JPH06264144A - 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 <=900 deg.C. The resulting steel plate is hardened, without delay, from >=750 deg.C down to ordinary temp., further reheated up to 700-850 deg.C, hardened, and tempered at a temp. not higher than the Ac1 transformation point. The resulting steel plate in which yield ratio YR is controlled so that the relation in YR<=80-0.8Xt/D [wherein (t) means plate thickness and D means the outside diameter of steel tube] is satisfied, is cold-formed, thus 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] In order to overcome the above-mentioned problems and achieve the object, the concrete means are as follows (1),
It shows in (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
After reheating to a temperature range of 00 to 1200 ° C. and rolling so that the cumulative rolling reduction of 900 ° C. or less is 30% or more, immediately quenching from a temperature of 750 ° C. or more to room temperature,
Reheat and quench in the temperature range of 0 to 850 ° C, and Ac ₁
Tempered in the temperature range below the transformation point, and YR ≦ 8
T / D ≦ 1 using a steel plate controlled to 0-0.8 × t / D
A method for producing a low yield ratio 600N / mm ² class steel pipe for construction having a plate thickness of 100 mm or less and a YR of 80% or less in the tube axis direction, which is characterized in that a steel pipe is manufactured by cold forming in a range of 0%.

(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 Cu 0.05 to 0.5%, Ni
0.05-1.0%, Cr 0.05-1.0%, Mo
0.05-1.0%, Nb 0.005-0.03%,
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 unavoidable impurities is reheated to a temperature range of 900 to 1200 ° C., and a cumulative reduction amount of 900 ° C. or less is 30
%, And then immediately quenched from a temperature of 750 ° C. or higher to normal temperature, reheated to a temperature range of 700 to 850 ° C., quenched, and tempered in a temperature range of the Ac ₁ transformation point or lower. A steel pipe having a thickness of 100 mm or less, characterized by producing a steel pipe by cold forming within a range of t / D ≦ 10% using a steel plate that has been subjected to a treatment and controlled to YR ≦ 80-0.8 × t / D, Manufacturing method for low yield ratio 600N / mm ² class steel pipe for construction with YR of 80% or less in the pipe axis direction.

[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 00 ° C or less is 30% or more. This is because the austenite grains are made finer by rolling in the unrecrystallized region.

Next, the cooling conditions after rolling are as follows. After the rolling, the steel sheet is quenched from a temperature of 750 ° C or higher to room temperature, reheated and quenched to a temperature range of 700 to 850 ° C, and then Ac _It is necessary to perform tempering treatment within a temperature range of ₁ transformation point or lower. 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, the quenching temperature after rolling is 7
If the temperature is less than 50 ° C, the transformation proceeds and the desired strength cannot be obtained.
Quenching from a reheating temperature of more than 0 ° C. has few ferrite phases, and the yield ratio reduction effect cannot be expected. Quenching from less than 700 ° C. reduces the strength and fails to achieve the object.

[0012] 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.

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 not preferable in terms of 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 deoxidation is also performed by Si and Ti, 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%.

Although N is generally contained in steel as an unavoidable impurity, it is combined with Nb and V to form a carbonitride to increase the strength, and TiN is formed to form an alloy 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 purpose, 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 explained. 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 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%, its precipitation effect is remarkable, and it becomes difficult to lower YR due to an excessive precipitation effect in the heat treatment after cold forming. The amount is limited to 0.05-0.5%.

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 weak 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, and the Cr content is 0.5.
%, The weather resistance is improved, but if it exceeds 1.0%, the weldability and HAZ toughness are deteriorated, and if it is 0.05% or less, the effect is weak. Therefore, the Cr content is 0.05 to 1.0%.

Nb forms fine carbonitrides, increases strength and improves HAZ toughness. But 0.0
If it is less than 5%, there is no effect, and if it exceeds 0.03%, the YR of the steel sheet is hindered by the excessive precipitation effect in the heat treatment.
V is an element having almost the same effect as Nb, but its precipitation hardening ability is slightly inferior to that of Nb. If it is less than 0.005%, the hardening is small, and if it exceeds 0.05%, excessive precipitation hardening in the heat treatment hinders reduction of YR of the steel sheet.

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%.

[0025]

[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 shows the chemical composition of comparative steel. Table 2 shows the steel plate manufacturing conditions and the mechanical properties of the present invention steel and the comparative steel.

Steels 1 to 9 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. In contrast, Comparative Steel 10 lacks strength because the reheating temperature during quenching is too low. Comparative steel 11
Since the rolling reduction of 950 ° C. or lower is low, the crystal grains are not sufficiently refined, and the toughness is deteriorated. Comparative steel 1
In No. 2, 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. Comparative steel 1
No. 4, the yield ratio (YR) of the steel sheet is high (> 80-0.8x).
Because of t / D), the YR of the steel pipe is high. In Comparative Steel 15, the reheating temperature at the time of quenching was too high, and the YR of the steel plate was high, and the YR of the steel pipe was also high. N for comparative steel 16
Since b is high, the YR of the steel plate is high and the YR of the steel pipe is also high. Comparative steel 17 has a high V, so that Y of the steel plate
The R is high and the YR for steel pipe is also high.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

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 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 900 ° C.
After rolling such that the cumulative rolling reduction below is 30% or more, quenching is immediately performed from a temperature of 750 ° C. or higher to room temperature,
Reheat to a temperature range of 700-850 ° C, quench,
_A tempering process is performed in a temperature range of ₁ transformation point or lower, and a yield ratio (YR) ≤ 80-0.8 x t / D (t: plate thickness, D: steel pipe outer diameter) is used to cool the steel plate. Manufacturing method of low yield ratio 600N / mm ² class steel pipe for construction, characterized by manufacturing steel pipe by hot forming. 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: 0.001% to 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: 0.00
5 to 0.05% Ca: 0.001 to 0.006% of 1 type or 2 types or more, and the remainder is steel and iron and inevitable impurities are reheated to a temperature range of 900 to 1200 ° C. After rolling at a cumulative rolling reduction of 900 ° C or lower to be 30% or higher, it is immediately quenched from a temperature of 750 ° C or higher to room temperature, reheated to a temperature range of 700 to 850 ° C, and quenched. Tempering is performed in the temperature range of ₁ transformation point or less, and yield ratio (YR) ≤ 80-0.8 x t / D
A method for manufacturing a low yield ratio 600 N / mm ² class steel pipe for construction, characterized by producing a steel pipe by cold forming using a steel plate controlled to (t: plate thickness, D: steel pipe outer diameter).