JPH04165019A - Manufacture of high corrosion-resistant seamless two-phase stainless steel tube - Google Patents

Abstract

PURPOSE:To stably mass-produce high strength two-phase stainless steel seamless tubes having excellent corrosion resistance and good toughness by hot-rolling a two-phase stainless steel ingot of specified compsn. in specified conditions to obtain a raw tube material, forming this raw tube material into a seamless steel tube by a conventional method, and then applying the tube to a solution heat treatment. CONSTITUTION:A two-phase stainless steel ingot incorporating, by wt.%, <=0.03% C, <=0.7% Si, <=2.0% Mn, <=0.030% P, <=0.003% S, 5-8% Ni, 22-26% Cr, 2.5-4.5% Mo, 0.14-0.30% N, and 0.010-0.040% Al is prepared. This ingot is formed into a raw tube material by hot working process in which soaking treatment at 1220-1270 deg.C held for 24 hours or longer is applied before the finish stage. Then the raw tube material is formed into a seamless steel tube by a conventional method. This tube is heated and maintained at the temp. for usual solution heat treatment, and then rapidly cooled at >=5X10<4> deg.C/hr cooling rate from 950 deg.C to 750 deg.C to obtain the objective seamless steel tube.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is applicable to heat exchanger tubes and associated piping that come into contact with seawater, oil and natural gases containing corrosive gases such as H, S, and CO2. The present invention relates to a method for manufacturing a high-strength seamless duplex stainless steel pipe with excellent corrosion resistance and toughness, which is suitable for use as oil country tubular goods for extracting gas, transportation pipes for transporting the above-mentioned oil and natural gas, and the like.

(Prior art and its problems) In recent years, marine equipment such as heat exchangers that use seawater have been manufactured using JISSUS329J1 class (Pe-3 to 6), which has relatively good corrosion resistance even in such severe corrosive environments.
wtXNi -23~28wtχCr-1~3wtχM
Duplex stainless steels such as O-series steels have come to be used. In addition, it has been pointed out that the above standard steel does not have sufficient corrosion resistance against high-temperature seawater, so in addition to the basic components of Cr, Ni, Mo, and N, w, v, Ti, Zr, Nb+Ta, etc. A duplex stainless steel was also proposed in which the above characteristics were improved by adding one or more of the following (Japanese Patent Publication No. 43807/1983). This steel type has excellent corrosion resistance against high-temperature seawater and has relatively good workability, so it is used not only in various chemical plant equipment, but also in H and S steels, which have more severe corrosive environments.

Applications are also expanding to pipes for drilling and transporting oil and natural gas, including CO□.

Note that the process shown in FIG. 4 is normally employed to manufacture seamless duplex stainless steel pipes for such uses. However, duplex stainless steel has poor hot workability compared to other steel types, and as a result, cracks are more likely to occur during tube manufacturing. Therefore, a proposal was made to improve the manufacturing stability of seamless duplex stainless steel pipes, such as ``adjusting the billet heating temperature and added strain during processing to a specific range during pipe manufacturing using the plug mill method'' (Special Publication No. 61-2743). ), and expectations for increased demand for seamless duplex stainless steel pipes are growing stronger.

However, recently, the environment in which marine equipment and equipment are used, oil and
The natural gas drilling environment is becoming increasingly harsh, and as a result, the requirements for corrosion resistance and strength for the steel pipes used are becoming even more stringent, and there are concerns that conventional duplex stainless steel will no longer be able to meet these requirements. It's here.

In general, if a cost balance can be achieved, "
It is known that increasing the content of the main components Cr+Ni, Mo and N is effective. However, C
When the amounts of r, Mo, and Ni are increased, intermetallic compounds mainly consisting of sigma phase are generated during tube manufacturing (during billet rolling and forging, during hot tube manufacturing, and during cooling after tube heat treatment, etc.). This causes the problem of further worsening hot and cold workability.

In particular, when the material is cooled to around room temperature during pipe manufacturing, the toughness deteriorates significantly, and depending on the amount of sigma phase precipitation and processing strain rate, the workability is extremely reduced and subsequent cold processing is difficult. There was also a fear that it would become impossible. Moreover, the presence of the sigma phase improves corrosion resistance (mainly J
- resistance to pitting corrosion and crevice corrosion in aqueous solutions containing
It was difficult to secure desired performance due to deterioration in toughness and toughness.

