JPS635461B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a high-strength seamless pipe steel material for oil country tubular goods with excellent resistance to sulfide stress corrosion cracking. Low P and Nn,
By regulating the quantitative relationship between P and C, we can eliminate the non-uniformity of the steel structure and hardness caused by the segregation of these elements, and at the same time, we can add Ca in a state of low S content to create MnS, etc. This invention relates to a steel material whose sulfide stress corrosion cracking resistance is significantly improved by dispersing sulfide inclusions and making them spheroidized. Conventionally, high-strength steel used in humid environments containing hydrogen sulfide is known to suffer from a deterioration and destruction phenomenon called sulfide stress corrosion cracking (hereinafter referred to as SSCC). 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 include materials with excellent SSCC resistance that can withstand harsh usage conditions, and
The premise is the development of seamless oil country tubular goods with high tensile strength. Originally, SSCC occurs when hydrogen generated by the corrosion reaction of steel in a hydrogen sulfide-containing environment penetrates into the steel, and is one of the hydrogen embrittlement phenomena. Although much of the mechanism of SSCC is still unknown, 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. It is known that this SSCC susceptibility has a strong correlation with HIC susceptibility in materials with relatively low strength (tensile strength 40 to 70 Kg/mm ² ). On the other hand, as shown in the examples below, for example, in high-strength products (tensile strength of 70 kg/mm ² or more), SSCC without HIC accounts for the majority, so the correlation between the two is It is denied. In addition, the composition and heat treatment of steel have long made it more resistant.
Methods have been proposed to improve SSCC performance, but
Furthermore, these methods are not perfect solutions, and require the addition of expensive alloying elements and complicated heat treatment, which significantly increases manufacturing costs, making them impracticable in practice. Therefore, the present inventors conducted a detailed study on the mechanism of occurrence of the SSCC phenomenon, and as a result, found that the SSCC phenomenon is closely related to the segregation of steel and the morphology of inclusions, and found that the segregation and morphology of the steel were properly controlled. control to improve SSCC resistance. In other words, the areas where Mn, P, etc. are segregated have high hardness, and the hydrogen that has entered the steel concentrates in these areas with high hardness, causing inclusions, especially around NnS inclusions that tend to elongate in the rolling direction. Also, 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 tried to prevent the hardness of the steel structure from varying due to the segregation of Mn, P, etc.
If hydrogen is prevented from concentrating by reducing NnS-based inclusions or making them spheroidal, SSCC resistance can be significantly improved.
Based on this point, the present invention has been completed. 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, P, and C. Researched in detail. Based on this research, firstly, P should be reduced to 0.009% or less to eliminate segregated areas with high hardness, and secondly, Mn should be determined by the relational formula between P and C.
When the conditions of Mn≦3.5―250P―2.5C are satisfied, Mn,
The structure and hardness do not become uneven in the segregated areas of P, etc., and the structure and hardness can maintain uniformity, and this uniformity improves SSCC resistance. Third, hydrogen is concentrated. Elongates in the easy rolling direction
It was found that MnS-based inclusions can be converted into 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, and the amount of Mn is reduced by P and C.
By adding Ca, inclusions are dispersed and spheroidized, and are not affected by other alloying elements or heat treatment, while improving the mechanical properties of steel and welding. Improves SSCC resistance without adversely affecting performance. Hereinafter, the present invention will be specifically explained through the reasons for limiting the ingredients. C: If C is less than 0.25%, it is difficult to obtain the required strength for seamless oil country tubing, and if it exceeds 0.45%, it impairs the toughness of the steel and causes quench cracking, which is undesirable, so it should be in the range of 0.25 to 0.45%. limited to. Si: Si is an element necessary for deoxidation, but if it is less than 0.01%, there is no deoxidation effect, and if it exceeds 0.35%, there is a risk of deterioration of toughness, so it was limited to a range of 0.010 to 0.35%. Mn: If Mn is less than 0.4%, the required strength cannot be ensured, but Mn, together with P (described later), increases the hardness of its segregated parts, and furthermore, combines with S to form MnS inclusions. Therefore, from the viewpoint of SSCC resistance, it is preferable to have less content, and if it exceeds 2.0%, toughness will be impaired. For this reason, Mn is restricted in the range of 0.5 to 2.0% in relation to P and C as described below, and S, which contributes to the formation of MnS, is also restricted to a low sulfur region as described later. P: When P exceeds 0.009%, segregated areas with high hardness appear and the steel structure becomes non-uniform, resulting in poor SSCC resistance.
sex deteriorates. Although it is preferable to not include P as much as possible, it is almost impossible to not include P at all, and some traces will inevitably remain, so this is set as the lower limit. S: Together with the addition of Ca described later, S has an effect on the dispersion and spheroidization of inclusions. However, if it exceeds 0.010%, MnS inclusions will not be dispersed even if Ca is added.
It becomes difficult to form spheroids, and SSCC resistance cannot be improved. It should be noted 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: Al is necessary for deoxidation, and is important as an element for improving the Ca yield in the present invention. But 0.01
If it is less than 0.1%, it has no effect, and if it exceeds 0.1%, the crystal grains will become coarser and the material will deteriorate. Ca: Ca is one of the important components along with the above-mentioned Mn, P, S, Al, etc., and the addition of Ca causes the MnS inclusions to be dispersed and spheroidized. This effect requires at least 0.0010%, and since it is technically difficult to add more than 0.010%, the upper limit was set at 0.010%. B: B improves hardenability and improves strength. However, if it is less than 0.0005%, it is ineffective, and if it exceeds 0.005%, the toughness is impaired, so it was set at 0.0005 to 0.005%. Note that the remainder consists of Fe and unavoidable impurities. As described above, in addition to containing each component, among these components, Mn must satisfy the following conditional expression in relation to the amounts of P and C. Mn≦3.5-250P-2.5C The reason for this is that if Mn is outside the range of this conditional expression, the structure and hardness will be non-uniform, resulting in insufficient durability.
This is because SSCC properties cannot be obtained. Next, examples will be described. First, inclusions are dispersed by uniformizing the structure and hardness by regulating P, Mn, and C, and by adding Ca.
In order to clarify the improvement in SSCC resistance due to spheroidization, based on the composition of the comparative steel (sample steel code 1), P was lowered to 0.002 to 0.008% in this base steel, and Ca was added. As shown in Table 1, the steels of the present invention (sample steel symbols 2, 3, 4, 7,
8) was adjusted. Also, for comparison, comparative steel 1
Although P and S are high, the Mn level is made the same as that of the steel of the present invention, Comparative Steel 5 simply has lower P, and Comparative Steel 6 has only Ca added. Next, each of these test steels 1 to 8 was heat treated to determine the mechanical properties, and the results were as shown in Table 2.The quenching conditions were 950℃30
The conditions were water quenching. In addition, API standard C-75, C-
90, three types of heat treatment were performed to obtain a strength equivalent to C-110, and these are shown in the standard section in Table 2. Each of the test steels that underwent these heat treatments was subjected to, for example, C-75 heat treatment. Those subjected to heat treatment are shown as Saffix A, those subjected to heat treatment of C-90 are shown as Safix B, and those subjected to heat treatment of C-110 are indicated as Safix C.

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[Table] Therefore, the following SSCC resistance test was conducted on each of the sample steels that had been heat treated as described above, and the results shown in Table 3 were obtained. That is, the SSCC resistance test used a rectangular test piece with the dimensions shown in Figure 1 (t = 5 mm, W = 15 mm, l = 105 mm), and this test piece l was 4 mm as shown in Figure 2.
It was supported at four points by a book glass rod 2 and measured by a constant strain four-point bending method. In addition, in Figure 2
W ₁ = 100 mm, W ₂ = 40 mm. In addition, the test solution was hydrogen sulfide saturated 0.5% acetic acid + 5%
Using a saline solution, a stressed sample is placed in this solution.
After 720 hours of immersion, the presence or absence of cracks was determined. Note that the added stress is based on the 0.5% proof stress of each sample.
A stress of 0.2 to 1.2 times is added, and the term of added stress in Table 3 is expressed as n0.5σv, where n is 0.5% proof stress (Kg/
mm ² ), 〇 indicates no cracks, and × indicates cracks. The test results shown in Table 3 are based on API standard C-75, C
-90 and C-110 are shown together. As is clear from Table 3, the steel of the present invention exhibits excellent SSCC resistance. In contrast, simply
Comparative Steel 5, which was subjected only to low P treatment, and Comparative Steel 6, which was treated only to Ca, show superior SSCC resistance compared to other comparative steels, but are inferior to the steel of the present invention. Recognize. In addition, the steel of the present invention exhibits excellent SSCC resistance regardless of the Mn level, but among these, the steel with a lower Mn level has better resistance.
Shows SSCC property. In short, it can be seen that the steel of the present invention does not lose its effectiveness even with different heat treatments and compositional systems, has improved SSCC resistance, and is extremely effective. As explained in detail above, although the steel of the present invention has a high C content and has the necessary strength as a seamless steel pipe, the amount of Mn, P, and C is regulated in conjunction with the reduction in P, and these This eliminates the nonuniform structure and hardness caused by element segregation, and at the same time achieves dispersion and spheroidization of sulfide-based inclusions by reducing the S content and adding Ca. Therefore, it has extremely excellent SSCC resistance and has the strength necessary for seamless oil country tubular goods, and this effect is not impaired even by the addition of other alloying elements or heat treatment.

[Brief explanation of the drawing]

Figure 1 is a perspective view of the test piece used for the SSCC resistance test, and Figure 2 is the specimen used for the SSCC resistance test.
It is an explanatory view of a jig for a point bending test. Code 1...test piece, 2...glass rod.

Claims (1)

[Claims] 1 C: 0.25-0.45%, Si: 0.010-0.35%,
Mn: 0.4-2.0%, P: 0.009% or less, S: 0.010%
Below, Al: 0.01~0.10%, Ca: 0.0010~0.010%,
B: Sulfide stress corrosion cracking resistant, characterized by containing 0.0005 to 0.005%, with the remainder consisting of Fe and unavoidable impurities, and satisfying the conditional expression Mn≦3.5-250P-2.5C regarding Mn, P, and C. Seamless pipe steel material for high-strength oil country tubular goods with excellent durability.