Production method of alloy tube blank of high-pressure boiler

Abstract

The invention relates to the technical field of high-pressure boiler alloy tube blank production, and discloses a high-pressure boiler alloy tube blank production process, which comprises the steps of molten iron and scrap steel preparation, converter smelting, refining station control, converter control, continuous casting control, continuous casting blank judgment, rolling, cooling after rolling, and finished product inspection and acceptance. In the molten iron and scrap steel preparation stage, the content of As and the content of Sn in the molten iron in a furnace are not larger than 0.012% respectively, self-produced high-quality scrap steel is added into the scrap steel in the furnace as much as possible, and it is guaranteed that the content of As and the content of Sn in finished products in the steel are not larger than 0.015% respectively. And the crystal structure of the metal can be changed. Along with the rise of the temperature, the activity ability of atoms is enhanced, the spacing of atoms in crystal lattices is increased, the resistance of dislocation movement is reduced, and carbon atoms in ferrite have more activity spaces, so that dislocation can move in the crystal lattices more easily, and the plasticity of the blank is improved.

Description

Technical Field

[0001] The invention relates to the field of alloy tube blank production, in particular to a method for producing high-pressure boiler alloy tube blanks. Background Art

[0002] As a key equipment in industrial production, high-pressure boilers have extremely harsh operating environments and need to withstand high temperatures, high pressures, and complex chemical media corrosion. As the basic raw material for manufacturing high-pressure boiler tubes, the quality of alloy tube billets is directly related to the safety, reliability, and service life of the boiler.

[0003] In the production of high-pressure boiler alloy tube billets, thermal stress is easily generated, causing the billet to crack. It will also affect the plasticity and uniformity of the billet, and then affect the dimensional accuracy and surface quality of the tube billet after rolling, which is easy to cause the curvature of the tube billet to not meet the requirements, and the defects such as surface scratches and cracks increase. In addition, it is impossible to timely discover and adjust the problems such as uneven temperature during the cooling process, which affects the performance stability of the tube billet. In addition, in the steelmaking process, the removal effect of impurity elements such as phosphorus is not good, which will seriously affect the toughness of the steel, making the tube billet prone to brittle cracks and other problems during subsequent processing and use. The molten steel is easily exposed to air during the casting process, resulting in secondary oxidation of the molten steel, generating a large amount of oxide inclusions, which seriously affects the purity and surface quality of the tube billet. Improper control of the pulling speed will cause uneven growth of the billet shell, which is easy to cause defects such as cracks. Imprecise control of the superheat of the molten steel in the tundish will cause uneven solidification structure and serious segregation, further reducing the quality of the tube billet. Summary of the invention

[0004] The object of the present invention is to provide a method for producing a high-pressure boiler alloy tube billet to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solution: a method for producing high-pressure boiler alloy tube billets, including molten iron and scrap steel preparation, converter smelting, refining station control, converter control, continuous casting control, continuous casting billet determination, rolling, post-rolling cooling, finished product inspection and acceptance; the specific steps are as follows:

[0006] S1, in the preparation stage of molten iron and scrap steel, the contents of the five harmful elements As, Sn, Sb, Pb and Bi in the molten iron entering the furnace are controlled, and high-quality scrap steel is added to the scrap steel entering the furnace;

[0007] S2. During the converter smelting stage, the target of steel tapping end point is controlled at C≥0.04%, and the number of re-blowing is ≥2 times to ensure C-T coordinated slag tapping. When 1 / 4 of the steel is tapped, deoxidation alloy is added to the ladle for deoxidation alloying. Carbon is the most basic element in steel, and it has an important influence on the strength and hardness of steel. A certain carbon content can make the alloy tube billet have sufficient strength to withstand external forces such as pressure during subsequent processing and use. If the carbon content is too low, the alloy tube billet may be too soft and cannot meet the use requirements. If the carbon content is too high, it will cause the alloy tube billet to be too hard and brittle, and it is easy to break.

[0008] S3, LF furnace control, total argon blowing time ≥ 40min, use Al particles for diffusion deoxidation, slag formation within 25min, rapid slag deoxidation and desulfurization, open pouring furnace feed Ca line 70±20~50m, continuous pouring furnace feed Ca line 50±20~50m for calcification treatment before molten steel leaves the station;

[0009] S4, VD / RH furnace control, vacuum target below 0.5tor, holding time ≥10min, soft blowing time after molten steel breaks through the air ≥15min, hydrogen is set for molten steel out of the station, and the temperature of molten steel out of the station on the platform is ≤35℃ for the start-casting furnace and ≤30℃ for the continuous casting furnace;

[0010] S5. During the continuous casting control stage, the casting is protected throughout the process, the casting machine speed is 0.70-0.95m / min, and the typical casting speed is 0.70m / min; the superheat of the molten steel in the middle package is ≤25℃ for the continuous casting furnace, ≤30℃ for the open casting furnace, and the superheat of the typical casting is 15-25℃. The crystallizer is electromagnetically stirred, and the billet stack is slowly cooled. The slow cooling time is ≥24h; during the continuous casting process, if the molten steel is exposed to the air, the active elements (such as aluminum, titanium, etc.) in it can easily react with oxygen in the air. For example, aluminum in the molten steel will react with oxygen to form aluminum oxide inclusions. The full protection casting can effectively prevent the molten steel from contacting with the air, reduce the formation of oxide inclusions in the steel, and thus improve the purity of the steel.

[0011] S6. In the continuous casting billet determination stage, the size, weight, shape and surface quality of rectangular billets and square billets are determined;

[0012] S7. During the rolling stage, the billet must first be heated. The temperature of the preheating section is controlled below 850°C, the temperature of the heating section is controlled between 1160 and 1220°C, and the temperature of the soaking section is controlled between 1180 and 1220°C. When the billet enters the heating furnace from room temperature, if the temperature rises too quickly, the temperature difference between the inside and outside of the billet will increase sharply, thereby generating greater thermal stress. Keeping the temperature of the preheating section below 850°C can slowly heat up the billet, allowing the internal structure of the billet enough time to adapt to the temperature change and reduce the risk of billet cracking due to thermal stress. For example, for some billets containing alloy elements, the thermal expansion coefficients of alloy elements are different at different temperatures. Slow preheating can make the organization of these elements distributed evenly heated to avoid local stress concentration. For the control of heating furnace time, when cold loading, the time of 300×430 square rectangular billet in the furnace shall not be less than 200 minutes, 240 square shall not be less than 120 minutes; 150 square shall not be less than 90 minutes; if the accident time is greater than 20 minutes, the heating temperature should be reduced in time, and when hot delivery and hot loading, the furnace temperature is required to be ≥300℃, the time of 300×430 square rectangular billet in the furnace shall not be less than 150 minutes, 240 square shall not be less than 100 minutes; 150 square shall not be less than 60 minutes, and the rolling start temperature shall be ≥1000℃, and the continuous rolling temperature shall be ≥950℃;

[0013] S8. During the cooling stage after rolling, the round steel is cooled slowly on the cooling bed with a heat preservation cover;

[0014] S9, inspecting and accepting the finished products obtained in step S8 in batches, each batch consisting of steel products with the same furnace number, the same processing method and the same size, and finally delivering them in a hot-rolled state;

[0015] The alloy components are C: 0.08%-0.15%, Si: 0.17%-0.37%, Mn: 0.40%-0.70%, P≤0.020%, S≤0.010%, Al≤0.020%, Cr: 0.90-1.20%, Mo: 0.25-0.35%, and V: 0.15-0.30%.

[0016] Furthermore, step S1, the molten iron and scrap steel preparation stage, includes the following specific steps:

[0017] S1.1, blast furnace ironmaking, iron ore, coke and limestone are loaded into the blast furnace through a feed car or a belt conveyor. The ore is pre-treated by crushing and screening to make its particle size meet the requirements of blast furnace smelting. Coke is required to have high strength and low ash content, and limestone has a moderate particle size;

[0018] S1.2, molten iron pretreatment, molten iron flows from the blast furnace into a torpedo tank car or a ladle, and is desulfurized by injection. The injection equipment sprays the desulfurizer into the molten iron, and the sulfur content of the treated molten iron is controlled below 0.01% to 0.03%.

[0019] Furthermore, in step S1, the contents of the five harmful elements in the molten iron are As≤0.012%, Sn≤0.012%, Pb≤0.008%, Sb≤0.010%, and Bi≤0.010%, and the contents of As and Sn in the finished steel are not more than 0.015% respectively.

[0020] Furthermore, in the LF furnace control stage, the power-on time is ≥20min, the outgoing activity [O] is ≤5ppm, the Fe-Ca wire is fed ≥400m or the pure Ca wire is fed ≥200m after refining, and the soft argon blowing time after wire feeding is ≥5min. Long-term power-on can maintain good reaction conditions in the furnace, which is conducive to the desulfurization reaction. Desulfurization is mainly achieved by the reaction of calcium (Ca), magnesium (Mg) and other elements in the molten steel with sulfur (S), generating sulfides that enter the slag phase and are removed; sufficient power-on time can fully mix the molten steel and the slag-making agent.

[0021] Furthermore, in the VD furnace control stage, argon blowing is started throughout the process when the molten steel enters the VD furnace. The total argon blowing time in the VD furnace is ≥30 minutes. The molten steel is hydrogenated when it leaves the station, and the target value [H] is controlled at ≤2.0ppm. The hydrogen setting is not less than 1 furnace per pouring. Argon blowing can cause strong convection in the molten steel. Argon blowing starts when the molten steel enters the VD (vacuum degassing furnace) / RH (vacuum circulating degassing furnace), which can ensure that the various components in the molten steel (such as alloy elements, deoxidation products, etc.) are quickly mixed and evenly mixed. At the same time, it can also make the temperature distribution of the molten steel more uniform. For example, after adding alloys to adjust the composition, the stirring effect of argon blowing can avoid local solidification or segregation caused by uneven composition and temperature, creating good conditions for the subsequent refining process.

[0022] Furthermore, in the continuous casting control stage, the current intensity of the crystallizer is 150-200A, the frequency is 2.5Hz; the water volume of the crystallizer is 2720-2920L / min.

[0023] Furthermore, the continuous casting slab determination stage includes the following specific steps:

[0024] S6.1. Inspectors use a strong flashlight to carefully observe the length and circumference of the continuous casting billet or the edges. For small cracks, a magnifying glass is needed to assist in the inspection.

[0025] S6.2. During inspection, pay attention to whether there are granular or blocky inclusions on the surface of the continuous casting billet;

[0026] S6.3. Surface pores appear as small round or oval pits on the surface of the continuous casting billet. When inspecting, pay attention to the size and distribution density of the pores.

[0027] S6.4. The ultrasonic flaw detector transmits high-frequency ultrasonic waves into the continuous casting billet through the probe. Before flaw detection, a coupling agent needs to be applied to the surface of the continuous casting billet to ensure that the ultrasonic waves can be effectively transmitted into the continuous casting billet. When the ultrasonic waves propagate inside the continuous casting billet, they will be reflected, refracted and scattered when encountering the interface between different media, such as the interface between the defect and the matrix.

[0028] S6.5. Cut samples from the continuous casting billet. The locations of the samples usually include the head, middle and tail. The cut samples must be rough ground, fine ground and polished to make the surface flat and smooth. Then, the samples are corroded with a corrosive agent to show their macrostructure.

[0029] Furthermore, finished product inspection and acceptance stages require inspection through surface quality inspection, ultrasonic testing, low-magnification structure inspection, sampling, and experimental methods. Qualified tube blanks must be correctly labeled, including product model, specification, production date, and inspection pass mark.

[0030] Furthermore, the iron ore is hematite or magnetite, and the desulfurizer is magnesium powder or lime-fluorite mixed powder.

[0031] Compared with the prior art, the present invention provides a high-pressure boiler alloy tube production process, which has the following beneficial effects:

[0032] 1. The production process of high-pressure boiler alloy tube billets uses precise heating controlled by the rolling furnace. During the heating process, the temperature of the billet increases and the crystal structure of the metal changes. As the temperature rises, the activity of atoms increases, the atomic spacing in the lattice increases, the resistance to dislocation movement decreases, and the carbon atoms in the ferrite have more activity space, making it easier for dislocations to move in the lattice, thereby improving the plasticity of the billet.

[0033] 2. The high-pressure boiler alloy tube production process uses a heat preservation cover on the cooling bed to slow down the round steel, so that the steel is directly cooled in the air after rolling. Adding a heat preservation cover on the cooling bed to slow down the cooling rate of the round steel surface can slow down the surface and the temperature of the surface and the inside can be reduced more evenly, thereby effectively reducing thermal stress and reducing the possibility of cracks.

[0034] 3. The high-pressure boiler alloy tube production process prevents the molten steel from being exposed to the air and causing oxidation reaction to generate alumina inclusions through full-process protective casting during the continuous casting stage. Full-process protective casting can effectively prevent the molten steel from contacting with the air, reduce the generation of oxide inclusions in the steel, and thus improve the purity of the steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1 This is a macrostructure diagram of the ingot of Example 1;

[0037] Figure 2 This is the non-metallic inclusion organization diagram of Example 2. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The present invention provides a high-pressure boiler alloy tube blank production process, including: molten iron and scrap steel preparation, converter smelting, refining station control, converter control, continuous casting control, continuous casting blank determination, rolling, cooling after rolling, finished product inspection and acceptance.

[0040] Embodiment 1:

[0041] The high-pressure boiler alloy tube production process of the present invention comprises the following steps:

[0042] S1. In the preparation stage of molten iron and scrap steel, the contents of As and Sn in the molten iron entering the furnace shall not be greater than 0.012% respectively, and the scrap steel entering the furnace shall be added with self-produced high-quality scrap steel as much as possible to ensure that the contents of As and Sn in the finished steel are not greater than 0.015% respectively; the preparation stage of molten iron and scrap steel includes the following specific steps:

[0043] S1.1, Blast furnace ironmaking, first of all, is the preparation and loading of raw materials. Iron ore (such as hematite, magnetite), coke and limestone are loaded into the blast furnace through a feed car or a belt conveyor. The ore is pre-treated by crushing, screening and other pretreatments to make its particle size meet the requirements of blast furnace smelting. Coke is required to have high strength and low ash content, and limestone has a moderate particle size.

[0044] S1.2, hot metal pretreatment, desulfurization pretreatment is the key link. When hot metal flows from the blast furnace into the torpedo tank car or the hot metal ladle, it is desulfurized by injection. The injection equipment sprays the desulfurizer (such as magnesium powder, lime-fluorite mixed powder) into the hot metal. Taking magnesium desulfurization as an example, magnesium powder reacts with sulfur in the hot metal to form magnesium sulfide, which floats up into the slag phase. The sulfur content of the treated hot metal is generally required to be controlled below 0.01% to 0.03%.

[0045] S2. During the converter smelting stage, the target of steel tapping endpoint is controlled at C≥0.04%, and the number of re-blowing times is ≥2 times to ensure C-T coordinated slag tapping. When 1 / 4 of the steel is tapped, deoxidation alloy is added to the ladle for deoxidation and alloying. Carbon is the most basic element in steel. It has an important influence on the strength and hardness of steel. A certain carbon content can make the alloy tube billet have sufficient strength to withstand external forces such as pressure during subsequent processing and use. If the carbon content is too low, the alloy tube billet may be too soft and cannot meet the use requirements. If the carbon content is too high, the alloy tube billet will be too hard and brittle, and it will be easy to break.

[0046] S3. In the LF furnace control stage, the power-on time is ≥20min, and the total argon blowing time is ≥40 minutes. Long-term power-on can maintain good reaction conditions in the furnace, which is conducive to the desulfurization reaction. In the LF furnace, desulfurization is mainly achieved through the reaction of calcium (Ca), magnesium (Mg) and other elements in the molten steel with sulfur (S), generating sulfides that enter the slag phase and are removed; sufficient power-on time can make the molten steel and slag-making agent fully mixed. The outgoing activity [O] ≤5ppm, after refining, the Fe-Ca wire is fed ≥400m or the pure Ca wire is fed ≥200m, and the soft argon blowing time after feeding the wire is ≥5min,

[0047] S4. During the VD / RH furnace control stage, argon blowing is started throughout the process when the molten steel enters the VD / RH furnace. The total argon blowing time is ≥30 minutes, the vacuum target is below 0.5tor, and the holding time is not less than 10 minutes. The soft blowing time before the molten steel leaves the VD / RH furnace is not less than 15 minutes. The molten steel is hydrogenated when it leaves the station, and the target value [H] is controlled at ≤2.0ppm. The hydrogen setting is not less than 1 furnace per pouring. The temperature of the continuous casting table on the molten steel leaving the station should ensure that the superheat of the tundish is controlled at ≤35℃ for the opening furnace and ≤30℃ for the continuous casting furnace. Argon blowing can cause strong convection in the molten steel. Argon blowing starts when the molten steel enters the VD (vacuum degassing furnace) / RH (vacuum circulating degassing furnace), which can ensure that the various components in the molten steel (such as alloy elements, deoxidation products, etc.) are quickly mixed and evenly.

[0048] S5. In the continuous casting control stage, the casting needs to be protected throughout the process. The casting machine speed is 0.80-0.90m / min, and the typical casting speed is 0.70m / min. The superheat of the molten steel in the middle ladle is ≤25℃ for the continuous casting furnace, ≤30℃ for the open casting furnace, and the superheat of the typical casting is 15-25℃. The crystallizer is electrically stirred, with a current intensity of 200A and a frequency of 2.5Hz. The water volume of the crystallizer is 2920L / min. The liquid level of the middle ladle should be greater than 600mm for normal casting and greater than 650mm for ladle replacement. During the continuous casting process, if the molten steel is exposed to the air, the active elements (such as aluminum, titanium, etc.) in it can easily react with oxygen in the air. For example, aluminum in the molten steel will react with oxygen to form aluminum oxide inclusions. The full protection of the casting can effectively prevent the molten steel from contacting with the air, reduce the formation of oxide inclusions in the steel, and thus improve the purity of the steel.

[0049] S6. In the continuous casting billet determination stage, it is necessary to determine the size, weight, shape and surface quality of the rectangular billet and square billet. The continuous casting billet determination stage includes the following specific steps:

[0050] S6.1. Inspectors use tools such as strong flashlights to carefully observe the length and circumference (for round billets) or edges (for square billets) of the continuous casting billet. For fine cracks, a magnifying glass is needed to assist in the inspection. Surface cracks may be caused by uneven growth and cooling of the billet shell in the crystallizer, or excessive stress during the billet drawing process. For example, in the crystallizer, if the performance of the protective slag is poor and it cannot lubricate the billet shell and the crystallizer wall well, it is easy to cause the local temperature of the billet shell to be too high, uneven growth, and then cracks. At the same time, it is important to distinguish different types of cracks. Longitudinal cracks usually extend along the length of the continuous casting billet. This type of crack may be caused by uneven thickness of the billet shell in the early stage of solidification, which is caused by the static pressure of the molten steel. Transverse cracks are generally perpendicular to the length of the continuous casting billet. It may be related to inappropriate vibration parameters of the crystallizer. For example, if the vibration frequency and amplitude do not meet the requirements, the billet shell is subjected to periodic stress and cracks during demolding;

[0051] S6.2. During the inspection, pay attention to observe whether there are granular or block inclusions on the surface of the continuous casting billet. These slag inclusions may be deoxidation products in the molten steel, refractory erosion products, etc. that have not floated up in time and are attached to the surface of the billet shell. For example, during the molten steel refining process, if the amount and method of adding the deoxidizer are inappropriate, more deoxidation products, such as alumina inclusions, may be produced. When these inclusions enter the crystallizer with the molten steel, they may be wrapped on the surface during the solidification process of the billet shell to form slag inclusions. The color and shape of the slag inclusions can also provide some clues. For example, alumina slag inclusions are generally white or grayish white, while slag inclusions produced by refractory erosion may be related to the color of the refractory material; through the observation and analysis of slag inclusions, the cause of its generation can be traced back, and then corresponding improvement measures can be taken.

[0052] S6.3. Surface pores appear as small round or oval pits on the surface of the continuous casting billet. When inspecting, pay attention to the size and distribution density of the pores. The formation of pores is mainly due to the escape of dissolved gases in the molten steel, such as hydrogen and nitrogen, during the solidification process.

[0053] S6.4. The ultrasonic flaw detector transmits high-frequency ultrasonic waves into the continuous casting billet through the probe. Before flaw detection, a coupling agent, such as glycerin, needs to be applied to the surface of the continuous casting billet to ensure that the ultrasonic waves can be effectively transmitted into the continuous casting billet. When the ultrasonic waves propagate inside the continuous casting billet, they will reflect, refract and scatter when encountering interfaces of different media, such as the interface between the defect and the matrix; the flaw detector receives the reflected waves and determines the location, size and nature of the internal defects based on the characteristics of the reflected waves, such as time, intensity and waveform.

