JPH06128691A - Duplex stainless steel excellent in toughness and thick-walled steel tube using same as stock - Google Patents

Abstract

PURPOSE:To make it possible to use a duplex stainless steel in the fields hitherto considered to be impossible. CONSTITUTION:This steel is a duplex stainless steel used for the purposes where the steel is exposed to the condition of holding at 700-950 deg.C for >=3sec or the conditions of <=1000 deg.C/min average cooling rate from solution heat treatment temp. down to 500 deg.C. By incorporating 0.0005-0.05% B, the duplex stainless steel capable of maintaining superior toughness can be formed. Further, this steel tube is a thick-walled steel tube of >=10mm thickness prepared by means of butt girth welding, and this steel tube can be formed by using, as a stock, the duplex stainless steel containing 0.0005-0.05% B. This B-containing duplex stainless steel can be used under no apprehension of deterioration in toughness even under embrittlement conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is suitable as a duplex stainless steel that can be used in applications where the use of the steel has been avoided due to the fear of a decrease in toughness. It relates to duplex stainless steel.

[0002]

2. Description of the Related Art Duplex stainless steel is excellent in strength and corrosion resistance, so that it is registered as SUS 329 steel in JIS and widely used. Also, SUS 329
A number of inventions relating to duplex stainless steels with further improved corrosion resistance based on steel have also been proposed (for example, Japanese Patent Laid-Open No. Hei 1).
-48345, 1-100248 and 2-258956).

However, duplex stainless steels are
Since it contains a large amount of Mo, intermetallic compounds such as σ phase, χ phase, and Laves phase are likely to precipitate, and the toughness is greatly reduced depending on the use conditions. For example, when the cooling rate after solutionizing cannot be high, in other words, when the cooling rate after being kept at the solutionizing temperature is small, or when it is kept for a certain period of time within a certain temperature range, it is exposed to a so-called aging state. When this happens, the duplex stainless steel becomes significantly brittle. Therefore, it has been common sense to avoid the use of duplex stainless steel for applications that may be exposed to such conditions.

To give one specific example, a duplex stainless steel pipe is desirable as a pipe for pipelines in which the transportation of highly corrosive crude oil and gas has increased in recent years. However, with a tube having a thickness of 10 mm or more, the cooling rate after solutionizing cannot be increased, and thus the embrittlement occurs. Further, when butt-welded and circumferentially welded, the heat-affected zone of the weld rises to a solution temperature of 1000 to 1300 ° C, and the cooling rate thereafter is small, so that embrittlement occurs. Even in such a part that does not reach a high temperature, the part exposed to the temperature range of 700 to 950 ° C for 3 seconds or more by welding heat becomes brittle by aging.
Therefore, it has been considered that duplex stainless steel cannot be used as a material for the thick-walled tube as described above.

Inventions aimed at improving the toughness of duplex stainless steels are disclosed in JP-A-61-564 and 61-19764. In these inventions, it is intended to improve the toughness by controlling the added amount of Al, but it is not intended to deal with the brittleness under the special conditions as described above, and the steels of these inventions are Nor can it be used for purposes that have been avoided in the past.

[0006]

As described above, in the conventional duplex stainless steel, when it is aged at a high temperature or when the cooling rate after solution treatment cannot be made large,
In a thick-walled material where the heat-affected zone of welding is gradually cooled, intermetallic compounds are likely to precipitate and the toughness is feared to be greatly reduced, which greatly limits the application.

The present invention has been made to solve the problem that the duplex stainless steel cannot be used in the field.

[0008]

The gist of the present invention is the following duplex stainless steel (1) and steel pipe (2).

(1) Used in a state where the temperature is kept in the temperature range of 700 to 950 ° C for 3 seconds or more, or in a state where the average cooling rate from the solution temperature to 500 ° C is 1000 ° C / min or less. Duplex stainless steel which contains 0.0005 to 0.05% of B and maintains good toughness even when exposed to any of the above conditions.

(2) Thickness used by butt circumference welding
It is a thick-walled steel pipe with a diameter of 10mm or more, and the material is 0.0005-
Steel pipe made of duplex stainless steel containing 0.05%.

In the above inventions (1) and (2), the duplex stainless steel is a duplex structure of ferrite and austenite, roughly speaking, a ferrite is 20 to 80% and the rest is austenite. It has stainless steel. There are a wide variety of chemical compositions for obtaining such a structure, and among them, the composition may be selected so as to satisfy the material properties such as corrosion resistance and mechanical properties required depending on the application. A typical composition range is as follows. (% Is weight%
Indicates. ) C: 0.05% or less, Si: 0.05 to 2.0%, Mn:
0.05 to 2.0%, Ni: 2 to 10% Cr: 17 to 30%, Mo: 0.1 to 7.00%, N: 0.01 to 0.
5%, B: 0.0005 to 0.05%.

Further, the following elements are components that can be selectively added as needed.

Cu: 0.1-2.0%, sol.Al: 0.005-0.5
%, W: 0.1 to 5.0%, Ti, Nb, V: 0.01 to each
0.5%, Ca, Mg, rare earth elements: 0.0001 to 0.01 respectively
%.

[0014]

The present invention is a new finding obtained by conducting a detailed study on the effect of various additive elements on the precipitation of intermetallic compounds in duplex stainless steel, that is, B (added to duplex stainless steel). (Boron) has the effect of suppressing the precipitation of intermetallic compounds and suppresses embrittlement even under the above-mentioned conditions that are generally said to cause embrittlement, and greatly improves toughness based on the finding. .

The addition of B to duplex stainless steel is known per se. For example, the above-mentioned Japanese Patent Laid-Open No. 1-100248
No. 2-258956, JP-A-56-142855, JP-A-58-224155, JP-A-59-21556, and the like, disclose duplex stainless steel containing B. . But,
The purpose of adding B in these steels is mainly to improve the hot workability, and is not an addition considering the toughness. That is, in the invention of each of the above publications, there is no recognition that the duplex stainless steel containing B can maintain excellent toughness even under special use conditions, and therefore, it has been conventionally considered that it cannot be used. It does not imply that it can also be used.

According to the research results of the present inventor, the toughness of the duplex stainless steel is greatly improved by the addition of B. Although this mechanism has not been clarified yet, B segregates at the grain boundaries, lowers the energy of the crystal grain boundaries of ferrite and austenite, reduces the precipitation nucleation sites of the intermetallic compounds, and the intermetallic compounds form grains. It is presumed that the precipitation from the boundary is suppressed.

The above-mentioned effects are remarkable when the B content is 0.0005% or more.

On the other hand, if the B content exceeds 0.05%, borocarbide is precipitated at the grain boundaries, rather reducing the toughness. Therefore, the suitable range of the B content is 0.0005 to 0.05%.

In the present invention of the above (1), “700 to 950 ° C.
Is used for 3 seconds or more in the temperature range, or exposed to an average cooling rate of 1000 ° C / minute or less from the solution temperature to 500 ° C ”for the following reasons.

Usually, duplex stainless steel is kept in the temperature range of 700 to 950 ° C for 3 seconds or more, or the solution temperature is kept.
It is considered that when the temperature is maintained at (about 1000 to 1300 ° C) and then cooled up to a temperature of 500 ° C at an average cooling rate of 1000 ° C / min or less, intermetallic compounds precipitate and the toughness decreases significantly. There is. In the duplex stainless steel which does not actually contain B, this decrease in toughness is remarkable. Also, as above, 700 ~
If it is kept in the temperature range of 950 ° C for 3 seconds or more, it becomes brittle by aging. Then, regardless of whether or not B is contained, the duplex stainless steel has been avoided from being used for the application in which the above heat history is received.

The present invention intends to expand the use to the application where the duplex stainless steel has not been used conventionally, by making use of the new knowledge about the action and effect of B as described above. That is, the specification of the above-mentioned use is for clarifying the present invention as a use invention except the use of the conventional duplex stainless steel. Incidentally, as a specific example of such an application, in addition to the thick-walled steel pipe described in detail later, steel plates having a thickness of 10 mm or more, forged steel products, dies, tools such as dies,
There is.

A typical new application is a thick-walled steel pipe. As mentioned above, for steel pipes with a wall thickness of 10 mm or more, even if heated to 1000 to 1300 ° C for solution treatment, 1000 ° C
It is difficult to cool at an average cooling rate of more than 1 minute.
Further, when such a steel pipe is used as a line pipe, for example, it is butt-welded by a method such as a gas-tungsten-arc welding method (GTAW), but since the welding heat input is large, the welding heat-affected zone is affected. Is heated to the solution temperature described above, and the cooling rate thereafter is slower than 1000 ° C./min. Furthermore, in the part affected by welding heat, 7
It will be held at 00-950 ℃ for more than 3 seconds. Therefore, duplex stainless steel has hitherto been unsuitable as a material for such a thick-walled pipe. The present invention is characterized by positively using the B-containing duplex stainless steel for such applications.

[0023]

[Example 1] Duplex stainless steels having various chemical compositions were melted in a vacuum melting furnace at a rate of 150 kg, respectively, and the hot water was divided into 75 kg each.
By adding B to one of them, as shown in Table 1, components other than B are almost the same, and ingots of B-containing steels (A to I) and B-free steels (a to i) are obtained. It was The same alphabet indicates a combination of separated water, the uppercase letter is B-containing steel, and the lowercase letter is B-undoped steel.

The above ingot was heated to 1250 ° C., forged and rolled to obtain a steel plate having a thickness of 12 mm. The following tests (a) and (b) were performed using this steel sheet.

(A) Test for changing the cooling rate after solution heat The steel sheet was heated at 1100 ° C. for 30 minutes, and then 500
The average cooling rate up to ° C was varied from 5 to 1000 ° C / min.

(B) Test for changing aging conditions The steel sheet was heated at 1100 ° C. for 30 minutes, water-quenched, and then heat-treated by high-frequency induction heating at 700 to 950 ° C. for 3 to 3600 seconds.

In order to clarify the effect of B addition in both (a) and (b), the heat treatment conditions were the same for the B-containing steel and the B-free steel having the same basic composition. (A) and (b) above
After the above treatment, a 2 mm V-notch Charpy test piece of 10 mm × 10 mm × 55 mm was sampled from the T direction of the steel sheet (direction perpendicular to the rolling direction), and the impact value was measured.

Table 2 shows the test results of (a), that is, the relationship between the average cooling rate up to 500 ° C and the impact value. Table 3 shows (b)
The test results, that is, the relationship between the aging condition and the impact value are shown.

FIG. 1 shows a summary of the effects of B addition on the toughness after heat treatment shown in Tables 2 and 3. In the figure, for example, ○ in B has an impact value of about 35 kgf-m / cm ² (horizontal axis) for B-containing steel, but about 3 kgf-m / cm ² (vertical axis) for B-free steel. Is shown. That is, FIG. 1 shows that even if the same heat treatment is applied, the impact value of the B-free steel is much lower than that of the B-containing steel, and a small amount of B addition is an extremely important element for ensuring the toughness after the heat treatment. Is shown.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Example 2] The steel used in Example 1 was forged into a round billet, which was subjected to solution treatment with water cooling at 1100 ° C for 30 minutes and then bored to obtain the outer diameter and the wall thickness shown in Table 4. A steel pipe was produced. Using this steel pipe as a test material, butt welding was carried out by the welding method shown in Table 4, and a 2 mm V-notch Charpy test piece of 10 mm x 10 mm x 55 mm was taken from the longitudinal direction of the pipe so that the weld heat affected zone became the notch part. The impact value was measured.

Table 4 shows the measurement results of the impact value. Further, FIG. 2 collectively shows the effects of addition and non-addition of B as in the case of FIG.

As is clear from Table 4 and FIG. 2, in the steel pipe using the B-containing steel, high toughness is maintained even in the weld heat affected zone, whereas in the case of the B-free steel, the toughness decreases. Remarkable.

[0036]

[Table 4]

[0037]

Industrial Applicability The present invention is an epoch-making use of B-containing duplex stainless steel in the field where duplex stainless steel cannot be used conventionally. The B-containing duplex stainless steel can be manufactured at almost the same cost as the conventional duplex stainless steel, and the basic properties such as corrosion resistance and mechanical properties are superior to those of the B-free duplex stainless steel. Expanding its applications greatly contributes to industrial development. In addition, it can be used in the above new applications without fear of deterioration of toughness.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship of toughness of a B-containing duplex stainless steel and a B-free duplex stainless steel after heat treatment.

FIG. 2 is a diagram showing the toughness of a weld heat affected zone of a steel pipe made of a B-containing duplex stainless steel and a B-free duplex stainless steel.

Claims (2)

[Claims] 1. Used in a state of being kept in a temperature range of 700 to 950 ° C. for 3 seconds or more, or in a state of being exposed to a state in which the average cooling rate from the solution temperature to 500 ° C. is 1000 ° C./minute or less. Duplex stainless steel containing 0.0005 to 0.05% of B and maintaining good toughness even when exposed to any of the above states. 2. A wall thickness of 10 mm which is used by butt circumference welding.
The thick steel pipe above, whose material is B 0.0005-0.05
% Steel tube made of duplex stainless steel.