Method for reducing internal defects of die steel, application of method and die steel

Abstract

The invention discloses a method for reducing internal defects of die steel, application of the method and the die steel, and relates to the technical field of steel production. Comprising the steps that refined molten steel is conveyed to a continuous casting machine to be continuously cast into a casting blank, and the rolling reduction between two adjacent fan-shaped sections of the continuous casting machine is 0.01-1 mm. By controlling the rolling reduction between two adjacent fan-shaped sections of a continuous casting machine to be 0.01-1mm, continuous transfer of pressure between the sections can be realized in the continuous casting process of a casting blank, and the stress uniformity of the casting blank in a transition area of the fan-shaped sections is effectively improved, so that the bulging risk of the casting blank is reduced, the internal stress concentration of the casting blank is reduced, and the casting quality of the casting blank is improved. Therefore, the defects such as cracks or shrinkage cavities in the casting blank are reduced, and the stability and consistency of the overall internal quality of the casting blank are improved.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, and more specifically, to a method for reducing internal defects in mold steel and its application, and mold steel. Background Technology

[0002] Plastic mold steel is an important material in industrial production, widely used in plastic products, medical devices, and small household appliance manufacturing. Currently, there is a significant market demand for grades such as P20 / L, 1.2311 / H, and 738 / H. However, during the production process, plastic mold steel is prone to internal voids and cracks, leading to substandard steel sheet quality upon flaw detection.

[0003] To reduce internal defects such as voids and cracks in mold steel, the compression ratio is typically increased to effectively compact these defects during rolling. However, an excessively high compression ratio implies a larger reduction, which not only limits the development of thicker plates but also reduces production efficiency and increases the difficulty of process control. Due to mill capacity constraints, the conventional compression ratio for mold steel is generally controlled between 3 and 5, within which process stability is relatively good. Under this compression ratio limitation, to produce mold steel with a thickness of 140mm or more, the casting section thickness of the continuously cast billet needs to reach over 400mm. As the section increases, internal defects in the billet become more difficult to control, thus requiring continuous casting machines to be equipped with electromagnetic stirring functions. However, configuring a continuous casting unit with electromagnetic stirring specifically for a single product category is difficult to achieve in actual production. Therefore, how to effectively reduce internal defects in the continuous casting process of thick-section mold steel without electromagnetic stirring has become a pressing technical challenge in this field.

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

[0005] The purpose of this invention is to provide a method for reducing internal defects in mold steel and its application, and mold steel.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a method for reducing internal defects in mold steel, comprising feeding refined molten steel to a continuous casting machine for continuous casting into a billet, wherein the reduction between two adjacent sector sections of the continuous casting machine is 0.01~1mm.

[0007] In an optional implementation, the sector section of the continuous casting machine corresponding to the solidification end of the billet is the main reduction section, and the reduction between two adjacent sector sections within the main reduction section is 0.03~1mm; the reduction between two adjacent sector sections within the non-main reduction section is 0.01~0.05mm.

[0008] In an optional implementation, the reduction amount of each sector segment within the non-main reduction section is 0.3~0.7mm; the reduction amount of each sector segment within the main reduction section is 1~5mm.

[0009] In an optional embodiment, the continuous casting machine is provided with 16 fan-shaped sections along the direction of billet movement, and the 9th to 11th sections are the main reduction sections. The reduction amount of the 9th section is 1~3mm, the reduction amount of the 10th section is 3~5mm, and the reduction amount of the 11th section is 1~3mm.

[0010] In an optional implementation, each sector segment has a stand, and the main power system of the continuous casting machine controls the reduction of each stand; methods for reducing internal defects in the mold steel also include reducing the opening pressure of the stand and / or increasing the total pressure of the main power system of the continuous casting machine.

[0011] In an optional implementation, the billet is further subjected to flaw detection using a flaw detector, with a flaw detection depth of 0~300mm.

[0012] Secondly, the present invention provides a mold steel, the preparation method of which includes any of the methods described in the foregoing embodiments.

[0013] Thirdly, the present invention provides the application of the method as described in any of the foregoing embodiments in the preparation of mold steel.

[0014] The present invention has the following beneficial effects: This invention provides a method for reducing internal defects in mold steel and its application. By controlling the reduction between two adjacent sector sections of the continuous casting machine to be 0.01~1mm, continuous pressure transmission between sections can be achieved during the continuous casting process, effectively improving the uniformity of stress on the casting in the transition area of ​​the sector section, thereby reducing the risk of bulging of the casting and reducing stress concentration inside the casting, thus reducing defects such as internal cracks or shrinkage cavities, and improving the stability and consistency of the overall internal quality of the casting. Attached Figure Description

[0015] 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.

