JP5141440B2 - High-strength steel pipe excellent in workability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel pipe having a seam portion, and more particularly to improvement of workability. The term “high strength” as used herein means a strength level at which the tensile strength TS is 590 MPa or more, preferably 780 MPa or more, more preferably 980 MPa or more.

    In recent years, from the viewpoint of protecting the global environment, efforts are being made to reduce the weight of automobile bodies and improve the fuel efficiency of automobiles. And the improvement in fuel efficiency of this car has become mandatory by law. Recently, it has been studied to reduce the weight by reducing the thickness of the material for automobile bodies using a high-strength material, and to increase the rigidity of the member as a closed cross-sectional structure. The use of high-strength steel pipes has also begun in response to the increased rigidity of automobile parts.

  Since steel pipes have a closed cross-sectional structure unlike steel sheets, they have the feature of excellent bending rigidity and torsional rigidity, and can greatly contribute to weight reduction of members. For example, even if the steel pipe is processed into a flat shape and the inner surfaces of the pipes are in contact with each other, the rigidity can be increased more than doubled due to the closed cross-sectional structure. A similar closed cross-sectional structure can be produced by press-forming and welding thin steel sheets, but there are extra steps such as press-forming processes, welding processes with long weld lengths, and processes such as cutting the flange after welding. Need. For this reason, also from a viewpoint of member manufacturing cost, the advantage of the member manufacturing method using the steel pipe which does not require such an extra process is large.

  However, in order to produce a member having a complicated shape using a steel pipe, it is necessary to improve both the processing technology of the steel pipe and the workability of the steel pipe itself. In particular, high-strength steel pipes with a tensile strength of TS: 590 MPa or more often have problems such as buckling due to the latest steel pipe processing technology. In addition, for example, a welded steel pipe has a seam portion with high hardness and reduced workability, and therefore, consideration is given to processing by placing the seam portion on a neutral shaft for processing in advance. However, there are many cases in which the above-mentioned considerations cannot always be made when processing members, and there are problems that processing troubles such as thinning occur frequently despite the above-mentioned considerations. . For these reasons, a high-strength steel pipe having excellent ductility such as elongation and improved workability is desired.

  In response to such a demand, for example, Patent Document 1 proposes a steel pipe excellent in strength ductility balance. In the technique described in Patent Document 1, C: 0.05 to 0.25%, Si: 0.3 to 2.5%, Mn: 0.10 to 3.00%, Al: 0.005 to 0.100%, N: 0.0050% or less, and Ti, Nb, A steel pipe having a composition containing one or more of V and B is heated to a two-phase temperature range, and then at a cooling rate of 0.5 ° C./s or higher (Ms point + 100 ° C.) to a temperature range of Ms point. A composite structure comprising at least one of bainite, ferrite, and pearlite, with the structure containing 5 to 20% residual austenite, and maintained at that temperature for 30 to 300 seconds and then air-cooled to room temperature. To do. Thereby, the ductility is dramatically improved, and it is possible to obtain a steel pipe excellent in strength and ductility balance while having high strength, and a hydroformed product having a complicated shape can be easily manufactured. However, the technique described in Patent Document 1 employs a complicated process and has a problem that productivity is lowered. Further, in the technique described in Patent Document 1, in actual operation, there is no guarantee that the entire pipe is given a uniform heat history during heat treatment, and there is also a guarantee that the same heat history is given for each heat treatment. However, there is a concern that the characteristics may vary in the obtained steel pipe or in each obtained cut single pipe.

Patent Document 2 describes a high-strength steel pipe excellent in all-around pipe formability and a manufacturing method thereof. In the technique described in Patent Document 2, C: 0.0005 to 0.30%, Si: 0.001 to 2.0%, Mn: 0.01 to 3.0%, P: 0.001 to 0.20%, S: 0.0001 to 0.01%, N: 0.0001 to 0.05%, Al: a steel containing from 0.0001 to 0.50 percent by pipe formation as a base tube, the mother tube (a _C3 transformation point -50 ° C.) or higher After heating (a _C3 transformation point + 200 ° C.) or less, Reduce the diameter at 650 to 900 ° C .: Reduce the diameter to 10 to 70%, or introduce sufficient strain at the time of pipe making and heat at (AC ₃ transformation point + 200 ° C.) or less and 500 ° C. or less, Alternatively, by performing light processing pipe making, the minimum thickness of the entire circumference: t _min and the minimum hardness value HV _min from the 1/2 thickness to 20 μm below the surface layer in the part,
4.0 ≧ (t _min × HV _min ) / (t ₀ × HV ₀ ) ≧ 0.94
(Where t _{0 is} the average thickness (mm) of the entire circumference, HV _{0 is} the average value of the Vickers hardness of the entire circumference)
It is going to become a steel pipe that satisfies Thereby, the non-uniformity of the plate thickness and the non-uniformity of the hardness are suppressed, and the all-round expansion formability of a high strength steel pipe having a tensile strength of 350 MPa or more is improved. However, in the technique described in Patent Document 2, the obtained steel pipe is a steel pipe mainly composed of a ferrite phase, and has a high strength level with a tensile strength TS of 590 MPa or more, preferably 780 MPa or more, more preferably 980 MPa or more. There is a problem that it is difficult to secure a stable steel pipe.

In Patent Document 3, C: 0.10 to 0.19%, Si: 0.01 to 0.5%, Mn: 0.8 to 2.2%, P: 0.02% or less, S: 0.003% or less, N: 0.005% or less, Al: 0.01 -0.06%, Nb: 0.005-0.03%, B: 0.0005-0.0030% included, P: 0.02% or less, S: 0.003% or less, N: 0.005% or less, Ti: 0.015% or less, and 80- A method for producing an electric resistance welded steel pipe having a structure composed of 100% tempered martensite and the balance composed of ferrite and having a tensile strength TS of 980 N / mm ² or more is described. However, the electric resistance welded steel pipe manufactured by the technique described in Patent Document 3 has a martensite fraction of 80% or more and high strength of TS of 980 N / mm ² or more. It is considered difficult to ensure, and it must be said that it is difficult to maintain excellent workability that can withstand bending and hydroforming.

