A heat-resistant steel for boiler tubes, its production method and application

Abstract

The present invention provides a heat-resistant steel for boiler tubes, its production method and application. The composition is C 0.06% - 0.11%, Si 0.20% - 0.50%, Mn 0.25% - 0.85%, Cr 8.4% - 9.4%, W 2.45% - 3.55%, B 0.008% - 0.017%, V 0.05% - 0.35%, Co 2.45% - 3.55%, Nb 0.05% - 0.12%, Nd 0.015% - 0.065%, N 0.005% - 0.015%, Ti ≤ 0.01%, Ni ≤ 0.15%, Al ≤ 0.020%, P ≤ 0.010%, S ≤ 0.006%, and the balance is Fe and other inevitable impurities. Compared with the prior art, through the design of the composition and heat treatment process, the steel obtained in the present invention is suitable for manufacturing heat-resistant steel for boiler tubes at 625°C - 650°C.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat-resistant steels, and particularly relates to a heat-resistant steel for boiler tubes, a production method thereof and an application thereof, which can be applied to the manufacture of ultra-supercritical boiler tubes with a temperature of 625°C to 650°C. Background Art

[0002] With the steady development of China's economy, the demand for electricity in people's daily lives is increasing continuously, and thermal power generation occupies a mainstream position in China's power supply structure. Therefore, developing efficient, energy-saving, clean and large-capacity thermal power generation technologies is an important trend in China's power development, which can not only meet the increasing electricity demand caused by the rapid development of the national economy, but also save resources and energy consumption and reduce pollutant emissions.

[0003] The operating parameters of the world's main thermal power units reach 600 - 620°C. In China, ultra-supercritical units at the 600°C level have been operating on a large scale, which makes China's high-parameter thermal power technology reach the world advanced level. If we want to further improve the thermal efficiency of power plants and reduce energy consumption, one of the most effective methods is to increase the capacity of generating units and steam parameters. With the increase of steam parameters of generating units, the requirements for heat-resistant steels used in main steam boiler pipes are also increasing continuously, which puts forward higher requirements for the service performance of materials.

[0004] In the use of ultra-supercritical boiler tube steel with a steam parameter of 600°C, the most used material is P92 heat-resistant steel. Practice has proved that the upper limit of the use temperature of P92 is about 625°C and does not exceed 630°C. At the same time, the materials required for ultra-supercritical boiler tubes at 650°C - 750°C mainly include: Inconel740H is mainly used for the manufacture of steel pipes at 700 - 750°C, and the new austenitic heat-resistant steel Sanicro25 is commonly used in the temperature range of 670 - 700°C. However, there is no corresponding material for large-diameter heat-resistant steel pipes in the temperature range of 625 - 650°C. If nickel-based alloys and austenitic heat-resistant steels are applied to the manufacture of large-diameter steel pipes, the power generation cost will increase significantly, which does not conform to China's national conditions. The most economical and feasible method is to increase the use temperature of common ferritic heat-resistant steels to 650°C and improve the performance of the steel through the design of chemical composition of the material. The heat-resistant steel at 625 - 650°C is proposed under this background.

[0005] The steel grade G115 developed by the Central Iron and Steel Research Institute on the basis of the research of MARBN steel has carried out industrial trial production and small-scale application. However, in order to ensure the high-temperature performance of the material, the carbon content control range of G115 steel is relatively low, which causes certain difficulties for actual smelting. Summary of the Invention

[0006] The object of the present invention is to provide a heat-resistant steel for boiler tubes and its production method. By mainly adding elements such as W, Co, and rare earth element Nd, the creep strength and high-temperature stability of the steel are improved. Through optimized composition design, the optimal chemical composition control range of the steel is proposed, and the obtained steel has good toughness and high-temperature creep properties.

[0007] Another object of the present invention is to provide an application of the heat-resistant steel for boiler tubes, which is used to manufacture ultra-supercritical boiler tubes at a temperature of 625°C to 650°C. During use, the highest temperature it can withstand reaches 650°C, and it can withstand the high temperature, high pressure, and corrosion of the main steam boiler pipeline with a pressure of up to 35 MPa.

[0008] The specific technical solution of the present invention is as follows:

[0009] A heat-resistant steel for boiler tubes includes the following components by mass percentage:

[0010] C 0.06% - 0.11%, Si 0.20% - 0.50%, Mn 0.25% - 0.85%, Cr 8.4% - 9.4%, W 2.45% - 3.55%, B 0.008% - 0.017%, V 0.05% - 0.35%, Co 2.45% - 3.55%, Nb 0.05% - 0.12%, Nd 0.015% - 0.065%, N 0.005% - 0.015%, Ti ≤ 0.01%, Ni ≤ 0.15%, Al ≤ 0.020%, P ≤ 0.010%, S ≤ 0.006%, and the balance is Fe and other inevitable impurities.

[0011] The composition of the heat-resistant steel for boiler tubes also satisfies:

[0012] 10.98% ≤ chromium equivalent ≤ 15.02%, chromium equivalent = Cr + 2Si + 5V + 1.75Nb + 0.75W;

[0013] 4.50% ≤ nickel equivalent ≤ 7.80%, nickel equivalent = Ni + Co + 30C + 0.5Mn + 25N;

[0014] log[N] = -0.40log[B] - 2.77;

[0015] In the above formulas, each component represents the content of the component in the steel × 100%;

[0016] The design idea of each component of the present invention is as follows:

[0017] C: C is an austenite - forming element that can dissolve in the matrix or exist in the form of carbides, playing a role in improving the high - temperature strength of the material. In heat - resistant steels, the higher the carbon content, the worse the high - temperature performance. At the same time, considering its solution strengthening effect on the room - temperature mechanical properties and its influence on the Ni equivalent, the C content is controlled at 0.06% - 0.11%.

[0018] Si: Si is beneficial to improving the strength and corrosion resistance of the material. Reducing the Si content can improve the toughness of the steel because Si can promote the formation of Laves phases in the steel. However, too high Si content will increase the brittleness of the steel. Therefore, the Si content is controlled at 0.20% - 0.50%.

[0019] Mn: The main role of adding Mn to heat - resistant steels is to improve the thermal strength. It can also stabilize P, S, etc. Research shows that when the Mn content is less than 0.2%, Mn does not play an obvious role. When the Mn content is higher than 1%, it is harmful to the impact toughness of the material. So, the Mn content is controlled at 0.25% - 0.85%.

[0020] Cr: Its main role is to improve the corrosion resistance and oxidation resistance of heat - resistant steels. At the same time, Cr is also a strong carbide - forming element, which can promote the formation of M 23 C6. At the same time, the Cr element is also the main element to increase the Cr equivalent of the material. Excessive Cr elements will cause the appearance of high - temperature ferrite in the steel, reducing the material's performance. For heat - resistant steels, when Cr is about 9%, its creep strength can reach the maximum value. Too high will promote the precipitation of the Z phase. Therefore, the Cr content should be controlled at 8.4% - 9.4%.

[0021] W: W will enter M 23 C6. Due to the low thermal diffusion coefficient of W, it inhibits the coarsening of M 23 C6 carbides. When the W content exceeds 3.6%, the Laves phase gradually grows, which will also damage the creep strength of the steel. Therefore, W is controlled at 2.45% - 3.55%.

[0022] Co: The role of Co in heat - resistant steels is similar to that of Ni. It also belongs to an austenite - forming element, which can expand the austenite phase region. Adding Co element to martensitic heat - resistant steels can promote the precipitation of precipitation phases of other alloy elements, thereby improving the high - temperature strength of the steel. Research shows that adding about 3% Co to the steel has the best effect on the creep strength of the steel. Therefore, the Co content is controlled at 2.45% - 3.55%.

[0023] V and Nb: Both V and Nb are the main carbide-forming elements. The formed carbides inhibit the recovery of martensite and simultaneously play a role in precipitation strengthening. There is a certain proportional relationship between V and Nb in martensitic heat-resistant steels. Research shows that in martensitic heat-resistant steels used above 600 °C, the optimal contents of V and Nb are 0.2% and 0.05% respectively. Therefore, the V content is controlled within 0.05% - 0.35%, and the Nb content is controlled within 0.05% - 0.12%.

[0024] B and N: In ferritic heat-resistant steels, the N content is usually above 0.0080%. Since N can strengthen austenite in a solid-solution manner. Trace amounts of B mainly strengthen the grain boundaries in heat-resistant steels, thereby improving the high-temperature properties of the steel, improving the creep ductility of the steel, and reducing the tendency of creep brittleness. At the same time, B can also replace some C in M 23 C6 to form M 23 (C 0.85 B 0.15 )6, which plays a role in precipitation strengthening. Research shows that a B content of about 0.010% can inhibit the coarsening of carbides, and type IV cracks will not appear in heat-resistant steels during the welding process. Therefore, the N content is controlled within 0.0050% - 0.0150%, and the B content is controlled within 0.008% - 0.017%.

[0025] Nd: Nd can improve the creep ductility and fatigue properties of the steel, and has strong deoxidation and desulfurization capabilities, which can purify the molten steel; delay the precipitation of carbides along the grain boundaries in heat-resistant steels, hinder the aggregation and growth of carbides, and change the shape and distribution of carbides. In the present invention, the Nd content is controlled within 0.015% - 0.065%.

[0026] Al: Al is an element that improves oxidation resistance. At high temperatures, Al forms Al2O3 on the surface of the iron oxide scale, becoming a strong protective film, which plays a role in improving oxidation resistance. However, too much Al element in the steel is likely to combine with the N element, which is not conducive to the N element playing its role in the steel. Therefore, the Al element content is controlled within 0.020%.

[0027] Ni: Ni is an element that stabilizes austenite and expands the austenite region. However, too much Ni content is not beneficial to the high-temperature properties of heat-resistant steels. Therefore, in the present invention, the Ni content is controlled within 0.15%.

[0028] Ti: Ti is a strong carbide-forming element, which easily affects the precipitation of M(CN), and Ti will also combine with nitrogen in the steel to form TiN inclusions, affecting the final properties of the steel. Therefore, the Ti content in the steel of the present invention is controlled within 0.01%.

[0029] P, S: To ensure the cleanliness of molten steel and the final service performance of the product, the P content in the steel is controlled within 0.010%, and the S content in the steel is controlled within 0.0060%.

[0030] In the use of heat-resistant steel, δ-ferrite is generally considered an important factor affecting its mechanical properties and high-temperature stability. To control the appearance of δ-ferrite in the steel, the elements in the steel are normalized by means of the Ni equivalent and Cr equivalent calculation formulas. According to the Cr equivalent and Ni equivalent calculation formulas, elements such as Cr, V, Si, and W are ferrite-forming elements that expand the ferrite phase region and promote the formation of δ-ferrite, while elements such as C, Mn, and Co are austenite-forming elements, and increasing their amounts can expand the austenite phase region. At the same time, considering the influence of each element on the equivalent separately, for every 0.1% increase in the amount of Cr, Si, W, V, and Nb in the steel, their Cr equivalents increase by 0.1, 0.2, 0.075, 0.5, and 0.175 respectively, and contribution coefficients 1, 2, 0.75, 5, and 1.75 are added before their respective formulas. At the same time, calculate the influence of elements C, Mn, Ni, Co, and N on the Ni equivalent, and multiply them by contribution coefficients 30, 0.5, 1, 1, and 25 respectively according to the calculation results.

[0031] Therefore, the equivalent calculation formulas are obtained as 10.98% ≤ chromium equivalent ≤ 15.02%, chromium equivalent = Cr + 2Si + 5V + 1.75Nb + 0.75W; 4.50% ≤ nickel equivalent ≤ 7.80%, nickel equivalent = Ni + Co + 30C + 0.5Mn + 25N.

[0032] To ensure the high-temperature creep performance of heat-resistant steel and inhibit the coarsening of the precipitated phase M 23 C6, it is necessary to reasonably control the contents of B and N. N in the steel will reduce the solid solubility of B in the matrix, thus promoting B to enter the carbides and precipitate in the form of complex B-containing compounds. When the contents of B and N are both very high, it will lead to the precipitation of BN, and even the formation of coarse BN inclusions, which is harmful to the creep performance of the material. Therefore, to prevent the formation of BN precipitates, the relationship log[N] = -0.40log[B] - 2.77 should be satisfied between B and N.

[0033] A production method of heat-resistant steel for boiler tubes provided by the present invention includes heat treatment, and the heat treatment includes normalizing and tempering;

[0034] For the normalizing, heat to 1070 - 1090 °C, hold for a certain time, and air cool to room temperature;

[0035] For the normalizing, the holding time t of the normalizing is: t is determined by the wall thickness S of the steel pipe, t = 0.5 - 1.5 × S, the unit of t is min, and the unit of S is mm; just substitute the values under the above unit conditions into the formula for calculation.

[0036] In the normalizing, the steel pipe is heated to 1070-1090°C at a rate of 100°C / h±10°C;

[0037] The tempering is to heat the steel pipe to 750-770° C., keep the temperature, and air-cool to room temperature.

[0038] The tempering, heating and holding time t is determined by the steel pipe wall thickness S, t = 1.2 ~ 4.5 × S, t is in min, S is in mm, and then air-cooled to room temperature; the values under the above unit conditions can be substituted into the formula for calculation.

[0039] The tempering is to heat the steel pipe to 750-770°C at a rate of 110°C / h±10°C.

[0040] The wall thickness of the steel pipe is between 50 mm and 140 mm.

[0041] The room temperature mechanical properties of the steel pipe after the above heat treatment meet the following requirements: hardness ≤ 25HRC, yield strength ≥ 620MPa, tensile strength ≥ 760MPa, elongation ≥ 20%, impact energy (longitudinal) > 160J; at 650℃, the extrapolated value of the 100,000-hour endurance strength is ≥ 110Mpa.

[0042] The production method of the heat-resistant steel for boiler tubes includes the following process flow:

[0043] Electric arc furnace or converter smelting → LF furnace refining → vacuum degassing (RH / VD) → round billet continuous casting → round billet punching → round billet heating → pipe threading → sizing → heat treatment → steel pipe grinding → flaw detection → packaging and warehousing.

[0044] The invention provides an application of heat-resistant steel for boiler tubes, which is used for manufacturing ultra-supercritical boiler tubes with a temperature of 625°C to 650°C.

[0045] Compared to existing technologies, this invention, through component design and matching heat treatment processes, produces steel with excellent toughness and high-temperature durability. Room-temperature mechanical properties meet the following requirements: hardness ≤ 25 HRC, yield strength ≥ 620 MPa, tensile strength ≥ 760 MPa, elongation ≥ 20%, and impact energy (longitudinal) > 160 J. The extrapolated 100,000-hour durability at 650°C is ≥ 110 MPa. Tensile properties at 650°C are yield strength ≥ 280 MPa, tensile strength ≥ 320 MPa, and elongation ≥ 25%, making it suitable for manufacturing heat-resistant steel for boiler tubes in temperatures between 625°C and 650°C. Furthermore, this invention contains no copper, reducing production costs and avoiding the problem of copper embrittlement and reduced yield associated with the addition of copper during subsequent hot deformation processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is the heat-resistant steel structure after heat treatment of the present invention. DETAILED DESCRIPTION

[0047] Examples 1 - 3

[0048] A heat - resistant steel for boiler tubes, comprising the following components by mass percentage: As shown in Table 1, the balance not shown in Table 1 is Fe and unavoidable impurities.

[0049] Comparative Examples 1 - 3

[0050] A heat - resistant steel for boiler tubes, comprising the following components by mass percentage: As shown in Table 1, the balance not shown in Table 1 is Fe and unavoidable impurities.

[0051] Table 1 Chemical composition of the examples of the present invention (wt%)

[0052]

[0053] The production methods of the heat - resistant steel for boiler tubes in each example and comparative example include the following technological processes:

[0054] Electric arc furnace or converter smelting → LF furnace refining → Vacuum degassing (RH / VD) → Round billet continuous casting → Drilling of round billet → Heating of round billet → Pipe piercing → Sizing → Heat treatment → Grinding of steel pipe → Detection of steel pipe flaw → Finishing → Detection → Packaging and warehousing.

[0055] Specifically:

[0056] Electric furnace smelting: Control the steel tapping to [C] ≤ 0.04%, [P] ≤ 0.004%, [S] ≤ 0.010%. Determine the oxygen before steel tapping, and adopt the steel - retaining operation during the steel - tapping process to avoid slagging;

[0057] LF furnace refining: Adjust elements such as C, Si, Mn, Cr, Ni, W, Co, V to the target values, and control the tapping temperature at 1645°C - 1665°C;

[0058] RH vacuum degassing: The pure degassing time ≥ 15 minutes. Ensure that the [H] content ≤ 1.0 ppm after breaking the vacuum and sampling;

[0059] Round billet continuous casting: Adopt the three - stage electromagnetic stirring technology in the mold, casting stream, and solidification end to reduce the composition segregation of the continuous casting billet;

[0060] Pipe manufacturing route: Drilling of round billet → Heating of round billet → Pipe piercing → Sizing;

[0061] Steel pipe processing route: Heat treatment of steel pipe → Grinding of steel pipe → Detection of steel pipe flaw → Finishing → Detection → Packaging and warehousing;

[0062] The specific heat treatment method is as follows: Normalizing: Heat the steel pipe to 1070 - 1090 °C at a rate of 100 ± 10 °C / h, hold for t hours, and then air cool to room temperature; the normalizing holding time t is determined by the wall thickness S of the steel pipe, t = 0.5 - 1.5 × S, with t in minutes and S in millimeters; then the tempering process is: Heat the steel pipe to 750 - 770 °C at a rate of 110 ± 10 °C / h, hold for t, and the tempering holding time t is determined by the wall thickness S of the steel pipe, t = 1.2 - 4.5 × S, with t in minutes and S in millimeters, and then air cool to room temperature. The wall thickness of the steel pipes in the examples and comparative examples is 50 mm, the normalizing holding time is 1 h, and the tempering holding time is 3 h. The heat treatment parameters of each specific example and comparative example are shown in Table 2.

[0063] The performance detection methods for the steel pipes produced in the above examples and comparative examples are as follows:

[0064] Microstructure: Take samples from the 1 / 2 wall thickness of the cross-section at the end of the steel pipe for metallographic analysis.

[0065] Performance: Take tensile, impact, and creep specimens from the 1 / 2 wall thickness of the cross-section at the end of the steel pipe, and conduct mechanical property tests with reference to GB / T228, GB / T 229, and GB / T 2039. The mechanical properties of each example and comparative example are shown in Tables 2 - 4.

[0066] Table 2 Heat treatment parameters and properties of each example and comparative example

[0067]

[0068] Table 3 List of creep performance detection situations for each example and comparative example

[0069] Steel grade Extrapolated value of 100,000-hour creep rupture strength at 650°C / MPa Example 1 120 Example 2 118 Example 3 115 Comparative example 1 100 Comparative example 2 100 Comparative example 3 92

[0070] Table 4 650 °C tensile performance detection list of the examples and comparative examples of the present invention

[0071]

[0072]

[0073] The properties of the trial-produced steel using the present invention were compared with those of the most advanced and mature, as well as the steel grades of the same kind that are being developed and applied currently. It can be seen from the above data that the strength, impact toughness, high-temperature tensile properties, and creep rupture properties of the steel in Examples 1 to 3 controlled according to the present invention are all good. In Comparative Example 1, the nickel equivalent was controlled, but due to improper control of the heat treatment process and low chromium equivalent and N content, the high-temperature creep rupture property of the steel was lower than that of the examples; in Comparative Example 2, the chromium equivalent and the heat treatment process were controlled, but the low nickel equivalent and N content resulted in insufficient high-temperature creep rupture property of the steel compared with the examples, and the mechanical properties were reduced. In Comparative Example 3, due to the low control of the nickel equivalent and B content, the strength and toughness of the material were insufficient, and the high-temperature creep rupture property was poor.

[0074] It can be seen from the data results in Tables 2 to 4 that the room-temperature mechanical properties and high-temperature properties of the steel of the present invention after heat treatment are superior to those of the widely used heat-resistant steels of the same kind currently. Therefore, the steel of the present invention meets the use requirements of heat-resistant steel pipes at 625 to 650 °C.

Claims

1. A heat-resistant steel for boiler tubes, characterized in that, The heat-resistant steel for boiler tubes comprises the following components by mass percentage: C 0.06% - 0.11%, Si 0.20% - 0.50%, Mn 0.25% - 0.85%, Cr 8.4% - 9.4%, W 2.45% - 3.55%, B 0.008% - 0.017%, V 0.05% - 0.35%, Co 2.45% - 3.55%, Nb 0.05% - 0.12%, Nd 0.015% - 0.065%, N 0.005% - 0.015%, Ti ≤ 0.01%, Ni ≤ 0.15%, Al ≤ 0.020%, P ≤ 0.010%, S ≤ 0.006%, and the balance is Fe and other inevitable impurities; The composition of the heat-resistant steel for boiler tubes further satisfies: log[N] = -0.40log[B] - 2.77; After heat treatment of the heat-resistant steel for boiler tubes, the extrapolated value of the creep rupture strength at 650°C for 100,000 hours is ≥ 110 Mpa.

2. The heat-resistant steel for boiler tubes according to claim 1, characterized in that, The composition of the heat-resistant steel for boiler tubes further satisfies: 10.98% ≤ chromium equivalent ≤ 15.02%, and chromium equivalent = Cr + 2Si + 5V + 1.75Nb + 0.75W.

3. The heat-resistant steel for boiler tubes according to claim 1 or 2, characterized in that, The composition of the heat-resistant steel for boiler tubes further satisfies: 4.50% ≤ nickel equivalent ≤ 7.80%, and nickel equivalent = Ni + Co + 30C + 0.5Mn + 25N.

4. A production method of heat-resistant steel for boiler tubes according to any one of claims 1-3, characterized in that, The heat treatment is included, and the heat treatment includes normalizing and tempering.

5. The production method according to claim 4, characterized in that, For the normalizing, heat to 1070 - 1090°C, hold for heat preservation, and air cool.

6. The production method according to claim 5, characterized in that, For the normalizing, the heat preservation time t of the normalizing is: t is determined by the wall thickness S of the steel pipe, t = 0.5 - 1.5 × S, t is in minutes, and S is in mm.

7. The production method according to claim 5, characterized in that, For the tempering, heat the steel pipe to 750 - 770°C, hold for heat preservation, and air cool to room temperature.

8. The production method according to claim 7, characterized in that, For the tempering, the heat preservation time t of the heating and tempering is determined by the wall thickness S of the steel pipe, t = 1.2 - 4.5 × S, t is in minutes, and S is in mm.

9. Use of the heat-resistant steel for boiler tubes according to any one of claims 1-3, characterized in that, It is used to manufacture ultra-supercritical boiler tubes at a temperature of 625°C - 650°C.

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