A method for reducing bulging defects of low-carbon boron-containing steel continuous casting slab

Abstract

This invention discloses a method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel, mainly addressing the technical problem of high bulging defect incidence in existing continuously cast slabs of low-carbon boron-containing steel. The technical solution is as follows: a method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel includes: 1) Converter smelting, using a top-and-bottom blowing converter, with the mass percentage of the input metal raw materials being 75%-90% molten iron and the remainder being scrap steel; 2) Converter tapping, tapping steel immediately after converter blowing, using a ladle that is not newly constructed or has not undergone minor repairs; 3) Continuous casting, where the molten steel in the ladle is distributed to the crystallizer of the slab continuous casting machine via an intermediate ladle and then poured into the slab continuous casting machine to obtain the continuously cast slab; 4) The casting speed of the continuous casting machine during the continuous casting process is 0.8-2.0 m / min. The method of this invention reduces the hydrogen content of the molten steel in the intermediate ladle to 0.000281%, and reduces the bulging defect incidence of the continuously cast slab to 0.78 times / month.

Description

Technical Field

[0001] This invention relates to a method for producing low-carbon boron-containing steel, and more particularly to a method for reducing bulging in continuously cast slabs of low-carbon boron-containing steel, belonging to the field of steelmaking continuous casting technology. Background Technology

[0002] Boron-containing steel is a widely used steel grade. Because boron delays the nucleation process of ferrite, thus improving the hardenability of the steel, it is widely used, especially in the export sector. However, continuously cast slabs made of boron-containing steel are prone to surface quality defects. Under the same process conditions, boron-containing steel produces more bubbles, and in severe cases, slab bulging defects can occur during continuous casting solidification or in the hot rolling furnace.

[0003] In recent years, with the increase in the production of low-carbon boron-containing steel, the frequency of slab bulging defects has increased significantly, sometimes exceeding 5 times per month. This has led to adverse consequences such as slab scrapping and damage to heating furnace equipment, seriously affecting product quality and production operations. Data indicates that slab bulging is related to the hydrogen content in molten steel; furthermore, inspections of heats with slab bulging defects have revealed that the hydrogen content in the tundish steel sometimes exceeds 10 ppm.

[0004] The main sources of hydrogen in molten steel are moisture in the air, raw materials and auxiliary materials used in smelting, and the containers used to hold the steel. Vacuum degassing is an effective method to reduce the hydrogen content in molten steel; RH vacuum degassing can control the hydrogen content in molten steel to below 2 ppm, but it also significantly increases steelmaking costs. To control the hydrogen content in molten steel, it is essential to minimize the introduction of moisture throughout the entire steelmaking process.

[0005] Chinese patent CN103741007A discloses a production method for reducing the gas content in low-carbon aluminum killed steel. This invention reduces the gas content in molten steel by adjusting the baking temperatures of the ladle and tundish, controlling the timing of alloy addition during tapping, and adjusting the amount of argon blown in each process, thereby reducing bubble cracking in deep-drawing steel rolling and bending cracking in automotive beam plates. However, this patent does not consider the moisture control of converter raw materials and the influence of tundish refractory materials on the hydrogen content of molten steel.

[0006] Chinese patent CN102554148A discloses a method for controlling hydrogen increase in the first ladle of molten steel during casting. This invention reduces the high hydrogen content in the first ladle of special steel castings by optimizing the tundish baking process and drying the tundish covering agent. However, this patent only considers the impact of the continuous casting process on the hydrogen content of the molten steel in the first ladle and cannot fundamentally solve the problem of bulging in continuously cast slabs.

[0007] The literature "Analysis of the Causes of Bulging in Low-Carbon Boron-Containing Steel Slabs" (Liu Huan, *Baosteel Technology*, 2019, No. 5, pp. 35-38) discloses the causes of bulging in low-carbon boron-containing steel. It suggests controlling the bulging problem by establishing moisture control standards for the materials, adjusting the timing of pelletizing, and regularly maintaining the continuous casting machine. However, this patent, while controlling the moisture content of the raw materials fed into the furnace, does not consider the timing of adding lump ore, cold-pressed pellets, lime, and other major auxiliary materials to the converter. Therefore, it cannot completely remove hydrogen from the molten steel and does not address the issue of hydrogen enrichment in the refractory materials of the tundish, thus failing to fundamentally solve the bulging problem in continuously cast slabs. Summary of the Invention

[0008] The purpose of this invention is to provide a method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel, mainly addressing the technical problem of high incidence of bulging defects in existing continuously cast slabs of low-carbon boron-containing steel. This invention can effectively reduce bulging quality defects in low-carbon boron-containing steel slabs and improve the quality of continuously cast slabs of low-carbon boron-containing steel.

