Martensitic stainless steel plate for supporting key equipment of nuclear power plant and its manufacturing method

Abstract

The present invention discloses a martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant and a manufacturing method thereof. The steel plate comprises the following components: C: 0.07%-0.15%; Si: 0.1%-1.0%; Mn: 0.3%-1.00%; P≤0.010%; S: 0.02%-0.06%; Ni: 0.5%-2.0%; Cr: 12%-15%; Mo: 0.02%-0.10%; N: 0.03%-0.15%; Al: 0.01%-0.20%; B: 0.0001%-0.0011%; Cu: 0.01%-0.2%; and ZrO2: 0.0001%-0.0010%. This steel plate utilizes controlled rolling combined with a special quenching and tempering heat treatment process, with an initial rolling temperature of 1100-1200°C and a final rolling temperature of 950-1050°C. Quenching temperatures range from 900°C to 1050°C, with a holding time of 1-6 min / mm; and tempering temperatures range from 680°C to 780°C, with a holding time of 1-10 min / mm. This plate exhibits excellent room-temperature and high-temperature mechanical properties, fully meeting the requirements for martensitic stainless steel used in support equipment of key third-generation nuclear power plants. This martensitic stainless steel medium and thick plate is primarily suitable for high-strength and toughness support components.

Description

Technical Field

[0001] The present invention relates to the technical field of martensitic stainless steel plate rolling, and in particular to a martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant and a manufacturing method thereof. The martensitic stainless steel medium and thick plate is mainly suitable for high-strength and toughness supporting components of a nuclear power plant. Background Art

[0002] The steam generator is one of the main equipment of nuclear power and belongs to the first-level nuclear equipment. There are tens of thousands of parallel pipes in the steam generator for steam transmission. The support plates of these parallel pipes are a special martensitic stainless steel.

[0003] The working temperature of the pipes in the steam generator is about 300℃. Therefore, the martensitic stainless steel used in the pipe support plates of nuclear power steam generators must have good high-temperature performance at high temperatures. A notable feature of martensitic stainless steel is that its high-temperature tensile properties decrease significantly with the increase of steel plate thickness. How to produce martensitic stainless steel that can ensure high-temperature performance is the main technical problem. The original technology generally uses short-time heat treatment to ensure the high-temperature performance of the steel plate as much as possible, but the toughness of the steel plate produced by this method is very poor. It can be seen that how to develop martensitic stainless steel with good high-temperature strength and toughness is the key.

[0004] At present, many patents have been formed for martensitic stainless steel at home and abroad, and the main ones related to this patent include the following:

[0005] The patent application number 201910821783.2 discloses "a method for eliminating surface cracks in ultra-wide ferritic stainless steel medium and thick plates". The proportion of equiaxed crystals in the continuous casting billet is not less than 75%; the billet homogenization temperature is 750-850℃, the furnace time is 1-2h, and it is water-cooled to room temperature; the surface roughness of the billet after grinding is Ra≤70μm, and high-temperature anti-oxidation coating is sprayed; the billet is sequentially treated in the preheating section, heating section and soaking section before being taken out of the furnace; the preheating section temperature is 950-1180℃, the heating section temperature is 1180-1260℃, the soaking section temperature is 1220-1250℃, and the furnace time is 4-5.5h; the rough rolling start temperature is ≥1150℃; the finishing rolling start temperature is ≥1150℃, the final rolling temperature is ≥950℃, the rolling single-pass reduction rate is ≤20%, the annealing temperature is 750-880℃, the furnace time is 2-5min / mm, and air cooling is performed. The product width is 2500-4000mm and the thickness is 8-30mm. The scrap rate due to surface cracks has been reduced from over 6% to below 0.5%. The steel plate of this invention has a yield strength of ≥230MPa, a tensile strength of ≥450MPa, and an elongation of ≥28%. However, its shortcomings are low mechanical properties and the fact that the invention does not consider the high-temperature mechanical properties of the steel plate, which cannot meet the demand for steel used in nuclear power plants.

