A heat treatment method for refining grains of g115 steel

Abstract

A heat treatment method for refining grains of G115 steel, comprising the following steps: 1) quenching and tempering heat treatment, heating G115 finished steel pipe, quenching and tempering temperature is 745-790 DEG C, holding for 0.2D+8 hours-0.2D+20 hours, air cooling; 2) normalizing treatment, normalizing temperature is 1050-1120 DEG C, holding for 0.2D+1 hour-0.2D+4 hours, water quenching to room temperature; 3) tempering treatment, tempering temperature is 745-790 DEG C, holding for 0.2D+3 hours-0.2D+7 hours, air cooling; D is the wall thickness of G115 finished steel pipe, unit is decimeter. The application can be used as a conventional production heat treatment process, can refine grains and improve mechanical properties of materials, and can also realize grain refinement and mechanical property improvement of G115 finished material due to unqualified grain size and mechanical properties. The grain size of the G115 steel after treatment is finer than 1 level, the room temperature tensile strength is greater than or equal to 660 MPa, the yield strength is greater than or equal to 480 MPa, and the 630 DEG C yield strength is greater than or equal to 271 MPa.

Description

Technical Field

[0001] This invention relates to the field of steel heat treatment, and more particularly to a heat treatment method for refining the grain size of G115 steel. Background Technology

[0002] G115 steel is a martensitic heat-resistant steel independently developed in China, as disclosed in Chinese patent CN103045962B. It aims to solve the material bottleneck in high-temperature components of efficient and clean coal-fired power plants. Its characteristics include high strength, high toughness, excellent high-temperature oxidation resistance and corrosion resistance. At 650℃, its creep strength is 1.5 times that of P92 steel, allowing for thinner component walls and extended service life. It is mainly used in key pressure-bearing components of ultra-supercritical thermal power units, such as boiler headers and steam pipes.

[0003] Because G115 steel has high high-temperature strength and resistance to high-temperature deformation is much higher than that of conventional steel grades, it is generally prone to insufficient hot working deformation and incomplete dynamic recrystallization when producing large-diameter thick-walled pipes and large-specification forgings. Therefore, during finished product inspection, its grain size is generally -4 to -2 grade, which reduces its room temperature tensile strength, high temperature tensile strength and high temperature creep performance, and affects its actual service life.

[0004] Generally, thick-walled pipes and large-size forgings have a large unit weight, and the process cost of modifying a single piece of material for production is estimated to be 50,000 to 150,000 yuan. If modification is not possible, the product will be scrapped, with a scrap loss of 400,000 to 800,000 yuan per piece. Therefore, developing a method to refine the grain size of G115 steel and improve its room temperature and high temperature mechanical properties without affecting its dimensional specifications is of great significance. Summary of the Invention

[0005] The purpose of this invention is to provide a heat treatment method for refining the grain size of G115 steel. This method can be used as a conventional production heat treatment process to refine the grain size and improve the mechanical properties of the material. It can also be used to salvage G115 finished products that do not meet the requirements for grain size and mechanical properties, thereby refining the grain size and improving the mechanical properties. The method is easy to operate, requires no secondary deformation, does not affect the dimensional specifications of G115 steel, and can significantly refine its grain size, thereby improving its room temperature and high temperature mechanical properties. The grain size of the G115 steel treated by this invention is finer than grade 1, with a room temperature tensile strength ≥660MPa, a yield strength ≥480MPa, and a yield strength ≥271MPa at 630℃.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: A heat treatment method for refining the grain size of G115 steel includes the following steps: 1) Quenching and tempering heat treatment: Heat the finished G115 steel pipe to a quenching and tempering temperature of 745~790℃, hold for 0.2D+8 hours to 0.2D+20 hours, and then air cool; 2) Normalizing heat treatment: Heat the quenched and tempered G115 finished steel pipe at a normalizing temperature of 1050~1120℃ and a holding time of 0.2D+1 hour to 0.2D+4 hours, then water quench to room temperature; 3) Tempering heat treatment: Heat the normalized G115 finished steel pipe to a tempering temperature of 745~790℃, hold for 0.2D+3 hours to 0.2D+7 hours, and then air cool. Where D represents the wall thickness of the G115 finished steel pipe, in decimeters.