[0009] The technical idea of ​​this invention is to control the moisture content of key raw materials and refractory materials in the smelting and continuous casting processes, optimize the types of raw materials and refractory materials fed into the furnace, and reduce the introduction of moisture from the source; at the same time, adjust the timing of adding auxiliary materials to the converter and optimize the baking system of the tundish to remove moisture introduced into the molten steel as effectively as possible, significantly reduce the hydrogen content in the molten steel in the tundish, and avoid slab bulging defects caused by high hydrogen content.

[0010] The low-carbon boron-containing steel of the present invention has the following chemical composition by mass percentage: C: 0.026-0.049%, Si≤0.034%, Mn: 0.15%-0.25%, P≤0.02%, S≤0.0154%, Al: 0.025%-0.05%, B: 0.001%-0.002%, with the balance being Fe and other alloys and unavoidable impurities.

[0011] The technical solution adopted in this invention is a method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel, comprising the following steps:

[0012] 1) Converter smelting: Top-and-bottom blowing converter smelting is adopted. The mass percentage of the metal raw materials input is 75%-90% iron and the remainder is scrap steel. Argon is blown into the bottom throughout the smelting process, and the argon flow rate is 0.02-0.1 cubic meters / (ton steel·minute). The converter cooling material is ore with a water mass percentage of ≤5%. The ore is added into the converter before the end of the converter blowing process. At the end of the converter blowing process, samples are taken to test the w[C] and temperature of the molten steel. When the w[C] in the molten steel at the end of the converter blowing process is ≥0.03% and the temperature of the molten steel at the end of the converter blowing process is 1620-1660℃, the converter blowing process ends.

[0013] 2) Converter tapping: Steel should be tapped immediately after converter blowing is completed. The ladle should not be a newly constructed ladle or a ladle that has not undergone minor repairs. During converter tapping, argon gas should be blown into the ladle at a flow rate of 21-40 liters / hour, and the blowing time should be ≥8 minutes. When the amount of molten steel tapped from the converter reaches 20-30% of the total molten steel, ferromanganese and lime should be added to the ladle. When the amount of molten steel tapped from the converter reaches 40-50% of the total molten steel, lime and ladle slag modifier should be added to the ladle. The moisture content of the lime should be ≤2%, and the moisture content of the ladle slag modifier should be ≤2%, with an addition amount ≤1.5 kg / t of steel.

[0014] 3) Continuous casting: Molten steel in the ladle is distributed to the crystallizer of the slab continuous casting machine via the tundish and then poured into the slab continuous casting machine to obtain continuously cast slabs. The casting speed of the continuous casting machine during the continuous casting process is 0.8-2.0 m / min. The refractory material of the tundish working layer is dry material, accounting for 94%-97% of the total mass of the tundish working layer, and the binder accounts for 3%-6% of the total mass of the tundish working layer. The binder is a mixture of sodium silicate and phenolic resin. After the tundish working layer is prepared using dry material, the tundish is baked with a mold for 60-90 minutes at a temperature of 200-300℃. The tundish is baked before receiving molten steel. The tundish working layer is baked at a temperature ≥1250℃ for ≥30 minutes. The chemical composition of the continuously cast slab by weight percentage is as follows: C: 0.026-0.049%, Si≤0.034%, Mn: 0.15%-0.25%, P≤0.02%, S≤0.0154%, Al: 0.025%-0.05%, B: 0.001%-0.002%, with the balance being Fe and other alloys and unavoidable impurities.

[0015] Furthermore, in step 1) of the present invention, the converter cooling material is converter dust ash cold-pressed balls, and the water content in the converter dust ash cold-pressed balls is ≤8% by mass. The converter dust ash cold-pressed balls are added into the converter before the total oxygen blowing volume of the converter reaches 75%.

[0016] In step 3) of this invention, the weight percentage of the binder components is: sodium silicate 35%-65%, phenolic resin 35%-65%, and the sum of the contents of each component is 100%.

[0017] The dry material has the following chemical composition by mass percentage: MgO ≥ 88%, with the remainder being impurities.

[0018] Furthermore, in step 3) of the present invention, the refractory material of the tundish working layer is a coating material. When preparing the tundish working layer, the coating material accounts for 82%-84% of the total mass of the tundish working layer, and water accounts for 16%-18% of the total mass of the tundish working layer. During the preparation of the tundish working layer with the coating material, the temperature of the permanent layer of the tundish is controlled at 80-120℃. After the tundish working layer is prepared with the coating material, the natural curing time of the tundish working layer is ≥20 hours.

[0019] The chemical composition of the coating material is as follows (by mass percentage): SiO2: 17%-25%, MgO: 65%-75%, with the remainder being impurities.

[0020] The reasons for determining the process control parameters in this invention are as follows:

[0021] 1. Setting of converter cooling material

[0022] The converter cooling material is ore. If the ore has a high moisture content and is added after the converter smelting is complete, the hydrogen decomposed by the moisture at high temperatures cannot completely escape from the molten steel. When the hydrogen content in boron-containing steel is high, it accumulates in the center of the slab, and the hydrogen inside the slab easily expands in volume. When the gas pressure exceeds the slab strength, it forms a bulging defect in the slab. This invention limits the moisture content of the ore to ≤5% and adds it before the converter blowing is completed.

[0023] The converter cooling material uses converter dust ash cold-pressed briquettes, which are formed by pressing converter dust ash, water, and binder into blocks. When the briquettes are fed into the furnace, they still contain a certain amount of moisture. These briquettes are added in the early stages of converter smelting, and the hydrogen produced by the decomposition of this moisture at high temperatures can be removed by the intense molten pool reaction and stirring during the early stages of converter smelting. This invention requires the cold-pressed briquettes to have a moisture content ≤8%, and they must be added before the total oxygen blowing volume in the converter reaches 75%.

[0024] 2. Setting of auxiliary materials during converter tapping process

[0025] Lime, added during the converter tapping process, can increase the basicity of the ladle slag and modify Al2O3 inclusions. However, if the moisture content of the lime is too high, the stirring degree of the molten steel in the ladle will be insufficient, and the generated hydrogen cannot be completely removed, easily leading to a high hydrogen content in the molten steel. This invention limits the moisture content of the lime to ≤2%.

[0026] A ladle slag modifier is added to molten steel during converter tapping to reduce turbulence, foaming, and oxidizing properties. The applicant's research has shown that increasing the amount of ladle slag modifier significantly increases the hydrogen content in the molten steel. To balance reducing oxidation and hydrogen accumulation, this invention limits the moisture content of the ladle slag modifier to ≤2% and the addition amount to ≤1.5 kg / t of steel.

[0027] 3. Setting of refractory material for the tundish working layer and baking parameters for the tundish working layer.

[0028] The refractory material for the working layer of the tundish uses dry-type refractory. Since dry-type refractory is non-aqueous, a certain amount of binder needs to be added. The selection of the binder must consider that it is anhydrous, does not react with the dry-type refractory at room temperature, and also ensures the high-temperature performance of the dry-type refractory. When resin is used as a binder, the residual carbon after curing can easily contaminate the molten steel, making it unsuitable for smelting steels with low carbon content. Furthermore, the decomposition of phenolic resin-based organic binders can easily lead to hydrogen enrichment in the molten steel. Using inorganic salts as binders often results in large shrinkage and makes ladle turning difficult. The applicant's research has found that using a binder containing a composite of inorganic salts and resin can prevent hydrogen enrichment in the molten steel and avoid the problem of difficult ladle turning. This invention specifies the binder components by weight percentage as follows: sodium silicate 35%-65%, phenolic resin 35%-65%, and the sum of the contents of all components is 100%.

[0029] The refractory coating for the tundish working layer has advantages such as relatively long service life and no contamination of molten steel. However, it requires the addition of a certain amount of water during construction. To prevent water from entering the molten steel and causing hydrogen enrichment, sufficient curing time for the tundish working lining must be ensured. This invention limits the natural curing time of the working lining to ≥20 hours.

[0030] Compared with existing technologies, this invention has the following positive effects: 1. This invention significantly reduces the moisture content of raw and auxiliary materials fed into the converter, prioritizing the selection of raw and auxiliary materials with lower moisture content and adding them in the early to mid-stages of converter smelting, utilizing the intense carbon-oxygen reaction in the early stage of converter smelting to remove hydrogen from the molten steel. 2. This invention reduces hydrogen enrichment in molten steel from the ladle, limiting the amount of ladle slag modifier used during tapping to reduce the introduction of moisture from refining slag. 3. This invention reduces the introduction of moisture from refractory materials in the continuous casting tundish. For example, when using special dry materials in continuous casting, it reduces the decomposition of binders and reduces hydrogen enrichment in the molten steel; when using coating materials in continuous casting, it controls the addition of moisture to the coating materials while ensuring sufficient natural curing time to ensure complete moisture removal. 4. This invention reduces hydrogen enrichment during the continuous casting process, ensuring sufficient baking and holding time in the tundish, which is conducive to the rapid removal of moisture during high-temperature baking. 5. The method of the present invention reduces the incidence of bulging defects in continuously cast slabs. The hydrogen content of molten steel in the tundish is reduced from an average of 0.000573% to 0.000281%, and the incidence of bulging defects in continuously cast slabs is reduced from 5 times / month to 0.78 times / month. Detailed Implementation

[0031] The present invention will be further described below with reference to Examples 1 to 5, as shown in Tables 1 to 10.

