JPH045741B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a steel for high-temperature and high-pressure containers that has excellent high-temperature strength and toughness. [Prior Art] Steels that have higher high-temperature strength than carbon steel and are lower in cost than Cr-Mo steel have been widely used in high-temperature, high-pressure vessels such as boilers as steels for high temperatures exceeding 300°C. The problem with this type of steel is that a large amount of C is added to ensure high-temperature strength, and especially in extremely thick steel plates with a thickness exceeding 100 mm, the upper limit of the Mn standard is low, and the ASTM , 0.25 for A204 steel
%, and A302 with increased Mn content and added Ni
Even steel has an added amount of C close to 0.20%. As is generally well known, addition of such a large amount of C causes deterioration of weldability and toughness. The inventors have developed a steel that has improved these problems.
Steels with a combination of Cr, Cu, Ni, Nb, and V additions and low carbon content were disclosed in JP-A-59-153866 and JP-A-59-153867. However, depending on the thickness of the plate, it is difficult to achieve an extremely low carbon content in order to ensure strength, and there is a problem in ensuring toughness, especially when the plate is extremely thick and accelerated cooling after normalization cannot be performed. . Recently, boilers have become increasingly hot and pressurized, and as a result, it has become desirable to improve the toughness and weldability of the steel materials used, as well as to further increase their high-temperature strength. In addition to the above-mentioned improvements, it is necessary to establish a new composition system for steel that satisfies these requirements. [Problems to be Solved by the Invention] The present invention is intended to solve the above-mentioned problems of the prior art. The object of the present invention is to provide a steel for high-temperature and high-pressure vessels that can significantly improve toughness and high-temperature strength without deteriorating weldability by adding C. [Means for Solving the Problems] In order to solve the above-mentioned problems, the present invention provides a thick steel material with a plate thickness exceeding 100 mm that has superior weldability compared to conventional steel, and also requires post-weld heat treatment. (Post
Welding Heat Treatment：PWHT or Stress
Relieving (hereinafter referred to as SR) is a steel with high high temperature strength and high toughness, and is constructed as follows. That is, the first invention steel has C0.07~0.20wt%, Si0.12~1.00wt%, Mn0.7~
1.8wt%, P0.020wt% or less, S0.010wt% or less
Mo0.35~0.70wt%, SoA0.020~0.05wt%,
Cr0.10~0.40wt%, V0.01~0.02wt%, and 2P+S≦[0.07−0.05(Cr+10V)]wt%, preferably [2P+S]0.012wt% or less, and
It is a high-temperature, high-pressure vessel steel with excellent high-temperature strength and SR cracking resistance, consisting of basic components of 0.0003 to 0.0015wt% B, 40ppm or less of T.N, and the balance Fe and unavoidable impurities. Furthermore, the second to fourth invention steels each contain the following alloying elements in addition to the basic components of the first invention, with the balance consisting of Fe and inevitable impurities, and have high temperature strength and SR cracking resistance. (reheat cracking resistance)
This is a high-temperature, high-pressure container steel with excellent toughness. That is, the second invention steel contains 0.003 to 0.05wt% Nb, and the third invention steel contains 0.05 to 1.00wt% Ni and 0.05 to 0.05wt% Cu.
The fourth invention steel contains one or two of 0.30wt%, Ni0.05~1.00wt%, Cu0.05~
One or two of 0.30wt% and Nb0.003~
It is characterized by containing 0.05wt%. [Function] The present invention has high temperature strength and resistance with the composition as described above.
It is a high-toughness steel for high-temperature and high-pressure vessels with excellent SR cracking resistance, but as a guideline for high-temperature strength and toughness in this case, the tempering parameter after PWHT, that is, TP = T × (ogt + 20) T: Tempering temperature 〓 t: (Tensile strength at 400°C: TS≧50kgf/mm ² , 50% toughness fracture surface transition temperature: vTs≦−20°C within the range of time h of 20.0×10 ³ or less. Furthermore, weldability is based on the premise that SR cracking does not occur. The characteristics of the steel of the present invention are 0.5Mo steel with Cr content of 0.10 to 0.40%
By adding a small amount of 0.01 to 0.02% of V, toughness can be improved.
The strength was improved, and the addition of a small amount of B significantly improved the hardenability.
It also stipulates that the amount of nitrogen must be controlled to a low level of 40 ppm. Furthermore, in order to improve the SR cracking susceptibility by adding Cr and V, the impurity elements S and P are regulated according to the amounts of Cr and V added. The effects of these various alloying elements make it possible to increase the strength, reduce the required amount of C, and improve toughness and weldability. Next, the process that led to the present invention will be described with reference to figures. Figure 1 shows preliminary results obtained by the present inventors in the course of the present invention.
- In Mo steel and Mn-Ni-Mo steel, Cr (0.4% max), V (0.3%
%max) and 400 in case of B (2-10ppm) addition
It is a graph showing the relationship between tensile strength TS and fracture surface transition temperature vTs at °C. In Figure 1, ○ indicates basic component steel, ◇ indicates
Single addition of Ni, △ means addition of 0.1 to 0.4% Cr, □ means Cr
and Ni addition, 〓 is Cr-V (0.01~0.02%), 〓 is Cr and
Addition of Ni and V; * indicates addition of B. Here, the base components of Mn-Mo steel and Mn-Ni-Mo steel are: C: 0.08-0.20%, Si: 0.15-0.45%, Mn: 0.8
~1.2%, P: 0.010%, S: 0.005%, Ni: 0~
0.8%, Mo: 0.4-0.6%, SolA: 0.01-0.06%
It is. The heat treatment conditions are 900°C x 2hN, 650°C x 2hT, then 650°C x 30h.SR when 50mm thick steel is N: normalized and T: tempered. In the figure, the balance between high-temperature strength and toughness is shown by bands, and bands are used for Mn-Mo steel and
The base components of Mn-Ni-Mo steel are: Bandoro is the base component with the addition of Cr, Bando is the Bandoro component with V further added, and Bandoni is the Bandoro component with the addition of B. It is something. As is clear from FIG. 1, the balance between high-temperature strength and toughness is improved in the following order: Bandoy → Ro → Ha → Ni. That is, in Mn-Mo steel and Mn-Ni-Mo steel with 0.5% Mo as a basic component, it is shown that the higher the Cr addition, the higher the high temperature strength and the higher toughness. Moreover, the combined addition of Cr and 0.01 to 0.02% of V further improves the balance between high temperature strength and toughness. However, although the addition of 0.03% V is effective in increasing high-temperature strength, the toughness deteriorates, so we found that it is preferable to add it in a small amount. By adding B to such a Cr-V composite additive material, the hardenability, that is, the structure, is improved, and both the high temperature strength and toughness are further improved. In particular, it has been confirmed that the thicker the plate, the greater the effect. Here, in order to secure solid solution B, which is effective in improving hardenability, by adding A, BN+A
→The reaction of B + AN needs to proceed while being maintained at a normalization temperature above the A _c3 point, and while the amount of SoA added is important, it is also effective to reduce the amount of N itself. In the present invention, in order to improve high temperature strength and toughness,
This assumes the addition of Cr-V composite. However, as is generally known, Cr and V are SR
It is an element that promotes intergranular cracking (SR cracking) in the weld heat affected zone through precipitation hardening during (PWHT).
Therefore, in the case of a composition system based on the addition of Cr-V, consideration must be given to SR cracking. Figure 2 shows 2P on the vertical axis in the example described later.
It is an explanatory diagram in which the effects of Cr, V, P, and S amounts on the SR cracking characteristics were investigated, with +S% as the horizontal axis and Cr+10V%. As is clear from Fig. 2, increasing the amount of Cr and V added increases the SR cracking susceptibility.
It can be organized relatively well. In the range of Cr+10V<0.6%, even if the addition of Cr and V is increased, SR cracking can be suppressed by reducing the amounts of P and S, which are grain boundary segregation elements. In this case, 2P+S can be arranged relatively well.
In addition, the ● mark in the figure indicates that cracking occurred even in the SR cracking test using the WES3005 type, which is relatively loosely constrained;
Although cracks did not occur in the formula, cracks did occur in the SR cracking test using JIS diagonal y-type specimens, which are subject to severe constraints.
Indicates that no cracks occurred in any of the models. Considering the actual structure, the diagonal y type constraint is too strict, so it would be better to evaluate using the WES type. In other words, when evaluated using the WES formula, 2P+S≦0.07−0.05 (Cr+
It was found that if the impurities were reduced to 10V)%, SR cracking would not occur. Diagonal y as a more severe condition
If evaluated using the formula, it means that by suppressing 2P+S to 0.012%, there is no need to worry about SR cracking at all. The reasons for limiting the components in the steel of the present invention will be described. C: 0.07wt% or more is required to ensure high temperature strength. However, if it is too large, not only the toughness but also the weldability (especially cold cracking resistance) deteriorates, so the upper limit was set at 0.20 wt%. however,
0.11 to have good high temperature strength and weldability
It is desirable to control it within a narrower range of ~0.16wt%. Si: An element necessary as a deoxidizing agent and an element necessary to ensure high-temperature strength, but if it is less than 0.12wt%, it has no effect, and on the other hand, if it is added in excess of 1.00wt%, it will reduce the toughness of the base material. The range was set to 0.12 to 1.00wt% to cause deterioration. Mn: An element necessary to improve hardenability, strength, and toughness. If it is less than 0.7 wt%, it will not be possible to satisfy the specified strength and the toughness will be low. On the other hand, addition of more than 1.8wt% greatly inhibits weldability and significantly increases the hardness of the weld heat affected zone, so the range is limited to 0.8~1.8wt%.
%. P, S: Elements that are unavoidably contained as impurity elements, but they not only deteriorate toughness but also segregate at grain boundaries and directly cause SR cracking, so their upper limits are set at 0.020wt% and 0.010wt%, respectively. And so. In addition, as mentioned above, depending on the amount of Cr-V added, 2P
+S≦[0.07−0.05(Cr+10V)]wt% The value of 2P+S is desirably controlled to 0.012wt% or less. Mo: For steel materials used at high temperatures, graphitization prevention,
Mo is added to ensure high-temperature strength, but if it is less than 0.35 wt%, its effect is small.
On the other hand, addition of more than 0.70 wt% inhibits weldability and reduces the toughness of welded joints, so 0.35 wt%
The range was set to ~0.70wt%. SoA: It is effective in fixing solid solution N and securing solid solution B necessary for improving toughness and hardenability by refining the structure, but if it is less than 0.020wt%, the effect is small. On the other hand, if it is added in excess of 0.050wt%, it will cause a decrease in ductility during hot working.
The range was 0.020 to 0.050wt%. Cr: As mentioned earlier, Cr is an essential element for improving the strength and toughness balance, but the effect is small at 0.1wt%, and adding more than 0.40wt% increases manufacturing costs and SR cracking susceptibility. , in the range of 0.10 to 0.40wt%. V: V is an effective element for increasing high-temperature strength without deteriorating toughness, but if it is less than 0.01wt%, the increase in strength is small, and if it is added more than 0.02wt%, like Cr, it increases SR cracking susceptibility. amount added
The range was 0.01-0.02wt%. B: B is an element that improves hardenability in small amounts, and is effective in terms of ensuring high temperature strength and toughness.
Adding less than 0.0003wt% has no effect on improving hardenability, while adding more than 0.0015wt% causes deterioration of weldability.
