JP6398585B2 - Steel pipe manufacturing method and steel pipe - Google Patents

Description

  The present invention relates to a method of manufacturing a steel pipe that is useful in the fields of architecture, civil engineering, etc., has high strength, has a low yield ratio, and has low temperature toughness, and a steel pipe having such characteristics.

  In recent years, with the increase in size of building structures, steel pipes used for the structures are required to have higher strength and buckling strength. For example, as a technique for increasing the buckling strength of a steel pipe, the existing steel pipe is made thicker without changing the outer diameter, that is, the ratio of the thickness t of the steel pipe to the outer diameter D (t / D ) Can be considered large. However, in the pipe making process of forming a steel pipe from a rolled steel plate that has been increased in thickness in this way, the yield strength and tensile strength are increased by work hardening, but among these the improvement in yield strength is particularly significant, The yield ratio tends to increase.

  Here, the yield ratio is a value represented by a ratio (YS / TS) between the yield strength YS and the tensile strength TS. The yield ratio is an index of durability until a stress greater than the yield strength is applied to the material and the material yields and then buckles or breaks.

  A high yield ratio of a steel pipe means that the amount of plastic change after elastic deformation is small when stress is excessively applied to the steel pipe. For this reason, such a steel pipe has a high possibility of breaking, and is not preferable as a member used for a building structure requiring earthquake resistance. Therefore, in order to reduce the yield ratio, a technique has been proposed in which the steel structure is a multiphase structure composed of a soft phase and a hard phase (see Patent Documents 1 and 2).

JP 2007-270304 A JP 05-117746 A

  By the way, a bending stress and a bending back stress may be applied to a steel pipe used for a building structure when an earthquake occurs. In particular, if bending stress or the like is suddenly applied to the steel pipe during plastic deformation after elastic deformation, the steel pipe may be destroyed. For this reason, the steel pipe is also required to have excellent toughness so that the structure does not collapse.

However, a steel pipe manufactured using the techniques disclosed in Patent Documents 1 and 2 and having a ratio of wall thickness t to outer diameter D (t / D) exceeding 6% has a good yield ratio of 85% or less. However, it is unclear whether good results can be obtained for low-temperature toughness (Charpy absorbed energy vE- ₃₀ ). Therefore, it cannot be said that the low yield ratio and the low temperature toughness are always compatible with the steel pipe.

The present invention has been made in view of the above circumstances, is useful in the fields of architecture, civil engineering, etc., has high strength, has a low yield ratio (YR ≦ 85%), and low temperature toughness (νE ₋ It is an object of the present invention to provide a method of manufacturing a steel pipe in which ₃₀ ≧ 50 J) is ensured, and a steel pipe having such characteristics.

The present inventors have refined the structure by setting the rolling ratio to 2 or more in addition to the temperature condition as the rolling condition of the steel sheet, and by temper embrittlement by setting the tempering temperature of the steel sheet to 350 ° C. or more. Knowledge that the steel sheet toughness reduction due to the formation of the embrittlement phase is suppressed, and as a result, these actions combine to ensure low temperature toughness (the steel pipe has Charpy absorbed energy vE- ₃₀ of 50 J or more). Got.

  Moreover, since the hard phase, such as a martensite, does not soften too much by making the heating temperature at the time of tempering a steel plate into less than 500 degreeC, these inventors suppress the raise of the yield ratio of a steel plate ( The yield ratio of the steel pipe is 0.85 or less).

  Based on the above findings, the present inventors have completed the present invention. The summary is as follows.

[1] A method of manufacturing a steel pipe in which the ratio t / D between the wall thickness t and the outer diameter D is more than 6%,
% By mass
C: 0.02 to 0.20%,
Si: 0.05-0.5%, and Mn: 0.5-2.0%
Containing
Al: 0.10% or less,
P: 0.03% or less,
S: 0.01% or less, and N: 0.007% or less, the steel slab with the balance being Fe and unavoidable impurities is heated, and the rolling ratio in the recrystallization temperature range is 2 or more, and hot rolling Then, the obtained steel sheet was quenched by heating to more than 850 ° C., further heated to a temperature range of 700 to 850 ° C., and further tempered in a temperature range of 350 ° C. or more and less than 500 ° C., Cold forming the steel sheet into a tubular shape to form a steel pipe,
A method for producing a steel pipe, characterized in that the steel pipe has a yield strength of 440 MPa or more, a tensile strength of more than 590 MPa and 740 MPa or less, a yield ratio of 0.85 or less, and a Charpy absorbed energy vE- ₃₀ of 50 J or more.

[2] The method for producing a steel pipe according to the above [1], wherein the steel pipe is heated to a temperature of less than ₁ Ac and air-cooled.

[3] The steel slab is in% by mass, Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
The method for producing a steel pipe according to the above [1] or [2], comprising at least one of Ti: 0.025% or less and Ca: 0.006% or less.

[4] In a steel pipe in which the ratio t / D of the wall thickness t to the outer diameter D is more than 6%,
% By mass
C: 0.02 to 0.20%,
Si: 0.05-0.5%, and Mn: 0.5-2.0%
Containing
Al: 0.10% or less,
P: 0.03% or less,
S: 0.01% or less, and N: 0.007% or less, the balance consisting of Fe and inevitable impurities,
The tissue includes hard tissue and soft tissue;
The effective crystal grain size of the hard structure is 1 to 15 μm, the effective crystal grain size of the soft structure is 1 to 15 μm, the yield stress is 440 MPa or more, the tensile strength is more than 590 MPa and 740 MPa or less, and the yield ratio is 0.1. 85 or less, Charpy absorbed energy vE- ₃₀ is 50 J or more, A steel pipe characterized by the above-mentioned.

[5] By mass%,
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
The steel pipe according to [4] above, which contains at least one of Ti: 0.025% or less and Ca: 0.006% or less.

  In the method for manufacturing a steel pipe according to the present invention, on the premise that the ratio (t / D) of the wall thickness t to the outer diameter D exceeds 6%, in particular, the rolling conditions (rolling ratio) of the steel plate and the steel plate The tempering conditions (heating temperature) are improved. As a result, according to the method for manufacturing a steel pipe according to the present invention, it is possible to obtain a steel pipe that is useful in the fields of construction, civil engineering, etc., has high strength, has a low yield ratio, and ensures low-temperature toughness. .

  In addition, the steel pipe according to the present invention is improved with respect to the equivalent circle diameter of the structure, on the premise that the steel pipe has a predetermined component composition and structure. As a result, the steel pipe according to the present invention can ensure strength, low yield ratio, and low temperature toughness that are useful in the fields of architecture, civil engineering, and the like.

It is a graph which shows the relationship between the yield ratio of a steel plate, and the heating temperature in a tempering process. It is a graph which shows the relationship between the Charpy absorbed energy vE- ₃₀ in the heat affected zone of a steel pipe, and the heating temperature in a tempering process.

<Manufacturing method of steel pipe>
Hereinafter, an embodiment of a method for manufacturing a steel pipe according to the present invention will be described in detail. In addition, embodiment shown below does not limit this invention. The constituent elements of the embodiment include those that can be easily replaced by those skilled in the art or those that are substantially the same. Furthermore, various forms included in the following embodiments can be arbitrarily combined within a range obvious to those skilled in the art.

[Ingredients of the base material]
Below, the component of the base material (rolled steel plate) used in the manufacturing method of the steel pipe which concerns on this invention is demonstrated. Note that “%” shown below represents “% by mass” unless otherwise specified.

(C: 0.02 to 0.20%)
C is an element that increases the strength of the base material. In order to ensure the strength of the base material, the lower limit is made 0.02%. On the other hand, if the amount of C exceeds 0.20%, the weldability decreases and the toughness decreases, so the upper limit is made 0.20%. From the viewpoint of further increasing the strength, the C content is preferably 0.03% or more, and more preferably 0.05% or more. Further, from the viewpoint of further increasing the toughness, the C content is preferably 0.15% or less, and more preferably 0.12% or less.

(Si: 0.05 to 0.50%)
Si is an effective element as a deoxidizer and contributes to an increase in strength, so the lower limit is made 0.05%. From the viewpoint of further increasing the strength, the Si content is preferably 0.10% or more, and more preferably 0.20 or more. On the other hand, if Si is added in an amount exceeding 0.50%, the toughness of the base metal and the heat-affected zone (hereinafter sometimes referred to as “HAZ”) and also the weldability are impaired, so the upper limit is made 0.50%. And From the viewpoint of further securing toughness, the Si content is preferably 0.40% or less, and more preferably 0.30% or less.

(Mn: 0.5-2.0%)
Mn is an element that simultaneously improves strength and toughness, and the lower limit is 0.5%. On the other hand, if Mn is added excessively, the toughness of the base material and the HAZ is lowered, and further the weldability is impaired, so the upper limit is made 2.0%. From the viewpoint of further increasing the strength, the Mn content is preferably 1.10% or more, and more preferably 1.20% or more. On the other hand, from the viewpoint of further suppressing the decrease in toughness, the Mn content is preferably 1.50% or less, and more preferably 1.40% or less.

