JP4900639B2 - Ferritic heat resistant steel having tempered martensite structure and method for producing the same - Google Patents

Description

  The invention of this application relates to a ferritic heat resistant steel having a tempered martensite structure suitable as a high-temperature structural member for boilers, thermal power generators, nuclear power generators, chemical industrial equipment, etc., and particularly compared with conventional ferritic heat resistant steels. In particular, the present invention relates to a ferritic heat-resistant steel and a method for producing the same, in which creep strength is hardly lowered even when used for a long time in a high temperature environment.

  It is said that the cost of fuel, such as coal, consumed by one unit of the latest high-efficiency thermal power plant per day is about 100 million yen less than a low-efficiency power plant built 20-30 years ago. In this way, the fact that the fuel consumption is small means that the carbon dioxide emission is also reduced accordingly.

Reducing emissions of carbon dioxide, a greenhouse gas, is an urgent issue, and there is a need to improve the energy efficiency of thermal power plants, which are the main source of carbon dioxide. The key to realizing this is high-strength ferrite It is said that it is the development of heat resistant steel, and research and development for improving high Cr ferritic heat resistant steel is being actively carried out all over the world (Patent Documents 1 to 4). As a method of improving the creep strength of high-strength ferritic heat-resistant steel, it is well known that the effect of precipitating and dispersing a second phase such as carbide in steel is great, and it is a steel that generates a second phase in steel. These strengthening methods are widely used in practical heat resistant steels as precipitation strengthening methods. For example, since Nb and V MX carbonitrides are finely precipitated and have a low coarsening rate, a method has been developed in which many high-strength ferritic heat resistant steels are precipitation strengthened by Nb and V MX carbonitrides.
JP-A-10-259542 JP 2003-253402 A JP-A-9-291308 JP-A-8-337813

  Conventional ferritic heat-resisting steel manufacturing methods use MX carbonitrides precipitated in the martensite structure during tempering heat treatment after normalizing heat treatment. In this case, MX carbonitride was preferentially precipitated. However, in order to enhance the effect of precipitation strengthening, it is important to uniformly disperse a large amount of fine particles to narrow the particle interval. However, in the conventional method, MX carbonitride precipitated during tempering heat treatment is not lath or Since it preferentially precipitates on the block boundary, it was difficult to deposit a large amount of MX carbonitride inside the lath.

  Therefore, in the present invention, the MX carbonitride is uniformly deposited inside the lath instead of the lath or block boundary to optimize the effect of precipitation strengthening by the MX carbonitride, thereby improving the creep strength and improving the lath or block. An object of the present invention is to provide a ferritic heat resistant steel excellent in long-term creep strength characteristics by suppressing material deterioration caused by precipitation and growth of the Z phase preferentially in the vicinity of the boundary.

As the invention of this application solves the above problems, first, the composition range of the steel material is C: 0.04 to 0.2 mass%, Cr: 8.0 to 13.5 mass%, Mo: 0 to 2.0 mass%, W: 0 to 4.0 mass%, V: 0.02 to 0.35 mass%, Nb: 0.01 to 0.2 mass%, Co: 0 to 4.0 mass% %, Ni: 0-3.0 mass%, N: 0.002-0.15 mass%, B: 0-0.02 mass%, Si: 0-0.5 mass%, Mn: 0-1. 0% by mass, Al: 0 to 0.05% by mass and inevitable impurities and ferritic heat-resistant steel as Fe are kept at 1000 ° C. or higher, and then cooled to a temperature of less than 700 ° C. within 10 hours, and then 400 ° C. Hold at least 10 minutes at a constant temperature in the temperature range of 650 ° C or lower, let it cool naturally, and then baked at 730 ° C or higher. To provide a method of manufacturing a ferritic heat-resistant steel having a tempered martensite structure, wherein contribute.

Second , ferritic heat-resistant steel having the above tempered martensite structure is produced by setting the time required for cooling from a temperature lower than 650 ° C. to 400 ° C. for 1 hour or more.

Third , the above manufacturing method ferritic heat resistant steel is provided.

  According to the first aspect of the invention, by specifying the cooling condition after quenching or normalizing, the creep strength is improved, and the material resulting from the precipitation and growth of the Z phase that occurs preferentially in the vicinity of the lath or block boundary It is possible to produce a ferritic heat resistant steel that suppresses deterioration and has excellent long-term creep strength characteristics.

  According to the second aspect of the invention, the ferritic heat-resistant steel having excellent long-term creep strength characteristics can be produced more practically and efficiently by using a steel material having a specific composition range.

  According to the third aspect of the invention, it is possible to efficiently produce a ferritic heat resistant steel that is further excellent in long-term creep strength characteristics.

  According to the fourth invention, it is possible to obtain a ferritic heat resistant steel with little decrease in creep strength even when used at a high temperature for a long time by the above production method.

The conventional ferritic heat-resistant steel that uses the effect of precipitation strengthening by MX carbonitrides has MX MXNitride deposited in the martensite structure during tempering heat treatment after normalizing heat treatment. It was preferentially deposited on the boundaries of blocks and blocks. Therefore, if MX carbonitride can be precipitated before transformation into martensite, MX carbonitride can be finely and uniformly dispersed and precipitated. In the invention of this application, the amount of MX carbonitride that is finely and uniformly dispersed and precipitated by holding for a certain time in a temperature range higher than the temperature at which it transforms into martensite during cooling from the normalizing heat treatment temperature. To increase the creep strength. That is, in the high-strength ferritic heat-resisting steel according to the invention of this application, the steel material is kept at a temperature of 1000 ° C. or higher, and then cooled to a temperature of less than 700 ° C. within 10 hours, and thereafter a temperature range of 400 ° C. or more and less than 650 ° C. At a constant temperature of at least 10 minutes, it is allowed to cool naturally after standing for at least 10 minutes to control the precipitation behavior of MX carbonitride, which is the main strengthening factor, and to exert the effect of precipitation strengthening by MX carbonitride. Ferritic steel with suppressed material deterioration due to long-term use in

  Compared with the prior art, the creep speed can be reduced to 1/2 or less, and the creep rupture time can be increased to twice or more.

In the invention of this application, it is preferably considered that the time required for cooling from a temperature lower than 650 ° C. to 400 ° C. is 1 hour or more. As a result, the amount of precipitates is increased 1.5 times or more and the creep rate is reduced to ½ or less as compared with the prior art.

  The invention of this application provides important new knowledge not only in the practical aspect of material development but also in the field of basic studies related to the precipitation behavior of the second phase and the strengthening mechanism, and its technical effect is enormous. The effect of the invention of this application is not strictly limited to the composition range of the steel material, but by making it a specific composition range, the ferrite in which the material deterioration associated with long-term use at high temperatures is further suppressed. Heat-resistant steel can be manufactured. For example, the following is considered when the composition range is set to a specific range.

C: Carbide or carbonitride is formed, and addition of 0.04% by mass or more is effective to improve the strength, but addition exceeding 0.2% by mass lowers the strength in the long time range. Let

Cr: Addition of 8.0% by mass or more is necessary to ensure oxidation resistance, but addition exceeding 13.5% by mass generates a delta ferrite phase and lowers the strength.

Mo: Although addition is considered for solid solution strengthening, addition exceeding 2.0 mass % promotes embrittlement.

W: Addition is considered for strengthening solid solution, but addition exceeding 4.0% by mass promotes embrittlement.

V: Carbonitride is formed, and addition of 0.02% by mass or more is effective for improving the strength. However, since insoluble precipitates increase, addition exceeding 0.35% by mass is It is not effective for strength improvement.

Nb: Carbonitride is formed, and the addition of 0.01% by mass or more is effective for improving the strength. However, since undissolved precipitates increase, addition exceeding 0.2% by weight It is not effective for strength improvement.

Co: Addition is considered in order to suppress the formation of the delta ferrite phase and ensure high strength, but in order to reduce the strength for a long time, addition exceeding 4.0 mass % is not effective.

Ni: Addition is considered to suppress the formation of the delta ferrite phase and ensure high strength. However, addition of more than 3.0% by mass is not effective in order to lower the transformation temperature of ferrite and austenite.

N: Nitride or carbonitride is formed, and addition of 0.002% by mass or more is effective for improving the strength, but addition exceeding 0.15% by mass is difficult in production.

B: In order to refine the precipitate and improve the stability at high temperature, the addition up to about 0.02% by mass is considered for improving the strength.

Si: Considered as a deoxygenated component or the like, but when it exceeds 0.5 mass %, the coarsening of the precipitate proceeds.

Although it considers similarly to Mn: Si, when it exceeds 1 mass %, the coarsening of a precipitate will advance and ductility will fall.

Al: Addition of 0.05% by mass or less is considered.

  Therefore, the invention of this application will be described in more detail below using examples. Of course, the invention is not limited by the following examples.

  About the test material which has a composition of Table 1, it heat-processed on the conditions shown in Table 2. FIG.

  In each case, tempering was performed at 765 ° C. for 30 minutes, and then natural air cooling was performed.

The standard (No. 1) is cooled from the normalizing temperature of 1050 ° C., which is carried out in the normal normalizing heat treatment, to room temperature by natural cooling in the air. In this case, the time until it becomes 700 ° C. or less is within 15 minutes. The comparative example (No. 2) was held at 765 ° C. during cooling from the normalizing temperature for 24 hours and then cooled to room temperature by natural cooling in the air. On the other hand, the present invention (No 4 ~No5) is during the cooling from the normalizing temperature and, after holding for 30 minutes each at each temperature of 600 ° C. and 500 ° C., allowed to cool to room temperature by natural cooling in air Is.

After the normalizing heat treatment, the electrolytic extraction residue was collected from the specimen cooled to room temperature, and the chemical components of the precipitated phase were analyzed. FIG. 1 shows the mass % of V and Nb contained in the precipitated phase of each specimen. Compared with the standard (No. 1) that is cooled directly from the normalizing temperature to room temperature, the amount of V and Nb contained in the precipitated phase is increased in the sample held at a specific temperature during cooling. It can be seen that the MX carbonitride precipitates by holding during the cooling from the normalizing temperature.

  The martensitic transformation temperature of the test material was about 380 ° C. Therefore, it is an austenite phase in the temperature range of 500 to 765 ° C. maintained during cooling from the normalizing temperature. From the above, it can be seen that MX carbonitride is precipitated in the austenite phase at the holding temperature in the test material held at a predetermined temperature during cooling from the normalizing temperature.

  FIG. 2 shows the relationship between the creep rate and time obtained by performing a creep test at a test temperature of 600 ° C. and a stress of 120 MPa after tempering heat treatment.

The rapid change in the creep rate in the comparative example (No. 2) is considered to be due to the transformation of the austenite phase to a low strength ferrite phase during 24 hours at 765 ° C. For this reason, in order to avoid transformation of the austenite phase to the ferrite phase, it is necessary to limit the time required for cooling from the normalizing temperature to 700 ° C. or less to 10 hours or less. On the other hand , the present invention (No. 4 to No. 5 ) held at 600 ° C. and 500 ° C. for 30 minutes shows that the creep rate is smaller than that of the standard heat treatment. The creep speed is ½ or less, and the creep rupture time is twice or more.

  From this result, it is understood that the creep strength is improved by applying the heat treatment condition of the present invention during the cooling from the normalizing temperature.

It is the figure which showed the mass % of V and Nb contained in the precipitation phase after the normalization heat processing of each test material. It is the figure which showed the creep rate-time curve in the stress of 120MPa in 600 degreeC of a test material.

Claims (3)

The composition range of steel materials is C: 0.04 to 0.2 mass%, Cr: 8.0 to 13.5 mass%, Mo: 0 to 2.0 mass%, W: 0 to 4.0 mass%, V : 0.02 to 0.35 mass%, Nb: 0.01 to 0.2 mass%, Co: 0 to 4.0 mass%, Ni: 0 to 3.0 mass%, N: 0.002 to 0 .15 mass%, B: 0 to 0.02 mass%, Si: 0 to 0.5 mass%, Mn: 0 to 1.0 mass%, Al: 0 to 0.05 mass%, unavoidable impurities and Fe The ferritic heat resistant steel is maintained at 1000 ° C. or higher, cooled to a temperature of less than 700 ° C. within 10 hours, and then kept at a constant temperature in the temperature range of 400 ° C. or higher and lower than 650 ° C. for at least 10 minutes. It has a tempered martensite structure characterized by being allowed to cool and then tempering at 730 ° C. or higher. Manufacturing method of ferritic heat resistant steel. Method for producing a ferritic heat-resistant steel having been tempered martensite structure, wherein the time from a temperature below 650 ° C. to cool to 400 ° C. to claim 1, characterized in that a 1 hour or more. A ferritic heat-resistant steel having a tempered martensite structure, which is manufactured by the method according to claim 1 or 2 .