A method for improving the recovery rate of alloy elements in 1Mn18Cr18N retaining ring steel

Abstract

The invention relates to a method for improving the alloy element recovery rate of 1Mn18Cr18N guard ring steel, comprising the following steps: S1, smelting crude molten steel in an electric arc furnace; S2, hoisting a stainless steel ladle into a vacuum oxygen decarburization furnace station for vacuum oxygen decarburization, using aluminum as a deoxidizer in the reduction period, making new slag, and entering a ladle refining furnace after slagging; S3, melting electrolytic manganese in a medium frequency induction furnace; S4, blowing nitrogen in place for refining with the stainless steel ladle; S5, hoisting the stainless steel ladle into a transfer vehicle, keeping the ladle blowing soft nitrogen, using an electrode ingot mold, and hanging protective slag in the electrode ingot mold; S6, after the electrode ingot solidifies, demoulding and hot transporting, and annealing in an annealing furnace; S7, cutting off the riser and ingot body of the electrode ingot; S8, preheating the electrode rod to above 400°C; S9, cleaning the surface of a bottom water tank, placing a sealing ring, and cleaning a crystallizer; putting an electrode holder in place, and putting an upper protective cover in place; S10, electroslag remelting. The recovery rate of manganese in the steel prepared by the invention is increased to more than 90%, the recovery rate of chromium in the steel is more than 93%, and the recovery rate of nitrogen in the steel is increased to more than 50%.

Description

Technical Field

[0001] The invention relates to the technical field of metal materials, in particular to a method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel. Background Art

[0002] The guard ring is one of the most important parts of the turbine generator rotor and is also the part that bears the highest stress on the turbine generator. It bears the huge centrifugal force of the rotor winding end and itself, as well as the superimposed load of the slot end coil winding on the guard ring. In order to ensure the safe and reliable operation of the unit, the guard ring is required to have high strength, good plasticity and toughness, and minimum residual stress. In order to avoid the guard ring heating due to electromagnetic induction during operation in a strong magnetic field, which affects the engine efficiency and safety, the guard ring must be made of austenitic non-magnetic steel - austenitic stainless steel. At the same time, the guard ring works in moisture and various corrosive media, and the guard ring steel is particularly required to have good stress corrosion resistance.

[0003] In recent years, with the continuous growth of my country's electricity demand and the adjustment of the power industry development plan, the safety performance requirements of the units are getting higher and higher, especially the development of supercritical units and the transformation of old units, which have increasingly larger requirements for the diameter of the retaining ring. The materials 50Mn18 carbon r5 and 50Mn18 carbon r4WN are gradually replaced by 1Mn18Cr18N.

[0004] At present, the commonly used process routes for steelmaking of 1Mn18Cr18N guard ring steel are: electric arc furnace EAF + vacuum oxygen decarburization furnace VOD + ladle refining furnace LF + electroslag remelting furnace ESR or induction furnace IF + electroslag remelting furnace ESR.

[0005] The main problems are:

[0006] 1) After adding metallic manganese to EAF, the burnout is too large during the vacuum oxygen decarburization in VOD, and a large amount of strong deoxidizers such as aluminum blocks need to be added during the reduction process after VOD; manganese elements evaporate during the vacuum carbon deoxidation VD and vacuum deoxidation VD processes, which seriously increases the manufacturing cost. Usually, the manganese recovery rate is about 80%.

[0007] 2) If it is added to LF, it will result in the addition of chromium nitride alloy and a large amount of electrolytic manganese alloy during the refining process. The amount of alloy added is too large, refining and slag adjustment are difficult, the refining time is too long, and the refractory material of the ladle lining is seriously corroded.

[0008] 3) In the middle and late stages of LF, ferrochrome nitride is added by blowing argon. Due to the low nitrogen partial pressure in the atmosphere, the nitrogen recovery rate in the ferrochrome nitride is less than 50%.

[0009] 4) Choose medium frequency or industrial frequency IF and other methods to smelt electrode ingots. Since the amount of alloy added to this type of steel reaches more than 50%, and the carbon content in the steel is required to be ≤0.10%, and the general internal control is ≤0.06%, the requirements for furnace charges are extremely strict and demanding. It is necessary to use expensive raw materials and alloys such as industrial pure iron and low-carbon ferrochrome, etc., and the manufacturing cost is high.

[0010] Therefore, it is desired to provide a method for improving the alloy element yield of 1Mn18Cr18N guard ring steel, which can solve the above technical problems. Summary of the invention

[0011] The purpose of the present invention is to provide a method for improving the alloy element recovery rate of 1Mn18Cr18N guard ring steel, which adopts medium frequency IF and other equipment to melt electrolytic manganese; EAF blows oxygen for dephosphorization and adjusts carbon, silicon, chromium and other components; VOD blows oxygen for decarburization, and appropriately increases the vacuum degree and vacuum time during VCD and VD to reduce the oxygen content and hydrogen content in the steel; nitrogen is blown into LF, and only alloy fine-tuning and chromium nitride are added; nitrogen is used to replace air in the electrode ingot mold, and argon is used for protection during the casting process; the electric slag remelting furnace uses a special quaternary slag system, adopts a large current, low voltage, low melting rate process, etc., and the surface of the prepared high-nitrogen guard ring steel electric slag ingot is flat, the gas overflow is less, and the composition is uniform.

