JP7063169B2 - Seamless steel pipe for hot forging - Google Patents

Description

The present disclosure relates to a seamless steel pipe, and more particularly to a seamless steel pipe for hot forging, which is processed into a pressure vessel such as a pressure accumulator by hot forging.

A pressure vessel typified by a pressure accumulator may be manufactured using a seamless steel pipe. When manufacturing a pressure vessel using a seamless steel pipe, the following steps are carried out. First, hot forging is performed on the seamless steel pipe to produce an intermediate product obtained by molding the shape of the seamless steel pipe into the shape of a pressure vessel. After that, the manufactured intermediate product is heat-treated to adjust the strength and hardness of the intermediate product, and the local strain introduced into the intermediate product by hot forging is reduced to improve the mechanical properties. Make it uniform. Through the above manufacturing process, a pressure vessel is manufactured from a seamless steel pipe.

In the conventional pressure vessel, high strength and high hardness are required for the seamless steel pipe after heat treatment, which is carried out after hot forging.

By the way, recently, there is a demand for the use of pressure vessels in cold regions. When the pressure vessel is used in a cold region, it is expected that excellent low temperature toughness is required as well as high strength and high hardness.

Techniques for achieving both strength and workability in steel pipes include JP-A-11-80899 (Patent Document 1), JP-A-2000-96143 (Patent Document 2), and JP-A-2000-9409 (Patent Document 1). It is proposed in Patent Document 3).

The high-strength steel pipe disclosed in Patent Document 1 has a weight% of C: more than 0.30 to 0.70%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.0%, Al: Contains 0.001 to 0.10%, and has a composition in which the balance is composed of Fe and unavoidable impurities. The structure of this steel pipe is composed of ferrite and a second phase other than ferrite having an area ratio of more than 30%, and the average crystal grain size of the cross section perpendicular to the longitudinal direction of the steel pipe is 2 μm or less. Patent Document 1 describes that by miniaturizing the average crystal grain size to 2 μm or less, a high-strength steel pipe having excellent workability can be obtained even if the tensile strength is 600 MPa or more.

The method for producing a steel pipe disclosed in Patent Document 2 is, in weight%, C: 0.005 to 0.70%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, A steel pipe containing Al: 0.001 to 0.10% is heated or heated to an _Ac3 transformation point to 400 ° C. Then, drawing rolling with a cumulative diameter reduction ratio of 20% or more is carried out at the _Acc3 transformation point to 400 ° C. After drawing and rolling, the temperature is kept so that the temperature θ (° C.) × time τ (min) is 1200 or more, and then the material is cooled. Patent Document 2 describes that this manufacturing process can obtain a steel pipe having an excellent balance between strength and ductility and having a small variation in mechanical properties.

The method for producing a steel pipe disclosed in Patent Document 3 is, in weight%, C: 0.005 to 0.70%, Si: 0.01 to 3.0%, Mn: 0.01 to 4.0%, A steel pipe containing Al: 0.001 to 0.10% is heated or heated to an _Ac3 transformation point to 400 ° C. Then, drawing rolling with a cumulative diameter reduction ratio of 20% or more is carried out at the _Acc3 transformation point to 400 ° C. After drawing and rolling, it is rapidly cooled to room temperature at a cooling rate of 1.5 ° C./s or higher. Patent Document 3 describes that this manufacturing process can advantageously suppress grain coarsening after rolling and can obtain a steel pipe having an excellent balance between ductility and strength.

Japanese Unexamined Patent Publication No. 11-80899 Japanese Unexamined Patent Publication No. 2000-96143 Japanese Unexamined Patent Publication No. 2000-94009

However, the steel pipes disclosed in Patent Documents 1 to 3 described above have not been studied for low temperature toughness and have not been studied for use in pressure vessels. Further, the steel pipes disclosed in Patent Documents 1 to 3 do not assume that hot forging is subsequently performed on the manufactured steel pipes. The steel pipes disclosed in Patent Documents 1 to 3 aim to achieve both strength and ductility of the steel pipe itself by miniaturizing the crystal grains of the steel pipe itself. On the other hand, in the seamless steel pipe for hot forging used for pressure vessels and the like, the mechanical properties after the heat treatment (that is, in the final product) performed after the hot forging are important.

In this way, in the seamless steel pipe for hot forging, which is hot forged as a post-process, we focused on the three characteristics of strength, hardness, and low temperature toughness after the heat treatment performed after hot forging. There is no example of study before.

After hot forging, for example, if quenching and tempering are performed as heat treatment, excellent low temperature toughness may be obtained as well as high strength and high hardness. However, when quenching and tempering are carried out, the heat treatment is carried out in two steps (quenching step and tempering step), which increases the manufacturing cost. Therefore, it is preferable that the heat treatment after the hot forging step can be suppressed by one heat treatment such as a normalizing treatment step. That is, in a seamless steel pipe for hot forging, even if the heat treatment after hot forging is a normalizing treatment, it is preferable that excellent low temperature toughness can be obtained together with high strength and high hardness.

