JP2012021214A - Hot-rolled high tensile steel sheet for high strength welded steel pipe for line pipe, and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-rolled high tensile steel sheet having both of high strength being a tensile strength TS of 520 Mpa or higher and high toughness, and to provide a method of producing the same.SOLUTION: The hot-rolled steel sheet has: a composition containing, by mass%, 0.04-0.08% C, ≤0.4% Si, 1.0-1.8% Mn, ≤0.1% Al, 0.02-0.08% Nb, 0.3-0.8% Cr; and a structure of mulitiple layers in a thickness direction in which a surface layer composed of a bainitic single phase and an inner layer composed of a bainitic phase as a main phase, a second phase in which a martensitic phase having an average grain diameter of ≤3 μm is dispersed in an amount of 1-4% in a volume ratio, and as a third phase, a ferrite phase in an amount of 10-30% in a volume ratio. The hot-rolled steel sheet may further contain one or two or more elements among Cu, Ni, Mo, V, Ti and B in a total of ≤1.0%. in addition to the above composition. The hot-rolled steel may further contain Ca and REM. When a pipe is made by using the hot-rolled steel sheet having such composition and structure, a high strength welded steel pipe having a low yield ratio of ≤85% and high toughness after being made into the pipe, is obtained.

Description

INDUSTRIAL APPLICABILITY The present invention is suitable as a material for welded steel pipes, such as welded steel pipes that require high strength and high toughness, especially for welded steel pipes such as high-strength ERW steel pipes or high-strength spiral steel pipes, for line pipes that transport crude oil, natural gas, etc. In particular, the present invention relates to a high-tensile hot-rolled steel sheet and a method for producing the same, and particularly to lowering the yield in the longitudinal direction of the pipe after pipe making.
Here, “high strength” means high strength of API5L-X65 grade or more and X80 grade or less. The “steel plate” includes a steel plate and a steel strip.

  In recent years, oil and natural gas mining and pipeline construction have been actively carried out in extremely cold regions such as the North Sea, Canada and Alaska due to soaring crude oil since the oil crisis and the demand for diversified energy supply sources. It has come to be. Furthermore, in the pipeline, in order to improve the transportation efficiency of natural gas and oil, there is a tendency to perform high-pressure operation with a large diameter. In order to withstand the high-pressure operation of the pipeline, the transport pipe (line pipe) must be a thick steel pipe, UOE steel pipe made of thick steel plate is used, and steel pipe of high strength grade such as API5L standard X80 is used. It is becoming used. However, recently, due to the strong demand for further reduction of pipeline construction costs and the lack of supply capacity of UOE steel pipes, there is a strong demand for reducing the material cost of steel pipes. Instead of UOE steel pipes, high-strength ERW steel pipes or high-strength spiral steel pipes made of coil-shaped hot-rolled steel sheets (hot-rolled steel strips), which are more productive and cheaper, have come to be used.

  These high-strength grade steel pipes are required to maintain excellent low-temperature toughness at the same time from the viewpoint of preventing line pipe breakage. In order to produce a steel pipe having both such high strength and high toughness, in steel sheets that are steel pipe materials, transformation strengthening using accelerated cooling after hot rolling and alloying elements such as Nb, V, Ti, etc. Strengthening by precipitation strengthening using precipitates and toughness by microstructure refinement using controlled rolling have been attempted.

  For example, in Patent Document 1, steel containing C: 0.01 to 0.07%, Si: 0.5% or less, Mn: 0.5 to 2.0%, one or more of Nb, V, and Ti is hot-rolled. A method for producing high-strength ERW steel pipe steel that is subjected to accelerated cooling that is cooled at a cooling rate of 20 ° C./s or higher and wound at a temperature of 250 ° C. or lower is described. However, in the technique described in Patent Document 1, winding at an extremely low temperature of 250 ° C. or lower is an indispensable requirement in order to increase the strength by strengthening with a hard low-temperature transformation phase. For this reason, there are many cases where it is difficult to wind in a coil shape, which causes a problem that the coil shape is deteriorated and productivity is extremely lowered.

