JPH03229839A - Manufacture of duplex stainless steel and its steel material - Google Patents

Abstract

PURPOSE:To show high strength and high corrosion resistance in a duplex stainless steel without requiring special soln. heat treatment by subjecting a duplex stainless steel contg. specified components to hot rolling under a specified condition and subjecting it to air cooling. CONSTITUTION:The compsn. of a steel is formed of, by weight, <=0.03% C, <=2.0% Si, <=5.0% Mn, 17.0 to <20.0% Cr, 2.0 to 7.0% Ni, 2.5 to 4.0% Mo, 0.005 to 0.05% Al, 0.01 to 0.2% N and the balance Fe with inevitable impurities, and its structure is regulated to a dual phase of austenite and ferrite. This steel is heated to >=1100 deg.C and is hot-worked under the condition of 900 to >=1100 deg.C finishing temp. After the completion of the working, the steel is air cooled. If required, the above components of the steel are incorporated with total <=3.0% of one or more kinds among Ti, Nb, V, Cu and W or total 0.001 to 0.05% of one or more kinds among Ca, Mg and B. This steel is suitable for use in a severe environment such as oil wells.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is an austenite-ferritic duplex stainless steel, which has excellent corrosion resistance, especially against carbon dioxide gas and hydrogen sulfide, and has high strength. The present invention relates to a duplex stainless steel that has been passed through for use in an oil well environment, etc., and a method for manufacturing steel materials made from the steel.

(Conventional technology) Duplex stainless steel is characterized by its high strength even when it is in solution form compared to normal austenitic stainless steel and ferritic stainless steel, and along with its excellent corrosion resistance, it is suitable for line pipes and oil country tubular products. Its uses are expanding. In the normal manufacturing process of these duplex stainless steel materials, carbonitrides, which are generated during the cooling process after hot forming,
Corrosion resistance is ensured by performing heat treatment to solutionize intermetallic compounds and redissolving them into solid solution. If high strength is desired, additional cold working is usually performed. As described above, the normal manufacturing method for duplex stainless steel materials requires steps such as solution heat treatment and cold working, so the manufacturing cost is extremely high.

In recent years, as a manufacturing method that omits solution heat treatment, a process (direct solution treatment) has been proposed in which ten analytes are dissolved in solid solution at high temperature, followed by hot forming and immediately quenched. For example, JP-A-59-182918 and JP-A-60
Publication No. 89519 discloses that by heating the material sufficiently at a high temperature,
There is a manufacturing method in which heat-opening is performed after the precipitates are completely dissolved, and carbonitrides and intermetallic compounds that reduce corrosion resistance are not precipitated by cooling before the introduced hot working strain has recovered. According to these methods, not only can solution treatment be omitted, but also high strength steel materials can be obtained.

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

One object of the present invention is to be able to develop high strength and high corrosion resistance without requiring special solution heat treatment on a separate line after manufacturing steel materials, as well as without requiring rapid cooling after hot working. Our goal is to provide phase stainless steel.

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

(Means for Solving the Problems) The present inventor has developed a duplex stainless steel that has good corrosion resistance and strength even at a cooling rate of, for example, air cooling, without particularly performing two cooling after hot working. , mainly aimed at obtaining by improving its chemical composition, carbonitrides, which reduce corrosion resistance,
The influence of added elements in duplex stainless steel on the precipitation of intermetallic compounds such as σ phase was investigated in detail. As a result, the following conclusions were obtained.

(2) Since the addition of Cr, Mo, and Ni significantly accelerates the precipitation of the σ phase, it is desirable to suppress the content of these to the minimum necessary level.

(2) Increase in C and N promotes precipitation of carbonitrides.

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

■Precipitation of carbides reduces corrosion resistance in environments containing carbon dioxide gas and trace amounts of hydrogen sulfide, but precipitation of nitrides has almost no negative effect.

(2) Addition of N increases the strength after direct solution treatment.

The present invention, which has been made by integrating the above knowledge, has the gist of the following duplex stainless steel and a method of manufacturing steel materials using the same.

