Chromium steel plate with excellent creep strength and high-temperature ductility and its manufacturing method

Abstract

The present invention relates to (Fe, Cr) 23 The present invention provides a chromium steel sheet with excellent creep strength and high-temperature ductility, which completely suppresses the formation of coarse precipitates such as C6 carbides and reduces crack sensitivity due to excellent high-temperature ductility as well as creep strength, thereby expanding the range of material applications, and a manufacturing method thereof. [Solution] The present invention provides a ferrous alloy containing, by weight, C: 0.04 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.1 to 0.6%, S: 0.01% or less (excluding 0%), P: 0.03% or less (excluding 0%), Cr: 1.9 to 2.6%, Mo: 0.05 to 1.5%, W: 1.4 to 2.0%, V: 0.4 to 1.0%, Ni: 0.4% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), Ti: 0.10% or less (excluding 0%). %, excluding Cr, N: 0.015% or less (excluding 0%), Al: 0.06% or less (excluding 0%), B: 0.007% or less (excluding 0%), with the balance being Fe and inevitable impurities, satisfying Relational Formula 1, the LMP value defined by Relational Formula 2 is 20,000 or more at an applied stress of 200 MPa and 21,000 or more at an applied stress of 125 MPa, and the area reduction rate at high temperature fracture is 20% or more.

Description

[Technical Field]

[0001] The present invention relates to a chromium steel sheet having excellent creep strength and high-temperature ductility and a manufacturing method thereof, and more particularly to a chromium steel sheet having excellent creep strength and high-temperature ductility due to the alloying of elements, in which only fine carbonitrides are precipitated inside a martensite / bainite microstructure, which is a constituent phase of a steel material, and at grain boundaries, thereby reducing crack sensitivity and providing excellent high-temperature ductility. [Background technology]

[0002] The thermal / nuclear power generation and oil refinery / refining industries must consider the construction of environmentally friendly facilities and the efficient use of energy. First, to increase power generation efficiency, there is a need to increase the temperature and pressure of steam supplied to turbines. This means that it is important to improve the heat resistance of boiler materials so that steam with even higher temperatures and pressures can be generated. In addition, in the oil refinery / refining industry, stricter environmental regulations have recently required higher efficiency, and the application of steel with excellent high-temperature properties to facilities is being considered.

[0003] Among steels used at high temperatures, austenitic stainless steels, which contain large amounts of expensive alloying elements, have poor physical properties such as low thermal conductivity and a high coefficient of thermal expansion, making it difficult to manufacture large parts and limiting their use. In contrast, chromium steels are more commonly used in areas requiring excellent physical properties such as creep strength, weldability, corrosion resistance, and oxidation resistance. In the case of nuclear power plants, safety is ensured by substituting chromium steels, which can ensure long-term integrity, instead of austenitic stainless steels to prevent swelling caused by neutron irradiation.

[0004] To maintain the high-temperature creep strength of heat-resistant chromium steel for a long period of time, solid solution strengthening and precipitation strengthening methods are used. For this purpose, vanadium, niobium, and titanium, which are solid solution strengthening elements and M(C,N) carbonitride (M = metal element, C = carbon, N = nitrogen) forming elements, are mainly alloyed. At the same time, by extremely reducing the carbon content to 0.002 wt%, it becomes thermodynamically unstable and easily coarsens, reducing creep properties (Fe, Cr). 23 Heat-resistant steels that suppress the formation of C6 carbides and precipitate fine carbonitrides to significantly improve creep properties have also been proposed. However, commercial mass production of heat-resistant steels with reduced carbon content is nearly impossible. Furthermore, it is important to reduce the formation of surface cracks that can occur during continuous casting or welding in the steel production process. This effectively reduces the frequency of cracks as the material's high-temperature ductility increases. Therefore, it is essential to develop alloy designs and manufacturing methods that allow for the development of steels with excellent creep strength while fully considering high-temperature ductility. Summary of the Invention [Problem to be solved by the invention]

[0005] Unlike the prior art, the present invention utilizes alloy design and heat treatment to achieve the desired results without excessively reducing the carbon content (Fe, Cr). 23 The present invention aims to provide a chromium steel sheet having excellent creep strength and high-temperature ductility, which not only has excellent creep strength by completely suppressing the formation of coarse precipitates such as C6 carbide and forming only fine carbonitrides, but also reduces crack sensitivity due to its excellent high-temperature ductility, thereby expanding the range of application of the material, and a method for manufacturing the same.

