CN111534738A - Small-batch manufacturing method of tens of kilogram-level nuclear reactor pressure vessel steel - Google Patents
Small-batch manufacturing method of tens of kilogram-level nuclear reactor pressure vessel steel Download PDFInfo
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Abstract
The invention relates to the technical field of pressure vessel steel manufacturing, in particular to a small-batch manufacturing method of tens of kilogram-level nuclear reactor pressure vessel steel. The method obtains the material with moderate strength, high low-temperature toughness and low yield ratio, so that the material obtained by production has the structure and mechanical properties consistent with those of a steel body of a nuclear reactor pressure vessel, and comprises the following steps. S100, carrying out component proportioning according to components of the industrial nuclear reactor pressure vessel steel, then carrying out vacuum induction melting, and obtaining an ingot through vacuum arc melting. S200, carrying out primary forging on the cast ingot to obtain a forged piece, and then carrying out upsetting-stretching-secondary forging on the forged piece to obtain a forging blank. S300, performing preliminary heat treatment on the forging stock; the preliminary heat treatment includes the following steps. S400-performance heat treatment. The invention can simply realize the development and production of small batches of nuclear reactor pressure vessel steel, and is greatly beneficial to the technical development and production improvement of the pressure vessel.
Description
Technical Field
The invention relates to the technical field of pressure vessel steel manufacturing, in particular to a small-batch manufacturing method of tens of kilogram-level nuclear reactor pressure vessel steel.
Background
The nuclear reactor pressure vessel is one of the main and critical components of a pressurized water reactor nuclear power plant, and has a height of about 10 m, an inner diameter of about 4 m, and a wall thickness of about 250 mm. The industrially adopted nuclear reactor pressure vessel steel is a large-scale ring forging, and the heat treatment process of the large-scale ring forging is double heat treatment of preliminary heat treatment and final heat treatment, wherein the preliminary heat treatment process comprises the following steps: normalizing (880-900 ℃) + high-temperature tempering (630-650 ℃), wherein the final heat treatment process is quenching and tempering: quenching (850-925 ℃, water immersion quenching or spray quenching cooling) + high-temperature tempering (635-665 ℃, cooling in static air).
However, the pressure vessel is bulky and weighs 256.6 tons. Therefore, when the composition, structure and performance of the steel are researched and improved, the industry needs to first perform trial production and research on small-batch pressure vessel steel. Compared with the pressure vessel body, the small-batch trial-produced material has smaller size and higher cooling speed under the same cooling process, so that the performance and the structure obtained by the small-batch trial-produced material after the same processing and heat treatment are different from those of the pressure vessel steel body. Therefore, it is very important to develop a method for manufacturing a small amount of steel for a nuclear reactor pressure vessel, which is consistent with the structure and mechanical properties of the steel body of the pressure vessel, by exploring the processes for processing and heat treatment of the small amount of steel for a nuclear reactor pressure vessel.
Disclosure of Invention
The invention provides a small-batch manufacturing method of dozens of kilograms of nuclear reactor pressure vessel steel, which aims to obtain a material with moderate strength, high low-temperature toughness and low yield ratio through the design of smelting, processing, heat treatment processes and the like, so that the produced material has the structure and mechanical properties consistent with those of a nuclear reactor pressure vessel steel body.
The invention adopts the following technical scheme: a method for manufacturing a small batch of steel for a pressure vessel of a nuclear reactor of tens of kilograms comprises the following steps.
S100, carrying out component proportioning according to components of the industrial nuclear reactor pressure vessel steel, then carrying out vacuum induction melting, and obtaining an ingot through vacuum arc melting.
S200, carrying out primary forging on the cast ingot to obtain a forged piece, and then carrying out upsetting-stretching-secondary forging on the forged piece to obtain a forging blank.
S300, performing preliminary heat treatment on the forging stock; the preliminary heat treatment includes the following steps.
S301-high temperature austenitization: and (3) carrying out austenitization on the forge piece by heating the forge piece in a furnace with the furnace temperature of 880-900 ℃, and taking the heat preservation time according to 1-1.5 min/mm.
S302-controlling cooling rate: in order to reach the cooling speed of the pressure container body during normalizing, the furnace temperature of the heating furnace is cooled at the cooling speed of 100-150 ℃/h and then discharged.
S303-high temperature tempering: and then, feeding the forged piece into a furnace for high-temperature tempering, wherein the tempering temperature is 630-650 ℃, the heat preservation time is 2-3 min/mm, and taking out of the furnace.
S400-performance heat treatment; the property heat treatment comprises the following steps.
S401-high temperature austenitization: and (3) feeding the forged piece into a furnace for austenitizing, wherein the austenitizing heat preservation temperature is 880-900 ℃, and the heat preservation time is 1-1.5 min/mm, so that the material is fully austenitized and the components and the structure are homogenized.
S402, heat preservation and cooling: and (3) carrying out heat preservation and cooling after austenitizing, wherein the cooling atmosphere is 100-200 ℃, and discharging after cooling.
S403-high temperature tempering: tempering and heat preservation temperature of the heat treatment after forging is 630-650 ℃, heat preservation time is taken according to 2-3 min/mm, and the forged piece is discharged from a furnace after tempering.
In the step S100, before smelting, 0.02 and 0.03 of burning loss is added to the carbon element and the manganese element respectively, smelting power is increased step by step during smelting so that furnace burden is melted layer by layer, temperature is controlled to be 1600-1650 ℃ during refining, vacuum degree is controlled to be 0.5-1 Pa, and refining time is 10-20 min.
The step S200 includes the steps of,
s201, cutting off a riser: and (4) cutting off a riser and the bottom of an ingot obtained by smelting, and reserving an ingot body for forging.
S202-primary forging: the heating temperature before forging is 1180-1250 ℃, the heat preservation time is taken according to 1-2 min/mm so as to ensure that the ingot casting temperature is uniform, the ingot is sent by heat for forging for one time, and the compression ratio during forging is more than 2.5.
S203-upsetting: and (3) carrying out primary upsetting on the cast ingot, then conveying the cast ingot into a heating furnace with the furnace temperature of 1180-1250 ℃, preserving the heat for 1-2 min/mm, and carrying out high-temperature diffusion annealing and reheating on the cast ingot.
S204-secondary forging: and heating, then carrying out hot feeding on a steel ingot, carrying out drawing out, carrying out secondary upsetting, and forging to obtain a forging stock, wherein the final forging temperature is not lower than 900 ℃.
Referring to the heat treatment process of the pressure vessel steel of the industrial nuclear reactor, the heat treatment process of the small-batch nuclear reactor pressure vessel steel adopts a double heat treatment process of heat treatment after forging and performance heat treatment, wherein the tempering temperature adopts a common tempering temperature interval of the reactor pressure vessel steel: 630-650 ℃. However, since the pressure vessel body weighs 256.6 tons and has a thickness of 200mm, while the steel for the pressure vessel of the nuclear reactor produced in small quantities weighs only tens of kilograms and has a thickness of only tens of mm, the quenching cooling rate of the steel in the heat treatment is significantly different: the reactor pressure vessel steel produced in small batches has small volume and high cooling speed, the quenching can cause the reactor pressure vessel steel to form a martensite structure, and the material has high strength, hardness and low toughness; the reactor pressure vessel steel body is used as a large-scale ring forging, the cooling speed is low during quenching, a bainite structure can be formed, and the material has moderate strength, hardness and high toughness. Therefore, the core problem of the invention is to successfully prepare the small-batch nuclear reactor pressure vessel steel with the structure and the performance consistent with those of the pressure vessel body by properly adjusting the heat treatment process of the material so that the small-batch refined steel and the large-volume pressure vessel steel body have similar cooling rates.
Compared with the prior art, the invention has the following beneficial effects: in the forging process, secondary forging is adopted, and the as-cast structure is fully crushed to obtain fine grains so as to prevent coarse columnar grains from generating tissue inheritance, thereby obtaining excellent performance; in the heat treatment process, the cooling speed of the small-batch steel is close to that of the industrial large-scale nuclear reactor pressure vessel steel by adopting a method of heat preservation cooling after austenitizing, and finally the consistency of the cooling speed, the structure and the performance of the small-batch steel and the industrial steel is ensured. The technology can simply realize the development and production of small batches of nuclear reactor pressure vessel steel, and is greatly beneficial to the technical development and production improvement of the pressure vessel.
Drawings
FIG. 1 is a micrograph of an industrial harbor;
FIG. 2 is a micrograph of example 1;
FIG. 3 is a micrograph of example 2;
FIG. 4 is a micrograph of example 3;
FIG. 5 is a micrograph of example 4;
FIG. 6 is a micrograph of comparative example 1;
FIG. 7 is a micrograph of comparative example 2;
FIG. 8 is a micrograph of comparative example 4;
FIG. 9 is a micrograph of comparative example 6;
FIG. 10 is a micrograph of comparative example 7;
FIG. 11 is a micrograph of comparative example 8;
FIG. 12 is a drawing graph of examples and comparative examples.
Detailed Description
A small-batch manufacturing method of tens of kilograms grade nuclear reactor pressure vessel steel is characterized in that: the method comprises the following steps:
s100, carrying out component proportioning according to components of the industrial nuclear reactor pressure vessel steel, then carrying out vacuum induction melting, and obtaining an ingot through vacuum arc melting.
S101, batching: the ingredients are mixed according to the national standard. Preparing components: the chemical components of the nuclear reactor pressure vessel steel in the national standard are defined as follows according to the mass percentage: c: 0.16 to 0.19%, Mn: 1.20 to 1.40%, Si: 0.20-0.30%, Ni: 0.70-0.85%, Cr is less than or equal to 0.15%, Mo: 0.45-0.50%, Cu not more than 0.03%, Al not more than 0.03%, Co not more than 0.03%, N not more than 0.02%, P not more than 0.003%, S not more than 0.003%, and the balance of Fe. According to the national standard component regulation, the chemical components and the mass percentage adopted in the invention are as follows: c: 0.87%, Mn: 1.34%, Si: 0.26%, Ni: 0.81%, Cr is less than or equal to 0.12%, Mo: 0.48 percent of Cu, less than or equal to 0.03 percent of Al, less than or equal to 0.03 percent of Co, less than or equal to 0.02 percent of N, less than or equal to 0.003 percent of P, less than or equal to 0.003 percent of S and the balance of Fe. According to the steps of smelting and forging in the invention, 0.02 and 0.03 of burning loss is added to the carbon element and the manganese element respectively before smelting.
S102-smelting: during smelting, the smelting power is increased step by step to melt the burden layer by layer, so that the problem that the unmelted and semi-melted burden is adhered above the furnace hearth and cannot enter the molten pool is avoided. The melting point of the steel of the nuclear reactor pressure vessel is about 1500 ℃, the temperature is controlled in a region with a slightly higher melting point during refining, namely 1600-1650 ℃, the vacuum degree is controlled at 0.5-1 Pa, and the refining time is 10-20 min, so that furnace burden is subjected to sufficient electromagnetic stirring, the uniformity of molten metal components and temperature is promoted, and the progress of carbon-oxygen reaction and the decomposition and volatilization of impurities are promoted.
S200, carrying out primary forging on the cast ingot to obtain a forged piece, and then carrying out upsetting-stretching-secondary forging on the forged piece to obtain a forging blank;
s201, cutting off a riser: because the riser of the ingot has the defects of looseness, shrinkage cavity and the like, and more inclusions are deposited at the bottom of the ingot, the riser and the bottom of the ingot obtained by smelting need to be cut off, and the ingot body is reserved for forging.
