JP7230454B2 - Steel materials for seamless steel pipes - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel material for seamless steel pipes that is particularly suitable as a material for seamless steel pipes used, for example, for oil country tubular goods and line pipes. It is.

Oil country tubular goods and line pipes are used to produce oil and natural gas (hereinafter referred to as oil). Oil country tubular goods are steel pipes that are used to drill wells for oil and natural gas, and line pipes are steel pipes that are laid on the seabed, under the sea, or on the ground to transport oil or natural gas.
As the development of oil fields and gas fields (hereafter referred to as oil fields) progresses, deep oil fields that are deep and subject to high pressure, high-pressure transportation of natural gas, highly corrosive oil fields containing a large amount of H ₂ S gas, seabed and The development of oil fields with severe conditions, such as oil fields in cold regions, is increasing, and steel materials with high strength, high corrosion resistance, and high toughness are required.

In order to solve these problems, inclusions contained in steel materials are harmful and cause various problems. First, inclusions, which are non-metallic, tend to serve as starting points for fracture, so if there are many coarse inclusions, the toughness of the steel material, especially the low-temperature toughness, is lowered. In a sour environment containing a large amount of H ₂ S gas, sulfide stress-induced cracking (SSC) and hydrogen-induced cracking (HIC) are likely to occur starting from inclusions in the steel material.

A typical harmful inclusion is MnS. In addition to being a starting point for pitting corrosion and causing SSC, it is easily elongated during rolling, which causes HIC and a decrease in toughness. MnS is generated in the micro-segregation between the dendrite trees of the cast slab and the central segregation. In particular, the MnS generated in the center segregation part is coarse, so that it is stretched during rolling, and the length in the rolling direction may reach several hundred μm or more, which greatly affects the product characteristics. rice field.

Therefore, for example, techniques for reducing the amount of MnS generated by reducing sulfur content and Ca treatment, techniques for preventing center segregation by light reduction at the final stage of solidification during continuous casting, and the like have been developed. On the other hand, microsegregation between dendrite trees has not been regarded as a problem so far. However, in recent years, since the Mn concentration has been increasing in order to increase the strength and hardenability, it has become necessary to take countermeasures against MnS in the micro-segregation part.

By the way, the degree of influence of inclusions, including MnS in the center segregation, varies depending on the steel pipe manufacturing method. Steel pipes are broadly classified into welded steel pipes, which are manufactured by forming steel materials into a cylindrical shape and then butt-welding them in the axial direction (seam welding), and seamless steel pipes, which are manufactured by making a hole in the center of a billet. . Welded steel pipes have the advantage of high dimensional accuracy because they are made of steel, but the center segregation of the steel, especially the stretched MnS remaining in the center segregation, greatly affects the properties of the steel pipe. On the other hand, with seamless steel pipes, since the central portion of the billet is perforated, center segregation is less of a problem than with welded steel pipes, and the problems of HIC and SSC are relatively small. In general, the weld zone of a welded steel pipe has inclusions in the steel that are extruded from the weld zone exposed on the inner and outer surfaces, inclusions in the steel that are stretched out during pressure welding after welding, and the metal structure of the weld zone. As a result of being different from the base material, there are problems such as inferior corrosion resistance and toughness compared to the base material. On the other hand, seamless steel pipes have no seam welds.

Thus, seamless steel pipes are less affected by inclusions such as MnS than welded steel pipes. Therefore, even if the [S] in the steel is high, the properties are good, and the desulfurization load during refining is low.
On the other hand, in the case of welded steel pipes, it is possible to refine the quality of the material by performing heat treatment during steel production, whereas in the case of seamless steel pipes, heat treatment is performed after pipe making, so the treatment conditions are restricted. As a countermeasure, for example, a large amount of Cr, Mo, or the like is often added in order to improve hardenability. Since there is a difference in the manufacturing process and the degree of influence of inclusions, there is a problem in applying the technology for steel materials used for welded steel pipes to the steel materials used for seamless steel pipes as it is in the technology related to inclusion control of steel materials.

Here, conventionally, various techniques have been proposed in order to improve the cleanliness of steel materials and improve their material properties.
For example, in Patent Document 1, Ca: 0.001 to 0.030% and one or more of rare earth elements: 0.001 to 0.030% are contained, S: 0.010% or less, steel Techniques have been proposed to improve the toughness in the direction perpendicular to the rolling direction by spheroidizing the inclusions and to reduce the susceptibility to sulfide stress corrosion cracking.
In addition, in Patent Document 2, one or more of Ca, Mg and REM are contained in a total of 0.0002 to 0.005%, S is 0.005% or less, and inclusion morphology control have proposed techniques for improving toughness and corrosion resistance.
Furthermore, in Patent Document 3, S is 0.005% or less, O is 0.005% or less, and the total of one or two types of Ca and REM: 0.0002 to 0.007% have proposed a technique for preventing the precipitation of MnS and improving the toughness and corrosion resistance of steel by controlling the morphology of inclusions.

JP-A-62-010241 JP 2010-242222 A Republished WO2007/023804

By the way, Ca and REM are not only desulfurizing elements, but also strong deoxidizing elements. Therefore, the amount of oxygen t. When [O] is high, oxides CaO and REM ₂ O ₃ are preferentially formed, so S is not sufficiently fixed as sulfide CaS and REMS (or oxysulfide REM ₂ O ₂ S), As a result, MnS may be generated. In order to control the composition of inclusions and thus suppress MnS formation, t. It is necessary to define [O].
Here, in Patent Documents 1 and 2, t. [O] is not specified. Moreover, in Patent Document 3, t. Only the upper limit is defined as [O]≦0.005%, which may be insufficient for MnS suppression. In addition, in Patent Document 3, since it is assumed that a material with a thickness of 30 mm or more is targeted, the Mn is limited to a high concentration range of 1.5 to 3.0%, and only the central segregation part In addition, MnS in the micro-segregation part may also become a problem.

Next, there is a concern that inclusions with a low melting point other than MnS may form. In any of the above Patent Documents 1 to 3, at least one of Ca and REM may be contained, and combined addition of Ca and REM is not essential. Therefore, when Ca alone is contained, a CaO—Al ₂ O ₃ -based oxide is produced, but t. Depending on [O], there is a concern that the composition may become a low-melting oxide composition and may be stretched during rolling. The low-melting point CaO--Al ₂ O ₃ -based oxide is easily elongated during rolling, and has a notch shape with a sharp tip, which lowers the toughness. In Patent Documents 1 and 2, t. Since [O] is not defined, t. When [O] is high, there is a high possibility that a low-melting-point oxide that is easily stretched will be formed. Patent document 3 discloses t. Although the upper limit is specified as [O]≤0.005%, t. Unless [Ca] and [REM] are specified according to [O], the generation of low-melting-point oxides cannot be sufficiently avoided.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel material for seamless steel pipes that suppresses the formation of MnS and low-melting-point oxides and has excellent toughness.

In order to solve the above-described problems, the present inventors have made intensive studies and obtained the following findings.
Inclusions extending in the rolling direction are particularly harmful because their tips are notched. Moreover, even non-stretchable inclusions are highly harmful if they are coarse, and it is important to reduce them. In addition to MnS described above, low-melting oxides can be used as elongated inclusions.

Low-melting-point oxides generally refer to oxides that are in a liquid phase in molten steel (normally assumed to be 1600° C.). Although it is in a solid phase at the rolling temperature, it easily deforms and stretches due to its low melting point. Although the degree of elongation (aspect ratio) is lower than that of MnS, it is regarded as a problem in high-strength steel materials. For example, a CaO—Al ₂ O ₃ -based oxide having a low melting point composition is generated during Ca treatment, and an example of stretching is observed. As a means of avoiding low melting point compositions, t. regulate and control [O], or t. It is effective to control [Ca] according to [O] to control the oxide to a high melting point composition.

