JP5460099B2 - High strength steel pipe excellent in corrosion resistance and impact bending toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength steel pipe excellent in corrosion resistance and impact bending toughness, and a method for producing the same.

  Steel pipes used for structural members such as automobiles and bicycles are required to have high strength and high fatigue characteristics. Recently, there is an increasing demand for weight reduction of structural members, and structural steel pipes are required to have further increased strength accompanying thinning. However, it is strongly desired to suppress as much as possible the cost increase necessary for increasing the strength.

  Examples of methods for strengthening steel materials include solid solution strengthening, transformation structure strengthening, and process strengthening. Among these, in the case of solid solution strengthening and transformation structure strengthening, it is necessary to adopt steel containing special alloy components in order to effectively generate these mechanisms, which inevitably increases the material cost. . In this respect, according to the processing strengthening, it is possible to increase the strength without adding a special element, which is advantageous in suppressing an increase in cost. Patent Documents 1 and 2 disclose a technique for increasing the strength of an electric resistance welded steel pipe by using a work-strengthened material steel sheet.

  On the other hand, the steel pipe used for the structural member is desired to have good corrosion resistance in order to withstand long-term use. Patent Document 3 discloses a technique in which P and Cu are added in combination as a technique for improving the corrosion resistance of an ERW steel pipe in the bare state.

JP 2002-327245 A JP 2005-29882 A JP 2004-225137 A

In structural steel pipes used in vehicles such as automobiles, high strength characteristics and corrosion resistance are important, and the property that cracks do not easily occur when subjected to impact during collision, that is, `` impact bending toughness '' It is desirable to be excellent. However, in conventional high-strength steel pipes, sufficient consideration is not given to improving impact bending toughness. According to the inventors' investigation, it has been found that it is not easy to improve the impact bending toughness while maintaining good corrosion resistance. In particular, it is currently difficult to improve the corrosion resistance and the impact bending toughness simultaneously while maintaining the tensile strength of the steel pipe at a high level of, for example, 980 N / mm ² or more.

  In view of such a current situation, an object of the present invention is to provide a welded steel pipe with improved corrosion resistance and impact bending toughness by an inexpensive method. In particular, it is an object of the present invention to provide a welded steel pipe that simultaneously improves corrosion resistance and impact bending toughness and has a high strength level.

In order to achieve the above object, in the present invention, in mass%, C: 0.01 to 0.2%, Si: 1.5% or less, Mn: 2.5% or less, P: 0.005 to 0 0.03%, S: 0.005% or less, Cu: 0.05-0.5%, acid-soluble Al: 0.005-0.1%, and Ti: 0.15% as necessary Hereinafter, Nb: containing at least one kind of 0.15% or less, having a chemical composition composed of the balance Fe and inevitable impurities, and the matrix is a ferrite single phase, or the matrix is ferrite phase + 50% by volume or less The second phase means martensite, bainite, and pearlite, has a metal structure in which the amount of pearlite in the matrix is 10% by volume or less, and is determined by the following formula (1). pressure the unannealed cold-rolled steel sheet ferrite grain elongation rate E _F is 5.0 or more Provided is a high-strength steel pipe excellent in corrosion resistance and impact bending toughness, which is formed by welding so that the extending direction becomes the longitudinal direction.
E _F = N ₁ / N ₂ (1)
here,
E _F : Extension degree of ferrite crystal grains of the rolled plate.
N ₁ : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscopic field of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction. However, the microscope field of view and the length X of the line segment are set so that N ₁ is 10 or more.
N ₂ : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field.

The above N ₁ and N ₂ can be determined according to the measurement method defined in the cutting method of Annex 2 of JIS G0551: 2005 (former JIS G0552). However, the point that the number of ferrite crystal grains cut by one line segment is 10 or more applies only to the measurement of N ₁ . An “unannealed cold rolled steel sheet” is a steel sheet that has not been annealed after cold rolling.

  This high-strength steel pipe has, for example, a pipe thickness of 0.6 to 2.4 mm and an outer diameter of 22 to 70 mm.

Further, as a steel pipe having a particular tensile strength 980 N / mm ² or more high strength above, is provided having a chemical composition of the following [1] or [2].
[1] By mass%, C: 0.1 to 0.2%, Si: 1.5% or less, Mn: more than 1.5 to 2.5%, P: 0.005 to 0.03%, S : Chemical composition consisting of 0.005% or less, Cu: 0.05-0.5%, acid-soluble Al: 0.005-0.1%, balance Fe and inevitable impurities.
[2] By mass%, C: 0.03 to 0.2%, Si: 1.5% or less, Mn: more than 1.5 to 2.5%, P: 0.005 to 0.03%, S : 0.005% or less, Cu: 0.05 to 0.5%, acid-soluble Al: 0.005 to 0.1%, Ti: 0.15% or less, Nb: 0.15% or less A chemical composition containing at least one kind, the balance being Fe and inevitable impurities, and the total content of Ti and Nb being 0.03% or more.

  A chemical composition further containing at least one of Ni: 1% or less, Cr: 1% or less, Mo: 0.3% or less, V: 0.3% or less, Zr: 0.3% or less as an optional element. Can be adopted.

As a manufacturing method of the high-strength steel pipe, when manufacturing a steel pipe by the steps of hot rolling, descaling treatment, cold rolling, and welded pipe making, the hot rolling finishing temperature is set to (Ar ₃ point-20 ° C in hot rolling). ) or more, after the finish rolling final pass finish, the average cooling rate to the winding 20 to 100 ° C. / sec, the coiling temperature of 450 to 650 ° C. (1) is ferrite grain elongation rate E _F determined by the equation It makes 2.0 or more hot-rolled steel sheet, the ferrite grain elongation rate E _F a reduction ratio as 40% to 80% in the cold rolling to make a 5.0 or more cold-rolled steel sheet, an unannealed in welding pipe There is provided a method of making a cold-rolled steel sheet as it is so that its rolling direction is the longitudinal direction of the pipe.

According to the present invention, it has become possible to stably provide a welded steel pipe with significantly improved corrosion resistance and impact bending toughness by an inexpensive method. In particular, those having a high strength with a tensile strength of 980 N / mm ² or more can be provided. The steel pipe of the present invention is superior in impact bending toughness compared to ERW steel pipe made of transformation-strengthened DP steel or TRIP steel, and the side surface of the curved portion is extremely cracked when subjected to impact at the time of collision. Hard to occur. Corrosion resistance is also improved. Therefore, the steel pipe of the present invention is particularly suitable for structural members used for vehicles such as automobiles and bicycles.

As a result of investigations, the impact bending toughness is remarkably improved in a high-strength steel pipe obtained by welding and pipe-forming a work-hardened steel sheet that has been sufficiently stretched with ferrite crystal grains to have a fibrous structure. I found. However, even if the method is applied to steel with improved corrosion resistance by adding P and Cu as disclosed in, for example, Patent Document 3, it is difficult to stably improve impact bending toughness. . As a result of detailed studies, the following findings were obtained.
(I) Reducing P is extremely effective for improving impact bending toughness.
(Ii) Even when P is reduced, by sufficiently controlling the P content to a narrow content range of 0.005 to 0.03 mass%, the effect of improving corrosion resistance by adding Cu can be sufficiently obtained. Is possible.

However, P is also an element responsible for increasing the strength. P reduction of a very disadvantageous in obtaining a high strength level steel pipe. As a result of further investigations, it was found that increasing the Mn content is effective for obtaining an extremely high strength level with P-reduced steel, and the following knowledge was obtained.
(Iii) It is preferable to define the C content as high as 0.1% or more after the Mn content exceeds 1.5% by mass.
(Iv) Alternatively, when the lower limit of the C content is relaxed to 0.03 mass%, the Mn content is set to an amount exceeding 1.5 mass%, and an appropriate amount of at least one of Ti and Nb is contained. It is desirable.
The present invention has been completed based on such new findings.
Hereinafter, matters for specifying the present invention will be described.

[Chemical composition]
“%” In the chemical composition means “% by mass” unless otherwise specified.

  C is an element effective for increasing the strength of steel. If it is less than 0.01%, it is difficult to obtain the effect sufficiently. However, in order to obtain an extremely high strength level with P-reduced steel, it is desirable to ensure a C content of 0.03% or more in combination with the increase of Mn and the content of one or more of Ti and Nb. It is. Moreover, when not containing Ti and Nb, it is preferable to set it as 0.1% or more of C content with the increase in Mn. On the other hand, a large amount of C content decreases ductility and weld toughness. As a result of various studies, it is necessary to set the C content within a range of 0.01 to 0.2%.

  Si is an element effective for increasing the strength of steel. Further, when the P-reduced steel containing Cu has an extremely low S, Si also effectively works to improve the corrosion resistance in the bare state. That is, the effect of improving the corrosion resistance by the combined addition of Si and Cu is exhibited. In order to obtain this function sufficiently, it is more effective to secure a Si content of 0.05% or more. However, if Si is contained in a large amount exceeding 1.5%, the strength increases, but the cold workability and the surface properties deteriorate. Accordingly, the Si content needs to be 1.5% or less, and more preferably 1.0% or less. Since the above-mentioned corrosion resistance improving action is almost saturated at a Si content of 0.5%, the upper limit of the Si content may be managed at 0.5%.

  Mn is an element effective for increasing the strength of steel. In order to exhibit the effect, it is more effective to secure a Mn content of 0.1% or more. In the present invention, since the P content is reduced to 0.03% or less, in order to stably obtain an extremely high strength level, it is important to set the Mn content exceeding 1.5%. More preferably, it is made .55% or more. However, if the Mn content exceeds 2.5%, the weldability is remarkably deteriorated although the strength increases as the content increases. In addition, if Mn is contained in a large amount, quench hardening is likely to occur in the welded portion, which deteriorates the workability of the welded portion and causes cracking. Therefore, the Mn content needs to be 2.5% or less, and may be controlled within a range of 2% or less.

P has the effect of improving the bare steel corrosion resistance by the combined addition with Cu. It is also an element that contributes to increasing the strength of steel pipes. However, it has become clear that P has a factor that impairs impact bending toughness. According to the study by the inventors, it is possible to avoid an adverse effect on impact bending toughness by suppressing the P content to 0.03% or less. On the other hand, with respect to corrosion resistance, it has been conventionally considered that the P content needs to be 0.05% or more in order to sufficiently obtain the effect of improving corrosion resistance by the combined addition with Cu (see Patent Document 3). However, a detailed study has revealed that an effect of improving corrosion resistance can be obtained by optimizing the content of other component elements even at a lower P content. Specifically, when the P content is 0.005% or more in the case of extremely low S as described later, the effect of improving the corrosion resistance by adding Cu can be brought out. When Si is set to 0.05% or more as described above, a more remarkable effect can be easily obtained. In addition, the decrease in strength accompanying the reduction of P can be compensated by the C content, Mn content, Ti, Nb content, etc. Even when P is reduced, the tensile strength is not less than 980 N / mm ² or more. It was confirmed that it was possible to manufacture a strength steel pipe. For this reason, in the present invention, the corrosion resistance and the impact bending toughness are both achieved by strictly controlling the P content in the range of 0.005 to 0.03%.

  S is an element having a factor that degrades the corrosion resistance of the steel pipe in the bare state. In the steel pipe of the present invention with reduced P, it is desirable that the S content be as low as possible in order to ensure sufficient corrosion resistance. As a result of various studies, the S content is limited to 0.005% or less.

  Cu is an element effective for improving the corrosion resistance of steel pipes barely, and its action is usually brought out by the combined addition with P. In the present invention, although the P content is reduced, Cu exhibits a remarkable corrosion resistance improving action due to a synergistic effect with extremely low S or further containing Si of 0.05% or more. For this purpose, a Cu content of at least 0.05% is required. However, even if Cu is contained in a large amount exceeding 0.5%, the effect of improving the corrosion resistance is saturated and uneconomical. Therefore, the Cu content is set to 0.05 to 0.5%.

  Al is an element added as a deoxidizer, and 0.005% or more of acid-soluble Al is required to obtain a sufficient deoxidation effect. The effect of Al deoxidation is saturated at about 0.1%, and addition beyond that causes an increase in the cost of the steel material. Therefore, in the present invention, the content of acid-soluble Al is set in the range of 0.005 to 0.1%.

  Ti is an element that forms precipitates with C, S, and N and contributes to increasing the strength of steel by precipitation strengthening. In addition, recovery of processing strain in the weld heat affected zone is suppressed by these precipitates, and softening of the heat affected zone can be prevented by solid solution and reprecipitation during welding heating. For this reason, Ti can be added as needed. It is more effective that the amount of Ti added is 0.005% or more. However, even if added over 0.15%, the above effect is saturated and the manufacturing cost is increased. Therefore, when adding Ti, it is performed within a range of 0.15% or less.

  Nb forms carbides like Ti, and contributes to increasing the strength of steel by precipitation strengthening. Moreover, the metal structure of the steel sheet is refined to improve the strength. Further, in the welded portion, similarly to the effect of Ti, recovery of work strain of the weld heat affected zone is suppressed by the precipitate, and softening of the heat affected zone can be prevented by solid solution and reprecipitation. For this reason, Nb can be added as needed. It is more effective to add Nb to 0.005% or more. However, even if added over 0.15%, the above effect is saturated and the manufacturing cost is increased. Therefore, when adding Nb, it is performed within a range of 0.15% or less.

As described above, when obtaining a high-strength steel pipe having a tensile strength of 980 N / mm ² or more, after setting C to 0.03% or more and Mn to a content exceeding 1.5%, Ti, A technique of containing one or more of Nb can be employed. In that case, the total content of Ti and Nb should be 0.03% or more (when any one of Ti and Nb is added alone, the content of the additive element should be 0.03% or more. Is very effective.

  Ni is an element effective for increasing the strength and improving the toughness of the welded portion, and can be added as necessary. In order to sufficiently obtain the effect, it is more preferable to secure a Ni content of 0.05% or more. However, even if added in excess of 1%, the above effect is saturated and the manufacturing cost is increased. Therefore, when adding Ni, it is performed within a range of 1% or less.

  Mo and V are elements that form carbides similarly to Ti and Nb and are effective in increasing the strength of steel by precipitation strengthening. Further, in the welded portion, it is effective for preventing the heat affected zone from being softened in the same manner as the effects of Ti and Nb. For this reason, one or both of Mo and V can be added as needed. It is more effective to secure a content of 0.01% or more for both Mo and V. However, even if it adds excessively, the said effect will be saturated and the raise of manufacturing cost will be caused. Addition of a large amount of Cr increases the hardenability and deteriorates the workability of the weld. As a result of various investigations, when adding Mo and V, each is performed within a range of 0.3% or less.

  Cr and Zr are effective elements for increasing the strength and improving the toughness of the welded portion. Therefore, one or both of Cr and Zr can be added as necessary. It is more effective to secure a content of 0.05% or more in the case of Cr and 0.01% or more in the case of Zr. However, even if it adds excessively, the said effect will be saturated and the raise of manufacturing cost will be caused. Addition of a large amount of Cr increases the hardenability and deteriorates the workability of the weld. As a result of various studies, when Cr is added, it is performed within a range of 1% or less, and when Zr is added, it is performed within a range of 0.3% or less.

[Metal structure]
The steel pipe of the present invention has a work-hardened structure and exhibits a fibrous structure in which ferrite crystal grains extend in the longitudinal direction. Specifically, one in which the (1) ferrite grain elongation rate E _F determined by the equation with 5.0 or more unannealed cold-rolled steel sheet as a material for pipe production. According to the study by the inventors, it has been found that impact bending toughness is remarkably improved in a steel pipe having the above-described chemical composition and having such a fibrous structure in the longitudinal direction. In addition, the unannealed cold-rolled steel sheet used for pipe making has a matrix of a ferrite single phase or a second phase of ferrite phase + 50% by volume or less. If the amount of the second phase is too large, the effect of improving the impact bending toughness by setting the ferrite crystal grain extension to 5.0 or more may not be sufficiently exhibited. The second phase is martensite, bainite, pearlite, etc., but if a large amount of pearlite is generated, the strength and impact bending toughness are reduced, so the amount of pearlite in the matrix is suppressed to 10% by volume or less. It is desirable.

[Strength level]
According to the present invention, it is possible to obtain a steel pipe having a high tensile strength of 980 N / mm ² or more in the longitudinal direction. This is a so-called 100 kg-class high-strength steel pipe, which is particularly suitable for members that require high strength in various mechanical structures including automobiles.

〔Manufacturing process〕
The high-strength steel pipe of the present invention is manufactured by melting the steel whose component composition has been adjusted as described above, and performing the steps of hot rolling, descaling (for example, pickling), cold rolling, and welded pipe making. Can do.

