Method for accurately controlling carbon content at bottom of TP316H stainless steel ingot through electroslag remelting

Abstract

The invention relates to a method for accurately controlling the carbon content of the bottom of a TP316H stainless steel ingot through electroslag remelting, and belongs to the field of ferrous metallurgy. According to the method, the TP316H stainless steel electrode blank smelting and electroslag remelting arc ingot plate combination mode and the arc starting current and voltage and slag material matching mode are broken through, accurate control over the C content of the bottom of the TP316H stainless steel electroslag steel ingot after electroslag remelting is achieved, and the C content of the bottom of the electroslag steel ingot after atmosphere protection electroslag remelting is controlled to be 0.04%-0.05%; and the cutting amount of the electroslag steel ingot before downward rotation is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of iron and steel metallurgy, and in particular relates to a method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot through electroslag remelting. Background Art

[0002] To ensure the stability of the properties of TP316H stainless steel for nuclear power plants, the material composition range is extremely narrow, with carbon content limited to 0.04-0.05%. Domestically, ultra-low-carbon stainless steel is typically produced using VOD or AOD methods. An existing electroslag remelting method for ultra-pure, ultra-low-carbon, nitrogen-controlled austenitic stainless steel for nuclear power plants controls the carbon content to ≤0.035%. In a protective atmosphere electroslag furnace, the melting rate is set at 0.80-1.0 kg / h. Another ultra-low-carbon, ultra-high-strength stainless steel and electroslag process, under protective atmosphere, produces an electroslag ingot with a chemical composition nearly identical to that of the electrode blank. However, the carbon content at the bottom of the ingot often exceeds the parent material requirements, even exceeding those in the technical agreement. This is because single-arm, atmosphere-shielded electroslag furnaces place a Q235 ingot guard plate between the mold and the bottom water tank. The arc starting method in the electroslag furnace is a Q235 ingot guard plate combined with a fast arc start. The following issues arise after each electroslag remelting process.

[0003] After electroslag remelting, the center of the ingot guard plate is melted due to the high temperature. The C content of Q235 steel plate is generally around 0.20%, while the C content in TP316H electrode billet is generally controlled at around 0.045%. Since the electrode billet is die-cast, the C content of the ingot riser often has positive segregation. In the early stage of electroslag remelting arc starting, since the arc starting method is direct arc starting, the early molten solid slag is mainly melted by the molten steel formed by the electrode billet and the arc starting block. This causes the Q235 ingot guard plate to melt and enter the early molten steel. With the stirring of the early metal molten pool, the C content at a certain position at the bottom of the electroslag ingot is ≥0.050%. In order to ensure that the chemical composition of the bottom of the final electroslag ingot meets the technical requirements, the tail end of the electroslag ingot is cut off, and the cutting amount is ≥150mm, which greatly reduces the utilization rate of the electroslag ingot. Summary of the Invention

[0004] The present invention provides a method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting, thereby solving the problem of increased carbon content at the bottom of the TP316H stainless steel ingot.

[0005] In order to solve the above technical problems, the present invention provides a method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot through electroslag remelting, which is characterized by:

[0006] Step 1: When smelting TP316H stainless steel in an electric furnace, before entering VOD, the carbon content is controlled at 0.3% to 0.5%, the Si content is controlled at 0.35% to 0.45%, the Mn content is controlled at ≤0.30%, and the temperature is controlled at ≥1580°C for VOD treatment;

[0007] Step 2: After blowing oxygen, the vacuum pump is started to perform vacuum carbon deoxidation reaction at a pressure of less than 280 Pa to 300 Pa, and the carbon content of the electrode blank is controlled to be 0.043% to 0.045%;

[0008] Step 3: Place the crystallizer on the arc starting plate and start melting.

[0009] Beneficial effects: The present invention achieves a C content of 0.04% to 0.05% in the TP316H stainless steel electrode blank, and controls the C content at the bottom of the electroslag ingot to be 0.04% to 0.05% after atmosphere-protected electroslag remelting, thereby reducing the amount of resection before the electroslag ingot is turned downward. DETAILED DESCRIPTION

[0010] In order to make the purpose, content and advantages of the present invention more clear, the specific embodiments of the present invention are further described in detail below.

