JPH11131177A - Steel plate for medium and normal temperature pressure vessel which can omit post-weld heat treatment and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a steel plate for a medium- and normal-temperature pressure vessel in which post-weld heat treatment can be omitted. SOLUTION: C: 0.02 to 0.10% by weight%, S
i: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P:
0.030% or less, S: 0.020% or less, Nb: 0.0
03-0.030%, Ti: 0.005-0.020%,
Al: 0.01 to 0.08%, N: 0.002 to 0.00
6C + Mn: 1.2 to 2.0, Ceq: 0.25 to 5%, with the balance being iron and unavoidable impurity elements.
0.38, Ppr: a steel plate for a medium-pressure container satisfying 2.0 to 40, and a steel plate slab of the above chemical composition at 950 to 13
Heat to 00 ° C, and reduce the cumulative rolling reduction in the recrystallization temperature range from 10 to 90.
% Rough rolling, and finish rolling at a cumulative reduction rate of 10 to 90% in a non-recrystallization temperature region of ₃ points or more of Ar, and the cooling rate is 1
The method for manufacturing the steel sheet, wherein the steel sheet is controlled and cooled at a temperature of 100 ° C./s.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon steel plate for a medium-pressure container (for example, in accordance with JIS G 3118 (1987)) used in a temperature range of about 350.degree. In particular, the present invention relates to a steel plate for a medium- and normal-temperature pressure vessel in which post-weld heat treatment can be omitted.

[0002]

2. Description of the Related Art In recent years, power plants have been increasing in size with increasing power demand. For example, also in the case of a nuclear power plant, an increase in the size of the reactor pressure vessel body is required to increase the size of the reactor containment vessel. When the thickness of the steel plate is increased with the increase in the size of the containment vessel, the plate thickness is 38 m.
It is stipulated by the Ministry of International Trade and Industry that the post-weld heat treatment must be performed when the distance exceeds m. Since the welding of the PCV is performed at the site of the power plant, post-weld heat treatment is a major disadvantage in terms of equipment, workability, and construction period. Therefore, there is a strong demand for the development of a steel sheet which can be used as it is even after the thickness exceeds 38 mm and which can be omitted from post-weld heat treatment.

[0003] In the post-weld heat treatment, the weld joint is made of metallic Ac.
₍₁₎ Heating uniformly to a temperature below the transformation point and sufficient to improve the performance of the welded part and remove harmful effects such as welding residual stress, hold for a certain period of time, and then cool uniformly Therefore, in order to omit this, even in a state where the residual tensile stress due to welding is applied, the weld heat affected zone (Heat Affected Z) can prevent brittle fracture.
one: Hereinafter, it is necessary to ensure the toughness of HAZ).

Conventionally, steel plates for containment vessels include:
A carbon steel plate for a medium-temperature and pressure vessel (JIS G3118 SGV480), which has a tensile strength of 480 to 590 MPa and is manufactured by a normalizing method, is often used. When the wall thickness is increased, the toughness of the welded portion is reduced, so that the post-weld heat treatment cannot be omitted.

[0005] A technique for obtaining high toughness with a steel sheet for a pressure vessel is described in Japanese Patent Publication No. 2-4673.
The technology disclosed herein regulates the amount of N as P, S, and AlN to provide normalizing (N) -quenching (Q) -tempering (T) -stress relief annealing (SR). The present invention relates to a high-toughness pressure vessel steel characterized by reducing austenite grains and improving the toughness of a steel sheet.

[0006] A technique for improving the toughness of a welded portion with a steel plate for a pressure vessel is described in Japanese Patent Publication No. 56-29951. The technology disclosed herein is based on Nb, M
By performing the normalizing (N) -tempering (T), which contains o, improves the toughness of the base material and the bond portion and can secure the base material strength at the operating temperature by defining Ceq and Pcm. TECHNICAL FIELD The present invention relates to a steel plate for a pressure vessel, which is characterized in that the steel plate is used in a pressure vessel.

