Low cost production of g115 heat resistant steel using return material

Abstract

A low-cost production method of smelting G115 heat-resistant steel with return materials, comprising the following steps: 1) smelting, melting smelting raw materials containing G115 same steel grade return materials in a medium-frequency induction furnace, wherein the G115 same steel grade return materials account for 60-90% of the smelting raw materials, the P content in the G115 same steel grade return materials is less than or equal to 0.020%, and the S content is less than or equal to 0.015%; the smelting temperature is controlled at 1600-1650 DEG C; 2) refining the molten steel in the medium-frequency induction furnace in an AOD furnace to decarburize, remove gas and alloy; 3) LF refining to perform final deoxidation, desulfurization, fine-tune the composition and uniform temperature; 4) VD vacuum furnace treatment, the vacuum degree reaches less than or equal to 67 Pa and is maintained for more than or equal to 20 minutes, argon is introduced to stir, and the argon pressure is 0.20-0.60 MPa; the tapping temperature is 1520-1560 DEG C; 5) mold casting to obtain a steel ingot.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, specifically relating to a low-cost production method for smelting G115 heat-resistant steel using recycled materials. Background Technology

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

[0003] G115 is a novel martensitic heat-resistant steel (see Chinese patent CN103045962B). This heat-resistant steel adopts a composite strengthening principle, containing multiple strengthening elements such as W, Co, Cu, B, and N. Its high-temperature long-term aging stability, high-temperature creep strength, and oxidation resistance are significantly higher than P92. It is suitable for long-term use under high-pressure conditions at temperatures below 650℃. Comparative studies have found that G115's creep strength and oxidation resistance at 650℃ are superior to P92's performance at 600℃. The existing process route of ingot casting (new material method) + (electroslag) + forging + pipe making is basically mature. G115 has also achieved its first engineering application in the world's first 630℃ ultra-supercritical demonstration project.

[0004] However, the existing production mainly uses the new material method, which is costly and not conducive to the large-scale promotion and application of G115. Therefore, a low-cost production process using recycled materials of the same steel grade as G115 has been developed.

[0005] In the prior art, Chinese patent CN103045962B mainly involves the composition design, strengthening concept, manufacturing method and performance characteristics of G115, but this technology does not involve the specific smelting method of G115.

[0006] Chinese patent CN108998650A relates to a method for manufacturing G115 large-diameter thick-walled seamless steel pipes for 630℃ ultra-supercritical units. This patent only relates to the pipe-making process of G115 large-diameter thick-walled seamless steel pipes and does not include the smelting process.

[0007] Chinese patent CN108950148A relates to a method for improving the radial microstructure and performance uniformity of G115 large-diameter thick-walled tubes, but this technology does not involve the smelting method of G115.

[0008] Chinese patent CN11463507A discloses a method for smelting martensitic heat-resistant steel, which describes the smelting of G115 using new materials, but does not involve the smelting of G115 using recycled materials. Summary of the Invention

[0009] The purpose of this invention is to provide a new method for smelting G115 martensitic heat-resistant steel using a recycled material method, which significantly reduces the production cost of G115 by more than 40%, and the performance meets the requirements of standards such as "Q / OAPD 2253-2022 Seamless steel pipe of new martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants" and "CSTM 00017-2021 Seamless steel pipe of martensitic heat-resistant steel 08Cr9W3Co3VNbCuBN (G115) for power plants".

[0010] To achieve the above objectives, the technical solution of the present invention is as follows: A low-cost production method for smelting G115 heat-resistant steel using recycled materials, comprising the following steps: 1) Smelting: The smelting raw materials containing recycled G115 steel grade are melted in a medium-frequency induction furnace. The recycled G115 steel grade accounts for 60-90% of the smelting raw materials. The recycled G115 steel grade contains P≤0.020 wt% and S≤0.015 wt%. The smelting tapping temperature is controlled at 1600-1650℃. 2) The molten steel from the medium-frequency induction furnace is decarburized, gas removed, and alloyed in an AOD furnace. During this process, oxygen and argon are blown in stages. In the main decarburization stage when C ≥ 0.4%, the gas ratio is O2:Ar = 3:1~4:1. In the fine decarburization stage when C < 0.4%, the gas ratio is O2:Ar = 1:2~1:4, ultimately decarburizing to C ≤ 0.03%. In the later stage of decarburization, a strong reducing atmosphere is introduced. 100~200 kg of ferrosilicon and 1000~1200 kg of aluminum granules are added to reduce the high-valence chromium and manganese oxides back to their metallic state. Low-carbon ferrochrome with a C content ≤ 0.15 wt% and electrolytic manganese main alloy are added to bring the Cr content to 8.5~9.0 wt% and Mn content to 0.3~0.7 wt%. 3) LF furnace refining: final deoxidation and desulfurization are performed. Low-carbon lime with a carbon content of less than 1% is added in batches at 0.25~0.75wt% / batch to form a slag layer with good fluidity, covering the molten steel surface for final deoxidation and desulfurization. The slag surface is deoxidized with SiFe powder. During this process, the Ar gas stirring intensity is increased, with an Ar gas pressure of 0.2~0.8MPa. The Ar gas pressure is controlled to prevent the molten steel from overflowing the slag surface. According to the G115 composition, micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, and ferroboron are added in batches with a C content ≤0.15wt%. Electrode heating is used to raise the temperature of the molten steel to the temperature required for VD treatment and casting, i.e., 1680~1720℃. 4) VD vacuum furnace treatment, vacuum degree reaches ≤67Pa and is maintained for ≥20 minutes, argon gas is introduced for stirring, argon gas pressure is 0.20~0.60MPa; tapping temperature is 1520~1560℃; 5) Casting to obtain steel ingots.

