High-strength austenitic stainless steel for high-pressure boiler and preparation method thereof

Abstract

The present invention provides high-strength austenitic stainless steel for high-pressure boilers and a preparation method thereof. The chemical composition of the stainless steel, by weight, is as follows: C 0.01-0.05%, Mn 0.50-1.00%, Si 0.50-1.00%, P ≤ 0.01%, S ≤ 0.01%, Cr 19.00-21.00%, Ni 24.00-26.00%, Al ≤ 0.03%, Mo ≤ 0.01%, B ≤ 0.0005%, Ti ≤ 0.0015%, Nb 0.50-1.00%, N 0.01-0.10%, O ≤ 0.002%, with the remainder being Fe and unavoidable impurities. The high-strength austenitic stainless steel for high-pressure boilers is prepared by vacuum induction, pouring, vacuum consumables, forging, and solution treatment. The high-strength austenitic stainless steel obtained by the present invention can effectively improve element segregation, carbide precipitation and structural uniformity by strictly controlling the content of each element and cooperating with each other, thereby improving the mechanical properties of the alloy.

Description

Technical Field

[0001] The present invention relates to the field of stainless steel materials, and in particular to high-strength austenitic stainless steel for high-pressure boilers and a preparation method thereof. Background Art

[0002] Austenitic heat-resistant stainless steel is made by adding niobium and nitrogen to nickel-chromium steel to precipitate finely dispersed NbCrN phases and Nb-rich carbonitrides, thereby further improving the mechanical properties of high-strength austenitic stainless steel. It is widely used in high-pressure boilers, power station boiler superheaters and reheaters.

[0003] The commonly used austenitic heat-resistant stainless steel for boilers is 07Cr25Ni21. Its chemical composition, by mass percentage, is as follows: Carbon (C) 0.04-0.10%, Silicon (Si) ≤ 0.75%, Manganese (Mn) ≤ 2.00%, Chromium (Cr) 24.00-26.00%, Nickel (Ni) ≤ 19.00-22.00%, Phosphorus (P) ≤ 0.03%, Sulfur (S) ≤ 0.015%, with the remainder being iron and unavoidable impurities. After heat treatment, 07Cr25Ni21 has a tensile strength of 561 MPa and a yield strength of 246 MPa.

[0004] However, with the development of industry, the working environment and working intensity of high-pressure boilers have put forward higher strength requirements for austenitic heat-resistant stainless steel alloy materials. The strength of 07Cr25Ni21 can no longer meet the needs of austenitic heat-resistant stainless steel for high-pressure boilers. Therefore, the development of a high-strength austenitic heat-resistant stainless steel material for high-pressure boilers is a technical problem that needs to be solved urgently. Summary of the Invention

[0005] In view of the defect of low strength of existing austenitic heat-resistant stainless steel materials for high-pressure boilers, the purpose of the present invention is to provide a high-strength austenitic stainless steel for high-pressure boilers and a preparation method thereof.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A high-strength austenitic stainless steel for high-pressure boilers, the chemical composition of which is as follows by weight: C 0.01-0.05%, Mn 0.50-1.00%, Si 0.50-1.00%, P≤0.01%, S≤0.01%, Cr 19.00-21.00%, Ni 24.00-26.00%, Al≤0.03%, Mo≤0.01%, B≤0.0005%, Ti≤0.0015%, Nb 0.50-1.00%, N 0.01-0.10%, O≤0.002%, and the remainder is Fe and unavoidable impurities.

[0008] The high-strength austenitic stainless steel for the high-pressure boiler is prepared through vacuum induction, pouring, vacuum self-consumption, forging, and solid solution treatment.

[0009] C 0.02-0.03%, Mn 0.56-0.68%, Si 0.59-0.60%, P 0.006%, S 0.003%, Cr20.09-20.16%, Ni 24.82-24.85%, Al 0.02%, Mo 0.006%, B 0.0005%, Ti 0.001-0.0011%, Nb 0.69-0.71%, N 0.064-0.065%, O 0.0011-0.0013%, and the rest are Fe and unavoidable impurities.

[0010] The high-strength austenitic stainless steel for high-pressure boilers has the following properties: average tensile strength of 662-678 MPa, and average yield strength of 370-382 MPa.

[0011] A method for preparing high-strength austenitic stainless steel for high-pressure boilers comprises the following steps:

[0012] (1) Raw material preparation: The raw materials are prepared according to the following mass percentage chemical composition of high-strength austenitic stainless steel: C 0.01-0.05%, Mn 0.50-1.00%, Si 0.50-1.00%, P ≤ 0.01%, S ≤ 0.01%, Cr 19.00-21.00%, Ni 24.00-26.00%, Al ≤ 0.03%, Mo ≤ 0.01%, B ≤ 0.0005%, Ti ≤ 0.0015%, Nb 0.50-1.00%, N 0.01-0.10%, O ≤ 0.002%, and the rest is Fe and unavoidable impurities;

[0013] (2) Vacuum induction melting: All raw materials are placed in a vacuum induction furnace and melted at 1450-1500°C, and then refined at 1500-1540°C under a vacuum degree of 1-5 Pa for 30-120 minutes to obtain molten steel for high-pressure boiler steel;

[0014] (3) Casting: The high-pressure boiler steel molten steel prepared in step (2) is cast into electrode rods of high-pressure boiler steel at a temperature of 1540-1570° C. and a casting speed of 150-300 kg / min. The casting process is carried out in an argon atmosphere with an argon pressure of 30,000-80,000 Pa.

[0015] (4) Vacuum consumable steel: The surface of the electrode rod obtained in step (3) is polished and then vacuum consumable steel is obtained. The current in the initial stage is 2.5-7.5KA, the voltage is 22.5-24.5V, and the melting rate is 1-9 drops / s; the melting rate in the smelting stage is 2.5-3.5kg / min, and the droplet rate is 2-4 drops / s; the current in the hot capping stage is 1.5-4.0KA, and the melting rate is 2-10 drops / s; and a consumable steel ingot for high-pressure boiler steel is obtained;

[0016] (5) Forging: The consumable ingot obtained in step (4) is subjected to forging hot working, with the starting forging temperature being 1050-1150°C and the final forging temperature being 840-920°C; after three piers and three draws and seven fires, the ingot is forged, and air-cooled to room temperature after forging to obtain high-pressure boiler steel;

[0017] (6) Solution treatment: heating the high-pressure boiler steel prepared in step (5) to 1030-1130°C at a heating rate of 80-100°C / h, keeping the temperature for 1-4h, and cooling the temperature to no more than 80°C to obtain high-strength austenitic stainless steel for high-pressure boilers.

[0018] In the step (1), C 0.02-0.03%, Mn 0.56-0.68%, Si 0.59-0.60%, P 0.006%, S 0.003%, Cr 20.09-20.16%, Ni 24.82-24.85%, Al 0.02%, Mo 0.006%, B 0.0005%, Ti 0.001-0.0011%, Nb 0.69-0.71%, N 0.064-0.065%, O 0.0011-0.0013%, and the remainder is Fe and unavoidable impurities; each element is added to the furnace through the following alloys or pure metals: metallic Mn, metallic Cr, metallic Ni, metallic Nb; Al, Ti, B, S, P and O elements are all impurities introduced by pure metal raw materials.

[0019] Preferably, in step (5), the starting forging temperature is 1050-1100°C, and the final forging temperature is 840-880°C.

[0020] The obtained high-strength austenitic stainless steel for high-pressure boilers has the following properties: average tensile strength of 662-678 MPa and average yield strength of 370-382 MPa.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The high-strength austenitic stainless steel provided by the present invention, by strictly controlling the contents of chemical elements, particularly N, Cr, and Nb, can produce a uniformly distributed, Nb-containing, dispersed strengthening precipitate, the NbCrN phase. This prevents improper combinations of N, Cr, and Nb. Excessive N will result in the precipitation of large amounts of CrN at the grain boundaries of the high-strength austenitic stainless steel, causing chromium depletion on both sides of the grain boundaries and reducing the corrosion resistance of the stainless steel. Low N content will result in less precipitation of the dispersed strengthening precipitate, the NbCrN phase, resulting in lower strength. By strictly controlling the contents of each element and ensuring the coordination of these elements, the high-strength austenitic stainless steel can effectively improve chemical element segregation, precipitate phase segregation, and structural uniformity in the stainless steel. This in turn improves the mechanical properties of the stainless steel, particularly its strength. Compared to existing austenitic heat-resistant steels for high-pressure boilers, the tensile strength and yield strength of the high-strength austenitic stainless steel provided by the present invention are increased by 18% and 50%, respectively, meeting the high-strength performance requirements of stainless steel materials in current high-pressure boiler operating environments while also extending the service life of high-pressure boilers. DETAILED DESCRIPTION

[0023] The present invention will be further described below with reference to the embodiments.

[0024] A high-strength austenitic stainless steel for high-pressure boilers comprises the following components, measured by mass percentage: C 0.01-0.05%, Mn 0.50-1.00%, Si 0.50-1.00%, P≤0.01%, S≤0.01%, Cr 19.00-21.00%, Ni 24.00-26.00%, Al≤0.03%, Mo≤0.01%, B≤0.0005%, Ti≤0.0015%, Nb 0.50-1.00%, N 0.01-0.10%, O≤0.002%, and the remainder being Fe and unavoidable impurities.

[0025] The present invention provides a method for preparing the high-strength austenitic stainless steel for high-pressure boilers, comprising the following steps: subjecting the raw materials to vacuum induction melting, pouring, vacuum self-consumable molding, forging, and solution treatment to obtain the high-strength austenitic stainless steel for high-pressure boilers.

[0026] The ingredients are prepared according to the following mass percentage chemical composition of high-strength austenitic stainless steel: C 0.01-0.05%, Mn 0.50-1.00%, Si 0.50-1.00%, P≤0.01%, S≤0.01%, Cr19.00-21.00%, Ni 24.00-26.00%, Al≤0.03%, Mo≤0.01%, B≤0.0005%, Ti≤0.0015%, Nb 0.50-1.00%, N 0.01-0.10%, O≤0.002%, and the rest are Fe and unavoidable impurities.

[0027] Among them, each element is added into the furnace through the following alloys or pure metals: metal Mn, metal Cr, metal Ni, metal Nb; Al, Ti, B, S, P and O elements are all impurities introduced by pure metal raw materials.

[0028] In the vacuum induction melting, the refining time is 30-120 minutes and the vacuum degree is 1-5 Pa.

[0029] In the vacuum induction melting, the temperature in the melting period is 1450-1500°C, and the temperature in the refining period is 1500-1540°C.

[0030] During the pouring process, the pouring temperature is 1540-1570°C, the pouring speed is 150-300 kg / min, and the pouring process is carried out in an argon atmosphere with an argon pressure of 30,000-80,000 Pa. By controlling the pouring speed and pouring temperature, the two work together to improve the ingot yield of the cast electrode rod and reduce production costs. If the pouring speed is too fast or the temperature is too high, energy loss will increase; if the pouring speed is too slow or the temperature is too low, the element distribution will be uneven.

[0031] In the vacuum consumables, the current in the initial stage is 2.5-7.5KA, the voltage is 22.5-24.5V, and the melting rate is 1-9 drops / s; the melting rate in the smelting stage is 2.5-3.5kg / min, and the melting drop rate is 2-4 drops / s; the current in the hot capping stage is 1.5-4.0KA, and the melting rate is 2-10 drops / s.

[0032] Preferably, in the vacuum consumable furnace, the melting rate in the melting stage is fixed.

[0033] The forging is to subject the vacuum consumable ingot to three-stack, three-draw and seven-fire processes to form the material.

[0034] Among them, the starting forging temperature of three-pier and three-draw is 1050-1150℃, and the final forging temperature is 840-920℃.

[0035] Example 1:

[0036] (1) In this embodiment, the high-strength austenitic stainless steel is prepared according to the following mass percentage chemical composition: C 0.02%, Mn 0.56%, Si 0.59%, P 0.006%, S 0.003%, Cr 20.09%, Ni 24.85%, Al 0.02%, Mo 0.006%, B 0.0005%, Ti 0.001%, Nb 0.69%, N 0.064%, O 0.0013%, and the remainder is Fe and unavoidable impurities.

[0037] (2) Vacuum induction melting: All raw materials are placed in a vacuum induction furnace and melted at 1450-1470°C. They are then refined at 1500-1520°C and 1-5 Pa for 60 minutes. Samples are then taken for full element analysis. Based on the results of the full element analysis, the materials are adjusted to ensure that all elements meet the chemical composition requirements of high-strength austenitic stainless steel. After melting in a vacuum induction furnace, molten steel for high-pressure boilers is formed.

[0038] (3) Casting: The high-pressure boiler steel molten steel obtained in step (2) is cast into high-pressure boiler steel electrode rods (diameter 250 mm) at a pouring rate of 150 kg / min at 1540-1550°C. Argon protection is used throughout the casting process, and the argon filling is: 40,000 Pa.

[0039] (4) Vacuum consumable steel: After the surface of the electrode rod is polished, vacuum consumable steel is produced. In the initial stage of vacuum consumable steel production, the smelting current is controlled at 2.5-7.5KA, the smelting voltage is 22.5-24.5V, and the melting rate is 1-9 drops / s; the melting rate in the smelting stage is 2.7kg / min, and the droplet rate is 2 drops / s; the current in the hot capping stage is 1.5-4.0KA, and the melting rate is 2-10 drops / s. A consumable steel ingot (diameter 305mm) for high-pressure boiler steel is obtained;

[0040] (5) Forging: The consumable ingot obtained in step (4) is subjected to forging hot working, with the starting forging temperature being 1050-1080°C and the final forging temperature being 840-860°C, after three piers and three draws, and seven fires, and then air-cooled to room temperature after forging;

[0041] (6) Solution treatment: The high-pressure boiler steel prepared in step (5) is heated to 1050°C at a heating rate of 80°C / h, kept at this temperature for 2h, and then water-cooled to room temperature to obtain high-pressure boiler steel.

[0042] Example 2:

[0043] (1) In this embodiment, the high-strength austenitic stainless steel is prepared according to the following chemical composition by mass percentage: C 0.03%, Mn 0.68%, Si 0.60%, P 0.006%, S 0.003%, Cr 20.16%, Ni 24.82%, Al 0.02%, Mo 0.006%, B 0.0005%, Ti 0.0011%, Nb 0.71%, N 0.065%, O 0.0011%, and the remainder is Fe and unavoidable impurities.

[0044] (2) Vacuum induction melting: All raw materials are placed in a vacuum induction furnace and melted at 1470-1490°C. They are then refined at 1520-1540°C and 1-5 Pa for 90 minutes. Samples are then taken for full element analysis. Based on the results of the full element analysis, the raw materials are adjusted to meet the chemical composition requirements of high-strength austenitic stainless steel. After melting in the vacuum induction furnace, molten steel for high-pressure boilers is formed.

[0045] (3) Casting: The high-pressure boiler steel molten steel obtained in step (2) is cast into high-pressure boiler steel electrode rods (diameter 250 mm) at a pouring rate of 200 kg / min at 1560-1570°C. Argon protection is used throughout the casting process, and the argon filling is: 60,000 Pa.

[0046] (4) Vacuum consumable steel: The surface of the electrode rod is polished and then vacuum consumable steel is produced. In the initial stage of the vacuum consumable steel production process, the smelting current is controlled at 2.5-7.5KA, the smelting voltage is 22.5-24.5V, and the melting rate is 1-9 drops / s; the melting rate in the smelting stage is 3kg / min, and the droplet rate is 2 drops / s; the current in the hot capping stage is 1.5-4.0KA, and the melting rate is 2-10 drops / s. A consumable steel ingot (diameter 305mm) for high-pressure boiler steel is obtained;

[0047] (5) Forging: The consumable ingot obtained in step (4) is subjected to forging hot working, with the starting forging temperature being 1080-1100°C and the final forging temperature being 860-880°C, after three piers and three draws, and seven fires, and then air-cooled to room temperature after forging;

[0048] (6) Solution treatment: The high-pressure boiler steel prepared in step (5) was heated to 1080°C at a heating rate of 100°C / h, kept at that temperature for 3h, and then water-cooled to room temperature to obtain high-pressure boiler steel.

[0049] Table 1 Test results of Example 1 and Example 2

[0050]

[0051] As shown in Table 1, the high-pressure boiler steel provided by the present invention can meet the high-strength performance requirements of stainless steel in the existing high-pressure boiler working environment through the synergistic effect of various components, and significantly improve the service life of the high-pressure boiler.

[0052] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A high-strength austenitic stainless steel for high-pressure boilers, characterized in that: The chemical composition of the high-strength austenitic stainless steel for high-pressure boilers is as follows by weight: C 0.02-0.03%, Mn 0.56-0.68%, Si 0.59-0.60%, P 0.006%, S 0.003%, Cr 20.09-20.16%, Ni 24.82-24.85%, Al 0.02%, Mo 0.006%, B 0.0005%, Ti 0.001-0.0011%, Nb 0.69-0.71%, N 0.064-0.065%, O 0.0011-0.0013%, and the remainder is Fe and unavoidable impurities; The high-strength austenitic stainless steel for the high-pressure boiler is prepared by vacuum induction, pouring, vacuum self-consumption, forging, and solution treatment; The high-strength austenitic stainless steel for high-pressure boilers obtains a uniformly distributed Nb-containing dispersed strengthening precipitation phase NbCrN phase, and has the following properties: an average tensile strength of 662-678 MPa and an average yield strength of 370-382 MPa.

2. A method for preparing high-strength austenitic stainless steel for high-pressure boilers according to claim 1, characterized in that: The preparation method specifically comprises the following steps: (1) Raw material preparation: The raw materials are prepared according to the following mass percentage chemical composition of high-strength austenitic stainless steel: C 0.02-0.03%, Mn 0.56-0.68%, Si 0.59-0.60%, P 0.006%, S 0.003%, Cr 20.09-20.16%, Ni 24.82-24.85%, Al 0.02%, Mo 0.006%, B 0.0005%, Ti 0.001-0.0011%, Nb 0.69-0.71%, N 0.064-0.065%, O 0.0011-0.0013%, and the rest is Fe and unavoidable impurities; (2) Vacuum induction melting: All raw materials are placed in a vacuum induction furnace and melted at 1450-1500°C, and then refined at 1500-1540°C under a vacuum degree of 1-5 Pa for 30-120 minutes to obtain molten steel for high-pressure boiler steel; (3) Casting: The high-pressure boiler steel molten steel prepared in step (2) is cast into electrode rods of high-pressure boiler steel at a temperature of 1540-1570° C. and a casting speed of 150-300 kg / min. The casting process is carried out in an argon atmosphere with an argon pressure of 30,000-80,000 Pa. (4) Vacuum consumable steel: The surface of the electrode rod obtained in step (3) is polished and then vacuum consumable steel is obtained. The current in the initial stage is 2.5-7.5KA, the voltage is 22.5-24.5V, and the melting rate is 1-9 drops / s; the melting rate in the smelting stage is 2.5-3.5kg / min, and the droplet rate is 2-4 drops / s; the current in the hot capping stage is 1.5-4.0KA, and the melting rate is 2-10 drops / s; and a consumable steel ingot for high-pressure boiler steel is obtained; (5) Forging: The consumable ingot obtained in step (4) is subjected to forging hot working, with the starting forging temperature being 1050-1150°C and the final forging temperature being 840-920°C; after three piers and three draws and seven fires, the ingot is forged, and air-cooled to room temperature after forging to obtain high-pressure boiler steel; (6) Solution treatment: heating the high-pressure boiler steel prepared in step (5) to 1030-1130°C at a heating rate of 80-100°C / h, keeping the temperature for 1-4h, and cooling to no more than 80°C to obtain high-strength austenitic stainless steel for high-pressure boilers; The obtained high-strength austenitic stainless steel for high-pressure boilers has a uniformly distributed Nb-containing dispersed strengthening precipitation phase NbCrN phase, and has the following properties: average tensile strength of 662-678 MPa and average yield strength of 370-382 MPa.

3. The preparation method according to claim 2, characterized in that In the step (1), each element is added into the furnace through the following alloys or pure metals: metal Mn, metal Cr, metal Ni, metal Nb; Al, Ti, B, S, P and O elements are all impurities introduced into the pure metal raw materials.

4. The preparation method according to claim 2, characterized in that In the step (5), the starting forging temperature is 1050-1100°C, and the final forging temperature is 840-880°C.

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