JP2005002385A - Steel tube having excellent formability and toughness, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel tube having formability such as ductility, bendability and hydroform workability, and toughness high in an isotropic manner, and to provide its production method. <P>SOLUTION: The steel tube has a composition comprising 0.0002 to 0.70% C, 0.003 to 3.0% Si, 0.003 to 3.0% Mn, 0.002 to 2.0% Al, ≤0.15% P, ≤0.05% S and ≤0.015% N, or, further comprising 0.0002 to 0.01% B, one or more kinds of metals selected from Ti, Nb, V and Zr by 0.005 to 1% in total, one or more kinds of metals selected from Cr, Mo, Cu and Ni by 0.005 to 3% in total, 0.0001 to 0.005% Ca or 0.0001 to 0.2% rare earth metals. Further, ≥50% by the area ratio in the structure consists of ferritic phases, the average crystal grain size thereof is ≤40 μm, the aspect ratio is ≤3.0, and the ratio of the large angle grain boundaries occupied in the ferrite grain boundaries is ≥70%. The average crystal grain size of the second phases in which the area ratio is the maximum among the balance phases is ≤40 μm, and the second phases in which the distance between the closest second phases is the double of the minimum size of the second phases or above cover ≥50% in the area ratio thereof. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００６９】
【表２】

【００７０】
【表３】

【００７１】
【表４】

【００７２】
【発明の効果】
本発明により、均一微細で等方的なフェライト単相組織または複相（フェライト相＋第二相）組織が得られ、成形性（延性、曲げ性、ハイドロフォーム加工性）と靱性が等方的に優れた鋼管を製造することができる。
【図面の簡単な説明】
【図１】本発明を実施する電縫鋼管の製造工程の一例を示す模式的説明図である。
【符号の説明】
１： アンコイラー ２： 帯鋼
３： 加熱炉 ４： 成形および誘導加熱溶接装置
５： 管再加熱炉 ６： 絞り圧延機
７： 冷却装置 ８： 管切断装置
９： 電縫鋼管[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel pipe excellent in formability, specifically ductility, bendability, hydroform formability, and toughness, in particular, an electric resistance steel pipe and a method for producing the same.
[0002]
[Prior art]
In recent years, steel materials are desired to have not only strength but also excellent secondary workability. In particular, steel pipes are required to be maintained at a high level without deterioration in formability and toughness such as ductility, bendability and hydroformability even when the strength is increased. In addition, since steel pipes are processed in various directions including the pipe axis direction and pipe circumferential direction, it is desired that the formability and toughness are isotropically high so that they may be processed in any direction. Yes. As a means for improving the characteristics corresponding to such needs, there is crystal grain refinement, and several proposals have been made.
[0003]
Patent Document 1 discloses Ac. ₃ After heating or soaking to a transformation point to 400 ° C., grain rolling can be suppressed by carrying out drawing rolling with a cumulative diameter reduction rate of 20% or more and then rapidly cooling to room temperature at a cooling rate of 1.5 ° C./s or more. This technique is disclosed. However, this technique has a problem that it becomes a structure (mixed grain structure) in which fine grains having a large particle size are mixed in fine grains. Furthermore, since the rolling is mainly performed in the ferrite region or (ferrite + austenite) two-phase region, the ferrite phase and the remaining phase are stretched to form a band-like structure in which the remaining phase is connected, and the formability and toughness are tube axes. There is a problem that the molding direction is restricted because the direction of the pipe and the pipe circumferential direction are different, that is, the characteristic anisotropy is large and isotropic.
[0004]
Patent Document 2 discloses that 400 ° C. to Ar ₃ A method of manufacturing a steel pipe is disclosed in which drawing rolling is performed at a transformation point of less than + 50 ° C. and a reduction rate of 30% or more, and cooling is performed at a cooling rate of 30 ° C./s or more within 0.5 seconds after the completion of rolling. . However, this prior art also has a problem that the characteristic anisotropy is large and the molding direction is restricted, as in Patent Document 1.
[0005]
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-94009
[Patent Document 2] Japanese Patent Laid-Open No. 2001-162305
[0006]
[Problems to be solved by the invention]
In the prior art, there are problems such as the mixing of the microstructure as described above, an increase in the anisotropy of the microstructure and characteristics, and the restriction of the molding direction due to them.
[0007]
The object of the present invention is to solve the above-mentioned problems of the prior art, and even if the strength is increased, formability such as ductility and bendability and toughness are not deteriorated, and these are maintained at an isotropically high level. It is an object of the present invention to provide a steel pipe having a high formability and toughness that can meet the needs to be made and a method for manufacturing the steel pipe.
[0008]
[Means for Solving the Problems]
The present inventor has repeatedly studied to solve the above problems. And in the prior art, the idea that the above-mentioned problem arose because the drawing rolling end temperature was low was obtained. As a result, we have obtained the knowledge that steel pipes with isotropic and excellent toughness can be obtained by prescribing the chemical composition and structure of the steel, as well as the drawing and cooling conditions as the manufacturing method. The present invention has been completed.
