Preparation method for improving rolling and perforating yield of round steel for ultra-supercritical boiler pipe

Abstract

The invention discloses a preparation method for improving rolling and perforating yield of round steel for an ultra-supercritical boiler tube, which comprises the following steps: step a, taking 14-inch ingot type austenitic heat-resistant steel with the trademark of TP310HCbN to obtain a product A; step b, heating the product A to 900 + / -20 DEG C, keeping the temperature for 1.5-2.5 hours, then heating to 1250 + / -10 DEG C, and keeping the temperature for 8-10 hours to obtain a product B; c, the product B is subjected to multi-pass cogging and round rolling, in the rolling process, the temperature of the product B is kept between 1000 DEG C and 1260 DEG C, and a product C is obtained; and step d, peeling, sizing, heating and perforating the product C to obtain a finished product. The method has the advantages of low energy consumption, high production efficiency, high yield and high finished product quality.

Description

Technical Field

[0001] This invention belongs to the field of improving the yield of round steel for ultra-supercritical boiler tubes, and particularly relates to a preparation method for improving the rolling and piercing yield of round steel for ultra-supercritical boiler tubes. Background Technology

[0002] Ultra-supercritical boilers are high-parameter power plant boilers used in thermal power plants. The main steam pressure reaches 25-31 MPa, the temperature is between 580-610℃, and the power generation efficiency can reach over 41%.

[0003] TP310HCbN (25Cr-20Ni-N-Nb-N) austenitic heat-resistant steel is a core material for manufacturing key pipe components in ultra-supercritical power plant boilers. Its production process typically involves first shaping 16-20 inch steel ingots into rounds through a single-fire rolling process, followed by intermediate billet grinding and a second intermediate billet firing. Then, it is hot-pierced to form tubes, resulting in a long process and low yield. Furthermore, due to its high alloy content, high resistance to high-temperature deformation, and the presence of easily segregating niobium (Nb), this steel is highly susceptible to cracking during round steel rolling and piercing, further reducing the yield to only 82%. Summary of the Invention

[0004] The purpose of this invention is to provide a method for improving the yield of rolled and pierced round steel for ultra-supercritical boiler tubes. This invention has the advantages of low energy consumption, high production efficiency, high yield, and high quality of the finished product.

[0005] The technical solution of this invention: a method for improving the yield of rolled and pierced round steel for ultra-supercritical boiler tubes, comprising the following steps: Step a: Take a 14-inch ingot of austenitic heat-resistant steel, grade TP310HCbN, to obtain product A; Step b: Heat product A to 900±20℃ and hold for 1.5-2.5 hours, then heat to 1250±10℃ and hold for 8-10 hours to obtain product B; Step c: After multiple passes of blanking, product B is rolled into a circle. During the rolling process, the temperature of product B is maintained between 1000-1260℃ to obtain product C. Step d: Peel, measure, heat, and pierce product C to obtain the finished product.

[0006] In the aforementioned method for improving the yield of rolled and pierced round steel for ultra-supercritical boiler tubes, step a involves heating product A to 900±20℃, holding it at that temperature for 2 hours, then heating it to 1250±10℃ and holding it at that temperature for 9 hours to obtain product B.

[0007] In the aforementioned method for improving the yield of round steel for ultra-supercritical boiler tubes through rolling and piercing, in step c, the B-grade billet is rolled 15-19 times to obtain a square billet intermediate billet of 180±5mm. The square billet intermediate billet is then rolled into a round steel, i.e., C-grade, with a diameter of 110±3mm.

[0008] In the aforementioned method for improving the yield of rolled and pierced round steel for ultra-supercritical boiler tubes, in step d, the outer diameter of the finished product is 110±3mm and the wall thickness is 12-15mm.

[0009] In the aforementioned method for improving the yield of rolled and pierced round steel for ultra-supercritical boiler tubes, in step b, product A is heated using a pusher-type heating furnace.

[0010] Compared with existing technologies, this invention achieves one-fire production by optimizing the ingot shape, the ingot heating temperature, and the rolling process, thereby reducing energy consumption, shortening the process steps, improving production efficiency, increasing the yield by 7%, and reaching a yield of 89%. In addition, the niobium (Nb) compound level in the resulting product is ≤1.0, indicating high quality. Attached Figure Description

[0011] Figure 1 This is the heating curve for step b.

[0012] Figure 2 This is a photograph of product C obtained by hot rolling in step c.

[0013] Figure 3 This is a picture of the finished product's exterior.

[0014] Figure 4 This is an internal photograph of the finished product.

[0015] Figure 5 This is a schematic diagram showing the uniform distribution of niobium (Nb) compounds in the round steel of the embodiment. Figure 6 This is a schematic diagram of niobium (Nb) compound accumulation in round steel under existing processes.

[0016] Figure 7 It is a thermo-calc graph. Detailed Implementation

[0017] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0018] Example: A method for improving the yield of rolled and pierced round steel for ultra-supercritical boiler tubes includes the following steps: Step a: Take 1.1 tons of 14-inch ingot-shaped austenitic heat-resistant steel, grade TP310HCbN, with the following composition: C: 0.04~0.10%, Si: 0.2~0.6%, Mn: 0.8~1.5%, S≤0.005%, P≤0.030%, Cr: 25.0~27.0%, Ni: 20.0~21.0%, Mo: ≤0.5%, N: 0.22~0.28%, Nb: 0.40~0.50%, to obtain grade A.

[0019] Step b: Heat product A in a pusher-type heating furnace. Heat product A to 900±20℃ and hold for 2 hours. Then heat it to 1250±10℃ and hold for 9 hours to obtain product B.

