JP2008007805A - Production method of ferritic steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a high-chromium ferritic steel excellent in creep ductility. <P>SOLUTION: The high-chromium ferritic steel is produced by conducting a step of heating a ferritic steel with a thickness th[mm] at ≥T<SB>1</SB>[°C] for ≥t<SB>1</SB>[min], a step of cooling the ferritic steel under a condition yielding a cooling rate of ≥10°C/sec at the center of thickness, a step of subsequently heating the same at 450-600°C and a step of subsequently heating the same at 650-780°C, wherein T<SB>1</SB>is a temperature defined by the equation 1: T<SB>1</SB>=3427×(C+N)×(Nb+V)+1,050 and t<SB>1</SB>is a time defined by the equation 2: t<SB>1</SB>=0.3th+10. The ferritic steel comprises, by weight, 0.01-0.05% C, 0.01-0.05% N, 8-13% Cr, 0.03-0.07% Nb, 0.05-0.20% V and the balance being Fe and unavoidable impurities. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a ferritic steel manufacturing method for manufacturing a high chromium ferritic steel in which fine precipitates stable at high temperature are dispersed and precipitated at a high density.

  Heat resistant steels used for high temperature equipment are roughly classified into high chromium ferritic steels and austenitic stainless steels.

  Among these, high chromium ferritic steel is considered to be used as a heat and pressure resistant member for boilers, thermal power generation and chemical plants because it has excellent strength, corrosion resistance and oxidation resistance in the temperature range of 400 ° C to 600 ° C. (For example, refer to Patent Document 1 and Patent Document 2).

By the way, the structure of a fast breeder reactor (FBR) using liquid sodium as a coolant, which is currently being developed and put to practical use, is repeatedly subjected to thermal stress accompanying temperature fluctuations due to operation / stop / operation control. Therefore, a steel material for heat stress design is required.
For this reason, creep ductility is required rather than creep strength for the material used for the structure of the fast breeder reactor. Moreover, since it becomes a large-sized welded structure, toughness is also required.

  Austenitic stainless steels having excellent creep ductility and toughness have been used for FBR in the development stage so far.

However, since austenitic stainless steel has a larger coefficient of thermal expansion than that of high chromium ferritic steel and a low thermal conductivity, it has a drawback that thermal stress generated during operation is increased.
Therefore, from the viewpoint of thermal stress, if high-chromium ferritic steel can be used for FBR structures, it will be possible to greatly rationalize the design and greatly improve the economic efficiency, which is the biggest problem in practical use. it can.

JP-A-6-10041 JP-A-8-337813

However, conventional high-chromium ferritic steel has been developed and put to practical use as a steel material for pressure-resistant design such as thermal power boilers that become high-pressure environments. Creep ductility and toughness are inferior to austenitic stainless steel.
For this reason, the high-chromium ferritic steel for thermal power boilers currently developed and put into practical use cannot satisfy the application conditions for FBR structures.

  In order to solve the above-described problems, the present invention provides a method for producing ferritic steel, which has excellent creep ductility and can be used for producing a high chromium ferritic steel suitable for use in a structure of a fast breeder reactor. It is to provide. In addition, since it is a structural material, a predetermined level of creep strength is required.

The reason for the low creep ductility and toughness of high chromium (Cr) ferritic steels for thermal power boilers is that carbon (C), niobium (Nb), vanadium (V), molybdenum ( Mo) and tungsten (W) are contained in large quantities.
These elements added in a large amount form coarse precipitates during a manufacturing process such as hot rolling, a heat treatment process, or high temperature use, thereby reducing creep ductility and toughness.

The present invention is a new component system (low carbon and Mo) that eliminates Cr carbide, which is a deterioration factor of material properties of high-chromium steel for thermal power oriented at a high temperature for a long time, and precipitation of intermetallic compound Laves phase. And W are limited to additions below the solid solubility limit) MX (MX is a carbonitride of V and Nb, M is a symbol representing Nb and V, and X is a symbol representing C and N, respectively. ) Is pursuing high temperature long time stabilization of precipitation strengthening mechanism.
Precipitation strengthening depends on the size and density (number) of the precipitates, and thus maintaining the strengthening mechanism requires preventing the precipitates from agglomerating and coarsening during high temperature use.
The precipitate produced at a temperature sufficiently higher than the use temperature is suppressed from agglomeration and coarsening at the use temperature. However, since the size of the precipitate is large and the density is low, a sufficient strengthening action cannot be obtained.
On the other hand, in the heat treatment near the use temperature, a precipitation form effective for fine and high-density strengthening can be obtained, but aggregation and coarsening during use cannot be suppressed.