[0011] Preferably, in step 1), the smelting raw materials include 60-90% of G115 steel grade return material and 10-40% of low-carbon scrap steel with a C content ≤0.1wt%; it is strictly forbidden to mix in non-ferrous metals, Ni-containing metals, mud, sand, refractory materials and other impurities; the return material needs to be crushed and screened to remove oil and moisture.

[0012] Preferably, before smelting and charging in step 1), a layer of dry scrap steel or steel chips with a thickness of 300-500 mm is evenly laid on the bottom of the medium-frequency induction furnace to form a furnace bed.

[0013] Preferably, in the medium-frequency induction furnace smelting described in step 1), a high-power (85-100% of maximum power) and rapid melting power supply principle is adopted. While ensuring equipment safety, the maximum power should be used as much as possible to shorten the melting time. The melting time after feeding should be controlled within 2.5 hours. The cumulative high-power power supply time should not exceed 5 hours to reduce excessive burning loss of alloying elements.

[0014] Preferably, in step 1), during the smelting process, the furnace must be sealed to prevent heat loss and the intrusion of nitrogen and oxygen from the air; after the molten steel is melted and cleared, 2.5~3.0 kg / t of carbonized rice husks should be added immediately for gasification protection.

[0015] Preferably, before smelting and tapping steel in step 1), the ladle must be fully baked to ensure dryness; a layer of 3.0~5.0 kg / t of flushing pre-melted slag (with the following chemical composition by weight percentage: CaO 45~55%, Al2O3 34~42%, SiO2≤6%, MgO≤8%, S≤0.05%) is pre-laid at the bottom of the ladle to protect the ladle lining and to form a reducing atmosphere in advance.

[0016] Preferably, in step 5) of the ingot casting process, a protective cover is used to protect the casting gate, and argon gas is used for protective casting during the casting process, with an argon gas flow rate of 2~5 Nm³. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; After casting, 3~4 kg / ton of exothermic agent and 5~6 kg / ton of rice ash are added; The protective cover is a sealing device for argon gas.

[0017] Preferably, the heating agent is at least one of coke, aluminum, silicon, and calcium silicon alloy.

[0018] Preferably, the G115 heat-resistant steel has the following composition by weight percentage: C 0.060~0.100%, Si≤0.55%, Mn0.27~0.73%, P≤0.020%, S≤0.010%, Cr 8.40~9.60%, W 2.33~3.17%, Co 2.80~3.25%, Cu0.40~1.20%, V 0.13~0.27%, Nb 0.03~0.10%, N 0.005~0.019%, B 0.008~0.022%, Ni≤0.13%, Ti≤0.02%, Al≤0.015%, O≤0.0040%, As≤0.015%, Sb≤0.015%, Bi≤0.005%, Sn≤0.020%, Pb≤0.015%, As+Sb+Bi+Sn+Pb≤0.035%, balance being Fe and unavoidable impurity elements.

[0019] In the production method described in this invention: The steelmaking process for G115 virgin material is EAF+LF+VD+ingot casting, but the return material method cannot use an EAF electric arc furnace to melt the return material. Because G115 return material contains 8.40–9.60% Cr, the oxygen blowing during EAF melting leads to a significant loss of Cr, typically exceeding 20%, resulting in low alloy element recovery and contradicting the goal of low-cost steelmaking. Therefore, a medium-frequency induction furnace must be used for melting.

[0020] The content of phosphorus (P) and sulfur (S) in returned materials must be strictly controlled, requiring P ≤ 0.020% and S ≤ 0.015%. The mixing of non-ferrous metals, Ni-containing metals, mud, sand, refractory materials, and other impurities is strictly prohibited.

