JPH03219019A - Production of square tube having yield point elongation, reduced in yield ratio, and excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a square tube having high strength and low yield ratio at a low cost by heating a low carbon steel tube at specific temp., forming this steel tube into a square tube, air-cooling this square tube, and successively carrying out rapid cooling, application of cold working strain, and tempering under respectively specified conditions. CONSTITUTION:A low carbon steel tube is heated up to >=Ac3, formed into a square tube at >=Ac3, air-cooled, and cooled rapidly from a temp. region between (Ar3-250 deg.C) and (Ar3-20 deg.C) at >=15 deg.C/sec cooling rate. Subsequently, cold working strain is applied to this steel tube so that the total amount of strain equivalent to that in the longitudinal direction is regulated to >=0.05%, and further, tempering is performed at 200-600 deg.C. By this method, the square tube having yield point elongation, reduced in yield ratio, and excellent in toughness at low temp. can be obtained.

Description

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

[Prior Art] In recent years, there has been an increasing demand for various types of yuzu, such as improvements in usage characteristics and reductions in manufacturing costs, in order to improve competitiveness in all fields that handle steel materials.

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
i: 0.26-0.48%), Can from the viewpoint of weldability
It cannot be applied to architectural structures where upper limits are 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 the work hardening of the steel, the amount of strain is considerably limited.
In actual operation, it is quite difficult.

[Problems to be Solved by the Invention] There is a requirement for a low yield ratio angle tube for construction to have a tensile strength of 40 kg or more and a yield ratio of 75% or less, but it is impossible to manufacture with the current manufacturing method. In other words, in the non-tempered type, the so-called azurol type, which is manufactured by simply forming a hot coil into a round shape, the structure is due to work hardening during forming, and in the tempered type, so-called QT type, the structure becomes tempered martensite. , a yield ratio of 75% or less has not been achieved.

Furthermore, recently, not only low yield ratios but also the shape of stress-strain curves have been specified for use in earthquake-resistant structures. In other words, the larger the area of 80 shown in Figures 1 and 2, the greater the plastic elongation ability and the harder it is to reach fracture. At this time, it is clear that the one shown in Fig. 2 is better for use in earthquake-resistant structures, but in this case, it can be said that Ae is almost determined by the yield ratio and the elongation at yield point. In other words,
Steel materials with low yield ratio and elongation at yield point are beginning to be required for earthquake-resistant structures.

[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 of carbide was confirmed. Furthermore, it was confirmed that in order to lower the yield ratio, it is important to lower the yield point and increase the tensile strength.

The present invention is based on this knowledge and has made it possible to manufacture square tubes with a low yield ratio. Form into a square tube, then air cool ^, 3'-250 ~
^, Cooled to 3-20℃, then rapidly cooled to e15℃ at a cooling rate of ec or more, then cold strain of 0.05% or more was applied, and then the temperature was increased to 200-600℃. This is a method for manufacturing a square tube having a yield point elongation, a low yield ratio, and excellent low-temperature toughness, which is characterized by tempering within a range.

[Function] In the present invention, the distortion caused by vibrator molding is completely removed by raising the heating temperature to 0 or more, and by forming the square tube at that temperature, the distortion is instantly removed.
By air-cooling to a high temperature between the ^cl and ^c3 transformation points, and then rapid cooling, they have succeeded in achieving duplex steel.

Furthermore, by applying cold processing strain,
Dislocations are introduced into the structure (ferrite), and then tempered to fix the dislocations with solid solution nitrogen and solid solution carbon.
We have succeeded in creating yield point elongation.

Furthermore, by softening the second phase part through tempering, we succeeded in sufficiently recovering the low-temperature toughness.
By lowering this tempering temperature, the synergistic effect of not softening the second phase part more than necessary makes it possible to manufacture square tubes with yield point elongation, low yield ratio, and excellent low-temperature toughness. It made it possible.

Next, the conditions for manufacturing, heating, square tube forming, cooling, imparting cold strain, and tempering the steel pipe of the present invention will be described.

First, there are no particular regulations regarding the manufacture of steel pipes, and any method is acceptable. For example, steel pipes can be classified into seamless steel pipes, electric resistance welded steel pipes, UO steel pipes, spiral steel pipes, forge-welded pipes, etc., depending on the manufacturing method. The present invention is acceptable with any of these manufacturing methods. 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 heating temperature was set to Ac3 or higher was to completely eliminate forming distortion in steel pipe manufacturing. Also, forming a square tube at Ar3 or higher is also aimed at instantaneous removal of distortion in forming a square tube. After that, it is air cooled to bring the cooling start temperature to A.
, -250~A, 3~20°C is because by cooling from this temperature range, it was aimed to achieve duplex steel after cooling. In other words, when quenching directly above Art, it becomes a two-phase steel, but the area ratio of ferrite increases and the yield ratio decreases, but as the area ratio of the second phase decreases, the tensile strength decreases and the desired strength is not achieved. You will not be able to obtain Higher temperature than the middle between Arl and Ar3, that is, A
By cooling from a temperature higher than 3-250° C., this temperature was set as the lower limit because both the dual-phase steel structure and the strength could be achieved. As the cooling start temperature is increased, the yield ratio passes through the lowest limit and the yield ratio increases. This is because the area ratio of ferrite decreases, and as it approaches hardness, the yield ratio increases rapidly. This is because the area ratio of ferrite approaches zero. From this, the upper limit of the heating temperature was set at 8r3-20°C.

A, rapid cooling from 3-250 to ^r3-20 is made into austenite after heating and air cooling, and the C-enriched part becomes a quenched structure, which hardens it sufficiently to increase the tensile strength and obtain a low yield ratio. It's for a reason. If the rapid cooling is insufficient, the quenched structure will not harden sufficiently, resulting in a low yield ratio.
The cooling rate was set at 15° C./sec or higher. As for cooling, water cooling is usually used, but the method does not matter as long as the cooling rate can be secured.

There are no particular regulations regarding imparting cold forming strain after rapid cooling. Usually, this is a sizing process, but any method may be used as long as it introduces a strain of 0.05% or more and can secure the shape of a square tube. Here, the strain of 0.05% is the total amount of longitudinal equivalent strain.

Tempering is the process of replacing dislocations introduced during cold square tube forming with solid solution nitrogen,
This is done to fix it with solid solution carbon to give it elongation at yield point and to improve toughness. At this time, the tempering temperature should be determined as follows: Regarding the two-phase structure of ferrite and second phase carbide, if the second phase part, which has been sufficiently hardened by the previous rapid cooling, is tempered at too high a temperature, it will become too soft, resulting in a decrease in tensile strength. Since this causes an increase in yield ratio, the upper limit was set at 600°C. 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 with good results. A preferable component composition is C: 0. (13~
0.30% St: 0.02-0.50% Mn: 0.20-2.00%^1: 0.001-0.100% N: 0.0005-0.0100% as basic components Depending on the required characteristics of low carbon steel or strength steel in addition to the above basic components, Cu: 2.0% or less Ni: 9.5% or less Cr: 5.5% or less Mo: 2.0% or less Nb: 0. 15% or less V: 0.3% or less Ti: 0.15% or less B: 0.0003 to 0.0030% Ca:
One or more types may be added in an amount of 0.0080% or less.

Cu is added because it is useful for increasing strength and improving corrosion resistance, but 2
．． Even if it is added in excess of 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%.

Mo is useful for increasing strength, but if too much Mo inhibits weldability, the upper limit of the content is 2.0%.

Nb is added because it is useful for refining austenite grains and increasing strength, but if too much, it impedes weldability, so the upper limit of the content is set to 0.15%.

(2) is useful for precipitation strengthening, but in large amounts it inhibits welding, so the upper limit of content is 0.3%.

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 %.

B has the effect of significantly increasing the hardenability of steel when added in a small amount. In order to effectively obtain this effect, it is necessary to add at least 0.0003%. However, if added in excess, B compounds are generated and the toughness is deteriorated, so the upper limit is set to 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, A1, BL, CI, D
I, El, Fl.

G1. )11.11. Jl, Kl, Ll, Ml, N1.
Of, Pl, Ql, R1, Sl, Tl.

01 and Vl are steels according to the present invention, which have achieved the low yield ratio (yield ratio of 70% or less) targeted by the present invention.

On the other hand, since the amount of strain in A2 is zero, the yield ratio is low, but no elongation at the yield point has been achieved. A3 has a high yield ratio because the cooling start temperature is too high. A4 has no strength because the cooling start temperature is too low. A5 has a high yield ratio due to insufficient cooling rate. A6 has a high yield ratio because the tempering temperature is too high.

In addition, B2 has a too low tempering temperature, so low-temperature toughness is not improved. C2 has a high yield ratio due to insufficient cooling rate. B2 has a high yield ratio because the superheating temperature is too low.

[Effects of the Invention] As explained in detail above, the present invention makes it possible to inexpensively manufacture a low yield angle tube having a high strength of 40 kgf/mm" or more without using any particularly expensive alloying elements. And, industrially, the effect is great.

[Brief explanation of drawings]

Figure 1 shows that Ac
FIG. 2 is a diagram showing an example of the SS curve when the area of 8c is large due to low YR and yield point elongation. 4 other people

Claims (1)

[Claims] 1. Heat a low carbon steel pipe to a temperature of A_c_3 or higher, and heat it to A_r.
Form the steel pipe into a square pipe at _3 or higher, then air cool it to A_r
_3-250~A_r_3-20℃ to 15℃/sec
After rapid cooling at a cooling rate of 0.05% or more, cold processing strain is applied, and further tempering is performed in a temperature range of 200 to 600°C. A method for manufacturing square tubes with low toughness and excellent low-temperature toughness.