Production method for improving NDT flaw detection qualified rate of steel rail

Abstract

The invention discloses a production method for improving the NDT flaw detection qualification rate of a steel rail. The production method comprises the steps of converter primary smelting, LF refining, RH vacuum refining and continuous casting. In the RH vacuum refining step, after the RH high vacuum degree reaches 25 pa and below, the high vacuum time is kept for 7-9 min; in the continuous casting step, semi-ladle casting is adopted; the continuous casting submersed nozzle is divided into two halves, one half of the continuous casting submersed nozzle is adopted for casting in the first half period of casting, and the other half of the continuous casting submersed nozzle is adopted for casting in the second half period; the continuous casting submersed nozzle is continuously vibrated in the whole casting process. The method can achieve the purpose of improving the NDT flaw detection qualification rate of the steel rail, can be applied to the steelmaking process on a large scale, is easy and convenient to operate and stable in effect, improves the internal quality of the steel rail and meets the high-standard requirements of railways; the steel rail NDT flaw detection alarm problem caused by large inclusions brought by process control can be effectively reduced, and the product quality and the social and economic benefits are improved.

Description

Technical Field

[0001] The invention relates to the technical field of iron and steel metallurgy, and in particular to a production method for improving the qualified rate of NDT flaw detection of rails. Background Art

[0002] In recent years, high-speed rail is gradually going global as a beautiful business card of the country. At the same time, high-speed rail also provides great convenience for people's travel. But behind the high-speed and stable operation of high-speed rail, there are stringent requirements for the quality of rails, which are mainly reflected in the appearance surface quality, dimensional accuracy and internal quality. Among them, the surface quality and dimensional accuracy are obtained by the high-precision rolling process; the internal quality of the rails is guaranteed by the high-cleanliness steelmaking process. In order to ensure that each rail meets the safety standard requirements for online service, after the rail ingot is rolled into finished products, nondestructive testing (NDT) is used to check whether there are defects inside each rail. Those that meet the quality requirements can be released. However, in the actual production process, affected by factors such as raw materials, refractory materials or process changes, rail NDT flaw detection alarm problems often occur, which not only brings great quality losses, but also threatens the quality and safety of rails in online service. Therefore, how to systematically analyze the factors affecting NDT flaw detection alarms in the smelting process, formulate corresponding process improvement measures, and improve the NDT flaw detection pass rate is a technical problem that the rail industry needs to solve urgently.

[0003] In the prior art, patent CN202210304562.X provides a method for distinguishing the effects of steelmaking and rolling on the ultrasonic flaw detection results of rails. This method mainly identifies the NDT flaw detection alarms caused by different factors in steelmaking and rolling. Patent application CN202410254565.6 provides a method for controlling magnesium-aluminum spinel inclusions in heavy rail steel production. This method mainly reduces the amount of magnesium-aluminum spinel in steel by strictly controlling the process parameters before continuous casting. The above two methods are both aimed at improving the cleanliness of rails, and are achieved by optimizing or strengthening certain process parameters, but neither has achieved a substantial breakthrough. Overall, the above two methods are not the best choice. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a production method for improving the qualified rate of NDT flaw detection of rails.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: comprising the steps of converter primary refining, LF refining, RH vacuum refining and continuous casting; The RH vacuum refining step: after the RH high vacuum degree reaches 25 Pa or less, the high vacuum time is maintained for 7 to 9 minutes; The continuous casting steps include: half-bag casting; the continuous casting immersion nozzle is divided into two halves, one half of the nozzle is used for casting in the first half of the casting, and the other half of the nozzle is used for casting in the second half of the casting; the continuous casting immersion nozzle is vibrated uninterruptedly during the entire casting process.

[0006] Furthermore, in the continuous casting step, the half-ladle casting is cast to 40% to 60% of the weight of the large ladle.

[0007] Furthermore, in the continuous casting step, the remaining molten steel from the half-ladle casting is returned to the LF refining step for recovery.

[0008] Furthermore, in the continuous casting step, the vibration frequency is 2.5HZ to 3.0HZ.

[0009] The beneficial effect of adopting the above technical solution is that compared with the optimization of certain parameters in traditional processes, the present invention takes a unique approach and creatively reduces the source of large and medium-sized inclusions in steel through technical innovations such as continuous casting half-bag casting, RH high vacuum holding time, and increasing vibration of continuous casting nozzles. The present invention can achieve the purpose of improving the qualified rate of NDT flaw detection of rails, can be applied on a large scale in the steelmaking process, is easy to operate, has stable effects, improves the internal quality of rails, and meets the high standards of railways. Compared with traditional steelmaking processes, the smelting of the present invention can effectively reduce the problem of NDT flaw detection alarms of rails caused by large inclusions caused by process control, and improve product quality and social and economic benefits. DETAILED DESCRIPTION

[0010] The present invention is further described in detail below in conjunction with specific implementation modes.

[0011] The rail smelting process is relatively long. From the pretreatment of molten iron to the primary refining in the converter, LF refining, RH vacuum refining, and finally to the continuous casting of large square billets, the impact on NDT flaw detection is mainly concentrated in the follow-up after RH refining. After a certain period of time under RH high vacuum conditions, there is basically no large slag in the molten steel. However, during the continuous casting process, especially in the late stage of casting of large ladles of molten steel, the risk of slag in large ladles increases sharply. In the actual production process, the project team found through tracking that not only is there a risk of slag in the late stage of the ladle, but the probability of slag begins to appear after the middle stage of casting. Therefore, how to reduce or avoid slag in large ladles is one of the important links in solving NDT flaw detection alarms. On the other hand, during the continuous casting process, due to the decrease in temperature, endogenous inclusions precipitate and react with the nozzle and refractory to form magnesium-aluminum composite inclusions, which will gather and grow on the inner wall of the immersion nozzle. When they grow to a certain size, they will be flushed into the molten steel and cause the rail NDT flaw detection alarm. The above technical difficulties in the industry cannot effectively solve the problem of rail NDT flaw detection alarm under traditional process conditions, and great constraints have been encountered when the NDT flaw detection pass rate reaches a certain level and is further improved.

[0012] In order to further improve the internal cleanliness of the rails, the project team of the present invention conducted a large number of experimental studies and creative analysis, and found that the improvement of the qualified rate of NDT flaw detection of rails can be carried out from the following three aspects: first, controlling the casting volume of the large ladle to prevent large inclusions brought by the slag from entering the middle ladle from the source; second, maintaining the extremely high vacuum degree of RH for a certain period of time to minimize the large inclusions in the steel; third, by improving the continuous casting immersion nozzle, the growth and aggregation of magnesium-aluminum composite inclusions precipitated during the casting of molten steel in the immersion nozzle can be reduced.

[0013] In order to improve the qualified rate of rail NDT flaw detection, this production method for improving the qualified rate of rail NDT flaw detection combines a large amount of production test data for process innovation. The methods and processes adopted are as follows: 1) Continuous casting: Continuous casting adopts half-bag casting, that is, when the weight of the large bag is 40% to 60%, the casting is stopped and replaced with the next bag; the remaining molten steel is returned to the LF refining step for heat recovery, and after re-slag making, it enters the next process according to the normal process; In order to realize the return of the remaining molten steel from the half-ladle casting to the LF refining step for heat recovery, the converter primary refining step is divided into two ladles for steel tapping, and the molten steel in each ladle accounts for half of the total steel tapping amount; the remaining space in the two ladles is used to receive the molten steel that has not been cast in the continuous casting step, so that the remaining molten steel from the half-ladle casting can be returned to the LF refining step for heat recovery.

[0014] 2) RH vacuum refining: After the RH high vacuum degree reaches 25Pa or below from atmospheric pressure, the high vacuum time is maintained for 7 to 9 minutes.

[0015] 3) Continuous casting: The continuous casting immersion nozzle is divided into two halves based on the original nozzle. One half of the nozzle is used for casting in the first half of the pouring, and the other half of the nozzle is used for casting in the second half of the pouring.

[0016] 4) Continuous casting: The continuous casting submerged nozzle is connected to one or more vibration sources, and uninterrupted vibration is used in the whole casting process with a vibration frequency of 2.5HZ~3.0HZ. At the same time, it is ensured that the molten steel surface in the crystallizer is in a creeping state and the molten steel surface cannot be exposed.

[0017] 5) The production organization aims to ensure smooth continuous casting, control the rhythm of heat recovery of the remaining molten steel, and match the continuous casting production after the converter taps out the steel.

[0018] 6) This method can increase the NDT flaw detection pass rate by at least 0.3%, and is particularly suitable for R260, U71Mn, U75V, 900A, R350LHT, and R350HT rail steels. Example 1

[0019] To smelt R260 rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 16 hours, and the nominal capacity of the large ladle is 120 tons. In actual production, after the converter taps out steel, two ladles are used, each with a tapping capacity of 60 tons. The RH high vacuum degree is 20-25pa, and the high vacuum time is 7-8min. During continuous casting, 8 hours before the casting, when the large ladle molten steel is 48 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 8 hours of casting, and the other half is used for casting in the remaining 8 hours. After the casting of this casting, the NDT flaw detection pass rate was 99.8%, which was 0.7% higher than 99.1% before the process improvement. Example 2

[0020] To smelt U71Mn rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 22 hours, and the nominal capacity of the large ladle is 150 tons. In actual production, after the converter taps out steel, two ladles are put in, and the output of each ladle is 75 tons. The RH high vacuum degree is 20-25pa, and the high vacuum time is 7-9min. During continuous casting, 11 hours before the casting, when the large ladle molten steel is cast 60 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 11 hours of casting, and the other half is used for casting in the remaining 11 hours. After the casting of the billet, the NDT flaw detection pass rate was 99.9%, which was 0.8% higher than 99.1% before the process improvement. Example 3

[0021] To smelt U75V rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 18 hours, and the nominal capacity of the large ladle is 120 tons. In actual production, after the converter taps out steel, two ladles are used, each with a tapping capacity of 60 tons. The RH high vacuum degree is 20-25pa, and the high vacuum time is 8-9 minutes. During continuous casting, 9 hours before the casting, when the large ladle of molten steel is cast at 60 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 9 hours of casting, and the other half is used for casting in the remaining 8 hours. After the casting of this casting, the NDT flaw detection pass rate was 99.6%, which was 0.5% higher than 99.1% before the process improvement. Example 4

[0022] To smelt R260 rail steel, continuous casting uses 5-machine 5-stream large square billet casting, the middle ladle casting time is 18 hours, and the large ladle nominal capacity is 100 tons. In actual production, after the converter taps out steel, two ladles are put into the ladle, each with a tapping capacity of 50 tons. The RH high vacuum degree is 20-24pa, and the high vacuum time is 7.5-8.5min. During continuous casting, 9 hours before the casting, when the large ladle molten steel is cast to 40 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 9 hours of casting, and the other half is used for casting in the remaining 9 hours. After the casting of the billet, the NDT flaw detection pass rate was 99.6%, which was 0.5% higher than 99.1% before the process improvement. Example 5

[0023] To smelt 900A rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 12 hours, and the nominal capacity of the large ladle is 120 tons. In actual production, after the converter taps out steel, two ladles are put into the billet, and the output of each ladle is 60 tons. The RH high vacuum degree is 20-22pa, and the high vacuum time is 7-6min. During continuous casting, 6 hours before the casting, when the large ladle molten steel is cast 72 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 6 hours of casting, and the other half is used for casting in the remaining 6 hours. After the billet is rolled, the NDT flaw detection pass rate is 99.5%, which is 0.4% higher than 99.1% before the process improvement. Example 6

[0024] To smelt R260 rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 20 hours, and the nominal capacity of the large ladle is 120 tons. In actual production, after the converter taps out steel, two ladles are used, each with a tapping capacity of 60 tons. The RH high vacuum degree is 22-25pa, and the high vacuum time is 8-9min. During continuous casting, 10 hours before the casting, when the large ladle of molten steel is cast at 60 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 10 hours of casting, and the other half is used for casting in the remaining 10 hours. After the casting of this casting, the NDT flaw detection pass rate was 99.5%, which was 0.4% higher than 99.1% before the process improvement. Example 7

[0025] To smelt R350LHT rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 14 hours, and the nominal capacity of the large ladle is 100 tons. In actual production, after the converter taps out steel, two ladles are put into the billet, and the output of each ladle is 50 tons. The RH high vacuum degree is 24-25pa, and the high vacuum time is 7-9 minutes. During continuous casting, 7 hours before the casting, when the large ladle molten steel is cast 60 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 7 hours of casting, and the other half is used for casting in the remaining 7 hours. After the billet is rolled, the NDT flaw detection pass rate is 99.4%, which is 0.3% higher than 99.1% before the process improvement. Example 8

[0026] To smelt R350HT rail steel, continuous casting uses 5-machine 5-stream billet casting, the middle ladle casting time is 18 hours, and the nominal capacity of the large ladle is 150 tons. In actual production, after the converter taps out steel, two ladles are put in, and the output of each ladle is 75 tons. The RH high vacuum degree is 24-25pa, and the high vacuum time is 8-9min. During continuous casting, 9 hours before the casting, when the large ladle molten steel is cast 90 tons, the next ladle is replaced for casting, and the remaining molten steel is returned to LF refining for heat recovery; the immersion nozzle adopts vibration mode throughout the process, and half of the nozzle is used for casting in the first 9 hours of casting, and the other half is used for casting in the remaining 9 hours. After the casting of the billet, the NDT flaw detection pass rate was 100%, which was 0.9% higher than 99.1% before the process improvement.

Claims

1. A production method for improving the qualified rate of rail NDT flaw detection, characterized in that: It includes converter primary refining, LF refining, RH vacuum refining and continuous casting steps; The RH vacuum refining step: after the RH high vacuum degree reaches 25 Pa or less, the high vacuum time is maintained for 7 to 9 minutes; The continuous casting steps include: half-bag casting; the continuous casting immersion nozzle is divided into two halves, one half of the nozzle is used for casting in the first half of the casting, and the other half of the nozzle is used for casting in the second half of the casting; the continuous casting immersion nozzle is vibrated uninterruptedly during the entire casting process.

2. A production method for improving the qualified rate of rail NDT flaw detection according to claim 1, characterized in that: In the continuous casting step, half-ladle casting is performed to cast to 40% to 60% of the weight of the large ladle.

3. A production method for improving the qualified rate of rail NDT flaw detection according to claim 1, characterized in that: In the continuous casting step, the remaining molten steel from the half-ladle casting is returned to the LF refining step for recovery.

4. A production method for improving the qualified rate of rail NDT flaw detection according to claim 1, 2 or 3, characterized in that: In the continuous casting step, the vibration frequency is 2.5HZ to 3.0HZ.