JPH0121849B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-strength line pipe steel that is suitable for transporting sour oil and sour gas and has excellent hydrogen-induced cracking resistance. In recent years, new oil and gas fields have been actively developed in line with changes in _the energy situation. As development attention has turned to oil (oil) and gas (sour gas), the demand for line pipes to transport these products has also continued to increase. However, steel materials used in such sour environments may crack due to the influence of wet H ₂ S, which poses a high risk of leading to serious accidents. This had become a major problem in gas field development. Among them,
For relatively low strength steels such as line pipe materials,
In particular, hydrogen-induced cracking (hereinafter abbreviated as “HIC”)
This is a problem, and as usage environments have become increasingly harsh in recent years, even higher HIC resistance has been required. However, until now it has been extremely difficult to stably and economically produce steel with the desired good HIC resistance. Currently, there is competition in research and development for mass production on a large scale. By the way, HIC is a type of hydrogen embrittlement that occurs due to internal pressure when hydrogen generated by corrosion in a sour environment containing wet H ₂ S penetrates into steel and accumulates at the interface of non-metallic inclusions. It is known that HIC occurs even in the absence of external stress, and the occurrence of HIC depends on the usage environmental conditions, such as H ₂ S concentration,
The risk of HIC occurrence in recent line pipe usage environments is increasing, as it has been clarified that HIC is controlled by many factors such as CO ₂ concentration, chloride ion concentration, and temperature. it is conceivable that. In other words, the degree of severity of environmental conditions for HIC generation is generally evaluated using the amount of hydrogen that has penetrated into steel as an index. Therefore, when evaluating the HIC resistance of steel materials, the level of hydrogen generated by corrosion reaction is evaluated. Various test baths with different PH values (specifically, liquids with different PH values) are used, but due to the situation described above, it is necessary to test baths with a PH value that is even lower than what was previously allowed.
Conditions (PH3.0-3.5, 0.5% acetic acid saturated with _H2S )
HIC resistance has come to be required under conditions such as immersion in a 5% salt aqueous solution. On the other hand, reflecting the recent trend toward high-pressure operation, line pipe materials are now required to have even higher strength than before, and in general, the higher the strength of steel materials, the more susceptible they are to HIC. In other words, in order to increase the strength of steel materials, it is common to add various alloying elements, but these alloying elements tend to segregate in steel materials, increasing the hardness of that part and causing deterioration of HIC resistance. The higher the strength of the steel, the more likely it is that the HIC resistance will deteriorate, and it is extremely difficult to completely prevent HIC, especially in situations where the environment becomes harsher and the amount of hydrogen penetrating into the steel increases. This had become a major issue. Conventional measures to prevent HIC include adding a small amount of Cu to suppress the amount of hydrogen that enters from the environment, reducing non-metallic inclusions that become crack initiation points, and
Attempts have been made to reduce the cracking susceptibility of the steel itself by controlling the morphology of non-metallic inclusions by adding Ca or REM (rare earth elements), but the effectiveness of the former has a strong environmental dependence. However, they would lose their effectiveness under the harsh environmental conditions that have recently been required (for example, conditions with a pH of 4 or less). On the other hand, in the latter case, it is not necessarily sufficient to withstand harsh environmental conditions (for example, NACE conditions), and there is a problem in that the higher the strength of the material, the more difficult it is to prevent cracking. As mentioned above, cracks in high-strength steel are strongly affected by component segregation within the steel material. For example, in products manufactured through large steel ingots or continuously cast steel slabs, cracks may occur due to V segregation or center segregation. Because of the segregation of Mn, P, etc., the cracking susceptibility is extremely high. For this reason, attempts have been made to reduce segregation by applying slab re-king treatment, which heats and holds the steel billet at a high temperature before rolling, but the effect is not sufficient and it also results in a significant increase in costs. It was hot. This invention is based on the above-mentioned problems,
Excellent HIC resistance that can withstand NACE conditions
In order to find a way to mass-produce line pipe steel on an industrial scale at a low cost, with a strength higher than API standard X-52 steel, we focused in particular on eliminating segregation, which is considered to be effective in reducing HIC susceptibility. This was done as a result of the research conducted by the present inventors on the component system with small segregation and the optimal conditions for uniform refinement of the structure, and its characteristics are as follows: % expressed as weight%), Si: 0.01 to 0.50%, Mn: 0.8 to 2.0%, Al: 0.01 to 0.10%, Ca: 0.0005 to 0.0050%, P: 0.015% or less, S: 0.002% or less , Cu: 0.05-0.50%, Ni: 0.05-0.50%, Cr: 0.05-0.50%, Mo: 0.05-0.50%, Nb: 0.01-0.10%, V: 0.01-0.10%, A steel containing at least one of Ti: 0.005 to 0.050%, B: 0.0005 to 0.0080%, and the remainder containing Fe and other unavoidable impurities is heated to the Ac ₃ transformation point or higher and then rolled and finished. The temperature is within the range of [Ar ₃ transformation point ±50℃], and
Hot rolling with a reduction rate of 50% or more at 950°C or lower, then cooling from the pearlite formation temperature or higher to any temperature below 600°C at a cooling rate of 3 to 15°C/sec, or after that. Furthermore [500℃～
The purpose of this method is to manufacture high-strength line pipe steel with excellent resistance to hydrogen-induced cracking by tempering it at a temperature of [Ac ₁ transformation point]. That is, the present invention is based on a high Mn-based steel to ensure strength, reduces segregation by lowering C and P, and creates line pipe steel with a composition system that limits strength-adjusting elements to specific ones. On the other hand, the cooling conditions after rolling are controlled to specific conditions to achieve a uniform and fine structure, and the influence of the center segregation area is further reduced by subsequent tempering. By combining multiple measures, we can achieve extremely high durability that cannot be estimated by simply adding up the effects obtained from each individual measure.
This is a high-strength line pipe steel with HIC properties that fully satisfies the NACE conditions.
Moreover, it has become possible to mass-produce line pipe steel with a strength higher than APIX-52 in an industrial scale, and the effects on the energy industry are immeasurable. Hereinafter, in the method of the present invention, the reason why the composition ratio of the steel and the rolling/heat treatment conditions are numerically limited as described above will be explained. (A) Component composition of steel (a) C C is an element that is involved in increasing segregation of steel and deteriorates HIC resistance, and the lower the content, the better. And the C content is 0.03
If the content exceeds 0.03%, segregation increases rapidly and the desired HIC resistance cannot be achieved, so the content was set at less than 0.03%. Thus, from the perspective of improving HIC resistance, it is recommended to suppress the C content as much as possible, but from the perspective of ensuring strength, it is recommended to limit the C content to the minimum.
It is desirable to set it at around 0.005%. (b) Si Although the Si component is effective as a deoxidizing agent for steel, if its content is less than 0.01%, the desired effect as a deoxidizing agent cannot be obtained, and if the content exceeds the other 0.50%, Since Si content causes deterioration in the toughness of steel, the Si content was set at 0.01 to 0.50%. (c) Mn The Mn component has the effect of improving the strength of steel and is also effective as a deoxidizing agent, but if its content is less than 0.8%, it will not ensure the desired strength of the steel. can't do the other 2.0
If the content exceeds %, segregation will increase and the resistance will decrease.
In addition to deteriorating HIC properties,
The Mn content was determined to be 0.8 to 2.0% since this would lead to deterioration of toughness or weldability. (d) Al Al component is effective as an acid deoxidizing agent, but if its content is less than 0.01%, the desired deoxidizing effect cannot be obtained, and if it is contained in excess of 0.10%, The Al content was set at 0.01 to 0.10% since this would lead to deterioration of toughness. (e) Ca Ca component is used to make inclusions spheroidal and prevent them from becoming crack initiation points, thereby increasing the durability of steel.
It has the effect of improving HIC resistance, but if the content is less than 0.0005%, the desired effect cannot be obtained, while if the content exceeds 0.0050%, the HIC resistance will deteriorate. Moreover, since it also causes toughness deterioration, the Ca content was set at 0.0005 to 0.0050%. (f) PP Since P is an impurity that causes segregation and deteriorates the HIC resistance of steel, it is an element that is preferably reduced as much as possible. In particular, the P content is 0.015
%, segregation will rapidly increase and the desired HIC resistance cannot be secured, so
It is set at 0.015% or less. (g) S S is an impurity that forms nonmetallic inclusions and deteriorates the HIC resistance of steel, so it must be reduced as much as possible. In particular, the S content
If it exceeds 0.002%, the desired HIC resistance cannot be ensured due to an increase in nonmetallic inclusions, so the S content was set at 0.002% or less. (h) Cu, Ni, Cr, Mo, Nb, V, Ti, and B These components all have the effect of improving the strength of steel without promoting segregation, so they further improve the strength of steel. In case it is necessary to B: 0.005 to 0.050% and B: 0.0005 to 0.0080%. However, if the content is less than the lower limit, the strength improvement effect will not be significant, whereas if it exceeds the upper limit, The amount of each component added was limited as described above, since the strength-improving effect would be saturated even if the amount of each component was added, and it would also cause economic disadvantage. (B) Rolling and heat treatment conditions (a) Rolling heating temperature If the heating temperature during rolling is below the Ac ₃ transformation point, uniform solutionization will not occur and α+
Since the product has a γ structure, the structure of the product after rolling and heat treatment will not be uniform, so the rolling heating temperature was set at the Ac ₃ transformation point or higher. (b) Rolling finishing temperature If the rolling finishing temperature is lower than [Ar ₃ transformation point - 50℃], it is not possible to achieve a uniform structure in the steel material, while on the other hand, if it is finished at a temperature exceeding [Ar ₃ transformation point + 50℃], Since the desired microstructure cannot be achieved and the HIC resistance will deteriorate in both cases, the rolling finishing temperature should be set to [Ar ₃ transformation point ±50°C].
