A production method for improving the qualified rate of rail NDT flaw detection

Abstract

The application discloses a production method for improving the qualified rate of rail NDT flaw detection, which comprises the steps of converter initial smelting, LF refining, RH vacuum refining and continuous casting pouring; the RH vacuum refining step: after the high vacuum degree of the RH reaches 25 Pa or below, the high vacuum time is kept for 7-9 min; the continuous casting pouring step: semi-enclosed pouring is adopted; the continuous casting submerged entry nozzle is divided into two halves, one half of the nozzle is used for pouring in the first half of the pouring period, and the other half of the nozzle is used for pouring in the second half of the pouring period; the submerged entry nozzle is vibrated uninterruptedly in the whole pouring process. The method can realize the purpose of improving the qualified rate of rail NDT flaw detection, can be applied to the steelmaking process in a large scale, is simple and convenient to operate, stable in effect, improves the internal quality of rails, meets the high standard requirements of railways, effectively reduces the rail NDT flaw detection alarm problem caused by large inclusions in process control, and improves the quality of products and social and economic benefits.

Description

TECHNICAL FIELD

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

[0002] In recent years, high-speed rail is gradually becoming a beautiful card of the country and is gradually moving towards the world. At the same time, high-speed rail also provides great convenience for people's travel. However, behind the high-speed and smooth operation of high-speed rail is the stringent requirement for the quality of steel rails, mainly in terms of appearance surface quality, dimensional accuracy and internal quality. Among them, the surface quality and dimensional accuracy are obtained by high-precision rolling process; the guarantee of the internal quality of the steel rail is guaranteed by high-purity steelmaking process. In order to ensure that each steel rail meets the safety standard requirements in online service, after the steel rail billet is rolled into finished product, nondestructive testing (NDT flaw detection) is performed to detect whether there are defects in each steel rail. The steel rail that meets the quality requirements can be released. However, in the actual production process, due to factors such as raw and auxiliary materials, refractory materials or process changes, the NDT flaw detection alarm of the steel rail occurs from time to time, which not only brings great quality loss, but also threatens the quality and safety of the steel rail in online service. Therefore, how to analyze the influencing factors of NDT flaw detection alarm in the smelting process, develop corresponding process improvement measures and improve the NDT flaw detection qualified rate is a technical problem to be solved in the steel rail industry.

[0003] In the prior art, patent CN202210304562.X provides a method for distinguishing the influence of steelmaking and rolling on the ultrasonic flaw detection results of steel rails. The method mainly identifies the NDT flaw detection alarm caused by different factors of steelmaking and rolling. Patent application CN202410254565.6 provides a method for controlling magnesium aluminum spinel in heavy rail steel production. The method mainly controls the process parameters before continuous casting to reduce the number of magnesium aluminum spinel in the steel. The above two methods are both aimed at improving the cleanliness of steel rails and are realized by optimizing or strengthening certain process parameters, but neither of them is a substantial breakthrough. In summary, the above two methods are not the best choice. SUMMARY

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

[0005] To solve the above technical problems, the technical solution adopted by the present application is: including converter preliminary smelting, LF refining, RH vacuum refining and continuous casting steps.

[0006] The RH vacuum refining step: after the RH high vacuum degree reaches 25pa and below, the high vacuum time is kept for 7-9min;

[0007] The continuous casting pouring step is semi-pouring, and the continuous casting submerged nozzle is divided into two halves, one half of the nozzle is used for pouring in the first half of the pouring period, and the other half of the nozzle is used for pouring in the second half of the pouring period; the continuous casting submerged nozzle is vibrated continuously in the whole pouring process.

[0008] Further, in the continuous casting pouring step, the semi-pouring is pouring to 40% to 60% of the weight of the ladle.

[0009] Further, in the continuous casting pouring step, the remaining molten steel in the semi-pouring is returned to the LF refining step for recycling.

[0010] Further, in the continuous casting pouring step, the vibration frequency is 2.5 HZ to 3.0 HZ.

[0011] The beneficial effects generated by the above technical solution are that, compared with the optimization of some parameters in the traditional process, the application creates a new path and creatively innovates the continuous casting semi-pouring, the RH high vacuum holding time, and the vibration of the continuous casting nozzle, thereby fundamentally reducing the source of large inclusions in the steel. The application can achieve the purpose of improving the NDT flaw detection qualification rate of the steel rail, can be applied on a large scale in the steelmaking process, is easy to operate, has stable effects, improves the internal quality of the steel rail, and meets the high standard requirements of the railway. Compared with the traditional steelmaking process, the application can effectively reduce the problem of NDT flaw detection alarm of the steel rail caused by large inclusions in the process control, and improve the quality of the product and the social and economic benefits. DETAILED DESCRIPTION

[0012] The application will be further described in detail below in combination with specific embodiments.

[0013] The steel rail smelting process is long, from the hot metal pretreatment to the converter initial smelting, the LF refining, the RH vacuum refining, and finally to the whole process of the bloom continuous casting pouring, the main influence on the NDT flaw detection is concentrated in the subsequent after the RH refining. After a certain time under the RH high vacuum condition, there are basically no large inclusions in the molten steel. However, in the continuous casting pouring process, especially in the late stage of the ladle molten steel pouring, the risk of ladle slagging increases sharply. In the actual production process, the project team found through tracking that not only is there a risk of ladle slagging in the late stage of the ladle, but there is also a risk of ladle slagging after the mid-pouring stage. Therefore, how to reduce or avoid ladle slagging is one of the important links to solve the NDT flaw detection alarm. On the other hand, in the continuous casting pouring process, due to the decrease in temperature, the magnesium-aluminum composite inclusions formed after the reaction of the precipitated endogenous inclusions with the nozzle and the refractory material will gather and grow on the inner wall of the submerged nozzle. When the inclusions grow to a certain extent, they will be washed into the molten steel and cause the NDT flaw detection alarm of the steel rail. The above industry technical problems cannot effectively solve the problem of NDT flaw detection alarm of the steel rail under the traditional process condition, and when the NDT flaw detection qualification rate reaches a certain level, it encounters great restrictions in further improvement.

[0014] To solve the problem of further improving the cleanliness of the steel rail, the project team has carried out a large number of experimental research and creative analysis, and found that the qualified rate of rail NDT flaw detection can be improved from the following three aspects: first, control the ladle casting volume, avoid large inclusions from entering the tundish due to ladle slag from the source; second, RH extremely high vacuum degree and keep for a certain time, to the greatest extent reduce the large inclusions in the steel; third, through the improvement of continuous casting submerged entry nozzle, reduce the magnesium-aluminum composite inclusions precipitated in the submerged entry nozzle during casting.

[0015] For the problem of improving the qualified rate of rail NDT flaw detection, the production method for improving the qualified rate of rail NDT flaw detection combines a large number of production test data to carry out process innovation, and the method and process adopted are as follows:

[0016] 1) Continuous casting: semi-batch casting is adopted for continuous casting, that is, when casting to 40% to 60% of the ladle weight, stop casting and replace the next ladle for casting; the remaining molten steel is returned to the LF refining step for heat recovery, and after re-slaging, it enters the next process according to the normal process;

[0017] In order to realize the heat recovery of the remaining molten steel returned to the LF refining step for semi-batch casting, the converter preliminary refining step is divided into two ladles when tapping, and the amount of molten steel in each ladle accounts for half of the total tapping amount; the remaining space of the two ladles is used to accept the molten steel that is not cast in the continuous casting step, so that the remaining molten steel of semi-batch casting can be returned to the LF refining step for heat recovery.

[0018] 2) RH vacuum refining: the high vacuum degree of RH is from atmospheric pressure to 25 pa and below, and the high vacuum time is kept for 7 to 9 minutes.

[0019] 3) Continuous casting: the continuous casting submerged entry nozzle is divided into two halves on the basis of the original nozzle, one half of the nozzle is used for casting in the first half of the casting period, and the other half of the nozzle is used for casting in the second half of the casting period.

[0020] 4) Continuous casting: one or several vibration sources are connected to the continuous casting submerged entry nozzle, and uninterrupted vibration is adopted in the whole casting process, the vibration frequency is 2.5 HZ to 3.0 HZ, and at the same time, the molten steel liquid level in the mold is kept in peristalsis state, and the molten steel liquid level cannot be exposed.

[0021] 5) The continuous casting sequence is taken as the target in production organization, the heat recovery rhythm of the remaining molten steel is controlled, and the continuous casting production is matched after the converter tapping.

[0022] 6) The method can improve the qualified rate of NDT flaw detection by at least 0.3%, and is especially suitable for R260, U71Mn, U75V, 900A, R350LHT and R350HT steel rails. Example 1

[0023] Smelt the R260 rail steel, continuous casting adopts 5 machine 5 flow big square billet casting, tundish casting time is 16 hours, the public nominal capacity of big ladle is 120 tons, in actual production, after the converter tapping to two ladles, each ladle tapping amount is 60 tons, RH high vacuum degree is 20-25pa, high vacuum time is 7-8min, when casting in continuous casting, 8 hours before pouring, when 48 tons of molten steel in big ladle is cast, the next ladle is cast, the remaining molten steel returns to LF refining for heat recovery; The whole process of submerged entry nozzle adopts vibration mode, 8 hours before casting, half of the nozzle is cast, and the other half is cast for the remaining 8 hours. After the casting billet is rolled, the NDT flaw detection qualified rate is 99.8%, which is improved by 0.7% compared with 99.1% before process improvement. Example 2

[0024] Smelt the U71Mn rail steel, continuous casting adopts 5 machine 5 flow big square billet casting, tundish casting time is 22 hours, the public nominal capacity of big ladle is 150 tons, in actual production, after the converter tapping to two ladles, each ladle tapping amount is 75 tons, RH high vacuum degree is 20-25pa, high vacuum time is 7-9min, when casting in continuous casting, 11 hours before pouring, when 60 tons of molten steel in big ladle is cast, the next ladle is cast, the remaining molten steel returns to LF refining for heat recovery; The whole process of submerged entry nozzle adopts vibration mode, 11 hours before casting, half of the nozzle is cast, and the other half is cast for the remaining 11 hours. After the casting billet is rolled, the NDT flaw detection qualified rate is 99.9%, which is improved by 0.8% compared with 99.1% before process improvement. Example 3

[0025] Smelt the U75V rail steel, continuous casting adopts 5 machine 5 flow big square billet casting, tundish casting time is 18 hours, the public nominal capacity of big ladle is 120 tons, in actual production, after the converter tapping to two ladles, each ladle tapping amount is 60 tons, RH high vacuum degree is 20-25pa, high vacuum time is 8-9min, when casting in continuous casting, 9 hours before pouring, when 60 tons of molten steel in big ladle is cast, the next ladle is cast, the remaining molten steel returns to LF refining for heat recovery; The whole process of submerged entry nozzle adopts vibration mode, 9 hours before casting, half of the nozzle is cast, and the other half is cast for the remaining 8 hours. After the casting billet is rolled, the NDT flaw detection qualified rate is 99.6%, which is improved by 0.5% compared with 99.1% before process improvement. Example 4

[0026] Smelt R260 rail steel, continuous casting adopts 5 machine 5 flow large square billet casting, tundish casting time is 18 hours, open hearth ladle public nominal capacity is 100 tons, in actual production, after converter tapping to two ladles, each ladle tapping quantity is 50 tons, RH high vacuum degree is 20-24pa, high vacuum time is 7.5-8.5min, when casting in continuous casting, at 9 hours before pouring, when casting 40 tons of molten steel in ladle, change another ladle to cast, remaining molten steel returns to LF refining to carry out heat recovery;Immersion nozzle adopts vibration mode all the way, before casting 9 hours, half of the nozzle is used for casting, the other half is used for casting for remaining 9 hours. After the casting billet is rolled, the NDT flaw detection qualified rate is 99.6%, which is improved by 0.5% compared with 99.1% before process improvement. Example 5

[0027] Smelt 900A rail steel, continuous casting adopts 5 machine 5 flow large square billet casting, tundish casting time is 12 hours, open hearth ladle public nominal capacity is 120 tons, in actual production, after converter tapping to two ladles, each ladle tapping quantity is 60 tons, RH high vacuum degree is 20-22pa, high vacuum time is 7-6min, when casting in continuous casting, at 6 hours before pouring, when casting 72 tons of molten steel in ladle, change another ladle to cast, remaining molten steel returns to LF refining to carry out heat recovery;Immersion nozzle adopts vibration mode all the way, before casting 6 hours, half of the nozzle is used for casting, the other half is used for casting for remaining 6 hours. After the casting billet is rolled, the NDT flaw detection qualified rate is 99.5%, which is improved by 0.4% compared with 99.1% before process improvement. Example 6

[0028] Smelt R260 rail steel, continuous casting adopts 5 machine 5 flow large square billet casting, tundish casting time is 20 hours, open hearth ladle public nominal capacity is 120 tons, in actual production, after converter tapping to two ladles, each ladle tapping quantity is 60 tons, RH high vacuum degree is 22-25pa, high vacuum time is 8-9min, when casting in continuous casting, at 10 hours before pouring, when casting 60 tons of molten steel in ladle, change another ladle to cast, remaining molten steel returns to LF refining to carry out heat recovery;Immersion nozzle adopts vibration mode all the way, before casting 10 hours, half of the nozzle is used for casting, the other half is used for casting for remaining 10 hours. After the casting billet is rolled, the NDT flaw detection qualified rate is 99.5%, which is improved by 0.4% compared with 99.1% before process improvement. Example 7

[0029] Smelt R350LHT rail steel, continuous casting adopts 5 machine 5 flow large square billet casting, tundish casting time is 14 hours, open hearth ladle nominal capacity is 100 tons, in actual production, after converter tapping to two ladles, each ladle tapping quantity is 50 tons, RH high vacuum degree is 24-25 pa, high vacuum time is 7-9 min, when casting in continuous casting, 7 hours before pouring, when casting 60 tons of molten steel in open hearth ladle, change another ladle for casting, remaining molten steel returns to LF refining for heat recovery; whole process of submerged entry nozzle adopts vibration mode, 7 hours before casting, cast by half of nozzle, cast by another half for remaining 7 hours. After rolling of the cast billet of this pouring, NDT flaw detection qualified rate is 99.4%, compared with 99.1% before process improvement, it is improved by 0.3%. Example 8

[0030] Smelt R350HT rail steel, continuous casting adopts 5 machine 5 flow large square billet casting, tundish casting time is 18 hours, open hearth ladle nominal capacity is 150 tons, in actual production, after converter tapping to two ladles, each ladle tapping quantity is 75 tons, RH high vacuum degree is 24-25 pa, high vacuum time is 8-9 min, when casting in continuous casting, 9 hours before pouring, when casting 90 tons of molten steel in open hearth ladle, change another ladle for casting, remaining molten steel returns to LF refining for heat recovery; whole process of submerged entry nozzle adopts vibration mode, 9 hours before casting, cast by half of nozzle, cast by another half for remaining 9 hours. After rolling of the cast billet of this pouring, NDT flaw detection qualified rate is 100%, compared with 99.1% before process improvement, it is improved by 0.9%.

Claims

1. A production method for improving the NDT flaw detection qualification rate of steel rails, characterized in that: The method comprises 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 below, the high vacuum time is kept for 7-9 min; The continuous casting step: semi-enclosed casting is adopted, that is, when the ladle weight reaches 40%-60%, the casting is stopped and the next ladle is cast, and the remaining molten steel is returned to the LF refining step for heat recovery; the continuous casting submerged entry 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 submerged entry nozzle is vibrated continuously in the whole casting process.

2. A method of increasing the production of NDT qualified rail according to claim 1, wherein: The frequency of the vibration is 2.5-3.0 Hz.