JPH04168218A - Manufacture of steel tube excellent in earthquake resistance, fire resistance and low temperature toughness - Google Patents

Abstract

PURPOSE:To improve the low temp. toughness of a steel tube by specifying heat treating conditions for a low carbon steel pipe having a specified chemical compsn. CONSTITUTION:A low carbon steel tube contg., by weight, 0.1 to 2.5% Mo and furthermore contg. 0.010 to 0.15% Nb and 0.010 to 0.30% V is heated to Ac3-250 to Ac3-20 deg.C, is successively subjected to rapid cooling at >=15 deg.C/sec cooling rate, is thereafter provided with >=0.05% cold working strain and is furthermore tempered in the temp. range of 200 to 600 deg.C. In this way, the steel tube excellent in earthquake resistance, fire resistance and low temp. toughness can be manufactured. It is effectively utilized for the manufacture of a steel having resistance till its collapse in case of generating an eqrthquake, having strength in case of a fire without fireproofing and furthermore withstanding use in a cold district.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is directed to steel pipes used for manufacturing structures,
It has seismic properties that have sufficient resistance to collapse in the event of an earthquake, sufficient strength in the event of a fire even if the fireproof coating is simplified or omitted, and that it can withstand use in cold regions. This invention relates to a method for producing steel pipes with excellent fire resistance and low-temperature toughness.

(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 increase 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 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
Amount: 0.26-0.48%), C0 from the viewpoint of weldability
It cannot be used for architectural structures where the upper limit of Q is regulated. 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.

On the other hand, in structures such as steel structures, in order to ensure sufficient strength even in the event of a fire, it was required that the steel be coated with rock wool or the like to prevent the temperature of the steel from rising above 350°C.

In recent years, the Building Standards Act has been revised, and fireproof coatings can now be simplified or omitted depending on the strength of the steel material at high temperatures. That is, it is said that if a steel material has sufficient strength (2/3 or more of the standard yield strength at room temperature) at a high temperature such as 600° C., it is possible to omit the fireproof coating and use it bare.

The high-temperature strength of steel materials has been well studied, and the developed materials have been standardized as boiler steel or pressure vessel steel. In addition, special public service No. 51-15188
As shown in the issue, various improvements and developments are still being carried out even now. These steel materials have high strength when used for long periods of time, such as tens of thousands or hundreds of thousands of hours at high temperatures, that is, they have high creep strength.

(Problems to be Solved by the Invention) As a low yield ratio steel 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 larger the area of Ao shown in Figures 1 and 2, the greater the plastic elongation capacity and the harder it is to reach failure. It is clear that Ae is excellent as a material, 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.

In addition, the fire resistance property that is the subject of the present invention is strength within a few hours at most in the event of a fire, which is completely different from high-temperature strength, which has been the subject of development for a long time. The object of the present invention is to provide a steel pipe whose strength at 600° C., which is an important property when a fireproof coating is omitted, is significantly improved compared to conventional steel.

Furthermore, the problem to be solved by the present invention is to provide a steel pipe that satisfies both earthquake resistance and fire resistance.

(Means for Solving the Problem) The inventors conducted numerous experiments and detailed studies to reduce the yield ratio, and found that in order to reduce the yield ratio, it is necessary to It was confirmed that it is necessary to have a two-phase structure consisting of ferrite and a second phase of carbide. 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, the steel pipe that has become a two-phase structure is cold strained to introduce dislocations into the ferrite, and then tempered to remove these introduced dislocations from solid solution carbon and solid solution. It was confirmed that it was necessary to immediately fix with nitrogen.

Furthermore, in order to impart fire resistance to steel pipes with excellent seismic resistance obtained from the above findings, the present inventors conducted numerous experiments and detailed studies, and as a result of the alloying elements, ■Mo and Nb
Combined addition of Mo, Nb and V, ■Combined addition of Mo, Nb and V
It was found that the combined addition of In other words, when the material is exposed to high temperatures, the effect of Mo delays the disappearance of stuck dislocations, and conversely, the dislocations are used as nuclei to form Mo, Nb, and M
This has the effect that composite carbides of o and - are finely dispersed and precipitated, inhibiting the movement of dislocations generated at high temperatures.

Based on this knowledge, the present invention has made it possible to manufacture steel pipes with earthquake resistance and fire resistance. %, and Nb: 0.010% to 0.15%, V:
A low carbon steel pipe containing one or two types of 0.010% to 0.30% was heated to -20°C, and then rapidly cooled at a cooling rate of 15°C/sec or more. After that, a strain of 0.05% or more is applied by cold, and further 2
This is a method for producing steel pipes with excellent seismic resistance, fire resistance, and low-temperature toughness, characterized by tempering at a temperature range of 00 to 600°C.

(Function) The present invention is characterized by specifying the heat treatment conditions and applying strain between two types of heat treatment: rapid cooling after heating in a two-phase region and tempering. First, let's talk about the heat treatment conditions.The heating temperature ranges from AC5-250 to Ac5-2.
By cooling the steel to 0°C and then rapidly cooling it, they succeeded in creating a duplex steel while eliminating the effects of work hardening during pipe forming.

Furthermore, by applying cold strain, 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 yield elongation. has been successful in

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 has made it possible to manufacture steel pipes with excellent earthquake resistance, fire resistance, and low-temperature toughness.

Next, the conditions for manufacturing, heating, cooling, imparting 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 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 Aea-2 (1℃) because by heating to this temperature range, we aimed to simultaneously achieve duplex steel after cooling and eliminate forming distortion. In other words, when heated immediately above Act and then rapidly cooled,
Although it is made into a dual-phase steel, the strength of the ferrite is high due to residual processing strain in the ferrite, and as a result, a low yield ratio cannot be achieved. By heating the steel to a temperature higher than the middle of AC3-AC3, that is, 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 increases,
After passing the lowest limit of yield ratio, the yield ratio increases. This is because the area ratio of ferrite decreases, and as AC3 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 AC3-20°C.

A e, -250 ~ A C3 - Rapid cooling after heating to -20°C is austenitized during reheating, and the C-enriched area becomes a quenched structure, which hardens the area sufficiently, increasing tensile strength and achieving a low yield ratio. This is to obtain. If the rapid cooling is insufficient, the quenched structure will not harden sufficiently, resulting in a low yield ratio. Therefore, the cooling rate is set to 15°C/sec.
As stipulated above. 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 the imparting of cold strain after rapid cooling. Usually, sizing processing is used, but any method is acceptable as long as distortion can be imparted. Moreover, 0.05% or more is calculated based on the total strain converted in the longitudinal direction.

Tempering is performed to fix dislocations introduced by cold straining with solid solute nitrogen and solid solute 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 will hardly harden and the toughness may not be improved, so the lower limit was set at 200°C.

In addition, performing tempering treatment after applying cold strain is IAo
This is essential for imparting fire-resistant properties to m-processed steel. In other words, when the material is exposed to high temperatures, the disappearance of dislocations is delayed due to the hardening of Mo, and conversely, composite carbides of Mo, Nb, and This has the effect of inhibiting the movement of

If strain imparting and tempering are omitted, since there are no precipitation nuclei, carbides will coarsely precipitate, and the precipitates will no longer contribute to high-temperature strength.

The method of the present invention can be applied to low carbon steel or low carbon low alloy steel to which special elements are added with good results. A preferred component composition is C: 0.03 to 0.30.
%St: 0.02-0.50% Mn: 0.20-2.00% Al: 0.001-0.100% N: 0.0005-0.0100% Mo:
0.10~2.5% Nb: 0.010~0.15%V: 0
．． It is a low carbon steel containing 0.010 to 0.3%.

However, regarding Nb and ■, it is sufficient if either one of them is added (of course, both may be added). Furthermore, in addition to the above ingredients, Cu: 2.0%
Below Ni・9.5% or less Cr: 5.5% 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.

As mentioned above, when steel is exposed to high temperatures, Mo
It is essential to inhibit the movement of dislocations that are finely dispersed and precipitated as carbides in the ferrite into which fixed dislocations have been introduced, and are generated at high temperatures, and in order to achieve this effect, 0.1
0% or more is required, but if the amount added is too large, weldability will be impaired, so the upper limit of the content is 2.5%.

It is added to generate a composite carbide of Nb and Mo, and is also useful for refining austenite grains and increasing strength. 0.010% or more is required to achieve this result, but the amount added is If it is too large, weldability will be impaired, so the upper limit of the content is set at 0.15%.

Similar to Nb, ■ is added to form a composite carbide with Mo, and is also useful for precipitation strengthening, and if it is necessary to achieve this effect by 0.10% or more, or if the amount added is too large, weldability will deteriorate. In order to inhibit this, the upper limit of the content is 0.3%.

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%.

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, heat treatment conditions, and mechanical properties of the obtained steel pipe.

Steel pipe No. shown in Table 2, A1, B1, cl, Dl,
Hl, 11, Jl, K1, Ll, Ml, N1, ol, P
1, Ql, R1, Sl, T1, Ul, and V1 steels, which achieve the low yield ratio (yield ratio 70% or less) and high AC (AC > 75 kgf/mm2・%) that are the aim of the present invention. and high high temperature strength (YSao./Y S >0
．． 500) at the same time.

On the other hand, in A2, since the strain was applied before heat treatment, the yield ratio was low, but no elongation at the yield point was achieved, and the Ao was low. A3 has a high yield ratio due to its high heating temperature.

A4 has a high yield ratio because the heating temperature is too low. A5 has a high yield ratio due to an 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.

Since Mo is not added to El and Fl, their high temperature strength is low.

G1 has Mo added, but neither Nb nor ■ is added, so its high temperature strength is low.

(Effects of the Invention) As explained in detail above, the present invention provides a steel pipe that has high strength of 40 kgf/mm or more, excellent seismic resistance, fire resistance, and low-temperature toughness without using any particularly expensive alloying elements. It can be manufactured at a low cost, and its industrial effects are outstanding.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams showing the stress-strain relationship of steel materials. As mentioned in the text, steel materials with the stress-strain relationship shown in Figure 2 have better plastic elongation capacity. This indicates that it has excellent seismic properties. Figure 1 Figure 2

Claims (1)

[Claims] 1% by weight, contains 0.1% to 2.5% of Mo, and Nb: 0.010% to 0.15%, V: 0.010%
Low carbon steel pipes containing ~0.30% of type 1 or type 2,
A_c_3-250~A_c_3-20℃, followed by rapid cooling at a cooling rate of 15℃/sec or more, cold processing strain of 0.05% or more, and further 200~
A method for producing steel pipes with excellent seismic resistance, fire resistance, and low-temperature toughness, characterized by tempering in a temperature range of 600°C.