JPH0368740A - Thick and small-diameter electric welded steel tube having uniform width of white layer and production thereof - Google Patents

Abstract

PURPOSE:To uniformize the width of the white layer at the cross section of a weld zone and to produce the thick and small-diameter electric welded steel tube having excellent cold workability and fatigue characteristic by specifying the groove shape and the ratio between the vertical diameter and horizontal diameter of an open pipe prior to welding at the time of producing the thick and small-diameter electric welded steel tube. CONSTITUTION:A steel sheet contg., by weight %, 0.01 to 0.40% C, 0.01 to 0.50% Si, 0.20 to 1.80% Mn, 0.01 to 0.10% Al, or further one or two kinds of 0.015 to 0.1% Nb, 0.01 to 0.10% Ti, 0.02 to 0.10% V, and 0.0005 to 0.005% Ca is used as the stock steel sheet at the time of producing the thick and small-diameter electric welded steel tube having 2 to 10mm thickness (t), 20 to 65mm outside diameter D and 0.10 to 0.25 t/D. The groove height X1 (X2) of the open pipe prior to the production of the electric welded steel tube by welding is specified to 0.80 to 0.95 of the thickness (t) the groove angle theta1 (theta2) to 20 to 45 deg. and the ratio Y/X between the vertical diameter Y and the horizontal diameter X to 0.85 to 0.97. The ratio WS/WM between the width WS in the front and rear part of the white layer at the cross section of the weld zone after the groove welding and the width WM of the central part is made into the uniform width of 1 to 4. The thick and small-diameter electric welded steel tube having the excellent cold workability and fatigue characteristic is thus produced.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a thick-walled small-diameter electric resistance welded steel pipe suitable for automobiles, machine structures, etc. that require good cold workability and fatigue properties, and a method for manufacturing the same. It is.

(Prior Art) Recently, the following requirements have been increasing for bar-shaped steel materials used in automobiles or machine structures. Specifically, as the output of equipment increases, each component is required to have higher rigidity and durability than ever before.

Second, as equipment becomes more multi-functional, the number of parts tends to increase significantly, and there is a growing demand for weight reduction. In order to meet these demands, wide consideration has begun to be given to the application of thick-walled, small-diameter steel pipes.

In this field, thick-walled small-diameter steel pipes have a plate thickness of 2 to 10 mm,
Outer diameter is 20~65+am, t/D ratio is 10~25%
It refers to something. There are two types of thick-walled, small-diameter steel pipes: seamless steel pipes and ERW welded steel pipes, but ERW welding is used for quality reasons such as less uneven wall thickness and good surface quality, and for economical reasons that it is inexpensive. The production of thick-walled, small-diameter steel pipes has been progressing.

However, the groove shape of conventional small-diameter ERW steel pipes was such that the opposite end surfaces of the oven tube were parallel or V-shaped with a slight opening outward (toward the surface side). When we actually manufactured thick-walled small-diameter ERW steel pipes, we found that
A new phenomenon has emerged. That is, with the same groove shape, the shape of the white layer of the welded portion is significantly different between a thin-walled small-diameter ERW steel pipe and a thick-walled small-diameter ERW steel pipe. This phenomenon will be explained below.

Figure 2 shows a schematic diagram of the white layer of the weld between a thin-walled small-diameter ERW steel pipe and a thick-walled small-diameter ERW steel pipe. An example of a welded steel pipe is shown, where l indicates a white layer and 2 indicates a weld heat-affected zone. In the thin-walled small-diameter ERW steel pipe in (a), the width of the welded white layer is almost uniform in the plate thickness direction, but ( In the thick-walled small-diameter ERW steel pipe b), the white layer of the welded portion has a conspicuously constricted central portion of the plate thickness. This drum-shaped shape is expressed numerically as follows. FIG. 3 is a schematic diagram for quantifying the shape of the welded white layer of a thick-walled, small-diameter electric resistance welded steel pipe. WS in the mouth indicates the width of the white layer on the front and back sides, and WH indicates the width of the white layer at the center of the thickness, where W represents the shape of the white layer.