(Hot rolling)
In the hot rolling process, the hot rolling finishing temperature is set to (Ar ₃ point−20 ° C.) or higher. When the temperature is lower than the Ar ₃ point, a two-phase mixed structure of ferrite and austenite is obtained. When rolling with such a mixed structure, the deformation resistance becomes non-uniform inside the plate and the thickness accuracy of the hot-rolled steel strip tends to decrease. However, if it is (Ar ₃ point-20 ° C) or more, the ferrite phase Since the amount is relatively small, it is possible to keep the plate thickness accuracy within a general allowable range. However, when the hot rolling finishing temperature is lower than (Ar ₃ point−20 ° C.), so-called gauge hunting in which the plate thickness fluctuation in the rolling direction fluctuates easily occurs. For this reason, it is more preferable that the hot rolling finishing temperature is Ar ₃ or higher. If the hot-rolling finishing temperature is too high, dynamic recrystallization tends to occur, and if dynamic recrystallization occurs excessively, the elongation of ferrite crystal grains in the hot-rolled sheet may be stabilized to 2.0 or more. It becomes difficult. As a result of the examination, it is more effective to set the hot rolling finishing temperature to a range of (Ar ₃ point + 50 ° C.) or lower.

  After the final pass of finish rolling, the average cooling rate until winding is 20 to 100 ° C./second. When the average cooling rate is less than 20 ° C./second, pearlite is easily generated, and it becomes difficult to obtain the intended strength and impact bending toughness. On the other hand, when the temperature exceeds 100 ° C./second, the amount of bainite generated increases and the ferrite phase decreases, and the effect of the present invention due to the expansion of ferrite crystal grains cannot be enjoyed.

  The coiling temperature is 450 to 650 ° C. The higher the coiling temperature, the more pearlite is generated and the strength level of the steel material is lowered. In addition, the predetermined degree of spread cannot be obtained, and the impact bending toughness tends to be lowered. As a result of the examination, the coiling temperature is set to 650 ° C. or lower. It is more preferable to manage the coiling temperature at 600 ° C. or lower particularly for member applications requiring high strength. On the other hand, when the coiling temperature is lower than 450 ° C., the strength is remarkably increased due to the transformation structure strengthening.

Within the range of the above hot rolling conditions, a hot rolled steel sheet having a ferrite grain extension E _F determined by the above formula (1) of 2.0 or more is made. If the elongation is not set to 2.0 or more at this stage, a high cold rolling rate is required to obtain a degree of elongation of 5.0 or more after cold rolling, and the extension is achieved at a desired product thickness. It tends to be difficult to adjust to 5.0 or higher.

(Cold rolling)
In the cold rolling step, the cold rolling rate is 40 to 80%. Cold rolling ratio said to be a stable 5.0 or more ferrite grain elongation rate E _F determined by (1) becomes difficult when the steel pipe is less than 40%. Further, when aiming for a tensile strength of 980 N / mm ² or more, the strength tends to be insufficient. On the other hand, if the cold rolling rate exceeds 80%, the number of cold rolling passes may increase significantly, resulting in a significant increase in manufacturing cost.

The range of the cold-rolling reduction (1) ferrite grain elongation rate E _F determined by the equation to obtain 5.0 or more cold-rolled steel sheet. That is, the hot rolling conditions and the cold rolling rate so that a ferrite grain extension of 5.0 or more can be obtained in the range of the cold rolling rate of 40 to 80, depending on the product plate thickness that is known in advance. It is important to set a combination. This combination of conditions can be determined by conducting a preliminary experiment in advance according to the chemical composition.

[Pipe making]
A cold-rolled steel sheet having a ferrite grain extension degree of 5.0 or more is used as it is in an unannealed state for welding. At that time, the pipe is formed such that the rolling direction, that is, the direction in which the ferrite crystal grains are extended becomes the longitudinal direction of the pipe. For pipe making, a general welding pipe making method in which a steel strip having a predetermined width is formed into a cylindrical shape by roll forming, and both edge portions of the steel strip are attached to each other and welded can be applied. Examples of the welding include high-frequency welding, plasma welding, TIG welding, and the like, but it is preferable to use an ERW steel pipe by high-frequency welding. The steel pipe obtained in this way exhibits excellent impact bending toughness.

