JPH07316651A - Production of sour resistant high strength steel plate excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce the steel plate for line pipe by specifying the conditions of heating, rolling, and cooling of a steel slab having an Nb-free specific compo sition and performing inhibition of gamma-grain coarsening, formation of ferrite by working refining of gamma-structure, securing of sour resistance, and strengthening of transformation. CONSTITUTION:A steel slab, consisting of, by weight, 0.04-0.14% C, <=0.6% Si, 0.6-1.6% Mn, <=0.015% P, <=0.001% S, 0.005-0.03% Ti, <=0.06% Al, 0.001-0.005% Ca, 0.001-0.005% N, =0.003% O, and the balance iron and satisfying 0.5<=[Ca](1-124[O]/1.25[S]<=8.0, is used. This steel slab is heated to 900-1150 deg.C. After rolling under the conditions of >=60% cumulative rolling reduction at <=1000 deg.C and >=20% cumulative rolling reduction at a temp. between (Ar3-50 deg.C) and Ar3 is finished, the resulting rolled plate is subjected to accelerated cooling down to 350-600 deg.C at (5 to 40) deg.C/sec cooling rate and then to air cooling. Further, one or more kinds among 0.1-0.5% Ni, 0.1-0.5% Cr, 0.1-0.5% Cu, and 0.01-0.1% V can be incorporated into this steel.

Description

Detailed Description of the Invention

[0001]

The present invention relates to excellent hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking resistance (S resistance).
(SC property) and strength of American Petroleum Institute (API) standard X65 or more, and is related to the manufacture of steel sheets for line pipes having significantly better low temperature toughness than before. In the steel industry, it is used for thick plate mills. It is desirable to apply.

[0002]

2. Description of the Related Art In recent years, the sourness of the pipeline environment is progressing with the injection of seawater into crude oil and natural gas wells and the development of poor resources, and linepipes have excellent HIC resistance and SSC resistance. Came to be demanded. In addition, demand for higher strength is increasing year by year from the viewpoint of improving transport efficiency and improving the efficiency of on-site welding by reducing the wall thickness. More recently, there is a demand for a line pipe applicable to a sour environment in a cold region of -45 ° C or lower.

Conventionally, excellent HIC resistance has been found, for example, in Japanese Examined Patent Publication No. 63-001369 and Japanese Unexamined Patent Publication No. 62-112722.
As shown in Japanese Patent Publication, high purity and high cleanliness of steel,
It has been achieved by making full use of technologies such as morphology control by adding Ca of sulfide inclusions, reduction of center segregation by light reduction during continuous casting, and improvement of microstructure of center segregation part by accelerated cooling. In particular, the application of accelerated cooling after rolling suppresses the formation of a hardened structure in the central segregation portion and is a very effective means for improving the HIC resistance.

Further, sour-resistant steel sheets having high strength are disclosed in, for example, JP-A-61-147813, JP-A-63-134647, JP-A-1-096329, and JP-A-2-008322. As shown, high strength has been achieved by positively utilizing precipitation hardening of Nb. However, as a result of earnest research by the present inventors, N
In the b-added steel, coarse Nb precipitates (Nb carbonitrides) that did not form a solid solution and remained when the slab was heated form clusters in the steel sheet, and these become the starting points of HIC and are resistant to HI resistance.
It became clear that the C property was deteriorated. It was necessary to heat the Nb precipitates to a considerably high temperature in order to form a solid solution completely within the limited time during the heating of the slab, and the heating γ grains were coarsened and the energy cost was increased.

On the other hand, as disclosed in Japanese Patent Laid-Open No. 62-182221, there is a technique for producing a high-strength sour-resistant steel sheet by applying accelerated cooling to steel to which Nb is not added. The low temperature toughness of line pipes is BDWTT (Battelle Drop), which shows brittle crack propagation stopping properties.
The Weight Tear Test characteristic is important. BDW shown in JP-A-62-182221
The TT characteristic is a level that guarantees -30 ° C, and constant temperature toughness cannot be guaranteed in a cold region of -45 ° C or lower and a sour environment which is recently required.

