JP4474729B2 - Structural electric resistance welded steel pipe with excellent hydroforming properties and low weld softening - Google Patents

Description

【００３０】
【表２】

【００３１】
また、表１中の管No. ３の鋼管について、サイジングの際の絞り率を0.1 〜12％の間で変化させた場合の、拡管試験結果を表３に示す。
表３より、サイジング時の絞り率が0.3 〜10％の範囲内であると、ＴＳ×拡管率が9000 MPa・％以上となることがわかる。
【００３２】
【表３】

【００３３】
【発明の効果】
以上説明したように、本発明によれば、ハイドロフォーミング性に優れ、しかも溶接部の軟化が少なく溶接による強度低下が少ない電縫鋼管を提供できる。したがって、本発明は、ハイドロフォーム後に、溶接することにより製造される構造部材の品質、安定生産に大きく貢献する。
【図面の簡単な説明】
【図１】自由バルジ試験に用いる金型を示す斜視図である。
【図２】自由バルジ試験に用いる金型を示す断面図である。
【図３】自由バルジ試験に用いるハイドロフォーミング加工装置の構成の例を示す断面図である。
【図４】被溶接材の形状を示す斜視図である。[0001]
BACKGROUND OF THE INVENTION
This invention provides a suitable steel using such a structural member or chassis member of a motor vehicle, particularly excellent in workability (hydroforming property) in hydroforming, electric resistance welded steel pipe for softening a small structure of the weld about the.
[0002]
[Prior art]
In order to manufacture hollow members having various cross-sectional shapes used as structural members for automobiles, a conventional method is to join parts formed by press working of steel plates by spot welding at the flange portion which is the welding allowance. Have been adopted, but improvements have been demanded both in terms of quality and production efficiency.
On the other hand, recently, higher impact absorbing ability at the time of collision has been demanded for structural hollow members, and steel sheets used as raw materials have been further strengthened. For this reason, it is becoming increasingly difficult to manufacture a member having no molding defects and having a good shape and dimensional accuracy in the conventional press molding method.
[0003]
Recently, hydroforming has attracted attention as a new molding method for solving such problems. Hydroforming is a method of plastic processing by injecting high-pressure liquid into the inside of a steel pipe, and by changing the cross-sectional dimension of the steel pipe by pipe expansion processing, etc., it is possible to integrally form complex shaped members and increase strength and rigidity. It is an excellent molding method with functions.
By the way, as a steel pipe used for hydroforming, generally, an electric resistance steel pipe manufactured with a material made of low carbon steel in C: 0.20 to 0.10% is often used.
[0004]
[Problems to be solved by the invention]
However, even when hydroforming is performed on an ERW steel pipe containing such an amount of C, there is a problem that a sufficient tube expansion rate cannot be obtained because the workability of the material itself is not good. In order to increase the tube expansion ratio in hydroforming, there is a method of applying axial force in the tube axis direction, but this method is not effective when forming a tube with a design in which the tube is greatly bent in the longitudinal direction. Absent.
On the other hand, in order to improve the workability of the material itself of the electric resistance welded steel pipe, it is conceivable to use an ultra-low carbon steel with a significantly reduced carbon content as the material. However, in the case of an ultra-low carbon ERW steel pipe, although it has high ductility, another problem caused by welding occurs. In other words, in ultra-low carbon ERW steel pipes, the crystal grains of the welded part due to electric resistance welding during steel pipe production are coarsened and softened, and deformation during tube expansion is concentrated locally, so that the high ductility of the material is sufficient. In addition, when a hydroformed member is welded to another member, similar softening occurs, and static strength and fatigue strength necessary for the member cannot be obtained.
[0005]
Therefore, in view of these problems that the above-described prior art has, the present invention proposes a new proposal for an electric resistance welded steel pipe suitable for hydroforming. In particular, an object of the present invention is to propose a structural electric-welded steel pipe that has excellent hydroforming properties and does not cause softening due to welding.
The specific target characteristics of the steel pipe of the present invention are as follows: Hydroforming property expressed by (TS of steel pipe) x (pipe expansion ratio under pipe end fixing conditions) is 9000 MPa ·% or more, and softening resistance of welds It is assumed that the ratio Hv (min) / Hv (material) between the minimum hardness Hv (min) of the welded portion used as an index indicating the hardness and the hardness Hv (material) of the steel pipe material is 0.9 or more.
