JP2529044B2 - Manufacturing method of low yield ratio steel pipe for building by cold forming. - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low yield ratio steel pipe by cold forming for various structures in the fields of construction and civil engineering.

[0002]

2. Description of the Related Art Generally, when cold working is applied to steel, YP and TS rise due to work hardening, and YP and TS are higher than TS.
The yield ratio (hereinafter referred to as YR) also increases due to the large increase in the steel sheet, and the cold-formed steel pipe has a drawback that it is difficult to apply to a building structure because the plastic deformation ability after yielding is small.

On the other hand, there are centrifugal casting method, heat treatment (quenching and tempering) in the steel tube, etc. as the manufacturing method of the low YR steel pipe, but the centrifugal casting method has low productivity and economical efficiency. The heat treatment of the steel pipe was inferior to the steel pipe manufactured by cold forming the steel plate in terms of its economical efficiency and dimensional accuracy of the steel pipe.

[0004]

SUMMARY OF THE INVENTION The present invention provides a technique for manufacturing a steel pipe having a low YR by cold forming a steel plate. The steel pipe manufactured based on the method of the present invention has low YR and high productivity, economy and dimensional accuracy.

[0005]

Means for Solving the Problems The present invention overcomes the above-mentioned problems and achieves the object, and its concrete means are shown in (1) and (2) below.

(1) C 0.01 to 0.20 by weight
%, Si 0.5% or less, Mn 0.5 to 1.6%, P
0.03% or less, S 0.01% or less, Ti 0.0
05-0.025%, Al 0.1% or less, N 0.0
After containing 0.06% or less of steel and the balance of iron and unavoidable impurities, the steel is reheated and rolled, and then the Ac ₃ transformation point to 1
Reheat to the temperature range of 000 ℃, quench and continue to 70
A steel sheet reheated and hardened in a temperature range of 0 to 850 ° C. and tempered in a temperature range of Ac ₁ transformation point or lower is t / D.
(T: plate thickness, D: outer diameter of steel pipe) ≦ 10%, a steel pipe is manufactured by cold forming, and then annealed by holding at a temperature of 500 to 650 ° C. for 1 hour per plate thickness of 25 mm. And plate thickness 100 mm or less, YR in the tube axis direction is 8
Manufacturing method of low yield ratio 60kgf / mm ² class steel pipe for construction with 0% or less.

(2) C 0.01 to 0.20 by weight
%, Si 0.5% or less, Mn 0.5 to 1.6%, P
0.03% or less, S 0.01% or less, Ti 0.0
05-0.025%, Al 0.1% or less, N 0.0
06% or less, Cu 0.05 to 0.5%, Ni
0.05-1.0%, Cr 0.05-1.0%, Mo
0.05-1.0%, Nb 0.005-0.05%,
V 0.005-0.05%, Ca 0.001-0.
Steel containing 006% of one kind or two or more kinds and the balance consisting of iron and unavoidable impurities is reheated and rolled, and then reheated and heated to a temperature range of Ac ₃ transformation point to 1000 ° C. Then, the steel pipe which is reheated and tempered in the temperature range of 700 to 850 ° C. and tempered in the temperature range below the Ac ₁ transformation point is manufactured into a steel pipe by cold forming in the range of t / D ≦ 10%. Then, after that, annealing is performed in a temperature range of 500 to 650 ° C. for 1 hour per 25 mm of plate thickness, the plate thickness is 100 mm or less, and the YR in the tube axis direction is 80%.
The following is a method for manufacturing a low yield ratio 60 kgf / mm ² class steel pipe for construction.

[0008]

The present invention will be described below.

According to the research conducted by the inventors, Y after cold working
To lower R, control the material of the steel plate before cold working, especially Y
It has been found that it is necessary to control R to be low and to combine appropriate heat treatment (annealing) after cold working.

Therefore, the point of the present invention lies in a manufacturing method for controlling a steel sheet to be cold-worked to a required YR value or less, and a material control technique by heat treatment after cold-working the steel sheet.

The first quenching is for re-transforming to γ by reheating after hot rolling and quenching to make the structure bainitic and fine, and therefore the lower limit of the reheating temperature after hot rolling is set. Ac ₃ transformation point. If the reheating temperature is too high, the crystal grains become large and the low temperature toughness deteriorates, so the upper limit temperature must be set to 1000 ° C.

Next, it is necessary to reheat the second quenching to a temperature range of 700 to 850 ° C., quench it, and then temper it in a temperature range below the Ac ₁ transformation point. The reason for this is that the ferrite-austenite two-phase coexisting region is reheated so that C is enriched from ferrite to austenite, and the austenite enriched in C and the ferrite phase depleted in C are made to undergo quenching from that state. Is to obtain a two-phase mixed structure of a phase having extremely fine carbide and a phase having coarse carbide. The reduction of the yield ratio is achieved by this two-phase mixed structure.

However, quenching from a reheating temperature of more than 850 ° C. has few ferrite phases, and the effect of reducing the yield ratio cannot be expected, and quenching from less than 700 ° C. lowers the strength and the object cannot be achieved.

The tempering treatment is essential for improving the toughness of steel and preventing softening due to welding, stress relief treatment and the like. However, if the temperature exceeds the Ac ₁ point, the strength is remarkably reduced, so the temperature must be set to the Ac ₁ point or less.

Next, the reason for limiting the component range will be described. C is necessary in order to secure the strength of the base material, but if it is contained in a large amount, toughness or weldability is impaired, so an appropriate amount of C must be added. From such a viewpoint, C is 0.0
It was set to 1 to 0.2%.

Si is an element that is inevitably contained in steel for deoxidation. However, since Si is an unfavorable element in HAZ toughness and weldability, its upper limit was made 0.5%.

Mn is an extremely important element that simultaneously improves strength and toughness, and 0.5% or more is necessary, but
If a large amount is added, the weldability and the toughness of the base material and HAZ are deteriorated, so the upper limit was made 1.6%.

The reason why the impurities P and S in the steel of the present invention are 0.03% and 0.01% or less, respectively, is to further improve the low temperature toughness of the base material and the welded portion. Reduction of P prevents grain boundary destruction, and reduction of S amount reduces MnS
To prevent deterioration of toughness. The preferred P and S contents are 0.01% and 0.005% or less, respectively.

Ti forms carbonitrides and improves HAZ toughness. When the amount of Al is small, Ti oxide is formed to improve the HAZ toughness, but if it is less than 0.005%, it has no effect, and if it exceeds 0.025%, it has an unfavorable effect on the HAZ toughness. Limited to ~ 0.025%.

Al is generally an element contained in the deoxidized upper steel, but since Si and Ti also perform deoxidation, the lower limit of the steel of the present invention is not limited. However, if the amount of Al increases, the cleanliness of the steel deteriorates and the toughness of the welded portion deteriorates, so the upper limit was made 0.06%.

N is generally contained in steel as an unavoidable impurity, but it is combined with Nb and V to form a carbonitride to increase strength, and TiN is formed to form the above-mentioned material. Enhances the properties of HAZ. Therefore, the N content must be at least 0.001%. However, if the amount of N increases, it deteriorates the HAZ toughness and the occurrence of surface flaws in the continuously cast slab, so the upper limit was made 0.006%.

The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved. However, in order to further improve the characteristics for the object, the elements described below, namely Cu, Ni and C, are used.
Strength is obtained by selectively adding r, Mo, Nb, V, and Ca.
Further favorable results are obtained with respect to improvement in toughness.

Next, the above-mentioned additional element and its addition amount will be described. Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but 0.0
If it is 5% or less, the effect is weak, and if it is 1.0% or more, the cost is extremely high and the economy is lost, so the upper limit was made 1.0%.

Cu has almost the same effect as Ni,
It also has the effect of increasing strength and improving corrosion resistance and weather resistance due to Cu precipitates. In this case, when the amount of Cu exceeds 0.5%, the precipitation effect is remarkable, and it becomes difficult to reduce YR due to the excessive precipitation effect in the heat treatment after cold forming.
If it is less than 05%, there is no effect, so the Cu content is 0.05-0.
Limited to 5%.

Mo is an element that improves both the strength and toughness of the base material, but if the content is 0.05% or less, the effect is small and 1.0
%, The toughness and weldability of the welded part are deteriorated, which is not preferable.

Cr is an element which enhances the strength of the base material and the welded portion, and when the amount of Cr is 0.5% or more, the weather resistance is also improved.
If it exceeds 1.0%, the weldability and HAZ toughness are deteriorated, and if it is 0.05% or less, the effect is small. Therefore, the Cr amount is 0.
05 to 1.0%.

Nb forms fine carbonitrides to increase strength and HAZ toughness. But 0.0
No effect if less than 05%, HAZ if more than 0.05%
It has an unfavorable effect on toughness.

V is an element having almost the same effect as Nb, but its precipitation hardening ability is slightly inferior to that of Nb. 0.005
% Or less, the hardening is small, and if it exceeds 0.05%, the YR is hindered by the excessive precipitation effect in the heat treatment after cold forming.

Ca controls the morphology of sulfide (MnS),
It has the effect of increasing Charpy absorbed energy and improving low temperature toughness. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, CaO, C
A large amount of aS is generated and becomes large inclusions, which not only impairs the toughness of steel but also the cleanliness and adversely affects the weldability and lamella tear resistance. Therefore, the range of Ca addition amount is 0.001 to 0.0
It is set to 06%.

Next, the heat treatment (annealing) temperature after cold forming (t / D ≦ 10%) is set so that the strain in cold working is sufficiently released and Y
The lower limit temperature is 5 in order to reduce R and recover toughness.
Set to 00 ° C. Also, annealing at too high a temperature not only releases the cold strain but also softens the steel material itself, resulting in insufficient strength, Y
The upper limit temperature is set to 650 ° C. because R is increased.

[0031]

EXAMPLE A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and then a steel pipe was manufactured by cold forming and annealed and heat-treated, and its strength and toughness were investigated.

Tables 1 to 6 show the chemical compositions of the present invention steels, and 7 to 13 show the chemical compositions of the comparative steels. Table 2 shows the steel plate manufacturing conditions and the mechanical properties of the present invention steel and the comparative steel.

Steels 1 to 6 of the present invention shown in Table 2 are strengths in steel pipes,
The toughness is achieved in a well-balanced manner, and the YR is 80% or less.

On the other hand, in Comparative Steel 7, the cold workability (t /
Since D) is too large as 12%, YR is high. Comparative Steel 8 lacks strength because the second quenching temperature is low. In Comparative Steel 9, since the first quenching temperature was high, the crystal grains were not sufficiently refined, and the toughness was deteriorated. Comparative steel 10 has a low annealing temperature, so YR
Is high and the toughness is deteriorated. Comparative Steel 11 has a high YR because the second quenching temperature is high. Comparative Steel 12 has a high YR because the annealing temperature is high. Comparative Steel 13 has a high V and therefore a high YR.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

INDUSTRIAL APPLICABILITY The steel pipe manufactured by the chemical composition and manufacturing method of the present invention has a low YR and an excellent plastic deformation ability after yielding. As a result, the safety of structures such as buildings and bridges can be greatly improved.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication C22C 38/14 C22C 38/14 (72) Inventor Akira Minami 1 Kimitsu, Kimitsu-shi Nippon Steel shares Company Kimitsu Works (56) References JP-A-5-105946 (JP, A) JP-A-3-20408 (JP, A)

Claims (2)

(57) [Claims] 1. By weight ratio, C: 0.01 to 0.20%, Si: 0.5% or less, Mn: 0.5 to 1.6%, P: 0.03% or less, S: 0. Steel containing 0.1% or less, Ti: 0.005 to 0.025%, Al: 0.1% or less, N: 0.006% or less, and the balance being iron and inevitable impurities was hot-rolled. After that, the steel sheet is reheated and quenched to a temperature range of Ac ₃ transformation point to 1000 ° C, subsequently reheated and quenched to a temperature range of 700 to 850 ° C, and tempered in a temperature range of Ac ₁ transformation point or less. Steel plate is manufactured by cold forming in the range of t / D ≦ 10%, where t is the plate thickness and D is the outer diameter of the steel pipe, and then annealed in the temperature range of 500 to 650 ° C. Yield ratio Steel pipe manufacturing method. 2. By weight ratio, C: 0.01 to 0.20%, Si: 0.5% or less, Mn: 0.5 to 1.6%, P: 0.03% or less, S: 0. 01% or less, Ti: 0.005 to 0.025%, Al: 0.1% or less, N: 0.006% or less, further Cu: 0.05 to 0.5%, Ni: 0.05 to 1 0.0%, Cr: 0.05 to 1.0%, Mo: 0.05 to 1.0%, Nb: 0.005 to 0.05%, V: 0.005 to 0.05%, Ca: After hot-rolling steel containing 0.001 to 0.006%, one or more of which is the balance of iron and inevitable impurities, the Ac ₃ transformation point
Reheat to the temperature range of 1000 ℃, quench and continue to 7
Reheat and quench in the temperature range of 0 to 850 ° C, and Ac ₁
A steel pipe that has been tempered in a temperature range below the transformation point is manufactured by cold forming within a range of t / D ≦ 10%, where t is the plate thickness and D is the outer diameter of the steel pipe, and then 500 to 650 ° C. A method for manufacturing a low yield ratio steel pipe for construction, characterized in that it is annealed in the above temperature range.