(Al: 0.10% or less)
Al is an element effective as a deoxidizer. When Si is added, the addition of Al is not essential. If Al is added in an amount exceeding 0.10%, the cleanliness of the base material is lowered and the toughness of the base material and the welded portion is lowered, so the upper limit is made 0.10%. From the viewpoint of further improving toughness, the Al content is preferably 0.05% or less. On the other hand, in the present embodiment, since Si is added for deoxidation, the lower limit value of the Al amount may be 0%. When Al is used as a deoxidizer, it is costly to remove. Therefore, Al may be contained in an amount of 0.001% or more for economic reasons. To further enhance the deoxidation effect, 0 Addition of 0.005% or more is more preferable.

(P: 0.03% or less)
Since P is an impurity, the upper limit of the content is set to 0.03%. Since grain boundary destruction is prevented by reducing the amount of P, the amount of P is preferably 0.010% or less. On the other hand, since it is preferable that the amount of P is as small as possible, there is no lower limit. However, since it takes a high cost to remove, the P content may be 0.001% or more for economic reasons.

(S: 0.01% or less)
Since S is an impurity like P, the upper limit of the content is 0.01%. Since the amount of MnS can be reduced and the toughness can be improved by reducing the amount of S, the amount of S is preferably 0.005% or less. On the other hand, since it is preferable that the amount of S is as small as possible, no lower limit is provided.

(N: 0.007% or less)
N is also an impurity, and excessive inclusion causes a reduction in the toughness and surface properties of the base material and HAZ, so the upper limit of the content is made 0.007%. On the other hand, when Ti, Nb, V, or the like is added, Ti forms nitrides with these and contributes to an increase in strength and an improvement in HAZ toughness, so the lower limit is preferably made 0.002%. From the viewpoint of improving the strength, the N amount is more preferably 0.003% or more.

(Ni: 1.0% or less)
Ni is an element that improves the strength of the base material and contributes to the improvement of the toughness of the base material without adversely affecting the weldability and the toughness of the HAZ. Is preferably 1.0%. From an economical viewpoint, the Ni content is more preferably 0.5% or less. On the other hand, from the viewpoint of improving the strength or the like, the Ni content is preferably 0.01% or more.

(Cu: 1.0% or less)
Cu is an element that improves the strength and toughness of the base material, and further contributes to the increase in strength and corrosion resistance due to Cu precipitates, but when Cu is added excessively, the yield ratio increases due to precipitation hardening, The upper limit is preferably 1.0%. On the other hand, from the viewpoint of improving the strength of the base material, the Cu content is preferably 0.01% or more.

(Cr: 1.0% or less)
Cr is an element that is effective in improving the strength of the base material and improves the corrosion resistance of the base material. However, if Cr is added excessively, the weldability of the base material and the toughness of the HAZ decrease, so the upper limit is set to 1. 0.0% is preferable. On the other hand, from the viewpoint of improving strength, the Cr content is preferably 0.01% or more, and from the viewpoint of improving corrosion resistance, the Cr content is preferably 0.1% or more.

(Mo: 1.0% or less)
Mo is an element that contributes to increasing the strength and toughness of the base metal. However, excessive addition of Mo causes a decrease in toughness and weldability of the weld zone, so the upper limit should be 1.0%. Is preferred. On the other hand, from the viewpoint of increasing strength and increasing toughness, the Mo content is preferably 0.01% or more.

(Nb: 0.1% or less)
Nb is an element that forms fine carbonitrides to improve the strength of the base material and improve the toughness of the HAZ. However, excessive addition of Nb may reduce the toughness of the base material. The upper limit is preferably 0.1%. On the other hand, from the viewpoint of improving the strength of the base material and improving the toughness of the HAZ, the Nb content is preferably 0.003% or more.

(V: 0.1% or less)
V is an element that generates carbides and nitrides to improve the strength of the base material and improve the toughness of the HAZ. However, if V is added excessively, the yield ratio may increase due to precipitation hardening. Therefore, the upper limit is preferably 0.1%. On the other hand, from the viewpoint of improving the strength of the base material and improving the toughness of the HAZ, the V content is preferably set to 0.003% or more.

(Ti: 0.025% or less)
Ti is an element that forms carbides and nitrides and contributes to improving the toughness of HAZ. However, excessive addition of Ti may reduce the toughness of HAZ, so the upper limit should be 0.025%. Is preferred. In addition, it is more preferable to make Ti amount into 0.020% or less from a viewpoint of further suppressing the fall of the toughness of HAZ. On the other hand, from the viewpoint of improving the toughness of HAZ, the Ti content is preferably 0.003% or more, and more preferably 0.008% or more.

