JP2017008391A - Flexure electroseamed steel pipe for boiler excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance corrosion resistance, especially stress corrosion crack resistance in a flexure electroseamed steel pipe for boiler manufactured by bending a flexure electroseamed steel pipe for boiler (linear pipe) to a predetermined shape by cold processing or hot processing.SOLUTION: There is provided a flexure electroseamed steel pipe for boiler excellent in corrosion resistance, having a component composition containing, by mass%, C:0.10 to 0.50%, Si:0.35% or less, Mn:0.30 to 1.50%, Ca:0.0001 to 0.0050%, S:0.005% or less, P:0.02% or less, Al:0.05% or less, N:0.01% or less, O:0.01% or less and the balance:Fe with inevitable impurities, and having hardness of whole area of cross sections in a thickness direction of the steel pipe of 250 Hv or less.SELECTED DRAWING: None

Description

  The present invention relates to a bent electric resistance welded steel pipe for boilers which is excellent in corrosion resistance, particularly stress corrosion cracking resistance.

  Conventionally, one of the characteristics required for boiler piping is corrosion resistance (for example, see Patent Document 1). However, in recent years, low-impurity coal is expensive, so cheap coal with many impurities such as sulfur is used, but when boiler piping is exposed to such cheap coal combustion gas and mixed with water, Stress corrosion cracking occurs from the outer surface of the boiler piping.

  As one of the countermeasures, there is a method of quenching only the surface layer portion of the steel pipe for boiler manufactured by hot rolling, but the hardness of the steel tube surface layer portion exceeds the limit hardness and stress corrosion cracking may occur.

  Boiler piping has many bent parts, and the bent parts are also required to have required corrosion resistance. For example, Patent Document 2 discloses a method in which heat treatment is performed on a bent portion of boiler piping to suppress deterioration of high temperature corrosion resistance. However, there is no disclosure about suppression of stress corrosion cracking.

Japanese Patent Laid-Open No. 06-287678 JP 2002-363710 A

  In recent years, stress corrosion cracking has occurred in boiler bent piping, and it has become a big problem that boiler piping blows out. Therefore, the boiler bending pipe is required to have excellent corrosion resistance, particularly excellent stress corrosion cracking resistance.

  Therefore, the present invention provides a corrosion resistance, particularly stress corrosion cracking resistance, in a bent electric resistance steel pipe for a boiler manufactured by bending an electric resistance steel pipe (straight pipe) for a boiler into a predetermined shape by cold working or hot working. An object of the present invention is to provide a bent electric resistance steel pipe for boilers and a method for manufacturing the same that solves the problem.

  The inventors of the present invention have intensively studied a method for solving the above-described problems. As a result, it is possible to secure excellent corrosion resistance, particularly excellent stress corrosion cracking resistance, by adjusting the hardness of the cross section in the thickness direction of the steel pipe to the required range in the ERW steel pipe for boilers with the required composition. I found.

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

(1) Component composition is mass%, C: 0.10 to 0.50%, Si: 0.35% or less, Mn: 0.30 to 1.50%, Ca: 0.0001 to 0.0050 %, S: 0.005% or less, P: 0.02% or less, Al: 0.05% or less, N: 0.01% or less, O: 0.01% or less, balance: Fe and inevitable impurities Become
A bending electric-welded steel pipe for boilers having excellent corrosion resistance, characterized in that the hardness is 250 Hv or less in the entire cross-section in the thickness direction of the steel pipe.

  (2) The above component composition is further mass%, Cr: 0.05 to 1.00%, Mo: 0.05 to 1.00%, Ni: 0.10 to 2.00%, Cu: 0 .10 to 2.00%, Nb: 0.005 to 0.10%, V: 0.005 to 0.10%, Ti: 0.01 to 0.20%, B: 0.0005 to 0.0030 %, And Mg: 0.0050% or less of one or two or more kinds, the bent electric resistance steel pipe for boilers having excellent corrosion resistance according to the above (1).

