Method for controlling cracks of weathering H-shaped steel 09CuPCrNiA

Abstract

The application discloses a method for controlling cracks of weather-resistant H-shaped steel 09CuPCrNiA; in order to reduce the occurrence rate of surface longitudinal cracks, weak cooling technology is adopted in continuous casting and secondary cooling, a suitable mold flux is selected, and meanwhile, a suitable and constant drawing speed is maintained in production, so that the generation of defects such as center segregation, internal cracks and surface longitudinal cracks of the casting blank is effectively prevented. Through the production practice of the weather-resistant steel, it is shown that the production process parameters and the various measures can reach the conditions for continuously developing the steel at the present stage, can provide the rail beam factory with high-quality casting blanks with good surface quality and qualified internal quality, can meet the requirements of the quality plan, and can meet the performance requirements of the weather-resistant steel of the user.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking technology, and in particular relates to a method for controlling cracks in weathering H-beam 09CuPCrNiA. Background Technology

[0002] Weathering steel refers to low-alloy high-strength steel that exhibits excellent corrosion resistance in the atmosphere by adding a small amount of alloying elements. The atmospheric corrosion resistance of weathering steel is 2 to 8 times that of ordinary carbon steel, and its corrosion resistance becomes more pronounced with longer service life. In addition to its excellent weather resistance, weathering steel also possesses superior mechanical and weldability properties, and is widely used in railway vehicles, bridges, and containers.

[0003] Weathering steel is a major trend in railway vehicle applications. High corrosion resistance, high strength, and low cost are the main development directions for steel used in railway vehicles. A steel plant has successfully developed weathering H-beams for railway carriages for the first time, using a shaped billet continuous casting machine in the steelmaking plant and a universal rolling mill in the rail beam plant for trial production. The shaped billet continuous casting machine is a domestically advanced shaped billet continuous casting production line with technical overall responsibility provided by Voestalpine. Products include H-beams for offshore oil platforms, mining, and shipbuilding, as well as H-beams for bridges and railway vehicles.

[0004] This patent describes the 09CuPCrNiA steel plate. Because the carbon content of this steel grade is mostly in the sub-peritectic reaction zone and contains Nb and relatively high levels of P, it exhibits poor high-temperature plasticity and high sensitivity to surface cracking. The production process presents the following challenges: 1) Reasonably controlling the phosphorus content at the end of the oxygen top-and-bottom blowing converter, ensuring the final phosphorus content is between 0.007% and 0.015%; 2) To ensure corrosion resistance, copper and nickel are added, requiring strict heating conditions for copper-containing steel during rolling; 3) The carbon content of this steel grade is in the peritectic reaction zone, making it highly susceptible to steel leakage during production. Optimization of the continuous casting protective slag performance and the secondary cooling water supply process is necessary. Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling cracks in weather-resistant H-beams 09CuPCrNiA. The process optimization of high-strength weather-resistant steel plate 09CuPCrNiA for railway carriages is introduced. Results show that the steel's chemical composition, mechanical properties, and other technical indicators all meet the standard requirements and can satisfy the needs of steel used in railway carriages.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] This invention discloses a method for controlling cracking in weather-resistant H-beams 09CuPCrNiA, comprising:

[0008] Converter process:

[0009] Reduce the amount of lime added to 25-30 kg / t to keep the slag basicity within 2.0-2.5 to ensure smelting effect; it is required to blow at the conventional lance position, and once the slag melts, the lance should be lowered in time to reduce the iron oxide in the slag and inhibit dephosphorization;

[0010] Refining process:

[0011] Add 400-500 kg / furnace of quicklime when tapping steel from the converter. Then, when the molten steel reaches the LF furnace, add 400-500 kg of lime, 100-150 kg of fluorite, 80-100 kg of aluminum granules, 25-35 kg of calcium silicate powder (preferably 30 kg), and 15-25 kg of carbon powder per furnace for slag deoxidation (preferably 20 kg), depending on the slag condition. During the process, add an appropriate amount of 10-15 kg of calcium silicate powder and 5-10 kg of carbon powder in one or two batches, depending on the slag condition, to maintain a good reducing atmosphere.

