JP2010196155A - Thick, high tensile-strength hot-rolled steel sheet having excellent low temperature toughness and manufacturing method therefor - Google Patents

Abstract

The present invention provides a thick, high-tensile hot-rolled steel sheet having high strength of TS: 510 MPa or more and excellent low-temperature toughness and excellent material uniformity.
SOLUTION: C: 0.02 to 0.08%, Nb: 0.01 to 0.10%, Ti: 0.001 to 0.05%, and C, Ti and Nb satisfy (Ti + (Nb / 2)) / C <4 The steel material with the composition contained in is heated, scale-removed before rough rolling, rough-rolled into a sheet bar, further scale-removed into the sheet bar, FET set to 800-1050 ° C, and FDT Finish rolling to 750-950 ° C, then cool down to 10 ° C / s or more at the average cooling rate at the center position of the plate thickness to below the specific cooling stop temperature depending on the alloy element amount and cooling rate, and then alloy Winding below a specific winding temperature depending on the amount of elements. As a result, a thick hot rolled steel sheet having an appropriate range of black skin thickness, excellent material uniformity in the steel sheet, and excellent structure uniformity in the sheet thickness direction is obtained.
[Selection] Figure 1

Description

  The present invention is a thick-walled, high-tensile hot-rolled steel sheet suitable for use as a material for high-strength ERW steel pipes or high-strength spiral steel pipes that require high toughness for line pipes that transport crude oil, natural gas, and the like, and production thereof In particular, it relates to the improvement of low temperature toughness. The “steel plate” includes a steel plate and a steel strip. As used herein, “high-tensile hot-rolled steel sheet” refers to a hot-rolled steel sheet having a high strength of tensile strength TS: 510 MPa or more, and “thick-walled” steel sheet refers to a steel sheet having a thickness of 11 mm or more. It shall be said.

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. Also, once the development has been abandoned, the development of a corrosive sour gas field, etc., has become active.
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 has been 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 steel pipes are required to maintain excellent low-temperature toughness 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.

In addition, in line pipes used for transporting crude oil and natural gas containing hydrogen sulfide, in addition to characteristics such as high strength and high toughness, so-called hydrogen-induced crack resistance (HIC resistance), stress corrosion crack resistance, and so on It is required to have excellent sour resistance.
In response to such a request, for example, Patent Document 1 includes C: 0.005 to less than 0.030%, B: 0.0002 to 0.0100%, Ti: 0.20% or less, and Nb: 0.25% or less. Two kinds are included so as to satisfy (Ti + Nb / 2) / C: 4 or more, and further steel containing an appropriate amount of Si, Mn, P, S, Al, N is hot-rolled, and then 5 to 20 ° C./s. Toughness, cooled at a cooling rate of 550 ° C and wound up in a temperature range of more than 550 ° C to 700 ° C, the structure is composed of ferrite and / or bainitic ferrite, and the amount of solid solution C in the grain is 1.0 to 4.0 ppm A method for producing a high-strength hot-rolled steel sheet having a low yield ratio and an excellent strength has been proposed. With the technique described in Patent Document 1, a high-strength hot-rolled steel sheet having excellent toughness, weldability, and sour resistance and having a low yield ratio is obtained without causing material unevenness in the thickness direction and the length direction. You can do that. However, in the technique described in Patent Document 1, since the amount of solid solution C in the grains is 1.0 to 4.0 ppm, crystal grain growth is likely to occur due to heat input during circumferential welding, and the weld heat affected zone is coarse. There exists a problem that it becomes a grain and the toughness fall of the welding heat affected zone of a circumferential welded part tends to occur.

In Patent Document 2, C: 0.01 to 0.12%, Si: 0.5% or less, Mn: 0.5 to 1.8%, Ti: 0.010 to 0.030%, Nb: 0.01 to 0.05%, Ca: 0.0005 to 0.0050%, carbon equivalent: 0.40, Ca / O: 1.5 to 2.0 so as to satisfy, the steel slab comprising, exit hot rolled at Ar _{3 +} 100 ° C. or higher, 20 seconds After cooling, the above Ar ₃ point There has been proposed a method for producing a high-strength steel sheet excellent in hydrogen-induced cracking resistance, which is cooled from temperature, cooled to 550 to 650 ° C. within 20 seconds, and then wound up at 450 to 500 ° C. According to the technology described in Patent Document 2, API standard X60 to X70 grade steel plates for line pipe having hydrogen-induced crack resistance can be manufactured. However, in the technique described in Patent Document 2, there is a problem that it is impossible to secure a desired cooling time with a thick steel plate, and further cooling capacity needs to be improved in order to secure desired characteristics. there were.

Moreover, although it is a thick steel plate, in patent document 3, C: 0.03-0.06%, Si: 0.01-0.5%, Mn: 0.8-1.5%, S: 0.0015% or less, Al: 0.08% or less, Ca: 0.001 The steel containing up to 0.005%, O: 0.0030% or less, and containing Ca, S, O so as to satisfy a specific relationship is heated to a cooling rate of 5 ° C./s or more from the temperature above the Ar ₃ transformation point. Accelerated cooling to 400 to 600 ° C, and then immediately reheats to a steel plate surface temperature of 600 ° C or higher and a plate thickness center position temperature of 550 to 700 ° C at a heating rate of 0.5 ° C / s or higher. There has been proposed a method for producing a steel sheet for high-strength line pipe excellent in hydrogen-induced cracking resistance, in which the temperature difference between the center of the plate thickness is 20 ° C. or more. In the technique described in Patent Document 3, a steel sheet is obtained in which the fraction of the second phase in the metal structure is 3% or less, and the hardness difference between the surface layer and the center of the plate thickness is Vickers hardness within 40 points. The thick steel plate is excellent in hydrogen-induced crack resistance. However, the technique described in Patent Document 3 has a problem that a reheating process is required, the manufacturing process becomes complicated, and further arrangement of a reheating facility or the like is required.

