JP2952929B2 - Duplex stainless steel and method for producing the same - Google Patents

Description

The present invention relates to an austenitic-ferritic duplex stainless steel having excellent corrosion resistance to carbon dioxide gas and hydrogen sulfide, and high strength. The present invention relates to a duplex stainless steel suitable for use in an oil well environment and the like, and a method for producing a steel material using the steel as a material.

(Prior art) Duplex stainless steel is characterized by its high strength even in solution as compared with ordinary austenitic stainless steel and ferritic stainless steel. Applications are expanding. In the normal manufacturing process of these duplex stainless steel materials, after hot forming, heat treatment for solutionizing carbonitrides and intermetallic compounds generated in the process of cooling is performed, and corrosion resistance is ensured by re-dissolving them. doing. When high strength is desired, it is common to perform further cold working. As described above, in the ordinary method for producing a duplex stainless steel material, a solution heat treatment and a cold working step are required, and thus the production cost is extremely high.

In recent years, as a manufacturing method that omits the solution heat treatment, a process of directly forming a solid solution at a high temperature, performing hot forming, and immediately quenching (direct solution treatment) has been proposed. For example, JP-A-59-182918 and JP-A-60-89519
Japanese Patent Application Publication No. 2004-214,1992 discloses a method of heating a material sufficiently at a high temperature, completely dissolving the precipitates, performing hot forming, rapidly cooling before the introduced hot working strain is recovered, and reducing the corrosion resistance. Manufacturing methods that do not precipitate nitrides or intermetallic compounds have been proposed. According to these methods, not only the solution treatment can be omitted, but also a high-strength steel material can be obtained.

(Problems to be Solved by the Invention) In each of the manufacturing methods proposed so far, a process of rapidly cooling from 800 ° C. or more after hot forming is essential,
In order to carry out this method, rapid cooling equipment such as water cooling is required in a place near the rolling line. Therefore, there is a problem that this technique can be implemented only in a limited specific line having a quenching facility.

One object of the present invention is not only to eliminate the need for a special solution heat treatment on a separate line after steel production, but also to achieve high strength and high corrosion resistance without requiring rapid cooling of hot working heat. It is to provide a duplex stainless steel.

Another object of the present invention is to provide a method for producing a high-strength, high-corrosion-resistant steel material by a simple process using the above duplex stainless steel.

(Means for Solving the Problems) The present inventor, after hot working, does not particularly need to perform rapid cooling,
For example, we aim to obtain a duplex stainless steel that has good corrosion resistance and strength even at a cooling rate of about air cooling, mainly by improving its chemical composition, and reduce the corrosion resistance of metals such as carbonitride and σ phase. The effect of the added elements of the duplex stainless steel on the precipitation of intermetallic compounds was investigated in detail. as a result,
The following conclusions were obtained.

Since the addition of Cr, Mo, and Ni significantly promotes the precipitation of the σ phase, their contents are desirably suppressed to the minimum necessary.

The increase in C and N promotes carbonitride precipitation.

In order to maintain corrosion resistance in an environment containing carbon dioxide gas and trace amounts of hydrogen sulfide, Ni and Mo are important and cannot be reduced so much.

The precipitation of carbides reduces the corrosion resistance in an environment containing carbon dioxide gas and trace amounts of hydrogen sulfide, but the precipitation of nitrides has almost no adverse effect.

The addition of N increases the strength after direct solution treatment.

The gist of the present invention based on the above findings is the following duplex stainless steel and a method of manufacturing a steel material using the same.

(I) C: 0.03% or less, Si: 2.0% or less, Mn: 5.0% by weight
%: Cr: 17.0 to less than 20.0%, Ni: 2.0 to 7.0%, Mo: 2.5
~ 4.0%, A: 0.005 ~ 0.05%, N: 0.01 ~ 0.2%,
Austenitic and ferritic duplex stainless steels with the balance being Fe and inevitable imperfections.

(Ii) In addition to the components described in (1) above, Ti, Nb,
In the total amount of one or more of V, Cu and W
Duplex stainless steel containing not more than 3.0%, or 0.001 to 0.05% in total of one or more of Ca, Mg and B.

