JPH06264143A - Production of steel tube with low yield ratio for construction use by cold forming - Google Patents

Abstract

PURPOSE:To produce a steel tube with low yield ratio for construction use, having high productivity, cost effectiveness, and dimensional accuracy, by subjecting a steel containing specific amounts of C, Si, Mn, P, S, Ti, Al, and N to respectively specified hot rolling, hardening, tempering, and cold forming. CONSTITUTION:A steel, which has a composition consisting of, by weight, 0.01-0.20% C, <=0.5% Si, 0.5-1.6% Mn, <=0.03% P, <=0.01% S, 0.005-0.025% Ti, <=0.06% Al, <=0.006% N, and the balance iron with inevitable impurities and further containing, if necessary, prescribed amounts of Ni, Cu, Cr, Mo, Nb, V, Ca, etc., is reheated up to 900-1200 deg.C and rolled at >=30% cumulative rolling reduction at <=950 deg.C at a finishing temp. between (Ar3+120 deg.C) and (Ar3-20 deg.C). The resulting steel plate is air-cooled down to (Ar3-20 deg.C) to (Ar3-100 deg.C), hardened down to <=200 deg.C, and tempered at a temp. not higher than the Ac1 transformation point. The resulting steel plate having yield ratio YR satisfying the relation in YR<=80-0.8t/D [where (t) means plate thickness and D means the outside diameter of steel tube] is cold-formed, by which the 600N/mm<2> class steel tube with low yield ratio for construction use can be obtained.

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, and the cold-formed steel pipe has a drawback that it is difficult to apply to a building structure because the plastic deformation capacity after yielding is small.

On the other hand, there are centrifugal casting method, heat treatment (quenching and tempering) in 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] Specific means for overcoming the above problems and achieving the object are shown in the following (1) and (2). (1) C 0.01 to 0.20% by weight and 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 0.02
Steel containing 5%, Al 0.06% or less, N 0.006% or less, and the balance iron and unavoidable impurities
It is reheated to a temperature range of 00 to 1200 ° C., the cumulative reduction amount of 950 ° C. or less is 30% or more, and the finishing temperature is Ar ₃ +12.
After rolling to 0 ° C. or lower and Ar ₃ −20 ° C. or higher, the steel sheet is air-cooled to Ar ₃ −20 ° C. to Ar ₃ −100 ° C., and subsequently quenched from this temperature to 200 ° C. or lower,
Tempered in the temperature range below the Ac ₁ transformation point, and Y
Using a steel plate controlled to R ≦ 80−0.8 × t / D, t /
Steel pipes are manufactured by cold forming within the range of D ≦ 10%, the plate thickness is 100 mm or less, and the YR in the pipe axis direction is 80.
% Yield 600N / mm ² class steel pipe for building with low yield ratio.

(2) C 0.01 to 0.20 in weight ratio
%, 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 0.06% or less, N 0.
006% or less, Ni 0.05 to 1.0%, Cu
0.05-0.5%, Cr 0.05-1.0%, Mo
0.05-1.0%, Nb 0.005-0.03%,
V 0.005-0.05%, Ca 0.001-0.
Steel containing 006% of 1 type or 2 types or more and the balance consisting of iron and inevitable impurities is reheated to a temperature range of 900 to 1200 ° C., and a cumulative reduction amount of 950 ° C. or less is 30
% Or more and the finishing temperature is Ar ₃ + 120 ° C. or less Ar ₃ -20
After rolling to a temperature of ℃ or more, the steel plate is Ar ₃ -2
Air cooling from 0 ° C to Ar ₃ -100 ° C, followed by quenching from this temperature to 200 ° C or lower, tempering treatment in the temperature range of Ac ₁ transformation point or lower, and YR ≤ 80-0.8xt.
A steel plate manufactured by cold forming in the range of t / D ≦ 10% using a steel plate controlled to be / D.
Manufacturing method for low yield ratio 600N / mm ² class steel pipe for construction with 0mm or less and YR in the axial direction of 80% or less.

[0007]

The present invention will be described below. According to the research conducted by the inventors, it has been found that in order to reduce YR after cold working, it is necessary to control the material quality of the steel sheet before cold working, and particularly to control YR low. Therefore, the point of the present invention resides in (1) a YR value required for a steel sheet to be subjected to cold working, and (2) a manufacturing method in which the YR value is controlled to be the YR value or less.