(Ca: 0.006% or less)
Ca is an element that controls the form of sulfide inclusions (MnS), increases Charpy absorbed energy, and improves low-temperature toughness. However, if Ca is added excessively, oxides (CaO) and sulfides (CaS) are produced in large quantities and large inclusions are present, and the toughness and cleanliness of the base material are reduced. Since the lamellar resistance is lowered, the upper limit is preferably made 0.006%. Here, “lamellatea” refers to a weld crack that occurs in a heat-affected zone or its adjacent portion in a peeled manner parallel to the surface of the base material. On the other hand, from the viewpoint of improving low-temperature toughness, the Ca content is preferably 0.001% or more.

  The balance is iron and inevitable impurities. Inevitable impurities refer to components that are included in the raw material or components that are mixed in during the manufacturing process and that are not intentionally included in the base material.

  Specifically, P, S, O, Sb, Sn, W, Co, As, Mg, Pb, Bi, B, and H are listed as inevitable impurities. Of these, as described above, P and S must be controlled to be 0.03% or less and 0.01% or less, respectively. It is preferable to control O to be 0.006% or less.

  For other elements, inevitable impurities are usually 0.1% or less for Sb, Sn, W, Co, and As, 0.005% or less for Mg, Pb, and Bi, and 0.0005% or less for B and H. As long as it is within the normal range, it is not necessary to control.

  Further, Si, Al, Ni, Cu, Cr, Mo, V, and Ca, which are components of the base material (rolled steel plate) used in the method for manufacturing a steel pipe according to the present invention, are unavoidable impurities even if they are not intended to be contained. Can be mixed in as However, these components do not adversely affect the desired steel pipe in the present invention as long as they are below the upper limit of the content when intentionally contained. Further, N is generally handled as an inevitable impurity in the base material, but in the present invention, it is preferable to control N within a certain range as described above.

[Manufacturing process]
Next, each manufacturing process for the method for manufacturing a steel pipe according to the present invention, that is, (A) a manufacturing process of a hot-rolled steel sheet, (B) a first quenching process that contributes to improvement of low-temperature toughness by refinement of the structure, (C) Second quenching process (two-phase zone quenching process) for forming a multiphase structure, (D) Suppressing the formation of the embrittled phase and contributing to the improvement of low temperature toughness and suppressing the softening of the hard phase A tempering process for improving the yield ratio and (E) a pipe forming process for cold-working a steel plate to make a steel pipe, and (F) a pipe making for removing the processing strain of the steel pipe and further improving the low temperature toughness. A subsequent annealing process is demonstrated in order.

(A) Manufacturing process of hot-rolled steel sheet After melting the steel having the above-mentioned components, it is cast into a steel slab, the steel slab is heated and hot-rolled, controlled and cooled, and air-cooled. To manufacture. In addition, melting and casting of steel are performed by a conventional method.

From the viewpoint of rolling in the recrystallization temperature range, the heating temperature of the steel slab needs to be higher than the austenite single-phase temperature (Ac ₃ points or higher), preferably 900 ° C or higher if possible. More preferably, the temperature is set to 1000 ° C. or higher. On the other hand, the heating temperature of the steel slab is preferably 1250 ° C. or lower in order to prevent the crystal grains from becoming coarse. Also,

Here, Ac ₃ points can be obtained by the following (Formula 1) from the component (mass%) of the base steel plate. In addition, when not adding each following component intentionally, it calculates as 0 in the same type | formula.

Ac ₃ (° C.) = 937.2-436.5C-19.7Mn-16.3Cu
-26.6Ni-4.9Cr + 38.1Mo + 124.8V
+ 136.3Ti-19.1Nb + 198.4Al + 3315B
... (Formula 1)

  In order to further refine the crystal grains of the steel slab and further increase the low temperature toughness, the reduction ratio in the recrystallization temperature region is set to 2 or more. Here, the rolling ratio is an amount representing the degree of processing in rolling, and is a value defined by (h1 / h2) when the thicknesses of the material before and after rolling are h1 and h2, respectively. In addition, it is preferable that the rolling ratio is 3 or more from the viewpoint of refining the crystal grains of the steel slab.

(B) 1st hardening process After hot rolling, in order to produce | generate a low temperature transformation structure | tissue (a martensite, a bainite etc.) for the purpose of refinement | miniaturization of a structure | tissue, it hardens.

  In this quenching treatment, the structure of the steel sheet is once transformed into austenite and then converted into a low temperature transformation structure. The lower limit of the heating temperature is more than 850 ° C. from the viewpoint of austenizing the structure of the steel sheet. The intensity | strength after a quenching process falls that it is 850 degrees C or less. On the other hand, the upper limit of the heating temperature is preferably set to 1000 ° C. or less in order to suppress the coarsening of the crystal grains of the steel sheet and suppress the decrease in the low temperature toughness.