[0009]
That is, the present inventors' knowledge is as follows.
(1) It has a specific chemical composition, and in an area ratio, 50% or more of the metal structure is a ferrite phase and, in some cases, is substantially composed of a ferrite phase, the average crystal grain size is 40 μm or less, and the aspect ratio is 3.0. Further, a steel pipe in which 70% or more of ferrite grain boundaries are composed of large-angle grain boundaries exhibits excellent formability and toughness, and is isotropic with small anisotropy in their properties.
[0010]
(2) Furthermore, the second phase is present, the average crystal grain size of the second phase having the largest area ratio among the remaining phases excluding the ferrite phase and the precipitates is 40 μm or less, and the closest A steel pipe that satisfies that the distance between the two phases is more than twice the minimum diameter of the second phase and that the second phase occupies 50% or more of the area ratio of the second phase has excellent formability and toughness. And isotropic with a small anisotropy of their properties.
[0011]
(3) In addition to the above (1) or (2), X-ray integrated intensity of all crystal orientations in a cross section perpendicular to the tube axis direction, a cross section perpendicular to the circumferential direction, and a cross section perpendicular to the radial direction of the steel pipe The steel pipe satisfying the ratio of 3.0 or less, and the hardness ratio of the second phase and the ferrite phase (the value obtained by dividing the Vickers hardness of the second phase by the Vickers hardness of the ferrite phase) is 7 or less. A satisfactory steel pipe, or a steel pipe satisfying that the area ratio of precipitates having a minimum diameter of 1 nm or more is 2% or less of the metal structure is isotropic, and has excellent formability and toughness. Show.
[0012]
(4) A method of manufacturing a steel pipe by forming a heated steel strip into an open pipe, welding the edge portion of the open pipe to form a base steel pipe, and then performing drawing rolling. The steel strip or base metal pipe ₃ Ae after heating to above 1300 ° C ₃ Point + 100 ° C to Ae ₃ The total cross-sectional area reduction rate at the point is 10% or more, and the rolling end temperature is (Ae ₃ Point −50 ° C.) After the drawing rolling, cooling is started within 5 s after drawing rolling, cooling to 650 ° C. at 1.0 ° C./s or more, and then cooling at 0.5 ° C./s or more. Accordingly, the steel pipe having excellent formability and toughness described in the above (1) to (3) can be manufactured.
[0013]
Among the technical terms defined in the present invention, the “aspect ratio” refers to the maximum value among the (maximum diameter) / (minimum diameter) values of the crystal grains of the phase.
The “maximum diameter” of a crystal grain refers to the longest diameter of the crystal grain, and the “minimum diameter” of the crystal grain refers to the shortest diameter of the crystal grain. For example, an optical microscope or a scanning electron microscope (SEM) The structure was photographed with several fields of view, and the “maximum diameter” and “minimum diameter” obtained by linear cutting using this structure photograph were multiplied by 1.13 to obtain the “maximum diameter” and “ "Minimum diameter".
[0014]
Similarly, the “average crystal grain size” of the phase is, for example, an average section length measured by a linear cutting method using several photographs of a tissue taken with an optical microscope or a scanning electron microscope (SEM). The value multiplied by 13 was adopted.
[0015]
“Large-angle grain boundaries” refer to those having an orientation difference of 15 ° or more between adjacent ferrite crystal grains. The crystal orientation difference between adjacent ferrite crystal grains can be measured by, for example, an electron beam backscattering method (EBSP).
[0016]
“Phase area ratio” is obtained, for example, by taking several views of a tissue with an optical microscope or a scanning electron microscope, analyzing a tissue photograph using an image analyzer to determine an area ratio in each field, and calculating an average value thereof. It was.
[0017]
“L value” indicates the proportion of the second phase that satisfies the fact that the distance between the closest second phases is at least twice the minimum diameter of the second phase in all the second phases. This was obtained by analyzing a tissue photograph using an image analyzer after taking several fields of view of the tissue.
[0018]
“Precipitate” refers to carbides (excluding cementite), nitrides, sulfides, oxides, phosphides, borides and composite products thereof, and the minimum diameter is the shortest diameter as described above. Say. The minimum diameter of the precipitate is obtained, for example, by taking a few fields of view of the structure with a transmission electron microscope (TEM) and obtained directly from the structure photograph. The area ratio of the precipitate is also the number of the structures with the transmission electron microscope (TEM). The field of view is photographed, the tissue photograph is subjected to image analysis, and the phase area ratio is obtained in the same manner as described above.
[0019]
“Second phase” refers to various phases such as cementite other than ferrite, pearlite, bainite, martensite, and austenite remaining without transformation (hereinafter referred to as “residual austenite”). Furthermore, each temperature prescribed | regulated by this invention points out the surface temperature of a to-be-measured material.
