KR20190014081A - Sealed steel pipe for boiler excellent in stress corrosion cracking resistance and manufacturing method thereof - Google Patents

Abstract

Provided is a steel pipe for a boiler having a ferritic grain boundary strength in a welded portion and suppressing cracking in a welded portion and having excellent stress corrosion cracking resistance. Wherein the composition comprises, by mass%, C: 0.05 to 0.35%, Si: 0.10 to 0.35%, Mn: 0.25 to 1.50%, S: not more than 0.035%, P: not more than 0.035% 0.010% or less; and O: 0.010% or less; and optionally, optionally, Cr, not more than 1.00%, Mo: not more than 1.00%, Ni: not more than 2.00%, Cu: not more than 2.00%, B: : At most 0.20%, V: at most 0.20%, Ti: at most 0.20%, Ca: at most 0.0050%, and Mg: at most 0.0050%, the balance being Fe and inevitable impurities, The difference in hardness between the hardness and the hardness at the depth of one-half of the thickness of the base material is not more than 20 Hv and the difference between the hardness of the welded portion to 1 mm from the outer surface and the hardness of the base material up to 1 mm from the outer surface is 20 Hv or less.

Description

Sealed steel pipe for boiler excellent in stress corrosion cracking resistance and manufacturing method thereof

The present invention relates to a seamless steel pipe for a boiler excellent in the stress corrosion cracking resistance of the outer surface of a steel pipe. Further, the present invention relates to a method of manufacturing such an electropolished steel pipe.

Boiler piping composed of a carbon steel heat transfer pipe may be used in a nitrate environment or the like. Therefore, such a boiler pipe may be damaged due to stress corrosion cracking caused by ammonium nitrate. Therefore, the boiler piping is required to have stress corrosion cracking resistance (hereinafter sometimes referred to as " internal SCC property ").

Damage to the boiler piping is caused by tensile residual stress caused by ammonium nitrate generated by dissolving NO ₂ or NH ₃ in the exhaust gas and condensation condensation on the pipe when the plant is started and bending or pin winding, It is a synergistic phenomenon.

In a general plant, there are about 200 to 500 boiler pipes in which stress corrosion cracking (hereinafter also referred to as " SCC ") may occur due to ammonium nitrate. For this reason, it is difficult to inspect all the pipes at high precision during the regular inspection, which is usually performed for about 50 days. In addition, the heat exchanger including such a boiler pipe also has a portion (piping) which can not be inspected due to its structure.

In recent years, the interval of periodic inspection has become longer, and depending on the plant, it may be operated without inspection for 4 years. In such a case, there is a high possibility that SCC occurs in the boiler pipe during the operation of the plant, and in some cases, the pipe may be ruptured.

Techniques relating to boiler piping are disclosed in, for example, Patent Documents 1 to 3 below.

Japanese Patent Application Laid-Open No. 2005-091029 Japanese Patent Application Laid-Open No. 06-287678 Japanese Patent Application Laid-Open No. 10-096050

Patent Document 1 discloses a method for evaluating and diagnosing SCC damage caused by nitrate in a carbon steel heat transfer pipe of a waste heat recovery boiler with high accuracy. Specifically, the number of times of starting and stopping of each mode of the plant and the wetting time And the concentration of nitrogen oxides in the exhaust gas in each mode are calculated and the concentration of the accumulated nitrate in all the wetting times is calculated and the concentration of the accumulated nitrate is calculated based on the nitrate concentration at the time of occurrence of the predetermined stress corrosion crack And the damage degree is evaluated by calculating the operation period until the temperature reaches the predetermined temperature.

Patent Document 2 discloses a seamless steel pipe for a high-strength boiler excellent in high-temperature characteristics and corrosion resistance. More specifically, it comprises 0.15 to 0.30% of C, 0.05 to 0.50% of Si, 0.25 to 1.5% of Mn, 0.001 to 0.010% of N, 0.02 to 0.10% of Cu, 0.001 to 0.004% of Ca, 0.01 to 0.10% of Mo and 0.003% of S or less as a base component and the balance Fe and inevitable elements, And a welding portion and a base material portion are uniformly structured by welding after welding.

Patent Document 3 discloses a steel material having excellent nitric acid stress corrosion cracking resistance. Specifically, the steel material contains 0.005 to 0.05% of C, 0.1 to 0.8% of Si, 0.2 to 1.5% of Mn, 0.015 %, S: not more than 0.015%, 3.5 to 5.0% of Cr, 0.2 to 1.2% of Mo, 0.01 to 0.15% of Nb, 0.01 to 0.20% of Al and 0.015% or less of N, Which is excellent in the resistance to nitric acid stress corrosion cracking.

Damage to the boiler pipe is caused by expansion of the crack along the ferrite grain boundaries of the welded portion when used in a nitrate environment or the like. This damage is caused in the welded portion because the carbon concentration is low in the welded portion as compared with the base material portion and the ferritic grain boundary strength is not sufficiently obtained in the welded portion so as to withstand use in a nitrate environment.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a seamless steel pipe for a boiler, which has high ferrite grain boundary strength in a welded portion to suppress cracking in a welded portion and has excellent stress corrosion cracking resistance. The purpose is to provide.

The present inventors have found that, in order to solve the above problems, the inventors of the present invention have found that, in order to increase the ferrite grain boundary strength of a welded portion, to suppress cracking in a welded portion and further to exhibit excellent stress corrosion cracking resistance By way of example, the following findings (a) and (b) were obtained.