A method of omitting post-weld heat treatment for a pressure vessel steel sheet is described in Japanese Patent Publication No. 6-35618. The technology disclosed herein uses low carbon Cr-
The Mo-based low alloy steel is hot-rolled under a specific temperature condition, and then quenched (Q) and tempered (T).
The present invention relates to a method for producing pressure vessel steel that does not require heat treatment after welding, which suppresses an increase in hardness of a welded portion.

[0008]

However, omitting the post-weld heat treatment involves the following problems.
Regarding the technique for obtaining high toughness, the technique disclosed in Japanese Patent Publication No. 2-4673 discloses that the amount of C added is 0.15 to 0.15.
It is 0.23%, and the range of the chemical component is different from that of the present invention which is 0.02 to 0.10%. Further, this is a technique relating to a base material of a steel sheet, there is no description about a welded portion, and it is not possible to omit post-weld heat treatment by this method.

Regarding the toughness of the weld, Japanese Patent Publication No. 56-29
No. 951 discloses a technology in which Ceq is 0.4.
The range is 5 to 0.54, which is different from the range of the present invention which is 0.25 to 0.38. Further, since there is no description about heat treatment after welding and the preheating temperature for preventing low-temperature cracking is 100 ° C., it is substantially impossible to omit the heat treatment after welding by this method.

Regarding a method of omitting heat treatment after welding, the technique disclosed in Japanese Patent Publication No. 6-35618
The content of r is 0.6 to 2.5%, and the range of the chemical component is different from that of the present invention containing 0.05 to 0.50% of Cr as a selective element. Also, the point that it is necessary to add Ti together with B is different from the present invention in which Ti is added alone and B is an inevitable impurity. Further, as for the welded portion, only the hardness is described, and the HAZ toughness is not known.
The problem was that it was unclear whether the post-weld heat treatment could be omitted for steel plates up to 6 mm and 38 mm or more.

[0011] In view of these problems of the prior art, the present invention has a good base metal strength at room temperature and medium temperature and excellent welded joint characteristics, so that a post-weld heat treatment can be omitted. An object of the present invention is to provide a steel sheet and a method for manufacturing the same.

[0012]

Means for Solving the Problems In order to omit the heat treatment after welding, the present inventors have conducted various studies on the HAZ toughness, medium temperature strength, and normal temperature strength according to the components and the manufacturing method of the steel sheet for a medium-pressure container. As a result, the following new findings were discovered.

First, regarding the HAZ toughness, Nb and Ti, which have the effect of reducing the crystal grain size of HAZ, are added.
Further, after adding N so as to have a ratio with Ti effective for grain refinement, the amounts of C and Mn which greatly affect HAZ toughness are limited, and Ceq is reduced in the base material strength in a conventional medium-temperature pressure vessel. It has been found that by lowering to a low level, which was impossible due to a shortage, a high HAZ toughness that cannot be obtained with conventional materials can be achieved.

Next, with respect to the decrease in the intermediate temperature strength due to the recovery of dislocations introduced by controlled rolling, Nb, Mo, and V, which have the effect of improving the base metal strength by precipitation, are added within a certain range. It has been found that the medium temperature strength equivalent to the material can be secured.

Further, as for the decrease in the room temperature strength due to the reduction of the alloying elements, it has been found that the room temperature strength is increased by the addition of Cu, Ni and Cr which improve the strength of the base material in addition to the above-mentioned components. In addition, after performing recrystallization zone rolling and non-recrystallization zone rolling, control cooling, or stop cooling halfway,
Alternatively, it has been found that by performing tempering after cooling to near normal temperature, a normal temperature strength equivalent to that of the conventional material can be obtained. The present invention achieves, for the first time, the omission of the post-weld heat treatment by implementing the above-mentioned limited conditions in combination.

