Smelting method of G115 martensite heat-resistant steel

Abstract

The invention discloses a smelting method of G115 martensite heat-resistant steel. The smelting method comprises the steps of slag-free tapping of an electric arc furnace, LF large-slag-amount white slag refining, VD deep degassing, B / N coordinated regulation and control, soft argon blowing pouring and the like. The method is characterized in that low-carbon low-phosphorus chromium alloy and high-purity electrolytic cobalt are selected and added in batches at controlled temperature; nitrogen is supplemented with chromium nitride alloy, Fe-B alloy wrapped by pure iron foil is inserted in batches, and the content of N and the content of B are accurately controlled; and Ca-Si line deoxidation and soft argon blowing for inclusion removal are matched with argon injection protection in the atmosphere. According to the process, redundant procedures are omitted, the production cycle is shortened, the content of sensitive elements such as carbon, boron, nitrogen and cobalt is effectively controlled, O in the molten steel is smaller than or equal to 25 PPm, the inclusion removal rate is larger than or equal to 95%, and the endurance strength at the high temperature of 650 DEG C is larger than or equal to 135 Mpa.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of smelting, and particularly relates to a smelting method of G115 martensitic heat-resistant steel. BACKGROUND

[0002] G115 steel is a new type of martensitic heat-resistant steel, which is a high-grade material independently developed by China to meet the demand of key components of 650 DEG C ultra-supercritical thermal power generating units. The steel requires that the stress-rupture strength after serving at 650 DEG C for 10^5 hours is still higher than 100 MPa, which puts extremely stringent requirements on the purity of molten steel, the uniformity of chemical composition, the gas content and the control of harmful trace elements. The key chemical composition control range is shown in Table 1: Table 1 Key chemical composition requirements (wt%) of G115 martensitic heat-resistant steel Meanwhile, the harmful residual elements (As+Sn+Sb+Pb+Bi≤0.035%), gas content (O≤40ppm) and inclusion level need to be controlled to ensure the long-term safe service of the unit.

[0003] Because the composition system of G115 steel is complex, the existing smelting process cannot take into account the following problems: 1. Difficulty in controlling sensitive elements: G115 steel has strict requirements on carbon content, which needs to be controlled at 0.06-0.10%. In the LF refining stage, a large amount of ferroalloy needs to be added to adjust the main elements such as chromium and tungsten, which will bring in carbon elements. At the same time, the traditional slag making process is slow and the slag layer is not well covered, resulting in significant electrode carbon increase in the early stage of LF refining, usually reaching 0.02-0.05%, which finally causes the carbon content of the finished product to easily exceed the upper limit of the specification.

[0004] Unstable recovery of boron and nitrogen elements and easy formation of harmful inclusions: Boron is crucial to improving the high-temperature strength of steel, but its chemical properties are active. The traditional one-time addition of B iron has a recovery rate fluctuation of 50%-90%, which is poor in controllability. More seriously, free boron and nitrogen elements in the steel are extremely easy to combine to form stable B-N inclusions, which seriously deteriorate the toughness and fatigue performance of the steel. How to achieve precise addition and stable recovery of boron and nitrogen elements, and avoid their interaction to form inclusions, is a core problem in the smelting process.

[0005] Introduction of cobalt element leads to excessive nickel element: G115 steel requires strict control of nickel content ≤0.10%, however, in order to achieve cobalt alloying, the traditional cobalt-iron alloy usually contains a high proportion of nickel impurities. A large amount of cobalt-iron alloy will introduce excessive nickel, causing the nickel element to exceed the standard, affecting the organizational stability and high-temperature performance of the steel.

[0006] 2. High process cost or poor applicability: Foreign patents (such as JP2010255024A) use VOD and other deep refining technologies. Although this technology can effectively control carbon and gas content, the equipment investment and operating cost is extremely high, which will increase the overall production cost by about 40%, and is not suitable for cost control requirements of large-scale industrial production.

[0007] Domestic patents (such as CN114635071B): Through die casting, forging, electroslag remelting process to improve steel purity, but new forging and electroslag remelting steps are added, and 2-3 days are additionally required from smelting to finished product, which not only prolongs the production cycle, but also increases the cost per ton of steel by 800-1000 yuan, making it difficult to meet the demand of large-scale production.

[0008] Therefore, it is urgent to develop a new smelting method based on existing mainstream equipment, which can stably and economically smelt high-purity G115 martensitic heat-resistant steel, to solve the series of problems in carbon control, accurate addition of boron and nitrogen, separation of cobalt and nickel, and high-purity control. SUMMARY

[0009] The purpose of the present application is to solve the above technical problems, and a G115 martensitic heat-resistant steel smelting method is proposed, which is based on conventional electric arc furnace (EF), ladle refining furnace (LF), and vacuum degassing device (VD) through a series of precise process control. Under the premise of not greatly increasing the cost, the high purity of molten steel and the accurate control of composition are realized, especially the stable control of carbon content, the avoidance of boron and nitrogen inclusions, and the prevention of excessive nickel, so as to smelt G115 steel which fully meets the performance requirements of 650℃ ultra-supercritical unit.