Moreover, although it is a thick steel plate, in patent document 4, C: 0.01-0.3%, Si: 0.6% or less, Mn: 0.2-2.0%, Al: 0.06% or less, Ti: 0.005-0.035%, N: 0.001 in Ac ₁ -50 ° C. below the temperature of the slab course the after hot rolling cooling containing 0.006%, performs rolling over 2% cumulative, then heated to a temperature of less than Ac ₁ super Ac ₃ A method of manufacturing a steel material having a coarse ferrite layer on the front and back surfaces is proposed. In the technique described in Patent Document 4, it is said that it contributes to the improvement of SCC sensitivity, weather resistance, and corrosion resistance of steel materials, and further suppression of material deterioration after cold working. However, the technique described in Patent Document 4 has a problem in that a reheating process is required, the manufacturing process becomes complicated, and further installation of a reheating facility or the like is required.

Furthermore, recently, steel pipes for extremely cold regions are often required to have excellent fracture toughness, particularly CTOD characteristics and DWTT characteristics, from the viewpoint of preventing burst fracture of pipelines.
In response to such a requirement, for example, Patent Document 5 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 includes Nb. : Rolling ratio of the first rolling performed at 1100 ° C or higher after heating the steel slab containing 0.01 to 0.1%: 15-30%, Total rolling ratio at 1000 ° C or higher: 60% or higher, Rolling ratio of final rolling : After rough rolling under the condition of 15-30%, once the surface layer is cooled to a temperature of ₁ point or less at a cooling rate of 5 ° C / s or more, then the surface layer is recovered by reheating or forced heating. Finishing rolling is started when the temperature of (Ac ₃ −40 ° C.) to (Ac ₃ + 40 ° C.) reaches 950 ° C. or less, the total rolling reduction: 60% or more, and the rolling end temperature: Ar ₃ points or more Finishing finish rolling under conditions, starting cooling within 2 s after finishing rolling, cooling to 600 ° C. or less at a rate of 10 ° C./s or more, and winding in a temperature range of 600 to 350 ° C. Method of manufacturing a degree electric resistance welded steel pipe for hot rolled steel sheet is described. The steel sheet manufactured by the technique described in Patent Document 5 has a refined structure of the steel sheet surface layer without adding an expensive alloy element or 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, the technique described in Patent Document 5 has a problem that a steel plate with a large thickness cannot secure a desired cooling rate, and further cooling capacity needs to be improved in order to secure desired characteristics. there were.

Patent Document 6 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 cooled immediately after rolling and wound up and cooled at a temperature of 700 ° C. or lower is described.

Japanese Unexamined Patent Publication No. 08-319538 Japanese Unexamined Patent Publication No. 09-296216 JP 2008-056962 JP Japanese Patent Laid-Open No. 2001-240936 Japanese Patent Laid-Open No. 2001-207220 JP 2004-315957 A

Recently, however, steel sheets for high-strength ERW steel pipes are required to further improve low-temperature toughness, particularly CTOD characteristics and DWTT characteristics. The technique described in Patent Document 6 has a problem that the low-temperature toughness is not sufficient, and the excellent low-temperature toughness cannot be provided to the extent that the required CTOD characteristics and DWTT characteristics are sufficiently satisfied.
Further, from the prior art, there has been a problem that hot rolled steel sheets often have large variations in material properties at respective positions in the plate longitudinal direction and the plate width direction.

  The present invention solves the above-mentioned problems of the prior art, and does not require the addition of a large amount of alloy elements, and has a high strength of TS: 510 MPa or more, excellent low-temperature toughness, particularly excellent CTOD characteristics, and DWTT characteristics. The present invention aims to provide a thick-walled high-tensile hot-rolled steel sheet suitable for high-strength ERW steel pipe or high-strength spiral steel pipe, and a method for producing the same. It aims at the further improvement of the material uniformity of the width direction.

  The “excellent CTOD characteristics” referred to here is when the critical opening displacement CTOD value in a CTOD test conducted at a test temperature of −10 ° C. is 0.30 mm or more in accordance with the provisions of ASTM E 1290. It shall be said. The “excellent DWTT property” here is a DWTT test conducted in accordance with the provisions of ASTM E 436, and the minimum temperature (DWTT temperature) at which the ductile fracture surface ratio is 85% is −35 ° C. or less. This shall be the case.

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting low temperature toughness, particularly DWTT characteristics and CTOD characteristics. As a result, it came to mind that the DWTT characteristic and the CTOD characteristic, which are toughness tests at the full thickness, are greatly influenced by the structure uniformity in the thickness direction. And it discovered that the influence of the structure nonuniformity of the sheet thickness direction on the DWTT characteristic and the CTOD characteristic, which are toughness tests at the full thickness, was manifested by a thick material having a sheet thickness of 11 mm or more.

  Further, according to further studies by the present inventors, “excellent DWTT characteristics” and “excellent CTOD characteristics” are the average of ferrite as the main phase at a position 1 mm (surface layer portion) in the thickness direction from the surface. Difference between the crystal grain size and the average crystal grain size of ferrite as the main phase at the plate thickness center position (plate thickness center portion), ΔD is 2 μm or less, and 1 mm from the steel plate surface in the plate thickness direction (surface layer portion) The difference between the second phase structure fraction (volume ratio) and the second phase structure fraction (volume ratio) at the plate thickness center position (plate thickness center), which is ensured when ΔV is 2% or less. I found out that I can do it.

First, the experimental results on which the present invention is based will be described.
A slab composed of 0.037% C-0.20% Si-1.59% Mn-0.016% P-0.0023% S-0.041% Al-0.061% Nb-0.013% Ti-balance Fe in mass% was used as a steel material. Note that (Ti + Nb / 2) / C is 1.18.
The steel material having the above composition is heated to 1230 ° C and hot rolled to a finish rolling start temperature of 980 ° C and a finish rolling finish temperature of 800 ° C to obtain a hot rolled sheet having a thickness of 14.5 mm. After the end of rolling, accelerated cooling is applied to various cooling stop temperatures at a cooling rate of 18 ° C / s in the temperature range where the temperature at the center of the plate thickness exceeds 750 ° C, and then at various winding temperatures. Winding and hot rolled steel sheet (steel strip) were used.

