Melting method of martensitic stainless steel

Abstract

The invention belongs to the technical field of steel casting smelting, and particularly relates to a smelting method of martensitic stainless steel, which adopts an EAF + LF + VOD smelting process and comprises the following specific steps: material preparation: feeding a furnace material by using martensitic return scrap of the same material, and removing various impurities on the surface of the return scrap; adding lime, ferrosilicon, carbon powder and fluorite in set amount; smelting in an EAF furnace: pre-blowing oxygen after the furnace temperature reaches 1570-1600 DEG C, and then blowing oxygen in batches; smelting in an LF furnace: controlling the element C to be 0.3 to 0.4 percent, the element Cr to be 12 to 12.5 percent, the element Ni to be 4 to 4.2 percent, the element Mo to be 0.45 to 0.55 percent and the element S to be below 0.005 percent; smelting in a VOD furnace: controlling the oxygen blowing vacuum degree to be 6000-8000Pa, controlling the oxygen flow to be 500-800Nm < 3 > / h, and starting to blow oxygen; by adopting the method disclosed by the invention, not only can the requirements of technical specifications be met, but also the problem of overstock of furnace charges (return charges) faced by enterprises can be solved.

Description

Technical Field

[0001] The invention belongs to the technical field of steel casting smelting, and particularly relates to a smelting method for martensitic stainless steel. Background Art

[0002] When designing castings in foundries, risers, runners, and allowances are usually set up to improve the casting quality of the castings. After the castings cool and solidify, these parts will be cut off to form recyclable materials. Generally, the yield rate of large steel castings is about 50%, which means that recyclable materials equal to the weight of the castings will be produced. If these materials are not used in time, it will lead to inventory backlogs, and a large amount of alloy materials will need to be purchased, which will increase costs. Therefore, effective measures must be taken to improve the utilization rate of recyclable materials and reduce the demand for raw materials such as alloys to reduce the burden of production costs. However, the large-scale use of recyclable materials may cause the content of residual elements such as phosphorus (P) and vanadium (V) in steel to exceed the standard. In view of this, in the process of smelting martensitic stainless steel, how to improve the process flow and optimize the smelting scheme to ensure that the residual element content does not exceed the standard while using a large amount of recyclable materials needs to be solved urgently. Summary of the invention

[0003] In order to solve the problems in the prior art, the present application provides a method for smelting martensitic stainless steel, which adopts the EAF+LF+VOD smelting process, and the specific steps include:

[0004] Material preparation: Use martensitic return materials of the same material as the furnace charge, and remove all kinds of impurities on the surface of the return materials; add a set amount of lime, ferrosilicon, carbon powder and fluorite;

[0005] EAF furnace smelting: pre-blow oxygen after the furnace temperature reaches 1570-1600℃, and then blow oxygen in batches. Take samples for analysis during the process. If C≤0.3%, Si≤0.5%, V≤0.03%, stop blowing oxygen. Otherwise, continue blowing oxygen until C≤0.3%, Si≤0.5%, V≤0.03% are met;

[0006] LF furnace smelting: C element is controlled at 0.3-0.4%, Cr element is controlled at 12-12.5%, Ni element is controlled at 4-4.2%, Mo element is controlled at 0.45-0.55%, and S element is controlled below 0.005%;

[0007] VOD furnace smelting: oxygen vacuum degree is controlled at 6000~8000Pa, oxygen flow rate is controlled at 500~800Nm 3 / h, argon flow rate is controlled at 200~400Nm 3 / h, start blowing oxygen.

[0008] In one embodiment, the chemical composition of the martensitic stainless steel is C≤0.04%, Si≤0.60%, Mn≤1.0%, P≤0.028%, S≤0.008%, 12.0%≤Cr≤13.5%, 0.4%≤Mo≤1.0%, 3.8%≤Ni≤5.0%, V≤0.018%, N≤0.018%, H≤0.0003%, O≤0.008%, Cu≤0.5%, and W≤0.1%.

[0009] In one embodiment, in the material preparation step, since the diameter of the furnace shell is 5500mm, the size of the return material is controlled to be less than or equal to 500mm. If the size of the return material is too large, it is easy to form bridges during the smelting process, causing material collapse and electrode breakage. At the same time, the return material is large in size and the melting speed is slow. Therefore, the size of the return material must be qualified to ensure that it can be melted quickly during the smelting process.

[0010] In one embodiment, in the material preparation step, the set amount of lime added is 1-1.5% of the total weight of the molten steel. If too little lime is added, the purpose of slag making and dephosphorization cannot be achieved. If too much lime is added, the basicity of the slag is too high, it is difficult to melt, the viscosity increases, and the slag cannot play its role. The set amount of ferrosilicon is 0.6-1.5% of the total weight of the molten steel. If too little ferrosilicon is added, the reduction effect cannot be achieved. If too much ferrosilicon is added, the SiO2 content in the oxidized slag increases, the basicity decreases, and it is difficult to remove phosphorus. The set amount of carbon powder is 0.03-0.06% of the total weight of the molten steel, and the set amount of fluorite is 0.002-0.005% of the total weight of the molten steel.

[0011] In one embodiment, in the EAF furnace smelting step, oxygen is pre-blown after the furnace temperature reaches 1570-1600°C, and the oxygen flow rate of the pre-blowing oxygen is controlled at 1000-2000 Nm 3 / h, oxygen blowing volume is 150~200m 3 Since the added ferrosilicon, lime, etc. are unevenly distributed in the furnace, if the main blowing is carried out directly, the oxygen flow rate will be large and the reaction will be fierce, which may easily cause explosion and safety accidents. Therefore, a small flow rate should be pre-blown and stirred to make the molten steel uniform.

[0012] In one embodiment, in the EAF furnace smelting step, when oxygen is blown in batches, the time of each oxygen blowing is less than or equal to 5 minutes. If the continuous oxygen blowing time is too long, the reaction will be intense and may easily cause large boiling, so it is necessary to adopt batch oxygen blowing operation.

[0013] In one embodiment, in the EAF furnace smelting step, after stopping the oxygen blowing operation, lime is added for slag formation, and the amount of lime added is 1-1.5% of the total amount of molten steel. If too little lime is added, the basicity is low, phosphorus is easily regenerated, and the protective slag cannot be formed; if too much lime is added, the basicity of the slag is too high, the melting point is high, and it is difficult to melt quickly, which also causes cost waste.

[0014] In one embodiment, in the LF furnace smelting step, the molten steel is first heated after reaching the LF furnace, and the slag is removed after the temperature reaches 1620-1650° C. If the temperature is too low, the slag is difficult to melt and has poor fluidity, and it is difficult to remove the slag cleanly; if the temperature is too high, the erosion of the ladle refractory is aggravated, posing a safety hazard.

[0015] In one embodiment, in the LF furnace smelting step, 50-100 kg of heat preservation agent is added to cover the surface of the molten steel before slagging, which plays a role of heat preservation and prevents the slag from condensing into a shell, making the slagging process difficult to operate.

[0016] In one embodiment, in the LF furnace smelting step, 3-5 kg / ton of steel Al is added for reduction, and 15-20 kg / ton of steel active lime is added for slag making of molten steel. The reduction slag is made by adding Al and active lime to improve the deoxidation and desulfurization capacity of the slag. Too little Al will not have a reduction effect, and too much Al will lead to a high residual Al content in the steel, affecting the performance of the casting. Active lime should also be kept in an appropriate amount. Too little Al will cause insufficient alkalinity of the slag, making it difficult to desulfurize, and too much Al will waste costs.

[0017] In one embodiment, in the VOD furnace smelting step, after the oxygen blowing is completed, the vacuum degree is increased to below 67 Pa and maintained for 15 minutes for vacuum degassing refining. If the high vacuum is maintained for a short time, the degassing and removal of inclusions are insufficient; if the time is too long, the temperature of the molten steel is low, and the degassing and removal of inclusions capabilities are reduced.

[0018] Compared with the conventional control process, the present invention has the following beneficial effects:

[0019] The martensitic stainless steel smelting method involved in the present invention creatively adds Si (silicon), C (carbon) and lime in the EAF (electric arc furnace) smelting stage, and implements an oxygen blowing process to create an oxidizing atmosphere. This process helps to remove part of the P (phosphorus) in the steel, and at the same time, part of the V (vanadium) is oxidized during the oxygen blowing process, effectively preventing the vanadium content from exceeding the standard. The oxygen blowing operation promotes the stirring of the molten steel in the molten pool, accelerates the melting speed of the furnace bottom material, and reduces the bonding phenomenon of the high-alloy material in the furnace. The method described in the present invention can not only meet the requirements of technical specifications, but also solve the problem of backlog of furnace charge (return material) faced by enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] none. DETAILED DESCRIPTION

[0021] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively below, and preferred embodiments of the present invention are provided. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thoroughly and comprehensively understood.

[0022] This embodiment provides a smelting method for ZG04Cr13Ni4Mo martensitic stainless steel castings, whose chemical composition is: C≤0.04%, Si≤0.60%, Mn≤1.0%, P≤0.028%, S≤0.008%, 12.0%≤Cr≤13.5%, 0.4%≤Mo≤1.0%, 3.8%≤Ni≤5.0%, V≤0.018%, N≤0.018%, H≤0.0003%, O≤0.008%, Cu≤0.5%, W≤0.1%. The EAF+LF+VOD smelting process is adopted, and the specific smelting steps include:

[0023] Material preparation: Use martensitic return materials of the same material as the charge, and remove all kinds of impurities on the surface of the return materials to avoid impurities being brought into the molten steel and reduce the oxidation of Cr, Fe and other elements in the molten steel; add a set amount of lime, ferrosilicon, carbon powder and fluorite; place them at the bottom and middle of the charge respectively, with a ratio of 1:1, and evenly spread them on the surface of the charge to avoid concentrated reactions during the smelting process, which will cause the local temperature of the molten steel to be too high, thereby damaging the furnace lining.

[0024] EAF furnace smelting: This process is crucial to the completion of smelting castings. The key to achieving return dephosphorization rather than rephosphorization in an electric arc furnace lies in the precise control of the basicity and temperature of the slag. In addition, during the oxygen blowing process, the oxidation heat release of the elements will cause a sharp increase in temperature, which will not only affect the dephosphorization process, but also aggravate the erosion of the furnace lining. Process control is crucial. After the molten steel is placed in the EAF furnace, it is powered for melting. After the furnace temperature reaches 1570-1600℃, oxygen is pre-blown, and then oxygen is blown in batches. Samples are taken for analysis during the process. If C≤0.3%, Si≤0.5%, and V≤0.03%, oxygen blowing is stopped. Otherwise, oxygen blowing is continued until C≤0.3%, Si≤0.5%, and V≤0.03% are met.

[0025] LF furnace smelting: the C element is controlled at 0.3-0.4%, the Cr element is controlled at 12-12.5%, the Ni element is controlled at 4-4.2%, the Mo element is controlled at 0.45-0.55%, and the S element is controlled below 0.005%.

[0026] VOD furnace smelting: oxygen vacuum degree is controlled at 6000~8000Pa, oxygen flow rate is controlled at 500~800Nm 3 / h, argon flow rate is controlled at 200~400Nm 3 / h, start blowing oxygen.

[0027] Specifically:

[0028] (1) In the material preparation step: to ensure rapid melting during the smelting process, the size of the return material is less than or equal to 500 mm; the set amount of lime added is 1-1.5% of the total weight of the molten steel, the set amount of ferrosilicon is 0.6-1.5% of the total weight of the molten steel, the set amount of carbon powder is 0.03-0.06% of the total weight of the molten steel, and the set amount of fluorite is 0.002-0.005% of the total weight of the molten steel.

[0029] (2) In the EAF furnace smelting step: after the furnace temperature reaches 1570-1600°C, pre-blow oxygen, and the oxygen flow rate of pre-blow oxygen is controlled at 1000-2000Nm 3 / h, oxygen blowing volume is 150~200m 3 , mainly plays the role of stirring the molten steel and making it uniform and sufficient. Then take samples to test the chemical composition of the molten steel. The purpose of sampling at this time is mainly to analyze elements such as C, P, and V. Control the temperature of the molten steel at 1580-1600℃, and increase the oxygen flow rate to 1500-2500Nm 3 / h, main blowing and decarburization are carried out, and the Si element and part of the V element in the molten steel are oxidized through the decarburization process. The generated SiO2 has a low melting point, which can increase the viscosity of the slag and reduce the sticking furnace phenomenon.

