JP7155702B2 - Thick steel plate for sour linepipe and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a thick steel plate for sour line pipes and a method for manufacturing the same.

Various performances are required for steel pipes used in pipelines for oil and natural gas transportation. One example is sour resistance (hereinafter also referred to as “SSC resistance”). The environment in which steel pipes for line pipes are used is an acidified, severely corrosive environment containing hydrogen sulfide and the like. Corrosion occurs in the steel pipe due to the flow of crude oil inside the pipe, and sulfide stress cracking (hereinafter also referred to as "SSC") may occur starting from the corrosion that has occurred. In order to prevent the occurrence of such SSC, steel pipes for line pipes are required to have excellent sour resistance as a basic performance, and in addition, "HIC resistance" and "DWTT characteristics" are also required. .

"HIC resistance" is a property that indicates the resistance to hydrogen-induced cracking (hereinafter referred to as "HIC") that occurs even when external stress does not act. "HIC" is known to occur in an environment where hydrogen sulfide or the like coexists with moisture, such as the environment in which steel pipes for line pipes are used. Further, "DWTT properties" are properties obtained when a drop weight tearing test (DWTT), which is one of the methods for evaluating toughness at low temperature, especially brittle crack arrestability, is performed.

Furthermore, when used in the deep sea at a depth of 1000 m or more, the steel pipe is required to have a thicker material, that is, a thicker material, from the viewpoint of pressure resistance.

JP 2012-241270 A

HIC resistance and DWTT properties are generally contradictory properties. In addition to increasing the wall thickness of steel pipes for line pipes, it is necessary to equip them with these conflicting characteristics. Therefore, it is necessary to appropriately control the manufacturing method, structure, and the like. As a technology for appropriately controlling the manufacturing method, structure, etc., for example, Patent Literature 1 discloses a sour-resistant line pipe having excellent resistance to crushing.

In the sour resistant line pipe disclosed in Patent Document 1, descaling is performed after the slab is hot-rolled, and immediately thereafter accelerated cooling is performed to 300 to 600° C. by adjusting the cooling rate of the surface layer and the average cooling rate of the steel plate. ing. Subsequently, it is cooled to room temperature, formed into a tube, and welded to adjust the microstructure and hardness at a predetermined site.

However, in Patent Document 1, it is necessary to perform descaling after increasing the impingement pressure of the jet against the steel sheet, which requires a special manufacturing apparatus. Further, in Patent Document 1, it cannot be said that the correlation among plate thickness, HIC resistance, DWTT characteristics, and SSC resistance is sufficiently studied.

An object of the present invention is to solve the above problems and to provide a thick steel plate for sour line pipes having excellent HIC resistance, DWTT resistance, and SSC resistance, and a method for producing the same.

The present invention has been made to solve the above problems, and the gist of the present invention is the following thick steel plate for sour line pipes and a method for producing the same.

(1) chemical composition, in mass %,
C: 0.03 to 0.06%,
Si: 0.10 to 0.60%,
Mn: 1.30-1.80%,
P: 0.010% or less,
S: 0.0010% or less,
Nb: 0.003 to 0.040%,
Al: 0.0010 to 0.050%,
Ti: 0.005 to 0.020%,
N: 0.002 to 0.006%,
Ca: 0.0010 to 0.0050%,
O: 0.0030% or less,
B: 0.0003% or less,
Cu: 0-0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.30%,
Mo: 0-0.20%,
V: 0 to 0.10%,
Mg: 0-0.010%,
REM: 0-0.010%,
balance: Fe and impurities,
and satisfying the following formula (i),
The metal structure in the surface layer is the area ratio,
10-40% ferrite and 3.0% or less hard phase,
the balance is bainite, and
The average crystal grain size is 15.0 μm or less,
The area ratio of the metal structure at the center of the sheet thickness is
30% or less ferrite and 2.0% or less hard phase,
the balance is bainite, and
The average crystal grain size is 20.0 μm or less,
The surface layer hardness is 200 or less in terms of Vickers hardness,
A thick steel plate for sour line pipes, having a plate thickness of 25 to 40 mm.
0.05≦Cu+Ni+Cr+Mo+V≦0.80 (i)
However, each element symbol in the above formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

(2) the chemical composition, in mass %,
Mg: 0.003-0.010%, and REM: 0.005-0.010%,
containing one or more selected from
The thick steel plate for sour line pipes according to (1) above.

