JP5206676B2 - Ferritic heat resistant steel - Google Patents

Description

  The present invention relates to a ferritic heat resistant steel that is excellent in high temperature strength and weld crack resistance of a weld heat affected zone, which is used for a member used at high temperatures such as a thermal power generation boiler.

  In recent years, in thermal power generation, high-temperature and high-pressure steam conditions have been promoted in order to increase thermal efficiency. In the future, operation under ultra-supercritical conditions of 650 ° C. and 350 atm is planned. Ferritic heat resistant steels are widely used because they are less expensive than austenitic stainless steels and have the advantage of heat resistant steels having a low coefficient of thermal expansion.

Ferritic heat-resisting steels are being strengthened to cope with severe future steam conditions. For example, Patent Document 1 and Patent Document 2 propose that the contents of W and Mo are optimized and that Co and B are contained. Patent Document 3 proposes a steel that utilizes the strengthening by a fine intermetallic compound phase by adding W and Mo. Patent Document 4 proposes a steel having high strength utilizing M ₂₃ C ₆ carbides and intermetallic compound phases precipitated at the martensitic lath interface.

  However, when these ferritic heat resistant steels are used as a welded structure, for example, as shown in Non-Patent Document 1, the creep strength is low in a weld heat affected zone (hereinafter referred to as HAZ) that has undergone a thermal cycle by welding. It may be greatly reduced. Therefore, there exists a problem that the advantage of steel which aimed at strengthening cannot fully be utilized. Therefore, not only steel but also steel aimed at improving the creep strength of HAZ subjected to welding heat cycle has been proposed.

  For example, Patent Document 5 generates Ti, Zr, and Hf-based nitrides that are stable against welding heat input, and Patent Document 6 adds W to (Nb, Ta) carbonitrides. In addition, in Patent Document 7 and Patent Document 8, the formation of Cr carbide is suppressed, and the long-term stability of carbonitrides such as fine V and Nb is improved. Steels with improved long term creep strength are disclosed. As described above, various HAZ strength improvement methods utilizing carbonitrides have been proposed, but further improvement in HAZ strength is desired from a practical aspect.

  Furthermore, Patent Document 9 proposes a method in which 0.003 to 0.03% B is contained to suppress the fine graining in the HAZ and to improve the creep strength in the HAZ. However, while B is known to be an element having such an effect, it is widely known that during welding, it is an element that enhances the susceptibility to solidification cracking of weld metal and liquefaction cracking of HAZ. Therefore, when it is used as a thick member such as a main steam pipe for a boiler or a pressure vessel, there is a problem that sufficient weldability (weld crack resistance) cannot be obtained.

JP-A-4-371551 JP-A-4-371552 JP 2001-152293 A JP 2002-241903 A JP-A-8-85848 JP-A-9-71845 JP 2001-279391 A JP 2002-69588 A JP 2004-300532 A Science and Technologyof Welding and Joining, 1996, Vol.1, No.1, p.36〜42

  Thus, ferritic heat-resistant steel has the advantage of having a low coefficient of thermal expansion in addition to being inexpensive, so it is used as a welded structure in thermal power generation boilers and the like where steam conditions are being increased at high temperatures and pressures. It is expected that

  As described above, various proposals have been made in order to further increase the strength and improve the creep strength of the HAZ of the welded joint so that it can be used even under high temperature and high pressure conditions. However, there is a problem that not only the strength of HAZ is increased, but also sufficient weld crack resistance during welding cannot be obtained.

  In view of such circumstances, an object of the present invention is to provide a ferritic heat resistant steel having excellent weld crack resistance of HAZ and excellent creep strength.

  In order to improve the creep strength of HAZ, it has become clear that it is effective to restrict Cr, Co, V, Nb to a predetermined range and add B. However, it has been clarified that, when an amount of B necessary for increasing the strength of HAZ is added, the cracking susceptibility of HAZ and weld metal increases, and there is a problem in weld crack resistance.

