JPH07188748A - Manufacturing method of high strength and low yield ratio steel pipe for construction - Google Patents

Abstract

(57) [Summary] [Structure] C: 0.03 to 0.20% Si: 0.01 to 0.50% M by weight%
n: 0.5-2.0%, solAl: 0.005-0.10%, balance F
e and unavoidable impurities, and is shown below (1)
Parameters representing the island martensite tendency in the two-phase region heating which is calculated by the formula: a steel value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, bending the steel plate cold working Then, the seam is welded into a steel pipe and reheated to a temperature range of Ac ₁ or more and Ac ₃ or less in the two-phase region, and then cooled at a cooling rate of air cooling or more and 30 ° C./sec or less, or slow cooling. To produce a high strength and low yield ratio steel pipe for construction. [Effect] It has become possible to economically manufacture a high-strength, low-yield ratio steel pipe for construction without lowering the toughness, with good productivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel pipe having a wall thickness of about 12 to 150 mm, a tensile strength of 500 to 650 N / mm ² and a high strength and a low yield ratio, which is used for construction of high-rise buildings. Manufacturing method.

[0002]

2. Description of the Related Art Steel pipes used for construction of high-rise buildings may require a low yield ratio in addition to performance such as strength and toughness or weldability. A treatment method has been proposed.

For example, a steel plate manufactured by rolling is
Reheat to a temperature range of ₃ or higher and quench, then 700
Method of reheating to ~ 850 ° C, quenching, tempering in the temperature range of Ac ₁ or less, manufacturing a steel pipe by cold forming, and then annealing in the temperature range of 500-650 ° C No. 5-117746), and 750 after rolling.
Immediately quench from the temperature of ℃ to room temperature, 700 ~ 850 ℃
The reheating and quenching, after performing tempering at a temperature range of Ac ₁ or less, to manufacture a steel tube by cold forming, followed method of annealing in the temperature range of 500 to 650 ° C. (JP-5
No. 117,747) has been proposed.

Further, the steel pipe is made to have a temperature of Ac ₃ −250 ° C. to Ac ₃ −.
A method of heating in a temperature range of 20 ° C. and water cooling at a cooling rate of 30 ° C./sec or more (JP-A-3-87318),
After heating to Ac ₃ or more, air-cooling and Ar ₃ −250 ° C.
A method of cooling with water at a cooling rate of 30 ° C./sec or more from a temperature range of Ar ₃ −20 ° C. (JP-A-3-87317),
Alternatively Ac ₃ -250 was heated to ° C. to Ac ₃ -20 temperature range of ° C., after quenching with 15 ° C. / sec or more cooling rate,
A method of imparting cold working strain and tempering (Japanese Patent Application Laid-Open No. 3-219
No. 017), and after air-cooling after heating to Ac ₃ or more, the temperature range from Ac ₃ −250 ° C. to Ac ₃ −20 ° C.
After quenching at a cooling rate of ° C / sec or more, a method of imparting cold working strain and tempering (JP-A-3-219018), and further heating the steel pipe to Ac ₃ 3-200 ° C or more, Ac ₃ Strain starts to be applied at -200 ° C or higher, and Ac _3-
Straining is completed in the temperature range of 200 ° C to Ac ₃ -20 ° C,
Method of tempering after cooling with water (JP-A-4-321)
Have been proposed.

[0005]

Although various methods have been proposed for producing a steel pipe having a low yield ratio as described above, "reheating quenching after rolling-two-phase region quenching-tempering-"
As in "cold forming-annealing" or "direct hardening-two-phase region quenching-tempering-cold forming-annealing", reheating is required 3 to 4 times, or "30 ° C after heating to the two-phase region temperature". / Se
After water cooling with "c or more", or "after air cooling with heating to Ac ₃ or more,
When the plate thickness is 40 mm or more, it is necessary to perform water cooling at an extremely high cooling rate that exceeds the theoretical limit value of water cooling, such as "water cooling from the two-phase region temperature to 30 ° C / sec or more". "Temperature after heating to the phase region temperature, quenching after cold working strain is imparted", "After air-cooling by heating to Ac ₃ or higher, quenching from the two phase region temperature, cold working strain is applied and then tempering" The manufacturing cost rises because the steel pipe needs to be further strained by applying strain to the steel pipe, such as "heating above the two-phase region temperature, applying strain at the two-phase region temperature, immediately quenching and then tempering". Therefore, a large capital investment is required, and a manufacturing method having higher productivity and economical efficiency is demanded.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a high-strength low-yield ratio steel pipe for construction, which is excellent in productivity and economy and has high toughness.

