to 35% per rolling pass; A step of subjecting the hot-rolled steel sheet to a primary heat treatment at 1,020 to 1,070°C; A step of cooling the steel plate subjected to the first heat treatment at 1 to 30 ° C. / sec; and subjecting the cooled steel plate to a secondary heat treatment at 820 to 845°C.

4. The method for manufacturing a steel plate for a pressure vessel according to claim 1, wherein the time (T2) for the secondary heat treatment is defined by the following Relation 2. [Relationship 2] 1.6×t+10≦T2≦1.6×t+30 (In the above Relational Formula 2, T2 means the time (min) for performing the secondary heat treatment, and t means the thickness of the hot-rolled steel sheet.)

5. 5. The method for manufacturing a steel plate for a pressure vessel according to claim 4, further comprising a post-weld heat treatment (PWHT) step at 750 to 850° C. for 10 to 50 hours after the secondary heat treatment step.

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