[0032] In this embodiment of the invention, the ladle used to hold molten steel has a capacity of 250 tons, and the steel grade produced is DC51D+Z. Five heats of molten steel are produced using the process of this invention; the production process route is: converter - argon blowing station - slab continuous casting.

[0033] The chemical composition of the steel in the example, by weight percentage, is as follows: C: 0.026-0.049%, Si≤0.034%, Mn: 0.15%-0.25%, P≤0.02%, S≤0.0154%, Al: 0.025%-0.05%, B: 0.001%-0.002%, with the balance being Fe and other alloys and unavoidable impurities.

[0034] A method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel includes the following steps:

[0035] The smelting process employs a top-and-bottom blown converter, with 75%-90% of the main metal raw material being molten iron and 10-25% being scrap steel. Argon is blown into the bottom throughout the smelting process, with an argon flow rate of 0.02-0.10 cubic meters per ton of steel per minute. Converter cooling material is added before and during the middle stages of converter blowing. The converter blowing process ends when the w[C] in the molten steel reaches ≥0.03% at the end of the converter blowing process and the temperature of the molten steel reaches 1620-1660℃.

[0036] Steel is tapped immediately after converter blowing, and the ladle is either not newly constructed or not a ladle that has undergone minor repairs; when the amount of molten steel tapped from the converter reaches 20-30% of the total molten steel, ferromanganese and lime are added to the ladle; when the amount of molten steel tapped from the converter reaches 40-50% of the total molten steel, lime and ladle slag modifier are added to the ladle; the ladle is bottom-blown with argon gas throughout the process, with an argon gas flow rate of 20-40 liters / hour and an argon blowing time of ≥8 minutes;

[0037] The molten steel in the ladle is transported to the argon blowing station for deoxidation. First, argon gas is introduced into the molten steel to stir it for 3-4 minutes, controlling the argon gas flow rate at 40-50 liters / hour. Then, the argon gas is stopped, and the temperature of the molten steel is measured. Boron wire is added to the molten steel. Argon gas is introduced into the molten steel again, controlling the argon gas flow rate at 20-30 liters / hour, and the temperature of the molten steel is adjusted to 1580-1600℃. Aluminum wire is then fed into the molten steel to deoxidize it.

[0038] Molten steel, after being treated by the argon blowing station, flows through the tundish into the continuous casting slab crystallizer for continuous casting of slabs. The slab casting speed during the continuous casting process is 0.8-2.0 m / min. When the refractory material for the tundish working lining is dry refractory, the tundish dry refractory strip mold is baked at a lower temperature of 200-300℃ for 60-90 minutes. When the refractory material for the tundish working lining is coated refractory, the surface temperature of the permanent layer on the inner wall of the tundish is between 80-120℃ during the coating application. After coating, the working lining is allowed to cure naturally for ≥20 hours. Before use, the tundish needs to be baked and kept at a temperature of ≥1250℃ for ≥30 minutes.

[0039] Table 1 Parameters of Metal Materials for Converter Smelting

[0040]

[0041] Table 2. Amount of converter cooling material added, unit: kg / ton of steel

[0042]

[0043] In Examples 1-2 and Examples 4-5, the water content in the converter dust ash cold-pressed balls was 7.4%, 4.5%, 4.1%, and 4.5% by mass, respectively; the converter dust ash cold-pressed balls were added to the converter before the total oxygen blowing volume of the converter reached 75%.

[0044] In Examples 1 and 3, the water content in the ore was 4.5% and 1.8% by mass, respectively; the ore was added to the converter before the converter blowing process was completed.

[0045] Table 3 Chemical composition and temperature of molten steel tapped from the converter

[0046]

[0047] Table 4. Charging during converter tapping process, unit: kg / ton of steel

[0048]

[0049] In Examples 1-5, the water content in the lime was 1.5%, 1.3%, 1.5%, 1.1%, and 1.3%, respectively; the water content in the ladle slag modifier was 1.0%, 0.5%, 0.8%, 1.0%, and 0.5%, respectively.

[0050] Table 5. Temperature and Argon Blowing Parameters of the Ladle During Converter Tapping Process

[0051]

[0052] Table 6. Composition of molten steel entering the argon blowing station, unit: weight percentage

[0053]

[0054] Table 7 Composition of molten steel leaving the argon blowing station, unit: weight percentage

[0055]

[0056] Table 8 Control parameters for continuous casting process

[0057]

[0058] Table 9. Mass percentage of components in the intermediate tundish working layer during preparation.

[0059]

[0060] In Examples 3-4, the refractory material of the tundish working layer is a coating material. After the tundish working layer is prepared with the coating material, the natural curing time of the tundish working layer is 21 hours, 20 hours and 20 hours respectively.

[0061] Table 10 Hydrogen content data of molten steel in tundish

[0062]

[0063] In Examples 1-5, the average hydrogen content of the molten steel in the tundish was 0.000164%, and the bulging rate of the continuously cast slab was 0.

[0064] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel, characterized in that, The method includes the following steps: 1) Converter smelting: Top-and-bottom blowing converter smelting is adopted. The mass percentage of the metal raw materials input is 75%-90% iron and the remainder is scrap steel. Argon is blown into the bottom throughout the smelting process, and the argon flow rate is 0.02-0.1 cubic meters / (ton steel·minute). The converter cooling material is ore with a water mass percentage of ≤5%. The ore is added into the converter before the end of the converter blowing process. At the end of the converter blowing process, samples are taken to test the w[C] and temperature of the molten steel. When the w[C] in the molten steel at the end of the converter blowing process is ≥0.03% and the temperature of the molten steel at the end of the converter blowing process is 1620-1660℃, the converter blowing process ends. 2) Converter tapping: Steel should be tapped immediately after converter blowing is completed. The ladle should not be a newly constructed ladle or a ladle that has not undergone minor repairs. During converter tapping, argon gas should be blown into the ladle at a flow rate of 21-40 liters / hour, and the blowing time should be ≥8 minutes. When the amount of molten steel tapped from the converter reaches 20-30% of the total molten steel, ferromanganese and lime should be added to the ladle. When the amount of molten steel tapped from the converter reaches 40-50% of the total molten steel, lime and ladle slag modifier should be added to the ladle. The moisture content of the lime should be ≤2%, and the moisture content of the ladle slag modifier should be ≤2%, with an addition amount ≤1.5 kg / t of steel. 3) Continuous casting: Molten steel in the ladle is distributed to the crystallizer of the slab continuous casting machine via the tundish and then poured into the slab continuous casting machine to obtain continuously cast slabs; the casting speed of the continuous casting machine during the continuous casting process is 0.8-2.0 m / min; the refractory material of the tundish working layer is a dry-mix material, accounting for 94%-97% of the total mass of the tundish working layer, and the binder accounts for 3%-6% of the total mass of the tundish working layer. The binder is a mixture of sodium silicate and phenolic resin; after the tundish working layer is prepared using dry-mix material, the tundish is baked with a mold for 60-90 minutes at a temperature of 200-300℃; the refractory material of the tundish working layer is a coating material, accounting for 82%-84% of the total mass of the tundish working layer, and water accounts for 16%-18% of the total mass of the tundish working layer; during the preparation of the tundish working layer using the coating material... The temperature of the permanent layer in the tundish is controlled at 80-120℃; after the working layer of the tundish is prepared with a coating material, the natural curing time of the working layer is ≥20 hours; before the tundish accepts molten steel, the tundish is baked and kept warm, the baking and keeping temperature of the working layer is ≥1250℃, and the baking and keeping time of the working layer is ≥30 minutes; the weight percentage of the chemical composition of the continuous casting slab is: C: 0.026-0.049%, Si≤0.034%, Mn: 0.15%-0.25%, P≤0.02%, S≤0.0154%, Al: 0.025%-0.05%, B: 0.001%-0.002%, with the balance being Fe and other alloys and unavoidable impurities.

2. The method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel as described in claim 1, characterized in that, The converter cooling material is converter dust ash cold-pressed briquettes. The water content in the converter dust ash cold-pressed briquettes is ≤8% by mass. The converter dust ash cold-pressed briquettes are added into the converter before the total oxygen blowing volume of the converter reaches 75%.

3. The method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel as described in claim 1, characterized in that, The binder components are: sodium silicate 35%-65%, phenolic resin 35%-65%, and the sum of the contents of each component is 100%.

4. The method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel as described in claim 1, characterized in that, The dry material has the following chemical composition by mass percentage: MgO ≥ 88%, with the remainder being impurities.

5. The method for reducing bulging defects in continuously cast slabs of low-carbon boron-containing steel as described in claim 1, characterized in that, The chemical composition of the coating material is as follows (by mass percentage): SiO2: 17%-25%, MgO: 65%-75%, with the remainder being impurities.

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