An object of the present disclosure is to provide a seamless steel pipe for hot forging, which can obtain excellent low temperature toughness as well as high strength and high hardness after normalizing treatment.

The seamless steel pipe for hot forging according to the present disclosure has a mass% of C: 0.25 to 0.33%, Si: 0.01 to 0.35%, Mn: 1.20 to 1.60%, P. : 0.020% or less, S: 0.010% or less, Ni: 0.11 to 0.30%, Cr: 0.01 to 0.30%, V: 0.01 to 0.08%, Ti: 0 to 0.005%, B: 0 to 0.0010%, Al: 0.01 to 0.10%, N: 0.0080% or less, Mo: 0 to 0.05%, Nb: 0 to 0. It consists of 010% and the balance: Fe and impurities, and has a hardness low temperature toughness index defined by the formula (1) of 0.60 to 0.68.
Hardness low temperature toughness index = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The seamless steel pipe for hot forging according to the present disclosure can obtain excellent low temperature toughness as well as high strength and high hardness after normalizing treatment. Specifically, after the normalizing treatment at 860 ° C., the tensile strength obtained by the tensile test based on ASTM E8 is 620 to 795 MPa, which is obtained by the Brinell hardness test based on ASTM E10-08. In addition, the Brinell hardness at the center position of the wall thickness is 187 HBW or more, and the impact value at −20 ° C. obtained by the Charpy impact test based on ASTM E23 is 48 J / cm ² or more.

FIG. 1 is a vertical sectional view showing an example of a pressure vessel manufactured by using the seamless steel pipe for hot forging of the present embodiment.

The present inventors have obtained a seam for hot forging that can obtain excellent low-temperature toughness as well as high strength and high hardness after the normalizing treatment even when the normalizing treatment is performed after the hot forging. A study was conducted on steelless pipes.

The conventional seamless steel pipe for hot forging does not require low temperature toughness. That is, in the conventional seamless steel pipe for hot forging, high strength and high hardness are required, and excellent low temperature toughness is not particularly required. This is because the demand for use in cold regions was not great. Therefore, in the conventional seamless steel pipe for hot forging, the C content is 0.35% or more in order to secure high strength and high hardness.

Therefore, the present inventors first performed normalizing treatment at 860 ° C. on a conventional seamless steel pipe for hot forging having a C content of 0.35% or more, and then conformed to ASTM E23. A Charpy impact test was carried out to determine the impact value at −20 ° C. As a result, in the case of the conventional seamless steel pipe for hot forging with a C content of 0.35% or more, the impact value at -20 ° C is less than 48 J / cm ² after the normalizing treatment, and sufficient low temperature toughness is obtained. It turned out that it could not be obtained.

Therefore, the present inventors have studied a seamless steel pipe for hot forging, which is different from the conventional seamless steel pipe for hot forging and has excellent low temperature toughness. As described above, the seamless steel pipe for hot forging is subjected to hot forging and heat treatment (normalizing treatment) after hot forging when applied to a pressure vessel. Therefore, even if the microstructure such as the crystal grain size of the seamless steel pipe for hot forging or the precipitate such as carbides is controlled, the microstructure of the seamless steel pipe and the microstructure of the seamless steel pipe and the subsequent manufacturing process (hot forging and heat treatment) are performed. The morphology of the precipitate changes. Therefore, the present inventors attempted to control the strength, hardness, and low-temperature toughness with a chemical composition that is not affected by the microstructure or the morphology of precipitates.

As described above, a conventional seamless steel pipe for hot forging having a C content of 0.35% or more cannot obtain sufficient low temperature toughness. Therefore, the present inventors set the C content to 0.25 to 0.33% in the chemical composition of the seamless steel pipe for hot forging, and further set the Ni content, which is an element for enhancing low temperature toughness, to 0.11. I tried to make it ~ 0.30%. Specifically, in terms of mass%, C: 0.25 to 0.33%, Si: 0.01 to 0.35%, Mn: 1.20 to 1.60%, P: 0.020% or less, S: 0.010% or less, Ni: 0.11 to 0.30%, Cr: 0.01 to 0.30%, V: 0.01 to 0.08%, Ti: 0 to 0.005%, B: 0 to 0.0010%, Al: 0.01 to 0.10%, N: 0.0080% or less, Mo: 0 to 0.05%, Nb: 0 to 0.010%, and the balance: It was considered that a seamless steel tube for hot forging having a chemical composition consisting of Fe and impurities could obtain not only high strength and high hardness but also excellent low temperature toughness.

However, even with a seamless steel pipe for hot forging within the above-mentioned chemical composition range, sufficient low-temperature toughness may not be obtained, or sufficient hardness may not be obtained at the center position of the wall thickness. A case has occurred. Therefore, as a result of further studies by the present inventors, it is a parameter formula relating to C, Si, Mn, Ni, Cr, Mo and V, which are elements affecting hardness and low temperature toughness among the above-mentioned chemical compositions. If the hardness low temperature toughness index defined by the formula (1) is in the range of 0.60 to 0.68, it conforms to ASTM E8 after the seamless steel pipe for hot forging is tempered at 860 ° C. The tensile strength obtained by the tensile test was 620 to 795 MPa, and the center of wall thickness obtained by the Brinell hardness test based on ASTM E10-08 after the seamless steel pipe for hot forging was tempered at 860 ° C. The Brinell hardness at the position was 187 HBW or more, and the impact value at -20 ° C obtained by the ASTM E23-compliant Sharpy impact test after the seamless steel tube for hot forging was tempered at 860 ° C was 48 J / cm. I found that it would be ² or more.
Hardness low temperature toughness index = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The seamless steel pipe for hot forging of this embodiment was completed based on the above findings. The seamless steel pipe for hot forging of the present embodiment has C: 0.25 to 0.33%, Si: 0.01 to 0.35%, Mn: 1.20 to 1.60% in mass%. P: 0.020% or less, S: 0.010% or less, Ni: 0.11 to 0.30%, Cr: 0.01 to 0.30%, V: 0.01 to 0.08%, Ti : 0 to 0.005%, B: 0 to 0.0010%, Al: 0.01 to 0.10%, N: 0.0080% or less, Mo: 0 to 0.05%, Nb: 0 to 0 It consists of 010% and the balance: Fe and impurities, and has a hardness low temperature toughness index defined by the formula (1) of 0.60 to 0.68.
Hardness low temperature toughness index = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The above-mentioned seamless steel pipe for hot forging has a tensile strength of 620 to 795 MPa obtained by a tensile test compliant with ASTM E8 after a normalizing treatment at 860 ° C., and a Brinell hardness compliant with ASTM E10-08. The Brinell hardness at the center position of the wall thickness obtained by the test is 187 HBW or more, and the impact value at −20 ° C. obtained by the Charpy impact test according to ASTM E23 is 48 J / cm ² or more.

Hereinafter, the seamless steel pipe for hot forging of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.

[About hot forging applications]
The seamless steel pipe for hot forging of the present embodiment is hot forged in a subsequent process. That is, it is a seamless steel pipe for hot forging. For example, a seamless steel pipe for hot forging is hot forged and then subjected to normalizing treatment after hot forging to become a pressure vessel. A pressure vessel is a container in which a high-pressure fluid is stored. The pressure vessel is, for example, a pressure vessel, a container for a pump and a compressor, a container for a shock absorber or other shock absorber, a container for an automobile airbag gas generator, a container for a storage battery, and the like.

FIG. 1 is a vertical sectional view of an example of a pressure vessel. With reference to FIG. 1, the pressure vessel 1 includes a body portion 2 and a pair of mirror portions 3A and 3B. The body portion 2 is a tubular member. High-pressure fluid (gas or the like) is stored in the body 2. The body portion 2 has a cylindrical shape, and the cross section perpendicular to the axial direction of the body portion is an annular shape. The pair of mirror portions 3A and 3B are arranged at both ends of the body portion 2 and are connected to both ends of the body portion 2 to seal the body portion 2. The pressure vessel 1 may be provided with a base (not shown) in the mirror portion 3A or 3B. The shape of the pressure vessel 1 is not limited to FIG. However, the pressure vessel 1 usually includes a body portion 2 and a pair of mirror portions 3A and 3B.

When manufacturing a pressure vessel using a seamless steel pipe for hot forging, first, the seamless steel pipe for hot forging is hot forged, and the seamless steel pipe is used for the body 2 and mirrors 3A and 3B. Mold into the shape of the including pressure vessel 1. The seamless steel pipe after hot forging locally accumulates strain. Therefore, for the purpose of reducing local strain and making the mechanical properties uniform, heat treatment (for example, normalizing treatment) is performed on the seamless steel pipe after hot forging. A pressure vessel is manufactured by the above manufacturing process.

The seamless steel pipe for hot forging of the present embodiment is not limited to the use of pressure vessels, and is not limited to the use of parts formed by hot forging the seamless steel pipe for hot forging of the present embodiment. Not particularly limited.

Hereinafter, the seamless steel pipe for hot forging of the present embodiment will be described in detail.

[Chemical composition]
The chemical composition of the seamless steel pipe for hot forging of the present embodiment contains the following elements.

C: 0.25 to 0.33%
Carbon (C) enhances the strength and hardness of the steel material. If the C content is less than 0.25%, even if the content of other elements is within the range of the present embodiment, sufficient tensile strength cannot be obtained or sufficient tensile strength is obtained after the normalizing treatment. However, sufficient hardness may not be obtained at the central portion of the wall thickness of the seamless steel pipe having a wall thickness of 10 to 40 mm. On the other hand, if the C content exceeds 0.33%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.25 to 0.33%. The lower limit of the C content is preferably 0.26%, more preferably 0.28%, still more preferably 0.30%. The preferred upper limit of the C content is 0.32%.

Si: 0.01-0.35%
Silicon (Si) deoxidizes the steel and further increases the strength and hardness of the steel material. If the Si content is less than 0.01%, even if the content of other elements is within the range of this embodiment, sufficient tensile strength cannot be obtained or sufficient tensile strength is obtained after the normalizing treatment. However, sufficient hardness may not be obtained in the central part of the wall thickness. On the other hand, if the Si content exceeds 0.35%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 0.01 to 0.35%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%.

Mn: 1.20 to 1.60%
Manganese (Mn) deoxidizes steel. Mn further increases the strength and hardness of the steel material. If the Mn content is less than 1.20%, even if the content of other elements is within the range of the present embodiment, sufficient tensile strength cannot be obtained or sufficient tensile strength is obtained after the normalizing treatment. However, sufficient hardness may not be obtained in the central part of the wall thickness. On the other hand, if the Mn content exceeds 1.60%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the Mn content is 1.20 to 1.60%. The preferred lower limit of the Mn content is 1.25%, more preferably 1.30%, still more preferably 1.35%.

P: 0.020% or less Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P segregates at the grain boundaries and reduces the low temperature toughness after the normalizing treatment. Therefore, the P content is 0.020% or less. The preferred upper limit of the P content is 0.018%, more preferably 0.015%. It is preferable that the P content is as low as possible. However, an extreme reduction in P content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the P content is 0.0001%, and more preferably 0.001%.

S: 0.010% or less Sulfur (S) is an impurity that is inevitably contained. That is, the S content is more than 0%. S segregates at the grain boundaries and lowers the low temperature toughness after the normalizing treatment. Therefore, the S content is 0.010% or less. The preferred upper limit of the S content is 0.005%, more preferably 0.003%. It is preferable that the S content is as low as possible. However, an extreme reduction in S content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the S content is 0.0001%, and more preferably 0.001%.

Ni: 0.11 to 0.30%
Nickel (Ni) enhances the low temperature toughness of the steel material after normalizing. Ni further enhances the strength and hardness of the steel material after normalizing. If the Ni content is less than 0.11%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ni content exceeds 0.30%, the effect is saturated. If the Ni content exceeds 0.30%, seamless steel pipes for hot forging fall into the category of carbon steel to alloy steel, so customer requests are requested for reasons other than performance (material standards, etc.). There is a risk that it will not be satisfied. Therefore, the Ni content is 0.11 to 0.30%. The preferable lower limit of the Ni content is 0.12%, more preferably 0.13%, still more preferably 0.15%, still more preferably 0.17%, still more preferably 0.20. %. The preferred upper limit of the Ni content is 0.29%, more preferably 0.28%, still more preferably 0.26%, still more preferably 0.25%.

Cr: 0.01-0.30%
Chromium (Cr) enhances the strength and hardness of the steel material after normalizing. If the Cr content is less than 0.01%, even if the content of other elements is within the range of the present embodiment, sufficient tensile strength cannot be obtained or sufficient tensile strength is obtained after the normalizing treatment. However, sufficient hardness may not be obtained in the central part of the wall thickness. On the other hand, if the Cr content exceeds 0.30%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the Cr content is 0.01 to 0.30%. The lower limit of the Cr content is preferably 0.05%, more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20%. The preferred upper limit of the Cr content is 0.28%, more preferably 0.26%, still more preferably 0.25%.

V: 0.01-0.08%
Vanadium (V) combines with carbon (C) and / or nitrogen (N) to form carbides, nitrides or carbonitrides (hereinafter referred to as "carbonitrides and the like"). By strengthening the precipitation of these carbonitrides and the like, the strength and hardness of the steel material after the normalizing treatment are increased. If the V content is less than 0.01%, even if the content of other elements is within the range of the present embodiment, sufficient tensile strength cannot be obtained or sufficient tensile strength is obtained after the normalizing treatment. However, sufficient hardness may not be obtained in the central part of the wall thickness. On the other hand, if the V content exceeds 0.08%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the V content is 0.01 to 0.08%. The lower limit of the V content is preferably 0.02%, more preferably 0.04%, still more preferably 0.05%.

Ti: 0 to 0.005%
Titanium (Ti) is an impurity in this embodiment. Ti combines with N in the steel to form a coarse nitride, which reduces the low temperature toughness of the steel after normalizing. If the Ti content exceeds 0.005%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the Ti content is 0 to 0.005%. The Ti content is preferably as low as possible. Therefore, the preferred upper limit of the Ti content is 0.004%, more preferably 0.003%, still more preferably 0.002%. The Ti content may be 0%.

B: 0 to 0.0010%
Boron (B) is an impurity in this embodiment. When B is contained, Ti must be contained in order to secure the effective solid solution B. As mentioned above, Ti is an impurity in this embodiment. Therefore, in the present embodiment, it is preferable that the B content is as low as possible. Therefore, the B content is 0 to 0.0010%. The preferred upper limit of the B content is 0.0005%, more preferably 0.0003%. The B content may be 0%.

Al: 0.01-0.10%
Al deoxidizes the steel. If the Al content is less than 0.01%, this effect may not be obtained even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.10%, Al-based oxides are excessively generated, and even if the content of other elements is within the range of the present embodiment, there are defects in the material such as slabs. Is more likely to occur. Therefore, the Al content is 0.01 to 0.10%. The lower limit of the Al content is preferably 0.02%, more preferably 0.025%. The preferred upper limit of the Al content is 0.08%, more preferably 0.06%, still more preferably 0.055%. In the chemical composition of the seamless steel pipe for hot forging of the present embodiment, the Al content means the content of acid-soluble Al (so-called “sol.Al”).

N: 0.0080% or less Nitrogen (N) is an unavoidable impurity. That is, N is more than 0%. N combines with Ti and B to form a nitride. Nitride reduces the low temperature toughness of the steel material after the normalizing treatment described later. Therefore, the N content is 0.0080% or less. The preferred upper limit of the N content is 0.0070%, more preferably 0.0060%. The N content is preferably as low as possible. However, an extreme reduction in N content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the N content is 0.0001%, and more preferably 0.0010%.

Mo: 0-0.05%
Molybdenum (Mo) is an optional element and may not be contained. That is, the Mo content may be 0%. When contained, Mo dissolves or produces fine carbides to increase the strength and hardness of the steel after normalizing. If the Mo content is contained even in a small amount, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.05%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the Mo content is 0 to 0.05%. The lower limit of the Mo content is preferably 0.01%, more preferably 0.02%. The preferred upper limit of the Mo content is 0.04%, more preferably 0.03%.

Nb: 0 to 0.010%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with carbon (C) and / or nitrogen (N) to form carbonitrides and the like. By strengthening the precipitation of these carbonitrides and the like, the strength and hardness of the steel material after the normalizing treatment are increased. However, if the Nb content exceeds 0.010%, sufficient low temperature toughness cannot be obtained after the normalizing treatment even if the content of other elements is within the range of the present embodiment. Therefore, the Nb content is 0 to 0.010%. The preferable lower limit of the Nb content is 0.001%. The preferred upper limit of the Nb content is 0.008%, more preferably 0.006%.

The balance of the chemical composition of the seamless steel pipe for hot forging according to this embodiment is composed of Fe and impurities. Here, the impurities are those mixed from ore, scrap, or the manufacturing environment as a raw material when industrially manufacturing a seamless steel pipe for hot forging, and are hot in the present embodiment. It means a pipe that is allowed as long as it does not adversely affect the seamless steel pipe for forging.

[Hardness and low temperature toughness index]
The chemical composition of the seamless steel pipe for hot forging of the present embodiment further has a hardness low temperature toughness index of 0.60 to 0.68 defined by the formula (1).
Hardness low temperature toughness index = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

When the seamless steel pipe for hot forging of the present embodiment is applied as a pressure vessel in a cold region, the seamless steel pipe for hot forging is hot forged and then subjected to normalizing treatment to manufacture a pressure vessel. .. In the normalizing treatment, first, the seamless steel pipe after hot forging is charged into a heat treatment furnace and heated at the _Ac3 transformation point or higher. The heat treatment temperature is, for example, 850 to 900 ° C. Hold at heat treatment temperature for a predetermined time. The holding time is not particularly limited, but is, for example, 10 to 180 minutes. After the holding time has elapsed at the normalizing temperature, the steel material is cooled. The cooling here is, for example, air cooling and forced air cooling. In the cooling of the normalizing treatment, cooling may be carried out at the initial stage of cooling, and after the temperature becomes equal to or less than the _Ar1 transformation point, slow cooling or forced air cooling may be carried out.

In the chemical composition of the seamless steel pipe for hot forging, the hardness low temperature toughness index defined by the formula (1) is the hardness at the center of the wall thickness of the seamless steel pipe after the above-mentioned normalizing treatment and the low temperature toughness. It is an index showing. Here, the normalizing treatment at 860 ° C. is defined as "standard normalizing treatment". In the present specification, the normalizing treatment at 860 ° C. is specifically that the normalizing temperature is 860 ° C., the holding time is 25 to 125 minutes, and the cooling method after the holding time has elapsed is to allow cooling ( It means normalizing treatment (natural cooling). If the hardness low temperature toughness index exceeds 0.68, excellent low temperature toughness cannot be obtained in the steel material after the above-mentioned normalizing treatment. More specifically, if the hardness low temperature toughness index defined by the formula (1) exceeds 0.68, the Charpy impact conforming to ASTM E23 after the standardizing treatment of the seamless steel pipe for hot forging. The impact value obtained in the test at -20 ° C is less than 48 J / cm ² .

If the low temperature toughness index defined by the formula (1) is 0.68 or less in the chemical composition of the seamless steel pipe for hot forging, it can be obtained by the Charpy impact test based on ASTM E23 after the standard normalizing treatment. The impact value at −20 ° C. is less than 48 J / cm ² .

Further, if the hardness low temperature toughness index defined by the formula (1) is less than 0.60, sufficient low temperature toughness can be obtained after the standard normalizing treatment, but sufficient hardness cannot be obtained. On the other hand, when the hardness low temperature toughness index is 0.60 or more, the tensile strength obtained by the tensile test based on ASTM E8 is 620 to 795 MPa after the standard normalizing treatment. Further, the Brinell hardness at the center position of the wall thickness obtained by the Brinell hardness test according to ASTM E10-08 is 187 HBW or more.

The preferred lower limit of the low temperature toughness index is 0.61. The preferred upper limit of the low temperature toughness index is 0.65.

[Tensile strength after standard normalizing treatment]
As described above, the tensile strength according to ASTM E8 after the seamless steel pipe for hot forging of the present embodiment is subjected to the reference normalizing treatment is 620 to 795 MPa. Here, the tensile strength is obtained by the following method.

Standard normalizing treatment is carried out for seamless steel pipes for hot forging. A round bar tensile test piece is collected from the central part of the wall thickness of the seamless steel pipe for hot forging after the standard normalizing treatment. For example, the diameter of the round bar tensile test piece is 6.35 mm, the length of the parallel portion is 35 mm, and the central axis of the round bar tensile test piece substantially coincides with the center position of the wall thickness of the seamless steel pipe for hot forging. The parallel portion of the round bar tensile test piece shall be parallel to the longitudinal direction (axial direction) of the seamless steel pipe for hot forging.

The above-mentioned round bar tensile test piece is collected from any four locations at a 90 ° pitch in the circumferential direction of the seamless steel pipe for hot forging. Tensile tests at room temperature (25 ° C.) and in the atmosphere are performed on four round bar tensile test pieces in accordance with ASTM E8 to obtain tensile strength (MPa). The average of the four obtained tensile strengths is defined as the tensile strength (MPa) of the seamless steel pipe for hot forging after the standard normalizing treatment.

[Brinell hardness at the center position of the wall thickness after standard normalizing treatment]
The Brinell hardness at the center position of the wall thickness of the seamless steel pipe for hot forging after the standard normalizing treatment is 187 HBW or more. Here, the Brinell hardness at the center position of the wall thickness is obtained by the following method.

Standard normalizing treatment is carried out for seamless steel pipes for hot forging. A seamless steel pipe for hot forging after the standard normalizing treatment is cut vertically in the longitudinal direction. A Brinell hardness test is carried out on the cut surface at four wall thickness center positions with a 90 ° pitch in the circumferential direction, in accordance with ASTM E10-08, with a test force of 750 kgf and an indenter diameter of 5 mm. The average of the obtained four Brinell hardnesses is defined as the Brinell hardness (HBW) at the center position of the wall thickness of the seamless steel pipe for hot forging after the standard normalizing treatment.

The preferred lower limit of Brinell hardness is 190 HBW, more preferably 195 HBW. The preferred upper limit of Brinell hardness is not particularly limited, but is, for example, 240 HBW, more preferably 230 HBW.

[Low temperature toughness after standard normalizing treatment]
The impact value of the seamless steel pipe for hot forging after the standard normalizing treatment at −20 ° C. is 48 J / cm ² or more. Here, the impact value at −20 ° C. is obtained by the following method.

Standard normalizing treatment is carried out for seamless steel pipes for hot forging. A V-notch test piece is collected from the central portion of the wall thickness of the seamless steel pipe for hot forging after the standard normalizing treatment. The V-notch test piece has a width of 10 mm and a length of 55 mm. The length direction of the V-notch test piece shall be parallel to the longitudinal direction of the seamless steel pipe for hot forging. The V notch is made so that the crack grows in the wall thickness direction. Three of the above-mentioned V-notch test pieces are prepared. Three V-notch test pieces are subjected to a Charpy impact test at −20 ° C. in accordance with ASTM E23. The minimum value of the three impact values (J / cm ² ) obtained by the test is defined as the impact value (J / cm ² ) at −20 ° C.

The preferable lower limit of the impact value at −20 ° C. is 50 J / cm ² , and more preferably 55 J / cm ² .

[Outer diameter and wall thickness of seamless steel pipe for hot forging]
The outer diameter and wall thickness of the seamless steel pipe for hot forging are not particularly limited. The lower limit of the outer diameter of the seamless steel pipe for hot forging is preferably 100.0 mm, more preferably 150.0 mm, still more preferably 170.0 mm. The upper limit of the outer diameter of the seamless steel pipe for hot forging is preferably 400.0 mm, more preferably 350.0 mm, still more preferably 320.0 mm. The preferable lower limit of the wall thickness of the seamless steel pipe for hot forging is 10.0 mm, more preferably 12.0 mm, still more preferably 12.5 mm. The upper limit of the wall thickness of the seamless steel pipe for hot forging is preferably 40.0 mm, more preferably 38.0 mm, still more preferably 36.0 mm.

[Production method]
An example of a method for manufacturing a seamless steel pipe for hot forging according to the present embodiment will be described. The seamless steel pipe for hot forging according to the present embodiment may be manufactured by a manufacturing process other than the following manufacturing process. The following manufacturing process is an example of a suitable manufacturing process for the seamless steel pipe for hot forging of the present embodiment. An example of a suitable manufacturing process includes a material manufacturing process and a hot working process. Hereinafter, each step will be described in detail.

[Material manufacturing process]
In the material manufacturing process, a material is manufactured using molten steel having the above-mentioned chemical composition and having a hardness low temperature toughness index of 0.60 to 0.68 defined by the formula (1). Specifically, slabs (slabs, blooms, or billets) are manufactured by a continuous casting method using molten steel. An ingot may be manufactured by an ingot method using molten steel. If necessary, slabs, blooms or ingots may be lump-rolled to produce billets. The material (slab, bloom, or billet) is manufactured by the above steps.

[Hot working process]
In the hot working process, the prepared material is hot-worked to produce a seamless steel pipe for hot forging. First, the material is heated in a heating furnace. The heating temperature is not particularly limited, but is, for example, 1100 to 1300 ° C. The Mannesmann method is carried out on the material extracted from the heating furnace to manufacture the raw pipe. Specifically, a raw pipe is manufactured by drilling and rolling a material with a drilling machine. Stretch rolling with a mandrel mill is carried out on the raw pipe after drilling and rolling. Further, if necessary, constant diameter rolling is carried out by a reducer. A seamless steel pipe is manufactured by the above hot working process. Cool the manufactured seamless steel pipe. The cooling method is not particularly limited. The cooling method may be free cooling, forced air cooling, or quenching. However, in order to reduce the manufacturing cost, the cooling method is preferably allowed to cool. In the hot working step described above, hot working by the Mannesmann method has been described, but instead of the Mannesmann method, a seamless steel pipe for hot forging may be manufactured by a hot extrusion method.

Through the above manufacturing process, the seamless steel pipe for hot forging of the present embodiment is manufactured. As described above, the seamless steel pipe for hot forging of the present embodiment may be an as-rolled (As-Rolled) material.

[Manufacturing method of pressure vessel]
A method for manufacturing a pressure vessel using the above-mentioned seamless steel pipe for hot forging will be described. The accumulator shown in FIG. 1 will be described as an example of the pressure vessel. The method for manufacturing a pressure vessel includes a hot forging step and a normalizing treatment step. Hereinafter, each step will be described in detail.

[Hot forging process]
First, hot forging is performed on a seamless steel pipe for hot forging. By hot forging, mirror portions 3A and 3B shown in FIG. 1 are molded at the end of a seamless steel pipe for hot forging to form a pressure vessel shape. Hereinafter, the seamless steel pipe after hot forging is defined as an "intermediate product". The heating temperature of the seamless steel pipe for hot forging during the hot forging step is, for example, 1000 to 1300 ° C. Cool the seamless steel pipe after hot forging. The cooling method is not particularly limited. The cooling method may be free cooling or forced air cooling. An intermediate product is manufactured by the above process.

[Normalizing process]
Strain is locally accumulated in the intermediate product after hot forging. Therefore, in order to reduce local strain and make the mechanical properties uniform, normalizing treatment is performed on the intermediate product. In the normalizing treatment, the intermediate product after hot forging is inserted into a heat treatment furnace and heated at the _Ac3 transformation point or higher. The normalizing temperature is, for example, 850 to 900 ° C. Hold at the above normalizing temperature for a predetermined time. The retention time is, for example, 10 to 180 minutes. After the holding time has elapsed at the normalizing temperature, the steel material is cooled. The cooling here is, for example, air cooling and forced air cooling. In the cooling of the normalizing treatment, cooling may be carried out at the initial stage of cooling, and after the temperature becomes equal to or less than the _Ar1 transformation point, slow cooling or forced air cooling may be carried out.

A pressure vessel is manufactured by the above manufacturing process. The pressure vessel not only has high strength and high hardness, but also exhibits excellent low temperature toughness even when used in cold regions. Specifically, when the above-mentioned normalizing treatment is the standard normalizing treatment, the tensile strength obtained by the tensile test based on ASTM E8 is 620 to 795 MPa, and the Brinell hardness test based on ASTM E10-08. The Brinell hardness at the center position of the wall thickness obtained in the above method is 187 HBW or more, and the impact value at −20 ° C. obtained by the Charpy impact test based on ASTM E23 is 48 J / cm ² or more.

A molten steel having the chemical composition shown in Table 1 was produced.

The "hardness low temperature toughness index" in Table 1 indicates the hardness low temperature toughness index defined by the formula (1) in each steel number. In Table 1, "A _c1 point" indicates the _Ac1 point of each steel number, and " _Ac3 point" indicates the _Ac3 point of each steel number. "-" In Table 1 indicates that the component is not contained (not detected).

A steel material (ingot) was manufactured by the ingot method using the above-mentioned molten steel. Hot rolling was carried out on the ingot to produce a steel plate having a plate thickness of 13 to 35 mm. The steel sheet produced by hot rolling was allowed to cool to room temperature (25 ° C.). Through the above manufacturing process, a test material (steel plate) simulating a seamless steel pipe for hot forging was manufactured.

For each test material, the following treatment was carried out by simulating the manufacturing process of the pressure vessel (hot forging step and normalizing treatment step). First, the hot forging process was simulated, and the test material of each test number shown in Table 2 was heated at 1100 ° C. for 5 minutes, and the heated test material was allowed to cool to room temperature (25 ° C.). Then, the normalizing treatment was carried out at the normalizing temperature (° C.) and the holding time (minutes) shown in Table 2.

The following tensile test, Brinell hardness test, and low temperature toughness test were carried out on the test materials of each test number after the normalizing treatment.

[Tensile test]
A round bar tensile test piece was collected from the central portion of the plate thickness of the test material of each test number after the normalizing treatment (corresponding to the central portion of the wall thickness in the seamless steel pipe). The diameter of the round bar tensile test piece was 6.35 mm, the length of the parallel portion was 35 mm, and the central axis of the round bar tensile test piece was substantially aligned with the center position of the plate thickness of the test material. The parallel portion of the round bar tensile test piece was parallel to the longitudinal direction (axial direction) of the test material. The prepared round bar tensile test pieces were collected from any four locations. Tensile strength (MPa) was obtained by conducting a tensile test in the air at room temperature (25 ° C.) in accordance with ASTM E8 on the four round bar tensile test pieces collected. The average of the four obtained tensile strengths was defined as the tensile strength TS (MPa) at the test number.

[Brinell hardness test]
The test material of each test number after the normalizing treatment was cut vertically in the longitudinal direction. A Brinell hardness test compliant with ASTM E10-08 was performed on the cut surface at any four central positions of the plate thickness. The test force in the test was 750 kgf, and the indenter diameter was 5 mm. The average of the four Brinell hardnesses obtained was defined as the Brinell hardness (HBW) at that test number.

[Low temperature toughness test]
A V-notch test piece was collected from the central portion of the plate thickness of the test material of each test number after the normalizing treatment. The V-notch test piece had a width of 10 mm and a length of 55 mm. The length direction of the V-notch test piece was parallel to the longitudinal direction (rolling direction) of the test material. The V-notch was made so that cracks would grow in the thickness direction of the test material. Three V-notch test pieces were prepared for the test materials of each test number. Three V-notch test pieces were subjected to a Charpy impact test at −20 ° C. according to ASTM E23. The minimum of the three impact values (J / cm ² ) obtained by the test was defined as the impact value (J / cm ² ) of the test number at −20 ° C. Table 2 shows all the impact values obtained from the three V-notch test pieces.

[Test results]
The test results are shown in Table 2. With reference to Table 2, the chemical compositions of test numbers 3, 4, 7, and 8 were appropriate and the hardness low temperature toughness index was in the range of 0.60 to 0.68. Therefore, the tensile strength TS after the standard normalizing treatment is 620 to 795 MPa, the Brinell hardness at the center position of the plate thickness corresponding to the center position of the wall thickness is 187 HBW or more, and the impact value at -20 ° C is 48 J /. It was cm ² or more.

On the other hand, in Test No. 1, although the content of each element was appropriate, the hardness low temperature toughness index was less than 0.60. Therefore, although sufficient tensile strength and sufficient low-temperature toughness were obtained, the Brinell hardness at the center position of the plate thickness corresponding to the center position of the wall thickness was less than 187 HBW, and sufficient hardness could not be obtained. ..

In test number 2, the Ni content was low. Therefore, the impact value at −20 ° C. was less than 48 J / cm ² .

In test numbers 5 and 6, the C content was too high. Therefore, in all of the test numbers 5 and 6, the impact value at −20 ° C. was less than 48 J / cm ² .

In Test No. 9, the chemical composition was equivalent to that of a conventional seamless steel pipe for hot forging. In test number 9, the C content was high and the Ti content was also high. On the other hand, the Ni content was low and V was not contained. Therefore, the impact values at −20 ° C. obtained in the three V-notch tests were all less than 48 J / cm ² .

In test number 10, the C content was low and the Ni content was also low. Therefore, the Brinell hardness was less than 187 HBW, and sufficient hardness could not be obtained.

In Test No. 11, although the content of each element was appropriate, the hardness low temperature toughness index exceeded 0.68. Therefore, the impact value at −20 ° C. was less than 48 J / cm ² .

The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.

1 Pressure vessel 2 Body 3A, 3B Mirror

Claims (2)

By mass%,
C: 0.25 to 0.33%,
Si: 0.01-0.35%,
Mn: 1.20 to 1.60%,
P: 0.020% or less,
S: 0.010% or less,
Ni: 0.11 to 0.30%,
Cr: 0.01-0.30%,
V: 0.01-0.08%,
Ti: 0 to 0.005%,
B: 0 to 0.0010%,
Al: 0.01-0.10%,
N: 0.0080% or less,
Mo: 0-0.05%,
Nb: 0 to 0.010% and
Remaining: Fe and impurities,
The hardness low temperature toughness index defined by the formula (1) is 0.60 to 0.68.
Seamless steel pipe for hot forging.
Hardness low temperature toughness index = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). The seamless steel pipe for hot forging according to claim 1.
In the seamless steel pipe for hot forging after normalizing at 860 ° C.
The tensile strength obtained by the tensile test according to ASTM E8 is 620 to 795 MPa.
The Brinell hardness at the center position of the wall thickness obtained by the Brinell hardness test according to ASTM E10-08 is 187 HBW or more.
The impact value at −20 ° C. obtained by the Charpy impact test according to ASTM E23 is 48 J / cm ² or more.
Seamless steel pipe for hot forging.

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