JP4542361B2 - Ferritic ERW boiler tube with excellent reheat cracking resistance and its manufacturing method - Google Patents

Description

  The present invention relates to a ferritic electric resistance boiler steel pipe excellent in reheat cracking resistance of a welded portion and a manufacturing method thereof, and more specifically, excellent in creep rupture strength used in a high temperature / high pressure environment and electric resistance welding. The present invention relates to a ferritic ERW boiler steel pipe having excellent joint characteristics and excellent weld-heat-resistant reheat cracking resistance and a method for producing the same.

  Generally, for high temperature heat and pressure resistant members for boilers, chemical industries, nuclear power, etc., austenitic stainless steel, high Cr ferritic steel with Cr content of 9-12% (hereinafter,% representing component ratio is weight%), Materials such as low Cr ferritic steel or carbon steel having a Cr content of 2.25% or less are used. These are appropriately selected in consideration of the use environment such as the use temperature and pressure of the target member and the economy.

  By the way, among these materials, as a feature of low Cr ferritic steel having a Cr content of 2.25% or less, since it contains Cr, it is superior in oxidation resistance, high temperature corrosion resistance and high temperature strength compared to carbon steel. In addition, it is much cheaper than austenitic stainless steel, has a low thermal expansion coefficient and does not cause stress corrosion cracking, and is also cheaper than high Cr ferritic steel, toughness, thermal conductivity and welding It is excellent in property.

  As typical examples of such low Cr ferritic steels, STBA20, STBA21, STBA22, STBA23, STBA24, etc., which are standardized by JIS, are known, and are generally collectively referred to as Cr-Mo steel. Further, low Cr ferritic steels added with precipitation strengthening elements V, Nb, Ti, Ta, and B for the purpose of improving high temperature strength are disclosed in Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document. 5, Patent Literature 6, Patent Literature 7, Patent Literature 8, Patent Literature 9, Patent Literature 10, and the like.

  Further, as precipitation strengthening type low Cr ferritic steel, 1Cr-1Mo-0.25V steel, which is a turbine material, 2.25Cr-1Mo-Nb steel, which is a structural material for fast breeder reactors, and the like are well known. . However, these low Cr ferritic steels are inferior in oxidation resistance and corrosion resistance at high temperatures and low in high temperature strength compared to high Cr ferritic steels and austenitic stainless steels. .

  Therefore, Patent Documents 11 and 12 propose low Cr ferritic steels to which a large amount of W is added in order to improve the creep strength at a high temperature of 550 ° C. or higher. Patent Document 13 discloses a low Cr ferrite in which a small amount of B is added after limiting the amount of N in order to improve the creep strength at a high temperature of 550 ° C. or higher and to suppress the decrease in toughness accompanying the increase in strength. Steel has been proposed.

  When these materials are electro-welded, a large number of high-melting-point oxides are generated in the electro-welded weld and are taken into the inner surface during electro-welding. Therefore, the properties such as creep rupture strength and toughness of ERW welds cannot be satisfied under a high temperature environment of 550 ° C. or higher. Therefore, the low Cr ferritic steel that can be used at a high temperature of 550 ° C. or higher is a seamless steel pipe. Seamless steel pipes are expensive to manufacture and are not economically useful materials.

  Conventionally, precipitation strengthening with carbides and solid solution strengthening with Mo or the like have been used to improve high temperature strength and creep strength. However, when exposed to a high temperature for a long time, the precipitates become coarse and solid solution elements form precipitates. Therefore, the strengthening by these metallurgical factors has a limit in improving the creep strength. In addition, attempts have been made to add B in order to improve the creep strength, but there is a problem that the hardenability is increased and the strength fluctuation in the hot rolled coil is increased.

  In addition, low Cr ferritic steel and high Cr ferritic steel have high susceptibility to reheat cracking in the welds, so control of heat input during welding and control of the cooling rate can prevent coarsening of the weld heat affected zone. ing. However, when considering from the viewpoint of welding workability, the work efficiency is very poor, and heat input control and cooling rate management may be very difficult.

Further, the present inventors have previously considered the mass of Si , Mn, and Cr contained in the steel as an electric-welded boiler steel pipe having few welding defects and excellent creep rupture strength and toughness. Patent Document 14 proposes a technique for adjusting the ratio , lowering the melting point of mixed oxides of SiO ₂ , MnO, and Cr ₂ O ₃ present in steel, and squeezing out these oxides as slag components during welding. Patent Document 15 proposed a technique for regulating the average number density of Mg-based oxides.

JP 57-131349 A JP-A-57-131350 JP 61-166916 A Japanese Patent Laid-Open No. 62-54062 JP 63-18038 A Japanese Unexamined Patent Publication No. 63-62848 Japanese Unexamined Patent Publication No. 64-68451 JP-A-1-29853 Japanese Patent Laid-Open No. 3-64428 JP-A-3-87332 JP-A-2-217438 JP-A-2-217439 JP-A-4-268040 JP 2000-234140 A JP 2002-146482 A

  Although the present invention is a low Cr ferritic steel having a Cr content of 3.5% or less, it exhibits high creep rupture strength on the high temperature and long time side, and has excellent ERW weldability and excellent resistance to reheat cracking in welds. Another object of the present invention is to provide a ferritic ERW boiler steel pipe and a manufacturing method thereof.

  The present invention shields the welded part from the outside air during ERW welding, thereby preventing the formation of oxides that cause welding defects in the ERW abutting part and showing high creep rupture strength on the high temperature and long time side. To provide a ferritic ERW boiler steel pipe excellent in sewing weldability and excellent in resistance to reheat cracking at a welded portion and a manufacturing method thereof.

Specifically, it is a condition in which an oxide that causes welding defects is not formed in the electro-welding abutting portion by shielding the welded portion from the outside air during electro-welding welding, and an oxide is formed. In addition, it is possible to prevent welding defects from occurring in the electro-resisting joint by adjusting the amounts of Si, Mn, and Cr and controlling the melting point of the mixed oxide. More specifically, when considering a ternary phase diagram of SiO ₂ , MnO, and Cr ₂ O ₃ that are oxides formed from Si, Mn, and Cr, the melting point of these mixed oxides is low. As much as possible, it does not remain as an oxide that becomes a defect in an ERW weld during ERW welding, but the oxide melts and is extruded as a slag component, and does not remain in the ERW weld, making it difficult to produce a welding defect. Considering the ternary phase diagram of these oxides, the more the SiO ₂ is added, the lower the melting point of the ternary oxide. On the other hand, the more MnO and Cr ₂ O ₃ are added, the higher the melting point of the ternary oxide. Taking these into account, SiO ₂ , MnO, Cr ₂ O ₃ , that is, the amount of Si, Mn, Cr added is distributed to the numerator and denominator coefficients to generate oxides that affect the characteristics of ERW welds. To control. The additive amount of Si, Mn, and Cr, which are the formation elements of SiO ₂ , MnO, and Cr ₂ O ₃ , which affect the characteristics of ERW welds, is the weight ratio of Si, Mn and Cr (Si%) / (Mn% + Cr %) Is controlled to be 0.005 or more and 1.5 or less, so that the weld defect area ratio of the ERW weld zone is extremely low and deterioration of the ERW weld zone creep characteristics, toughness and the like is prevented. Further, by blocking the outside air around the ERW weld with a non-oxygen-based gas, it becomes more difficult to form oxides. As a result, weld defects in the ERW weld are reduced, and the material characteristics are improved. Furthermore, Mg-based oxides have an average particle size of 0.002 μm to 3 μm in steel, and the heat density is controlled during welding by controlling the number density to 0.01 / μm ^{2 to} 10 / μm ^2. It is possible to prevent coarsening of the weld heat affected zone at the time of joint welding without performing the above, and as a result, it is possible to prevent weld reheat cracking caused by heat treatment of the joint weld.

That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.005 to 0.20%,
Si: 1.04 to 1.52 %,
Mn: 0.05 to 2.0%,
Cr: 0.25 to 2.52 %,
Mo: 0.01 to 2.0%,
O: 0.001 to 0.3%,
Al: 0.01% or less,
N: 0.001 to 0.1%,
Nb: 0.001 to 0.5%,
V: 0.02 to 1.0%,
B: 0.0002 to 0.02%,
W: 0.01-4.0%,
Cu: 0.1 to 2.0%,
Ni: 0.1 to 2.0%,
Co: 0.1 to 2.0%,
Ti: 0.001 to 0.05%,
Mg: 0.0002 to 0.05%,
Containing one or more of
P: 0.030% or less,
S: 0.010% or less,
The mass ratio of Si, Mn and Cr is 0.357 to 1.042 in terms of (Si%) / ((Mn%) + (Cr%)), and the balance consists of Fe and inevitable impurities. And the base material structure is one or more of tempered martensite, tempered bainite, tempered ferrite-pearlite, and the electric resistance welded steel pipe has a microstructure as welded, A ferritic ERW boiler pipe excellent in reheat cracking resistance against welded portions, wherein the ERW welded portion defect area ratio of the ERW boiler steel tube is 0.1% or less.
( 2 ) The Mg-based oxide has an average particle size of 0.002 μm to 3 μm in the steel and a number density of 0.01 / μm ^{2 to} 10 / μm ² in (1 ). The ferritic ERW boiler steel pipe having excellent reheat cracking resistance against welding as described.
( 3 ) In mass%,
C: 0.01-0.20%
Si: 1.04 to 1.52 %,
Mn: 0.05 to 2.0%,
Cr: 0.25 to 2.52 %,
Mo: 0.01 to 2.0%,
O: 0.001 to 0.3%,
Al: 0.01% or less,
N: 0.001 to 0.1%
Nb: 0.001 to 0.5%,
V: 0.02 to 1.0%,
B: 0.0002 to 0.02%,
W: 0.01-4.0%,
Cu: 0.1 to 2.0%,
Ni: 0.1 to 2.0%,
Co: 0.1 to 2.0%,
Ti: 0.001 to 0.05%,
Mg: 0.0002 to 0.05%,
1 type or 2 types of
P: 0.030% or less,
S: 0.010% or less,
The mass ratio of Si, Mn and Cr is 0.357 to 1.042 in terms of (Si%) / ((Mn%) + (Cr%)), with the balance being Fe and inevitable impurities. A method of manufacturing a ferritic ERW boiler steel pipe excellent in reheat cracking resistance of a welded portion, characterized by tempering a hot-rolled steel plate, pipe-forming it into a cylindrical shape, and subjecting the butt portion to ERW welding.
(4) electric resistance welding during the inert gas from the weld top and bottom surfaces in the weld vicinity, nitrogen gas, wherein the blow non-oxygen-containing gas consisting of one or more of hydrogen gas (3) Symbol mounting Ferritic ERW boiler steel pipe manufacturing method with excellent reheat cracking resistance of welds.
(5) (3) or (4) Symbol in addition to the production method of mounting further, quenching - tempering or Shojun, - tempering excellent ferrite applying heat treatment-resistant weld reheat cracking characterized by the Electric ERW boiler steel pipe manufacturing method .

  According to the present invention, a ferrite-based electrode having a high creep rupture strength on a high temperature and long time side used in a high temperature / high pressure environment, excellent in an electric resistance welded portion, and excellent in resistance to reheat cracking in a welded portion. The present invention relates to a sewn boiler steel pipe and a manufacturing method, is an economical material with a low manufacturing cost, and contributes greatly to industrial development.

  Hereinafter, the present invention will be described in detail.

The steel of the present invention improves the high-temperature long-term stability of Nb, V precipitates and fine carbides (M ₂₃ C ₆ and M ₇ C ₃ ), and coarse precipitates (M ₆ C) mainly composed of W and Mo. And the formation of intermetallic compounds), Mg-containing, Mg-based oxides are finely dispersed in the steel, and the metal structure is stably maintained for a long time even in a high temperature range. The creep rupture strength on the time side is improved. Furthermore, by controlling these precipitates, it is possible to prevent in advance the weld reheat cracking that occurs in the joint weld heat treatment after welding.

Furthermore, by controlling the addition amount of Si, Mn, Cr, which are the generation elements of SiO ₂ , Cr ₂ O ₃ , and MnO that affect the characteristics of the ERW weld, the weld defect area ratio of the ERW weld is extremely low Deterioration of creep characteristics, toughness, etc. of ERW welds is prevented.

  Further, by blocking outside air in the vicinity of the ERW weld with non-oxygen-based gas, the weld defect area ratio of the ERW weld is extremely low, and deterioration of the creep characteristics and toughness of the ERW weld is prevented.

  Furthermore, by finely dispersing the Mg-based oxide in the steel, it becomes possible to suppress the grain coarsening of the weld heat-affected zone of the joint weld, and as a result, a weld that occurs in the heat treatment of the joint weld after welding. It becomes possible to prevent reheat cracking in advance. In order to sufficiently disperse Mg, it is necessary to sufficiently stir the molten steel.

  The reason why the component composition of the steel of the present invention is limited as described above is as follows.

  C forms carbides with Cr, Fe, W, Mo, V, and Nb, contributes to the improvement of the high temperature strength, and stabilizes the structure as an austenite stabilizing element.

  The steel of the present invention becomes a mixed structure of ferrite, martensite, bainite and pearlite by normalizing / tempering treatment, and the C content is also important for controlling the balance of these structures.

  When the C content is less than 0.01%, the amount of precipitated carbide is insufficient, and the amount of δ ferrite is excessively increased, which impairs strength and toughness. On the other hand, if it exceeds 0.20%, carbides are excessively precipitated and the steel is markedly hardened, thereby impairing workability and weldability. Therefore, the C content is set to 0.01% or more and 0.20% or less.

Si is an element that acts as a deoxidizer and enhances the steam oxidation resistance of steel. If the Si content is less than 0.01%, it is not sufficient, and if it exceeds 1.52 %, the toughness is remarkably lowered, which is also harmful to the creep rupture strength. Therefore, the Si content is set to 0.01% or more and 1.52 % or less. The lower limit of the Si content is 1.04% based on the examples.

  Mn is an element necessary not only for deoxidation but also for maintaining strength. In order to obtain a sufficient effect, 0.05% or more must be added, and if it exceeds 2.0%, the creep rupture strength may decrease. Therefore, the Mn content is set to 0.05% or more and 2.0% or less.

Cr is an indispensable element for improving the oxidation resistance and high temperature corrosion resistance of the low Cr ferritic steel. If the Cr content is less than 0.5%, these effects cannot be obtained. However, if the Cr content exceeds 3.5%, the toughness, weldability and thermal conductivity are lowered, and the advantages of the low Cr ferritic steel are reduced. Therefore, the Cr content is set to 0.5% to 3.5%. In addition, the upper limit of Cr content shall be 2.52% based on an Example.

  Nb combines with C and N to form a fine carbonitride of Nb (C, N) and contributes to the creep rupture strength. In particular, at 625 ° C. or lower, there is an effect of remarkably improving the creep rupture strength by forming a stable fine precipitate. Furthermore, it is effective in making crystal grains fine and improving toughness. However, if the Nb content is less than 0.001%, the above effect cannot be obtained. On the other hand, if the Nb content exceeds 0.5%, the steel is markedly hardened and the toughness, workability and weldability are impaired. Therefore, the Nb content is set to be 0.001% or more and 0.5% or less.

  V, like Nb, combines with C and N to form Nb (C, N) fine carbonitride and contributes to the improvement of creep rupture strength on the high temperature and long time side, but its content is 0. If it is less than 02%, the effect is not sufficient. However, if V is added in excess of 1.0%, the amount of precipitation of V (C, N) becomes excessive, and on the contrary, the strength and toughness are impaired. Therefore, the V content is set to 0.02% or more and 1.0% or less.

  N precipitates in the matrix as a solid solution or as a nitride or carbonitride, and mainly takes the form of VN, NbN or the respective carbonitrides and contributes to solid solution strengthening and precipitation strengthening. In this invention, it couple | bonds with Ti and precipitates as TiN, and also couple | bonds with B as BN, and each contributes to improvement in creep rupture strength. Addition of less than 0.001% hardly contributes to strengthening, and if added over 0.08%, the base material toughness and strength are significantly reduced. Therefore, the N content is set to be 0.001% or more and 0.08% or less.

B is an element added to ensure the following effects. By co-segregating with C, fine carbides (specifically, M ₂₃ C ₆ carbides) are stabilized. In low Cr ferritic steel, when heated for a long time at a high temperature, W and Mo concentrate in M ₂₃ C ₆ carbide, which changes to coarse M ₆ C carbide, leading to a decrease in creep strength and toughness. . However, the addition of B stabilizes M ₂₃ C ₆ , so that precipitation of coarse carbide M ₆ C is suppressed, and a decrease in creep strength is suppressed. However, when the B content is less than 0.0003%, the above effect cannot be obtained. On the other hand, when the B content exceeds 0.01%, B is segregated excessively at the crystal grain boundary, and co-segregation with C results. Carbide may agglomerate and coarsen, and as a result, workability, toughness and weldability are significantly impaired. Therefore, the B content is set to 0.0003% or more and 0.01% or less.

  Al is effective as a deoxidizing agent. However, when it exceeds 0.01%, the high-temperature strength decreases, so the content was made 0.01% or less.

  Mo has an effect of strengthening by solid solution strengthening and fine carbide precipitation, and is an element effective for improving the creep rupture strength, and can be contained as necessary. However, if the Mo content is less than 0.01%, the above effect cannot be obtained. On the other hand, if it exceeds 2.0%, the effect is saturated, and weldability and toughness are impaired. Therefore, when adding Mo, 0.01% or more and 2.0% or less are preferable. In addition, when Mo and W are added in combination, the strength of the steel is further improved as compared with the case where it is added alone, and particularly, the high temperature creep rupture strength is improved.

  W is an element effective for improving the creep rupture strength because it exerts a strengthening action by solid solution and a precipitation action of fine carbides, but these effects are obtained when the W content is less than 0.01%. Absent. On the other hand, if the W content exceeds 3.0%, the steel is markedly hardened and the toughness, workability and weldability are impaired. Therefore, the W content is set to 0.01 to 3.0%. In addition, as already described, W is combined with Mo and the effect of improving the strength of steel becomes remarkable.

  Although P and S are mixed as impurities in the steel of the present invention, P and S decrease the strength in order to exert the effects of the present invention, so the upper limit values are 0.030% and 0.010, respectively. %.

  O is limited to 0.001 to 0.3%. If O is less than 0.001%, even if Mg is contained in a necessary amount, the number of oxides is insufficient and the strength cannot be increased. On the other hand, if O exceeds 0.3%, the oxide becomes extremely coarse and becomes the starting point of brittle fracture, which is not preferable. Therefore, O is limited to 0.001 to 0.3%.

  When the Mg content is less than 0.0002%, the number of dispersed oxides is not sufficient. When the Mg content is more than 0.05%, Mg is excessive and uneconomical, and the oxide becomes coarse and adversely affects mechanical properties. Therefore, the Mg content is set to 0.0002 or more and 0.05% or less.

  Further, Ti combines with C and N to form Ti (C, N). In particular, since the binding force with N is strong, it is effective for fixing solute N. Of course, as described later, B also has an action of fixing solute N, but the bonding form with C is greatly different from Ti. That is, B tends to segregate in carbides mainly composed of Fe and CrW, and when excessive B is present, aggregation of these carbides may be promoted. On the other hand, Ti binds to C alone and forms a composite precipitate with TiN, but coagulation coarsening does not proceed further. Therefore, Ti is preferable in that it fixes N effectively and does not affect the phase stability of the carbide at the same time.

  Ti improves hardenability by suppressing the amount of dissolved N, and improves toughness and creep strength. However, if the Ti content is less than 0.001%, the above effect cannot be obtained. On the other hand, if the Ti content exceeds 0.05%, the amount of Ti (C, N) precipitated increases and the toughness is significantly impaired. It comes to be. Therefore, the Ti content is preferably 0.001 to 0.05%.

  Cu, Ni, and Co are all strong austenite stabilizing elements, and particularly when a large amount of ferrite stabilizing elements, that is, Cr, W, Mo, Ti, Si, or the like is added, the quenched structure or the quenching- Necessary and useful for obtaining tempered tissue. At the same time, Cu is effective in improving high-temperature corrosion resistance, Ni is effective in improving toughness, and Co is effective in improving strength. In any case, the effect is insufficient at 0.1% or less, and when added over 2.0%, embrittlement due to precipitation of coarse intermetallic compounds or segregation at grain boundaries is inevitable. . Therefore, the Cu, Ni, and Co contents are 0.1% or more and 2.0% or less, respectively.

  In the present invention, one or more of La, Ca, Y, Ce, Zr, Ta, Hf, Re, Pt, Ir, Pd, and Sb can be added as necessary. These elements are added for the purpose of controlling the form of impurity elements (P, S, O) and their precipitates (inclusions). By adding at least one of these elements in an amount of 0.001% or more for each element, the impurities are fixed as stable and harmless precipitates, and the strength and toughness are improved. If the content is less than 0.001%, the effect is not obtained. If the content exceeds 0.2%, inclusions increase and the toughness is deteriorated. Therefore, the respective contents are set to 0.001 to 0.2%.

  Further, the number density of the oxide in the present invention is obtained by observing and photographing the extracted replica with an optical microscope or an electron microscope. That is, 100 or more oxides are measured for 3 or more fields of an extracted replica photograph having an appropriate magnification of 1000 to 50000 times, and the average particle diameter and the number per observation area are obtained. When the shape of the oxide is not a circle, the equivalent circle diameter is defined as the particle size from the area of the oxide.

If the average particle size of the Mg-based oxide is less than 0.002 μm, the dislocation easily overcomes the oxide, so there is little improvement in strength, and a pinning effect for preventing grain coarsening in the weld heat affected zone can be exhibited. When the weld zone reheat cracking occurs and the thickness exceeds 3 μm, the oxide itself tends to be a starting point of fracture and the toughness is lowered, which is not preferable. Also, if the number density is less than 0.01 per 1 μm ² , the high temperature creep strength will not be improved, and the pinning effect for preventing grain coarsening in the weld heat affected zone will not be exhibited, resulting in weld reheat cracking. . Further, the strength increases as the number density of oxides increases. However, since dispersion of more than 10 is difficult to control in steelmaking, the number density of oxides is limited to 0.01 to 10 per 1 μm ² in the present invention. .

Furthermore, in the present invention described above, in order to obtain good characteristics such as creep rupture strength, toughness, and ERW weldability of the ERW welded part, Si, which is a generation element of SiO ₂ , MnO, Cr ₂ O ₃ , It is necessary to control the weight ratio of Mn and Cr and the melting point of these oxides. Considering the ternary phase diagram of SiO ₂ , MnO, Cr ₂ O ₃ , the lower the melting point of these mixed oxides, the more the oxide that becomes a defect in an ERW weld part during ERW welding does not remain. The oxide melts and is extruded as a slag component, and does not remain in the ERW weld, so that welding defects are less likely to occur. Therefore, the melting point of the mixed oxide needs to be 1600 ° C. or lower. Further, when considering the ternary phase diagram of these oxides, the more the SiO ₂ is added, the lower the melting point of the ternary oxide. On the other hand, the higher the MnO and Cr ₂ O ₃ are added, the higher the melting point of the ternary oxide. Taking these into account, SiO ₂ , MnO, Cr ₂ O ₃ , that is, the amount of Si, Mn, Cr added is distributed to the numerator and denominator coefficients to generate oxides that affect the characteristics of ERW welds. At the same time, it is necessary to control the melting point of these mixed oxides to 1600 ° C. or lower. The weight ratio of Si, Mn, and Cr, which are the generation elements of SiO ₂ , MnO, and Cr ₂ O ₃ that affect the electric resistance welded portion characteristics, is limited by the value of (Si%) / (Mn% + Cr%). These ideas are the same in the case of binary oxides. For the measurement of the melting point of the mixed oxide, a method is usually used in which a change in the steel material being heated in the heating furnace is observed with a telescope and the melted temperature is set as the melting point.

  Charpy specimens were collected along the direction C of the ERW steel pipe, and the weld defect area ratio was measured with an optical microscope using the specimens subjected to the Charpy test at 100 ° C. In the measurement method, the area of the oxide is measured by observation with an optical microscope, and the weld defect area ratio is calculated.

That is, when the value of (Si%) / (Mn% + Cr%) is less than 0.005, MnO or Cr ₂ O ₃ oxides remain in the ERW weld, causing weld defects, Properties such as strength and toughness deteriorate. Also, if the value of (Si%) / (Mn% + Cr%) exceeds 1.5, the oxide of SiO ₂ remains in the ERW weld, causing weld defects, and the strength and toughness of the weld. The characteristics of the will deteriorate. In addition, the lower limit and upper limit of the value of (Si%) / (Mn% + Cr%) are set to 0.357 or more and 1.042 or less, respectively, based on the examples. When these oxides melt at the time of upsetting and are extruded as a slag component, weld defects do not occur.However, if they are present as oxides, weld defects occur in the ERW welds, and the creep rupture strength and toughness of the ERW welds are reduced. Deteriorate significantly. In order to prevent these problems from occurring, the weld defect area ratio needs to be 0.1% or less.

