A slag system and smelting method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel

Abstract

This invention relates to a slag system and smelting method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, belonging to the field of special metallurgical technology. The slag system comprises: CaF2: 45-65%; Al2O3: 11-20%; CaO: 15-27%; MgO: 2-5%; B2O3: 0.5-5.0%; ZrO2: 2-7%; SiO2 ≤ 0.5%; FeO ≤ 0.7%, and the B2O3 and ZrO2 in the slag system satisfy 2.0 < lg((%ZrO2)). 3 / (%B2O3) 2 <2.7. This invention controls the content of B and Zr in steel by reacting the dissolved B and Zr in molten steel with B2O3 and ZrO2 in slag: 4[B]+3(ZrO2)=3[Zr]+2(B2O3), so that the content of B and Zr in steel is within the target range.

Description

Technical Field

[0001] This invention relates to the field of special metallurgical technology, and in particular to a slag system and smelting method for electroslag remelting of low-Si, low-Al, B-containing, Zr-containing martensitic heat-resistant steel. Background Technology

[0002] The development trend of martensitic heat-resistant steel is towards high strength, high toughness, good heat resistance, and resistance to thermal fatigue, in order to meet the requirements of high-temperature, high-pressure, and long-term stable operation. In recent years, advanced martensitic heat-resistant steel composition design generally adopts a low-Si, low-Al design. By controlling Si ≤ 0.1% and Al ≤ 0.015%, the precipitation of brittle Laves phase can be significantly delayed, toughness improved, and hot workability enhanced. Simultaneously, AlN formation is avoided, as it consumes nitrogen elements for solid solution strengthening and impairs high-temperature creep resistance. Furthermore, the addition of B and Zr to alloying elements has also become a trend. By actively adding 0.010–0.015% B and 0.005–0.02% Zr, grain boundary strength can be improved and martensitic phases suppressed. 23 C6 coarsening improves high-temperature creep resistance; meanwhile, Zr carbonitrides reduce grain growth tendency, refine grain size, and improve toughness.

[0003] Si and Al are the most common oxygen-removing elements in steel. However, for martensitic heat-resistant steels with Al content below 0.015% and Si content below 0.1%, controlling the oxygen content during the refining process poses a challenge. Martensitic heat-resistant steels are usually manufactured using electroslag remelting. For low-Si, low-Al martensitic heat-resistant steels containing Zr and B, electroslag remelting can reduce the S content in the alloy, adsorb and remove large-sized inclusions, and significantly improve the forging performance of the ingot. Since Zr and B are both easily oxidized elements, remelting with conventional slag systems containing high levels of Al2O3 will cause burn-off due to the steel-slag reaction: 3[Zr] + 2(Al2O3) = 4[Al] + 3(ZrO2) or 2[B] + (Al2O3) = 2[Al] + (B2O3), significantly reducing the yield of Zr and B and the quality stability of the electroslag ingot. At the same time, it leads to an increase in the Al content in the steel, exceeding the upper limit of the composition. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a slag system and smelting method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, ensuring the yield and quality of Zr and B in electroslag ingots.

[0005] On one hand, this invention provides a slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel. By mass percentage, it comprises: CaF2: 45-65%; Al2O3: 11-20%; CaO: 15-27%; MgO: 2-5%; B2O3: 0.5-5.0%; ZrO2: 2-7%; SiO2 ≤ 0.5%; FeO ≤ 0.7%. In this slag system, B2O3 and ZrO2 satisfy the condition 2.0 < lg((%ZrO2)). 3 / (%B2O3) 2 <2.7. Where (%ZrO2) represents the mass percentage of ZrO2 in the slag, and (%B2O3) represents the mass percentage of B2O3 in the slag.

[0006] Further, by mass percentage, CaF2: 45–55%; Al2O3: 15–18%; CaO: 20–25%; MgO: 3–5%; B2O3: 0.5–3.0%; ZrO2: 3.0–6.0%; SiO2 ≤ 0.5%; FeO ≤ 0.7%.

[0007] Furthermore, the melting temperature of the slag system is 1200–1350°C.

