JPH03219017A - 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 and to rapid cooling under respectively specified conditions, applying cold working strain to this steel tube, and then carrying out tempering. CONSTITUTION:A low carbon steel tube is heated up to a temp. region between (Ac3-250 deg.C) and (Ac3-20 deg.C) and cooled rapidly at >=15 deg.C/sec cooling rate. Subsequently, cold working strain is applied to this steel so that the total amount of strain expressed in terms of 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 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 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 producing 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 Japanese Patent Application Laid-Open No. 57-11i118 as a method for manufacturing electric resistance welded steel pipes for oil wells with a low yield ratio, but in this method, a considerable amount of C is added to achieve a low yield ratio (
C content: 0.26 to 0.48%), from the viewpoint of weldability
It cannot be used for architectural structures where eq upper limit is specified. Similarly, as a manufacturing method for low yield ratio high tensile resistance welded steel pipes,
There is Japanese Unexamined Patent Publication No. 57-16119, which manufactures extremely low YR steel at the hot coil stage and considerably limits the amount of strain in order to suppress work hardening when manufacturing ERW steel pipes. , which is quite difficult in actual operation.

[Problems to be Solved by the Invention] There is a requirement for a low yield ratio steel pipe 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.

Recently, not only the yield ratio but also the shape force of the stress-strain curve has become the focus of attention as a material characteristic of steel materials required for earthquake-resistant structures. In other words, it is beginning to be said that in order for steel materials to have sufficient desired 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 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. , followed by rapid cooling at a cooling rate of 15°C/sec or more, then applying cold processing strain of 0.05% or more, and then tempering at a temperature range of 200 to 600°C. , has a yield point elongation and a low yield ratio;
It is also a method for producing steel pipes with excellent low-temperature toughness.

In the present invention, by raising the heating temperature to a high range between Act and Ae3 transformation points and then rapidly cooling, we succeeded in achieving duplex steel while eliminating the influence of work hardening during pipe forming. ing.

Furthermore, by applying cold processing strain,
By introducing dislocations into the structure (ferrite) and then performing tempering, they have succeeded in fixing the dislocations with solid solution nitrogen and solid solution carbon, thereby creating elongation at the yield point.

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 results in a high yield point elongation, a low yield ratio, and excellent low-temperature toughness. This is what made it possible.

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 specific regulations regarding the manufacturing of steel pipes, and any method is acceptable.For example, steel pipes can be classified into seamless steel pipes, ERW steel pipes, UO steel pipes, spiral steel pipes, forge-welded pipes, etc. based 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 heating temperature was set to AC3-250 to AC3-20°C because by heating to this temperature range, the aim was to simultaneously achieve duplex steel after cooling and remove forming strain. That is, when heated directly above Ael and then rapidly cooled, 2
Although it becomes a phase steel, the strength of the ferrite is high because of residual processing strain in the ferrite, and as a result, a low yield ratio cannot be achieved. Higher temperature than the middle between Ael and Ac3,
In other words, by heating the steel to a temperature higher than AC3-250°C, it is possible to achieve both duplex steel formation and strain removal, so this temperature was set as the lower limit. As the heating 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 Ac3, the yield ratio increases rapidly. This is because the area ratio of ferrite approaches zero. From this, AC3-20°C was set as the upper limit of the heating temperature. Ac3
The purpose of rapid cooling after heating to -250 to AC3-20 is to fully harden the part by turning it into austenite during reheating and making the C-enriched part a quenched structure, increasing the tensile strength and obtaining a low yield ratio. . 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 specified to be 15° C./sec or more. As for cooling, water cooling is usually used, but as long as the cooling rate is secured, the method does not matter.

There are no particular regulations regarding imparting cold processing strain after quenching. Usually, sizing processing is used, but any method may be used as long as distortion can be imparted. Furthermore, 0.005% or more is calculated based on the total strain converted in the longitudinal direction.

Tempering is performed to fix dislocations introduced by applying cold working strain with solid solute nitrogen and solid solute carbon to give yield point 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 with good results. A preferred component composition is C, 0.03-030
% St: 0.02-050% Mn + 0.20-2.00% AN + 0.001-0.100% N ・Low carbon steel with 0.0005-0.0100% as a basic component, or the above-mentioned In addition to the basic components, depending on the required properties 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 less V: 0 .3% or lessTi・0.15% or lessB: 0.0003 to 0.0030%Ca: 0.
0080% or less of one type or two or more types may be added.

Is Cu useful for increasing strength and improving corrosion resistance?
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 since too much MO impedes weldability, the upper limit of the content is 20%.

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
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 at 0.0030%. .

Ca is added because it is useful for controlling the form of sulfide-based inclusions, or 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, Al, BL, C1, DI,
El, Fl.

G1]1.11. Jl, 1. Ll, Ml, Nl, 01
．． PI, Ql, R1,51, T101, and Vl are the steels according to the present invention, and have achieved the low yield ratio (yield ratio of 70% or less) that is the aim of the present invention.

On the other hand, since the amount of strain in A2 is zero, the yield ratio is low or there is no elongation at the yield point at all.

A3 has a high yield ratio because the heating temperature is too high. A4 has a high yield ratio because the heating temperature is too low. A5 has a high yield ratio due to insufficient cooling rate after heating. 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 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 that although YR is low, the yield point does not increase.
FIG. 2 is a diagram showing an example of an SS curve with a small area of Ao, and FIG. 2 is a diagram showing an example of an SS curve with a large area of AC due to low YR and elongation at yield point. 4 other people

Claims (1)

[Claims] 1. Low carbon steel pipes A_c_3-250 to A_c_3
It is characterized by heating to -20°C, followed by rapid cooling at a cooling rate of 15°C/sec or more, applying cold processing strain of 0.05% or more, and further tempering at a temperature range of 200 to 600°C. A method for manufacturing a steel pipe that has elongation at yield point, low yield ratio, and excellent low-temperature toughness.