[0010] To achieve the above purpose, the technical scheme adopted by the present application is as follows: a G115 martensitic heat-resistant steel smelting method, comprising the following steps: Step 1) The forged steel head, heavy scrap steel, steel scrap and high-quality pig iron are put into the electric arc furnace for smelting, the end point C is controlled to be ≤0.05wt%, the end point P is controlled to be ≤0.005wt%, and the slag-free tapping is performed; Step 2) After the rough refining in the electric arc furnace, the rough refined molten steel is sent to the ladle refining furnace for refining, large amount of slag is used, and the lime and slag-making material are added in 2-3 batches, the total slag amount is controlled to be 3-5wt% of the total molten steel amount, the thickness is controlled to be 200-250mm, the slag composition is controlled to be CaO:55-65wt%, Al2O3:25-30wt%, SiO2≤5wt%, FeO+MnO≤0.8wt%, and the carbon increase during the refining period is controlled to be ≤0.02wt%; after the first batch of slag is burned into white slag, electrolytic cobalt and low-carbon low-phosphorus chromium iron alloy are added; Step 3) The molten steel obtained through the refining in step 2) is vacuum degassed under a protective atmosphere; Step 4) The molten steel after vacuum degassing is added with a chromium nitride alloy to control the nitrogen content to be 0.008-0.010wt%, and then is added with a silicon-calcium cored wire for deoxidation, and after the oxygen content reaches the set standard, an FeB alloy (ferroboron) is added to control the boron content to be 0.010-0.012wt%; Step 5) The molten steel after the treatment in step 4) is subjected to soft argon blowing and then is poured to obtain the G115 martensitic heat-resistant steel.

[0011] The application provides a core smelting framework of EF refining, LF refining, vacuum degassing, secondary refining and pouring. First, the end-point composition (C≤0.05wt%, P≤0.005wt%) is controlled from the source in the roughing stage, and the slag tapping operation is avoided to prevent the arc furnace slag from carrying impurities such as P and C into the ladle. In the first refining stage, a large amount of slag operation and batch slagging are beneficial to the thick slag layer formed quickly, which can effectively isolate air and absorb floating inclusions. The contact of the molten steel with the electrode and air is reduced, so that the carbon pick-up amount in the refining period is controlled to be ≤0.02%, and the problem of carbon element fluctuation in the refining period due to the electrode and carbon-containing slag is solved. The B and N elements are supplemented in the secondary refining, the loss of B and N elements in the vacuum degassing process is avoided, the B element is stabilized in a narrow interval of 0.010-0.012wt%, and the N element is controlled in the interval of 0.008-0.010wt%, so that the excellent high-temperature strength of the steel is ensured, and the formation of BN harmful inclusions is effectively avoided. Soft argon blowing promotes the floating of fine inclusions, and pouring under argon protection avoids secondary oxidation, so that the purity of the molten steel and the as-cast quality are ensured. Preferably, the step 1) comprises: The forged steel head, heavy scrap steel, steel scrap and high-quality pig iron are matched and smelted in the electric arc furnace, and the pig iron is finally added; The carbon-containing powder is sprayed through the carbon-oxygen gun of the furnace door to form foamed slag and make the furnace boil, and the end-point C is controlled to be ≤0.05wt%; After melting, when the carbon content is 0.08-0.12wt%, 600-800Kg / t of steel is added to form high-alkalinity foamed slag, and the end-point P is controlled to be ≤0.005wt%; During the tapping process, 1-1.5Kg / t of steel is added to aluminum ingot, 3-3.5Kg / t of steel is added to Si-Mn alloy, and 0.8-1.2Kg / t of steel is added to Si-Fe alloy for diffusion deoxidation, and after the tapping is completed, 6-8t of molten steel is retained in the electric furnace.

[0012] The pig iron is finally added to delay the melting of the pig iron (high carbon source), to melt the scrap steel first and form a molten pool, to optimize the smelting rhythm by using the melting point difference, to be conducive to the rapid pre-heating and slagging and dephosphorization. The aluminum ingot, Si-Mn alloy and Si-Fe alloy are used for synergistic deoxidization, and the deoxidization products are easy to float up, so as to avoid the residual oxidation inclusions in the molten steel.

[0013] Preferably, the raw material ratio of the electric arc furnace smelting is as follows: 30-45wt% of forged steel head and heavy scrap steel, 25-30wt% of steel scrap, and 30-35wt% of high-quality pig iron, and the total is 100wt%.

[0014] The CaO / SiO2 in the lime is greater than or equal to 3.0; The aluminum content in the aluminum ingot is greater than or equal to 99.7wt%, the Mn content in the Si-Mn alloy is 65-72wt%, the Si content is 17-20wt%, and the P content is less than or equal to 0.1wt%, and the Si content in the Si-Fe alloy is 72-75wt%, and the P content is less than or equal to 0.04wt%.

[0015] The raw material ratio is accurate to control the clean carbon in a reasonable range, to provide enough reactants for dephosphorization and decarburization in the oxidation period, and to consider the smelting fluidity and decarburization and dephosphorization efficiency. The high basicity of the lime improves the dephosphorization and desulfurization capacity of the slag, reduces the pollution of the slag system to the molten steel, the high purity of the alloy reduces the additional harmful elements brought by the alloy, and reduces the pressure of the refining process.

[0016] Preferably, the step 2) comprises: The first batch of lime is added according to 1.5-2.5wt% of the electric furnace tapping amount, the Si-Fe powder is added according to 1-1.5Kg / t of steel, the Si-Ca powder is added according to 0.5-0.8 Kg / t of steel for deoxidization and slagging, when the first batch of slag is burned into white slag, electrolytic cobalt is added according to 28.5-32 Kg / t of steel at 1620-1640℃, low-carbon low-phosphorus chromium iron alloy is added in batches according to 140-160 Kg / t of steel at 1640-1660℃; The second batch of lime is added according to 0.5-1.0wt% of the electric furnace tapping amount, the Si-Fe powder is added according to 0.5-0.8 Kg / t of steel, the Si-Ca powder is added according to 0.3-0.5 Kg / t of steel, and the carbon powder is added according to 0.5-1.0 Kg / t of steel for slag adjustment. After the second batch of slag is burned into white slag, the white slag is kept for more than or equal to 30 minutes, the slag sample in the furnace is analyzed, if the slag composition meets: CaO: 55-65wt%, Al2O3: 25-30wt%, SiO2≤5wt%, FeO+MnO≤0.8wt%, and the total slag amount is 3-5wt% of the total molten steel amount, the slagging operation is ended; If any condition is not met, the third batch of lime is added at 0.3-0.5wt% of the tapping amount of the electric furnace, and Si-Fe powder is added at 0.2-0.3Kg / t of steel to adjust the slag.

