JPS62151545A - Thick-walled, high-strength, low-pcm bended steel pipe and its production - Google Patents

Abstract

PURPOSE:To manufacture a thick-walled, high-strength, low-PCM curved pipe by subjecting a thick-walled steel pipe which has a specific composition and in which PCM is regulated to a specific value or below to heating in a specific temp. range, to bending at that temp., to cooling, and then to tempering in a specific temp. range. CONSTITUTION:The thick-walled steel pipe has a composition consisting of, by weight, 0.04-0.12% C, 0.2-0.6% Si, 0.8-1.6% Mn, 0.05-0.5% Ni, 0.1-0.25% Mo, 0.03-0.1% V, 0.005-0.05% Ti, 0.02-0.08% Nb, 0.05-0.5% Cu, 0.01-0.1% Al, 0.002-0.006% N, and the balance Fe with inevitable impurities, in which the value of PCM represented by an equation is regulated to <=0.19%. This thick-walled steel pipe is subjected to heating to a temp. between AC3 point and 1,100 deg.C, bending at the same temp., cooling, and then tempering at 550-650 deg.C.

Description

Detailed Description of the Invention (Field of Industrial Application) Regarding steel pipes used for thick-walled, high-strength curved pipes used for bends and branching parts of oil and gas transportation vibrating lines, X60 grade ( Y, S 42.7-52.
7kgf/mm2. T, S, 52.8~65.9k
The purpose of this paper is to propose a thick-walled, high-strength, low-Pcm curved pipe that can maintain high strength and high toughness equivalent to (gf/mm2) and a method for manufacturing the same.

BACKGROUND OF THE INVENTION Bent pipes used for bends and branches in oil and gas transmission line pipes have been manufactured by casting, forging or welding assembly methods.

On the other hand, in recent years, the use of high frequencies has become popular industrially, and even in the manufacture of this type of bent pipe, straight pipes such as UOE steel pipes, ERW steel pipes, and seamless steel pipes are processed by hot bending using high frequency heating. At the same time, the environment in which bent pipes are used has become harsher, and curved pipes with high toughness and high performance are being used in cold regions and deep seas. There is a growing demand for increased strength and thicker walls.

In addition, it goes without saying that a low P, curved pipe with good weldability is required.

Here, since high-frequency heating during molding is heating from the tube surface, a temperature difference occurs between the outer and inner surfaces, and it is estimated that this temperature difference will increase as the wall thickness of the tube increases. Ru.

Furthermore, it is water cooled immediately after heating and bending, but it is estimated that the thicker the wall, the lower the cooling capacity, the lower the cooling rate, and the larger the difference in cooling rate between the surface and the center of the thickness (1/2). Ru.

In this way, when forming a bent pipe using high-frequency heating, the bending heating temperature and cooling rate vary depending on the part in the thickness direction of the steel pipe, so the thicker the wall, the more uneven the material becomes, and the mechanical properties of each part of the steel pipe vary. This results in a different, non-uniform curved pipe.

In addition, as the thickness increases, the cooling rate decreases, so in order to increase the strength, it is necessary to increase the amount of added alloying elements, resulting in a decrease in weldability.

(Prior Art) In order to reduce the non-uniformity of these curved pipe materials, the following operational measures have been considered. That is, for the purpose of reducing the heating temperature difference, processing is performed by lowering the heating frequency so as to increase the heating penetration depth. However, this also increases the width of the heating zone and causes large deformations during bending, causing new problems in terms of roundness, etc., and essentially does not eliminate the heating temperature difference.

In addition, in order to improve the cooling capacity, the water density is increased and the inner and outer surfaces of the tube are simultaneously cooled. However, in induction heating bending equipment, the cooling zone and heating zone are adjacent to each other, so the cooling effect on the heating zone is also reduced. As the temperature increases, the problem arises that it becomes difficult to obtain a predetermined heating temperature.

Furthermore, in order to eliminate material non-uniformity, post-processing is sometimes performed in which the bent pipe after induction heating bending is placed in a large heat treatment furnace, heated uniformly, and quenched in a large water tank, but this is not possible due to cost considerations. Another disadvantage is that dimensional changes occur, making it difficult to obtain a curved pipe of a predetermined shape.

Incidentally, some of the inventors previously proposed the invention of Japanese Patent Application No. 59-248190 as a steel that always has stable toughness even after various heating and cooling processes during bent pipe forming, but this The focus is only on the toughness of the steel, and no other prior art has been found regarding steel for curved pipes with low PCM in thickness and high strength and high toughness as in the present invention.

(Problems to be Solved by the Invention) As described above, special consideration has been required for forming thick-walled, high-strength, low-PC, curved pipes, but the copper itself for curved pipes has
If properties such as high strength, high toughness, and excellent homogeneity can be imparted even after hot bending, the above-mentioned special considerations will not be necessary, and by combining these properties, it will be possible to manufacture bent pipes of even higher quality. becomes.

Therefore, when manufacturing thick-walled, high-strength curved pipes with excellent weldability using the induction heating bending/forming method, the curved pipes with stable strength and toughness can be produced after forming without any special process consideration. It is the purpose of your invention to provide.

The specific goals of this invention are mainly to have a plate thickness of 0.5 inches or more, a PCM of 0.19% or less, a strength of X60 grade or more, and a VE-zo of 20 kgf m or more. The purpose is to provide a PCM high-strength, high-toughness curved pipe.

(Means for solving the problems) This invention has C: 0.04 to 0.12 wt%, Si: 0.2
0~0.60wt%Mn: 0.80~0.6Owt
%, Ni: 0.05-1.60wt%Mo:
0.10~0.25wt%, V: 0.030~
0.100 wt%Ti: 0.005 to 0.050
wt%Nb: 0.020-0.080 wt%C
u: 0.05-1.60wt% A1: 0.010-0.100wt% N: 0
．． A thick-walled, high-strength, low-Pl:M curved pipe containing 0.020 to 0.0060 t%, pc+(!0.19% or less, given by the following formula, and the balance consisting of iron and impurities.

PCH and C: 0.04-0.12wt%, Si: 0.2
0~0.60 L%Mn: 0.80~0.60wt
%, old: 0.05-1.60wt%Mo: 0.1
0-0.25wt%, V: 0.030-0.10
0 wt%Ti: 0.005 to 0.050 wt
%Nb: 0.020 ~ 0.080 wt%Cu
: 0.05~1.60wt% Al: 0.010~0.100yoket%N
: A thick-walled steel pipe having a composition containing 0.0020 to 0.0060 wt% with a PCM value of 0.19% or less given by the following formula is heated to a temperature above the 3 point temperature and below 1100''C, and bent at that temperature. After processing, cool to 550-650℃
Thick wall high strength low PCM characterized by tempering
Method for manufacturing bent pipes.

Record cH It is.

In this invention, changes in austenite grain size due to changes in heating temperature are reduced. As shown in Figure 1, by minimizing the growth of austenite grains as the heating temperature increases, the difference in austenite grain size in the thickness direction of the steel pipe plate during heating for manufacturing bent pipes is reduced, resulting in excellent material homogeneity. It becomes like this.

Therefore, the addition of C+Mn, Cu, Ni, Cr, and Nb lowers the temperature by one point (lower heating limit), and the addition of Nb, Ti, Zr, and RE
The addition of M creates a component system that can maintain fine grains even when the heating temperature increases. It is necessary. On the other hand, in order to ensure strength at a low cooling rate with a low PC□, Nb and M are used due to the effective use of C.

From addition and tempering precipitation strengthening, Nb, V, Mo,
It is also necessary to consider the addition of Ti.

Now, as a problem with the conventional technology, when a thick-walled steel pipe exceeding 20 mm thick is heated by induction heating and processed into a curved pipe, the material quality varies in the thickness direction, and the cooling rate is slow, resulting in low It has been difficult to manufacture high-strength bent pipes using PCM components.

The inventors investigated the materials by heating a large number of steel types to various temperatures from 850 to 1150°C, then cooling at a cooling rate of 2 to 400°C between 800 and 400°C to reproduce the thermal history in the thickness direction.

Also, the tempering temperature is 400°C to 700°C.
The effect was investigated after 90 minutes of treatment.

As a result, heating in the two-phase region below the 3-point temperature results in low toughness and uneven material quality, and although the limiting temperature for heating depends on the steel type, high-temperature heating causes coarsening of austenite grains. It was found that the strength increases and the toughness deteriorates due to an increase in the solid solution amount of each added element.

As an example, C10,08,5i10.25. Mn/
0.45. Nb10.030. Vlo, 029.
A A10.030. FIG. 2 shows changes in material properties depending on heating temperature for steel with Pc1ll=O, 16%. The holding time during heating is approximately 10 seconds, and the cooling rate is 15°C.
/s, and the tempering temperature is 600°C.

As shown in Figure 2, the strength and toughness are stable in the heating temperature range of 850 to 950℃, but the target
It can be seen that it is not a 60 grade material.

Next, Figure 3 shows the relationship between PCM value, strength, and toughness.
It can be seen that it is quite difficult to produce 0 grade.

It goes without saying that raising the heating temperature in order to increase the strength will simply increase the material quality variation as described above and deteriorate the toughness of the welded part.

Through these numerous experiments, the inventors discovered that Mo and Nb could be effectively used to increase strength while reducing PCM.
In addition, we believe that each of the above issues can be overcome by designing the components in an appropriate balance, focusing on Ti and Nb from the perspective of suppressing a large increase in strength due to high-temperature heating, and Nb and V from the perspective of precipitation strengthening during tempering. This knowledge was obtained and led to this invention.

(for production) First, the reasons for limiting each component will be explained.

C is a component that most easily helps increase the strength of steel, but if it is less than 0.04%, it is difficult to obtain the specified strength and the net making cost becomes relatively high, while if it exceeds 0.12%, it is hard to harden. The content is set in the range of 0.04 to 0.12% because it increases the toughness and deteriorates the weldability (increase in PcPI value).

Si is 0.20% from the point of view of deoxidizing effect and hardening hardening.
However, if it exceeds 0.60%, hardenability increases and low temperature toughness deteriorates, so 0.20-0.60%
The range shall be .

Mn needs to be 0.80% or more to ensure the specified strength, but if it exceeds 0.60%, toughness will deteriorate significantly during quenching, and it will also harm weldability, workability, and low PCM. , ao% to 0.60%.

Ni improves toughness. 0.05% or more is required for a decrease of 1 point, but if it exceeds 1.60%, there will be no noticeable difference in the effect, and it will be disadvantageous in terms of economy, so 0.05% or more is necessary.
The range is 1.60%.

Mo is used in o and i in terms of improvement in hardening 11 and grain size regulating effect.
.

% or more, but if it is 0.25% or more, the toughness deteriorates significantly, and from the point of view of economic efficiency, it is set in the range of 0.10-0.25%. This component is a particularly important element in order to ensure a predetermined strength in low PCM steel.

(2) is an element that can be strengthened in a small amount, and needs to be present in an amount of 0.030% or more; however, if it exceeds 0.100%, weldability deteriorates, so it should be in the range of 0.030 to 0.100%.

Ti becomes a nitride and suppresses grain growth, and further improves toughness by reducing nitrogen in the steel.
．． 0.005% or more is required, but if it exceeds 0.050%, the effect will be saturated, and if it is too much, the toughness will deteriorate, so it is set in the range of o, oos to o, oso%.

Nb needs to be 0.020% or more because it becomes Nb carbonitride and suppresses grain growth during high-temperature heating and improves toughness, but if it exceeds 0.080%, weldability decreases and hardening Since the improvement in toughness will lead to a deterioration in toughness,
The range is 0.020% to 0.080%.

Cu is required to be added in an amount of 0.05% or more in order to increase the strength by one point, but if it exceeds 1.60%, the effect will be saturated and weldability and hot workability will decrease. ，
The range is from OS to 1.60%.

Aj2 has a deoxidizing effect and needs to be 0.010% or more, but if it exceeds 0.100%, weldability and toughness will deteriorate, so it should be in the range of 0.010 to 0.100%.

N is an element that is unavoidably mixed during steel manufacturing, and in order to improve toughness, it is preferable to have a small amount, and the upper limit is set to 0.0
060%.

Next, regarding the PCM value, even if it exceeds 0.19%,
Ensuring the strength of the grade is relatively easy and no special consideration is required, but when the pc is 0.19% or less, as in the case of the steel of the present invention, its significance is demonstrated and the weldability toughness is also significantly improved. In this invention, the PCM value is set to 0.19% or less.

That is, with PCM of 0.19% or less, high strength and high toughness of the X60 grade cannot be achieved without special consideration according to the present invention.

Furthermore, regarding variations in material quality due to heating temperature, the invention steel has a grain refining effect and is a low-component type, and its strength and toughness are stable in the heating range from above the temperature to 1100°C. If the heating temperature exceeds 1100°C, grain coarsening will inevitably lead to an increase in strength and a decrease in toughness. In addition, in actual processes, a heating temperature of around 950°C is often used as a guideline, and the fact that the material is stable up to 1100°C is an advantageous factor for operation.

In addition, the strength of low PCM steel, especially Y, S, (yield stress)
In order to ensure that N
A temperature of 550° C. to 650° C. is optimal in order to effectively bring out precipitation strengthening due to b + V + Mo + Ti.

Here, when the temperature is lower than 550°C, the increase in Y and S is small, and the effect of improving toughness is also small. On the other hand, if the temperature exceeds 650℃, the material will soften and the strength may not meet the specified value.
The temperature shall be 0°C.

(Example) Comparative steels 1 to 6° and invention steels 7 to 10 containing the chemical components shown in Table 1 were melted into 100 kg steel ingots by vacuum melting, cut into slab steel pieces with a thickness of 110 mm, and then 115° After heating to C, a rolled steel plate with a thickness of 15 mm was obtained by controlled rolling at a finishing temperature of 730°C.

After heating this rolled steel plate as a test material to 950°C and 1100°C, the cooling rate from 800 to 400°C was 3°C/s+.
Cooling was performed to 10°C. This cooling rate corresponds to the cooling rate of the inner and outer surfaces of a 38 mm thick steel pipe when the outer surface is cooled.

Thereafter, tempering treatment was performed at 500°C and 600°C for 40 minutes. A round bar tensile test piece and a Charpy impact test piece were taken from the steel plate subjected to heat cycles assuming induction heating bending, and the materials were compared.

The results of this accuracy test are shown in Table 2.

Comparative Steel 1 has a too low cl, so it is not possible to secure the desired strength. Comparative steels 2.3 and 4.6 do not have the compositional system of this invention, so they have strength and strength at both heating temperatures of 950°C and 1100°C.
It can be seen that the change in toughness is large. moreover,
Comparative steel 5 is within the composition range of this invention, but has Pc5 (!
Since the steel has a higher toughness, its toughness value is lower than that of the invention steel, and its weldability is also lower due to its high PCM.

However, the material quality variation between each heat treatment is smaller than that of other comparative steels, and some of the characteristics of the composition system according to the present invention are lost.

On the other hand, it can be seen that invention steels 7 to 10 all have high weldability and high toughness in the x60 grade after each heat treatment, regardless of the variation in the element content of the added alloy. In addition,
In tempering at 500℃, Y and S have the specified strength (42,
7kgf/n+m”) may be at the very limit,
It is clear that the tempering temperature should be between 550°C and 650°C.

Furthermore, a material investigation was conducted after the bent pipe was manufactured using Comparative Steel 4 and Inventive Steel 9, which have approximately the same PcM value.

A 0.5-inch thick steel plate was made using the normal hot rolling method, and a U with an outer diameter of 22 inches and a length of 6 m was made using the υOE manufacturing method.
We made OE steel pipes. This steel pipe was heated using an induction heating method and bent to produce a bent pipe. The heating temperature was measured using a pyrometer and was set at 1100° C. on the outer surface of the tube.

After that, a tempering treatment was performed at 630° C., and then test pieces were taken from each part of the bent pipe and a material test was conducted. The results are shown in Table 3.

Table 3 It can be seen that compared to the comparative steel, the invented steel has less variation in material quality depending on each part, resulting in a high-strength, high-toughness curved pipe.

The test piece sampling positions A-C and D-F in Table 3 distinguish between the outside and inside of the bending process, with A and F being the outside surface of the bent pipe, C and D being the inside surface, and B and E being the inside surface. A test piece taken from the center of the wall thickness is shown.

(Effects of the invention) The thick-walled, high-strength #, pcM curved pipe of the present invention does not require special consideration during hot bending, has excellent material homogeneity after bending, and has good weldability, and can be welded in practice. It also has great benefits during construction.

Furthermore, since the method of the present invention does not require strict heating temperature control or consideration for temperature variations, it improves workability and productivity.

[Brief explanation of drawings]

Fig. 1 is a graph of austenite grain size and heating temperature, Fig. 2 is a graph of material change depending on heating temperature, and Fig. 3 is a graph of the relationship between PCM and material based on an induction heating bending simulation experiment. Figure 2

Claims (1)

[Claims] 1. C: 0.04~0.12wt%, Si: 0.20~
0.60wt%Mn: 0.80-1.60wt%, Ni
:0.05~0.50wt%Mo:0.10~0.25
wt%, V: 0.030 to 0.100 wt% Ti: 0.
005-0.050wt% Nb: 0.020-0.080wt% Cu: 0.05-0.50wt% Al: 0.010-0.100wt% and N: 0.0020-0.0060wt% as follows. Thick-walled, high-strength, low-P_C_ containing the P_C_M value given by the formula 0.19% or less, with the balance consisting of iron and impurities.
M curved pipe. P_C_M =C+(Si/30)+(1/20)(Mn+Cu+C
r) + (Ni/60) + (MO/15) + (V/10)
+5B(%)2, C: 0.04-0.12wt%, Si
:0.20~0.60wt%Mn:0.80~1.60
wt%, Ni: 0.05-0.50 wt% Mo: 0.1
0-0.25wt%, V: 0.030-0.100wt
%Ti: 0.005-0.050wt% Nb: 0.020-0.080wt% Cu: 0.05-0.50wt% Al: 0.010-0.100wt% and N: 0.0020-0. 0060wt% with a P_C_M value of 0.19% or less given by the following formula at a temperature of A_C_3 points or higher.
A method for manufacturing a thick-walled, high-strength, low-P_C_M curved pipe, which comprises heating to 100°C or lower, bending at that temperature, cooling, and tempering at 550 to 650°C. P_C_M =C+(Si/30)+(1/20)(Mn+Cu+C
r) + (Ni/60) + (Mo/15) + (V/10)
+5B (%)