JPS63206452A - Electric resistance welded steel pipe for boiler and heat exchanger - Google Patents

Abstract

PURPOSE:To permit local restriction of a normalizing treatment of an electric resistance welded steel pipe for boilers and heat exchangers and to reduce the heat treatment cost thereof by decreasing the content of impurity S in said steel pipe as far as possible and incorporating a specific ratio of Ca therein so as to comply with the content of S. CONSTITUTION:Steel materials having the compsn. contg., by weight %, <0.20% C, <0.35% Si, <1.70% Mn, <0.015% P, <0.003% S, and <0.008% Ca, and having 1.5-3.0 ratio of Ca/S or further contg. 1 or 2 kinds of 0.03-0.20% Cu, and <0.05% Mo are used as the steel materials for producing the electric resistance welded steel pipe for boilers and heat exchangers. The content of S is decreased as far as possible as this element induces groove-like corrosion in a weld zone in the case of forming the pipe by using such steel sheet and subjecting the same to electric resistance welding. The defect thereof is eliminated by adding Ca to the steel to form a stable sulfide therein. This steel pipe is usable as the steel pipe for boilers and heat exchangers in the as-welded state or after the part near the weld zone is subjected to the normalizing alone.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to electric resistance welded steel pipes used for boilers and heat exchangers.

(Prior technology) Conventionally, for electric resistance welded steel pipes other than hot finished and cold finished used for heat exchangers, in the case of carbon steel pipes, the entire steel pipe should be normalized as a heat treatment. is defined in standards such as JIS G-3461.

(Problems to be Solved by the Invention) However, in order to perform the heat treatment as described above, a heat treatment furnace, energy resources necessary for heating, and manpower for operation are required. In addition, in order to prevent surface quality deterioration of steel pipes due to oxidation and decarburization due to heat treatment, more expensive equipment such as non-oxidation-reduction atmosphere control is required, and significant manufacturing costs are required due to straightening to prevent deformation due to heating. This is forcing costs to rise.

This is clearly a waste of resources if there are other alternatives, and considering the current economic situation in our country, it is an essential requirement for manufacturers to reduce manufacturing costs as much as possible.

The purpose of the present invention is to reduce the waste of resources and reduce the cost. Our objective is to provide steel pipes for heat exchangers.

(Precautions to Solve Problems) The present invention has the following features: (1) C0.2% or less, sx 0.35 or less, Mn1.
705 or less, P 0.015% or less, 80.003% or less, Ca 0.0080% or less,
) is 1,5-3.0, Cu 0.03-3.0 as necessary
0.20%, Mo alone or in combination with 0.05% or less, and the balance is iron and unavoidable elements -! It is characterized by being used up to l? Electric resistance welded steel pipe for heat exchanger (2) C0, 25% or less, Si 0.35% or less, Mn1
．． The ratio of Ca and S (C&/
S) is 1.0 to 2. O, Cu0.03~ as necessary
Electric resistance welding for boilers and heat exchangers used by locally normalizing the welded part and its vicinity, in which 0o15%, Mo 0.05% or less is used alone or in combination, and the remainder is iron and unavoidable elements. It is a steel pipe.

Generally, steel pipes for boilers and heat exchangers are used for boilers, heat exchangers,
When used as a material for manufacturing, various processes such as cold and hot bending, tube expansion, tube shrinkage, cutting, and explosion welding are performed. Therefore, the ease with which these materials can be processed, that is, their workability, is greater than a certain standard value, which is an important point in determining the superiority or inferiority of the material.

In addition, after production is completed, it is operated under the usage conditions set based on the design conditions, and the high-temperature properties (creep properties, high-temperature tensile properties, etc.) and corrosion resistance under those environmental conditions exceed the standard values. This also serves as a guideline for determining the superiority or inferiority of the material.

In light of the above-mentioned circumstances, for the steel pipe of the present invention to be put into practical use, it is essential that it has the same properties as conventional heat-treated steel pipes.

Below, specific devices and means for the present invention to satisfy the above-mentioned requirements will be explained.

C is an effective element that increases strength (including high-temperature strength, creep properties, etc.) at a low cost. There is a limit. Furthermore, during electrical resistance welding, which is the ninth step in pipe manufacturing, the welded portion is significantly hardened due to the heat effect, impairing workability. Therefore, the permissible upper limit of C is 0.20% or less in the case of explosive contact, and 0.25% in the case of normalizing the welded part and its vicinity.

Sl is an essential element for deoxidation, but if too much it deteriorates ductility, its upper limit is 0.35%.

Mn has the same tendency as C, but its influence is much milder than that of C, so it is desirable to actively increase its addition amount, and the addition amount is allowed up to 1.70%. It is possible.

P is an element that reduces not only workability and corrosion resistance but also creep strength, and as an impurity, the smaller the amount, the better.
From a practical standpoint, it is 0.015% if the explosion weld is left as is, and 0.020% if the welded part and its vicinity are normalized.
is the upper limit.

Although it is similar to 84P, it is an element that further promotes the occurrence of groove-like corrosion in electric resistance welds for pipe manufacturing. Therefore, its content must be strictly limited. The content must be 0.003% or less when as-welded, and 0.005% or less when the welded portion and its vicinity are normalized.

Ca is added to actively eliminate the adverse effects of S, and to form more stable sulfides in the steel. In this case, if the amount of Ca added is too large, the absolute amount of nonmetallic inclusions in the steel will increase and have an adverse effect, so the amount added is 60. oos% or less, and C
This effect was achieved by limiting the ratio of the amounts of a and S added to 1.5 to 3.0. Further, when normalizing the welded portion and its vicinity, the ratio of the amounts of Ca and S added may be 1.0 to .2.0. As for Ca, there are other elements that have similar effects; for example, REM, Zr, Ti, etc. can be used in place of or in combination with Ca.

Cu and Mo, which are added as necessary, are as good as conventional ones in terms of usability due to the above-mentioned measures, but they are added for further improvement.

Cu is added to improve corrosion resistance, and a minimum content of 0.03% is required to exhibit its effectiveness, but if it is too much, defects such as cracks are likely to occur during the manufacturing stage. , 0.20% is the upper limit. In addition, when normalizing the welded part and its vicinity, the upper limit value is also 0.1.
5% is sufficient.

Mo is particularly effective in improving high-temperature properties (high-temperature strength, creep properties, etc.), but if too much Mo deteriorates workability, it should be kept at an upper limit of t-0.05%.

It goes without saying that when the welded part and its vicinity are normalized, the quality of the weld is improved, so the upper limit of the chemical composition is slightly wider.

By combining the above measures, we were able to normalize the as-welded area or the welded area and its vicinity, which has sufficient performance as shown in the following examples. Finally, electrical resistance welded steel pipes for heat exchangers are realized.

(Example) Examples are shown below.

Table 1 shows the gills that can be used as welded.

An example of the chemical composition of steel pipes for heat exchangers is shown in comparison with comparative steel, and its mechanical properties are shown in Table 2. In terms of chemical composition, the steel of the present invention has slightly lower C1, higher Mn, and lower carbon than comparative steel.
, low S content, and addition of Ca and some of Cu and Mo. In terms of mechanical properties, the strength is slightly higher due to work hardening due to molding, and the ductility is accordingly slightly lower, but in practical terms they are almost the same and there is no difference.

In terms of hardness, the air resistance welded part hardens under the influence of heat, and its hardness is higher than that of the base material. However, as shown in the results of the practical tests in Table 3, it can be seen that in practice there is no problem in terms of workability and corrosion properties, as there is no difference from the comparison steel.

In particular, since the structure is used for a long period of time, its corrosion resistance is important, but the structure is good, with no groove corrosion, which is said to be apt to occur in electric resistance welded steel pipes.

Next, the electrical resistance welded part and its vicinity are normalized before use. Now, an example of steel pipes for heat exchangers will be described.

Table 4 shows examples of its chemical composition, Table 5 shows its mechanical properties, and Table 6 shows the results of investigations regarding workability and corrosion resistance as practical test examples in comparison with comparative steels. All of them show good results, similar to the as-welded steel mentioned above, with no difference from the comparison steel. In particular, it can be said that the weld maintainability is excellent in that the hardness of the heat-affected zone of the weld where the welded part and its vicinity are normalized is low.

(Effects of the Invention) As can be seen from the above examples, the present invention is comparable in performance to conventional heat-treated steel pipes.

The electrical resistance welded steel pipe for heat exchangers of the present invention does not require heat treatment such as normalizing the entire pipe as in the past.
Since it can be used as welded or by only locally normalizing the welded part and its vicinity, it is very advantageous in terms of cost.

1-1

Claims (2)

[Claims] (1) C 0.20% or less, Si 0.35% or less, Mn 1.70% or less, P 0.015% or less, S 0.003% or less, Ca 0.0080% or less, and the ratio of Ca to S (Ca/S) is 1.5 to 3.0. If necessary, 0.03 to 0.20% of Cu and 0.05% or less of Mo are used singly or together, and the remainder is iron and unavoidable elements. Electric resistance welded steel pipes for boilers and heat exchangers. (2) C 0.25% or less, Si 0.35% or less, Mn 1.70% or less, P 0.020% or less, S 0.005% or less, Ca 0.0080% or less, and the ratio of Ca to S (Ca/S) 1.0 to 2.0, if necessary, Cu 0.03 to 0.15% and Mo 0.05% or less may be used alone or in combination, with the remainder being iron and other unavoidable elements. Electric resistance welded steel pipes for boilers and heat exchangers that are normalized and used for use in boilers and heat exchangers.