JP2008111162A - Hot-rolled steel sheet for electroseamed steel pipe having high toughness and showing low yield ratio after having been painted, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet from which an electroseamed steel pipe that shows a low yield ratio after having been painted and baked, and has high toughness, can be stably obtained. <P>SOLUTION: This hot-rolled steel sheet has a component composition that includes a predetermined content range of C, Si, Mn, P, S, Al and N, and further includes Cr and Mo in amounts of less than 1.5% and less than 0.3% respectively, on a condition of satisfying the expression [-1.11×(Cr)<SP>2</SP>+0.1<Mo<-0.13×(Cr)<SP>2</SP>+0.3]; and has a dual-structure that includes mainly ferrite and one or more second phases selected from bainite, martensite and retained austenite. Ferrite particles with aspect ratios of 1 to less than 4 occupy 50% or more by number of all crystal grains in the structure. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet for an electric-resistance-welded steel pipe, which is particularly suitable for pipelines such as crude oil and gas, water pipes, columns for construction and civil engineering, and a method for producing the same.

ERW steel pipes are widely used in transportation pipes such as line pipes and water pipes, but in recent years, as a countermeasure against deformation stress associated with landslides during earthquakes when laying transportation pipes, preventing buckling after laying, There is a growing need for ERW steel pipes having a low yield ratio (hereinafter referred to as “YR”) in the longitudinal direction of the pipe.
In general, ERW steel pipes used for line pipes, etc. are subjected to heavy anti-corrosion coating (paint baking) for the purpose of anti-corrosion after pipe formation. If, for example, baking is performed at about 250 ° C. × 10 minutes at that time, the solid solution element remaining in the steel is fixed, strain aging occurs, and YR increases.
On the other hand, line pipes are often laid on the sea floor or frozen land, and excellent toughness is required for the entire ERW steel pipe including welds.

However, it is not easy to achieve both low YR after paint baking and high toughness. In general, a fine structure is advantageous for obtaining high toughness. However, for example, when a fine ferrite structure is formed, the yield point increases and YR increases. That is, generally, low YR and high toughness are contradictory.
As a hot rolled steel sheet for electric resistance steel pipes aiming at low YR, for example, a steel sheet containing Mo + Cr in a specific range and having a structure composed of martensite and ferrite having an area ratio of 1 to 20% is disclosed in Patent Document 1. Has been proposed.
Japanese Patent Laid-Open No. 10-176239

However, since the technology of Patent Document 1 aims to reduce the YR of the hot-rolled steel sheet simply by utilizing martensite and to suppress the YS decrease after pipe forming, High toughness cannot be achieved at the same time.
Accordingly, an object of the present invention is to provide a hot-rolled steel sheet for an electric-resistance-welded steel pipe and a method for producing the same, which can stably obtain an electric-welded steel pipe having a low YR after coating and baking and having high toughness.

  The present inventors should provide a hot-rolled steel sheet for electric-resistance-welded steel pipes in order to obtain an electric-welded steel pipe having a low YR in the longitudinal direction of the pipe and maintaining the low-YR even after paint baking and having high toughness. The chemical composition / structure and manufacturing conditions were examined. As a result, in order to obtain the desired hot-rolled steel sheet, (1) it is necessary to add Cr and Mo in an optimal balance, and (2) two-phase separation actively during the cooling process after finishing rolling. Therefore, the formation of polygonal ferrite is important, and (3) the bainite transformation is actively used, and the second phase (one or more of bainite, martensite, and retained austenite) is used. We have learned that it is necessary to generate them.

The present invention has been made on the basis of such knowledge and has the following gist.
[1] By mass%, C: 0.01 to 0.1%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.05% or less, Al: 0.01 to 0.10%, N: 0.01% or less, and Cr and Mo, Cr: less than 1.5%, Mo: less than 0.3%, and (1) It contains under the conditions that satisfy the formula, and the balance has a component composition consisting of iron and inevitable impurities,
It has a composite structure mainly composed of ferrite and one or more second phases selected from bainite, martensite, and retained austenite, and the number of ferrite grains having an aspect ratio of 1 or more and less than 4 is all crystal grains in the structure. A hot-rolled steel sheet for high-toughness ERW steel pipes with a low yield ratio after coating, characterized by being over 50%.
−1.11 × (Cr) ² +0.1 <Mo <−0.13 × (Cr) ² +0.3 (1)
However, Cr: Cr content (mass%)
Mo: Mo content (% by mass)