(a) At the time of complete welding, decarburization occurs in the welded welded portion, thereby lowering the ferrite grain boundary strength of the welded welded portion. The inventors of the present invention have obtained the knowledge that by performing a predetermined heat treatment after welding, it is possible to improve the carbon concentration of the welded portion, improve the ferrite grain boundary strength of the welded portion, and further improve the inner SCC property. The inventors of the present invention evaluated the difference in hardness between different depth positions from the surface of the base material portion and evaluated the difference between the hardness of the welded portion and the hardness of the base material portion to evaluate the decarburization affecting the ferrite grain boundary strength (Evaluation Method 1) or a method of evaluating the pearlite area ratio of the welded portion and comparing and evaluating the pearlite area ratio of the welded portion and the pearlite portion of the base material (Evaluation Method 2) .

(b) Further, the inventors of the present invention found that the improvement of the internal SCC property can be attained by a method (evaluation method 3) of evaluating the residual stress of the steel pipe surface layer portion in addition to the evaluation methods 1 and 2 concerning the decarburization shown in the knowledge I got the knowledge that I can judge.

The present invention is based on the above findings, and its main points are as follows.

[1] A composition comprising, in% by mass,

C: 0.05 to 0.35%

Si: 0.10 to 0.35%,

Mn: 0.25 to 1.50%

S: 0.035% or less,

P: not more than 0.035%

Al: 0.005 to 0.050%

N: 0.010% or less, and

O: 0.010% or less

Lt; / RTI >

Optionally, at least one selected from the group consisting of Cr: not more than 1.00%, Mo: not more than 1.00%, Ni: not more than 2.00%, Cu: not more than 2.00%, B: not more than 0.0030%, Nb: not more than 0.20% % Or less, Ca: 0.0050% or less, and Mg: 0.0050% or less,

Balance: Fe and inevitable impurities,

The difference between the hardness at the surface layer of the base material portion and the hardness at the half thickness depth position of the base material portion is 20 Hv or less,

The difference between the hardness of the welded fitting portion from the outer surface to 1 mm and the hardness of the base material portion up to 1 mm from the outer surface is 20 Hv or less

Which is excellent in stress corrosion cracking resistance.

[2] The composition according to any one of [1] to [

C: 0.05 to 0.35%

Si: 0.10 to 0.35%,

Mn: 0.25 to 1.50%

S: 0.035% or less,

P: not more than 0.035%

Al: 0.005 to 0.050%

N: 0.010% or less, and

O: 0.010% or less

Lt; / RTI >

Optionally, at least one selected from the group consisting of Cr: not more than 1.00%, Mo: not more than 1.00%, Ni: not more than 2.00%, Cu: not more than 2.00%, B: not more than 0.0030%, Nb: not more than 0.20% % Or less, Ca: 0.0050% or less, and Mg: 0.0050% or less,

Balance: Fe and inevitable impurities,

A welded steel pipe for a boiler excellent in stress corrosion cracking resistance, having a pearlite area ratio of 5% or more in a welded joint portion of 1 mm from the outer surface.

[3] A welded steel pipe for a boiler excellent in stress corrosion cracking resistance as described in [1] or [2], wherein a residual stress at the surface layer portion of the welded portion is 200 MPa or less.

[4] The steel strip for boiler according to any one of the above [1] to [3], wherein the area ratio of dendrites in the welded joint portion from the outer surface to 1 mm is 1% or less.

[5] A method for producing a seamless steel pipe for a boiler excellent in stress corrosion cracking resistance according to any one of [1] to [4]

A step (i) of molding a steel material having the composition described in the above [1] or [2]

(Ii) a step of subjecting the tube to a heat treatment in an atmosphere furnace for 30 seconds or more at a temperature of Ac 3 point to 1250 ° C

Wherein said method comprises the steps of:

[6] The method according to the above [5], wherein the step (ii) further comprises a step (iii) of correcting the bending of the tube and then performing a heat treatment at a temperature of 400 ° C or higher and Ac1 point or lower. A method of manufacturing a seamless steel pipe for a boiler excellent in cracking property.

[7] A method for manufacturing a seamless steel pipe for boilers, which is excellent in stress corrosion cracking resistance as described in [5] or [6], wherein the atmosphere in the atmosphere does not contain oxygen.

[8] The method for producing a seamless steel pipe for boiler according to [7], wherein the atmosphere in the atmosphere is at least one of argon, nitrogen, carbon dioxide, and hydrogen.

In setting the specific component composition, the hardness difference between the different depth positions from the surface of the base material portion is set to a predetermined range, and the difference between the hardness of the welded portion and the hardness of the base material portion is set to And is set to a predetermined range. In general, in the tubular steel pipe for a boiler according to the present invention, the pearlite area ratio of the welded portion is set to a predetermined range after setting a specific component composition. In order to realize these hardnesses and pearlite area ratios, in the method for producing a tubular steel pipe for boilers according to the present invention, a predetermined atmosphere furnace is used and the holding temperature and the holding time in the heat treatment are set within a predetermined range . Therefore, according to the technique relating to the present invention, it is possible to increase the carbon concentration of the welded joint portion to increase the ferrite grain boundary strength to suppress cracking in the welded portion and to provide a welded steel pipe for boiler having excellent stress corrosion cracking resistance Can be obtained.

Fig. 1 (a) is a cross-sectional view showing a welded portion of a steel pipe, and Fig. 1 (b) is an enlarged view of a portion .
Fig. 2 is a cross-sectional photograph of a welded fitting portion of a steel pipe; Fig. 2 (a) is an example in which cracks did not occur;
Fig. 3 is a graph showing the relationship between the hardness of the annealed steel pipe for the boiler and the distance from the welded surface, wherein (a) shows a steel pipe in which stress corrosion cracking did not occur, and Fig. 3 (b) shows a steel pipe in which stress corrosion cracking occurred.
Fig. 4 is a schematic view showing a carbon concentration in a welded portion of a pipe, in which (a) shows a state before heat treatment, and Fig. 4 (b) shows a state after heat treatment.