[0007] The grain size of the microstructure of the G115 steel is finer than grade 1.

[0008] The G115 steel has a room temperature tensile strength ≥660MPa, a yield strength ≥480MPa, and a yield strength ≥271MPa at 630℃.

[0009] The weight percentage composition of the G115 steel is as follows: C 0.060~0.100%, Si≤0.55%, Mn 0.27~0.73%, P≤0.020%, S≤0.010%, Cr 8.40~9.60%, W 2.33~3.17%, Co 2.80~3.25%, Cu 0.40~1.20%, V 0.13~0.27%, Nb 0.03~0.10%, N 0.005~0.019%, B 0.008~0.022%, Ni≤0.13%, Ti≤0.02%, Al≤0.015%, O≤0.0040%, As≤0.015%, Sb≤0.015%, Bi≤0.005%, Sn≤0.020%, Pb≤0.015%, As+Sb+Bi+Sn+Pb≤0.035%, balance is Fe and unavoidable impurities.

[0010] Because the grain size of G115 steel large-diameter pipes is generally between -4 and -2 during production, it cannot meet product standard requirements. Conventional treatment methods include normalizing, normalizing + tempering, or austenitizing + tempering (Chinese Patent CN103045962B). Normalizing or austenitizing can transform ferrite into austenite, during which austenite nucleates at grain boundaries, thereby refining the grain size. However, in actual production, normalizing or austenitizing alone only improves the grain size of G115 steel large-diameter pipes from -4 to -2 to -4 to -1, which is insufficient to meet product performance requirements.

[0011] Meanwhile, G115 is a martensitic steel. After normalizing or austenitizing, its microstructure will transform from austenite to martensite, increasing its strength and hardness, but also increasing its cracking tendency and decreasing its plasticity. Therefore, tempering is necessary to reduce the cracking tendency and improve the material's plasticity while ensuring that the material's strength and hardness meet the requirements. However, neither normalizing + tempering nor austenitizing + tempering can refine the austenite grains.

[0012] As is well known, the larger the grain size, the smaller the grain boundary area. Furthermore, grain boundaries are nucleation sites for austenite; the more nucleation sites there are, the more new grains are formed from the original austenite grains, resulting in a better grain refinement effect. Because the grains of G115 steel are too large to be directly observed with the naked eye, their grain boundary area is relatively small, leading to poor grain refinement during the normalizing process.

[0013] This invention innovatively designs a heat treatment process route based on the characteristics of G115 steel: quenching and tempering + normalizing + tempering. This approach is not simply adding quenching and tempering to normalizing and tempering heat treatment; rather, it refines the grain size of G115 steel and improves its mechanical properties by fully integrating the three heat treatments, taking into account the precipitation and microstructure characteristics of G115 steel.

[0014] The core key point of the process design of this invention is the quenching and tempering treatment. The normalizing treatment and tempering treatment are designed specifically for the precipitation characteristics and microstructure characteristics of G115 steel after quenching and tempering treatment.

[0015] Thermodynamic calculations show that during tempering, G115 steel precipitates dispersed, fine carbides, transforming its matrix structure into a carbide + ferrite structure. A concentration gradient exists at the carbide-ferrite interface, providing favorable conditions for austenite nucleation. If the steel has a large number of carbide-ferrite interfaces, when the G115 steel is heated to its full austenitization temperature, austenite can nucleate not only at grain boundaries but also at the carbide-ferrite interface, thus increasing the number of austenite nucleation sites. The more austenite nucleation sites there are, the better the austenite grain boundary segmentation effect, resulting in better grain refinement.

[0016] However, during the quenching and tempering process, the alloying elements dissolved in G115 steel are fully desoluble, and a large amount of carbides precipitate, resulting in a significant decrease in the strength of G115. The room temperature tensile strength is <660MPa, the yield strength is <480MPa, and the yield strength at 630℃ is <271MPa. Furthermore, quenching and tempering alone cannot refine the grain size of G115, resulting in a grain size coarser than grade 1, which cannot meet the actual application requirements of G115 steel. Therefore, normalizing treatment is necessary to raise the temperature of G115 steel above the complete austenitizing temperature. During this process, carbides dissolve back into the matrix, and the matrix structure transforms from ferrite to austenite. Because a large number of carbides are formed during quenching and tempering, there are more carbide-ferrite phase interfaces, resulting in more austenite nucleation sites and better austenite grain boundary segmentation, thus achieving better grain refinement.

[0017] After normalizing, the grain size of G115 steel is finer than grade 1, with a room temperature tensile strength ≥660MPa, a yield strength ≥480MPa, and a yield strength at 630℃ ≥271MPa, meeting product requirements. However, after normalizing, G115 steel transforms from austenite to martensite, and a large amount of alloying elements are dissolved in the interstitial spaces, placing it in a supersaturated and unstable state. At this point, G115 steel is highly prone to cracking. To reduce the cracking tendency of G115 steel, avoid secondary hardening peaks as much as possible, and ensure that its room temperature tensile strength is ≥660MPa, yield strength is ≥480MPa, and yield strength at 630℃ is ≥271MPa, a tempering heat treatment process was specifically designed.

[0018] Specifically, in the process design of quenching and tempering + normalizing + tempering described in this invention: ① The quenching and tempering heat treatment process fully utilizes the characteristics of G115 steel to improve the phase interface between carbides and ferrite, increase the number of austenite nucleation sites, and thus achieve the effect of segmenting the original austenite grain boundaries and refining the grains. The temperature of the quenching and tempering heat treatment is determined by the characteristics of G115 steel. The purpose of quenching and tempering heat treatment is to rapidly precipitate a large amount of carbides. Increasing the quenching and tempering heat treatment temperature can increase the precipitation rate and amount of carbides. Therefore, the lower limit of the quenching and tempering heat treatment temperature is set at 745℃. However, the temperature of the quenching and tempering heat treatment should not be too high. Since the austenite transformation temperature of G115 steel is approximately 795℃, if the quenching and tempering temperature is higher than 795℃, a certain amount of austenite will form in G115 steel. During the subsequent cooling process, the austenite will transform into martensite, increasing the cracking risk of G115. Considering the temperature control capability of conventional heat treatment equipment, a 5℃ allowance is made, so the upper limit of the quenching and tempering heat treatment temperature is set at 790℃. The holding time for quenching and tempering heat treatment also needs to be specified. If the holding time is too short, carbide precipitation will be insufficient, the phase interface area between carbides and ferrite will be small, and the original austenite grain segmentation effect will be poor. If the holding time is too long, small-sized carbides will dissolve and large-sized carbides will grow during the holding process of G115 steel, reducing the phase interface area between carbides and ferrite. Therefore, the holding time for quenching and tempering heat treatment should be controlled between 0.2D+8 hours and 0.2D+20 hours, followed by air cooling, where D represents the wall thickness of the finished G115 steel pipe in decimeters.

[0019] ② The normalizing heat treatment process, combined with the dissolution characteristics of precipitated phases in G115 steel, allows the matrix structure of G115 to fully transform into austenitic grains. The carbides precipitated during the quenching and tempering heat treatment are fully dissolved, and the grain size does not significantly increase. Based on the above design concept, the normalizing heat treatment temperature and holding time are determined. Since G115 steel has a high chromium content, and this invention has already precipitated a large amount of chromium-based carbides through quenching and tempering, if the heating temperature is below 1050℃, the superheat for dissolution of chromium-based carbides will be insufficient, resulting in a longer holding time and poorer dissolution effect. If the heating temperature is above 1120℃, a large amount of molybdenum-based carbides in G115 steel will dissolve, significantly reducing the number of carbides pinning grain boundaries, making it easier for grains to merge and grow. Therefore, the normalizing heating temperature of this invention is 1050~1120℃. This invention has corresponding regulations on the holding time; if the holding time is insufficient, the dissolution of G115 steel at 1050℃ will be incomplete. If the holding time is too long, a large amount of molybdenum carbides pinning the grain boundaries in G115 steel will dissolve at 1120℃, and the grains will grow rapidly. Therefore, the normalizing holding time of this invention is 0.2D+1 hours to 0.2D+4 hours, followed by water cooling, where D represents the wall thickness of the finished G115 steel pipe in decimeters.

[0020] ③ The tempering heat treatment process, combined with the precipitation characteristics of carbides and precipitates in G115 steel, significantly improves the mechanical properties of G115 steel by secondary precipitation of dispersed fine carbides and precipitates from the matrix through precipitation strengthening. Based on the above design concept, the tempering heat treatment temperature and holding time are determined. If the tempering temperature is below 745℃, the carbide precipitation rate decreases, and incomplete precipitation of precipitates may occur. If the tempering temperature is above 790℃, a certain amount of austenite will form in G115 steel, which will transform into martensite during subsequent cooling, increasing the cracking risk of G115. Therefore, this invention sets the tempering temperature to 745-790℃. If the tempering time is too low, at 745℃, the precipitation of carbides and precipitates will be incomplete, failing to fully improve the mechanical properties of G115 steel. If the tempering time is too long, small-sized carbides and precipitates will dissolve and large-sized carbides and precipitates will grow during the heat preservation process of G115 steel at 790℃. As the carbides and precipitates continue to grow, the interface relationship between the carbides, precipitates and the matrix will gradually change from a coherent interface to a semi-coherent interface and an incoherent interface, reducing the precipitation strengthening effect of carbides and precipitates. Therefore, this invention sets the tempering temperature to 745~790℃, the heat preservation time to 0.2D+3 hours~0.2D+7 hours, and air cooling, where D represents the wall thickness of the finished G115 steel pipe in decimeters.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Chinese Patent CN121228235A discloses a method for preparing titanium alloy tubing. This method refines grains by optimizing the composition and utilizing the pinning effect of nano-yttrium oxide to hinder grain growth during heat preservation. This invention refines grains by increasing the nucleation points of austenite transformation and enhancing the segmentation effect of austenite grain boundaries during the austenite transformation process; the design approach is inconsistent.

[0022] 2. Chinese patent CN121289240A discloses a rolling method for hot-rolled Q450NQR1 weathering H-beams, which refines the grain structure through large reduction and low-temperature rolling. This invention refines the grains through heat treatment, a design approach that differs from the present invention.

[0023] 3. Chinese patent CN121065445A discloses a heat treatment method for optimizing the microstructure and refining the grains of gear steel. This method precipitates AlN at grain boundaries during heat treatment, pinning the grain boundaries and inhibiting austenite grain growth during quenching. This invention refines the grains by increasing the nucleation points of austenite transformation and improving the grain boundary segmentation effect during the austenite transformation process; however, the design approach is inconsistent.

[0024] 4. Chinese patent CN106119469A discloses a heat treatment process for refining grains in large forgings, which refines grains through normalizing heat treatment; however, the grain size of the G115 large-diameter thick-walled pipe and large-specification forgings involved in this invention is -4 to -2 grade, with fewer austenite nucleation points. It has been confirmed that the patented method cannot effectively divide the original austenite grain boundaries, so the improvement effect on grain size is limited. See the comparative example for details.

[0025] 5. Chinese patent CN105385822A discloses a heat treatment method that breaks the genetic structure of large P91 / P92 material forgings and refines the grains. It refines the grains through multi-stage heat preservation + normalizing treatment, with a heat preservation temperature range of 660~720℃. However, the design mechanism is not explained in this patent.

