Method for judging occurrence reasons and positions of transverse cracks on surface of continuous casting billet

Abstract

The invention discloses a method for judging the occurrence reason and position of a transverse crack on the surface of a continuous casting billet, which comprises the following steps of: (1) cutting the transverse crack part on the surface of the continuous casting billet, cutting the continuous casting billet in the middle of the transverse crack, and polishing the cut section; (2) carrying out dendritic crystal erosion on the section, and observing whether dendritic crystal segregation exists or not through a metallographic microscope; (3) if dendritic segregation is observed and the minimum distance between the dendritic segregation part and the surface of the blank shell is smaller than the thickness of the blank shell out of the crystallizer, judging that the transverse crack occurs in the crystallizer; and (3) if dendritic segregation is not observed, judging that the transverse crack occurs in the secondary cooling area of the continuous casting machine. According to the method, the position and the reason of the transverse crack of the continuous casting surface blank can be accurately judged, and different control measures are adopted in a targeted manner, so that the occurrence of the transverse crack on the surface of the casting blank is effectively controlled, the quality of the casting blank is improved, and the production cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of continuous casting, in particular to a method for judging the cause and position of transverse cracks on the surface of a continuous casting billet. Background Art

[0002] Transverse cracks are one of the common quality defects of continuous casting billets. Over the years, although metallurgical workers at home and abroad have conducted a lot of research on transverse cracks on the surface of billets, it is difficult to control due to the complex influencing factors. Transverse cracks are not easy to be found in the production process, which can easily cause batch quality degradation or even scrapping; after the appearance of surface transverse cracks, in order to control the batch quality, the billets will be forced to be taken offline for cold inspection, cut corners and then cold loaded, resulting in huge cost losses. Surface transverse cracks have long been a major quality problem that has plagued domestic and foreign steel companies.

[0003] At present, the traditional cognition and general theory of the formation mechanism of transverse cracks on the surface of the ingot is: steel has a third brittle zone in the temperature range of 700-900℃. During the bending and straightening process of the ingot in the secondary cooling zone, if its surface temperature enters the third brittle zone, transverse cracks will occur at the corresponding position on the surface of the ingot. According to the above theory, the method of optimizing and adjusting the secondary cooling water of continuous casting so that the ingot avoids the third brittle zone during bending and straightening is usually adopted to control the occurrence of surface transverse cracks. However, it is found in actual production that the method of adjusting the secondary cooling can only control and reduce the occurrence of some surface transverse cracks, and is ineffective in controlling the occurrence of other surface transverse cracks, indicating that there are other causes and formation mechanisms of transverse cracks on the surface of the ingot. Therefore, it is urgent to find a new formation mechanism of transverse cracks on the surface of the ingot, as well as a method to determine the cause and location of transverse cracks on the surface of the ingot. In this way, targeted measures can be taken according to the different formation mechanisms of surface transverse cracks to control the occurrence of surface transverse cracks, improve the quality of the ingot, reduce production costs, and create conditions for hot charging and hot delivery of the ingot, continuous casting and rolling, and realizing green, low-carbon and efficient production at the interface of continuous casting and hot rolling. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for accurately judging the cause and position of transverse cracks on the surface of a continuous casting billet.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention adopts the following steps: (1) cutting the transverse crack position on the surface of the continuous casting billet, cutting the continuous casting billet in the middle of the transverse crack, and polishing the cut section;

[0006] (2) Perform dendrite erosion on the cross section and observe whether there is dendrite segregation using a metallographic microscope;

[0007] (3) If dendrite segregation is observed in step (2), and the minimum distance between the dendrite segregation site and the shell surface is less than the shell thickness out of the crystallizer, it is determined that the transverse crack at that location occurs in the crystallizer;

[0008] (3) If no dendritic segregation is observed in step (2), it is judged that the transverse crack occurs in the secondary cooling area of ​​the continuous casting machine.

[0009] Furthermore, between steps (1) and (2), the following is performed: observing whether there are holes and / or cracks around the surface transverse cracks.

[0010] Furthermore, if holes and / or cracks are found, the distance between the holes and / or cracks and the surface of the continuous casting billet is measured to determine whether it is less than the billet shell thickness of the continuous casting billet out of the crystallizer.

[0011] Furthermore, in the step (2), if dendrite segregation is observed, an electron probe is used to check whether there is element enrichment around the crack.

[0012] The beneficial effects of adopting the above-mentioned technical scheme are: the present invention has discovered a new mechanism for the generation of transverse cracks on the surface of the ingot, enriched and expanded the technical understanding of the causes of transverse cracks on the surface of the ingot, and proposed a method for judging the causes and positions of transverse cracks on the surface of the continuous casting ingot, which can accurately judge the positions and causes of transverse cracks on the surface of the continuous casting ingot, and take different control measures accordingly, thereby effectively controlling the occurrence of transverse cracks on the surface of the ingot, improving the quality of the ingot, and reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0014] Figure 1 is a schematic diagram of holes or cracks observed around transverse cracks in Example 1 of the present invention;

[0015] Figure 2 is a schematic diagram of dendrite segregation observed in Example 1 of the present invention;

[0016] Figure 3 It is a schematic diagram of the detection range of the electronic probe inspection in Example 1 of the present invention;

[0017] Figure 4 This is a partial schematic diagram of the electronic probe inspection of Example 1 of the present invention;

[0018] Figure 5 is another partial schematic diagram of the electronic probe inspection of Example 1 of the present invention;

[0019] Figure 6 It is a schematic diagram of dendrite fracture observed in Example 2 of the present invention. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below in conjunction with specific implementation modes.

[0021] The inventors found in actual production that by adjusting the secondary cooling method, only the occurrence of some surface transverse cracks can be controlled and reduced, and the occurrence of other surface transverse cracks is ineffective, indicating that there are other causes and formation mechanisms of surface transverse cracks on the ingot. Therefore, it is urgent to find a new formation mechanism of surface transverse cracks on the ingot, and to find a method to determine the cause and location of surface transverse cracks on the ingot. In this way, targeted measures can be taken according to different surface transverse crack formation mechanisms to effectively control the occurrence of surface transverse cracks.

[0022] The method for determining the cause and location of transverse cracks on the surface of continuous casting billets adopts the following steps:

[0023] (1) Cut the transverse cracks on the surface of the continuous casting billet, cut the continuous casting billet in the middle of the transverse cracks, and polish the cut section with a roughness of ≤0.4μm;

[0024] (2) Observe whether there are holes and / or cracks around the transverse cracks, especially whether there are holes and / or cracks around the bottom of the cracks. If there are holes and / or cracks, measure the distance range between the holes and / or cracks and the surface of the continuous casting billet to determine the minimum distance between the holes and / or cracks and the surface of the continuous casting billet;

[0025] (3) Determine whether the minimum distance between the holes and / or cracks found and the surface of the ingot is less than the shell thickness H of the continuous casting ingot out of the crystallizer. The shell thickness is calculated using the following formula: H = k (L / V) 0.5 , where H is the shell thickness, mm; k is the mold solidification coefficient, K ranges from 15 to 21 mm / min 1 / 2 The specific value depends on the actual situation of each factory and the variety; L is the effective length of the crystallizer, m; V is the billet drawing speed, m / s.

[0026] (4) performing dendrite erosion on the cross section, observing the dendrites around the holes or cracks through a metallographic microscope, and searching for dendrite segregation around the holes or cracks. The dendrite segregation can be more easily found by locating the holes or cracks. If no holes or cracks are found in step (2), observing the dendrites of the entire cross section for dendrite segregation. The dendrite segregation refers to whether the dendrite spacing is abnormally large relative to the dendrite spacing in normal areas.

[0027] (5) If dendrite segregation exists, an electron probe is used to check whether there is obvious element enrichment around the dendrite segregation, such as abnormal segregation of carbon, sulfur, manganese, phosphorus and other elements; if dendrite segregation exists, it can be checked that there is obvious element enrichment around the dendrite segregation, which can further verify the existence of dendrite segregation.

