A manufacturing method for solving the surface quality of high-silicon low-manganese pc steel bar steel

Abstract

The application discloses a manufacturing method for solving the surface quality of high-silicon low-manganese PC steel rod steel, in the process of smelting PC steel rod steel, according to the change of the ambient temperature, the secondary cooling water temperature of continuous casting is adjusted, the secondary cooling water temperature is set to 35-40 DEG C; in production, the specific water quantity is determined according to the steel grade characteristics, the automatic water distribution is adopted in continuous casting, the total water quantity of the secondary cooling changes with the drawing speed, when the drawing speed is 2.1 m / min, the specific water quantity is 1.31 L / kg; when the drawing speed is 2.2 m / min, the specific water quantity is 1.33 L / kg; when the drawing speed is 2.4 m / min, the specific water quantity is 1.41 L / kg; the operation mode of the spray number after the end part of the billet is cooled by water spraying at the offline cooling bed is changed into the spray number operation after the billet is naturally cooled. Through the above technical scheme, the probability of the subcutaneous crack defect of the casting blank is reduced, the wire rod surface quality is improved, and the influence on the quality of the subsequent prestressed steel wire is reduced.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, specifically relating to a manufacturing method for improving the surface quality of high-silicon, low-manganese PC steel bars. Background Technology

[0002] PC700 prestressed steel bars are a specially customized product of Yingkou Northern Company, replacing 30MnSi for the manufacture of PC steel bars for pipe piles. The original high-Mn, low-Si steel of 30MnSi was adjusted to high-Si, low-Mn steel of PC700; by increasing the online tempering temperature, the occurrence of delayed brittle fracture was greatly reduced. However, customers reported surface defects in some PC steel bars, such as... Figure 5 As shown.

[0003] To minimize the impact of wire rod surface defects on the quality of subsequent prestressed PC steel wire, the company's rolling mill has strengthened its supervision of wire rod surface quality and traced the production process to find the root cause of the problem. Currently, the rolling process of this product has resulted in scabbing on the surface of 99 round billets and coiled wire rods after rough rolling. Analysis of defective samples revealed that the scabbing defect in the wire rod was caused by subsurface cracks in the cast billet. These subsurface cracks were caused by thermal stress resulting from uneven cooling and a large temperature rise when the cast billet passed through the secondary cooling zone. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention adjusts the temperature and specific water volume of the secondary cooling water in continuous casting according to changes in ambient temperature during the smelting of PC steel bars. It also changes the operation method of spraying water to cool the billet end on the cooling bed after it leaves the production line to spraying the billet after it has cooled naturally. These measures reduce the probability of subcutaneous crack defects in the billet, improve the surface quality of the wire rod, and minimize the impact on the quality of subsequent prestressed steel wire.

[0005] To achieve the above-mentioned objective, this invention provides a manufacturing method for improving the surface quality of high-silicon, low-manganese PC steel bars. The method includes the following steps: hot metal pretreatment, converter smelting, LF refining, and continuous casting; wherein the secondary cooling water temperature of the continuous casting is set to 35℃~40℃; the continuous casting adopts automatic water distribution, and the total secondary cooling water volume varies with the casting speed.

[0006] "Specific water volume" refers to the amount of cooling water used per unit weight of molten steel, expressed in L / kg. Once the cooling water distribution ratio of each section of the secondary cooling and the steel casting rate are determined, the cooling water volume of each cooling zone can be calculated. Different continuous casting machines have different specific water volume values.

[0007] The specific water content is determined based on the steel grade characteristics. This product has a carbon content of 0.30~0.35%, classifying it as medium carbon steel. Initially, conventional low-carbon steel cooling methods are used, i.e., strong cooling to increase the solidification coefficient and shorten the solidification time. Later, due to the product's high susceptibility to cracking, weak cooling is employed to suppress columnar crystal growth and increase the equiaxed crystal area ratio on the cross-section. The coefficient K is a function related to the casting cross-section and specific water content; K values ​​are set as shown in Table 1. Table 1. Specific values ​​set for the K value

[0008] The specific water volume of the secondary cooling varies with the pulling speed. When the pulling speed is 2.1 m / min, the specific water volume is 1.31 L / kg; when the pulling speed is 2.2 m / min, the specific water volume is 1.33 L / kg; and when the pulling speed is 2.4 m / min, the specific water volume is 1.41 L / kg.

