JP4887506B2 - Method for producing ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a method for producing a ferritic heat resistant steel used as a heat resistant metal material, for example.

  As heat-resistant materials used in various high-temperature devices such as boilers, turbines, engines, chemical reactor materials, and fuel cell members, only metal materials are used from the viewpoint of reliability against tensile stress. Further, the heat-resistant metal material is limited to steel or Ni-based alloy from the viewpoint of economy. Among these, ferritic heat resistant steels are highly expected because of their excellent thermal conductivity and low thermal expansion coefficient, and even now, great efforts are being made to improve the performance of ferritic heat resistant steels.

  Historically speaking, 2.25Cr-1Mo steel and stainless steel have been the boiler's hottest part for quite some time. It is an exaggeration to say that improved 9Cr steel was invented in the early 1990s, replacing the ferritic steel that has been researched and put to practical use, and that improved 9Cr steel is now used in the hottest parts of boilers. is not. In addition, as a part of global warming countermeasures, research on high chromium ferritic steel aiming at high temperature and high pressure in steam conditions is still in progress. However, since the service temperature of these materials is just over 620 ° C., investigation of expensive Ni-based alloys (for example, austenitic stainless steel) has been started to further improve the steam conditions.

  Ferritic heat-resistant steel is cheaper than Ni-based alloys, and is excellent in terms of heat expansion and thermal conductivity. On the other hand, ferritic heat-resistant steel has a bcc structure (body-centered cubic lattice structure) at room temperature, and therefore generally has a problem that the high-temperature strength is small compared to Ni-based alloys having an fcc structure (face-centered cubic lattice structure) at room temperature. is there.

  Therefore, the present invention is a ferritic heat-resistant steel that can increase the service temperature (high-temperature strength) of a ferritic heat-resistant steel that is inexpensive, excellent in thermal expansion and thermal conductivity compared to a Ni-based alloy. An object is to provide a manufacturing method.

One embodiment of the present invention is a method for producing a ferritic heat resistant steel. This manufacturing method is
A heating step of heating a steel material mainly containing Fe and containing Cr to a temperature equal to or higher than the _AC3 transformation point;
A cooling step of cooling the steel material heated in the heating step to a predetermined temperature range higher than room temperature below the martensitic transformation start temperature at a speed at which pearlite transformation does not occur;
The steel material cooled in the cooling step possess a tempering step is performed tempering treatment without cooling to room temperature,
When the predetermined temperature range is T and the martensite transformation start temperature is Ms, the T is 50 ° C. ≦ T ≦ Ms−20 ° C.
A ferritic heat-resisting steel comprising a structure in which martensite formed in the predetermined temperature range is tempered and a structure formed when untransformed austenite is cooled from the tempering temperature as a final form, To do .

  In the manufacturing method of a certain aspect, the steel material is 0 <C ≦ 0.5; 0 <Si ≦ 2; 0 <Mn ≦ 2; 5 ≦ Cr ≦ 13; 0 <N ≦ 0.1; It is preferable to contain Al ≦ 0.05.

  In the manufacturing method of a certain aspect, it is good that the said steel material contains 0.2 or more and 4 or less by mass% of Mo, W, or Mo + W.

  In the manufacturing method of a certain aspect, the said steel material is good to contain 0.02 or more and 0.6 or less by mass% of V, Nb, Ta, or these composite additives.

  In the manufacturing method of a certain aspect, it is good in the said steel material containing 0.002 or more and 0.02 or less by mass%.

  In the manufacturing method of a certain aspect, the steel material may contain Ni, Co, Cu, or a composite additive thereof in an amount of 0.05 to 4 in mass%.

  It should be noted that any combination of the above components is also effective as an aspect of the present invention.

  In conventional ferritic steel manufacturing methods, heat treatment combining quenching / normalizing and tempering / annealing has been performed. According to the present invention, the steel material heated in the heating step is cooled to a predetermined temperature range below the martensite transformation start temperature and higher than room temperature, and then subjected to a tempering process without cooling to room temperature, thereby performing Ni treatment. Compared to the base alloy, it is possible to increase the service temperature (high temperature strength) of the ferritic heat resistant steel which is inexpensive and excellent in thermal expansion and thermal conductivity.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the embodiments do not limit the invention but are exemplifications, and all features and combinations described in the embodiments are not necessarily essential to the invention. It is not always true.

(Basic steel composition)
The most basic steel in the present embodiment contains the following in addition to the main component Fe. In addition to the following, impurities that are inevitable in steelmaking are also contained.
C (carbon): 0 <C ≦ 0.5 in mass%. Carbon is usually added in an amount of 0.05 to 0.2 in mass% to form a martensite structure. However, if it exceeds 0.5 mass%, the toughness decreases due to the precipitation of a large amount of carbide. In addition, since the structure | tissue similar to a martensite is formed even if it is a carbon-free material, carbon may be 0 in a modification.
Si (silicon): 0 <Si ≦ 2 by mass%. Silicon is necessary for improving corrosion resistance and as a deoxidizer. However, if it exceeds 2% by mass, the toughness is impaired.
Mn (manganese): 0 <Mn ≦ 2 by mass%. Manganese is necessary as a desulfurizing agent and a deoxidizing agent, but if it exceeds 2% by mass, the structure becomes unstable.
Cr (chromium): 5 ≦ Cr ≦ 13 by mass%. Chromium needs to be added in an amount of 5% by mass or more in order to ensure oxidation resistance, but if it exceeds 13% by mass, it becomes difficult to form a martensite structure.
N (nitrogen): 0 <N ≦ 0.1 in mass%. Nitrogen is an element necessary for stabilizing martensite, like carbon, but it is difficult to add more than 0.1% by mass in vacuum melting or atmospheric melting, which impairs economic efficiency.
Al (aluminum): 0 <Al ≦ 0.05 by mass%. Aluminum can be added as a deoxidizing agent, but 0.05% is the upper limit because it impairs the high-temperature strength.

(Description of heat treatment)
FIG. 1 is an explanatory view of heat treatment in a method for producing a ferritic heat resistant steel according to an embodiment of the present invention. In this figure, the horizontal axis indicates time and the vertical axis indicates temperature. The heat treatment of the present embodiment has the following steps.
Heating step: The steel material is heated to a temperature equal to or higher than the _AC3 transformation point. After heating, the temperature is maintained (soaked) for a predetermined time (for example, about 30 minutes to 2 hours).
Cooling step: The steel material heated in the heating step is cooled to a predetermined temperature range that is lower than the martensitic transformation start temperature and higher than room temperature at a speed at which pearlite transformation does not occur. The predetermined temperature range will be described later.
Tempering process: The steel material cooled in the cooling process is tempered without cooling to room temperature. That is, it heats from the said predetermined temperature range to the predetermined temperature below an _AC1 transformation point, and hold | maintains the temperature only for predetermined time (for example, about 30 minutes-2 hours), Then, it cools to room temperature.

  The basic heat treatment of conventional steel is to maintain it in the austenite region, then to quench and air-cool and to martensite, that is, after quenching or normalizing, tempering to balance strength and toughness Is to do. This embodiment follows this basic idea. However, quenching or tempering is based on austenitizing and then cooling to near room temperature to martensite. On the other hand, the heat treatment according to the present embodiment does not cool to room temperature after austenitization, but at the time when martensite is formed to some extent (some bainite may be formed depending on the steel type) (hereinafter, bainite). In order to soften the martensite formed at that temperature, the temperature is raised directly without cooling to room temperature and kept at the normal tempering temperature for air cooling. To ensure excellent high temperature strength.

The martensite treatment temperature is naturally below the martensite transformation start temperature (hereinafter also referred to as “Ms point”), and an upper limit is 20 ° C. below the Ms point in order to secure an appropriate amount of martensite. More preferably, the martensite treatment temperature has an upper limit of 50 ° C. below the Ms point. The Ms point may be described in the handbook, but can be easily obtained by actual measurement. In addition, when the martensite processing temperature is sufficiently low, almost causing a martensitic transformation, since not change the normal hardening, normalizing, the lower limit of the martensite processing temperature is set to 50 ℃. More preferably, the lower limit of the martensite treatment temperature is 70 ° C.

  The cooling rate to the martensite temperature depends on the furnace structure, material size, etc., so it is not particularly determined.However, if the cooling rate is sufficiently low, pearlite transformation occurs, so the cooling rate is at least that which does not cause pearlite transformation. To do. In addition, the retention time after reaching the martensite temperature is considered to cause a minute change in the microstructure, but due to the nature of the martensite transformation, even if the temperature is raised immediately after reaching the martensite temperature. Even if the temperature is maintained, there is no difference in the basic martensite formation reaction, and therefore the retention time of the martensite treatment temperature is not particularly defined. However, the retention time of the martensite treatment temperature is preferably 5 minutes or more.

  There are many historical studies on heat treatment in steel manufacturing. However, in many cases, when the product is cooled from a high temperature to martensite, the temperature of the product may become non-uniform and bending or cracking may occur. To prevent this, the quenching temperature (from room temperature) After reaching the temperature of about 300 ° C., there is a method of maintaining the temperature for a while or gradually cooling from the temperature. For example, pulling and quenching, martemper, etc. are examples. As a special example, there is a process called austemper for the purpose of making bainite instead of martensite. In any of these processes, the temperature is lowered or kept from the quenching / normalizing temperature, but the temperature is not raised during the temperature lowering (the temperature is not raised to the room temperature). Ferritic steel is also known as constant temperature annealing, and there is a heat treatment that combines normalizing and tempering. However, this does not raise the temperature during the temperature reduction (the temperature rises without lowering to room temperature). The treatment is for promoting the formation of a ferrite pearlite structure and is not intended for martensite as in the present embodiment.

  The structure formed by the heat treatment of the present embodiment is a structure (tempered martensite) obtained by tempering martensite (including some bainite) formed at the martensite treatment temperature, and untransformed austenite is cooled from the tempering temperature. It sometimes becomes a mixed structure of tissues that are sometimes formed (called latent martensite). However, unlike martensite, which is formed during cooling from quenching and normalizing temperatures, latent martensite partially precipitates charcoal and nitride at the tempering temperature, so it does not dissolve so much supersaturated carbon. Because the amount is limited, this latent martensite does not make the material brittle. In addition, a sufficiently high strength can be obtained even with the martensite structure formed by quenching and normalizing from a high temperature. However, when used at a high temperature, the martensite structure recovers and softens in a very short time, making it a practical heat-resistant material. It is a well-known fact that it is not suitable. That is, the structure of the present embodiment is completely different from the structure obtained by conventional heat treatment. Depending on the composition and manufacturing method, delta ferrite may be included slightly, but if it is 20% by mass or less, the effect of improving the high temperature strength is not lost.

(Description of additives)
In addition to the composition of the basic steel described above, the following additives may be included.
Mo (molybdenum) and W (tungsten): Usually, Mo and W are added individually or in combination in an amount of 0.2 to 4% by mass for the purpose of increasing the high temperature strength. When these elements are added in a large amount, delta ferrite is generated. However, as long as the amount of delta ferrite does not exceed 20% by mass, the effect of the heat treatment of the present embodiment is not lost.
V (vanadium), Nb (niobium), Ta (tantalum): Usually, V, Nb, and Ta combine with carbon and nitrogen to form fine precipitates, increasing the high temperature strength. In combination, 0.02 or more and 0.6 or less are added by mass%. When these elements are added in a large amount, delta ferrite is produced. However, even if these elements are contained in the basic steel, the heat treatment of this embodiment is performed as long as the amount of delta ferrite does not exceed 20 mass%. The effect of does not disappear.
-B (boron): If B is not added in an amount of 0.002% by mass or more, the effect of increasing the high-temperature strength cannot be expected, and if added over 0.02%, a compound is formed and the toughness is impaired.
Ni (nickel), Co (cobalt), and Cu (copper): Usually, Ni, Co, and Cu are used alone to stabilize austenite and to ensure toughness. .05 or more and 4 or less are added.

(Creep test)
Here, the results of creep tests on several steels will be described. Table 1 below shows the chemical compositions of the steels (# 1 to # 8) used in the test.

Table 2 below shows the normalization temperature, martensite treatment temperature, time, HV (Biccus hardness) of each steel shown in Table 1 above, and the creep rate (%) when the creep test was performed at 650 ° C. and 110 MPa. / H) and specific creep rate (described later).

  In Table 2, the creep speed is the minimum creep speed, but when the creep speed is hardly deformed and does not reach the minimum creep speed, the creep speed at 1000 h is indicated (the martensite treatment temperatures of # 2, # 3, and # 5) 200 ° C). All tempering conditions are 750 ° C. and 2 hours. In order to compare the strength of each material, Table 2 shows the creep rate of the material subjected to normal normalizing / tempering treatment (with a martensite treatment temperature of 25 ° C.). The divided value (specific creep speed) is shown. Table 2 also shows the target creep rates of stainless steel (# 7, # 8).

  # 1 steel is the basic steel described above. Even if the martensite treatment temperature is changed from room temperature to 350 ° C., the HV (Bickus hardness) does not change so much as 225 to 245 after tempering. However, there are significant differences in creep speed. In particular, the effect when the martensite treatment temperature is 200 ° C. is remarkable. When the martensite temperature is 450 ° C., the hardness is large, but there is no effect of improving the creep strength.

  # 2 steel is steel obtained by adding W, V, and Ta to the basic steel described above. There is no significant change in hardness even when the martensite treatment temperature is changed. When the martensite treatment is performed at 200 ° C., a remarkable improvement effect is recognized in the creep rate.

  # 3 steel is a steel obtained by adding Mo, V, Nb, and N to the above-mentioned basic steel, and a significant improvement in creep rate is recognized by the addition of a strong carbide forming element as compared with the basic steel. Even if the martensite treatment temperature is changed from 100 to 350 ° C., no significant change in hardness is observed, but when a 200 ° C. martensite treatment is performed, a remarkable effect of improving the creep rate is recognized. Table 2 shows the hardness and creep speed of C steel as it is normalized. When the martensite temperature is 450 ° C., the hardness is large, but there is no effect of improving the creep strength. Further, the hardness of C steel is extremely high until normalizing, but the creep speed is larger (deteriorates) than that of normal normalizing and tempering materials.

  # 4 steel is obtained by adding Mo, W, V, Ni, Cu, Co, and N to the above-mentioned basic steel. By performing martensite treatment at 200 ° C., a remarkable effect of improving the creep rate can be confirmed. .

  # 5 steel is obtained by adding Mo, W, V, Nb, Co, N, and B to the basic steel, but by performing martensite treatment at 200 ° C., a remarkable effect of improving the creep rate can be confirmed.

  # 6 steel is produced by intentionally producing 15% delta ferrite by reducing carbon, but by performing martensite treatment at 200 ° C., a remarkable effect of improving the creep rate can be confirmed. The effect of is not lost.

  # 7 and # 8 steels were austenitic stainless steels and exhibited creep rates with the standard solution treatment. Comparing these with the creep rates of materials obtained by martensite treatment of # 1 to # 6 steel at 200 ° C., the creep rate of the steel of the present embodiment is similar to that of austenitic steel, and some steel types, # 3, # It can be seen that steels # 4 and # 5 are superior to austenitic steels.

  Since the creep rate of steel changes depending on the composition and the basic heat treatment temperature, it will be complicated to explain directly. Therefore, for the purpose of comparing the improvement effect of each steel, the specific creep rate described above is defined, and each steel is The relationship between the specific creep rate and the martensite treatment temperature is shown in FIG. As shown in the figure, it can be seen that the creep rate of each steel is remarkably improved by the heat treatment obtained in the present embodiment. The effect is remarkable between 70 ° C. and 350 ° C., and the effect is particularly remarkable in the treatment at 200 ° C. On the other hand, it can be said that there is a certain improvement effect even at 50 to 380 ° C. In addition, the martensitic transformation start temperature of these steels is said to be about 400 ° C.

  Thus, according to the manufacturing method of the ferritic heat resistant steel of the present embodiment, the high temperature strength of the ferritic heat resistant steel can be dramatically improved by a new heat treatment different from the conventional one. These techniques are applied to various high-temperature devices such as boilers, turbines, engines, chemical reactor materials, fuel cell members, fastening members related to them, and cast forged steel parts.

  The present invention has been described above by taking the embodiment as an example, but it will be understood by those skilled in the art that various steps can be made within the scope of the claims for each step of the embodiment.

It is explanatory drawing of the heat processing in the manufacturing method of the ferritic heat-resistant steel which concerns on embodiment of this invention. It is explanatory drawing which shows the relationship between the specific creep rate of each steel shown by Table 1, and a martensite process temperature.

Claims (6)

A heating step of heating a steel material mainly containing Fe and containing Cr to a temperature equal to or higher than the _AC3 transformation point;
A cooling step of cooling the steel material heated in the heating step to a predetermined temperature range higher than room temperature below the martensitic transformation start temperature at a speed at which pearlite transformation does not occur;
The steel material cooled in the cooling step possess a tempering step is performed tempering treatment without cooling to room temperature,
When the predetermined temperature range is T and the martensite transformation start temperature is Ms, the T is 50 ° C. ≦ T ≦ Ms−20 ° C.
A ferritic heat-resisting steel comprising a structure in which martensite formed in the predetermined temperature range is tempered and a structure formed when untransformed austenite is cooled from the tempering temperature as a final form, A method for producing a ferritic heat resistant steel. 2. The manufacturing method according to claim 1 , wherein the steel material is in mass% 0 <C ≦ 0.5; 0 <Si ≦ 2; 0 <Mn ≦ 2; 5 ≦ Cr ≦ 13; 0 <N ≦ 0.1. ; Ferritic heat resistant steel production method containing 0 <Al ≦ 0.05. The manufacturing method according to claim 1 or 2 , wherein the steel material contains Mo, W, or Mo + W in a mass% of 0.2 or more and 4 or less. In the manufacturing method in any one of Claim 1 to 3 , the said steel material contains 0.02 or more and 0.6 or less by mass% of V, Nb, Ta, or these composite additives. Manufacturing method of heat-resistant steel. The manufacturing method according to any one of claims 1 to 4 , wherein the steel material contains B in an amount of 0.002 to 0.02 by mass%. The ferritic heat resistant steel according to any one of claims 1 to 5 , wherein the steel material contains Ni, Co, Cu or a composite additive thereof in an amount of 0.05% to 4% by mass. Manufacturing method.

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