That is, the gist of the present invention is as follows. (1) C: 0.02 to 0.10% by weight, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.030% or less, S: 0 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008% The balance consisting of iron and unavoidable impurity elements, and satisfying 6C + Mn: 1.2 to 2.0, Ceq: 0.25 to 0.38, and Ppr: 2.0 to 40. A steel plate for medium-pressure containers that can be omitted from post-heat treatment. However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2

(2) C: 0.02-0.10%, Si: 0.05-0.5%, Mn: 0.8-1.6%, P: 0.030% or less by weight%, S: 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008 Mo: 0.03 to 0.50%, V: 0.005 to 0.10%, containing one or two kinds, and the balance consisting of iron and unavoidable impurity elements , 6C + Mn: 1.2 to 2.0, Ceq: 0.25 to 0.38, Ppr: 2.0 to 40. . However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2

(3) C: 0.02-0.10%, Si: 0.05-0.5%, Mn: 0.8-1.6%, P: 0.030% or less by weight%, S: 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008 %, And one or more of Cu: 0.10 to 1.0%, Ni: 0.10 to 1.0%, and Cr: 0.05 to 0.50%. The balance being iron and unavoidable impurity elements, and satisfying 6C + Mn: 1.2 to 2.0, Ceq: 0.25 to 0.38, Ppr: 2.0 to 40, after welding A steel plate for medium and normal pressure vessels that can be omitted from heat treatment. However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2

(4) C: 0.02 to 0.10%, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.030% or less by weight%, S: 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008 %, Mo: 0.03 to 0.50%, V: 0.005 to 0.10%, one or two of them, and Cu: 0.10 to 1.0% , Ni: 0.10 to 1.0%, Cr: 0.05 to 0.50%, containing one or more of them, with the balance being iron and unavoidable impurity elements, 6C + Mn: 2 to 2.0, Ceq: 0.25 to 0.38, and Ppr: 2.0 to 40. Steel sheet for cold pressure vessel in an optional post heat treatment. However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27 Nb ^1/2 +55 Mo ^1/2 +14 V ^1/2 .

(5) A steel slab having the chemical composition described in any one of (1) to (4) is heated to 950 to 1300 ° C., and has a cumulative rolling reduction of 10 to 90% in a recrystallization temperature range. Rough rolling is performed, followed by finish rolling with a cumulative rolling reduction of 10 to 90% in the non-recrystallization temperature range of ₃ points or more, and immediately controlled cooling at a cooling rate of 1 to 100 ° C / s to 650 to 500 ° C. And a method for producing a steel plate for a medium- or normal-temperature pressure vessel, wherein heat treatment after welding can be omitted.

(6) The steel slab having the chemical composition described in any one of (1) to (4) is heated to 950 to 1300 ° C., and has a cumulative rolling reduction of 10 to 90% in a recrystallization temperature range. Rough rolling is performed, followed by finish rolling with a cumulative rolling reduction of 10 to 90% in the non-recrystallization temperature range of Ar ₃ points or more, and immediately controlled cooling to 200 ° C or less at a cooling rate of 1 to 100 ° C / s. ,
Thereafter, tempering is performed at 500 ° C to 650 ° C.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The technical concept of the present invention and the reasons for limitation will be described in detail below. In the present invention, as described above, in order to omit the post-weld heat treatment, it is necessary to secure HAZ toughness to the extent that brittle fracture can be prevented even when residual tensile stress due to welding acts.
The effects of alloying elements on Z toughness were studied from a microstructural point of view. Generally, in the HAZ of mild steel, bainetic ferrite, grain boundary ferrite, and upper bainite are generated. However, in the HAZ of 490 MPa class steel, the hardenability increases due to an increase in the amount of alloying elements, and the ratio of upper bainite increases. Due to the increase, the HAZ toughness decreases.

Therefore, first, among the added elements, the amounts of C and Mn which greatly affect hardenability were examined.
It contains 0.02% Nb and 0.01% Ti. The amounts of C and Mn are C: 0.02 to 0.10% and Mn: 0.8 to 1.6.
% Was heated to 1150 ° C.
After performing the recrystallization zone rolling from 00 ° C. and the non-recrystallization zone rolling from 830 ° C., the temperature was controlled and cooled to room temperature.
A 50 mm thick steel sheet obtained by tempering at 0 ° C. was subjected to a tensile test at room temperature.
A Charpy impact test was performed after applying a welding reproduction thermal cycle under the condition of cooling at 400 ° C. and 800 to 500 ° C. for 23 seconds. FIG. 1 shows the results of various studies on the effects of the amounts of C and Mn on the tensile strength and HAZ toughness.

As can be seen from FIG.
0.10%, Mn: in the range of 0.8 to 1.6%, and 6
When the value of C + Mn is 1.2 to 2.0, good H
AZ toughness and base metal strength can be obtained. Even if the amounts of C and Mn are respectively within the above ranges, the base metal strength is insufficient if 6C + Mn is less than 1.2, and the quenching is more than 2.0. It has been found that the HAZ toughness deteriorates due to an increase in the ratio of the upper bainite due to an increase in the properties.

Next, the effect of other alloying elements including C and Mn on hardenability was examined. As the hardenability increases, the maximum hardness of the HAZ also increases. It is known that the maximum hardness of the HAZ is determined by the chemical component when the cooling rate is constant, and the effect of the added element can be evaluated by a carbon equivalent formula evaluated by a ratio to carbon. In Japan, the WE
The carbon equivalent formula proposed by S is often used. Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5 +
Mo / 4 + V / 14 Therefore, the results of the Charpy impact test and the tensile strength used in FIG. 1 are rearranged for Ceq, and this is shown in FIG.

As can be seen from this figure, Ceq is 0.2
In the range of 5 to 0.38, good HAZ toughness and base material strength can be obtained, but if it is less than 0.25, sufficient base material strength cannot be obtained, and if it exceeds 0.38, upper bainite increases. It became clear that sufficient HAZ toughness could not be obtained. Here, more preferably, the upper limit is 0.3
6 or less.

In the present invention, the base material strength is ensured by utilizing the dislocation strengthening by controlled rolling. However, at a medium temperature of about 350 ° C., the recovery of the dislocation occurs. Yield stress tends to decrease slightly.
Therefore, a method of improving the strength of the base material at 350 ° C. above the tempering temperature by precipitating carbide during tempering at 500 to 650 ° C. was studied.

As a result of studying various precipitation strengthening elements, Nb and M which are considered to be effective for securing the strength at 350 ° C.
As for o and V, the steel ingot with these added amounts changed was 1
After heating to 150 ° C., performing recrystallization zone rolling from 1100 ° C., performing non-recrystallization zone rolling from 830 ° C., controlled cooling to room temperature, and further tempering at 500 ° C. The steel plate was subjected to a tensile test at 350 ° C. FIG. 3 shows the relationship between the yield of the yield stress at 350 ° C. and the amount of the alloy element due to the addition of the alloy element.

As can be seen from this figure, it is clear that the yield stress at 350 ° C. is improved by adding a small amount of Nb, Mo, and V, and that the effect is saturated as the added amount is increased. It became. From this figure, the rise rate of the yield stress by each element is Nb: 27 MPa ·% ^−1/2 , M
^{o: 55MPa ·% -1/2, V} : 14MPa ·% -1/2
Met. Therefore, the equivalent equation for the 350 ° C. yield stress due to these elements is expressed as the precipitation strengthening parameter Pp
Assuming that r, it can be expressed by Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2 .

If Ppr is less than 2.0, improvement of the yield stress at 350 ° C. due to the precipitation strengthening element cannot be expected, and if it exceeds 40, not only is it easy to form island martensite in HAZ, but also In multi-pass welding, local precipitation occurs due to tempering in a subsequent pass, and HAZ
The toughness decreases. Therefore, it has been found that by limiting Ppr to the range of 2.0 to 40, good intermediate temperature strength and HAZ toughness can be obtained.