[0021] Returned materials need to be crushed and screened to remove oil and moisture. Damp materials will generate a large amount of water vapor during smelting, which may lead to risks such as hydrogen enrichment in molten steel and explosion. Unlike the virgin material method, the cleanliness of returned materials is worse and may contain foreign matter, so they must be strictly treated before they can be used to smelt G115.

[0022] Before smelting and charging, a layer of dry scrap steel or steel chips about 300-500mm thick is evenly spread on the bottom of the furnace to form a furnace bed, which prevents the furnace bottom from burning through and helps to quickly start the arc and melt.

[0023] This invention employs a material distribution method in medium-frequency induction furnace smelting, using large pieces of material at the bottom and small pieces filling the gaps, to ensure a tight packing and reduce air ingress. First, large pieces of scrap steel and recycled material are placed into the furnace, and then small pieces of material are used to fill the gaps.

[0024] In medium-frequency induction furnace smelting, the principle of "high power, rapid melting" is adopted. While ensuring equipment safety, the maximum power should be used as much as possible to shorten the melting time. The melting time after feeding should be controlled within 2.5 hours. The cumulative high-power supply time should not exceed 5 hours to reduce excessive burning loss of alloying elements.

[0025] In medium-frequency induction furnace smelting, the furnace must be sealed during the smelting process to prevent heat loss and the intrusion of nitrogen and oxygen from the air. After the molten steel has melted and cleared, 2.5~3.0 kg / t of carbonized rice husks is immediately added for gas generation and protection, further isolating air and reducing gas absorption.

[0026] In medium-frequency induction furnace smelting, the ladle must be thoroughly baked and dried before tapping. A layer of 3.0~5.0 kg / t of flushing pre-melted slag is pre-laid at the bottom of the ladle to protect the ladle lining and to create a reducing atmosphere in advance. When tapping, the ladle should be directly facing the center of the furnace mouth to ensure a smooth steel flow and avoid excessive impact of molten steel on the ladle wall, which could cause splashing and air intake.

[0027] In step 2) of AOD furnace refining, oxygen and argon are blown in stages. In the main decarburization stage where C ≥ 0.4%, the gas ratio is O2:Ar = 3:1~4:1. In the fine decarburization stage where C < 0.4%, the gas ratio is O2:Ar = 1:2~1:4, ultimately decarburizing to C ≤ 0.03%. Unlike the virgin material method, the virgin material method produces clean steel with low carbon content (≤ 0.04%), which can be directly fed into the LF furnace for refining after EAF melting. However, the recycled material method, due to contamination such as oil in the recycled material, results in a higher carbon content (1.0~1.5%) in the molten steel after melting compared to the virgin material method. Therefore, it requires AOD decarburization to reach the target carbon content before entering the LF furnace for refining.

[0028] In the AOD furnace refining process, during the later stages of decarburization, a strong reducing atmosphere is adopted. Reducing agents such as ferrosilicon and aluminum granules are added to reduce high-valence chromium and manganese oxides back to their metallic state. This is a crucial step in ensuring the performance of G115.

[0029] In the AOD furnace refining process, under a reducing atmosphere, low-carbon ferrochrome and electrolytic manganese main alloy are added to bring the Cr content to 8.5~9.0wt% and the Mn content to 0.3~0.7wt%.

[0030] In step 3) LF furnace refining, low-carbon lime (carbon content less than 1%) is added in batches at 0.25~0.75wt% / batch for slag formation, with 1~3 batches added to form a reducing slag layer with good fluidity, which covers the molten steel surface for final deoxidation and desulfurization. SiFe powder is used for slag surface deoxidation, and C powder is strictly prohibited.

[0031] In LF furnace refining, the Ar gas stirring intensity should be increased, and the Ar gas pressure (0.2~0.8MPa) should be controlled so that the molten steel does not overflow onto the slag surface.

[0032] In the LF furnace refining process, the composition is finely adjusted based on the sampling analysis results, and appropriate amounts of micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, and ferroboron are added in batches.

[0033] In LF furnace refining, electrode heating is used to raise the temperature of molten steel to the temperature required for VD treatment and casting (1680~1720℃).

[0034] In step 4) VD vacuum furnace treatment, after closing the furnace door, start evacuating until the vacuum degree reaches ≤67Pa and maintain it for ≥20 minutes to remove gaseous elements such as nitrogen, hydrogen, and oxygen from the molten steel.

[0035] In the VD vacuum furnace process, under vacuum conditions, a small amount of argon gas is introduced through the permeable bricks installed at the bottom of the ladle for stirring (argon gas pressure 0.20~0.60MPa), which causes inclusions and gases in the molten steel to float to the surface.

