JP3933089B2 - Low alloy steel - Google Patents

Description

【００８６】
【表２】

【００８７】
このようにして得た継目無鋼管から、１辺が１０ｍｍの立方体状の試験片をそれぞれ２個ずつ切り出し、いずれか１つの面が観察面となるように樹脂に埋め込んで鏡面研磨し、ＳＥＭの倍率を５０倍又は２００倍としてＮｂ系介在物及びＮｂ−Ｚｒ系介在物の観察を各１０視野について実施し、既に述べた方法で、上記介在物の長径を測定するとともに、介在物の中心部をＥＤＸによって組成分析した。そして、各視野において、長径が１μｍ以上のＮｂ系介在物又はＮｂ−Ｚｒ系介在物のうち、前記(1)式又は(3)式を満たすものの１ｍｍ²あたりの個数、及び、前記(2)式又は(4)式を満たすものの１ｍｍ²あたりの個数を計数し、その計数した値を１０視野で平均した。
【００８８】
また、上述の継目無鋼管から３ｍｍ×１０ｍｍ×４０ｍｍの寸法の試験片を長手方向に各２個ずつ採取し、６００番エメリー紙で研磨後、腐食試験を実施して耐孔食性を調査した。
【００８９】
すなわち、上記の研磨した試験片を脱気した２５℃の「５％食塩＋０．５％酢酸」水溶液中に７２０時間浸漬し、発生した孔食の最大径を測定した。なお、発生した孔食の最大直径が１００μｍを超えると孔食を起点としたＳＳＣが発生しやすくなるので、耐孔食性の目標を、孔食の最大直径が１００μｍ以下であることとした。
【００９０】
表３に、調査結果を整理して示す。
【００９１】
【表３】

【００９２】
表３から、鋼の化学組成が本発明で規定する範囲内にあり、しかも、長径が１μｍ以上のＮｂ系介在物のうち、前記の(1)式を満たすものを１ｍｍ²あたり１０個以上含み、しかも、前記の(2)式を満たすＮｂ系介在物が断面積１ｍｍ²あたり１０個以下であるか、長径が１μｍ以上のＮｂ−Ｚｒ系介在物のうち、前記の(3)式を満たすものを１ｍｍ²あたり１０個以上含み、しかも、前記の(4)式を満たすＮｂ−Ｚｒ系介在物が断面積１ｍｍ²あたり１０個以下である試験番号１〜１６の場合には、発生した孔食の最大直径は高々５０μｍで、極めて良好な耐孔食性を有することが明らかである。
【００９３】
これに対して、鋼の化学組成が本発明で規定する範囲内にあっても、長径が１μｍ以上のＮｂ系介在物のうち、前記の(1)式を満たすものが断面積１ｍｍ²あたり１０個に満たず、しかも、前記の (2) 式を満たすものが、断面積１ｍｍ ² あたり１０個を超える場合（試験番号３３〜３５）、更には、鋼の化学組成が本発明で規定する範囲内にあっても、長径が１μｍ以上のＮｂ−Ｚｒ系介在物のうち、前記の(3)式を満たすものが断面積１ｍｍ²あたり１０個に満たず、しかも、前記の (4) 式を満たすものが、断面積１ｍｍ ² あたり１０個を超える場合（試験番号３６）には、最大直径で１００μｍを超える２３０〜３８０μｍの孔食が発生し、耐孔食性に劣ることが明らかである。
【００９４】
また、その成分のいずれかが本発明で規定する含有量の範囲から外れた比較例の鋼１１〜１９を用いた試験番号３７〜４５の場合も、最大直径で１００μｍを超える孔食が発生し、耐孔食性に劣ることが明らかである。
【００９５】
すなわち、Ｍｎ、Ｐ及びＳの含有量がそれぞれ本発明で規定する含有量の範囲から高めに外れた比較例の鋼１１〜１３を用いた試験番号３７〜３９の場合には、これらの合金元素が鋼材の溶出を促進して最大直径で１００μｍを超える１８０〜２６０μｍの孔食が発生し、耐孔食性に劣ることが明らかである。
【００９６】
Ｔｉ、Ｎｂ、Ｚｒ及びＮの含有量がそれぞれ本発明で規定する含有量の範囲から高めに外れた比較例の鋼１４〜１６及び鋼１８を用いた試験番号４０〜４２及び試験番号４４の場合には、粗大なＮｂ系介在物又はＮｂ−Ｚｒ系介在物が生成し、これらが孔食の起点となるため最大直径で１００μｍをはるかに超える４５０〜６００μｍの孔食が発生し、耐孔食性に極めて劣っていた。
【００９７】
Ｃａの含有量が本発明で規定する含有量の範囲から高めに外れた比較例の鋼１７を用いた試験番号４３の場合には、溶解性のＣａ酸硫化物が多く生成し、これが孔食の起点となるため最大直径で１００μｍを超える２５０μｍの孔食が発生し、耐孔食性に劣っていた。
【００９８】
更に、Ｏ（酸素）の含有量が本発明で規定する含有量の範囲から高めに外れた比較例の鋼１９を用いた試験番号４５の場合には、粗大な酸化物が残留し、これが孔食の起点となるため最大直径で１００μｍを超える２１０μｍの孔食が発生し、耐孔食性に劣っていた。
【００９９】
【発明の効果】
本発明の低合金鋼は、介在物を起点とする孔食の発生が抑制されるため、孔食を起点とするＳＳＣを誘発することがなく、油井やガス井用のケーシングやチュービング、掘削用のドリルパイプ、ドリルカラーやサッカーロッド、更には、化学プラント用の鋼板や配管の素材として用いることができる。
【図面の簡単な説明】
【図１】 介在物の長径の測定方法及び介在物中の質量％でのＮｂやＺｒの含有量を求めるためのＥＤＸによる分析箇所について説明する図である。
【符号の説明】
ａ１、ａ２：介在物の長径、
ｂ１、ｂ２：ＥＤＸで分析する介在物の中心部[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a low alloy steel, and in particular, to a low alloy steel that has excellent pitting corrosion resistance in a corrosive environment and can suppress the occurrence of stress corrosion cracking starting from pitting corrosion. More specifically, the present invention relates to a low alloy steel suitable as a material for casings and tubing for oil wells and gas wells, drill pipes for drilling, drill collars and soccer rods, and steel plates and pipes for chemical plants.
