JP2006233292A - Corrosion resistant steel and producing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the corrosion resistance and the irradiation resistance of a metallic material by grain-refining of a metal crystal with a working and heat treatment method. <P>SOLUTION: A method for producing a corrosion resistant steel includes a crystal grain-refining process 110 performing cold-working 107, heating treatment 108 and cooling treatment 109; and the crystal grain-refining process 110 is characterized in that the crystal grain-refining is performed so that the grain diameter of the crystal grain becomes ≤5 μm by rapidly heating to a recrystallization temperature at ≥30°C/sec heating speed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a corrosion-resistant steel capable of improving the corrosion resistance and irradiation resistance of a metal material used in high-temperature water such as a reactor material by processing and heat treatment, and a method for producing the same.

  Metal materials used in high-temperature water such as nuclear equipment are desired to further improve corrosion resistance and irradiation resistance from the viewpoint of improving thermal efficiency. Moreover, it is estimated that the specification regarding the metal material in the next generation nuclear reactor will be a higher temperature and higher pressure condition. For this reason, there is a further demand for a material having improved corrosion resistance and irradiation resistance under high temperature and pressure as the nuclear reactor material.

  On the other hand, in order to increase the strength and toughness of carbon steel, crystal grain refinement is performed. It has been reported that this crystal refinement effect can be expected in terms of corrosion resistance in the case of stainless steel. Furthermore, it is known that the effect of crystal refinement can be expected to improve irradiation resistance by refinement of crystal of reactor material from the analysis of irradiation damage mechanism of metal material.

  This metal crystal refinement method can be broadly classified into a processing heat treatment method using phase transformation and recrystallization, and a special process method such as mechanical alloying, ultrafine particle sintering, and amorphous crystallization. In the latter method of refining metal crystals, nano-sized fine particles can be obtained, but the shape and the like are limited, and the range of use is limited. The former method of refining metal crystals by the thermomechanical method can be applied to a wide range of shapes such as plates and bars, and can also produce relatively large materials. As described above, since the metal crystal refinement method by the former process heat treatment method having a wide application range is excellent, this process heat treatment method will be examined.

  Conventionally, crystal grain refinement of a low alloy steel has been performed via processes by processing and heat treatment. In the case of low alloy steel, the temperature range and processing rate of hot rolling are optimized, and the growth of crystal grains is suppressed when the austenite (γ) phase is transformed to the ferrite (α) phase by the subsequent rapid cooling treatment. In this way, the crystal is made fine by suppressing the growth of crystal grains.

  On the other hand, in the austenitic stainless steel that does not cause a phase transformation from the austenite (γ) phase to the ferrite (α) phase, crystal grains are refined by the following steps.

  First, the solution-treated austenitic stainless steel is transformed from an austenite (γ) system to a martensite (α ′) system by cold working. Subsequent to this phase transformation, heating is performed to reversely transform the martensite (α ′) system to the austenite (γ) system to produce refined crystal grains. In a stainless steel or nickel-base alloy with a stable austenite (γ) phase, the crystal grains are refined by recrystallization without phase transformation into the martensite (α ′) system.

  On the other hand, when austenitic stainless steel is subjected to neutron irradiation, the Cr concentration at the grain boundaries decreases, and the susceptibility to stress corrosion cracking (irradiation-induced SCC: IASCC) increases. In order to cope with this, it is known that the resistance against IASCC increases when the Cr concentration in the grain boundary is increased in advance by applying a certain heat treatment (see, for example, Patent Documents 1 and 2).

  That is, the austenitic stainless steel is heated to a high temperature of 1050 ° C. or higher and then cooled to suppress Cr deficiency at the grain boundaries due to irradiation-induced segregation. In addition, the austenitic stainless steel is intended to suppress Cr deficiency at grain boundaries by heat treatment or machining.

  Thus, not only the sensitivity of stress corrosion cracking can be increased by the thermomechanical processing method, but also the heat resistance can be applied to refine the crystal to improve the corrosion resistance.

Also, future high-performance nuclear reactors are desired to operate at higher temperatures than current light water reactors, and higher temperature strength and creep strength are further desired. It is required not only to increase the strength of the material by refining the crystal grains, but also to further increase the strength by dispersing fine precipitates that suppress the movement of dislocations that can be placed in the fine-grained crystals. ing.
JP 2001-32045 A JP-A-11-293337

  However, the above-mentioned conventional austenite-based material crystallizing by the thermomechanical processing method goes through a process of refining crystal grains by recrystallization, so the condition is set more than the refinement of low alloy steel crystal grains. There is a problem that is difficult.

