A normalized 420mpa grade steel for offshore converter station and a manufacturing method thereof

Abstract

This invention discloses a normalized 420MPa grade steel for offshore converter stations, whose chemical composition and mass percentages include: C: 0.13-0.16%, Mn: 1.40-1.70%, Si: 0.20-0.40%, P≤0.012%, S≤0.020%, V: 0.030-0.050%, Al: 0.030-0.050%, Nb: 0.020-0.040%, Ti: 0.005-0.020%, with the balance being Fe and incidental impurities. The advantages of this invention are that it ensures the strength and toughness of the steel plate while possessing good weldability, and significantly reduces raw material costs.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and in particular relates to a normalized 420MPa grade steel for offshore converter stations and its manufacturing method. Background Technology

[0002] Offshore converter stations are key equipment for long-distance transmission of offshore wind power, and are developing towards ultra-large-scale capacity and deep-sea deployment. To adapt to this trend, the performance requirements for structural steel are also increasing. Replacing traditional 360MPa grade steel plates with higher-strength 420MPa grade steel plates is not simply a strength upgrade, but a systematic optimization choice considering load adaptability, structural efficiency, and total life-cycle cost. The 420MPa grade offers 18% to 20% higher strength than the 360MPa grade, meaning that DH420 has a higher load-bearing capacity for the same cross-sectional dimensions; or, for the same load requirements, thinner steel plates can be used, reducing the amount of steel used.

[0003] Currently, common 420MPa grade steel plates are mainly produced using controlled rolling and controlled cooling TMCP process or quenching and tempering QT process. However, due to the extremely stringent requirements for the weldability and service stability of materials in offshore converter stations, normalized steel plates are typically preferred for construction. Compared to TMCP steel plates, normalized steel plates have more uniform microstructure and properties along the thickness direction, and a lower risk of performance degradation in the weld heat-affected zone; compared to QT steel plates, their production process is simpler and the cost is lower.

[0004] However, for traditional steel grades and processes, it is difficult to achieve a yield strength of 420 MPa in steel plates through simple normalizing treatment. This is because after conventional normalizing, the microstructure of the steel is typically ferrite and pearlite, which is insufficient to provide sufficiently high strength. Therefore, how to stably manufacture 420 MPa grade steel plates that meet the requirements of high strength and high toughness through normalizing while ensuring uniform performance and low cost is a current technical challenge in this field. Summary of the Invention

[0005] The purpose of this invention is to solve the problem that it is difficult to obtain 420MPa grade steel plates with high strength, high toughness and excellent weldability through the normalizing process. It provides a normalized 420MPa grade steel for offshore converter stations, which ensures the strength and toughness of the steel plate, also gives it good weldability, and significantly reduces the cost of raw materials.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A normalized 420MPa grade offshore converter station steel has the following chemical composition and mass percentages: C: 0.13–0.16%, Mn: 1.40–1.70%, Si: 0.20–0.40%, P≤0.012%, S≤0.020%, V: 0.030–0.050%, Al: 0.030–0.050%, Nb: 0.020–0.040%, Ti: 0.005–0.020%, with the balance being Fe and incidental impurities. The steel is strengthened by adding V, Al, Nb, and Ti microalloying elements, but does not contain Ni, Mo, Cr, Cu, or other alloying elements.

[0007] Furthermore, the steel is produced in a normalized state, and the finished product thickness is 15-50 mm.

[0008] Furthermore, the steel plate has a microstructure of bainitic and ferritic, with a yield strength of not less than 420 MPa, a tensile strength of 520–680 MPa, and an impact toughness (Akv) greater than 100 J at -50°C at one-quarter of the plate thickness. Moreover, after PWHT treatment, its strength and toughness remain at levels comparable to those in the normalized state.

[0009] To further achieve the objectives of this invention, a method for manufacturing 420MPa grade offshore converter station steel in a normalized state is also provided. This method includes using 220 / 260mm continuously cast billets, with the billet entering the furnace at a temperature of 1080–1180℃. During the rolling stage, a controlled rolling and cooling process is adopted, with a rolling compression ratio of not less than 4.5, a second-stage compression ratio of not less than 2.5, a finishing rolling start temperature of 880±30℃, and a finishing rolling finish temperature of 820±30℃. After finishing rolling, the reheating temperature is 650±30℃.

[0010] Furthermore, during the rolling process, the reduction in each roughing pass is not less than 30 mm.

[0011] Furthermore, during the rolling process, the reduction rate of the last three passes of finishing rolling is not less than 12%, and the cumulative reduction rate of the last three passes is not less than 30%, in order to refine the microstructure.

[0012] Furthermore, after controlled rolling and cooling, normalizing heat treatment is performed at a temperature of 880±30℃, with the furnace time measured in minutes as (2.5~3.0) multiplied by the steel plate thickness.

