JP2705416B2 - Martensitic stainless steel and manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high yield ratio martensitic stainless steel having a low susceptibility to sulfide stress cracking and a martensitic stainless steel suitable for use as a material for oil country tubular goods and oil well equipment, for example. The present invention relates to a method for producing a high yield ratio martensitic seamless stainless steel pipe using steel.

[0002]

2. Description of the Related Art Martensitic stainless steel exhibits extremely excellent corrosion resistance against corrosion in an environment containing wet carbon dioxide (CO ₂ ) due to the action of Cr, one of its components. Are known. Further, martensitic stainless steel also has the property of easily obtaining high strength, and is therefore widely used, for example, as a material for oil country tubular goods and oil well equipment.

However, martensitic stainless steel, in an environment containing hydrogen sulfide (H ₂ S) is known from experience that susceptible to sulfide stress cracking, that is in an environment containing H ₂ S Limited use. By the way, the environment of wells for extracting oil and natural gas has become increasingly severe in recent years, and even wells containing CO ₂ sometimes contain trace amounts of H ₂ S. In addition, initially CO
_Even a well containing only ₂ may contain a small amount of H ₂ S due to aging.

[0004] For this reason, as described above, martensitic stainless steel used as a material for oil country tubular goods and the like is required to exhibit excellent corrosion resistance even in an environment containing H ₂ S. It has become. Hitherto, it has been empirically known that reducing the hardness of martensitic stainless steel is effective in reducing the susceptibility to sulfide stress cracking. For example, in API standard 5CT, L
−80 13Cr is specified, and its hardness is limited to HRC ≦ 23.

[0005] However, martensitic stainless steel generally has a low yield ratio (ratio of 0.5% proof stress / tensile strength), and tends to have too high a hardness as compared with other materials. For example, martensite compared to a relatively more corrosion resistant steel for oil country tubular goods for the AISI4130 system used was based in an environment (such as steel for L-80 having the composition shown in Table 1 to be described later) comprising H ₂ S Series stainless steel has a yield ratio of 10 to
It is about 20% lower and relatively harder than proof stress.

[0006]

[Table 1]

For this reason, in a martensitic stainless steel having an allowable proof stress near the upper limit of the specified range, the hardness often exceeds the upper limit of the specified range, and on the other hand, the hardness should be surely within the above range. In this case, the proof stress width after tempering becomes small. As described above, conventionally, it has been quite difficult to produce a martensitic stainless steel satisfying both the specified values of the proof stress and the hardness on an actual production line.

However, from the user's point of view, martensitic stainless steel has a higher susceptibility to sulfide stress cracking than a corrosion-resistant oil well pipe steel based on AISI 4130, and is therefore required to reduce sulfide stress cracking sensitivity. There is a demand to increase the hardness of martensitic stainless steel to just below the upper limit while maintaining the above proof stress. For example, in the 80 grade martensitic stainless steel, the upper limit of the hardness is HRC: 22, but if the hardness is set to a value near the upper limit, the targeted proof stress width after tempering becomes narrower. Since the production becomes more difficult, improvement has been desired.

Therefore, conventionally, a high yield ratio has been ensured by limiting the composition so that the V value and the N value in the martensitic stainless steel satisfy the relationship defined by the following equation.

Effective V (%) = (%) V−3.6 × (%) N + 0.01 ≧ 0 FIG. 1 shows that the Nb content is 0.005% (hereinafter, there is no particular limitation in this specification) As long as "%" related to the content means "% by weight", the effective V (%) on the yield ratio YR (%) after tempering at 740 ° C for martensitic stainless steels less than ) Is shown in a graph. As is clear from FIG. 1, it is understood that as the effective V (%) increases, the yield ratio also increases.

However, in order to increase the effective V (%), it is necessary to lower the N as apparent from the equation. However, the lower N causes deterioration of hot workability due to an increase in δ-ferrite. In addition, it induces flaws on the inner surface of the pipe, which is not preferable in quality. FIG. 2 shows the relationship between the amount of δ-ferrite obtained from the relationship between the amount of N and Si in the steel, and FIG. The relationship between the incidences is shown by a graph. As is clear from FIGS. 2 and 3, when the N content is reduced, the number of flaws on the inner surface of the tube increases due to an increase in δ-ferrite.