[0020] Step c: Product B undergoes 17 passes of continuous rolling to produce a square intermediate billet of 180±5mm. This intermediate billet is then rolled into a round bar (Product C) with a diameter of 110mm through 6 passes of continuous rolling. Throughout the rolling process, the hot rolling deformation temperature is maintained at 1000-1260℃, with a further optimized range of 1100-1150℃.

[0021] Step d: Remove the oxide scale from product C, cut it to the required length according to the weight of the finished product, generally 1-2m, heat it to 1150-1200℃, pierce it, and obtain the finished product. The outer diameter of the finished product is 110mm and the wall thickness is 12-15mm.

[0022] Compared to existing methods for rolling and piercing round steel bars, the improvements made in this embodiment include the following: First, in step a, a smaller ingot with a single weight of 1.1 tons was used, which is smaller than the traditional ingot of 1.7 tons. A comparison of these two ingot types shows that the smaller ingot facilitates rapid solidification of molten steel during casting, reducing the segregation and aggregation of niobium (Nb). Furthermore, the smaller ingot size reduces its cross-sectional diameter, making it easier to achieve a single-fire forming process, reducing energy consumption, minimizing finishing processes required by multiple firings, and improving production efficiency.

[0023] Second, in step b, the heating method was optimized. For example... Figure 5 and Figure 6 As shown, by homogenizing high-temperature diffusion simulation and extending the high-temperature heating and holding time, the segregation and aggregation of niobium (Nb) can be improved, thereby achieving a uniform distribution of niobium (Nb) compounds and reducing internal crack defects caused by niobium compound aggregation during the perforation process.

[0024] Third, in step c, the rolling method was optimized. For example... Figure 7As shown, calculations based on the thermo-calc thermodynamic phase diagram indicate that this type of heat-resistant steel is at risk of σ phase precipitation when held at temperatures between 450-850℃ for an extended period. The σ phase degrades hot working plasticity and is detrimental to hot working performance, therefore, it is necessary to strictly control its hot working temperature and final rolling temperature.

[0025] This type of heat-resistant steel has an austenitic structure between 1000-1300℃, which is beneficial for hot working. However, when the ingot temperature exceeds 1280℃, the structure tends to coarsen, leading to poor hot working plasticity. At excessively low temperatures, the resistance to hot deformation increases, resulting in more rolling passes and deformation cracks. Therefore, considering all factors, the hot working rolling temperature of this steel should be controlled between 1000-1260℃.

[0026] The surface quality of this type of heat-resistant steel is related not only to the heating temperature but also to the amount of hot working deformation. While ensuring the hot working temperature, reducing the amount of rolling deformation, especially decreasing the initial rolling reduction and increasing the number of rolling passes, can improve the surface crack defects of stainless steel. Examples were obtained by adjusting the process according to Table 1 below, resulting in multiple experimental cases. These verified that after segmented heating of the steel ingot, after 15-19 rolling passes to form an intermediate billet with dimensions of approximately 180*180*Lmm, the final rolled C-grade product had a good surface without cracks.

[0027] Table 1 Table 1 shows the corresponding preheating section, heating section 1, heating section 2, heating section 3, and soaking section. Figure 1 The heating curve shows that the first heating stage is held for 2 hours, and the total holding time for the second heating stage, the third heating stage, and the homogenization stage is 8-10 hours.

[0028] In Experiment 1, the final heating temperature of the steel ingot was relatively low, resulting in cracks on the surface of product C.

[0029] In Experiment 3, the final heating temperature of the steel ingot was relatively high, resulting in cracks on the surface of product C.

[0030] In Experiment Example 4, the final heating temperature of the steel ingot was low, and there were too many rolling passes, resulting in excessive rolling temperature drop and causing cracks on the surface of the C-grade product.

[0031] In Experiment Example 5, the number of rolling passes was too small, and the deformation per single pass was too large, resulting in cracks on the surface of the C-grade product.

[0032] In Example 6, too many rolling passes resulted in excessive rolling temperature drop, leading to cracks on the surface of the C-grade product.

Claims

1. A method for improving the yield of round steel rolling and piercing for ultra-supercritical boiler pipes, characterized in that: The method comprises the following steps: Step a, taking 14-inch ingot austenitic heat-resistant steel, the grade of the austenitic heat-resistant steel is TP310HCbN, obtaining A product; Step b, heating the A product to 900±20 DEG C and keeping for 1.5-2.5 hours, then heating to 1250±10 DEG C and keeping for 8-10 hours, obtaining B product; Step c, after multi-pass blooming of the B product, rolling the B product into round steel, keeping the temperature of the B product between 1000-1260 DEG C during the rolling process, obtaining C product; Step d, peeling, sizing, heating and piercing the C product, obtaining finished product.

2. The method of claim 1, wherein the method further comprises: In the step a, the A product is heated to 900±20 DEG C and kept for 2 hours, then heated to 1250±10 DEG C and kept for 9 hours, obtaining B product. ​ 3. The method of claim 1, wherein the method further comprises: In the step c, the B product is bloomed for 15-19 passes, obtaining 180±5 mm square billet intermediate billet, the square billet intermediate billet is rolled into round steel for 5-7 passes, obtaining C product, the diameter of the C product is 110±3 mm. ​ 4. The method of claim 1, wherein the method further comprises: In the step d, the outer diameter of the finished product is 110±3 mm, and the wall thickness is 12-15 mm. ​ 5. The method of claim 1, wherein the method further comprises: In the step b, the A product is heated by a pusher-type heating furnace. ​

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