JP5347540B2 - Thick high-tensile hot-rolled steel sheet excellent in low-temperature toughness and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a thick and high tension hot-rolled steel plate which is suitable as a raw metal for high strength electroseamed steel pipe having X65-class or higher, and excellent in low temperature toughness. <P>SOLUTION: To a steel raw material containing, by mass, 0.02-0.25% C, 0.3-2.3% Mn, 0.03-0.25% Nb, 0.001-0.10% Ti so as to satisfy (Ti+Nb/2)/C&lt;4, hot-rolling composed of rough-rolling and finish-rolling, is applied, and after completing the finish-rolling, a first process which performs accelerated cooling till becoming &le;500&deg;C at &ge;30&deg;C/s surface cooling speed, a second process which performs air-cooling within 10s after the first process, and successively, a third process which acceleratively cools till becoming in the temperature range of 350&deg;C to &lt;600&deg;C of the center of the plate, are applied in order, and this cooled steel plate is wound at the winding temperature of 350&deg;C to &lt;600&deg;C. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a thick-walled high-tensile hot-rolled steel sheet suitable for use as a raw material for high-strength welded steel pipes that require high toughness as a line pipe for transporting crude oil, natural gas, etc., and a method for producing the same. Regarding toughness improvement. Here, the “thick steel plate” refers to a steel plate having a thickness of 8.7 mm or more and 35.4 mm or less. The “steel plate” includes a steel plate and a steel strip.

  In recent years, oil and natural gas mining and pipeline laying in active areas such as the North Sea, Canada, and Alaska have become active due to soaring crude oil since the oil crisis and demands for diversifying energy supply sources. To be done. 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) needs to be a thick steel pipe, and a UOE steel pipe made of a thick steel plate is used.

Recently, however, there is a strong demand for reducing the material cost of steel pipes in accordance with the strong demand for further reduction of pipeline construction costs. For this reason, instead of UOE steel pipes made of thick steel plates, high-strength welded steel pipes made of coil-shaped hot-rolled steel sheets (hot-rolled steel strips) are used as transport pipes. It has become.
These high-strength welded 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 manufacture steel pipes that combine such strength and toughness, steel sheets that are steel pipe materials are produced by transformation strengthening using accelerated cooling after hot rolling and precipitation of alloy elements such as Nb, V, and Ti. Strengthening by precipitation strengthening using sapphire and high toughness by refinement of structure using control rolling or the like have been attempted.

  For example, Patent Document 1 relates to a steel plate for high-strength ERW steel pipe of API X70 or higher, but C: 0.03-0.08%, Mn: 1.0-1.9%, Nb: 0.005-0.05%, Ti: 0.005- Steel slabs containing 0.02%, Al: 0.01-0.07%, Ca: 0.0005-0.0040% are slab heated at 1000-1200 ° C to accelerate cooling of the steel sheet after hot rolling, and the surface temperature of the steel sheet is 500 Accelerated cooling is temporarily interrupted until the temperature falls below ℃, then reheated until the steel sheet surface temperature reaches 500 ℃ or higher, and then accelerated cooling to a temperature below 600 ℃ at a cooling rate of 3-50 ℃ / s. The manufacturing method of the steel plate for high-strength line pipes to perform is described. In the technique described in Patent Document 1, intermittent accelerated cooling is employed, whereby the temperature distribution in the plate thickness direction is made uniform, and the hardened structure generated on the surface side is subjected to a tempering process. It is supposed that the HIC resistance of the high-strength steel sheet can be improved while suppressing an increase in hardness near the surface.

Patent Document 2 relates to steel plates for high-strength steel pipes of API X60 or higher, but C: 0.03-0.08%, Mn: 1.0-1.8%, Nb: 0.005-0.05%, Ti: 0.005-0.02% After heating a steel slab containing Ca: 0.0005 to 0.0025% to 1000 to 1200 ° C. and rolling it at an austenite temperature range of 950 ° C. or less and a reduction rate of 60% or more, (Ar ₃ −50 ° C.) or more Cooling at an average cooling rate of 5-20 ° C / s at the center of the steel plate until the surface temperature of the steel plate is 500 ° C or less, and further cooling to 600 ° C or less at an average cooling rate of 5-50 ° C / s at the center of the steel plate A method for producing high strength steel is described. The technique described in Patent Literature 1 employs two-stage cooling that changes the cooling rate during cooling, and secures a desired strength while suppressing the hardness near the surface of the steel sheet.