[0006] However, the problems that the present invention aims to solve are not limited to the problems mentioned above, and further problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0007] The chromium steel plate of the present invention having excellent creep strength and high-temperature ductility is In weight percent, C: 0.04 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.1 to 0.6%, S: 0.01% or less (excluding 0%), P: 0.03% or less (excluding 0%), Cr: 1.9 to 2.6%, Mo: 0.05 to 1.5%, W: 1.4 to 2.0%, V: 0.4 to 1.0%, Ni: 0.4% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), Ti: 0.10% or less (excluding 0%), The material is characterized by containing N: 0.015% or less (excluding 0%), Al: 0.06% or less (excluding 0%), B: 0.007% or less (excluding 0%), with the balance being Fe and inevitable impurities, satisfying the following relational expression 1, having an LMP value defined by relational expression 2 of 20,000 or more at an applied stress of 200 MPa and 21,000 or more at an applied stress of 125 MPa, and having an area reduction rate at high temperature fracture of 20% or more. [Equation 1] 0.3≦(V-10SUM)≦1 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb. [Equation 2] LMP = T × (20 + log(tr)) where T is the absolute temperature in Kelvin and tr is the time to rupture in hours.

[0008] The steel sheet is characterized by having a chemical composition that satisfies the following relational expression 3, and at the same time having an LMP value of 20,000 or more as defined by the above relational expression 2 at an acting stress of 250 MPa, and an area reduction rate at high temperature fracture of 40% or more. [Equation 3] 35≦|(V-10SUM)×(Mo-10SUM)×(Ni-10SUM)×10 3 |≦600 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb.

[0009] The steel sheet is characterized by having a microstructure containing tempered martensite / bainite.

[0010] The microstructure of the above steel sheet contains (Fe, Cr) 23 Precipitates containing C6 with a diameter of 200 nm or more are 1 / μm 2 It is characterized by being present in the following number range.

[0011] The microstructure of the above steel sheet contains precipitates with a diameter of 20 nm or less at 20 counts / μm. 2 It is characterized in that it exists in the above number range.

[0012] The precipitates having a diameter of 20 nm or less are characterized by being (V, Mo, Nb, Ti)(C, N).

[0013] Further, the method for producing a chromium steel sheet having excellent creep strength and high-temperature ductility according to the present invention comprises the steps of: A process of producing a hot-rolled steel sheet by hot-rolling the steel slab having the above-mentioned composition at a finish rolling temperature of Ar3 or higher, and then cooling the hot-rolled steel sheet; a step of reheating the cooled hot-rolled steel sheet at a temperature in the range of 1000 to 1100°C for at least 30 minutes to austenitize it; A step of normalizing or quenching the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 0.1 ° C. / s or more; The method is characterized by including a step of tempering the cooled hot-rolled steel sheet at a temperature in the range of 700 to 800°C for at least 30 minutes. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a chromium steel plate having excellent creep strength and high-temperature ductility, with an LMP value of 20,000 or more at an acting stress of 200 MPa and 21,000 or more at an acting stress of 125 MPa, which can be quenched and tempered to have an excellent creep life at high temperatures, a creep life longer than that of ASTM A213 92 grade steel containing a large amount of chromium (9 wt %), and an excellent reduction of area at high temperature fracture of 20% or more.

[0015] Furthermore, it is possible to provide an excellent chromium steel sheet having an LMP value of 20,000 or more at an applied stress of 250 MPa, a creep life of 1,000 hours or more at a temperature of 600°C, and an area reduction rate at high temperature fracture of 40% or more. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram showing a comparison of creep test results for steel types 1 to 6 used in the experiments of the present invention and conventional materials. [Figure 2] 1 is a graph showing creep deformation rates over time measured using an extensometer for steels 3-1 and 4-1 used in the experiments of the present invention and steel 1 as a comparative example under conditions of 600°C / 125 MPa. [Figure 3] 1 is a scanning electron microscope (SEM) photograph of steel type 1 and steel type 4-1 steel plates used in an experiment of the present invention. [Figure 4] 1 is a transmission electron microscope (TEM) photograph of steel type 1 and 4-1 steel sheets used in an experiment of the present invention. [Figure 5] 1 shows a photograph of a test piece of steel type 1 used in an experiment of the present invention, fractured under conditions of 600° C. / 200 MPa, and photographs of test pieces of steel types 2 to 6 fractured under conditions of 600° C. / 275 MPa. [Figure 6] 1 is a graph summarizing the area ratios of test specimens of steel types 1 to 6 that were used in experiments of the present invention and ultimately fractured. DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention will be described in detail below. Conventional heat-resistant chromium steels mainly use molybdenum and vanadium, niobium, and titanium, which are M(C,N) carbonitride (M = metal element, C = carbon, N = nitrogen) forming elements, as alloying elements. However, these heat-resistant chromium steels are thermodynamically unstable and easily coarsen, resulting in poor creep properties (Fe, Cr).23 The formation of C6 carbides was unavoidable, making it difficult to ensure good creep properties. In order to solve these problems of the prior art, the inventors of the present invention have conducted extensive research and experiments. As a result, they have confirmed that a heat-resistant chromium steel having excellent creep properties and high-temperature ductility can be obtained by optimizing the amounts of vanadium, molybdenum, and nickel added to a heat-resistant chromium alloy containing 1.9 to 2.6% Cr, and simultaneously optimizing the austenitizing temperature, cooling rate, tempering temperature, and other processes. Based on this, the present inventors present the present invention.