[0017] By batch slagging, the fluctuation of slag quantity caused by one-time slagging is avoided, the target slag system is gradually built, and the slag phase stability is improved. The first batch of materials builds the slag base, and in the second batch of slagging process, carbon powder is added for strengthening reduction to quickly reduce FeO in the slag to below 0.8%. After the formation of white slag in the first batch of slag, Co and Cr are added at a specific temperature to ensure high recovery rate. White slag means strong reducing atmosphere, which can prevent Co and Cr from being oxidized to the greatest extent, and high temperature of 1620-1660℃ ensures that Co and Cr with high melting point can quickly melt and dissolve.

[0018] Preferably, the CaO / SiO2 in the lime is ≥3.0; The Si content in Si-Fe powder is 72-75wt%, and the P content is ≤0.04wt%; The Ca content in Si-Ca powder is ≥31wt%, the Si content is 58-65wt%, the total content of Si and Ca is ≥90wt%, and the P content is ≤0.04% The C content in carbon powder is ≥90wt%.

[0019] The purity of lime basicity and various powders is limited to ensure the quality of refining environment. High-basicity lime is the basis for building high-basicity slag system, and low-phosphorus auxiliary materials prevent the back phosphorus to molten steel during slag adjustment.

[0020] Preferably, the low-carbon low-phosphorus chromium iron alloy is added in batches according to the mass ratio of 5:3:2, and the time interval of each batch is 15-20min. The Cr content in the low-carbon low-phosphorus chromium iron alloy is 60-65wt%, the C content is ≤0.03wt%, the P content is ≤0.03wt%, and the Si content is ≤1.0wt%. Adding chromium iron alloy in large quantities will bring in certain C, P and Si elements, causing carbon, phosphorus and silicon to rise, so low-carbon low-phosphorus chromium iron alloy is selected. Adding in three batches according to the ratio of 5:3:2 avoids the sudden drop of molten steel temperature caused by the addition of a large amount of cold charge at one time, and the uneven melting and composition segregation caused by the local concentration being too high, so that the molten steel has enough time to realize the homogenization of composition and temperature through argon stirring.

[0021] The cobalt content in electrolytic cobalt is ≥99.97wt%, and the nickel content is ≤0.020wt%. Traditional cobalt-iron alloy generally contains nickel element, in order to prevent the addition of Co element from causing Ni element to exceed the standard, electrolytic cobalt is used instead of cobalt-iron alloy.

[0022] Preferably, the step 3) is vacuum degassing for 20-23 min under the condition of argon flow 40-60 L / min and vacuum degree ≤67 Pa.

[0023] The high vacuum degree is beneficial to greatly reduce the gas partial pressure, accelerate the diffusion and escape of H and O atoms in the molten steel, break the boundary layer of the molten steel by argon stirring, promote the nucleation and floating of gas bubbles, and improve the degassing effect in cooperation with the high vacuum.

[0024] Preferably, the step 4) is adding chromium nitride alloy at 1580-1600 ℃ according to 3.0-3.6 Kg / t steel, controlling the N content target value to be 0.010wt%, controlling the N yield to be 92-95%, and controlling the N content to be 0.008-0.010wt%.

[0025] Sampling detection meets the requirement that the N content in the molten steel is 0.008-0.010wt%, and the silicon-calcium cored wire is added according to 0.4-0.6 Kg / t steel. Sampling detection meets the requirement that the O content in the molten steel is ≤25PPm, and the FeB alloy is added in batches according to 0.65-0.72 Kg / t steel at 1580-1590 ℃.

[0026] The temperature range of 1580-1600 ℃ is suitable for the dissolution rate of N in the molten steel, avoids the N escape caused by too high temperature and the insufficient dissolution caused by too low temperature, and guarantees the N yield. The O content ≤25ppm before adding B is a necessary condition, because B has a strong affinity with O, and if the O content in the molten steel is high, B will be oxidized into the slag first, resulting in a sharp drop in yield and the formation of inclusions. The Ca-Si wire is fed first to perform deep deoxidation, creating a chemical environment in which B can exist stably.

[0027] Preferably, the FeB alloy is wrapped with pure iron foil and inserted into the molten steel in three batches with an interval of 5 min, the first batch is added according to the B content target value of 0.005wt% and the recovery rate of 80%; the second batch is added according to the B content target value of 0.0035wt% and the recovery rate of 85%; and the third batch is added according to the B content target value of 0.003wt% and the recovery rate of 90%. The interval time between each time of inserting B iron is 5 min, so that the B element can be controlled in the range of 0.010-0.012wt%.

[0028] The pure iron foil wrapping is a physical protection method, which is to prevent the B iron from being oxidized by the oxidizing slag when it passes through the slag layer and directly reaches the inside of the molten steel, so as to further improve and stabilize the yield. The addition in three batches according to different recovery rates is based on experience, and with the gradual reduction of the O content in the molten steel and the optimization of conditions, the recovery rate of B will gradually increase.

