A production method for improving segregation of continuous casting billet of sulfur-containing steel

Abstract

The present application relates to a kind of production methods for improving the segregation of continuous casting billet of sulfur-containing steel grade, comprising step one, sampling in the columnar crystal region of the billet of steel grade, obtaining hot plasticity curve by high temperature plasticity detection;Step two, according to the hot plasticity curve obtained in step one, analogous to a steel grade without sulfur and using light press-down process to cast billet;Step three, temperature measurement is carried out, including entering the temperature of straightening machine and straightening zone temperature;Step four, finally, the total amount of light press-down and the distribution of each press-down roller of the light press-down of continuous casting are set according to the sulfur content of steel grade, the total amount of light press-down is finally designed to be relatively reduced to 90% or less of the total amount of light press-down in the continuous casting stage of the analogous steel grade without sulfur, and the higher the sulfur content, the smaller the total amount of light press-down is set, and the distribution of press-down roller from front to back follows the principle of small-large-small.The purpose of the method is to improve the segregation of continuous casting billet, while avoiding light press-down crack.

Description

A production method for improving segregation in continuously cast billets of sulfur-containing steel grades Technical Field

[0001] This invention relates to a production method for improving segregation in continuously cast billets of sulfur-containing steel grades. It belongs to the technical field of light reduction in continuous casting of iron and steel metallurgy. Background Technology

[0002] With the development of special steel technology, free-cutting steels are becoming increasingly common, and adding sulfur is one of the most economical methods. However, adding sulfur reduces the plasticity of the steel, making it impossible to directly use the light reduction technique to improve segregation, just like with high-carbon steels. Therefore, segregation in sulfur-containing steels has always been a challenge in the industry. Many companies, in order to prevent light reduction cracks, choose to abandon the light reduction technique and use other methods such as electromagnetic stirring to improve it, but the effects are not significant, resulting in a persistent problem in improving the segregation quality of sulfur-containing steel billets. Our company, like other companies in the industry, previously did not use the light reduction technique when producing sulfur-containing steels. However, some products could not meet the needs of high-end customers, therefore, the segregation of sulfur-containing steel billets needed to be improved. Summary of the Invention

[0003] In the past, some attempts have been made to use light reduction technology in the continuous casting straightening process of sulfur-containing steel grades. However, insufficient reduction or inappropriate distribution of the reduction amount resulted in insignificant improvement in segregation and even serious light reduction cracks, ultimately leading to failure. Through systematic research, a production method for improving segregation in continuously cast billets of sulfur-containing steel grades has been successfully designed, which not only improves the segregation of the billets but also avoids light reduction cracks.

[0004] The technical solution adopted by this invention to solve the above problems is: a production method for improving segregation in continuously cast billets of sulfur-containing steel, comprising:

[0005] Step 1: Take samples from the columnar crystal region of the steel billet and obtain the thermoplasticity curve of the sample within the test temperature range using the high-temperature plasticity test of the steel billet sample.

[0006] Step 2: Based on the thermoplasticity curve obtained in Step 1, draw an analogy with a steel grade that does not contain sulfur and is cast using a light-pressure process.

[0007] Step 3: Conduct on-site temperature measurements, including the temperature entering the straightening machine and the temperature in the straightening zone, and set the temperature range for continuous casting under light pressure.

[0008] Step 4: Finally, based on the sulfur content of the steel grade, set the total reduction amount of the continuous casting light reduction and the distribution of the reduction amount of each reduction roller. The final design of the total reduction amount of the light reduction should be reduced to less than 90% of the total reduction amount of the continuous casting stage of the sulfur-free steel grade. The higher the sulfur content, the smaller the total reduction amount should be. The distribution of the reduction amount of the reduction rollers from front to back follows the principle of small-large-small.

[0009] Preferably, in step one, the billet sample is taken from the columnar crystal region. The sample used in this application is a billet sample. Although the thermoplasticity results obtained using rolled steel are better, considering that rolling improves the plasticity of the steel grade, the plasticity of rolled steel samples will be inconsistent with that of billet samples. Using the thermoplasticity curve of rolled steel to study the light reduction process of billets would be misleading. The billet sample is preferably taken from the columnar crystal region, where defects are relatively fewer and will not interfere with the test results.

[0010] In step one, the billet samples are subjected to high-temperature plasticity testing on a thermal simulation testing machine. The reduction of area of ​​the steel is measured at 50°C intervals within the temperature range of 650°C-1300°C, and a thermoplastic curve of the reduction of area as a function of temperature is plotted.

[0011] In step two, based on the thermoplasticity curve of the sample, sulfur-free steel grades whose thermoplasticity curve trend is infinitely close to or consistent with the thermoplasticity curve of the sample are found from the thermoplasticity curve database. The thermoplasticity law refers to the first brittle temperature range, the second brittle temperature range, the third brittle temperature range, and the reduction of area within the brittle temperature range.

[0012] In step two, the sulfur content of the sulfur-free steel grade being compared is ≤0.005wt%.

[0013] In step two, the sulfur-free steel and the sulfur-containing steel have temperature ranges in which their thermoplasticity is infinitely close or consistent within the first brittle temperature range, the second brittle temperature range, and the third brittle temperature range, respectively.

[0014] In step four, the total reduction of the sulfur-free steel grade being compared with that of the sulfur-containing steel grade is multiplied by 70% as the total reduction of the light reduction process in the continuous casting stage. Furthermore, let 'a' be the total reduction of the sulfur-free steel grade using the light reduction process, and 'b' be the total reduction of the sulfur-containing steel grade, where b = k * a. According to statistics: when the sulfur content of the steel grade is within 0.02 wt%, k is 0.9; when the sulfur content is in the range of (0.02 wt%, 0.05 wt%), k is 0.7; when the sulfur content is in the range of (0.05 wt%, 0.08 wt%), k is 0.5; when the sulfur content is in the range of (0.08 wt%, 0.1 wt%), k is 0.3; when the sulfur content exceeds 0.1%, k ≤ 0.1, and the light reduction process is not recommended in this case.

[0015] Compared with the prior art, the advantages of this invention are as follows: For sulfur-containing steel grades, a production method to improve the segregation of continuously cast billets of sulfur-containing steel grades has been successfully designed. Based on sampling the columnar crystal region of the billet of sulfur-containing steel grades, a thermoplasticity curve is obtained by testing. By analogy with the thermoplasticity curve of sulfur-free steel grades, the total reduction of the continuous casting stage of sulfur-containing steel grades is designed based on the reduction of the continuous casting of sulfur-free steel grades. The reduction of each roll is then redistributed based on the total reduction. The production method of sulfur-containing steel billets determined in this way not only improves the segregation of the continuously cast billets, but also avoids the generation of slight reduction cracks. Detailed Implementation

[0016] Taking a steel grade with a sulfur content of 0.04% as an example, the method for improving segregation in continuously cast billets in this application will be explained.

[0017] (1) Selected steel billet samples were used for thermoplasticity testing using a thermal simulation testing machine. Although the thermoplasticity results obtained by using rolled steel are better, they are inconsistent with the plasticity of the billet samples. Rolling can improve the plasticity of the steel grade, which can mislead the subsequent light reduction process design. The billet samples are taken from the columnar crystal region of the billet, where there are relatively few defects, which will not interfere with the test results. High-temperature plasticity testing refers to testing at 50°C intervals within the temperature range of 650°C-1300°C.

[0018] (2) By analogy with sulfur-free steel grades, a thermoplasticity curve is generated based on the thermoplasticity test results. A steel grade whose thermoplasticity trend is consistent with that of sulfur-containing steel grades is then found from the thermoplasticity curve database. The plasticity of this sulfur-free steel grade in the first, second, and third brittle temperature ranges is consistent with that of the sulfur-containing steel grade. The analogous steel grade should also utilize a mature light reduction process to improve segregation, thus providing a reference for the design of light reduction parameters for sulfur-containing steel grades. To avoid cracking, 70% of the total reduction of sulfur-free steel grades using a mature light reduction process can be used as the total reduction of the sulfur-containing steel grade.

