JP6597901B2 - ERW steel pipe for boiler excellent in stress corrosion cracking resistance and its manufacturing method - Google Patents

Description

  The present invention relates to an electric resistance welded steel pipe for boilers having excellent resistance to stress corrosion cracking on the outer surface of the steel pipe. Moreover, this invention relates to the manufacturing method of such an electric-resistance-welded steel pipe.

  Boiler pipes made of carbon steel heat transfer tubes are sometimes used in nitrate environments. For this reason, such boiler piping 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 “SCC resistance”).

Boiler piping damage is caused by ammonium nitrate, which is generated when NO ₂ or NH ₃ in the exhaust gas dissolves in condensed condensed water on the piping when starting up the plant, and tensile residual stress due to bending or fin winding welding. It is a phenomenon that these causes occur in synergy.

  In a general plant, there are about 200 to 500 boiler pipes in which stress corrosion cracking (hereinafter sometimes referred to as “SCC”) may occur due to ammonium nitrate. For this reason, it is difficult to accurately inspect all the pipes during the regular inspection that is normally performed in about 50 days. Moreover, the heat exchange apparatus including such boiler piping also has a part (pipe) that cannot be inspected due to its structure.

  In recent years, the interval between periodic inspections has become longer, and some plants may operate without inspection for four years. In such a case, there is a high possibility that SCC will occur in the boiler piping during plant operation, and in some cases, the piping may blow out.

  The technique regarding boiler piping is disclosed by the following patent documents 1-3, for example.

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

  Patent Document 1 discloses a method for evaluating and diagnosing SCC damage due to nitrate in a carbon steel heat transfer tube of a waste heat recovery boiler with high accuracy. Specifically, the number of start / stop times in each mode of the plant and each The wet time in each mode and the concentration of nitrogen oxides in the exhaust gas in each mode are calculated, the concentration of accumulated nitrate during the entire wet time is calculated, and the calculated concentration of accumulated nitrate is determined by stress corrosion cracking. A nitrate stress corrosion cracking diagnosis method is disclosed that evaluates the degree of damage by calculating the operating period until a possible generation limit nitrate concentration is reached.

  Patent Document 2 discloses a high-strength electric resistance welded steel pipe excellent in high temperature characteristics and corrosion resistance. Specifically, the component composition is C: 0.15 to 0.30% by weight%, Si: 0.05-0.50%, Mn: 0.25-1.5%, N: 0.005-0.010%, Cu: 0.02-0.10%, Ca: 0.001-0. 004%, Mo: 0.01 to 0.10%, S: 0.003% or less as the basic components, the balance consisting of Fe and inevitable elements. A boiler ERW steel pipe having a uniform structure is disclosed.

  Patent Document 3 discloses a steel material excellent in resistance to nitric acid stress corrosion cracking, and specifically, by weight%, C: 0.005 to 0.05%, Si: 0.1 to 0.3. 8%, Mn: 0.2 to 1.5%, P: 0.015% or less, S: 0.015% or less, Cr: 3.5 to 5.0%, Mo: 0.2 to 1.2 %, Nb: 0.01 to 0.15%, Al: 0.01 to 0.20%, N: 0.015% or less, and the remainder is composed of Fe containing unavoidable components for resistance to nitric acid stress corrosion cracking Excellent steel materials are disclosed.

  The above-described boiler piping damage is caused by cracks extending along the ferrite grain boundary of the welded portion when used in a nitrate environment or the like. This damage occurs in the weld due to the lower carbon concentration in the weld compared to the base metal, and the ferrite grain boundary strength is such that the weld can withstand use in a nitrate environment. This is because it is not sufficiently obtained.

  The present invention has been made in view of the above circumstances, and provides an electric resistance welded steel pipe for a boiler that has excellent stress corrosion cracking resistance by increasing the ferrite grain boundary strength of the weld and suppressing cracks in the weld. And providing a method for manufacturing the ERW steel pipe.

  In order to solve the above-mentioned problems, the inventors of the present invention can increase the ferrite grain boundary strength of the welded portion, suppress cracking at the welded portion, and thus exhibit excellent stress corrosion cracking resistance (SCC resistance). As a result of earnest examination of the electric resistance steel pipe for boilers, the following findings (a) and (b) were obtained.

  (A) At the time of ERW welding, decarburization occurs at the weld abutting portion, thereby reducing the ferrite grain boundary strength at the welding abutting portion. The inventors of the present invention can increase the carbon concentration of the weld abutting portion and improve the ferrite grain boundary strength of the weld abutting portion by performing a predetermined heat treatment after the electric resistance welding, and thus improve the SCC resistance. The knowledge that it can be made was acquired. In addition, the present inventors evaluated the hardness difference between different depth positions from the surface of the base material part and the hardness of the weld contact part for the presence or absence of decarburization affecting the ferrite grain boundary strength. A method of evaluating the difference between the hardness of the base metal part (Evaluation Method 1), or evaluating the pearlite area ratio of the weld interface, and the pearlite area ratio of the weld interface and the pearlite area ratio of the base material part The knowledge that it can judge with the method (evaluation method 2) of comparative evaluation is obtained.

  (B) In addition to the evaluation methods 1 and 2 for the presence or absence of decarburization shown in the knowledge (a), the present inventors evaluate the residual stress of the steel pipe surface layer portion in order to improve the SCC resistance. The knowledge that it can judge by the method (evaluation method 3) was acquired.

  The present invention has been made based on each of the above findings, and the gist thereof is as follows.