[0006] Invention No. 201010151832.5 discloses a heat treatment process for 1Cr13 thick-walled pipes. The process utilizes a tubular protective atmosphere heat treatment furnace for heat treatment. The process parameters are: holding at 1000-1050°C for 1-2 hours, air cooling to room temperature, and a cooling rate of less than 500°C / h; holding at 650-700°C for 3-5 hours, air cooling to room temperature, and a cooling rate of less than 500°C / h. The process produces a ferrite content of less than 10%. The 1Cr13 pipes produced using this heat treatment method utilize air cooling, which cannot guarantee uniform steel plate properties. Furthermore, the specification does not provide information on the material's high-temperature resistance and impact toughness at specific temperatures, making it incompatible with its intended application.

[0007] The invention with application number 201010582850.9 discloses a heat treatment method for martensitic stainless steel forgings for compression springs of nuclear power reactor internal components. The heat treatment comprises the following steps: a preliminary heat treatment, in which the workpiece is rapidly heated to a high temperature of 990-1030°C at a rate of 100±10°C / hour, kept warm, and then air-cooled; a second step, quenching, in which the workpiece is rapidly heated to a high temperature of 950-990°C at a rate of 100±10°C / hour, kept warm, and then oil-cooled; and a third step, high-temperature tempering, in which the workpiece is rapidly heated to a high temperature of 630-670°C at a rate of 100±10°C / hour, kept warm, and then air-cooled. The invention improves the yield strength and impact energy by adding a high-temperature preheating treatment, but the high-temperature preheating treatment increases the strength resulting in coarse grain size of the steel plate, and this heat treatment process has obvious limitations on the thickness of the steel plate. The comparative invention does not specify the available thickness range of the material. At the same time, the steel plate does not have good high-temperature tensile properties, -20°C impact energy, and bending properties under this process. Summary of the Invention

[0008] The purpose of the present invention is to provide a martensitic stainless steel plate for supporting key equipment in third-generation nuclear power plants and a manufacturing method thereof. Through chemical composition design and appropriate continuous casting, heating, rolling, heat treatment and other processes, the produced steel plate not only has good room temperature mechanical properties and high temperature mechanical properties, but can fully meet the requirements of martensitic stainless steel for supporting key equipment in third-generation nuclear power plants. The martensitic stainless steel medium and thick plate is mainly suitable for high-strength and high-toughness supporting components.

[0009] In order to achieve the purpose of the present invention, the technical solution adopted by the present invention is as follows:

[0010] The present invention provides a martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant and a manufacturing method thereof. The specific technical scheme is as follows: the plate comprises the following components by weight: C: 0.07%-0.15%; Si: 0.1%-1.0%; Mn: 0.3%-1.00%; P≤0.010%; S: 0.02%-0.06%; Ni: 0.5%-2.0%; Cr: 12%-15%; Mo: 0.02%-0.10%; N: 0.03%-0.15%; Al: 0.01%-0.20%; B: 0.0001%-0.0011%; Cu: 0.01%-0.2%; ZrO2: 0.0001%-0.0010%, and the balance is Fe and unavoidable impurities.

[0011] The reasons for adopting the above components are as follows:

[0012] (1) Carbon: Carbon is the most effective element for strengthening structural steel. It directly affects the strength, plasticity, toughness, and weldability of steel, and also helps improve hardenability. Therefore, the steel composition of the present invention requires a carbon content of 0.07% to 0.15%.

[0013] (2) Silicon: Si is a ferrite phase-forming element. When the Si content increases, the stability of the ferrite phase becomes higher and the oxidation resistance, nitric acid and sulfuric acid corrosion resistance are changed. The formation of ferrite is conducive to softening and the toughness of steel. Therefore, Si: 0.1%-1.0%.

[0014] (3) Manganese: Mn can inhibit the effect of sulfur in steel and improve thermoplasticity. However, when the Mn content becomes high, MnS is formed, which easily causes pitting corrosion and reduces the corrosion resistance of stainless steel. Therefore, Mn: 0.3%-1.00%.

[0015] (4) Phosphorus: Phosphorus is considered a harmful element in stainless steel and should be controlled as low as possible. Considering the cost, the P content is controlled below 0.010%.

[0016] (5) Sulfur: Sulfur easily forms sulfide inclusions in steel, which reduces the impact toughness of steel and impairs welding performance. It also aggravates defects such as central segregation and porosity and increases radiation embrittlement. However, an appropriate amount of sulfur is beneficial to the cutting effect of steel plates. Therefore, the present invention requires S: 0.02%-0.06%.

[0017] (6) Nickel: Ni is an austenite-forming element. When the austenite content increases, it promotes the formation of martensite during air cooling after hot rolling, thereby increasing strength and hardness, and also improving toughness. Therefore, Ni: 0.5%-2.0%.

[0018] (7) Chromium: Cr is the most important alloying element in stainless steel. Chromium forms a dense oxide film of Cr2O3, which hinders the diffusion of oxygen and metal ions, thereby improving the oxidation resistance and corrosion resistance of steel. However, when the Cr content is too high, the elongation decreases and the forming performance deteriorates. In the present invention, the Cr content is controlled between 12% and 15%.

[0019] (8) Molybdenum: Mo can improve the corrosion resistance and high-temperature performance of steel. Through specific heat treatment process design, it can form composite cementite precipitation to hinder high-temperature grain boundary migration, thereby improving heat resistance and reducing temper brittleness. Therefore, the present invention requires Mo: 0.02%-0.10%.

[0020] (9) Nitrogen: N is an austenite-forming element. Nitrogen is easily soluble in steel and can expand the high-temperature austenite phase region. N: 0.03%-0.15%.

[0021] (10) Aluminum: Al is added as a deoxidizing element and also improves oxidation resistance. It is also useful for improving strength as a solid solution strengthening element. However, excessive addition leads to hardening, significantly reducing uniform elongation and toughness. For these reasons, Als: 0.01%-0.20%.

[0022] (11) Boron: Boron is the only intergranular strengthening element. A trace amount of Boron can also improve high-temperature strength. Boron has a strong neutron absorption capability, but it has a slight tendency to promote temper brittleness. Therefore, the present invention requires that the Boron content in the steel be controlled to 0.0001%-0.0011%.

[0023] (12) Copper: Cu is an austenite-forming element. At the same time, the use of copper in the core of the steel plate can compensate for the loss of strength due to the increase in thickness. Cu: 0.01%-0.2%

[0024] (13) Zirconia: ZrO2 has the effect of refining grains, which is beneficial to the low-temperature toughness of steel. When added during the pouring of molten steel, extremely small amounts of ZrO2 can play a role in intergranular strengthening. Therefore, the ZrO2 content added to the steel is controlled at 0.0001%-0.0010%.

[0025] The following technical measures are taken in the production process to realize the present invention:

[0026] (1) Smelting: Using molten iron + scrap steel, or using molten iron alone, through the three-step process of electric furnace steelmaking, AOD decarburization, and VOD deoxidation, steel can be obtained when the comprehensive composition meets the design requirements;

[0027] (2) Continuous casting: By controlling the casting speed during the continuous casting process to 1.0-2.5 m / min and strengthening electromagnetic stirring during the secondary cooling stage, the stirring mode is unidirectional stirring, wherein the unidirectional stirring time is 10-15 s, the current is 1300-2500 A, and the frequency is 5-25 Hz, so that the molten steel is continuously cast to obtain a continuous casting billet, and the proportion of columnar crystals in the continuous casting billet is not less than 70%;

[0028] (3) Heating of ingots: The ingots that have been ground and sprayed with an anti-oxidation coating are sent to a walking beam furnace for heating. The ingots are sequentially processed through the preheating section, heating section, and soaking section before being taken out of the furnace. The temperature range of the preheating section is 900-1100°C, the temperature range of the heating section is 1100-1250°C, and the temperature range of the soaking section is 1200-1250°C. The furnace time is 4-5.5 hours.

[0029] (4) High-pressure water dephosphorization: Before rolling, high-pressure water is used to descale the ingot after it is discharged from the furnace twice. The first dephosphorization lasts 10 to 30 seconds, and the descaling machine pressure is 20 to 25 MPa. The second dephosphorization lasts 10 to 30 seconds, and the descaling machine pressure is 20 to 24 MPa.

[0030] (5) Rolling: Billet heating temperature 1150-1250℃, start rolling temperature: 1100-1200℃, final rolling temperature: 950-1050℃. After rolling, the steel plate is subjected to a quenching and tempering process to obtain a fine and uniform tempered martensite structure and to give the steel plate good comprehensive mechanical properties.

[0031] (6) The quenching and tempering process is: quenching temperature 900℃~1050℃, holding time 1-6min / mm;

[0032] Tempering temperature is 680℃~780℃, and holding time is 1-10min / mm.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] Table 1 shows the chemical composition of the example steel; Table 2 shows the smelting process system of the example steel; Table 3 shows the heating and dephosphorization method of the example steel ingot; Table 4 shows the rolling and heat treatment method of the example steel; Table 5 shows the properties of the example stainless steel.

[0035] (1) The foundation for meeting the mechanical properties of stainless steel is achieved through the design of special chemical composition, in which special elements such as B, Cu and ZrO2 are introduced to ensure the special performance requirements of the steel plate, laying the foundation for the production of martensitic stainless steel for supporting key equipment of third-generation nuclear power plants;

[0036] (2) Through the specially designed electromagnetic stirring process and casting speed during the continuous casting stage, the molten steel is continuously cast to obtain continuous casting billets, and the proportion of columnar crystals in the continuous casting billets is not less than 70%, laying the foundation for the production of high-quality martensitic stainless steel hot-rolled medium and thick plate products from the source;

[0037] (3) Billet heating: The billet after grinding and spraying with anti-oxidation coating is sent to a walking beam heating furnace for heating. The billet is sequentially processed through the preheating section, heating section and soaking section before being taken out of the furnace. The temperature range of the preheating section is 900-1100°C to avoid heating in the thermal stress concentration section. The temperature range of the heating section is 1100-1250°C to completely remove the thermal stress of the billet. The temperature range of the soaking section is 1200-1250°C to maximize the austenite content. The furnace time is 4-5.5 hours.

[0038] (5) Two-stage high-pressure dephosphorization can avoid excessive surface temperature drop of the billet caused by a long-term high-pressure dephosphorization. At the same time, the two dephosphorizations can better remove the surface iron oxide scale, which is beneficial to the surface quality of the finished steel plate.

[0039] (6) The present invention adopts controlled rolling combined with a special quenching + tempering heat treatment process. The combination of the two can ensure that the austenite content in the steel is the highest and can be refined to the greatest extent during the rolling process. After combined heat treatment, a uniform and fine tempered martensite structure can be obtained.

[0040] (7) The present invention finally produces wide medium and thick martensitic stainless steel plates with a width of 4300-5000 mm and a thickness of 6-70 mm, which solves the difficult problem of producing ultra-wide medium and thick martensitic stainless steel plates. The room temperature tensile yield strength is ≥578 MPa, the tensile strength is ≥689 MPa, and the elongation is ≥20%; the high temperature tensile yield strength at 350°C is ≥492 MPa, the tensile strength is ≥569 MPa, the impact energy at -20°C is ≥50 J, the Brinell hardness is about 168 HB, and all 180° bending tests are qualified. DETAILED DESCRIPTION

[0041] A martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant and a manufacturing method thereof, comprising smelting, continuous casting, heating of the ingot, rolling and heat treatment.

[0042] Ingot heating: The ingot is sent into the heating furnace for heating. The ingot is processed in the preheating section, heating section and soaking section in sequence before being taken out of the furnace. The temperature range of the preheating section is 900-1100℃, the temperature range of the heating section is 1100-1250℃, and the temperature range of the soaking section is 1200-1250℃. The ingot is kept in the furnace for 4-5.5 hours.

[0043] Rolling: starting rolling temperature: 1100-1200℃, finishing rolling temperature: 950-1050℃;

[0044] Quenching and tempering treatment: quenching temperature 900℃~1050℃, holding time 1-6min / mm; tempering temperature 680℃~780℃, holding time 1-10min / mm.

[0045] Further; smelting: using molten iron + scrap steel, or using molten iron alone, through the three-step steelmaking process of electric furnace steelmaking, AOD decarburization, and VOD deoxidation;

[0046] Further; continuous casting: by controlling the pulling speed of the continuous casting process to 1.0~2.5m / min and strengthening the electromagnetic stirring in the secondary cooling stage, the stirring mode is unidirectional stirring, wherein the unidirectional stirring time is 10~15s, the current is 1300~2500A, and the frequency is 5~25Hz. The molten steel is continuously cast to obtain a continuous casting billet, and the proportion of columnar crystals in the continuous casting billet is not less than 70%.

[0047] Further; high-pressure water dephosphorization: before rolling, high-pressure water is used to descale the ingot after it is discharged from the furnace twice. The first dephosphorization is 10 to 30 seconds, the descaling machine pressure is 20 to 25 MPa, and the second dephosphorization is 10 to 30 seconds, the descaling machine pressure is 20 to 24 MPa.

[0048] Furthermore, the heating furnace is a walking-beam heating furnace, and the billet is ground and sprayed with an anti-oxidation coating before being fed into the walking-beam heating furnace.

[0049] The specific implementation is as follows:

[0050] Table 1 shows the chemical composition of the example steel; Table 2 shows the smelting process system of the example steel; Table 3 shows the heating and dephosphorization method of the example steel ingot; Table 4 shows the rolling and heat treatment method of the example steel; Table 5 shows the properties of the example stainless steel.

[0051] Table 1 Chemical composition of steel in each example (wt%)

[0052]

[0053] Table 2 Smelting process system of example steel

[0054]

[0055] Table 3 Heating and dephosphorization method of steel castings according to the embodiment

[0056]

[0057] Table 4 Rolling and heat treatment methods of example steel

[0058]

[0059] Table 5 Stainless steel properties of the examples

[0060]

[0061] It can be seen from the examples that the average mechanical properties of the steel plates are: room temperature tensile yield strength ≥578MPa, tensile strength ≥689MPa, elongation ≥20%; high temperature tensile yield strength ≥492MPa, tensile strength ≥569MPa, -20℃ impact energy ≥50J, Brinell hardness approximately 168HB, and all 180° bending tests are qualified.

[0062] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant, characterized in that: The components are as follows by weight: C: 0.07%-0.12%; Si: 0.1%-1.0%; Mn: 0.7%-1.00%; P: 0.003%-0.008%; S：0.02%-0.045%； Ni: 0.6%-2.0%; Cr: 13.5%-15%; Mo: 0.02%-0.10%; N: 0.12%-0.15%; Als: 0.01%-0.20%; B: 0.0001%-0.0011%; Cu: 0.01%-0.2%; ZrO2: 0.0001%-0.0010%, the balance is Fe and unavoidable impurities; The steel plate has a width of 4300-5000mm and a thickness of 6-70mm. The room temperature tensile yield strength is ≥578MPa, the tensile strength is ≥689MPa, and the elongation is ≥20%; the high temperature tensile yield strength at 350℃ is ≥492MPa, the tensile strength is ≥569MPa, the impact energy at -20℃ is ≥50J, the Brinell hardness is ≥168HB, and the 180° bending is all qualified. The manufacturing method of the steel plate includes smelting, continuous casting, billet heating, rolling and heat treatment, among which, Ingot heating: The ingot is sent into the heating furnace for heating. The ingot is processed in the preheating section, heating section and soaking section in sequence before being taken out of the furnace. The temperature range of the preheating section is 900-970℃, the temperature range of the heating section is 1100-1200℃, and the temperature range of the soaking section is 1200-1225℃. The ingot is kept in the furnace for 4-5.5 hours. Rolling: starting rolling temperature: 1100-1200℃, finishing rolling temperature: 950-1050℃; Quenching and tempering treatment: quenching temperature 975℃~1020℃, holding time 1.0-3.5min / mm; tempering temperature 710℃~750℃, holding time 2-5min / mm.

2. The martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant according to claim 1, characterized in that: Smelting: Using molten iron + scrap steel, or using molten iron alone, steel is made through a three-step process of electric furnace steelmaking, AOD decarburization, and VOD deoxidation.

3. The martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant according to claim 1, characterized in that: Continuous casting: By controlling the casting speed at 1.0-2.5 m / min during the continuous casting process and strengthening electromagnetic stirring in the secondary cooling stage, the stirring mode is unidirectional stirring, wherein the unidirectional stirring time is 10-15 s, the current is 1300-2500 A, and the frequency is 5-25 Hz. The molten steel is continuously cast to obtain a continuous casting billet, and the proportion of columnar crystals in the continuous casting billet is not less than 70%.

4. The martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant according to claim 1, characterized in that: High-pressure water dephosphorization: Before rolling, high-pressure water is used to descale the ingot twice after it comes out of the furnace. The first dephosphorization takes 10 to 30 seconds, and the descaling machine pressure is 20 to 25 MPa. The second dephosphorization takes 10 to 30 seconds, and the descaling machine pressure is 20 to 24 MPa.

5. The martensitic stainless steel plate for supporting key equipment of a third-generation nuclear power plant according to claim 1, characterized in that: The heating furnace is a walking-beam heating furnace. Before the ingot is sent into the walking-beam heating furnace, the ingot is ground and sprayed with an anti-oxidation coating.

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