Further, Patent Document 4 describes a method for manufacturing an electric resistance welded steel pipe for forming hydroform that is formed while maintaining a state of an elastic region having a strain amount of 0.5% or less and is subjected to electric resistance welding. In the technique described in Patent Document 4, since the amount of strain applied during pipe making is reduced (within the elastic range) by the cage roll method, the amount of decrease in elongation of the steel pipe after pipe forming is small and suitable for hydroforming. It is said that it will be possible to secure the elongation and improve the hydroformability of the steel pipe.
JP2003-342687 JP 2002-146469 A JP 09-104921 A JP 2000-15349

  However, the technique described in Patent Document 4 is a pipe making method for low-strength materials. By applying this technique, it is completely elastic and has high strength such as TS: 590 MPa class and TS: 780 MPa class. There is a problem that it is difficult to make a steel pipe. This is because, normally, in pipe making by roll forming, the forming is performed in the plastic region, and it is impossible to form the entire region completely in the elastic region. This is clear from the fact that even if the pipe is cut in the longitudinal direction, it does not return to a flat plate shape.

  The object of the present invention is to solve the problems of the prior art, and to provide a high-strength steel pipe having a seam portion and excellent workability, and a method for producing the same. The term “excellent workability” as used herein refers to an arc specimen (JIS 12 arc specimen: 50 mm distance between gauge points) or ASTM arc specimen (sample distance is 50 mm) taken from the tube so that the circumferential direction is the tensile direction. When the distance between the gauges is 25 mm), when TS is 980 MPa or more, the elongation El is 12% or more. When TS is 780 MPa or more, the elongation El is 15% or more. In some cases, TS: 590 MPa or more, elongation El: 20% or more.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting the workability of a high-strength steel pipe having a seam portion. As a result, first of all, the steel pipe base material has a high Si content and a martensite phase fraction adjusted to an appropriate range. For example, a steel plate is used, or, further, for example, by adopting a pipe making method in which the processing strain during pipe making is in an appropriate low strain range, the residual stress in the circumferential direction is reduced as much as possible, or the seam portion By adopting seam annealing to minimize the difference in hardness between the seam part and the other base material part, the excellent workability of the steel sheet is inherited to the steel pipe as much as possible. In addition, the present inventors have found that a steel pipe having excellent workability can be obtained.

The present invention has been completed based on the above findings and further studies.
That is, the gist of the present invention is as follows.
(1) A steel pipe having a seam portion, in mass%, C: 0.05 to 0.25%, Si: 0.5 to 2.0%, Mn: 1.0 to 2.5%, P: 0.020% or less, S: 0.002% or less, Al : 0.005 to 0.08%, N: 0.004% or less, the composition composed of the remaining Fe and unavoidable impurities, and the structure of the region of 15 ° to 345 ° in the circumferential direction based on the center position of the seam portion is martensite A high-strength steel pipe excellent in workability, characterized by having a structure containing a phase of 20 to 60% by volume.

(2) The high strength steel pipe according to (1), wherein the region has a tensile residual stress of 450 MPa or less in a circumferential direction.
(3) The high-strength steel pipe according to (1) or (2), wherein a hardness difference ΔHV between the region and the other region is 250 points or less in terms of Vickers hardness.
(4) The high strength steel pipe according to (1) or (2), wherein a hardness difference ΔHV between the region and the other region is 50 points or less in terms of Vickers hardness.

(5) In any one of (1) to (4), in addition to the above-mentioned composition, the mass is selected from B: 0.0005 to 0.0020%, Nb: 0.005 to 0.03%, and V: 0.02 to 0.20%. A high-strength steel pipe having a composition containing one or more of them.
(6) In any one of (1) to (5), a high-strength steel pipe having a composition containing Ti: 0.01 to 0.02% by mass% in addition to the above composition.

(7) In any one of (1) to (6), a high-strength steel pipe having a composition containing Cu: 0.08 to 0.25% by mass% in addition to the above composition.
(8) In any one of (1) to (4), the composition is an impurity, and any one or more of Ti, B, Nb, V, and Cu is contained in mass%, and Ti is less than 0.01%. B: A high strength steel pipe having a composition adjusted to less than 0.0005%, Nb: less than 0.005%, V: less than 0.02%, and Cu: less than 0.08%.

  (9) A steel pipe having a roll forming step of roll-forming the raw material into an open tube shape using a strip-shaped steel plate as a raw material, and a joining step of joining both end faces of the open pipe shape to have a seam portion The steel sheet is, in mass%, C: 0.05 to 0.25%, Si: 0.5 to 2.0%, Mn: 1.0 to 2.5%, P: 0.020% or less, S: 0.002% or less, Al: A high-strength steel pipe containing 0.005 to 0.08%, N: 0.004% or less, having a composition composed of the balance Fe and inevitable impurities, and a structure having a martensite fraction of 20 to 60% by volume. Manufacturing method.

(10) The method for producing a high-strength steel pipe according to (9), wherein the steel sheet is a cold-rolled steel sheet subjected to cold-rolled sheet annealing.
(11) The method for producing a high-strength steel pipe according to (9) or (10), wherein the open tube shape is a vertically long oval shape and further bent back into an elliptical shape. (12) The method for producing a high strength steel pipe according to any one of (9) to (11), wherein the roll forming is cage roll type roll forming.

(13) In any one of (9) to (12), following the joining step, a seam annealing process is performed to heat the seam part to an austenite single-phase temperature range, a two-phase temperature range, or a ferrite temperature range. A method for producing a high-strength steel pipe, comprising performing a seam annealing step.
(14) In any one of (9) to (13), in addition to the above-mentioned composition, the mass is selected from B: 0.0005 to 0.0020%, Nb: 0.005 to 0.03%, and V: 0.02 to 0.20%. The manufacturing method of the high strength steel pipe characterized by having the composition containing 1 type, or 2 or more types.

(15) The method for producing a high-strength steel pipe according to any one of (9) to (14), further comprising a composition containing Ti: 0.01 to 0.02% by mass% in addition to the above composition.
(16) The method for producing a high-strength steel pipe according to any one of (9) to (15), further having a composition containing Cu: 0.08 to 0.25% by mass% in addition to the above composition.
(17) In any one of (9) to (13), the composition is an impurity, and any one or more of Ti, B, Nb, V, and Cu is contained in mass%, and Ti is less than 0.01%. B: Less than 0.0005%, Nb: less than 0.005%, V: less than 0.02%, Cu: less than 0.08%.

  ADVANTAGE OF THE INVENTION According to this invention, the high strength steel pipe excellent in workability which has a seam part can be manufactured easily and stably, and there is a remarkable industrial effect. Further, according to the present invention, the tensile strength TS has a strength level of 590 MPa or more, preferably 780 MPa or more, more preferably 980 MPa or more, and when the tensile strength TS is 590 MPa or more, elongation: 20% or more, In the case of 780 MPa or more, 15% or more, and in the case of 980 MPa or more, the elongation: 12% or more, which has the effect of easily producing a high-strength steel pipe having excellent ductility.

  The steel pipe of the present invention is a steel pipe having a seam portion, and a composition in a predetermined range and a structure in a region of 15 ° to 345 ° in the circumferential direction based on the center position of the seam portion has a martensite phase in an area ratio of 20 to 60. % High strength steel pipe. The steel pipe having a seam portion according to the present invention includes, for example, a welded steel pipe such as an electric resistance steel pipe or a laser welded steel pipe, a forged steel pipe, or the like other than a seamless steel pipe.

The steel pipe of the present invention is a roll forming step in which a strip-shaped steel plate is used as a raw material, and the raw material is roll-formed into an open pipe shape, and a joining step in which both end faces of the open pipe shape are joined to have a seam portion. Although it is preferable to manufacture through these, it is not limited to this.
First, the reason for limiting the composition of the steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.05-0.25%
C is an effective element that increases the hardenability and facilitates the formation of a martensite phase, and is adjusted to a martensite fraction in an appropriate range to ensure a tensile strength TS: 590 MPa or more. Containing is required. On the other hand, when the content exceeds 0.25%, the strength increases excessively, elongation decreases, workability decreases, and properties such as delayed fracture resistance due to hydrogen deteriorate. For this reason, C was limited to the range of 0.05 to 0.25%. In addition, Preferably, it is 0.05 to 0.20%.

Si: 0.5-2.0%
Si is a ferrite-forming element. In the present invention, Si is actively contained in an amount of 0.5% or more to promote the formation of a ferrite phase and ensure excellent workability. On the other hand, if the content exceeds 2.0%, seam defects such as penetrators are likely to be generated, and the workability of the seam is reduced. For this reason, Si was limited to 0.5 to 2.0%. From the viewpoint of workability, it is preferably 1.0% or more.

Mn: 1.0-2.5%
Mn is an element that increases the hardenability, facilitates the formation of a martensite phase, and increases the strength of the steel, and in the present invention, it needs to be contained in an amount of 1.0% or more from the viewpoint of securing a desired strength. On the other hand, if the content exceeds 2.5%, the elongation property is lowered. For this reason, Mn was limited to the range of 1.0 to 2.5%. In order to secure a good balance between workability and strength, the content is preferably 1.5% or more from the viewpoint of interaction with Si.

P: 0.020% or less P is desirable to reduce as much as possible in order to maintain the workability, delayed fracture resistance, etc. in a desired range together with S, but it is acceptable up to 0.020%. For this reason, P was limited to 0.020% or less. In addition, since extreme reduction leads to an increase in steelmaking cost, it is preferable to set it to 0.001% or more from the viewpoint of economical production.

S: 0.002% or less It is desirable to reduce S as much as possible in order to maintain the workability, delayed fracture resistance, etc. in the desired ranges together with P, but it is acceptable up to 0.002%. For this reason, S was limited to 0.002% or less. In addition, since extreme reduction leads to a rise in steelmaking cost, it is preferable to make it 0.0005% or more from the viewpoint of economical production.

Al: 0.005-0.08%
Al acts as a deoxidizer, is added to the molten metal for deoxidation, and floats and is removed as slag, but a part is dissolved in the molten metal. In addition, Al acts as a nitride forming element and fixes nitrogen. In order to sufficiently function as such a deoxidizer, it is necessary that 0.01% or more remain in the steel. On the other hand, if the content exceeds 0.08%, oxide inclusions increase, causing cracks during processing and lowering workability. For this reason, Al was limited to the range of 0.005-0.08%.

N: 0.004% or less N is an element that causes age hardening. In the present invention, N is preferably reduced as much as possible. In the present invention, Al or the like, which is a nitride-forming element, is contained in a predetermined range, but N is 0.004% or less in order to reduce age hardening to a level at which no problem arises due to the inclusion of a nitride-forming element such as Al. Limited. From the viewpoint of steelmaking technology, the lower limit is about 0.0005%.

The above components are basic components. In addition to the above basic composition, B: 0.0005 to 0.0020%, Nb: 0.005 to 0.03%, V: 0.02 to 0.20%, Ti: 0.01 to 0.02%, Cu : One or two or more selected from 0.08 to 0.25% and / or REM: 0.1% or less may be contained.
One or more selected from B: 0.0005 to 0.0020%, Nb: 0.005 to 0.03%, V: 0.02 to 0.20% B, Nb, and V are all listed above in order to ensure excellent materials. It is an element that complements the action of the element, and can be selected and contained as necessary.

B supplements the effect of adjusting the martensite fraction to a predetermined range through improvement in hardenability. In order to acquire such an effect, it is preferable to contain 0.0005% or more, but even if it contains exceeding 0.0020%, an effect will be saturated and it becomes economically disadvantageous. For this reason, when it contains, it is preferable to limit B to 0.0005 to 0.0020% of range.
Nb supplements the effect of adjusting the martensite fraction to a predetermined range through the refinement of crystal grains. Nb forms carbides and carbonitrides, and contributes to increasing strength by precipitation strengthening. In order to obtain such an effect, the content is preferably 0.005% or more. However, if it exceeds 0.03%, the strength may become too high due to precipitation strengthening, and the elongation may decrease. For this reason, when it contains, it is preferable to limit Nb to 0.005 to 0.03% of range.

