JPH07233415A - Method for producing high-strength steel sheet excellent in sour resistance and low temperature toughness - Google Patents

Abstract

(57) [Abstract] [Objective] The object of the present invention is to provide a method for producing a steel sheet characterized by excellent sour resistance and low-temperature toughness, particularly low-temperature toughness of extra-thick steel of 30 mm or more. It is to provide an excellent manufacturing method of. [Composition] C: 0.01 to 0.08% by weight, Nb:
0.005-0.040%, Ti: 0.005-0.0
Steel of 25% and Al: 0.008% or less was made into a slab by a continuous casting method, which was reheated in a temperature range of 1100 to 1250 ° C., then controlled rolling, quenched at 900 ° C. or more, and 500 to 700 ° C. A method for producing a thick high-strength steel sheet excellent in sour resistance and low-temperature toughness, which is characterized by tempering.

Description

Detailed Description of the Invention

[0001]

This invention relates to petroleum containing H ₂ S,
Involved in the method of manufacturing steel sheets with excellent sour resistance and low temperature toughness, which are useful for line pipes used for natural gas, and further, from heat input welding to heat input welding (HAZ) The present invention relates to a method for producing an extremely low-temperature toughness of, particularly, an extremely thick steel having a thickness of 30 mm or more and an excellent low-temperature toughness.

[0002]

_2. Description of the Related Art Steel pipe materials such as line pipes for transporting sour oil and sour gas containing hydrogen sulfide (H ₂ S) and its auxiliary equipment or steel pipe materials such as chemical plant pipes handling fluids containing H ₂ S are sour resistant. Property (HIC resistance and SSC resistance) is required. In this case, as for HIC resistance, artificial seawater + saturated H ₂ S solution (pH ~ 5.0) usually specified in NACE TM-02-84 or NACE is used.
5% NaCl + 0.5% specified in TM-01-77
CH ₃ COOH + saturated H ₂ S solution (pH ~ 3.5) is used in the assessment. In particular, as a HIC resistant steel material in the low pH environment such as the latter, measures have been taken by controlling the morphology of inclusions by extremely reducing S and adding Ca, and by controlling the hardness of the segregated portion by reducing Mn and P.

However, when the strength of the steel material is increased, the concentration of components in the segregated portion increases and the HIC resistance in a low pH environment may not always be satisfied. For example, the SSC resistance when stress is applied is, for example, Constant load SSC test according to NACE TM-01-77 standard (a round bar test piece of 6.35 mmφ is 5% NaCl + 0.5% CH ₃ COOH + saturated H ₂ S
When a tensile stress is applied by a certain load in the liquid and a rupture time at various stresses is evaluated), the above conventional steel materials have a critical stress value for rupture (maximum stress that does not rupture for 720 hr).
Is about 0.5 to 0.8 × yield stress (σ _y ).

On the other hand, by reducing the carbon content of the steel material and compensating for the decrease in strength by adding an alloy such as Mn and Nb, the microstructure becomes a uniform low carbon bainite structure.
Generally, excellent HIC resistance is obtained even in a low pH environment even if the strength is relatively high as compared with C-low Mn type steel. further,
Controlled rolling for steel sheets of 30 mm or less,
By combining also controlled cooling, a fine and uniform bainite structure can be obtained, and excellent HIC resistance and SSC resistance of the base material can be obtained.

On the other hand, the operating conditions of oil wells and the like containing H ₂ S gas have recently become more severe, and higher toughness has been demanded. To secure HAZ toughness of low alloy steel, various factors such as crystal grain size, dispersed state of hardened structure of high carbon island martensite (MA), upper bainite (Bu), presence of grain boundary embrittlement, element microsegregation, etc. Controlled by metallurgical factors. Above all, it is known that the size of the crystal grains of HAZ has a great influence on the low temperature toughness.
Many technologies for making the structure finer have been developed and put into practical use. The technology of finely dispersing a relatively stable nitride such as TiN even in a high temperature in steel and thereby suppressing coarsening of austenite (γ) grains of HAZ is particularly famous.