(Iii) In addition to the components described in (1) above, Ti, Nb,
In the total amount of one or more of V, Cu and W
A duplex stainless steel containing not more than 3.0% and 0.001 to 0.05% in total of one or more of Ca, Mg and B.

(Iv) Heating temperature of 1100 ° C or more, finishing temperature using the duplex stainless steel of any of (1) to (3) above
A method for producing a duplex stainless steel material having high strength and high corrosion resistance, wherein hot working is performed at a temperature of 900 to 1100 ° C., followed by cooling.

The duplex stainless steel of the present invention exhibits excellent characteristics by the comprehensive action of the above-mentioned components, and its main features are as follows.

That is, of the Cr, Ni and Mo that promote the precipitation of the σ phase, Ni and Mo that enhance the corrosion resistance to trace amounts of hydrogen sulfide are added at the same level as or more than ordinary duplex stainless steel, and
Is in the range of 17% to less than 20% of the minimum value for obtaining a two-phase structure of ferrite-austenite. Since the precipitation of carbides greatly reduces the corrosion resistance, the amount of carbon is limited to a low level in order to prevent the precipitation of carbides even in a direct solution heat treatment process such as air cooling. The precipitation of nitride does not reduce the corrosion resistance as described above, in particular, the corrosion resistance to trace amounts of hydrogen sulfide,
Since the strength after the direct solution solution is greatly increased, N is not particularly suppressed.

The duplex stainless steel of the present invention has the strength and strength necessary for ordinary hot forming, such as rolling, forging, extruding, drawing, etc., and then simply allowing it to cool without passing through a quenching process such as water cooling. It comes to have corrosion resistance. Therefore, if this duplex stainless steel is used as a material, not only is there no need for off-line solution heat treatment, but there is no need to attach a quenching facility to the hot working line, and plates and rods can be produced economically. , Pipes and other shapes of steel.

The method (iv) described above is suitable as a method for processing a steel material made of the duplex stainless steel.

(Operation) First, the components constituting the duplex stainless steel of the present invention will be described. In addition, all the percentages related to the component contents mean weight%.

C: FIG. 1 shows the result of examining the corrosion rate of the basic steel of 18% Cr-5% Ni-3% Mo-0.1% N while changing the C content.

The test piece was prepared by heating a solution (thickness: 60 mm) at a heating temperature of 1200 ° C.
2x10x75 (mm) shown in Fig. 3 (a) cut out from hot-rolled and air-cooled at a finishing temperature of 1000 ° C to a thickness of 15mm
And has a notch of 0.25mmR at the center. The test piece 1 was subjected to bending stress by the bending jig 2 so that the stress σ shown in FIG. 2C became 1σ _y (σ _y : 0.2% proof stress) as shown in FIG. , 5% NaCl + 0.1atmH ₂
It was immersed in an environment of S + 30 atm CO ₂ (120 ° C.) for 366 hours.

From FIG. 1, it is clear that the lower the C content, the lower the corrosion rate. This is because when C is as low as 0.03% or less, carbides that deteriorate corrosion resistance do not precipitate even when cooled at a cooling rate of about air cooling after hot working. For this reason, in the present invention, the C content is set to 0.03% or less.

Si: Si promotes precipitation of the σ phase and lowers toughness. Also,
It is also an undesirable component from the viewpoint of corrosion resistance. Therefore, in the present invention, 2.0% is set as the allowable upper limit. In addition, Si is 1.0%
It is desirable to keep it below.

Mn: Mn stabilizes austenite and suppresses martensite formation. There is also an advantage of increasing the strength. However, when the content of Mn exceeds 5.0%, the hot workability decreases, so the upper limit of the content is 5.0%.

Cr: Most directly related to precipitation of the σ phase, the amount of Cr at which the content is reduced so that the σ phase does not precipitate is less than 20.0%.

FIG. 2 shows the results of a corrosion test under the same test conditions as in FIG. 1 when the Cr content was changed based on the basic composition of 0.01% C-5% Ni-3% Mo-0.15% N.

As shown in FIG. 2, when the Cr content is less than 20%, no cracking occurs, and the corrosion rate rapidly decreases. That is, Cr
Although the corrosion resistance in an environment containing carbon dioxide gas and a trace amount of hydrogen sulfide does not decrease even if the content is reduced, if less than 17.0%, martensite is formed to lower the corrosion resistance. Therefore, the range of the Cr content is set to 17.0% to less than 20.0%.

Ni: An important element for ensuring corrosion resistance in environments containing carbon dioxide gas and trace amounts of hydrogen sulfide. In order to reduce the cracking susceptibility to trace amounts of hydrogen sulfide, a content of 2.0% or more is required. However, if the content exceeds 7.0%, the effect of improving corrosion resistance is saturated, and the σ phase is easily precipitated, so that the appropriate content of Ni is 2.0 to 7.0%.

Mo: Like Ni, it is an element that greatly contributes to the improvement of corrosion resistance in environments containing carbon dioxide gas and trace amounts of hydrogen sulfide. If it is less than 2.5%, the effect cannot be expected, and if it exceeds 4.0%, the σ phase is likely to precipitate, which is not suitable for the direct solution treatment.

N: Nitride precipitated in the direct solution process does not reduce corrosion resistance in an environment containing carbon dioxide gas and trace amounts of hydrogen sulfide. Therefore, it is not necessary to suppress N to an extremely low content unlike the duplex stainless steel to which the conventional direct solution process is applied. Rather, N has a large effect of directly increasing the strength after solution treatment. Less than 0.1% has no effect, 0.2%
If N exceeds N, the hot workability decreases, so the N content is
0.01 to 0.2%.

A: It is added for deoxidation, and its content is required to be 0.005% or more to ensure its effect. However, when the content of A exceeds 0.05%, AN precipitates and the corrosion resistance decreases.

Ti, Nb, V, Cu, W: one or two
More than one species can be included as needed. If one kind is used, and if two or more kinds are used, the hot workability is reduced when the total content exceeds 3.0%.

Ca, Mg, B: may be added to improve hot workability when severe working is performed. One or two for that purpose
It is necessary that the total content of the elements is 0.001% or more. If one or more of them exceeds 0.05% in total, the corrosion resistance decreases.

Ti, Nb, V, Cu and W groups with Ca, Mg and B
May contain both in the above range.

S and P are both undesirable impurities that degrade hot workability. S is 0.002% or less, P is 0.0
It is better to reduce each to 5% or less as much as possible.

Next, a desirable method for producing a steel material using the duplex stainless steel of the present invention will be described.

As described above, this method is characterized in that hot working is performed under the conditions of a heating temperature of 1100 ° C. or more and a finishing temperature of 900 to 1100 ° C., and after the working, cooling is performed.

Heating temperature: This heating is performed to facilitate the hot working and to completely dissolve the carbonitride and the σ phase to homogenize the material. To achieve these objectives, a heating temperature of 1100 ° C or higher is required. The upper limit of the heating temperature is desirably about 1350 ° C. The heating time depends on the size of the material and the heating temperature, but it is sufficient that the heating time is sufficient to completely homogenize the core of the material.

Finishing temperature: The lower this temperature, the easier the work hardening and the greater the strength increase, but when working at a constant temperature of more than 900 ° C., the precipitation of carbides and σ phase becomes remarkable and the corrosion resistance decreases. Finish temperature
If the temperature exceeds 1100 ° C., work hardening due to recrystallization recovery does not occur, so that an increase in strength cannot be obtained.

It is a great advantage of the duplex stainless steel of the present invention that it is not necessary to perform special quenching after performing processing such as rolling (including Mannesmann piercing rolling), forging, extrusion, and drawing under the above conditions. . That is, by adjusting the alloy components described above, in the duplex stainless steel of the present invention, carbides and intermetallic compounds hardly precipitate even at a cooling rate of about air cooling. Therefore, after hot working, it is sufficient to perform cooling in the air (so-called air cooling) from the working end temperature. However, the thickness is 50mm
If the member exceeds the above, the cooling rate in the central portion may be slowed and the corrosion resistance may not be sufficient. Therefore, forced cooling such as water cooling may be performed.