  Test pieces were collected from the obtained hot-rolled steel sheet, and the structure and DWTT characteristics were investigated. The microstructure is the average crystal grain size (μm) of ferrite that is the main phase at the 1 mm position (surface layer part) in the sheet thickness direction from the steel sheet surface, and the center position (sheet thickness center part). The rate (volume%) was determined. From the measured values obtained, the average crystal grain size difference ΔD of the ferrite, which is the main phase, at the 1 mm position (surface layer portion) and the plate thickness center position (plate thickness center portion) in the plate thickness direction from the steel plate surface and the second The difference ΔV in the phase tissue fraction was calculated. The second phase is pearlite, martensite, MA (also called island martensite) or the like.

The obtained results are shown in FIG. 1 as the relationship between ΔD and ΔV on DWTT.
From FIG. 1, it was found that “excellent DWTT characteristics” in which DWTT is −35 ° C. or less can be reliably maintained when ΔD is 2 μm or less and ΔV is 2% or less.
Next, FIG. 2 shows the relationship between ΔD and ΔV and the cooling stop temperature, and FIG. 3 shows the relationship between ΔD and ΔV and the coiling temperature.

2 and 3, it is necessary to adjust the cooling stop temperature to 620 ° C. or lower and the coiling temperature to 647 ° C. or lower in order to make ΔD 2 μm or less and ΔV 2% or less. I understand that.
According to further studies by the present inventors, the cooling stop temperature and the coiling temperature necessary for ΔD to be 2 μm or less and ΔV to be 2% or less include the inclusion of alloy elements mainly affecting the bainite transformation start temperature. It was found that it was determined depending on the amount and the cooling rate from the end of hot rolling. That is, in order to set ΔD to 2 μm or less and ΔV to 2% or less, the cooling stop temperature is the temperature at the center position of the plate thickness of the steel plate,
BFS (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR
(Here, C, Mn, Cr, Mo, Cu, Ni: content of each element (mass%), CR: average cooling rate at the center position of the plate thickness of the steel sheet (° C./s))
The temperature is equal to or lower than the BFS defined in, and the coiling temperature is the temperature at the center of the plate thickness of the steel sheet.
BFS0 (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni
(Here, C, Mn, Cr, Mo, Cu, Ni: content of each element (mass%))
It is important to set the temperature below BFS0 as defined in.

Further, according to further studies by the present inventors, in order to improve the material uniformity in the longitudinal direction and width direction of the steel sheet, the thickness of the black skin (scale) formed on the surface of the hot-rolled steel sheet is within an appropriate range. I found that I needed to adjust.
Next, the experimental results that led to this finding will be described.
A slab consisting of 0.053% C-0.20% Si-1.60% Mn-0.012% P-0.0026% S-0.035% Al-0.061% Nb-0.013% Ti-0.0032% N-balance Fe is used as a steel material. did. Note that (Ti + Nb / 2) / C is 0.82.

  The steel material having the above composition was heated to 1200 ° C. and subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot rolled steel sheet (steel strip). In addition, the scale removal process was performed with the scale breaker (RSB) before the rough rolling. Also, in finish rolling, the surface blackening is performed by variously changing the descaling process by the scale breaker (FSB) before finish rolling, finishing rolling entry temperature FET, and finishing rolling exit temperature FDT. The thickness of the sheet was 15.6 mm. In addition, after the hot rolling is completed, the cooling at a cooling rate of 50 ° C./s in the temperature region where the temperature at the center position of the steel sheet is 750 ° C. or less is subjected to accelerated cooling to a cooling stop temperature: 540 ° C., Subsequently, it wound up at winding-up temperature: 520 degreeC.

Tensile test pieces (plate thickness 1 mm thickness x width 12.5 mm: GL = 25 mm) were sampled from a position 1 mm from the surface of the obtained hot-rolled steel plate in the plate thickness direction, and the tensile properties were investigated.
The obtained results are shown in FIG. 4 in relation to the tensile properties (tensile strength TS, elongation El) and black skin thickness (μm).
FIG. 4 shows that the change in the tensile properties (TS, El) of the surface layer is reduced when the thickness of the black skin is in the range of 5 to 30 μm. From this, if the black skin thickness can be adjusted to an appropriate range, the variation in the tensile properties of the surface layer will be reduced, and as a result, the variation in the material in the longitudinal direction and the width direction of the steel sheet will be reduced, and the material uniformity will be further increased. I thought it would improve.

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) By mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.5 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.01 ˜0.10%, Ti: 0.001˜0.05%, and C, Ti, Nb is expressed by 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, the average crystal grain size of the ferrite phase, which is the main phase at a position 1 mm from the steel sheet surface, and the main phase at the center position of the steel sheet The difference ΔD from the average grain size of the ferrite phase is 2 μm or less, and the second phase structure fraction (volume%) at a position 1 mm from the steel sheet surface in the thickness direction and the second thickness at the center of the thickness of the steel sheet. Thickness excellent in low temperature toughness characterized by having a structure in which the difference ΔV from the phase structure fraction (volume%) is 2% or less, and further having a black skin with a thickness of 3 to 30 μm on the steel sheet surface High tensile hot rolled steel sheet.

(2) In (1), in addition to the above composition, in terms of mass%, V: 0.01 to 0.10%, Mo: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 A thick-walled, high-tensile hot-rolled steel sheet characterized by comprising one or more of ˜0.50%.
(3) A thick-walled, high-tensile hot-rolled steel sheet according to (1) or (2), further comprising, in addition to the above composition, Ca: 0.0005 to 0.005% by mass.

(4) By mass%, C: 0.02 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.5 to 1.8%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.01 ˜0.10%, Ti: 0.001˜0.05%, and C, Ti, Nb is expressed by the following formula (1) (Ti + (Nb / 2)) / C <4 (1)
(Here, Ti, Nb, C: content of each element (mass%))
In order to heat the steel material of the composition consisting of the remaining Fe and inevitable impurities, hot rolling consisting of rough rolling and finish rolling to form a hot rolled steel sheet, before the rough rolling and Before the finish rolling, a scale removal process is performed by a scale breaker, and the finish rolling entry temperature FET is set to 800 to 1050 ° C., and the finish rolling exit temperature FDT is set to 750 to 950 ° C. After the hot rolling, cooling at 10 ° C./s or more at the average cooling rate at the plate thickness center position of the steel plate is performed at the temperature at the plate thickness center position by the following equation (2):
BFS (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR (2)
(Here, C, Mn, Cr, Mo, Cu, Ni: content of each element (mass%), CR: average cooling rate at the center position of the plate thickness of the steel sheet (° C./s))
Accelerated cooling is performed to the cooling stop temperature below BFS defined in, and then the temperature at the center of the plate thickness of the steel plate,
BFS0 (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni (3)
(Here, C, Mn, Cr, Mo, Cu, Ni: content of each element (mass%))
A method for producing a thick-walled, high-tensile hot-rolled steel sheet excellent in low-temperature toughness, characterized by winding at a winding temperature of BFS0 or less as defined in 1.