[0012] In order to achieve the above object of the invention, a method for improving the alloy element recovery rate of 1Mn18Cr18N retaining ring steel is provided, comprising the following steps:

[0013] S1. Electric arc furnace smelts crude molten steel, using scrap steel and pig iron; removes reducing slag before tapping; uses stainless steel ladle to connect steel;

[0014] S2. The stainless steel ladle is hoisted into the vacuum oxygen decarburization furnace station, and the temperature is measured by sampling. The splash cover and vacuum cover are covered to start oxygen decarburization. The pre-slow blowing flow rate and the main blowing oxygen flow rate are controlled. After the temperature of the molten steel is raised at the end of oxidation, the oxygen blowing flow rate is reduced. Aluminum is used as a deoxidizer in the reduction period, and lime and fluorite are added to make new slag. Then vacuum deoxidation is carried out, and the vacuum degree is ≤ 66.7Pa for 15 to 20 minutes. After slagging, it enters the ladle refining furnace;

[0015] S3. Use a medium frequency induction furnace to melt electrolytic manganese; add lime to cover the liquid surface, pour the steel into a ladle, and add 1 kg / t aluminum block into the ladle;

[0016] S4. Stainless steel is refined by nitrogen blowing in a ladle. Manganese metal molten steel is poured into the molten steel of the ladle furnace by turning the ladle over. Selected lime and fluorite are used to make slag, and the slag is adjusted. Ferrosilicon or ferrosilicon powder is used for deoxidation. Temperature measurement and sampling are fully analyzed. The composition is adjusted according to the analysis results. Electrolytic manganese and ferrochrome nitride are added in batches. After the last batch of ferrochrome nitride is melted, temperature measurement and sampling are fully analyzed. The composition is adjusted according to the analysis results. After the composition is qualified, the steel is tapped as soon as possible. The ladle temperature is 1465-1485℃, and the upper limit of the ladle is controlled.

[0017] S5. Load the stainless steel into the transfer car with a hanging bag, and operate the transfer car to cross the span. During the process, the ladle is kept soft and blown with nitrogen. Use electrode ingot mold, hang protective slag in the electrode ingot mold, flush nitrogen in the electrode ingot mold in advance before pouring, use argon to protect pouring, and control the pouring speed to first fast and then slow;

[0018] S6. After pouring, add rice husk into the cap. After the electrode ingot solidifies, demould and heat it into an annealing furnace for annealing at 650±10℃.

[0019] S7. Before electroslag remelting, the outer surface of the electrode ingot is cleaned by a combination of sandblasting and manual grinding; the riser and the body of the electrode ingot are removed by electric spark or cutting;

[0020] S8. The surface of the electrode ingot is required to be polished, without oxide scale and slag inclusion. The crystallizer is Φ660mm, and the inner surface is required to be clean. The quaternary slag system is used, and the components of the quaternary slag system are CaF2, Al2O3, CaO, and MgO; the slag volume is 4% to 5% of the mass of the electroslag ingot; the slag material is used after being baked at 680℃ for ≧6h, and the electrode rod is preheated to above 400℃;

[0021] S9. Clean the surface of the bottom water tank, put the polished arc-starting plate and Bengang steel plate on it, place the sealing ring, clean the inner surface of the crystallizer, blow it with compressed air, put it on the bottom water tank, tighten it with bolts, and hoist it into the electroslag furnace station; hoist the electrode into the crystallizer and align it, put the protective cover in place, put the electrode holder in place, fine-tune the position of the electrode in the crystallizer, put the upper protective cover in place, connect the cooling water and return water pipelines, and connect the nitrogen protection system;

[0022] S10. The entire electroslag remelting process is sealed with a protective cover and protected with nitrogen.

[0023] Specifically, in step S1, high carbon content, oxygen blowing for decarburization and dephosphorization are adopted, the sampling temperature is controlled at ≥1550°C, high basicity slag is produced during the oxidation period, oxygen blowing for decarburization and dephosphorization is performed, and the slag removal conditions are: phosphorus ≤0.005%, and the slag removal temperature is ≥1650°C. After slag removal, new slag is produced, and the element composition is adjusted after deoxidation and reduction; the steelmaking temperature is ≥1650°C.

[0024] Specifically, in step S2, the start-up temperature of oxygen decarburization is 1550-1580°C, the start-up carbon is 0.3-0.5%, and the start-up silicon is ≤0.4%; the start-up oxygen pressure is 0.4-0.8MPa, the gun position is 1000-1200mm, the pre-slow blowing flow rate and the main blowing oxygen flow rate are controlled, and the oxygen blowing flow rate is reduced after the steel liquid temperature is raised at the end of oxidation. The main blowing oxygen flow rate is about 260-320m3 / h, the pre-slow blowing flow rate is about 220-260m3 / h, the vacuum degree is ≤16KPa, the endpoint carbon is controlled ≤0.03%, and vacuum carbon deoxidation is carried out, the vacuum degree is ≤1000Pa, and the holding time is 10-15min.

[0025] Preferably, in step S4, the nitrogen cylinder group is connected to the bus bar to ensure that argon can be replaced during the entire process of the ladle furnace, and nitrogen is blown in to make the molten steel absorb nitrogen and increase the nitrogen partial pressure in the refining atmosphere of the ladle refining furnace, so as to increase the nitrogen content in the steel and reduce nitrogen loss during the addition of ferrochrome nitride; all bulk alloys in the ladle furnace should be baked and dried in advance.

[0026] Preferably, in step S5, the pouring speed is halved after the cap is installed.

[0027] Preferably, in step S6, the starting temperature is 550-600°C, and the temperature is slowly increased to 650°C at a heating rate of ≤50°C / hour, and the heating and heat preservation time is 15-20 hours to eliminate the temperature difference between the surface and the inside and outside of the core of the ingot, and then the ingot is cooled to 200°C at a cooling rate of ≤30°C / hour and taken out of the furnace.

[0028] Specifically, in step S7, according to the solidification of the electrode ingot, the removal amount is generally 8-15%, but the cavity defect on the incision plane must be ﹤￠5mm, the arc-starting section surface is polished, and a 30mm thin slice is cut from the water inlet end of the electrode ingot for the bottom plate of the electroslag ingot to check the end surface condition.

[0029] Specifically, in step S10, N gas is filled throughout the whole process, and the N gas is required to be filtered and dried to remove moisture. The arc starting method is solid slag arc starting, the slag melting stage time is ≥60 minutes, and the shrinkage feeding time in the later stage of electroslag remelting is controlled to be ≥80 minutes; after the shrinkage feeding and capping, the electroslag ingot is demolded after cooling time of more than 100 minutes, and put into the hood for hood cooling, and the hood cooling is ≥30 hours before it is air-cooled out of the hood.

[0030] Specifically, after the electroslag remelting in step S10, the chemical composition of the guard ring steel electroslag ingot is: C≤0.10, Si≤0.80, Mn17.50~20.00, S≤0.015, P≤0.050, Cr 17.50~20.00, N≥0.50, AL≤0.025, Ni≤1.00, Ti≤0.10, V≤0.15, and B≤0.001.

[0031] The method for improving the alloy element recovery rate of 1Mn18Cr18N retaining ring steel of the present invention has the following advantages compared with the prior art:

[0032] (1) Use medium frequency IF and other special melting electrolytic manganese to avoid oxidation of manganese in the VOD process and evaporation in the VCD and VD processes, thereby improving the recovery rate of electrolytic manganese;

[0033] (2) The nitrogen bottle is connected to nitrogen through the busbar, and nitrogen is blown into the LF to make the molten steel absorb nitrogen and increase the nitrogen partial pressure in the refining atmosphere of the ladle refining furnace, thereby increasing the nitrogen content in the steel and reducing nitrogen leakage during the process;

[0034] (3) Nitrogen protection is used during the pouring process to prevent secondary oxidation and nitrogen precipitation in the steel;

[0035] (4) The whole process of electroslag remelting adopts protective cover sealing + nitrogen protection measures;

[0036] (5) For the first time, medium frequency IF was used to melt electrolytic manganese, which prevented oxidation of manganese in the VOD process and evaporation of manganese in the VCD and VD processes, significantly improving the recovery rate of electrolytic manganese; a self-made special device was used to blow nitrogen into the molten steel in the LF through a breathable plug to achieve uniform stirring of the chemical composition of the molten steel, floating of inclusions, nitrogen adsorption by the molten steel to increase nitrogen, and increase the nitrogen partial pressure of the molten steel to prevent nitrogen from escaping during the refining process; the following effects were achieved: the recovery rate of manganese in steel increased from the original 80.4% to more than 90%; the recovery rate of chromium in steel reached more than 93%; the recovery rate of nitrogen in steel increased from the original 43.5% to more than 50%; other elements in the steel met the internal control requirements of the material; the surface quality of the electrode ingot and the shrinkage of the riser were good;

[0037] (6) The high nitrogen guard ring steel electroslag ingot manufactured by the method of the present invention has a smooth surface, less gas overflow, and uniform composition. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a structural schematic diagram of a steel ingot with a riser in the background technology;

[0039] Figure 2 A structural schematic diagram of a method for improving the alloy element recovery rate of 1Mn18Cr18N guard ring steel in this embodiment Figure 1 . DETAILED DESCRIPTION

[0040] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0041] Example 1

[0042] A method for improving the alloy element yield of 1Mn18Cr18N guard ring steel in this embodiment comprises the following steps:

[0043] 1) Smelting

[0044] The arc furnace is used to smelt crude steel, using scrap steel and high-quality pig iron with clear chemical composition. The first and second furnaces of a new furnace body and the end of the furnace life are not in good condition. The steel used to smelt titanium alloys, high-temperature alloys, or steels that are strictly controlled in the smelting table or have high harmful element content should be washed before the furnace is allowed to smelt this steel. High carbon content, oxygen decarburization and dephosphorization, sampling temperature of 1550℃, high alkalinity slag during oxidation, oxygen decarbonization and dephosphorization, slag removal conditions: P≤0.005%, slag removal temperature of 1650℃. After slag removal, new slag is made; aluminum ingots, ferrosilicon, etc. are used for deoxidation. After reduction, low-carbon ferrochrome and electrolytic manganese are added to adjust the carbon, silicon, chromium and other components to meet the VOD process requirements; the tapping temperature is ≥1650℃; before tapping, the reducing slag is removed to minimize the amount of slag entering the VOD furnace; a special ladle furnace is used to connect the steel.

[0045] The ladle is hoisted into the VOD station, the temperature is measured by sampling, the splash cover and vacuum cover are covered and oxygen decarburization is started; the blowing temperature is 15501580℃, the blowing C is 0.4%, and the blowing Si is ≤0.4%; the blowing oxygen pressure is 0.5MPa, the gun position is 1000mm, the pre-slow blowing flow rate and the main blowing oxygen flow rate are controlled, and the oxygen blowing flow rate is reduced after the molten steel temperature is raised at the end of oxidation. The main blowing oxygen flow rate is about 3000m3 / h, the pre-slow blowing flow rate is about 260m3 / h, the vacuum degree is ≤16Kpa, the end point carbon control is ≤0.03%, the vacuum degree of vacuum carbon deoxidation VCD is ≤1000pa, and the holding time is 10 minutes. The amount of reducing agent added is recalculated according to the amount of oxygen blowing. Al is used as the deoxidizer in the reduction period, and lime fluorite is added to make new slag; the vacuum degree of vacuum deoxidation VD is ≤66.7pa, and the time is 15min. After slagging, enter the LF furnace.

[0046] A 2t medium frequency induction furnace is used to melt electrolytic manganese. When the furnace is new or at the end of its service life and the furnace is in poor condition, this type of steel shall not be smelted. The steel used to smelt titanium alloy, high-temperature alloy, or steel strictly controlled in the smelting table or with high harmful element content should be cleaned before smelting this type of steel. During the melting process, the relatively low melting point of electrolytic manganese is 1245℃, and the temperature difference with the molten steel in the ladle furnace should be taken into consideration. Attention should be paid to the tapping temperature control. An appropriate amount of lime should be added to cover the liquid surface to prevent manganese oxidation. The tapped steel is poured into the ladle. In order to prevent the oxidation of metallic manganese, 1Kg / t aluminum blocks are added to the ladle.

[0047] The ladle is in place for nitrogen blowing and refining. The molten manganese steel is slowly poured into the molten steel in the ladle furnace by turning the ladle over, and be careful of splashing. A self-made facility - a nitrogen bottle group is used to merge into the bus to ensure that argon can be replaced during the entire process in the ladle furnace. Nitrogen is blown in to make the molten steel absorb nitrogen and increase the nitrogen partial pressure in the LF refining atmosphere; this increases the nitrogen content in the steel and reduces nitrogen loss during the addition of ferrochromium nitride. All bulk alloys in the ladle furnace should be baked and dried in advance, and selected lime and fluorite should be used to make slag. Bensteel is a low-carbon austenitic stainless steel. Pay attention to the problem of carbon increase during the power-on and temperature-raising process. Adjust the slag and use ferrosilicon or ferrosilicon powder for deoxidation. It is forbidden to use aluminum blocks or aluminum powder for deoxidation to ensure that the slag turns white. The temperature is measured and sampled for full analysis (including N), and the composition is adjusted according to the analysis results. Electrolytic manganese and ferrochrome nitride are added in batches. After the last batch of ferrochrome nitride is melted, the temperature is measured and sampled for full analysis (including N), and the composition is adjusted according to the analysis results. After the composition is qualified, the steel is tapped as soon as possible; considering that the liquidus temperature of this steel is between 1340 and 1360°C, which is relatively low, pay attention to controlling the power supply operation of the refining furnace to prevent the ladle temperature from being too high; the ladle temperature is 1465°C, and the upper limit of the ladle is controlled.

[0048] 2) Ingot casting and annealing

[0049] Lift the ladle into the transfer vehicle, operate the transfer vehicle to cross the span as quickly as possible, and keep the ladle softly blown with nitrogen during the process. Use a special mold for electrode ingots, with 4 or 2 ingots in a tray. Use an iron brush to clean the inner wall of the mold and bake it; bake and suction the pouring system to ensure that the system is clean and dry; hang special protective slag in the mold; flush nitrogen into the mold 15 minutes before pouring, use argon to protect the pouring, control the pouring speed to be fast first and then slow, especially after the cap is put on, the pouring speed is halved; add enough rice husks to the cap mouth after pouring. After the electrode ingot is solidified, demold it and send it to the annealing furnace for annealing at 650±10℃; the starting temperature is 550℃, and the temperature is slowly increased to 650℃ at a heating rate of 45℃ / hour. The heating and heat preservation time is 20 hours to eliminate the temperature difference between the surface and the inside and outside of the core of the ingot, and then the furnace is cooled to 200℃ at a cooling rate of ≦25℃ / hour to reduce thermal stress. Annealing process such as Figure 1 .

