JPS648686B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] This invention is suitable as a steel pipe material for oil or gas wells, and has high resistance to sulfide corrosion cracking (hereinafter abbreviated as SSCC) in a wet hydrogen sulfide (H ₂ S) environment. High-strength steel with resistance, especially high-strength steel with a yield strength level of 70 Kgf/mm ² or higher, and
This invention relates to a method for manufacturing steel with excellent SSCC resistance, such as an SSCC cracking limit stress ratio (cracking limit stress/yield strength) of 80% or more. [Prior art] SSCC is a phenomenon that occurs when stress is applied to steel materials used in a humid environment containing hydrogen sulfide.
Generally, it is known that the higher the material strength (for example, yield strength), the worse the SSCC resistance. Based on numerous research results and experiences from the past, it has been found that the manufacturing method of high-strength steel using normal quenching and tempering processes has a yield strength level of 70 Kgf/mm ² or higher and also has good SSCC resistance. Therefore, steel materials used in humid environments containing hydrogen sulfide must be limited to those with a yield strength of less than 70Kgf/ ^mm2 . It was. However, the energy situation in recent years has required the expansion of development to oil and gas fields located deep underground. There was also an urgent need to increase the strength of steel for oil country tubular goods. Therefore, in order to meet these demands, Ti-B
Additive steel is used and rapid heating is applied during quenching (Japanese Unexamined Patent Publication No. 52-52114), Cr-
Means for rapidly heating and quenching Mo steel (Japanese Patent Application Laid-Open No. 54
A method for manufacturing high-strength steel with excellent SSCC resistance was proposed in Japanese Patent Publication No. 119324). The effect of improving SSCC resistance by these methods is thought to be brought about through the refinement of crystal grains and uniform dispersion of precipitated carbides.
Although it is acknowledged that this is an effective means for improving SSCC resistance, it is difficult to say that it necessarily provides a sufficient effect of improving SSCC resistance based on what has been reported so far. On the other hand, steel products for oil wells (for example, steel pipes for oil wells) have relatively strict requirements for shape such as bending and roundness, so the shape is usually corrected in the cold using a straightener or press. Since cold working after the final heat treatment is prohibited for corrosion-resistant applications, the products after the cold straightening are subjected to stress relief annealing. However, applying such post-treatment not only complicates the process and increases costs, but it has also been observed that the SSCC resistance of the steel material tends to deteriorate, so rapid heating is used during the final heat treatment. Endurance
The inventors' studies have revealed that even if the SSCC property is improved, the effect is lost or reduced in subsequent steps. [Problems to be solved by the invention] As mentioned above, the conventional technology has a yield strength of 70 Kgf/mm ² or more and an excellent cracking stress ratio of 80% or more. Not only was it not possible to stably manufacture high-strength steel materials with SSCC properties, but the addition of a product shape correction process further deteriorated SSCC resistance, and the process was forced to become more complicated and costs increased. I couldn't solve the problem. [Means for solving the problem] From this perspective, the present inventors have developed ^a
In order to find a method to efficiently and inexpensively manufacture high-strength steel products that have the above-mentioned yield strength and excellent SSCC resistance shown by a cracking limit ratio of 80% or more and have good shape accuracy, As a result of many years of research, we have found that by carefully adjusting the composition of the steel and the heat treatment conditions, especially the tempering temperature, and by correcting the shape in the warm state during the cooling process during tempering, it is possible to achieve cold straightening. A high-strength preparation with sufficiently satisfactory strength, SSCC resistance, and shape and dimensions can be stably obtained without processing and subsequent stress relief annealing. This led us to the following knowledge. This invention was made based on the above findings, and the weight percentages are as follows: C: 0.20-0.40%, Si: 0.05-0.60%, Mn: 0.30-0.80%, P: 0.015% or less, S: 0.010 % or less, Cr: 0.30-1.50%, Mo: 0.10-0.70%, Nb: 0.015-0.050%, Ti: 0.005-0.025%, B: 0.0005-0.0025%, Al: 0.01-0.10%, and Mo/ After heating a steel whose composition satisfies Cr: 0.3 to 0.6 and the balance consists of Fe and other unavoidable impurities to 850 to 1050℃ at an average heating rate of 2.5 to 50℃/sec over 700℃. , quenched from the temperature, then 650-730