[0011] The present invention proposes a method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting, and the specific steps are as follows:

[0012] Step 1: When smelting TP316H stainless steel in an electric furnace, to ensure the carbon content of the final electrode blank, the carbon content is controlled at 0.3% to 0.5%, the Si content is controlled at 0.35% to 0.45%, and the Mn content is controlled at ≤0.30% before entering VOD. The VOD treatment is performed at a temperature of ≥1580°C.

[0013] Step 2: After blowing oxygen, the vacuum pump is started to perform vacuum carbon deoxidation reaction at a pressure of less than 280 Pa to 300 Pa, and the carbon content of the electrode blank is controlled to be 0.043% to 0.045%;

[0014] Step 3: Before electroslag remelting, design the arc starting guard plate, including the main hollow guard plate (material: Q235, diameter 1150mm), the auxiliary guard plate (material TP316H, specification is electrode blank riser × 25mm) and the TP316H arc starting block (30mm × 30mm);

[0015] A hollow circle is placed in the center of the main ingot guard plate, and the diameter of the circle is 20mm smaller than the diameter of the electrode blank riser;

[0016] Cut a 25mm round piece from the riser of the TP316H electrode blank as the auxiliary ingot guard plate, put it on the hollow of the main ingot guard plate, and weld the two together by spot welding;

[0017] Cut five TP316H stainless steel arc-starting blocks of 30mm×30mm and weld them to the 1 / 2R radius of the auxiliary ingot guard plate;

[0018] Step 4: Place the crystallizer on the arc starting plate and power on for melting: In order to prevent the electrode blank from being inserted too deeply into the slag pool in the early stage, which would cause the arc starting plate to melt, the arc starting current is controlled at 8000A-12000A and the voltage is controlled at 40V-50V in the first 20 minutes;

[0019] Step 5: During power transmission, the slag adding speed is controlled as follows: 10% of the slag weight is added in the first 5 minutes, 25% of the total slag weight is added in 5-10 minutes, 50% of the total slag weight is added in 10-20 minutes, and the remaining slag is added within 40 minutes.

[0020] The present invention can control the carbon content of the bottom of the TP316H stainless steel electroslag ingot to be 0.04% to 0.05%.

[0021] Example:

[0022] When smelting TP316H stainless steel in an electric furnace, to ensure the final carbon content, the carbon content is controlled at 0.4%, the Si content is controlled at 0.44%, and the Mn content is controlled at 0.20% before entering VOD. The VOD treatment is carried out at a temperature of 1600°C. After blowing oxygen, the vacuum pump is started and vacuum carbon deoxidation reaction is carried out at a pressure of less than 303Pa. At this time, the argon flow rate is controlled at 3-10Nm3 / h, and the reaction time is about 18 minutes (i.e., vacuum carbon deoxidation ends when the oxygen potential rises again and drops to 0). The carbon content of the electrode blank is controlled to 0.044%.

[0023] The electroslag remelting process uses a mold with a diameter of 780 mm. A 650 x 25 mm sheet is cut from the riser end of the electrode blank to serve as the secondary guard plate. The arc starter blocks are cut into 30 mm x 30 mm blocks using the remaining filler. Five arc starter blocks (made of TP316H, 30 mm x 30 mm) are then welded to the secondary guard plate (made of TP316H, sized to meet the electrode blank's riser x 25 mm). The arc starter blocks are welded at a radius of 1 / 2R from the circle cut from the electrode blank's riser. A circle (630 mm) is cut around the center of the main guard plate (1150 mm diameter). The secondary guard plate (electrode blank's riser) is placed on the center circle of the main guard plate. The mold is placed between the main guard plate and the secondary starter block. The main and secondary guard plates are then connected using spot welding around their perimeters. After electroslag remelting, since the arc starter block and auxiliary plate of the same steel grade were melted in the early stage, and the center of the main plate was hollowed out, no melted Q235 material melted into the molten steel, so the C content at the bottom of the electroslag remelted ingot is similar to that of the base material, controlled at 0.043% to 0.045%;

[0024] The slag material is 240 kg, and the arc starting current and voltage are set to 4500 A and 38 V. After 20 minutes, the current and voltage are gradually increased to 11000 A and 42 V. 5 minutes before the slag material is added, 24 kg of the slag weight is added, 10 minutes after the slag material is added, 120 kg of the slag weight is added after 20 minutes, and the remaining slag is added within 40 minutes.

