KR20120074706A - High strength austenitic high frequency welding steel pipe and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high-strength austenite steel pipe for high-frequency welding and a manufacturing method thereof are provided to improve the formability of a weld zone by controlling the projection degree of inner and outer beads. CONSTITUTION: A high-strength austenite steel pipe for high-frequency welding comprises base metal(1) and a weld zone. The base metal contains less than 1.5wt% C, Mn 5~35wt%, Al 0.01~3wt%, less than 3wt% Si, residual Fe, and inevitable impurities. The projection degree of inner and outer beads(3,4) of the weld zone is 0.1~0.3mm. The strength ratio of the weld zone to the base metal is more than 1.

Description

High Strength Austenitic High Frequency Welded Steel Pipe and Manufacturing Method thereof

The present invention relates to a high-strength austenitic high frequency welded steel pipe and a method of manufacturing the same, and more particularly, to a high-strength austenitic high frequency welded steel pipe improved in workability of a welded part by improving the softening phenomenon of the welded metal part and a method of manufacturing the same. .

Recently, the automobile industry is increasing the adoption of high-strength steel for the purpose of lightening the body weight due to strengthening 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.

The tailored blank processing method is a method of welding a blank obtained by cutting steel sheets having different thicknesses, strengths, and materials into appropriate sizes and shapes, and pressing the blanks into a component. 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 automobiles are generally manufactured by welding after roll forming, and are continuously manufactured. Examples of the welding welded pipes include TIG, plasma, high frequency, and laser welding.

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 weld core heated by an induction coil is pressed by a squeezing 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. Moreover, in the case of high strength steel of 980 MPa or more, the hardening phenomenon of the welded part is remarkable.

Recently, steel grades such as TWIP (Twinning Induced Plasticity) steel, a high-strength steel of austenitic structure having 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 austenitic structure and does not generate phase transformation even after welding. However, the welded part of the TWIP steel has a problem that softening occurs because coarse grains and TWIP effects are lost compared to the base metal. In the case of the welded steel pipe for hydroforming, there is a problem in that the softening phenomenon is remarkable as compared to the plate welded portion because the hardness of the welded portion is increased by processing the material by roll forming before welding.

A representative technique for improving these problems is Japanese Patent Application Laid-open No. Hei 8-134609, which has a high temperature for the purpose of improving the softening of the welding heat affected zone (HAZ) of 780MPa class high Mn nonmagnetic steel. In addition, a technique of complex addition of Ti and Nb to form a stable carbonitride on HAZ has been proposed. As another technique, Japanese Patent Application Laid-Open No. 8-013092 discloses a technique in which Ca and S are strictly controlled by high Mn nonmagnetic steel excellent in weldability and machinability. However, these techniques do not improve the hydrogen embrittlement resistance.

In addition, Japanese Laid-Open Patent Publication No. 6-192788 discloses an arc welding method for securing a high-quality, healthy welding metal of a high Mn non-magnetic steel welding joint, in which the amount of oxygen is limited and C, Si, Mn is disclosed. Flux, Cr, Ni, and N are specified to include welding wire and PbO, and fluxes are specified for 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, Japanese Laid-Open Patent Publication No. 2000-084614 discloses a method for producing a high strength high frequency welded steel pipe having a tensile strength of 690 to 1180 MPa excellent in weld toughness. However, the above technique has a problem that it is difficult to apply the welding wire and flux in the thin weld steel pipe.

In addition, Japanese Laid-Open Patent Publication No. Hei 11-172376 discloses a method for producing a high frequency welded steel pipe excellent in hydroforming formability used for materials of automobile structural parts. However, the technique is preheated to 700 ~ 1200 ℃ before tube welding, there is a disadvantage that requires additional equipment according to the preheating and management is not easy.

In addition, Japanese Patent Application Laid-open No. Hei 10-183292 discloses a high-strength high frequency welded steel pipe for automobiles and a manufacturing method thereof, and Japanese Patent Application Laid-open No. Hei 08-319517 discloses high frequency induction heating in a weld portion of a thick high frequency welded steel pipe. A method of continuously heat treating twice with an apparatus is disclosed. However, the above techniques require high-frequency welded steel pipes and post-heating, which requires additional equipment and management necessary for post-heating.

One aspect of the present invention is to provide a high-strength austenitic high frequency welded steel pipe and a method of manufacturing the same by controlling the heat input amount of the weld portion and the bead protrusion amount of the inner and outer surfaces of the weld to an appropriate level.

The present invention is a high frequency welded steel pipe including a base material and a welded portion, wherein the base material is by weight, C: 1.5% or less, Mn: 5 to 35%, Al: 0.01 to 3%, Si: 3% or less, balance Fe and Provided with other inevitably contained impurities, it provides a high-strength austenitic high-frequency welded steel pipe of the bead protrusion amount of the inner and outer surfaces of the weld portion 0.1 ~ 0.3mm.

At this time, it is preferable that the strength ratio between the welded portion of the steel pipe and the base metal is 1 or more, and the expansion ratio of the steel pipe is preferably 30% or more.

