A method for controlling white spot defects in steel

Abstract

A method for controlling white spot defects in steel, belonging to the field of metallurgical technology, which includes the processes of electric furnace steelmaking, RH vacuum refining, continuous casting, rolling and cooling. In the electric furnace steelmaking process, according to the different injected gases, the oxygen-burning lance is divided into a burner mode and a nozzle mode, and is divided into low, medium and high gears according to the flow rate of the injected gas; starting from the first batch of scrap steel being powered on, all oxygen-burning lances adopt the burner mode, and each oxygen-burning lance executes the combustion sequence from low gear to high gear, and all enter the high burner mode within 4 minutes; 10 minutes after power-on, all are switched to the low nozzle mode; at 6 minutes after power-on, slag-making materials are added; at 12 minutes after power-on, lime is added, and at 15 minutes after power-on, the carbon lance is started to send carbon into the furnace to achieve accurate control of the carbon content. By using the method of the present invention, the H element in the steel grade after rolling and cooling is lower than 0.82 ppm, greatly reducing the number of white spots in the steel grade.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a method for controlling white spot defects in steel. Background Art

[0002] White spots are caused by the fact that the undissipated hydrogen atoms in the steel are easily transformed from the atomic state to the molecular state due to external factors such as the cooling rate being too fast, causing their volume to increase sharply. When the stress exceeds the ultimate strength of the steel, cracks are formed, which are white spots, also known as cracking.

[0003] (1) When hydrogen is contained in steel, the plasticity of the steel is reduced. When the hydrogen content reaches a certain value, the plasticity drops sharply. When the stress is sufficient, microcracks will form, white spots will be produced, and hydrogen embrittlement will occur. Especially when stress exists in the steel for a long time, hydrogen can diffuse into the stress concentration area (the hydrogen atoms dissolved in the gap tend to concentrate in the lattice that bears tensile stress), and the plasticity of the steel will drop to almost zero. When the stress is large enough, brittle fracture will occur.

[0004] (2) Hydrogen absorbed into molten steel during steelmaking; it precipitates when the molten steel solidifies due to reduced solubility.

[0005] (3) Under certain conditions, the structural stress caused by phase change during the cooling process of the steel billet can reach a considerable value (the more severe the dendritic segregation, the faster the cooling rate, and the better the hardenability of the steel, the greater the structural stress). Therefore, hydrogen embrittlement of the steel loses its plasticity, and under the combined action of structural stress and internal stress caused by hydrogen precipitation, tiny cracks are generated in the steel, which form white spots.

[0006] The external characteristics of white spots are: round or oval silver-white spots with smooth and clean surface on the longitudinal fracture of the steel billet. The average diameter of the white spots is several millimeters to tens of millimeters, which destroy the continuity of the steel and make the steel easy to break brittlely; because the white spots seriously destroy the continuity of the metal matrix.

[0007] The temperature range for the formation of white spots is between 250℃ and 100℃, while the maximum range of hydrogen diffusion coefficient is between 650℃ and 300℃. Therefore, hydrogen that has no time to diffuse below 300℃ exists in the steel and causes stress, thus creating the necessary conditions for the formation of self-spots. It can be seen that hydrogen is the main factor in the formation of white spots. If slow cooling can be performed after rolling, the appearance of white spots can be avoided. Because white spots are mainly caused by the combined action of hydrogen and organizational stress in steel, trying to remove hydrogen and eliminate organizational stress can avoid the formation of white spots. White spots are often produced after the steel has cooled to room temperature for several hours or dozens of hours, or even longer.

[0008] In recent years, in response to the white spot defect problem of ingots, patent CN102363855B proposed a method of using the second phase in steel to improve the sensitivity of white spots. By selecting and adding appropriate alloy elements and controlling the rolling process to precipitate fine dispersed second phase particles in steel, the critical hydrogen content for the formation of white spots is increased. Patent CN111451425B mainly controls the preset temperature of the forging process and adopts a two-step material forming method to reduce white spot defects. Summary of the invention

[0009] In order to solve the above technical problems, the present invention provides a method for controlling white spot defects in steel, which can significantly reduce the number of white spots in continuous casting billets.

[0010] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is:

[0011] A method for controlling white spot defects in steel, comprising electric furnace steelmaking, RH vacuum refining, continuous casting, rolling and cooling processes;

[0012] Electric furnace steelmaking process: According to the different gases injected, the oxygen-burning gun is divided into burner mode and nozzle mode. The gas injected by the burner mode is natural gas and oxygen with a volume ratio of 1:10. According to the injected gas flow rate, it is divided into three levels: low, medium and high. The maximum natural gas flow rate is 610Nm 3 / h; The gas sprayed by the nozzle mode is pure oxygen, which is divided into three levels: low, medium and high according to the sprayed gas flow rate. The maximum oxygen flow rate is 2500Nm 3 / h; the operating steps to control the melting period are:

[0013] (1) Starting from the first batch of scrap steel being powered on, all oxygen-fueled lances adopt the burner mode, and each oxygen-fueled lance executes the combustion sequence from low to high; the burner low mode lasts for 1 to 2 minutes, the burner medium mode lasts for 1 to 3 minutes, and within 4 minutes, all oxygen-fueled lances are ensured to enter the burner high mode;

[0014] (2) 10 minutes after power is supplied, all oxygen-burning guns are switched to low-nozzle mode to spray oxygen;

[0015] (3) Add slag-making material 6 minutes after power supply;

[0016] (4) When the power is supplied for 12 minutes, lime is added, and when the power is supplied for 15 minutes, the carbon gun is turned on to deliver carbon into the furnace to achieve accurate control of the carbon content;

[0017] (5) Start preheating scrap steel when the power supply is less than 24000Kwh;

[0018] Rolling and cooling process: After rolling, the steel is cooled by pit cooling or pile cooling.

[0019] Furthermore, in the electric furnace steelmaking process, 8 to 12 kg / ton of steel of slag material is added when power is supplied for 6 minutes.

[0020] Furthermore, in the electric furnace steelmaking process, 8 - 12 kg of lime per ton of steel is added 12 minutes after power supply.

[0021] Furthermore, in the RH vacuum refining process, the vacuum degree ≤ 67 Pa and the vacuum time is 6 - 30 min.

[0022] Furthermore, in the continuous casting process, the method of using a long tundish nozzle + blowing Ar for sealing and casting with an integral nozzle in the middle tundish is adopted. The long tundish nozzle is connected to the lower tundish nozzle with a sealing ring, and there is an Ar - blowing joint on the long tundish nozzle for full - process Ar - blowing protection, and the Ar gas flow rate is 3 - 7 m 3 / h.

[0023] Furthermore, in the rolling and cooling process, it is taken out of the pit after being pit - cooled to ≤ 200 °C.

[0024] Furthermore, before entering the electric furnace, the raw materials for furnace charging are baked, and the baking temperature ≥ 200 °C.

[0025] The beneficial effects of adopting the above - mentioned technical solution are as follows: The present invention can make the continuous casting billet in the temperature range where the hydrogen diffusion rate is the fastest under the condition of reducing various stresses (phase - change structure stress, deformation residual stress, cooling temperature stress, etc.), and the H element in the steel grade after rolling is lower than 0.82 ppm, greatly reducing the number of white spots in the steel grade. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a diagram of the white - spot situation after rolling of 40Mn2 steel in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0027] The following further describes the present invention in detail in conjunction with the specific embodiments.

[0028] The method for controlling white - spot defects in steel of the present invention includes the processes of electric - furnace steelmaking, RH vacuum refining, continuous casting, rolling and cooling, and the steps of each process are as follows:

[0029] (1) Selection and processing of raw materials: Before entering the electric furnace, the raw materials are selected and processed. High - quality scrap steel with less rust is used to ensure that the raw material alloy is dry. The raw materials for furnace charging are baked by a hot - blast stove, and the baking temperature ≥ 200 °C is ensured to effectively reduce the H content brought into the steel by the alloy materials.

[0030] (2) Electric - furnace steelmaking process: According to the different gases blown, the oxy - fuel lance is divided into a burner mode and a nozzle mode. Among them, the gas blown in the burner mode is natural gas and oxygen with a volume ratio of 1:10. According to the gas - blowing flow rate, it is divided into low, medium, and high gears, and the maximum natural - gas flow rate is 610 Nm 3 / h; The gas blown in the nozzle mode is pure oxygen, which is divided into three grades of low, medium and high according to the gas flow rate of blowing, and the maximum oxygen flow rate is 2500 Nm 3 / h; The operation steps for controlling the melting period are as follows:

[0031] ① Starting from the power-on of the first batch of scrap steel, all the oxy-fuel burners adopt the burner mode, and each oxy-fuel burner executes the combustion sequence from low grade to high grade; the low burner mode lasts for 1 - 2 minutes, the medium burner mode lasts for 1 - 3 minutes, and it is ensured that all the oxy-fuel burners enter the high burner mode within 4 minutes, so that the surrounding scrap steel reaches the preheating temperature and remove the H element as much as possible first;

[0032] ② 10 minutes after power-on, all the oxy-fuel burners enter the low nozzle mode to blow oxygen to complete the purpose of decarburizing the molten steel and the stirring task, promote the flow of the molten steel, and accelerate the removal of the H element;

[0033] ③ When the power is on for 6 minutes, add 8 - 12 kg / ton of steel slag-making materials;

[0034] ④ When the power is on for 12 minutes, add 8 - 12 kg / ton of lime, and start feeding carbon into the furnace with a carbon gun when the power is on for 15 minutes to achieve accurate control of the carbon content;

[0035] ⑤ Start preheating the scrap steel when the power consumption < 24000 Kwh.

[0036] (3)RH vacuum refining process: The vacuum degree ≤ 67 Pa, and the vacuum time is 6 - 30 min;

[0037] Control the vacuum time according to the steel type, bearing steel: 20 - 30 min; free-cutting non-quenched and tempered steel, gear steel: 15 - 25 min; alloy structural steel, spring steel: 10 - 20 min; nitrogen-preserving steel, peritectic steel: 6 - 10 min.