(Hereinafter sometimes referred to as " the present invention seamless steel pipe ") and the production method thereof (hereinafter sometimes referred to as " the present invention production method ") excellent in stress corrosion cracking resistance of the present invention The form will be explained in detail. The following embodiments are not intended to limit the present invention. Also, the constituent elements of the embodiment include those which can be easily replaced by persons skilled in the art, or substantially the same. In addition, various forms included in the above embodiments can be arbitrarily combined within a range that is obvious to a person skilled in the art.

<Our main steel pipe>

[Composition of ingredients]

First, the reason for limiting the composition of the present invention is explained in detail. In addition, the unit [%] of the composition shown below means% by mass.

(Essential element)

The steel pipe of the present invention contains C, Si, Mn, S, P, Al, N and O, Fe and inevitable impurities in the respective ranges shown below.

C: 0.05 to 0.35%

C is an element necessary for securing the strength of the steel pipe. If it is less than 0.05%, the strength of the steel pipe is insufficient, so C is set to 0.05% or more. It is preferably at least 0.06%. On the other hand, if it exceeds 0.35%, the hardness of the steel pipe increases and the workability deteriorates. Therefore, C is set to 0.35% or less. It is preferably 0.32% or less.

Si: 0.10 to 0.35%

Si is an element contributing to improvement of the strength of the steel pipe by solid solution strengthening. If it is less than 0.10%, the effect of addition is not sufficiently manifested, so Si should be 0.10% or more. It is preferably at least 0.15%. On the other hand, if it exceeds 0.35%, the strength of the steel pipe is excessively increased and the workability is lowered, so Si is set to 0.35% or less. It is preferably 0.30% or less.

Mn: 0.25 to 1.50%

Mn is an element contributing to the improvement of the strength of the steel pipe by securing quenching. If it is less than 0.25%, the effect of addition is not sufficiently manifested, so Mn should be 0.25% or more. It is preferably at least 0.27%. On the other hand, if it exceeds 1.50%, the hardness of the steel pipe is excessively increased and the workability is deteriorated, so that Mn is 1.50% or less. It is preferably 1.45% or less.

S: not more than 0.035%

S is an element forming MnS which is a starting point of crack generation. For this reason, S is preferably as small as possible, and therefore, it is set to 0.035% or less. It is preferably 0.030% or less. The lower limit includes 0%, but if the S is reduced to less than 0.0001%, the manufacturing cost increases sharply, so 0.0001% is the practical lower limit in practical use.

P: not more than 0.035%

P is an element that causes grain boundary segregation or center segregation which is a cause of soft deterioration. Therefore, P is preferably as small as possible, and therefore, it is set to 0.035% or less. It is preferably 0.030% or less. The lower limit includes 0%, but if the P is reduced to less than 0.0010%, the manufacturing cost increases greatly, so 0.0010% is practically the lower limit in practical use.

Al: 0.005 to 0.050%

Al is a deoxidizing element. When the content is less than 0.005%, deoxidation is not sufficient, so the content of Al is 0.005% or more. It is preferably 0.010% or more. On the other hand, if it exceeds 0.050%, the workability is deteriorated due to the coarsening of the alumina-based oxide, so that the content of Al is 0.050% or less. Preferably 0.040% or less.

N: 0.010% or less

N is an inevitably present element. If it exceeds 0.010%, inclusions are coarsened and workability deteriorates, so N is set to 0.010% or less. It is preferably 0.008% or less. The lower limit includes 0%, but if the N is reduced to less than 0.0010%, the manufacturing cost increases sharply, so 0.0010% is practically the lower limit in practical use.

O: 0.010% or less

O is an inevitable element after deoxidation. If it exceeds 0.010%, the inclusions are coarsened and the workability deteriorates. Therefore, O is set to 0.010% or less. It is preferably 0.005% or less. The lower limit includes 0%, but if the O is reduced to less than 0.0005%, the manufacturing cost increases sharply, so 0.0005% is practically the lower limit in practical use.

Balance: Fe and inevitable impurities

The remainder is Fe and inevitable impurities. Here, inevitable impurities are not intentionally mixed impurities but impurities contained in raw materials or impurities that can be incorporated in each manufacturing process. As the inevitable impurities in the present invention, for example, As, Na, Zr and Sb can be mentioned. In the present invention, these inevitable impurities are specifically limited in the content (upper limit value) as described below.

That is, the content of As is 0.01% or less, the content of Na is 0.01% or less, the content of Zr is 0.01% or less, and the content of Sb is 0.01% or less. By limiting the respective contents of these elements to the above-mentioned respective ranges, it is possible to effectively prevent the ferrite grain boundary strength of the welded portion, which is an effect inherent to the present invention, from being increased and inhibiting the effect of suppressing cracking in the welded portion.

(Optional optional element)

At least one of Cr, Mo, Ni, Cu, B, Nb, V, Ti, and Mg may be used in the range shown below in addition to the elements shown above in the scope of the present invention, .

Cr: 0.05 to 1.00%

Cr is an element contributing to improvement of the strength of a steel pipe by ensuring quenching. If it is less than 0.05%, the effect of addition is not sufficiently manifested, and therefore it is preferable that Cr is 0.05% or more. More preferably, it is at least 0.10%. On the other hand, when it exceeds 1.00%, the hardness of the steel pipe excessively rises and the workability deteriorates. Therefore, it is preferable that the Cr content is 1.00% or less. More preferably, it is 0.80% or less.

