JPH06256852A - Production of square pipe having low yield ratio and excellent weatherability - Google Patents

Abstract

PURPOSE:To produce the square pipe having a low yield ratio and excellent weatherability. CONSTITUTION:The corner parts of a low-alloy steel square pipe contg. one or two kinds of 0.10 to 2.0% Cu and 0.070 to 0.150% P are heated to Ac3 or above and is then cooled down to <=200 deg.C at <=10 deg.C/sec cooling rate and is subjected to tempering in a 200 to 600 deg.C range at need, by which the square pipe having the low yield ratio and the excellent weatherability is obtd. As a result, the square pipe having high strength of >=40kgf/mm<2>, low yield ratio and the excellent weatherability is inexpensively produced without specifically using costly 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 electric resistance welded steel pipes, it 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.
Attempts have also been made to reduce the yield ratio of the entire square tube by heat-treating only the corners of the square tube. For example, in Japanese Patent Laid-Open No. 4-323319, a corner portion of a square tube is formed into a γ1 phase structure after forming, and then air-cooled into a microstructure of steel to form a two-phase structure of ferrite and pearlite, and a ferrite structure is yielded as strain-free clean ferrite. By lowering the points, the yield ratio is reduced. This method has an advantage that a low yield ratio can be stably obtained regardless of the manufacturing method and treatment of the steel pipe before the heat treatment.

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 for manufacturing a square tube having a low yield ratio has not been established, and a method for manufacturing a square tube having weather resistance added thereto has not been established.

[0007]

As a seismic and weather-resistant square tube for construction, it is required to have a composite property of weather resistance with a tensile strength of 40 kg or more and a yield ratio of 85% or less. In other words, the steel materials used for bridges are required to have low YR characteristics from the viewpoint of seismic resistance, and when preventing corrosion in the atmosphere, enormous coating costs and Cr
In order to reduce alloying costs such as addition, there is a demand for improved corrosion resistance in ordinary steel.

[0008]

Means for Solving the Problems The present inventors have conducted numerous experiments and detailed studies in order to impart weather resistance, and as a result, it was found that addition of Cu or P is effective for improving weather resistance. confirmed.

At the same time, in order to add a yield ratio, the microstructure of the steel has a two-phase structure of ferrite and a second phase (pearlite), and the ferrite structure is made of clean ferrite without strain to lower the yield point. .

Based on the above findings, the present invention has made it possible to manufacture a square tube having a low yield ratio and excellent weather resistance. The gist of the invention is, in terms of% by weight, C
u: 0.10 to 2.0%, P: 0.070 to 0.150
%, The corner portion of a low alloy steel square tube containing one or two kinds is heated to Ac ₃ or more, and subsequently cooled to a temperature of 200 ° C. or less at a cooling rate of 10 ° C./sec or less, and then as necessary. A method of manufacturing a square tube having a low yield ratio and excellent weather resistance, which is characterized by tempering in a temperature range of 200 to 600 ° C.

The reason why Cu or P yields to 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.

[0012]

In the present invention, the heating temperature of the corner portion of the square tube is set to Ac ₃ or higher to form a complete austenite phase, and the effect of work hardening in pipe forming and subsequent square tube forming is completely removed. By cooling to room temperature by air, the structure achieved a dual phase steel of clean ferrite + pearlite. Further, by lowering the tempering temperature, it is possible to manufacture a square tube having a low yield ratio due to 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 for the production of steel pipes and the subsequent forming of square pipes (including square steel pipes having a square cross-section and other wide-ranging deformed 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, forged pipes, etc. according to their manufacturing methods, but the present invention is allowed by any of these manufacturing methods. It is of course acceptable that a plate such as a hot coil is directly formed into a square tube and welded. This is to regulate the heating temperature in the subsequent corner heat treatment to the temperature at which the processing strain is removed.

Next, the heating temperature of the corner portion after molding is set to Ac ₃
The reason for the above is that by heating the corner portion to this temperature, the aim is to completely remove the forming strain while achieving the dual phase steeling after cooling. If the heating temperature is below Ac ₃ ,
Although it becomes a dual-phase steel, the strength of the ferrite is high because the work strain remains in the ferrite, and as a result, a low yield ratio cannot be achieved.

The cooling rate after heating Ac ₃ is 10 ° C./sec.
The reason for the following is that from the portion where the austenite is formed during heating and the concentration of C is increased, clean ferrite and pearlite are
This is because the yield ratio is lowered by forming a phase structure. If the cooling rate is large and exceeds 10 ° C / sec, the transformation from austenite to ferrite + pearlite becomes insufficient and the second phase has a structure containing bainite and martensite, resulting in a large variation in material at the corners. Some parts have extremely high strength, and it is difficult to achieve a low yield ratio after all. When the cooling rate is further increased, the rectangular tube is deformed after cooling, and it is difficult to put it in a predetermined size. Cooling rate is 10 ℃ / sec
Air cooling is usually adopted because the following conditions are acceptable, but any method may be used as long as the cooling rate is satisfied. 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 ferrite structure will not be sufficiently clean due to rapid cooling in the middle, and as a result, a low yield ratio cannot be obtained.

Regarding the heating method, for example, by heating the rectangular steel pipe with an induction heating device having a round cross section, only the corner portion of the rectangular pipe close to the induction coil is heated, and the flat portion is not heated, and then cooled. The above heat treatment can be performed only on the corners.

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 after rapid cooling is necessary to improve toughness. At that time, regarding the tempering temperature, regarding the two-phase structure of ferrite and pearlite of the second phase, if the second phase portion hardened to some extent by the preceding quenching is tempered at too high temperature, it is excessively softened, which causes a decrease in tensile strength. The upper limit is 6 because it causes an increase in the yield ratio.
It was set to 00 ° C. However, if 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 the respective components will be described below.

C has the effect of increasing the strength of the steel material, and C.
The content is 0.05% or more, but if it exceeds 0.30%, the toughness is significantly deteriorated.
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%.

Like C, Mn 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 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, when the amount of N increases, the weldability of the steel material deteriorates, and it promotes the occurrence 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, but if it 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.
That is, the rust layer formed when ordinary steel is exposed to the atmosphere is composed of base iron and the outer layer FeOOH, but when Cu is added to this, the amorphous iron oxide is formed between the base iron and the outer layer FeOOH. Is generated in a thickness of about 50 to 100 μm, and Cu is concentrated in it, and it is considered that this layer contributes to weather resistance.

Similar to Cu, P also 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, but if too much 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 its content is 9.
The upper limit was 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 its 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 refining the austenite grains and increasing the strength, but it increases the weldability if it increases, so the upper limit of the content is 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%.

Ti is useful for grain refinement of austenite grains and is added, but if it increases, the weldability is impaired. Therefore, the upper limit of the content is 0.15%.

B has an effect of remarkably enhancing the hardenability of steel by the addition of 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 deteriorates, the upper limit was made 0.0030%.

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%.

[0035]

[Examples] Table 1 shows the chemical composition of the test steel, Table 2 shows the size of the square tube, the heat treatment conditions, and the mechanical properties of the obtained steel tube.
Also, the corrosion weight loss when the obtained steel pipe is subjected to an atmospheric exposure test for 4 years in an industrial area is shown. 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 low yield ratios (yield ratio 85% or less) which are the targets of the present invention.
And high weather resistance (corrosion loss <4.
0 g / 100 cm ² ) is achieved at the same time.

On the other hand, in A2 and A3, the heating ratio is too low and the yield ratio is high (the ferrite is not completely clean). A4 has a high yield ratio because the cooling rate after heating is too high. 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 too high. Since the heating temperature of D2 is too low, the processing strain is not removed, and the yield ratio is high. Further, in E1, F1, and G1, neither Cu nor P satisfied the necessary amount, and therefore the weather resistance did not satisfy the target value.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

As described above in detail, according to the present invention, 40 kgf / m can be obtained without using any 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.

Claims (3)

[Claims] 1. Cu: 0.10 to 2.0 in weight%.
%, P: 0.070 to 0.150%, a corner portion of a low alloy steel square tube containing one or two kinds is heated to Ac ₃ or more, and subsequently 200 ° C. at a cooling rate of 10 ° C./sec or less.
A method for manufacturing a square tube having a low yield ratio and excellent weather resistance, characterized by cooling to the following temperature. 2. Cu: 0.10 to 2.0 at% by weight.
%, P: 0.070 to 0.150%, a corner portion of a low alloy steel square tube containing one or two kinds is heated to Ac ₃ or more, and subsequently 200 ° C. at a cooling rate of 10 ° C./sec or less.
A method for producing a rectangular tube having a low yield ratio and excellent weather resistance, which comprises cooling to the following temperature and then tempering in a temperature range of 200 to 600 ° C. 3. The low alloy steel square tube according to claim 1 or 2, wherein the low alloy steel square tube comprises any one of the following first group and second group components.
A method for manufacturing a low alloy steel square tube according to. 1st group weight%, C: 0.05-0.30% Si: 0.02-0.50% Mn: 0.30-2.00% Al: 0.001-0.100% N: 0 0.0005 to 0.0100% Cu: 0.10 to 2.00% P: 0.070 to 0.150%, the low alloy steel second group consisting of the balance Fe and unavoidable impurities. 0.05-0.30%, Si: 0.02-0.50%, Mn: 0.30-2.00%, Al: 0.001-0.100%, N: 0.0005-0. 0100%, Cu: 0.10 to 2.00%, P: 0.070 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 0.00 It contains one or two or more of the 0% or less, low alloy steel and the balance Fe and unavoidable impurities.