%. TN: Since N has a strong affinity with B or A, it forms a nitride. When BN is formed, the hardenability improving effect of B is lost, so it is important to reduce the amount of N in addition to adding A. Therefore, in order to improve hardenability, TN was limited to 40ppm. If further improvement in high-temperature strength and toughness is required for the above component system, use one or two of the solid solution strengthening elements Ni and Cu, and precipitation strengthening elements Nb or Nb alone within the following ranges. may be added. Ni: As a solid solution strengthening element, it is an effective element for increasing high temperature strength and toughness.
If it is less than 0.05 wt%, the effect will be small, and on the other hand, adding a large amount will impair economic efficiency, so it is set in the range of 0.05 to 1.00 wt%. Cu: Similar to Ni, it is an element that is effective in solid solution to increase high-temperature strength, but if it is less than 0.05wt%, the effect is small, and on the other hand, if it is added in large amounts, it may cause surface flaws during steel plate manufacturing and heat generation during container manufacturing. This will cause cracks during machining.
The range was 0.05 to 0.30wt%. Nb: Forms fine carbonitrides, which improves toughness by refining the structure and improves high-temperature strength by precipitation strengthening. Addition of less than 0.003wt% has no effect, while addition of more than 0.05wt% 0.003~ because it impairs weldability and deteriorates toughness.
The range was 0.05wt%. Note that the addition of sulfide form control elements such as Ca, Mg, REM, and Zr does not impair the present invention, so they may be added as necessary. The steel of the present invention is suitable not only for boilers but also as steel for pressure vessels such as chemical plants.It is heated to a temperature above the A _c3 point, hot worked, and then air-cooled or water-cooled (in some cases, the steel is further heated to a temperature above the A _c3 point). After reheating, air cooling or water cooling), then tempering below A _c1 point,
It is used by welding and SR (PWHT), and conventional methods are used for manufacturing from melting to assembly. Next, examples of the present invention will be described. [Example] Table 1 shows the chemical composition (wt%) of the sample material.
Table 2 shows the results regarding the mechanical properties and weldability of the test materials. From these Tables 1 and 2, comparative materials 1 to 4 are as follows:
Tensile strength at 400℃ TSat400℃≧50k
gf/mm ² is not satisfied. Also, comparative material 5.6
has high SR cracking susceptibility because the P and S contents are high relative to the amounts of Cr and V added. Comparative material 7 is P, S
Although the content is low to some extent, the susceptibility to SR cracking is high due to the large amounts of V and Cr added.In contrast, inventive steels 8 to 25 have excellent strength and toughness.
SR cracking susceptibility and low temperature cracking susceptibility are also low. In particular, invention steels 16 to 22 have 2P+S≦0.012%, so SR cracking does not occur even under severe restraint conditions. Inventive steels 18 to 21 have almost the same components other than C, and have the same plate thickness and heat treatment. Here, invention steel 21 with high C has high strength but slightly high cold cracking susceptibility, whereas invention steel 20 with low C has low cold cracking susceptibility but slightly low strength. 19 not only has excellent toughness and SR cracking resistance, but also has an excellent balance of strength and cold cracking resistance. Therefore, C:
Although the specified characteristics can be obtained at 0.07 to 0.20%,
C: 0.11 to 0.16 is a more desirable component range. The heat treatment symbols and test pieces in Table 2 are as follows. N (normalizing): 900 to 930℃ x 1 to 4h → air cooling Q (quenching): 〃 × 〃 → water cooling T (tempering): 620 to 700℃ x 0.5 to 5h → air cooling SR (stress relief annealing) :600~650℃×5~
40h → Furnace cooling TP (tempering parameter): T (20 + ogt) T: Holding temperature (k) t: Holding time (h) Test piece: Tensile JIS Z 2201 No. 4 Impact JIS Z 2202 No. 4 WES type SR cracking WES3005 40mm Diagonal Y type SR crack JISZ3158 40mm Diagonal Y crack (low temperature crack) 〃 〃 Figure 2 also shows the test piece (C0.08~0.20wt%
Si0.18~0.40wt% Mn0.8~1.0wt%, P0.003~
0.030wt% S0.001~0.012wt% Ni0~0.8wt%
Mo0.4~0.6wt% Cr0.1~0.5wt% V0.01~
0.35wt% SoA0.01~0.06wt% B0.0001~
The effects of Cr, V, P, and S on the SR crackability of a 40 mm thick specimen at 625°C x 2 h PWHT with a heat input of 22 kg/cm and 625°C x 2 h PWHT are shown below. Note that the numbers are the numbers of the steels in the examples.

