JP2021021087A - Manufacturing method of stainless steel tube - Google Patents

Abstract

To provide a manufacturing method of a stainless steel tube capable of appropriately removing a Cr-removal layer.SOLUTION: A manufacturing method of a stainless steel tube includes: a preparation step (step S1) of preparing an element tube having a prescribed chemical composition tempered by having been held at 540 to 710°C (temperature T1) for 5 to 180 minutes (time t1) and cooled; and an acid cleaning step (step S3) of immersing the element tube in an acid solution with a hydrogen-ion concentration C2 at temperature T2 for a duration of time t2. A Cr-removal index J and an acid cleaning index L calculated from expressions below satisfy a relation J≤L<J+230: J=(%Cr+%Cr1)×d1×0.5×1000; L=C2×(T2+273)2×t2/100000 [In each of the above expressions, %Cr is a Cr content (mass%) of the element tube and %Cr1 is a surface Cr concentration (mass%). A unit of T1 or T2 is °C, a unit of t1 is second, a unit of t2 is minute and a unit of C2 is mol/kg.].SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for manufacturing a stainless steel pipe.

Petroleum and natural gas produced from oil wells and gas wells contain corrosive gases such as carbon dioxide and hydrogen sulfide as accompanying gases. Martensitic stainless steel pipes containing about 13% by mass of Cr (hereinafter referred to as "13% Cr steel pipes") have an excellent balance between corrosion resistance and economic efficiency, and are widely used as steel pipes for oil wells and line pipes. (See, for example, JP-A-2015-161010, JP-A-2006-144069, JP-A-2010-242162, etc.).

Japanese Patent No. 3430661, Japanese Patent No. 3550996, and Japanese Patent No. 3915235 describe a method for pickling stainless steel.

JP 2015-161010 Japanese Unexamined Patent Publication No. 2006-144069 JP-A-2010-242162 Japanese Patent No. 3430661 Japanese Patent No. 3550996 Japanese Patent No. 3915235

The 13% Cr steel pipe is usually manufactured after undergoing a heat treatment such as quenching and tempering. At this time, Cr may be incorporated into the oxide scale formed by the heat treatment, and the Cr concentration near the surface of the steel pipe may decrease (hereinafter, the region where the Cr concentration has decreased is referred to as a “de-Cr layer”). If the de-Cr layer is present, the required stress corrosion cracking resistance (SCC resistance) may not be obtained.

An object of the present invention is to provide a method for producing a stainless steel pipe capable of appropriately removing a deCrected layer.

The method for producing a stainless steel pipe according to an embodiment of the present invention is, in terms of mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%. , P: 0.030% or less, S: 0.0020% or less, Cu: less than 0.50%, Cr: 11.50 to less than 14.00%, Ni: more than 5.00% to 7.00%, Mo: Over 1.00% to 3.00%, Ti: 0.02 to 0.50%, Al: 0.001 to 0.100%, Ca: 0.0001 to 0.0040%, N: 0. 0001 to less than 0.0200%, V: 0 to 0.500%, Nb: 0 to 0.500%, Co: 0 to 0.500%, balance: Fe and impurities having a chemical composition of 540 to 0 At a temperature T ₁ which is a temperature of 710 ° C., a preparatory step of preparing a tempered raw tube which is held for a time t ₁ which is a time of 5 to 180 minutes and then cooled, and a hydrogen ion concentration of the raw tube. It is provided with a pickling step of immersing in an acid solution at C ₂ and temperature T ₂ for time t ₂ . The de-Cr index J and the pickling index L calculated from the following formula satisfy the relationship of J ≦ L <J + 230.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
_{L = C 2 × (T 2} +273) 2 × t 2/100000
However,
% Cr ₁ =% Cr-% Cr ₂
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
d ₁ = (2 x D ₁ x t ₁ ) ^0.5
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
In each of the above equations, R is the gas constant, Q and A are the activation energies, and R = 8.31J / K, A = 5000J, and Q = 21000J are substituted. The contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively. The unit of T ₁ and T ₂ is ° C, the unit of t ₁ is seconds, the unit of t ₂ is minutes, and the unit of C ₂ is mol / kg.