(5) In (4), in addition to the above composition, in terms of mass%, V: 0.01 to 0.10%, Mo: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 A method for producing a thick-walled, high-tensile hot-rolled steel sheet, characterized in that the composition contains one or more of ˜0.50%.
(6) In (4) or (5), in addition to the said composition, it is set as the composition which contains further Ca: 0.0005-0.005% by the mass%, The manufacturing method of the thick-wall high tension hot-rolled steel plate characterized by the above-mentioned. .

  According to the present invention, there is little material variation in the plate longitudinal direction and plate width direction, excellent material uniformity, and there is little structure variation in the plate thickness direction, and low-temperature toughness, especially DWTT characteristics and CTOD characteristics are excellent. A high-tensile hot-rolled steel sheet can be manufactured easily and at a low cost, and there is a remarkable industrial effect. In addition, according to the present invention, there is also an effect that an ERW steel pipe for line pipe and a spiral steel pipe for line pipe, which are excellent in low temperature toughness and circumferential weldability when laying a pipeline, can be easily manufactured.

It is a graph which shows the relationship between (DELTA) D and (DELTA) V which acts on DWTT. It is a graph which shows the relationship between (DELTA) D and (DELTA) V and the cooling stop temperature of accelerated cooling. It is a graph which shows the relationship between (DELTA) D and (DELTA) V and coiling temperature. It is a graph which shows the influence of the black skin thickness which acts on the tensile characteristic of a surface layer.

First, the reasons for limiting the composition of the thick-walled high-tensile hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.
C: 0.02 to 0.08%
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 0.08% 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 0.08%. In addition, Preferably it is 0.02 to 0.05%. More preferably, it is 0.04 to 0.06%.

Si: 0.01-0.50%
Si has an action of increasing the strength of steel through solid solution strengthening and improvement of hardenability. Such an effect is recognized when the content is 0.01% or more. On the other hand, Si has the effect of concentrating C in the γ phase during the γ → α transformation and promoting the formation of the martensite phase as the second phase, resulting in an increase in ΔD and a decrease in the toughness of the steel sheet. . 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 up to 0.50% is acceptable. For these reasons, Si was limited to 0.01 to 0.50%. Preferably it is 0.40% or less.

In addition, since hot rolled steel sheets for ERW welded steel pipes contain Mn, Si forms a low-melting point Mn silicate and facilitates oxide discharge from the weld zone, so Si is contained in an amount of about 0.10 to 0.30%. You may let them.
Mn: 0.5-1.8%
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 acquire such an effect, 0.5% or more of content 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 0.5 to 1.8%. In addition, Preferably it is 0.9 to 1.7%.

P: 0.025% 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 0.025% and it contains excessively, weldability will fall. For this reason, P was limited to 0.025% or less. In addition, Preferably it is 0.015% or less.
S: 0.005% or less S is inevitably contained as an impurity in steel like P, but if it exceeds 0.005% and excessively contained, slab cracking occurs and coarse MnS is contained in the hot-rolled steel sheet. Forming and causing a reduction in ductility. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.004% or less.

Al: 0.005-0.10%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.005% or more. On the other hand, the content exceeding 0.10% significantly impairs the cleanliness of the welded part during ERW welding. For this reason, Al was limited to 0.005 to 0.10%. In addition, Preferably it is 0.08% or less.

Nb: 0.01-0.10%
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 acquire such an effect, 0.01% or more of content is required. On the other hand, an excessive content exceeding 0.10% 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 0.01 to 0.10%. In addition, Preferably it is 0.03-0.09%.

Ti: 0.001 to 0.05%
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 0.05%, the yield point is remarkably increased by precipitation strengthening. For this reason, Ti was limited to the range of 0.001 to 0.05%. In addition, Preferably it is 0.005-0.035%.

In the present invention, Nb, Ti, and C in the above ranges are included, and the following formula (1) (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. However, the drastic decrease in the amount of C dissolved in ferrite grains adversely affects the circumferential weldability during pipeline construction. The reason is that when circumferential welding is performed using a steel pipe manufactured using a steel sheet in which the amount of dissolved C in the ferrite grain is extremely reduced as a line pipe, the grain in the heat-affected zone of the circumferential welded part This is because the growth becomes remarkable and the heat-affected zone toughness of the circumferential weld may be lowered. For this reason, in the present invention, Nb, Ti, and C are contained so as to satisfy the formula (1). 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 heat affected zone toughness of a circumference welded part. In order to suppress the strength reduction of the welded portion, Nb, Ti, and C are expressed by the following formula (1a) (Ti + (Nb / 2)) / C ≦ 3 (1a)
It is preferable to make it contain so that it may satisfy.

In the present invention, the above components are basic components. In addition to this basic composition, V: 0.01 to 0.10%, Mo: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Cu: One or more of 0.01 to 0.50%, Ni: 0.01 to 0.50%, and / or Ca: 0.0005 to 0.005% can be selected and contained as necessary.
One or more of V: 0.01 to 0.10%, Mo: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50% V, Mo, Cr, Cu , Ni 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 0.10% deteriorates weldability. For this reason, V is preferably 0.01 to 0.10%. Further, it is more preferably 0.03 to 0.08%.
Mo is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet. In order to obtain such an effect, it is preferable to contain 0.01% or more. On the other hand, a large content exceeding 0.50% reduces weldability. For this reason, it is preferable to limit Mo to 0.01 to 0.50%. In addition, More preferably, it is 0.05 to 0.30%.

