JP3177555B2 - Method for producing stainless steel seamless steel pipe containing high Si with excellent corrosion resistance and ductility - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Si-containing stainless steel seamless pipe having excellent ductility and corrosion resistance in high-temperature, high-concentration sulfuric acid, which is useful as a device material for a drying tower and an absorption tower of a sulfuric acid production plant. And its manufacturing method.

[0002]

2. Description of the Related Art In the absorption, drying and cooling steps which are important in the contact type sulfuric acid production method, the equipment material generally has a concentration of 95%.
~ 99%, exposed to a sulfuric acid environment at a temperature of 65-120 ° C.
Above all, for pipes, conventionally, Cr cast iron, high Si cast iron, stainless steel, high Ni alloy and the like have been used. However, cast iron is not only restricted in the design of the apparatus, but also has difficulties in maintenance due to many internal defects. On the other hand, stainless steel and high Ni alloy are suitable as structural materials, but general-purpose stainless steel such as SUS316L does not withstand the above-mentioned environment, and even high Ni alloy such as UNS N10276 cannot be used at a temperature of 100 ° C. or higher.

[0003] Generally, the operating environment in a drying tower has a concentration of 95%.
% In sulfuric acid at a temperature of about 65 ° C., but the temperature may rise to about 100 ° C. in some of the piping.
Furthermore, absorption towers with 98% sulfuric acid environment are currently 100-
Although the operation is performed at 120 ° C., it is possible to improve the operation efficiency by increasing the temperature.
There is a need for a pipe that can withstand the above use.

As stainless steels intended for use in the above environment, Japanese Patent Application Laid-Open Nos.
No. 290949 discloses that by increasing the Si content of stainless steel, good corrosion resistance up to high temperatures can be obtained at both 95% and 98% sulfuric acid concentrations.

[0005]

The high Si content stainless steel becomes harder (HV: 500 to 500) as the Si content increases.
1000) An embrittlement phase such as an intermetallic compound or δ ferrite is formed. The formation of this embrittlement phase during the production of a high Si content stainless steel seamless steel pipe by cold rolling, drawing or the like not only increases the deformation resistance but also causes cracking. However, in the above-mentioned JP-A-52-4418 and JP-A-2-290949, this point is not sufficiently considered.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems in the prior art as described above, and the range and composition of a high Si-containing stainless steel in which a seamless steel pipe can be easily manufactured by cold rolling, drawing or the like. By defining the conditions, a stainless steel pipe having good corrosion resistance in an environment of 65 ° C. or more in 95% sulfuric acid and 150 ° C. or more in 98% sulfuric acid and having excellent ductility as a structural material is obtained. The purpose is to:

[0007]

The above objects can be attained by the following component limitation and production conditions. The first invention is
In terms of% by weight, C: 0.08% or less and Si: 5.0-8.
0%, Mn: 2.0% or less, Ni: 10 to 35%
And Cr: 10 to 25% Mo: 0.2 to 2.0%
And Pd: selected from the group consisting of 0.005 to 1.0%
Consisting of at least one of the above, and the balance Fe and unavoidable impurities,
And the contents of Cr, Mo, Si and Ni satisfy the expression (1).
Upon manufacturing a cold-finishing seamless steel pipe plus, corrosion resistance and stress relief annealing and performing in a temperature range of 1,050-1,150 ° C., it is the manufacturing method of the high Si-containing stainless seamless steel pipe having excellent ductility properties .

According to a second invention, C: 0.08% or less, Si: 5.0 to 8.0%, Mn: 2.0% or less, Ni: 10 to 35% by weight. Cr: 10 to 25%, C
u: 0.5 to 3.0%, Mo: 0.2 to 2.0%, and
Pd: selected from the group consisting of 0.005 to 1.0%
In producing a cold-finished seamless steel pipe composed of at least one kind and the balance of Fe and unavoidable impurities and having Cr, Mo, Si, and Ni contents satisfying the expression (1), strain relief annealing is performed at 1050 to 1150. This is a method for producing a high Si-containing stainless steel seamless steel pipe having excellent corrosion resistance and ductility, which is performed in a temperature range of ° C. Cr (%) + Mo (%) + 3 × Si (%)-Ni (%)
-14 <5. . . (1)

[0009]

The reasons for adding and limiting the components of the high Si content stainless steel pipe according to the present invention will be described below. If the content of C increases, carbides are formed and the corrosion resistance is degraded, so the upper limit is made 0.08%.