[0007]

Means for Solving the Problems The inventors of the present application did not perform heat treatment a number of times, did not require an extremely high cooling rate, and did not need to impart strain to a steel pipe. As a result of various studies to obtain a low steel pipe manufacturing method, by controlling the composition of components to a specific range, even if it is heated to the two-phase temperature of ferrite + austenite and formed from that temperature range, island-shaped martensite by suppressing the generation so as not to cause deterioration of toughness, the steel to produce a steel sheet by hot rolling using, which was a molded and welded to the steel pipe between the cold, Ac ₁ transformation point or more, Ac _By heating in a temperature range of ₃ transformation points or less and then cooling at a cooling rate of air cooling or more and 30 ° C./sec or less, or by gradually cooling, the problem of deterioration of toughness that has been a problem in conventional two-phase region heating occurs. High strength and low yield without It was found that the steel pipe is obtained. That is, it has been conventionally known that a low yield ratio can be obtained by heating to a two-phase region, but at the same time, it is accompanied by a remarkable deterioration of toughness, so that after heating to this temperature region, after tempering or stress relief annealing, etc. By adopting the above-described component composition of the heat-treated material, deterioration of the toughness is less likely to occur even if it is heated in the two-phase region.

The present invention has been completed on the basis of such findings. Firstly, C: 0.03% by weight,
0.20%, Si: 0.01 to 0.50%, Mn: 0.
5% to 2.0%, solAl: 0.005 to 0.10%
Containing the balance consisting of Fe and unavoidable impurities, and parameters representative of the (1) shown island martensite tendency in the two-phase region heating which is calculated by the formula below: P values of _{the MA} P _MA ≦ 120 The steel to be made into a steel sheet by hot rolling, this steel sheet is cold-bent, the seam portion is welded into a steel pipe, and reheated to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less, The present invention provides a method for manufacturing a high-strength low-yield ratio steel pipe for construction, characterized by cooling at a cooling rate of not less than air cooling and not more than 30 ° C / sec.

Secondly, C: 0.03 to 0.2 by weight.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, containing 1 or 2 or more kinds, consisting of the balance Fe and unavoidable impurities, and calculated by the formula (1) shown below. parameters representing the island martensite tendency in the two-phase region heating: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, bending a steel sheet by cold, the joint portion Welded into a steel pipe, reheated to the temperature range of Ac ₁ or more and Ac ₃ or less in the two-phase region, and then air cooled or more, 30 ° C / se
c A method for producing a high-strength, low-yield ratio steel pipe for construction, which is characterized by cooling at the following cooling rate.

Thirdly, in% by weight, C: 0.03 to 0.2.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, further Cu: 1.5% or less, Ni: 1.0% or less,
Cr: 1.0% or less, Mo: 0.5% or less, one or more kinds are contained, the balance is Fe and inevitable impurities, and two-phase calculated by the formula (1) shown below. Parameters indicating the tendency of island martensite formation during zone heating:
Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, the steel sheet was bent with cold, and the steel pipe by welding the seam portions, and Ac ₃ following two at Ac ₁ or more It is intended to provide a method for producing a high-strength low-yield ratio steel pipe for construction, which comprises reheating to a phase region temperature range and then cooling at a cooling rate of not less than air cooling and not more than 30 ° C./sec.

Fourth, C: 0.03 to 0.2 by weight%.
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and Cu: 1.5% or less, Ni:
1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more types are contained, the balance is Fe and inevitable impurities, and the formula (1) is shown below. parameters representing the island martensite tendency in the two-phase region heating in the calculation: the steel value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, working and bending the steel plate cold ,
Weld the seam to make a steel pipe, and if it is Ac ₁ or more and Ac ₃
Reheat to the following two-phase temperature range, then air cool or more, 3
It is intended to provide a method for manufacturing a high-strength low-yield ratio steel pipe for construction, which is characterized by cooling at a cooling rate of 0 ° C./sec or less.

