JPH08144011A - Chromium-molybdenum steel excellent in weldability - Google Patents

Abstract

PURPOSE: To produce a Cr-Mo steel for pressure vessel, excellent in weldability, by specifying a composition consisting of C, Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Ti, B, Al, Ca, N, O, and Fe. CONSTITUTION: This Cr-Mo steel for pressure vessel has a composition which consists of, by weight, 0.03-0.08% C, 0.01-0.8% Si, 0.45-1% Mn, <=0.015% P, <=0.003% S, 0.05-0.6% Cu, 0.3-1% Ni, 0.8-1.5% Cr, 0.3-0.8% Mo, 0.02-0.15% V, 0.005-0.015% Ti, 0.0003-0.002% B, 0.005-0.035% S.Al, 0.0005-0.005% Ca, <=0.005% N, <=0.003% O, and the balance essentially Fe and in which 0.5<=Cu+Ni<=1.2% and 1<=Ca/S<=10 are satisfied and further S.Al content and Ca content are regulated to the values within the shaded region of a figure. The steel is excellent in reheat crack resistance during post weld heat treatment as well as in cold crack resistance at the time of welding and also has high strength and high toughness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in petrochemical, petroleum refining, power plant, etc. in a medium and high temperature hydrogen environment, etc., and has strength and toughness secured, and resistance to cold cracking during welding. And subsequent heat treatment after welding (hereinafter referred to as SR)
The present invention relates to a Cr-Mo steel for thick wall pressure vessels, which has excellent weldability such as resistance to reheat cracking that occurs inside.

[0002]

2. Description of the Related Art Cr-Mo steel sheets for pressure vessels are subjected to normalizing or quenching-tempering after hot rolling and / or hot working, or direct quenching after hot rolling or air cooling-tempering. Welding is performed and then SR processing is performed. This steel type requires preheating at 150 ° C to 250 ° C during welding, and it has been conventionally desired to reduce the load.

Therefore, as disclosed in Japanese Examined Patent Publication No. 62-50547, in thick-walled materials, improvement of hardenability by addition of Ti and B and regulation of N content, and strength and toughness by addition of Cu, Ni and the like. It has been proposed to secure the temperature, reduce C, and improve the weld cold cracking property.

Further, 1-1 / 4Cr-1 / 2Mo steel and 1
Cr-1 / 2Mo steel has conventionally been known to have high reheat cracking susceptibility, so C is lowered to improve weldability, and C
High reheat cracking susceptibility is a problem even in low C—Cu, Ni, V-added Ti—B systems that secure strength by increasing the amounts of u and Ni and carbide forming elements such as Nb and V.

Therefore, low C-Cu, Ni, V-added Ti-
As for the improvement of the weld cold cracking resistance and reheat cracking resistance of B-based Cr-Mo steel sheets, as disclosed in Japanese Patent Laid-Open No. 5-1351, the regulation of (Cu + Ni / 2 + V) / Cr amount and extremely low Al It is said that the reheating cracking resistance is improved by the chemical conversion.

[0006]

However, Japanese Unexamined Patent Publication No.
When the extremely low Al content disclosed in Japanese Patent No. 1351 is used,
With the current manufacturing method, the oxygen level tends to be relatively high,
The deterioration of steel cleanliness and the deterioration of toughness pose problems. The plate thickness has become thicker with the recent enlargement of the apparatus, but as the plate thickness becomes thicker, it is generally difficult to secure strength and toughness, and in particular, ultra-low Al steel has a relatively low Cr content ( The content of Cu, Ni, and V in the <1.5%) region is limited to a low level, and it cannot be said that the composition is appropriate as a thick steel material.

In fact, the plate thickness of the embodiment disclosed in Japanese Patent Laid-Open No. 51351/1993 is 25 mm. That is, the requirements for low temperature crack resistance and reheat crack resistance have become stricter as the wall thickness has increased, but at present, there is no product that satisfies these requirements sufficiently and further improvement of both is required. . Therefore, the inventors of the present invention aim to further improve the low temperature crack resistance and the reheat crack resistance of the thick-walled Cr—Mo steel material while securing strength and toughness. The present invention has been completed.

[0008]

[Means for Solving the Problems]

(1) The invention of claim 1 is a Cr-Mo steel having the following composition (the composition is wt%) and having excellent weldability. (A) C: 0.03 to 0.08%, Si: 0.01
~ 0.8%, Mn: 0.45 to 1%, P: ≤
0.015%, S: 0.003%, Cu: 0.
05-0.6%, Ni: 0.3-1%, Cr:
0.8-1.5%, Mo: 0.3-0.8%, V:
0.02-0.15%, Ti: 0.005-0.015
%, B: 0.0003 to 0.002%, S.I. Al: 0.
005-0.035%, Ca: 0.0005-0.00
5%, N: ≤ 0.005%, O: ≤ 0.003%
And (b) 0.5% ≦ Cu + Ni ≦ 1.2%, 1 ≦ Ca / S ≦ 10, and (c) S. Al content and Ca
The content is included in the shaded area in FIG. (D) Alternatively, Nb is further contained in the range of 0.02 ≦ Nb + V ≦ 0.15%.

(2) According to the invention of claim 2, in addition to the composition of claim 1, Cr-Mo steel containing Nb in the range of 0.02% ≤Nb + V≤0.15% and having excellent weldability. Is.

(3) The invention of claim 3 is the composition of claim 1, further comprising Mg: 0.0003 to 0.5%, and one kind of a rare earth element (REM) having an atomic number of 57 to 71. Alternatively, it is a Cr-Mo steel excellent in weldability and containing either or both of two or more kinds in total of 0.001 to 0.3%.

(4) The invention of claim 4 is the composition of claim 2, further comprising Mg: 0.0003 to 0.5% and one kind of a rare earth element (REM) of atomic number 57 to 71. Alternatively, it is a Cr-Mo steel excellent in weldability and containing either or both of two or more kinds in total of 0.001 to 0.3%.

[0012]

The present invention will be described in detail below. The basic structure of the present invention is as follows. (1) By reducing the carbon content to an extremely low level, the weld cold cracking property is greatly improved, and preheating is unnecessary even for thick materials.

(2) In order to secure the strength and toughness of a thick material, a trace amount B is added, N is regulated to a low level, and Ti is added to secure the hardenability. Further, a slight amount of V (or Nb in addition to this) is added to finely precipitate carbonitrides and increase the strength. Add an appropriate amount of Cu + Ni (0.5 to 1.2
%) Improves hardenability and secures strength and toughness without reducing reheat cracking resistance.

(3) The oxygen level is kept low and the amount of inclusions is controlled. (4) S content is suppressed to a low level, and Ca is added to fix the solid solution S to improve reheat crack resistance.

For the above purposes, 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 was made 0.08%. Si: Si is effective in securing strength and improving oxidation resistance, but at the same time, increases toughness deterioration and temper embrittlement susceptibility, so 0.01% ≦ Si ≦ 0.8%.

Mn: Mn is effective in increasing strength and toughness, but at the same time increases temper embrittlement susceptibility, so 0.45 ≦ Mn ≦ 1%. P: P is 0.015% or less in order to promote temper embrittlement, impair toughness, and enhance reheat cracking susceptibility.

S: S increases susceptibility to reheat cracking, so
It was set to 0.003% or less. Cu: Cu increases the hardenability and increases the strength as a solid solution strengthening element, so 0.05 or more is added, but if added excessively, reheat cracking susceptibility is increased, and creep ductility and hot workability are also increased. The upper limit was made 0.6% in order to lower the temperature. Ni: Ni is effective in improving hardenability and improving toughness, so 0.3% or more is added, but if added excessively, reheat cracking susceptibility is increased, so the upper limit was 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 1.2 because of adverse effects on reheat cracking susceptibility. % Or less. Cr: Cr is effective for high-temperature strength, hydrogen attack resistance, and oxidation resistance, which are important performances for pressure vessel steel, and it is necessary to contain 0.8% or more, but on the other hand, cost and From the viewpoint of weldability, 1.5% was made the upper limit.

Mo: Mo forms a stable carbide and is effective for high temperature strength, creep strength, and hydrogen attack resistance as Cr, but if added excessively, it impairs economical efficiency. % ≦ Mo ≦ 0.8%.

V: V is a carbide-forming element and can increase the strength, but if added in excess, it increases the reheat cracking susceptibility, so 0.02≤V≤0.015%. Ti: Ti is a strong nitride-forming element, and Ti enhances the effect of improving the hardenability of B by fixing N, but if it is present in excess, it decreases toughness, so 0.005 ≦ Ti
≦ 0.015%.

Nb: Nb is a stable carbonitride forming element and can improve the high temperature strength, but if it is contained excessively, toughness and weldability are impaired. Therefore, 0.02% ≤ Nb + V ≤ 0.15% when contained if necessary.
And

S. Al: S. Al (Soluble A
1) improves the reheat cracking susceptibility by keeping it low, 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 by 0.005% ≦ S. ． Al ≦ 0.035%.