By tempering at ℃ and performing straightening at a processing rate of 10% or less in the temperature range of 650 to 450℃ during the cooling process, high-strength steel with excellent SSCC resistance and good shape and dimensions is produced. It is characterized by the following points. Next, in this invention, the reason why the chemical composition, heat treatment conditions, and straightening conditions of the steel are limited as described above will be explained. A Chemical composition of steel C C component has the effect of ensuring the strength of the steel, as well as the effect of ensuring a uniform high-temperature tempered martensitic structure that is effective in improving SSCC resistance by improving hardenability and tempering resistance. However, if the content is less than 0.20%, the desired effect cannot be obtained, and on the other hand, 0.40%
The C content was set at 0.20 to 0.40% because if the C content exceeds 0.2%, the toughness deteriorates, and bending and cracking are likely to occur during heat treatment. Si In addition to acting as a deoxidizing agent for steel, the Si component also has the effect of ensuring cleanliness, so 0.05%
Although it requires addition of more than 0.60
The Si content was set at 0.05 to 0.60% because Si content exceeding 0.05% causes deterioration of toughness and workability, and also tends to coarsen crystal grains and cause deterioration of SSCC resistance. However, in practice
0.20-0.40% is appropriate. Mn The Mn component has the effect of improving the hardenability of steel and increasing its strength and toughness.
If the Mn content is less than 0.30%, the desired effect cannot be obtained, while if the Mn content exceeds 0.80%, segregation will increase and corrosion resistance will deteriorate. Established. 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%, segregation will be promoted and corrosion resistance will deteriorate. The content was set at 0.015% or less. S S is also an impurity that inevitably accompanies steel, and the less it is, the better it is, but if its content exceeds 0.010%, nonmetallic inclusions will increase, leading to deterioration of corrosion resistance. ,
The S content was set at 0.010% or less. However, it is desirably 0.005% or less. Cr The Cr component has the effect of improving the hardenability of steel and increasing its strength, but if the content is less than 0.1%, the desired effect cannot be obtained, while if the content exceeds 1.50%, the desired effect cannot be obtained. Since Cr content causes deterioration of steel toughness and workability, the Cr content was set at 0.30 to 1.50%. Mo The Mo component has the effect of improving SSCC resistance by improving the hardenability and temper softening resistance of steel, but if its content is less than 0.10%, the desired effect cannot be obtained.
On the other hand, if Mo content exceeds 0.70%, the precipitation state of carbides changes and corrosion resistance deteriorates, so the Mo content was set at 0.10 to 0.70%. Nb Nb component has the effect of improving the hardenability and temper softening resistance of steel and improving SSCC resistance, but if its content is less than 0.015%, the desired effect cannot be obtained;
If the Nb content exceeds 0.050%, the toughness and workability deteriorate, so the Nb content was reduced to 0.015%.
It was set at ~0.050%. Ti The Ti component has the effect of greatly improving the hardenability of steel when added in combination with B, but if its content is less than 0.005%, the desired effect cannot be obtained; If the content exceeds 0.025%, toughness will deteriorate.
The Ti content was determined to be 0.005% to 0.025%. B The B component has the effect of greatly improving the hardenability of steel when added in combination with Ti, but if its content is less than 0.0005%, the desired effect cannot be obtained; If the B content exceeds 0.0025%, not only will no further improvement effect be obtained, but the toughness will deteriorate, so the B content was set at 0.0005 to 0.0025%. Al The Al component has a grain refining effect in addition to deoxidizing the steel, but its content is 0.01
If the Al content is less than 0.1%, the desired effect cannot be obtained, while if the Al content exceeds 0.10%, inclusions will increase and cause embrittlement of the steel. Established. Mo/Cr ratio Even if the amounts of Mo and Cr added are within the above range, the ratio of Mo content to Cr content is
Good SSCC resistance should be in the range of 0.3~0.6
This is an essential requirement to ensure safety. That is, even if the Mo/Cr value is less than 0.3, or even if the value exceeds 0.6, the carbide precipitation distribution will be adversely affected, leading to deterioration in corrosion resistance. In addition, when the product wall thickness is 25 mm or more, the combined addition of Ti and B is essential as mentioned above, but when the product wall thickness is thin, less than 25 mm, the combined addition of Ti and B is required. It was also confirmed that the desired strength and SSCC resistance could be ensured even without this process. B Heat treatment conditions Heating rate during quenching Refining of crystal grains, which is effective for SSCC resistance, can be achieved by rapid heating in a temperature range of 700℃ or higher during quenching, and 700℃
Heating rates in the temperature range below do not have a significant effect on grain refinement. If the average heating rate is less than 2.5°C/sec, the desired SSCC resistance improvement effect cannot be obtained, while if the average heating rate exceeds 50°C/sec, mixed grains will occur and the desired SSCC resistance improvement will not be achieved. Since no effect could be obtained, heating during quenching was limited to a heating rate of 2.5 to 50°C/sec in the temperature range of 700°C or higher. Heating temperature during quenching If the heating temperature during quenching is less than 850°C, there is a risk that the steel will not be sufficiently austenitized, while heating above 1050°C will result in coarsening of crystal grains. The heating temperature was limited to 850-1050°C. Tempering Temperature If the tempering temperature is less than 650℃, the tempering will be insufficient and carbide spheroidization will not be sufficient, thus reducing the durability of the steel.
SSCC properties will be inferior. On the other hand, if the tempering temperature exceeds 730°C, transformation occurs in the segregated parts, leading to deterioration of SSCC resistance. For this reason, the tempering temperature is 650~
The temperature was set at 730℃. C Straightening conditions Straightening temperature One of the features of this invention is that the shape is straightened during the cooling process in the tempering process, but if the straightening temperature is lower than 450°C, the strain in the plastically deformed part due to straightening will not recover. Endurance
In addition to deteriorating the SSCC property, residual stress remains and reduces the crushing strength. On the other hand, if straightening is performed in a temperature range exceeding 650°C, the strength variation of the resulting product will increase. For the above reasons, the straightening temperature was set at 650 to 450°C. Processing rate during straightening If the processing rate during straightening in the above temperature range exceeds 10%, the strength variation of the obtained product will increase.
% or less. Next, the present invention will be explained using examples and comparing with comparative examples. [Example] First, steel having the chemical composition as shown in Table 1 was melted using a conventional method, and using these as raw materials, a tube with an outer diameter of 127 mm and an inner diameter of 127 mm was produced using the ordinary Mannesmann pipe manufacturing method.
A 108.6mm steel pipe was manufactured. Next, the steel pipe was heated to 950°C by induction heating at a heating rate of 25°C/sec, and then water quenched. Subsequently, the steel pipe was heated and tempered to the tempering temperature shown in Table 2, and test numbers 18 and 20 were obtained.
The material shown in was cooled to room temperature as it is, but
For the other steel pipes, straightening was performed using a straightener at a processing rate of 3% when the temperature of the steel pipe after tempering reached the straightening temperature shown in Table 2. In addition, for the steel pipes of test numbers 18 and 20 that were cooled to room temperature as they were, straightening was performed using a cold straightener at a working rate of 3% after cooling, and then stress relief annealing was performed. The yield strength and SSCC resistance performance of the steel pipe products obtained in this way were investigated, and the results are shown in Table 2.

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

[Overall effect]

As described above, according to the present invention, excellent durability is achieved.
Industrially useful effects include the ability to efficiently produce high-strength steel materials with good shape and SSCC properties at low cost, creating a situation that can further promote the development of natural resources that exist under harsh conditions. is brought about.

Claims (1)

[Claims] 1 C: 0.20-0.40%, Si: 0.05-0.60%, Mn: 0.30-0.80%, P: 0.015% or less, S: 0.010% or less, Cr: 0.30-1.50%, Mo: 0.10 ~0.70%, Nb: 0.015~0.050%, Ti: 0.005~0.025%, B: 0.0005~0.0025%, Al: 0.01~0.10%, and satisfies Mo/Cr: 0.3~0.6, balance: Fe and Steel with a composition (more than % by weight) consisting of other unavoidable impurities is heated to 700℃.
850~ with average heating rate of 2.5~50℃/sec
After heating to 1050℃, quenching from that temperature,
Then, it is tempered at 650 to 730℃, and the processing rate is reduced in the temperature range of 650 to 450℃ during the cooling process:
A method for manufacturing high-strength steel with excellent sulfide corrosion cracking resistance, which is characterized by applying straightening of 10% or less.