JPH05339637A - Production of steel pipe or square pipe having low yield ratio and excellent weatherability - Google Patents

Abstract

PURPOSE:To provide the process for production of the steel pipe or square pipe having a low yield ratio and excellent weatherability. CONSTITUTION:The low-alloy steel pipe or square pipe contg., by weight %, one or two kinds of 0.10 to 2.0% Cu and 0.070 to 0.150% P and contg. one or >=2 kinds among Ni, Cr, Mo, Nb, V, Ti, B and Ca at need is heated to Ac3-250 deg.C to Ac3-20 deg.C and is cooled at >=15 deg.C/sec rate in succession thereto; thereafter, the steel pipe or the square pipe is tempered in a 200 to 600 deg.C range at need.

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 and has sufficient resistance to collapse when an earthquake occurs, and even in the atmosphere corrosive environment even if coating is omitted. The present invention relates to a method for manufacturing a steel pipe or a square pipe having excellent corrosion resistance and excellent earthquake 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, Japanese Patent Laid-Open No. 3-87318 discloses a steel pipe (α +
γ) It is known that the yield ratio is reduced by heating the steel to a two-phase region and then rapidly cooling it to make the microstructure of the steel a two-phase structure of ferrite and carbide of the second phase to lower the yield point and increase the tensile strength. ing.

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 composite properties such as a yield ratio of 75% or less and a weather resistance at a tensile strength level of 40 to 60 kg.

That is, 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 alloying costs such as Cr addition are required. In order to reduce it, there is a demand for improving the weather resistance of ordinary steel.

[0009]

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.

Based on such knowledge, the present invention enables the production of a steel pipe or a square pipe having a low yield ratio and excellent weather resistance. The gist of the invention is the weight. %
At Cu: 0.10 to 2.0%, P: 0.070 to
The low alloy steel tube or square tube comprises one or 0.150% and at Ac ₁ or Ac ₃ -250~Ac ₃ -
A low yield ratio, characterized by heating to 20 ° C., subsequently cooling at a cooling rate of 15 ° C./sec or more, and then tempering in a temperature range of 200 to 600 ° C. if necessary,
It is also a method of manufacturing a steel pipe or a square pipe having excellent weather resistance.

[0012]

In the present invention, the heating temperature is set to Ac ₁ to Ac ₃
By increasing the transformation point and then quenching, the effect of work hardening in pipe forming and subsequent square tube forming is eliminated, and a dual phase steel is achieved.

Further, by lowering the tempering temperature, it is possible to manufacture a steel pipe (or square pipe) having a low yield ratio 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 for the production of steel pipes and subsequent forming of square pipes (including square steel pipes having a square cross section, and deformed steel pipes in other broad senses), 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 because the heating temperature in the subsequent heat treatment is regulated to the temperature at which the working strain is removed.

Next, the heating temperature after molding is set to Ac ₃ -250 to A.
The reason why the temperature is set to c ₃ -20 ° C. is that heating to this temperature range is aimed at simultaneously removing the forming strain while achieving dual-phase steeling after cooling. That is, if water is cooled immediately above Ac ₁ and then water-cooled, a two-phase steel is formed, but since the processing strain remains in the ferrite, the strength of the ferrite is high, and as a result, a low yield ratio cannot be achieved. Ac _{1 to} Ac ₃
By heating to a temperature higher than the middle of the above, that is, a temperature higher than Ac ₃ -250 ° C., this dual phase steeling and strain removal can be compatible, so this temperature was made the lower limit. As the heating temperature is increased, the yield ratio lower limit is passed and this time the yield ratio increases conversely. This is because the area ratio of ferrite decreases, and the yield ratio sharply 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. Rapid cooling after heating to Ac ₃ -250~Ac ₃ -20 is reheated at by austenitizing was a sufficiently cured that the thickened portion of the C and hardened structure in order to obtain a low yield ratio increases the tensile strength Is. 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.