(3) A method for manufacturing the thick steel plate for sour line pipes according to (1) or (2) above,
(a) a step of heating the steel slab to a temperature range of 1100 to 1250° C. for soaking;
(b) a step of rough rolling the steel slab, starting finish rolling from a temperature range where the surface temperature is 900 to 760° C., and completing finish rolling at 700° C. or higher to obtain a steel plate;
(c) starting to cool the finish-rolled steel sheet within 80 seconds after the completion of the finish rolling, and cooling the surface of the steel sheet at a cooling rate of 15 to 150° C./s; A step of stopping the cooling in a temperature range in which the surface temperature of the steel plate is 650 ° C. or lower and the temperature at 1/4 part of the plate thickness is 680 ° C. or higher;
(d) a step of reheating the surface temperature of the steel plate whose cooling has been stopped in (c) above to 700° C. or higher;
(e) cooling the double-heated steel plate so that the cooling rate at the center of the plate thickness is 3 to 40 ° C./s, and stopping cooling when the surface temperature of the steel plate is in the temperature range of 500 to 300 ° C.; followed by a step of air cooling to room temperature,
A method for manufacturing thick steel plates for sour line pipes.

According to the present invention, it is possible to obtain a thick steel plate for sour line pipes that is excellent in HIC resistance, DWTT resistance, and SSC resistance.

The present inventors assumed use in deep seas of 1,000 m or more, and studied steel pipes for sour line pipes having the required HIC resistance, DWTT resistance, and SSC resistance. As a result, the following findings (a) to (d) were obtained.

(a) In order to obtain the desired HIC resistance, it is preferable to suppress the formation of ferrite at the thickness center of the steel sheet where center segregation is likely to occur, and to have a bainite-based structure.

(b) DWTT properties improve as the amount of ferrite in the steel increases. On the other hand, if the amount of ferrite is excessive, the strength and HIC resistance deteriorate. Therefore, it is effective to generate a relatively large amount of ferrite in the surface layer while securing a predetermined amount of bainite in the central portion of the plate thickness of the steel plate. As a result, the DWTT properties can be improved, and deterioration in strength and HIC resistance properties can be suppressed.

(c) Since crude oil or the like directly flows through the steel pipe, SSC may occur. In order to prevent the occurrence of SCC in a steel plate, which is a steel pipe material, the SSC resistance of the surface is important. Since ferrite has the effect of improving the SSC resistance, it is preferable that the surface layer has a structure with a relatively large amount of ferrite.

(d) In order to obtain a metal structure such as (a) to (c) above, it is effective to carry out two stages of cooling in the cooling process during production of the steel sheet. That is, in the first-stage cooling, only the surface layer is cooled rapidly to maximize the temperature difference between the surface layer and the inside of the steel sheet. As a result, the steel sheet has a significantly different metallographic structure at each portion of the plate thickness. After the first-stage cooling, the water cooling is temporarily stopped, and the surface temperature of the steel sheet rises due to the internal heat, causing so-called multiple heat. After that, when the surface temperature of the steel sheet becomes close to the temperature at the center of the sheet thickness, the second-stage cooling is performed to control the structure.

The present invention has been made based on the above findings. Each requirement of the present invention will be described in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, "%" about content in the following description means "mass %."

C: 0.03-0.06%
C is an element necessary for ensuring strength. If the C content is less than 0.03%, the strength described later cannot be obtained. Therefore, the C content is 0.03% or more, preferably 0.04% or more. However, if the C content exceeds 0.06%, the formation of carbides is promoted, impairing the HIC resistance. Therefore, the C content is set to 0.06% or less, preferably 0.05% or less.

Si: 0.10-0.60%
Si has a deoxidizing effect and also has an effect of strengthening steel. If the Si content is less than 0.10%, deoxidation will be insufficient, so the Si content should be 0.10% or more, preferably 0.20% or more. However, when the Si content exceeds 0.60%, a large amount of martensite is generated in the weld heat affected zone (hereinafter referred to as "HAZ"), resulting in extremely poor toughness. Therefore, the Si content is 0.60% or less, preferably 0.50% or less.

Mn: 1.30-1.80%
Mn has the effect of strengthening steel and increasing toughness. If the Mn content is less than 1.30%, the strength described later cannot be obtained. Therefore, the Mn content is 1.30% or more, preferably 1.40% or more. However, when the Mn content exceeds 1.80%, the center segregation in the slab increases and HIC tends to occur. Therefore, the Mn content is 1.80% or less, preferably 1.70% or less.

P: 0.010% or less P is an impurity and is preferably reduced as much as possible. When the P content becomes excessive, especially exceeding 0.010%, the center segregation in the slab increases and the hardness increases locally. Therefore, the P content should be 0.010% or less.

S: 0.0010% or less S, like P, is an impurity and is preferably reduced as much as possible. When the S content exceeds 0.0010%, a large amount of MnS, which is an inclusion harmful to steel, is generated. Therefore, the S content should be 0.0010% or less.

Nb: 0.003-0.040%
Nb expands the non-recrystallized region and facilitates the introduction of dislocations during rolling. As a result, Nb has the effect of forming a fine structure in the steel sheet. Therefore, the Nb content is set to 0.003% or more. The Nb content is preferably 0.004% or more, more preferably 0.005% or more.

On the other hand, Nb forms Nb carbonitrides in the slab, and these Nb carbonitrides do not dissolve in the matrix and form clusters. Specifically, when the Nb content becomes excessive and exceeds 0.040%, the Nb carbonitrides form clusters with a size exceeding 10 μm. Then, this Nb carbonitride becomes a starting point and causes the generation of HIC. Therefore, the content of Nb is set to 0.040% or less. The Nb content is preferably 0.035% or less, more preferably 0.030% or less.

Al: 0.0010-0.050%
Al is an element necessary for deoxidation. Therefore, the Al content is set to 0.0010% or more, preferably 0.0030% or more. However, when the Al content exceeds 0.050%, toughness tends to deteriorate in the HAZ. This is because coarse cluster-like alumina-based inclusion particles are likely to be formed. Therefore, the Al content is set to 0.050% or less, more preferably 0.040% or less.

Ti: 0.005-0.020%
Ti acts as a deoxidizing agent and forms nitrides to contribute to grain refinement of steel. Therefore, the Ti content is set to 0.005% or more. The Ti content is preferably 0.006% or more, more preferably 0.007% or more. However, when the Ti content exceeds 0.020%, coarse carbonitrides are formed and the toughness is lowered. Therefore, the Ti content should be 0.020% or less. The Ti content is preferably 0.018% or less, more preferably 0.016% or less.

N: 0.002-0.006%
N forms nitrides (TiN, NbN, etc.) with Ti, Nb, and the like. The formed nitride suppresses the growth of austenite grains during heating and refines the crystal grains. If the N content is less than 0.002%, sufficient nitrides are not formed and the grains of the steel sheet cannot be refined. Therefore, the N content is set to 0.002% or more, preferably 0.003% or more. However, when the N content exceeds 0.006%, carbonitrides in which Ti or Nb is combined with N and C accumulate, resulting in a significant decrease in toughness. Therefore, the N content is 0.006% or less, preferably 0.005% or less.

Ca: 0.0010-0.0050%
Ca has an effective effect on modifying sulfide inclusions and spheroidizing alumina inclusions. If the Ca content is less than 0.0010%, these effects cannot be obtained, and the occurrence of HIC due to MnS and/or alumina clusters cannot be suppressed. Therefore, the Ca content is set to 0.0010% or more, preferably 0.0020% or more. However, when the Ca content exceeds 0.0050%, CaS clusters may be generated. Therefore, the Ca content is set to 0.0050%, and the Ca content is preferably 0.0040% or less.

O: 0.0030% or less O (oxygen) is present in steel as an impurity, and when the content is large, it adversely affects the toughness of the base material. Specifically, when the O content exceeds 0.0030%, the toughness of the base material deteriorates significantly. Therefore, the O content is set to 0.0030% or less.

B: 0.0003% or less When B is contained, hardenability can be improved, but ferrite formation may be suppressed and toughness may be lowered. Therefore, even if B is contained as an impurity, the B content should be 0.0003% or less, preferably 0.0002% or less.

Cu: 0-0.50%
Ni: 0-0.50%
Cr: 0-0.30%
Mo: 0-0.20%
V: 0-0.10%
Cu, Ni, Cr, Mo and V have the effect of increasing strength. Therefore, in addition to the above elements, the steel plate of the present invention further contains one or more elements selected from Cu, Ni, Cr, Mo and V, and the total content of these elements is as follows (i ) satisfies the expression

0.05≦Cu+Ni+Cr+Mo+V≦0.80 (i)
However, each element symbol in the above formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained.

In the steel plate according to the present invention, the median value of the above formula (i) is set in the range of 0.05 to 0.80%. If the median value in the formula (i) is less than 0.05%, sufficient strength cannot be ensured. Therefore, the median value in formula (i) is set to 0.05% or more, preferably 0.10% or more. On the other hand, when the median value in the above formula (i) exceeds 0.80%, the effect is saturated. Therefore, the median value in the above formula (i) is set to 0.80% or less, preferably 0.70% or less. Each element will be specifically described below.

Cu has the effect of improving the strength. When Cu is contained, the strength can be further increased by age hardening due to Cu, especially when the heat treatment of quenching and tempering is performed. Cu also has the effect of improving corrosion resistance. Therefore, it may be contained as necessary. However, even if the Cu content exceeds 0.50%, the improvement in performance commensurate with the increase in cost is not observed. Therefore, the Cu content is 0.50% or less, preferably 0.40% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.10% or more.

When Ni is contained, the hardenability is enhanced and the strength can be improved. In addition to improving the strength, Ni solid-soluted in the matrix has the effect of increasing the toughness of the steel matrix. Therefore, it may be contained as necessary. However, even if the Ni content exceeds 0.50%, the improvement in performance commensurate with the increase in cost is not observed. Therefore, the Ni content is 0.50% or less, preferably 0.40% or less. On the other hand, in order to obtain the above effects, the Ni content is preferably 0.10% or more.

Inclusion of Cr can increase the strength. Cr is difficult to concentrate in the center segregation part during the solidification process of the slab. Therefore, when the hot-rolled steel plate is water-cooled, the transformation from austenite to ferrite and/or pearlite is retarded to enhance hardenability. Thereby, Cr increases the strength of the steel sheet. Cr also produces a precipitation hardening effect of fine special carbides during tempering or high temperature SR processing. As a result, Cr increases the softening resistance and retards the softening of the base ferrite.

Thus, Cr is very effective in ensuring both HIC resistance and high strength. Therefore, it may be contained as necessary. However, when the Cr content exceeds 0.30%, workability during welding is extremely lowered and the cost is increased. Therefore, the Cr content is 0.30% or less, preferably 0.25% or less. On the other hand, in order to obtain the above effects, the Cr content is preferably 0.10% or more.

Mo has the effect of improving the strength. Mo also has the effect of improving the toughness. Therefore, it may be contained as necessary. However, if the Mo content exceeds 0.20%, the HAZ, in particular, increases in hardness and impairs toughness and SSC resistance. Therefore, the Mo content is set to 0.20% or less.

V has the effect of improving the strength. Specifically, V mainly precipitates carbonitrides during tempering, thereby improving the strength. Therefore, it may be contained as necessary. However, even if the V content exceeds 0.10%, the effect of improving the strength is saturated and the cost increases. Moreover, deterioration of toughness also occurs. Therefore, the V content is 0.10% or less, preferably 0.06% or less.

The steel plate according to the present invention may contain one or more elements selected from Mg and REM in addition to the above elements.

Mg: 0-0.010%
Mg has the effect of improving hot workability. Mg also has the effect of forming Mg-containing oxides and serving as TiN generation nuclei to finely disperse TiN. Therefore, it may be contained as necessary. However, when the Mg content exceeds 0.010%, excessive oxides are produced, resulting in a decrease in ductility. Therefore, the Mg content should be 0.010% or less. In order to obtain the above effects, the Mg content is preferably 0.003% or more.

REM: 0-0.010%
REM has the effect of improving hot workability. REM also has the effect of refining the HAZ structure. Therefore, REM may be contained as necessary. However, excessive REM content causes the formation of inclusions and reduces cleanliness. Inclusions formed by containing REM have a relatively small effect on the reduction in toughness, but if the REM content exceeds 0.010%, the reduction in toughness of the base metal due to the inclusions cannot be ignored.

Therefore, the REM content is 0.010% or less, preferably 0.009% or less. On the other hand, in order to obtain the above effects, the REM content is preferably 0.005% or more.

In addition, "REM" in the present invention is a generic term for a total of 17 elements of Sc, Y and lanthanoids, and the content of REM refers to the total content of one or more elements among REM.

The balance of the steel plate according to the present invention is Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when steel is manufactured industrially, and are allowed within a range that does not adversely affect the steel plate of the present invention. means to be

2. Metal structure In the steel sheet according to the present invention, the main phase is bainite throughout the sheet thickness. However, bainite alone cannot provide all of the desired DWTT properties, HIC resistance, and SSC resistance. For this reason, in the present invention, by taking advantage of the thick plate thickness, each portion of the plate thickness is provided with the required functions, specifically, the DWTT characteristics, the HIC resistance characteristics, and the SSC resistance characteristics.

2-1. Metal structure in surface layer 2-1-1. Ferrite The area ratio of ferrite in the surface layer is in the range of 10 to 40%. The ferrite formed on the surface layer of the steel sheet stabilizes the DWTT characteristics. Also, the ferrite in the surface layer improves the SSC resistance. Therefore, in the surface layer, the area ratio of ferrite is 10% or more, preferably 15% or more. On the other hand, excessive ferrite in the surface layer degrades the HIC resistance. Therefore, the area ratio of ferrite in the surface layer is 40% or less, preferably 35% or less. In addition, in the steel sheet according to the present invention, the surface layer refers to a position at a depth of 0.5 mm from the surface of the steel sheet.

2-1-2. Hard Phase The metal structure of the surface layer basically consists of bainite and ferrite, but it may partially contain a hard phase. It is preferable that the hard phase is not contained as much as possible, but if it is contained, the area ratio is 3.0% or less. This is because if the surface layer contains more than 3.0% of the hard phase, it becomes a starting point for SSC and deteriorates the SSC resistance. Therefore, the area ratio of the hard phase in the surface layer is 3.0% or less, preferably 1.0% or less. The hard phase refers to island-shaped martensite, pearlite, and the like.

2-1-3. The metallographic structure of the bainite surface layer is composed of bainite as the main phase and the balance other than ferrite and hard phase as bainite phase. The bainite includes so-called “bainitic ferrite” and “acicular ferrite”. Bainite has a higher strength structure than ferrite. Therefore, by using bainite as the main phase, it is possible to easily achieve a strength grade of 520 MPa or more in TS and 415 MPa or more in YS, which will be described later, when a steel pipe is manufactured.

Therefore, the metallographic structure of the surface layer should contain 10 to 40% ferrite, 3.0% or less hard phase, and the balance bainite in terms of area ratio.

In addition, in this invention, a metal structure is observed in the following procedures. Specifically, a sample of the full thickness of the steel plate is cut from the center of the width of the steel plate, and the L cross section (vertical cross section in the rolling direction and the thickness direction) is mirror-polished. Grain size and ferrite fraction were measured using EBSD.

2-1-4. Average Grain Size The smaller the grain size of the metal structure, the better the DWTT characteristics. For this reason, the average grain size is preferably as small as possible in order to stabilize the DWTT characteristics. In a thick steel plate, the crystal grain size in the surface layer is smaller than that in the central portion of the plate thickness. Therefore, considering the average crystal grain size at the center of the plate thickness, the average crystal grain size of the metal structure in the surface layer is set to 15.0 μm or less.

The average crystal grain size is defined by the equivalent circle diameter of a region surrounded by grain boundaries with a large tilt angle of 15° or more when measured by electron backscatter diffraction (EBSD). Moreover, the measurement by EBSD shall be performed in a range of 200 μm×300 μm with a magnification of 400 times.

2-2. Metal structure at center of plate thickness 2-2-1. Ferrite It is preferable not to form ferrite as much as possible in the thickness center of the thick steel plate in order to ensure HIC resistance. However, if the area ratio of ferrite in the central portion of the sheet thickness is 30% or less, there is no effect on the HIC resistance. For this reason, the area ratio of ferrite is 30% or less, preferably 25% or less, in the central portion of the sheet thickness.

2-2-2. Hard Phase In addition to ferrite, the central portion of the sheet thickness may contain a hard phase within a range that does not affect the HIC resistance characteristics, that is, within an area ratio of 2.0% or less. For this reason, the area ratio of the hard phase in the central portion of the plate thickness is set to 2.0% or less, preferably 1.7% or less. The hard phase is as described above.

2-2-3. Bainite The metal structure at the center of the plate thickness is mainly bainite, and the remainder other than ferrite and hard phase is bainite. In addition, bainite is as described above.

Therefore, the metal structure at the central portion of the plate thickness is a structure containing 30% or less ferrite, 2.0% or less hard phase, and the balance bainite, which is the main phase, in terms of area ratio. The method for observing the metal structure is as described above.

2-2-4. Average grain size As described above, the smaller the grain size of the metal structure, the better the DWTT characteristics. The DWTT characteristics are stabilized by setting the average grain size at the central portion of the plate thickness, which is the most difficult to cool and where the crystal grain size is large, to be 20.0 μm or less. For this reason, the average grain size of the metal structure at the central portion of the plate thickness is set to 20.0 μm or less. Incidentally, the measurement of the average crystal grain size is as described above.

3. Surface Hardness The surface hardness of the steel sheet according to the present invention is Vickers hardness (hereinafter referred to as “HV hardness”) of 200 or less. When the surface layer hardness of the steel sheet exceeds 200 in terms of HV hardness, the cracking susceptibility increases and the SSC resistance deteriorates. Although the lower limit of the surface layer hardness is not specified, the HV hardness is usually 170 or more.

In addition, in the present invention, as described above, the surface layer refers to a position at a depth of 0.5 mm from the surface of the steel sheet. The surface layer hardness is measured by a Vickers hardness test with a pressing load of 100 g as follows. Specifically, the manufactured steel plate is cut in the direction parallel to the rolling, and the surface to 1 mm below the surface in the cross section is measured at a pitch of 0.1 mm, and the maximum hardness is defined as the surface layer hardness.

4. Plate Thickness In order to obtain the desired properties, the plate thickness of the thick steel plate according to the present invention is in the range of 25-40 mm.

5. Target characteristics In the steel plate according to the present invention, the strength when manufactured into a steel pipe is the strength of X60 grade of American Petroleum Institute standard API 5L (hereinafter simply referred to as “API 5L”), that is, TS 520 MPa or more, The target is to satisfy YS415 MPa or more. In addition, when the DWTT ductile fracture surface ratio is 85% or more, it is judged that the DWTT characteristics are good. Also, the HIC resistance and the SSC resistance are tested individually, and if cracks are not observed, it is determined that both characteristics are good.

6. Manufacturing Method A steel slab having the above chemical composition is manufactured by a continuous casting method. A method for manufacturing a thick steel plate according to the present invention will be described below.

6-1. Heating process The billet is heated in a temperature range of 1100-1250°C. The main purpose of heating the billet is to enable the rolling process to be carried out smoothly due to the softening effect of heating. Also, by dissolving the Nb carbonitrides present in the steel billet and making the Nb into a solid solution, it is possible to prevent the occurrence of HIC. For this reason, the billet is heated to 1100° C. or higher, preferably 1120° C. or higher.

On the other hand, if the heating temperature is too high, the energy required for heating will be wasted. For this reason, the billet is heated at 1250° C. or less, preferably at 1200° C. or less. Also, after heating, it is necessary to perform sufficient soaking. If soaking is insufficient and the temperature of the slab differs greatly in each part, not only will the rolling process become uneven in the subsequent rolling process, but it will also be difficult to control the structure well in the cooling process after the rolling process. can't As a result, a desired metal structure cannot be obtained.

6-2. Rolling process Rolling performs rough rolling on the steel plate, starts finish rolling from a temperature range where the surface temperature of the object to be rolled (thick steel plate) is 900 to 760 ° C., completes finish rolling at 700 ° C. or higher, and forms a steel plate. . If the finish rolling start temperature is higher than 900°C, grain refinement will be insufficient and the DWTT properties will not be stabilized. Therefore, the finish rolling start temperature is set to 900° C. or lower, preferably 880° C. or lower. On the other hand, if the finish rolling start temperature is lower than 760° C., the amount of ferrite increases in the central portion of the sheet thickness, and the HIC resistance deteriorates. Therefore, the finish rolling start temperature is set to 760° C. or higher, preferably 780° C. or higher. Moreover, when the finish rolling completion temperature is less than 700° C., the amount of ferrite increases in the central portion of the plate thickness, and the HIC resistance deteriorates. Therefore, the finish rolling completion temperature (surface temperature) is set to 700° C. or higher, preferably 750° C. or higher.

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

6-3. Cooling process After completion of finish rolling, cooling is performed. Cooling may be performed, for example, by water cooling. A case where cooling is performed by water cooling will be described below as an example. Cooling is divided into two stages, cooling is stopped between each cooling, and the surface temperature of the steel plate rises due to internal heat. In the following description, the initial cooling may be referred to as first-stage cooling, and the cooling after multiple heating may be referred to as second-stage cooling.

6-3-1. First Stage Cooling In the first stage cooling, cooling is started as soon as possible after the completion of finish rolling. Specifically, water cooling of the steel sheet is started within 80 seconds after completion of finish rolling. If water cooling is started more than 80 seconds after the completion of finish rolling, ferrite formation is accelerated before the start of water cooling after rolling, resulting in unstable HIC resistance. Therefore, the time from the completion of finish rolling to the start of water cooling is set within 80 seconds, preferably within 60 seconds. In addition, cooling by water cooling during the first stage cooling is performed so that the cooling rate on the surface of the steel plate is 15 to 150 ° C./s, and the surface temperature of the steel plate is 650 ° C. or less, and the thickness of the steel plate is 1/4. Cooling by water cooling is stopped when the temperature in the part is 680° C. or higher.

If the cooling rate during the first stage cooling is less than 15° C./s, it becomes difficult to secure the cooling rate inside the plate thickness. Therefore, the cooling rate is set to 15° C./s or more, preferably 20° C./s or more. On the other hand, when the cooling rate is more than 150° C./s, a hard structure having a HV hardness of more than 200 is formed. Therefore, the cooling rate is set to 150° C./s or less, preferably 100° C./s or less. By rapidly cooling only the surface of the thick steel plate in this manner, the temperature difference between the surface and the inside of the plate can be increased, and the area ratio of ferrite in the plate thickness direction can be controlled.

When the cooling is stopped, if the surface temperature of the steel sheet is over 650°C, it is difficult to secure the desired amount of ferrite in the surface layer. For this reason, the surface temperature of the steel sheet when stopping the cooling in the first stage cooling is set to 650° C. or lower, preferably 600° C. or lower. Further, when the cooling is stopped, if the temperature of the ¼ part of the thickness of the steel sheet is less than 680° C., it is difficult to secure the bainite structure inside the thickness. For this reason, when the cooling is stopped, the temperature at 1/4 part of the thickness of the steel sheet should be 680° C. or higher, preferably 700° C. or higher.

When the cooling is stopped, if there is a temperature difference between the surface of the steel sheet and the part of the thickness of 1/4, the surface temperature rises due to multiple heat. Here, the steel sheet whose cooling has been stopped is allowed to stand until the surface temperature reaches 700° C. or higher, and is subjected to multi-heating. Although the maximum surface temperature after reheating of the steel sheet whose cooling has been stopped is not specified, it is usually 800° C. or less.

6-3-2. Second Stage Cooling In the subsequent second stage cooling, if water cooling is started when the surface temperature of the steel sheet after reheating is less than 700° C., securing of the ferrite area ratio in the surface layer becomes insufficient.

In the second-stage cooling, the steel sheet is cooled at a cooling rate of 3 to 40° C./s at the thickness center. If the cooling rate of the second-stage cooling is less than 3°C/s, it becomes difficult to secure a bainite structure inside the sheet thickness. Therefore, the cooling rate is 3° C./s or more, preferably 5° C./s or more. On the other hand, if the cooling rate of the cooling is more than 40° C./s, formation of a hard structure becomes remarkable inside the plate thickness, and it becomes difficult to secure the HIC resistance characteristics. Therefore, the cooling rate of the above cooling is set to 40° C./s or less, preferably 30° C./s or less.

After that, when the surface temperature of the steel sheet falls within a temperature range of 500 to 300° C., the water cooling is stopped. When the cooling stop temperature exceeds 500°C, it becomes difficult to secure a bainite structure inside the sheet thickness. Therefore, the cooling stop temperature is set to 500° C. or lower, preferably 480° C. or lower. On the other hand, if the cooling stop temperature is less than 300° C., hard structures are significantly formed inside the sheet thickness, making it difficult to ensure HIC resistance. Therefore, the cooling stop temperature is set to 300° C. or higher, preferably 350° C. or higher. After that, it may be left as it is and air-cooled to room temperature.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to these examples.

Steel slabs with a thickness of 300 mm having compositions shown in Tables 1 and 2 were 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 thick steel plate.

The metal structure and properties of the manufactured steel plates were investigated by the methods described below. Specifically, a sample of the full thickness of the steel plate is cut from the center of the width of the steel plate, the L cross section is mirror-polished, the sample is adjusted with colloidal silica, and the average grain size and ferrite fraction are measured using EBSD. measured by Measurements were made at a position of 0.5 mm on the surface layer and at the center of the plate thickness. The ferrite grain size was measured by setting the magnification to 400 times and measuring a range of 200 μm×300 μm at a pitch of 0.25 μm. The crystal grain size was determined based on the inclination angle of 15°. As for the ferrite fraction, the threshold value of GAM was set to 0.5, and 0.5 or less was determined as a ferrite structure.

Two full-thickness test specimens conforming to API 5L were sampled from the center of the width of the steel sheet with the width direction as the longitudinal direction, and a tensile test was performed at room temperature to determine the yield stress and tensile strength. Considering the use in the deep sea, those satisfying TS of 520 MPa or more and YS of 415 MPa or more were judged to be good.

The hardness measurement is a Vickers hardness test, in which the pressed load is set to 100 g, the produced steel plate is cut in the direction parallel to the rolling, and the surface to 1 mm below the surface in the cross section is measured at a pitch of 0.1 mm. The maximum hardness was defined as the surface layer hardness.

A full-thickness DWT test piece with the width direction as the longitudinal direction was taken from the center of the width of the steel plate. The DWT test was also conducted twice at -30°C according to API 5L, and the lowest value was measured as the DWTT ductile fracture surface ratio. Here, when the DWTT ductile fracture surface ratio was 85% or more, the DWTT characteristics were considered to be good.

For HIC resistance, an HIC test was performed in accordance with NACE Standard TM 0284, with a immersion time of 96 hours in A solution. is indicated by x.

In the SSC test, a rectangular test piece with a thickness of 5 mm, a width of 15 mm, and a length of 115 mm was taken, and a stress equivalent to 90% of the yield strength was applied to the center of the test piece by four-point bending. were mixed to adjust the pH to 3.0, the immersion liquid was saturated with 100% hydrogen sulfide gas, and immersed for 720 hours. After immersion, the test piece was removed from the jig, washed with water, and then checked for SSC on the surface of the test piece at a magnification of 100 times. , and the case where cracking occurred was indicated by x.

Their textures and properties are shown in Tables 5 and 6.

Steel type no. Nos. 1 to 27 were able to obtain good strength, DWTT properties, SSC resistance properties and HIC resistance properties because they satisfied the provisions and preferred manufacturing conditions of the present invention. On the other hand, steel type No. Nos. 28 to 46 did not satisfy the composition specified in the present invention, resulting in inferiority in at least one of strength, DWTT characteristics, SSC resistance characteristics, and HIC resistance characteristics. Then, No. 1 satisfies the composition specified in the present invention but does not satisfy the preferred manufacturing conditions. The results of Nos. 47 to 59 were also inferior in at least one of the strength, DWTT characteristics, SSC resistance characteristics, and HIC resistance characteristics.

Claims (3)

The chemical composition, in mass %,
C: 0.03 to 0.06%,
Si: 0.10 to 0.60%,
Mn: 1.30-1.80%,
P: 0.010% or less,
S: 0.0010% or less,
Nb: 0.003 to 0.040%,
Al: 0.0010 to 0.050%,
Ti: 0.005 to 0.020%,
N: 0.002 to 0.006%,
Ca: 0.0010 to 0.0050%,
O: 0.0030% or less,
B: 0.0003% or less,
Cu: 0-0.50%,
Ni: 0 to 0.50%,
Cr: 0 to 0.30%,
Mo: 0-0.20%,
V: 0 to 0.10%,
Mg: 0-0.010%,
REM: 0-0.010%,
balance: Fe and impurities,
and satisfying the following formula (i),
The metal structure at a position 0.5 mm from the surface of the steel plate has an area ratio of
10-40% ferrite and 3.0% or less hard phase,
the balance is bainite, and
The average crystal grain size is 15.0 μm or less,
The area ratio of the metal structure at the center of the sheet thickness is
30% or less ferrite and 2.0% or less hard phase,
the balance is bainite, and
The average crystal grain size is 20.0 μm or less,
The maximum hardness measured at a pitch of 0.1 mm from the surface to 1 mm below the surface is defined as the surface layer hardness, and the surface layer hardness is 200 or less in terms of Vickers hardness,
A thick steel plate for sour line pipes, having a plate thickness of 25 to 40 mm.
0.05≦Cu+Ni+Cr+Mo+V≦0.80 (i)
However, each element symbol in the above formula represents the content (% by mass) of each element contained in the steel, and is zero when not contained. The chemical composition, in mass %,
Mg: 0.003-0.010%, and REM: 0.005-0.010%,
containing one or more selected from
The thick steel plate for sour line pipes according to claim 1. A method for manufacturing the thick steel plate for sour line pipes according to claim 1 or 2,
(a) a step of heating the steel slab to a temperature range of 1100 to 1250° C. for soaking;
(b) a step of rough rolling the steel slab, starting finish rolling from a temperature range where the surface temperature is 900 to 760° C., and completing finish rolling at 700° C. or higher to obtain a steel plate;
(c) starting to cool the finish-rolled steel sheet within 80 seconds after the completion of the finish rolling, and cooling the surface of the steel sheet at a cooling rate of 15 to 150° C./s; A step of stopping the cooling in a temperature range in which the surface temperature of the steel plate is 650 ° C. or lower and the temperature at 1/4 part of the plate thickness is 680 ° C. or higher;
(d) a step of reheating the surface temperature of the steel plate whose cooling has been stopped in (c) above to 700° C. or higher;
(e) cooling the double-heated steel plate so that the cooling rate at the center of the plate thickness is 3 to 40 ° C./s, and stopping cooling when the surface temperature of the steel plate is in the temperature range of 500 to 300 ° C.; followed by a step of air cooling to room temperature,
A method for manufacturing thick steel plates for sour line pipes.