JPH06279941A - Low alloy steel tube of preweld and postweld heat treatment omitted type - Google Patents

Abstract

PURPOSE:To obtain a low alloy steel tube of preweld and postweld heat treatment-omitted type, capable of facilitating site welding, by specifying a composition consisting of C, Si, Mn, P, S, Cr, Mo, Al, N, and Fe and also specifying tube thickness. CONSTITUTION:A composition, consisting of, by weight, 0.06-0.10% C, 0.60-0.80% Si, 0.35-0.50% Mn, <=0.020% P, <=0.007% S, 1.10-1.30% Cr, 0.45-0.55% Mo, 0.002-0.010% Al, 0.002-0.010% N, and the balance Fe with inevitable impurities, is provided and the thickness of a tube is regulated to <=25mm to obtain the low alloy steel tube of preweld and postweld heat treatment omitted type. By specifying the contents of the constituent alloy components, limiting the contents of the impurities, P and S in particular and reducing tube thickness, proper joint characteristics can be obtained in this steel tube even if preweld heat treatment and post weld heat treatment in a welding site are omitted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low alloy steel pipe for omitting welding preheating and post heat treatment, and relates to STPA23 used in a power plant or the like, for example, a combined cycle steam pipe in LNG thermal power generation, a in-house steam pipe in nuclear power generation. It is intended to facilitate on-site welding of such steel pipes.

[0002]

2. Description of the Related Art 1.25Cr-0.5Mo steel pipes such as STPA23 have extremely high hardenability during welding. In the past, their hardness was lowered, and cold cracking during welding was prevented to prevent corrosion during use. Pre-heat treatment and post-heat treatment are required for the purpose of preventing force corrosion cracking.

That is, in order to prevent the occurrence of weld cracks, preheating is necessary, while the as-welded energy absorbed by the Charpy impact test at room temperature is 2 to 7 kgm, which is post-heat treated to 550 to 700 ° C. By 10
It is essential to improve it to about 15 kgm.

[0004] For example, on page 371 of the Welding Society of Japan, Volume 3, No. 2, published in 1985, C, Mn, Ni, Cr and Mo are given.
Pre-heating to 100 ° C. or higher in the coated arc welding of a steel plate containing a steel within a specific range, and a multi-layer SAW
It has been announced that preheating to 225 ° C. or higher before welding and the temperature between passes to 225 ° C. or higher do not require post heat treatment. A similar description is disclosed in Japanese Examined Patent Publication No. 61-56309.

Further, in the stress annealing standard and its explanation in HIPS E-111, SCMV3, STP
Steel with standard composition of A23 and STBA23 is 150 to 3
It is described that the temperature is preheated to 00 ° C. and the temperature between passes is set to 150 to 300 ° C. to eliminate the need for post-heating, and the same technique is disclosed in Japanese Patent Publication No. 61-56309.

[0006]

[Problems to be Solved by the Invention] As described above, 1.25
Since the heat-affected zone of Cr-0.5Mo steel is extremely hardened during welding, welding cold cracking is likely to occur. To prevent this, preheat treatment at 200 to 350 ° C is performed to It is necessary to reduce hardening and reduce the amount of diffusible hydrogen that causes weld cracking. In many cases, post-heat treatment at 620 to 700 ° C. is necessary for the purpose of softening the heat-affected zone of the weld and reducing the amount of diffusible hydrogen to restore toughness and at the same time prevent stress corrosion cracking.

It is clear that performing these treatments is complicated, has many disadvantages in terms of construction cost and construction period, and is not preferable in quality control. These disadvantages cannot be completely avoided even by the technique disclosed in the above-mentioned JP-A-61-56309.

[0008]

Means for Solving the Problems The present invention has been studied to solve the problems in the prior art as described above, and regarding the 1.25Cr-0.5Mo steel as described above, C, M
We have succeeded in obtaining favorable weld joint characteristics by omitting both the preheating and the post heat treatment by setting the composition range and the material tube thickness to specific ranges such as n, P, and S. Is.

In wt%, C: 0.06 to 0.10%, Si: 0.6
0-0.80%, Mn: 0.35-0.50%, P: 0.020%
Below, S: 0.007% or less, Cr: 1.10 to 1.30%, M
o: 0.45 to 0.55%, Al: 0.002 to 0.010%,
N: 0.002 to 0.010%, the balance consisting of Fe and unavoidable impurities, and the tube thickness thereof is set to 25 mm or less.

[0010]

The reason for limiting the component range of the material according to the present invention will be explained by wt% (hereinafter simply referred to as%). First, C is 0.06 to 0.10%. That is, C is an element that most affects the hardening of welds and eventually the occurrence of low temperature cracking, and the upper limit of 0.10% provides both crack prevention and hardness reduction, while the lower limit is 0.06%, Ensure strength.

Si: 0.60 to 0.80% Si must be contained as a deoxidizing agent and in an amount of 0.60% or more for the purpose of improving the heat resistance of steel. Further, by setting the upper limit to 0.80%, the toughness is secured and the hardness of the welded portion is lowered.

Mn: 0.35 to 0.50% Mn is required 0.35% for ensuring room temperature strength or for deoxidation and desulfurization. Further, in order to prevent the hardness of the welded portion from increasing, it is necessary to set the upper limit to 0.50%.

