A 150-250mm corrosion-resistant low-weld-crack-sensitivity high-strength steel Q620CF and a production method thereof

Abstract

The application discloses a 150-250mm corrosion-resistant high-strength steel Q620CF with low welding crack sensitivity and a production method thereof. The chemical components of the steel are as follows: C: 0.05-0.08, Si: 0.2-0.3, Mn: 0.6-1.0, P: 0.010-0.015, S: less than or equal to 0.003, Nb: 0.020-0.030, V: 0.020-0.030, Als: 0.015-0.030, Cr: 0.50-0.60, Ni: 0.40-0.50, Mo: 0.30-0.35, Cu: 0.30-0.35 and B: 0.0014-0.0020. The steel plate is obtained through smelting, water-cooling mold casting, ingot heating, hot rolling, repeated cooling, slow cooling, quenching and tempering air cooling. The maximum thickness of the obtained steel plate reaches 250mm, the low welding crack sensitivity index Pcm of the steel plate is 0.17-0.22%, and the corrosion-resistant coefficient I of the steel plate is 6.5-7.0, so that the corrosion resistance of the large-thickness low welding crack sensitivity high-strength steel Q620CF in a severe use environment is solved.

Description

Technical Field

[0001] This invention relates to the field of molded plate production technology, specifically to a 150-250mm corrosion-resistant, low-weld-crack-susceptibility high-strength steel Q620CF and its production method. Background Technology

[0002] Q620CF is a high-strength steel with low weld crack sensitivity, possessing high strength, high hardness, good plasticity, and toughness. It exhibits excellent weldability, with minimal deformation and low residual stress during welding, which contributes to improved weld quality and structural safety. In recent years, with the increasing scale of hydropower, bridges, ships, and marine engineering projects, the research and development of this type of high-strength steel with low weld crack sensitivity has gradually expanded to greater thicknesses. Currently, steel plates with higher strength and low weld crack sensitivity exceeding 150mm in thickness are relatively rare in the market, and their performance has always been a challenge to control. Furthermore, these steel plates are generally used in harsh environments. If the steel plate also possesses certain corrosion resistance, the equipment manufactured would be safer, more reliable, and have a longer service life. However, existing Q620CF production technology cannot simultaneously achieve ultra-thickness, low weld crack sensitivity, and corrosion resistance.

[0003] Using the I-coefficient (corrosion resistance index) to determine the corrosion resistance of steel plates is a common and effective method. The I-value is a comprehensive indicator that considers various aspects of the steel plate's performance, including oxidation resistance, corrosion resistance, and wear resistance, under specific environmental conditions. Generally, an I-value of 6.0 or higher is considered to indicate good corrosion resistance. A higher I-value indicates better corrosion resistance, enabling the steel plate to maintain stability and service life for longer periods in harsher environments.

[0004] For example, Chinese patent CN202211291567X discloses "a 690MPa grade hydrogen-induced delayed fracture resistant low weld crack sensitive quenched and tempered steel and its manufacturing method", whose chemical composition (unit, wt%) is: C: 0.065~0.09, Mn: 1.00~1.20, Ni: 0.50~0.65, Mo: 0.20~0.30, V: 0.3~0.4, Pcm: ≤0.20; This invention uses a low C content composition design, a special rolling controlled cooling mode and a specific quenching and tempering process to obtain a 18~50mm thick 690MPa grade hydrogen-induced delayed fracture resistant low weld crack sensitive quenched and tempered steel with yield strength ≥550MPa, tensile strength 690~820MPa, elongation ≥16%, and low temperature impact at -50℃ ≥100J. Although the steel has a weld crack sensitivity index Pcm:≤0.20 and good weldability, its thickness is only 18-50mm and its corrosion resistance coefficient I:2.1-2.7, so it does not have corrosion resistance characteristics.

