A CrMo series high-pressure boiler pipe steel bloom and its preparation method

Abstract

This application relates to the field of metallurgy and specifically discloses a CrMo-based high-pressure boiler pipe steel bloom and a preparation method thereof. The CrMo-based high-pressure boiler pipe steel bloom comprises the following elements: carbon, silicon, manganese, chromium, and molybdenum, with the remainder being iron and unavoidable impurities. The resulting CrMo-based high-pressure boiler pipe steel bloom has coarse / fine A, B, C, and D inclusion levels as low as 0 / 0.5, 0 / 0, 0 / 0, and 0 / 0, respectively, reducing the content of A, B, C, and D inclusions in the bloom. Furthermore, the DS inclusion level and banded structure level of the CrMo-based high-pressure boiler pipe steel bloom are both as low as 0, enhancing the cleanliness and quality of the CrMo-based high-pressure boiler pipe steel bloom.

Description

Technical Field

[0001] The present application relates to the field of metallurgy technology, and more specifically, to a CrMo series high-pressure boiler tube steel bloom and a preparation method thereof. Background Art

[0002] High-pressure boiler tubes are primarily used in the manufacture of seamless steel tubes for heating surface tubes, headers, economizers, and other equipment used in high-pressure steam boilers in thermal power plants. They are often exposed to high temperatures and high pressures during use, and oxidation and corrosion can occur under the influence of high-temperature flue gas and water vapor. Therefore, high requirements are placed on the steel tubes' strength, resistance to oxidation and corrosion, and structural stability. The strength, resistance to oxidation and corrosion, and structural stability of steel tubes are closely related to their cleanliness. The purity of steel tubes is primarily affected by non-metallic inclusions, which disrupt the continuity and density of the steel tubes, thereby affecting their strength, resistance to oxidation and corrosion, and structural stability.

[0003] In related technologies, the LF refining process is usually used to change the composition or morphology of inclusions to improve the ability to remove inclusions, but the cleanliness of the resulting steel is still low, making it difficult to meet the requirements for use under harsh conditions such as high temperature and high pressure. Summary of the Invention

[0004] In order to improve the cleanliness of large square blooms of high-pressure boiler tube steel and improve the uniformity of the steel tube structure so that the steel tube has higher endurance strength and anti-oxidation and corrosion performance, the present application provides a large square bloom of CrMo series high-pressure boiler tube steel and a preparation method thereof.

[0005] In a first aspect, the present application provides a CrMo series high-pressure boiler tube steel, which adopts the following technical solution:

[0006] A CrMo series high-pressure boiler tube steel bloom comprises the following elements in percentage by weight: 0.13-0.17% carbon, 0.20-0.35% silicon, 0.50-0.68% manganese, 0.90-1.10% chromium, 0.42-0.55% molybdenum, ≤0.025% phosphorus, ≤0.010% sulfur, and the balance being iron and unavoidable impurities.

[0007] The raw material of the CrMo series high-pressure boiler tube steel of the present application may include 0.13-0.17% carbon, 0.20-0.35% silicon, 0.50-0.68% manganese, 0.90-1.10% chromium, 0.42-0.55% molybdenum, 0-0.002% yttrium, 0-0.007% lanthanum, ≤0.025% phosphorus, ≤0.010% sulfur, with the balance being iron and inevitable impurities. Any value within the respective ranges may be selected, and the low-temperature impact resistance of the CrMo series high-pressure boiler tube steel can be improved. The best effect is achieved when the carbon content is 0.15%, the silicon content is 0.27%, the manganese content is 0.60%, the chromium content is 0.95%, the molybdenum content is 0.50%, the phosphorus content is 0.015%, the sulfur content is 0.005%, and the balance is iron and inevitable impurities.

[0008] By employing this technical solution, carbon can form precipitates with elements such as chromium and molybdenum. These precipitated carbides can improve the material's creep resistance through dispersion strengthening and other methods. Silicon, dissolved in ferrite and austenite, can increase the steel's strength and hardness, particularly its yield strength. Furthermore, silicon acts as a deoxidizer, enhancing the cleanliness of steel used in high-pressure boiler tubes.

