Martensitic Stainless Steel Material

The present disclosure relates to a steel material, and more particularly relates to a martensitic stainless steel material.

The environments of oil wells and gas wells (hereinafter, oil wells and gas wells are collectively referred to as “oil wells”) include environments which contain large amounts of corrosive substances. The corrosive substances are, for example, corrosive gases such as hydrogen sulfide (HS) gas and carbon dioxide (CO) gas. It is known that chromium (Cr) is effective for improving the carbonic-acid gas corrosion resistance of a steel material. Therefore, in an oil well in an environment containing a large amount of carbon dioxide gas, martensitic stainless steel materials containing about 13% by mass of Cr that are typified by API L80 13Cr steel material (normal 13Cr steel material) and Super 13Cr steel material in which the content of C is reduced are used according to the partial pressure and temperature of the carbon dioxide gas.

In recent years, with deeper oil wells, there is a demand to enhance the strength of steel materials for oil wells. Specifically, steel materials for oil wells of 80 ksi grade (yield strength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa) and 95 ksi grade (yield strength is 95 to less than 110 ksi, that is, 655 to less than 758 MPa) are being widely utilized. Furthermore, recently steel materials for oil wells of 110 ksi grade or more (yield strength is 758 MPa or more) have also started to be demanded.

Here, in the present description, an environment containing hydrogen sulfide and carbon dioxide gas is referred to as a “sour environment”. Steel materials for oil wells to be used in a sour environment are required to have sulfide stress cracking resistance (hereunder, referred to as “SSC resistance”). That is, in recent years, steel materials for oil wells are required to have both high strength and excellent SSC resistance.

Japanese Patent Application Publication No. 2000-192196 (Patent Literature 1), Japanese Patent Application Publication No. 2012-136742 (Patent Literature 2), and International Application Publication No. WO2008/023702 (Patent Literature 3) each proposes a steel material that has high strength and excellent SSC resistance.

The steel material proposed in Patent Literature 1 is a martensitic stainless steel for oil wells consisting of, in weight %, C: 0.001 to 0.05%, Si: 0.05 to 1%, Mn: 0.05 to 2%, P: 0.025% or less, S: 0.01% or less, Cr: 9 to 14%, Mo: 3.1 to 7%, Ni: 1 to 8%, Co: 0.5 to 7%, sol. Al: 0.001 to 0.1%, N: 0.05% or less, O (oxygen): 0.01% or less, Cu: 0 to 5%, and W: 0 to 5%, with the balance being Fe and unavoidable impurities. When a steel material contains Mo, the Ms point decreases. Therefore, because this steel material contains Co as well as Mo, a decrease in the Ms point is suppressed, and the microstructure is made a martensitic single-phase structure. It is described in Patent Literature 1 that, as a result, in this steel material, the SSC resistance can be increased while maintaining the strength at 80 ksi or more (552 MPa or more).

The steel material proposed in Patent Literature 2 is a martensitic stainless steel seamless pipe consisting of, in mass %, C: 0.01% or less, Si: 0.5% or less, Mn: 0.1 to 2.0%, P: 0.03% or less, S: 0.005% or less, Cr: 14.0 to 15.5%, Ni: 5.5 to 7.0%, Mo: 2.0 to 3.5%, Cu: 0.3 to 3.5%, V: 0.20% or less, Al: 0.05% or less, and N: 0.06% or less, with the balance being Fe and unavoidable impurities. The steel material has a yield strength of 655 to 862 MPa, and a yield ratio of 0.90 or more. It is described in Patent Literature 2 that by setting the content of C to 0.01% or less, adjusting Cr, Ni and Mo to within a preferable range, and also containing suitable amounts of Cu and V or a suitable amount of W, excellent SSC resistance is obtained while also having a strength of 655 MPa or more.

The steel material proposed in Patent Literature 3 is a martensitic stainless steel that consists of, in mass %, C: 0.010 to 0.030%, Mn: 0.30 to 0.60%, P: 0.040% or less, S: 0.0100% or less, Cr: 10.00 to 15.00%, Ni: 2.50 to 8.00%, Mo: 1.00 to 5.00%, Ti: 0.050 to 0.250%, V: 0.25% or less, N: 0.07% or less, and one or more kinds of element among Si: 0.50% or less and Al: 0.10% or less, with the balance being Fe and impurities, and that satisfies the formula (6.0≤Ti/C≤10.1). The yield strength is 758 to 862 MPa. There is a correlation between a ratio (Ti/C) of the content of Ti to the content of C in the steel and a value obtained by subtracting the yield strength from the tensile strength. Further, when there are large hardness variations in a steel material, the SSC resistance of the steel material decreases. Therefore, it is described in Patent Literature 3 that in this steel material, by adjusting Ti/C to within a preferable range, hardness variations are suppressed and the yield strength is made 758 to 862 MPa.

The aforementioned Patent Literatures 1 to 3 propose techniques for increasing the yield strength and improving the SSC resistance of a steel material. However, a martensitic stainless steel material that has excellent SSC resistance while also increasing the yield strength may be obtained by a technique other than the techniques proposed in the aforementioned Patent Literatures 1 to 3.

In addition, in recent years, oil wells with higher concentrations of hydrogen ions than in the past are being actively developed. In general, SSC is liable to occur in an environment in which there is a high hydrogen ion concentration (that is, the pH is low). Therefore, there is a need for a martensitic stainless steel material that has excellent SSC resistance even in a sour environment with a pH of 3.0 in which the hydrogen ion concentration is higher than has been the case in the past. However, the aforementioned Patent Literatures 1 to 3 contain no discussion regarding the SSC resistance of the steel materials in sour environments with a pH of 3.0.

An objective of the present disclosure is to provide a martensitic stainless steel material that can achieve both a high yield strength, and excellent SSC resistance in a sour environment with a pH of 3.0.

A martensitic stainless steel material according to the present disclosure consists of, in mass %,

The martensitic stainless steel material according to the present disclosure can achieve both a high yield strength, and excellent SSC resistance in a sour environment with a pH of 3.0.

First, the present inventors conducted studies from the viewpoint of the chemical composition with respect to a martensitic stainless steel material that can achieve both a high yield strength, and excellent SSC resistance in a sour environment with a pH of 3.0. As a result, the present inventors considered that if a martensitic stainless steel material contains, in mass %, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 7.50%, Mo: 1.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, and Nb: 0 to 0.500%, there is a possibility that both a yield strength of 758 MPa (110 ksi) or more, and excellent SSC resistance in a sour environment with a pH of 3.0 can be obtained.

Next, with respect to a martensitic stainless steel material containing the contents of elements described above, the present inventors conducted detailed studies regarding means for increasing the SSC resistance while maintaining a yield strength of 758 MPa or more. As a result, the present inventors discovered that in a martensitic stainless steel material containing the contents of elements described above, there is a possibility that tin (Sn), arsenic (As) and antimony (Sb), which are elements on which attention had not been focused heretofore, increases the SSC resistance. As a result of further detailed studies conducted by the present inventors, it has been found that in a martensitic stainless steel material containing the contents of elements described above, there is a possibility that Sn, in particular, markedly increases the SSC resistance, and that As and Sb assist the effect of increasing SSC resistance produced by Sn.

Therefore, the present inventors conducted detailed studies regarding contents of Sn, As and Sb that can sufficiently increase the SSC resistance of a martensitic stainless steel material. As a result, it has been clarified that, in addition to the contents of elements described above, by the martensitic stainless steel material according to the present embodiment also containing Sn in an amount of 0.0005 to 0.0500%, As in an amount of 0 to 0.0100%, and Sb in an amount of 0 to 0.0100%, the SSC resistance of the steel material can be increased. That is, if a martensitic stainless steel material consists of, in mass %, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, Cu: 0.01 to 3.50%, Cr: 10.00 to 14.00%, Ni: 4.50 to 7.50%, Mo: 1.00 to 4.00%, Ti: 0.050 to 0.300%, V: 0.01 to 1.00%, Al: 0.001 to 0.100%, Co: 0.010 to 0.500%, Ca: 0.0005 to 0.0050%, Sn: 0.0005 to 0.0500%, N: 0.0010 to 0.0500%, O: 0.050% or less, W: 0 to 0.50%, Nb: 0 to 0.500%, As: 0 to 0.0100%, and Sb: 0 to 0.0100%, with the balance being Fe and impurities, there is a possibility that both a yield strength of 758 MPa or more, and excellent SSC resistance in a sour environment with a pH of 3.0 can be obtained.

On the other hand, the present inventors have found that even in the case of a martensitic stainless steel material having the aforementioned chemical composition, when the martensitic stainless steel material has a yield strength of 758 MPa or more, there are some cases where the SSC resistance is not stably increased in a sour environment with a pH of 3.0. Therefore, with respect to a martensitic stainless steel material having the aforementioned chemical composition, the present inventors conducted detailed studies regarding means for increasing the SSC resistance in a sour environment with a pH of 3.0 while maintaining a yield strength of 758 MPa or more. As a result, the present inventors obtained the following findings.

It has been clarified as a result of the detailed studies conducted by the present inventors that in a martensitic stainless steel material having the aforementioned chemical composition and in which the yield strength is 758 MPa or more, if the contents of elements and the yield strength satisfy Formula (1), the SSC resistance of the steel material is markedly increased in a sour environment with a pH of 3.0.

Where, in Formula (1), a content of a corresponding element in percent by mass is substituted for each symbol of an element, and a yield strength in MPa is substituted for YS. Note that, if a corresponding element is not contained, “0” is substituted for the symbol of the relevant element.

Let F1 be defined as F1=(Sn+As+Sb)/{(Cu+Ni)/YS}. As described above, As and Sb assist the effect of increasing the SSC resistance of the steel material produced by Sn. In addition, the SSC resistance of the steel material markedly increases by making a ratio of the contents of Sn, As and Sb to the contents of Cu and Ni fall within a certain range. On the other hand, the higher that the yield strength of the steel material is, the more that the SSC resistance of the steel material tends to decrease. Therefore, the denominator of F1 is set to a ratio of the contents of Cu and Ni to the yield strength. Thus, a ratio of the contents of Sn, As and Sb to the contents of Cu and Ni that is adjusted according to the yield strength is defined as F1. That is, F1 is an index of an increase in the SSC resistance in a sour environment with a pH of 3.0 obtained by a synergetic effect between Sn, As and Sb, and Cu and Ni that is adjusted according to the yield strength. The relation between F1 and the SSC resistance in a sour environment with a pH of 3.0 is described specifically hereunder using the accompanying drawing.

is a view illustrating the relation between F1 and SSC resistance in the present Examples.was created using F1 and the number of specimens in which pitting occurred (specimens) that is an index of SSC resistance, with respect to Examples which had the aforementioned chemical composition and in which the yield strength was 758 MPa or more among Examples that are described later. Note that, the number of specimens in which pitting occurred was obtained by performing an SSC resistance evaluation test that assumed a sour environment with a pH of 3.0, which will be described later.

Referring to, when F1 was too low, pitting occurred in one or more specimens. Similarly, when F1 was too high, pitting also occurred in one or more specimens. On the other hand, when F1 was within the range of 0.15 to 1.00, pitting did not occur in even one specimen. That is, referring to, in a steel material having the aforementioned chemical composition and in which the yield strength is 758 MPa or more, when F1 is within the range of 0.15 to 1.00, excellent SSC resistance is obtained in a sour environment with a pH of 3.0.

Note that, with respect to a steel material having the aforementioned chemical composition and a yield strength of 758 MPa or more, the detailed mechanism whereby the SSC resistance of the steel material in a sour environment with a pH of 3.0 is increased by adjusting F1 to within the range of 0.15 to 1.00 has not been clarified. However, as illustrated also in, the fact that the SSC resistance in a sour environment with a pH of 3.0 of a martensitic stainless steel material having the aforementioned chemical composition and a yield strength of 758 MPa or more is increased by adjusting F1 to within the range of 0.15 to 1.00 has been proven by the Examples.

As described above, the martensitic stainless steel material according to the present embodiment has the aforementioned chemical composition, has a yield strength of 758 MPa or more, and furthermore, within the ranges of the contents of the elements and the yield strength, the contents of the elements and the yield strength satisfy Formula (1). As a result, the martensitic stainless steel material according to the present embodiment can achieve both a high yield strength of 758 MPa or more, and excellent SSC resistance in a sour environment with a pH of 3.0.

The gist of the martensitic stainless steel material according to the present embodiment, which has been completed based on the above findings, is as follows.

[1]

A martensitic stainless steel material consisting of, in mass %,

The martensitic stainless steel material according to [1], containing one or more elements selected from a group consisting of:

The shape of the martensitic stainless steel material according to the present embodiment is not particularly limited. The martensitic stainless steel material according to the present embodiment may be a pipe, may be a round steel bar (solid material), or may be a steel plate. Note that, the term “round steel bar” refers to a steel bar in which a cross section in a direction perpendicular to the axial direction is a circular shape. Further, the pipe may be a seamless pipe or may be a welded pipe.

Hereunder, the martensitic stainless steel material according to the present embodiment is described in detail. The symbol “%” in relation to an element means mass percent unless otherwise stated. Further, in the following description, the martensitic stainless steel material is also referred to as simply “steel material”.

The martensitic stainless steel material according to the present embodiment contains the following elements.

Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C increases hardenability of the steel material and increases strength of the steel material. On the other hand, if the content of C is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high. As a result, the SSC resistance of the steel material will decrease. Therefore, the content of C is to be 0.030% or less. A preferable upper limit of the content of C is 0.028%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.018%. The content of C is preferably as low as possible. However, extremely reducing the content of C will increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of C is 0.001%, more preferably is 0.003%, and further preferably is 0.005%.

Silicon (Si) is unavoidably contained. That is, the lower limit of the content of Si is more than 0%. Si deoxidizes the steel. On the other hand, if the content of Si is too high, even if the contents of other elements are within the range of the present embodiment, hot workability of the steel material will decrease. Therefore, the content of Si is to be 1.00% or less. A preferable lower limit of the content of Si for effectively obtaining the aforementioned advantageous effect is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%. A preferable upper limit of the content of Si is 0.80%, more preferably is 0.60%, further preferably is 0.50%, and further preferably is 0.45%.

Manganese (Mn) is unavoidably contained. That is, the lower limit of the content of Mn is more than 0%. Mn increases hardenability of the steel material and increases strength of the steel material. On the other hand, if the content of Mn is too high, even if the contents of other elements are within the range of the present embodiment, in some cases Mn will segregate at grain boundaries together with impurity elements such as P and S. In such a case, the SSC resistance of the steel material will decrease. Therefore, the content of Mn is to be 1.00% or less. A preferable lower limit of the content of Mn for effectively obtaining the aforementioned advantageous effect is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%. A preferable upper limit of the content of Mn is 0.80%, more preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.

Phosphorus (P) is an impurity that is unavoidably contained. That is, the lower limit of the content of P is more than 0%. P segregates at grain boundaries and facilitates the occurrence of SSC. Therefore, if the content of P is too high, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will markedly decrease. Therefore, the content of P is to be 0.030% or less. A preferable upper limit of the content of P is 0.025%, more preferably is 0.020%, and further preferably is 0.018%. The content of P is preferably as low as possible. However, extremely reducing the content of P will raise the production cost. Accordingly, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.

Sulfur (S) is an impurity that is unavoidably contained. That is, the lower limit of the content of S is more than 0%. Similarly to P, S segregates at grain boundaries and facilitates the occurrence of SSC. Therefore, if the content of S is too high, even if the contents of other elements are within the range of the present embodiment, the SSC resistance of the steel material will markedly decrease. Therefore, the content of S is to be 0.0050% or less. A preferable upper limit of the content of S is 0.0040%, more preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%. The content of S is preferably as low as possible. However, extremely reducing the content of S will raise the production cost. Accordingly, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

Copper (Cu) is an austenite forming element and causes the microstructure after quenching to become martensitic. Cu also increases the SSC resistance of the steel material in a sour environment with a pH of 3.0, by a synergetic effect with Sn, As and Sb. If the content of Cu is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Cu is too high, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will be saturated, and furthermore, hot workability of the steel material will markedly decrease. In this case, in addition, the production cost will rise. Therefore, the content of Cu is to be 0.01 to 3.50%. A preferable lower limit of the content of Cu is 0.02%, more preferably is 0.03%, and further preferably is 0.05%. A preferable upper limit of the content of Cu is 3.30%, more preferably is 3.10%, and further preferably is 2.90%.

Chromium (Cr) forms a passive film on the surface of the steel material and thereby increases the SSC resistance of the steel material. If the content of Cr is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Cr is too high, even if the contents of other elements are within the range of the present embodiment, in some cases ferrite will be included in the microstructure, and it will be difficult to secure sufficient strength. If the content of Cr is too high, in addition, even if the contents of other elements are within the range of the present embodiment, intermetallic compounds or Cr carbo-nitrides will easily form in the steel material. As a result, the SSC resistance of the steel material will decrease. Therefore, the content of Cr is to be 10.00 to 14.00%. A preferable lower limit of the content of Cr is 10.30%, more preferably is 10.50%, and further preferably is 11.00%. A preferable upper limit of the content of Cr is 13.80%, more preferably is 13.60%, further preferably is 13.50%, further preferably is 13.45%, further preferably is 13.40%, and further preferably is 13.35%.

Nickel (Ni) is an austenite forming element and causes the microstructure after quenching to become martensitic. Ni also forms sulfides on the passive film in a sour environment. The Ni sulfides inhibit chloride ions (Cl) and hydrogen sulfide ions (HS) from coming into contact with the passive film, thereby suppressing destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel material increases. Furthermore, Ni increases the SSC resistance of the steel material in a sour environment with a pH of 3.0 by a synergetic effect with Sn, As and Sb. If the content of Ni is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, the diffusion coefficient of hydrogen in the steel material may decrease. In such case, the SSC resistance of the steel material will decrease. Therefore, the content of Ni is to be 4.50 to 7.50%. A preferable lower limit of the content of Ni is 4.80%, more preferably is 5.00%, and further preferably is 5.50%. A preferable upper limit of the content of Ni is 7.30%, more preferably is 7.00%, and further preferably is 6.50%.

Molybdenum (Mo) forms sulfides on the passive film in a sour environment. The Mo sulfides inhibit chloride ions (Cl) and hydrogen sulfide ions (HS) from coming into contact with the passive film, thereby suppressing destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel material increases. Mo also dissolves in the steel material, and thereby increases strength of the steel material. If the content of Mo is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Mo is too high, even if the contents of other elements are within the range of the present embodiment, it will be difficult to stabilize austenite. As a result, in some cases a large amount of ferrite will be included in the microstructure after tempering. If the content of Mo is too high, even if the contents of other elements are within the range of the present embodiment, in some cases a large amount of intermetallic compounds such as Laves phase-based intermetallic compounds may form, and the yield strength of the steel material may become too high. Therefore, the content of Mo is to be 1.00 to 4.00%. A preferable lower limit of the content of Mo is 1.30%, more preferably is 1.50%, and further preferably is 1.80%. A preferable upper limit of the content of Mo is 3.80%, more preferably is 3.60%, and further preferably is 3.40%.

Titanium (Ti) combines with C and/or N to form carbides or nitrides. In such case, coarsening of grains is suppressed by the pinning effect, and the yield strength of the steel material increases. If the content of Ti is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high, and the SSC resistance of the steel material will decrease. Therefore, the content of Ti is to be 0.050 to 0.300%. A preferable lower limit of the content of Ti is 0.060%, and more preferably is 0.080%. A preferable upper limit of the content of Ti is 0.250%, more preferably is 0.200%, and further preferably is 0.180%.

Vanadium (V) increases hardenability of the steel material and raises the yield strength of the steel material. If the content of V is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of V is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and the SSC resistance of the steel material will decrease. Therefore, the content of V is to be 0.01 to 1.00%. A preferable lower limit of the content of V is 0.02%, and more preferably is 0.03%. A preferable upper limit of the content of V is 0.80%, more preferably is 0.60%, and further preferably is 0.50%.

Aluminum (Al) deoxidizes the steel. If the content of Al is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Al is too high, even if the contents of other elements are within the range of the present embodiment, coarse oxides will form and the SSC resistance of the steel material will decrease. Therefore, the content of Al is to be 0.001 to 0.100%. A preferable lower limit of the content of Al is 0.005%, more preferably is 0.010%, and further preferably is 0.015%. A preferable upper limit of the content of Al is 0.080%, more preferably is 0.060%, further preferably is 0.055%, and further preferably is 0.050%. As used in the present description, the term “content of Al” means the content of sol. Al (acid-soluble Al).

Cobalt (Co) forms sulfides on the passive film in a sour environment. The Co sulfides inhibit chloride ions (Cl) and hydrogen sulfide ions (HS) from coming into contact with the passive film, thereby suppressing destruction of the passive film by chloride ions and hydrogen sulfide ions. As a result, the SSC resistance of the steel material increases. Co also increases hardenability of the steel material, and particularly during industrial production, ensures consistent high strength of the steel material. Specifically, Co suppresses the formation of retained austenite, and suppresses the occurrence of variations in strength of the steel material. If the content of Co is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effects will not be sufficiently obtained. On the other hand, if the content of Co is too high, even if the contents of other elements are within the range of the present embodiment, toughness of the steel material will decrease. Therefore, the content of Co is to be 0.010 to 0.500%. A preferable lower limit of the content of Co is 0.015%, more preferably is 0.020%, further preferably is 0.030%, further preferably is 0.050%, and further preferably is 0.100%. A preferable upper limit of the content of Co is 0.450%, more preferably is 0.400%, and further preferably is 0.350%.

Calcium (Ca) immobilizes S in the steel material as a sulfide to make it harmless, and thereby improves hot workability of the steel material. If the content of Ca is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, coarse inclusions will be formed in the steel material and the SSC resistance of the steel material will decrease. Therefore, the content of Ca is to be 0.0005 to 0.0050%. A preferable lower limit of the content of Ca is 0.0006%, more preferably is 0.0008%, and further preferably is 0.0010%. A preferable upper limit of the content of Ca is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.

Tin (Sn) increases the SSC resistance of the steel material in a sour environment with a pH of 3.0. If the content of Sn is too low, even if the contents of other elements are within the range of the present embodiment, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Sn is too high, even if the contents of other elements are within the range of the present embodiment, Sn will segregate at grain boundaries and, on the contrary, the SSC resistance of the steel material will decrease. Therefore, the content of Sn is to be 0.0005 to 0.0500%. A preferable lower limit of the content of Sn is 0.0008%, more preferably is 0.0010%, and further preferably is 0.0015%. A preferable upper limit of the content of Sn is 0.0400%, more preferably is 0.0300%, further preferably is 0.0200%, further preferably is 0.0100%, and further preferably is 0.0080%.