Patent Document 2 contains appropriate amounts of C, Si, Mn, and N, and further contains Si and Mn in a range where Mn / Si satisfies 5 to 8, and further contains Nb: 0.01 to 0.1%. After rolling the steel slab, the rolling ratio of the first rolling performed at 1100 ° C or higher: 15-30%, the total rolling ratio at 1000 ° C or higher: 60% or higher, the final rolling reduction ratio: 15-30% After performing rough rolling under, once at 5 ° C. / s or more cooling rate, cooling temperature of the surface layer portion to below ₁ point Ar then the temperature of the surface layer portion at recuperation or forced heating (Ac ₃ -40 C.) to (Ac ₃ + 40 ° C.), and finish rolling was started, and finish rolling was completed under the conditions of a total rolling reduction at 950 ° C. or less: 60% or more and a rolling end temperature: Ar of ₃ points or more, Manufacture of hot-rolled steel sheets for high-strength ERW steel pipes, starting cooling within 2 s after finishing rolling, cooling to 600 ° C or lower at a rate of 10 ° C / s or higher, and winding in a temperature range of 600 to 350 ° C The method has been described. The steel sheet manufactured by the technique described in Patent Document 2 has a refined structure of the steel sheet surface layer without adding an expensive alloy element and without heat treating the entire steel pipe, resulting in low temperature toughness, particularly DWTT characteristics. An excellent high-strength ERW steel pipe can be manufactured. However, with the technique described in Patent Document 2, it is impossible to secure a desired cooling rate with a thick steel plate, and there is a problem that further improvement in cooling capacity is required to secure desired characteristics. there were.

Patent Document 3 contains appropriate amounts of C, Si, Mn, Al, and N, and further includes Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, Ti: 0.001 to 0.1%, Cu, Ni After heating a steel slab containing one or more of Mo and having a Pcm value of 0.17 or less, finish rolling is finished under conditions where the surface temperature is (A _r3 −50 ° C.) or more, A method for producing a hot-rolled steel strip for a high-strength ERW pipe excellent in low-temperature toughness and weldability that is immediately cooled after rolling and wound at a temperature of 700 ° C. or less is described.

JP 63-25916 A Japanese Patent Laid-Open No. 2001-207220 JP 2004-315957 A

Recently, steel sheets for high-strength welded steel pipes have been required to further improve low-temperature toughness, and the technique described in Patent Document 3 has a problem that the required low-temperature toughness cannot be sufficiently satisfied. It was.
Generally, with increasing strength, the yield strength increases and the yield ratio tends to increase. Therefore, there is a concern that the deformability until the structural member is broken is lowered. From the viewpoint of ensuring the desired deformability and ensuring the safety of structures such as pipelines, high strength welded steel pipes for line pipes are also required to have a reduced yield ratio, and the low yield ratio and high strength welded steel pipes are strong. It was sought after.

The present invention solves the above-mentioned problems of the prior art, combines high strength of X65 grade or higher and excellent low temperature toughness without the need for adding a large amount of alloying elements, and further yield ratio in the longitudinal direction of the pipe Is a high strength hot-rolled steel sheet suitable for use as a material for high-strength ERW steel pipes with a low yield ratio of 85% or less, and a high-strength steel sheet intended to provide a method for producing the same, has a tensile strength of TS: 520 MPa or more. Is a high-strength hot-rolled steel sheet that has both high strength and fracture surface transition temperature Trs ₅₀ : high toughness of −80 ° C. or less and can achieve a yield ratio of 85% or less after pipe forming.

  In order to achieve the above-mentioned object, the present inventors diligently studied the influence of various factors on the yield ratio. As a result, in order to realize a low yield ratio after pipe making, it was thought that it was necessary to introduce movable dislocations inside with strain introduced during pipe making. In order to introduce movable dislocations into the interior with strain introduced during pipe making, the inventors have conceived that the internal structure needs to be a structure in which an appropriate amount of fine martensite phase is dispersed. Furthermore, in order to combine the low yield ratio after pipe making with high strength and high toughness, it has been found that the steel sheet structure needs to be a multi-layer structure different in the sheet thickness direction.