[0036] In VD vacuum furnace processing, after the composition is qualified, the temperature is controlled at 1520~1560℃ for tapping.

[0037] In step 5), during the ingot casting process, a protective cover is used to protect the casting gate, and argon gas is used for protective casting during the casting process, with an argon gas flow rate of 2~5 Nm³. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; after casting, 3~4kg / ton of exothermic agent and 5~6kg / ton of rice ash are added.

[0038] The protective cover is an argon-sealed device that ensures that the molten steel does not come into contact with air after flowing out of the nozzle and before entering the injection pipe, and is always in an argon protective atmosphere to prevent the molten steel from absorbing oxygen and nitrogen from the air.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Compared with the new material method, the present invention uses the recycled material method to smelt G115, which can reduce the cost by more than 40%.

[0040] 2. Since G115 recycled material contains 8.40 to 9.60% Cr, oxygen blowing during the melting of raw materials in the EAF electric arc furnace will cause a large amount of Cr element to be burned off, resulting in a low recovery rate of alloying elements. Therefore, this invention uses a medium-frequency induction furnace for melting, which can reduce the Cr burning rate from 20% to less than 5%.

[0041] 3. During the melting stage of the medium-frequency induction furnace, adopt the principle of high power (85-100% of the maximum power) and rapid melting, and control the melting time within 2.5 hours to further reduce the burning loss of alloying elements by 2-3%.

[0042] 4. Before tapping steel from the medium-frequency induction furnace, the ladle must be fully baked to ensure dryness; a layer of 3.0~5.0 kg / t of flushing pre-melted slag (chemical composition CaO: 45-55%, Al2O3: 34-42%, SiO2: ≤6%, MgO≤8%, S≤0.05%) should be pre-laid at the bottom of the ladle to protect the ladle lining and to form a reducing atmosphere in advance.

[0043] 5. Using AOD refining, oxygen and argon are blown in stages. In the main decarburization stage with C≥0.4%, the gas ratio is O2:Ar=3:1~4:1. In the fine decarburization stage with C<0.4%, the gas ratio is O2:Ar=1:2~1:4. This can decarburize the carbon content of crude steel (1.0~1.5%) to below 0.03%.

[0044] 6. During LF refining, increase the stirring intensity of Ar gas, with an Ar gas pressure of 0.2~0.8MPa. The Ar gas pressure should be controlled so that the molten steel does not overflow the slag surface. Thorough stirring helps inclusions float to the surface, and the various inclusions in the finished product should be ≤0.5 grade.

[0045] 7. VD vacuum furnace treatment, vacuum degree reaches ≤67Pa and is maintained for ≥20 minutes, argon gas of 0.20~0.60MPa is introduced for stirring, which helps degassing. The finished product has N≤150ppm, O≤20ppm, and H≤2ppm. Detailed Implementation

[0046] The present invention will be further described below with reference to the embodiments.

[0047] Example 1 A low-cost production method for G115 martensitic heat-resistant steel using a recycled material process includes the following steps: 1) Melt the raw materials into crude steel in a medium-frequency induction furnace; the raw materials are 60% G115 recycled material + 40% low-carbon scrap steel, with P 0.005% and S 0.002% in the G115 recycled material; before charging, spread a layer of dry scrap steel or steel chips 400~500mm thick evenly on the bottom of the furnace; first put the large pieces of scrap steel and recycled material into the furnace, and then fill the gaps with small pieces of material; the melting time after adding materials is within 2 hours; after the molten steel is clear, immediately add 135kg of carbonized rice husks, and control the tapping temperature at 1620℃; 2) The crude molten steel obtained in step 1) is added to an AOD furnace for decarburization and refining; oxygen and argon are blown in stages to decarburize to 0.025%; in the later stage of decarburization, reducing agents such as ferrosilicon and aluminum particles are added, and major alloys (such as low-carbon ferrochrome and electrolytic manganese) are added under a reducing atmosphere. 3) The molten steel obtained in step 2) is refined in an LF ladle furnace; In this process, low-carbon lime (carbon content less than 1%) is added in batches at 0.25~0.75wt% / batch to form a reducing slag layer with good fluidity, which covers the molten steel surface for final deoxidation and desulfurization. The slag surface is deoxidized with SiFe powder. The stirring intensity of Ar gas is increased, with an Ar gas pressure of 0.6MPa. The Ar gas pressure is controlled so that the molten steel does not overflow the slag surface. Micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, ferroboron, etc. are added in batches in appropriate amounts to fine-tune the composition. Electrode heating is used to raise the temperature of the molten steel to 1680℃, which is required for VD treatment and casting. 4) The refined molten steel obtained in step 3) is degassed in a VD furnace; The vacuum level reached 67 Pa and was maintained for 25 minutes. A small amount of argon gas was introduced for stirring, with an argon gas pressure of 0.40 MPa, which caused inclusions and gases in the molten steel to float to the surface. The tapping temperature was 1540℃. 5) The molten steel that has been refined and degassed in step 4) is cast into four steel ingots, each weighing 13.5 tons. During the casting process, a specially designed protective cover is used to protect the casting gate, and argon gas is used for protective pouring with a flow rate of 3 Nm. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; After casting, 3.5 kg / ton of exothermic agent and 5.5 kg / ton of rice ash are added.

[0048] The chemical composition of the martensitic heat-resistant steel obtained in this embodiment is shown in Table 1, the low-magnification test results are shown in Table 2, and the inclusion rating results are shown in Table 3. The mechanical properties of the large-diameter pipes made from the steel ingots through billet forging and hot extrusion are shown in Table 4. It can be seen that its tensile and impact properties are far higher than the standard requirements.

[0049] Example 2 A low-cost production method for G115 martensitic heat-resistant steel using a recycled material process includes the following steps: 1) Melt the raw materials into crude steel in a medium-frequency induction furnace; the raw materials are 70% G115 recycled material + 30% low-carbon scrap steel, with P 0.005wt% and S 0.002wt% in the G115 recycled material; before charging, spread a layer of dry scrap steel or steel chips 400~500mm thick evenly on the bottom of the furnace; first put the large pieces of scrap steel and recycled material into the furnace, and then fill the gaps with small pieces of material; the melting time after adding materials is within 2 hours; after the molten steel is clear, immediately add 135kg of carbonized rice husks, and control the tapping temperature at 1630℃; 2) The crude steel obtained in step 1) is added to an AOD furnace for decarburization and refining; oxygen and argon are blown in stages to decarburize to 0.024%; in the later stage of decarburization, reducing agents such as ferrosilicon and aluminum particles are added, and major alloys (such as low-carbon ferrochrome and electrolytic manganese) are added under a reducing atmosphere. 3) The molten steel obtained in step 2) is refined in an LF ladle furnace; low-carbon lime (carbon content less than 1%) is added in batches at 0.25~0.70wt% / batch to form a reducing slag layer with good fluidity, which covers the surface of the molten steel for final deoxidation and desulfurization. The slag surface is deoxidized with SiFe powder; the Ar gas stirring intensity is increased, the Ar gas pressure is 0.4MPa, and the Ar gas pressure is controlled so that the molten steel does not overflow the slag surface; appropriate amounts of micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, and ferroboron are added in batches to fine-tune the composition. The temperature of the molten steel is raised to 1700℃, the temperature required for VD treatment and casting, by electrode heating. 4) The refined molten steel obtained in step 3) is degassed in a VD furnace; the vacuum degree reaches 63 Pa and is maintained for 20 minutes, and a small amount of argon gas is introduced for stirring (argon gas pressure 0.50 MPa) to promote the floating of inclusions and gases in the molten steel; the tapping temperature is 1520℃. 5) The molten steel refined and degassed in step 4) is then cast into four 13.5-ton steel ingots. During the casting process, a protective cover is used to protect the casting nozzle, and argon gas is used for protective pouring at a flow rate of 4 Nm³. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; After casting, 3kg / ton of exothermic agent and 5kg / ton of rice ash are added.

[0050] The chemical composition of the martensitic heat-resistant steel obtained in this embodiment is shown in Table 1, the low-magnification test results are shown in Table 2, and the inclusion rating results are shown in Table 3. The mechanical properties of the large-diameter pipes made from the steel ingots through billet forging and hot extrusion are shown in Table 4. It can be seen that its tensile and impact properties are far higher than the standard requirements.

[0051] Example 3 A low-cost production method for G115 martensitic heat-resistant steel using a recycled material process includes the following steps: The raw materials (80% G115 recycled material + 20% low-carbon scrap steel) were sequentially processed through a medium-frequency induction furnace, an AOD argon-oxygen decarburization furnace, an LF furnace for refining, a VD vacuum degassing furnace, and ingot casting to obtain four steel ingots weighing 13.5 tons each. 1) Melt the raw materials into crude steel in a medium-frequency induction furnace; the raw materials are 80% G115 recycled material + 20% low-carbon scrap steel, and the P of the G115 recycled material is 0.005% and the S is 0.002%; before charging, spread a layer of dry scrap steel or steel chips 400~500mm thick evenly on the bottom of the furnace; first put the large pieces of scrap steel and recycled material into the furnace, and then fill the gaps with small pieces of material; the melting time after adding materials is within 2 hours; after the molten steel is clear, immediately add 135kg of carbonized rice husks, and control the tapping temperature at 1600℃; 2) The crude steel obtained in step 1) is added to an AOD furnace for decarburization and refining; oxygen and argon are blown in stages to decarburize to 0.024%; in the later stage of decarburization, reducing agents such as ferrosilicon and aluminum particles are added, and major alloys (such as low-carbon ferrochrome and electrolytic manganese) are added under a reducing atmosphere. 3) The molten steel obtained in step 2) is refined in an LF ladle furnace; In this process, low-carbon lime (carbon content less than 1%) is added in batches at 0.25~0.74wt% / batch to form a reducing slag layer with good fluidity, which covers the molten steel surface for final deoxidation and desulfurization. The slag surface is deoxidized with SiFe powder. The stirring intensity of Ar gas is increased, with an Ar gas pressure of 0.2MPa. The Ar gas pressure is controlled so that the molten steel does not overflow the slag surface. Micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, ferroboron, etc. are added in batches in appropriate amounts to fine-tune the composition. Electrode heating is used to raise the temperature of the molten steel to the temperature of 1720℃ required for VD treatment and casting. 4) The refined molten steel obtained in step 3) is degassed in a VD furnace; the vacuum degree reaches 60 Pa and is maintained for 24 minutes, and a small amount of argon gas is introduced for stirring (argon gas pressure 0.20 MPa, which promotes the floating of inclusions and gases in the molten steel; the tapping temperature is 1560℃). 5) The molten steel refined and degassed in step 4) is then cast into four 13.5-ton steel ingots. During casting, a protective cover is used to protect the casting nozzle, and argon gas is used for protective pouring with a flow rate of 2-5 Nm³. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; After casting, 4 kg / ton of exothermic agent and 6 kg / ton of rice ash are added.

[0052] The chemical composition of the martensitic heat-resistant steel obtained in this embodiment is shown in Table 1, the low-magnification test results are shown in Table 2, and the inclusion rating results are shown in Table 3. The mechanical properties of the large-diameter pipes made from the steel ingots through billet forging and hot extrusion are shown in Table 4. It can be seen that its tensile and impact properties are far higher than the standard requirements.

[0053] Example 4 A low-cost production method for G115 martensitic heat-resistant steel using a recycled material process includes the following steps: 1) Melt the raw materials into crude steel in a medium-frequency induction furnace; the raw materials are 90% G115 recycled material + 10% low-carbon scrap steel, and the P of the G115 recycled material is 0.005% and the S is 0.002%; before charging, spread a layer of dry scrap steel or steel chips 400-500mm thick evenly on the bottom of the furnace; first put the large pieces of scrap steel and recycled material into the furnace, and then fill the gaps with small pieces of material; the melting time after adding materials is within 2 hours; after the molten steel is clear, immediately add 135kg of carbonized rice husks, and control the tapping temperature at 1650℃; 2) The crude molten steel obtained in step 1) is added to an AOD furnace for decarburization and refining; oxygen and argon are blown in stages to decarburize to 0.023%; in the later stage of decarburization, reducing agents such as ferrosilicon and aluminum particles are added, and major alloys (such as low-carbon ferrochrome and electrolytic manganese) are added under a reducing atmosphere. 3) The molten steel obtained in step 2) is refined in an LF ladle furnace; low-carbon lime (carbon content less than 1%) is added in batches at 0.25~0.75wt% / batch to form a reducing slag layer with good fluidity, which covers the surface of the molten steel for final deoxidation and desulfurization. The slag surface is deoxidized with SiFe powder; the Ar gas stirring intensity is increased, the Ar gas pressure is 0.8MPa, and the Ar gas pressure is controlled so that the molten steel does not overflow the slag surface; appropriate amounts of micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, and ferroboron are added in batches to fine-tune the composition. The temperature of the molten steel is raised to 1690℃, the temperature required for VD treatment and casting, by electrode heating. 4) The refined molten steel obtained in step 3) is degassed in a VD furnace; the vacuum degree reaches 63 Pa and is maintained for 27 minutes, and a small amount of argon gas is introduced for stirring (argon gas pressure 0.60 MPa) to promote the floating of inclusions and gases in the molten steel; the tapping temperature is 1550℃. 5) The molten steel refined and degassed in step 4) is then cast into four 13.5-ton steel ingots. During the casting process, a protective cover is used to protect the casting nozzle, and argon gas is used for protective pouring at a flow rate of 5 Nm³. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; After casting, 3.8 kg / ton of exothermic agent and 5.7 kg / ton of rice ash are added.

[0054] The chemical composition of the martensitic heat-resistant steel obtained in this embodiment is shown in Table 1, the low-magnification test results are shown in Table 2, and the inclusion rating results are shown in Table 3. The mechanical properties of the large-diameter pipes made from the steel ingots through billet forging and hot extrusion are shown in Table 4. It can be seen that its tensile and impact properties are far higher than the standard requirements.

[0055] Comparative Example In the comparative example, steelmaking using virgin materials was carried out. The raw materials were sequentially processed through electric arc furnace smelting, ladle refining, VD vacuum furnace treatment, and ingot casting. The specific steps are as follows: 1) During electric arc furnace smelting, low-carbon and low-nickel raw materials such as metallic Cr and pure Fe are used; argon is used as the inert gas for stirring at the bottom of the electric arc furnace; the carbon content is controlled to not exceed 0.04% and the phosphorus content to not exceed 0.003% at the end point; slag is blocked at 100% of the tapping; the oxygen activity of the tapping is 1000~1400ppm; Al deoxidation is used, and 0.25~0.35% aluminum ingots are added according to the oxygen activity; the target temperature is 1660~1680℃. 2) During refining in the ladle refining furnace, add low-carbon lime in batches of 0.25~0.75wt% / batch according to the slag condition, adding 1~3 batches; use SiFe powder for slag surface deoxidation; increase the Ar gas stirring intensity, and control the Ar gas pressure so that the molten steel does not overflow the slag surface; feed Al for deoxidation according to the sampling analysis results, with the amount of Al added ≤0.035wt%; add metallic Cr, metallic Co, copper plates and ferrotungsten in batches at a temperature not lower than 1600℃, with each batch not exceeding 0.6 tons; add ferroboron 0.10~0.18% according to the analysis results before ladle hoisting, and measure the boron content in the molten steel to be 0.012~0.020%; the ladle hoisting temperature is 1680~1720℃; 3) During VD vacuum furnace treatment, the vacuum degree is 66.7 Pa and the holding time is 20 min; Ar gas is introduced to enhance stirring, and the Ar gas pressure is 0.40 MPa; the hydrogen concentration is constant, and the H content is 1.0 ppm; before hoisting the ladle, the Ar gas supply pressure is controlled at 0.30 MPa to prevent the molten steel from being exposed to air; the ladle temperature is 1560℃. 4) During the casting process, a special protective cover is used to protect the casting nozzle. Argon gas is used to protect the casting process, with an argon gas flow rate of 4 Nm3 / h. The casting is done by bottom pouring. Before casting, Ar is filled into the steel ingot mold. After pouring, 3.4 kg / ton of exothermic agent and 5.3 kg / ton of rice ash are added.

[0056] The chemical composition of the martensitic heat-resistant steel obtained in the comparative example is shown in Table 1, the low-magnification test results are shown in Table 2, and the inclusion rating results are shown in Table 3. The mechanical properties of the large-diameter pipes made from steel ingots through billet forging and hot extrusion are shown in Table 4.

[0057] Table 1 shows that the chemical composition of G115 produced using this invention meets the requirements of the comparative example. Table 2 shows that the low-magnification test results of G115 produced by this invention are consistent with those of the comparative example, indicating that the low-magnification test results of G115 produced by the recycled material method are consistent with those of G115 produced by the virgin material method. Table 3 shows that the inclusion level of G115 produced by this invention is better than that of G115 in the comparative example. The coarse inclusions in the embodiments of this invention are all 0, and the fine inclusions are ≤0.5 grade, of which the inclusions of type A and type C are 0, which is better than the inclusions in the comparative example. The fine inclusions in the comparative example are ≤0.5 grade, but the inclusion grades of type A and type C are higher than those in the embodiments of this invention. Table 4 shows that the mechanical properties of G115 produced by this invention are basically equivalent to those of G115 produced by the virgin material method in the comparative example. The yield strength, tensile strength, elongation, reduction of area, impact energy, and hardness all meet the standard requirements.

[0058] In summary, these are merely preferred embodiments of the present invention and are not intended to limit the scope of the invention. All equivalent variations and modifications made in accordance with the shape, structure, features and spirit described in the claims of the present invention should be included within the scope of the claims of the present invention.

[0059]

[0060]

[0061]

[0062]

Claims

1. A low-cost production method for smelting G115 heat-resistant steel using recycled materials, characterized in that, Includes the following steps: 1) Smelting: The smelting raw materials containing recycled material of the same steel grade as G115 are melted in a medium-frequency induction furnace. Among them, the recycled material of the same steel grade as G115 accounts for 60~90wt% of the smelting raw materials, and P≤0.020wt% and S≤0.015wt% of the recycled material of the same steel grade as G115 are controlled at 1600~1650℃. 2) The rough steel from the medium-frequency induction furnace is decarburized, gas removed, and alloyed in an AOD furnace. During this process, oxygen and argon are blown in stages. In the main decarburization stage when C ≥ 0.4%, the gas ratio is O2:Ar = 3:1~4:

1. In the fine decarburization stage when C < 0.4%, the gas ratio is O2:Ar = 1:2~1:

4. The final decarburization is achieved to C ≤ 0.03%. In the later stage of decarburization, 100~200 kg of ferrosilicon and 1000~1200 kg of aluminum granules are added, along with low-carbon ferrochrome with a C content ≤ 0.15 wt% and electrolytic manganese main alloy to bring the Cr content to 8.5~9.0 wt% and Mn to 0.3~0.7 wt%. 3) LF furnace refining: final deoxidation and desulfurization are performed. Low-carbon lime with a carbon content of less than 1% is added in batches at 0.25~0.75wt% / batch to form a slag layer with good fluidity, covering the molten steel surface for final deoxidation and desulfurization. The slag surface is deoxidized with SiFe powder. During this process, the Ar gas stirring intensity is increased, with an Ar gas pressure of 0.2~0.8MPa. The Ar gas pressure is controlled to prevent the molten steel from overflowing the slag surface. According to the G115 composition, micro-carbon ferrochrome, metallic cobalt, ferrotungsten, electrolytic copper, ferrosilicon, ferroniobium, and ferroboron are added in batches with a C content ≤0.15wt%. Electrode heating is used to raise the temperature of the molten steel to the temperature required for VD treatment and casting, i.e., 1680~1720℃. 4) VD vacuum furnace treatment, vacuum degree reaches ≤67Pa and is maintained for ≥20 minutes, argon gas is introduced for stirring, argon gas pressure is 0.20~0.60MPa; tapping temperature is 1520~1560℃; 5) Casting to obtain steel ingots.

2. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, characterized in that, In step 1), the smelting raw materials include 60-90% of G115 steel grade return material and 10-40% of low carbon scrap steel with a C content of ≤0.1%; non-ferrous metals, Ni-containing metals, mud and sand, and refractory materials are strictly prohibited from being mixed in; the return material must be crushed and screened to remove oil and moisture.

3. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1 or 2, characterized in that, Step 1) Before smelting and charging, a layer of dry scrap steel or steel chips with a thickness of 300-500mm is evenly laid on the bottom of the medium frequency induction furnace to form a furnace bed.

4. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, 2, or 3, characterized in that, In the medium-frequency induction furnace smelting described in step 1), high power (85-100% of the maximum power) is used, and the melting time after feeding should be controlled within 2.5 hours; the cumulative high-power supply time should not exceed 5 hours.

5. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, 2, 3, or 4, characterized in that, Step 1) During the smelting process, the furnace must be sealed to prevent heat loss and the intrusion of nitrogen and oxygen from the air.

6. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, 2, 3, 4, or 5, characterized in that, Step 1) During smelting, immediately after the molten steel is melted and cleared, add 2.5~3.0 kg / t of carbonized rice husks for gasification protection.

7. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, 2, 3, 4, 5, or 6, characterized in that, Step 1) Before smelting and tapping steel, the ladle must be fully baked to ensure dryness; a layer of 3.0~5.0 kg / t of flushing pre-melted slag is pre-laid at the bottom of the ladle, with the following chemical composition by weight percentage: CaO 45~55%, Al2O3 34~42%, SiO2≤6%, MgO≤8%, S≤0.05%.

8. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, characterized in that, Step 5) During ingot casting, a protective cover is used to protect the casting gate, and argon gas is used for protective casting during the casting process, with an argon gas flow rate of 2~5 Nm³. 3 / h; The casting method is adopted. Before casting, Ar is filled into the steel ingot mold; after casting, 3~4kg / ton of exothermic agent and 5~6kg / ton of rice ash are added.

9. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 8, characterized in that, The heating agent is at least one of coke, aluminum, silicon, and calcium silicon alloy.

10. The low-cost production method for smelting G115 heat-resistant steel using recycled materials as described in claim 1, characterized in that, The weight percentage composition of the G115 heat-resistant steel is as follows: C 0.060~0.100%, Si≤0.55%, Mn 0.27~0.73%, P≤0.020%, S≤0.010%, Cr 8.40~9.60%, W 2.33~3.17%, Co 2.80~3.25%, Cu 0.40~1.20%, V 0.13~0.27%, Nb 0.03~0.10%, N 0.005~0.019%, B 0.008~0.022%, Ni≤0.13%, Ti≤0.02%, Al≤0.015%, O≤0.0040%, As≤0.015%, Sb≤0.015%, Bi≤0.005%, Sn≤0.020%, Pb≤0.015%, As+Sb+Bi+Sn+Pb≤0.035%, balance being Fe and unavoidable impurity elements.

Images