CN1668768A - Martensitic stainless steel seamless pipe and manufacturing method thereof - Google Patents

Abstract

The present invention provides a martensitic stainless steel seamless pipe which has no inner surface defects and can suppress the occurrence of delayed fracture in an impact-worked portion, and a method of manufacturing the seamless steel pipe. A martensitic stainless steel seamless tube, characterized by consisting of: c: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O (oxygen): 0.0060% or less, at least one alloying element selected from 0.005 to 0.200% of V, Nb and Ti and 0.0005 to 0.0100% of B, and the balance Fe and impurities, satisfying the following inequalities (1), (2), (4) and (5) or the following inequalities (1), (3), (4) and (5): c^*+10N^*≤0.45，(1)；H1≤－0.003(C^*+10N^*)+0.0016，(2)；H2≤－0.0018(C^*+10N^*)+0.00096，(3)；Cr^*≤9.0，(4)；S≤0.088N^*+0.00056，(5)。

Description

Martensitic stainless steel seamless pipe and manufacturing method thereof

technical field

The present invention relates to a martensitic stainless steel seamless pipe, such as a steel pipe for oil wells, which can ensure that cracks by delayed fracture do not occur. The present invention also relates to a method for preparing a martensitic stainless steel pipe free from any internal surface defects such as internal scarring.

Background technique

Martensitic stainless steel pipes such as API-13%Cr used as oil well conduits generally include about 0.2% carbon content, which requires a high yield strength of 80 ksi (552 MPa) or more and hot workability. Due to the high C and Cr content, the stainless steel pipe after rolling has very high hardness, and thus the toughness is reduced. Therefore, the conventional martensitic stainless steel pipe after rolling may have cracks generated by hysteresis fracture in the "impact-worked portion" subjected to impact-load or static-load working before heat treatment. Therefore, it is necessary to limit the plate height within the "stand" and/or the drop into the steel pipe support during transport or storage. Also, it is necessary to shorten the idle time before heat treatment after hot rolling.

The above limitations during transport or storage can lead to various drawbacks such as large stockyards, because of plate stack height and/or tube drop limitations, reduced work efficiency due to careful handling of stainless steel tubes without excessive load shocks , the limited schedule from hot rolling to heat treatment is reduced in order to complete heat treatment within a limited working cycle.

Japanese Patent Unexamined Application H8-120415 discloses a martensitic stainless steel with a limited N content. In this patent specification, only the improvement of toughness after heat treatment is described. However, neither the relationship between the N content in the impact-worked portion of the rolled stainless steel pipe and the hysteresis fracture nor the measures for suppressing inner surface defects such as internal scars due to poor hot workability due to increased N content are described. . It is not practical to make a seamless steel pipe that does nothing to suppress internal scarring.

Japanese Patent Unexamined Application H6-306551 discloses an invention in which hydrogen content is limited to improve toughness in a heat-affected area by welding martensitic stainless steel pipes having a low carbon content. Furthermore, Japanese Patent Unexamined Application H5-255734 describes an invention of dehydrogenating a martensitic stainless steel pipe having a low carbon content to prevent hysteresis fracture. These inventions relate to martensitic stainless steels with low carbon content. However, no description is given on the relationship between the hydrogen content and delayed fracture in the shock worked portion of the as-rolled martensitic stainless steel pipe containing such a high C as about 0.2%.

Contents of the invention

A first object of the present invention is to provide a martensitic stainless steel pipe containing about 0.2% C, which suppresses delayed fracture of impact-worked portions before post-rolling heat treatment and does not generate internal scars.

A second object of the present invention is to provide a method for producing a martensitic stainless steel pipe that suppresses hysteresis fracture of impact-worked portions before post-rolling heat treatment without generating internal scars.

The present inventors achieved the first object by restricting the relationship of the contents of C (carbon), H (hydrogen), N (nitrogen) and S (sulfur) in addition to appropriately specifying the contents of various elements in steel.

Furthermore, the present invention achieves the second object by specifying the conditions for rolling steel pipes.

The present invention is characterized by the following martensitic stainless steel (A) and the following method (B) for producing martensitic stainless steel. In this specification, "%" means "mass%" with respect to the content of each element. In addition, "rolled steel pipe" means a steel pipe formed by hot rolling and not subjected to heat treatment.

(A) A martensitic stainless steel seamless pipe, characterized in that it consists of the following: by mass %, C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00 ~14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O (oxygen): 0.0060% or less, Al: 0-0.1%, Ni: 0-0.5%, Cu : 0 to 0.25%, Ca: 0 to 0.0050% and at least one alloying element selected from at least one group of those mentioned below (if two or more of these alloying elements are contained, a total of 0.005 to 0.200 mass%), and balance Fe and impurities:

The first group: 0.005-0.200% by mass of V, Nb and Ti, respectively,

The second group: 0.0005 to 0.0100% by mass of B, and is further characterized by satisfying the following inequalities (1), (2), (4) and (5) or the following inequalities (1), (3), ( 4) and (5):

C ^* +10N ^* ≤0.45, (1)

H1≤-0.003(C ^* +10N ^* )+0.0016, (2)

H2≤-0.0018(C ^* +10N ^* )+0.00096, (3)

Cr ^* ≤9.0, (4)

S≤0.088N ^* +0.00056, (5) wherein C ^* is the effective solute carbon content (mass %) defined by the following equation (6), N ^* is the effective solute nitrogen content (mass %) defined by the equation (7) , and Cr ^* is the Cr equivalent defined by the equation (8), H1 of the inequality (2) is the residual hydrogen (mass %) in the steel pipe after rolling, and H2 of the inequality (3) is the hydrogen in the steel pipe after heat treatment The amount of residual hydrogen (mass %), and the element symbol in each equation or inequality is the content (mass %) of the corresponding element:

C ^* ＝C-[12{(Cr/52)×(6/23)}/10], (6)

N ^* ＝N-[14{(V/51)+(Nb/93)}/10]

-[14{(Ti/48)+(B/11)+(Al/27)}/2], (7)

Cr ^* = Cr+4Si-(22C+0.5Mn+1.5Ni+30N) (8).