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) In mass%, C: 0.04 to 0.08%, Si: 0.4% or less, Mn: 1.0 to 1.8%, Al: 0.1% or less, Nb: 0.02 to 0.08%, Cr: 0.3 to 0.8%, the balance Composition composed of Fe and unavoidable impurities, surface layer is single phase of bainite phase, inner layer is bainite phase as main phase, and second phase is martensite phase with average particle size of 3 μm or less dispersed by 1 to 4% by volume A high-tensile hot-rolled steel sheet for welded steel pipes for line pipes, characterized in that it has a multilayered structure in the thickness direction including a ferrite phase having a volume ratio of 0 to 30% as a third phase.

(2) In (1), in addition to the above composition, in terms of mass%, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B : A high-tensile hot-rolled steel sheet characterized by containing a total of 1.0% or less of one or more selected from 0.0010% or less.
(3) In (1) or (2), in addition to the above-mentioned composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less by mass% A high-tensile hot-rolled steel sheet characterized by:

(4) In mass%, C: 0.04 to 0.08%, Si: 0.4% or less, Mn: 1.0 to 1.8%, Al: 0.1% or less, Nb: 0.02 to 0.08%, Cr: 0.3 to 0.8%, the balance After heating a steel material composed of Fe and inevitable impurities, the cumulative rolling reduction in the temperature range of 950 ° C or lower is 45% or higher, and the finish rolling finish temperature is (Ar ₃ transformation point -50 ° C) or higher and 900 ° C The following hot rolling is performed, and immediately after the hot rolling is finished, the sheet is cooled to a temperature range of 400 to 600 ° C. at an average cooling rate in the range of 5 to 20 ° C./s at the cooling rate at the center position of the plate thickness. A method for producing a high-strength hot-rolled steel sheet for welded steel pipes for line pipes, which is subjected to an accelerated cooling treatment, and is subjected to a cooling treatment for cooling for at least 15 s after completion of the accelerated cooling treatment, and then wound into a coil shape .

(5) In (4), in addition to the above composition, in terms of mass%, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B : A method for producing a high-tensile hot-rolled steel sheet, characterized in that the composition contains 1.0% or less in total of one or more selected from 0.0010% or less.
(6) In (4) or (5), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less by mass%. A method for producing a high-tensile hot-rolled steel sheet.

According to the present invention, it is suitable as a material for high-strength welded steel pipes for line pipes, and has both high strength of tensile strength TS: 520 MPa or higher and fracture surface transition temperature Trs ₅₀ : high toughness of −80 ° C. or lower. A high-tensile hot-rolled steel sheet that can realize a low yield ratio of 85% or less after pipe making can be manufactured easily and inexpensively, and has a remarkable industrial effect. According to the present invention, a high-strength welded steel pipe for line pipe (electrical pipe) having both high strength of X65 grade or higher and excellent low-temperature toughness, and at least a yield ratio of 85% or less in the longitudinal direction of the pipe. There is also an effect that the sewn steel pipe) can be easily manufactured.

First, the reasons for limiting the composition of the high-tensile hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.
C: 0.04-0.08%
C is an element having an action of increasing the strength of steel. In the present invention, it is necessary to contain 0.04% or more in order to ensure a desired high strength. On the other hand, an excessive content exceeding 0.08% lowers the base metal toughness and the weld heat affected zone toughness. For this reason, C was limited to 0.04 to 0.08% of range. In addition, Preferably it is 0.05 to 0.07%.

Si: 0.4% or less
Si is an element that acts as a deoxidizer, and such an effect is recognized with a content of 0.01% or more. Moreover, Si forms an oxide containing Si during ERW welding, lowers the weld zone quality, and lowers the weld heat affected zone toughness. From this point of view, it is desirable to reduce Si as much as possible, but it is acceptable up to 0.4%. For these reasons, Si is limited to 0.4% or less.

