JPH0138851B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing large-diameter welded steel pipes, etc., which are welded by submerged arc welding and bent by induction heating, and in particular, relates to a method for producing bent pipes with excellent low-temperature toughness and strength at welded parts. Regarding the method. [Prior Art] As shown in Fig. 3, conventionally, a large-diameter welded steel pipe obtained by welding by submerged arc welding is bent while being induction heated, and then the entire steel pipe is quenched.
A bent pipe is then manufactured by tempering the steel pipe. [Problems to be Solved by the Invention] However, in this method, the entire length of the steel pipe is quenched by high-frequency induction heating, so the heat treatment time is long, and it is necessary to attach a dummy pipe during heat treatment, which reduces work efficiency. It's bad. Therefore, the conventional method has a problem in that a large amount of time and man-hours are required to manufacture the bent pipe. [Means for Solving the Problems] This invention was made in view of the above circumstances, and it shortens the heat treatment time by hardening only the curved portion of the raw tube and then tempering the entire raw tube. Another object of the present invention is to provide a method for manufacturing a bent pipe that can improve work efficiency and produce a high-strength bent pipe with excellent low-temperature toughness at the welded part. The method for manufacturing a bent pipe according to the present invention includes:
So, C is 0.05 to 0.15%, Si is 0.5% or less, and Mn is
1.0 to 2.0%, Ni 0.5 to 3.0%, Cu 0.02 to 2.0%
1.0%, O 0.03% or less, Nb 0.01 to 0.10%,
Ti is 0.005 to 0.05%, and further V is 0.1% or less, Mo is 1.0% or less, and B is 0.0005 to 0.005%.
A raw pipe welded with a weld metal containing one or more of these components, with the remainder consisting of Fe and unavoidable impurities, is bent while heating, and the bent portion is
Hardened at a temperature of 1100℃, then the entire raw tube is
It is characterized by being tempered by heating at a temperature of 550 to 700°C for 5 minutes or more. As shown in Figure 2, a large-diameter welded raw pipe obtained by welding by submerged arc welding is bent by induction heating, then only the bent part QT is quenched, and then the entire raw pipe is In the method of tempering in a gas furnace or the like, since the hardening of the straight portion T can be omitted, the heat treatment time is shortened, and there is no need to attach a dummy tube, so the work efficiency is high. However, in this method of quenching only the curved part QT and tempering the entire part, the weld metal part QT, which undergoes the heat coverage history of quenching and tempering, coexists with the weld metal part T, which receives only the heat coverage history of tempering. It is necessary to satisfy the low temperature toughness and strength of both parts. Especially when using bent pipes in cold regions such as Alaska,
The low-temperature toughness of weld metal becomes a problem. As a result of various studies to improve low-temperature toughness and strength in the above method, the inventors of the present application found that
It has been found that by appropriately selecting the composition of weld metal and heat treatment conditions, it is possible to improve the low-temperature toughness and strength of curved parts and straight parts. This invention was made based on such knowledge, and by adjusting the composition of weld metal and heat treatment conditions within the range specified in the present invention, 10 kgf · Absorbed energy of 50 to 70 Kgf/mm ² can be obtained. Therefore, according to the present invention, a high-strength bent pipe with excellent low-temperature toughness can be manufactured. This invention will be explained in detail below. In induction quenching, the heating rate is usually
100 to 500°C/min, the holding time at the highest heating temperature is within 1 minute, the cooling rate is 10 to 50°C/sec, and in furnace tempering, the holding time at the tempering temperature is 5 to 120 minutes, The thermal history of such bent pipes is different from that obtained by ordinary heat treatment methods. In weld metal that undergoes such a thermal history,