In addition, steel pipes having a C content of 0.18 to 0.21%, a Si content of 0.20 to 0.35%, a Cr content of 12.40 to 13.10%, an S content of 0.003% or less, and a N content of 0.035% or less are preferred.

(B) A method for producing a martensitic stainless steel seamless pipe, characterized by piercing and rolling the stainless steel with an inclined roll type piercing mill under the condition that the following inequality (9) is satisfied. The stainless steel seamless pipe is composed of the following: by mass %, C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O (oxygen): 0.0060% or less, Al: 0-0.1%, Ni: 0-0.5%, Cu: 0-0.25%, Ca: 0-0.0050 % and at least one alloying element selected from at least one group of those mentioned below (if two or more of these alloying elements are contained, a total of 0.005 to 0.200 mass %), and the balance Fe and impurities:

The first group: 0.005-0.200% by mass of V, Nb and Ti, respectively,

The second group: 0.0005-0.0100% by mass of B,

And also satisfy all of the following inequalities (1), (4) and (5):

C ^* +10N ^* ≤0.45, (1)

Cr ^* ≤9.0, (4)

S≤0.088N ^* +0.00056, (5)

Cr ^* ＜0.00009(CA+FA) ³ -0.0035(CA+FA) ²

+0.0567(CA+FA)+8.0024 (9) where C ^* is the effective solute carbon content (mass %) defined by the following equation (6), and N ^* is the effective solute nitrogen content (mass %) defined by the equation (7) ), and Cr ^* is the Cr equivalent defined by equation (8), and CA (≥0°) and FA in inequality (9) represent cross angle (toe angle) and feed angle (feed angle) respectively, and each The element symbol in the equation or inequality is the content (mass %) of the corresponding element:

C ^* =C-[12{(Cr/52)×(6/23))/10], (6)

N ^* ＝N-[14{(V/51)+(Nb/93)}/10]

-[14{(Ti/48)+(B/11)+(Al/27)}/2], (7)

Cr ^* = Cr+4Si-(22C+0.5Mn+1.5Ni+30N) (8).

In addition, a stainless steel pipe having a C content of 0.18 to 0.21%, a Si content of 0.20 to 0.35%, a Cr content of 12.40 to 13.10%, an S content of 0.003% or less and a N content of 0.035% or less is preferable, And preferably the method for manufacturing martensitic stainless steel seamless pipe comprises the following steps (10) and (11) after piercing and rolling:

(10) soaking seamless pipes at a temperature of 920°C or higher,

(11) Perform hot rolling.

Brief description of the drawings

Figure 1 shows a graph of the relationship between cracks produced by delayed fracture and two parameters: available solute carbon content (C ^* ) and available solute nitrogen content (N ^* ).

Fig. 2 is a graph showing the relation between the residual hydrogen amount (H1) in the steel pipe after rolling and the residual hydrogen amount (H2) in the steel pipe after heat treatment.

Fig. 3 is a graph showing the relationship between cracks generated by delayed fracture and two parameters: "C ^* + 10N ^* " and the amount of residual hydrogen (H1) in the steel pipe after rolling.

Fig. 4 is a graph showing the relationship between cracks generated by delayed fracture and two parameters: "C ^* + 10N ^* " and the amount of residual hydrogen (H2) in the steel pipe after heat treatment.

Figure 5 is a graph of the appearance of internal scars in the correlation of available solute nitrogen content (N ^* ) and sulfur content.

FIG. 6 is a graph showing appearance of internal scars and external defects in the correlation of "crossing angle (CA)+feeding angle (FA)" and Cr equivalent (Cr ^* ).

Best Mode for Carrying Out the Invention

The present inventors consider that hysteresis cracking of impact-worked portions in martensitic stainless steel depends on solute C (carbon), solute N (nitrogen) and solute H (hydrogen), which are interstitial elements. The following many experiments and the following facts (a) to (d) are confirmed:

(a) The hysteresis fracture sensitivity in the impact-worked portion of the steel pipe after rolling depends on the amounts of solute C and solute N, particularly the amount of solute N.

(b) The amount of solute C strongly affects the mechanical strength after heat treatment, while the amount of solute N has little effect. However, N significantly reduces the delayed fracture resistance for the impact-worked portion of the steel pipe after rolling.

(c) When the N content is lowered in order to improve the hysteresis fracture resistance of the impact-worked portion of the steel pipe after rolling, the austenite structure becomes unstable at high temperature, and the austenite structure becomes unstable due to poor hot workability in the process of manufacturing the steel pipe Resulting in massive internal scarring. Therefore, scarring must be suppressed.

(d) To solve this problem, in order to minimize the amount of work strain in the material, the piercing angle (crossing angle) and feeding horn. Thus, the procedure prevents internal scarring.

Various conditions according to the present invention, such as the chemical composition of the steel pipe and the manufacturing method, will be explained in detail below.

1. Chemical composition of steel pipe

The chemical composition of the martensitic stainless steel pipe of the present invention is determined as follows:

C:

Together, C and N provide solution hardening of the steel pipe after rolling. In order to suppress hysteresis fracture of impact-worked portions by solid solution hardening, the C content should be 0.22% or less, and preferably 0.21% or less. However, such a reduction in the C content makes it difficult to obtain the intended mechanical strength after heat treatment. Also, an excessive reduction in C content results in scarring after fabrication into a steel pipe due to delta ferrite because C is an austenite forming element. Therefore, the C content should be 0.15% or more, and the effective solute C content should satisfy the above inequality (1). The reason for this will be explained later. Preferably the C content is 0.18% or more.

Si:

Si is added as an oxygen scavenger during steel manufacturing. A content of less than 0.1% does not exert an effect on oxygen removal, while more than 1.0% results in low toughness. Therefore, the content should be 0.1 to 1.0%. The preferred content is 0.75% or less to obtain high toughness. A more preferable content is 0.20 to 0.35%.

Mn:

Mn is an element effective for improving the mechanical strength of steel, and is added as an oxygen scavenger during the steel manufacturing process. In addition, it fixes S in steel by forming MnS, and imparts good hot workability. A content of less than 0.10% has no effect on hot workability, while exceeding 1.00% results in low toughness. Therefore, the content should be 0.1 to 1.0%. The Mn content is preferably 0.7% or less.

Cr:

Cr is a basic element for improving the corrosion resistance of steel. Specifically, the content higher than 12.00% can improve the corrosion resistance of plaques, but also greatly improve the corrosion resistance under _CO2 environment. On the other hand, since Cr is a ferrite-forming element, a Cr content higher than 14.00% tends to generate delta ferrite in a high-temperature process, resulting in reduced hot workability. Also, an excessive Cr content leads to high production costs. Therefore, the content should be 12.00% to 14.00%, more preferably 12.40% to 13.10%.

P:

P is an impurity contained in steel. Excessive P content leads to low toughness of the product after heat treatment. The allowable upper limit of the P content should be 0.020%. It is preferred that the P content is minimized to as little as possible.

S:

Since S is an impurity that degrades hot workability, the S content should be minimized. The allowable upper limit of the S content is 0.010%. The S content should satisfy the above inequality (5). The S content is preferably 0.003% or less.

N:

N is an austenite stabilizing element that improves the hot workability of steel. However, N causes delayed fracture of the impact-worked portion of the steel pipe after rolling. Therefore, the upper limit of the N content should be 0.05%. The decrease in hot workability caused by reducing the N content is compensated by other elements, so the N content should be minimized. The N content is preferably 0.035% or less.

O (oxygen):

If oxygen is not completely removed during the steel manufacturing process, the number of cracks or streaks on the surface of the slab increases, and external scarring occurs in hot-rolled steel. Therefore, the O content should be minimized to 0.0060% or less.

V, Ti, Nb and B:

These elements combine with N to form nitrides. The inclusion of more than one element selected from these elements reduces the solubility amount of the solute N if the N content is reduced. However, an excessive N content leads to very high hardness through nitrides formed after heat treatment, and leads to a decrease in corrosion resistance and toughness. Therefore, the V, Ti or Nb content should be 0.005-0.200%, respectively, and the B content should be 0.0005-0.0100%. If two or more of these alloying elements are included, the total content of these elements should be 0.005 to 0.20%.

Al, Ni, Cu and Ca

These elements can be included if desired. A value of "0" in the content for one of these elements indicates that the element was not intentionally added to the steel.

Al:

Al can be used to remove oxygen during the steel manufacturing process, and can effectively suppress external scarring in steel pipes. However, an excessive Al content causes a decrease in the cleanliness of the steel and also causes clogging of submerged nozzles during continuous casting. Therefore, the Al content is preferably 0 to 0.1%.

Ni:

Ni is an austenite stabilizing element and improves hot workability of steel. However, an excessive Ni content causes a decrease in the resistance to sulfide stress corrosion cracking. Therefore, the Ni content is preferably 0 to 0.5%.

Cu:

Cu is effective for improving corrosion resistance and is an austenite stabilizing element for improving hot workability of steel. However, Cu has a low melting point, and an excessive Cu content causes a decrease in hot workability. Therefore, the Cu content is preferably 0 to 0.25%.

Ca:

Ca combines with S in steel and prevents segregation of sulfur in grain boundaries, which can lead to a decrease in hot workability. However, an excessive Ca content causes large streaks. Therefore, the Ca content is preferably 0 to 0.0050%.

2. About inequalities (1)～(5)

First, inequality (1) is described. In order to suppress cracks at impact-worked portions, it is necessary to improve hysteresis fracture resistance. Interstitial elements such as C and N increase the mechanical strength of the steel, but degrade the hysteresis fracture resistance of impact-worked parts. In the steel pipe after rolling, there is residual stress caused by hot rolling through a sizing mill or a tension reducer, which can reduce the hysteresis fracture resistance more.

The present inventors studied the effects of C and N on the delayed fracture of the impact processed portion of API-13%Cr steel pipe after rolling. In the hysteresis fracture test, an impact load is applied to the steel pipe, and the conditions of the impact load will be described in "Examples". The results are shown in Figure 1 and Tables 1-4, where available solute carbon (C ^* ) and available solute nitrogen (N ^* ) were used. The reasons for using C ^* and N ^* are described below.

Some C atoms combine with Cr atoms to form carbides. The content of C as an interstitial element can be obtained by subtracting the C content in carbides from the total C content. Therefore, the effective solute carbon content (C ^* ) is defined by equation (6).

Similarly, some N atoms combine with V, Nb, Ti, B, and Al atoms to form nitrides. The content of N as an intermittent element can be obtained by subtracting the N content in nitrides from the total N content. Therefore, the effective solute nitrogen content (N ^* ) is defined by equation (7). In equation (7), the coefficient is 1/10 for Nb and V nitrides due to the lower precipitation temperature, and 1/2 for Ti, B and Al nitrides due to the higher due to the high precipitation temperature.

Both C and N are interstitial elements in steel. If their contents are the same, their effects on mechanical strength and hardness are approximately the same. However, the C content in the 13% Cr martensitic stainless steel seamless pipe specified in the API-L80 grade, which is used for oil wells, is limited within 0.18~0.21%. On the contrary, if the N content is limited only with "0.1% or less", then the N content has wide selectivity. Usually, the N content is 0.01-0.05%, which is less than one-tenth of the C content. Therefore, the properties of the steel were studied by the relationship between the effective solute carbon content (C ^* ) and the 10-fold effective solute nitrogen content (N ^* ).

As shown in FIG. 1, the hysteresis fracture (crack) at the impact worked portion decreases as the contents of both C ^* and N ^* decrease. Inequality (1) above is determined by applying linear interpolation to the result.

Intermittent elements such as C and N affect work hardening due to cold working when the steel pipe is subjected to impact working. Specifically, N provides dislocation pinning to enhance work hardening. From the experimental results, the present inventors found that when the amount of "C ^* +10N ^* " was limited to 0.45 or less, work hardening and delayed fracture due to hydrogen were significantly suppressed.

Delayed fracture of impact processed parts is influenced by hydrogen content and part hardening. The effective solute carbon content (C ^* ) and the effective solute nitrogen content (N ^* ), and thus the hardness, must be reduced to suppress cracking. When steel is work hardened by cold working due to treatment shock, hydrogen cracking occurs even if the initial hardness is low. Therefore, the residual hydrogen content in the steel pipe should be reduced to prevent hydrogen cracking.

The amount of residual hydrogen in the steel pipe after rolling is different from that after heat-treating the steel pipe. In 13%Cr steel, there is a correlation between the residual hydrogen amount in the steel pipe after rolling and the residual hydrogen amount after heat treatment of the steel pipe because the heat treatment temperature is substantially fixed. The quenching temperature is 920-980°C, and the tempering temperature is 650-750°C.

Fig. 2 is a graph showing the relationship between the amount of residual hydrogen between H1 (after rolling) and H2 (after heat treatment) in 13% Cr steel pipes used in the following examples. For example, at the point marked by "a", the residual hydrogen amount (H1) of the steel pipe after rolling is about 3 ppm, and the residual hydrogen amount (H2) after heat treatment is about 2 ppm.

The above inequality (2) limits the relationship between "C ^* +10N ^* " and H1, and the above inequality (3) limits the relationship between "C ^* +10N ^* " and H2. As mentioned above, the increased amount of C ^* and N ^* causes an increase in strength and a decrease in toughness, and then increases the susceptibility to hysteresis fracture due to hydrogen in the shock-worked portion. As a result, in order to suppress hysteresis fracture, the total relationship of the content of C ^* and N ^* and the amount of residual hydrogen must be considered.

Fig. 3 shows the results obtained by studying the hysteresis fracture sensitivity of the impact-worked portion of steel pipes after rolling of 13% Cr martensitic stainless steel with a C content of 0.19% and plotting the results on "C ^* + 10N ^* " and the relationship between H1. Figure 4 shows similar findings and plots "C ^* +10N ^* " versus H2 after heat treatment. These results are obtained in the Examples below.

From the illustrations in Fig. 3 and Fig. 4, it can be recognized that if the above inequalities (1) and the following inequalities (2) and (3) are satisfied, the hysteresis fracture (crack) in the impact processed part will no longer occur, where H1 is the residual hydrogen in the steel pipe after rolling, and H2 is the residual hydrogen after heat treatment:

H1≤-0.003(C ^* +10N ^* )+0.0016, (2)

H2≤-0.0018(C ^* +10N ^* )+0.00096, (3)

On the other hand, the following inequalities (4) and (5) represent the ranges of Cr and S contents effective for suppressing internal surface defects called internal scars. Satisfying the above inequalities (2) and (3) can suppress delayed fracture in steel pipes after rolling and impact worked portions after heat treatment. However, there is still the possibility of internal scarring during the steel pipe manufacturing process.

The generation of internal scars is caused by the circumferential shear deformation during the piercing and rolling process of the piercing and rolling mill. In slabs, shear stresses cause cracks at portions with different deformation resistance due to ferrite/austenite grain boundaries, sulfur precipitation and inclusions. These cracks deform during rolling and cause internal scarring.

To suppress cracking in ferrite/austenite grain boundaries, the content of delta-ferrite should be minimized. The content of delta-ferrite depends on the Cr equivalent (Cr ^* ), and in fact an increase in Cr ^* leads to an increase in ferrite. Cr ^* can be represented by the following equation (8), which expresses the linear relationship between ferrite-forming elements and austenite-forming elements:

Cr ^* ＝Cr+4Si-(22C+0.5Mn+1.5Ni+30N) (8)

As can be seen in equation (8), N makes a large contribution to Cr ^* . When the N content is reduced to improve the toughness of the steel pipe after rolling, the Cr equivalent increases and the amount of ferrite increases, which leads to internal scarring. Due to these conditions, the following inequality (4) is satisfied to suppress ferrite and internal scarring:

Cr ^* ≤9.0 (4)

The sulfur-precipitated portion also becomes a cause of cracks. In order to suppress this precipitation, it is therefore necessary to minimize the sulfur content. For this reason, the S content should be 0.010% or less, preferably 0.003% or less. In order to reduce inclusions, large streaks, and S content in steel during steel manufacturing, it is preferable that the oxygen (O) content is 0.0060% or less.

When N ^* is lowered to suppress cracks in order to satisfy Inequality (1), Cr ^* represented by Equation (8) increases. This causes an increase in the ferrite phase leading to a decrease in hot workability. In order to restore hot workability, the S content should be reduced.

Fig. 5 is a graph showing the occurrence of internal scars below 2% (indicated by symbol ○) or not less than 2% (indicated by symbol X) in the relation of N ^* on the abscissa and S content on the ordinate. This figure leads to the recognition that limiting the S content suppresses internal scarring by the following inequality (5). Considering the work efficiency of uninterrupted processing, the standard line is determined to be 2% of internal scars:

S≤0.088N ^* +0.00056, (5)

3. About the manufacturing method

In the method of manufacturing the seamless steel pipe of the present invention, the steel having the above-mentioned chemical composition and satisfying the inequalities (1), (4) and (5) is passed through a cross-roller type piercing mill under the conditions restricted by the inequalities (9). With the help of piercing and rolling.

In order to suppress internal scarring during piercing and rolling, it is important to select appropriate rolling conditions in consideration of the hot workability of the rolled steel.

Various factors contribute to the development of internal scarring. Among these factors, the feed angle and crossing angle of the main rolls in the piercing mill play an essential role. In general, increasing the feed angle and crossing angle reduces additional shear deformation during piercing and rolling, and steel can be rolled without cracking even if the steel has poor hot workability.

However, the feed and intersection angles cannot always be easily increased. To obtain these angle increases, the main motor needs to be replaced and even the rolling mill needs to be replaced. If the steel has suitable hot workability during rolling, relatively small feed and cross angles can be selected. Considering the economics of manufacturing, the relationship between the index on hot workability during rolling and the index on the suppression of internal scarring, i.e. additional shear deformation, can lead to the best possible preparation conditions for the design of the steel material as well as the piercing and rolling conditions .

The present inventors have studied the experimental data of studying the influence of feeding angle and crossing angle on additional shear deformation in the past, and further studied the relationship between Cr ^* and the sum of "CA (crossing angle) + FA (feeding angle)" Relationship. As a result, a definite correlation between Cr ^* and "CA+FA" was found on the basis that the feed angle and cross angle contribute to the additional shear stress to the same extent.

Fig. 6 shows internal scars and external defects of less than 2% (indicated by ○) or not less than 2% (indicated by symbol ●) in the correlation of abscissa "CA+FA" and ordinate Cr ^* Appearance diagram. This figure leads to consider that the boundary line whether internal scars or external defects is less than 2% (indicated by ○) or not less than 2% (indicated by symbol ●) can be represented by a cubic curve. Satisfying the condition of the following inequality (9) results in suppression of internal scarring.

Cr ^* ＜0.00009(CA+FA) ³ -0.0035(CA+FA) ²

+0.0567(C.A.+F.A.)+8.0024 (9)

The right side of the inequality (9) is determined by interpolating the obtained data, and represents the above boundary.

The manufacturing method of the present invention may include a reheating process prior to finish rolling using a tension reducer. In this case, it is preferable that the soaking treatment during reheating is kept at 920° C. or above. During reheating, lowering the soaking temperature causes a reduction in the toughness of the rolled steel in the T direction perpendicular to the rolling direction, due to incomplete recrystallization of flat grains formed during processing. Furthermore, C and N-rich regions are created around Nb and/or V carbides/nitrides due to incomplete solid solution or diffusion of carbides and/or nitrides. Hardening and brittleness then develop in this region, which leads to delayed fracture. Preferably, the lower limit of the soaking temperature during reheating is 920°C, or more preferably 1000°C, while the preferred upper limit of the soaking temperature is about 1100°C.

Example

Seamless pipes having an outer diameter of 60.3 mm and a thickness of 4.83 mm were produced from 43 kinds of steels having the chemical compositions shown in Tables 1 and 2. Then, these steel pipes were tested downwards.

(1) Hysteresis fracture test

Drop weight specimens with a length of 250 mm were prepared from rolled steel pipes. A gravimetric test part with a weight of 150 kg and a curvature of 90 mm at the tip is dropped from a height of 0.2 m onto the specimen, which deforms under the impact load (294J). After one week, each sample was tested for cracks. Crack inspection was performed visually, and also by ultrasonic testing (UST). The results obtained are listed in Table 3 and Table 4.

FIG. 1 shows a graph of the relationship between the generated cracks and the effective solute carbon content (C ^* ) and effective solute nitrogen content (N ^* ). As shown in the figure, a straight line "a" indicates a boundary where a crack is generated. The straight line "a" can be represented by "C ^* +10N ^* "=0.45". Therefore, the condition that no hysteresis fracture occurs is represented by "C ^* +10N ^* "≤0.45".

(2) Determination of residual hydrogen (H1 and H2)

The amount of residual hydrogen in steel pipes after rolling and the amount of residual hydrogen in the same steel pipes after heat treatment were measured using the analysis method specified in JIS Z2614. In the heat treatment, the samples were water quenched at 950°C and then at 700°C. The test results are listed in Table 3 and Table 4.

Fig. 2 is a graph showing the relationship between H1 and H2 of the sample. It can be confirmed that there is a linear relationship that can be approximately represented by "H2=0.6H1".

(3) Hysteresis fracture and the relationship between three parameters: C ^* , N ^* and residual hydrogen

The data listed in Table 3 and Table 4 for whether hysteresis fracture occurs are respectively shown in Figure 3 for steel pipes after rolling and Figure 4 for steel pipes after heat treatment, and the abscissa in the figure represents "C ^* +10N ^* ", And the ordinate represents the amount of residual hydrogen. The straight lines of whether or not crack boundaries are generated are represented by the following equations (2)-1 and (3)-1, respectively. Therefore, the condition that hysteresis fracture does not occur satisfies the above-mentioned inequality (2) or (3). Also, even if the inequalities (2) and (3) are satisfied, hysteresis fracture may occur when "C ^* +10N ^* " is higher than 0.45. Then, the above inequality (1) should be satisfied.

H1＝-0.003(C ^* +10N ^* )+0.0016 (2)-1

H2=-0.0018(C ^* +10N ^* )+0.00096(3)-1.

Table 1 serial number Chemical composition (remainder: Fe and impurities, mass %) C Si mn P S Cr Ni Cu V al N Nb Ti B Ca o 12345678910111213141516171819202122232425 0.190.180.190.210.200.190.200.210.200.190.180.200.180.200.200.210.200.210.180.190.180.200.200.200.21 0.270.290.280.290.310.280.240.270.300.260.270.290.310.300.280.230.270.290.250.270.280.280.270.220.25 0.850.890.820.760.720.910.940.880.760.770.820.840.790.830.870.840.780.860.880.760.910.840.890.920.89 0.0140.0150.0180.0170.0160.0190.0140.0180.0170.0140.0170.0180.0130.0160.0180.0170.0180.0170.0160.0160.0180.0140.0180.0140.017 0.0010.0020.0030.0010.0020.0010.0010.0020.0010.0030.0010.0020.0010.0030.0020.0010.0020.0010.0020.0010.0020.001 12.8012.7012.9012.6012.6012.8012.9013.1012.8013.0012.8012.4012.7012.8012.8012.5012.8012.9012.6012.8012.7012.9012.6012.7013.00 0.080.140.120.070.340.210.090.340.450.210.260.350.210.270.120.080.040.050.030.060.090.140.1 90.070.33 0.040.020.060.020.110.140.150.120.050.030.020.010.060.130.210.160.220.140.060.050.020.030.120.140.13 0.0400.0300.0800.0400.0900.0600.1100.0800.1600.0900.0500.1200.1100.0800.0900.0700.1300.0300.0200.0400.0300.0500.0500.1000.090 0.00130.00220.00190.00080.00140.023002400.0300.0015001500.00900.00200.00300.00700.00700.0018001600.0222200.0110110 0.0350.0340.0280.0280.0220.0330.0210.0210.0270.0300.0280.0390.0410.0370.0390.0370.0390.0410.0440.0440.0450.0430.0320.0330.021 0.0010.0020.0050.0030.0060.0040.0030.0050.0080.0090.0020.0010.0010.0030.010.0150.0160.0060.0090.0040.0060.0020.0020.0040.004 0.0030.0020.0040.0010.0030.0020.0010.0010.0040.0030.0040.0030.0020.0010.0040.0030.0020.0060.0050.0020.0010.0020.0010.0020.002 0.00020.00010.00030.00040.00020.00050.00060.00070.00020.00060.00030.00030.00070.00020.00070.00040.00010.00030.00010.00040.00050.00055 0.00020.00030.00050.00060.00080.00210.00170.00180.00340.00320.00160.00160.00180.00150.00190.00060.00170.00390.00250.00190.002020 0.00200.00300.00500.00400.00200.00100.00300.00300.00300.00300.00300.00300.00200.00100.00300300.00200.00200200.00200.00300.00550

Table 2 serial number Chemical composition (remainder: Fe and impurities, mass %) C Si mn P S Cr Ni Cu V Al N Nb Ti B Ca o 262728293031323334353637383940414243 0.200.200.200.210.200.180.180.200.160.180.190.210.210.190.180.180.180.17 0.280.250.270.270.250.240.280.260.210.270.290.310.270.220.250.260.260.26 0.790.750.870.880.860.780.920.850.520.380.780.850.880.860.770.740.730.44 0.0150.0120.0170.0160.0150.0160.0170.0130.0140.0170.0180.0220.0130.0130.0140.0120.0110.012 0.0020.0010.0020.0010.0020.0020.0020.0010.0010.0020.0040.0060.0030.0010.0010.0010.0010.001 12.9012.9012.6012.8012.5012.6012.5012.8013.1012.9012.8012.7013.0012.9012.7012.9013.1013.20 0.420.200.230.070.040.050.070.120.040.050.180.060.140.080.140.190.120.11 0.040.050.040.120.070.040.050.040.040.030.050.040.030.130.020.020.020.06 0.1500.0800.0400.0200.0400.0500.0400.0600.0800.0400.0200.0300.0800.1400.0700.1100.0900.070 0.00400.01400.01200.01300.00800.01500.01800.01700.00180.01500.01300.00300.01200.01400.02200.02000.02900.0022 0.0260.0290.0270.0380.0430.0450.0470.0420.0290.0200.0310.0410.0320.0310.0250.0270.0280.020 0.0070.0060.0040.0030.0070.0030.0050.0030.0020.0040.0070.0050.0040.0030.0030.0020.0030.004 0.0030.0030.0040.0050.0060.0030.0020.0040.0030.0040.0070.0020.0010.0020.0050.0030.0050.002 0.00030.00070.00040.00050.00020.00020.00010.00030.00030.00050.00070.00020.00040.00020.00020.00020.00040.0003 0.00320.00280.00140.00040.00130.00250.00310.00220.00110.00080.00210.00180.00090.00120.00190.00240.00290.0028 0.00200.00300.00100.00500.00300.00200.00100.00400.00300.00400.00500.00200.00800.00200.00200.00200.00300.0040

table 3 serial number C ^* N ^* C ^* +10N ^* ① Residual hydrogen after rolling (H1) ② ③ Residual hydrogen (H2) after heat treatment ④ ⑤ hysteresis fracture Cr ^* ⑥ ⑦ ⑧ internal scar external scar evaluate 12345678910111213141516171819202122232425 0.1130.1040.1120.1340.1240.1130.1220.1310.1230.1120.1030.1250.1040.1230.1230.1350.1230.1320.1040.1130.1040.1220.1240.1240.132 0.03300.03220.02450.02620.01850.02470.015001500.022702190.03803620.034903280370.03702488848888888840240240240240240240240240248024 0.4430.4260.3570.3970.3090.3600.2340.2810.3330.3390.3220.4530.4650.4590.4720.4510.4510.4950.5110.4900.5000.4960.3720.3620.282 ○○○○○○○○○○○○××××××××××××○○○ 0.000050.000070.0050.00005050.000250.000270.000300.000200.000100.000200.000220.000050.0000.000170.000190.000240.00000080808.0090.00700700700700700707070706070607060.00700706 0.000270.000320320.000530.000410.000670.000520.000900.000760.000600.000580.000240200.000220.000180.00025070.000130.00010.000110.000110.000110.00010.0001010101010101.00010101010138.000100100130130001000100010001001k00130 that000 that ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × ○ 0.0000300.00400400400400.0000300.0001400.0001600.0001900.0001100.00400900.0001300.0001200.00400.00900900.00800800680370.0077700.0050027002700270027003800380038003800380038388002738383800530027005200200200200200200700700700700700 -. 0.0001630.0001940.0003170.0002460.0004030.0003120.0005380.0004530.0003610.0003510.0003800.0001440.0001220.0001330.0001110.0001480.0001480.0000700.0000400.0000780.0000600.0000670.0002910.0003080.000453 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × ○ ○○○○○○○○○○○○×××××××××××○×○ 8.1058.2258.4107.8157.9107.9808.2257.9807.7358.2608.2807.0458.0407.6707.7357.1507.8607.7057.8357.9107.9207.7007.5907.6257.810 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ 0.0034630.0033950.0027130.0028700.0021890.0027350.0015440.0018820.0024080.0025560.0024900.0034460.0037440.0035210.0036300.0033440.0034490.0037480.0041420.0038770.0040510.0038470.0027380.0026620.001881 ○○×○○○○×○×○○○○○○○○○○○○○○○○ ○○×○○○○×○×○○○○○○○○○○○○○○○○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○○×○○○○×○×○×××××××××××○×○ No. 1, 2, 4-7, 9, 11, 23 and 25: No. 3, 8, 10, 12-22 and 24 of the present invention: comparison ①: whether inequality (1) is satisfied (○) or not (×) ②: Calculated value on the right side of inequality (2) ③: Whether inequality (2) is satisfied (○) or not (×) ④: Calculated value on the right side of inequality (3) ⑤: Whether inequality (3) is satisfied (○) or not Satisfy (×) ⑥: Whether the inequality (4) is satisfied (○) or not (×) ⑦: The calculated value on the right side of the inequality (5) ⑧: Whether the inequality (5) is satisfied (○) or not (×)

Table 4 serial number C ^* N ^* C ^* +10N ^* ① Residual hydrogen after rolling (H1) ② ③ Residual hydrogen (H2) after heat treatment ④ ⑤ hysteresis fracture Cr ^* ⑥ ⑦ ⑧ external scar internal scar evaluate 262728293031323334353637383940414243 0.1220.1220.1240.1330.1250.1040.1050.1230.0810.1020.1130.1340.1320.1120.1040.1020.1010.091 0.02010.02220.02190.03300.03870.03910.04080.03510.02570.01410.02550.03890.02620.02310.01650.01820.01700.0170 0.3230.3440.3430.4630.5120.4950.5130.4740.3380.2430.3680.5230.3940.3430.2680.2840.2710.260 ○○○×××××○○○×○○○○○○ 0.000700.000600.000650.000200.000050.000050.000050.000100.000050.000110.000080.000240.000120.000040.000080.000190.000140.0000 0.000630.000570.000570.000210.000060.000110.000060.000180.000590.000870.000500.000030.000420.000570.000800.000750.000790.00082 ×××○○○○○○○○×○○○○○○ 0.0004100.0003500.0004100.0001000.0000200.0000300.0000360.0000600.0000100.0000700.0000400.0001500.0000600.0000100.00004000000.00001 0.0003780.0003400.0003420.0001270.0000380.0000680.0000370.0001060.0003520.0005230.0002980.0000190.0002510.0003430.00070480.00002 ×××○○○○○○○○×○○○○○○ ××××××××○○○×○○○○○○ 7.8157.9557.6907.5757.3207.7857.6857.5759.2309.1558.1907.5757.8508.1208.3958.5158.7959.515 ○○○○○○○○××○○○○○○○× 0.0023300.0025140.0024870.0034630.0039670.0040030.0041510.0036510.0028200.0017970.0028050.0039840.0028680.0025900.50201002.40002 ○○○○○○○○○××××○○○○○ ○○○○○○○○×××××○○○○× ○○○○○○○○○○○○○×○○○○○ ×××××××××××××○○○○× No. 39-42: No. 26-38 and 43 of the present invention: comparison ①: whether the inequality (1) is satisfied (○) or not (×) ②: the calculated value on the right side of the inequality (2) ③: whether the inequality (2) is satisfied (○) or not (×) ④: Calculated value on the right side of inequality (3) ⑤: Whether inequality (3) is satisfied (○) or (×) not satisfied ⑥: Whether inequality (4) is satisfied (○) or (× ) does not satisfy ⑦: the calculated value on the right side of inequality (5) ⑧: whether inequality (5) is satisfied (○) or (×) is not satisfied

(4) Internal scar detection

By selecting several steels with various effective solute N and sulfur contents in Tables 1 and 2, 500 steel pipes were prepared under the condition of "C.A.+F.A."=9, and whether internal scars were produced was detected. The results are shown in FIG. 5 . The sloped line indicates whether internal scarring was below or above the 2% border. It can be represented by the following equation (5)-1. Therefore, internal scarring can be suppressed by satisfying the above inequality (5).

S＝0.088N ^* +0.00056 (5)-1

By selecting several steels in Tables 1 and 2, 50 steel pipes were prepared from billets under the following conditions with different Cr equivalents (Cr ^* ) listed in Table 5, and then tested to determine whether internal scarring occurred:

(1) Billet heating temperature: 1200～1250℃

(2) The reduction rate of billet diameter at the tip of the plug: 5.0 to 8.0%

(3) C.A.+F.A.: 10, 17, 21 and 30

Table 5 shows the relationship between internal scarring and two parameters: Cr ^* and "CA+FA". In Table 5 and Figure 6, the ○ symbol indicates that both internal and external scars are below 2%, while the symbol ● indicates that both internal and external scars are above 2%.

Figure 6 is a line graph of the results using the parameters in Table 5: "CA+FA" and Cr ^* . The cubic curve in this figure is represented by Equation (9)-1 below. Therefore, the condition for suppressing the generation of internal scars is to satisfy the above-mentioned inequality (9).

Cr ^* = 0.00009(CA+FA) ³ -0.0035(CA+FA) ²

+0.0567(C.A.+F.A.)+8.0024 (9)-1

table 5 serial number Cr ^* CA+FA 10 17 twenty one 30 9 7.735 ○ ○ ○ ○ 4 7.815 ○ ○ ○ ○ 6 7.980 ○ ○ ○ 39 8.120 ○ ○ ○ ○ 7 8.225 ○ 11 8.280 ○ ○ ○ 40 8.395 ● ○ 41 8.515 ● ● ● ○ 42 8.795 ● ● ● ○ 35 9.155 ● ● ○ 34 9.230 ● ● 43 9.515 ●

Industrial applicability

When the 13% Cr martensitic seamless steel pipe of the present invention is subjected to impact cold working during the treatment after the pipe is manufactured, it can prevent hysteresis fracture. The steel pipe has excellent corrosion resistance and is especially suitable for oil wells. According to the manufacturing method of the present invention, a 13% Cr martensitic seamless steel pipe without internal scars can be manufactured.

Claims (5)

1. A martensitic stainless steel seamless pipe, characterized in that it consists of the following: by mass %, C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00- 14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O (oxygen): 0.0060% or less, Al: 0-0.1%, Ni: 0-0.5%, Cu: 0 to 0.25%, Ca: 0 to 0.0050% and at least one element selected from at least one group mentioned below (if two or more of these elements are contained, 0.005 to 0.200% by mass in total), and Balance Fe and impurities: The first group: 0.005-0.200% by mass of V, Nb and Ti, respectively, The second group: 0.0005-0.0100% by mass of B, And it is also characterized in that the following inequalities (1), (2), (4) and (5) or the following inequalities (1), (3), (4) and (5) are satisfied: C ^* +10N ^* ≤0.45, (1) H1≤-0.003(C ^* +10N ^* )+0.0016, (2) H2≤-0.0018(C ^* +10N ^* )+0.00096, (3) Cr ^* ≤9.0, (4) S≤0.088N ^* +0.00056, (5) where C ^* is the effective solute carbon content (mass %) defined by the following equation (6), N ^* is the effective solute nitrogen content (mass %) defined by the equation (7), and Cr ^* is the effective solute nitrogen content (mass %) defined by the equation (8) Cr equivalent, H1 of inequality (2) is the residual hydrogen (mass %) in the steel pipe after rolling, and H2 of inequality (3) is the residual hydrogen (mass %) in the steel pipe after heat treatment, and each The element symbol in the equation or inequality is the content (mass%) of the corresponding element: C ^* ＝C-[12{(Cr/52)×(6/23)}/10], (6) N ^* ＝N-[14{(V/51)+(Nb/93)}/10] -[14{(Ti/48)+(B/11)+(Al/27)}/2], (7) Cr ^* = Cr+4Si-(22C+0.5Mn+1.5Ni+30N) (8). 2. The martensitic stainless steel seamless pipe according to claim 1, wherein the C content is 0.18 to 0.21% by mass, the Si content is 0.20 to 0.35% by mass, the Cr content is 12.40 to 13.10% by mass, and the S content is 0.003% by mass % or less and the N content is 0.035% by mass or less. 3. A method for manufacturing a martensitic stainless steel seamless pipe, characterized in that the stainless steel is pierced and rolled using an inclined roll type piercing and rolling mill under the condition that the following inequality (9) is satisfied. The stainless steel seamless pipe is composed of the following: by mass %, C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, P: 0.020% or less, S: 0.010% or less, N: 0.05% or less, O (oxygen): 0.0060% or less, Al: 0-0.1%, Ni: 0-0.5%, Cu: 0-0.25%, Ca: 0-0.0050 % and at least one alloying element selected from at least one group mentioned below (if two or more of these alloying elements are contained, the total is 0.005 to 0.200% by mass), and the balance Fe and impurities: The first group: 0.005-0.200% by mass of V, Nb and Ti, respectively, The second group: 0.0005-0.0100% by mass of B, And it also satisfies all of the following inequalities (1), (4) and (5): C ^* +10N ^* ≤0.45, (1) Cr ^* ≤9.0, (4) S≤0.088N ^* +0.00056, (5) Cr ^* ＜0.00009(CA+FA) ³ -0.0035(CA+FA) ² +0.0567(C.A.+F.A.)+8.0024 (9) where C ^* is the effective solute carbon content (mass %) defined by the following equation (6), N ^* is the effective solute nitrogen content (mass %) defined by the equation (7), and Cr ^* is the effective solute nitrogen content (mass %) defined by the equation (8) Cr equivalent, CA (≥0°) and FA in inequality (9) represent crossing angle and feeding angle respectively, and the element symbol in each equation or inequality is the content (mass %) of corresponding element: C ^* ＝C-[12{(Cr/52)×(6/23)}/10], (6) N ^* ＝N-[14{(V/51)+(Nb/93)}/10] -[14{(Ti/48)+(B/11)+(Al/27)}/2], (7) Cr ^* = Cr+4Si-(22C+0.5Mn+1.5Ni+30N) (8). 4. The method for manufacturing a martensitic stainless steel seamless pipe as claimed in claim 3, wherein the C content is 0.18 to 0.21 mass%, the Si content is 0.20 to 0.35 mass%, and the Cr content is 12.40 to 13.10 mass%, The S content is 0.003% by mass or less and the N content is 0.035% by mass or less. 5. The method for manufacturing martensitic stainless steel seamless pipe as claimed in claim 3 or 4, characterized in that, the method also comprises the following steps (10) and (11) after piercing and rolling: (10) soaking seamless pipes at a temperature of 920°C or higher, (11) Perform hot rolling.

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