A steel slab having the composition shown in Table 1 was heated to 1250 ° C., extracted and hot-rolled. The hot rolling conditions were as shown in Table 2. Table 2 also shows the Ar ₃ point of each steel. Subsequently, after removing the scale by pickling, cold rolling was performed at the cold rolling rate shown in Table 2. The cold-rolled steel sheet was passed through a pipe making line in the state of an unannealed steel strip, and piped by high frequency welding to obtain an electric resistance steel pipe having a wall thickness of 1.0 mm and an outer diameter of 31.8 mm. In addition, the average cooling rate until the winding after the finish rolling final pass is within the range of 20 to 100 ° C./second for all of the examples of the present invention shown in Table 2.

Samples were taken from the hot-rolled steel sheet and cold-rolled steel sheet after pickling, the L section was observed with a microscope, and the ferrite crystal grain elongation of the hot-rolled steel sheet and the cold-rolled steel sheet was examined by the method described above. At that time, the L cross section of the rolled material was polished and etched, and the vicinity of the center of the plate thickness was observed.
Incidentally, those of the invention examples were grain elongation rate E _F is 2.0 or more at the hot-rolled steel sheet either. In all of the examples of the present invention, the matrix comprised 70% by volume or more of the ferrite phase, the second phase consisted of bainite or martensite, and the amount of pearlite was 10% by volume or less.

  A No. 11 tensile test piece of JIS Z2201 was produced from the obtained steel pipe, a tensile test was performed, and a yield stress YS and a tensile strength TS were measured. The obtained steel pipe was subjected to a low temperature drop test and a corrosion resistance test by the following methods.

[Low temperature drop test]
A sample with a length of 900 mm was taken from the steel pipe, cooled to −30 ° C., and immediately placed horizontally on two supporting jigs fixed at intervals of 300 mm, and a weight of 114 kg was dropped. It was dropped and collided at a central position between the support jigs of the sample steel pipe at 300 mm. The support jig has an inverted U-shaped cross section with R = 30 mm, and the weight has a punch with a tip R = 25 mm at the collision part. The weld bead position was 0 ° from the upper end. After the test, no cracks occurred on the side surface of the curved specimen, and no cracks were observed in the welded portion, ○ (impact bending toughness; good), and other specimens × (impact bending toughness; Bad).

[Corrosion resistance test]
A sample having a length of 150 mm was cut out from the steel pipe, and the cut end face and the inner face of the steel pipe were coated and sealed, and then subjected to an accelerated corrosion test in a state where the weld bead portion faced up to 75 °. The corrosion test was performed up to 50 cycles, with 12 hours of “salt spray: 4 h → drying: 8 h” being 1 cycle. After the corrosion test, the seal was removed and the maximum erosion depth of the outer surface of the steel pipe was measured. The salt water spray used an aqueous NaCl solution having a concentration of 0.5% at 35 ° C. Drying was performed by introducing outside air adjusted to 20 ° C. at a humidity of 60%. Samples after the test were observed with an optical microscope, and those having a maximum erosion depth of 0.2 mm or less were evaluated as ○ (good corrosion resistance), and those exceeding 0.2 mm were evaluated as × (poor corrosion resistance).
These results are shown in Table 2.

As can be seen from Table 2, the steel pipe of the example of the present invention has excellent impact bending toughness in a low temperature drop test by using a cold rolled steel sheet having a ferrite grain elongation of 5.0 or more as a material. And it was confirmed that having a predetermined chemical composition has good corrosion resistance. Moreover, it was confirmed that what has high intensity | strength more than tensile strength 980N / mm < ² > is obtained by adjustment of a chemical composition.

In contrast, Comparative Example No.31~33, by either cold rolling reduction is too low, ferrite grain elongation rate E _F of cold-rolled steel sheet did not reach 5.0. As a result, cracks are generated in the curved portion in the low temperature drop test, and the impact bending toughness is poor. Since No. 34 has a high C content and No. 35 has a high P content, they all have poor impact bending toughness. No. 36 has a low Cu content, and Nos. 37 and 38 have a high S content, so these are all inferior in corrosion resistance, and among these, No. 38 has a high Mn content and inferior in impact bending toughness.

Claims (6)

In mass%, C: 0.01-0.2%, Si: 1.5% or less, Mn: 2.5% or less, P: 0.005-0.03%, S: 0.005% or less, Cu: 0.05-0.5%, acid-soluble Al: 0.005-0.1%, a metal composition having a chemical composition comprising the balance Fe and inevitable impurities, and the matrix is a single phase of ferrite, or The matrix is composed of a ferrite phase + 50 volume% or less of the second phase, the second phase means martensite, bainite, pearlite, and has a metal structure in which the amount of pearlite in the matrix is 10 volume% or less, and corrosion resistance and impact flexural toughness below (1) unannealed cold-rolled steel sheet rolling direction determined ferrite grain elongation rate E _F is 5.0 or more by equation formed by welding pipe such that the longitudinal direction Excellent high strength steel pipe.
E _F = N ₁ / N ₂ (1)
here,
E _F : Extension degree of ferrite crystal grains of the rolled plate.
N ₁ : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscopic field of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction. However, the microscope field of view and the length X of the line segment are set so that N ₁ is 10 or more.
N ₂ : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field.   The high-strength steel pipe according to claim 1, further comprising at least one of Ti: 0.15% or less and Nb: 0.15% or less. By mass%, C: 0.1 to 0.2%, Si: 1.5% or less, Mn: more than 1.5 to 2.5%, P: 0.005 to 0.03%, S: 0.0. 005% or less, Cu: 0.05 to 0.5%, acid-soluble Al: 0.005 to 0.1%, balance Fe and chemical composition consisting of inevitable impurities, and matrix is a single phase of ferrite Metal structure or a matrix composed of a ferrite phase and a second phase of 50% by volume or less, wherein the second phase means martensite, bainite and pearlite, and the amount of pearlite in the matrix is 10% by volume or less has, and the corrosion resistance of the following (1) unannealed cold-rolled steel sheet rolling direction determined ferrite grain elongation rate E _F is 5.0 or more by equation formed by welding pipe such that the longitudinal direction High-strength steel pipe with a tensile strength of 980 N / mm ² or more with excellent impact bending toughness.
E _F = N ₁ / N ₂ (1)
here,
E _F : Extension degree of ferrite crystal grains of the rolled plate.
N ₁ : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscopic field of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction. However, the microscope field of view and the length X of the line segment are set so that N ₁ is 10 or more.
N ₂ : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field.   Chemical composition is mass%, C: 0.03-0.2%, Si: 1.5% or less, Mn: more than 1.5-2.5%, P: 0.005-0.03%, S: 0.005% or less, Cu: 0.05-0.5%, acid-soluble Al: 0.005-0.1%, Ti: 0.15% or less, Nb: 0.15% or less 4. The high-strength steel pipe according to claim 3, wherein the high-strength steel pipe comprises at least one of the following: the balance Fe and inevitable impurities, and the total content of Ti and Nb is 0.03% or more.   Furthermore, it has a chemical composition containing one or more of Ni: 1% or less, Cr: 1% or less, Mo: 0.3% or less, V: 0.3% or less, Zr: 0.3% or less. The high-strength steel pipe in any one of -4. After hot rolling the slab having the chemical composition according to any one of claims 1 to 5, when producing a steel pipe by the steps of scale removal treatment, cold rolling, and welded pipe forming, hot rolling finish in hot rolling The temperature is (Ar ₃ point−20 ° C.) or more, and after the final pass of finish rolling, the average cooling rate until winding is 20 to 100 ° C./second and the winding temperature is 450 to 650 ° C. ferrite grain elongation rate E _F is made a 2.0 or more hot-rolled steel sheet, as 40% to 80% of rolling reduction in cold rolling, the metal structure, or matrix ferrite phase +50 volume matrix is ferrite single phase % consists of the following second phase, the second phase means martensite, bainite, pearlite, abundance of pearlite in the matrix has a metal structure is 10 vol% or less, and the Fe High strength Ito grain elongation rate E _F is made 5.0 or more cold-rolled steel sheet, a cold rolled steel sheet remains unannealed its rolling direction to pipe-making so that the longitudinal direction of the pipe in welding pipe Steel pipe manufacturing method.
E _F = N ₁ / N ₂ (1)
here,
E _F : Extension degree of ferrite crystal grains of the rolled plate.
N ₁ : Number of crystal grains cut by a line segment of a certain length X in the plate thickness direction in a microscopic field of a cross section (L cross section) parallel to the rolling direction and the plate thickness direction. However, the microscope field of view and the length X of the line segment are set so that N ₁ is 10 or more.
N ₂ : Number of crystal grains cut by the line segment of the length X in the rolling direction in the visual field.