  The cooling rate before the austenite / ferrite transformation in the center of the plate thickness in this step can be 1 to 90 ° C./s, and from the viewpoint of toughness, it is preferably 10 to 90 ° C./s. Moreover, quenching stop temperature can be made into room temperature-400 degreeC, and it is preferable to set it as room temperature-300 degreeC from a viewpoint of toughness.

(C) Second quenching step (two-phase quenching step)
Subsequently, in order to make the structure of the steel sheet into a multiphase structure, a two-phase region quenching is performed. In this two-phase quenching treatment, the heating temperature is set to 700 to 850 ° C. so that the structure of the steel sheet has two phases of ferrite and austenite. In this process, a structure composed of austenite enriched with C and ferrite composed of reduced C produces a multiphase structure composed of a phase containing extremely fine carbides and a phase containing coarse carbides. The yield ratio can be reduced.

  When the heating temperature exceeds 850 ° C., the same treatment as that in the first quenching step described above is repeated, and the strength is excessively increased and the yield ratio is increased. On the other hand, when the heating temperature is less than 700 ° C., the strength is not sufficiently increased even if quenching is performed.

  The cooling rate before the austenite / ferrite transformation in the center of the plate thickness in this step can be 1 to 90 ° C./s, and from the viewpoint of toughness, it is preferably 10 to 90 ° C./s. Moreover, quenching stop temperature can be made into room temperature-400 degreeC, and it is preferable to set it as room temperature-300 degreeC from a viewpoint of toughness.

(D) Tempering process Furthermore, in order to improve the toughness of a steel plate, tempering is performed in the temperature range of 350 degreeC or more and less than 500 degreeC.

  FIG. 1 is a graph showing the relationship between the yield ratio of a steel sheet and the heating temperature in the tempering step. In the example shown in the figure, the heating temperature in the tempering step is set to 470 ° C., 500 ° C., assuming that the heating temperature in the two-phase region quenching step before the tempering step is one of 750 ° C., 770 ° C. and 800 ° C. Either ℃ or 550 ℃. In addition, the component of the base material in the example shown in FIG. 1 includes 0.07% C, 0.3% Si, 1.7% Mn, 0.03% Al, and 0.004% N.

  In order to set the yield ratio in a steel pipe after pipe making described below to a target value of 0.85 or less, it is usually necessary to set the yield ratio in a steel sheet before pipe making to 0.80 or less. From the results shown in FIG. 1, if the heating temperature in the two-phase zone quenching process is any of the above three temperatures (two-phase zone quenching process), if the heating temperature in the tempering step is less than 500 ° C., low yielding It can be seen that the ratio (0.80 or less in the steel plate state) can be realized.

FIG. 2 is a graph showing the relationship between Charpy absorbed energy vE- ₃₀ in the heat-affected zone of the steel pipe and the heating temperature in the tempering step. In the same figure, the results of samples with heating temperatures in the tempering process from around 50 ° C. to 500 ° C. are shown. 2 are the same as the components of the base material shown in FIG. 1 for the circles, and 0.01% for the □ marks in addition to the components of the base material shown in FIG. Nb is included.

From the results shown in FIG. 2, the low temperature toughness (Charpy absorbed energy vE- ₃₀ is 50 J or more) in the heat-affected zone of the steel pipe if the heating temperature in the tempering step is 350 ° C. or higher regardless of whether or not Nb is added. It can be seen that it is secured.

  From the above results, in the tempering step, by setting the heating temperature to 350 ° C. or higher, it is possible to suppress a reduction in the toughness of the steel sheet due to the formation of an embrittled phase due to temper embrittlement. On the other hand, by setting the heating temperature to less than 500 ° C., it is possible to suppress an increase in the yield ratio of the steel sheet without excessively softening a hard phase such as martensite.

  In consideration of variation in temperature distribution, the lower limit of the heating temperature in the tempering step is preferably 370 ° C., more preferably 390 ° C. On the other hand, the upper limit of the heating temperature is preferably 470 ° C., more preferably 450 ° C. The cooling after the heating may be air cooling (air cooling) as in the normal tempering process. That is, the average cooling rate may be 1 to 50 ° C./min.

(E) Pipe-forming process Next, it is set as a steel pipe by the press bend cold forming method. Usually, when a steel pipe is formed from a rolled steel plate, the yield strength and the tensile strength are increased by work hardening, but the yield ratio tends to increase because the yield strength is particularly improved.

  For this reason, this manufacturing method which suppresses a raise of a yield ratio by the tempering process mentioned above especially, when the process distortion which generate | occur | produces in a pipe making process is large, in other words, plate | board thickness (t) and steel pipe outer diameter (D) This is suitable for manufacturing a steel pipe having a large ratio (t / D). In the production method according to the present invention, the case where the ratio (t / D) is large refers to the case where the ratio (t / D) exceeds 6%.