The manufacturing method is as follows.
Similar effects can be obtained not only in the case where the steel sheet is manufactured by tempering at 0 ° C., but also in the case where the controlled cooling is stopped at 650 to 500 ° C. after rolling and air-cooled.

In the present invention, in order to secure base material strength and HAZ toughness, it is necessary to limit not only the above-mentioned conditions but also the range of chemical components. C is an important element for securing the base material strength. If it is less than 0.02%, it is difficult to secure base material strength, and if it exceeds 0.10%, HA
The Z hardness increases, the amount of island martensite formed increases, and the HAZ toughness deteriorates.
Range.

[0033] Si is an element effective for deoxidation and improvement of base material strength. If it is less than 0.05%, the base material strength cannot be improved, and if it exceeds 0.5%, the base material toughness and the HAZ toughness are deteriorated.

Mn is an important element for improving the strength of the base material and fixing S as MnS. If it is less than 0.8%, sufficient base material strength cannot be obtained, and if it exceeds 1.6%, the hardness of HAZ increases, and island-like martensite is formed.
Since the toughness is reduced, the content is set in the range of 0.8 to 1.6%.

P is an element that segregates microscopically in steel and easily causes grain boundary embrittlement. If the content exceeds 0.030%, the effect becomes significant, so the content is set to 0.030% or less.

S is an element that forms coarse and elongated MnS in steel, causes anisotropy of the material, and lowers toughness. If it exceeds 0.020%, the effect becomes remarkable,
0.020% or less.

Nb suppresses recrystallization by hindering movement of crystal grain boundaries during rolling, enables rolling in a non-recrystallization temperature range, reduces the crystal grain size and increases the base material strength,
It is an essential element in the present invention, since the HAZ structure is refined to improve HAZ toughness, and further, NbC is generated during tempering to improve the yield stress at a medium temperature of about 350 ° C. If the content is less than 0.003%, these effects cannot be sufficiently obtained. If the content is more than 0.030%, island martensite is formed to lower the HAZ toughness.
The range was 30%.

Ti combines with N to form TiN, and H
In order to refine the AZ structure and improve the HAZ toughness,
It is an essential element in the present invention. If Ti is less than 0.005%, these effects cannot be sufficiently obtained, and if it exceeds 0.030%, carbides are formed and the toughness is reduced.
-0.030%. In addition, the addition ratio of Ti and N is preferably in the range of 2.0 to 3.4.

Al is an element effective for fixing solid solution N and deoxidizing. If it is less than 0.01%, these effects cannot be sufficiently obtained, and if it exceeds 0.08%, coarse inclusions are formed and the toughness is reduced. .

N is an essential element in the present invention because it produces TiN as described above. If it is less than 0.002%, TiN cannot be sufficiently generated, and if it exceeds 0.008%, solid solution N
0.002% to 0.008%
Range.

Mo not only improves the hardenability and improves the strength of the base material at room temperature, but also generates Mo ₂ C at the time of tempering.
It is an important element in the present invention for improving the yield stress at a medium temperature of about 50 ° C. If it is less than 0.03%, the effect cannot be sufficiently obtained, and if it exceeds 0.50%, island-like martensite is formed in the HAZ, and the HAZ toughness is reduced.
The range was 0.03 to 0.50%.

V is an important element in the present invention because it not only improves the strength of the base material at normal temperature but also generates VC during tempering and improves the yield stress at a medium temperature of about 350 ° C. If it is less than 0.005%, the effect cannot be obtained sufficiently,
If it exceeds 0.10%, island-like martensite is formed in the HAZ to reduce the HAZ toughness.
The range was 0%.