Mn: 1.0-1.8%
Mn has the effect of improving hardenability, and increases the strength of the steel sheet through the improvement of hardenability. Further, Mn forms MnS and fixes S, thereby preventing segregation of S grain boundaries and suppressing slab (steel material) cracking. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, if the content exceeds 1.8%, solidification segregation during slab casting is promoted, Mn-concentrated portions remain in the steel sheet, and the occurrence of separation increases. In order to eliminate this Mn enriched part, it is necessary to heat to a temperature exceeding 1300 ° C., and it is not practical to carry out such a heat treatment on an industrial scale. For this reason, Mn was limited to the range of 1.0 to 1.8%. In addition, Preferably it is 1.4 to 1.8%.

Al: 0.1% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, a content exceeding 0.1% significantly impairs the cleanliness of the welded part during ERW welding. For these reasons, Al is limited to 0.1% or less.
Nb: 0.02 to 0.08%
Nb is an element that has the effect of suppressing austenite grain boundary migration, austenite grain coarsening and recrystallization, and has the effect of enabling austenite non-recrystallization temperature range rolling in hot finish rolling. It is. Moreover, Nb has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing weldability by carrying out fine precipitation as a carbonitride (precipitate). In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, an excessive content exceeding 0.08% results in an increase in rolling load during hot finish rolling, which may make hot rolling difficult. For this reason, Nb was limited to the range of 0.02 to 0.08%.

Cr: 0.3-0.8%
Cr is an element having an action of increasing the strength of the steel sheet through improvement of hardenability. In the present invention, a content of 0.3% or more is required in order to secure a desired high strength. On the other hand, a content exceeding 0.8% tends to cause frequent welding defects during resistance welding. For this reason, Cr was limited to the range of 0.3 to 0.8%.

  The above-mentioned components are basic components. If necessary, in addition to the basic composition described above, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, One or two or more selected from Ti: 0.03% or less, B: 0.0010% or less, and 1.0% or less in total, and / or Ca: 0.005% or less, REM: 0.005% or less Alternatively, one or two of them may be contained.

Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.0010% or less, or one or more selected from a total of 1.0 %Less than
Cu, Ni, Mo, V, Ti, and B are all elements that have an action of increasing the strength of steel, and can be selected and contained as necessary.
Cu is an element that has the effect of improving the hardenability and increasing the strength of the steel sheet by solid solution strengthening or precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.3% reduces hot workability. For this reason, it is preferable to limit Cu to 0.3% or less.

Ni is an element that has the effect of increasing the strength of steel through the improvement of hardenability and also improving the toughness of the steel sheet. In order to obtain such an effect, Ni should be contained in an amount of 0.01% or more. desirable. On the other hand, if the content exceeds 0.3%, the weldability is lowered and the material cost is increased. For this reason, it is preferable to limit Ni to 0.3% or less.
Mo is an element that improves hardenability and forms carbonitrides (precipitates) to increase the strength of the steel. To obtain such effects, it is contained in an amount of 0.01% or more. Although it is desirable, the content exceeding 0.3% lowers the weldability. For this reason, it is preferable to limit Mo to 0.3% or less.

V has the effect of increasing the strength with a small content without impairing the weldability by solid solution or fine precipitation as carbonitride (precipitate) in the steel. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.1% reduces weldability. For this reason, when it contains, it is preferable to limit V to 0.1% or less.
Ti has the effect of increasing strength by forming carbides and finely precipitating. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.03% degrades the toughness of the weld. For this reason, when it contains, it is preferable to limit Ti to 0.03% or less.

B is an element having an action of increasing the strength of steel through improvement of hardenability. In order to obtain such an effect, B is preferably contained in an amount of 0.0003% or more. On the other hand, if the content exceeds 0.0010%, the hardenability deteriorates. For this reason, when it contains, it is preferable to limit B to 0.0010% or less.
In addition, when Cu, Ni, Mo, V, Ti, and B are contained in combination, the total content of those elements within the above-described content range of each element is 1.0% or less. It is preferable from the viewpoint of deterioration of toughness of the weld zone.

One or two selected from Ca: 0.005% or less, REM: 0.005% or less
Both Ca and REM are elements that contribute to the control of the morphology of the sulfide, in which the expanded coarse sulfide is a spherical sulfide, and can be selected and contained as necessary. In order to acquire such an effect, it is desirable to contain 0.001% or more, but if it contains more than 0.005%, the cleanliness of the steel sheet is lowered. For this reason, it is preferable to limit both Ca and REM to 0.005% or less.
The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include P: 0.025% or less, S: 0.005% or less, N: 0.008% or less, and O: 0.005% or less.

