JP5533024B2 - Manufacturing method for thick-walled high-tensile hot-rolled steel sheet with excellent low-temperature toughness - Google Patents

Description

  The present invention relates to a method for producing a thick, high-tensile hot-rolled steel sheet suitable for use as a material for high-strength welded steel pipes that require high toughness, such as for line pipes that transport crude oil, natural gas, etc., and in particular, low-temperature toughness. Regarding improvements. The “thick steel plate” used herein refers to a steel plate having a thickness of 8.7 mm or more, preferably more than 20.0 mm 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 Document 1 adopts 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 also become more stringent, and there has been a demand for higher strength, and it has been required to ensure high strength of X65 grade or higher and excellent low temperature toughness. . 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 As used herein, “high strength” steel sheet refers to a steel sheet having a tensile strength of 535 MPa or more, and “excellent in low temperature toughness” refers to the absorbed energy vE _{− at} −80 ° C. in the Charpy impact test. ₈₀ (J) means 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-0.95-1.05% 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 vE _-80 (J) of Charpy impact test and the amount of grain boundary cementite. The grain boundary cementite amount was expressed as the ratio (%) of the grain boundary cementite length to the total grain boundary length.

FIG. 1 shows that by setting the grain boundary cementite amount to 10% or less, vE- ₈₀ (J) is 200 J or more, and the low temperature toughness can be remarkably improved. 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.
In addition, for thick steel plates with a plate thickness exceeding 20.0 mm, in order to ensure the desired material properties described above over the entire plate thickness, the structure at the center of the plate thickness should be the desired structure as described above. For this purpose, it has been found that it is particularly important to adjust the cooling time to be within a predetermined 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, C is 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 comprising the balance Fe and inevitable impurities to hot rolling consisting of rough rolling and finish rolling, A first cooling step in which the plate surface is accelerated and cooled at an average cooling rate of 20 ° C./s or more and less than a martensite formation critical cooling rate until the surface temperature becomes equal to or higher than the _Ar3 transformation point and Ms point; and the first cooling step After the completion, the second cooling step of rapidly cooling until the thickness center reaches a temperature in the temperature range of 350 ° C. or higher and lower than 600 ° C., the total cooling time of the first cooling step and the second cooling step It is adjusted so that it becomes 60 s or less, and is applied in sequence. Further, after the second cooling step, the coil is wound in a coil shape at a coiling temperature in the temperature range of 350 ° C. or more and less than 600 ° C. at the center of the plate thickness. The position of 1 / 4T to 3 / 4T in the thickness direction is 350 to 600 ° C. In in a third cooling step of performing cooling to hold or dwell more 30min, and facilities, tissue at 1mm position in the thickness direction from the surface, bainite phase or a bainitic ferrite phase is 98% or more 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, tensile strength: 535 MPa or more, Charpy impact test temperature: -80 A method for producing a thick, high-tensile hot-rolled steel sheet having excellent low-temperature toughness : 8.7 mm or more, characterized in that the hot-rolled steel sheet has an absorbed energy vE- ₈₀ (J) at 200 ° C. of 200 J or more .

(2) The thick-walled, high-tensile hot-rolled steel sheet according to (1), wherein the rapid cooling in the second cooling step is cooling with a whole surface nucleate boiling and a heat flow rate of 1.0 Gcal / m ² hr or more. Manufacturing method.
(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.

According to the present invention, it is suitable as a material for high-strength ERW steel pipe, high strength with tensile strength TS of 535 MPa or more, and high absorbed energy vE- ₈₀ (J) at −80 ° C. in Charpy impact test of 200 J or more. A thick, high-tensile hot-rolled steel sheet having toughness can be stably produced, and has a remarkable industrial effect. Further, by using the hot-rolled steel sheet produced according to the present invention as a raw material, there is also an effect that a high-strength electric resistance welded steel pipe of X65 grade or more excellent in low-temperature toughness 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 vE- _{80 of a} Charpy impact test.

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. 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, a content exceeding 2.3% decreases weldability and corrosion resistance. 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 the steel, but has the effect of increasing the strength of the 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 0.10%. In addition, Preferably it is 0.01 to 0.08%.

In the present invention, the following formula (1) (Ti + (Nb / 2)) / C <4 (1) within the above-mentioned range.
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 plate with extremely reduced amount of dissolved C in ferrite grains as a line pipe, grain growth in the heat-affected zone (HAZ) of the circumferential weld zone Becomes noticeable, and the HAZ toughness of the circumferential weld may 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.

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. 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 0.1 to 1.0%.
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 one or two of them 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, the steel material having the above composition is heated and hot-rolled to obtain a hot-rolled sheet (hot-rolled steel sheet or hot-rolled steel strip).
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.

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 sheet.
The heating temperature of the steel material is not particularly limited as long as the steel material can be rolled into a hot-rolled sheet having a desired shape and size, but is preferably set to 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.

