JPH10140280A - Low cr ferritic steel excellent in high temperature strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low Cr ferritic steel excellent in high temp. strength and having superior weldability. SOLUTION: This ferritic steel has a composition which consists of, by weight, 0.03-0.12% C, 0.05-0.7% Si, 0.05-1% Mn, 0.002-0.025% P, 0.002-0.015% S, 0.8-3% Cr, 0.01-1% Ni, 0.1-1% Mo, 0.05-0.5% V, 0.1-3% W, 0.01-0.16% Nb, 0.01-0.16% Ta, 0.003-0.05% Al, 0.0001-0.01% B, 0.003-0.03% N, and the balance iron with inevitable impurities and in which the additive quaritities of Nb and Ta satisfy the condition of 0.02%<=Nb+Ta<=0.17%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat generator such as a steam generator and a boiler used under high temperature and high pressure, and
Low C suitable for use in high-temperature environments of over ℃
r For ferritic steel.

[0002]

2. Description of the Related Art Austenitic stainless steel, high Cr ferritic steel having a Cr content of 9 to 12%, and high temperature heat and pressure resistant members for boilers, chemical industries, nuclear power, and the like.
-There is a low Cr ferrite steel represented by a 1 / 4Cr-1Mo steel, a carbon steel, and the like (in this specification, the content of all alloy components is% by weight). These are selected according to the use temperature, pressure, use environment and the like of the target member and in consideration of economic efficiency. In recent years, steam conditions have become higher temperature and pressure, and high temperature strength characteristics are required rather than corrosion resistance. Therefore, high strength high Cr ferritic steel is used even when it is disadvantageous in cost. Low Cr ferritic steel is inexpensive because it has a lower Cr content than high Cr ferritic steel, and if its high temperature strength is equal to or higher than that of high Cr ferritic steel, it is expected to be used in places where corrosion resistance does not pose a problem.

[0003]

SUMMARY OF THE INVENTION Therefore, the present invention relates to 50
Low Cr ferritic steel that significantly improves high temperature creep strength at temperatures of 0 ° C or higher and has toughness, workability, and weldability equal to or higher than existing low alloy steels, and can be used as a substitute for high Cr ferritic steel. Is provided.

[0004]

Means for Solving the Problems The present invention provides (1) weight%
, C: 0.03-0.12%, Si: 0.05-0.
7%, Mn: 0.05-1%, P: 0.002-0.0
25%, S: 0.002 to 0.015%, Cr: 0.8
-3%, Ni: 0.01-1%, Mo: 0.1-1%,
V: 0.05 to 0.5%, W: 0.1 to 3%, Nb:
0.01 to 0.16%, Ta: 0.01 to 0.16%,
Al: 0.003-0.05%, B: 0.0001-
0.01%, N: 0.003 to 0.03%, the balance being iron and unavoidable impurities, and Nb and Ta
(2) In addition to the above components, 0.01 to 0.2% by weight in addition to the above components.
%, One or two of La, Ce, Y and Ca
An object of the present invention is to provide a low Cr ferrite steel characterized by having excellent high-temperature strength containing at least one or more kinds. The function of each component and the reason for selecting the content will be described below.

Embedded image 0.02% ≦ Nb + Ta ≦ 0.17% (a)

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION C combines with Cr, Fe, W, V, and Nb to form a carbide and contributes to high-temperature strength, and itself stabilizes the structure as an austenite stabilizing element. If it is less than 0.03%, carbide precipitation is insufficient, and if it exceeds 0.12%, carbides are excessively precipitated and the steel is extremely hardened to deteriorate workability. That is, C
Is 0.03 to 0.12%.

[0006] Si acts as a deoxidizing agent and is an element that enhances the steam oxidation resistance. However, if it exceeds 0.7%, the toughness is significantly reduced, and is harmful to the strength. Particularly in large forged products, tempering embrittlement is promoted, so the Si content is set to 0.05 to 0.7%.

Mn is effective for improving the hot workability of steel and stabilizing the structure. However, if it is less than 0.05%, a sufficient effect cannot be obtained. If it exceeds 1%, the steel is hardened to improve workability. In addition to damaging, it increases the temper embrittlement susceptibility like Si. Therefore, the content of Mn is set to 0.05 to 1%.

