JPH101739A - Low alloy heat resistant steel, excellent in high temperature strength and weldability, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low alloy heat resistant steel excellent in weldability while maintaining sufficient high temp. strength required of a steel for high temp. use. SOLUTION: The low alloy heat resistant steel, excellent in high temp. strength and weldability, has a composition consisting of, by weight ratio, 0.02-0.10% C, 0.05-0.40% Si, 0.1-1.0% Mn, 2.0-3.0% Cr, 0.1-1.0% Mo, 0.5-2.0% W, 0.2-0.5% V, 0.02-0.12% Ti, 0.0010-0.0050% B, and the balance Fe with inevitable impurities and satisfying Mo+W=0.6 to 2.5%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-strength ferritic low-alloy heat-resistant steel excellent in high-temperature creep rupture strength and weldability and a method for producing the same.

[0002]

2. Description of the Related Art Various steels, from carbon steel to austenitic stainless steel, are used as high-temperature steels according to operating temperature, stress and environment. Among them, boiler steel is used under severe conditions.

[0003] In recent years, boilers are being planned to have high temperature and high pressure steam conditions, and materials having higher strength than conventional materials are required. At temperatures below 600 ° C, 2.25C
r-1Mo steel is mainly used. If the steam pressure is increased at the same temperature, it is possible to cope with the problem by increasing the wall thickness of the current 2.25Cr-1Mo steel, but the weight of the plant increases, which necessitates a drastic design change. .

As higher strength materials, for example, STBA
28 (9Cr-1Mo-Nb-V) is standardized to the thermal power standard, but high Cr materials are expensive and therefore have economical difficulties.

On the other hand, as a high-strength low-alloy steel, one characterized by the addition of Nb has been proposed (JP-A-2-21).
No. 7438, JP-A-2-217439, JP-A-3-8
No. 7333, JP-A-4-268040, JP-A-5-3
No. 45949).

However, the working steel and the low alloy steels mentioned above have insufficient strength. That is, when increasing the current steam pressure from 246 kg / cm ² to 350 kg / cm ² , in order to design with the same wall thickness, the strength is required to be about 1.5 times, and the current 2.25Cr-1Mo is used. 60
0 ℃ the allowable stress 4.2kgf / mm ² things allowable stress is 1.5 times of 2.8 kgf / mm ² but is required, it is not possible to also achieve such strength in any of the above steel There is a demand for higher strength steel. Further, since the boiler steel is used by welding, not only high strength at such a high temperature but also weldability is required.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and a low-alloy heat-resistant steel having excellent weldability while maintaining sufficient high-temperature strength as a high-temperature steel and its production. It is an object to provide a method.

[0008]

The present inventors have studied the effects of various additional elements on the creep rupture strength, toughness and weldability of a low alloy steel, and have obtained the following findings. (1) Mo and W as solid solution strengthening elements improve the creep rupture strength, but the effect is saturated and the toughness is reduced even if added by 2% or more by weight.

(2) Nb, Ti,
When V is added alone or in combination, there is steel in an appropriate amount range, but excessive addition decreases the creep rupture strength conversely. (3) Tensile and creep rupture strength is remarkably improved by sufficiently dissolving the carbide precipitation element at a high temperature of 1000 ° C. or higher and then tempering.

(4) By setting the amount of C to 0.1% or less and optimizing the amounts of elements for solid solution strengthening and precipitation strengthening, good weldability is exhibited without impairing creep rupture strength. The present invention has been completed based on such findings, and the weight ratio of C: 0.02 to 0.10% and Si: 0.05 to
0.40%, Mn: 0.1-1.0%, Cr: 2.0-
3.0%, Mo: 0.1-1.0%, W: 0.5-2.
0%, V: 0.2-0.5%, Ti: 0.02-0.1
2%, B: 0.0010 to 0.0050%, the balance consisting of Fe and inevitable impurities, Mo + W: 0.6 to
It is intended to provide a low-alloy heat-resistant steel excellent in high-temperature strength and weldability characterized by being 2.5%.

Further, C: 0.02 to 0.10 by weight ratio.
%, Si: 0.05 to 0.40%, Mn: 0.1 to 1.%.
0%, Cr: 2.0 to 3.0%, Mo: 0.1 to 1.0
%, W: 0.5 to 2.0%, V: 0.2 to 0.5%, T
i: 0.02 to 0.12%, B: 0.0010 to 0.0
Steel containing 050%, the balance being Fe and unavoidable impurities, Mo + W: 0.6-2.5%,
Normalized at a temperature in the range of ~ 1100 ° C and 700 ° C or more
An object of the present invention is to provide a method for producing a low-alloy heat-resistant steel excellent in high-temperature strength and weldability, characterized by tempering at a temperature within _one point or less.

Further, C: 0.02 to 0.1 in weight ratio.
0%, Si: 0.05 to 0.40%, Mn: 0.1 to
1.0%, Cr: 2.0 to 3.0%, Mo: 0.1 to
1.0%, W: 0.5 to 2.0%, V: 0.2 to 0.5
%, Ti: 0.02 to 0.12%, B: 0.0010
Steel containing 0.0050%, the balance being Fe and unavoidable impurities, Mo + W: 0.6-2.5%
After heating at a temperature of at least 00 ° C. and processing at least 30% at a temperature in the range of 1000 to 800 ° C., cooling to a temperature of at most 200 ° C. and baking at a temperature of at least 700 ° C. and at most _one point of Ac. An object of the present invention is to provide a method for producing a low-alloy heat-resistant steel excellent in high-temperature strength and weldability characterized by returning.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. The low-alloy heat-resistant steel according to the present invention has a C:
0.02 to 0.10%, Si: 0.05 to 0.40%,
Mn: 0.1-1.0%, Cr: 2.0-3.0%, M
o: 0.1 to 1.0%, W: 0.5 to 2.0%, V:
0.2-0.5%, Ti: 0.02-0.12%, B:
0.0010 to 0.0050%, and Mo +
W: 0.6 to 2.5%.

The reasons for limiting each component will be described below. C: 0.02 to 0.10% C forms carbides of the MC type with Ti, V, and carbides of Cr and M ₂₃ C ₆ , and also forms carbides of Mo and W, thereby reducing the tensile strength and creep rupture strength. It is an element to improve. However, if the content is 0.02% or less, the amount of carbide precipitated is small and sufficient strength cannot be obtained. On the other hand, if it exceeds 0.10%, toughness is reduced and weld cracks occur. Therefore, the C content is in the range of 0.02 to 0.10%.

Si: 0.05 to 0.40% Si is added as a deoxidizing agent, but its content is 0.1%.
If it is less than 05%, deoxidation will not be sufficient and the toughness will decrease. Further, Si is effective from the viewpoint of oxidation resistance,
If it exceeds 0%, it promotes agglomeration and coarsening of carbides, lowers creep rupture strength and lowers toughness after tempering.
Therefore, the Si content is in the range of 0.05 to 0.4%.

Mn: 0.1-1.0% Mn is added as a deoxidizing and desulfurizing agent, but if its content is less than 0.1%, its effect is not sufficient.
Reduces toughness as well. Therefore, the Mn content is set in the range of 0.1 to 1.0%.

Cr: 2.0-3.0% Cr is effective for oxidation resistance, forms M ₂₃ C ₆ and improves the high-temperature strength. Is not sufficient, and if it exceeds 3.0%, the amount of MC carbide formed is reduced, and the creep rupture strength is lowered. Therefore, the amount of Cr is set in the range of 2.0 to 3.0%.

Mo: 0.1-1.0%, W: 0.5-
2.0%, Mo + W: 0.6-2.5% Mo, W is effective for high-temperature strength as a solid-solution strengthening element and a carbide-forming element, but excessive addition increases economic efficiency because it is an expensive element. Spoil. Further, it is an element that is easily segregated at the time of solidification, and causes non-uniformity of the material to lower toughness and the like. Therefore, these contents are set to Mo: 0.1 to 1.0%, W:
The range is 0.5 to 2.0%, and the total content is 0.6 to 2.5%.

V: 0.2 to 0.5% V is an element which forms VC and is effective for improving high-temperature strength, particularly creep rupture strength on the high-temperature side. However, if the content is less than 0.2%, the amount of precipitation as VC is small, so that sufficient creep rupture strength cannot be obtained. On the other hand, 0.5
%, Even if high-temperature heating of 1000 ° C. or more is carried out, undissolved coarse VC remains, causing a decrease in creep rupture strength and a decrease in toughness. Therefore, the V content is set in the range of 0.2 to 0.5%.