[0006]
[Means for Solving the Problems]
In order to achieve the above-mentioned problems, the inventors have made various studies on the composition of the ERW steel pipe, the manufacturing method, and the like. As a result, using a low carbon to semi-very low carbon material with a C content in the range of 0.01 to 0.05%, adding an appropriate amount of Ti to this, drawing the pipe joint by electrical resistance welding of the seam portion It was found that sizing (reducing diameter) of 0.3 to 10% is effective. The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[0007]
(1) Steel composition in mass% (hereinafter simply referred to as%) C: 0.01 to 0.08%, Si: 1.0% or less, Mn: 3.0% or less, P: 0.15% or less , S: 0.015% or less, Al: 0.01~0.10%, Ti: 0.21 ~0.50%, and Ti has a C and ^N, Ti *: -0.05% or more, However, Ti ^* = Ti (%)-(48/14) N (%)-(48/12) C (%) is satisfied and contained, and the balance is a slab composed of a steel composition of Fe and inevitable impurities. Hot-rolled steel sheet that has been hot-rolled or cold-rolled steel sheet that has been cold-rolled and annealed after hot-rolling is formed into a cylindrical shape, and the seam part where both width ends are butted together is resistance-welded and, then, a electric resistance welded steel pipe subjected 0.3 to 10% of the sizing aperture ratio of the peripheral length, Hydro under tube end fixed conditions Omingu TS of expansion ratio (%) and steel (MPa) is less than the TS (MPa) × expansion ratio (%) ≧ 9000 MPa ·% by minimum weld hardness Hv (min) and of the steel pipe material hardness Hv A structural electric resistance welded steel pipe having excellent hydroforming properties and low softening of welds, characterized in that the ratio Hv (min) / Hv (material) to (material) is 0.9 or more .
[0008]
(2) In the above (1), the steel composition contains one or more selected from the following group A to group C, in addition to the above components, and is excellent in hydroforming properties, and has a welded portion. ERW steel pipe for structural use with little softening.
Serial Group A: Nb: 0.005~0.040%, B: 0.0005~0.020% 1 kind of or two or Group B: N i: 0.02~1.0%
Group C: Ca: 0.0020-0.02%, REM: One or two of 0.0020-0.02%
In the present invention, N is not actively added, but when obtaining the above Ti ^* , an N content of less than 0.005% contained as an unavoidable impurity is used.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The reason for limiting the steel components in the present invention, the method for producing the ERW steel pipe, etc. will be described.
C: 0.01-0.08%
C contributes to the strengthening of the steel, but is an element that lowers the formability. In particular, when the C content is 0.08% or more, the formability is greatly reduced. On the other hand, when the content is less than 0.01%, a welded part (welding heat affected zone) is produced by resistance welding during the manufacture of ERW pipes and by arc welding when assembling parts manufactured by machining ERW pipes. The crystal grains become coarse, resulting in non-uniform deformation as well as a decrease in static strength and fatigue strength. For this reason, C amount is taken as 0.01 to 0.08% of range.
[0012]
Si: 1.0% or less
Si is an element useful for strengthening steel, and is added depending on the desired strength. However, if added over 1.0%, the surface properties of the steel pipe material are significantly deteriorated, and as a result, the burst resistance during hydroforming is lowered. Therefore, it is added in the range of 1.0% or less.
[0013]
Mn: 3.0% or less
Mn is an element effective for improving the strength of the steel sheet and thus the hydroforming member without deteriorating the surface properties and weldability. However, the addition of more than 3.0% lowers the tube expansion rate during hydroforming. Therefore, the Mn content is 3.0% or less.
[0014]
P: 0.15% or less P is an element effective for improving the strength, but if added over 0.15%, the weldability is remarkably deteriorated. In particular, when the strengthening action by P is not so necessary, or when there is a concern about a decrease in weldability due to a high C content, it is desirable to limit it to 0.02% or less.
[0015]
S: 0.015% or less S is present as a non-metallic inclusion in steel, and this may be the starting point and break the steel pipe during hydroforming. For this reason, the lower the amount of S, the better the burst resistance. If the S content is 0.015% or less, the effect is exhibited. In order to further improve the burst resistance, it is preferably limited to 0.010% or less, more preferably 0.005% or less.
[0016]
Al: 0.01-0.10%,
In addition to deoxidizing steel, Al is a useful element for suppressing the coarsening of crystal grains, so it is necessary to add 0.01% or more. On the other hand, adding a large amount exceeding 0.1% not only saturates these effects but also causes surface defects of the steel sheet. Therefore, Al is contained in the range of 0.01 to 0.10%, preferably 0.02 to 0.10%.
[0017]
Ti: 0.21 to 0.50% and Ti ^* : −0.05 or more, provided that Ti ^* = Ti (%) − (48/14) N (%) − (48/12) C (%)
In the present invention, Ti is a particularly important element that contributes to the improvement of the formability of the steel pipe through the improvement of the ductility and r value of the material and also contributes to the improvement of the hydroforming property. Such an effect is obtained when the Ti content is 0.21 % or more and the Ti ^* content is −0.05% or more. However, even if the Ti content exceeds 0.50%, not only the effect is saturated, but also the hot deformation resistance of the steel is increased to impair the productivity. Therefore, these elements are added in the above range.
[0018]
Nb: 0.005 to 0.040%, B: 0.0005 to 0.020%
Nb and B are both useful elements for crystal grain refinement. Such an effect appears when Nb is 0.005% or more and B is 0.0005% or more. On the other hand, the addition of Nb exceeding 0.040% and B: 0.020% not only saturates the effect, but also increases the hot deformation resistance of the steel and impairs manufacturability. Therefore, these elements are added in the above range.
[0019]
N i: 0.02~1.0%
Ni is an element useful for improving the strength without impairing the ductility of the steel pipe. Such an effect can be obtained by adding 0.02% or more. On the other hand, even if Ni is added in excess of 1.0% , the effect is saturated, resulting in an increase in cost, and the hot workability and cold workability of the steel are lowered. Therefore, Ni is added in the above range.
[0020]
Ca: 0.0020-0.02%, REM: 0.0020-0.02%
Ca and REM are useful elements for improving the burst resistance by reducing the notch action of the non-metallic inclusions mainly composed of S in the steel and making them spherical. Such an effect can be obtained by adding 0.0020% or more of both Ca and REM, but even if added over 0.02%, the effect tends to be saturated or slightly reduced. Therefore, these elements are added in the above range. When both Ca and REM are used in combination, it is preferable to add in a total amount of 0.03% or less.
[0021]
Next, necessary characteristics of the steel pipe of the present invention will be described.
If the tensile strength of the steel pipe is small, a high impact absorbing ability cannot be obtained, and if the expansion ratio by hydroforming is small, the shape that can be formed by hydroforming is limited. In the present invention, it is necessary that these two characteristics are balanced. In hydroforming, a liquid such as water is supplied from both ends of the tube, and deformation is performed from the inner surface of the tube by hydraulic pressure. When both ends of the tube are fixed (hereinafter referred to as tube end fixing), both ends of the tube are fixed. In some cases, the compression force is applied (hereinafter referred to as tube end compression). In general, the tube expansion rate can provide a higher tube expansion rate.
In the present invention, since it is necessary to balance strength and hydroforming properties, the tensile strength (MPa) x tube expansion rate (%) is 9000 MPa ·% or more when the tube end is fixed. To do.
Here, the pipe expansion rate is the maximum outer diameter when a steel pipe with an outer diameter do is deformed part length l c = 2 do, liquid is supplied from the pipe end to the inner surface of the pipe, the hydraulic pressure is applied, the circular cross section is freely deformed, and bursts. From the diameter dmax, it is defined as (dmax -do) / do x100.
[0022]
The pipe expansion rate can be measured by, for example, expanding the pipes 2a and 2b shown in FIGS. 1 and 2 using the hydroforming apparatus having the configuration shown in FIG.
FIG. 1 is a perspective view of a mold, and FIG. 2 is a cross-sectional view of the mold. Each of the molds 2a and 2b has a steel pipe holding portion 3 composed of a semi-cylindrical surface having a diameter substantially equal to the outer diameter do of the steel pipe at both ends in the length direction. a deformable portion 6 comprising a semicylindrical deformable portion 4 with dc = 2 do and a tapered deformable portion 5 with an inclination angle θ = 45 °, and the length lc of the deformable portion 6 is twice as long as do. It consists of an upper mold 2a and a lower mold 2b. As shown in FIG. 3, the steel pipe 1 is sandwiched between the upper mold 2a and the lower mold 2b so that the steel pipe 1 fits in the steel pipe holding portion 3 of each mold. In this state, when a liquid such as water is supplied from both ends of the steel pipe 1 to the inner surface side of the steel pipe 1 via the axial cylinder 7a, and a hydraulic pressure P is applied, and a circular cross section free bulge is deformed and burst. The maximum outer diameter dmax is measured. In addition, 8 and 9 in FIG. 4 are a mold holder and an outer ring for holding the molds 2a and 2b in a state of sandwiching the steel pipe, respectively.
[0023]
In the present invention, the tube expansion rate is measured under tube end fixing conditions. For this reason, it is necessary to prevent the tube end from moving in the axial direction. For example, both ends of the steel pipe 1 are fixed to the shaft pressing cylinders 7a and 7b by bolting or the like, and the shaft pressing cylinder 7a is subjected to a hydraulic load. , 7b can be carried out by fixing the steel pipe 1 so as not to move in the axial direction.
[0024]
Next, the manufacturing method of this invention steel pipe is demonstrated. First, after melting steel in accordance with the above-described component composition, a slab is formed by a continuous casting method or an ingot-bundling method. Subsequently, this slab is made into a hot-rolled steel sheet by hot rolling or, after hot rolling, is further made into a cold-rolled steel sheet by cold rolling-annealing. Using the hot-rolled steel sheet or cold-rolled steel sheet obtained in this way as a raw material, it is formed into a substantially cylindrical shape by roll forming or bending, and the seam part where both width end parts are butted together is electrically resistance welded (electrical Join together by sewing).
Following electrical resistance welding, sizing of 0.3 to 10% is performed with the drawing ratio of the outer peripheral length. The purpose of sizing is to obtain sufficient shape accuracy and to ensure uniformity of material deformation in order to use the ERW steel pipe for hydroforming. In order to achieve such an objective, at least 0.3% is required for the drawing ratio of the outer peripheral length, but if it exceeds 10%, the steel pipe is remarkably work-hardened and the hydroforming property is lowered. Therefore, after electric resistance welding, sizing of 0.3 to 10% is performed at the drawing ratio of the outer peripheral length.
[0025]
【Example】
Steels having chemical components shown in Tables 1 and 2 were melted to form slabs. This slab is heated to 1220 ° C. and then hot rolled to obtain a hot rolled steel sheet having a thickness of 2.0 mm, or, following hot rolling, a thickness of 2.0 mm is obtained by pickling, cold rolling, and continuous annealing processes. The cold-rolled steel sheet.
After forming these hot-rolled steel sheets or cold-rolled steel sheets into a cylindrical shape, the seam is electrically resistance welded to form a steel pipe having a diameter of 63.5 mm and a wall thickness of 2.0 mm, and then a sizing of 2% with a drawing ratio of the outer peripheral length. (Shrinking) was performed.
[0026]
From the obtained electric resistance welded steel pipe, a JIS No. 12 test piece was collected, and the tensile strength of the steel pipe was investigated. Moreover, the steel pipe was cut | disconnected to the length of 500 mm, and it was set as the test body for hydroforms. As shown in FIGS. 1 to 3, this test body was constrained at both ends, water was supplied from the end of the pipe, a circular cross section free bulge was deformed by water pressure, and the maximum tube expansion rate when bursting was measured. . Here, the mold dimensions in FIG. 2 are that lc is 127.0 mm, dc is 127.0 mm, rd is 5 mm, lo is 550 mm, and θ is 45 °. Further, in deforming the test body, a lubricant having a viscosity of 800 cst was interposed between the mold and the surface (outer peripheral surface) of the test body. And the characteristic of each electric resistance welded steel pipe was expressed not only by the pipe expansion ratio but also by the strength × the pipe expansion ratio in consideration of the balance with the steel pipe strength.
The characteristics of each electric resistance welded steel pipe are expressed not only by the pipe expansion ratio but also by the steel pipe strength x the pipe expansion ratio in consideration of the balance with the steel pipe strength.
[0027]
Furthermore, the strength of the welded part by arc welding was evaluated on the assumption that the part was assembled by welding after hydroforming. The welding conditions were as follows.
-Welding wire: JIS YGW15 equivalent-Welding wire diameter: 1.2 mm
Shield gas: Ar-20% by volume CO ₂
・ Gas flow rate: 40 l / min
・ Welding current: 200 to 220 A
・ Arc voltage: 18-19V
-Welding speed: 80-100 cm / min
-Material to be welded: Butt weld between ERW steel pipe and cold-rolled steel sheet with a thickness of 2 mm (steel composition is the same as ERW steel pipe) (see Fig. 4)
Here, since it was confirmed that both static strength and fatigue strength depend on the ratio of the hardness Hv (min) of the softest part of the weld heat affected zone (HAZ) to the hardness Hv (material) of the material, The hardness ratio was evaluated by the hardness ratio, Hv (min) / Hv (material).
[0028]
The obtained results are also shown in Tables 1 and 2. From these tables, the ERW steel pipe according to the present invention has a high steel pipe strength × expansion rate, excellent hydroforming properties, high Hv (min) / Hv (material), static strength and fatigue strength. It turns out that it is favorable.
On the other hand, the comparative example in which the chemical component is not appropriate has poor hydroforming properties or has a large softening of the welded portion and has a difficulty in static strength and fatigue strength.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
Table 3 shows the results of the pipe expansion test when the sizing ratio of the steel pipe No. 3 in Table 1 was changed between 0.1 and 12%.
From Table 3, it can be seen that if the drawing ratio during sizing is within the range of 0.3 to 10%, the TS × tube expansion ratio is 9000 MPa ·% or more.
[0032]
[Table 3]

[0033]
【The invention's effect】
As described above, according to the present invention, it is possible to provide an electric-welded steel pipe that is excellent in hydroforming properties and that has little softening of the welded portion and less strength reduction due to welding. Therefore, the present invention greatly contributes to the quality and stable production of structural members manufactured by welding after hydroforming.
[Brief description of the drawings]
FIG. 1 is a perspective view showing a mold used for a free bulge test.
FIG. 2 is a cross-sectional view showing a mold used for a free bulge test.
FIG. 3 is a cross-sectional view showing an example of the configuration of a hydroforming apparatus used for a free bulge test.
FIG. 4 is a perspective view showing a shape of a material to be welded.

Claims (2)

Steel composition is C: 0.01-0.08% by mass%,
Si: 1.0% or less,
Mn: 3.0% or less,
P: 0.15% or less,
S: 0.015% or less,
Al: 0.01 to 0.10%,
Ti: 0.21 to 0.50%,
And Ti is C and N, Ti ^* : -0.05% or more,
However, Ti ^* = Ti (%)-(48/14) N (%)-(48/12) C (%)
The balance is contained, and the balance is a hot-rolled steel sheet hot-rolled with a slab comprising a steel composition of Fe and inevitable impurities, or a cold-rolled steel sheet further cold-rolled and annealed after hot rolling, An electric resistance welded steel pipe formed into a cylindrical shape, subjected to electrical resistance welding at the joint part where both width ends are butted together, and then subjected to sizing of 0.3 to 10% at the drawing ratio of the outer peripheral length, Tube expansion rate (%) by hydroforming under tube end fixing conditions and TS (MPa) of steel pipe satisfy TS (MPa) x tube expansion rate (%) ≧ 9000 MPa ·%, and minimum hardness Hv (min ) And the hardness Hv (material) of the steel pipe material Hv (min) / Hv (material) is 0.9 or more, and has excellent hydroforming properties and less softening of welds Sewn steel pipe. 2. The structure according to claim 1, wherein the steel composition contains one or more selected from the following group A to group C in addition to the above components, and has excellent hydroforming properties and little softening of the welded portion. ERW steel pipe.
Serial Group A: Nb: 0.005~0.040%, B: 0.0005~0.020% 1 kind of or two or Group B: N i: 0.02~1.0%
Group C: Ca: 0.0020 to 0.02%, REM: One or two of 0.0020 to 0.02%

Images