[2] In the steel sheet according to [1] above, in mass%, Nb: 0.5% or less, Ti: 0.5% or less, V: 0.5% or less, Zr: 0.5% or less A hot-rolled steel sheet for a high-toughness ERW steel pipe having a low yield ratio after coating, comprising at least one selected from the group consisting of:
[3] In the steel sheet according to [1] or [2], further, by mass, Cu: 0.5% or less, Ni: 0.5% or less, Sn: 0.5% or less, Sb: 0.5 % Hot-rolled steel sheet for high-toughness ERW steel pipe having a low yield ratio after coating, characterized by containing at least one selected from the group consisting of 1% or less.
[4] The steel sheet according to any one of [1] to [3] further includes at least one selected from Ca: 0.01% or less and REM: 0.01% or less by mass. A hot-rolled steel sheet for high-toughness ERW steel pipes with a low yield ratio after coating.

[5] In the steel sheet of any one of [1] to [4], the composite structure has a pearlite volume fraction of 3% or less (including 0%), and the balance is ferrite, bainite, martensite. A hot-rolled steel sheet for a high-toughness ERW steel pipe having a low post-painting yield ratio, comprising at least one second phase selected from sites and retained austenite.
[6] The steel having any of the component compositions described in [1] to [4] is heated and hot-rolled, and in the hot rolling, a temperature range of Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C. Finish rolling with a reduction ratio of 50% or more at the end, and after cooling has been started at least 0.5 seconds, cooling using a refrigerant is started, and the average from the start of cooling to winding A method for producing a hot-rolled steel sheet for a high toughness ERW steel pipe having a low post-painting yield ratio, wherein the cooling rate is 3 to 40 ° C / s and winding is performed at 400 to 600 ° C.
[7] A high-toughness electric resistance welded steel pipe having a low yield ratio after coating, obtained by pipe-making the hot-rolled steel sheet according to any one of [1] to [5].

  According to the hot rolled steel sheet for electric resistance welded steel pipe of the present invention, an electric resistance welded steel pipe having a low yield ratio after baking and having high toughness can be obtained stably. For this reason, it is possible to effectively prevent, for example, buckling prevention at the time of laying or after laying of the transportation piping, and damage due to deformation stress accompanying landslide at the time of earthquake occurrence.

Hereinafter, the component composition of the hot-rolled steel sheet of the present invention will be described. In the following description, the content of each element is “mass%”.
If the C content is less than 0.01%, the desired strength cannot be obtained, and the toughness is deteriorated due to the coarsening of the crystal grains. On the other hand, when it exceeds 0.1%, pearlite is easily generated, and the toughness is deteriorated. Therefore, the C amount is set to 0.01 to 0.1%.
Si is very useful for promoting two-phase separation, making ferrite polygonal, and stabilizing the bainite transformation. However, if the amount of Si exceeds 1.0%, oxides are likely to be generated during ERW welding, and the weld toughness deteriorates. Therefore, the Si content is 1.0% or less, preferably 0.5% or less, and more preferably 0.3% or less. In order to effectively promote the two-phase separation, the Si amount is desirably 0.01% or more.

Mn is an element effective for securing the strength of steel, but if added over 2.0%, not only does the yield ratio (hereinafter referred to as “YR”) increase due to solid solution strengthening, but also the toughness of the weld zone. Deteriorate. For this reason, the amount of Mn is made 2.0% or less.
P is also effective in securing the strength and contributes to the formation of the second phase. However, P is an element that is very easily segregated at the grain boundary. Therefore, if the amount of P exceeds 0.05%, YR increases and toughness deteriorates, which is not preferable. Therefore, the P content is 0.05% or less, preferably 0.03% or less.

If the amount of S exceeds 0.05%, coarse sulfides are easily generated, and the toughness of the welded portion is deteriorated. Therefore, the S content is 0.05% or less, preferably 0.01% or less.
Al is used as a deoxidizing agent in the steelmaking stage and needs to be added so as to contain 0.01% or more. However, if the Al content exceeds 0.10%, the oxide in the steel is increased, and the toughness of the base metal and the welded portion is lowered, which is not preferable. For this reason, the Al content is 0.01 to 0.10%, preferably 0.01 to 0.05%.
If the amount of N exceeds 0.01%, in addition to the deterioration of toughness due to the formation of coarse nitrides, the solid solution N causes strain aging after coating baking and deteriorates YR. Therefore, the N content is 0.01% or less.

