JP3501882B2 - Manufacturing method of ferritic stainless steel pipe for exhaust gas heat transfer member - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a ferritic stainless steel pipe which is excellent in high-frequency pipe forming properties, intergranular corrosion resistance, oxidation resistance and sulfuric acid resistance and is suitable as an exhaust gas heat transfer member.

[0002]

2. Description of the Related Art Steels used for exhaust gas such as boiler air preheaters include high temperature steels for which oxidation is a problem and low temperature steels for which sulfuric acid dew point corrosion is a problem. Heat resistant steel such as cyclomal is used for high temperatures, and low C, N SUS410L and 13Cr stainless steel seamless pipes are used for low temperatures. The seamless pipe does not include a welded portion and has excellent structural reliability, but the manufacturing cost is high. Therefore, when there is a cost constraint, a plain steel boiler steel pipe is used as a consumable item. When using welded steel pipe for exhaust gas applications,
Depending on the conditions of use, it is exposed to the environment of sulfuric acid dew point corrosion, so the intergranular corrosion resistance of the weld becomes a problem. It is known that the intergranular corrosion resistance of the welded portion is improved by adding Nb or Ti that fixes C and N. Japanese Patent Laid-Open No. 5-1535
The publication discloses stainless steel whose Nb addition improves the high temperature strength. The inventors of the present invention have also disclosed a stainless steel having improved intergranular corrosion resistance and high-frequency pipe forming property by adding an appropriate amount of Nb corresponding to the contents of C, N and Cr. Introduced in.

[0003]

In the exhaust gas heat transfer member used as an air preheater and the like of the boiler [0006] is resistant since the SO _x contained in the exhaust gas is exposed to the sulfuric acid dew-point corrosion environment condensation Sulfate is required. Further, even a member used in a low temperature part is required to have excellent oxidation resistance since the exhaust gas temperature and pressure during the boiler operation are high. Furthermore, high-temperature corrosion resistance to V ₂ O ₅ -containing combustion ash generated by trace amounts of V and Na contained in heavy oil is also required. The present invention has been devised to solve such a problem, and in addition to the pipe-forming property for processing into a pipe and the intergranular corrosion resistance of a welded portion, the addition of P and Cu improves resistance. An object of the present invention is to provide a ferritic stainless steel suitable for an exhaust gas heat transfer member, which has improved sulfuric acid resistance and improved oxidation resistance and corrosion resistance by annealing after pipe fabrication.

[0004]

In order to achieve the object of the method for producing a ferritic stainless steel pipe for an exhaust gas heat transfer member of the present invention, C: 0.03% by weight or less, Si: 1% by weight or less, Mn. 1% by weight or less, P: 0.04 to 0.1
5% by weight, S: 0.03% by weight or less, Ni: 0.6% by weight or less, Cr: 10-18% by weight, Nb: 0.2-1.
0% by weight, Cu: 0.1 to 0.4% by weight, N: 0.03
A weld containing less than or equal to wt%, the balance consisting essentially of Fe, and having a composition in which the X value defined by X = Nb-7 × (C + N) -P-0.15 is adjusted to 0.103 or more. 9 in a combustion gas atmosphere with an oxygen concentration of 10% by volume or less
It is characterized in that it is annealed by heating to 00 to 1100 ° C. The content of Cu and P is preferably 0.15 to 0.
3% by weight and 0.04 to 0.08% by weight, respectively. It is preferable that the steel pipe of the present invention is further regulated to V: 0.1% by weight or less. Furthermore,
It can also contain up to 1.5% by weight Mo and / or up to 0.5% by weight Al.

[0005]

The present inventors have investigated various characteristics required for the exhaust gas heat transfer member, and as a result, have found that the combined addition of P and Cu is effective for improving the sulfuric acid resistance, and the high temperature corrosion resistance. It was found that it is necessary to regulate the V content, and that the annealing conditions after pipe making are effective in improving the oxidation resistance and the corrosion resistance. The present invention has been completed based on such findings, and simultaneously satisfies sulfuric acid resistance, high temperature oxidation resistance, and intergranular corrosion resistance as an exhaust gas heat transfer member. Cu is an alloying element that plays an important role in improving the sulfuric acid resistance, which is the most important property in the stainless steel of the present invention. Generally, stainless steel undergoes general corrosion due to active dissolution in an environment where corrosiveness such as acid is severe. That is, F near -0.4V (vs. SCE)
Corrosion proceeds in a region where e is melted. However, when Cu is added to stainless steel, Cu eluted along with main alloying elements such as Fe and Cr makes the potential noble and changes toward the passivation region existing on the high potential side of the active region, so that corrosion is caused. It is speculated that it will be suppressed.