  Thus, it has been found that in order to improve the creep strength in HAZ and achieve both excellent weldability, the problem can be solved by optimizing the contents of C and B as follows. In the ferritic heat resistant steel according to the present invention, the target value is that the creep strength in the HAZ is a rupture time that is at least three times the rupture time of the general-purpose steel, and preferably a rupture time that is five times or more.

  As a result of investigation, a ferritic steel having a composition range of Cr: 7 to 13%, Co: 1 to 8%, V: 0.05 to 0.4% and Nb: 0.01 to 0.09%. In Table 1, it was confirmed that when B was contained, HAZ was strengthened.

The reason why the creep strength in the HAZ is lower than that of the base material is partly due to refinement by heating to a temperature between the Ac ₁ transformation point and the Ac ₃ transformation point by the welding heat cycle. When the ferrite phase (tempered martensite phase), which is the original structure, is heated to this temperature range, the fine graining is caused by newly nucleating and growing an austenite phase at the grain boundary. B is an element that easily segregates at the grain boundary. When heated to this temperature range, it segregates at the grain boundary of the original ferrite phase to reduce the energy of the grain boundary and suppress or delay the nucleation of the austenite phase. This prevents fine graining. As a result, it was considered that the creep strength in HAZ was improved.

  However, it has been found that when more than the necessary amount of B that can improve the creep strength is contained, the solidification cracking of the weld metal and the liquefaction cracking sensitivity of the HAZ increase.

  This is partly because B is an element that easily segregates at grain boundaries and at the same time an element that greatly reduces the melting point. In addition, S and P are also elements that are likely to segregate at the grain boundaries as well as B and greatly reduce the melting point. Therefore, in the HAZ closest to the melting line, P and S grain boundary segregations are superimposed on the B grain boundary segregation, resulting in grain boundary melting, opening due to thermal stress or external stress, and liquefaction cracking. It was.

  Solidification cracking of the weld metal can be prevented by adjusting the components of the weld material. On the other hand, liquefaction cracking of HAZ is a problem related to the composition of the steel used, and is a major limitation in practical use. In light of these problems, the present inventors have intensively investigated the requirements that enable prevention of liquefaction cracking of HAZ and increase the strength of HAZ.

  As a result of repeated studies, new knowledge was obtained that liquefaction cracking can be prevented only when the C content is defined within a predetermined range. And this reason was considered as follows.

  That is, C, like B, acts as a melting point lowering element, superimposes on the above-described melting point lowering action by B, and increases the liquefaction cracking sensitivity of HAZ. Therefore, it was considered that the decrease in the melting point can be reduced by reducing the C content according to the B content. And the solid alloy brittleness in the range of Cr: 7-13%, Co: 1-8%, V: 0.05-0.4%, Nb: 0.01-0.09% which is the basic alloy component of the present invention The upper limit of the content (%) of C that reduces the temperature range (BTR) to 100 ° C. or less, which can sufficiently prevent liquefaction cracking in practice, is (−5/3) × [% B] from thermodynamic theoretical calculation. +0.085 could be specified. Here, [% B] represents the B content (mass%) in the steel (the same applies hereinafter).

  In addition, C affects the free energy of formation of sulfides and vicinals by its interaction. That is, at high temperatures, the solubility of sulfides or phosphides such as Cr and Nd decreases with an increase in the C content, and when the C content further increases, the solubility tends to increase again. When the solubility of sulfide or phosphide increases, the amount of S or P segregated at the grain boundary due to the thermal effect of welding or the like increases, and the sensitivity to liquefaction cracking increases. Therefore, in the case of the C content within the range of the present invention in which the C content is reduced, the solubility of sulfide and phosphide is reduced, and a stable compound is formed. Along with this, S and P at the grain boundary decreased, and it was considered that liquefaction cracking of HAZ was prevented by a synergistic effect with the suppression of melting point lowering.

  Further, when the content of C is reduced after containing B, not only liquefaction cracking can be prevented, but also the creep strength of HAZ is further improved as compared with the case where only B is contained. I got new knowledge.

