JPH06104849B2 - Method for producing low alloy high strength oil well steel excellent in sulfide stress cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has a high yield strength of 56 kg / mm ² or more and excellent sulfide stress crack resistance (hereinafter referred to as SSC resistance),
Steel pipes, especially those used in oil and gas wells, such as drill pipes for drilling and tubing and casing for production,
Further, the present invention relates to the production of oil well steel suitable for valves for oil wells, transportation pipes, storage facilities, and the like.

(Prior Art) In recent years, with the urgent need for energy, steel materials have been increasingly used for drilling, transportation, and storage of crude oil containing hydrogen sulfide. In particular, the steel used for oil well pipes used for crude oil drilling is exposed to severe corrosive environments due to the tendency to deeper wells, and steel that has both high yield strength and excellent SSC resistance is required. ing. Also, due to economic requirements, it is necessary to use low alloy steel to meet most requirements.

(Problems to be solved by the invention) Sulfide stress cracking due to hydrogen sulfide is caused by hydrogen embrittlement caused by diffusion of hydrogen generated when a steel surface is corroded into steel. ing. In a steel material having a chemical composition based on low alloy, as the strength of the steel material increases, the susceptibility to embrittlement increases, so that it is difficult to simultaneously provide the steel material strength and the excellent SSC resistance.

Conventionally, the production of steel materials with excellent SSC resistance has been achieved by controlling individual alloying elements. For example, Japanese Patent Publication Sho 56
As described in JP-A-33459, reduction of Mn amount, reduction of impurities such as P which are harmful to grain boundary embrittlement, or addition of a large amount of Mo has been carried out. However, as a result, they have unnecessarily narrowed the range of alloy components that can be used and increased the manufacturing cost.

In addition, since SSC resistance deteriorates as the strength of the material increases, producing relatively low-strength steel with chemical components that can produce high-strength steel can lead to a significant increase in production costs. is there.

However, the role of alloying elements with respect to SSC resistance characteristics essentially has an interaction, and by considering the interaction or the role of alloying elements corresponding to strength, steel with excellent SSC resistance characteristics can be obtained. Expands the manufacturable range of
It is possible to give the possibility of manufacturing a steel material having excellent SSC resistance at a lower cost.

(Means for Solving Problems) The present inventor conducted research from the above viewpoint, and found that SSC resistance
The characteristics revealed that the crack propagation path was deteriorated by passing through the grain boundaries of the former austenite grains (that is, the appearance of grain boundary fracture surfaces).

By the way, when the steel made into martensite by quenching is tempered, the yield strength decreases as the tempering temperature is increased. The crack morphology of these SSC (sulfide stress cracking) is
When the yield strength is higher than the value of “critical strength σc” defined by the present inventors, it becomes intergranular cracking of old austenite grains,
If it is low, intragranular cracking will occur. In addition, this "limit strength σ
The value of "c" varies depending on the steel composition.

Therefore, in the range where the yield strength is the critical strength σc or less, the SSC resistance is determined by the fact that SSC is the intragranular cracking of prior austenite grains, and this steel has excellent SSC resistance. On the other hand, in the range where the yield strength exceeds the limit strength σc, intergranular cracking is a determining factor for SSC resistance, and this steel has poor SSC resistance.

In addition, even if the Mn, P, Mo content of the steel is changed in the range where the critical strength σc is higher than the desired yield strength of the steel, the SSC resistance is determined by grain cracking, so the SSC resistance does not change. It is good.

The inventors conducted an experiment and found that the limit strength σc decreases as the content of the elements Mn and P that lowers the grain boundary strength increases, and increases as the content of the element Mo that increases the grain boundary strength increases. Found, calculate the influence coefficient of each element, σc = -3.9 (Mn + 4.3P + 17.0Mn x P) + 5 (Mo-0.1)
The formula of +83.2 is derived.

Based on such knowledge, the present invention has determined the constitutional requirements in consideration of having high quenching capable of producing a steel material having a sufficient thickness.