[0009] Manganese has a solid solution strengthening effect, expanding the austenite region and lowering the austenite-to-ferrite transition temperature. This, in turn, refines the ferrite grains, increases the steel's strength and toughness, and compensates for the strength loss caused by low carbon. Chromium improves the steel's high-temperature mechanical properties, increasing its strength and hardness, and forms a passivation film on the steel surface. Chromium also offers antioxidant and corrosion resistance, improving carbon dioxide corrosion resistance and inhibiting sulfur adsorption. Chromium also combines with carbon to enhance dispersion strengthening and reduce graphitization tendency, thereby improving the oxidation resistance and corrosion resistance of high-pressure boiler tube steel. Molybdenum has a solid solution strengthening effect on ferrite, increasing the steel's creep strength and tempering stability. It also forms a dense passivation film on the surface. Molybdenum also resists hydrogen sulfide corrosion and reduces pitting corrosion.

[0010] Harmful elements phosphorus and sulfur will increase the segregation of steel at grain boundaries or subgrain boundaries, which can significantly reduce the high-temperature fatigue strength, stress corrosion resistance and thermal creep properties of alloy steel, increase its embrittlement tendency, and make the alloy steel's fracture performance under hydrogen corrosion conditions more obvious. Therefore, this application strictly reduces the phosphorus and sulfur contents in the alloy bars.

[0011] Preferably, the CrMo series high-pressure boiler tube steel bloom further comprises 0.001-0.002% of yttrium and 0.005-0.007% of lanthanum.

[0012] By adopting the above technical solution, yttrium and lanthanum are both rare earth metals. Adding yttrium and lanthanum can play a role in deoxidation and desulfurization, and can change the morphology of inclusions, refine grains, reduce creep rate, and improve the cleanliness of molten steel.

[0013] Preferably, the weight percentage of yttrium to lanthanum is 1:(3-6).

[0014] By adopting the above technical solution and adjusting the weight percentages of yttrium and lanthanum, the cleanliness of the molten steel can be further improved.

[0015] In a second aspect, the present application provides a method for preparing a CrMo series high-pressure boiler tube steel bloom, which is specifically achieved through the following technical solutions:

[0016] A method for preparing a CrMo series high-pressure boiler tube steel bloom comprises the following steps:

[0017] Converter smelting: molten iron and scrap steel are charged into a converter and blown. Aluminum bars are added before tapping to adopt a strong deoxidation method. After deoxidation, further alloying is carried out in the ladle. The aluminum content at the outlet is controlled at 0.020-0.040%, the carbon content at the converter end is ≥0.05%, and the end temperature is ≥1620℃. LF refining: A CaO-Al2O3-SiO2 slag system is used, and the binary basicity Ca / SiO2 is controlled at 4-8. Argon is blown into the ladle, the slag is treated, heated, and the argon blowing is stopped. The slag basicity after refining is 5-10, the white slag retention time is ≥15 minutes, and a deep desulfurization process is used. The sulfur content at the outlet is ≤0.005%.

[0018] VD vacuum: deep vacuum degree ≤67Pa, vacuum holding time ≥15min, feed iron-calcium wire to modify non-metallic inclusions, feed amount 300-400m / furnace, perform soft blowing, soft blowing time ≥15min;

[0019] Bloom continuous casting: control the tundish superheat at 20-35°C and the billet casting speed at 0.80-1.0 m / min to obtain billets;

[0020] Ingot heating: The ingot is heated by high-temperature diffusion technology. The preheating section temperature is ≤780℃, the heating section temperature is 1200-1280℃, the soaking section temperature is 1210-1270℃, the rolling start temperature is 1150±30℃, and the total heating time is ≥280min.

[0021] slab rolling;

[0022] sawing and collecting;

[0023] Slow cooling after entering the pit: the temperature entering the pit is ≥500℃, the cooling rate is 6-8℃ / h, and the temperature leaving the pit is ≤100℃;

[0024] Combined flaw detection: adopts the combined flaw detection method of magnetic leakage and ultrasonic.

