JPH07278660A - Manufacture of steel plate for line pipe excellent in low temperature toughness and co2 corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a manufacturing method of the high tensile steel plate for the line pipe which is used for petroleum and natural gas containing CO2 and excellent in the CO2 corrosion. CONSTITUTION:The steel containing, by weight, 0.01-0.09% C, 0.5-1.2% Cr and Nb-Ti, is heated to the temperature of 1100-1250 deg.C after casting, the rolling is started after pulling-out, and the rolling is once stopped at the surface temperature of >=900 deg.C, Then, the steel is water cooled at the cooling speed of 5-40 deg.C-/sec until the surface temperature reaches <=700 deg.C, and air-cooled until the surface temperature is below Ac3 and the temperature of the thickness center is <=950 deg.C. The rolling reduction is successively executed where the accumulated draft is >=60% and the rolling true strain per mean pass is <=0.2, and the rolling is completed when the mean temperature of the plate is above Ar3. The steel is immediately cooled at the cooling speed of 5-40 deg.C/sec until the mean temperature of the plate is 350-500 deg.C, and then, air-cooled. This constitution manufactures the steel plate excellent in the low temperature toughness and the CO2 corrosion resistance.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to petroleum containing CO ₂ .
High-strength steel sheet for line pipes with excellent CO ₂ corrosion resistance used in natural gas (tensile strength: 500 MPa or more, sheet thickness 40
mm or less). Further, since the steel of the present invention is excellent in low temperature toughness and field weldability, it can be used in cold regions and offshore. In the steel industry, it is most preferable to apply it to thick plate mills, but it can also be applied to hot coils. The steel sheet produced by this method is also excellent in low-temperature toughness and field weldability, and is most suitable for use in cold regions and offshore.

[0002]

2. Description of the Related Art High-strength and excellent low-temperature toughness are required for large-diameter line pipes for oil and gas transportation in cold regions and offshore. Furthermore, in recent years,
Due to the decrease in the inhibitor effect due to the injection of CO ₂ and the deep well formation in the third recovery, the corrosion of the line pipe due to the CO ₂ gas became a big problem, and the CO ₂ corrosion resistance became required.

Since addition of a large amount of Cr hinders weldability,
Pre-heating at high temperature and post-heat treatment are indispensable from the viewpoint of welding crack prevention during on-site welding, which significantly reduces the work efficiency. In addition, if a large amount of Cr is added, the base metal and welding heat affected zone (HAZ)
Deteriorates toughness. Therefore, development of a steel sheet for a line pipe having excellent CO ₂ corrosion resistance, good low temperature toughness, and field weldability is strongly desired.

In Japanese Patent Laid-Open No. 03-212130, there is a C resistance.
A low-temperature CO ₂ corrosion resistant line pipe steel excellent in O ₂ corrosion resistance, low temperature toughness and field weldability is disclosed. However, in an environment containing CO ₂ , if the Cr content is 1.2% or less, the passive state region cannot be formed by Cr, so that the Cr content is 10%.
0.5 at NaCl + 1 atmCO ₂ (pH 5, 80 ° C.)
It is finally possible to secure mm / y. Therefore, C resistance in the organization
Development of steel for O ₂ corrosion line pipe is desired.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a steel sheet for line pipes having excellent CO ₂ corrosion resistance and low temperature toughness.

[0006]

The gist of the present invention is as follows. % By weight, C: 0.01 to 0.09%, Si: 0.
5% or less, Mn: 0.7 to 1.5%, P: 0.03% or less, S: 0.005% or less, Nb: 0.01 to 0.06.
%, Cr: 0.5 to 1.2%, Ti: 0.005 to 0.
03%, Al: 0.05% or less, N: 0.001 to 0.
005% and contains 0.35 ≦ C + Mn / 6 + (Cr +
V) / 5 + (Ni + Cu) /15≦0.48, steel consisting of balance Fe and unavoidable impurities, or the above alloy composition, and further V: 0.005
0.060%, Ni: 0.05 to 1.0%, Cu: 0.
05-1.0%, Mo: 0.05-0.30%, Ca:
0.001-0.005%, Zr: 0.005-0.0
25%, REM: One or more of 0.0008 to 0.01%, and 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Ni + C
u) /15≦0.48, steel made of residual Fe and unavoidable impurities is not cast into cold pieces or 1100 cold pieces after casting.
After heating to a temperature of ~ 1250 ° C, starting the rolling after extraction and cooling with water until the surface temperature becomes 700 ° C or less, the surface temperature is allowed to cool to Ac ₃ or less and the plate thickness center temperature becomes 950 ° C or less. Then, the rolling reduction is applied so that the cumulative rolling reduction is 60% or more and the rolling true strain per average pass is 0.2 or less, and the rolling is finished at the sheet thickness average temperature of Ar ₃ or more, and immediately after 5 to 40 The average temperature of the plate thickness is 350 to 5 at the cooling rate of ° C / s.
A method for producing a steel sheet excellent in low temperature toughness and CO ₂ corrosion resistance, which comprises cooling to 00 ° C. and then air cooling.

The present invention will be described in detail below. The present inventors have made detailed investigations mainly on the chemical composition and structure of the effect of non-uniform corrosion on CO ₂ corrosion resistance, and as a result, the following facts have been found. Addition of Cr improves CO ₂ corrosion resistance and suppresses uneven corrosion. However, addition of an excessive amount of Cr deteriorates low temperature toughness and field weldability. When the structure of the plate surface is a very fine ferrite and the structure of C is finely dispersed, the immersion potential is higher than that of the conventional controlled rolling + controlled cooling surface structure (which tends to be a band structure such as ferrite and bainite). 20mV rise,
CO ₂ corrosion resistance is improved.

That is, regarding CO ₂ corrosion, Cr addition is effective, but addition of an excessive amount of Cr causes deterioration of low temperature toughness and field weldability. In contrast, after regulating the upper limit of the Cr content, it is possible to reduce the effective cathode site by a further surface tissue is very fine ferrite a and C of finely dispersed tissue, resistance to CO ₂ The property is improved.

It will be described below that high temperature rolling of Ar ₃ or more is effective for improving low temperature toughness. Generally, there is a temperature difference of about 100 to 200 ° C. between the transformation temperature range generated in the temperature lowering process by cooling from high temperature and the transformation temperature region generated in the temperature rising process by heating from low temperature, and the transformation generated in the temperature rising process. The temperature range is higher. Therefore, as shown in FIG. 1, in the process in which the thick steel plate is once cooled to an appropriate temperature range and then reheated, the surface layer part of the plate thickness transforms from ferrite to austenite during temperature rise, but the central part of the plate thickness is The ferrite transformation has not yet started and the austenite single phase is still present.

Even when the temperature difference between the two is reduced due to excessive progress of recuperation, the surface metal part of the plate thickness has a metal structure mainly composed of ferrite, and the center part of the plate thickness has a metal structure mainly composed of austenite. A large difference in deformation resistance occurs between the two, and the deformation resistance of the surface layer portion of the plate thickness is extremely large. This is because, as shown in FIG. 2, the stress-strain relationship is different between the metal structure mainly composed of ferrite and the metal structure mainly composed of austenite, and the metal structure mainly composed of ferrite is more preferable in the range of rolling true strain of 0.2 or less. This is because the deformation resistance is large when the same strain is applied.

In the present invention, a reduction is applied with a temperature difference intentionally applied in the plate thickness direction to cure the plate thickness surface layer from the center of the plate thickness to increase the deformation resistance difference in the plate thickness direction. The central part is strongly thickened to improve the low temperature toughness.

The reason for limiting the chemical components in the present invention is as follows. C: The lower limit of the amount of C is 0.01%, which is the minimum amount for ensuring the strength of the base material and the welded portion and for exerting hardening of Nb, V and the like when added. But,
If the amount of C is too large, not only the HAZ toughness is adversely affected, but also the base metal toughness and weldability are deteriorated, so the upper limit is 0.0.
It was set to 9%. When the amount of C is large, martensite is generated, and the low temperature toughness is significantly deteriorated. From the viewpoint of preventing CO ₂ corrosion resistance, excessive addition of C produces cathode sites such as carbides, so a lower C content is desirable.

Si: 0.05% or more is necessary for steel in terms of deoxidation, but if a large amount is added, the weldability and the toughness of the weld will deteriorate, so the upper limit was made 0.5%. Mn: an element essential for ensuring strength and toughness,
The lower limit is 0.7%. In order to improve the HAZ toughness, it is necessary to prevent the coarse first-order ferrite that is refined to the γ grain boundary, but addition of Mn has hardening that suppresses this. However, if the Mn content is too large, the hardenability increases, which not only deteriorates the weldability and HAZ toughness, but also increases the Mn content of the slab.
Since the center deviation of S and the like is promoted to deteriorate the HIC resistance, the upper limit of Mn addition is set to 1.5%.

P: In the present invention, P as an impurity is set to 0.03% or less. This is to further improve the low temperature toughness of the base material and HAZ and reduce the center deviation of the slab. Reduction of the amount of P tends to reduce the tendency of intergranular fracture in HAZ. Preferably, the P amount is 0.01% or less. S: If the amount of S is set to 0.05% or more of the upper limit, MnS deteriorates the low temperature toughness. Therefore, in the present invention, the amount of S is set to 0.05% or less. Nb: In high strength steel, it is difficult to obtain excellent HAZ toughness without adding Nb. Nb suppresses the refined ferrite at the γ grain boundary, refines the crystal grains, and strengthens the steel. To obtain this effect, at least 0.02% Nb
Quantity is needed. However, if the amount of Nb is too large,
On the contrary, since the formation of a fine structure is hindered, its upper limit is set to 0.
It was set to 06%.

Cr: An important element from the viewpoint of preventing CO ₂ corrosion resistance. The lower limit value of 0.5% is the minimum value for obtaining the effect of CO ₂ corrosion resistance, but if it is too large, the field weldability and HAZ toughness deteriorate, so the upper limit was made 1.2%. Ti: TiN is formed, the HAZ structure is refined, and HA
Improves Z toughness. The lower limit of 0.005% is the minimum amount for obtaining this effect, and the upper limit of 0.03% is for preventing HAZ plastic deterioration due to TiC formation. Al: An element generally contained in deoxidized upper steel, but addition of an excessive amount impairs steel cleanliness, so its upper limit is 0.05.
%. N: 0.001% or more is necessary to secure HAZ toughness due to TiN or the like. Further, if it exceeds 0.005%, the HIC resistance deteriorates, so the upper limit is 0.005%.
And

In the present invention, if desired, V, Ni, Cu, Mo, Ca, Zr, and
At least one kind of REM is added. V: An element having almost the same effect as Nb, but 0.0
If it is less than 05%, there is no effect, and the upper limit is acceptable up to 0.060%. Addition of Ni: 0.05% or more improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness. On the other hand, if it exceeds 1.0%, it is not preferable for economical sticking, so the upper limit was made 1.0%. Almost the same effect as Cu: Ni can be obtained by adding 0.05% or more. However, if 1.0% or more is added, Cu-cracks are generated during hot rolling, which makes manufacturing difficult, so the upper limit was made 1.0%.

Mo: An element that improves the strength and toughness of the base material by adding 0.05% or more, but if it is too large, it deteriorates the base material, HAZ toughness, and weldability, which is not preferable. The upper limit is 0.40%. Ca: Controls the morphology of sulfides (MnS) to provide low temperature toughness. If the amount of Ca is less than 0.001%, it has no practical effect, and if added over 0.005%, CaO, Ca
A large amount of S is generated and becomes large inclusions, which not only impairs the cleanliness of steel but also adversely affects toughness and field weldability. Zr: An element having almost the same effect as Ti. The upper and lower limits are 0.01% and 0.005%. REM: An element having almost the same effect as Ti, and its upper and lower limit values are 0.025% and 0.005%.

However, it is not sufficient to limit the amount of each element, and the following formula 0.35≤C + (Mn + Cr + V) / 5 + (Ni + Cu) /
It must satisfy 15 ≦ 0.48. This is because the low temperature toughness and field weldability are determined by the total amount of chemical components including Cr.
The lower limit of 0.35% is the minimum amount for obtaining the necessary strength of the base material and the welded portion, and 0.48% is the upper limit for obtaining excellent low temperature toughness and weldability.

Next, the manufacturing conditions of the present invention will be described. The slab heating temperature after casting is 1000 to 1250 ° C.
However, this is necessary to secure the strength and low temperature toughness of the base material. If the heating temperature is less than 1000 ° C., solid solution of Nb, V, Ti, etc. becomes insufficient, and good strength and toughness cannot be obtained. However, the reheating temperature is 1250 ° C
If it exceeds, the austenite grains become coarse and the low temperature toughness deteriorates. After casting, the slab may be directly rolled without forming a cold piece.

After heating, rolling is carried out to bring the surface temperature to 90
It is necessary to suspend the rolling once at 0 ° C or higher and cool it at a cooling rate of 5 to 40 ° C / s until the surface temperature becomes 700 ° C or lower. Rough rolling is required in order to make the structure finer and more uniform by rolling in the austenite recrystallization region. If the surface temperature of the slab is cooled from less than 900 ° C., the temperature of the central portion of the steel plate will also decrease, and it will be difficult to secure the plate thickness average temperature at Ar ₃ or higher at the end of rolling.

The temperature range reached by cooling is 700 at the surface temperature.
The reason why the temperature is not more than 700 ° C. is that at a temperature exceeding 700 ° C., the temperature in the central portion of the plate thickness does not fall to the non-recrystallized temperature range in the recuperation process. If the cooling rate is too low, α
The amount of transformation into a small amount becomes so small that it is impossible to carry out a strong reduction in the center of the plate thickness, so it was set to 5 ° C / s. If the cooling rate is too high, the sheet thickness average temperature decreases, and at the end of rolling, the sheet thickness average temperature becomes A
The upper limit was set to 40 ° C./s because it cannot be secured to r ₃ or more.

After cooling, it is left standing until the surface temperature is Ar ₃ or less and the plate thickness center temperature is 950 ° C. or less, and then the rolling reduction is 60% or more and the average rolling per pass. It is necessary to apply a reduction so that the true strain becomes 0.2 or less. The reason why the surface temperature at the start of rolling after cooling is Ac ₃ or less is that when Ac ₃ is exceeded, α in the surface layer portion of the plate pressure undergoes reverse transformation to γ, and the toughness due to the difference in deformation resistance between α and γ It is difficult to improve.

After cooling, the temperature at the center of the plate pressure is 950 ° C.
The reason for allowing the temperature to fall to the following temperature range is to reduce the temperature of the central part of the plate pressure to the austenite unrecrystallized temperature range and then apply the reduction. If the cumulative rolling reduction after cooling is less than 60%, the effect of strong reduction due to the difference in deformation resistance between the plate thickness surface layer part (α) and the plate thickness center part (γ) is small, and the processing introduced into the plate thickness center part Since the amount of strain is small, the cumulative rolling reduction was set to 60% or more. The true rolling strain per pass is 0.
If it exceeds 2, as shown in FIG. 2, the difference in deformation resistance between the center part of the plate thickness and the surface layer part of the plate thickness is reversed. Therefore, the true rolling strain is controlled to 0.2 or less.

Further, the rolling after cooling has a sheet thickness average temperature of A
It is completed at r ₃ or more, and immediately thereafter, accelerated cooling is performed at a cooling rate of 5 to 40 ° C./s until the plate thickness average becomes 350 to 550 ° C., and then cooled. When the rolling after cooling is completed when the sheet thickness average temperature is less than Ar ₃ , if the MnS-based inclusions remain, it is easy to stretch, and due to the deviation of Mn, the central deviation of Ar ₃ is lower than that of the surroundings. Concentration of C due to the γ-α transformation occurs in the folded portion, and a hardened structure such as MA (high C island martensite) is formed in the central bent portion, so that the low temperature toughness decreases.

Accelerated cooling + cooling after completion of rolling are carried out in order to secure good strength and toughness. In acceleration cooling, cooling rate of 5 ℃ / s or less or 550 ℃
When the cooling start temperature exceeds the range, C accompanying the γ-α transformation
Occurs, and a hardened structure such as MA is formed in the centrally bent portion, so that the low temperature toughness decreases. At a cooling rate of 40 ° C./s or more and at a cooling start temperature of less than 350 ° C., a hardened structure such as martensite having high hardenability is formed, and it becomes difficult to secure low temperature toughness. Air cooling after accelerated cooling has the effect of tempering, and can achieve high strength and high toughness.

[0026]

EXAMPLES The present invention will be further described below based on examples. Using the test steels having the chemical components shown in Table 1, CC slabs were reheated and hot-rolled under the production conditions shown in Table 2, and then accelerated cooling was performed. Table 3 shows the mechanical properties and CO ₂ corrosion resistance of the steel sheet thus obtained. FIG. 3 shows a schematic view of the sampling situation of the test piece. The Charby test piece was sampled from a plate thickness of 1/2 t. The CO ₂ corrosion test piece was collected from the surface layer.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

From the results shown in Tables 1 to 3, the following can be seen. Steels 11 to 24 are not suitable manufacturing conditions within the scope of the present invention, and thus low temperature toughness and CO ₂ corrosion resistance are deteriorated.

Steels 25 to 30 are not within the range of the chemical composition of the steel of the present invention, and thus have not obtained appropriate mechanical properties. That is, in Steel 25, since the Mn content is excessive, the hardenability of the welded portion is increased and the toughness is reduced. Steel 26 and steel 27 have a low toughness due to lack of Nb and Ti, respectively. Since the steel 28 has a large amount of C, its toughness decreases, and the number of cathode sites such as cementite increases, resulting in deterioration of CO ₂ corrosion resistance. Steel 29 is C
r amount is low, resistance CO ₂ corrosion resistance is deteriorated can not be suppressed cathode reaction under CO ₂ corrosive environments. Steel 30 has a large amount of Cr and the toughness of the welded portion is reduced.

[0033]

As described above, according to the present invention, according to the present invention, to improve the resistance to CO ₂ corrosion in an environment containing CO _2, in particular properly improve toughness of the weld not only the base material, linepipe The characteristics as a steel pipe material for use can be effectively enhanced.

[Brief description of drawings]

FIG. 1 is a diagram showing rolling and water cooling patterns in the present invention.

FIG. 2 is a diagram showing a relationship between rolling true strain and deformation resistance in a structure mainly composed of ferrite and austenite.

FIG. 3 is a schematic view showing a sampling position of a test piece.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirohiro Hiroshi Kimitsu, Chiba Prefecture Kimitsu No. 1 Nippon Steel Corp. Kimitsu Works Ltd.

Claims (2)

[Claims] 1. C .: 0.01 to 0.09% by weight, S:
i: 0.5% or less, Mn: 0.7 to 1.5%, P: 0.
03% or less, S: 0.005% or less, Nb: 0.01 to
0.06%, Cr: 0.5-1.2%, Ti: 0.00
5 to 0.03%, Al: 0.05% or less, N: 0.00
1-0.005% and the following formula 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Ni + C
u) /15≦0.48, a steel consisting of balance Fe and unavoidable impurities is cast into a cold piece without casting, or a cold piece is made into 1100 to 1
After heating to a temperature of 250 ° C., starting the rolling after extraction and cooling with water until the surface temperature becomes 700 ° C. or less, the surface temperature is A
c ₃ or less, and allowed to cool until the temperature at the center of the plate thickness became 950 ° C. or less, after which the cumulative rolling reduction was 60% or more and an average of 1
Rolling is finished when the true strain of rolling per pass is 0.2 or less, and the sheet thickness average temperature is Ar ₃ or more.
Plate thickness average temperature is 350 to 500 ° C at a cooling rate of 0 ° C / s.
The method for producing a steel sheet excellent in low temperature toughness and CO ₂ corrosion resistance, comprising: 2. C: 0.01 to 0.09% by weight, S
i: 0.5% or less, Mn: 0.7 to 1.5%, P: 0.
03% or less, S: 0.005% or less, Nb: 0.01 to
0.06%, Cr: 0.5-1.2%, Ti: 0.00
5 to 0.03%, Al: 0.05% or less, N: 0.00
1 to 0.005%, V: 0.005 to 0.060%, Ni: 0.0
5 to 1.0%, Cu: 0.05 to 1.0%, Mo: 0.
05-0.30%, Ca: 0.001-0.005%,
Zr: 0.005-0.025%, REM: 0.000
It contains at least one of 8 to 0.01% and has the following formula 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Ni + C
u) /15≦0.48, the steel consisting of the balance Fe and unavoidable impurities is not cast into cold pieces after casting, or the cold pieces are made from 1100 to 1
After heating to a temperature of 250 ° C., rolling after extraction is started, the rolling is temporarily stopped when the surface temperature is 900 ° C. or higher, and then 5 to 40
After cooling with water at a cooling temperature of ℃ / s until the surface temperature becomes 700 ° C or lower, the surface temperature is allowed to cool to Ac ₃ or lower and the plate pressure center temperature becomes 950 ° C or lower, after which the cumulative rolling reduction is 60 % Or more and the average rolling true strain per pass is 0.2
The following rolling reduction is applied, rolling is terminated when the plate pressure average temperature is Ar ₃ or more, and immediately the plate thickness average temperature is cooled to 350 to 500 ° C. at a cooling rate of 5 to 40 ° C./s, and then air cooling is performed. A method for producing a steel sheet having excellent low temperature toughness and CO ₂ corrosion resistance, characterized by: