JPS6027731B2 - Method for manufacturing steel pipes for steam transport that exhibit high strength in the medium temperature range of 200 to 450°C - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing steel pipes for steam transport that exhibit high strength in medium temperature steel pipes of 200 to 45 pi. Alternatively, the present invention relates to a method for producing a steel pipe that exhibits high strength in the temperature range of 200 to 450 qo (hereinafter referred to as medium temperature castle) and has good weldability during pipeline construction.

Conventionally, steel pipes for various uses have been manufactured. Recently, as a new property, it does not soften when heated to a medium temperature like 200 to 45,000, but rather,
Properties that can maintain or exhibit strength equal to or higher than that at room temperature are required in new energy development fields such as oil sand development and geothermal development. In addition, since steel pipes for this purpose have not been developed, generally JIS, STPG 38-42, STS 35-49,
SmT paper~49 or ASTM, A I06G de
Steel pipes for pressure arrangement such as A to C are intended for the above applications. However, the reason why these steel pipes are used is mainly for economic reasons, and even though there are no problems with workability or oxidation properties, they are made from carbon steel with a high C content, and the above-mentioned However, sufficient strength cannot be developed in the medium temperature range, and this problem becomes especially noticeable in thick-walled products. In addition to this, weldability is poor, and although this problem does not often occur in small-diameter, thin-walled pipes, this problem becomes fatal when the wall becomes thicker, and this problem is common in large-diameter, thick-walled pipes. On the other hand, the pace of new energy development has recently increased, and in response to this new demand, steam piping for oil sand development using the steam injection method and geothermal development are replacing these carbon steel pipes. There is a need for new steel pipes that are compatible with pipelines that transport produced steam, with large outer diameters, wall thicknesses, and long pipe lengths. It is necessary to equip.

That is, when using a normal pipe material, even if the working pressure is constant, if the outside diameter of the pipe is doubled, the wall thickness of the pipe must be approximately doubled accordingly.

However, as the wall thickness increases, the cooling rate after the root bath during circumferential welding of the tube increases, which increases the risk of hydrogen cracking. In order to reduce the risk of hydrogen cracking, it is necessary to increase the preliminary temperature before welding, but if the piping length is long, increasing the core heat temperature is undesirable as it will significantly impede welding efficiency. Furthermore, the thicker the pipe, the more multi-layered the circumferential welding, so welding efficiency is significantly reduced in this respect as well. On the other hand, when a high-strength steel pipe with high allowable stress is used instead of a normal pipe material, it is possible to reduce the thickness of the pipe.

However, when using alloy steel such as STPA12-26, heat treatment is required after welding, which not only reduces the overall welding efficiency but also increases the construction cost due to the increase in the unit price of L steel. . As mentioned above, the optimal material for new applications has not yet been discovered, and if conventional materials are used, construction costs will increase, which will impair the economic efficiency of new energy, and eventually This is also one of the reasons for delaying the development itself.

The present invention was developed in view of the above circumstances, and specifically, despite the low C content, it is possible to dramatically increase the strength of pipes without softening in the medium temperature range. We propose a method for manufacturing steel pipes for steam transport that can be made thinner and thicker without any problems, and also exhibit high strength in medium-temperature steel pipes that have good weldability during pipeline construction.

That is, in the method of the present invention, C: 0.03 to 0.03 in weight percentage
0.15%, Si: 0.01-0.80%, Mn: 0.
50-2.00%, Mo: 0.01-0.30%, V: 0.005-0.10 and A: 0.
001 to 0.10%, with the balance consisting of Fe and impurities, is heated to a temperature of 3 Ac or higher, placed next to it, and then tempered at a temperature of 600 qo to 600 qo or lower, Ac, or lower. do. First, in the method of the present invention, Mo and V are added to the material of the steel pipe in an appropriate range, and this is heated, quenched, and re-bowled, so it has high strength in the medium temperature range despite its low C. manifest.

In other words, conventional steel pipes are given properties such as normal temperature through heat treatment such as quenching and reheating, but when they are reheated to medium temperature by transport steam, their properties change, making it difficult to use them in such heated conditions. Not applicable. Accordingly, the inventors conducted research on this point and found that when a steel pipe is heated and subjected to a hardening process, adding Mo and V in combination can maintain sufficiently high strength in the operating temperature range of 200 to 450 qo. Furthermore, it was discovered that the joint maintains sufficiently high strength in the above temperature range not only during use but also when reheated during welding. According to the method of the present invention, the reason why steel pipes have high strength in the medium temperature range is due to the introduction of high dislocation density through quenching treatment, the precipitation of fine carbides through competitive treatment, and the pinning of dislocations by these precipitates. This is based on the fact that an appropriate dislocation density can be maintained. However, the inventors further investigated and found that the high strength achieved in this way is 450
They found that it is possible to maintain the weld heat-affected zone satisfactorily at temperatures up to about 100°F, and that even in the weld heat-affected zone where the carbides have been remelted by reheating, high strength is maintained due to the reprecipitation of the carbides in the medium-temperature castle. First, in the steel pipes manufactured by the method of the present invention, C, which is the most harmful in terms of susceptibility to hydrogen cracking during welding, is significantly reduced compared to conventional steel pipes, so the preheating temperature can be lowered, making it possible to Weldability is extremely good.

・Next, the amount of C in the component is reduced, but from 200 to 4
Since it shows significantly high strength when used in the temperature range of 50°C, it can be made thinner than conventional products, and it also reduces the core heat temperature and the number of welding passes required for one joint. This can significantly improve welding efficiency.

The method of the present invention will be explained below in order from the composition of the steel pipe.

C: C is effective in increasing the tensile strength of around 300 qo, and also causes fine precipitation of carbides during recovery to increase the yield strength, so a certain amount is necessary.

Therefore, in the present invention, the lower limit is also set at 0.03%. However, if the amount is too high, it is the most harmful element in terms of weldability, and if it exceeds 0.15%, the risk of hydrogen cracking increases rapidly, so this is the upper limit. Si: Si is a useful element for deoxidation, and usually requires a content of 0.01% or more. It also has a hardening effect and is effective in terms of oxidation resistance, but if it exceeds 0.80% The range of Sj is 0.01 to 0.80% because the plowability and weldability will be significantly deteriorated.
And so.

Mn: Mn contributes to strengthening not only by strengthening hard ports but also by improving impregnability, so at least 0.50% is necessary.

Furthermore, since Mn has a strong polarization tendency and an excessive amount increases the risk of weld cracking, the upper limit is set at 2.0%. Mo:
Mo is an element that plays an important role as described above in the method of the present invention.The presence of Mo improves hardenability and strengthens precipitation during bran resetting. Achieves high strength in In addition, Mo is effective in maintaining strength on the high temperature side, and if it is less than 0.01%, it has no effect, and if it exceeds 0.30%, weldability is impaired, and since it is an expensive element, the upper limit is 0. .30%. V: A feature of the present invention is that V is added together with Mo, and high strength in medium temperature castles is achieved by the combined addition of these. However, if V is less than 0.005%, there is no effect, and if it exceeds 0.10%, it not only impairs weldability but also deteriorates rutting. 0.005 to 0.10%. A: A is a useful element for deoxidizing, fixes impurity nitrogen in steel, and effectively contributes to grain formation of steel, so it should be at least 0.
001% is necessary.

However, too much content causes deterioration of toughness, so the upper limit was set at 0.10%. Contains each component as described above, with the remainder being Fe.
If a steel pipe consisting of impurities is heated to Ac3 point or higher, quenched, and then tempered at a temperature of 600 qo or higher and Ac, or lower, a steel pipe that maintains strength at medium temperature and has excellent weldability can be manufactured.

In addition, the reason why the heating temperature during quenching of the steel pipe with the above composition was set to 3 Ac or higher is because if the heating temperature is less than 3 Ac, a uniform austenitic state cannot be obtained and the strength after quenching becomes unstable. It is.

In addition, the range of the dawn reheating temperature was 600oo or more Ac, or less than 1 point. This is because in the case of phosphorus rehydration, due to the two-phase structure, a sudden decrease in strength occurs, making it impossible to obtain the required strength.

Next, examples will be described.

First, Table 1 shows the composition of seamless steel pipes used as test materials, among which steel pipes A to C are steel pipes with compositions that fall within the scope of the present invention, and steel pipes J to L are steel pipes with compositions that fall within the scope of the present invention. This is the composition of the steel pipe for use.

Next, among these, steel pipes A to C were heated to 92% by induction heating.
After heating from 0 to 940 o'clock, and then water quenching,
The steel pipes J to L were tempered by induction heating at a temperature of 650 to 670 oo, whereas the steel pipes J to L were either normalized by a conventional method or further tempered after normalization. The steel pipe obtained in this way was tested at room temperature and at 3500°C.
Each strength at 0, and cellulose welding rod (E601
The cracking rate of the battel type beat bottom cracking test according to 0) was determined and was as shown in Table 2. The conditions for the cracking test under the beat are: initial temperature of the specimen 000, welding current 100
A. The welding speed was 25 arc/min, and the beat length was long. Table 1 Table 2 However, QT: Adjacent (920-940℃) Phosphorus reversion (6
50 to 670°C) N: Dawn-washed (910°C) NT: Dawn-washed (925°C) Phosphorus rehydrated (675°C) As is clear from Table 2, when producing by the method of the present invention, the conventional method is used. The strength at 350℃ is significantly higher than that of
In terms of weldability, the cracking rate in the under-beat cracking test is extremely low.

This shows that the method of the present invention provides high strength and good weldability in the medium temperature range. Next, from among the steel pipes manufactured as described above, steel pipe A was selected as a typical medium-temperature castle according to the present invention, and steel pipe K was selected as a typical steel pipe according to the conventional example, and the strength of these pipes was determined over a temperature range of room temperature to 550q0. , Comparing these, the first
It was as shown in the diagram.

This high-temperature tensile test was conducted according to JIS. This was also done based on G0567. Further, in FIG. 1, the solid line indicates the steel pipe A, and the dotted line indicates the steel pipe K. From the results shown in Figure 1,
It can be seen that in the case of the method of the present invention, the strength of the medium-temperature castle is significantly higher than that of the conventional example, and the yield strength is particularly high.

Moreover, since creep becomes noticeable when the temperature exceeds 450oo, it is preferable that the range in which the steel pipe A produced by the method of the present invention is used does not exceed 450. It can be seen that it is well maintained. Furthermore, the tensile strength in the case of the conventional example. The peak temperature of steel pipe B is around 25,000, whereas the peak temperature of steel pipe A manufactured by the method of the present invention at 08 is around 300,000.
It can be seen that the method of the present invention is superior in this respect as well. Next, a circumferentially welded joint was manufactured using steel pipe A manufactured by the method of the present invention, and the strength of the joint including the heat affected zone and the strength of the base metal were compared, as shown in Figure 2. Ta. The test pieces and test method were the same as those shown in FIG. 1, and the conditions for circumferential welding are shown in Table 3. In addition, in FIG. 2, the symbol A indicates the strength of the center portion, and the symbol A indicates the strength of the welded joint. From Table 3 and Figure 2, the welded joint part is at room temperature to 25°C compared to the base metal part.
Although the strength decreases slightly at the relatively low temperature side of 0℃, 300℃
In the region of ~450:0, the strength is the same as that of the base material,
It can be seen that the overall strength is sufficiently high.

In other words, in the steel pipe manufactured by the method of the present invention, some of the carbides that were once re-dissolved in the heat-affected zone during welding remain in a solid solution state even after welding, and either re-precipitate at the test temperature or interact with dislocations. Therefore, it is thought that sufficient high strength is maintained at the joint. As explained in detail above, the method of the present invention has a C:0.
03-0.15%, Si: 0.01-0.80%, Mn
:0.50~2.00%, Mo:0.01~0.30%
, V: 0.005-0.10% and A 0.001
A steel pipe containing ~0.10%, with the remainder being iron and impurities, is heated to a temperature of Ac 3 or higher, then turned on, and then heated to 6,000 yen.
Steel pipes are manufactured by tempering at a temperature of 0 or higher and Ac or lower.

For this reason, when steel pipes for steam transportation are manufactured by the method of the present invention,... Since the steel pipe has a low C content, it has excellent susceptibility to hydrogen cracking during welding, allows for low preheating temperatures, and has good weldability.

In addition, despite the low C content, it has significantly high strength at medium temperature, and the steel pipe can be made thinner than in the conventional example. In addition, in the above, the method of the present invention was mainly explained using examples using seamless steel pipes.
This is because in the case of seamless steel pipes, there are no welds in the longitudinal direction of the pipe, and the only welding required is circumferential welding during construction. However, even if there is a weld in the longitudinal direction of the pipe, the method of the present invention involves heating the entire steel pipe, including the weld, to AC3 points or higher before welding, so that the weld metal consists of the above-mentioned components. As long as welding materials are appropriately selected, the present invention can be applied to welded steel pipes other than seamless.

Furthermore, in the case of steel pipes for steam transportation, low-temperature ballability is not particularly required from the perspective of the purpose of use, but the construction site is in a cold region, so a certain degree of toughness may be required.

The steel pipe for cold castles manufactured by the method of the present invention automatically has good toughness due to the deep tempering treatment, and is superior to conventional steel in this respect as well.

[Brief explanation of drawings]

Fig. 1 is a graph comparing the strengths of typical examples of steel pipes manufactured by the method of the present invention at room temperature to 550do, and Fig. 2 is a graph showing typical examples of steel pipes manufactured by the method of the present invention. It is a graph showing a comparison between the strength of the joint part and the main village part at room temperature to 550 qo when the joint part is melted. A: Strength of the base metal; Mouth: Strength of the welded joint. Figure 2 Figure 1

Claims (1)

[Claims] 1 C: 0.03-0.15%, Si: 0 in weight percentage
．． 01-0.80%, Mn: 0.50-2.00%, M
Contains o: 0.01 to 0.30%, and V: 0.00
Ac
A method for producing a steel pipe for steam transport that exhibits high strength in the medium temperature range of 200 to 450°C, characterized by heating and quenching to a temperature of 3 points or more, and then tempering at a temperature of 600°C or more and less than 1 Ac_point. .