JP3463617B2 - Austenitic heat-resistant steel for seamless steel pipes with excellent hot workability - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a boiler, nuclear power,
In a wide range of industrial fields such as the chemical industry,
Suitable for high pressure strength, high temperature corrosion resistance and
And austenitic heat resistant steel with excellent weldability, especially seams
When manufacturing steelless pipes, especially by using an inclined roll type piercing and rolling mill
The present invention relates to an austenitic heat-resistant steel for seamless steel pipes, which is excellent in hot workability during pipe manufacturing .

[0002]

2. Description of the Related Art SUS304H and SUS are used as materials for equipment used in high temperature environments of boilers and chemical plants.
Austenitic stainless steels represented by 18-8 series such as 316H, SUS321H and SUS347H have been used.

However, in recent years, the operating conditions of the apparatus under such a high temperature environment have become remarkably severe, and the required performance for the materials used has become stricter accordingly, and the austenite type represented by the conventionally used 18-8 type is used. High temperature strength is becoming insufficient for stainless steel.

As an austenitic steel having improved high temperature strength without adding a large amount of expensive elements, Japanese Patent Application Laid-Open No. 62-62 is known.
-1333048 (Japanese Patent Publication No. 8-30247)
As disclosed in JP-A-8-13102 and C
There is a composite addition of u, Nb and N. But,
Such Cu, Nb, N composite addition steel is the conventional 18
The hot workability is insufficient as compared with the austenitic stainless steel represented by the -8 series, and an urgent improvement thereof has been desired.

[0005]

A small-diameter seamless steel pipe such as a boiler tube made of austenitic heat-resistant steel to which Cu, Nb, and N are added in combination is generally manufactured by a hot pipe manufacturing method. And, the tube making method is a hot extrusion tube making method typified by the Eugene Sejournet method, and a hot rolling typified by a Mannesmann-plug mill method or a Mannesmann-mandrel mill method using an inclined roll type piercing and rolling machine. There are roughly two types of pipe making methods.

In general, since a material having poor hot workability is difficult to produce by a hot rolling pipe making method, a seamless steel pipe is conventionally produced by a hot extrusion pipe making method in many cases. In this case, a pipe can be manufactured as long as the material has excellent hot workability in a high temperature range of 1000 ° C. or higher.

On the other hand, from the viewpoint of pipe manufacturing efficiency and cost, the hot rolling pipe manufacturing method is overwhelmingly advantageous.
When the austenitic heat-resisting steel to which u, Nb, and N are added together is produced by the hot rolling pipe making method, the pipe making end temperature range is lower than that in the hot extrusion pipe making method. Therefore, in order to stably manufacture, it is necessary to secure excellent hot workability in a wide temperature range of about 800 ° C. or higher.

An object of the present invention is to provide a seamless pipe having excellent hot workability capable of easily producing a pipe by a hot rolling pipe making method.
It is to provide an austenitic heat resistant steel for steel pipes .

[0009]

The gist of the present invention resides in the following austenitic heat-resistant steel for seamless steel pipes, which is excellent in hot workability.

% By weight, C: 0.03 to 0.15%, S
i: 1.0% or less, Mn: 0.10 to 2.0%, P:
0.030% or less, S: 0.0010% or less, Cr: 1
5.0 to 25.0%, Ni: 6.0 to 25.0%, C
u: 2.0 to 6.0%, Nb: 0.10 to 0.80%,
N: more than 0.050 % and 0.25% or less , B: 0.00
05-0.010%, Al: 0.001-0.10%,
O (oxygen): 0.0050% or less, and Ca: 0.
0010-0.010% and Mg: 0.0010
0.010% of either or both of them, and the balance consisting essentially of Fe, and Ca, M
Austenitic heat-resistant steel for seamless steel pipes with excellent hot workability in which the relationship between g, O (oxygen) and S satisfies the following formula (1).

3.0 ≦ {(Ca + Mg) −0.1 × O} /S≦15.0 (1) where the element symbols in the formula are the contents of each element contained in the steel. (% By weight) is meant.

In the above-mentioned steel of the present invention, Mo: 0.3-2.0% and W: 0.5-4.
Either or both of the 0% can be added.

The present inventor has conducted earnest experimental research in order to improve the hot workability of austenitic heat-resisting steel containing Cu, Nb, and N in combination. As a result, the inventors have found that if the following three conditions are satisfied, the hot workability is dramatically improved, and pipes can be produced by the hot rolling pipe production method, and the present invention has been completed.

The contents of S and O (oxygen) as impurity elements in steel are 0.0010% or less and 0.0050, respectively.
It must be less than or equal to%.

