KR101207707B1 - Method for producing austenitic welded steel tube having excellent formability and austenitic welded steel tube prepared by the same - Google Patents

Abstract

The present invention relates to a method for producing an austenitic welded steel pipe excellent in workability, and to an austenitic welded steel pipe produced by the present invention, and more particularly, to providing austenitic high strength steel; Manufacturing an open pipe; Manufacturing an austenitic welded steel pipe by welding; And a shape correction step of the steel pipe in which the deformation amount of the final point in time is in the range of 2% to 5% in the temperature range of 100 to 300 ° C. of the welded portion, and the austenite manufactured by the austenitic welded steel pipe. It relates to a knight welded steel pipe.
According to the present invention, by adjusting the temperature range and deformation amount in the shape correction step of the steel pipe after welding, the softening phenomenon of the weld portion is improved and the workability of the welded steel pipe is greatly improved, it can contribute significantly to the productivity improvement without additional equipment.

Description

Method for producing austenitic welded steel pipe having excellent processability and austenitic welded steel tube manufactured thereby

The present invention relates to a method for producing an austenitic welded steel pipe excellent in workability and a welded steel pipe manufactured by the same, and to a method for manufacturing a welded steel pipe for improving softening of welds and improving workability of a welded steel pipe, Relates to a welded steel pipe.

In recent years, the automobile industry has been increasingly adopting high-strength steels for the purpose of lightening the body weight due to tightening environmental regulations. In terms of processing methods, tailored blanks (TWB, Tailored Blank) and hydroforming (Hydroforming) and other methods are actively examined to replace the existing press process.

The tailored blank 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 blank to form a component.

Hydroforming technology is a technology for forming parts by applying high pressure to the inside of the tube, and it is possible to reduce the process of assembling parts by manufacturing a large number of parts into a single part. It is a technology having.

Automotive welded steel sheets are generally manufactured continuously by welding after roll forming. In this case, the welding may be applied to TIG welding, plasma welding, high frequency welding, laser welding, and the like.

The characteristics of such a processing method is that the welding is formed under the same conditions as the material, so it is very important to secure the quality of the weld in order to be applied as a component material. However, in the case of high-strength steel, a large amount of alloying elements are usually added, so that the hardenability increases during welding, thereby decreasing the workability of the weld. Particularly, in the case of high strength steel of 980 MPa or higher, the hardening phenomenon of the weld is remarkable.

On the other hand, high-strength TWIP (Twinning Induced Plasticity) steel of austenitic structure having a high elongation by developing twin hardening during deformation by adding a large amount of manganese and carbon to increase work hardening rate has been developed. Unlike conventional high strength steels such as DP and TRIP, TWIP steels have a single-phase austenitic structure and do not cause phase transformation even after welding. In general, steel products have high workability, while TWIP steel has high workability at ultra high strength level.

However, the welded portion of the TWIP steel has a problem that coarse grains, and the softening phenomenon occurs due to the loss of the TWIP effect compared to the base material. Furthermore, in the case of the welded steel pipe for hydroforming, since the material is processed by roll forming before welding and the hardness of the welded portion is increased, the softening phenomenon is more remarkable compared to the plate welded portion.

Accordingly, an aspect of the present invention is to provide a method for producing a welded steel pipe in which the weldability of austenitic high strength steel is improved to ensure quality even in harsh processing conditions.

Another aspect of the present invention is to provide an austenitic high strength steel produced by the above method.

According to one aspect of the invention, providing an austenitic high strength steel; Manufacturing an open pipe; Manufacturing an austenitic welded steel pipe by welding; And a shape correction step of the steel pipe in which the deformation amount of the final point in time in the range of 100 to 300 ° C. in the weld is performed in a range of 2% to 5%.

The high strength steel preferably contains 5 to 35% by weight of manganese (Mn), 0.01 to 3% by weight of aluminum (Al), more than 0 to 1.5% by weight of carbon (C) and the balance of iron (F).

The high strength steel preferably further comprises 0.0005 to 0.5% by weight of titanium (Ti) and more than 0 to 0.5% by weight of vanadium (V).

The high strength steel is preferably a TWIP (Twinning Induced Plasticity) steel having a tensile strength of 980 MPa or more.

The step of manufacturing the open pipe is preferably carried out by a continuous roll forming method.