[0010] Si is a component that significantly improves the corrosion resistance in high-temperature, high-concentration sulfuric acid. However, in order to obtain good corrosion resistance in the above environment, it is necessary to contain Si in an amount of 5.0% or more. Also,
If it is added in excess of 8.0%, solidification cracking occurs during casting due to generation of a large amount of intermetallic compounds, making it impossible to produce a steel ingot. Therefore, the content of Si is 5.0 to 8.0%.
And

Mn is a component having a deoxidizing effect, and is also an austenite-forming element. However, if the content exceeds 2.0%, the corrosion resistance deteriorates. Therefore, M
The upper limit of the n content is 2.0%.

Ni is an essential component for obtaining an austenite structure. If the content is less than 10%, the embrittlement phase such as δ ferrite and intermetallic compound increases, cracks are generated at the time of cold working, and steel pipes are formed. Deteriorates the ductility of steel. Further, it is necessary to increase the Ni content as the Cr, Mo and Si contents increase, and the details will be described later. However,
Increasing its content not only increases costs, but also
Since the temperature range in which hot working is possible is restricted by the formation of the low melting point compound, and the production of a raw tube becomes impossible, the upper limit value is 3
5%.

[0013] Cr is the most important element for the general corrosion resistance of stainless steel, and the content of high Si stainless steel must be 10% or more.
On the other hand, the corrosion resistance in high-temperature, high-concentration sulfuric acid also increases with an increase in the Cr content, but if it exceeds 25%, the effect on corrosion resistance is saturated. Further, when the Cr content increases, precipitation of the embrittlement phase is promoted. Therefore, the Cr content is 10
To 25%.

The inventors have found that Cu is a component effective for improving corrosion resistance in 95% sulfuric acid. In particular, the effect becomes more remarkable as the temperature becomes higher.
% Is not exhibited. Further, the effect on corrosion resistance is saturated even if it is added in excess of 3.0%, so the Cu content is set to 0.5 to 3.0%.

The inventors have found that Mo is an effective component for improving corrosion resistance in 95% sulfuric acid, but its effect is not exhibited if the content is less than 0.2%. Also, 2.
Even if it is added in excess of 0%, the effect on corrosion resistance is saturated,
In addition, since the formation of the embrittlement phase is promoted as the content increases, the upper limit is set to 2.0%.

The present inventors have found that Pd is a component effective for improving corrosion resistance in sulfuric acid. However, if the content is less than 0.005%, the effect is not exhibited, and
Even if it is added in excess of 1.0%, the effect on corrosion resistance saturates and costs increase. Therefore, the Pd content is 0.0
05 to 1.0%.

The present inventors have examined in detail the relationship between the occurrence of cracks during cold working and the volume fraction of the embrittlement phase. As a result, in this steel, the volume fraction of the embrittlement phase was (3) Expression value Fp
(%), And when this value is 5 or more,
It has been found that not only remarkable cracks occur during cold working, but also the ductility of the product is significantly deteriorated. Therefore, the contents of Cr, Mo, Si and Ni are set in a range satisfying the expression (2) in addition to the above-mentioned limits.

Fp = Cr (%) + Mo (%) + 3 × Si (%) − Ni (%) − 14 (3) Cr (%) + Mo (%) + 3 × Si (%) − Ni (%) − 14 <5 (2) The steel having the above component range is adjusted by adding a predetermined additive component to the molten steel in the form of a master alloy or a simple substance in a conventional manner.

Next, the reasons for limiting the manufacturing conditions will be described. In the process of manufacturing a seamless steel pipe by cold working, softening annealing is performed several times in order to remove working strain. The present inventors have examined the relationship between the volume fraction of the embrittlement phase of the steel and the soaking temperature during annealing. As a result, when the soaking temperature is less than 1050 ° C., the intermetallic compound is formed. It has been found that the ferrite is re-precipitated, and the volume fraction of the embrittlement phase increases from the value Fp (%) expressed by the equation (3). Therefore, the soaking temperature of the strain relief annealing is set to a temperature range of 1050 to 1150 ° C.

[0020]

