A seamless steel pipe for high-pressure boiler with high cleanliness and good toughness and a manufacturing method thereof

Abstract

The invention discloses a seamless steel pipe for high-pressure boilers with high cleanliness and good toughness and a manufacturing method thereof, and belongs to the technical field of steel pipes for high-pressure boilers. The invention adopts converter smelting, LF+VD refining outside the furnace, continuous casting of square billets, cooling to room temperature and then entering a heating furnace, and continuous rolling to obtain a tube billet route, strictly controls steelmaking process parameters, obtains a high-clean seamless steel pipe for high-pressure boilers, makes the number and size of inclusions in the steel extremely small, and is cleaner near the inner and outer walls where cracks are easily generated, and then through optimized rolling process and heat treatment technology, effectively optimizes the metallographic structure phase composition, refines the grain size, and generates a structure ratio that can effectively improve the toughness of the steel pipe. The obtained steel pipe has a room temperature impact performance of more than 230J, an elongation of more than 25%, a yield strength of more than 480MPa, a tensile strength of more than 610MPa, excellent high-temperature tensile performance, and a yield strength of more than 320MPa at 550℃. The uniformity and stability of product performance are good, and it can be widely used in high-pressure boilers in different fields, and has broad market prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of high-pressure boiler steel tubes, and in particular relates to a high-pressure boiler seamless steel tube with high cleanliness and good toughness and a manufacturing method thereof. Background Art

[0002] Seamless steel pipe for high-pressure boiler is a kind of low-alloy heat-resistant steel. Through solid solution strengthening of chromium and nickel alloy elements and dispersion strengthening of vanadium carbides, it has a good and stable organizational structure and good thermal strength, long-lasting plasticity, oxidation resistance and welding performance. It is widely used in superheater tubes (tube temperature <580℃) and header tubes, steam pipes, etc. (tube temperature <570℃) of high-pressure boiler tubes. Its working pressure is subcritical and supercritical pressure, but due to its long-term use under high temperature and high pressure, it is easy to cause fatigue, creep, corrosion, oxidation and stress failure, especially creep caused by long-term overheating is a common failure form. Therefore, it is required to have high lasting strength and lasting plasticity, good oxidation resistance, and no hot brittle tendency. This requires the steel to have good room temperature toughness, high impact performance and elongation, excellent metallographic structure and optimized inclusion morphology and distribution requirements, which plays an extremely important role in improving the toughness of seamless steel pipe for high-pressure boiler.

[0003] Chinese patent number CN106367689A discloses "a high-pressure boiler tube steel and its preparation method", Chinese patent number CN109930064A discloses "a corrosion-resistant high-pressure boiler tube heat-resistant steel and its production method", Chinese patent number CN104651754A discloses "a high-pressure boiler tube low-alloy steel and its preparation method", and Chinese patent number CN109023063A discloses "a small-diameter high-pressure boiler tube steel used at 550°C and its production method". None of them have control requirements on the inclusion distribution and metallographic structure composition of the products. After inclusions are formed near the inner and outer walls of the steel pipe, cracks on the inner and outer walls are likely to occur, and the production process has no obvious effect on improving the elongation and toughness of the product, and failure is prone to occur during use in a high-pressure environment.

[0004] In view of the above situation, it is urgent to research and develop high-cleanliness seamless steel pipes for high-pressure boilers with good toughness and their manufacturing methods, which have good toughness and strict control over inclusions and metallographic structure, thereby improving the service life and safety of the steel pipes. Summary of the invention

[0005] In view of this, the object of the present invention is to provide a seamless steel tube for high-pressure boilers with high cleanliness and good toughness and a method for manufacturing the same. The present invention has carried out a large number of systematic experimental studies from the aspects of metallographic structure phase composition, steel cleanliness control, process optimization and parameter selection, and finally determined the metallographic structure ratio, inclusion morphology and manufacturing process that can meet the purpose of the present invention. The high-cleanliness seamless steel tube prepared by the present invention has excellent toughness, good metallographic structure phase composition and optimized inclusion morphology, has high impact performance and elongation, and can be widely used in high-pressure boilers in different fields.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A seamless steel pipe for a high-pressure boiler with high cleanliness and good toughness. The chemical composition of the seamless steel pipe is as follows by weight percentage: C: 0.11%-0.13%, Si: 0.27%-0.32%, Mn: 0.52%-0.62%, Mo: 0.28%-0.32%, Cr: 1.00%-1.10%, V: 0.20%-0.25%, P≤0.010%, S≤0.008%, As≤0.010%, Sb≤0.010%, Bi≤0.010%, Sn≤0.010%, Pb≤0.010%, As+Sb+Bi+Sn+Pb≤0.025%, and the rest is Fe and unavoidable impurities.

