A method for improving slab segregation

Abstract

The application discloses a method for improving slab segregation, and belongs to the technical field of continuous casting. The method comprises the following steps: performing dynamic pressing on a slab with the mass fraction of Ni being greater than or equal to 3%, wherein the pressing amount is 0.5-3 mm when the slab is in the interval of fs=1.0-1.1. According to the method, dynamic light pressing is performed on the slab of high-nickel steel in the interval of fs=1.0-1.1 during continuous casting, and the pressing amount is controlled to be 0.5-3 mm, so that the molten metal of the slab can be fully pressed into the peripheral dendrites near the position of one fourth and three fourths, solute redistribution is realized, shrinkage holes are pressed together, the segregation and porosity problems of the slab of high-nickel steel near the position of one fourth and three fourths are reduced, and the internal quality of the slab of high-nickel steel is improved.

Description

Technical Field

[0001] This invention belongs to the field of continuous casting technology, and specifically relates to a method for improving slab segregation. Background Technology

[0002] The internal quality of a slab includes central segregation and porosity. Central segregation occurs when the element content is high at the center of the slab's cross-section, gradually decreasing outwards. Currently, dynamic light reduction in continuous casting is a common method to improve the internal quality of slabs. Dynamic light reduction involves applying a certain amount of reduction force at the end of the slab's solidification process.

[0003] Currently, in China, dynamic light reduction processes are generally performed in the solid-liquid two-phase range where fs is less than 1, where fs refers to the solid fraction at the center of the continuously cast billet. Patent [CN101648263B] proposes a reduction range of fs = 0.35-1 for high-quality cord steel billets, with a total reduction of 10-15.5 mm. Patent [CN101036921A] proposes a reduction range of fs = 0.3-1 for heavy rail steel billets, with a total reduction of 1.6-7 mm. Patent [CN105839002A] proposes a reduction range of fs = 0.2-0.8 for bainitic steel billets, with a total reduction of 10-15 mm.

[0004] The above-mentioned reduction method is used to reduce high-nickel steel, but the interior of the high-nickel steel slab still has serious element segregation and porosity problems. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention provides a method for improving slab segregation by pressing down the solid phase region of high-nickel steel, thereby improving the central segregation and porosity of high-nickel steel at the quarter and three-quarter positions of the cross-section and improving the internal quality of high-nickel steel.

[0006] The technical solution of this invention is as follows:

[0007] This invention provides a method for improving slab segregation, comprising:

[0008] A slab with a Ni mass fraction ≥ 3% is dynamically pressed down, wherein the pressing amount of the slab is 0.5-3 mm in the range of fs = 1.0-1.1.

[0009] In some embodiments, the reduction of the slab in the range of fs = 0.6-0.95 is greater than the reduction in the range of fs = 1.0-1.1.

[0010] In some embodiments, the slab is pressed down by 4-9 mm in the range of fs = 0.6-0.95.

[0011] In some embodiments, the slab is pressed down by 0.5-2 mm when fs = 1.0-1.1; and by 5-8 mm when fs = 0.6-0.95.

[0012] In some embodiments, the total reduction amount of the dynamic pressing is 5.5-10 mm.

[0013] In some embodiments, the total reduction amount of the dynamic compression is 6-9 mm.

[0014] In some embodiments, the dynamic pressing rate is 0.5-1 mm / m.

[0015] In some embodiments, the width-to-thickness ratio of the slab is ≥5.

[0016] In some embodiments, during the dynamic pressing process, the secondary cooling water volume is 0.3-0.8 L / kg.

[0017] In some embodiments, during the dynamic pressing process, the slab drawing speed is 0.8-1.0 m / min.

[0018] The beneficial effects of the present invention include at least the following:

[0019] The present invention provides a method for improving slab segregation, comprising the following steps: dynamically reducing a slab with a Ni mass fraction ≥ 3%, wherein the reduction is 0.5-3 mm within the range of fs = 1.0-1.1 for the slab. High-nickel steel slabs have a high nickel alloy content, a low liquidus temperature (1380-1430℃), and poor thermal conductivity. Therefore, during solidification, high-nickel steel exhibits a long solid-liquid two-phase region and a large temperature gradient, resulting in severe segregation and porosity at the solidification end. Because of the large width-to-thickness ratio of high-nickel steel slabs, meaning the width of the high-nickel steel is very wide, the center of the slab along the width direction is already solidified, i.e., fs = 1.0-1.1 at the center. However, liquid phase still exists near the quarter and three-quarter positions, i.e., fs < 1 near the quarter and three-quarter positions. Therefore, applying a dynamic light reduction of 0.5-3 mm to the slab in the fs = 1.0-1.1 position range can force the liquid metal near the quarter and three-quarter positions into the nearby dendrites, compressing the shrinkage cavities and realizing the redistribution of the solute. This improves the elemental segregation and porosity problems near the quarter and three-quarter positions of the high-nickel steel, thereby improving the internal quality of the high-nickel steel slab. Attached Figure Description

[0020] Figure 1 A low-magnification view of Example 1 is shown.

[0021] Figure 2A low-magnification view of Example 4 is shown.

[0022] Figure 3 A low-magnification image of Comparative Example 2 is shown. Detailed Implementation

[0023] To enable those skilled in the art to better understand this application, the technical solution of this application will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] Explanation: fs represents the solid fraction at the center of the continuously cast billet. The calculation method for fs is: fs = (TL - T) / (TL - Ts), where TL is the liquidus temperature of the high-nickel steel, Ts is the solidus temperature of the high-nickel steel, and T is the temperature of the high-nickel steel molten steel at the local time. The position where fs = 0-1 represents the solid-liquid two-phase region, and the position where fs > 1 represents the solid phase region.

[0025] High-nickel steel ([%Ni]≥3%) is widely used in the manufacture of storage tanks for liquefied natural gas (LNG), liquefied ethylene gas (LEG), and liquefied petroleum gas (LPG). As a high-end steel product, it requires ultra-low temperature toughness, high yield strength, high alloy content, high purity, and high crack sensitivity. Therefore, the continuous casting slab must have a center segregation grade of C below 1.0 and a center porosity grade of 1.0 below. The solidification characteristics of high-Ni steel continuous casting slabs are: 1) Due to the high nickel content, high-nickel steel has a low liquidus temperature, a long solid-liquid two-phase region, and a large temperature gradient, leading to severe elemental segregation at the end of solidification. 2) The steel has poor thermal conductivity; excessive cooling water intensity and overheating during solidification can easily cause quality problems, and the process control window is narrow. 3) Due to the large width-to-thickness ratio, continuous casting slabs are prone to asynchronous solidification in the width direction at the end of the solidification process. Specifically, on the cross-section of the slab perpendicular to the length direction, i.e., the cross-section formed by the thickness and width, on the center line perpendicular to the thickness direction, the central part, which is near the halfway point, has already solidified, i.e., fs > 1, while the area near the quarter and three-quarters points is still in the solid-liquid two-phase region, i.e., fs < 1 at the quarter and three-quarters points of the slab. When the central part has not yet solidified, for example, fs = 0.9 at the central part, fs at the quarter and three-quarters points may be 0.5, which is commonly referred to as the bone type, or W type. Traditional dynamic light reduction processes use the fs value at the center as a benchmark, and the areas where dynamic light reduction is applied are generally where fs = 0.6-1. When this method is used to apply light reduction to high-nickel steel, on the one hand, for the area where fs = 0.6-1 at the center, dynamic light reduction is applied, but fs < 0.6 near the one-quarter and three-quarters positions, so dynamic light reduction has no effect on the area near the one-quarter and three-quarters positions; on the other hand, for the area where fs > 1 at the center, but fs < 1 near the one-quarter and three-quarters positions, no light reduction is applied, and severe segregation and porosity will occur near the one-quarter and three-quarters positions.

[0026] To address the issues of center segregation and porosity near the quarter- and three-quarter-point positions in high-nickel steel, this application provides a method for improving the internal quality of slabs. This method involves dynamically applying light reduction during continuous casting of the high-nickel steel slab within the fs = 1.0-1.1 range, with the reduction amount controlled at 0.5-3 mm. This allows the molten metal near the quarter- and three-quarter-point positions to be fully pressed into the surrounding dendrites, achieving solute redistribution and simultaneously closing shrinkage cavities. This reduces segregation and porosity in the high-nickel steel slab near the quarter- and three-quarter-point positions, thereby improving the internal quality of the high-nickel steel slab.

[0027] The method for improving slab segregation provided in this application includes the following steps:

[0028] Dynamic reduction is performed on slabs with a Ni mass fraction ≥ 3%, wherein the reduction amount is 0.5-3 mm in the range of fs = 1.0-1.1 for the slab.

[0029] High-nickel steel slabs have a high nickel alloy content, and the liquidus temperature of high-nickel steel is 1380-1430℃. The low liquidus temperature, coupled with the poor thermal conductivity of high-nickel steel, results in a long solid-liquid two-phase region and a large temperature gradient during solidification, leading to severe segregation and porosity at the end of solidification. Because of the large width-to-thickness ratio of high-nickel steel slabs, meaning the width of the high-nickel steel is very wide, the center of the slab along the width direction is already solidified, i.e., fs = 1.0-1.1 at the center. However, liquid phase still exists near the quarter and three-quarter positions, i.e., fs < 1 near the quarter and three-quarter positions. Therefore, applying a dynamic light reduction of 0.5-3 mm to the slab in the fs = 1.0-1.1 position range can force the liquid metal near the quarter and three-quarter positions into the nearby dendrites, compressing the shrinkage cavities and realizing the redistribution of the solute. This improves the elemental segregation and porosity problems near the quarter and three-quarter positions of the high-nickel steel, thereby improving the internal quality of the high-nickel steel slab.

[0030] Excessive reduction under dynamic light pressing may cause cracks in the high-nickel steel slab, and in severe cases, breakage. Insufficient reduction under dynamic light pressing will not achieve the purpose of improving elemental segregation and porosity. Regarding the pressing position, if the fs at the pressing position exceeds 1.1, the corresponding section may have already formed a solid near the one-quarter and three-quarters positions, resulting in porosity and segregation problems that cannot be improved. If the fs at the pressing position is less than 1.0, the effect of using a reduction of 0.5-3mm to improve porosity and segregation is not good.

[0031] In some embodiments, the reduction in the slab range of fs = 0.6-0.95 is greater than the reduction in the range of fs = 1.0-1.1. The liquid core size at the center of the high-nickel steel is larger than the dimensions near the aforementioned one-quarter and three-quarters positions; therefore, the reduction in the high-nickel steel slab range of fs = 0.6-0.95 is greater than the reduction in the high-nickel steel slab range of fs = 1.0-1.1.

[0032] In some embodiments, the reduction is 4-9 mm in the range of fs = 0.6-0.95 for the slab. Reducing the slab in the range of fs = 0.6-0.95 at the center of the high-nickel steel slab can improve segregation and porosity in the center of the slab; if the slab is reduced in the range of fs below 0.6, the improvement effect on segregation and porosity is poor.

[0033] In some embodiments, preferably, the reduction is 0.5-2 mm in the range of fs = 1.0-1.1 for the slab, and 5-8 mm in the range of fs = 0.6-0.95 for the slab.

[0034] In some embodiments, the total reduction during dynamic pressing is 5.5-10 mm. Here, the total reduction refers to the sum of the reduction of the high-nickel steel slab in the range of fs = 1.0-1.1 and the reduction of the high-nickel steel slab in the range of fs = 0.6-0.95. Excessive total reduction will further increase the tensile force on the slab, potentially leading to a major production quality accident such as a horizontally laid slab; excessive total reduction can also cause internal cracks in the cast slab. Insufficient total reduction will not effectively compress and concentrate the molten steel, thus failing to reduce segregation.

[0035] Preferably, the total reduction under dynamic pressure is 6-9 mm.

[0036] In some embodiments, the dynamic reduction rate is 0.5-1 mm / m. Controlling the reduction rate ensures that the reduction amount is applied evenly to the high-nickel steel slab, thus mitigating the surface cracks caused by excessively high instantaneous reduction.

[0037] In some embodiments, the width-to-thickness ratio of the slab is ≥5. High-nickel steel slabs with an excessively large width-to-thickness ratio will exhibit the aforementioned W-shaped or bone-shaped morphology, i.e., solid-liquid two-phase regions appear near the one-quarter and three-quarters positions of the cross-section, while the central position is a solid phase region. This is because the cross-section of the high-nickel steel slab is too large, and high-nickel steel has poor thermal conductivity.

[0038] In some embodiments, during the dynamic pressing process, the secondary cooling water volume is 0.3-0.8 L / kg.

[0039] In some embodiments, during the dynamic pressing process, the slab drawing speed is 0.8-1.0 m / min.

[0040] The method for improving slab segregation provided in this application will be further described below with reference to specific embodiments.

[0041] Example 1

[0042] The production process involves using 3Ni steel (3% nickel content) for cryogenic containers with a continuous casting slab size of 300mm × 1800mm. The narrow-face copper plate of the crystallizer has a taper coefficient of 1.15% / m, a secondary cooling water ratio of 0.7L / kg, and a slab casting speed of 0.95m / min. A combined reduction process is implemented in the solid-liquid two-phase region and the solid phase region: 5mm reduction in the solid-liquid two-phase region when fs = 0.6-0.95, and 0.5mm reduction in the solid phase region when fs = 1.0-1.1, with a reduction rate of 0.5mm / min.

[0043] After continuous casting, samples were taken and prepared according to GB / T226 and graded. In this embodiment, the slab center segregation was grade C1.0 and the center porosity was grade 1.0.

[0044] Example 2

[0045] The production process involves using 5Ni steel (5% nickel content) for cryogenic vessels with a continuous casting slab size of 400mm×2000mm. The narrow-face copper plate of the crystallizer has a taper coefficient of 1.2% / m, a secondary cooling water flow rate of 0.65L / kg, and a slab casting speed of 0.9m / min. A combined reduction process is implemented in the solid-liquid two-phase region and the solid phase region: 6.5mm reduction in the solid-liquid two-phase region (fs = 0.6-0.95) and 1mm reduction in the solid phase region (fs = 1.0-1.1), with a reduction rate of 0.8mm / min.

[0046] After continuous casting, samples were taken and prepared according to GB / T226 and graded. In this embodiment, the slab center segregation was grade C1.0 and the center porosity was grade 1.0.

[0047] Example 3

[0048] The production process involves using 9Ni steel (9% nickel content) for cryogenic containers with a continuous casting slab size of 300mm×2000mm. The narrow-face copper plate of the crystallizer has a taper coefficient of 1.25% / m, a secondary cooling water ratio of 0.75L / kg, and a slab casting speed of 0.85m / min. A combined reduction process is implemented in the solid-liquid two-phase region and the solid phase region: 8mm reduction in the solid-liquid two-phase region (fs=0.6-0.95) and 2mm reduction in the solid phase region (fs=1.0-1.1), with a reduction rate of 0.7mm / min.

[0049] After continuous casting, samples were taken and prepared according to GB / T226 and graded. In this embodiment, the slab center segregation was grade C1.0 and the center porosity was grade 1.0.

[0050] Example 4

[0051] The production process involves using 9Ni steel (9% nickel content) for cryogenic containers with a continuous casting slab size of 250mm×2000mm. The narrow-face copper plate of the crystallizer has a taper coefficient of 1.25% / m, a secondary cooling water ratio of 0.5L / kg, and a slab casting speed of 1.0m / min. A combined reduction process is implemented in the solid-liquid two-phase region and the solid phase region: 7mm reduction in the solid-liquid two-phase region (fs=0.6-0.95) and 1.5mm reduction in the solid phase region (fs=1.0-1.1), with a reduction rate of 0.9mm / min.

[0052] After continuous casting, samples were taken and prepared according to GB / T226 and graded. In this embodiment, the slab center segregation was grade C1.0 and the center porosity was grade 1.0.

[0053] Comparative Example 1

[0054] Comparative Example 1, referring to Example 1, produced 3Ni steel (nickel content 3%) for cryogenic containers with a continuous casting slab size of 300mm×1800mm. The inverted taper coefficient of the narrow copper plate of the crystallizer was 1.15% / m, the secondary cooling water volume was 0.7L / kg, the slab drawing speed was 0.95m / min, and the pressing process was implemented in the solid-liquid two-phase region: where fs=0.6-0.95, the pressing in the solid-liquid two-phase region was 3.5mm, and the pressing rate was 0.5mm / min.

[0055] After continuous casting, samples were taken and prepared according to GB / T226 and graded. In this embodiment, the slab center segregation was grade C1.0 and the center porosity was grade 2.0.

[0056] Comparative Example 2

[0057] Comparative Example 2 is based on Example 1. The difference between Comparative Example 2 and Example 1 is that fs = 1.0-1.1 is reduced by 4 mm in the solid phase region.

[0058] After continuous casting, samples were taken and prepared according to GB / T226 and graded. In this embodiment, the slab center segregation was grade C1.5 and the center porosity was grade 1.5.

[0059] It should be noted that center segregation is classified into three categories: A, B, and C. Category A indicates severe center segregation, with the slab exhibiting a string-like connection in its low-magnification morphology. Category B indicates moderate center segregation, with the slab exhibiting a discontinuous connection in its low-magnification morphology. Category C indicates excellent center segregation, with the slab exhibiting a dotted, discontinuous morphology in its low-magnification morphology. Therefore, a category A rating indicates severe center segregation, while a category C rating indicates moderate center segregation. The grade in center segregation indicates the width of the segregation band. For example, grade 1.0 means the width of the segregation band is 1.0 mm, and similarly, grade 0.5 means the width of the segregation band is 0.5 mm. The higher the grade, the more severe the center segregation. Therefore, it can be inferred that category A (grade 1.0) center segregation is more severe than category C (grade 0.5). Similarly, a higher grade for center porosity indicates more severe center porosity.

[0060] In addition, it should be noted that the high-nickel steel slab samples were all prepared using the hot acid etching method mentioned in GB / T226. The composition of the etching solution was 1:1 (volume ratio) hydrochloric acid aqueous solution, the etching temperature was 70-80℃, and the etching time was 10min.

[0061] As can be seen from the above rating results, the center segregation of the high-nickel steel slab provided in this application embodiment is Class C 0.5 grade and the center porosity is Class C 0.5 grade. The center segregation of the high-nickel steel slab in Comparative Example 1 is Class C 1.5 grade and the center porosity is Class C 1.0 grade. Both the center segregation and center porosity are more severe than those in this application embodiment.

[0062] This invention provides a method for improving slab segregation. The method involves dynamically applying light reduction during continuous casting of high-nickel steel slabs in the fs = 1.0-1.1 range, with the reduction amount controlled at 0.5-3 mm. This allows the molten metal near the one-quarter and three-quarters mark of the slab to be fully pressed into the surrounding dendrites, achieving solute redistribution and simultaneously closing shrinkage cavities. This reduces segregation and porosity in the one-quarter and three-quarters mark areas of the high-nickel steel slab, improving its internal quality. Using the method provided by this invention, the center segregation of the high-nickel steel slab is grade C 0.5, and the center porosity is grade 0.5, resulting in good internal quality.

[0063] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0064] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for improving slab segregation, characterized in that, include: A slab with a Ni mass fraction ≥3% is subjected to dynamic pressing, wherein the pressing amount of the slab in the range of fs=0.6-0.95 is greater than the pressing amount in the range of fs=1.0-1.1; the pressing amount of the slab in the range of fs=0.6-0.95 is 4-9 mm; the pressing amount of the slab in the range of fs=1.0-1.1 is 0.5-3 mm; and the total pressing amount of the dynamic pressing is 5.5-10 mm.

2. The method for improving slab segregation according to claim 1, characterized in that, The slab is pressed down by 0.5-2 mm when fs = 1.0-1.1; and by 5-8 mm when fs = 0.6-0.

95.

3. The method for improving slab segregation according to any one of claims 1-2, characterized in that, The total reduction under dynamic compression is 6-9 mm.

4. The method for improving slab segregation according to any one of claims 1-2, characterized in that, The dynamic compression rate is 0.5-1 mm / m.

5. The method for improving slab segregation according to any one of claims 1-2, characterized in that, The width-to-thickness ratio of the slab is ≥5.

6. The method for improving slab segregation according to any one of claims 1-2, characterized in that, During the dynamic pressing process, the secondary cooling water volume is 0.3-0.8 L / kg.

7. The method for improving slab segregation according to any one of claims 1-2, characterized in that, During the dynamic pressing process, the slab drawing speed is 0.8-1.0 m / min.

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