Fifth, C: 0.03 to 0.2 by weight%
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005-0.10%, C
a: 0.0005 to 0.0050%, balance Fe
And becomes unavoidable impurities, and represents the island martensite tendency in the two-phase region heating is calculated by the following equation (1) Parameters: Steels value of P _MA is P _MA ≦ 120, heat A steel plate is formed by hot rolling, the steel plate is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less, and then air cooling or more, 30 ° C. The present invention provides a method for manufacturing a high-strength, low-yield ratio steel pipe for construction, which is characterized by cooling at a cooling rate of not more than / sec.

Sixth, in% by weight, C: 0.03 to 0.2
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and B: 0.0005 to 0.003.
0%, Ca: 0.0005 to 0.0050%, containing 1 or 2 kinds, consisting of the balance Fe and inevitable impurities, and island-like when heating in the two-phase region calculated by the formula (1) Parameter indicating martensite formation tendency: The value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, which is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises heating and then cooling at a cooling rate of not less than air cooling and not more than 30 ° C./sec.

Seventh, in% by weight, C: 0.03 to 0.2
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, further Cu: 1.5% or less, Ni: 1.0% or less,
One or more of Cr: 1.0% or less and Mo: 0.5% or less, and B: 0.0005 to 0.0030%,
Ca: 1 or 2 out of 0.0005 to 0.0050%
A parameter that contains at least one species, consists of the balance Fe and unavoidable impurities, and represents the island-like martensite formation tendency during heating in the two-phase region calculated by equation (1): the value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, which is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises heating and then cooling at a cooling rate of not less than air cooling and not more than 30 ° C./sec.

Eighth, C: 0.03 to 0.2 by weight%
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, Cu: 1.5% or less, Ni: 1.0
% Or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more, and B: 0.0005 to 0.00
30%, Ca: 1 out of 0.0005 to 0.0050%
Containing species or two, and the balance Fe and unavoidable impurities, and (1) the parameters representative of the island martensite tendency in the two-phase region heating which is calculated by the formula: The value of P _MA is P _MA ≦ The steel to be 120 is made into a steel plate by hot rolling, the steel plate is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less. Then, the present invention provides a method for producing a high-strength, low-yield ratio steel pipe for construction, characterized by cooling at a cooling rate of not less than air cooling and not more than 30 ° C / sec.

Ninth, in% by weight, C: 0.03 to 0.2
0%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, consisting of balance Fe and unavoidable impurities, and island martensite at the time of heating in the two-phase region calculated by the following formula (1) parameters representing the generated trend: the value of P _MA is P _MA ≦
The steel to be 120 is formed into a steel plate by hot rolling, the steel plate is cold-bent, and the seam portion is welded into a steel pipe.
It is intended to provide a method for producing a high-strength low-yield ratio steel pipe for construction, which comprises reheating to a two-phase region temperature range of c ₁ or more and Ac ₃ or less and then gradually cooling.

Tenth, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
.About.2.0%, solAl: 0.005 to 0.10%, and Nb: 0.005 to 0.05%, V: 0.
01 to 0.10%, Ti: 0.005 to 0.10%, containing one or more kinds, consisting of the balance Fe and unavoidable impurities, and calculated by the formula (1) shown below. parameters representing the island martensite tendency in the two-phase region heating: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, bending a steel sheet by cold, the joint portion A method for producing a high-strength, low-yield ratio steel pipe for construction, which comprises welding to form a steel pipe, reheating to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less, and then gradually cooling. is there.

Eleventh, in% by weight, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
˜2.0%, solAl: 0.005 to 0.10%, Cu: 1.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5 % Out of 1
Contains more species or two or the balance Fe and unavoidable consists impurities, and parameters representative of the island martensite tendency in the two-phase region heating is calculated by the following equation (1): The value of P _MA Steel with P _MA ≦ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, and the temperature range of the two-phase region is between Ac ₁ and Ac ₃ inclusive. The present invention provides a method for producing a high-strength low-yield-ratio steel pipe for construction, which is characterized in that the steel pipe is reheated to room temperature and then slowly cooled.

