A martensitic heat-resistant steel ingot, its smelting method and application

Abstract

This invention relates to a martensitic heat-resistant steel ingot, its smelting method, and its application, belonging to the field of smelting technology. The smelting method for the martensitic heat-resistant steel ingot includes electric furnace melting, an LF refining stage, a first VD degassing stage, and the molten steel after the first VD degassing being transferred to an LF furnace for further refining; a second VD degassing stage; after the VD refining is completed, the nitrogen content of the molten steel is tested, carbon is added to reach the target composition, and casting is performed under vacuum. After cooling and solidification, the martensitic heat-resistant steel ingot is obtained. This invention employs multi-stage vacuum refining and degassing, leveraging the advantages of both vacuum carbon deoxidation increasing the nitrogen removal reaction interface area and high-purity refining increasing the nitrogen removal reaction rate. This achieves the goal of stably controlling the nitrogen content of the martensitic heat-resistant steel to below 0.012% in a limited number of refining processes.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to a martensitic heat-resistant steel ingot, its smelting method, and its application. Background Technology

[0002] Martensitic heat-resistant steel typically contains 8–17% Cr, which acts as an antioxidant and strengthener. However, a high Cr content can reduce the degassing reaction rate during vacuum refining. When producing martensitic heat-resistant steel using an electric furnace + LF + VD or similar methods, the finished product usually has an O content higher than 0.002% and a N content higher than 0.015%. Simultaneously, to improve creep strength, grain boundary strengthening elements such as Zr and B are added to martensitic heat-resistant steel. With a high N content, large-sized liquid precipitated primary nitrides are easily formed at the end of solidification, impairing the material's plasticity and creep strength.

[0003] To reduce the gas content in martensitic heat-resistant steel, many methods have been proposed, including extending the vacuum refining time and increasing the number of vacuum refining cycles. However, vacuum refining accompanied by argon stirring leads to a continuous decrease in the temperature of the molten steel, requiring the temperature of the molten steel to be increased in the preceding LF refining stage, or multiple heating cycles through LF refining. This has adverse effects on the quality of the molten steel and the refractory materials in the ladle, resulting in a decrease in the purity of the molten steel.

[0004] Furthermore, from the perspective of metallurgical reaction mechanisms, reducing the O and S content in molten steel helps to improve the surface activity of the steel and promote the denitrification reaction. However, to avoid the precipitation of harmful phases during long-term high-temperature service, martensitic heat-resistant steel has strict limits on the content of deoxidizing elements such as Si and Al, requiring Al content to be below 0.015% and Si content to be below 0.2%, making it impossible to effectively reduce the O content through precipitation deoxidation. The O content in molten steel can also be increased by adding iron oxide scale, utilizing the vacuum carbon deoxidation reaction to generate CO bubbles, thereby increasing the surface area for the denitrification reaction and promoting denitrification. However, iron oxide scale contains a relatively high content of impurity elements, and while this method can improve degassing efficiency in the early stages of the reaction, the degassing reaction efficiency does not change significantly once the O content is reduced to the same level as before the addition of iron oxide scale.

[0005] Therefore, in order to achieve ultra-low nitrogen control in martensitic heat-resistant steel, it is necessary to consider multiple aspects such as process, equipment and metallurgical reaction mechanism. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a martensitic heat-resistant steel ingot, its smelting method and application, in order to solve the problem of difficult denitrification in existing martensitic heat-resistant ingots.

[0007] On one hand, the present invention provides a method for smelting martensitic heat-resistant steel ingots, comprising the following steps:

[0008] S1: Add the raw materials to the electric furnace for melting. After the temperature rises to 1620-1650℃, tap the steel and add C to 0.06% for diffusion deoxidation.

[0009] S2: In the LF refining stage, active lime and alumina are used to make reducing slag, and ferrosilicon alloy is added to make white slag. The temperature is controlled at 1580-1620℃, and the slag is transferred to the VD station to wait for vacuum degassing.

[0010] S3: VD first degassing, the first degassing treatment is carried out under high vacuum <133Pa, and the denitrification is accelerated by vacuum carbon deoxidation reaction. After refining, the sample is taken, the slag is removed and high-alkalinity protective slag is remade.

[0011] S4: Transfer the molten steel after the first degassing in VD to the LF furnace for further refining. Heat the furnace to 1580-1600℃, add C to the lower limit of the composition range of 0.08% according to the sampling results, and adjust the composition of other alloying elements except N to the target value. Then add at least one of Zr, Ce and La in the form of ferroalloy for deep deoxidation. After argon purging for ≥10min, transfer the steel to the VD station for secondary degassing.