Addition of Cr and Mo in an optimal balance is one of the most important requirements for ensuring low YR and high toughness after baking. The hot-rolled sheet structure of the present invention as described later, that is, the number of ferrite grains (polygonal ferrite grains) having an aspect ratio of 1 or more and less than 4 occupies more than 50% of the total crystal grains in the structure, and as the second phase In order to obtain a composite structure having one or more selected from bainite, martensite, and retained austenite, the amount of Cr and Mo needs to satisfy the following formula (1). Although related to the hot rolling conditions described later, the balance of the amount of Cr and Mn as described above is to pass the ferrite nose at the optimum position in the hot rolling cooling (runout cooling) process to form polygonal ferrite. It turned out to be an essential requirement.
−1.11 × (Cr) ² +0.1 <Mo <−0.13 × (Cr) ² +0.3 (1)
However, Cr: Cr content (mass%)
Mo: Mo content (% by mass)

FIG. 1 shows the relationship between the Cr and Mo contents of a hot-rolled steel sheet and the post-painting yield ratio and weld seam toughness of an ERW steel pipe obtained by pipe-making the steel sheet. This hot-rolled steel sheet has C: 0.03-0.07%, Si: 0.01-0.5%, Mn: 0.9-1.7%, P: 0.005-0.03%, A steel slab containing S: 0.001 to 0.03%, Al: 0.01 to 0.07%, N: 0.001 to 0.006% is hot-rolled, and Ar ₃ points to Ar ₃ After finishing rolling at a reduction rate of 50% or more in the temperature range of the point + 100 ° C., cooling is started using a refrigerant after at least 0.5 second has elapsed after the rolling is finished, and this cooling The average cooling rate from the start of winding to winding is 3 to 40 ° C./s, and winding is performed at 400 to 600 ° C. In addition, the pipe making conditions, the yield ratio after coating, and the evaluation method of the weld seam toughness were the same as those in Examples described later.

According to FIG. 1, in order to achieve both low YR after paint baking (YP <90%) and high toughness (vTrs ≦ −30 ° C.) aimed by the present invention, the addition amounts of Cr and Mo are the above ( 1) It is necessary to be within the range of the formula, and if it is outside this range, it can be seen that YR and / or toughness after paint baking are not sufficient.
On the other hand, if the Cr content is 1.5% or more, an oxide is generated during welding and the toughness deteriorates, so the Cr content is less than 1.5%, more preferably 1.0% or less, and particularly preferably 0.8%. The following. On the other hand, from the viewpoint of obtaining a desired structure, the lower limit of Cr is preferably 0.01%. Further, if the Mo amount is 0.3% or more, not only will the cost increase significantly, but a desired structure cannot be obtained in the hot rolling cooling process, and a low YR cannot be achieved. %. On the other hand, from the viewpoint of obtaining a desired structure, the Mo amount is 0.05% or more, desirably 0.10% or more, and particularly desirably 0.15% or more. However, from the viewpoint of cost, the upper limit of the Mo amount is preferably about 0.20%.
For these reasons, the Cr content is less than 1.5% (desirably 1.0% or less, particularly preferably 0.8% or less), and the Mo content is less than 0.3% (desirably 0.05% or more, more Desirably, 0.10% or more, particularly desirably 0.15% or more), and a range satisfying the above formula (1).

In the present invention, the following elements may be further contained according to the strength and toughness of the target steel pipe.
First, one or more selected from Nb: 0.5% or less, Ti: 0.5% or less, V: 0.5% or less, and Zr: 0.5% or less can be added. These elements are elements that precipitate carbides mainly during the hot rolling cooling process, and are useful not only for ensuring strength but also for reducing solid solution C in ferrite. Contributes to lower YR. Moreover, a part contributes to refinement | miniaturization of an austenite grain by precipitating during hot rolling finish rolling, and it acts effectively on structure formation of desired polygonal ferrite + second phase. However, if both are added in excess of 0.5%, YR increases due to excessive precipitation strengthening.

Moreover, 1 or more types chosen from Cu: 0.5% or less, Ni: 0.5% or less, Sn: 0.5% or less, and Sb: 0.5% or less can be added. The addition of these elements is effective for increasing the strength. Moreover, these elements have the property of concentrating on the steel sheet surface layer in the hot rolling heating stage. However, when adding over 0.5%, hot workability may be deteriorated due to the concentration of these elements along the grain boundary.
Furthermore, 1 or more types chosen from Ca: 0.01% or less and REM: 0.01% or less can be added. By adding appropriate amounts of these elements, it is possible to control the form of the sulfide. Thereby, improvement in toughness can be expected. However, if both are added in excess of 0.01%, a large amount of oxide remains in the steel, and on the contrary, the toughness is deteriorated.