Mo: 0.05 to 1.00%

Mo is an element contributing to the improvement of the strength of a steel pipe by ensuring quenching. If it is less than 0.05%, the effect of addition is not sufficiently manifested, so that Mo is preferably 0.05% or more. More preferably, it is at least 0.10%. On the other hand, if it exceeds 1.00%, the hardness of the steel pipe is excessively increased and the workability is deteriorated. Therefore, Mo is preferably 1.00% or less. More preferably, it is 0.80% or less.

Ni: 0.10 to 2.00%

Ni is an element contributing to the improvement of strength of a steel pipe by ensuring quenching. If it is less than 0.10%, the effect of addition is not sufficiently manifested, and therefore, it is preferable that Ni is 0.10% or more. More preferably, it is at least 0.15%. On the other hand, if it exceeds 2.00%, the hardness of the steel pipe is excessively increased and the workability is deteriorated. Therefore, the Ni content is preferably 2.00% or less. More preferably, it is 1.50% or less.

Cu: 0.10 to 2.00%

Cu is an element contributing to the improvement of the strength of a steel pipe by ensuring quenching. If it is less than 0.10%, the effect of addition is not sufficiently manifested, so that Cu is preferably 0.10% or more. More preferably, it is at least 0.15%. On the other hand, if it exceeds 2.00%, the hardness of the steel pipe is excessively increased and the workability is deteriorated. Therefore, it is preferable that Cu is 2.00% or less. More preferably, it is 1.50% or less.

B: 0.0003 to 0.0030%

B is an element contributing to improvement of strength of steel pipe by securing quenching. If it is less than 0.0003%, the effect of addition is not sufficiently manifested, and therefore B is preferably 0.0003% or more. More preferably, it is 0.0005% or more. On the other hand, if it exceeds 0.0030%, grain boundary embrittlement may be caused, and B is preferably 0.0030% or less. And more preferably 0.0020% or less.

Nb: 0.005 to 0.20%

Nb is an element that has a strong affinity between C and N to precipitate NbCN and contributes to improvement of the strength of the steel pipe. If it is less than 0.005%, the effect of addition is not sufficiently manifested, and therefore Nb is preferably 0.005% or more. More preferably, it is 0.010% or more. On the other hand, if it exceeds 0.20%, precipitation hardening by NbC becomes remarkable and workability deteriorates. Therefore, Nb is preferably 0.20% or less. More preferably, it is 0.10% or less.

V: 0.005 to 0.20%

V is an element that contributes to the improvement of the strength of the steel pipe by the strong affinity between C and N to precipitate VN or VC. If it is less than 0.005%, the effect of addition is not sufficiently manifested, so V is preferably 0.005% or more. More preferably, it is 0.010% or more. On the other hand, if it exceeds 0.20%, precipitation hardening by VN or VC becomes remarkable, and the workability is deteriorated. Therefore, V is preferably set to 0.20% or less. More preferably, it is 0.10% or less.

Ti: 0.005 to 0.20%

Ti has strong affinity with N and precipitates TiN, contributing to the refinement of the structure. If it is less than 0.005%, the effect of addition is not sufficiently manifested, and therefore Ti is preferably 0.005% or more. More preferably, it is 0.008% or more. On the other hand, if it exceeds 0.20%, the precipitation hardening by TiC becomes remarkable and the workability is deteriorated. Therefore, Ti is preferably set to 0.20% or less. More preferably, it is 0.10% or less.

Ca: 0.0001 to 0.0050%

Ca is an element that contributes to the improvement of workability by adjusting the inclusion form of the base metal and the weld zone. If it is less than 0.0001%, the effect of addition is not sufficiently manifested, so Ca is preferably 0.0001% or more. More preferably, it is 0.0005% or more. On the other hand, when it exceeds 0.0050%, inclusions in the steel increase and workability deteriorates, so Ca is preferably set to 0.0050% or less. And more preferably 0.0040% or less.

Mg: not more than 0.0050%

Mg is a deoxidizing element, and the resulting oxide functions as precipitation nuclei of MnS, and is an element contributing to miniaturization and uniform dispersion of MnS. If it exceeds 0.0050%, the yield is lowered and the effect of addition is saturated, so that Mg is preferably 0.0050% or less. And more preferably 0.0040% or less.

[Limited reason for hardness]

Hereinafter, in the present invention, it is essential that (1) the hardness is limited. The limitation on this hardness,

(1-1) The difference between the hardness at the surface layer of the base material portion and the hardness at the half thickness depth position of the base material portion is 20 Hv or less, and

(1-2) the difference between the hardness of the welded joint portion from the outer surface to 1 mm and the hardness of the base material portion up to 1 mm from the outer surface is 20 Hv or less,

It includes two items.

Here, the welded filler portion refers to a portion in the vicinity of the welded surface where a distance from the welded surface is a distance of 0 占 퐉 to about 100 占 퐉 among the portions to which the microstructure is thermally affected by the welding. In addition, the base material refers to a portion where the microstructure is not affected by heat by welding, from the welding surface to a portion spaced from the welding surface, rather than the welded portion.

The inventors of the present invention have found that, in order to investigate the occurrence of SCC in the welded joint portion and the causal relationship between the above items (1-1) and (1-2), steel tubes having no stress corrosion crack (SCC) And the welded joints were photographed with respect to the steel pipe where SCC occurred in the nitrate environment, and the occurrence of cracks was examined in the steel pipe where SCC occurred.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing a portion of a steel pipe to be inspected in the irradiation of a crack-generated portion, and is a drawing showing a scale in the left-right direction with respect to the scale in the vertical direction. 1 (a) shows a part of a cross-sectional area that widens in the depth direction from the outer surface 12 of the unshrouded steel pipe 10, and Fig. 1 (b) Fig.

In this embodiment, as shown in Fig. 1 (b), the steel pipe 10 is a region extending from the surface 12 to a position of 1 mm in the depth direction (there is a possibility of cracking) ) Is referred to as a welding filler portion. The welded fitting portion can be defined by specifying the welding face by performing the metal flow etching and then specifying the positions of 100 μm each on both sides of the welding face with an indentation or the like.

Fig. 2 is a cross-sectional photograph of a welded fitting portion of a steel pipe; Fig. 2 (a) is an example in which cracks did not occur; Further, it was confirmed that the cracks were propagated from the outer surface to the core portion in the welded portion of the steel pipe, specifically, in the region from the outer surface to the depth of 1 mm.

Next, in the present investigation, it is assumed that the unsealed unsealed steel pipe and the unsealed unsealed steel pipe have cracks

(1-1) Difference in hardness between the hardness in the surface layer of the base material portion and the half thickness depth position of the base material portion (difference in hardness in the depth direction of the base material portion), and

(1-2) Difference between the hardness of the welded joint portion from the outer surface to 1 mm and the hardness of the base material portion from the outer surface to 1 mm (hardness difference between the welded portion and the base material portion at a specific depth position)

Respectively.

In particular, the hardness of the welded joint at a specific depth position was measured at a position 0.5 mm deep from the outer surface of the steel pipe at a Vickers hardness (Hv) of 0.6 mm on both sides beyond the weld surface. This hardness is an arcuate shape centered on the weld surface and measured at an angular position of 0.1 mm pitch at a test load of 100 g. On the other hand, the hardness of the member at a specific depth position is determined so that the Vickers hardness (Hv) at a depth of 0.5 mm from the outer surface of the steel pipe is 0.6 mm at both sides of a position rotated 90 ° around the center of the steel pipe from the weld surface Lt; / RTI &gt;

3 is a graph showing the results of the above item (1-2). That is, FIG. 3 is a graph showing the relationship between the hardness (Vickers hardness at a depth of 0.5 mm from the outer surface of the steel pipe) and the distance from the weld surface, and FIG. 3 (a) (B) shows a steel pipe in which a crack occurred. In Figures (a) and (b), the number of the positive (negative) side of the abscissa means that the distance from the welding surface to one side (the other side) is a predetermined distance. In the figures (a) and (b), the respective figures show the results of two examples in which the measurement positions are different although they are the same steel pipe.

In Figs. 3 (a) and 3 (b), the hardness at a distance of 0 (mm) from the weld surface is the hardness of the weld fill portion, and the distance from the weld surface is ± 0.1, 0.3, ± 0.4, ± 0.5, ± 0.6 (mm) "is the hardness of the base material portion.

As shown in Fig. 3 (a), as for the hardness distributions of the two kinds of tubular steel tubes for boilers in which stress corrosion cracks (SCC) did not occur, the difference between the hardness of the welded joint portion and the hardness of the base material portion converged to 20 Hv or less have. On the other hand, as shown in Fig. 3 (b), there are cases where the difference between the hardness of the welded joint portion and the hardness of the base material portion exceeds 20 Hv in all of the hardness distributions of the two kinds of tubular steel tubes for boilers in which SCC occurs.

From the results of these investigations and measurements,

(1-1) the hardness difference in the depth direction of the base material portion is 20 Hv or less, and

(1-2) When the hardness difference between the welded portion and the base material portion at a specific depth position is 20 Hv

, It was confirmed that stress corrosion cracking (SCC) did not occur.

It is considered that, if both of the above items (1-1) and (1-2) are satisfied, the carbon concentration is not so low in the welding portion as compared with the base portion, and furthermore, This is because it is considered that the ferrite grain boundary strength is sufficiently obtained to withstand use in the nitrate environment diagram.

As described above, according to the present invention, in the steel-wrapped steel pipe according to the present embodiment, after setting a specific component composition, the hardness difference between the depth direction hardness difference of the base material portion and the hardness difference between the weld- It is possible to increase the carbon concentration of the welded portion (more specifically, the welded welded portion) to increase the ferrite grain boundary strength, suppress cracking in the welded portion, and realize excellent stress corrosion cracking resistance.

Particularly, it can be said that the smaller the difference in hardness of the item (1-2) is, the more the occurrence of stress corrosion cracking (SCC) is suppressed, and furthermore, it is a seamless steel pipe for a boiler having excellent stress corrosion cracking resistance. For this reason, when the hardness difference is 15 Hv or less, it is preferable because it can be said to be a more excellent seamless steel pipe for a boiler.

[Reason for limitation on pearlite area ratio]

In the present invention, it is necessary to substitute for the reason for the above hardness, and (2) to limit the pearlite area ratio. More specifically,

The pearlite area ratio of the welded joint to 1 mm from the outer surface is 5% or more

Is essential.

The inventors of the present invention have found that in order to investigate the causal relationship between the occurrence of SCC in the welded joint and the above item (2)

(2) Pearlite area ratio of the welded joint to 1 mm from the outer surface

Respectively.

The pearlite area ratio of the welded portion was determined by defining the welded surface after the metal flow etching was performed to define the welded portion, further subjected to the nontraditional etching, and the structure of the welded portion was observed under an optical microscope (magnification: 200 times) , And the captured image was analyzed by image analysis.

It was confirmed that in the steel pipe in which SCC occurred, most of the microstructure of the welded fitting portion in the depth region from the outer surface to 1 mm was ferrite and almost no pearlite existed. On the contrary, in the steel pipe in which no SCC was generated, it was confirmed that the microstructure of the welded joint in the depth region of 1 mm from the outer surface had pearlite of 5% or more in area ratio in addition to ferrite.

In other words,

(2) It was confirmed that stress corrosion cracking (SCC) did not occur when the pearlite area ratio of the welded joint portion to the outer surface of 1 mm was 5% or more.