[0008] Furthermore, at 1500℃, the viscosity is ≤0.04 Pa·s, the electrical conductivity is 1~2 / (Ω·cm), and the density is 2.90~3.10 g / cm³. 3 The surface tension is 350–500 mN / m.

[0009] On the other hand, the present invention provides a method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, which employs the slag system described in the present invention for smelting, and includes the following steps:

[0010] S1: Consumable electrode preparation: The surface of the consumable electrode rod is thoroughly polished, and there must be no defects such as iron oxide scale, shrinkage cavities, or scars on the surface;

[0011] S2: Preparation of liquid slag: Pre-melted slag is prepared in advance, or high-purity powders of CaF2, Al2O3, CaO, B2O3, MgO and ZrO2 are mixed in proportion and melted into liquid. After the melting begins, the liquid slag is added to the crystallizer in the manner of arc initiation.

[0012] 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 energizer is started at the same time as the liquid slag is added.

[0013] 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.

[0014] Further, the chemical composition of the martensitic heat-resistant steel electroslag ingot, by mass percentage, includes: C: 0.08–0.15%; Si: ≤0.1%; 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%; Zr: 0.005–0.02%; B: 0.010–0.017%; N: 0.006–0.014%; O: ≤0.003%.

[0015] Further, in step S4, the filling ratio is 0.6 to 0.9.

[0016] Further, the chemical composition of the consumable electrode rod, by mass percentage, includes: C: 0.08–0.14%; Si: ≤0.1%; Al: ≤0.012%; 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%; Zr: 0.005–0.02%; B: 0.010–0.017%; N: 0.006–0.014%; O: ≤0.0035%.

[0017] Furthermore, the melting rate during the remelting process is determined by the following formula:

[0018] v = (0.6 ~ 0.8) × D

[0019] Where D represents the crystallizer diameter in mm; v represents the melting rate in kg / h.

[0020] Furthermore, the martensitic heat-resistant steel electroslag ingot contains 1g ([%Zr]×[%O]). 2 )≤-7.4; where [%Zr] represents the mass percentage of Zr in the steel, and [%O] represents the mass percentage of O in the steel.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] 1. The present invention adds B2O3 to the slag, which can inhibit the burning loss of B in the steel through the slag-steel equilibrium reaction, improve the yield of B in the steel, and ensure that the B content in the martensitic heat-resistant steel electroslag ingot is stably controlled at 0.010-0.017%.

[0023] 2. This invention, by adding ZrO2 to the slag, can suppress the burn-off of Zr in the steel through slag-steel balance, so that the Zr content in the martensitic heat-resistant steel electroslag ingot is within the target range; Zr: 0.005~0.02%;

[0024] 3. In this invention, 0.5-5% B2O3 and 3-7% ZrO2 are added to the slag, and the contents of B2O3 and ZrO2 in the slag satisfy 2 < lg((%ZrO2)). 3 / (%B2O3) 2 If the content of B and Zr in the steel is less than 2.7, the content of B and Zr in the steel can be controlled by the reaction of B and Zr dissolved in the molten steel with B2O3 and ZrO2 in the slag: 4[B]+3(ZrO2)=3[Zr]+2(B2O3), so that the content of B and Zr in the steel is within the target range.

[0025] 4. The slag system of this invention has a melting temperature range of 1200-1350℃, which is 100-200℃ lower than the melting temperature of this martensitic heat-resistant steel. This prevents crystallization during the slag shell formation process, facilitates the formation of a thin slag shell, and improves the surface quality of the steel ingot. The viscosity is ≤0.04Pa·s, which is beneficial for the removal of inclusions, improves the cleanliness of the steel ingot, and provides good lubrication performance, which is conducive to the forming of the steel ingot.

[0026] 5. This invention employs a liquid slag arc initiation method to improve the uniformity of the molten slag, thereby improving the surface quality of the electroslag ingot. The melting rate during the electroslag remelting process is controlled at v = (0.6~0.8) × D, which reduces the segregation of alloying elements in the ingot, shortens the time for removing inclusions from the molten metal pool, lowers the inclusion level of the electroslag ingot, and improves the cleanliness of the electroslag ingot.

