JP2020012169A - Thick steel sheet for linepipe and manufacturing method therefor - Google Patents

Abstract

To provide a thick steel sheet for linepipe excellent in DWTT property and HIC resistance and a manufacturing method therefor.SOLUTION: There is provided a thick steel sheet for linepipe having a chemical composition of a steel sheet containing, by mass%, C:0.03 to 0.08%, Si:0.10 to 0.50%, Mn:1.00 to 1.80%, P:0.020% or less, S:0.0010% or less, Cu:0.10 to 0.50%, Ni:0.10 to 0.50%, Nb:0.005 to 0.050%, Ti:0.005 to 0.030%, Al:0.010 to 0.040%, N:0.0010 to 0.0050%, B:0.001% or less, optional elements or the like, and the balance:Fe with impurities, and having Pcm of 0.15 to 0.23, and Ceq of 0.35 to 0.43, a metallic structure at a sheet thickness center part containing, by area percentage, 20 to 35% of ferrite, and 2.0% or less of a hard phase, and the balance bainite, average crystal particle diameter of ferrite at a sheet thickness center part of 20.0 μm or less, difference between area percentage of ferrite in a steel sheet surface layer and area percentage of ferrite at the sheet thickness center part of 5.0% or less, maximum longer diameter of an inclusion of 30.0 μm or less, and sheet thickness of 30 to 45 mm.SELECTED DRAWING: None

Description

  The present invention relates to a thick steel plate for a line pipe and a method for producing the same.

  Various performances are required for steel pipes used for oil and natural gas pipelines. As one example, “HIC resistance characteristics” and “DWTT characteristics” are exemplified.

  The “HIC resistance characteristic” is a characteristic that indicates the difficulty of hydrogen-induced cracking (hereinafter referred to as “HIC”) that occurs even when no external stress is applied. It is known that “HIC” occurs in an environment where hydrogen sulfide and the like coexist with moisture, such as an environment in which a steel pipe for a line pipe is used. The “DWTT characteristic” is a characteristic obtained by performing a drop-weight tear test (DWTT), which is one of the evaluation methods of toughness at low temperature, particularly, a brittle crack propagation stopping characteristic.

  Furthermore, in recent years, the line pipe may be used in a region having a deep water depth, specifically, a region having a water depth of 1000 mm or more. In such a case, from the viewpoint of the pressure resistance, the steel pipe for a line pipe is required to be thick, that is, so-called thickened. Conventionally, the wall thickness of a steel pipe for a line pipe was approximately 25 mm or less. On the other hand, as the use environment becomes an area having a deep water depth, it is required that the wall thickness be 30 to 45 mm. Therefore, it is required to increase the thickness of a steel plate used as a material of a steel pipe for a line pipe.

WO 2015/030210

  However, as the thickness of the steel sheet increases, it becomes difficult to appropriately control the metal structure. That is, if a thick steel plate as a steel pipe material is manufactured under the same conditions as a conventional steel pipe material steel plate having a thickness of 25 mm or less, it is difficult to obtain a desired metallographic structure. As a result, it may be difficult for the line pipe steel pipe to have required characteristics, for example, DWTT characteristics.

  Patent Literature 1 discloses a steel plate for a sour-resistant line pipe having a plate thickness of 25 to 45 mm assuming use in the deep sea. The steel sheet disclosed in Patent Literature 1 controls the metal structures of the surface layer and the center of the sheet thickness to be different from each other, and forms a processed ferrite in the surface layer, so that not only the DWTT characteristics and the HIC resistance characteristics but also the pressure crush resistance. It has characteristics.

  An object of the present invention is to provide a thick steel plate for a line pipe having excellent DWTT characteristics and HIC resistance, and a method for producing the same.

  The present invention has been made to solve the above-described problems, and has a gist of the following steel plate for a line pipe and a method of manufacturing the same.

(1) The chemical composition is expressed in mass%
C: 0.03 to 0.08%,
Si: 0.10 to 0.50%,
Mn: 1.00 to 1.80%,
P: 0.020% or less,
S: 0.0010% or less,
Cu: 0.10 to 0.50%,
Ni: 0.10 to 0.50%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.030%,
Al: 0.010-0.040%,
N: 0.0010 to 0.0050%,
B: 0.001% or less,
Cr: 0 to 0.50%,
Mo: 0 to 0.10%,
V: 0 to 0.05%,
Ca: 0 to 0.02%,
Mg: 0 to 0.02%,
REM: 0-0.02%,
The balance: Fe and impurities,
Pcm represented by the following formula (i) is 0.15 to 0.23,
Ceq represented by the following formula (ii) is 0.35 to 0.43,
The metal structure at the center of the sheet thickness is the area ratio,
20 to 35% ferrite, and no more than 2.0% hard phase,
The rest is bainite, and
The average crystal grain size of the ferrite at the center of the plate thickness is 20.0 μm or less,
The difference between the area ratio of ferrite in the surface layer of the steel sheet and the area ratio of ferrite in the center part of the sheet thickness is 5.0% or less;
The maximum major axis of the inclusion is 30.0 μm or less;
Thick steel plate for line pipes having a thickness of 30 to 45 mm.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (i)
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (ii)
However, each element symbol in the above formulas (i) and (ii) represents the content (% by mass) of each element contained in the steel, and is zero if not contained.

(2) The chemical composition is represented by mass%
Cr: 0.10 to 0.50%,
Mo: 0.05-0.10%, and V: 0.01-0.05%,
The thick steel plate for a line pipe according to the above (1), comprising at least one member selected from the group consisting of:

(3) the chemical composition is expressed in mass%;
Ca: 0.0005-0.02%,
Mg: 0.0005-0.02%, and REM: 0.0005-0.02%,
The thick steel plate for a line pipe according to the above (1) or (2), comprising at least one member selected from the group consisting of:

(4) A method for producing a thick steel plate for a line pipe according to any one of the above (1) to (3),
(A) heating a billet to a temperature range of 1100 to 1250 ° C. and soaking;
(B) a step of subjecting the slab to rough rolling to form a rolled body;
(C) Finish rolling is started when the surface temperature of the rolled body is in the range of Ar ₃ points −10 ° C. to Ar ₃ points + 60 ° C., and the finish rolling is completed at a higher surface temperature than at the start of the finish rolling. And the process of
(D) water cooling is started at a surface temperature of the steel sheet in the range of Ar ₃ points + 10 ° C. to Ar ₃ points + 50 ° C. and higher than the surface temperature at the start of finish rolling;
Stopping the water cooling in a temperature range of 400 to 500 ° C., and air cooling to room temperature;
A method for producing a thick steel plate for a line pipe, comprising:

  ADVANTAGE OF THE INVENTION According to this invention, the thick steel plate for line pipes excellent in DWTT characteristic and HIC resistance can be obtained.

  The present inventors have made the following studies in order to manufacture a steel pipe for line pipe that satisfies desired characteristics even when a steel plate having a thickness of 30 to 45 mm is used. Specifically, the present inventors manufactured the steel plate having a thickness of 30 to 45 mm under the conditions for manufacturing a steel plate having a thickness of 25 mm or less, and investigated differences in the structure. As a result, the following findings (a) to (e) were obtained.

  (A) In a steel plate having a thickness of 25 mm or less, the difference in the metal structure between the surface layer and the center of the plate thickness was small. On the other hand, in a steel plate having a thickness of 30 mm or more, the crystal grains in the central portion of the plate thickness changed, and the metal structures in the surface layer and the central portion of the plate thickness were greatly different.

  (B) With a steel plate having a thickness of 30 mm or more, the DWTT characteristics decreased at the center of the plate thickness. This is presumably because the ferrite content was reduced and the crystal grains were coarsened in the center of the plate thickness.

  (C) Based on the above, in a steel plate having a thickness of 30 mm or more, in order to obtain desired characteristics, the difference in the metal structure between the surface layer and the central portion of the thickness is reduced, and the variation in the characteristics in both portions is reduced. There is a need.

  (D) In order to reduce the difference in the metal structure between the surface layer and the central part of the sheet thickness, it is preferable to reduce the temperature difference between the two parts and follow the same thermal history in the manufacturing process.

  (E) For this purpose, it is effective to reduce the temperature difference between the surface layer and the center of the sheet thickness by utilizing recuperation during rolling. Here, the recuperation refers to a phenomenon in which, when cooling is stopped, heat inside the steel sheet is transferred, and the surface temperature of the steel sheet rises. In the case of a thick steel plate, water is directly sprayed on the surface of the steel plate by water cooling, so that the steel plate surface is relatively easily cooled. On the other hand, the inside of the steel sheet cannot be cooled and is hardly cooled. Once the cooling is stopped, the heat inside the steel sheet is transferred to the steel sheet surface and the temperature of the steel sheet surface rises, so that the temperature difference between the surface layer and the center of the sheet thickness can be reduced. As a result, the same thermal history cannot be traced between the surface layer and the center of the sheet thickness, but the metal structure at the center of the sheet thickness becomes similar to the metal structure of the surface layer. In addition, the crystal grains in the center portion of the plate thickness, which tends to be coarsened, can be made finer.

  The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition Reasons for limiting each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.03 to 0.08%
C improves the strength of the steel. Therefore, the C content is set to 0.03% or more, and preferably 0.04% or more. However, when C is excessively contained, segregation of C is caused at the time of pearlite transformation in a rolling step or a cooling step after rolling, and the C is locally hardened. Such local curing reduces HIC resistance. Therefore, the C content is set to 0.08% or less, and preferably 0.06% or less.

Si: 0.10 to 0.50%
Si is effective as a deoxidizing agent, and if its content is too small, deoxidizing becomes insufficient. Further, Si improves the strength of the steel and the HIC resistance. Therefore, the Si content is 0.10% or more, and preferably 0.12% or more. However, if the Si content is excessive, coarse inclusions are generated instead, and the HIC resistance decreases. In addition, martensite is generated in a heat-affected zone (HAZ), which causes a decrease in toughness. Therefore, the Si content is set to 0.50% or less, and preferably 0.30% or less.

Mn: 1.00 to 1.80%
Mn contributes to improvement in the strength and toughness of steel. If the Mn content is insufficient, other alloying elements must be added to improve the strength. This increases manufacturing costs. Therefore, the Mn content is set to 1.00% or more, and preferably 1.10% or more. However, if the Mn content is excessive, central segregation is caused in the continuously cast slab, and C is concentrated in the Mn segregated portion in the rolling step or the water cooling step after rolling, and the steel sheet is locally hardened. As a result, the HIC resistance decreases. Therefore, the Mn content is set to 1.80% or less, and preferably 1.50% or less.

P: 0.020% or less P is an impurity. P segregates in the center of the slab, hardens the structure, and causes HIC. P also lowers the DWTT characteristics. Therefore, the P content is set to 0.020% or less.

S: 0.0010% or less S is an impurity. S combines with Mn to form MnS. Since this MnS becomes the starting point of HIC, it is preferable that S is reduced. S lowers the DWTT characteristics. Therefore, the S content is set to 0.0010% or less.

Cu: 0.10 to 0.50%
Cu contributes to improvement in toughness and HIC resistance. Therefore, the Cu content is 0.10% or more, and preferably 0.12% or more. However, excessive Cu content increases the production cost. For this reason, the Cu content is set to 0.50% or less, and preferably 0.40% or less.

Ni: 0.10 to 0.50%
Ni contributes to improvement in strength and toughness. Further, when the content of Ni is 1/2 or more of that of Cu, the effect of preventing Cu checking is obtained. Therefore, the Ni content is 0.10% or more, and preferably 0.12% or more. However, excess Ni increases manufacturing costs. Also, the weldability is reduced. Therefore, the Ni content is 0.50% or less, and preferably 0.45% or less.

Nb: 0.005 to 0.050%
Nb refines crystal grains and improves strength and toughness. Nb is rolled in an austenite unrecrystallized region and then rapidly cooled from a temperature not lower than the Ar ₃ transformation point, thereby turning the metal structure into a fine lower bainite structure. Therefore, the Nb content is 0.005% or more, and preferably 0.010% or more. However, when the Nb content is excessive, when the slab is heated, the Nb carbonitride in the slab is not completely dissolved, and the undissolved Nb carbonitride becomes the starting point of HIC. For this reason, the Nb content is set to 0.050% or less, and preferably 0.040% or less.

Ti: 0.005 to 0.030%
Ti forms Ti carbonitride and contributes to the improvement of steel strength. Therefore, the Ti content is 0.005% or more, and preferably 0.007% or more. However, if the Ti content is excessive, the Ti carbonitride coarsens and the toughness in the HAZ decreases. In addition, excess Ti carbonitride forms in the center of the continuously cast slab, causing HIC. For this reason, the Ti content is set to 0.030% or less, preferably 0.025% or less.

Al: 0.010-0.040%
Al is a deoxidizing agent and contributes to improvement in toughness. Therefore, the Al content is set to 0.010% or more, and preferably 0.015% or more. However, if the Al content is excessive, inclusions are excessively formed, and the cleanliness of the steel is reduced. As a result, the HIC resistance decreases. Excessive Al also lowers toughness. For this reason, the Al content is set to 0.040% or less, and preferably 0.035% or less.

N: 0.0010 to 0.0050%
N is an element that inevitably penetrates into steel when the steel is melted in an air atmosphere such as a converter. N is an element that forms a nitride with Al and / or Ti in a steel material. These nitrides have the effect of refining crystal grains as pinned particles during hot working. As a result, N affects not only the mechanical properties of the steel material but also the formation of the metal structure, and improves the toughness. For this reason, the N content is set to 0.0010% or more, preferably 0.0015% or more. However, when the N content is excessive, the nitride is dynamically precipitated at the austenite grain boundary during continuous casting, which causes a slab surface crack. Therefore, the N content is 0.0050% or less, and preferably 0.0040% or less.

B: 0.001% or less B is an impurity. If B is contained in a large amount in steel, the toughness is reduced. Therefore, even if B is contained as an impurity, the B content is set to 0.001% or less, preferably 0.0005% or less. In addition, among the compositions of the steel slabs shown in Tables 1 and 2 described below, a steel slab in which B is indicated by "-" indicates that B was not detected when the composition of the steel slab was analyzed.

  The steel sheet according to the present invention may contain one or more elements selected from Cr, Mo, V, Ca, Mg and REM, in addition to the above elements.

Cr: 0 to 0.50%
Since Cr contributes to improvement in strength and corrosion resistance, it may be contained as necessary. However, when the Cr content is excessive, the production cost increases and the weldability decreases. Therefore, the Cr content is set to 0.50% or less, and preferably 0.45% or less. On the other hand, in order to reliably obtain the above effects, the Cr content is preferably 0.10% or more, and more preferably 0.12% or more.

Mo: 0 to 0.10%
Mo contributes to improvement in strength by improving hardenability. Specifically, Mo delays the transformation of austenite to ferrite and / or pearlite. As described above, the steel sheet may be contained as necessary to obtain a desired strength. However, when Mo is contained excessively, the production cost increases. For this reason, the Mo content is set to 0.10% or less, preferably 0.08% or less. On the other hand, in order to reliably obtain the above effects, the Mo content is preferably 0.05% or more, and more preferably 0.06% or more.

V: 0 to 0.05%
V contributes to the improvement in strength by precipitation hardening. For this reason, you may make it contain as needed. However, an excessive V content increases the production cost. For this reason, the V content is set to 0.05% or less, and preferably 0.04% or less. On the other hand, the V content is preferably 0.01% or more, more preferably 0.02% or more, in order to surely obtain the above effects.

Ca: 0 to 0.02%
Ca has an effect of improving hot workability. In addition, Ca reacts with S in steel to form acids and sulfides (hereinafter also referred to as “oxysulfides”) in molten steel. In general, inclusions such as MnS may be elongated during hot working, for example, rolling, and the tip of the elongated inclusion may be a starting point of cracking. On the other hand, the above-mentioned oxysulfide does not extend in the rolling direction in the rolling process, and its shape remains spherical. For this reason, Ca has an effect of suppressing welding cracks or HIC, and Ca may be contained as necessary. However, if the Ca content exceeds 0.002%, the toughness may deteriorate. Therefore, the Ca content is set to 0.02% or less, preferably 0.005% or less. On the other hand, in order to reliably obtain the above effects, the Ca content is preferably at least 0.0005%, more preferably at least 0.001%.

Mg: 0 to 0.02%
Mg has an effect of improving hot workability. In addition, Mg has an effect of generating an Mg-containing oxide, serving as a nucleus for generating TiN, and finely dispersing TiN. For this reason, you may make it contain as needed. However, if the Mg content exceeds 0.02%, the amount of oxide formation becomes excessive, resulting in a decrease in ductility. For this reason, the Mg content is set to 0.02% or less, and preferably 0.005% or less. On the other hand, in order to obtain the above effects, the Mg content is preferably 0.0005% or more, and more preferably 0.001% or more.

REM: 0-0.02%
REM has the effect of increasing hot workability. REM also has the effect of making the HAZ structure finer. For this reason, you may make it contain as needed. However, if the REM content is excessive, it becomes an inclusion and reduces cleanliness. In addition, inclusions formed by adding REM are relatively hard to deteriorate toughness. However, if the REM content is more than 0.02%, the decrease in toughness of the base material due to inclusions cannot be ignored. For this reason, the REM content is set to 0.02% or less, and preferably 0.005% or less. On the other hand, in order to obtain the above effects, the REM content is preferably 0.0005% or more, and more preferably 0.001% or more.

  In the present invention, “REM” is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM indicates the total content of one or more elements of REM.

  The balance of the steel sheet according to the present invention is Fe and impurities. Here, the impurities are those that are mixed from the ore, scrap, or the production environment as raw materials when industrially producing steel, and are allowed in a range that does not adversely affect the steel sheet of the present invention. Means something.

2. Weld crack susceptibility index Pcm: 0.15 to 0.23
Pcm is a so-called “weld crack susceptibility index” and is generally and widely used as an index for evaluating the likelihood of low-temperature cracking. Pcm is represented by the following equation (i).
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (i)
However, each element symbol in the above formula (i) represents the content (% by mass) of each element contained in the steel, and is zero if not contained.

  In the steel sheet according to the present invention, the Pcm is set to 0.15 to 0.23. By the way, in order to use in a deep water region, it is a standard to have a tensile strength of 460MP or more. When Pcm is less than 0.15, it becomes difficult to obtain the tensile strength in the above range. For this reason, Pcm is set to 0.15 or more, preferably 0.16 or more, and more preferably 0.17 or more. On the other hand, if Pcm exceeds 0.23, welding cracks are likely to occur when the steel sheet is formed into a tube and welded. For this reason, Pcm is set to 0.23 or less, preferably 0.22 or less, and more preferably 0.21 or less.

3. Carbon equivalent Ceq: 0.35-0.43
Ceq is a so-called “carbon equivalent” and is generally widely used as an index for evaluating the hardenability or weldability of a steel sheet. Ceq is represented by the following formula (ii).
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (ii)
However, each element symbol in the above formula (ii) represents the content (% by mass) of each element contained in the steel, and is zero if not contained.

  In the steel sheet according to the present invention, the Ceq is set to 0.35 to 0.43. If Ceq is less than 0.35, it becomes difficult to obtain a tensile strength of 460 MPa or more, as with Pcm. Therefore, Ceq is set to 0.35 or more, preferably 0.36 or more, and more preferably 0.37 or more. On the other hand, when Ceq exceeds 0.43, hardenability is excessively increased, causing deterioration of weldability and making welding work difficult. Therefore, Ceq is set to 0.43 or less, preferably 0.42 or less, and more preferably 0.41 or less.

4. Metal structure 4-1. Metal Structure at the Center of Sheet Thickness In the steel sheet according to the present invention, the metal structure at the center of the sheet thickness that is harder to cool than the surface layer of the steel sheet and difficult to control the structure is first defined. More specifically, the metal structure at the center of the plate thickness contains 20 to 35% of ferrite and 2.0% or less of a hard phase in area ratio, and the rest is bainite.

4-1-1. Ferrite In order to achieve both sour resistance and strength, the structure of the steel sheet according to the present invention at the center of the thickness is bainite. However, bainite lacks low-temperature toughness, so that a bainite single phase cannot provide desired DWTT characteristics. In order to provide the DWTT characteristic, a metal structure having a ferrite at the center of the thickness is required.

  In the steel sheet according to the present invention, the ferrite has an area ratio of 20% to 35% at the center of the thickness. If the area ratio of ferrite at the center of the thickness is less than 20%, sufficient DWTT characteristics cannot be obtained. For this reason, the area ratio of ferrite at the center of the thickness is set to 20% or more, and preferably 25% or more.

  On the other hand, when the area ratio of ferrite at the center of the thickness exceeds 35%, not only the strength is reduced but also the HIC resistance is reduced. For this reason, the amount of ferrite at the center of the plate thickness is 35% or less in area ratio, and preferably 30% or less.

4-1-2. Hard Phase It is preferable that the steel sheet according to the present invention basically has a structure composed of bainite and ferrite at the center of the thickness. However, when ferrite is formed, solid solution C (carbon) is discharged to the surroundings, and a hard phase enriched in C may be formed. Since this hard phase serves as a starting point of HIC, it is desirable that the hard phase is not generated as much as possible. If the hard phase has an area ratio of 2.0% or less, it does not affect the characteristics as a line pipe. Therefore, the area ratio of the hard phase is 2.0% or less, and preferably 1.5% or less. In addition, as a hard phase, a cementite, MA, etc. are illustrated, for example.

4-1-3. Bainite As described above, in order to achieve both the sour resistance and the strength, the structure of the steel sheet according to the present invention in the central part of the thickness is bainite except for the ferrite and the hard phase. Here, the bainite includes so-called “bainitic ferrite” and “acicular ferrite”.

4-1-4. Average crystal grain size at the center of the plate thickness In the center of the plate thickness, the crystal grain size is most easily coarsened, and the toughness is apt to be reduced. Such coarsening of crystal grains in the center portion of the plate thickness lowers DWTT characteristics. For this reason, in the DWTT characteristic, the average crystal grain size at the center of the plate thickness is particularly important. Specifically, when the average crystal grain size of the ferrite at the center of the plate thickness exceeds 20.0 μm, the DWTT characteristics deteriorate. Therefore, in the steel sheet according to the present invention, the average crystal grain size of ferrite at the center of the sheet thickness is set to 20.0 μm or less, and preferably 18.0 μm or less.

  The lower limit of the average crystal grain size of the ferrite is not particularly defined, but usually, the average crystal grain size of the ferrite at the center of the plate thickness is 10.0 μm or more.

4-2. In the steel sheet according to the present invention, in order to reduce the characteristic difference between the surface layer and the center of the sheet thickness, the area ratio of the ferrite in the surface layer and the area of the ferrite in the center of the sheet thickness are reduced. (Hereinafter simply referred to as “difference in ferrite area ratio”) is set to 5.0% or less.

  That is, the value obtained by subtracting the area ratio of the ferrite at the center of the thickness from the area ratio of the ferrite in the surface layer of the steel sheet is in the range of -5.0 to 5.0%. The surface layer in the present invention refers to a position at a depth of 1.0 mm from the surface of the steel sheet in the thickness direction.

  The DWTT characteristic is required to be good not only in a part such as a surface layer or a central part of a sheet thickness but also in the whole steel sheet. If the difference in the ferrite area ratio is more than 5.0%, a large difference occurs in the elongation at impact between the surface layer and the central part of the sheet thickness, and the DWTT characteristics deteriorate. Therefore, the difference in the ferrite area ratio is set to 5.0% or less, and preferably 4.0% or less. Note that the difference in the ferrite area ratio is derived by calculating the ferrite area ratio in the surface layer and the center portion of the plate thickness by the above-described method, and calculating the difference.

4-3. Maximum long diameter of inclusions Since coarse inclusions cause HIC, it is preferable to suppress generation as much as possible. Here, examples of the inclusion include MnS (manganese sulfide) and NbC (niobium carbide). In the steel sheet according to the present invention, the maximum major axis of the inclusions is 30.0 μm or less, and preferably 25.0 μm or less. As described above, if the maximum major axis of the inclusions exceeds 30.0 μm, HIC is likely to occur.

  The major axis of the inclusion is measured by setting the magnification to 1000 times, photographing the inclusion with an electronic scanning microscope (SEM), identifying the inclusion from the image data, and measuring the longest linear end as the major axis. In addition, the number of measurement visual fields is five.

5. The thickness of the steel sheet according to the present invention is in the range of 30 to 45 mm depending on the use.

6. Characteristics In the steel sheet according to the present invention, the strength when manufactured into a steel pipe has a strength of X52 to 65 grade of American Petroleum Institute Standard API 5L (hereinafter, simply referred to as “API 5L”), that is, tensile strength (hereinafter, referred to as “API 5L”). The target is to satisfy the range of 460 to 760 MPa and the yield strength (hereinafter also referred to as “YS”) of 360 to 600 MPa. When the DWTT ductile fracture rate is 85% or more, the DWTT characteristic is judged to be good, and the HIC resistance is evaluated by performing an HIC test. to decide.

7. Manufacturing Method A steel slab having the above chemical composition is manufactured by a continuous casting method. Hereinafter, a method for manufacturing a steel sheet according to the present invention will be described.

7-1. Heating process The steel slab is heated in a temperature range of 1100 to 1250 ° C. and soaked. The main purpose of heating the steel slab is to smoothly perform the rolling process by the softening effect of the heating. Further, HIC can be prevented from being generated by dissolving Nb carbonitride present in the steel slab and dissolving Nb in solid solution. For this reason, it is preferable to heat the slab to 1100 ° C. or higher, and to heat to 1130 ° C. or higher.

  On the other hand, if the heating temperature is too high, energy used for heating is wasted. Also, the occurrence of scale flaws increases. On the other hand, if the heating temperature is too high, the crystal grains become coarse, and the DWTT characteristics may decrease. For this reason, it is preferable to heat the slab at 1250 ° C. or lower and at 1200 ° C. or lower. Further, after heating, it is necessary to perform sufficient soaking. If the soaking temperature is insufficient and the temperature of the billet greatly differs in each part, not only the rolling becomes uneven in the subsequent rolling process, but also the structure control is well performed in the cooling process after the rolling process. Can not do. As a result, a desired metal structure cannot be obtained.

7-2. Rolling process In rolling, rough rolling is performed on a steel slab, and the steel slab is used as a rolled body. Subsequently, finish rolling is performed on the above-mentioned rolled body to obtain a steel sheet. Note that the finish rolling is started from a temperature lower than the surface temperature at the end of the rough rolling. In the finish rolling, the finish rolling is started when the surface temperature of the rolled body is in the range of Ar ₃ points −10 ° C. to Ar ₃ points + 60 ° C., and the finish rolling is completed at a higher surface temperature than at the start of the finish rolling.

In finish rolling, the inside of the rolled body is brought into a non-recrystallization temperature range, and the grains are refined by rolling down. If the finish rolling start temperature is lower than the Ar ₃ point −10 ° C., the rolled body does not sufficiently reheat during the finish rolling as described later. On the other hand, if the surface temperature of the rolled body at the start of the finish rolling is more than the _three points of Ar + 60 ° C., the pressure is reduced in the recrystallization temperature range, and the crystal grains are coarsened. As a result, the DWTT characteristics deteriorate. As a result, ferrite is excessively generated, sufficient strength cannot be obtained, and the HIC resistance decreases.

Note that, in the present invention, the Ar ₃ points are defined by the following formula (a).
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo + 0.35 (t-8) (a)
However, the symbols in the above formula are defined as follows, and the element symbols represent the contents (% by mass) of each element, and are 0 when not contained.
t (mm): thickness of steel sheet after rolling is completed

  In the finish rolling, the finish rolling is completed at a higher surface temperature than at the start of the finish rolling. In the finish rolling, recuperation in which the surface temperature of a steel sheet rises due to internal heat is used. Usually, due to heat removal during rolling, the surface temperature at the time of finish rolling is lower than at the start of finish rolling. However, in the production of the steel sheet according to the present invention, heat stored inside the steel sheet is used, and the surface temperature is increased by recuperation. Thereby, it is possible to control so that the surface temperature is higher at the time of finishing rolling than at the time of starting finishing rolling. As described above, by increasing the surface temperature by the recuperation, the temperature difference between the surface layer of the steel sheet and the central part of the sheet thickness is reduced, and as a result, the structure in the entire sheet thickness can be made uniform.

  In the rough rolling, the pressure is reduced to approximately 3 to 5 times the final thickness of the steel sheet. Then, it is further reduced by finish rolling to obtain a steel plate having a thickness of 25 to 40 mm.

7-3. Cooling process Even after rolling, the steel sheet is reheated as it is, and then the steel sheet is water-cooled. Water cooling is started from a temperature range where the surface temperature of the steel sheet is Ar ₃ points + 10 ° C. to Ar ₃ points + 50 ° C. In addition, water cooling is started at a temperature at which the surface temperature of the steel sheet is higher than the surface temperature at the start of finish rolling. If the water-cooling start temperature (surface temperature) is lower than the Ar ₃ point + 10 ° C., the production of ferrite becomes excessive and sufficient strength cannot be obtained. In addition, the HIC resistance is deteriorated due to an excessive amount of ferrite. On the other hand, if the water-cooling start temperature is more than the Ar ₃ point + 50 ° C., the crystal grains of the steel sheet become coarse, and the desired DWTT characteristics cannot be obtained.

  As described above, water cooling is started at a temperature at which the surface temperature of the steel sheet is higher than at the start of finish rolling. Since the steel sheet according to the present invention has a large thickness, as described above, the surface temperature increases due to internal heat of the steel sheet immediately after finish rolling from finish rolling due to reheating. Then, after a certain time has passed after the finish rolling, the surface temperature of the steel sheet gradually decreases. When water cooling is started after the temperature of the steel sheet decreases, the temperature difference between the surface layer of the steel sheet and the center of the thickness increases. Therefore, in order to equalize the temperature of the steel sheet and to equalize the structure of the entire steel sheet, water cooling is started at a temperature higher than that at the start of finish rolling of the steel sheet.

  Then, water cooling is stopped in a temperature range of 400 to 500 ° C., and air cooling is performed to room temperature. If the temperature at which the water cooling is stopped is higher than 500 ° C., the amount of ferrite generated increases, and the desired strength and HIC resistance cannot be obtained. On the other hand, if the water cooling stop temperature is less than 400 ° C., the amount of ferrite decreases, the bainite phase increases too much, and the DWTT characteristics deteriorate.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

  A 300 mm thick slab having the composition shown in Tables 1 and 2 was produced by a continuous casting method. It was manufactured under the manufacturing conditions shown in Tables 3 and 4, and then air-cooled to room temperature to manufacture a steel sheet.

  The metal structure and properties of the manufactured steel sheet were examined by the following methods. From the center of the width of the steel sheet, cut out a sample of the entire thickness of the steel sheet, mirror-polished the L cross section, repeller eroded, 1.0 mm deep in the thickness direction from the surface of the steel sheet, and the center of the thickness, The microstructure was photographed at a magnification of 500 times with an optical microscope, and the area ratio of MA was determined by image analysis. On the other hand, for the microstructure of the hard phase other than ferrite and MA, the repeller-corroded sample is reused, mirror-polished again, nital-corroded, and the microstructure is photographed at a magnification of 500 times with an optical microscope, and the area is determined by image analysis. The rate was determined. In calculating the area ratio, five photographing visual fields were set, the average value was defined as the area ratio of MA, ferrite, and a hard phase other than MA, and the remainder calculated from the sum of these area ratios was defined as the bainite area ratio.

  The average grain size of the ferrite at the center of the plate thickness was obtained by obtaining the equivalent circle grain size of each ferrite grain using the result of image analysis from a sample subjected to nital corrosion, and using the average value as the average grain size. Also in this case, the photographing visual field was set to 5 visual fields. The inclusions were photographed at a magnification of 1000 times by SEM, the inclusions were identified from the image data, and the longest straight end was measured as the major axis.

  From the center of the width of the steel sheet, two full-length test specimens each conforming to the American Petroleum Institute Standard API 5L were sampled, and the tensile test was performed at room temperature to determine the yield stress and the tensile strength. Considering use in the deep sea, those satisfying X52 to X65 grades (tensile strength of 460 to 760 MPa, yield strength of 360 to 600 MPa) were judged as good.

  From the center of the steel sheet in the width direction, a DWT test piece having the entire thickness in the width direction was sampled. The DWT test was also performed at −20 ° C. based on API 5L, and the DWTT ductile fracture rate was measured. Here, when the DWTT ductile fracture rate was 85% or more, the DWTT resistance was determined to be good.

  The HIC resistance was evaluated by performing a HIC test for 96 hours in an A solution immersion time according to NACE Standard TM 0284. If no cracks were observed, the HIC resistance was judged to be good, and a circle was given. Indicated by x.

  These properties are shown in Tables 5 and 6.

Steel type No. Samples Nos. 1 to 20 satisfied the requirements of the present invention and preferred production conditions, and were thus able to obtain good strength, DWTT characteristics, and HIC resistance. On the other hand, steel type No. Samples Nos. 21 to 37 did not satisfy the chemical composition defined in the present invention and resulted in inferior strength, at least one of DWTT characteristics and HIC resistance. No. 1 satisfies the chemical composition specified in the present invention but does not satisfy the preferable production conditions. As for 38 to 47, at least one of the strength, the DWTT characteristic, and the HIC resistance was inferior.

Claims (4)

The chemical composition of the steel sheet is expressed in mass%
C: 0.03 to 0.08%,
Si: 0.10 to 0.50%,
Mn: 1.00 to 1.80%,
P: 0.020% or less,
S: 0.0010% or less,
Cu: 0.10 to 0.50%,
Ni: 0.10 to 0.50%,
Nb: 0.005 to 0.050%,
Ti: 0.005 to 0.030%,
Al: 0.010-0.040%,
N: 0.0010 to 0.0050%,
B: 0.001% or less,
Cr: 0 to 0.50%,
Mo: 0 to 0.10%,
V: 0 to 0.05%,
Ca: 0 to 0.02%,
Mg: 0 to 0.02%,
REM: 0-0.02%,
The balance: Fe and impurities,
Pcm represented by the following formula (i) is 0.15 to 0.23,
Ceq represented by the following formula (ii) is 0.35 to 0.43,
The metal structure at the center of the sheet thickness is the area ratio,
20 to 35% ferrite, and no more than 2.0% hard phase,
The rest is bainite, and
The average crystal grain size of the ferrite at the center of the plate thickness is 20.0 μm or less,
A difference between the area ratio of ferrite in the surface layer of the steel sheet and the area ratio of ferrite in the center part of the sheet thickness is 5.0% or less;
The maximum major axis of the inclusion is 30.0 μm or less;
Thick steel plate for line pipes having a thickness of 30 to 45 mm.
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (i)
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (ii)
However, each element symbol in the above formulas (i) and (ii) represents the content (% by mass) of each element contained in the steel, and is zero if not contained. The chemical composition is expressed in mass%;
Cr: 0.10 to 0.50%,
Mo: 0.05-0.10%, and V: 0.01-0.05%,
The thick steel plate for line pipes according to claim 1, comprising at least one selected from the group consisting of: The chemical composition is expressed in mass%;
Ca: 0.0005-0.02%,
Mg: 0.0005-0.02%, and REM: 0.0005-0.02%,
The thick steel plate for a line pipe according to claim 1, comprising at least one selected from the group consisting of: A method for producing a thick steel plate for a line pipe according to any one of claims 1 to 3,
(A) heating a billet to a temperature range of 1100 to 1250 ° C. and soaking;
(B) a step of subjecting the slab to rough rolling to form a rolled body;
(C) Finish rolling is started when the surface temperature of the rolled body is in the range of Ar ₃ points −10 ° C. to Ar ₃ points + 60 ° C., and the finish rolling is completed at a higher surface temperature than at the start of the finish rolling. And the process of
(D) water cooling is started at a surface temperature of the steel sheet in the range of Ar ₃ points + 10 ° C. to Ar ₃ points + 50 ° C. and higher than the surface temperature at the start of finish rolling;
Stopping the water cooling in a temperature range of 400 to 500 ° C., and air cooling to room temperature;
A method for producing a thick steel plate for a line pipe, comprising: