Vacuum arc remelting method for nitrogen-containing stainless steel

Abstract

The invention discloses a vacuum arc remelting method for nitrogen-containing stainless steel with the nitrogen content of an electrode being about 0.05%, the concentration of N2 between the electrode and a molten pool is reduced through short arc control, and melting parameter fluctuation caused by glow discharge is eliminated, so that the solidification defect is avoided. In the steady state and filling stage, inert gas argon is filled into the furnace, the vacuum degree in the furnace is reduced, volatilization and escape of N in molten steel are restrained to a certain extent, and the N content of the molten steel is larger than or equal to 0.02% according to the technical standard requirement.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a vacuum arc remelting method for nitrogen-containing stainless steel. Background Art

[0002] As an environmentally friendly and inexpensive alloying element, N is often added to stainless steel to refine the structure, enhance toughness, improve corrosion resistance, and increase strength. Stainless steel containing N is widely used in important fields such as severely corrosive marine environments and ultra-low temperature nuclear power cooling environments. At present, high-end special-purpose stainless steel containing N is usually smelted using a double vacuum process of vacuum induction + vacuum arc remelting.

[0003] Vacuum arc remelting technology uses metal vapor arc to melt electrodes under extreme vacuum conditions. The electrodes are dripped into a water-cooled crystallizer in the form of molten droplets and directionally solidified to obtain high-end metal materials with low gas content, fine-grained solidification structure, and low inclusion content. Vacuum arc remelting is the terminal of the smelting process in the production of high-end special-purpose N-containing stainless steel. The stability of the smelting process even directly determines the product quality.

[0004] Traditional technology uses long arc and high vacuum atmosphere to smelt nitrogen-containing stainless steel. Since N in molten steel volatilizes severely under high vacuum conditions, these volatilized N generate N2 at the gas-liquid interface and enter the arc discharge area, resulting in severe glow discharge of the metal vapor arc, causing huge fluctuations in various parameters of the smelting process, resulting in poor surface quality of the ingot, internal solidification defects and other problems. Moreover, excessive volatilization of N makes the remelted ingot unable to meet the technical requirements for the minimum N content of nitrogen-containing stainless steel. Summary of the invention

[0005] The purpose of the present invention is to solve the outstanding problems existing in the above-mentioned conventional technology, such as poor stability of the smelting process (large fluctuations in melting rate, molten droplet, voltage and current) leading to solidification defects and excessive volatilization of N.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A vacuum arc remelting method for nitrogen-containing stainless steel comprises the following steps:

[0008] Step 1: Pretreatment of induction furnace electrode rods

[0009] N-containing stainless steel refers to an electrode rod with a N content of 0.05%; the riser end and the bottom of the electrode rod are cut off to ensure that both ends are flat and rust-free clean surfaces to facilitate welding of dummy electrodes and vacuum arc remelting furnace loading operations; the oxidized black skin on the side surface of the electrode rod is removed (the black skin on the electrode side surface is composed of oxides and inclusions, and the ability of vacuum arc remelting to remove inclusions is very limited); a dummy electrode is welded to the bottom of the electrode rod, so that the riser end of the electrode rod is downward, and it is loaded into a vacuum arc remelting furnace.

[0010] Step 2: Set the smelting parameters of the vacuum arc remelting, which include: setting the remelting control mode of the vacuum arc remelting furnace, the remelting ingot size, the electrode size, the vacuum degree, the argon pressure, the cooling parameters, the current size, the voltage size, the droplet parameters, the arc starting stage parameters, the steady state stage parameters and the filling stage parameters.

[0011] The electrode rod formed by vacuum induction casting is used as a vacuum arc remelting electrode. The electrode is connected to the negative pole of the power supply, and the copper crystallizer is connected to the positive pole of the power supply. When the power is turned on, a metal vapor arc is generated between the bottom of the electrode and the copper plate at the bottom of the crystallizer. The arc melts the bottom of the electrode. The melted electrode drips into the water-cooled crystallizer in the form of molten droplets to form a molten pool and directionally solidifies into a steel ingot. Helium, a highly heat-conductive gas, is filled into the contraction gap between the steel ingot and the crystallizer to strengthen the cooling of the remelting ingot. Argon is filled into the vacuum system to suppress the volatilization of N in the molten steel.

