KR101309921B1 - Welded Steel Pipe Having Excellent Spinning Formability And Method of Manufacturing The Same - Google Patents

Abstract

The present invention is to improve the toughness of the weld by reducing impurities such as nitrogen (N), oxygen (O) of the welded joint, in weight%, Cr: 14.0 ~ 20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04% or less, S: 0.01% or less, 0 (oxygen): 0.01% or less, residual Fe and other unavoidable impurities It relates to a welded steel pipe having excellent spinning workability, which has a Ducttile Brittle Transition Temperature (DBTT) of -20 ° C. or less, and a manufacturing method thereof.

Description

Welded Steel Pipe Having Excellent Spinning Formability And Method of Manufacturing The Same}

The present invention relates to a welded steel pipe and a method for manufacturing the same, and more particularly, to improve the toughness of the welded joint by reducing impurities such as nitrogen (N) and oxygen (O) of the welded joint, thereby having excellent spinning processability. It relates to a welded steel pipe and a method of manufacturing the same.

Recently, the automobile industry has been required to improve fuel efficiency by tightening exhaust gas regulations and reducing vehicle weight. In addition, automobile manufacturers are using ferritic stainless steel having excellent corrosion resistance and heat resistance in place of the existing casting or aluminum plated steel sheet for exhaust system parts.

The converter, which is an exhaust system component, is used for purifying harmful components in exhaust gas generated during combustion of an automobile engine by using a catalyst device. In the past, upper and lower plate materials are press-molded, and catalyst is charged and manufactured by welding. Recently, a method of charging a catalyst into a welded steel pipe and spinning a method for reducing the number of welding works has been proposed. Spinning is a rotary plastic processing method for processing while pushing a welded steel pipe into a rotating roller set according to the shape.

The ferritic stainless steel welded steel pipe for spinning processing is formed by roll bending a sheet material cut to a predetermined size and is made by TIG welding, plasma welding, laser welding, or the like. Although the initial investment cost is higher than TIG welding and plasma welding, the laser welded steel pipe uses the high-efficiency laser beam, and the welded part is narrow, so the quality characteristics are excellent and high-speed production can be expected. TIG welding and plasma welding, which are gas shielded arc welding, have the advantages of low initial installation cost and relatively simple operation compared to laser welding.

Meanwhile, the welded steel pipe needs to remove the tabs TAB attached to both ends of the steel pipe for spinning. At this time, cracks are generated at the welded joint during cutting. In addition, despite the excellent workability of the base material due to cracking in the welded part during spinning, the application of parts is insufficient due to difficulty in securing the workability of the welded part. This phenomenon occurs when brittle cracking is prominent in the welded part when forming in winter or at low processing temperature.

As described above, the use of ferritic stainless steel of medium and high Cr is generally adopted to secure high temperature characteristics of the steel used as the temperature of the exhaust gas increases due to strengthening environmental regulations. It is pointed out as a problem that weldability is further reduced.

Various techniques have been proposed to secure the processability of the above-described medium and high Cr ferritic stainless steel pipes, which will be described below.

Japanese Laid-Open Patent Publication No. 1997-125209 discloses a method of welding a heat-resistant ferritic stainless steel containing Nb and the like by electric resistance welding (ERW), followed by post-heat treatment at a high temperature of 850 to 1000 ° C. and rapidly cooling to 1 ° C./sec or more. Japanese Laid-Open Patent Publication No. 2006-193770 discloses a Vickers hardness HV _W and a base metal of a ferritic single-phase stainless steel welded steel tube containing 0.1 to 0.5% of Ti or Nb as one or two types by mass, respectively. Vickers hardness HV _M with the hardness difference △ negative HV (HV = _W -HV _M) ratio of the RT 10 to the width of the weld bead in the range of 40 T _W and thickness of base metal _M T (= T _W / T _M) is 1.05 to It is set to 1.3, after molding and welding (ERW, GTAW), the calibration is carried out with a deformation amount of 0.5 to 2.0% in the longitudinal direction, and post-heat treatment in the temperature range of 700 ~ 850 ℃ is disclosed, but the work hardening of welded steel pipe The application of annealing heat treatment after welding of pipes to reduce the Although effective in improving workability, there are problems such as cost increase and surface oxidation due to annealing heat treatment.

In addition, Japanese Laid-Open Patent Publication No. 1993-277769 preheats above 250 ° C before laser tube welding of ferritic stainless steel, welds the protruding height of the inner bead to 0.15mm or more, and then reduces the welding part in the plate thickness direction to improve workability. However, the method of installing the preheater and laser welding are difficult to secure the inner bead protrusion height more than 0.15mm by the low heat input welding method.

In addition, Japanese Laid-Open Patent Publication No. 1999-256286 discloses a heat-resistant ferritic stainless steel for the purpose of omitting the post-heat treatment of ERW steel pipes, C: 0.02% or less, Si: 0.7-1.0%, Mn: 1.0-1.5%, Cr: 13.5 to 15.5, N: 0.02% or less, Nb: 0.3 to 0.6%, Cu: 0.02 to 0.24%, Al: 0 to 0.03%, and satisfy the relationship of 1.45≥Nb + Si and 1.35≤Nb + 1.2Si Although an annealed welded steel pipe for processing is disclosed, it is difficult to secure sufficient workability for deep processing applications such as bending part expansion pipes.

In addition, Japanese Laid-Open Patent Publication No. 1995-266072 discloses a method for preventing nitrogen absorption of stainless steel laser welds to improve workability. During laser welding, the concentration of atmospheric nitrogen in the vicinity of the weld seam is reduced to Cr concentration [% Cr] and melting of stainless steel. [% N], which is determined by the allowable temperature T (℃) and the weld metal of the metal nitrogen content _{[% N] WM at, log} ([% N] at) ≤ 2log ([% N] WM) - 2 * (518 / T + 1.068) -2 * (0.046 [% Cr] -0.00028 [% Cr] ² ), but it is difficult to control the process variables in real time for each piping process.

The present invention relates to a welded steel pipe and a method for manufacturing the same, the welded steel pipe with improved spinning processability, which improves the low temperature toughness of the welded joint by controlling impurities in the welded joint of the medium and high Cr welded steel tubes with poor weldability, and It is to provide a manufacturing method.

One aspect of the present invention is by weight, Cr: 14.0-20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04 % Or less, S: 0.01% or less, 0 (oxygen): 0.01% or less, welded steel pipe containing residual Fe and other unavoidable impurities, and having a DBTT (Ductile Brittle Transition Temperature) of the welded joint at -20 ° C or less Provided is a welded steel pipe having excellent spinning workability in which the N + O content of the base material of the steel pipe and the welded joint is 0.025% or less.

The welded steel pipe may further include one or two or more of Mo: 0.1 to 2.0%, Ni: 0.1 to 2.0%, and Cu: 0.1 to 2.0% by weight.

The welded steel pipe may further include one or two of Nb: 0.01 to 0.5% and Ti: 0.01 to 0.5% by weight.

The welded steel pipe may further include Ca: 0.0005 to 0.002% by weight.

The N + O content of the base metal and the weld joint of the welded steel pipe may be 0.0185 wt% or less.

One aspect of the present invention is by weight, Cr: 14.0-20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04 % Or less, S: 0.01% or less, 0 (oxygen): 0.01% or less, shield gas flow rate for supplying steel materials containing residual Fe and other unavoidable impurities to the torch portion: 15 to 20 L / min, shield gas behind the torch portion Provided is a method for producing a welded steel pipe having excellent spinning workability, characterized in that the pipe is welded by a gas shield arc welding method for supplying a flow rate of 15 to 20 L / min and a shield gas flow rate of 5 to 10 L / min. It is preferable that the N + O content of the base material and the weld joint part of the said welded steel pipe is 0.0185 weight% or less.