[0027] 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 what is particularly pointed out in the description. Detailed Implementation

[0028] To meet the requirements of high temperature, high pressure, and long-term stable operation, martensitic heat-resistant steel needs to possess high strength, high toughness, good heat resistance, and resistance to thermal fatigue. Currently, these objectives are mainly achieved through composition design, which employs low Si and low Al content and incorporates B and Zr elements.

[0029] A low-Si, low-Al design can delay the precipitation of brittle Laves phase, improve toughness, and enhance hot workability; at the same time, it avoids AlN formation, which consumes nitrogen elements for solid solution strengthening and impairs high-temperature creep resistance. However, Si and Al are the most common oxygen-removing elements in steel, posing a challenge to controlling the oxygen content during the refining process for martensitic heat-resistant steels with low Si and Al content.

[0030] The addition of boron (B) and zirconium (Zr) elements can improve grain boundary strength and suppress nitrogen (M). 23 C6 carbides coarsen the grains, thereby improving high-temperature creep resistance; meanwhile, Zr carbonitrides reduce grain growth tendency, refine grain size, and improve toughness. However, B and Zr are easily oxidized elements, making their content control difficult.

[0031] Furthermore, the smelting process for martensitic heat-resistant steel employs either electric furnace melting followed by ladle refining and electroslag remelting, or vacuum induction melting followed by electroslag remelting. During electroslag remelting, the slag system contains a high level of Al₂O₃, which reacts with B and Zr elements. Reactions such as 3[Zr] + 2(Al₂O₃) = 4[Al] + 3(ZrO₂) or 2[B] + (Al₂O₃) = 2[Al] + (B₂O₃) result in burn-off, significantly reducing the yield of Zr and B and the stability of the electroslag ingot quality. Simultaneously, it leads to an increase in the Al content in the steel, exceeding the allowable upper limit for composition. Therefore, it is impossible to obtain martensitic heat-resistant steel with low Si and low Al content while meeting the requirements for B and Zr elements.

[0032] Therefore, this invention provides a slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, comprising, by mass percentage: CaF2: 45-65%; Al2O3: 11-20%; CaO: 15-27%; MgO: 2-5%; B2O3: 0.5-5.0%; ZrO2: 2-7%; SiO2 ≤ 0.5%; FeO ≤ 0.7%, wherein the B2O3 and ZrO2 in the slag system satisfy 2.0 < lg((%ZrO2)) 3 / (%B2O3) 2 <2.7.

[0033] Compared with existing technologies, this invention provides a slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel. Based on the CaF2-CaO-Al2O3 ternary phase diagram, the CaF2-CaO-Al2O3 ratio of the basic components in the slag is obtained. Then, a small amount of MgO is added to prevent the entry of oxygen from the air. Finally, B2O3 and ZrO2 are added to balance the B and Zr content in the steel. The designed slag system can suppress the loss of B and Zr content in the steel through burning, ensuring the yield of B and Zr.

[0034] The addition of boron (B) and zirconium (Zr) to martensitic heat-resistant steel can improve grain boundary strengthening and yield higher-performance martensitic heat-resistant steel. Specifically:

[0035] Botanicals (B) are interstitial solid solution elements. During quenching, they segregate near grain boundaries, combining with grain boundary vacancies to stabilize the grain boundaries and thus increase their strength. On the other hand, B atoms enter the grain boundaries of heat-resistant steel, and M atoms precipitate near these boundaries. 23 In C6 carbides, M is formed 23 (C 0.85 B 0.15 )6 carbon boride, greatly delayed M 23 C6 coarsening. However, when the B content is too high, BN inclusions will form, severely deteriorating the material's hot working and welding properties.

[0036] Zr segregation at grain boundaries can reduce grain boundary defects, improve grain boundary bonding, decrease grain boundary diffusion rate, slow 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. However, Zr has an extremely strong affinity for oxygen. When Zr combines with oxygen to form large-sized ZrO2 inclusions, microcracks will occur, which is detrimental to strength and toughness, impairs the fatigue resistance of the material, and furthermore, Zr reacts with O to form ZrO2 after being added to molten steel, causing nozzle nodules and affecting castability.