Ca: Ca is a strong sulfide-forming element, which fixes solid solution S in steel and improves reheat cracking susceptibility. However, if added excessively, the toughness and weldability are deteriorated, so 0.0005 ≦ Ca ≦ 0.005% was set. Also,
In order to obtain the effect of adding Ca, the ratio of Ca / S is set to 1 or more and 10 or less.

Mg, REM (rare earth elements): Mg and R
Both EMs are sulfide-forming elements and have the effect of improving reheat cracking susceptibility, but when added in excess, they lower toughness, hydrogen corrosion resistance, and weldability, so an upper limit was defined. Mg is 0.0003 to 0.5%, and REM is 0.001 to 0.3% in total of one or more kinds. All of these elements have the effect of controlling the morphology of sulfides, so Mg,
Alternatively, either one or both of REM can be contained.

N: N easily binds to B, reduces the amount of solid solution B, and deteriorates hardenability, so it is kept to 0.005% or less. O: O prevents the deterioration of ductility and weldability due to the formation of oxide inclusions, and maximizes the effect of Ca, so it is kept as low as 0.003% or less. Other Sn, A
Impurity elements such as s and Sb deteriorate SR cracking resistance, so it is preferable to reduce the content to 0.01% or less.

Furthermore, the present inventors have found that Ca, S, S. As a result of examining the relationship of Al, as shown in FIG. Al
Regarding the relationship between the content and the Ca content, the reheat cracking rate can be set to 20% or less in the range of the diagonal line. At this time C
Regarding the a content, Ca is added so that Ca / S is 1 or more and 10 or less, and it is necessary to set certain limits on the Cu + Ni amount and V amount added to secure the strength and toughness.

The reheat cracking rate is the cracking rate of the cross section after the oblique Y cracking test specified in JIS Z3158 and the SR treatment are performed. Further, in FIG. 2, Ca, S, S. Al
In the relationship of, the fracture surface transition temperature vTs in the Charpy impact test is shown, but within the same shaded area as in FIG.
It was extremely good.

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

[0029]

EXAMPLES Examples will be shown below, but the present invention is not limited to these examples. Table 1 shows the composition of the present invention steel and comparative steel, and Table 2 shows the material properties of the steels shown in Table 1. All examples are normalizing-tempering-SR
T. P. = T (20 + log t) = 20.1
It was set to × 10 ³ . Here, T is the SR processing temperature (K), t
Is the processing time (hr).

Steels A, B and I are invention steels having a plate thickness of 60 mm, Steel C is thin (plate thickness 30 mm), Steels D and E are thick (1
It is an invention steel of 00 mm). Steel F is an invention steel to which Nb is further added, and Steel G, Steel H and Steel J are invention steels to which Mg and REM are added individually or in combination.

Steel K is a conventional steel and has good resistance to reheat cracking, but has a high cracking stop preheating temperature of 200 ° C. Steels L and W are welded low temperature cracking-improved steels to which V is not added, but have a higher welding preheating temperature than the invention steels and high reheat cracking susceptibility. Steel M has a low Cu + Ni content (0.30%)
Therefore, the strength and toughness are low.

Steel N is S. Al, Ca, (Ca /
S) is in a proper region, but Cu + Ni is high (1.3
5%), high reheat cracking susceptibility. Steel O to Steel V are S. A
The amount of l and the amount of Ca are changed and deviate from the optimum region for reheat cracking susceptibility. Also, S. Steel with extremely low Al content or steel with high Ca content or high Ca / S (steel M, P, Q, R, T, U) shows low toughness.

Based on these examples and steel A, the Ca content and the S. 1 and 2 show the reheat cracking resistance and toughness when the Al content was changed. It can be seen that the shaded portion has excellent reheat cracking resistance and toughness. As described above, it was confirmed that the steel of the present invention is excellent in weld cold crack resistance and reheat crack resistance.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

As described above, the steel of the present invention is excellent in the following points. (1) Due to the extremely low carbon content, the weld cold cracking property is greatly improved, and preheating is unnecessary even for thick-walled materials. (2) Since a small amount of B is added, N is controlled to be low, and Ti is added, the hardenability for securing the strength and toughness of a thick material is secured. (3) Further, since a small amount of V (or Nb in addition to this) is added and these carbonitrides are finely precipitated, the strength is high. (4) The amount of Cu + Ni is in the range of 0.5 to 1.2%,
Hardenability can be improved and strength and toughness can be secured without reducing reheat cracking resistance. (5) The oxygen level is low and the amount of inclusions is small. (6) Since S is low and Ca is added to fix the solid solution S, reheat cracking resistance is improved.

[Brief description of drawings]

FIG. 1 shows the amount of Ca and S. It is a figure which shows the reheat cracking rate after SR without preheating in relation to the amount of Al.

FIG. 2 shows the amount of Ca and S. It is a figure which shows the fracture surface transition temperature in the Charpy impact test in relation to the amount of Al.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ken Takishima 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (4)

[Claims] 1. A Cr-Mo steel having the following composition (the composition of the composition is wt%) having excellent weldability. (A) C: 0.03 to 0.08%, Si: 0.01
~ 0.8%, Mn: 0.45 to 1%, P: ≤
0.015%, S: ≤ 0.003%, Cu:
0.05-0.6%, Ni: 0.3-1%,
Cr: 0.8-1.5%, Mo: 0.3-0.8%,
V: 0.02-0.15%, Ti: 0.005-
0.015%, B: 0.0003 to 0.002%, S.I.
Al: 0.005-0.035%, Ca: 0.0005
~ 0.005%, N: ≤ 0.005%, O: ≤
0.003% is contained, and the balance is substantially Fe, and (b) 0.5% ≦ Cu +
Ni ≦ 1.2%, 1 ≦ Ca / S ≦ 10, and (c) S. Al content and Ca
The content is included in the shaded area in FIG. 2. A Cr-Mo steel having excellent weldability, which has the following component composition (the component composition is wt%). (A) C: 0.03 to 0.08%, Si: 0.01
~ 0.8%, Mn: 0.45 to 1%, P: ≤
0.015%, S: ≤ 0.003%, Cu:
0.05-0.6%, Ni: 0.3-1%,
Cr: 0.8-1.5%, Mo: 0.3-0.8%,
V: 0.02-0.15%, Ti: 0.005-
0.015%, B: 0.0003 to 0.002%, S.I.
Al: 0.005-0.035%, Ca: 0.0005
~ 0.005%, N: ≤ 0.005%, O: ≤
0.003%, 0.02% ≦ Nb + V ≦ 0.15%
Nb in the range of, the balance consisting essentially of Fe, (b) 0.5% ≤ Cu +
Ni ≦ 1.2%, 1 ≦ Ca / S ≦ 10, and (c) S. Al content and Ca
The content is included in the shaded area in FIG. 3. A Cr-Mo steel having the following composition (the composition is wt%) and having excellent weldability. (A) C: 0.03 to 0.08%, Si: 0.01
~ 0.8%, Mn: 0.45 to 1%, P: ≤
0.015%, S: ≤ 0.003%, Cu:
0.05-0.6%, Ni: 0.3-1%,
Cr: 0.8-1.5%, Mo: 0.3-0.8%,
V: 0.02-0.15%, Ti: 0.005-
0.015%, B: 0.0003 to 0.002%, S.I.
Al: 0.005-0.035%, Ca: 0.0005
~ 0.005%, N: ≤ 0.005%, O: ≤
0.003% is contained, and the balance is substantially Fe, and (b) 0.5% ≦ Cu +
Ni ≦ 1.2%, 1 ≦ Ca / S ≦ 10 are satisfied, and (c) Mg: 0.0003 to
0.5%, rare earth element with atomic number 57 to 71 (REM)
One or more of 0.001 to 0.3% in total, or both, and (d) S. Al content and Ca
The content is included in the shaded area in FIG. 4. A Cr-Mo steel having the following composition (the composition of the composition is wt%) and having excellent weldability. (A) C: 0.03 to 0.08%, Si: 0.01
~ 0.8%, Mn: 0.45 to 1%, P: ≤
0.015%, S: ≤ 0.003%, Cu:
0.05-0.6%, Ni: 0.3-1%,
Cr: 0.8-1.5%, Mo: 0.3-0.8%,
V: 0.02-0.15%, Ti: 0.005-
0.015%, B: 0.0003 to 0.002%, S.I.
Al: 0.005-0.035%, Ca: 0.0005
~ 0.005%, N: ≤ 0.005%, O: ≤
0.003%, 0.02% ≦ Nb + V ≦ 0.15%
Nb in the range of, the balance consisting essentially of Fe, (b) 0.5% ≤ Cu +
Ni ≦ 1.2%, 1 ≦ Ca / S ≦ 10 are satisfied, and (c) Mg: 0.0003 to
0.5%, rare earth element with atomic number 57 to 71 (REM)
One or more of 0.001 to 0.3% in total, or both, and (d) S. Al content and Ca
The content is included in the shaded area in FIG.