One aspect of the present invention is by weight, Cr: 14.0-20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04 % Or less, S: 0.01% or less, 0 (oxygen): 0.01% or less, shield gas flow rate for supplying steel materials containing residual Fe and other unavoidable impurities to the laser head: 15 to 20 L / min, shield behind the laser beam Provided is a method for producing a welded steel pipe having excellent spinning workability, characterized in that the tube is welded by a laser welding method which is supplied at a gas flow rate of 100 to 200 L / min and a shield gas flow rate of 5 to 10 L / min on the back side of the weld bead. It is preferable that the N + O content of the base material and the weld joint part of the said weld steel pipe is 0.025 weight% or less.

According to one aspect of the present invention, it is possible to control the impurity of the weld joint and the base material at the same level, thereby improving the weldability and welding quality irrespective of the welding method, it is possible to provide a low-cost welded steel pipe.

1 is a graph showing the correlation between steel grade and DBTT which is an embodiment of the present invention,
Figure 2 is a graph showing the correlation between the steel grade and DBTT is another embodiment of the present invention,
Figure 3 is a graph showing the correlation between the steel grade and DBTT which is another embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The present inventors have tried to minimize the impurities in the welded joint generated during welding. In the case of laser welding, the temperature of molten metal increases with the use of a high density heat source as compared to the normal arc welding, and the mixing of N and O increases from the atmosphere. It was confirmed that there is a problem of increasing the mixing of N and O from the atmosphere, it was recognized that it can be solved by the manufacturing conditions according to the present invention has led to the present invention.

First, the welded steel pipe which is one side of the present invention will be described in detail.

The base metal and the welded joint of the welded steel pipe by weight%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 14.0-20.0%, Al: 0.15% or less , Balance Fe and other unavoidable impurities.

Cr (chrome): 14.0-20.0 wt%

If the Cr content is less than 14% by weight, the oxidation resistance and fatigue resistance of the steel may be inferior. On the other hand, when the Cr content exceeds 20% by weight, the toughness of the weld portion may deteriorate.

C (carbon), N (nitrogen): 0.01% by weight or less each

In the present invention, since C and N play a role of degrading the workability of the base metal and the welded part, it is preferable to be included in the smallest amount possible. In theory, it is advantageous to limit the content of C and N to 0%, but inevitably contained in the manufacturing process. Therefore, it is important to manage the upper limit, and the upper limit of the C and N content in the present invention is preferably limited to 0.01% by weight, respectively.

Si (silicon), Mn (manganese), Al (aluminum), P (phosphorus), S (sulfur)

These elements are invariably present in the steel, but when present in a large amount, they lower workability and lower corrosion resistance, which is characteristic of stainless steel, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, Al: 0.15 wt% or less, It is preferable to limit it to P: 0.040 weight% or less and S: 0.010 weight% or less.

O (oxygen): 0.01 wt% or less

Since O is an element which lowers the workability of the base metal and the welded part, it is preferable to make it as small as possible, but it is preferable to limit it to 0.01% by weight or less in consideration of the increase in the manufacturing price in steelmaking technology.

Mo (molybdenum), Ni (nickel), Cu (copper): 0.1 to 2.0 wt% each

Such an element is an element that can improve the corrosion resistance of steel. Therefore, it is preferable to further include 1 type, or 2 or more types of Mo, Ni, and Cu in the case of the steel which acts as an important use for corrosion resistance. The upper limit of the content of each element is preferably limited to 0.1% by weight or more. However, if the content exceeds 2.0% by weight, the workability may deteriorate and the manufacturing price may increase.

Ti (titanium) and Nb (niobium): 0.01 to 0.5 wt% each

Such an element is added as needed as an element which can improve workability. An effect is exhibited by adding in Nb: 0.01 weight% or more and Ti: 0.01 weight% or more. However, when added in excess of Nb: 0.5% by weight and Ti: 0.5% by weight, respectively, there is a problem in that workability deteriorates due to an increase in the amount of solid solution Nb and Ti.

Ca (calcium): 0.0005 ~ 0.002 wt%

Ca is an element which can improve the weldability of steel. In order to improve weldability, it is necessary to contain 0.0005% by weight or more. However, if the content exceeds 0.002% by weight, the size of the oxidation inclusion may increase, which may adversely affect the corrosion resistance.