It was determined that (c) Reduction rate In order to refine the structure, it is necessary to ensure a reduction rate of 50% or more in the low temperature range (finishing temperature ~ 950℃), but the reduction rate in the low temperature range is
If it is less than 50%, the structure will become coarse and the desired HIC resistance will not be achieved.
Not only would it not be possible to achieve the desired toughness, but also the toughness would deteriorate, so the reduction rate at temperatures below 950°C should be reduced to 50°C.
% or more. (d) Cooling conditions In order to avoid pearlite formation and obtain a uniform structure with good HIC resistance, the cooling conditions must be from above the pearlite formation temperature to below 600℃ (including room temperature).
During this period, it is necessary to cool at a rate of 3 to 15°C/sec. This is because if the cooling rate is less than 3℃/sec, the desired microstructure and strength cannot be secured;
Cooling at a rate exceeding .degree. C./sec will cause the structure of the segregated portion to become non-uniform. (e) Tempering temperature If the tempering temperature is less than 500°C, it will not be possible to achieve a uniform structure in the segregated area, and therefore the desired durability will not be achieved.
The effect of improving HIC property cannot be obtained. On the other hand, when the tempering temperature exceeds the Ac ₁ transformation point, the strength of the steel changes significantly and HIC resistance also deteriorates. For this reason, the tempering temperature was set in the range of [500°C to Ac ₁ transformation point]. Next, the present invention will be explained by examples and in comparison with comparative examples. Example 1 First, steels A to P having the compositions shown in Table 1 were melted by a conventional method. Next, after heating these steels to 1200℃, hot rolling was performed at a total reduction rate of 90%, a reduction rate between 950 and 800℃: 70%, and a finishing temperature of 800℃, and immediately after finishing,
After cooling to room temperature by water spray at a cooling rate of 10°C/sec, it was further tempered at 650°C.

[Table] The mechanical properties and HIC resistance characteristics of the thus obtained steel materials were investigated, and the results are also shown in Table 1. In the HIC test, a test piece with dimensions of 15 ^t x 20 ^w x 100 ^l is cut out and placed in a NACE bath (0.5% acetic acid +
96 in a 5% aqueous solution of common salt saturated with _H2S )
A method was adopted in which the HIC was immersed for a period of time and then the HIC was detected using ultrasonic flaw detection. And the result is
The symbols are shown in Table 1 as ○: No cracking, ×: Cracking occurred. From the results shown in Table 1, it is clear that none of the steels A to H satisfying the conditions of the present invention were cracked and exhibited good HIC resistance. On the other hand, those with a high content of any one of C, Mn, P, and S, and those with a Ca content outside the range of the present invention, are resistant to
It can be seen that the HIC property is inferior. Furthermore, it is clear that a material with a low Mn content outside the range of the present invention results in insufficient strength. In this example, the strength adjusting element is
Among Cu, Ni, Cr, Mo, Nb, V, Ti and B,
Although we did not show examples of individual additions, we were able to confirm that a sufficiently satisfactory strength improvement effect was confirmed whether these elements were added individually or in any combination of two or more. Of course. Example 2 Steel F in Table 1, melted by a conventional method,
The mechanical properties and resistance of the steel obtained by hot rolling and heat treatment under the conditions shown in Table 2 are as follows:
HIC properties were investigated in the same manner as in Example 1. The results are also shown in Table 2 (The display of HIC resistance is the same as in Table 1, 〇: No cracking, ×:
cracking occurred). From the results shown in Table 2, it is clear that if the hot rolling and heat treatment conditions are within the range of the present invention, good durability can be achieved.