In order to satisfy the low-temperature toughness of both the part that has undergone quenching and tempering treatment (hereinafter referred to as the quenched and tempered part) and the part that has undergone only tempering (hereinafter referred to as the tempered part), it is necessary to change the composition of the weld metal. is defined as follows. First, addition of nickel Ni, copper Cu, titanium Ti, boron B, etc. is effective in improving the low-temperature toughness of both the quenched and tempered part and the tempered part. Ni content is 0.5 to 3.0% by weight. Ni stabilizes austenite during quenching to increase hardenability, and does not cause precipitation hardening during tempering, so it is an effective component for improving the low-temperature toughness of the quenched and tempered part. On the other hand, in the tempered part, Ni has the effect of strengthening the ferrite base and improving its low-temperature toughness because it does not harden by precipitation even during tempering. However, in the case of submerged arc welding, if the weld metal contains more than 3% of Ni, hot cracking will occur, so the Ni content must be limited to 3% or less. On the other hand, if the Ni content is less than 0.5%, the aforementioned effect of increasing low-temperature toughness cannot be obtained. In Figure 1, the horizontal axis shows Ni
The content of
50° C.), and is a graph showing the relationship between the Ni content and the Charpy absorbed energy in the quenched and tempered portion and the tempered portion. According to this, it can be seen that the low-temperature toughness increases as the Ni content increases in both the quenched and tempered part and the tempered part of the weld metal. As is clear from FIG. 1, in order to ensure sufficient low-temperature toughness in both the quenched and tempered portion and the tempered portion, the Ni content must be 0.5% or more. Cu content is 0.02 to 1.0% by weight. When the Cu content is 1% or less, the low-temperature toughness of both the quenched and tempered portion and the tempered portion is improved. However, the effect is small when the Cu content is lower than 0.02%. For this reason, the Cu content is defined as 0.02 to 1.0%. The Ti content is 0.005 to 0.05% by weight. Ti refines the structure of the weld metal and improves the toughness of the tempered part of the weld metal. However, Ti in the weld metal
If the content exceeds 0.05%, the toughness will decrease due to precipitation hardening, while if it is less than 0.005%, the grain size of the structure will not be sufficiently refined. B content is 0.0005 to 0.005% by weight. B
When added in combination with Ti, it has the effect of refining the structure of the weld metal and increases the toughness of the tempered part of the weld metal. In addition, in a portion that undergoes rapid heating and rapid cooling, such as the quenched and tempered portion of a bent pipe bend, the toughness is improved by a small amount of B. However, if the B content exceeds 0.005%, the toughness of the quenched and tempered part of the weld metal decreases, and if it is less than 0.0005%, the above effects cannot be obtained. For this reason, the B content is 0.0005 to 0.005.
Set to weight%. Next, carbon C and manganese Mn greatly influence the low temperature toughness and strength of both the quenched and tempered parts and the tempered parts. The C content is 0.05 to 0.15% by weight. In the quenched and tempered part of the weld metal, if C is less than 0.05%, sufficient quenching effect cannot be obtained;
If it exceeds %, the tempered martensitic structure will be hardened, and hot welding cracks will occur. In the tempered part, fine carbides precipitate when the C content is about 0.10%, resulting in the highest toughness, but low-temperature toughness is also good in the range of 0.05 to 0.15%. Therefore, the C content is set at 0.05 to 0.15%. The Mn content is 1.0 to 2.0% by weight. If the Mn content is lower than 1.0%, hardenability decreases, making it impossible to obtain sufficient toughness in the quenched and tempered part. Furthermore, in the tempered portion, coarse ferrite precipitates at grain boundaries, resulting in poor low-temperature toughness. On the other hand, when Mn exceeds 2.0%, the strength increases, but the susceptibility to temper embrittlement of the quenched and tempered portion increases. Moreover, in the tempered part, when the Mn content exceeds 2.0%, upper bainite crystallizes, resulting in deterioration of toughness. Therefore, in order to ensure sufficient toughness in both the quenched and tempered part and the tempered part, the Mn content needs to be 1.0 to 2.0%. The silicon content must be 0.5% by weight or less. This is because Si enters the weld metal from the base metal, welding wire, and flux, but if Si exceeds 0.5%, the toughness of the weld metal decreases in both the quenched and tempered portion and the tempered portion. The oxygen O content is 0.03% by weight or less. O affects the notch toughness of weld metal. In other words, most of the O in the weld metal exists as inclusions, but as O decreases, the number of inclusions decreases accordingly. Since the notch effect is reduced when the inclusions are reduced, the notch toughness of both the quenched and tempered portion and the tempered portion is improved due to the reduction of O. Further, particularly in the quenched and tempered portion, inclusions containing O become nucleation sites for ferrite, which inhibits hardenability. Therefore, reducing O is effective to ensure hardenability. For the above reasons, in order to obtain a bent pipe with excellent low-temperature toughness in both the quenched and tempered portion and the tempered portion, it is necessary to reduce the O content to 0.03% or less. On the other hand, niobium Nb, vanadium V, molybdenum
Components such as Mo enter the weld metal by dilution from the steel material. In submerged arc welding, the dilution rate of the base metal is 60 to 70%, so the content of Nb, V, Mo, etc. in the base metal also affects the toughness of the quenched and tempered parts of the weld metal. give. Nb content is 0.10% by weight or less. Nb is 0.10
%, precipitation hardening occurs in the weld metal due to tempering, resulting in a decrease in toughness. Therefore, the Nb content needs to be 0.10% or less. but,
In order to ensure the strength and toughness of steel materials, Nb is added to steel materials.
is added, which causes Nb to be added to the weld metal.
Contains more than 0.01%. For this reason,
The Nb content will typically be between 0.01 and 0.10%. The V content is 0.1% by weight or less. This is because V has the same effect as Nb, and if its content exceeds 0.10%, the toughness decreases. Mo content is 1.0% by weight or less. Mo is a component that increases the strength of the weld metal in the quenched and tempered part, and by adding a small amount, a high-strength weld metal can be obtained. However, as the Mo content increases, the toughness decreases, so in order to ensure the low temperature toughness of the quenched and tempered part, the Mo content needs to be 1.0% or less.
Furthermore, in the tempered part, the addition of Mo refines the structure and improves the low-temperature toughness, but if it is added in excess, the upper bainite structure becomes mixed and the toughness decreases. Therefore, taking into account that Mo is expensive, the Mo content is set to 1.0% or less. Phosphorus P and sulfur S are components that enter as impurities from the base metal and the welding material, and since they tend to cause welding defects such as weld hot cracking, it is better to reduce them as much as possible. Next, the reason for limiting the heat treatment conditions will be explained. In this invention, the quenching temperature of the bent pipe portion is set in a temperature range of 900 to 1100°C. this is,
If the quenching temperature is lower than 900°C, the strength of the weld metal will decrease because the quenching will not be done sufficiently, and if it exceeds 1100°C, the strength of the weld metal will be sufficient, but the toughness will deteriorate. The tempering temperature of the curved portion and the straight portion is 550 to 700°C. This means that if the tempering temperature is lower than 550℃,
This is because the tempering effect is insufficient, resulting in low toughness, and when the temperature exceeds 700°C, the toughness rapidly decreases as it enters a two-phase region. The holding time at the above tempering temperature needs to be 5 minutes or more. This is because if the tempering retention time is less than 5 minutes, a sufficient tempering effect cannot be obtained. The upper limit of the tempering retention time is determined from the viewpoint of processing efficiency and energy consumption.
It is desirable that the duration be 120 minutes or less. [Examples] Examples of the present invention will be described below. Test example 1 0.09C−1.50Mn−0.02Nb with pipe thickness of 20 mm
−0.25Mo UOE steel pipe A and 0.10C−1.50Mn−
Both sides of the 0.02Nb-0.03V UOE steel pipe B were welded in a single layer by submerged arc welding. The welding conditions are
Regarding the inner side, the leading one is 940A−36V, and the trailing one is 940A−36V.
740A−40V, welding speed 80cm/min, welding heat input
47KJ/cm, and for the outer surface, the leading
960A-38V, trailing 700A-44V, welding speed 75
cm/min, welding heat input is 54KJ/cm. In addition, as a welding rod wire for submerged arc welding,
The compositions shown in W-1 to W-5 shown in Table 1 below were used, and the fluxes were as follows:
The compositions shown in F-1 to F-3 shown in the table were used.