By the way, depending on the type of steel, the toughness is not good only by rapid cooling after heating, and there is a case where a tempering process after rapid cooling is necessary to improve the 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, 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 basis of the defined range of each component 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 action, has an action 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 thereof 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 steel as an unavoidable impurity. However, if a too low N is aimed at, the cost for steelmaking remarkably increases, 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, but if it is added in excess of 2.0%, there is a margin of 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 also needs to be added in an amount of 0.070% or more in order to improve the weather resistance in an atmospheric corrosive environment, that is, the weather resistance, but if too much is added, the toughness and weldability of steel deteriorate, The upper limit of the content is 0.150%
And 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. since it is an expensive element.
The upper limit was 5%.

[0026] 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%.

[0028] Nb is useful for refining austenite grains and increasing strength, but if it increases, it impairs weldability, so the upper limit of the content is made 0.15%.

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

Ti is useful for grain refinement of austenite and is added, but if it increases, the weldability is impaired. Therefore, the upper limit of the content is 0.3%.

B has the 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,
The upper limit was made 0.0030% because it deteriorates the toughness.

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. Therefore, the upper limit of the content is 0.006%.

[0033]

[Examples] Table 1 shows the chemical composition of the test steel, and Table 2 shows the size of the steel pipe or the 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 pipe Nos. Shown in Table 2 A1, B1, C
1, D1, H1, I1, J1, K1, L1, M1, N
1, O1, P1, Q1, R1, S1, T1, U1, V1
Is the low yield ratio (yield ratio 70%)
The following) and high weather resistance (corrosion weight loss in atmospheric exposure test for 4 years <8.0 g / 100 cm ² ) are simultaneously achieved.

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 due to insufficient cooling rate. Since the heating temperature of D2 is too low, the yield ratio is high. Also, E1, F1 and G1 are C
Since neither u nor P satisfies the required amount, the weather resistance does not satisfy the target value.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

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 steel pipe or a square pipe having a high yield strength 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 (6)

[Claims] 1. Cu: 0.10 to 2.00 in% by weight.
%, P: 0.070 to 0.150% of a low alloy steel steel pipe containing one or two kinds, Ac ₃ −250 to Ac ₃ −20 ° C.
A method for producing a steel pipe having a low yield ratio and excellent weather resistance, which comprises heating the steel pipe to a low temperature and subsequently cooling at a cooling rate of 15 ° C./sec or more. 2. Cu: 0.10 to 2.00 at% by weight.
%, P: 0.070 to 0.150% of a low alloy steel steel pipe containing one or two kinds, Ac ₃ −250 to Ac ₃ −20 ° C.
A method for producing a steel pipe having a low yield ratio and excellent weather resistance, which comprises heating the steel pipe to a high temperature, subsequently cooling at a cooling rate of 15 ° C./sec or more, and then tempering in a temperature range of 200 to 600 ° C. 3. Cu: 0.10 to 2.00 in% by weight.
%, P: 0.070 to 0.150% of a low alloy steel square tube containing one or two kinds, Ac ₃ −250 to Ac ₃ −20 ° C.
A method for producing a square tube having a low yield ratio and excellent weather resistance, characterized by comprising heating to 50 ° C./sec and subsequently cooling at a cooling rate of 15 ° C./sec or more. 4. Cu: 0.10 to 2.0 in weight%.
%, P: 0.070 to 0.150% of a low alloy steel square tube containing one or two kinds, Ac ₃ −250 to Ac ₃ −20 ° C.
A method for producing a steel pipe having a low yield ratio and excellent weather resistance, which comprises heating the steel pipe to a high temperature, subsequently cooling at a cooling rate of 15 ° C./sec or more, and then tempering in a temperature range of 200 to 600 ° C. 5. The low alloy steel pipe or square pipe according to claim 1, 2 or 3 or 4, wherein the low alloy steel pipe or the square pipe comprises the following components: 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: 0.070 to 0.150%, low alloy steel consisting of balance Fe and unavoidable impurities 6. The low alloy steel pipe or square pipe according to claim 1, 2 or 3 or 4, wherein the low alloy steel pipe or the square pipe comprises the following components: 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: 0.070 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
Low alloy steel containing one or two or more of 0.0030% and Ca 0.0080% or less, and the balance Fe and unavoidable impurities.