  (3) In the thickness direction of the steel pipe, the difference between the hardness on the outer surface side and the hardness on the inner surface side is 50 Hv or less, and the corrosion resistance according to (1) or (2) is excellent Bending ERW steel pipe for boiler.

  (4) The electric resistance steel pipe for boilers having the composition described in the above (1) or (2) is bent into a predetermined shape by cold working or hot working, and the hardness in the entire cross section in the thickness direction of the steel pipe A method for producing a bent electric resistance steel pipe for a boiler excellent in corrosion resistance, characterized by producing a bent electric resistance steel pipe for a boiler having a Hb of 250 Hv or less.

  (5) The difference in hardness between the outer surface side and the inner surface side in the thickness direction of the bent electric resistance welded steel pipe for boiler is 50 Hv or less, and has excellent corrosion resistance according to (4) A method of manufacturing bent electric resistance steel pipes for boilers.

  ADVANTAGE OF THE INVENTION According to this invention, the bending steel pipe for boilers excellent in corrosion resistance, especially stress corrosion cracking resistance, and its manufacturing method can be provided.

The boiler electric resistance welded steel pipe excellent in corrosion resistance of the present invention (hereinafter sometimes referred to as “the present steel pipe”) has a component composition of mass%, C: 0.10 to 0.50%, Si: 0. .35% or less, Mn: 0.30 to 1.50%, Ca: 0.0001 to 0.0050%, S: 0.005% or less, P: 0.02% or less, Al: 0.05% or less N: 0.01% or less, O: 0.01% or less, balance: Fe and inevitable impurities,
The hardness is 250 Hv or less over the entire cross-section in the thickness direction of the steel pipe.

The method of manufacturing a bent electric resistance steel pipe for boilers of the present invention having excellent corrosion resistance (hereinafter sometimes referred to as “the present invention manufacturing method”) includes cold-working an electric resistance steel pipe for a boiler having the composition of the steel pipe of the present invention. Alternatively, it is characterized in that it is bent into a predetermined shape by hot working, and a bent electric-welded steel pipe for boiler having a hardness of 250 Hv or less in the entire cross section in the thickness direction of the steel pipe is produced.

  Hereinafter, the steel pipe of the present invention and the manufacturing method of the present invention will be described.

  The steel pipe of the present invention will be described. First, the reasons for limiting the component composition of the steel pipe of the present invention will be described. Hereinafter,% is mass%.

C: 0.10 to 0.50%
C is an element necessary for ensuring strength as a steel pipe and ensuring induction hardenability. If it is less than 0.10%, the strength of the final product is insufficient, and induction hardenability cannot be ensured, so C is made 0.10% or more. Preferably it is 0.15% or more. On the other hand, if it exceeds 0.50%, the cold workability at the time of pipe making is lowered, and further the weldability is lowered, so C is made 0.50% or less. Preferably it is 0.45% or less.

Si: 0.35% or less Si is an element that contributes to improvement in strength by solid solution strengthening. However, if it exceeds 0.35%, the cold workability deteriorates, so Si is made 0.35% or less. Preferably it is 0.25% or less. The lower limit includes 0%, but if Si is reduced as much as possible, the manufacturing cost increases, so about 0.01% is a practical lower limit on practical steel.

Mn: 0.30 to 1.50%
Mn is an element effective for ensuring induction hardenability. If it is less than 0.30%, the effect of addition is not sufficiently exhibited, so Mn is 0.30% or more. Preferably it is 0.50% or more. On the other hand, if it exceeds 1.50%, the cold workability at the time of pipe forming is lowered, so Mn is made 1.50% or less. Preferably it is 1.40% or less.

Ca: 0.0001 to 0.0050%
Ca is an element that controls the form of inclusions in the base metal and the ERW weld and contributes to the improvement of cold workability. If it is less than 0.0001%, the effect of addition does not appear, so Ca is made 0.0001% or more. Preferably it is 0.0005% or more. On the other hand, if it exceeds 0.0050%, inclusions increase and cold workability decreases, so Ca is made 0.0050% or less. Preferably it is 0.0040% or less.