Twelfth, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
.About.2.0%, solAl: 0.005 to 0.10%, and Nb: 0.005 to 0.05%, V: 0.
01 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and Cu: 1.5% or less, Ni:
1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more types are contained, the balance is Fe and inevitable impurities, and the formula (1) is shown below. parameters representing the island martensite tendency in the two-phase region heating in the calculation: the steel value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, working and bending the steel plate cold ,
Weld the seam to make a steel pipe, and if it is Ac ₁ or more and Ac ₃
It is intended to provide a method for producing a high-strength, low-yield ratio steel pipe for construction, which comprises reheating to the following two-phase region temperature range and then gradually cooling.

Thirteenth, in% by weight, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
~ 2.0%, solAl: 0.005-0.10%, C
a: 0.0005 to 0.0050%, balance Fe
And becomes unavoidable impurities, and represents the island martensite tendency in the two-phase region heating is calculated by the following equation (1) Parameters: Steels value of P _MA is P _MA ≦ 120, heat A steel plate is formed by hot rolling, the steel plate is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less, and then gradually cooled. The present invention provides a method for producing a characteristic high-strength, low-yield ratio steel pipe for construction.

Fourteenth, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
.About.2.0%, solAl: 0.005 to 0.10%, and Nb: 0.005 to 0.05%, V: 0.
01 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and B: 0.0005 to 0.00
30%, Ca: 1 out of 0.0005 to 0.0050%
Containing species or two, and the balance Fe and unavoidable impurities, and (1) the parameters representative of the island martensite tendency in the two-phase region heating which is calculated by the formula: The value of P _MA is P _MA ≦ The steel to be 120 is made into a steel plate by hot rolling, the steel plate is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less. The present invention provides a method for producing a high-strength low-yield ratio steel pipe for construction, which is characterized by slow cooling.

Fifteenth, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
˜2.0%, solAl: 0.005 to 0.10%, Cu: 1.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5 % Out of 1
Or two or more, and B: 0.0005 to 0.0030
%, Ca: 0.0005 to 0.0050%, containing one or more kinds, consisting of the balance Fe and inevitable impurities, and island-like when heating in the two-phase region calculated by the formula (1) parameter represents the martensite tends: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, the steel sheet was bent with cold, and the steel pipe by welding the seam portions, Ac ₁ The present invention provides a method for producing a high-strength, low-yield ratio steel pipe for construction, which comprises reheating to a temperature range of two-phase region of Ac ₃ or less and then gradually cooling.

Sixteenth, C: 0.03 to 0.
20%, Si: 0.01 to 0.50%, Mn: 0.5%
.About.2.0%, solAl: 0.005 to 0.10%, and Nb: 0.005 to 0.05%, V: 0.
01 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, Cu: 1.5% or less, Ni: 1.
0% or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more, and B: 0.0005 to 0.0
030%, Ca: 0.0005 to 0.0050%, containing 1 or 2 kinds, consisting of balance Fe and unavoidable impurities, and island-like when heating in the two-phase region calculated by the formula (1) parameter represents the martensite tends: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling,
This steel sheet is cold-bent, welded at the joint to form a steel pipe, reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less, and then gradually cooled. The present invention provides a method for manufacturing a high strength low yield ratio steel pipe.

P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) Next, the construction according to the present invention A method of manufacturing a high-strength, low-yield ratio steel pipe for use will be described in detail by dividing it into component compositions and manufacturing conditions.

[Component composition] In the present invention, C, Si, M
n and solAl as basic components, (a) Nb, V,
One or more of Ti, (b) Cu, Ni, C
One or two or more of r and Mo, and one or two of (c) Ca and B are selected as the selected components, and the target steel is the basic component alone or the basic components and these (a) to (). One or more of c) are to be contained. In addition, when only the above (c) is added to the basic component, Ca
Limited to a single person.

Further, in the present invention, as will be described later, P _MA = 325 × [C%] + 9 × [Si%] + 40 × [M
By setting the parameter represented by [n%] + 3 × [solAl%] + 380 × [Nb%] to 120 or less, the tendency of island martensite formation during heating in the two-phase region can be significantly reduced.

The reasons for limiting the range of each component will be described below. In the following explanation, all percentages are% by weight.
Indicates.

C: To secure the strength of this type of steel inexpensively and effectively, C is required to be 0.03%. However, if it exceeds 0.20%, the weldability such as cold cracking and hot cracking is impaired. Therefore, the C content is 0.03 to 0.2.
Specify in the range of 0%.