[0037] Therefore, it is proposed to add small amounts of B2O3 and ZrO2 to the CaF2-CaO-Al2O3 basic slag system to control the B and Zr content in the steel. The roles of each component in the slag are as follows:

[0038] CaF2 is used to lower the melting temperature, viscosity, and surface tension of slag, facilitating the dissolution of inclusions in the slag and improving the internal and surface quality of steel ingots. However, excessive CaF2 content leads to severe volatilization during the smelting process, resulting in significant variations in slag composition. This is detrimental to the stable control of the composition of large electroslag ingots and causes severe pollution. Therefore, the CaF2 content is controlled between 45% and 65%.

[0039] CaO is used to increase the basicity of slag, reduce electrical conductivity, and improve desulfurization efficiency. However, excessive CaO content can raise the melting point of the slag; therefore, the CaO content is controlled between 15% and 27%.

[0040] Al₂O₃ serves two purposes: firstly, it reduces the electrical conductivity of slag; secondly, it balances with dissolved Al in the steel, controlling the Al content. Excessive Al₂O₃ content in the slag leads to a steel-slag reaction, causing Zr loss through combustion, and simultaneously increasing the Al content beyond the recommended range. Furthermore, Al₂O₃ increases the melting point and viscosity of the slag; therefore, the Al₂O₃ content is controlled at 11–20%.

[0041] MgO can form a semi-solid film in the slag pool, reducing the absorption of oxygen and hydrogen from the slag surface and reducing thermal radiation. Excessive MgO content can affect the melting point, viscosity, and crystallization characteristics of the slag; therefore, the MgO content should be controlled at 2–5%. FeO in the slag is unavoidable and will transfer oxygen into the molten steel; therefore, its content should be kept as low as possible.

[0042] B₂O₃ and ZrO₂ are used to precisely control the composition of electroslag ingots and improve their purity and compositional uniformity. The addition of B₂O₃ and a small amount of ZrO₂ helps to lower the melting temperature of the slag and improve surface quality. Simultaneously, B and Zr react with their corresponding oxides in the slag via equations 1 to 6. Therefore, the contents of B₂O₃ and ZrO₂ in the slag are controlled to be 0.5–5% and 2–7%, respectively, satisfying the condition 2 < lg((%ZrO₂)). 3 / (%B2O3) 2 <2.7, ensuring that both elements are within the control range simultaneously.

[0043]

[0044] In addition, the slag may contain small amounts of SiO2 and FeO introduced from the raw materials, which will aggravate the loss of elements such as B and Zr. Their content should be reduced as much as possible, and SiO2 should be controlled to ≤0.5% and FeO to ≤0.7%.

[0045] Specifically, by mass percentage: CaF2: 45–55%; Al2O3: 15–18%; CaO: 20–25%; MgO: 3–5%; B2O3: 0.5–3.0%; ZrO2: 3.0–6.0%; SiO2 ≤ 0.5%; FeO ≤ 0.7%.

[0046] Specifically, the melting temperature of the slag system is 1200–1350℃.

[0047] Specifically, at 1500℃, the viscosity is ≤0.04 Pa·s, the electrical conductivity is 1~2 / (Ω·cm), and the density is 2.90~3.10 g / cm³. 3 The surface tension is 350–500 mN / m.

[0048] It should be noted that the slag system of this invention has a melting temperature 100-200°C lower than that of the martensitic heat-resistant steel, and the viscosity of the liquid slag after complete melting is low, which is beneficial for forming a thin slag shell and improving the surface quality of the electroslag ingot. If the conductivity is too low, the electrode gap will be too small, causing short circuits in the molten droplets and reducing the stability of the melting process. Conversely, if the conductivity is too high, heat loss will be significantly increased, which is detrimental to the melting of the electrodes.

[0049] Furthermore, the lower the surface tension of the liquid slag, the less adhesion work is between the slag and steel, making it easier for the slag skin to detach from the steel ingot surface, and reducing the adsorption and removal effect of the liquid slag on inclusions. The density of the liquid slag also affects the flow of the molten metal. High slag density restricts the flow of the liquid slag, reduces heat transfer efficiency, and decreases metallurgical reaction efficiency; conversely, excessively low slag density may lead to increased molten pool fluctuations, excessively rapid slag flow, and an overly thin or uneven slag shell, thus affecting the surface quality and internal structure of the electroslag ingot.