Therefore, the inventors of the present invention disclosed in Japanese Patent Application Laid-Open No. 2-104639 that C: 0.1 to 0.30%, Si: 0.25% or less, Mn: 0.2
5 to 1%, P: 0.05% or less, S: 0.005% or less, Cr: 11 to
A high yield ratio martensitic stainless steel comprising 15%, Ni: 0.1% or less, Al: 0.005 to 0.1%, balance: Fe and unavoidable impurities and having a structure mainly composed of tempered martensite was proposed. According to this proposal, the yield ratio can be improved by promoting the coarsening of the carbide during tempering by reducing the Si. However, according to the results of subsequent studies by the present inventors, this technique is not preferable in terms of quality because of low Si, resulting in insufficient deoxidation during melting.

[0013]

On the other hand, the present inventors have
According to Japanese Patent Application No. 2-413804, C: 0.15-0.25%, S
i: 1.00% or less, Mn: 1.00% or less, P: 0.050% or less,
S: 0.005% or less, Cr: 12.0 to 13.5%, Ni: 0.10% or less, V: 0.05 to 0.50%, sol. Al: 0.05 to 0.100%, N:
In order to suppress the inner surface flaw of a martensitic seamless steel pipe having a composition of 0.1% or less and the balance of Fe and unavoidable impurities, a technique for controlling the steel composition so as to satisfy the following equation was proposed. F ₁ (Si, N) = 9 × (%) Si−25 × (%) N <1.3 ・ ・ ・ F ₁ : δ-ferrite formation index (%) Si: Si weight% (%) N: N weight% .

Further, in this proposal, taking into account the annealing heating time of the billet or the ingot and the drilling heating temperature,
It is assumed that the following formula is satisfied. F (Si, N, t, T) = 9 × (%) Si−25 × (%) N−0.13 × (Σt _i ) ^{1/2
-0.14 × (1200-T) 1/2} <1.3 F: δ- ferrite index (%) Si: Si wt% (%) N: N wt% t _i: annealing heating time in the billet or ingot _(i is , Heat No.) Σt _i : t ₁ + t ₂ + ... (total of all heating times) T: Perforation heating temperature Annealing heating temperature: 1200 to 1300 ° C Perforation heating temperature: 1200 ° C or less Assuming that 0.20%, the minimum value (%) Nmin satisfying the equation is 200 ppm.

FIG. 4 is a graph showing the relationship between the N amount and the Si amount, and FIG. 5 is a graph showing the relationship between the V amount and the N amount. To obtain an effective V (%) of 0 or more in order to obtain a high yield ratio, V is 0.06
% Must be added. As described above, according to the conventional technique, the effective V (%)> 0 is suppressed while suppressing the generation of the inner surface flaw of the pipe.
In order to secure the cost, the cost increase due to the addition of V
There were various problems, such as lowering the heating temperature and deteriorating the tube-forming properties, and further lowering the Si and causing insufficient deoxidation. Therefore, there has been a need for a technical development capable of easily obtaining a martensitic stainless steel having excellent pipe inner surface properties and a high yield ratio.

Here, an object of the present invention is to provide a martensitic stainless steel which can be easily manufactured industrially and has a lower hardness than the yield strength, ie, a high yield ratio, and a martensitic stainless steel. It is an object of the present invention to provide a method for producing a martensitic seamless stainless steel pipe utilizing the above method.

[0017]

The above problems are attributable to the fact that martensitic stainless steels generally have a higher hardness than their yield strength. Since the hardness corresponds to the tensile strength, this problem can be solved by increasing the ratio between the proof stress and the tensile strength, that is, the yield ratio.

The inventors of the present invention have investigated various factors affecting the yield ratio, and have found that it is possible to increase the yield ratio by forming a structure strengthened by precipitation with carbides. That is, Nb is added, quenching is performed, and Nb is added.
After forming a solid solution of Nb-C by performing a tempering treatment, it was possible to increase the yield strength while suppressing an increase in hardness, and completed the present invention.