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 sheet.
In finish rolling, from the viewpoint of increasing toughness, the finish rolling finish temperature is preferably set to a surface temperature (A _r3 −50 ° C.) or less, and the total reduction amount (%) in a temperature range of 1000 ° C. or less is set. 60% or more is preferable.

The hot-rolled sheet that has been finish-rolled is then sequentially subjected to a first cooling step, a second cooling step, and a third cooling step.
In the first cooling process, immediately after finishing rolling, the surface of the hot-rolled sheet is accelerated at an average cooling rate of 20 ° C./s or more and less than the martensite formation critical cooling rate until the surface temperature reaches the _Ars transformation point or less and the Ms point or more. Apply cooling. Here, “immediately after finishing rolling” means starting cooling within 10 seconds after finishing rolling.

In the accelerated cooling in the first cooling step, the surface temperature is controlled. When the average cooling rate on the surface of the hot-rolled sheet is less than 20 ° C./s, polygonal ferrite is precipitated, and desired high strength and high toughness cannot be achieved. The upper limit of the average cooling rate on the surface of the hot-rolled sheet is less than the martensite formation critical cooling rate for the purpose of preventing the formation of surface martensite (in the composition range of the present invention, about 100 ° C./s to about 500 ° C./s). ) Is preferable. A preferable average surface cooling rate is 50 to 100 ° C./s. In the first cooling step, the cooling stop temperature of the accelerated cooling is a surface temperature that is not higher than the _Ar3 transformation point and not lower than the Ms point. When the cooling stop temperature exceeds the _Ar3 transformation point, the transformation in the surface layer region may not be completed, and in the subsequent cooling step, it is transformed into a low-temperature transformation product, and the hardness of the surface layer cannot be reduced. Moreover, if the cooling stop temperature is lower than the Ms point, it is impossible to expect a reduction in the hardness of the desired surface layer.

In the second cooling step, after completion of the first cooling step, rapid cooling is performed until the thickness center reaches a temperature in the temperature range of 350 ° C. or higher and lower than 600 ° C. In addition, it is preferable that the cooling rate in rapid cooling shall be 10 degrees C / s or more by the average cooling rate of a plate | board thickness center position. If the average cooling rate at the center position of the plate thickness is less than 10 ° C./s, pearlite is 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 300 ° C./s or less without causing deterioration of the steel plate shape such as warpage. In addition, from the viewpoint of improving toughness, a preferable average cooling rate at the thickness center position 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 temperature at the center position of the plate thickness and the cooling rate are calculated from the plate thickness, surface temperature, and heat flow rate.

The rapid cooling as described above is performed until the temperature at the center of the plate thickness reaches a temperature of 350 ° C. or higher and lower than 600 ° C. (cooling stop temperature). If the cooling stop temperature is less than 350 ° C., subsequent normal winding is impossible. On the other hand, when the coiling temperature is 600 ° C. or higher, the crystal grains become coarse, and desired high strength and high toughness cannot be ensured.
In the present invention, the total cooling time of the first cooling step and the second cooling step is limited to 60 s or less. The total cooling time is from the end of finish rolling, which is the start time of the first cooling process, to the end of the second cooling process. Further, “after finishing rolling” means the time when the hot-rolled sheet leaves the final stand of finishing rolling.

  When the total cooling time of the first cooling step and the second cooling step is longer than 60 s, high-temperature transformation ferrite is likely to be formed at the center of the plate thickness where the cooling is delayed compared to the surface layer. The amount of cementite increases and the toughness decreases. This is particularly noticeable for thick steel plates with a thickness exceeding 20.0 mm. For this reason, the total cooling time of the first cooling step and the second cooling step was limited to 60 s or less.

Next, after the second cooling step, the hot-rolled sheet is wound in the form of a coil by adjusting the coiling temperature to a temperature of 350 to 600 ° C. at the sheet thickness center temperature, A third cooling step of holding or staying for 30 min or more in a temperature range of 350 ° C. or more and less than 600 ° C. at a position of 1 / 4T to 3 / 4T in the coil thickness direction is performed.
When the coiling temperature is less than 350 ° C., the plate temperature becomes too low, and it becomes difficult to wind into an appropriate coiling shape. On the other hand, when the coiling temperature is higher than 600 ° C., the crystal grains become coarse and desired high strength and high toughness cannot be ensured. For this reason, the coiling temperature is a temperature in the range of 350 to less than 600 ° C. at the plate thickness center temperature. In addition, Preferably it is 450-550 degreeC.

  In the third cooling step, the hot-rolled sheet wound in a coil shape is held at least at a position of 1 / 4T to 3 / 4T in the thickness direction of the coil for at least 30 min in a temperature range of 350 ° C. or higher and lower than 600 ° C. Alternatively, cooling is performed so as to stay. The rapid cooling is stopped at the above-described cooling stop temperature, and the coil is wound into the coil shape at the above-described winding temperature, so that the position of 1 / 4T to 3 / 4T in the coil thickness direction is 350 ° C. or higher just by allowing it to cool. Cooling that can be held or retained for 30 minutes or more in a temperature range of less than 600 ° C is possible, but in order to ensure such retention or retention, the coil should be heated after being wound into a coil. Or it is preferable to store in a coil box.

