JPS6151009B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing seamless steel pipes for steam transportation that exhibit high strength in the temperature range of 200 to 450°C, and more specifically, to a method for manufacturing seamless steel pipes for steam transport that exhibit high strength in the temperature range of 200 to 450°C. Manufacturing a seamless steel pipe for steam transport that does not soften even when exposed to the steam temperature range (hereinafter referred to as the medium temperature range) and exhibits high strength and also has good weldability. Regarding the method. In general, seamless steel pipes used in the medium temperature range of 200 to 450°C have not been developed to suit the new application, but they are usually JIS, STPG 38 to 42, JIS, STS 35. ~49,
JIS, STPT 38-49 or ASTM, A 106
So-called carbon steel pipes such as grades A to C are used. These steel pipes are mainly used for economic reasons rather than workability and oxidation resistance, and they have a high carbon content and do not have very good weldability. Furthermore, these steel pipes were mainly developed to have small diameters and thin walls, and welding rods that generate a small amount of hydrogen are used for welding during construction. There are many problems when applying this method to conditions such as welding with a large amount of welding rods. In other words, in relation to recent new energy development,
New demands have arisen for steel pipes for steam transport, and along with this, strength in the medium temperature range has become a new issue, and weldability has also become an issue due to the thick walls. For example, steam piping in oil sand development using the steam injection method, or steam pipelines connecting steam wells and power plants in geothermal development, require high strength steel pipes at the transport steam temperature, and at the same time, the piping is relatively It is necessary to run thick and long pipes, and only cellulose-based welding rods, which generate a large amount of hydrogen, are used in overseas pipeline construction, so weldability that meets these conditions has become an important issue. . However, if the conventional steel pipes mentioned above are used for this new purpose of steam transportation,
Due to prolonged exposure to temperatures of °C, it softens and its strength decreases, so even if the working pressure is not increased,
If the outer diameter of the tube is doubled, the wall thickness of the tube must be increased to approximately double. Furthermore, as the wall thickness increases, the cooling rate after the root pass during circumferential welding increases, increasing the risk of hydrogen cracking. In order to avoid this risk of hydrogen cracking, it is necessary to increase the preheating temperature before welding, but if the length of the pipe is long, increasing the preheating temperature is not preferable as it will significantly impede welding efficiency. Furthermore, the thicker the pipe, the more multi-layered the circumferential welding, and in this respect as well, the welding efficiency is significantly reduced. On the other hand, instead of regular pipe materials, it is possible to use high-strength steel pipes with high allowable stress, such as STPA 12 to 26 alloy steel, and make the walls thinner, but these alloy steels have high allowable stress in high temperature ranges. However, it does not have a high allowable stress in the medium temperature range, and if welded during construction, it requires heat treatment afterwards, which reduces welding efficiency, and since it is originally expensive, construction costs increase due to the rise in the unit price of steel materials. . As mentioned above, the optimal steel pipe for new uses has not yet been found, and if conventional steel pipes are used, the construction cost will increase, which will impair the economic efficiency of new energy and even delay the development itself. This is one of the reasons why. The present invention aims to solve the above problem, and specifically, the present invention aims to solve the above problem, and specifically, it
We propose a method for manufacturing seamless steel pipes that maintain high strength without softening even when heated to a medium temperature range such as 200 to 450°C, and exhibit good weldability during pipeline construction. In other words, by applying quenching treatment immediately after forming seamless steel pipes by hot rolling, we are able to provide significantly higher strength in the medium temperature range, achieve thinner pipes, and provide good weldability.
Through these methods, steam transport steel pipes with good weldability during construction can be manufactured, and in particular, when one or both of Mo0.01~0.30% or V0.005~0.10% is added, the medium temperature range of 200~450℃ This was established based on the knowledge that it exhibits high yield strength and tensile strength. Furthermore, when a steel pipe is manufactured by the method of the present invention,
It is an indicator of the susceptibility to hydrogen cracking during welding of steel pipes.
Because the Pcm value can be significantly reduced, on-site weldability is extremely good, and even though the amount of C in the composition is reduced, based on the above knowledge, in the medium temperature range around 350℃, Significantly higher strength than conventional steel pipes can be obtained. Therefore, the steel pipe can be made thinner, the preheating temperature is lowered, and the number of welding passes required per joint is reduced, thereby improving welding efficiency. In addition, when producing seamless steel pipes by the method of the present invention, the steel pipes are quenched immediately after hot rolling and then tempered, so even if the components have low Pcm values, Pcm High strength can be maintained without increasing the value, and since small amounts of Mo and V are added, high strength in the medium temperature range can be maintained even when heat treated in this way. That is, in the normal quenching and tempering treatment, a steel pipe after hot rolling is reheated to an Ac point of ₃ or higher, quenched, and then tempered. Even with such normal quenching and tempering treatment, it is possible to achieve a certain degree of increase in strength and weldability for medium temperature range steel pipes. However, for new applications such as steam pipelines in oil sands development,
