JPH0751737A - Production of high-si-containing seamless stainless steel tube having excellent corrosion resistance and ductility and toughness - Google Patents

Abstract

PURPOSE:To provide a stainless steel tube having good corrosion resistance in environment in concd. sulfuric acids and excellent ductility and toughness by specifying the component range and production conditions of a high-Si-contg. stainless steel so as to facilitate production of the seamless steel tube by hot press piercing and extruding. CONSTITUTION:A steel ingot having the compsn. components consisting of, by weight%, <=0.08% C, 5.0 to 8.0% Si, <=2.0% Mn, 10 to 35% Ni, 10 to 25% Cr and the balance Fe and inevitable impurities and contg. the Cr, Sr and Ni in the contents satisfying equation I is used. This steel ingot is soaked in 1050 to 1150 deg.C and a temp. region satisfying equation II and thereafter the hot press piercing and extruding are ended in a >=950 deg.C temp. region. As a result, the high-Si-contg. seamless stainless steel tube having the corrosion resistance in the environment of >=65 deg.C in 95% sulfuric acid and >=150 deg.C in 98% sulfuric acid and the excellent ductility and toughness is obtd. This steel tube is used to piping of drying columns, absorption columns, etc., of a sulfuric acid production plant.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art In the absorption, drying and cooling processes, which are important in the catalytic sulfuric acid production method, the equipment material generally has a concentration of 95%.
Exposed to sulfuric acid environment of ~ 99%, temperature 65-120 ° C.
Among them, for the pipes, Cr cast iron, high Si cast iron, stainless steel, high Ni alloy, etc. have been conventionally used. However, cast iron is not only restricted by the design of the apparatus, but also has a lot of internal defects, which makes maintenance difficult. On the other hand, although stainless steel and high Ni alloy are suitable as structural materials, general-purpose stainless steel such as SUS316L cannot withstand the above environment, and even high Ni alloy such as UNS N10276 cannot be used at a temperature of 100 ° C. or higher.

Generally, the operating environment in a drying tower is a concentration of 95
%, The temperature is about 65 ° C., but the temperature may rise to about 100 ° C. in some of the piping.
Furthermore, the absorption tower which is 98% sulfuric acid environment is currently 100-
It is operated at 120 ° C, but it is possible to improve operating efficiency by raising the temperature, so 150 ° C
Piping that can withstand the above needs is required.

As stainless steel intended for use in the above environment, Japanese Patent Laid-Open No. 4418/1982 and Japanese Patent Laid-Open No.
Japanese Patent No. 290949 discloses that by increasing the Si content of stainless steel, good corrosion resistance can be obtained up to high temperatures at both sulfuric acid concentrations of 95% and 98%.

[0005]

However, in the high Si content stainless steel, as the Si content increases, it becomes hard (HV:
(500 to 1000) An embrittlement phase such as an intermetallic compound or δ ferrite is generated. In particular, in the casting process, since the segregation is more prominent in the region closer to the center of the steel solidified finally, the embrittlement phase increases, and not only the hot workability is deteriorated, but also the ductility of the product steel pipe is significantly deteriorated. Therefore, a steel ingot obtained by a normal casting method has a problem that it is difficult to manufacture a steel pipe by hot extrusion and the ductility which is important as a structural material is poor. However, the above-mentioned Japanese Patent Laid-Open Nos. 52-4418 and 2-290949 do not sufficiently consider this point.

The present invention has been made in order to solve the problems in the prior art as described above, and the composition range and production of a high Si content stainless steel which facilitates the production of a seamless steel pipe by hot press punching and extrusion. By specifying the conditions, a stainless steel pipe having good corrosion resistance in an environment of 65 ° C. or higher in 95% sulfuric acid and 150 ° C. or higher in 98% sulfuric acid and excellent in ductility as a structural material is obtained. The purpose is to

[0007]

The above-mentioned problems can be solved by the following component limitation and manufacturing conditions. The first invention is
% 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%, balance Fe and unavoidable impurities, and a steel ingot having Cr, Si and Ni contents satisfying the expression (1), satisfying the expression (2) to 1050 to 1150 ° C. After soaking in a temperature range T (° C.), the method for producing a high Si-containing stainless seamless steel pipe excellent in corrosion resistance and ductility, characterized in that hot press punching and extrusion are completed in a temperature range of 950 ° C. or higher. Is.

