JPH0539542A - Steel for pressure vessel excellent in electron beam weldability - Google Patents

Abstract

PURPOSE:To provide a steel for pressure vessel excellent in electron beam weldability. CONSTITUTION:The objective steel for a pressure vessel is constituted of a compsn. contg., as essential components, 0.17 to 0.35% C, 0.05 to 0.45% Si, 0.6 to 1.7% Mn, <=0.010% P, <=0.010% S, 0.005 to 0.040% Al, 0.0003 to 0.0012% B and <=0.006% N and contg., at need, one or >= two kinds among the elemental group for improving its strength of Cu, Ni, Cr, Mo, Nb and V. This steel has good toughness by electron beam welding, laser beam welding and flash butt welding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure vessel steel having excellent electron beam welding characteristics.

[0002]

2. Description of the Related Art Sufficient consideration has been given to the safety of boiler / pressure vessel equipment, especially the safety at the time of water pressure test, and a certain toughness is also required for steel for pressure vessels. The demand is naturally made also on the welded portion forming a part of the structure. Conventional welding of steel for pressure vessels is performed by latent welding (SA
W) is the main subject. In these weldings, the number of laminated layers increases at an accelerating rate as the plate thickness increases. For example, with a material having a plate thickness of 100 mm, it is necessary to stack 20 passes or more in SAW welding even if a narrow groove is applied. The construction time associated therewith becomes enormous. In order to improve the welding efficiency and meet the toughness requirements, application of electron beam welding has come to be considered. Electron beam welding is 50m thicker than conventional arc welding (SAW welding)
In the range exceeding m, it becomes a region advantageous in terms of cost, and the effect increases as the plate thickness increases. However, unlike conventional welding methods, electron beam welding involves melting and joining the steel sheet itself, and therefore in the production of the steel sheet, it is necessary to design the composition in consideration of this welded portion, particularly toughness. Heretofore, as publicly known documents relating to structural steel, there are JP-A-2-77557, JP-A-2-77561, JP-A-2-77562 and JP-A-2-277743, but the electron beam welded portion is further improved. Structural steel with excellent toughness is required.

[0003]

The object of the present invention was made in view of the above points, and a pressure vessel having excellent electron beam welding characteristics with good low temperature toughness of the welded portion even if welding by electron beam welding is performed. It is to provide steel materials for use.

[0004]

The present invention, in% by weight, comprises C:
0.17 to 0.35%, Si: 0.05 to 0.45%,
Mn: 0.6 to 1.70%, P: 0.010% or less,
S: 0.010% or less, Al: 0.005-0.040
%, B: 0.0003 to 0.0012%, N: 0.00
6% or less, balance Fe, and inevitable impurities, steel material for pressure vessel having excellent electron beam welding characteristics, and C: 0.17 to 0.35% by weight, S
i: 0.05 to 0.45%, Mn: 0.6 to 1.70
%, P: 0.010% or less, S: 0.010% or less, A
1: 0.005-0.040%, B: 0.0003-
0.0012%, N: 0.006% or less as a basic component, and Cu ≦ 2.0%, Ni ≦ 4.0%, Cr ≦
1.0%, Mo ≦ 1.0%, Nb ≦ 0.1%, V ≦ 0.
A steel material for a pressure vessel having excellent electron beam welding characteristics, characterized in that it contains one or more kinds of a strength improving element group consisting of 1% and the balance is Fe and inevitable impurities.

[0005]

In the electron beam welding, unlike the conventional welding method, another material is supplied to the welded portion to improve the characteristics of the welded portion, and the steel sheet itself is melted and welded. Therefore, a steel plate having high toughness is adjusted by a method such as grain refinement in the production of a steel plate, but since this is melted at a high temperature, the toughness becomes low. JP-A-2-77
557, JP-A-2-77561, and JP-A-2-775.
No. 62 and Japanese Patent Laid-Open No. 2-277743 disclose that coarse carbides and nitrides precipitated in the grain of the electron beam weld and at grain boundaries reduce the toughness of the electron beam weld. is there. As a result of various investigations to develop a steel material having a better toughness in the electron beam welded portion, the inventors have found that, in the case where the toughness is low, it is close to the point of failure of the Charpy test for evaluating the toughness of the welded portion. Microcracks are generated along with solidification segregation. This significantly reduces toughness. That is, it has been found that it is important to eliminate microcracks in order to improve the toughness of the electron beam welded portion, and for this purpose, solidification segregation reduction is necessary. In order to reduce this solidification segregation,
It has been found that the P and N contents are kept within a certain range, that is, the superposition of these effects significantly improves the toughness of the electron beam weld. Further, it was found that uniform precipitation of cementite in the grain by adding B is effective for preventing solidification segregation of C.

FIG. 1 is a diagram showing the influence of the amounts of P and N on the Charpy impact test value vE_ ₁₀ of the electron beam welded portion. The amount of C is 0.23%. By setting the P content to 0.010% or less and the N content to 0.006% or less, vE_ ₁₀
A toughness of ≧ 6 kgf · m can be obtained. Moreover, the effects of the individual components are not linear. For example, when N is 0.009% and P is decreased from 0.015 to 0.010%, v
To E_ ₁₀ is not only to 1.3kgf · m from 0.8, N: P is from 0.015 at 0.006% or 0.010
If you drop in%, 6.5 kg from VE_ ₁₀ 1.0
Greatly improved to f ・ m. The reasons for limiting the components will be described below. C is an element necessary to secure the strength, and is at least 0.
17% is required. However, if it exceeds 0.35%, the toughness of the electron beam welded portion is remarkably reduced, so the upper limit is set to 0.
35%. Since Si is an element that lowers the low temperature toughness and weldability, it is reduced as much as possible and 0.45% is made the upper limit. However, 0.05% is necessary for steelmaking. Mn is an element necessary to secure the strength, and it is necessary to add it in an amount of 0.6% or more. However, if it exceeds 1.7%, not only the weldability is deteriorated but also the cost is increased. Since it is not economical, the upper limit is 1.7%. As mentioned above, P is
It is necessary to reduce the toughness of the electron beam welded portion to 0.010% or less in order to reduce the toughness of the electron beam welded portion due to the generation of microcracks due to solidification segregation due to the overlapping action with C and N. S
Is an element harmful to toughness and is limited to 0.010% or less.

Al is required to be 0.005% or more for deoxidation, but if it exceeds 0.040%, the ratio with N becomes too small and the AlN precipitates become coarse and the toughness deteriorates, so the upper limit is 0. 0.040%. B is an essential element for improving the toughness of electron beam welds, and 0.0003%
Although it is added as described above, if it exceeds 0.0012%, the base material strength is excessively increased, so the upper limit is made 0.0012%. As described above, N reduces the toughness of the electron beam welded portion due to the generation of microcracks due to solidification segregation due to the synergistic action with P, so the upper limit is 0.006%. Cu, N
i, Cr, Mo, Nb and V have an equal effect of increasing the strength of steel, and if necessary, one or more of them may be contained. Cu: 2.0%, N
i: 4.0%, Cr: 1.0%, Mo: 1.0%, N
Even if the content exceeds the upper limit of the content of b: 0.1% and V: 0.1%, the effect is saturated, the cost is increased, and it is not economical. The content of each component was determined as described above. Either an electric furnace or a converter may be used for melting this steel. In forming the steel sheet, either forging or rolling may be used. Further, as the heat treatment of the steel sheet, it is possible to use normalizing, normalizing-tempering, or accelerating cooling in the case of thick material, as it is rolled.

[0008]

EXAMPLES Among the chemical components shown in Table 1, 1 to 10 are steels of the present invention, and 11 to 20 are comparative steels. Smelting of steel was carried out by a converter, and a slab was formed by a conventional method, and then rolled into a plate thickness shown in Table 1. In the heat treatment of the steel sheet, 1, 2, 11 to 13 are as-rolled, 3, 4, 6, 7, 14, 15 are normalized at 910 ° C., 5 and 16 are accelerated cooling, and 8 to 10 and 17 to 910 are 910. C. Normalization-Temperature of -650.degree. Table 1 shows the results of the tensile test, the Charpy impact test, and the electron beam welded Charpy impact test of the base materials of these steels. However, electron beam welding conditions are voltage 150 kV, current 180
mA, speed 20 cm / min. The notch position of the Charpy impact test of the electron beam weld is the center of the weld metal.

[0009]

[Table 1A]

[0010]

[Table 1B]

The steels 1 to 10 according to the present invention have good low temperature toughness of the electron beam welded portion due to their synergistic effect by adjusting the amounts of P and N in the appropriate ranges. The base material properties are also good. Next, the steels 11 and 12 have high P, the steel 13 has high N, the steels 14 and 15 have high P, the steel 16 has high N, the steel 17 has high P, and the electron beam welded portions have low toughness. .. Steel 18 has a low strength because of its low C content.
Steel 19 has a low B and the toughness of the electron beam weld is low. Steel 20 has high B, too high base metal strength, and low base metal toughness and electron beam welded toughness.

[0012]

As described above, according to the present invention, C,
By limiting the components of P and N to an appropriate range, it is possible to prevent the generation of microcracks due to solidification segregation, and to economically provide a steel material for pressure vessels with high toughness in the electron beam welded portion. It has a great effect.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of P amount and N amount on a Charpy impact test value of an electron beam welded portion.

Claims (2)

[Claims] 1. By weight%, C: 0.17 to 0.35% Si: 0.05 to 0.45% Mn: 0.6 to 1.70% P: 0.010% or less S: 0.0. 010% or less Al: 0.005 to 0.040% B: 0.0003 to 0.0012% N: 0.006% or less The balance of Fe and inevitable impurities is excellent in electron beam welding characteristics. Steel for pressure vessels. 2. C .: 0.17 to 0.35% Si: 0.05 to 0.45% Mn: 0.6 to 1.70% P: 0.010% or less S: 0.0. 010% or less Al: 0.005 to 0.040% B: 0.0003 to 0.0012% N: 0.006% or less as a basic component, and further Cu ≤ 2.0% Ni ≤ 4.0% Cr. ≦ 1.0% Mo ≦ 1.0% Nb ≦ 0.1% V ≦ 0.1% One or more members of the strength improving element group is contained, and the balance Fe and unavoidable impurities are included. A steel material for pressure vessels with excellent electron beam welding characteristics.