Here, the gist of the present invention is as follows.
C: 0.15 to 0.30%, Si: 1.00% or less, Mn: 0.25 to 1%,
P: 0.05% or less, S: 0.005% or less, Cr: 11 to 15%,
Ni: 0.1% or less, Al: 0.003 to 0.1%, Nb: 0.005 to 0.500
% Balance: a high yield ratio martensitic stainless steel with low sulfide stress cracking susceptibility, having a steel composition of Fe and unavoidable impurities and having a structure mainly composed of tempered martensite.

In the above-mentioned present invention, the steel composition contains
In addition, one or more elements selected from one or both of the following first group and second group may be further contained, and tempered martensite may have an area ratio of 80% or more.

[Group 1] Mo: 0.5 to 2%, V: 0.01 to 0.5%, Ti: 0.01 to 0.5%,
Zr: 0.01 to 0.5%, B: 0.0005 to 0.01%, N: 0.001 to 0.
15% [Group 2] Ca: 0.001 to 0.05%, La: 0.001 to 0.05%, Ce: 0.001 to 0.05
%.

Further, in the above-mentioned present invention, the steel composition may satisfy the following relationship. F ₁ (Si, N) = 9 × (%) Si−25 × (%) N <1.3 F ₁ : δ-ferrite formation index (%) Si: Si weight% (%) N: N weight% A martensitic stainless steel pipe utilizing a martensitic stainless steel having a composition of the following formula is obtained by annealing a billet made of a martensitic stainless steel having the above-described composition in an annealing heating temperature range for a predetermined time and then performing a piercing heating temperature. After performing drilling, rolling and diameter reduction by adjusting or reheating the region, heating to form a structure having austenite of at least 80% in area ratio, then cooling to 80% or more of martensite in area ratio , And reheated in a temperature range not higher than the Ac ₁ transformation point where substantially no austenite is generated.

Further, at this time, it is desirable to control the steel composition, the annealing heating time of the billet or the steel ingot, and the drilling heating temperature so as to satisfy the following equations. F (Si, N, t, T) = 9 × (%) Si−25 × (%) N−0.13 × (Σt _i )
^1/2 −0.14 × (1200−T) ^1/2 <1.3 F: δ-ferrite formation index (%) Si: Si weight% (%) N: N weight% t _i : Annealing heating time in billet or ingot (i
Is the heat No.) Σ t _i : t ₁ + t ₂ + ... (total of all heating times) T: Perforation heating temperature Annealing heating temperature: 1200-1300 ° C Perforating heating temperature: 1200 ° C or less

[0024]

Hereinafter, the present invention will be described in detail together with the functions and effects. First, the reason why the component composition of the martensitic stainless steel according to the present invention is limited as described above will be described.

C: C effectively acts to increase the strength of the martensitic stainless steel and to suppress the formation of δ-ferrite, but if its content exceeds 0.30%, it considerably lowers the toughness. On the other hand, if the content is less than 0.15%, the generation ratio of δ-ferrite that appears during quenching increases, and it becomes difficult to homogenize the material. Therefore,
In the present invention, the C content is limited to 0.15% or more and 0.30% or less.

Si: Si is used as a deoxidizing material.
When the content exceeds 1.00%, δ-ferrite increases and hot workability deteriorates. Therefore, the Si content is limited to 1.00% or less. Desirably, it is more than 0.25% and 1.00% or less.

Mn: Mn widens the austenite range and has an effective action for improving the strength and toughness. However, if it is less than 0.25%, the desired effect cannot be obtained. Worsen. Therefore, in the present invention, the Mn content is limited to 0.25% or more and 1% or less.

P and S: P and S are both impurity elements, and the lower the content, the better. If it is too high, the toughness and stress corrosion cracking resistance are impaired. The upper limit of the allowable content is P: 0.05% and S: 0.005%, respectively.
It is. Therefore, in the present invention, the P content is 0.05% or less,
The S content is limited to 0.005% or less.

[0029] Cr: Cr is, CO ₂ - trace H ₂ S -Cl ^- are extremely effective element for reducing the corrosion rate in an environment,
If the content is less than 11%, the effect is insufficient, while
%, The amount of δ-ferrite is increased, which impairs hot workability. Therefore, in the present invention, the Cr content is limited to 11% or more and 15% or less.