(F) Annealing process after pipe making In the manufacturing method of the steel pipe concerning the present invention, although each process of the above-mentioned (A) process to (E) process is indispensable, The annealing process will be described in detail.

That is, the steel pipe can be subjected to heat treatment (annealing) in order to sufficiently remove the distortion in the cold forming in the step (E), thereby further reducing the yield ratio and improving the toughness. The annealing temperature is preferably less than Ac ₁ point in order to suppress a decrease in strength due to softening of the steel pipe.

Here, the Ac ₁ point can be obtained by the following (Formula 2) from the component (mass%) of the base steel plate. In addition, when not adding each following component intentionally, it calculates as 0 in the same type | formula.

_{Ac 1 (℃) = 750.8-26.6C-} 17.6Si-11.6Mn-22.9Cu
-23Ni + 24.1Cr + 22.5Mo-39.7V-5.7Ti
+ 232.4Nb-169.4Al-894.7B (Formula 2)

Of the steps (A) to (F) shown above, the steel pipe obtained through at least the steps (A) to (E) has a ratio (t / D) between the wall thickness t and the outer diameter D of 6 (t / D). Even though the strength is increased to exceed%, the yield ratio can be reduced and the low-temperature toughness can be ensured. As a result, excellent earthquake resistance can be exhibited. Specifically, the steel pipe has yield strength: 440 MPa or more, tensile strength: more than 590 MPa and 740 MPa or less, yield ratio: 0.85 or less, and Charpy absorbed energy vE- ₃₀ : 50 J or more. Therefore, according to the manufacturing method according to the present invention, a steel pipe (for example, wall thickness: 50 to 100 mm) having high strength, low yield ratio and low temperature toughness, which is useful in the field of construction, civil engineering, etc. ) Can be obtained.

<Steel pipe>
Next, an embodiment of a steel pipe according to the present invention will be described in detail. The embodiment described below is not intended to limit the present invention as in the above-described embodiment of the method for manufacturing a steel pipe according to the present invention, and its constituent elements can be easily replaced by those skilled in the art. Or substantially the same. Further, various forms included in the following embodiments can be arbitrarily combined within a range obvious to those skilled in the art.

[Components of steel pipe base material]
The components of the steel pipe according to the present invention are as described in detail in the method for manufacturing a steel pipe according to the present invention.

[Steel pipe structure]
The structure of the steel pipe according to the present invention is a multiphase structure composed of a plurality of soft structures (ferrite) and hard structures (martensite, bainite ). The effective crystal grain size of the hard tissue is 1 to 15 μm, and the effective crystal grain size of the soft tissue is 1 to 15 μm. Here, the effective crystal grain size is a crystal grain having a circle-equivalent diameter measured by an EBSP (Electron Backscatter Diffraction Pattern) method and surrounded by a structure boundary having an orientation difference of 15 ° or less. It means the diameter.

  Moreover, the low yield ratio of a steel pipe is realizable by making the structure of a steel pipe into a double phase structure.

  Under such a premise, by setting the effective crystal grain size of the hard structure to 1 μm or more, this multiphase structure can sufficiently contribute to the reduction of the yield ratio of the steel pipe. On the other hand, when the effective crystal grain size of the hard structure is 15 μm or less, the steel pipe can exhibit excellent low temperature toughness. In addition, when the effective crystal grain diameter of a hard structure | tissue is 2-10 micrometers, since these effects are show | played by a higher level, it is preferable.

  By setting the effective crystal grain size of the soft structure to 1 μm or more, this multiphase structure can sufficiently contribute to the reduction of the yield ratio of the steel pipe. On the other hand, by setting the effective crystal grain size of the soft structure to 15 μm or less, the steel pipe can exhibit excellent low temperature toughness. In addition, when the effective crystal grain diameter of the soft tissue is 2 to 10 μm, these effects are achieved at a higher level, which is preferable.

The steel pipe according to the present invention having the above-described configuration has a reduced yield ratio and low-temperature toughness despite the fact that the ratio of the wall thickness t to the outer diameter D (t / D) is increased to more than 6% and the strength is increased. Can be secured, and as a result, excellent earthquake resistance can be exhibited. Specifically, the steel pipe (for example, wall thickness: 50 to 100 mm) has a yield strength of 440 MPa or more, a tensile strength: more than 590 MPa and 740 MPa or less, a yield ratio: 0.85 or less, and a Charpy absorbed energy vE- ₃₀ : 50 J or more. Have Therefore, the steel pipe according to the present invention is useful in the fields of construction, civil engineering and the like because of its high strength, low yield ratio and low temperature toughness.