[0026] This invention increases the number of austenite nucleation sites, adjusts the microstructure through a single tempering process, and effectively refines the grains through normalizing. The design concept and process are simpler, and the holding temperature range is 745~790℃. The concepts and processes of the two are different. Attached Figure Description

[0027] Figure 1 This is a 50x magnified metallographic image of the structure before processing in Example 1 of the present invention; Figure 2 This is a 100x magnified metallographic image of the structure after processing according to Example 1 of the present invention; Figure 3 This is a 50x magnified metallographic image of the structure before processing in Example 2 of the present invention; Figure 4 This is a 100x magnified metallographic image of the structure after processing in Example 2 of this invention; Figure 5 This is a 100x magnified metallographic image of the structure before processing in Example 3 of the present invention; Figure 6 This is a 100x magnified metallographic image of the structure after processing in Example 3 of this invention; Figure 7 This is a 50x magnified metallographic image of the structure before processing in Example 4 of the present invention; Figure 8 This is a 200x magnified metallographic image of the structure after processing in Example 4 of this invention; Figure 9 This is a 100x magnified metallographic image of the structure before processing in Comparative Example 1 of this invention; Figure 10 This is a 100x magnified metallographic image of the structure after processing, as shown in Comparative Example 1 of this invention. Detailed Implementation

[0028] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0029] The heat treatment process parameters of the G115 large-diameter thick-walled tube in this embodiment of the invention are shown in Table 1. Table 2 shows the grain size grade of the G115 large-diameter thick-walled tube. Table 3 shows the mechanical properties of the G115 large-diameter thick-walled tube in this embodiment of the invention.

[0030] Example 1 The specifications for G115 large-diameter thick-walled tubes are Φ530mm × 120mm, where 120mm is the wall thickness. Their heat treatment methods include: 1) Quenching and tempering heat treatment: hold at 745℃ for 20 hours, then air cool; 2) Normalizing heat treatment: hold at 1050℃ for 4 hours, then water quench; 3) Tempering heat treatment: hold at 760℃ for 6 hours, then air cool.

[0031] Upon testing, the G115 photos before and after processing according to the method of this invention are as follows: Figure 1 and Figure 2 As shown in Table 1, the heat treatment process is shown in Table 2, the grain size grade is shown in Table 3, and the mechanical properties are shown in Table 3. It can be seen that after heat treatment by the method of the present invention, the grains are significantly refined and the grain size grade is significantly improved.

[0032] Example 2 The specifications for G115 large-diameter thick-walled tubes are Φ762mm × 105mm, where 105mm is the wall thickness. Their heat treatment methods include: 1) Tempering heat treatment: hold at 790℃ for 12 hours, then air cool; 2) Normalizing heat treatment: hold at 1120℃ for 2.5 hours, then water quench; 3) Tempering heat treatment process: hold at 745℃ for 6 hours, then air cool.

[0033] Upon testing, the G115 photos before and after processing according to the method of this invention are as follows: Figure 3 and Figure 4 As shown in Table 1, the heat treatment process is shown in Table 2, the grain size grade is shown in Table 3, and the mechanical properties are shown in Table 3. It can be seen that after heat treatment by the method of the present invention, the grains are significantly refined and the grain size grade is significantly improved.

[0034] Example 3 The specifications for G115 large-diameter thick-walled tubes are Φ625mm × 88mm, where 88mm is the wall thickness. Their heat treatment methods include: 1) Tempering heat treatment: hold at 760℃ for 8.5 hours, then air cool; 2) Normalizing heat treatment: hold at 1080℃ for 1.5 hours, then water quench; 3) Tempering heat treatment: hold at 780℃ for 3.5 hours, then air cool.

[0035] Upon testing, the G115 photos before and after processing according to the method of this invention are as follows: Figure 5 and Figure 6 As shown in Table 1, the heat treatment process is shown in Table 2, the grain size grade is shown in Table 3, and the mechanical properties are shown in Table 3. It can be seen that after heat treatment by the method of the present invention, the grains are significantly refined and the grain size grade is significantly improved.

[0036] Example 4 The dimensions of the G115 large-size forging are 910mm × 550mm × 780mm, with a minimum thickness of 550mm. Its heat treatment methods include: 1) Quenching and tempering heat treatment: hold at 770℃ for 21.5 hours, then air cool; 2) Normalizing heat treatment: hold at 1080℃ for 5.5 hours, then water quench; 3) Tempering heat treatment: hold at 790℃ for 8.5 hours, then air cool.