The reason for limiting the reheating temperature to the range of 900 to 1200 ° C. is to keep the austenite grains at the time of heating small and to make the rolling structure finer. 1200 ° C is the upper limit temperature at which the austenite grains do not become extremely coarse during heating, and if the heating temperature exceeds this temperature, the austenite grains become coarsely mixed grains, and the structure after transformation becomes a coarse bainite structure. Is significantly deteriorated. On the other hand, if the heating temperature is too low, the rolling finish temperature will be too low, and a sufficient material improvement effect cannot be expected. Also, when precipitation hardening elements such as Nb and V are added, they do not form a solid solution sufficiently and the balance of strength and toughness deteriorates. Therefore, it is necessary to set the lower limit to 900 ° C.

The slab heated under the above conditions was
Rolling is performed so that the cumulative reduction amount in the unrecrystallized region of 50 ° C. or less is 30% or more and the finishing temperature is Ar ₃ + 120 ° C. or less and Ar ₃ −20 ° C. or more. This is because the austenite grains are made finer by rolling in the unrecrystallized region. The lower limit of the finishing temperature was set to Ar ₃ -20 ° C. is
This is because the toughness is not deteriorated by rolling in the (γ + α) region below the excessive transformation point. On the other hand, if the finishing temperature is too high, the effect of refining the austenite grains cannot be expected and the toughness deteriorates. Therefore, it is necessary to set the upper limit to Ar ₃ + 120 ° C.

Next, the cooling conditions after rolling are as follows. This is air cooling after completion of rolling and the steel sheet temperature is Ar ₃ -20 ° C. to Ar ₃ -1.
It is necessary to quench from a temperature of 00 ° C. to room temperature and then to perform tempering treatment in a temperature range below the Ac ₁ transformation point. The reason for this is that an appropriate amount of pro-eutectoid ferrite is precipitated and then untransformed austenite enriched with carbon is quenched to obtain a final structure of ferrite-bainite-martensite. The lower limit of the cooling start temperature is set to Ar ₃ -100 ° C., because if the temperature is lower than this, the precipitation amount of ferrite increases and the strength decreases. Further, the upper limit is set to Ar ₃ −20 ° C., because at a temperature higher than this, the precipitation amount of ferrite is small and the yield strength is not lowered, so that a low yield ratio steel cannot be obtained.

Further, the YR value of the steel sheet before cold forming (t / D ≦ 10%) is controlled to be (80-0.8 × t / D) or less. This is to control the YR value after cold forming to 80% or less, and for steel plates with YR values higher than this, the Y by cold forming is used.
The increase in R causes YR in the steel pipe to exceed 80%.

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 0.20%. Si is an element that is inevitably contained in steel for deoxidation, but Si is an element that is unfavorable for HAZ toughness and weldability, so its upper limit was made 0.5%. Mn is an extremely important element that improves strength and toughness at the same time, and 0.5% or more is necessary. However, if added in a large amount, weldability, deterioration of toughness of the base metal and HAZ are deteriorated, so the upper limit is 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 deoxidized upper steel, but since Si and Ti also perform deoxidation, the lower limit is not limited for the steel of the present invention. 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 the strength, and TiN is formed to form the same as described above. 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.
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.
% Or less, the effect is thin, and if 1.0% or more, the cost is extremely high and the economy is lost. Therefore, the upper limit was made 1.0%. C
u has an effect similar to that of Ni, and also has an effect of increasing strength due to Cu precipitates and improving corrosion resistance and weather resistance. In this case, when the amount of Cu exceeds 0.5%, the precipitation effect is remarkable, and it becomes difficult to lower YR due to the excessive precipitation effect in the heat treatment. Limited to 0.5%.

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 weld zone toughness and weldability are deteriorated, which is not preferable, so the content is limited to 0.05 to 1.0%.

Cr is an element that enhances the strength of the base material and the welded portion. 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 improve HAZ toughness. But 0.0
If it is less than 05%, there is no effect, and if it exceeds 0.03%, the YR of the steel sheet is hindered due to the excessive precipitation effect in the heat treatment. V is an element that has almost the same effect as Nb, but Nb
The precipitation effect is slightly inferior to that of. If it is less than 0.005%, the effect is small, and if it exceeds 0.05%, the YR of the steel sheet is hindered by the excessive precipitation effect in the heat treatment.

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%.

[0023]

[Example] A steel plate is manufactured by a well-known converter, continuous casting, and thick plate process, and then a steel pipe is manufactured by cold forming.
The toughness was investigated. Inventive steels 10 to 1 to 9 in Table 1
18 to 18 show the chemical composition of comparative steels. Table 2 shows the steel plate manufacturing conditions and the mechanical properties of the present invention steel and the comparative steel. The steels 1 to 9 of the present invention in Table 2 have achieved a well-balanced strength and toughness in the steel pipe, and the YR is also 80% or less.

On the other hand, in Comparative Steel 10, since the finishing temperature is high, the grain size of the crystal grains is not sufficiently reduced, and the toughness is deteriorated. Comparative Steel 11 has a low rolling reduction of 950 ° C. or lower, so that the crystal grains are not sufficiently refined and the toughness is deteriorated. In Comparative Steel 12, since the heating temperature is high, the grain size of the crystal grains is not sufficiently reduced, and the toughness is deteriorated. In Comparative Steel 13, the strength is lowered because the cooling start temperature is low. In Comparative Steel 14, since the cooling start temperature is high, the YR of the steel plate is high and the YR of the steel pipe is also high. In Comparative Steel 15, the toughness is deteriorated because the finishing temperature is low. In Comparative Steel 16, the YR of the steel pipe is high because the yield ratio (YR) of the steel plate is high (> 80−0.8 × t / D). Since the comparative steel 17 has a high Nb, the YR of the steel plate is high and the YR of the steel pipe is also high. Comparative steel 18
However, since V is high, YR of the steel plate is high and YR of the steel pipe is also high.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

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.

Claims (2)

[Claims] 1. A weight ratio of C: 0.01 to 0.20% and Si: 0.5%.
Hereinafter, Mn: 0.5 to 1.6%, P: 0.03
% Or less, S: 0.01% or less, Ti: 0.00
5 to 0.025%, Al: 0.06% or less, N: 0.00
Steel containing 6% or less and the balance consisting of iron and unavoidable impurities is reheated to a temperature range of 900 to 1200 ° C. to 950 ° C.
The following cumulative reduction is 30% or more and the finishing temperature is Ar ₃ +1
After rolling to 20 ° C. or lower Ar ₃ −20 ° C. or higher, the steel sheet is air-cooled to Ar ₃ −20 ° C. to Ar ₃ −100 ° C., and subsequently quenched from this temperature to 200 ° C. or lower, Ac _A tempering process is performed in a temperature range of ₁ transformation point or lower, and a yield ratio (YR) ≦ 80−0.8 × t / D (t: plate thickness,
D: Steel pipe outer diameter controlled) to produce a steel pipe by cold forming using a steel plate, and a low yield ratio for construction 600
N / mm ² class steel pipe manufacturing method. 2. C: 0.01 to 0.20% and Si: 0.5% by weight.
Hereinafter, Mn: 0.5 to 1.6%, P: 0.03
% Or less, S: 0.01% or less, Ti: 0.00
5 to 0.025%, Al: 0.06% or less, N: 0.00
6% or less Ni: 0.05 to 1.0%, Cu: 0.05
~ 0.5%, Cr: 0.05-1.0%, Mo: 0.05
-1.0%, Nb: 0.005-0.03%, V: 0.00
5 to 0.05%, Ca: 0.001 to 0.006%, containing 1 or 2 or more kinds, and the balance consisting of iron and unavoidable impurities, the steel is reheated to a temperature range of 900 to 1200 ° C. Then, after rolling such that the cumulative reduction amount of 950 ° C. or less is 30% or more and the finishing temperature is Ar ₃ + 120 ° C. or less Ar ₃ −20 ° C. or more, the steel sheet is Ar ₃ −20 ° C. to A.
Air cool to r ₃ -100 ° C., then continue from this temperature to 200
Quenched to ℃ or less, tempered in the temperature range of Ac ₁ transformation point or less, and yield ratio (YR) ≦ 80-0.8 ×
A method for manufacturing a low yield ratio 600N / mm ² class steel pipe for construction, characterized by manufacturing a steel pipe by cold forming using a steel sheet controlled to t / D.