P: 0.020% or less By limiting P, which is an impurity element, to 0.020% or less, impact characteristics are improved and hardness of the welded portion is reduced.

S: 0.007% or less In order to improve the impact characteristics, it is necessary to set S to 0.007% or less.

Cr: 1.10 to 1.30% Cr is an element necessary for ensuring corrosion resistance, and for this purpose, 1.10% or more is required. However, when welding is assumed as in the present invention, it is necessary to set the upper limit to 1.30% in order to prevent hardening of the welded portion.

Mo: 0.45 to 0.55% Mo is an element necessary to secure high temperature strength, and for this purpose, it is necessary to contain 0.45% or more. However, when the content exceeds 0.55%, the weldability deteriorates, so it is necessary to set this to the upper limit.

Al: 0.002-0.010% Al has a function of improving deoxidation and oxidation resistance, and for these, it is necessary to contain 0.002% or more. However, if a large amount of this Al is contained, the workability is impaired,
Further, since it lowers the creep strength, it is necessary to set the upper limit to 0.10%.

N: 0.002-0.010% N is 0.002% to obtain high temperature strength by solid solution strengthening.
It is necessary to include the above. On the other hand, 0.010%
The effect will be saturated to some extent, so this is the upper limit.

The steel pipe of the present invention having the above-described composition is required to have a thickness of 25 mm or less.
That is, as the pipe thickness increases during welding, the cooling rate of the welding heat cycle increases and the weld hardness increases. Hv250
In order to obtain the following prescribed hardness, the pipe thickness must be 25 mm or less, and welding cracks are also due to the restraint stress generated during welding. This restraint stress is proportional to the pipe thickness of the joint. Grows. Therefore, in order to prevent excessive restraint stress from being generated in the joint portion, it is necessary to make the pipe thickness 25 mm or less.

[0020]

EXAMPLE 250A × having the chemical composition shown in Table 1 below
CMB96MB with 25 mm steel pipes A to J as welding materials
(Kobe Steel Co., Ltd. welding rod) 4mmφ 170A, 24
The result of multi-layer coating arc welding under the conditions of V and 8 to 15 cm / min is shown in FIG.

[0021]

[Table 1]

That is, when the C content is 0.10 wt% or less, the maximum weld hardness Hv10 is 250 or less.
The comparative steels of the steel pipes I and J show hardness exceeding Hv250.

Further, the relationship between the maximum hardness of the welded portion and the pipe thickness is as shown in FIG. 2, and by setting the pipe thickness to 25 mm or less, the hardness Hv of 250 or less is secured.

Further, the result of y-constraint cracking test for weld cracking is shown in FIG. 3, and no cracking occurred even without preheating.

The A steel pipe (250A × 2) shown in Table 1 is used.
The mechanical properties of the steel pipe are shown in Table 2 below, and the impact property values of this steel pipe are as summarized in FIG.

[0026]

[Table 2]

Regarding joint performance, as shown in FIG.
Covered arc welding was performed at a V groove of 0 °, and the welding conditions were as shown in Table 3 below, but the preheating and postheating were not performed.

[0028]

[Table 3]

The tensile test results for the joints thus obtained are shown in Table 4 below. The tensile strength shows a sufficiently large value, and the fracture position is the base metal portion.

[0030]

[Table 4]

The measurement results of the impact characteristics of each joint are shown in FIG. 6, and the test was conducted at each position of the weld metal center and the heat-affected zone. Good impact characteristic values were obtained at any notch positions. It is clear that it has.

Further, FIG. 7 shows the hardness distribution of the joint part.
A Vickers hardness test (load 10 kg) was performed at a position 2 mm from the upper surface of the surface layer, the center of the tube thickness and 2 mm of the lower surface layer, and the measurement pitch was 1.0 mm for the base metal and weld metal, and for the heat affected zone
Although it is 0.5 mm, Hv250 or less is secured in all parts.

[0033]

According to the present invention as described above,
In the field welding of 1.25Cr-0.5Mo steel pipe, neither preheating nor post heat treatment can be omitted to obtain proper joint characteristics, which is advantageous in terms of quality control and construction cost. This is an invention that has a large effect industrially, such as obtaining a preferable joint.

[Brief description of drawings]

FIG. 1 is a chart summarizing the relationship between the C content in steel and the maximum hardness of a welded portion.

FIG. 2 is a chart summarizing the relationship between the thickness of a steel pipe and the maximum hardness of a welded portion.

FIG. 3 is a chart summarizing the results of the y-restraint cracking test.

FIG. 4 is a chart summarizing the results of a tube impact test.

FIG. 5 is an explanatory diagram of a V groove in an example of the present invention.

FIG. 6 is a chart summarizing the impact test results of the joint portion.

FIG. 7 is a chart showing the measurement results regarding the hardness distribution at each position of the joint portion.

Claims (1)

[Claims] 1. W: wt%, C: 0.06 to 0.10%, Si: 0.0.
60 to 0.80%, Mn: 0.35 to 0.50%, P: 0.020
% Or less, S: 0.007% or less, Cr: 1.10 to 1.30%,
Mo: 0.45 to 0.55%, Al: 0.002 to 0.010%,
N: 0.002 to 0.010%, the balance consisting of Fe and unavoidable impurities, and the tube thickness thereof is set to 25 mm or less.