[0012] S5: After the second degassing of VD and the completion of VD refining, the N content of the molten steel is tested, C is added to the target composition, and it is poured in a vacuum environment. After cooling and solidification, a martensitic heat-resistant steel ingot is obtained.

[0013] Furthermore, in step S2, the basicity of the reduction residue is not less than 5, and the (FeO) content in the residue is not higher than 0.6%.

[0014] Furthermore, in step S3, the high-alkalinity protective slag is composed of CaO, Al2O3, SiO2 and MgO, and the basicity of the protective slag, CaO / SiO2, is not less than 8.

[0015] Furthermore, in step S4, the amounts of Zr, Ce, and La added satisfy -8 <lg[%Si]+4lg[%Zr]+3.8lg[%Ce]+3.7lg[%La]<-7。

[0016] Furthermore, in step S5, if the composition does not meet the target requirements, Zr, Ce, and La deoxidizers are added again, and degassing is performed 1 to 2 more times.

[0017] Furthermore, in step S2, the Si content in the molten steel is not less than 0.08%.

[0018] Furthermore, deep deoxidation is performed using at least one of Zr, Ce, and La, and the residual content of a single element in the molten steel after VD refining does not exceed 0.003%.

[0019] On the other hand, the present invention provides a martensitic heat-resistant steel ingot, the chemical composition of which, by mass percentage, is: C: 0.08-0.14%; Si: ≤0.2%; Al: ≤0.015%; Mn: 0.35-0.80%; Cr: 8.6-9.5%; W: 1.80-2.55%; Mo: 0.20-0.60%; Co: 2.5-3.2%; V: 0.17-0.27%; Nb: 0.04-0.07%; B: 0.010-0.017%; N: 0.006-0.012%; O: ≤0.002%, with the remainder being Fe.

[0020] Furthermore, the smelting method or the application of the martensitic heat-resistant steel ingot can be described, in which the martensitic heat-resistant steel ingot is used as a consumable electrode for electroslag remelting or direct forging.

[0021] Furthermore, the martensitic heat-resistant steel ingot is used as a consumable electrode for electroslag remelting. Before remelting begins, the crystallizer is baked for no less than 20 minutes. Ar is charged until the O2 content in the furnace is <0.2% before power is supplied. Remelting is carried out under a protective atmosphere. The slag system adopts the CaF2-Al2O3-CaO-B2O3-MgO pre-melted slag system.

[0022] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0023] 1. This invention employs multi-stage vacuum refining and degassing, leveraging the advantages of both vacuum carbon deoxidation to increase the N removal reaction interface area and high-purity refining to improve the N removal reaction rate. This achieves the goal of stably controlling the N content of martensitic heat-resistant steel to below 0.012% in a limited number of refining processes.

[0024] 2. In the refining process, the present invention uses elements such as Zr, Ce, and La for deoxidation, achieving ultra-low O control. At the same time, the added deoxidizing elements are automatically removed through inclusion flotation or steel slag reaction. The residual amount of a single element does not exceed 0.0030%, which is actually beneficial to the performance of martensitic heat-resistant steel.

[0025] 3. The martensitic heat-resistant steel ingot provided by the present invention contains elements such as B and Cr, and controls the content of Si, Al and O elements, especially controlling the N content in the ingot to be 0.006 to 0.012%, which helps to exert the grain boundary strengthening effect of alloying elements, improve processing plasticity, and improve the long service performance of the material.

[0026] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objectives and other advantages of this invention can be realized and obtained from the content of the description. Detailed Implementation

[0027] Martensitic heat-resistant steel typically contains 8–17% Cr, which acts as an antioxidant and strengthening agent. However, a high Cr content can reduce the degassing reaction rate during vacuum refining. When producing martensitic heat-resistant steel using an electric furnace + LF + VD or similar methods, the finished product typically has an O content higher than 0.002% and a N content higher than 0.015%.

[0028] Meanwhile, in order to improve creep strength, grain boundary strengthening elements such as Zr and B are added to martensitic heat-resistant steel. When the nitrogen content is high, large-sized liquid precipitated primary nitrides are easily formed at the end of solidification, which impairs the material's plasticity and creep strength.

[0029] To reduce the gas content in martensitic heat-resistant steel, the main existing methods include: long vacuum refining time and increasing the number of vacuum refining times. However, most of these methods will lead to a decrease in the purity of the molten steel and the formation of inclusions and other defects in the ingot.

[0030] For martensitic heat-resistant steels with high Cr content, removing nitrogen (N) is quite difficult, and since they also contain boron (B), controlling the N content in the ingot between 0.006% and 0.012% helps to exert the grain boundary strengthening effect of alloying elements, improve processing plasticity, and enhance the long-term service performance of the material.