As non-stretchable inclusions, sulfides such as TiS and (carbo)nitrides such as Ti(C)N may be generated depending on the steel type. I will assume and explain. Conventionally, the size of non-stretchable inclusions has been viewed as a problem, and emphasis has been placed on measures to remove coarse inclusions (oxides). However, against the background of increasing the strength of steel materials and emphasizing safety, further reduction (purification) of the size and number (quantity), which were not regarded as problems in the past, has come to be demanded. Therefore, in order to float and remove the oxides from the molten steel, it is widely practiced to extend the secondary refining stirring time. The total amount of oxides in the steel is t. [O] is an index. For example, in bearing steel that requires a high degree of cleanliness, t. [O] is strictly controlled, and by extending the stirring time, t. [O] steadily decreases.

The present invention has been made based on the above findings, and the steel material for seamless steel pipes according to the present invention contains, by mass%,
C: 0.20% or more and less than 0.34%,
Si: 0.15% or more and 0.40% or less,
Mn: 0.30% or more and less than 1.50%,
Al: 0.031% or more and 0.050% or less,
Ti: 0.002% or more and 0.060% or less,
Cr: 0.02% or more and 1.50% or less,
Mo: 0% or more and 1.0% or less,
Ca: 0.0003% or more and 0.0050% or less,
REM: 0.0003% or more and 0.0050% or less,
and the balance consists of iron and impurities, and among the impurities, P, S, O, and N,
P: 0.020% or less,
S: 0.0050% or less,
O: 0.0050% or less,
N: 0.0075% or less,
limited to
When the mass ratio of each component element M in the steel is represented by [M], [REM] is a first threshold value [REM_1] indicated by the following formula (1) and a second threshold value indicated by the following formula (2) [REM_2] is divided into three regions, and in each region, the amount of Ca [Ca] in the steel is a1 shown in the following formula (3), b1 shown in the following formula (4), and shown in the following formula (5) Using a2, b2 shown in formula (6), and b3 shown in formula (7), the following formulas (8), (9), and (10) are satisfied,
With a cross-section parallel to the rolling direction and the thickness direction of the steel pipe as an observation plane, MnS, CaS, Al ₂ O ₃ -CaO-REM ₂ O having an equivalent circle diameter of 1.0 μm or more contained in the steel The number density of inclusions containing at least one of _ternary oxides is 100/mm ² or less.
(1) Formula: [REM_1] = 0.400 x [S] - 0.0006
(2) Formula: [REM_2] = 0.005 x [Al] + 0.300 x [S] + 1.590 x [O] - 0.0014
(3) Formula: a1 = 5.000 x [Al] -35.714 x [S] -29.870 x [O] -0.0675
(4) Formula: b1 = -0.005 x [Al] + 0.400 x [S] + 1.155 x [O] -0.0007
(5) Formula: a2 = 2.143 x [Al] + 53.571 x [S] + 31.169 x [O] - 1.0471
(6) Formula: b2 = -0.002 x [Al] + 0.618 x [S] + 1.100 x [O] -0.0010
(7) Formula: b3 = 0.475 × [S] - 0.0005
(8) Formula: When [REM] < [REM_1], [Ca] ≥ a1 × [REM] + b1
(9) Formula: If [REM_1] ≤ [REM] < [REM_2], [Ca] ≥ a2 × [REM] + b2
(10) Formula: When [REM_2] ≤ [REM] ≤ 0.0050%, [Ca] ≥ b3

According to the steel material for seamless steel pipes having such a configuration, as described above, t. Since the relationship between [REM] and [Ca] is defined in consideration of [O], even if Ca and REM form oxides, the amount of Ca and REM that binds to S is secured, and MnS It becomes possible to reduce system inclusions.
In addition, [Ca], [REM], t. [Al], t. Since the relationship between the amounts of [O] and [S] is defined as described above, the ratio of Al ₂ O ₃ in the inclusions can be reduced, and the formation of low melting point oxides can be suppressed. Become. Furthermore, Ca and REM are added in combination, and the main composition of the oxide is a ternary system containing REM ₂ O ₃ , an Al ₂ O ₃ -CaO-REM ₂ O ₃ system, thereby improving the crushability during rolling. is increased, and the size after rolling becomes finer, so the harmfulness can be reduced.
Furthermore, since the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is specified to be 100/mm ² or less, the number of inclusions that cause fracture is sufficiently reduced.
Therefore, it is possible to provide a high-quality steel material for seamless steel pipes in which the occurrence of cracks or the like during use or processing is suppressed.

Here, in the steel material for seamless steel pipes of the present invention, further, in mass %,
Cu: 0% or more and 0.05% or less,
Nb: 0% or more and 0.05% or less,
V: 0% or more and 0.10% or less,
Ni: 0% or more and 0.10% or less,
B: 0% or more and 0.0050% or less,
It is good also as a structure containing 1 type, or 2 or more types selected from the group which consists of.
In this case, it is possible to improve various properties of the steel material for seamless steel pipes by further including the above elements.

In the steel material for seamless steel pipes of the present invention, among the inclusions present in the steel pipe, the mass of each compound MX in inclusions containing at least Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxides When the ratio is (MX), it is preferable to satisfy both the following formulas (11) and (12).
(11) Formula: (Al ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≦0.40
(12) Formula: (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0.05
In this case, since the composition of the inclusions is defined as described above, it is possible to suppress a decrease in the melting point of the inclusions and further reduce the number of drawn inclusions caused by the low-melting-point oxide. Become.

As described above, according to the present invention, it is possible to suppress the formation of MnS and low-melting-point oxides and to provide a steel material for seamless steel pipes with excellent toughness.

4 is a graph showing how stretched inclusions are generated in laboratory experiments. t. FIG. 10 is a graph showing the results stratified by [O]. FIG. (a) is t. [O]: 0.0005% or more and less than 0.0012%, (b) is t. [O]: 0.0012% or more and less than 0.0017%, (c) is t. [O]: 0.0017% or more and less than 0.0022%, (d) is t. [O]: 0.0022% or more and less than 0.0030%. 4 is a graph showing conditions for suppressing the formation of stretched inclusions in laboratory experiments. 4 is a graph showing the relationship between the number density of stretched inclusions and the Charpy absorbed energy. 4 is a graph showing the relationship between the number density of inclusions having an equivalent circle diameter of ≧1.0 μm and the number density of stretched inclusions.

Steel materials for seamless steel pipes according to embodiments of the present invention will be described below with reference to the accompanying drawings. In addition, this invention is not limited to the following embodiment.
Here, the steel material for seamless steel pipes of the present embodiment is used as seamless steel pipes that can be used for oil well pipes and line pipes. Furthermore, the steel material for seamless steel pipes according to the present embodiment is a cast material such as a billet.

In the steel material of the present embodiment, in mass%, C: 0.20% or more and less than 0.34%, Si: 0.15% or more and 0.40% or less, Mn: 0.30% or more and 1.50% less than, Al: 0.003% to 0.050%, Ti: 0% to 0.060%, Cr: 0% to 1.50%, Mo: 0% to 1.0%, Ca: 0.0003% or more and 0.0050% or less, REM: 0.0003% or more and 0.0050% or less, the balance being iron and impurities, and further, among the impurities, P, S, O and N are limited to P: 0.020% or less, S: 0.0050% or less, O: 0.0050% or less, and N: 0.0075% or less.

Then, when the mass ratio of each component element M in the steel is indicated by [M], [REM] is a first threshold value [REM_1] indicated by the following formula (1), and a first threshold value [REM_1] indicated by the following formula (2) Three regions are distinguished by two threshold values [REM_2], and in each region, the amount of Ca [Ca] in the steel is a1 shown in the following formula (3), b1 shown in the following formula (4), and formula (5) is defined to satisfy the following expressions (8), (9), and (10) using a2 shown in (6), b2 shown in expression (6), and b3 shown in expression (7).
(1) Formula: [REM_1] = 0.400 x [S] - 0.0006
(2) Formula: [REM_2] = 0.005 x [Al] + 0.300 x [S] + 1.590 x [O] - 0.0014
(3) Formula: a1 = 5.000 x [Al] -35.714 x [S] -29.870 x [O] -0.0675
(4) Formula: b1 = -0.005 x [Al] + 0.400 x [S] + 1.155 x [O] -0.0007
(5) Formula: a2 = 2.143 x [Al] + 53.571 x [S] + 31.169 x [O] - 1.0471
(6) Formula: b2 = -0.002 x [Al] + 0.618 x [S] + 1.100 x [O] -0.0010
(7) Formula: b3 = 0.475 × [S] - 0.0005
(8) Formula: When [REM] < [REM_1], [Ca] ≥ a1 × [REM] + b1
(9) Formula: If [REM_1] ≤ [REM] < [REM_2], [Ca] ≥ a2 × [REM] + b2
(10) Formula: When [REM_2] ≤ [REM] ≤ 0.0050%, [Ca] ≥ b3

Furthermore, in the steel material of the present embodiment, the number density of inclusions having an equivalent circle diameter of 1.0 μm or more contained in the steel is 100/mm ² or less.

In addition, in the steel material of the present embodiment, if necessary, in terms of mass%, Cu: 0% or more and 0.05% or less, Nb: 0% or more and 0.05% or less, V: 0% or more and 0% 10% or less, Ni: 0% or more and 0.10% or less, B: 0% or more and 0.0050% or less, one or more selected from the group consisting of 0% or more and 0.0050% or less.

Furthermore, in the steel material of the present embodiment, where (MX) is the mass ratio of each compound MX in the inclusions present inside, both the following formulas (11) and (12) are satisfied: may be
(11) Formula: (Al ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≦0.40
(12) Formula: (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0.05

In the following, the reasons for defining each component in the steel material of the present embodiment as described above will be described.

(C: 0.20% or more and less than 0.34%)
C (carbon) is an important element for ensuring the strength and hardenability of steel materials. By setting the C content to 0.20% or more, sufficient strength for the application is ensured. On the other hand, when the C content is 0.34% or more, the toughness deteriorates. Therefore, the C content is controlled within a range of more than 0.20% and less than 0.34%.

(Si: 0.15% or more and 0.40% or less)
Si (silicon) is an element that acts as a deoxidizing agent and is effective in improving hardenability and strength of steel materials. On the other hand, if it exceeds 0.40%, the toughness deteriorates. Therefore, the Si content is controlled within the range of 0.15% to 0.40%.

(Mn: 0.30% or more and less than 1.50%)
Mn (manganese) is an element that acts as a deoxidizing agent and is also an effective element for enhancing hardenability and improving the strength and toughness of steel materials. The strength after quenching is affected by the cooling rate during quenching as well as the amount of alloying elements such as Mn. The thicker the steel material, the lower the cooling rate at the center of the thickness, so the thicker the material, the lower the hardenability. Therefore, it is preferable to change the amount of alloying elements that affect hardenability depending on the thickness of the steel material.
The steel material of the present embodiment was determined by assuming a material for a general thick steel pipe, a steel pipe with a thickness of approximately ten and several mm to twenty and several mm. (Therefore, for extremely thick materials exceeding this range, for example, 30 mm or more, hardenability at the center of the thickness cannot be ensured unless the amount of alloying elements such as Mn is increased.)

In the case of this embodiment, if the Mn content is less than 0.30%, the effect of increasing strength and toughness cannot be sufficiently obtained. On the other hand, when the Mn content is 1.50% or more, the steel material with the assumed thickness of 10 mm to 20 mm has an excessively high strength and deteriorates toughness, and coarse MnS is generated in the segregation part. As a result, the HIC resistance of the steel material may deteriorate. Therefore, the Mn content is controlled within the range of 0.30% or more and less than 1.50%.
The lower limit of the Mn content is preferably 0.80% or more, and the upper limit of the Mn content is preferably 1.35% or less.

(Al: 0.003% or more and 0.050% or less)
Al (aluminum) is an element that acts as a deoxidizing agent and is an element that is effective in improving the toughness of steel materials by fixing N. In the present embodiment, it refers to total Al (hereinafter referred to as t.Al), which is the sum of not only so-called acid-soluble Al that is dissolved in the steel matrix, but also Al that forms oxides and nitrides. . If the Al content is less than 0.003%, the above effects cannot be sufficiently obtained, so the Al content must be 0.003% or more. On the other hand, if the Al content exceeds 0.050%, the effect of the inclusion is saturated, and coarse inclusions increase. These coarse inclusions degrade the toughness or tend to cause surface flaws on the outer and inner surfaces of the steel pipe. Therefore, the Al content is controlled within the range of 0.003% or more and 0.050% or less.
The lower limit of the Al content is preferably 0.010% or more, and the upper limit of the Al content is preferably 0.040% or less.

(Ti: 0% or more and 0.060% or less)
Ti (titanium) has the effect of increasing the strength by forming carbonitrides and also has the effect of refining grains, so it may be contained within the range of 0.060% or less as necessary. On the other hand, when the Ti content exceeds 0.060%, coarse and angular carbonitrides are likely to be formed, degrading the toughness. Therefore, the Ti content is limited to 0.060% or less. The lower limit of the Ti content may be 0%. Moreover, considering current general refining (including secondary refining), the lower limit of the Ti content may be 0.0005% or more.

(Cr: 0% or more and 1.50% or less)
Cr (chromium) is an element effective in improving hardenability and strength of steel materials, as well as improving corrosion resistance. Therefore, if necessary, Cr may be contained within a range of 1.50% or less. Moreover, when the lower limit of the Cr content is set to 0.40%, the above effect can be preferably obtained. If the Cr content exceeds 1.50%, the toughness deteriorates. Furthermore, the inclusion effect saturates while the cost increases. Therefore, the Cr content is controlled to 1.50% or less.

(Mo: 0% or more and 1.0% or less)
Mo (molybdenum) is an element that has the effect of improving the strength of steel materials by improving hardenability and temper softening resistance, and also has the effect of improving corrosion resistance. Therefore, Mo may be contained within the range of 1.0% or less as needed. Moreover, when the lower limit of the Mo content is 0.10% or more, the above effects can be preferably obtained. On the other hand, when the Mo content exceeds 1.0%, the cost increases, but the effect of containing Mo saturates. Furthermore, if the Mo content exceeds 1.0%, the toughness of the steel deteriorates. For the above reasons, the upper limit of Mo content is set to 1.0%. In addition, the preferable range of Mo content is 0.30% or more and 0.80% or less.

(Ca: 0.0003% or more and 0.0050% or less)
Ca (calcium) is an effective element for reducing MnS and controlling the morphology of inclusions, thereby improving the toughness of steel materials. If the Ca content is less than 0.0003%, the above effect cannot be sufficiently obtained. On the other hand, when the Ca content exceeds 0.0050%, oxides, CaS-based inclusions, and the like become coarse, which may deteriorate the toughness of the steel material. Furthermore, when the Ca content exceeds 0.0050%, the nozzle refractory is likely to be eroded, and the continuous casting operation may become unstable. Therefore, the Ca content is controlled within the range of 0.0003% or more and 0.0050% or less.
The lower limit of the Ca content is preferably 0.0010% or more, and the upper limit of the Ca content is preferably 0.0035% or less.

(REM: 0.0003% or more and 0.0050% or less)
REM (Rare Earth Metal) means rare earth elements, 17 elements of scandium Sc (atomic number 21), yttrium Y (atomic number 39) and lanthanoids (15 elements from lanthanum with atomic number 57 to lutecium with atomic number 71) is a generic term for The steel material according to the present embodiment contains at least one element selected from these elements. In general, REM is often selected from among Ce (cerium), La (lanthanum), Nd (neodymium), Pr (praseodymium), etc., because of its availability. As a method of addition, for example, adding misch metal, which is a mixture of these elements, to steel is widely practiced. The main components of misch metal are Ce, La, Nd, and Pr. In the steel material according to this embodiment, the total amount of these rare earth elements contained in the steel material is defined as the REM content. In this embodiment, since the average atomic weight of misch metal is about 140, the atomic weight of REM is set to 140.

REM is an element effective for reducing MnS, controlling the morphology of inclusions, and improving the toughness of steel materials. If the REM content is less than 0.0003%, the above effect cannot be sufficiently obtained. On the other hand, when the REM content exceeds 0.0050%, nozzle clogging tends to occur during continuous casting. In addition, when the REM content exceeds 0.0050%, the number density of the REM inclusions (oxides and oxysulfides) generated is relatively high, so the lower surface of the cast slab that is curved during continuous casting of the cast slab. These REM inclusions are deposited on the surface. This may cause internal defects in the product obtained by rolling the slab, and further deteriorate the toughness of the steel material. Therefore, the REM content is controlled within the range of 0.0003% or more and 0.0050% or less.
The lower limit of the REM content is preferably 0.0010% or more, and the upper limit of the REM content is preferably 0.0030% or less.

Furthermore, the content of Ca and REM was determined by S, t. O, t. It is necessary to control according to the content of Al. Specifically, it is necessary to control the mass % content of each element in the chemical components within the range represented by the above formulas (1) to (10). The reasons for formulas (1) to (10) will be described later.

The steel material according to the present embodiment contains impurities in addition to the basic components described above. Here, the impurities refer to auxiliary raw materials such as scrap, P, S, O, N, Cd, Zn, Sb, W, Mg, Zr, As, Co, Sn, Pb, etc. mixed in from the manufacturing process. means an element of Since the content of these elements is not essential, the lower limit of the content of these elements is 0%. Among these, P, S, O, and N are restricted as follows in order to preferably exhibit the above effects. Moreover, it is preferable to limit the above impurities other than P, S, O, and N to 0.01% or less. However, even if these impurities are contained in an amount of 0.01% or less, the above effect is not lost. Here, % described is mass %.

(P: 0.020% or less)
An excessive amount of P deteriorates the toughness of the steel material. Therefore, the P content is limited to 0.020% or less. The lower limit of the P content may be 0%. Moreover, considering current general refining (including secondary refining), the lower limit of the P content may be 0.005% or more.

(S: 0.0050% or less)
S (sulfur) is an impurity element that deteriorates the toughness of steel materials for seamless steel pipes by forming non-metallic inclusions mainly composed of MnS. Therefore, the S content is limited to 0.0050% or less, preferably 0.0040% or less. In addition, considering the current general refining load (including secondary refining), the lower limit of the S content may be 0.0020% or more. may be 0.0035% or more.

(O: 0.0050% or less)
O (oxygen) forms oxides (non-metallic inclusions), which agglomerates and coarsens, and depending on the composition of the oxides, elongates during rolling. It is an impurity element that deteriorates toughness. Therefore, the O content is limited to 0.0050% or less. The lower limit of the O content may be 0%. Moreover, considering current general refining (including secondary refining), the lower limit of the O content may be 0.0005% or more. The O content of the steel material for seamless steel pipes according to the present embodiment is the total O content ( t.O). The total O content (t.O) was analyzed by the commonly used “inert gas fusion-infrared absorption method” described in JIS G 1239 (established in 2014).

t. Controlling O is very important because it greatly affects the composition of the oxide and the total amount of oxide. Therefore, the t.c. O is included together with S and Al in the above formulas (1) to (10) defining the contents of Ca and REM. Qualitatively, in order to reduce low-melting-point oxides that are easily stretched and to reduce the total amount of oxides, t. It is preferable to reduce O. t. In order to reduce O, for example, the stirring time of secondary refining is extended to promote the removal of inclusions.

(N: 0.0075% or less)
N (nitrogen) is an impurity element that forms nitrides (non-metallic inclusions) and reduces the toughness of steel materials for seamless steel pipes. Therefore, the N content is limited to 0.0075% or less. The lower limit of the N content may be 0%. Also, considering current general refining (including secondary refining), the lower limit of the N content may be 0.0010%.

The steel material for seamless steel pipes according to the present embodiment has the above-described basic components controlled, and the remainder consists of iron and the above-described impurities. However, the steel material for seamless steel pipes according to the present embodiment may contain, in addition to these basic components, the following optional components in place of part of the remaining Fe, if necessary.

That is, the steel material for seamless steel pipes according to the present embodiment may further contain one or more of Cu, Nb, V, Ni, and B as selective components in addition to the basic components and impurities described above. good. The numerical limitation ranges of the selected components and the reasons for the limitations will be described below. Here, % described is mass %.

(Cu: 0% or more and 0.05% or less)
Cu (copper) is a selective element that has effects of improving strength and corrosion resistance of steel materials for seamless steel pipes. Therefore, Cu may be contained within a range of 0.05% or less, if necessary. Moreover, when the lower limit of the Cu content is set to 0.01% or more, the above effects can be preferably obtained. On the other hand, when the Cu content exceeds 0.05%, the effect of the Cu content is saturated, and hot working cracks may occur during pipe making due to molten metal embrittlement (Cu cracks). In addition, the preferable range of Cu content is 0.02% or more and 0.04% or less.

(Nb: 0% or more and 0.05% or less)
Nb (niobium) forms carbonitrides and is a selective element effective in preventing coarsening of crystal grains and improving the toughness of steel materials for seamless steel pipes. Therefore, Nb may be contained within a range of 0.05% or less, if necessary. Moreover, when the lower limit of the Nb content is 0.01% or more, the above effect can be preferably obtained. On the other hand, if the Nb content exceeds 0.05%, coarse Nb carbonitrides may be precipitated to deteriorate the toughness of the steel material for seamless steel pipes. In addition, the preferable range of Nb content is 0.02% or more and 0.04% or less.

(V: 0% or more and 0.10 or less)
V (vanadium), like Nb, forms carbonitrides and is a selective element effective in preventing grain coarsening and improving toughness. Therefore, V may be contained within the range of 0.10% or less, if necessary. Moreover, when the lower limit of the V content is 0.01% or more, the above effect can be preferably obtained. On the other hand, if the V content exceeds 0.10%, coarse V carbonitrides are formed, which may lead to deterioration of the toughness of the steel material for seamless steel pipes. In addition, the preferable range of V content is 0.02% or more and 0.04% or less.

(Ni: 0% or more and 0.10% or less)
Ni (nickel) is a selective element that is effective in improving the strength and toughness of steel materials for seamless steel pipes by improving hardenability. In addition, it is a selective element that also has the effect of preventing molten metal embrittlement (Cu cracking) when Cu is contained. Therefore, Ni may be contained within a range of 0.10% or less, if necessary. Moreover, when the lower limit of the Ni content is 0.01% or more, the above effect can be preferably obtained. On the other hand, when the Ni content exceeds 0.10%, the cost increases and the effect of the Ni content saturates, so the upper limit of the Ni content is made 0.10% or less. In addition, the preferable range of Ni content is 0.01% or more and 0.08% or less.

(B: 0% or more and 0.0050% or less)
B (boron) is an optional element that has the effect of increasing the hardenability and improving the strength of steel materials for seamless steel pipes. Therefore, B may be contained within the range of 0.0050% or less as needed. On the other hand, if the B content exceeds 0.0050%, nitrides are formed and the toughness of the steel material for seamless steel pipes is lowered, so the upper limit is made 0.0050% or less. In addition, it is preferable to make the upper limit of B content into 0.0020% or less.

Next, the results of laboratory experiments that provided knowledge for defining the number of inclusions and formulas (1) to (12) as described above will be described.

In a vacuum melting furnace, in mass%, C: 0.23% to 0.28%, Si: 0.23% to 0.28%, Mn: 0.90% to 1.00%, P: 0.006% or more and 0.009% or less, S: 0.0037% or more and 0.0043% or less, t. Al: 0.027% or more and 0.033% or less, Cr: 0.47% or more and 0.53% or less, Ti: 0.004% or more and 0.007% or less, t. A plurality of types of molten steel containing O: 0.0012% or more and 0.0030% or less were melted, and REM and Ca were added in varying amounts to produce 50 kg ingots. REM added misch metal containing Ce, La and Nd.
These ingots were hot-rolled to a thickness of 15 mm at a final rolling temperature of 920°C, water-cooled, and then tempered at 550°C to obtain hot-rolled steel sheets. It should be noted that each component value described above and hereinafter in the present invention is not the added amount but the analytical value (content) of the hot-rolled steel sheet.

In this embodiment, the content in steel and the composition of inclusions are described, but the units are all expressed in mass %. The content in steel is expressed as [M] with the element symbol M sandwiched between rectangular brackets. For example, the content of Ca in steel is expressed as [Ca]. It should be noted that the total oxygen amount t. O for t. Denoted as [O] Further, the content of the compound MX in the inclusion is denoted as (MX) by enclosing the compound MX in parentheses. For example, the content of CaO in inclusions is expressed as (CaO).

With a cross section (L cross section) parallel to the rolling direction and thickness direction of the hot-rolled steel sheet as an observation plane, inclusions in the hot-rolled steel sheet are observed with an optical microscope at a magnification of 400 (however, the shape of the inclusions is measured in detail. Observations were made at a total of 60 fields of view at a magnification of 1000 times. In each observation field, inclusions with a grain size (in the case of inclusions with a spherical shape) or major diameters (in the case of inclusions with a non-spherical shape) of 1 μm or more are observed. They were classified into those having an aspect ratio of 3.0 or less and those exceeding 3.0, and their number densities were measured. A sample with an inclusion number density of more than 10 inclusions/mm ² with an aspect ratio of >3.0 was evaluated as “poor”, and a sample with an inclusion number density of 10 inclusions/mm ² or less was evaluated as “good”.

In addition, EPMA (Electron Probe Micro Analysis), or EDX (Energy Dispersive X-ray Analysis, Energy Dispersive X-Ray Analysis) SEM (Scanning Electron Microscope) hot rolled using Inclusions in the steel plate were analyzed.

Then, the Charpy impact value at room temperature (0° C.) was measured as an index of the toughness of the hot-rolled steel sheets obtained above. A full-size Charpy test piece was taken from the C direction of the steel plate. That is, a test piece having a length of 55 mm in the width direction of the steel sheet, a height of 10 mm in the longitudinal (rolling) direction of the steel sheet, and a width of 10 mm in the thickness direction of the steel sheet was sampled. A 2 mm V notch was processed on a surface of 55 mm length×10 mm in the steel plate thickness direction. If the test piece is taken in this direction, the L cross section of the steel sheet becomes the fracture surface, so the influence of the stretched inclusions can be easily evaluated.

The composition of inclusions observed in hot-rolled steel sheets is MnS, CaS, Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxides. In addition to the case where each of the above three types exists alone, for example, when MnS or CaS adheres to a part around the Al ₂ O ₃ —CaO—REM ₂ O ₃ oxide, a plurality of types may exist. coexisting phases in many cases.

First, the results of investigations on the cleanliness of hot-rolled steel sheets will be described. FIG. 1 shows the content of Ca and REM slabs and the formation of elongated inclusions. "○" in Fig. 1 indicates a good material with an aspect ratio > 3.0 and the number of inclusions is 10/mm ² or less, and "△" in Fig. 1 indicates a poor material with more than 10/mm ² . indicates
Here, in FIG. 1, in which all the samples are plotted, the good material and the inferior material are mixed, and the boundary conditions for preventing the generation of harmful stretched inclusions are unclear.

As a result of analyzing the composition of inclusions stretched with an aspect ratio exceeding 3.0 by the EDS attached to the SEM, (i) the case where MnS alone is stretched and (ii) Al ₂ O ₃ --CaO-REM ₂ O In the case of a _ternary oxide with a low melting point composition (MnS and CaS may adhere to part of the surroundings. As will be described later, from the EDS analysis results, MnS and CaS adhering to the surroundings and oxides can be separated. It is possible to calculate the composition by dividing it.).

As a result of detailed analysis of the composition of the latter (ii), an oxide with (Al ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}>0.40 has an aspect ratio of >3.0.
On the other hand, when (Al ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≦0.40, the oxide aspect ratio≦3.0. This is probably because the melting point of the oxide rises, making it difficult to stretch during rolling.

The compositions of inclusions (CaO), (Al ₂ O ₃ ), and (REM ₂ O ₃ ) can be calculated based on inclusion analysis results by EDS attached to SEM, taking into account the mass balance of the elements. . EDS analysis results are output in mass % for each element. In the following description, the content of the element M detected from inclusions is expressed as (M).
From each of (Ca), (Al), and (REM), the atomic weight is used to calculate the amount of oxides, and formulas (11) and (12) can be calculated. For example, (Al ₂ O ₃ ) can be calculated as (27×2+16×3)/(27×2)×(Al), using the atomic weight of 27 for Al and 16 for oxygen. The inclusions were observed with an SEM to confirm that AlN was not generated in the inclusions. Since AlN has a characteristic angular shape, it can be easily identified by SEM observation. Here, the problem is the generation of AlN having a size of submicron order or larger, which affects the calculation of the inclusion composition.
In a similar manner, (CaO) can be obtained from (Ca), and (REM ₂ O ₃ ) can be obtained from (REM). Here, (REM) is the total amount of rare earth elements detected, specifically the total amount of Ce, La, and Nd detected in this experiment.

In addition, since Ca and REM form sulfides in addition to oxides, considering the mass balance of S, Ca and REM that combine with S to form sulfides are subtracted to form oxides. Care should be taken in determining the amount of Ca and REM.
Specifically, first, the amount of S forming MnS is calculated from 32/55×(Mn) using the atomic weights of Mn and S, and is excluded from the total amount of S. This is because, since this experiment was performed on Al-deoxidized steel, Mn detected from inclusions can be considered to have been detected from MnS. This residual amount of S is the amount of S forming CaS, REM ₂ O ₂ S (oxysulfide), and REMS (sulfide in which REM atoms and S atoms are bonded at a molar ratio of 1:1). Thermodynamically, CaS is most likely to form, followed by REM ₂ O ₂ S, and finally REMS. In this order, the amounts of sulfides and oxysulfides produced can be calculated in consideration of the mass balance of S.
The amounts of Ca and REM that form oxides are obtained by subtracting the Ca and REM that form sulfides and oxysulfides thus obtained from the respective total amounts. Using the atomic weights of Ca, REM, and O, (CaO) and (REM ₂ O ₃ ) can be calculated. In many cases, S existed as MnS and CaS, and few were bound to REM in terms of mass balance. That is, Ca often existed as CaS and CaO, and REM as an oxide. In particular, when the amount of S was large, there were examples in which REM ₂ O ₂ S and REMS were generated.

The oxide composition is t. It changes with [O]. Therefore, t. [O] By organizing the data separately, it was found that the conditions (hereinafter referred to as generation prevention conditions) that can prevent the number of stretched inclusions with an aspect ratio > 3.0 from 10/mm ² or less can be clarified (Fig. 2). . The symbols in the figure are the same as in FIG. 1, ○: number of drawn inclusions with aspect ratio >3.0 ≤ 10 / mm ² (range of generation prevention), △: number of drawn inclusions with aspect ratio > 3.0 > 10 / ^mm2 . The generation prevention condition (boundary line) is divided into three areas, area I, area II, and area III, according to [REM] (FIG. 3).

(Region I)
Region I is a region where [REM]<[REM_1] and [REM] is low.
First, consider the case where the content of [REM] is fixed at a low level and [Ca] is increased. Since both [REM] and [Ca] are low at the beginning, MnS is formed and elongated during rolling. As [Ca] increases, S binds to Ca and MnS decreases. On the other hand, an Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxide is produced, but when [REM] is low, it is a low-melting-point oxide mainly composed of Al ₂ O ₃ —CaO, so it is stretched during rolling. . While MnS is reduced, low-melting-point oxides are formed, so when [Ca] is low, the total amount of stretched inclusions does not decrease significantly. If [Ca] is further increased, the melting point of the oxide rises, and the degree of drawing of the oxide gradually decreases.
Next, consider the case where [REM] is increased from the above. Similarly, as [Ca] increases, S binds to Ca and MnS decreases. On the other hand, _{the melting point of the Al 2 O 3 --CaO--REM 2 O 3 -based oxide is the} (Even if REM ₂ O ₃ ) increases), it does not change much. Therefore, the slope of the boundary line is relatively small.

In this way, in region I, if it contains at least the lower limit [Ca] defined by formula (8), MnS can be sufficiently reduced, and the oxide composition is expressed by formula (11) and formula (12) are satisfied at the same time, it is possible to avoid forming a composition of a low-melting-point oxide and prevent stretching during rolling.
If the formula (8) is not satisfied, MnS cannot be sufficiently reduced, or an Al ₂ O ₃ —CaO—REM ₂ O ₃ -based low-melting oxide that does not satisfy at least one of the formulas (11) and (12) is produced. These are easily elongated during rolling, and many inclusions having an aspect ratio of >3.0 are generated, resulting in poor toughness of the steel material for seamless steel pipes.

(Region II)
Region II is in the range of [REM_1]≦[REM]<[REM_2], and the lower limit [Ca] defined by Equation (9) decreases as [REM] increases. The decreasing slope is greater than region I. This is because as [REM] increases and (REM ₂ O ₃ ) in the oxide increases, the melting point of the Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxide rises relatively greatly, and stretching This is because it becomes difficult to
When region II contains at least the lower limit [Ca] defined by formula (9), MnS can be sufficiently reduced, and the composition of the oxide satisfies formulas (11) and (12).
If the formula (9) is not satisfied, MnS cannot be sufficiently reduced, or an Al ₂ O ₃ —CaO—REM ₂ O ₃ -based low-melting oxide that does not satisfy at least one of the formulas (11) and (12) is produced. This easily elongates during rolling, and many inclusions having an aspect ratio of >3.0 are generated, resulting in poor toughness of the steel material for seamless steel pipes.

(Region III)
Region III is in the range of [REM_2] ≤ [REM] ≤ 0.0050%, and the lower limit [Ca] defined by formula (10) is lower than regions I and II, and is lower than [REM]. It is a constant area regardless of the When [REM_2] ≤ [REM], (Al 2 O ₃ ) in the Al ₂ O ₃ —CaO—REM ₂ O ₃ -based oxide calculated by Equation (11) is reduced to 40% or less, so the melting point _is High and difficult to stretch during rolling. Since it is high [REM], equation (12) is also satisfied at the same time. Therefore, [Ca] necessary for MnS prevention is the lower limit [Ca] of this region. In this region, the t.V in steel is high because of the high [REM]. Since [O] binds mainly to REM (and Al) and the available [Ca] to bind S is sufficient, the lower limit [Ca] is constant regardless of [REM].
In region III, when the content is at least the lower limit [Ca], MnS can be sufficiently reduced, and the composition of the oxide satisfies the formulas (11) and (12), so it has a high melting point and is difficult to stretch during rolling. .
In region III, if the formula (10) is not satisfied, MnS cannot be sufficiently prevented, so MnS that has been stretched to an aspect ratio of >3.0 during rolling remains in the steel material for seamless steel pipes, and the steel material for seamless steel pipes is reduced. The toughness is inferior.

From FIGS. 2(a) to (d), [REM_1], [REM_2] dividing the regions I to III, and the boundary (lower limit) [Ca] of each region are represented by [Al], [S], t. A regression equation for [O] was obtained to define the above equations (1) to (10).

Next, FIG. 4 shows the relationship between the number density of elongated inclusions with an aspect ratio>3.0 and the Charpy impact value. When the number of stretched inclusions exceeds 10/mm ² , the impact value drops sharply and falls below the standard value of 80 (J/cm ² ). As shown in FIG. 5, the number of drawn inclusions has a correlation with the number of inclusions having an equivalent circle diameter of ≧1.0 μm. This is because when the number of all inclusions (the number of inclusions with equivalent circle diameter ≥ 1.0 µm as an index) is small, the inclusion size distribution shifts to the small size side as a whole. This is thought to be due to the decrease in the low-melting-point oxide that stretches when the aspect ratio exceeds 3.0. When inclusions stretched after rolling are observed, inclusions with a smaller equivalent circle diameter, that is, inclusions with a smaller volume tend to have a smaller aspect ratio than inclusions with a larger volume. The reason for this is thought to be that the thickness of inclusions after rolling does not become infinitely thin, but converges to a certain thickness, for example, about 0.1 μm. Since there is a lower limit to the thickness after rolling, assuming that the volume is preserved, it is thought that the greater the volume, the greater the stretching and the greater the aspect ratio. That is, it can be interpreted that inclusions having an aspect ratio of >3.0 increase during rolling as the composition has a lower melting point and the size before rolling is coarser.
In this way, by controlling the composition of inclusions and reducing the size and number of inclusions, it is possible to reduce harmful stretched inclusions with an aspect ratio >3.0 that serve as starting points for fracture. For this reason, in the present embodiment, the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is specified to be 100/mm ² or less.

In addition, in the steel material for seamless steel pipes according to the present embodiment, only inclusions with a grain size (in the case of substantially spherical inclusions) or major axis (in the case of non-spherical inclusions) of 1.0 μm or more are considered. Inclusions having a grain size or a major diameter of less than 1.0 μm, even if contained in the steel, have little effect on the toughness of the steel material for seamless steel pipes, so such inclusions are not considered in the present embodiment. . Further, the major axis is defined as a line segment having the maximum length among line segments connecting non-adjacent vertices in the cross-sectional profile of inclusions on the viewing surface. Similarly, the above minor axis is defined as a line segment having the minimum length among line segments connecting non-adjacent vertices in the cross-sectional profile of inclusions on the viewing surface. Hereinafter, the description of "particle size (in the case of substantially spherical inclusions) or long diameter (in the case of non-spherical inclusions)" may be abbreviated as "grain size or long diameter".

Here, in this embodiment, in particular, t. It is important to control the composition of inclusions by controlling the components in steel such as [Ca] and [REM] according to [O].
As shown in formulas (1) to (10), t. including [O], t. Appropriate combination conditions of [REM] and the lower limit [Ca] are determined according to [Al] and [S]. Therefore, before adding [REM] and [Ca], t. [O], t. If [Al] and [S] are known, the target ranges of [REM] and [Ca] can be set accurately. However, in production sites where minutes are a matter of constant operation and efficient operation is required, all necessary ingredients, especially t. It is often difficult for [O] to be known during operation.

Therefore, as a realistic measure for implementing the present invention in actual operation, based on the results of conventional operation, t. The target range of [REM] and [Ca] can be set in advance considering the variation of components including [O]. By collecting as many existing operating results as possible with steel grades as similar as possible, it is actually sufficiently possible to predict the range of each composition, including the range of variation.

Below, t. [O] will be described as an example. In some steel grades, with a fairly high probability, t. Assume that [O] is found to fall within the range of α to β. t. The combination conditions of [REM] and [Ca] can be calculated for [O]=α and β respectively. The range of [REM] and [Ca] that are common to these two conditions may be set as the target range for actual operation. t. The wider the variation range of [O] is assumed, that is, the larger the difference between α and β, the higher the probability that the actual concentration will fall between α and β. On the other hand, the ranges of [REM] and [Ca], which are common to the two conditions α and β, are narrowed. Therefore, if the component variation is assumed to be too wide, it becomes difficult for [REM] and [Ca] to hit the target range. Therefore, measures for narrowing the range of variation of the components are effective in widening the target range of [REM] and [Ca] and facilitating the operation. t. The variation of [O] is considered to include the extension of the stirring time after the secondary refining treatment for promoting the floating of inclusions, the extension of the standing time of the molten steel, and the like.
above, t. Although the variation of [O] has been described as an example, the target ranges of [REM] and [Ca] can be set by similarly considering the variation ranges of other components.

Next, an example of a method for manufacturing a steel material for seamless steel pipes according to the present embodiment will be described.
The steel material for seamless steel pipes according to the present embodiment uses, for example, molten iron from a blast furnace as a raw material, and is produced by performing converter refining and secondary refining, in the same manner as general steel materials. A seamless steel pipe can be manufactured by piercing the central part and performing heat treatment as necessary. At that time, after the decarburization treatment in the converter, secondary refining in the ladle is performed to adjust the composition of the steel and to control inclusions by adding Ca and REM. In addition to blast furnace molten iron, molten steel obtained by melting iron scrap as a raw material in an electric furnace may be used as the raw material.

Ca and REM are preferably added after adjusting the components of other contained elements and floating Al ₂ O ₃ generated by Al deoxidization from molten steel. If a large amount of Al ₂ O ₃ remains in the molten steel, Ca and REM will reduce the Al ₂ O ₃ , so that amount will be consumed. Therefore, the contents of Ca and REM used for fixing S are reduced, and the generation of MnS cannot be sufficiently prevented.

Since Ca has a high vapor pressure, it is preferable to add Ca--Si alloy, Fe--Ca--Si alloy, Ca--Ni alloy, or the like in order to increase the yield. Alloy wires composed of these alloys may be used for these alloy additions. REM may be added in the form of Fe--Si--REM alloy, misch metal, or the like. A misch metal is a mixture of rare earth elements, and more specifically, it often contains about 40 to 50% Ce and about 20 to 40% La. For example, misch metal consisting of 45% Ce, 35% La, 9% Nd, 6% Pr, and other impurities can be obtained.

The addition order of Ca and REM is not particularly limited. However, when Ca is added after adding REM, the size of inclusions tends to decrease. Therefore, it is preferable to add Ca after adding REM.

Al ₂ O ₃ is produced after Al deoxidation and partly clustered. Next, REM or Ca is added. If REM is added before Ca, some of the Al ₂ O ₃ clusters are reduced and decomposed during the addition of REM, and the size of the clusters can be reduced. On the other hand, when Ca is added before REM, Al ₂ O ₃ is reformed into CaO—Al ₂ O ₃ system inclusions, and these inclusions have a low melting point and are in liquid phase in molten steel, so they easily agglomerate. There is a risk that coarse CaO--Al ₂ O _3- based inclusions may result. Therefore, it is preferable to add Ca after adding REM.

Here, in order to sufficiently float Al ₂ O ₃ generated when Al is added, it is preferable to hold for 5 minutes or more after adding Al. Then REM is added. At this time, it is preferable to hold the mixture for 3 minutes or longer so that the REM is uniformly mixed. Then, it is preferable to add Ca while REM is uniformly mixed.

As described above, the steel material for seamless steel pipes according to the present embodiment is manufactured.

According to the steel material for seamless steel pipes of the present embodiment configured as described above, t. Since the relationship between REM and Ca is defined in consideration of [O], even if Ca and REM form oxides, the amount of Ca and REM that bond with S is ensured, and MnS-based inclusions are formed. can be reduced.
In addition, [Ca], [REM], t. [Al], t. Since the relationship between the amounts of [O] and [S] is defined as described above, the content of Al ₂ O ₃ in the inclusions can be reduced, and the formation of low-melting oxides can be suppressed. becomes. Furthermore, Ca and REM are added in combination, and the main composition of the oxide is Al ₂ O ₃ —CaO—REM ₂ O ₃ system, which is a ternary system containing REM ₂ O ₃ , so that crushing during rolling is reduced. Since the hardness increases and the size after rolling becomes finer, the harmfulness can be reduced.
Furthermore, since the number density of inclusions having an equivalent circle diameter of 1.0 μm or more is specified to be 100/mm ² or less, the number of inclusions that cause fracture is sufficiently reduced.
Therefore, it is possible to provide a steel material for a seamless steel pipe excellent in toughness that can be used as a high-quality seamless steel pipe in which cracks or the like are suppressed during pipemaking and use.

Further, in the steel material for seamless steel pipe of the present embodiment, Cu: 0% or more and 0.05% or less, Nb: 0% or more and 0.05% or less, V: 0% or more and 0.10% or less, Ni: 0% % or more and 0.10% or less and B: 0% or more and 0.0050% or less. characteristics can be improved.

Furthermore, in the steel material for seamless steel pipes according to the present embodiment, when the mass ratio of each compound MX in the inclusions present inside is (MX), the above-mentioned formulas (11) and (12) are satisfied. In this case, it is possible to further reduce the number of elongated inclusions.

Although the steel materials for seamless steel pipes, which are the embodiments of the present invention, have been specifically described above, the present invention is not limited to this, and can be appropriately modified without departing from the technical idea of the invention. .
For example, in the embodiments, an example of the method for manufacturing the steel material for seamless steel pipe of the present invention is shown, but the present invention is not limited to this, and other manufacturing methods may be applied. good.

Further, in the present embodiment, the use of seamless steel pipes that can be used for oil country tubular goods and line pipes is assumed, but the applications are not limited to these.
Furthermore, in the present embodiment, as described above, the components and inclusions contained in the steel material for seamless steel pipes are defined, but the metal structure is not particularly defined. Can be made separately. At that time, since the composition of inclusions defined in the present embodiment does not change, even those subjected to heat treatment are effective.

The results of experiments conducted to confirm the effects of the present invention will be described below.

Using blast furnace molten iron as a raw material, 250 tons of molten steel was melted after preliminary treatment of molten iron and decarburization treatment in a converter, followed by component adjustment in ladle refining. In ladle refining, Al is first added for deoxidation, then after adjusting the components of other elements such as Ti, the ladle is held for 5 minutes or more to float Al ₂ O ₃ generated by Al deoxidization. Afterwards REM was added and held for 3 minutes to mix homogeneously before Ca was added. REM used misch metal. The REM elements contained in this misch metal were Ce 50%, La 25%, Nd 10%, and the remainder was impurities. Therefore, the ratio of each REM element contained in the obtained steel sheet was almost the same as the ratio of each REM element described above. Since Ca has a high vapor pressure, a Ca—Si alloy was added to increase the yield.

The molten steel after refining was poured into a mold having a round cross section, continuously cast, and bloomed to produce a billet (rolling material). The billet was heated, subjected to hot piercing, and pipe-making by drawing and rolling to produce a seamless steel pipe having an outer diameter of about 340 mm and a wall thickness of about 16 mm. The obtained steel pipe was heated to 950° C. and water quenched, and then tempered in the range of 550 to 650° C. depending on the composition to uniform the strength of the steel pipe.

The obtained steel materials for seamless steel pipes were examined for composition of inclusions and deformation behavior (ratio of major axis/minor axis after rolling; aspect ratio). Using an optical microscope, a cross section parallel to the rolling direction and the steel pipe thickness direction is used as an observation surface. bottom. In each observation field, inclusions with a grain size (in the case of spherical inclusions) or a major axis (in the case of non-spherical inclusions) of 1.0 μm or more are observed, and those inclusions are observed with an aspect ratio of 3.0. were classified according to the criteria. A sample with an inclusion number density of more than 10 inclusions/mm ² with an aspect ratio of >3.0 was evaluated as “poor”, and a sample with an inclusion number density of 10 inclusions/mm ² or less was evaluated as “good”.

In addition, 10 representative inclusions having a grain size (in the case of spherical inclusions) or a major diameter (in the case of non-spherical inclusions) of 1.0 μm or more were randomly selected, and the entire cross section of each inclusion was was analyzed using an EDX device attached to the SEM to determine the average composition of 10 samples.
As for the composition of each inclusion, the average composition of the entire cross section was measured instead _of representing, for example, the composition at the center of the inclusion _. This is because a plurality of phases coexist in many cases, such as when MnS or CaS adheres to a portion of the periphery of the -CaO-REM ₂ O ₃ oxide. The average composition of such inclusions can be obtained by analyzing the entire cross section of the inclusions within the range of EDX analysis. Alternatively, point analysis may be performed for each inclusion phase, and the average composition may be calculated by multiplying the area ratio of each phase obtained from the SEM image.

The Charpy impact value at 0°C was measured as an index of the toughness of the steel materials for seamless steel pipes obtained above. Full-size specimens were taken from the direction perpendicular to the longitudinal (rolling) direction of the steel pipe (T-direction specimens). That is, a test piece having a length of 55 mm was sampled from the circumferential direction of the steel pipe, a test piece having a height of 10 mm in the longitudinal (rolling) direction of the steel pipe, and a width of 10 mm in the wall thickness direction of the steel pipe. A 2 mm V notch was processed on a surface of 55 mm length x 10 mm width in the thickness direction. If a test piece is taken in this direction, the L cross section of the steel pipe (the cross section parallel to the rolling direction and the thickness direction of the steel pipe) becomes the fracture surface, so the influence of elongated inclusions can be easily evaluated. The Charpy absorbed energy (impact value) (J/cm ² ) improves as the number of inclusions, which are crack initiation points, particularly those extending in the rolling direction, decreases in the steel sheet.

No. 51 to 55 are comparative examples.
No. From equation (1), 51 is [REM]<[REM_1] and is in region I, but [Ca] does not satisfy equation (8). Therefore, the MnS cannot be sufficiently reduced, the number of stretched inclusions exceeds 10/mm ² , the number of inclusions with an equivalent circle diameter of 1.0 μm or more exceeds 100/mm ² , and the impact value is the standard value of 80 J/mm. cm ² was not reached.

No. 52, from equations (1) and (2), [REM_1]≤[REM]<[REM_2] and is in region II, but [Ca] does not satisfy equation (9). Therefore, MnS is not sufficiently reduced, the number of stretched inclusions exceeds 10/mm ² , the number of inclusions with an equivalent circle diameter of 1.0 μm or more exceeds 100/mm ² , and the impact value is the standard value of 80 J/mm. cm ² was not reached.

No. 53 satisfies [REM_2]≦[REM]<0.0050% from equation (2) and is in region III, but [Ca] does not satisfy equation (10). Therefore, MnS is not sufficiently reduced, the number of stretched inclusions exceeds 10/mm ² , the number of inclusions with an equivalent circle diameter of 1.0 μm or more exceeds 100/mm ² , and the impact value is the standard value of 80 J/mm. cm ² was not reached.

No. No. 54 is an example in which no REM was added and only Ca was added. Since the inclusions did not contain REM ₂ O ₃ and did not satisfy the formula (12), an Al ₂ O ₃ —CaO-based low-melting-point oxide was generated and elongated during pipe making. As a result, the number of stretched inclusions exceeded 10/ ^mm2 , the number of inclusions with an equivalent circle diameter of 1.0 µm or more exceeded 100/ ^mm2 , and the impact value was below the standard value of 80 J/ ^cm2 . It was

No. 55 has [REM]=0.0002%, which is less than the lower limit of 0.0003%. Therefore, the amount of (REM ₂ O ₃ ) in the inclusions was small, and the formula ( ¹² ) was not satisfied. was over 100/mm ² , and the impact value did not reach the standard value of 80 J/cm ² .

On the other hand, Example No. of the present invention, which satisfies the component conditions defined by formulas (1) to (10). 1, 6, 7, 18 to 22, 26 to 29, and Reference Examples 2 to 5, 8 to 17, 23 to 25 , the number of stretched inclusions is 10/mm ² or less, and the equivalent circle diameter is 1.0 μm or more. The number of inclusions was 100/mm ² or less. The average composition of inclusions satisfied the formulas (11) and (12). As a result, the impact value (Charpy absorbed energy) was above the standard value of 80 J/cm ² , indicating that the material was good.

From the above, according to the present invention, it is possible to prevent the formation of elongated inclusions, including both MnS and low melting point oxides, and the number density of inclusions having an equivalent circle diameter of 1.0 μm or more was 100/mm ² or less, and steel materials for seamless steel pipes that could be used as seamless steel pipes with excellent cleanliness and toughness could be produced.

Claims (3)

in % by mass,
C: 0.20% or more and less than 0.34%,
Si: 0.15% or more and 0.40% or less,
Mn: 0.30% or more and less than 1.50%,
Al: 0.031% or more and 0.050% or less,
Ti: 0.002% or more and 0.060% or less,
Cr: 0.02% or more and 1.50% or less,
Mo: 0% or more and 1.0% or less,
Ca: 0.0003% or more and 0.0050% or less,
REM: 0.0003% or more and 0.0050% or less,
and the balance consists of iron and impurities, and among the impurities, P, S, O, and N,
P: 0.020% or less,
S: 0.0050% or less,
O: 0.0050% or less,
N: 0.0075% or less,
limited to
When the mass ratio of each component element M in the steel is represented by [M], [REM] is a first threshold value [REM_1] indicated by the following formula (1) and a second threshold value indicated by the following formula (2) [REM_2] is divided into three regions, and in each region, the amount of Ca [Ca] in the steel is a1 shown in the following formula (3), b1 shown in the following formula (4), and shown in the following formula (5) Using a2, b2 shown in formula (6), and b3 shown in formula (7), the following formulas (8), (9), and (10) are satisfied,
With a cross-section parallel to the rolling direction and the thickness direction of the steel pipe as an observation plane, MnS, CaS, Al ₂ O ₃ -CaO-REM ₂ O having an equivalent circle diameter of 1.0 μm or more contained in the steel A steel material for a seamless steel pipe, characterized in that the number density of inclusions containing at least one of _tri- based oxides is 100/mm ² or less.
(1) Formula: [REM_1] = 0.400 x [S] - 0.0006
(2) Formula: [REM_2] = 0.005 x [Al] + 0.300 x [S] + 1.590 x [O] - 0.0014
(3) Formula: a1 = 5.000 x [Al] -35.714 x [S] -29.870 x [O] -0.0675
(4) Formula: b1 = -0.005 x [Al] + 0.400 x [S] + 1.155 x [O] -0.0007
(5) Formula: a2 = 2.143 x [Al] + 53.571 x [S] + 31.169 x [O] - 1.0471
(6) Formula: b2 = -0.002 x [Al] + 0.618 x [S] + 1.100 x [O] -0.0010
(7) Formula: b3 = 0.475 × [S] - 0.0005
(8) Formula: When [REM] < [REM_1], [Ca] ≥ a1 × [REM] + b1
(9) Formula: If [REM_1] ≤ [REM] < [REM_2], [Ca] ≥ a2 × [REM] + b2
(10) Formula: When [REM_2] ≤ [REM] ≤ 0.0050%, [Ca] ≥ b3 Furthermore, in mass %,
Cu: 0% or more and 0.05% or less,
Nb: 0% or more and 0.05% or less,
V: 0% or more and 0.10% or less,
Ni: 0% or more and 0.10% or less,
B: 0% or more and 0.0050% or less,
The steel material for a seamless steel pipe according to claim 1, characterized by containing one or more selected from the group consisting of: _{Among the} inclusions _present in the steel pipe , the following ₍ 11) The steel material for seamless steel pipes according to claim 1 or 2, which satisfies both equations (11) and (12).
(11) Formula: (Al ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≦0.40
(12) Formula: (REM ₂ O ₃ )/{(Al ₂ O ₃ )+(CaO)+(REM ₂ O ₃ )}≧0.05

Images