It can be seen that while a steel material exhibiting HIC properties and mechanical properties can be obtained, when the above conditions are out of the scope of the present invention, a steel material with inferior properties can only be obtained.

[Table] (Note) * indicates that the conditions are outside the conditions of the present invention.
As described above, according to the present invention, it is possible to mass-produce high-strength line pipe steel with excellent HIC resistance that can withstand severe corrosive environments at a low cost, and to save energy resources in sour environments. This will bring about industrially useful effects, such as the expectation that it will greatly contribute to the prevention of structural destruction accidents associated with development.

Claims (1)

[Claims] 1. In weight percentage: C: less than 0.03%, Si: 0.01-0.50%, Mn: 0.8-2.0%, Al: 0.01-0.10%, Ca: 0.0005-0.0050%, P: 0.015% or less , S: 0.002% or less, Fe and other unavoidable impurities: the remainder, after heating the steel to above the Ac ₃ transformation point, the rolling finishing temperature is within the range of [Ar ₃ transformation point ± 50 ° C], and
It is characterized by hot rolling with a reduction rate of 50% or more at 950°C or lower, and then cooling from the pearlite formation temperature or higher to an arbitrary temperature of 600°C or lower at a cooling rate of 3 to 15°C/sec. A method for manufacturing high-strength line pipe steel with excellent resistance to hydrogen-induced cracking. 2 Weight percentage: C: less than 0.03%, Si: 0.01-0.50%, Mn: 0.8-2.0%, Al: 0.01-0.10%, Ca: 0.0005-0.0050%, P: 0.015% or less, S: 0.002% or less , and further contains Cu: 0.05-0.50%, Ni: 0.05-0.50%, Cr: 0.05-0.50%, Mo: 0.05-0.50%, Nb: 0.01-0.10%, V: 0.01-0.10%, Ti: 0.005 to 0.050%, B: 0.0005 to 0.0080%, and Fe and other unavoidable impurities: After heating the steel to above the _Ac3 transformation point, the rolling finish temperature is is within the range of [Ar ₃ transformation point ±50℃], and
It is characterized by hot rolling with a reduction rate of 50% or more at 950°C or lower, and then cooling from the pearlite formation temperature or higher to an arbitrary temperature of 600°C or lower at a cooling rate of 3 to 15°C/sec. A method for manufacturing high-strength line pipe steel with excellent resistance to hydrogen-induced cracking. 3 Weight percentage: C: less than 0.03%, Si: 0.01-0.50%, Mn: 0.8-2.0%, Al: 0.01-0.10%, Ca: 0.0005-0.0050%, P: 0.015% or less, S: 0.002% or less , Fe and other unavoidable impurities: after heating the steel to above the Ac ₃ transformation point, the rolling finishing temperature is within the range of [Ar ₃ transformation point ±50℃], and
After hot rolling with a reduction rate of 50% or more at 950°C or lower, and then cooling from the pearlite formation temperature or higher to an arbitrary temperature of 600°C or lower at a cooling rate of 3 to 15°C/sec, further [ A method for producing high-strength pipeline steel with excellent hydrogen cracking resistance, characterized by tempering at a temperature of 500°C to Ac ₁ transformation point]. 4 Weight percentage: C: less than 0.03%, Si: 0.01-0.50%, Mn: 0.8-2.0%, Al: 0.01-0.10%, Ca: 0.0005-0.0050%, P: 0.015% or less, S: 0.002% or less , and further contains Cu: 0.05-0.50%, Ni: 0.05-0.50%, Cr: 0.05-0.50%, Mo: 0.05-0.50%, Nb: 0.01-0.10%, V: 0.01-0.10%, Ti: 0.005 to 0.050%, B: 0.0005 to 0.0080%, the remainder being Fe and other unavoidable impurities: After heating the steel to the Ac ₃ transformation point or higher, the rolling finishing temperature is is within the range of [Ar ₃ transformation point ±50℃], and
After hot rolling with a reduction rate of 50% or more at 950°C or lower, and then cooling from the pearlite formation temperature or higher to an arbitrary temperature of 600°C or lower at a cooling rate of 3 to 15°C/sec, further [ A method for producing high-strength line pipe steel with excellent resistance to hydrogen-induced cracking, characterized by tempering at a temperature of 500°C to Ac ₁ transformation point].