Although it is a thick steel plate, Patent Document 3 includes C: 0.005 to 0.2%, N: 0.0001 to 0.01%, Al: 0.001 to 0.1%, Si: 0.01 to 1%, Mn: 0.1 to 2%. Furthermore, a steel slab or slab containing one or more of Ti: 0.001 to 0.1%, V: 0.001 to 0.2%, Nb: 0.001 to 0.1% is heated to 1050 to 1350 ° C, and the temperature range After holding for 20 minutes or more, rolling is started, and rolling is finished at _Ar3 point or more and 900 ° C or less, and then within 20 seconds, the steel sheet is cooled at an average cooling rate of 0.5 ° C / s or more and 300 ° C or more. Air cooling after finishing the cooling at the end of the cooling, or cooling at an _Ar3 point or less of 500 ° C. or more and further cooling the steel sheet surface with an average cooling rate of 0.5 ° C./s or more within 600 seconds. A manufacturing method is described.

JP 11-80833 A Japanese Unexamined Patent Publication No. 2000-160245 JP 2004-162076 A

  Recently, the demand for transport pipes (line pipes) has become stricter, and further enhancement of strength has been demanded, and it has been required to ensure excellent low temperature toughness with high strength of X65 grade or higher. . However, the techniques described in Patent Documents 1 to 3 produce a high-strength hot-rolled steel sheet that has both high strength capable of producing high-strength ERW steel pipes of X65 grade or higher and excellent low-temperature toughness. There was a problem that could not.

An object of the present invention is to solve the problems of the prior art and to provide a method for producing a thick-walled, high-tensile hot-rolled steel sheet capable of producing high-strength ERW steel pipe of X65 grade or higher and having excellent low-temperature toughness. And The “high strength” steel sheet here means a steel sheet having a tensile strength of 535 MPa or more, and “excellent in low temperature toughness” means the absorbed energy _v E at a test temperature of −80 ° C. in the Charpy impact test. The case where -80 is 200J or more.

  In order to achieve the above-mentioned object, the present inventors diligently studied the influence of various factors on low temperature toughness. As a result, it was newly found that cementite precipitated at the crystal grain boundaries (hereinafter also referred to as grain boundary cementite) has a great influence on the low temperature toughness. And it discovered that low temperature toughness was greatly improved by adjusting the grain boundary cementite amount to a specific value or less.

First, the experimental results on which the present invention is based will be described.
0.04-0.05% C-0.15-0.25% Si-0.90-1.20% Mn-0.005-0.007% P-0.0003-0.0006% S-0.035-0.045% Al-0.045-0.055% Nb-0.010-0.015% The steel material containing Ti was hot-rolled to obtain a hot-rolled sheet (thickness: 15.9 mm), and then various types of cooling were performed to wind it up in a coil shape. From the obtained hot-rolled sheet, a structure observation specimen, a tensile specimen, and an impact specimen were collected, and the relationship between the structure, strength, and toughness was investigated. The obtained results are shown in FIG. 1 in relation to the fracture surface transition temperature Trs ₅₀ in the Charpy impact test and the amount of grain boundary cementite. The grain boundary cementite amount was expressed as a ratio (%) of the grain boundary cementite length to the total grain boundary length.

From FIG. 1, it can be seen that by setting the grain boundary cementite amount to 10% or less, the fracture surface transition temperature Trs ₅₀ is −100 ° C. or less and the low temperature toughness can be remarkably improved. In this case, it has also been confirmed that the absorbed energy at a test temperature of −80 ° C. is 200 J or more. In addition, in order to make the grain boundary cementite amount 10% or less, it was found that it is important to adjust the cooling stop temperature of cooling to an appropriate range while setting the Nb, Ti, and C contents within an appropriate range.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.02 to 0.25%, Si: 1.0% or less, Mn: 0.3 to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03 to 0.25 %, Ti: 0.001 to 0.10%, and Nb, Ti and C are expressed by the following formula (1) (Ti + Nb / 2) / C <4 (1)
(Here, Ti, Nb, C: content of each element (mass%))
In order to obtain a hot-rolled sheet by subjecting a steel material having a composition composed of the balance Fe and inevitable impurities to hot rolling comprising rough rolling and finish rolling, the finish rolling is completed at 30 ° C. A first cooling step for accelerated cooling until the surface temperature becomes 500 ° C. or less at a surface average cooling rate of at least / s, and a second cooling step for air cooling within 10 s after completion of the first cooling step; In addition, a third cooling step is provided in which an accelerated cooling is performed to a temperature range of 350 ° C. or more and less than 600 ° C. at the center of the plate thickness at an average cooling rate of the plate thickness center of 10 ° C./s or more. later, the coiling temperature as less than 600 ° C. 350 ° C. or higher in thickness center, Ri taken up into a coil shape, the area from the surface to 1mm in thickness direction, contains martensite phase exceeding 50% by volume tensile excellent low temperature toughness characterized by be Rukoto a thick hot-rolled steel sheet having a tissue Is: In 535MPa or more, thickness: 8.7 mm or more 35.4mm thickness less than the production method of meat high strength hot-rolled steel sheet.
(2) In (1), the accelerated cooling in the third cooling step is cooling with a whole surface nucleate boiling and a heat flow rate of 1.0 Gcal / m ² hr or more. A method of manufacturing a steel sheet.
(3) In (1) or (2), in addition to the above composition, in addition to mass, V: 1.0% or less, Mo: 1.5% or less, Cr: 1.0% or less, Ni: 4.0% or less, Cu: 2.0 % Or less, a method of producing a thick high-tensile hot-rolled steel sheet.
(4) In any one of (1) to (3), in addition to the above composition, the composition further contains one or two of Ca: 0.01% or less and REM: 0.02% or less in mass%. A method for producing a thick, high-tensile hot-rolled steel sheet.
(5) By mass%, C: 0.02 to 0.25%, Si: 1.0% or less, Mn: 0.3 to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03 to 0.25 %, Ti: 0.001 to 0.10%, and Nb, Ti, C is the following formula (1)
(Ti + Nb / 2) / C <4 (1)
(Here, Ti, Nb, C: content of each element (mass%))
The composition consisting of the balance Fe and inevitable impurities, and the region from the surface to 1 mm in the thickness direction contains a martensite phase exceeding 50% by volume and precipitates at the grain boundaries. The tensile strength: excellent in low temperature toughness characterized by having a structure in which the amount of grain boundary cementite is 10% or less in terms of the ratio of grain boundary cementite length to total grain boundary length : Plate thickness: 535 MPa or more Thick, high-tensile hot-rolled steel sheet of 8.7mm to 35.4mm .
(6) In (5), in addition to the above composition, in mass%, V: 1.0% or less, Mo: 1.5% or less, Cr: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less A thick-walled high-tensile hot-rolled steel sheet characterized by having a composition containing one or more selected from the above.
(7) In (5) or (6), in addition to the above composition, the composition further comprises one or two of Ca: 0.01% or less and REM: 0.02% or less by mass%. Thick, high-tensile hot-rolled steel sheet.

According to the present invention, high strength and high toughness having both high strength of tensile strength: 535 MPa or more and high toughness of Charpy impact test temperature: absorbed energy _v E-80 at −80 ° C. of 200 J or more. Thick, high-tensile hot-rolled steel sheets can be easily manufactured, and have remarkable industrial effects. Further, by using the hot-rolled steel sheet produced according to the present invention as a raw material, there is an effect that a high-strength electric resistance welded steel pipe of X65 grade or more can be produced at low cost and stably.

It is a graph which shows the influence of the amount of grain boundary cementite which gives to low temperature toughness.

First, the reasons for limiting the composition of the steel material used will be described. Unless otherwise specified, mass% is simply expressed as%.
C: 0.02-0.25%
C is an element having an action of increasing the strength of steel, and in the present invention, it is necessary to contain 0.02% or more in order to ensure a desired high strength. On the other hand, an excessive content exceeding 0.25% increases the structural fraction of the second phase such as pearlite, and lowers the base metal toughness and the weld heat affected zone toughness. For this reason, C was limited to the range of 0.02 to 0.25%. In addition, Preferably it is 0.03-0.10%.

Si: 1.0% or less
Si acts as a deoxidizer and has the effect of increasing the strength of steel through solid solution strengthening and hardenability improvement. Such an effect is recognized when the content is 0.01% or more. On the other hand, if the content exceeds 1.0%, an oxide containing Si is formed during ERW welding, the quality of the welded portion is lowered, and the weld heat affected zone toughness is lowered. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.2 to 0.5%.

Mn: 0.3-2.3%
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 acquire such an effect, 0.3% or more of content is required. On the other hand, if the content exceeds 2.3%, weldability and HIC resistance are lowered. In addition, a large amount of Mn promotes solidification segregation during slab casting, leaving a Mn-concentrated portion in the steel sheet and increasing the occurrence of separation. 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 0.3-2.3%. In addition, Preferably it is 0.5 to 2.0%.

P: 0.03% or less P is inevitably contained as an impurity in steel, but has an effect of increasing the strength of steel. However, when it exceeds 0.03% and it contains excessively, weldability will fall. For this reason, P was limited to 0.03% or less. In addition, Preferably it is 0.01% or less.
S: 0.03% or less S is inevitably contained as an impurity in steel as with P, but if it exceeds 0.03%, it excessively causes slab cracking and coarse MnS in hot-rolled steel sheets. Forming and causing a reduction in ductility. For this reason, S was limited to 0.03% or less. In addition, Preferably it is 0.01% or less.

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 this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.08% or less.

Nb: 0.03-0.25%
Nb is an element that has the effect of suppressing the coarsening and recrystallization of austenite grains, enabling the austenite non-recrystallization temperature range rolling in hot finish rolling, and by precipitating finely as carbonitride, It has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing property. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, an excessive content exceeding 0.25% may cause 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 0.03-0.25%. In addition, Preferably it is 0.04-0.10%.

Ti: 0.001 to 0.10%
Ti has the effect of forming nitrides and fixing N to prevent cracking of slabs (steel material), and finely precipitates as carbides, thereby increasing the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.001% or more. However, when the content exceeds 0.10%, the yield point is remarkably increased by precipitation strengthening. For this reason, Ti was limited to the range of 0.001 to 0.1%. In addition, Preferably it is 0.01 to 0.08%.

In the present invention, the following formula (1) is within the above range.
(Ti + (Nb / 2)) / C <4 (1)
Nb, Ti, and C content are adjusted so as to satisfy the above.
Nb and Ti are elements that have a strong tendency to form carbides. When the C content is low, most of the C becomes carbides, and the amount of solid solution C in the ferrite grains is assumed to decrease drastically. The drastic reduction of the amount of C dissolved in the ferrite grains adversely affects the circumferential weldability of the steel pipe during pipeline construction. When circumferential welding is performed using a steel pipe manufactured using a steel sheet with extremely reduced solid solution C in the ferrite grains as a line pipe, grain growth in the heat-affected zone (HAZ) becomes significant. There is a risk that the HAZ toughness of the circumferential weld will be reduced. For this reason, in this invention, Nb, Ti, and C are adjusted and contained so that Formula (1) may be satisfied. Thereby, it becomes possible to make solid solution C amount in a ferrite grain 10 ppm or more, and can prevent the fall of the HAZ toughness of a circumference welded part. Moreover, Nb and Ti have the effect | action which suppresses precipitation of cementite to a crystal grain boundary, and, thereby, low temperature toughness improves remarkably.

The above components are basic components. In the present invention, in addition to this basic composition, V: 1.0% or less, Mo: 1.5% or less, Cr: 1.0% or less, Ni: 4.0% or less, Cu: One or two or more selected from 2.0% or less and / or one or two of Ca: 0.01% or less and REM: 0.02% or less can be selected and contained as necessary.
One or more selected from V: 1.0% or less, Mo: 1.5% or less, Cr: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less V, Mo, Cr, Ni, Cu Is an element that improves the hardenability and increases the strength of the steel sheet, and can be selected from one or more as required.

V is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet, and such an effect becomes remarkable when the content is 0.01% or more. On the other hand, excessive content exceeding 1.0% deteriorates weldability. For this reason, V is preferably 1.0% or less. In addition, More preferably, it is 0.1% or less.
Mo is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, a large content exceeding 1.5% reduces weldability. For this reason, it is preferable to limit Mo to 1.5% or less. More preferably, it is 0.1 to 1.0%.

Cr is an element that has the effect of improving hardenability and increasing the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, an excessive content exceeding 1.0% tends to cause frequent welding defects during ERW welding. For this reason, it is preferable to limit Cr to 1.0% or less. In addition, More preferably, it is 0.8% or less.
Ni is an element that has the effect of improving hardenability, increasing the strength of the steel, and improving the toughness of the steel sheet. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 4.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit Ni to 4.0% or less. More preferably, it is 0.1 to 1.0%.

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 2.0% will reduce hot workability. For this reason, it is preferable to limit Cu to 2.0% or less. More preferably, it is 0.1 to 1.0%.
One or two types of Ca: 0.01% or less, REM: 0.02% 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 obtain such an effect, it is desirable to contain Ca: 0.001% or more and REM: 0.001% or more, but a large content exceeding Ca: 0.01% and REM: 0.02% lowers the cleanliness of the steel sheet. Let For this reason, it is preferable to limit to Ca: 0.01% or less and REM: 0.02% or less.

The balance other than the components described above consists of Fe and inevitable impurities.
Inevitable impurities include O: 0.005% or less and N: 0.008% or less.
O: 0.005% or less O forms various oxides in steel and reduces hot workability, corrosion resistance, toughness and the like. For this reason, it is desirable to reduce it as much as possible. However, since extreme reduction leads to an increase in refining costs, it is acceptable up to 0.005%.

N: 0.008% or less N is an element inevitably contained in the steel, but excessive content frequently causes cracking during slab casting, so it is desirable to reduce it as much as possible, but up to 0.008% is acceptable.
In addition, as a manufacturing method of the steel material, it is preferable that the molten steel having the above composition is melted by a conventional melting method such as a converter and used as a steel material such as a slab by a conventional casting method such as a continuous casting method. However, the present invention is not limited to this.

In the present invention, a steel material having the above composition is heated and hot-rolled to obtain a hot-rolled steel sheet (steel strip).
Hot rolling is composed of rough rolling in which a steel material is heated to form a sheet bar, and finish rolling in which the sheet bar is used as a hot-rolled steel sheet.
The heating temperature of the steel material is not particularly limited as long as it can be rolled into a hot-rolled steel sheet, but it is preferably a temperature in the range of 1000 to 1300 ° C. When the heating temperature is less than 1000 ° C., the deformation resistance is high, the rolling load increases, and the load on the rolling mill becomes excessive. On the other hand, when the heating temperature is higher than 1300 ° C., the crystal grains are coarsened and the low-temperature toughness is reduced, the amount of scale generation is increased, and the yield is lowered. For this reason, it is preferable that the heating temperature in hot rolling shall be 1000-1300 degreeC. In addition, More preferably, it is 1050-1250 degreeC.

The heated steel material is roughly rolled into a sheet bar. The rough rolling conditions are not particularly limited as long as a sheet bar having a desired size and shape can be obtained.
The obtained sheet bar is further subjected to finish rolling to obtain a hot-rolled steel sheet.
In finish rolling, from the viewpoint of increasing toughness, it is preferable that the finish rolling finish temperature is (A _c3 −50 ° C.) or less, and the total reduction (%) in the temperature range of 1000 ° C. or less is 60% or more.

The hot-rolled steel sheet that has been finish-rolled is then subjected to a first cooling step, a second cooling step, and a third cooling step, and is wound into a coil after completion of the third cooling step. .
In the first cooling step, accelerated cooling is performed immediately after finishing rolling until the surface temperature reaches 500 ° C. or lower at a surface average cooling rate of 30 ° C./s or higher.
In the accelerated cooling in the first cooling step, the surface temperature is controlled. When the surface average cooling rate is less than 30 ° C./s, polygonal ferrite is precipitated, and desired high strength and high toughness cannot be achieved. A preferable surface average cooling rate is 100 to 300 ° C./s. In the first cooling step, the cooling stop temperature for accelerated cooling is set to a temperature of 500 ° C. or less at the surface temperature. If the cooling stop temperature exceeds 500 ° C., the transformation in the surface layer region may not be completed, and in the subsequent cooling step, the transformation is further transformed into a low-temperature transformation product, and the desired high toughness cannot be expected.

In the second cooling step, air cooling is performed for a time within 10 seconds after the end of the first cooling step.
During this air cooling, the surface layer is reheated by the heat held by the central portion, and the surface layer is tempered, so that the hardness of the surface layer can be reduced. Further, by air cooling, there is an effect that the cooling at the center of the plate thickness is promoted by the subsequent cooling. Even if the air cooling time is increased beyond 10 s, the effect is saturated and the productivity is lowered. For this reason, the air cooling time was limited to within 10 s. From the viewpoint of improving productivity, it is preferably 0.7 s or less.

In the third cooling step, after completion of the second cooling step, until the temperature at the center of the plate thickness reaches a temperature in the temperature range of 350 ° C. or higher and lower than 600 ° C. at an average cooling rate at the center of the plate thickness of 10 ° C./s or higher. Apply accelerated cooling. Note that the accelerated cooling in the third cooling step is center thickness temperature control.
If the average cooling rate at the center of the plate thickness is less than 10 ° C./s, polygonal ferrite and pearlite are likely to precipitate, and the desired high strength and high toughness cannot be achieved. The upper limit of the average cooling rate at the center of the plate thickness is determined depending on the ability of the cooling device to be used, but is preferably set to 100 ° C./s or less without causing deterioration of the steel plate shape such as warpage.

In addition, from the viewpoint of securing high toughness, a preferable average cooling rate at the center of the plate thickness is 25 ° C./s or more. Such cooling can be achieved by cooling (water cooling) with whole surface nucleate boiling and a heat flow rate of 1.0 Gcal / m ² hr or more.
The accelerated cooling as described above is performed until the temperature at the center of the plate thickness reaches a temperature in the temperature range of 350 ° C. or higher and lower than 600 ° C. (cooling stop temperature). If the cooling stop temperature is out of this range, the coil cannot be held for a predetermined time or more in a predetermined temperature range after being coiled after accelerated cooling, and desired high strength and high toughness cannot be ensured.

After the third cooling step, the hot-rolled steel sheet is wound in a coil shape with the coiling temperature set at 350 ° C. or higher and lower than 600 ° C. at the center of the plate thickness.
Accelerated cooling is stopped at the cooling stop temperature described above, and coiled at the winding temperature described above, it is possible to hold and stay for 30 minutes or more in a temperature range of 350 ° C or higher and lower than 600 ° C. Strengthening is promoted, and desired high strength and high toughness can be secured.

The hot-rolled steel sheet obtained by the above-described production method of the present invention has the above-described composition, and the region from the surface to the thickness direction of 1 mm contains a martensite phase exceeding 50% by volume, The grain boundary cementite amount precipitated at the grain boundary has a structure of 10% or less in terms of the ratio of the grain boundary cementite length to the total grain boundary length, and a single phase structure consisting of bainitic ferrite phase or bainite phase inside the plate ( Here, the single phase means that the volume ratio is 98% or more), tensile strength: high strength of 535 MPa or more, and absorption energy _v E-80 at a test temperature of −80 ° C. is 200 J or more. It is a thick-walled, high-tensile hot-rolled steel sheet having excellent low-temperature toughness. As used herein, “bainitic ferrite phase” includes needle-like ferrite and acicular ferrite.

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

The steel material having the composition shown in Table 1 is hot-rolled under the hot rolling conditions shown in Table 2, and after the hot rolling is completed, the steel material is cooled under the cooling conditions shown in Table 2 and coiled at the winding temperature shown in Table 2. A hot rolled steel sheet (steel strip) having a thickness shown in Table 2 was obtained.
From the obtained hot-rolled steel sheet, specimens were collected and subjected to structure observation, tensile test, impact test, and circumferential welding test to evaluate tensile properties, toughness, and circumferential weldability. 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, it is subjected to nital corrosion, and an optical microscope (magnification: 1000 times) or a scanning electron microscope (magnification: 1000). 10 times or more were observed at each position of the surface layer and the center of the plate thickness, and the type of tissue and the tissue fraction were measured. The grain boundary cementite amount was determined by measuring the total grain boundary length and the total cementite length precipitated at the grain boundary (total grain boundary cementite length) within each field of view. Using (total grain boundary length) × 100, the grain boundary cementite amount (%) was calculated.
(2) Tensile test From the obtained hot-rolled steel sheet, a tensile test was carried out at room temperature in accordance with the provisions of API-5L so that the direction perpendicular to the rolling direction (C direction) would be the longitudinal direction, yielding. Strength YS and tensile strength TS were determined.
(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, and an absorbed energy _v E-80 (J) at a test temperature: −80 ° C. was obtained.
(4) Circumferential welding test Circumferential weldability was evaluated using a y-type weld cracking test. From the obtained hot-rolled steel sheet, a test piece was sampled and subjected to test welding at room temperature in accordance with the provisions of JIS Z 3158 to produce a welded joint, and the presence or absence of cracks in the weld was examined. Circumferential weldability was evaluated with x when the crack occurred and ◯ when there was no crack.

  The obtained results are shown in Table 3.

Examples The present invention both, the tensile strength: and more high strength 535MPa, _{v E-80} has a high toughness that satisfies the above 200 J, and has a high strength and high toughness of the hot-rolled steel sheet. On the other hand, the comparative example which deviates from the scope of the present invention cannot secure desired characteristics as a material for high-strength electric resistance welded steel pipes because the desired high strength cannot be ensured or the low-temperature toughness is lowered.

Claims (7)

% By mass
C: 0.02 to 0.25%, Si: 1.0% or less,
Mn: 0.3-2.3%, P: 0.03% or less,
S: 0.03% or less, Al: 0.1% or less,
Nb: 0.03-0.25%, Ti: 0.001-0.10%
And Nb, Ti, C is contained so as to satisfy the following formula (1), and a steel material having a composition comprising the balance Fe and inevitable impurities is subjected to hot rolling consisting of rough rolling and finish rolling, In making a hot-rolled sheet,
After finishing the finish rolling, a first cooling step of accelerated cooling until the surface temperature becomes 500 ° C. or less at a surface average cooling rate of 30 ° C./s or more, and within 10 s after the completion of the first cooling step, A second cooling step for air cooling, and a third cooling step for accelerated cooling to a temperature range of 350 ° C. to less than 600 ° C. at the center of the plate thickness at an average cooling rate of the center of the plate thickness of 10 ° C./s or more. Giving,
After said third cooling step, the coiling temperature as less than 600 ° C. 350 ° C. or higher in thickness center, Ri taken up into a coil shape,
Region from the surface to 1mm in thickness direction, the low-temperature toughness excellent tensile strength, wherein to Rukoto a thick hot-rolled steel sheet having a tissue containing martensite phase exceeding 50% by volume: A method for producing thick, high-tensile hot-rolled steel sheets with a thickness of 535 MPa or more and a thickness of 8.7 mm to 35.4 mm .
Record
(Ti + Nb / 2) / C <4 (1)
Here, Ti, Nb, C: Content of each element (mass%) The accelerated cooling in the third cooling step is cooling with a whole surface nucleate boiling and a heat flow rate of 1.0 Gcal / m ² hr or more. Production method.   In addition to the above composition, one or two selected from V, 1.0% or less, Mo: 1.5% or less, Cr: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less in terms of mass% The method for producing a thick, high-tensile hot-rolled steel sheet according to claim 1 or 2, wherein the composition contains a seed or more.   The composition according to any one of claims 1 to 3, wherein in addition to the composition, the composition further contains one or two of Ca: 0.01% or less and REM: 0.02% or less by mass%. A method for producing thick, high-tensile hot-rolled steel sheets. % By mass
C: 0.02 to 0.25%, Si: 1.0% or less,
Mn: 0.3-2.3%, P: 0.03% or less,
S: 0.03% or less, Al: 0.1% or less,
Nb: 0.03-0.25%, Ti: 0.001-0.10%
Nb, Ti, and C are contained so as to satisfy the following formula (1), and the composition composed of the remaining Fe and unavoidable impurities and the region from the surface to 1 mm in the thickness direction are 50% by volume. And a structure in which the amount of grain boundary cementite precipitated in the grain boundary is less than 10% in terms of the ratio of the grain boundary cementite length to the total grain boundary length. Thick, high-tensile hot-rolled steel sheet with excellent toughness : tensile strength: 535 MPa or more and sheet thickness: 8.7 mm to 35.4 mm .
Record
(Ti + Nb / 2) / C <4 (1)
Here, Ti, Nb, C: Content of each element (mass%)   In addition to the above composition, one or two selected from V, 1.0% or less, Mo: 1.5% or less, Cr: 1.0% or less, Ni: 4.0% or less, Cu: 2.0% or less in terms of mass% The thick-walled high-tensile hot-rolled steel sheet according to claim 5, wherein the thick-walled high-tensile-rolled steel sheet has a composition containing seeds or more.   The thick high tension according to claim 5 or 6, further comprising one or two of Ca: 0.01% or less and REM: 0.02% or less in mass% in addition to the composition. Hot rolled steel sheet.

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