JPH09291308A - Production of high chrome ferritic heat resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high Cr ferritic heat resistant steel excellent in toughness by controlling the component ratio of a metallic structure. SOLUTION: This ferritic heat resistant steel is produced by applying hot working and further specific heat treatment to a steel having a composition which consists of, by weight, 0.01-0.15% C, 0.01-0.80% Si, 0.05-1.50% Mn, 8.00-13.00% Cr, 0.05-1.50% Mo, 0.05-4.00% W, 0.05-0.50% V, 0.02-0.15% Nb, 0.002-0.050% Al, 0.010-0.110% N, <=0.030% P, <=0.010%, S, <=0.015% O, 0.001-0.030% B, further one or more kinds among 0.01-3.00% Ni, 0.01-5.00% Co, and 0.01-5.00% Cu, and the balance Fe with inevitable impurities and in which Cr, Ni, Co, and Cu within the above compositional ranges satisfy a relational inequality, Cr-2Ni-2Co-Cu<=9 and further a relational inequality of Ni/59+ Co/59+N/(Nb+V)>=Mo/96+W/184 within the above compositional ranges is satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Cr ferritic heat-resistant steel, and more particularly to a method for producing a ferritic Cr-containing boiler steel pipe steel excellent in creep rupture characteristics and toughness at high temperatures. Is.

[0002]

2. Description of the Related Art In recent years, in thermal power generation, steam conditions have been promoted to high temperature and high pressure from the viewpoint of improving thermal efficiency, and a plan to increase the current supercritical pressure condition to an ultra-supercritical pressure condition through an intermediate step has been promoted. . With such trends in power generation conditions, 2.25 which is currently used from the viewpoint of oxidation resistance and high temperature strength in selecting materials for boiler tubes and the like.
It is difficult to apply Cr-1Mo steel. On the other hand, application of austenitic heat-resistant steel can be considered, but there is a problem such as an increase in cost. Therefore, development of a high-strength, high-toughness ferritic heat-resistant steel located between the two is desired.

In view of such circumstances, a new steel type having a creep rupture strength much higher than that of conventional materials has been developed and proposed. So far, 9Cr-1Mo steel and 9Cr-
High-Cr ferritic heat-resistant steels such as 2Mo steel have been proposed, but all of them are difficult to apply from the viewpoint of creep rupture strength under the above super-supercritical pressure steam conditions.

Steels having improved required characteristics have been developed, and the relationship between (Mo + W) and the amount of Nb has been established to improve the creep characteristics and toughness. Further, the optimum range of W for improving the creep strength is W. , Nb addition is known to be effective. However, the conventional heat treatment methods represented by the normalizing and tempering treatments have not been able to bring out sufficient characteristics.

Japanese Patent Publication No. 6-74452 discloses improvement in strength and toughness by precipitation of intermetallic compounds. However, since heat treatment for a long time precipitates not only intermetallic compounds but also carbides and causes coarsening, a significant improvement in strength cannot be expected, and there is a concern that toughness may decrease and this may lead to cost increase.

Further, Japanese Patent Publication No. 2-267217 discloses a method in which an intermediate heat treatment is carried out at a low temperature after quenching or normalizing to completely decompose residual austenite and to disperse carbonitrides by tempering to improve high temperature strength. Although disclosed, the improvement of high temperature strength cannot be expected only by complete decomposition of retained austenite.

Further, Japanese Patent Publication No. 6-306550 discloses a method for improving the high temperature strength by precipitating 2.5 to 7% of precipitates on grain boundaries, martensite lath boundaries and inside martensite lath by heat treatment in a steam turbine member. Although disclosed, when it precipitates at grain boundaries or boundaries, it is not possible to expect a significant improvement in high-temperature strength, and there is concern that toughness may decrease.

[0008]

DISCLOSURE OF THE INVENTION The present invention improves the conventional drawbacks as described above and controls the composition ratio of the metal structure so that it can be used in an ultra-supercritical boiler, etc. High Cr with improved impact absorption energy at 0 ° C and creep rupture strength in the test
The purpose is to provide ferritic heat-resistant steel.

[0009]

In order to achieve the above-mentioned object, the present invention optimizes the alloy components and optimizes the amounts of addition of Mo and W, and at the same time, Ni, Co and Cu.
The present invention provides a ferritic heat-resistant steel excellent in high-temperature strength and toughness by suppressing the formation of δ-ferrite by controlling the compositional ratio of the metal structure by optimizing heat treatment conditions.

