Thick-walled seamless steel pipe and method for controlling grain size thereof

Abstract

The present application belongs to the technical field of seamless steel pipe manufacturing, and particularly relates to a thick-walled seamless steel pipe and a grain size control method thereof. The technical scheme avoids using valuable elements such as Ti, Nb and V, has small product performance fluctuation, and greatly reduces production cost. In the scheme, the grain size control method needs to strictly control the soaking section temperature in pipe blank heating to be 1180-1200 DEG C and the heating time to be less than 45 min, so as to improve the impact toughness of the steel pipe. The final rolling temperature is strictly limited to be lower than 930 DEG C, and the cooling speed is controlled after sizing to avoid the formation of coarse austenite grains. After rapid cooling of the steel pipe, the residual temperature of the inner surface of the steel pipe is used to make the outer surface temperature of the steel pipe reach 750-800 DEG C, and then air cooling is performed, so that fine granular ferrite + pearlite structure is obtained, thereby meeting the safety service requirement of the boiler pipe.

Description

Technical Field

[0001] This invention belongs to the field of seamless steel pipe manufacturing technology, specifically relating to a thick-walled seamless steel pipe and its grain size control method. Background Technology

[0002] 20G high-pressure boiler tubes are widely used in high and low pressure boilers, petroleum, chemical and other industries. They are one of the most widely used materials in boiler manufacturing. They are often used in boiler water-cooled walls, economizers, superheaters and other components with a rated operating temperature of ≤450℃, and are subjected to the scouring of high-temperature flue gas or steam for a long time.

[0003] The GB / T-5310 standard for seamless steel tubes for high-pressure boilers clearly specifies the composition, final rolling temperature, and metallographic structure of 20G steel tubes. It requires the steel tube structure to be ferrite + pearlite, and that no elements not mentioned should be added except for C, Si, Mn, P, and S. However, the development of controlled rolling and controlled cooling technology in the field of seamless steel tubes is still insufficient. Therefore, when the steel tube wall thickness reaches a certain level, the grains become coarse, resulting in Widmanstätten structure, which leads to substandard metallographic structure and mechanical properties, significantly deteriorating weldability, easy high-temperature decomposition, rapid softening, and reduced creep life of 20G products. Substandard steel tubes require normalizing treatment, wasting time and energy. To reduce production costs, a reasonable hot rolling process needs to be designed to ensure the microstructure and mechanical properties of 20G seamless steel tubes for high-pressure boilers, while shortening the production cycle and saving energy.

[0004] CN 103114249A discloses a manufacturing method for producing medium-thickness 20G seamless steel pipes using controlled rolling instead of normalizing. This method adds 0.03%~0.07% V to the 20G steel, while adjusting the chemical composition of C, Mn, etc., to 0.17%~0.21% C, 0.50%~0.65% Mn, P≤0.015%, S≤0.010%, and P+S≤0.020% to improve the yield strength and impact toughness of the thick-walled hot-rolled pipe. The final rolling temperature is controlled at 850~950℃. Although this method eliminates the need for normalizing and its performance indicators fully meet the requirements of GB / T 5310 for 20G high-pressure boiler tubes, it requires the addition of V to improve the performance of the seamless steel pipe. The addition of V significantly increases the alloy cost, making it suitable only for a few boiler manufacturers. Furthermore, this method requires different rolling mills, relying on increasing the amount of metal deformation to achieve performance optimization. Summary of the Invention

[0005] To address the shortcomings of existing thick-walled seamless steel pipes, such as the narrow applicability of their composition design leading to coarse grains and Widmanstätten structure when the wall thickness reaches 14mm or more, resulting in substandard metallographic structure and mechanical properties, significantly deteriorated weldability, easy decomposition at high temperatures, and rapid softening, thus reducing the creep life of 20G products; and the need for additional normalizing treatment for substandard steel pipes, which is time-consuming, energy-intensive, and increases production costs, this invention provides a method for controlling the grain size of thick-walled seamless steel pipes. This method eliminates the need for adding microalloying components to refine the grains, controlling the microstructure and grain size of hot-rolled thick-walled steel pipes solely through the rolling process, refining the pearlite lamellar spacing (heating process and post-rolling cooling), and achieving rolling on the same unit. This improves the performance of thick-walled seamless steel pipes while reducing the need for normalizing treatment, achieving qualified hot-rolled performance.