[0012] The vacuum arc remelting control methods include: current + voltage control; melting speed + droplet short-circuit time control.

[0013] Current + voltage control, the arc starting stage parameters include arc starting current and arc starting voltage. The current range of the arc starting stage is 4KA-17.8KA-16.7KA, and the voltage range of the arc starting stage is 21-26V-24.5V;

[0014] The parameters of the arc starting stage are: melting rate 8.5Kg / min→0Kg / min, droplet level 0, droplet short circuit time 0.75S→0.12S, and argon pressure 20Pa.

[0015] After the arcing stage is over, the smelting enters the steady-state stage, and the control mode is changed to melting rate + droplet short-circuit time control; the steady-state stage parameters are: melting rate 8.5Kg / min, droplet level 0, droplet short-circuit time 0.75S, argon pressure 20Pa.

[0016] The remelting ingot size is 920mm in diameter and the electrode size is 840mm.

[0017] The vacuum degree includes: the gas leakage rate is less than 0.3Pa / min, and the argon pressure setting range is 10-20Pa after entering the steady state stage, which reduces the vacuum degree in the furnace, inhibits the volatilization and escape of N in the molten steel, and ensures that the N content of the remelted ingot meets the technical standards.

[0018] The cooling parameters include: the cooling water flow rate of the crystallizer is 700L / min, the temperature difference between the cooling water inlet and outlet is less than or equal to 3°C, and when the height of the remelting ingot reaches 300mm, helium is filled into the shrinkage gap between the ingot and the crystallizer, and the helium pressure is 400Pa. After the smelting is completed, the ingot is cooled in the furnace for 3 hours.

[0019] Step 3: After smelting is completed, continue cooling in the furnace until the remelted ingot is completely solidified, then lift the crystallizer together with the remelted ingot out of the vacuum arc remelting furnace, remove the crystallizer to obtain a stainless steel vacuum arc remelted ingot with a nitrogen content greater than 0.02%.

[0020] In the embodiment of the present invention, the control mode of the steady state stage is melting rate + droplet short-circuit time, the melting rate is set to 8.5Kg / min, the droplet short-circuit level is selected to level 0, and the droplet short-circuit time is set to 0.75S.

[0021] When the molten droplet contacts the molten pool, it appears as a short-circuit signal on the voltage curve, and the arc length is controlled by processing the voltage signal. The specific principle is that the droplet size is divided into 15 levels as shown in Table 1 according to the length of the short-circuit time of a single molten droplet in contact with the molten pool. For example, level 14 means that when a single molten droplet contacts the molten pool, the short-circuit time is greater than 100ms, and level 13 means that the short-circuit time of a single molten droplet in contact with the molten pool is greater than 50ms, which includes the molten droplets of level 14. By analogy, level 0 means that the short-circuit time of a single molten droplet in contact with the molten pool is greater than 0.1ms, which includes all molten droplets from levels 0 to 14. Therefore, the lower the short-circuit level selected, the smaller the molten droplet that can be detected by the voltage signal, which can also be understood as the shorter the arc length. Then the ratio of the time within 300 seconds to the number of molten droplets detected in the corresponding molten droplet level is defined as the molten droplet short-circuit time displayed on the smelting curve, which is expressed by the formula:

[0022] Table 1: Droplet level and corresponding single droplet short circuit time

[0023] Class 0 1 2 3 4 Drip short time(ms) ＞0.1 ＞0.2 ＞0.3 ＞0.5 ＞1.0 Class 5 6 7 8 9 Drip short time(ms) ＞2.0 ＞3.0 ＞4.0 ＞5.0 ＞6.0 Class 10 11 12 13 14 Drip short time(ms) ＞10.0 ＞20.0 ＞30.0 ＞50.0 ＞100.0

[0024] The control method of the filling stage is melting rate + molten droplet short-circuit time. The melting rate slowly decreases from 8.5Kg / min to 0Kg / min within 2.7h, and the molten droplet short-circuit time decreases from 0.75S to 0.12S, and then the power is cut off and the smelting is ended.