[0002]
[Prior art]
  With the recent tightening of energy situation, crude oil and natural gas containing a lot of corrosive gases such as hydrogen sulfide and carbon dioxide, which have been avoided, have been used. For drilling, transporting and storing such crude oil and natural gas, steel materials and pipes having resistance to corrosion and stress corrosion cracking are required.
[0003]
  Stress corrosion cracking in an environment containing hydrogen sulfide is particularly referred to as sulfide stress cracking (hereinafter referred to as SSC). When low alloy steel is used in an oil well environment, it is required to have excellent SSC resistance.
[0004]
  From the viewpoint of deep wells in oil wells and gas wells, improvement in transport efficiency, and cost reduction, steel materials and steel pipes are required to have higher strength, but in general, higher strength steels are more likely to generate SSC. For this reason, various techniques for achieving both high strength and SSC resistance have been studied.
[0005]
  As a result, SSC resistance strongly depends on the structure of the steel material itself, and it has been reported that the improvement of the structure of the steel material is most effective in preventing SSC. For example, in many literatures, (b) a fine grain structure, (b) high-temperature tempering treatment, (c) a structure containing a large amount of martensite phase are effective in improving SSC resistance. It has been reported.
[0006]
  In order to achieve both the high strength of steel and the above-mentioned desirable structure, it is effective to contain Ti, Nb, V, Mo, and in some cases, an alloy element such as Zr in some cases. The reason for this is that the above alloy elements have a high affinity with C, so they easily form carbides, and the fine alloy carbides produced in the heat treatment process contribute to the increase in strength by precipitation strengthening, and fine graining by pinning. This is to enable tempering at high temperatures. In general, sufficient effects can be obtained if the above elements are contained in an amount of about 0.03 to 0.05%.
[0007]
  On the other hand, there are inevitably impurity elements and non-metallic inclusions in steel (hereinafter, “non-metallic inclusions” are simply referred to as “inclusions”). The generation is promoted and SSC starting from pitting corrosion tends to occur. In particular, in thick steel plates and large-diameter steel pipes with a large amount of melting, segregation of impurity elements and coarse inclusions generated during the melting process often greatly reduce the SSC resistance. Mn, P, and S are well known as impurity elements that segregate in steel and promote pitting corrosion and SSC, and reducing these impurity elements is effective in preventing pitting corrosion and SSC.
[0008]
  Although the influence of inclusions on pitting corrosion has been investigated a lot in stainless steel, there are almost no investigation examples in low alloy steel, and Patent Document 1 contains a coarse Ti nitride even in low alloy steel. It is stated that this is the starting point of pitting corrosion and induces SSC, and it is pointed out that the SSC resistance can be improved by making the Ti nitride finer.
[0009]
  However, it is difficult to say that the above-described technology is sufficient to cope with the recent severe environment of steel materials and pipes and the demand for higher strength, and further improvement in corrosion resistance is required. In particular, it is difficult to say that a technique for preventing pitting corrosion by controlling inclusions has been sufficiently studied, and a technique for improving corrosion resistance from this viewpoint is desired.
[0010]
[Patent Document 1]
  JP 2001-131698 A
[0011]
[Problems to be solved by the invention]
  The present invention has been made in view of the above-described present situation, and its object is to prevent the occurrence of pitting corrosion starting from inclusions, thereby preventing the occurrence of SSC starting from pitting corrosion. It is to provide a low alloy steel having excellent corrosion resistance.
[0012]
[Means for Solving the Problems]
  The gist of the present invention is the following (1) to(5)It is in the low alloy steel shown in
[0013]
  (1) By mass%, C: 0.2 to 0.55%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.0%, Al: 0.005 to 0.05% Cr: 0.1-1.5%, Mo: 0.1-1.0%, Ti: 0.005-0.03% and Nb: 0.002-0.03%, the balance being Fe In the impurities, P is 0.03% or less, S is 0.01% or less, N is 0.01% or less, and O (oxygen) is 0.01% or less as an inclusion. Nb-based inclusions satisfying the following formula (1) whose major axis is 1 μm or more are cross-sectional area 1 mm²Contains 10 or more perMoreover, the following (2) Nb-based inclusions satisfying the equation have a cross-sectional area of 1 mm ² 10 or less perLow alloy steel characterized by that.
[0014]
  a_NBXb_NB≦ 150 ... (1), A _NB Xb _NB ≧ 300 ... (2). However, the above formula (1)as well as (2) formulaA in_NBIs the long diameter (μm) of the Nb inclusion, b_NBIs the content (% by mass) of Nb in the Nb-based inclusion.
[0015]
  (2) By mass%, C: 0.2 to 0.55%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.0%, Al: 0.005 to 0.05% Cr: 0.1-1.5%, Mo: 0.1-1.0%, Ti: 0.005-0.03%, Nb: 0.002-0.03% and Zr: 0.005 -0.06% is included, the balance is Fe and impurities, P in the impurities is 0.03% or less, S is 0.01% or less, N is 0.01% or less, and O (oxygen) is 0.0. Nb—Zr inclusions satisfying the following formula (3) with a chemical composition of 01% or less and a major axis of 1 μm or more as an inclusion have a sectional area of 1 mm²Contains 10 or more perMoreover, the following (Four) Nb-Zr inclusions satisfying the equation have a cross-sectional area of 1 mm ² 10 or less perLow alloy steel characterized by that.
[0016]
  a_NZ× (b_NZ+ 0.5c_NZ) ≦ 150 ... (3), A _NZ × (b _NZ + 0.5c _NZ ) ≧ 300 ... (Four). However, the above formula (3)as well as (Four) formulaA in_NZIs the long diameter (μm) of the Nb—Zr inclusion, b_NZIs the content (% by mass) of Nb in the Nb-Zr inclusion, c_NZIs the content (mass%) of Zr in the Nb-Zr inclusions.
[0017]
  (3)Instead of part of Fe,% By mass0.03 to 0.5% of V is contained in the above (1) or (2)Low alloy steel.
[0018]
  (4)Instead of part of Fe,% By mass0.0001 to 0.005% of B is contained in any one of said (1) to (3) characterized by the above-mentionedLow alloy steel.
[0019]
  (5) Instead of a part of Fe, in mass%,0.0003 to 0.005% CaFrom the above (1), characterized by containing(4)Low alloy according to any one ofsteel.
[0020]
  Less thanBelow, the above (1) ~(5)Inventions related to low alloy steels of (1) to(5)This invention is called.
[0021]
  here,(1)The “Nb-based inclusion” as used in the present invention is a composite inclusion containing 10% or more of Nb by mass%, with the balance being Ti, C, N, Al, Ca, Mg, O (oxygen), S or the like. Point to. Therefore, said b_NBThe value of is greater than or equal to 10, and as is clear from its definition, a_NBThe value of is 1 or more.
[0022]
  Also,(2)The “Nb—Zr-based inclusion” as used in the invention is mass% and contains 5% or more of Nb and Zr respectively, and the balance is Ti, C, N, Al, Ca, Mg, O (oxygen), S, etc. A composite inclusion consisting of Therefore, said b_NZAnd c_NZEach of the values is 5 or more. As is clear from the definition, a_NZThe value of is 1 or more.
[0023]
  Here, the “major axis” of the inclusion refers to the maximum diameter among the inner diameters of the inclusion in an arbitrary cut surface, as indicated by a1 or a2 in FIG.
[0024]
  In addition, the content of Nb and Zr in mass% in the inclusions is obtained by analyzing the composition of the central part of the inclusions with an energy dispersive X-ray analyzer (EDX) as shown as b1 or b2 in FIG. This is the calculated value. Specifically, since the measured values of C, N and O (oxygen) are low in reliability in accuracy, Nb, Zr, Ti, Al, Ca, Mg and S except these elements are analyzed, It calculated from the mass ratio.
[0025]
  The “inclusions” as used in the present invention refers to “non-metallic inclusions precipitated or entrained in steel during the solidification process of steel” described in JIS G 0202, for example, the major axis is 100 nm or less. Such fine carbides and nitrides that precipitate when tempering a quenched steel are not included.
[0026]
  In order to achieve the above-mentioned problems, the present inventors have investigated in detail the influence of various inclusions on the pitting corrosion of low alloy steel. As a result, it has been found for the first time that Nb-based inclusions or Nb-Zr-based inclusions unavoidably present in steel strongly affect pitting corrosion.
[0027]
  That is, as described above, conventionally, Nb and Zr have been used as elements for precipitation strengthening and grain refinement by precipitating as fine alloy carbides of 100 nm or less in the heat treatment process. However, on the other hand, a part of Nb and Zr may be concentrated in inclusions inevitably generated in the melting stage, but there has been no example in which the influence of such inclusions has been investigated in detail. It was. Therefore, the inventors investigated the metallurgical factors affecting the occurrence of pitting corrosion in steels containing Nb and Zr, as will be described in detail later, and Nb-based inclusions by the following actions (a) and (b): And Nb-Zr inclusions have been found to strongly affect pitting corrosion resistance.
[0028]
  (A) Occurrence of pitting corrosion is suppressed by uniform dispersion of fine Nb-based inclusions or Nb-Zr-based inclusions.
[0029]
  (B) Coarse Nb-based inclusions or Nb-Zr-based inclusions may cause pitting corrosion and reduce pitting corrosion resistance. Therefore, by eliminating such inclusions, pitting corrosion can be further generated. Deterred.
[0030]
  Therefore, for the reasons of (a) and (b), if the Nb-based or Nb-Zr-based fine inclusions are uniformly dispersed, the pitting corrosion resistance is improved, and the Nb-based or Nb-Zr-based fine inclusions are improved. It became clear that the pitting corrosion resistance can be further improved by preventing the formation of Nb-based and Nb-Zr-based coarse inclusions in addition to the uniform dispersion of materials.
[0031]
  In addition, in the conventional manufacturing method of steel materials and steel pipes, refinement of Nb-based inclusions or Nb-Zr-based inclusions as described above, coarse Nb-based inclusions, and coarse Nb-Zr-based inclusions. Removal could not be achieved. Accordingly, the present inventors have studied various production methods for stably obtaining a desirable inclusion form. As a result, it has been found that, for example, the forms of Nb-based inclusions and Nb-Zr-based inclusions can be controlled by optimizing the steel melting, casting and processing steps.
[0032]
    That is, for example,
  (I) Molten steel holding temperature in the tundish,
  (Ii) cooling rate after casting,
  (Iii) Total working degree after casting,
As a result, by combining these in an appropriate manner, a low alloy steel excellent in pitting corrosion resistance in which Nb inclusions and Nb-Zr inclusions, which could not be obtained in the past, are uniformly finely dispersed, or even Nb and Nb- It has been clarified that a low alloy steel excellent in pitting corrosion resistance that prevents the formation of Zr-based coarse inclusions can be obtained.
[0033]
  Said (1) ~(5)The invention has been completed based on the above findings.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, each requirement of the present invention will be described in detail.
[0035]
  (A) Chemical composition of steel
  First, the chemical composition in the low alloy steel of the present invention and the reason for limitation will be described. In the following description, “%” display of the content of each element means “mass%”.
[0036]
  C: 0.2-0.55%
  C is an element effective for improving the hardenability and improving the strength, and needs to be contained by 0.2% or more. However, if its content exceeds 0.55%, the sensitivity to fire cracking increases and the toughness also decreases. Therefore, the content of C is set to 0.2 to 0.55%.
[0037]
  Si: 0.05-0.5%
  Si is an element having a deoxidizing action, and it is necessary to contain 0.05% or more in order to obtain a sufficient deoxidizing effect. However, when the content exceeds 0.5%, the toughness and workability are reduced. Therefore, the Si content is set to 0.05 to 0.5%. The range of preferable content of Si is 0.05 to 0.35%.
[0038]
  Mn: 0.1 to 1.0%
  Mn is an element having an action of enhancing the hardenability of steel, and in order to obtain this effect, it is necessary to contain 0.1% or more. However, if the Mn content exceeds 1.0%, elution of the steel material is promoted and pitting corrosion tends to occur. Furthermore, Mn segregates at the grain boundaries, leading to a decrease in toughness and SSC resistance. Therefore, the Mn content is set to 0.1 to 1.0%. The range of preferable content of Mn is 0.1 to 0.5%.
[0039]
  Al: 0.005 to 0.05%
  Al is an element necessary for deoxidation of steel, and if the content is less than 0.005%, it is difficult to obtain the effect. On the other hand, even if the content exceeds 0.05%, the effect is saturated, and a large amount of coarse Al-based oxide is generated, resulting in a problem that the toughness is lowered. For this reason, the content of Al is set to 0.005 to 0.05%. In addition, Al in the present specification refers to so-called “sol. Al (acid-soluble Al)”.
[0040]
  Cr: 0.1 to 1.5%
  Cr has the effect of improving the SSC resistance by increasing the hardenability and increasing the temper softening resistance to enable high temperature tempering. In order to obtain this effect, the Cr content needs to be 0.1% or more. On the other hand, even if Cr is contained exceeding 1.5%, the above effect is saturated and the cost is increased. Therefore, the content of Cr is set to 0.1 to 1.5%.
[0041]
  Mo: 0.1 to 1.0%
  Mo has the effects of improving hardenability, increasing temper softening resistance, enabling high-temperature tempering, and improving SSC resistance. However, if the content is less than 0.1%, a sufficient effect cannot be obtained. On the other hand, even if Mo is contained in excess of 1.0%, the effect is saturated and the cost is increased. Therefore, the Mo content is set to 0.1 to 1.0%.
[0042]
  Ti: 0.005 to 0.03%
  Ti fixes N (nitrogen), which is an impurity in steel, as a nitride. This is effective for increasing the strength by refining crystal grains and strengthening precipitation. Further, the steel containing B has an action of suppressing the formation of B nitride and promoting the hardenability by B. In order to obtain these effects, it is necessary to contain 0.005% or more of Ti. On the other hand, when excessively contained, part of the Nb-based inclusions or Nb-Zr-based inclusions are solid-solved to promote the coarsening of the Nb-based inclusions or Nb-Zr-based inclusions. Reduces eating habits. In particular, when the Ti content exceeds 0.03%, the pitting corrosion resistance is significantly reduced. Therefore, the content of Ti is set to 0.005 to 0.03%.
[0043]
  Nb: 0.002 to 0.03%
  Nb is an important element in the present invention. Nb forms carbonitride by itself or forms composite carbonitride with Ti, Zr and Al, refines the structure by the pinning effect, and improves toughness and SSC resistance. Further, when it is uniformly dispersed in the steel as fine Nb-based inclusions, it works to enhance pitting corrosion resistance as will be described later. In order to acquire such an effect, it is necessary to contain Nb 0.002% or more. However, when Nb is contained in excess of 0.03%, a large amount of coarse Nb inclusions are formed as will be described later, resulting in a significant decrease in pitting corrosion resistance. Therefore, the Nb content is set to 0.002 to 0.03%.
[0044]
  In the present invention, the contents of P, S, N and O (oxygen) as impurity elements are defined as follows.
[0045]
  P: 0.03% or less
  P is unavoidably present in steel as an impurity, and is actively dissolved to lower pitting corrosion resistance, or segregates at grain boundaries to deteriorate toughness and SSC resistance. In particular, when the content exceeds 0.03%, the pitting corrosion resistance, toughness, and SSC resistance decrease significantly. Therefore, even if P is mixed as an impurity, its content needs to be 0.03% or less. Note that the P content is preferably as low as possible.
[0046]
  S: 0.01% or less
  S, like P, is inevitably present in the steel as an impurity, and reduces the pitting corrosion resistance and SSC resistance by generating a large amount of easily dissolved Mn-based sulfides. In particular, when the S content exceeds 0.01%, the pitting corrosion resistance and the SSC resistance are remarkably lowered. Therefore, even if S is mixed as an impurity, its content needs to be 0.01% or less. Note that the S content is desirably as low as possible.
[0047]
  N: 0.01% or less
  N is unavoidably present in steel as an impurity, and is concentrated in Nb-based inclusions or Nb-Zr-based inclusions to coarsen these inclusions and lower pitting corrosion resistance. In particular, when the content exceeds 0.01%, the pitting corrosion resistance is significantly reduced. Therefore, even if N is mixed as an impurity, its content needs to be 0.01% or less. Note that the N content is desirably as low as possible.
[0048]
  O (oxygen): 0.01% or less
  O is unavoidably present in steel as an impurity like N, and if it is excessively contained, it forms a coarse oxide and lowers pitting corrosion resistance. In particular, when the content exceeds 0.01%, the pitting corrosion resistance is remarkably lowered. Therefore, even if O is mixed as an impurity, its content needs to be 0.01% or less. Note that the O content is desirably as low as possible.
[0049]
  Said(1)The chemical composition of the low alloy steel according to the invention contains the above-described components from C to O, with the balance being Fe and impurities.
[0050]
  Said(2)The chemical composition of the low alloy steel according to the invention is that the grain refinement by the pinning effect, the effect of improving the hardenability of B by fixing the impurity N in the steel, and the resistance to resistance by fine dispersion of Nb-Zr inclusions. For the purpose of improving pitting corrosionAnd aboveRecord(1)Instead of a part of Fe of the steel of the invention, the following amount of Zr is contained.
[0051]
  Zr: 0.005 to 0.06%
  Zr may not be added. If added, carbonitrides are formed alone, or composite carbonitrides are produced together with Nb, Ti, and Al, and as with Nb, it has a finening effect due to the pinning effect. In addition, since N has a greater affinity with N than Nb and has a strong ability to form nitrides, N, which is an impurity in steel, is fixed as nitrides, and has an effect of promoting the effect of improving hardenability by B. Furthermore, as will be described later, when combined with Nb, Nb-Zr-based inclusions are formed, and the Nb-Zr-based inclusions are uniformly finely dispersed to enhance pitting corrosion resistance. Can do. In order to reliably obtain these effects, it is preferable that the content of Zr is 0.005% or more. On the other hand, when Zr is excessively contained, coarse Nb—Zr-based inclusions are formed as will be described later, and this serves as a starting point for pitting corrosion, thereby reducing pitting corrosion resistance. In particular, when the content exceeds 0.06%, the pitting corrosion resistance is remarkably lowered. Therefore, the content of Zr when added is preferably 0.005 to 0.06%.
[0052]
  Said(3)The chemical composition of the low alloy steel according to the invention of the above is intended to increase the toughness and SSC resistance by enabling high temperature tempering.(1) or (2)Instead of a part of Fe in the steel of the invention, the following amount of V is contained.
[0053]
  V: 0.03-0.5%
  V may not be added. If added, it precipitates as fine carbides during tempering to increase temper softening resistance and enables high temperature tempering to improve toughness and SSC resistance. In order to obtain this effect, the V content is preferably 0.03% or more. On the other hand, even if V is contained in excess of 0.5%, the above effect is saturated and the cost is increased. Therefore, when V is added, the content of V is preferably 0.03 to 0.5%.
[0054]
  Said(4)The chemical composition of the low alloy steel according to the present invention is from the above (1) for the purpose of enhancing the hardenability of the steel.(3)Instead of a part of Fe of the steel of any of the above inventions, an amount of B described below is contained.
[0055]
  B: 0.0001 to 0.005%
  B may not be added. If added, it has the effect of improving the hardenability of the steel in a small amount. In order to obtain this effect, the B content is preferably 0.0001% or more. On the other hand, even if B is contained in excess of 0.005%, the effect is saturated and the cost is increased. Therefore, when B is added, the B content is preferably 0.0001 to 0.005%, and more preferably 0.0001 to 0.003%.
[0056]
  Said(5)The chemical composition of the low alloy steel according to the present invention is from the above (1) for the purpose of preventing nozzle clogging during continuous casting.(4)Instead of a part of Fe of the steel of any of the above inventions, the following amount of Ca is contained.
[0057]
  Ca: 0.0003 to 0.005%
  Ca need not be added. If added, the coarse Al-based inclusions are changed to fine Al-Ca-based oxysulfides, and there is an effect of preventing nozzle clogging during continuous casting. In order to reliably obtain this effect, the Ca content is preferably 0.0003% or more. On the other hand, when the content of Ca exceeds 0.005%, coarse Al—Ca oxysulfide is generated and the pitting corrosion resistance is lowered. Therefore, when Ca is added, the content of Ca is preferably 0.0003 to 0.005%.
[0058]
  (B) Inclusion
  The steel containing Nb and the steel containing a composite of Nb and Zr inevitably contain inclusions in which Nb is dissolved and inclusions in which Nb and Zr are dissolved.
[0059]
  The present inventors have investigated in detail the effect of inclusions in which Nb is dissolved and inclusions in which Nb and Zr are dissolved in pitting corrosion, and have already obtained the findings of (a) and (b). Obtained.
[0060]
  The details of the investigation conducted by the present inventors will be described in detail below.