(Examples) Steels having the components shown in Table 1 were melted using a high-frequency induction heating vacuum melting furnace to form 100 kg ingots. These steel ingots are forged and divided into blocks and have a thickness of 75 mm x width 100 mm x length 200
mm blocks were made.

Next, hot rolling was performed under the processing conditions shown in Table 2,
After finishing to a thickness of mm, air-cooled direct solution treatment or ordinary reheating solution treatment was performed, and the yield strength and stress corrosion cracking resistance in an atmosphere containing a trace amount of hydrogen sulfide were investigated.

The tensile test was performed by sampling a tensile test piece having a diameter of 4 mm and a parallel portion of 34 mm. The corrosion test was performed under the same conditions as those of the test shown in FIGS. 1 and 2, and two test pieces were prepared. The result of the corrosion test is a 366 hour immersion test, the sample is taken out,
The presence or absence of cracks and the presence or absence of selective corrosion were investigated and evaluated by visual observation with the naked eye and observation of the cross section of the test piece with an optical microscope.

The test results are shown in Table 2. In the evaluation of corrosion resistance, ○ indicates that neither two test pieces had cracks and no selective corrosion, and XX indicates that both two pieces had cracks or selective corrosion observed.

Table 1 shows that steel types A and B are commonly used.
% Cr and 25% Cr duplex stainless steels. Steel types C, D, E and F are the 18% Cr-based duplex stainless steels of the present invention.

Comparative Examples 1 and 2 shown in Table 2 are examples of a conventional process in which a solution is formed by reheating after processing. The steel obtained by this method has no problem in corrosion resistance, but has a remarkably low strength.

Comparative Examples 3 and 4 are processes in which a solution is formed directly after rolling, and the cooling is applied to the conventional steels A and B in Table 1 by using air cooling. Since these steel types have a high Cr content, a σ phase precipitates during the air cooling, and although the strength is increased, the corrosion resistance is extremely poor.

In contrast to these comparative examples, in Examples 1 to 6 of the present invention, excellent corrosion resistance was obtained even though cooling for direct solution treatment was performed by cooling. Moreover, the strength is higher than that of the conventional reheated solution-treated material (Comparative Examples 1 and 2).

(Effects of the Invention) As specifically shown in the examples, the duplex stainless steel of the present invention is a steel material having high strength and high corrosion resistance even if it is processed by a simple process of cooling after hot working. Become. Therefore, the use of the duplex stainless steel of the present invention makes it possible to produce a steel material that can be used in a severe environment at a low production cost without requiring special equipment such as a quenching device.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the corrosion rate of a duplex stainless steel in an atmosphere containing a trace amount of hydrogen sulfide and the C content, and FIG. 2 is also the relationship between the Cr content. FIG. 3 (a) is a diagram showing the shape of a test piece used in a corrosion test, FIGS. 3 (b) and (c) are diagrams for explaining a method of applying stress to the test piece, It is.

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Claims (5)

(57) [Claims] (1) In terms of% by weight, C: 0.03% or less, Si: 2.0% or less,
Mn: 5.0% or less, Cr: 17.0 to less than 20.0%, Ni: 2.0 to 7.0%,
Duplex stainless steel of austenite and ferrite containing Mo: 2.5 to 4.0%, A: 0.005 to 0.05%, N: 0.01 to 0.2%, with the balance being Fe and unavoidable impurities. (2) In addition to the component described in (1), T
A duplex stainless steel containing 3.0% or less in total of one or more of i, Nb, V, Cu and W. (3) In addition to the component described in (1), C
a, Mg and B in one or more of the total amount
Duplex stainless steel containing 0.001 to 0.05%. 4. In addition to the components described in (1), T
One or more of i, Nb, V, Cu, and W in a total amount of 3.0% or less, and one of Ca, Mg, and B
2 containing 0.001 to 0.05% in total of two or more species
Phase stainless steel. 5. The duplex stainless steel according to any one of claims (1) to (4) is hot worked at a heating temperature of 1100 ° C. or more and a finishing temperature of 900 to 1100 ° C. A method for producing a duplex stainless steel material having high strength and excellent corrosion resistance, which is left to cool after completion.