JPS61174359A - Steel material for oil well pipe having superior resistance to sulfide stress corrosion cracking - Google Patents

Abstract

PURPOSE:To obtain a steel material for an oil well pipe having superior resistance to sulfide stress corrosion cracking by reducing the P content in a steel material and regulating the Mn content to a specified value in relation to the P and C contents. CONSTITUTION:A steel pipe used in an oil well or the like in a wet environment having a high H2S content is made of a steel material having a composition contg. 0.1-0.45% C, 0.010-0.35% Si, 0.4-2.0% Mn, <0.009% P, <0.010% S, 0.01-0.10% Al, 0.0010-0.020% Ca and 0.0005-0.005% B and satisfying relation represented by an equation Mn<=3.5-250P-2.5C among the Mn, P and C contents. The structure and hardness of the steel are made uniform by the reduced P content and the regulated Mn, P and C contents, and inclusions such as MnS are dispersed and spheroidized by the added Ca, so the steel material for an oil well pipe has superior resistance to sulfide stress corrosion cracking without deteriorating the weldability.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to steel materials for oil country tubular goods having excellent sulfide stress corrosion cracking resistance, and more specifically, by reducing P and regulating Mn, P, and +4, the present invention relates to steel materials that are produced by segregation of these elements. In addition, Ca is added in a low S state to disperse and spheroidize sulfide-based inclusions such as MnS, thereby significantly improving sulfide stress corrosion cracking resistance. Concerning improved steel materials.

BACKGROUND ART Conventionally, it has been known that a deterioration and destruction phenomenon called so-called sulfide stress corrosion cracking (hereinafter referred to as 5 SCC) occurs in high-strength steel used in a humid environment containing hydrogen sulfide.

In recent years, with the increase in energy consumption and the decrease in easily available high-quality oil resources, sour gas fields and sour oil fields with high hydrogen sulfide content and deep wells have been developed. These developments are predicated on the development of oil country tubular goods with excellent 5scc resistance that can withstand severe usage conditions.

Originally, 5scc occurs when hydrogen generated by a corrosion reaction of steel in a hydrogen sulfide-containing environment penetrates into the steel, and is one of the hydrogen embrittlement phenomena. Also, 5s
The mechanism of cc is still largely unknown, but it is a phenomenon in which various factors such as metallurgical factors such as the composition and structure of the steel and the state of stress applied to the steel are intricately related.

Additionally, methods have been proposed for improving the 5SCC resistance by changing the composition of the steel, heat treatment, etc., but these methods are not perfect, and furthermore, these methods do not require the addition of expensive alloying elements or complicated heat treatment. The manufacturing cost increases significantly due to the required openings, making it impracticable in reality.

Therefore, the present inventors conducted a detailed study on the mechanism of occurrence of the 5scc phenomenon, and as a result, found that the 5scc phenomenon is closely related to the segregation of steel and the morphology of inclusions, and that this segregation and morphology can be adjusted appropriately. control to improve the 5scc resistance. In other words, the areas where Mn, P, etc. are segregated have high hardness, and hydrogen that has entered the steel concentrates in these areas with high hardness, causing inclusions, especially around MnS-based inclusions that tend to elongate in the rolling direction. Also, this hydrogen is concentrated. Further, when hydrogen is concentrated in these structures or inclusions, cracks occur from the concentrated structures or inclusions as a starting point.

For this reason, the present inventors have determined that the hardness of the steel structure does not vary due to the segregation of Mn, P, etc., and that hydrogen concentration is prevented by reducing MnP-based inclusions or making them spheroidal. The present invention was completed based on this fact.

To explain in more detail, in the present invention, as mentioned above, it has been found that the components that cause non-uniform structure and hardness due to segregation are mainly three components such as Mn and P%C, and each of these components is described in detail. Researched. Based on this research, firstly, P should be reduced to 0.009% or less to eliminate the highly hard uniform part, and secondly, Mn should be reduced according to the relational formula between P and C. Mn≦
When the conditions of 3.5-250P-2,5G are satisfied, Mn
The structure and hardness do not become non-uniform in the segregated areas of , P, etc., and the uniformity of structure and hardness can be maintained, and this uniformity improves the 5scc resistance. Thirdly, hydrogen tends to concentrate. It was found that MnS-based inclusions that elongate in the rolling direction can be converted to inclusions that do not elongate during hot rolling by adding Ca.

Therefore, in the present invention, P, which causes non-uniformity of structure and hardness, is reduced, Mnl is regulated in relation to P and C peaks, inclusions are dispersed and spheroidized by the addition of Ca, and other There is no effect of alloying elements or heat treatment, etc., and at the same time there is no effect of adverse effects on the mechanical properties and weldability of the steel, improving the 5scc resistance.

Hereinafter, the present invention will be specifically explained through the reasons for limiting the ingredients.

C: If C is less than 0.10, it is difficult to obtain the necessary strength for oil country tubular goods, and if it exceeds 0.45%, it impairs the toughness of the steel.
0.10 to 0, since it is undesirable to cause quench cracking.
It was limited to a range of 45%.

Sl: Si is an element necessary for deoxidation, but if it is less than 0.01%, it has no deoxidizing effect, and if it exceeds 0.35%, there is a risk of toughness deterioration. limited to a range.

Mn: If Mn is less than 0.4%, the required strength cannot be secured, but Mn
As n increases with e mentioned below, the hardness of the segregated part increases,
Furthermore, it combines with S to generate MnS inclusions. Therefore, from the viewpoint of 5scc resistance, it is preferable to have less.
If it exceeds 2.0%, toughness will be impaired. Therefore, Mn is 0
．． The relationship between P and C is controlled as described below in the range of 5 to 2.0%, and S, which contributes to the formation of MnS, is also limited to a low sulfur range as described below.

P: When P exceeds 0.009%, segregated areas with high hardness are formed, the steel structure becomes non-uniform, and the 5scc resistance deteriorates. Although it is preferable to not include P as much as possible,
It is almost impossible to have a mouth that does not contain any substances, and some traces will inevitably remain, so this is set as the lower limit.

S: S, together with the addition of Ca described later, has an effect on the dispersion and spheroidization of inclusions. However, if it exceeds 0.010%, Ca
Even if MnS inclusions are added, the MnS inclusions are not dispersed, making it difficult to form balls, and the 5scc resistance cannot be improved. Note that, like P, it is preferable that %S is not contained at all, but some trace remains, and this is set as the lower limit.

Al: 8L is necessary for deoxidation, and in the present invention is important as an element for improving the Ca yield. However, if it is less than 0.01%, it has no effect, and if it exceeds o or i%, crystal grains become coarse and the material quality deteriorates.

Ca: Ca is one of the important components along with the above-mentioned Mn, P, S, AI, etc., and the addition of Ca causes the MnS inclusions to be dispersed and spheroidized. This effect is at least o
, ooio%, and it is technically difficult to add more than 0.020%, so the upper limit was set at 0.020%.

B: It improves hardenability and strength.

However, it is ineffective if it is less than 0.0005% (,0,00
If it exceeds 5%, toughness will be impaired, so o, ooos ~ 0.0
05%.

The remainder consists of "e" and unavoidable impurities.

As mentioned above, in addition to containing each component, among these components, Mn needs to satisfy the following conditional expression in relation to P and Ca.

The reason for M11≦3.5-250P-2,5G is that if Mn is outside the range of this conditional expression, the structure and hardness will be non-uniform and sufficient 5scc resistance cannot be obtained. .

Next, examples will be described.

First, to clarify the improvement of 5scc resistance by homogenizing the structure and hardness by regulating P, Mn, and C, and by dispersing and spheroidizing inclusions by adding Ca.

Based on the composition of the comparative steel (sample steel code 1), P was lowered to 0.002 to 0.008% and Ca was added to this base steel, and as shown in Table 1, the invention steel ( The test steel symbol 2.3.4) was adjusted. Also, for comparison, comparative mi has high P and S, but the Mn level is made to match that of the invention steel, and comparative tA5 has only high P and S.
Comparative steel 6 has only Ga added thereto.

Next, each of these test steels 1 to G was heat treated to determine its mechanical properties, as shown in Table 2, and the quenching conditions were: 950°C for 30 minutes → main quenching. there were.

In addition, API standards C-75 and C-90 were applied to each sample steel.
, c-ilo, and these are shown in the specifications section of Table 2. For example, the steel samples that were heat treated with C-75 are given the suffix A, Steels that were heat treated to C-90 are shown with suffix B, and those that were heat-treated to C-110 are shown with suffix C. Therefore, each test steel that was heat treated as described above was subjected to a 5 scc resistance test as follows. The results shown in Table 3 were obtained.

That is, in the 5scc resistance test, the dimensions (t=
5a+m, 11=15mm, j! = 105a), this test piece 1 has a diameter of 4 as shown in Figure 2.
It was supported at four points by a book glass rod 2 and measured using a constant strain four-point bending method. In addition, in Fig. 2, *, =10
0mm%*2=40mm.

Further, the test liquid used was 0.5% acetic acid + 5% saline saturated with hydrogen sulfide, and the presence or absence of cracks was determined after the stressed sample was immersed in this liquid for 720 hours.

In addition, the added stress is based on the 0.5% proof stress of each sample.
．． A stress of 2 to 1.2 times is added, and the term of added stress in Table 3 is shown as n065σy, where n is 0.5% proof stress (kg
/mm2), ○ indicates no cracks, and X indicates cracks.

The test results shown in Table 3 are based on API standards C-75 and C-9.
0 and C-110.

As is clear from Table 3, the steel of the present invention has an excellent resistance to 5 s.
It shows cc property. On the other hand, comparative vA5, which was simply subjected to low P reduction, and comparative WA6, which was only subjected to Ca treatment, exhibited superior 5scc resistance compared to other comparative steels, but was inferior compared to the steel of the present invention. I understand that. In addition, regardless of the Mn level, the steel of the present invention has excellent resistance to 5 s.
cc resistance, but among these, those with lower Mn levels exhibit more aggressive 5scc resistance.

In short, it can be seen that the steel of the present invention does not lose its effectiveness even if the heat treatment and composition are different, and the 5scc resistance is improved, making it extremely effective.

As explained in detail above, the steel of the present invention not only has low P content but also controls Mn, P, and O, and eliminates the non-uniformity of structure and hardness caused by the segregation of these elements. In addition, the addition of Ca achieves dispersion and spheroidization of sulfide inclusions. Therefore, extremely excellent 5scc resistance can be obtained and it is suitable as a steel material for oil country tubular goods, and this effect is not impaired even by the addition of other alloying elements or by heat treatment.

Although the present invention has been explained above with a focus on steel materials for oil country tubular goods, it can also be applied to all cases of embrittlement caused by hydrogen, such as high-tensile bolt materials and high-pressure vessel steel materials. It can also be applied to high-strength steel such as steel.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a test piece to be subjected to a 5 scc resistance test, and FIG. 2 is an explanatory diagram of a jig for a four-point bending test conducted as a 5 scc resistance test.

Claims (1)

[Claims] C: 0.1-0.45%, Si: 0.010-0.35
%, Mn: 0.4 to 2.0%, P: 0.009% or less,
S: 0.010% or less, Al: 0.01 to 0.10%,
Ca: 0.0010-0.020%, B: 0.0005
~0.005%, the remainder being Fe and unavoidable impurities, and the conditional expression Mn≦3 regarding Mn, P, and C is satisfied.
A steel material for oil country tubular goods having excellent resistance to sulfide stress corrosion cracking and satisfying 5-250P-2.5C.