[0013] Compared with the prior art, the advantages of the technical solution of the present invention are as follows: (1) By precisely designing the chemical composition, the present invention uses medium carbon content and adds micro-alloying elements such as V, Al, Nb, and Ti, while avoiding the use of expensive alloying elements such as Ni, Mo, and Cr. This not only ensures the strength and toughness of the steel plate, but also gives it good weldability, i.e., Pcm≤0.22, and significantly reduces the cost of raw materials. (2) The present invention refines the core structure by increasing the deformation amount in the non-recrystallization zone after controlled rolling and controlled cooling, and then combines it with normalizing heat treatment to obtain a structure of bainite and ferrite, thus obtaining a low-cost normalized EH420 grade offshore converter station steel plate. (3) By controlling a large rolling compression ratio and the reduction rate at the end of finishing rolling, the present invention effectively refines the rolled austenite grains, laying the foundation for obtaining a fine ferrite and bainite composite structure in subsequent normalizing treatment; (4) The present invention can stably produce normalized steel plates with a yield strength of not less than 420MPa, a tensile strength in the range of 520 to 680MPa, and excellent impact toughness at -50℃. After simulated post-weld heat treatment, the mechanical properties of the steel plate can still maintain a level comparable to that of the normalized state, indicating that it has excellent welded joint performance stability and fully meets the use requirements of harsh working conditions such as offshore converter stations. Attached Figure Description

[0014] Figure 1 This is a metallographic image of the steel plate produced in Example 1 of the present invention; Figure 2 This is a parameter table for the manufacturing method in Examples 1 to 3 of the present invention; Figure 3 This is a statistical table of mechanical performance results in Examples 1 to 3 of the present invention. Detailed Implementation Example 1

[0015] To make the present invention clearer, the following description, in conjunction with the accompanying drawings, further illustrates a normalized 420MPa grade offshore converter station steel and its manufacturing method. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the present invention.

[0016] The normalized 420MPa grade offshore converter station steel provided in this embodiment has the following chemical composition and mass percentage: C: 0.141%, Mn: 1.55%, Si: 0.32%, P: 0.005%, S: 0.0008%, V: 0.034%, Al: 0.042%, Nb: 0.034%, Ti: 0.010%, with the balance being Fe and incidental impurities.

[0017] See Figure 2 The manufacturing method of the aforementioned 420MPa-class offshore converter station steel in a normalized state includes: The billet is a 220 / 260mm continuous casting billet with a furnace temperature of 1150℃. The rolling stage adopts a controlled rolling and cooling process with a rolling compression ratio of 4.6 and a two-stage compression ratio of 2.7. The finishing rolling start temperature is 910℃ and the finishing rolling finish temperature is 840℃. After finishing rolling, the red-hot temperature is 650±30℃.

[0018] During the rolling process, the reduction in the roughing pass is 32mm; the reduction rate of the last three passes in the finishing roll is not less than 12%, and the cumulative reduction rate of the last three passes is 38%, in order to refine the microstructure.

[0019] After controlled rolling and cooling, normalizing heat treatment is carried out at a temperature of 905℃, with a furnace time of 60 minutes based on a plate thickness of 20mm.

[0020] The metallographic morphology of the steel plate prepared by the method of this embodiment is as follows: Figure 1 As shown in the figure, it can be clearly observed that the microstructure consists of very fine polygonal ferrite and dispersed bainite, with uniform and fine grain size. This fine-grained microstructure is the fundamental reason why the steel plate simultaneously achieves high strength and excellent toughness. Its mechanical properties after testing are as follows: Figure 3 As shown. Example 2

[0021] The normalized 420MPa grade offshore converter station steel provided in this embodiment has the following chemical composition and mass percentage: C: 0.138%, Mn: 1.67%, Si: 0.37%, P: 0.007%, S: 0.0009%, V: 0.047%, Al: 0.045%, Nb: 0.031%, Ti: 0.012%, with the balance being Fe and incidental impurities.

[0022] See Figure 2 The manufacturing method of the aforementioned 420MPa-class offshore converter station steel in a normalized state includes: The billet is a 220 / 260mm continuous casting billet with a furnace temperature of 1160℃. The rolling stage adopts a controlled rolling and cooling process with a rolling compression ratio of 4.8 and a two-stage compression ratio of 3.0. The finishing rolling start temperature is 885℃ and the finishing rolling finish temperature is 835℃. After finishing rolling, the red-hot temperature is 650±30℃.

[0023] During the rolling process, the reduction in the roughing pass is 35mm; the reduction rate of the last three passes in the finishing roll is not less than 12%, and the cumulative reduction rate of the last three passes is 42%, in order to refine the microstructure.