The method for producing a stainless steel pipe according to an embodiment of the present invention is, in terms of mass%, C: 0.001 to 0.050%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%. , P: 0.030% or less, S: 0.0020% or less, Cu: less than 0.50%, Cr: 11.50 to less than 14.00%, Ni: more than 5.00% to 7.00%, Mo: Over 1.00% to 3.00%, Ti: 0.02 to 0.50%, Al: 0.001 to 0.100%, Ca: 0.0001 to 0.0040%, N: 0. It has a chemical composition of 0001 to less than 00200%, V: 0 to 0.500%, Nb: 0 to 0.500%, Co: 0 to 0.500%, balance: Fe and impurities, and is tempered. A preparatory step for preparing a raw pipe and a tempering step of holding the raw pipe at a temperature T ₁ having a temperature of 540 to 710 ° C. for a time t ₁ which is a time of 5 to 180 minutes and then cooling the raw pipe. And the pickling step of immersing the tempered raw pipe in an acid solution having a hydrogen ion concentration of C ₂ and a temperature of T _{2 for} a time t ₂ . The de-Cr index J and the pickling index L calculated from the following formula satisfy the relationship of J ≦ L <J + 230.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
_{L = C 2 × (T 2} +273) 2 × t 2/100000
However,
% Cr ₁ =% Cr-% Cr ₂
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
d ₁ = (2 x D ₁ x t ₁ ) ^0.5
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
In each of the above equations, R is the gas constant, Q and A are the activation energies, and R = 8.31J / K, A = 5000J, and Q = 21000J are substituted. The contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively. The unit of T ₁ and T ₂ is ° C, the unit of t ₁ is seconds, the unit of t ₂ is minutes, and the unit of C ₂ is mol / kg.

According to the present invention, a stainless steel pipe from which the Cr-depleted layer is appropriately removed can be obtained.

FIG. 1A is a diagram schematically showing the distribution of the concentration of alloying elements when the de-Cr layer is deep. FIG. 1B is a diagram schematically showing the distribution of the concentration of alloying elements when the de-Cr layer is shallow. FIG. 2 is a diagram schematically showing the distribution of Cr concentration near the surface of the steel pipe. FIG. 3 is a flow chart of a method for manufacturing a stainless steel pipe according to an embodiment of the present invention.

The present inventors have investigated a method for appropriately removing the Cr-decrypted layer, and obtained the following findings.

The Cr-de-Cred layer can be removed by dissolving the surface of the steel pipe by pickling. On the other hand, the state of the de-Cr layer differs depending on the heat treatment conditions (specifically, tempering conditions), and specific alloying elements (specifically, Cr, Mo, Ni, and Cu. Hereinafter, simply It also depends on the content of "alloy element"). Therefore, the appropriate pickling conditions differ depending on the heat treatment conditions and the chemical composition of the steel pipe. If the pickling is insufficient, the Cr-de-cracked layer remains, which causes a decrease in SCC resistance. On the other hand, if excessive pickling is carried out, in addition to lowering the production efficiency, problems such as unevenness in the surface texture of the steel pipe occur.

FIG. 1A is a diagram schematically showing the distribution of the concentration of alloying elements when the de-Cr layer is deep and FIG. 1B is a diagram when the de-Cr layer is shallow. In FIGS. 1A and 1B, the concentration of the alloying element is indicated by the color density (dot size), and the darker the color (larger the dot), the higher the concentration of the alloying element. As shown in FIGS. 1A and 1B, the concentration of the alloying element in the de-Cr layer is not uniform, the concentration of the alloying element drops significantly just below the oxidation scale, and the base material (effect of the oxidation scale is affected toward the inner layer). It has a distribution close to the concentration of alloying elements of (referring to the unreceived part. The same applies hereinafter).

FIG. 2 is a diagram schematically showing the distribution of Cr concentration near the surface of the steel pipe. Line A in the figure shows the distribution of Cr concentration when a steel pipe with a high Cr content is heat-treated at low temperature for a short time. Similarly, the B line is when a steel pipe with a high Cr content is heat-treated at a medium temperature and a medium time, and the C line is when a steel pipe with a high Cr content is heat-treated at a high temperature for a long time. , D line is when a steel pipe with low Cr content is heat-treated at low temperature for a short time, and line E is when a steel pipe with low Cr content is heat-treated at medium temperature and medium time, F line. Is the distribution of Cr concentration when a steel pipe having a low Cr content is heat-treated at a high temperature for a long time.

