Antioxidant heat-resistant cast steel and preparation method thereof

Abstract

The invention discloses antioxidant heat-resistant cast steel and a preparation method thereof. The cast steel comprises the following components in percentage by mass: 0.08-0.15% of C; 0.1% to 0.8% of Zr; 0.05% to 0.8% of Hf; 0.1% to 0.8% of Re; 8.0% to 11.0% of Cr; 0.5% to 2.5% of Ni; 1.0% to 2.0% of Mo; 0 < Si < = 0.5%; 0 < Mn < = 1.0%; 0 < P < = 0.025%; 0 < S < = 0.015%. According to the antioxidant heat-resistant cast steel added with zirconium, hafnium and rhenium and the electroslag remelting preparation method of the antioxidant heat-resistant cast steel, the grain size of heat-resistant steel is larger than or equal to the ASTM7 grade; the service life is not less than 500 hours under the conditions of 650 DEG C and 170 MPa, the service life is not less than 150 hours under the conditions of 700 DEG C and 100 MPa, and the weight gain rate is not more than 0.1 mg / cm < 2 > after oxidation for 100 hours at 700 DEG C.

Description

Technical Field

[0001] The present invention relates to an oxidation-resistant heat-resistant cast steel and a preparation method thereof, in particular to an oxidation-resistant heat-resistant cast steel added with zirconium, hafnium and rhenium elements and an electroslag remelting preparation method thereof. Background Art

[0002] Since the 1970s, global industrialization and excessive exploitation of fossil fuels have increased the release of greenhouse gases, leading to rising global temperatures, environmental problems and energy crises. Therefore, it is imperative to improve the traditional industrial development model. In recent years, in order to achieve energy conservation and emission reduction, the country has reduced CO2, SO2, NO by improving the thermal efficiency of traditional coal-fired power plants. x Reducing the emission of gases such as CO2 and improving the economic efficiency of units have become an inevitable trend for thermal power generation units. Among them, increasing steam temperature and pressure is an effective way to improve the thermal efficiency of thermal power generation cycles. As the demand for unit efficiency increases, the demand for high-temperature alloys in the coal-fired power sector will gradually increase.

[0003] Heat-resistant steels are widely used in high-temperature components. Representative examples include 9CrMoCoB (CB2) steel and 9Cr-WCoVNb (MARBN) steel. CB2 steel, a product of the European COST 522 standard, is primarily used in turbine components such as cylinders and valves in ultra-supercritical thermal power plants. It is an ideal material for castings in ultra-supercritical units operating at temperatures between 620°C and 650°C. Nickel-based superalloys, however, are widely used in high-temperature components such as valves and piping operating above 700°C due to their superior high-temperature performance, excellent oxidation resistance, and corrosion resistance. However, their application in ultra-supercritical power generation units operating at 700°C is challenging, given the high cost of nickel-based alloys and the significant differences in service conditions and component dimensions. Therefore, to reduce costs and fill the gap in superalloys operating in the 650°C to 700°C range, research and development of oxidation-resistant, heat-resistant steels with high creep strength is of great practical significance.

[0004] High-temperature components are in complex working conditions for a long time during their service life and need to withstand the dual effects of high-temperature stress and oxidation corrosion. According to literature records, although the existing CB2 heat-resistant steel has improved its high-temperature strength and oxidation resistance by adding elements such as Cr, Mo, and Ni, it still faces problems such as insufficient long-term service life caused by grain boundary sliding and cracking and peeling of oxide films during long-term service, making it difficult to meet the long-life service requirements under extreme conditions. The grain boundary strengthening of CB2 heat-resistant steel mainly depends on the precipitation of carbides, but the carbides formed by a single element (such as Cr, Mo) are prone to coarsening or segregation, resulting in insufficient grain boundary stability. At the same time, there is a lack of design for the synergistic strengthening of grain boundaries and oxide films. The oxidation resistance of the grain boundary area is weak, and crack channels are easily formed during the oxidation process, accelerating matrix corrosion.

[0005] Electroslag remelting (ESR) is a key process for improving molten steel purity and controlling solidification structure. However, for steels containing Zr and Hf, traditional slag systems, such as single CaF2-based slags, struggle to balance element protection with inclusion removal, leading to oxidation loss of Zr and Hf or residual inclusions. Furthermore, improper control of cooling rates and aging parameters during heat treatment can lead to coarsening of strengthening phases or uneven precipitation at grain boundaries, further reducing the high-temperature performance of heat-resistant steels. Summary of the Invention

[0006] The present invention overcomes the deficiencies of the prior art and provides an oxidation-resistant and heat-resistant cast steel and an electroslag remelting preparation method thereof.