[0018] The chromium steel plate of the present invention, which is excellent in creep strength and high-temperature ductility, contains, by weight, C: 0.04 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.1 to 0.6%, S: 0.01% or less (excluding 0%), P: 0.03% or less (excluding 0%), Cr: 1.9 to 2.6%, Mo: 0.05 to 1.5%, W: 1.4 to 2.0%, V: 0.4 to 1.0%, Ni: 0.4% or less (excluding 0%), Nb: 0.10% or less (excluding 0%), Ti: 0.10% or less (excluding 0%). ), N: 0.015% or less (excluding 0%), Al: 0.06% or less (excluding 0%), B: 0.007% or less (excluding 0%), with the balance being Fe and inevitable impurities, and the chromium steel plate satisfies the following relational formula 1, has an LMP value defined by relational formula 2 of 20,000 or more at an acting stress of 200 MPa and 21,000 or more at an acting stress of 125 MPa, and has an area reduction rate at high temperature fracture of 20% or more, and is excellent in creep strength and high-temperature ductility. [Equation 1] 0.3≦(V-10SUM)≦1 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb. [Equation 2] LMP = T × (20 + log(tr)) where T is the absolute temperature in Kelvin and tr is the time to rupture in hours.

[0019] The reasons for limiting the components of the chromium steel sheet having excellent creep strength and high-temperature ductility will be explained below. Here, "%" indicates "% by weight" unless otherwise specified. ·Carbon (C): 0.04~0.15% Carbon is an austenite stabilizing element that can adjust the Ae3 temperature and martensite formation initiation temperature depending on its content. It is also an interstitial element that imparts asymmetric strain to the lattice structure of the martensite phase, making it highly effective in ensuring high strength. However, if the carbon content in steel exceeds 0.15%, excessive carbide formation occurs, significantly reducing weldability. Therefore, in the present invention, it is preferable to limit the carbon content to the range of 0.04 to 0.15%.

[0020] Silicon (Si): 0.5% or less (excluding 0%) Silicon is added not only for solid solution strengthening but also as a deoxidizer during casting. However, for the chromium steel sheet having excellent creep strength and high-temperature ductility according to one embodiment of the present invention, the formation of beneficial carbides such as fine carbides is essential, while silicon plays a role in suppressing the formation of carbides. Therefore, in the present invention, it is preferable to control the silicon content to 0.5% or less.

[0021] Manganese (Mn): 0.1-0.6% Manganese is an austenite-stabilizing element that significantly increases the hardening ability of steel, allowing hard phases such as martensite to form. It also reacts with sulfur to precipitate MnS, which is beneficial in preventing hot cracking due to sulfur segregation. However, as the manganese content increases, the austenite stability increases excessively. Therefore, in the present invention, the manganese content is preferably limited to the range of 0.1 to 0.6%, and more preferably to the range of 0.4 to 0.6%.

[0022] Sulfur (S): 0.010% or less (excluding 0%) Sulfur is an impurity element, and if its content exceeds 0.010%, the ductility and weldability of the steel will decrease. Therefore, it is preferable to limit the sulfur content to 0.010% or less.

[0023] Phosphorus (P): 0.03% or less (excluding 0%) Phosphorus is an element that has a solid solution strengthening effect, but like sulfur, it is an impurity element, and if its content exceeds 0.03%, it causes embrittlement in the steel and reduces weldability. Therefore, it is preferable to limit the phosphorus content to 0.03% or less.

[0024] Chromium (Cr): 1.9~2.6% Chromium is a ferrite-stabilizing element that increases hardening ability. Its amount controls the Ae3 temperature and the temperature range in which delta ferrite forms. Chromium also reacts with oxygen to form a dense and stable protective film of Cr2O3, improving high-temperature oxidation resistance and corrosion resistance, while also broadening the temperature range in which delta ferrite forms. During the casting process of steel with a high chromium content, delta ferrite may form, and this may remain after heat treatment, adversely affecting the properties of the steel. Therefore, in the present invention, the chromium content is preferably limited to a range of 1.9 to 2.6%, and more preferably to a range of 2.1 to 2.5%.

[0025] Molybdenum (Mo): 0.05-1.5% Molybdenum increases hardenability, effectively preventing the formation of ferrite and pearlite structures, which significantly reduces matrix strength. It also increases high-temperature creep life through strong solid solution strengthening. Molybdenum acts as a metal element that forms M(C,N) carbonitrides, stabilizing the carbonitrides and significantly reducing the rate of coarsening. Furthermore, the present invention has confirmed that molybdenum, as an element that strengthens grain boundaries, significantly contributes to increasing the high-temperature ductility of materials. While molybdenum should be added at least 0.05%, excessive molybdenum addition significantly increases manufacturing costs, so its addition is preferably limited to 1.5%, and more preferably to the range of 0.2 to 1.4%.

