JPS619519A - Manufacture of high strength steel superior in sulfide corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain the titled high strength steel having good shape and superior SSCC resistance by heting a steel having a prescribed component compsn. by a specified average heating rate, then quenching from said temp., next tempering and applying a suitable working to said material at a prescribed temp. range in the cooling process. CONSTITUTION:The steel contg. by weight % 0.20-0.40 C, 0.05-0.60 Si, 0.30- 0.80 Mn, <=0.015 P, <=0.010 S, 0.30-1.50 Cr, etc. while satisfying 0.3-0.6 Mo/Cr is prepared. Next, the steel is heated to 850-1,050 deg.C from 700 deg.C by 2.5-50 deg.C/sec average heating rate, then quenched from the temp., and tempered at 650-730 deg.C. Next, said steel is sizing worked by <=10% working ratio at 650-450 deg.C temp. range during the cooling process in the tempering treatment. In this way, the high strength steel material having good shape and superior SSCC resistance is obtained efficiently and at low cost.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a steel pipe material suitable for use in oil wells or gas wells.
High-strength steel with high resistance to sulfide corrosion cracking (hereinafter abbreviated as 5scc) in a wet hydrogen sulfide (H2S) environment, especially yield strength level 70 Kgf/ln
The present invention relates to a method for manufacturing a high-strength steel having a strength of 5SCC or more and having an excellent 5SCC resistance of 80% or more.

[Conventional technology]

5SCC is a phenomenon that occurs when stress is applied to steel materials used in a wet extraction environment containing hydrogen sulfide, but it is generally known that the higher the material strength (for example, yield strength), the worse the 5SCC resistance. It is being

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 treatments has a yield strength level of 70 Kgf/mm' or higher and good 5SCC resistance. It is said that it is difficult to create steel that has both of these properties, and therefore, steel materials that can be used in humid environments containing hydrogen sulfide have a 70-! If
Therefore, it was necessary to limit the yield strength to less than /mW.

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, we used Ti-B additive steel and adopted rapid heating during quenching (
There is a method for producing high-strength steel with excellent 5 SCC resistance by rapidly heating and quenching Cr-Mo steel (Japanese Patent Application Laid-open No. 54-119324). was suggested.

The effect of improving 5SCC resistance by these methods is thought to be brought about through the refinement of crystal grains and uniform dispersion of precipitated carbides, and in fact, it is acknowledged that they are effective means for improving 5SCC resistance. However, as far as what has been reported so far, it is difficult to say that a sufficient effect of improving 5SCC resistance can be obtained.

On the other hand, since steel products for oil wells (for example, steel pipes for oil wells) have relatively strict requirements for shape such as bending and roundness, 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, such post-treatment not only complicates the process and increases cost, but also reduces the steel's resistance to corrosion.
A tendency for SCC resistance to deteriorate has been observed, and even if rapid heating is employed in the final heat treatment to improve 5SCC resistance, the effect is lost or reduced in subsequent steps. This has become clear through studies by the present inventors.

[Problem that the invention seeks to solve]

As mentioned above, depending on the conventional technology, the yield strength is 70
K! It has a strength of 7f/mm” or more and an excellent 5sCC resistance with a cracking limit stress ratio of 80% or more.
Not only was it not possible to stably produce high-strength steel materials that had both good properties and properties, but the addition of a product shape correction process further deteriorated the SSC resistance, and further complicated the process and increased costs. Couldn't solve the problem.

[Means for solving problems]

From this point of view, the inventors have determined that 70KJf/mm
``To find a method to efficiently and at low cost manufacture high-strength steel products that have the above yield strength and excellent 5 SCC 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 steel and heat treatment conditions, especially the tempering temperature, and by correcting the shape in the warm during the cooling process during tempering, it is possible to correct the shape in the cold. Achieves sufficient strength without straightening and subsequent stress relief annealing.
High-strength preparations with 5SCC resistance and shape and dimensions can be stably obtained.

This led us to the following knowledge.

This invention was made based on the above knowledge, and in terms of weight percentage, 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: 0.10-070%.

Nb: 0.015-0.050%.

Ti: 01005-0025%.

B: 0.0005-0.0025%.

A steel containing Al: 0.01 to 0.10%, satisfying Mo/Cr: 0.3 to 0.6, and the balance consisting of Fe and other unavoidable impurities is heated at 700°C or higher. Average heating rate 2.5-50
After heating to 850 to 1050 °C in terms of °C/sec, quenching from this temperature, then tempering at 650 to 730 °C, and 650 to 4 °C in the cooling process.
It is characterized by the fact that by performing straightening at a processing rate of 10% or less in a temperature range of 50° C., a high-strength steel with excellent 5 SCC resistance and good shape and dimensions can be obtained.

Next, in this development, the reason why the composition of the steel, heat treatment conditions, and straightening conditions were 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 improving hardenability and tempering resistance, ensuring a uniform high temperature tempered martensitic structure that is effective in improving 5SCC resistance. However, if the content is less than 0.20%, the desired effect cannot be obtained;
If the C content exceeds 0.2, the toughness will deteriorate and bending and cracking will occur more easily during heat treatment.
It was set as 0 to 040%.

■ 5i The Si component has the function of deoxidizing the steel as well as ensuring cleanliness, so it is necessary to add 0.05% or more, but it is not allowed to contain more than 0.60%. This leads to deterioration of toughness and workability, and the crystal grains become coarser, resulting in
The Si content was determined to be 0.05 to 0860% since this tends to cause deterioration of SCC properties. However, in practice it is 0.
20-040% is appropriate.

■ Mn The Mn component has the effect of improving the hardenability of steel and increasing its strength and toughness, but its content is 0.30%.
If it is less than 0.8, the desired effect cannot be obtained;
If the Mn content exceeds 0%, segregation increases and corrosion resistance deteriorates, so the Mn content was set at 0.30 to 0.80%.

＠ P P is an impurity that inevitably accompanies steel, and the less it is, the better it is.
If it exceeds 15%, segregation is promoted and corrosion resistance deteriorates, so the P content is 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.
If the S content exceeds 0.010, nonmetallic inclusions increase and corrosion resistance deteriorates, so 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 its content is less than 0.1%, the desired effect cannot be obtained; on the other hand, 1. 5
If the Cr content exceeds 0%, the toughness and workability of the steel will deteriorate, so the Cr content is set at 030% to 150%.

■　M.

The Mo component has the effect of improving the 5SCC resistance by improving the hardenability and temper softening resistance of steel, but if its content is less than 0.1%, the desired effect cannot be obtained; If the content exceeds 0.70%, the precipitation state of carbides will change and corrosion resistance will deteriorate.
Mc) content was determined to be 0.10 to 0.70%.

■Nb The Nb component has the effect of improving the hardenability and temper softening resistance of steel, and improving the 5scc resistance, but if its content is less than 0.015%, the desired effect cannot be obtained, On the other hand, if the Nb content exceeds 0.050, it will cause deterioration of toughness and workability, so the Nb content should be reduced to 0.015.
~0050%.

■ Ti The Ti component has the effect of greatly improving the hardenability of steel when added in combination with B, but when its content is 0.
If the Ti content is less than 0.005%, the desired effect cannot be obtained, while if the Ti content exceeds 0.025%, the toughness will deteriorate.

■ B The B component has the effect of greatly improving the hardenability of steel when added in combination with Ti, but when its content is o, o
If it is less than 0.00%, the desired effect cannot be obtained; Even if the B content exceeds 0.0005% to OO'25%, not only will no further improvement effect be obtained, but the toughness will deteriorate.
It was determined that

(2) The Al Ad acid component has a grain refining effect in addition to deoxidizing the steel, but if its content is less than 0.01%, the desired effect cannot be obtained; If the content exceeds 10%, inclusions will increase and the steel will become brittle, so AJt'll is set at 10.01 to 0.10%.

■ Mo/CrO ratio Even if the amounts of Mo and Cr added are within the above range, C
It is essential to ensure good 5scc resistance that the ratio of Mo content to r content is in the range p''-03 to 0.6. That is, the value of Mo/Cr is 0.3. Even if the value is less than 0.06, or even if the value exceeds 0.6, the state of carbide precipitation distribution will be adversely affected,
This is because it leads to deterioration of corrosion resistance.゛Please note that the product wall thickness is 2
In the case of 5 dragons or more, as mentioned earlier, the combined addition of Ti and B is essential, but if the product wall thickness is as thin as less than 2511r1L, the combined addition of T1 and B may not be necessary. It was also confirmed that the strength and 5SCC resistance could be ensured.

B. Heat treatment conditions■ Refining of crystal grains, which is effective for 5scc heating rate resistance during quenching, can be achieved by rapid heating in a temperature range of 700°C or higher during quenching, and in a temperature range of less than 700°C. The heating rate at 100° C. does not have a significant effect on grain refinement. If the average heating rate is less than 2.5°C/sec, the desired resistance s s
It is possible to obtain the effect of improving cc property, while at 50℃/se
If the average heating rate exceeds 700℃, the grains will be mixed and the desired 5SCC resistance improvement effect cannot be obtained. ~50''C/sec.

■ Heating temperature during quenching If the heating temperature during quenching is less than 850℃, the steel may not be sufficiently austenitized;
The heating temperature during quenching was limited to 850 to 1050°C, since heating beyond this temperature would result in coarsening of crystal grains.

(2) Tempering Temperature If the tempering temperature is less than 650°G, the tempering will be insufficient and carbide spheroidization will not be sufficient, resulting in poor 5SCC resistance of the steel. On the other hand, when the tempering temperature exceeds 730℃,
Transformation occurs in the segregated portion, resulting in deterioration of 5SCC resistance.

For this reason, the tempering temperature was set at 650 to 730°C.

C0 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.
If the temperature is lower than 50°C, the strain in the plastically deformed portion due to straightening will not recover, leading to deterioration of 5SCC resistance, and residual stress will remain, reducing crushing strength. On the other hand, if straightening is performed in a temperature range exceeding 650°C, the strength variation of the obtained 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%,
Since the strength variation of the resulting product would increase, the processing rate during straightening was set at 10% or less.

Next, the present invention will be explained using examples and comparing with comparative examples.

〔Example〕

First, steel having the chemical composition shown in Table 1 was melted by a conventional method, and a steel pipe having an outer diameter of 1271 m and an inner diameter of 108.6 mm was manufactured using the steel as raw materials by a conventional Mannesmann pipe manufacturing method.

Next, the steel pipe was heated to 950° C. at a heating rate of 25° C./sec by induction heating, and then water quenched.

Subsequently, the steel pipes were heated and tempered to the tempering temperature shown in Table 2, and those shown in test numbers 18 and 20 were cooled to room temperature as they were, but the other pipes were
When the temperature of the steel pipe after tempering reached the straightening temperature shown in Table 2, straightening was performed using a straightener at a working rate of 3%. In addition, regarding the steel pipes of test numbers 18 and 20 that were cooled as they were to room temperature, straightening was performed using a cold straightener at a processing rate of 3% after cooling, and subsequently stress relief annealing was performed.

The yield strength and resistance 5 of the steel pipe products obtained in this way
The scc performance was investigated and the results are also shown in Table 2.

In addition, 5SCC resistance performance was measured using a smooth round bar tensile test piece cut from each steel pipe product, and applying a constant stress in 0.5% acetic acid-5% saline solution saturated with H2S until cracking occurred within 720 hours. The maximum stress at which no cracking occurred was determined, and the critical stress ratio for cracking was calculated and investigated.

From the results shown in Table 2, it is clear that the method of the present invention has a high yield strength of 70KIIf/mm" or more, and has excellent 5SC resistance with a cracking critical stress ratio of 80% or more.
It is clear that the desired properties cannot be achieved using comparative methods or conventional methods in which the manufacturing conditions (including the composition of the target steel) are outside the scope of the present invention, whereas steel with C properties can be obtained. .

[Overall effect]

As described above, according to the present invention, high-strength steel materials with excellent 5SCC resistance and good shape can be manufactured efficiently and at low cost, and the development of natural resources that exist under harsh conditions is facilitated. This will bring about industrially useful effects, such as creating conditions that can further promote the process.

Claims (1)

[Claims] 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/ Cr: 0.3 to 0.6, balance: Fe and other unavoidable impurities A steel with a composition (weight %) satisfying Cr: 0.3 to 0.6 is heated at an average heating rate of 2.5 to 50 °C/2.5 to 50 °C/700 °C or higher. 850 as sec
After heating to ~1050°C, quenching from this temperature,
Then, it is tempered at 650 to 730°C, and the processing rate is reduced in the temperature range of 650 to 450°C during the cooling process:
A method for manufacturing high-strength steel with excellent sulfide corrosion cracking resistance, characterized by applying straightening of 10% or less.