JPH0641635A - Production of square pipe excellent in earthquake resistance and weather resistance - Google Patents

Abstract

PURPOSE:To provide the method for manufacturing a square pipe excellent in earthquake resistance and weather resistance. CONSTITUTION:A low alloy steel pipe contg. one or two kinds of 0.10 to 2.0% Cu and 0.070 to 0.150% P is heated to the Ac1 or above as well as to the Ac3-200 deg.C or above and is formed into the square pipe in such a manner that the finishing temp. in the forming is regulated to the Ac3-200 to the Ac3-20 deg.C, which is successively cooled to 200 or lower temp deg.C at 15 deg.C/sec or a higher cooling rate and is tempered in the temp. range of 200 to 600 deg.C according to necessity. In this way, the square pipe having a high strength of 50kgf/mm<2> or more and excellent in earthquake resistance and weather resistance can inexpensively be manufactured without specially using expensive alloy elements.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is intended for a steel pipe used for manufacturing a structure, has a sufficient resistance to collapse when an earthquake occurs, and can be used in an atmospheric corrosive environment even if coating is omitted. The present invention relates to a method for manufacturing a square tube having excellent corrosion resistance, seismic resistance and weather resistance.

[0002]

2. Description of the Related Art Recently, in each field dealing with steel materials,
Various demands such as improvement of usage characteristics and reduction of manufacturing cost for increasing competitiveness are increasing.

Among them, in the construction field, it is desired to reduce the yield ratio in order to improve the safety of the structure, especially in order to improve the seismic resistance. Up until now, demands have been strong mainly in the field of thick plates, but recently, this requirement has increased in the field of steel pipes.

Regarding the reduction of the yield ratio of ERW steel pipes, there is a measure to suppress the increase of the yield ratio due to work hardening during forming.
There is a limit to the yield ratio reduction. On the other hand, attempts have been made to reduce the yield ratio by heat treatment after forming a steel pipe.
For example, in Japanese Patent Laid-Open No. 3-87318, after forming, a steel pipe or a square pipe is heated to a (α + γ) two-layer region and then rapidly cooled to lower the yield point by making the microstructure of steel into a two-phase structure of ferrite and a second-phase carbide. It is known that the yield ratio is reduced by increasing the tensile strength.

On the other hand, when a structure is exposed to the atmosphere, it is common to coat the structure to prevent corrosion in the atmosphere. The coating cost for corrosion prevention is enormous in large-scale steel structures. In order to prevent corrosion, for example, when stainless steel is used, the stainless steel usually has a C content of 11% or more.
Since it contains r, the corrosion resistance is excellent, but the price is high, and it is considered that it is more expensive than coating. For the above reasons, it is known that steel for structural members that is used without coating in an atmospheric corrosive environment is relatively inexpensive and has necessary and sufficient corrosion resistance for the operating environment.

That is, a method of manufacturing a steel pipe (square pipe) having a low yield ratio is known, but a method of manufacturing a steel pipe (square pipe) having weather resistance added thereto is not known.

[0007]

As a seismic-resistant weatherproof steel pipe or square pipe for construction, there is a demand for a composite property of weather resistance and a yield ratio of 75% or less at a tensile strength of 40 to 60 kg level.

Recently, not only the yield ratio but also the shape of the stress-strain curve has been attracting attention as the material property of the steel material required for the seismic resistant structure. In other words, it is beginning to be said that the increase in Ac shown in FIGS. 1 and 2 is necessary for the steel material to have a sufficient plastic elongation ability. For that purpose, YR can be lowered, but this can be achieved by further increasing the yield point elongation. As can be seen by comparing FIG. 1 and FIG.
It can be said that the steel material shown in Fig. 2 is suitable for earthquake-resistant structures.

In other words, the steel material used for the bridge has a low YR and a yield point elongation from the viewpoint of seismic resistance, and when preventing corrosion in the atmosphere, enormous coating costs and alloys such as Cr addition are required. There is a demand for improving the weather resistance of ordinary steel in order to reduce the cost of conversion.