[0016] Figure 1 This is a partial structural diagram of a sector segment provided in an embodiment of the present invention. Detailed Implementation

[0017] 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.

[0018] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0019] In a first aspect, the present invention provides a method for reducing internal defects in mold steel, comprising feeding refined molten steel to a continuous casting machine for continuous casting into a billet, wherein the reduction between two adjacent sector sections of the continuous casting machine is 0.01~1mm.

[0020] Understandably, the process of continuously casting molten steel into a billet in a continuous casting machine includes: after refining, the high-temperature molten steel is injected from the tundish into the crystallizer, where it initially solidifies to form a billet shell. The billet is then pulled out and enters the secondary cooling zone (i.e., the second cooling zone) of the continuous casting machine. The second cooling zone consists of multiple sector sections. Each sector section contains multiple pairs of pressure rollers. Each pair of pressure rollers includes an inner arc roller and an outer arc roller arranged vertically at intervals. These rollers support and guide the billet's movement during solidification and control the reduction amount by applying pressure to obtain the billet of the required thickness.

[0021] In the current mold steel production process, the reduction between two adjacent sector segments is 0. The thinning of the billet is mainly achieved by the pressure rollers within the sector segment. This is still applicable to thin plates with lower thickness. However, when used to produce thicker plates with higher thickness, there is no additional reduction compensation when the billet is pulled out from the previous sector segment and enters the next sector segment. This causes the billet to bulge in that area, which in turn leads to stress concentration and may cause defects such as internal cracks or shrinkage cavities.

[0022] This invention introduces a small amount of reduction between adjacent sector segments, ensuring continuous and gradual pressure on the billet throughout the casting path. This avoids localized expansion caused by insufficient reduction between segments, significantly reducing the risk of bulging and improving the internal density of the billet. The optimized reduction range ensures sufficient feeding pressure transmission while preventing excessive reduction from causing overloading of the roller system or billet deformation and instability. Therefore, by achieving continuous and uniform reduction between segments, this invention effectively reduces the formation of defects such as internal voids and shrinkage cavities in the mold steel, improving the overall stability and consistency of the billet's internal quality.

[0023] Please refer to Figure 1It should be noted that in some embodiments, each sector has seven pairs of pressure rollers, arranged sequentially along the direction of billet movement as the first pair, second pair, third pair, fourth pair, fifth pair, sixth pair, and seventh pair. The reduction between two adjacent sector segments in the continuous casting machine is the reduction between the seventh pair of pressure rollers in the first sector segment and the first pair of pressure rollers in the second sector segment. This principle applies to other positions as well. Even when the number of pressure rollers within a sector segment changes, the reduction between two adjacent sector segments in the continuous casting machine can still be deduced based on the above principle.

[0024] In an optional implementation, the sector section of the continuous casting machine corresponding to the solidification end of the billet is the main reduction section, and the reduction between two adjacent sector sections within the main reduction section is 0.03~1mm; the reduction between two adjacent sector sections within the non-main reduction section is 0.01~0.05mm.

[0025] During continuous casting, after the billet is pulled from the mold, its outer layer cools and solidifies first, while the interior remains liquid or semi-solid. As the billet moves forward, the liquid core gradually shortens and eventually solidifies completely. The solidification end of the billet refers to the region in the center of the billet where the liquid phase is about to completely transform into the solid phase. Since the fan-shaped section is the key area where the billet moves and gradually cools, the solidification end of the billet corresponds to at least one fan-shaped section of the continuous casting machine, meaning that within this fan-shaped section, the central liquid phase of the billet completely transforms into the solid phase.

[0026] This area is the key location for reduction. By applying appropriate pressure to this area, the residual molten steel can be encouraged to fill the shrinkage gaps in the solidified area, thereby reducing defects such as central shrinkage cavities and porosity. Therefore, the fan-shaped section containing this area is called the main reduction section.

[0027] By increasing the reduction amount between two adjacent sector sections within the main reduction section, the bulging phenomenon of the billet can be avoided, and internal defects of the billet can be reduced.

[0028] In an optional implementation, the reduction amount of each sector segment within the non-main reduction section is 0.3~0.7mm; the reduction amount of each sector segment within the main reduction section is 1~5mm.

[0029] By increasing the reduction amount of each sector within the main reduction section, sufficient compressive force can be provided in the critical solidification area, promoting the flow of residual liquid to the incompletely solidified area and reducing shrinkage cavities and sand holes caused by insufficient feeding. Simultaneously, the reduction amount in the non-main reduction section is controlled within the range of 0.3~0.7mm, which maintains the stability of the billet shape, avoids excessive deformation stress, and coordinates with the small reduction amounts between adjacent sector sections to ensure the continuity and uniformity of the overall reduction process, effectively suppressing internal defects in the mold steel.