[0020]
The “X-ray integral intensity ratio” of the crystal orientation relates to each surface such as {100}, {110}, {111}, {211}, {311}, {332} in a cross section perpendicular to the radial direction of the steel pipe, for example. It can be evaluated by measuring the X-ray random intensity ratio. The random intensity ratio is obtained as a ratio between the diffraction intensity of each crystal plane obtained from the measurement sample and the diffraction intensity of the sample having a random crystal orientation. It can also be evaluated by measuring an inverse pole figure for each orientation such as <100>, <110>, <111>, <211>, <311>, <332>, and the like.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The requirements defined by the present invention will be described by classifying them into chemical compositions, metal structures and manufacturing methods of steel pipes. In the following description,% display of the content of each element means mass%.
[0022]
(A) Chemical composition
Here, the reason for limiting the above-described steel composition in the present invention will be described.
C: 0.0002 to 0.70%
If C is less than 0.0002%, the crystal grains become extremely coarse, and high formability cannot be stably obtained, and cracks and rough surfaces are likely to occur during the formation of a steel pipe. Moreover, the plating adhesion is also reduced. Furthermore, in order to reduce the C content to less than 0.0002%, a special steelmaking technique is required, so the cost is increased. When the content exceeds 0.70%, the strength increases excessively and ductility and hot workability decrease, and defects are likely to occur in the welded joint when a steel pipe is manufactured from a steel sheet by roll forming. The welding situation becomes unstable, and the groove-like corrosion resistance of the weld is deteriorated. Therefore, in this embodiment, the C content is limited to 0.0002% or more and 0.70% or less, preferably 0.010% or more and 0.60% or less, and more preferably 0.020% or more and 0.000% or less. 50% or less.
[0023]
Si: 0.003 to 3.0%
Si has an action of improving the strength of steel without impairing workability. Further, it has the effect of promoting the generation of ferrite and increasing the amount of ferrite. In order to exert such an effect, at least 0.003% is contained. Moreover, although it acts as a deoxidation element by containing, there exists an effect | action of the alloy layer growth suppression of plating, and 3.0% or less is added. Preferably, 0.05% to 2.5% is effective. More preferably, it is 0.10% or more and 2.0% or less. In addition, when content exceeds 3.0%, bad influences, such as degrading a moldability and toughness, will arise.
[0024]
Mn: 0.003 to 3.0%
Mn has the effect of preventing hot brittleness of the steel due to S. Furthermore, it also has the effect of strengthening the solid solution. In order to exhibit such an effect, it is necessary to contain at least 0.003%. However, if the content exceeds 3.0%, the ductility and weldability are deteriorated and the periphery of MnS, which is a nonmetallic inclusion, is easily dissolved. Grooved corrosiveness deteriorates. Therefore, in the present invention, the Mn content is limited to 0.003% to 3.0%. From the viewpoint of harmony between strength and elongation, the lower limit value of the Mn content is preferably 0.10%, and the upper limit value is preferably 2.7%, more preferably 0.20% to 2.5%. is there.
[0025]
Al: 0.002 to 2.0%
Al also acts as a deoxidizing element when contained in an amount of 0.002% or more. However, if the Al content exceeds 2.0%, the amount of inclusions increases to lower the cleanliness of the steel and decrease the corrosion resistance. Invite. Therefore, in the present invention, the Al content is limited to 0.002% or more and 2.0% or less. Preferably, they are 0.005% or more and 1.0% or less, More preferably, they are 0.010% or more and 0.5% or less.
[0026]
P: 0.15% or less
P is an unavoidable impurity, and segregates at the grain boundaries to deteriorate both toughness and groove-like corrosion resistance. However, since the extreme reduction of P is accompanied by a corresponding increase in cost, the P content is set to 0.15% or less in the present invention. Preferably it is 0.10% or less, More preferably, it is 0.05% or less.
[0027]
S: 0.05% or less
S is an unavoidable impurity, and since it produces sulfides and degrades both the cleanliness and the groove-like corrosion resistance of steel, it is desirable that its content be small. However, since extreme reduction of S is accompanied by a corresponding increase in cost, in the present invention, the upper limit value of S content is preferably set to 0.05%. More preferably, it is 0.03% or less, More preferably, it is 0.01% or less.
[0028]
N: 0.015% or less
N is a steel strengthening element and is an unavoidable impurity. Although the amount usually contained as an impurity is about 0.003%, the content up to 0.015% is allowed without any particular harm. Therefore, in the present invention, the N content is preferably limited to 0.015% or less. More preferably, it is 0.010% or less, More preferably, it is 0.007% or less.
[0029]
These elements are basic components of the steel pipe according to the present embodiment. By further including at least one of the elements described below as an optional additive element in this basic component, even more excellent groove corrosion resistance and A steel pipe having other characteristics can be obtained. Therefore, these optional additive elements will be described below.
[0030]
In the present invention, the steel composition may further include one or more of the following first group to fourth group.
First group: B: 0.0002 to 0.01%
Second group: 0.005 to 1% in total of one or more of Ti, Nb, V and Zr,
Third group: one or more of Cr, Mo, Cu and Ni in a total of 0.005 to 3%,
Group 4: One element or more of Ca: 0.0001 to 0.005% and REM (rare earth element): 0.0001 to 0.2%.
