JPS59153867A - Steel for high toughness pressure vessel having excellent weldability - Google Patents

Abstract

PURPOSE:To provide a steel for a high toughness pressure vessel whch has excellent weldability and workability and has substantial strength and high toughness even when subjected to working or a post weld heat treatment by readucing an amt. of C and adding Cr and Nb/V in combination to a basic component system of an Mn-(Ni)-Mo steel. CONSTITUTION:A steel for a high toughness pressure vessel contg., by weight, 0.07-0.20% C, 0.10-0.50% Si, 0.90-1.60% Mn, 0.05-0.40% Cr, 0.40-0.65% Mo, <=0.025% P, <=0.006% S, and 0.005-0.050% SolAl, and contg. 0.005-0.07% V or/and 0.003-0.05% Nb. The steel has a positively improved weldability and toughness after post-weld heat treatment (PWHT) while having high strength and has an excellent high temp. characteristic and toughness.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the creation of a high-toughness steel for pressure vessels with excellent weldability. This is an attempt to improve weldability and toughness after post-weld heat treatment while maintaining high strength by adding a composite material.

Conventionally, pressure vessels designed around 350℃, such as ammonia converters and methanol converters,
Mn-(Ni)-Mo steel is widely used, and since it has excellent high-temperature properties, it is also used as steel for dealers. By the way, this Mn −(N
i) M.

It is standardized as A302 steel in the steel ASTM standard,
It is divided into four steel types: grades AXB, C, and D. Grades A and B are FiNi-free, grade C is 0.4 to 0.7 wt% Ni, and grade D is 0.7 to 1.
, Q-wt%Ni addition is specified, and the strength level of grades B, C, and D (these grades B to D are at the same level) is higher than that of grade A, and this steel type is a high-temperature steel. ASTM A2 widely used as
In order to improve the high temperature strength, creep properties and toughness of 04 steel (0.5 wt% MO steel), increasing the amount of Mn and increasing the amount of Ni
was added. However, in these steel types, a large amount of EC is added to ensure high-temperature strength, and especially in thick steel plates such as those with a thickness of 1'00y+m or more, A30
In the case of KO steel, the amount of C added is close to 20 wt %, so it has some problems. That is, as is generally well known, adding a large amount of C as described above leads to deterioration of weldability and workability, and it is necessary to increase the preheating temperature to prevent the occurrence of cold cracks during welding, and to prevent the occurrence of cold cracks during gas cutting. In order to prevent cracking, it is necessary to use warm cutting. Another major problem with this high C system is that it is difficult to ensure toughness, and pressure vessels generally undergo post-weld heat treatment (Po5) several times during construction.
t Weld HeatTreatment: Below P
However, it is difficult to ensure sufficient toughness due to the coarsening of carbides that occurs during the PWHT process. Grades C and D of the A302 steel have Ni
It is supplemented by K and has been applied as thick-walled steel for reactor pressure vessels, but even if Nl
It goes without saying that even this type of steel containing '6 is required to have excellent toughness since it is used in containers that require particularly strict safety.

The present invention was devised after repeated studies in view of the above-mentioned circumstances, and has excellent weldability and workability in thick-walled Mn-(Nl)-Mo steel, as well as processability, P, WHT. We succeeded in obtaining a steel that still has sufficient strength and has higher toughness than conventional steel. That is, the product according to the present invention is wt% (hereinafter simply referred to as %): (1) C: 0.07 to 0.20%, st: O
,io ~0.50%, Mausoleum exceeds 0.90%, 1.
60% or less, P: 0.025% or less, S: 0.006 or less, Mo: 0.40 to 0.65%, t+a11.
Al: 0.005-0.050%, Cr: 0.0
Contains 5 to 0.40 Ti and Nb: 0.003 to 0
．． A high-toughness steel for pressure vessels with excellent weldability, containing one or two of 0005 to 0.07%, the balance being iron and unavoidable impurities.

(2) C: 0.07-0.20%, SL: 0.10
~0.50%, Mn exceeds 0.90% and is 1.60% or less, P: 0.025 or less, S: 0.006 or less, M
o: 0.40-0.65%, 5o11. Al: 0.
005-0.050%, Cr: 005-0.40%, Nb: 0.003-0.05%, V:
Contains 0.005 to 0.07% of one or two types, further Cu: 0.05 to 0.30, Ni: 0.05 to 1
．． 00%, B: 0.0002 to 0.002% of one or more types, and the balance is iron and inevitable impurities. High toughness steel for pressure vessels with excellent weldability.

However, in the present invention, it is particularly difficult to obtain strength and toughness, and PWHT
The conditions are strict, and the target is plates with a thickness of 100 mm or more.

As a guideline for strength and toughness in the case of Naoto, the tempering parameter after PWHT [T, P, =T (log t
+20 ), where T is the processing temperature in TWHT (unit: ℃ + 273), t is processing time (in hr) at the PWHT) is within the range of 19.5XIQ" or less, and in terms of strength, it meets the above-mentioned A302 standard. In terms of toughness value, VTII≦0°C.For T in the tempering parameters T and P, if post-standard tempering is performed during steel plate manufacturing, only PWHT is required. T, P, in the tempering process rather than
Of course, we must also consider the T and P values, and just to be sure, if the T, P and values are 1.
9.5X10'' means, for example, that no tempering treatment is performed, and when a PWHT temperature of 600 to 650° C. is adopted, the treatment is performed at that temperature for approximately 13 to 217 hours.

The features of the present invention as described above are Mn −0,
5Mo steel and Mn-Ni-Q, 5Mo steel with C
Add a small amount of r within the range of 0.05 to 0.4%, and further add 0.
．． 003chi≦Nb≦0.05chi, 0.005chi≦■≦0
．． The effect of such a composite addition is to effectively utilize the phenomenon that strength increases without deterioration of toughness by adding either one or both of 0.07% or both, and the effect of such composite addition is to obtain the same strength as a result. This makes it possible to reduce the amount of C required for this purpose, resulting in improved weldability and workability as well as higher toughness.

That is, Fig. 1ic, Kn-0,5Mo steel and Mn
-N10,S The influence of single addition and combined addition of Cr and Ni on the strength and toughness balance of Mo steel is shown. In this Figure 1, △ indicates single addition of Ni,
Cr is added alone, ★ is Nl and Cr
Composite addition of . indicates the basic component, Ni and Cr
It is clear from Fig. 1 that when the PWHT conditions are at this level, KnO, SMo steel (the ASTM A302 grade AXB) ), the addition of Ni or Cr alone brings about high strength and high toughness, and furthermore, the combined addition of Ni and Cr brings about even higher strength and high toughness.

Also un-NI 0. S Mo steel [Above ASTM
This shows that even in A302 grade CXD (containing 04 to 1.0% Ni) grade steel), high strength and high toughness can be obtained by further adding Cr.

On the other hand, in FIG. 2, Mn-NI 0. SMo
NbX Cr and Cr-Nb when based on steel
When T and P are large, that is, when subjected to severe PWHT treatment as shown in the middle row of Fig. 2, when ij:cr is added alone and when Nb is added alone, the toughness is lower than that of Cr. It is clear that the combined addition of Cr and Nb is effective in improving the balance of strength and toughness, whereas it is at the same level as -free pace steel. In particular, when T and P exceed 20X10', as shown in the lower part of the figure, lowering C is as effective as adding Cr-Nb in combination to increase toughness, which indicates that the present invention It is clear that the steel exhibits an excellent strength-toughness balance.

As is clear from FIG. 3, the effect of Cr tends to be saturated when it exceeds 0.4%, and the effect is not observed when it is less than 0.05%.

However, the effect of Cr is explained by K. that the addition of Cr improves hardenability and solid solution of Cr into carbide (cementite) prevents the coarsening of carbide due to SR.

The reason for limiting the range of each element in the present invention is as follows.

C is Mn 0. 0.07% or more is required to satisfy the strength standards of S Mo steel (A302AXB) and Mn-Ni-0,5Mo steel (A302CXD),
Further, the upper limit is 0.20% from the viewpoint of ensuring low-temperature toughness, and is set at 0.07 to 0.20%. A particularly preferable range is 0
．． 07% to 0.17%. Figure 4 shows several types of Mn.
-9,5Mo steel and Mn -Ni 0.5Mo steel, the changes in toughness associated with the tempering parameter trap are shown, and as is clear from this figure, the combined addition of KCr and Nb/V and simultaneous KC≦
This shows that the condition of 0.20 inch is effective in securing low temperature toughness.

SL is an element necessary as a deoxidizing agent, and is also a necessary element to prevent high-temperature oxidation and ensure high-temperature strength.
．． If it is less than 10%, there is no effect, and if it is added in excess of 0.5%, the toughness of the base material deteriorates, so the range is set to 0.10 to 0.5%.

Mn iJ: is an element necessary to improve hardenability, as well as strength and toughness. If it is less than 0.9%, the strength of the specification cannot be satisfied and low toughness is exhibited. On the other hand, if it exceeds 1.60%, weldability will be significantly impaired and the hardness of the weld heat affected zone will increase significantly, so the range should be reduced to 0.90%.
~1.60%.

P is an element that is inevitably contained as an impurity element, but in order to promote SR embrittlement and temper embrittlement during use of the container, its upper limit is set to 0.025%.

S is an element that is unavoidably contained as an impurity element, but it is often applied especially to thick-walled materials because it causes deterioration of toughness, promotes material anisotropy, and deteriorates properties in the thickness direction. In the steel of the present invention, the upper limit is set to 0.006%.

Mo is added to steel materials used at high temperatures from the viewpoint of preventing graphitization, ensuring high-temperature strength, and preventing hydrogen corrosion, but if it is less than 0.40%, it has no effect, and it also prevents quenching. This leads to a decrease in strength and toughness due to a decrease in elasticity. On the other hand 0,6
Addition of more than 5% inhibits weldability and reduces the toughness of the weld joint, so the range is set to 0.40 to 0.65%.

Gol and Al fix solid solution N and improve toughness by refining the structure. Addition of less than 0.05% causes grain coarsening, while addition of more than 0.050% causes a decrease in ductility during hot working, so the range was set at 0.005 to 0.050%. .

Regarding Cr, it is an important element in the present invention, and as described above with reference to FIG. % as the lower limit and 0.4% as the upper limit.

Nb%V is an important addition element like Cr among the elements of the present invention, and it forms fine carbides, strengthens the structure by making it finer, and strengthens it by precipitation of carbonitrides at room temperature and high temperature. This results in improved strength. Further, the formation of these carbonitrides has the effect of suppressing the coarsening of cementite accompanying the SR treatment, and the effect is achieved when Nb is 0.003% or more and ▪ is 0.005% or more. However, adding more than 0.05% of Nb and more than 0.07% of V causes toughness deterioration and at the same time promotes the occurrence of SR cracking, so 0.003%≦Nb≦0
．． 0.05 inch and 0.005%≦V≦0.07 inch.

Note that the addition of sulfide form control elements such as Ca, REM, and Zr does not impair the present invention as described above, so one or more of them can be added as appropriate. .

In contrast to the above-mentioned basic invention, the present invention further incorporates Cu.
XNi XB can be added, and the reasons for limiting the range of these are as follows.

As a solid solution strengthening element, Cu has a lower limit of 0.05%, and when it is contained above this level, it is effective in increasing strength, but if it is contained in a large amount, it may cause surface flaws during steel plate manufacturing and hot processing during container manufacturing. Since time cracking is induced, the K to prevent this is 0.3
The upper limit is %.

As shown in Figure 1, Ni is an effective element for providing high strength and high toughness, but if it is less than 0.05, the effect is not clear, and adding a large amount leaves problems in economic efficiency. . Therefore, in A203 grade A and B steels, it is contained within a range that does not exceed O54, and in grade C and D grade steels, it is contained within a range of 0.4 to 1.00% as specified.

B is an element that improves hardenability like Cr,
Using B in place of a part of Cr or adding both Cr and B is effective in obtaining high hardenability and ensuring strength and toughness; On the other hand, addition of more than 0.002% deteriorates weldability, that is, promotes the occurrence of cold cracking and SR cracking, so the content was set at 0.0002 to 0.002%.

Specific manufacturing examples according to the present invention are shown below together with comparative examples.

Table 1 below shows the composition systems and comparative examples of Mn O, S Mo steels. Steels A and B are conventional and modified Mn-0,5 steels that do not contain Cr; Steel C and below are steels of the present invention, including steel C, Cr-V series, steel,
E is Cu-Ni-Cr-V system and Cu-N
i -Cr-Nb system, steel F f″1Cr-Nb system, steel G
is a 0r-V system, and steel H is a Cr-Nb-B system. In addition, those marked with * in the table are 150 am accelerated cooling standard simulation materials, those with a thickness of less than 100 am were normalized, and those with a thickness of ↓ or more were subjected to accelerated cooling normalization.

A comparative example of the composition system for Mn-Ni-0,5Mo steel is shown in Table 2 below, and steel I is the conventional Mn-Nl 70. S Mo steel, steel J and below are steels of the present invention, steel JViCr-V-Nb system, steel K is Cr
-Nb system, steel is Cr-V system, steel M is Cu-Cr-V system, steel N is Cu-Cr-Nb system, and steel O is Cr-V-B system. Note that the * marked Mononoke 150 Dragon t accelerated cooling normalization simulation materials are the same as those in Table 1, with normal normalization applied to plates less than 100 mm in thickness, and accelerated cooling normalized applied to plates larger than 100 mm. It is.

However, the strength and toughness of the steels listed in Table 1 above after tempering-PWHT and the cracking prevention preheating temperature determined from the diagonal Y-type weld cracking test with 25 plate thicknesses are as shown in Table 3 below. be.

*T, P, = T(log t +20) T
: Temperature, K1 t: Time, h** Diagonal Y-type weld cracking test (in order of plate thickness 25) That is, as is clear from Table 3, conventional steels generally have a low toughness level, whereas the steel of the present invention has high strength. It exhibits excellent toughness while maintaining its properties. In particular, low C steels such as steel are Mn-Ni as shown in Table 4 below.
0.5 It shows the same or better toughness than Mo steel, and from the viewpoint of weldability, the crack prevention preheating temperature in the diagonal Y-type weld cracking test is 125℃ or less, 18.5
× Maximum hardness of heat affected zone after short PWHT treatment of about 103 Heat input 40 KJ/(7+1 welding conditions) HV≦2
40 and exhibits a major feature not found in conventional steel.

Similar relationships as in Table 3 for each steel shown in Table 2 are shown in Table 4 below.

Table 4* T, P, =T(j?ogt+20) T:
Temperature, K10 hours, h** Diagonal Y-type weld cracking test (plate thickness 25 mm), that is, according to Table 4, although the conventional steel (@I) also showed a degree of toughness improvement due to the addition of Nl, the steel of the present invention Then, the Charpy absorbed energy at 0℃ is 20kg.
- A high toughness of more than m has been obtained, and the improvement in toughness is remarkable. Furthermore, since the steel of the present invention has a low C content, the preheating temperature for preventing cracking in the diagonal Y-type weld cracking test is 25 to 50°C lower than that of the conventional steel, and the weldability is sufficiently improved.

According to the present invention as explained above, Mn −(N
i) Low C and Cr for the basic component system of -Mo steel
It is possible to provide various pressure vessels with excellent high-temperature properties and toughness by accurately improving weldability and toughness after PWHT while maintaining high strength, and with a composite addition of Nb and /V. This is a highly effective invention.

[Brief explanation of drawings]

The drawings show the technical contents of the present invention. Figure 1 is a diagram showing the influence of Ni and Cr on the balance of strength and toughness, and Figure 2 is a diagram showing the influence of Cr on the balance of strength and toughness.
Chart showing the influence of XNb and Cr-7qb, Part 3
The figure is a diagram showing the relationship of the influence of Cr on tensile strength.
FIG. 4 is a chart showing the change in toughness due to tempering and gradation, that is, the influence of C.弗 / En No. 2 Old TS (Elf/lnm') 弗 J En Ct [%] No. 4 Cabinet Mei 1 Yadoshiha Rameg (xlo-li procedural amendment (Hakuchu Patent Office Commissioner Wakasugi Kazuinu)
. . 1. Indication of the incident Showa Rihi Patent Application No. 24Qo4 3.3
, Person making an amendment Patent applicant related to the case Name: Nippon Kokan Co., Ltd. 4, Agent address: 1-1-1 Toranomon, C8-ku, Tokyo 105 East S;
'l l Number 8] Gindai 10 Mori Pill 8th floor Phone +503)
3948 f generation) 5, 81 sun
'Knee'';15"5i/Showa year
Date of delivery Contents of the specification Medium temperature 7th line 4th line "T is 1' W I-I T
ni'' is corrected to ``'J゛ is PWIIi'' + ko''. Page 14, line 17, JA, 203j is replaced with “A302
Correct it as j. In "Table 2" on page 19, description A in [-note] is corrected as follows. Note: If the thickness of the plate is thick, it is permitted to cool the plate after standard cooling using a 1.5-liter cooling method instead of air cooling.The thickness of a 150mm plate is equivalent to the accelerated cooling standard for the same plate thickness. This shows that the speed has been reproduced and standardized. ,1

Claims (1)

[Claims] 1. C: 0.07 to 0.20wt, St: 0.
10-0.50 wt%, Mn: more than 0.90 wt% and 1.60 wt% or less, Cr: 0.05-0.
40 wt%, Mo: 0.40-0.65 wt
%, P: 0.025 wt% or less, S: 0.006 wt% or less, IIolJ, Al: 0.005 to 0.050 wt
%, and any one of V: 0.005 to 0.07 wt%, Nb: 01003 to 0.05 wt%
1. A high-toughness steel for pressure vessels with excellent weldability, characterized in that it contains at least one of the above two species, and the remainder consists of iron and unavoidable impurities. 2, C: 0.07-0.20 wt%, St: 0.
10-0.50 wt%, Mn: 0.90 wt%
exceeding 1.60 wt%, Cr: 0.0 5 to 0
．． 40 wt%, Mo: 0.40-0.
65 wt%, P: 0.025 wt% or less, S: 0.006 wt% or less, aol, Al: 0.0 0 5 to 0.0 5 0
wt%, and any one of v20, 005 to 0.07 wt%, Nb: 0.003 to 0.05 wt%
or two species, and Ni: 0.05 to 1.
Owt%, B: 0.0002-0.002wt%,
Cu: 0.05 to 0.30 wt%, for high toughness pressure vessels with excellent weldability, characterized by containing one or more of the following, with the remainder consisting of iron and inevitable impurities. steel.