[0029] Preferably, in the chromium nitride alloy, the content of Cr is 60-65wt%, the content of N is 3.0-3.5wt%, the content of C is ≤0.03wt%, and the content of P is ≤0.03wt%; In the FeB alloy, the content of B is 19-21wt%, and the content of P is ≤0.01wt%.

[0030] While supplementing the key elements N and B, no additional harmful impurities are introduced, the purity of molten steel is ensured, and the high-temperature strength and microstructure stability of molten steel are synergistically improved.

[0031] Preferably, in step 5), the stirring is continuously performed at an argon flow rate of 20-30L / min for 15-20min, then argon is introduced into the injection pipe at a flow rate of 40-60L / min to remove air, and the pouring is completed under the protection of argon at a flow rate of 30-40L / min.

[0032] The soft argon blowing process is to remove fine inclusions, and weak stirring avoids slag entrapment, but the mild argon bubbles can act as carriers to adsorb the tiny inclusions in the molten steel and carry them to the slag layer. The secondary oxidation of molten steel under the protection of argon throughout the pouring process is the last key barrier to ensure that the high-purity molten steel is finally formed into a high-purity steel ingot.

[0033] The beneficial effects of the present application are as follows: Accurate and stable component control is achieved: by using large amount of slag and batch slagging technology, the carbon content during LF refining is controlled to be ≤0.02%, solving the problem of carbon content fluctuation. By adopting the innovative method of sequential nitrogen, oxygen and boron, pure iron foil wrapping and three batches of addition, the recovery rate of easily oxidized boron element is stably controlled at 80%-90%, and the formation of BN harmful inclusions is effectively avoided. By selecting 99.97% high-purity electrolytic cobalt, the nickel element is prevented from exceeding the standard from the source.

[0034] The purity of molten steel is significantly improved: the optimized white slag process and soft argon blowing technology make the inclusions removal rate ≥95%, the content of gas and harmful elements in the finished steel is extremely low, and the level of various non-metallic inclusions is far superior to the standard requirements.

[0035] The process cost advantage is outstanding: the whole method is based on conventional EF+LF+VD equipment, and the expensive VOD technology is replaced by fine process innovation, which greatly reduces the production cost while ensuring the quality of top-grade products, and has excellent operability and market competitiveness. DETAILED DESCRIPTION

[0036] The principles and characteristics of the present application are described below in conjunction with examples, which are used to explain the present application and are not intended to limit the scope of the present application.

[0037] Example 1 A G115 martensitic heat-resistant steel smelting method comprises the following steps: Step 1: Arc furnace roughening 1.1 According to the weight, forge steel head and heavy scrap steel 35wt%, steel scrap 30wt%, high-quality pig iron 35wt% (the high-quality pig iron used in the example all meets the GB / T 718-2024 standard) are matched into the arc furnace for smelting; 1.2 The carbon powder is sprayed through the furnace door carbon oxygen gun to form foamed slag to make the furnace boil, and the end point C=0.05wt% is controlled; 1.3 When the carbon content is 0.09wt% after melting, add lime (CaO / SiO2=3.2) to form high-basicity foamed slag at 700Kg / t steel, and control the end point P=0.004wt%; 1.4 During the tapping process, add aluminum ingot (aluminum content=99.7wt%) at 1.3Kg / t steel, Si-Mn alloy (Mn content is 70wt%, Si content is 18wt%) at 3Kg / t steel, and Si-Fe alloy (Si content is 73wt%, P content=0.03wt%) at 1Kg / t steel for diffusion deoxidation, and after the tapping is completed, 7t of molten steel is retained in the electric furnace to ensure that the oxidized slag does not enter the refining ladle.

[0038] Step 2: First refining of ladle refining furnace 2.1 According to the electric furnace tapping amount 2.2wt%, add the first batch of lime, add Si-Fe powder at 1.2Kg / t steel, add Si-Ca powder at 0.6Kg / t steel for deoxidation and slagging, when the first batch of slag is burned into white slag, add electrolytic cobalt (cobalt content=99.98wt%, nickel content=0.020wt%) at 1625℃ at 30 Kg / t steel, add low-carbon low-phosphorus chromium iron alloy in three batches with a mass ratio of 5:3:2 at 1650℃ at 150 Kg / t steel, the time interval for each batch is 18min, the Cr content in the low-carbon low-phosphorus chromium iron alloy is 62wt%, the C content is 0.02wt%, the P content is 0.02wt%, and the Si content is 1.0wt%.

[0039] 2.2 According to the electric furnace tapping amount 1.0wt%, add the second batch of lime, add Si-Fe powder at 0.6Kg / t steel, add Si-Ca powder at 0.4Kg / t steel, and add carbon powder at 0.8 Kg / t steel for slag adjustment, after the second batch of slag is burned into white slag, the white slag is kept for 35 minutes, then the slag sample in the furnace is analyzed, the slag sample contains: CaO: 59wt%, Al2O3: 28wt%, SiO2: 5wt%, FeO+MnO=0.8wt%, and other unavoidable slag components; the total slag amount is 3.3wt% of the total molten steel amount, and the slag making operation is completed; The slag added above has CaO / SiO2=3.2 in lime; Si content of 73wt% and P content of 0.03wt% in Si-Fe powder; Ca content of 32wt%, Si content of 60wt%, total Si content and Ca content of 92wt%, and P content of 0.02% in Si-Ca powder; and C content of 92wt% in carbon powder.

[0040] Step 3: vacuum degassing The vacuum degassing was performed under the condition of argon flow of 45L / min and vacuum degree of 32Pa for 22min.