EXAMPLES Experimental examples and examples of the present invention will be described. Experimental Example 1 A 50 kg ingot having the chemical components shown in Table 1 was prepared.
Ingots of sample numbers 7, 8, 12, 13, 14, 17
Have components within the scope of the present invention and have sample numbers 2, 4, 5,
11 has reference components that are outside the scope of the invention, and the remaining ingots of sample numbers have comparison components that are outside the scope of the invention. These ingots were soaked at 1050 ° C. for 10 hours, and then hot-rolled to 12 mm ^t to obtain steel sheets. This steel sheet was subjected to a solution heat treatment at 1100 ° C., and 10 ^t × 10 ^w
× 200 ^L of material for cold rolling was collected. This material 6
cold rolling to mm ^t, while visually observing the presence or absence of cracks after stress relief annealing of 1100 ° C., the microstructure observation sample, corrosion specimen sample ^{(3 t × 40 w × 40} L),
Tensile test pieces (4φ, GL = 16 mm) and 2 mm V-notched Charpy impact test pieces (half size) were collected. In addition, samples of pitting potential measurement (JIS G0577) were also collected from steels of sample numbers 16 to 20. Further, for the steels of Sample Nos. 11 and 12, strain relief annealing after cold rolling was performed in a temperature range of 1000 to 1200 ° C., and a sample for micro observation was collected. In addition, in the case of a steel sheet in which cracks occurred during cold rolling, the above sample was collected from a sound part having no cracks. Further, in the case of the steel of Sample No. 21 having a Si content exceeding 8%, hot rolling was not possible because cracks occurred in the entire ingot as cast.

FIGS. 1 and 2 show 95%, 65 ° C. and 98 ° C.
% Shows the relationship between corrosion resistance in 150 ° C. sulfuric acid and Si content. According to FIG. 1 and FIG.
It can be seen that the corrosion rate is significantly reduced by the inclusion.

FIG. 3 shows the relationship between the corrosion resistance in 95% and 100 ° C. sulfuric acid and the pitting potential in 3.5% NaCl and the Cr content. FIG. 3 shows that when the Cr content is less than 10%, the pitting potential is significantly reduced even when the Si content is about 8%. Further, it is understood that the corrosion resistance in sulfuric acid increases with an increase in the Cr content, but the corrosion rate becomes constant when the content exceeds 25%.

FIGS. 4 and 5 show the relationship between the corrosion resistance in 95% sulfuric acid at 100 ° C. and the Cu content and the Mo content, respectively. According to FIGS. 4 and 5, when Cu is added at 0.5% or more or Mo is added at 0.2% or more, 95%, 100 ° C.
Corrosion rates in sulfuric acid are significantly reduced. However, when the content exceeds 3% for Cu and 2% for Mo, the corrosion rate becomes constant.

FIG. 6 shows that 95%, 100 ° C. and 98%, 2%
4 shows the relationship between the corrosion resistance in sulfuric acid at 20 ° C. and the Pd content.
FIG. 6 shows that the corrosion resistance in 95%, 100 ° C. and 98%, 220 ° C. sulfuric acid is improved by adding 0.005% or more of Pd. However, its content is 1.0%
If it exceeds, the corrosion rate becomes constant.

FIG. 7 shows the relationship between the volume fraction of the embrittlement phase, the presence or absence of cracks during cold rolling, the elongation in the tensile test, and the absorbed energy at 0 ° C. in the Charpy impact test. FIG.
According to the formula, the volume fraction of the embrittlement phase is the value Fp (%) of the above equation (3).
It turns out that it corresponds well. Further, when this value is 5 or more, cracks are generated during cold rolling, and the elongation and absorbed energy of the steel sheet are significantly reduced, so that it is understood that the steel sheet is not suitable as a structural material.

FIG. 8 shows the relationship between the volume percentage and hardness of the embrittlement phase of the steels of Sample Nos. 11 and 12, and the annealing temperature. According to FIG. 8, when the annealing temperature after cold rolling of both steels 11 and 12 is less than 1050 ° C. or more than 1150 ° C., the volume fraction of the embrittlement phase increases as compared to the as-rolled state, and the steels are not sufficiently softened. . Example 1 A hot-extruded 13 ^t × 140 φ steel tube having the chemical components shown in Table 2 ( sample numbers 1 and 2 are reference steel components, sample numbers 3 and 4 are invention steel components, and sample numbers 5 to 8 are comparative steel components. ) As a material, 7 ^t × 65 φ by cold drawing twice and cold rolling once
Manufactured a seamless steel pipe. In addition, after the cold working, the strain relief annealing was performed each time at the temperature shown in Table 3 ( Sample Nos.
B is the method of the present invention based on reference steel components, sample number c and d are
The method of the present invention using the inventive steel component , Sample No. H is a comparative steel component and has a strain relief annealing temperature of the method of the present invention. thing). Further, the seamless steel pipe was visually observed for cracks, and a corrosion test sample (3 ^t × 15 ^w × 5
0 ^L ), tensile test piece (4φ, GL = 16 mm) and 2 m
Charpy impact test specimen with mV notch (half size)
Was collected.