[0006]

The present invention provides excellent HI resistance.
C temperature and SSC resistance (both in NACE environment) and strength of API standard X65 or more, and outstanding low temperature toughness (B
DWTT 85% Shear FATT ≤-45 ° C)
It aims at providing the manufacturing method of the steel plate for line pipes which has.

[0007]

The present invention provides C:% by weight.
0.04 to 0.14%, Si: 0.6% or less,
Mn: 0.6 to 1.6%, P: 0.01
5% or less, S: 0.001% or less, Ti:
0.005-0.03%, Al: 0.06% or less,
 Ca: 0.001 to 0.005%, N: 0.
001 to 0.005%, O: 0.003% or less
Contains and 0.5 ≦ [Ca] (1-124 [O]) /
Satisfies 1.25 [S] ≦ 8.0, and if necessary
% By weight Ni: 0.1 to 0.5%, Mo: 0.
1 to 0.5%, Cr: 0.1 to 0.5%, C
u: 0.1 to 0.5%, V: 0.01 to 0.1%, one type
Contains the above, the balance consisting of iron and unavoidable impurities
The steel slab is heated to 900 to 1150 ° C and 1000 ° C or less.
The cumulative rolling reduction at 60% or more and Ar ₃-50 ℃ ~
Ar₃Rolling to achieve a cumulative rolling reduction of 20% or more
After completion, 350-60 at a cooling rate of 5-40 ° C / sec
Characterized by accelerated cooling to 0 ° C and then cooling.
Summary of manufacturing method for sour-resistant high-strength steel sheet with excellent low temperature toughness
It is what

The technical idea of the present invention will be described below. The idea of the present invention is to improve sour resistance by not adding Nb, suppress coarsening of heated austenite grains by lowering the heating temperature, and perform rolling in a two-phase region where austenite (γ) and ferrite (α) coexist. Is to produce processed ferrite and refine the γ structure, ensure sour resistance and accelerate transformation by accelerated cooling after rolling, and produce a sour-resistant high-strength steel sheet excellent in low-temperature toughness.

As a result of the research conducted by the present inventors, in the Nb-added steel, Nb having a size of more than 10 μm which remains unmelted during slab heating.
The carbonitrides form clusters in the region of more than 100 μm near the center segregation part, and these become the starting points of HIC and are resistant to H
It was revealed that the IC property was deteriorated. In order to completely form such a coarse Nb carbonitride in a solid solution within a limited time during heating of the slab, it is necessary to heat it to a high temperature exceeding 1200 ° C. Such an increase in the heating temperature brings about a significant coarsening of the heating γ grains, and also a significant increase in energy cost. Therefore, in the present invention, by not adding Nb, Nb
Preventing the formation of carbonitrides and improving the HIC resistance.

Increasing the slab heating temperature causes coarsening of the heated γ grains, leading to deterioration of low temperature toughness. In the present invention, N
Since it does not contain b, it is not necessary to heat it at a high temperature to form a solid solution with Nb. Therefore, in the present invention, the slab heating temperature is lowered to suppress the coarsening of the heated γ grains, and the low temperature toughness is improved by starting the rolling from the finest γ grains.

Active rolling in the γ / α two-phase region is a feature of the present invention. During rolling in the γ / α two-phase region, the pro-eutectoid ferrite transformed in the surface layer part in the plate thickness direction is work-hardened, and a deformation zone is introduced by γ processing inside the plate thickness direction to refine the microstructure after transformation. Try to change. This is an important technique for achieving both high strength and low temperature toughness. Accelerated cooling after rolling suppresses the formation of a hardened structure in the center segregation area to ensure sour resistance and also reduces the γ inside the plate thickness direction.
Higher strength is achieved by transformation strengthening by transforming to a bainitic ferrite.

The reasons for limiting the chemical components will be described below. The C content and Mn content inevitably increase in high strength steel of API standard X65 or more, but these elements have a strong degree of center segregation in the slab and form a hardened structure in the center of the plate thickness of the steel sheet to prevent HIC resistance. In order to remarkably deteriorate the properties, the upper limit of the amount of C was 0.14% and the upper limit of the amount of Mn was 1.6%. C
The lower limit of the amount and Mn amount is the base metal, welding heat affected zone (HAZ)
0.04 each to secure the strength and low temperature toughness of
% And 0.6%.

P is an element having a strong degree of center segregation and significantly deteriorating HIC resistance. Therefore, the upper limit is 0.0
It was set to 15%. The smaller the amount of P, the higher the HIC resistance. Ti forms fine TiN, suppresses the coarsening of the heating γ grains during slab heating and welding, and improves the base metal toughness and H
Improves AZ toughness. The lower limit of the Ti content is 0.005% in order to exert its effect, and the upper limit is 0.03% in order not to deteriorate the HAZ toughness and field weldability.

If a large amount of Si is added, local weldability and HAZ
In order to deteriorate the toughness, the upper limit was set to 0.6%. Only Al and Ti are sufficient for deoxidizing steel, and Si is not necessarily added. In the steel of the present invention, S which is an impurity is set to 0.001% or less, and Ca is added so that 0.5 ≦ [Ca] (1-124 [O]) / 1.25.
[S] ≦ 8.0. S forms MnS-based inclusions,
MnS is elongated by rolling and becomes a starting point of HIC. To prevent this, reduce the absolute amount of inclusions and
CaS (-
O). Therefore, the amount of S is 0.001%
The amount of Ca is 0.001 to 0.005% and C
In order to sufficiently control the morphology of sulfide by a, ESSP
= [Ca] (1-124 [O]) / 1.25 [S] ≧
It was set to 0.5. However, if the ESSP is too large, Ca-based inclusions increase and become the starting point of HIC generation, so the upper limit was made 8.0.

In relation to the above, the amount of O is limited to 0.003% or less. This is because oxide-based inclusions, which are the starting point of HIC, are reduced and the sulfide morphology is controlled by the amount of Ca.
Al is an element contained in steel as a deoxidizing element, but deoxidizing is also possible with Ti or Si, and it is not always necessary to add it. If the amount of Al exceeds 0.06%, Al-based nonmetallic inclusions increase and impair the cleanliness of steel, so the upper limit was made 0.06%.

N forms TiN and improves the base metal and HAZ toughness by suppressing coarsening of γ grains during slab reheating and welding. The minimum amount required for this is 0.001%. However, if it is too much, HA will be caused by slab surface defects and solid solution N.
Since it causes deterioration of Z toughness, its upper limit is 0.005%.
It is necessary to keep below. Next, select elements Ni and M
The reason for adding o, Cr, Cu, V will be described.
The main purpose of adding these elements to the basic composition is to improve the properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be limited by itself, and the lower limit is the minimum amount at which these substantial effects can be obtained.

Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but excessive addition is not preferable for weldability, so the upper limit is made 0.5%,
The lower limit was 0.1%. Mo improves both strength and toughness of the base metal, but excessive addition of Mo causes toughness of the base metal and HAZ.
Since the weldability is deteriorated, the upper limit is set to 0.5% and the lower limit is set to 0.1%.

Cr is less likely to segregate in the center in the CC slab and improves the strength of the base metal, but excessive addition deteriorates the toughness and weldability of the base metal and HAZ, so the upper limit is made 0.5% and the lower limit is made. Was 0.1%. Cu has almost the same effect as Ni, but excessive addition of C causes C during hot rolling.
Since u-cracks are generated and manufacturing becomes difficult, the upper limit was made 0.5% and the lower limit was made 0.1%.

V makes it possible to improve the toughness by refining the microstructure, increase the hardenability, and improve the strength by precipitation hardening. However, excessive addition causes deterioration of HAZ toughness and weldability, so the upper limit is set to 0.1% and the lower limit is set to 0.01%.
And Next, the reasons for limiting the manufacturing method will be described. The heating temperature of the billet must be 900 to 1150 ° C. This is to suppress the coarsening of the heated γ grains and to secure the rolling end temperature at Ar ₃ −50 ° C. or higher. If the heating temperature is less than 900 ° C., the rolling end temperature will be Ar ₃ −50.
It is difficult to secure the temperature above ℃. The heating temperature is 115
If the temperature exceeds 0 ° C., the heated γ grains are significantly coarsened, and it becomes difficult to refine the γ structure by subsequent rolling, and the microstructure after transformation is coarsened, and good low temperature toughness cannot be obtained. A desirable heating temperature is 950 to 1100 ° C., and it is very effective to improve the low temperature toughness by minimizing the heating γ grains.

[0020] The cumulative reduction ratio in the rolling at 1000 ° C. or less to 60% or more, and must the cumulative reduction ratio at Ar ₃ -50 ℃ ~Ar ₃ finished rolling as 20% or more. The cumulative rolling reduction at 1000 ° C or lower is 60% or more because the γ structure before transformation is sufficiently refined by the strong reduction in the γ low temperature region, and the microstructure after transformation is made fine and the low temperature toughness is good. Is to get. If the cumulative reduction amount at 1000 ° C. or less is less than 60%, the γ structure before transformation is not sufficiently refined, and the microstructure after transformation is insufficiently refined and good low temperature toughness cannot be obtained. . Ar ₃ -50 ° C ~
The reason why the cumulative rolling reduction with Ar ₃ is 20% or more is to increase the strength by work hardening of the proeutectoid ferrite transformed in the surface layer portion in the sheet thickness direction, and secondly, inside the sheet in the sheet thickness direction. This is because γ of γ is processed at a low temperature to introduce a large amount of deformation zones to refine the microstructure after transformation to the limit. Ar ₃ −
If the cumulative reduction amount at 50 ° C. to Ar ₃ is less than 20%, it is difficult to achieve a predetermined strength because the work hardening amount of ferrite in the surface layer portion in the plate thickness direction is small, and the inside in the plate thickness direction is difficult to achieve. Since the deformation band density in γ is low, the microstructure after transformation does not become extremely fine. Rolling end temperature is Ar ₃
When the temperature is lower than −50 ° C., the ferrite transformation progresses to the vicinity of the central portion of the plate thickness, so that C is concentrated in the central segregated portion which is an untransformed portion, a hardened structure is formed, and the HIC resistance deteriorates.

After rolling, the temperature should rise from Ar ₃ -50 ° C. or higher to 5
It must be accelerated cooled to 350-600 ° C at a cooling rate of -40 ° C / sec and then allowed to cool. Accelerated cooling improves the microstructure of the center segregated portion to improve the HIC resistance, and at the same time, enables higher strength by transformation strengthening. When the cooling start temperature is less than Ar ₃ −50 ° C., the cooling rate is less than 5 ° C./sec, or the cooling stop temperature exceeds 600 ° C., C is transferred to the central segregation portion along with the ferrite transformation.
Is thickened, a hardened structure is formed, HIC resistance is deteriorated, and transformation strengthening is insufficient, resulting in insufficient strength. On the other hand, if the cooling rate exceeds 40 ° C./sec or the water cooling stop temperature is lower than 350 ° C., a large amount of low-temperature transformation products are formed and HIC resistance and low-temperature toughness deteriorate. FIG. 1 shows a schematic diagram of a method for manufacturing a steel sheet according to the present invention.

Note that tempering the steel sheet according to the present invention to a temperature of Ac ₁ or lower does not impair the characteristics of the steel of the present invention. The CC slab may be hot-charged in a heating furnace and rolled for the purpose of energy saving. The steel sheet according to the present invention can be applied not only to sour resistant line pipes but also to sour resistant pressure vessels.

[0023]

EXAMPLES Table 1 shows the chemical composition of the steel slab. Table 2 shows steel plate manufacturing conditions, and Table 3 (continued from Table 2) shows mechanical properties and sour resistance. Steels 1 to 8 in Tables 1, 2 and 3 are steels of the present invention, and steels 9 to 25 are comparative steels. The present invention steel is AP
I5L-X65 and higher strength and excellent low temperature toughness (BDWTT 85% Shear FATT ≦ -4
5 ℃) and excellent HIC resistance and S resistance in NACE environment
It has SC property. On the other hand, the comparative steels are inferior in strength, low temperature toughness, and sour resistance due to inappropriate chemical composition or manufacturing conditions. Steel 9, 10, 11 is C
Content, Mn content, and P content are too large, center segregation is promoted, and HIC resistance and SSC resistance are inferior. Steel 12 has an ESSP of less than 0.5 because the amount of S is too large, resulting in insufficient morphology control of sulfide inclusions and HIC resistance and SS resistance.
The C property is inferior. Steel 13 has an excessively small amount of Ti, so that the control of coarsening of the heated γ grains by TiN becomes insufficient and B
DWTT characteristics are inferior. Steel 14 has an inadequate morphological control of sulfide inclusions due to an excessively small amount of Ca, and is inferior in HIC resistance and SSC resistance. Steel 15 has an excessive amount of Ca, so that Ca-based inclusions increase and HIC resistance increases.
SSC resistance is inferior. Steel 16 has a heating temperature of 900 ° C
Since the rolling end temperature of Ar ₃ −50 ° C. or higher cannot be ensured because the temperature is less than 50 ° C., a hardened structure is formed in the central segregation portion and HIC resistance is high.
And SSC resistance are inferior. Steel 17 has a heating temperature of 11
Since the temperature exceeds 50 ° C, the heated γ grains become coarse and BDW
The TT characteristics are inferior. Steel 18 has a small cumulative reduction amount of 1000 ° C. or less, so that the microstructure after transformation is not sufficiently refined and the BDWTT characteristic is inferior. Steel 19 is Ar ₃
The strength is insufficient because the cumulative reduction amount below is small. Steel 20 has a rolling end temperature of less than Ar ₃ −50 ° C., and Steel 21 has a water cooling start temperature of less than Ar ₃ −50 ° C., so that a hardened structure is formed in the central segregation portion and HIC resistance,
SSC resistance is inferior. Since Steel 22 has a cooling rate of less than 5 ° C./sec, HIC resistance and SSC resistance are deteriorated due to the formation of a hardened structure in the central segregation portion, and the strength is insufficient due to insufficient transformation strengthening. Steel 23 has a cooling rate of more than 40 ° C./sec, and Steel 24 has a water cooling stop temperature of less than 350 ° C., so that a large amount of hard low-temperature transformation products are generated and HIC resistance, SSC resistance, and BDWTT characteristics are high. Inferior Steel 25 has a water cooling stop temperature of over 600 ° C., so a hardened structure is formed in the center segregated portion, and HIC resistance,
SSC resistance is inferior.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

EFFECTS OF THE INVENTION The sour-resistant high-strength steel sheet produced according to the present invention has extremely low temperature toughness as compared with the conventional steel, and the pipeline safety in cold and sour environment is improved. Greatly improved.

[Brief description of drawings]

FIG. 1 is a schematic view of a method for manufacturing a steel sheet according to the present invention.

Claims (2)

[Claims] 1. C: 0.04 to 0.14% by weight,
Si: 0.6% or less, Mn: 0.6 to 1.6%, P: 0.01
5% or less, S: 0.001% or less, Ti: 0.005
~ 0.03%, Al: 0.06% or less, Ca: 0.00
1 to 0.005%, N: 0.001 to 0.005%, O: 0.003
% Or less, and 0.5 ≦ [Ca] (1-124
[O]) / 1.25 [S] ≦ 8.0, and the balance of the steel slab containing iron and unavoidable impurities is 900 to 115
When heated to 0 ℃, the cumulative reduction below 1000 ℃ is 60%
Or more, and after the cumulative reduction ratio at Ar ₃ -50 ℃ ~Ar ₃ has finished rolled such that at least 20%, 5 to 40 ° C.
A method for producing a sour-resistant high-strength steel sheet excellent in low-temperature toughness, which comprises accelerating cooling to 350 to 600 ° C. at a cooling rate of / sec and then cooling. 2. Ni: 0.1-0.5% by weight,
Mo: 0.1-0.5%, Cr: 0.1-0.5%, Cu: 0.1-0.
5%, V: 0.01-0.1% of 1 or more types are contained, The manufacturing method of the sour-proof high strength steel plate excellent in the low temperature toughness of Claim 1 characterized by the above-mentioned.