It is speculated that P, after being eluted from the steel, combines with O present in the solution to form phosphoric acid, and improves the sulfuric acid resistance by the corrosion inhibiting action of phosphoric acid. The effect of P on the sulfuric acid resistance is that in a steel containing Ti as a stabilizing element instead of Nb, the bonding force with oxygen is higher than that of P by Ti.
It is inferred from the fact that the effect of improving the sulfuric acid resistance of P cannot be exerted because it is larger. In the ordinary stainless steel defined by JIS G4304 or the like, the P content is severely limited to 0.04% by weight or less in terms of workability, toughness and the like. However, a material of a composition system containing a stabilizing element, such as the stainless steel according to the present invention, is added to 6.0 m.
When used with a plate thickness smaller than m, even if P is contained in an amount of 0.04 wt% or more, there is no problem of toughness deterioration, and the material can be supplied inexpensively without sacrificing corrosion resistance or mechanical properties. It is possible.

The phenomenon of V ₂ O ₅ corrosion is complicated, and the cause of corrosion has not been clarified. However, when the difference in the corrosiveness depending on the presence or absence of V is examined for steels containing other alloy components in almost the same contents, the V-containing steels have a larger corrosion weight loss. It is considered that this is because V contained in the steel cooperates with V in the exhaust gas to reduce the V attack resistance.
Therefore, in order to reduce V ₂ O ₅ corrosion, it is preferable to regulate the V content to a low level, specifically 0.1% by weight or less. When a stainless steel pipe is used for an exhaust gas heat transfer member, the pipe is generally left as it is, and if it is subjected to severe processing, the pipe is annealed and then the surface scale is removed by pickling. However, since it is heated by the exhaust gas even in the use environment, pickling is not always necessary. Therefore, in the present invention, pickling is omitted by regulating the annealing conditions.

The regulation of the annealing conditions is not limited to the cost merit that the pickling can be omitted, but also exerts a positive effect of improving the oxidation resistance. When annealed in an atmosphere having an oxygen concentration of 10% by volume or less, easily oxidizable elements such as Si and Mn are preferentially oxidized and diffuse into the surface layer to be concentrated as an oxide layer.
Therefore, even if it is subsequently exposed to an oxidizing atmosphere, further progress of oxidation is suppressed. Combustion gases such as city gas and butane are used in an atmosphere having an oxygen concentration of 10% by volume or less. The annealing temperature at this time needs to be set in a temperature range of 900 to 1100 ° C. in order to form a surface oxide layer effective for oxidation resistance. If the annealing temperature does not reach 900 ° C, not only the workability is deteriorated due to insufficient recovery and recrystallization of the material, but also Cr deficiency of the base material occurs due to the oxidation of Cr, and corrosion resistance and oxidation resistance Lower. On the contrary, if the annealing temperature is higher than 1100 ° C., the crystal grains are coarsened, the toughness of the material is lowered, and the sheet thickness is substantially reduced by the oxidation loss.

The alloying elements and contents contained in the stainless steel of the present invention will be described below. Cr: 10-18% by weight It is an alloying element essential for ensuring the corrosion resistance of stainless steel, and the effect of Cr becomes remarkable when the content is 10% by weight ≧. The characteristics required for the intended use of the present invention are 12
Although it is sufficiently expressed at the Cr level, the same effect can be expected in a material having a higher Cr level. However, C
When the r content exceeds 15% by weight, not only the cost increases but also the effect of improving the high temperature strength by adding Nb tends to decrease. Therefore, the Cr content is 10 to 18
The weight ratio is set to preferably 10 to 15% by weight.

Nb: 0.2 to 1.0% by weight Nb is an alloying element effective in improving high temperature strength, and 0.2% by weight.
With the above content, the effect of adding Nb appears, and becomes remarkable when the content is 0.3% by weight or more. Since Nb fixes C and N similarly to Ti, it also has an effect of improving intergranular corrosion resistance. Also, even if 0.3% by weight or more of Nb is added, the high-frequency pipe forming property does not deteriorate unlike Ti. However, if Nb in excess of 0.8 wt% is added, hot cracking tends to occur in the spot welds or TIG welds, and this tendency is 1.0
It becomes remarkable when the Nb content exceeds wt%. Therefore,
The Nb content is set in the range of 0.2 to 1.0% by weight, preferably 0.3 to 0.8% by weight. Si: 1% by weight or less This is an element added as a deoxidizer during steel making, and a high Si content improves the oxidation resistance. However, 1.0% by weight
If a large amount of Si exceeding 10 is included, the material is hardened by solid solution strengthening and the workability is deteriorated.

Mn: 1% by weight or less It is an element effective as a deoxidizing agent during steel making like Si, but when Mn in excess of 1.0% by weight is added, a soluble compound MnS is produced and corrosion resistance is improved. descend. Cu: 0.1-0.4% by weight, preferably 0.15-
0.3% by weight It is an alloying element effective in improving the sulfuric acid resistance and improving the high-frequency pipe forming property associated with the improvement in the toughness of the material. Even when the Cu content is 0.15% by weight or less, the sulfuric acid resistance and the high-frequency pipe forming property are improved as compared with the case of no addition, but in order to bring out the original effect, the content of 0.15% by weight or more It is preferable to contain Cu. However, if Cu is excessively added, not only the cost becomes high, but also the material becomes hard and the workability is deteriorated. The hot workability also deteriorates with the excessive addition of Cu. Therefore, the Cu content is
It is set in the range of 0.1 to 0.4% by weight, preferably 0.15 to 0.3% by weight.

P: 0.04 to 0.15% by weight, preferably 0.04 to 0.08% by weight It is an alloying element effective for improving the sulfuric acid resistance, and the form of corrosion is changed from local corrosion such as pitting corrosion. It is an element suitable for use as an exhaust gas heat transfer member in which functionality such as perforation becomes a problem by changing to general corrosion. In order to exhibit such actions and effects, it is necessary to contain 0.04% by weight or more of P. However, excessive addition promotes the segregation of P at the crystal grain boundaries and reduces the intergranular corrosion resistance of steel. Therefore, the upper limit of the P content is regulated to 0.15% by weight, preferably 0.08% by weight. X = Nb−7 × (C + N) −P−0.15 ≧ 0 The X value is obtained as an experimental result by the present inventors and is an index for evaluating the intergranular corrosion susceptibility at the welded portion of the material.
A similar formula for calculating the amount of Nb necessary for stabilizing C and N has been conventionally known, but in the steel containing a large amount of P as in the present invention, the influence of P cannot be ignored. That is, the cause of the grain boundary corrosion includes grain boundary precipitation of Cr-based carbide and grain boundary segregation of P, which have been conventionally pointed out. However, in the present invention, the addition of an appropriate amount of Nb gives priority to the grain boundary. It is to prevent precipitation of intergranular corrosion due to P segregation. From such a viewpoint, the amount of Nb is calculated by the X value that incorporates the effect of P on the intergranular corrosion, and the intergranular corrosion resistance is improved.

In the stainless steel of the present invention, in addition to the alloying elements listed above, Ti, V, C, S, Ni, M
It is preferable to regulate o, Al, N, O, etc. as follows. Ti: 0.2 wt% or less Generally, it is said that it is effective as a fixing element of C for corrosion resistance and workability, like Nb. However, in the stainless steel according to the present invention, addition of Ti is not preferable because it eliminates the effect of adding P. However, a small amount of Ti
Is allowed because it is effective in fixing N.
In this respect, the Ti content is restricted to 0.2% by weight or less, preferably 0.1% by weight or less. V: less than 0.1% by weight Generally, like Nb and Ti, it is treated as a fixed element of C as an alloy element effective for corrosion resistance and workability. However, V addition is not preferable for stainless steel exposed to an exhaust gas environment such as heavy oil, which is the object of the present invention, because it has an action of promoting V attack. However, it may be mixed as an impurity such as Cr raw material, and when the V content is severely limited, the restriction on the usable raw material becomes large. In this respect, in the present invention, the allowable amount of V is
The amount is regulated to 0.1% by weight, preferably 0.05% by weight.