The second aspect of the present invention is, in% by weight, C: 0.08% or less, Si: 5.0 to 8.0%, Mn: 2.0% or less, and Ni: 10 to 35%. Cr: 10 to 25%, C
u: 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%
C and more, and the balance Fe and unavoidable impurities, and C
After soaking a steel ingot whose r, Mo, Si and Ni contents satisfy the expression (3) to a temperature range T (° C) which satisfies the expression (2) and 1050 to 1150 ° C, hot press punching, 95 extrusion
Corrosion resistance, characterized by finishing in a temperature range of 0 ° C or higher,
It is a method for producing a high Si-containing stainless seamless steel pipe having excellent ductility.

Cr (%) + 3 × Si (%) −Ni (%) −14 <5 (1) T <1470−35 × Si (%) −5 × Ni (%) (2) Cr (% ) + Mo (%) + 3 × Si (%) − Ni (%) − 14 <5 (3)

[0010]

The reason for adding the components and the reason for limiting the components of the stainless steel pipe of the present invention will be described below. When the content of C is large, it forms carbides and deteriorates the corrosion resistance, so the upper limit is made 0.08%.

Si is a component that remarkably improves the corrosion resistance in high temperature and high concentration sulfuric acid, but in order to obtain good corrosion resistance in the above environment, it is necessary to contain Si by 5.0% or more. Also,
If it is added in excess of 8.0%, a large amount of intermetallic compound is generated, solidification cracking occurs during casting, and it becomes impossible to manufacture a steel ingot. Therefore, the Si content is 5.0 to 8.0%.
And

Mn is a component having a deoxidizing action 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, and if the content is less than 10%, the brittle phases such as δ ferrite and intermetallic compounds increase, and the hot workability and the ductility of the steel pipe are increased. Deteriorate. Also, Cr, Mo and Si
It is necessary to increase the Ni content as the content increases (details will be described later). However, if the content thereof is increased, not only the cost becomes high, but also the partial melting temperature described later is lowered, the temperature range in which hot working is possible is narrowed, and it becomes impossible to manufacture a steel pipe. %.

[0014] Cr is the most important element for the general corrosion resistance of stainless steel, and in high Si content stainless steel, its content must be 10% or more.
On the other hand, the corrosion resistance in high-temperature high-concentration sulfuric acid also improves as the Cr content increases, but if it exceeds 25%, the effect on the corrosion resistance becomes saturated. Further, as the Cr content increases, the deformation resistance during hot working increases and the precipitation of the embrittlement phase is promoted. Therefore, the Cr content is 10 to 25%.

The inventors have found that Cu is an effective component for improving corrosion resistance in 95% sulfuric acid. Especially, the effect becomes remarkable as the temperature rises, but the content is 0.5
If it is less than%, it will not be exhibited. Further, even if added over 3.0%, the effect on the corrosion resistance is saturated, so the Cu content is made 0.5 to 3.0%.

The inventors have found that Mo is an effective component for improving the corrosion resistance in 95% sulfuric acid, but if the content is less than 0.2%, the effect is not exhibited. Also, 2.
Even if added over 0%, the effect on corrosion resistance is saturated,
In addition, since the increase in deformation resistance and the formation of the embrittlement phase are promoted with the increase in the content, the upper limit value is made 2.0%.

The inventors have found that Pd is a component effective in improving corrosion resistance in sulfuric acid. However, if the content is less than 0.005%, the effect is not exhibited, and
Even if added over 1.0%, the effect on the corrosion resistance is saturated and the cost becomes high. Therefore, the Pd content is 0.0
05 to 1.0%.