  V complements the effect of adjusting the martensite fraction to a predetermined range through improvement in hardenability. V also has an action of fixing nitrogen. In order to obtain such an effect, the content is preferably 0.02% or more. On the other hand, if it exceeds 0.20%, coarse VN is formed, which may cause cracks during processing. . For this reason, when it contains, it is preferable to limit V to 0.02 to 0.20% of range.

Ti: 0.01-0.02%
Ti is an element that binds to nitrogen (N), fixes N, suppresses the occurrence of age hardening, and complements the effect of preventing the deterioration of workability, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, the upper limit was set to 0.02% which can fix the maximum N content of 0.004%.

Cu: 0.08-0.25%
Cu is an element that improves corrosion resistance and delayed fracture resistance, and can be contained if necessary. In order to acquire such an effect, it is desirable to contain 0.08% or more, while inclusion exceeding 0.25% causes hot brittleness. For this reason, when it contains, it is preferable to limit to 0.08 to 0.25% of range. Note that Cu is dissolved or precipitated as a precipitate. In the case of precipitation, Cu may be deposited as Cu-S so as to cover the outermost layer of MnS.

Total of one or two of REM and Ca: 0.1% or less
Both REM and Ca are elements that improve processability through the form control of inclusions, and can be selected and contained as necessary. In order to obtain such an effect, the total content is desirably 0.002% or more. However, if the total content exceeds 0.1%, the amount of inclusions is increased and the corrosion resistance is decreased.

The balance other than the above components is Fe and inevitable impurities.
As impurities, any one or more of Ti, B, Nb, V, and Cu is expressed by mass, Ti: less than 0.01%, B: less than 0.0005%, Nb: less than 0.005%, V: 0.02% Less than, Cu: It is preferable to adjust to less than 0.08%. At relatively low strength levels within the scope of the present invention, it is desirable to reduce Ti, B, Nb, V, and Cu as much as possible from the viewpoint of not increasing the strength excessively and reducing elongation.

Next, the reason for limiting the structure of the steel pipe of the present invention will be described.
In the steel pipe of the present invention, the martensite phase has the above composition, and the region of 15 ° to 345 ° in the circumferential direction excluding the range of ± 15 ° in the circumferential direction with reference to the center position of the seam portion represents the martensite phase. It has a structure containing 20 to 60% by volume. The remainder other than the martensite phase is mainly composed of a ferrite phase. If the martensite phase is less than 20%, the desired strength cannot be ensured. On the other hand, when the martensite phase exceeds 60%, the ductility is remarkably lowered and the desired processability cannot be secured. For this reason, the content of the martensite phase is limited to 20 to 60%. The TS780MPa class is preferably 35% or less, the TS980MPa class is 45% or less, and the TS1180MPa class is preferably 60% or less. The method for measuring the amount of martensite phase is not particularly limited. It is preferable to take an image and measure the area ratio of the martensite phase and convert it to a volume ratio. However, there is no problem with other methods such as measurement of saturation magnetization and X-ray diffraction. In this case, the volume ratio is obtained.

  The remainder other than the martensite phase is mainly the ferrite phase. By making the remainder other than the martensite phase mainly a ferrite phase, it has a desired high strength and can stably ensure excellent workability. In addition to the ferrite phase, the balance other than the martensite phase can be a bainite phase, a bainitic ferrite phase, and a retained austenite phase. It is preferable from the viewpoint of improving workability.

  Note that the range of ± 15 ° in the circumferential direction with respect to the center position of the seam portion is different from the base material structure because, for example, it is rapidly heated above the melting point and rapidly cooled at the time of joining by welding or the like. In order to form the bead portion and the heat affected zone of the structure, it was excluded from the limited region of the steel pipe structure. The bead part and the heat-affected zone are usually within a range of ± 10 ° in the circumferential direction based on the center position of the seam part under normal welding conditions, but depending on the joining conditions (welding conditions), The range of ± 15 ° was excluded in consideration of the possibility of spreading. Therefore, the structure of the above-described region (region excluding the range of ± 15 ° with respect to the center position of the seam portion) is directly inherited from the structure of the steel plate (steel strip) used as the material of the steel pipe.

In the steel pipe of the present invention, the residual stress in the circumferential direction in the above-mentioned region (excluding the range of ± 15 ° in the circumferential direction with reference to the center position of the seam portion) is limited to a tensile residual stress of 450 MPa or less. To do.
When a steel plate is formed using roll forming, or when the tube is processed with a sizer, a straightening machine, or the like, processing strain is added to the steel pipe. If processing strain is added more than necessary, the excellent properties of the original steel plate (steel strip) will deteriorate and the properties of the steel pipe will deteriorate, ensuring a steel pipe with excellent desired workability. become unable. Therefore, in the steel pipe of the present invention, the residual stress in the circumferential direction is limited to 450 MPa or less in terms of tensile residual stress. When the residual stress exceeds 450 MPa as the tensile residual stress, the workability of the steel pipe deteriorates. The pressure is preferably 250 MPa or less.

  In general, steel pipes are piped while being given some plastic strain according to the strength of the original plate (coil), the plate thickness, the pipe outer diameter after pipe making, and the pipe making process, etc. The residual stress tends to increase as the strength of the original plate increases. For example, when an original plate having a strength of TS490 MPa or more is used, a residual stress of about 100 to 200 MPa or more is introduced into the steel pipe, but it is preferably reduced as much as possible.

