Non-replacement tundish different steel grade pouring method
By adjusting the chemical composition and controlling the superheat of the original and different steel grades, and by using troughs and overflow troughs, the problem of casting different steel grades with large compositional deviations without changing the tundish was solved, achieving low scrap volume and high-efficiency production.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SHANDONG IRON & STEEL CO LTD
- Filing Date
- 2025-08-28
- Publication Date
- 2026-07-07
AI Technical Summary
How to perform continuous casting of dissimilar steel grades with large compositional deviations, no overlap in compositional ranges, and no possibility of downgrading or reclassification without equipment modification, thereby reducing scrap steel production and ensuring the compositional conformity of the cast billets?
By adjusting the chemical composition of the last heat of the original steel grade and the first heat of the different steel grade, controlling the superheat of the tundish, and reducing the liquid level and casting speed during the pouring operation, the swivel trough and overflow trough are used to separate molten steel with incompatible chemical compositions, thus preventing contamination of the billet.
This achieves reduced scrap, increased production efficiency, lower production costs, and ensured billet composition compliance without replacing the tundish.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of continuous casting technology in iron and steel metallurgy, specifically to a method for casting different steel grades without changing the tundish. Background Technology
[0002] There are currently four methods for casting dissimilar steel grades using continuous casting machines: First, for dissimilar steel grades with minor compositional deviations and overlapping compositional ranges, direct mixing and separate judging are used; second, for dissimilar steel grades with minor compositional deviations and no overlapping compositional ranges, but which can be downgraded and re-judged, direct mixing and downgrading to a higher grade is used; third, for dissimilar steel grades with large compositional deviations, no overlapping compositional ranges, and no possibility of downgrading and re-judgment, the tundish is replaced; fourth, for dissimilar steel grades with large compositional deviations, no overlapping compositional ranges, and no possibility of downgrading and re-judgment, direct mixing and rejecting is used. As can be seen from the above, for dissimilar steel grades with large compositional deviations, no overlap in compositional ranges, and no possibility of downgrading or reclassification, the method of replacing the tundish for casting dissimilar steel grades requires the use of a new tundish. Furthermore, the old tundish is shut down, the new tundish is opened for casting, and the amount of billet discarded at the beginning and end is large, resulting in high production costs. The method of directly mixing and rejecting dissimilar steel grades requires the rejection of a large number of billets for both mixed steel grades in order to ensure that the composition is qualified, and there is a significant quality risk.
[0003] Patent CN117583567A, "A Method for Mixing and Casting Dissimilar Steel Grades in Continuous Casting with Non-overlapping Compositions," describes a process for casting dissimilar steel grades with a liquidus temperature difference less than or equal to a preset temperature. This method allows for the mixing and casting of dissimilar steel grades in short castings into longer castings, reducing the length of the beginning and end cuts. However, this method requires that when mixing the molten steel, the first ladle after mixing is opened when the remaining steel in the tundish of the last heat reaches 35%-60% of the tundish's rated weight, continuing this process until the continuous casting of the round billets is complete. The drawback of this method is that the weight of the molten steel in the tundish is generally 10-30 tons. Analysis suggests that the actual weight of the mixed and cast billets should be over 20-60 tons, and the chemical composition of the remaining billets cannot be accurately guaranteed. Patent CN107186191B, "A Method for Continuous Casting of Different Steel Grades," determines the casting sequence based on the carbon content or alloy element content of the two steel grades, i.e., determining the first and second ladles. The molten steel from the first ladle is poured into the tundish. When the amount of molten steel from the first ladle in the tundish reaches 10% to 20% of the total capacity of the tundish, the molten steel from the second ladle is then added. While this method improves the casting yield and reduces production costs when casting two steel grades with significantly different compositions in the same tundish, it requires an isolation device in the crystallizer to reduce mixing between the molten steel in the ladle and the molten steel in the liquid phase cavity of the cast billet in the crystallizer, increasing equipment complexity and modification costs.
[0004] Therefore, how to cast dissimilar steel grades with large compositional deviations, no overlap in compositional ranges, and no possibility of downgrading and reclassification without equipment modification using a continuous casting machine and without changing the tundish is an urgent problem to be solved. Summary of the Invention
[0005] In view of the above-mentioned prior art, the purpose of this invention is to provide a method for casting different steel grades without changing the tundish.
[0006] To achieve the above objectives, the present invention adopts the following technical solution:
[0007] In a first aspect, the present invention provides a method for casting different steel grades without changing the tundish, wherein the chemical composition of the last heat of the original steel grade and the first heat of the different steel grade are adjusted, the superheat of the tundish is controlled, and the casting operation is controlled, so as to realize the casting of different steel grades without changing the tundish.
[0008] Regarding the adjustment of chemical composition: the chemical composition of different steel grades is divided. Under the premise of meeting the technical requirements, the chemical composition of the last heat of the original steel grade and the first heat of the different steel grade is controlled as close as possible to the technical requirements of the high content of alloy components. The harmful elements P and S are controlled according to the technical requirements of low content.
[0009] Regarding the control of superheat in the tundish, the superheat of the last heat of the original steel grade and the first heat of the different steel grade is increased;
[0010] In terms of casting operation control, the liquid level in the tundish is reduced and the casting speed is controlled.
[0011] In some embodiments, the tundish liquid level drops to 50-150mm, the molten steel volume is 2-5 tons, and about 2-4 tons of molten steel flow into the overflow trough through the swing trough within 2 minutes between the last heat of the original steel grade and the first heat of the different steel grade. About 4-10 tons of billets are discarded and scrapped in the mixed casting. The average scrap volume of a single mixed casting is about 10 tons, which is lower than the standard and eliminates the risk of inconsistencies in billet composition after mixed casting.
[0012] Furthermore, based on the original superheat used in normal smelting, the superheat of the last tundish of the original steel grade is controlled by increasing it by 10-20℃, and the superheat of the first tundish of the different steel grades is controlled by increasing it by 20-30℃.
[0013] In some embodiments, increasing the superheat can prevent the tundish from dropping the liquid level, and prevent the tundish from absorbing heat and the liquidus temperature from deviating significantly, causing freezing and stopping the casting process when different steel types are mixed.
[0014] Furthermore, the original steel grade and the different steel grade are controlled separately during the casting operation;
[0015] For the original steel grade:
[0016] Before the last ladle of the original steel grade is poured, the slag discharge operation is carried out in the tundish; after the last ladle of the original steel grade is poured, the tundish pouring operation continues; after the liquid level in the tundish drops, the pulling speed is returned to zero, and the trough is set for waiting.
[0017] For the section on different steel grades:
[0018] After the first heat of dissimilar steel is poured, the ladle flow rate is controlled to a low speed, and the liquid level in the tundish is slowly increased. Then, the ladle flow rate is increased, and the liquid level in the tundish is raised to the normal pouring level. During tundish pouring, the molten steel produced initially is not introduced into the crystallizer, but flows through the swing trough to the overflow trough. After the first heat of dissimilar steel is poured, a reconnection operation is performed, and the casting speed is restored to the normal casting speed for billet reconnection.
[0019] In some embodiments, the continuous casting machine is equipped with a swing chute and an overflow chute. The swing chute is located below each flow in the tundish, and the overflow chute is located on the outer arc side of the tundish. After the first heat of dissimilar steel grades is poured, the chemical composition of the mixed steel grades deviates significantly. For the first 2 minutes after the first heat of dissimilar steel grades is poured, the molten steel does not flow into the crystallizer. This mixed portion of molten steel flows out of the tundish and through the swing chute to the overflow chute, discharging the dissimilar steel grades and effectively preventing contamination of the previously cast billets, thus improving the compositional consistency of the billets before and after the mixing of dissimilar steel grades.
[0020] Between the last heat of the original steel grade and the first heat of the different steel grade, molten steel does not flow into the crystallizer for 2 minutes. The casting speed is then reduced to zero, and the molten steel in the crystallizer partially solidifies and forms a reconnection, preventing the molten steel from the first heat of the different steel grade from entering the last heat of the original steel grade. The advantages of this operation are: ① During mixing, molten steel with large deviations in chemical composition flows into the overflow trough through the chute instead of into the crystallizer, effectively preventing contamination of previous slabs; ② It can accurately distinguish between the last heat of the original steel grade and the first heat of the different steel grade (the slab before reconnection is the last heat of the original steel grade, and the slab after reconnection is the first heat of the different steel grade); ③ For the reconnected slabs, the slabs after reconnection are discarded for scrap, ensuring the consistency of the slab composition.