  In addition, there is a problem that uniform fine particles cannot be obtained unless conditions such as processing rate and heating / cooling are optimized. Depending on the component system, it is necessary to devise methods such as adding trace elements that promote recrystallization. Further, in an alloy such as stainless steel, an unnecessary phase often appears by processing and heat treatment.

  Therefore, the refinement of crystals by a thermomechanical processing method of an austenitic material is not limited to the refinement of crystal grains, but at the same time, it is important that they are in the same phase as normal grains. For this purpose, it is necessary to perform heat treatment that promotes only the nucleation of crystals and suppresses the appearance of other phases and the growth of crystals. Such a heat treatment method is possible only under very limited conditions, and there is a problem that it is difficult to determine the processing conditions.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a corrosion-resistant steel capable of improving corrosion resistance and irradiation resistance by atomizing a metal material by a thermomechanical processing method and a method for producing the same. And

  In order to achieve the above object, the present invention provides a method for producing a corrosion-resistant steel including a crystal grain refinement process that is processed by cold working, recrystallization heat treatment, and cooling process. It is characterized in that it is rapidly heated up to a temperature of 30 ° C./second or more to refine the crystal grain size to 5 μm or less.

    Further, in order to achieve the above object, the present invention provides a rapid heating to a recrystallization temperature at a heating rate of 30 ° C./second or more in a corrosion resistant steel that is atomized by cold working, recrystallization heating treatment and cooling treatment. Thus, it is characterized by being manufactured by refining the crystal grain size to be 5 μm or less.

  According to the corrosion-resistant steel of the present invention and the method for producing the same, grain refinement, grain boundary segregation, and structure improvement by intragranular fine precipitates are achieved.As a result, a metal material having excellent corrosion resistance and irradiation resistance can be easily obtained. It can be achieved inexpensively.

  Hereinafter, embodiments of a corrosion-resistant steel and a method for producing the same according to the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

  FIG. 1 is a process showing a fine graining process of an austenitic stainless steel according to an embodiment of the present invention.

  As shown in FIG. 1, first, the austenitic stainless steel is subjected to a solution heat treatment 105 and a rapid cooling treatment 106. The treatment conditions for the solution heat treatment 105 and the rapid cooling treatment 106 are the same as those for ordinary austenitic stainless steel, and after being heated to a temperature of 1050 ° C. or higher, they are quenched in water or the like. Next, the process proceeds to the fine grain manufacturing process 110. The fine grain manufacturing process 110 includes a cold working process 107 for cold working austenitic stainless steel, a recrystallization heating process 108 for recrystallizing the cold worked stainless steel, and a rapid cooling of the recrystallized stainless steel. Formed from the cooling step 109.

  FIG. 2 is a perspective view showing the cold working apparatus according to the embodiment of the present invention.

  As shown in FIG. 2, the cold working 107 of austenitic stainless steel is performed by a roll rolling mill or a pilger rolling mill. An austenitic stainless steel plate material 202 is sandwiched between two rollers 201 arranged in parallel in this rolling mill and cold rolled.

  The stainless steel is then rapidly heated 108 to the recrystallization temperature. Further, the recrystallized stainless steel 202 is rapidly cooled 109. This rapid heating 108 may be used if a rapid heating rate (heating time from room temperature to 1000 ° C. within 1 minute) can be achieved with a general heating apparatus. When molten salt or liquid alkali metal is used as the heat medium, the heating time from room temperature to 1000 ° C. is 25 seconds or less. The normal heating rate is 30 ° C./second or more, and preferably the average heating rate is 40 ° C./second or more. This is because when the heating rate is less than 30 ° C./second, recrystallization proceeds and recovery of processing strain is promoted to prevent atomization.

  Thereafter, in the rapid cooling step 109, the stainless steel 201 is cooled with water, oil, or a liquid alkali metal.

  In the present embodiment configured as described above, the heating rate is extremely fast as compared with a normal heat treatment in a gas atmosphere. For this reason, during the heating of austenitic stainless steel by rapid heating, precipitation of carbides or the like that adversely affect the material is suppressed, recovery of working strain is suppressed, and the cold working effect can be largely left.