However, in the region where HAZ is heated to 1400 ° C. or higher, TiN coarsens or dissolves, and the ability to suppress the coarsening of γ grains disappears. For this reason, the toughness is largely deteriorated in the vicinity of the melting line, and it is not possible to stably obtain high toughness in the entire HAZ. On the other hand, Ti oxide (mainly T
i ₂ O ₃ ) finely dispersed steel (Japanese Patent Laid-Open No. 61-7974).
No. 5) can refine the HAZ structure even in the vicinity of the melting line, and can obtain excellent low temperature toughness as compared with TiN steel.

Further, by adding Ca to the low S steel, fine Ca
By forming and dispersing an oxide (mainly CaO), thereby refining the structure and fixing S with residual Ca, a technique for inexpensively producing steel having excellent HAZ toughness was established. The steel produced by this method has a fine microstructure in the entire region from the vicinity of the melting line to the HAZ and exhibits excellent low temperature toughness. Although Ca oxide has a small ability to suppress coarsening of γ grains, CaO existing in γ grains at the time of γ-α transformation is used as a nucleus to form radially fine acicular ferrite (IFP).
Are generated, the HAZ structure becomes extremely fine. Ca
O is also stable in a region heated to 1400 ° C. or higher in the vicinity of the melting line, and also in this region, it has an effect on making the structure fine. As a result, the welded portion is miniaturized over the entire area, and extremely excellent low temperature toughness is obtained.

As a method of improving the HAZ toughness by using an oxide, there is a method of utilizing a Ti oxide as disclosed in Japanese Patent Laid-Open No. 61-79745, but a Ca oxide and a Ti oxide have a large IFP forming ability. There is no difference. CaO is Ti ₂ O ₃
Since it has a higher generation temperature and is less affected by the cooling rate at the time of solidification of the slab, it is excellent in that it enables uniform fine dispersion over the entire slab. Further, JP-A-3-23642
According to Japanese Patent No. 0, the improvement of low temperature toughness due to Ti oxide and C
By controlling inclusions by a, etc., it became possible to obtain a steel excellent in both low temperature toughness and sour resistance in all regions including HAZ. However, although the above technique is effective for steel of 30 mm or less, for extremely thick steel of 30 mm or more,
There is a large temperature difference between the plate surface and the center of plate thickness, making it difficult to secure sour resistance. That is, in the case of an extremely thick material having a plate thickness of 30 mm or more, it is not possible to have both sour resistance and low temperature toughness because the structure cannot be controlled in the center part of the plate thickness and the surface layer part of the plate in the prior art.

[0009]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for producing a steel plate for a high-strength line pipe excellent in sour resistance and low temperature toughness even at a thickness of 30 mm or more, as well as at a thickness of 30 mm or less. That is. Further, the object of the present invention is to prevent the formation of high carbon island martensite due to the reduction of C, to prevent the hardness increase of bainite, to add Ti and Ca to the reduction of S, to contain substantially no Al, and to control rolling, Low temperature toughness by quenching and tempering, SSC resistance and HI resistance
It is to provide a method for manufacturing a steel sheet having excellent C property.

[0010]

SUMMARY OF THE INVENTION The present invention is intended to produce a high toughness line pipe steel having a large plate thickness without impairing SSC resistance and HIC resistance.
As a result of various examinations by the inventors of the present invention on the manufacturing conditions, the SSC resistance, the HIC resistance, and the mechanical properties with respect to the chemical composition, the structure, and the plate thickness, the following facts were found.

In the case of a thin material (hereinafter referred to as a material having a plate thickness of 30 mm or less), a controlled rolling + accelerated cooling, or a controlled rolling + quenching and tempering material with a fine C and a hardness of 250 Hv or less, which is fine and uniform bainite (first The structure consisting of ferrite or IFP) as the two phases is most resistant to SSC and HIC
It has excellent properties. However, thick materials (hereinafter, thickness 3
In the case of 0 mm or more), the uniform structure described above cannot be obtained in the controlled rolling + controlled cooling material, and the sour resistance can be secured only by the controlled rolling (+ controlled cooling) + quenched and tempered material. Low S and C
Addition of a improves both sour resistance and low temperature toughness. When the S content is lowered and the Al content is reduced together with the addition of Ca, the production of IFP in the HAZ increases, the production of high carbon island martensite decreases, the low temperature toughness improves, and the sour resistance does not deteriorate. That is, in the rolling condition + accelerated cooling, even if the cooling conditions are variously changed, the structure cannot be made uniform in the thick material.