[0025] By adopting the above technical solution, the carbon content of molten steel is reduced, harmful impurities are removed, and phosphorus, sulfur, and oxygen are removed through converter smelting, thereby improving the cold brittleness of steel caused by phosphorus and the hot brittleness caused by sulfur. After the molten steel composition (aluminum 0.020-0.040%, carbon ≥0.05%) and temperature (≥1620°C) are adjusted to the specified values, the steel is tapped. Before tapping, aluminum bars are added to the furnace for deoxidation. Aluminum has a greater affinity for oxygen than iron, thus achieving a strong deoxidation effect. At the same time, converter smelting can also remove hydrogen and nitrogen gases, as well as non-metallic inclusions such as oxides, sulfides, phosphides, and nitrides. Secondly, due to the large amount of basic slag in the converter, complete deoxidation in the furnace is very difficult. Therefore, the molten steel is poured into a ladle below the converter, and aluminum bars are added to the ladle for further deoxidation and alloying.

[0026] LF refining utilizes a CaO-Al2O3-SiO2 slag system with a binary basicity of Ca / SiO2 controlled at 4-8, which achieves dephosphorization, desulfurization, and deoxidation. Because argon is insoluble in molten steel and does not react with other elements, it is used for ladle agitation. A constant flow of argon is blown into the molten steel through a lance. The bubbles attract inclusions, causing them to move upward due to buoyancy, removing them and reducing the hydrogen, nitrogen, and oxygen content in the molten steel. It also removes slag and inclusions, ensuring uniform composition and temperature, and reducing segregation. Furthermore, ladle argon blowing further prevents or minimizes secondary oxidation of the molten steel, improving its cleanliness. Controlling the white slag retention time to 15 minutes or longer reduces the oxygen, sulfur, and inclusion contents in the alloy bars, further improving the cleanliness of the molten steel.

[0027] VD vacuum control deep vacuum degree ≤67Pa, vacuum holding time ≥15min, feed iron-calcium wire to carry out non-metallic inclusion modification treatment, soft blowing time ≥15min, remove larger particle inclusions in molten steel.

[0028] The use of large square billet continuous casting can reduce non-metallic inclusions in the billet, prevent internal porosity, and increase casting speed.

[0029] The ingot heating adopts high-temperature diffusion technology to make the internal structure of the ingot more uniform. The temperature of the preheating section is controlled at ≤780℃, the temperature of the heating section is controlled at 1200-1280℃, the temperature of the soaking section is controlled at 1210-1270℃, and the starting rolling temperature is controlled at 1150±30℃, which can prevent defects such as overheating, overburning and decarburization.

[0030] Slow cooling in the pit helps to release the internal stress of the rolled material and prevent cracks in the rolled material. The pit temperature is controlled at ≥500℃, which can avoid internal cracking of the alloy bar and improve the qualified rate of subsequent flaw detection. The cooling rate is 6-8℃ / h, which reduces the organizational stress and thermal stress generated by the cooling process, avoids the generation of surface and internal cracks of the ingot, and maximizes the production rhythm.

[0031] Combined flaw detection uses magnetic flux leakage and ultrasonic testing to ensure the surface and internal quality of bars. Magnetic flux leakage testing involves magnetizing the alloy bar and detecting surface or near-surface defects, which generate a leakage magnetic field. This field can be used to detect changes in the magnetic field and thus reveal defects. Ultrasonic testing involves ultrasonic waves entering an object and encountering a defect, where a portion of the sound wave is reflected. The receiver analyzes the reflected wave, accurately detecting defects in the alloy bar and displaying their location and size. This combined testing method can accurately detect both internal and external defects in alloy bars.

[0032] Preferably, the LF refining step adopts narrow composition control technology, with carbon controlled at ±0.01%, and manganese and chromium controlled at ±0.02%.

[0033] By adopting the above technical solution, LF refining uses narrow composition control technology to accurately control the composition of various alloy elements in the alloy bar, reduce fluctuations, control carbon to ±0.01%, and control manganese and chromium to ±0.02%, which can improve the performance stability of the raw materials.

[0034] Preferably, in the VD vacuum step, the feeding amount of the iron-calcium wire is 300-400 m / furnace.

[0035] By adopting the above technical solution and controlling the appropriate feeding amount of iron-calcium wire, the effect of calcium-iron wire in modifying inclusions can be improved, thereby effectively removing inclusions.

[0036] Preferably, in the VD vacuum step, the soft blowing pressure is 0.20-0.23 MPa.