[0030] Ni: Ni is, CO ₂ - trace amounts of H ₂ S -Cl ^- under the environment
If the content exceeds 0.1%, remarkable generation of pudding and sulfide stress cracking are caused. Therefore, in the present invention, Ni
The content is limited to 0.1% or less.

Al: Al is added as a deoxidizing material in an amount of 0.003% or more. However, if the content exceeds 0.1%, the effect is saturated, and the toughness is rather reduced due to an increase in inclusions. Therefore, in the present invention, the Al content is limited to 0.003% or more and 0.1% or less.

Nb: Nb is added to increase the yield ratio, but if its content is less than 0.005%, its effect is small.
Addition of more than 500% deteriorates toughness. Therefore, in the present invention, the Nb content is limited to 0.005% or more and 0.500% or less. The martensitic stainless steel according to the present invention comprises, in addition to the above elements, the balance being Fe and unavoidable impurities. And, it is composed of a structure mainly composed of tempered martensite, preferably a structure having tempered martensite of 80% or more in area ratio.

In order to enhance the suitability of the high yield ratio martensitic stainless steel having low sulfide stress cracking susceptibility according to the present invention, for example, as a material for seamless stainless steel pipes, the first group is further improved. And one or more elements selected from one or both of the second group. Specific functions and effects of the elements belonging to the first and second groups are as follows.

Mo, V, Ti, Zr, B, N: These elements have an effective effect on increasing the strength. In particular, Mo,
V, Ti and Zr have the effect of improving the toughness and the effect of preventing the reduction of Cr in the substrate, which is effective for corrosion resistance. The effect of B also has the effect of refining the structure and improving the toughness and corrosion resistance. But,
Mo: less than 0.5%, V: less than 0.01%, T
i: less than 0.01%, Zr: less than 0.01%, B: less than 0.0005%,
When the content of N is less than 0.001%, these effects are small. On the other hand, Mo: more than 2%, V: more than 0.5%, Ti: more than 0.5%, Zr: more than 0.5%,
If B: more than 0.01% and N: more than 0.15%, the toughness and / or the corrosion resistance are rather reduced. Therefore, in the present invention, Mo: 0.5% to 2%, V: 0.01% to 0.5%, Ti: 0.01% to 0.5%, Zr: 0.01% to 0.5%
Below, B: 0.0005% to 0.01%, N: 0.001% to 0.
It is desirable to limit each to 15% or less.

Ca, La, Ce: These elements have the effect of improving the shape of the sulfide in the steel and improving the stress corrosion cracking resistance. If the content of each element is less than 0.001%, the effect cannot be obtained, while if it exceeds 0.05%, toughness and corrosion resistance are reduced. Therefore, in the present invention, Ca: 0.001% or more
It is desirable to limit the content to 0.05% or less, La: 0.001% to 0.05%, and Ce: 0.001% to 0.05%.

Further, in the present invention, from the viewpoint of reducing δ-ferrite, F ₁ (Si, N) = 9 × (%) Si−25 × (%) N <1.3 , F ₁ : δ-ferrite formation index (%) Si: Si weight% (%) It is desirable to limit to N: N weight%. Here, F ₁ is a δ- ferrite index.

In the present invention, the formation of δ-ferrite is required to be as small as possible. For this purpose, the steel composition as described above is selected. To do so, the following manufacturing process is employed.

First, as a preferred embodiment, a billet as a raw material is annealed and heated to a temperature of 1200 ° C. or more and 1300 ° C. or less for a predetermined time. If the annealing heating temperature is less than 12000 ° C., diffusion of segregation is difficult and δ− The formation of a ferrite phase cannot be sufficiently suppressed. On the other hand, when the temperature is higher than 1300 ° C., billet surface flaws are easily generated. On the other hand, such suppression of the formation of the δ-ferrite phase is also possible by annealing heating prior to billet production. Annealing heating is performed at a high temperature at which the diffusion rate increases, that is, in the present invention, it is desirable to perform annealing heating of the billet at 1200 ° C. or more and 1300 ° C. or less.

Next, the temperature is adjusted or reheated to the perforation heating temperature range, and the inclined roll type perforating mill (so-called Mannesmann perforating machine) or the extruding type perforating machine (so-called press piercing mill) is used in the same manner as a normal seamless steel pipe. And piercing, rolling and diameter reduction using a rolling mill as a subsequent step. Therefore, conventional means may be employed for such an operation.

The steel pipe after diameter reduction is heated to obtain a structure having austenite of 80% or more in area ratio, for example. At this stage, Nb forms a solid solution in the present invention. Then, by cooling, a structure mainly composed of martensite, for example, a structure in which the area ratio of martensite is 80% or more, is further reheated in a temperature range below the Ac ₁ transformation point where substantially no austenite is generated. Nb dissolved at this stage is Nb-C
Precipitates.

In consideration of the billet annealing heating temperature, the above equation is limited to the following equation. F (Si, N, t,) = 9 × (%) Si −25 × (%) N−0.13 × (Σt _i ) ^1/2 <1.3 t _i : annealing heating time in billet (or ingot)
(i is the heat number) Σ t _i : t ₁ + t ₂ + ... (total of all heating times) Annealing heating temperature: 1200 to 1300 ° C.

Further, since the heating temperature of the pipe making is greatly affected, it is more desirable to consider the heating temperature of the pipe making, that is, the drilling heating temperature T. Therefore, the above equation is limited as follows. F (Si, N, t, T) = 9 × (%) Si−25 × (%) N−0.13 × (Σt _i ) ^1/2 −0.14 × (1200−T) ^1/2 <1.3・ ・ T: Perforation heating temperature (1200 ° C or less).

As described above, according to the present invention, a martensitic stainless steel which can be easily manufactured industrially and has a lower hardness than the yield strength, that is, a high yield ratio, and a martensitic stainless steel A method for producing a martensitic seamless stainless steel pipe using steel is provided. Next, the present invention will be described more specifically with reference to examples.

[0044]

EXAMPLE Steels having the chemical compositions shown in Tables 2 and 3 were melted in a converter, subjected to continuous casting and slab rolling to produce round cast slabs (billets), and δ− at the center of the billet. The amount of ferrite was measured. Then, the material was heated to a heating temperature of 1,200 ° C. to perform perforation, rolling, and diameter reduction to obtain a martensitic seamless stainless steel pipe.

Table 4 shows some of the manufacturing conditions in each step. Thereafter, the thus produced martensitic seamless stainless steel pipe was subjected to a solution treatment for holding at a temperature of 980 ° C. for 15 minutes and air-cooled, and then cooled to a temperature shown in Table 1 (700, 720
, 740 and 760 ° C) for 30 minutes to produce a martensitic seamless stainless steel pipe.

The δ-ferrite formation index F of the thus produced martensitic seamless stainless steel pipe was calculated, and its mechanical properties (proof stress, tensile strength, yield ratio and strength) were investigated. The results are shown in the right columns of Tables 2 and 3, and the relationship between proof stress and hardness is shown graphically in FIG.

As is clear from FIG. 6, the HRC hardness can be reduced by about 2 compared with the same level of proof stress of the prior art, and a high yield ratio can be obtained. FIG.
FIG. 7 is a graph showing the relationship between the amount of N (%) and the amount of V (%) exerted on YR (%). As is clear from FIG. 7, this high yield ratio can be obtained without lowering the N. Could be achieved. Therefore, δ-ferrite can be reduced by maintaining high N, and the inner surface properties of the tube are not impaired.

FIG. 8 is a graph showing the relationship between the amount of N and Si on the amount of δ-ferrite produced, and FIG. 9 is a graph showing the relationship between the amount of N and Si on the rate of formation of flaws on the inner surface of the tube. However, it can be seen from both figures that the present invention was able to reduce δ-ferrite without deteriorating the inner surface properties of the tube.

[0049]

[Table 2]

[0050]

[Table 3]

[0051]

[Table 4]

[0052]

As described in detail above, according to the present invention, a martensitic stainless steel having a lower hardness than a proof stress, that is, a sulfide stress cracking susceptibility, which can be easily produced industrially, is provided. Thus, a method for producing a martensitic seamless stainless steel pipe using this martensitic stainless steel could be provided. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of effective V (%) on the yield ratio YR (%) after tempering at 740 ° C. for martensitic stainless steel having an Nb content of less than 0.005%.

FIG. 2 is a graph showing the relationship between the amount of δ-ferrite obtained from the relationship between the amount of N and the amount of Si in steel.

FIG. 3 is a graph showing a relationship between occurrence rates of inner surface flaws of a pipe obtained from a relationship between an N amount and a Si amount in steel.

FIG. 4 is a graph showing the relationship between the amount of N and the amount of Si.

FIG. 5 is a graph showing a relationship between a V amount and an N amount.

FIG. 6 is a graph showing a relationship between hardness (HRC) and proof stress (YS) in an example.

FIG. 7 is a graph showing the relationship between the amount of N (%) and the amount of V (%) on YR.

FIG. 8 shows the effect of N content and Si on the amount of δ-ferrite produced.
It is a graph which shows the relationship with quantity.

FIG. 9 is a graph showing the relationship between the amount of N and the amount of Si affecting the generation rate of the inner surface flaw of the pipe.

Claims (5)

(57) [Claims] C. 0.15 to 0.30%, Si: 1.00% or less, Mn: 0.25 to 1% by weight,
P: 0.05% or less, S: 0.005% or less, Cr: 11 to 15%, Ni: 0.1% or less,
Al: 0.003 to 0.1%, Nb: 0.005 to 0.500% Remainder: High susceptibility to sulfide stress cracking, characterized by having a steel composition composed of Fe and unavoidable impurities and having a structure mainly composed of tempered martensite. Yield ratio martensitic stainless steel. 2. The steel composition further contains one or more elements selected from one or both of the following first group and second group in weight%, and the area ratio of tempered martensite is The high-yield-ratio martensitic stainless steel having low sulfide stress cracking susceptibility according to claim 1, wherein the steel has 80% or more. [Group 1] Mo: 0.5 to 2%, V: 0.01 to 0.5%, Ti: 0.01 to 0.5%,
Zr: 0.01 to 0.5%, B: 0.0005 to 0.01%, N: 0.001 to 0.15% [Group 2] Ca: 0.001 to 0.05%, La: 0.001 to 0.05%, Ce: 0.001 to 0.05
% 3. The high yield ratio martensitic stainless steel with low susceptibility to sulfide stress cracking according to claim 1, wherein said steel composition satisfies the following formula: F ₁ (Si, N) = 9 × (%) Si−25 × (%) N <1.3 F ₁ : δ-ferrite formation index (%) Si: Si weight% (%) N: N weight% 4. A hole made by billet having a steel composition according to any one of claims 1 to 3 after being heated to an annealing heating temperature range for a predetermined time and then adjusted or reheated to a drilling heating temperature range. After rolling and reducing the diameter, heating is performed to form a structure having austenite of at least 80% in area ratio, and then cooling to form a structure occupied by martensite in an area ratio of 80% or more. A method for producing a martensitic stainless steel pipe having a low sulfide stress cracking susceptibility and a high yield ratio, wherein the pipe is reheated in a temperature range not higher than the Ac ₁ transformation point where no austenite is formed. 5. The high-yield-ratio martensitic seamless material having low sulfide stress cracking susceptibility according to claim 4, wherein the steel composition, the annealing heating time of the billet or the ingot, and the piercing heating temperature are controlled so as to satisfy the following expressions. Manufacturing method of stainless steel pipe. F (Si, N, t, T) = 9 × (%) Si−25 × (%) N−0.13 × (Σt _i )
^1/2 −0.14 × (1200−T) ^1/2 <1.3 F: δ-ferrite formation index (%) Si: Si weight% (%) N: N weight% t _i : Annealing heating time in billet or ingot (i
Is the heat No.) Σ t _i : t ₁ + t ₂ + ... (total of all heating times) T: Perforation heating temperature Annealing heating temperature: 1200-1300 ° C Perforating heating temperature: 1200 ° C or less