[0024] After controlled rolling and cooling, normalizing heat treatment is carried out at a temperature of 895℃, with a furnace time of 100 minutes based on a plate thickness of 35mm.

[0025] The mechanical property data of the steel plate prepared by the above method after testing are as follows: Figure 3 As shown. Example 3

[0026] The normalized 420MPa grade offshore converter station steel provided in this embodiment has the following chemical composition and mass percentage: C: 0.153%, Mn: 1.43%, Si: 0.29%, P: 0.006%, S: 0.009%, V: 0.041%, Al: 0.039%, Nb: 0.029%, Ti: 0.015%, with the balance being Fe and incidental impurities.

[0027] See Figure 2 The manufacturing method of the aforementioned 420MPa-class offshore converter station steel in a normalized state includes: The billet is a 220 / 260mm continuous casting billet with a furnace temperature of 1120℃. The rolling stage adopts a controlled rolling and cooling process with a rolling compression ratio of 5.2 and a two-stage compression ratio of 3.2. The finishing rolling start temperature is 870℃ and the finishing rolling finish temperature is 815℃. After finishing rolling, the red-hot temperature is 650±30℃.

[0028] During the rolling process, the reduction in the roughing pass is 40 mm; the reduction rate of the last three passes in the finishing roll is not less than 12%, and the cumulative reduction rate of the last three passes is 40%, in order to refine the microstructure.

[0029] After controlled rolling and cooling, normalizing heat treatment is carried out at a temperature of 880℃, with a furnace time of 145 minutes based on a plate thickness of 50mm.

[0030] The mechanical property data of the steel plate prepared by the above method after testing are as follows: Figure 3 As shown.

[0031] In summary, Examples 1, 2, and 3, corresponding to thicknesses of 20mm, 35mm, and 50mm respectively, all exhibited normalized yield strengths significantly higher than the target value of 420MPa, and tensile strengths falling within the target range of 520–680MPa. In the -50℃ low-temperature impact test, the average impact energy of all examples was well greater than 100J, demonstrating excellent low-temperature toughness. More importantly, after PWHT treatment at 580±10℃, the strength and impact toughness of each example did not show a significant decrease, remaining comparable to their normalized performance. This fully demonstrates the excellent post-weld performance stability of the steel grade of this invention.

[0032] In addition to the embodiments described above, the present invention may have other implementations. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A type of steel for offshore converter stations in a normalized state with a strength of 420MPa, characterized in that: The chemical composition and mass percentage of the steel include: C: 0.13-0.16%, Mn: 1.40-1.70%, Si: 0.20-0.40%, P≤0.012%, S≤0.020%, V: 0.030-0.050%, Al: 0.030-0.050%, Nb: 0.020-0.040%, Ti: 0.005-0.020%, with the balance being Fe and incidental impurities.

2. The 420MPa grade offshore converter station steel in normalized condition according to claim 1, characterized in that: The steel is produced in a normalized state, and the finished product thickness is 15-50 mm.

3. The 420MPa grade offshore converter station steel in normalized condition according to claim 2, characterized in that: The steel plate has a microstructure of bainite and ferrite, a yield strength of not less than 420 MPa, a tensile strength of 520 to 680 MPa, an impact toughness Akv greater than 100 J at -50℃ at one-quarter of the plate thickness, and its strength and toughness remain at a level comparable to those of the normalized state after PWHT treatment.

4. A method for manufacturing the 420MPa grade offshore converter station steel in normalized condition as described in claim 3, characterized in that: The process includes using 220 / 260mm continuously cast billets, with the billet entering the furnace at a temperature of 1080–1180℃. The rolling stage employs a controlled rolling and cooling process, with a rolling compression ratio of not less than 4.5 and a second-stage compression ratio of not less than 2.

5. The initial finishing rolling temperature is 880±30℃, and the final finishing rolling temperature is 820±30℃. After finishing rolling, the reheating temperature is 650±30℃.

5. The method for manufacturing the 420MPa grade offshore converter station steel in normalized state according to claim 4, characterized in that: During the rolling process, the reduction in each roughing pass is not less than 30 mm.

6. The method for manufacturing the 420MPa grade offshore converter station steel in normalized state according to claim 4, characterized in that: During the rolling process, the reduction rate of the last three passes of finishing rolling is not less than 12%, and the cumulative reduction rate of the last three passes is not less than 30%.

7. The method for manufacturing the 420MPa grade offshore converter station steel in normalized state according to claim 4, characterized in that: After controlled rolling and cooling, the steel plate undergoes normalizing heat treatment at a temperature of 880±30℃. The furnace time is measured in minutes and is calculated as (2.5~3.0) multiplied by the steel plate thickness.

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