[0021] Furthermore, when performing slag discharge from the tundish before the last ladle of the original steel grade is completed, the slag thickness in the tundish should be controlled within 1 mm.
[0022] In some embodiments, this can prevent tundish slag from entering the crystallizer when the tundish liquid level is low, thus preventing slag leakage.
[0023] Furthermore, after the last ladle of the original steel grade is poured, the tundish pouring operation continues until the liquid level in the tundish drops to 50-150mm.
[0024] Furthermore, when the liquid level in the tundish of the last heat of the original steel grade drops to 50-150mm, the casting speed is returned to zero, and the trough is set for waiting.
[0025] Furthermore, 4-5 minutes after the first batch of different steel grades is poured, when the liquid level in the tundish rises to 400-500mm, the ladle flow rate is increased.
[0026] Furthermore, after the first ladle pour of dissimilar steel grades, molten steel does not flow into the crystallizer for the first 2 minutes before the tundish pour; after the first ladle pour of dissimilar steel grades, a reconnection operation is performed 2 minutes after the ladle pour.
[0027] Furthermore, the cast billets at the rejoined joints are rejected.
[0028] Furthermore, the original steel billet was a qualified product before the reconnection, and it was shipped normally.
[0029] After reconnection, the billets are rejected by throwing them out at twice the amount of mixed casting, and the first batch of billets of different steel grades are placed on the ground; samples are taken from the head of the first billet of the first batch of different steel grades for chemical composition testing, and the qualified different steel grades are released normally, while the unqualified parts are rejected.
[0030] In some embodiments, to ensure the conformity of the billet composition, the first batch of billets of different steel grades is placed on the ground, and a sample is taken from the head of the first batch of billets of different steel grades for chemical composition testing. After the chemical composition test is qualified, the billets are produced normally according to the different steel grades, thus eliminating the risk of billet composition inconsistency after mixed casting.
[0031] The beneficial effects of this invention are:
[0032] This invention achieves casting of dissimilar steel grades without changing the tundish or requiring equipment modifications by controlling a series of parameters, including the chemical composition of the last heat of the original steel grade and the first heat of the dissimilar steel grade, the superheat of the tundish, the liquid level in the tundish, and the casting speed. This is achieved by controlling factors such as the chemical composition of the last heat of the original steel grade and the first heat of the dissimilar steel grade, the superheat of the tundish, the liquid level in the tundish, and the casting speed. This saves one tundish and significantly improves the production efficiency of the continuous casting machine. The method of this invention is simple, highly operable, and results in a small amount of scrap after casting dissimilar steel grades, reducing production costs and contributing to the improvement of quality and efficiency for enterprises. Detailed Implementation
[0033] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.
[0034] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.
[0035] The test materials used in the embodiments of the present invention, unless otherwise specified, are all conventional test materials in the art and can be purchased through commercial channels.
[0036] Example 1
[0037] A 320*420 cross-section rectangular billet continuous casting machine was used to sequentially cast different steel grades, Q235B and Q355B. The main controlled chemical compositions of Q235B and Q355B are shown in Table 1 below.
[0038] Table 1. Main Controlled Chemical Compositions of Different Steel Grades Q235B and Q355B
[0039]
[0040] As shown in Table 1, due to the significant differences in chemical composition between Q235B and Q355B steel grades, with no overlap in Mn element content, and the substantial performance differences, downgrading is not feasible. Therefore, a non-replacement tundish casting method using different steel grades is adopted, as detailed below:
[0041] 1. Ingredient control
[0042] For the last batch of Q235B steel, the Si content was controlled within the cross-range of 0.30-0.35%, the Mn content was controlled within the range of 0.45-0.50%, and the P and S contents were both controlled within the range of ≤0.035%.
[0043] For the first batch of Q355B steel, the Mn content was controlled at 1.35-1.40%, and the V content was controlled at ≥0.030%.