  The present invention realizes a stable precipitation form at high temperature by precipitating fine MX with high density by high-temperature tempering after distributing fine MX precipitation nuclei with high-density by low-temperature tempering, and a high-temperature precipitation strengthening mechanism. It was secured for a long time.

That is, the method for producing a ferritic steel according to the present invention is, in wt%, C: 0.01 to 0.05%, N: 0.01 to 0.05%, Cr: 8 to 13%, Nb: 0.03. A ferritic steel having a thickness of th [mm], containing 0.0 to 7%, V: 0.05 to 0.20%, the balance being Fe and inevitable impurities, is expressed by the temperature T ₁ [ [° C.] or more and a step of heating for a time t ₁ [min] or more shown in the following formula 2 (first heating step);
T ₁ = 3427 × (C + N) × (Nb + V) +1050 (Formula 1)
(However, C, N, Nb, and V are the content (% by weight) of each element)
t ₁ = 0.3th + 10 (Formula 2)
Thereafter, a cooling step under the condition that the thickness center position of the ferritic steel has a cooling rate of 10 ° C./sec or more, and
Thereafter, at a temperature T ₂ [° C.] in the range of 450 to 600 ° C., a step of heating t ₂ [hour] when the index 1 shown in the following formula 3 becomes 15500 to 17300 (second heating step);
Index 1 = (T ₂ +273) (logt ₂ +20) (Formula 3)
After that, at a temperature T ₃ [° C.] in the range of 650 to 780 ° C., a time t ₃ [hour] heating step (third heating step) at which the index 2 shown in the following formula 4 becomes 19600 to 21100 is performed. Index 2 = (T ₃ +273) (logt ₃ +20) (Formula 4)
Is.

  In the present invention, more preferably, ferritic steel is further contained by weight, Mo: 0.2 to 0.6%, W: 0.2 to 0.6%, or both as components. The configuration is as follows.

  Each component of the manufacturing method of the above-mentioned ferritic steel of the present invention is derived as follows.

(1) Chemical composition of ferritic steel (high chromium steel) used In the ferritic steel (high chromium steel) used, the content of each component element is defined as follows.
First, C and N require 0.01% or more from the role of precipitation strengthening by MX, which is a carbonitride of Nb and V, but if it contains more than 0.05%, MX This value was made the upper limit in order to cause coarsening of the steel and deteriorate toughness.
Cr needs to be 8% or more in order to ensure oxidation resistance and hardenability, but if it is excessively contained, an intermetallic compound precipitates and impairs toughness, so it was made 13% or less.
Nb and V are elements constituting MX for precipitation strengthening, but the effect is produced at 0.03% or more for Nb and 0.05% or more for V. Therefore, these values are set as lower limit values. . On the other hand, if added excessively, toughness deterioration occurs due to an increase in the amount of MX precipitated, so Nb was set to 0.07% and V was set to 0.20% as the upper limit.
Both Mo and W are elements that increase the strength by solid solution strengthening, and the effect is effective at 0.2% or more. If the amount of these elements added is below the solid solubility limit at the operating temperature, a very stable strengthening action can be obtained. However, if added above the solid solubility limit, it precipitates as an intermetallic compound during use at high temperatures, so that the solid solution strengthening action decreases to the level of solid solution addition steel after long-term use at high temperatures. Since the toughness deterioration due to the intermetallic compound occurs, the upper limit was made 0.6%.

(2) Precipitate formation conditions 1st heating process In order to precipitate fine MX, it is necessary to make the solid MX which precipitated in the manufacturing processes of raw materials, such as hot rolling, form a solid solution. For that purpose, it must be heated to a temperature equal to or higher than the solid solution temperature of MX, and the temperature rises as the amount of constituent elements increases. That is, in order to form a solid solution, it is necessary to raise the temperature corresponding to the solubility product of MX represented by (C + N) × (Nb + V).
Based on this concept, T ₁ depending on the amounts of C, N, Nb, and V as shown in Equation 1 was selected as an industrial temperature for re-dissolving coarse MX. The constant 1050 in Equation 1 is a temperature (° C.) for making various processing and transformation structures containing a large amount of lattice defects into a complete austenite structure, and the coefficient 3427 of the first term was obtained empirically. Is.
Such tissue control is basically based on the diffusion process, and the time t ₁ (minute) for diffusion shown in Equation 2 is required. The constant 10 in Equation 2 is the time (minutes) required to make a material manufactured with various histories into a complete austenite structure, and the coefficient 0.3 of the first term is increased with the increase in the thickness th. Since the size of coarse MX generated in the raw material manufacturing process is increased, the diffusion time necessary for solid solution is increased, and the coefficient is obtained empirically.

B. Cooling step Next, in cooling from the solid solution heating temperature, if the cooling rate is less than 10 ° C./sec, coarse precipitation occurs during cooling, and the desired high density dispersion of fine MX cannot be obtained. Therefore, in order to suppress this precipitation during cooling, it is necessary to set the cooling rate at the center of the thickness where the cooling rate is the slowest to 10 ° C./sec or more.

C. Second heating step C is a precipitate-forming elements in this manner, N, Nb, materials that are sufficiently solid solution V, then, 450 ° C. to 600 ° C. temperature range T ₂ time is shown in Equation 3 at t By performing the process of ₂ heating (2nd heating process), very fine MX can be deposited. That is, this heat treatment prepares precipitation nuclei for obtaining fine MX with high density. Between the temperature of the second heating step and the precipitation state, there is a relationship that the lower the temperature, the smaller the critical size of precipitation nuclei and the greater the degree of supersaturation, resulting in finer and higher density precipitation. . However, at 450 ° C. or lower, since diffusion is slow, the heating time becomes long and the economy is impaired. On the other hand, since the precipitation density decreases at 600 ° C. or higher, this temperature range and time were determined.
Since both the temperature and the time affect the phenomenon accompanied by the thermal activation process such as diffusion, the temperature T ₂ and the time t _{2 of the second} heating step are further determined by the Larson Miller index (Equation 3). The condition range of was set.

D. The third heating step followed by performing a 650 ° C. to 780 ° C. in the step of the time t ₃ heating depicted in Equation 4 in a temperature range T ₃ (third heating step), growth at high density has been prepared precipitation nuclei As a result, a fine and stable MX high-density dispersion state can be realized. Below 650 ° C., the diffusion rate is slow, so a long time is required and the economy is impaired. On the other hand, since the reverse transformation to austenite occurs and MX disappears at 780 ° C. or higher, this temperature range was determined. Further, since diffusion continues at a high temperature, the precipitates agglomerate and become coarse, that is, so-called Ostwald growth, as time increases. Therefore, in order to ensure a high-density dispersion state of fine MX, the condition range of the temperature T ₃ and the time t ₃ of this third heating step is further determined by the Larson Miller index (Equation 4) relating the temperature and time. It was set.

According to the manufacturing method of the present invention described above, fine MX can be precipitated at high density, so that a stable precipitation state can be realized at high temperature, and high creep ductility and precipitation strengthening mechanism can be achieved at high temperature for a long time. Can be secured.
Thereby, the high chromium ferritic steel excellent in creep ductility and toughness can be manufactured.
Therefore, according to the present invention, it is possible to realize a high chromium ferritic steel that can be applied to applications in which thermal stress accompanying temperature fluctuation is repeatedly applied, such as a structure of a fast breeder reactor (FBR).

  By weight, C (carbon) is 0.04%, N (nitrogen) is 0.02%, Cr (chromium) is 9.0%, Nb (niobium) is 0.05%, and V (vanadium) is 0. 10%, Mo (molybdenum) content of 0.5%, W (tungsten) content of 0.2%, the remainder using Fe (iron) and 15mm thick ferrite steel plate consisting of inevitable impurities, The ferritic steel sheet was subjected to heat treatment under different heat treatment conditions, and the microstructure after the heat treatment was examined.

As an example of the present invention, the first heating step is performed at 1100 ° C. for 30 minutes, the cooling step is performed at a cooling rate of 80 ° C./sec by water cooling, and the precipitation nucleus preparation process is performed at 550 ° C. for 1 hour (first step) 2), a process of heating at 750 ° C. for 1 hour (third heating process) was performed.
Since T ₁ = 1081 ° C. in (Equation 1) from the above-described content, the condition for performing the first heating step at a temperature equal to or higher than T ₁ is satisfied. Further, since the thickness of 15 mm becomes t ₁ = 14.5 minutes in (Expression 2), the condition of the heating time is satisfied. Furthermore, from T ₂ = 550 ° C. and t ₂ = 1, the index 1 in Equation 3 is 16460, which satisfies the condition of the second heating step. Then, from T ₃ = 750 ° C. and t ₃ = 1, the index 2 of Equation 4 becomes 20460, which satisfies the condition of the third heating step.

As an example of the present invention, when the step of heating at 550 ° C. for 1 hour (second heating step) is performed as the precipitation nucleus preparatory treatment, and then the step of heating at 750 ° C. for 1 hour (third heating step) is performed. In addition, the distribution of the particle diameter d of MX precipitated on each ferritic steel was examined in the case where only the step of heating at 750 ° C. for 1 hour was performed without performing the step of preparing the precipitation nuclei. The conditions were exactly the same except for the precipitation nucleus preparation process.
FIG. 1 shows a comparison of the distribution of the particle diameter d of MX precipitated in each case. FIG. 1 also shows a photograph of the electron microscope structure of the extracted replica sample.

  As can be seen from FIG. 1, when the precipitation nucleus preparation process (second heating process) is performed, fine particles can be precipitated at a high density.

Next, as an example of the present invention, after the first heating step is performed at 1100 ° C., the cooling step is performed at a cooling rate of 80 ° C./sec by water cooling, and as a comparative example, 1 cooling is performed from 1100 ° C. by air cooling. The distribution of the particle diameter d of MX precipitated on each ferritic steel was examined in the case of cooling at ° C / sec. In addition, it was set as the completely same conditions after the 2nd heating process after cooling.
FIG. 2 shows a comparison of the distribution of the particle diameter d of MX precipitated in each case. FIG. 2 also shows a photograph of the electron microscopic structure of the extracted replica sample.

As can be seen from FIG. 2, when the cooling step is performed with water cooling at a cooling rate of 10 ° C./sec or more and 80 ° C./sec, fine particles can be precipitated at high density.
On the other hand, it can be seen that in air cooling (AC) in which the cooling rate is as low as 1 ° C./sec, the precipitates become coarse and the density also decreases.

  The present invention is not limited to the above-described embodiments, and various other configurations can be taken without departing from the gist of the present invention.

  The high chromium ferritic steel obtained by the ferritic steel manufacturing method of the present invention can be used as a steel plate, a steel pipe, and a forged product for a fast breeder reactor (FBR) structure.

It is the figure which compared the distribution of the particle diameter of MX which precipitates on each ferritic steel with the case where it does not perform the process of the precipitation nucleus preparation process. It is the figure which compared distribution of the particle diameter of MX which precipitates in each ferritic steel, when the cooling process is performed at a cooling rate of 80 ° C./sec and when cooling is performed at 1 ° C./sec.

Claims (2)

By weight, C: 0.01 to 0.05%, N: 0.01 to 0.05%, Cr: 8 to 13%, Nb: 0.03 to 0.07%, V: 0.05 to A ferrite steel having a thickness of th [mm], containing 0.20%, the balance being Fe and inevitable impurities, is not less than the temperature T ₁ [° C.] shown in the following formula 1 and is shown in the following formula 2. heating for t ₁ [min] or more;
T ₁ = 3427 × (C + N) × (Nb + V) +1050 (Formula 1)
(However, C, N, Nb, and V are the content (% by weight) of each element)
t ₁ = 0.3th + 10 (Formula 2)
Then, the step of cooling under the condition that the thickness center position of the ferritic steel becomes a cooling rate of 10 ° C./sec or more,
Then, at a temperature T ₂ [° C.] in the range of 450 to 600 ° C., a time t ₂ [hour] in which the index 1 shown in the following formula 3 becomes 15500 to 17300;
Index 1 = (T ₂ +273) (logt ₂ +20) (Formula 3)
Then, at a temperature T ₃ [° C.] in the range of 650 to 780 ° C., a time t ₃ [hour] in which the index 2 shown in the following formula 4 becomes 19600 to 21100 is performed. Index 2 = (T ₃ +273 ) (Logt ₃ +20) (Formula 4)
A method for producing a ferritic steel, characterized in that   The ferritic steel is further configured to contain, by weight, either Mo: 0.2 to 0.6%, W: 0.2 to 0.6%, or both as components. The manufacturing method of the ferritic steel of Claim 1.