[0050] 3) Cleaning of electrode ingots and removal of water risers

[0051] Before electroslag remelting, the outer surface of the electrode ingot is cleaned by combining sandblasting and manual grinding to eliminate unfavorable factors such as oxide scale and slag inclusions that may affect the quality of electroslag remelting. The riser and ingot body of the electrode ingot are cut off by electric spark or cutting. According to the solidification of the electrode ingot, the cutting amount is 15%, but the cavity defect on the cut plane must be less than ￠5mm, and the surface of the arc-starting section must be polished. Black skin, oxide scale and other defects are not allowed to exist, so as to reduce inclusions and unallowed segregation in the electroslag ingot. A 30mm thin slice is cut off from the water inlet end of the electrode ingot for the bottom plate of the electroslag ingot to check the end face, but it must be ensured that the metal at this end, which accounts for about 5% of the total weight of the electrode ingot during electroslag remelting, is not remelted into the electroslag ingot.

[0052] 4) Electroslag remelting

[0053] The electrode ingot is required to be surface polished, free of oxide scale and slag inclusions. The crystallizer is Φ660mm, and the inner surface is required to be clean. Use a special quaternary slag system (CaF2, Al2O3, CaO, MgO); slag volume = 4% to 5% of the mass of the electroslag ingot, about 200kg; the slag is used after being baked at 680℃ for ≧6h, and the electrode rod is preheated to above 400℃.

[0054] Clean the surface of the bottom water tank, place the polished arc-starting plate and Bengang steel plate, place the sealing ring, clean the inner surface of the crystallizer, blow it with compressed air, place it on the bottom water tank, tighten it with bolts, and hang it in the electroslag furnace station. Hang the electrode into the crystallizer and align it, lower the protective cover into place, put the electrode holder into place, fine-tune the position of the electrode in the crystallizer, and put the upper protective cover into place. Connect the cooling water and return water pipelines, and connect the nitrogen protection system.

[0055] The electroslag remelting adopts a high current, low voltage, and low melting rate process. The melting rate is controlled at a constant rate during the melting period. The whole process of electroslag remelting adopts a protective cover closed + nitrogen protection measure. N gas is filled throughout the process, and the N gas is required to be filtered and dried to remove moisture. The arc starting method is solid slag arc starting. The slag melting stage time is ≥60 minutes. The arc starting (slag melting) stage is mainly controlled by current and voltage. Other parameters are referenced. The actual current and voltage are allowed to fluctuate within ±20% of the set value. The specific actual value is mainly based on the actual value. Constant melting rate control is adopted in the melting stage; the key parameter melting rate in the melting stage (steady-state stage) allows the actual melting rate to be controlled within ±10% of the set value. Other parameters such as current, voltage, swing, weight and other parameters will be automatically adjusted by the computer during the smelting process. The actual data shall prevail and no judgment basis shall be used. If it exceeds the fluctuation range, it shall be produced with the consent of the process engineer and recorded truthfully in the records. In the late stage of electroslag remelting, the shrinkage feeding time is controlled to be ≥80 minutes; in the hot capping stage, the current and voltage control is the main mode, and the actual current and voltage are allowed to fluctuate within ±20% of the set value, with the melting speed as a reference. After shrinkage feeding and capping, the electroslag ingot is demoulded after cooling for more than 100 minutes, and put into the hood for hood cooling. The hood cooling lasts ≥30 hours and then the hood is air-cooled.

[0056] Example 2

[0057] 1) Smelting

[0058] Electric arc furnaces are used to smelt crude steel, using scrap steel and high-quality pig iron with clear chemical composition. The first and second furnaces of a new furnace body and the end of the furnace life are not allowed to smelt this type of steel when the furnace condition is not good. The steel used to smelt titanium alloys, high-temperature alloys, or steels that are strictly controlled in the smelting table or have high harmful element content should be washed before smelting this type of steel. High carbon content, oxygen blowing for decarburization and dephosphorization, sampling temperature of 1530℃, high alkalinity slag during oxidation period, oxygen blowing for C and P removal, slag removal conditions: P≤0.005%, slag removal temperature of 1660℃. After slag removal, new slag is made; aluminum ingots, ferrosilicon, etc. are used for deoxidation. Low-carbon ferrochrome is added after reduction to adjust the carbon, silicon, chromium and other components to meet the VOD process requirements; the tapping temperature is ≥1650℃; the reducing slag is removed before tapping to minimize the amount of slag entering the VOD furnace; a special ladle furnace is used to connect the steel with a ladle.

[0059] The ladle is hoisted into the VOD station, the temperature is measured by sampling, and the splash cover and vacuum cover are covered to start oxygen decarburization. The opening temperature is 1570℃, the opening C is 0.35%, and the opening Si is 0.4%; the opening oxygen pressure is 0.6MPa, and the gun position is 1100mm. Control the pre-slow blowing flow rate and the main blowing oxygen flow rate, and reduce the oxygen blowing flow rate after raising the temperature of the molten steel at the end of oxidation. The main blowing oxygen flow rate is about 280m3 / h, the pre-slow blowing flow rate is about 250m3 / h, and the vacuum degree is ≦16Kpa. The end point carbon control is ≤0.03%, the vacuum degree of vacuum carbon deoxidation VCD is ≦1000pa, and the holding time is 10 minutes. Recalculate the amount of reducing agent added according to the oxygen blowing amount, use Al as the deoxidizer in the reduction period, and add lime fluorite to make new slag; the vacuum degree of vacuum deoxidation VD is ≦66.7pa, and the time is 20min. After slagging, enter the LF furnace.

[0060] A 2t medium frequency induction furnace is used to melt electrolytic manganese. When the furnace is new or at the end of its service life and the furnace is in poor condition, this type of steel shall not be smelted. The steel used to smelt titanium alloy, high-temperature alloy, or steel strictly controlled in the smelting table or with high harmful element content should be cleaned before smelting this type of steel. During the melting process, the relatively low melting point of electrolytic manganese is 1245℃, and the temperature difference with the molten steel in the ladle furnace should be taken into consideration. Attention should be paid to the tapping temperature control. An appropriate amount of lime should be added to cover the liquid surface to prevent manganese oxidation. The tapped steel is poured into the ladle. In order to prevent the oxidation of metallic manganese, 1Kg / t aluminum blocks are added to the ladle.

[0061] The ladle is in place for nitrogen blowing and refining. The molten manganese steel is slowly poured into the molten steel in the ladle furnace by turning the ladle over, and be careful of splashing. A self-made facility - a nitrogen bottle group is used to merge into the bus to ensure that argon can be replaced during the entire process in the ladle furnace. Nitrogen is blown in to make the molten steel absorb nitrogen and increase the nitrogen partial pressure in the LF refining atmosphere; this increases the nitrogen content in the steel and reduces nitrogen loss during the addition of ferrochromium nitride. All bulk alloys in the ladle furnace should be baked and dried in advance, and selected lime and fluorite should be used to make slag. Bensteel is a low-carbon austenitic stainless steel. Pay attention to the problem of carbon increase during the power-on and temperature-raising process. Adjust the slag and use ferrosilicon or ferrosilicon powder for deoxidation. It is forbidden to use aluminum blocks or aluminum powder for deoxidation to ensure that the slag turns white. The temperature is measured and sampled for full analysis (including N), and the composition is adjusted according to the analysis results. Electrolytic manganese and ferrochrome nitride are added in batches. After the last batch of ferrochrome nitride is melted, the temperature is measured and sampled for full analysis (including N), and the composition is adjusted according to the analysis results. After the composition is qualified, the steel is tapped as soon as possible; considering that the liquidus temperature of this steel is between 1340 and 1360°C, which is relatively low, pay attention to controlling the power supply operation of the refining furnace to prevent the ladle temperature from being too high; the ladle temperature is 1465°C, and the upper limit of the ladle is controlled.

[0062] 2) Ingot casting and annealing

[0063] Lift the ladle into the transfer vehicle, operate the transfer vehicle to cross the span as quickly as possible, and keep the ladle softly blown with nitrogen during the process. Use a special mold for electrode ingots, with 4 or 2 ingots in a tray. Use an iron brush to clean the inner wall of the mold and bake it; bake and suction the pouring system to ensure that the system is clean and dry; hang special protective slag in the mold; flush nitrogen into the mold 15 minutes before pouring, use argon to protect the pouring, control the pouring speed to be fast first and then slow, especially after the cap is put on, the pouring speed is halved; add enough rice husks to the cap mouth after pouring. After the electrode ingot is solidified, demold it and send it to the annealing furnace for annealing at 650±10℃; the starting temperature is 530℃, and the temperature is slowly increased to 650℃ at a heating rate of 40℃ / hour. The heating and insulation time is 18 hours to eliminate the temperature difference between the surface and the inside and outside of the core of the ingot, and then the furnace is cooled to 200℃ at a cooling rate of 25℃ / hour to reduce thermal stress. Among them, the annealing process is as follows Figure 2 ;

[0064] 3) Cleaning of electrode ingots and removal of water risers

[0065] Before electroslag remelting, the outer surface of the electrode ingot is cleaned by combining sandblasting and manual grinding to eliminate unfavorable factors such as oxide scale and slag inclusion that may affect the quality of electroslag remelting. The riser and ingot body of the electrode ingot are cut off by electric spark or cutting. According to the solidification of the electrode ingot, the cutting amount is generally 8-15%, but the cavity defect of the cut plane must be ﹤￠5mm, and the surface of the arc-starting section must be polished. Black skin, oxide scale and other defects are not allowed to exist, so as to reduce inclusions and unallowed segregation in the electroslag ingot. A 30mm thin slice is cut off from the water inlet end of the electrode ingot for the bottom plate of the electroslag ingot to check the end face, but it must be ensured that the metal at this end, which accounts for about 5% of the total weight of the electrode ingot during electroslag remelting, is not remelted into the electroslag ingot.

[0066] 4) Electroslag remelting

[0067] The electrode ingot is required to be surface polished, free of oxide scale and slag inclusions. The crystallizer is Φ660mm and the inner surface is required to be clean. Use a special quaternary slag system (CaF2, Al2O3, CaO, MgO); slag volume G slag = (4% ~ 5%) G ingot is about 200kg; the slag is used after being baked at 680℃≧6h, and the electrode rod is preheated to above 400℃.