[0054] S6.5. Cut the sample from the continuous casting billet. The positions of the sample usually include the head, middle and tail. The cut sample should be processed by rough grinding, fine grinding and polishing to make the surface flat and smooth. Then, use a suitable corrosive agent, such as nitric acid alcohol solution, to corrode the sample to show its macrostructure. When observing the macrostructure under a microscope, internal defects such as central looseness, shrinkage, segregation, etc. can be seen.

[0055] S7. During the rolling stage, the billet must first be heated. The temperature of the preheating section is controlled below 850°C, the temperature of the heating section is controlled between 1160 and 1220°C, and the temperature of the soaking section is controlled between 1160 and 1220°C; preferably 1180 to 1210°C. The billet enters the heating furnace from room temperature. If the temperature rises too quickly, the temperature difference between the inside and outside of the billet will increase sharply, thereby generating greater thermal stress. Controlling the temperature of the preheating section below 850°C can slowly heat up the billet, allowing the internal structure of the billet enough time to adapt to the temperature change and reduce the risk of billet cracking due to thermal stress. For example, for some billets containing alloy elements, the thermal expansion coefficients of alloy elements at different temperatures are different. Slow preheating can make the organization of these elements distributed evenly heated to avoid local stress concentration. For the control of heating furnace time, the time of 300×430 square rectangular billet in the furnace is not less than 200 minutes, 240 square is not less than 120 minutes, and 150 square is not less than 90 minutes during cold loading. If the accident time is greater than 20 minutes, the heating temperature should be reduced in time. During hot delivery and hot loading, the furnace temperature is required to be ≥300℃, the time of 300×430 square rectangular billet in the furnace is not less than 150 minutes, 240 square is not less than 100 minutes, and 150 square is not less than 60 minutes, and the rolling temperature is ≥1000℃, and the continuous rolling temperature is ≥950℃. Secondly, rolling is carried out. When rolling, the places with sharp angles and burrs such as rolling grooves, baffles, guides, rollers, etc. should be polished clean; if they cannot be polished clean, they should be replaced in time. The non-roundness and dimensional tolerance of round steel are strictly controlled during the rolling process.

[0056] S8. During the cooling stage after rolling, the round steel is slowly cooled on the cooling bed with a heat preservation cover. The operator must operate carefully, not bend the finished product, not scratch the surface, ensure that the bending degree of the steel meets the requirements, and conduct surface quality inspection. The ears, scratches, cracks, burrs and flash defects on the surface of the finished product must be polished clean;

[0057] S9, during the inspection and acceptance stage of the finished product obtained in step S8, the steel should be inspected and accepted in batches. Each batch consists of steel of the same furnace number, the same processing method, and the same size, and is finally delivered in the hot-rolled state.

[0058] The alloy components of this embodiment are C: 0.12%-0.13%, Si: 0.20%-0.28%, Mn: 0.57%-0.65%, P≤0.0015%, S≤0.006%, Al: 0.005-0.015%, Cr: 0.90-1.05%, Mo: 0.27-0.30%, and V: 0.175-0.20%.

[0059] The low magnification structure diagram of the ingot is as follows Figure 1 As shown, non-metallic inclusions such as Figure 2 shown.

[0060] Embodiment 2:

[0061] The high-pressure boiler alloy tube production process of the present invention comprises the following steps:

[0062] S1. In the preparation stage of molten iron and scrap steel, the contents of As and Sn in the molten iron entering the furnace shall not be greater than 0.012% respectively, and the scrap steel entering the furnace shall be added with self-produced high-quality scrap steel as much as possible to ensure that the contents of As and Sn in the finished steel are not greater than 0.015% respectively; the preparation stage of molten iron and scrap steel includes the following specific steps:

[0063] S1.1, Blast furnace ironmaking, first of all, is the preparation and loading of raw materials. Iron ore (such as hematite, magnetite), coke and limestone are loaded into the blast furnace through a feed car or a belt conveyor. The ore is pre-treated by crushing, screening and other pretreatments to make its particle size meet the requirements of blast furnace smelting. Coke is required to have high strength and low ash content, and limestone has a moderate particle size.

[0064] S1.2, hot metal pretreatment, desulfurization pretreatment is the key link. When hot metal flows from the blast furnace into the torpedo tank car or the hot metal ladle, it is desulfurized by injection. The injection equipment sprays the desulfurizer (such as magnesium powder, lime-fluorite mixed powder) into the hot metal. Taking magnesium desulfurization as an example, magnesium powder reacts with sulfur in the hot metal to form magnesium sulfide, which floats up into the slag phase. The sulfur content of the treated hot metal is generally required to be controlled below 0.01% to 0.03%.

[0065] S2. During the converter smelting stage, the target control point of steel tapping is C ≥ 0.04%, and the number of re-blowing times is ≥ 2 times to ensure C-T coordinated slag tapping. When 1 / 4 of the steel is tapped, deoxidation alloy is added to the ladle for deoxidation and alloying. Carbon is the most basic element in steel. It has an important impact on the strength and hardness of steel. A certain carbon content can make the alloy billet have sufficient strength to withstand external forces such as pressure during subsequent processing and use. If the carbon content is too low, the alloy billet may be too soft and cannot meet the use requirements. If the carbon content is too high, the alloy billet will be too hard and brittle, and it will be easy to break.

[0066] S3. In the LF furnace control stage, the power-on time is ≥20min, and the total argon blowing time is ≥40 minutes. Long-term power-on can maintain good reaction conditions in the furnace, which is conducive to the desulfurization reaction. In the LF furnace, desulfurization is mainly achieved through the reaction of calcium (Ca), magnesium (Mg) and other elements in the molten steel with sulfur (S), generating sulfides that enter the slag phase and are removed; sufficient power-on time can make the molten steel and slag-making agent fully mixed. The outgoing activity [O] ≤5ppm, after refining, the Fe-Ca wire is fed ≥400m or the pure Ca wire is fed ≥200m, and the soft argon blowing time after feeding the wire is ≥5min,

[0067] S4. During the VD / RH furnace control stage, argon blowing is started throughout the process when the molten steel enters the VD / RH furnace. The total argon blowing time is ≥30 minutes, the vacuum target is below 0.5tor, and the holding time is not less than 10 minutes. The soft blowing time before the molten steel leaves the VD / RH furnace is not less than 15 minutes. The molten steel is hydrogenated when it leaves the station, and the target value [H] is controlled at ≤2.0ppm. The hydrogen setting is not less than 1 furnace per pouring. The temperature of the continuous casting table on the molten steel leaving the station should ensure that the superheat of the tundish is controlled at ≤35℃ for the opening furnace and ≤30℃ for the continuous casting furnace. Argon blowing can cause strong convection in the molten steel. Argon blowing starts when the molten steel enters the VD (vacuum degassing furnace) / RH (vacuum circulating degassing furnace), which can ensure that the various components in the molten steel (such as alloy elements, deoxidation products, etc.) are quickly mixed and evenly.

[0068] S5. During the continuous casting control stage, the casting needs to be protected throughout the process. The casting machine speed is 0.70-0.75m / min, and the typical casting is 0.70m / min. The superheat of the molten steel in the middle ladle is ≤25℃ for the continuous casting furnace, ≤30℃ for the open casting furnace, and the typical casting superheat is 15-25℃. The crystallizer is electrically stirred, with a current intensity of 150A and a frequency of 2.5Hz. The water volume of the crystallizer is 2720L / min. The liquid level of the middle ladle should be greater than 600mm for normal casting and greater than 650mm for ladle replacement. During the continuous casting process, if the molten steel is exposed to the air, the active elements (such as aluminum, titanium, etc.) in it can easily react with oxygen in the air. For example, aluminum in the molten steel will react with oxygen to form aluminum oxide inclusions. The full protection of the casting can effectively prevent the molten steel from contacting with the air, reduce the formation of oxide inclusions in the steel, and thus improve the purity of the steel.

[0069] S6. In the continuous casting billet determination stage, it is necessary to determine the size, weight, shape and surface quality of the rectangular billet and square billet. The continuous casting billet determination stage includes the following specific steps:

[0070] S6.1. Inspectors use tools such as strong flashlights to carefully observe the length and circumference (for round billets) or edges (for square billets) of the continuous casting billet. For fine cracks, a magnifying glass is needed to assist in the inspection. Surface cracks may be caused by uneven growth and cooling of the billet shell in the crystallizer, or excessive stress during the billet drawing process. For example, in the crystallizer, if the performance of the protective slag is poor and it cannot lubricate the billet shell and the crystallizer wall well, it is easy to cause the local temperature of the billet shell to be too high, uneven growth, and then cracks. At the same time, it is important to distinguish different types of cracks. Longitudinal cracks usually extend along the length of the continuous casting billet. This type of crack may be caused by uneven thickness of the billet shell in the early stage of solidification, which is caused by the static pressure of the molten steel. Transverse cracks are generally perpendicular to the length of the continuous casting billet. It may be related to inappropriate vibration parameters of the crystallizer. For example, if the vibration frequency and amplitude do not meet the requirements, the billet shell is subjected to periodic stress and cracks during demolding;

[0071] S6.2. During the inspection, pay attention to observe whether there are granular or block inclusions on the surface of the continuous casting billet. These slag inclusions may be deoxidation products in the molten steel, refractory erosion products, etc. that have not floated up in time and are attached to the surface of the billet shell. For example, during the molten steel refining process, if the amount and method of adding the deoxidizer are inappropriate, more deoxidation products, such as alumina inclusions, may be produced. When these inclusions enter the crystallizer with the molten steel, they may be wrapped on the surface during the solidification process of the billet shell to form slag inclusions. The color and shape of the slag inclusions can also provide some clues. For example, alumina slag inclusions are generally white or grayish white, while slag inclusions produced by refractory erosion may be related to the color of the refractory material; through the observation and analysis of slag inclusions, the cause of its generation can be traced back, and then corresponding improvement measures can be taken.

[0072] S6.3. Surface pores appear as small round or oval pits on the surface of the continuous casting billet. When inspecting, pay attention to the size and distribution density of the pores. The formation of pores is mainly due to the escape of dissolved gases in the molten steel, such as hydrogen and nitrogen, during the solidification process.

[0073] S6.4. The ultrasonic flaw detector transmits high-frequency ultrasonic waves into the continuous casting billet through the probe. Before flaw detection, a coupling agent, such as glycerin, needs to be applied to the surface of the continuous casting billet to ensure that the ultrasonic waves can be effectively transmitted into the continuous casting billet. When the ultrasonic waves propagate inside the continuous casting billet, they will reflect, refract and scatter when encountering interfaces of different media, such as the interface between the defect and the matrix; the flaw detector receives the reflected waves and determines the location, size and nature of the internal defects based on the characteristics of the reflected waves, such as time, intensity and waveform.

[0074] S6.5. Cut the sample from the continuous casting billet. The positions of the sample usually include the head, middle and tail. The cut sample should be processed by rough grinding, fine grinding and polishing to make the surface flat and smooth. Then, use a suitable corrosive agent, such as nitric acid alcohol solution, to corrode the sample to show its macrostructure. When observing the macrostructure under a microscope, internal defects such as central looseness, shrinkage, segregation, etc. can be seen.

[0075] S7. During the rolling stage, the billet must first be heated. The temperature of the preheating section is controlled below 850°C, the temperature of the heating section is controlled between 1160 and 1220°C, and the temperature of the soaking section is controlled between 1160 and 1220°C; preferably 1180 to 1210°C. The billet enters the heating furnace from room temperature. If the temperature rises too quickly, the temperature difference between the inside and outside of the billet will increase sharply, thereby generating greater thermal stress. Controlling the temperature of the preheating section below 850°C can slowly heat up the billet, allowing the internal structure of the billet enough time to adapt to the temperature change and reduce the risk of billet cracking due to thermal stress. For example, for some billets containing alloy elements, the thermal expansion coefficients of alloy elements at different temperatures are different. Slow preheating can make the organization of these elements distributed evenly heated to avoid local stress concentration. For the control of heating furnace time, the time of 300×430 square rectangular billet in the furnace is not less than 200 minutes, 240 square is not less than 120 minutes, and 150 square is not less than 90 minutes during cold loading. If the accident time is greater than 20 minutes, the heating temperature should be reduced in time. During hot delivery and hot loading, the furnace temperature is required to be ≥300℃, the time of 300×430 square rectangular billet in the furnace is not less than 150 minutes, 240 square is not less than 100 minutes, and 150 square is not less than 60 minutes, and the rolling temperature is ≥1000℃, and the continuous rolling temperature is ≥950℃. Secondly, rolling is carried out. When rolling, the places with sharp angles and burrs such as rolling grooves, baffles, guides, rollers, etc. should be polished clean; if they cannot be polished clean, they should be replaced in time. The non-roundness and dimensional tolerance of round steel are strictly controlled during the rolling process.

[0076] S8. During the cooling stage after rolling, the round steel is slowly cooled on the cooling bed with a heat preservation cover. The operator must operate carefully, not bend the finished product, not scratch the surface, ensure that the bending degree of the steel meets the requirements, and conduct surface quality inspection. The ears, scratches, cracks, burrs and flash defects on the surface of the finished product must be polished clean;

[0077] S9, during the inspection and acceptance stage of the finished product obtained in step S8, the steel should be inspected and accepted in batches. Each batch consists of steel of the same furnace number, the same processing method, and the same size, and is finally delivered in the hot-rolled state.

[0078] The alloy components of this embodiment are C: 0.11%-0.12%, Si: 0.20%-0.28%, Mn: 0.50-0.58%, P≤0.0014%, S≤0.006%, Al: 0.005-0.015%, Cr: 1.00-1.05%, Mo: 0.25-0.29%, and V: 0.165-0.175% by mass percentage.

[0079] The surface of the tube billet shall comply with the provisions of YB / T5137. There shall be no scars, folds, pinholes, slag inclusions, inclusions or visible cracks on the surface of the tube billet.

[0080] Macrostructure: There shall be no visible white spots, residual shrinkage cavities, delamination, cracks, bubbles, inclusions, skin peeling and subcutaneous bubbles on the full cross-section acid immersion test piece. Macrostructure defects are allowed and shall meet the requirements of Table 1.

[0081] Table 1. Allowable levels of macrostructure

[0082] Serial number Generally loose, grade Loose center, grade Segregation, level Example 1 ≤2.0 ≤2.0 ≤2.0 Example 2 ≤1.5 ≤1.5 ≤1.5

[0083] Non-metallic inclusions: Steel should be inspected and graded according to the JK series rating chart in GB / T10561. The grade of non-metallic inclusions shall meet the requirements of Table 2.

[0084] Table 2: Qualified Levels of Non-metallic Inclusions

[0085]

[0086] Category A: sulfide type, Category B: oxide type, Category C: silicate type, Category D: spherical oxide type; among them, coarse series and fine series refer to: inspection of inclusions is carried out in accordance with GB / T 10561, and the width or diameter of the inclusions is measured and compared with the value specified in the standard to determine whether it is coarse or fine.

[0087] Flaw detection and inspection: Ultrasonic flaw detection is carried out on each steel bar in accordance with Class B standard of GB / T4162.

[0088] The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A method for producing a high-pressure boiler alloy tube blank, characterized in that: The production method comprises the following steps: S1, in the preparation stage of molten iron and scrap steel, the contents of the five harmful elements As, Sn, Sb, Pb and Bi in the molten iron entering the furnace are controlled, and high-quality scrap steel is added to the scrap steel entering the furnace; S2, converter smelting stage, the tapping endpoint target is controlled at C ≥ 0.04%, the number of re-blowing is ≥ 2 times, ensuring C-T coordination of slag tapping, and adding deoxidation alloy into the ladle for deoxidation alloying when 1 / 4 of the steel is tapped; S3, LF furnace control, total argon blowing time ≥ 40min, use Al particles for diffusion deoxidation, slag formation within 25min, rapid slag deoxidation and desulfurization, open pouring furnace feed Ca line 70±20~50m, continuous pouring furnace feed Ca line 50±20~50m for calcification treatment before molten steel leaves the station; S4, VD / RH furnace control, vacuum target below 0.5tor, holding time ≥10min, soft blowing time after molten steel breaks through the air ≥15min, hydrogen is set for molten steel out of the station, and the temperature of molten steel out of the station on the platform is ≤35℃ for the start-casting furnace and ≤30℃ for the continuous casting furnace; S5. During the continuous casting control stage, the casting is protected throughout the whole process. The casting speed of the casting machine is 0.70-0.95m / min, and the typical casting speed is 0.70m / min. The superheat of the molten steel in the tundish continuous casting furnace is ≤25℃, the open casting furnace is ≤30℃, and the typical casting superheat is 15-25℃. The crystallizer is electromagnetically stirred, and the billet stack is slowly cooled. The slow cooling time is ≥24h. S6. In the continuous casting billet determination stage, the size, weight, shape and surface quality of rectangular billets and square billets are determined; S7. In the rolling stage, the steel sheet must first be heated, with the preheating section temperature controlled below 850°C, the heating section temperature controlled between 1160 and 1220°C, and the soaking section temperature controlled between 1180 and 1220°C; S8. During the cooling stage after rolling, the round steel is cooled slowly on the cooling bed with a heat preservation cover; S9, inspecting and accepting the finished products obtained in step S8 in batches, each batch consisting of steel products with the same furnace number, the same processing method and the same size, and finally delivering them in a hot-rolled state; The alloy components are C: 0.08%-0.15%, Si: 0.17%-0.37%, Mn: 0.40%-0.70%, P≤0.020%, S≤0.010%, Al≤0.020%, Cr: 0.90-1.20%, Mo: 0.25-0.35%, and V: 0.15-0.30%.

2. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: Step S1: the molten iron and scrap steel preparation stage includes the following specific steps: S1.1, blast furnace ironmaking, iron ore, coke and limestone are loaded into the blast furnace through a feed car or a belt conveyor. The ore is pre-treated by crushing and screening to make its particle size meet the requirements of blast furnace smelting. Coke is required to have high strength and low ash content, and limestone has a moderate particle size; S1.2, molten iron pretreatment, molten iron flows from the blast furnace into a torpedo tank car or a ladle, and is desulfurized by injection. The injection equipment sprays the desulfurizer into the molten iron, and the sulfur content of the treated molten iron is controlled at 0.01% to 0.03%.

3. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: In step S1, the contents of the five harmful elements in the molten iron are As≤0.012%, Sn≤0.012%, Pb≤0.008%, Sb≤0.010%, and Bi≤0.010%, and the contents of As and Sn in the finished steel are not more than 0.015% respectively.

4. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: In the LF furnace control stage, the power-on time is ≥20 minutes, the outgoing activity [O] is ≤5ppm, the Fe-Ca wire is fed ≥400m or the pure Ca wire is fed ≥200m after refining, and the soft argon blowing time after wire feeding is ≥5 minutes.

5. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: During the VD furnace control stage, argon blowing is started throughout the process when the molten steel enters the VD furnace. The total argon blowing time in the VD furnace is ≥30 minutes. The molten steel is hydrogenated when it leaves the furnace and is controlled according to the target value [H]≤2.0ppm. The hydrogen setting for each pouring is not less than 1 furnace.

6. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: During the continuous casting control stage, the current intensity of the crystallizer is 150-200A, the frequency is 2.5Hz, and the water volume of the crystallizer is 2720-2920L / min.

7. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: The continuous casting slab determination stage includes the following specific steps: S6.

1. Inspectors use a strong flashlight to carefully observe the length and circumference of the continuous casting billet or the edges. For small cracks, a magnifying glass is needed to assist in the inspection. S6.

2. During inspection, pay attention to whether there are granular or blocky inclusions on the surface of the continuous casting billet; S6.

3. Surface pores appear as small round or oval pits on the surface of the continuous casting billet. When inspecting, pay attention to the size and distribution density of the pores. S6.

4. The ultrasonic flaw detector transmits high-frequency ultrasonic waves into the continuous casting billet through the probe. Before flaw detection, a coupling agent needs to be applied to the surface of the continuous casting billet to ensure that the ultrasonic waves can be effectively transmitted into the continuous casting billet. When the ultrasonic waves propagate inside the continuous casting billet, they will be reflected, refracted and scattered when encountering the interface between different media, such as the interface between the defect and the matrix. S6.

5. Cut samples from the continuous casting billet. The locations of the samples usually include the head, middle and tail. The cut samples must be rough ground, fine ground and polished to make the surface flat and smooth. Then, the samples are corroded with a corrosive agent to show their macrostructure.

8. The method for producing a high-pressure boiler alloy tube blank according to claim 1, characterized in that: The finished product inspection and acceptance stage require inspection through surface quality inspection, ultrasonic flaw detection, low-magnification structure inspection, sampling, and experimental methods. Qualified tube blanks must be correctly labeled, and the labeling content includes product model, specification, production date, and inspection pass mark.

9. The method for producing a high-pressure boiler alloy tube blank according to claim 2, characterized in that: The iron ore is hematite or magnetite, and the desulfurizing agent is magnesium powder or lime-fluorite mixed powder.

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