The remaining component of the present invention is Fe. However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

When welded steel pipe, which is one side of the present invention, is welded by laser welding, the N + O content of the welded joint is preferably controlled to 0.025% by weight or less within the range of securing the through-bead and the defect-free welded portion.

As a result of analyzing the correlation between the N + O content of the welded joint and the DBTT characteristics of the laser welded joint for welded steel pipes with a Cr content of 14-20%, the following empirical formula (regression analysis) can be derived. Could. In addition, graphs of these correlations are shown in FIGS. 1 and 2, respectively.

DBTT _429EM (° C) = -45.1368 + 1134.08 (N + O) -5852.07 (N + O) ²

(Correlation for 429EM, ferritic stainless steel with 14% Cr)

DBTT _430J1L (° C) = -93.9557 + 2466.96 (N + O) -8417.93 (N + O) ²

(Correlation for 19% Cr Ferritic Stainless Steel 430J1L)

As shown in Fig. 1 and Fig. 2, in order to control DBTT to -20 ° C or lower, the fact that the limit of N + O is required to be 0.025% by weight or less for the 429EM and 0.033% or less for the 430J1L is required. It can be seen. That is, when laser welding is applied to ferritic stainless steel in the range of 14 to 20% Cr, the N + O content of the welded joint must be controlled to 0.025% by weight or less in order to ensure low temperature workability of the welded joint. It can be seen. As described above, when the content of N + O exceeds 0.025% by weight, excessive oxidation inclusions and nitrides are formed to reduce impact toughness and increase cracking susceptibility of welded joints during spinning.

When welded steel pipe, which is one side of the present invention, is welded by gas shield arc welding, the N + O content of the welded joint is preferably controlled to 0.0185% by weight or less within the range of securing the through bead and the defect-free welded portion. .

As a result of analyzing the correlation between N + O content of welded joint and DBTT characteristics of gas shielded arc welded joint for welded steel pipe with 14 ~ 20% Cr content, the following empirical formula was obtained. Also, a graph of this correlation is shown in FIG. 3.

DBTT (° C) = -86.3753 + 0.359536 (N + O)

(N + O unit is ppm)

Ductile Brittle Transition Temperature (DBTT) is a soft brittle transition temperature, and it can be determined that low temperature toughness and processability improve as the value moves to a low temperature. From the above equation, it can be seen that the N + O content is required to be controlled to 185 ppm or less (0.0185% or less) by calculating the processable range up to -20 ° C, which is estimated to be a cold working temperature. N and O exist in the form of oxidation inclusions and nitrides in the welded joint and can act as a starting point for cracking during processing. The extra N can be dissolved in the matrix and harden the matrix, increasing the rate of growth when cracking occurs. That is, when the content of N + O exceeds 0.0185% by weight, excessive oxidation inclusions, nitrides are formed to reduce the impact toughness and increase the crack susceptibility of the weld portion during spinning.

Hereinafter, the production method of the present invention will be described.

In one aspect of the present invention, a steel material having the above-described component system can be piped by a gas shield arc welding method. Although a general gas shielded arc welding method can be applied, it is preferable to conduct piping according to the conditions described below.

The flow rate of the shield gas supplied to the torch nozzle is preferably limited to 15-20 L / min for arc stability and electrode oxidation prevention. If the shield gas flow rate is less than 15 L / min, the arc may be unstable and death bead may occur. On the other hand, when the flow rate of the shield gas exceeds 20L / min, the arc is cooled, the penetration characteristics are lowered, bead drooping phenomenon occurs, there is a problem in that the cost due to excessive use also increases.

In addition, it is possible to shield the arc rear, through which it is possible to prevent the inclusion of impurities. In the case of shielding gas, the flow rate is preferably limited to 15 to 20 L / min. If the flow rate is less than 15 L / min, a part of the weld is oxidized, and the N + O content exceeds the reference value. If the flow rate exceeds 20 L / min, the shaking of the arc occurs around the electrode. Underfill phenomenon may occur as the entire bead sags.

The shielding flow rate of the weld steel pipe inner surface bead is preferably controlled to 5 ~ 10L / min. If less than 5L / min, the bead surface is oxidized due to poor shielding, and the content of N and O increases, and if it exceeds 10L / min, the weld bead may be recessed in the steel thickness direction by gas pressure.

According to another aspect of the present invention, a steel material having the above-described component system can be welded by a laser welding method. Although a general laser welding method can be applied, it is preferable to be piped according to the conditions described below.

The flow rate of the shield gas supplied to the laser head is preferably controlled at 15 to 20 L / min in order to stabilize the laser beam and prevent oxidation of the optical system. If less than 15L / min, there is a problem that the beam output is lowered due to poor shielding, the optical system is oxidized, and the workability is lowered. On the other hand, if it exceeds 20L / min, the melted portion is cooled, the penetration characteristics are lowered, bead drooping phenomenon occurs, there is a problem that the cost due to overuse also increases.

In the case of a gas connected to the laser head to shield the beam back, it is preferable to control the gas to 100 to 200 L / min. If it is less than 100 L / min, a part of the welded joint is oxidized, and the N + O content exceeds the standard value. If it exceeds 200 L / min, the penetration characteristics may deteriorate and underfill phenomenon may occur due to gas pressure. have.

In addition, the inside of the welded steel pipe can shield the entire inner surface bead. The shielding flow rate of the weld bead inner surface bead is preferably controlled to 5 ~ 10L / min. If less than 5L / min, the bead surface is oxidized due to poor shielding, and the content of N and O increases, and if it exceeds 10L / min, the weld bead may be recessed in the steel thickness direction by gas pressure.

It is preferable to manage weld heat input amount to 1.5-6 kW * min / m. As the amount of heat input of welding decreases, the amount of impurity mixed from the outside decreases, thereby improving the shielding property. However, when the amount of heat input of the welding decreases, it is difficult to secure the penetration bead due to the lack of heat input. In addition, when the heat input amount exceeds 6kW · min / m, the shielding property is poor, and a hole is generated during welding, so that a healthy welded joint cannot be secured.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are for the understanding of the present invention, and the present invention is not limited thereto.

(Example 1)

The ferritic stainless steels having the components shown in Table 1 were manufactured into steel sheets having a thickness of 1.5 mm through hot rolling, cold rolling, and annealing. Using the steel sheet as a base material, a weld steel pipe having an outer diameter of 115.7 mm, a thickness of 1.5 mm, and a length of 600 mm was manufactured by a roll bending method of a TIG welding tube method.

The welding speed was 0.32 m / min, the pure argon gas was used for welding, and the welding current and gas amount were changed. Weldability was evaluated by using external bead, cross-sectional texture test and impact test. The DBTT characteristics of the welded portion were investigated by applying the Charpy impact test on a 1/4 Sub-size (1.5mm ^t × 10mm ^w × 55mm ^l ) test specimen in the range of -100 ~ 100 ℃. In the present invention, the test temperature corresponding to 1/2 of the upper limit impact energy was set to DBTT.

Table 2 below shows the results of evaluating the welding characteristics when the shielding method was changed for the lower limit and the upper limit of the appropriate welding current range. At this time, in case of A, only the shielding treatment of the torch portion is performed, and in case of B, the shielding of the torch portion and the welded steel tube backside bead shielding treatment are performed, and C is the torch portion shielding and the welded steel tube backside bead shielding and the torch rear shielding. As for the shield gas flow rate, the torch shield flow rate was 15 L / min, the torch rear shield flow rate was 20 L / min, and the back bead shield flow rate was 5 L / min, respectively.

Through-bead means that the molten part is formed over the entire range in the thickness direction of the steel. Melting occurs when excessive welding heat input is applied. The welding defect is generated by the hole appearance in the weld joint. It was. The occurrence of underfill and bead depression was considered as pass when the weld bead protruded outward from the steel surface in the cross section and failed when beads were formed inside the steel.

division Cr Cu C Mn Mo Nb Ni N P Si S Ti TotalAl Grade 1 11.3 - 0.007 0.30 - - 0.09 0.008 0.01 0.45 0.002 0.23 0.050 Gangjong 2 14 0.072 0.0068 0.283 0.02 0.397 0.13 0.0082 0.0237 0.892 0.0020 0.164 0.072 Gangjong 3 19.22 0.479 0.0117 0.302 0.01 0.442 0.19 0.0062 0.0203 0.537 0.0009 - 0.003

(However, the unit of Table 1 is weight%)

Steel grade Shielding
Way flux
(l / min) welding
electric current
(A) Penetrate
Bead
The presence or absence Entertainment
Occur
The presence or absence Underfill presence Bead
depression
The presence or absence Impurity amount of welding part (ppm) Weldment DBTT
(℃)
Remarks Backside shield Torch Rear Shield O N O + N Grade 1 - - - - - - - 38 56 94 -68 Base material Grade 1 A - - 120 ○ Not occurring Not occurring Not occurring 95 80 175 -25 Comparative Example 1 Grade 1 A - - 170 ○ Not occurring Not occurring Not occurring 120 100 220 -10 Comparative Example 2 Grade 1 B 5 - 120 ○ Not occurring Not occurring Not occurring 51 52 103 -70 Comparative Example 3 Grade 1 B 5 - 170 ○ Not occurring Not occurring Not occurring 74 54 128 -32 Comparative Example 4 Grade 1 C 5 20 120 ○ Not occurring Not occurring Not occurring 43 51 94 -67 Inventory 1 Grade 1 C 5 20 170 ○ Not occurring Not occurring Not occurring 51 48 99 -40 Inventive Example 2 Gangjong 2 - - - - - - - - 15 88 103 -42 Base material Gangjong 2 A 5 - 120 ○ Not occurring Not occurring Not occurring 140 260 400 40 Comparative Example 5 Gangjong 2 A 5 - 170 ○ Not occurring Not occurring Not occurring 190 180 370 45 Comparative Example 6 Gangjong 2 B 5 - 120 ○ Not occurring Not occurring Not occurring 98 200 298 17 Comparative Example 7 Gangjong 2 B 5 - 170 ○ Not occurring Not occurring Not occurring 110 160 270 33 Comparative Example 8 Gangjong 2 C 5 20 120 ○ Not occurring Not occurring Not occurring 24 87 111 -30 Inventory 3 Gangjong 2 C 5 20 170 ○ Not occurring Not occurring Not occurring 21 84 105 -32 Honorable 4 Gangjong 3 - - - - - - - - 40 63 103 -80 Base material Gangjong 3 A - - 120 ○ Not occurring Not occurring Not occurring 180 200 380 52 Comparative Example 9 Gangjong 3 A - - 170 ○ Not occurring Not occurring Not occurring 260 170 430 58 Comparative Example 10 Gangjong 3 B 5 - 120 ○ Not occurring Not occurring Not occurring 130 220 350 50 Comparative Example 11 Gangjong 3 B 5 - 170 ○ Not occurring Not occurring Not occurring 130 140 270 27 Comparative Example 12 Gangjong 3 C 5 20 120 ○ Not occurring Not occurring Not occurring 41 66 107 -44 Inventory 5 Gangjong 3 C 5 20 170 ○ Not occurring Not occurring Not occurring 36 62 98 -47 Inventory 6

Regardless of the steel type, the shielding methods A, B, and C decreased in the order of N, O, and the DBTT moved to the low temperature side, indicating that the low temperature toughness and workability of the welded part were improved. In case of steel grade 1, which is a low Cr type, the DBTT characteristics can be secured by B only shielding treatment.However, in the case of medium and high Cr ferritic stainless steels such as steel grades 2 and 3, only C is applied. It can be seen that the O and N content can be controlled and the DBTT characteristics are greatly improved. In the case of medium and high Cr ferritic stainless steel, O and N in the air can be easily mixed during welding, and it can be confirmed that blocking external air is a necessary factor for securing workability.

(Example 2)

A ferritic stainless steel that satisfies the composition of Steel Grades 1 and 2 of Table 1 was manufactured by hot rolling, cold rolling, and annealing to prepare a steel plate having a thickness of 1.5 mm.

Using the steel sheet as a base material, a weld steel pipe having an outer diameter of 115.7 mm, a thickness of 1.5 mm, and a length of 600 mm was manufactured by a laser welding welding method of a roll bending method. Laser welding has a maximum output of 12kW CO ₂ The laser welding machine was used in a laser output range of 5 to 9 kW and a welding speed of 0.5 to 10 m / min. The flow rate of the helium shield gas supplied to the laser head was 20 L / min, and the rear shield and the inner bead shield gas were welded with varying flow rates using Arcon.

Weldability was evaluated by using external bead, cross-sectional texture test, and impact test. The DBTT characteristics of the weld were investigated by applying the Charpy impact test on the 1/4 Sub-size (1.5mm ^t × 10mm ^w × 55mm ^l ) test specimens in the range of -100 ~ 100 ℃. In the present invention, the test temperature corresponding to 1/2 of the upper limit impact energy was set to DBTT. In addition, the crack susceptibility was evaluated by examining the weld crack in the weld section after press-cutting in the vertical direction of the weld. In addition, the weld heat input, shielding conditions and occurrence of weld defects, N + O content of the weld joint, DBTT characteristics, Table 3 shows the results of examining the correlation between crack susceptibility.

Steel grade welding
Heat input
(kWmin / m) Gas flow rate (l / min) Penetrate
Bead
The presence or absence Underfill
The presence or absence
Inside
Bead
depression
The presence or absence
N + O
(ppm) DBTT
(℃) Shear
cut
crack
Incidence
(%)
Remarks Torch Rear Shield
(Ar) Inner Bead Shield
(Ar) Gangjong 2 - - - - - - 0.0103 -42 0 Base material Gangjong 2 12 - - Penetrate
(Accept) - - - - - Comparative Example 13 Gangjong 2 6 - - Penetrate Not occurring Not occurring 0.103 11 20 Comparative Example 14 Gangjong 2 6 - 5 Penetrate Not occurring Not occurring 0.086 8 10 Comparative Example 15 Gangjong 2 6 - 10 Penetrate Not occurring Not occurring 0.083 8 10 Comparative Example 16 Gangjong 2 6 - 15 Penetrate Not occurring Occur 0.083 - - Comparative Example 17 Gangjong 2 6 50 5 Penetrate Not occurring Not occurring 0.053 -3 10 Comparative Example 18 Gangjong 2 6 100 5 Penetrate Not occurring Not occurring 0.0171 -26 0 Honorable 7 Gangjong 2 1.5 200 5 Penetrate Not occurring Not occurring 0.0121 -30 0 Inventive Example 8 Gangjong 2 1.5 - - Penetrate Not occurring Not occurring 0.025 -20 0 Comparative Example 19 Gangjong 2 1.5 - 5 Penetrate Not occurring Not occurring 0.02 -20 0 Comparative Example 20 Gangjong 2 1.5 100 5 Penetrate Not occurring Not occurring 0.0115 -32 0 Proposition 9 Gangjong 2 One - - Unpenetrating - - - - - Comparative Example 21 Gangjong 3 - - - - - - 0.0103 -80 0 Base material Gangjong 3 6 - - Penetrate Not occurring Not occurring 0.160 85 90 Comparative Example 22 Gangjong 3 6 - 5 Penetrate Not occurring Not occurring 0.096 68 70 Comparative Example 23 Gangjong 3 6 100 5 Penetrate Not occurring Not occurring 0.016 -31 0 Inventory 10 Gangjong 3 1.5 - - Penetrate Not occurring Not occurring 0.063 30 50 Comparative Example 24 Gangjong 3 1.5 - 5 Penetrate Not occurring Not occurring 0.057 10 20 Comparative Example 25 Gangjong 3 1.5 100 5 Penetrate Not occurring Not occurring 0.011 -78 0 Exhibit 11

In Comparative Example 13, the welding heat input was excessively applied at 12 kW · min / m, but the penetration welding part could be secured, but welding of the tubular pipe was difficult due to the melt phenomenon in which holes were generated during welding. Comparative Examples 14 and 19 show that the N + O content increases rapidly, the DBTT temperature rises and the cracking susceptibility increases when the rear shield and the inner bead shielding treatment are not performed. In Comparative Examples 15 to 17, the gas flow rate was changed when only the inner bead was shielded, and it was found that the impact toughness was reduced and the crack susceptibility was high because the N + O content was high regardless of the flow rate increase. In addition, in the case of the gas flow rate 15L / min, the bead shape in which the inner surface beads are recessed in the steel thickness direction due to the high gas pressure was poor. In Comparative Examples 19 and 20, laser welding was performed at low heat input, and the specific DBTT characteristics and crack prevention can be achieved without additional shielding treatment or by shielding only internal beads. Since it is difficult to secure, it was excluded from the present invention. In Comparative Example 21, when a small amount of welding input heat was applied at 1 kW · min / m, it was found that the penetration bead was difficult to be secured, and therefore, it was difficult to be applied to welding a tubular tube. Inventive Examples 7 to 9 correspond to the present invention, by applying the rear shield and the inner bead shielding at the same time, the N + O value is greatly reduced, thereby greatly improving the low temperature toughness of the weld and preventing cracking.

Claims (9)

By weight%, Cr: 14.0-20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04% or less, S: 0.01 % Or less, 0 (oxygen): 0.01% or less, remainder Fe and other unavoidable impurities, and the welded steel pipe having a DBTT (Ductile Brittle Transition Temperature) of the welded joint at -20 ° C or less, and the base metal and the weld of the welded steel pipe Welded steel pipe with excellent spinning processability with a content of N + O in the negative part of 0.0185% or less.
The method according to claim 1,
The welded steel pipe by weight%, Mo: 0.1 ~ 2.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1 ~ 2.0% of the welded steel pipe with excellent spinning processability, characterized in that it further comprises one or two or more. .
The method according to claim 1,
The welded steel pipe by weight%, Nb: 0.01 ~ 0.5%, Ti: 0.01 to 0.5% of the weld steel pipe with excellent spinning workability, characterized in that it further comprises one or two.
The method according to claim 1,
The welded steel pipe is a welded steel pipe with excellent spinning processability, characterized in that it further comprises Ca: 0.0005 to 0.002% by weight.
delete By weight%, Cr: 14.0-20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04% or less, S: 0.01 % Or less, 0 (oxygen): 0.01% or less, shield gas flow rate for supplying the steel containing the remaining Fe and other unavoidable impurities to the torch portion: 15-20 L / min, shield gas flow rate behind the torch portion: 15-20 L / A method for manufacturing a welded steel pipe with excellent spinning processability, characterized in that the pipe is welded by a gas shield arc welding method supplied at min and a shield gas flow rate at the back of the weld bead: 5 to 10 L / min.
The method of claim 6,
N + O content of the base metal and the welded joint of the welded steel pipe is 0.0185% by weight or less.
By weight%, Cr: 14.0-20.0%, C: 0.01% or less, N: 0.01% or less, Si: 1.0% or less, Mn: 1.0% or less, Al: 0.15% or less, P: 0.04% or less, S: 0.01 % Or less, 0 (oxygen): 0.01% or less, shield gas flow rate for supplying steel materials containing residual Fe and other unavoidable impurities to the laser head portion: 15 to 20 L / min, shield gas flow rate behind the laser beam: 100 to 200 L A manufacturing method of a welded steel pipe having good spinning workability, characterized in that the tube is welded by a laser welding method supplied at a flow rate of 5 to 10 L / min per minute / min and a weld bead.
The method according to claim 8,
N + O content of the base metal and the welded joint of the welded steel pipe is 0.0185% by weight or less.

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