  According to the present embodiment, recrystallization of cold-worked austenitic stainless steel by rapid heating promotes refinement of crystal grains and suppresses generation of grain boundary segregation and intragranular fine precipitates. The structure of austenitic stainless steel is greatly improved. As a result, by applying the thermomechanical processing method, it is possible to obtain a corrosion-resistant steel excellent in corrosion resistance and irradiation resistance at a simple and low cost.

  FIG. 3 is a front view showing the fine grain heat treatment apparatus according to the embodiment of the present invention. The recrystallization rapid heating 108 shown in FIG. 1 is performed in the molten salt 203, and the rapid cooling step 109 is an embodiment in which cooling is performed by quenching in the cooling water 208.

  As shown in FIG. 3, the molten salt 203 is supplied into the molten salt bath 204. Corrosion resistant steel 202 as an object is immersed in this. Instead of the molten salt 203, an alkali metal may be filled. The molten salt 203 in the molten salt bath 204 is heated by the heater 205. The corrosion resistant steel 202 is immersed in the molten salt 203 by the crane 206. The crane 206 is travel-controlled by a drive control device 207 installed on an overhead crane cart 207a that travels on the ceiling of the building.

  A cooling water tank 209 is installed in the vicinity of the molten salt bathtub 204. The cooling water tank 209 is filled with cooling water 208.

  The corrosion resistant steel 202 heated by the molten salt 203 in the molten salt bath 204 is moved to the cooling water tank 209 by the crane 206. By controlling the drive control device 207, the heated corrosion-resistant steel 202 is immersed in the cooling water 208 in the cooling water tank 209.

  In the present embodiment configured as described above, the heat treatment conditions are optimized by adjusting the temperature of the molten salt 203 and the immersion time. Generally, when the temperature of the molten salt 203 increases, the immersion time is shortened because crystal grain growth proceeds, but it is necessary to consider the influence of heat input to the corrosion resistant steel 202. In the case of austenitic stainless steel, the heating condition is 950 ° C. and within 1 minute with 70% cold working. The heating temperature is substantially 870 ° C. to 970 ° C. in consideration of recrystallization. This heating temperature is premised on rapid heating and rapid cooling. This is because if the heating rate is slower than 1 minute, the total amount of heat input increases, and the time varies depending on the thickness and shape of the material. However, since the crystal grain growth proceeds, it becomes difficult to control the structure of the crystal grains.

  In addition to austenitic stainless steel, this thermomechanical processing method is applicable to ferritic stainless steel, martensitic stainless steel, austenitic and ferritic duplex stainless steel, austenitic and martensitic duplex stainless steel and nickel base. It can be applied to at least one selected from alloys. Regardless of which material is selected, when the austenite structure is made to contain 80% or more, the structure of the austenitic stainless steel is greatly improved by applying the thermomechanical processing method. When the austenite structure is less than 80%, the effect of atomization of the austenitic stainless steel by the thermomechanical processing method is small.

  4A and 4B are photographs showing the structure of the fine grained stainless steel of the present embodiment, where FIG. 4A is a micrograph showing a normal material as a comparative example, and FIG. 4B is a micrograph showing a refined material.

  As shown in FIG. 4A, the average particle diameter of the normal material 211 is 75 μm. On the other hand, as shown in FIG. 4B, the average particle diameter of the stainless steel 212 finely granulated by performing the recrystallization rapid heating 108 with molten salt and quenching in the quenching step 109 with water is 1 μm. Thus, it can be seen that the crystal grains of stainless steel are remarkably refined by applying the thermomechanical processing method. As described above, when the particle diameter of the stainless steel 212 is substantially 5 μm or less, it can be said that the crystal grain diameter of the normal material 211 is significantly improved from 75 μm, and the crystal is practically refined.

  According to the present embodiment, recrystallization rapid heating is performed with a molten salt, and the quenching process is water-quenched to refine the crystal grains of the material, resulting in excellent corrosion resistance, irradiation resistance, and high temperature strength. Corrosion resistant steel can be obtained.

  FIG. 5 is a graph showing the refinement effect of the additive element of the present embodiment.

  In the embodiment in which the recrystallization 108 is difficult to occur, a trace element that becomes a nucleus of the crystal is added, and a heat treatment method is applied to obtain a corrosion-resistant steel having excellent corrosion resistance, irradiation resistance, and high-temperature strength. is there. Examples of trace additive elements include titanium, zirconium, and niobium.