[0012] The composition of the protective slag used, by mass percentage, includes: SiO2 31-32%, Al2O3 5.5-6%, CaO 31.5-32.5%, Fe2O3 0.95-1.05%, Na2O 6-6.2%, and R 1.0-1.2.

[0013] The chemical composition of the weather-resistant H-beam 09CuPCrNiA by mass percentage includes: C 0.07-0.15%, Si 0.40-0.60%, Mn 0.35-0.55%, P 0.010-0.20%, S ≤0.025%, Cu 0.25-0.30%, Ni 0.25-0.40%, Cr 0.45-0.75%, V 0.030-0.060%, with the remainder being Fe and unavoidable impurities.

[0014] Furthermore, the pulling speed is set to 0.9–1.0 m / min.

[0015] Furthermore, in the production of weathering steel, the cooling water volume of the crystallizer is controlled, the crystallizer adopts a weak cooling formula, and the inlet temperature of the cooling water is controlled at 28~31℃, so that the temperature difference between the inlet and outlet water is 4~5℃.

[0016] Furthermore, the Soft-160 low-cooling water meter is used, and the secondary cooling water volume is controlled at 0.55L / kg.

[0017] Furthermore, to achieve the desired melting effect of the protective slag, the insertion depth of the submerged nozzle is controlled within the range of 90 mm to 100 mm; the suitable pulling speed for this protective slag is 0.8 to 1.1 m / min.

[0018] Furthermore, before straightening, the target temperature for the web is ≥810℃, and the target temperature for the flange is ≥800℃. After the continuously cast billet is cut, it should be removed from the production line and allowed to cool slowly, with the top cover mold laid underneath, and the slow cooling time should be greater than 72 hours.

[0019] Furthermore, the composition of the protective slag by mass percentage is as follows: SiO2 31.6%, Al2O3 5.7%, CaO 31.8%, Fe2O3 1.02%, Na2O 6.1%, R 1.0-1.2.

[0020] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0021] Because the carbon content of this steel grade is mostly in the subperitectic reaction zone and contains Nb and relatively high levels of P, it exhibits poor high-temperature plasticity and high sensitivity to surface cracks. Simultaneously, the steel grade contains elements such as chromium, copper, and nickel, making the billet shell prone to adhesion to the copper plate of the crystallizer, resulting in weld line defects. To reduce the weld line defect rate, a low casting speed process was adopted during the continuous casting of irregularly shaped billets, leading to a high incidence of joint defects in the cast billets. This invention, through optimized adjustment of the protective slag, reasonable control of the casting speed, and control of the cooling water volume and straightening temperature, achieves smooth production and good product quality.

[0022] 1. To reduce the incidence of surface cracks in Cu-containing steel, the secondary cooling process was adjusted, using a Soft-160 weak cooling water meter, with the secondary cooling water volume controlled at 0.55 L / kg. Surface defects in the cast billet were effectively controlled.

[0023] 2. Weathering steel solidified billet shell shrinks greatly and is prone to longitudinal cracks. By selecting a protective slag with higher basicity and solidification temperature, the thermal resistance of the slag film is increased, ensuring uniform cooling of the billet and smooth production.

[0024] Through analysis of the protective slags from different manufacturers used on irregularly shaped billets, the Stöberg-specific protective slag was ultimately selected. To achieve the desired melting effect, the insertion depth of the submerged entry nozzle was controlled within the range of 90 mm to 100 mm; the suitable casting speed for this protective slag is 0.8 to 1.1 m / min. The main chemical composition and physical properties of this slag are shown in Table 6. During use, this slag exhibits strong Al2O3 adsorption capacity, minimal slag ring formation, a slag consumption of approximately 0.62 kg / t, a liquid slag layer between 8 and 10 mm, good billet surface quality, and no sticking or leakage, thus effectively meeting the production needs of weathering steel. Detailed Implementation

[0025] A method for controlling cracking in weather-resistant H-beam 09CuPCrNiA includes:

[0026] 1. Technical Requirements

[0027] 1.1 Chemical composition

[0028] The chemical composition of H-beams should meet the requirements of Table 1.

[0029] Table 1 Chemical composition (unit: %)

[0030]

[0031] 1.2 Mechanical Properties

[0032] The mechanical properties of H-beams should meet the requirements in Table 2.

[0033] Table 2 Mechanical Properties

[0034]

[0035] 2. Process Route

[0036] Based on the requirements for weather resistance and toughness of 09CUPCRNIA steel, and combined with the equipment and process conditions of a certain steel plant, a production process characterized by high-purity smelting, high-quality cast billets, and high-precision controlled rolling and cooling hot continuous rolling was determined.

[0037] Specific process route: Blast furnace molten iron --- KR desulfurization --- Converter top and bottom reblowing smelting --- LF refining → VD vacuum degassing → --- 7# casting machine for special-shaped billets → feeding → walking beam furnace → high-pressure water descaling → BD1 billet opening → CCS universal rolling → hot sawing sampling → walking beam cooling bed cooling → straightening → sawing → shape and surface quality inspection → packaging → warehousing and outsourcing.

[0038] 3 Smelting process

[0039] 3.1. Converter process

[0040] The main purpose of slag formation in general steel smelting is to remove phosphorus (P) and sulfur (S). However, 09CUPCRNIA steel requires a higher P content and a lower S content. Appropriately reducing the amount of lime added lowers the slag basicity; reducing the lime addition to 25-30 kg / t keeps the slag basicity within 2.0-2.5, ensuring optimal smelting results. It is required to start blowing at the conventional lance position, and once the slag has melted, the lance should be lowered promptly to reduce iron oxide in the slag and inhibit P removal.

[0041] 3.2 Refining Process

[0042] Because converters need to maintain phosphorus (P), the slag basicity is relatively low. Therefore, the slag basicity in the ladle after tapping is also low, generally around 1.5. It is required to add 400-500 kg / furnace of quicklime during tapping. Then, when the molten steel reaches the LF furnace, another 400-500 kg of lime, fluorite, alumina granules, calcium silicate powder, and carbon powder are added, depending on the slag condition, for slag deoxidation. During this process, depending on the slag condition, appropriate amounts of calcium silicate powder and carbon powder are added in one or two batches to maintain a good reducing atmosphere. Slag sample data are shown in Table 4.

[0043] 4. Process Optimization

[0044] Copper-containing weathering steel is prone to leakage during casting, and longitudinal cracks easily appear on the surface of the billet. These longitudinal cracks are mainly due to the fact that the carbon content of this steel grade is in the peritectic reaction zone. In the mold, δ-Fe is 100% converted to γ-Fe, resulting in large phase transformation volume shrinkage, i.e., large billet shell shrinkage. This leads to large gaps between the mold wall and the billet shell, making uneven heat flow highly likely. During continuous casting, because this steel grade contains elements such as chromium, copper, and nickel, the billet shell easily adheres to the copper plate of the mold, leading to leakage due to adhesion.

[0045] Currently, the main methods to avoid steel leakage accidents and surface longitudinal cracks are to select appropriate protective slag and reasonable primary and secondary cooling processes.

[0046] Table 3. Composition and Physical Properties of Special Protective Slag

[0047]

[0048] 3. Metallographic microstructure analysis of the rolled sample of weathering steel 09CUPCRNIA shows that, in the hot-rolled state, its microstructure consists of uniform and fine ferrite and pearlite, with banded structures generally around 1.0 mm and a grain size of around 9.0 mm. The content of non-metallic inclusions is low. This fully meets the user's requirements for inclusions, banded structures, and grain size. The metallographic structure and inclusion level are well controlled.

[0049] Table 4. Metallographic Structure and Inclusion Rating of Weathering Steel

[0050]

[0051] Example (The target chemical composition of the weathering H-beam 09CuPCrNiA by mass percentage includes: C < 0.1%, Si 0.50%, Mn 0.45%, P 0.07-0.1%, S < 0.02%, Cu 0.28%, Ni 0.3%, Cr 0.6%, V 0.045%, with the remainder being Fe and unavoidable impurities):

[0052] 1. Process optimization

[0053] 1.1 Control of pulling speed

[0054] Because weathering steel contains high levels of elements such as Cu, P, and Ni, these elements can easily cause grain boundary embrittlement during crystallization, resulting in very low high-temperature plasticity and a high risk of leaks. Therefore, the continuous casting speed cannot be too fast. Considering the continuous casting equipment of the No. 7 casting machine in the steel plant, the casting speed is set at 0.9–1.0 m / min.