Cr is an element that has the effect of improving hardenability and increasing the strength of the steel sheet. In order to acquire such an effect, it is desirable to contain 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 0.01 to 1.0%. In addition, More preferably, it is 0.01 to 0.80%.
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 0.50% reduces hot workability. For this reason, it is preferable to limit Cu to 0.01 to 0.50%. In addition, More preferably, it is 0.10 to 0.40%.

  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 0.50%, 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 0.01 to 0.50%. In addition, More preferably, it is 0.10 to 0.45%.

Ca: 0.0005 to 0.005%
Ca has the action of fixing S as CaS, spheroidizing sulfide inclusions, and controlling the form of inclusions, reducing the lattice strain of the matrix surrounding inclusions, and reducing the hydrogen trapping ability It is an element which has the effect | action to make. In order to acquire such an effect, it is desirable to make it contain 0.0005% or more, but if it contains more than 0.005%, CaO will increase and corrosion resistance and toughness will be reduced. For this reason, when it contains Ca, it is preferable to limit to 0.0005 to 0.005%. In addition, More preferably, it is 0.0009 to 0.003%.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include N: 0.005% or less, O: 0.005% or less, Mg: 0.003% or less, and Sn: 0.005% or less.
N: 0.005% or less N is inevitably contained in steel, but excessive inclusion frequently causes cracking during casting of a steel material (slab). For this reason, it is desirable to limit N to 0.005% or less. More preferably, it is 0.004% or less.

O: 0.005% or less O exists as various oxides in steel, and causes hot workability, corrosion resistance, toughness and the like to decrease. For this reason, it is desirable to reduce as much as possible in the present invention, but it is acceptable up to 0.005%. Since extreme reduction leads to an increase in refining costs, it is desirable to limit O to 0.005% or less.

Mg: 0.003% or less
Mg, like Ca, forms oxides and sulfides and has the effect of suppressing the formation of coarse MnS, but if it exceeds 0.003%, Mg oxide and Mg sulfide clusters occur frequently, and toughness Cause a decline. For this reason, it is desirable to limit Mg to 0.003% or less.
Sn: 0.005% or less
Sn is mixed from scraps used as steelmaking raw materials. Sn is an element that easily segregates at grain boundaries and the like, and if it is contained in a large amount exceeding 0.005%, the grain boundary strength is lowered and the toughness is lowered. For this reason, it is desirable to limit Sn to 0.005% or less.

  The thick-walled high-tensile hot-rolled steel sheet of the present invention has the above-described composition, and further, the average crystal grain size of the ferrite phase, which is the main phase at a position of 1 mm in the sheet thickness direction from the sheet surface, and the sheet thickness center position. The difference ΔD from the average crystal grain size (μm) of the ferrite phase, which is the main phase, is 2 μm or less, and the structure fraction (volume%) of the second phase at a position 1 mm from the steel sheet surface in the sheet thickness direction It has a structure having a difference ΔV of 2% or less with respect to the structure fraction (volume%) of the second phase at the plate thickness center position.

  The “ferrite phase as the main phase” here means that the main structure (main phase) in the present invention is composed of a hard low-temperature transformation ferrite phase, and the hard low-temperature transformation ferrite phase is bainitic. It refers to a ferrite phase, a bainite phase, or a mixed phase thereof. Here, the soft high temperature transformation ferrite phase is not included. The second phase other than the main phase is a pearlite phase, a martensite phase, an MA phase (also referred to as an island martensite phase) and a mixed phase thereof.

  Only when ΔD is 2 μm or less and ΔV is 2% or less, the low-temperature toughness of the thick, high-tensile hot-rolled steel sheet, in particular, the DWTT characteristics and CTOD characteristics using a full-thickness test piece are significantly improved. When any one of ΔD and ΔV falls outside the above range, as is apparent from FIG. 1, DWTT is higher than −35 ° C., DWTT characteristics are lowered, and low temperature toughness is deteriorated. For this reason, in the present invention, the structure of the ferrite phase, which is the main phase at the center position of the steel sheet thickness, and the average crystal grain size of the ferrite phase, which is the main phase at a position 1 mm from the steel sheet surface in the thickness direction, is obtained. The difference ΔD from the average crystal grain size (μm) is 2 μm or less, and the structure fraction (volume%) of the second phase at a position of 1 mm from the steel sheet surface in the sheet thickness direction and the second phase at the center position of the sheet thickness of the steel sheet. The difference ΔV with respect to the tissue fraction (volume%) was limited to a tissue of 2% or less.

  Note that a hot-rolled steel sheet having a structure in which ΔD is 2 μm or less and ΔV is 2% or less has a ferrite phase that is a main phase at a position of 1 mm in the sheet thickness direction from the sheet surface and a 1/4 position of the sheet thickness. The difference ΔD * in the average grain size (μm) is 2 μm or less, the difference ΔV * in the structure fraction (%) in the second phase is 2% or less, and the position 1 mm from the steel sheet surface in the thickness direction The difference ΔD ** in the average crystal grain size (μm) of the ferrite phase, the main phase, from the 3/4 thickness position is also 2 μm or less, and the difference ΔV ** in the second phase structure fraction (%) is also 2% or less. Make sure you are satisfied.

Furthermore, the thick high-tensile hot-rolled steel sheet of the present invention has a uniform black skin with a thickness in the range of 3 to 30 μm on the surface.
When the thickness of the black skin formed on the surface is less than 3 μm, the heat transfer coefficient is reduced as compared with the case where the thickness is larger than that, and as a result, the tensile strength is reduced as shown in FIG. This causes an increase in the cooling stop temperature at the center position of the plate thickness and causes a decrease in toughness. In addition, if there is a portion where the black skin having a thickness of less than 3 μm is thin, uneven cooling occurs, resulting in local strength reduction. On the other hand, when the thickness of the black skin exceeds 30 μm, the heat transfer coefficient increases as compared with the case where the thickness of the black skin is smaller than that, and the tensile strength TS is increased as shown in FIG. This leads to excessive increase in strength and causes a decrease in toughness. In addition, if there is a thick portion exceeding 30 μm, cooling unevenness occurs, causing a local increase in strength and a decrease in ductility. For this reason, the thickness of the black skin formed on the surface is limited to a range of 3 to 30 μm. By adjusting the thickness of the black skin formed on the surface within this range, variations in strength and ductility at each position in the steel sheet are reduced, and the uniformity of the material at each position in the steel sheet is improved.

Below, the preferable manufacturing method of this invention hot-rolled steel plate is demonstrated.
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 heated and hot-rolled. Hot rolling consists of rough rolling using a steel material as a sheet bar and finish rolling using the sheet bar as a hot-rolled sheet.

  The heating temperature of the steel material is not particularly limited as long as it can be rolled into a hot-rolled sheet, but it is preferably a temperature in the range of 1100 to 1300 ° C. When the heating temperature is less than 1100 ° 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 1100-1300 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. From the viewpoint of securing low temperature toughness, the rolling end temperature of rough rolling is preferably 1050 ° C. or lower. In the present invention, prior to rough rolling, a scale removal treatment is performed to remove the primary scale generated on the surface of the steel material by heating with a scale breaker RSB for a rough rolling mill. The scale removal treatment may be performed a plurality of times during rough rolling in addition to before rough rolling. In order to adjust the black skin thickness of the product (hot rolled sheet) to an appropriate range, it is desirable to avoid the use of an excessive scale breaker.

The obtained sheet bar is further subjected to finish rolling. The surface temperature is used as the temperature in finish rolling.
In finish rolling, it is preferable that the inlet temperature FET is 800 to 1050 ° C. and the outlet temperature FDT is 750 to 950 ° C. If the entry temperature FET of finish rolling is less than 800 ° C, the vicinity of the surface may be cooled too much to be less than the _Ar3 transformation point, and the structure in the plate thickness direction becomes non-uniform and the toughness decreases. On the other hand, when the FET exceeds 1050 ° C., a secondary scale may be generated in the finish rolling mill, and it becomes difficult to adjust the thickness of the black skin within a desired appropriate range. Further, when the exit side temperature FDT of finish rolling is less than 750 ° C., the vicinity of the surface may be less than the _Ar3 transformation point, and the structure in the plate thickness direction becomes non-uniform and the toughness decreases. On the other hand, when the FDT exceeds 950 ° C. and becomes a high temperature, a secondary scale is generated in the finish rolling mill, and it becomes difficult to adjust the thickness of the black skin within a desired appropriate range.

  In addition, it is preferable to adjust the entrance temperature of finish rolling by performing accelerated cooling on the sheet bar before finish rolling, or performing oscillation on a table. Thereby, the reduction rate in the temperature range effective for high toughness in the finish rolling mill can be increased. Moreover, in this invention, the scale removal process which removes the secondary scale formed in the sheet bar using the scale breaker FSB for finish rolling mills is performed on the sheet bar before finish rolling. The scale removal treatment may be performed a plurality of times by cooling between stands of the finishing mill in addition to finishing rolling. In addition, it is preferable that the temperature of the sheet bar at the time of performing the descaling process is a temperature in the range of 800 to 1050 ° C. In order to adjust the black skin thickness of the product (hot rolled sheet) to an appropriate range, it is desirable to avoid the use of an excessive scale breaker. Also by this scale removal process, the entry temperature of finish rolling can be adjusted.

In finish rolling, it is preferable that the effective rolling reduction is 20% or more from the viewpoint of increasing toughness. Here, the “effective reduction ratio” refers to the total reduction amount (%) in the temperature range of 950 ° C. or lower. In order to achieve the desired high toughness over the entire plate thickness, it is preferable that the effective rolling reduction at the plate thickness center position satisfies 20% or more.
After the hot rolling (finish rolling), the hot-rolled sheet is preferably subjected to accelerated cooling on a hot run table. It is desirable to start the accelerated cooling while the temperature at the center position of the plate thickness is 750 ° C. or higher. When the temperature at the center of the plate thickness is less than 750 ° C., high-temperature transformation ferrite (polygonal ferrite) is formed, and a second phase is formed around the polygonal ferrite due to C discharged during the γ → α transformation. For this reason, the structure fraction of the second phase becomes high at the plate thickness center position, and the above-described desired structure cannot be formed.

Accelerated cooling is preferably performed at a cooling rate of 10 ° C./s or more at the average cooling rate at the center position of the plate thickness up to a cooling stop temperature of BFS or less. In addition, let an average cooling rate be the average of a temperature range of 750-650 degreeC.
If the cooling rate is less than 10 ° C./s, high-temperature transformation ferrite (polygonal ferrite) is likely to be formed, and the second phase structure fraction becomes high at the center of the plate thickness, making it impossible to form the desired structure described above. For this reason, it is preferable to perform accelerated cooling after completion | finish of hot rolling with the cooling rate of 10 degrees C / s or more by the average cooling rate of a plate | board thickness center position. In addition, More preferably, it is 20 degrees C / s or more. In addition, although the upper limit of a cooling rate is determined depending on the capability of the cooling device to be used, it is preferable that it is slower than the martensite production cooling rate which is a cooling rate without the deterioration of steel plate shapes, such as curvature. In addition, such a cooling rate can be achieved by a water cooling device using a flat nozzle, a rod-like nozzle, a circular tube nozzle, or the like. In the present invention, the temperature at the center of the plate thickness, the cooling rate, the coiling temperature, and the like are those calculated by heat transfer calculation or the like.

Further, the cooling stop temperature of the above-described accelerated cooling is preferably set to a temperature at the center position of the plate thickness or lower than BFS. In addition, More preferably, it is (BFS-20 degreeC) or less. BFS is the following equation (2)
BFS (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR (2)
(Here, C, Mn, Cr, Mo, Cu, Ni: content of each element (mass%), CR: average cooling rate at the center position of the plate thickness (° C / s))
Defined by In addition, after accelerating cooling is stopped below the above-described cooling stop temperature, the hot-rolled sheet is wound in a coil shape at a coiling temperature equal to or lower than BFS0 at the temperature at the center of the sheet thickness. More preferably, it is (BFS 0-20 ° C.) or less. BFS0 is the following formula (3)
BFS0 (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni (3)
(Here, C, Mn, Cr, Mo, Cu, Ni: content of each element (mass%))
Defined by

  By setting the cooling stop temperature for accelerated cooling to BFS or lower and the coiling temperature to BFS0 or lower, as shown in FIGS. 2 and 3, ΔD is 2 μm or less and ΔV is 2% or less for the first time. In addition, the uniformity of the structure in the thickness direction becomes remarkable. Thereby, the excellent DWTT characteristic and the outstanding CTOD characteristic can be ensured, and it can be set as the thick-walled high-tensile-strength hot-rolled steel plate which markedly improved low-temperature toughness.

  In addition, it is preferable that the hot-rolled sheet wound up in a coil shape is cooled to room temperature at 20 to 60 ° C./hr at a cooling rate at the coil central part (coil longitudinal direction central part). If the cooling rate is less than 20 ° C./hr, the growth of crystal grains proceeds, so that the toughness may decrease. Further, at a cooling rate exceeding 60 ° C./hr, the temperature difference between the coil central portion and the coil outer peripheral portion or inner peripheral portion becomes large, and the coil shape tends to deteriorate.

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

  Using a slab (steel material) (thickness: 230 mm) having the composition shown in Table 1, hot rolling is performed under the hot rolling conditions shown in Table 2, and after completion of the hot rolling, cooling is performed under the cooling conditions shown in Table 2. And it wound up in coil shape at the coiling temperature shown in Table 2, and it was set as the hot-rolled steel plate (steel strip) of the board thickness shown in Table 2. Using these hot-rolled steel sheets as the raw material, open pipes were formed by continuous roll forming in the cold, and the end faces of the open pipes were electro-welded to form electric-welded steel pipes (outer diameter 660 mmφ).

Test pieces were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, impact test, DWTT test, and CTOD test. The DWTT test and CTOD test were also conducted on ERW steel pipes. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation was collected from the obtained hot-rolled steel sheet, the cross section in the rolling direction was polished and corroded, and the optical microscope (magnification: 1000 times) or scanning electron microscope (magnification: 1000 times) was used. Observe at least two fields of view, take an image, and use an image analyzer to determine the average grain size of the ferrite phase as the main phase and the fraction of the second phase other than the ferrite phase as the main phase (volume%) Was measured. The observation position was a position of 1 mm in the thickness direction from the surface of the steel plate and a central position of the thickness. The average crystal grain size of the ferrite phase as the main phase was determined by the cutting method, and the nominal grain size was defined as the average crystal grain size at this position.

  In addition, a test piece for measuring the thickness of the black skin is obtained from each position in the longitudinal direction of the obtained hot-rolled steel sheet (4 locations at intervals of 40 m in the length direction) and from each position in the width direction (4 locations at intervals of 0.4 m in the width direction). The sample was collected, the cross section in the rolling direction was polished, and the thickness of the black skin was measured using an optical microscope or a scanning electron microscope. Of the obtained average value of the black skin thickness, the average black skin thickness ts, and the black skin thickness at each position, a difference Δts between the maximum value and the minimum value was calculated.

(2) Tensile test The obtained hot-rolled steel sheet is orthogonal to the rolling direction from each position in the longitudinal direction (4 places at intervals of 40 m in the length direction) and each position in the width direction (4 places at intervals of 0.4 m in the width direction). Take a plate-shaped test piece (parallel part width: 25 mm, distance between gauge points: 50 mm) so that the direction (C direction) is the longitudinal direction, and pull it at room temperature in accordance with the provisions of ASTM E8M-04. A test was conducted to determine the tensile strength TS. The difference between the minimum value and the maximum value was obtained from the obtained tensile strength TS at each position, and the variation ΔTS was used to evaluate the variation in tensile strength at each position of the steel sheet. The case where ΔTS was 35 MPa or less was assumed to be uniform.

(3) Impact test From the center position of the plate thickness of each position in the longitudinal direction of the obtained hot-rolled steel sheet (4 places at intervals of 40 m in the length direction) and each position in the width direction (4 places at intervals of 0.4 m in the width direction), V-notch test specimens are collected so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction, Charpy impact test is performed in accordance with the provisions of JIS Z 2242, and the test temperature is absorbed at −80 ° C. The energy (J) was determined. The number of test pieces was three, and the arithmetic average of the obtained absorbed energy values was obtained to obtain the absorbed energy value vE- ₈₀ (J) of the steel sheet. The case where vE- ₈₀ was 300 J or more was evaluated as “good toughness”. Further, the vE _-80 at each position obtained, obtains a difference between the minimum and maximum values, and variations [Delta] VE _-80, it was evaluated variations in toughness at the steel sheet each position. The case where ΔvE- ₈₀ was 45 J or less was assumed to be uniform.

(4) DWTT test From the obtained hot-rolled steel sheet, a DWTT test piece (size: plate thickness x width 3 in. X length 12 in.) Was set so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction. The sample was collected and subjected to a DWTT test in accordance with ASTM E 436, and the lowest temperature (DWTT) at which the ductile fracture surface ratio was 85% was determined. The case where DWTT was −35 ° C. or less was evaluated as having [excellent DWTT characteristics].

In the DWTT test, a DWTT test piece was sampled from the base material portion of the ERW steel pipe so that the longitudinal direction of the test piece became the pipe circumferential direction, and tested in the same manner as the steel plate.
(5) CTOD test From the obtained hot-rolled steel sheet, a CTOD test piece (size: thickness t × width (2 × t) × length so that the direction orthogonal to the rolling direction (C direction) is the longitudinal direction. (10 × t)) was collected, and a CTOD test was conducted at a test temperature of −10 ° C. in accordance with the provisions of ASTM E 1290 to determine a critical opening displacement (CTOD value) at −10 ° C. The test load was applied by a three-point bending method, a displacement meter was attached to the notch, and the critical opening displacement CTOD value was obtained. The case where the CTOD value was 0.30 mm or more was evaluated as having “excellent CTOD characteristics”.

In addition, CTOD test is also taken from the ERW steel pipe, so that the direction orthogonal to the tube axis direction is the longitudinal direction of the test piece, CTOD test piece is taken, and the notch is introduced into the base metal part and the seam part, Tested in the same manner as the steel sheet.
The obtained results are shown in Table 3.

Each of the examples of the present invention has a black skin with an appropriate thickness and an appropriate structure, TS: high strength of 510 MPa or more, vE- ₈₀ of 300 J or more, CTOD value of 0.30 mm or more, −35 ° C. or less. DWTT and excellent low-temperature toughness, and it is a hot-rolled steel sheet that has a uniform material with little material variation in the longitudinal and width directions of the sheet, and has particularly excellent CTOD characteristics and excellent DWTT characteristics. is doing. The ERW steel pipe using the hot-rolled steel sheet of the present invention also has a CTOD value of 0.30 mm or more and a DWTT of −20 ° C. or less for both the base metal part and the seam part, and has excellent low temperature toughness. Yes.

On the other hand, comparative examples that are out of the scope of the present invention show that the low temperature toughness is reduced as vE- ₈₀ is less than 300 J, the CTOD value is less than 0.30 mm, or the DWTT exceeds -35 ° C. Furthermore, there is a variation in the thickness of the black skin, and the material variation in the plate longitudinal direction and the plate width direction is large. In the comparative example (steel plate No. 5) in which the cooling rate of accelerated cooling after the end of hot rolling is out of the range of the present invention is low, the difference ΔV in the structure fraction of the second phase exceeds 2%, and the low temperature toughness decreases. ing. Further, in the comparative example (steel plate No. 4) in which the cooling stop temperature of the accelerated cooling is out of the range of the present invention, the average black skin thickness exceeds 30 μm, and the black skin thickness varies, ΔD exceeds 2 μm and the low temperature toughness is reduced. Also, the tensile strength variation ΔTS is large. In the comparative example (steel plate No. 3) in which the cooling rate of accelerated cooling is out of the range of the present invention and the coiling temperature is out of the range of the present invention, the average black skin thickness is less than 3 μm and ΔV is 2 %, The low-temperature toughness is reduced. In addition, in the comparative example (steel plate No. 7) in which the scale breaker is not removed before the rough rolling, the average black skin thickness exceeds 30 μm, and the black skin thickness varies, and the tensile strength The variation ΔTS is also large. In addition, in the comparative example (steel plate No. 8) in which the scale removal treatment was not performed with the scale breaker before the finish rolling and the winding temperature was outside the range of the present invention, the average black skin thickness exceeded 30 μm. The black skin thickness varies, and the tensile strength variation ΔTS is also large. Furthermore, ΔD exceeds 2 μm, ΔV exceeds 2%, and low temperature toughness is reduced. Further, in the comparative example (steel plate No. 15) having a composition in which the Si and Nb contents are outside the scope of the present invention and does not satisfy the formula (1), ΔD exceeds 2 μm, and the low temperature toughness is lowered. Moreover, the low temperature toughness of the base material part and seam part of the electric resistance welded steel pipe manufactured using this steel plate is also lowered.

Claims (6)

% By mass
C: 0.02 to 0.08%, Si: 0.01 to 0.50%,
Mn: 0.5 to 1.8%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
Nb: 0.01-0.10%, Ti: 0.001-0.05%
And a composition comprising C, Ti, Nb so as to satisfy the following formula (1), the balance Fe and unavoidable impurities, and a ferrite phase which is a main phase at a position of 1 mm from the steel sheet surface in the thickness direction The difference ΔD between the average crystal grain size of the steel sheet and the average crystal grain size of the ferrite phase, which is the main phase at the center position of the steel sheet, is 2 μm or less, and the microstructure of the second phase at a position 1 mm from the steel sheet surface in the thickness direction. The difference ΔV between the rate (volume%) and the structure fraction (volume%) of the second phase at the center of the plate thickness of the steel sheet has a structure where the difference ΔV is 2% or less, and the steel sheet surface has a thickness of 3 to 30 μm. A thick, high-tensile hot-rolled steel sheet excellent in low-temperature toughness characterized by having a leather.
(Ti + (Nb / 2)) / C <4 (1)
Here, Ti, Nb, C: Content of each element (mass%)   In addition to the above composition, in addition to mass, V: 0.01 to 0.10%, Mo: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50% Or it is set as the composition containing 2 or more types, The thick high tension hot-rolled steel plate of Claim 1 characterized by the above-mentioned.   The thick-walled high-tensile hot-rolled steel sheet according to claim 1 or 2, further comprising Ca: 0.0005 to 0.005% by mass% in addition to the composition. % By mass
C: 0.02 to 0.08%, Si: 0.01 to 0.50%,
Mn: 0.5 to 1.8%, P: 0.025% or less,
S: 0.005% or less, Al: 0.005-0.10%,
Nb: 0.01-0.10%, Ti: 0.001-0.05%
And containing C, Ti, Nb so as to satisfy the following formula (1), heating a steel material having a composition composed of the balance Fe and inevitable impurities, and performing hot rolling consisting of rough rolling and finish rolling To make a hot-rolled steel sheet,
Before the rough rolling and before the finish rolling, a scale removal process is performed by a scale breaker, the finish-side entry temperature FET is set to 800 to 1050 ° C, and the finish-rolling exit temperature FDT is set to 750 to 950 ° C. After hot rolling, after the hot rolling is completed, cooling at 10 ° C / s or more at the average cooling rate at the plate thickness center position of the steel plate is performed at the temperature at the plate thickness center position of the steel plate by the following formula (2) Accelerated cooling is performed to the cooling stop temperature below the defined BFS, and then the coil is wound at the coil thickness below the BFS0 defined by the following equation (3) at the temperature at the center of the plate thickness. A method for producing a thick, high-tensile hot-rolled steel sheet with excellent low-temperature toughness.
(Ti + (Nb / 2)) / C <4 (1)
BFS (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR (2)
BFS0 (℃) = 770−300C−70Mn−70Cr−170Mo−40Cu−40Ni (3)
Here, C, Ti, Nb, Mn, Cr, Mo, Cu, Ni: Content of each element (mass%)
CR: Average cooling rate at the center position of the steel sheet thickness (° C / s)   In addition to the above composition, in addition to mass, V: 0.01 to 0.10%, Mo: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50% Or it is set as the composition containing 2 or more types, The manufacturing method of the thick high tension hot-rolled steel plate of Claim 4 characterized by the above-mentioned.   6. The method for producing a thick high-tensile hot-rolled steel sheet according to claim 4, wherein the composition further contains Ca: 0.0005 to 0.005% by mass% in addition to the composition.

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