Particularly, the above item (2) is an effect that the carbon is diffused by the heat treatment described later to raise the carbon concentration of the welded portion.

This is considered to be because if the above item (2) is satisfied, it is considered that the carbon concentration is not so low in the welding portion as compared with the base portion, and even in the welding portion, It can be said that the ferrite grain boundary strength is sufficiently obtained.

As described above, in the presently filed steel pipe according to the present embodiment, as described above, by setting the specific component composition and then setting the pearlite area ratio of the welded portion to 1 mm from the outer surface to a predetermined range, Specifically, the welded joint portion) can be increased, cracks in the welded portion can be suppressed, and excellent stress corrosion cracking resistance can be realized.

In particular, the larger the pearlite area ratio in the item (2), the more the occurrence of stress corrosion cracking (SCC) is suppressed, and moreover, it can be said to be a seamless steel pipe for a boiler having excellent stress corrosion cracking resistance. Therefore, when the pearlite area ratio is 8% or more, it is preferable from the viewpoint that it can be said to be a more excellent seamless steel pipe for a boiler. On the other hand, the upper limit of the pearlite area ratio is determined by the amount of carbon, and is not particularly limited.

[Residual stress in surface layer portion of welded portion]

Further, in the present invention, it is possible to arbitrarily selectively limit the residual stress in addition to the limitation on the hardness or the pearlite area ratio described above. The limitation on the residual stress is,

(3) The residual stress at the surface layer of the welded joint is 200 MPa or less.

In order to investigate the causal relationship between the occurrence of SCC in the welded joint and the above item (3)

(3) Residual stress at the surface layer of welded joint

Respectively.

The residual stress of the surface layer of the welded portion was measured by X-ray diffraction (irradiation diameter: 0.5 mm), and the external surface stress of the steel pipe was measured. The residual stress on the outer surface of the steel pipe is uniform in the circumferential direction, including the welded portion.

The residual stress in the surface layer portion of the welded joint portion is not more than 200 MPa in the steel pipe in which the SCC is not generated even when the welded joint portion of 1 mm from the outer surface and the base portion of 1 mm from the outer surface exceed 20 Hv in the hardness difference, In the resulting steel pipe, the residual stress in the surface layer portion of the welded portion exceeded 200 MPa.

In other words,

(3) Even if the difference in hardness between the welding filler portion from the outer surface to 1 mm and the base material portion up to 1 mm from the outer surface slightly exceeds 20 Hv, the residual stress in the surface layer portion of the weld-

, It was confirmed that stress corrosion cracking (SCC) did not occur.

Such low residual stress is realized because the residual stress generated when the heat-treated pipe is bent and corrected is sufficiently reduced by the subsequent heat treatment as described later. Thus, even if the difference in hardness between the welded portion and the base material exceeds 20 Hv, the occurrence of SCC can be prevented by setting the residual stress in the surface layer portion of the welded portion to an appropriate range.

Further, the smaller the residual stress in the surface layer portion of the welded portion, the more the occurrence of stress corrosion cracking (SCC) is suppressed, and furthermore, it can be said to be a seamless steel pipe for boiler having excellent stress corrosion cracking resistance. Therefore, when the residual stress is 70 MPa or less, it is preferable from the viewpoint that it can be said to be an excellent seamless steel pipe for a boiler, and when it is 30 MPa or less, it is extremely preferable for the same reason.

[Dendrite area ratio of welded portion]

Further, in the present invention, it is preferable that "the area ratio of dendrites in the welded joint portion from the outer surface to 1 mm is 1% or less". The reason for this will be described below.

As the name implies, this steel pipe is a steel pipe that is formed by welding. For this reason, dendrites do not exist in excess of 1% in area ratio at the welded joint portion at first. Therefore, the steel pipe according to the present embodiment, which is obtained by the continuous welding, is advantageous in productivity as compared with a steel pipe formed through beam welding or the like. The dendrite area ratio can be calculated by dividing the texture of the welded portion in the depth region from the outer surface of the steel pipe up to 1 mm by using an optical microscope (magnification: 200 times) in the same manner as the deriving method of the pearlite area ratio And the captured image can be analyzed and derived.

<Manufacturing method>

Next, the reasons for limitation of the manufacturing method of the present invention will be described in detail. The manufacturing method of the present invention includes at least a step of making a steel material into a pipe (step (i)) and a step of heat-treating the pipe (step (ii)). The manufacturing method of the present invention optionally includes, in addition to these steps, a step (step (iii)) of calibrating the bending of the tube and then subjecting the tube to the final heat treatment.

(The step of making the steel material into the pipe (step (i)))

This step is an essential step and is a step of forming a steel material (hot-rolled coil) having a predetermined component composition into a tube. Here, the predetermined component composition is the above-mentioned component composition. Further, the molding apparatus for the tube may be a commonly used apparatus, for example,

A molding machine (a machine having a plurality of roll pairs that press the steel material in the up-and-down direction or in the left-to-right direction to sequentially form an open pipe)

· Welding machines (machines for welding the ends of open tubes),

· Cutting machines (machines for removing weld beads),

· Cooling equipment (equipment that cools the pipes),

· Formulating machines (machines for shaping welded tubes),

· Non-destructive testing equipment (non-destructive inspection equipment of standardized tubing), and

· Driving cutting equipment (equipment to cut pipe to predetermined length after non-destructive inspection)

Can be used.

In this process, molding, welding, bead removal, cooling, shaping, nondestructive inspection, and traveling cutting are sequentially performed using each of the above-described devices sequentially.