[0019] (3) Conduct on-site temperature measurement, mainly the temperature of the tension straightener and the straightening temperature, in order to match the thermoplasticity and prevent the formation of light pressure cracks.

[0020] (4) Finally, the total reduction and the distribution of the reduction amount of each roll are set according to the sulfur content of the steel grade. The total reduction amount should be relatively reduced for sulfur-containing steel grades, and the higher the sulfur content, the smaller the total reduction amount. The reduction amount distribution of the rolls from front to back follows the principle of small-large-small.

[0021] Implementation Results: Following the above steps, a light reduction process was designed for a certain sulfur-containing steel grade (sulfur content 0.04%). The final total reduction was determined to be 10mm, with the reduction amounts of each light reduction roller distributed as 1mm-2mm-4mm-2mm-1mm. Experiments were then conducted, and segregation detection was performed on selected billet samples. The results were compared with those obtained without the light reduction process. The results are as follows:

[0022]

[0023] The light reduction process for sulfur-containing steel not only improves segregation but also effectively reduces central porosity during the reduction process.

[0024] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A production method for improving segregation in continuously cast billets of sulfur-containing steel grades, characterized in that: The process includes: Step 1: Taking billet samples from columnar crystal regions and conducting high-temperature plasticity tests on a thermal simulation testing machine. The reduction of area of ​​the steel is measured at 50°C intervals within a temperature range of 650°C-1300°C to obtain the thermoplasticity curve of the sample within the test temperature range. Step 2: Based on the thermoplasticity law reflected in the thermoplasticity curve obtained in Step 1, an analogy is made to a sulfur-free steel grade that uses a light-reduction casting process. Specifically, based on the thermoplasticity curve of the sample, a sulfur-free steel grade whose thermoplasticity curve trend is infinitely close to or consistent with the thermoplasticity curve of the sample is found from a thermoplasticity curve database. The thermoplasticity law refers to... The first brittle temperature range, the second brittle temperature range, the third brittle temperature range, and the reduction of area within the respective brittle temperature range; Step 3: Conduct on-site temperature measurements, including the temperature upon entering the straightening machine and the temperature in the straightening zone, and set the temperature range for implementing light reduction in continuous casting; Step 4: Finally, based on the sulfur content of the steel grade, set the total reduction amount of light reduction in continuous casting and the distribution of the reduction amount of each reduction roller. The final design of the total reduction amount of light reduction should be relatively reduced to less than 90% of the total reduction amount in the continuous casting stage of the analogous sulfur-free steel grade, and the higher the sulfur content, the smaller the total reduction amount should be. The distribution of the reduction amount of the reduction rollers from front to back follows the principle of small-large-small.

2. The production method for improving segregation in continuously cast billets of sulfur-containing steel according to claim 1, characterized in that: In step two, the sulfur content of the sulfur-free steel grade being compared is ≤0.005wt%.

3. The production method for improving segregation in continuously cast billets of sulfur-containing steel according to claim 1, characterized in that: In step two, the sulfur-free steel and the sulfur-containing steel, respectively, have temperature ranges in which their thermoplasticity is infinitely close or consistent within the first brittle temperature range, the second brittle temperature range, and the third brittle temperature range.

4. The production method for improving segregation in continuously cast billets of sulfur-containing steel according to claim 1, characterized in that: The total reduction of sulfur-containing steel grades during the continuous casting stage is calculated as 70% of the total reduction of the sulfur-free steel grades.

5. The production method for improving segregation in continuously cast billets of sulfur-containing steel according to claim 1, characterized in that: In step four, the total reduction amount for sulfur-free steel using the light reduction process is set as 'a', and the total reduction amount for sulfur-containing steel is set as 'b', where b = k * a. According to statistics: when the sulfur content of the steel is within 0.02 wt%, k is 0.9; when the sulfur content is in the range of (0.02 wt%, 0.05 wt%), k is 0.7; when the sulfur content is in the range of (0.05 wt%, 0.08 wt%), k is 0.5; when the sulfur content is in the range of (0.08 wt%, 0.1 wt%), k is 0.3; when the sulfur content exceeds 0.1%, k ≤ 0.1, and the light reduction process is not used.

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