  As shown in FIG. 5, in the case where the parent phase is the same component, in the case of the usual stainless steel (SUS304), the particle size is 75 μm. When the thermomechanical processing method is applied to this stainless steel, it can be refined to 3 μm. Furthermore, when titanium (Ti) is added as a trace element that becomes the nucleus of the stainless steel crystal of the normal material and the thermomechanical processing method is applied, it can be further refined to 1 μm.

  In this embodiment configured as described above, these additive elements are combined with carbon, oxygen, or nitrogen in the metal to generate these compounds, and this effect is called a scavenge effect. When a large amount of additive element is present, it often precipitates in the vicinity of the crystal grain boundary. However, when the additive element is in an appropriate amount, it has been measured that segregation occurs near the grain boundary. This amount depends on the amount of oxygen, carbon or nitrogen that is dissolved.

  As the amount of added element increases, intragranular precipitates are generated. In the case of manufacturing by applying this processing heat treatment method, it has been confirmed by experimental results that it does not precipitate in the vicinity of the grain boundary as generally called but spreads throughout the grain. In addition, when this processing heat treatment method is applied, the sizes of the precipitates are relatively uniform, and the size can be controlled by the amount of element added and is also reproducible.

  These precipitates become extinction points (sinks) of point defects due to irradiation, promote dispersion of voids, and are effective in suppressing chromium (Cr) diffusion. At the same time, grain refinement increases the total distance around the grain boundary and increases the sink. For this reason, the reduction | decrease per unit length of chromium deficiency by irradiation-induced segregation or thermal segregation is brought about.

  According to the present embodiment, the application of the thermomechanical processing method in a state where an appropriate amount of the additive element is added is effective in suppressing the grain boundary chromium deficiency, and by suppressing this grain boundary chromium deficiency, Corrosion resistant steel excellent in corrosion resistance, irradiation resistance and high temperature strength can be obtained. Furthermore, since the intragranular precipitate is precipitated, it has an effect of suppressing the movement of dislocations, so that the creep strength can be improved.

  Further, the strength of austenitic stainless steel can be defined by the cold descent rate. When the cold working is 60% or more, the crystal grain refining effect is constant at the maximum. For this reason, in the case of austenitic stainless steel, the processing rate of cold working can be defined as 60% or more.

  Corrosion resistant steel is mainly used as nuclear reactor equipment and has three types of utilization. One may be used with thin-walled material because it requires strong processing. Second, there is a case where a bulk material is multi-stage rolled and used as a thick material. In some cases, a thin material is clad into an unprocessed thick material. In consideration of corrosion resistance, the clad material is a sufficiently effective material if the wetted part is limited to the cladding.

  FIG. 6 is a graph showing the irradiation damage life extension (grain boundary Cr concentration analysis) by grain boundary segregation in the present embodiment, and FIG. 7 shows the irradiation damage life extension (grain boundary by grain boundary segregation in this embodiment). It is a graph which shows (Cr density | concentration analysis).

  In the present embodiment, an example of quenching with water, oil, or alkali metal in a quenching step 109 for quenching stainless steel recrystallized after being cold worked will be described.

  The stainless steel is cooled at a cooling rate of 5 ° C./second to 50 ° C./second. By cooling at this cooling rate, the Cr concentration at the grain boundary of stainless steel can be increased by 2% or more than the parent phase. By increasing the Cr concentration more than the parent phase, it is possible to suppress the occurrence of sensitization due to Cr deficiency.

  FIG. 6 shows the grain boundary Cr concentration of the austenitic stainless steel when the cooling rate is 10 ° C./second. When the cooling rate during quenching of the normal material is about 100 ° C./second, no change is observed in the grain boundary Cr concentration. On the other hand, when the cooling rate of the austenitic stainless steel is 10 ° C./second, the grain boundary Cr concentration is greatly increased. It can be seen that the Cr concentration is increased by about 5% when the cooling rate is set to 10 ° C./second as compared with the case of the normal material. As described above, even if the stainless steel is cooled practically at a cooling rate of 5 ° C./second to 50 ° C./second, there is no significant difference when compared with the cooling rate of 10 ° C./second.

  FIG. 7 shows a period until Cr at the grain boundary is deficient by irradiation with simulated ions of neutron irradiation to show sensitivity to irradiation induced stress corrosion cracking (IASCC). The developed material with an austenitic stainless steel cooling rate of 10 ° C./second has an approximately 5% increase in Cr concentration compared to the normal material, and as a result, until it shows sensitivity to irradiation-induced stress corrosion cracking (IASCC). It shows that the period of was stretched twice.