The present invention is a method for manufacturing a high frequency welded steel pipe including a welded portion by welding a base material by a high frequency welding, the weight of the base material, C: 1.5% or less, Mn: 5 ~ 35%, Al: 0.01 ~ 3%, Si: 3% or less, using a steel material composed of the remaining Fe and other unavoidable impurities, so that the welding heat input at the high frequency welding is 1.16 ~ 1.2KW ~ min / m, Provided is a method of manufacturing a high strength high frequency welded steel pipe for cutting beads using cutting tools such as bites such that the protrusion of each of the inner and outer beads formed by the welding portion is 0.1 to 0.3 mm.

At this time, the bead cutting is preferably made within 1 to 2 seconds after welding, it is possible to spray the coolant through the nozzle at the same time as the bead cutting.

According to one aspect of the present invention, it is possible to provide a 980MPa grade austenitic high frequency welded steel pipe that can be applied to a hydroforming welded steel pipe for deep seas for automobiles by securing productivity and workability at the same time.

Figure 1 shows a schematic diagram of the welded portion of the high frequency welded steel pipe.

The present inventors have recognized that in order to secure the workability of the high strength austenitic high frequency welded steel pipe welded part, it is necessary to secure a joint free of weld defects such as Penetrator, cold weld, and the like. Unlike other welded steel pipes, they are joined by pressure welding, so that the beads protruding by the pressure welding can be cut to an appropriate range using cutting tools such as a bite, thereby improving the processability of the welded part. It was completed.

Hereinafter, the present invention will be described in detail.

The steel pipe of the present invention includes a base material and a welded portion.

The base metal of the welded steel pipe of the present invention is C: 1.5% by weight or less, Mn: 5 to 35% by weight, Al: 0.01 to 3% by weight, Si: 3% by weight or less, the composition consisting of the balance Fe and other unavoidable impurities It is preferable to have a component, wherein the above-mentioned composition is a composition of conventional TWIP steel, and in the present invention, any TWIP steel having the above component system, that is, a conventional TWIP steel, can be applied.

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.

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 austenitic high frequency welded steel pipe proposed by the present invention is heated by melting the weld core using the resistance heat and then the base material (1) is welded by using a squeezing (Sqeezing Roll) By this pressure welding, the beads protrude to the inside and outside surfaces of the welded part. 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 the present invention, a great emphasis is placed on controlling the bead protrusion amount of each of the inner and outer surfaces of the above-described welded part. If the bead protrusion amount is not properly controlled, the weld interface 5 or the weld heat affected zone when the steel pipe is expanded Breakage occurs at (6).

More specifically, it is preferable to control the bead protrusion amount of the inner and outer surfaces of the weld portion to be 0.1 ~ 0.3mm, by controlling the bead protrusion amount as described above can ensure the ductility to a certain level or more, During expansion, breakage may occur on the substrate. However, if the bead protrusion amount is less than 0.1mm, the welded part is reduced in strength compared to the base material, which may cause breakage at the interface of the welded part during expansion, and if it exceeds 0.3mm, the weld heat affected part is notched by the notch effect. Cracking may occur in the vicinity, which is difficult to apply to the hydroforming process.

As described above, the steel pipe of the present invention can secure an excellent strength and ductility, that is, a strength ratio between the welded portion and the base material, in the range of 1 or more, and the expansion ratio can also be secured to 30% or more as described above.

Hereinafter, the manufacturing method of this invention is demonstrated.

In the present invention, when welding using the TWIP steel, in order to secure a healthy joint without welding defects, it is preferable to control the welding heat input amount to 1.16 ~ 1.2KW · min / m. If it is less than 1.16KW? Min / m, there is a possibility of cold welding defect due to insufficient welding heat input, and if it exceeds 1.2KW? Min / m, splash will be occurred during piping due to excessive input of heat input. There is a problem that the occurrence of, or the occurrence of chipping defect (Penetrator) is likely to occur in the weld. In the present invention, the welding heat input amount is defined as a welding output / connection.

As described above, since the base metal is joined by pressure welding due to the characteristics of high frequency welding, the molten metal existing inside the welded portion protrudes as a bead. The process of cutting the beads protruding on the inner and outer surfaces using cutting tools such as bite is required, and at this time, it is preferable to control the protrusion amount of the inner and outer beads to be 0.1 to 0.3 mm.

At this time, 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 bead or cutting tool, but is preferably sprayed on both the beads and the cutting tool.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are merely examples for describing the present invention in more detail, and do not limit the scope of the present invention.

(Example)

A steel sheet having a thickness of 1.45 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 0.57 0.282 18.31 0.026 0.007 0.346 0.09 1.26 0.021 0.10 0.005 Mechanical properties Hardness (Hv) Tensile Strength (MPa) Yield strength (MPa) Elongation (%) 240 993 520 60

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 34 mm was manufactured through forming, high frequency welding, straightening, and cutting. At this time, the welding speed was 25L / min, the steel pipe was manufactured while changing the welding output to 28 ~ 31kW as shown in Table 2. Evaluation of defects and workability of the welded part after the production of the steel pipe was performed, and the results are shown in Table 2 below.

The defects of the welded portion were examined by using an optical microscope for 5 welded cross sections, and then the protrusion amount of the welded bead was measured, and the tensile test of the base material and the welded portion of the steel pipe was made in accordance with JIS Z2201_13B to evaluate the tensile properties. At this time, the tensile speed was 20mm / min. The workability evaluation of the welded steel pipe was measured by the flaring expansion test to determine the maximum expansion rate up to the point of crack occurrence.

In addition, in order to examine the effect of improving the bite life by the cooling method, as shown in Table 2 below, when the cooling treatment is not performed (Type A), when only the cooling treatment is performed (Type B), the weld and the bite When the cooling treatment was performed (C type), the cooling treatment was performed, and the results are shown in Table 2. The cooling treatment consisted of installing a nozzle having a diameter of 5 mm on the squeegee roll and the inner face bite and spraying cooling water on the inner face and the bite of the welded part.

division welding
Print
(kW) welding
Heat input
(kW? min / m) Bead
Protrusion
(mm) Cooling
type welding
flaw
The presence or absence Strength ratio
(Welding part / base material) Expansion rate
(%) Breaking
location byte
life span Comparative Example 1 28 1.12 0.2 C U (C) - 20 Interface Good Existing example 29 1.16 0 C radish 0.98 27 HAZ Good Inventory 1 29 1.16 0.1 C radish 1.01 32 Base material Good Inventive Example 2 29 1.16 0.3 A radish 1.05 33 Base material Bad Inventory 3 29 1.16 0.2 B radish 1.03 32 Base material Bad Honorable 4 29 1.16 0.3 C radish 1.04 32 Base material Good Comparative Example 2 29 1.16 0.5 C radish 1.03 25 HAZ Good Inventory 5 30 1.20 0.1 C radish 1.04 32 Base material Good Inventory 6 30 1.20 0.3 C radish 1.04 32 Base material Good Comparative Example 3 31 1.24 0.1 C P - 24 HAZ Good However, in the determination of weld defects, C means cold weld defects and P means intrusion defects.

As can be seen from Table 2, Inventive Examples 1 to 6 satisfying the welding heat input range of the present invention was obtained a good joint without welding defects, Comparative Example 1 is cold welding defects due to lack of heat input, Comparative Example 3 It can be seen that the intrusion defect caused by the splash is caused by excessive heat input. As such, for Comparative Examples 1 and 3 in which welding defects occurred, the strength ratio was not measured, and it was confirmed that the expansion rate was also low.

In addition, the present invention can be seen that by securing the bead protrusion amount of the appropriate range, it is possible to secure at least one weld / base material strength ratio and 30% or more expansion ratio, in the case of the existing example with a small bead protrusion amount is sufficient strength ratio And the expansion ratio could not be secured, and in the case of Comparative Example 2 having a large amount of bead protrusion, the strength ratio could be secured, but the expansion ratio could not be secured to more than 30%.

In addition, the examples of the present invention showed that the fracture position of the steel pipe as a base material, through the results it can be seen that the mechanical properties of the weld and the base material is similar to have excellent processing characteristics. However, in the case of Comparative Examples 1 to 3 and the existing example where the fracture position is the interface or the weld heat affected zone (HAZ), it can be seen that the properties of the welded zone are considerably deteriorated compared to the base metal.

On the other hand, as shown in Table 2, C type, which is a case where the cooling treatment is performed on the welded part and the bite, rather than the case where the cooling process is not performed (Type A) or only the byte is cooled (B type), the life of the byte It can be seen that it is effective for improvement.

1: base material 2: inner and outer bead protrusion amount
3: weld inner bead 4: weld outer bead
5 weld interface 6 weld heat affected zone

Claims (6)

A high frequency welded steel pipe comprising a base material and a welded part,
The base material is, by weight, C: 1.5% or less, Mn: 5 to 35%, Al: 0.01 to 3%, Si: 3% or less, the balance Fe and other unavoidably contained impurities, and in the weld portion? High strength austenitic high frequency welded steel pipe with a bead protrusion of 0.1 ~ 0.3mm on the outside.
The high-strength austenitic high frequency welded steel pipe according to claim 1, wherein the weld portion and the base material have a strength ratio of 1 or more.
The high strength austenitic high frequency welded steel pipe according to claim 1, wherein the expansion ratio of the steel pipe is 30% or more.
As a method of manufacturing a high frequency welded steel pipe including a weld by welding a base material to a steel pipe by high frequency welding,
As the base material by weight, C: 1.5% or less, Mn: 5 to 35%, Al: 0.01 to 3%, Si: 3% or less, remainder Fe and other unavoidably contained steels composed of impurities, The high strength of the bead by cutting the bead using a bite so that the welding heat input amount is 1.16 ~ 1.2KW? Min / m during the high frequency welding, and the protrusion amount of each of the inner and outer beads of the weld formed by the welding is 0.1 ~ 0.3mm Method for manufacturing high frequency welded steel pipe.
The method of claim 4, wherein the bead cutting is performed within 1 to 2 seconds after welding.
The method of manufacturing a high strength high frequency welded steel pipe according to claim 5, wherein the coolant is sprayed on the bead and the bite through a nozzle simultaneously with cutting the bead.

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