[0028] (6) In step (4), as long as dendritic segregation is observed, it is determined whether the minimum distance D between the dendritic segregation and the surface of the ingot is less than the shell thickness H of the ingot exiting the crystallizer; the determination is made based on the following two situations:

[0029] The first case: in step (4), dendritic segregation is observed and D<H. D<H indicates that all or part of the dendritic segregation site falls within the thickness range of the crystallizer. In this case, the location and cause of the transverse crack can be determined: the transverse crack does not occur in the secondary cooling zone, but in the crystallizer. The shell formed by the ingot in the inner and outer layers of the crystallizer is thinner, with a large amount of liquid core inside, which mainly bears friction resistance. Because the shell is thinner at this time, when the friction force in the opposite direction of the billet drawing is too large and exceeds the limit that the ingot can withstand, the surface of the shell at the weak point and the interface between the solid-liquid two-phase zone will be torn. At this time, the steel liquid enriched with solute elements at the solidification front will replenish the small cracks, resulting in dendritic segregation. If the liquid phase replenishes the volume incompletely, that is, it solidifies rapidly, resulting in holes or cracks around it.

[0030] The second case: no dendritic segregation was observed in step (4); in this case, the dendritic erosion results only showed that the dendrites were broken or exceeded the thickness of the shell of the billet leaving the crystallizer, indicating that the shell was already very thick when the transverse surface crack occurred, and had exceeded the range of the crystallizer; since the shell was very thick at this time, the billet was only torn from the surface under the action of external force, and could not be supplemented by the molten steel enriched with solute elements inside, it can be judged that the transverse surface crack can only occur in the secondary cooling area of ​​the continuous casting machine. Under the action of the external force of bending and straightening, the surface temperature of the billet was in the third brittle zone, and the surface was torn to produce transverse cracks.

[0031] (7) According to the location and cause of the transverse cracks on the surface of the ingot, different control measures should be taken in a targeted manner. For the transverse cracks on the surface that occur in the crystallizer, the reason is that the ingot is subjected to excessive friction. Measures can be taken to improve the lubrication performance of the crystallizer protection slag and improve its melting and filling uniformity. For the transverse cracks on the surface that occur in the secondary cooling zone, the reason is that the ingot is bent and straightened in the third brittle zone. Measures can be taken to optimize the amount and distribution of the secondary cooling water to avoid the third brittle zone.

[0032] Example 1: The method for determining the cause and location of transverse cracks on the surface of a continuous casting billet is specifically described as follows.

[0033] The transverse cracks on the surface of the DP590 steel continuous casting billet were cut, the continuous casting billet was cut in the middle of the transverse cracks, and the cut section was polished to a roughness of 0.3μm; the area around the transverse cracks was observed, and holes or cracks were found around the bottom of the cracks, such as Figure 1 , measure its distance from the continuous casting billet surface 3mm;

[0034] When producing this ingot, the pulling speed is 1.5m / min, and the effective length of the casting machine crystallizer is 900mm. Then the shell thickness of the ingot when it leaves the crystallizer is: H = k (L / V) 0.5 =18(0.9 / 1.5)0.5=13.94mm.

[0035] The cross section is subjected to dendrite erosion, and the primary dendrite spacing around the crack hole or crack is observed through a metallographic microscope to be abnormally coarse than that of the normal part. Figure 2 As shown; and the minimum distance between the dendrite segregation and the surface of the ingot is D = 3mm, D < H; using the electron probe inspection, there is obvious abnormal segregation of carbon, sulfur, manganese, phosphorus and other elements around the crack, such as Figure 3 , Figure 4 and Figure 5 As shown, the darker the color, the more serious the segregation.

[0036] It can be judged that the transverse cracks on the surface occurred in the crystallizer, because the ingot was subjected to greater friction in the crystallizer. The control measures for the transverse cracks on the surface of the ingot of this steel grade are to improve the lubrication performance of the crystallizer protection slag and improve its melting and filling uniformity. By improving the lubrication performance of the crystallizer protection slag, the occurrence rate of transverse cracks in the ingot of this steel grade was reduced from an average of 35% to 7.1%.

[0037] Example 2: The method for determining the cause and location of transverse cracks on the surface of a continuous casting billet is specifically described as follows.

[0038] The transverse cracks on the surface of the Q420B steel continuous casting billet were cut, the continuous casting billet was cut in the middle of the transverse cracks, and the cut section was polished to a roughness of 0.2μm; the area around the transverse cracks was observed, the cracks extended inward from the billet surface, and there were no holes or cracks around the bottom of the cracks;

[0039] The cross section was dendrite eroded and the primary dendrite spacing of the cross section was observed under a metallographic microscope. It was found that the primary dendrite spacing of the cross section was not abnormally large compared with the primary dendrite spacing of the normal part. Only the fracture of the dendrite was found. Figure 6 As shown;

[0040] It can be judged that the transverse cracks occurred in the secondary cooling area because the billet was bent and straightened in the third brittle zone. We can take measures to optimize the amount and distribution of secondary cooling water to avoid the third brittle zone. By optimizing the amount and distribution of secondary cooling water, the occurrence rate of transverse cracks in the billet of this steel grade was reduced from an average of 7.1% to 0.5%.

[0041] Example 3: The method for determining the cause and location of transverse cracks on the surface of a continuous casting billet is specifically described as follows.

[0042] The transverse cracks on the surface of the DP590 steel continuous casting billet were intercepted, the continuous casting billet was cut in the middle of the transverse cracks, and the cut section was polished to a roughness of 0.4 μm; the area around the transverse cracks was observed, and holes or cracks were found around the bottom of the cracks, and the distance from the continuous casting billet surface to 14 mm was measured;

[0043] When producing this ingot, the pulling speed is 1.5m / min, and the effective length of the casting machine crystallizer is 900mm. Then the shell thickness of the ingot when it leaves the crystallizer is: H = k (L / V) 0.5 =18(0.9 / 1.5)0.5=13.94mm.

[0044] The cross section was subjected to dendrite erosion, and observation under a metallographic microscope showed that the primary dendrite spacing around the crack hole or crack was abnormally coarse than that in normal parts; and the minimum distance between the dendrite segregation and the surface of the ingot was D=2mm, D<H; inspection using an electron probe showed obvious abnormal segregation of elements such as carbon, sulfur, manganese, and phosphorus around the crack.

[0045] It can be judged that the transverse cracks occurred in the secondary cooling area because the billet was bent and straightened in the third brittle zone. We can take measures to optimize the amount and distribution of secondary cooling water to avoid the third brittle zone. By optimizing the amount and distribution of secondary cooling water, the occurrence rate of transverse cracks in the billet of this steel grade was reduced from an average of 35% to 8.3%.

Claims

1. A method for determining the cause and location of transverse cracks on the surface of a continuous casting billet, characterized in that: The following steps are adopted: (1) cutting the transverse crack portion of the surface of the continuous casting billet, cutting the continuous casting billet in the middle of the transverse crack, and polishing the cut section; (2) Perform dendrite erosion on the cross section and observe whether there is dendrite segregation using a metallographic microscope; (3) If dendrite segregation is observed in step (2), and the minimum distance between the dendrite segregation site and the shell surface is less than the shell thickness out of the crystallizer, it is determined that the transverse crack at that location occurs in the crystallizer; (3) If no dendritic segregation is observed in step (2), it is judged that the transverse crack occurs in the secondary cooling area of ​​the continuous casting machine.

2. A method for determining the cause and location of transverse cracks on the surface of a continuous casting billet according to claim 1, characterized in that: Between steps (1) and (2), the following is further performed: observing whether there are holes and / or cracks around the surface transverse cracks.

3. A method for determining the cause and location of transverse cracks on the surface of a continuous casting billet according to claim 2, characterized in that: If holes and / or cracks are found, measure the distance between the holes and / or cracks and the surface of the continuous casting billet and determine whether it is less than the thickness of the billet shell when the continuous casting billet leaves the crystallizer.

4. A method for determining the cause and location of transverse cracks on the surface of a continuous casting billet according to claim 1, 2 or 3, characterized in that: In the step (2), if dendrite segregation is observed, an electron probe is used to check whether there is element enrichment around the crack.

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