[0026] Tungsten (W): 1.4-2.0% Tungsten increases high-temperature creep life through its solid-solution strengthening effect. It also stabilizes carbonitrides by acting as a metal element, significantly slowing their coarsening rate. Meanwhile, increased tungsten content broadens the temperature range for delta ferrite formation, leading to the formation of delta ferrite during the steel casting process. Residual delta ferrite after heat treatment adversely affects creep properties. Therefore, the tungsten content is preferably limited to a range of 1.4 to 2.0%, and more preferably to a range of 1.5 to 1.8%.

[0027] Vanadium (V): 0.4-1.0% Vanadium is one of the elements that increases the hardening ability and forms M(C,N) carbonitrides, but as the vanadium content increases, (Fe, Cr) 23 The driving force for C6 carbide formation becomes smaller, resulting in (Fe, Cr) 23 The formation of C6 carbides can be completely suppressed. In steels with a chromium content of 1.9-2.6%, a tungsten content of 1.4-2.0%, and a molybdenum content of 0.05-1.5% (Fe, Cr) 23 To suppress the formation of C6 carbides, a vanadium alloy containing 0.4% or more is necessary. However, if the vanadium content exceeds 1.0%, there is a problem that production of the material becomes difficult. Therefore, the vanadium content is preferably limited to the range of 0.40 to 1.0%, and more preferably to the range of 0.5 to 0.9%.

[0028] Nickel (Ni): 0.4% or less (excluding 0%) Nickel is an element that improves the toughness of steel and is added to increase the strength of steel without deteriorating its low-temperature toughness. It also increases the hardening ability of steel when nickel is added, effectively preventing the problem of a significant decrease in matrix strength due to the formation of ferrite and pearlite structures. Furthermore, as an element that strengthens grain boundaries, it significantly contributes to increasing the high-temperature ductility of materials. However, if nickel is added in amounts exceeding 0.4%, the price of the material will increase. Therefore, it is preferable to limit the nickel content to 0.4% or less.

[0029] Niobium (Nb): 0.10% or less (excluding 0%) Niobium is one of the elements that form M(C,N) carbonitrides. It also dissolves in the slab when it is reheated, suppressing the growth of austenite grains during hot rolling, and then precipitates to improve the strength of the steel. However, excessive niobium content (over 0.10%) can reduce weldability and cause grains to become unnecessarily fine. Therefore, it is preferable to limit the niobium content to 0.10% or less.

[0030] Titanium (Ti): 0.10% or less (excluding 0%) Titanium is also an effective element for suppressing the growth of austenite grains in the form of TiN, but when titanium is added in excess of 0.10%, coarse Ti-based precipitates are formed, making the material difficult to weld. Therefore, it is preferable to limit the titanium content to 0.10% or less.

[0031] Nitrogen (N): 0.015% or less (excluding 0%) Since it is difficult to completely remove nitrogen from steel industrially, the upper limit is set at 0.015%, which is the allowable range in the manufacturing process. Nitrogen is known as an austenite stabilizing element, and when it forms M(C,N) carbonitrides rather than simple MC carbides, it significantly increases high-temperature stability and effectively increases the creep strength of steel. However, if it exceeds 0.015%, it combines with boron to form BN, increasing the risk of defects. Therefore, it is preferable to limit the nitrogen content to 0.015% or less.

[0032] Aluminum (Al): 0.06% or less (excluding 0%) Aluminum expands the ferrite region and is added as a deoxidizer during casting. In the case of chromium steel, many other ferrite-stabilizing elements are alloyed, and if the aluminum content increases, the Ae3 temperature can rise excessively. Also, in the current composition, if the aluminum content exceeds 0.06%, a large amount of oxide-based inclusions is formed, which impairs the material's physical properties. Therefore, it is preferable to limit the aluminum content to 0.06% or less.

[0033] Boron (B): 0.007% or less (excluding 0%) Boron is a ferrite stabilizing element, and even small amounts contribute significantly to increasing hardenability. It also easily segregates at grain boundaries, strengthening them. However, if added in excess of 0.007%, it can form BN, which can adversely affect the mechanical properties of the material. Therefore, it is preferable to limit the boron content to 0.007% or less.

[0034] In addition to these, if the balance includes Fe and unavoidable impurities, the impurities include, for example, Cu, Co, La, Y, Ce, Zr, Ta, Hf, Re, Pt, Ir, Pd, Sb, etc. These impurity elements are sometimes unavoidably mixed in from raw materials or the surrounding environment during normal manufacturing processes, and therefore cannot be eliminated.

[0035] The steel sheet of the present invention preferably has a chemical composition that satisfies the following relational expression 1. [Equation 1] 0.3≦(V-10SUM)≦1 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb. That is, the steel of the present invention not only needs to satisfy the condition of V: ​​0.4 to 1.0%, but also needs to be controlled so that impurity elements that may inhibit the beneficial effects of vanadium are not contained in the steel of the present invention. Specifically, after applying a weighting value by multiplying the above-defined "SUM" by the number 10, it has been confirmed that the effects of vanadium described in the present invention can be obtained when the value obtained by subtracting 10SUM from the vanadium content (wt%) in the steel is 0.4% to 1.0%, and the present technical configuration is presented.