By cooling the coil so that it is held or retained for 30 min or more in a temperature range of 350 ° C. or higher and lower than 600 ° C., precipitation strengthening is promoted inside the steel sheet and becomes high strength, while the steel sheet surface layer is hardened by self-annealing. Decreases. Thereby, desired high intensity | strength and high toughness are securable over the plate | board thickness direction whole region.
The hot-rolled steel sheet obtained by the above-described production method of the present invention has the above-described composition, and the structure at a position of 1 mm from the surface in the thickness direction is a single phase (here, a bainite phase or a bainitic ferrite phase) In this case, the single phase is the case where the volume ratio is 98% or more), and the grain boundary cementite amount precipitated at the grain boundaries is 10% or less in terms of the ratio of the grain boundary cementite length to the total grain boundary length. It is a thick- walled high-tensile hot-rolled steel sheet. The inside of the plate has a single-phase structure consisting of bainitic ferrite phase or bainite phase (where the single phase is 98% or more by volume), and the amount of grain boundary cementite precipitated at the grain boundaries is It has a structure in which the ratio of the grain boundary cementite length to the total grain boundary length is 10% or less.

The thick-wall high-tensile hot-rolled steel sheet according to the present invention has the above-described composition and structure, high strength of tensile strength: 535 MPa or more, and excellent low-temperature toughness of absorbed energy vE- ₈₀ (J) of 200 J or more. Have. As used herein, “bainitic ferrite phase” includes needle-like ferrite and acicular ferrite. The “surface layer” refers to a region within 1 mm from the steel plate surface in the plate thickness direction.

  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 welding test to evaluate tensile properties, toughness and 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 of each visual field was observed on the surface layer, and the type of tissue and its 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 evaluated.
(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 conducted at a test temperature of −80 ° C., and the absorbed energy vE- ₈₀ (J) was obtained.
(4) Welding test Circumferential weldability was evaluated using a y-type weld cracking test. A test plate was sampled from the obtained hot-rolled steel plate, subjected to test welding in accordance with the provisions of JIS Z 3158, and examined for weld cracking. Circumferential weldability was evaluated with x when the crack occurred and ◯ when there was no crack.

  The obtained results are shown in Table 3.

Each of the inventive examples is a hot-rolled steel sheet having high tensile strength: 535 MPa or more, high toughness satisfying vE- ₈₀ of 200 J or more, and excellent circumferential weldability. On the other hand, the comparative example out of the scope of the present invention is that the desired high strength cannot be ensured, the low temperature toughness is lowered, or the circumferential weldability is lowered. As a result, desired characteristics cannot be secured.

Claims (4)

% 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 forming a hot-rolled sheet, after finishing the finish rolling, the surface of the hot-rolled sheet is accelerated at an average cooling rate of 20 ° C./s or more and less than the martensite formation critical cooling rate until the surface temperature reaches the _Ars transformation point or less and the Ms point or more. A first cooling step for cooling;
After the completion of the first cooling step, a second cooling step of rapidly cooling until the thickness center reaches a temperature in the temperature range of 350 ° C. or higher and lower than 600 ° C., the first cooling step and the second cooling step The total cooling time is adjusted so that the total cooling time is 60 s or less. After the second cooling step, the coil is coiled at a coiling temperature in the temperature range of 350 ° C. or more and less than 600 ° C. at the center thickness. A third cooling step in which winding is performed so that at least a position of 1 / 4T to 3 / 4T in the coil thickness direction is held or retained for 30 minutes or more in a temperature range of 350 to 600 ° C.,
And facilities,
The structure at a position of 1 mm from the surface in the plate thickness direction is that the bainite phase or bainitic ferrite phase is 98% or more by volume, and the amount of grain boundary cementite precipitated at the grain boundaries is the grain boundary cementite relative to the total grain boundary length. A hot rolled steel sheet having a length ratio of 10% or less, tensile strength: 535 MPa or more, Charpy impact test temperature: absorbed energy vE- ₈₀ (J) at −80 ° C. is 200 J or more <br/> A method for producing a thick, high-tensile hot-rolled steel sheet having excellent low-temperature toughness : 8.7 mm or more .
Record
(Ti + Nb / 2) / C <4 (1)
Here, Ti, Nb, C: Content of each element (mass%) The rapid cooling in the second cooling step is cooling with a whole surface nucleate boiling and a heat flow rate of 1.0 Gcal / m ² hr or more. 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 the composition further contains one or two of Ca: 0.01% or less and REM: 0.02% or less in addition to the composition. A method for producing thick, high-tensile hot-rolled steel sheets.

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