[0041] Step 4: second refining in ladle refining furnace After breaking the vacuum, chromium nitride alloy (Cr content of 63wt%, N content of 3.2wt%, C content of 0.03wt%, and P content of 0.02wt%) was added at 1590℃ according to 3.2 Kg / t of steel, and the sampling detection met the requirement of nitrogen content of 0.009wt% in molten steel according to the target value of nitrogen content of 0.010wt%, and silicon-calcium cored wire was added according to 0.45Kg / t of steel. The sampling detection showed that the oxygen content in molten steel was 22PPm, FeB alloy was added in batches according to 0.67Kg / t of steel at 1590℃, and pure iron foil was used to wrap FeB alloy (B content of 20wt% in FeB alloy, P content of 0.01wt%) and inserted into molten steel. The insertion was performed in three batches, the target value of the first batch was 0.005%, the recovery rate was calculated according to 80% for addition; the target value of the second batch was 0.0035%, the recovery rate was calculated according to 85% for addition; and the target value of the third batch was 0.003%, the recovery rate was calculated according to 90% for addition. The interval time between each insertion of B iron was 5min, the analysis of B element was 0.011%, the analysis of B element was 0.011%, and the recovery rate was 87.3%.

[0042] Step 5: soft argon blowing The stirring was continuously performed at argon flow of 25L / min for 18min.

[0043] Step 6: pouring The argon was blown into the injection tube to remove air at 50L / min when the ladle was lifted, and the pouring was completed under the protection of argon at 35L / min, and G115 martensitic heat-resistant steel was obtained.

[0044] The composition of the obtained G115 martensitic heat-resistant steel was analyzed, and the results are shown in Tables 2-3, and the mechanical property test results are shown in Table 4.

[0045] Table 2 Chemical composition of G115 martensitic heat-resistant steel Residual elements As+Sn+Sb+Pb+Bi=0.023%, and oxygen content was 15PPm.

[0046] Table 3 Composition of non-metallic inclusions in G115 martensitic heat-resistant steel Table 4 Mechanical properties of G115 martensitic heat-resistant steel Example 2 A G115 martensitic heat-resistant steel smelting method, comprising the following steps: Step 1: Electric arc furnace roughening 1.1 According to the weight, the forge steel head and heavy scrap steel 35wt%, steel scrap 30wt%, high-quality pig iron 35wt% are matched into the electric arc furnace for smelting; 1.2 The carbon content is controlled at 0.05wt% by blowing carbon powder through the furnace door carbon oxygen gun to make foamed slag and make the furnace boil; 1.3 When the carbon content is 0.09wt% after melting, add lime (CaO / SiO2=3.2) to make high-alkalinity foamed slag at 700Kg / t steel, and control the end point P=0.004wt%; 1.4 During the tapping process, add aluminum ingot (aluminum content=99.7wt%) at 1.3Kg / t steel, Si-Mn alloy (Mn content is 70wt%, Si content is 18wt%) at 3Kg / t steel, and Si-Fe alloy (Si content is 73wt%, P content=0.03wt%) at 1Kg / t steel for diffusion deoxidation, and after the end of tapping, 7t of molten steel is retained in the electric furnace to ensure that the oxidized slag does not enter the refining ladle.

[0047] Step 2: First refining of ladle refining furnace 2.1 Add the first batch of lime according to 2wt% of the electric furnace tapping amount, add Si-Fe powder at 1Kg / t steel, and add Si-Ca powder at 0.5Kg / t steel for deoxidation and slagging, when the first batch of slag is burned into white slag, add electrolytic cobalt (cobalt content=99.98wt%, nickel content=0.020wt%) at 1620℃ according to 28.5Kg / t steel, add low-carbon low-phosphorus chromium iron alloy at 1640℃ according to 140Kg / t steel in three batches with a mass ratio of 5:3:2, the interval between each batch is 15min, the Cr content in the low-carbon low-phosphorus chromium iron alloy is 60wt%, the C content=0.03wt%, the P content=0.03wt%, and the Si content=1.0wt%.

[0048] 2.2 The second batch of lime is added at 1 wt% of the tapping amount of the electric furnace, Si-Fe powder is added at 0.5 Kg / t of steel, Si-Ca powder is added at 0.3 Kg / t of steel, and carbon powder is added at 0.5 Kg / t of steel for slag conditioning. After the second batch of slag is burned into white slag, the white slag is maintained for 30 minutes, and then a slag sample in the furnace is analyzed. The slag sample contains: CaO: 55 wt%, Al2O3: 25 wt%, SiO2: 5 wt%, FeO+MnO: 0.8 wt%, and other unavoidable slag components, and the total slag amount is 3 wt% of the total molten steel amount, and the slag making operation is completed. In the above-mentioned added slag, the CaO / SiO2 in the lime is 3.2; the Si content in the Si-Fe powder is 73 wt%, and the P content is 0.03 wt%; the Ca content in the Si-Ca powder is 32 wt%, the Si content is 60 wt%, the total Si content and Ca content is 92 wt%, and the P content is 0.02%; and the C content in the carbon powder is 92 wt%.

[0049] Step 3: Vacuum degassing Under the condition of argon flow rate 45 L / min and vacuum degree 32 Pa, vacuum degassing is performed for 22 min.