[0030] In an optional embodiment, the continuous casting machine is provided with 16 fan-shaped sections along the direction of billet movement, and the 9th to 11th sections are the main reduction sections. The reduction amount of the 9th section is 1~3mm, the reduction amount of the 10th section is 3~5mm, and the reduction amount of the 11th section is 1~3mm.

[0031] During the later stages of solidification, the central region of the billet still contains incompletely solidified liquid phase. Insufficient feeding can easily lead to internal defects such as shrinkage cavities and sand holes. The main reduction section is located in sections 9 to 11, corresponding to the critical feeding area at the end of the billet's solidification process. By applying a relatively large reduction in this range, especially in section 10 which provides a maximum reduction of 3-5 mm as the peak reduction section, sufficient compressive stress can be generated in the weakest solidification area, promoting the flow of residual liquid into the dendrite interstices and achieving adequate feeding. Simultaneously, sections 9 and 11 use reductions of 1-3 mm as transitions, forming a "low-high-low" reduction gradient. This avoids strain concentration or billet damage caused by sudden pressure changes, ensuring smooth and continuous pressure transmission. This reduction layout combines the actual needs of the billet solidification process with equipment control capabilities, concentrating force at critical locations while maintaining process stability.

[0032] In an alternative implementation, each sector segment has a frame, and the main power system of the continuous casting machine controls the movement of each frame to control the amount of reduction.

[0033] Methods to reduce internal defects in mold steel also include reducing the opening pressure of the stand and / or increasing the total pressure of the main power system of the continuous casting machine.

[0034] Currently, in actual production, there is a problem with the main pressing section not pressing down completely, meaning the set pressing amount is not fully achieved, and there is a deviation of 0~1.5mm between the actual pressing amount and the target value. The main reason is the limited capacity of the main machine power system, especially the excessively high opening pressure of the frame, which consumes the hydraulic system pressure resources available for pressing.

[0035] For example, in the actual production process of smelting 415mm thick mold steel XF2311, when inspecting the final product with an ultrasonic flaw detector, a point defect was found at a position 16cm away from the inner arc. After calculation using the solidification law, the location of the defect was determined to be the 6th sector segment. However, no abnormality was found in the pressure roller of the 6th sector segment. Therefore, the roller gap opening of the pressure roller of the 6th sector segment was calibrated. It was found that the exit calibration value of the 6th sector segment deviated from the actual handheld roller gap meter by 0.6mm. After recalibration, production continued, and no defect was found again at that position.

[0036] This invention reduces the opening pressure of the frame, allowing more available pressure to be used for the pressing action without changing the total system pressure, thereby improving the pressing response accuracy and execution force. At the same time, by increasing the total pressure of the main power system of the continuous casting machine, the system's ability to drive the frame to apply pressure can be directly enhanced, making the actual pressing amount in key areas such as the main pressing section closer to the set value, achieving sufficient and stable compression and feeding of the billet, and reducing internal defects such as shrinkage cavities and sand holes caused by insufficient feeding.

[0037] This invention improves the accuracy and reliability of pressing control by optimizing the pressure distribution and output capability of the main power system, thereby significantly improving the internal density and overall quality stability of the mold steel billet.

[0038] In an optional implementation, the billet is further subjected to flaw detection using a flaw detector, with a flaw detection depth of 0~300mm.

[0039] Current flaw detection primarily operates on the final mold steel product, making it difficult to pinpoint the process responsible for defects. This invention performs flaw detection on the cast billet after continuous casting, enabling early, comprehensive, and in-depth inspection of the billet's internal quality, thereby improving the timeliness and accuracy of defect identification.

[0040] In addition, existing flaw detection methods mostly involve cutting a section of the sample, observing the microstructure after surface treatment such as pickling. This method can only reflect the quality status of a single cross section and cannot reflect the continuous defect distribution inside the billet. It is not representative enough and it is difficult to detect hidden dangers such as shrinkage cavities and cracks that are deep or discretely distributed.

[0041] This invention employs a flaw detector to perform non-destructive testing on the smooth surface, with a detection depth of 0-300mm, covering the entire thickness range from the inner and outer arcs to the central region. This effectively detects internal point defects, continuous cracks, shrinkage cavities, and other abnormal structures that are difficult to detect using traditional pickling methods. By performing a depth scan of the entire cast billet, not only is the defect detection rate improved, but the representativeness and reliability of the test results are also enhanced. More importantly, by combining the distance information of the defect from the inner and outer arcs and applying the solidification law, the location of the fan-shaped segment where the defect occurred can be calculated backwards, providing precise feedback for adjusting process parameters.

[0042] Secondly, the present invention provides a mold steel, the preparation method of which includes any of the methods described in the foregoing embodiments.

[0043] Thirdly, the present invention provides the application of the method as described in any of the foregoing embodiments in the preparation of mold steel.

[0044] Example 1 This embodiment provides a method for reducing internal defects in mold steel, including feeding refined molten steel into a 415mm extra-thick slab continuous casting machine to cast it into a billet. The pressure of the main power system of the continuous casting machine is 2250KN.

[0045] The secondary cooling zone of the continuous casting machine is arranged in 16 fan-shaped sections along the direction of billet movement. Sections 9 to 11 are the main reduction sections, with a reduction of 2 mm for section 9, 4 mm for section 10, and 2 mm for section 11. The remaining sections 1 to 8 and 12 to 16 are non-main reduction sections, with a reduction of 0.5 mm for each section.

[0046] In this embodiment, among the 16 sector segments, the inter-segment compression amount between the 9th and 10th segments is 0.3mm, and the inter-segment compression amount between the 10th and 11th segments is 0.4mm; the compression amount between the remaining sector segments all satisfy the requirement that the compression amount between two adjacent sector segments is 0.1mm.

[0047] Please refer to Figure 1 In this embodiment, each sector has seven pairs of pressure rollers, arranged sequentially along the direction of billet movement: the first pair of pressure rollers, the second pair of pressure rollers, the third pair of pressure rollers, the fourth pair of pressure rollers, the fifth pair of pressure rollers, the sixth pair of pressure rollers, and the seventh pair of pressure rollers. The reduction between two adjacent sector segments in the continuous casting machine is the reduction between the seventh pair of pressure rollers in the first sector segment and the first pair of pressure rollers in the second sector segment, and so on for other positions.

[0048] In addition, each sector segment in this embodiment has a frame. The main power system of the continuous casting machine controls the movement of each frame to control the reduction amount. Specifically, the reduction amount and frame opening pressure of each sector segment are shown in Table 1.

[0049] Table 1 Parameters of the sector segment

[0050] Using the continuous casting parameters in Table 1 above, three tundishes were used to smelt 415mm thick mold steel XF2311. After rolling, the flaw detection pass rate of 160~180mm steel plates reached 95%.

[0051] Using the traditional method, the pressing amount of each sector segment and the frame opening pressure are shown in Table 2.

[0052] Table 2 Parameters of the sector segment

[0053] Using the continuous casting parameters in Table 2 above, 415mm thick mold steel XF2311 was prepared after three months of smelting. After being rolled by a rolling mill, the 160-180mm steel plate was inspected by a flaw detector. Multiple sand holes or pinholes exceeding the φ3 standard were found inside the steel plate, and the flaw detection pass rate was less than 70%.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the 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 for reducing internal defects in mold steel, characterized in that, This includes sending refined molten steel to a continuous casting machine for continuous casting into billets, with a reduction of 0.01~1mm between two adjacent sector sections of the continuous casting machine.

2. The method according to claim 1, characterized in that, The sector section area of ​​the continuous casting machine corresponding to the solidification end of the billet is the main reduction section. The reduction between two adjacent sector sections in the main reduction section is 0.03~1mm; the reduction between two adjacent sector sections in the non-main reduction section is 0.01~0.05mm.

3. The method according to claim 2, characterized in that, The reduction amount of each sector segment in the non-main reduction section is 0.3~0.7mm; the reduction amount of each sector segment in the main reduction section is 1~5mm.

4. The method according to claim 2, characterized in that, The continuous casting machine has 16 fan-shaped sections arranged sequentially along the direction of billet movement, and the 9th to 11th sections are the main reduction sections. The reduction amount of the 9th section is 1~3mm, the reduction amount of the 10th section is 3~5mm, and the reduction amount of the 11th section is 1~3mm.

5. The method according to claim 1, characterized in that, Each sector has a stand, and the main power system of the continuous casting machine controls the reduction of each stand; methods to reduce internal defects in the mold steel also include reducing the opening pressure of the stand and / or increasing the total pressure of the main power system of the continuous casting machine.

6. The method according to claim 1, characterized in that, It also includes using a flaw detector to inspect the billet, wherein the flaw detector has a detection depth of 0~300mm.

7. A mold steel, characterized in that, The preparation method includes the method described in any one of claims 1 to 6.

8. The application of the method as described in any one of claims 1 to 6 in the preparation of mold steel.

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