Finishing methods for crack-sensitive billets

Abstract

This invention discloses a finishing method for crack-sensitive billets, relating to the field of steel production technology. The method includes sampling billets obtained from continuous casting, heating and pickling them to obtain pickled billet samples, and then observing and marking the crack depths of the pickled billet samples under a microscope. When the crack depth of the pickled billet sample is ≤0.5mm, the continuously cast billet is directly rolled. When the crack depth of the pickled billet sample is >0.5mm, the continuously cast billet is first finished before rolling. By sampling the continuously cast billets and heating and pickling them, the iron oxide scale on their surface can be removed, improving the accuracy of crack identification. Microscopic observation of the pickled billet samples allows for accurate determination of the crack morphology and depth, and the depth can be used to determine whether the batch of continuously cast billets needs finishing, thereby avoiding unnecessary metal loss.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, and more specifically, to a finishing method for crack-sensitive square billets. Background Technology

[0002] In the steel production process, the form of steel products varies depending on their intended use, such as wire rod, plate, profile, bar, and pipe. Wire rod production primarily involves transforming steel billets into wire rods through deformation processes like drawing. When elements with high crack sensitivity, such as boron, are added to the steel billets used for rolling wire rods, cracks easily form on the billet surface. If these cracks are not treated, they will directly lead to linear defects on the wire rod surface, causing the wire rod to crack and become unusable during drawing and deformation processes.

[0003] Currently, the main method for inspecting and treating cracks in steel billets is visual inspection. If obvious cracks are observed, the billet is peeled and finished. However, because of the iron oxide scale on the billet surface, it is not possible to completely confirm whether the crack is surface-based. Furthermore, directly peeling and finishing the billet removes surface-defect-free steel as well, leading to increased metal loss and raising the smelting cost of steel production.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a finishing method for crack-sensitive square billets.

[0006] This invention is implemented as follows:

[0007] In a first aspect, the present invention provides a finishing method for a crack-sensitive billet, comprising taking a sample of the billet obtained by continuous casting and heating and pickling it to obtain a pickled billet sample, and observing and marking the depth of the cracks on the pickled billet sample under a microscope.

[0008] When the crack depth of the pickled billet sample is ≤0.5mm, the billet obtained by continuous casting is directly rolled.

[0009] When the crack depth of the pickled billet sample is >0.5mm, the billet obtained by continuous casting is first finished and then rolled.

[0010] In an optional implementation, the finishing method includes at least one of corner peeling, edge peeling, and full peeling.

[0011] In an optional implementation, the finishing method of the billet obtained by continuous casting is also determined based on the crack location of the pickled billet sample.

[0012] In an optional implementation, when the minimum distance between the crack location and the edge of the pickled billet sample is less than 3 / 40 of the side length of the pickled billet sample, the billet obtained by continuous casting is first peeled at the corners and then rolled.

[0013] When the minimum distance between the crack location and the edge of the pickled billet sample is less than 1 / 10 of the side length of the pickled billet sample, the billet obtained by continuous casting is first peeled off at the corners and edges, and then rolled.

[0014] When the minimum distance between the crack location and the edge of the pickled billet sample is greater than 1 / 10 of the side length of the pickled billet sample, the billet obtained by continuous casting is first fully peeled off and then rolled.

[0015] In an optional embodiment, the heating temperature for pickling is 70–80°C, and the time is 25–30 minutes.

[0016] In an optional embodiment, the pickling solution used for heating and pickling is a hydrochloric acid solution, wherein the volume ratio of hydrochloric acid to water in the hydrochloric acid solution is 1:1.

[0017] In an optional embodiment, the billet obtained by continuous casting is sampled, and the length of the sampled continuous casting billet is 300-400 mm.

[0018] In an optional implementation, the sampling method includes taking samples from different flows in each heat or between two heats of continuous casting.

[0019] In an optional embodiment, the continuous casting billet sample is further cut into at least two pieces before heating and pickling.

[0020] In an optional implementation, finishing also includes locally grinding the billet obtained from continuous casting.

[0021] The present invention has the following beneficial effects:

[0022] This invention provides a finishing method for crack-sensitive billets. By sampling the billets obtained from continuous casting, the billet samples are heated and pickled to remove the iron oxide scale on their surface, improving the accuracy of crack identification. Then, by observing the pickled billet samples under a microscope, the shape and depth of the cracks can be accurately determined. Based on the depth, it can be determined whether the batch of billets obtained from continuous casting needs to be finished, thereby avoiding unnecessary metal loss. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the edge cracks of the pickled billet sample provided in Embodiment 1 of the present invention;

[0025] Figure 2 Metallographic microscope image of the edge crack of the pickled billet sample provided in Embodiment 1 of the present invention;

[0026] Figure 3 This is a metallographic microscope image of the edge cracks of the pickled billet sample provided in Embodiment 1 of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0028] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0029] In a first aspect, the present invention provides a finishing method for a crack-sensitive billet, comprising taking a sample of the billet obtained by continuous casting and heating and pickling it to obtain a pickled billet sample, and observing and marking the depth of the cracks in the pickled billet sample under a microscope.

[0030] By sampling the billets obtained from continuous casting and then heating and pickling the billet samples, the iron oxide scale on their surface can be removed, thus improving the accuracy of crack identification.

[0031] When the crack depth of the pickled billet sample is ≤0.5mm, the billet obtained by continuous casting is directly rolled.

[0032] When the crack depth of the pickled billet sample is >0.5mm, the billet obtained by continuous casting is first finished and then rolled.

[0033] By observing the crack defects in the pickled billet samples under a microscope, the shape and depth of the cracks can be accurately determined. Based on the crack condition of the pickled billet samples, it can be determined whether the billets obtained from continuous casting in this batch need to be finished, thereby avoiding unnecessary metal loss.

[0034] In an optional implementation, the finishing method includes at least one of corner peeling, edge peeling, and full peeling.

[0035] In an optional implementation, the finishing method of the billet obtained by continuous casting is also determined based on the crack location of the pickled billet sample.

[0036] In an optional implementation, when the minimum distance between the crack location and the edge of the pickled billet sample is less than 3 / 40 of the side length of the pickled billet sample, the billet obtained by continuous casting is first peeled at the corners and then rolled.

[0037] When the minimum distance between the crack location and the edge of the pickled billet sample is less than 1 / 10 of the side length of the pickled billet sample, the billet obtained by continuous casting is first peeled off at the corners and edges, and then rolled.

[0038] When the minimum distance between the crack location and the edge of the pickled billet sample is greater than 1 / 10 of the side length of the pickled billet sample, the billet obtained by continuous casting is first fully peeled off and then rolled.

[0039] The four surfaces of the billet sample, excluding the two end faces, are divided into an inner arc surface, an outer arc surface, and two side surfaces according to the orientation of the continuous casting process. Based on the location of the crack, the distance between the crack and each edge on the plane where it is located is measured. The smallest distance obtained is the minimum distance between the crack location and the edge of the pickled billet sample.

[0040] In an optional embodiment, in order to better remove the iron oxide scale on the surface of the billet without damaging the morphology of the billet, the heating temperature for pickling is 70-80°C and the time is 25-30 minutes.

[0041] For example, the temperature for pickling can be any value among 70℃, 72℃, 74℃, 76℃, 78℃ or 80℃, or a range between any two of the above values ​​or any point within that range.

[0042] For example, the heating pickling time can be any value among 25 min, 26 min, 27 min, 28 min, 29 min, or 30 min, or a range between any two of the above values, or any point within that range.

[0043] In an optional embodiment, the pickling solution used for heating and pickling is a hydrochloric acid solution, wherein the volume ratio of hydrochloric acid to water in the hydrochloric acid solution is 1:1.

[0044] In an optional embodiment, the billet obtained by continuous casting is sampled, and the length of the sampled continuous casting billet is 300-400 mm.

[0045] For example, the length of the continuously cast billet sample can be any value of 300mm, 350mm or 400mm, or a range between any two of the above values ​​or any point value within that range.

[0046] By cutting the billet obtained from continuous casting to the aforementioned length, it is possible to quickly determine whether the continuous casting process of the billet meets product requirements and to rapidly confirm further processing methods. This method is convenient to operate and can quickly and accurately address the actual condition of the billet, reducing unnecessary metal loss without affecting production efficiency. Furthermore, the parameters of the continuous casting process can be adjusted based on crack detection results to reduce the number, depth, and length of cracks on the surface of billets obtained from subsequent continuous casting.

[0047] In optional embodiments, sampling methods include taking samples from different flows in each heat or between every two heats of continuous casting. That is, sampling can be performed continuously for each heat, or at intervals of one heat, for example, sampling heats 1, 3, 5, and 7. Each heat of continuous casting also has different flows, such as flows I, II, III, IV, and V of the first heat. To ensure the representativeness of the continuously cast billet samples, sampling should be performed on different flows, for example, sampling flows I of the first heat, II of the second heat, III of the third heat, and IV of the fourth heat for crack detection.

[0048] In an optional embodiment, in order to ensure that the surface of the continuously cast billet sample can be completely immersed in the hydrochloric acid solution during the heating and pickling process, the continuously cast billet sample is also cut into at least two pieces before heating and pickling.

[0049] Preferably, each continuously cast billet sample cut from the sample should be of similar size; for example, it can be cut in half along the center line of the inner and outer arcs of the entire continuously cast billet sample.

[0050] In an optional implementation, finishing also includes local grinding of the billet obtained from continuous casting to remove defects such as surface scars, slag pits, indentations and scratches, thereby ensuring product quality.

[0051] Example 1

[0052] This embodiment provides a finishing method for crack-sensitive square billets, including the following steps:

[0053] Steelmaking employs a 120t converter for smelting, followed by LF furnace refining and RH vacuum treatment to obtain refined molten steel. A 6-strand, 6-flow 160×160mm steel mill is used. 2 Continuous casting of billets produces 15 heats of boron-containing wire rod steel billets per casting cycle.

[0054] Take one 400mm long continuous casting billet sample from each of the following eight casting batches: Stream I (1st Heat), Stream II (3rd Heat), Stream III (5th Heat), Stream IV (7th Heat), Stream V (9th Heat), Stream VI (11th Heat), Stream I (13th Heat), and Stream I (15th Heat). Cut each of these continuous casting billet samples into two identical halves along the center lines of the inner and outer arcs using a saw.

[0055] Hydrochloric acid and water were added to a hot acid bath in a 1:1 volume ratio as the etching solution. The etching solution was heated to 75°C. The continuously cast billet sample was then placed in the hot acid bath with the cut surface facing down and immersed for 25 minutes. The etching solution was then drained, the continuously cast billet sample was rinsed with water, and dried with compressed air to obtain the pickled billet sample.

[0056] The top and sides (non-sawed surfaces) of the pickled billet samples were examined visually and under a microscope. A total of 7 edge cracks and 4 corner cracks were found, as detailed below:

[0057] There are two edge cracks on the side of the first furnace I flow (such as...) Figures 1-3 (As shown) and 1 corner crack; 1 edge crack on the outer arc surface of flow V in furnace 9; 2 edge cracks and 1 corner crack on the inner arc surface of flow I in furnace 13; 1 edge crack and 1 corner crack on the inner arc surface of flow I in furnace 15; 1 edge crack and 1 corner crack on the outer arc surface of flow I in furnace 15.

[0058] Among them, the minimum distances from the two edge cracks on the side of the first heat's I-flow to the edge of the pickled billet sample were 64 mm and 35 mm, respectively. The edge crack length on the inner arc surface of the 15th heat's I-flow was 9 mm. It was initially judged that the cracks were relatively severe and could not be eliminated by heating oxidation. Therefore, metallographic analysis was performed on the edge crack with a minimum distance of 35 mm on the side of the first heat's I-flow, the corner crack on the inner arc surface of the first heat's I-flow, and the edge crack on the inner arc surface of the 15th heat's I-flow.

[0059] Metallographic analysis revealed that the edge crack depths in furnaces 1 and 15 were 1.518 mm (e.g., Figure 3 (As shown) and 0.91mm, the depth of the corner crack in the first furnace is 0.65mm. The depth of the above cracks is all >0.5mm, and they need to be finished.

[0060] Furthermore, based on the eight continuously cast billet samples, which totaled 3.2m in length, the crack occurrence probability was 3.4 cracks per meter. This indicates a high crack occurrence probability in this casting batch. Considering the above crack parameters, it was determined that all billets obtained from this continuous casting batch underwent full peeling. The metal loss from full peeling was 1.75%. For the 15 heats of billets cast, totaling 1875t of steel, the metal loss from peeling was 32.8t. The wire rod rolled from the peeled billets in this batch did not experience any cracking issues due to billet defects during processing.

[0061] Example 2

[0062] This embodiment provides a finishing method for crack-sensitive square billets, including the following steps:

[0063] Steelmaking employs a 120t converter for smelting, followed by LF furnace refining and RH vacuum treatment to obtain refined molten steel. A 6-strand, 6-flow 160×160mm steel mill is used. 2 Continuous casting of billets produces 15 heats of boron-containing wire rod steel billets per casting cycle.

[0064] Ten continuous casting billet samples, each 400mm in length, were taken from each of the following casting batches: Stream I, Stream II, Stream III, Stream IV, Stream V, Stream VI, Stream I, Stream II, Stream III, and Stream IV. Each sample was then cut into two identical halves along the center lines of the inner and outer arcs using a saw.

[0065] Hydrochloric acid and water were added to a hot acid bath in a 1:1 volume ratio as the etching solution. The etching solution was heated to 78°C. The continuously cast billet sample was then placed in the hot acid bath with the cut surface facing down and immersed for 28 minutes. The etching solution was then drained, the continuously cast billet sample was rinsed with water, and dried with compressed air to obtain the pickled billet sample.

[0066] The top and sides (non-saved surfaces) of the pickled billet samples were examined visually and under a microscope. A total of 3 edge cracks and 4 corner cracks were found, as detailed below:

[0067] One edge crack was found on the inner arc surface of the first furnace I flow; one edge crack was found on the inner arc surface of the first furnace II flow; one edge crack and one corner crack were found on the inner arc surface of the 12th furnace I flow; one corner crack was found on the inner arc surface of the 12th furnace II flow; one corner crack was found on the inner arc surface of the 15th furnace III flow; and one corner crack was found on the outer arc surface of the 15th furnace IV flow.

[0068] Among them, the minimum distance between the edge crack on the inner arc surface of the I flow of the 12th furnace and the edge of the pickled billet sample was 28mm. Metallographic analysis was performed on the edge crack of the 1st furnace, the corner crack of the III and IV flows of the 15th furnace, which had relatively severe cracks.

[0069] Metallographic analysis revealed that the edge crack depths in the first furnace were 0.73 mm, and the corner crack depths in the third and fourth flows of the 15th furnace were 0.57 mm and 0.64 mm, respectively. Since the depths of all the above cracks were greater than 0.5 mm, finishing was required.

[0070] Furthermore, based on 10 continuously cast billet samples, totaling 4m in length, the probability of crack occurrence was 1.75 cracks / m, indicating a relatively low crack probability in this casting batch. Considering these crack parameters, it was determined that all square billets obtained from this casting batch underwent corner peeling. The metal loss from corner peeling was 0.9%. For 15 heats of square billets in this batch, totaling 1890t of steel, the metal loss from peeling was 16.9t. The wire rod rolled from the peeled square billets in this batch did not experience any cracking issues during processing due to billet defects.

[0071] Example 3

[0072] This embodiment provides a finishing method for crack-sensitive square billets, including the following steps:

[0073] Steelmaking employs a 120t converter for smelting, followed by LF furnace refining and RH vacuum treatment to obtain refined molten steel. A 6-strand, 6-flow 160×160mm steel mill is used. 2 Continuous casting of billets produces 16 heats of boron-containing wire rod steel billets in one casting cycle.

[0074] Ten continuous casting billet samples, each 400mm in length, were taken from each of the following casting batches: Stream I, Stream II, Stream III, Stream IV, Stream V, Stream VI, Stream I, Stream II, Stream III, and Stream IV. Each of these samples was then cut into identical halves along the center lines of the inner and outer arcs using a saw.

[0075] Hydrochloric acid and water were added to a hot acid bath in a 1:1 volume ratio to form an etching solution. The etching solution was heated to 80°C. The continuously cast billet sample was then placed in the hot acid bath with the cut surface facing down and immersed for 30 minutes. The etching solution was then drained, the continuously cast billet sample was rinsed with water, and dried with compressed air to obtain the pickled billet sample.

[0076] The top and sides (non-saved surfaces) of the pickled billet sample were examined visually and under a microscope. One edge crack and two corner cracks were found, as detailed below:

[0077] One edge crack was found on the inner arc surface of the first furnace I flow; one corner crack was found on the inner arc surface of the first furnace II flow; and one corner crack was found on the inner arc surface of the 16th furnace III flow. Metallographic analysis was performed on the above three cracks.

[0078] Metallographic analysis revealed that the edge crack depth in the first heat was 0.5 mm, the corner crack depth was 0.46 mm, and the corner crack depth in the 16th heat was 0.39 mm. The total length of the 10 continuously cast billet samples was 4 m, with a crack occurrence probability of 0.75 cracks / m. This indicates a low crack probability in this casting batch. The depths of the edge and corner cracks in the tested billet samples did not exceed the 0.5 mm burn-off during billet heating; therefore, finishing was unnecessary. The wire rod rolled from this batch of billets did not experience any cracking issues due to billet defects during processing.

[0079] Comparative Example 1

[0080] This comparative example provides a finishing method for crack-sensitive square billets. The same process as in Example 1 is used to produce 15 heats of boron-containing wire rod square billets in one casting. The billets are rolled directly without finishing. The produced wire rods are supplied to users for drawing and deformation processing. Five coils of steel experienced cracking, resulting in a quality issue with a weight of 8.16 tons and a quality issue rate of 4.5‰.

[0081] Comparative Example 2

[0082] This comparative example provides a finishing method for crack-sensitive billets. The same process as in Example 2 is used to produce 16 heats of boron-containing wire rod billets in one casting. After pickling, the number and depth of cracks observed by the naked eye in the billets are relatively high. Therefore, full peeling is directly performed. The metal loss during peeling is 32.9t, which is 16t more than that in Example 2.

[0083] Comparative Example 3

[0084] This comparative example provides a finishing method for crack-sensitive square billets. The same process as in Example 3 is used to produce 16 heats of boron-containing wire rod steel square billets in one casting. After pickling, the number and depth of cracks observed by the naked eye in the billets are relatively high. The corners are directly peeled off, and the metal loss from peeling is 17t, which is 17t more than that in Example 3.

[0085] In summary, relying on visual inspection to determine the finishing method may lead to unnecessary metal loss, thereby increasing smelting costs. The method provided in this invention can determine the billet processing technology based on the crack state of the billet, obtaining higher quality wire at a lower cost and preventing defects such as cracking during wire processing.

[0086] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of finishing a crack-sensitive square billet, characterized by, The method comprises the following steps: sampling the billet obtained by continuous casting, heating and pickling the billet sample to obtain a pickled billet sample, observing the pickled billet sample under a microscope and marking the depth of cracks; when the crack depth of the pickled billet sample is ≤0.5mm, directly rolling the billet obtained by continuous casting; when the crack depth of the pickled billet sample is >0.5mm, first finishing the billet obtained by continuous casting, and then rolling; The method further comprises determining the finishing method of the billet obtained by continuous casting according to the crack position of the pickled billet sample; the finishing method comprises at least one of corner skinning, edge skinning and full skinning; when the minimum distance between the crack position of the pickled billet sample and the edge of the pickled billet sample is <3 / 40 of the side length of the pickled billet sample, first performing corner skinning on the billet obtained by continuous casting, and then rolling; when the minimum distance between the crack position of the pickled billet sample and the edge of the pickled billet sample is <1 / 10 of the side length of the pickled billet sample, first performing corner skinning and edge skinning on the billet obtained by continuous casting, and then rolling; when the minimum distance between the crack position of the pickled billet sample and the edge of the pickled billet sample is >1 / 10 of the side length of the pickled billet sample, first performing full skinning on the billet obtained by continuous casting, and then rolling.

2. The method of claim 1, wherein, The heating temperature of the heating and pickling is 70-80℃, and the time is 25-30min.

3. The method according to claim 1 or 2, characterized in that, The pickling solution of the heating and pickling is a hydrochloric acid solution, and the volume ratio of hydrochloric acid to water in the hydrochloric acid solution is 1:

1.

4. The method of claim 1, wherein, The billet obtained by continuous casting is sampled, and the length of the billet sample obtained by sampling is 300-400mm.

5. The method of claim 4, wherein, Before the heating and pickling, the billet sample is sawn into at least two pieces.

6. The method of claim 1, wherein, The finishing further comprises locally grinding the billet obtained by continuous casting.

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