Both P and S are elements harmful to toughness and workability. Even if the amount of S is extremely small, it makes grain boundaries and Cr ₂ O ₃ scale films unstable, causing deterioration in strength, toughness and workability. Therefore, it is better to be as small as possible even within the above-mentioned allowable range. As an inevitable content, P is 0.002 to 0.0
25%, S was 0.002 to 0.015%.

[0009] Cr is an indispensable element from the viewpoints of oxidation resistance and high-temperature corrosion resistance of low alloy steel, and if its content is less than 0.8%, sufficient oxidation resistance and high-temperature corrosion resistance cannot be obtained. on the other hand,
If it exceeds 3%, strength and toughness are impaired. Therefore,
The content of Cr is set to 0.8 to 3%.

Ni is an austenite stabilizing element,
And it contributes to improvement of toughness, but if its content exceeds 1%, the high temperature creep strength is impaired. Also, in view of economy, large-scale addition is disadvantageous. Therefore, the content of Ni is 0.01 to
1%.

Mo is effective in improving the creep strength, but if it is less than 0.1%, a sufficient effect cannot be obtained. If it exceeds 1%, not only the toughness is lowered but also the effect on the strength is lost. Therefore, it is set to 0.1 to 1%.

V combines with C and N to form fine precipitates such as V (C, N). The precipitate greatly contributes to the improvement of the long-term creep strength at a high temperature. However, if it is less than 0.05%, a sufficient effect cannot be obtained, and if it exceeds 0.5%, the creep strength is rather deteriorated. Therefore, the appropriate content of V is 0.05 to 0.5%.

W is effective for improving creep strength as a solid solution strengthening element and a fine carbide precipitation strengthening element. When W is less than 0.1%, it is ineffective, and when it exceeds 3%, steel is hardened and workability is impaired. 0.1 to 3%.

Nb and Ta, like V, combine with C and N to form Nb (C, N) and Ta (C, N) and contribute to creep strength. Particularly at a relatively low temperature of 600 ° C. or lower, a remarkable strength improving effect is exhibited. In the present invention, as a result of various studies on the effect on creep strength when V and Nb are added alone, it has been found that when Nb and Ta are simultaneously added (hereinafter referred to as composite addition), a remarkable improvement in strength is achieved. However, when added in excess, not only does the creep strength decrease, but also the toughness significantly decreases. If each is less than 0.01%, the above-mentioned effects cannot be obtained, and 0.1% or less.
If it exceeds 17%, undissolved NbC and TaC increase, and creep strength and toughness are impaired. Therefore, the amounts of Nb and Ta added are respectively 0.01% to 0.16%,
And 0.02% ≦ Nb + Ta ≦ 0.17%.

Al is indispensable as a deoxidizing element. If its content is less than 0.003%, it has no effect.
If it exceeds, the creep strength and workability are impaired.
The content of 1 is 0.003 to 0.05%.

B has an effect of dispersing and stabilizing carbides by adding a very small amount. If it is less than 0.0001%, the effect is small, and if it exceeds 0.01%, processability is impaired. Therefore, the addition of B reduces the content to 0.0001 to 0.01%.
Should be within the range.

N is necessary for carbonitride formation with V and Nb.
Less than 0.003% has no effect. However, if it exceeds 0.03%, the structure becomes finer and the nitride becomes coarser, thereby impairing the strength, toughness and workability. Therefore, the content of N is set to 0.03% or less, and 0.003 to 0.0
3%

La, Ce, Y and Ca are added as necessary for the purpose of controlling the morphology of impurities P, S and O and their precipitates (inclusions). When one or more of these elements are contained in an amount of 0.01% or more, the toughness, strength, workability, and weldability of the steel are improved by the above-described actions. If these contents are less than 0.01%, there is no effect, and if added more than 0.2%, inclusions increase and, on the contrary, toughness and strength are impaired. Therefore, when these elements are added, the content is 0.01 to 0.2%.

[0019]

EXAMPLE A steel having the chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace, and the ingot was forged at 1150 ° C. to 950 ° C. to form a plate having a thickness of 20 mm.

No. 1 and No. 1 steel. No. 2 steel is a component of existing typical low Cr ferritic steel,
Is a component corresponding to STBA22 and STBA24. No. No. 3 steel-No. No. 17 steel is No. No. 1 steel, no. 2
This is a comparative steel in which V and Nb or Ta are independently added to steel. A to M steels are the steels of the present invention, and Nb and Ta are compounded. No. 1 and No. 1 steel. The steel No. 2 was subjected to a normal heat treatment at 920 ° C. × 1 h · AC, followed by a tempering at 740 ° C. × 1 h · AC. No.
No. 3 steel-No. 17 steel and A to M steel are 1050 ° C x 1
After normalizing h · AC, tempering at 770 ° C. × 1 h · AC was performed.

[0021]

[Table 1]

In order to compare the mechanical properties, the comparative steel and the steel of the present invention were subjected to a tensile test, a Charpy impact test, and a creep rupture test. In the tensile test and the creep test, test pieces having a diameter of 6 mm and a distance between gauge points of 30 mm were sampled in the longitudinal direction of the plate. The tensile test was conducted at room temperature and 600 ° C., and the creep test was performed at 500 ° C., 550 ° C., 600 ° C., and 650 ° C. by performing a long-term fracture test up to about 10,000 hours.
The creep rupture strength at 00 ° C. × 10 ⁵ h was determined. In the Charpy impact test, a No. 4 test piece was sampled in the longitudinal direction of the plate in accordance with JIS Z2202, and a ductile-brittle fracture transition temperature was determined. Further, in order to evaluate the weldability, the thickness of the test plate was 20 mm, and the preheating temperature was 20 ° C., 50 ° C., 100 ° C.
C., 150.degree. C., and 200.degree. C., a y-type welding crack test according to JIS Z3158 was performed. The temperature at which the cross-sectional cracking rate was 0% was defined as the crack prevention temperature.

Table 2 shows the test results. The steel of the present invention shows equal or higher strength than the comparative steel in the tensile strength and 0.2% proof stress, and the same tendency is observed in the results of the high-temperature tensile test at 600 ° C. FIG. 1 shows the relationship between the amounts of Nb and Ta added and the creep rupture strength. The creep rupture strength at 600 ° C., when Nb and Ta are added in combination, is much higher than that of existing steel, and it can be seen that the creep rupture strength is significantly higher than that in the case where Nb and Ta are added alone. It is understood that the steel of the present invention requires the complex addition of Nb and Ta, and the complex addition can significantly improve the creep strength which could not be achieved by the single addition. From the y-type welding crack test, it can be seen that all the comparative steels require preheating of 150 ° C. or more, whereas the steel of the present invention can prevent cracking by preheating at 20 to 100 ° C. As described above, the steel of the present invention has a high-temperature strength and excellent weldability that are significantly higher than those of conventional steel, so that it is a material that can reduce the thickness of the heat-resistant part and reduce the preheating caused by welding. is there.

[0024]

[Table 2]

[0025]

The present invention significantly improves the high-temperature strength of conventional low-alloy steels and provides a low-Cr ferrite steel having high-temperature strength equal to or higher than that of high-Cr ferrite steel and excellent welding characteristics. Since this steel has excellent high-temperature strength, it can be used as a substitute for high Cr ferritic steel, and because of its excellent weldability, preheating during welding may be omitted, which is an advantage of ferritic steel. As a material having both toughness, workability, and economy, pipes as heat-resistant pressure-resistant members used in industrial fields such as boilers, chemical industry, nuclear power, etc.
It can be widely applied to plates and other forged products of various shapes.

[Brief description of the drawings]

FIG. 1 is a chart showing the relationship between the amounts of Nb and Ta added and creep rupture strength.

Claims (2)

[Claims] C .: 0.03 to 0.12% by weight,
Si: 0.05 to 0.7%, Mn: 0.05 to 1%,
P: 0.002 to 0.025%, S: 0.002 to 0.
015%, Cr: 0.8-3%, Ni: 0.01-1
%, Mo: 0.1-1%, V: 0.05-0.5%,
W: 0.1 to 3%, Nb: 0.01 to 0.16%, T
a: 0.01 to 0.16%, Al: 0.003 to 0.0
5%, B: 0.0001 to 0.01%, N: 0.003
A low Cr ferritic steel containing -0.03%, the balance consisting of iron and unavoidable impurities, and the addition amount of Nb and Ta being excellent in high-temperature strength satisfying the condition of the following formula (a). Embedded image 0.02% ≦ Nb + Ta ≦ 0.17% (a) 2. The composition according to claim 1, further comprising 0.01 to 0.2% by weight of La, Ce, Y and Ca.
A low Cr ferritic steel comprising one or more of them and having excellent high-temperature strength.