[0005] For example, Chinese patent CN2014107536791 discloses "an 800MPa grade low-weld-crack-sensitivity quenched and tempered steel plate for hydropower," with the following chemical composition (wt%): C: 0.09–0.14, Mn: 0.7–1.45, Ni: 0.30–0.50, V: 0.035–0.060, Cr: 0.25–0.60, Mo: 0.25–0.55, Cu: 0.15. ~0.30, B:0.0008~0.002; its preparation method: smelting and casting into a billet according to the set composition, heating and rolling in two stages, and then performing heat treatment after rapid cooling to obtain steel plate properties: yield strength ≥690MPa, tensile strength 780~930MPa, elongation after fracture ≥17%, impact energy absorption at -40℃ ≥47J, welding cold crack sensitivity index Pcm=0.24, but its thickness is only 10~80mm.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a 150-250mm thick high-strength corrosion-resistant steel Q620CF with low weld crack sensitivity. This steel plate has a crack sensitivity coefficient Pcm ≤ 0.22% and a corrosion resistance coefficient I ≥ 6.5. The steel plate has a yield strength of 711-756MPa, a tensile strength of 786-810MPa, an elongation of 18-21%, and an impact energy of 155-220J at -60℃. While ensuring the low weld crack sensitivity Pcm index for a large thickness, it also meets the corrosion resistance coefficient I requirement, thus solving the corrosion problem of large-thickness low crack sensitivity steel plates in harsh operating environments.

[0008] Another objective of this invention is to provide a method for producing 150-250mm corrosion-resistant, low-weld-crack-susceptibility high-strength steel Q620CF.

[0009] To achieve the above objectives, the technical solution adopted by this invention is as follows: a 150-250mm thick high-strength corrosion-resistant steel Q620CF with low weld crack sensitivity, wherein the steel has the following chemical composition (wt%): C: 0.05-0.08, Si: 0.2-0.3, Mn: 0.6-1.0, P: 0.010-0.015, S≤0.003, Nb: 0.020-0.0 30. V: 0.020~0.030, Als: 0.015~0.030, Cr: 0.50~0.60, Ni: 0.40~0.50, Mo: 0.30~0.35, Cu: 0.30~0.35, B: 0.0014~0.0020, the others are Fe and residual elements. Its crack sensitivity coefficient Pcm: 0.17~0.22% and corrosion resistance coefficient I: 6.5~7.0.

[0010] The dosage and function of each element in this invention are explained below:

[0011] Carbon: Carbon is the most important strengthening element in this invention, improving the yield strength and tensile strength of the steel plate. However, as the carbon content increases, the toughness of the steel plate decreases. Excessive carbon content not only affects the impact performance of the steel plate but also compromises welding safety. Carbon content also significantly impacts the mechanical properties and corrosion resistance of the steel. Excessive carbon content reduces corrosion resistance and weldability. Based on the application range of this steel grade in forming and processing, it is necessary to ensure both good low weld crack sensitivity and good corrosion resistance. If the carbon content exceeds 0.08%, it significantly affects weldability and low-temperature toughness of the steel plate; if the carbon content is below 0.05%, the strength of the steel plate cannot be guaranteed. Therefore, the carbon content is controlled between 0.05% and 0.08%.

[0012] Chromium: Chromium is one of the important strengthening and corrosion-resistant elements in this invention. It is a medium-sized carbide-forming element, and among all carbides, chromium carbides are the finest. It can be uniformly distributed in the steel body, thus possessing high strength, hardness, yield point, and high wear resistance. Chromium also improves corrosion resistance, but its enhancing effect decreases with increasing carbon content. Based on the application and performance requirements of this steel grade, if the chromium content is higher than 0.6%, the Pcm low weld crack sensitivity index will be too high, affecting the weldability of the steel plate; if the chromium content is lower than 0.5%, the solid solution strengthening effect will decrease, the material strength may not be sufficient, and the corrosion resistance coefficient I will also decrease sharply, leading to a reduction in the corrosion resistance of the steel plate. Therefore, the chromium content is controlled at 0.5%–0.6%.