S: 0.005% or less S is an element that combines with Mn to form MnS that can be a starting point of cracking during cold working. For this reason, S is preferably as small as possible and is made 0.005% or less. Preferably it is 0.003% or less. The lower limit includes 0%, but if S is reduced as much as possible, the manufacturing cost increases, so about 0.0001% is a practical lower limit on practical steel.

P: 0.02% or less P is an element that inevitably remains in steel. If it exceeds 0.02%, grain boundary segregation and center segregation occur, resulting in a decrease in ductility, so P is made 0.02% or less. Preferably it is 0.015% or less. The lower limit includes 0%, but if P is reduced as much as possible, the manufacturing cost increases, so about 0.001% is a practical lower limit on practical steel.

Al: 0.05% or less Al is a deoxidizing element. If it exceeds 0.05% after deoxidation, the alumina-based oxide becomes coarse and causes a decrease in cold workability, so Al is made 0.05% or less. Preferably it is 0.03% or less. The lower limit includes 0%, but if Al is reduced as much as possible, the manufacturing cost increases, so about 0.001% is a practical lower limit on practical steel.

N: 0.01% or less N is an element that inevitably remains in steel and forms nitrides. If the content exceeds 0.01%, coarse nitrides are generated and cold workability may be lowered. Therefore, N is set to 0.01% or less. Preferably it is 0.005% or less. The lower limit includes 0%, but if N is reduced as much as possible, the manufacturing cost increases, so about 0.001% is a practical lower limit on practical steel.

O: 0.01% or less O is an element that inevitably remains in steel even after deoxidation. If it exceeds 0.01%, a coarse oxide is generated and cold workability may be lowered. Therefore, O is set to 0.01% or less. Preferably it is 0.003% or less. The lower limit includes 0%, but if O is reduced as much as possible, the manufacturing cost increases, so about 0.001% is a practical lower limit on practical steel.

  In addition to the above elements, the steel pipe of the present invention is Cr: 0.05-1.00%, Mo: 0.05-1.00%, Ni: 0.10-2.00%, Cu: 0.10-2 0.00%, Nb: 0.005-0.10%, V: 0.005-0.10%, Ti: 0.01-0.20%, B: 0.0005-0.0030%, Mg: You may contain 0.0050% or less of 1 type, or 2 or more types.

Cr: 0.05-1.00%
Cr is an element effective for ensuring induction hardenability. If it is less than 0.05%, the effect of addition is not sufficiently exhibited, so the content is made 0.05% or more. Preferably it is 0.10% or more. On the other hand, if it exceeds 1.00%, the hardness increases and the cold workability is lowered, so the content is made 1.00% or less. Preferably it is 0.50% or less.

Mo: 0.05-1.00%
Mo is an element effective for ensuring induction hardenability. If it is less than 0.05%, the effect of addition is not sufficiently exhibited, so the content is made 0.05% or more. Preferably it is 0.10% or more. On the other hand, if it exceeds 1.00%, the hardness increases and the cold workability is lowered, so the content is made 1.00% or less. Preferably it is 0.50% or less.

Ni: 0.10 to 2.00%
Ni is an element effective for ensuring induction hardenability. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so the content is made 0.10% or more. Preferably it is 0.15% or more. On the other hand, if it exceeds 2.00%, the hardness increases and the cold workability is deteriorated, so the content is made 2.00% or less. Preferably it is 1.50% or less.

Cu: 0.10 to 2.00%
Cu is an element effective for ensuring induction hardenability. If it is less than 0.10%, the effect of addition is not sufficiently exhibited, so the content is made 0.10% or more. Preferably it is 0.12% or more. On the other hand, if it exceeds 2.00%, the hardness increases and the cold workability is deteriorated, so the content is made 2.00% or less. Preferably it is 1.50% or less.

Ti: 0.01-0.20%
Since Ti has a strong affinity with N, Ti is an element that prevents precipitation of BN due to the addition of B and contributes to securing solid solution B. Ti is added as necessary in the case of addition of B. However, if less than 0.01%, the effect of addition is not sufficiently exhibited, so the content is made 0.01% or more. Preferably it is 0.015% or more. On the other hand, if it exceeds 0.20%, precipitation hardening of TiC becomes remarkable and the cold workability is deteriorated, so the content is made 0.20% or less. Preferably it is 0.05% or less.

Nb: 0.005 to 0.10%
Nb is an element necessary for ensuring hardenability. If it is less than 0.005%, the effect of addition is not sufficiently exhibited, so the content is made 0.005% or more. Preferably it is 0.010% or more. On the other hand, if it exceeds 0.10%, precipitation hardening due to NbCN becomes prominent, leading to deterioration of cold workability and toughness. Preferably it is 0.05% or less.

V: 0.005-0.10%
V is an element necessary for ensuring hardenability. If it is less than 0.005%, the effect of addition is not sufficiently exhibited, so the content is made 0.005% or more. Preferably it is 0.010% or more. On the other hand, if it exceeds 0.10%, precipitation hardening due to VCN becomes prominent, leading to deterioration of cold workability and toughness. Preferably it is 0.05% or less.

B: 0.0005 to 0.0030%
B is an element effective for ensuring induction hardenability. If it is less than 0.0005%, the effect of addition is not sufficiently exhibited. Preferably it is 0.0010% or more. On the other hand, if it exceeds 0.0030%, grain boundary embrittlement may be caused, so the content is made 0.0030% or less. Preferably it is 0.0020% or less.

Mg: 0.0050% or less Mg is a deoxidizing element, is an element that forms a precipitation nucleus of MnS and generates an oxide that contributes to fine uniform dispersion of MnS. However, if it exceeds 0.0050%, the yield is deteriorated and the effect of addition is saturated, so the content is made 0.0050% or less. Preferably it is 0.0030% or less. The lower limit includes 0%, but 0.0005% or more is preferable in that the effect of addition is surely obtained.

  In the steel pipe of the present invention, in order to ensure excellent corrosion resistance, in particular, excellent stress corrosion cracking resistance, the component composition is specified as described above, and the hardness in the entire cross section in the thickness direction of the steel pipe is 250 Hv or less. To do.

  An electric-resistance welded steel pipe (straight pipe) is bent into a predetermined shape by cold working or hot working to produce a bent electric-welded steel pipe. At this time, if the hardness of the outer surface exceeds 250 Hv in the bending ERW steel pipe, stress corrosion cracking may occur from the outer surface. Therefore, not only the outer surface of the steel pipe but also the hardness of the entire cross section in the thickness direction of the steel pipe is set to 250 Hv or less. Preferably it is 240 Hv or less.

  In order to further suppress the occurrence of stress corrosion cracking in the bending ERW steel pipe, the difference between the hardness on the outer surface side and the hardness on the inner surface side is set to 50 Hv or less in the thickness direction of the bending ERW steel pipe.

  After bending an electric resistance steel pipe (straight pipe) into a predetermined shape, the bent electric resistance steel pipe may be quenched. Even if the hardness of the outer surface is 250 Hv or less, the hardness of the outer surface and the hardness of the inner surface If the difference in thickness is 50 Hv or more, the difference in hardness causes strain in the thickness direction of the steel pipe, and stress corrosion cracking is likely to occur.

  Next, the manufacturing method of the present invention will be described.

  Basically, an electric resistance welded steel pipe (straight pipe) is bent into a predetermined shape by cold working or hot working, and a bent electric resistance welded steel pipe having a hardness of 250 Hv or less in the entire cross section in the thickness direction of the steel pipe is manufactured.

  The electric-resistance-welded steel pipe (straight pipe) may be manufactured according to a conventional method using a steel plate having the composition of the steel pipe of the present invention. Moreover, what is necessary is just to perform cold processing and hot processing according to a conventional method.

  However, the hardness of the outer surface of the steel pipe does not rise above 250 Hv when it is cooled after being bent into a predetermined shape by hot working, and the hardness and inner surface of the outer surface side in the thickness direction of the steel pipe It is preferable to cool by selecting a cooling condition in which the difference in hardness on the side does not exceed 50 Hv.

  The hardness in the thickness direction in the cross section of the steel pipe is measured from the inner surface to the outer surface with a load of 1 kg and a pitch of 0.5 mm.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on these one example conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
The steel strip was continuously formed into a tubular shape, the ends of the tubular steel strip were welded by high frequency welding, and then reheated to a temperature above the _Ac3 transformation point to produce an ERW steel pipe. Table 1 shows the component composition of the manufactured ERW steel pipe. In Table 1, the invention steel pipe is a steel pipe whose component composition is within the range of the component composition of the steel pipe of the present invention, and the comparative steel pipe is a steel pipe whose component composition is outside the range of the component composition of the steel pipe of the present invention.

  Table 2 shows the hardness of the outer surface and the inner surface of a bent steel pipe for boilers manufactured by bending an electric resistance steel pipe having the composition shown in Table 1 into a predetermined shape by cold working or hot working. The hardness was measured with a Vickers hardness meter at a load of 9.8 N.

  Moreover, the bending steel pipe for boilers (steel pipes 1 to 12) manufactured by bending the steel pipe according to the invention into a predetermined shape and the stress corrosion cracking of the bent steel pipes for boilers (steel pipes 13 to 16) manufactured by bending the comparative steel pipe into a predetermined shape. The occurrence was investigated. The results are also shown in Table 2.

  From Table 2, it can be seen that in the bent steel pipe for boilers of the present invention, stress corrosion cracking does not occur and it has excellent corrosion resistance.

  As described above, according to the present invention, it is possible to provide a bent steel pipe for a boiler excellent in corrosion resistance, particularly stress corrosion cracking resistance, and a method for manufacturing the same. Therefore, the present invention has high applicability in the steel pipe manufacturing industry.

Claims (5)

Ingredient composition is mass%,
C: 0.10 to 0.50%,
Si: 0.35% or less,
Mn: 0.30 to 1.50%,
Ca: 0.0001 to 0.0050%,
S: 0.005% or less,
P: 0.02% or less,
Al: 0.05% or less,
N: 0.01% or less,
O: 0.01% or less,
The balance: Fe and inevitable impurities,
A bending electric-welded steel pipe for boilers having excellent corrosion resistance, characterized in that the hardness is 250 Hv or less in the entire cross-section in the thickness direction of the steel pipe. The component composition is further in mass%,
Cr: 0.05-1.00%,
Mo: 0.05-1.00%,
Ni: 0.10 to 2.00%,
Cu: 0.10 to 2.00%,
Nb: 0.005 to 0.10%
V: 0.005-0.10%
Ti: 0.01-0.20%,
B: 0.0005 to 0.0030% and
Mg: 0.0050% or less of one type or two or more types, Bending electric resistance welded steel pipe for boiler excellent in corrosion resistance according to claim 1 characterized by the above-mentioned.   The difference between the hardness on the outer surface side and the hardness on the inner surface side in the thickness direction of the steel pipe is 50 Hv or less, and the bending electric-welded steel pipe for boilers with excellent corrosion resistance according to claim 1 or 2 .   A boiler having a component composition according to claim 1 or 2 bent to a predetermined shape by cold working or hot working, and having a hardness of 250 Hv or less across the entire cross section in the thickness direction of the steel pipe The manufacturing method of the bending electric resistance steel pipe for boilers which is excellent in corrosion resistance characterized by manufacturing a bending electric resistance steel pipe for electric machinery.   5. The bending for boiler with excellent corrosion resistance according to claim 4, wherein a difference in hardness between the outer surface side and the inner surface side is 50 Hv or less in the thickness direction of the bending electric resistance welded steel pipe for boiler. A method for manufacturing ERW steel pipes.