[0025] In the embodiment of the present invention, the N content of the electrode rod of the N-containing stainless steel is generally 0.05%-0.065%, and the N content after vacuum arc remelting is generally required to be greater than 0.02%. The following table lists several typical N-containing stainless steels, the N content of the electrode, and the technical standard requirements for the N content of the remelted ingot:

[0026] Table 2: Changes in N content of N-containing stainless steel during double vacuum process

[0027] Steel Type Electrode N content provided by induction furnace % N content % requirements after vacuum arc remelting 0Cr13Ni4Mo 0.04-0.06 0.02-0.035 2Cr10NiMoVNb 0.045-0.065 0.02-0.03 1Crl0Co6MoVNbN 0.045-0.065 0.02-0.035 1Cr12Ni3MoVN 0.05-0.07 0.02-0.04

[0028] After the smelting is completed, the cooling time in the furnace is 3 hours, and then the crystallizer is lifted out of the furnace and the crystallizer is dismantled to obtain a nitrogen-containing stainless steel vacuum arc remelting ingot.

[0029] The beneficial effects of the present invention are:

[0030] The present invention is suitable for vacuum arc remelting of nitrogen-containing stainless steel with an electrode nitrogen content of about 0.05%. The stability of the smelting process and the stability of the nitrogen content can be achieved. The smelting process is stable, and the melting rate, droplet short-circuit time, voltage, and current fluctuate very little, thereby avoiding scrapping caused by solidification defects. The filling of argon gas inhibits the volatilization of nitrogen to a certain extent, thereby ensuring that the nitrogen content meets the technical requirements of various nitrogen-containing stainless steels. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The present invention is a curve diagram of the melting rate and the droplet short-circuit time of smelting nitrogen-containing stainless steel by the method of the present invention.

[0032] Figure 2 The graph is a curve of voltage and current for smelting nitrogen-containing stainless steel using the method of the present invention.

[0033] Figure 3 This is a curve diagram of the melting rate and droplet short-circuit time of nitrogen-containing stainless steel smelted by traditional vacuum arc remelting.

[0034] Figure 4 This is a curve diagram of voltage and current for conventional vacuum arc remelting of nitrogen-containing stainless steel.

[0035] Figure 5 The melting rate and droplet short-circuit time curves are shown in Figure 2 when the method proposed by the present invention and the conventional technical solution are respectively implemented in the steady state stage during the vacuum arc remelting process.

[0036] Figure 6 The curve diagrams of voltage and current are respectively shown in the steady state stage of the vacuum arc remelting process when the method proposed by the present invention and the conventional technical solution are implemented.

[0037] Figure 7 The curves showing the change of the number of droplets when the method proposed by the present invention and the conventional technical solution are respectively implemented in the steady state stage during the vacuum arc remelting process. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is clearly and completely described below in conjunction with the embodiments of the present invention.

[0039] The present invention provides a vacuum arc remelting solution for N-containing stainless steel. In the arc starting stage, voltage and current control are adopted. The control idea is to quickly increase the current to a maximum current of 17.8KA under a low voltage state and maintain it for 1 hour, so as to achieve the purpose of quickly forming a molten pool.

[0040] The steady-state stage is the most critical and important part of the entire vacuum arc remelting. Its control ideas are as follows: First, use a short arc to reduce the N2 concentration between the electrode and the molten pool, thereby suppressing or weakening the glow discharge. Because the arc power is weakened during glow discharge, in order to increase the power to maintain the set value of the melting speed, the control system will automatically lower the electrode and increase the current, which leads to direct contact between the electrode and the molten pool, which is manifested in the voltage signal as follows. Figure 4 The short circuit shown also causes fluctuations in the melting rate and the droplet. A single glow discharge generally lasts for 3-5 seconds, but the periodic glow discharge during the entire smelting process will cause large fluctuations in the smelting curve, resulting in solidification quality defects. The lower the level of droplet short circuit, the shorter the arc length. Therefore, the droplet short circuit level selected in the steady state stage in the present invention is level 0. Second, 10-20Pa of inert gas argon is filled into the vacuum system to reduce the vacuum degree in the furnace and suppress the volatilization of N in the molten pool, so that the remelted ingot meets the technical requirements of various N-containing stainless steels regarding N (Table 2).

[0041] The control method in the filling stage is similar to that in the steady-state stage. It also uses molten droplet and melting rate control under argon filling. The difference is that the melting rate and molten droplet short-circuit time are gradually reduced until the end of smelting, thereby achieving the effect of shrinkage compensation.

[0042] Example 1

[0043] This embodiment uses electrodes provided by vacuum induction, the steel type is 0Cr13Ni4Mo, the electrode diameter is 840mm, the ingot diameter is 920mm, and the N content of the electrode in the middle of the longitudinal direction is 0.06%. Before smelting, a sawing machine is used to cut off the electrode head end and the bottom 300mm and 100mm respectively. Then a lathe is used to remove the black oxide skin on the electrode surface to make the electrode surface present a metallic luster. Then a dummy electrode is welded at the bottom of the electrode, and the electrode is turned over and loaded into the furnace. The furnace loading step should ensure that the electrode is in the middle of the crystallizer as much as possible, and the distance from the edge of the electrode in each direction to the crystallizer must be greater than 25-30mm to prevent arcing from piercing the crystallizer during the smelting process. After the furnace loading step is completed, the furnace is sealed and vacuumed to below 0.1Pa. Only after the leakage rate is tested and qualified can it be sent for power smelting. The specific smelting control parameters are listed in the following table:

[0044] 1. Arc starting stage:

[0045] step Current (KA) Time (h) Voltage (V) Time (h) 1 4→14.7 0.4 21→24.4 0.4 2 14.7→17.8 0.3 24.4→26 1.5 3 17.8→17.8 1 26→24.5 0.3 4 17.8→16.7 0.2 / /

[0046] 2. Steady state stage:

[0047] Melting rate (Kg / min) Droplet control level Droplet Short Circuit Time (DST) Argon(Pa) 8.5 0 0.75 20

[0048] 3. Filling stage:

[0049] Melting rate (Kg / min) Droplet control level Droplet Short Circuit Time (DST) Argon(Pa) 8.5→0 0 0.75→0.12 20

[0050] According to the above parameters, the smelting process curve is as follows: Figure 1 , Figure 2 The N contents of the samples taken from the head and tail of the remelting ingot were 0.023% and 0.024% respectively.

[0051] Comparative Example 1

[0052] In order to highlight the technical advantages of the present invention, the conventional method is used to smelt the same ingot type 0Cr13Ni4Mo, with the same N content of 0.06%. The control in the arc starting stage is the same as that of the present invention, and the difference lies in the steady state stage and the filling stage. The droplet control level used in the steady state stage and the filling stage is level 1, which means a longer arc length and no argon is filled. The smelting curve is as follows Figure 3 , Figure 4 As shown, compared with the technology of the present invention Figure 1 and Figure 2 It can be seen from the smelting curve that the traditional technology has large fluctuations in the control of melting rate, droplet short-circuit time, current and voltage, which will cause potential solidification defects. The N content of the head and tail samples of the remelting ingot was 0.019% and 0.020% respectively, which is significantly lower than the N content of the head and tail of the remelting ingot of the present invention, and the N content of the head does not meet the standard, which is 0.020% lower than the technical requirement.

[0053] Comparative Example 2

[0054] In order to further intuitively highlight the advantages of the method of the present invention over the traditional method, in the same furnace 0Cr13Ni4Mo consumable remelting process, the technology of the present invention and the traditional technical solution are respectively used to control half of the time in the steady state stage, and the smelting curve is as follows: Figure 5-7 As shown in the figure, it can be seen intuitively that the method of the present invention is far superior to the traditional method in controlling the melting rate, the short-circuit time of the molten droplet, the stability of the current and the voltage.

Claims

1. A method for vacuum arc remelting of nitrogen-containing stainless steel, characterized in that Follow these steps: Step 1: Pretreatment of induction furnace electrode rods Cut off the riser end and bottom of the electrode rod to ensure that both ends are flat and rust-free, so as to facilitate the welding of dummy electrodes and vacuum arc remelting furnace loading operations; remove the oxidized black skin on the side surface of the electrode rod; weld the dummy electrode at the bottom of the electrode rod, so that the riser end of the electrode rod is facing downward, and load it into the vacuum arc remelting furnace; Step 2: Setting the smelting parameters of vacuum arc remelting, which include: setting the remelting control mode of the vacuum arc remelting furnace, the size of the remelting ingot, the size of the electrode, the vacuum degree, the argon pressure, the cooling parameters, the current size, the voltage size, the droplet parameters, the arc starting stage parameters, the steady state stage parameters and the filling stage parameters; The electrode rod formed by vacuum induction casting is used as a vacuum arc remelting electrode. The electrode is connected to the negative pole of the power supply, and the copper crystallizer is connected to the positive pole of the power supply. When the power is turned on, a metal vapor arc is generated between the bottom of the electrode and the copper plate at the bottom of the crystallizer. The arc melts the bottom of the electrode. The melted electrode drips into the water-cooled crystallizer in the form of molten droplets to form a molten pool and directionally solidifies into a steel ingot. Helium, a highly heat-conductive gas, is filled into the contraction gap between the steel ingot and the crystallizer to strengthen the cooling of the remelting ingot. Argon is filled into the vacuum system to suppress the volatilization of N in the molten steel. Step 3: After smelting is completed, continue cooling in the furnace until the remelted ingot is completely solidified, then lift the crystallizer together with the remelted ingot out of the vacuum arc remelting furnace, remove the crystallizer to obtain a stainless steel vacuum arc remelted ingot with a nitrogen content greater than 0.02%.

2. A nitrogen-containing stainless steel vacuum arc remelting method according to claim 1, characterized in that: Nitrogen-containing stainless steel refers to an electrode rod with a nitrogen content of 0.05%.

3. A nitrogen-containing stainless steel vacuum arc remelting method according to claim 2, characterized in that: The vacuum arc remelting control modes include: current + voltage control; melting speed + droplet short circuit time control; current + voltage control, the arc starting stage parameters include arc starting current and arc starting voltage; the current range of the arc starting stage is 4KA-17.8KA-16.7KA, and the voltage range of the arc starting stage is 21-26V-24.5V; The parameters of the arc starting stage are: melting rate 8.5Kg / min→0Kg / min, droplet level 0, droplet short-circuit time 0.75S→0.12S, and argon pressure 20Pa; at the end of the arc starting stage, the smelting enters the steady-state stage, and the control mode is changed to melting rate + droplet short-circuit time control; the parameters of the steady-state stage are: melting rate 8.5Kg / min, droplet level 0, droplet short-circuit time 0.75S, and argon pressure 20Pa.

4. A nitrogen-containing stainless steel vacuum arc remelting method according to claim 2, characterized in that: The remelting ingot size is 920mm in diameter and the electrode size is 840mm.

5. A nitrogen-containing stainless steel vacuum arc remelting method according to claim 2, characterized in that: Vacuum levels include: The leakage rate is less than 0.3Pa / min. After entering the steady-state stage, the argon pressure setting range is 10-20Pa, which reduces the vacuum degree in the furnace, inhibits the volatilization and escape of N in the molten steel, and ensures that the N content of the remelted ingot meets the technical standards.

6. A nitrogen-containing stainless steel vacuum arc remelting method according to claim 2, characterized in that: The cooling parameters include: the crystallizer cooling water flow rate is 700L / min, the cooling water inlet and outlet temperature difference is less than or equal to 3°C, and when the height of the remelting ingot reaches 300mm, helium is filled into the shrinkage gap between the ingot and the crystallizer, and the helium pressure is 400Pa; after the smelting is completed, the ingot is cooled in the furnace for 3 hours.

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