For example, when a steel pipe having a high Cr content is heat-treated at a low temperature for a short time (line A in FIG. 2), the Cr concentration immediately below the oxide scale becomes high and the depth of the Cr-depleted layer becomes shallow. On the contrary, when a steel pipe having a low Cr content is heat-treated at a high temperature for a long time (line F in FIG. 2), the Cr concentration immediately below the oxide scale becomes low and the depth of the Cr de-Cred layer becomes deep.

The de-Cr layer to be removed by pickling is the alloy concentration changing layer hatched in FIG. The amount of Cr contained in this de-Cr layer is (% Cr +% Cr ₁ ) × d, where the Cr content of the base metal is% Cr, the surface Cr concentration of the steel pipe is% Cr ₁ , and the de-Cr layer depth is d _{1. It} is proportional to ₁ x 0.5. Here, each of the surface Cr concentration% Cr ₁ and the de-Cr layer depth d ₁ is a function of the chemical composition of the base material, the heat treatment temperature T ₁ , and the heat treatment time t ₁ .

On the other hand, the strength of pickling is a function of the hydrogen ion concentration C ₂ of the acid solution, the temperature T ₂ , and the immersion time t ₂ .

The present inventors carried out heat treatment and pickling under various conditions, and clarified the relationship between the heat treatment conditions and appropriate pickling conditions. Specifically, the de-Cr layer can be appropriately removed by performing pickling so that the de-Cr index J and the pickling index L calculated from the following formula satisfy the relationship of J ≦ L <J + 230. Clarified.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
_{L = C 2 × (T 2} +273) 2 × t 2/100000
However,
% Cr ₁ =% Cr-% Cr ₂
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
d ₁ = (2 x D ₁ x t ₁ ) ^0.5
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
In each of the above equations, R is the gas constant, Q and A are the activation energies, and R = 8.31J / K, A = 5000J, and Q = 21000J are substituted. The contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively. The unit of T ₁ and T ₂ is ° C, the unit of t ₁ is seconds, the unit of t ₂ is minutes, and the unit of C ₂ is mol / kg.

Based on the above findings, the present invention has been completed. Hereinafter, a method for manufacturing a stainless steel pipe according to an embodiment of the present invention will be described in detail.

[Manufacturing method of stainless steel pipe]
FIG. 3 is a flow chart of a method for manufacturing a stainless steel pipe according to an embodiment of the present invention. The method for manufacturing a stainless steel pipe according to the present embodiment includes a step of preparing a raw pipe (step S1), a step of removing the oxide scale of the raw pipe (step S2), and a step of pickling the raw pipe (step S3). It has. Hereinafter, each step will be described in detail.

[Preparation process]
A raw tube having the chemical composition described below and heat-treated under predetermined conditions is prepared (step S1). In the following description, "%" of the element content means mass%.

C: 0.001 to 0.050%
Carbon (C) is precipitated as Cr carbide in the weld heat affected zone (HAZ) during welding, which lowers the SCC resistance of HAZ. On the other hand, if the C content is excessively limited, the manufacturing cost increases. Therefore, the C content is 0.001 to 0.050%. The lower limit of the C content is preferably 0.005%, more preferably 0.008%. The upper limit of the C content is preferably 0.030%, more preferably 0.020%.

Si: 0.05 to 1.00%
Silicon (Si) deoxidizes steel. On the other hand, if the Si content is too high, the toughness of the steel decreases. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.10%, more preferably 0.15%. The upper limit of the Si content is preferably 0.80%, more preferably 0.50%.

Mn: 0.05 to 1.00%
Manganese (Mn) improves the strength of steel. On the other hand, if the Mn content is too high, the toughness of the steel decreases. Therefore, the Mn content is 0.05 to 1.00%. The lower limit of the Mn content is preferably 0.10%, more preferably 0.20%. The upper limit of the Mn content is preferably 0.80%, more preferably 0.60%.

P: 0.030% or less Phosphorus (P) is an impurity. P reduces the SCC resistance of steel. Therefore, the P content is 0.030% or less. The P content is preferably 0.025% or less.

S: 0.0020% or less Sulfur (S) is an impurity. S lowers the hot workability of steel. Therefore, the S content is 0.0020% or less.

Cu: Less than 0.50% Copper (Cu) is an impurity. Therefore, the Cu content is less than 0.50%. The Cu content is preferably 0.10% or less, more preferably 0.08% or less.

Cr: 11.50 to less than 14.00% Chromium (Cr) improves the carbon dioxide corrosion resistance of steel. On the other hand, if the Cr content is too high, the toughness and hot workability of the steel will decrease. Therefore, the Cr content is less than 11.50 to 14.00%. The lower limit of the Cr content is preferably 12.00%, more preferably 12.50%. The upper limit of the Cr content is preferably 13.50%, more preferably 13.20%.

Ni: Over 5.00% to 7.00%
Nickel (Ni) is an austenite-forming element and is contained to make the structure of steel martensite. On the other hand, if the Ni content is too high, the strength of the steel will decrease. Therefore, the Ni content is more than 5.00% to 7.00%. The lower limit of the Ni content is preferably 5.50%, more preferably 5.80%, and even more preferably 6.00%. The upper limit of the Ni content is preferably 6.80%, more preferably 6.60%.

Mo: Over 1.00% to 3.00%
Molybdenum (Mo) improves the sulfide stress corrosion cracking resistance of steel. Mo further forms carbides during welding to prevent the precipitation of Cr carbides and suppresses a decrease in HAZ SCC resistance. On the other hand, if the Mo content is too high, the toughness of the steel will decrease. Therefore, the Mo content is more than 1.00% to 3.00%. The lower limit of the Mo content is preferably 1.50%, more preferably 1.80%. The upper limit of the Mo content is preferably 2.80%, more preferably 2.60%.

Ti: 0.02 to 0.50%
Titanium (Ti) forms carbides during welding to prevent the precipitation of Cr carbides and suppresses the decrease in SCC resistance of HAZ. On the other hand, if the Ti content is too high, the toughness of the steel will decrease. Therefore, the Ti content is 0.02 to 0.50%. The lower limit of the Ti content is preferably 0.05%, more preferably 0.10%. The upper limit of the Ti content is preferably 0.40%, more preferably 0.30%.

Al: 0.001 to 0.100%
Aluminum (Al) deoxidizes steel. On the other hand, if the Al content is too high, the toughness of the steel will decrease. Therefore, the Al content is 0.001 to 0.100%. The lower limit of the Al content is preferably 0.005%, more preferably 0.010%. The upper limit of the Al content is preferably 0.080%, more preferably 0.060%. The Al content in the present specification means the content of acid-soluble Al (so-called Sol.Al).

Ca: 0.0001 to 0.0040%
Calcium (Ca) improves the hot workability of steel. On the other hand, if the Ca content is too high, the toughness of the steel will decrease. Therefore, the Ca content is 0.0001 to 0.0040%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0008%. The upper limit of the Ca content is preferably 0.0035%, more preferably 0.0030%.

N: 0.0001 to less than 0.0200% Nitrogen (N) forms nitrides and reduces the toughness of steel. On the other hand, if the N content is excessively limited, the manufacturing cost increases. Therefore, the N content is less than 0.0001 to 0.0200%. The lower limit of the N content is preferably 0.0010%, more preferably 0.0020%. The upper limit of the N content is preferably 0.0100%.

The rest of the chemical composition of the raw tube is Fe and impurities. The impurities referred to here refer to elements mixed from ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.

The chemical composition of the raw tube may contain one or more elements selected from the group consisting of V, Nb, and Co, instead of a part of Fe. V, Nb, and Co are all selective elements. That is, the chemical composition of the raw tube may not contain a part or all of V, Nb, and Co.

V: 0 to 0.500%
Vanadium (V) improves the strength of steel. This effect can be obtained if even a small amount of V is contained. On the other hand, if the V content is too high, the toughness of the steel will decrease. Therefore, the V content is 0 to 0.500%. The lower limit of the V content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%. The upper limit of the V content is preferably 0.300%, more preferably 0.200%.

Nb: 0 to 0.500%
Niobium (Nb) improves the strength of steel. This effect can be obtained if even a small amount of Nb is contained. On the other hand, if the Nb content is too high, the toughness of the steel will decrease. Therefore, the Nb content is 0 to 0.500%. The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%. The upper limit of the Nb content is preferably 0.300%, more preferably 0.200%.

Co: 0 to 0.500%
Cobalt (Co) is an austenite-forming element and may be contained to make the steel structure martensite. This effect can be obtained if even a small amount of Co is contained. On the other hand, if the Co content is too high, the strength of the steel will decrease. Therefore, the Co content is 0 to 0.500%. The lower limit of the Co content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%. The upper limit of the Co content is preferably 0.350%, more preferably 0.300%, and even more preferably 0.280%.

The raw tube is not limited to this, and can be manufactured, for example, as follows.

A material having the same chemical composition as the above-mentioned raw tube is prepared (step S1-1). For example, steel having the above-mentioned chemical composition is melted and continuously cast or lump-rolled to make billets. In addition to continuous casting or ingot rolling, hot working, cold working, heat treatment and the like may be carried out.

The material is hot-processed to produce a raw tube (step S1-2). Hot working is, for example, the Mannesmann method or the Eugene Sejurne method.

The hot-worked raw tube is quenched (step S1-3). Quenching may be direct quenching, in-line quenching, or reheating quenching. Direct quenching is a heat treatment in which a high-temperature raw tube after hot working is rapidly cooled. In-line quenching is a heat treatment in which a raw tube after hot working is heated in a heating furnace and then rapidly cooled. Reheating quenching is a heat treatment in which a hot-worked raw tube is once cooled to around room temperature, then reheated to a temperature of ₃ points or more, and then rapidly cooled.

The quenching temperature (temperature of the raw tube immediately before quenching) is preferably 850 to 1000 ° C. The cooling rate during quenching is preferably 300 ° C./min or higher.

The hardened raw tube is tempered (step S1-4). Specifically, the raw tube is held at a temperature T ₁ having a temperature of 540 to 710 ° C. for a time t ₁ which is a time in the range of 5 to 180 minutes, and then cooled. The time t ₁ is the holding time after the temperature at the center position of the wall thickness of the raw tube reaches the temperature T ₁ .

Tempering is carried out in order to remove the strain generated in the quenching step (step S1-3) and to adjust the mechanical properties of the steel pipe. In general, the higher the temperature T ₁ or the longer the time t ₁ , the lower the strength of the steel pipe and the higher the toughness. The temperature T ₁ and the time t ₁ are determined according to the required mechanical properties.

When the temperature T ₁ is out of the range of 540 to 710 ° C., it becomes difficult to satisfy the mechanical properties required for the 13% Cr steel pipe. The lower limit of the temperature T ₁ is preferably 550 ° C. The upper limit of the temperature T ₁ is preferably 700 ° C.

Even when the time t ₁ is less than 5 minutes, it becomes difficult to satisfy the mechanical properties required for the 13% Cr steel pipe. If the time t ₁ exceeds 180 minutes, the production efficiency is lowered and it becomes difficult to properly remove the Cr-de-Cred layer. The lower limit of the time t ₁ is preferably 10 minutes, more preferably 15 minutes, and even more preferably 30 minutes. The upper limit of the time t ₁ is preferably 150 minutes, more preferably 120 minutes.

[Oxidation scale removal process]
The oxidation scale generated in the tempering step is removed (step S2). The oxidation scale can be removed by, for example, sandblasting or shotblasting. The oxidation scale removal step (step S2) is an arbitrary step, and this step may be omitted. If the oxidation scale removing step is carried out, deterioration of the acid solution used in the next pickling step (step S3) can be suppressed.

[Pickling process]
The bare tube is pickled (step S3). Specifically, the raw tube is immersed in an acid solution having a hydrogen ion concentration of C ₂ and a temperature of T _{2 for} a time t ₂ .

The type of acid used for pickling is not particularly limited. For example, sulfuric acid, hydrochloric acid, and nitric acid can be used, but sulfuric acid is particularly preferable. A solution in which two or more kinds of acids are mixed may be used. The acid solution is not limited to this, but an aqueous solution is usually used.

The hydrogen ion concentration C ₂ of the acid solution is not limited to this, but is, for example, 1.00 to 6.00 mol / kg. In the case of sulfuric acid, this corresponds to about 5 to 30% by mass. The lower limit of the hydrogen ion concentration C ₂ is preferably 2.00 mol / kg, more preferably 2.50 mol / kg. The upper limit of the hydrogen ion concentration C ₂ is preferably 5.10 mol / kg, more preferably 4.50 mol / kg.

The temperature T _{2 of the} acid solution is, but is not limited to, 25 to 80 ° C., for example. The lower limit of the temperature T ₂ is preferably 30 ° C, more preferably 40 ° C. The upper limit of the temperature T ₂ is preferably 70 ° C., more preferably 65 ° C.

The time t ₂ of immersion in the acid solution is, for example, 10 to 90 minutes, but is not limited to this. The lower limit of the time t ₂ is preferably 20 minutes, more preferably 30 minutes. The upper limit of the time t ₂ is preferably 60 minutes, more preferably 50 minutes. When the raw tube is immersed in the acid solution multiple times, the time t ₂ is the total immersion time.

At this time, the de-Cr index J and the pickling index L described below satisfy the relationship of J ≦ L <J + 230.

[De-Cr index J]
The de-Cr index J is an index of the amount of Cr contained in the de-Cr layer. The amount of Cr contained in the de-Cr layer depends on the chemical composition of the raw tube and the tempering conditions as described at the beginning of the embodiment. The de-Cr index J is specifically expressed by the following equation.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
Here,% Cr is the Cr content (mass%) of the raw tube,% Cr ₁ is the surface Cr concentration (mass%), and d ₁ is the de-Cr layer depth (mm).

The surface Cr concentration% Cr ₁ (mass%) can be obtained from the difference between the Cr content% Cr (mass%) of the raw tube and the Cr weight loss% Cr ₂ (mass%) due to the generation of the oxide scale.
% Cr ₁ =% Cr-% Cr ₂

Cr weight loss% Cr ₂ (mass%) is a function of the chemical composition of the raw tube, the tempering temperature T ₁ (° C.) and the time t ₁ (sec), and is specifically expressed by the following formula.
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
Here, R is the gas constant, A is the activation energy, and R = 8.31J / K and A = 5000J are substituted.

h ₀ is a coefficient determined by the contents of Cr, Ni, Mo, and Cu in the raw tube, and is specifically expressed by the following formula.
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
Here, the contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively.

The de-Cr layer depth d ₁ (mm) is expressed as follows by the diffusion coefficient D ₁ and the time t ₁ (seconds).
d ₁ = (2 x D ₁ x t ₁ ) ^0.5

The diffusion coefficient D ₁ is a function of the temperature T ₁ (° C.), and is specifically expressed by the following equation.
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
Here, R is the gas constant, Q is the activation energy, and R = 8.31J / K and Q = 21000J are substituted.

D ₀ is a coefficient determined by the Cr content of the raw tube, and is specifically expressed by the following formula.
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
Here, the Cr content of the raw tube is substituted into% Cr in mass%.

Summarizing the above, the de-Cr index J can be calculated from the following formula.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
However,
% Cr ₁ =% Cr-% Cr ₂
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
d ₁ = (2 x D ₁ x t ₁ ) ^0.5
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
In each of the above equations, R is the gas constant, Q and A are the activation energies, and R = 8.31J / K, A = 5000J, and Q = 21000J are substituted. The contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively. The unit of T ₁ is ° C. and the unit of t ₁ is seconds.

[Pickling index L]
The pickling index L is an index of the strength of pickling, and is specifically expressed by the following formula.
_{L = C 2 × (T 2} +273) 2 × t 2/100000
The unit of T ₂ is ° C., the unit of t ₂ is minutes, and the unit of C ₂ is mol / kg.

If pickling is performed so that the de-Cr index J and the pickling index L satisfy the relationship of J ≦ L <J + 230, a stainless steel pipe in which the de-Cr layer is appropriately removed can be obtained. J ≦ L is a condition for not insufficient pickling, and L <J + 230 is a condition for not being over-pickled.

Theoretically, the deCring layer can be removed by making the pickling index L equal to the deCring index J. On the other hand, in order to stabilize the quality, it is preferable to provide a certain margin. The lower limit of the pickling index L is preferably J + 10, more preferably J + 15, and even more preferably J + 20.

When the pickling index L is less than the de-Cr index J + 230, it is possible to suppress the occurrence of unevenness in the surface texture of the steel pipe due to peracid washing. On the other hand, from the viewpoint of production efficiency, it is preferable that the pickling index L is as small as possible. The upper limit of the pickling index L is preferably J + 200, more preferably J + 150, and even more preferably J + 100.

The pickling may be divided into two or more times. In this case, conditions such as the type, concentration, and temperature of the acid may be changed between the first pickling and the second pickling. The pickling index in this case is the sum of the pickling index obtained by the first pickling and the pickling index obtained by the second pickling.

The method for manufacturing a stainless steel pipe according to an embodiment of the present invention has been described above. According to this embodiment, a stainless steel pipe from which the Cr-depleted layer is appropriately removed can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to these examples.

Steels having the chemical compositions shown in Table 1 were melted and hot forged and hot rolled to produce test materials.

The produced test material was subjected to quenching and tempering heat treatment. Specifically, after quenching by water cooling from 900 ° C. to room temperature, tempering was carried out by holding at the temperature T ₁ shown in Table 2 for a time t ₁ and then cooling. Subsequently, pickling was carried out under the conditions shown in Table 2.

From each test material after pickling, a 4-point bending test piece having a width of 10 mm, a length of 75 mm, and a thickness of 2 mm including the surface was collected and subjected to an SCC test. Specifically, each test piece was immersed in a test solution in a state in which a stress equal to the actual yield stress (0.2% proof stress) was applied in an autoclave filled with 10 atm of CO ₂ . The test solution was a 10 mass% NaCl aqueous solution, and the test temperature was 170 ° C. After 720 hours, the test piece was observed with an optical microscope to check for cracks (SCC).

The test results are shown in Table 2. The de-Cr depth d ₁ and the surface Cr concentration% Cr _{1 in} Table 2 are both calculated values.

In the test materials of test numbers 1, 5 to 8, 12, 13, 15, 16, and 18 to 21, the de-Cr index J and the pickling index L satisfied the relationship of J ≦ L <J + 230. These test materials had appropriate pickling levels and did not generate SCC.

SCC occurred in the test materials of test numbers 3, 4, 9, 14 and 17. It is considered that this is because the pickling was insufficient for the degree of Cr removal.

Color unevenness was observed on the surfaces of the test materials of test numbers 2, 10 and 11. It is considered that this is because the pickling was excessive with respect to the degree of Cr removal.

The embodiments of the present invention have been described above. The above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

The present invention is applied to a method for producing a stainless steel pipe, but is preferably applicable to a method for producing a martensitic stainless steel pipe, and more preferably to a method for producing a martensitic stainless seamless steel pipe.

Claims (4)

By mass%
C: 0.001 to 0.050%,
Si: 0.05 to 1.00%,
Mn: 0.05 to 1.00%,
P: 0.030% or less,
S: 0.0020% or less,
Cu: less than 0.50%,
Cr: 11.50 to less than 14.00%,
Ni: Over 5.00% to 7.00%,
Mo: Over 1.00% to 3.00%,
Ti: 0.02 to 0.50%,
Al: 0.001 to 0.100%,
Ca: 0.0001 to 0.0040%,
N: 0.0001 to less than 0.0200%,
V: 0 to 0.500%,
Nb: 0 to 0.500%,
Co: 0 to 0.500%,
Remaining: Has a chemical composition of Fe and impurities,
A preparatory step for preparing a tempered raw tube to be cooled after being held for a time t ₁ which is a time of 5 to 180 minutes at a temperature T ₁ which is a temperature of 540 to 710 ° C.
The base tube is provided with a pickling step of immersing the raw tube in an acid solution having a hydrogen ion concentration of C ₂ and a temperature of T _{2 for} a time t ₂ .
A method for manufacturing a stainless steel pipe, in which the de-Cr index J and the pickling index L calculated from the following formula satisfy the relationship of J ≦ L <J + 230.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
_{L = C 2 × (T 2} +273) 2 × t 2/100000
However,
% Cr ₁ =% Cr-% Cr ₂
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
d ₁ = (2 x D ₁ x t ₁ ) ^0.5
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
In each of the above equations, R is the gas constant, Q and A are the activation energies, and R = 8.31J / K, A = 5000J, and Q = 21000J are substituted. The contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively. The unit of T ₁ and T ₂ is ° C, the unit of t ₁ is seconds, the unit of t ₂ is minutes, and the unit of C ₂ is mol / kg. By mass%
C: 0.001 to 0.050%,
Si: 0.05 to 1.00%,
Mn: 0.05 to 1.00%,
P: 0.030% or less,
S: 0.0020% or less,
Cu: less than 0.50%,
Cr: 11.50 to less than 14.00%,
Ni: Over 5.00% to 7.00%,
Mo: Over 1.00% to 3.00%,
Ti: 0.02 to 0.50%,
Al: 0.001 to 0.100%,
Ca: 0.0001 to 0.0040%,
N: 0.0001 to less than 0.0200%,
V: 0 to 0.500%,
Nb: 0 to 0.500%,
Co: 0 to 0.500%,
Remaining: Has a chemical composition of Fe and impurities,
The preparatory process to prepare the hardened raw tube and
The mother tube, and tempering steps of the temperature _{T 1,} the tempering of cooling after holding during the time that is _{t 1} 5 to 180 minute time at a temperature of 540 to 710 ° C.,
The tempered raw tube is provided with a pickling step of immersing the tempered raw tube in an acid solution having a hydrogen ion concentration of C ₂ and a temperature of T _{2 for} a time t ₂ .
A method for manufacturing a stainless steel pipe, in which the de-Cr index J and the pickling index L calculated from the following formula satisfy the relationship of J ≦ L <J + 230.
J = (% Cr +% Cr ₁ ) x d ₁ x 0.5 x 1000
_{L = C 2 × (T 2} +273) 2 × t 2/100000
However,
% Cr ₁ =% Cr-% Cr ₂
% Cr ₂ = t ₁ ^0.5 × h ₀ × exp (−A / (R × (T ₁ +273))) × 100
h ₀ = 1 / (4 ×% Cr ² + 3 ×% Ni ² + (% Mo +% Cu) ² )
d ₁ = (2 x D ₁ x t ₁ ) ^0.5
D ₁ = D ₀ × exp (−Q / (R × (T ₁ +273)))
D ₀ = ^10-11 × (4-20 ×% Cr + 150 ×% Cr ² )
In each of the above equations, R is the gas constant, Q and A are the activation energies, and R = 8.31J / K, A = 5000J, and Q = 21000J are substituted. The contents of Cr, Ni, Mo, and Cu in the raw tube are substituted in% by mass for% Cr,% Ni,% Mo, and% Cu, respectively. The unit of T ₁ and T ₂ is ° C, the unit of t ₁ is seconds, the unit of t ₂ is minutes, and the unit of C ₂ is mol / kg. The method for manufacturing a stainless steel pipe according to claim 2.
The preparatory step
A material preparation step for preparing a material having the chemical composition according to claim 2,
The hot working process of hot-working the material to manufacture the raw tube, and
A method for manufacturing a stainless steel pipe, which comprises a quenching step of quenching the manufactured raw pipe. The method for manufacturing a stainless steel pipe according to any one of claims 1 to 3.
The chemical composition is mass%.
V: 0.001 to 0.500%,
Nb: 0.001 to 0.500%, and Co: 0.001 to 0.500%,
A method for producing a stainless steel pipe, which contains one or more elements selected from the group consisting of.

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