KR20160077240A - Manufacturing method for welded steel pipe having reduced hot crack - Google Patents

Abstract

The present invention relates to a manufacturing method for a welded steel pipe with reduced hot crack and, more specifically, relates to a manufacturing method for a welded steel pipe by applying a welding method to prevent hot cracks caused by an excessive tensile stress on a welded portion by a solidification contraction or a thermal contraction during a welding process. According to an embodiment of the present invention, the manufacturing method for the welded steel pipe comprises: a step of preparing a steel pipe to be welded; a step of preheating a welding portion and a heat-affected zone of the steel pipe during a retention time (tp) defined in a mathematical formula 1 of 0.7(D×t)/(Ts+200) <= tp <= (D×t)/(Ts+200) at a temperature range of 50-300°C; and a step of welding the steel pipe.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method for manufacturing a welded steel pipe with reduced high temperature cracks,

The present invention relates to a method of manufacturing a welded steel pipe with reduced high temperature cracks, and more particularly, to a welding method of preventing a high temperature crack from occurring due to excessive tensile stress acting on a welded portion by coagulation shrinkage or heat shrinkage during a welding process To a method for manufacturing a welded steel pipe.

2. Description of the Related Art In recent years, as the demand for high strength, high strength, non-magnetization, and the like of steel has been increased, the capacity for austenitic steel has been widening. These austenitic steels are used as they are in the form of plate materials and post materials, but they are often used in the form of steel pipes according to their use.

It is common to use a method of obtaining a steel pipe by butt welding between the plate material and the plate material after rounding the plate material except for the seamless steel pipe to form a circular cross section. However, due to the nature of the pipe-making process of flattening the plate material into a circular shape, compressive stress acts on the inner surface portion of the steel pipe, and tensile stress acts on the outer surface of the steel pipe.

In such a state, the seam portion where the end portion and the end portion of the steel plate are abutted against each other is formed through the process of solidifying after the base material or the welding material is dissolved. Especially, the steel material having high coagulation shrinkage ratio, such as austenite steel, During the cooling process, the welds are excessively shrunk, resulting in strong tensile stresses in the welds. These strong tensile stresses combine with the existing residual stresses on the outer surface of the steel pipe, causing high temperature cracks in the welds.

According to one aspect of the present invention, there is provided a method of manufacturing a novel welded steel pipe capable of preventing a high temperature crack from being formed in a welded portion when a steel pipe is manufactured by welding.

The object of the present invention is not limited to the above description. Those skilled in the art will understand that there is no difficulty in understanding the present invention from the overall contents of the specification.

A welding method according to one aspect of the present invention includes: preparing a steel pipe to be welded; Preheating at least the welded portion of the steel pipe and the weld heat affected portion in a temperature range of 50 to 300 占 폚 for a holding time tp defined by Equation 1 below; And welding the steel pipe.

[Equation 1]

0.7 (D x t) / (Ts + 200)? T p? (D x t) / (Ts + 200)

At this time, the solidification shrinkage ratio of the welded portion of the steel pipe may be 4% or more.

According to one aspect of the present invention, the steel pipe may be a high manganese steel having a Mn content of 10 wt% or more, or an austenitic stainless steel of SUS 300 series.

According to still another aspect of the present invention, the step of welding the steel pipe is performed using a welding material, and the coagulation shrinkage ratio of the welding material may be 4% or more.

The welding material having a coagulation shrinkage of 4% or more may be a high manganese steel having a Mn content of 10% by weight or more, or an austenitic stainless steel of the SUS 300 series.

The temperature for preheating the welded portion of the steel pipe and the weld heat affected portion is more preferably 50 to 200 占 폚.

As described above, in the welding method of the present invention, when the steel pipe is seam-welded using a steel material having a high coagulation shrinkage ratio, the welded portion is preheated as an appropriate method to minimize the residual stress acting on the welded portion during the solidification process, It is possible to effectively prevent the occurrence of cracks.

Fig. 1 is a schematic view showing that each welding layer is formed in the core part when multi-layer welding is performed on the inner and outer surfaces of the steel pipe.

Hereinafter, the present invention will be described in detail.

Fig. 1 shows an exemplary shape of a welded portion (a deep portion) to be formed when welding a steel pipe. Although not limited in this order, it is preferable to perform inner-layer super-layer welding to form the inner-layer super-layer bead 11, and then weld the inner-layer two layers 12. If necessary, three or more layers may be welded on the inside. Thereafter, outer surface welding is performed in the order of welding of the outer surface layer 21, welding of the outer surface two layers 22, and welding of the outer surface three layers 23. In some cases, the outer surface gouging process may be performed before the outer surface welding, and the number of layers may be more or less than that shown in the figure. In addition, the inner surface welding is not necessarily performed before the outer surface welding. In a very special case, the outer surface and the inner surface may be alternately welded.

One aspect of the present invention employs a method to prevent excessive tensile stress from acting on the welds through the process of preheating the welds prior to welding of the layers that are susceptible to high temperature cracks. In other words, when only a local welded portion is heated, the heat capacity of the welded portion is small, so that the speed of coagulation and cooling after heating becomes high, and the portion where residual stress due to shrinkage acts is limited to a narrow region. So that high-temperature cracks tend to occur. Therefore, the inventors of the present invention have found that when the periphery of the welded portion is widely heated, not only the portion where the stress acts at the time of solidification is widened, but also the coagulation and cooling progress slowly to easily solve the residual tensile stress, And came to the present invention.

Conventionally, it is generally considered that heating is not preferable because the speed at which the welded portion is cooled after welding is too slow to cause segregation in the welded portion. However, in the present invention, It is rather advantageous to preheat the welded portion and the periphery thereof to reach the technical idea peculiar to the present invention.

Coagulation and cooling of the welded portion are not particularly problematic in the case of a steel having a low coagulation shrinkage percentage, but may be a problem in a steel having a coagulation shrinkage ratio of 4% or more as in an austenitic steel. Therefore, one aspect of the present invention can be advantageously applied to a steel material having a coagulation shrinkage ratio of 4% or more of a welded portion or a welded material. In addition, any material can be applied as long as the coagulation shrinkage is 4% or more, and the kind thereof is not particularly limited. However, in one embodiment of the present invention, a high manganese steel having a Mn content of 10% by weight or more and an austenitic stainless steel of a SUS 300 series may be used. They may be dissolved in welding in the form of welding material to form a welded portion, and the base material (steel pipe) may be dissolved to form a welded portion. The higher the solidification shrinkage percentage, the higher the necessity of application of the present invention. Therefore, the upper limit of the solidification shrinkage percentage is not particularly limited. However, the currently marketed steels are usually determined to have a coagulation shrinkage ratio of 10% or less, but the present invention is not limited thereto.

Further, in the case of small heat input welding with small heat input, the welded portion formed by dissolving is not large, and thus the tensile stress to be applied is not large. Therefore, the present invention can be more effective at the heat input amount of 1 kJ / mm or more. The upper limit of the heat input amount is not specifically defined as the heat input amount used in the technical field to which the present invention belongs. However, when the heat input amount exceeds 5kJ / mm in a typical welding operation, high temperature cracks sometimes occur. Therefore, the upper limit of the heat input amount may be set at 5kJ / mm.

In the present invention, the welding portion (meaning that the portion to be welded is not yet welded, and the region to be welded in the future) and the region to be weld heat affected portion (HAZ) , And the area to be welded heat affected part in the welding will be hereinafter simply referred to as the welded heat affected part), is heated to a constant temperature range for a certain period of time. In the present invention, in order to improve the efficiency of operation, the entire steel pipe (including the steel pipe not yet welded) may be heated to a certain extent by heating the portion including the welded portion and the weld heat affected portion. Concept) may also be used.

In order to sufficiently obtain the effect of relaxing the residual tensile stress by heating, it is advantageous to set the heating temperature of the welded portion and the weld heat affected portion at 50 DEG C or higher. The effect may be inadequate at temperatures below that. However, even if the heating temperature of the welded portion is raised to more than 300 ° C, further improvement effect is not confirmed, and the cooling speed of the welded portion becomes too slow, so that a problem due to segregation may newly occur. Accordingly, in the present invention, the preheating temperature (Ts) before welding is limited to the range of 50 to 300 占 폚 based on the surface portion of the work. According to one embodiment, the preheating temperature (Ts) prior to welding may range from 50 to 200 &lt; 0 &gt; C. In the present invention, the preheating temperature is based on the temperature of the surface portion (the outer surface side of the steel pipe) of the weld line (abutting portion of the two end faces of the steel plate).

According to the results of the inventors' study, the time (tp) required for sufficient preheating can be determined according to the diameter of the steel pipe and the thickness of the steel pipe Specifically, it is required to be not less than 0.7 (D x t) / (Ts + 200). Where D is the diameter of the steel pipe (outer diameter, mm), t is the thickness of the steel pipe (mm), and Ts is the preheating temperature (캜). It is not necessary to limit the upper limit particularly. However, when the holding time tp reaches (D x t) / (Ts + 200), the effect of the present invention can be obtained. It is not economically advantageous because it is difficult to expect an increase in the effect, and the energy intensity required for preheating rises. Therefore, the holding time tp has the following relationship.

[Equation 1]

0.7 (D x t) / (Ts + 200)? T p? (D x t) / (Ts + 200)

In the present invention, the preheating step may be performed before any step of the inner surface and the outer surface welding, but it is more preferable that the preheating is performed immediately before the outer surface welding because the residual tensile stress of the steel pipe greatly acts on the outer surface. In addition, when the outer surface welding is carried out by multi-layer welding, it is more advantageous to be performed immediately before the multi-layer welding in the multi-layer welding. On the other hand, when welding the outer layer, the effect of the preheating can be obtained to some extent owing to the residual heat due to the immediately preceding welding, but the welding of the other layers is not excluded.

In addition, according to one aspect of the present invention, the temperature of the region where the highest temperature is preheated during welding is 20% or more or 15% or more , Preferably at least 10%, before cooling. If the degree of cooling of the preheating region is excessive, it is difficult to sufficiently reduce the residual tensile stress due to the preheating. In order to satisfy such a condition, one aspect of the present invention allows welding to be performed immediately after preheating.

Accordingly, one aspect of the present invention provides a method of manufacturing a steel pipe, comprising: preparing a steel pipe to be welded; Preheating at least the welded portion of the steel pipe and the weld heat affected portion in a temperature range of 50 to 300 占 폚 for a holding time tp defined by Equation 1; And a step of welding the steel pipe.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following embodiments are intended to illustrate and describe one aspect of the present invention and are not intended to limit the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

(Example)

A steel sheet having the composition shown in Table 1 and a welding material were prepared. The steel plate was processed into a steel pipe shape, and the inner surface of the steel pipe was welded in two layers. The content of each element shown in Table 1 is expressed in terms of% by weight. Both the steel sheet and the welding material exhibited a 100% austenite single phase structure, and the coagulation shrinkage rate was about 5%. Thereafter, the whole steel plate including the portion to be welded of the steel sheet and the portion to form the weld heat affected portion was preheated to the preheating condition shown in Table 2 below, and then the outer layer was welded under the welding conditions shown in Table 3 without delay. As shown in Table 3, the rate of high-temperature cracking, which represents the ratio of the length of the high-temperature cracked portion to the total weld length, is shown together. In Table 2, Comparative Example 9 means that it was maintained at room temperature without preheating.

division C Si Mn P S S.Al N Steel plate 1.17 0.08 17.5 0.023 0.0019 0.0019 0.0088 Welding materials
(Special clad metal) 0.43 0.56 18.1 0.019 0.0081 - -

division Pipe diameter
(mm) Pipe thickness
(mm) Preheat temperature
(° C) Warm-up time
(minute) 0.7 (D x t) / (Ts + 200) (D x t) / (Ts + 200) Inventory 1 762 20 60 50 41.0 58.6 Inventory 2 762 20 120 35 33.3 47.6 Inventory 3 762 20 185 35 27.7 39.6 Honorable 4 609 20 200 25 21.3 30.5 Inventory 5 609 20 150 25 24.4 34.8 Inventory 6 609 20 70 44 31.6 45.1 Honorable 7 508 15 65 25 20.1 28.8 Honors 8 508 15 100 20 17.8 25.4 Proposition 9 508 15 195 15 13.5 19.3 Inventory 10 508 10 52 20 14.1 20.2 Exhibit 11 508 10 110 15 11.5 16.4 Comparative Example 1 762 20 40 50 44.5 63.5 Comparative Example 2 762 20 35 100 45.4 64.9 Comparative Example 3 762 20 33 60 45.8 65.4 Comparative Example 4 609 20 150 15 24.4 34.8 Comparative Example 5 609 20 145 20 24.7 35.3 Comparative Example 6 609 20 137 100 25.3 36.1 Comparative Example 7 508 15 40 20 22.2 31.8 Comparative Example 8 508 15 150 12 15.2 21.8 Comparative Example 9 508 15 23 0 23.9 34.2

division Welding current
(A) Welding voltage
(V) Welding speed
(mm / min) Heat input
(kJ / mm) High Temperature Crack Initiation Rate
(%) Inventory 1 520 30 450 2.08 One Inventory 2 520 30 750 1.248 0 Inventory 3 520 30 800 1.17 0 Honorable 4 520 30 500 1.872 0 Inventory 5 520 30 500 1.872 One Inventory 6 520 30 600 1.56 0 Honorable 7 520 30 600 1.56 2 Honors 8 520 30 600 1.56 0 Proposition 9 520 30 500 1.872 0 Inventory 10 520 30 500 1.872 0 Exhibit 11 520 30 500 1.872 0 Comparative Example 1 520 30 450 2.08 25 Comparative Example 2 520 30 750 1.248 23 Comparative Example 3 520 30 800 1.17 28 Comparative Example 4 520 30 500 1.872 27 Comparative Example 5 520 30 500 1.872 35 Comparative Example 6 520 30 600 1.56 2 Comparative Example 7 520 30 600 1.56 6 Comparative Example 8 520 30 600 1.56 8 Comparative Example 9 520 30 500 1.872 9

As described above, Inventive Example 1 to Inventive Example 11 satisfied all the conditions defined in the present invention, and the area where high-temperature cracking occurred was only 2% or less of the entire welded length, and thus showed very good results.

However, in Comparative Examples 1 to 3, the preheating temperature was less than 50 占 폚 and the preheating was not sufficiently performed, so that the effect of reducing the high temperature cracking was not sufficient. In Comparative Example 4 and Comparative Example 5, the preheating time was comparatively short, and it was confirmed that a large amount of high temperature cracks were generated. In Comparative Example 6, the preheating time was longer than that specified in the present invention, but the high temperature cracking rate was 2%, which means that the preheating time was prolonged. In Comparative Example 7, it was confirmed that the preheat temperature and the preheat time did not fall within the range defined by the present invention, and it was confirmed that the high temperature crack occurred about 6%. In Comparative Example 8 and Comparative Example 9, the preheating time was very short, or when the preheating was not carried out at room temperature, the high temperature cracking rates were 8% and 9%, respectively.

Therefore, the advantageous effects of the present invention can be confirmed.

11: Inner super-layer bead
12: inner two-layer bead
13: inner three-layer bead
21: outer superfine bead
22: outer two-layer bead
33: Outer three layer bead

Claims (6)

Preparing a steel pipe to be welded;
Preheating at least the welded portion of the steel pipe and the welded heat affected portion in a temperature range of 50 to 300 占 폚 for a holding time tp defined by Equation 1 below; And
And welding the steel pipe, wherein the high-temperature crack is reduced.
[Equation 1]
0.7 (D x t) / (Ts + 200)? T p? (D x t) / (Ts + 200)
The method of manufacturing a welded steel pipe according to claim 1, wherein a high-temperature crack having a coagulation shrinkage of 4% or more is reduced in the welded portion of the steel pipe.
The method of manufacturing a welded steel pipe according to claim 1, wherein the steel pipe is a high manganese steel having a Mn content of 10 wt% or more, or austenitic stainless steel of SUS 300 series.
The method of manufacturing a welded steel pipe according to claim 1, wherein the step of welding the steel pipe is performed using a welding material, and the high temperature crack having a coagulation shrinkage of 4% or more is reduced.
5. The method of manufacturing a welded steel pipe according to claim 4, wherein the welding material is a high manganese steel having a Mn content of 10 wt% or more, or an austenitic stainless steel of SUS 300 series.
The method of manufacturing a welded steel pipe according to any one of claims 1 to 5, wherein a high temperature crack at a temperature of 50 to 200 占 폚 is preheated at a welded portion of the steel pipe and a weld heat affected portion.

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