KR20120074636A - High strength austenitic laser welding steel pipe and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high-strength austenite steel pipe for the laser welding and a manufacturing method thereof are provided to improve productivity by omitting a thermal process of the welded steel pipe and controlling steel components. CONSTITUTION: A laser welding steel pipe with a weld portion contains less than 1.5wt% C, 5~35wt% Mn, 0.01~3wt% Al, less than 3wt% Si, residual Fe, and inevitable impurities. The nitrogen content of the weld portion is 0.01~0.03wt%. The hardness of the weld portion of the laser welding steel pipe is more than 240Hv. A bead width of the weld portion of the laser welding steel pipe is 0.7~1mm.

Description

High-strength austenitic laser welded steel pipe and its manufacturing method {HIGH STRENGTH AUSTENITIC LASER WELDING STEEL PIPE AND METHOD FOR MANUFACTURING THE SAME}

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

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 this method is continuous. Types of welding applied here include TIG, plasma, high frequency, and laser welding.

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 are 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. 9-168879 discloses a method for manufacturing a laser welded steel tube with fewer welding defects by controlling the amount of carbon and oxygen as trace elements, but has a disadvantage in that it does not improve the softening characteristics of the welded portion. .

In addition, Japanese Laid-Open Patent Publication No. 2010-029938 discloses a method of measuring the presence or absence of a keyhole penetrating the inner surface side of an open pipe during welding in order to precisely manage the tube welding in the manufacture of a laser welded steel tube. However, the above technique has a problem in that, in the case of welded steel pipes of small and medium diameters, measurement equipment is installed inside and difficult to measure.

Further, Japanese Laid-Open Patent Publication No. 2001-179472 is a manufacturing method of martensitic stainless steel laser welded steel tube, which is formed into an open pipe by roll forming, and then preheated the weld core of the open pipe by using a high frequency heating method. Although a method of welding is disclosed, a production cost increase due to preheating is a problem.

In addition, Japanese Laid-Open Patent Publication No. 2000-326079, alone or in combination, forms a ferritic stainless steel containing Ti and Nb of 1% or less in an open pipe, and when welding the butt face to laser welding, the weld tube is calibrated. Disclosed is a method of controlling the temperature when passing through a roll in a temperature range where the strength of the material becomes 80% or more of the room temperature strength at 150 ° C or less in order to control the hardness difference between the Vickers hardness HVw of the welded portion and the Vickers hardness HVb of the base metal portion. Although it does not improve the softening characteristic of the welded part, there is a problem that the heat cycle control of the welded part due to rapid heating and quenching is difficult.

Further, Japanese Laid-Open Patent Publication No. 8-155665 discloses a method for miniaturizing the crystal grains of a ferritic stainless steel welded portion, which is subjected to penetration welding with a first laser beam, and the welded portion has a temperature of 400 ° C. or lower, Although a method of partially welding by irradiating a heat input second laser beam is disclosed, there is a problem that the manufacturing cost increases because the weld steel pipe is manufactured using two lasers.

Further, Japanese Patent Laid-Open No. 5-277769 discloses preheating at 250 ° C. or higher before laser tube welding of ferritic stainless steel, welding the protruding height of the inner bead to 0.15 mm or more, and reducing the welded portion in the plate thickness direction to form workability. A method of improving is disclosed. However, as described above, the production cost is increased according to the preheating process, and a predetermined space is required inside the welded steel pipe for forced pressure reduction, but it is difficult to apply in small and medium diameters.

One aspect of the present invention is to provide a high-strength austenitic laser welded steel pipe and a method of manufacturing the same by controlling the amount of nitrogen in the weld to an appropriate level, thereby improving the processing characteristics of the weld.

The present invention is a laser welded steel pipe including a welded portion, in weight percent, C: 1.5% or less, Mn: 5 to 35%, Al: 0.01 to 3%, Si: 3% or less, remainder Fe and other unavoidably contained It is made of an impurity, the weld provides a high-strength austenitic laser welded steel pipe having a nitrogen content of 0.01 ~ 0.03% by weight.

At this time, the weld weld hardness of the laser welded steel pipe is preferably 240 Hv or more, the weld bead width is 0.7 ~ 1mm, and the expansion ratio of the steel pipe is preferably 40% or more.