[0031]
B: 0.0002 to 0.01%
Since B has the effect of enhancing the hardenability of the steel, it may be utilized when controlling the crystal grain size of the ferrite phase during the cooling process. If the B content is less than 0.0002%, it is difficult to obtain the effect. However, if the B content exceeds 0.010%, both formability and toughness deteriorate. Therefore, when B is added, the content is made 0.0002% or more and 0.01% or less.
[0032]
One or more of Ti, Nb, V and Zr in total 0.005 to 1%
Ti, Nb, V, and Zr combine with C, N, and S in the steel to form precipitates, and have the effect of increasing the strength of the steel without significantly impairing ductility and bendability. Moreover, it has the effect | action which couple | bonds with C, N, and S, and makes these harmless. Therefore, in order to increase the strength of the steel, one or more of Ti, Nb, V, and Zr may be contained. However, when the content is less than 0.005%, it is difficult to obtain such an effect. When the total exceeds 1%, the above effects are saturated, and conversely, ductility and bendability are lowered. Therefore, when adding one or more of Ti, Nb, V, and Zr, the total content thereof is preferably 0.005% or more and 1% or less.
[0033]
One or more of Cr, Mo, Cu and Ni in total 0.005 to 3%
Since Cr, Mo, Cu and Ni have an effect of improving hardenability, it becomes easy to control the crystal grain size and area ratio of the ferrite phase and the remaining phase in the cooling process. In addition to enhancing the hardenability, Cu also has the effect of enhancing corrosion resistance. For this reason, one or more of Cr, Mo, Cu and Ni may be contained for the above-mentioned purpose. However, when the content is less than 0.005%, it is difficult to obtain such an effect. If it exceeds 3%, the above effect is saturated and, on the contrary, ductility and bendability are lowered. Therefore, when one or more of Cr, Mo, Cu and Ni are added, the total content thereof is preferably 0.005% or more and 3% or less.
[0034]
Ca: 0.0001 to 0.005%, and REM (rare earth element): 0.0001 to 0.2%
Ca and REM are elements of 0.0001% or more, respectively, and have the effect of improving the workability by controlling the form of inclusions, and improve the resistance to groove corrosion of the ERW weld. For this reason, Ca and REM may be contained for the above-mentioned purpose, but such an effect is difficult to obtain when the content is less than 0.0001%. On the other hand, when the Ca content exceeds 0.005% or the REM exceeds 0.2%, the cleanliness of the steel decreases and the ductility deteriorates. Therefore, when Ca and REM are added, the content of Ca is preferably 0.0001% or more and 0.005% or less, and REM is preferably 0.0001% or more and 0.2% or less.
[0035]
(B) Metallographic structure
(1) Having a specific chemical composition, that is, that is, in an area ratio, 50% or more of the metal structure is a ferrite phase, the average crystal grain size is 40 μm or less, the aspect ratio is 3.0 or less, and the ferrite When 70% or more of the grain boundaries are composed of large-angle grain boundaries and the second phase is present, the average crystal grain size of the second phase having the largest area ratio among the remaining phases excluding the ferrite phase and the precipitate is 40 μm. A steel pipe that satisfies the following conditions exhibits excellent formability and toughness.
[0036]
(2) In addition to the above (1), the second phase satisfying that the distance between the closest second phases is at least twice the minimum diameter of the second phase accounts for 50% or more of the area ratio of the second phase. Furthermore, a steel pipe satisfying that the area ratio of precipitates having a minimum diameter of 1 nm or more is 2% or less of the metal structure, and the hardness ratio between the second phase and the ferrite phase (the Vickers hardness of the second phase is the same as that of the ferrite phase). A steel pipe satisfying that the value divided by Vickers hardness) is 7 or less, or, further, all the crystals in the cross section perpendicular to the pipe axis direction, the cross section perpendicular to the circumferential direction, and the cross section perpendicular to the radial direction. A steel pipe satisfying an X-ray integral intensity ratio of azimuth of 3.0 or less exhibits extremely excellent formability and toughness.
[0037]
When the area ratio of ferrite in the structure of the steel pipe is less than 50%, a high strength is obtained because the second phase having a higher strength than ferrite increases, but the formability and toughness such as ductility and elongation of the steel pipe are improved. It will deteriorate significantly. Therefore, the area ratio of ferrite in the steel pipe structure is set to 50% or more. The area ratio of ferrite is preferably 60% or more, and more preferably 70% or more. The area ratio of ferrite in the steel pipe structure may be a value close to 100%.
[0038]
The area ratio of the ferrite phase is determined by many factors such as alloy elements, drawing rolling conditions, cooling conditions, and the like, but can be changed depending on, for example, the amount of C, drawing rolling completion temperature, and cooling rate.