[0050] Using this slag system ratio, the corresponding physical properties help to improve the metallurgical performance of the liquid slag in the electroslag remelting process, improve the purity of the electroslag ingot, and enhance the surface quality of the electroslag ingot.

[0051] This invention provides a method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, using the slag system described in this invention for smelting, and includes the following steps:

[0052] S1: Consumable electrode preparation: The surface of the consumable electrode rod is thoroughly polished, and there must be no defects such as iron oxide scale, shrinkage cavities, or scars on the surface. Avoid oxygenation or the introduction of large-sized foreign inclusions.

[0053] S2: Preparation of liquid slag: Pre-melted slag is prepared in advance, or high-purity powders of CaF2, Al2O3, CaO, B2O3, MgO and ZrO2 are prepared in proportion and mixed evenly, and then completely melted into liquid state in a medium frequency induction furnace. After the melting begins, the liquid slag is added to the crystallizer in the manner of arc initiation.

[0054] S3: Remelting: Electroslag remelting is performed using high-purity argon as a protective atmosphere, with the O2 volume fraction in the atmosphere controlled to be no higher than 0.02%. The consumable electrode rod is inserted into the crystallizer, and energization is started simultaneously with the addition of liquid slag. The melting rate during the remelting process is determined by the following formula:

[0055] v = (0.6 ~ 0.8) × D

[0056] Where D represents the crystallizer diameter in mm; v represents the melting rate in kg / h.

[0057] 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.

[0058] Compared with existing technologies, the slag system designed in this invention exhibits superior metallurgical properties, improving the surface quality and cleanliness of electroslag ingots during the smelting process. Furthermore, the addition of small amounts of B₂O₃ and ZrO₂ to the slag can balance the B and Zr content in the steel, thereby controlling the B and Zr content and ensuring it reaches the target range. The use of pre-melted slag during the remelting process ensures uniform slag composition, which is beneficial for improving the uniformity of ingot composition. Pre-baking the slag before smelting guarantees the internal quality of the electroslag ingot, preventing defects such as porosity.

[0059] Specifically, the chemical composition of the consumable electrode rod, by weight percentage, includes: C: 0.08–0.14%; Si: ≤0.1%; Al: ≤0.012%; 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%; Zr: 0.005–0.02%; B: 0.010–0.017%; N: 0.006–0.014%; O: ≤0.0035%.

[0060] It should be noted that, in order to avoid the increase of Al in the electroslag remelting process due to the addition of Zr, the Al content in the consumable electrode rod should be appropriately reduced.

[0061] Specifically, the chemical composition of the martensitic heat-resistant steel electroslag ingot, by weight percentage, includes: C: 0.08–0.14%; Si: ≤0.1%; 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%; Zr: 0.005–0.02%; B: 0.010–0.017%; N: 0.006–0.014%; O: ≤0.003%. And it satisfies lg([%Zr]×[%O]). 2 )≤-7.4, to avoid the formation of a large number of large-sized ZrO2 inclusions that deteriorate the performance of martensitic heat-resistant steel electroslag ingots.

[0062] It should be noted that during the smelting stage, the composition design of low Si, low Al, and containing B and Zr is highly difficult to smelt, especially in the electroslag remelting stage, where Al is easily increased, leading to excessive Al content. Furthermore, B and Zr are easily lost through the reaction between slag and steel, causing fluctuations in the yield at both ends.

[0063] Specifically, in step S4, the filling ratio is 0.6 to 0.9, and the melting rate v = (0.6 to 0.8) × D.

[0064] 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.

[0065] To more clearly describe the present invention, the following embodiments and comparative examples are provided for further illustration.

[0066] Example 1

[0067] A method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, using the slag system described in this invention, includes the following steps:

[0068] S1: Consumable electrode preparation: The surface of the consumable electrode rod is thoroughly polished, and there must be no defects such as iron oxide scale, shrinkage cavities, or scars on the surface. Avoid oxygenation or the introduction of large-sized foreign inclusions.