[0044] The control principle is as follows:
[0045] Grades Q235B and Q355B are different types of steel that are cast sequentially.
[0046] For the C content, since the Q235B and Q355B have the same range, no special control is required.
[0047] For Si content, the range is 0.10-0.35% for Q235B and 0.20-0.50% for Q355B, with an overlap range of 0.20-0.35%. The Si content should be controlled according to the overlap range of 0.30-0.35% for the later-cast grade Q355B.
[0048] For Mn content, the range is 0.40-0.50% for Q235B and 1.25-1.40% for Q355B, with no overlap in the 0.20-0.35% range. For the last heat of Q235B steel, the Mn content is controlled at 0.45-0.50%; for the first heat of Q355B steel, the Mn content is controlled at 1.35-1.40% according to the range of Q355B used in subsequent castings, in order to appropriately increase the element content and alleviate dilution.
[0049] For P and S content, the range is ≤0.040% for Q235B and ≤0.035% for Q355B. There is an overlap range of ≤0.035%. For the last heat of Q235B steel, the P and S content is controlled according to the overlap range of ≤0.035%.
[0050] Regarding the V content, Q235B steel does not contain vanadium, while Q355B has a range of ≥0.020%. When Q355B is mixed with Q235B, the V content of the first heat of Q355B steel may drop to the unqualified level. To be on the safe side, the V content of the first heat of Q355B steel should be appropriately increased, that is, the V content should be controlled at ≥0.030%.
[0051] 2. Superheat control
[0052] When Q235B and Q355B steels are poured normally, the superheat of the tundish is 15-25℃. In this method, this superheat is used for all steels except the last heat of Q235B steel and the first heat of Q355B steel.
[0053] For the last heat of Q235B steel, the superheat of the tundish is controlled by increasing the temperature by 10°C, with the tundish superheat being 25-35°C. Meanwhile, for the first heat of Q355B steel, the superheat of the tundish is controlled by increasing the temperature by 25°C, with the tundish superheat being 40-50°C.
[0054] 3. Pouring operation method and control of main parameters
[0055] (1) During the middle of the last heat of Q235B steel, the tundish slag discharge operation is carried out, and the slag thickness in the tundish is controlled within 1mm. After the last heat of Q235B steel is poured, the tundish pouring operation continues until the liquid level in the tundish drops to 100mm.
[0056] (2) When the liquid level in the tundish of the last heat of Q235B steel drops to 100mm, the casting speed is returned to zero, and the trough is set for waiting. When the liquid level in the tundish of the last heat of Q235B steel drops to 100mm, the first heat of Q355B steel is poured from the ladle. The ladle flow rate is controlled, and the liquid level in the tundish is slowly increased. After 4-5 minutes, the liquid level in the tundish is increased to 400-500mm. Then, the ladle flow rate is increased, and the liquid level in the tundish is increased to the normal pouring level.
[0057] (3) After the first ladle of Q355B steel is poured, the molten steel does not flow into the crystallizer in the first 2 minutes before the tundish is poured, but flows to the overflow trough through the swing trough.
[0058] (4) Two minutes after the first batch of Q355B steel is poured, a reconnection operation is carried out, and the casting speed is restored to the normal casting speed according to the reconnection technical requirements.
[0059] 4. Judgment of invalidity
[0060] Before the reconnection, the last batch of Q235B steel billets was issued normally. At the reconnection point, billets were judged as scrap if each flow was 3-5 meters long. When the tundish liquid level dropped to 100mm, the steel volume was 5.5 tons. After the reconnection, billets were judged as scrap if they were more than twice the amount of mixed pouring (≥11 tons), and the first batch of Q355B steel billets was placed on the ground.
[0061] 5. Component analysis
[0062] Chemical composition analysis was performed on the head of the first billet from the first furnace of Q355B steel. After the chemical composition analysis was passed, billets were produced normally according to the Q355B steel standard.
[0063] Example 2
[0064] A 160*160mm cross-section small square billet continuous casting machine was used to sequentially cast different steel grades, HRB600E and HRB400E. The main controlled chemical compositions of HRB600E and HRB400E are shown in Table 2 below.
[0065] Table 2 shows the main controlled chemical compositions of HRB600E and HRB400E steel grades.
[0066]
[0067] As shown in Table 2, due to the significant deviation in chemical composition between HRB400E and HRB600E steels, with no overlap in Mn and V elements, and the substantial performance differences, downgrading is not feasible. Therefore, a non-replacement tundish casting method using different steel grades is adopted, as detailed below:
[0068] 1. Ingredient control
[0069] For the last heat of HRB600E steel, the Si content is controlled at 0.75-0.80%, and the V content is controlled at 1.45-1.50%.
[0070] For the first heat of HRB400E steel, the C content is controlled at 0.24-0.25%, the Si content at 0.50-0.55%, the Mn content at 1.40-1.45%, the P and S contents at ≤0.035%, and the V content at 0.040-0.043%.
[0071] The control principle is as follows:
[0072] Grades Q235B and Q355B are different types of steel that are cast sequentially.
[0073] For the C content, since the Q235B and Q355B have the same range, no special control is required.
[0074] For Si content, the range is 0.10-0.35% for Q235B and 0.20-0.50% for Q355B, with an overlap range of 0.20-0.35%. The Si content should be controlled according to the overlap range of 0.30-0.35% for the later-cast grade Q355B.
[0075] For Mn content, the range is 0.40-0.50% for Q235B and 1.25-1.40% for Q355B, with no overlap in the 0.20-0.35% range. For the last heat of Q235B steel, the Mn content is controlled at 0.45-0.50%; for the first heat of Q355B steel, the Mn content is controlled at 1.35-1.40% according to the range of Q355B used in subsequent castings, in order to appropriately increase the element content and alleviate dilution.
[0076] For P and S content, the range is ≤0.040% for Q235B and ≤0.035% for Q355B. There is an overlap range of ≤0.035%. For the last heat of Q235B steel, the P and S content is controlled according to the overlap range of ≤0.035%.
[0077] Regarding the V content, Q235B steel does not contain vanadium, while Q355B has a range of ≥0.020%. When Q355B is mixed with Q235B, the V content of the first heat of Q355B steel may drop to the unqualified level. To be on the safe side, the V content of the first heat of Q355B steel should be appropriately increased, that is, the V content should be controlled at ≥0.030%.
[0078] 2. Superheat control
[0079] When HRB600E and HRB400E steels are poured normally, the superheat of the tundish is 15-25℃. In this method, this superheat is used for all heats except the last heat of HRB600E steel and the first heat of HRB400E steel.
[0080] For the last heat of HRB600E steel, the superheat of the tundish was controlled by increasing the temperature by 10°C, with the tundish superheat being 25-35°C. Meanwhile, for the first heat of HRB400E steel, the superheat of the tundish was controlled by increasing the temperature by 25°C, with the tundish superheat being 40-50°C.
[0081] 3. Pouring operation method and control of main parameters
[0082] (1) During the middle of the last heat of HRB600E steel, the tundish slag discharge operation is carried out, and the slag thickness in the tundish is controlled within 1mm. After the last heat of HRB600E steel is poured, the tundish pouring operation continues until the liquid level in the tundish drops to 100mm.
[0083] (2) When the liquid level in the tundish of the last heat of HRB600E steel drops to 100mm, the casting speed is returned to zero and the trough is set for waiting. When the liquid level in the tundish of the last heat of HRB600E steel drops to 100mm, the first heat of HRB400E steel is poured from the ladle. The ladle flow rate is controlled, and the liquid level in the tundish is slowly increased. The liquid level in the tundish is increased to 400-500mm in 4-5 minutes. Then the ladle flow rate is increased, and the liquid level in the tundish is increased to the normal pouring level.
[0084] (3) After the first heat of HRB400E steel is poured, the molten steel does not flow into the crystallizer in the first 2 minutes before the tundish is poured, but flows to the overflow trough through the swing trough.
[0085] (4) Two minutes after the first batch of HRB400E steel is poured, the reconnection operation is carried out and the casting speed is restored to the normal casting speed according to the reconnection technical requirements.
[0086] 4. Judgment of invalidity
[0087] Before the reconnection, the last batch of HRB600E steel billets was cast normally. At the reconnection point, billets were judged as scrap if each flow was 3-5 meters long. When the tundish liquid level dropped to 100mm, the steel volume was 4 tons. After the reconnection, the billets were judged as scrap if the volume was more than twice the amount of mixed pouring (≥8 tons), and the first batch of HRB400E steel billets was placed on the ground.
[0088] 5. Component analysis
[0089] Chemical composition analysis was performed on the head of the first billet of the first furnace of HRB400E steel. After the chemical composition analysis was qualified, the billets were produced normally according to the HRB400E steel grade.
[0090] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.
Claims
1. A method for casting dissimilar steel grades without changing the tundish, characterized in that, Chemical composition adjustment, tundish superheat control, and casting operation control are carried out for the last heat of the original steel grade and the first heat of the different steel grade to achieve casting of different steel grades without changing the tundish. Regarding the adjustment of chemical composition: the chemical composition of different steel grades is divided. Under the premise of meeting the technical requirements, the chemical composition of the last heat of the original steel grade and the first heat of the different steel grade is controlled as close as possible to the technical requirements of the high content of alloy components. The harmful elements P and S are controlled according to the technical requirements of low content. Regarding the control of superheat in the tundish, the superheat of the last heat of the original steel grade and the first heat of the different steel grade is increased; In terms of casting operation control, the liquid level in the tundish is reduced and the casting speed is controlled; Based on the original superheat used in normal smelting, the superheat of the last tundish of the original steel grade is controlled by increasing it by 10-20℃, and the superheat of the first tundish of the different steel grades is controlled by increasing it by 20-30℃. The original steel grade and the different steel grade are controlled separately during the casting operation; For the original steel grade: Before the last ladle of the original steel grade is poured, the slag discharge operation is carried out in the tundish; after the last ladle of the original steel grade is poured, the tundish pouring operation continues; after the liquid level in the tundish drops, the pulling speed is returned to zero, and the trough is set for waiting. For the section on different steel grades: After the first heat of dissimilar steel is poured, the ladle's flow rate is controlled to a low speed, and the tundish liquid level is slowly raised. Then, the ladle's flow rate is increased, and the tundish liquid level is raised to the normal pouring level. During tundish pouring, the molten steel initially produced is not introduced into the crystallizer but flows through the swing trough to the overflow trough. After the first heat of dissimilar steel is poured, a reconnection operation is performed, and the casting speed is restored to the normal casting speed for billet reconnection. When the liquid level in the tundish of the last heat of the original steel grade drops to 50-150mm, the casting speed is returned to zero, and the trough is set for waiting. 4-5 minutes after the first batch of different steel grades is poured, when the liquid level in the tundish rises to 400-500 mm, increase the flow rate of the ladle.
2. The method for casting dissimilar steel grades without changing the tundish according to claim 1, characterized in that, When performing slag discharge from the tundish before the last ladle of the original steel grade is poured, the slag thickness in the tundish should be controlled within 1 mm.
3. The method for casting dissimilar steel grades without changing the tundish according to claim 1, characterized in that, After the last ladle of the original steel grade is poured, the tundish pouring operation continues until the liquid level in the tundish drops to 50-150mm.
4. The method for casting dissimilar steel grades without changing the tundish according to claim 1, characterized in that, After the first ladle pour of different steel grades is started, the molten steel does not flow into the crystallizer for the first 2 minutes before the tundish pour; 2 minutes after the first ladle pour of different steel grades is started, the reconnection operation is performed.
5. The method for casting dissimilar steel grades without changing the tundish according to claim 1, characterized in that, The cast billet at the joint is rejected.
6. The method for casting dissimilar steel grades without changing the tundish according to claim 5, characterized in that, Before the reconnection, the original steel grade billet was a qualified product and was shipped normally. After reconnection, the billets were rejected by throwing them out at twice the amount of mixed casting, and the first batch of billets of different steel grades were placed on the ground. Chemical composition testing was conducted on the head of the first billet from the first furnace of the dissimilar steel grade. Qualified dissimilar steel grades were processed normally, while unqualified portions were rejected.