[0037] By adopting the above technical solution, when the soft blowing pressure is 0.20-0.23 MPa, the molten steel is weakly stirred, which facilitates further floating of inclusions, removes the inclusions, and improves the cleanliness of the molten steel.

[0038] As a preference: in the bloom continuous casting step, the crystallizer electromagnetic stirring, the end electromagnetic stirring, the secondary cooling weak cooling electromagnetic stirring, the low superheat casting method are adopted, the plasma heating method is adopted, the superheat is controlled at 15-30 ° C; the billet casting speed is 0.45m / min, the crystallizer water volume is 180m 3 / h, secondary cooling water volume 0.22L / kg.

[0039] By adopting the above technical solution, electromagnetic stirring of the crystallizer can gradually solidify the molten steel, increase the billet drawing speed, improve the surface quality of the ingot, clean the bubbles and inclusions on the surface of the solidified shell, which is beneficial to reduce the superheat of the molten steel and improve the cleanliness of the molten steel.

[0040] Secondary cooling and weak electromagnetic stirring can eliminate bridging between columnar branches, reduce or eliminate center porosity and center shrinkage, reduce center segregation and the accumulation of inner arc inclusions, and improve the internal quality of the ingot. Combining secondary cooling and weak electromagnetic stirring with end-stage electromagnetic stirring can further reduce center segregation and reduce or eliminate center porosity and center shrinkage.

[0041] Low superheat casting can increase the percentage of equiaxed crystals, reduce or avoid porosity and segregation within the ingot, and thus improve the internal quality of the ingot. Plasma heating technology can avoid contamination of the ingot at high temperatures, making the casting temperature more stable and thus improving the quality of the ingot.

[0042] Preferably, the slab rolling is performed through 9 reciprocating rolling passes with a maximum reduction of 80-90 mm.

[0043] By adopting the above technical solution, the slab rolling adopts 1100 slab rolling machine, and undergoes 9 reciprocating rolling passes with a maximum reduction of 80-90mm. The large reduction is used to improve the grain size crushing ability, refine the grain size of the organization, and improve the impact performance of the raw materials.

[0044] In summary, this application includes at least one of the following beneficial technical effects:

[0045] (1) By controlling the type and dosage of each element component, the present application makes the coarse / fine A, B, C and D inclusion levels of CrMo series high-pressure boiler tube steel large square blooms 0.5 / 1, 1 / 1, 0.5 / 0.5 and 0 / 0.5, and the DS inclusion level and banded structure level 1.5, with high cleanliness.

[0046] (2) In the present application, by adding yttrium and lanthanum elements to the CrMo series high-pressure boiler tube steel bloom and adjusting the ratio of the two elements, the fine A-type inclusion grade of the CrMo series high-pressure boiler tube steel bloom is set to 0.5, the DS inclusion grade and the banded structure grade are set to 1, thereby further improving the cleanliness of the molten steel.

[0047] (3) The present application adopts narrow composition control technology in the LF refining step, controlling carbon to ±0.01%, manganese and chromium to ±0.02%, so that the fine A, coarse / fine B, and coarse / fine C inclusion levels of large square billets of CrMo series high-pressure boiler tube steel are 0.5, 0 / 0.5, and 0 / 0.5, respectively, thereby improving the cleanliness of the molten steel.

[0048] (4) In the present application, the feed rate of the iron-calcium wire is adjusted to 300-400 m / furnace in the VD vacuum step, so that the coarse Class A inclusion level of the CrMo series high-pressure boiler tube steel bloom is 0, the DS inclusion level and the banded structure level are 0.5, thereby further improving the cleanliness of the molten steel.

[0049] (5) In the present application, the soft blowing pressure is controlled to be 0.2-0.23 MPa in the VD vacuum step, so that the fine C and fine D inclusion levels of the CrMo series high-pressure boiler pipe steel blooms are reduced to level 0, thereby further improving the cleanliness of the molten steel.

[0050] (6) This application adopts a crystallizer, end electromagnetic stirring, secondary cooling, low superheat casting technology, plasma heating technology, and superheat control at 25 ° C in the large square billet continuous casting step; the casting speed is 0.45m / min, the crystallizer water volume is 180m 3 / h, and the secondary cooling water volume is 0.22L / kg, which makes the fine B-type inclusion level of CrMo series high-pressure boiler tube steel blooms level 0, further improving the cleanliness of the molten steel.

[0051] (7) The present application controls the bloom rolling process through 9 reciprocating rolling passes with a final reduction of 90 mm, so that the coarse / fine A, B, C and D inclusion levels of the CrMo series high-pressure boiler pipe steel bloom are 0 / 0.5, 0.5 / 1, 0 / 0 and 0 / 0, respectively, and the bloom has a higher cleanliness, thereby improving the quality of the CrMo series high-pressure boiler pipe steel bloom. DETAILED DESCRIPTION

[0052] The present application is further described in detail below with reference to specific embodiments.

[0053] The following raw materials in this application are all commercially available products. These materials are provided for the purpose of providing sufficient disclosure of the raw materials in this application and should not be construed as limiting their sources. Specifically, they are: silicomanganese, FeMn68Si18; ferrochrome, FeCr65C0.10; ferromolybdenum, FeMo60; ferroydttrium, FeY65; and ferrolanthanum, LCF80.

[0054] Example 1

[0055] A CrMo series high-pressure boiler tube steel bloom is prepared by the following steps:

[0056] Converter smelting: according to the dosage in Table 1, molten iron and scrap steel are charged into the converter, blown, and aluminum bars are added before tapping to adopt strong deoxidation method. After deoxidation, alloys (silicon manganese, ferrochrome, ferromolybdenum) are further added. The aluminum content at the exit is controlled at 0.030%, the carbon content at the converter end is 0.05%, and the end temperature is 1620℃.

[0057] LF refining: using CaO-Al2O3-SiO2 slag system, binary basicity CaO / SiO2 controlled at 6, ladle slag making, deoxidation, desulfurization, heating, alloying, the slag basicity after refining is about 7, white slag holding time 15min;

[0058] VD vacuum: deep vacuum degree 67Pa, vacuum holding time 15min, after vacuum treatment, feed iron-calcium wire for non-metallic inclusion modification treatment, feed amount is 250m / furnace, soft blowing is carried out, soft blowing pressure is 0.18MPa, soft blowing time is 15min;

[0059] Bloom continuous casting: the tundish superheat of molten steel is 30°C, the casting speed is 0.9m / min, and the billet is obtained;

[0060] Ingot heating: The ingot is heated using high-temperature diffusion technology. The preheating section temperature is 780°C, the heating section temperature is 1240°C, the soaking section temperature is 1240°C, the start rolling temperature is 1150°C, and the total heating time is 280 minutes.

[0061] Bloom rolling: After 9 rounds of rolling, the final reduction is 40mm;

[0062] sawing and collecting;

[0063] Slow cooling after entering the pit: the temperature entering the pit is 500℃, the cooling rate is 7℃ / h, and the temperature out of the pit is 100℃;

[0064] Combined flaw detection: adopts the combined flaw detection method of magnetic leakage and ultrasonic.

[0065] Examples 2-6

[0066] The preparation methods of the CrMo series high-pressure boiler tube steel blooms of Examples 2-6 are the same, except that the alloy bars further contain yttrium and lanthanum elements, that is, the alloy added after deoxidation also contains yttrium iron and lanthanum iron. The specific addition amounts are shown in Table 1.

[0067] Table 1 Content of various raw material components in the CrMo series high pressure boiler tube steel blooms of Examples 1-6 (unit: %)

[0068]

[0069] The balance is Fe and inevitable impurities, and the inevitable impurities are ≤0.03%.

[0070] Example 7

[0071] The CrMo series high-pressure boiler tube steel bloom of Example 7 has the same elemental composition as that of Example 4, except that a narrow composition control technology is used in the LF refining step, with carbon controlled at ±0.01%, and manganese and chromium controlled at ±0.02%. The remaining operations are the same as those of Example 4.

[0072] Examples 8-11

[0073] The CrMo series high-pressure boiler tube steel blooms of Examples 8-11 have the same elemental composition as that of Example 7, except that in the VD vacuum step, the feeding amounts of the iron-calcium wire are 300 m / furnace, 350 m / furnace, 400 m / furnace, and 450 m / furnace, respectively. The remaining operations are the same as those of Example 7.

[0074] Examples 12-15

[0075] The CrMo series high-pressure boiler tube steel blooms of Examples 12-15 have the same elemental composition as that of Example 9, except that in the VD vacuum step, the soft blowing pressures are 0.2 MPa, 0.22 MPa, 0.23 MPa, and 0.24 MPa, respectively. The remaining operations are the same as those of Example 9.

[0076] Example 16

[0077] The CrMo series high pressure boiler tube steel bloom of Example 16 has the same elemental composition as that of Example 13. The difference is that in the bloom continuous casting step, the crystallizer electromagnetic stirring, the end electromagnetic stirring, the secondary cooling weak cooling electromagnetic stirring, and the low superheat casting method are adopted. The plasma heating technology is adopted and the superheat is controlled at 25°C. The casting speed is 0.45m / min and the crystallizer water volume is 180m 3 / h, the secondary cooling water volume is 0.22L / kg, and the other operations are the same as those in Example 13.

[0078] Example 17

[0079] The high-strength, low-temperature impact-resistant alloy bar of Example 17 has the same elemental composition as that of Example 16, except that the blanking rolling is performed through 9 reciprocating rolling passes with a maximum reduction of 80 mm, and the remaining operations are the same as those of Example 16.

[0080] Example 18

[0081] The preparation method of the high-strength, low-temperature impact-resistant alloy rod of Example 18 is exactly the same as that of Example 16, except that the blanking rolling is carried out through 9 reciprocating rolling passes with a maximum reduction of 90 mm, and the remaining raw materials and admixtures are the same as those of Example 16.

[0082] Comparative Example 1

[0083] The high-strength, low-temperature impact-resistant alloy rod of Comparative Example 1 is prepared in exactly the same manner as in Example 1, except that an equal amount of silicon in the alloy rod is replaced by iron, and the remaining raw materials and admixtures are the same as in Example 1.

[0084] Performance test (I)

[0085] The basic properties of the CrMo series high-pressure boiler tube steel blooms obtained in Examples 1-18 and Comparative Example 1 were tested using the following testing standards or methods. The test results are shown in Table 2.

[0086] A, B, C, D non-metallic inclusion grades: Refer to GB / T 10561-2005 "Standard for determination of non-metallic inclusion content in steel - Microscopic examination method with rating chart" to test the A, B, C, D non-metallic inclusion grades of CrMo series high-pressure boiler tube steel blooms.

[0087] DS inclusion grade: The DS inclusion grade of CrMo series high-pressure boiler tube steel blooms is tested with reference to GB / T 10561-2005 "Standard for determination of non-metallic inclusion content in steel - Microscopic examination method using a rating chart".

[0088] Banded structure grade: Refer to GB / T 34474.1-2017 "Evaluation of banded structure in steel Part 1: Standard rating chart method" to test the basicity of the banded structure grade of CrMo series high-pressure boiler tube steel blooms.

[0089] Table 2 Performance test results of different CrMo series high pressure boiler tube steel blooms

[0090]

[0091] The test results in Table 2 indicate that the coarse / fine inclusion levels of A, B, C, and D in the CrMo-based high-pressure boiler pipe steel blooms obtained in this application are as low as 0 / 0.5, 0 / 0, 0 / 0, and 0 / 0, respectively, reducing the content of A, B, C, and D inclusions in the blooms. Furthermore, the DS inclusion level and banded structure level in the CrMo-based high-pressure boiler pipe steel blooms are both as low as 0, improving the cleanliness and quality of the CrMo-based high-pressure boiler pipe steel blooms.

[0092] Combined with the performance test data of the CrMo-based high-pressure boiler tube steel bloom obtained in Example 1, it was found that the coarse / fine A, B, C, and D inclusion grades of the CrMo-based high-pressure boiler tube steel bloom obtained in Example 1 were 0.5 / 1, 1 / 1, 0.5 / 0.5, and 0 / 0.5, and the DS inclusion grade and banded structure grade were 1.5, indicating high cleanliness.

[0093] In Examples 2-6, the CrMo-based high-pressure boiler tube steel blooms obtained in Examples 3-5 had a fine Class A inclusion rating of 0.5, and a DS inclusion rating and banded structure rating of 1, all lower than those in Examples 2 and 6. This indicates that a yttrium-to-lanthanum ratio of 1:3-6 (by weight) in the blooms improves the cleanliness of the blooms. This may be due to the fact that adjusting the weight ratio of yttrium to lanthanum can further improve the cleanliness of the molten steel.

[0094] Combining the performance test data of the CrMo-based high-pressure boiler pipe steel blooms of Examples 3-5 and 7, it was found that the fine A, coarse / fine B, and coarse / fine C inclusion levels of the CrMo-based high-pressure boiler pipe steel bloom obtained in Example 7 were 0.5, 0 / 0.5, and 0 / 0.5, respectively, which were all lower than those of Examples 3-5. This indicates that the use of narrow composition control technology in the LF refining step, with carbon controlled to ±0.01% and manganese and chromium controlled to ±0.02%, can improve the cleanliness of the molten steel. This may be related to the use of narrow composition control technology in LF refining, which accurately controls the composition of various alloying elements in the alloy bar, reduces fluctuations, and controls carbon to ±0.01% and manganese and chromium to ±0.02%, which can improve the performance stability of the raw materials.

[0095] Combining the performance test data of the CrMo-based high-pressure boiler pipe steel blooms obtained in Examples 7 and 8-11, it was found that the coarse Class A inclusion grade of the CrMo-based high-pressure boiler pipe steel blooms obtained in Examples 8-10 was 0, and the DS inclusion grade and banded structure grade were 0.5, all lower than those in Examples 7 and 11. This indicates that a feed rate of 300-400 m3 / furnace of ferrocalcium wire during the VD vacuum process can improve the cleanliness of the blooms. This may be related to the fact that controlling the appropriate ferrocalcium wire feed rate can enhance the effect of the calcium-iron wire on inclusion modification, thereby effectively removing inclusions.

[0096] Combining the performance test data of the CrMo-based high-pressure boiler pipe steel blooms of Examples 8-10 and Examples 12-15, it was found that the fine C and fine D inclusion levels of the CrMo-based high-pressure boiler pipe steel blooms obtained in Examples 12-14 were 0, which were lower than those of Examples 8-10 and Example 15. This indicates that in the VD vacuum step, a soft blowing pressure of 0.2-0.23 MPa can improve the cleanliness of the bloom. This may be related to the fact that a soft blowing pressure of 0.20-0.23 MPa results in weaker stirring of the molten steel, which facilitates further floating of the inclusions, removing the inclusions, and improving the cleanliness of the molten steel.

[0097] Combining the performance test data of the CrMo series high-pressure boiler pipe steel blooms of Examples 12-14 and 16, it was found that the fine B-type inclusion level of the CrMo series high-pressure boiler pipe steel blooms obtained in Example 16 was 0, which was lower than that of Examples 12-14, indicating that in the bloom continuous casting step, a crystallizer, end electromagnetic stirring, secondary cooling, low superheat casting technology was used, plasma heating technology was used, and the superheat was controlled at 25°C; the casting speed was 0.45m / min, and the crystallizer water volume was 180m 3 / h, the secondary cooling water volume is 0.22L / kg, which can improve the cleanliness of the large square bloom.

[0098] Combining the performance test data of the CrMo-based high-pressure boiler pipe steel blooms from Examples 17 and 18, it was found that the coarse / fine A, B, C, and D inclusion levels of the CrMo-based high-pressure boiler pipe steel blooms obtained in Example 18 were 0 / 0.5, 0 / 0, 0 / 0, and 0 / 0, the same as those in Example 17. This indicates that bloom rolling with nine reciprocating passes and a maximum reduction of 90 mm does not improve the cleanliness of the blooms. This may be because the high reduction can improve grain size crushing and refine the microstructure, but has no effect on internal inclusions.

[0099] Combining the CrMo series high-pressure boiler pipe steel blooms of Comparative Example 1 with that of Example 1, it was found that adding silicon to the CrMo series high-pressure boiler pipe steel blooms can improve the cleanliness of the CrMo series high-pressure boiler pipe steel blooms.

[0100] Performance Testing (II)

[0101] The basic properties of the CrMo-based high-pressure boiler tube steel blooms obtained in Example 1, Example 4, Example 7, Example 9, Example 13, Examples 16-18, and Comparative Example 1 were tested using the following testing standards or methods. The test results are detailed in Table 3.

[0102] Tensile strength: The tensile strength of the alloy bar is tested in accordance with GB / T 228.1-2010 "Metallic Materials Room Temperature Tensile Test Methods".

[0103] Yield strength: The yield strength of alloy bars is tested in accordance with GB / T 228.1-2010 "Metallic Materials Room Temperature Tensile Test Methods".

[0104] Table 3 Performance test results of different CrMo series high pressure boiler tube steel blooms

[0105]

[0106] The test results in Table 3 indicate that the tensile strength, yield strength, and surface hardness of the CrMo-based high-pressure boiler pipe steel bloom obtained in the present application can reach a maximum of 783 MPa, 580 MPa, and 319 HB, respectively. This improves the mechanical properties of the CrMo-based high-pressure boiler pipe steel bloom, indicating that high cleanliness of the CrMo-based high-pressure boiler pipe steel bloom can improve its quality.

[0107] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A CrMo series high pressure boiler tube steel bloom, characterized in that: The invention has the following elemental composition in percentage by weight: carbon 0.13-0.17%, silicon 0.20-0.35%, manganese 0.50-0.68%, chromium 0.90-1.10%, molybdenum 0.42-0.55%, yttrium 0.001-0.002%, lanthanum 0.005-0.007%, phosphorus ≤0.025%, sulfur ≤0.010%, and the balance is iron and unavoidable impurities; the weight percentage of yttrium to lanthanum is 1:(3-6); The method for preparing a CrMo series high-pressure boiler tube steel bloom comprises the following steps: Converter smelting: put molten iron and scrap steel into converter, blow, add aluminum bars for strong deoxidation before tapping, alloy in ladle after deoxidation, control the aluminum content at the outlet to be 0.020-0.040%, converter end point carbon ≥0.05%, and end point temperature ≥1620℃; LF refining: Using CaO-Al2O3-SiO2 slag system, binary basicity CaO / SiO2 is controlled at 4-8, ladle slag making, deoxidation, desulfurization, heating, alloying, the slag basicity after refining is 5-10, white slag holding time ≥15min, deep desulfurization process is adopted, sulfur at the station is ≤0.005%; VD vacuum: deep vacuum degree ≤67Pa, vacuum holding time ≥15min, feed iron-calcium wire to modify non-metallic inclusions, perform soft blowing, soft blowing time ≥15min; Bloom continuous casting: Control the tundish superheat at 20-35°C and the billet casting speed at 0.80-1.0m / min to obtain billets; Ingot heating: The ingot is heated by high-temperature diffusion method, with the preheating section temperature ≤ 780°C, the heating section temperature 1200-1280°C, the soaking section temperature 1210-1270°C, the start rolling temperature 1150±30°C, and the total heating time ≥ 280min; slab rolling; sawing and collecting; Slow cooling after entering the pit: the temperature entering the pit is ≥500℃, the cooling rate is 6-8℃ / h, and the temperature out of the pit is ≤100℃; Combined flaw detection: adopts the combined flaw detection method of magnetic leakage and ultrasonic.

2. The CrMo series high-pressure boiler tube steel bloom according to claim 1, characterized in that: The LF refining step adopts a narrow composition control technology, with carbon controlled within ±0.01%, and manganese and chromium controlled within ±0.02%.

3. The CrMo series high pressure boiler tube steel bloom according to claim 1, characterized in that: In the VD vacuum step, the feeding amount of the iron-calcium wire is 300-400m / furnace.

4. The CrMo series high pressure boiler tube steel bloom according to claim 1, characterized in that: In the VD vacuum step, the soft blowing pressure is 0.20-0.23 MPa.

5. The CrMo series high pressure boiler tube steel bloom according to claim 1, characterized in that: In the bloom continuous casting step, a crystallizer electromagnetic stirring, a terminal electromagnetic stirring, a secondary cooling weak cooling electromagnetic stirring, and a low superheat casting method are adopted; The crystallizer water volume during electromagnetic stirring of the crystallizer is 180m 3 / h, the secondary cooling water volume in the secondary cooling weak cooling electromagnetic stirring is 0.22L / kg, and the billet pulling speed is 0.45m / min.

6. The CrMo series high pressure boiler tube steel bloom according to claim 1, characterized in that: The slab rolling is performed through 9 reciprocating rolling passes, with a final reduction of 80-90 mm.

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