That is, in% by weight, C: 0.01 to 0.15%, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13.00. %, Mo: 0.05 to 1.50%, W: 0.05 to 4.00%, V: 0.05 to 0.50%, Nb: 0.02 to 0.15%, Al: 0. 002 to 0.050%, N: 0.010 to 0.110%, P: 0.030% or less, S: 0.010% or less, O: 0.015% or less, further necessary B: 0.001 to 0.030%, and N
i: 0.01 to 3.00%, Co: 0.01 to 5.0
0%, Cu: 0.01 to 5.00% containing one or more,
Further, Cr, Ni, Co and Cu in the above component range satisfy the relational expression of Cr-2Ni-2Co-Cu≤9, and further Ni / 59 + Co / 59 + N / (Nb + V) ≥Mo / 9 in the above component range.
After satisfying the relational expression of 6 + W / 184 and hot-working the steel with the balance being Fe and unavoidable impurities, after normalizing by keeping the temperature at [Ac ₃ points + 50 ° C] or higher and lowering to room temperature, Ac _A method for producing a ferritic heat-resistant steel characterized by performing intermediate heat treatment in a temperature range of ₁ point or higher and [Ac ₁ point + 40 ° C.] or lower, and then tempering it in a temperature range of 650 ° C. or higher and 730 or lower. In the above steel, Ac ₁ and Ac ₃ can be calculated by the following formula. _{Ac 1 = 723 + 16.9Cr + 29.1Si} + 6.38W-15Mn -250C-250N-60Ni-20Co-15Cu Ac 3 = 910 + 50Si + 45Mo + 40W + 120V + 400Al -11Cr-30Mn-15Ni-20Cu-100C-5Co

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention provides a method for producing a high Cr ferritic heat resistant steel having excellent toughness and high creep rupture strength by introducing untempered martensite. The present inventors have focused on the heat treatment characteristics of high Cr ferritic heat-resistant steel used for headers of thermal power generation boilers, etc., by performing intermediate heat treatment between normalizing and tempering, untempered martensite is removed after intermediate heat treatment. It was found that by introducing a part of them, it is possible to improve the creep rupture strength by changing the composition ratio of the metal structure.

Further, according to the present invention, in order to improve creep rupture strength and toughness, a martensite single phase structure free of δ-ferrite is used to form Cr, Ni, Co.
And an appropriate balance of C content is defined. The present inventors have found that if Cr, Ni, Co and Cu within the range of the components of the present invention satisfy Cr-2Ni-2Co-Cu ≦ 9, the formation of δ ferrite can be suppressed and the creep strength and toughness can be improved. It was

Further, Ni / 59 + Co / 59 + N / (Nb + V) ≧ Mo / 9
It was found that untempered martensite can be introduced by the intermediate heat treatment by satisfying the relational expression of 6 + W / 184.

The reason why each condition of the heat treatment is limited as described above in the present invention will be described below. In order to sufficiently obtain the effect of solid solution of the elements by normalizing, the lower limit temperature of normalizing is set to [Ac ₃ point + 50 ° C.] or higher. The cooling temperature was completed at room temperature in order to complete the martensite transformation and obtain a martensite single-phase structure.

In the intermediate heat treatment after normalizing, 5% to 20% of untempered martensite structure is introduced, and at the same time the holding temperature of the intermediate heat treatment is set to Ac ₁ point or more [Ac _{1 in} order to deposit precipitates in the tempered martensite. Point + 40 ° C]
The temperature range was as follows. When the temperature is below this temperature range, the tempered martensite structure is entirely formed, the untempered martensite structure is not introduced, and only the conventional creep rupture properties and toughness are obtained. In addition, although the creep rupture strength also improves with an increase in the composition ratio of untempered martensite, in the temperature range above this, the composition ratio of untempered martensite exceeds 20% and the toughness decreases, so the upper limit temperature Was set to [Ac ₁ point + 40 ° C.] or less.

The temperature range for tempering is set to a temperature range of 650 ° C. or higher and 730 ° C. or lower in order to precipitate finely and in large quantities a precipitate and an intermetallic compound which enhance the effect of precipitation strengthening.

The reasons for limiting the range of each component of the steel used in the present invention as described above will be described below. C is mainly M
C (M is an alloying element, the same applies below) and M ₂₃ C ₆
It deposits as a type of charcoal and has a great effect on strength and toughness. If it is less than 0.01%, the amount of precipitation is small and it is insufficient for precipitation strengthening, and if it exceeds 0.15%, the toughness is reduced and the agglomeration and coarsening of carbides are promoted, and the creep rupture strength at high temperature and long time side is reduced. 0.01 to 0.15
%.