Next, the structure of the hot rolled steel sheet of the present invention will be described.
The hot-rolled steel sheet of the present invention has a composite structure mainly composed of ferrite as a first phase and one or more second phases selected from bainite, martensite, and retained austenite, and an aspect ratio of 1 The number of ferrite grains less than 4 occupies more than 50% of all crystal grains in the structure.
Here, the ferrite grains having an aspect ratio of 1 or more and less than 4 are so-called polygonal ferrites, and the present inventors configure the structure of a steel sheet to ensure low YR and high toughness after paint baking. It has been found that more than 50% of all crystal grains to be converted into polygonal ferrite.
In the present invention, the aspect ratio of the ferrite grain is defined as the ratio between the long side and the short side (= long side / short side) of the ellipse obtained by obtaining the maximum ellipse inscribed in each grain.

  Generally, in order to obtain high toughness, it is preferable to make ferrite as fine as possible. However, when trying to simultaneously obtain low YR and high toughness after baking, it is not appropriate to make the ferrite grains too fine, and the transformed ferrite grains are grown to some extent to promote two-phase separation, from ferrite to austenite. It was found that it is important to exhale the constituent elements. Therefore, the ferrite grains should not be made too fine, and the main component is polygonal ferrite having an aspect ratio of 1 or more and less than 4 so that distribution of component elements at the γ / α interface is likely to occur (that is, all It is important that more than 50% of the crystal grains). From the above viewpoint, the average particle size of the polygonal ferrite is preferably 1 μm or more. Further, as the polygonal ferrite, it is more preferable that the number of ferrite grains having an aspect ratio of 1 or more and less than 3.5, and desirably an aspect ratio of 1 or more and less than 3 occupy more than 50% of all crystal grains in the structure.

The hot-rolled steel sheet of the present invention has at least one selected from bainite, martensite, and retained austenite as the second phase, and it is desirable that pearlite is not included in principle, but is inevitably included. Even in this case, the upper limit should be about 3% by volume. These second phases have an effect of increasing the toughness as the crystal grains become finer. The volume ratio of the entire second phase is preferably less than 15%.
Therefore, a preferable composite structure of the hot-rolled steel sheet of the present invention has a pearlite volume ratio of 3% or less (including 0%), and the balance is ferrite (however, ferrite grains having an aspect ratio of 1 to less than 4). The number of which is more than 50% of the total crystal grains in the structure) and one or more second phases selected from bainite, martensite, and retained austenite, and the volume fraction of the second phase is less than 15% It is a complex organization.
In the present invention, the number of grains and the volume ratio of each structure can be measured by observing the microstructure of the cross section in the rolling direction of the steel sheet with a scanning electron microscope. For example, the number of grains of each structure existing in a 100 mm square area set arbitrarily by image analysis is counted using a cross-sectional structure photograph with a magnification of 3000 times obtained with a scanning electron microscope. Moreover, the measurement of the volume ratio of the whole 2nd phase calculates | requires the 2nd phase occupation area rate by image processing using said photograph, and makes it the volume ratio of a 2nd phase. Further, the aspect ratio of the ferrite grain is obtained as the longest / shortest side of the ellipse by obtaining the maximum ellipse inscribed in each grain using the same photograph.

Next, the manufacturing conditions of the hot rolled steel sheet of the present invention will be described.
In the present invention, the steel (slab) having the above-described component composition is heated and hot-rolled. In the finish rolling of this hot rolling, 50% or more in the range of Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C. It is necessary to ensure a reduction rate of 60% or more. The reason for securing a high rolling reduction in such a specific temperature range is to reduce the austenite grain size before transformation and increase the number of transformation nuclei in the γ → α transformation in the subsequent hot rolling cooling process (runout cooling). is there. Also, the second phase can be finely dispersed, contributing to improved toughness. In the finish rolling, there is no particular limitation on the rolling reduction in a temperature range other than Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C.

  After finishing rolling, at least 0.5 seconds after rolling is finished, cooling using a refrigerant (usually cooling with cooling water in contact) is started, and the average from the start of cooling to winding It is very important to control the cooling rate in the range of 3 to 40 ° C./s. A more preferable average cooling rate is 5 to 25 ° C./s. The reason for controlling the cooling rate after rolling in this way is to generate polygonal ferrite from recrystallized austenite grains and promote two-phase separation (γ → α transformation). Here, when the cooling rate is too low, the ferrite grains become coarse and pearlite is precipitated to deteriorate toughness. On the other hand, when the cooling rate is too large, the ratio of acicular ferrite that exceeds the above aspect ratio is increased. In this case, the component elements are not sufficiently distributed by the two-phase separation, which is disadvantageous for the generation of the second phase, and YR after baking is deteriorated due to strain aging. That is, since solid solution elements remain in the ferrite, strain aging occurs after baking and the desired low YR cannot be obtained.

Further, it is important that the cooling (runout cooling) using the refrigerant after the finish rolling is started after 0.5 seconds or more, preferably 1 second or more has elapsed after the rolling, and thus recrystallized austenite. From this, ferrite transformation is started and polygonal ferrite is easily formed. However, if the time from the end of rolling to the start of cooling with a refrigerant is too long, pearlite will precipitate and the toughness will deteriorate, so the time until the start of cooling will be regulated so that the volume fraction of pearlite can be kept below 3%. It is preferable to do. Therefore, it is usually preferable to start cooling using a refrigerant within 3 seconds after the end of rolling. In addition, after finishing rolling, it is natural cooling until it starts the cooling using a refrigerant | coolant.
The cooling rate after the end of rolling can be easily controlled, for example, by adjusting the density of the amount of cooling water poured into the steel plate in runout cooling.

  The coiling temperature is 400 to 600 ° C., and this coiling temperature is important for actively utilizing the bainite transformation. A more preferable winding temperature is 450 to 570 ° C. The bainite transformation from austenite enriched with component elements such as carbon discharged from polygonal ferrite is very effective for lowering YR after coating baking. That is, when a large number of polygonal ferrites are formed and then the bainite transformation is started, component elements are concentrated in austenite, so martensite and retained austenite are likely to be generated. Low YR after paint baking can be realized.

The ERW steel pipe using the hot-rolled steel sheet of the present invention can be basically produced by a normal pipe making method. That is, for example, a hot-rolled steel sheet is formed by cage roll forming, electric resistance welding is performed, bead grinding of the inner and outer surfaces is performed, heat treatment is applied by a post-annealer, and sizing is performed. Usually, paint baking for heavy anticorrosion is performed after pipe making, and the baking condition of this heavy anticorrosion coating is, for example, about 250 ° C. × 10 minutes.
When the pipe forming process as described above is performed, tensile strain or compression strain generally remains in the longitudinal direction of the tube, but the effect of the present invention is not impaired even if any strain remains.

After heating the steel slab of the component composition shown in Table 1 to 1100-1250 degreeC, it hot-rolled on the conditions shown in Table 2, and manufactured the hot rolled steel plate of 20 mm in thickness. In addition, No. 3 and no. In Examples other than 4, after finishing rolling, cooling with cooling water was started after 0.5 to 1.5 seconds had elapsed.
About the manufactured hot-rolled steel sheet, the number of grains of each structure and the volume ratio of the second phase were measured as follows. The microstructure of the cross section in the rolling direction of each steel sheet was observed with a scanning electron microscope, and a cross-sectional structure photograph with a magnification of 3000 times was obtained. Using this cross-sectional structure photograph, the number of grains of each structure existing in a 100 mm square area set arbitrarily by image analysis was counted. Moreover, the second phase occupation area ratio was calculated | required by image processing using the said photograph, and it was made into the volume ratio of the second phase.
Each hot-rolled steel sheet was piped into an electric resistance steel pipe (outer diameter 20 inches), and the weld seam toughness and YR after coating were measured by the following methods. The results are shown in Table 2 together with the production conditions and structure of the hot-rolled steel sheet.

(1) Weld seam part toughness The weld seam part toughness was obtained by taking No. 4 test piece of JIS Z 2202 from the circumferential direction of the ERW steel pipe so that the notch is parallel to the ERW weld part and centered. Using this test piece, the fracture surface transition temperature vTrs (temperature at which the ductile fracture surface ratio becomes 50%) was evaluated.
(2) YR after painting
After applying a heavy anticorrosion coating (250 ° C. × 10 minutes) to the ERW steel pipe, the YR value in the longitudinal direction of the steel pipe was evaluated. Tensile properties were measured using a small round bar tensile tester. Here, YR is defined as the ratio of stress / tensile strength at 0.5% tension.
In this example, the targets were YR <90% and vTrs ≦ −30 ° C.

  In Table 2, no. No. 3, although the component composition and finish rolling conditions are appropriate, the time until finishing cooling with cooling water after finishing rolling is too short, the ratio of polygonal ferrite grains having an aspect ratio of 1 or more and less than 4 is small. , YR after painting is high. No. 4, the component composition and finish rolling conditions are appropriate, but after finishing finish rolling, the time until cooling with cooling water is started is too long, so that pearlite is precipitated, weld toughness is low, and YR after coating is also low. high. No. No. 9, although the component composition and finish rolling conditions are appropriate, the polygonal shape having an aspect ratio of 1 or more and less than 4 since the average cooling rate from the start of cooling with cooling water to the winding after finishing finish rolling is too high Since the ratio of ferrite grains is small, that is, the two-phase separation does not proceed sufficiently, the YR after coating is high. Similarly, no. No. 10 has a too low rolling reduction during finish rolling, so the ratio of polygonal ferrite grains is still small and the YR after coating is high. No. In No. 11, after finishing rolling, since the average cooling rate from the start of cooling with cooling water to winding is too small, pearlite is deposited and YP is high after coating. No. 12 and no. No. 13 cannot obtain a desired microstructure because of an inappropriate winding temperature, so that the weld toughness is low and the YR after coating is also high. No. No. 15-18 cannot obtain a desired microstructure because the component composition is inappropriate. For this reason, weld zone toughness is low, and YR after coating is also high.

A graph showing the relationship between the Cr and Mo contents of a hot-rolled steel sheet and the post-painting yield ratio and seam toughness of an ERW steel pipe obtained by pipe-making the steel sheet

Claims (7)

In mass%, C: 0.01 to 0.1%, Si: 1.0% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.05% or less, Al: 0 0.01 to 0.10%, N: 0.01% or less, and Cr and Mo, Cr: less than 1.5%, Mo: less than 0.3%, and the following (1) Containing under the conditions satisfying the formula, the balance has a component composition consisting of iron and inevitable impurities,
It has a composite structure mainly composed of ferrite and one or more second phases selected from bainite, martensite, and retained austenite, and the number of ferrite grains having an aspect ratio of 1 or more and less than 4 is all crystal grains in the structure. A hot-rolled steel sheet for high-toughness ERW steel pipes with a low yield ratio after coating, characterized by being over 50%.
−1.11 × (Cr) ² +0.1 <Mo <−0.13 × (Cr) ² +0.3 (1)
However, Cr: Cr content (mass%)
Mo: Mo content (% by mass)   Furthermore, it contains at least one selected from the group consisting of Nb: 0.5% or less, Ti: 0.5% or less, V: 0.5% or less, Zr: 0.5% or less in mass%. The hot-rolled steel sheet for high-toughness ERW steel pipes having a low post-painting yield ratio according to claim 1.   Furthermore, it contains at least one selected from Cu: 0.5% or less, Ni: 0.5% or less, Sn: 0.5% or less, and Sb: 0.5% or less by mass%. The hot-rolled steel sheet for high toughness ERW steel pipe having a low post-painting yield ratio according to claim 1 or 2.   The coating according to any one of claims 1 to 3, further comprising at least one selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. Hot rolled steel sheet for high tough ERW steel pipe with low post-yield ratio.   The composite structure has a pearlite volume fraction of 3% or less (including 0%), and the balance is composed of ferrite and one or more second phases selected from bainite, martensite, and retained austenite. The hot-rolled steel sheet for high-toughness electric resistance welded steel pipes having a low post-painting yield ratio according to any one of claims 1 to 4. The steel having the component composition according to any one of claims 1 to 4 is heated and hot-rolled, and in the hot rolling, the rolling reduction in the temperature range of Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C is 50. After finishing the rolling, at least 0.5 second has passed and the cooling using the refrigerant is started, and the average cooling rate from the start of the cooling to the winding is 3 to 40%. A method for producing a hot-rolled steel sheet for a high toughness ERW steel pipe having a low post-painting yield ratio, characterized by being wound at 400 to 600 ° C.   A high toughness electric-welded steel pipe having a low post-painting yield ratio, obtained by pipe-making the hot-rolled steel sheet according to any one of claims 1 to 5.

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