[0050] Step 4: Second refining in ladle refining furnace After breaking the vacuum, chromium nitride alloy (Cr content 63 wt%, N content 3.2 wt%, C content 0.03 wt%, P content 0.02 wt%) is added at 3.2 Kg / t of steel at 1590℃, and the nitrogen content target value is adjusted to 0.010 wt%. Sampling detection shows that the nitrogen content in the molten steel is 0.009 wt%. Silicon-calcium cored wire is added at 0.45 Kg / t of steel. Sampling detection shows that the oxygen content in the molten steel is 22 PPM. FeB alloy is added in batches at 0.67 Kg / t of steel at 1590℃, and pure iron foil is used to wrap the FeB alloy (B content 20 wt% in FeB alloy, P content 0.01 wt%). The FeB alloy is inserted into the molten steel in three batches. The target value of the first batch is 0.005%, the recovery rate is calculated as 80%; the target value of the second batch is 0.0035%, the recovery rate is calculated as 85%; and the target value of the third batch is 0.003%, the recovery rate is calculated as 90%. The interval time between each insertion of B-iron is 5 minutes.

[0051] Step 5: Soft argon blowing The argon flow rate is 25 L / min for continuous stirring for 18 min.

[0052] Step 6: Pouring When the ladle is lifted, argon is blown into the injection pipe at a flow rate of 50 L / min to remove air. The pouring is completed under the protection of argon at a flow rate of 35 L / min, and G115 martensitic heat-resistant steel is obtained.

[0053] The composition of the obtained G115 martensitic heat-resistant steel was analyzed, and the results are shown in Tables 5-6, and the mechanical property test results are shown in Table 7.

[0054] Table 5 Chemical composition of G115 martensitic heat-resistant steel Residual elements As + Sn + Sb + Pb + Bi = 0.023%, and the oxygen content is 20 PPM.

[0055] Table 6 Composition of non-metallic inclusions in G115 martensitic heat-resistant steel Table 7 Mechanical properties of G115 martensitic heat-resistant steel Example 3 A G115 martensitic heat-resistant steel smelting method, comprising the following steps: Step 1: Electric arc furnace refining 1.1 According to the weight, the forged steel head and heavy scrap steel 35wt%, steel scrap 30wt%, high-quality pig iron 35wt% are mixed into the electric arc furnace for smelting; 1.2 Make the furnace boil by blowing carbon powder through the carbon-oxygen gun of the furnace door to form a foamed slag, and control the end point C=0.05wt%; 1.3 After refining, when the carbon content is 0.09wt%, add lime (CaO / SiO2=3.2) to form high-alkalinity foamed slag at 700Kg / t steel, and control the end point P=0.004wt%; 1.4 During the tapping process, add aluminum ingots (aluminum content=99.7wt%) at 1.3Kg / t steel, Si-Mn alloy (Mn content is 70wt%, Si content is 18wt%) at 3Kg / t steel, and Si-Fe alloy (Si content is 73wt%, P content=0.03wt%) at 1Kg / t steel for diffusion deoxidation, and after the end of tapping, 7t of molten steel is retained in the electric furnace to ensure that the oxidized slag does not enter the refining ladle.

[0056] Step 2: First refining of ladle refining furnace 2.1 The first batch of lime is added at 2.5wt% of the tapping amount of the electric furnace, Si-Fe powder is added at 1.5 Kg / t of steel, Si-Ca powder is added at 0.8 Kg / t of steel for deoxidation and slagging, when the first batch of slag is burned into white slag, electrolytic cobalt (cobalt content = 99.98wt%, nickel content = 0.020wt%) is added at 32 Kg / t of steel at 1640℃, low-carbon low-phosphorus chromium iron alloy is added in three batches with a mass ratio of 5:3:2 at 160 Kg / t of steel at 1660℃, the interval between each batch is 15-20 min, the Cr content in the low-carbon low-phosphorus chromium iron alloy is 65wt%, the C content = 0.03wt%, the P content = 0.03wt%, and the Si content = 1.0wt%.

[0057] 2.2 The second batch of lime is added at 1.0wt% of the tapping amount of the electric furnace, Si-Fe powder is added at 0.8 Kg / t of steel, Si-Ca powder is added at 0.5 Kg / t of steel, carbon powder is added at 1.0 Kg / t of steel for slag adjustment, after the second batch of slag is burned into white slag, the white slag is kept for 30 minutes, then the slag sample in the furnace is analyzed, the slag sample contains: CaO: 65wt%, Al2O3: 30wt%, SiO2: 5wt%, FeO+MnO = 0.8wt%, and other unavoidable slag components; the total slag amount is 3.5wt% of the total molten steel amount, and the slagging operation is ended; In the above-mentioned slag, the CaO / SiO2 in the lime is 3.2; the Si content in the Si-Fe powder is 73wt%, the P content = 0.03wt%; the Ca content in the Si-Ca powder is 32wt%, the Si content is 60wt%, the total Si content and Ca content is 92wt%, the P content = 0.02%; the C content in the carbon powder is 92wt%.

[0058] Step 3: vacuum degassing Under the condition of argon flow 45L / min and vacuum degree 32Pa, vacuum degassing is carried out for 22 min.

[0059] Step 4: second refining in ladle refining furnace After breaking the vacuum, chromium nitride alloy (Cr content 63wt%, N content 3.2wt%, C content = 0.03wt%, P content = 0.02wt%) is added at 3.2 Kg / t of steel at 1590℃, the nitrogen content target value is adjusted to 0.010wt%, sampling detection meets the nitrogen content in molten steel of 0.009wt%, and silicon-calcium cored wire is added at 0.45 Kg / t of steel; Sampling detection meets the oxygen content in molten steel = 22PPm, at 1590℃, according to 0.67Kg / t steel, FeB alloy is added in batches, pure iron foil is used to wrap FeB alloy (B content in FeB alloy is 20wt%, P content = 0.01wt%) and inserted into molten steel. It is inserted in three batches, the target value of the first batch is 0.005%, the recovery rate is calculated as 80% to add; the target value of the second batch is 0.0035%, the recovery rate is calculated as 85% to add; the target value of the third batch is 0.003%, the recovery rate is calculated as 90% to add. The interval time between each insertion of B iron is 5 minutes, and the B element is analyzed as 0.011%.