Cu is an element effective for improving the strength of the base material. If it is less than 0.10%, the effect cannot be obtained sufficiently,
If it exceeds 1.0%, a large amount of ε-Cu is precipitated, and the HAZ toughness is reduced.

Ni is an element effective for improving the strength of the base material by increasing the hardenability and for improving the base material toughness and the HAZ toughness. If it is less than 0.10%, a sufficient effect of improving the base material strength and toughness cannot be obtained, and if it exceeds 1.0%, it is not economical.

Cr is an element effective in improving the strength of the base material by increasing the hardenability. 0.05%
If it is less than 0.50%, the effect cannot be sufficiently obtained. If it exceeds 0.50%, HAZ is hardened and HAZ toughness is reduced.
-0.50%.

In the present invention, the amounts of C and Mn are limited to low values,
The eq is also limited to a low value of 0.25 to 0.38, and it is impossible to secure a sufficient base material strength by the conventional normalizing method. Also needs to be limited.

After the steel slab is melted in a converter and an electric furnace,
Ladle scouring and vacuum degassing are performed as necessary, and the product is manufactured by a continuous casting method. Alternatively, the slab may be formed by lumping with a mold or a one-way solidification mold and then lumping.

Heating is performed to reduce the deformation resistance during rolling and to form a solid solution of part or all of Nb and V contained in the steel. If the temperature is lower than 950 ° C., they cannot be dissolved. If the temperature is higher than 1300 ° C., the austenite grain size becomes coarse, and it becomes difficult to refine the crystal grains by rolling, so that sufficient base material toughness cannot be secured. , Heating temperature 95
The range was 0 to 1300 ° C. In addition, the holding time of heating is preferably 60 to 180 minutes.

The rough rolling is performed to gradually reduce the austenite grain size by recrystallization in each rolling pass. Since it is necessary to recrystallize at the time of rolling, the temperature is defined as a recrystallization temperature range. The cumulative rolling reduction of the rough rolling is 10
%, Sufficient reduction of austenite grains cannot be obtained, and if it exceeds 90%, the rolling reduction of finish rolling cannot be ensured. Therefore, the cumulative rolling reduction is in the range of 10 to 90%. Here, the cumulative draft is ((slab thickness) − (transfer thickness)) /
(Slab thickness).

The finish rolling is performed to roll austenite in an unrecrystallized state and to introduce a deformation zone and a twin boundary in austenite grains. At low temperatures than Ar ₃ point, since the ferrite is formed, can not rolling in the austenite single phase and non-recrystallization for the state needs to be rolled, the temperature of the Ar ₃ point or more pre-recrystallization temperature region And Also,
If the cumulative rolling reduction of the finish rolling is less than 10%, a sufficient effect of refining the ferrite grains cannot be obtained, and if it exceeds 90%, it takes a long time for the finish rolling, and it is difficult to secure the temperature. The cumulative rolling reduction was in the range of 10 to 90%. Here, the cumulative draft is ((transfer thickness) − (finish thickness)) /
(Transfer thickness).

The controlled cooling is carried out in order to promote the deformation band in the austenite grains in the non-recrystallized state and the nucleation of ferrite transformation from the twin boundaries, to make the ferrite grains finer, and to obtain a sufficient base material strength. . If the cooling rate is less than 1 ° C./s, these effects cannot be obtained, and it is difficult to obtain a cooling rate of 100 ° C./s or more due to facility restrictions.
The range was 100 ° C./s. When the tempering described below is not performed, the above effect cannot be sufficiently obtained when the cooling stop temperature is higher than 650 ° C, and when the cooling stop temperature is lower than 500 ° C, the tempering effect during air cooling is insufficient and the base material toughness is reduced. Therefore, the cooling stop temperature was set to 650 to 500 ° C. In the case of performing tempering, sufficient base metal strength cannot be obtained unless the temperature is lowered to 200 ° C. or lower.
0 ° C. or less. The cooling rate and the tempering temperature are set at the center of the sheet thickness, 1/4 t according to the sheet thickness part of the required material value.
Any of the (t: plate thickness) portions may be used.

