JP2006241482A - Ferritic heat resistant steel having tempered martensitic structure and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic heat resistant steel in which deterioration in creep strength is reduced even when used at high temperature for a long time compared with the conventional ferritic heat resistant steel, and which is suitable as a high temperature structural member for a boiler, a thermal power generation device, a nuclear power generation device, a chemical industry device or the like. <P>SOLUTION: A steel is subjected to quenching or normalizing at ≥1,000°C, and, within 10 hr, is cooled to ≤700°C. Further, in a cooling stage, the steel is held at a fixed temperature in the temperature range of 800 to 400°C for at least ≥10 min, and is thereafter naturally let to cool, and tempering is performed at ≥730°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

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, but the key to realizing this is high-strength ferrite. It is also said that this 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 for improving the creep strength of high-strength ferritic heat-resistant steel, it is well known that the effect of precipitating and dispersing the second phase such as carbides in the steel is great, and the steel that generates the second phase in the steel. This strengthening method is used in practical heat resistant steels as a precipitation strengthening method. 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.

The invention of this application solves the above-mentioned problems. First, a steel material containing 8 wt% to 13.5 wt% of Cr is quenched or normalized at 1000 ° C. or higher. Is cooled to 700 ° C. or lower within 10 hours, and in this cooling process, it is naturally cooled after being held at a constant temperature in the temperature range of 800 ° C. to 400 ° C. for at least 10 minutes, and then 73
Provided is a method for producing a ferritic heat resistant steel having a tempered martensite structure characterized by tempering at 0 ° C. or higher.

  Second, the composition range of the steel material is C: 0.04 to 0.2 wt%, Cr: 8.0 to 13.5 wt%, Mo: 0 to 2.0 wt%, W: 0 to 4. 0 wt%, V: 0.02 to 0.35 wt%, Nb: 0.01 to 0.2 wt%, Co: 0 to 4.0 wt%, Ni: 0 to 3.0 wt%, N: 0.002 to 0.15 wt%, B: 0 to 0.02 wt%, Si: 0 to 0.5 wt%, Mn: 0 to 1.0 wt%, Al: 0 to 0.05 wt% A ferritic heat resistant steel having the above tempered martensite structure which is an inevitable impurity and Fe is produced.

  Third, the time required for cooling from 800 ° C. to 400 ° C. is set to 1 hour or longer to produce a ferritic heat resistant steel having the above tempered martensite structure.

  Fourth, 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 resisting steel that utilizes the effect of precipitation strengthening by MX carbonitrides has MX MXNitrides deposited in the martensite structure during tempering 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, a steel material that has been quenched or normalized at a temperature of 1000 ° C. or higher is cooled to 700 ° C. or lower within 10 hours. Controlling the precipitation behavior of MX carbonitride, which is the main strengthening factor, by allowing it to cool naturally after being held for at least 10 minutes at a constant temperature in the temperature range, thereby exhibiting the effect of precipitation strengthening by MX carbonitride At the same time, it is a ferritic steel in which material deterioration due to long-term use at high temperatures is suppressed.

  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 800 ° C. to 400 ° C. is 1 hour or more. As a result, the amount of precipitates is 1.
The creep speed will be reduced to 1/2 or less.

  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 weight or more is effective to improve the strength, but addition exceeding 0.2% by weight reduces the strength in the long-term range. Let

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

  Mo: Addition is considered for solid solution strengthening, but addition exceeding 2.0% by weight promotes embrittlement.

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

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

  Nb: Carbonitride is formed, and addition of 0.01% by weight or more is effective to improve the strength. However, since insoluble 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% by weight is not effective.

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

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

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

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

  Al: Addition of 0.05% by weight 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, in the present invention (No. 3 to No. 5), during the cooling from the normalizing temperature, after being held at each temperature of 765 ° C., 600 ° C. and 500 ° C. for 30 minutes, it is cooled to room temperature by natural cooling in the air. It has been made.

  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 weight percentages of V and Nb contained in the precipitated phase of each test material. 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, in this invention (No3-No5) which was hold | maintained for 30 minutes at each temperature of 765 degreeC, 600 degreeC, and 500 degreeC, it is shown that it is a small creep rate compared with the case of standard heat processing. 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 weight% 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 (4)

  The steel containing 8 wt% to 13.5 wt% of Cr is quenched or normalized at 1000 ° C. or higher and cooled to 700 ° C. or lower within 10 hours. A method for producing a ferritic heat resistant steel having a tempered martensite structure, characterized by holding at a constant temperature in a temperature range of 400 ° C for at least 10 minutes, allowing to cool naturally, and then tempering at 730 ° C or higher.   The composition range of the steel materials is C: 0.04 to 0.2 wt%, Cr: 8.0 to 13.5 wt%, Mo: 0 to 2.0 wt%, W: 0 to 4.0 wt%, V : 0.02-0.35 wt%, Nb: 0.01-0.2 wt%, Co: 0-4.0 wt%, Ni: 0-3.0 wt%, N: 0.002-0 .15 wt%, B: 0 to 0.02 wt%, Si: 0 to 0.5 wt%, Mn: 0 to 1.0 wt%, Al: 0 to 0.05 wt%, unavoidable impurities and Fe The method for producing a heat-resistant ferritic steel having a tempered martensite structure according to claim 1.   The method for producing a ferritic heat resistant steel having a tempered martensite structure according to claim 1 or 2, wherein the time required for cooling from 800 ° C to 400 ° C is 1 hour or more. A ferritic heat-resistant steel having a tempered martensite structure, which is produced by the method according to any one of claims 1 to 3.

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