Method for eliminating surface cracks of round steel for Ni-free corrosion-resistant pipe

Abstract

The application discloses a method for eliminating surface cracks of round steel for Ni-free corrosion-resistant pipes and belongs to the field of steel production. The method for eliminating surface cracks of round steel for Ni-free corrosion-resistant pipes provided by the application adopts a BOF or EAF smelting-LF refining-VD vacuum degassing-continuous casting-steel billet pit cooling-heating rolling process, the composition of molten steel is designed, the continuous casting process conditions are controlled, the enrichment and precipitation of low-melting-point elements Cu and Sb at the grain boundary in the cooling process of the casting blank are inhibited, and the surface quality of the casting blank is improved; the heating rolling process conditions are controlled, the temperature region with the strongest Cu and Sb permeability is avoided, and the secondary enrichment of low-melting-point elements such as Cu and Sb along the austenite grain boundary in the heating process is relieved. The method greatly improves the surface cracks of the round steel for Ni-free corrosion-resistant pipes and can reduce the flaw detection and grinding costs.

Description

Technical Field

[0001] This invention relates to the field of steel preparation, and specifically to a method for eliminating surface cracks in round steel for Ni-free corrosion-resistant pipes. Background Technology

[0002] To enhance the acid corrosion resistance of 09CrCuSb round steel for corrosion-resistant pipes, appropriate amounts of alloying elements such as Cu, Cr, and Sb are added during the steelmaking process, in addition to conventional alloying elements (C, Si, Mn, P, S). The high content of Cu and Sb in this steel grade leads to a greater tendency for segregation during crystallization, causing grain boundary embrittlement. The carbon content of this steel grade is located in the peritectic reaction zone, resulting in significant shrinkage of the billet shell in the crystallizer. This easily causes uneven heat flow between the crystallizer wall and the billet shell, leading to uneven solidification and cracking of the cast billet. The surface cracks of the continuously cast billet expand with rolling, and the penetration and enrichment of low-melting-point elements such as Cu and Sb along the austenite grain boundaries during the heating process further embrittles the grain boundaries, resulting in surface cracks in the round steel.

[0003] To improve surface cracks in round steel, the nickel content in the molten steel is usually increased to suppress copper embrittlement. However, adding a large amount of nickel not only increases costs but also degrades the corrosion resistance of the round steel.

[0004] Therefore, it is necessary to take corresponding measures to improve the surface quality of the hot-rolled steel and reduce the occurrence of cracks. Summary of the Invention

[0005] The purpose of this invention is to provide a method for eliminating surface cracks in Ni-free corrosion-resistant round steel pipes, thus solving the problem that 09CrCuSb round steel is prone to surface cracks.

[0006] This invention is achieved through the following technical solution:

[0007] This invention provides a method for eliminating surface cracks in round steel bars used for Ni-free corrosion-resistant pipes, the method comprising:

[0008] BOF or EAF smelting: The steel composition in the smelting process, by weight percentage, includes: C: 0.06-0.09%, Si: 0.30-0.37%, Mn: 0.45-0.55%, P≤0.020%, S≤0.006%, Cr: 0.85-0.95%, Ni≤0.10%, Cu: 0.26-0.32%, Alt: 0.005-0.030%, Sb: 0.045%-0.075%, Ti: 0.030-0.045%, N≤0.006%, with the balance being iron and unavoidable impurity elements;

[0009] LF refining: Bottom-blown argon gas stirring is used, and after submerged arc stabilization, the slag fluidity is adjusted and deoxidation is performed;

[0010] VD vacuum degassing: High vacuum degree ≤67Pa, holding time ≥10min;

[0011] Continuous casting: Low superheat casting is adopted, with the superheat of continuous casting between 14-22℃; the crystallizer vibration adopts high frequency and small amplitude, with a frequency of 120-140HZ and an amplitude control of ±2.5mm; the electromagnetic stirring current of the crystallizer is controlled at 250-300A.

