JPS61231137A - Steel for steel pipe for high frequency heat bending having superior toughness - Google Patents

Abstract

PURPOSE:To allow a steel pipe to show superior strength and toughness without carrying out any treatment after high CONSTITUTION:The composition of a steel is composed of, by weight, <=0.12% C, %h0.5% Si, <=1.5% Mn, 0.1-1% Mo, one or more.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to steel for high-frequency heated bent steel pipes, particularly steel pipes used in power generation plants and chemical plant piping through which heating fluid such as steam is circulated. The present invention relates to a steel for steel pipes, and more specifically, to a steel for high-frequency heated bent steel pipes that is suitable for ensuring excellent toughness even after being bent by an arm-rotating high-frequency bending machine.

[Background of the invention]

In conventional high-frequency heated bent steel pipes used in piping for circulating heating fluids such as steam, welded steel pipes made of JIS standard carbon steel for boilers and pressure vessels are used to ensure high strength. . The amount of carbon contained in commercially available welded steel pipes belonging to this category is kept low at about 0.1% in the welded metal part to prevent weld cracking, but in the base metal steel plate it is kept at 0.1% to ensure tensile strength. .22~
Contains 0.24 inches. If such a steel pipe is bent using a high-frequency heating bending machine with an arm rotation method, the toughness of the base material on the pipe surface side will be significantly reduced, and the collision absorption energy 2 normally required for steel structures will be reduced. ．． ．．
1 to fm cannot be satisfied with OC. To satisfy this requirement, it is necessary to perform annealing treatment after bending. The maximum temperature reached on the surface of the steel pipe during bending reaches a high temperature of 750-950C, and to ensure the accuracy and efficiency of bending, water cooling with jet water or spray is performed immediately after the heating and bending process. It is necessary to
Therefore, when performing this kind of bending process.

This is because a base material with a high carbon content becomes extremely hardened and hardened on the tube surface side.

In addition, the mechanical properties required for carbon steel piping that circulates heated fluids such as water vapor vary depending on the wall thickness of the pipe, etc., but carbon steel has the highest strength level among carbon steels for boilers and pressure vessels according to JIS standards. Considering steel, 0.2% can resistance 213 K9 f/dark 2, tensile strength: 49-63 h
It may be set that f/rm2 and elongation ≧21 inches. In this case, steel plates containing 0.22 to 0.24% carbon are usually used to form welded steel pipes, and when this is subjected to high-frequency bending, the toughness decreases on the pipe surface side of the base material, as described above. However, it had the disadvantage of requiring annealing treatment after bending.

[Purpose of the invention]

An object of the present invention is to provide a steel for high-frequency bending steel pipes that has excellent strength and toughness as it is after bending even when subjected to bending using an arm-rotating high-frequency bending machine.

[Summary of the invention]

As mentioned above, the carbon equivalent of SB steel, which is a JIS standard carbon steel for boilers and pressure vessels, is extremely high, and when steel pipes made of this steel are subjected to high-frequency bending, the pipe surface side becomes significantly hardened and its toughness deteriorates. Because of this, it is necessary to perform an annealing treatment after bending to improve toughness.

The hardenability of steel is generally evaluated using carbon equivalent, and this carbon equivalent is, for example, (C+(Mn/6)+(Cu
/15)+(Ni/15)+(Cr15)+(Mo15
)+(V15))o formula.

Therefore, in order to suppress the decrease in toughness due to quench hardening during induction bending, the carbon equivalent should be kept low, that is,
It is possible to reduce the carbon content, but if the carbon content is reduced, the strength will decrease, making the steel pipe unusable as a high-strength steel pipe such as a high-pressure pipe.

As a result of intensive research into these problems, the present inventors found that in the composition of the steel constituting the steel pipe, MO, which is basically a precipitation hardening alloy element, and MO, which is also a precipitation hardening alloy element, may be mixed. By adding a predetermined amount of Nb or V, which has the effect of refining crystal grains, and B, which has a remarkable solid solution strengthening effect, it is possible to compensate for the decrease in strength due to the reduction in carbon content, and it is possible to compensate for the decrease in strength due to the reduction in carbon content. When processed, there is no need for annealing, and the impact absorption energy required for normal steel structures is achieved as is; LIK
The invention was completed after discovering that toughness satisfying 9f-m can be obtained with OC. Specifically, the composition of the steel constituting the steel pipe is C0,1 by weight.
2 or less, Si 0.5% or less, Mn 1.5 or less, MO
0.1-1ch and Nb0. o5% or less, V 0.05%
Below is O! It has a composition containing at least one or more of 0.001% or less of Fe, the balance being substantially Fe,
and MotO, the value of C+- should be 0.133 or more and 0.180 or less, and the composition should further include Cr, Cu, or Ni, which is a solid solution strengthening type alloying element, and for Cr, 065
% or less or 0.2 each for Cu and Ni
The above-mentioned problems can be solved by containing the above-mentioned elements in amounts of up to %, singly or in combination.

Next, the technical reason for determining the composition range as described above in the present invention will be explained.

C content is the most important requirement in the present invention. As mentioned above, in order to suppress hardenability, the lower the C content, the better. However, if the C content exceeds 0.12%, the hardenability of the steel increases, and the tube surface during high-frequency bending becomes difficult.・Quenching hardening on the side becomes significant. Therefore, the amount of C is set to 0.12% or less. Among them, 0.03 to 0.10% is preferable, and 0.0%
Particularly preferred is 3 to 0.07 inches.

Si has a deoxidizing effect and is an effective element for increasing the strength of steel materials at room and high temperatures, but if it exceeds 0.5%, it reduces weldability by 1 m. , especially 0.15-0
．． 30- is preferred.

Mn has deoxidizing and desulfurizing effects, and is required in an amount of 1.5 or less in order to provide strength and ductility to steel. If it exceeds 1.5 inches, the hardenability of the steel increases, resulting in hardening of the tube surface during high-frequency bending. In the steel having the composition of the present invention, M0. N
Since the re-added elements b, V, and B make a large contribution to ensuring strength, there is no need to use a large amount of Mn from the viewpoint of strength, and it is preferable to use Mn in an amount of 1.15% or less. In particular, 0.30-
〇, 90% is preferable.

MO is a precipitation hardening element, and together with C, is an important element in ensuring strength, but in the present invention, if it is less than 0.1 inch, its effect is small. On the other hand, if it exceeds 1 inch, weldability will deteriorate. In particular, 0.2 to 0.6 inches is preferable.

Cr contributes to the formation of a high-strength structure and has a solid solution strengthening effect, but Cr itself increases the hardenability and causes quench hardening on the Iff surface side during high-frequency bending, so it is better to have less, and it is set to 0.5 or less. .

In particular, 0.10% to 0.25% is preferable.

Cu and Ni also have a solid solution strengthening effect, and by adding them alone or in combination, it is possible to increase the strength and improve the corrosion resistance. However, since these elements themselves improve hardenability and harden the tube surface side during high-frequency bending, it is better to have a smaller amount, and each content is set to 0.2% or less. Particularly preferred is 0.1 to 0.2.

B is an alloying element that has a remarkable solid solution strengthening effect, and its addition, especially when 0
．． 0005 to 0.0010 is preferable.

Nb, 9 and V have a precipitation strengthening effect and are important for ensuring strength.
effective. Nb further promotes grain refinement and contributes to strengthening the steel. These additions make it possible to reduce the C content while maintaining the strength level. However, if each of Nb and V exceeds 0.05%, it will cause hardening of the tube surface side of the high-frequency bent tube.
5% or less.

0.02~0. It is particularly preferable to use an OS.

・It is related to the toughness of the quenching zone, and the content of C and MO is expressed in weight (ch), and the value of the first figure described later is 0.1
By making it 33 or more, 49 Kg f at room temperature
It is possible to obtain a tensile strength of /Na'' or higher.Furthermore,
By making the coefficient obtained by the equation 0.180 or less, it is possible to make the V-notch Charpy absorption energy in the OC of the rapid heating/quenching section by high frequency 2.1 Kgf-rn or more.

0.180 or less, the strength required for steel pipes for high frequency bending is 49 Kg f /ax",
7 f-m or more requirement for OC Charpy impact value 2.1 #
We have discovered that it can be done.

Normal # in the present invention! Unavoidable impurities included in the steel process are acceptable. The general limit for S and P is 0.025% for both to increase the susceptibility to cracking during high-frequency bending and the high-temperature cracking susceptibility of welds when high-frequency bent steel pipes are welded with other steel pipes. It is preferable to do the following. Furthermore, N coexists with At to refine crystal grains and improve toughness, so N is included in the normal steelmaking process.
0.0150% is effective, but if it exceeds 0.0150%, the properties of the steel ingot deteriorate due to the occurrence of blowholes, etc., so it is preferable to keep it in the above range.

The reasons for limiting the composition of the steel constituting the high-frequency bent steel pipe of the present invention have been explained above, but the steel of the present invention is melted with the above-mentioned composition and then subjected to a normal rolling or forging process. It is manufactured as a steel material by subjecting it to normalizing. The normalizing here refers to the AC point or above, especially the AC
This is a process of heating at 3 points plus around 50C and then air cooling.

Does the structure contain a small amount of ferrite less than 10 t7?1
．． is a total bainite organization. The latter is particularly preferred.

[Example of the Invention] Steel having the composition shown in Table 1 was melted and hot rolled, and then normalized at 930C for 1 hour. In terms of material, conventional steel of JI611 has a carbon content of 0 to 2.
The 3% point is outside the composition range defined in the present invention.

(a) Strength test A material having the composition shown in Table 1 and normalized as described above, with a diameter of 610I and a parallel part of 30mm,
A test piece with a cage length of 25 was prepared and a tensile test was performed at room temperature. Tensile strength at normal temperature 49 Kg f
/lea” does not satisfy the strength requirements above.-Test specimens I62 to 410 that satisfy the composition range specified in the Strength Wood Invention
It can be seen that all of them have a tensile strength of 49 Kff/■2 or more.

Furthermore, since the steel of the present invention is used in power plants and chemical plants at temperatures up to 400C, it was verified whether the steel had sufficient high-temperature strength. The test piece used had the same dimensions as those used in the room temperature tensile test described above.

Among the invention steels, black 5 steel and flat 1 are particularly preferred as test pieces.
Figure 2 shows the results of a high-temperature property test on selected conventional steel No. 1. In addition, since the conventional high-frequency bending steel requires annealing treatment of 650 to 700 tons after bending to improve toughness, the conventional steel was subjected to an annealing treatment of 675CX1h before being subjected to the test.

As is clear from Part 2, compared to conventional steel, which exhibits a rapid decline in strength at high temperatures of 350C or higher, the steel of the present invention exhibits more stable high-temperature properties and is higher in strength. .

Furthermore, in order to investigate the differences in these strengths metallographically,
The microscopic structure of each test piece was observed.

Figure 3 shows the microstructures of 41.49 and 11.

Compared to the ferrite + pearlite structure of Grave 11, which is a conventional steel, the ferrite crystal grains of 41 are slightly finer, and some payinite is observed, and although the structure is improved, it is the first.
As is clear from the figure, the target strength was not achieved. On the other hand, the invention steel 49 exhibits a fine bainite structure and is significantly improved in structure than the former, and it can be understood from the structure point of view that the target strength was achieved despite the low C content. Ru.

(b) Toughness Test As mentioned above, the steel produced from the steel of the present invention is used for bending with a high-frequency bending machine, and at that time, rapid heating and water cooling are performed. Therefore, a test for toughness after rapid heating and water cooling was conducted under the following conditions close to actual high-frequency bending. The test pieces were made from materials with the composition shown in Table 1, with a plate thickness of 51111 and a plate width of 1offI.
After cutting out a board with a length of 100mm and a length of 100cm, it was heated to 950C by high-temperature waves, immediately cooled with a fountain, and a V-notch was machined so that the fracture surface progressed in the width direction of the board.
A so-called half-size Charpy impact test piece having a plate thickness of 1/2 of the No. 202 No. 4 test piece was used.

The relationship with shock absorption energy (t) can be summarized as the fourth factor. As is clear from the fourth cause, C+/O in the quenching section
The Charpy absorbed energy of C is 1.
8 Ky f·m or more, and when converted to a full-size test piece with a plate thickness of 10 m, it can be seen that zIV4f-m or more can be secured.

FIG. 5 is a diagram showing the composition range of the present invention determined from the relationship between the amount of C, M a il, and the toughness of the rapid heating/quenching section by strong and high frequency waves. Charpy absorbed energy of OC is lIKpf-m and tensile strength is 49 h f /m”
The composition to achieve the above is (CO*.

MO0,665%), (C0.12%, Mo0.06
5%), (C0,12%, Mo0.315'J) and (00%, MO0.90%).

(c) High-frequency bending test From the above test results, the steel made using the steel of the present invention was compared with the conventional steel in terms of strength and toughness. , 14f, but we also created an actual tube and conducted a test in which it was subjected to high-frequency bending.

For the test, conventional steels of Reduction 2, A5 and All were selected as those according to the present invention from among the compositions shown in Table 1,
This was cold-worked to a thickness of 17.4cm and an outer diameter of 750m.
It was formed into a tubular shape and welded using a lower C wire than conventional wire having the composition of the weld metal shown in Table 2.

Table 2 Maximum heating temperature by high frequency for these steel pipes 750
It was bent into the shape shown in Section 6 under the conditions of ~950 cm, fountain cooling bending radius of 750 mm, and bending speed of 0.5 mm/h. Note that annealing treatment was not performed after bending.

The results of measuring the hardness distribution in the plate thickness direction of the base material in the bending area of the steel pipe after the above-mentioned high-frequency bending are shown in Figure 7 for a steel pipe made from conventional steel Al. Steel pipes made of No. 2 and A5 steel are shown in FIG.

Comparison of the two figures shows that hardening on the tube surface side of the high-frequency bent steel pipe made of conventional steel is remarkable, but hardening of the high-frequency bent steel pipe made of the steel of the present invention is much less than the former.

Moreover, FIG. 9 shows the absorbed energy of OC measured by taking a V-notch Charpy impact test piece of size -5 from the base material in the bending region of the above-mentioned high-frequency bent steel pipe. It is clear that the high frequency bent steels made of Steel &3 and A9 of the present invention have far superior toughness as bent as compared to the high frequency bent steel pipes made of conventional steels. ), the absorbed energy of OC in the base metal of welded steel pipes made of conventional steel is originally as low as about 4 Kpf-m, and even when bent at a relatively low temperature of about 7,500 psi,
The target value of 2.1Kgf-m is as low as 3Kgf-m, unless annealing is performed when bending at high temperatures.
It is impossible to always satisfy m.

〔Effect of the invention〕

As explained above in detail 4a, the high-frequency bending steel made from the steel of the present invention does not require annealing treatment after bending, has excellent toughness in the as-bent state, and Since the strength at room temperature and high temperature is also higher than that of conventional steel pipes, if high-frequency bent steel pipes are manufactured using the steel of the present invention, piping for chemical plants, nuclear power plants, and thermal power plants can be easily installed. At the same time, the remarkable effect of increasing the reliability of ・g can be obtained.

[Brief explanation of the drawing]

Figure s1 is a diagram showing the relationship between C and MO contents and tensile strength, Figure 2 is a diagram showing the mechanical properties at high temperatures of the steel used in the present invention and conventional steel, and Figure 3 is a diagram showing the mechanical properties of the steel used in the present invention and the conventional steel. and a micrograph of a cross section of conventional steel. Figure 4 is a diagram showing the relationship between C and MO contents and V-notch/Charpy absorbed energy in the OC of the rapid heating/cooling section by high frequency. Figure 5 is a diagram showing the relationship between C content and Charpy absorbed energy. M.O.
A diagram showing the composition range of the present invention determined from the relationship between quantity, strength, and toughness of the rapid heating/quenching part due to high frequency. Figure 8 is a diagram showing the hardness distribution in the thickness direction of high frequency bent steel pipes made of conventional steel and steel of the present invention, respectively.
The cause is V-notch in OC. FIG. 3 is a diagram showing the distribution of Charpy absorbed energy in the plate thickness direction. 1... High frequency bent steel pipe, R... bending radius, D.
...Pipe diameter, t...Pipe plate thickness.

Claims (1)

[Claims] 1. In copper for bending steel pipes produced by an arm rotation type high-frequency bending machine, the steel has a carbon content of C0.12 in weight%.
% or less, Si 0.5% or less, Mn 1.5% or less, Mo0
．． 1 to 1%, and at least one of Nb 0.05% or less, V 0.05% or less, and B 0.001% or less, and has a composition in which the balance is substantially Fe, and with respect to the amount of C and Mo. C+Mo/5 value is 0.133 or more, 0
．． 180 or less, copper for high frequency bent steel pipes. 2. The steel contains 0.07-0.12% C, Si
0.15-0.30%, Mn0.3-1.15%, Mo
0.1-0.6%, Nb 0.05% or less, V0.05
% or less and B0.001% or less, and the balance is substantially Fe.
Copper for high-frequency bending steel pipes as described in . 3. In copper for bending steel pipes manufactured with an arm rotation type high-frequency bending machine, the steel has a weight percent of C0.07.
% or less, Si0.5R or less, Mn1.5% or less, Mo0
．． 1 to 1%, at least one of Cu0.2% or less, Ni0.2% or less, and Cr0.5% or less, and Nb0
．． 0.05% or less, V0.05% or less and B0.001%
Contains at least one of the following, the remainder being substantially Fe
and has a composition of C+M in terms of C and Mo content.
Copper for high frequency bent steel pipes, characterized in that the value of o/5 is 0.133 or more and 0.180 or less. 4. The steel pipe contains 0.03 to 0.07% C and Si by weight.
0.15-0.30%, Mn0.3-1.15%, Mo
0.1-0.6%, Cu0.2%, Ni0.2% or less, Cr0.25% or less, and Nb0.02-0.05%
, V0.02 to 0.05%, and B0.001% or less, and the remainder is substantially Fe. 5. The steel for high-frequency bent steel pipes according to any one of items 1 to 4, wherein the steel pipe is an arc-welded steel pipe.