JPS6354765B2 - - Google Patents

Description

[Detailed description of the invention]

This invention is suitable as a steel pipe material for oil or gas wells, and is particularly effective against sulfide corrosion cracking (hereinafter abbreviated as SSCC) when applied to so-called sour oil wells and sour gas wells. The present invention relates to a method for manufacturing high-strength steel having high resistance. SSCC is a phenomenon that occurs when stress is applied to steel materials used in humid environments containing hydrogen sulfide (sour environments) such as sour oil wells and sour gas wells, and generally, material hardness (e.g. yield strength) is It is known that SSCC resistance deteriorates as the strength increases, and it has been confirmed that high-strength materials crack even when subjected to extremely small stress. Based on many previous studies and actual experience, it has been found that in order to prevent SSCC in steel materials, it is effective to limit the strength to a Rockwell C scale hardness (H _R C) of 20 to 22 or less. has been
Therefore, steel materials used in sour environments have usually been limited to those with a yield strength of 70 Kgf/mm ² or less, which corresponds to the above-mentioned hardness. However, the energy situation in recent years has required development to extend to oil and gas fields deep underground, and as the depth of oil and gas wells increases, the oil country pipes used in these wells, etc. At present, there is an urgent need to increase the strength of steel materials. In order to meet these demands, we developed a method for producing steel that combines high strength and excellent SSCC resistance by rapidly heating Ti-B-added steel, quenching it, and then tempering it (Japanese Patent Laid-Open No. 52-52114). Public bulletin), Cr-
A method for producing steel with both high strength and excellent SSCC resistance by rapidly heating and then quenching Mo-added steel (Japanese Unexamined Patent Publication No. 119324/1982) has been proposed, but However, there is a limit to how high the strength can be increased (in a sour environment, SSCC will occur unless the yield strength is lower than 75 Kgf/ ^mm2 ), and the critical stress ratio for cracking can only reach less than 75%. What is the "cracking limit ratio" in a sour environment?
It is the ratio of the maximum stress that does not cause SSCC to the yield strength of the material, expressed as a percentage. From the above-mentioned viewpoints, the present inventors have developed a material that has a yield strength of 75 Kgf/mm ² or more, a cracking limit ratio of 90% or more, which is extremely high strength, and has excellent SSCC resistance. As a result of many years of research in order to produce high-strength steel at a low cost, which was previously thought to be impossible, we have (a) suppressed the P and S contents to specific values to eliminate grain boundary segregation and inclusions; When the reduced C-Mn-Cr-Mo steel is repeatedly quenched twice and then tempered, the strength increases due to Cr and Mo, the corrosion resistance improves due to low P and low S,
The dispersion of segregated elements and homogenization of the structure by repeated quenching, the spheroidization of inclusions by repeated quenching, the internal stress relief effect of martensite by tempering, the spheroidization of cementite, and the corrosion resistance improvement effect are intertwined with each other. In addition, an extremely excellent SSCC-resistant steel material having a high yield strength of 75 Kgf/mm ² or more and a cracking critical stress ratio of 90% or more can be obtained; (b) during the repeated quenching, If tempering is performed after the first quenching, the distortion caused by the first quenching will be sufficiently removed and cracking will be prevented, and the second quenching effect will be further improved, creating a more uniform structure. After repeated experiments and studies, we found that (c) During the repeated quenching, the heating during the second quenching was performed using high-frequency induction heating. If the average heating rate in the temperature range from Ac ₁ transformation point to Ac ₃ transformation point is set to 10 to 50℃/sec, the crystal grains of the steel material become even finer, resulting in better SSCC resistance. We have confirmed that it is possible to grant This invention was made based on the above findings, and includes C: 0.15 to 0.40% (hereinafter, % indicating component composition ratio is expressed as weight %), Si: 0.1 to 1.0%, Mn: 0.3 to 1.0%. , Cr: 0.1 to 1.5%, Mo: 0.1 to 1.0%, Al: 0.01 to 0.10%, P: 0.015% or less, S: 0.005 or less, N: 0.003 to 0.015%, and further if necessary. , Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050%, B: 0.0001 to 0.0050%, Ca: 0.0005 to 0.0100%, Cu: 0.1 to 0.5%. , Fe and other unavoidable impurities: the remainder, quenching steel from a temperature range above the Ac ₃ transformation point and below the crystal grain coarsening start temperature, and if necessary [Ac ₁ transformation point - 50℃] After tempering at the following temperature, heat again to a temperature range above the Ac ₃ transformation point and below the crystal grain coarsening start temperature, quenching again from this temperature range, and then tempering at a temperature below the Ac ₁ transformation point. As a result, it has a yield strength of 75Kgf/ ^mm2 or more and a cracking limit ratio of 90% or more.
This method is characterized by obtaining high-strength steel with SSCC properties. Next, in this invention, the reason why the chemical composition and heat treatment conditions of the steel are limited as described above will be explained. A Chemical composition of steel a○ C C component has the effect of ensuring the strength of steel, as well as
By improving hardenability and tempering resistance, it also has the effect of promoting uniform hardening structure and high temperature tempering uniform structure, and improving SSCC resistance.
If the content is less than 0.15%, the desired effect cannot be obtained, while if the content exceeds 0.40%, it will cause quench cracking during heat treatment or deterioration of toughness. Therefore, the C content should be reduced to 0.15%. It was set at ~0.40%. b○ Si In addition to acting as a deoxidizer for steel, the Si component also has
Since it has the effect of improving the strength of steel materials, it is necessary to add 0.1% or more, but if it is added in excess of 1.0%, the toughness will deteriorate, so the Si content should be reduced to 0.1 to 1.0%. It was determined that c○ Mn The Mn component also acts as a deoxidizing agent for steel, and also has the effect of improving strength and toughness, but if its content is less than 0.3%, the desired effect will not be achieved. On the other hand, if the Mn content exceeds 1.0%, the toughness of the steel deteriorates, so the Mn content was set at 0.3 to 1.0%. d○ Cr The Cr component has the effect of increasing the strength and tempering resistance of steel, but if its content is 0.1
If the content is less than 1.5%, the desired effect cannot be obtained, while if the content exceeds 1.5%, the toughness of the steel will deteriorate.
It was set at 0.1-1.5%. e○ Mo The Mo component has the effect of increasing the tempering resistance of steel, but if its content is less than 0.1%, the desired effect cannot be obtained, and on the other hand, if it is contained in excess of 1.0%, Since this causes deterioration of the toughness of the steel, the Mo content is set at 0.1 to 1.0%. f○ Al Al component is an element useful as a deoxidizing agent for steel,
In addition, B combines with N in steel to form nitrides, thereby making the action of B more effective, but if the content is less than 0.01%, the desired effect cannot be obtained; If the Al content exceeds 0.01% to 0.10%, the Al content is set at 0.01% to 0.10% because inclusions increase and the steel becomes brittle. g○ P P is an impurity that inevitably accompanies steel, and the less it is, the better it is, but if its content exceeds 0.015%, it not only deteriorates SSCC resistance but also increases quench cracking susceptibility. The content was set at 0.015% or less. h○ S S is also an impurity that inevitably accompanies steel, and the less it is, the better it is, but especially if its content exceeds 0.005%, it will significantly deteriorate the SSCC resistance. , S content 0.005
% or less. i○ N The N component has the effect of forming nitrides, suppressing the grain growth of steel, and making the structure uniform, but if its content is less than 0.003%, the desired effect cannot be obtained. This will deteriorate the toughness of the steel material, and on the other hand,
If the N content exceeds 0.015%, the effect of B addition will be reduced and hardenability will deteriorate, so the N content was set at 0.003 to 0.015%. j○ Nb and V These components have the effect of refining austenite grains and improving tempering resistance, so they can be added as necessary.
If the content of Nb and V is less than 0.01% each, the desired effect cannot be obtained in the above action, and on the other hand, if the content of each exceeds 0.10%, not only can no further improvement effect be obtained, but also the toughness decreases. The content of Nb and V was set at 0.01 each.
It was set at ~0.10%. k○ Ti The Ti component, together with B, has the effect of improving the hardenability of steel and increasing the tempering resistance, so it can be added as necessary, but if its content is less than 0.005%, the above effects will be reduced. However, if the Ti content exceeds 0.050%, the toughness will deteriorate due to an increase in carbonitride precipitates, so the Ti content was set at 0.005 to 0.050%. l○ B The B component has the effect of improving the hardenability of steel, making the austenite grains finer, and improving the tempering resistance, and it can be contained in an amount of 0.0001% or more if necessary, but its content If the B content exceeds 0.0050%, not only no further improvement effect can be obtained, but also a decrease in toughness occurs, so the B content was set at 0.0001 to 0.0050%. m○ Ca Ca component has sulfide-based inclusions spheroidized and
Since it has the effect of improving SSCC properties, it is added at 0.0005% or more as necessary, but if the content exceeds 0.0100%, inclusions will increase and toughness will deteriorate. The Ca content was determined to be 0.0005 to 0.0100%. n○ Cu The Cu component has the effect of improving the corrosion resistance of steel, so it can be contained at 0.1% or more if necessary, but if it is contained in excess of 0.5%, hot cracking will occur. Therefore, the Cu content was set at 0.1 to 0.5%. B Heat treatment conditions a○ Quenching temperature In both the first and second quenching, if the quenching temperature is lower than the Ac ₃ transformation point, the steel structure will be non-uniform because it will be quenched in the two-phase region of (α + γ), and the other quenching temperature If the temperature exceeds the crystal grain coarsening start temperature, it becomes impossible to obtain the desired fine grain structure in the resulting steel material, resulting in deterioration of _SSCC resistance. It is set as less than. In addition, during the second quenching, Ac ₁ transformation point ~
It is preferable to set the average temperature increase rate at the Ac ₃ transformation point to 10 to 50° C./sec in order to improve the SSCC resistance performance. At this time, if the temperature increase rate is less than 10℃/sec, the degree of grain refinement will be small and the effect on improving SSCC resistance will be somewhat lower, and the temperature increase rate will be less than 10℃/sec.
This is because when the temperature exceeds 50° C./sec, the austenite grains become coarse and mixed, and the SSCC resistance tends to decrease. b○ Tempering temperature after the first quenching If the tempering temperature after the first quenching exceeds the value of [Ac ₁ transformation point - 50℃], the grain refining effect of the product will be small, resulting in insufficient strength. Therefore, the tempering temperature was set to be below [Ac ₁ transformation point - 50°C]. c○ Tempering temperature after the second quenching The tempering after the second quenching removes the internal stress of the martensite and makes the cementite spheroidized by sufficiently tempering the martensite generated by quenching. This is to improve corrosion resistance, and the lower limit of the tempering temperature is not particularly limited, but the higher the tempering temperature, the better the results.
However, if the temperature exceeds the Ac ₁ transformation point, C
Concentrated austenite such as Mn and Mn is generated,
Since island-shaped martensite is formed during cooling, which has a negative effect on SSCC resistance, the tempering temperature after the second quenching was determined to be below the Ac ₁ transformation point. Next, the present invention will be explained using Examples and in comparison with Comparative Examples. Example 1 First, the subject steels A to M of the present invention and the comparative steels N to M having the chemical compositions shown in Table 1 were prepared by a conventional method.
S was melted. Then, hot rolling is applied to each of these steels.

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.

【table】

[Table] Plate materials having the thickness shown in Table 2 were then heat treated under the conditions shown by symbols X and Y in Table 2, and their yield strengths and cracking limit stress ratios were investigated. The results are also shown in Table 2. In Table 2, symbol This shows a series of heat treatments shown in steps i) to (iii), where the symbol Y is: (i) water quenching after heating at 920°C for 30 minutes, (ii) heating rate: 30°C/sec to 920°C This shows a series of heat treatments shown in steps (i) to (iii) above: water quenching immediately after heating, (iii) tempering at 700°C for 30 minutes followed by air cooling. The cracking limit stress ratio was measured as follows. That is, as shown in FIG. 1, a round bar tensile test piece 1 with a parallel part of 6.4 minφ cut out from each steel plate after heat treatment is held in a test container 2, and the inside of this test container 2 is heated with H ₂ S. Fill with saturated 0.5% acetic acid-5% saline solution and place 3 weights on round bar tensile test piece 1.
A method was used in which a constant load was applied to the specimen, the maximum stress at which cracking did not occur within 720 hours was determined, and this was taken as the SSCC critical stress. In addition, what is indicated by the symbol 4 in FIG. 1 is a hole in the H ₂ S gas flow path, and the symbol 5
6 is the aqueous solution circulation pump.
What is shown is the aqueous solution storage tank. The results shown in Table 2 also show that the method of the present invention has high yield strength of 75 Kgf/mm ² or more and excellent durability with a cracking critical stress ratio of 90% or more.
It is clear that while a steel with SSCC properties is obtained, the desired properties cannot be achieved with the comparative process in which the composition of the steel is outside the scope of the present invention. Example 2 Steel A in Table 1 in Example 1 was
Heat treatments were performed as shown in the table. Yield strength and cracking of the steel thus obtained

【table】

[Table] (Note) * indicates that it is outside the scope of the present invention.
The critical stress ratio was investigated in the same manner as in Example 1, and the results are also shown in Table 3. The results shown in Table 3 also show that the method of the present invention has a high yield strength of 75 Kgf/mm ² or more and excellent durability with a cracking limit stress of 90% or more.
It can be seen that while a steel having SSCC properties can be obtained, the comparative method in which the heat treatment conditions are outside the scope of the present invention cannot achieve the desired properties. As described above, the present invention provides excellent durability.
It is possible to produce high-strength steel with SSCC properties at a low cost, and it brings about industrially useful effects, such as making it possible to exploit natural resources that exist in deep layers and under harsh conditions in sour environments. .

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an apparatus for measuring the cracking critical stress ratio of steel. In the drawings, 1...Round bar tensile test piece, 2...
...Test container, 3... Weight, 4... Kotuku, 5...
Pump, 6...solution storage tank.

Claims (1)

[Claims] 1. In weight percentage: C: 0.15-0.40%, Si: 0.1-1.0%, Mn: 0.3-1.0%, Cr: 0.1-1.5%, Mo: 0.1-1.0%, Al: 0.01- Contains 0.10%, P: 0.015% or less, S: 0.005% or less, N: 0.003 to 0.015%, and further contains, if necessary, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, Ti: 0.005 ~0.050%, B: 0.0001~0.0050%, Ca: 0.0005~0.0100%, Cu: 0.1~0.5%, and contains one or more of the following, Fe and other unavoidable impurities: the remainder. is quenched from a temperature range above the Ac ₃ transformation point and below the crystal grain coarsening start temperature, and then heated again to a temperature range above the Ac ₃ transformation point and below the crystal grain coarsening start temperature. A method for producing high-strength steel with excellent sulfide corrosion cracking resistance, which is characterized by quenching the steel again at a temperature below the Ac 1 transformation point and then tempering it at a temperature below the Ac ₁ transformation point. 2 When heating during re-quenching, Ac ₁ transformation point ~
Average heating rate for temperature range up to Ac ₃ transformation point: 10~
A method for producing high-strength steel with excellent sulfide corrosion cracking resistance according to claim 1, which comprises raising the temperature at 50° C./sec. 3 In terms of weight percentage, C: 0.15-0.40%, Si: 0.1-1.0%, Mn: 0.3-1.0%, Cr: 0.1-1.5%, Mo: 0.1-1.0%, Al: 0.01-0.10%, P: 0.015 % or less, S: 0.005% or less, N: 0.003 to 0.015%, and further contains, if necessary, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, Ti: 0.005 to 0.050%, B: 0.0001 to 0.0050%, Ca: 0.0005 to 0.0100%, Cu: 0.1 to 0.5%, containing one or more of the following, Fe and other unavoidable impurities: the remainder: Ac ₃ transformation point After quenching from a temperature range below the crystal grain coarsening starting temperature,
Tempering at a temperature below [Ac ₁ transformation point - 50°C], then heating this again to a temperature range of at least the Ac ₃ transformation point and below the crystal grain coarsening start temperature, and then quenching again from this temperature range, A method for producing high-strength steel with excellent sulfide corrosion cracking resistance, which is then tempered at a temperature below the Ac ₁ transformation point. 4 Ac ₁ transformation point ~ during heating during re-quenching
Average heating rate for temperature range up to Ac ₃ transformation point: 10~
A method for producing high-strength steel with excellent sulfide corrosion cracking resistance according to claim 1, which comprises raising the temperature at 50° C./sec.