On the other hand, by properly selecting the controlled rolling + quenching and tempering material, a fine and uniform bainite structure (ferrite, IFP or tempered martensite as the second phase) of 250 Hv or less is obtained. In addition, the formation of martensite is suppressed, and the fine and uniform bainite structure improves SSC resistance and HIC resistance. Furthermore, low S
Addition of Ca to the composition improves the HIC resistance by controlling the morphology of inclusions such as MnS.

The most effective structure for improving the HAZ toughness is IFP. However, by reducing S, adding Ti and Ca, and reducing Al, the nuclei of IFP (C
a oxide, Ti oxide) is increased, and high toughness is achieved.
Further, the reduction of Al makes it easy to add a strengthening element because it suppresses the formation of high carbon island martensite, and the HAZ toughness does not deteriorate even if the strength level increases. The importance of the present invention is that SSC resistance and H resistance are high even for materials with a plate thickness of 30 mm or more
The point is to secure an important characteristic as a sour-resistant line pipe that improves the IC property and low temperature toughness.

Therefore, the gist of the present invention is as follows. (1) C: 0.01 to 0.08% by weight, Si: 0.
05-0.5%, Mn: 0.8-1.5%, P: 0.0
15% or less, S: 0.0010% or less, Nb: 0.00
5 to 0.040%, Ti: 0.005 to 0.025%,
Al: 0.008% or less, Ca: 0.001 to 0.00
5%, N: 0.001 to 0.005%, O: 0.001
Steel containing ˜0.005% and consisting of residual unavoidable impurities and iron was made into a slab by a continuous casting method.
After reheating in the temperature range of 100 to 1250 ° C., controlled rolling, quenching at a reheating temperature of 900 ° C. or higher, 500 to 70
A method for producing a thick high-strength steel sheet having excellent sour resistance and low-temperature toughness, which is characterized by tempering at 0 ° C.

(2) C by weight%: 0.01 to 0.08
%, Si: 0.05 to 0.5%, Mn: 0.8 to 1.5
%, P: 0.015% or less, S: 0.0010% or less,
Nb: 0.005-0.040%, Ti: 0.005-
0.025%, Al: 0.008% or less, Ca: 0.0
01-0.005%, N: 0.001-0.005%,
O: 0.001 to 0.005% is included, V: 0.00
5 to 0.060%, Ni: 0.05 to 0.50%, M
o: 0.05 to 0.40%, Cu: 0.05 to 1.0
%, Cr: 0.05 to 1.0% of one or two or more, and steel made of residual unavoidable impurities and iron is made into a slab by a continuous casting method.
After reheating in the temperature range of 0 ° C, control rolling is performed and the reheating temperature is 9
A method for producing a thick high-strength steel sheet having excellent sour resistance and low-temperature toughness, which comprises quenching at 00 ° C or higher and tempering at 500 to 700 ° C.

[0016]

The reason for limiting the chemical components as described above in the present invention is as follows. C: The lower limit of the amount of C is set to 0.01%, which is the minimum amount for ensuring the strength of the base material and the welded portion and for exerting these effects when Nb, V and the like are added. But C
If it is too large, not only the HAZ toughness is adversely affected, but also the base metal toughness, weldability, and sour resistance are deteriorated.
The upper limit was 0.08%. Si: Si is required for steel in an amount of 0.05% or more in terms of deoxidation, but if added in a large amount, the weldability and the toughness of the welded portion deteriorate, so the upper limit was made 0.5%.

Mn: Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.8%. HAZ
In order to improve the 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 the Mn content is too large, the hardenability increases, which not only deteriorates the weldability and HAZ toughness but also promotes the central segregation of MnS and the like in the slab and deteriorates the sour resistance. Was set to 1.5%.

P: P which is an impurity in the present invention is 0.
It was set to 015% 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. P content is preferably 0.010%
The following is desirable.

S: When the upper limit of the amount of S is set to 0.001% or more, MnS whose morphology cannot be controlled by Ca is produced, and H
IC is the starting point. Therefore, in the present invention, the S content is 0.00
It was set to 1% or less. Nb: Nb is an important element in the steel of the present invention, and it is difficult to obtain excellent HAZ toughness in high strength steel without adding Nb. Nb has a function of suppressing ferrite generated at the γ grain boundary and promoting generation of fine IFP having CaO as a nucleus. To obtain this effect, a minimum Nb content of 0.005% is required. However, N
If the amount of b is too large, on the contrary, the production of fine IFPs is hindered. Further, the production of MA in HAZ is promoted and HAZ is deteriorated, so the upper limit was made 0.040%. Especially CT
In a steel required to have OD characteristics, Nb precipitates can serve as a starting point of occurrence of brittle cracks, so 0.030% or less is preferable.

Ti: When Ti is added to the steel of the present invention, TiO
And TiN are formed to refine the HAZ structure,
Improves Z toughness. The lower limit of 0.005% is the minimum amount for obtaining this effect, and the upper limit of 0.025%.
Is to prevent HAZ toughness deterioration due to TiC formation. Al: Al is an element that is generally contained in deoxidized upper steel, but is an element that is not preferred in the present invention, and its upper limit was made 0.008%. This is because when Al is contained in the steel, it is combined with O and Ti oxide and Ca oxide are not generated. Preferably, the amount of Al is 0.003% or less.

Ca: In order to control the morphology of MnS, which is an inclusion in steel, to improve the HIC resistance, and to generate CaO for improving the toughness in HAZ,
Add 001% or more. However, when it exceeds 0.005%, HIC resistance and SSC resistance are deteriorated by large Ca-based inclusions and clusters, so 0.005% was made the upper limit. N: In order to secure HAZ toughness due to TiN or the like, 0.
001% or more is necessary. Further, if it exceeds 0.005%, the HIC resistance deteriorates, so the upper limit was made 0.005%. In order to produce CaO and TiO in O: HAZ, the amount of O needs to be 0.001% or more. The upper limit of the amount of O is set to 0.005% in order to prevent deterioration of cleanliness and toughness of steel due to the formation of non-metallic inclusions.

In the present invention, at least one kind of V, Cu and Cr is further added as a strength adjusting element, if desired. V: V is an element having almost the same effect as Nb, but 0.
If it is 005% or less, there is no effect, and the upper limit is 0.060%. 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 0.5%, the effect of improving toughness due to the reduction of Al decreases, so the upper limit was made 0.5%. Cu: Cu has almost the same effect as Ni, and is 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. Therefore, the upper limit is set to 1.0%.

Cr: Addition of 0.05% or more of Cr increases the strength of the base material and the welded portion, but if it is too large, the weldability and HAZ toughness deteriorate. Therefore, the upper limit is 1.0%
And 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, but if it is too large, it deteriorates HAZ toughness and weldability like Ni, which is not preferable.
The upper limit is 0.40%.

The CC slab having such a composition is hot-rolled (+ accelerated cooling) + quenched and tempered, and FIG. 1 shows a water cooling pattern at this time. That is, 1100
After reheating in a temperature range of ˜1250 ° C., controlled rolling, 90
Quench at 0 ° C or higher and temper at 500 to 700 ° C. The reasons for limiting the manufacturing conditions in the present invention as described above are as follows. First, the upper limit of the reheating temperature was 1250 ° C. This is because the γ grains become coarse and the toughness deteriorates. On the other hand, if the temperature is lower than 1100 ° C., precipitates such as Nb (CN) are coarsened to deteriorate the HIC resistance. Controlled rolling is necessary to make the γ grains fine, but is not particularly limited. Preferably, the cumulative reduction amount at 900 ° C or lower is 40%
As described above, it is desirable to perform rolling at the rolling end temperature of 750 to 900 ° C. In addition, if necessary, accelerated cooling does not cause any problem.

Further, the structure is refined by quenching at a temperature of 900 ° C. or higher after controlled rolling. If the temperature is 900 ° C. or less, quenching cannot be performed from fine-grained austenite, and the base material toughness and sour resistance deteriorate. To temper the low temperature transformation structure after hardening, martensite, and bainite of 250 Hv or more, 50
If the temperature is not higher than 0 ° C, the material does not decompose and the toughness and sour resistance of the base material deteriorate. Further, when the temperature is 700 ° C. or higher, MA is generated although the aforementioned structure is decomposed, which deteriorates the base material toughness and sour resistance. Therefore, the quenching temperature should be 900 ° C or higher,
The tempering temperature was 500 to 700 ° C.

[0026]

[Examples] Using test steels having the chemical compositions shown in Table 1, CC slabs were manufactured under the manufacturing conditions shown in Table 2. Mechanical properties, HIC resistance and SS resistance of the steel sheet thus obtained
C property is shown in Table 2. The test piece was taken from the position shown in FIG.

[0027]

[Table 1]

[0028]

[Table 2]

Steels 13 to 16 are not under proper manufacturing conditions, so that HIC resistance and SSC resistance are deteriorated. Steel 1
This is because the reheating temperature of No. 3 is too low, the tempering temperature of Steel 14 is high, and the tempering temperature of Steel 15 is low, and the quenching temperature of Steel 16 is less than 900 ° C. Further, in steels 14 to 16, the base material toughness also decreased. Steel 1
Nos. 7 to 25 have an inappropriate chemical composition and cannot obtain mechanical properties. Steel 17 has a large amount of C and cannot obtain a fine and uniform bainite structure. Therefore, Steel 18 has a high Mn content and a large amount of MnS.
Since steel 19 has a large amount of S and cannot control the morphology of MnS, HIC resistance and SSC resistance cannot be obtained. Steel 20 had a small amount of Ti, and Steel 21 had a large amount of Al, so that the HAZ toughness deteriorated. Further, Steel 22 is an example in which the O content is large, the cleanliness is impaired, and the toughness and sour resistance are deteriorated. Steel 23 had a small amount of Ca, so its toughness and sour resistance decreased. Steel 24 is an example in which the strength of the base material and the toughness of the base material cannot be ensured because the amount of Nb is small. Further, in Steel 25, the HAZ toughness is deteriorated because the Nb content is large.

[0030]

EFFECTS OF THE INVENTION According to the present invention, about pH 3 containing H ₂ S is used.
Hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance in a low pH environment, such as that of the base metal, as well as the toughness of the welded part are properly improved, and high strength sour resistance even with a plate thickness of 30 mm or more. The characteristics as a steel pipe material for line pipes can be effectively enhanced. It is an invention that is industrially highly effective.

[Brief description of drawings]

FIG. 1 is a diagram showing rolling and water cooling patterns according to the method of the present invention.

2A to 2D are explanatory views of a test piece sampling position.

Front Page Continuation (72) Inventor Akihiro Date 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Steel Works

Claims (2)

[Claims] 1. C: 0.01 to 0.08% by weight, Si: 0.05 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S : 0.0010% or less, Nb: 0.005 to 0.040%, Ti: 0.005 to 0.025%, Al: 0.008% or less, Ca: 0.001 to 0.005%, N: A steel containing 0.001 to 0.005%, O: 0.001 to 0.005%, and consisting of residual unavoidable impurities and iron was made into a slab by a continuous casting method.
After reheating in the temperature range of 0 ° C, control rolling is performed and the reheating temperature is 9
A method for producing a thick high-strength steel sheet having excellent sour resistance and low-temperature toughness, which comprises quenching at 00 ° C or higher and tempering at 500 to 700 ° C. 2. By weight%, V: 0.005-0.060%, Ni: 0.05-0.50%, Mo: 0.05-0.40%, Cu: 0.05-1.0. %, Cr: 0.05 to 1.0%, or a combination of two or more thereof. The method for producing a thick high-strength steel sheet having excellent sour resistance and low temperature toughness according to claim 1.