JPH03219016A - 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 subjecting a low carbon steel tube to heating, to air cooling, and to raid cooling under respectively specified conditions, cold-forming this steel tube into a square tube, and then carrying out tempering in a specific temp. region. CONSTITUTION:A low carbon steel tube or a low carbon low alloy steel tube is heated up to >=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, this steel tube is cold-formed into a square rube so that elongation in the longitudinal direction is regulated to about + or -3.0%, 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 (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. 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: 0.26-048%), Ceq 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) 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.

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 area of Ac shown in Figures 1 and 2 is larger,
It has a greater plastic elongation capacity and is less likely to break. 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 Ac is almost determined by the yield ratio and the elongation at yield point. In other words, steel materials with a low yield ratio and elongation at yield point are beginning to be required for use in 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.

Based on this knowledge, the present invention has made it possible to manufacture square tubes with a low yield ratio.The gist of the present invention is to heat low carbon steel or low carbon low alloy steel tubes to Ac3 or higher. , then air-cooled to A r3-250 to A r3-2
Rapid cooling from 0°C at a cooling rate of 15°C/sec or more, then cold forming into a square tube, and then 200 to 600°C
This is a method for producing a square tube having a yield point elongation, a low yield ratio, and excellent low-temperature toughness, which is characterized by tempering in a temperature range of .

(Function) The present invention is characterized in that the heat treatment conditions are specified and square tube forming is performed between two types of heat treatment: quenching after heating in a two-phase region and tempering. First, let's talk about the heat treatment conditions.The heating temperature is A. By setting the value to 3 or more, distortion during pipe forming can be completely removed, and A (1 to Ac!
By air cooling to a temperature higher than the transformation point and then rapidly cooling, we have succeeded in achieving duplex steel.

Furthermore, by cold forming a square tube, dislocations are introduced into the structure (ferrite), and then tempering is performed to fix the dislocations with solid solution nitrogen and solid solution carbon, giving it yield point elongation. It has been extremely successful.

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 produces square tubes with yield point elongation, low yield ratio, and excellent low-temperature toughness. This is what made it possible.

Next, we will discuss the advantages of forming square tubes between two types of heat treatment: rapid cooling after heating in the two-phase region and tempering. First of all, by forming a rectangular tube between the two types of heat treatments, elongation at yield point can be imparted. There is also a method of performing these two types of heat treatment after forming a square tube, but this method lowers the yield ratio, but does not allow the product to have elongation at the yield point. Secondly, it is advantageous that the square tube is formed cold. There is a method of achieving both a low yield ratio and yield point elongation by rapidly cooling after forming a square tube in the two-phase region, but this method involves forming a square tube at a warm temperature, making temperature control extremely difficult. Since the present invention can distinguish between heat treatment and square tube forming, it is easy to apply industrially.

Next, the conditions for manufacturing, heating, square tube forming, cooling, 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. based on the manufacturing method, and the present invention is acceptable in 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. The reason why the steel was air cooled and the cooling start temperature was set to Ar3-250 to Ar3-20°C was to achieve a two-phase steel after cooling by cooling from this temperature range. That is, Ar
If it is rapidly cooled directly above I, it becomes a two-phase steel, but the area ratio of ferrite increases and the yield ratio decreases, or the area ratio of the second phase decreases, so the tensile strength decreases and the desired strength is achieved. I won't be able to do that.

Higher temperature than the middle between Ar1 and Ar3, that is, A, 3-25
By cooling from a temperature higher than 0° 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 Ar3 approaches, 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 A, 3-20°C.

A , 3-250~A , Rapid cooling from 3-20℃ is sufficient to harden the part by turning it into austenite after heating and air cooling, and making the C-enriched part into a quenched structure, increasing the tensile strength and achieving a low yield ratio. This is to obtain. If the rapid cooling is insufficient, the quenched structure will not be sufficiently hardened and, as a result, a low yield ratio will not be obtained. Therefore, the cooling rate was set at 15° C./sec or more. As for cooling, it usually doesn't matter whether it is water cooling or as long as the cooling rate can be ensured.

There are no particular regulations regarding cold square tube forming after quenching, but if strain is applied in the longitudinal direction at the same time as square tube forming, too many dislocations will be introduced, resulting in a high yield ratio. Therefore, the elongation in the longitudinal direction during square tube forming is specified to be ±3.0%.

Tempering is the process of replacing dislocations introduced during cold square tube forming with solid solution nitrogen,
It is fixed with solid solution carbon to provide yield elongation 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, which will cause 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 or low carbon low alloys to which special elements are added with good results. Preferred component compositions include: 0 003-030% Si: 0.02-050% Mn + 0.20-2.00% A-text + 0.001-0100% N: 0
．． Depending on the required characteristics of low carbon steel whose basic component is 0005 to 0.0100%, 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 ■ = 03% 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 20%.

Since Ni is added because it is useful for improving low-temperature toughness and 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 since too much MO impedes 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 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%.
is the upper limit.

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, ^1. Bl, C1, DI, E
l, Fl.

Gl, Hl, 11. Jl, 1. Ll, Ml, N1.0
1. Pl, Ql, R1,51, Tl.

1]1 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 A2 was formed into a square tube before heat treatment, the yield ratio was low, but no elongation at the yield point was observed at all.

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. D2 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/mm2 or more without using any particularly expensive alloying elements. , the effect on the industry is a dog.

[Brief explanation of drawings]

FIG. 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. FIG. 2 shows an example of the SS curve when the area of Ac is increased due to low YR and yield point elongation. 4 other people

Claims (1)

[Claims] 1 A low carbon steel pipe is heated to A_c_3 or higher, then air cooled and rapidly cooled from A_r_3-250 to A_r_3-20°C at a cooling rate of 15°C/sec or higher, then cold formed into a square tube, and further heated to 200°C. A method for producing a square tube having a yield point elongation, a low yield ratio, and excellent low-temperature toughness, the method comprising tempering at a temperature range of ~600°C.