The precipitation temperature of the sigma phase is 950 to 750°C, and it is known that the precipitation occurs due to the diffusion of Cr and Mo elements. In the normal seamless pipe manufacturing process, it is during cooling after billet rolling and forging.

Precipitation of the sigma phase, which has an adverse effect, is likely to occur during cooling after hot pipe forming and during cooling after heat treatment, but especially during cooling after hot working, the above elements are easily diffused due to residual processing strain. Therefore, the precipitation of the sigma phase is accelerated compared to cooling after heat treatment without strain.

In addition, although the addition of N is effective in improving corrosion resistance and strength as described above, it also has the disadvantage of increasing deformation resistance during hot working, and when heated to high temperatures. Nitride is formed due to the solid solubility of N, and C
This also led to behavior that caused an r-deficient layer and deteriorated corrosion resistance and toughness.

Therefore, the purpose of the present invention was to establish a means for industrially stably mass-producing high-strength duplex stainless steel seamless pipes with sufficiently excellent corrosion resistance and toughness. .

<Means for Solving the Problems> Therefore, as a result of intensive research to achieve the above object, the present inventors found that [Cr,
Duplex stainless steel with increased Ni, Mo, and N contents has the problem that intermetallic compounds, mainly sigma phase, are easily formed and impede workability, as described above. If the component segregation is reduced by reducing the amount and performing soaking heat treatment during billet making, the precipitation of sigma phase and nitrides during hot forming of seamless steel pipes will be significantly suppressed, and the workability will be significantly improved. In addition to this, by adjusting the cooling rate during solution heat treatment after pipe manufacturing, high-strength seamless duplex stainless steel pipes that exhibit extremely excellent corrosion resistance and toughness with minimal sigma phase and nitrides can be produced with high efficiency. We were able to obtain the knowledge that "it will be possible to obtain stable results under good workability."

The present invention was made based on the above-mentioned findings, etc. rc: 0.03% or less (hereinafter, % representing the component ratio is expressed as weight %).

Si: 0.7% or less, Mn: 2.0%
below.

P: 0.030% or less, S: 0.003
%below.

Ni: 5-8%, Cr: 22-26%.

Mo: 2.5-4.5%, N: 0.14
~0.30%.

1200-127 from duplex stainless steel slab containing A170.010-0.040%
After obtaining a pipe-making material by hot working, which involves applying a soaking treatment in a temperature range of 0°C for 24 hours or more before the final process, the material is used to form a seamless steel pipe of a predetermined size by a conventional method, Next, 950-7 after heating and holding
By performing solution heat treatment with a cooling rate of 50°C adjusted to 5 x 104°C/hr or more, seamless duplex stainless steel pipes with excellent corrosion resistance and toughness can be stably manufactured. It has great characteristics.

In other words, the present invention aims to further improve corrosion resistance by using high Cr.
, Ni, Mo, and N, and a duplex stainless steel with reduced Si content to suppress precipitation of sigma phase is used as a material, and a) a material for pipe making is manufactured from the slab by hot working. In this process, if the slab is made into a billet by one hot working, it is directly applied to the slab, or if the slab is made into a billet by multiple hot workings, at least the final hot working is performed. Before that, a soaking treatment under specific conditions is carried out, followed by final hot working to obtain a material for pipe making. The "hot working" method is used in the rolling pipe manufacturing method, such as the Eugene-Séjournet method, or the extrusion pipe manufacturing method, such as the Eugene-Séjournet method. - Forming a seamless steel pipe of predetermined dimensions by "cold working" such as rolling with a mill), and c) Further, after heating and holding the formed seamless steel pipe at a solution temperature. Temperature range between 950-750℃ 5 x 104℃
Through a series of processes in which solution heat treatment is performed under conditions of cooling at a cooling rate of /hr or more, seamless steel pipe products are manufactured while effectively preventing precipitation of sigma phase and nitrides due to component segregation. The main point of this invention is to provide a high-strength seamless duplex stainless steel pipe that has both high corrosion resistance and high toughness without causing problems such as cracking. The reason why the composition and processing conditions are limited as described above will be explained together with their effects.

<Function> A) Component composition of steel material DanC has the effect of ensuring the strength of steel pipes, but if its content exceeds 0.030%, the formation of carbides in the steel becomes noticeable and corrosion resistance deteriorates. Due to deterioration, the C content is 0.
．． It was set as 0.30% or less.