Si is added for the purpose of deoxidizing effect, ensuring strength and oxidation resistance, but is an element which adversely affects toughness. Therefore, the lower limit is 0.01% from the viewpoint of deoxidation, strength, and oxidation resistance, and the upper limit is 0.80% from the viewpoint of toughness. Mn is an element required not only for deoxidation but also for improving strength, and it is necessary to add at least 0.05% or more. However, excessive addition lowers the high temperature strength and toughness, so the upper limit was made 1.50%.

Cr is an essential element for ensuring high temperature oxidation resistance, has an effect of precipitating M ₂₃ C ₆ type carbide in the matrix, and enhances high temperature strength.
If it is less than 8.0%, the oxidation resistance at high temperature becomes insufficient and the high temperature strength also decreases. On the other hand, if the content is 13.0% or more, it becomes difficult to suppress δ ferrite, and the strength and toughness decrease.
The amount of Cr is limited to 8.0 to 13.0%.

Mo provides solid solution strengthening, and at the same time M ₂₃
Stabilizes C ₆ and provides high temperature strength. If it is less than 0.05%, the effect is small, and if it exceeds 1.50%, the formation of δ ferrite is promoted, and at the same time, the precipitation of M ₆ C and the Laves phase and the coarsening of aggregation are promoted.
%.

W contributes to solid solution strengthening and fine precipitation of M ₂₃ C ₆ , and at the same time suppresses agglomeration and coarsening of carbides and remarkably improves creep rupture strength at high temperature and long time. At least 0.
It is necessary to be 05% or more, but if it exceeds 4.0%, δ ferrite and a coarse Laves phase are likely to be formed and the high temperature strength and toughness are deteriorated, so the content was made 0.05 to 4.0%.

V precipitates fine carbonitrides as a precipitation strengthening element and enhances high temperature strength. If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.50%, not only V (C, N) is coarsened, but also the amount of C that can be precipitated as M ₂₃ C ₆ is decreased, and high temperature strength is increased. Of 0.05 to 0.
Limited to 50%.

Nb is precipitated as a carbonitride to increase the high temperature strength and to improve the toughness due to the effect of the refinement of the structure, so Nb must be at least 0.02%. But 0.15
If it is excessively added in excess of%, it cannot be completely dissolved in the matrix at the normalizing temperature, and a sufficient strengthening effect cannot be obtained, so the content is limited to 0.02 to 0.15%.

N is one of the important elements for precipitating nitrides or carbonitrides and increasing the high temperature strength. The effect is exhibited by addition of 0.01% or more, but if it exceeds 0.11%, not only does it cause coarsening of the nitride and decrease in toughness, but also it becomes difficult in manufacturing, so 0.01 to 0 is added. Limited to 11%.

Al is used as a deoxidizer, and its amount has a great influence on the crystal grain size and mechanical properties. 0.00
If it is less than 2%, it is insufficient as a deoxidizing material, and if it exceeds 0.05%, the creep rupture strength decreases, so 0.002 to 0.05
Limited to 0%.

Since P adversely affects temper embrittlement and reheat cracking susceptibility, the upper limit was made 0.030%. Since S causes deterioration in toughness, anisotropy and increase in reheat cracking susceptibility, the upper limit was made 0.010%. Since O causes the formation of oxides that adversely affect toughness, the upper limit is 0.015.
%.

Ni is an austenite forming element, and δ
It has an effect of suppressing the formation of ferrite and is also effective in improving the toughness, and 0.01% is the minimum. But,
If it exceeds 3.00%, the coarsening of precipitates will occur, and the creep rupture strength on the long-term side will decrease, so the upper limit is 3.00%.
And

The positive use of Co is one of the major features of the present invention. Co is an austenite forming element,
At the same time as suppressing the formation of δ ferrite, it stabilizes the precipitates and enhances the high temperature strength. If it is less than 0.01%, the effect is small, and if it exceeds 5.00%, the cost is high and embrittlement easily occurs, so the content is limited to 0.01 to 5.00%.

Cu is an austenite forming element, and δ
Suppress the formation of ferrite. If it is less than 0.01%, the effect is small, and if it exceeds 5.00%, embrittlement easily occurs, so the content is limited to 0.01 to 5.00%.

The steel according to the method of the present invention can be provided not only as a steel pipe but also in the form of a thick plate and a thin plate, and can be used in the form of various heat-resistant materials by using a heat-treated plate. is there. Rolling is an example of hot working of the steel of the present invention, but the effect of the present invention does not change in forging or the like, and does not depend on the hot working method.

[0031]

EXAMPLES Table 1 shows the chemical components of the test steel. These steels were melted in a vacuum furnace, and a plate having a plate thickness of 15 mm was manufactured by hot rolling, and heat treatment was performed under the conditions shown in FIGS. 1, 2 and Table 2. 1 is a drawing showing the heat treatment method of the present invention, and FIG. 2 is a drawing showing the conventional heat treatment method.

[0032]

[Table 1]

[0033]

[Table 2]

A test piece was taken from the center of the thickness of each heat-treated plate material, and a creep rupture test and an impact test were carried out. Figure 3 shows the effect of heat treatment conditions on the estimated creep rupture strength at 600 ° C for 100,000 hours obtained by linear extrapolation using data up to 600 ° C for 10,000 hours. When steel is produced by the method of the present invention, the creep rupture strength does not decrease and exceeds the target value of 150 MPa.

[0035]

As described above, the steel produced by the method of the present invention is a steel that achieves an increase in high-temperature strength that can cope with higher temperatures and higher pressures of equipment, as compared with conventional ferritic heat-resistant steels, and has practical toughness. It has excellent characteristics, can be applied to many fields such as ultra-supercritical thermal power generation, nuclear power generation, etc., and has a great contribution to the industrial world.

[Brief description of drawings]

FIG. 1 is a chart showing a heat treatment method of the present invention.

FIG. 2 is a chart showing a conventional heat treatment method.

FIG. 3 is a table showing a comparison of creep rupture strength between steel according to the method of the present invention and conventional steel.

Claims (2)

[Claims] 1. By weight%, C: 0.01 to 0.15%, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13.00. %, Mo: 0.05 to 1.50%, W: 0.05 to 4.00%, V: 0.05 to 0.50%, Nb: 0.02 to 0.15%, Al: 0. 002 to 0.050%, N: 0.010 to 0.110%, P: 0.030% or less, S: 0.010% or less, O: 0.015% or less, and further Ni : 0.01 to 3.00%, Co: 0.01 to 5.00%, Cu: 0.01 to 5.00%, with the balance being Fe and inevitable impurities, and Cr, Ni, Co and C in the composition range
u satisfies the relational expression of Cr-2Ni-2Co-Cu ≦ 9, and further, Ni / 59 + Co / 59 + N / (Nb + V) ≧ Mo / 9 in the above component range.
After satisfying the relational expression of 6 + W / 184 and hot-working steel with the balance being Fe and unavoidable impurities, after normalizing by keeping the temperature above [Ac ₃ points + 50 ° C] and lowering to room temperature, Ac _A method for producing a ferritic heat-resistant steel, which comprises performing intermediate heat treatment in a temperature range of ₁ point or higher and [Ac ₁ point + 40 ° C.] or lower, and then tempering it in a temperature range of 650 ° C. or higher and 730 or lower. 2. C: 0.01 to 0.15% by weight, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13.00. %, Mo: 0.05 to 1.50%, W: 0.05 to 4.00%, V: 0.05 to 0.50%, Nb: 0.02 to 0.15%, Al: 0. 002 to 0.050%, N: 0.010 to 0.110%, P: 0.030% or less, S: 0.010% or less, O: 0.015% or less, and further B : 0.001 to 0.030%, Ni: 0.01 to 3.00%, Co: 0.01 to 5.00%, Cu: 0.01 to 5.00% Including Cr, the balance consisting of Fe and unavoidable impurities, and Cr, Ni, Co and Cu in the above component range are Cr-2Ni-2Co-C. Satisfy ≦ 9 relationship, further the component range of the Ni / 59 + Co / 59 + N / (Nb + V) ≧ Mo / 9
After satisfying the relational expression of 6 + W / 184 and hot-working steel with the balance being Fe and unavoidable impurities, after normalizing by keeping the temperature above [Ac ₃ points + 50 ° C] and lowering to room temperature, Ac _A method for producing a ferritic heat-resistant steel, which comprises performing intermediate heat treatment in a temperature range of ₁ point or higher and [Ac ₁ point + 40 ° C.] or lower, and then tempering it in a temperature range of 650 ° C. or higher and 730 or lower.