CN111944953A

CN111944953A - Method for reducing number of small-size nonmetallic inclusions in steel

Info

Publication number: CN111944953A
Application number: CN202010920824.6A
Authority: CN
Inventors: 金友林; 陆强; 龚志翔; 沈昶; 高振波; 王尚; 程方波
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2020-09-04
Filing date: 2020-09-04
Publication date: 2020-11-17
Anticipated expiration: 2040-09-04
Also published as: CN111944953B

Abstract

The invention provides a method for reducing the number of small-size nonmetallic inclusions in steel, which utilizes the principle of separating out tiny bubbles in molten steel to remove inclusions, fully considers the thermodynamic and kinetic conditions of nitrogen increase and nitrogen separation, adopts the operation processes of LF furnace process nitrogen increase, controls the content of oxygen and sulfur in the steel and RH process denitrification to ensure the stability of the nitrogen increase and nitrogen separation reaction rate of the molten steel, and meets the requirement of low-cost continuous and stable production while effectively reducing the number of nonmetallic inclusions in the high-quality special steel.

Description

Method for reducing number of small-size nonmetallic inclusions in steel

Technical Field

The invention belongs to the technical field of steel making, and relates to a method for reducing the number of small-size nonmetallic inclusions in steel.

Background

The non-metallic inclusions are important factors influencing the fatigue life of metallurgical products such as bearings, springs and cords, and the quantity and the size of the non-metallic inclusions in steel are important indexes for measuring the quality of the products. In the existing production process flow of high-quality special steel, the non-metallic inclusions are mostly controlled by LF, RH or VD procedures for refining, and refining links such as LF, RH or VD need a long time for removing the non-metallic inclusions. Moreover, these refining processes, although capable of producing high quality special steels, have a low efficiency of removing small size (20 μm or less) inclusions.

The ability of bubbles in molten steel to adsorb and remove inclusions has been studied and used by metallurgists, but the bubbles formed in molten steel by externally supplied gas have large size, are unevenly distributed, have limited inclusion adsorption ability, and have poor inclusion removal effect, especially on small-size inclusions.

At present, a method for removing nonmetallic inclusions by separating out tiny bubbles in steel is provided by a patent, and the main operation key points are that nitrogen elements or hydrogen elements or mixed elements of hydrogen and nitrogen are added in molten steel in advance, and then the molten steel is subjected to vacuum treatment to separate out bubbles in the molten steel so as to achieve the effect of removing the inclusions. The main patents involved are: publication No. CN104404205, published 3/11/2015, a method for removing microscopic non-metallic inclusions in molten steel by nitrogen increasing and nitrogen separating methods, publication No. CN105779699B, published 1/11/2019, a method for removing metallic inclusions by a dissolved gas float process and a refining device of a pressurized vacuum induction furnace, and publication No. CN106086315, published 4/24/2018, a method for generating micro-bubbles in molten steel. Although the above patent can reduce the amount of non-metallic inclusions in steel, it has several problems: 1. the operation of adding hydrogen to high-temperature liquid molten steel (hydrogen, coke oven gas, natural gas and other hydrogen-containing gases are blown in) has greater potential safety hazard, the requirements on the sealing performance of equipment and the leakage detection of a daily gas pipeline are strict, the management cost is higher for enterprises, and the application adaptability in industrial mass production is poorer; 2. the dynamics factors of the nitrogen increasing and separating reactions of molten steel are not fully considered, particularly the influence of the content of oxygen and sulfur in steel on the mass transfer of nitrogen in steel is not fully considered, the reaction rate of nitrogen increasing and separating cannot be effectively controlled, the difference of the nitrogen increasing and separating effects is large, and the impurity removing effect cannot be stabilized. 3. The target requirement of the nitrogen content in the steel after nitrogen increase is clarified in the patents, but the nitrogen increase and nitrogen separation time is inevitably unstable because no requirement is made on other factors influencing the thermodynamics and kinetics of the nitrogen increase reaction in the steel (the time for nitrogen increase and nitrogen separation of the molten steel cannot be controlled because effective deoxidation and sulfur control are not carried out on the molten steel). At present, more than 90% of steel enterprises in China are in high-efficiency continuous production, and the nitrogen increasing and separating time is unstable, so that the production rhythm is disturbed, the stable continuous production is influenced, and the cost of each ton of steel is increased. In addition, special pressurizing equipment is needed in part of the invention, and the cost investment is high.

Disclosure of Invention

The invention aims to provide a method for reducing the number of small-size nonmetallic inclusions in steel, which utilizes the principle of removing the inclusions by separating out tiny bubbles from molten steel, fully considers the thermodynamic and kinetic conditions of nitrogen increase and nitrogen separation, simultaneously combines the requirements of efficient continuous stable production on process design, comprehensively considers the cost and the quality, designs LF and RH processes aiming at the problems of high requirements of high-quality special steel inclusions and difficult effective removal of small-size inclusions, effectively controls the contents of oxygen and sulfur in the steel, realizes stable and effective nitrogen increase and hierarchical efficient nitrogen removal in the steel, can efficiently remove the small-size inclusions in the steel, and can also meet the requirements of low-cost stable continuous production.

The specific technical scheme of the invention is as follows:

a method for reducing the number of small-size nonmetallic inclusions in steel comprises the following steps:

1) deoxidizing the molten steel in the tapping process;

2) carrying out LF refining on the tapped molten steel;

3) denitrification is carried out in the RH process, low vacuum treatment is firstly carried out, and the lifting gas is nitrogen; then high vacuum treatment is carried out, and the lifting gas is argon.

Further, in the step 1), the molten steel is deoxidized in the tapping process, so that the content of [ O ] in the molten steel entering the LF furnace is less than or equal to 30ppm, and the content of [ S ] in the steel is required to be less than or equal to 0.030%.

In the step 2), in the LF refining process, controlling the oxygen and sulfur contents in steel and carrying out nitrogen increasing treatment, specifically comprising the following steps: before the top slag is converted into the white slag, argon is used as bottom blowing gas, and preferably, the flow rate of the bottom blowing gas is 150-450 NL/min; when the top slag is converted into white slag, the bottom blowing gas is argon, preferably, the flow rate of the bottom blowing gas is 250-550NL/min, and is kept for 4-8min, the holding time is long when the S content of the molten steel entering the station is higher, and the holding time is short when the S content is lower; then the bottom blowing gas is switched to nitrogen, preferably, the flow rate is adjusted to 150-; and 5-8min before the LF furnace is out of the station, and switching bottom blowing gas into argon. The process comprehensively considers the three factors of desulfurization, deoxidation (secondary oxidation prevention) and nitrogen increase. When LF is out of the station, the [ O ] in the molten steel is less than 15ppm, the [ S ] is less than 20ppm, the nitrogen content in the molten steel is stabilized at 250ppm of 200-.

Further, in the step 2), if the nitrogen content in the steel does not reach the target range, nitrogen content adjustment can be carried out by using a nitrogen-containing alloy or feeding a nitrogen-containing wire.

In the step 3), a hierarchical two-stage degassing is adopted, wherein a low vacuum shallow treatment stage is adopted: the lifting gas of the dipping pipe is nitrogen, preferably, the vacuum degree of the vacuum tank is controlled at 200-; then, a high vacuum deep treatment stage: the lifting gas of the dipping pipe is switched into argon, preferably, the vacuum degree is less than 100 Pa, the flow rate of the lifting gas is 800-.

The purpose of the graded step-by-step treatment degassing is two points: a) n, H, O gas elements in molten steel are prevented from being removed simultaneously in the early stage of vacuum, H and O are mainly removed in the shallow treatment stage, N is mainly removed in the deep treatment stage, and splashing is prevented; b) the staged degassing operation prolongs the floating time of bubbles in the molten steel in the vacuum treatment process, and strengthens the effect of removing impurities by utilizing the micro bubbles precipitated in the molten steel.

After the treatment by the method, the removal rate of the inclusions with the size of 20 mu m or more reaches more than 99 percent, and the removal rate of the inclusions with the size of less than 20 mu m reaches more than 85 percent.

The invention utilizes the principle of separating out tiny bubbles in the molten steel to remove inclusions, fully considers the thermodynamics and dynamics conditions of nitrogen increase and nitrogen separation, and aims at the problems of high requirements of high-quality special steel inclusions and difficult effective removal of small-size inclusions.

The invention is used for obviously reducing the number of small-size inclusions in steel, improving the quality of products and simultaneously meeting the requirements of low-cost continuous and stable production in the production process of high-quality special steel with high requirements on inclusions, including bearing steel, spring steel, cord steel and the like.

Compared with the prior art, the method fully considers the influence of factors such as oxygen and sulfur content, bottom blowing gas flow and time and the like on the nitrogen increasing thermodynamic and kinetic conditions of the molten steel in the LF refining process, improves the influence of factors such as gas types, flow, time and vacuum degree in the RH vacuum treatment process on the nitrogen removing thermodynamic and kinetic conditions of the molten steel, combines the requirements of efficient continuous stable production on process design, comprehensively considers cost and quality, and designs the LF and RH process aiming at the problems of high requirements of high-quality special steel inclusions and difficulty in effectively removing small-size inclusions, so that the oxygen and sulfur content in the steel is stably and effectively controlled, stable molten steel, accurate nitrogen increasing and continuous effective fractional nitrogen removal are realized, and the effect of adsorbing and removing the inclusions by separating out small dispersed nitrogen bubbles is fully exerted. The method can stably reduce the quantity of inclusions in the molten steel by more than 90 percent before and after an RH process, wherein the removal rate of the inclusions with the size of 20 mu m and more than 99 percent and the removal rate of the inclusions with the size less than 20 mu m reach more than 85 percent, thereby obviously improving the cleanliness of the molten steel and simultaneously realizing low-cost stable continuous production.

Detailed Description

The specific embodiment of the invention is as follows:

example 1

The invention is used in the process of producing a group of 6-furnace bearing steel in the process of electric furnace-LF refining-RH vacuum treatment-continuous casting in a certain factory, adopts the operation process of LF furnace procedure nitrogen increasing, controlling the content of oxygen and sulfur in steel and RH procedure nitrogen removal, and specifically comprises the following steps:

1) deoxidizing the molten steel in the tapping process: deoxidizing the molten steel in the tapping process to ensure that the content of [ O ] in the molten steel entering the LF furnace is less than or equal to 30ppm and the content of [ S ] in the steel is required to be less than or equal to 0.030%.

2) And (3) refining the tapped molten steel through LF: the flow rate of bottom blowing gas is 150-450NL/min before the top slag is converted into white slag, and the bottom blowing gas is argon; when the top slag is converted into white slag, the flow rate of bottom blowing gas is 250-550NL/min, the flow rate of bottom blowing gas is argon, and the flow rate is kept for 4-8 min; the holding time is long when the S content of molten steel entering the station is higher, and the holding time is short when the S content is lower; then, the bottom blowing gas is switched to nitrogen, the flow rate is adjusted to 150-450NL/min, and the holding time is 15-20 min; and 5-8min before the LF furnace is out of the station, and switching bottom blowing gas into argon. When LF is out of the station, the [ O ] in the molten steel is less than 15ppm, the [ S ] is less than 20ppm, the nitrogen content in the molten steel is stabilized at 250ppm of 200-.

3) RH vacuum degassing treatment: performing denitrification in an RH process, namely performing two-stage degassing in a grading manner, wherein a low-vacuum shallow treatment stage is adopted, the vacuum degree of a vacuum tank is controlled to be 500 Pa, the lifting gas of a dipping pipe is nitrogen, the lifting gas flow is 300 NL/min, the stage mainly takes dehydrogenation and deoxidation as the main step, and the treatment time is 8-12 min; then, a high vacuum deep treatment stage: the vacuum degree is less than 100 Pa, the lifting gas of the dipping pipe is switched to argon, the lifting gas flow is 800-1000NL/min, and the treatment time is 15-20 min.

Specific process control parameters are shown in the following table, wherein the LF procedure is shown in the table 1, and the RH procedure is shown in the table 2. During the production process, the removal conditions of inclusions with different sizes in molten steel before and after RH are inspected, compared and analyzed, and the comparison result is shown in Table 3.

TABLE 1 LF refining procedure implementation

In the tables No. 5 and No. 6, which are comparative, argon gas was blown into the molten steel throughout the entire process.

The flow units in the surface are NL/min, and the mass fractions of TFeO and MnO in the top slag of the 6-furnace molten steel when the LF is discharged from the station are all less than or equal to 0.8 percent.

TABLE 2 RH refining procedure implementation

The flow units in the table are NL/min.

TABLE 3 foreign matter removal before and after RH refining

Serial number	Removing ratio of inclusions larger than or equal to 20 mu m	Impurity removal ratio of < 20 [ mu ] m
			1	100％	93％
2	100％	88％
			3	100％	85％
4	99％	90％
			5	35％	30％
6	40％	35％

The method can be obtained from the process control condition of the embodiment and the conditions of impurity removal before and after the RH refining process, the process control and impurity removal effects of the invention are good, and the molten steel cleanliness and product quality improvement effects are obvious.

Claims

1. A method for reducing the number of small-sized non-metallic inclusions in steel, characterized in that the method comprises the steps of:

1) deoxidizing the molten steel in the tapping process;

2) carrying out LF refining on the tapped molten steel;

2. The method as claimed in claim 1, wherein in the step 1), the molten steel is deoxidized during tapping so that the [ O ] content in the molten steel entering the LF furnace is less than or equal to 30ppm and the [ S ] content in the steel is required to be less than or equal to 0.030%.

3. The method according to claim 1 or 2, wherein in the step 2), the bottom blowing gas is argon before the top slag is converted into white slag; when the top slag is changed into white slag, the bottom blowing gas is argon; and then, switching the bottom blowing gas to nitrogen, and switching the bottom blowing gas to argon 5-8min before the LF is discharged.

4. The method as claimed in claim 1 or 3, characterized in that after the treatment of step 2), the quality fraction of TFeO + MnO in the top slag is less than or equal to 0.8% when the LF is outbound and the [ O ] in the molten steel is less than 15ppm and the [ S ] is less than 20ppm and the nitrogen content in the molten steel is stabilized at 250ppm and 200-.

5. The method as claimed in claim 3, wherein the bottom-blown gas flow rate is 150-450NL/min before the top slag is converted into white slag.

6. The method as claimed in claim 3, wherein the flow rate of the bottom-blown gas is 550NL/min and is maintained for 4-8min when the top slag is converted into the white slag.

7. The method as claimed in claim 3, wherein the subsequent bottom-blown gas is switched to nitrogen, the flow rate is adjusted to 150-450NL/min, and the holding time is 15-20 min.

8. The method as claimed in claim 1 or 3, wherein the low vacuum treatment in step 3), the vacuum degree of the vacuum tank is controlled at 200-.

9. The method as claimed in claim 1 or 8, wherein the high vacuum treatment in step 3) is performed under a vacuum degree of less than 100 Pa, a lift gas flow rate of 800-1000NL/min, and a treatment time of 15min or more.

10. The method according to claim 1 or 3, wherein after the treatment of step 3), the removal rate of inclusions with a size of 20 μm or more is 99% or more, and the removal rate of inclusions with a size of less than 20 μm is 85% or more.