The tempering is performed to improve the base material toughness reduced by the controlled cooling. If the tempering temperature is lower than 500 ° C., the base metal toughness is not sufficiently improved. If the tempering temperature is higher than 650 ° C., the base metal toughness is improved.
The temperature range was from 00 to 650 ° C. In addition, the holding time of tempering is preferably 20 to 120 minutes.

Based on the above technical concept, the present invention has both good base material strength and excellent HAZ toughness,
An object of the present invention is to propose a steel plate for a medium- or normal-temperature pressure vessel in which post-weld heat treatment can be omitted, and a method for manufacturing the same.

In order to prevent low-temperature cracking in the welded portion as it is, the low-temperature cracking susceptibility evaluated by the following equation is also 0.11 to 0.11.
It is preferred to be in the range of 0.20. Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + N
i / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B The present invention is particularly effective when submerged arc welding (SAW) is performed. Arc welding using carbon dioxide gas (MAG), arc using inert gas, etc. Welding (MI
G), the same effect can be obtained by a welding method such as covered arc welding (SMAW) if the welding heat input is 50 kJ / cm or less.

[0055]

Embodiments of the present invention will be described below. In order to clarify the relationship between the chemical composition of steel and the manufacturing conditions and the properties of the base metal and the properties of the welded joint, steels having various chemical compositions shown in Table 1 were melted, and slabs obtained by continuous casting were prepared as shown in Table 2. Under the conditions shown in (1), a steel plate having a thickness of 9 to 120 mm was manufactured.

In order to investigate the strength of the base material at room temperature and medium temperature,
A tensile test was performed at normal temperature and 350 ° C. In addition, in order to evaluate the HAZ toughness of the welded joint, the X groove was machined, welded under the conditions shown in Table 3, and a Charpy impact test at −40 ° C. at the bond boundary was performed. Furthermore, to reproduce low stress fracture due to welding residual stress, 30 × 40
Create a 0x500mm cross weld joint, cut a notch along the bond boundary of a 400mm long bead at the intersection of the weld bead, and apply a tensile load in the weld line direction of the 500mm long bead. Cruciform joint wide test (Japan Welding Association, supervised product report, “The practice of high tensile steel welding” 198
Fracture toughness values were determined by 4 years, pp. 73-74, and it was confirmed whether post-weld heat treatment could be omitted.

Inventive steels 1 to 15 all exhibited good values for the base metal strength at room temperature and the yield stress at medium temperature. In addition, since high values were also obtained for the Charpy impact value and the fracture toughness value of the welded portion, it was confirmed that post-weld heat treatment could be omitted.

On the other hand, the comparative steel 16 has the components within the scope of the present invention, but the joint method is good because the manufacturing method is different, but the base metal strength at room temperature is low, and the medium temperature strength is accordingly low. Low. Since both the composition and the production method of the comparative steel 17 are out of the range of the present invention, the intermediate temperature strength is particularly reduced with respect to the base metal strength, and the joint properties are also reduced. Comparative steel 18 is 6
Since C + Mn and production conditions are out of the scope of the present invention,
Low joint properties and base material strength at normal and medium temperatures. Ceq and Ppr of the comparative steels 19 and 20 are out of the range, but since the manufacturing conditions are within the scope of the present invention, the joint properties are low although the base metal strength is high. From the above examples, it can be understood that the steel of the present invention, when satisfying the claims, has both good base metal strength at room temperature and medium temperature and excellent welded joint properties.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

[0062]

As described in detail above, according to the present invention,
With respect to a carbon steel sheet for a medium-pressure chamber used at a temperature range of about 350 ° C. to a normal temperature, it is possible to provide a steel sheet for a medium-temperature pressure vessel that can omit heat treatment after welding. Pressure vessels such as furnace containment vessels can be made larger. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 Tensile strength and HAZ in Charpy impact test
It is a figure which shows the relationship of C and Mn which affect toughness.