[0008] Based on the above technical solution, further, the outer diameter of the seamless steel pipe is 88.9mm~168mm, the wall thickness is 5mm~20mm, the metallographic phase composition is 30~50% ferrite, 30~40% pearlite, 20~30% bainite, the difference between the maximum grain size and the minimum grain size per square centimeter is not higher than level 1; the banded structure is not higher than level 1.

[0009] Based on the above technical solution, further, the number of inclusions in the range of 1 / 4 of the outer wall of the seamless steel pipe to the center is no more than 4 / cm 2 The number of inclusions in the range of 1 / 4 of the inner wall of the steel pipe from the center is no more than 4 / cm 2 The maximum single inclusion size is less than 10μm, the oxide inclusions in the overall inclusion composition are less than 20wt%, and the remainder are other non-hard inclusion particles.

[0010] Ferrite maintains sufficient plasticity and toughness of the steel, while pearlite and bainite provide sufficient strength for the steel. Under the action of external forces such as processing and impact deformation, the inner and outer surfaces of the steel are respectively subjected to compressive stress or tensile stress, and the stress value is the largest. Due to the existence of multiple tissue morphologies in the sub-surface layer, the microscopic stress is uneven, which makes it easier to generate shear stress and cause tangential cracking. Under the action of stress, inclusions at 1 / 4 are more likely to generate stress concentration. Therefore, the present invention requires strict control of the number and size of inclusions at 1 / 4 of the inner and outer walls of the steel pipe.

[0011] Based on the above technical scheme, further, the mechanical properties of the steel pipe at room temperature are as follows: tensile strength 600-650MPa, yield strength 450-550MPa, elongation 20%-30%, impact energy 200-270J; high temperature tensile properties are as follows: at 300°C, yield strength 520-600MPa; at 350°C, yield strength 500-600MPa; at 400°C, yield strength 430-560MPa; at 450°C, yield strength 450-500MPa; at 500°C, yield strength 350-510MPa; at 550°C, yield strength 300-450MPa.

[0012] The present invention also provides a method for manufacturing the seamless steel tube for high-pressure boiler with high cleanliness and good toughness, comprising the following steps:

[0013] (1) Converter smelting, LF+VD refining outside the furnace, billet continuous casting, cooling to room temperature, the billet enters the heating furnace, and continuous rolling to obtain a tube billet;

[0014] Among them, the tapping temperature of the converter is greater than 1630℃; the high temperature of the converter prevents the subsequent cooling of the molten steel and avoids the heating of the aluminothermic reaction by adding Al, which will increase the risk of alumina inclusions. The oxygen content of the converter tapping is 300-600ppm, which ensures that the oxygen in the molten steel can combine with C and S elements when the LF furnace is charged, automatically remove C and S, and avoid additional oxygen blowing to increase other inclusions;

[0015] The LF furnace inlet temperature is greater than 1610℃, the oxygen content is 300ppm~600ppm, the top slag basicity is 3.0~3.5, and the top slag basicity is above 3.0, which can better absorb the inclusions floating on the molten steel and prevent the inclusion particles from remelting and refluxing. Argon blowing and stirring are adopted throughout the process; the VD inlet temperature is greater than 1590℃, argon blowing and stirring are adopted throughout the process, the outlet temperature is 1560℃~1570℃, and the outlet active oxygen content is less than 5ppm; VD enters the station at a high temperature to prevent forced aluminum heat reaction caused by temperature drop and increase the risk of inclusions. The vacuum treatment time is as long as possible to ensure that the molten steel is fully degassed and prevent the formation of oxide inclusions in the subsequent sequence. The outlet active O content is less than 5ppm to ensure the full cleanliness of the molten steel;

[0016] The temperature of the molten steel in the tundish is between 1540 and 1550°C. The immersion pouring is adopted. The nozzle is fully protected by argon. The amount of remaining steel in the tundish is greater than 10% of the amount of steel contained in the tundish. The fluctuation of the liquid level in the crystallizer is less than 3mm. The billet drawing speed is 0.5 to 0.8m / min, and the billet drawing speed is kept constant for each tank. The nozzle is fully protected by argon to prevent oxidation. If the temperature of the tundish is too high, it will easily lead to crystallization difficulties. If the temperature is too low, the fluidity of the molten steel will be poor, and the nozzle and the furnace wall will stick to produce large particles. The temperature is controlled at 1540 to 1550°C. If the amount of remaining steel in the tundish is too small, a strong vortex will form in the ladle, which will draw in the top slag and cause inclusions. The crystallizer liquid level is less than 3mm to prevent the liquid level from fluctuating and drawing in the top slag. The constant billet drawing speed of 0.5 to 0.8m / min is also to prevent the fluctuation of the crystallizer liquid level, ensure the quality of the billet, and prevent cracking.

[0017] (2) The tube blank obtained in step (1) is heated in an annular furnace and rolled into a steel tube by a perforation, PQF continuous rolling and sizing process; wherein the heating temperature of the annular furnace is 1270-1280° C., the perforation temperature is 1250-1270° C., and the sizing temperature is 830-850° C.;

[0018] (3) The steel pipe obtained in step (2) is subjected to normalizing + tempering heat treatment, wherein the normalizing heating temperature is 955-975° C., the normalizing holding time is 45-60 min; the tempering heating temperature is 765-780° C., and the tempering holding time is 60-80 min.

[0019] Based on the above technical solution, further, in step (1), the tapping temperature of the converter is 1630-1660°C; the inlet temperature of the LF furnace is 1610-1635°C, and the inlet temperature of the VD furnace is 1590-1610°C;

[0020] Based on the above technical solution, further, in step (1), the LF treatment process time is 40 to 60 minutes.

[0021] Based on the above technical solution, further, in step (1), the vacuum treatment time of the VD treatment process is greater than 20 minutes, and the total time is 35 to 50 minutes.

[0022] Based on the above technical solution, further, in step (1), the tundish is covered throughout the process before entering the station to isolate it from external air.

[0023] Based on the above technical solution, further, in step (1), the process of continuously rolling the square billet to obtain the tube billet increases the deformation by more than 74%, which has the effect of refining the grains, and the rolling temperature of the square billet is not lower than 1070°C.

[0024] Based on the above technical solution, further, in step (2), the rolling elongation coefficient is controlled at 8.0 to 8.5. A larger elongation coefficient increases the rolling deformation, which is beneficial to grain refinement and improving product toughness.

[0025] Based on the above technical solution, further, step (3) is carried out using a step heat treatment furnace.

[0026] Based on the above technical solution, further, in step (3), air cooling is performed after normalizing treatment, and the cooling rate is 2 to 6°C / s.

[0027] The beneficial effects of the present invention compared with the prior art are as follows:

[0028] The present invention provides a high-cleanliness seamless steel pipe for high-pressure boilers with good toughness and a manufacturing method thereof. Compared with existing products, the present invention adopts a route of converter smelting, LF+VD refining outside the furnace, continuous casting of square billets, cooling to room temperature and then entering a heating furnace, and continuous rolling to obtain a tube billet, strictly controls steelmaking process parameters, and obtains a high-cleanliness seamless steel pipe for high-pressure boilers, so that the number and size of inclusions in the steel are extremely small, and the inner and outer walls where cracks are easily generated are cleaner, have good toughness, and have high cleanliness. The present invention also effectively optimizes the metallographic structure phase composition, refines the grain size, and generates a structure ratio that can effectively improve the toughness of the steel pipe through optimized rolling process and heat treatment technology. The obtained steel pipe has a good metallographic structure phase composition, excellent mechanical properties, room temperature impact performance of more than 230J, elongation of more than 25%, yield strength of more than 480MPa, and tensile strength of more than 610MPa. The product performance has good uniformity and stability, can be widely used in high-pressure boilers in different fields, and has broad market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention, the drawings related to the embodiments are briefly introduced below.

[0030] Figure 1 This is the metallographic structure diagram of the high-clean seamless steel pipe prepared in the example. DETAILED DESCRIPTION

[0031] The present invention is described in detail below with reference to the embodiments, but the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only partial embodiments of the present invention. For those skilled in the art, other similar embodiments obtained without creative labor all fall within the protection scope of the present invention.

[0032] Examples 1-5

[0033] This embodiment provides a high-cleanliness seamless steel pipe for high-pressure boilers with good toughness. The thickness, chemical composition and weight percentage of the steel pipe are shown in Table 1.

[0034] Table 1 Main chemical composition and weight percentage (wt%) of high clean seamless steel pipe of Examples 1-5

[0035]

[0036] Among them, As+Sb+Bi+Sn+Pb≤0.025%.

[0037] The manufacturing method comprises the following steps:

[0038] Converter smelting - refining LF+VD outside the furnace - continuous casting of square billets - cooling to room temperature and then entering the heating furnace - continuous rolling to obtain tube billets - heating in annular furnace - rolling into steel pipes using piercing, PQF continuous rolling and sizing processes - normalizing (air cooling) + tempering heat treatment;

[0039] Among them, the parameters of converter tapping temperature, LF inlet temperature, LF inlet O content, top slag basicity, VD inlet temperature, vacuum treatment time, VD outlet temperature, VD outlet active O content, tundish residual steel tonnage, mold liquid level fluctuation range, and billet drawing rate are shown in Table 2;

[0040] Table 2 Steelmaking and billet rolling process parameters of high clean seamless steel pipes of Examples 1-5

[0041]

[0042] The parameters of the ring furnace heating temperature, piercing temperature, sizing temperature, and rolling elongation coefficient during the continuous rolling process are shown in Table 3;

[0043] The parameters of normalizing heating temperature, normalizing holding time, tempering heating temperature, and tempering holding time in the heat treatment process are shown in Table 3;

[0044] Table 3 Rolling and heat treatment process parameters of high clean seamless steel pipes of Examples 1-5

[0045]

[0046] The room temperature mechanical properties of the prepared high-cleanliness seamless steel pipe for high-pressure boilers with good toughness are shown in Table 4, the high temperature tensile properties are shown in Table 5, and the inclusions and metallographic structure of the seamless steel pipe are shown in Table 6.

[0047] Table 4 Room temperature mechanical properties of high clean seamless steel pipes of Examples 1-5

[0048]

[0049] Table 5 High temperature tensile properties of high clean seamless steel pipes of Examples 1-5

[0050]

[0051] Table 6 Inclusions and metallographic structure of high clean seamless steel pipes of Examples 1-5

[0052]

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for manufacturing a seamless steel tube for a high pressure boiler having high cleanliness and good toughness, characterized in that: The chemical composition of the seamless steel pipe is as follows by weight percentage: C: 0.11%~0.13%, Si: 0.27%~0.32%, Mn: 0.52%~0.62%, Mo: 0.28%~0.32%, Cr: 1.00%~1.10%, V: 0.20%~0.25%, P≤0.010%, S≤0.008%, As≤0.010%, Sb≤0.010%, Bi≤0.010%, Sn≤0.010%, Pb≤0.010%, As+Sb+Bi+Sn+Pb≤0.025%, and the rest is Fe and unavoidable impurities; The outer diameter of the seamless steel pipe is 88.9 mm to 168 mm, the wall thickness is 5 mm to 20 mm, the metallographic structure is 30% to 50% ferrite, 30% to 40% pearlite, 20% to 30% bainite, the difference between the maximum grain size and the minimum grain size per square centimeter is not higher than 1 level; the banded structure is not higher than 1 level; The steps include: (1) Converter smelting, LF+VD refining outside the furnace, continuous casting of billets, cooling to room temperature, the billets enter the heating furnace, and continuous rolling to obtain tube billets; Among them, the converter tapping temperature is greater than 1630℃; the converter tapping oxygen content is 300~600ppm, The inlet temperature of LF furnace is greater than 1610℃, the top slag basicity is above 3.0, and argon blowing is used for stirring throughout the process; the inlet temperature of VD furnace is greater than 1590℃, argon blowing is used for stirring throughout the process, the outlet temperature is 1560℃~1570℃, and the outlet active oxygen content is less than 5ppm; The temperature of molten steel in the tundish is 1540~1550℃, immersion casting is adopted, the nozzle is fully protected by argon, the remaining steel in the tundish is greater than 15 tons; the crystallizer liquid level fluctuation is less than 3mm, and the billet drawing speed is 0.5~0.8m / min; (2) The tube blank obtained in step (1) is heated in an annular furnace and rolled into a steel tube by a perforation, PQF continuous rolling and sizing process; wherein the heating temperature of the annular furnace is 1270-1280°C, the perforation temperature is 1250-1270°C, and the sizing temperature is 830-850°C; (3) The steel pipe obtained in step (2) is subjected to normalizing + tempering heat treatment, wherein the normalizing heating temperature is 955-975°C, and the normalizing holding time is 45-60 min; the tempering heating temperature is 765-780°C, and the tempering holding time is 60-80 min.

2. The manufacturing method according to claim 1, characterized in that: In step (1), the tapping temperature of the converter is greater than 1630°C and less than or equal to 1660°C; the inlet temperature of the LF furnace is greater than 1610°C and less than or equal to 1635°C, and the inlet temperature of the VD furnace is greater than 1590°C and less than or equal to 1610°C.

3. The manufacturing method according to claim 1, characterized in that: In step (1), the LF treatment process takes 40 to 60 minutes.

4. The manufacturing method according to claim 1, characterized in that In step (1), the vacuum treatment time of the VD treatment process is greater than 20 minutes, and the total time is 35 to 50 minutes.

5. The manufacturing method according to claim 1, characterized in that: In step (1), the tundish is covered throughout the process before entering the station to isolate it from the outside air; the amount of steel remaining in the tundish is greater than 10% of the amount of steel contained in the tundish.

6. The manufacturing method according to claim 1, characterized in that: In step (1), the process of continuously rolling the square billet to obtain the tube billet increases the deformation amount by more than 74%, and the rolling temperature of the square billet is not less than 1070°C.

7. The manufacturing method according to claim 1, characterized in that: In step (2), the rolling elongation coefficient is controlled within a range of 8.0 to 8.

5.

8. The manufacturing method according to claim 1, characterized in that: In step (3), after normalizing, the steel is air-cooled at a cooling rate of 2-6°C / s.

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