Method for producing martensite heat-resistant steel G115 for ultra-supercritical power station based on electroslag remelting

Abstract

The invention discloses a method for producing martensite heat-resistant steel G115 for an ultra-supercritical power station based on electroslag remelting. The method comprises the following steps that S1, an electrode bar is obtained through EAF electric furnace smelting, LF refining, VD vacuum treatment and electrode die casting; s2, the electrode bar is subjected to electroslag remelting to obtain an electroslag ingot, a CaF2-Al2O3-CaO ternary slag system is adopted in the electroslag remelting process, Al powder, Si-Ca powder and B-Fe powder are added into the slag system, and argon protection is adopted in the whole process; and S3, after the electroslag ingot is demolded, annealing treatment is conducted. Smelting is conducted through the EAF electric furnace smelting technology, the LF refining technology, the VD vacuum treatment technology, the electrode die casting technology and the electroslag remelting technology, the problems of precise control over the easily-burnt element B and the gas content N and solidification structure segregation are solved, the product performance is further improved, and the welding process window is improved.

Description

Technical Field

[0001] This invention relates to the field of metallurgy, specifically to a method for producing G115 martensitic heat-resistant steel for ultra-supercritical power plants based on electroslag remelting, and more particularly to an electroslag remelting method for producing G115 martensitic heat-resistant steel for ultra-supercritical power plants at 630℃. Background Technology

[0002] With rapid economic development, the demand for energy is increasing. Thermal power generation is an important part of my country's energy mix. However, the environmental pollution problems caused by thermal power generation are increasingly conflicting with my country's ecological civilization construction and environmental protection concepts. Therefore, the demand for high-efficiency, low-emission, high-parameter thermal power units is becoming increasingly urgent. The higher the steam temperature and pressure parameters of coal-fired power generation, the lower the coal consumption and the less pollutant emissions, but the higher the performance requirements for materials. The P92 used in 600℃ ultra-supercritical units can no longer meet the requirements of higher parameter units; and to build ultra-supercritical units with parameters of 630℃ or higher, the high-temperature resistance of materials must reach 650℃.

[0003] 9Cr-3W-3CoVNbCuBN (G115) is a novel martensitic heat-resistant steel invented by Academician Liu Zhengdong's team at the Iron and Steel Research Institute (Chinese Patent No. CN103045962B). This heat-resistant steel adopts a composite strengthening principle, containing multiple strengthening elements such as W, Co, Cu, B, and N. Its high-temperature long-term aging microstructure stability, high-temperature creep strength, and oxidation resistance are significantly higher than P92. It is suitable for long-term use under high-pressure conditions at temperatures below 650℃. Comparative studies have found that G115's creep strength and oxidation resistance at 650℃ are superior to P92's performance at 600℃. As early as 2007, our company collaborated with a partner team to develop and trial-produce G115. After more than ten years of systematic research and development, many problems in industrial production have been solved. However, to address the segregation problem, the control of easily burnable elements such as boron and gaseous elements such as nitrogen after electroslag remelting still requires further solutions.

[0004] To address the control issues of easily burnable elements boron and gaseous elements nitrogen during the G115 electroslag process, a series of process developments have been carried out. This application can solve the above problems and produce G115 products that meet the standards of CSTM 00017-2021, Q / OAPD 2753-2022, and Q / OAPD 2253-2022, thus meeting the requirements of engineering applications.

[0005] In the prior art, Chinese patent CN103045962B is an invention patent for G115, mainly involving the composition design, strengthening concept, manufacturing method, and performance characteristics of G115. However, this technology does not involve the specific smelting method of G115. Chinese patent publication number CN108998650A involves a manufacturing method for large-diameter, thick-walled seamless G115 steel pipes for 630℃ ultra-supercritical units. This patent only involves the pipe-making process of large-diameter, thick-walled seamless G115 steel pipes and does not include the smelting process. Chinese patent publication number CN108950148A involves a method for improving the radial microstructure and performance uniformity of large-diameter, thick-walled G115 pipes; similarly, this technology also does not involve the smelting method of G115. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting. The method employs EAF electric furnace smelting + LF refining + VD vacuum treatment + ingot casting electrode + electroslag remelting technology. This not only solves the problems of precise control of easily burnable element B and gas content N, as well as solidification structure segregation, but also further improves product performance and expands the welding process window.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention provides a method for producing G115 martensitic heat-resistant steel for ultra-supercritical power plants based on electroslag remelting, comprising the following steps:

[0009] S1, electrode rods are obtained by EAF electric furnace smelting + LF refining + VD vacuum treatment + die casting.

