KR101359085B1 - High frequency welding steel pipe having high strength and method for manufacturing the same - Google Patents

Abstract

High-strength high frequency welded steel pipe, which is one aspect of the present invention, is a high frequency welded steel pipe including a weld formed by welding a base material into a steel pipe by high frequency welding, wherein the base material is 0.3% by weight (C) of 0.3 to 0.9% of manganese (Mn). ) 15 ~ 30%, Aluminum (Al) 0.01 ~ 4.0%, Silicon (Si) 0.61% or less (excluding 0%), Phosphorus (P) 0.05% or less, Sulfur (S) 0.01% or less, Nitrogen (N) 0.01 Contains% or less (excluding 0%),
Niobium (Nb) 0.02 to 0.10%, titanium (Ti) 0.01 to 0.10% and vanadium (V) 0.02 to 0.50% of one or two or more, including the remaining Fe and inevitable impurities,
Mn-Al-based oxidation inclusions having a major axis length of less than 3 µm are included at the remaining interface after welding.
Another aspect of the present invention is a method of manufacturing a high-strength high frequency welded steel pipe, a method of manufacturing a high frequency welded steel pipe including a welded portion by heating and melting a welded portion of a base material and welding the base material by an upset roll, wherein the base material is weight percent. , 0.3 to 0.9% of carbon (C), 15 to 30% of manganese (Mn), 0.01 to 4.0% of aluminum (Al), 0.61% or less of silicon (Si) (excluding 0%), phosphorus (P) of 0.05% or less, 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N) (excluding 0%), 0.02 to 0.10% of niobium (Nb), 0.01 to 0.10% of titanium (Ti) and 0.02 to 0.50 of vanadium (V) Preparing a steel sheet including one kind or two or more kinds of% and including a balance Fe and unavoidable impurities; And manufacturing a steel pipe having a welded portion by welding the steel sheet at a high heating rate of 1.026 to 1.077 KW · M / MIN and a welding speed of 40 M / MIN or less, and injecting an inert gas into the welded portion during the high frequency welding. Mn-Al-based oxidation inclusions having a major axis length of less than 3 µm are included at the remaining interface after welding.

Description

High-strength high frequency welded steel pipe and its manufacturing method {HIGH FREQUENCY WELDING STEEL PIPE HAVING HIGH STRENGTH AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a high strength high frequency welded steel pipe and a method of manufacturing the same.

Recently, in the automobile industry, the adoption of high-strength steels has been increasing for the purpose of lightening the weight of the vehicle due to the reinforcement of environmental regulations. In addition, in terms of processing methods, tailored blanks (TWB, Tailor Blank), hydroforming, and the like are actively examined to replace existing press processes.

Hydroforming technology is a technology for forming parts by applying high pressure inside the tube, and manufacturing a number of parts into a single part reduces the assembly process of parts, thereby reducing process costs, lightening parts, and reducing material prices. There is an advantage to see. Welded steel pipes for hydroforming are mainly used in sizes of 48.6 ~ 165.2mm in outer diameter, and are generally manufactured by welding after roll forming.The types of welding applied here are TIG, plasma, high frequency, and laser. Welding and the like.

TIG welded steel pipes are often used by small manufacturers because they have low productivity but low initial installation cost, and facility operation is easier than other welding tanks. The laser welded steel pipe is excellent in welded quality because welding is performed using a laser, which is a high density heat source. However, due to the characteristics of the laser heat source, precise management and control of the cutting surface of the raw material, the weld seam, and the like are necessary. The high frequency welding method is a welding method in which a welding core heated by an induction coil is pressed by an upset roll to release molten metal to the outside, and finally, a solid structure of the base material remains. The welding speed is much faster than other welding, which is advantageous in terms of productivity, but it is difficult to secure quality in high strength steel.

The characteristics of the new processing method is that the welding is formed under the same conditions as the material, so that the quality of the weld is very important to be applied as a component material. In particular, in the case of high strength steel, since a large amount of alloying elements are typically added, there is a problem in that the hardenability increases during welding, thereby degrading the workability of the weld. In addition, in the case of high strength steel of 780MPa grade or more, the hardening phenomenon of the welded part is remarkable, so that it is difficult to apply for deep processing.

On the other hand, steel grades such as TWIP (Twinning Induced Plasticity) steel, which is a high-strength steel of austenitic structure with high elongation, have been developed by inducing twinning during deformation by adding a large amount of manganese, carbon, etc. Unlike conventional high strength steels (DP, TRIP, etc.), it has a single-phase austenite structure and does not generate phase transformation even after welding. However, the welded part of TWIP steel is softened due to coarsening of grains and loss of TWIP effect compared to the base material, and it contains a large amount of alloying elements that are easy to form oxidation inclusions such as Mn and Al. There is a problem of deterioration. In addition, due to the spring back phenomenon after welding, it is difficult to secure the proper tube welding conditions. In particular, the thin welded steel pipe is difficult to restrain by the roll spring spring phenomenon is more remarkable, there is a difficulty in processing.

Representative techniques for improving these problems is to form a stable carbonitride in HAZ even at high temperature for the purpose of improving the softening phenomenon of the welding heat affected zone (HAZ) of 780MPa-class high Mn nonmagnetic steels as in Patent Document 1 For this, a technique of combining Ti and Nb has been proposed. Another technique is Patent Document 2, which discloses a technique in which Ca and S are strictly controlled by a high Mn nonmagnetic steel excellent in weldability and machinability. However, these techniques do not improve the hydrogen embrittlement resistance.

In addition, Patent Document 3 discloses an arc welding method for securing a high-quality, healthy welding metal of a high Mn non-magnetic steel welded joint, in which the amount of oxygen is limited and the amount of C, Si, Mn, Cr, Ni, and N is disclosed. Flux is proposed to include specific welding wire and PbO, and to specify the amount of CaF ₂ , CaO, MgO, SiO ₂ , and Al ₂ O ₃ . However, the above technique relates to welding wire and flux for submerged arc welding applied to thick plate high Mn steel, and has a problem that it is difficult to apply to thin weld steel pipe.

In addition, Patent Document 4 discloses a method of manufacturing a high strength high frequency welded steel pipe having a tensile strength of 690 to 1180 MPa excellent in weldability. However, this method has a problem in that it is difficult to apply the welding wire and the flux in the thin weld steel pipe.

In addition, Patent Document 5 discloses a method for producing a high frequency welded steel pipe excellent in hydroforming formability used for materials for automobile structural parts. However, the method is preheated to 700 ~ 1200 ° before the tube welding, there is a disadvantage that it requires additional equipment according to the preheating and management is not easy.

In addition, Patent Document 6 discloses a high-strength high frequency welded steel pipe for automobiles and a manufacturing method thereof, and Patent Document 7 discloses a method of continuously heat treating twice in a welded portion of a thick high frequency welded steel pipe by a high frequency induction heating apparatus. . However, the above methods require high-frequency welded steel pipes and post-heat treatment, and thus require additional equipment and management for post-heat treatment.

Japanese Patent Laid-Open No. 8-134609 Japanese Patent Laid-Open No. 8-013092 Japanese Patent Laid-Open No. 6-192788 Japanese Laid-Open Patent Publication No. 2000-084614 Japanese Patent Laid-Open No. 11-172376. Japanese Patent Laid-Open No. 10-183292 Japanese Patent Laid-Open No. 08-319517

According to one aspect of the invention, to provide a high-strength high frequency welded steel sheet and a method of manufacturing the improved weldability.

High-strength high frequency welded steel pipe, which is one aspect of the present invention, is a high frequency welded steel pipe including a weld formed by welding a base material into a steel pipe by high frequency welding, wherein the base material is 0.3% by weight (C) of 0.3 to 0.9% of manganese (Mn). ) 15 ~ 30%, Aluminum (Al) 0.01 ~ 4.0%, Silicon (Si) 0.61% or less (excluding 0%), Phosphorus (P) 0.05% or less, Sulfur (S) 0.01% or less, Nitrogen (N) 0.01 Contains% or less (excluding 0%),

Niobium (Nb) 0.02 to 0.10%, titanium (Ti) 0.01 to 0.10% and vanadium (V) 0.02 to 0.50% of one or two or more, including the remaining Fe and inevitable impurities,

Mn-Al-based oxidation inclusions having a major axis length of less than 3 µm are included at the remaining interface after welding.

Another aspect of the present invention is a method of manufacturing a high-strength high frequency welded steel pipe, a method of manufacturing a high frequency welded steel pipe including a welded portion by heating and melting a welded portion of a base material and welding the base material by an upset roll, wherein the base material is weight percent. , 0.3 to 0.9% of carbon (C), 15 to 30% of manganese (Mn), 0.01 to 4.0% of aluminum (Al), 0.61% or less of silicon (Si) (excluding 0%), phosphorus (P) of 0.05% or less, 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N) (excluding 0%), 0.02 to 0.10% of niobium (Nb), 0.01 to 0.10% of titanium (Ti) and 0.02 to 0.50 of vanadium (V) Preparing a steel sheet including one kind or two or more kinds of% and including a balance Fe and unavoidable impurities; And manufacturing a steel pipe having a welded portion by welding the steel sheet at a high heating rate of 1.026 to 1.077 KW · M / MIN and a welding speed of 40 M / MIN or less, and injecting an inert gas into the welded portion during the high frequency welding. Mn-Al-based oxidation inclusions having a major axis length of less than 3 µm are included at the remaining interface after welding.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, by suppressing the formation of oxidation inclusions of a predetermined size or more at the interface remaining after welding, or simultaneously controlling the amount of protrusions of the inner and outer surfaces of the welded portion and controlling the springback phenomenon, it is possible to secure the same level of strength as that of the base metal and 780 MPa class. It is possible to provide a high strength austenitic high frequency welded steel pipe having a expansion ratio of 40% or more that cannot be obtained from the above general high strength steel, and a manufacturing method thereof.

1 is a schematic diagram of a welded portion of a high frequency welded steel pipe.

The present inventors have confirmed that the joint part without oxidation inclusions of a predetermined size or more is required in order to secure the processability of the high-strength high frequency welded steel pipe welded part, and in the case of the high frequency welded steel pipe, unlike other welded steel pipes, it is joined by pressure welding. By cutting the bead projecting by an appropriate range using a cutting tool such as a bite, it is confirmed that the processability of the welded part can be improved, and also it is confirmed that the spring back phenomenon can be controlled by optimizing the upset roll spacing and thus, the present invention. It became.

Hereinafter, a high strength high frequency welded steel pipe which is one side of the present invention will be described in detail.

High-strength high frequency welded steel pipe, which is one aspect of the present invention, is a high frequency welded steel pipe including a weld formed by welding a base material into a steel pipe by high frequency welding, wherein the base material is 0.3% by weight (C) of 0.3 to 0.9% of manganese (Mn). ) 15 ~ 30%, Aluminum (Al) 0.01 ~ 4.0%, Silicon (Si) 0.61% or less (excluding 0%), Phosphorus (P) 0.05% or less, Sulfur (S) 0.01% or less, Nitrogen (N) 0.01 Contains% or less (excluding 0%),

Niobium (Nb) 0.02 to 0.10%, titanium (Ti) 0.01 to 0.10% and vanadium (V) 0.02 to 0.50% of one or two or more, including the remaining Fe and inevitable impurities,

Mn-Al-based oxidation inclusions having a major axis length of less than 3 µm are included at the remaining interface after welding.

Carbon (C): 0.3-0.9 weight%

Since carbon (C) contributes to the stabilizer of the austenite phase, it is advantageous as the amount added thereof increases. Since the a '(alpha) -marthecite phase is produced during deformation, carbon is added in an amount of 0.3% by weight or more in order to prevent cracking and ductility deterioration during processing. In addition, in order to prevent the workability from being lowered due to the transition of the deformation behavior due to slip deformation due to a significant increase in the austenite phase, on the other hand, the addition amount of carbon is added at 0.9% or less. Therefore, the content of carbon is preferably 0.3 to 0.9% by weight.

Manganese (Mn): 15-30 wt%

Manganese (Mn) is also an essential element for stabilizing the austenite phase, but below 15% manganese is produced because an a '(alpha) -marthecite phase, which degrades formability, is produced and the strength increases but the ductility decreases rapidly. It is preferable to add more than weight%. On the other hand, as the amount of manganese is increased, twinning is suppressed to increase strength but decrease ductility. In addition, hot rolling cracks are more likely to occur as the amount of manganese is increased, and the upper limit of the amount of manganese is preferably limited to 30% by weight because steel sheet manufacturing costs increase due to the addition of a large amount of manganese having a high raw material cost. Therefore, the content of Mn is preferably 15 to 30% by weight.

Aluminum (Al): 0.01 to 4.0 wt%

Aluminum (Al) is usually added for deoxidation of steel, but in the present invention it is added to increase the work hardening index. High strength, high ductility, high manganese steel, aluminum is a ferrite stabilizing element, but increasing the stacking fault energy (stacking fault energy) in the slip surface of the steel to suppress the formation of ε-martesite phase to improve the ductility. In addition, aluminum suppresses the formation of the ε-martesite phase even in the case of low manganese addition, thereby minimizing the amount of manganese and improving workability. In addition, when aluminum is added, segregation of components is reduced, so that the material becomes uniform and moldability is increased. Therefore, the smaller the amount added, the more epsilon-martensite is produced and the strength is increased, but the depth of drawing is drastically reduced, so the lower limit is limited to 0.01% by weight. On the other hand, as the content of aluminum increases, ductility is reduced by reducing the ductility, worsening castability during continuous casting, and the surface quality of the product is degraded due to severe surface oxidation during hot rolling, so the upper limit of aluminum is 4.0% by weight. It is preferable to limit to. Therefore, the content of aluminum is preferably 0.01 to 4.0% by weight.

Silicon (Si): 0.61 wt% or less (excluding 0%)

Silicon (Si) is an element that enhances solid solution and increases yield strength by reducing grain size by a solid solution effect. In general, when excessively added, it is known to form a silicon oxide layer on the surface to lower the melt plating property. However, in a steel with a large amount of manganese, when an appropriate amount of silicon is added, a thin silicon oxide layer is formed on the surface to suppress the oxidation of manganese, thereby preventing formation of a thick manganese oxide layer formed after rolling in a cold rolled steel sheet. After annealing to prevent the corrosion of the cold rolled steel to improve the surface quality, it is possible to maintain the excellent surface quality as a steel plate of the electroplating material.

However, when the addition amount of silicon is increased, silicon oxide is formed on the surface of the steel sheet during hot rolling, which results in poor pickling property, thereby degrading the surface quality of the hot rolled steel sheet. In addition, silicon is concentrated on the surface of the steel sheet during high temperature annealing in the continuous annealing process and the continuous hot dip plating process, thereby reducing the wettability of the molten zinc on the surface of the steel sheet. In addition, the addition of large amounts of silicon greatly reduces the weldability of the steel. Therefore, the upper limit content of silicon is preferably included as 0.61% by weight.

Nitrogen (N): 0.01 wt% or less (excluding 0%)

Nitrogen (N) in the austenite grains acts as aluminum during the solidification process to precipitate fine nitrides to promote twin formation, thus improving strength and ductility when forming steel sheet, but with increasing nitrogen content, nitrides are excessively precipitated. To reduce hot workability and elongation, the nitrogen content was limited to 0.01 wt% or less.

According to one aspect of the present invention, the precipitated phase forming element is preferably made of one or two or more of niobium (Nb), titanium (Ti) and vanadium (V).

Niobium (Nb): 0.02 to 0.10 wt%

Niobium (Nb) is a strong carbide forming element that combines with carbon to form carbides in the same form as titanium. Also, the carbide formed at this time is an element effective in miniaturizing grain size by preventing grain growth and forming a precipitated phase at a lower temperature than titanium, which is an element having a large precipitation strengthening effect due to grain size refinement and precipitation phase formation. However, when the content of niobium is small, the effect intended for the present invention cannot be secured, and the lower limit of niobium is limited to 0.02% by weight. On the other hand, as the content of niobium increases, an excess of niobium segregates at grain boundaries, causing grain boundary odors, or excessively coarsening of precipitated phases, thereby decreasing grain growth effects. Therefore, the upper limit of niobium is limited to 0.10% by weight. It is preferable. Therefore, the content of niobium is preferably included in 0.02 ~ 0.1% by weight.

Titanium (Ti) 0.01 ~ 0.10 wt%

Titanium (Ti) is a strong carbide forming element that combines with carbon to form carbide, and the carbide formed at this time is an element effective in miniaturizing grain size by preventing grain growth. When the content of titanium is small, the effect intended for the present invention cannot be secured, and the lower limit of the titanium is limited to 0.01% by weight. On the other hand, as the content of titanium increases, excess titanium segregates at the grain boundaries, causing grain boundary odors or excessively coarsening of the precipitated phases, thereby decreasing grain growth effects. Therefore, the upper limit of titanium is limited to 0.10% by weight. It is preferable. Therefore, the content of titanium is preferably included in 0.01 ~ 0.10%.

Vanadium (V) 0.02 ~ 0.50 wt%

Vanadium (V) is a strong carbide forming element that combines with carbon to form carbides, such as titanium and niobium. Vanadium is usually an element having a large precipitation strengthening effect because it forms a fine precipitated phase at a low temperature. However, when the content of vanadium is small, the effect intended to be secured in the present invention cannot be secured, and the lower limit of vanadium is limited to 0.02% by weight. On the other hand, as the content of vanadium increases, excessive vanadium segregates at the grain boundaries, causing grain boundary odors, or excessively coarsening of the precipitated phases, thereby lowering the grain growth effect. Therefore, the upper limit of vanadium is limited to 0.5% by weight. It is preferable. Therefore, it is preferable to include the content of vanadium in 0.025 to 0.50% by weight.

The remainder of the present invention is iron (Fe). However, in the ordinary 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 manufacturing.

However, since phosphorus and sulfur are generally referred to as impurities, a brief description thereof is as follows.

Phosphorus (P): 0.05 wt%

Phosphorus (P) is an element that is inevitably contained in the manufacturing process, and it is preferable to include the addition range of 0.05% or less because it degrades quality such as performance crack formation.

Sulfur (S): 0.01 wt% or less

Sulfur (S) is an element that is inevitably contained in manufacturing, and forms coarse manganese sulfide (MnS), which causes defects such as flange cracks, and decreases the hole expandability of the steel sheet. It is preferable to include the range of 0.01% or less.

The steel pipe of the present invention includes a base material and a welded part, and the above-mentioned composition is a composition of conventional TWIP steel, and in the present invention, any TWIP steel having the above-described component system, that is, a conventional TWIP steel, can be applied.

In addition, TWIP (Twinning Induced Plasticity) steel, which is the object of the present invention, is a high-strength steel of austenitic structure having a high elongation by inducing twins during deformation by adding a large amount of manganese, carbon, and increasing the work hardening rate. Unlike high-strength steel (DP steel, TRIP steel, etc.), it has a single-phase austenitic structure and does not cause phase transformation even after welding.

It is preferable to include an oxidation inclusion having a major axis length of less than 3 µm at the remaining interface after welding of the steel pipe. If the size of the oxidation inclusion is 3 µm or more, there is a problem of acting as a starting point as a crack during the hydroforming process. Therefore, it is preferable that the oxidation inclusions formed at the remaining interface after welding of the steel pipe have a long axis length of less than 3 µm.

In addition, the steel pipe of the present invention contains a large amount of deoxidation elements such as Mn, Al and reacts with oxygen during welding, and is likely to remain as oxidation inclusions at the remaining interface after welding. Mn-Al-based oxidation inclusions lower the bonding strength of the welded portion and act as a factor that causes cracking during the bending or hydroforming process. Therefore, in the present invention, it is preferable to secure a joint part without an Mn-Al-based oxidation inclusion having a long axis length of 3 µm or more at the remaining interface after welding of the steel pipe.

By doing so, it is possible to provide a highly workable steel pipe having, for example, an expansion ratio of 40% or more. If the expansion ratio of the steel pipe is less than 40%, there is a fear that breakage occurs in the weld. Therefore, the expansion rate of the steel pipe is preferably 40% or more, more preferably 40 to 60%.

Moreover, in this invention, it is preferable to control so that the bead protrusion amount of the inside and outside surfaces of the said welding part may be set to 0.1-0.3 mm.

Figure 1 shows a schematic diagram of the welded portion of the high frequency welded steel pipe. As can be seen in Figure 1, the high-strength high frequency welded steel pipe proposed by the present invention is a bead protrudes on the inner and outer surfaces of the weld. The weld inner and outer bead protrusion amount 2 is defined as the protrusion amount of the weld inner surface bead 3 and the protrusion amount of the weld outer bead 4. In Fig. 1, reference numeral 1 denotes a base material, 5 denotes an interface remaining after welding, and 6 denotes a welded heat affected zone.

By controlling the amount of bead protrusion as described above it can ensure the ductility to a certain level or more, and when the expansion of the welded steel pipe can be broken in the base material. However, when the bead protrusion amount is less than 0.1mm, the welded part decreases in strength compared to the base material, and the breakage may occur at the interface remaining after welding during expansion, and when it exceeds 0.3mm, the weld is notched due to the notch effect. Cracking may occur in the vicinity of the heat affected zone, and it is difficult to apply to the hydroforming process.

Hereinafter, a method of manufacturing a high strength high frequency region steel pipe which is one aspect of the present invention will be described in detail.

Another aspect of the present invention is a method of manufacturing a high-strength high frequency welded steel pipe, a method of manufacturing a high frequency welded steel pipe including a welded portion by heating and melting a welded portion of a base material and welding the base material by an upset roll, wherein the base material is weight percent. , 0.3 to 0.9% of carbon (C), 15 to 30% of manganese (Mn), 0.01 to 4.0% of aluminum (Al), 0.61% or less of silicon (Si) (excluding 0%), phosphorus (P) of 0.05% or less, 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N) (excluding 0%), 0.02 to 0.10% of niobium (Nb), 0.01 to 0.10% of titanium (Ti) and 0.02 to 0.50 of vanadium (V) Preparing a steel sheet including one kind or two or more kinds of% and including a balance Fe and unavoidable impurities; And manufacturing a steel pipe having a welded portion by welding the steel sheet at a high heating rate of 1.026 to 1.077 KW · M / MIN and a welding speed of 40 M / MIN or less, and injecting an inert gas into the welded portion during the high frequency welding. Mn-Al-based oxidation inclusions having a major axis length of less than 3 µm are included at the remaining interface after welding.

TWIP (Twinning Induced Plasticity) steel, which is the object of the present invention, is a high-strength steel of austenitic structure having a high elongation by inducing twins during deformation by adding a large amount of manganese, carbon, etc. Unlike steels (DP steel, TRIP steel, etc.), it has a single-phase austenitic structure and does not cause phase transformation even after welding.

First, in the present invention, a steel pipe having a welded portion is manufactured by high frequency welding the steel sheet at a heat input amount of 1.026 to 1.077 kW · MIN / M and a welding speed of 40 m / min or less.

In high frequency tube welding of high strength steel, the welding speed is affected by the same heat input. In other words, if the pipe speed is increased above a certain speed, the wave is generated severely, resulting in an irregular welding heat generation, and the upset process is also difficult to press normally. Therefore, the welding speed is preferably limited to 40 m / min or less, and more preferably 15 to 40 m / min. In addition, the welding heat input amount is set to 1 kW · MIN / M and the corresponding welding output is controlled. In the present invention, the welding heat input amount is defined as a welding output / connection.

In the present invention, the inert gas is blown into the welding portion during the high frequency welding so that the Mn-Al-based oxide inclusion having a major axis length of 3 µm or more is not formed at the remaining interface after the welding.

Preferably, the inert gas blowing is performed locally on only the shielding box or the welded portion of the welded portion.

Although it does not specifically limit as said inert gas, It is preferable to use argon gas, helium gas, etc.

The blowing amount of the inert gas is preferably limited to 15 ~ 20L / MIN range to effectively control the reaction with the molten metal and the atmosphere. According to the measurement result of this inventor, it was confirmed that the oxygen concentration of the part by which welding is made is 0.03% or less. When the inert gas blowing amount is small, the oxygen concentration of the welding part is increased to oxidize the welding part, and an oxidation inclusion is formed on the welding interface. Therefore, the lower limit of the gas blowing amount is preferably controlled to 15 L / MIN. On the other hand, as the gas blowing amount increases, it is difficult to secure the normal welded joint due to severe fluctuations in the melted portion due to the flow, and therefore, the upper limit of the gas blowing amount is preferably limited to 20L / MIN.

As in the present invention, in the high frequency welding, the base metal is joined by pressure welding, so that the molten metal existing inside the weld portion protrudes as a bead, and in the case of the steel pipe including the base metal, during welding to maintain the shape of the pipe. The process of cutting the bead which protruded in the inside and outside of a weld part using cutting tools, such as a bite, is needed. In this invention, it is preferable to control so that protrusion amount of each inner and outer surface bead may be 0.1-0.3 mm. It is possible to add a process of cutting the beads to have such a protrusion amount.

The bead cutting is preferably made immediately after welding, that is, within 1 to 2 seconds after welding. By removing the beads immediately after welding as described above, the welding beads in a high temperature state can be more easily cut.

In cutting the beads as described above, in the case of TWIP steel, since it has excellent high temperature strength and ductility compared with other high strength steel, when cutting, the wear or damage of cutting tools such as bite is serious. Accordingly, in order to solve the above problems, the coolant may be sprayed through the nozzle at the same time as the bead cutting. It is preferable to use water or cutting oil as said cooling water, and it is preferable to use the copper pipe whose outer diameter is about 5 mm as a nozzle. The cooling water may be sprayed on each of the beads or cutting tools, but is more preferably sprayed on both the beads and the cutting tool.

In addition, immediately after welding, the liquid phase of the inner and outer surfaces melted and discharged due to the springback phenomenon is re-incorporated in the form of oxide in the welding portion, thereby deteriorating the characteristics of the welding portion. The springback phenomenon is a phenomenon in which the welded welded part is opened as the material expands due to the welding heat, and the higher strength steel becomes more remarkable. In particular, austenitic steels such as TWIP steels have a higher thermal expansion coefficient than general high-strength steels, which makes springback more problematic.

In the present invention, it is preferable to control the springback phenomenon by installing the upset roll at a point 60 to 70 mm away from the welding spot between the upset rolls at the welding spot. As the distance from the welding point to the upset roll gets closer, damage of the roll occurs due to spatter and heat generated during welding, so the lower limit of the distance from the welding point to the upset roll is preferably controlled to 60 mm. On the other hand, the springback phenomenon is reproduced as the distance from the welding point to the upset roll increases, so the upper limit of the distance is preferably controlled to 70 mm. Therefore, it is preferable to make the space | interval between upset rolls at a welding point 60-70 mm apart from a welding point.

Hereinafter, the present invention will be described in more detail with reference to examples. However, it is necessary to note that the following examples are intended to illustrate the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

A steel sheet having a thickness of 1.5 mm was manufactured by hot rolling, cold rolling, and annealing of 980 MPa grade TWIP steel having the composition and mechanical properties shown in Table 1 below.

Chemical composition (% by weight) C Si Mn P S Cr Ni Al Ti V N 0.62 0.61 15.1 0.026 0.007 0.346 0.09 2.09 0.021 0.10 0.005 Mechanical property Hardness (Hv) Tensile Strength (MPa) Yield strength (MPa) Elongation (%) 240 964 545 54

As a high frequency welded tube, a continuous roll forming method generally applied to automotive welded steel tubes was adopted, and a welded steel tube having an outer diameter of 76.2 mm was manufactured through a process of forming, high frequency welding, straightening, and cutting. From the preliminary test results, the welding heat input amount was set to 1 kW · MIN / M, and the predetermined welding speed and corresponding welding output were changed. In addition, a shield box was installed near the induction coil where high frequency welding was performed, and argon gas was supplied. At this time, concentration measurement was performed using an oxygen concentration near the welding point. Evaluation of defects and workability of the welded part after steel pipe production was performed, and the results are shown in Table 2 below.

Oxide analysis of the weld was investigated by scanning electron microscopy and EDX elemental element analyzer for five weld cross-sections. The machinability evaluation of welded steel pipe was determined by flaring expansion test. Was measured and the wave unit value was investigated.

division welding
Print
(kW) Welding speed
(m / min) welding
Heat input
(kW · in / m) Shielding
flux
(L / min) Oxygen concentration
(%) Weld-Upset roll spacing (mm) Oxidation dog
Riches Expansion rate
(%) Break position Inventory 1 40 38 1.026 15 0.03 70 radish 40 Base material Inventive Example 2 40 38 1.026 20 0.01 70 radish 41 Base material Inventory 3 40 38 1.026 15 0.02 60 radish 43 Base material Honorable 4 40 38 1.026 15 0.02 70 radish 41 Base material Inventory 5 42 39 1.077 15 0.02 70 radish 40 Base material Comparative Example 1 40 38 1.026 0 0.25 70 7 26 Weld Comparative Example 2 40 38 1.026 10 0.25 70 7 30 Weld Comparative Example 3 40 38 1.026 15 0.01 90 4 34 Weld Comparative Example 4 45 45 1,000 15 0.03 70 3 34 Weld Comparative Example 5 50 51 0.980 15 0.03 70 4 33 Weld

As shown in Table 2, Inventive Examples 1 and 2 and Comparative Examples 1 and 2 were used to change the shielding flow rate to 15, 20, 10, and 0 L / MIN, respectively, at constant heat input and upset roll intervals. The weld characteristics in the case were evaluated. Inventive Examples 1 and 2, in which the shielding flow rate is injected into the 15L / MIN and 20L / MIN welding parts, show excellent processing characteristics with no oxidation inclusions remaining at the interface remaining after welding, and an expansion ratio of 40% or more, compared to Comparative Examples 1 and 2. . From this fact, a good correlation is found between the number of oxidation inclusions present in the welding interface and the expandability, and it is understood that the oxidation inclusions are reduced by welding of the welded portion, thereby improving the expansion and workability.

Inventive Examples 3, 4 and Comparative Example 3 are characteristics evaluation results for welded steel pipes when the distance between welding and upset roll was changed to 60, 70 and 90 mm, respectively, under constant heat input and gas flow conditions. In the case of Inventive Examples 3 and 4, in which the interval between the upset rolls was 60 and 70 mm, the expansion ratio was greatly increased as compared with Comparative Example 3 of 90 mm, and the result of fracture in the base metal was obtained. This may be due to the formation of oxidized inclusions due to the spring-backed phenomenon in high-strength steel as the gap between welding and upset roll increases.

Invention Example 5 and Comparative Examples 4 and 5 show the results of examining the influence of the welding speed when the welding heat input is kept constant at around 1 KW · MIN / M. Inventive Example 5 with a welding speed of 39 M / MIN can achieve relatively good processing characteristics. In Comparative Examples 4 and 5 in which the welding speed exceeded 40 M / MIN, it was found that the welding characteristics were poor. This is because the welding wave, that is, the pipe speed increases, the edge wave phenomenon deepens and the welding start point is unstable.

1. Base material
2. Inner and Outer Bead Projection
3. Weld inner bead
4. Welding exterior bead
5. Remaining interface after welding
6. Welding heat affected zone

Claims (8)

High-strength high frequency welded steel pipe is a high frequency welded steel pipe including a weld formed by welding a base material into a steel pipe by high frequency welding, wherein the base material is 0.3% by weight (C) 0.3% to 0.9%, manganese (Mn) 15% to 30%, 0.01% to 4.0% of aluminum, 0.61% or less of silicon (excluding 0%), 0.05% or less of phosphorus (P), 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N) (0% Exclusions),
Niobium (Nb) 0.02 to 0.10%, titanium (Ti) 0.01 to 0.10% and vanadium (V) 0.02 to 0.50% of one or two or more, including the remaining Fe and inevitable impurities,
A high-strength high frequency welded steel pipe having an Mn-Al-based oxidation inclusion having a long axis length of less than 3 µm at a remaining interface after welding.
The method of claim 1,
Expansion rate of the steel pipe is 40% or more high-strength high frequency welded steel pipe.
The method of claim 1,
High-strength high frequency welded steel pipe, characterized in that the bead protrusion amount of the inner and outer surfaces of the weld portion is 0.1 ~ 0.3mm.
A method of manufacturing a high frequency welded steel pipe including a welded portion by heating and melting the welded portion of the base material and welding the base material by an upset roll, wherein the base material is 0.3% by weight (C) 0.3% to 0.9% and manganese (Mn) 15. ~ 30%, Aluminum (Al) 0.01 ~ 4.0%, Silicon (Si) 0.61% or less (excluding 0%), Phosphorus (P) 0.05% or less, Sulfur (S) 0.01% or less, Nitrogen (N) 0.01% or less (Excluding 0%),
Preparing a steel sheet containing one or two or more of 0.02 to 0.10% of niobium (Nb), 0.01 to 0.10% of titanium (Ti), and 0.02 to 0.50% of vanadium (V), and the balance of Fe and unavoidable impurities ; And manufacturing a steel pipe having a welded portion by welding the steel sheet at a high heating rate of 1.026 to 1.077 KW · M / MIN and a welding speed of 40 M / MIN or less, and injecting an inert gas into the welded portion during the high frequency welding. A method of manufacturing a high strength high frequency welded steel pipe, characterized by having an Mn-Al-based oxidation inclusion having a long axis length of less than 3 µm at the remaining interface after welding.
5. The method of claim 4,
The inert gas is at least one of argon gas or helium gas, and the manufacturing method of high-strength high frequency welded steel pipe, characterized in that blown into the welding portion at a rate of 15 ~ 20L / MIN.
5. The method of claim 4,
The method of manufacturing a high strength high-frequency welded steel pipe, characterized in that it further comprises the step of cutting the bead of the welding portion so that the inner and outer beads have a protruding size of 0.1 ~ 0.3 mm.
The method according to claim 6,
The bead cutting uses a bite, the bead cutting is made within 1 to 2 seconds after welding, and the production of high-strength high-frequency welded steel pipe, characterized in that to spray the coolant to the bead and bite through a nozzle at the same time as the bead cutting Way.
5. The method of claim 4,
The upset roll is a high-frequency high-frequency welded steel pipe manufacturing method, characterized in that located 60 ~ 70mm away from the welding point.

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