When expressed as the ratio δ of and W, it is thick and small diameter! The δ of the sewn steel pipe reaches 5 or more. Incidentally, the δ of a thin-walled small-diameter electric resistance welded steel pipe is 2 or less.

(Problems to be Solved by the Invention) As described above, when δ exceeds 4, the softened regions on the front and back surfaces increase, and the oxides generated during welding become insufficiently discharged. Furthermore, if W is made smaller as an extension of conventional welding conditions, WM becomes 0 and a sound weld cannot be obtained. Therefore, if the δ of a thick-walled small-diameter welded steel pipe exceeds 4, the cold workability will be affected by the effects of oxides generated during welding, and the fatigue properties will deteriorate due to the increase in the softened area on the front and back surfaces, which is described above. There was a problem that it could not be used for this purpose.

(Means for Solving the Problems) Therefore, as a result of extensive research in order to effectively utilize thick-walled small-diameter ERW steel pipes, which are newly finding new uses, as an industrial material, the present inventors have developed a groove shape. The present invention was developed based on the knowledge that δ can be reduced by adjusting the shape of the oven tube.
C: 0.01-0.40%, Sj: 0.01-0.50
%, Mr+: 0.20-1.8%, AI: 0, O1-0
．． When manufacturing a steel strip containing 1% Fe and unavoidable impurities into a thick-walled small-diameter ERW steel pipe, the width of the white layer on the front and back sides of the steel pipe and at the center of the wall thickness in the white layer on the cross section of the welded part. are respectively W5, W, and l, and the ratio δ=Ws/
It is a thick-walled small-diameter fi-sewn steel pipe with a uniform white layer width and W and l of 1 to 4.

The second invention has Nb: 0.045 to 0.1%, Ti: 0.
01-001%, V: 0.02-0.1% and Ca:
The thick-walled small-diameter electric resistance welded steel pipe according to claim 1, which contains one or more selected from 0.0005 to 0.005%.

A third aspect of the invention is that when welding the thick-walled small-diameter ERW steel pipe of claim (1) or (2), the groove height in the oven pipe immediately before welding is set to 0.80 to 0.95 of the wall thickness t. Bevel angle 20~
This is a method for manufacturing thick-walled small-diameter electric resistance welded steel pipes that is controlled at 45 degrees.

A fourth aspect of the invention is that when welding the thick-walled small-diameter ERW steel pipe of claim (1) or (2), the groove height in the oven pipe immediately before welding is set to 0.80 to 0.95 of the wall thickness t. Bevel angle 20~
45 degrees, and the ratio γ=y/X of the vertical diameter y and the horizontal diameter x of the oven tube is controlled to 0.85 to 0.97.

(effect) Hereinafter, the effects of the present invention will be explained in detail.

First, the reasons for limiting the chemical components in the present invention will be explained.

C is an effective element for increasing the strength of steel, and the amount of oil added is 0.
．． If it is less than 0.01%, the required strength cannot be secured,
On the other hand, if it exceeds 0.40%, ductility decreases, impairing cold workability and weldability. Therefore, the amount of C added is in the range of 0.01 to 0.40%.

Si is an element necessary for deoxidation during steel manufacturing, and is also an element useful for increasing the strength of steel through solid solution strengthening. If the amount added is less than 0.01%, deoxidation will be insufficient and clean steel will not be obtained. ! It is easy to produce oxides during welding, and if the amount added exceeds 0.50%, cold workability decreases, and so-called Si red scale is likely to occur during hot rolling, which deteriorates the surface of ERW steel pipes. It also causes deterioration of the properties and decrease of fatigue strength. Therefore, the amount of Si added is 0
, O1 to 0.50%.

Mn is a basic element that improves hardenability and strength, and at the same time is useful as an element that prevents hot embrittlement caused by S during hot rolling. If the amount added is less than 0.620%, the increase in strength will be insufficient, and if the amount added exceeds 1.8%, segregation during casting will increase, impairing cold workability. Therefore, the amount of Mn added is in the range of 0.20 to 1.8%.

A1 needs to be added in an amount of at least 0.01% as a deoxidizing element. However, excessive addition of more than 0.1% results in an increase in nonmetallic inclusions. Therefore, the amount of AI added is 0. The range is O1 to 0.1%.

Nb is an effective element for improving strength and toughness in combination with hot rolling conditions. Addition amount is 0.015%
If it is less than that, a sufficient increase in strength cannot be obtained.

On the other hand, if added in excess of more than 0.1%, the hardness of the weld will increase and cold workability will decrease. I wanted to.

Therefore, the amount of Nb added is in the range of o, ois to 0.1%.

Like Nb, Ti is an element effective in improving strength and toughness, and is also an element that controls the shape of sulfide from an elongated shape to a spherical shape. If the amount added is less than 0.01%, sufficient strength and ability to control sulfide morphology cannot be obtained;
．． If added in excess of more than 1%, nonmetallic inclusions in the weld zone will increase and cold workability will deteriorate. Therefore, the amount of Ti added is in the range of 0.01 to 0.1%.

V is an element effective in increasing the strength of steel. Added amount is 0
．． If it is less than 0.02%, the required strength cannot be secured,
On the other hand, if the amount added exceeds 0.1%, ductility will decrease and cold workability will be impaired. Therefore, the amount of V added is in the range of 0.02 to 0.1%.

Ca is an element that has the effect of controlling the shape of sulfides in steel to change them from an elongated shape to a spherical shape, thereby reducing mechanical anisotropy and improving ductility and toughness. In order to effectively obtain such effects, it is necessary to add o, oos% or more, but
If it is added in an amount exceeding 0.005%, non-metallic inclusions in the steel will increase, resulting in a decrease in ductility and toughness. Therefore, the amount of Ca added is in the range of 0°0005 to 0.000%.

Note that impurities in steel such as P, S, 0, N, etc. are harmful to cold workability, so it is desirable that they be as low as possible.

Next, the reason for limiting the width of the white layer in the welded portion will be described.

In the white layer width of the weld cross section, the ratio of the steel pipe front and back sides W and the wall thickness center WM, δ-W, / W8, is 1 to 4.
The reason for limiting this is that δ is 4 in thick wall small diameter ERW steel pipes.
If the
Oxide emissions during welding become insufficient. As a result, a sound weld cannot be obtained, and cold workability and fatigue properties deteriorate. In addition, δ is never less than 1 in normal electric resistance welding, so δ is limited to a range of 1 to 4.Furthermore, the main point of the present invention method, which explains the reason for the limitation of the manufacturing method, is that electric resistance welding The purpose of this method is to create a bevel on the opposing end faces of the open pipe.

The reason why δ becomes large in thick-walled small-diameter ERW steel pipes is that when the opposing end faces of the open pipe are parallel or slightly opened outward, the welding current during welding is concentrated at the corner of the end face of the open pipe. This is because preferential melting at the corners progresses, promoting the formation of white layers on the front and back surfaces. moreover,
The thick wall makes it difficult for the molten metal in the center to be discharged, causing oxides generated during welding to remain in the center. In order to prevent such a phenomenon, grooves are provided on opposing end surfaces of the oven tube.

Figure 4 shows the groove shape of the end face of the oven tube before welding. In the figure, t is the wall thickness of the steel pipe, X, , X is the groove height, θ5
, θ2 respectively indicate the groove angle.

The reason why the groove angles θ1 and θ2 are limited to 20 to 45 degrees is as follows.
This is because if the angle is less than 20 degrees, preferential melting of the corner portions cannot be prevented. Furthermore, if the angle exceeds 45 degrees, bead formation will be insufficient and a sound welded part will not be obtained. Therefore, the groove angle is 20 to 45 degrees and the groove height X, ,X,
The reason for limiting the wall thickness t to 0980 to 0.95 is that 0.
This is because if it is less than 80, bead formation is insufficient and a sound weld cannot be obtained. Moreover, if it exceeds 0.95, preferential melting of the corner portions cannot be prevented and oxides generated during welding cannot be discharged. Therefore, the groove heights X, X2 are set to 0.80 to 0.95 of the wall thickness t.

Next, the reason for limiting the ratio γ between the longitudinal diameter y and the lateral displacement X of the oven tube will be explained.

Figure 5 shows the cross-sectional shape of the oven tube when the back side is not beveled. Let be γ.

Regarding the groove on the back side, the groove may damage the mandrel for supporting the invider, and as a practical matter, it may be difficult to process the groove. We have obtained knowledge from various test results that as an alternative to this, the cross-sectional shape of the open tube may be made elliptical. FIG. 6 shows an example of the results.

The horizontal axis in FIG. 6 shows the ratio T between the vertical diameter and the horizontal width, and the vertical axis shows the ratio δ between the back side width of the white layer and the thickness center width. By controlling the ratio T between the width and width to 0.97 or less, the ratio δ between the back side width of the white layer and the thickness center width can be made 4 or less. That is, in order to make δ 4 or less, the ratio T between the vertical diameter y and the lateral shift X shown in FIG. 5 should be 0.97 or less.

The reason for limiting γ=y/x to 0.85 to 0.97 is that T
is less than 0.85, the rise of the metal flow at the weld on the surface side becomes steep, ductility and toughness decrease, and 0.85 is less than 0.85.
This is because if it exceeds 97, the width of the white layer on the back side becomes wider as described above. Therefore, the ratio γ between the vertical diameter y and the lateral shift X is set to 0.85 to 0.97.

The welding conditions are not limited to long as long as the appearance of a white layer, which is a prerequisite for sound electric resistance welding, is observed, but the welding speed (pipe making speed) is preferably 20 to 80 m/win.

Note that the method for performing the groove processing is not particularly limited, and any method such as cutting, polishing, rolling, etc. may be used.

(Example) Example 1 Thickness 3.0-10.0 containing the chemical components shown in Table 1
A thick-walled small-diameter electric resistance welded steel pipe with an outer diameter of 25.4 to 65.0 m was manufactured using #i4 band of m-porcelain. The details of the manufacturing conditions are shown in Table 2, and the observation results of the white layer width of the welded part are also shown in Table 2.

As shown in Table 1 and Table 2, Example 1 is an example in which grooves were formed on both the front and back surfaces, and the ratio T of the longitudinal diameter to the lateral diameter was 1.0 in both cases. Production Nos. 1 to 9 were made by the method of the present invention, and the ratio δ of the white layer width between the front and back sides and the thick center portion was all within the range of 1 to 4. On the other hand, production numbers 10 to 15 are based on the comparative method; production numbers 10, 13, and 14 have groove height and groove angle; production numbers 11 and 11 have groove height;
, 12.15, the groove angles are outside the limited range of the method of the present invention, so the ratio δ of the white layer width between the front and back sides and the thickness center portion is all over 4.

Furthermore, test pieces were taken from each of the steel pipes with manufacturing numbers 1 to 15 and subjected to a torsional fatigue test. The results are shown in Figure 1.
The horizontal axis in the figure represents the ratio (δ) of the width of the front and back surfaces of the white layer to the thickness center width, and the vertical axis represents the fatigue strength of 10' cycles.
. indicates the method of the present invention, and . indicates the comparative method.

As is clear from the figure, when δ exceeds 4, the width of the white layer on the front and back sides becomes large and the fatigue strength is extremely reduced.

Example 2 Thickness 3.0-10.0 containing chemical components shown in Table 1
Using the I band of ms+, the outer diameter is 25.4 to 65. A thick-walled small-diameter ERW steel pipe with an O-opening was manufactured. Details of the manufacturing conditions are shown in Table 3. Table 3 also shows the observation results of the white layer width of the welded portion.

(The following is a margin) As shown in Table 3, production numbers 16 to 23 of Example 2
These are examples in which groove processing was performed only on the front side by the method of the present invention, and in both cases, the ratio δ of the white layer width between the front and back sides and the thickness center part is within the range of 1 to 4. On the other hand, production No. 24
~26 is a comparative method, and is an example in which no beveling is performed on both the front and back sides, and the groove heights are X, , and X.

is 1.0, the groove angles θ1 and θ2 are 0 degrees, the ratio T between the longitudinal diameter and the lateral shift is in the range of 1.05 to 1.10, and the groove shape and the ratio T between the longitudinal diameter and the lateral shift are in the range of 1.05 to 1.10. Since the method of the present invention is out of the limit range, the ratio δ of the white layer width between the front and back sides and the thickness center portion is both greater than 4.

As is clear from the above two examples, the thick-walled, small-diameter 111 steel pipe with a uniform white layer width and the method for manufacturing the same according to the present invention are as follows: The fatigue strength can be improved by setting the value in the range of 1 to 4.

(Effects of the Invention) As explained above, the thick-walled small-diameter ERW steel pipe with a uniform white layer width and the method for manufacturing the same according to the present invention have the above configuration, so that the width of the white layer at the welded portion is made uniform. It is possible,
This has the excellent effect of producing thick-walled, small-diameter electric resistance welded steel pipes with improved cold workability and fatigue properties.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the ratio of the front and back side widths to the thickness center width of the white layer and the fatigue strength. FIG. 2 shows a schematic diagram of a white layer of a welded part, in which (a) shows an example of a thin-walled small-diameter ERW steel pipe, and (b) shows an example of a thick-walled, small-diameter ERW steel pipe. FIG. 3 is a schematic diagram for quantifying the shape of the welded white layer of a thick-walled, small-diameter electric resistance welded steel pipe. FIG. 4 is a diagram showing the groove shape of the open tube end face before welding. FIG. 5 is a diagram showing the cross-sectional shape of the oven tube before welding when the back side is not beveled. FIG. 6 is a graph showing the relationship between the ratio of the longitudinal diameter and the lateral shift and the ratio of the back side width of the white layer to the thickness center width. 1 - white layer, 2 - heat affected zone, W, - width of front and back white layer, w, - width of white layer at center of wall thickness, XXt - groove height, θ1 θ2 - opening Tip angle, --- wall thickness, y - longitudinal diameter, X - lateral displacement.

Claims (4)

[Claims] (1) C: 0.01-0.40%, Si: 0.01-0
．． 50%, Mn: 0.20-1.8%, Al: 0.01
When manufacturing a steel strip containing ~0.1% and the balance consisting of Fe and unavoidable impurities into a thick-walled small-diameter ERW steel pipe, in the white layer of the cross section of the weld, the front and back sides of the steel pipe and the center of the wall thickness are Let the white layer widths be W_S and W_M, respectively, and the ratio δ=W_
A thick-walled small-diameter electric resistance welded steel pipe with a uniform white layer width, characterized in that S/W_M is 1 to 4. (2) Nb: 0.015~0.1%, Ti: 0.01~
0.1%, V: 0.02-0.1% and Ca: 0.0
The thick-walled small-diameter electric resistance welded steel pipe according to claim 1, containing one or more selected from 0.005% to 0.005%. (3) When welding the thick-walled small-diameter ERW steel pipe of claim (1) or (2), the groove height of the open pipe immediately before welding is set to 0.80 to 0.95 of the wall thickness t, and the groove height A method for producing a thick-walled, small-diameter electric resistance welded steel pipe, the method comprising controlling the angle between 20 and 45 degrees. (4) When welding the thick-walled, small-diameter ERW steel pipe of claim (1) or (2), the groove height of the open pipe immediately before welding is set so that the wall thickness is 0.80 to 0.95 of t, and the open pipe is Tip angle 20-45
degree, the ratio γ = y/x of the vertical diameter y and the horizontal diameter x of the open pipe is 0.
85 to 0.97. A method for manufacturing a thick-walled small-diameter electric resistance welded steel pipe.