JPS6046318A - Preparation of steel excellent in sulfide cracking resistance - Google Patents

Abstract

PURPOSE:To obtain high strength steel excellent in sulfide cracking resistance, by applying two-stage hot processing to steel, wherein the amount of C, Nb and Mn and the amount of P and S in impurities are specified and Cu, V and one of Ca, REM or the like are added, under specific condition before applying direct hardening. CONSTITUTION:The composition of steel consists of, on a wt% basis, 0.15-0.45 C, 0.08 or less Si, 0.01-0.30 Mn, 0.20-1.50 Cr, 0.05-0.80 Mo, 0.01-0.10 Nb, 0.01-0.10 Al, according to necessity, 0.0005-0.0050 B, 0.05-0.50 Cu, 0.01-0.10 V, and one or more of 0.001-0.030 Ca and 0.001-0.050 REM and the remainder of Fe and inevitable impurities. In this case, P in said impurities is set to 0.015 or less and S therein to 0.010 or less. This steel is subjected to primary hot processing and, thereafter, held or re-heated to Ac3-Ac3+200 deg.C to be subjected to secondary hot processing. Succeedingly, the processed steel is directly hardened from an austenite state and subsequently heated in the above mentioned temp. range to be subjected to hardening treatment one or more times. Thereafter, annealing treatment is applied at a temp. of Ac3 transformation point or less.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a steel with excellent sulfide cracking resistance, particularly a steel having high yield strength of 65 kgf/- or more and excellent resistance in a wet sulfide water environment. It exhibits excellent sulfide cracking properties and is suitable for use as structural parts used in oil and gas wells, such as oil country tubular goods and line pipes, as well as structural members used around oil and gas wells. This invention relates to a method of manufacturing oil well steel at low cost.

A notable feature of the development of new oil and gas fields in recent years is that they are being developed in deep layers, where oil and gas are contaminated with hydrogen sulfide (lFI2s) and are in so-called sour environments, which were previously abandoned. I can point out that the focus of development has now turned to this area.

Therefore, in recent years, in the field of oil and natural gas production, it has become necessary to withstand tensile loads due to the pressure of the earth's formations, gas pressure, or the own weight of steel materials, and also to withstand the effects of glaciation even when used in sour environments. J) There is a growing demand for steel that has high strength and strong resistance to sulfide cracking (hereinafter referred to as 5SCC) (iif+).

1950 for steel drama 5scc sexual improvement means
Various studies have been carried out over the years, and currently, for example, NACE 5 standard MR-01-75 (
It is said that keeping the strength of steel below the upper limit of hardness (strength) shown in the 1977 Revision) is the most effective way to prevent 5SCC, and based on this, <L-80 [
The lower limit of yield strength is 80,000psi (56,2kgf
/mu)) has been added to the API standard to meet the demands of consumers.

However, in acidic deep wells such as those mentioned above, if oil country tubular goods with reduced strength are used, the required wall thickness will inevitably be increased, resulting in significant disadvantages in terms of economy and workability. The problem was that they started asking for money. For this reason, there has been a strong demand for steel materials that are even stronger than L-80 and have excellent 5scc resistance.In particular, recently, the lower limit of yield strength has been reduced to 9.
(The demand for high-strength oil country tubular goods exceeding JOU (Jpsi (63.3 kgf/mj) is also increasing.

Conventionally, in order to meet such demands for high strength and high 5SCC resistance, Al5I4 having a uniform tempered martensite group*k formed by quenching and tempering treatment was used.
Attempts have been made to use all 130 series steel or to add CO to prevent hydrogen absorption in the steel, but even so, Al5I 4130 thread steel still fails to achieve fully satisfactory corrosion resistance. I can't, ○
In Co-added steel, in order to fully utilize the hydrogen absorption prevention effect of CO, it is necessary to avoid combined addition with Mo.
It is necessary to strengthen the steel by adding large amounts of reinforcing elements such as r or V, which leads to deterioration of toughness.
It is possible to avoid the problem that the cost of preparation increases significantly because it is necessary to add elements in an amount far exceeding 1% of the total (hereinafter, % representing the component ratio is expressed as weight %). One.

By the way, as a method to fully confirm the 5SCC resistance, H2S
Jl8 consisting of a 0.5% CH3CO0H solution saturated with
゜The shell type test method, which is carried out in a food solution, was developed through extensive research and comparison with numerous field experiences.
By this test i! The S S CC critical stress value (Sc value) to be ++jw is given by the formula, Sc > (SMYSlo, 75) x I O-'
1. If all satisfaction is met, no cracks will occur. Teike Jl
However, none of the conventional low-cost, low-alloy, high-strength steels satisfies all of these strict standards.

In addition, the shell type test method is negative 'S 1 riko/I
A test specimen 1 with a drilled hole in the center of its length, as shown in FIG. ~, cracking rate is 50%
The stress of the crab [ke? ! -11111 is added to make the total Sc value. In Figure 2, Dish I No. 2
What is indicated by is a glass round rod with a diameter of 4 sections, and what is indicated by code 3 is a bolt for applying load (stress).

In addition, attempts have been made to improve the materials themselves, such as coating the entire steel material or injecting inhibitors into the corrosive environment, but none of these methods have been expected to be sufficiently effective.

From the above-mentioned viewpoints, the present inventors have determined that the SC value satisfies the formula, Sc> (SMYSlo, 75) As a result of conducting research to obtain a relatively low-cost steel material that has sufficiently satisfactory performance as oil country tubular goods used in It is. That is, (a) K that satisfies the Sc value of the predetermined value or more and achieves high strength of yield strength: 65 kgf/vuj or more.
(b) It is effective to make the steel structure into an ultra-fine tempered low-temperature transformation structure, that is, an ultra-fine tempered martensite structure, or a mixed structure of ultra-fine tempered martensite and ultra-fine tempered low-bainite; In order to obtain the ultra-fine tempered low-temperature transformed structure, the low-temperature transformed structure before tempering must be extremely fine. structure, or a mixed structure of Qmartensite and low-temperature bainite,
It is also martensite and low-temperature bainite lath.
(h) This cannot be achieved without a fine-grained structure with little collapse; (c) In order to refine the crystal grains of fA, it is recommended to perform quenching one or more times using rapid heating means such as induction heating. Although it is known to be effective, for copper with an 11-foot component composition, it is gradually inscribed in hot working, and then the temperature range is at least 3 AC and below the austenite grain coarsening starting temperature. It is difficult to repeat the heating and quenching process one or more times, even if the heating rate during the quenching process is set at a slow rate of 1' cz44 or less, as in electric furnace heating. (d) As mentioned above, direct quenching and one or more quenchings at a slow heating rate (n and r) are used to refine the grain of the steel. In order to achieve this, the steel must contain all the components, especially C component of 0.15% or more and Nb component of 0.01% or more at the same time. The processing is divided into two stages: first, the entire first hot working is carried out in the austenite region, and then the process is immediately carried out in the fine-grained austenite region (temperature range in which austenite does not coarsen: that is, from the Ac3 transformation point) without starting transformation. [Ac
It is effective to perform a second hot rolling by holding or reheating at a temperature range of 3 transformation point + 200°C) (7) temperature range, and directly quenching from the austenitic state after this second hot rolling. , (e) Furthermore, the content of P and S, which are unavoidable impurities in steel, is suppressed to below the specified value in the wire, and (si + I
VIn) amount, especially the formation amount, is also controlled below a certain value r$:
(f) When one or more of K, K, Cu and V are contained in the steel, the 5 SCC resistance and strength of the steel are further improved, and Ca and When 1 s or more of rare earth elements is added, the inclusions in the steel become spheroidized and the copper is cleaned, resulting in a UX-i with 5scc resistance, and when a small amount of B is added, the steel becomes The strength, 5SCC resistance, and sheath attachment are further improved.

This invention was made based on the above knowledge, and includes: C: 0.15 to 0.45% + Si: 0.80%
below.

to4r1: o, ox% or more and less than 0.30%.

Cr: 0.20-1.50%, h'Lo: 0
．． 05-0.80%.