Further, as a result of investigating the relationship between the occurrence of cracks during hot extrusion and the ductility of the steel pipe and the volume fraction of the embrittlement phase, the present inventors have found that the volume fraction of the embrittlement phase is expressed by equation (4). It can be expressed by the value Fp (%), and it has been found that when this value is 5 or more, not only cracks occur during hot extrusion, but also the ductility of the steel pipe is significantly deteriorated. Therefore, the Cr, Mo, Si, and Ni contents are in the range that satisfies the formula (3) in addition to the above-mentioned limitations.

Fp = Cr (%) + Mo (%) + 3 × Si (%) − Ni (%) − 14 (4) Cr (%) + Mo (%) + 3 × Si (%) − Ni (%) − 14 <5 (3) The steel having the above composition range is prepared by adding a predetermined additive component in molten steel in the form of a master alloy or a simple substance according to a conventional method.

Next, the reason for limiting the manufacturing conditions will be described. In the as-cast steel ingot of this steel, the volume ratio of the embrittlement phase becomes larger than the value Fp of the equation (4). In particular, the more the center of the ingot where the segregation is remarkable, the more the embrittlement phase increases, which causes defects on the inner surface, so that a larger amount of Ni must be added. But,
Increasing the Ni content leads to a lowering of the partial melting temperature, which will be described later, resulting in a narrower temperature range in which hot working is possible.

As a result of a detailed study of the effect of long-time soaking on the volume fraction of the embrittlement phase, the inventors of the present invention performed soaking treatment in a specific temperature range to obtain a surface layer portion even in the central portion of the steel ingot. It has been found that the volume ratios are almost the same and the value is reduced to Fp (%) that can be expressed by the equation (4). However, if the soaking temperature is less than 1050 ° C, the above effect cannot be obtained even if soaking is performed for 100 hours or more, and if it exceeds 1150 ° C, the volume ratio increases conversely. In addition, this steel forms low melting point compounds with increasing Si content. Therefore, if the soaking temperature is too high, partial melting occurs and cracks occur during hot working. The present inventors have found that the minimum temperature at which this steel partially melts is the value Tm (° C) of the equation (5). Therefore, the hot press perforation for the steel ingot and the soaking condition before extrusion are set to a temperature range T (° C) that satisfies 1050 to 1150 ° C and the above expression (2).

Tm = 1470-35 × Si (%)-5 + Ni (%) (5) T <1470-35 × Si (%)-5 × Ni (%) (2) Further, in this steel, 950 In the temperature range below ℃, hot ductility decreases and surface cracking occurs. Therefore, the end temperature of hot press punching and extrusion is 950 ° C or higher.

[0023]

EXAMPLES Experimental examples and examples according to the present invention will be described. Example 1 A 150 kg ingot of the chemical composition shown in Table 1 was prepared (Sample Nos. 2 to 5, 7, 8, 11 to 13, 15, and 17 are ingots of the composition of the present invention, and others are ingots other than the composition of the present invention. ). These ingots were subjected to soaking at 1050 ° C. for 10 hours, and hot tensile samples were collected. Further, in order to investigate the relationship between the volume ratio of the embrittlement phase in the steel ingots of sample numbers 11 and 15 in Table 1 and the soaking conditions, soaking is performed in the temperature range of 1000 to 1200 ° C. and micro observation samples are collected. did. Further, a hot extrusion steel ingot having an inner diameter of 29 mm and an outer diameter of 80 mm was carved out from the ingot after the soaking treatment, heated at 1050 ° C. as in the soaking treatment, and then hot extruded at an end temperature of 950 ° C. to obtain 10 ^t × 44.
A φ seamless steel pipe was manufactured. For the steels of Samples 11 and 15, steel pipes having different finishing temperatures from 900 to 1000 ° C were also prepared. After visually observing the presence or absence of cracks in the steel pipe and subjecting it to solution heat treatment at 1100 ° C., a corrosion test sample (3 ^t × 15 ^w × 50 ^l ) and a tensile test piece (6φ,
GL = 24 mm) and 2 mm V notched Charpy impact test pieces (half size) were taken. Further, with the steels of sample numbers 16 to 20, pitting corrosion potential measurement (JIS G057
The sample for 7) was collected. However, in the case of a cracked steel pipe, the above sample was taken from a sound part without cracks. Further, in the steel of Sample No. 21 having a Si content of more than 8%, cracking occurred in the entire ingot as cast, and thus hot extrusion could not be performed.

1 and 2, 95%, 65 ° C. and 98
%, Shows the relationship between the corrosion resistance in 150 ° C. sulfuric acid and the Si content. According to FIGS. 1 and 2, in this environment, 5% or more of Si
It can be seen that the corrosion rate remarkably decreases due to the inclusion.

FIG. 3 shows the relationship between the corrosion resistance in 95% sulfuric acid at 100 ° C., the pitting potential in 3.5% NaCl, and the Cr content. From FIG. 3, it can be seen that when the Cr content is less than 10%, the pitting potential is significantly lowered even when the Si content is about 8%. Further, it is understood that the corrosion resistance in sulfuric acid improves as the Cr content increases, but the corrosion rate becomes constant when it exceeds 25%.

4 and 5 show the relationship between the corrosion resistance in 95% and 100 ° C. sulfuric acid and the Cu content and Mo content, respectively. According to FIG. 4 and FIG. 5, when Cu is added by 0.5% or more or Mo is added by 0.2% or more, 95%, 100 ° C.
The corrosion rate in sulfuric acid is significantly reduced. However, it can be seen that the corrosion rate becomes constant when the content of Cu exceeds 3% and the content of Mo exceeds 2%.

In FIG. 6, 95%, 100 ° C. and 98%, 2
The relationship between the corrosion resistance in 20 ° C. sulfuric acid and the Pd content is shown.
From FIG. 6, it can be seen 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%
Above, the corrosion rate becomes constant.

FIG. 7 shows the relationship between the minimum temperature at which the ingot is partially melted and the drawing becomes 0% and the Ni and Si contents by the hot tensile test of the ingot. It can be seen from FIG. 7 that the minimum temperature at which partial melting occurs can be represented by the value Tm (° C.) of the equation (5).

Tm = 1470−35 × Si (%) − 5 × Ni (%) (5) FIG. 8 shows the volume fraction Fp of the embrittlement phase in the steel ingots of sample numbers 11 and 15 and the soaking condition. Shows the relationship.
According to FIG. 8, with the steels of sample numbers 11 and 15, when the hot temperature was less than 1050 ° C., the volume fraction of the embrittlement phase did not change as it was even after soaking for 100 hours, and especially in the surface layer portion of the ingot. Compared with this, the number is much higher in the center. In contrast,
When soaking in the temperature range of 1050 to 1150 ° C., the volume ratios of both the central portion and the surface layer portion decrease and become almost the same value.
Further, it can be seen that the volume ratio increases at a temperature higher than 1150 ° C.

FIG. 9 shows the relationship between the presence or absence of cracks and the finishing temperature during hot extrusion of the steels of sample numbers 11 and 15. According to FIG. 9, it can be seen that when the finishing temperature is lower than 950 ° C., cracks occur in both samples 11 and 15 during hot extrusion.

FIG. 10 shows the relationship between the volume fraction of the embrittlement phase, the presence or absence of cracks during hot extrusion, the elongation in the tensile test, and the absorbed energy at 0 ° C. in the Charpy impact test and the components. According to FIG. 10, the volume ratio of the embrittlement phase is the value Fp (%) of the equation (4).
You can see that it corresponds well. Further, it is understood that when this value is 5 or more, cracking occurs during hot extrusion and the elongation and absorbed energy of the steel pipe remarkably decrease, which makes it unsuitable as a structural material.

Fp = Cr (%) + Mo (%) + 3 × Si (%)-Ni (%)-14 (4) Example 2 A steel ingot having a chemical composition shown in Table 2 having an inner diameter of 50 mm and an outer diameter of 210 mm was prepared. As a raw material, a 13 ^t × 140φ seamless steel pipe was manufactured by hot press punching and hot extrusion under the conditions shown in Table 3. The soaking time before punching the press was 3 hours in all cases.
And Similar to Example 1, the steel pipe was visually observed for cracks, subjected to solution heat treatment at 1100 ° C., and then subjected to a corrosion test sample,
A tensile test piece and a Charpy impact test piece with a 2 mm V notch (full size) were collected.