[0010] S2, the electrode rod is electroslag remelted to obtain an electroslag ingot. In the electroslag remelting process, a ternary slag system of CaF2-Al2O3-CaO is used, and Al powder, Si-Ca powder and B-Fe powder are added to the slag system. Argon gas protection is used throughout the process.

[0011] S3, after the electroslag ingot is demolded, it undergoes annealing treatment.

[0012] Preferably, in step S1:

[0013] The electrode rod contains 0.0130–0.0180% B and ≤0.012% N.

[0014] The specifications of the electrode rod are as follows:

[0015] Preferably, in step S2, the ratio of CaF2:Al2O3:CaO in the ternary slag system of CaF2-Al2O3-CaO is 60-70:20-30:10-20.

[0016] Preferably, the amount of the ternary slag system of CaF2-Al2O3-CaO added is 400-700 kg / furnace.

[0017] Preferably, the amount of Al powder added is 300-800g / furnace, the amount of Si-Ca powder added is 1000-3000g / furnace, and the amount of B-Fe powder added is 500-1500g / furnace.

[0018] Preferably, in step S2, during the electroslag remelting process, the smelting voltage is 40-55V, the smelting current is 19000-22000A, and the melting rate is controlled at 8.0-15.0Kg / min.

[0019] Preferably, in step S2, during the electroslag remelting process, the argon gas flow rate is constant, and the argon gas flow rate is 20L / min to 50L / min.

[0020] Preferably, in step S2, before electroslag remelting, the electrode rod is peeled until the surface is bright and the metal color is visible.

[0021] Preferably, in step S2, after electroslag remelting, the cooling water is turned on to the maximum for rapid cooling of the electroslag ingot, with a cooling water flow rate ≥ 200 m³ / h. 3 / h.

[0022] Preferably, in step S3, the electroslag ingot is annealed in an annealing furnace within 4 hours after demolding, the annealing temperature is 750-780℃, the annealing time is determined according to the annealing rate ≥1.5min / mm, and after annealing, the furnace is cooled to below 100℃ and then air-cooled after being removed from the furnace.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. In the electroslag remelting process, the present invention adopts a CaF2-Al2O3-CaO ternary slag system and mixes Al powder, Si-Ca powder and B-Fe powder into the slag material to reduce the amount of B in the early stage of smelting. At the same time, argon gas is continuously introduced into the crystallizer to keep the remelting process under argon protection, preventing the loss of B and the increase of the content of gases such as N, O and H.

[0025] 2. In the method of the present invention, the loss of B element in electroslag ingot is greatly improved; the increase in gas content, especially N content, of electroslag ingot is greatly reduced; and the solidification structure and segregation of electroslag ingot are significantly improved.

[0026] 3. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting provided by this invention is a method that can be successfully implemented in large-scale industrial production. The G115 martensitic heat-resistant steel smelted according to the above method meets the CSTM standards and enterprise standards such as "CSTM 00017-2021 Seamless steel pipe of martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants", "Q / OAPD 2753-2022 Tube blank and profile of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants" and "Q / OAPD 2253-2022 Seamless steel pipe of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants" after welding evaluation by users, and has begun to be applied in a 630℃ ultra-supercritical demonstration project. Detailed Implementation

[0027] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way.

[0028] This invention provides a method for producing G115 martensitic heat-resistant steel for ultra-supercritical power plants based on electroslag remelting, comprising the following steps:

[0029] S1, the raw materials are smelted in an EAF electric furnace, refined by LF, treated by VD vacuum treatment, and die-cast to obtain electrode rods;

[0030] Based on the raw material composition of G115, electrode rods are obtained through EAF electric furnace smelting + LF refining + VD vacuum treatment + ingot casting. The boron (B) content should fully consider the electroslag burn-off situation; the B content in the electrode rods should be controlled within the range of 0.0130–0.0180%. Furthermore, the nitrogen (N) content should be strictly controlled during the smelting process; the N content in the electrode rods should be controlled within the range of ≤0.012%. The specifications of the electrode rods obtained by casting during the smelting process are as follows:

[0031] S2, the electrode rod is electroslag remelted to obtain an electroslag ingot. In the electroslag remelting process, a ternary slag system of CaF2-Al2O3-CaO is used, and Al powder, Si-Ca powder and B-Fe powder are added to the slag system. Argon gas protection is used throughout the process.

[0032] Before electroslag remelting, the electrode rods must be peeled until the surface is bright and the metal color is visible. They must not be black or blue to prevent increased boron burn-off. An argon protective atmosphere is used throughout the process. That is, a constant flow of argon gas is continuously introduced into the crystallizer before and during electroslag remelting, so that the entire smelting process is carried out under an argon protective atmosphere. The argon gas flow rate should be set between 20 L / min and 50 L / min to reduce boron burn-off and nitrogen increase.

[0033] In the electroslag remelting process, a ternary slag system of CaF2-Al2O3-CaO is used, and Al powder, Si-Ca powder and B-Fe powder are added to the slag system to prevent the loss of boron. In the CaF2-Al2O3-CaO ternary slag system, the ratio of CaF2:Al2O3:CaO is 60-70:20-30:10-20. Depending on the weight of the electrode rod, the amount of CaF2-Al2O3-CaO ternary slag system added is 400-700 kg / furnace. Among them, the amount of Al powder added is 300-800 g / furnace, the amount of Si-Ca powder added is 1000-3000 g / furnace, and the amount of B-Fe powder added is 500-1500 g / furnace.

[0034] During the electroslag remelting process, the smelting voltage is 40-55V, the smelting current is 19000-22000A, and the melting rate is controlled at 8.0-15.0kg / min (the melting rate depends on the electrode rod).

[0035] After electroslag remelting, the cooling water is turned on to maximum for rapid cooling of the electroslag ingot, with a cooling water flow rate ≥200 m³ / h. 3 / h, so as to facilitate rapid forced water cooling of electroslag ingots, which can reduce component segregation and promote the redistribution of inclusions. The slag skin on the surface of the electroslag ingot will also form a thin and uniform slag skin due to forced water cooling, resulting in better surface quality of the electroslag ingot.

[0036] S3, after the electroslag ingot is demolded, it undergoes annealing treatment.

[0037] After the electroslag ingot is demolded, it is placed in an annealing furnace for annealing within 4 hours. The annealing temperature is 750-780℃. The annealing time can be determined according to the size of the electroslag ingot. Specifically, the annealing time can be determined according to the annealing speed ≥1.5min / mm. After annealing, the furnace is cooled to below 100℃ and then air-cooled after being removed from the furnace.

[0038] The properties of the above-mentioned annealed electroslag ingots after tube forging and hot extrusion are as follows: yield strength Rp0.2 is 600-610 MPa, tensile strength R m Its strength is 750–760 MPa, its elongation at break (A) is 23–25%, its reduction of area (Z) is 65–70%, and its hardness is 215–220 HBW.

[0039] The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting according to the present invention will be further described below with specific examples.

[0040] Example 1

[0041] The process involves primary refining in a 50-ton EAF electric furnace, followed by LF refining, VD vacuum treatment, and ingot casting of 5 pieces. Each electrode rod weighs 10.6 tons. The composition of the electrode rods is shown in Table 1. The electrode rods are subjected to electroslag remelting to obtain Φ900 electroslag ingots. During the electroslag remelting process, a ternary slag system of CaF2:Al2O3:CaO = 63:24:13 is used, with an addition amount of 400 kg. 350 g of Al powder, 1000 g of Si-Ca powder, and 500 g of B-Fe powder are added to the slag system. The smelting voltage for electroslag remelting is 40–45 V, the smelting current is 19000–20000 A, and the melting rate is 8.0–10.0 kg / min. A constant flow of argon gas is continuously introduced throughout the process, with the argon gas flow rate set to 22 L / min. After electroslag remelting, the cooling water is turned on to maximum. The electroslag ingots are then placed in an annealing furnace within 4 hours and annealed at 770℃ for 25 hours.

[0042] Table 1 shows a comparison of the chemical composition before and after electroslag remelting. It can be seen that after electroslag remelting, easily burned elements such as B and N, as well as gaseous elements, are well controlled, with significant burning or increase.

[0043] The low-magnification inspection results of the annealed electroslag ingots are shown in Table 2, and the inclusion rating results are shown in Table 3.

[0044] The mechanical properties of the large-diameter pipes made from annealed electroslag ingots through tube blank forging and hot extrusion are shown in Table 4. It can be seen that their tensile and impact properties are far higher than the standard requirements.

[0045] Example 2

[0046] The process involves primary refining in a 50-ton EAF electric furnace, followed by LF refining, VD vacuum treatment, and ingot casting of four pieces. Each electrode rod weighs 12.5 tons. The composition of the electrode rods is shown in Table 1. The electrode rods are subjected to electroslag remelting to obtain electroslag ingots with a diameter of Φ1000. During the electroslag remelting process, a ternary slag system of CaF2:Al2O3:CaO = 65:25:10 is used, with an addition amount of 400 kg. 580 g of Al powder, 1700 g of Si-Ca powder, and 900 g of B-Fe powder are added to the above slag system. The smelting voltage used for electroslag remelting is 45–50 V, the smelting current is 20000–21000 A, and the melting rate is 10.0–12.0 kg / min. A constant flow of argon gas is continuously introduced throughout the process, with the argon gas flow rate set to 31 L / min. After the electroslag remelting is completed, the cooling water is turned on to maximum. The electroslag ingots are then placed in an annealing furnace within 4 hours and annealed at 770℃ for 25 hours.

[0047] Table 1 shows a comparison of the chemical composition before and after electroslag remelting. It can be seen that after electroslag remelting, easily burned elements such as B and N, as well as gaseous elements, are well controlled, with significant burning or increase.

[0048] The low-magnification inspection results of the annealed electroslag ingots are shown in Table 2, and the inclusion rating results are shown in Table 3.

[0049] The mechanical properties of the large-diameter pipes made from annealed electroslag ingots through tube blank forging and hot extrusion are shown in Table 4. It can be seen that their tensile and impact properties are far higher than the standard requirements.

[0050] Example 3

[0051] The process involves primary refining in a 50-ton EAF electric furnace, followed by LF refining, VD vacuum treatment, and ingot casting of three pieces. Each electrode rod weighs 17 tons. The composition of the electrode rods is shown in Table 1. The electrode rods are then subjected to electroslag remelting to obtain electroslag ingots with a diameter of Φ1200. During the electroslag remelting process, a ternary slag system of CaF2:Al2O3:CaO = 65:25:10 is used, with an addition amount of 650 kg. Additionally, 800 g of Al powder, 3000 g of Si-Ca powder, and 1500 g of B-Fe powder are added to the slag system. The smelting voltage for electroslag remelting is 50–55 V, the smelting current is 21000–22000 A, and the melting rate is 12.0–15.0 kg / min. A constant flow of argon gas is continuously introduced throughout the process, with the argon gas flow rate set to 48 L / min. After electroslag remelting, the cooling water is turned on to maximum. The electroslag ingots are then placed in an annealing furnace within 4 hours and annealed at 770℃ for 25 hours.

[0052] Table 1 shows a comparison of the chemical composition before and after electroslag remelting. It can be seen that after electroslag remelting, easily burned elements such as B and N, as well as gaseous elements, are well controlled, with significant burning or increase.

[0053] The low-magnification inspection results of the annealed electroslag ingots are shown in Table 2, and the inclusion rating results are shown in Table 3.