Si is added for the purpose of deoxidizing during melting, but if the Si content exceeds 0.7%, precipitation of sigma phase becomes easy, leading to a decrease in corrosion resistance, toughness, and workability. However, when the Si content is reduced to 0.7% or less, the precipitation of sigma phases and nitrides is delayed, and the formation of sigma phases and nitrides during cooling after hot working is suppressed, resulting in desired corrosion resistance, toughness, and Since it is possible to ensure workability, the Si content is 0.7.
% or less.

In addition, Fig. 1 shows Fe-0,02χC-0,5χMn
6XNi -25χCr-4χMo-0.28χN-0
．． Soaking (1200
℃×24hr) → Forging → Solution treatment (1080℃×3
Cooling at lXl0'°C/hr after m1n) → Heat treatment under conditions where sigma phase is likely to precipitate (820°C x air cooling after 1 m1n).

2 is a graph showing the results of an investigation on the "relationship between Si content and sigma phase precipitation state" for test pieces obtained by subjecting them to the following treatments. Here, since the state of precipitation of the sigma phase closely corresponds to the impact value, in FIG. 1, the "state of precipitation of the sigma phase" is shown in place of the "impact value."

This figure 1 also shows that by suppressing the Si content to 0.7% or less, the precipitation of the sigma phase can be sufficiently suppressed, and a practically satisfactory "impact value: 5 kg-m/c+J or more" can be secured. I understand.

Mn is added to improve desulfurization and hot workability during melting of duplex stainless steel, but when it is added in an amount exceeding 2.0%, it improves corrosion resistance (pitting corrosion resistance, crevice corrosion resistance).

Since the Mn content (general corrosion resistance) deteriorates, the Mn content should be set to 2.
It was set as 0% or less.

Although P is an impurity element that inevitably mixes into steel, if its content exceeds 0.030%, the toughness deteriorates significantly, so the P content was set at 0.030% or less.

S is also an impurity element that inevitably mixes into steel, but if its content exceeds 0.003%, hot workability will deteriorate, so the S content should be 0.003% or less. Established.

Ni is an essential component for stabilizing austenite, but if it is contained in an amount exceeding 8%, the precipitation of the sigma phase becomes extremely easy, while if its content is less than 5%, the ferrite phase increases. The Ni content was determined to be 5 to 8% because nitrides are easily formed and both lead to deterioration of corrosion resistance.

Cr has the effect of improving strength and corrosion resistance, but if the content is less than 22%, the desired effect cannot be obtained, while if the content exceeds 26%, the precipitation of sigma phase may occur. The Cr content is 22.
It was set at ~26%.

Isao Mo also has the effect of improving corrosion resistance and strength, but
If the Mo content is less than 2.5%, the desired effect due to the above action cannot be obtained, while if the Mo content exceeds 4.5%, precipitation of the sigma phase is likely to occur.
．． It was set at 5 to 4.5%.

N also exhibits the effect of improving corrosion resistance and strength, but if its content is less than 0.14%, the desired effect of the above effect cannot be obtained, while on the other hand, if the content exceeds 0.30%, nitridation occurs. The N content was determined to be 0.14 to 0.30% since the precipitation of substances would have an adverse effect on corrosion resistance and toughness.

M is essential for deoxidizing low-oxygen stainless steel, and in the case of low St, it is necessary to ensure a content of 0.010% or more. On the other hand, if the M content exceeds 0.040%, AfN will be generated and the quality will deteriorate, so the M content is set at 0.010 to 0.040%.

In addition to the above components, the material steel also contains W, V, and Cu.

One or more of Zr, Ti+Nb, Ta, and Ca may be contained. Here, although Cu improves corrosion resistance, if it is contained in an amount exceeding 1.0%, hot workability deteriorates. In addition, W and ■ improve corrosion resistance, but if they are contained up to 1%, a sufficient effect can be ensured, but if they are contained in excess of 1%, the cost will only increase and no significant improvement in the effect will be observed. do not have. Furthermore, Zr, Ti+Nb,
Ta improves the hot and cold workability of the steel by combining with oxygen in the steel, and also improves the corrosion resistance, but a content of about 0.001 to 0.05% is appropriate for both. . It is desirable to add Ca to improve hot workability, but if it is added in excess, Ca-Al-0 type inclusions will increase and corrosion resistance will deteriorate, so the upper limit is set at 0.05. It is best to keep it at around %.