[0068] Clean the surface of the bottom water tank, place the polished arc-starting plate and Bengang steel plate, place the sealing ring, clean the inner surface of the crystallizer, blow it with compressed air, place it on the bottom water tank, tighten it with bolts, and hang it in the electroslag furnace station. Hang the electrode into the crystallizer and align it, lower the protective cover into place, put the electrode holder into place, fine-tune the position of the electrode in the crystallizer, and put the upper protective cover into place. Connect the cooling water and return water pipelines, and connect the nitrogen protection system.

[0069] The electroslag remelting adopts a high current, low voltage, and low melting rate process. The melting rate is controlled at a constant rate during the melting period. The whole process of electroslag remelting adopts a protective cover closed + nitrogen protection measure. N gas is filled throughout the process, and the N gas is required to be filtered and dried to remove moisture. The arc starting method is solid slag arc starting. The slag melting stage time is ≥60 minutes. The arc starting (slag melting) stage is mainly controlled by current and voltage. Other parameters are referenced. The actual current and voltage are allowed to fluctuate within ±20% of the set value. The specific actual value is mainly based on the actual value. Constant melting rate control is adopted in the melting stage; the key parameter melting rate in the melting stage (steady-state stage) allows the actual melting rate to be controlled within ±10% of the set value. Other parameters such as current, voltage, swing, weight and other parameters will be automatically adjusted by the computer during the smelting process. The actual data shall prevail and no judgment basis shall be used. If it exceeds the fluctuation range, it shall be produced with the consent of the process engineer and recorded truthfully in the records. In the late stage of electroslag remelting, the shrinkage feeding time is controlled to be ≥80 minutes; in the hot capping stage, the current and voltage control is the main mode, and the actual current and voltage are allowed to fluctuate within ±20% of the set value, with the melting speed as a reference. After shrinkage feeding and capping, the electroslag ingot is demoulded after cooling for more than 100 minutes, and then put into the hood for hood cooling. The hood cooling lasts ≥30 hours and then the hood is air-cooled.

[0070] The chemical composition of the high nitrogen retaining ring steel electroslag ingot prepared in Example 1 and Example 2 is: C≤0.12, Si≤0.80, Mn17.50~20.00, S≤0.050, P≤0.015, Cr17.50~20.00, N≥0.47, AL≤0.030.

[0071] The mechanical properties of the high nitrogen retaining ring steel electroslag ingots prepared in Example 1 and Example 2 are as follows: ultimate strength Rm≥970MPa, yield strength Rp0.2 970-1100MPa, elongation A4.52≥17%, end face shrinkage Z2≥55%, and test temperature 95-105°C.

[0072] It can be seen that the retaining ring steel prepared by the preparation method significantly reduces the manufacturing cost because of the greatly improved recovery rate of manganese and nitrogen elements; at the same time, it meets the chemical composition requirements of the user, the S and P contents in the steel are low, especially the N content in the steel reaches about 0.60%; on the basis of meeting the user's material strength limit, the plasticity and toughness of the material are much higher than the basic requirements.