This is because when the C content is reduced to a predetermined range, the carbides present at the grain boundaries are reduced. Therefore, even when the austenite phase is heated to a temperature between the Ac ₁ transformation point and the Ac ₃ transformation point and the austenite phase is nucleated at the grain boundary, the pinning effect is small, so that the crystal grains are easily coarsened. As a result, the effect of suppressing grain refinement in the HAZ is increased due to the superposition effect with the suppression of nucleation due to the B content. Furthermore, since the growth rate of V and Nb fine carbonitrides in the grains that contribute to strengthening of ferritic steel is suppressed, the allowance for increasing the creep strength is larger than when only B is contained. It was considered.

  However, when the C content is extremely reduced, the amount of fine carbonitrides of V and Nb contributing to intragranular strengthening is small, and the strengthening effect cannot be obtained sufficiently, so the strength improving effect is small. It was thought to be. Therefore, the lower limit of the C content is set to 0.005% or more.

  From the results of these studies, in order to enable prevention of liquefaction cracking of HAZ and to improve the creep strength of HAZ, B: 0.005 to 0.025% and 0.005 ≦ C ≦ ( It was found that the condition of −5/3) × [% B] +0.085 must be satisfied.

  The present invention has been completed based on these new findings, and the gist of the ferritic heat resistant steel according to the present invention is as shown in the following (1) to (3). Hereinafter, these are referred to as the present inventions (1) to (4). These are collectively referred to as the present invention.

(1) By mass%, Si: more than 0.1% and 1.0% or less, Mn: 2.0% or less, Co: 1-8%, Cr: 7-13%, V: 0.05- 0.4%, Nb: 0.01 to 0.09%, one or both of Mo and W in total 0.5 to 4%, B: more than 0.01% and 0.025% or less , Al: 0.03% or less, N: 0.003 to 0.06% and Nd: 0.005 to 0.08%, including C in an amount satisfying the following formula (1), with the balance being Fe and impurities O, P, and S as impurities are O: 0.02% or less, P: 0.03% or less, and S: 0.02% or less, respectively. .
0.005 ≦ C ≦ (−5/3) × B + 0.085 (1)
Here, C and B show content (mass%) of each element.

(2) The high Cr ferritic heat-resisting steel according to (1) above, which contains, by mass%, Ta: 0.08% or less instead of part of Fe.

(3) The above (1), characterized in that it contains one or two of Ca: 0.02% or less and Mg: 0.02% or less in place of part of Fe by mass% Alternatively, the high Cr ferritic heat resistant steel according to (2) .

  The ferritic heat resistant steel according to the present invention is excellent in the HAZ weld crack resistance and has an excellent HAZ creep strength.

  Next, the reasons for limiting the steel of the present invention will be described. In addition,% display represents mass%.

C: 0.005% or more and {(−5/3) × [% B] +0.085}% or less
C together with B is an important element in the present invention. C is an essential element because it forms carbides, contributes to securing high-temperature strength and is effective in obtaining a martensite structure. However, if segregated at the grain boundaries, it will overlap with B, S, and P to promote a decrease in the melting point of the grain boundaries, and indirectly participate in the formation of sulfides and vicinals in the coarse grain HAZ, affecting the liquefaction cracking susceptibility. Give it. Further, when the content of C is reduced, the fine-grained HAZ has an effect of improving the creep strength due to the effect of promoting the coarsening of crystal grains at the time of transformation and suppressing the growth of fine carbides. Suppressing the lowering of the melting point of the grain boundary due to C itself and forming stable sulfides and phosphides with coarse-grained HAZ, reducing the lowering of the melting point due to segregation of S and P grain boundaries, and preventing liquefaction cracking In order to improve the creep strength of the fine-grained HAZ, as described later, the B content is specified in a specific range, and C is 0.005% or more, and {(−5/3) × [ % B] +0.085}% or less. The minimum with preferable C content is 0.010%.

Si: more than 0.1% and not more than 1.0% Si is contained as a deoxidizer in an amount exceeding 0.1%, but excessive addition causes a decrease in creep ductility and toughness. %. Desirably, it is 0.8% or less. More desirably, it is more than 0.2% and 0.7% or less.