[0013] Nickel: Nickel is one of the important elements in this invention for strength, toughness, and corrosion resistance. Nickel possesses high strength, high toughness, and good hardenability and corrosion resistance. On the one hand, it significantly enhances the strength of the steel; on the other hand, it consistently maintains a very high level of toughness. Based on the application and performance requirements of this steel grade, if the nickel content is below 0.4%, the strength and low-temperature toughness of the thick steel plate of this invention cannot be guaranteed, and the corrosion resistance coefficient will not meet the requirements. If the nickel content is above 0.5%, it will adversely affect the weldability of the steel plate. Furthermore, nickel is a relatively expensive alloying element, and a high nickel content will increase the production cost of the steel, which will not only put economic pressure on the production enterprise but may also affect the market competitiveness of the product. Therefore, the nickel content is controlled at 0.4% to 0.5%.

[0014] Molybdenum: Molybdenum is one of the important strengthening and corrosion-resistant elements in this invention. It can significantly improve the hardenability of the steel plate and has a certain degree of corrosion resistance. Combined with chromium and nickel, it can greatly improve hardenability, refine grains, increase toughness, and facilitate forging. Based on the application and performance requirements of this steel grade, if the molybdenum content is below 0.3%, the hardenability of the thick steel plate of this invention will be significantly reduced, making it impossible to guarantee the strength performance of the steel plate; if the molybdenum content is above 0.35%, it will reduce the weldability of the steel plate, and excessively high levels will also make the high-temperature tempered water-cooled steel brittle, affecting impact toughness. Therefore, the molybdenum content is controlled at 0.30-0.35%.

[0015] Copper: Copper is one of the important corrosion-resistant elements in this invention. Copper can improve the stability of austenite in steel, thus improving hardenability and hardenability, and strengthening ferrite. Adding about 0.20% copper to low-alloy steel, especially when used in combination with phosphorus, can improve the steel's resistance to atmospheric corrosion. Based on the application range of this steel grade in forming and processing, if the copper content exceeds 0.35%, it easily causes "copper brittleness" in the steel plate, which will worsen the material's processing performance, increase processing difficulty and cost; it will also lead to a significant decrease in the material's hardness, strength, toughness, and other mechanical properties, affecting the product's service life and safety; if the copper content is below 0.30%, the material's corrosion resistance will be significantly reduced; therefore, the copper content is controlled at 0.30-0.35%.

[0016] Boron: Boron is the most important hardenability element in this invention. Adding trace amounts of boron (0.0005% to 0.005%) to steel can significantly improve its hardenability, with little or no effect on other properties. This can, to some extent, replace nickel, chromium, and molybdenum. Based on the application range of this steel grade in forming and processing, and due to the large thickness and poor hardenability of the steel plates, boron is added to improve its hardenability and ensure that an ideal bainitic structure can be obtained after quenching. However, when the boron content exceeds 0.005%, the hardenability deteriorates, easily causing brittleness and affecting the strength and impact toughness of the steel plate. Therefore, the boron content is controlled at 0.0014% to 0.0020%.

[0017] Phosphorus: Phosphorus is one of the important corrosion-resistant elements in this invention because it exists in steel in the form of phosphides. Phosphates form brittle phases at grain boundaries, and excessive content can significantly reduce the low-temperature toughness of the steel, making it prone to brittle fracture. However, phosphorus can also act as a beneficial alloying element, working with copper to promote the zoning of the internal rust layer, which is beneficial for enhancing the corrosion resistance of the steel. In this invention, it is mainly used to combine with copper to improve the corrosion resistance of the steel plate; therefore, the phosphorus content is controlled at 0.010–0.015%.

[0018] The production method of the above-mentioned 150-250mm corrosion-resistant, low-weld-crack-susceptibility high-strength steel Q620CF includes the following steps:

[0019] 1) The smelting process of converter + LF refining + VD vacuum degassing is adopted to control the P content at 0.010~0.015%, H content ≤1.2PPm, and N content ≤50PPm;

[0020] 2) Water-cooled mold casting is used, with the initial casting temperature controlled at 1550-1555℃, the main body casting time being 18-21 minutes, and the riser casting time being 5-8 minutes;

[0021] 3) Heat the steel ingots to a holding temperature of 1230–1250℃ for a total heating time of 18–22 hours;

[0022] 4) After descaling, the steel ingot is hot rolled. The initial rolling temperature is 1000-1150℃, the reduction per pass is 50-55mm, and the final rolling temperature is 950-1000℃. The large reduction is beneficial to crushing the coarse grains that may be generated during the heating process, so as to achieve the purpose of refining the grains.

[0023] 5) After rolling, the steel plate is rapidly cooled multiple times using ACC. The temperature of the hot-rolled steel plate is controlled at 530-580℃. Rapid and repeated cooling after hot rolling can prevent the broken grains from growing back and achieve the purpose of refining the grains.

[0024] 6) Stack the steel plates for slow cooling, control the stacking temperature to ≥450℃, and slow cooling time to 24-48h; slow cooling by stacking can reduce the cooling rate and suppress the risk of crack formation.

[0025] 7) After slow cooling, quenching is performed at a temperature of 920–940℃, with a holding time of 1.8–2.0 min / mm. Water cooling is then performed for 15–35 min to room temperature. During the quenching process, a chiller is used to control the water temperature to <16℃, with a cooling rate of 1–2℃ / s. It should be noted that the hardenability of extra-thick plates is relatively poor. Studies have found that 150–250 mm thick Q620CF steel plates require cooling to 556–456℃ at a cooling rate of 0.05–40℃ / s to obtain the desired bainitic structure.

[0026] 8) Tempering and air cooling: The steel plate is placed in a tempering furnace and rapidly heated at high power to a tempering temperature of 470–530℃, with a holding coefficient of 4.0 min / mm, followed by air cooling to room temperature. The nickel and copper alloys added to this steel grade cause temper brittleness, which occurs in the approximately 400–450℃ range. This tempering temperature setting of 470–530℃, through rapid heating, quickly overcomes the brittle region. During cooling, air cooling is used at a rate of approximately 0.06–0.09℃ / s. Therefore, air cooling increases the cooling rate, avoiding and reducing temper brittleness. This avoids the traditional air-cooling tempering method with a cooling rate of only 0.01–0.02℃ / s, effectively avoiding the temper brittle range and ensuring that tempering effectively avoids this brittle zone. Simultaneously, it imparts a low-carbon bainitic tempered structure to the steel plate, improving its toughness and stability.

[0027] According to the test, the metallographic structure of the steel plate obtained according to this method is: low carbon bainite + ferrite + a small amount of pearlite, with a grain size of grade 9.

[0028] Compared with existing technologies, the 150-250mm thick Q620CF high-strength steel produced by this invention not only ensures a low weld crack sensitivity index (Pcm) for thick steel plates, but also meets the corrosion resistance coefficient (I) requirement, thus solving the corrosion problem of thick steel plates with low crack sensitivity in harsh operating environments. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the metallographic structure of the steel plate at 1 / 4 thickness obtained by the present invention, magnified by 100x.

[0030] Figure 2 This is a schematic diagram of the metallographic structure of the steel plate at 1 / 4 thickness obtained by the present invention, magnified at 200x.

[0031] Figure 3 This is a schematic diagram of the metallographic structure of the steel plate at 1 / 4 thickness obtained by the present invention, magnified at 500x.

[0032] Figure 4 This is a schematic diagram of the metallographic structure at another location with a magnification of 500x at 1 / 4 thickness of the steel plate obtained by the present invention. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, a detailed description will be provided below in conjunction with specific embodiments.

[0034] Table 1 is a list of chemical components for each embodiment and comparative example of the present invention.

[0035] Table 2 lists the main process parameters of each embodiment and comparative example of the present invention.

[0036] Table 3 lists the performance test results of various embodiments and comparative examples of the present invention.

[0037] The present invention is manufactured according to the following steps:

[0038] 1) The smelting process of converter + LF refining + VD vacuum degassing is adopted to control the P content at 0.010~0.015%, H content ≤1.2PPm, and N content ≤50PPm;

[0039] 2) Water-cooled mold casting is used, with the initial casting temperature controlled at 1550-1555℃, the main body casting time being 18-21 minutes, and the riser casting time being 5-8 minutes;

[0040] 3) Heat the steel ingots to a holding temperature of 1230–1250℃ for a total heating time of 18–22 hours;

[0041] 4) After descaling, the steel ingots are hot rolled. The initial rolling temperature is 1000-1150℃, the reduction per pass is 50-55mm, and the final rolling temperature is 950-1000℃.