(i) In 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%,
Mo: 2.5-4.0%, I/! :0.005~0
．． A duplex stainless steel of austenite and ferrite, containing 0.05%, N: 0.01 to 0.2%, and the remainder consisting of Fe and unavoidable impurities.

(ii) In addition to the components described in (1) above, Ti,
The total amount of one or more of Nb, V, Cu and W is 3.0% or less, or Ca, Mg and B
The total amount of one or more of the following: 0.001 to 0
．． Duplex stainless steel containing 0.05%.

(iii) In addition to the components described in (1) above, Ti
, the total amount of one or more of Nb, V, Cu and W is 3.0% or less, and the total amount of one or more of Ca, Mg and B is 0.001
Duplex stainless steel containing ~0.05%.

(1v) Using any of the two-phase stainless steels described in (11 to (3) above) as a material, hot working at a heating temperature of 1100°C or higher and a finishing temperature of 900 to 1100'C, followed by cooling. A method for producing a duplex stainless steel material having high strength and high corrosion resistance.

The duplex stainless steel of the present invention exhibits excellent properties due to the comprehensive effects of the above-mentioned components, and its main characteristics are as follows.

That is, Cr, Ni and M promote the precipitation of the σ phase.
Of O, N1 and Mo, which increase the corrosion resistance against trace amounts of hydrogen sulfide, are added at the same level or more than normal two-phase stainless steel, and C is added from 17%, the minimum value to form a ferrite-austenite two-phase structure. The range should be less than 20%.Since the precipitation of carbides greatly reduces corrosion resistance, the amount of C should be kept low (limited) to prevent carbide precipitation even in a direct solution treatment process such as air cooling. As mentioned above, N is not particularly kept low because it does not reduce the corrosion resistance, especially the corrosion resistance against trace amounts of hydrogen sulfide, and greatly increases the strength after direct solution treatment.

The above-mentioned duplex stainless steel of the present invention can be produced by simply cooling it by normal hot forming processing such as rolling, forging, extrusion, wire bowing, etc. without going through a rapid cooling process such as water cooling. This results in strength and 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 also no need to attach quenching equipment to the hot processing line, and it is possible to produce plates in an economical manner.
It can produce steel products in the form of rods, tubes, and other shapes.

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

(Function) First, the components constituting the duplex stainless steel of the present invention will be explained. All percentages regarding component content are by weight.
means.

Figure 1 shows 18χCr-5χNi-3χMo-0,1χ
These are the results of investigating the corrosion rate of N basic steel by changing the C content.

The test piece was made of melted lumber (thickness 60II 1m) heated at +
It was hot-rolled to a thickness of 15 mm at a finishing temperature of 1,000°C and air-cooled, and then cut into a 2 x 10 x 75 (mm) piece as shown in Fig. 3(a) with 0.25n+ in the center.
It has a notch of mR. The test piece 1 was bent by the bending jig 2 as shown in FIG. 2(b) until the stress σ shown in FIG. 1(c) was 1σ.

(σ, : 0.2% proof stress), 5% NaCj! +0.1atmHzS
The test was carried out by immersion in a +30 atm COz environment (+20"C) for 366 hours.

It is clear from FIG. 1 that the lower the C content, the lower the corrosion rate. This is because when the C content is as low as 0.03% or less, carbides that reduce corrosion resistance will not precipitate even if the steel is cooled at a cooling rate comparable to 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 reduces toughness.

It is also an unfavorable component from the viewpoint of corrosion resistance.

Therefore, in the present invention, 2.0% is set as the allowable upper limit.

Furthermore, it is desirable to suppress Si to 1.0% or less.

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

Cr: It is most directly related to the precipitation of the σ phase, and the amount of Cr at which the σ phase no longer precipitates when its content is reduced is 20.0%.
less than

Figure 2 shows 0.01χC=5χNi-3χMo-0,1
These are the results of a corrosion test under the same test conditions as in FIG. 1 above, when the basic composition was 5χN and the Cr content was varied.

As shown in FIG. 2, when the Cr content is less than 20%, no cracking occurs and the corrosion rate decreases rapidly. That is,
Even if Cr is reduced, corrosion resistance in an environment containing carbon dioxide gas and trace amounts of hydrogen sulfide will not deteriorate, but if it is less than 17.0%, martensite will be produced and corrosion resistance will be reduced. Therefore, Cr
The content range was from 17.0% to less than 20.0%.

Ni: An important element for ensuring corrosion resistance in an environment 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, even if the N content exceeds 7.0%, the corrosion resistance improvement effect is saturated and the σ phase tends to precipitate.
The appropriate content of i is 2.0 to 7.0%.

MO= Like N1, it is an element that greatly contributes to improving corrosion resistance in environments containing carbon dioxide gas and trace amounts of hydrogen sulfide.

If it is less than 2.5%, no effect can be expected, and if it exceeds 4.0%, the σ phase tends to precipitate, making it unsuitable for the direct solution treatment process.

N: Nitride precipitated in the direct solution treatment process does not reduce corrosion resistance in an environment containing carbon dioxide gas and trace amounts of hydrogen sulfide. Therefore, unlike duplex stainless steel to which the conventional direct solution treatment process is applied, it is not necessary to suppress the N content to an extremely low level.

Rather, N has a great effect of increasing the strength after direct solution treatment.

If it is less than 0.01%, there is no effect, and if it is introduced to 0.2%, hot workability will decrease, so the N content should be 0.01%.
~0.2%.

8L: Added for deoxidation, and to ensure its effect, the content must be 0.005% or more. However, if the Al content exceeds 0.05%, AIN will precipitate and the corrosion resistance will decrease.

Ti, Nb, V, Cu, and W: All have the effect of increasing strength, so one or more of them can be contained as needed. If the content of each component exceeds 3.0% when one type is used, or if the total content exceeds 3.0% when two or more types are used, hot workability deteriorates.

Ca, Mg, B: May be added to improve hot workability when severe processing is performed. For that purpose, it is necessary to contain one or more types in a total amount of 0.001% or more. Moreover, if the total content of one or more types exceeds 0.05%, corrosion resistance will decrease.

Groups of Ti, Nb, V, Cu and W and Ca,
The Mg and B groups may contain both within the above range.

Note that both S and P are undesirable impurities that reduce hot workability. It is preferable to reduce S to 0.002% or less and P to 0.05% or less, respectively, as much as possible.

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

As mentioned above, this method includes a heating temperature of 1100°C or higher,
It is characterized by performing hot working at a finishing temperature of 900 to 1100° C. and allowing it to cool after completion of the working.

Heating temperature: This heating is performed in order to make hot working acceptable and to completely dissolve carbonitrides and 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 in short, it may be set to a time sufficient to completely homogenize the core of the material.

Finishing temperature: The lower the temperature, the easier the work hardening and the greater the increase in strength.However, if the temperature is lower than 900°C, the precipitation of carbides and σ phase will be significant, resulting in a decrease in corrosion resistance. If the finishing temperature exceeds 1100°C, work hardening will not occur due to recrystallization recovery, so no increase in strength will be obtained.

Under the above conditions, rolling (including Mannesmann piercing rolling),
After processing such as forging, extrusion, drawing, etc., the special elephant,
A major advantage of the duplex stainless steel of the present invention is that it does not require refrigeration. That is, by adjusting the alloy components as described above, the duplex stainless steel of the present invention is difficult to precipitate carbides and intermetallic compounds even at a cooling rate comparable to that of air cooling. Therefore,
After hot working, it is sufficient to allow the material to cool in the atmosphere (so-called air cooling) from the working end temperature. However, for members whose wall thickness exceeds 50 mm, the cooling rate at the center will be slow.
Since there is a risk that the material will not have sufficient corrosion resistance, forced cooling such as water cooling may be used.

(Example) Steels having the components shown in Table 1 were melted using a high-frequency induction heating (vacuum) melting furnace to obtain steel ingots weighing 100 kg each. These steel ingots were forged and bloomed to a thickness of 75n+m x width of 10mm.
A block of 0 mm x 200 mm in length was prepared.

Next, hot rolling was performed under the processing conditions shown in Table 2, and 30
After finishing to a thickness of mm, air-cooled direct solution treatment or normal reheating solution treatment was performed, and the yield strength and stress corrosion resistance in an atmosphere containing a trace amount of hydrogen sulfide were investigated.

The tensile test was conducted by taking a tensile test piece with a diameter of 4 mm and a parallel portion 341. The corrosion test was carried out under the same conditions as the tests shown in FIGS. 1 and 2, and two test pieces were prepared.

The results of the corrosion test were evaluated by taking out the sample after the 366-hour immersion test and observing the appearance with the naked eye and observing the cross section of the test piece with an optical microscope to check for cracks and selective corrosion.

These test results are also shown in Table 2. In the evaluation of corrosion resistance, ○○ indicates that there was no cracking or selective corrosion in both test pieces, and XX indicates that cracking or selective corrosion was observed in both test pieces.

Steel types A and B in Table 1 are commonly used 22
% Cr, and 25% Cr duplex stainless steel.

Steel types C, D, E, and F are 18% Cr duplex stainless steels of the present invention.

Comparative Examples 1.2.3 shown in Table 2 are examples of conventional processes in which solution treatment is performed by reheating after processing.

Steel materials obtained by this method have no problem with corrosion resistance, but have significantly low strength.

Comparative Example 4.5 is a process in which solution treatment is performed directly after rolling, but air cooling is applied to the conventional methods @A and B in Table 1. Since these steel types have a high Cr content, the σ phase precipitates during air cooling, and although the strength is increased, the corrosion resistance is extremely poor. Comparative Example 6 was made of the steel of the present invention, but because the finishing temperature of hot rolling was too high, there was no work hardening effect and the strength was low.

In contrast to these comparative examples, in Examples 1 to 6 of the present invention, excellent corrosion resistance was obtained even though the cooling for direct solution treatment was performed by standing to cool. Furthermore, the strength is higher than that of conventional reheated solution-treated materials (Comparative Examples 1 to 3) (hereinafter referred to as the margin) (Effects of the invention) As specifically shown in the examples, the duplex stainless steel of the present invention has Even if treated through a simple process of cooling after hot working, it can be made into a steel material with high strength and high corrosion resistance. Therefore, by using the duplex stainless steel of the present invention, it becomes possible to produce a steel material that can be used in harsh environments at low production cost without requiring special equipment such as a quenching device.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the corrosion rate of duplex stainless steel in an atmosphere containing a trace amount of hydrogen sulfide and the C content, Figure 2 is a diagram showing the relationship with the Cr content, and Figure 3 Figure (a) is a diagram showing the shape of the test piece used in the corrosion test,
FIGS. 3(b) and 3(C) are diagrams illustrating a method of applying stress to a test piece.

Claims (5)

[Claims] (1) In weight%, C: 0.03% or less, Si: 2.0%
Below, Mn: 5.0% or less, Cr: 17.0 to 20.0
%, Ni: 2.0-7.0%, Mo: 2.5-4.
0%, Al: 0.005~0.05%, N: 0.01~
A duplex stainless steel of austenite and ferrite containing 0.2% and the remainder consisting of Fe and unavoidable impurities. (2) In addition to the components described in claim (1), Ti, N
Duplex stainless steel containing one or more of V, Cu and W in a total amount of 3.0% or less. (3) In addition to the components described in claim (1), Ca, M
The total amount of one or more of g and B is 0.
Duplex stainless steel containing 0.001 to 0.05%. (4) In addition to the components described in claim (1), Ti, N
The total amount of one or more of b, V, Cu, and W is 3.0% or less, and the total amount of one or more of Ca, Mg, and B is 0.001 to 0. .05
Duplex stainless steel containing %. (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 higher and a finishing temperature of 900 to 1100°C, and after the completion of this working. , a method for producing a duplex stainless steel material having high strength and excellent corrosion resistance, characterized by allowing it to cool.