Table 3 also shows the presence or absence of cracks during cold working of this steel pipe, elongation in a tensile test, absorbed energy at 0 ° C. in a Charpy impact test, and corrosion rate in high-temperature, high-concentration sulfuric acid. According to Table 3, the seamless stainless steel pipe containing high Si content manufactured by the method of the present invention had no cracks,
It can be seen that it has good corrosion resistance in sulfuric acid at 65 ° C. or more, 98%, sulfuric acid at 150 ° C. or more, and also has excellent ductility. In particular, it is understood that a steel type containing at least one of Cu, Mo, and Pd has excellent corrosion resistance even at 95% and 100 ° C. sulfuric acid.

[0028]

As described above, according to the present invention, 95
A high-Si content stainless steel seamless steel tube having good corrosion resistance in an environment of 65 to 100 ° C. in 100% sulfuric acid and 150 to 220 ° C. in 98% sulfuric acid and having excellent ductility as a structural material is used. There is an effect easily obtained by rolling or drawing. Therefore, it is possible to provide a stainless steel pipe that can be used for piping such as a drying tower and an absorption tower of a sulfuric acid production plant.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between corrosion resistance of 95% steel at 65 ° C. in sulfuric acid and Si content according to Experimental Example 1 of the present invention.

FIG. 2 is a diagram showing the relationship between corrosion resistance of 98% steel in 150 ° C. sulfuric acid and Si content according to Experimental Example 1.

FIG. 3 is a graph showing the relationship between the corrosion resistance of 95% steel in 100 ° C. sulfuric acid, the pitting potential in 3.5% NaCl, and the Cr content in Experimental Example 1;

FIG. 4 is a graph showing the relationship between corrosion resistance of 95% steel in 100 ° C. sulfuric acid and Cu content in Experimental Example 1.

FIG. 5 is a graph showing a relationship between corrosion resistance of 95% steel in sulfuric acid at 100 ° C. and Mo content according to Experimental Example 1;

FIG. 6 shows 95% of steel according to Example 1 at 100 ° C. and 98%.
%, A diagram showing the relationship between corrosion resistance in 220 ° C. sulfuric acid and Pd content.

FIG. 7 is a graph showing the relationship between the volume fraction of the embrittlement phase, the presence or absence of cracks during cold rolling, the elongation in a tensile test, and the absorbed energy at 0 ° C. in the Charby impact test according to Experimental Example 1. .

FIG. 8 is a view showing the relationship between the volume ratio and hardness of the embrittlement phase of steels 11 and 12 according to Experimental Example 1 and the annealing temperature.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yasuo Kobayashi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Ryuichiro Ehara 4-2-2 Kanon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Inside Mitsubishi Heavy Industries, Ltd.Hiroshima Research Center (72) Inventor Hideo Nakamoto 4-22, Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Inside Mitsubishi Heavy Industries, Ltd.Hiroshima Research Center No. 6-22, Mitsubishi Heavy Industries, Ltd., Hiroshima Research Laboratory (72) Inventor Hajime Nagano 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Inside Mitsubishi Heavy Industries, Ltd. (72) Inventor Makoto Nakamura 2-5-2, Marunouchi, Chiyoda-ku, Tokyo No. 1 Inside Mitsubishi Heavy Industries, Ltd. (56) References JP-A-5-51633 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/00-9/44 C21D 9/50 C22C 38/00-38/60

Claims (2)

(57) [Claims] (1) C: 0.08% or less by weight, and
i: 5.0 to 8.0%; Mn: 2.0% or less;
i: 10 to 35%, Cr: 10 to 25%, Mo:
0.2-2.0% and Pd: 0.005-1.0%
And at least one selected from the group consisting of Fe and unavoidable impurities and containing Cr, Mo, Si and Ni.
In producing a cold-finished seamless steel pipe having a weight satisfying the formula (1) , the strain relief annealing is performed in a temperature range of 1050 to 1150 ° C., characterized by high corrosion resistance and high ductility.
A method for producing an i-containing stainless steel seamless steel pipe. Cr (%) + Mo (%) + 3 × Si (%)-Ni (%)
-14 <5. . . (1) 2. C: 0.08% or less in terms of% by weight;
i: 5.0 to 8.0%; Mn: 2.0% or less;
i: 10 to 35%, Cr: 10 to 25%, Cu:
0.5 to 3.0%, Mo: 0.2 to 2.0% and P
d: 1 selected from the group consisting of 0.005 to 1.0%
In producing a cold-finished seamless steel pipe consisting of at least the seed and the balance of Fe and unavoidable impurities and having Cr, Mo, Si and Ni contents satisfying the expression (1) , the strain relief annealing is performed at 1050 to 1150 ° C. A method for producing a high Si-containing stainless steel seamless steel pipe having excellent corrosion resistance and ductility, which is performed in a temperature range of Cr (%) + Mo (%) + 3 × Si (%)-Ni (%)
-14 <5. . . (1)