Nil: 0.01 to 0.10%, lt:
0.01 to 0.10%.

Contains all of the ingredients, and if necessary, 1st category... B: 0.0005 to 0.0050%.

Second category... Cu: 0.05-0.50%.

V: 0.01-0.10%.

Third category... Ca: 0.001-0.030% REM: 0.001-0.050%.

Fe and unavoidable impurities: formed from the rest, and P and 8 of the impurities are respectively P: 0.015% or less, S: 0.010%
below.

After the first hot working of the steel in the austenitic state, the steel is immediately maintained or reheated in the temperature range of AC3 transformation point to [Aca transformation point + 200°C] in the austenitic state without completely starting the transformation. All secondary hot working is performed, followed by direct quenching from the austenitic state, and then this kAcsAC3 transformation point ~ 3 transformation point + 200℃
] By repeating the process of heating to the temperature range and then quenching one or more times, and then performing the tempering process at a temperature below the Act transformation point, it has excellent 5SCC resistance and a weight of 65 kg.
This method is characterized in that a steel having a yield strength of f/mJM or more is obtained.

In addition, in the method of this invention, when reheating to the fine-grained austenite region for performing the second hot working, the first
One of the major features is that heating energy can be saved by effectively utilizing the heat held by the steel material after the next hot working. JjlJ, after the first hot working, transformation should not be started, therefore, the austenitic state should be immediately turned off.
Since the steel is heated to or maintained in the grain austenite region, all of the large thermal energy possessed by the steel material can be used effectively, resulting in savings in heating energy.

Conventionally, when taking such a two-step hot working process, after the first hot working, the material is once cooled to below the Arl transformation point to complete the transformation, and then reheated to above the Ac3 transformation point. It was customary to carry out hot working, but this aimed to make the grain finer by passing through transformation, and it required a large amount of thermal energy to heat from low temperature to AC 3 points or more, VC, pJ. there was.

In addition, conventionally, after this second hot working, the structure was cooled down to room temperature by air cooling, then hardened and tempered.In this case, the structure before quenching was ferrite or pearlite. Alternatively, it may be a high-temperature transformation product such as high 1711'1 bainite, and such a pre-structure cannot be refined even if a subsequent quenching treatment is performed.

However, in the present invention, even if it is directly reheated or retained in the fine-grained austenite region after the first hot stress is completed, the next second
In the next hot working, a sufficient fine-grain effect is produced by the working, and the low-temperature transformed structure that is directly quenched after this is not a fine-grained structure.The entire extremely fine-grained structure can be easily realized by the subsequent quenching treatment, so it is not necessarily the case that the first There is no need to terminate the transformation after hot working, which makes it possible to effectively utilize the thermal energy of the steel material.

Furthermore, in the method of the present invention, when the quenching treatment at a slow heating rate is repeated two or more times, the austenitizing heating temperature i1:, ('n -1 ) It is preferable that the heating temperature be lower than the austenitizing temperature during the second quenching, and by doing so, the structure of the steel will be finer and more uniform9, and the 5SCC resistance will be further improved. becomes.

In other words, this invention proposes that ``For steel containing a specific amount of C component and Nb component at the same time, the hot working step is carried out in two stages, first, the first hot working is performed, and then the transformation process is carried out. Either the λ-stenite state is maintained in the fine-grained austenite region without starting, or the second stage is heated again.
If the next hot heating is carried out, followed by direct quenching from the austenite state, the austenite crystal grains will begin to coarsen at AC3 points or higher even if heated at a slow heating rate such as in an electric furnace. After at least one quenching process after heating to a temperature below that temperature, the steel becomes extremely fine-grained. Therefore, this is all A
When tempered at a temperature below the cr point, a very fine tempered low-temperature transformation structure is created, and a steel material having both sufficient strength and excellent 5scc resistance is obtained. ” is a complete outline of the technical matters. Of course, an even finer grain structure can be obtained by fully employing rapid heating during quenching after direct quenching, but with the method of this invention, the desired fine grain structure can be achieved even without such rapid heating. can be achieved.

In addition, after direct quenching or 1Q quenching treatment, tempering treatment (hereinafter referred to as rough tempering) is always performed to prevent 1h' cracking, etc., before heating for the next quenching. When implementing the law, Al calculated using the formula At = T (A2 + log t) as the rough tempering condition is Al< 19. It is preferable to set it to satisfy OX 103, and such VC
By doing so, the collapse of martensite laths and low-temperature bainite laths due to rough tempering is suppressed to a small extent, and a finer-grained structure can be obtained in the mth quenching than in the (m-1)th time.

Next, in the method of the present invention, the reason why the chemical composition of copper and the rolling and heat treatment conditions are numerically limited as described above will be explained.

A. Chemical composition of steel C Component C is an essential element for increasing the hardenability and strength of steel, as well as for refining the grains, but its content is 0.15
If the heating rate is less than 20%, the strength will decrease and the hardenability will be reduced. Even if the quenching treatment is performed one or more times, grain refinement cannot be achieved, and the 5SCC quality is poor. -force 0.4
If C is contained in excess of 5%, cracking susceptibility or strength increase during quenching may occur, leading to deterioration of shoe resistance.
The C content was determined to be 0.15 to 0.45%.

■ 5I Si component is an immobile element as a deoxidizing agent for steel, but if its content J and t exceeds 80% of C1, it will cause a total deterioration of toughness and increase the 5scc sensitivity. Therefore, the Si content was determined to be trs, o8o% or less.

In addition, in order to further improve the 5scc resistance, p-ts
, or with the reduction of 1' blood (S+ +1!/In
) is preferably less than 0.16%.

■ The Mn excited component has the effect of promoting the grain boundary segregation of P and S and deteriorating the sscc resistance of high-strength materials, and this effect becomes noticeable at a formation content of 0.30% or more. There is a tendency.

In addition, in high-strength materials, it is effective to reduce the amount of P and S as much as possible and to reduce the P content to less than 0.16% (Si + Mn), but it is effective to prevent 5SCC. (2) Since it is extremely difficult to make the total stone content less than 0.01% in terms of steel production and increases costs, the total seed content is set at 0.01% or more and less than 0.30%.

■ Cr The Cr component has the effect of increasing the hardenability, strength, and resistance to temper softening of steel, and is extremely effective for increasing the strength. It also has the effect of improving 5SCC resistance, but its content If the amount is less than 20%, the desired effect cannot be obtained, while if it is more than 1.50%, the toughness will deteriorate and the susceptibility to quench cracking will increase.
The Cr content was determined to be 0.20 to 1.50%.

■　M.

The MOC component increases the hardenability, strength, and tempering softening resistance of steel, and is an effective component for improving toughness.It also suppresses grain boundary segregation of impurities during the tempering process and improves 5SCC resistance.
It has the effect of improving sex, but its content is 0.05%
If it is less than 7 hours, will the above effect have the desired effect? On the other hand, even if the Mo content exceeds 0.80%, the above effect will be saturated, and Mo is a very expensive element, so the Mo content should be reduced to 0.05 to 0.8%. It was set at 80%.

■ Nb Nb component is an essential element for increasing steel strength, increasing temper softening resistance, and improving 5scc resistance, as well as for grain refinement, but if its content is less than o, o i%. If the heating rate is particularly slow, the desired grain refinement cannot be achieved even if the quenching treatment is repeated two or more times;
The Nb content was set at 0.01 to 0.10% since the above effect would not be saturated and it would also cause deterioration of toughness. .

■ At The At component is added to stabilize the deoxidation of steel, homogenize it, and make the grains finer.
If the content is less than 0.0%, the desired effect cannot be obtained.
If the At content exceeds 10%, the deoxidizing effect will be saturated, and this will also lead to the occurrence of defects and deterioration of toughness due to the increase in inclusions, so the At content is set at 0.01 to 0.10%. Ta.

■B Component B improves hardenability when added in a small amount, and has the effect of improving strength, toughness, and 5scc resistance, so it can be added when it is necessary to further improve these properties. However, the content is 0.0005
If the content is less than 0.0050φ, the desired effect cannot be obtained, and if the content exceeds 0.0050φ, no further improvement effect will be observed, and on the contrary, it may cause deterioration of toughness. , B content is 0.0005-0.0050%
It was determined that

■Cu, and V These components include the strength of the sow and rjSSC, respectively.
Since it has the effect of improving carbon properties, one or more types of necessary C (one or more types are added and contained depending on the case), but if the Cu content ht is 0,
If the V content is less than 0.05% and the V content is less than 1.01%, the desired effect cannot be obtained;
If more than 0.50% of V is contained, α, machinability will deteriorate, and if more than 0.10% of V is contained, toughness will be deteriorated. k (Lo
5 to 0.50%, ■ Content amount (T” (1,01
0.10%, respectively.

■ Ca, and REM Ca, R, and EM (rare earth elements) all spheroidize inclusions in steel and clean the steel.
Type 1 J depending on the type of fire because it has the effect of reducing C sensitivity.
However, if the Ca content is less than 0.001%, the desired effect on the production Jtl cannot be achieved.
If it is contained in excess of 0.050%, not only will the effect of its addition be saturated, but it will also increase the number of non-metallic inclusions such as oxides, which will increase the quality of the steel. decreases, changing the 5scc sensitivity and increasing the C.
The a content of tfO was determined to be 001 to 0.030%, and the REM content was determined to be 0.001 to 0.050%.

■ P, and S Improving the toughness of steel Full diagram 91 Also note 5 To improve the SCC property, K is the impurity P and S content' (c-flJ) It is desirable to reduce it as much as possible, but it is Considering the balance, the upper limit of P content 'tf: 0-015 article, the upper limit of S content'k 0.010% and 1°B, respectively.
After rolling and heat treatment conditions, this invention was constructed as described above.
Thick plates, shaped steel, steel pipes, etc. are hot worked. At this time, the hot working process is carried out to completely refine the austenite grains, followed by the first hot working process, followed by the complete initiation of transformation. The process is divided into a second hot working process, which is carried out immediately after the grain is cooled and held in the austened/iron region or reheated.

After the second hot working, a fine low-temperature transformed structure can be obtained by directly quenching the austenitic state with an appropriate cooling medium. This also has the secondary effect of being effective in reducing the susceptibility to quench cracking i''L during direct quenching, but its primary meaning is Even if the speed is low, the point is to create a base that can achieve fine grain refinement of the steel by subsequently performing heating and quenching treatment on the fine grain austenite region one or more times.

In other words, if the pre-structure during quenching after direct quenching is a fine low-temperature transformed structure, if it is also a fine-grained structure with little lath collapse, the entire extremely fine structure can be removed by the next quenching. Therefore, it is an advantage of the method of the present invention to hold or reheat the fine-grained austenite region and perform the second hot working so as to obtain a fine low-temperature transformed structure by direct quenching. This is one of its major features.

Then, the lifting force of the secondary processing [1 is natural when the thermal temperature is less than the AC3 transformation point, so it becomes austenitic 75
<Unable to achieve, - When heated above [AC3 transformation point + 200°C], the austenite crystal grains become coarse, and the treatment of the present invention i/C cannot obtain the desired fine grain structure. become unable to do so.

Repeated quenching following direct quenching process ~ Directly quenched material or rough tempered material after direct quenching to ACs transformation point ~
After heating to a temperature of [Ac3 transformation point + 200°C] to completely austenite the structure, quenching with an appropriate cooling medium failed to achieve austenitization despite the low-temperature transformed structure; on the other hand, [Ac3 transformation point + 200°C] If the temperature exceeds 1-1, the austenite crystal grains will become coarse, making it impossible to obtain the desired fine structure even by the treatment of the present invention. Therefore, the heating temperature in the second quenching treatment following the direct quenching treatment was determined to be between the total Ac3 transformation point and [Ac3 transformation point +200°C].

As mentioned above, it is preferable that the heating temperature for the second and subsequent quenching be lower than that of the previous quenching, thereby achieving an even finer grain structure and improving the performance of the steel material. I can do it.

In addition, when all steel components containing B are considered, it is desirable to keep the austenitizing temperature below 1075°C in order to fully exhibit the hardenability improvement effect of the B component. . , and when rough tempering is performed prior to each quenching treatment, as mentioned earlier, T: rough tempering temperature (0K).

t: Retention time (hr) at Rough Ten 71 temperature.

A2=22-4 xC (%) ], OXNb (support))
.

It is preferable to set all the conditions so that the AI calculated by the formula At-T (A2 + log t) becomes a value of [19, OXl 03] or less.

When the thin low-temperature transformed U4 weave, which has been quenched by the above-mentioned quenching process, is then tempered at a temperature below the ACl transformation point, the desired strength and SCC property will be imparted to the copper. . Only by tempering at a temperature below the A Cs transformation point can it be possible to obtain a material with excellent yield strength of 65 kgf/- or more and 5 sec properties suitable for each application. It is.

Although it is not necessary to impose a special lower limit on the tempering temperature,
Considering that high-temperature tempering removes the internal stress of martensite and low-temperature bainite generated by quenching, and also improves the performance of the steel by turning cementite into spheroidized particles, tempering at a temperature of 650°C or higher is recommended if possible. It is desirable to do so.

In this case, if the tempering temperature exceeds the ACl transformation point, the strength of the steel material changes significantly and the 5scc resistance deteriorates, so the temperature was determined to be below the 'kACt transformation point.

Next, all embodiments of this invention will be explained in detail while being compared with comparative examples.

Example 1 First, steels (1) to (2) having the compositions shown in Table 1 were melted by a conventional method.

Next, all of these steel slabs were soaked at 1200° C. and then hot rolled and heat treated under the conditions shown in Table 2.

The yield strength (yield strength), tensile strength, and resistance to 5scce were measured for the rolled steel plate. The method of the present invention involves heating to 1200°C and performing the first hot rolling in the austenite region, followed by a transformation process. Therefore, the austenitic state is immediately held in the fine-grained austenite region or - reheated to complete the second hot rolling to finish the final product dimensions, and then directly quenched from the austenitic state, and then further quenched.
All tempering processing is performed.

Regarding the sscc resistance, a test piece 1 as shown in Fig. 1 was cut out from each end, and was heated at a liquid temperature of 20°C while applying full stress using a jig as shown in Fig. 2. ) (The Sc value was measured by immersing it in a 2S fully saturated 0.5% CH3COOH solution for 20 days, and the value is shown.

In FIG. 2, the reference numeral 2 indicates a glass round rod, and the reference numeral 3 indicates a stress-applying bolt.

From the results shown in Table 2, it is clear that according to the method of the present invention, it is possible to reliably obtain a steel material with excellent 5 sccu despite high strength.

Example 2 The steel subject to the present invention in Table 1 was soaked at a temperature of 1220°C, then subjected to 7'51 hot rolling in the austenite region, and then subjected to secondary hot rolling under the conditions shown in Table 3. After hot rolling, direct quenching, quenching, and tempering, the strength and 1fil (SS CCiJ) of the obtained steel were measured, and the results are also shown in Table 3. The properties were measured in the same test as in Example 1 by calculating the Sc value.

From the results shown in Table 3, it is clear that the method of the present invention can improve the strength and corrosion resistance of the copper phase. SCC-resistant cut steel *J't, i'm not asked about it' white.

Example 3 After soaking copper (1), which is the steel subject to the present invention in Table 1, to 1230°C, it was subjected to the first hot rolling process in the austenite region, and
Subsequently, from 870°C to 102°C, which exhibits an austenitic state
It is reheated to 0°C and subjected to a second hot rolling to finish it into the final product dimensions, then directly quenched at the temperature shown in Table 4, and then further quenched and tempered under the conditions shown in Table 4. The strength and 5scc resistance (test conditions were the same as in Example 1) were measured.

The results thus obtained are also shown in Table 4.

It is clear from the results shown in Table 4 that steel materials with excellent strength and 5 SCC resistance can be obtained by the method of the present invention.

Example 4 After soaking the steel of the present invention in Table 1 to 1200'C, a temporary phase was produced under the treatment conditions shown in Table 5, and the obtained plate material had an austenia degree of B Number (AS
TMNα) was measured.

(Note) The East mark indicates that there is a deviation from the conditions of the present invention.

Table 5 The drafting process in Table 5 is as follows: ■ After performing the first hot rolling in the austenite region, cooling to 320°C to complete the transformation, and then reheating to 930°C. 2nd heat 1*1 Roll grapes to final product size,
After cooling with air at JL Temperature 1, it was heated to 930℃ by 0.75℃/
Hardening treatment is performed by heating at a heating rate of seconds. ■ After the first heating in the austenite region, the transformation is started by reheating from le<87°C to 950°C and the second After hot-rolling the grapes and finishing them to the final product dimensions, they were quenched at 850°C for 10 minutes, and then heated to 930°C at a heating rate of 0.75°C/Ishi for quenching. .

From the results shown in Table 5, it is clear that the treatment that satisfies the conditions of the present invention increases the fine grain structure by 4 cm,t.

Example 5 The steel subject to the present invention in Table 1 above was soaked to 1220°C, then subjected to the first υ hot pressure IA in the austenitic region, and then heated from 860°C to 940°C until the austenitic state was achieved.
The product is reheated to 200°C and subjected to a second hot rolling to finish it to the final product dimensions, followed by direct quenching at 850°C, followed by intermediate rough tempering under the conditions shown in Table 6, followed by quenching and tempering. went.

Examples of strength and SSCC resistance of products obtained in this way
(Test conditions are the same as in Example 1) were measured, and the results are shown in Table 6.

The results shown in Table 6 are as follows: 11) Even if rough tempering is applied to prevent cracking due to cracking before the quenching after contact hardening, a steel material with excellent strength and 5SCC properties can be obtained. It is clear that if the rough tempering condition is set to A, (19,0 x 10"), the steel material will have an even better balance of strength and 5SCC resistance.

As mentioned above, this invention combines @i-quenching treatment and conventional 1
This invention combines a high heat quenching treatment to obtain a fine grain structure and achieves a steel with excellent strength and resistance to 5 SCC.According to this invention, no complicated treatment is required for grain refinement. It is possible to manufacture high-strength steel easily and at low cost, which is suitable for oil country tubular goods and other equipment used in the development of oil fields and gas fields located in deep layers and under sour environments. , industrially useful effects are brought about.

[Brief explanation of drawings]

Figure 1 shows an example of a shell-type corrosion test piece. Figure 1 (a) is its front view, Figure 1 (b) is its side view, and Figure 2 shows the test piece used in the shell-type corrosion test. FIG. 2 is a schematic diagram showing a state in which it is supported by a support jig. In the drawings, 1... test piece, 2... glass round bar, 3... stress-applied bolt. Figure 1 (Q) (b) Second Calendar

Claims (1)

[Claims] (1) C: 0.15 to 0.45% by weight. Si: 0.80% or less. Mn: 0.01% or more and less than 0.30%. Cr: 0.20-1.50%. Mo: 0.05-0.80%. Nb: o, ot~0.10%. ht: 0.01-0.10%. Contains all of the ingredients, and if necessary, the first category... B: 0.0005 to 0.0050. Second category... Cu: 0.05-0.50%. V: 0.01 to 0.10%. Third category... Ca: 0.001-0.030%. REM: 0.001-0.050%. Contains one or more of the following: Fe and unavoidable impurities: composed of residues, and the contents of P and S in the impurities are P: 0.0] and 5% or less, respectively. S: After the first hot working of steel with a S content of 0.010% or less in an austenitic state, the steel is reduced to 1 in an austenitic state without starting transformation.
1 directi/CAc, metamorphosis point ~ [Ac3 metamorphosis point + 200
The second hot working is carried out by heating to a temperature range of 0G], followed by direct quenching from the austenitic state, and then the temperature range from K Ac3 transformation point to (ACa transformation point + 20 (1℃) A method for producing steel with excellent sulfide cracking resistance, characterized by repeating the process of heating and quenching in a humidity range of 9 times or more, followed by tempering at a temperature below the Ac transformation point. .