JPH09296249A - Chromium-molybdenum steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remarkably improve the welding low temp. cracking resistance of a Cr-Mo steel and to jointly improve its reheating cracking resistance while its strength and toughness are secured. SOLUTION: This Cr-Mo steel excellent in welding low temp. cracking resistance and reheating cracking resistance has a compsn. contg., by weight, 0.03 to 0.08% C, 0.01 to 0.8% Si, 0.05 to 1% Mn, <=0.015% P, <=0.003% S, 0.05 to 1% Cu, 0.05 to 1% Ni, 2.0 to 3.5% Cr, 0.5 to 1.5% Mo+1/2W, 0.05 to 0.5% V, 0.005 to 0.05% Ti, 0.0003 to 0.003% B, 0.005 to 0.035% Al, <=0.006% N, <=0.005% O and 0.0005 to 0.005% Ca, satisfying the relations of 0.5<Cu%+Ni%<1.2 and 1<Ca%/S%<10, and the balance iron with inevitably intruded impurities.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure vessel or the like used in a high temperature and high pressure hydrogen environment such as petrochemical and petroleum refining, or a large diameter thick wall tube used under high temperature and high pressure in a power plant facility such as a boiler. Used for such purposes as ensuring strength and toughness, cold cracking during welding and subsequent heat treatment after welding (hereinafter PW
Cr-Mo with improved reheat cracking that occurs in (HT)
Regarding steel.

[0002]

2. Description of the Related Art 2 1/4 Cr for pressure vessels and large diameter thick tubes
Cr represented by -1Mo steel (JIS SCMV4 etc.)
-Mo steel is produced by hot rolling / or hot working, followed by normalization-tempering, or quenching-tempering, or immediately after hot rolling / or hot working or by air cooling-tempering. Combined with cold, warm, hot working etc., finally assembled into welding, then PWHT
Is applied. With this type of steel, it is usually 150 when welding.
Preheating from ℃ to 250 is required, and reduction of the load has been desired conventionally.

Therefore, as disclosed in JP-B-64-7128, strength and toughness are secured by a combination of addition of Ti and B and regulation of N content and increase of precipitation strengthening by addition of V and Nb. In addition, a method has been proposed in which the C content can be reduced and the welding cold cracking property is improved. Further, JP-B-64-7127 and JP-B1-29853 describe that the strength and toughness can be improved by further adding Cu, Ni or the like or performing direct quenching-tempering.

However, these Cu, Ni and Ti,
It is well known that the addition of B, Nb and V enhances the susceptibility to reheat cracking that occurs during PWHT after welding. Two one
/ 4Cr-1Mo steel is generally said to be less prone to reheat cracking than 1 1 / 4Cr-1 / 2Mo steel and 1Cr-1 / 2Mo steel, but when a large amount of these elements is added, In particular, reheat cracking may become a serious problem in construction in places where the constraint conditions are severe.

Therefore, low C-Cu, Ni, and V-added Ti-
Regarding the improvement of the weld cold cracking resistance and reheat cracking resistance of the B-based Cr-Mo steel sheet, as disclosed in JP-A-5-1351, (Cu + Ni / 2 + V) / Cr amount regulation and extremely low Al content It is said that the reheating cracking resistance is improved by the chemical conversion. Further, in Japanese Patent Laid-Open No. 4-184842, when the V content is higher than that of the same steel type (0.09 to 0.50w).
t. %), It is stated that the resistance to reheat cracking is improved by making extremely low Al.

[0006]

[Problems to be Solved by the Invention] However, JP-A-5-13
When the ultra-low Al disclosed in Japanese Patent No. 51 and Japanese Patent Application Laid-Open No. 4-183842 is used, the oxygen level tends to be relatively high in the current manufacturing method, and deterioration of steel cleanliness and deterioration of toughness are problems. Become. In addition, with the recent trend toward higher efficiency of plants and higher temperature and pressure of operating conditions and larger size of equipment, the thickness of steel sheets tends to become thicker. However, it is difficult to secure strength and toughness, and
In 1351 gazette, it cannot be said that the component system is designed specifically for thick wall. In fact, the plate thickness of the example disclosed in Japanese Patent Laid-Open No. 5-1351 is 25 mm. Also, it is limited to less than 0.09% of V addition amount, and V is 0.10%.
With the high strength steels contained above, sufficient reheat cracking resistance cannot be ensured.

On the other hand, in Japanese Patent Application Laid-Open No. 4-183842, the V content is as high as 0.09 to 0.50 wt%, which is for thick materials, but when compared with Japanese Patent Application Laid-Open No. 5-1351. However, there is no regulation of the amount of (Cu + Ni / 2 + V) / Cr, but according to Japanese Patent Laid-Open No. 5-3511, there is a contradiction that the reheat cracking susceptibility cannot be improved by this component system. Therefore, it cannot be said that the reheat cracking property is secured in the component system for thick wall with the V content of 0.10% or more.

The present invention has been made in view of the above circumstances, and in a thick-walled Cr-Mo steel material, while securing strength and toughness, the cold cracking resistance to welding is significantly improved and the reheat cracking resistance is also improved. The purpose is also to improve sex.

[0009]

That is, according to the present invention, in% by weight, C: 0.03 to 0.08%, Si: 0.01 to.
0.8%, Mn: 0.05 to 1%, P: ≤ 0.015
%, S: 0.003%, Cu: 0.05 to 1%, N
i: 0.05 to 1%, Cr: 2.0 to 3.5%, Mo +
1 / 2W: 0.5-1.5%, V: 0.05-0.5
%, Ti: 0.005 to 0.03%, B: 0.0003
~ 0.003%, Al: 0.003 to 0.035%,
N: ≦ 0.006%, O: ≦ 0.005%, Ca: 0.
0005 to 0.005%, and if necessary N
b: contains ≦ 0.10% and 0.5 <Cu% + Ni
% Welding cold cracking resistance and reheat cracking resistance, which are characterized by satisfying the relationships of% <1.2 and 1 <Ca% / S% <10, and the balance consisting of inevitable impurities and iron. Cr-M
o Steel.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.

The basic structure of the present invention is as follows.

(1) The extremely low carbon content greatly improves the weld cold cracking property even in thick-walled materials.

(2) In order to secure the strength and toughness of thick-walled materials, a trace amount of B is added, N is regulated to a low level, and Ti is added to secure solid solution B during normalization or quenching. Secure. (3) V (or a small amount of Nb in addition thereto) is added in an appropriate amount to finely precipitate the carbonitride and increase the strength without deteriorating the reheat cracking resistance. (4) By adding an appropriate amount of Cu + Ni (0.5% to 1.2%), the hardenability is improved and the strength is increased without deteriorating the reheat cracking resistance. (5) S is suppressed to a low level, Ca is added to fix the solid solution S, and the reheat cracking property is improved by controlling the form of oxide inclusions. Here, in order to maximize the latter effect of Ca, the oxygen level is kept low and the amount of Al added is controlled. For the above purpose, the chemical components of the present invention are limited as follows.

C: C is required to be 0.03% or more from the viewpoint of securing the strength, but since the weld cold cracking property increases with the increase, the upper limit is made 0.08%.

Si: Si is effective for securing strength and improving oxidation resistance, but at the same time increases toughness deterioration and susceptibility to temper embrittlement, so 0.01% ≤Si≤0.8%.

Mn: Mn is effective in increasing the strength and toughness, but at the same time increases the back embrittlement susceptibility, so that 0.05% ≦ Mn ≦ 1%.

P: P is 0.015% or less in order to promote temper embrittlement, impair toughness and enhance reheat cracking susceptibility.

S: S enhances reheat cracking susceptibility,
It is made 0.003% or less.

Cu: Cu enhances hardenability and strengthens as a solid solution strengthening element, so 0.05% or more is added. However, if added in excess, reheat crackability is enhanced, and creep ductility and hot workability are increased. Since the workability is reduced, the upper limit is 1%.

Ni: Ni is effective in improving hardenability and toughness, so 0.05% or more is added.
If added excessively, reheat cracking susceptibility is increased, so the upper limit is made 1%.

Further, in the present invention, in order to secure the strength and toughness of a thick material having a low carbon content, the amount of Cu + Ni is set to 0.5% or more, and the upper limit is 1. Limit to 2% or less.

Cr: Cr is effective for high temperature strength, hydrogen attack resistance and oxidation resistance, which are important performances for high temperature steel, and it is necessary to contain Cr in an amount of 2.0% or more. From the viewpoint of cost and weldability, the upper limit is 3.5%.

Mo and W: Mo and W are effective in high temperature strength, creep strength and hydrogen attack resistance like Cr because they stably form carbides, but extreme addition impairs weldability. 0.5% ≦ M to impair economy
o + 1 / 2W ≦ 1.5%. Here, since W has twice the atomic weight of Mo, it is considered that Mo and W / 2 are equivalent. Although Mo alone and W alone are effective for high temperature strength and the like, the higher the W content, the greater the effect.

V: V is a carbide-forming element and can increase the strength, but when it is added in excess, it increases the reheat cracking susceptibility, so 0.05% ≤V≤0.5%.

Ti: Ti is a strong nitride forming element, and N
The effect of improving the quenching of B is enhanced by fixing B. However, if it is present excessively, the toughness decreases, so 0.005% ≦ T
i ≦ 0.05%.

S. Al: S. By suppressing Al to a low level, the reheat cracking susceptibility is improved, but if it is less than 0.005%, not only the toughness is impaired, but also the effect of improving the reheat cracking property of Ca is reduced, so 0.005% or more. To do. Further, if added excessively, the reheat cracking susceptibility is increased and the high temperature strength is lowered, so the upper limit is 0.035%.
And

N: N easily binds to B, reduces the amount of solid solution B, and deteriorates hardenability, so it is kept to 0.006% or less.

O: O prevents deterioration of ductility and weldability due to the formation of oxide inclusions, and maximizes the effect of improving reheat cracking resistance by controlling oxide inclusions of Ca. Therefore, it is kept low at 0.005% or less.

Nb: Nb is a carbonitride-forming element that is stable and can increase the strength. However, if it is contained excessively, the toughness and weldability are impaired, and the reheat cracking susceptibility is increased. ≦ Nb ≦ 0.10%.

Ca: Ca is a strong sulfide-forming element, which fixes solid solution S in steel and improves reheat cracking susceptibility. Furthermore, as will be described later, S. By adjusting the amount of Al appropriately, oxide inclusions are controlled and reheat cracking property is improved. In this case, the latter effect is very large. 0.0005%
≦ Ca ≦ 0.005%.

Ca / S: In order to sufficiently obtain the effect of improving the reheat cracking resistance of Ca, the ratio of Ca / S is set to 1 or more and 10 or less.
That is, the present inventors have found that Ca, S, S. The relationship of Al was examined. The result is shown in FIG. In the figure, the numbers circled indicate the values of Ca / S, and the shaded areas indicate that the reheat cracking rate in the y cracking test is 10% or less. From Figure 1
S. Al content and Ca content are within a predetermined range, and
It was found that in 1 <Ca / S <10, the reheat cracking ratio by the y cracking test can be made 10% or less. Here, the reheat cracking rate is determined by performing a y-type weld cracking test specified in JIS Z3158, performing PWHT under severe conditions of 620 ° C. × 12 h, which is a temperature at which reheat cracking easily occurs, and then evaluating the cross-sectional cracking rate. did.

In order to produce the Cr-Mo steel according to the present invention, depending on the required mechanical properties, hot rolling and / or hot working is followed by normalizing or quenching to perform tempering, or hot rolling. After rolling or hot working, direct quenching or air cooling and tempering may be performed.

Other impurity elements such as Sn, As, and Sb increase the susceptibility to reheat cracking and the susceptibility to creep embrittlement, and are therefore preferably set to 0.01 or less.

[0034]

EXAMPLES Examples of the present invention will be described below.

(Manufacturing conditions) Plate thickness: 50 mm Rolling conditions: 1200 ° C. heating-1000 ° C. finishing (air cooling after rolling) Normalizing: Cooling rate controlled (150 mm of air-cooled material)
Cooling rate of 1/2 t) Tempering: 720 ° C. A TMCP-T type in which only normalizing of the invention steel 1 was omitted was also performed. Post-weld heat treatment (PWHT): 700 ° C. × 24 h (base material properties) 620 ° C. × 12 h (reheat cracking test) (test conditions) Reheat cracking test method According to JIS Z3158 (y-type weld cracking test) 50 mm thick test bead → PWHT → Observation of cracks (cross-section crack) The reheat cracking test is performed on a y-crack type test piece with severe restraint conditions, and the PWHT condition is gradual heating at the temperature (600 to 650 ° C) where reheat cracking is most likely keeping. Even if the holding time is made longer than this, the HAZ is softened and the crack does not progress. This is a very stringent test condition. The compositions of the present invention steel and comparative steel are shown in Tables 1 to 3, and the test conditions and test results are shown in Tables 4 to 7.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

[Table 6]

[0042]

[Table 7]

[0043]

As is clear from the above results, according to the present invention, in the Cr-Mo steel material, while the strength and toughness are secured, the welding cold crack resistance is greatly improved and the reheat crack resistance is improved. In addition, it can also be improved. Pressure vessels, etc. used in high-temperature and high-pressure hydrogen environments such as petrochemical and petroleum refining, and large-diameter thick-walled tubes used under high-temperature and high-pressure in power plant facilities such as boilers. It is extremely effective for

[Brief description of drawings]

FIG. 1 shows Ca, S, S. The figure which shows the relationship between the amount of addition of Al, and the cross-section cracking rate by the reheat cracking test of a y-cracking test type.

Claims (2)

[Claims] 1. C: 0.03 to 0.08% by weight,
Si: 0.01 to 0.8%, Mn: 0.05 to 1%,
P: ≤ 0.015%, S: ≤ 0.003%, Cu: 0.
05-1%, Ni: 0.05-1%, Cr: 2.0-
3.5%, Mo + 1 / 2W: 0.5-1.5%, V: 0.
05-0.5%, Ti: 0.005-0.05%, B:
0.0003 to 0.003%, Al: 0.005 to 0.
035%, N: ≤ 0.006%, O: ≤ 0.005%,
Ca: 0.0005 to 0.005% is contained, and 0.
5 <Cu% + Ni% <1.2, and 1 <Ca% / S% <
Cr-Mo steel satisfying the relationship of 10 and being excellent in weld cold cracking resistance and reheat cracking resistance, characterized in that the balance consists of inevitable impurities and iron. 2. C: 0.03 to 0.08% by weight,
Si: 0.01 to 0.8%, Mn: 0.05 to 1%,
P: ≤ 0.015%, S: ≤ 0.003%, Cu: 0.
05-1%, Ni: 0.05-1%, Cr: 2.0-
3.5%, Mo + 1 / 2W: 0.5-1.5%, V: 0.
05-0.5%, Ti: 0.005-0.03%, B:
0.0003 to 0.003%, Al: 0.003 to 0.
035%, N: ≤ 0.006%, O: ≤ 0.005%,
Ca: 0.0005 to 0.005% is contained, and further N
b: contains ≦ 0.10% and 0.5 <Cu% + Ni
% Welding cold cracking resistance and reheat cracking resistance, which are characterized by satisfying the relationships of% <1.2 and 1 <Ca% / S% <10, and the balance consisting of inevitable impurities and iron. Cr-M
o steel.