[0069] The chemical composition of the consumable electrode rod is as follows: C: 0.12%; Si: 0.08%; Al: 0.01%; Mn: 0.50%; Cr: 9.0%; W: 2.1%; Mo: 0.48%; Co: 3.0%; V: 0.2%; Nb: 0.05%; Zr: 0.018%; B: 0.015%; N: 0.008%; O: 0.003%.

[0070] S2: Preparation of liquid slag: 48.3% CaF2, 17% Al2O3, 23% CaO, 5% MgO, 0.9% B2O3 and 5.2% ZrO2 were prepared and mixed thoroughly, then completely melted into a liquid state using a medium-frequency induction furnace. After the melting process began, the liquid slag was added to the crystallizer using an arc-starting method; 1g((% ZrO2)) 3 / (%B2O3) 2 =2.24;

[0071] 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.

[0072] 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.

[0073] The melting temperature of this slag system is 1289℃, and its electrical conductivity at 1500℃ is 1.72 / (Ω·cm). -1 The viscosity is 0.035 Pa·s, and the surface tension is 475.83 N / m;

[0074] Electroslag remelting was performed using the above slag system. The required B and Zr contents for the remelted steel were B: 0.010–0.017% and Zr: 0.005–0.02%, respectively. The specific remelting steps are described above. Sampling and analysis of the remelted steel ingot revealed a B content of 0.0140%, a Zr content of 0.0181%, an Al content of 0.010%, and an O content of 0.001%. lg([%Zr]×[%O]) 2 = -7.74 < -7.4. Yield: B: 93.3%, Al: 100%, Zr: 100%. Class B inclusions: 0.5.

[0075] Example 2

[0076] Example 2 is prepared in a largely similar manner to Example 1, except that the chemical composition of the slag used in Example 2 is 54.0% CaF2, 17.8% Al2O3, 20.1% CaO, 3.0% MgO, 0.5% B2O3, and 4.5% ZrO2 by mass percentage, along with unavoidable impurities. The impurities contain 0.38% SiO2 and 0.28% FeO; 1g((% ZrO2)) 3 / (%B2O3) 2 =2.56;

[0077] The melting temperature of this slag system is 1344℃, and its electrical conductivity at 1500℃ is 1.75 / (Ω·cm). -1 The viscosity is 0.024 Pa·s, and the surface tension is 429.99 N / m;

[0078] Electroslag remelting was performed using the above slag system. The required B and Zr contents for the remelted steel were B: 0.010–0.017% and Zr: 0.005–0.02%, respectively. The specific remelting steps are described above. Sampling and analysis of the remelted steel ingot revealed the following contents: B 0.0110%, Zr 0.0165%, Al 0.011%, and O 0.0015%. lg([%Zr]×[%O]) 2 = -7.44 < -7.4. Yield: B: 73.3%, Al: 110%, Zr: 91.7%. Class B inclusions: 0.5.

[0079] Comparative Example 1

[0080] The preparation process of Comparative Example 1 is largely the same as that of Example 1, except that the chemical composition of the slag system used in Comparative Example 1 is 49.9% CaF2, 17.6% Al2O3, 23.5% CaO, 2.8% MgO, 4.2% B2O3, and 1% ZrO2 by mass percentage, along with unavoidable impurities. The impurities contain 0.43% SiO2 and 0.37% FeO; 1g ((% ZrO2)) 3 / (%B2O3) 2 ) = -1.25;

[0081] Electroslag remelting was performed using the above slag system. The required B and Zr contents for the remelted steel were B: 0.010–0.017% and Zr: 0.005–0.02%, respectively. The specific remelting steps are described above. Sampling and analysis of the remelted steel ingot revealed the following contents: B 0.0400%, Zr 0.0023%, Al 0.013%, and O 0.0030%. lg([%Zr]×[%O]) 2 = -7.68 < -7.4. Yield: B: 267%, Al: 130%, Zr: 12.8%. Class B inclusions: 0.5.

