JPH03219018A - Production of steel tube having yield point elongation, reduced in yield ratio, and excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a steel tube having high strength and low yield ratio at a low cost by subjecting a low carbon steel tube to heating up to specific temp. and then to air cooling 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, air-cooled, and cooled rapidly from a temp. region between (Ar3-250 deg.C) and (Ar3-15 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 steel 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 of manufacturing a steel pipe with a low yield ratio.

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

Among these, in the construction field, it is desired to lower the yield ratio in order to improve the safety of structures, especially 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 in actual operation.

[Problems to be Solved by the Invention] There is a requirement for a tensile strength of 40 kg or more and a yield ratio of 75% or less as a low yield ratio steel pipe for construction, 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 a comparison of Figures 1 and 2, the steel material shown in Figure 2 is suitable for use in earthquake-resistant structures. In other words, for earthquake-resistant structures, steel pipes 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.

Based on this knowledge, the present invention has made it possible to manufacture steel pipes with a low yield ratio. ,, -250~Ar3-15℃/sec from 20℃
It has a yield point elongation and is characterized by being rapidly cooled at a cooling rate of 0.05% or more, and then tempered in a temperature range of 200 to 600°C. This is a method for manufacturing steel pipes with a low ratio and excellent low-temperature toughness.

[Function] In the present invention, the heating temperature is ^. By doing the above, distortion in pipe forming can be completely removed and AC1 to Ac
By air-cooling to a high point between the three transformation points and then rapidly cooling, we have succeeded in achieving duplex steel.

Furthermore, by applying processing strain afterwards,
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 providing yield elongation.

Furthermore, by tempering, we succeeded in sufficiently recovering the low-temperature toughness by converting the 'MS2 phase into a partial phase. However, by lowering this tempering temperature, it is important not to soften the second phase part more than necessary. This synergistic effect has made it possible to manufacture steel pipes that have elongation at yield, a low yield ratio, and excellent low-temperature toughness.

Next, the conditions for manufacturing, heating, cooling, applying 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. according to their manufacturing methods, and the present invention allows 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 -250 to Ar3-20°C was that by cooling from this temperature range, it was possible to achieve a two-phase steel after cooling. That is, hert
If it is rapidly cooled from directly above, 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, making it difficult to obtain the desired strength. become unable to do so.

Higher temperature than the middle between Art and Ar3, that is, 8r3-25
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 8r3-20°C. ^,, -250~Ar,
The purpose of the rapid cooling from -20 is to austenitize after heating and air cooling, and make the C-enriched portion into a quenched structure, thereby sufficiently hardening the steel to increase the tensile strength and obtain a low yield ratio. If the rapid cooling is insufficient, the quenched structure will not harden sufficiently, resulting in a low yield ratio.
c or more. 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 a distortion of 0.05% or more is introduced. Here 0.05%
The strain is the total amount of longitudinal equivalent strain.

Tempering is performed to fix dislocations introduced by cold straining with solid solute nitrogen and solid solute carbon to give yield point elongation and to improve toughness. Regarding the two-phase structure of the 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 becomes too soft, which causes a decrease in tensile strength and an increase in the 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 preferred component composition is C: 0.03-03
Low carbon steel whose basic components are 0% Si: 0.02-0.50% Mn: 0.20-2.00% Al: 0.001-0100% N: 0.0005-0.0100%, Or, in addition to the above basic components, depending on the required characteristics of the strength steel: Cu: 2.0% or less Ni: 9.5% or less Cr: 5.5% or less Mo: 2.0% or less Nb: 0.15% or lessV : 0.3% or less Ti: 0.15% or less B: 0.0003-0.0030% 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, 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, adding too much will generate B compounds and deteriorate toughness, 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, heat treatment conditions, and mechanical properties of the obtained steel pipe.

Steel pipe No. shown in Table 2, ^1. Bl, C1, DI, E
1. F1°Gl, Hl, 11. Jl, 1. Ll, old;
Nl, 01. PI, Ql, R1, St.

T1. Old steel and Vl are steels according to the present invention, which have achieved the low yield ratio (yield ratio 70% or less) that is the aim of the present invention.

On the other hand, A2 has zero strain, so although the yield ratio is low, there is no elongation at the yield point 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. Since the tempering temperature of A6 is high, the yield ratio is 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 enables low-yield ratio steel pipes having a high strength of 40 kgf/mm2 or more to be manufactured at low cost without using particularly expensive alloying elements. The industrial effect is great.

[Brief explanation of drawings]

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

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_3-15°C at a cooling rate of 15°C/sec or higher, and then cooled to 0 in the cold.
．． Applying processing strain of 0.05% or more, and further 200 to 600
A method for producing a steel pipe having a yield point elongation, a low yield ratio, and excellent low-temperature toughness, the method comprising tempering at a temperature range of °C.