The steam pressure is as high as 150 Kg/cm ² , and even if the strength in the medium temperature range is increased to some extent, the wall thickness must become considerably thicker as the outer diameter of the tube increases. Even if such thick-walled steel pipes are subjected to normal quenching after reheating, the entire wall thickness will not be sufficiently quenched, and in particular, in order to ensure high strength, the wall thickness may need to be increased. Therefore, the Pcm value will be increased. In this respect, as in the method of the present invention, after hot rolling, the austenite crystal grains are larger than the austenite crystal grains after reheating, and quenching from this state results in quenching over the entire wall thickness of the steel pipe. It is extremely easy to penetrate, and despite the low Pcm value, the same strength as that obtained by quenching after reheating can be obtained. This reduction in Pcm value also improves weldability. In addition, when manufacturing steel pipes for medium temperature ranges, there has never been an example of applying quenching and tempering treatment, and this point in particular is that the method of the present invention goes one step further without performing the usual quenching and tempering treatment. , a seamless steel pipe is formed by hot rolling and then directly quenched and tempered.
This is based on a highly advanced technological idea that has never been seen before, but when the steel pipes manufactured in this way are reheated with steam, their properties change, and the strength at room temperature is maintained in the medium temperature range. It has the disadvantage that it cannot be made and becomes soft. Therefore, the present inventors conducted various studies on this point and found that if a small amount of either Mo or V or both is added, steel pipes can be produced by direct quenching and tempering as described above. , the steel pipe can maintain sufficiently high strength in the medium temperature range of around 350℃;
As shown in the examples below, the temperature was higher than that at room temperature. The method of the present invention will be explained in detail below, starting from the composition of the steel pipe. C: C is effective in increasing tensile strength at around 300°C, and also causes fine precipitation of carbides during tempering to increase yield strength, so a certain amount is necessary. Therefore, in the present invention, 0.03%
Let be its lower limit. However, if too much, it is the most harmful element in terms of weldability, and 0.15%
If this value is exceeded, the risk of hydrogen cracking increases rapidly, so this is set as the upper limit. Si: Si is a useful element for deoxidation, and usually 0.01% or more is required. It also has a solid solution strengthening effect,
It is also effective in terms of oxidation resistance, but if it exceeds 0.80%, toughness and weldability will deteriorate significantly.
The range of Si was 0.01% to 0.80%. Mn: Mn contributes to strengthening not only by solid solution strengthening but also by improving hardenability, so at least 0.50%
is necessary. In addition, Mn has a strong tendency to segregate, and too much Mn increases the risk of weld cracking.
The upper limit is 2.0%. Mo: Mo is an element that plays an important role as described above in the method of the present invention.
When the steel is then manufactured by direct quenching and tempering, the presence of this Mo improves hardenability and achieves precipitation strengthening during tempering, thereby achieving high strength in the medium temperature range. At this time, Mo is effective in maintaining strength on the high-temperature side of the medium temperature range, and if it is less than 0.01%, it has no effect, but if it exceeds 0.30%, it not only impairs weldability but also has an economical impact. Since it is an expensive element, adding more than necessary was avoided, and the upper limit was set at 0.30%. V: Similar to Mo, V is an indispensable element for achieving high strength in the medium temperature range.
It is particularly effective when added in combination with Mo. However, if V is less than 0.005%, there is no effect. Moreover, if it exceeds 0.10%, it not only impairs weldability but also deteriorates toughness, so the range was set to 0.005 to 0.10%. Al: Al is a useful element for deoxidation, fixes impurity nitrogen in steel, and improves toughness, so it is necessary to have at least 0.001%. Also, Al is at 300℃
It has the effect of suppressing a decrease in ductility due to blue brittleness before and after, and in this sense as well, it is preferable to add it. However, too much can lead to deterioration of toughness.
The upper limit was set at 0.10%. As described above, a seamless steel pipe containing each component with the remainder consisting of Fe and impurities is formed by hot rolling, and then immediately quenched,
Subsequently, it is tempered at a temperature of 600°C or higher and Ac ₁ point or lower to produce a seamless steel pipe with excellent strength in the medium temperature range. Further, each component added as described above is limited so that the composition has a Pcm value expressed by the following formula of 0.21% or less. Pcm=C+Si/30+Mn/20+Mo/15+V
/10≠(%) In other words, hydrogen cracking susceptibility during circumferential welding of steel pipes
There is a good correlation between Pcm values; for example, even in under-bead cracking tests using cellulose rods to check weldability, if the Pcm value of the pipe material is 0.21% or less, hydrogen cracking will not occur. In addition, in the underbead cracking test, circumferential welding is possible even if cracking occurs, but if the Pcm value is limited in this way, circumferential welding can be performed with almost no preheating even in cold regions, and oil side development Considering that the construction site for steam pipes, etc. may be in a cold region, the Pcm value must be 0.21% or less. In addition, the tempering temperature range was set to 600℃ or higher and Ac ₁ point or lower because insufficient tempering below 600℃ will cause a large change in strength when reheated.
Furthermore, tempering in the two-phase region exceeding Ac ₁ point causes a rapid decrease in strength, making it impossible to obtain the required strength. Next, examples will be described. First, in Table 1, among the various seamless steel pipes, steel pipes A to C are manufactured by the method of the present invention, and steel pipes X and Y are medium-temperature range steel pipes manufactured by the conventional method. .

[Table] Of these, steel pipes A to C are manufactured by hot rolling on a seamless rolling line, followed by direct quenching treatment by putting them into a water tank immediately after completion, and then tempering at 630℃. Ta. On the other hand, steel pipes X and Y were manufactured by normalizing at 910°C or by normalizing at 925°C and then tempering at 675°C. In addition, for comparison purposes, steel pipes with the same composition as A, B, and C were air-cooled to around room temperature after hot rolling, and then reheated to a temperature above Ac ₃ according to the normal process. A steel pipe of the same size was manufactured by quenching and tempering. (However, these are indicated by symbols A', B', and C', respectively, in Table 2.) In addition, the reheating and quenching and tempering conditions of the comparative example were as follows: After heating and quenching at 900℃ in a heat treatment furnace, One of two methods was adopted: tempering, or heating and quenching to 920-940°C using an induction heating method, followed by tempering at 650-670°C. For the various steel pipes obtained in this way, we determined the respective strengths at room temperature and 350°C, as well as the cracking rate in a buttel type under-bead cracking test using a cellulose welding rod (E6010), as shown in Table 2. It was hot.

【table】

[Table] As is clear from Table 2, when manufacturing by the method of the present invention, 350°C
In terms of weldability, the cracking rate in the under-bead cracking test was almost 0%. In addition, steel pipes manufactured by direct quenching and tempering as in the method of the present invention had even higher strength compared to steel pipes with the same composition that were subjected to normal reheating and quenching and tempering, that is, comparative examples. . That is, in order to obtain the same strength as a normal quenched and tempered material in the medium temperature range, the Pcm value could be further reduced component-wise by the method of the present invention. Next, from among the steel pipes manufactured by various methods as described above, steel pipe B was selected as a representative steel pipe manufactured by the method of the present invention, and steel pipe B was selected from among the steel pipes manufactured by the conventional method. Steel pipe Y was selected as a representative pipe, and its strength was determined over a temperature range from room temperature to 550°C, and the results were compared as shown in Figure 1. (However, the solid line indicates steel pipe B, and the dotted line indicates steel pipe Y.) In addition, this high temperature tensile test was conducted using a parallel part diameter of 6 mmφ,
Using a round bar test piece with a gauge length of 30 mm, JISGO567
It was based on this. From the results shown in FIG. 1, the method of the present invention has significantly high strength over a wide temperature range from room temperature to 550°C, and particularly high yield strength (σ _y ). Furthermore, while the temperature at which the tensile strength (σ _B ) peaks in the case of the conventional example is around 250°C, the peak temperature of σ _B in steel pipe B manufactured by the method of the present invention increases to around 300°C. However, the present invention was also superior in this respect. As is clear from the above, when manufactured by the method of the present invention, the steel pipe not only has higher strength and better weldability in the medium temperature range than conventional steel pipes, but also can be directly quenched and tempered. Because of this process, the strength required to obtain the same medium temperature strength is higher than that of steel pipes that have been subjected to normal quenching and tempering treatment.
The Pcm value can be reduced and even better weldability can be achieved. In other words, Figure 2 is a graph showing the results of maximum welding hardness tests conducted at various heat inputs for steel pipes B and A', which have approximately the same strength in Table 2. represents the maximum hardness (Vickers hardness) of the heat-affected zone (near the bond), and the lower horizontal axis is a parameter representing the cooling rate after welding calculated by the following formula. 1/√=(E+1000N)/ {54T(1+T/1000)(N+0.5)} R: Cooling rate at 300℃ E: Heat input (J/in) T: 300−To To: Preheating temperature (20℃) N: 8 x wall thickness (in) The upper horizontal axis shows the correspondence between the cooling rate calculated by the above formula and the welding heat input for wall thicknesses of 14.3 mm and 25.4 mm. First, in Table 2, steel pipe B that was directly quenched and tempered by the method of the present invention has the same strength as steel pipe A' that was manufactured by reheating and quenching and tempering. First, looking at FIG. 2, the maximum welding hardness is significantly lower for steel pipe B, which indicates that the risk of hydrogen cracking during circumferential welding is even smaller. By the way, the heat input in the root pass during circumferential welding is usually around 8KJ/cm, and the maximum hardness of the heat affected zone due to this heat input is
It is said that if the temperature is 350Hv or less, there is no risk of hydrogen cracking even if a cellulose welding rod is used. As can be seen from Figure 2, in the case of steel pipe A, the wall thickness
If it is 14.3mm, the maximum hardness is 350Hv even with this heat input.
However, if the wall thickness becomes thicker than this, there is a risk of cracking. On the other hand, in the case of mounted pipe B, even if the wall thickness is 25.4 mm, the maximum hardness for a heat input of around 8 KJ/cm is 300 HV.
It can be seen that there is no risk of cracking. In short, if a particularly thick steel pipe for steam transport is manufactured by the method of the present invention, its effects can be further exhibited. As explained in detail above, according to the method of the present invention, it is possible to economically and inexpensively manufacture steel pipes that have significantly higher strength than conventional methods and excellent weldability in the medium temperature range, and are useful for industrial progress and alternative energy sources. This will greatly contribute to the promotion of development.

[Brief explanation of the drawing]

FIG. 1 shows typical steel pipes manufactured by the method of the present invention and steel pipes manufactured by the conventional method.
This is a graph showing a comparison of the strength from room temperature to 550°C. Figure 2 shows an example of a steel pipe produced by the method of the present invention, and a steel pipe with the same strength as that produced by ordinary reheating, quenching and tempering. 1 is a graph showing a comparison of the results of a maximum welding hardness test for a steel pipe of this comparative example manufactured by the method of the present invention and a steel pipe manufactured by the method of the present invention.

Claims (1)

[Claims] 1. In weight percentage, C: 0.03 to 0.15%, Si: 0.01
~0.80%, Mn: 0.50~2.00%, Al: 0.001~0.10
%, and one or both of Mo or V is included as Mo0.01 to 0.30% and
Contains in the range of V0.005 to 0.10%, with the balance consisting of iron and impurities, Pcm=C+Si/30+Mn/2
A seamless steel pipe whose Pcm value given by 0+Mo/15+V/10 (%) is 0.21% or less is formed by hot rolling, and then
Direct quenching immediately, then 600℃ or more Ac ₁
Characterized by tempering at a temperature below the transformation point
A method for producing seamless steel pipes for steam transportation that exhibit high strength in the medium temperature range of 200 to 450°C.