Table 4 also shows the presence or absence of cracks during hot working, the elongation in the tensile test, the absorbed energy at 0 ° C. in the Charpy impact test and the corrosion rate in high temperature concentrated sulfuric acid. According to Table 4, the high Si content stainless seamless steel pipe produced by the method of the present invention has 95%
It can be seen that it has good corrosion resistance in sulfuric acid of 65 ° C. or higher, 98%, and sulfuric acid of 150 ° C. or higher, and has excellent ductility and toughness. In particular, it is understood that the steel type containing any one or more of Cu, Mo and Pd has excellent corrosion resistance even in 95% and 100 ° C. sulfuric acid.

[0034]

As described above, according to the present invention, 95
% High-Si stainless steel seamless steel pipe that has good corrosion resistance in an environment of 65 to 100 ° C in 98% sulfuric acid and 150 to 220 ° C in 98% sulfuric acid and has excellent ductility as a structural material There is an effect that can be easily obtained by press punching and extrusion. 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 manufacturing plant.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[Brief description of drawings]

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

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

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

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

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

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

FIG. 7 is a diagram showing the relationship between the minimum temperature at which the steel partially melts and the Ni and Si contents according to the first embodiment.

FIG. 8 is a diagram showing the relationship between the volume ratio of the embrittlement phase of Samples 11 and 12 according to Example 1 and the soaking condition.

9 is a graph showing the relationship between the presence or absence of cracks and the finishing temperature during hot extrusion of Samples 11 and 15 according to Example 1. FIG.

10 is a diagram showing the relationship between the volume fraction of the embrittlement phase of steel according to Example 1, the presence or absence of cracks during hot extrusion, the elongation in the tensile test, and the absorbed energy at 0 ° C. in the Charpy impact test and the components.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuo Kobayashi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Ryuichiro Ehara 4-22, Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima No. Mitsubishi Heavy Industries, Ltd. Hiroshima Research Institute (72) Inventor Hideo Nakamoto 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries Ltd. Hiroshima Research Institute (72) Inventor Yoshikazu Yamada Kannon Shinmachi, Nishi-ku, Hiroshima Prefecture 4-6-22 Mitsubishi Heavy Industries Ltd. Hiroshima Research Institute (72) Inventor Hajime Nagano 2-5-1-5 Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. (72) Makoto Nakamura 2 Marunouchi, Chiyoda-ku, Tokyo 5th-1th Sanryo Heavy Industries Co., Ltd.

Claims (2)

[Claims] 1. C: 0.08% or less by weight% and S
i: 5.0 to 8.0%, Mn: 2.0% or less, N
i: 10 to 35%, Cr: 10 to 25%, balance Fe
And an inevitable impurity, and a Cr, Si, and Ni content satisfying the formula (1),
After soaking at 0 ° C. and a temperature range T (° C.) satisfying the formula (2), hot press punching and extrusion are finished at a temperature range of 950 ° C. or higher, which is excellent in corrosion resistance and ductility. High S
Method for producing i-containing stainless seamless steel pipe. Cr (%) + 3 * Si (%)-Ni (%)-14 <5 ... (1) T <1470-35 * Si (%)-5 * Ni (%) ... (2) 2. C: 0.08% or less by weight% and S
i: 5.0 to 8.0%, Mn: 2.0% or less, N
i: 10-35%, Cr: 10-25%, Cu:
0.5-3.0%, Mo: 0.2-2.0% and Pd:
One or more selected from the group consisting of 0.005 to 1.0% and the balance Fe and inevitable impurities, and Cr, M
After soaking the steel ingot having the contents of o, Si and Ni satisfying the expression (3) to a temperature range T (° C) satisfying the expression (2) at 1050 to 1150 ° C, hot press punching and extrusion To 95
Corrosion resistance, characterized by finishing in a temperature range of 0 ° C or higher,
A method for producing a high Si-containing stainless seamless steel pipe having excellent ductility. Cr (%) + Mo (%) + 3 * Si (%)-Ni (%)-14 <5 ... (3) T <1470-35 * Si (%)-5 * Ni (%) ... (2)