(Heat treatment process of the pipe (process (ii)))

This step is an essential step, and is a step of performing heat treatment on the pipe after cutting the running. In the case of a sewage pipe for boiler, since there is a possibility of cracks occurring in a welded portion of low carbon concentration when used in a nitrate environment or the like, this process effectively diffuses the carbon in the base material portion into the welded portion in a short time, To increase the ferrite grain boundary strength of SCC to suppress the occurrence of SCC and to increase the stress corrosion cracking resistance. Specifically, the tube is subjected to a heat treatment (hereinafter, simply referred to as &quot; present heat treatment &quot;) for holding the tube at a temperature of Ac 3 point or more and 1250 ° C or lower for 30 seconds or longer in an atmosphere. After the heat treatment, the steel pipe for cooling the boiler is obtained as a final product through cooling.

Fig. 4 is a schematic view showing a carbon concentration in a welded portion of a pipe, in which (a) shows a state before heat treatment, and Fig. 4 (b) shows a state after heat treatment. As shown in the figure, carbon is diffused into the welded joint by the present heat treatment, and as a result, the ferrite grain boundary strength of the welded welded portion is increased, the generation of SCC is suppressed, and the stress corrosion cracking resistance have.

When the heat treatment temperature in this step is less than Ac3 point, the structure is not homogenized and the diffusion of carbon into the welded portion becomes insufficient, so the heat treatment temperature is set to Ac3 or higher. In order to diffuse the carbon in a larger amount by the welded portion, the heat treatment temperature is preferably set to (Ac 3 point + 10) ° C or higher.

On the other hand, if the heat treatment temperature in this step exceeds 1250 DEG C, a scale is generated at the time of welding and an adverse effect is given to the welded portion due to excessive decarburization in the welded portion, 1250 캜 or less. In order to further suppress the adverse effect in the welded portion, it is preferable to set the heat treatment temperature to 1150 DEG C or lower.

The holding time in the heat treatment is 30 seconds or more. Here, the holding time refers to the elapsed time since the internal temperature of the furnace reached (target temperature -20 占 폚). By setting the holding time to 30 seconds or longer, carbon is sufficiently diffused in the welded portion in the base material portion. When the heat treatment temperature is 60 seconds or more, diffusion of carbon is further promoted in the welded portion, which is preferable.

By employing the predetermined heat treatment temperature and the holding time, it is possible to obtain the full product as a final product in which the item (1) (limited in terms of hardness) or item (2) (limited in terms of pearlite area ratio) A steel pipe is obtained. As a result, the carbon concentration of the welded portion, that is, the ferrite grain boundary strength, is increased, the occurrence of SCC is suppressed, and the stress corrosion cracking resistance is improved.

During the heat treatment, scale is generated and decarburization occurs in the tube surface layer. Therefore, in order to suppress these scale formation and decarburization, this heat treatment is preferably carried out in an atmosphere containing no oxygen. For example, the atmosphere in the atmosphere may be an atmosphere composed of at least one of argon, nitrogen, carbon dioxide, and hydrogen. However, it is inevitable that oxygen is actually mixed inevitably. The present inventors have found out that item (1) (limitation on hardness) or item (2) (limitation on pearlite area ratio) is not within the above-mentioned predetermined range And further, it does not inhibit the heat treatment effect. Therefore, the oxygen concentration in the atmosphere in this step is preferably 1% or less.

(Step (iii)) of correcting the bending of the pipe and thereafter performing the final heat treatment on the pipe,

The present step is a step of performing the above-described main heat treatment using a steel material as a tube in an optional step, then correcting the bending of the pipe, and performing a finishing heat treatment. There is a possibility that the tube is not straight enough to be suitable for a desired use in the longitudinal direction thereof and is slightly curved while the steel material is in a tubular state. With this bending, the residual stress in the surface layer portion of the welded portion of the welded steel pipe after the end of manufacturing becomes high, the residual stress becomes large due to the correction of bending, and stress corrosion cracking (SCC) tends to occur. In addition, steel pipes having bending can not be distributed to the market in the first place. Therefore, by adopting the present step, it is possible to suppress the occurrence of bending, and moreover, to effectively suppress the occurrence of SCC. The correction of the bending can be performed using a conventional calibrating device.

After the bending correction, a final heat treatment is further performed. Thereby, the residual stress of the steel pipe is reduced, so that occurrence of SCC is further suppressed, and the stress corrosion cracking resistance can be further enhanced. Concretely, the final heat treatment is performed at a temperature of 400 ° C or higher and Ac 1 point or lower.

When the heat treatment temperature in this step is less than 400 占 폚, the residual stress is not effectively reduced. Therefore, the heat treatment temperature is 400 占 폚 or higher. Further, in order to further reduce the residual stress, the heat treatment temperature is more preferably 450 DEG C or higher.

On the other hand, if the heat treatment temperature in this step exceeds Ac1 point, it transforms to impair texture uniformity, so that the heat treatment temperature is set to Ac1 or less. In order to further suppress the adverse effect in the welded portion, it is preferable to set the heat treatment temperature to (Ac1 point -10) 占 폚 or lower.

[Example]

Next, the effects of the present invention are demonstrated in comparison with the characteristics of the present invention (inventive steel pipe) and the comparative example (comparative steel pipe). The various conditions used in manufacturing the inventive steel pipe are examples of conditions employed to confirm the feasibility and effect of the present invention, and therefore the present invention is not limited to these conditions. In addition, the present invention can employ various conditions as long as it does not depart from the gist of the present invention and the object thereof is attained.

[Manufacture of a steel pipe]

Each of the steel strips shown in Table 1 having 20 kinds of component compositions was formed. In the lower part of Table 1, calculation method of Ac3 point for each steel strip is described. Also, in Table 1, the blank part means that its element is not included.

