JPH0539540A - Steel for structure excellent in electron beam weldability - Google Patents

Abstract

PURPOSE:To provide a steel for structure excellent in electron beam weldability. CONSTITUTION:The objective steel for structure is constituted of a compsn. contg., as essential components, 0.04 to 0.13% C, 0.05 to 0.45% Si, 0.6 to 1.7% Mn, <=0.007% P, <=0.005% S, 0.005 to 0.040% Al and 0.003 to 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]

FIELD OF THE INVENTION The present invention relates to a structural steel suitable for electron beam welding, laser beam welding or flash butt welding. In the iron and steel industry, it is most preferable to apply it to a thick plate mill, but it is also applicable to hot coils, shaped steel and the like.

[0002]

2. Description of the Related Art Interest in the safety of nuclear power generation facilities, boilers, pressure vessel facilities, etc. has been increasing in recent years, and the demands on the toughness values for structural steels used in these facilities are extremely strict. Is becoming The demand is naturally made also on the welded portion forming a part of the structure.
The conventional welding of structural steel is mainly performed by latent welding (SAW). 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. Compared with conventional arc welding (SAW welding), electron beam welding is a region that is more cost effective in the range where the plate thickness exceeds 50 mm, 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. So far
As a known document concerning structural steel, Japanese Patent Laid-Open No. 2-775
57, JP-A-2-77561, and JP-A-2-7756.
No. 2 and Japanese Patent Laid-Open No. 2-277743, there is a demand for a structural steel having even higher toughness in an electron beam weld.

[0003]

The object of the present invention was made in view of the above points, and a structure having good low temperature toughness of a welded portion even if welding by electron beam welding, laser beam welding or flash butt welding is performed. To provide steel for use.

[0004]

The present invention, in weight percent, comprises:
C: 0.04% or more and less than 0.13%, Si: 0.05 to
0.45%, Mn: 0.6% or more and less than 1.70%, P:
0.007% or less, S: 0.005% or less, Al: 0.
005 to 0.040%, N: 0.003 to 0.006
%, The balance Fe, and unavoidable impurities, which is a structural steel having excellent electron beam welding characteristics, and C: 0.04% or more and less than 0.13%, Si: 0.
05-0.45%, Mn: 0.6% or more and less than 1.70%, P: 0.007% or less, S: 0.005% or less, A
1: 0.005 to 0.040%, N: 0.003 to 0.
006% as a basic component, and Cu ≦ 2.0%,
Ni ≦ 9.5%, Cr ≦ 1.0%, Mo ≦ 1.0%, N
Electron beam welding characteristics characterized by containing one or two or more of a strength improving element group consisting of b ≦ 0.1% and V ≦ 0.1%, and the balance Fe and unavoidable impurities. It is an excellent structural steel.

[0005]

[Function] In electron beam welding, laser beam welding, flash butt welding, etc., unlike the conventional welding method, instead of supplying another material to the welded part to improve the characteristics of the welded part, the steel sheet itself is melted and welded. To do. for that reason,
In manufacturing a steel sheet, a steel sheet having high toughness is adjusted by a method such as grain refinement, but since this is melted at a high temperature, the toughness becomes low. Japanese Patent Laid-Open No. 2-77557
The findings disclosed in JP-A-2-77561, JP-A-2-77562, and JP-A-2-27743 are that coarse carbides and nitrides precipitated in the grain of the electron beam welded portion and at grain boundaries. Reduces the toughness of the electron beam weld. 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 the solidification segregation of the electron beam welded portion, the content of C, P and N is kept within a certain range, that is, the toughness of the electron beam welded portion is remarkably improved by the superposition of these effects. Was discovered.

As shown in FIGS. 1A to 1D, C
Content of 0.04% or more and less than 0.13%, P content of 0.007
% Or less, good toughness of vE_ ₄₀ ≧ 5kgf · m is obtained by putting N amount in the range of 30 or more 60ppm or less. The reasons for limiting the components will be described below. C is an element that is harmful to toughness. As described above, when it is 0.13% or more, superposition of P and N causes microcracking due to solidification segregation, which significantly reduces the toughness of the electron beam weld. However, it is difficult to satisfy the specifications of the structural steel, so the content is set to less than 0.13%. However, if less than 0.04%, cracks occur in the welded portion, so the lower limit is 0.04%. 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 that increases the hardenability, and it is necessary to add 0.60% or more to secure the strength, but if it is contained 1.70% or more, the weldability decreases, so 1.70%. The upper limit was less than. As described above, since P causes microcracks due to solidification segregation, it is necessary to reduce P to 0.007% or less. S is an element harmful to toughness and is limited to 0.005% or less. Al is required to be 0.005% or more in terms of deoxidation, but if it exceeds 0.040%, the ratio with N becomes too small and the AlN precipitates become coarse and the toughness rather deteriorates, so the upper limit is 0.040%. And As described above, N causes microcracks due to solidification segregation due to the overlapping action with C and P, so N is made 0.006% or less.
However, if it is too low, grain refinement by AlN cannot be achieved, so the lower limit is made 0.003%. Cu, Ni, 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%, Ni: 9.5%,
Even if the content exceeds the upper limit content of Cr: 1.0%, Mo: 1.0%, Nb: 0.1% and V: 0.1%, the effect is saturated and the cost is increased. However, since it is not economical, the content of each component of the above-mentioned strength improving element group 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.

[0007]

EXAMPLES Among the chemical components shown in Table 1, 1 to 7 are the steels of the present invention, and 8 to 15 are comparative steels. Steel was melted by a converter, made into a slab by a conventional method, and then rolled into a plate having a thickness shown in Table 1. The heat treatment conditions of the steel sheet are steel 1 to 3 and 8 to 1
0 is quenching: 910 ° C water cooling, tempering: 650 ° C, steel 4 ~
Nos. 7, 11 to 15 are as-rolled. Table 1 shows 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 150kV, current 180mA, speed 20c.
m / min. Is. The notch position of the Charpy test of the electron beam weld is the center of the weld metal.

[0008]

[Table 1A]

[0009]

[Table 1B]

The steels 1 to 7 of the present invention have good low temperature toughness of the electron beam welded portion due to the superposition effect of C, P and N by setting the amounts of C, P and N in appropriate ranges. The base material toughness is also good. Next, Steel 8 has a high P and the toughness of the electron beam weld is low. Steel 9 has a high C, and has low toughness in the base material and particularly in the electron beam welded portion. Steel 10 has high P and steel 1
1 has a high N, steel 12 has a high Si, steel 13 has a high Mn, steel 14 has a high S, steel 15 has a high Al, and the toughness of the electron beam welds is low.

[0011]

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 structural steel with high toughness for electron beam welds. 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: 0.04% or more and less than 0.13% Si: 0.05 to 0.45% Mn: 0.6% or more and less than 1.7% P: 0.007% or less S: 0.005% or less Al: 0.005 to 0.040% N: 0.003 to 0.006% A balance Fe, and an unavoidable impurity, for a structure having excellent electron beam welding characteristics steel. 2. In wt%, C: 0.04% or more and less than 0.13% Si: 0.05 to 0.45% Mn: 0.6% or more and less than 1.7% P: 0.007% or less S: 0.005% or less Al: 0.005 to 0.040% N: 0.003 to 0.006% as a basic component, and further Cu ≤ 2.0% Ni ≤ 9.5% Cr ≤ 1. 0% Mo ≤1.0% Nb ≤0.1% V ≤0.1% One or more of the strength improving element group is contained, and the balance Fe and unavoidable impurities are contained. Structural steel with excellent electron beam welding characteristics.