[0031] This invention provides a method for smelting martensitic heat-resistant steel ingots, comprising the following steps:

[0032] S1: Add the raw materials to the electric furnace for melting. After the temperature rises to 1620-1650℃, tap the steel and add C to 0.06% for diffusion deoxidation.

[0033] S2: In the LF refining stage, active lime and alumina are used to make reducing slag, and ferrosilicon alloy is added to make white slag. The temperature is controlled at 1580-1620℃, and the slag is transferred to the VD station to wait for vacuum degassing.

[0034] S3: VD first degassing, the first degassing treatment is carried out under high vacuum <133Pa, using vacuum decarburization reaction to generate CO bubbles, increasing the interface area of ​​denitrification reaction, and playing a role in accelerating denitrification. After refining, samples are taken, the slag is removed and high-alkalinity protective slag is remade.

[0035] S4: The molten steel after the first degassing in VD is transferred to the LF furnace for further refining. The temperature is raised to 1580-1600℃. According to the sampling results, C is added to the lower limit of the composition range of 0.08%. The composition of other alloying elements is adjusted to the target value. Then, at least one of Zr, Ce, and La is added in the form of ferroalloys for deep deoxidation. After argon purging and quenching for 10 minutes, it is transferred to the VD station for secondary degassing.

[0036] S5: After the second degassing of VD and the completion of VD refining, the N content of the molten steel is tested, C is added to the target composition, and it is poured in a vacuum environment. After cooling and solidification, a martensitic heat-resistant steel ingot is obtained.

[0037] Compared with existing technologies, this invention provides a smelting method comprising three stages: smelting, refining, and vacuum casting. The refining process employs a combination of LF (sulfuric acid) and VD (vacuum deoxidation). The first LF refining does not involve additional precipitation deoxidation, controlling the Si and Al content of the molten steel to be as low as possible, and utilizing vacuum carbon deoxidation to quickly remove the easily removable N from the molten steel. The second LF refining utilizes at least one of Zr, Ce, and La for precipitation deoxidation. Based on thermodynamic reactions and steel-slag balance calculations, the O content of the molten steel is reduced to below the conventional refining level, improving the degassing efficiency of subsequent vacuum refining. If the N content does not meet the standard after the second VD degassing, Zr, Ce, and La deoxidizers are added again, followed by 1-2 more VD vacuum refining cycles until the N content is reduced to below 0.012%.

[0038] Specifically, in step S1, the raw materials include scrap steel, molten iron, and ferrochrome, etc., and the nitrogen content brought into the molten steel from the ferrochrome raw materials should be less than 0.003%.

[0039] It should be noted that the low-grade ferrochrome raw material has a high nitrogen content. In order to reduce the gas content of the molten steel before vacuum refining, shorten the degassing reaction time as much as possible, and give full play to the advantages of the first-stage degassing reaction, the N content in the ferrochrome raw material needs to be strictly controlled in this invention to ensure that the N content brought into the molten steel by the ferrochrome does not exceed 0.003%.

[0040] Specifically, in step S2, the basicity of the reducing slag is not less than 5, and the (FeO) content in the slag is ≤0.6%, to avoid oxidation and burn-off of elements such as Cr and Mn in the molten steel.

[0041] It should be noted that in this invention, during the LF refining stage, the molten steel or alloy melt after electric furnace melting is transferred to the LF furnace, and active lime and alumina are added to the molten steel or alloy melt to form a reducing slag, the purpose of which is to reduce the content of sulfur (S) and oxygen (O). When forming the reducing slag, its basicity must not be lower than 5. When the basicity is lower than this, the desulfurization and deoxygenation effect of the reducing slag is weakened, thus affecting the final ingot composition.

[0042] It should be noted that during the first LF refining stage, after the addition of ferrosilicon alloy, the Si content in the molten steel is not less than 0.08% to ensure that other elements such as Nb and Cr in the molten steel will not be burned off due to excessive O content.

[0043] Specifically, in step S3, the high-alkalinity protective slag is composed of CaO, Al2O3, SiO2 and MgO, and the basicity of the protective slag, CaO / SiO2, is not less than 8.

[0044] Preferably, the components of the high-basicity refining slag are CaO: 50-65%; Al₂O₃: 30-45%; SiO₂: 1-7%; MgO: 3-7%.

[0045] It should be noted that when the basicity is lower than 8, adding deoxidizers Zr, Ce and La cannot achieve an effective deep deoxidation effect. These deoxidizers are burned out into the slag mainly through the steel-slag reaction, and increase the contents of Si and Al in the molten steel.

[0046] Specifically, in step S4, the addition amount satisfies -8<lg[%Si]+4lg[%Zr]+3.8lg[%Ce]+3.7lg[%La]<-7.

[0047] Specifically, at least one of Zr, Ce and La is used for deep deoxidation, and the residual content of a single element in the molten steel after VD refining does not exceed 0.003%.

[0048] It should be noted that in the present invention, elements such as Zr, Ce and La are added as deoxidizers to reduce the oxygen content in molten steel, and the addition of deoxidizers must satisfy -8<lg[%Si]+4lg[%Zr]+3.8lg[%Ce]+3.7lg[%La]<-7. When the addition amount corresponds to a value greater than -7, reactions 3Zr+2(Al₂O₃)=3(ZrO₂)+4Al, 2Ce+(Al₂O₃)=(Ce₂O₃)+2Al and 2La+(Al₂O₃)=(La₂O₃)+2Al may cause the Al element content in the molten steel to exceed the standard, and large-size oxide inclusions will appear in the ingot, which adversely affects its performance; when the addition amount corresponds to a value less than -8, the content of deoxidizing elements in the molten steel is too low to achieve deep deoxidation and improve denitrification efficiency.

[0049] In addition, elements such as Zr, Ce and La are extremely prone to segregate at grain boundaries, which can reduce grain boundary defects, improve grain boundary bonding force, reduce grain boundary diffusion rate, slow down dislocation climb and strengthen grain boundaries. Zr-containing carbonitrides can inhibit austenite grain growth, refine grain size, and improve the short-term strength and toughness of steel. Ce can accelerate the migration rate of Si in steel to the surface, promote the formation of a dense SiO₂ film on the surface, and improve oxidation resistance. However, the three elements have extremely strong affinity with oxygen. When large-size inclusions are formed, microcracks will be generated, which is detrimental to strength and toughness, impairs the fatigue resistance of the material, causes nozzle clogging and affects castability.

[0050] Preferably, in the Zr, Ce, and La ferroalloy, the zirconium ferroalloy composition is: Zr: 25-35%, Fe: 65-75%, with the remainder being difficult-to-remove impurities; the iron content in cerium ferroalloy and lanthanum ferroalloy is not less than 40%. After the deoxidation process and the reaction with the steel slag, Zr, Ce, La, and other elements are essentially completely removed, with the residual content of a single element not exceeding 0.0030%, which plays a certain role in improving the service performance of heat-resistant steel.

[0051] Specifically, in step S5, if the components do not meet the target requirements, repeat step S4 1 to 2 times.

[0052] It should be noted that a single VD vacuum refining degassing process cannot achieve the target N content of less than 0.012%. By adding strong deoxidizers such as Zr, Ce, and La, the O content in the molten steel is reduced, thereby increasing the surface activity of the molten steel and improving the N removal rate of the second VD degassing reaction. After the second VD, the N content needs to be tested, and the C element adjusted to the target composition. Once the composition meets the requirements, the steel is transferred to a vacuum environment (such as a vacuum casting chamber) for casting. After cooling, solidification, and demolding, a martensitic heat-resistant steel ingot is obtained. However, if the tested composition does not meet the requirements, especially if the N content exceeds the standard, the deoxidizer is added again (at an S4 ratio), and the degassing process is repeated 1-2 times until the N content meets the requirements before casting.

[0053] This invention provides a martensitic heat-resistant steel ingot, whose chemical composition by mass percentage is as follows: C: 0.08–0.14%; Si: ≤0.2%; Al: ≤0.015%; Mn: 0.35–0.80%; Cr: 8.6–9.5%; W: 1.80–2.55%; Mo: 0.20–0.60%; Co: 2.5–3.2%; V: 0.17–0.27%; Nb: 0.04–0.07%; B: 0.010–0.017%; N: 0.006–0.012%; O: ≤0.002%, with the remainder being Fe.

[0054] Specifically, the martensitic heat-resistant steel ingot obtained by this invention can be remelted as an electroslag remelting electrode, or as a vacuum consumable remelting electrode, or directly forged.

[0055] When preparing electroslag ingots from martensitic heat-resistant steel ingots, the slag system contains Al₂O₃, which reacts with boron (B) in the reaction 2[B] + (Al₂O₃) = 2[Al] + (B₂O₃), causing burn-off and significantly reducing the yield of boron and the quality stability of the electroslag ingot. Simultaneously, it leads to an increase in the Al content in the steel, exceeding the allowable upper limit of the composition. The requirements for electroslag ingots are: Al ≤ 0.015%; B 0.010–0.017%. Furthermore, during the electroslag remelting process, nitrogen (N₂) in the environment may enter the protective slag through a dissolution reaction, leading to nitrogen enrichment in the molten steel and causing the final N content in the electroslag ingot to also exceed the acceptable range.