  In order to set the weld defect area ratio as described above to 0.1% or less, the hot-rolled steel sheet having the optimum amount of the above-described steel components, Si, Mn and Cr is tempered, preferably at a temperature of 700 to 760 ° C. Therefore, it is necessary to temper in a heat-retaining furnace or a heating furnace in a state of a hot rolled coil of 1 to 10 hours. The hot-rolled steel sheet according to the present invention may have a maximum strength variation of ± 300 MPa in tensile strength, but the tempered hot-rolled steel plate of the present invention falls within a maximum strength variation of about ± 150 MPa. If such a tempered hot-rolled steel sheet is piped into an electric-welded boiler steel pipe, the strength of the hot-rolled steel sheet becomes uniform, the pipe-forming shape is kept constant, and as a result, the electric-welding welding conditions are constant. Therefore, welding defects are reduced.

  Furthermore, at the time of ERW welding, a box is put on the welded portion, and an inert gas is blown into and filled with the box to block the ERW abutting portion from outside air, or an inert gas (for example, argon gas), Si, Mn, and Cr-based oxides that contribute to welding defects are not formed in the electro-welding joints by shutting off from the outside air with one or more non-oxygen gases such as nitrogen gas and hydrogen gas. As a result, it is possible to obtain a material with good electro-welded welded portion characteristics.

  Each steel having chemical components shown in Tables 1 and 2 (continued in Table 1) was heated and rolled at 1050 to 1300 ° C. to obtain a hot-rolled steel sheet having a thickness of 8 mm. The rolling end temperature was all controlled to be between 850 and 1050 ° C. Next, the heat treatment was tempered by heating to 740 ° C. × 1 hr and then air cooling. Thereafter, an ERW steel pipe having an outer diameter of 45 mm was manufactured. At that time, a non-oxygen-based gas shield such as an argon gas shield was also applied to the ERW weld. Moreover, the steel pipe was joint-welded, and the reheat cracking characteristics of the welded part of the test piece subjected to heat treatment after heating at 720 ° C. for 1 hour and air cooling were evaluated. The electric resistance welded portion characteristics were determined by measuring the weld defect area ratio of the electric resistance welded portion of each specimen, and determining that the weld defect area ratio was 0.1% or more and that welding was impossible. The weld reheat cracking characteristics were evaluated by a side bending test of the steel pipe joint.

  The weld defect area ratio was measured with an optical microscope using a test piece subjected to a Charpy test at 100 ° C.

  The average particle size and number density of the oxides in the developed steel were obtained by observing and photographing the extracted replica sample with an optical microscope or an electron microscope. The magnification at the time of observation was set to an appropriate magnification of 1000 to 50000 times, 100 or more oxides were measured for 3 or more fields of the extracted replica photograph, and the average particle diameter and the number per observation area were obtained. When the shape of the oxide was not a circle, the equivalent circle diameter was defined as the particle size from the area of the oxide.

Table 2 (continuation of Table 1) shows chemical components and evaluation results of the steels of the present invention and the comparative steel. It turns out that this invention steel (No. 2, 5, 7, 8, 10, 11, 16, 18 ) is excellent in all the characteristics compared with comparative steel (No. 101-113 ).

  In the case of the steel No. 102 of the comparative steel, when the Mn content exceeds 2.0%, the creep rupture strength may be lowered.

  In the case of the steel No. 113 of the comparative steel, Mg for forming an Mg-based oxide having an Mg content of less than 0.0002% is insufficient, and the welding heat affected zone cannot be suppressed, and welding is not possible. Partial reheat cracking occurs.

  In the case of steel Nos. 101, 104 and 107, which are comparative steels, if the O content exceeds 0.3%, coarse oxides are formed, causing toughness reduction and intergranular embrittlement of the weld heat affected zone. Causes thermal cracking.

  In the case of steel No. 103, which is a comparative steel, if the Mo content exceeds 2%, a large amount of coarse Mo carbide precipitates, which causes a decrease in creep strength and causes embrittlement, and causes for grain boundary embrittlement in the weld heat affected zone. It becomes.

  In the case of steel Nos. 102 and 111, which are comparative steels, if the W content exceeds 4%, a large amount of coarse W carbides and intermetallic compounds are precipitated, and the solid solution W amount is also reduced. It causes the grain boundary embrittlement of the weld heat affected zone.

  In the case of comparative steel numbers 106 and 108, if the Nb content exceeds 0.5%, coarse Nb carbonitride precipitates, causing intergranular embrittlement in the weld heat affected zone and causing reheat cracks in the weld. Arise.

  In the case of steel Nos. 109 and 112, which are comparative steels, if the V content exceeds 1%, coarse V carbonitrides precipitate, causing grain boundary embrittlement in the weld heat-affected zone, resulting in weld reheat cracking.

  In the case of the steel No. 110 of the comparative steel, if the N content exceeds 0.1%, coarse Nb and V nitrides are formed, causing intergranular embrittlement in the weld heat affected zone and causing reheat cracks in the weld. .

  In the case of steel No. 105, which is a comparative steel, if the B content exceeds 0.02%, coarse B carbonitride precipitates, causing grain boundary embrittlement in the weld heat affected zone and causing reheat cracks in the weld.

In the case of the steel numbers 112 and 113 of the comparative steel, if the number density of the Mg-based oxide is less than 0.01 pieces / μm ² , the oxide density necessary for suppressing the grain coarsening of the weld heat affected zone is insufficient, As a result, weld reheat cracking occurs.

  In the case of the steel No. 110 of the comparative steel, if the area ratio of the ERW weld defect exceeds 0.1%, the ERW welded portion characteristics deteriorate, and the welded portion creep and mechanical characteristics deteriorate.

Claims (5)

% By mass
C: 0.005 to 0.20%,
Si: 1.04 to 1.52 %,
Mn: 0.05 to 2.0%,
Cr: 0.25 to 2.52 %,
Mo: 0.01 to 2.0%,
O: 0.001 to 0.3%,
Al: 0.01% or less,
N: 0.001 to 0.1%,
Nb: 0.001 to 0.5%,
V: 0.02 to 1.0%,
B: 0.0002 to 0.02%,
W: 0.01-4.0%,
Cu: 0.1 to 2.0%,
Ni: 0.1 to 2.0%,
Co: 0.1 to 2.0%,
Ti: 0.001 to 0.05%,
Mg: 0.0002 to 0.05%,
Containing one or more of
P: 0.030% or less,
S: 0.010% or less,
The mass ratio of Si, Mn and Cr is 0.357 to 1.042 in terms of (Si%) / ((Mn%) + (Cr%)), and the balance consists of Fe and inevitable impurities. And the base material structure is one or more of tempered martensite, tempered bainite, and tempered ferrite-pearlite, and the ERW boiler steel pipe has a microstructure in which the ERW weld is welded, A ferritic ERW boiler pipe excellent in reheat cracking resistance against welded portions, wherein the ERW welded portion defect area ratio of the ERW boiler steel tube is 0.1% or less. Resistance of Claim 1 Symbol placement Mg-based oxide is equal to or not more than 3μm mean particle size 0.002μm or more in the steel, and the number density of 0.01 units / [mu] m ² or more to 10 / [mu] m ² or less Ferritic ERW boiler pipe with excellent weld reheat cracking. % By mass
C: 0.01-0.20%
Si: 1.04 to 1.52 %,
Mn: 0.05 to 2.0%,
Cr: 0.25 to 2.52 %,
Mo: 0.01 to 2.0%,
O: 0.001 to 0.3%,
Al: 0.01% or less,
N: 0.001 to 0.1%
Nb: 0.001 to 0.5%,
V: 0.02 to 1.0%,
B: 0.0002 to 0.02%,
W: 0.01-4.0%,
Cu: 0.1 to 2.0%,
Ni: 0.1 to 2.0%,
Co: 0.1 to 2.0%,
Ti: 0.001 to 0.05%,
Mg: 0.0002 to 0.05%,
1 type or 2 types of
P: 0.030% or less,
S: 0.010% or less,
The mass ratio of Si, Mn and Cr is 0.357 to 1.042 in terms of (Si%) / ((Mn%) + (Cr%)), with the balance being Fe and inevitable impurities. A method of manufacturing a ferritic ERW boiler steel pipe excellent in reheat cracking resistance of a welded portion, characterized by tempering a hot-rolled steel plate, pipe-forming it into a cylindrical shape, and subjecting the butt portion to ERW welding. Inert gas from the weld top and bottom surfaces in the weld vicinity during electric resistance welding, nitrogen gas, according to claim 3 Symbol placement of resistance welding, wherein the blowing non-oxygen-containing gas consisting of one or more of hydrogen gas Ferritic ERW boiler steel pipe manufacturing method with excellent reheat cracking. In addition to the manufacturing method according to claim 3 or 4, the heat treatment of quenching-tempering or tempering-tempering is further performed to produce a ferritic ERW boiler steel pipe excellent in reheat cracking resistance of welded parts Law.