Si: Si is added for deoxidation, but if it is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, while if it exceeds 0.50%, toughness and weldability are deteriorated. Therefore, the Si content is specified in the range of 0.01 to 0.50%.

Mn: Mn is added as a basic element of steel effective in improving the strength and toughness of the steel, but if it is less than 0.5%, its effect is small, and if it exceeds 2.0%, the weldability is remarkable. to degrade. Therefore, the Mn content is 0.5 to 2.
Specify in the range of 0%.

SolAl: SolAl is added for deoxidation of steel, but if it is less than 0.005%, a sufficient effect cannot be obtained, while at 0.10%, the effect is saturated. Therefore, Al is specified in the range of 0.005 to 0.10%.

The reasons for limiting the basic components have been described above. The reasons for limiting the selective components shown in the above (a) to (c) will be described below.

(A) Nb, V, Ti: These elements bring about an increase in strength due to precipitation strengthening and an improvement in toughness due to the refinement of crystal grains, but when the addition amount increases, the toughness of the welded portion and the like deteriorate. Nb: 0.005-0.05
%, V: 0.01 to 0.10%, Ti: 0.005 to
It is specified in the range of 0.10%.

(B) Cu, Ni, Cr, Mo: These elements can be strengthened without impairing toughness through solid solution strengthening and structural change due to increase in hardenability, but Cu is 1 from the viewpoint of weldability and economy. 0.5% or less, Ni, Cr 1.0
%, And Mo is specified to be 0.5% or less.

(C) B, Ca: B increases the hardenability of steel and exerts a great effect on the strength increase, but 0.0
If it is less than 005%, this effect is small, and 0.0030
%, The weldability is impaired, so 0.0005 to 0.
Specified within the range of 0030%. Further, Ca is effective in preventing hydrogen-induced cracking and lamellathea by spheroidizing the form of inclusions, but if less than 0.0005%, the effect cannot be obtained, and if added in excess of 0.0050%. However, the effect is saturated, so the range is defined as 0.0005 to 0.0050%.

P _MA : The effect of alloying elements on the tendency of island martensite formation when heating to the two-phase region and starting the molding from that temperature was quantitatively examined, and as shown in FIG. It became clear that it has a good correlation with the parameter P _MA calculated by the formula.

P _MA = 325 × [C%] + 9 × [Si%]
+40 x [Mn%] +3 x [solAl%] +380 x
[Nb%] Further, there is a good correlation between the toughness and the area fraction of island martensite. For example, as shown in FIG. 2, by setting the area fraction of island martensite to 5% or less, the Charpy impact test was performed at 0 ° C. The absorbed energy at 100 J is 100 J or more and good toughness is achieved. Here, in order to reduce the area fraction of island martensite to 5% or less, from FIG. 1, P _MA ≦ 1
It should be 20. Therefore, in the present invention, P _MA ≦ 12
It is defined as 0.

[Manufacturing conditions] In the present invention, after hot rolling, cold bending is performed, and the seam is welded into a steel pipe, and then Ac is obtained.
₁ above Ac ₃ was reheated to below the so-called two-phase region temperature region is heated to Ac ₁ or Ac ₃ following a so-called two-phase region temperature, after holding an arbitrary time, air cooling or more, 30 ° C. / sec
Cooling at the following cooling rate or slow cooling is an essential requirement.

The temperature of the two-phase region is heated to a value that yields a high yield ratio of usually 80% or more due to work hardening in the cold-formed state, but is 80% due to recovery and change in microstructure.
This is to obtain a low yield ratio of not more than%. As a method of heating to the two-phase region temperature, there is a method of putting the entire steel pipe in a heat treatment furnace, or a method of winding a ring-shaped high-frequency induction heating furnace device around the steel pipe and continuously heating it while moving it. It is selected in consideration of productivity.

Whether the cooling rate after heating in the two-phase region is air cooling, rapid cooling of 30 ° C./sec or less, or slow cooling, the low yield ratio targeted by the present invention is obtained. However, it depends on the required strength level and facility capacity. Here, the air cooling refers to one that is naturally cooled in the atmosphere, and is usually about 0.05 ° C./sec or more. The rapid cooling at a cooling rate of 30 ° C./sec or less means forced cooling by water cooling or gas cooling. Further, gradual cooling means cooling at a cooling rate lower than air cooling by some heat retention or heating. Then, the absolute value of the cooling rate is determined by the thickness of the steel pipe to be cooled.

For the hot rolling, a general method usually used in this field may be adopted. Further, in the production of a steel pipe, the above-mentioned steel plate may be bent by a normal method such as UOE forming or press bend forming in a cold state, and the seam may be welded by an appropriate method.

[0042]

By using the present invention, the following action does not cause deterioration of toughness even if the material is heated in the two-phase region and molded from that temperature region. That is, C, Si, Mn, solAl, and Nb are extracted as five elements that promote the formation of island martensite by adding a large amount of chemical components, and island martensite of these elements during heating in the two-phase region is extracted. The relative contribution to the generation is obtained and expressed as a parameter P _MA . By setting this value to 120 or less, the area fraction of island martensite can be set to 5% or less. Even if heating is performed, significant deterioration of toughness can be prevented.

[0043]

EXAMPLE Steels having the chemical compositions shown in Table 1 were formed into steel plates by hot rolling, and then cold-formed into a cylindrical shape by UOE forming or press bending, and the joints were welded to form a steel pipe. Then, it heated at 780 degreeC and water-cooled, air-cooled, or furnace-cooled. Tensile test pieces were sampled so that the longitudinal direction of these steel pipes would be the longitudinal direction of the test pieces, and a tensile test was performed. A Charpy test was also conducted. These results are also shown in Table 1. Also, P _MA = 325 × [C%] + 9 × [Si%] + 40 × [M
n%] + 3 × [solAl%] + 380 × [Nb%] value of parameter P _MA and plate thickness 40 m
The results of the oblique y-type weld cracking test performed at m and a preheating temperature of 100 ° C. are also shown.

In Table 1, steel numbers 1 to 17 are steels of the present invention, and steel numbers 18 to 22 are comparative steels.

[0045]

[Table 1] As shown in Table 1, in the steels of the present invention, cracks did not occur in the weld crack test, good weldability was exhibited, and all tensile strengths were high values of 500 N / mm ² or more.
In addition, the yield ratios were all low values of 70% or less, and a sufficient effect of the two-phase region thermoforming was confirmed. Furthermore, despite the two-phase region hot forming, the Charpy impact values at 0 ° C all show good toughness of 100 J or more, and high strength /
It was confirmed that high toughness / weldability and low yield ratio were simultaneously achieved.

On the other hand, in the case of the comparative steels, although both high strength and low yield ratio have been achieved, there is a problem in weldability and toughness, and particularly regarding toughness, the Charpy impact value at 0 ° C. is extremely low at 24 J or less. It's getting low. This is due to the coarse island-like martensite formed during the two-phase region hot forming, suggesting the importance of limiting _PMA to 120 or less.

[0047]

As described above, according to the present invention,
Wall thickness is about 12 to 150 mm, tensile strength is 500 to 65
It has become possible to economically manufacture a high-strength low-yield ratio steel pipe for construction with 0 N / mm ² and a yield ratio of 70% or less without lowering the toughness, with good productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a correlation between a parameter P _MA and an area fraction of island martensite.

FIG. 2 is a diagram showing a relationship between an area fraction of martensite and absorbed energy at 0 ° C. in a Charpy impact test.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Takayuki Honda Inventor Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Junichiro Kawasaki Inventor 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (16)

[Claims] 1. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, the balance Fe and unavoidable impurities, and the island-like martensite formation tendency at the time of heating in the two-phase region calculated by the formula (1) shown below. represents parameters: the value of P _MA is P _MA ≦ 12
0 become steel, and the steel sheet by hot rolling, the steel sheet was bent with cold, and the steel pipe by welding the seam portions, Ac ₁
A method for producing a high-strength low-yield ratio steel pipe for construction, comprising reheating to a temperature range of two phases of Ac ₃ or less and then cooling at a cooling rate of air cooling or more and 30 ° C./sec or less. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 2. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, containing one or more kinds, consisting of the balance Fe and inevitable impurities, and a two-phase calculated by the following formula (1) parameters representing the island martensite propensity to generate time-frequency heating: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, the steel sheet was bent with cold welded seam portion Steel tube, reheated to the temperature range of Ac ₁ or more and Ac ₃ or less in the two-phase region, and then air cooling or more, 30 ° C / sec
A method for producing a high-strength, low-yield ratio steel pipe for construction, which comprises cooling at the following cooling rate. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 3. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, Cu: 1.5% or less, Ni: 1.0% or less, Cr:
1.0% or less, Mo: 0.5% or less, one or more kinds are contained, and the balance is Fe and unavoidable impurities.
And parameters representative of the island martensite tendency in the two-phase region heating is calculated by the following equation (1): Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, the The steel sheet is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less, and then air cooling or more at a cooling rate of 30 ° C / sec or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which is characterized by cooling. 4. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, one or more kinds, and Cu: 1.5% or less, Ni: 1.0
% Or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more of them are contained, the balance is Fe and unavoidable impurities, and calculated by the following formula (1). parameters representing the island martensite tendency in the two-phase region heating that: the steel value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, bending a steel sheet by cold joint portion Welded to form a steel pipe, reheated to the temperature range of Ac ₁ or more and Ac ₃ or less in the two-phase region, and then air cooled or more, 30 ° C /
A method for manufacturing a high-strength, low-yield ratio steel pipe for construction, which comprises cooling at a cooling rate of sec or less. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 5. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, Ca: 0.
A parameter indicating the tendency of island-like martensite formation during heating in the two-phase region, which is composed of the balance Fe and unavoidable impurities and contains 0005 to 0.0050%, and is calculated by the following formula (1): P _MA A steel having a value of P _MA ≦ 120 is formed into a steel sheet by hot rolling, and the steel sheet is cold-bent,
Weld the seam to make a steel pipe, and if it is Ac ₁ or more and Ac ₃
Reheat to the following two-phase temperature range, then air cool or more, 3
A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises cooling at a cooling rate of 0 ° C / sec or less. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 6. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, one or more kinds, and B: 0.0005 to 0.0030.
%, Ca: 0.0005 to 0.0050% of 1 type or 2 types, the balance Fe and unavoidable impurities, and island martens during heating in the two-phase region calculated by the formula (1). Parameter indicating the site generation tendency: The value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, which is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises heating and then cooling at a cooling rate of not less than air cooling and not more than 30 ° C./sec. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 7. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, Cu: 1.5% or less, Ni: 1.0% or less, Cr:
1.0% or less, one or more of Mo: 0.5% or less, and B: 0.0005 to 0.0030%, Ca:
One or more of 0.0005 to 0.0050% is contained, the balance is Fe and unavoidable impurities, and the island-like martensite formation tendency at the time of heating in the two-phase region is calculated by the formula (1). parameters representing the: value of P _MA is P _MA ≦ 12
0 become steel, and the steel sheet by hot rolling, the steel sheet was bent with cold, and the steel pipe by welding the seam portions, Ac ₁
A method for producing a high-strength low-yield ratio steel pipe for construction, comprising reheating to a temperature range of two phases of Ac ₃ or less and then cooling at a cooling rate of air cooling or more and 30 ° C./sec or less. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 8. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005-0.10%, Nb: 0.005-0.05%, V: 0.01-
0.10%, Ti: 0.005 to 0.10%, one or more, Cu: 1.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1 out of 0.5% or less
Or two or more, and B: 0.0005 to 0.0030
%, Ca: 0.0005 to 0.0050% of 1 type or 2 types, the balance Fe and unavoidable impurities, and island martens during heating in the two-phase region calculated by the formula (1). Parameter indicating the site generation tendency: The value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, which is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises heating and then cooling at a cooling rate of not less than air cooling and not more than 30 ° C./sec. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 9. C: 0.03 to 0.20% by weight,
Si: 0.01 to 0.50%, Mn: 0.5% to 2.0
%, SolAl: 0.005 to 0.10%, the balance Fe and unavoidable impurities, and the island-like martensite formation tendency at the time of heating in the two-phase region calculated by the formula (1) shown below. represents parameters: the value of P _MA is P _MA ≦ 12
0 become steel, and the steel sheet by hot rolling, the steel sheet was bent with cold, and the steel pipe by welding the seam portions, Ac ₁
A method for manufacturing a high-strength low-yield ratio steel pipe for construction, which comprises reheating to a temperature range of two-phase region of Ac ₃ or less and then gradually cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 10. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, containing 1 or 2 or more kinds, consisting of the balance Fe and unavoidable impurities, and calculated by the formula (1) shown below. parameters representing the island martensite tendency in the two-phase region heating: steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, bending a steel sheet by cold, the joint portion A method for producing a high-strength, low-yield ratio steel pipe for construction, which comprises welding to form a steel pipe, reheating to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less, and then gradually cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 11. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, further Cu: 1.5% or less, Ni: 1.0% or less,
Cr: 1.0% or less, Mo: 0.5% or less, one or more kinds are contained, the balance is Fe and inevitable impurities, and two-phase calculated by the formula (1) shown below. Parameters indicating the tendency of island martensite formation during zone heating:
Steels value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, the steel sheet was bent with cold, and the steel pipe by welding the seam portions, and Ac ₃ following two at Ac ₁ or more A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises reheating to a phase region temperature range and then gradually cooling. 12. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and Cu: 1.5% or less, Ni:
1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more types are contained, the balance is Fe and inevitable impurities, and the formula (1) is shown below. parameters representing the island martensite tendency in the two-phase region heating in the calculation: the steel value of P _MA is P _MA ≦ 120, and the steel sheet by hot rolling, working and bending the steel plate cold ,
Weld the seam to make a steel pipe, and if it is Ac ₁ or more and Ac ₃
A method for producing a high-strength, low-yield ratio steel pipe for construction, comprising reheating to the temperature range of the following two-phase region and then gradually cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 13. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005-0.10%, C
a: 0.0005 to 0.0050%, balance Fe
And becomes unavoidable impurities, and represents the island martensite tendency in the two-phase region heating is calculated by the following equation (1) Parameters: Steels value of P _MA is P _MA ≦ 120, heat A steel plate is formed by hot rolling, the steel plate is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less, and then gradually cooled. A method for producing a high-strength, low-yield ratio steel pipe for construction which is characterized. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 14. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, and B: 0.0005 to 0.003.
0%, Ca: 0.0005 to 0.0050%, containing 1 or 2 kinds, consisting of the balance Fe and inevitable impurities, and island-like when heating in the two-phase region calculated by the formula (1) Parameter indicating martensite formation tendency: The value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, which is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises heating and then gradually cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 15. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, further Cu: 1.5% or less, Ni: 1.0% or less,
One or more of Cr: 1.0% or less and Mo: 0.5% or less, and B: 0.0005 to 0.0030%,
Ca: 1 or 2 out of 0.0005 to 0.0050%
A parameter that contains at least one species, consists of the balance Fe and unavoidable impurities, and represents the island-like martensite formation tendency during heating in the two-phase region calculated by equation (1): the value of P _MA is P
Steel with _MA ≤ 120 is hot-rolled into a steel plate, and this steel plate is cold-bent and welded at the joint to form a steel pipe, which is reheated to a temperature range of two phases of Ac ₁ or more and Ac ₃ or less. A method for producing a high-strength low-yield ratio steel pipe for construction, which comprises heating and then gradually cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1) 16. C: 0.03 to 0.20 in% by weight.
%, Si: 0.01 to 0.50%, Mn: 0.5% to
2.0%, solAl: 0.005 to 0.10%, Nb: 0.005 to 0.05%, V: 0.0
1 to 0.10%, Ti: 0.005 to 0.10%, one or more kinds, Cu: 1.5% or less, Ni: 1.0
% Or less, Cr: 1.0% or less, Mo: 0.5% or less, one or more, and B: 0.0005 to 0.00
30%, Ca: 1 out of 0.0005 to 0.0050%
Containing species or two, and the balance Fe and unavoidable impurities, and (1) the parameters representative of the island martensite tendency in the two-phase region heating which is calculated by the formula: The value of P _MA is P _MA ≦ The steel to be 120 is made into a steel plate by hot rolling, the steel plate is cold-bent, the seam is welded to form a steel pipe, and the steel pipe is reheated to a two-phase region temperature range of Ac ₁ or more and Ac ₃ or less. And a method of manufacturing a high-strength low-yield ratio steel pipe for construction, characterized by slow cooling. P _MA = 325 × [C%] + 9 × [Si%] + 40 × [Mn%] + 3 × [solAl%] + 380 × [Nb%] (1)