Mn: 2.0% or less Mn is contained as a deoxidizing agent in the same manner as Si. However, when excessively contained, Mn causes creep embrittlement and a decrease in toughness. Therefore, it is set to 2.0% or less. Desirably, it is 1.8% or less. However, excessive reduction does not provide a sufficient deoxidizing effect and deteriorates the cleanliness of the steel, and increases the manufacturing cost. For this reason, although there is no particular lower limit, it is desirable to contain Mn 0.01% or more.

Co: 1-8%
Co is an austenite-forming element and is an element necessary for the martensite formation of the matrix. In order to acquire the effect, it is necessary to contain 1% or more. However, if the content exceeds 8%, the creep ductility is significantly reduced. Desirably, it exceeds 2% and is 7% or less.

Cr: 7-13%
Cr is an indispensable element in order to ensure oxidation resistance and high temperature corrosion resistance in a heat resistant steel and to stably obtain a martensitic structure of a matrix. In order to acquire the effect, it is necessary to make it contain 7% or more. However, if it is contained excessively, the stability of the carbide is reduced due to the generation of a large amount of Cr carbide, and the creep strength is lowered and the toughness is also deteriorated. Therefore, the Cr content needs to be 13% or less. is there. Desirably, it is 8 to 12%. More desirably, it is 8 to 10%.

V: 0.05-0.4%
V is an element that forms fine carbonitrides in the grains together with Nb and greatly contributes to the improvement of creep strength. In order to acquire the effect, it is necessary to make it contain at least 0.05% or more. However, when excessively contained, the growth rate of carbonitrides is increased, the dispersion strengthening effect disappears early, and the toughness is reduced, so the V content is 0.4% or less. There is a need. Desirably, it is 0.10 to 0.35%.

Nb: 0.01 to 0.09%
Nb is an element that forms fine carbonitride that is stable up to a high temperature in the grains together with V and contributes greatly to the improvement of creep strength. In order to obtain the effect, it is necessary to contain at least 0.01% or more. However, when excessively contained, the growth rate of carbonitrides is increased, the dispersion strengthening effect disappears early, and the toughness is reduced. Therefore, the Nb content is 0.09% or less. There is a need.

One or both of Mo and W: 0.5 to 4% (total)
Mo and W are elements that solid-solution strengthen the matrix and contribute to the improvement of creep strength. In order to acquire this effect, it is necessary to contain 0.5% or more of one or both of Mo and W in total. However, if it exceeds 4% and contains excessively, a coarse intermetallic compound will be produced | generated and the extreme fall of toughness will be caused. In addition, when making W contain independently, it is preferable that the minimum of content of W shall be 1%.

B: 0.005-0.025%
B is an important element in the present invention together with C. B segregates at grain boundaries in HAZ and lowers grain boundary energy, thereby delaying nucleation of the austenite phase and suppressing grain refinement. In order to sufficiently obtain the effect, it is necessary to contain at least 0.005% or more. However, in coarse-grained HAZ, grain boundary segregated B promotes a decrease in the melting point of the grain boundary and overlaps with the segregation of S and P to cause liquefaction cracks. In order to prevent this, it is necessary to define C within the aforementioned range. However, if the B content exceeds 0.025%, the effect of improving the creep strength of HAZ is saturated, and liquefaction cracking cannot be prevented even if the C content is specified in the above range. The lower limit of the B content is preferably 0.007% or more. A more desirable range is more than 0.01% and 0.02% or less.

N: 0.003-0.06%
N is an element that forms fine carbonitrides containing V and Nb and is effective in ensuring creep strength. In order to acquire the effect, it is necessary to make it contain 0.003% or more. However, if it is contained excessively, the amount of carbonitride deposited increases and causes embrittlement. Therefore, the upper limit of the N content is 0.06%.

Al: 0.03% or less Al is contained as a deoxidizing agent. However, if excessively contained, the creep ductility and toughness are reduced, so the upper limit is made 0.03%. Desirably, it is 0.02% or less. However, excessive reduction does not provide a sufficient deoxidizing effect and deteriorates the cleanliness of the steel, and increases the manufacturing cost. For this reason, although there is no particular lower limit, it is desirable to contain Al in an amount of 0.001% or more.

O: 0.02% or less O is present as an impurity, but when it is contained in a large amount, a large amount of oxide is generated, and workability and ductility are deteriorated. Therefore, the O content needs to be 0.02% or less.
P: 0.03% or less P is contained as an impurity, but segregates at grain boundaries in coarse-grained HAZ together with S and B, lowers the melting point, and causes liquefaction cracking. In order to prevent this, it is necessary to define C, Nb, S and B within a predetermined range and to make the P content 0.03% or less.

S: 0.02% or less S is contained as an impurity in the same manner as P, and segregates at the grain boundary in the coarse grain HAZ to lower the melting point and cause liquefaction cracking. In order to prevent this, it is necessary to define C, Nb, S and P within a predetermined range and to make the S content 0.02% or less.

  The steel of the present invention can contain a predetermined amount of the following elements as required.

Nd: 0.08% or less Nd has strong affinity with P and S, and forms a compound with S and P at the grain boundary of coarse HAZ, thereby suppressing a decrease in melting point due to S and P. In addition to preventing liquefaction cracking of HAZ, it is effective in reducing grain boundary embrittlement during use at high temperatures due to S and P and improving the creep ductility of HAZ. Good. However, since the affinity with oxygen is strong, if it is contained excessively, an excess oxide is generated and the toughness of the HAZ is reduced, so the upper limit is made 0.08%. A desirable upper limit is 0.06%. In addition, in order to acquire the said effect by containing Nd reliably, it is desirable to contain Nd 0.005% or more. More desirably, the content is 0.015% or more.

Ta: 0.08% or less Ta, like V and Nb, forms fine carbides that are stable up to a high temperature and greatly contributes to the improvement of creep strength. Therefore, Ta may be contained as necessary. However, when it is contained excessively, the growth rate of carbides is increased, so that the dispersion strengthening effect disappears early and the toughness is lowered. Therefore, the upper limit is made 0.08% or less. In addition, in order to acquire said effect by Ta containing, it is desirable to make it contain 0.005% or more.

Ca: 0.02% or less Ca is an element that improves the hot workability of steel, and can be contained when it is necessary to improve the hot workability. However, if its content exceeds 0.02%, inclusions become coarse and conversely, workability and toughness are impaired, so the upper limit is made 0.02%. In addition, in order to acquire said effect by Ca containing, it is desirable to make it contain 0.0003% or more. A more desirable range for the Ca content is 0.001 to 0.01%.

Mg: 0.02% or less Mg, like Ca, is an element that improves the hot workability of steel, and when it is necessary to improve the hot workability, Mg alone or Mg alone, It can be included. However, if its content exceeds 0.02%, inclusions become coarse and conversely, workability and toughness are impaired, so the upper limit is made 0.02%. In addition, in order to acquire said effect by Mg containing, it is desirable to make it contain 0.0003% or more. A more desirable range for the Ca content is 0.001 to 0.01%.

Sixteen kinds of steels having the chemical compositions shown in Table 1 were melted in a vacuum melting furnace, cast and rolled, held at 1150 ° C. for 1 hour, air-cooled normalizing, and 770 ° C. for 1.5 hours. After holding, heat treatment of air-cooled tempering was performed. In addition, the symbol 13 is steel equivalent to fire SUS410J3TB, which is a general-purpose steel, and was used as a comparative steel relating to creep strength. A steel plate having a plate thickness of 12 mm, a width of 50 mm and a length of 300 mm and a plate thickness of 10 mm, a width of 100 to 120 mm and a length of 300 to 500 mm were produced by machining. A steel plate having a thickness of 12 mm was subjected to the longibarestrain test to evaluate the liquefaction cracking susceptibility of HAZ. Incidentally, the symbols 9, 10 and 14 are reference examples.

  As shown schematically in Fig. 1, the Longi Ballest Train test performs bead-on-plate welding in the longitudinal direction of a steel plate by GTA welding, and applies a force F to the end during the welding to add strain due to bending. In this method, the cracking of HAZ is forcibly generated and the total length thereof is measured to evaluate the liquefaction cracking sensitivity of HAZ. The welding conditions were 200 A × 15 V × 10 cm / min, the amount of added strain was 4%, and the one in which liquefaction cracking did not occur in HAZ was considered acceptable.

  For a steel type in which liquefaction cracking did not occur in the HAZ, a test material having a thickness of 10 mm, a width of 10 mm and a length of 100 mm was taken from a 10 mm thick steel sheet, and the temperature was increased to 1000 ° C., which is a particularly remarkable temperature at which HAZ strength decreases. A HAZ reproducible welding heat cycle was applied that was heated for 2 seconds. Thereafter, the test material was subjected to heat treatment after welding at 740 ° C. for 30 minutes and air cooling, and a creep test piece was collected, and a creep test was performed under conditions of a temperature of 650 ° C. and a stress of 117.7 MPa.

  Table 2 shows the weld crack length (mm) in the longibarestrain test and the rupture time (hr) in the creep test.

As is apparent from Table 2, the materials of the alternatives 3-6, 11 and 15 in which the contents of C and B satisfy the specified range of the present invention and the formula (1) are as in the Longibarestrain test. Even in a severe cracking test, HAZ liquefaction cracking did not occur, and the HAZ creep rupture time was more than three times the break time of the symbol 13. In particular, the materials of the symbols 3 to 5, 11 and 15 had a HAZ creep rupture time of 5 times or more that of the symbol 13.

However , in the materials of the symbols 1, 2 and 16 where the C content exceeds the upper limit of the formula (1), the melting point of the grain boundary of the coarse HAZ is remarkably lowered, and liquefaction cracking occurred in the HAZ in the longibarestrain test .

  On the other hand, although the materials of the symbols 7, 8 and 12 did not cause liquefaction cracking in the HAZ in the longibarestrain test, none of the HAZ creep rupture time satisfied the target value.

  That is, the content of B is within the specified range of the present invention, but the mark 12 in which the content of C is less than the lower limit of the expression (1) did not satisfy the target value of the creep rupture time of HAZ. On the other hand, the material of the symbol 8 whose B content is less than the specified range of the present invention does not satisfy the target value of the HAZ creep rupture time although the C content satisfies the formula (1). It was. Further, in addition to the content of B being less than the range of the present invention, the material of surrogate 7 in which the content of C exceeds the upper limit of the formula (1), the creep rupture time of HAZ compared to surrogate 8 It was even lower.

  From the above results, it can be seen that a material having a chemical component satisfying the scope of the present invention has excellent liquefaction cracking resistance and creep strength in HAZ.

  Since the ferritic heat resistant steel according to the present invention provides ferritic heat resistant steel having excellent weld cracking resistance and creep strength of HAZ, it is a welded structure in a thermal power generation boiler and the like where steam conditions are being increased at high temperature and pressure. Can be used as

The Longi Ballest Train test method is shown.

Claims (3)

In mass%, Si: more than 0.1% and 1.0% or less, Mn: 2.0% or less, Co: 1-8%, Cr: 7-13%, V: 0.05-0.4 %, Nb: 0.01 to 0.09%, one or both of Mo and W in total 0.5 to 4%, B: more than 0.01% and 0.025% or less , Al: 0.03 %, N: 0.003 to 0.06% and Nd: 0.005 to 0.08%, C is contained in an amount satisfying the following formula (1), and the balance consists of Fe and impurities, O, P, and S as impurities are O: 0.02% or less, P: 0.03% or less, and S: 0.02% or less, respectively.
0.005 ≦ C ≦ (−5/3) × B + 0.085 (1)
Here, C and B show content (mass%) of each element.   2. The high Cr ferritic heat resistant steel according to claim 1, wherein, in mass%, Ta: 0.08% or less is included instead of a part of Fe. The mass% includes one or two of Ca: 0.02% or less and Mg: 0.02% or less in place of part of Fe. High Cr ferritic heat resistant steel.

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