[0006] According to one aspect of the present invention, a thick-walled seamless steel pipe is provided, the composition of which is as follows: C: 0.17%~0.23%, Si: 0.26%~0.37%, Mn: 0.48%~0.65%, P≤0.015%, S≤0.010%, Si+Mn≥0.80%, with the remainder being Fe and trace impurities.

[0007] As a basic strengthening element in steel, carbon (C) is used in this invention to ensure the strength of the steel pipe. Excessive C content can exacerbate the banded structure on the inner wall of thick-walled steel pipes, reduce the low-temperature impact toughness of the steel pipe, and also mitigate the tendency of steel to graphitize under long-term high-temperature conditions. Therefore, the preferred C content in this invention is 0.17%~0.20%.

[0008] Si is a strong deoxidizer that removes oxygen from molten steel at the end of the steelmaking process, ensuring the purity of the steel. It also plays a role in solid solution strengthening, increasing the hardness and strength of the steel. Furthermore, less than 0.40% Si has minimal impact on plasticity, toughness, and weldability. Therefore, this invention precisely controls the Si content to be between 0.26% and 0.37%.

[0009] Mn can improve the strength and impact toughness of steel pipes, compensate for the strength loss after reducing carbon content, lower the critical transformation temperature of steel, and inhibit austenite grain growth, thereby refining ferrite grains. To ensure the hot-rolling performance of the steel pipe, the preferred Mn content in this invention is 0.48%~0.65%.

[0010] P and S are harmful elements. They readily accumulate at grain boundaries, reducing the low-temperature toughness of steel. This invention precisely controls P ≤ 0.015% and S ≤ 0.010%.

[0011] Based on the above technical solution, the outer diameter of the thick-walled seamless steel pipe is Φ140mm~Φ180mm, and the wall thickness is 14~25mm; Based on the above technical solution, the hot-rolled steel pipe has a yield strength ≥245MPa, a yield strength at 450℃ ≥150MPa, a tensile strength of 410~550MPa, Charpy V-type impact toughness: Charpy impact energy at 20℃ longitudinal full-size ≥40J, elongation ≥24%, and grain size of grade 7 or above.

[0012] Based on the above technical solution, the microstructure of the thick-walled seamless steel pipe is granular ferrite + pearlite, without coarse acicular ferrite, i.e. Widmanstätten structure.

[0013] According to another aspect of the present invention, a method for controlling the grain size of a thick-walled seamless steel pipe is provided, comprising the following steps: preparation of a continuously cast round billet - billet heating - hot rolling - post-rolling cooling - heat preservation followed by cooling; Preparation of continuously cast round billets; Billet heating: includes a heating section and a soaking section, with a total heating time of 180~240min; the temperature of the heating section is 1130~1180℃; the heating time in the soaking section at 1180~1200℃ is less than 45min (when 20G steel is heated to 1200℃, it is completely transformed into coarse austenite grains, in order to avoid grain boundary coarsening and the formation of hot corrosion grooves). Hot rolling: After the round billet passes through piercing, rolling, and sizing, the final rolling temperature of the thick-walled steel pipe is controlled to be below 930℃ (if heated to the traditional 1250℃, the final rolling temperature of the thick-walled steel pipe can reach 980~1000℃, dynamic recrystallization occurs fully, resulting in severe grain growth after rolling). Cooling after rolling.

[0014] Keep warm and then cool.

[0015] Based on the above technical solution, the preparation of the continuously cast round billet involves: pretreatment of molten iron, converter smelting, LF refining, and superheating of molten steel to ≤25℃ to obtain the continuously cast round billet.

[0016] Based on the above technical solution, the post-rolling cooling specifically involves: uniformly cooling the steel pipe circumferentially using an annular water ring spray method. The steel pipe advances at a speed of 1.0~1.2m / s in the water ring, with a cooling rate of 25~35℃ / s, thereby reducing the outer surface temperature of the steel pipe after final rolling to 700~750℃, preventing static recrystallization and the formation of coarse austenite grains.