S202-primary forging: the heating temperature before forging is 1180-1250 ℃, and the heat preservation time is taken according to 1-2 min/mm, so as to ensure the uniform ingot casting temperature. And (3) carrying out primary forging on the hot-fed steel ingot, wherein the compression ratio in the forging process needs to be more than 2.5 so as to fully break columnar crystals and dendrites in the ingot, accumulate high-density dislocation and deformation zones in the material, provide high-density nucleation positions in the heat treatment process and refine grains.
S203-upsetting: carrying out primary upsetting on the cast ingot, then conveying the cast ingot into a heating furnace with the furnace temperature of 1180-1250 ℃, preserving the heat for 1-2 min/mm, carrying out high-temperature diffusion annealing and reheating on the cast ingot, fully recovering steel materials, and refining the structure;
s204-secondary forging: and (4) heating, then carrying out hot feeding on the steel ingot, carrying out drawing out, and carrying out secondary upsetting. As the transformation of proeutectoid ferrite of the nuclear reactor pressure vessel steel into austenite is 860 ℃, the final forging temperature is required to be not lower than 900 ℃ in order to avoid the phenomenon of mixed crystals. And forging to obtain a forging stock.
S300, performing preliminary heat treatment on the forging stock; the preliminary heat treatment includes the steps of,
s301-high temperature austenitization: and (3) carrying out austenitization on the forge piece by heating the forge piece in a furnace with the furnace temperature of 880-900 ℃, and taking the heat preservation time according to 1-1.5 min/mm.
S302-controlling cooling rate: in order to reach the cooling speed of the pressure container body during normalizing, the furnace temperature of the heating furnace is cooled at the cooling speed of 100-150 ℃/h and then discharged.
S303-high temperature tempering: and then, feeding the forged piece into a furnace for high-temperature tempering, wherein the tempering temperature is 630-650 ℃, the heat preservation time is 2-3 min/mm, and taking out of the furnace.
S400-Performance Heat treatment, which includes the following steps.
S401-high temperature austenitization: sending the forged piece into a furnace for austenitizing, wherein the austenitizing heat preservation temperature is 880-900 ℃, and the heat preservation time is 1-1.5 min/mm, so that the material is fully austenitized and the components and the structure are homogenized;
s402, heat preservation and cooling: carrying out heat preservation cooling after austenitizing, wherein the cooling atmosphere is 100-200 ℃, and discharging the product after cooling;
s403-high temperature tempering: tempering and heat preservation temperature of the heat treatment after forging is 630-650 ℃, heat preservation time is taken according to 2-3 min/mm, and the forged piece is discharged from a furnace after tempering.
According to the present invention, 4 examples (examples 1 to 4) in accordance with the heat treatment process of the present invention and 8 comparative examples (comparative examples 1 to 8) in accordance with the heat treatment process of the present invention were prepared and subjected to preliminary heat treatment and final heat treatment. The austenitizing heat preservation time of the examples and the comparative examples is 120 min, the tempering temperature is 650 ℃, the heat preservation time is 240min, and the steel is discharged from a furnace and cooled in air. Key parameters such as austenitizing temperature, cooling manner, etc. are compared in detail in comparative examples, in which the cooling manner of the preliminary heat treatment and the final heat treatment of comparative examples 1 and 2 is not within the process range of the present invention, the cooling manner of the final heat treatment of comparative examples 3 and 4 is not within the process range of the present invention, the cooling manner of the preliminary heat treatment of comparative examples 5 and 6 is not within the process range of the present invention, and the austenitizing temperature of the preliminary heat treatment and the final heat treatment of comparative examples 7 and 8 is not within the process range of the present invention.
The microstructure, room temperature tensile properties (including yield strength Rel, tensile strength Rm and elongation percentage after fracture A) and impact properties (including room temperature impact energy Ak, ductile-brittle transition temperature DBTT and upper plateau energy USE) of the examples and comparative examples were tested, and the metallographic structure and the tensile curve were shown in FIG. 1 and FIG. 2, respectively. As can be seen from FIG. 1, the structures of the nuclear reactor pressure vessel steels prepared in examples 1-4 of the present invention are consistent with those of industrial steels, and are granular bainite structures, examples 1-4 have appropriate strength (Rel ≥ 400 MPa), high toughness (room temperature Ak ≥ 140J) and low ductile-brittle transition temperature (≤ 20 ℃), and the mechanical properties of examples 1-4 all satisfy the performance requirements of industrial steels, and have high room temperature impact energy and upper platform energy, low ductile-brittle transition temperature, and excellent toughness of materials. Wherein, the best embodiment is that the toughness of the embodiment 2 is excellent, the impact energy at room temperature reaches 238J, and the ductile-brittle transition temperature reaches-42.7 ℃. In contrast, in comparative examples 1 to 8, since the production process is out of the range of the present invention, the structure and properties of the prepared material could not satisfy the requirements of the industrial nuclear reactor pressure vessel steel.
Wherein the structures of the comparative examples 1 and 2 are tempered martensite structures, the corresponding tensile strengths are 1020MPa and 760 MPa respectively, the upper limit of the industrial steel requirement is exceeded, the elongation after fracture is lower than the lower limit of the industrial steel requirement, the room temperature impact energy is 16.0J and 14.2J respectively, and the lower limit of the industrial steel requirement is obviously lower. This is because the cooling method of water cooling and air cooling in the preliminary heat treatment and the final heat treatment results in the sample being cooled too fast, the strength is increased and the toughness is reduced, which shows that the slow cooling method adopted in the present invention has better effect.
The structure of comparative example 3 is a tempered lower bainite structure, the room temperature tensile properties of which meet the requirements of industrial steel, but the tensile strength is 630 MPa, which is higher in the range of the requirements of industrial steel, and the room temperature impact energy is 105.7J, which is still lower than 120J required by industrial steel. Compared with the comparative example 2, the final heat treatment of the preliminary heat treatment of the comparative example 3 adopts the cooling speed of 100 ℃/h to effectively improve the toughness of the material, but the impact energy at room temperature is still lower than that of the examples 1 to 4, which shows that the final heat treatment of the invention adopts the heat preservation cooling mode of 100 to 200 ℃ and has better effect.
The final heat treatment of the comparative example 4 adopts the heat preservation and cooling at 400 ℃, the obtained structure is granular bainite with a large amount of blocky ferrite, the obtained mechanical property meets the requirement of industrial steel, but the yield strength and the tensile strength are only 410 MPa and 560MPa, which are partially lower limits of the requirement of the industrial steel. The reason is that the material is cooled too slowly due to the heat preservation and cooling at the temperature of 400 ℃, and massive ferrite is precipitated, which shows that the final heat treatment in the invention has better effect by adopting a heat preservation and cooling mode at the temperature of 100-200 ℃.
The preliminary heat treatment of comparative example 5 employs cooling at 200 ℃ to obtain granular bainite as a structure, and the mechanical properties obtained meet the requirements of industrial steel, but the impact energy at room temperature is lower than the lower limit of the requirements of industrial steel. The reason is that the cooling at 200 ℃ can cause the material to be cooled too fast, and the toughness of the material is reduced, which shows that the preheating treatment has better effect by adopting a 100-200 ℃ heat preservation cooling mode.
The preliminary heat treatment of comparative example 6 employs cooling at a cooling rate of 50 c, the resulting structure is granular bainite but coarse, the resulting mechanical properties meet the requirements of industrial steel, but the yield strength, tensile strength, and room temperature impact energy are all below the lower limits of the requirements of industrial steel. The reason is that the material is cooled too slowly due to the cooling speed of 50 ℃, and the strength of the material is reduced, which shows that the preheating treatment has better effect by adopting a heat preservation cooling mode of 100-200 ℃.
The austenitizing temperatures of comparative examples 7 and 8 are out of the range of the present invention, wherein the structure of comparative example 7 has coarse primary grains, because the austenitizing temperature is too high, which causes the primary austenite grains to grow significantly and the toughness of the material to be reduced. The non-austenitized region remaining in the as-forged structure is present in the structure of comparative example 8 because the austenitizing temperature is too low, resulting in incomplete recrystallization of the material. This shows that the austenitizing temperature of 880-900 ℃ adopted in the invention has better effect.
The experimental results of the above examples show that the structure of the above 4 invention examples is a granular bainite structure, and the performance is also close to that of an industrial finished product, so that the invention successfully realizes the preparation of small-batch nuclear reactor pressure vessel steel, and can be used for the research, development and production of small batches of the material.
Claims (3)
1. A small-batch manufacturing method of tens of kilograms grade nuclear reactor pressure vessel steel is characterized in that: comprises the following steps of (a) carrying out,
s100, carrying out component proportioning according to components of industrial nuclear reactor pressure vessel steel, then carrying out vacuum induction melting, and obtaining an ingot through vacuum arc melting;
s200, carrying out primary forging on the cast ingot to obtain a forged piece, and then carrying out upsetting-stretching-secondary forging on the forged piece to obtain a forging blank;
s300, performing preliminary heat treatment on the forging stock;
the preliminary heat treatment includes the steps of,
s301-high temperature austenitization: the forge piece is heated and sent to a furnace with the furnace temperature of 880-900 ℃ for austenitizing, the heat preservation time is taken according to 1-1.5 min/mm,
s302-controlling cooling rate: in order to reach the cooling speed of the pressure container body during normalizing, the furnace temperature of the heating furnace is cooled at the cooling speed of 100-150 ℃/h and then discharged;
s303-high temperature tempering: then, feeding the forge piece into a furnace for high-temperature tempering, wherein the tempering temperature is 630-650 ℃, the heat preservation time is taken according to 2-3 min/mm, and discharging the forge piece from the furnace;
s400-performance heat treatment;
the property heat treatment comprises the following steps,
s401-high temperature austenitization: the forging is sent into a furnace for austenitizing, the austenitizing heat preservation temperature is 860-900 ℃, the heat preservation time is 1-1.5 min/mm, and the material is fully austenitized and the components and the structure are homogenized;
s402, heat preservation and cooling: carrying out heat preservation cooling after austenitizing, wherein the cooling atmosphere is 100-200 ℃, and discharging the product after cooling;
s403-high temperature tempering: tempering and heat preservation temperature of the heat treatment after forging is 630-650 ℃, heat preservation time is taken according to 2-3 min/mm, and the forged piece is discharged from a furnace after tempering.
2. A method for manufacturing a small batch of steel for a pressure vessel of a nuclear reactor of the order of tens of kilograms according to claim 1, characterized in that: in the step S100, before smelting, 0.02 and 0.03 of burning loss is added to the carbon element and the manganese element respectively, smelting power is increased step by step during smelting so that furnace burden is melted layer by layer, temperature is controlled to be 1600-1650 ℃, vacuum degree is controlled to be 0.5-1 Pa during refining, and refining time is 10-20 min.
3. A method for manufacturing a small batch of steel for a pressure vessel of a nuclear reactor of the order of tens of kilograms according to claim 1, characterized in that: the step S200 includes the following steps,
s201, cutting off a riser: cutting off a dead head and the bottom of an ingot obtained by smelting, and keeping an ingot body for forging;
s202-primary forging: heating temperature before forging is 1180-1250 ℃, heat preservation time is taken according to 1-2 min/mm to ensure that ingot casting temperature is uniform, hot-feeding steel ingots for primary forging, and the compression ratio during forging is more than 2.5;
s203-upsetting: carrying out primary upsetting on the cast ingot, then conveying the cast ingot into a heating furnace with the furnace temperature of 1180-1250 ℃ for heat preservation, taking the heat preservation time according to 1-2 min/mm, and carrying out high-temperature diffusion annealing and reheating on the cast ingot;
s204-secondary forging: and heating, then carrying out hot feeding on a steel ingot, carrying out drawing out, carrying out secondary upsetting, and forging to obtain a forging stock, wherein the final forging temperature is not lower than 900 ℃.
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