JPS5996216A - Manufacture of high strength steel with superior sulfide cracking resistance - Google Patents

Abstract

PURPOSE:To improve the sulfide cracking resistance and yield strength of a steel contg. prescribed percentages of C, Si, Mn, P, S, Cr, Mo, Ti, Al and B by heat treating the steel under prescribed conditions. CONSTITUTION:A steel consisting of, by weight, 0.15-0.45% C, 0.05-0.8% Si, 0.3-1.7% Mn, <=0.015% P, <=0.01% Si, 0.2-1.5% Cr, 0.05-0.8% Mo, 0.005- 0.05% Ti, 0.01-0.1% Al, 0.0005-0.003% B and the balance Fe and satisfying Ti(%)<3.5XN(%) and 2.5XB(%)-1.5X10<-3N(%)<3XB(%)+1.2X10<-2> is refined. The steel is rapidly heated to (the Ac3 point+30 deg.C) or above in a short time and quenched. The heat treatment is repeated once or more to make the austenite grains fine up to ASTM grain size No.10 or above. The steel is then tempered at the Ac1 point or below.

Description

Detailed Description of the Invention This invention has a high yield strength of 701 gf/rrui or more and exhibits excellent sulfide cracking resistance in a humid hydrogen sulfide environment, and is particularly suitable for use in oil and gas wells. mechanical structural parts, such as oil country tubular goods and line pipes,
Furthermore, the present invention relates to a method for producing oil well steel suitable for use as a member for equipment used around oil and gas wells.

In recent years, new oil and gas fields have been actively developed in order to cope with the deteriorating energy situation, and oil and gas are now being developed in deep layers, where they were previously left unattended. Development efforts are now turning to oil and gas fields that are contaminated with sour gas (H2S). Therefore, in recent years in the field of oil and natural gas production (l), it has become possible to withstand the pressure of crude oil or gas in deep wells, or the tensile load due to the weight of steel, and also to maintain the desired performance even when used in sour environments. There is an increasing demand for steel products that exhibit high strength and have strong resistance to sulfide cracking.

By the way, various studies have been conducted since around 1950 on ways to improve the sulfide cracking resistance of 4111, and currently, for example, N A CE Standard M
R-01-'75 (1977Revision)
[It is recognized that suppressing the strength of steel below the upper limit of strength is the most effective way to prevent granite cracking. s O
J (lower limit of yield strength is 800.00 psi (!5
'6.2 kgf/mt7)] has been added to the API standard to meet user requests.

However, as mentioned above, the world's energy situation is accelerating the development of acidic deep wells, and if low-strength oil country tubing is used for deep wells, the required wall thickness will inevitably increase, making it difficult to improve economic efficiency or Because it causes a significant disadvantage in terms of workability,
There is a growing demand for steel materials that are even stronger than "L-80" and have excellent sulfide cracking resistance.9 Especially recently, the lower limit of yield strength has been increased to 90,000 psi (63,
Demand for high-strength oil country tubular goods exceeding 3 kgf/m+D is also increasing.

Therefore, r L −80J, J of the API standard:
, There has been no research on the steel for casing pipes of Rimo f Moe Jν.At first, attention was focused on martensitic chains, which relatively easily achieve high strength, but when used as casings, it is relatively difficult to use martensitic chains. They are often made of thick-walled materials and a seamless pipe manufacturing method is used to manufacture them, making it extremely difficult to harden the pipe from the inside, making it difficult to obtain a uniform martensitic structure over the entire wall thickness of the product. Unfortunately, it was inevitable that a bainite structure would occur on the inner surface. This bainite structure does not bring good results to the corrosion resistance of steel, so one problem that remains is that good corrosion resistance cannot be ensured after all.

Therefore, we added B, an element that improves hardenability, and in order to make effective use of B, we added T1 to the steel.
Attempts have also been made to obtain a combination of spring strength and corrosion resistance by adding 1L, but it has still not been possible to achieve sufficiently satisfactory corrosion resistance in such a steel with high tensile strength. In other words, in order to confirm corrosion resistance,
The shell-type test method shown in Figures 2 and 2 has been developed as a result of repeated research by comparing the actual results at many sites, and the V゛υ deviation limit value (33c value) measured by this test is , expression, i'i! It is said that if 44 is added, cracks will not occur, but there was no high-strength steel that met such a strict standard.

In addition, the shell type test method is as shown in Fig. 1, in which a test piece L is provided with a drilled hole in the center of its length, and as shown in Fig. The apparent stress at which the cracking rate is 50% is measured by adding stress and dipping into the corrosive liquid, and this is calculated as the sulfide cracking limit stress value (hereinafter referred to as
Sc value), and in Figure 2, 2
is a glass round rod with a diameter of 4 mm, and 3 is a bolt for applying load (stress).

In addition to improving these materials themselves, methods such as coating the steel material and injecting inhibitors into the corrosive environment have also been taken, but none of these methods have been able to produce satisfactory results.

From the above-mentioned viewpoint, the present inventors have determined that the Sc value is determined by the formula, Sc (S MY S / 0.75) X 1
0-' and the yield strength? OkgfAn
- We conducted research to obtain a steel material with the following high strength and satisfactory performance as oil country tubular goods used in sour environments. In order to satisfy this value and achieve high strength of 70 kgf/mm or more, ■ the 1rliiii material structure must be a martensite structure, ■ the austenite grain size must be A S T M
The grain must have a hardness of A of 10 or more; ■ Coarse carbide containing B added to improve hardening and properties must not be precipitated at grain boundaries.

It was discovered that the following three conditions are essential, and in order to obtain a thick steel material that satisfies these three conditions, (a) j
The liω material composition is low in P and has good hardenability with the addition of B and T1; (b) Usually, B
In addition steel, the content of N is reduced to T to maximize hardenability.
In this case, the J3'5 insertion property is certainly very good, but the crystal grains are no longer fine. Therefore, in order to make the austenite crystal grains fine, it is necessary to leave N that is not fixed at T1, and the amount of Ti added is determined by the formula, Tj, (%) < 3.5XN (%). (C) In order to make Austenai I. finer,
It is necessary to heat rapidly in quenching 11-1, and in order to obtain the desired crystal grains for extremely thick-walled products, it is necessary to repeat the upper curving process twice, (d)
In steel, the formula N(%l>2.5X B(%l-1,5X 10-
If there is N that satisfies the following, coarse borocarbide will not precipitate and the corrosion resistance of the material will not deteriorate. (e) The amount of N in the steel is expressed by the formula, If ×8 (%) + 1.2X10 is satisfied, the hardenability effect of B will be exhibited and corrosion resistance will increase.

The findings shown in (a) to (el) have been obtained above.

This invention was made based on the findings, C: ○15-0.45%, Si: 0.05-080%.
%, Mn: 0.30-1.70%, P: 0.015
% Rabbit, bottom, S: 0.01.0% or less, Cr: 0.2
0-1.50%, Mo', 0. 0 5
~080%, Tj: 0.0 0 5
~0.050%, Afl: 0.01~0.10
%, B:O,0O05~0. Contains 30% OO, or further contains Nb: 001-0.10%,
V: 0.01~O1O%, Cu: 0.10~05
It also contains one or more of the coefficients 0 and 25X B(
%)-1,5X 10- (N(% to 3XB(%)+1
．． 2X10-”.

Steel consisting of Fe and unavoidable impurities (remaining) is subjected to a process of rapid short-term heating to a temperature range of [AC 3 points + 30°C] or higher and quenching at least once to reduce the austenite grain size to A'S T. M
It is characterized in that it produces 6+hi having a value of /ma or less.

Next, in this invention, the amount of compositional components of steel.

The reason why the heating temperature during quenching and the degree of austenite grain size are limited as described above will be explained.

A1 Composition component a) C The C component is an element that is effective in increasing the hardenability, increasing the strength, and refining the grains of steel. However, if its content is less than 0.15%, the strength decreases and the hardening process deteriorates. Therefore, it is necessary to temper at a low temperature to achieve the desired strength, and it is difficult to obtain a fine grain structure of ASTIV4A10 or higher even with repeated rapid heating quenching treatments, resulting in increased susceptibility to sulfide cracking. On the other hand, if the content exceeds 0.45%,
In addition to increasing the susceptibility to quench cracking during quenching and making it difficult to form a uniform solid solution in austenite during rapid heating, B's hardenability increasing effect is extremely small. It was set as 0.15 to 0.4.5.

b) Si $1 is an effective element as a deoxidizing component of jJlYj and also has the effect of increasing the strength and hardenability, but if its content is less than 0.005%, the desired effect cannot be obtained in the above effect. On the other hand, if the content exceeds 0.80,
Since sulfide increases susceptibility to cracking and significantly deteriorates toughness, its content is set at 0.05 to 0.80%.

C) Mn The Mn component has the effect of increasing hardenability and improving strength and toughness, but if its content is less than 0.30%, the desired effect cannot be obtained; If the content exceeds 0.30 to 1.70, segregation bands will be formed resulting in deterioration of toughness, increased susceptibility to sulfide cracking, and increased susceptibility to quench cracking.
%.

d) P The P content is an impurity element that increases the sulfide cracking susceptibility of steel, or if its content exceeds 0.015%, the desired Sc
Since the values cannot be reached, the content is set at 15% or less of O and O'.

e) S In order to improve toughness and sulfide cracking resistance, it is desirable to reduce the impurity element 8 as much as possible, but considering the balance with the manufacturing cost of steel, The content was set at 0.010% or less.

f) Cr The Cr component has the effect of improving corrosion resistance as well as increasing hardenability and temper softening resistance, but if its content is less than 0.20%, the desired effect is not achieved.
4) On the other hand, if the content exceeds 1.50%, it will be difficult to form a uniform solid solution in austenite during rapid heating, as well as deterioration of toughness and increased susceptibility to quench cracking. Since it also causes the content, the content is 0.20 =
It was set at 1.50%.

g) M.

The Mo component has the effect of increasing the hardenability and strength of steel, increasing the resistance to hardening, and improving toughness, but if its content is less than 0.05%, the above-mentioned The desired effect cannot be obtained in the action, while 08
Even if the content exceeds 0%, not only will no further improvement effect be obtained, but also it will be difficult to achieve a uniform solid solution in austenite during rapid heating, so the content should be reduced to 0%.
．． The fissure was determined to be 05-08°.

h) Ti The Ti component has the effect of fixing N in the jll as TiN and fully exhibiting the hardenability effect of the coexisting B, but if its content is less than 0.005%, the desired effect may not be achieved. On the other hand, if the content exceeds ○050, the toughness of the steel deteriorates, so the content was set as O,OO5~0050.

1) AQ A66 is added to stabilize the deoxidation of steel, but if its content is less than 0.01%, its effect will be small; on the other hand, if it is contained in excess of 0.10%, However, the deoxidizing effect does not show any further deoxidation effect, and furthermore, it may cause cracks due to inclusions and deterioration of toughness.
The content was determined to be between 0.01 and 0.10.

J) B ■3 composition has the effect of improving hardenability with a small amount of 7d' addition, and also improving strength, toughness, and sulfide cracking resistance, but if the content is O , OOO less than 5%, the desired effect cannot be obtained; on the other hand, O, .
Even if the content exceeds 0.030%, no further improvement in the effect can be expected, and on the contrary, the toughness may deteriorate. Therefore, the content was set at 5% to 30%.

k) Nb and V Nb and component (2) both have the effect of refining austenite grains and increasing temper softening resistance.
In particular, it is added as necessary to improve these properties, but the content 'is 0.0.
If the content is less than 1%, the desired effect cannot be obtained, while if the content exceeds 0.10%, toughness will deteriorate.
It was set at 10%.

l) The C1 Cu component has the effect of suppressing hydrogen absorption of steel and improving sulfide cracking resistance, so it is included as necessary when it is desired to further improve sulfide cracking resistance. but,
If the content is less than 0.10%, the desired effect cannot be obtained, while if the content exceeds 0.10%, hot workability will deteriorate. It was set at ~0.50%.

In addition, as explained earlier, when the Ti content (%) is 3 or 5
If it is more than X N (%), the amount of free N that is not fixed by T1 will decrease, making it impossible to obtain the desired fine-grained structure, and the N content (%) will still be lower in B-treated steel with T1 added. If it is less than 2.5 x B (%) - 1.5 x 10, coarse borocarbide will precipitate and ASTM7
ii Even if a fine-grained steel with a grain size of 10 or more is obtained, the corrosion resistance deteriorates and fJ: (j cannot be obtained), and the N content (the hook is 3 x B (%) +12×
If it is more than 2, the hardening tendency of B cannot be obtained and the corrosion resistance will be reduced. Therefore, Ti (%) < 3.5
,5X B (%l −1,5X I O<
A restriction was set: N(%)<3XB(%)+1.2×YO2.

B. Heating temperature: When the heating temperature is (Ac3 points + 30°C) or less than /i14, it is difficult to uniformly and completely austenite the steel structure using the rapid short-time heating method as in the method of this invention. Increase the heating temperature by CAc3 points +3
0℃] or above.

As a method for rapid heating, an induction heating method, a direct current heating method, or the like can be employed, but it is common to use a high frequency induction heating method. In this case, high-speed heating using a c:6 frequency is possible for U-thick products, but for thick-walled products, the heating rate becomes lower and the temperature gradient becomes larger, so it is necessary to lower the frequency. l+A 4th degree is inevitably lower. In any case, a heating rate of 1 to 12j5℃/sec can usually be applied, but in the case of a relatively slow heating rate of 1 to b, a heating rate of 2 degrees or less may be applied, although it varies depending on the j'l:rl component. By repeating the above-described quenching process, it is possible to stably and uniformly obtain fine grains of J11zlO or less, which is desirable.

C. Austenite grain size If the austenite grain size becomes coarse grains less than ASTM AIO, the sulfide cracking resistance becomes extremely poor, so the grain size was determined to be ASTM j6.10 or more.

As described above, the method of the present invention is to melt steel composed of predetermined components and then process it into steel bars, bars, buttons, etc. by the usual method.
1, etc., and heat it to a temperature of CAc 3 points + 30°C or higher using a rapid short-time heating method such as induction heating or direct current heating to completely change the structure to austenite. The quenching treatment with a cooling medium is repeated at least once to uniformly dissolve the alloying elements in the austenite and reduce the austenite grain size to AST.
Sulfide cracking resistance is imparted by creating an ultra-fine grain structure with an M diameter of 1o or more, and then sufficiently tempering the martensite generated by quenching. That is, Ac1
By tempering the steel to a temperature below 11.0, a steel with a strength of 701 sof/m77 or higher and excellent resistance to sulfide cracking, which is suitable for each use, is manufactured. ``The back crystal tempering process relieves the internal stress of martensite generated by quenching and spheroidizes cementite I, thereby giving the steel the desired performance.

Next, the present invention will be explained with reference to examples and in comparison with comparative examples.

EXAMPLE First, steels A to P having the compositions shown in Table 1 were melted by a conventional method. Steel A to King have compositions within the scope of this invention, and Steel J-T has the composition within the scope of this invention.
The points marked with * in the table have compositions that are outside the scope of the present invention.

Next, these were quenched under the conditions shown in Table 2,
Ac, reconstitution was carried out at a temperature of -25°C.

For each steel treated in this way, the austenite grain size number (ASTMA), strength, and corrosion resistance were measured,
The results are also shown in Table 2.

Regarding corrosion resistance, a test piece as shown in Fig. 1 was cut out from each piece, and the test piece was supported with a test piece support as shown in Fig. 2, and a corrosive liquid composition: 05% CH3C
S in a test under the conditions of OOCH3C00H-4-2200-3200pp, temperature: 20℃, oil immersion time: 20 days.
The c value was measured and expressed as that value. And in Table 2 *
The mark indicates that the conditions deviate from the conditions of the present invention.

From the results shown in Table 2, the test Mf No. 1-9, whose Qrtj component composition and heat treatment conditions are within the scope of the present invention, has ultrafine austenite crystal grains of 0 or more. However, the composition of the component J↓, the heating temperature when inserting the tube, the chair of the heating means, and the l'1. Test numbers 10 to 26, which are outside the scope of the present invention, are
Even if there is not that much difference in strength, it is clear that the sulfide cracking resistance is significantly inferior.

As mentioned above, according to the present invention, the yield strength is '70
It is relatively easy to produce a preparation that has an extremely high strength of 4 kgf/mr or more and is resistant to abrasion and sulfide cracking, and can be used in oil fields under sour environments or It may/will have industrially useful effects, such as solving major problems in gas field development and making it possible to make a significant contribution to energy resource development.

[Brief explanation of drawings]

Figure 1 shows an example of a shell type corrosion resistance test piece.
Figure 1(a) is its front view, Figure 1(b) is its side iJ.
11, and FIG. 2 is a schematic configuration diagram showing a state in which a test piece is supported on a test piece support. In the drawings: ] Test piece, 2. Glass round bar, 3. Stressed bolt. Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] K) C: O, 15-045%. Si: 0.05-080%. Mu: 0.30 to 1.70%. F): 0.015 inch or less. S: 0.0IO% or less. Cr: 0.20-1.50%. Mo: 005-0.80%. T1.0005-0.050%. AC3001-010%. B: O,0O05~O,OO30chi. and the formula 'I'i (%) (3,5
xB(%)-1-1, 2x1o-2 is satisfied. Fe and inevitable impurities: Remaining. The austenite grain size is changed by applying one or more times of rapid short-term heating to a temperature range of [Ac 3 points + 50°C] or higher, followed by quenching, to a steel (with a weight greater than or equal to the above).
Miniaturization to M A 10 or more, then AC. A method for producing high-strength steel with excellent sulfide cracking resistance, which is characterized by tempering to a temperature below 100 ml. (2) C: 0.15-045%. Si: 0.05-0.80%. Mn, '0.30-1.70%. P: 0.015% or less. S: 0.010% or less. Cr: 0.20-1.50%. Mo: 0.05-0.80%. Ti: 0.005-0.050%. AQ: 0.01-010%. 8.00005~O, OO30%. and Nl):O,Ol ~010%. V: 001-0.10%. Cu: 0.10-050%. It also contains one or more of the following, and further has the formula, T, H, (
%) (3,5X 'N (%). And 2.5XB (%) - 1,5X 10 <N (%)
(Add 614 of 3XB (%) j-1゜2X10. Fe and unavoidable impurities: the rest.) To the steel (weight ratio), [AC 3 points + 30℃]
The austenite grain size is changed to AS'T by performing rapid heating to a temperature range of 2 or more times and then quenching.
M/) '10 or more, and then Ac. A method for producing high-strength steel with excellent sulfide cracking resistance, which is characterized by tempering to a temperature below the dot plate.