[0073] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel, characterized in that: The following steps are included: S1. Electric arc furnace is used to smelt crude molten steel, using scrap steel and pig iron; before tapping, the reducing slag is removed and the steel is connected using a stainless steel ladle; S2. The stainless steel ladle is hoisted into the vacuum oxygen decarburization furnace station, and the temperature is measured by sampling. The splash cover and vacuum cover are covered to start oxygen decarburization. The pre-slow blowing flow rate and the main blowing oxygen flow rate are controlled. After the temperature of the molten steel is raised at the end of oxidation, the oxygen blowing flow rate is reduced. Aluminum is used as a deoxidizer in the reduction period, and lime and fluorite are added to make new slag. Then vacuum deoxidation is carried out, and the vacuum degree is ≤ 66.7Pa for 15 to 20 minutes. After slagging, it enters the ladle refining furnace; S3. Use a medium frequency induction furnace to melt electrolytic manganese; add lime to cover the liquid surface, pour the steel into a ladle, and add 1 kg / t aluminum block into the ladle; S4. Stainless steel is refined by nitrogen blowing in a ladle. Manganese metal molten steel is poured into the molten steel of the ladle furnace by turning the ladle over. Selected lime and fluorite are used to make slag, and the slag is adjusted. Ferrosilicon or ferrosilicon powder is used for deoxidation. Temperature measurement and sampling are fully analyzed. The composition is adjusted according to the analysis results. Electrolytic manganese and ferrochrome nitride are added in batches. After the last batch of ferrochrome nitride is melted, temperature measurement and sampling are fully analyzed. The composition is adjusted according to the analysis results. After the composition is qualified, the steel is tapped as soon as possible. The ladle temperature is 1465-1485℃, and the upper limit of the ladle is controlled. In step S4, the nitrogen cylinder group is connected to the busbar to ensure that argon can be replaced during the entire process of the ladle furnace, and nitrogen is blown in to make the molten steel absorb nitrogen, increase the nitrogen partial pressure in the refining atmosphere of the ladle refining furnace, increase the nitrogen content in the steel, and reduce nitrogen loss during the addition of ferrochrome nitride; all bulk alloys in the ladle furnace are baked and dried in advance; S5. Load the stainless steel into the transfer car with a hanging bag, and operate the transfer car to cross the span. During the process, the ladle is kept soft and blown with nitrogen. Use electrode ingot mold, hang protective slag in the electrode ingot mold, flush nitrogen in the electrode ingot mold in advance before pouring, use argon to protect pouring, and control the pouring speed to first fast and then slow; S6. After pouring, add rice husk into the cap. After the electrode ingot solidifies, demould and heat it into an annealing furnace for annealing at 650±10℃. S7. Before electroslag remelting, the outer surface of the electrode ingot is cleaned by a combination of sandblasting and manual grinding; the riser and the body of the electrode ingot are removed by electric spark or other removal methods according to the solidification condition of the electrode ingot; S8. The surface of the electrode ingot is required to be polished, without oxide scale and slag inclusion. The crystallizer is Φ660mm, and the inner surface is required to be clean. The quaternary slag system is used, and the components of the quaternary slag system are CaF2, Al2O3, CaO, and MgO; the slag volume is 4% to 5% of the mass of the electroslag ingot; the slag material is used after being baked at 680℃ for ≥6h, and the electrode rod is preheated to above 400℃; S9. Clean the surface of the bottom water tank, put the polished arc-starting plate and Bengang steel plate on it, place the sealing ring, clean the inner surface of the crystallizer, blow it with compressed air, put it on the bottom water tank, tighten it with bolts, and hoist it into the electroslag furnace station; hoist the electrode into the crystallizer and align it, put the protective cover in place, put the electrode holder in place, fine-tune the position of the electrode in the crystallizer, put the upper protective cover in place, connect the cooling water and return water pipelines, and connect the nitrogen protection system; S10. The entire electroslag remelting process is sealed with a protective cover and protected with nitrogen.

2. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: In step S1, high carbon content, oxygen blowing for decarburization and dephosphorization are adopted, the sampling temperature is controlled at ≥1550℃, high basicity slag is produced during the oxidation period, oxygen blowing for decarburization and dephosphorization, slag removal conditions: phosphorus ≤0.005%, slag removal temperature ≥1650℃, after slag removal, new slag is produced, and the element composition is adjusted after deoxidation and reduction; the steelmaking temperature is ≥1650℃.

3. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: In step S2, the oxygen decarburization start-up temperature is 1550-1580°C, the carbon start-up is 0.3-0.5%, and the silicon start-up is ≤0.4%; the oxygen start-up pressure is 0.4-0.8MPa, the gun position is 1000-1200mm, the pre-slow blowing flow rate and the main blowing oxygen flow rate are controlled, and the oxygen blowing flow rate is reduced after the steel liquid temperature is raised at the end of oxidation, and the main blowing oxygen flow rate is 260-320m 3 / h, pre-slow blowing flow rate 220~260m 3 / h, vacuum degree ≦16KPa, endpoint carbon control ≤0.03%, vacuum carbon deoxidation, vacuum degree ≦1000Pa, holding time 10~15min.

4. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: In step S5, after the cap is installed, the pouring speed is halved.

5. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: In step S6, the starting temperature is 550-600°C, and the temperature is slowly raised to 650°C at a heating rate of ≤50°C / hour, and the heating and heat preservation time is 15-20 hours to eliminate the temperature difference between the surface and the inside and outside of the core of the ingot, and then the ingot is cooled to 200°C at a cooling rate of ≤30°C / hour and taken out of the furnace.

6. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: In step S7, according to the solidification of the electrode ingot, the removal amount is 8-15%, but the cavity defect on the cut plane must be less than ￠5mm, the arc-starting section surface is polished, and a 30mm thin slice is removed from the nozzle end of the electrode ingot for the bottom plate of the electroslag ingot to check the end surface condition.

7. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: In step S10, N gas is filled throughout the whole process. The N gas is required to be filtered and dried to remove moisture. The arc starting method is solid slag arc starting. The slag melting stage time is ≥60 minutes. In the later stage of electroslag remelting, the shrinkage feeding time is controlled to be ≥80 minutes. After the shrinkage feeding and capping, the electroslag ingot is demoulded after cooling time of more than 100 minutes, and put into the hood for hood cooling. The hood cooling lasts ≥30 hours and then the hood is air-cooled.

8. A method for improving the alloy element yield of 1Mn18Cr18N retaining ring steel as claimed in claim 1, characterized in that: After the electroslag remelting in step S10, the chemical composition of the electroslag ingot of the guard ring steel is as follows in terms of mass percentage: C≤0.10%, Si≤0.80%, Mn17.50%~20.00%, S≤0.015%, P≤0.050%, Cr 17.50%~20.00%, N≥0.50%, Al≤0.025%, Ni≤1.00%, Ti≤0.10%, V≤0.15%, B≤0.001%, and the remainder is Fe and unavoidable impurities.

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