[0082] Comparative Example 2

[0083] Comparative Example 2 was prepared in a largely similar manner to Example 1, except that the slag used in Comparative Example 2 contained the following chemical composition by mass percentage: 46.4% CaF2, 16.5% Al2O3, 24.0% CaO, 4.0% MgO, 0.3% B2O3, and 7.8% ZrO2, along with unavoidable impurities. The impurities contained 0.39% SiO2 and 0.26% FeO. (lg((% ZrO2)) 3 / (%B2O3) 2 =3.72;

[0084] Electroslag remelting was performed using the above slag system. The required B and Zr contents for the remelted steel were B: 0.010–0.017%; Zr: 0.005–0.02%. The specific remelting steps are described above. Sampling and analysis of the remelted steel ingot revealed the following contents: B 0.0040%, Zr 0.0220%, Al 0.010%, and O 0.0015%. lg([%Zr]×[%O]) 2 = -7.30 > -7.4. Yield: B: 22.2%, Al: 100%, Zr: 122%. Class B inclusions: Grade 1.0.

[0085] Comparative Example 3

[0086] The preparation process of Comparative Example 3 is largely the same as that of Example 1, except that the chemical composition of the slag system used in Comparative Example 3 is 52.3% CaF2, 17.8% Al2O3, 23.6% CaO, 5.0% MgO and unavoidable impurities, among which the SiO2 content is 0.38% and the FeO content is 0.30%.

[0087] Electroslag remelting was performed using the above slag system. The required B and Zr contents for the remelted steel were B: 0.010–0.017% and Zr: 0.005–0.02%, respectively. The specific remelting steps are described above. Sampling and analysis of the remelted steel ingot revealed a B content of 0.0039%, a Zr content of 0.0025%, an Al content of 0.014%, and an O content of 0.0030%. lg([%Zr]×[%O]) 2 = -7.64 < -7.4. Yield: B: 26%, Al: 140%, Zr: 13.9%. Class B inclusions: 0.5 grade.

[0088] Comparative Example 4

[0089] Comparative Example 4 was prepared in a largely similar manner to Example 1, except that the slag used in Comparative Example 4 contained the following chemical composition by mass percentage: 45.4% CaF2, 25.7% Al2O3, 20.8% CaO, 3.0% MgO, 0.5% B2O3, and 3.5% ZrO2, along with unavoidable impurities. The impurities contained 0.45% SiO2 and 0.33% FeO. (lg((% ZrO2)) 3 / (%B2O3) 2 =2.23;

[0090] Electroslag remelting was performed using the above slag system. The required B and Zr contents for the remelted steel were B: 0.010–0.017% and Zr: 0.005–0.02%, respectively. The specific remelting steps are described above. Analysis of the remelted steel ingot revealed the following: B content was 0.0080%, Zr content was 0.0050%, Al content was 0.025% (exceeding the specified range), and O content was 0.0025%. The formula is: lg([%Zr]×[%O]) 2 = -7.51 < -7.4. Yield: B: 53.3%, Al: 250%, Zr: 27.8%. Class B inclusions: 0.5 grade.

[0091] Comparative Example 5

[0092] The preparation process of Comparative Example 5 is largely the same as that of Example 1, except that the melting rate used in Comparative Example 5 is v = 0.85D.

[0093] Electroslag remelting was performed using the above process. The required ranges for B and Zr content in the remelted steel were B: 0.010–0.017%; Zr: 0.005–0.02%. The specific remelting steps are detailed above. Sampling and analysis of the remelted steel ingot revealed a B content of 0.0142% and a Zr content of 0.0185%. However, the ZrO2 inclusions in the steel exceeded the standard, with an Al content of 0.010% and an O content of 0.0019%. (lg([%Zr]×[%O])) 2 = -7.13 > -7.4. Yield: B: 94.7%, Al: 100%, Zr: 102.8%. Class B inclusions: Grade 1.0.

[0094] Component testing

[0095] The components and inclusions of Examples 1-2 and Comparative Examples 1-5 were analyzed, and the specific results are shown in Table 1.

