JPH04319A - 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 yield point elongation, reduced in yield ratio, and excellent in toughness at low temp. by successively applying heating, square tube forming, cooling, and tempering to a low carbon steel tube under respectively specified temp. conditions. CONSTITUTION:After a low carbon steel tube is heated to a temp. not lower than (Ac3-200 deg.C), square tube forming is started at a temp. not lower than (Ac3-200 deg.C) and finished at a temp. in the region between (Ac3-200 deg.C) and (Ac3-20 deg.C). After forming, the resulting square tube is cooled, without delay, at >=15 deg.C/sec cooling rate and tempered at 200-600 deg.C. Further, the preferred basic composition of the low carbon steel is about 0.03-0.30% C, about 0.02-0.50% Si, about 0.20-2.00% Mn, about 0.001-0.100% Al, and about 0.0005-0.0100% N. By this method, the square tube reduced in yield ratio and having strength as high as >= about 40kgf/mm<2> can be obtained without especially using expensive alloying elements.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a rectangular tube 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 to improve competitiveness and reducing manufacturing costs.

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 this requirement 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.

(Problems to be Solved by the Invention) As a low yield ratio angle pipe for construction, there is a requirement for 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.

Recently, not only the yield ratio but also the shape of the stress-strain curve has attracted attention as a material characteristic of steel materials necessary for earthquake-resistant structures. In other words, it is beginning to be said that in order for steel materials to have sufficient plastic elongation ability, it is necessary to increase Ac as shown in FIGS. 1 and 2. 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, square tubes with elongation at yield point and 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 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.

Furthermore, in order to have a yield point elongation, strain (square tube forming) is applied in the two-phase region between A and Ac3, and the solid solution carbon and nitrogen formed in the ferrite are immediately fixed, and then It was confirmed that a two-phase structure consisting of ferrite and a second phase of carbide is formed by quenching, and a rectangular tube having both a yield point elongation and a low yield ratio can be obtained.

Based on this knowledge, the present invention has a yield point elongation,
It also made it possible to manufacture square tubes with a low yield ratio.
The gist of this is that low carbon steel pipes are made of Ac5-2
Heating to 00°C or above, starting square tube forming at A as-200°C or above, finishing square tube forming at AC3-200 to Ac3-20°C, immediately after forming, or after air cooling after forming,
From the temperature range of Ac5-250 to Ac3-70℃, 15
Cool at a rate of ℃/sec or higher, then cool to 200 to 600℃
This is a method for producing a square tube that has a yield point elongation, a low yield ratio, and excellent low-temperature toughness, characterized by tempering in a temperature range of .degree.

(Function) In the present invention, by raising the heating temperature to AC5-200°C or higher and then performing square tube forming and water cooling, the effects of work hardening in pipe forming can be removed, while new distortion can be created in square tube forming. By immediately fixing the dislocations generated at that time and then rapidly cooling them, they succeeded in achieving a dual-phase steel.

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

First, there are no specific regulations regarding the manufacture of steel pipes, and any type of steel pipe is acceptable. In other words, seamless steel pipe, UO
Any pipe manufacturing method can be used, such as steel pipes, spiral steel pipes, electric resistance welded steel pipes, forge welded steel pipes, etc. 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 after forming was set above Ac3-200 was that by heating to this temperature range, we aimed to achieve duplex steel after cooling and at the same time eliminate forming distortion in steel pipe manufacturing. .

Forming a square tube in that temperature range involves applying strain in the two-phase region, introducing an appropriate amount of dislocations into the ferrite, immediately fixing it with solid solution carbon and solid solution nitrogen, and then forming two by rapid cooling.
This is to give the composite steel an elongation at the yield point.

Square tube forming end temperature A C5-200~A cs -2
The reason for the 0t difference was that the aim was to create a dual-phase steel after cooling, and also to optimize the amount of processing strain in the ferrite. In other words, if A(1) is formed into a square tube and then water cooled, it becomes a two-phase steel, but the ferrite has too much processing strain and the strength of the ferrite is high, making it impossible to achieve a low yield ratio.AcI By cooling from a temperature higher than the middle of ~Ac3, that is, higher than AC3-200℃, this temperature was set as the lower limit because it is possible to achieve both duplex steel production and appropriate strain. As the value is increased, the yield ratio passes the lowest limit and the yield ratio increases.This is because the area ratio of ferrite decreases, and as it approaches Ac3, the yield ratio increases rapidly. This is because the area ratio of ferrite approaches zero.For this reason, the upper limit of the square tube forming temperature is set to Ac3-20.
℃ was set.

A C5-200~A C320℃ After forming a square tube, cooling is performed to austenite during reheating, and the C-enriched portion becomes a quenched structure, which hardens the tube sufficiently, increasing tensile strength and achieving a low yield ratio. It's to get it. If the cooling is insufficient, the quenched structure will not be hardened sufficiently and a low yield ratio will not be obtained. As long as the speed is secured, the method is not a problem.

By the way, depending on the type of steel, there are some steels whose toughness is not good only by heating and then quenching, and it may be necessary to perform a tempering treatment after quenching in order to improve the toughness. At that time, the tempering temperature should be determined for 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. In other words, since this causes an increase in the yield ratio, the upper limit was set at 600 t. However, if the tempering temperature is low and is less than 200t, the tempering effect will be almost gone and the toughness may not be improved, so the adjustment was made to be 200t.

In addition, during the period after forming the square tube until quenching (for example, due to equipment constraints)
In some cases, it may be necessary to use air cooling. In that case, if you air-cool too much, the introduced dislocations will disappear,
There is no point in forming square tubes in the two-phase region. Therefore, if air cooling treatment is used, AC3-200-A cs 20
Complete strain imparting at ℃ and after air cooling ACs-250~AC
It was decided to rapidly cool from 3-70°C.

The present invention can be applied to low carbon steel with good results. A preferred component composition is C: 0.03 to 0.3.
Low carbon steel whose basic components are 0% Si: 0.02-0.50% Mn: 0.20-2.00% A-cross, 0.001-0.100% N: 0.0005-0.0100 , or depending on the required characteristics of the 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
．． The upper limit of the content is set at 2.0% because there is almost no increase in strength even if it is added in excess of 0%.

N1 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 impedes weldability, so the upper limit of the content is 2.0%.

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

(2) is useful for precipitation strengthening, but if too large, it impedes weldability, 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 square tube, heat treatment conditions, and mechanical properties of the obtained steel tube.

Steel pipe No. shown in Table 2, A1. B1. C1, Di,
El, Fl.

G1. Hl, 11. Jl, 1. Ll, Ml, N1,0
1. PI, Ql, R1, Sl, TI.

IJI 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, A2 has a high yield ratio because the heating temperature is too high. A3 has a high yield ratio because the heating temperature is too low. A4 has a high yield ratio because the cooling rate after heating is insufficient. A5 has a high yield ratio because the tempering temperature is too high.

Further, since the tempering temperature of B2 is too low, the 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 heating 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/nun2 or more without using any particularly expensive alloying elements. Therefore, the industrial effect is great.

[Brief explanation of drawings]

Figure 1 shows an example of the SS curve when the area of Ac is small due to low YR but no elongation at yield point, and Figure 2 shows an example of the SS curve when the area of Ac is small due to low YR and elongation at yield point. It is a figure which shows the example of the SS curve when it becomes large.

Claims (1)

[Claims] 1. Heating a low carbon steel pipe to A_c_3-200°C or higher, starting square tube forming at A_c_3-200°C or higher,
_c_3-200~A_c_3-Yield point characterized by finishing square tube forming at 20°C, cooling at a rate of 15°C/sec or more immediately after forming, and then tempering at a temperature range of 200-600°C. A method for manufacturing square tubes that have elongation, a low yield ratio, and excellent low-temperature toughness. 2 Heat the low carbon steel pipe to A_c_3-200°C or higher, start square tube forming at A_c_3-200°C or higher, and
Finish the square tube forming at _c_3-200 to A_c_3-20℃, then air cooling, and continue to A_c_3-250.
~A_c_3 - Cooling at a rate of 15 °C/sec or more from a temperature range of 70 °C, and then tempering at a temperature range of 200 to 600 °C, having a yield point elongation and a low yield ratio, A method for manufacturing a square tube that also has excellent low-temperature toughness.