[0036] On the other hand, in the present invention, copper (Cu), an element that constitutes "SUM," is likely to have a negative effect on the surface sporadic cracking of chromium steel. Furthermore, cobalt (Co) reduces hardening ability, so if it is present in steel, a bainite / martensite structure may not be obtained when a hot-rolled steel sheet that has been austenitized by reheating is normalized or quenched at a cooling rate of 0.1°C / s or more and then cooled to room temperature. Among the remaining impurities, if rare earth elements, which are very expensive, are present in the steel grade, the price may increase significantly and the mechanical properties may be deteriorated. Therefore, the SUM is the sum of the weight percent of alloying elements that are preferably not present in the steel grade of the present invention.

[0037] In the present invention, a steel plate satisfying Relational Formula 1 has an LMP (Larson-Miller Parameter) value defined by Relational Formula 2 of 20,000 or more at an applied stress of 200 MPa and 21,000 or more at an applied stress of 125 MPa, and has an area reduction rate at high temperature fracture of 20% or more. [Equation 2] LMP = T × (20 + log(tr)) where T is the absolute temperature in Kelvin and tr is the time to rupture in hours.

[0038] Moreover, it is preferable that the steel sheet has a chemical composition that satisfies Relational Formula 3. [Equation 3] 35≦|(V-10SUM)×(Mo-10SUM)×(Ni-10SUM)×10 3 |≦600 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb. In the present invention, a steel sheet satisfying relational expression 3 has an LMP value defined by relational expression 2 of 20,000 or more at an acting stress of 250 MPa, and an area reduction rate at break at high temperature of 40% or more.

[0039] In order to provide a chromium steel sheet according to the present invention that has an LMP value of 20,000 or more at an acting stress of 250 MPa, as defined by Relation 2, and that has excellent creep strength and high-temperature ductility, with a reduction in area at high-temperature fracture of 40% or more, it is preferable to appropriately control the contents of vanadium, molybdenum, and nickel in the steel. Therefore, it is necessary to avoid the inclusion of impurity elements that may impair the beneficial effects of the addition of these elements in the steel of the present invention, and from this perspective, the above-mentioned Relation 3 has been derived.

[0040] The microstructure and precipitates of the chromium steel sheet of the present invention, which has excellent creep strength and high-temperature ductility, will be described in detail below. First, the steel sheet of the present invention contains a tempered martensite / bainite structure as its base microstructure. The microstructure of the steel sheet of the present invention contains (Fe, Cr) 23 Precipitates containing C6 with a diameter of 200 nm or more are 1 / μm 2 If the number of precipitates with a diameter of 200 nm or more is 1 / μm 2 If it exceeds this value, coarse carbides may result in a deterioration in creep properties.

[0041] On the other hand, the microstructure of the steel sheet of the present invention has precipitates with a diameter of 20 nm or less at a density of 20 particles / μm. 2 It is preferable that the number of precipitates with a diameter of 20 nm or less is 20 / μm. 2 If the thickness is less than 1 / 2 mm, the distance between fine carbonitrides becomes too large, and therefore the displacement and subgrain movement at high temperatures cannot be effectively prevented, and the creep property may not be significantly improved. Precipitates with a diameter of 20 nm or less contain (V, Mo, Nb, Ti)(C, N).

[0042] Next, a method for producing the precipitation hardening chromium steel sheet of the present invention, which is excellent in creep strength and high-temperature ductility, will be described. The method for producing a precipitation hardened chromium-molybdenum steel sheet excellent in creep strength and high-temperature ductility of the present invention includes the steps of: hot rolling a steel slab having the above-described composition at a finish rolling temperature of Ar3 or higher to produce a hot-rolled steel sheet, followed by cooling; reheating the cooled hot-rolled steel sheet in a temperature range of 1000 to 1100°C for at least 30 minutes to austenitize it; normalizing or quenching the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 0.1°C / s or more; and tempering the cooled hot-rolled steel sheet in a temperature range of 700 to 800°C for at least 30 minutes.

[0043] First, a steel slab having the above-mentioned composition is hot-rolled to a finish rolling temperature of Ar3 or higher to obtain a hot-rolled steel sheet. The reason for performing hot rolling in the austenite single phase region is to increase the uniformity of the structure. Then, the produced hot-rolled steel sheet is cooled to room temperature. Furthermore, the cooled hot-rolled steel sheet is reheated to austenitize it, preferably at a reheating temperature range of 1000 to 1100°C for at least 30 minutes. If the reheating temperature is less than 1000°C, it is difficult to completely redissolve unnecessary carbides formed during the cooling process after hot rolling, whereas if the reheating temperature exceeds 1100°C, the crystal grains may become coarse, resulting in poor properties. The reheating time is preferably at least 30 minutes. If the reheating time is less than 30 minutes, it is difficult to completely redissolve unnecessary carbides formed during the cooling process after hot rolling.