[1] Ingredient composition is 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: 0.035% or less,
Al: 0.005 to 0.050%,
N: 0.010% or less, and O: 0.010% or less,
Further optionally, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 2.00% or less, Cu: 2.00% or less, B: 0.0030% or less, Nb: 0.0. Including at least one of 20% or less, V: 0.20% or less, Ti: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less,
The remainder: Fe and inevitable impurities
The difference between the hardness at the surface layer of the base material part and the hardness at the 1/2 thickness depth position of the base material part is 20 Hv or less,
For boilers with excellent resistance to stress corrosion cracking, characterized in that the difference between the hardness of the weld interface from the outer surface to 1 mm and the hardness of the base material from the outer surface to 1 mm is 20 Hv or less. ERW steel pipe.

[2] Ingredient composition is 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: 0.035% or less,
Al: 0.005 to 0.050%,
N: 0.010% or less, and O: 0.010% or less,
Further optionally, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 2.00% or less, Cu: 2.00% or less, B: 0.0030% or less, Nb: 0.0. Including at least one of 20% or less, V: 0.20% or less, Ti: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less,
The remainder: Fe and inevitable impurities
An electric resistance welded steel pipe for boilers with a pearlite area ratio of 5% or more at the weld interface from the outer surface to 1 mm and excellent in stress corrosion cracking resistance.

  [3] The electric resistance welded steel pipe for boilers according to the above [1] or [2], wherein the residual stress in the surface layer portion of the welding contact portion is 200 MPa or less.

  [4] The boiler excellent in stress corrosion cracking resistance according to any one of the above [1] to [3], wherein the dendrite area ratio of the weld abutting portion from the outer surface to 1 mm is 1% or less. ERW steel pipe.

[5] A method for producing an electric-welded steel pipe for boilers having excellent stress corrosion cracking resistance according to any one of [1] to [4],
A step (i) of forming a steel material having the composition described in [1] or [2] into a pipe;
And a step (ii) of subjecting the tube to a heat treatment for 30 seconds or more at a temperature of not less than Ac3 and not more than 1250 ° C. in an atmosphere furnace. Manufacturing method of sewn steel pipe.

  [6] Stress corrosion resistance according to the above [5], including the step (iii) of correcting the bending of the tube after the step (ii) and then performing a heat treatment at a temperature of 400 ° C. or higher and Ac1 point or lower. A method of manufacturing an electric resistance welded steel pipe for boilers with excellent cracking properties.

  [7] The method for producing an electric-welded steel pipe for boilers according to [5] or [6] above, wherein the atmosphere in the atmosphere furnace does not contain oxygen and has excellent stress corrosion cracking resistance.

  [8] The method for producing an electric-welded steel pipe for a boiler excellent in stress corrosion cracking resistance according to [7] above, wherein the atmosphere in the atmosphere furnace is at least one of argon, nitrogen, carbon dioxide, and hydrogen.

  In the electric resistance welded steel pipe for a boiler according to the present invention, after setting a specific component composition, the hardness difference between different depth positions from the surface of the base material part is set to a predetermined range, and the welding abutting part The difference between the hardness and the hardness of the base material portion is set within a predetermined range. Alternatively, in the electric resistance welded steel pipe for boilers according to the present invention, the pearlite area ratio of the welding contact portion is set within a predetermined range after setting a specific component composition. Further, in order to achieve these hardness and pearlite area ratio, the method for manufacturing an electric resistance welded steel pipe for boiler according to the present invention uses a predetermined atmosphere furnace and sets a holding temperature and a holding time in a predetermined range in the heat treatment. Yes. Therefore, according to the technology related to the electric-welded steel pipe for boilers according to the present invention, the ferrite grain boundary strength is increased by increasing the carbon concentration of the weld abutting portion, cracking at the welded portion is suppressed, and excellent stress corrosion resistance is achieved. An electric resistance welded steel pipe for boiler having cracking properties can be obtained.

FIG. 1 is a schematic view showing a portion of a steel pipe targeted in the investigation of a crack occurrence portion, (a) is a cross-sectional view showing a weld abutting portion in the steel pipe, and (b) is a drawing of (a). It is an enlarged view of a circled part. FIG. 2 is a cross-sectional photograph of a welded contact portion of an ERW steel pipe, where (a) is an example in which no cracking occurred and (b) is an example in which cracking has occurred. Fig. 3 is a graph showing the relationship between the hardness of the ERW steel pipe for boiler and the distance from the weld surface. (A) shows a steel pipe where stress corrosion cracking has not occurred, and (b) shows stress corrosion cracking. The steel pipe which is doing is shown. 4A and 4B are schematic views showing the carbon concentration at the weld abutting portion of the pipe. FIG. 4A shows a state before the heat treatment, and FIG. 4B shows a state after the heat treatment.

  Below, the electric resistance steel pipe for boilers which is excellent in the stress corrosion cracking resistance according to the present invention (hereinafter may be referred to as “the present electric resistance welded steel pipe”), and the manufacturing method thereof (hereinafter may be referred to as “the present application manufacturing method”). ) Will be described in detail. The following embodiments do not limit the present invention. The constituent elements of the above embodiment include those that can be easily replaced by those skilled in the art or those that are substantially the same. Furthermore, various forms included in the above-described embodiments can be arbitrarily combined within a range obvious to those skilled in the art.

<Electrically welded steel pipe>
[Ingredient composition]
First, the reasons for limiting the component composition of the electric resistance welded steel pipe will be described in detail. The unit [%] of the component composition shown below means mass%.

(Essential element)
The ERW steel pipe of the present application contains C, Si, Mn, S, P, Al, N, and O, and Fe and unavoidable impurities in the following ranges.

C: 0.05 to 0.35%
C is an element necessary for ensuring the strength of the steel pipe. If it is less than 0.05%, the strength of the steel pipe is insufficient, so C is made 0.05% or more. Preferably it is 0.06% or more. On the other hand, if it exceeds 0.35%, the hardness of the steel pipe increases and the workability deteriorates, so C is made 0.35% or less. Preferably it is 0.32% or less.

Si: 0.10 to 0.35%
Si is an element that contributes to improving the strength of the steel pipe by solid solution strengthening. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so Si is made 0.10% or more. Preferably it is 0.15% or more. On the other hand, if it exceeds 0.35%, the strength of the steel pipe will increase too much and the workability will deteriorate, so Si is made 0.35% or less. Preferably it is 0.30% or less.

Mn: 0.25 to 1.50%
Mn is an element that ensures hardenability and contributes to improving the strength of the steel pipe. If it is less than 0.25%, the effect of addition is not sufficiently exhibited, so Mn is made 0.25% or more. Preferably it is 0.27% or more. On the other hand, if it exceeds 1.50%, the hardness of the steel pipe increases so much that the workability deteriorates, so Mn is made 1.50% or less. Preferably it is 1.45% or less.

S: 0.035% or less S is an element that forms MnS that is a starting point of cracking. For this reason, it is preferable that S is as small as possible. Preferably it is 0.030% or less. The lower limit includes 0%, but if it is attempted to reduce S to less than 0.0001%, the manufacturing cost increases significantly, so practically 0.0001% is the practical lower limit.

P: 0.035% or less P is an element that causes grain boundary segregation or center segregation which causes ductile deterioration. For this reason, it is preferable that P is as small as possible. Preferably it is 0.030% or less. The lower limit includes 0%, but if P is reduced to less than 0.0010%, the manufacturing cost increases significantly. Therefore, in practice, 0.0010% is the practical lower limit.

Al: 0.005 to 0.050%
Al is a deoxidizing element. If it is less than 0.005%, deoxidation becomes insufficient, so Al is made 0.005% or more. Preferably it is 0.010% or more. On the other hand, if it exceeds 0.050%, the workability deteriorates due to the coarsening of the alumina-based oxide, so Al is made 0.050% or less. Preferably it is 0.040% or less.

N: 0.010% or less N is an unavoidable element. If it exceeds 0.010%, inclusions become coarse and workability deteriorates, so N is made 0.010% or less. Preferably it is 0.008% or less. The lower limit includes 0%, but if N is reduced to less than 0.0010%, the manufacturing cost increases significantly, so practically 0.0010% is the practical lower limit.

O: 0.010% or less O is an element unavoidably present after deoxidation. If it exceeds 0.010%, inclusions become coarse and workability deteriorates, so O is made 0.010% or less. Preferably it is 0.005% or less. The lower limit includes 0%, but if O is reduced to less than 0.0005%, the manufacturing cost is significantly increased, so practically 0.0005% is the practical lower limit.

The balance: Fe and inevitable impurities The balance is Fe and inevitable impurities. Here, the inevitable impure part is not an impurity that is intentionally mixed, but also an impurity contained in a raw material or an impurity that can be mixed in each manufacturing process. Examples of inevitable impurities in the ERW steel pipe of the present application include As, Na, Zr, and Sb. In the present application, the content (upper limit value) of these inevitable impurities is particularly limited as follows.

  That is, each content is limited to As is 0.01% or less, Na is 0.01% or less, Zr is 0.01% or less, and Sb is 0.01% or less. By limiting the content of each of these elements to the above ranges, the ferrite grain boundary strength of the welded portion, which is an effect unique to the present application, is increased, and the effect of suppressing cracking at the welded portion is inhibited. Can be efficiently prevented.

(Optional element)
In addition to the elements shown above, the present electric resistance welded steel pipe is at least one of Cr, Mo, Ni, Cu, B, Nb, V, Ti, and Mg, as long as the characteristics of the present electric resistance welded steel pipe are not impaired. You may contain in each range shown.

Cr: 0.05-1.00%
Cr is an element that ensures hardenability and contributes to improving the strength of the steel pipe. If it is less than 0.05%, the effect of addition is not sufficiently exhibited, so Cr is preferably made 0.05% or more. More preferably, it is 0.10% or more. On the other hand, if it exceeds 1.00%, the hardness of the steel pipe will increase too much and the workability will deteriorate, so Cr is preferably made 1.00% or less. More preferably, it is 0.80% or less.

Mo: 0.05-1.00%
Mo is an element that ensures hardenability and contributes to improving the strength of the steel pipe. If it is less than 0.05%, the effect of addition is not sufficiently exhibited, so Mo is preferably made 0.05% or more. More preferably, it is 0.10% or more. On the other hand, if it exceeds 1.00%, the hardness of the steel pipe will increase excessively and the workability will deteriorate, so Mo is preferably made 1.00% or less. More preferably, it is 0.80% or less.

Ni: 0.10 to 2.00%
Ni is an element that ensures hardenability and contributes to improving the strength of the steel pipe. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so Ni is preferably made 0.10% or more. More preferably, it is 0.15% or more. On the other hand, since the hardness of the steel pipe exceeding 2.00% increases excessively and the workability deteriorates, Ni is preferably made 2.00% or less. More preferably, it is 1.50% or less.

Cu: 0.10 to 2.00%
Cu is an element that ensures hardenability and contributes to improving the strength of the steel pipe. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so Cu is preferably made 0.10% or more. More preferably, it is 0.15% or more. On the other hand, if it exceeds 2.00%, the hardness of the steel pipe will increase too much and the workability will deteriorate, so Cu is preferably made 2.00% or less. More preferably, it is 1.50% or less.

B: 0.0003 to 0.0030%
B is an element that ensures hardenability and contributes to improving the strength of the steel pipe. If it is less than 0.0003%, the effect of addition is not sufficiently exhibited, so B is preferably made 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, so B is preferably made 0.0030% or less. More preferably, it is 0.0020% or less.

Nb: 0.005 to 0.20%
Nb is an element that has a strong affinity between C and N, precipitates NbCN, and contributes to improving the strength of the steel pipe. If it is less than 0.005%, the effect of addition is not sufficiently exhibited, so 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, so Nb is preferably 0.20% or less. More preferably, it is 0.10% or less.

V: 0.005-0.20%
V is an element that has a strong affinity between C and N, precipitates VN and VC, and contributes to improving the strength of the steel pipe. If it is less than 0.005%, the effect of addition is not sufficiently exhibited, so V is preferably set to 0.005% or more. More preferably, it is 0.010% or more. On the other hand, if it exceeds 0.20%, precipitation hardening due to VN or VC becomes remarkable and workability deteriorates. Therefore, V is preferably 0.20% or less. More preferably, it is 0.10% or less.

Ti: 0.005 to 0.20%
Ti is an element that has a strong affinity for N and precipitates TiN to contribute to the refinement of the structure. If it is less than 0.005%, the effect of addition is not sufficiently exhibited, so Ti is preferably made 0.005% or more. More preferably, it is 0.008% or more. On the other hand, if it exceeds 0.20%, precipitation hardening by TiC becomes remarkable and workability deteriorates, so Ti is preferably 0.20% or less. More preferably, it is 0.10% or less.

Ca: 0.0001 to 0.0050%
Ca is an element that adjusts the form of inclusions in the base material and the ERW weld and contributes to improvement in workability. If it is less than 0.0001%, the effect of addition is not sufficiently exhibited, so Ca is preferably made 0.0001% or more. More preferably, it is 0.0005% or more. On the other hand, if it exceeds 0.0050%, the inclusions in the steel increase and the workability deteriorates, so Ca is preferably 0.0050% or less. More preferably, it is 0.0040% or less.

Mg: 0.0050% or less Mg is a deoxidizing element, and the generated oxide functions as a precipitation nucleus of MnS. Therefore, Mg is an element contributing to miniaturization and uniform dispersion of MnS. If it exceeds 0.0050%, the yield decreases and the effect of addition is saturated. Therefore, Mg is preferably 0.0050% or less. More preferably, it is 0.0040% or less.

[Limitation reason for hardness]
In the following, in the ERW steel pipe of the present application, (1) it is essential to limit the hardness. This hardness limitation is
(1-1) The difference between the hardness at the surface layer of the base material part and the hardness at the 1/2 thickness depth position of the base material part is 20 Hv or less, and (1-2) from the outer surface to 1 mm The difference between the hardness of the weld contact portion and the hardness of the base metal portion from the outer surface to 1 mm is 20 Hv or less,
The following two matters are included.

  Here, the welding abutting portion is a portion in the vicinity of the welding surface, and a portion whose distance from the welding surface is a distance of 0 μm to about 100 μm among the portions where the microstructure is thermally affected by the electric resistance welding. means. Further, the base material portion is a portion that is further away from the welding surface than the welding abutting portion, and means a portion in which the microstructure is not affected by heat due to the electric resistance welding.

  In order to investigate the causal relationship between the occurrence of SCC at the weld interface and the above item (1-1) and the above item (1-2), the inventors of the present invention have stress corrosion cracking (SCC) in a nitrate environment. Welds were photographed for steel pipes where no SCC occurred and steel pipes where SCC occurred in a nitrate environment, and the cracked portions of the steel pipe where SCC occurred were investigated.

  FIG. 1 is a schematic diagram showing a portion of a steel pipe that is a target in the investigation of a crack occurrence portion, and is a diagram drawn by increasing the horizontal scale relative to the vertical scale. Fig.1 (a) has shown a part of cross-sectional area | region which spreads in the depth direction from the outer surface 12 of the ERW steel pipe 10, and FIG.1 (b) is an enlarged view of the encircled part X of Fig.1 (a). It is.

  In this embodiment, as shown in FIG.1 (b), it is an area | region from the surface 12 in the steel pipe 10 to the position of 1 mm in a depth direction (it may generate | occur | produce a crack), and a welding surface A region 16 of 100 μm on both sides of 14 is defined as a welding contact portion. The weld abutting portion can be defined by specifying the positions of 100 μm on both sides of the weld surface by making indentations after specifying the weld surface by performing metal flow etching.

  FIG. 2 is a cross-sectional photograph of a welded contact portion of an ERW steel pipe, where (a) is an example in which no cracking occurred and (b) is an example in which cracking has occurred. In addition, it was confirmed that the crack propagated from the outer surface to the deep part at the weld abutting portion of the steel pipe, and specifically, occurred in a region from the outer surface to a depth of 1 mm.

Next, in this survey, ERW steel pipes that did not crack and ERW steel pipes that cracked,
(1-1) The difference between the hardness of the surface of the base material part and the hardness at the 1/2 thickness depth position of the base material part (the difference in hardness in the depth direction of the base material part), and (1-2) outside The difference between the hardness of the weld contact part from the surface to 1 mm and the hardness of the base material part from the outer surface to 1 mm (the difference in hardness between the weld contact part and the base material part at a specific depth position)
Was investigated.

In particular, the hardness of the weld abutting portion at a specific depth position is measured by measuring the Vickers hardness (Hv) at a depth position of 0.5 mm from the outer surface of the steel pipe in a range of 0.6 mm on both sides across the weld surface. did. This hardness is measured at a test load of 100 g at each position of 0.1 mm pitch in an arc shape around the weld surface. On the other hand, the hardness of the base material portion at a specific depth position is a position obtained by rotating the Vickers hardness (Hv) at a depth position of 0.5 mm from the outer surface of the steel pipe by 90 ° around the center of the steel pipe from the weld surface. Measurements were made in the range of 0.6 mm on both sides.

  FIG. 3 is a graph showing the results of item (1-2) in particular. That is, FIG. 3 is a graph showing the relationship between the hardness of the ERW steel pipe for boiler (Vickers hardness at a depth of 0.5 mm from the outer surface of the steel pipe) and the distance from the weld surface. The steel pipe which has not generate | occur | produced is shown, (b) shows the steel pipe in which the crack has generate | occur | produced. In FIGS. 1A and 1B, a positive (negative) number on the horizontal axis means that the welding surface is separated from the welding surface by a predetermined distance. Moreover, in FIG. (A), (b), each figure shown in figure shows the result about two examples from which the measurement position differs although it is the same steel pipe.

  3A and 3B, the hardness when the distance from the weld surface is “0 (mm)” is the hardness of the weld abutting portion, and the distance from the weld surface is “± 0.1, ± 0. .2, ± 0.3, ± 0.4, ± 0.5, ± 0.6 (mm) ”is the hardness of the base material portion.

  As shown in FIG. 3 (a), regarding the hardness distribution of the two types of boiler electric resistance welded steel pipes in which stress corrosion cracking (SCC) does not occur, both the hardness of the weld contact portion and the hardness of the base metal portion. The difference is within 20 Hv. On the other hand, as shown in FIG. 3B, regarding the hardness distribution of the two types of boiler electric resistance welded steel pipes in which SCC occurred, the difference between the hardness of the weld contact portion and the hardness of the base material portion is the same. May exceed 20 Hv.

From the results of these surveys and measurements,
(1-1) The hardness difference in the depth direction of the base material portion is 20 Hv or less, and (1-2) the hardness difference between the weld contact portion and the base material portion at the specific depth position is 20 Hv.
Then, it was confirmed that stress corrosion cracking (SCC) does not occur.

  This is because if the above item (1-1) and the above item (1-2) are satisfied at the same time, it is considered that the carbon concentration in the welding contact portion is not so low as compared with the base metal portion, and as a result, welding is performed. This is because it is considered that the ferrite grain boundary strength is sufficiently obtained at the abutting portion to the extent that it can be used in a nitrate environment degree.

  By the above, in the ERW pipe according to the present embodiment, as described above, after setting a specific component composition, the depth difference in hardness in the base material portion and the weld abutting portion at the specific depth position By setting the hardness difference from the base metal part within a predetermined range, the carbon concentration of the welded part (more specifically, the weld abutting part) is increased to increase the ferrite grain boundary strength, and cracks at the welded part are prevented. It is possible to suppress and realize excellent stress corrosion cracking resistance.

  In particular, it can be said that the smaller the hardness difference of the above item (1-2), the more the occurrence of stress corrosion cracking (SCC) is suppressed, and thus the electric resistance steel pipe for boiler having excellent stress corrosion cracking resistance. For this reason, when this hardness difference is 15 Hv or less, it is preferable at the point which can be said to be the more excellent electric-welding steel pipe for boilers.

[Reason for limitation regarding pearlite area ratio]
In the ERW steel pipe of the present application, in place of the above-described reason for limitation on hardness, (2) limitation on pearlite area ratio is essential. More specifically,
The pearlite area ratio of the weld abutting part from the outer surface to 1 mm is 5% or more,
Is essential.

In order to investigate the causal relationship between the occurrence of SCC and the above item (2) in the welding contact portion,
(2) The pearlite area ratio of the weld abutting portion from the outer surface to 1 mm was investigated.

  The pearlite area ratio of the weld contact area is determined by performing metal flow etching to identify the weld surface, demarcating the weld contact area, further performing nital etching, and analyzing the structure of the weld contact area with an optical microscope ( (Magnification 200 times) was taken continuously, and the photographed image was derived by image analysis.

  In the steel pipe in which SCC was generated, it was confirmed that most of the microstructure of the weld abutting portion in the depth region from the outer surface to 1 mm was ferrite, and almost no pearlite was present. On the other hand, in the steel pipe in which SCC is not generated, it was confirmed that the microstructure of the weld contact portion in the depth region from the outer surface to 1 mm has pearlite in an area ratio of 5% or more in addition to ferrite.

In other words,
(2) The pearlite area ratio of the weld abutting portion from the outer surface to 1 mm is 5% or more,
Then, it was confirmed that stress corrosion cracking (SCC) does not occur.
In particular, the item (2) is an effect that carbon is diffused by a heat treatment to be described later, and the carbon concentration of the weld contact portion is increased.

  If the above item (2) is satisfied, it is considered that the carbon concentration is not so low in the weld contact portion as compared with the base metal portion. This is because it can be said that the ferrite grain boundary strength is sufficiently obtained to withstand the above.

  By the above, in the ERW steel pipe according to the present embodiment, as described above, after setting a specific component composition, the pearlite area ratio of the weld abutting portion from the outer surface to 1 mm is set within a predetermined range. As a result, the ferrite grain boundary strength of the welded portion (more specifically, the weld abutting portion) can be increased, and cracking at the welded portion can be suppressed to achieve excellent stress corrosion cracking resistance.

  In particular, the larger the pearlite area ratio of the above item (2), the more the occurrence of stress corrosion cracking (SCC) is suppressed, and it can be said that this is an electric-welded steel pipe for boilers having excellent stress corrosion cracking resistance. For this reason, when the pearlite area ratio is 8% or more, it is preferable in that it can be said to be an excellent electric-welded steel pipe for boilers. On the other hand, the upper limit of the pearlite area ratio is not particularly limited because it is determined by the amount of carbon.

[Residual stress in the surface layer of the weld contact]
Furthermore, in the ERW steel pipe of the present application, in addition to the above-described limitation on hardness or limitation on pearlite area ratio, limitation on residual stress can be optionally adopted. The limitation on this residual stress is
(3) The residual stress of the surface layer portion of the welding contact portion is 200 MPa or less.

In order to investigate the causal relationship between the occurrence of SCC and the above item (3) in the welding contact portion,
(3) The residual stress in the surface layer portion of the weld contact portion was investigated.

  As the residual stress in the surface layer portion of the welding contact portion, the stress on the outer surface of the steel pipe was measured using X-ray diffraction (irradiation diameter: 0.5 mm), and this value was adopted. The residual stress on the outer surface of the steel pipe is uniform in the circumferential direction including the welded portion.

  Even if the steel pipe has a hardness difference of more than 20 Hv between the weld interface from the outer surface up to 1 mm and the base metal from 1 mm to the outer surface, the surface layer of the weld interface is not used in steel pipes where no SCC has occurred. On the other hand, in the steel pipe in which SCC was generated, the residual stress in the surface layer portion of the weld contact portion was over 200 MPa.

In other words,
(3) Even if the hardness difference between the weld contact portion from the outer surface to 1 mm and the base metal portion from the outer surface to 1 mm slightly exceeds 20 Hv, the residual stress of the surface portion of the weld contact portion is 200 MPa or less. It was confirmed that no stress corrosion cracking (SCC) occurred.

  As will be described later, such low residual stress is realized because the residual stress generated when the heat-treated pipe is bent and straightened is sufficiently reduced by the subsequent final heat treatment. Thereby, even if the hardness difference between the weld contact portion and the base material portion exceeds 20 Hv, the occurrence of SCC can be prevented if the residual stress of the surface layer portion of the weld contact portion is within an appropriate range. .

  In addition, it can be said that it is an electric-welded steel pipe for boilers that has less stress corrosion cracking (SCC) as the residual stress in the surface layer portion of the above-mentioned welding contact portion is smaller, and consequently has excellent stress corrosion cracking resistance. For this reason, when this residual stress is 70 MPa or less, it is preferable in that it can be said to be a further excellent electric-welded steel pipe for boilers, and when it is 30 MPa or less, it is extremely preferable for the same reason.

[Dendrite area ratio of welding contact]
In addition, in the ERW steel pipe of the present application, it is preferable that “the dendrite area ratio of the weld contact portion from the outer surface to 1 mm is 1% or less”. The reason will be described below.

  As the name suggests, the ERW steel pipe is a steel pipe formed through ERW welding. For this reason, dendride does not exist in the welding contact portion in an area ratio exceeding 1% in the first place. Therefore, the steel pipe of the present embodiment obtained by electric resistance welding has an advantage that the productivity is extremely high as compared with a steel pipe formed through beam welding or the like. In addition, the dendrite area ratio is continuously photographed using an optical microscope (200 magnifications) of the structure of the weld abutting portion in a depth region from the outer surface of the steel pipe to 1 mm in the same manner as the method for deriving the pearlite area ratio described above. The captured image can be derived by image analysis.

<Production method>
Next, the reasons for limiting the manufacturing method of the present application will be described in detail. The manufacturing method of the present application includes at least a process using a steel material as a pipe (process (i)) and a heat treatment process of the pipe (process (ii)). In addition to these steps, the manufacturing method of the present application optionally includes a step (step (iii)) of correcting the bending of the tube and then subjecting the tube to a final heat treatment.

(Process using steel as a pipe (process (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-described component composition. In addition, the apparatus for forming a pipe is a commonly used apparatus, for example,
・ Forming equipment (equipment with a plurality of roll pairs that press steel from its up-down direction or left-right direction to form an open tube in sequence),
・ Welding machine (equipment for welding the ends of open pipes),
・ Cutting equipment (equipment that removes weld beads),
・ Cooling equipment (equipment for cooling pipes),
-Stylization equipment (equipment for shaping welded pipes),
・ Non-destructive inspection equipment (equipment for non-destructive inspection of standardized pipes), and traveling cutting equipment (equipment for cutting pipes after non-destructive inspection to a predetermined length)
Can be used.
In this step, each of the above devices is sequentially used to sequentially perform forming, welding, bead removal, cooling, shaping, nondestructive inspection, and traveling cutting.

(Pipe heat treatment process (process (ii)))
This step is an essential step and is a step in which heat treatment is performed on the above-described tube after traveling cutting. Since ERW steel pipes for boilers may crack at welds with low carbon concentration when used in nitrate environments, etc., this process is effective in converting the base material carbon to the weld contact area in a short time. This is a process for increasing the stress corrosion cracking resistance by suppressing the generation of SCC by increasing the ferrite grain boundary strength of the weld abutting portion. Specifically, the tube is subjected to a heat treatment (hereinafter sometimes simply referred to as “main heat treatment”) for 30 seconds or more at a temperature of Ac3 or higher and 1250 ° C. or lower in an atmosphere furnace. In addition, after this heat processing, the electric-resistance-welded steel pipe for boilers as a final product is obtained through standing to cool.

  4A and 4B are schematic views showing the carbon concentration at the weld abutting portion of the pipe. FIG. 4A shows a state before the heat treatment, and FIG. 4B shows a state after the heat treatment. As shown in the figure, by performing this heat treatment, carbon diffuses in the weld abutting portion, and as a result, the ferrite grain boundary strength of the weld abutting portion is increased and the occurrence of SCC is suppressed, and as a result, the stress corrosion cracking resistance. Can be increased.

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

  On the other hand, if the heat treatment temperature in this step exceeds 1250 ° C., it will adversely affect the weld abutting part due to the generation of scale during welding and excessive decarburization at the welding abutting part. The temperature is 1250 ° C. or lower. In order to further suppress the adverse effects at the weld abutting portion, the heat treatment temperature is preferably set to 1150 ° C. or lower.

  The holding time in this heat treatment is 30 seconds or more. Here, the holding time refers to a time elapsed after the furnace temperature reaches (target temperature −20 ° C.). By setting the holding time to 30 seconds or more, the carbon is sufficiently diffused from the base material portion to the welding contact portion. When the heat treatment temperature is set to 60 seconds or more, it is preferable because carbon diffusion is further promoted to the weld abutting portion.

  By adopting these predetermined heat treatment temperature and holding time, as the final product in which item (1) (limitation on hardness) or item (2) (limitation on pearlite area ratio) explained in the section of ERW steel pipe is realized The ERW steel pipe is obtained. As a result, the carbon concentration of the weld abutting portion and, consequently, the ferrite grain boundary strength is increased, the occurrence of SCC is suppressed, and the stress corrosion cracking resistance is improved.

  In addition, a scale is produced | generated at the time of heat processing, and decarburization generate | occur | produces in a pipe | tube surface layer. For this reason, it is preferable to perform this heat processing in an atmosphere furnace which does not contain oxygen in order to suppress these scale formation and decarburization. For example, the atmosphere in the atmosphere furnace may be an atmosphere composed of at least one of argon, nitrogen, carbon dioxide, and hydrogen. However, in practice, it is inevitable that oxygen is inevitably mixed. The present inventors have an item (1) (limitation on hardness) or item (2) (limitation on pearlite area ratio) if the amount of oxygen is unavoidably mixed (oxygen concentration is about 1%). It was experimentally confirmed that the above-mentioned predetermined range was obtained, and that the heat treatment effect was not hindered. For this reason, it is preferable that the oxygen concentration in the atmospheric furnace in this step is 1% or less.

(Step of correcting the bending of the tube and then subjecting the tube to a final heat treatment (step (iii))
This step is an optional step, and is a step in which a steel material is used as a pipe, the pipe is bent after being subjected to the main heat treatment described above, and a finish heat treatment is performed. If the steel material remains in a tubular state, the tube may not be straight enough to fit the desired use in its longitudinal direction and may be somewhat curved. If this bending occurs, the residual stress of the surface layer of the welded joint becomes high for the ERW steel pipe after production is finished, and the residual stress increases due to straightening of the bending, and stress corrosion cracking (SCC) is likely to occur. Become. Further, a steel pipe having a bend cannot be distributed to the market in the first place. For this reason, by adopting this step, the bending can be suppressed, and the occurrence of SCC can be effectively suppressed. In addition, the correction | amendment of the said curvature can be performed using a normal correction apparatus.

  Then, after the bend correction, a final heat treatment is further performed. Thereby, the residual stress of a steel pipe can be reduced, generation | occurrence | production of SCC can be suppressed further, and stress corrosion cracking resistance can be improved further. Specifically, the final heat treatment is performed at a temperature of 400 ° C. or higher and Ac1 point or lower.

  If the heat treatment temperature in this step is less than 400 ° C., the residual stress is not effectively reduced, so the heat treatment temperature is set to 400 ° C. or higher. In order to further reduce the residual stress, the heat treatment temperature is more preferably 450 ° C. or higher.

  On the other hand, if the heat treatment temperature in this step exceeds the Ac1 point, transformation occurs and the structure uniformity is impaired, so the heat treatment temperature is set to the Ac1 point or lower. In order to further suppress the adverse effects at the weld abutting portion, the heat treatment temperature is preferably set to (Ac1 point−10) ° C. or lower.

  Next, the effect of the present invention is demonstrated by comparing the characteristics of the inventive example (invented steel pipe) with the characteristics of the comparative example (comparative steel pipe). Since various conditions used when manufacturing the steel pipe of the invention are examples of conditions adopted to confirm the feasibility and effects of the present invention, the present invention is not limited to these example conditions. In addition, the present invention can adopt various conditions as long as it does not depart from the gist and achieve the object.

[Manufacture of ERW steel pipe]
Each steel strip shown in Table 1 having 20 kinds of component compositions was formed. In addition, the calculation method of Ac3 point regarding each steel strip is written together in the lower part of Table 1. In Table 1, a blank part means that the 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 edges of the tubular steel strips were welded by high-frequency welding to produce 20 types of tubes (outer diameter 38 mm, meat A thickness of 2.5 mm) was obtained. Thereafter, each pipe was heat-treated under the conditions shown in Table 2 to obtain invention steel pipes 1 to 13 and comparative steel pipes 14 to 20. In Table 2, a blank part means that the final heat treatment was not performed.

Subsequently, for all the steel pipes (invention steel pipes 1 to 13 and comparative steel pipes 14 to 20), the following indices A to D were investigated.
Index A: Difference between the hardness at the surface layer of the base material part and the hardness at the 1/2 thickness depth position of the base material part (Hv)
Index B: Difference (Hv) between the hardness of the weld contact portion 0.5 mm from the outer surface and the hardness of the base metal portion 0.5 mm from the outer surface
Index C: Perlite area ratio (%) of the weld contact portion 0.5 mm from the outer surface
Index D: Residual stress (MPa) at the surface layer of the weld contact

  Here, each of the indicators A to D was measured by the method described above or according to the method described above.

  Separately from these, the presence of occurrence of stress corrosion cracking was investigated. For stress corrosion cracking, steel fins were joined to each ERW pipe to produce a boiler pipe, which was attached to an actual machine and tested, and the occurrence of stress corrosion cracking was investigated after two years. Table 3 shows these results.

  As is apparent from Tables 1 to 3, all of the inventive steel pipes adopting the component composition and heat treatment conditions of the present invention have at least the predetermined hardness and the pearlite area ratio within the predetermined range of the present application, and hence stress corrosion. It was confirmed that no cracks occurred. On the other hand, for comparative steel pipes that do not employ at least one of the component composition and heat treatment conditions of the present invention, at least one of the predetermined hardness and the pearlite area ratio is outside the predetermined range of the present application. Therefore, it was confirmed that stress corrosion cracking occurred.

  In addition, since the heat treatment atmosphere of the samples of steel pipe number 13 and steel pipe number 14 in Table 3 was air as shown in Table 2, the surface layer part was decarburized both in the base metal part and in the welding contact part. Since the surface layer hardness is reduced, the values of the index A and the index B in Table 3 are not any desired 20 Hv or less. For this reason, the sample of the steel pipe number 13 and the steel pipe number 14 is not an invented steel pipe but a comparative steel pipe.

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

Claims (5)

Ingredient composition is 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: 0.035% or less,
Al: 0.013 to 0.050%,
N: 0.010% or less, and O: 0.010% or less,
Further optionally, Cr: 1.00% or less, Mo: 0.12% or less, Ni: 2.00% or less, Cu: 2.00% or less, B: 0.0030% or less, Nb: 0.00%. Including at least one of 20% or less, V: 0.20% or less, Ti: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less,
The remainder: Fe and inevitable impurities
The difference between the hardness at the surface layer of the base material part and the hardness at the 1/2 thickness depth position of the base material part is 20 Hv or less,
And hardness of the weld abutment of 0.5mm from the outer surface, and hardness of the base metal of 0.5mm from the outer surface, the difference of Ri der less 20 HV,
An electric resistance welded steel pipe for a boiler excellent in stress corrosion cracking resistance, wherein a residual stress in a surface layer portion of the weld contact portion is 200 MPa or less . Ingredient composition is 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: 0.035% or less,
Al: 0.013 to 0.050%,
N: 0.010% or less, and O: 0.010% or less,
Further optionally, Cr: 1.00% or less, Mo: 0.12% or less, Ni: 2.00% or less, Cu: 2.00% or less, B: 0.0030% or less, Nb: 0.00%. Including at least one of 20% or less, V: 0.20% or less, Ti: 0.20% or less, Ca: 0.0050% or less, Mg: 0.0050% or less,
The remainder: Fe and inevitable impurities
Der pearlite area ratio of 5% or more of the weld abutment of 0.5mm from the outer surface is,
An electric resistance welded steel pipe for a boiler excellent in stress corrosion cracking resistance, wherein a residual stress of a surface layer portion of the welding contact portion is 200 MPa or less . The electric resistance steel pipe for boilers excellent in stress corrosion cracking resistance according to claim 1 or 2 , wherein a dendrite area ratio of a welding contact portion 0.5 mm from the outer surface is 1% or less. It is a manufacturing method of the electric resistance steel pipe for boilers excellent in stress corrosion cracking resistance given in any 1 paragraph of Claims 1-3 ,
A step (i) of forming a steel material having the composition according to claim 1 or 2 into a pipe;
The tube, in an atmosphere furnace, viewed including the step (ii) to heat treatment to hold 30 seconds or more at a temperature of 1250 ° C. or less than Ac3 point, and
The method for producing an electric-welded steel pipe for a boiler excellent in stress corrosion cracking resistance, wherein the atmosphere in the atmosphere furnace is at least one of argon, nitrogen, carbon dioxide, and hydrogen . The method for producing an electric resistance welded steel pipe for boilers according to claim 4 , wherein the oxygen concentration in the atmosphere furnace is 1% or less .