[0060] Step 5: soft argon blowing With argon flow rate of 25L / min, stirring for 18min.

[0061] Step 6: pouring When the ladle is lifted, argon is blown into the injection tube at a flow rate of 50L / min to remove air, and pouring is completed under the protection of argon at a flow rate of 35L / min, obtaining G115 martensitic heat-resistant steel.

[0062] The composition of the obtained G115 martensitic heat-resistant steel is analyzed, and the results are shown in Tables 8-9, and the mechanical property test results are shown in Table 10.

[0063] Table 8 Chemical composition of G115 martensitic heat-resistant steel Residual elements As+Sn+Sb+Pb+Bi = 0.023%, oxygen content is 22PPm.

[0064] Table 9 Composition of non-metallic inclusions in G115 martensitic heat-resistant steel Table 10 Mechanical properties of G115 martensitic heat-resistant steel Comparative Example 1 In order to explore the effect of B addition in molten steel, in this comparative example, FeB alloy is added in the second refining of the ladle refining furnace in step 4 using another different process condition, and the other steps are consistent with Example 1.

[0065] Process condition a of FeB alloy addition in the second refining Sampling detection meets the oxygen content in molten steel = 22PPm, at 1590℃, according to 0.67Kg / t steel, FeB alloy is added in batches, pure iron foil is used to wrap FeB alloy inserted into molten steel, 10 minutes later, the B element content is analyzed as 0.0065wt%, and the recovery rate is 51.5%.

[0066] Process condition b of FeB alloy addition in the second refining The oxygen content in the molten steel is 22PPm, FeB alloy is added into the argon spouting of the molten steel at 1590℃ according to 0.67Kg / t steel in three batches, the target value of the first batch is 0.005%, the recovery rate is calculated according to 80%; the target value of the second batch is 0.0035%, the recovery rate is calculated according to 85%; the target value of the third batch is 0.003%, the recovery rate is calculated according to 90%. The interval time between each time of inserting B iron is 5 minutes, the B element content is 0.0052 wt%, and the recovery rate is 41.2%.

[0067] The process condition c of adding FeB alloy in the second refining: The oxygen content in the molten steel is 48PPm, FeB alloy is added into the argon spouting of the molten steel at 1590℃ according to 0.67Kg / t steel in three batches, the target value of the first batch is 0.005%, the recovery rate is calculated according to 80%; the target value of the second batch is 0.0035%, the recovery rate is calculated according to 85%; the target value of the third batch is 0.003%, the recovery rate is calculated according to 90%. The interval time between each time of inserting B iron is 5 minutes, the B element content is 0.0043 wt%, and the recovery rate is 34.2%.

[0068] Compared with example 1, the difference of process condition a is that all FeB alloy is added at one time, which leads to uneven dispersion in the molten steel, and high concentration area will react with N in the molten steel to generate BN inclusions, resulting in a decrease in yield. Compared with example 1, the difference of process condition b is that FeB is not wrapped with iron foil, but is directly added into the molten steel. When the unwrapped FeB alloy is added, the surface will directly contact the air on the surface of the molten steel or the residual oxygen in the molten steel, and the oxidation reaction will occur rapidly, resulting in a decrease in yield. Compared with process condition b, process condition c is that FeB alloy is added when the oxygen content is high. When the oxygen content in the molten steel is greater than 25PPm, B in the added FeB alloy will preferentially react with oxygen, and most of it will be oxidized and consumed, resulting in a decrease in yield.

[0069] The above is only a preferred embodiment of the present application, and does not limit the present application in any form; any ordinary skilled person in the industry can easily implement the present application according to the description and the above; however, any equivalent changes, modifications and evolution made by the skilled person in the professional field without departing from the technical solution of the present application, using the disclosed technical content, are equivalent embodiments of the present application; at the same time, any equivalent changes, modifications and evolution of the above embodiments according to the essential technology of the present application are still within the protection scope of the technical solution of the present application.

Claims

1. A method for smelting G115 martensitic heat-resistant steel, characterized in that, Includes the following steps: Step 1) The forged steel scrap, heavy scrap steel, steel chips, and high-quality pig iron are fed into the electric arc furnace for smelting. The final C is controlled to be ≤0.05wt% and the final P is controlled to be ≤0.005wt%, and the steel is tapped without slag. Step 2) The molten steel obtained after roughing in the electric arc furnace is sent to the ladle refining furnace for refining. Lime and slag-forming materials are added in 2-3 batches, and the total slag amount is controlled to be 3-5 wt% of the total molten steel amount. The slag composition is controlled to maintain CaO: 55-65 wt%, Al2O3: 25-30 wt%, SiO2 ≤ 5 wt%, FeO + MnO ≤ 0.8 wt%, and the carbon increase during the refining period is controlled to be ≤ 0.02 wt%. After the first batch of slag is burned into white slag, electrolytic cobalt and low-carbon, low-phosphorus ferrochrome alloy are added. Step 3) The molten steel obtained after refining in Step 2) is subjected to vacuum degassing; Step 4) The vacuum-degassed molten steel is fed into a ladle refining furnace for a second refining process. Chromium nitride alloy is added, and the nitrogen content is controlled at 0.008-0.010 wt%. Then, silicon-calcium cored wire is added to deoxidize the molten steel. FeB alloy is added, and the boron content is controlled at 0.010-0.012 wt%. Step 5) After the molten steel obtained from the secondary refining in Step 4) is soft-blown with argon, it is then cast to obtain the G115 martensitic heat-resistant steel.