[0012] The cooling water for the crystallizer is set as follows: the primary cooling water flow rate is 110-115 m³ / h. 3 / h, the inlet temperature of the primary cooling water is 30±3℃, the temperature difference between the primary and secondary cooling water is 8-10℃, the continuous casting speed is 1.1±0.5m / min, and the specific water volume is set to 0.27-0.30L / kg;

[0013] Pit cooling of steel billets: Pit cooling furnace entry temperature 500-580℃; holding time ≥24 hours, temperature ≤100℃ when exiting the pit;

[0014] Heating and rolling: Under a reducing atmosphere, a high-temperature rapid heating regime is adopted, with a soaking furnace temperature of 1180-1220℃ and a total heating time of 120-180min; the initial rolling temperature is 1050-1070℃.

[0015] Furthermore, in the BOF or EAF smelting process, the blowing endpoint is controlled, with the tapping endpoint C ≤ 0.04% and the tapping endpoint P ≤ 0.012%; the tapping temperature ≥ 1600℃.

[0016] Furthermore, in the BOF or EAF smelting process, the order of auxiliary materials for steelmaking is: aluminum ingots, alloys, calcium aluminate, and lime.

[0017] Furthermore, in the LF refining process, aluminum briquettes and calcium carbide are added in the early stage of deoxidation treatment, and ferrosilicon powder, calcium carbide and silicon carbide are added in the later stage; the addition is done in batches and multiple times, with a total usage of ≥100kg / furnace.

[0018] Furthermore, in the LF refining process, Ti iron is added 10-20 minutes before the end of refining.

[0019] Furthermore, in the VD vacuum degassing process, calcium treatment is carried out 5-10 minutes before leaving the station. 20m of pure calcium wire is fed into the start-up furnace, and 1-20m of pure calcium wire is fed into the remaining furnaces. The soft blowing time is ≥20 minutes.

[0020] Furthermore, in the continuous casting process, a protective slag with a viscosity of 0.7-1.0 Pas and a melting point of 1160±20℃ is used.

[0021] Furthermore, in the continuous casting process, the final electromagnetic stirring current is 300-400A and the frequency is 10HZ.

[0022] Furthermore, during the heated rolling process, the air-fuel ratio is controlled at 0.85-0.95.

[0023] Furthermore, in the heating and rolling process, after rolling, the round steel is stacked in a sheltered place to cool. After 12 hours of cooling and at a temperature below 100°C, it is transferred.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] The method for eliminating surface cracks in Ni-free corrosion-resistant round steel tubing provided by this invention improves the surface quality of the billet by designing the steel composition and controlling the continuous casting process conditions to suppress the enrichment and precipitation of low-melting-point elements Cu and Sb at grain boundaries during the billet cooling process. Furthermore, by controlling the heating and rolling process conditions, the method avoids the temperature range where Cu and Sb have the strongest penetrating power, thus mitigating the secondary enrichment of low-melting-point elements such as Cu and Sb along the austenite grain boundaries during heating. This method significantly improves the surface crack reduction of Ni-free corrosion-resistant round steel tubing and can reduce the costs of flaw detection and grinding.

[0026] The method for eliminating surface cracks in Ni-free corrosion-resistant round steel provided by this invention effectively controls surface cracks in Ni-free weather-resistant round steel, eliminating the need for flaw detection and grinding processes for cast billets and round steel. Based on a flaw detection cost of 100 yuan / ton of steel for cast billets and a grinding cost of 100 yuan / ton of steel for cast billets, and with an annual output of 20,000 tons, this method can save 4 million yuan annually, demonstrating high application potential. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0028] Figure 1 The flaw detection results of the round steel obtained in Example 2 of this invention;

[0029] Figure 2 The flaw detection results are shown for the round steel prepared in Comparative Example 1 of this invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and accompanying drawings. The illustrative embodiments and descriptions of this invention are for explanation only and are not intended to limit the invention. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0031] The technical solution of this invention is as follows:

[0032] A method for eliminating surface cracks in Ni-free corrosion-resistant round steel pipes, the method comprising:

[0033] Step (1) BOF or EAF smelting: The performance and manufacturability of Ni-free corrosion-resistant steel pipe need to be considered, including atmospheric corrosion resistance, strength and crack sensitivity factors. Therefore, the steel composition in the smelting process includes the following by weight percentage: C: 0.06-0.09%, Si: 0.30-0.37%, Mn: 0.45-0.55%, P≤0.020%, S≤0.006%, Cr: 0.85-0.95%, Ni≤0.10%, Cu: 0.26-0.32%, Alt: 0.005-0.030%, Sb: 0.045%-0.075%, Ti: 0.030-0.045%, N≤0.006%, with the balance being iron and unavoidable impurity elements.

[0034] Among them, the blowing endpoint control is as follows: the tapping endpoint C ≤ 0.04% and the tapping endpoint P ≤ 0.012%; the tapping temperature ≥ 1600℃;

[0035] The order of auxiliary materials for steelmaking is as follows: aluminum ingots → alloys → calcium aluminate 200-300kg → lime 400-500kg.

[0036] The steel composition used in this invention has the following effects:

[0037] Carbon (C) is an essential element for ensuring the strength of steel. Too low a C content results in insufficient strength, while excessively high C content is detrimental to the ductility and toughness of steel and also reduces its corrosion resistance. This invention controls the C content to 0.06-0.09% to ensure material performance while reducing peritectic reactions and lowering surface crack sensitivity.

[0038] Si (Si) is a deoxidizing element in steel, improving its strength through solid solution strengthening and also enhancing its corrosion resistance. When the Si content is below 0.10%, the deoxidizing effect is poor, while higher Si content reduces toughness. In this invention, the Si content is controlled at 0.30-0.37%.

[0039] Mn is a strengthening element in steel, playing a role in solid solution strengthening to compensate for the strength loss caused by the decrease in carbon content. However, excessively high Mn content leads to poor corrosion resistance. Therefore, the Mn content in this invention is controlled at 0.45-0.55%.

[0040] Cu (Cu) is a fundamental element for improving corrosion resistance and can promote anodic passivation in steel, thereby reducing the corrosion rate. Cu enrichment in rust layers can significantly improve their protective properties; however, excessive Cu content can lead to copper embrittlement. In this invention, the Cu content is controlled at 0.26-0.32%.

[0041] Cr: It has a passivation tendency and, when used in combination with Cu and Si elements in steel, can significantly improve the corrosion resistance of steel. In this invention, the Cr content is controlled at 0.85-0.95%.

[0042] The addition of Sb increases the hardness and corrosion resistance of steel, but also increases its brittleness. The Sb content is controlled at 0.045-0.075%.

[0043] Ni: As a precious metal, its cost is too high, so it is not added intentionally during smelting and its content is controlled to be below 0.10%.

[0044] Alt (Al) plays a role in nitrogen fixation and deoxidation, and also helps to form a passivation film on the steel surface, thereby improving sulfuric acid resistance. The AlN formed by the combination of Al and N can effectively refine the grains, but excessive content will impair the toughness of the steel and worsen its hot workability. This invention controls its content (Al) within the range of 0.005-0.03%.

[0045] Ti (Ti) helps refine the microstructure of steel, reduces the matrix potential difference, improves corrosion resistance, and compensates for the strength reduction caused by low carbon content. By reducing oxygen in the steel and adding Ti, sulfide inclusions are dispersed, increasing the amount of MnS, allowing more antimony to accumulate and precipitate on MnS inclusions. This reduces antimony precipitation at the original austenite grain boundaries, eliminates antimony segregation at grain boundaries in subsequent heating furnaces and during rolling, and mitigates the tendency for cracking caused by antimony precipitation at grain boundaries. However, excessive Ti promotes the precipitation of coarse TiN, deteriorating the corrosion resistance of the steel. This invention controls the Ti content within the range of 0.030-0.045%.

[0046] Nitrogen (N), phosphorus (P), and sulfur (S) are impurity elements in steel, which easily lead to defects such as segregation and inclusions. These defects negatively impact the toughness and hot workability of the steel. This invention controls N to ≤ 0.006%, P to ≤ 0.020%, and S to ≤ 0.006%.

[0047] Step (2) LF refining: Bottom-blown argon gas is used for stirring, and after the submerged arc is stabilized, the slag fluidity is adjusted and deoxidized.

[0048] LF refining is performed with bottom-blown argon stirring throughout. After the submerged arc stabilizes, slag is added to adjust slag fluidity and for deoxidation. Initially, aluminum pellets and calcium carbide are the main components. After the first sampling, 10-50 kg of calcium carbide is added. After the second sampling, ferrosilicon powder is the main component, supplemented by calcium carbide and silicon carbide. Diffusion deoxidation requires small, multiple batches of addition, with a total usage ≥100 kg / furnace. The white slag retention time is ≥20 minutes. Copper plates and antimony ingots are added after the white slag is formed. Ti ferrophosphate is added 10-20 minutes before the end of refining.

[0049] This deoxidation method reduces the oxygen and sulfur content in the steel, which helps to refine the manganese sulfide inclusions in the steel. Adding a certain amount of Ti as nucleation sites for MnS inclusions makes the sulfides smaller and more dispersed, reducing the impact of MnS inclusions on the steel properties. This allows more antimony to be enriched and precipitated on the MnS inclusions, thereby greatly reducing the precipitation of antimony at the original austenite grain boundaries. At the same time, it effectively reduces or eliminates the segregation of antimony at grain boundaries in the subsequent heating furnace and rolling process, greatly reducing the tendency for cracking caused by antimony precipitation at grain boundaries.

[0050] Step (3) VD vacuum degassing: high vacuum degree ≤67Pa, holding time ≥10min.

[0051] Calcium treatment is performed 5 minutes before leaving the station. 20m of pure calcium wire is fed into the furnace when it is started. For the remaining furnaces, 1-20m of pure calcium wire is fed into the furnace. The soft blowing time is ≥20min.

[0052] Step (4) Continuous casting: Low superheat casting is adopted, with the superheat of continuous casting between 14-22℃; the crystallizer vibration adopts high frequency and small amplitude, with a frequency of 120-140HZ and an amplitude control of ±2.5mm; the electromagnetic stirring current of the crystallizer is controlled at 250-300A; low superheat and strong electromagnetic stirring current are beneficial to reduce the segregation and enrichment of Cu, Sb and other substances during the solidification process; increasing the vibration frequency and reducing the amplitude can reduce the vibration mark depth of the continuous casting billet and improve the surface quality of the billet.

[0053] The flow rate of the first cold water is 110-115m³. 3 The continuous casting speed is 1.1±0.5 m / min, and the water inlet temperature is 30±3℃, with a temperature difference of 8-10℃. The specific water volume is set at 0.27-0.30 L / kg. This ensures that the surface and corner temperatures of the billet are within the range of 980-1020℃ before entering the straightening machine. Since the precipitation temperature of Cu is around 1100℃ and the melting point of FeSb is 1019℃, the above measures can reduce the precipitation and aggregation of Cu and Sb at grain boundaries, thus reducing the probability of straightening cracks.

[0054] The end electromagnetic stirring current is 300-400A and the frequency is 10HZ. Using crystallizer electromagnetic stirring + end electromagnetic stirring reduces the columnar crystal region, increases the equiaxed crystal region, and reduces the segregation of Cu and Sb during solidification.

[0055] High-viscosity, low-melting-point protective slag is used, with a viscosity of 0.7-1.0 Pa·s and a melting point of 1160±20℃. The high-viscosity protective slag ensures uniform heat transfer of the billet within the crystallizer and reduces surface pitting; the low-melting-point protective slag increases slag consumption, improves lubrication between the billet and the crystallizing wall, and mitigates surface cracks in the billet.

[0056] Step (5) Pit cooling of steel billets: Pit cooling furnace temperature is 500-580℃, and the melting point of antimony is 600℃; the holding time is ≥24 hours, and the temperature is ≤100℃ when the billets are removed from the pit.

[0057] The initial temperature of the pit-cooled furnace should be 500-580℃. Antimony has a melting point of 600℃. If the initial temperature is too high, antimony is prone to segregation at the grain boundaries; if it is too low, it is also prone to thermal stress cracking. The holding time should be ≥24 hours, and the temperature should be ≤100℃ when removing the pit.

[0058] Step (6) Heating and rolling: Under a reducing atmosphere, a high-temperature rapid heating system is adopted, with a soaking furnace temperature of 1180-1220℃ and a total heating time of 120-180min; the initial rolling temperature is 1050-1070℃.

[0059] A high-temperature, rapid-heating process is employed, with a soaking furnace temperature of 1180-1220℃. This improves heating efficiency, resulting in rapid temperature rise and a relatively short heating time, with a total heating time of 120-180 min (target ≤140 min). This reduces antimony segregation at grain boundaries near 600℃, while also reducing the precipitation and enrichment of FeSb and Cu in the 1020-1100℃ range. A reducing atmosphere is maintained within the furnace during heating, with the air-fuel ratio adjusted to 0.85-0.95.

[0060] The initial rolling temperature is 1050-1070℃; after rolling, the round steel should be stacked in a sheltered place to cool, and it can only be transported after 12 hours of cooling and when the temperature is below 100℃.

[0061] To further illustrate the present invention, the following describes a method for removing surface cracks in round steel for Ni-free corrosion-resistant pipes provided by the present invention in conjunction with embodiments. However, it should be understood that these embodiments are implemented under the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. They are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention. The scope of protection of the present invention is not limited to the following embodiments.

[0062] Example 1

[0063] This embodiment provides a method for eliminating surface cracks in round steel pipes used for Ni-free corrosion-resistant tubing, the method comprising:

[0064] Step (1) Converter smelting:

[0065] The smelting composition by mass percentage is controlled as follows: C: 0.07%, Si: 0.34%, Mn: 0.50%, P: 0.017%, S: 0.005%, Cr: 0.90%, Ni: 0.02%, Cu: 0.28%, Alt: 0.020%, Sb: 0.050%, Ti: 0.040%, N: 0.005%, with the remainder being Fe.

[0066] High-quality molten iron is used, and the required composition of the molten iron entering the furnace is as follows: Si: 0.45%, P: 0.090%, S: 0.050%, and the temperature of the molten iron entering the furnace is T=1310℃.

[0067] The converter smelting process controls the tapping endpoint C=0.04%, the endpoint P=0.012%, and the tapping temperature 1605℃. The tapping process adopts the sliding plate slag-blocking operation, and the tapping time is 4.0 minutes. When 1 / 4 of the steel is tapped, deoxidizer, alloy and slag are added sequentially with the molten steel. The added deoxidizer is calcium carbide and aluminum cake 1 kg / t each; the added alloy is silicon manganese 5.8 kg / t, ferrosilicon 2.5 kg / t, ferrochrome 15.8 kg / t; the added slag is calcium aluminate 260 kg and lime 440 kg / furnace.

[0068] Step (2) LF Refining:

[0069] Argon stirring is applied throughout the process. After the submerged arc stabilizes, lime and other slag materials are added to adjust the slag fluidity. Initially, aluminum briquettes and calcium carbide are the main components: 20 kg calcium carbide + 10 kg aluminum briquettes + 30 kg ferrosilicon powder, added in two batches (approximately 80% of the total amount is added after about 5 minutes, and the remaining 20% ​​after about 10 minutes). After the first sampling, 10 kg of calcium carbide is added. After the second sampling, ferrosilicon powder is the main component, supplemented by calcium carbide and silicon carbide. Diffusion deoxidation requires small, frequent applications, with a total usage of 150 kg / furnace. The white slag is maintained for 25 minutes, after which copper plates and antimony ingots are added. Ti ferrophosphate is added 10 minutes before the end of refining.

[0070] Step (3) VD vacuum degassing

[0071] High vacuum of 60 Pa, held for 12 minutes. Calcium treatment is performed 5 minutes before leaving the station. 20 m of pure calcium wire is fed into the furnace at startup, and 10 m of pure calcium wire is fed into the furnace for the remaining furnaces. Soft blowing time is 25 minutes.

[0072] Step (4) Continuous casting

[0073] Low superheat casting is adopted, with the superheat of the initial casting furnace at 22℃ and the continuous casting furnace at 15℃. The crystallizer vibration adopts high-frequency, low-amplitude vibration, with a frequency of 120Hz and an amplitude controlled at 2.5mm. The electromagnetic stirring current of the crystallizer is controlled at 250A. The end electromagnetic stirring current is 300A, with a frequency of 10Hz. Both crystallizer electromagnetic stirring and end electromagnetic stirring are used.

[0074] Cold water flow rate: 110m³ 3 / h, primary cooling water inlet temperature: 30℃, primary cooling water temperature difference: 8℃, continuous casting speed: 1.1m / min, secondary cooling adopts air mist cooling, specific water volume is set to 0.28L / kg, the surface and corner temperature of the billet before entering the straightening machine is within 1000℃.

[0075] A high-viscosity, low-melting-point protective slag is used, with a viscosity of 0.8 Pa·s and a melting point of 1160℃.

[0076] Step (5) Pit cooling of steel billet

[0077] The pit is cooled and then fed into the furnace at 550℃, held for 25 hours, and removed from the pit at 80℃.

[0078] Step (6) Heating and rolling

[0079] A high-temperature, rapid-fire heating system is adopted, with a soaking furnace temperature of 1220℃; this improves heating efficiency and allows for rapid temperature rise, resulting in a relatively short heating time, with a total heating time of 160 minutes. A reducing atmosphere is maintained inside the furnace during heating, and the air-fuel ratio is adjusted to 0.95.

[0080] The initial rolling temperature is 1075℃; after rolling, the round steel is stacked in a sheltered place to cool, and after 12 hours of cooling, it is transferred at a temperature of 80℃.

[0081] The above-prepared round steel was subjected to flaw detection. The results showed that the surface quality of the rolled material was good, the first-pass flaw detection pass rate was 96.5%, and the user feedback after use was good.

[0082] Example 2

[0083] The method for eliminating surface cracks in Ni-free corrosion-resistant round steel pipes in this embodiment is the same as the method in Example 1, except that the smelting composition is different and the viscosity of the protective slag is adjusted to 1.0 Pa s and the melting point to 1140℃.

[0084] In this embodiment, the smelting composition by mass percentage is as follows: C: 0.08%, Si: 0.32%, Mn: 0.50%, P: 0.017%, S: 0.0055%, Cr: 0.90%, Ni: 0.03%, Cu: 0.30%, Alt: 0.020%, Sb: 0.045%, Ti: 0.043%, N: 0.0055%, with the remainder being Fe.

[0085] The above-prepared round steel was subjected to flaw detection, and the results are shown in the figure. Figure 1 The results showed that the surface quality of the rolled material was good, the first-pass inspection pass rate was 95.8%, and the user feedback after use was positive.

[0086] Example 3

[0087] The method for eliminating surface cracks in Ni-free corrosion-resistant round steel pipes in this embodiment is the same as the method in Embodiment 1, except that the conditions for cooling the billet in step (4) and heating and rolling in step (5) are different.

[0088] In step (4) of this embodiment, the pulling speed is adjusted to 1.3 m / min, and the cold water flow rate is 115 m³ / min. 3 / h, the secondary cooling water ratio is adjusted to 0.3L / kg;

[0089] The temperature of the soaking furnace in step (5) is 1200℃; the total heating time is 150min.

[0090] The above-prepared round steel was subjected to flaw detection. The results showed that the surface quality of the rolled material was good, the first-pass flaw detection pass rate was 94.8%, and the user feedback after use was good.

[0091] Comparative Example 1

[0092] In this comparative example, the Ti content of the finished product was controlled at 0.015%, the N content at 0.0070%, and the remaining operations were the same as those in Example 2.

[0093] The obtained round steel bars were inspected, and the results are shown below. Figure 2 The results showed that the surface quality of the round steel was poor, with multiple deep cracks. The first-time flaw detection pass rate was 41.8%, which was far worse than the surface quality of the steel prepared in the embodiments of the present invention.

[0094] Comparative Example 2

[0095] In this comparative example, the continuous casting speed in step (4) of the method in Example 3 is 1.7 m / min, the specific water content is set to 0.26 L / kg, and the rest of the operation is the same as in Example 3.

[0096] The obtained round steel was inspected, and the results showed that the surface quality of the round steel was poor, with many deep cracks. The first-time flaw detection pass rate was 56.6%, which was far worse than the surface quality of the steel prepared in the embodiment of the present invention.

[0097] Comparative Example 3

[0098] In this comparative example, the total heating time of the homogenizing furnace in step (5) of the method of Example 1 is 240 min, the rolling temperature is 1100℃, and the rest of the operation is the same as in Example 1.

[0099] The obtained round steel was inspected, and the results showed that the surface quality of the round steel was poor, with many deep cracks. The first-time flaw detection pass rate was 61.3%, and multiple tumbling processes were required. The surface quality was far worse than that of the steel prepared in the embodiment of the present invention.

[0100] Analysis of the round steel quality of Examples 1-3 and Comparative Examples 1-3 shows that the method of the present invention, through the design of steel composition and the control of continuous casting and hot rolling process conditions, greatly improves the surface cracks of Ni-free corrosion-resistant round steel pipes, reduces the cost of flaw detection and grinding, and has high application prospects.

[0101] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. 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 eliminating surface cracks in round steel bars used for Ni-free corrosion-resistant pipes, characterized in that, The method includes: BOF or EAF smelting: The steel composition in the smelting process, by weight percentage, includes: C: 0.06-0.09%, Si: 0.30-0.37%, Mn: 0.45-0.55%, P≤0.020%, S≤0.006%, Cr: 0.85-0.95%, Ni≤0.03%, Cu: 0.26-0.32%, Alt: 0.005-0.030%, Sb: 0.045%-0.075%, Ti: 0.030-0.045%, N≤0.006%, with the balance being iron and unavoidable impurity elements; LF refining: Bottom-blown argon gas stirring is used, and after the submerged arc is stabilized, the slag fluidity is adjusted and deoxidation is performed; among them, Ti iron is added 10-20 minutes before the end of the LF refining process. VD vacuum degassing: High vacuum degree ≤67Pa, holding time ≥10min; Continuous casting: Low superheat casting is adopted, with the superheat of continuous casting between 14-22℃; the crystallizer vibration adopts high frequency and small amplitude, with a frequency of 120-140HZ and an amplitude control of ±2.5mm; the electromagnetic stirring current of the crystallizer is controlled at 250-300A. The cooling water for the crystallizer is set as follows: the primary cooling water flow rate is 110-115 m³ / h. 3 / h, the inlet temperature of the primary cooling water is 30±3℃, the temperature difference between the primary and secondary cooling water is 8-10℃, the continuous casting speed is 1.1±0.5m / min, and the specific water volume is set to 0.27-0.30L / kg; Pit cooling of steel billets: Pit cooling furnace entry temperature 500-580℃; holding time ≥24 hours, temperature ≤100℃ when exiting the pit; Heating and rolling: Under a reducing atmosphere, a high-temperature rapid heating regime is adopted, with a soaking furnace temperature of 1180-1220℃ and a total heating time of 120-180min; the initial rolling temperature is 1050-1070℃; the air-fuel ratio is controlled at 0.85-0.95; after rolling, the round steel is stacked in a sheltered place to cool, and after 12 hours of stacking and cooling at a temperature below 100℃, it is transferred.

2. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, In the BOF or EAF smelting process, the blowing endpoint is controlled, with the tapping endpoint C ≤ 0.04% and the tapping endpoint P ≤ 0.012%; the tapping temperature ≥ 1600℃.

3. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 2, characterized in that, In the BOF or EAF smelting process, the order of auxiliary materials for steelmaking is: aluminum ingots, alloys, calcium aluminate, and lime.

4. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, In the LF refining process, aluminum briquettes and calcium carbide are added in the early stage of deoxidation, and ferrosilicon powder, calcium carbide and silicon carbide are added in the later stage. Add in batches multiple times, with a total usage of ≥100kg / furnace.

5. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, In the VD vacuum degassing process, calcium treatment is carried out 5-10 minutes before leaving the station. 20m of pure calcium wire is fed into the furnace during startup, and 1-20m of pure calcium wire is fed into the remaining furnaces. The soft blowing time is ≥20 minutes.

6. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, In the continuous casting process, a protective slag with a viscosity of 0.7-1.0 Pas and a melting point of 1160±20℃ is used.

7. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, In the continuous casting process, the final electromagnetic stirring current is 300-400A and the frequency is 10HZ.

8. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, During the heated rolling process, the air-fuel ratio is controlled between 0.85 and 0.

95.

9. The method for eliminating surface cracks in round steel bars for Ni-free corrosion-resistant pipes according to claim 1, characterized in that, In the heated rolling process, after rolling, the round steel is stacked in a sheltered place to cool. After 12 hours of cooling and at a temperature below 100°C, it is transferred.

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