The present invention is a method for manufacturing a laser welded steel pipe including a welded portion, in weight%, C: 1.5% or less, Mn: 5 to 35%, Al: 0.01 to 3%, Si: 3% or less, balance Fe and other unavoidable The laser is welded while supplying a steel material composed of impurity contained at a flow rate of 5 to 10 L / min to a rear surface of the weld bead of the steel, at a flow rate of 5 to 10 L / min. It provides a method for producing a high strength austenitic laser welded steel pipe, characterized in that.

The welding heat input during the welding is preferably controlled in the range of 0.7 ~ 1kW · min / m.

According to one aspect of the present invention, it is possible to provide a high-strength austenitic laser welded steel pipe that can be applied to hydroforming welded steel pipes for deep seam use for automobiles by improving the workability of the welded portion, and control the steel components and manufacture welded steel pipes. The heat treatment during the process can be omitted, and the productivity can also be improved.

The welded portion of the TWIP steel has a problem in that workability is poor, such as coarsening of grains during melting and coarsening of TWIP, and loss of TWIP effect. Accordingly, the present inventors have recognized that it is necessary to improve the softening phenomenon of the weld in order to manufacture a high strength austenitic laser welded steel tube having excellent workability of the weld. Attention was drawn to (N).

The present inventors are applicable to TWIP steel in which nitrogen does not cause phase transformation as an austenite forming element, and as a substance capable of exerting strength due to precipitation strengthening and solid solution strengthening effects due to the formation of carbonitride mixed in the weld metal, It was recognized that the optimum component in accordance with the present invention, by controlling the nitrogen content of the weld to improve the softening properties of the weld, it has been completed the present invention on the basis that can also secure the workability.

Hereinafter, the present invention will be described in detail.

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

The austenitic laser welded steel pipe proposed by the present invention preferably has a nitrogen content of 0.01 to 0.03% by weight, thereby improving the softening of the weld and ensuring an expansion rate over a certain level. Of course, the inclusion of predetermined nitrogen in the weld metal part can be realized even during steel fabrication through nitrogen content control, but the control is not easy, and in the case of a large amount of addition, the crack of the slab surface and the etch shape defect during rolling There is a problem in reality because it can cause problems.

Therefore, in this invention, it is proposed to mix nitrogen as a shield gas during welding. More specifically, it is preferable to install a shield gas nozzle inside the welded steel pipe and to control and spray the type and flow rate of the shielded gas. Through the above control, the laser welded steel pipe of the present invention has a weld hardness of 240 Hv or more and 40% or more. Steel pipe expansion rate can be secured, through which there is little hardness difference with the base material, and it is possible to manufacture a steel pipe having excellent ductility.

In addition, the steel pipe of the present invention can ensure a weld bead width of 0.7 ~ 1mm, by securing the weld bead width in the above range, it is possible to improve the softening phenomenon of the weld, and improve the quality of the weld. When the weld bead width is less than 0.7 mm, deformation may occur in the weld softened during processing, and when the weld bead width exceeds 1 mm, quality degradation such as cracking or bead sagging may occur in the weld.

As described above, when injecting shield gas to the back surface of the weld bead, it is preferable to use a shield gas composed of 10 to 20% by volume of nitrogen, residual argon and other unavoidable impurities, but less than 10% by volume of pure argon or argon. When using a shield gas containing nitrogen, the nitrogen content in the weld is insufficient and the effect of strength improvement is insufficient. When the nitrogen content exceeds 20% by volume, the nitrogen content of the weld is increased but the weldability of the weld and the arc are unstable. there is a problem. Here, the back side of the weld bead means a lower portion of the welded portion, and it is preferable to use helium (He) or the like as a protective gas on the upper portion of the welded portion.

The flow rate of the shield gas is preferably limited to the range of 5 ~ 10L / min, if less than 5L / min, the welding beads are incompletely shielded due to insufficient flow rate, oxidation occurs in some, exceeding 10L / min In this case, the phenomenon that the molten metal portion is recessed into the steel material due to the excessive supply of the shield gas occurs.

Of course, in addition to the shielding gas mixing method, there are other means for incorporating nitrogen into the welding part through a nozzle of the laser welding head, but in this case, there is a problem that application is not easy due to the influence on the laser beam.

On the other hand, in order to further improve the softening phenomenon, it is preferable to control the amount of welding heat input. In general, when the welding heat input amount is increased, the weld grain size is coarsened, and the weld width is increased to decrease the weld hardness. However, in the case of laser welding, the width of the welded portion increases in a predetermined heat input range as a characteristic of a high density heat source, but the hardness change is small.

In the case of high-strength steel in which the weld is hardened, the smaller the weld bead width, the more the workability increases because the strain applied to the weld is reduced. However, when the weld is softened, such as TWIP steel, since the weld is preferentially deformed at the time of processing, if the weld width is small, the deformation may be concentrated and preferentially broken. That is, it is necessary to control the welding heat input amount to secure the predetermined bead width of the weld in the range that the weld hardness decreases, the shape of the weld steel pipe and the weld defect does not occur, and in the present invention, the amount is 0.67 to 1 kW? It is limited to the range of / m. When the welding heat input amount is less than 0.67kW? Min / m, the weld width is excessively reduced due to the decrease in the welding heat input amount, so that deformation may be concentrated in the soft welded portion during processing, and thus it may be difficult to secure a certain expansion rate. If the calorific value exceeds 1 kW · min / m, the underfill, which is a phenomenon of cracking and bead deflection, may occur in the weld. In the present invention, the heat input of welding is defined as the laser power / tubing speed.

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 1)

Steel coils shown in Table 1 were manufactured by hot rolling, cold rolling, and annealing to form a coil having a thickness of 1.45 mm. Table 1 below describes the composition and range and mechanical properties of the 980MPa grade TWIP steel.

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

In order to examine the effect of the shielding gas, laser welding was performed on the steel materials having the conditions of Table 1, wherein the welding was performed using a 12kW CO ₂ laser welding machine and a bead at a laser power of 5 kW and a welding speed of 6 m / min. On plate welding was performed. At the time of welding, shield gas was injected to the back surface of the weld bead as shown in Table 2 below, and the bead shape and defects of the weld part, the nitrogen content of the weld part, and the like were analyzed and the results are shown in Table 2. At this time, the characteristics of the welds were evaluated using a tensile test, and the test piece shape was manufactured in the weld line length direction according to JIS Z2201_13B. Tensile speed was 20 mm / min.

division Welding part
Shield Gas flux
(L / min) Arc stability Backside shielding property Bead shape Weld
Nitrogen
(weight%) Tensile Strength Ratio
(Welding part / base material) Comparative Example 1 No treatment 5 Good Poor (oxidation) Good 0.0185 0.97 Existing example Ar 5 Good Good Good 0.0055 0.95 Inventory 1 Ar + 10% N ₂ 5 Good Good Good 0.0175 0.97 Inventive Example 2 Ar + 10% N ₂ 10 Good Good Good 0.0215 0.98 Comparative Example 2 Ar + 10% N ₂ 15 Good Good Bad
(Backside bead depression) 0.025 0.98 Inventory 3 Ar + 20% N ₂ 5 Good Good Good 0.0271 0.99 Comparative Example 3 N ₂ 5 Instability Bad
(Some discoloration) Good 0.032 0.99

As can be seen from Table 2, it can be seen that the nitrogen content of the weld portion increases as the ratio of nitrogen gas in the shield gas increases. Comparative Example 1, which is a case where the shield gas is not treated, is able to secure predetermined nitrogen, but it can be seen that the vicinity of the weld is severely oxidized, so that it is difficult to actually apply.

Existing example using pure Ar as the shielding gas has excellent shielding property of the weld bead, but it can be seen that the strength improvement effect is insignificant due to the lack of nitrogen content of the welded part. In case of Comparative Example 3 in which excessively applied nitrogen gas, the nitrogen content is increased, but the arc is unstable and the shielding property is insufficient, causing a problem of discoloration of the weld part.

As a result, when the shield gas flow rate exceeds 10L / min, it can be seen that the phenomenon that the weld bead is recessed inside the steel material, it can be seen that the appropriate flow rate is 5 ~ 10L / min.

Inventive Examples 1 to 3 satisfying the conditions of the present invention can be seen that a good arc stability, shielding properties, bead shape is secured, and the nitrogen contained in the appropriate range in the weld portion, ensuring a certain level or more strength.

(Example 2)

The piping process was performed about the steel material which has the conditions of the said Table 1. In the tube manufacturing process, a continuous roll forming method generally applied to a welded steel pipe for automobiles was adopted, and a welded steel pipe having an outer diameter of 35 mm was manufactured through a process of forming, laser welding, straightening, and cutting. 6 kW CO ₂ laser welding machine was used for laser welding. When welding, the laser power is set to 6kW, the maximum power, and as shown in Table 3 below, the welding speed is changed to a range of 5 to 14 m / min, and the welding heat input is changed to a range of 0.43 to 1.2 kW / m / min. Was prepared. At this time, the upper protective gas was He gas at a flow rate of 15-20 L / min, Ar + 10% N ₂ was used as the protective gas at the bottom of the bead, and the flow rate was treated at 5 L / min.

The defects and mechanical properties of the welded portion prepared as described above were measured, and the results are shown in Table 3 below. When measuring the hardness of the weld, the average was calculated by measuring three times at 0.2 mm intervals with a load of 500 g and a holding time of 10 seconds for the area including the base metal and the weld. Nitrogen analysis in the weld was measured using a N (nitrogen), O (oxygen) analyzer after the weld was cut to 2mm in width, 8mm in length, and then polished and washed. 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.

division Welding speed
(m / min) Welding heat input
(kW? min / m) Penetration Weld Crack Generation Underfill
Occurrence Weld part bead width
(mm) Weld hardness
(Hv) Expansion rate
(%) Existing example 5 1.2 U U U 1.12 - - Inventory 1 6 1.0 U radish radish 0.97 260 42 Inventive Example 2 7 0.86 U radish radish 0.91 256 41 Inventory 3 8 0.75 U radish radish 0.83 256 41 Honorable 4 9 0.67 U radish radish 0.78 245 40 Comparative Example 1 10 0.60 U radish radish 0.69 241 39.5 Comparative Example 2 11 0.55 U radish radish 0.60 244 39 Comparative Example 3 12 0.50 U radish radish 0.58 249 38 Comparative Example 4 13 0.46 U radish U
(Inner bead) 0.51 240 38 Comparative Example 5 14 0.43 radish - - - - -

As can be seen in Table 3, when the welding heat input condition of the present invention is satisfied, it is possible to manufacture a steel pipe having excellent surface quality without generating a welding crack, and can secure excellent hardness and ductility at the same time.

However, as in the conventional example, when the excessive amount of welding heat input is applied, it is judged that it is difficult to actually apply the welding crack along with the shape defect of the welding bead. If not, an appropriate level of weld bead width could not be secured, and if the welding speed was faster than that of Comparative Examples 1 to 3, an underfill phenomenon occurred in the weld as in Comparative Example 4. In Comparative Example 5, the welding heat input amount was insufficient, and the welding was incompletely performed.

Claims (6)

A laser welded steel pipe comprising a welded portion,
By weight%, C: 1.5% or less, Mn: 5 to 35%, Al: 0.01 to 3%, Si: 3% or less, balance Fe and other unavoidably contained impurities, the weld portion is nitrogen content of 0.01 High strength austenitic laser welded steel pipe of ~ 0.03% by weight.
The high-strength austenitic laser welded steel pipe according to claim 1, wherein the welded hardness of the laser welded steel pipe is 240 Hv or more.
The high intensity austenitic laser welded steel pipe according to claim 1, wherein the weld bead width of the laser welded steel pipe is 0.7 to 1 mm.
The high-strength austenitic laser welded steel pipe according to claim 1, wherein the expansion ratio of the laser welded steel pipe is 40% or more.
As a method of manufacturing a laser welded steel pipe comprising a welded part,
By weight%, C: 1.5% or less, Mn: 5 ~ 35%, Al: 0.01 ~ 3%, Si: 3% or less, remainder Fe and other unavoidable impurities, A method of manufacturing a high strength austenitic laser welded steel pipe, wherein the part is laser welded while supplying a shield gas composed of 10 to 20% by volume of nitrogen, residual argon and other unavoidable impurities at a flow rate of 5 to 10 L / min.
The method of manufacturing a high strength austenitic laser welded steel pipe according to claim 5, wherein the welding heat input amount during the welding is controlled to be 0.67 to 1 kW · min / m.