[0056] Therefore, the present invention provides a method for electroslag remelting of martensitic electroslag ingots, comprising the following steps:

[0057] S1: Electrode preparation: The martensitic heat-resistant steel ingot is processed and used as an electrode by electroslag remelting;

[0058] S2: Pre-melted slag baking: Baking the CaF2-Al2O3-CaO-B2O3-MgO pre-melted slag to remove internal water of crystallization, at a temperature of not less than 800℃ and for a time of not less than 8 hours;

[0059] S3: Remelting: Electroslag remelting is performed using high-purity argon as a protective atmosphere. The volume fraction of O2 in the atmosphere is controlled to be no higher than 0.02%. The consumable electrode rod is inserted into the crystallizer, and the pre-melted slag is added while the current is turned on.

[0060] S4: Feeding: Reduce the melting rate before the remelting is finished to feed the steel. After the molten steel has completely solidified, demold to obtain a martensitic heat-resistant steel electroslag ingot.

[0061] Compared with existing technologies, the electroslag remelting process designed in this invention can further reduce the risk of nitrogen increase during the remelting process. Adding a small amount of B2O3 and MgO to the slag can further improve the purity of the molten steel and increase the yield of B element in electroslag remelting, ensuring the optimal ratio of B, N and Al elements and improving the service performance of martensitic heat-resistant steel.

[0062] Specifically, the crystallizer is fully baked before the remelting stage begins, Ar is charged until the O2 content in the furnace is <0.2%, and power is supplied to carry out melting under a protective atmosphere.

[0063] Specifically, the remelting slag system is selected from the CaF2-Al2O3-CaO-B2O3-MgO premelted slag system, with CaF2 content of 45-58%, Al2O3 content of 15-22%, CaO content of 18-25%, MgO content of 1.5-3%, and B2O3 content of 0.5-2.5%.

[0064] It should be noted that during the electroslag remelting stage, nitrogen (N2) may enter the protective slag through a dissolution reaction, leading to increased nitrogen content in the molten steel. Therefore, the crystallizer must be baked before remelting begins, and Ar should be added until the O2 content in the furnace is <0.2% before remelting can commence. 1.5–3% MgO needs to be added to the electroslag remelting slag system to form a protective layer on the surface of the liquid slag, reducing the risk of N2 dissolution. Additionally, 0.5–2.5% B2O3 should be added to increase the yield of boron in the electroslag ingot, while simultaneously reducing the activity of dissolved nitrogen in the slag, further preventing increased nitrogen content, and ensuring that the Al content does not exceed the required range (≤0.015%).

[0065] This invention proposes adding MgO to the CaF2-CaO-Al2O3 basic slag system to lower the melting point of the slag system and form a protective film to inhibit air from entering the molten steel; adding a small amount of B2O3 is used to finely control the composition of the electroslag ingot, improve the purity and compositional uniformity of the electroslag ingot, and help to lower the melting temperature of the slag and improve the surface quality.

[0066] Specifically, in step S4, the filling ratio is 0.6 to 0.9, and the melting rate v = (0.6 to 0.8) × D.

[0067] It should be noted that the filling ratio refers to the ratio of the consumable electrode to the crystallizer diameter. An excessively high filling ratio will reduce the slag refining effect and decrease the cleanliness of the steel, while an excessively low filling ratio will increase the molten pool depth, reduce inclusion removal efficiency, and decrease production efficiency. The melting rate described here is the optimized melting rate for martensitic heat-resistant steel electroslag ingots. Using this melting rate to prepare electroslag ingots results in good surface quality and a low risk of internal quality defects.

[0068] To more clearly describe the present invention, the following embodiments and comparative examples are provided for further illustration.

[0069] Example 1

[0070] A method for smelting martensitic heat-resistant steel ingots includes the following steps:

[0071] S1: Steel is smelted in an electric furnace using scrap steel and low-carbon ferrochrome as the main raw materials. After the temperature is raised to 1630℃, the steel is tapped and C is added to 0.06% for diffusion deoxidation.

[0072] The N content in the low-carbon ferrochrome is 0.023%, and the amount of N element introduced from the ferrochrome raw material is 0.0017% based on the amount added.

[0073] S2: In the LF refining stage, active lime and alumina are used to make reducing slag, and ferrosilicon alloy is added to make white slag. The temperature is controlled at 1580-1620℃, and the slag is transferred to the VD station to wait for vacuum degassing.

[0074] The basicity of the reducing slag is 4.0; the Si content of the molten steel is 0.07%, and the Al content is 0.0006%.

[0075] S3: First degassing by VD. The first degassing treatment is carried out under high vacuum <133Pa, the vacuum decarburization reaction is used to generate CO bubbles to accelerate denitrification, argon is continuously charged for stirring, sampling is performed after refining, slag is skimmed and high-basicity protective slag is newly prepared;

[0076] The components of said high-basicity refining slag are CaO: 54.52%; Al₂O₃: 35.6%; SiO₂: 4.7%; MgO: 5.18%, and the basicity CaO / SiO₂ of the protective slag is 11.6; after the completion of the first VD refining, the N content in molten steel is 0.0127%;

[0077] S4: Transfer the molten steel after first VD degassing into an LF furnace for secondary refining, heat the temperature to 1590°C, add C to 0.08% according to the sampling results, adjust and replenish other alloy element components to the specified range, then add 0.02% metallic Zr into the molten steel in the form of ferrozirconium alloy (zirconium Zr: 30%, iron Fe: 70%) for deep deoxidation -8<lg[%Si]+4lg[%Zr]=-7.95<-7, perform argon blowing and calming for 10 min, then transfer to the VD station for secondary degassing; the N content in molten steel is reduced to 0.0097%;

[0078] S5: Second degassing by VD. After VD refining is completed, detect the N content in molten steel, replenish C to the target component, perform pouring in a vacuum environment, and obtain a martensitic heat-resistant steel ingot after cooling and solidification.

[0079] The chemical composition of the martensitic heat-resistant steel ingot is: C: 0.011%; Si: 0.12%; Al: 0.010%; Mn: 0.45%; Cr: 9.0%; W: 2.2%; Mo: 0.40%; Co: 2.7%; V: 0.20%; Nb: 0.06%; B: 0.014%; N: 0.0092%; O: 0.0014%, and the balance is Fe.

[0080] Example 2

[0081] Example 2 is substantially the same as the preparation process of Example 1, the difference is that in Example 2, 0.018% Ce is added (-8<lg[%Si]+3.8lg[%Ce]=-7.78<-7), after two times of VD refining, the N content in molten steel is reduced to 0.0108%, after vacuum pouring, the N content is 0.0105%, the O content is 0.0015%, the Ce content is 0.0005%, which meets the target requirements.

[0082] Example 3

[0083] Example 3 is substantially the same as the preparation process of Example 1, except that in Example 3, 0.02% Zr is added before the second LF refining. After two times of VD refining, the N content is 0.011%. After reheating via an LF furnace and adding 0.015% Zr, the N content is reduced to 0.0089% after the third VD refining. After vacuum casting, the N content is 0.0090%, the O content is 0.0011%, and the Zr content is 0.0008%.

[0084] Comparative Example 1

[0085] Comparative Example 1 is substantially the same as the preparation process of Example 1, except that in Comparative Example 1, scrap steel and high-carbon ferrochromium with high N content are used as raw materials, and martensitic heat-resistant steel ingots are smelted through electric furnace + LF + VD + protective atmosphere casting process. After LF refining, the alloy composition is adjusted to the target value, wherein the Si content is 0.16% and the Al content is 0.011%; after the first VD refining, the N content in the molten steel is 0.016%. After reheating via LF again, the N content in the molten steel is reduced to 0.014% after the second VD refining, and further reduced to 0.0135% after the third VD refining. However, after protective atmosphere casting, the N content of the ingot increases to 0.0150%, and the O content reaches 0.0032%.

[0086] Comparative Example 2

[0087] Comparative Example 2 is substantially the same as the preparation process of Example 1, except that Comparative Example 2 smelts martensitic heat-resistant steel through electric furnace + LF + VD + vacuum casting process, and the refining process of molten steel is the same as that of Comparative Example 1. After 3 times of VD refining, the N content of molten steel is reduced to 0.0132%, after vacuum casting, the N content of the ingot is 0.0128%, and the O content is 0.0026%.

[0088] Comparative Example 3

[0089] Comparative Example 3 is substantially the same as the preparation process of Example 1, except that Comparative Example 3 smelts martensitic heat-resistant steel through electric furnace + LF + VD + protective atmosphere casting process. After LF refining, the alloy composition is adjusted to the target value, wherein the Si content is 0.13% and the Al content is 0.0010%; after the first VD refining, the N content in molten steel is 0.013%. After reheating via LF again, 0.04% of metallic Zr is added to the molten steel in the form of ferrozirconium, (-7<lg[%Si]+4lg[%Zr]=-6.47). After the second VD refining, the N content in molten steel is reduced to 0.011%, after vacuum casting, the N content of the ingot is 0.0108%, the O content is 0.0013%, and the Zr content is 0.0012%, but the Al content reaches 0.0019%, which exceeds the target range.

[0090] Performance Testing

[0091] The components and inclusions of Examples 1-3 and Comparative Examples 1-3 were analyzed, and the results are shown in Table 1.

[0092] Table 1 Performance Test Results

[0093] Example 1 0.0092％ 0.0014％ 1.0 0.5 Example 2 0.0105％ 0.0015％ 1.0 0.5 Example 3 0.0090％ 0.0011％ 0.5 0.5 Comparative Example 1 0.0150％ 0.0032％ 1.5 1.0 Comparative Example 2 0.0128％ 0.0026％ 1.0 0.5 Comparative Example 3 0.0108％ 0.0013％ 0.5 0.5

[0094] Application examples

[0095] The ingot obtained in Example 1 was subjected to electroslag remelting to ensure that the contents of B, N and Al in the resulting electroslag ingot met the target values, as detailed below:

[0096] Application Example 1

[0097] The electroslag remelting method for martensitic heat-resistant steel includes the following steps:

[0098] S1: Electrode preparation: The electrode surface is thoroughly polished, and there must be no defects such as iron oxide scale, shrinkage cavities, or scale on the surface. Avoid oxygenation or the introduction of large-sized foreign inclusions.

[0099] The chemical composition of the electrode is shown in Example 1:

[0100] S2: Preparation of pre-melted slag: Mix 50.3% CaF2, 20.1% Al2O3, 23.2% CaO, 2.5% MgO and 2.5% B2O3, heat and melt at a temperature above 1500℃, then cool with water to obtain pre-melted slag. Crush the slag and bake it at about 800℃ for 10 hours before use.

[0101] S3: Remelting: Electroslag remelting is performed using high-purity argon as a protective atmosphere. The volume fraction of O2 in the atmosphere is controlled to be no higher than 0.02%. The consumable electrode rod is inserted into the crystallizer, and the current is applied at the same time as the liquid slag is added. The melting rate during the remelting process is v = 0.7 × D.

[0102] S4: Feeding: Reduce the melting rate before the remelting is finished to feed the steel. After the molten steel has completely solidified, demold to obtain a martensitic heat-resistant steel electroslag ingot.

[0103] The electroslag ingot obtained after remelting was sampled and analyzed. The B content was 0.0160%, the Al content was 0.0115%, and the N content was 0.0095%.

[0104] Application Example 2

[0105] Application Example 2 is prepared in a similar manner to Application Example 1. The difference is that the chemical composition of the slag used in Application Example 2 is 54.0% CaF2, 17.8% Al2O3, 21.1% CaO, 3.0% MgO and 1.5% B2O3 by mass percentage, as well as unavoidable impurities. The impurities contain 0.38% SiO2 and 0.28% FeO.

[0106] The electroslag ingot obtained after remelting was sampled and analyzed. The B content was 0.0120%, the Al content was 0.0101%, and the N content was 0.0095%.

[0107] Comparative Application Example 1

[0108] The preparation process of Comparative Application Example 1 is largely the same as that of Application Example 1. The difference is that the chemical composition of the slag system used in Comparative Application Example 1 is 51.9% CaF2, 18.6% Al2O3, 21.5% CaO, 2.5% MgO, 4.3% B2O3 by mass percentage, as well as unavoidable impurities. Among the impurities, the SiO2 content is 0.43% and the FeO content is 0.37%.

[0109] The electroslag ingot obtained after remelting was sampled and analyzed. The B content was 0.0250%, the Al content was 0.0104%, and the N content was 0.0098%.

[0110] Comparative Application Example 2

[0111] Comparative Application Example 2 is largely the same as Application Example 1 in terms of preparation process. The difference is that the chemical composition of the slag system used in Comparative Application Example 2 is 52.4% CaF2, 19.5% Al2O3, 24.0% CaO, 3.0% MgO and 0.3% B2O3 by mass percentage, as well as unavoidable impurities. Among the impurities, the SiO2 content is 0.39% and the FeO content is 0.26%.

[0112] The electroslag ingot obtained after remelting was sampled and analyzed. The B content was 0.0038%, the Al content was 0.010%, and the N content was 0.0100%.

[0113] Comparative Application Example 3

[0114] The preparation process of Comparative Application Example 3 is largely the same as that of Example 1. The difference is that the chemical composition of the slag system used in Comparative Application Example 3 is 53.4% ​​CaF2, 19.8% Al2O3, 22.8% CaO and 2.3% B2O3 by mass percentage, as well as unavoidable impurities. The impurities contain 0.38% SiO2 and 0.30% FeO.

[0115] The electroslag ingot obtained after remelting was sampled and analyzed. The B content was 0.0149%, the Al content was 0.0125%, and the N content was 0.0150%.