  In order to achieve such a residual stress state, for example, as shown in FIG. 2 (a), pipes are formed by cage roll type roll forming, the addition of processing strain is reduced as much as possible, and warpage and bending of the steel pipe are reduced in advance. In addition to the contrivance to be applied, it is necessary to further reduce the processing strain applied by a sizer or a straightening machine. In addition, you may pipe-form by CBR type roll forming (for example, refer to Kawatetsu technical report 32 (2000) 1, P49-53) shown in FIG.2 (b). Thereby, the processing strain introduced into the steel pipe can be further reduced.

In addition, when forming the material into an open tube shape by roll forming, in order to reduce the addition of processing strain as much as possible, the formed flower in the roll forming is formed into a vertically long egg shape as shown in FIG. The oval shape is preferable. Thereby, springback can be suppressed as much as possible, and the applied processing strain can be reduced.
In the steel pipe of the present invention, the above-described regions (regions of 15 ° to 345 ° in the circumferential direction excluding a range of ± 15 ° in the circumferential direction with respect to the center position of the seam portion) and other regions (the seam) The hardness difference ΔHV with respect to the center position of the portion in a range of ± 15 ° in the circumferential direction) is preferably 250 points or less, more preferably 50 points or less in terms of Vickers hardness.

  Since the seam portion is rapidly heated above the melting point and rapidly cooled at the time of joining by welding or the like after pipe making, it tends to have higher hardness than the base material portion or the heat affected zone. . Seam part (region of ± 15 ° in the circumferential direction with reference to the center position of the seam part) and base material part (region of 15 ° to 345 ° in the circumferential direction with reference to the center position of the seam part) When the hardness difference is large, the heat-affected zone tends to be specifically thinned, and therefore there is a high risk that the portion will break early. Therefore, in the present invention, the range of ± 15 ° in the circumferential direction with respect to the center position of the seam portion, and the range of ± 15 ° in the circumferential direction with respect to the circumferential direction based on the center position of the seam portion. It is preferable to limit the hardness difference ΔHV between 15 ° and 345 ° except for Vickers hardness to 250 points or less. When ΔHV increases beyond 250 points, the part with low hardness preferentially deforms compared to the part with high hardness (bead part), including the heat affected part, and the thickness is reduced. It is not processed uniformly. More preferably, it is 50 points or less. If ΔHV is 50 points or less, the risk of partial thinning will be significantly reduced even if somewhat severe processing is performed.

It is desirable that the seam part has a higher hardness than the heat-affected part and the base material part. This is because the seam part has a high risk of causing defects due to the entry of microscopic objects during ERW welding, so by setting the hardness higher than the surrounding area, stress concentration does not occur on the seam part and the seam part This is because the risk of cracks and the like can be reduced.
In the vicinity of the seam portion where the change in hardness is larger than that of the base material portion, it is necessary to keep in mind that the hardness is measured with a smaller test force. For example, in the base material portion, the load (test force) may be 5 to 10 kgf, but in the vicinity of the seam portion, the load (test force) is preferably measured at about 300 to 500 gf. Note that it is important to measure the hardness in the vicinity of the seam portion after accurately grasping the position of the bead portion by etching or the like.

    In order to adjust the hardness difference ΔHV between the region near the seam part and the other region within the above range, seam annealing is performed on-line or off-line following the joining process. It is preferable to apply a process. The seam annealing treatment is preferably a treatment in which the seam portion is heated to an austenite single-phase temperature range and then gradually cooled or air-cooled. Thereby, the hardness of a seam part can be easily reduced to the hardness in a desired range. The seam annealing treatment may be a treatment in which heating is performed in a two-phase temperature range (austenite + ferrite mixed temperature range) or a ferrite single-phase temperature range, followed by slow cooling or air cooling.

Below, the preferable manufacturing method of this invention steel pipe is demonstrated.
Using a strip-shaped steel plate having the above composition as a raw material, a roll forming step of roll forming the raw material into an open tube shape, and a joining step of joining both end surfaces of the open pipe shape to have a seam portion Apply sequentially to make a steel pipe.
The steel plate used as a raw material has the above-described composition, and further includes a martensite phase in a volume ratio of 20 to 60%, and preferably the balance other than the martensite phase has a structure mainly composed of a ferrite phase. . As long as the steel sheet has the above-described composition and the above-described structure, either a hot-rolled sheet or a cold-rolled sheet may be used, but the cold-rolled sheet is more likely to achieve improved workability. In addition, although a hot-rolled sheet may be hot-rolled, it is preferable that the hot-rolled sheet is further subjected to pickling treatment.

First, the preferable manufacturing method of the steel plate used as a raw material is demonstrated.
The hot-rolled sheet is pickled and cold-rolled to form a cold-rolled sheet, and then the cold-rolled sheet is heated in a continuous annealing furnace to a temperature range of 750 to 870 ° C and subjected to a soaking process. After that, it cools naturally, then starts cooling from a temperature of 600-730 ° C, performs a cooling process that rapidly cools to a cooling stop temperature of 300 ° C or less at a cooling rate of 100 ° C / s or more, and then 150-500 ° C It is preferable to perform an overaging treatment in the temperature range.

  Heat treatment in the range of 750 to 870 ° C, soaking soaking, the strain added by cold rolling is released, recrystallization occurs, and austenite (γ) in two-phase region or austenite single-phase region C is concentrated in the phase. The γ phase enriched with C is transformed into a martensite phase by subsequent rapid cooling, and a predetermined amount of the martensite phase can be dispersed in the structure. When the heating temperature is less than 750 ° C., the α-γ two-phase temperature region cannot be reached, and C cannot be concentrated in the γ phase in the two-phase region. On the other hand, even if the temperature exceeds 870 ° C., further improvement in characteristics cannot be expected.

In addition, after soaking, natural cooling is performed until the start of cooling. Natural cooling here refers to cooling in which the cooling start temperature is adjusted by cooling by gas jet or by contacting with a roll and adjusting the temperature appropriately by simply letting it through, or by adjusting the temperature appropriately. . During natural cooling, C is further concentrated in the γ phase in a two-phase region or a γ single-phase region.
Natural cooling is performed, the cooling start temperature is adjusted, cooling is started from a temperature of 600 to 730 ° C., and a cooling process is performed to rapidly cool to a cooling stop temperature of 300 ° C. or lower at a cooling rate of 100 ° C./s or higher. The rapid cooling rate is preferably 100 ° C./s or more. By this rapid cooling, the γ phase enriched with C is transformed into a martensite phase. The martensite phase fraction can be adjusted to an appropriate range by a combination of the cooling start temperature, the cooling rate, and the cooling stop temperature. If the cooling rate is less than 100 ° C./s or the cooling stop temperature exceeds 300 ° C., a desired martensite phase fraction cannot be secured. In order to secure a predetermined cooling rate, it is important that this cooling process is a process of immersing the steel sheet in water or an aqueous refrigerant bath. In cooling by mist cooling or roll contact, it is often difficult to ensure a predetermined cooling rate. Also, in cooling by mist cooling or roll contact, etc., the cooling rate is often slow, and it is difficult to adjust to a two-phase structure of martensite phase and ferrite phase, inevitably bainite phase etc. increases, elongation decreases May become prominent.

  After the cooling treatment, it is preferable to perform an overaging treatment in a temperature range of 150 to 500 ° C. in an overaging furnace. By performing an overaging treatment after cooling, the steel sheet is tempered and softened, and the workability is remarkably improved. If the overaging temperature is less than 150 ° C., the desired effect cannot be expected. On the other hand, if the temperature exceeds 500 ° C., the degree of softening becomes too large, and it becomes difficult to ensure the desired strength, and film-like coarse carbides are generated at the grain boundaries, resulting in a decrease in workability or exceeding 980 MPa. In the case of high strength, delayed fracture resistance may decrease.

In the present invention, a steel pipe having a composition and a structure as described above is used as a raw material, and a roll forming step and a joining step are sequentially performed on the raw material to obtain a steel pipe.
The roll forming step is preferably a cage roll type roll forming. According to the roll forming method of the cage roll method, since the diameter of the forming roll to be used is small and the interval between the forming roll groups is short, it is not necessary to add an extra plastic working, and there is little deterioration of the characteristics, which is close to the steel sheet characteristics. The steel pipe characteristics can be retained. On the other hand, for example, breakdown-type roll forming uses several stages of large forming rolls and a long roll group interval, so it is necessary to take into account the return due to springback, and the added processing strain is also Increased and the deterioration from the steel sheet characteristics increases.

  In addition, in order to minimize the addition of processing strain when forming a raw material (original plate) into an open tube shape by cage roll type roll forming, in the present invention, a formed flower in roll forming is, for example, as shown in FIG. It is preferable to make a vertically long oval shape and further bend back (add a strain in the crushing direction) to make it an oval shape close to a perfect circle. Thereby, the springback at the time of roll forming can be suppressed as much as possible, and the applied processing strain can be reduced.

  Subsequent to the roll forming process, a joining process is performed to obtain a pipe. The joining step is preferably an ordinary step in which both ends of the open tube shape are heated to the melting point or higher by resistance heating, induction heating, laser, or the like, both ends are butted and welded with a squeeze roll. Note that welding may be performed in a shield gas atmosphere using a hood as shown in FIG. 4 in place of the air atmosphere in order to improve the soundness of the seam portion.

  Subsequent to the joining step, a seam annealing step in which the seam portion is heated to perform a seam annealing process may be performed online or offline. Depending on the component composition, the seam hardness as it is by electro-welding may exceed a desired range of hardness. In this case, by performing a seam annealing treatment, the hardness of the seam portion can be easily reduced to a hardness within a desired range.

  The heating temperature of the seam annealing treatment is preferably variously changed depending on the composition of the original plate used, the structure, or the cooling conditions after heating so that the hardness of the seam portion is within a desired range. γ) A temperature in a single-phase temperature range, a two-phase temperature range, or a ferrite temperature range can be exemplified. When heated to the temperature of the austenite single phase temperature range, the hardened structure can be eliminated thereby, but it is important not to increase the hardness of the seam portion by slightly cooling the subsequent cooling. Further, when heating to a temperature in the ferrite temperature range, a tempering effect can be expected, but it is necessary to set a line speed and a line length for ensuring a required tempering time. In the seam annealing treatment, it is important to take care not to cause significant softening or hardening of the heat-affected zone and the base material zone adjacent to the seam zone.

  The seam annealing treatment is preferably performed in-line or offline using, for example, a seam annealing apparatus using induction heating. The seam annealing treatment is performed with the goal of annealing a region including the seam portion, that is, a region within a range of ± 5 °, preferably ± 10 ° in the circumferential direction with respect to the center position of the seam portion. Is preferred.

A steel plate (strip steel) having the composition shown in Table 1 and having the thickness and structure shown in Table 2 was used as the material. The steel plate used as a raw material was a hot rolled plate or a cold rolled plate. The hot-rolled sheet was subjected to pickling after hot-rolling. Further, the cold-rolled sheet was subjected to an annealing treatment after cold rolling.
These materials are sequentially subjected to a roll forming process for forming into an open pipe shape by cage roll type roll forming, a joining process for joining both ends of the open pipe by electro-welding welding, and a sizer process for adjusting the cross-sectional shape by drawing. Alternatively, a seam annealing process in which the seam part is subjected to a seam annealing process using an induction heating apparatus, and a straightening process in which bending is corrected with a straightening machine, were used to obtain steel pipes having the dimensions shown in Table 2.

In the joining process, joining was performed by electric seam welding in an air atmosphere. In some cases, Ar gas is used as shielding gas, and through a hood as shown in Fig. 4, in shielding gas atmosphere (shielding gas pressure: 2 MPa, shielding gas flow rate: 0.5 l / min (converted to 1 atm. 0 ° C)) Then, both ends of the open pipe were joined by electric resistance welding.
In the roll forming process, the CBR type roll forming shown in FIG. 2 (b) is performed so that the formed flower has a vertically long oval shape as shown in FIG. 1, and bent back (distortion is applied in the crushing direction). And) formed into an oval shape. In some cases, breakdown-type roll forming was performed as a comparison.

In addition, the seam annealing treatment was performed by heating to 870 ° C. in the γ single phase temperature range and air cooling. For comparison, some parts were cold-worked to reduce the outer diameter from 48.6 mmφ to 38.1 mmφ. For comparison, in some cases, about 5% drawing was intentionally performed by a straightening machine.
Test pieces were collected from each of the obtained steel pipes and subjected to structure observation, tensile test, residual stress measurement, hardness measurement, and flatness test, respectively. The test method is as follows.
(1) Structure observation A specimen for structure observation was collected from a position 180 ° apart in the circumferential direction with reference to the center position of the seam portion of the obtained steel pipe. Polishing the 1/2 T position surface of the plate thickness, corroding the polished surface with a corrosive liquid mainly composed of bilera liquid and adding ferric chloride, and observing the structure with a scanning electron microscope (1000 to 2000 times), Images were taken to determine the fraction (area%) of the martensite phase. In addition, about some steel pipes, it calculated | required by the method by a saturation magnetization measurement. The obtained value is the martensite phase fraction of the base material part (excluding the range of ± 15 ° in the circumferential direction relative to the circumferential direction with respect to the center position of the seam portion). did. This value can be regarded as reflecting the status of the material (steel plate).
(2) Tensile test From the obtained steel pipe, so that the center of the test piece is at a position of ± 90 ° in the circumferential direction with respect to the center position of the seam part, and the tensile direction is the pipe axis direction, JIS No. 12 test piece (arc-shaped test piece) or ASTM arc-shaped test piece (sub-size) is sampled, and a tensile test is performed in accordance with JIS Z 2201, JIS Z 2241 or ASTM A370-97a. The tensile properties (YS, TS, El) of the steel pipe base material were determined. The workability was evaluated from the relationship between TS and El. In other words, El is 20% or more for TS: 590MPa class, El is 15% or more for TS: 780MPa class, El is 12% or more for TS: 980MPa class, El is 12% or more for TS: 1270MPa class In each case, the processability was set to ○ (good). In other cases, the workability was set to x.
(3) Residual stress measurement A test piece (steel pipe: length 250 mm) was collected from the obtained steel pipe, a strain gauge was attached to the center position of the seam portion, and the residual stress in the circumferential direction was measured. The residual stress was measured by a so-called strain gauge method, in which the residual stress was calculated from the difference between the value of the strain gauge in the state of the steel pipe and the value of the strain gauge after cutting.
(4) Hardness measurement The test piece for measuring the hardness of the base metal part from the base material part at 180 ° in the circumferential direction with respect to the center position of the seam part of the obtained steel pipe, and the center position of the seam part. A test piece for seam hardness measurement including a seam portion was collected from within a range of ± 15 ° in the circumferential direction with reference to the above.

For the test piece for measuring the base metal part hardness, the Vickers hardness HV was measured at 10 points or more at the 1 / 2T position of the plate thickness, and the average value was defined as the base metal part hardness of the steel pipe. For the seam hardness test piece, measure five or more points at the center position in the thickness direction of the seam area excluding the outermost layer (thickness 1/5) of the inner and outer layers. The value was the seam hardness of the steel pipe. The seam hardness was measured by changing the test force according to the width of the seam up to a test force with which the seam hardness could be accurately measured with a test force of 0.5 kgf as a reference. And the difference (DELTA) HV of seam part hardness and base material part hardness was computed.
(5) Flat test A test piece (outer diameter Dmmφ x length 50-100mm) is taken from some of the obtained steel pipes and set between the pushing jig 2 and the receiving jig 3 as shown in FIG. Then, the pressing jig 2 was lowered so as to have a deformation height h (mm), and the test piece (steel pipe) was flattened. In addition, as shown in FIG. 3, the test piece (steel pipe) was set so that the seam portion would be at a position 90 ° away from the pushing jig 2. The flattening rate was set to be 0.3 in h / D. The test piece length was 100 mm as a standard, and an appropriate length of 50 mm or more was selected according to the capability of the testing machine. If it is 50mm or more, it has been confirmed that there is no effect on the test results. After the test, the presence or absence of cracks was visually inspected, and when no cracks were found, the seam portion was judged to be sound and was accepted. The test piece was lightly polished in a range of ± 15 mm including the seam portion before the test so that visual judgment was easy. In addition, after the test, the thickness change in the test piece (steel pipe) is investigated over the entire circumference with an ultrasonic thickness gauge. When the rate of change in thickness exceeds 15%, × As a result, the presence or absence of local deformability was evaluated.

  The obtained results are shown in Table 3.

Each of the examples of the present invention is a steel pipe having a desired high strength, a desired elongation value, and excellent workability. On the other hand, in the comparative example that is outside the scope of the present invention, the desired high strength cannot be secured, the elongation value according to the strength level cannot be secured, the residual stress is high, or the seam portion and the base material portion The hardness difference is large and workability is reduced.
Steel pipe No. 2 (example of the present invention) is a roll-down (BD) method, so the residual stress is slightly higher, the strength is higher, and the elongation El is slightly lower than other examples of the present invention. Yes. However, since the joining method is the same as that of the present invention examples (steel pipes No. 1 and No. 3), the difference in hardness ΔHV between the seam part and the base material part is almost the same as that of the other present invention examples.

Steel pipes No.4 and No.9 (comparative example) were subjected to special cold drawing or straightening after pipe making, and the residual stress in the base metal became higher than the specified value, resulting in an elongation value. The decline of
In steel pipe No. 10 (comparative example), the martensite fraction (including residual γ and bainite) is out of the predetermined range, and the strength, particularly the yield strength YS, is reduced. This is because a large amount of residual γ exists. Steel pipe No. 10 (comparative example) shows a large elongation El value because the residual γ undergoes plastic deformation and causes martensitic transformation. The presence of the residual γ causes variations in the forming process during pipe making, and there is a concern that the steel pipe has material variations in the circumferential direction. In particular, there is a high possibility of causing a problem when the tube is locally deformed.

  Steel tube No. 12 (comparative example) has a hardness difference ΔHV between the seam part and the base metal part that is outside the specified range, has a high thickness variation rate in the flat test, and is likely to cause local thinning during processing. It has become. Steel pipe No. 13 (comparative example) has a martensite fraction that is out of the predetermined range and the desired high strength can be ensured, but the Si content deviates from the scope of the present invention, so depending on the strength level. The desired elongation value cannot be secured, and the workability is deteriorated. In Steel Pipe No. 14 (Comparative Example), the C and Si contents are out of the range of the present invention, the desired martensite fraction cannot be ensured, and the desired high strength (TS: 590 MPa or more) cannot be ensured.

  Steel pipe No. 15 (comparative example) has a Mn content outside the range of the present invention, has high strength, cannot secure a desired elongation value, and has low workability. Steel pipe No. 16 (comparative example) has a C content that deviates from the scope of the present invention and a martensite fraction that is far from the predetermined range, and a desired high strength can be ensured. Since the scope of the invention deviates low, a desired elongation value corresponding to the strength level cannot be secured, and workability is deteriorated. In Steel Pipe No. 17 (Comparative Example), the Si content deviates from the range of the present invention, the desired elongation value cannot be secured, and the workability is lowered.

  Steel pipe No. 19, No. 20 (comparative example), the martensite fraction is higher than the predetermined range, the Si content is out of the scope of the present invention, the desired elongation value can not be secured, Processability is degraded. Steel pipe No. 21 (comparative example) has a martensite fraction within a predetermined range, but because the Si content deviates from the range of the present invention, the desired elongation value cannot be ensured and the workability decreases. ing. Even if the martensite fraction is set within a predetermined range and a desired strength is ensured, the elongation decreases when the Si content is low.

  In Steel Pipe No. 22 (Comparative Example), the Si and Mn contents deviate from the scope of the present invention, the martensite fraction deviates from a predetermined range, and the desired high strength cannot be ensured. Although the P content is far from the range of the present invention, a sufficient increase in strength has not been obtained.

It is explanatory drawing which shows typically an example of the shaping | molding flower suitable in this invention. It is explanatory drawing which shows typically an example of the roll forming apparatus suitable for implementation of this invention. It is explanatory drawing which illustrates a flat test method typically. It is explanatory drawing which shows an example of the food | hood for shield gas atmospheres suitable for seam part welding.

Explanation of symbols

1 Steel pipe (test body)
2 Pushing jig 3 Receiving jig

Claims (13)

A steel pipe having a seam portion,
C: 0.05-0.25%, Si: 0.5-2.0%,
Mn: 1.0 to 2.5%, P: 0.020% or less,
S: 0.002% or less, Al: 0.005-0.08%,
N: 0.004% or less, the composition composed of the remainder Fe and inevitable impurities, and the structure of the region of 15 ° to 345 ° in the circumferential direction with respect to the center position of the seam portion is a martensite phase in an area ratio of 20 High-strength steel pipe with excellent workability characterized by a structure containing ~ 60%.   The high-strength steel pipe according to claim 1, wherein the region has a tensile residual stress of 450 MPa or less in a circumferential direction.   3. The high-strength steel pipe according to claim 1, wherein a hardness difference ΔHV between the region and the other region is 250 points or less in terms of Vickers hardness.   3. The high-strength steel pipe according to claim 1, wherein a hardness difference ΔHV between the region and the other region is 50 points or less in terms of Vickers hardness. In addition to the above composition,
Composition containing one or more selected from Ti: 0.01 to 0.02%, B: 0.0005 to 0.0020%, Nb: 0.005 to 0.03%, V: 0.02 to 0.20%, Cu: 0.08 to 0.25% The high-strength steel pipe according to any one of claims 1 to 4, characterized by comprising:   As the impurities, any one or more of Ti, B, Nb, V, and Cu in terms of mass%, Ti: less than 0.01%, B: less than 0.0005%, Nb: less than 0.005%, V: 0.02 The high-strength steel pipe according to any one of claims 1 to 4, which has a composition adjusted to less than% and Cu: less than 0.08%. A steel pipe manufacturing method comprising: a roll forming step of roll-forming the raw material into an open tube shape using a strip-shaped steel plate as a raw material; and a joining step of joining both end surfaces of the open pipe shape to have a seam portion Because
The steel sheet is in mass%,
C: 0.05-0.25%, Si: 0.5-2.0%,
Mn: 1.0 to 2.5%, P: 0.020% or less,
S: 0.002% or less, Al: 0.005-0.08%,
N: A method for producing a high-strength steel pipe comprising a composition comprising 0.004% or less, the balance being Fe and inevitable impurities, and a structure having a martensite fraction of 20 to 60% by volume.   The method for producing a high-strength steel pipe according to claim 7, wherein the steel sheet is a cold-rolled steel sheet that has been subjected to cold-rolled sheet annealing.   The method for manufacturing a high-strength steel pipe according to claim 7 or 8, wherein the roll forming is cage roll type roll forming.   The method for producing a high-strength steel pipe according to any one of claims 7 to 9, wherein the open pipe shape is a vertically long oval shape and further bent back into an elliptical shape.   11. A seam annealing process for performing a seam annealing process for heating the seam portion to an austenite single-phase temperature range, a two-phase temperature range, or a ferrite temperature range following the joining step. The manufacturing method of the high strength steel pipe in any one of. In addition to the above composition,
Composition containing one or more selected from Ti: 0.01 to 0.02%, B: 0.0005 to 0.0020%, Nb: 0.005 to 0.03%, V: 0.02 to 0.20%, Cu: 0.08 to 0.25% The method for producing a high-strength steel pipe according to any one of claims 7 to 11, wherein:     As the impurities, any one or more of Ti, B, Nb, V, and Cu in terms of mass%, Ti: less than 0.01%, B: less than 0.0005%, Nb: less than 0.005%, V: 0.02 The method for producing a high-strength steel pipe according to any one of claims 7 to 11, wherein the composition is adjusted to less than% and Cu: less than 0.08%.

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