KR20140019483A - Method of manufacturing sheet steel for sour-resistant line pipe - Google Patents

Abstract

Since the temperature tends to drop during hot rolling when the heating temperature of the steel sheet is reduced or when the sheet thickness of the steel sheet is thin, the line excellent in sour resistance that can cope with fluctuations in heating temperature and sheet thickness of hot rolling Provided is a method for producing a steel sheet for pipes. Contain C, Si, Mn, Nb, Ti, limit Al, P, N, and also contain Ca: 0.001% to 0.004%, S: 0.0008% or less, O: 0.0030% or less, Ca After reheating the steel pieces satisfying the contents of [Ca] (1-l24 [O]) / 1.25 [S]> 3.0 at 1000 to 1250 ° C., rough rolling, finish rolling and accelerated cooling are performed. . The ratio [C / Mn] between the cooling start temperature Tc of the accelerated cooling and the amount of C and the amount of Mn satisfies 4 ≦ Tc × [C / Mn] ≦ 32, and the cooling rate of the accelerated cooling is 10 to 40 ° C./s, Stop temperature shall be 200-500 degreeC.

Description

Manufacturing method of steel plate for sour line pipe {METHOD OF MANUFACTURING SHEET STEEL FOR SOUR-RESISTANT LINE PIPE}

The present invention relates to a method for producing a steel sheet excellent in hydrogen cracking resistance, that is, sour resistance, in an environment containing hydrogen sulfide (H ₂ S).

Sour resistance is required for the sour oil containing hydrogen sulfide, the steel pipe used for the line pipe which conveys sour gas, and the steel plate used for the installation equipment of a pipeline. In addition, sour resistance is hydrogen organic crack resistance (HIC resistance) in the corrosive environment containing hydrogen sulfide.

It is known that sour resistance deteriorates by production | generation of MnS extended | stretched in the rolling direction, and generation | occurrence | production of the inclusion of a cluster form. In addition, in order to improve the sour resistance in an extremely corrosive environment, the method of lowering the content of P, S, O, and N, adding Ca, and controlling the form of MnS to control rolling and water-cooling This is proposed (for example, patent document 1). In addition to MnS, there are Nb-Ti-CN-based inclusions other than MnS, and a method of completely solidifying Nb-Ti-CN-based inclusions has been proposed with appropriate heat treatment conditions for steel materials (for example, Patent Document 2).

In addition, from the viewpoint of improving the transport efficiency of the pipeline and reducing the cost by thinning, the high strength of the steel sheet for line pipe is required. With respect to such a request, for example, a method for producing a steel sheet having a strength of about X70, a metal structure uniform in the plate thickness direction, and fine fine bainite, having excellent sour resistance (for example, Patent Document 3).

In addition, when laying a pipeline in a cold district, it is necessary to improve the low temperature toughness of the steel plate for line pipes. With respect to such a problem, a method for producing a high strength steel sheet having improved low temperature toughness and sour resistance has been proposed (for example, Patent Documents 4 to 6). They reduce the increase in hardness by reducing the amount of C, control the form of MnS by reducing the amount of S and addition of Ca, and control the form of the oxide by reducing the amount of Al, sour resistance and low temperature toughness. Is intended to be compatible. On the other hand, by containing a certain amount of carbon, high strength is achieved without adding toughness by addition of Cr, and crystal grains are refined by specifying manufacturing conditions (patent document 7).

Patent Document 1 Japanese Laid-Open Patent Publication No. 62-112722 Patent Document 2 Japanese Unexamined Patent Publication No. 2006-63351 Patent Document 3 Japanese Unexamined Patent Publication No. 61-165207 Patent Document 4 Japanese Unexamined Patent Publication No. 03-236420 Patent document 5 Unexamined-Japanese-Patent No. 05-295434 Patent Document 6 Japanese Unexamined Patent Publication No. 07-242944 Patent Document 7 Japanese Unexamined Patent Publication No. 06-136440

In order to improve sour resistance, it is necessary to suppress the production of polygonal ferrite. For that purpose, it is good to start accelerated cooling after hot rolling at the temperature (Ar ₃ point or more) whose metal structure is austenite single phase. However, when the heating temperature of the steel slab is lowered or when the sheet thickness of the steel sheet is thin, the temperature decreases during hot rolling, and the water-cooling start temperature becomes less than Ar ₃ point, and polygonal ferrite is produced. It may hurt sourness.

This invention solves such a problem, and makes it a subject to provide the manufacturing method of the steel plate for line pipes excellent in sour resistance which can respond to the fluctuation | variation of the heating temperature and plate | board thickness of hot rolling.

The present invention strictly limits the content of S and O, by adding Ca,

(Ca) x (1-124 x [O]) / (1.25 x [S])

The temperature and rolling reduction ratio of the hot rolling are controlled so as to control the ESSP value represented by, to control the form of the sulfide, and to make the grain size before starting the accelerated cooling after the hot rolling to be fine. By increasing the amount of Mn and defining the relationship between the ratio of the amount of C to the amount of Mn [C / Mn] and the start temperature Tc of the accelerated cooling, the generation of polygonal ferrite is suppressed even if the cooling start temperature Tc decreases. The present invention is made based on the knowledge that deterioration of sour resistance can be prevented, and the gist thereof is as follows.

(1) in mass%

C: 0.01 to 0.06,

Si: 0.1 to 0.5%,

Mn: 1.0 to 1.4%,

Nb: 0.010 to 0.040%,

Ca: 0.001 to 0.004%,

Ti: 0.005 to 0.030%

≪ / RTI >

Al: 0.08% or less,

P: 0.015% or less,

S: 0.0008% or less,

O: 0.0030% or less,

N: 0.0050% or less

Limited to the content of Ca, O and S

[Ca] x (1-124 x [O]) / (1.25 x [S])> 3.0

After reheating the steel piece consisting of the balance Fe and unavoidable impurities to 1000 to 1250 ° C, rough rolling is performed, and further, the finishing temperature of the finish rolling is 800 ° C or more, and the rolling of the finish rolling of 950 ° C or less. Finishing rolling is performed with the hobby R above 3.125, and the ratio [C / Mn] of the amount of C to the amount of Mn and the cooling start temperature Tc

4≤Tc × [C / Mn] ≤32

, The reduction ratio R and the cooling start temperature Tc of the finish rolling

20 / (R-3) + 640≤Tc≤800

Accelerated cooling with a cooling rate of 10 to 40 ° C / s is started from the cooling start temperature Tc to stop the cooling at 200 to 500 ° C,

The structure of a steel plate is ash ferrite and bainitic ferrite, and HIC breaking rate is 5% or less, The manufacturing method of the steel plate for sour line pipes characterized by the above-mentioned.

(2) Said cooling start temperature Tc exists in the range of 650-800 degreeC, The manufacturing method of the steel plate for sour line pipes of said (1) characterized by the above-mentioned.

(3) the said steel piece further, in mass%,

Ni: 0.5% or less,

Cu: 0.5% or less,

Cr: 0.5% or less,

Mo: 0.3% or less,

V: 0.10% or less,

B: 0.0020% or less,

Mg: 0.01% or less

It contains 1 type, or 2 or more types of, The manufacturing method of the steel plate for sour line pipes as described in said (1) or (2) characterized by the above-mentioned.

According to this invention, even when the heating temperature and plate | board thickness of hot rolling are fluctuate | varied, it becomes possible to provide the steel plate for line pipes excellent in sour resistance.

From the content of Ca, S, and O,

(Ca) x (1-124 x [O]) / (1.25 x [S])

The steel plate which controlled the ESSP value calculated | required by was manufactured, and the sour resistance was evaluated.

The sour resistance was tested in accordance with TM0284 of the National Association of Corrosion and Engineers (NACE), and evaluated by the presence or absence of the occurrence of HIC (hydrogen organic crack). When the HIC fracture rate is about 5% or less, the sour resistance is good.

NACE test is a test method to saturate hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH2.7, and crack after 96 hours. As a result of examining the structure of the sample in which the HIC occurred, it was found that polygonal ferrite was formed in the steel sheet in which sour resistance was deteriorated.

Moreover, as a result of summarizing the relationship between the structure of the steel sheet and the manufacturing conditions, it was found that even if the start temperature of accelerated cooling was lowered, the steel sheet in which the polygonal ferrite was not produced was qualitatively high in C and low in Mn.

Therefore, the present inventors considered that the cooling start temperature Tc of accelerated cooling can be reduced if the ratio [C / Mn] of C amount and Mn amount is reduced, and summarized experimental data. As a result, Tc and [C / Mn]

When the relationship of 4? Tc x [C / Mn]? 32 is satisfied, it has been found that cooling start temperature Tc can be lowered without producing polygonal ferrite.

Moreover, the present inventors produced the steel plate with the rolling reduction ratio to 950 degreeC or less and the finish rolling more than 3.125, and investigated the relationship between a reduction ratio and cooling start temperature Tc. As a result, when a reduction ratio is made high, even if a grain size becomes small and cooling start temperature Tc falls, it turns out that a metal structure does not become a layered structure and sour resistance improves.

Thus, the present inventors have summarized the experimental data on the relationship between the conditions of finish rolling and the cooling start temperature Tc of accelerated cooling and sour resistance. As a result,

20 / (R-3) + 640≤Tc≤800

When the relationship with the resin was satisfied, it was found that the metal structure did not become a layered structure and that good sour resistance could be obtained.

Hereinafter, the present invention will be described in detail. In addition,% means the mass% below.

C: 0.01% to 0.06%

C: C is an element which improves the strength of the steel, and addition of 0.01% or more is necessary as the effective lower limit. On the other hand, if the amount of C is increased, the formation of carbides is accelerated and the HIC resistance is impaired, and therefore, the present invention aims at low carbonization. In order to suppress deterioration of HIC property, weldability, toughness and the like, the amount of C may be 0.06% or less, preferably 0.05% or less, and more preferably 0.038% or less.

Si 0.1% to 0.5%

Si: Si is a deoxidation element and 0.1% or more of addition is required. On the other hand, when the amount of Si exceeds 0.5%, the toughness of the weld heat affected zone HAZ decreases, so the upper limit is made 0.5%. The preferred range is 0.15 to 0.35%.

Mn 1.0% to 1.4%

Mn: Mn is an element that improves strength and toughness, and addition of 1.0% or more is required. On the other hand, since Mn is an element that generates MnS and deteriorates sour resistance, in order to suppress HIC, it is necessary to set the upper limit of Mn amount to 1.4%. The preferred range is 1.1 to 1.4%.

Nb : 0.010% to 0.040%

Nb: Nb is an element which enlarges the unrecrystallized temperature range to refine the grain size, forms carbides and nitrides, and contributes to the improvement of strength, and requires addition of 0.010% or more. On the other hand, in the present invention, it is extremely important to prevent the formation of coarse carbides, so the upper limit needs to be 0.040% or less. The preferred range of Nb is 0.011 to 0.025%, and more preferably 0.012 to 0.020%.

Ca : 0.001% to 0.004%

Ca: Ca is an element which produces sulfide CaS, suppresses the production of MnS which extends in the rolling direction, and contributes significantly to the improvement of HIC resistance. If the added amount of Ca is less than 0.001%, no effect can be obtained, so the lower limit is made 0.001%. On the other hand, when the addition amount of Ca exceeds 0.004%, oxides accumulate and impair HIC resistance, so the upper limit is made 0.004%. The preferred range is 0.0025 to 0.0035%.

Ti : 0.005% to 0.030%

Ti: Ti is an element used for refining grains as a deoxidizer or a nitride forming element, and it is necessary to add 0.005% or more. On the other hand, when Ti is excessively added, the toughness decreases due to the formation of coarse nitride, so the upper limit is made 0.030%. The preferred range is 0.010 to 0.020%.

Al : 0.08% or less

Al: Al is a deoxidation element. However, when the amount of addition exceeds 0.08%, an integrated cluster of Al oxide is formed and the sour resistance is impaired, so it is limited to 0.08% or less. In addition, when toughness is calculated | required, it is good to make Al amount 0.03% or less. The upper limit of more preferable Al amount is 0.01%. Although the minimum of Al amount is not specifically limited, In order to reduce the amount of oxygen in molten steel, it is good to add 0.0005% or more.

P: not more than 0.015%

P: P is an impurity. When the content exceeds 0.015%, the HIC resistance is impaired. Therefore, the upper limit of content of P is made into 0.015%.

S: 0.0008% or less

S: S is an element which produces MnS extending | stretching to a rolling direction at the time of hot rolling, and reduces HIC resistance. Therefore, in this invention, it is necessary to reduce the amount of S, and let an upper limit be 0.0008%. The smaller the amount of S, the better, but it is difficult to be less than 0.0001%. Moreover, it is good to set it as 0.0001% or more also from a manufacturing cost viewpoint.

O: 0.0030% or less

O: O is an impurity. In order to suppress the accumulation of oxides and to improve HIC resistance, the upper limit needs to be 0.0030%. In order to suppress the formation of oxides and improve the toughness, the amount of 0 is preferably made 0.0020% or less.

N: 0.0050% or less

N: N is an impurity, and when the content of N exceeds 0.0050%, carbonitrides of Ti and Nb tend to accumulate, and HIC resistance is impaired. Therefore, the upper limit of N amount is made into 0.0050%. When toughness or the like is required, the amount of N is preferably made 0.0035% or less in order to suppress coarsening of TiN. When nitrides such as TiN and NbN are used and the austenite grain size during heating is reduced, it is preferable to contain N or more than 0.0010%.

[ Ca ] x (1-124 x [O]) / (1.25 x [S])> 3.0

In the present invention, it is necessary to increase [Ca] (1-124 [O]) / 1.25 [S], that is, the ESSP value. The ESSP value is a ratio of the amount of Ca to the amount of S necessary to generate CaS, taking into account that Ca forms an oxide. In order to form CaS by adding Ca, and to fix S, it is necessary to make ESSP value exceed 3.0.

When the amount of S becomes 0, the ESSP value becomes infinity, but in this case, there is no generation of MnS. Therefore, if Ca amount is in the above-mentioned range, it is not necessary to define the upper limit of ESSP value.

In this invention, it is good to add 1 type, or 2 or more types of elements among Ni, Cu, Cr, Mo, V, and B as an element which improves strength and toughness.

Ni : 0.5% or less

Ni: Ni is an effective element for improving toughness and strength, and contributes to improvement of corrosion resistance. Therefore, it is preferable to add Ni: 0.01% or more. On the other hand, Ni is an expensive element, and it is preferable to limit the upper limit to 0.5% in order to reduce the manufacturing cost.

Cu : 0.5% or less

Cu: Cu is an effective element for increasing the strength and contributes to the improvement of the corrosion resistance. Therefore, it is preferable to add Cu by 0.01% or more. On the other hand, Cu is also an expensive element, and in order to reduce manufacturing costs, it is preferable to limit the upper limit to 0.5%.

Cr : 0.5% or less

Cr: Cr is an element effective for increasing the strength, and preferably 0.01% or more. On the other hand, when a large amount is added, hardenability becomes high and toughness may fall, so an upper limit is 0.5%.

Mo 0.3% or less

Mo: Mo is an element that improves hardenability and at the same time forms carbonitrides and improves strength. To obtain the effect, Mo is preferably added at least 0.01%. On the other hand, Mo is an expensive element, and in order to reduce manufacturing cost, it is good to set an upper limit to 0.30%. Moreover, since the HIC property and toughness may fall when the strength of steel rises, a preferable upper limit is 0.20%.

V: 0.10% or less

V: V is an element which forms carbides and nitrides and contributes to the improvement of strength, and in order to obtain the addition effect, it is preferable to add 0.01% or more. On the other hand, if V exceeding 0.10% is added, the toughness may be reduced, so the upper limit is preferably 0.10%.

B: 0.0020% or less

B: B is an element which segregates at grain boundaries of the steel and contributes significantly to the improvement of hardenability. In order to acquire this addition effect, it is good to add B 0.0001% or more. Meanwhile. When B is excessively added, segregation to the grain boundaries becomes excessive, which may cause a decrease in toughness, so the upper limit is preferably 0.0020%.

Mg : 0.01% or less

Mg: Mg is an element that acts as a deoxidizer and a desulfurization agent, and is particularly effective for improving toughness because it produces fine oxides and suppresses coarsening of particle diameters. In order to acquire this addition effect, it is good to add 0.0001% or more. On the other hand, when Mg is added exceeding 0.01%, an oxide will become easy to aggregate and coarsen, and HIC property and toughness may fall, and an upper limit may be 0.01%.

The steel containing the above-mentioned components is made into steel pieces by continuous casting after melting in the steelmaking step, and the steel pieces are heated, followed by thick plate rolling consisting of rough rolling and finish rolling to produce steel sheets.

Heating temperature: 1000 ° C to 1250 ° C

If the heating temperature of a steel piece is less than 1000 degreeC, NbC precipitated in the steel piece will not be dissolved and coarse NbC will remain | survive on a steel plate, and sour resistance falls. On the other hand, when the heating temperature of a steel piece exceeds 1250 degreeC, the grain size of a steel plate will become coarse, the metal structure after finishing rolling will become a layered structure, and sour resistance falls. Therefore, the heating temperature of the steel piece is in the range of 1000-1250 degreeC.

After finish rolling, accelerated cooling is performed. Accelerated cooling is preferably carried out immediately after finishing rolling. However, when the plate | board thickness of a steel plate becomes thin, temperature will fall easily. Therefore, in the present invention, the conditions of accelerated cooling are extremely important in order to suppress the formation of polygonal ferrite and to prevent the formation of layered structures of ferrite and low-temperature transformation phase (bainite or martensite).

Cooling start temperature of accelerated cooling Tc :

Accelerated cooling is performed in order to make the structure of a steel plate into fine ash ferrite or bainitic ferrite. When the cooling start temperature of accelerated cooling falls, polygonal ferrite transformation is accelerated. On the other hand, polygonal ferrite transformation is suppressed by decreasing the amount of C and increasing the amount of Mn.

Therefore, in this invention, in order to suppress generation | occurrence | production of polygonal ferrite, and to lower the cooling start temperature of accelerated cooling, the ratio of C amount and Mn amount [C / Mn] and cooling start temperature Tc (degreeC) Product: Tc x [C / Mn] is 32 or less. In addition, 30 is good and 27 is more preferable for the upper limit of Tcx [C / Mn] from a viewpoint of reducing the starting temperature Tc of accelerated cooling.

On the other hand, if the cooling start temperature Tc becomes too low, even if [C / Mn] is lowered within the range of the component composition of the present invention, generation of polygonal ferrite cannot be avoided, so the lower limit of Tc × [C / Mn] is It is 4. In order to suppress the production of polygonal ferrite, Tc × [C / Mn] is preferably 4.5 or more, more preferably 10 or more.

When the cooling start temperature of the accelerated cooling is less than 650 ° C, the production of polygonal ferrite is promoted. Therefore, the cooling start temperature of the accelerated cooling is preferably set to 650 ° C or more to ensure sour resistance.

On the other hand, in order to improve sour resistance, it is good to reduce the finishing temperature of hot rolling to about 900-800 degreeC, and to homogenize a structure. Since the cooling start temperature of accelerated cooling becomes below the finishing temperature of hot rolling, it is good to make cooling start temperature of accelerated cooling into 800 degrees C or less.

Cooling rate of accelerated cooling: 10 ° C./s to 40 ° C./s

Accelerated cooling is performed in order to make the structure of a steel plate into fine ash ferrite or bainitic ferrite. In order to suppress a polygonal ferrite transformation and to prevent generation of pearlite, it is necessary to make cooling rate 10 degreeC / s or more.

On the other hand, when the cooling rate of accelerated cooling exceeds 40 degree-C / s, martensite will produce | generate excessively, hardness will become nonuniform, and sour resistance and toughness will fall. Therefore, the cooling rate of accelerated cooling shall be 10-40 degreeC / s. In addition, a cooling rate is a speed in the center of the sheet thickness of a steel plate.

Stop temperature of accelerated cooling: 200 ° C to 500 ° C

The stop temperature of accelerated cooling is in the range of 200-500 degreeC in order to suppress generation | occurrence | production of martensite. In order to suppress polygonal ferrite transformation and to prevent generation of pearlite, it is necessary to set the stop temperature of accelerated cooling to 500 degrees C or less.

On the other hand, when the stop rate of accelerated cooling is less than 200 ° C, martensite is excessively generated, the hardness becomes uneven, and sour resistance and toughness are reduced.

In addition, in order to suppress formation of a layered structure, it is good to control the temperature of finish rolling, the reduction ratio, and the cooling start temperature of accelerated cooling after rolling.

Finishing temperature: 800 ℃ or higher

In order to make a structure homogeneous, the finishing temperature of hot rolling should be 800 degreeC or more. This is because ferrite is produced at less than 800 ° C depending on the component composition, and the structure of the steel sheet after rolling may be layered, and sour resistance may be impaired. In addition, depending on the conditions of finish rolling, processed ferrite may remain in a steel plate and the toughness may be impaired.

Below 950 ℃ Abhabi  R: greater than 3.125

In finish rolling, in order to refine a grain size, it is necessary to control rolling temperature and a reduction ratio. In particular, the structure of the steel sheet can be made fine by increasing the reduction ratio at low temperature and performing finish rolling. When a rolling temperature exceeds 950 degreeC, since recrystallization arises, the reduction ratio R in 950 degreeC or less is important.

Moreover, when the reduction ratio in 950 degreeC or less is 3.125 or less, a structure may not become homogeneous and sour resistance may fall. Therefore, it is good to make the reduction ratio until 950 degreeC or less and finish rolling complete | finished more than 3.125, and 4 or more is more preferable. Although the upper limit of the reduction ratio in 950 degrees C or less is not prescribed | regulated, in consideration of slab plate thickness and the plate thickness after finish rolling, 20 is a preferable upper limit. The reduction ratio from 950 degrees C or less to completion of finishing rolling is a ratio of the plate thickness after rolling with respect to the plate thickness in 950 degreeC.

20 / (R-3) + 640≤ TC ≤800

In this invention, since accelerated cooling is performed as it is after hot rolling, the cooling start temperature of accelerated cooling becomes below the finishing temperature of hot rolling. In order to improve sour resistance, it is preferable to lower the finishing temperature of hot rolling to about 900-800 degreeC. Therefore, the cooling start temperature of accelerated cooling is also good to be 800 degrees C or less.

On the other hand, when the rolling reduction ratio of finish rolling is enlarged, since plate | board thickness becomes thin, the start of accelerated cooling will be delayed and cooling start temperature will fall. However, since the grain size becomes finer by increase of the reduction ratio of finish rolling, even if cooling start temperature falls, it can prevent that it becomes a layered structure. Therefore, when the reduction ratio is increased and the cooling start temperature Tc is lowered, the allowable range of the manufacturing conditions when manufacturing a thin steel sheet excellent in sour resistance becomes wider.

Therefore, the relationship between the reduction ratio R up to finishing rolling and cooling start temperature Tc is 950 degrees C or less,

20 / (R-3) + 640≤Tc≤800

When the relationship is satisfied, the metal structure does not become a layered structure, so that good sour resistance can be obtained, and the allowable range of manufacturing conditions can be widened.

<Examples>

The steel with the chemical component shown in Table 1 was melted by converter, secondary refining, and the 250 mm-thick steel piece was produced by continuous casting. The obtained steel strip was hot-rolled on the conditions shown in Table 2, and it was made into the steel plate. HIC property of the steel plate after manufacture was evaluated by the NACE test. The conditions of the NACE test were to saturate hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH2.7, immersion time 96 hours, observe the presence of cracks, and measure the HIC wave rate (CAR). .

The results are shown in Table 2. No. 4-11 are the components and manufacturing conditions of a steel plate in the range of this invention, CAR becomes 5% or less, and has favorable sour resistance. On the other hand, 12 is an example in which the ESSP value is lower than the range of the present invention and the sour resistance is lowered. In addition, 13 and 14 are examples where C amount is large, Tc x [C / Mn] is also large, and HIC resistance is lowered. No. 15 is an example in which cooling start temperature is low and sour resistance deteriorates.

As mentioned above, according to this invention, even when the heating temperature and plate | board thickness of hot rolling are fluctuate | varied, it becomes possible to provide the steel plate for line pipes excellent in sour resistance. Accordingly, the present invention is extremely remarkable in industrial contribution, and has great industrial applicability.

Claims (3)

In terms of% by mass,
C: 0.01% to 0.06%,
Si: 0.1% to 0.5%,
Mn: 1.0% to 1.4%,
Nb: 0.010% to 0.040%,
Ca: 0.001% to 0.004%,
Ti: 0.005% to 0.030%
&Lt; / RTI &gt;
Al: 0.08% or less (not including 0),
P: 0.015% or less,
S: 0.0008% or less (not including 0),
O: 0.0030% or less,
N: 0.0050% or less
Limited to the content of Ca, O and S
[Ca] x (1-124 x [O]) / (1.25 x [S])> 3.0
After reheating the steel piece composed of the balance Fe and unavoidable impurities at 1000 ° C to 1250 ° C, rough rolling is performed, and the finish temperature of the finish rolling is 800 ° C or more, and the finish rolling of 950 ° C or less Finish rolling is performed with the reduction ratio R exceeding 3.125, and the ratio [C / Mn] of the amount of C to the amount of Mn and the cooling start temperature Tc
4≤Tc × [C / Mn] ≤32
, The reduction ratio R and the cooling start temperature Tc of the finish rolling
20 / (R-3) + 640≤Tc≤800
Accelerated cooling with a cooling rate of 10 ° C./s to 40 ° C./s is started from the cooling start temperature Tc, and stops the accelerated cooling at 200 ° C. to 500 ° C.,
The structure of a steel plate is ash ferrite and bainitic ferrite, and HIC breaking rate is 5% or less, The manufacturing method of the steel plate for sour line pipes characterized by the above-mentioned. The said cooling start temperature Tc exists in the range of 650 degreeC-800 degreeC, The manufacturing method of the steel plate for sour line pipes of Claim 1 characterized by the above-mentioned. The method according to claim 1 or 2, wherein the steel sheet is further, in mass%,
Ni: 0.5% or less (not including 0),
Cu: 0.5% or less (not including 0),
Cr: 0.5% or less (not including 0),
Mo: 0.3% or less (not including 0),
V: 0.10% or less (not including 0),
B: 0.0020% or less (not including 0),
Mg: 0.01% or less (does not contain 0)
It contains 1 type, or 2 or more types of, The manufacturing method of the steel plate for sour line pipes characterized by the above-mentioned.