JPH04176818A - Production of steel tube or square tube excellent in earthquake resistance - Google Patents

Abstract

PURPOSE:To inexpensively produce a steel tube or square tube having high strength and low yield ratio and excellent in earthquake resistance by subjecting a steel tube or square tube of low carbon steel or low carbon log alloy steel to forming at a temp. not lower than the Ac3 point and successively to controlled cooling. CONSTITUTION:A steel tube or square tube of low carbon steel or low carbon low alloy steel is heated to >=Ac3 and formed into a round tube or square tube in the above temp. range. Successively, this steel tube or square tube is cooled slowly at <=10 deg.C/sec cooling rate and subjected, if necessary, to tempering at 200-600 deg.C. The structure can be formed into a dual phase structure of clean ferrite + pearlite while removing the influence of forming by subjecting the above steel tube of square tube to slow cooling after forming, and further, the excessive softening of the secondary phase can be prevented by lowering the subsequent tempering temp. By this method, the steel tube or square tube having strength as high as >= about 40kgf/mm<2>, reduced in yield ratio, and excellent in earthquake resistance can be obtained while obviating the necessity of using particularly expensive alloying elements.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for manufacturing steel pipes or square pipes with a low yield ratio.

[Prior Art] In recent years, various demands have been increasing across various fields that handle steel materials, such as improving usage characteristics and reducing manufacturing costs in order to improve competitiveness.

Among these, in the field of construction, it is desired to reduce the yield ratio in order to improve the safety of structures, especially in order to improve their seismic resistance. Until now, this requirement has been strong mainly in the thick plate field, but recently it has become stronger in the steel pipe field. Various methods have been studied for manufacturing thick steel plates with a low yield ratio, but unfortunately, in the field of steel pipes, there are currently almost no examples of such methods being studied, at least for construction purposes. For example, ERW steel pipes are manufactured by forming hot coils, but the yield ratio increases due to work hardening during forming.
It is considered to be a disadvantageous manufacturing method for manufacturing steel pipes with a low yield ratio. For example, there is a method for manufacturing ERW steel pipes for oil wells with a low yield ratio in JP-A-57-16118, but in this method a considerable amount of C is added to achieve a low yield ratio (C
amount: 0.26-0.48%), from the viewpoint of weldability
It cannot be applied to architectural structures where the upper limit of q is specified. Similarly, there is Japanese Patent Application Laid-Open No. 57-16119 as a manufacturing method for low yield ratio, high tensile resistance welded steel pipes, but this method involves manufacturing extremely low YR steel at the hot coil stage, and then manufacturing ERW steel pipes. In order to suppress work hardening, the amount of strain is considerably limited, but this is quite difficult or difficult in actual operation.

[Problem to be solved by the invention] As a low yield ratio steel pipe for construction or a square pipe, the tensile strength is 4
There is a requirement for a yield ratio of 80% or less at a temperature of 0 kg or more, but this is impossible with current manufacturing methods. In other words, in the non-thermal type, so-called azurol type, which is manufactured by simply molding a hot coil into a round shape, due to work hardening during molding,
In addition, in the tempered type so-called QT type, the structure becomes tempered martensite, so a yield ratio of 80% or less cannot be achieved.

In addition, recently, attention has been paid not only to the yield ratio but also to the shape of the stress-strain curve as a material characteristic of steel materials required for earthquake-resistant structures. In other words, in order for the steel material to have sufficient desired elongation ability, A shown in Figures 1 and 2 must be achieved. It is beginning to be said that there is a need for an increase in This can be achieved not only by lowering YR, but also by increasing elongation at yield point. As is clear from comparing Figures 1 and 2, it can be said that steel materials as shown in Figure 2 are used for earthquake-resistant structures. In other words, for earthquake-resistant structures, steel pipes or square pipes that have elongation at the yield point and a low yield ratio are required.

[Means for Solving the Problems] Therefore, the present inventors conducted numerous experiments and detailed studies in order to reduce the yield ratio. The necessity of creating a two-phase structure consisting of ferrite and a second phase (pearlite) was confirmed. moreover,
It was also confirmed that in order to lower the yield ratio, it is important to lower the yield point and increase the tensile strength. It was also confirmed that in order to lower the yield point and achieve sufficient elongation at the yield point, it is important to make the ferrite structure a clean ferrite without distortion.

Based on this knowledge, the present invention has made it possible to manufacture steel pipes or square pipes with excellent earthquake resistance. ) is heated to AC3 or above and formed into a round tube in that temperature range, or from a round tube to a square tube, and then cooled at a cooling rate of 10°C/sec or less, and then heated to 200°C as necessary. This is a method for manufacturing steel pipes or square pipes with excellent seismic properties, characterized by tempering at a temperature range of ~600°C.

[Function] In the present invention, the heating temperature is set to the AC3 transformation point or higher,
By then slowly cooling it, we succeeded in creating a dual-phase steel of clean ferrite and pearlite while removing the effects of pipe forming and subsequent hot square tube forming.

Furthermore, by lowering the tempering temperature, the synergistic effect of not softening the second phase part more than necessary,
This makes it possible to manufacture steel pipes or square pipes with excellent seismic properties that have a low yield ratio and elongation at yield point.

Next, the conditions for manufacturing steel pipes, forming square pipes, heating, cooling, and tempering according to the present invention will be described.

First, regarding the manufacturing of steel pipes and the subsequent forming of square pipes,
There are no particular regulations and any method is acceptable. For example, steel pipes are manufactured using different manufacturing methods, such as seamless steel pipes, electric resistance welded steel pipes,
It can be classified into UO steel pipes, spiral steel pipes, ferruled pipes, etc.
The present invention is applicable to any of these manufacturing methods. Of course, it is also acceptable to directly form a square tube from a plate such as a hot coil and weld it. This is to set the heating temperature in the subsequent heat treatment to a temperature at which processing strain is removed.

Next, the reason why the post-forming heating temperature was set to AC3 or higher was that by heating to this temperature range, it was possible to achieve duplex steel after cooling and simultaneously eliminate forming distortion. In other words, the heating temperature is 8. .

If the steel is made below, although it becomes a dual-phase steel, the strength of the ferrite is high because of residual working strain in the ferrite, and as a result, a low yield ratio cannot be achieved.

Molding at AC3 or higher eliminates processing strain instantly, so there is no problem with the material quality of the final product. Therefore, any type of molding, such as sizing treatment, drawing treatment, and molding from a round tube to a square tube, is acceptable.

The reason why the cooling rate after heating to AC3 or higher is set to 10℃/sec or less is that the tensile strength is increased by converting the C-enriched part into austenite during reheating to create a two-phase structure of clean ferrite and pearlite. This is to obtain a low yield ratio. If the cooling rate is high, the transformation from austenite to ferrite and pearlite will be insufficient, resulting in a structure containing bainite and martensite in the $2 phase.
Although the yield ratio is low, there is no elongation at the yield point, resulting in a material with deteriorated seismic properties. Air cooling is usually adopted as it is sufficient if the cooling rate is 10° C./sec or less, but any method may be used as long as the cooling rate is satisfied.

By the way, depending on the type of steel, there are some steels whose toughness is not good only by slow cooling after heating, and it may be necessary to perform tempering treatment after cooling in order to improve the toughness. At this time, the tempering temperature is determined as follows: Regarding the two-phase structure of ferrite and second phase pearlite, if pearlite, which has been hardened to some extent by air cooling, is tempered at too high a temperature, it will become too soft, resulting in a decrease in tensile strength, or a decrease in yield ratio. The upper limit was set at 600°C to avoid this.

However, if the tempering temperature is too low to be less than 200°C, the tempering effect will be almost gone and the toughness may not be improved, so the lower limit was set at 200°C.

The method of the present invention can be applied to low carbon steel or low carbon low alloy steel to which special elements are added with good results. A preferred component composition is C: 0.03 to 0.30.
% St: 0.02-0.50% Mn: 0.2 (1-2.00% An: 0.001-0.100% N: 0.0005-0.0100% Low carbon steel , or depending on the required properties of the strength steel in addition to the above components: Cu: 2.0% or less Ni: 9.5% or less Cr: 5.5% or less Ti: 0.15% or less B: 0.0003-0. 0030%Ca: 0.
0080% or less of one type or two or more types may be added.

Cu is added because it is useful for increasing strength and improving corrosion resistance, but
Even if it is added in excess of 2.0%, there is almost no increase in strength, so the upper limit of the content is set at 2.0%.

Ni is added because it is useful for improving low-temperature toughness, but since it is an expensive element, the upper limit of the content is 9.5%.

Cr is added because it is useful for increasing strength and improving corrosion resistance, but if it increases, it impedes low temperature toughness and weldability, so the content should be 5.
The upper limit is 5%.

Ti is added because it is useful for refining austenite grains, but if it increases, it inhibits weldability, so the content is 0.15
The upper limit is %.

When added in a small amount, B has the effect of significantly increasing the hardenability of steel. In order to effectively obtain this effect, it is necessary to add at least 0.0003%. However, if added in excess, B compounds will be generated and the toughness will deteriorate, so the upper limit is set at 0.0030%.

Ca is added because it is useful for controlling the form of sulfide-based inclusions, but if it increases, it forms inclusions in the steel and deteriorates the properties of the steel, so the upper limit of the content is set at o, ooao%.

[Example] Table 1 shows the chemical composition of the test materials, and Table 2 shows the size of the steel pipe or square pipe, heat treatment conditions, and mechanical properties of the obtained steel pipe.

Steel pipe No. shown in Table 2, AI, Bl, C1, Dl,
El, Fl, Gl, Hl, T1, Jl, K1, Ll,
Ml, N1.01, Pl, Ql, R1,51, T1, U
1 and vl are steels according to the present invention, which have achieved the low yield ratio (yield ratio of 80% or less) targeted by the present invention.

On the other hand, A2. A3 has a high yield ratio because the heating temperature is too low (ferrite is not completely clean). A4 has a low Ac because the cooling rate after heating is too high (because no elongation at yield point has been achieved).

A5 has a high yield ratio because the tempering temperature is too high.

Further, B2 has a high yield ratio because the tempering temperature is too high. C2 has a low Ac because the cooling rate is too high. B2 has a high yield ratio because the heating temperature is too low.

[Effects of the Invention] As explained in detail above, the present invention enables low-yield ratio steel pipes or rectangular pipes with high strength of 40 kgf/mm' or more to be manufactured at low cost without using particularly expensive alloying elements. As a result, the industrial effects are significant.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are diagrams showing the stress-strain relationship of steel materials. Figure 1 Figure 2

Claims (1)

[Claims] 1. Low carbon steel or low carbon low alloy steel pipe or square pipe,
After heating to A_C_3 or higher and molding in that temperature range,
A method for producing a steel pipe or square pipe with excellent seismic properties, characterized by successively cooling at a cooling rate of 10° C./sec or less. 2 Low carbon steel or low carbon low alloy steel pipe or square pipe,
After heating to A_C_3 or higher and molding in that temperature range,
A method for manufacturing a steel pipe or square pipe with excellent seismic properties, which comprises subsequently cooling at a cooling rate of 10°C/sec or less, and then tempering at a temperature range of 200 to 600°C. 3 Low carbon steel or low carbon low alloy steel pipe, A_C_3
A method for producing a square tube with excellent seismic properties, which comprises heating to a temperature above and forming a round tube into a square tube within that temperature range, followed by cooling at a cooling rate of 10° C./sec or less. 4 Low carbon steel or low carbon low alloy steel pipe, A_C_3
Earthquake-resistant, characterized by heating the round tube to a temperature above and forming the round tube into a square tube within that temperature range, followed by cooling at a cooling rate of 10°C/sec or less, and then tempering at a temperature range of 200 to 600°C. A method for manufacturing square tubes with excellent properties.