[0010]

Means for Solving the Problems As a result of numerous experiments and detailed studies for imparting weather resistance, the present inventors have found that
It was confirmed that the addition of Cu or P is effective for improving the weather resistance.

At the same time, in order to reduce the yield ratio, the microstructure of the steel has a two-phase structure of ferrite and carbide of the second phase,
It is designed to lower the yield point and increase the tensile strength. Further, in order to have a yield point elongation, Ac _{1 to} Ac ₃
Strain (square tube forming) is applied in the two-phase region, and dislocations generated in ferrite are immediately fixed by solid solution carbon and solid solution nitrogen,
It was confirmed that the subsequent quenching yielded a square tube having both a yield point elongation and a low yield ratio as a ferrite and a second phase structure.

The present invention has made it possible to manufacture a square tube excellent in seismic resistance and weather resistance based on such knowledge.
The gist is that Cu: 0.10% by weight.
To 2.0%, P: 0.070 to 0.150% of a low alloy steel steel tube or a square tube containing one or two kinds, and heating at Ac ₁ or more and Ac ₃ -200 ° C. or more, and the molding is completed. Temperature is Ac ₃ −
Square tube molding is performed at 200 to Ac ₃ -20 ° C, followed by cooling to a temperature of 200 ° C or less at a cooling rate of 15 ° C / sec or more, and then tempering in a temperature range of 200 to 600 ° C if necessary. A method for producing a steel pipe or a square pipe having excellent seismic resistance and weather resistance, which is characterized in that

The reason why Cu and P improve the weather resistance is considered as follows. That is, the rust layer that is usually formed when steel is exposed to the atmosphere is composed of base iron and the outer layer FeOOH, but when Cu or P is added to this, the rust layer is amorphous between the base iron and the outer layer FeOOH. About 50 to 1 of iron oxide
It is considered that a thickness of 00 μm was generated and Cu and P were concentrated in it, and this layer contributes to weather resistance.

[0014]

In the present invention, the heating temperature is set to not lower than Ac ₁ and not lower than Ac ₃ -200 ° C., and then rapidly cooled to eliminate the influence of work hardening in pipe forming and subsequent square tube forming, and It has succeeded in achieving dual-phase steel by newly imparting strain by pipe forming, immediately fixing dislocations generated at that time, and then rapidly cooling.

Further, by lowering the tempering temperature, it is possible to manufacture a rectangular tube having excellent seismic resistance by the synergistic effect of not softening the second phase portion more than necessary.

Next, the steel pipe manufacturing, square pipe forming, heating,
The conditions for cooling and tempering will be described.

First, there is no particular regulation on the production of the steel pipe, and any method is acceptable. For example, steel pipes are manufactured according to their manufacturing methods, including seamless steel pipes, electric resistance welded steel pipes, UO steel pipes,
Although it can be classified into a spiral steel pipe, a forged pipe, etc., the present invention is acceptable in any of these manufacturing methods. This is to regulate the heating temperature in the subsequent heat treatment to the temperature at which the processing strain is removed. Further, although the shape after forming is a square tube, this includes a square steel tube having a square cross-sectional shape and other wide-ranging deformed steel tubes.

Next, the heating temperature of the pipe is Ac ₁ or more and A
The reason for setting c ₃ -200 or more is that heating at this temperature range is aimed at simultaneously removing the forming strain while achieving dual-phase steel after cooling. Forming a square tube in that temperature range gives strain in the two-layer region, introduces an appropriate amount of dislocations into ferrite, and immediately fixes with solid solution carbon and solid solution nitrogen,
This is because the duplex stainless steel produced by the subsequent quenching has a yield point elongation.