[0017] Based on the above technical solution, the cooling after insulation is specifically as follows: after the steel pipe is rapidly cooled, it enters a 40m long steel pipe insulation tunnel at a forward speed of 1.0~1.2m / s. At this time, the residual heat on the inner surface of the steel pipe gradually returns to the outer surface, making the outer surface temperature of the steel pipe 750~800℃. After passing through the insulation tunnel, the steel pipe is air-cooled to room temperature on a cooling bed at a cooling rate of 0.25~0.35℃ / s.

[0018] Beneficial effects The technical solution disclosed in this invention avoids the use of precious elements such as Ti, Nb, and V, resulting in smaller fluctuations in product performance and a significant reduction in production costs. Since 20G steel transforms entirely into coarse austenite grains when heated to 1200℃, to prevent grain boundary coarsening and the formation of hot corrosion grooves, this invention strictly controls the temperature of the soaking zone during billet heating to 1180~1200℃, and the heating time to be less than 45 minutes, thereby improving the impact toughness of the steel pipe. Simultaneously, the technical solution disclosed in this invention strictly limits the final rolling temperature to below... After sizing at 930℃, the cooling rate is controlled to avoid the formation of coarse austenite grains. After rapid cooling of the steel pipe, the residual heat of the inner surface of the steel pipe is used to make the outer surface temperature of the steel pipe reach 750~800℃. Then, the cooling rate is controlled at 0.2~0.5℃ / s to obtain a fine granular ferrite + pearlite structure. This ensures that 20G achieves the best comprehensive balance of strength, plasticity, toughness, weldability, creep resistance and structural stability under medium temperature (≤450℃) and high pressure conditions, thus meeting the requirements for safe service of boiler tubes. Attached Figure Description

[0019] Figure 1 Metallographic image of the thick-walled seamless steel pipe prepared in Example 6; Figure 2 Metallographic image of the thick-walled seamless steel pipe prepared for Comparative Example 1. Detailed Implementation

[0020] To make the objectives and technical solutions of this invention clearer, the following embodiments are provided for further explanation. However, the scope of protection of this invention is not limited to these embodiments; the embodiments are merely for illustrative purposes. Those skilled in the art should understand that any changes or equivalent substitutions that do not depart from the concept of this invention are included within the scope of protection of this invention.

[0021] Unless otherwise specified, all reagents and raw materials used in this invention are obtained through purchase.

[0022] Example A thick-walled seamless steel pipe, wherein the composition of the thick-walled seamless steel pipe is as follows: C: 0.17%~0.23%, Si: 0.26%~0.37%, Mn: 0.48%~0.65%, P≤0.015%, S≤0.010%, Si+Mn≥0.80%, with the remainder being Fe and trace impurities. The specific elemental contents of the steel in Examples 1~7 and Comparative Examples 1~2 are shown in Table 1.

[0023] The microstructure of the thick-walled seamless steel pipe is granular ferrite + pearlite, without coarse acicular ferrite, i.e. Widmanstätten structure.

[0024] The method for controlling the grain size of the thick-walled seamless steel pipe includes the following steps: preparation of continuously cast round billet - billet heating - hot rolling - cooling after rolling - cooling after heat preservation; Preparation of continuously cast round billets: hot metal pretreatment, converter smelting, LF refining, steel superheat ≤25℃, to obtain continuously cast round billets; Billet heating: includes a heating section and a soaking section, with a total heating time of 180~240min; the temperature of the heating section is 1130~1180℃; the heating time in the soaking section at 1180~1200℃ is less than 45min (when 20G steel is heated to 1200℃, it is completely transformed into coarse austenite grains, in order to avoid grain boundary coarsening and the formation of hot corrosion grooves). Hot rolling: After the round billet passes through piercing, rolling, and sizing, the final rolling temperature of the thick-walled steel pipe is controlled to be below 930℃ (if heated to the traditional 1250℃, the final rolling temperature of the thick-walled steel pipe can reach 980~1000℃, dynamic recrystallization occurs fully, resulting in severe grain growth after rolling). Post-rolling cooling: The steel pipe is uniformly cooled circumferentially by an annular water ring spray. The steel pipe advances at a speed of 1.0~1.2m / s in the water ring, and the cooling rate is 25~35℃ / s, so that the outer surface temperature of the steel pipe after final rolling is reduced to 700~750℃, avoiding static recrystallization and the formation of coarse austenite grains.