[0096] Table 1. B, Zr, and Al contents in steel after electroslag remelting

[0097]

[0098] Based on Examples 1-2 and Comparative Examples 1-5, and referring to Table 1, it can be seen that the slag system provided by this invention, comprising CaF2: 45-65%; Al2O3: 11-20%; CaO: 15-27%; MgO: 2-5%; B2O3: 0.5-5.0%; ZrO2: 2-7%; SiO2 ≤ 0.5%; FeO ≤ 0.7%, satisfies the condition that B2O3 and ZrO2 in the slag system satisfy 2.0 < lg((%ZrO2)) 3 / (%B2O3) 2 The content of B, Zr and Al in the obtained electroslag ingot is less than 2.7, and the content of B, Zr and Al in the obtained ingot all meet the requirements.

[0099] 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 slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, characterized in that, By mass percentage, the composition includes: CaF2: 45~65%; Al2O3: 11~20%; CaO: 15~27%; MgO: 2~5%; B2O3: 0.5~5.0%; ZrO2: 2~7%; SiO2 ≤ 0.5%; FeO ≤ 0.7%, with B2O3 and ZrO2 in the slag system satisfying 2.0 < <2.7; where This indicates the mass percentage of ZrO2 in the slag. This indicates the mass percentage of B2O3 in the slag; the chemical composition of martensitic heat-resistant steel is: C: 0.08~0.14%; Si: ≤0.1%; 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%; Zr：0.005~0.02%； B：0.010~0.017%； N：0.006~0.014%； O ：≤0.003%。 2. The slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 1, characterized in that, By mass percentage, CaF2: 45~55%; Al2O3: 15~18%; CaO: 20~25%; MgO: 3~5%; B2O3: 0.5~3.0%; ZrO2: 3.0~6.0%; SiO2≤0.5%; FeO≤0.7%.

3. The slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 1 or 2, characterized in that, The melting temperature of the slag system is 1200~1350℃.

4. The slag system for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 1, characterized in that, 1500℃, viscosity ≤0.04 Pa·s, conductivity 1~2 / (Ω·cm), density 2.90~3.10 g / cm 3 , surface tension 350~500 mN / m.

5. A method for electroslag remelting of low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel, characterized in that, Smelting using the slag system described in any one of claims 1-4 includes the following steps: S1: Consumable electrode preparation: The surface of the consumable electrode rod is thoroughly polished, and there must be no defects such as iron oxide scale, shrinkage cavities, or scars on the surface; S2: Preparation of liquid slag: Pre-melted slag is prepared in advance, or high-purity powders of CaF2, Al2O3, CaO, B2O3, MgO and ZrO2 are mixed in proportion and melted into liquid. After the melting begins, the liquid slag is added to the crystallizer in the manner of arc initiation. 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 energizer is started at the same time as the liquid slag is added. 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.

6. The electroslag remelting method for low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 5, characterized in that, The chemical composition of the martensitic heat-resistant steel electroslag ingot, by mass percentage, includes: C: 0.08~0.15%; Si: ≤0.1%; 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%; Zr: 0.005~0.02%; B: 0.010~0.017%; N: 0.006~0.014%; O: ≤0.003%.

7. The electroslag remelting method for low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 5, characterized in that, In step S4, the filling ratio is 0.6~0.

9.

8. The electroslag remelting method for low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 5, characterized in that, The chemical composition of the consumable electrode rod, by mass percentage, includes: C: 0.08~0.14%; Si: ≤0.1%; Al: ≤0.012%; 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%; Zr: 0.005~0.02%; B: 0.010~0.017%; N: 0.006~0.014%; O: ≤0.0035%.

9. The electroslag remelting method for low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 5, characterized in that, The melting rate during remelting is determined by the following formula: v = (0.6~0.8) × D Where D represents the crystallizer diameter in mm; v represents the melting rate in kg / h.

10. The electroslag remelting method for low-Si, low-Al, B- and Zr-containing martensitic heat-resistant steel according to claim 5, characterized in that, The martensitic heat-resistant steel electroslag ingot contains lg([%Zr]×[%O)). 2 )≤-7.4; where [%Zr] represents the mass percentage of Zr in the steel, and [%O] represents the mass percentage of O in the steel.

Images