P is inevitably contained as an impurity in the steel, but has an effect of increasing the strength of the steel. However, when it exceeds 0.025% and it contains excessively, weldability will fall. For this reason, it is preferable to limit P to 0.025% or less.
S is inevitably contained as an impurity in the steel as in the case of P. However, when it exceeds 0.005%, S excessively causes slab cracks and segregates at the grain boundaries to deteriorate the base metal toughness of the steel sheet. Moreover, coarse MnS is formed in a hot-rolled steel sheet, and separation is generated. For this reason, it is preferable to limit S to 0.005% or less.

N: 0.008% or less N is an element inevitably contained in steel, but excessive inclusion frequently causes cracks during slab casting. For this reason, it is preferable to limit N to 0.008% or less.
The high-tensile hot-rolled steel sheet of the present invention has the above-described composition, and the surface layer is a bainite phase single phase, the inner layer is a bainite phase as a main phase, and the second phase has a martensite phase with an average particle size of 3 μm or less. It is a steel sheet having a multilayer structure in the thickness direction including 1 to 4% by volume and containing a ferrite phase having a volume ratio of 0 to 30% as a third phase. As used herein, “surface layer” refers to a region of approximately 20% of the total thickness in the thickness direction from the surface, and “inner layer” refers to an internal region surrounded by the surface layer.

  The structure of the steel sheet surface layer is a single bainite phase in order to combine high strength and high toughness. The term “single phase” here refers to a case where the phase is 98% or more. On the other hand, the structure of the inner layer of the steel sheet is a structure in which a bainite phase is a main phase and a martensite phase having an average particle diameter of 3 μm or less is dispersed as a second phase. By making the main phase a bainite phase, it is possible to combine high strength and high toughness. In addition, the main phase here shall mean the phase which exists 70% or more. In addition, by dispersing a martensite phase with an average particle size of 3 μm or less, movable dislocations can be introduced due to strain during pipe forming, which reduces the yield ratio after pipe forming to a low yield ratio of 85% or less. can do. When the martensite phase becomes larger than the average particle size: 3 μm, the starting point of the movable dislocation decreases due to strain during pipe making, so that a low yield ratio becomes difficult. Further, if the fine martensite phase is less than 1%, it becomes difficult to introduce a desired amount of movable dislocations due to strain during pipe forming. On the other hand, if the content exceeds 4%, the toughness of the steel sheet decreases.

The “bainite phase” includes a bainitic ferrite phase.
In the present invention, 0 to 30% of a ferrite phase may be included as a third phase other than the main phase and the second phase. If the ferrite phase is contained in a large amount exceeding 30%, it becomes difficult to ensure a desired high strength.
The structure identification and the structure fraction are performed by polishing and corroding the cross section in the rolling direction and observing the structure with an optical microscope or the like.

Below, the preferable manufacturing method of this invention high tension hot-rolled steel plate is demonstrated.
The steel material having the above composition is hot-rolled to obtain a hot-rolled steel sheet.
As a manufacturing method of the steel material, it is preferable to melt the molten steel having the above composition by a conventional melting method such as a converter, and to make a steel material such as a slab by a conventional casting method such as a continuous casting method, The present invention is not limited to this.

The steel material having the above composition is preferably heated to 1050 to 1250 ° C.
When the heating temperature is less than 1050 ° C., the amount of increase in strength due to the solid solution of Nb and precipitation after rolling decreases, and it becomes difficult to secure a desired high strength. On the other hand, when the heating temperature is higher than 1250 ° C., the crystal grains are coarsened and the low-temperature toughness is reduced, the amount of scale generation is increased, the yield is lowered, and the surface properties may be lowered. For this reason, it is preferable that the heating temperature in hot rolling shall be 1050-1250 degreeC.

The heated steel material is then hot rolled.
Hot rolling is rolling consisting of rough rolling and finish rolling. In rough rolling, it is only necessary to secure a sheet bar having a desired size and shape, and it is not necessary to limit the conditions. Finish rolling is a rolling with a cumulative rolling reduction in a temperature range of 950 ° C. or lower of 45% or more and a finish rolling finish temperature of (Ar ₃ transformation point −50 ° C.) or higher, preferably 900 ° C. or lower.

If the cumulative reduction in the temperature range of 950 ° C. or lower in finish rolling is less than 45%, the desired high toughness cannot be achieved. In addition, Preferably it is 50 to 75%. Further, when the finish rolling finish temperature becomes a low temperature less than (Ar ₃ transformation point −50 ° C.), ferrite transformation proceeds in the surface layer during finish rolling, and the surface layer structure cannot be made into a desired bainite single phase structure. . Further, if the finish rolling finish temperature is higher than 900 ° C., the structure cannot be refined and the toughness is lowered. For this reason, the finish rolling finish temperature is preferably (Ar ₃ transformation point −50 ° C.) or more, preferably 900 ° C. or less. The temperature during rolling is the steel sheet surface temperature.

Immediately after the hot rolling is finished (within 15 seconds), cooling (accelerated cooling treatment) is performed on the runout table. Note that the cooling start temperature of the accelerated cooling treatment is preferably 750 ° C. or higher at the center thickness position. If the cooling start temperature is less than 750 ° C., at least the surface layer undergoes ferrite transformation, a desired surface layer structure cannot be ensured, and the strength and toughness decrease.
Moreover, it is preferable to make the average cooling rate of accelerated cooling processing into the range of 5-20 degrees C / s by the cooling rate of a plate | board thickness center position. Thereby, the production | generation of an excess ferrite phase and the production | generation of a pearlite phase are suppressed, and the coarsening of a crystal grain can be prevented. When the average cooling rate exceeds 20 ° C./s, it becomes difficult to form a fine martensite phase in the subsequent treatment. On the other hand, if it is less than 5 ° C./s, ferrite is excessively generated, and it becomes difficult to ensure desired high strength and high toughness. When a large amount of ferrite formed at a high temperature is formed, it becomes difficult to form a fine martensite phase.

  The cooling stop temperature in the accelerated cooling process is preferably set to a temperature in the temperature range of 600 to 400 ° C. (temperature at the center of the plate thickness). When the cooling stop temperature is a low temperature of less than 400 ° C., the formation of fine martensite phase is reduced in the subsequent treatment. On the other hand, at a high temperature exceeding 600 ° C., ferrite and pearlite are remarkably precipitated by subsequent cooling (cooling), and a desired structure cannot be secured. The cooling stop temperature is more preferably 470 to 600 ° C.

After the accelerated cooling process, a cooling process is performed. The cooling treatment time is set to 15 s or more and 40 s or less after the accelerated cooling is stopped. By performing the cooling treatment after the accelerated cooling treatment, the effect that the steel plate surface is reheated and the temperature distribution in the thickness direction is uniform can be expected. Prolonged cooling treatment causes a decrease in productivity.
After cooling for a predetermined time, the steel strip (steel plate) is wound into a coil shape and allowed to cool.

  Hereinafter, the present invention will be described in detail based on examples.

  Using the steel material having the composition shown in Table 1, hot rolling is performed under the hot rolling conditions shown in Table 2, and after the hot rolling, accelerated cooling treatment and cooling treatment are performed under the cooling conditions shown in Table 2, The coil was wound and allowed to cool.

From the obtained hot-rolled steel sheet (steel strip), specimens were collected and subjected to structure observation, tensile test, and impact test, and the structure, tensile characteristics, and toughness were investigated. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet, the cross section in the rolling direction is polished and corroded, and the optical microscope (magnification: 1000 times) or scanning electron microscope (magnification: 2000 times) is used. Two or more visual fields were observed at the surface layer (position 1 mm from the surface) and the center position of the plate thickness, and the type of tissue and the tissue fraction were measured.
(2) Tensile test JIS No. 5 test specimens were collected from the obtained hot-rolled steel sheet in accordance with the provisions of JIS Z 2201 so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction. A tensile test was performed in accordance with the provisions of Z 2241 to determine the yield strength YS and tensile strength TS.
(3) Impact test V-notch test specimens were taken from the center of the thickness of the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and conformed to the provisions of JIS Z 2242 Then, a Charpy impact test was performed to determine the fracture surface transition temperature Trs ₅₀ (° C.) and toughness was evaluated.

Furthermore, using the obtained hot-rolled steel sheet, an open pipe is formed by cold forming using a plurality of rolls, both ends of the open pipe are butt-welded and electro-welded, and an electric-welded steel pipe having an outer diameter of 508 mmφ and did. From the obtained electric resistance welded steel pipe, an arc-shaped API tensile test piece was taken so that the pipe axis direction was the tensile direction, the tensile properties (YS, TS) were measured, and the yield ratio was calculated.
The obtained results are shown in Table 3.

Each of the examples of the present invention has an appropriate structure, tensile strength: high strength of 520 MPa or more, and high toughness of Trs ₅₀ of −80 ° C. or less, and high strength ERW steel pipe of X65 class or more. It has sufficient characteristics as a hot-rolled steel sheet. Moreover, the yield ratio of the electric resistance welded steel pipe (welded steel pipe) obtained by using the hot-rolled steel sheet of the present invention was 85% or less, which was a low yield ratio steel pipe.
On the other hand, in the comparative example outside the scope of the present invention, an appropriate structure cannot be secured, and either or all of strength and toughness are lowered, and the desired characteristics as a hot rolled steel sheet for high-strength ERW steel pipe material are obtained. It is not secured.

Claims (6)

% By mass
C: 0.04 to 0.08%, Si: 0.4% or less,
Mn: 1.0 to 1.8%, Al: 0.1% or less,
Nb: 0.02-0.08%, Cr: 0.3-0.8%
And the composition consisting of the remainder Fe and inevitable impurities, the surface layer is a bainite phase single phase, the inner layer is a bainite phase as a main phase, and the second phase is a martensite phase with an average particle size of 3 μm or less in volume ratio of 1 A high-tensile hot-rolled steel sheet for welded steel pipes for line pipes, characterized by having a multi-layered structure in the thickness direction containing 4% to 3% and a ferrite phase having a volume ratio of 0 to 30% as a third phase.   In addition to the above-mentioned composition, it is further selected by mass% from Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.0010% or less. The high-tensile hot-rolled steel sheet according to claim 1, wherein the composition contains 1.0% or less of one or two or more of the total.   The composition according to claim 1 or 2, further comprising one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less in mass% in addition to the composition. The high-tensile hot-rolled steel sheet described. % By mass
C: 0.04 to 0.08%, Si: 0.4% or less,
Mn: 1.0 to 1.8%, Al: 0.1% or less,
Nb: 0.02-0.08%, Cr: 0.3-0.8%
And a steel material having a composition comprising the balance Fe and inevitable impurities, and after heating, the cumulative rolling reduction in the temperature range of 950 ° C. or lower is 45% or more, and the finish rolling finish temperature is (Ar ₃ transformation point −50 ° C. ) After performing hot rolling at 900 ° C. or less, and immediately after the hot rolling is finished, immediately after the completion of the hot rolling, a temperature range of 400 to 600 ° C. at an average cooling rate in the range of 5 to 20 ° C./s at the cooling rate at the center position of the plate thickness. A high-tensile hot-rolled steel sheet for welded steel pipes for line pipes, which is subjected to an accelerated cooling process to cool down to 15 seconds after the accelerated cooling process is completed, and is then allowed to cool for at least 15 s and then wound into a coil shape Manufacturing method.   In addition to the above-mentioned composition, it is further selected by mass% from Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.0010% or less. The method for producing a high-tensile hot-rolled steel sheet according to claim 4, wherein the composition contains 1.0% or less of one or more of the above-described one or more.   The composition according to claim 4 or 5, further comprising, in addition to the above composition, one or two selected from Ca: 0.005% or less and REM: 0.005% or less by mass%. The manufacturing method of the high tension hot-rolled steel sheet as described.