FIG. 2 is a diagram rearranging the results of the Charpy impact test and the tensile strength used in FIG. 1 for Ceq.

FIG. 3 is a diagram showing a relationship between a rise in yield stress due to addition of an alloy element and an amount of an alloy element in a tensile test at 350 ° C.

Claims (6)

[Claims] 1. C: 0.02 to 0.10% by weight, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.030% or less by weight%, S: : 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008% And the balance consists of iron and unavoidable impurity elements, and satisfies 6C + Mn: 1.2 to 2.0, Ceq: 0.25 to 0.38, and Ppr: 2.0 to 40. A steel plate for medium and normal pressure vessels that can be omitted from post-weld heat treatment. Here, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2 2. C: 0.02 to 0.10%, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.030% or less by weight%, S: : 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008% Mo: 0.03 to 0.50%, V: 0.005 to 0.10%, containing one or two of the following, and the balance consisting of iron and unavoidable impurity elements: 6C + Mn: A steel plate for a medium-pressure container that can be omitted from post-weld heat treatment, satisfying 1.2 to 2.0, Ceq: 0.25 to 0.38, and Ppr: 2.0 to 40. However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2 3. C: 0.02 to 0.10% by weight, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.030% or less by weight%, S: : 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008% And further contains one or more of Cu: 0.10 to 1.0%, Ni: 0.10 to 1.0%, and Cr: 0.05 to 0.50%, with the balance being Is composed of iron and an unavoidable impurity element, and satisfies 6C + Mn: 1.2 to 2.0, Ceq: 0.25 to 0.38, and Ppr: 2.0 to 40. Optional steel plate for medium and normal temperature pressure vessels. However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2 4. In% by weight, C: 0.02 to 0.10%, Si: 0.05 to 0.5%, Mn: 0.8 to 1.6%, P: 0.030% or less, S: : 0.020% or less, Nb: 0.005 to 0.030%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.08%, N: 0.002 to 0.008% Mo: 0.03 to 0.50%; V: 0.005 to 0.10% One or two of the following: Cu: 0.10 to 1.0%, Ni : 0.10 to 1.0%, Cr: 0.05 to 0.50% Contains one or more of the following, and the balance consists of iron and inevitable impurity elements. 6C + Mn: 1.2 to 2 0.0, Ceq: 0.25 to 0.38, Ppr: 2.0 to 40 Steel sheet for cold pressure vessel in an optional. However, Ceq = C + Si / 24 + Mn / 6 + Ni / 60 + Cr / 5
+ Mo / 4 + V / 14 Ppr = 27Nb ^1/2 + 55Mo ^1/2 + 14V ^1/2 5. A steel slab having the chemical composition according to any one of claims 1 to 4, which is heated to 950 to 1300 ° C.
Rough rolling is performed in the recrystallization temperature range with a cumulative draft of 10 to 90%, followed by finish rolling with a cumulative draft of 10 to 90% in the non-recrystallization temperature range of _three or more Ar points. 1
A method for manufacturing a steel plate for a medium- or normal-temperature pressure vessel, in which post-weld heat treatment can be omitted, comprising controlled cooling to 650 to 500 ° C. at １００100 ° C./s and air cooling to room temperature. 6. A steel slab having the chemical composition according to claim 1 is heated to 950 to 1300 ° C.,
Rough rolling is performed in the recrystallization temperature range with a cumulative draft of 10 to 90%, followed by finish rolling with a cumulative draft of 10 to 90% in the non-recrystallization temperature range of _three or more Ar points. 1
Controlled cooling to 200 ° C or less at ~ 100 ° C / s, then
A method for producing a steel plate for a medium-pressure container capable of eliminating heat treatment after welding, wherein tempering is performed at 500 ° C to 650 ° C.