  As a result, this effect could be obtained by gas cooling. However, it is easy to optimize the cooling rate by selecting a liquid metal such as sodium (Na) or lithium (Li) depending on the size and type of stainless steel.

  Moreover, if it is small stainless steel, a fine-grain material can be obtained by one heat processing. However, there are cases where large stainless steel does not provide the heating rate that is expected. In such a case, a predetermined fine-grain material can be manufactured by repeatedly performing the cold working 107 and the heat treatment 108 twice or more in the process related to the fine-graining process shown in FIG. is there.

  Further, the present invention is not limited to the embodiments described above, and the recrystallization rapid heating may be changed to an alkali metal instead of the molten salt, and does not depart from the gist of the present invention. Various modifications can be made within the range.

The process figure which shows the fine graining process of the austenitic stainless steel of embodiment of this invention. The perspective view which shows the cold processing apparatus of embodiment of this invention. The front view which shows the fine graining heat processing apparatus of embodiment of this invention. It is the photograph which shows the structure | tissue of the fine grained stainless steel of this Embodiment, (a) is a microscope picture which shows the normal material as a comparative example, (b) is a microscope picture which shows a refined material. The graph which shows the refinement | miniaturization effect by inclusion of the additive element of this Embodiment. The graph which shows the irradiation damage lifetime extending | stretching (grain boundary Cr density | concentration analysis) by the grain boundary segregation of this Embodiment. The graph which shows the irradiation damage lifetime extension (change of the irradiation damage by Cr segregation) by the grain boundary segregation of this Embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 105 ... Solution heating, 106 ... Rapid cooling process, 107 ... Cold working, 108 ... Recrystallization heating, 109 ... Rapid cooling process, 201 ... Roller, 202 ... Heat-resistant steel, 203 ... Molten salt, 204 ... Bath, 205 ... Heater, 206 ... Crane, 207 ... Drive control device, 208 ... Cooling water, 209 ... Cooling water tank, 211 ... Normal material structure, 212 ... Fine grain material structure.

Claims (13)

In the manufacturing method of corrosion-resistant steel including a grain refinement process processed by cold working, recrystallization heating treatment and cooling treatment,
The crystal grain refining step is to rapidly heat to a recrystallization temperature at a heating rate of 30 ° C./second or more to refine the crystal grain size to 5 μm or less,
A method for producing corrosion-resistant steel, characterized by:   The method for producing corrosion-resistant steel according to claim 1, wherein the cold working has a cold working rate of 60% or more.   The method for producing corrosion-resistant steel according to claim 1, wherein the recrystallization heat treatment is performed at a heating temperature of 870 ° C to 970 ° C.   The said recrystallization heat processing is a manufacturing method of the corrosion-resistant steel of Claim 3 whose heating time is 1 minute or less.   The said recrystallization heat processing is what heats with molten salt, The manufacturing method of the corrosion-resistant steel in any one of the Claims 1 thru | or 4 characterized by the above-mentioned.   The said recrystallization heat processing is what heats with the liquid metal of sodium or lithium, The manufacturing method of the corrosion-resistant steel in any one of the Claims 1 thru | or 4 characterized by the above-mentioned.   2. The cooling process according to claim 1, wherein the cooling treatment is performed at a cooling rate of 5 ° C./second to 50 ° C./second to increase the chromium concentration at the grain boundary by 2% or more than the parent phase. A method for producing corrosion-resistant steel.   The said cooling process is what cools with the liquid metal of sodium or lithium, The manufacturing method of the corrosion-resistant steel of Claim 1 or 7 characterized by the above-mentioned.   The method for producing corrosion-resistant steel according to claim 1, wherein the cold working and the heat treatment are repeated twice or more. Corrosion-resistant steel that is atomized by cold working, recrystallization heat treatment and cooling treatment is rapidly heated to a recrystallization temperature at a heating rate of 30 ° C./second or more to refine the crystal grain size to 5 μm or less. To be manufactured,
Corrosion resistant steel.   Must be at least one selected from austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, austenitic and ferritic duplex stainless steel, austenitic and martensitic duplex stainless steel and nickel-base alloy The corrosion-resistant steel according to claim 10.   The corrosion-resistant steel according to claim 10, wherein at least one additive selected from titanium, zirconium and niobium is contained.   The corrosion-resistant steel according to claim 12, wherein the additives are each 3% by weight or less.

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