2. The method for smelting G115 martensitic heat-resistant steel according to claim 1, characterized in that, Step 1) includes: Forged steel scrap, heavy scrap steel, steel chips, and high-quality pig iron are mixed into an electric arc furnace for smelting, with the high-quality pig iron being added last. Carbonaceous powder is sprayed into the furnace door through a carbon-oxygen lance to create foamy slag, causing the furnace to boil, with the endpoint C ≤ 0.05 wt% controlled. After melting and cleaning, when the carbon content is 0.08-0.12wt%, add lime at a rate of 600-800 kg / t of steel to create a high-alkalinity foamy slag, controlling the endpoint P≤0.005%; During the tapping process, aluminum ingots are added at a rate of 1-1.5 kg / t of steel, Si-Mn alloy is added at a rate of 3-3.5 kg / t of steel, and Si-Fe alloy is added at a rate of 0.8-1.2 kg / t of steel for diffusion deoxidation. After tapping, 6-8 tons of molten steel are retained in the electric furnace.

3. The method for smelting G115 martensitic heat-resistant steel according to claim 2, characterized in that, The raw material ratio for the electric arc furnace smelting is as follows: 30-45 wt% forged steel scrap and heavy scrap steel, 25-30 wt% steel chips, and 30-35 wt% high-quality pig iron, totaling 100 wt%.

4. The method for smelting G115 martensitic heat-resistant steel according to claim 1, characterized in that, Step 2) includes: Add the first batch of lime at 1.5-2.5 wt% of the electric furnace steel output, add Si-Fe powder at 1-1.5 kg / t of steel, and add Si-Ca powder at 0.5-0.8 kg / t of steel for deoxidation and slag formation. After the first batch of slag is burned into white slag, add electrolytic cobalt at 28.5-32 kg / t of steel at 1620-1640℃, and add low-carbon, low-phosphorus ferrochrome alloy in batches at 140-160 kg / t of steel at 1640-1660℃. Add a second batch of lime at 0.5-1.0 wt% of the electric furnace steel output, add Si-Fe powder at 0.5-0.8 kg / t steel, add Si-Ca powder at 0.3-0.5 kg / t steel, and add carbon powder at 0.5-1.0 kg / t steel for slag conditioning. After the second batch of slag turns into white slag, and the white slag is maintained for ≥30 minutes, analyze the slag sample in the furnace. If the slag composition meets the following conditions: CaO: 55-65 wt%, Al2O3: 25-30 wt%, SiO2 ≤ 5 wt%, FeO + MnO ≤ 0.8 wt%, and the total slag amount is 3-5 wt% of the total molten steel, the slag-making operation ends. If any condition is not met, a third batch of lime and Si-Fe powder is added for slag conditioning.

5. The method for smelting G115 martensitic heat-resistant steel according to claim 4, characterized in that, The lime contains CaO / SiO2 ≥ 3.0; The Si-Fe powder contains 72-75 wt% Si and ≤0.04 wt% P. The Si-Ca powder contains ≥31 wt% Ca, 58-65 wt% Si, ≥90 wt% total Si and Ca content, and ≤0.04% P content. The carbon content in the toner is ≥90wt%.

6. The method for smelting G115 martensitic heat-resistant steel according to claim 4, characterized in that, The low-carbon, low-phosphorus ferrochrome alloy is added in batches at a mass ratio of 5:3:2, with an interval of 15-20 minutes between each batch. The low-carbon, low-phosphorus ferrochrome alloy contains 60-65 wt% Cr, ≤0.03 wt% C, ≤0.03 wt% P, and ≤1.0 wt% Si.

7. The method for smelting G115 martensitic heat-resistant steel according to claim 1, characterized in that, In step 3), vacuum degassing is performed for 20-23 minutes under conditions of argon flow rate of 40-60 L / min and vacuum degree of 5-67 Pa.

8. The method for smelting G115 martensitic heat-resistant steel according to claim 1, characterized in that, In step 4), chromium nitride alloy is added at 1580-1600℃ at a rate of 3.0-3.6 Kg / t steel. Sampling and testing shall meet the requirement that the nitrogen content in the molten steel is 0.008-0.010 wt%, and silicon-calcium cored wire shall be added at a rate of 0.4-0.6 kg / t of steel; Sampling and testing ensured that the oxygen content in the molten steel was ≤25 ppm. FeB alloy was added in batches at 1580-1590℃, at a rate of 0.65-0.72 Kg / t steel.

9. A method for smelting G115 martensitic heat-resistant steel according to claim 8, characterized in that, FeB wrapped in pure iron foil was inserted into molten steel in three batches at 5-minute intervals. The first batch was added with a target B content of 0.005 wt% and a recovery rate of 80%; the second batch was added with a target B content of 0.0035 wt% and a recovery rate of 85%; and the third batch was added with a target B content of 0.003 wt% and a recovery rate of 90%.

10. A method for smelting G115 martensitic heat-resistant steel according to claim 1, characterized in that, In step 5), the mixture is stirred continuously for 15-20 minutes at an argon flow rate of 20-30 L / min. Then, during the hoisting process, argon is introduced into the injection pipe at a flow rate of 40-60 L / min to remove air. The casting is then completed under the protection of argon at a flow rate of 30-40 L / min.

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