JPH09324216A - Manufacture of high strength steel or line pipe, excellent in hic resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacture of a high strength steel for line pipe, excellent in HIC resistance, capable of stably and inexpensively preventing the occurrence of HIC while having strength as high as X80 grade. SOLUTION: This steel can be manufactured by subjecting a steel slab, having a composition which contains, by weight, 0.02-0.08% C, 0.05-0.50% Si, 1.0-1.8% Mn, <=0.010% P, <=0.002% S, 0.005-0.05% Nb, 0.005-0.10% Ti, 0.01-0.07% Al, and 0.0005-0.0040% Ca and in which the carbon equivalent represented by (carbon equivalent)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 is regulated to >=0.32%, to heating and to rolling and then subjecting the resultant steel plate or a steel pipe, prepared by using the steel plate, to hardening treatment from >=850 deg.C at (5 to 40) deg.C/sec cooling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel plate or a steel pipe which is excellent in hydrogen-induced cracking resistance and is used as a material for a line pipe having a strength level of API standard X80 grade. In the manufacturing method of the present invention, a steel pipe is formed into a steel plate by a thick plate mill or a hot rolling mill, formed by UOE forming, press bend forming, roll forming, etc., and welded by submerged arc welding or electric resistance welding. It is used as a line pipe for transporting crude oil and natural gas.

[0002]

2. Description of the Related Art In general, line pipes used for transporting crude oil and natural gas containing hydrogen sulfide have properties such as strength, toughness, and weldability required for line pipes, as well as hydrogen-induced cracking resistance (HIC resistance). Resistance) and stress corrosion cracking resistance (SS resistance)
So-called sour resistance performance such as CC property) is required.

In HIC, hydrogen ions generated by the corrosion reaction are adsorbed on the steel surface and penetrate into the steel as atomic hydrogen, and non-metallic inclusions such as MnS in the steel and a hard second phase structure are present. It is said that it diffuses and accumulates around and is generated by the internal pressure. Therefore, the following methods have been proposed so far as methods for preventing the occurrence of HIC.

(1) By reducing the S content in steel and adding Ca, REM, and the like in appropriate amounts, the formation of long-stretched MnS is suppressed, and finely dispersed spherical inclusions with low stress concentration are formed. To suppress the generation and propagation of cracks (for example, Japanese Patent Laid-Open No. 54-110119).

(2) In order to suppress the formation of island-like martensite which can be a starting point for cracks at the central segregation portion, and to suppress the formation of a hardened structure such as martensite or bainite which easily becomes a propagation path of cracks, When reducing the content of elements with a high segregation tendency such as C, Mn, and P in steel, or performing soaking treatment to eliminate alloy element segregation in the slab heating stage before rolling, or during cooling after rolling In order to prevent the formation of a hardened structure due to the diffusion of C during the transformation of (1), accelerated cooling is performed (for example, JP-A-61-60866 and JP-A-61-165207).

(3) A microstructure with low cracking susceptibility is obtained by performing heat treatment such as quenching and tempering, or by setting the rolling finish temperature to the recrystallization lower limit temperature of austenite or higher (for example, JP-A-54-12782). Japanese Laid-Open Patent Publication No. 62-7
819, JP-A-6-73450, etc.).

(4) By adding Cu to the steel, a protective film is formed on the surface to prevent hydrogen from penetrating into the steel (Japanese Patent Laid-Open No. 52-11118). The HIC resistance is improved by these various methods, and line pipes requiring sour resistance have also been mass-produced up to API standard X65 grade or X70 grade.

However, in recent years, there has been an increasing demand for high-strength steel pipes in order to increase transportation efficiency and reduce laying costs, and line pipes used in sour environments are also required to have high strength up to X80 grade. It is like this.

However, since HIC is likely to occur as the strength increases, HIC is reduced by the methods (1) to (4).
It has become impossible to completely suppress the generation of IC. Specifically, cracks are generated from inclusions whose shape is controlled by the method (1), and cracks are generated at portions other than the central portion where the countermeasure for central segregation is taken by the method (2). become. Further, among the methods of (3), quenching and tempering treatments are not suitable for mass production of line pipes in terms of cost and efficiency, and the effect of microstructure control by recrystallization temperature region finishing is not sufficient. Come on. In addition, the Cu coating formed by the method (4) cannot be expected to be effective in an environment having a low pH, and is actually a 5% NaCl + 0.5% CH ₃ COOH aqueous solution saturated with hydrogen sulfide having a pH of about 3 (commonly called NACE solution). In (), the effect of the coating is not obtained.

Recently, X80 having sour resistance
A method of manufacturing grade steel sheets for line pipes has been proposed one after another. The skeleton controls the morphology of inclusions by adding low S and Ca, suppresses central segregation as low C and low Mn, and reduces the accompanying strength reduction of Cr, Mo, N.
The addition of i or the like and accelerated cooling after rolling make up for this, and the addition of Cr is disclosed in JP-A-5-9575, and the addition of Cr-in JP-A-271766 and 7-109519. The addition of Mo is disclosed in JP-A-7-173536, and the addition of Ni-Cr-Mo is disclosed.

However, these are all H of the central segregation part.
This is a method for preventing the generation of IC, and no specific measures against cracking are applied to the parts other than the central segregation part. That is, when the tensile strength is increased, HIC is likely to occur not only in the central segregated portion but also in the entire plate thickness direction, but these techniques do not show a measure for preventing HIC in the entire plate thickness direction.
Further, all of these methods are intended to obtain a steel sheet of X80 grade by performing tempering treatment as it is or after accelerated cooling.

As a heat treatment method for obtaining good HIC resistance, Japanese Patent Application Laid-Open No. 62-7819 discloses a heat treatment method of heating and quenching, but it has not been shown to obtain high strength such as X80.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a high strength such as X80 grade, while at the same time, it is possible to stably and inexpensively prevent the occurrence of HIC. An object of the present invention is to provide a method for manufacturing a high-strength line pipe steel excellent in HIC property.

[0014]

[Means for Solving the Problems] The inventors of the present invention produced a base material having various component systems and microstructures by changing the additive elements of steel and the heat treatment conditions to obtain HIC resistance and strength / toughness. investigated. As a result, sufficient hardness and toughness as X80 grade and good HIC resistance performance can be obtained by performing specific quenching treatment on the steel in which the amount of added elements and the carbon equivalent shown by the following formula are specified. Found. Carbon equivalent = C + Mn / 6 + (Cu + Ni) / 15 + (C
r + Mo + V) / 5 The present invention has been completed on the basis of such findings. Firstly, in% by weight, C: 0.02 to 0.08%, S
i: 0.05 to 0.50% or less, Mn: 1.0 to 1.8
%, P: 0.010% or less, S: 0.002% or less, N
b: 0.005 to 0.05%, Ti: 0.005 to 0.
10%, Al: 0.01 to 0.07%, Ca: 0.00
05-0.0040%, the carbon equivalent is 0.3
For steel plates obtained by heating and rolling steel slabs of 2% or more, or steel pipes using the steel plates, from 850 ° C or higher to 5-40
It is intended to provide a method for producing a high-strength line pipe steel having excellent HIC resistance, which is characterized by performing quenching at a cooling rate of ° C / sec.

Secondly, in% by weight, C: 0.02 to 0.0
8%, Si: 0.05 to 0.50% or less, Mn: 1.0
~ 1.8%, P: 0.010% or less, S: 0.002%
Hereinafter, Nb: 0.005 to 0.05%, Ti: 0.00
5 to 0.10%, Al: 0.01 to 0.07%, Ca:
0.0005 to 0.0040%, and further C
u: 0.50% or less, Ni: 0.50% or less, Cr:
0.50% or less, Mo: 0.50% or less, V: 0.10.
% Or less, one or two or more, and the carbon equivalent of the steel slab having a carbon slab of 0.32% or more is heat-rolled to a steel plate or a steel pipe using the steel plate. The present invention provides a method for producing high-strength line pipe steel excellent in HIC resistance, characterized by performing quenching at a cooling rate of / sec.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the composition of the steel sheet according to the present invention is, in weight%,
C: 0.02 to 0.08%, Si: 0.05 to 0.50
% Or less, Mn: 1.0 to 1.8%, P: 0.010% or less, S: 0.002% or less, Nb: 0.005 to 0.0
5%, Ti: 0.005 to 0.10%, Al: 0.01
~ 0.07%, Ca: 0.0005 to 0.0040%, C + Mn / 6 + (Cu + Ni) / 15 + (Cr +
The carbon equivalent represented by Mo + V) / 5 is 0.32% or more. Further, as a selective component, Cu: 0.50% or less, N
i: 0.50% or less, Cr: 0.50% or less, Mo:
One or two or more of 0.50% or less and V: 0.10% or less may be contained.