B) Processing conditions In the present invention, a slab having the above-mentioned composition is hot-worked under specific conditions to produce a pipe-making material, and then used to form a seamless steel pipe according to a conventional method, and then further hardened under specific conditions. Fig. 2 is a drawing showing an example of the seamless steel pipe manufacturing process according to the present invention, which is characterized by performing a solution heat treatment. The reason why the processing conditions in each step were limited to specific conditions is as follows.

As shown in Figure 2, when a slab (ingot, continuous casting material) is made into a billet material by one-time hot processing (rolling, forging, extrusion, drawing), ,on the other hand,
When a billet material is obtained by hot working multiple times, soaking heat treatment is performed before the final hot working.

The reason why hot working is performed at least once after the soaking heat treatment is to make the grain growth structure produced by the soaking heat treatment into fine grains by this hot working. If the crystal grains are made fine, hot workability during hot tube manufacturing will be improved.

In addition, when soaking heat treatment is performed during billet making, the precipitation of sigma phase and nitrides, which tend to be generated in areas where Cr and Mo components are segregated, is delayed, and the formation of sigma phases and nitrides is suppressed accordingly. .

Note that soaking heat treatment after billet making (after the final hot working) is also effective, but in this case, grain growth occurs and the workability during hot pipe forming deteriorates, so the final It is important to perform soaking heat treatment when the slab is in the form of a slab before hot working or during forging and rolling.

In this case, if the soaking heat treatment temperature is less than 1200° C., the treatment will require a long time and even then, a sufficient soaking effect will not be obtained, which is not practical. On the other hand, if the temperature in the soaking heat treatment exceeds 1270° C., a single ferrite phase will be formed and significant grain growth will occur, resulting in frequent occurrence of hot worked bottoms during billet making. In addition, if the soaking time is less than 24 hours, segregation will not be sufficiently eliminated and the effect will be insufficient.

In addition, Fe-0,023χC-0,05χ5i-1,5
0χMn-6.3χNi-22.7χCr-3,4χM
o-0.170χN-0,025χAf-0,026χ
P - Forging a 20kg ingot of 0,001χS steel as it is or by soaking (1200℃ x 24hr) L.
Rolling-solution treatment (1080°C x 3ml after 1 x 1
When we investigated the incidence of pitting corrosion on specimens obtained by cooling at 04°C/hr), we found that the incidence of pitting corrosion was 13% in the specimens without soaking heat treatment, while the pitting corrosion incidence in specimens without soaking heat treatment was 13%. It was confirmed that the pitting corrosion occurrence rate was 0% in the case where the coating was applied. Here, the "pitting corrosion occurrence rate" is defined as the 10x
After being immersed in a Fe (J3.6HzO aqueous solution for 24 hours), the presence or absence of pitting corrosion on the entire surface of the test piece was investigated and expressed as the ratio [number of test pieces with pitting corrosion/total number of test pieces].

I-manufacturing ■ The pipe-making material (billet) produced by hot processing is then made into seamless steel pipes by a well-known hot pipe-making method such as hot extrusion method (e.g. Eugene method), Mannesmann method, etc. .

In the case of hot-finished pipes, the obtained seamless steel pipe is subjected to solid solution treatment as it is, and in the case of cold-finished pipes, the pipe obtained by the above-mentioned hot pipe-making is used as a raw pipe and cold-processed ( After cold drawing, cold rolling) is performed once or multiple times, solution treatment is performed. In addition, if cold working is performed multiple times,
Intermediate heat treatment (softening heat treatment) is performed between cold working and the next cold working.

The temperature in the solution treatment for 1 hour is not particularly limited, and the temperature for 1 hour in the solution treatment is as usual, for example, at 1050 to 1100°C for 2 hours.
The process takes about 30 minutes.

However, during cooling, it is necessary to rapidly cool the temperature between 950 and 750°C at a cooling rate of 5 x 104°C/hr or more.

The reason for rapidly cooling between 950 and 750°C during cooling during solution treatment is to prevent sigma phase precipitation in this temperature range where sigma phase precipitation is significant, and to prevent deterioration of corrosion resistance and toughness due to sigma phase precipitation. It is.

In this case, the cooling rate in the temperature range is 5×104”C.
If it is less than /hr, since the material is high Cr, high Mo, high N duplex stainless steel, a sigma phase on an electron microscope level will be generated even in a non-segregating material, which will have a negative effect on corrosion resistance and toughness. A specific method for ensuring a cooling rate of 5 x 104°C/hr or higher is "method of cooling the inside and outside of the pipe with water."
etc. can be mentioned.

Figure 3 is a graph showing the influence of cooling rate during solution heat treatment on the corrosion resistance of duplex stainless steel.
023χC-0, 05χ5i-1, 50χMn-6,
32:Ni-22,7χCr-3,4χMo-0.1
70χN-0,025χAf-0,026χP-0,0
20kg ingot of 01χsl was soaked (1200℃ x 24hr) and forged → rolled → - cold worked - solution treatment (1080℃ x 3min)
The results of investigating the number of pitting corrosion on specimens obtained by applying various treatments and changing the cooling rate by changing the plate thickness during cooling in this solution treatment, water cooling for soaking, and shower cooling are shown. There is.

It can also be seen from FIG. 3 that excellent corrosion resistance can be ensured by setting the cooling rate to 5×10 4 C/hr or more.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> First, a duplex stainless steel slab (upper end diameter: 250 m, lower end diameter: 2
10 m, length: 520+n), they were subjected to soaking heat treatment under the conditions shown in Table 2, followed by one hot forging treatment (1200° C.) to produce a solid billet of 185 mφ.

Next, by machining, the outer diameter was 177fiφ and the inner diameter was 40w.
After forming a hollow billet with a diameter of 450 mm and a length of 450 mm, it was subjected to hot extrusion using the Eugene method (extrusion temperature: 1150'C),
A seamless steel pipe with an outer diameter of 89 nφ and a wall thickness of 8+n was obtained.

Subsequently, each of the obtained seamless steel pipes was bent and removed with glass as a lubricant, and then subjected to solution heat treatment to obtain a product seamless steel pipe. In addition, in solution heat treatment, 1060
Conditions were adopted in which the sample was heated and maintained at 0.degree. C. for 3 minutes and then immediately cooled at the cooling rate shown in Table 2.

Then, test pieces were taken from each product seamless steel pipe manufactured in this way and the corrosion resistance and toughness (impact value) were measured.
The results are also shown in Table 2.

The corrosion resistance test was carried out by cutting out a test piece of 6 mt x 20 m'' M x 40 cranes and immersing it in a 10χFeCl3-H20 aqueous solution at 35°C for 24 hours, and investigating the number of occurrences of pitting corrosion at this time. The impact test was conducted using JIS-22202 4 from seamless steel pipes heat-treated for 820"cxI.
As is clear from the results shown in Table 2, if the method of the present invention is followed, the Charpy impact value at 20°C is measured by cutting out a No. 1 test piece (width 5 m). However, it is clear that it is possible to stably manufacture ``seamless duplex stainless steel pipes that have both excellent corrosion resistance and toughness'' that can exhibit sufficient performance.

Summary of the Effects> As explained above, according to the present invention, it is possible to stably mass-produce high-strength duplex stainless steel seamless pipes with extremely excellent corrosion resistance and good toughness, and it is suitable for marine equipment.・Very useful industrial effects can be brought about, such as greatly contributing to the further performance improvement of equipment, oil and natural gas extraction and transportation equipment, equipment, etc.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the results of an investigation into the "relationship between S+ content and sigma phase precipitation" in duplex stainless steel. FIG. 2 is a diagram illustrating an example of the manufacturing process of a seamless duplex stainless steel pipe according to the present invention. FIG. 3 is a graph showing the influence of the cooling rate during solution heat treatment on the corrosion resistance of duplex stainless steel. FIG. 4 is a diagram illustrating an example of the manufacturing process of a conventional seamless duplex stainless steel pipe.

Claims (1)

[Claims] C: 0.03% or less, Si: 0.7% or less, Mn: 2.
0% or less, P: 0.030% or less, S: 0.003% or less, Ni: 5-8%, Cr: 22-26%, Mo: 2.5-4.5%, N: 0
．． 14-0.30%, Al: 0.010-0.040%
1200-127 from duplex stainless steel slab containing
After obtaining a pipe-making material by hot working, which involves applying a soaking treatment in a temperature range of 0°C for 24 hours or more before the final process, the material is used to form a seamless steel pipe of a predetermined size by a conventional method, Next, 950-7 after heating and holding
A method for manufacturing a seamless duplex stainless steel pipe with excellent corrosion resistance and toughness, characterized by performing solution heat treatment with a cooling rate of 50°C adjusted to 5 x 10^4°C/hr or more.