Immediately by weight%, C: 0.10 to 0.35%, Si: 0.35% or less, S:
0.005% or less, Nb: 0.01 to 0.10%, Cr: 0.2 to 0.95%, N: 0.008
% Or less, Ti: 0.028% or less, and -0.005% ≤ Ti-3.4N ≤
0.01%, Al: 0.01 to 0.10%, B: 0.0007 to 0.0020% Further, M
o: 0.1% or more, and the limit strength was found by the following formula _{σc σ c = -3.9 (Mn +} 4.3P + 17.0Mn × P) +5 (Mo-0.1)
+83.2 compared with the yield strength YS of the steel actually manufactured, YS ≤ σ _c
In combination such that, Mn, P, containing Mo, the steel and the balance of iron and unavoidable impurities, after hot working A _c3 point +
At a temperature of 20 ° C or more and 1020 ° C or less, it is austenitized and then subjected to quenching treatment to obtain a martensite structure of 90% or more, and subsequently, tempered at a temperature of 560 ° C or more and _{Ac 1} point or less This is a method for producing an excellent low alloy, high strength oil well steel.

The present invention will be described in detail below.

In the present invention, the content range of the steel composition is specified for the following reasons.

c; The content is 0.1% or less as an essential element for ensuring the strength of the low alloy steel and increasing the hardenability. However
A large content of more than 0.35% may cause cracking during quenching, so 0.35% was made the upper limit.

Si: Since it is an element that lowers the grain boundary strength, it is desirable to reduce the amount, and the maximum content was 0.35%.

S; Impurities that cannot be completely removed in steelmaking.
Therefore, the upper limit of the content is set to 0.005%.

Mb; Nb has the effect of making the grain size of the reheat-quenched steel finer, but if it is less than 0.01%, the effect is not sufficient, and even if added in a large amount, further grain refining effect cannot be expected, The upper limit of the content is set to 0.1% because it may easily cause scratches during processing.

Cr; Cr has the effect of enhancing the hardenability and keeping most of the added Mo in a solid solution state, so 0.2% or more is added. When the Cr content is small, the SSC resistance is not deteriorated, but when added in a large amount, the grain boundary strength is lowered and the SSC resistance is obviously lowered, so the upper limit was made 0.95%.

Al; Al is necessary for sufficiently producing killed steel in the steelmaking process, and is contained in an amount of 0.01% or more. However, addition of a large amount increases the amount of alumina-based inclusions and may become the starting point of cracking, so the upper limit of the content was made 0.1%.

B; B is an element that significantly improves hardenability, but 0.0007
% Or less, the effect is not sufficient, and even if a large amount is added, the effect is not only saturated, but cracks and scratches may occur during hot working, so the upper limit was made 0.0020%. In addition, the content of B reduces the contents of Mn and Cr, and has the effect of suppressing the tendency for the SSC resistance to decrease.

Ti; Ti is added to fix N as TiN and prevent the formation of BN and maintain the hardenability improving function of B. Where Ti
-3.4N theoretically indicates excess or deficiency with respect to the Ti amount (%) required to fix N. That is, all N in the steel is TiN TiN
The amount of Ti required to be fixed as
It is 3.4N. If the Ti addition amount is too small, N cannot be sufficiently fixed as TiN, and excessive addition of Ti promotes precipitation of coarse TiN and reduces SSC resistance. Therefore, the Ti addition amount is Ti-3.4N.
In this relation, -0.005% to 0.01%, and the upper limit of the total Ti amount was 0.028%.

It is desirable that the N; N content be low in order to reduce the total Ti content,
Since it is unavoidably contained in steelmaking, the upper limit was made 0.008%.

The SSC resistance is improved by lowering the Mn and P contents and increasing the Mo content, but as a result of an experimental detailed examination, a Mo content of 0.1% or more and σ _c =- 3.9
(Mn + 4.3P + 17.0Mn x P) + 5 (Mo-0.1) + 83.2
Compared with the actual yield strength of steel YS, if Mn, P, and Mo are contained in a combination that satisfies YS ≤ σ _c , grain boundary fracture surfaces do not appear and excellent SSC resistance is provided. Revealed that you can.

That is, under the condition that YS ≤ σ _c , what kind of Mn, P, M
It is possible to design steel materials having almost the same SSC resistance even if the content of o is the same, and it is possible to select the combination that is the easiest to manufacture and the combination that has the lowest cost. This is the main point of the present invention.

When these relationships are illustrated, the range of Mn, P, Mo and the strength at which the grain boundary fracture surface appears is as shown in Fig. 1. As an example, when 0.2% Mo, the yield strength is 77 kg / mm ² or less, The allowable contents of Mn and P that do not cause intergranular cracking are within the hatched range in FIG.

Steel having the above-mentioned chemical composition is melted and cast in a converter, an electric furnace or the like, and a desired shape is obtained through an ordinary hot working process.

Next, it is necessary to perform quenching after austenitization in order to obtain fine martensite with uniform grain size and size. For this reason, in order to perform sufficient solution treatment, the heating temperature is set to A _c3 + 20 ° C. or higher, and at 1020 ° C. or higher, the crystal grain suppressing effect due to Nb is lost, so the upper limit of the heating temperature was set to 1020 ° C.

In addition, the structure of the steel obtained by such quenching treatment,
The reason why the martensite structure is 90% or more is as follows.

The presence of a structurally inhomogeneous portion of the SSC resistance deteriorates due to stress concentration and partial yielding phenomenon, so it is desirable that the proportion of martensitic structure during quenching is high. Furthermore, coarse carbides are generally formed in the portions other than the martensite structure, and they tend to be the starting points for cracking. Therefore, it is desirable that the ratio of martensite structure is high,
Considering the level of industrially stable production of low alloy steel, 90
% And above.

Furthermore, tempering is necessary to increase the SSC resistance of the quenched steel and obtain the desired strength. Regarding the tempering temperature, if the temperature is 560 ° C. or lower, the segregation of P to the grain boundaries is remarkable, which lowers the SSC resistance, and when heated to A _c1 or higher, the austenite phase precipitates and transforms into ferrite after cooling, resulting in a non-uniform structure. Therefore, it is not preferable for SSC resistance. Therefore, the tempering temperature was set to 560 ° C. to A _c1 point.

The above low alloy steel has extremely excellent SSC resistance.

(Example) The chemical composition shown in Table 1 and the steel of the present invention 1 to 1 under the heat treatment conditions.
7 and comparative steels 9 to 13 were produced respectively.

The SSC resistance was evaluated by using a constant load type stress corrosion cracking evaluation tester in a parallel bar tensile test piece with a diameter of 6 mm in 0.5% CH ₃ COOH + 5% NaCl aqueous solution saturated with 1 atmosphere of hydrogen sulfide. Then, a stress of 75% of the yield strength was applied, and the test was performed with or without breakage at 720 hr.

In the column of SSC resistance of Table 1, those marked with ◯ are excellent in SSC resistance and marked with × are inferior, and the steel according to the present invention has excellent SSC resistance. Therefore, it is clear that the characteristics are inferior when the content is out of the range of the present invention.

(Effect of the Invention) According to the present invention, a steel excellent in SSC resistance and the like required as a steel for oil wells can be obtained, and the industrial effect is great.

[Brief Description of Drawings] FIG. 1 is a graph showing the range of grain boundary fracture surface formation in relation to the contents of Mn, P and Mo and the yield strength, and FIG. 7 is a graph showing an allowable range in which SSC characteristics are obtained.

Claims (1)

[Claims] 1. In weight%, C: 0.10 to 0.35% Si: 0.35% or less S: 0.005% or less Nb: 0.01 to 0.10% Cr: 0.2 to 0.95% N: 0.008% or less Ti: 0.028% or less and -0.005 % ≦ Ti-3.4N ≦ 0.01% Al: 0.01~0.1% B: 0.0007~0.0020% further Mo: 0.1% or more, and advance limit strength determined by the following calculation _{σ c σ c = -3.9 (Mn} + 4. 3P + 17.0Mn x P) + 5 (Mo-0.1)
+83.2 compared to the yield strength YS of the steel actually manufactured, YS
A steel containing Mn, P, and Mo with the balance iron and unavoidable impurities in a combination such that ≦ σ _{c
Characterized} by austenitizing and quenching at a temperature of _c3 point + 20 ° C or more and 1020 ° C or less to obtain a martensite structure of 90% or more, and subsequently tempering at a temperature of 560 ° C or more and A _c1 point or less A method for producing a low alloy, high strength oil well steel excellent in sulfide stress cracking resistance.