【table】

【table】

【table】

[Table] − Not implemented ○ No cracks ● Cracks [Effects of the invention] The steel of the present invention has a low C
By reducing the impurities and adding trace amounts of Cr, V, and B, we have achieved significant improvements in toughness and high-temperature strength without deteriorating weldability, making it suitable for use not only in boilers but also as steel for high-temperature and high-pressure vessels. It is the most suitable steel for pressure vessels such as chemical plants.

[Brief explanation of drawings]

Figure 1 shows the effects of Cr on high temperature strength and toughness balance.
A graph showing the relationship between tensile strength TS and fracture surface transition temperature vTs showing the influence of Ni, V, and B. Figure 2 shows Cr, V (Cr+10V)% and P,
It is a relationship graph of S(2P+S)%.

Claims (1)

[Claims] 1 C0.07-0.20wt%, Si0.12-1.00wt%, Mn0.7
~1.8wt%, P0.020wt% or less, S0.010wt% or less,
Mo0.35~0.70wt%, SoA0.020~0.05wt%,
Cr0.10~0.40wt%, V0.01~0.02wt%, and 2P+
S≦[0.07−0.05(Cr+10V)]wt%, and
A high-toughness, high-temperature, high-pressure vessel steel with excellent high-temperature strength and SR cracking resistance, characterized by having B0.0003 to 0.0015wt%, T.N40ppm or less as its basic components, and the balance consisting of Fe and unavoidable impurities. 2 C0.07~0.20wt%, Si0.12~1.00wt%, Mn0.7
~1.8wt%, P0.020wt% or less, S0.010wt% or less,
Mo0.35~0.70wt%, SoA0.020~0.05wt%,
Cr0.10~0.40wt%, V0.01~0.02wt%, and 2P+
S≦[0.07−0.05(Cr+10V)]wt%, and
The basic components are B0.0003~0.0015wt%, T.N40ppm or less, and further contains Nb0.003~0.05wt%, and the balance is Fe and unavoidable impurities.It has high temperature strength and SR cracking resistance. Excellent toughness steel for high temperature and high pressure vessels. 3 C0.07~0.20wt%, Si0.12~1.00wt%, Mn0.7
~1.8wt%, P0.020wt% or less, S0.010wt% or less,
Mo0.35~0.70wt%, SoA0.020~0.05wt%,
Cr0.10~0.40wt%, V0.01~0.02wt%, and 2P+
S≦[0.07−0.05(Cr+10V)]wt%, and
The basic components are B0.0003~0.0015wt%, T.N40ppm or less, and Ni0.05~1.00wt%, Cu0.05~
Contains one or two of 0.30wt%, the balance being Fe
A high-toughness, high-temperature, high-pressure vessel steel with excellent high-temperature strength and SR cracking resistance, which is characterized by containing unavoidable impurities. 4 C0.07~0.20wt%, Si0.12~1.00wt%, Mn0.7
~1.8wt%, P0.020wt% or less, S0.010wt% or less,
Mo0.35~0.70wt%, SoA0.020~0.05wt%,
Cr0.10~0.40wt%, V0.01~0.02wt%, and 2P+
S≦[0.07−0.05(Cr+10V)]wt%, and
The basic components are B0.0003~0.0015wt%, T.N40ppm or less, and Ni0.05~1.00wt%, Cu0.05~
One or two of 0.30wt% and Nb0.003~
A high-toughness, high-temperature, high-pressure vessel steel with excellent high-temperature strength and SR cracking resistance, containing 0.05wt% with the balance consisting of Fe and unavoidable impurities.