The welding is preferably selected from TIG welding, plasma welding, laser beam welding, and high frequency welding.

According to another aspect of the invention, the step of providing an austenitic high strength steel; Manufacturing an open pipe; Manufacturing an austenitic welded steel pipe by welding; And an austenitic welded steel pipe manufactured by the shape correction step of the steel pipe in which the amount of deformation of the final point in time in the range of 100 to 300 ° C. of the weld is performed in a range of 2% to 5%.

The high strength steel preferably contains 5 to 35% by weight of manganese (Mn), 0.01 to 3% by weight of aluminum (Al), more than 0 to 1.5% by weight of carbon (C) and the balance of iron (F).

The high strength steel preferably further comprises 0.0005 to 0.5% by weight of titanium (Ti) and more than 0.5% by weight of vanadium (V).

The austenitic welded steel pipe preferably has an expansion ratio of 32 to 35%.

The austenitic welded steel pipe preferably has a roundness of internal and external diameter tolerances of maximum (+) 0.2 mm and minimum (−) 0.2 mm.

The austenitic welded steel pipe has a welded part hardness of 267 to 301 Hv, a difference in hardness between the welded part and the base material is 17 to 51 Hv, and preferably has an expansion ratio of 32 to 35%.

The austenitic welded steel pipe has a welded hardness of 267 to 301 Hv, a difference in hardness between the welded portion and the base metal, and a hardness of 17 to 51 Hv, and an internal and external diameter tolerance having a maximum (+) 0.2 mm and a minimum (-) 0.2 mm roundness. It is preferable.

According to the present invention, by adjusting the temperature range and deformation in the shape correction step of the steel pipe after welding, the softening phenomenon of the weld portion is improved and the workability of the welded steel pipe is greatly improved, in the manufacture of austenitic welded steel pipe without additional equipment It can contribute greatly to productivity improvement.

The present invention relates to a method of manufacturing austenitic welded steel pipes in which the quality of the welded parts is improved by improving the workability of the welded part, and the quality thereof is guaranteed even in harsh processing conditions. Manufacturing an open pipe; Manufacturing an austenitic welded steel pipe by welding; And it relates to a method for producing austenitic welded steel pipe comprising the step of correcting the shape of the steel pipe is carried out in the range of 2% to 5% of the deformation amount of the final point in the temperature range of 100 to 300 ℃.

High strength steel that can be used in the present invention is preferably a TWIP (Twinning Induced Plasticity) austenitic steel having a tensile strength of 980 MPa or more, the high strength steel is 5 to 35% by weight of manganese (Mn), 0.01 to 3 aluminum (Al) It is preferred to include by weight, greater than zero carbon (C) 1.5% by weight or less and the balance of iron (F).

The high strength steel preferably further comprises 0.0005 to 0.1% by weight of titanium (Ti) and more than 0 to 0.5% by weight of vanadium (V).

The manganese (Mn) forms S and sulfide (MnS), which may be incorporated as impurities, and improves hot workability. However, when less than 5% by weight, such an effect cannot be obtained, and when it exceeds 35% by weight, the hardness is made high to destabilize the steel, and rather, the workability or weldability may be impaired.

The aluminum is usually added for deoxidation of steel, but in the present invention, aluminum is added for ductility improvement. That is, aluminum is a stabilizing element in the ferrite phase, but the stacking fault energy (stacking fault energy) is increased in the slip surface of the steel to suppress the generation of ε-marthecite phase to improve the ductility. In addition, aluminum suppresses the formation of ε-martesite phase even in the case of low manganese addition, thus minimizing the amount of manganese added and improving processability. Therefore, when the addition amount is less than 0.01% by weight, ε-martensite is formed to increase the strength, but the ductility decreases rapidly.When the addition amount is more than 3% by weight, the ductility is suppressed by reducing the ductility and continuous roughening. The castability is poor, the surface oxidation during hot rolling is severe, there is a problem to lower the surface quality of the product.

On the other hand, since the carbon (C) contributes to the stabilizer phase of the austenite phase, it is advantageous as the addition amount thereof is increased, and therefore, it is preferable to be added to exceed 0 wt%. However, when the amount of carbon added is more than 1.5% by weight, the stability of the austenite phase is greatly increased, and workability is lowered due to the transition of deformation behavior due to slip deformation.

Further, the high strength steel is Fe-18Mn-1.5Al-0.6C as the basic composition (wt%), and other steels containing a small amount of titanium (Ti), vanadium (V), hot rolled, It can be prepared through the process of cold rolling, annealing, and preferably further comprises 0.0005 to 0.1% by weight of titanium (Ti) and more than 0 to 0.5% by weight of vanadium (V).

Titanium (Ti) is dissolved in the columnar grain boundary to raise the melting temperature of the aluminum-rich low melting point compound to prevent the formation of the liquid film below 1300 ℃, and has a high affinity with nitrogen to cause columnar grain embrittlement. It acts as a nucleus for the precipitation of aluminum nitride, which strengthens the columnar grain boundary. However, it is ineffective at 0.005% by weight or less, and when it exceeds 0.10% by weight, excess titanium segregates at the grain boundaries, causing grain boundary odor.

On the other hand, vanadium (V) is a component that is added to increase the strength is preferably added in excess of 0% by weight, when the amount of vanadium added is more than 0.5% by weight to produce a low melting point compound to impair the hot workability, the addition amount is It is preferable to limit to 0.5 weight% or less.

The chemical composition of the most preferred high strength austenitic steels that can be used in the present invention is as shown in Table 1 below, the remainder consisting of iron.

Chemical composition (wt%) 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

On the other hand, the mechanical properties of the most preferred high strength austenitic steel that can be used in the present invention are as described in Table 2 below.

Mechanical properties Hardness Tensile Strength (TS) Yield Stress (YS) Elongation (EL) 240Hv 980 MPa 520 MPa 60%

The manufacturing process of welded steel pipe generally includes the steps of providing a steel material, forming a plate to produce a circular open pipe, manufacturing an austenitic welded steel pipe by welding, and then correcting the shape of the steel pipe. It includes four steps of steps.

The step of manufacturing the open pipe may be performed by a continuous roll forming method or a roll bending method, it is preferable to be carried out by a continuous roll forming method.

In the manufacturing process of the open pipe, the hardness of the material increases due to work hardening. Therefore, when the subsequent welding is applied, grains are coarsened and the TWIP effect is lost. Furthermore, the work hardened material part is softened by the heat treatment effect of welding heat. This tends to increase.

The kind of welding that can be used in the step of manufacturing the austenitic welded steel pipe by the welding can be selected from TIG welding, plasma welding, laser beam welding, and high frequency welding, and is preferably performed by TIG welding. TIG welding (Tungsten Inert Gas welding), also known as tungsten inert arc welding, is a welding method that prevents oxidation and nitriding by covering an arc with an inert gas (inert gas such as helium or argon gas).

The softened welds perform a step for correcting the shape of the steel pipe, wherein the shape correction of the steel pipe includes the reduction and expansion of the steel pipe, and by manufacturing the temperature and the deformation amount in performing the shape correction step as described above. Intensive work hardening effect on the weld can be obtained.

The steel pipe shape correction step of the present invention can ensure the characteristics of the steel pipe quality by performing the shape correction of the pipe material in the range of 2% to 5% of the deformation amount of the final point of time to arrive at the temperature range of 100 to 300 ℃ of the weld. have.

If the temperature of the weld is less than 100 ℃, the load load for imparting a predetermined deformation is increased. If the temperature of the weld is 300 ℃ or more, it is difficult to maintain the temperature by natural cooling method after welding the pipe, thus adding preheating and post-heat treatment. A process may be necessary. In addition, when the amount of deformation exceeds 5%, commercial production is difficult due to a poor shape of the welded steel pipe, and when the amount of deformation is less than 2%, there is a problem in securing a predetermined hardness due to a lack of processing strain in the weld.

On the other hand, after manufacturing the steel pipe in the present invention, before the shape correction step in a separate shape correction line may further comprise a preheating step in the range of 100 to 300 ℃. When the preheating step is additionally included, the softened welds may be processed to increase the hardness, and the preheating step may be performed for 5 to 10 minutes.

However, the cost increase due to additional process equipment investment, such as pre-heating and post-heating treatment may be a problem, and also because it is difficult to secure a predetermined shape because the welded steel pipe is deformed again when cooled at high temperature. Thus, it is not essential for the present invention.

Furthermore, according to the present invention, an austenitic welded steel pipe manufactured by the above method is provided, and the high-strength steel is 5 to 35 wt% of manganese (Mn), 0.01 to 3 wt% of aluminum (Al), and carbon (C). ) More than 0 and 1.5% by weight or less and the balance of iron (F), preferably 0.0005 to 0.5% or less by weight (Ti) and vanadium (V) more than 0 and 0.5% by weight may be further included. have.

On the other hand, the welded steel pipe is evaluated by using the maximum expansion rate up to the time of crack generation by the flaring expansion test has a expansion ratio of 32 to 35%.

In addition, the austenitic welded steel pipe according to the present invention has a roundness included within an internal and external diameter tolerance maximum (+) 0.2 mm, minimum (−) 0.2, which is a required level of the weld steel pipe for hydroforming.

As described above, the austenitic welded steel pipe according to the present invention can improve the softening phenomenon of the welded part and greatly improve the workability of the welded steel pipe, and can greatly contribute to productivity improvement without requiring additional equipment.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are for illustration of the present invention, and the scope of the present invention is not intended to be limited by the following examples.

<Example>

1. Manufacturing of austenitic welded steel pipe

0.57 wt% carbon (C), 0.282 wt% silicon (Si), 18.31 wt% manganese (Mn), 0.026 wt% phosphorus (P), 0.007 wt% sulfur (S), 0.346 wt% chromium (Cr), nickel ( Ni) 980 MPa grade TWIP steel consisting of 0.09% by weight, 1.26% by weight of aluminum (Al), 0.021% by weight of titanium (Ti), 0.10% by weight of vanadium (V), 0.005% by weight of nitrogen (N) and% by weight of remainder (78.983) As a coil, a coil having a thickness of 1.45 mm was manufactured through a process of casting, hot rolling, cold rolling and annealing.

In general, the tubing process adopts a continuous roll forming method applied to a welded steel pipe for automobiles, and manufactured a welded steel pipe having an outer diameter of 34 mm by forming, TIG welding, straightening, and cutting.

In welding, a welding current of 125 A, a pipe speed of 1.33 m / min, and 9% of hydrogen gas (H ₂ ) was added to Ar gas as a shield gas to produce a welded steel pipe.

The temperature and deformation amount in the post-weld calibration process were controlled by the flow rate and pressing force of the cooling water for the rolls and the tubular material.

2. Evaluation of mechanical properties of steel pipe

(1) tensile test

Warm tensile tests were conducted to quantitatively examine the effects of temperature and strain on the weld hardening properties of TWIP steel.

Tensile test of the weld is to produce the test piece shape in the vertical direction of the weld line according to JIS Z2201_13B, the test temperature is at room temperature, 100 levels, 100, 200, 300, 400 ℃ four levels, the strain rate is changed to 2, 5, 7% while the strain rate It carried out at 20 mm / min.

(2) Hardness measurement of welding part

When measuring the hardness of the welded part, the area including the base material and the welded part was measured three times at 0.2 mm intervals with a load of 500 g and a holding time of 10 s.

(3) Evaluate the roundness of welded steel pipes

The roundness of the welded steel pipe was considered to be within the maximum (+) 0.2mm tolerance and the minimum (-) 0.2 tolerance, which is the required level of the welded steel pipe for hydroforming.

(4) Evaluation of workability of welded steel pipe

Workability evaluation of the welded steel pipe was evaluated using the maximum expansion rate up to the time of crack generation by the flaring expansion test.

Table 3 below is a result of evaluating the properties of the weld plate and welded steel pipe of the TWIP steel.

Sheet welding Welded steel pipe Temperature
(℃) Strain (%) Weld
Hardness (Hv) △ Hv
(Plate welding part-Pipe material base material) Roundness
Pass or fail Expansion rate
(%) Comparative Example 1 Room temperature 0 210 -40 pass 30 Example 1 100 2 275 25 pass 32 Example 2 100 5 301 51 pass 35 Comparative Example 2 100 7 294 44 Bad - Example 3 200 2 267 17 pass - Example 4 200 5 285 25 pass 34 Comparative Example 3 200 7 314 64 Bad - Example 5 300 2 289 39 pass 34 Example 6 300 5 284 34 pass 33 Comparative Example 4 300 7 328 78 Bad - Comparative Example 5 400 2 287 37 - - Comparative Example 6 400 5 301 51 - - Comparative Example 7 400 7 302 52 - -

In Table 3, ΔHv (Plate Weld-Pipe Material) indicates the level of hardness of the weld against the base material, △ Hv is positive, the weld is hardened, and the negative value is negative. It means. In other words, when the welded steel pipe is processed, as the value of ΔHv increases, the weld portion is firm, so that the expansion rate can be increased. In the case of the present invention, a target expansion ratio of 32% or more, which is necessary when expanding the banding end portion after bending, is set.

Looking at the hardness distribution of the weld plate, it can be seen that the softened weld is significantly increased in hardness by adding a deformation amount of about 2 to 7% at 100 to 400 ° C. in the weld compared to the conventional method. However, in the case of 7% deformation amount, it was confirmed that the weld part was severely deformed after the tensile test.

On the other hand, if the amount of deformation of about 2 to 5% at 100 to 300 ° C. is added after welding of the pipe, the softening phenomenon is greatly improved, and the expansion rate of the welded steel pipe is also increased.

On the other hand, in the case of spontaneous cooling after welding of pipes, it is difficult to maintain the temperature up to 400 ° C in the calibration line.

On the other hand, in relation to '-' in the above numerical value, it is difficult to secure a welded steel pipe at 400 ° C or higher, and when the expansion test is difficult due to a shape defect, it is a case in which the expansion ratio of the welded steel pipe is normally difficult and omitted. .

Claims (13)

Providing austenitic high strength steel;
Manufacturing an open pipe;
Manufacturing an austenitic welded steel pipe by welding; And
The shape correction step of the steel pipe is carried out in the range of 2 to 5% of the deformation amount of the final point in the temperature range of 100 to 300 ℃ of the weld, the high-strength steel is 5 to 35% by weight of manganese (Mn), aluminum (Al) 0.01 to 3% by weight, carbon (C) more than 0 to 1.5% by weight and the remainder of the iron (F) method of producing an austenitic welded steel pipe.
delete The method of claim 1, wherein the high strength steel further comprises 0.0005 to 0.5 wt% of titanium (Ti) and 0.5 wt% or less of vanadium (V).
The method of claim 1, wherein the high-strength steel is a twinning induced plasticity (TWIP) steel having a tensile strength of 980 MPa or more.
The method of claim 1, wherein the manufacturing of the open pipe is performed by a continuous roll forming method.
The method of claim 1, wherein the welding is selected from TIG welding, plasma welding, laser beam welding, and high frequency welding.

Providing austenitic high strength steel;
Manufacturing an open pipe;
Manufacturing an austenitic welded steel pipe by welding; And
It is manufactured by the shape correction step of the steel pipe is carried out in the range of 2% to 5% of the deformation amount of the final point in the temperature range of 100 to 300 ℃ of the weld, the high-strength steel is 5 to 35% by weight of manganese (Mn), An austenitic welded steel pipe comprising 0.01 to 3% by weight of aluminum (Al), more than 0 to 1.5% by weight of carbon (C) and the balance of iron (F).
delete The austenitic welded steel pipe according to claim 7, wherein the high strength steel further comprises 0.0005 to 0.5 wt% of titanium (Ti) and more than 0.5 wt% of vanadium (V).
The austenitic welded steel pipe according to claim 7, wherein the austenitic welded steel pipe has an expansion ratio of 32 to 35%.
The austenitic welded steel pipe according to claim 7, wherein the austenitic welded steel pipe has a roundness having an internal and external diameter tolerance of at most (+) 0.2 mm and at least (-) 0.2 mm.
8. The austenitic welded steel pipe according to claim 7, wherein the austenitic welded steel pipe has a welded hardness of 267 to 301 Hv, a difference in hardness between the welded portion and the base material is 17 to 51 Hv, and has an expansion ratio of 32 to 35%.
The method of claim 7, wherein the austenitic welded steel pipe has a welded hardness of 267 to 301 Hv, the hardness difference between the welded portion and the base material is 17 to 51 Hv, the internal and external diameter tolerance is a maximum (+) 0.2mm, minimum (-) Austenitic welded steel pipe with a roundness of 0.2 mm.