[0044] The hot-rolled steel sheet, which has been austenitized by reheating, is normalized or quenched at a cooling rate of 0.1°C / s or more to room temperature, and then cooled to room temperature to obtain a bainite / martensite structure. Care must be taken to prevent the formation of ferrite and pearlite structures during the cooling of the matrix structure, which would significantly reduce the strength of the matrix. Since the steel of the present invention contains elements such as V, Mo, and Ni, which have high hardenability, ferrite and pearlite structures are not formed if the steel is normalized or quenched at a cooling rate of 0.1°C / s or more. Therefore, it is preferable to control the upper limit of the cooling rate to 50°C / s. The normalized or quenched hot-rolled steel sheet is tempered, preferably at a tempering temperature of 700 to 800°C for at least 30 minutes, followed by air cooling. If the tempering temperature is less than 700°C, the low temperature may not induce the precipitation of fine carbonitrides in time. On the other hand, if the tempering temperature exceeds 800°C, tempering may cause the material to soften, significantly reducing the creep life. If the tempering time is less than 30 minutes, the desired precipitates may not form. [Example]

[0045] The present invention will be described in detail below with reference to examples. (Example) A hot-rolled steel sheet having the alloy composition shown in Table 1 and a thickness of 12 mm was prepared. The hot-rolled steel sheet was then reheated at various temperatures within the range of 1000 to 1100°C for at least 30 minutes, normalized or quenched, and cooled to room temperature. The cooled steel sheet was then tempered at various temperatures within the range of 700 to 800°C for at least 30 minutes, and then air-cooled to room temperature to produce a steel sheet. Meanwhile, steel type 1 in Table 1 has a general ASTM A213 23 grade steel composition, and the remaining steel types all satisfy the steel compositional composition of the present invention. Specifically, steel types 2 to 4 have chemical compositions that satisfy relational formula 1 but not relational formula 3, and steel types 5 and 6 have chemical compositions that simultaneously satisfy relational formulas 1 and 3.

[0046] For the alloy steels manufactured as described above, creep test specimens with a gauge length of 15 mm and a gauge diameter of 6 mm were prepared in the hot rolling direction according to the ASTM E139 standard. The high temperature creep life of these test specimens was evaluated using an ATS 2320 creep testing machine, and the results are shown in Figure 1. For comparison, the creep results for ASTM A213 grades 23, 91, and 92 steel provided by the Japan Institute for Materials Science (NIMS) are also shown in Figure 1. The creep deformation rates of steel types 1, 3-1, and 4-1 were also measured using an extensometer, and the results are shown in Figure 2.

[0047] The microstructure of the manufactured alloy steel specimens was observed using a scanning electron microscope (SEM), and the results are shown in Figure 3. The distribution of precipitates was accurately observed using a transmission electron microscope (TEM) and energy spectroscopy, and the results are shown in Figure 4. Additionally, reduction in area (RA) was used as a measure of whether a steel exhibited ductile rupture when it finally creep-ruptured at high temperatures. If a creep test specimen with an initial gauge diameter of R0 (6 mm) has a surface diameter of R where creep rupture occurred at high temperatures, the reduction in area is [(RO-R) / RO] x 100. The microstructure, creep test conditions (temperature and stress), rupture time, and reduction in area of ​​the steels are shown in Table 2 below. Figure 5 shows photographs of test specimens that allow intuitive comparison of the reduction in area of ​​the actual ruptured material. The sulfur content of all steels in Table 1 is 30 ppm or less, the boron content is 70 ppm or less (except for 0%), and the remainder consists of Fe and unavoidable impurities.

[0048] [Table 1]

[0049] *In Table 1, N stands for normalizing, Q for quenching, and T for tempering, and the numbers before the letters indicate the temperature at which the heat treatment was performed. The normalizing / quenching and tempering heat treatment times were at least 30 minutes. A* indicates the value calculated using Relation 1, and B* indicates the value calculated using Relation 3. On the other hand, the "SUM" content of impurity elements used in the calculation of Relational Formulas 1 and 2 is in weight percent. For steel type 1, it is Cu (0.004%), Co (0.003%), and the sum of other rare earth elements (0.003%). For steel type 2, it is Cu (0.002%), Co (0.004%), and the sum of other rare earth elements (0.004%). For steel type 3, it is Cu (0.003%), Co (0.02%), and the sum of other rare earth elements (0.004%). The composition is the sum of rare earth elements (0.007%), in the case of steel type 4, Cu (0.005%), Co (0.01%), and the sum of other rare earth elements (0.01%), in the case of steel type 5, Cu (0.015%), Co (0.01%), and the sum of other rare earth elements (0.01%), and in the case of steel type 6, Cu (0.01%), Co (0.015%), and the sum of other rare earth elements (0.01%).

[0050] [Table 2] TIFF0007734653000003.tif242164TIFF0007734653000004.tif101164

[0051] As shown in Tables 1 and 2 and Figure 1, the chromium steel sheets of the present invention have a creep life that is superior to that of ASTM A213 Grades 91 and 92, which contain 9% by weight of chromium, when compared with the results provided by NIMS. It can also be seen that Steels 2 to 6, which satisfy the steel compositional components of the present invention, have significantly superior creep properties compared to Steel 1, which does not. In particular, Steels 5 and 6 have an even longer creep life than Steels 2 to 4. Specifically, they exhibit excellent creep deformation suppression ability at a temperature of 600°C and an applied stress of 250 MPa, and can withstand high temperatures and applied stresses even after 1,000 hours.

[0052] Figure 2 shows the creep deformation rate over time measured for steels 1, 3-1, and 4-1 at a temperature of 600°C and an applied stress of 125 MPa. In the case of steel 1, a comparative example, creep deformation occurred quickly, ultimately resulting in creep rupture at 6,427 hours. However, steels 3-1 and 4-1, which are inventive examples, demonstrate a higher creep deformation suppression ability than steel 1, and are able to withstand high temperatures and applied stresses even after tens of thousands of hours.

[0053] Figure 3 is a scanning electron microscope photograph showing the microstructure of steel types 1 and 4-1 steel sheets that were reheated at 1000°C for 30 minutes, normalized, cooled to room temperature, and then tempered at 700°C for 30 minutes. Figure 4 is a transmission electron microscope photograph showing the precipitate distribution of steel types 1 and 4-1 steel sheets. As an example of the invention, steel grade 4-1 exhibits only fine carbonitride precipitation within all grains and along subgrain boundaries, and it can be seen from Table 2 that these carbonitrides not only effectively prevent dislocation migration at high temperatures, but also effectively prevent the migration of subgrains in steel grades with martensite / bainite, ensuring stability and thereby significantly improving creep properties compared to conventional chromium steels. In other words, it can be seen that precipitation of only fine carbonitrides is extremely effective in extending creep life in all steel grades containing martensite and bainite, which are microstructures with subgrains. Furthermore, it is expected that the creep strength of steel types 5 and 6 will increase not only due to the effect of fine carbonitrides but also due to the additional solid solution strengthening effect of molybdenum. In contrast, steel type 1 contains coarse (Fe, Cr) 23 It can be seen that the creep properties of steels 2 to 6 are not good due to the formation of C6 carbide.

[0054] In the case of high-temperature ductility, which can determine the probability of surface cracks occurring during continuous casting or welding (increasing high-temperature ductility reduces the probability of surface cracks occurring), as shown in Table 2 and Figures 5 and 6, increasing the content of vanadium, nickel, and molybdenum increases the area reduction rate and high-temperature ductility. Vanadium forms coarse grains at the grain boundaries (Fe, Cr). 23 By preventing the formation of C6 carbides, steels 2-1 to 4-4 of the present invention satisfied Relation 1, resulting in reductions of area of ​​20% or more. Steels 5-1 to 6-4 of the present invention had chemical compositions that simultaneously satisfied Relation 1 and Relation 3, resulting in reductions of area of ​​40% or more, demonstrating significantly higher ductility than other steels. As a result, it was confirmed that the steels manufactured by the proposed heat treatment method in the present invention, which suppresses the formation of coarse carbides, introduces fine carbonitrides, and uses additional solid solution elements such as nickel and molybdenum, exhibit excellent high-temperature creep strength and high-temperature ductility.

[0055] The present invention is not limited to the above-described implementation examples and embodiments, and can be implemented in various different forms, and a person skilled in the art to which the present invention pertains can understand that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, the implementation examples and embodiments described above are illustrative in all respects and not limiting.

Claims

1. In weight percent, C: 0.04 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.1 to 0.6%, S: 0.01% or less (excluding 0%), P: 0.03% or less (excluding 0%), Cr: 1.9 to 2.6%, Mo: 0.05 to 1.5%, W: 1.4 to 2.0%, V: 0.4 to 1.0%, Ni: 0.4% or less (excluding 0%), Nb: 0.10% or less (excluding 0%) 1. A chromium steel plate having excellent creep strength and high-temperature ductility, comprising: 0.10% or less (excluding 0%) of Ti, 0.015% or less (excluding 0%) of N, 0.06% or less (excluding 0%) of Al, 0.007% or less (excluding 0%) of B, and the balance consisting of Fe and inevitable impurities; 2. A chromium steel plate having excellent creep strength and high-temperature ductility, comprising: 0.10% or less (excluding 0%) of Ti, 0.015% or less (excluding 0%) of N, 0.06% or less (excluding 0%) of Al, 0.007% or less (excluding 0%) of B, and the balance consisting of Fe and inevitable impurities; 3. A chromium steel plate having excellent creep strength and high-temperature ductility, comprising: 0.10% or less (excluding 0%) of Ti, 0.015% or less (excluding 0%) of N, 0.015% or less (excluding 0%) of N, 0.06% or less (excluding 0%) of Al, 0.007% or less (excluding 0%) of B, and the balance consisting of Fe and inevitable impurities; 4. A chromium steel plate having excellent creep strength and high-temperature ductility, comprising: 0.10% or less (excluding 0%) of Ti, 0.10% or less (excluding 0%) of N, 0.015% or less (excluding 0%) of N, 0.06% or less (excluding 0%) of Al, 0.007% or less (excluding 0%) of B, and the balance consisting of Fe and inevitable impurities; 5. A chromium steel plate having excellent creep strength and high-temperature ductility, comprising: 0.10% or less (excluding 0%) of Ti, 0.10% or less (excluding 0%) of N, 0.015% or less (excluding 0%) of N, 0 Here, the reduction in area is a value calculated by [(R0-R) / R0] x 100, where R is the diameter of the surface where a creep test specimen having an initial gauge diameter R0 (6 mm) creep ruptures in a 600°C high-temperature creep test under an applied stress of 125 to 275 MPa. [Relationship 1] 0.3≦(V-10SUM)≦1 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb. [Relationship 2] LMP=T×(20+log(tr)) where T is the absolute temperature in Kelvin and tr is the time to rupture in hours.

2. 2. The chromium steel plate according to claim 1, wherein the steel plate further has a chemical composition that satisfies the following relational expression 3, an LMP value defined by the relational expression 2 at an acting stress of 250 MPa is 20,000 or more, and a reduction in area at high temperature fracture is 40% or more. [Relationship 3] 35≦|(V-10SUM)×(M / -10SUM)×(Ni-10SUM)×10 ３ |≦600 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb.

3. 2. The chromium steel plate according to claim 1, wherein the steel plate has a microstructure containing tempered martensite / bainite.

4. The microstructure of the steel sheet contains (Fe, Cr) ２３ C ６ Precipitates with a diameter of 200 nm or more, including ２ 2. The chromium steel plate having excellent creep strength and high-temperature ductility according to claim 1, characterized in that the following number ranges are present:

5. 2. The chromium steel plate according to claim 1, wherein the microstructure of the steel plate contains (V, Mo, Nb, Ti)(C, N) precipitates with a diameter of 20 nm or less.

6. In weight percent, C: 0.04 to 0.15%, Si: 0.5% or less (excluding 0%), Mn: 0.1 to 0.6%, S: 0.01% or less (excluding 0%), P: 0.03% or less (excluding 0%), Cr: 1.9 to 2.6%, Mo: 0.05 to 1.5%, W: 1.4 to 2.0%, V: 0.4 to 1.0%, Ni: 0.4% or less (excluding 0%), Nb: 0.10% or less (excluding 0%) a process of producing a hot-rolled steel sheet by hot-rolling a steel slab having a composition that satisfies the following relational expression 1, comprising: 1.0% or less of Ti (except 0%), 0.015% or less of N (except 0%), 0.06% or less of Al (except 0%), 0.007% or less of B (except 0%), and the balance being Fe and unavoidable impurities, at a finish rolling temperature of Ar3 or higher, followed by cooling; a step of reheating the cooled hot-rolled steel sheet at a temperature in the range of 1000 to 1100°C for at least 30 minutes to austenitize it; A step of normalizing or quenching the austenitized hot-rolled steel sheet to room temperature at a cooling rate of 0.1 ° C. / s or more; and tempering the cooled hot-rolled steel sheet in a temperature range of 700 to 800°C for at least 30 minutes, wherein the LMP value, as defined by the following relational expression 2, is 20,000 or more at an acting stress of 200 MPa and 21,000 or more at an acting stress of 125 MPa, and the area reduction at high temperature fracture is 20% or more. Here, the reduction in area is a value calculated by [(R0-R) / R0] x 100, where R is the diameter of the surface where a creep test specimen having an initial gauge diameter R0 (6 mm) creep ruptures in a 600°C high-temperature creep test under an applied stress of 125 to 275 MPa. [Relationship 1] 0.3≦(V-10SUM)≦1 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb. [Relationship 2] LMP=T×(20+log(tr)) where T is the absolute temperature in Kelvin and tr is the time to rupture in hours.

7. 7. The method for producing a chromium steel plate excellent in creep strength and high-temperature ductility according to claim 6, wherein the steel slab further has a chemical composition that satisfies the following relational expression 3, and the produced chromium steel plate has an LMP value defined by relational expression 2 at an acting stress of 250 MPa of 20,000 or more and an area reduction at high-temperature fracture of 40% or more. [Relationship 3] 35≦|(V-10SUM)×(M / -10SUM)×(Ni-10SUM)×10 ３ |≦600 Here, SUM means the total content of specific impurity elements, specifically the total content of Cu+Co+La+Y+Ce+Zr+Ta+Hf+Re+Pt+Ir+Pd+Sb.

8. 7. The method for producing a chromium steel plate having excellent creep strength and high-temperature ductility according to claim 6, wherein the produced chromium steel has a microstructure containing tempered martensite / bainite.

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