[0030] During the EAF furnace smelting process, oxygen blowing should be carried out in batches, and the oxygen blowing time should be controlled within 5 minutes each time. Oxygen blowing should not be concentrated to prevent the rapid temperature rise of the molten steel. After 20 minutes, the furnace door should be tilted downward by about 30 degrees to let the oxidized slag flow out. At the same time, the height of the furnace door should be continuously monitored to ensure that the molten steel does not overflow. When the oxygen blowing volume reaches 500m 3 Afterwards, sampling is carried out for testing. If the test results show that the carbon content C≤0.3%, the silicon content Si≤0.5%, and the vanadium content V≤0.03%, stop blowing oxygen; if the standards are not met, continue blowing oxygen until all indicators are qualified, and continue to tilt the furnace door to discharge the oxidized slag until the oxidized slag is completely discharged.

[0031] After stopping the oxygen blowing operation, a total amount of 1 to 1.5% lime is added to the molten steel to form new slag, which is used to increase the basicity of the slag and thus prevent rephosphorization.

[0032] The temperature measurement program is executed in real time, and the steel tapping operation is carried out when the temperature of the molten steel rises to 1650-1680℃. The steel tapping method is adopted with large flow and fast speed, and after the steel tapping is completed, check whether there is any molten steel adhering to the bottom of the furnace. If adhesion is found, the power should continue to be supplied to melt the adhering molten steel and ensure that all the molten steel is completely transferred to the ladle.

[0033] (3) In the LF furnace smelting step, the molten steel is first heated after reaching the LF furnace, and slag is removed after the temperature reaches 1620-1650°C. Before slag removal, 50-100 kg of a heat preservation agent is added to prevent the steel slag from solidifying and forming a crust.

[0034] 3-5 kg / ton of Al particles and 15-20 kg / ton of active lime are added to the molten steel for slag making and reduction treatment. Sampling and analysis are then carried out to determine the chemical composition, and appropriate amounts of alloys are added according to the actual composition of the molten steel.

[0035] The C content is controlled between 0.3 and 0.4%, the Cr content is set to 12 to 12.5%, the Ni content is 4 to 4.2%, the Mo content is 0.45 to 0.55%, and the S content is controlled below 0.005%. Easily oxidizable elements such as Si and Mn should be added after VOD (vacuum oxygen decarburization) treatment. Finally, the temperature is controlled within the range of 1610 to 1630°C before smelting in the VOD furnace.

[0036] (4) In the VOD furnace smelting step, the main purpose is to "remove carbon and maintain chromium content". The oxygen blowing amount must be strictly controlled to avoid excessive oxidation of the molten steel, and the composition needs to be adjusted quickly to quickly complete steel tapping.

[0037] The oxygen blowing vacuum should be controlled between 6000 and 8000 Pa, and the oxygen flow rate should be adjusted to 500 to 800 Nm 3 / h, at the same time, the argon flow rate should be set at 200~400Nm 3 / h, then start blowing oxygen. Calculate the oxygen blowing amount according to the formula: oxygen blowing amount = (actual carbon content - target carbon content) * molten steel weight / oxygen recovery rate, and the oxygen recovery rate is between 0.3% and 0.4%.

[0038] After the oxygen blowing process is completed, the vacuum degree needs to be increased to below 67 Pa and maintained at this state for 15 minutes, followed by VD (vacuum degassing refining).

[0039] Add 0.5-1kg / ton of active lime, 2-3kg / ton of Al particles, and 0.05-0.1kg / ton of fluorite to the molten steel for slag making and reduction. Take samples for testing. All alloys can be added at one time according to the composition of the molten steel. After the composition test is qualified, perform temperature measurement and steel pouring.

[0040] It should be noted that, by using the above smelting method, the chemical composition of the martensitic stainless steel finally obtained is: C: 0.028%, Si: 0.46%, Mn: 0.88%, P: 0.021%, S: 0.006%, Cr: 12.15%, Mo: 0.51%, Ni: 3.88%, V: 0.041%, N: 0.01%, H: 0.0002%, O: 0.006%, Cu: 0.05%, W: 0.01, all of which meet the standard requirements.

[0041] In the process of smelting high-alloy martensitic steel, the use of return material smelting technology can effectively reduce the inventory of return materials and reduce the amount of alloy required to be added during the smelting process, thereby reducing the economic burden of the enterprise. However, excessive use of return materials may lead to excessive residual element content. In addition, due to the high chromium content and high viscosity of molten steel, the phenomenon of adhesion to the furnace bottom and furnace wall is prone to occur during the smelting process, which will have an adverse effect on the continuous smelting of the enterprise. The above embodiment mainly improves the electric arc furnace smelting process by implementing oxygen blowing operation during the smelting process to improve the fluidity of the molten steel, prevent adhesion to the furnace bottom and furnace wall, and reduce the content of residual elements such as phosphorus and vanadium to meet the requirements of technical specifications.

[0042] The above embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the invention patent. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A method for smelting martensitic stainless steel, characterized in that: The following steps are involved: Material preparation: Use martensitic return materials of the same material as the furnace charge, and remove all kinds of impurities on the surface of the return materials; add a set amount of lime, ferrosilicon, carbon powder and fluorite; EAF furnace smelting: pre-blow oxygen after the furnace temperature reaches 1570-1600℃, and then blow oxygen in batches. Take samples for analysis during the process. If C≤0.3%, Si≤0.5%, V≤0.03%, stop blowing oxygen. Otherwise, continue blowing oxygen until C≤0.3%, Si≤0.5%, V≤0.03% are met; LF furnace smelting: C element is controlled at 0.3-0.4%, Cr element is controlled at 12-12.5%, Ni element is controlled at 4-4.2%, Mo element is controlled at 0.45-0.55%, and S element is controlled below 0.005%; VOD furnace smelting: oxygen vacuum degree is controlled at 6000~8000Pa, oxygen flow rate is controlled at 500~800Nm 3 / h, argon flow rate is controlled at 200~400Nm 3 / h, start blowing oxygen.

2. The method for smelting martensitic stainless steel according to claim 1, characterized in that: The chemical composition of the martensitic stainless steel is C≤0.04%, Si≤0.60%, Mn≤1.0%, P≤0.028%, S≤0.008%, 12.0%≤Cr≤13.5%, 0.4%≤Mo≤1.0%, 3.8%≤Ni≤5.0%, V≤0.018%, N≤0.018%, H≤0.0003%, O≤0.008%, Cu≤0.5%, and W≤0.1%.

3. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the material preparation step, the set amount of lime added is 1-1.5% of the total weight of the molten steel, the set amount of ferrosilicon is 0.6-1.5% of the total weight of the molten steel, the set amount of carbon powder is 0.03-0.06% of the total weight of the molten steel, and the set amount of fluorite is 0.002-0.005% of the total weight of the molten steel.

4. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the EAF furnace smelting step, oxygen is pre-blown after the furnace temperature reaches 1570-1600°C, and the oxygen flow rate of the pre-blowing oxygen is controlled at 1000-2000Nm 3 / h, oxygen blowing volume is 150~200m 3 .

5. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the EAF furnace smelting step, when oxygen is blown in batches, the time of each oxygen blowing is less than or equal to 5 minutes.

6. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the EAF furnace smelting step, after the oxygen blowing operation is stopped, lime is added for slagging, and the amount of lime added is 1-1.5% of the total amount of molten steel.

7. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the LF furnace smelting step, the molten steel is first heated up after arriving at the LF furnace, and the slag is removed after the temperature reaches 1620-1650°C.

8. The method for smelting martensitic stainless steel according to claim 7, characterized in that: In the LF furnace smelting step, 50-100 kg of heat preservation agent is added before slagging.

9. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the LF furnace smelting step, 3-5 kg / ton of steel Al and 15-20 kg / ton of steel active lime are added to slag and reduce the molten steel.

10. The method for smelting martensitic stainless steel according to claim 1, characterized in that: In the VOD furnace smelting step, after the oxygen blowing is completed, the vacuum degree is increased to below 67 Pa and maintained for 15 minutes for vacuum degassing and refining.