[0038] (4)Continuous casting process: Adopt the method of tundish long nozzle + blowing Ar seal and casting with the integral nozzle of the tundish. The tundish long nozzle is connected to the tundish lower nozzle with a sealing ring, and there is an Ar blowing joint on the tundish long nozzle for full-process Ar blowing protection. The length of the tundish long nozzle for continuous casting is 1370 mm, and the Ar gas flow rate is 3 - 7 m 3 / h, which can reduce the hydrogen content in the molten steel by 15% - 40%, effectively solving the problem of H increase during continuous casting.

[0039] (5)Rolling and cooling process: After rolling, the rolled material is never allowed to be directly air-cooled to room temperature. After rolling, it is cooled by pit cooling or stacking cooling. It is taken out of the pit after being pit-cooled to ≤ 200℃; the cooling time ≥ 36 h;

[0040] The larger the slab specification, the longer the cooling time, which can more effectively reduce the surface temperature gradient of the slab, prevent the phase transformation stress from increasing, accelerate the effective diffusion of H element. After slow cooling to ≤200°C and taking out of the pit, if annealing is required, annealing should be carried out within three days. When the slow cooling pit position is insufficient, it should be immediately transferred to the annealing furnace for annealing. The cooling methods and cooling times for different specification slabs are as follows:

[0041]

[0042] The parameter controls of each process in Examples 1-7 are shown in Tables 1 and 2, and the H element contents of the steel grades after cooling in each example are shown in Table 2.

[0043] Table 1: Parameters of the electric furnace steelmaking and RH vacuum refining processes in each example

[0044]

[0045] Table 2: Parameters of the continuous casting, rolling and cooling processes and the H content of the steel grades in each example

[0046]

[0047] It can be seen from Figure 1 that for the steel grade 40Mn2 produced according to the present invention, the macrostructure quality is good and there are no obvious white spot defects.

Claims

1. A method for controlling white spot defects in steel, characterized in that, The method includes the processes of electric furnace steelmaking, RH vacuum refining, continuous casting, rolling and cooling; Electric arc furnace steelmaking process: According to the different gases injected, the oxy-fuel lance is divided into a burner mode and a nozzle mode. Among them, the gas injected in the burner mode is natural gas and oxygen with a volume ratio of 1:

10. According to the gas flow rate of injection, it is divided into low, medium, and high grades, and the maximum natural gas flow rate is 610 Nm 3 / h; The gas injected in the nozzle mode is pure oxygen. According to the gas flow rate of injection, it is divided into low, medium, and high grades, and the maximum oxygen flow rate is 2500 Nm 3 / h; The operating steps for controlling the melting period are as follows: (1) Starting from the power-on of the first batch of scrap steel, all oxygen burners are in the burner mode, and each oxygen burner executes the combustion sequence from low to high gear; the low burner mode lasts for 1 - 2 minutes, the medium burner mode lasts for 1 - 3 minutes, and it is ensured that all oxygen burners enter the high burner mode within 4 minutes; (2) 10 minutes after power-on, all oxygen burners are switched to the low nozzle mode for oxygen injection; (3) Flux materials are added 6 minutes after power-on; (4) Lime is added 12 minutes after power-on, and a carbon lance is started to supply carbon into the furnace 15 minutes after power-on to achieve accurate control of the carbon content; (5) Scrap steel is preheated when the power consumption < 24000 Kwh; Rolling and cooling process: After rolling, it is cooled by pit cooling or stockpiling cooling.

2. The method for controlling the white spot defect in steel according to claim 1, wherein, In the electric furnace steelmaking process, 8 - 12 kg / ton of steel of flux materials are added 6 minutes after power-on.

3. The method for controlling the white spot defect in steel according to claim 2, wherein In the electric furnace steelmaking process, 8 - 12 kg / ton of steel of lime are added 12 minutes after power-on.

4. The method for controlling the white spot defect in steel according to claim 3, characterized in that, In the RH vacuum refining process, the vacuum degree ≤ 67 Pa and the vacuum time is 6 - 30 min.

5. The method for controlling the white spot defect in steel according to claim 4, wherein, For the continuous casting process, the method of using a long tundish nozzle + Ar blowing for sealing and a monolithic tundish nozzle for casting is adopted. The long tundish nozzle is connected to the lower tundish nozzle with a sealing ring, and there is an Ar blowing joint on the long tundish nozzle for full-process Ar blowing protection, with an Ar gas flow rate of 3 - 7 m 3 / h.

6. The method for controlling the white spot defect in steel according to claim 5, characterized in that, In the rolling and cooling process, it is taken out of the pit after pit cooling to ≤ 200 °C.

7. The method for controlling the white spot defect in steel according to claim 6, characterized in that, Before entering the electric furnace, the raw materials entering the furnace are baked at a baking temperature ≥ 200 °C.

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