[0039]
When the average crystal grain size of ferrite in the steel pipe exceeds 40 μm, even if the area ratio of ferrite in the structure is 50% or more, deformation is concentrated on specific coarse crystal grains and the strain is likely to be localized. For this reason, at the time of processing such as tensile molding and bending molding, it becomes impossible to stably obtain a high moldability and a good strength-formability balance. Furthermore, since the ferrite crystal grains are large, toughness deterioration and characteristic fluctuation increase occur. In addition, the surface of the steel pipe is roughened during processing, and the surface roughness tends to increase, resulting in poor surface properties. Therefore, the average crystal grain size of ferrite in the steel pipe is set to 40 μm or less. The average crystal grain size of ferrite is preferably 30 μm or less, and more preferably 20 μm or less. The smaller the average crystal grain size of this ferrite, the better. However, in order to make the average crystal grain size of ferrite 1 μm or less, a special technique is required and the cost increases, so the lower limit on the industrial scale is about 1 μm. .
[0040]
If the aspect ratio of the ferrite of the steel pipe exceeds 3.0, it is impossible to obtain a substantially isotropic characteristic as an equiaxed microstructure. In such a structure, the formability and toughness are different between the pipe axis direction and the pipe circumferential direction, that is, the characteristic anisotropy is large and isotropic, and therefore, there is a problem that molding is possible only in a specific direction. Therefore, the aspect ratio of ferrite needs to be 3.0 or less. The aspect ratio should be small, and is preferably 2.0 or less. It is even more preferable that the aspect ratio is a value close to 1.0.
[0041]
If the ratio of “large-angle grain boundaries” in the ferrite phase grain boundaries of the steel pipe is less than 70%, that is, if the number of small-angle grain boundaries increases, the effect as a ferrite grain boundary is substantially reduced and surrounded by the large-angle grain boundaries. As a result, there are many coarse grains as ferrite grains. In this case, not only the high formability cannot be stably obtained, but also the toughness is deteriorated, the characteristic variation is increased, and the surface roughness during processing is likely to occur.
[0042]
The average grain size, aspect ratio, and large-angle grain boundary ratio of such ferrite can be adjusted by the alloying elements, drawing rolling conditions, cooling conditions, etc., as with the ferrite phase ratio. The rolling can be adjusted by the rolling temperature, degree of processing, and cooling rate.
[0043]
In the present invention, it includes the case where there is substantially no other phase other than the ferrite phase, but when such a second phase is present, the maximum area of the remaining phase other than the ferrite phase and precipitates. If the average crystal grain size of the second phase, which is the phase occupying the ratio, exceeds 40 μm, cracks are likely to occur at the interface between the ferrite phase and the second phase in the steel pipe, and propagation at the ferrite grain boundary is difficult to prevent. Become. Furthermore, since the distribution of the hard second phase tends to be non-uniform, moldability and toughness are reduced.
[0044]
For this reason, the average crystal grain size of the second phase is set to 40 μm or less. Preferably it is 30 micrometers or less, More preferably, it is 20 micrometers or less. A preferred lower limit is 0.1 μm.
The ratio of the area occupied by the second phase in the second phase satisfying that the distance between the closest second phases is at least twice the minimum diameter of the second phase, that is, the L value is less than 50%, or When the distance between the second phases is less than twice the minimum diameter of the second phase, the proportion of so-called band-like structures in which the second phases are connected and distributed increases. Steel pipes often have a band-like structure in the pipe axis direction. In this case, the formability and toughness in the pipe axis direction are good, but the formability and toughness in the pipe circumferential direction are greatly deteriorated because cracks are generated and propagated from the interface between the ferrite and the second phase. The anisotropy of becomes large. Accordingly, the area ratio of the second phase in the second phase that satisfies that the distance between the closest second phases is at least twice the minimum diameter of the second phase needs to be 50% or more. The distance between the closest second phases is preferably at least 3 times the minimum diameter of the second phase, more preferably at least 4 times. The second phase satisfying that the distance between the closest second phases is at least twice the minimum diameter of the second phase is preferably 60% or more of the area ratio of the second phase, and preferably 70% or more. Even more preferred.
[0045]
The amount and refinement state of the second phase can be adjusted in the same manner as the ferrite phase.
The strength ratio increased when the X-ray integral intensity ratio of any crystal orientation in the cross section perpendicular to the tube axis direction, the cross section perpendicular to the circumferential direction, and the cross section perpendicular to the radial direction of the steel pipe exceeded 3.0. Although the characteristic in a specific direction is improved, the degree of deterioration of the characteristic in another direction is increased, and the anisotropy of the characteristic is increased. For example, if the X-ray integral intensity ratio of {111} texture in the cross section perpendicular to the circumferential direction exceeds 3.0, the r value in the tube axis direction increases and the ductility in that direction is improved. Ductility in other directions deteriorates. Therefore, in order to obtain isotropic characteristics, the X-ray integral intensity ratio of all crystal orientations in the cross section perpendicular to the tube axis direction, the cross section perpendicular to the circumferential direction, and the cross section perpendicular to the radial direction is 3. It is preferably 0 or less. More preferably, it is 2.5 or less, and most preferably 2.0 or less.
[0046]
When forming a steel pipe, cracks often occur from the interface between the second phase and the ferrite phase. If the hardness ratio (Hv2 / Hvf) between the second phase Vickers hardness (Hv2) and the ferrite phase Vickers hardness (Hvf) exceeds 7, cracks frequently occur at the interface during molding, and ductility and bendability deteriorate. . Therefore, the hardness ratio between the second phase and the ferrite phase is preferably 7 or less. More preferably, it is 6 or less, and most preferably 5 or less.