[0042] 5) After rolling, the steel plate is rapidly cooled multiple times using ACC to control the reddening temperature to 530-580℃;

[0043] 6) The steel plates are stacked and cooled slowly, with the stacking temperature controlled at ≥450℃ and the cooling time at 24-48h, to fully diffuse the hydrogen contained in the steel plates and reduce stress;

[0044] 7) After slow cooling, quenching is performed at a temperature of 920-940℃ and a holding time of 1.8-2.0 min / mm. Water cooling is performed for 15-35 min to room temperature. During the quenching process, a chiller is used to control the water temperature to <16℃ and the cooling rate to 1-2℃ / s.

[0045] 8) Tempering and air cooling: The steel plate is loaded into the tempering furnace, heated rapidly with high power, tempering temperature 470~530℃, heat preservation coefficient 4.0min / mm, and then air cooled to room temperature.

[0046] Table 1. Chemical composition (wt%) of the embodiments and comparative examples of the present invention.

[0047]

[0048]

[0049] Table 2. Main process parameters of the embodiments and comparative examples of the present invention.

[0050]

[0051] Table 3 Performance testing results of embodiments and comparative examples of the present invention

[0052]

[0053] In summary, the steel plates obtained using this method have a thickness of 150–250 mm, a crack sensitivity coefficient Pcm ≤ 0.22%, a corrosion resistance coefficient I ≥ 6.5, a yield strength of 711–756 MPa, a tensile strength of 786–810 MPa, an elongation of 18–21%, and an impact energy of 155–220 J at -60℃. This approach can meet the requirements of extra-thickness, low weld crack sensitivity, and corrosion resistance, making it more suitable for use in harsh environments.

Claims

1. A method for producing 150-250mm corrosion-resistant, low-weld-crack-susceptibility high-strength steel Q620CF, characterized in that, This steel is composed of the following chemical composition by mass percentage: C: 0.05–0.08, Si: 0.2–0.3, Mn: 0.6–1.0, P: 0.010–0.015, S≤0.003, Nb: 0.020–0.030, V: 0.020–0.030, Als: 0.015–0.030, Cr: 0.50–0.60, Ni: 0.40–0.50, Mo: 0.30–0.35, Cu: 0.30–0.35, B: 0.0014–0.0020, with the remainder being Fe and residual elements. Its crack sensitivity coefficient Pcm: 0.17–0.22%, and corrosion resistance coefficient I: 6.5–7.

0. The production method of the above-mentioned 150-250mm corrosion-resistant, low-weld-crack-susceptibility high-strength steel Q620CF includes the following steps: 1) The smelting process of converter + LF refining + VD vacuum degassing is adopted to control the P content at 0.010~0.015%, H content ≤1.2PPm, and N content ≤50PPm; 2) Water-cooled mold casting is used, with the initial casting temperature controlled at 1550-1555℃, the main body casting time being 18-21 minutes, and the riser casting time being 5-8 minutes; 3) Heat the steel ingots to a holding temperature of 1230–1250℃ for a total heating time of 18–22 hours; 4) After descaling, the steel ingots are hot rolled. The initial rolling temperature is 1000-1150℃, the reduction per pass is 50-55mm, and the final rolling temperature is 950-1000℃. 5) After rolling, the steel plate is rapidly cooled multiple times using ACC to control the reddening temperature to 530-580℃; 6) The steel plates are stacked and cooled slowly, with the stacking temperature controlled at ≥450℃ and the cooling time at 24-48 hours; 7) After slow cooling, perform quenching at a temperature of 920–940℃, holding time of 1.8–2.0 min / mm, followed by water cooling for 15–35 min to room temperature. During the quenching process, use a chiller to control the water temperature to <16℃, with a cooling rate of 1–2℃ / s. 8) Tempering and air cooling: Load the steel plate into the tempering furnace, heat it rapidly with high power, tempering temperature 470~530℃, holding time 4.0min / mm, and air cool to room temperature.

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