  Hereinafter, the effects of the present invention will be specifically described with reference to invention examples. The present invention is not limited to the conditions used in the following invention examples. Moreover, the blank in Table 1 indicates that the element is not intentionally added. Invention Examples 1 to 20 are steel pipes within the scope of the present invention, and Conventional Examples (examples based on the technique disclosed in Patent Document 1) and Comparative Examples 1 to 16 are steel pipes outside the scope of the present invention.

  Steel having chemical components shown in Table 1 was cast into a steel slab. These steel slabs are heated to the heating temperature 1 shown in Table 2 and hot-rolled at a reduction ratio in the recrystallization temperature range shown in Table 2, and then heated at the heating temperatures 2 and 3 shown in Table 2. First quenching and second quenching (two-phase quenching) were sequentially performed, and further tempering was performed at a heating temperature 4 shown in Table 2 to obtain a base steel plate.

  In addition, the cooling rate before the austenite / ferrite transformation at the center of the plate thickness in the first quenching was 20 ° C./s, and the quenching stop temperature was 150 ° C. The cooling rate in the second quenching was 20 ° C./s, and the quenching stop temperature was 150 ° C.

  Subsequently, after the obtained hot-rolled steel sheet was air-cooled, the steel sheet was formed into a tubular shape by a press bend cold forming method, and ends were welded to form a steel pipe. Then, about the invention examples 4, 7-9, 13, 17, and the comparative examples 3, 7, and 16, the annealing heat processing was performed with the heating temperature 5 shown in Table 2. FIG.

  Next, a sample for observing the structure was taken from the base material portion of the obtained steel pipe, and the cross-section parallel to the longitudinal direction of the steel pipe was subjected to nital etching, the structure was observed with an optical microscope, and a photograph was taken. The observation position was t / 4 position from the outer surface (t indicates thickness). Using these structure photographs, it was confirmed that structures other than martensite and ferrite were not generated. Thereafter, the effective crystal grain sizes of martensite and ferrite were measured by image analysis. About the effective crystal grain diameter of each structure | tissue, 5 places of visual fields of 100 micrometers x 200 micrometers were measured, and the average value was calculated | required.

  A two-phase structure composed of ferrite and martensite of the steel pipe (Invention Example 1) according to the present invention is confirmed, and the effective crystal grain sizes thereof are 10 μm (ferrite), 10 μm (martensite), respectively. It was confirmed that.

  On the other hand, although a two-phase structure composed of ferrite and martensite of a conventional steel pipe (conventional steel pipe) has been confirmed, these effective crystal grain sizes are 16 μm (ferrite) and 17 μm (respectively). Martensite).

Furthermore, from the base material of the steel pipe, an arc-shaped tensile test piece was taken in the longitudinal direction of the steel pipe in accordance with JIS Z 2241, a tensile test was performed at room temperature, yield strength and tensile strength were obtained, and a yield ratio was calculated. Further, a V-notch test piece was collected from the base material of the steel pipe in accordance with JIS Z 2242 and subjected to a Charpy test at -30 ° C. to obtain Charpy absorbed energy vE- ₃₀ , and toughness was evaluated. In addition, the V notch test piece was extract | collected considering the steel pipe circumferential direction as a longitudinal direction. When a full-size test piece having a thickness of 10 mm could not be collected, a sub-size test piece was used and converted to a thickness of 10 mm. These results are shown in Table 3 together with the thickness of the steel pipe and the ratio (t / D) between the thickness t of the steel pipe and the outer diameter D.

As shown in Table 3, all of the steel pipes of Invention Examples 1 to 20 are steel plates obtained by an appropriate manufacturing method within the scope of the present invention. For these steel pipes, the yield strength is 440 MPa or more, the tensile strength is more than 590 MPa and 740 MPa or less, the yield ratio is 0.85 or less, and the Charpy absorbed energy vE- ₃₀ is 50 J or more. For this reason, all of the steel pipes of Invention Examples 1 to 20 have high strength, low yield ratio and low temperature toughness, in other words, excellent in earthquake resistance, in the fields of architecture, civil engineering, etc. It turns out that it is useful. In addition, the upper limit of the preferable value of the tensile strength was set to 740 MPa, because when the tensile strength is excessively increased, the yield strength is improved as well as the improvement of the tensile strength, and thus the yield ratio is excessively increased. This is because it was intended to exclude such cases.

On the other hand, none of the steel pipes of the conventional example and the comparative examples 1 to 16 are steel plates obtained by an appropriate manufacturing method within the scope of the present invention. For these steel pipes, it can be seen that at least one of yield strength, tensile strength, yield ratio, and Charpy absorbed energy vE- ₃₀ is out of the desired range.