If either or both of Ca and Mg are added, the hot workability is improved, but this is not sufficient, and the combined addition with B is essential.

The contents of Ca, Mg, O and S in the steel must each be within the above ranges and satisfy the above formula (1).

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the chemical composition of the steel of the present invention is defined as described above will be described in detail below. In addition,
In the following, "%" means "% by weight".

C: 0.03 to 0.15% C is an element effective for ensuring the tensile strength and creep rupture strength required when used in a high temperature environment. However, even if the content exceeds 0.15%, only the amount of undissolved carbide in the solution state increases, which does not contribute to the improvement of high temperature strength, but also deteriorates mechanical properties such as toughness. Therefore, the upper limit of the C content is 0.15%. In the present invention, the C content may be low in order to contain N as described later, but the lower limit is made 0.03% in order to exert the above effects. The lower limit for exerting a more sufficient effect is 0.05%.

Si: 1.0% or less Si is added as a deoxidizing agent and is an element effective for improving the oxidation resistance. However, if its content increases, the weldability and hot workability deteriorate. . Further, since N is also contained in the present invention, if a large amount of Si is added, the amount of nitrides that precipitate during use at high temperature increases, leading to a decrease in toughness and ductility. Therefore, in the present invention, since the hot workability is particularly emphasized, the Si content is set to 1.0% or less. When the hot workability, toughness and ductility are further emphasized, the content is preferably 0.5% or less, more preferably 0.3% or less. In addition, when deoxidation is sufficiently performed with other elements, it may be substantially 0.

Mn: 0.10 to 2.0% Mn has a deoxidizing action similar to the above Si, and in the present invention, Cu fixes grain boundary segregation-promoted S to fix hot It is an important element that improves workability. In order to obtain the effect sufficiently, the content of 0.10% or more is necessary. However, if its content exceeds 2.0%, precipitation of intermetallic compounds such as σ phase is caused, and high temperature strength,
Mechanical properties deteriorate. Therefore, the Mn content is 0.
10 to 2.0%. A preferred range is 0.15 to 1.
0%, and a more preferable range is 0.15 to 0.50%.

P: 0.030% or less P is contained in steel as an unavoidable impurity, but if it is contained in excess, hot workability deteriorates. Therefore, it is better to make it as low as possible, but 0.0 in consideration of the P removal cost.
30% or less.

S: 0.0010% or less S, which is contained in the steel as an unavoidable impurity as in the case of P, remarkably deteriorates the hot workability. for that reason,
From the viewpoint of hot workability, it is better to make it as low as possible,
In particular, when the pipe is manufactured by a hot rolling pipe manufacturing method such as the Mannesmann-mandrel mill method, it is essential to limit the content to 0.0010% or less. It is preferably 0.0006% or less.

Cr: 15.0 to 25.0% Cr is an element necessary for improving the oxidation resistance and corrosion resistance at high temperatures, and the performance thereof improves as the content increases. However, if its content is less than 15.0%, a sufficient effect cannot be obtained, while if it exceeds 25.0%, the austenite structure becomes unstable. Therefore, the Cr content is set to 15.0 to 25.0%. The preferred range is 1
6.0 to 23.0%, more preferably 17.0 to 2
It is 0.0%.

Ni: 6.0 to 25.0% Ni is an essential component for ensuring a stable austenite structure, and the optimum content thereof is Cr contained in steel,
It is determined by the contents of ferrite-forming elements such as Mo, W and Nb and austenite-forming elements such as C and N. However, if the content is less than 6.0%, it is difficult to stabilize the austenite structure, while if the content exceeds 25.0%, it is economically disadvantageous. Therefore, the Ni content is set to 6.0 to 25.0%. The preferred range is 7.
0 to 15.0%, more preferable range is 7.5 to 13.0
%.

Cu: 2.0 to 6.0% Cu is coherently precipitated as a fine Cu phase in the austenite matrix during use at high temperature, and contributes greatly to the improvement of creep rupture strength, but exhibits its effect. To achieve this, it is necessary to contain 2.0% or more. However, if the content exceeds 6.0%, creep rupture ductility and workability deteriorate. Therefore, the Cu content is set to 2.0 to 6.0%. A preferable range is 2.5 to 5.0%, and a more preferable range is 2.5 to 4.0%.

Nb: 0.10 to 0.80% Nb is an element that improves creep rupture strength by dispersion precipitation strengthening of fine carbonitrides. However, if its content is less than 0.10%, a sufficient effect cannot be obtained, while
If the content exceeds 0.80%, the weldability and hot workability deteriorate, and in N-added steel such as the steel of the present invention, the amount of undissolved carbonitride increases and the mechanical properties also deteriorate. Therefore, the Nb content is set to 0.10 to 0.80%. In particular, the preferable upper limit is 0.6 when the hot workability is important.
It is 0%.

Al: 0.001 to 0.10% Al is an element added as a deoxidizing agent, and is 0.00
It is necessary to contain 1% or more. However, if the content exceeds 0.10%, when used for a long time under high temperature conditions,
Precipitation of intermetallic compounds such as σ phase is promoted and toughness deteriorates. Therefore, the Al content is 0.001 to 0.10.
%. A preferred range is 0.001 to 0.06%, and a more preferred range is 0.001 to 0.03%. The term "Al" used in the present invention means sol. Al (acid-soluble A
l).

N: more than 0.050 % and 25% or less N is an element effective for improving the tensile strength and the creep rupture strength as in the case of the above-mentioned C, but its content is 0.050.
If it is less than %, the sufficient effect cannot be exerted. On the other hand, N has a larger solid solution limit than C, so that even if it is contained in a relatively large amount, it is sufficiently dissolved in the solution state, and the decrease in toughness associated with the precipitation of nitride during aging is relatively small. But,
If the content is more than 0.25%, the toughness after aging decreases and the hot workability also deteriorates. Especially, when it is used for pipe making by a hot rolling pipe making method such as a Mannesmann-mandrel mill method. However, there arises a problem that the deformation resistance increases.
Therefore, the N content exceeds 0.050 % and 0.25%
Below . The preferred range is more than 0.050 % to 0.15.
%, And a more preferable range is from more than 0.050 % to 0.13%.

B: 0.0005 to 0.010% B is an important element that contributes to the improvement of creep rupture strength by fine dispersion precipitation strengthening of carbonitrides and grain boundary strengthening. Further, B has an effect of improving high temperature ductility, and is an element indispensable for improving hot workability at the time of pipe making by a hot rolling pipe making method such as the Mannesmann-mandrel mill method which is an object of the present invention. is there. In order to exert the effect, 0.0005% or more is required. But,
If the content exceeds 0.010%, the weldability deteriorates.
Therefore, the B content is 0.0005% to 0.010%.
And A preferred range is 0.0015 to 0.008%,
A more preferable range is 0.0020 to 0.006%.

Ca, Mg: Ca and Mg are effective elements for reducing solid solution S in steel and improving hot workability. However, the Mannesmann-
In order to sufficiently bring out the effect at the time of pipe making by a hot rolling pipe making method such as a mandrel mill method, the O (oxygen) content is set to 0.0050% or less, and either one or both of them is set to 0. It is necessary to contain at least 0.0010%. However, if any of the elements exceeds 0.010%, the effect of improving the hot workability is saturated, and the amount of inclusions increases, and particularly the weldability deteriorates significantly. Therefore, the contents of Ca and Mg are
It was set to 0.0010 to 0.010%.

O (oxygen): 0.0050% or less O (oxygen) is contained in steel as an unavoidable impurity.
In order to improve the hot workability during the Mannesmann pipe making, which is the object of the present invention, it is necessary to reduce it to at least 0.0050% or less, preferably 0.0040% or less.

Between Ca, Mg, O and S, the expression "3.0≤ {(Ca + Mg) -0.1xO} / S≤
15.0 ”must be satisfied. This is because if the value obtained by the formula “{(Ca + Mg) -0.1 × O} / S” is less than 3.0 or more than 15.0, desired hot workability cannot be ensured. is there. This is clear from the results of Examples described later.

Mo: 0.3-2.0%, W: 0.5-
4.0% These elements may not be added. However, all of these elements have the effect of improving the high temperature strength, and either one or both of them can be added to obtain the effect. The effect becomes remarkable at 0.3% or more in the case of Mo and 0.5% or more in the case of W.
However, 2.0% for Mo and 4.0 for W
If it is contained in excess of%, the effect tends to be saturated, and the structure stability and hot workability deteriorate. For this reason,
When added, the Mo content is preferably 0.3 to 2.0% and the W content is preferably 0.5 to 4.0%.

The austenitic heat-resistant steel of the present invention having the above chemical composition is melted in a steelmaking furnace such as an electric furnace, and if necessary, the molten metal is used in a smelting furnace such as an AOD furnace or a VOD furnace. It can be easily manufactured by smelting by smelting and then making a slab of a predetermined size by an ingot making method or a continuous casting method. Further, the seamless steel pipe, if the slab is a round billet for pipe making, as it is, in the case of bloom etc., after forming a round billet for pipe making by hot rolling or hot forging,
Mannestmann-plug mill system and Mannesmann-mandrel mill system are used to form hot-rolled pipes to form a seamless steel pipe with a predetermined size, and then a predetermined solution heat treatment can be applied to produce the product. Since the hot workability is excellent, no problems occur.