[0009] The marking operation method on the cooling bed has been changed from spraying water to cool the billet end before marking to allowing the billet end to cool naturally before marking.

[0010] In the above technical solution, further, the molten iron pretreatment process is as follows: if the molten iron S≤0.050%, no molten iron pretreatment is performed; if the raw molten iron S>0.050%, passivating magnesium powder and lime powder are sprayed into the molten iron ladle for desulfurization, and the desulfurization slag in the ladle is removed to ensure that the molten iron S≤0.050%.

[0011] Further, in converter smelting: to stabilize the charge, the scrap steel ratio is 10-20%, and the molten iron ratio is 80-90%; oxygen is blown from both the top and bottom; the tapping temperature is 1630-1660℃; the final tapping concentration (C) is 0.18-0.24%; the target per-cycle charge (P) is ≤0.015%; the auxiliary materials added per ton of steel are: 27-35 kg / t of active lime, 5.9-7.9 kg / t of dolomite, and 10-15 kg / t of ore. For ladle alloying: lime and ferroalloys are added for deoxidation and alloying when the ladle is 1 / 4 to 1 / 3 full; argon is blown throughout the tapping process; the target alloy addition per ton of steel is: 10-11.5 kg / t of ferrosilicon and 13.4-14.9 kg / t of ferrosilicon. Ensure the final carbon content and temperature are achieved on the first attempt, and that the ladle and alloy are thoroughly baked. Pre-purge the ladle with argon for 3-4 minutes, prevent slag from falling into the ladle, and maintain a circular flow during tapping to prevent molten steel from spreading. Tapping time should be controlled at 4-6 minutes. Static argon blowing time should be ≥5 minutes. Pre-treatment temperature should be 1570℃~1610℃, and post-treatment temperature should be 1540℃~1580℃.

[0012] Further, LF furnace refining requires white slag operation, controlling slag basicity and oxygen potential while ensuring slag fluidity; continuous monitoring of molten steel and fine-tuning of alloys to ensure the steel composition meets standard requirements; LF refining time is 60-80 minutes, and the full analysis temperature is 1560℃-1580℃. Auxiliary material addition per ton of steel: active lime 4-6.6 kg / t, fluorite 0.40-0.60 kg / t, carbonized rice husk 0.35-0.5 kg / t, manganese silicon 1.0-1.4 kg / t, low-calcium ferrosilicon 3.3-6.2 kg / t. Continuous monitoring of molten steel and fine-tuning of alloys are also required to ensure the steel composition meets standard requirements, and soft blowing time is ≥15 minutes.

[0013] Furthermore, in continuous casting: the electromagnetic stirring current in the crystallizer is 260~300A, frequency 4~6Hz, rotating in both directions; the electromagnetic stirring current at the end is 230~270A, frequency 6~10Hz, rotating continuously. The platform temperature is 1575±5℃, the tundish temperature is 1525℃~1550℃, low superheat casting is used, with superheat maintained at 20℃~35℃, and the casting speed is 2.1~2.4 m / min. Full-process protective casting is employed, using medium carbon steel protective slag from square billets in the crystallizer; billet cutting is done using a combination of automatic and manual methods.