Ti: 0.02 to 0.12% Ti, like V, is an MC carbide forming element and precipitates as TiC to improve creep rupture strength. But,
If less than 0.02%, the effect is not exhibited, and 0.12%
If the temperature is higher than that of V, undissolved Ti
C remains and reduces the creep rupture strength. Therefore, the Ti content is in the range of 0.02 to 0.12%.

B: 0.0010 to 0.0050% B is an element effective for improving the creep rupture strength.
If less than 0.0010%, the effect is not recognized, and 0.0
If it exceeds 050%, the hot workability will be reduced, which may cause scratches and the like. Therefore, the B content is 0.0010
The range is 0.0050%.

Next, the manufacturing method will be described. First, in the manufacturing method according to the second aspect of the present invention, the steel
Normalization is performed at a temperature in the range of 00 to 1100 ° C., and tempering is performed at a temperature in the range of 700 ° C. or more and _one Ac or less.

The normalization is performed for the purpose of homogenizing the structure and dissolving the carbide forming element. If the temperature is lower than 1000 ° C., the carbide forming element does not sufficiently form a solid solution. Is carried out at a temperature in the range of 1000 to 1100 ° C.

The tempering is carried out for softening and intensity adjustment of the tissue, the temperature toughness is insufficient in strength not sufficiently softened is less than 700 ° C., cooling after tempering exceeds Ac ₁ point , A hardened phase is formed and toughness is reduced. Therefore, tempering is performed at a temperature in the range of 700 ° C. or more and the Ac ₁ point or less.

In the method according to the third aspect of the present invention, the steel is heated at a temperature of 1000 ° C. or more,
After processing at least 30% at a temperature in the range of 0 ° C.,
It is cooled to a temperature of 0 ° C. or less and tempered at a temperature in a range of 700 ° C. or more and _one Ac or less.

The reason why the heating temperature is set to 1000 ° C. or higher is to homogenize the structure and dissolve the carbide forming element. The reason why the processing of 30% or more is performed at a temperature in the range of 1000 to 800 ° C. is to cause dynamic recrystallization by the processing and to make the structure and the crystal grains uniform.

The purpose of cooling to a temperature of 200 ° C. or less is to complete the bainite transformation. Thereafter, tempering is performed to soften the structure and adjust the strength. The tempering temperature range is 700 ° C. or more and Ac ₁ point or less, for the same reason as in the case of the manufacturing method according to the second embodiment.

[0028]

Embodiments of the present invention will be described below. Table 1
A steel having the chemical composition shown in Table 1 was melted in vacuum to form a 10 kg steel ingot, and then heated to 1000 to 1200 ° C.
It was a 2 mm thick plate. The hot working ratio was 85%.

In Table 1, 1 to 6 are steels of the present invention, and 7 to 1
3 is a comparative steel. Comparative steel No. 7 is a standardized and currently used STBA24 (2.25Cr-1Mo steel).
Comparative steel No. 8 is a steel obtained by adding Nb to the composition in the range of the present invention;
Comparative steels 9 and 10 are steels to which Mo and W are excessively added, 11 of comparative steels are steels to which C is excessively added, 12 of comparative steels are steels to which V is excessively added, and 13 of comparative steels are Ti. Excessive addition of steel.

In the heat treatment, the comparative steel 7 was austenitized to a normal temperature of 930 ° C., gradually cooled, and then subjected to isothermal annealing at 690 ° C., and the other steels were heated at 1030 ° C. × 1 hour, and air-cooled. Tempering treatment was performed at 760 ° C. × 1 hour.

[0031]

[Table 1]

Test pieces were taken from the heat-treated material of each steel, and subjected to a normal temperature tensile test, a creep rupture test and a Charpy impact test. The creep rupture test was performed at 600 ° C. and 650 ° C., and the breaking strength at 600 ° C. for 10,000 hours was determined. In addition, the weldability was evaluated by performing a diagonal y-shaped restraint welding crack test (JIS Z3158) and a preheating temperature capable of preventing cracking. Table 2 shows the results.