[0007] The present invention adds Zr, Hf and a trace amount of rare earth element Re to the original composition of CB2 heat-resistant steel. The addition of Re plays a certain role in solid solution strengthening, improves grain boundaries, reduces the segregation of impurity elements at grain boundaries, inhibits local weakening, and improves the fatigue performance of cast steel. At the same time, the addition of Re can improve the stability of the oxide film of heat-resistant cast steel at high temperatures, reduce the isothermal oxidation rate, and improve the high-temperature oxidation resistance of heat-resistant cast steel. In addition, high-melting-point elements such as Zr and Hf can form fine and dispersed MC-type carbides (such as ZrC and HfC). However, due to their high chemical activity and easy burning during the smelting process, it is difficult to accurately control their content and distribution in traditional preparation processes, which limits their application in heat-resistant steel.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] An oxidation-resistant and heat-resistant cast steel, the components and mass ratios of which are as follows:

[0010] C: 0.08% ~ 0.15%; Zr: 0.1% ~ 0.8%; Hf: 0.05% ~ 0.8%; Re: 0.1% ~ 0.8%; Cr: 8.0% ~ 11.0%; Ni: 0.5% ~ 2.5%; Mo: 1.0% ~ 2.0%; 0<Si≤0.5%; 0<Mn≤1.0%; 0<P≤0.025%; 0<S≤0.015%; the balance is Fe and unavoidable impurities.

[0011] The total content of the unavoidable impurities is ≤0.5%, and the content of a single impurity element is ≤0.1%, including N, O, Cu, Sn or As.

[0012] The method for preparing the oxidation-resistant heat-resistant cast steel of the present invention comprises the following steps:

[0013] S1: A master alloy containing Zr, Hf, and Re is prepared by vacuum induction melting, forged into blanks, and then rolled into consumable electrodes. The surface is polished to remove oxide scale, and one end of the consumable electrode is inserted into a slag pool;

[0014] S2: Electroslag remelting is performed using a slag system consisting of 60% to 70% CaF2, 15% to 25% Al2O3, 5% to 10% CaO, 3% to 8% MgO, and 2% to 5% TiO2 in a mass ratio, with the slag pool depth controlled at 150 to 200 mm;

[0015] S3: In the first 25 to 30 minutes of electroslag remelting, add 0.5 to 1.0 kg / t of TiO2 powder to the slag pool;

[0016] S4: The electroslag remelted ingot is subjected to subsequent heat treatment, solution treatment at 1050-1100°C for 2 hours, air cooling, and then aging treatment at 750-800°C for 3.5-4 hours to obtain the final oxidation-resistant and heat-resistant cast steel.

[0017] The melting temperature of the master alloy S1 is 1550-1650°C, and the melting time is 30-60 minutes.

[0018] The forging temperature of the above S1 is 1100°C.

[0019] The consumable electrode diameter of S1 is 150-200mm

[0020] The above-mentioned S2 remelting current is 800-1200A, the voltage is 28-35V, and the melting rate is 3-5kg / min.

[0021] The cooling rate after the above-mentioned S4 solution treatment is ≥50°C / min.

[0022] The method for preparing the oxidation-resistant heat-resistant cast steel of the present invention comprises the following steps:

[0023] The technical solution adopted by the present invention solves the defects existing in the background technology by adding zirconium, hafnium and rhenium elements to the oxidation-resistant heat-resistant cast steel and the electroslag remelting preparation method thereof. The grain size of the heat-resistant steel described in the above method is ≥ ASTM grade 7; the endurance life is ≥ 500 hours under the conditions of 650°C and 170MPa, and the endurance life is ≥ 150 hours under the conditions of 700°C and 100MPa; the weight gain rate after 100 hours of oxidation in a 700°C environment is ≤ 0.1mg / cm 2 .

[0024] The present invention has the following beneficial effects:

[0025] (1) By adding 0.1% to 0.8% Zr and 0.05% to 0.8% Hf, fine dispersed ZrC and HfC carbides are formed, which pin the grain boundaries to inhibit grain growth, making the grain size ≥ ASTM grade 7, significantly improving the anti-slip ability of the grain boundaries, and achieving a durability life of ≥ 500h at 650℃ and 170MPa, and ≥ 150h at 700℃ and 100MPa.

[0026] (2) Zr and Hf form a dense ZrO2 / HfO2 oxide film during the oxidation process, which cooperates with Cr2O3 to form a composite protective layer to prevent O 2- Diffusion, making the oxidation weight gain rate ≤0.1mg / cm3 at 700℃ / 100h 2 , the oxide layer thickness is reduced from 20μm to below 5μm without cracks, significantly delaying substrate corrosion. In addition, Re can increase the stability of the alloy's oxide film, reduce the isothermal oxidation rate, and improve the alloy's oxidation resistance.

[0027] (3) A multi-component slag system with 60% to 70% CaF2 as the main component is used. The high conductivity and desulfurization ability of CaF2 are utilized, combined with the initial addition of TiO2 to promote the floating removal of inclusions, reduce the oxidation loss of Zr and Hf, and ensure the stable element recovery rate.

[0028] (4) By controlling the remelting current, melting rate and solution treatment cooling rate, the coarsening of the strengthening phase and grain boundary segregation are avoided, and a fine and dispersed distribution of the precipitated phase is achieved, providing a uniform supersaturated solid solution for aging treatment and optimizing the matching of high-temperature strength and plasticity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a metallographic photograph of the preferred embodiment 1 of the present invention after heat treatment. DETAILED DESCRIPTION

[0030] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0031] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0032] An oxidation-resistant heat-resistant cast steel, the chemical composition of the heat-resistant steel comprising, by mass percentage:

[0033] C: 0.08%-0.15%; Zr: 0.1%-0.8%; Hf: 0.05%-0.8%; Re: 0.1%-0.8%; Cr: 8.0%-11.0%; Ni: 0.5%-2.5%; Mo: 1.0%-2.0%; 0 < Si ≤ 0.5%; 0 < Mn ≤ 1.0%; 0 < P ≤ 0.025%; 0 < S ≤ 0.015%; the balance is Fe and unavoidable impurities. The total content of unavoidable impurities is ≤ 0.5%, and the content of any individual impurity element is ≤ 0.1%, including but not limited to N, O, Cu, Sn, As, etc.

[0034] The process for preparing the above-mentioned heat-resistant steel comprises the following steps:

[0035] S1: A master alloy containing Zr, Hf, and Re is prepared by vacuum induction melting, forged into blanks, and then rolled into consumable electrodes. The surface is polished to remove oxide scale, and one end of the consumable electrode is inserted into a slag pool;

[0036] In step S1, the master alloy is smelted at a temperature of 1550-1650° C. for a time of 30-60 minutes.

[0037] S2: Electroslag remelting is performed using a slag system consisting of 60% to 70% CaF2, 15% to 25% Al2O3, 5% to 10% CaO, 3% to 8% MgO, and 2% to 5% TiO2 in a mass ratio. The remelting current is controlled at 800 to 1200 A, the voltage is 28 to 35 V, the melting rate is 3 to 5 kg / min, and the slag pool depth is controlled at 150 to 200 mm.

[0038] S3: In the first 25 to 30 minutes of electroslag remelting, add 0.5 to 1.0 kg / t of TiO2 powder to the slag pool;

[0039] S4: The electroslag remelted ingot is heat treated by solution treatment at 1050-1100°C for 2 hours, followed by air cooling, and then aging treatment at 750-800°C for 3.5-4 hours to obtain the final oxidation-resistant and heat-resistant cast steel.

[0040] In step S4, the cooling rate after the solution treatment is ≥50°C / min.

[0041] Example 1

[0042] The chemical composition of heat-resistant steel includes, by mass percentage: C: 0.115%; Zr: 0.3%; Hf: 0.175%; Cr: 9.25%; Ni: 1.0%; Mo: 1.5%; Si: 0.25%; Re: 0.5%; Mn: 0.5%; P: 0.02%; S: 0.01%, and the balance is Fe and impurities.

[0043] Preparation process

[0044] S1: Use vacuum induction melting furnace, vacuum degree ≤1×10 -3 Pa, smelting temperature of 1600℃, smelting time of 45min. Add industrial pure Fe, C, Zr, Hf, Cr, Ni, Mo raw materials according to the composition ratio, smelt and cast into ingots, forge at 1100℃, and then roll into consumable electrodes with a diameter of 175mm. The surface is polished to remove oxide scale.

[0045] S2: The chemical composition of the slag system includes, by mass percentage, 65% CaF2, 20% Al2O3, 7.5% CaO, 5.5% MgO, and 2% TiO2. A single-phase monopolar electroslag furnace is selected, with a set current of 1000A, a voltage of 31.5V, a melting rate of 4kg / min, and a slag pool depth of 180mm.

[0046] S3: In the first 28 minutes after remelting begins, add 0.75 kg / t of TiO2 and evenly spread it into the slag pool through an automatic feeder to prevent local slagging;

[0047] S4: Set the solution treatment temperature to 1075°C, hold for 2 hours, and then force air cooling with a fan after exiting the furnace, controlling the cooling rate to ≥50°C / min. Reduce to 775°C, hold for 3.5 hours, and then cool to room temperature with the furnace. Figure 1 1 is a metallographic photograph of Example 1 after heat treatment.

[0048] Example 2

[0049] The chemical composition of heat-resistant steel includes, by mass percentage: C: 0.08%; Zr: 0.1%; Hf: 0.05%; Cr: 9.0%; Ni: 0.5%; Mo: 1.0%; Si: 0.5%; Re: 0.1%; Mn: 1.0%; P: 0.025%; S: 0.015%, and the balance is Fe and impurities.

[0050] Preparation process

[0051] S1: Use vacuum induction melting furnace, vacuum degree ≤1×10 -3 Pa, melting temperature of 1550℃, melting time of 45min. Add industrial pure Fe, C, Zr, Hf, Cr, Ni, and Mo raw materials according to the composition ratio, smelt and cast into ingots, forge at 1100℃, and then roll into consumable electrodes with a diameter of 175mm. The surface is polished to remove oxide scale.

[0052] S2: The chemical composition of the slag system, by mass percentage, is: 60% CaF2, 25% Al2O3, 10% CaO, 3% MgO, and 2% TiO2. A single-phase, single-pole electroslag furnace was used, with a current of 1000A, a voltage of 31.5V, a melting rate of 3kg / min, and a slag pool depth of 180mm.

[0053] S3: In the first 28 minutes after remelting begins, add 0.75 kg / t of TiO2 and evenly spread it into the slag pool through an automatic feeder to prevent local slagging.

[0054] S4: Set the solution treatment temperature to 1050°C, hold for 2 hours, and then force air cooling with a fan after exiting the furnace, controlling the cooling rate to ≥50°C / min. Reduce the temperature to 750°C, hold for 3.5 hours, and then cool to room temperature with the furnace.

[0055] Example 3

[0056] The chemical composition of heat-resistant steel includes, by mass percentage: C: 0.15%; Zr: 0.8%; Hf: 0.8%; Cr: 11.0%; Ni: 2.5%; Mo: 2.0%; Si: 0.5%; Re: 0.8%; Mn: 1.0%; P: 0.025%; S: 0.015%, and the balance is Fe and impurities.

[0057] Preparation process

[0058] S1: Use vacuum induction melting furnace, vacuum degree ≤1×10 -3 Pa; melting temperature is 1650℃; melting time is 45min

[0059] Industrial pure Fe, C, Zr, Hf, Cr, Ni and Mo raw materials are added according to the composition ratio, smelted and cast into ingots, forged at 1100℃, and then rolled into consumable electrodes with a diameter of 175mm. The surface is polished to remove the oxide scale.

[0060] S2: The chemical composition of the slag system, by mass percentage, is: 70% CaF2, 15% Al2O3, 5% CaO, 8% MgO, and 2% TiO2. A single-phase, single-pole electroslag furnace was used, with a current of 1000A, a voltage of 31.5V, a melting rate of 5kg / min, and a slag pool depth of 180mm.

[0061] S3: In the first 28 minutes after remelting begins, add 0.75 kg / t of TiO2 and evenly spread it into the slag pool through an automatic feeder to prevent local slagging.

[0062] S4: Set the solution treatment temperature to 1100°C, hold for 2 hours, and then force air cooling with a fan after exiting the furnace, controlling the cooling rate to ≥50°C / min. Reduce the temperature to 800°C, hold for 3.5 hours, and then cool to room temperature with the furnace.

[0063] Comparative Example 1 uses 9CrMoCoB (CB2) steel, whose chemical composition, by mass percentage, includes C: 0.115%; Cr: 9.14%; Ni: 0.15%; Mo: 1.5%; Si: 0.25%; Mn: 0.84%; P: 0.02%; S: 0.01%, and the balance is Fe and impurities.

[0064] No Zr or Hf is added, that is, Zr=0%, Hf=0%.

[0065]

[0066] Based on the data in the above table, the following analysis is made:

[0067] The oxidation-resistant and heat-resistant steels prepared by electroslag remelting in the three examples all meet the above-mentioned requirements of long-lasting life and oxidation weight gain.

[0068] The heat-resistant steel produced by electroslag remelting in Example 1 achieves a grain size of ASTM grade 8, with carbonitrides such as ZrC and HfC uniformly distributed across the grain boundaries. Furthermore, the size and quantity of the ZrC and HfC precipitates remain stable during extended high-temperature treatment, resulting in excellent creep resistance. Furthermore, at high temperatures, ZrO2 and HfO2 oxide films form on the heat-resistant steel surface, synergistically protecting the substrate with Cr2O3 and effectively reducing oxidation weight gain. At 700°C, the oxidation weight gain is only one-sixth that of Comparative Example 1, demonstrating excellent oxidation resistance.

[0069] The grain size of the heat-resistant steel prepared by electroslag remelting in Example 2 reaches ASTM grade 7.5, HfC is dispersed on the grain boundaries, and a small amount of ZrC is precipitated. The endurance life reaches 527 hours under high-temperature endurance conditions of 650°C and 170 MPa, showing strong high-temperature endurance performance, an intact oxide layer without obvious cracks, and excellent antioxidant properties.

[0070] The grain size of the heat-resistant steel prepared by electroslag remelting in Example 3 reached ASTM grade 7.5, and slightly coarsened ZrC and HfC particles were observed to be evenly distributed on the grain boundaries. The endurance life reached 169 hours under high-temperature endurance conditions of 700°C and 100 MPa. It had strong creep resistance at high temperatures, and the oxide layer had good density at high temperatures, showing excellent oxidation resistance.

[0071] The grain size of the CB2 heat-resistant steel prepared by conventional technology in Comparative Example 1 is only level 7. During the high-temperature endurance process, the grain boundaries are prone to structural degradation due to the lack of carbonitride strengthening phases such as ZrC and HfC, which intensifies the grain boundary sliding and leads to insufficient high-temperature endurance performance. As a result, the endurance under the experimental conditions of 650℃ and 170MPa is far less than 500h, and under the experimental conditions of 700℃ and 100MPa is only 75h, which is less than 1 / 3 of that in Example 1. At the same time, the oxide film is loose and porous, which cannot effectively block the O 2- Diffusion and oxidation resistance are insufficient, and the weight gain rate after oxidation at 700°C for 100 hours is not ≤0.1mg / cm 2 requirements.

[0072] The above description is based on the ideal embodiment of the present invention. Based on the above description, relevant personnel can make various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the content of the specification and must be determined according to the scope of the claims.

Claims

1. An oxidation-resistant heat-resistant cast steel, characterized in that: The components and mass percentages are as follows: C: 0.08% ~ 0.15%; Zr: 0.1% ~ 0.8%; Hf: 0.05% ~ 0.8%; Re: 0.1% ~ 0.8%; Cr: 8.0% ~ 11.0%; Ni: 0.5% to 2.5%; Mo: 1.0% to 2.0%; 0<Si≤0.5%; 0<Mn≤1.0%; 0<P≤0.025%; 0<S≤0.015%; the balance is Fe and unavoidable impurities.

2. The oxidation-resistant heat-resistant cast steel according to claim 1, characterized in that: The total content of the unavoidable impurities is ≤0.5%, and the content of a single impurity element is ≤0.1%, including N, O, Cu, Sn or As.

3. The method for preparing the oxidation-resistant heat-resistant cast steel according to claim 1, wherein the cast steel is prepared by electroslag remelting, wherein: The steps include: S1: A master alloy containing Zr, Hf, and Re is prepared by vacuum induction melting, forged into a blank, and then rolled into a consumable electrode. The surface is polished to remove oxide scale, and one end of the consumable electrode is inserted into a slag pool; S2: Electroslag remelting is performed using a slag system consisting of 60% to 70% CaF2, 15% to 25% Al2O3, 5% to 10% CaO, 3% to 8% MgO, and 2% to 5% TiO2 in a mass ratio, with the slag pool depth controlled at 150 to 200 mm; S3: In the first 25 to 30 minutes of electroslag remelting, add 0.5 to 1.0 kg / t of TiO2 powder to the slag pool; S4: The electroslag remelted ingot is subjected to subsequent heat treatment, solution treatment at 1050-1100°C for 2 hours, air cooling, and then aging treatment at 750-800°C for 3.5-4 hours to obtain the final oxidation-resistant and heat-resistant cast steel.

4. The preparation method according to claim 3, wherein: The melting temperature of the master alloy S1 is 1550-1650°C, and the melting time is 30-60 minutes.

5. The preparation method according to claim 3, wherein: The forging temperature of the above S1 is 1100°C.

6. The preparation method according to claim 3, wherein: The consumable electrode of S1 has a diameter of 150 to 200 mm.

7. The preparation method according to claim 3, wherein: The above-mentioned S2 remelting current is 800-1200A, the voltage is 28-35V, and the melting rate is 3-5kg / min.

8. The preparation method according to claim 3, wherein: The cooling rate after the above-mentioned S4 solution treatment is ≥50°C / min.

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