[0025] Cooling after insulation: After rapid cooling, the steel pipe enters a 40m long steel pipe insulation tunnel at a forward speed of 1.0~1.2m / s. At this time, the residual heat on the inner surface of the steel pipe gradually returns to the outer surface, so that the temperature of the steel pipe reaches 750~800℃. After passing through the insulation tunnel, the steel pipe is air-cooled to room temperature on a cooling bed at a cooling rate of 0.25~0.35℃ / s. The key process parameters are shown in Table 2.

[0026] Table 1 Chemical composition (wt%) of thick-walled seamless steel pipes

[0027] Table 2 Key Process Parameters

[0028] Table 3 Mechanical properties of thick-walled seamless steel pipes

[0029] Figure 1 The image shows a microstructure of the thick-walled seamless steel pipe prepared in Example 6 of the present invention, illustrating that the technical solution disclosed in the present invention successfully prepared a thick-walled seamless steel pipe with a metallographic structure of granular ferrite + pearlite and no coarse acicular ferrite. Figure 2 The seamless steel pipe prepared for Comparative Example 1 clearly shows Widmanstätten structure, with an elongation of 28.5% and a longitudinal full-size Charpy impact energy of 36J.

[0030] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A thick-walled seamless steel pipe, characterized in that, The composition of the thick-walled seamless steel pipe is as follows: C: 0.17%~0.23%, Si: 0.26%~0.37%, Mn: 0.48%~0.65%, P≤0.015%, S≤0.010%, Si+Mn≥0.80%, with the remainder being Fe and trace impurities.

2. The thick-walled seamless steel pipe according to claim 1, characterized in that, The outer diameter of the thick-walled seamless steel pipe is Φ140mm~Φ180mm, and the wall thickness is 14~25mm.

3. The thick-walled seamless steel pipe according to claim 1, characterized in that, Hot-rolled steel pipes have a yield strength ≥245MPa, a yield strength ≥150MPa at 450℃, a tensile strength of 410~550MPa, Charpy V-type impact toughness: Charpy impact energy ≥40J at 20℃ longitudinal full-size, elongation ≥24%, and grain size of grade 7 or above.

4. The thick-walled seamless steel pipe according to claim 1, characterized in that, The microstructure of the thick-walled seamless steel pipe is granular ferrite + pearlite, without coarse acicular ferrite, i.e. Widmanstätten structure.

5. A method for controlling the grain size of thick-walled seamless steel pipes, characterized in that, The process includes the following steps: preparation of continuously cast round billets - billet heating - hot rolling - post-rolling cooling - heat preservation followed by cooling; Preparation of continuously cast round billets; Tube blank heating: includes a heating section and a soaking section, with a total heating time of 180~240min; the temperature of the heating section is 1130~1180℃; the heating time in the soaking section at 1180~1200℃ is less than 45min; Hot rolling: Final rolling temperature below 930℃; Post-rolling cooling: The steel pipe is uniformly cooled circumferentially by an annular water ring spray. The steel pipe advances at a speed of 1.0~1.2m / s in the water ring, and the cooling rate is 25~35℃ / s, so that the outer surface temperature of the steel pipe after final rolling is reduced to 700~750℃. Keep warm and then cool.

6. The grain size control method according to claim 5, characterized in that, The preparation of the continuously cast round billet involves: hot metal pretreatment, converter smelting, LF refining, and steel superheating ≤25℃ to obtain the continuously cast round billet.

7. The grain size control method according to claim 5, characterized in that, The cooling process after insulation is as follows: the steel pipe enters the insulation tunnel at a forward speed of 1.0~1.2m / s. At this time, the residual heat on the inner surface of the steel pipe gradually returns to the outer surface, so that the outer surface temperature of the steel pipe when it exits the insulation tunnel is 750~800℃. After passing through the insulation tunnel, the steel pipe is air-cooled to room temperature on a cooling bed at a cooling rate of 0.25~0.35℃ / s.

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