The reasons for limiting these constituent elements are as follows. C: 0.02 to 0.08% When the C content is less than 0.02%, it becomes difficult to secure the predetermined strength of X80, while when it is added in excess of 0.08%, the toughness and HIC resistance of steel are increased. Cause deterioration of sex. From the viewpoint of weldability and sulfide stress corrosion cracking resistance, it is desirable to reduce the C content. Therefore, the amount of C is set to the range of 0.02 to 0.08%.

Si: 0.05 to 0.50% Si is added for deoxidation, but if it is less than 0.05%, a sufficient deoxidizing effect cannot be obtained. On the other hand, if it exceeds 0.50%, deterioration of toughness and weldability is caused. Therefore, S
The i amount is in the range of 0.05 to 0.50%.

Mn: 1.0 to 1.8% Mn is added as a basic component of steel effective for improving the strength and toughness of steel, but if it is less than 1.0%, its effect is small. On the other hand, if it exceeds 1.8%, the weldability and HIC resistance are significantly deteriorated. Therefore, the Mn content is 1.0 to 1.8%.
Range.

P: 0.010% or less In the case of the present invention, P is an impurity element that deteriorates weldability and HIC resistance, and it is desirable to reduce it as much as possible, but excessive P removal causes cost increase. , P upper limit of 0.0
10%.

S: 0.002% or less Even if Ca is added to control the morphology from MnS to CaS-based inclusions, finely dispersed CaS-based inclusions are also included in the case of the X80 grade high-strength material. Since it can be a starting point of cracking, it is necessary to reduce the S content to 0.002% or less.

Nb: 0.005 to 0.05% Nb is an essential component necessary for imparting sufficient toughness as a line pipe by refining the microstructure by suppressing grain growth during rolling. The effect is effectively exhibited at 0.005% or more. However, if it exceeds 0.05%, the effect is almost saturated and the toughness of the weld heat affected zone is deteriorated. Therefore, the amount of Nb is set to 0.005 to 0.05%.

Ti: 0.005 to 0.10% Ti forms TiN and suppresses grain growth during heating of the slab. As a result, the microstructure is refined and toughness is improved. Appears at 0.005% or more. However, if it exceeds 0.10%, the toughness is deteriorated. Therefore, the Ti content is 0.005 to 0.1.
The range is 0%.

Al: 0.01 to 0.07% Al is added as a deoxidizing agent, and the effect is effectively exhibited at 0.01% or more. However, if it exceeds 0.07%, the cleanliness is lowered and the HIC resistance is deteriorated. Therefore, the amount of Al is set to the range of 0.01 to 0.07%.

Ca: 0.0005 to 0.0040% Ca is an element indispensable for controlling the morphology of sulfide inclusions, and its effect is effectively exhibited at 0.0005% or more. However, if it exceeds 0.0040%, the effect is saturated, and conversely, the cleanliness is lowered and the HIC resistance is deteriorated.
Therefore, the amount of Ca is set to the range of 0.0005 to 0.0040%.

Carbon equivalent: 0.32% or less Carbon equivalent is 0.3 in order to obtain sufficient strength as X80.
Since it is necessary to be 2% or more, it is set to 0.32% or more. The upper limit need not be specified.

Next, the limitation of the optional additive components will be described. Cu: 0.05% or less Cu is one of the elements effective in improving the toughness and increasing the strength, but if it exceeds 0.05%, the weldability is impaired.
When adding, it is necessary to regulate the amount to 0.05% or less.

Ni: 0.50% Ni is one of the elements effective in improving the toughness and increasing the strength, but if it exceeds 0.50%, the effect is saturated and stress corrosion cracking easily occurs. . Therefore, when it is added, its amount is set to 0.50%.

Cr: 0.50% or less Cr, together with Mn, is an effective element for obtaining sufficient strength as an X80 grade even with low C, but if added in excess of 0.50%, the weldability is adversely affected. When added, the upper limit is 0.50%.

Mo: 0.50% or less Mo is one of the elements effective for improving the toughness and increasing the strength, but if it exceeds 0.50%, the effect is saturated and the weldability and HIC resistance are improved. If added, the amount is made 0.50% or less for inhibiting.

V: 0.10% or less Addition of an appropriate amount of V enhances strength without deteriorating toughness, weldability and sour resistance, so that even with low Cr, X80 can be obtained.
It is an element effective for obtaining sufficient strength as a grade, but if it exceeds 0.10%, the weldability is significantly impaired. Therefore, if it is added, its upper limit is made 0.10%.

Next, the reasons for limiting the manufacturing conditions will be described. In the present invention, the rolling and accelerated cooling conditions need not be particularly limited. The heating temperature during quenching is set to 850 ° C. or higher in order to obtain sufficient toughness and HIC resistance. Also,
The cooling rate at that time must be 5 ° C./sec or more in order to obtain sufficient strength, and the upper limit of the cooling rate that does not deteriorate the toughness and HIC resistance is 40 ° C./sec. 5 to 40 ° C./sec. In addition, such quenching may be performed in the state of a steel plate, or may be performed after forming into a steel pipe. With the composition and manufacturing conditions as described above, X having good toughness in addition to good HIC resistance
80 grade high strength steel can be produced.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described. Steel having the chemical composition shown in Table 1 was hot-rolled into a steel sheet and heat-treated under the heat-treatment conditions shown in Table 2. Tensile test, Charpy impact test, HIC
The test was performed. The HIC resistance test was performed in a 5% NaCl + 0.5% CH ₃ COOH aqueous solution (commonly called NACE solution) saturated with hydrogen sulfide having a pH of about 3. Table 2 also shows these results. Here, it is assumed that good performance is obtained when the strength is sufficient as X80, and when vTrs in the Charpy test is −70 ° C. or less. Also, H
In the IC test, it was determined to be good when the crack length ratio was 15% or less.

[0034]

[Table 1]

[0035]

[Table 2]

As shown in Table 2, the examples of the present invention having the composition within the range of the present invention and subjected to the heat treatment within the range of the present invention obtained sufficient strength as X80 and good HIC resistance performance. Was given.

On the other hand, even if a steel having a composition satisfying the range of the present invention is used, the steel sheets A-1, A-5 which are not subjected to the heat treatment of the present invention,
In A-6, sufficient performance was not obtained. Steel plates L-1, M-1, N-1, O-1, and P- which are heat-treated steels having compositions not satisfying the scope of the present invention satisfying the present invention.
No satisfactory performance was obtained even with 1, Q-1, R-1, or steel sheet R-2 whose composition and heat treatment conditions did not satisfy the present invention.

[0038]

As described above, according to the present invention,
It has become possible to inexpensively and stably manufacture steel for line pipes of API standard X80 grade having excellent HIC resistance.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/58 C22C 38/58

Claims (2)

[Claims] 1. C: 0.02 to 0.08% by weight,
Si: 0.05 to 0.50% or less, Mn: 1.0 to 1.
8%, P: 0.010% or less, S: 0.002% or less,
Nb: 0.005-0.05%, Ti: 0.005-
0.10%, Al: 0.01 to 0.07%, Ca: 0.
For steel plates obtained by heating and rolling steel slabs containing 0005 to 0.0040% and having carbon equivalents of 0.32% or more, or steel pipes using the steel plates, from 850 ° C or higher to 5 to 40 ° C / sec. A method for producing high strength line pipe steel excellent in HIC resistance, characterized by performing quenching treatment at a cooling rate of 1. Carbon equivalent = C + Mn / 6 + (Cu + Ni) / 15 + (C
r + Mo + V) / 5 2. C: 0.02 to 0.08% by weight,
Si: 0.05 to 0.50% or less, Mn: 1.0 to 1.
8%, P: 0.010% or less, S: 0.002% or less,
Nb: 0.005-0.05%, Ti: 0.005-
0.10%, Al: 0.01 to 0.07%, Ca: 0.
0005-0.0040%, and further Cu:
0.50% or less, Ni: 0.50% or less, Cr: 0.5
A steel slab containing one or more of 0% or less, Mo: 0.50% or less, and V: 0.10% or less and having a carbon equivalent of 0.32% or more shown below was heated and rolled. A method for producing a steel for high strength line pipe excellent in HIC resistance, which comprises subjecting a steel plate or a steel pipe using the steel plate to a quenching treatment at a cooling rate of 850 ° C or higher to 5 to 40 ° C / sec. Carbon equivalent = C + Mn / 6 + (Cu + Ni) / 15 + (C
r + Mo + V) / 5