【table】

【table】

【table】

[Table] Table 3 shows the chemical composition of the weld metal when steel pipes A and B are welded and the mechanical properties of the weld metal after heat treatment. Regarding the heat treatment conditions, the bent part of the steel pipe was heated and held at 950℃ for 5 seconds, and then heated at 20℃/second (from 800℃
The tube was quenched at a cooling rate of 400℃ (up to 400℃), and the entire steel tube was heated and held at 650℃ for 30 minutes to temper it.
In the table, QT indicates the quenched and tempered part, and T indicates the tempered part. In Table 3, Examples 1 to 4 are within the composition range defined by the present invention, and Comparative Examples 1 to 4 are outside of that range. Table 4 shows the combinations of wire and flux compositions of Examples 1 to 4 and Comparative Examples 1 to 4.

【table】

[Table] In all of Examples 1 to 4, the shear py absorbed energy of both the quenched and tempered part and the tempered part of the weld metal is 10 Kgf·m or more at -50°C. Also,
Each example has sufficient tensile strength. On the other hand, in Comparative Example 1, Ni is less than 0.5%, in Comparative Examples 2 and 4, Ti is more than 0.05%, and in Comparative Example 3, Ti is more than 0.05%.
Since Ni is less than 0.5% and Cu is more than 1.0%, both of which are outside the composition range specified in this invention, the absorbed energy in the Charpy impact test at -50°C is or 10 in the tempering section
The value is lower than Kgf・m. Test Example 2 Table 5 shows the mechanical properties of the quenched and tempered part QT and the tempered part T when the heat treatment conditions were changed for UOE steel pipes welded with weld metals having the compositions shown in Examples 1 and 4 in Table 3. It shows the characteristics of

[Table] As heat treatment conditions, the quenching temperature Q is 850 to 850.
The tempering temperature T was varied within a range of 1150°C, and the tempering temperature T was varied within a range of 550 to 650°C. The cooling rate in the quenching process was 20°C/sec (speed from 800°C to 400°C), and the holding time in the tempering process was 30 minutes.
In Table 5, Examples 5 to 7 were heat treatment conditions within the range defined by the present invention, and Comparative Examples 5 to 6 were outside the range. In Examples 5 to 7, the quenching temperature is in the range of 900 to 1100°C, and the tempering temperature is in the range of 550 to 700°C. The absorbed energy of both the quenched and tempered parts of the weld metal in the Sharpie impact test at -50°C was
It has a high toughness of 10Kgf・m or more. On the other hand,
Comparative Example 5 has a quenching temperature of 850°C, and Comparative Example 6 has a quenching temperature of 1150°C, both of which are outside the range of heat treatment conditions specified in this invention. The absorbed energy in the impact test was less than 10 kgf・m, and the toughness was low. [Effects of the Invention] According to the present invention, the manufacturing process of a bent pipe is extremely simplified, the productivity of the bent pipe is significantly improved, and a bent pipe having excellent low-temperature toughness and strength can be obtained. Therefore, this invention has extremely high industrial value.

[Brief explanation of drawings]

Figure 1 is a graph showing the influence of Ni content on the toughness of weld metal, Figure 2 is a schematic diagram showing the heat treatment method for bent pipes in this invention, and Figure 3 is a schematic diagram showing the conventional heat treatment method for bent pipes. It is a diagram. QT: curved section, T: straight section.

Claims (1)

[Claims] 1% by weight, C 0.05 to 0.15%, Si 0.5% or less, Mn 1.0 to 2.0%, Ni 0.5 to 3.0%, Cu
0.02 to 1.0%, O 0.03% or less, Nb 0.01 to 1.0%
0.10%, Ti is 0.005 to 0.05%, and furthermore, 0.1
% or less V, 1.0% or less Mo, and 0.0005 to
Contains one or more components of 0.005% B,
A raw pipe welded with weld metal with the balance consisting of Fe and unavoidable impurities is bent while being heated, the bent portion is quenched at a temperature of 900 to 1100°C, and then,
A method for manufacturing a bent pipe, which comprises heating and tempering the entire raw pipe at a temperature of 550 to 700°C for 5 minutes or more.