[0035]

EXAMPLES 47 kinds of test steels having the chemical compositions shown in Tables 1 to 3 were melted using a 50 kg vacuum induction melting furnace,
After forming an ingot with an outer diameter of 144 mm, it was hot forged into a plate material having a thickness of 40 mm.

In Tables 1 to 3, Steel Nos. 1 to 22 are steels of the present invention, Steel Nos. A1 to A7, B1 to B7, C1 to C2,
D1-D2, E1-E2, F1, G1, H1, I1 and J1 are comparative steels.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

Regarding the hot workability of each test steel, a greeble test piece having an outer diameter of 10 mm and a length of 130 mm is taken from each of the above-mentioned plate materials and subjected to a greeble test under the following conditions, and the drawing value after the test is examined. It was evaluated by.

Gleeble test conditions; Heating of test piece: 1250 ° C for 3 minutes Tensile test temperature: 800 ° C, Applied strain rate: 1 / S (where S is seconds), Test method: High speed tensile test.

The above test conditions are Mannesmann-
As a result of a preliminary experiment carried out in a tensile test temperature range of 750 to 1200 ° C. by simulating heating conditions and working conditions during pipe manufacturing by the mandrel mill method, the present invention steel shows that the reduction value is the smallest near 800 ° C. Was confirmed and that the pipe forming end temperature range was lowered to about 800 ° C.

The above test results are shown in Tables 1 to 3 as well, and five kinds of the same component systems (No. 1 to 6 and A1 to A1) are shown.
A7, No. 7-11 and B1-B6, No. 12-13 and C
1 to C2, No. 14 to 15 and D1 to D2 and No. 16
17 and E1 to E2), the results of the present invention example steel and the comparative example steel are shown in comparison with FIG. Further, the results of the example steel of the present invention and the comparative example steel for each of the other component systems are shown in comparison with FIG.

As can be seen from the results shown in Tables 1 to 3 and FIGS. 1 and 2, all of the steels of the present invention have a reduction value of 80% or more except the steel of No. 19. The workability was extremely good.

On the other hand, the drawing value of the steel of the comparative example is the highest, which is 74% of steel Nos. A7, D2 and J1, and the hot workability was poor.

Although the data are omitted, the creep strength, corrosion resistance, oxidation resistance, toughness and weldability of the steel of the present invention were equal to or higher than those of the conventional steels shown in the above-mentioned respective publications. Further, the result of the manufacturing experiment by the Mannesmann-mandrel mill system of the actual machine was also very good, and the small diameter boiler tube could be manufactured without any problems.

[0047]

INDUSTRIAL APPLICABILITY The austenitic heat-resistant steel for seamless steel pipes according to the present invention has extremely high hot workability in the hot rolling pipe forming method . Therefore, even a small-diameter boiler tube can be manufactured with high efficiency and high yield by the hot rolling tube manufacturing method using the inclined roll type piercing and rolling machine, and an inexpensive product can be provided. .

[Brief description of drawings]

FIG. 1 is a diagram showing, in contrast, some results of Examples.

FIG. 2 is a diagram showing a comparison of another part of the results of Examples.

Claims (2)

(57) [Claims] 1. By weight%, C: 0.03 to 0.15%, S
i: 1.0% or less, Mn: 0.10 to 2.0%, P:
0.030% or less, S: 0.0010% or less, Cr: 1
5.0 to 25.0%, Ni: 6.0 to 25.0%, C
u: 2.0 to 6.0%, Nb: 0.10 to 0.80%,
N: more than 0.050 % and 0.25% or less , B: 0.00
05-0.010%, Al: 0.001-0.10%,
O (oxygen): 0.0050% or less, and Ca: 0.
0010-0.010% and Mg: 0.0010
0.010% of one or both of them, the balance consisting essentially of Fe, and Ca, Mg, O
Austenite heat-resistant steel for seamless steel pipes having excellent hot workability, characterized in that the relationship between (oxygen) and S satisfies the following expression (1). 3.0 ≦ {(Ca + Mg) −0.1 × O} /S≦15.0 (1) where the element symbols in the formula are the contents of each element contained in the steel (% by weight). ) Means. 2. Further, in% by weight, Mo: 0.3 to 2.0.
% And W: 0.5 to 4.0%, either or both of them are contained , The austenitic heat resistant steel for a seamless steel pipe having excellent hot workability according to claim 1.