[0061]
  First, in order to investigate the electrochemical effect of various inclusions on corrosion, deaeration was performed using sintered bodies of various carbides, nitrides, sulfides and oxides having a purity of 99% or more. The electrochemical test was performed in a 25% aqueous solution of “5% sodium chloride + 0.5% acetic acid”. Here, the sintered body of the above-mentioned “carbide” includes not only a sintered body of a single element carbide (for example, “Nb carbide”) but also a composite carbide of two or more elements (for example, Nb and Zr). In addition, a sintered body of composite carbide ("Nb-Zr carbide") of Nitride, sulfide and oxide is also the same.
[0062]
  As a result, carbides, nitrides, sulfides and oxides containing Nb alone or containing Nb and Zr in combination form a non-conductive film on the surface layer and hardly elute themselves. It was revealed that the film acts as a strong n-type semiconductor, accumulates positive charges, and has an effect of promoting anodic dissolution of adjacent Fe.
[0063]
  That is, for each steel shown in Table 1, a cube-shaped test piece having a side of 10 mm was sampled, 5 surfaces of the 6 surfaces of the cube were sealed with silicon resin, and the remaining surface was mirror-polished. The Nb-based inclusions and Nb-Zr-based inclusions were observed with a scanning electron microscope (SEM). Next, a cube-shaped test piece in which five of the six surfaces subjected to the SEM observation described above were sealed with silicon resin was placed in a degassed 25 ° C. “5% salt + 0.5% acetic acid” aqueous solution for 24 hours. After immersion for 720 hours, the occurrence of pitting corrosion was observed.
[0064]
  As a result, in steel in which fine Nb-based inclusions or fine Nb-Zr-based inclusions were uniformly dispersed, these inclusions had an effect of suppressing pitting corrosion. This is because the above-mentioned finely dispersed Nb-based inclusions and Nb-Zr-based inclusions uniformly dispersed make the anodic dissolution of the entire test piece uniform and suppress local anodic dissolution, that is, the growth of pitting corrosion.
[0065]
  On the other hand, in steel containing coarse Nb-based inclusions or coarse Nb-Zr-based inclusions, pitting corrosion occurred starting from these coarse inclusions. This is due to the fact that the anodic dissolution of the substrate adjacent to the coarse inclusion is promoted and progresses from here to pitting corrosion.
[0066]
  Further examination based on the above results revealed that the effect of suppressing the occurrence of pitting corrosion and the action of promoting the occurrence of pitting corrosion were only the size of Nb-based inclusions and Nb-Zr-based inclusions. However, it became clear that it also depends on the composition.
[0067]
  Then, next, the major axis of the Nb-based inclusion or the major axis of the Nb-Zr-based inclusion, the Nb content (%) in the Nb-based inclusion, or the contents of Nb and Zr in the Nb-Zr-based inclusion (Nb ( %) + 0.5Zr (%)) and examined in detail.
[0068]
  As a result, among the Nb-based inclusions whose major axis is 1 μm or more, those satisfying the above formula (1) are 1 mm.²Nb-based inclusions containing 10 or more per electrode and satisfying the above formula (2) are 1 mm in cross-sectional area²Per101 mm or less satisfying the above-mentioned formula (3) among Nb-Zr inclusions whose major axis is 1 μm or more.²Nb-Zr inclusions containing 10 or more per piece and satisfying the above formula (4) have a cross-sectional area of 1 mm.²Per10It is found that the resistance to pitting corrosion is improved when the number is less thanIt was.
[0069]
  ThisHere, as already described, the “major axis” of the inclusion indicates the largest diameter among the inner diameters of the inclusion in any one cut surface, as indicated by a1 or a2 in FIG. The content of Nb and Zr in mass% is a value obtained by analyzing the composition of the central part of the inclusion with an energy dispersive X-ray analyzer (EDX), as shown as b1 or b2 in FIG. Specifically, since the measured values of C, N, and O have low reliability in accuracy, Nb, Zr, Ti, Al, Ca, Mg, and S except these elements are analyzed, and the mass ratio thereof is analyzed. The value calculated from
[0070]
  Therefore, it is a low alloy steel containing Nb.(1)In the present invention, 1 mm of Nb-based inclusions having a major axis of 1 μm or more on any one cut surface satisfying the above-mentioned formula (1).²10 or more per case, and satisfies the above equation (2)Sumo1mm²It was decided that there should be no more than 10.
[0071]
  Moreover, it is a low alloy steel containing a composite of Nb and Zr.(2)In the invention, 1 mm of Nb—Zr inclusions having a major axis of 1 μm or more at any one cut surface satisfying the above-mentioned expression (3).²10 or more per case, and satisfies the above equation (4)Sumo1mm²It was decided that there should be no more than 10.
[0072]
  As already described, a in the above formulas (1) and (2)_NBThe upper limit value is preferably about 15 μm from the viewpoint of ensuring pitting corrosion resistance.
[0073]
  Similarly, a in formulas (3) and (4)_NZThe upper limit value is preferably about 15 μm from the viewpoint of ensuring pitting corrosion resistance.
[0074]
  Note that b in the above equations (1) and (2)_NBMay be a value close to 100.
[0075]
  In addition, b in the equations (3) and (4)_NZThe value of may be as close to 95 as possible, and similarly c in the above formulas (3) and (4)_NZThe value of may be as close to 95 as possible.
[0076]
  The low alloy steel having the chemical composition described in the item (A) is, for example, an Nb-based inclusion and an Nb-Zr-based inclusion described above by optimizing the melting, casting and processing steps of the steel material. It can be made the form.
[0077]
  Low alloy steel is usually melted in a converter or electric furnace, and after being subjected to treatments such as addition of alloy components and so-called “secondary refining”, it is transferred to a ladle, and then from the ladle through a tundish. The steel ingot is poured into a mold to form a steel ingot, and the steel ingot undergoes various processes and is processed into a desired shape.
[0078]
  In the above manufacturing process, the invention of (1)(2)DepartureClearlyIn order to obtain the inclusion form of the low alloy steel, for example, the following conditions (i) to (iii) may be satisfied.
[0079]
  (I) The molten steel holding temperature in the tundish is set to 1520 ° C. or more, and coarse inclusions are aggregated and floated in the tundish to be removed.
  (Ii) The cooling rate after casting, in particular, the cooling rate in the temperature range from 1500 ° C. to 1000 ° C. is set to 50 ° C./min or more to prevent the inclusions from coarsening and uniformly finely disperse.
  (Iii) The total degree of processing after casting is set to 60% or more. That is, the cast round billet, bloom, slab, ingot, etc. are hot or cold processed into a desired shape such as a steel plate or steel pipe. In order to uniformly and finely disperse the Nb—Zr inclusions, the processing after casting is performed with the total degree of processing at the cross-section reduction rate being 60% or more.
[0080]
  Hereinafter, the present invention will be described in more detail with reference to examples.
[0081]
【Example】
  Table 1And Table 2It has the chemical composition shown in29A low-alloy steel of the kind is melted in the converter, and after secondary refining and component adjustment according to each steel, the molten steel is transferred to a ladle, and then a round billet with a diameter of 220 to 360 mm by continuous casting through a tundish. did. In addition, the molten steel holding temperature in the tundish is changed variously using a heater, and after casting, the surface temperature of the billet is measured while adjusting the amount of water for cooling the slab, and the cooling rate after casting is varied. Changed.
[0082]
  The round billet obtained as described above was subjected to piercing with a piercing and rolling mill, hot rolling with a mandrel mill, and dimension adjustment with a stretch reducer by a conventional method to obtain a seamless steel pipe. In addition, the total workability from the cast round billet to the final seamless steel pipe was changed variously by changing the diameter of the billet and the dimensions of the seamless steel pipe.
[0083]
  Table 1And Table 2In steel A~ P and steel 7-10 is a steel of the present invention within the range defined by the present invention in chemical composition, and steels 11-19 are steels of comparative examples in which any of the components deviates from the content range defined by the present invention. .
[0084]
  Here, Table 1And Table 2Steel A inPFor steels 11 to 19, the cooling rate in the temperature range from 1500 ° C. to 1000 ° C. after casting was set to 50 ° C./min or more, and the total workability after casting was set to 60% or more.Also,SpecialAndThe molten steel holding temperature in the dish was set to 1520 ° C. or higher. on the other hand,Table 2For steels 7 to 10, the cooling rate after casting was less than 50 ° C./min, or the total workability after casting was less than 60%.
[0085]
[Table 1]

[0086]
[Table 2]

[0087]
  ThisFrom the seamless steel pipe obtained as described above, two cube-shaped test pieces each having a side of 10 mm are cut out, embedded in a resin so that any one surface becomes an observation surface, and mirror-polished, The observation of Nb inclusions and Nb-Zr inclusions was carried out for each 10 visual fields at a magnification of 50 times or 200 times, the major axis of the inclusions was measured by the method already described, and the center of the inclusions The composition was analyzed by EDX. In each field of view, 1 mm of the Nb-based inclusions or Nb-Zr-based inclusions having a major axis of 1 μm or more satisfying the formula (1) or (3).²1mm that satisfies the above formula (2) or (4)²The number per unit was counted, and the counted values were averaged over 10 fields.
[0088]
  Further, two test pieces each having a size of 3 mm × 10 mm × 40 mm were collected from the seamless steel pipe in the longitudinal direction, polished with No. 600 emery paper, and then subjected to a corrosion test to investigate pitting corrosion resistance.
[0089]
  That is, the polished specimen was immersed in a degassed 25 ° C. “5% salt + 0.5% acetic acid” aqueous solution for 720 hours, and the maximum diameter of pitting corrosion that occurred was measured. In addition, when the maximum diameter of the generated pitting corrosion exceeds 100 μm, SSC starting from the pitting corrosion is likely to occur. Therefore, the target of the pitting corrosion resistance is set to 100 μm or less.
[0090]
  tableTo 3The survey results are organized and shown.
[0091]
[Table 3]

[0092]
  Table 3From the above, the Nb-based inclusions having a chemical composition of steel within the range specified by the present invention and having a major axis of 1 μm or more satisfying the above formula (1) is 1 mm.²Nb-based inclusions containing 10 or more per electrode and satisfying the above formula (2) are 1 mm in cross-sectional area²Per101 mm of Nb-Zr inclusions having a major axis of 1 μm or more and satisfying the above formula (3).²Nb-Zr inclusions containing 10 or more per piece and satisfying the above formula (4) have a cross-sectional area of 1 mm.²Per10Test number 1 to less than16In the case of the maximum diameter of pitting corrosion that occurredIs highVery good pitting corrosion resistance at 50μmHaveIt is clear to do.
[0093]
  In contrast, the chemical composition of steel is within the range specified in the present invention.EvenAmong the Nb inclusions whose major axis is 1 μm or more,(1)Satisfy the expressionThe cross section is 1mm²PerLess than 10 and the above (2) One that satisfies the equation has a cross-sectional area of 1 mm. ² More than 10 per(Test numbers 33 to 35), and the chemical composition of the steel is within the range specified by the present invention.EvenAmong the Nb-Zr inclusions whose major axis is 1 μm or more,(3)Satisfy the expressionThe cross section is 1mm²PerLess than 10 and the above (Four) One that satisfies the equation has a cross-sectional area of 1 mm. ² More than 10 perIn the case (test number 36), pitting corrosion of 230 to 380 μm exceeding the maximum diameter of 100 μm occurs, and it is clear that the pitting corrosion resistance is inferior.
[0094]
  Moreover, also in the case of the test numbers 37-45 using the steels 11-19 of the comparative example from which any of the components remove | deviated from the range of content prescribed | regulated by this invention, pitting corrosion exceeding 100 micrometers in maximum diameter generate | occur | produced. It is apparent that the pitting corrosion resistance is inferior.
[0095]
  That is, in the case of test numbers 37 to 39 using steels 11 to 13 of comparative examples in which the contents of Mn, P and S deviate from the range of contents specified in the present invention, these alloy elements However, the elution of the steel material is promoted, and the pitting corrosion of 180 to 260 μm exceeding the maximum diameter of 100 μm occurs, which is clearly inferior in pitting corrosion resistance.
[0096]
  In the case of Test Nos. 40 to 42 and Test No. 44 using Steels 14 to 16 and Steel 18 of Comparative Examples in which the contents of Ti, Nb, Zr and N deviate from the range of contents specified in the present invention, respectively. In this case, coarse Nb-based inclusions or Nb-Zr-based inclusions are generated, and these serve as starting points for pitting corrosion, so that pitting corrosion of 450 to 600 μm, which exceeds the maximum diameter of 100 μm, occurs, and pitting corrosion resistance It was extremely inferior.
[0097]
  In the case of the test number 43 using the steel 17 of the comparative example in which the Ca content deviates from the content range specified in the present invention, a large amount of soluble Ca oxysulfide is generated, which is pitting corrosion. Therefore, pitting corrosion of 250 μm exceeding the maximum diameter of 100 μm occurred and the pitting corrosion resistance was inferior.
[0098]
  Further, in the case of the test number 45 using the steel 19 of the comparative example in which the content of O (oxygen) deviates from the content range specified in the present invention, coarse oxide remains, which is a pore. Since it became the starting point of corrosion, pitting corrosion of 210 μm exceeding the maximum diameter of 100 μm occurred, and the pitting corrosion resistance was poor.
[0099]
【The invention's effect】
  Since the low alloy steel of the present invention suppresses the occurrence of pitting corrosion starting from inclusions, it does not induce SSC starting from pitting corrosion, and is used for casings and tubing for oil wells and gas wells, for drilling. Drill pipes, drill collars and soccer rods, as well as steel plates and piping materials for chemical plants.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram for explaining a method for measuring a major axis of inclusions and an analysis portion by EDX for obtaining the contents of Nb and Zr in mass% in the inclusions.
[Explanation of symbols]
  a1, a2: long diameter of inclusions,
  b1, b2: central part of inclusions analyzed by EDX

Claims (5)

In mass%, C: 0.2 to 0.55%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.0%, Al: 0.005 to 0.05%, Cr: 0.1 to 1.5%, Mo: 0.1 to 1.0%, Ti: 0.005 to 0.03% and Nb: 0.002 to 0.03%, with the balance being Fe and impurities P in the impurity has a chemical composition of 0.03% or less, S is 0.01% or less, N is 0.01% or less, O (oxygen) is 0.01% or less, and the major axis is 1 μm as an inclusion. 10 or more Nb inclusions satisfying the following formula (1) per 1 mm ² in cross-sectional area, and 10 Nb inclusions satisfying the following formula (2) per 1 mm ^{2 in} cross-sectional area Low alloy steel characterized by
a _NB × b _NB ≦ 150 (1)
_{a NB × b NB ≧ 300 ···} (2)
In the above formulas (1) and (2), a _NB is the major axis (μm) of the Nb-based inclusion, and b _NB is the Nb content (% by mass) in the Nb-based inclusion. In mass%, C: 0.2 to 0.55%, Si: 0.05 to 0.5%, Mn: 0.1 to 1.0%, Al: 0.005 to 0.05%, Cr: 0.1-1.5%, Mo: 0.1-1.0%, Ti: 0.005-0.03%, Nb: 0.002-0.03% and Zr: 0.005-0. The remaining portion is composed of Fe and impurities, P in the impurities is 0.03% or less, S is 0.01% or less, N is 0.01% or less, and O (oxygen) is 0.01% or less. Nb-Zr containing 10 or more Nb-Zr type inclusions having a major axis of 1 μm or more and satisfying the following formula (3) per 1 mm ² in cross-sectional area, and satisfying the following formula (4) A low alloy steel characterized in that the number of system inclusions is 10 or less per 1 mm ^{2 in} cross-sectional area.
a _NZ x (b _NZ + 0.5c _NZ ) ≤ 150 (3)
a _NZ × (b _NZ + 0.5c _NZ ) ≧ 300 (4)
However, equation (3) and (4) the long diameter of a _NZ is Nb-Zr-based inclusions in the formula ([mu] m), the content of b _NZ is Nb-Zr system Nb of inclusions (mass%), c _NZ is the content (mass%) of Zr in the Nb-Zr inclusions. Instead of a part of Fe, by mass%, low alloy steel according to claim 1 or 2, characterized in that it contains from 0.03 to 0.5% of V. The low alloy steel according to any one of claims 1 to 3 , characterized by containing 0.0001 to 0.005% B in mass% instead of a part of Fe. The low alloy steel according to any one of claims 1 to 4 , characterized by containing 0.0003 to 0.005% Ca in mass% instead of a part of Fe.

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