[0014] A high-silicon, low-manganese PC steel bar steel manufactured by the above method, wherein the chemical composition of the PC steel bar steel, by mass percentage, is: C 0.30~0.35%; Si 1.20~1.40%; Mn 0.70~1.00%; P≤0.025%; S≤0.025%; Cr≤0.25%; Ni≤0.25%; Cu≤0.20%, with the balance being Fe and unavoidable impurities.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: In the process of smelting PC steel bars, this invention adjusts the temperature and specific water volume of the secondary cooling water in continuous casting according to changes in ambient temperature; and changes the operation method of spraying water to cool the end of the billet off the cooling bed and then spraying the numbering operation to the billet after natural cooling. These measures reduce the probability of subcutaneous crack defects in the billet, improve the surface quality of the wire rod, and reduce the impact on the quality of subsequent prestressed steel wire. Attached Figure Description

[0016] Figure 1 Low-magnification micrograph of the steel sample used for PC steel bars prepared for the example; Figure 2 Micrographs of steel samples used for making PC steel bars for the example; Figure 3 Images showing the surface quality of PC700 wire rods for PC steel bars manufactured according to the technical solution of this invention; Figure 4Image showing the surface quality of PC700 wire rods for PC steel bars manufactured using the existing technical solution; Figure 5 Images showing surface defects in PC steel bars manufactured using existing technology. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments, but this does not limit the invention in any way. To avoid redundancy, unless otherwise specified, the raw materials used in the following embodiments are all commercially available products, and the methods used are all conventional methods unless otherwise specified.

[0018] The production process of PC700 prestressed steel bar wire rod in the embodiment includes: blast furnace molten iron → molten iron pretreatment (desulfurization, slag removal) → 120t converter (top and bottom blowing) → ladle bottom blowing argon → LF furnace refining → 150 150mm square billet continuous casting (crystallizer electromagnetic stirring and solidification end electromagnetic stirring) → billet slow cooling → walking beam furnace heating → wire rod rolling → wire cutting → inspection → coiling → heat preservation channel slow cooling → sampling, inspection, and judgment → packaging and delivery.

[0019] Example A manufacturing method for improving the surface quality of high-silicon, low-manganese PC steel bars, the method comprising the following steps: hot metal pretreatment, converter smelting, LF refining, and continuous casting.

[0020] Hot metal pretreatment process: If the hot metal S≤0.050%, no hot metal pretreatment is required; if the raw hot metal S>0.050%, passivating magnesium powder and lime powder are sprayed into the hot metal ladle for desulfurization, and the desulfurization slag in the ladle is removed to ensure that the hot metal S≤0.050%.

[0021] Converter smelting: To stabilize the charge, the scrap steel ratio is 15%, and the molten iron ratio is 85%; oxygen is blown from both the top and bottom; the tapping temperature is 1630~1660℃; the final tapping concentration (C) is 0.18~0.24%; the target per-cycle charge (P) is ≤0.015%; the auxiliary materials added per ton of steel are: 27~35 kg / t of active lime, 5.9~7.9 kg / t of dolomite, and 10~15 kg / t of ore. Ladle alloying: Lime and ferroalloys are added for deoxidation and alloying when the steel is 1 / 4 to 1 / 3 full; argon is blown throughout the tapping process; the target alloy addition per ton of steel is: 10~11.5 kg / t of ferrosilicon and 13.4~14.9 kg / t of ferrosilicon. Ensure the final carbon content and temperature are achieved on the first attempt, and that the ladle and alloy are thoroughly baked. Pre-purge the ladle with argon for 3-4 minutes, prevent slag from falling into the ladle, and maintain a circular flow during tapping to prevent molten steel from spreading. Tapping time should be controlled at 4-6 minutes. Static argon blowing time should be ≥5 minutes. Pre-treatment temperature should be 1570℃~1610℃, and post-treatment temperature should be 1540℃~1580℃.

[0022] LF furnace refining: White slag operation is required. While ensuring slag fluidity, the slag basicity and oxygen potential must be controlled. The molten steel must be continuously monitored and the alloy composition fine-tuned to ensure it meets standard requirements. LF refining time is 60-80 minutes, and the full analysis temperature is 1560℃-1580℃. Auxiliary material addition per ton of steel: active lime 4-6.6 kg / t, fluorite 0.40-0.60 kg / t, carbonized rice husk 0.35-0.5 kg / t, manganese silicon 1.0-1.4 kg / t, low-calcium ferrosilicon 3.3-6.2 kg / t. Soft blowing time is ≥15 minutes.

[0023] Continuous casting: The electromagnetic stirring current in the crystallizer is 260~300A, frequency 4~6Hz, rotating in both directions; the electromagnetic stirring current at the end is 230~270A, frequency 6~10Hz, rotating continuously. The platform temperature is 1575±5℃, the tundish temperature is 1525℃~1550℃, low superheat casting is used, and the superheat is maintained at 20℃~35℃. The casting speed is 2.1~2.4 m / min. Full-process protective casting is employed, using medium carbon steel protective slag for the crystallizer; billet cutting is done using a combination of automatic and manual methods.

[0024] The secondary cooling water temperature for continuous casting is set at 35℃~40℃; the continuous casting adopts automatic water distribution, and the total amount of secondary cooling water varies with the casting speed.

[0025] The specific water content is determined based on the characteristics of the steel grade and different continuous casting speeds. The specific water content of the technical scheme before and after adjustment is shown in Table 2.

[0026] Table 2. Set values ​​for specific water content

[0027] The marking operation method on the cooling bed has been changed from spraying water to cool the billet end before marking to allowing the billet end to cool naturally before marking.

[0028] The prestressed steel bar wire rod PC700 prepared in the examples has the following chemical composition, depending on the smelting number, as shown in Table 3. The remainder consists of Fe and unavoidable impurities. Micrographs of the PC steel bar samples prepared in the examples are shown below. Figure 1-2 As shown, the quality is good; Figure 3 This is a surface quality image of the PC steel bar manufactured using the technical solution of this invention. Figure 4 The images show the surface quality of PC steel bars produced using existing technologies, illustrating that the present invention overcomes the surface defects caused by the prior art.

[0029] Table 3 Chemical composition of finished products prepared by different smelting batches in the examples

[0030] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A manufacturing method for improving the surface quality of high-silicon, low-manganese PC steel bars, characterized in that, The method includes the following steps: hot metal pretreatment, converter smelting, LF refining, and continuous casting of 150mm. A 150mm square billet; wherein, the secondary cooling water temperature of the continuous casting is set to 35℃~40℃; the specific water volume of the secondary cooling in the continuous casting varies with the casting speed: when the casting speed is 2.1 m / min, the specific water volume is 1.31 L / kg; when the casting speed is 2.2 m / min, the specific water volume is 1.33 L / kg; when the casting speed is 2.4 m / min, the specific water volume is 1.41 L / kg; The aforementioned molten iron pretreatment process requires that the sulfur content (S) in the molten iron be ≤ 0.050%. The converter smelting process is as follows: scrap steel ratio 10-20%, molten iron ratio 80-90%; oxygen top and bottom blowing, tapping temperature 1630-1660℃, tapping endpoint C 0.18-0.24%; static argon blowing time ≥5 min, pre-treatment temperature 1570℃-1610℃, post-treatment temperature 1540℃-1580℃. The LF refining process includes: refining time of 60-80 min and full analysis temperature of 1560℃-1580℃; auxiliary material addition per ton of steel: active lime 4-6.6 kg / t, fluorite 0.40-0.60 kg / t, carbonized rice husk 0.35-0.5 kg / t, manganese silicon 1.0-1.4 kg / t, and low-calcium ferrosilicon 3.3-6.2 kg / t. The continuous casting process is as follows: the electromagnetic stirring current in the crystallizer is 260~300A, the frequency is 4~6 Hz, and it rotates in both directions; the electromagnetic stirring current at the end is 230~270 A, the frequency is 6~10 Hz, and it rotates continuously; the platform temperature is 1575±5℃, the tundish temperature is 1525℃~1550℃, low superheat casting is adopted, and the superheat is maintained at 20℃~35℃; the casting speed is 2.1~2.4 m / min. The chemical composition of the steel used for PC steel bars, by mass percentage, is: C 0.30~0.35%; Si 1.20~1.40%; Mn 0.70~1.00%; P≤0.025%; S≤0.025%; Cr≤0.25%; Ni≤0.25%; Cu≤0.20%, with the balance being Fe and unavoidable impurities.

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