[0054] The mechanical properties of the large-diameter pipes made from annealed electroslag ingots through tube blank forging and hot extrusion are shown in Table 4. It can be seen that their tensile and impact properties are far higher than the standard requirements.

[0055] Table 1 shows the composition of the alloy before and after electroslag remelting in the examples, with the balance being iron (wt%).

[0056]

[0057] Table 2 shows the low-magnification test results of the alloys in the examples.

[0058] Generally loose Central loose Ingot segregation Point segregation Other defects that are visible to the naked eye Example 1 0.5 0 0 0 0 Example 2 0.5 0 0 0 0 Example 3 0.5 0 0 0 0

[0059] Table 3 Inclusion ratings of the alloys in the examples

[0060]

[0061]

[0062] Table 4 Mechanical properties of alloys from the examples

[0063]

[0064] As shown in Tables 2-4, the G115 martensitic heat-resistant steel for ultra-supercritical power plants prepared in the embodiments of the present invention has achieved the expected chemical composition. In particular, the control of easily burnable elements boron and gaseous elements nitrogen is significantly better than the standard requirements. The low-magnification test results and inclusion results meet the standard requirements and are significantly better than the standard requirements. The tensile, impact and hardness mechanical properties meet the standard requirements and are significantly better than the standard requirements.

[0065] In summary, the martensitic heat-resistant steel G115 prepared by this invention meets the CSTM standards and enterprise standards such as "CSTM 00017-2021 Seamless steel pipe of martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants", "Q / OAPD2753-2022 Tube blank and profile of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants" and "Q / OAPD2253-2022 Seamless steel pipe of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants". After welding evaluation by users, its performance has been further improved, and it has begun to be applied in a 630℃ ultra-supercritical demonstration project.

[0066] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for producing G115 martensitic heat-resistant steel for ultra-supercritical power plants based on electroslag remelting, characterized in that, Includes the following steps: S1, electrode rods are obtained by EAF electric furnace smelting + LF refining + VD vacuum treatment + die casting. S2, the electrode rod is electroslag remelted to obtain an electroslag ingot. In the electroslag remelting process, a ternary slag system of CaF2-Al2O3-CaO is used, and Al powder, Si-Ca powder and B-Fe powder are added to the slag system. Argon gas protection is used throughout the process. S3, after the electroslag ingot is demolded, it undergoes annealing treatment.

2. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S1: The electrode rod contains 0.0130–0.0180% B and ≤0.012% N. The electrode rod has a diameter of φ700mm to φ1000mm.

3. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S2, in the ternary slag system of CaF2-Al2O3-CaO, the ratio of CaF2:Al2O3:CaO is 60-70:20-30:10-20.

4. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 3, characterized in that, The amount of the ternary slag system of CaF2-Al2O3-CaO added is 400-700 kg / furnace.

5. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 4, characterized in that, The amount of Al powder added is 300-800g / furnace, the amount of Si-Ca powder added is 1000-3000g / furnace, and the amount of B-Fe powder added is 500-1500g / furnace.

6. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S2, during the electroslag remelting process, the smelting voltage is 40-55V, the smelting current is 19000-22000A, and the melting rate is controlled at 8.0-15.0Kg / min.

7. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S2, during the electroslag remelting process, the argon gas flow rate is constant, and the argon gas flow rate is 20L / min to 50L / min.

8. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S2, before electroslag remelting, the electrode rod is peeled until the surface is bright and the metal color is visible.

9. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S2, after electroslag remelting, the cooling water is turned on to maximum for rapid cooling of the electroslag ingot, with a cooling water flow rate ≥ 200 m³ / h. 3 / h.

10. The method for producing ultra-supercritical martensitic heat-resistant steel G115 for power plants based on electroslag remelting as described in claim 1, characterized in that, In step S3, the electroslag ingot is annealed in an annealing furnace within 4 hours after demolding. The annealing temperature is 750-780℃, and the annealing time is determined according to the annealing rate ≥1.5min / mm. After annealing, the furnace is cooled to below 100℃, and the ingot is then air-cooled after being removed from the furnace.

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