[0116] As can be seen from Application Examples 1-2 and Comparative Application Examples 1-3, the B, N and Al contents in the electroslag ingots obtained by using the slag system provided by the present invention all meet the requirements.

[0117] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for smelting martensitic heat-resistant steel ingots, characterized in that, Includes the following steps: S1: Add the raw materials to the electric furnace for melting. After the temperature rises to 1620~1650℃, tap the steel and add C to 0.06% for diffusion deoxidation. S2: In the LF refining stage, active lime and alumina are used to make reducing slag, and ferrosilicon alloy is added to make white slag. The temperature is controlled at 1580~1620℃, and the slag is transferred to the VD station to wait for vacuum degassing. S3: VD first degassing, the first degassing treatment is carried out under high vacuum <133Pa, and the denitrification is accelerated by vacuum carbon deoxidation reaction. After refining, the sample is taken, the slag is removed and high-alkalinity protective slag is remade. S4: Transfer the molten steel after the first degassing in VD to the LF furnace for further refining. Heat the furnace to 1580~1600℃, add C to the lower limit of the composition range of 0.08% according to the sampling results, and adjust the composition of other alloying elements except N to the target value. Then add at least one of Zr, Ce and La in the form of ferroalloy for deep deoxidation. After argon purging and quenching for ≥10min, transfer the steel to the VD station for secondary degassing. S5: After the second degassing in VD refining, the nitrogen content of the molten steel is tested, and carbon is added to reach the target composition. The steel is then cast under vacuum, and after cooling and solidification, a martensitic heat-resistant steel ingot is obtained. The chemical composition of the martensitic heat-resistant steel ingot, by mass percentage, is: C: 0.08~0.14%; Si: ≤0.2%; Al: ≤0.015%; Mn: 0.35~0.80%; Cr: 8.6~9.5%. W：1.80~2.55%； Mo: 0.20~0.60%; Co: 2.5~3.2%; V: 0.17~0.27%; Nb: 0.04~0.07%; B：0.010~0.017%； N：0.006~0.012%； O ≤0.002%, the remainder is Fe.

2. The smelting method for martensitic heat-resistant steel ingots according to claim 1, characterized in that, In step S2, the basicity of the reduction residue is not less than 5, and the (FeO) content in the residue is not higher than 0.6%.

3. The smelting method for martensitic heat-resistant steel ingots according to claim 1, characterized in that, In step S3, the high-alkalinity protective slag is composed of CaO, Al2O3, SiO2 and MgO, and the basicity of the protective slag, CaO / SiO2, is not less than 8.

4. The smelting method for martensitic heat-resistant steel ingots according to claim 1, characterized in that, In step S4, the amounts of Zr, Ce, and La added satisfy -8 <lg[%Si]+4lg[%Zr]+3.8lg[%Ce]+3.7lg[%La]<-7。 5. The smelting method for martensitic heat-resistant steel ingots according to claim 1, characterized in that, In step S5, if the composition does not meet the target requirements, Zr, Ce, and La deoxidizers are added again, and degassing is performed 1-2 more times.

6. The smelting method for martensitic heat-resistant steel ingots according to claim 1, characterized in that, In step S2, the Si content in the molten steel is not less than 0.08%.

7. The smelting method for martensitic heat-resistant steel ingots according to claim 1, characterized in that, Deep deoxidation is performed using at least one of Zr, Ce, and La, and the residual content of a single element in the molten steel after VD refining does not exceed 0.003%.

8. A martensitic heat-resistant steel ingot, characterized in that, The martensitic heat-resistant steel ingot, prepared by the smelting method according to any one of claims 1-7, has the following chemical composition by mass percentage: C: 0.08~0.14%; Si: ≤0.2%; Al: ≤0.015%; Mn: 0.35~0.80%; Cr: 8.6~9.5%. W：1.80~2.55%； Mo: 0.20~0.60%; Co: 2.5~3.2%; V: 0.17~0.27%; Nb: 0.04~0.07%; B：0.010~0.017%； N：0.006~0.012%； O ≤0.002%, the remainder is Fe.

9. The application of a martensitic heat-resistant steel ingot obtained by the smelting method according to any one of claims 1-7 or the martensitic heat-resistant steel ingot according to claim 8, characterized in that, The martensitic heat-resistant steel ingot is used as a consumable electrode for electroslag remelting or direct forging.

10. The application according to claim 9, characterized in that, Martensitic heat-resistant steel ingots are used as consumable electrodes for electroslag remelting. Before remelting begins, the crystallizer is baked for no less than 20 minutes. Ar is charged until the O2 content in the furnace is <0.2% before power is supplied. Remelting is carried out under a protective atmosphere. The remelting slag system adopts the CaF2-Al2O3-CaO-B2O3-MgO pre-melted slag system.