[0037] Upon testing, the G115 photos before and after processing according to the method of this invention are as follows: Figure 7 and Figure 8 As shown in Table 1, the heat treatment process is shown in Table 2, the grain size grade is shown in Table 3, and the mechanical properties are shown in Table 3. It can be seen that after heat treatment by the method of the present invention, the grains are significantly refined and the grain size grade is significantly improved.

[0038] Comparative Example The specifications for G115 large-diameter thick-walled tubes are Φ530mm × 120mm, where 120mm is the wall thickness. Their heat treatment methods include: 1) Normalizing heat treatment, holding at 1080℃ for 4 hours, followed by water quenching; 2) Tempering heat treatment: hold at 780℃ for 6.5 hours, then air cool.

[0039] The images of G115 before and after heat treatment were examined as follows: Figure 9 and Figure 10 As shown in Table 1, the heat treatment process is shown in Table 2, the grain size grade is shown in Table 3, and the mechanical properties are shown in Table 3. It can be seen that normalizing alone cannot refine the grains of G115 large-diameter thick-walled pipes and large-size forgings.

[0040] Depend on Figures 1 to 10 As can be clearly seen from Tables 1 and 2, normalizing alone cannot refine the grains of G115 large-diameter thick-walled pipes and large-size forgings; at the same time, after processing with the solution provided by the present invention, the grains of G115 large-diameter thick-walled pipes and large-size forgings are significantly refined.

[0041] As can be clearly seen from Table 3, the room temperature and high temperature mechanical properties of G115 large-diameter thick-walled pipes and large-size forgings are significantly improved after treatment by the solution provided by the present invention.

[0042] In summary, this invention fully utilizes the carbides precipitated during the high-temperature tempering process of G115 to improve the phase interface between carbides and ferrite, and increase the number of austenite nucleation points, thereby achieving the function of dividing the original austenite grain boundaries and refining the grains. At the same time, the normalizing and tempering heat treatment processes are specifically designed to achieve the redistribution of carbides and improve the room temperature and high temperature mechanical properties of G115 large-diameter thick-walled tubes and large-size forgings.

[0043]

[0044]

[0045]

Claims

1. A heat treatment method for refining the grain size of G115 steel, characterized in that, Includes the following steps: 1) Quenching and tempering heat treatment: Heat the finished G115 steel pipe to a quenching and tempering temperature of 745~790℃, hold for 0.2D+8 hours~0.2D+20 hours, and then air cool. 2) Normalizing heat treatment: Heat the quenched and tempered G115 finished steel pipe at a normalizing temperature of 1050~1120℃ and a holding time of 0.2D+1 hour to 0.2D+4 hours, then water quench to room temperature; 3) Tempering heat treatment: Heat the normalized G115 finished steel pipe to a tempering temperature of 745~790℃, hold for 0.2D+3 hours to 0.2D+7 hours, and then air cool. Where D represents the wall thickness of the G115 finished steel pipe, in decimeters.

2. The heat treatment method for refining the grain size of G115 steel as described in claim 1, characterized in that, The grain size of the microstructure of the G115 steel is finer than grade 1.

3. The heat treatment method for refining the grain size of G115 steel as described in claim 1 or 2, characterized in that, The G115 steel has a room temperature tensile strength ≥660MPa, a yield strength ≥480MPa, and a yield strength ≥271MPa at 630℃.

4. The heat treatment method for refining the grain size of G115 steel as described in claim 1, 2, or 3, characterized in that, The weight percentage composition of the G115 steel is as follows: C 0.060~0.100%, Si≤0.55%, Mn 0.27~0.73%, P≤0.020%, S≤0.010%, Cr 8.40~9.60%, W 2.33~3.17%, Co 2.80~3.25%, Cu 0.40~1.20%, V 0.13~0.27%, Nb 0.03~0.10%, N 0.005~0.019%, B 0.008~0.022%, Ni≤0.13%, Ti≤0.02%, Al≤0.015%, O≤0.0040%, As≤0.015%, Sb≤0.015%, Bi≤0.005%, Sn≤0.020%, Pb≤0.015%, As+Sb+Bi+Sn+Pb≤0.035%, balance is Fe and unavoidable impurities.

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