Next, each of the 20 types of steel strips obtained as described above was continuously formed into a tubular shape, and the edge portions of the tubular steel strips were welded by high frequency welding to obtain 20 types of pipes (outer diameter (38 mm, thickness 2.5 mm). Thereafter, the tubes were subjected to heat treatment under the conditions shown in Table 2 to obtain the inventive steel tubes 1 to 13 and the comparative steel tubes 14 to 20. In Table 2, the blank portion means that the final heat treatment was not performed.

Subsequently, all the steel pipes (inventive steel pipes 1 to 13 and comparative steel pipes 14 to 20) were examined for the following indices A to D, respectively.

(A): Difference in hardness between the hardness in the surface layer of the base material portion and the depth of the base material portion at the depth depth (Hv)

Indicator B: The difference (Hv) between the hardness of the welded fitting portion of 0.5 mm from the outer surface and the hardness of the base material portion of 1 mm from the outer surface,

Indicator C: Area percentage (%) of pearlite in the welding portion 0.5 mm from the outer surface

· Indicative D: Residual stress (MPa) at the surface layer of welded joint

Here, the indices A to D were measured by the above-mentioned methods or by the methods described above, respectively.

Separately from these, the presence or absence of stress corrosion cracking was investigated. For stress corrosion cracking, pipes for boilers were manufactured by joining forcible steel pipes to each steel pipe, installed on actual equipment, and tested for the occurrence of stress corrosion cracking after two years. Table 3 shows the results thereof.

As is clear from Tables 1 to 3, all of the inventive steel pipes employing the component composition and heat treatment conditions of the present invention are all at a predetermined hardness and pearlite area ratio converged to a predetermined range in the present invention, Was not generated. On the other hand, for the comparative steel pipe which does not employ at least any of the component composition and the heat treatment conditions of the present invention, at least one of the predetermined hardness and the pearlite area ratio is out of the predetermined range of the present invention, .

As shown in Table 2, the samples of the steel pipe No. 13 and the steel pipe No. 14 of Table 3 were all of the atmosphere in the heat treatment atmosphere, so that the surface layer was decarbonized with respect to the base material portion and the welded portion, , The values of the index A and the index B in Table 3 are not all the desired 20Hv or less. Therefore, the samples of the steel pipe No. 13 and the steel pipe No. 14 are not the inventive steel pipes but the comparative steel pipes.

However, no SCC occurred for steel pipe No. 13. This is presumably because the residual stress is lowered to 30 MPa (index D of Table 3) by tempering. Therefore, if the residual stress is lowered as in steel pipe No. 13, SCC may not occur even if the indices A and B in Table 3 are not within the desired range.

Claims (8)

The composition of matter, in% by mass,
C: 0.05 to 0.35%
Si: 0.10 to 0.35%,
Mn: 0.25 to 1.50%
S: 0.035% or less,
P: not more than 0.035%
Al: 0.005 to 0.050%
N: 0.010% or less, and
O: 0.010% or less
Lt; / RTI &gt;
Optionally, at least one selected from the group consisting of Cr: not more than 1.00%, Mo: not more than 1.00%, Ni: not more than 2.00%, Cu: not more than 2.00%, B: not more than 0.0030%, Nb: not more than 0.20% % Or less, Ca: 0.0050% or less, and Mg: 0.0050% or less,
Balance: Fe and inevitable impurities,
The difference between the hardness at the surface layer of the base material portion and the hardness at the half thickness depth position of the base material portion is 20 Hv or less,
The difference between the hardness of the welded fitting portion from the outer surface to 1 mm and the hardness of the base material portion up to 1 mm from the outer surface is 20 Hv or less
Which is excellent in stress corrosion cracking resistance. The composition of matter, in% by mass,
C: 0.05 to 0.35%
Si: 0.10 to 0.35%,
Mn: 0.25 to 1.50%
S: 0.035% or less,
P: not more than 0.035%
Al: 0.005 to 0.050%
N: 0.010% or less, and
O: 0.010% or less
Lt; / RTI &gt;
Optionally, at least one selected from the group consisting of Cr: not more than 1.00%, Mo: not more than 1.00%, Ni: not more than 2.00%, Cu: not more than 2.00%, B: not more than 0.0030%, Nb: not more than 0.20% % Or less, Ca: 0.0050% or less, and Mg: 0.0050% or less,
Balance: Fe and inevitable impurities,
A welded steel pipe for a boiler excellent in stress corrosion cracking resistance, having a pearlite area ratio of 5% or more in a welded joint portion of 1 mm from the outer surface. 3. The method according to claim 1 or 2,
Wherein the residual stress of the surface layer portion of the welded fitting portion is 200 MPa or less and is excellent in stress corrosion cracking resistance. 4. The method according to any one of claims 1 to 3,
A welded steel pipe for a boiler excellent in stress corrosion cracking resistance, having a dendritic area ratio of 1% or less in the welded joint portion from the outer surface to 1 mm. A method of manufacturing a seamless steel pipe for a boiler excellent in stress corrosion cracking resistance according to any one of claims 1 to 4,
(I) a step of molding a steel material having the compositional formula as set forth in claim 1 or 2 into a tube,
(Ii) a step of subjecting the tube to a heat treatment in an atmosphere furnace for 30 seconds or more at a temperature of Ac 3 point to 1250 ° C
Wherein said method comprises the steps of: 6. The method of claim 5,
(Iii) a step of calibrating the bending of the tube after the step (ii) and then performing a heat treatment at a temperature of not lower than 400 DEG C and not higher than Ac1 point, Gt; The method according to claim 5 or 6,
Wherein the atmosphere in the atmosphere does not contain oxygen and is excellent in stress corrosion cracking resistance. 8. The method of claim 7,
Wherein the atmosphere in the atmosphere is at least one of argon, nitrogen, carbon dioxide, and hydrogen, and is excellent in stress corrosion cracking resistance.

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