JP2000144308A - Low alloy heat resistant steel excellent in high temperature strength and oxidation resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow steel to correspond to the increase of temp. and pressure of the steam condition while its oxidation resistance is maintained by alloying it to have a specified compsn. contg. C, Si, Mn, Cr, Mo, W, Ti, B, Nb, and the balance Fe with inevitable impurities and satisfying the total content of Mo and W to a specified range. SOLUTION: This steel has a compsn. composed of, by weight, 0.05 to 0.20% C, 0.05 to 0.40% Si, 0.1 to 1.0% Mn, 4.5 to 5.5% Cr 0.1 to 1.0% Mo, 0.1 to 1.0% W, 0.10 to 0.40% Ti, 0.0010 to 0.0060% B, <=0.01 Nb, and the balance Fe with inevitable impurities. Then, the content of Mo+W is controlled to 0.2 to 1.5%. The steel whose components are controlled to the above is normalized at 1000 to 1150 deg.C and is tempered at 700 deg.C to the Ac1 point. In this way, the excellent steel capable of correspoinding to the increase of the temp. and pressure in vapor while its weldability and workability required as those of a steel pipe for a boiler and for chemical plant piping can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-alloy heat-resistant steel excellent in high-temperature strength and oxidation resistance, particularly used for steel pipes for boilers and steel pipes for chemical plant piping, and a method for producing the same.

[0002]

2. Description of the Related Art As steel for boilers, carbon steel to austenitic stainless steel is used depending on the operating temperature and pressure. In recent years, high temperature and high pressure steam conditions are planned, and higher strength than conventional materials is used. Material is required.

When the current steam pressure is increased from 246 kgf / cm ² to 350 kgf / cm ² , it takes about 1.
Three times the strength is required. That is, the current 2.25Cr-1Mo
Allowable stress 2.8 kgf / 1.3 times the mm ² of the allowable stress of 600 ° C. Steel 3.6k
It should have a gf / mm ^2. Since the creep rupture strength of low alloy steel tends to sharply decrease at high temperature and long time,
In order to satisfy this allowable stress, a creep rupture strength at 600 ° C. for 10,000 hours of 12 kgf / mm ² or more is required.
One cause of the decrease in the creep rupture strength of the low alloy steel on the high temperature and long time side is the wall thinning due to oxidation. At temperatures below 600 ° C, 2.25Cr-1Mo steel is currently mainly used. Considering the future high temperature and pressure of steam temperature and pressure, increasing the wall thickness of the current 2.25Cr-1Mo steel will cause a significant increase in plant weight, and will require a major design change. Cost increases.

As a material having high strength and oxidation resistance, STBA28 (9Cr-1Mo-Nb-V) steel has been standardized in thermal power technical standards. It is expensive and has poor economy. STBA25 (5Cr-0.5Mo) steel, which has an appropriate amount of Cr added to provide oxidation resistance, has oxidation resistance, but is inferior in creep rupture strength to 2.25Cr-1Mo steel.

[0005] As a high-strength 5% Cr heat-resistant steel for improving the above-mentioned drawbacks of conventional steel, addition of V, Nb, etc., which is effective for creep rupture strength (Japanese Patent Laid-Open No. 3-20440, Kaihei 6-4
9601, JP-A-4-65043), addition of W (JP-A-6-88167), and composite addition thereof (JP-A-7-28624)
No. 8, JP-A-8-134584).

[0006]

However, the above-mentioned V, N
The high-strength 5% Cr heat-resistant steel characterized by the addition of b, W, etc., does not sufficiently satisfy the creep rupture strength that can cope with high temperature and high pressure under the above-mentioned steam conditions. The purpose of the present invention is
An object of the present invention is to provide a low-alloy heat-resisting steel containing 5% Cr and having excellent high-temperature strength capable of coping with high-temperature and high-pressure steam conditions while maintaining oxidation resistance and a method for producing the same.

[0007]

In order to solve the above problems and achieve the object, the present invention uses the following means.

(1) The low-alloy heat-resistant steel of the present invention, which has excellent high-temperature strength and oxidation resistance, has a weight percentage of C: 0.05 to 0.20% and Si: 0.1%.
05 ~ 0.40%, Mn: 0.1 ~ 1.0%, Cr: 4.5 ~ 5.5%, Mo: 0.
1 to 1.0%, W: 0.1 to 1.0%, Ti: 0.10 to 0.40%, B: 0.00
10 to 0.0060% and Nb: 0.01% or less (including 0%), the balance consists of Fe and unavoidable impurities, (Mo% + W%): 0.2 to 1.
It is excellent in high temperature strength and oxidation resistance characterized by being 5%.

(2) The method of the present invention for producing a low-alloy heat-resistant steel excellent in high-temperature strength and oxidation resistance comprises normalizing the steel described in (1) above at 1000 ° C. to 1150 ° C. It is a method of tempering with _one or less points.

(3) The method for producing a low-alloy heat-resistant steel excellent in high-temperature strength and oxidation resistance according to the present invention comprises heating the steel described in (1) above at 1000 ° C. or more, % Or more, then cooled to 200 ° C or less, and tempered at 700 ° C or more and _one Ac or less.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has proposed a conventional steel (2.25Cr-1Mo).
Cr in order to improve oxidation resistance compared to
At the same time, the precipitation strengthening elements Ti and Nb are added together with the solid solution strengthening elements Mo and W, which are effective for creep rupture strength. A study was made on steels that could respond to as a result,
In 5% Cr steel, Nb significantly lowers the toughness when added in combination with Ti, and the finding that it is better not to add it in combination with Ti, and B has a role to assist the precipitation strengthening action of Ti, It has been found that composite addition must be positively performed. Based on these findings, the present inventor has proposed a method that can cope with high-temperature and high-pressure steam conditions by limiting the composite addition ratio of Nb, B, and Ti while containing Cr having appropriate oxidation resistance. The present inventors have found the steel of the invention and a method for producing the same, and have completed the present invention.

That is, the present invention restricts the steel composition and the production conditions to the following ranges, thereby maintaining the weldability and workability required for a steel pipe for a boiler or a steel pipe for a chemical plant piping, while maintaining high steam and high temperature conditions. A steel excellent in high-temperature strength and oxidation resistance that can cope with high pressure can be obtained.

The reasons for adding the components of the present invention, the reasons for limiting the range of the components, and the reasons for limiting the manufacturing conditions will be described below.

(1) Component composition range C: Ti and MC type carbides, Cr and M ₂₃ C ₆ type carbides, and Mo and W are elements that form carbides and improve tensile and creep rupture strength. If the addition amount is less than 0.05%, the amount of carbide precipitated is small and sufficient strength cannot be obtained. If it exceeds 0.20%, the toughness is reduced, and hot cracking due to welding occurs, so the content is made 0.05 to 0.20%.

Si: It is added as a deoxidizing agent, but if added less than 0.05%, deoxidizing is not sufficient and the toughness is reduced. Although it is an effective element from the viewpoint of oxidation resistance, if it exceeds 0.40%, it promotes agglomeration and coarsening of carbides, reduces creep rupture strength, and also reduces toughness after tempering.
Be within the range of 0.40%.

Mn: added as a deoxidizing and desulfurizing agent,
If it is less than 1%, sufficient deoxidation and desulfurization effects cannot be obtained, and 1.
If it exceeds 0%, the toughness is reduced as in the case of Si.
Be within the range of 1.0%.

Cr: an element that is effective for oxidation resistance, forms an M ₂₃ C ₆ type carbide, and improves high-temperature strength.
If it is less than 5.5%, the oxidation resistance is not sufficient.If it exceeds 5.5%, MC
The range is 4.5 to 5.5% in order to reduce the amount of carbide generated and lower the creep rupture strength.

Mo, W: Effective as a solid solution strengthening element or a carbide forming element for high-temperature strength, but is an expensive element, so excessive addition impairs economic efficiency. Further, it is an element that is easily segregated at the time of solidification, and causes non-uniformity of the material to reduce toughness and the like. Therefore, the addition amount is each set to 0.1 to 1.0%, and the total content is limited to 0.2 to 1.5%.

Ti: An element that forms TiC and is effective for improving high-temperature strength, particularly creep rupture strength on the high-temperature side. 0.10%
If it is less than 1, the target creep rupture strength cannot be obtained because the amount of precipitation as TiC is small. 1000 over 0.40%
Even when heating at a high temperature of not less than ℃, it remains as undissolved coarse TiC, which causes a decrease in creep rupture strength. Also,
In order to reduce toughness, the range is set to 0.10 to 0.40%.

B: An element that assists the precipitation strengthening effect of Ti and is effective for improving the creep rupture strength. However, if it is less than 0.0010%, the effect is not recognized, and if it exceeds 0.0060%, the hot workability is reduced. Therefore, the amount of addition is set to 0.0010 to 0.0060% to cause generation of scratches.

Nb: an element effective for improving the creep rupture strength, but when added in combination with Ti, if the upper limit exceeds 0.01%, the toughness is remarkably reduced, so that the range is 0.01%.
% Or less (including 0%).

By adjusting to the above component ranges, it is possible to obtain a high temperature strength and an excellent oxidation resistance which can cope with high temperature and high pressure of steam conditions.

The steel having such characteristics can be manufactured by the following two manufacturing methods. (2) Manufacturing process a) In the first manufacturing method, steel adjusted to the above-mentioned suitable components is used for one step.
In this method, normalization is performed in the temperature range of 000 ° C to 1150 ° C, and tempering is performed at 700 ° C or higher and _one point of Ac or lower.

Here, if normalizing is performed at a temperature exceeding 1150 ° C., the crystal grains become coarse and the toughness is reduced. Because of this,
It must be 1150 ° C. If the temperature is lower than 1000 ° C., the carbide precipitation element does not sufficiently form a solid solution, and the balance between strength and toughness is deteriorated.

The tempering treatment is indispensable for improving the toughness of the steel and preventing softening due to welding, stress removal treatment and the like. However, the temperature is greater than ₁ point Ac, because the re-dissolution of C into the austenite phase occurs strength is lowered, the upper limit Ac ₁
Need to be a point. If the temperature is lower than 700 ° C., the strength is increased and the toughness is deteriorated.

B) In the second production method, the steel adjusted to the above-mentioned preferred components is heated at 1000 ° C. or more to sufficiently dissolve the carbide precipitation element, and is processed at 1000 ° C. to 800 ° C. at 30% or more. After the addition, it is cooled to 200 ° C or lower and tempered at 700 ° C or higher and _one point of Ac or lower.

Here, when the heating temperature is less than 1000 ° C.,
Since the carbide precipitation element does not sufficiently form a solid solution and the creep rupture strength deteriorates, the lower limit must be set to 1000 ° C.

If the amount of processing at the time of rolling is less than 30% even when the heating temperature is set to 1000 ° C. or higher, the grain size cannot be sufficiently reduced, and the toughness is deteriorated. There is a need to.

If the cooling after processing is stopped at a temperature higher than 200 ° C., the bainite transformation is not completed and the strength is reduced. Therefore, it is necessary to set the upper limit to 200 ° C.

The tempering treatment is indispensable for improving the toughness of the steel and preventing the steel from being softened by welding, stress removing treatment and the like. However, the temperature is greater than ₁ point Ac, because the re-dissolution of C into the austenite phase occurs strength is lowered, the upper limit Ac ₁
Need to be a point. If the temperature is lower than 700 ° C., the strength is increased and the toughness is deteriorated.

Hereinafter, the effects of the present invention will be proved by giving examples of the present invention.

[0032]

EXAMPLES Steels having the chemical compositions shown in Table 1 (Steel Nos. 1 to N of the present invention)
o.8, Comparative steel No.9 ~ No.15) is melted in vacuum to make a 10kg steel ingot, then heated to 1000 ℃ ~ 1200 ℃ and hot rolled to 12mm thick
Of the board. The hot working ratio is 85%. No. of comparative steel
9 is a standardized and working STBA24 (2.25Cr-1Mo) steel. STBA25 (5Cr-0.5), which is standardized and has oxidation resistance but lacks high-temperature strength, is also standardized.
Mo) steel. No. 11 is a steel obtained by adding Nb to the steel of the present invention, No. 1
Nos. 2, 13, and 14 are steels to which Mo and W are excessively added, and No. 15 is a steel to which Ti is excessively added.

[0033]

[Table 1]

In the heat treatment, No. 9 and 10 steels were austenitized to a normal temperature of 930 ° C., then gradually cooled, and subjected to constant temperature annealing at 690 ° C., except for No. 9 and 10 steels, which were heated at 1050 ° C. × 1 hour. After air cooling 7
Tempering at 80 ° C. × 1 h was performed.

A test piece was taken from the heat-treated material of each steel and subjected to a room temperature tensile test, a creep rupture test, a Charpy impact test and a steam oxidation 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. The steam oxidation test was performed at 600 ° C x 1000h.
And the total thickness of the formed oxide scale was measured.
Table 2 shows the results.

[0036]

[Table 2]

The working 2.25Cr-1Mo steel of Comparative steel No. 9 has a creep rupture strength of 12 kgf / mm ² or less. In addition, a large amount of steam oxidation scale is generated, and the oxidation resistance is poor. Comparative steel No.10
No. 5Cr-0.5Mo steel has good oxidation resistance, but has a lower creep rupture strength than 2.25Cr-1Mo steel. On the other hand, the comparative steel Nos. 11 to 14 had a creep rupture strength of 12 kgf / mm ²
As described above, all of the impact characteristics are 10 kgf · m or less, and the toughness is poor. In particular, Nb-added comparative steel No. 11 has 2 kgf
It shows only a very low absorption energy of the order of m.

[0038] Comparative steel No. 15 has relatively good impact properties, but has a remarkable decrease in creep rupture strength.
mm ² or less.

On the other hand, the steels of the present invention were all good.
It has a creep rupture strength of 12 kgf / mm ² or more, impact properties and oxidation resistance.

Table 3 shows the results when the heat treatment conditions were changed using No. 2 of the steel of the present invention. The heating and rolling conditions of the material are 85% processing at 1150 ° C heating, and the rolling finish temperature is 970 ° C. When the normalizing temperature is high, the creep rupture strength is improved, but when normalizing at a temperature higher than 1150 ° C., the crystal grains are coarsened, and the impact characteristics are reduced. On the other hand, when the temperature is lower than 1000 ° C., the creep rupture strength is clearly reduced, and 12 kgf / mm ²
It is as follows.

[0041]

[Table 3]

FIG. 1 shows the relationship between normalizing temperature and creep rupture strength at 600 ° C. for 10,000 hours. The creep rupture strength increases to 12 kgf / mm ² or more at a normal temperature of 1000 ° C. or more.

Table 4 shows the results when the heating temperature, the finished rolling temperature, and the working ratio were changed using No. 2 of the steel of the present invention. The comparative steel 3E having a heating temperature lower than 1000 ° C. has a creep rupture strength of 12 kgf / mm ² or less. Heating temperature up to 1000 ℃
Even with the above, if the processing amount during rolling is less than 30%,
As shown in Comparative Steel 3D, the creep rupture strength is high, but the impact properties are reduced.

[0044]

[Table 4]

[0045]

According to the present invention, by specifying the steel composition and the manufacturing conditions, it is possible to cope with high temperature and high pressure steam conditions by adding an appropriate amount having oxidation resistance without adding a large amount of Cr. Heat resistant steel with excellent high temperature strength and excellent oxidation resistance can be manufactured. This steel has excellent high-temperature strength and oxidation resistance, and also has excellent toughness, weldability, and workability, making it possible to realize thermal power generation equipment with high steam and high temperature steam conditions for boiler steel pipes. .

Also, as piping for an oil refinery plant,
Due to its excellent high-temperature strength, the allowable range of the design thickness is widened, which can contribute to cost reduction of the whole plant.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between normalization temperature and creep rupture strength at 600 ° C. for 10,000 hours according to an example of the present invention.

Continued on the front page F term (reference) 4K032 AA02 AA04 AA05 AA12 AA16 AA19 AA22 AA31 AA35 AA37 BA01 CA02 CB01 CB02 CC03 CC04 CF02 CL01

Claims (3)

[Claims] (1) C: 0.05 to 0.20% by weight and Si: 0.05 to
0.40%, Mn: 0.1 to 1.0%, Cr: 4.5 to 5.5%, Mo: 0.1 to
1.0%, W: 0.1 to 1.0%, Ti: 0.10 to 0.40%, B: 0.0010
~ 0.0060%, Nb: 0.01% or less (including 0%), the balance consists of Fe and unavoidable impurities, (Mo% + W%): 0.2 ~ 1.5%
A low-alloy heat-resistant steel having excellent high-temperature strength and oxidation resistance. 2. C: 0.05 to 0.20% by weight and Si: 0.05 to 2% by weight.
0.40%, Mn: 0.1 to 1.0%, Cr: 4.5 to 5.5%, Mo: 0.1 to
1.0%, W: 0.1 to 1.0%, Ti: 0.10 to 0.40%, B: 0.0010
~ 0.0060%, Nb: 0.01% or less (including 0%), the balance consists of Fe and unavoidable impurities, (Mo% + W%): 0.2 ~ 1.5%
A method for producing a low-alloy heat-resistant steel excellent in high-temperature strength and oxidation resistance, characterized by normalizing steel at 1000 ° C to 1150 ° C and tempering at 700 ° C to ₁ point Ac. 3. The method according to claim 1, wherein C: 0.05 to 0.20% and Si: 0.05 to 0.2% by weight.
0.40%, Mn: 0.1 to 1.0%, Cr: 4.5 to 5.5%, Mo: 0.1 to
1.0%, W: 0.1 to 1.0%, Ti: 0.10 to 0.40%, B: 0.0010
~ 0.0060%, Nb: 0.01% or less (including 0%), the balance consists of Fe and unavoidable impurities, (Mo% + W%): 0.2 ~ 1.5%
The steel is heated at 1000 ℃ or more, processed at 1000 ℃ ~ 800 ℃ for 30% or more, then cooled to 200 ℃ or less, 700 ℃ or more
A method for producing a low-alloy heat-resistant steel excellent in high-temperature strength and oxidation resistance, characterized by tempering at _one point or less of Ac.