Hereinafter, Comparative Examples 1 to 16 will be specifically examined.
Comparative Examples 1, 3, and 4 are examples in which the desired low-temperature toughness could not be ensured because the amounts of C, Si, and Mn were too large. Comparative Example 2 is an example in which the desired strength could not be ensured because the amount of C was too small. Comparative Examples 5 to 8 are examples in which the desired low-temperature toughness could not be ensured due to the presence of inclusions and the like because the amounts of P, S, Al and N were too large.

  Comparative Example 9 is an example in which the desired low-temperature toughness could not be ensured because the rolling in the recrystallization temperature range could not be performed because the heating temperature in the hot rolling was too low. Comparative Example 10 is an example in which the desired low-temperature toughness could not be ensured because the reduction ratio in the recrystallization temperature range was too low. Comparative Example 11 is an example in which the desired strength could not be ensured because the heating temperature in the first quenching was too low. In Comparative Example 12, the heating temperature in the second quenching (two-phase quenching) was too low, so that the structure became a single phase only of martensite, and a desired yield ratio could not be ensured. Comparative Example 13 is an example in which the heating temperature in the second quenching (two-phase quenching) was too high, and the structure became a single phase only of martensite, and a desired yield ratio could not be ensured. Comparative Example 14 is an example in which the desired yield ratio could not be ensured because the tempering temperature was too high. Comparative Example 15 is an example in which since the tempering temperature was too low, an embrittled phase due to temper embrittlement was generated, and the desired low-temperature toughness could not be ensured.

  Comparative Example 16 is an example in which annealing was further performed after the pipe making process of Invention Example 20, but because the heating temperature in annealing after pipe forming was too high, a desired yield ratio could not be ensured.

  According to the present invention, it is possible to ensure a low yield ratio and low temperature toughness for a high strength steel pipe having a ratio (t / D) of the wall thickness t to the outer diameter D useful for large buildings and the like (t / D) exceeding 6%. it can. Therefore, the present invention is particularly promising in the fields of architecture and civil engineering.

Claims (5)

A method for producing a steel pipe, wherein the ratio t / D of the wall thickness t to the outer diameter D is more than 6%,
% By mass
C: 0.02 to 0.20%,
Si: 0.05-0.5%, and Mn: 0.5-2.0%
Containing
Al: 0.10% or less,
P: 0.03% or less,
S: 0.01% or less, and N: 0.007% or less, the steel slab with the balance being Fe and unavoidable impurities is heated, and the rolling ratio in the recrystallization temperature range is 2 or more, and hot rolling Then, the obtained steel plate is heated to Ac3 point or higher and quenched, and further heated to a temperature range of 700 to less than Ac3 point , and further tempered in a temperature range of 350 ° C or higher and lower than 500 ° C. , The steel sheet is cold formed into a tube to form a steel pipe,
A method for producing a steel pipe, characterized in that the steel pipe has a yield stress of 440 MPa or more, a tensile strength of more than 590 MPa and 740 MPa or less, a yield ratio of 0.85 or less, and a Charpy absorbed energy vE- ₃₀ of 50 J or more. The method of manufacturing a steel pipe according to claim 1, wherein the steel pipe is heated to a temperature of less than _one point Ac and air-cooled. The steel slab is in mass%, and
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
It contains at least 1 sort (s) of Ti: 0.025% or less and Ca: 0.006% or less, The manufacturing method of the steel pipe of Claim 1 or 2 characterized by the above-mentioned. In a steel pipe in which the ratio t / D between the wall thickness t and the outer diameter D is more than 6%,
% By mass
C: 0.02 to 0.20%,
Si: 0.05-0.5%, and Mn: 0.5-2.0%
Containing
Al: 0.10% or less,
P: 0.03% or less,
S: 0.01% or less, and N: 0.007% or less, the balance consisting of Fe and inevitable impurities,
The tissue includes hard tissue and soft tissue;
The effective crystal grain size of the hard structure is 1 to 15 μm, the effective crystal grain size of the soft structure is 1 to 15 μm, the yield stress is 440 MPa or more, the tensile strength is more than 590 MPa and 740 MPa or less, and the yield ratio is 0.1. 85 or less, Charpy absorbed energy vE- ₃₀ is 50 J or more, A steel pipe characterized by the above-mentioned. In mass%,
Ni: 1.0% or less,
Cu: 1.0% or less,
Cr: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.1% or less,
V: 0.1% or less,
The steel pipe according to claim 4, comprising at least one of Ti: 0.025% or less and Ca: 0.006% or less.

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