[0025] After electroslag remelting, carbon content analysis at the bottom of the electroslag ingot at 30mm, 60mm, 90mm, and 120mm diameters revealed carbon contents of 0.046%, 0.045%, 0.043%, and 0.044%, respectively. Analysis of carbon contents at the edge, 1 / 2R, and center of a cross-section at 60mm revealed carbon contents of 0.047%, 0.045%, and 0.045%, respectively. These results demonstrate that the present invention solves the problem of carbon accumulation at the bottom of TP316H stainless steel after electroslag remelting, eliminating the need for sawing the bottom of the ingot before lowering.

[0026] To address the increased carbon content in the bottom of TP316H stainless steel, the present invention improves the arc starting method of the electroslag furnace. Specifically, the original method of a guard plate (material: Q235) + an arc starting block is changed to a hollow guard plate (material: Q235) + a secondary guard plate of the same steel grade + an arc starting block of the same steel grade. The guard plate and the secondary guard plate of the same steel grade are spot welded together.

[0027] In the original arc starting method, since the C content of Q235 steel plate is 0.14% to 0.22%, the early stage of electroslag slag making is mainly achieved by first melting the arc starting fast and then melting the solid slag, and then gradually forming a liquid slag pool. Since the electrode blank is close to the ingot guard plate in the early stage, it will cause local melting of the ingot guard plate. The fast arc starting and the melting of the arc ingot plate increase the C content at the bottom of the TP316H electroslag steel ingot, making the C content at the bottom of the ingot 130mm exceed 0.05%, which does not meet the technical requirements.

[0028] According to the arc starting method of the arc starting ingot protection plate adopted by the present invention, after electroslag remelting, since the arc starting block and the auxiliary plate of the same steel grade are melted in the early stage, and the center of the main plate is hollowed out, no melted Q235 material melts into the molten steel, so that the C content at the bottom of the steel ingot after electroslag remelting is close to that of the base material and is controlled at 0.043% to 0.045%.

[0029] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot during electroslag remelting, characterized in that: Step 1: When smelting TP316H stainless steel in an electric furnace, before entering VOD, the carbon content is controlled at 0.3% to 0.5%, the Si content is controlled at 0.35% to 0.45%, the Mn content is controlled at ≤0.30%, and the temperature is controlled at ≥1580°C for VOD treatment; Step 2: After blowing oxygen, the vacuum pump is started to perform vacuum carbon deoxidation reaction at a pressure of less than 280 Pa to 300 Pa, and the carbon content of the electrode blank is controlled to be 0.043% to 0.045%; Step 3: Place the crystallizer on the arc starting plate and start melting.

2. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 1, characterized in that: The carbon content in the bottom of TP316H stainless steel obtained by electroslag remelting is 0.04% to 0.05%.

3. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 1, characterized in that: Before electroslag remelting, an arc-starting guard plate is designed, including a main hollow guard plate, an auxiliary guard plate and a TP316H arc-starting block; a hollow circle is placed in the center of the main guard plate, and a disc is cut as the auxiliary guard plate, which is placed on the hollow of the main guard plate and fixed as a whole; the TP316H stainless steel arc-starting block is welded to the auxiliary guard plate.

4. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 3, characterized in that: The main hollow ingot guard plate is made of Q235, and the auxiliary ingot guard plate is made of TP316H.

5. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 3, characterized in that: The diameter of the hollow circle in the center of the main ingot guard plate is smaller than the diameter of the electrode blank riser.

6. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 3, characterized in that: Cut 5 TP316H stainless steel arc starting blocks of 30mm×30mm and weld them to the 1 / 2R radius of the auxiliary ingot guard plate.

7. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 1, characterized in that: In step 3, when power is supplied for smelting, in the first 20 minutes, the arc starting current is controlled at 8000A to 12000A, and the voltage is controlled at 40V to 50V.

8. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 1, characterized in that: During the power transmission process, the slag addition speed is controlled as follows: 10% of the slag weight is added in the first 5 minutes, 25% of the total slag weight is added from 5 minutes to 10 minutes, 50% of the total slag weight is added from 10 minutes to 20 minutes, and the remaining slag is added within 40 minutes.

9. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 3, characterized in that: The crystallizer is placed on the main ingot guard plate and the auxiliary ingot dummy plate.

10. The method for accurately controlling the carbon content at the bottom of a TP316H stainless steel ingot by electroslag remelting according to claim 3, characterized in that: The main ingot guard plate and the auxiliary ingot guard plate are connected around by spot welding.