C: 0.03% by weight or less It is an alloying element inevitably contained in steel. As the content decreases, the material softens and the workability is improved, and the formation of carbides is reduced, and the weldability is improved. , Intergranular corrosion resistance is improved. Further, in the Nb and Ti-added steel, the consumption of Nb and Ti is suppressed by reducing the C content, and the high temperature strength is improved and the cost is reduced. Because of this, C
The content is preferably 0.03% by weight or less. N: 0.03 wt% or less It is an element contained in steel as an unavoidable impurity like C, and when the N content is high, the material becomes hard and the workability is deteriorated, and Nb such as Nb is used as a nitride. Consume a large amount of fixed elements. From this point, it is preferable to set the upper limit of the N content to 0.03% by weight.

The intergranular corrosion resistance results from the intergranular precipitation of Cr-based carbide. Therefore, in order to prevent intergranular corrosion, it is most important to reduce the V content. However, when a fixed element is added as in the steel of the present invention, N is also bonded to C in the same manner as C to fix the fixed element. Is consumed. Therefore, it is necessary to control C and N with the sum of C + N. Also,
To improve the high temperature strength, it is effective to increase the amount of solute Nb, in other words, decrease the amounts of C and N. With the current refining technology, C
It is almost impossible to make + N less than 0.005% by weight. However, when C + N exceeds 0.04% by weight, the intergranular corrosion susceptibility increases and the high temperature strength decreases. Therefore, it is preferable to set the amount of C + N in the range of 0.005 to 0.04% by weight. S: 0.03% by weight or less An element contained in steel as an unavoidable impurity.
If the content is high, hot workability and corrosion resistance deteriorate. Therefore, the upper limit of the S content is specified to be 0.03% by weight. Ni: 0.6 wt% or less It is an alloying element effective in improving the toughness of ferritic stainless steel, but excessive Ni content causes a rise in steel material cost. In the present invention, the Ni content is specified to 0.6% by weight or less, which is specified for ordinary ferritic stainless steel.

Mo: 1.5% by weight or less This is an alloying element added as necessary, and has the same effect as Cr, which improves the corrosion resistance and the high temperature strength. But,
Since excessive addition increases the cost of steel materials, the content is set to 1.5 wt% or less when Mo is added. Al: 0.5 wt% or less Like Si, it is an element added as a deoxidizing agent in the steelmaking stage, but since it has extremely high reactivity with oxygen, Al remaining in the steel is Ti during high-frequency pipe forming. It forms an oxide similar to that and causes pinholes. Therefore, A
The upper limit of the 1 content is preferably set to 0.5% by weight. O: 0.01 wt% or less Like C and N, it is an element mixed in steel as an unavoidable impurity, and if the O content is high, the workability is significantly impaired. In addition, when forming a high-frequency pipe, it combines with Ti, Al, etc. to form an oxide, which causes pinholes. Therefore, the O content is regulated to 0.01% by weight or less.

[0017]

Example A ferritic stainless steel having the composition shown in Table 1 was melted in a laboratory and hot-rolled to a plate thickness of 4.5 mm.
The hot rolled sheet was manufactured. The hot-rolled sheet was cold-rolled to a sheet thickness of 2 mm and finish-annealed at 900 to 1050 ° C to prepare a test material. In Table 1, Group A is stainless steel according to the present invention, and N is a stabilizing element.
b is X [= Nb−7 × (C + N) −P−0.15] ≧ 0
Is added under the conditions of. Group B is a comparative steel. In B1, the contents of Nb, Cu and P satisfy the range specified in the present invention, but the X value is out of the range specified in the present invention. B2 does not contain Cu. B3 does not contain Cu and has a low P content. B4 and B5 are
Includes Ti and V above the levels specified in the present invention. B6, B7
Does not satisfy the ranges specified in the present invention for the contents of Nb, Cu and P. B8 has a P content exceeding the range specified in the present invention. As the A1, B6, and B7 stainless steels, those manufactured under the same conditions using a real line were prepared.

[0018]

[Table 1]

The corrosion resistance of each test material was investigated as follows. Sulfuric acid resistance test: A test piece was immersed in a 50% sulfuric acid aqueous solution kept at 70 ° C for 2 hours, and the weight change before and after the immersion was measured. Electrochemical test: 6000 ppm Cl ^- and 60,000
A solution containing ppm SO ₄ ²⁻ was adjusted to pH 3 with hydrochloric acid at a temperature of 80 ° C., and the maximum current density of the anodic polarization curve was measured as a measure of active dissolution. Sulfuric acid-copper sulfate test: TIG without using a welding core wire
After heat treating the welded test piece at 500 ° C. for 10 hours,
A test of immersing in a sulfuric acid-copper sulfate solution adjusted according to JIS G0575 at 60 ° C. for 16 hours was performed, and the occurrence of intergranular corrosion was investigated by bending and observing the cross-sectional structure. V ₂ O ₅ corrosion test: V ₂ O ₅ ash is applied to the surface of the test piece,
It heated at 900 degreeC for 3 hours, and measured the corrosion weight loss before and after heating. As shown in Table 2 showing the investigation results, the comparative steels containing Cu also satisfy the sulfuric acid resistance, but the characteristics were insufficient in the electrochemical test. On the other hand, P
The comparative steels containing a showed the opposite trend. In contrast,
The stainless steel of the present invention to which Cu and P were added in combination showed better corrosion resistance than the other comparative steels in both tests.
Regarding V ₂ O ₅ corrosion, comparative steel B5 containing V
Corrosion weight loss is large compared to A1 and existing B6 and B7. This indicates that V contained in the steel had an adverse effect. In addition, looking at the sulfuric acid-copper sulfate test that investigated the occurrence of intergranular corrosion,
The occurrence of intergranular corrosion was not detected in the steels of the present invention in which the value exceeded 0, but the intergranular corrosion occurred in Comparative Steels B1 and B6 to 8 below 0.

[0020]

[Table 2]

A sulfuric acid resistance test and an oxidation test were carried out on the raw material portion, the welded portion of the steel pipe manufactured from the steel A1 of the present invention, and the steel pipe annealed at various annealing temperatures after the pipe manufacturing using a high-frequency pipe forming machine. In the oxidation test, continuous heating at 900 ° C. for 100 hours in an oxidizing atmosphere was conducted to examine whether abnormal oxidation occurred. As can be seen from Table 3 showing the test results, the steel A1 of the present invention having the adjusted X value exhibited the same corrosion resistance as the base metal part in the welded part. The annealed steel pipe was slightly inferior in sulfuric acid resistance to the as-made steel pipe, but a remarkable improvement effect was observed in the oxidation resistance. Regarding the sulfuric acid resistance, the steel pipe annealed at a temperature exceeding 900 ° C. also showed a smaller degree of increase in corrosion weight loss than the as-made steel pipe.

[0022]

[Table 3]

[0023]

As described above, in the stainless steel of the present invention, the intergranular corrosion resistance, high frequency pipe forming property and In addition to high temperature strength properties, sulfuric acid resistance and high temperature oxidation properties are also improved. This ferritic stainless steel has a high yield in the pipe forming process due to its excellent high-frequency pipe forming characteristics, and since it is not necessary to perform pickling after annealing, it can be manufactured relatively inexpensively. Further, by utilizing the fact that a welded portion having excellent intergranular corrosion resistance is obtained and excellent sulfuric acid resistance, an exhaust gas heat transfer member which is exposed to a severe corrosive environment as it is during welding or as a structural steel sheet, Used as a structural material for various flues and chimneys.

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C21D 9/08 C21D 6/00 102 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. C: 0.03% by weight or less, Si: 1% by weight
% Or less, Mn: 1% by weight or less, P: 0.04 to 0.15
% By weight, S: 0.03% by weight or less, Ni: 0.6% by weight
Below, Cr: 10 to 18% by weight, Nb: 0.2 to 1.0
% By weight, Cu: 0.1 to 0.4% by weight, N: 0.03 weight
% Or less, the balance consisting essentially of Fe, and X
= X defined by Nb-7 * (C + N) -P-0.15
Welded steel pipe having a composition whose value is adjusted to 0.103 or more
90 in a combustion gas atmosphere with an oxygen concentration of 10% by volume or less.
Characterized by performing annealing by heating at 0 to 1100 ° C
Manufacturing method of ferritic stainless steel pipe for exhaust gas heat transfer member
Law. 2. The steel pipe composition further comprises V: 0.1% by weight.
Exhaust Den of claim 1, wherein those which are regulated fully
A method for manufacturing a ferritic stainless steel pipe for a heat member. 3. The steel pipe composition further comprises 1.5% by weight or less.
Contains Mo and / or 0.5 wt% or less of Al
3. The ferrite system for exhaust gas heat transfer member according to claim 1 or 2.
Manufacturing method of stainless steel pipe.