[0047]
In steel pipes, when the area ratio of precipitates with a minimum diameter of 1 nm or more exceeds 2%, the rate of increase in strength due to precipitation strengthening, which is a strengthening mechanism that reduces formability and toughness, increases, and formability and toughness are reduced. May decrease. Therefore, it is preferable that the area ratio of precipitates having a minimum diameter of 1 nm or more is defined as 2% or less of the structure. The minimum diameter of the precipitate is preferably 5 nm or more, and more preferably 10 nm or more. The upper limit of the minimum diameter of the precipitate may be about 5 μm. The maximum diameter of the precipitate is preferably about 10 μm. The upper limit of the area ratio occupied by the precipitate is preferably 1%, and more preferably 0.2%.
[0048]
(C) Steel pipe manufacturing method
The production method of the present invention will be described.
FIG. 1 is a schematic explanatory view showing an example of a manufacturing process for manufacturing an electric resistance welded steel pipe according to one embodiment of the present invention.
[0049]
According to this embodiment, the steel strip 2 unwound from the uncoiler 1 is first heated to a predetermined temperature in the heating furnace 3. Next, in the forming and induction heating welding device 4, the heated steel strip is formed into an open pipe, and the edge portion of the open pipe is welded to form a base steel pipe. The base steel pipe is reheated by the pipe reheating furnace 5 and then drawn by the drawing mill 6 to obtain a steel pipe. The obtained steel pipe is subjected to predetermined processing by the cooling device 7 and the pipe cutting device 8, and is further subjected to a plating treatment as necessary to become an electric-welded steel pipe 9.
[0050]
Here, according to this embodiment, the above-described strip steel or base steel pipe is connected to Ac. ₃ Ae after heating to above 1300 ° C ₃ Point + 100 ° C to Ae ₃ The total cross-sectional area reduction rate at the point is 10% or more, and the rolling end temperature is (Ae ₃ Point −50 ° C.) After the drawing rolling, cooling is started within 5 s after drawing rolling, cooling to 650 ° C. at 1.0 ° C./s or more, and then cooling at 0.5 ° C./s or more. .
[0051]
The heating temperature of the strip steel or base steel pipe is Ac ₃ When the temperature is less than the point, that is, when the microstructure is not sufficiently austenitized, the drawing rolling is started in the (ferrite + austenite) two-phase region, and the drawing rolling end temperature is set to (Ae). ₃ Point −50 ° C.) or more. As a result, the microstructure becomes a mixed grain structure or a band-like structure extending in the tube axis direction.
[0052]
In this case, for example, the average crystal grain size of the ferrite phase is 40 μm or less, or the aspect ratio is 3.0 or less, and the ratio of the large-angle grain boundary of the ferrite grain boundary is 70% or more. The X-ray integral of the second phase whose particle size is 40 μm or less and whose distance between the closest second phases is at least twice the minimum diameter of the second phase is 50% or more of the area ratio of the second phase or all crystal orientations A microstructure with a strength ratio of 3.0 or less is not obtained, and formability and toughness are reduced, and anisotropy is increased. In addition, the base steel pipe is made to Ac using a pipe reheating furnace before drawing rolling. ₃ By raising the temperature to a point or more, austenitization of the microstructure can be promoted, but productivity is lowered because heating for a long time is required.
[0053]
When manufacturing the base steel pipe from the strip steel, when the heating temperature exceeds 1300 ° C., it is difficult to completely remove the surface scale at the butt end face at the time of solid phase bonding at a high temperature because the heating temperature is high. Welding defects such as scale biting into the joint portion occur, the joint strength of the joint portion tends to be inferior to that of the base material portion, and the thermal efficiency is large. In addition, when heating either the steel strip or the base steel pipe, if the heating temperature exceeds 1300 ° C., the austenite grains become coarse, and even if strong processing is performed by drawing rolling, the crystal grains of the final product become coarse, Sufficient strength cannot be obtained, and the scale of the surface of the steel strip or the base steel tube is liable to occur, leading to deterioration of the surface properties of the steel tube after drawing. Furthermore, the starting temperature of drawing rolling becomes higher, and Ae ₃ Point + 100 ° C to Ae ₃ Since it becomes difficult to ensure the total cross-sectional area reduction rate at 10% or more in terms of points, a desired microstructure cannot be obtained.
[0054]
Therefore, in the present embodiment, the heating temperature of the strip steel or the base steel pipe is Ac. ₃ It is set as 1300 degrees C or less from a point. What is necessary is just to select a heating time suitably according to the dimension of a base material steel pipe in the range in which an austenite crystal grain does not become coarse.
[0055]
Drawing Ae ₃ Point + 100 ° C to Ae ₃ When the total cross-sectional area reduction rate in terms of points is less than 10%, the desired microstructure cannot be obtained, and isotropically excellent moldability and toughness cannot be obtained. Therefore, in this embodiment, Ae of drawing rolling ₃ Point + 100 ° C to Ae ₃ The total cross-sectional area reduction rate at points is set to 10% or more.
[0056]
Drawing rolling finish temperature is (Ae ₃ If the temperature is less than −50 ° C., the ferrite phase produced by transformation from the austenite phase during drawing rolling increases. The ferrite that has been processed after transformation becomes coarse, and the ferrite that transforms after being processed in the austenite region becomes finer. Therefore, the more ferrite phase that is produced during drawing rolling, the more the mixed grain structure in the final product. The ratio to the whole becomes large.
[0057]
Further, the microstructure becomes a band-like structure in which the ferrite phase and the residual phase are expanded in the rolling direction and the residual phases are connected.
In this way, when it becomes a mixed grain structure in the final product, when the obtained ERW steel pipe is used as underground pipe, etc., local batteries are generated due to particle size difference in a wet or corrosive environment, and the corrosion resistance is improved. to degrade. When it becomes a band-like structure, the anisotropy of formability and toughness increases. The drawing rolling end temperature is set to (Ae ₃ By setting the temperature to -50 ° C. or higher, the ferrite phase generated and coarsened during drawing rolling can be reduced, and the ratio of the mixed grain structure and band-like structure in the final product can be reduced. In addition, it has been found that since the proportion of the fine grain structure can be increased, the corrosion resistance can be improved and the moldability and toughness can be improved isotropically. Therefore, in this embodiment, the drawing rolling end temperature is set to (Ae ₃ Point −50 ° C.) or higher.
[0058]
Further, the upper limit of the start temperature and the end temperature of drawing rolling need not be specified, but the upper limit value of the start temperature is preferably 1300 ° C. or less. This is because when the starting temperature exceeds 1300 ° C., the austenite grains become coarse, and even if strong processing by drawing rolling is performed, the crystal grains of the final product become coarse, and sufficient strength cannot be obtained. This is because it tends to occur and may deteriorate the tube surface properties after drawing.
[0059]
Cooling is started within 5 s after drawing rolling, and after cooling to 650 ° C. at 1.0 ° C./s or more, cooling at 0.5 ° C./s or more can prevent coarsening of the crystal grains, and the ferrite phase And the desired microstructure with the remainder phase formed. When the cooling start time after rolling is over 5 s, the cooling rate to 650 ° C. is less than 1.0 ° C./s, or the cooling rate after 650 ° C. is less than 0.5 ° C./s, Becomes coarse and a desired microstructure cannot be obtained. If the cooling rate exceeds 300 ° C./s, bainite or martensite becomes the main phase, so that a microstructure with the desired ferrite as the main phase cannot be obtained. Accordingly, the cooling rate is desirably 300 ° C./s or less. More preferably, it is 200 degrees C / s or less, More preferably, it is 100 degrees C / s or less.
[0060]
In addition, the steel plate for the strip steel and the base steel pipe may be any of a hot rolled steel plate, a cold rolled steel plate, and a cold rolled annealed steel plate, and is manufactured by a normal casting, hot rolling, cold rolling, annealing method. Should be used.
[0061]
As a welding method for manufacturing the base steel pipe, any method such as forging welding, seam welding, submerged arc welding, MIG welding, TIG welding, laser welding, etc. may be used as well as electric seam welding. Also, there is no problem even if a seamless steel pipe is used as the base steel pipe.
[0062]
The heating method of the strip steel or the base steel pipe is a method of heating by high-frequency induction heating, a direct current heating method of heating by direct current flowing through the steel material through a roll, a gas heating method of heating the steel material by a gas burner using combustion gas Various methods can be used.
[0063]
【Example】
A steel pipe was manufactured under the conditions shown in Table 2 using the steel having the chemical composition shown in Table 1 by the manufacturing process shown in FIG.
[0064]
A steel strip having a width of 290 to 480 mm and a thickness of 2.8 to 4.5 mm is heated by a steel strip heating furnace, and formed and electro-welded by a forming and induction heating welding apparatus, and the outer diameter is 90 to 150 mm. By making the steel pipe continuously reheated with a pipe reheating device, this steel pipe is subjected to drawing rolling with a 3-roll type stretch reducer (drawing rolling mill), and further cut into a predetermined pipe length with a pipe cutting device, An electric resistance welded steel pipe having an outer diameter of 20 to 120 mm and a wall thickness of 2.5 to 4.5 mm was obtained.
[0065]
Tables 3 and 4 show the results of investigations on the microstructure and mechanical properties of these electric resistance welded steel pipes. The tensile properties in the tube axis direction were evaluated by taking a JIS 12B test piece, and the tensile properties in the pipe circumferential direction were obtained by developing a steel pipe and collecting a test piece having a gauge length of 50 mm and a gauge width of 25 mm. The bendability was evaluated by the presence or absence of surface cracking by conducting a test in which a steel pipe was bent to an inner radius of 90 degrees, which is four times the outer diameter. For hydroform formability, the steel pipe subjected to drawing rolling (d: the diameter of the test steel pipe) is placed in an upper and lower mold forming a space having a length of 4d (four times the diameter of the test steel pipe), and the space A steel pipe part of 5d (5 times the diameter of the test steel pipe) is gripped on both sides from the part, and internal pressure is applied to the steel pipe with water, causing the steel pipe to bulge into the mold space in the circumferential direction, causing cracks. The peripheral length of the broken part was measured, and evaluated by the limit tube expansion ratio using the following formula.
Limit pipe expansion rate = {(Break circumference-pipe circumference) / (element circumference)} x 100
No. 12 (steel L), no. 15 (steel O), and No. 18 (steel R) has a large amount of C and a large amount of the second phase. 19 and no. For No. 22, since drawing rolling is completed in a temperature range where a large amount of ferrite phase is generated, the area ratio of the connected second phase is large and the L value is low. The average particle size of the second phase in these cases was measured for the unconnected second phase. No. 19 and no. No. 22 has a mixed grain structure in which a part of the ferrite phase is coarsened, and the average grain size is measured including coarse grains.
[0066]
No. which is an example of the present invention. 1 (steel A) -No. For 14 (N), the desired microstructure was obtained, the tensile properties and toughness were isotropically good, and the bendability and hydroform moldability were also excellent.
[0067]
No. having a component deviating from the chemical composition defined in the present invention. 15 (steel O) -No. No. 18 (steel R) and No. 18 manufactured under conditions deviating from the manufacturing conditions specified in the present invention. 19-No. For No. 25, the desired microstructure and characteristics were not obtained.
[0068]
[Table 1]

[0069]
[Table 2]

[0070]
[Table 3]

[0071]
[Table 4]

[0072]
【The invention's effect】
According to the present invention, a uniform fine and isotropic ferrite single phase structure or a double phase (ferrite phase + second phase) structure can be obtained, and formability (ductility, bendability, hydroform workability) and toughness are isotropic. It is possible to manufacture a steel pipe excellent in
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic explanatory view showing an example of a manufacturing process of an ERW steel pipe embodying the present invention.
[Explanation of symbols]
1: Uncoiler 2: Strip steel
3: Heating furnace 4: Molding and induction heating welding equipment
5: Tube reheating furnace 6: Drawing mill
7: Cooling device 8: Pipe cutting device
9: ERW steel pipe

Claims (7)

% By mass
C: 0.0002 to 0.70%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0%,
Al: 0.002 to 2.0% and P: 0.15% or less, S: 0.05% or less,
N: 0.015% or less,
The balance is composed of Fe and impurities, and is substantially composed of a ferrite phase. The average crystal grain size of the ferrite phase is 40 μm or less, the aspect ratio is 3.0 or less, and the proportion of the large-angle grain boundary in the ferrite grain boundary is A steel pipe excellent in formability and toughness characterized by being 70% or more. % By mass
C: 0.0002 to 0.70%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0%,
Al: 0.002 to 2.0% and P: 0.15% or less, S: 0.05% or less,
N: 0.015% or less,
The balance consists of Fe and impurities, and 50% or more of the metal structure in the area ratio is the ferrite phase, and the average crystal grain size of the ferrite phase is 40 μm or less, the aspect ratio is 3.0 or less, And the average grain size of the second phase having the largest area ratio among the remaining phases excluding the ferrite phase and the precipitate is 40 μm or less, and A steel pipe excellent in formability and toughness characterized in that the second phase whose distance between the closest second phases is at least twice the minimum diameter of the second phase occupies 50% or more of the area ratio of the second phase . Furthermore, in mass%,
First group: 0.0002 to 0.01% of B,
Second group: 0.005 to 1% in total of one or more of Ti, Nb, V and Zr,
Third group: one or more of Cr, Mo, Cu and Ni in a total of 0.005 to 3%,
The fourth group according to claim 1 or 2, which contains any one or more elements of Ca: 0.0001 to 0.005% and REM (rare earth element): 0.0001 to 0.2%. Steel pipe. The steel pipe according to any one of claims 1 to 3, wherein an X-ray integral intensity ratio of all crystal orientations is 3.0 or less. The steel pipe according to any one of claims 1 to 4, wherein a hardness ratio between the second phase and the ferrite phase (a value obtained by dividing the Vickers hardness of the second phase by the Vickers hardness of the ferrite phase) is 7 or less. Features steel pipe. The steel pipe according to any one of claims 1 to 5, wherein an area ratio of precipitates having a minimum diameter of 1 nm or more is 2% or less of a metal structure. When manufacturing the steel pipe according to any one of claims 1 to 6, after heating the steel strip to form an open pipe and welding the edge of the open pipe to form a base steel pipe, a method for producing a steel pipe by performing rolling, after heating the steel strip or matrix steel tube 1300 ° C. or less than ₃ points Ac, the total cross-sectional area reduction ratio of at Ae ₃ point +100 ℃ ~Ae _3-point 10 %, The rolling finish temperature is (Ae ₃ point-50 ° C) or more, and after the drawing rolling, cooling is started within 5 s, and after cooling to 650 ° C at 1.0 ° C / s or more, A method for producing a steel pipe excellent in formability and toughness, characterized by cooling at 0.5 ° C./s or more.

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