[0055] 1.2 Research on Cooling System

[0056] Weathering steel has a carbon content in the subperitetic range, which easily leads to crack initiation. It also contains high levels of elements such as Cu, P, and Ni, resulting in significant thermal and structural stresses during cooling. Strong cooling during continuous casting can easily cause crack propagation, while weak cooling can lead to P segregation and internal cracks in the billet. Therefore, a moderate cooling method is recommended for this steel during continuous casting.

[0057] In the production of weathering steel, the cooling water volume of the crystallizer is controlled by employing a weak cooling formula, with the inlet water temperature maintained at 28-31℃. This ensures a temperature difference of 4-5℃ between the inlet and outlet water, guaranteeing stable and uniform heat flow across the wide surface of the crystallizer, thereby reducing the probability of crack formation. The actual crystallizer water volume (l / min) and temperature difference are shown in Table 5.

[0058] Table 5 Actual water flow rate (L / min) and temperature difference (°C) in the crystallizer

[0059]

[0060] To reduce the incidence of surface cracks in Cu-containing steel, the secondary cooling process was adjusted, using a Soft-160 weak cooling water meter and controlling the secondary cooling water volume at 0.55 L / kg.

[0061] 1.3 Selection of protective slag

[0062] Weathering steel billets shrink significantly upon solidification, making them prone to longitudinal cracks. Therefore, protective slags with higher basicity and solidification temperature should be selected to increase the thermal resistance of the slag film and ensure uniform cooling of the billet. However, higher solidification temperature reduces the thickness of the liquid slag film, increases the friction during billet pulling, and leads to poor lubrication of the billet, resulting in sticking and leakage of steel.

[0063] By analyzing the protective slags from different manufacturers used on irregularly shaped billets, the Stöberg-specific protective slag was ultimately selected. To achieve the desired melting effect, the insertion depth of the submerged nozzle was controlled within the range of 90 mm to 100 mm; the suitable casting speed for this protective slag is 0.8 to 1.1 m / min. The main chemical composition and physical properties of this slag are shown in Table 6.

[0064] Table 6. Composition and Physical Properties of Special Protective Slag

[0065]

[0066] During use, the slag has a strong ability to adsorb Al2O3, with very little slag ring formation. The slag consumption is about 0.62 kg / t, the liquid slag layer is between 8 and 10 mm, the surface quality of the cast billet is good, and there is no sticking or leakage of steel, which can well meet the production needs of weathering steel.

[0067] 1.4 Straightening process

[0068] Because weathering steel contains copper, to avoid the influence of alloying elements on the mechanical properties of the cast billet, and to prevent straightening cracks during straightening, the target temperature for the web is ≥810℃ and the target temperature for the flange is ≥800℃ before straightening. After cutting, the continuously cast billet is promptly removed from the production line for slow cooling, and then laid on top of the mold. The slow cooling time is greater than 72 hours. Straightening temperature parameters are shown in Table 7.

[0069] Table 7 Straightening Temperature Parameters

[0070]

[0071] 1.5 Inspection of Cast Billets

[0072] The surface quality of the cast billet was good during this trial production, with no cracks or inclusions found. The gas content of the cast billet met the quality plan requirements, and the low-magnification inspection of the billet was satisfactory, meeting the quality and performance requirements for rolling. The results of the low-magnification inspection are shown in Table 8.

[0073] Table 8 Low-magnification test analysis

[0074] project center segregation center porosity centerline crack corner crack triangular zone crack alumina inclusion grade 0 0 0 0 0.5 0

[0075] 2. Product Performance

[0076] Metallographic microstructure analysis of the 09CUPCRNIA rolled sample showed that, in the hot-rolled state, the microstructure consisted of uniform and fine ferrite and pearlite, with banded structures generally around 1.0 mm and a grain size of around 9.0 mm. Non-metallic inclusions were minimal. This fully meets the user's requirements for inclusions, banded structures, and grain size. The metallographic structure and inclusion levels are shown in Table 9.

[0077] Table 9. Metallographic Structure and Inclusion Rating of Weathering Steel

[0078]

[0079] Through this trial production, the yield strength, tensile strength, and elongation of the rolled material fully meet the user's requirements. The mechanical properties of 09CUPCRNIA are shown in Table 10.

[0080] Table 10 Mechanical properties of weathering steel

[0081]

[0082] Longitudinal cracking on the surface of weathering steel shaped billets is the main cause of surface cracking defects in H-beams. To reduce the incidence of surface longitudinal cracking, a weak cooling process is adopted for the primary and secondary cooling stages of continuous casting, and a suitable mold flux is selected. Simultaneously, a suitable and constant casting speed is maintained during production, effectively preventing defects such as center segregation, internal cracks, and surface longitudinal cracking in the billets. Production practice of weathering steel shows that the production process parameters and various measures can meet the conditions for continued development of this steel grade at this stage, providing rail beam manufacturers with high-quality billets with good surface and internal quality. This meets the requirements of the quality plan and satisfies users' performance needs for weathering steel.

[0083] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for controlling cracking in weathering H-beam 09CuPCrNiA, characterized in that, include: Converter process: Reduce the amount of lime added to 25-30 kg / t to keep the slag basicity within 2.0-2.5 to ensure smelting effect; it is required to blow at the conventional lance position, and once the slag melts, the lance should be lowered in time to reduce the iron oxide in the slag and inhibit dephosphorization; Refining process: Add 400-500 kg / furnace of quicklime when tapping steel from the converter. Then, when the molten steel reaches the LF furnace, add 400-500 kg of lime, 100-150 kg of fluorite, 80-100 kg of aluminum granules, 25-35 kg of calcium silicate powder, and 15-25 kg of carbon powder per furnace for slag deoxidation, depending on the slag condition. During the process, add an appropriate amount of 10-15 kg of calcium silicate powder and 5-10 kg of carbon powder in one or two batches, depending on the slag condition, to maintain a good reducing atmosphere. The composition of the protective slag used, by mass percentage, includes: SiO2 31-32%, Al2O3 5.5-6%, CaO 31.5-32.5%, Fe2O3 0.95-1.05%, Na2O 6-6.2%, and R 1.0-1.

2. The chemical composition of the weather-resistant H-beam 09CuPCrNiA by mass percentage includes: C 0.07-0.15%, Si 0.40-0.60%, Mn 0.35-0.55%, P 0.010-0.20%, S ≤0.025%, Cu 0.25-0.30%, Ni 0.25-0.40%, Cr 0.45-0.75%, V 0.030-0.060%, with the remainder being Fe and unavoidable impurities.

2. The method for controlling cracks in weathering H-beam 09CuPCrNiA according to claim 1, characterized in that, The pulling speed is set to 0.9–1.0 m / min.

3. The method for controlling cracks in weathering H-beam 09CuPCrNiA according to claim 1, characterized in that, When producing weathering steel, the cooling water volume of the crystallizer is controlled, and a weak cooling formula is used in the crystallizer. The inlet temperature of the cooling water is controlled at 28~31℃, so that the temperature difference between the inlet and outlet water is 4~5℃.

4. The method for controlling cracks in weathering H-beam 09CuPCrNiA according to claim 1, characterized in that, The Soft-160 low-cooling water meter is used, and the secondary cooling water flow rate is controlled at 0.55L / kg.

5. The method for controlling cracks in weathering H-beam 09CuPCrNiA according to claim 1, characterized in that, To achieve the desired melting effect of the protective slag, the insertion depth of the submerged nozzle should be controlled within the range of 90 mm to 100 mm; the suitable pulling speed for this protective slag is 0.8 to 1.1 m / min.

6. The method for controlling cracks in weathering H-beam 09CuPCrNiA according to claim 1, characterized in that, Before straightening, the target temperature for the web is ≥810℃ and the target temperature for the flange is ≥800℃. After cutting, the continuously cast billet should be removed from the production line and allowed to cool slowly, with the top cover mold laid underneath, and the slow cooling time should be greater than 72 hours.

7. The method for controlling cracks in weathering H-beam 09CuPCrNiA according to claim 1, characterized in that, The composition of the protective slag by mass percentage is: SiO2 31.6%, Al2O3 5.7%, CaO 31.8%, Fe2O3 1.02%, Na2O 6.1%, R 1.0-1.2.

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