[0033]

[Table 2]

As shown in Table 2, the current 2.25Cr-1
Comparative steel 7, which is a Mo steel, had a low creep rupture strength. The creep rupture strengths of Comparative Steels 9 and 10 were high, but the impact characteristics were all 30 J or less. The comparative steel 8 to which Nb was added showed a remarkably low absorbed energy of about 10 J. Since the comparative steel 11 had a high C content, the preheating temperature for preventing the y-crack was 200 ° C. or more. Furthermore, the comparative steels 12 and 13 had relatively good impact properties, but had a marked decrease in creep rupture strength. In contrast, all of the steels of the present invention had good impact properties, high creep rupture strength, and good weldability.

Next, the heat treatment conditions (normalization temperature) were changed using the steel indicated by reference numeral 2 in Table 1. Table 3 shows the results. The heating and rolling conditions of the material here were 85% processing by heating at 1150 ° C., and the rolling finish temperature was 960 ° C.

[0036]

[Table 3]

As shown in the table, when the normalizing temperature is high, the creep rupture strength is improved.
When normalization was performed at a temperature exceeding 100 ° C., the crystal grains became coarse and the impact characteristics were reduced. When the normalizing temperature is 100
When the temperature is lower than 0 ° C., the creep rupture strength clearly decreases as in the conditions E and F. FIG. 1 shows the normalizing temperature and 600 ° C., 10 ° C.
9 shows the relationship between the creep rupture strength for 000 hours. This figure also confirms that the creep rupture strength is improved at a normal temperature of 1000 ° C. or higher. The preheating temperature for preventing y cracking is
In each case, it was as good as 100 ° C. or less. Next, the heating temperature, the finished rolling temperature, and the working ratio were changed using steel No. 2 in Table 1. Table 4 shows the results.

[0038]

[Table 4]

As shown in this table, the creep rupture strength was as low as 9.6 kg / mm ^{2 under the} condition K where the heating temperature was low. Further, even when the heating temperature was set to 1000 ° C. or higher, the creep rupture strength was high but the impact characteristics were reduced under the condition J in which the amount of processing during rolling was small. The preheating temperature for preventing the occurrence of y cracks was 100 ° C. or less in all cases.

[0040]

As described above, according to the present invention,
A low-alloy heat-resistant steel excellent in weldability while maintaining sufficient high-temperature strength as a high-temperature steel and a method for producing the same are provided. For this reason, the present invention makes it possible to realize a thermal power plant in which steam conditions are set to high temperature and high pressure.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between normalizing temperature and creep rupture strength.

Claims (3)

[Claims] 1. C: 0.02 to 0.10% by weight,
Si: 0.05 to 0.40%, Mn: 0.1 to 1.0
%, Cr: 2.0 to 3.0%, Mo: 0.1 to 1.0
%, W: 0.5 to 2.0%, V: 0.2 to 0.5%, T
i: 0.02 to 0.12%, B: 0.0010 to 0.0
A low-alloy heat-resistant steel excellent in high-temperature strength and weldability, characterized by containing 050%, the balance being Fe and inevitable impurities, and Mo + W: 0.6 to 2.5%. 2. C: 0.02 to 0.10% by weight,
Si: 0.05 to 0.40%, Mn: 0.1 to 1.0
%, Cr: 2.0 to 3.0%, Mo: 0.1 to 1.0
%, W: 0.5 to 2.0%, V: 0.2 to 0.5%, T
i: 0.02 to 0.12%, B: 0.0010 to 0.0
Steel containing 050%, the balance being Fe and unavoidable impurities, Mo + W: 0.6-2.5%,
Normalized at a temperature in the range of ~ 1100 ° C and 700 ° C or more
_A method for producing a low-alloy heat-resistant steel having excellent high-temperature strength and weldability, characterized by tempering at a temperature within _one point or less. 3. C: 0.02 to 0.10% by weight,
Si: 0.05 to 0.40%, Mn: 0.1 to 1.0
%, Cr: 2.0 to 3.0%, Mo: 0.1 to 1.0
%, W: 0.5 to 2.0%, V: 0.2 to 0.5%, T
i: 0.02 to 0.12%, B: 0.0010 to 0.0
Steel containing 050%, the balance being Fe and unavoidable impurities, Mo + W: 0.6-2.5%,
After heating at a temperature of at least 100 ° C. and applying a processing of at least 30% at a temperature within the range of 1000 to 800 ° C., cooling to a temperature of at most 200 ° C. and tempering at a temperature of at least 700 ° C. and at most _one point of Ac. A method for producing a low-alloy heat-resistant steel excellent in high-temperature strength and weldability, characterized in that: