JPH08209241A - Production of steel plate for line pipe excellent in carbon dioxide corrosion resistance and low temperature toughness - Google Patents

Abstract

PURPOSE: To improve the CO2 corrosion resistance and low temp. toughness of a steel plate by casting a steel having a specified compsn., extracting the same, thereafter executing rolling and cooling so as to regulate its surface temp. to a specified one and moreover executing rolling and cooling under specified conditions. CONSTITUTION: A steel having a compsn. contg., by weight 0.01 to 0.09% C, <=0.5% Si, 0.7 to 1.5% Mn, <=0.03% P, <=0.005% S, 0.01 to 0.06% Nb, 0.4 to 1.2% Cr, 0.05 to 0.4% Cu, 0.005 to 0.03% Ti, <=0.05% Al and 0.001 to 0.005% N and satisfying the conditions shown by the inequality is cast. This slab not formed into a cold slab or the one formed into a cold slab is heated to 1100 to 1250 deg.C, is extracted and is thereafter rolled. This rolling is stopped at >=900 deg.C surface temp. and is water-cooled to <=700 deg.C at 5 to 40 deg.C/sec rate. It is air-cooled to the Ac3 point or below of surface temp. and <=950 deg.C intermediate temp. and is thereafter applied with rolling reduction at >=60% cumulative draft and <=0.2 rolling true strain per pass. It is finished at the Ar3 or above of plate thickness temp., and cooling is executed to 350 to 550 deg.C at 5 to 40 deg.C/sec rate. After that, it is air-cooled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. 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 also 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 a thick plate mill, but it can also be applied to a hot coil. Also,
The steel sheet produced by this method is excellent in low temperature toughness and field weldability, and is most suitable for use in cold regions and offshore.

[0003]

2. Description of the Related Art High strength, excellent low temperature toughness and field weldability are required for large diameter line pipes for oil and gas transportation in cold regions and offshore. Further, in recent years, due to a decrease in the inhibitor effect due to CO ₂ injection and deep well formation in the secondary and tertiary recovery of crude oil, corrosion of line pipes due to CO ₂ gas has become a major problem, and CO ₂ corrosion resistance is required. became.

Since a large amount of Cr impairs 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. Further, addition of a large amount of Cr deteriorates the toughness of the base material and the weld heat affected zone (HAZ). 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.

Japanese Unexamined Patent Publication No. 3-211230 discloses a C-resistant material.
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 , it is C if 1.2% Cr or less.
Since a passive state cannot be formed by r, 10% NaC
0.5 mm / at 1 + 1 atm CO ₂ (pH 5, 80 ° C)
Only y can be secured. Therefore, it is desired to develop a steel for line pipes which is excellent in CO ₂ corrosion resistance in terms of structure.

[0006]

In view of the present situation as described above, an object of the present invention is to provide a method for producing a steel sheet for a line pipe having excellent CO ₂ corrosion resistance and low temperature toughness.

[0007]

The gist of the present invention is as follows. (1) Weight% C: 0.01 to 0.09% Si: 0.5% or less Mn: 0.7 to 1.5% P: 0.03%
Hereinafter S: 0.005% or less Nb: 0.01 to
0.06% Cr: 0.4 to 1.2% Cu: 0.05 to
0.4% Ti: 0.005-0.03% Al: 0.05%
Below N: 0.001 to 0.005% is contained, and the following formula 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Cu +
Ni) /15≦0.50 and casting steel with the balance iron and unavoidable impurities, without forming cold pieces, or cooling the cold pieces from 1100 to 125
After extraction by heating to a temperature of 0 ° C., rolling is started and the surface temperature is 900 ° C. or higher, the rolling is temporarily stopped, and then 5 to 40 ° C. /
After water cooling at a cooling rate of s until the surface temperature becomes 700 ° C. or less, the surface temperature is Ac ₃ or less and the plate thickness center temperature is 95
Allow to cool to 0 ° C or below, then the cumulative rolling reduction is 6
Rolling is terminated at 0% or more and the average rolling true strain per pass is 0.2 or less, the sheet thickness average temperature is Ar ₃ or more, and the sheet is immediately rolled at a cooling rate of 5 to 40 ° C./s. A method for producing a steel sheet excellent in CO ₂ corrosion resistance and low temperature toughness, which comprises cooling until a thickness average temperature reaches 350 to 550 ° C. and then air cooling.

(2) C: 0.01 to 0.09% Si: 0.5% by weight
Below Mn: 0.7 to 1.5% P: 0.03
% Or less S: 0.005% or less Nb: 0.01
~ 0.06% Cr: 0.4-1.2% Cu: 0.05
~ 0.4% Ti: 0.005-0.03% Al: 0.05
% Or less N: 0.001 to 0.005% is contained, and further V: 0.005 to 0.060% Ni: 0.05
~ 1.0% Mo: 0.05-0.30% Ca: 0.00
1 to 0.005% Zr: 0.005 to 0.025% REM: 0.00
It contains one or more of the above-mentioned formula 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Cu +
Ni) /15≦0.50, and after casting the steel consisting of the balance iron and unavoidable impurities, without casting cold pieces, or cooling the cold pieces from 1100 to 125
After extraction by heating to a temperature of 0 ° C., rolling is started and the surface temperature is 900 ° C. or higher, the rolling is temporarily stopped, and then 5 to 40 ° C. /
After allowing to cool at a cooling rate of s until the surface temperature becomes 700 ° C. or less, the surface temperature is Ac ₃ or less and the plate thickness center temperature is 95
Water-cool until the temperature falls below 0 ° C, after which the cumulative rolling reduction is 6
Rolling is terminated at 0% or more and the average rolling true strain per pass is 0.2 or less, the sheet thickness average temperature is Ar ₃ or more, and the sheet is immediately cooled at a cooling rate of 5 to 40 ° C./s. A method for producing a steel sheet excellent in CO ₂ corrosion resistance, low temperature toughness and sour resistance, characterized by cooling until a thickness average temperature reaches 350 to 550 ° C. and then air cooling.

The present invention is an object of the invention to produce a high toughness line pipe steel which suppresses corrosion due to CO ₂ . The inventors of the present invention have made detailed investigations mainly on the chemical composition and structure of the effect of non-uniform corrosion on the CO ₂ corrosion resistance, and have found the following facts. The combined addition of Cr and Cu improves CO ₂ corrosion resistance and suppresses uneven corrosion. However, addition of excessive amounts of Cr and Cu deteriorates low temperature toughness and field weldability.

Since the steel sheet obtained by the present invention has a structure in which the surface is very fine ferrite and C is finely dispersed, the surface of the steel sheet obtained by conventional controlled rolling + controlled cooling +20 in immersion potential compared to the structure (prone to band structure such as ferrite and bainite)
It becomes noble about mV and CO ₂ corrosion resistance is improved. That is, Cr addition is effective for CO ₂ corrosion, but excessive Cr addition causes deterioration of low temperature toughness and field weldability. On the other hand, after limiting the upper limit of the Cr amount,
Further, by making the surface structure a structure in which ferrite is extremely fine and C is finely dispersed, effective cathode sites are reduced and CO ₂ resistance is improved. Further, by adding Cu to the Cr-added steel, the corrosion product is stabilized and the corrosion resistance is further improved.

Further, it will be described below that adopting high temperature rolling of Ar ₃ or more in the present invention is effective in improving low temperature toughness. Generally, 100 to 100 is provided 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.
There is a temperature difference of about 200 ° C., and the transformation temperature range that occurs during the temperature rising process is higher.

Therefore, as in the case of the present invention, as shown in FIG. 1, in the process of once cooling the thick steel plate to an appropriate temperature range and then reheating it, the surface portion of the plate thickness transforms from ferrite to austenite during temperature rise. However, the central part of the plate thickness is still in the austenite single phase state without the ferrite transformation starting. Therefore, even when the temperature difference between the two is reduced due to the progress of recuperation to some extent, the surface layer 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 them, and the deformation resistance of the surface layer portion of the plate thickness is extremely large.

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 in the range of 0.2 or less in true rolling strain. This is because the deformation resistance is larger when the same strain is applied. In the present invention, a reduction is applied intentionally with a temperature difference in the plate thickness direction, the plate thickness surface layer part is hardened from the plate thickness center part, and the plate thickness center part is increased by increasing the deformation resistance difference in the plate thickness direction. It is intended to improve the low temperature toughness by performing strong reduction.

[0014]

The reason for limiting the range of chemical components in the present invention will be explained. C: The lower limit of the amount of C is set to 0.01% because it is the minimum amount for ensuring the strength of the base material and the welded portion and for exerting these effects when Nb, V, etc. are added.
However, too much C not only adversely affects the HAZ toughness, but also deteriorates the base material toughness and weldability, so the upper limit was made 0.09%. When the amount of C is large, martensite is generated, and the low temperature toughness is significantly deteriorated. Further, since excessive addition of C produces cathode sites such as carbides from the viewpoint of preventing CO ₂ corrosion, it is desirable that the C content be low.

Si: Si is preferably 0.05 in terms of deoxidation.
% Or more, but if a large amount is added, the weldability and the toughness of the welded portion deteriorate, so the upper limit was made 0.5%. Mn: Mn is an element essential for securing strength and toughness, and its lower limit is 0.7%. In order to improve the HAZ toughness, it is necessary to prevent the coarse proeutectoid ferrite generated at the γ grain boundary, but the addition of Mn has the effect of suppressing this. However, if Mn is too much, the hardenability increases,
Not only the weldability and HAZ toughness are deteriorated, but also the center segregation of MnS of the slab is promoted and the HIC resistance is deteriorated, so the upper limit of Mn addition is made 1.5%.

P: P that is an impurity in the present invention is 0.
It was set to 03% or less. This is to further improve the low temperature toughness of the base material and HAZ and reduce the center segregation of the slab. Reduction of the amount of P tends to reduce the tendency of intergranular fracture in the HAZ. Preferably, the P amount is 0.01% or less.

S: When the amount of S exceeds 0.005%, M
The low temperature toughness deteriorates due to nS. Therefore, in the present invention, S
The amount was 0.005% or less. Nb: In high strength steel, it is difficult to obtain excellent HAZ toughness without adding Nb. Nb suppresses the pro-eutectoid ferrite generated at the γ grain boundary, refines the crystal grains, and increases the toughness of the steel. To obtain this effect, a minimum Nb content of 0.02% is required. However, if the amount of Nb is too large, on the contrary, the formation of a fine structure is hindered, so the upper limit was made 0.06%.

Cr: Cr is CO₂Heavy from the viewpoint of corrosion prevention
It is an essential element. The lower limit of 0.4% is CO resistant ₂Corrosive effect
It is the minimum value to obtain the result. However, if there is too much, local weldability
And deteriorates HAZ toughness. Therefore, the upper limit is 1.2%
And Cu: Cu stabilizes the corrosion product of Cr-added steel
In order to achieve this, addition of 0.05% or more is necessary. But,
If added in excess of 0.4%, Cu-clad during hot rolling.
This causes burrs and makes manufacturing difficult. Therefore, the upper limit is 0.4%
And

Ti: Ti forms TiN to refine the HAZ structure and improve HAZ toughness. Lower limit of 0.0
05% is the minimum amount for obtaining this effect, and the upper limit of 0.03% is for preventing HAZ toughness deterioration due to TiC formation. Al: Al is an element generally contained in deoxidized upper steel, but the addition of an excessive amount impairs the cleanliness of the steel, so the upper limit was made 0.05%.

N: N is required to be 0.001% or more in order to secure the HAZ toughness of TiN or the like. Also, 0.
If it exceeds 005%, the HIC resistance deteriorates, so the upper limit was made 0.005%. If desired, at least one of V, Ni, Mo, Ca, Zr, and REM is further added to the starting steel of the present invention as a strength adjusting element.

V: V is an element having almost the same effect as Nb, but if it is less than 0.005%, it has no effect, and the upper limit is 0.1.
Acceptable up to 060%. Ni: Addition of 0.05% or more of Ni results in weldability and H
The strength and toughness of the base material are improved without adversely affecting the AZ toughness. On the other hand, if it exceeds 1.0%, it is not preferable in terms of economy, so the upper limit was made 1.0%.

Mo: Mo is an element that improves the strength and toughness of the base material when added in an amount of 0.05% or more. However, if it is too large, it deteriorates the base material, HAZ toughness and weldability, which is not preferable. The upper limit is 0.30%. Ca: Ca controls the morphology of sulfide (MnS) and improves low temperature toughness. If the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.005%, Ca
A large amount of O and CaS are formed to form large inclusions, which not only impairs the cleanliness of steel but also adversely affects toughness and field weldability.

Zr: Zr is an element having almost the same effect as Ti. The upper and lower limits are 0.005%,
It is 0.025%. REM: REM is an element that has almost the same effect as Ti. The lower limit is 0.0005% and the upper limit is 0.01%.

It is not sufficient to limit the individual chemical components as described above, and the following formula 0.35≤C + Mn / 6 + (Cr + V) / 5 + (Ni +
Cu) /15≦0.50 must be satisfied. 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 0.50% is the upper limit for obtaining excellent low temperature toughness and weldability.

Next, manufacturing conditions will be described. In the present invention, the slab heating temperature after casting is 1000 to 1250 ° C.
Range. This is necessary to secure the strength and low temperature toughness of the base material. When the heating temperature is less than 1000 ° C, solid solution of Nb, V, Ti, etc. becomes insufficient, resulting in good strength,
Toughness cannot be obtained. However, if the reheating temperature exceeds 1250 ° C., 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. The rough rolling is required in order to make uniform the refinement of the structure 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 sheet will also decrease, making it difficult to secure the average sheet thickness 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 higher than 700 ° C. is that the temperature at the center of the plate thickness does not drop to the non-recrystallized temperature range in the recuperating process at a temperature higher than 700 ° C. If the cooling rate is too low, the amount of transformation into α in the surface layer portion of the sheet thickness becomes small, and strong reduction cannot be performed at the center portion of the sheet thickness, so it was set to 5 ° C / s or more. If the cooling rate is too high, the sheet thickness average temperature decreases, and the sheet thickness average temperature cannot be kept above Ar ₃ at the end of rolling, so the upper limit is 40 ° C / s.
And

After cooling, the steel sheet was allowed to stand until the surface temperature was Ac ₃ or less and the sheet thickness center temperature was 950 ° C. or less, and then the rolling reduction was 60% or more in cumulative reduction and the average rolling pass per pass. It is necessary to apply reduction so that the strain becomes 0.2 or less. The reason why the surface temperature at the start of rolling after cooling is set to Ac ₃ or less is that when Ac ₃ is exceeded, α in the surface layer portion of the plate thickness reversely transforms to γ, and in the present invention, the difference in deformation resistance between α and γ. This makes it difficult to improve the toughness.

The temperature at the center of the plate thickness after cooling 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 thickness to the austenite non-recrystallization temperature range and then apply 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 sheet thickness surface layer portion (α) and the sheet thickness center portion (γ) is small, and processing introduced into the sheet thickness center portion Since the amount of strain is small, it was set to 60% or more.

When the average rolling true strain per one pass exceeds 0.2, the difference in deformation resistance between the central portion of the sheet thickness and the surface layer portion of the sheet thickness reverses (as shown in FIG. 2), so it is set to 0.2 or less. Limited Further, the rolling after cooling is completed when the sheet thickness average temperature is Ac ₃ or more, and immediately thereafter, accelerated cooling is performed at a cooling rate of 5 to 40 ° C./s until the sheet thickness average becomes 350 to 550 ° C., and then air cooling is performed.

When the rolling after cooling is completed when the sheet thickness average temperature is less than Ac ₃ , if the MnS inclusions remain, it tends to be stretched. The center segregation in which Ac ₃ is lower than the surroundings due to segregation of Mn and the like. Since C is concentrated in the portion due to the γ-α transformation and a hardened structure such as martensite is formed in the central segregation portion, the low temperature toughness is reduced. Accelerated cooling + cooling after completion of rolling are performed to ensure good strength and toughness. In accelerated cooling, if the cooling rate is less than 5 ° C./s or the cooling start temperature is more than 550 ° C., the concentration of C occurs due to the γ-α transformation, and a hardened structure such as martensite is formed in the central segregation portion. Therefore, the low temperature toughness decreases.

Cooling rate over 40 ° C./s or 350
At a cooling start temperature of less than ° 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.

[0033]

EXAMPLE The CC slab of the test steel having the chemical composition shown in Table 1 was reheated, hot rolled and accelerated cooled under the production conditions shown in Tables 2 and 3 (Table 2). Table 4 shows the mechanical properties and CO ₂ corrosion resistance of the obtained steel sheet. Note that FIG. 3 shows a schematic view of the situation of collecting the test pieces. The tensile test was a full-thickness tensile test, and the Charpy test piece was sampled from the center of the plate thickness, and the corrosion test piece was sampled from the fine ferrite structure of the surface layer.

Since the steels 11 to 24 are not under appropriate manufacturing conditions, the low temperature toughness and the CO ₂ corrosion resistance are deteriorated. Steels 25 to 30 have an unsuitable chemical composition and cannot obtain desired mechanical properties. In Steel 25, since the Mn content exceeds the range of the present invention, the hardenability of the welded portion increased and the toughness decreased. Steel 26 is an example in which Ti and steel 27 are inferior in toughness due to lack of Nb in the range of the present invention, respectively. Steel 28 has a higher amount of C than the range of the present invention, so that the toughness is reduced and the number of cathode sites such as cementite is increased, resulting in CO ₂ resistance.
This is an example of deterioration in corrosiveness. Steel 29 has a low Cr content than the present invention range, resistance CO ₂ corrosion resistance is deteriorated can not be suppressed cathode reaction under CO ₂ corrosive environments. Steel 30 had a larger amount of Cr than the range of the present invention, and the toughness of the welded portion decreased.

In Table 1, Pc represents the value of the following formula. Pc = C + Mn / 6 + (Cr + V) / 5 + (Cu + N
i) / 15

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

Effect of the Invention] above, obtained steel sheet by the present invention as described, excellent resistance to CO ₂ corrosion in an environment containing CO _2, moreover particularly excellent in toughness of the weld not only preform Therefore, the applicability as a steel pipe material for line pipe can be effectively enhanced, and the effect of the present invention on the industry is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a rolling and water cooling pattern of the present invention.

FIG. 2 is a diagram showing a relationship between rolling true strain and deformation resistance in a ferrite and austenite-based structure.

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

[Explanation of symbols]

 1 Plate thickness 2 Surface layer 3 Central part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroshi Tamehiro 2-3-6 Otemachi, Chiyoda-ku, Tokyo Within Nippon Steel Corporation (72) Inventor Hiroyuki Ogawa 2-6, Otemachi, Chiyoda-ku, Tokyo No. 3 within Nippon Steel Corporation

Claims (2)

[Claims] 1. C: 0.01 to 0.09% Si: 0.5% or less by weight% Mn: 0.7 to 1.5% P: 0.03%
Hereinafter S: 0.005% or less Nb: 0.01 to
0.06% Cr: 0.4 to 1.2% Cu: 0.05 to
0.4% Ti: 0.005-0.03% Al: 0.05%
Below N: 0.001 to 0.005% is contained, and the following formula 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Cu +
Ni) /15≦0.50 and casting steel with the balance iron and unavoidable impurities, without forming cold pieces, or cooling the cold pieces from 1100 to 125
After extraction by heating to a temperature of 0 ° C., rolling is started and the surface temperature is 900 ° C. or higher, the rolling is temporarily stopped, and then 5 to 40 ° C. /
After water cooling at a cooling rate of s until the surface temperature becomes 700 ° C. or less, the surface temperature is Ac ₃ or less and the plate thickness center temperature is 95
Allow to cool to 0 ° C or below, then the cumulative rolling reduction is 6
Rolling is terminated at 0% or more and the average rolling true strain per pass is 0.2 or less, the sheet thickness average temperature is Ar ₃ or more, and the sheet is immediately rolled at a cooling rate of 5 to 40 ° C./s. A method for producing a steel sheet excellent in CO ₂ corrosion resistance and low temperature toughness, which comprises cooling until a thickness average temperature reaches 350 to 550 ° C. and then air cooling. 2. C: 0.01 to 0.09% Si: 0.5% by weight
Below Mn: 0.7 to 1.5% P: 0.03
% Or less S: 0.005% or less Nb: 0.01
~ 0.06% Cr: 0.4-1.2% Cu: 0.05
~ 0.4% Ti: 0.005-0.03% Al: 0.05
% Or less N: 0.001 to 0.005% is contained, and further V: 0.005 to 0.060% Ni: 0.05
~ 1.0% Mo: 0.05-0.30% Ca: 0.001
-0.005% Zr: 0.005-0.025% REM: 0.00
It contains one or more of the above-mentioned formula 0.35 ≦ C + Mn / 6 + (Cr + V) / 5 + (Cu +
Ni) /15≦0.50 and casting steel with the balance iron and unavoidable impurities, without forming cold pieces, or cooling the cold pieces from 1100 to 125
After extraction by heating to a temperature of 0 ° C., rolling is started and the surface temperature is 900 ° C. or higher, the rolling is temporarily stopped, and then 5 to 40 ° C. /
After allowing to cool at a cooling rate of s until the surface temperature becomes 700 ° C. or less, the surface temperature is Ac ₃ or less and the plate thickness center temperature is 95
Water-cool until the temperature falls below 0 ° C, after which the cumulative rolling reduction is 6
Rolling is terminated at 0% or more and the average rolling true strain per pass is 0.2 or less, the sheet thickness average temperature is Ar ₃ or more, and the sheet is immediately cooled at a cooling rate of 5 to 40 ° C./s. A method for producing a steel sheet excellent in CO ₂ corrosion resistance, low temperature toughness and sour resistance, characterized by cooling until a thickness average temperature reaches 350 to 550 ° C. and then air cooling.