[0019] The square tube forming finishing temperature Ac ₃ -200~Ac ₃ -
The reason for setting the temperature to 20 ° C. is to aim at the dual phase steel after cooling, and also to aim at the proper amount of work strain in ferrite. That is, if a square tube is formed immediately above Ac ₁ and then rapidly cooled, a two-phase steel is formed, but the ferrite has a high strength due to excessive work strain of ferrite, and as a result, a low yield ratio cannot be achieved. Ac ₁ to Ac intermediate temperature higher than the _3, i.e. by molding completed at above Ac ₃ -200 ° C., since it is possible to achieve both the two-phase steel of the distortion elimination, and the temperature and the lower limit. When the temperature for forming the rectangular tube is increased, the yield ratio passes through the lower limit and the yield ratio increases. This is because the area ratio of ferrite decreases, and the yield ratio rapidly increases as it approaches Ac ₃ . This is because the area ratio of ferrite approaches zero. From this, Ac ₃ −20 ° C. was set as the upper limit of the heating temperature.

[0020] Ac ₃ -200~Ac ₃ heating & quenching after molding to -20, the reheating time and austenite is sufficiently cured by the thickened portion of the C and quenched structure, the low elevated tensile strength This is to obtain the yield ratio. If the cooling is insufficient, the quenched structure will not be sufficiently hardened, and as a result, a low yield ratio cannot be obtained. Therefore, the cooling rate was defined as 15 ° C / sec or more. The cooling method is usually water cooling, but any method can be used as long as a cooling rate can be secured. The upper limit of the cooling stop temperature is set to 200 ° C. This is because if the cooling stop temperature is too high, the quenching structure does not harden sufficiently and, as a result, a low yield ratio cannot be obtained.

By the way, there is a case where the air-cooling process is inevitably required after the square tube is formed and before the rapid cooling (for example, due to equipment restrictions). In that case, if the air cooling is excessively performed, the introduced dislocations disappear and the meaning of forming the square tube in the two-phase region becomes meaningless. Therefore, when air cooling is applied, Ac ₃ -200 to A
Completed square tube molding at c ₃ -20 ℃, and after air cooling Ac ₃ -250〜
It was decided to cool rapidly from Ac ₃ -70 ° C.

By the way, depending on the type of steel, toughness is not good only by rapid cooling after heating, and there is a case where tempering treatment after rapid cooling is necessary to improve toughness. At that time, regarding the tempering temperature, regarding the two-phase structure of ferrite and the carbide of the second phase, if the second phase portion sufficiently hardened by the preceding quenching is tempered at too high temperature, it is excessively softened, which causes a decrease in tensile strength. The upper limit was set to 600 ° C because it causes an increase in the yield ratio. However, when the tempering temperature is low and the temperature is less than 200 ° C, the tempering effect is almost lost and the toughness may not be improved. Therefore, the lower limit is set to 200 ° C.

The definition of the components is as described in the claims, and the grounds for the defined ranges of each component are described below.

C has the effect of increasing the strength of the steel material, and C.
Although it is added in an amount of 0.05% or more, if added in excess of 0.30%, the toughness deteriorates significantly.
Was set to 0.30%.

Si has a solid solution strengthening effect and an effect of improving the strength and ductility of the steel material, and is added in an amount of 0.02% or more, but if added in excess of 0.50%, the toughness of the steel material is deteriorated. Therefore, the content is set to 0.02 to 0.50%.

Mn, like C, also has the effect of increasing the strength of the steel material, and is added in an amount of 0.30% or more. However, if the content exceeds 2.0%, not only the steelmaking work becomes difficult, but also the economy. Therefore, its content is 0.30
It was set to 2.0%.

Al is necessary for deoxidation in the steelmaking stage, and its lower limit was made 0.001%. Also, 0.100
%, The amount of inclusions increases, the cleanliness of the steel is lost, and the steelmaking work is hindered. Therefore, the range is set to 0.001 to 0.100%.

N is generally contained in the steel as an unavoidable impurity, but if the amount of N is too low, the cost for steelmaking increases significantly, so the lower limit was made 0.0005%. Further, if the amount of N increases, the weldability of the steel material deteriorates, and it also promotes the generation of surface flaws in the continuous cast slab.
The upper limit was 0.0100%.

Cu is required to be added in an amount of 0.10% or more in order to improve the corrosion resistance in an atmospheric corrosive environment, that is, the weather resistance. However, even if Cu is added in excess of 2.0%, there is an increase in the weather resistance. Since it almost disappears, the upper limit of the content is set to 2.0%.
The reason why Cu improves the weather resistance is considered as follows. In other words, the rust layer produced when ordinary steel is exposed to the atmosphere is composed of base iron and the outer layer FeOOH,
When Cu is added to this, amorphous iron oxide is generated between the base iron and the outer layer FeOOH to a thickness of about 50 to 100 μm, and Cu is concentrated in it, and this layer contributes to weather resistance. I think it is because there is.

Similar to Cu, P needs to be added in an amount of 0.070% or more in order to improve the corrosion resistance in an atmospheric corrosive environment, that is, the weather resistance. However, if too much P is added, the toughness and weldability of steel deteriorate. Therefore, the upper limit of the content is set to 0.150%. The reason why P improves weather resistance is that, like Cu, a rust layer enriched in P is formed between the base iron and FeOOH, and this layer contributes to weather resistance.

Ni is useful for improving low temperature toughness and corrosion resistance and is added, but since it is an expensive element, the upper limit of its content is 9.5%.

Cr is useful for increasing strength and improving corrosion resistance, but if it increases, it impairs low temperature toughness and weldability, so the upper limit of the content is 5.5%.

Mo is useful for increasing the strength, but if it increases, the weldability is impaired. Therefore, the upper limit of the content is 2.0%.

Nb is useful for making the austenite grains finer and for increasing the strength, but if it increases, it deteriorates the weldability, so the upper limit of the content is made 0.15%.

V is useful for precipitation strengthening, but if it increases, it impairs weldability, so the upper limit of the content is 0.3%.

Although Ti is useful for grain refinement of austenite grains and is added, if it increases, the weldability is impaired, so the content is made 0.3% as the upper limit.

B has an effect of remarkably enhancing the hardenability of steel by adding a trace amount. In order to effectively obtain this effect, it is necessary to add at least 0.0003%. However, if added in excess, it will produce B compound,
Since the toughness is deteriorated, the upper limit was made 0.0030%.

[0038] Ca is useful for controlling the morphology 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 its content is 0.0080%.

[0039]

[Examples] Table 1 shows the chemical composition of the test steel, and Table 2 shows the size of the steel pipe or square pipe, the heat treatment conditions, the mechanical properties of the obtained steel pipe, and the obtained steel pipe in the industrial zone. Shows the corrosion weight loss during an annual atmospheric exposure test. At this time,
When the corrosion weight loss was more than 8.0 g / 100 cm ² , it was judged that the weather resistance was not exhibited.

Steel pipes NoA1, B1, C1, shown in Table 2
D1, H1, I1, J1, K1, L1, M1, N1, O
1, P1, Q1, R1, S1, T1, U1, and V1 are the seismic resistance characteristics (yield ratio 70% or less &
At the same time, it has achieved a yield point elongation of 1.0% or more) and high weather resistance (corrosion weight loss of <8.0 g / 100 cm ² in a 4-year atmospheric exposure test).

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, B2 has a high yield ratio because the tempering temperature is too high. C2 has a high yield ratio because the cooling rate is insufficient. Since the heating temperature of D2 is too low, the yield ratio is high. Also, E1, F1, G1 are
Since neither Cu nor P satisfies the required amount, the weather resistance does not satisfy the target value.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described above in detail, according to the present invention, it is possible to use 50 kgf / m without using an expensive alloying element.
A square tube having a high yield ratio of m ² or more and a low yield ratio and excellent weather resistance can be manufactured at low cost, and the effect is industrially great.

[Brief description of drawings]

FIG. 1 is a diagram showing a stress-strain relationship of a steel material.

FIG. 2 is a diagram showing a stress-strain relationship of a steel material.

Claims (5)

[Claims] 1. Cu: 0.10 to 2.0 in weight%.
%, P: 0.070 to 0.150% of a low alloy steel steel pipe containing one or two kinds is heated to Ac ₁ or more and Ac _{3 to} 200 ° C. or more, and the molding end temperature is Ac _{3 to} 200 to. Ac ₃ -20
A method for producing a rectangular tube having excellent seismic resistance and weather resistance, which comprises forming a rectangular tube at a temperature of 20 ° C and then cooling it to a temperature of 200 ° C or less at a cooling rate of 15 ° C / sec or more. 2. Cu: 0.10 to 2.0 at% by weight.
%, P: 0.070 to 0.150% of a low alloy steel steel pipe containing one or two kinds is heated to Ac ₁ or more and Ac _{3 to} 200 ° C. or more, and the molding end temperature is Ac _{3 to} 200 to. Ac ₃ -20
Seismic resistance and weather resistance, characterized in that it is formed into a square tube so that the temperature becomes ℃, subsequently cooled to a temperature of 200 ℃ or less at a cooling rate of 15 ℃ / sec or more, and then tempered in a temperature range of 200 to 600 ℃. A method of manufacturing square tubes with excellent properties. 3. Cu: 0.10 to 2.0 in weight%.
%, P: 0.070 to 0.150% of a low alloy steel steel pipe containing one or two kinds is heated to Ac ₁ or more and Ac _{3 to} 200 ° C. or more, and the molding end temperature is Ac _{3 to} 200 to. Ac ₃ -20
It is formed into a square tube so that the temperature becomes ℃, then air-cooled, and then from the temperature range of Ac ₃ −250 to Ac ₃ −70 ° C.
A method of manufacturing a rectangular tube having excellent seismic resistance and weather resistance, which comprises cooling to a temperature of 200 ° C. or lower at a cooling rate of not less than / sec. 4. Cu: 0.10 to 2.0 in weight%.
%, P: 0.070 to 0.150% of a low alloy steel steel pipe containing one or two kinds is heated to Ac ₁ or more and Ac _{3 to} 200 ° C. or more, and the molding end temperature is Ac _{3 to} 200 to. Ac ₃ -20
It is formed into a square tube so that the temperature becomes ℃, then air-cooled, and then from the temperature range of Ac ₃ −250 to Ac ₃ −70 ° C. to 15 ° C.
A method for producing a rectangular tube having excellent seismic resistance and weather resistance, which comprises cooling to a temperature of 200 ° C. or lower at a cooling rate of / sec or more and then tempering in a temperature range of 200 to 600 ° C. 5. The low alloy steel square tube according to claim 1, claim 2, claim 3 or claim 4, wherein the low alloy steel square tube is a low alloy steel square tube composed of a component of one of the following first group and second group. Method for manufacturing square tube having excellent seismic resistance and weather resistance as described 1st group, by weight%, C: 0.05 to 0.30%, Si: 0.02 to 0.50%, Mn: 0.30 2.00%, Al: 0.001 to 0.100%, N: 0.0005 to 0.0100%, Cu: 0.10 to 2.00%, P: 0.070 to 0.150% However, the low alloy steel consisting of the balance Fe and unavoidable impurities, 2nd group, by weight%, C: 0.05 to 0.30%, Si: 0.02 to 0.50%, Mn: 0.30 to 2.00 %, Al: 0.001 to 0.100%, N: 0.0005 to 0.0100%, Cu: 0.10 to 2.00%, P: 0. 70 to 0.150%, 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: 0.0080% or less, and one or more kinds of low alloy steel containing the balance Fe and inevitable impurities.