JPH08232041A - High toughness hot rolled steel plate for square steel tube, excellent in galvanizing crack resistance, and high toughness square steel tube - Google Patents

Abstract

PURPOSE: To produce a square steel tube excellent in galvanizing crack resistance and having high strength and toughness by applying hot rolling to a steel having a composition in which respective contents of C, Si, Mn, Nb, V, and Ti are specified. CONSTITUTION: A steel, having a composition consisting of, by weight, 0.04-0.12% C, <=0.05% Si, 0.2-2.0% Mn, further one or >=2 kinds among <=0.08% Nb, <=0.08% V, and <=0.05% Ti, and the balance Fe with inevitable impurities, is hot-rolled, by which a hot rolled steel plate for use in square steel tube forming is prepared. This hot rolled steel plate is formed into square-shaping roll, and the resulting square steel tube is hot-dip-galvanized. The resulting square steel tube has excellent strength and toughness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet for a high toughness square steel pipe and a high toughness square steel pipe which are excellent in Zn plating cracking resistance.

[0002]

2. Description of the Related Art Square steel pipes are widely used mainly as steel materials for structures. This square steel pipe may be subjected to hot dip Zn plating if aesthetics or rust prevention is required.
When immersed in an n-plating bath, cracks may occur on the inner surface of the corners, which is a problem.

Generally, a square steel pipe having a problem of Zn plating cracking is manufactured by first forming a hot-rolled steel sheet into a circular shape by roll forming into an electric resistance welded steel pipe, and then continuously forming the electric resistance welded steel pipe into a square shape. To be done. As a result, residual tensile stress is generated in the circumferential direction of the inner surface of the pipe corner. And
The above-mentioned cracks occur due to the residual tensile stress at the corners of the tube and the liquid metal embrittlement at the grain boundaries due to Zn invading during Zn plating.

As a similar phenomenon, there is cracking in the weld heat affected zone (HAZ) which occurs when the steel for welded structure is assembled by welding and then hot-dip Zn plating is performed.
This crack also causes residual stress due to welding and Z to the grain boundary.
It is caused by the action of liquid metal embrittlement due to the penetration of n. However, this crack is different from the crack of a square steel pipe, in which the stress acting is the residual stress due to welding, which is caused by the residual stress generated on the inner surface of the pipe corner portion due to the pipe making.

Although many measures have been taken against plating cracks that occur in the HAZ of steel for welded structures, there are still few measures against cracks in square steel pipes. As a countermeasure currently taken, there is a method of removing residual stress in the corner portion by heat treatment. Further, as described in JP-A-64-56853, it is said that zinc plating cracks can be prevented by limiting the amounts of C, Si, and Mn in steel materials.

[0006]

However, the method of removing the residual tensile stress on the inner surface of the pipe corner by heat treatment is expensive.

As a method for preventing Zn plating cracks due to residual tensile stress generated on the inner surface of a pipe corner during square roll molding, cracking occurs even if the conventional method described in JP-A-64-56853 is diverted. I have something to do. Also,
Since not only Zn plating cracking property but also good toughness are required in the structural design, the conventional method of limiting the C content to 0.13% by weight or more has a problem that sufficient toughness cannot be secured especially at the corners of the pipe. is there.

According to the present invention, a hot rolled steel sheet is formed into a square roll,
Further, it is an object of the present invention to provide a square steel pipe manufactured by performing hot dip galvanizing, which can be manufactured at low cost, has excellent Zn plating cracking resistance, strength, and toughness.

[0009]

The present invention according to claim 1 is a hot-rolled steel sheet used for forming a square steel pipe, wherein C:
0.04 to 0.12% by weight, Si: 0.05% by weight or less, Mn: 0.2
~ 2.0 wt%, further Nb: 0.08 wt% or less,
V: 0.08% by weight or less and Ti: 0.05% by weight or less are contained by one or two or more kinds, and are made by hot rolling a steel containing the balance Fe and unavoidable impurities.

A high toughness square steel pipe according to a second aspect is a hot-rolled steel sheet according to the first aspect, which is formed into a square roll, and after the square steel pipe is manufactured, hot dip galvanizing is performed.

[0011]

The present inventor studied the composition of the hot-rolled steel sheet that affects the resistance to Zn plating cracking, and obtained the following findings. C: 0.04 to 0.12 wt% C is a component that helps increase the strength of steel most simply, and if it is less than 0.04 wt%, its effect cannot be expected.
On the other hand, if the content exceeds 0.12% by weight, the toughness decreases, so C
Is 0.04% by weight or more and 0.12% by weight or less.

That is, C was changed in the range of 0.01 to 0.18% by weight, Si: 0.02% by weight, Mn: 1.0% by weight, P: 0.01
3% by weight, S: 0.005% by weight, Nb: 0.03% by weight, hot rolled steel sheet (sheet thickness 16 mm) is roll-formed into square electric resistance welded steel pipes (one side length is 350 mm), and the pipe corners are manufactured. Part of 0 ℃
The relationship between the Charpy absorbed energy vEO and the amount of C (Fig. 1) was investigated. In addition, the tensile properties of the flat portion of the tube in the L direction (Fig. 1) were investigated. From Figure 1, C: 0.04 to 0.12 wt%
As a result, the square steel pipe corner has a v of approximately 30 J or more.
It is possible to secure a tensile strength of 500 MPa or more in the EO and the flat portion of the pipe.

Si: 0.05 wt% or less Si is the most important element in the present invention, and if the content of Si is 0.05 wt% or less, the Zn plating crack resistance is improved. % Or less.

That is, Si is changed in the range of 0.01 to 0.08% by weight, C: 0.08% by weight, Mn: 1.0% by weight, P: 0.01
3% by weight, S: 0.005% by weight, Nb: 0.03% by weight, hot rolled steel sheet (sheet thickness 16 mm) is roll-formed into a square electric resistance welded steel pipe (one side length is 350 mm), and the pipe corner portion is manufactured. 0 ℃
The relationship between the Charpy absorbed energy vEO and the amount of Si (Fig. 2) was investigated. At the same time, this square steel pipe was subjected to hot dip Zn plating at 455 ° C., and the presence or absence of cracking was investigated (FIG. 2). From FIG. 2, it is possible to improve both the toughness of the corner portion of the square steel pipe and the resistance to Zn plating cracking by setting Si: 0.05% by weight or less.

Mn: 0.2-2.0 wt% Mn is added to secure the strength, and to obtain its effect.
0.2 wt% or more is required. On the other hand, if it exceeds 2.0% by weight, weldability and toughness are adversely affected, so Mn is 0.2
~ 2.0% by weight.

Nb: 0.08% by weight or less, V: 0.08% by weight or less, Ti: 0.05% by weight or less Nb, V, and Ti have the effects of increasing the strength and toughness by making the crystal grains finer. However, Nb and V are 0.08% by weight, Ti
When the content of Ni exceeds 0.05% by weight, the precipitates coarsen and the toughness rather decreases, so Nb and V are 0.08% by weight.
Hereinafter, Ti is set to 0.05% by weight or less.

[0017]

EXAMPLE Using the hot rolled steel sheets of Examples and Comparative Examples of the present invention, square steel pipes were manufactured by a roll forming method. A tensile test in the L direction of the flat portion and a Charpy test in the L direction of the corner portion at 0 ° C were performed. In addition, a plating cracking test was conducted in which the actual pipe was pickled and fluxed, and then immersed in a molten Zn plating tank at 455 ° C for 180 seconds.

As shown in the results summarized in Table 1, no Zn plating cracks occurred in the examples of the present invention. In addition, the tensile strength of the flat part is 500 MPa or more and 490 MP
It has strength as a grade, and the Charpy absorbed energy at 0 ° C of the corners is 30 J or more, and the toughness is good.

[Table 1]

The embodiment of the present invention has been described in detail above with reference to the drawings. However, the specific structure of the present invention is not limited to this embodiment, and changes in design within the scope not departing from the gist of the present invention can be made. Even if it exists, it is included in the present invention. For example, the square steel pipe of the present invention is not limited to one in which the hot-rolled steel plate is made into a circular electric resistance welded steel pipe and then made into a square steel pipe, and the hot-rolled steel plate may be immediately formed into a square shape.

[0020]

As described above, according to the present invention, in a square steel pipe manufactured by forming a square-rolled hot-rolled steel sheet and further subjecting it to hot-dip Zn plating, the cost is excellent and the Zn plating cracking resistance is excellent. It is possible to manufacture a square steel pipe having excellent strength and toughness.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the toughness of a corner portion of a square steel pipe and the C content.

FIG. 2 is a diagram showing the relationship between the toughness of the corner portion of a square steel pipe, the resistance to Zn plating cracking, and the amount of Si.

Claims (2)

[Claims] 1. A hot-rolled steel sheet used for forming a square steel pipe, wherein C: 0.04 to 0.12 wt%, Si: 0.05 wt% or less, and Mn:
Steel containing 0.2 to 2.0% by weight, Nb: 0.08% by weight or less, V: 0.08% by weight or less, Ti: 0.05% by weight or less, and a balance Fe and inevitable impurities. Z resistance, characterized by being manufactured by rolling
n High-toughness hot-rolled steel sheet for square steel pipe with excellent cracking resistance. 2. A high toughness square steel pipe excellent in Zn plating cracking resistance, which is obtained by forming the square steel pipe by hot-rolling the hot-rolled steel sheet according to claim 1 and then performing hot dip galvanizing.